Redefining Construction: An In-Depth Review of Sustainable Polyurethane Applications

Abstract

The construction sector is a prominent resource-intensive industry on a global scale, contributing significantly to environmental challenges through material production and construction operations. Selecting sustainable and energy-efficient building materials is crucial, considering green sustainable construction. Over the past two decades, polyurethane (PU) technology has experienced remarkable progress and is emerging as a versatile alternative to traditional building materials, but concerns still arise due to the petrochemical origins of the PU feedstocks. This increasing demand for eco-friendly building materials has spurred significant advancements in the research, development, and utilization of bio-based polyurethane (BPU) within the construction industry. Responding to the global shift towards sustainable development, this study aims to systematically identify bio-based and sustainable sources of PU, analyse advancements in production processes, and evaluate their properties compared to conventional materials. The purpose of this study is addressed by conducting a literature review, wherein findings from a diverse range of current studies in the field are compiled and the specific application areas covered are PU foam, coating, sealants, concrete systems, adhesives and road construction. The results suggest that BPU present various possibilities, benefits, and challenges. The review underscores BPU as a sustainable alternative with comparable properties to traditional counterparts with insights into BPU market highlighting a promising future in construction. By highlighting potential challenges, this review emphasises avenues for new research. Providing a comprehensive analysis of BPU sources, performance, and applications in construction, it would serve as a valuable resource for researchers, aiding in identifying unexplored research areas, fostering industry collaborations, and recognizing the expanding scope of BPU in the field.

Graphical Abstract

Introduction

Polyurethanes (PU) constitute approximately 8% of the total plastics production, ranking as the 7th most widely utilized polymer globally, with a production of 25.78 million metric tons in 2022 and anticipated growth of 5.88% in future [1]. The widespread adoption of PU is attributed to its outstanding properties, encompassing flexibility, corrosion resistance, chemical resistance, high tensile strength, low-temperature comfort, adhesiveness, and adaptability in chemical structure, making it a preferred choice among researchers [2]. PU features urethane groups as the primary repeating units along its main backbone chains, formed through the reaction of hydroxyl (–OH) terminated compounds with isocyanate (–NCO) terminated compounds. Additionally, other functional groups may be present in the end structures of PU chains, including ethers, esters, urea, biuret, and aromatic moieties [3,4,5].

Amongst innumerable applications of polymers, the construction sector employs a variety of polymeric materials. Fossil fuels, metals and energy are a few non-renewable sources frequently employed in construction [6]. The building and construction sector is undoubtedly one of the most resource-intensive sectors on a global scale [7], and cement manufacture is one of the world’s massive industries [8]. Concrete, which comprises mainly of aggregate, water, and often cement as a binder, is one of the most significant building materials. The chemical activation of the binder by the water results in the formation of a paste, which hardens the inactive aggregate into a stone-like mass. Its widespread use stems from the fact that it is inexpensive and easily accessible, can be moulded into diverse element shapes and sizes, and is moisture resistant [9].

In the last century, building material use has grown proportionally with productivity [7]. Figure 1 shows the production of cement for years. In the year 2021, the global production of cement was estimated to be approximately 4.4 billion tons [6]. Since 1950, worldwide cement manufacturing has more than tripled and almost quadrupled since 1990, growing faster than fossil energy production [10]. The demand for building materials has tripled from 6.7 billion tons in 2000 to 17.5 billion tons in 2017 [11].

Fig. 1

Global cement production over years [12]

Whether via the production of building materials or actual building operations, construction causes a variety of additional environmental issues. Each year, building materials create one million tonnes of trash worldwide [13]. The usage of concrete in construction is responsible for the consumption of approximately 20 billion tonnes of aggregates, 1.5 billion tonnes of cement, and 800 million tonnes of water annually [14]. Concrete is widely utilised in constructing various structures, such as bridges, roads, dams, and buildings. However, the production of concrete emits an excessive quantity of carbon dioxide (CO₂) annually, thereby contributing to environmental degradation. According to the National Ready Mixed Concrete Association, each pound of concrete released into the atmosphere emits 0.93 pounds of carbon dioxide [15]. Cement manufacturing is a significant source of carbon dioxide emissions (CO₂) [16]. According to research by the IEA, the direct CO₂ intensity of cement manufacturing grew by around 1.5% between 2015 and 2021 (Fig. 2) [17]. Although cement makes up just 12–14% of the final concrete mix, the transport and mining of aggregates and steel production for reinforced concrete add more energy inputs. Road transport infrastructure, notably asphalt paving, emits 1% more dioxin emissions annually. Sand and gravel mining in stream channels damages public and private property [7].

Fig. 2

CO₂ emissions by different sectors [17, 18]

Clearly, steps are required to increase the sustainability of the building environment and construction activities [19]. Eventually, the construction sector must adopt a new way of operation that places environmental considerations at the heart of its operations, as opposed to its traditional, environmentally unconcerned approach. Therefore, the sustainability of constructions must be thoroughly investigated [20]. Selecting eco-friendly and energy-efficient building materials may significantly reduce CO₂ emissions and improve the sustainability and energy efficacy of our structures; hence green sustainable construction is crucial [19]. Polymeric material has played a significant role in the building industry over the last 50 years, and there are many ongoing studies in different countries to employ polymers for diverse uses in construction [21]. Diverse construction-related studies are focusing on the use of new, sustainable materials to replace cement as the binding material in conventional concrete in order to minimise carbon dioxide emissions and generate green concrete [22]. It is anticipated that construction-related applications of polymers will dominate the market for polymers [23]. Table 1 below shows different polymers employed in the construction sector along with their advantages. Polymer materials possess immense potential for application in the construction industry, primarily due to their superior features in comparison to inorganic materials. These features include excellent waterproofing capabilities, anti-corrosion properties, high wear resistance, anti-seismic abilities, lightweight nature, good strength, insulation, heat insulation, strong electrical insulation, and vibrant colours [21, 23].

Table 1 Construction applications of different polymers

Out of all polymers, in the previous two decades, PU technology has seen incredible advancements [23, 24]. Within infrastructure industries, PU has several uses. PU products are widely used across various applications. The most commonly utilised products include rigid and flexible foams, coatings, castable elastomers, fibres, textiles, adhesives, sealants, thermoplastics, millable gums, hybrids, and composites. These products exhibit versatility and offer a range of properties, making them suitable for various applications [3]. The characteristics of PU that make it a functional building material are shown in Fig. 3. Many companies worldwide are interested in investing in PU because of its promising future. However, toxic, costly, and made from dwindling supplies of petrochemical-based raw materials, synthetic PU materials are a major environmental problem. Thus, sustainability must be sought not just by replacing construction materials with new materials but by selecting sustainable materials. This would mean completely replacing unhealthy traditional practices with greener alternatives that are nature friendly.

Fig. 3

Properties of polyurethane

Constantly considering environmentally friendly choices is essential for many industry types due to rising worries about greenhouse gas emissions, fast fossil fuel depletion, tight monitoring of carbon dioxide footprint in large-scale manufacturing units, etc. One of the most versatile materials on the market today, bio-based polyurethane (BPU) has the potential to significantly advance environmental goals linked to waste reduction and sustainability. The construction sector is one of the most significant users of plastics [3]. Thus, BPU has considerable potential in this market sector as a preferred alternative that offers additional benefits, such as low-cost constructions, a long service life, corrosion resistance, and lightweight properties. Bio-based materials are derived from existing matter, such as biomass, and can either occur naturally or result from products developed through advanced technologies that utilise biomass. The use of bio-based materials in innovative and current ways is still growing and is expected to surpass traditional materials shortly. The incorporation of sustainable elements in the construction industry has been proven to reduce climate-related issues, including global warming and the depletion of resources on a global scale, according to various studies [25, 26].

Bio-based alternatives to petrochemical based raw materials have become a need due to the worldwide trend toward green goods and sustainable development. Despite the fact that the BPU sector is still in its infancy on a worldwide scale, the growing acceptance of bio-based products by a variety of industrial segments guarantees exponential development in the future. The building sector was the biggest end, consumer of BPU in 2015 [26]. It is predicted that the worldwide market for BPU would expand by more than 6% during the next several years (2021–2027). Numerous companies, including Lubrizol Corporation, Huntsman International LLC, Covestro AG, BASF SE, Mitsui Chemicals Inc, and others, have already entered the market for BPU [27]. With its huge and constantly expanding end-use sector, notably in countries like China and India, Asia-Pacific held the central position in the worldwide BPU market. In terms of infrastructure development, for instance, between 2020 and 2030, India is predicted to have the world’s highest growth rate of 9.8%, as reported by Oxford Economics [26, 28]. A breakdown of the BPU market by industry is presented in Fig. 4.

Fig. 4

Global market share of various sectors utilising bio-based polyurethane [27]

PU is multi-functional. Because of its versatility, availability, energy-saving properties and recycling, many sectors favour it. As a result of all of these features and benefits, PU is quickly becoming a more popular alternative to traditional building materials. It replaces them, either partly or entirely, as aggregates, binders, composites, additives, and coatings [2, 29, 30]. On a global scale, the PU revolution in civil engineering leads to a reduction in the carbon footprint, which in turn improves the environment.

Thus, the primary objective of this review is to conduct an in-depth analysis of the most recent advancements and uses of PU materials, which make them such an environmentally friendly option in the building and construction business. This study aims to systematically identify bio-based and sustainable sources of PU, analyse advancements in production processes, and evaluate their properties compared to conventional materials. The purpose of this study is addressed by conducting a literature review, wherein findings from a diverse range of current studies in the field are compiled and the specific application areas covered are PU foam, coating, sealants, concrete systems, adhesives and road construction. Figure 5 depicts the general flow of the review. First, this review briefly introduces PU and its various properties and applications. Various novel bio-based sources of PU are explored in brief. Further, an explanation for various novel applications of BPU is given in detail. Thus, this review mainly focuses on the application perspective of PU developed from bio-based sources. Further the environmental impact of using BPU in place of conventional materials is evaluated and future scope is explored. Through this review, we wish to highlight the difficulties encountered in applying BPU for specific applications and novel research areas that can be explored in the sustainable applications of natural materials-based PU. This review will provide an detailed evaluation of BPU materials’ advancements, potential uses, and challenges in the building and construction sector. Providing a comprehensive analysis of BPU sources, performance, and applications in construction, it would serve as a valuable resource for researchers, aiding in identifying unexplored research areas, fostering industry collaborations, and recognizing the expanding scope of BPU in the field. The methods for data extraction and utilization is described in the “Methodology for Data Extraction” section below.

Fig. 5

Flow of the review

Methodology for Data Extraction

Inclusion and exclusion criteria Total of 26 studies are thoroughly analysed and included in this review paper. Studies included in this review paper are limited to use of polyurethane from sustainable sources for construction related applications. Two criteria were considered for inclusion of studies.

• 1. The study must have used or synthesised polyurethane from bio-based and sustainable sources that included nature-based materials and waste products.

• 2. The utilised or synthesized polyurethane must be developed for some application within the construction sector.

Any studies out of these criteria were not considered. Example: Papers demonstrating application of polyurethane for construction but the polyurethane is not from bio-based source or polyurethane is from sustainable source but its applications are not in construction.

Search strategy Comprehensive search was carried out utilising databases like Science direct, PubMed, Google Scholar. Keywords used were “Polyurethane Waste”, “Bio-based Polyurethane”, “Construction”, “Sustainable” and related words. The access to the journal articles and databases was provided by the Institute. Data Points of Interest: Data extracted from the included studies focused on source and synthesis of Polyurethane, Its Physical, chemical, mechanical and thermal properties and its successful application in the field of Construction.

Data extraction A standardised data extraction outline was created which included predefined categories for each data point. Example: If a study reports use of polyurethane from particular source for let’s say sealant application, then the outline included fields for Source of Polyurethane, Synthesis method used (if applicable), characterization techniques used and the application intended.

Data synthesis The data extracted from the included studies were synthesized using a qualitative approach. This involved summarizing key findings related to the synthesis approach, properties and limitations of Polyurethane used in specific application area. A brief outline was developed covering all potential application fields within Construction sector and the summaries from the studies analysed were the included in corresponding sections.

Data presentation The data extracted are presented in figures, tables, or graphs, with labels and captions explaining the significance of each data point. The software used for this purpose includes Microsoft PowerPoint, ChemDraw, Wondershare Edrawmax and Canva.

Polyurethane

Structure, Properties and Applications

PU has grown into one the most promising polymers with adaptive properties that suit practically all the fields of polymer applications, like foams, adhesives, fibres, sealants, coatings, elastomers, etc. The chemical structure of PU comprises a diol with hydroxyl (–OH group) and a di or polyisocyanate (–NCO group) joined by urethane linkages. An exothermic reaction between polyol and isocyanate gives PU, as shown in Fig. 6. Different materials with desirable mechanical properties can be obtained from the synthesis method adopted for PU [31, 32]. The three primary elements in the production of PU are polyol, diisocyanate, and chain extender. The polyols and diisocyanates used for producing PU significantly impact the material’s mechanical and physical characteristics. The backbone of PU is characterised by hard (isocyanate part) and soft (polyol part) segments (Fig. 6). The hard component has a higher glass transition temperature, giving PU better mechanical properties and crystalline behaviour. In contrast, the soft segments have a low glass transition temperature providing flexibility to the structure. This difference in glass transition temperature and other characteristics of the two parts causes microphase separation in the system, which is the main feature accountable for PU’s wide variation of properties. As a result, the tensile characteristics of PU can be adjusted by changing the type and concentration of isocyanate and the amount and molecular weight of polyol.

Fig. 6

General structure of polyurethane

PU technology has advanced dramatically over the past two decades. PU is a well-chosen multi-functional material due to its excellent properties and morphology, such as the ability to change the microstructure. It results in diverse types with contrasting mechanical properties, including stiffness, flexibility, damping properties, and resistance to impact, wear and climatic conditions. The molecular composition of PU allows it to be altered to become the toughest plastic and the most malleable and softest rubber. According to their uses and practicality, PU products can be split into two major groups: rigid and elastic. Rigid PU products include solid foams, structural foams, rigid polymers, and wood alternatives, while elastic PU products include elastomers, flexible foams, coatings, adhesives, and fibres [4, 33]. Because of this, PU has become an essential component of the building sector. Numerous technologies are readily available to the construction industry, including protective coatings, seals, flooring, waterproofing, adhesives, insulating foams, and leak sealers or crack-injecting devices [2]. Table 2 lists the features that make PU acceptable for specific applications.

Table 2 Construction applications and properties of polyurethane

Bio-Based Polyurethane: Sources

Using traditional construction materials has four major environmental impacts: greenhouse gas emissions, energy expenditure, and natural resource depletion, which results in massive waste generation. Utilising waste materials and finding substitutes for natural resources can help reduce the overall effect. Plant-based products offer a superior alternative that benefits the environment, human health, and a pleasant environment. The building industry has used various plant-based products, including wood, hemp fibres, mycelium, mussel shells, straws, reed, etc. [26]. BPUs are being designed to propel various industries to new heights of sustainability and environmental considerations. BPUs are rapidly acquiring appeal in various sectors, including coatings and paints, electronics, building, transportation and housing [34, 35].

Plant-based substances serve as the fundamental building blocks for the development of bio-based monomers. Oils, proteins, lignin, cellulose, glucose, sugar, and biomass like cooking oil and cashew nut shell oil have all been employed. A variety of methods, including thermochemical, chemical catalysts, and microbial processes, have been used to transform these renewable raw materials into monomers. Figure 7 depicts the various sources of BPU [36, 37]. The derived bio-based monomers are utilised in numerous conventional PU production procedures to yield a partially bio-based PU. Bio-polyol is an essential component in the production of PU [28, 38]. Natural oil-based polyols have similar origins and uses, but their compositions differ depending on the manufacturing process. PUs, which are better in terms of chemical adaptability and practicality, are produced due to the chemical reaction between bio-based polyols and synthetic diisocyanates. Researchers have been looking into more ecologically responsible ways to make PUs and use renewable resources. Several publications have outlined non-isocyanate methods and bio-based isocyanates to avoid using sensitising synthetic isocyanates and dangerous phosgene to produce isocyanates [39,40,41,42].

Fig. 7

Bio-based sources of polyurethane

Most bio-based projects expanded to commercial levels involve the polyol component. An existing industrial source of polyols is the use of precursors obtained from plant oils, and research is presently being done to create novel synthetic pathways [43, 44]. Many vegetable oils (VOs) have been used for many years as agrochemicals, varnishes, dyes, lubricants, and plasticisers. Due to their affordability, renewable nature, high purity, and abundance have drawn the attention of various industries [45, 46]. A number of methods are used for synthesising these polyols from VOs. The most popular and sustainable sources of oil polyols are castor oil (CO), soy oil, peanut oil, canola oil, rapeseed oil, and sunflower oil [36, 39]. Fibre reinforcement is explored to improve the properties of bio-based PU [47].

Numerous researchers have written comprehensive reviews on synthesising PU from bio-based polyols and isocyanates [48, 49]. Hai et al. have thoroughly reviewed the biological sources of producing isocyanates and polyols. It highlights the pathways to synthesise completely green precursors for PU [50]. Apart from these mentioned bio-based sources, another viable solution to mitigate the detrimental effects of PUs on the environment and combat the challenges associated with their end-of-life disposal a circular economy strategy centred on chemical and biological depolymerisation of PUs can be adopted [51, 52]. The underlying principle of chemical and biological recycling involves the utilisation of energy-efficient and environmentally sustainable processes to obtain materials with properties similar to virgin materials, which can be effectively integrated into the production of PUs or other types of polymers. Fonseca et al. [53] have done a comprehensive study of PU depolymerisation, highlighting the environmental benefits of reducing waste and detailing various pathways of achieving depolymerisation.

In conclusion, using plant-based materials, including PU, and recycling PU waste into newer products offers a promising alternative to reduce the environmental impacts. Using different methods, PU can be produced from various plant-based sources, including oils, proteins, lignin, cellulose, glucose, and sugar. The use of VOs as a source of polyols for PU has gained particular attention due to their affordability, renewable nature, high purity, and abundance.

Bio-Based Polyurethane: Applications

Traditionally, both residential and commercial buildings extensively employed non-natural materials, such as asbestos in the past, petroleum-based paints, coatings, and adhesives, and conventional PU in panel and roofing systems. Over time, experts and end users have progressively advocated for safer, recyclable, and bio-based alternatives. This increasing demand for eco-friendly building materials has spurred significant advancements in the research, development, and utilization of BPU within the construction industry, reflecting a broader shift towards more sustainable practices [54].

Polyols and isocyanates serve as crucial building blocks for the creation of polyurethanes. Notably, bio-polyols, derived from VOs, are gaining prominence as a sustainable substitute for fossil fuel-based PUs. As polyols represent a key constituent in PU formulations, the integration of bio-based polyols derived from natural sources enhances the renewable carbon component in the end product [55]. Consequently, the utilization of such polyols is progressively contributing to the expanding global bio-economy, aligning with the contemporary drive for more sustainable materials. For concerns relating to isocyanate part, there is substantial research going on in developing bio-based isocyanates and non-isocyanate polyurethane (NIPU) which do not involve the use of isocyanate in their synthesis. In 2022, the estimated market size for BPU globally was USD 36.39 million and is expected to grow even more in coming years [27]. Government initiatives and increasing customer preference for sustainable materials have propelled the commercial adoption of BPU. Various companies have started developing BPU based counterparts of conventional PU based materials and various projects are being planned to implement BPU based materials for practical applications. For example,

• 1. The BIOPURFIL project [56], funded by the European Union, brought together a collaborative effort of experts from Europe and South America. This initiative has harnessed cutting-edge technologies to produce bio-based composites featuring PU matrices reinforced with natural fillers. To achieve this, the project partners have synthesized a range of natural polyols sourced from various materials, including rapeseed oil, palm oil, tall oil (a by-product of wood pulp and paper mill operations), castor oil, linseed oil, and soybean oil. The project employs commercially available micro-cellulose and sisal as natural fillers and incorporated glycerol as a reactive modifier to facilitate chemical reactions. These bio-based polyols are utilized to craft a diverse array of materials, including porous and rigid foams and non-foam bio-polyurethanes. Scientific investigations examine the impacts of bio-polyols, natural fillers, and glycerol on foam density, physicochemical and mechanical properties, and the materials’ underlying structure. Their research provides valuable insights into the suitability of these materials for various application methods, such as spraying, molding, and pouring systems. Formulations are further optimized to produce a diverse range of bio-based composites. Notably, the preparation processes for these bio-composites predominantly avoided the use of solvents and minimize the utilization of catalysts. The BIOPURFIL project stands as a crucial step in developing environmentally friendly alternatives to conventional fossil fuel-based PUs.

• 2. The BIOMAT project [57] is a pioneering 4-year European endeavour focused on creating innovative bio-based materials. The consortium identifies a substantial demand for nano material incorporated Bio-Polyurethane foams in key markets, including building, construction, automotive, and furniture & bedding. To fulfil this mission, BIOMAT aims to develop technologies for making bio-based PUR foams, incorporating high-performance nano-fillers, adhesives, and bio-polyols and providing support for businesses. The project aims to create materials with exceptional heat insulation properties, improved thermal and mechanical performance, noise insulation, flame resistance, hydrophobicity, and hydrolysis stability, while reducing energy consumption and offering antifungal and anti-rotting attributes. Specifically, BIOMAT-TB will facilitate the development of PU foams, tailored for applications in waterproofing, construction, and building industries. They’re also working on making the manufacturing process more efficient. This will help companies quickly use these materials in their products. They are working towards microwave assisted and enzyme degradation of BPU based foams and composites, to promote use of recycled polymers. The initiative stands as a remarkable stride towards enhancing sustainable materials and fostering innovation across industries.

• 3. To address the challenge of recycling Polyurethane foam (PUF), Circularise, a Netherlands based company working on model of circular economy has partnered with Circular Foam [58], an interdisciplinary research project focused on closing the loop in the PU foam value chain through the development of an efficient recycling process. This project encompasses the construction and appliance sectors, where PU foam is commonly used in building and refrigeration applications. Covestro, a polymer company, leads Circular Foam, and various other key partners contribute their expertise to the initiative. These partners include Interzero, a recycling agency working on dismantling strategies; Unilin, a producer of PU insulation boards, conducting tests on alternative recycled materials; Ruhr Universität Bochum and TU Dortmund, providing research on the blueprint for assessing systemic solutions; and IZNAB, a consultancy agency responsible for dissemination activities. By collaborating with these stakeholders, Circular Foam aims to revolutionize PU foam recycling and reduce its environmental impact.

• 4. Covestro [59], a prominent part of the plastics industry, has made significant strides in the development of BPU at an industrial scale. They work on innovative Mass balance approach [certified by International Sustainability and Carbon Certification (ISCC)] which is like a way to keep track of how much of a product comes from renewable or recycled materials when they’re mixed in with other materials during manufacturing. It helps determine the portion of the final product that is environmentally friendly. Through this approach they have successfully synthesized eco-friendly alternatives to conventional isocyanates and polyols by incorporating renewable precursors from biowaste and residual materials. Notably, these include renewable toluene diisocyanate and climate-neutral methylene diphenyl diisocyanate, along with sustainable polyols, maintaining high product quality and meeting industry demands. The distinctive “CQ” suffix in the product name, denoting “Circular Intelligence”, underscores Covestro’s commitment to circularity while facilitating a seamless transition to a circular economy without the need for technical alterations or increased risk at same processing conditions. Their product, Desmodur CQ MS is employed in the production of a new range of PU insulation boards, boasting a 43% reduction in embodied carbon emissions compared to fossil-based foams, while maintaining durability and heat transfer performance. Key benefits include net zero CO₂ emissions throughout life cycle, 60% share of sustainable materials, low maintenance, weather, chemical and UV resistant and saves up to 100 times more energy that what is used to prepare it. This innovation addresses the growing demand for environmentally sustainable insulation solutions in the construction industry, ultimately contributing to lower life cycle emissions in buildings and aligning with the industry’s push for net zero emissions by 2050.

• 5. In 2021, WeylChem [60] introduced Velvetol, a product derived from renewable 1,3-propanediol. As demonstrated by a peer-reviewed International Organization for Standardization (ISO) 14,000-compliant life cycle assessment, when compared to petroleum-based poly-(tetramethylene-ether) glycol (PTMEG) production, Velvetol exhibits notable environmental advantages. It offers a 50% reduction in global warming potential and a 33% decrease in non-renewable energy consumption. Velvetol is entirely bio-based, characterized by high reactivity and excellent hydrolysis resistance. Additionally, it enables high abrasion resistance in final PU components, enhances flexibility, particularly at lower temperatures, improves softness, and contributes to overall toughness in PU parts. Its lower viscosity and melting point, compared to PTMEG, which facilitates easy processing and handling.

• 6. There are companies working towards recycling of PU. BASF [61] is actively advancing circularity initiatives by recycling mattresses to reclaim raw materials of a quality equivalent to virgin resin. Their process involves the breakdown of flexible PU to capture polyols. They anticipate bio-based foams and recyclability of PU to become more prevalent in future. Additionally, Dow’s [62] Renuva mattress recycling program employs chemical recycling to transform discarded mattress foam into raw polyols.

Thus, the construction sector significantly utilizes BPU, driven by the increasing demand for energy-efficient buildings. BPU finds applications in insulation, coatings, and adhesives, among others. The market revenue for BPU is experiencing growth, propelled by the escalating demand for eco-friendly construction materials. Inspite of such a development the used of BPU in actual construction is seldom touched. Although there is very limited data that shows use of BPU in real applications in buildings, there is huge data wherein many researchers have shown that plant-based counterparts have performed well and similar to the traditional counterparts which is explored further in this review paper under individual sections. While bio-based projects involving polyols are widely expanding, researchers are continuously exploring new synthetic pathways to enhance PU production’s sustainability and environmental impact. The following sections will explore sustainable construction applications of PU products, and Table 3 summarises the studies analysed under various applications within the Construction sector.

Table 3 Summary of the studies analyzed under various applications

Polyurethane Foams

Overview

Polyurethane foam (PUF) is a multipurpose chemical product utilised in a wide variety of conventional building applications, including bonding, filling, sealing, and insulating. It is ideal for water pipe insulation, roof and wall bonding, sealing, and door and window frame installation because of its superior temperature and sound insulation capabilities [24, 88].

There are two primary varieties of PU foams (PUFs): flexible foam (F-PUF) with an open cell structure and rigid foam (R-PUF) with a closed cell structure. By adjusting (i) the monomer structure and amount, (ii) the catalysts, (iii) the surfactants, and (iv) the blowing agents (BA), the structures and characteristics of PUFs may be modified. In addition to this fundamental list, several additional substances, including fillers, flame retardants (FR), pigments, and dyes, are also used [89]. Nowadays, it is possible to manufacture foams with flame-retardant qualities, a subject of great interest [90]. The very recent advancements in the field of sustainable PUFs have been very well highlighted in the review done by Peyorton et al. [28]. Alinejad et al. [91] have given comprehensive details on using lignin to produce PUFs intended for various applications within the construction, including elastomers, adhesives, etc. There is an increasing trend of reinforcing PUF with natural fibres due to their eco-friendliness and enhanced properties [92].

Thermal insulation is nowadays, an important research topic due to the growing energy demands of heating and cooling the building structures [93]. One of the major applications of R-PUF is thermal insulation. In cooler climates, PUs can significantly minimise heat loss in houses and workplaces and keep buildings cool in the summer, reducing air conditioning use. Although various new materials are being developed, R-PUFs remain a preferred choice. Therefore, it is a present need and a challenge to establish PU foams from sustainable sources with better properties than the traditional fossil fuel-based equivalents.

Traditional materials have a comparatively high heat transfer coefficient $\lambda$ (measure of how quickly heat can move through a material), which can be only compensated by increasing the thickness of the material to achieve better insulation, which adds to material requirements and, in turn, construction costs. This has drawn attention towards using R-PUFs in a metal framework termed as sandwich panel [35]. PU is a common sandwich panel core material because of its low heat transfer coefficient, $\lambda$ = 0.23 (W/mK). Sandwich panels with a PU core are inexpensive, lightweight, easy to handle and carry, and resistant to pest damage and chemical degradation [94]. Consequently, they are adaptable to all climates and seasons and have several applications.

Applications

As discussed above, there is increasing interest in producing PU starting from oil-based polyols. Andersons et al. [63] developed R-PUFs starting from tall oil obtained from the side stream of cellulose pulping. Two different polyols were produced, one was a low functionality polyol obtained by esterification of tall oil carboxylic acid with trimethanolamine, and another was a high functionality polyol obtained by epoxidation followed by esterification of the carboxylic acid group and finally, ring opening by trimethyolpropane. A two-step process was employed: first, the homogenous polyol component was developed by mixing and mechanically stirring for 60 s at 2000 rpm, the aforementioned polyols, glycerol as a crosslinking agent, tris(1-chloro-2-propyl) phosphate as a flame retardant, catalysts PC CAT NP10 (tertiary amine-based) and PC CAT TKA30 (30 wt% of potassium acetate in diethylene glycerol), Niax Silicone L-6915 as a surfactant, and water as a blowing agent. This mixture was conditioned for 2 h to remove air. Further, the Methylene diphenyl diisocyanate (MDI)-based isocyanate component Desmodur 44 V20 L was added to the polyol system with mechanical stirring for 15 s at 2000 rpm. This mixture was then cast into stainless steel moulds of dimensions 145 $\times$ 145 $\times$ 50 mm, preheated to 50 $^\circ$C. Finally, after post-curing and conditioning procedures, a PU foam called TMP PU based on tall oil and trimethylolpropane was obtained, which had 21.3% renewable material by weight. When water was not used as a blowing agent, the samples were called monolithic TMP PU. Different tests were employed for PU foams and monolithic PU samples as they had different densities and dimensions, as revealed by scanning electron microscopy (SEM) images. Thermal conductivity of PU polymer that was milled, crushed, and compressed was found to be $\lambda _p$ = 223 mW/mK, PU stiffness and yield strength were done by nanoindentation of foams, which yielded values of ca. 2.5 GPa and 110 MPa, respectively, and tensile tests gave a modulus of ca. 3 GPa and a strength of 64 MPa. The TMP PU foams revealed thermal conductivity, compressive strength, and stiffness comparable to their commercially available counterparts. The Young’s modulus values (measure of how stiff a material is, used to understand how much a material will deform under stress) of these isotropic PU foams were lower than those of the anisotropic commercial PU foams. However, because the thermal conductivity and strength properties, which are fundamental for thermal break applications, are in the predicted range, it can be said that these foams made from renewable resources can be a better substitute for conventional, non-sustainable materials. It is important to note that although PU materials contain a renewable polyol component, they cannot be classified as completely sustainable as the isocyanates used still pose a question.

Sandwich panel is being used more frequently for a variety of structural purposes, particularly in the civil engineering and rehabilitation fields. This is due to the high ratios of stiffness to weight and strength to weight, as well as the flexibility provided by the wide variety of geometrical choices and materials accessible. Given its low heat conductivity, affordable expense, and ease of processing and free-form shaping, R-PUF is one of the most frequently used core materials in sandwich panels for use in structural engineering applications [95]. Numerous examples in the literature use PUFs and their modifications in sandwich composites [96]. One such interesting application is a modular sandwich panelised system for post-disaster housing. A prefabricated composite wall system has been developed by Manalo et al. [64] from glass fibre-reinforced PUFs and MgO board, which showed potential for real-life applications with further improvements. In a research done by Sharafi et al. [65], two 3D high-density polyethylene (HDPE) sheets serve as the system’s exterior, and a R-PUF centre serves as the system’s inner structure. Under different pressure circumstances, HDPE sheets made with a notched surface significantly improve the sandwich panel’s stress distribution, buckling efficiency, and strength. The findings demonstrate that the method complies with the requirements of the American Society for Testing and Materials (ASTM) standards for semipermanent dwellings and temporary accommodations and meets their needs for post-disaster housing. Research is being carried out to develop foams from plant-based polyols or make natural fibre-reinforced PU composites and using nanoparticles to improve the performance of the PUFs; however, practical applications have not been studied or demonstrated yet [35, 97]. Although newer composites and materials are being developed, more research is required to obtain sustainable systems. The above mentioned innovations surely pave the way for use of PUFs but their origin hinders their classification as sustainable alternative. Exploring the use of PU from renewable sources in above mentioned applications holds significant potential for further investigation.

Coating Applications

Overview

The widespread use of reinforced concrete in infrastructure frameworks such as buildings, bridges, roads, etc., necessitates sufficiently robust materials to sustain decades of operation in harsh conditions with little performance degradation. However, with time, the penetration of reactive substances such as chloride, water, carbon dioxide, and oxygen damages concrete buildings, especially the steel inside them. This may lead to concrete cracking and enable corrosion and oxidation, ultimately leading to the eventual breakdown of the entire structure. Consequently, organic coatings offer versatile and efficient steel and reinforced concrete protection. Bio-based coating’s primary purpose is to prevent water, oxygen, and soluble salts from permeating concrete and has proven to lower the pace of steel bar corrosion in concrete significantly. Because of their chemical compositions, including urethane groups, PU is ideal for use as a coating material in various industries, including the pharmaceutical, building, construction, automobile, packaging, marine, furniture, and technological sectors. Owing to potential renewability, affordability, and broad accessibility, using VO’s in producing polyols and, subsequently, PU has attracted significant attention. Vegetable oil-based PU coatings have shown their worth by exhibiting remarkable features such as high strength, wear resistance, corrosion and chemical resistance, thermal flexibility, prospering industrial uses, and reducing or avoiding utilising volatile organic compounds (VOCs). A significant study is being done on waterborne polyurethane (WPU) dispersions, which use water as the solvent media as an alternative to the common PU that uses petrochemical solvents. The inherent advantages of WPU dispersions over traditional PU are their eco-friendliness, sustainability, availability, reduced toxicity, and safety [98]. Interest in WPU solutions in the coatings sector emerges due to increasingly meeting the quality of solvent-borne coatings concurrently with the introduction of stricter legislative restrictions.

Despite this, organic coatings are often damaged under the cumulative impacts of environmental elements, such as ultraviolet (UV) radiation, humidity, oxygen, heat, and wear and tear, through photo-chemical, thermal-oxidative, and hydrolytic degradation reactions that diminish overall protective efficiency. Numerous researchers have investigated how environmental stresses lead to the deterioration and, eventually, the impairment of the protective efficacy of coatings [99]. The degree to which a coating adheres to concrete is one of the important factors to consider. Given the significance of coatings on wood, concrete, steel, etc., in the building industry, researchers are working to enhance the effectiveness of bio-based sustainable coating alternatives.

Wood Coating

Wood is the earliest material utilised by humans for construction. It distinguishes itself by being readily accessible, easy to use or remodel, and delivering a comfortable indoor atmosphere. It enhances the energy efficiency of buildings as a result of its superior insulating characteristics. It controls moisture and removes contaminants from the air in the space. For wood to retain its aesthetic value, it requires a covering—especially when used exterior—to preserve it from water, UV radiation, and chemical and biological impacts. As a result of its expanding and shrinking characteristics, wood provides a formidable task for coatings. Appropriate wood coatings may be utilised as protective measures to fulfil present structural requirements for durability and aesthetic soundness. They are intended to prevent the infiltration of moisture and the degradation of lignin by UV radiation.

Ayuso et al. [66] synthesised WPU coating utilising green co-solvents and studied its functional characteristics. The structure and, thus, the performance of the coating film depends upon the hard and soft segment content, structure of the isocyanate group, amount of emulsifier, chain extender and the ratio of NCO and OH groups. The polyol part with multiple –OH groups is considered the soft segment, and the isocyanate group, along with chain extender and internal emulsifiers, is regarded as the hard segment. The authors explore the influence of three different green co-solvents $\gamma$-valerolactone (GVL), Dihydrolevoglucosenone (Cyrene, CY) and propylene carbonate (PC) on the synthesis, dispersion process and the performance of two PU formulations. The rigid PU formulations had 1,3-butanediol as the chain extender and a higher amount of hard segment (49%) suited for wood coating. The flexible PU formulations had ethylenediamine as the chain extender and lesser hard segment content (40 wt%), suitable for packaging or textile uses. All the formulations had polycarbonate diol Duranol™ T5651 as the polyol, isophorone diisocyanate (IPDI) as the isocyanate, Dibutyltin dilaurate (DBTDL) as a catalyst, 7% on internal emulsifier dimethylolpropionic acid (DMPA) which was neutralised by Triethylamine (TEA). The resulting PU dispersions included between 10 and 13% weight per cent of organic co-solvent. PU coating performance was evaluated using chemical stability, pendulum hardness, abrasion resistance, gloss measurement, adhesion, cross-cut test, contact angle, surface quality, dynamic mechanical analysis (DMA), film manufacturing, and Attenuated total reflectance-Fourier transform Infrared spectroscopy (ATR-FT-IR). The particle size (diameter) of the monomodal dispersions obtained was 180 mm, and the dispersions were stable for a month. The ATR-FT-IR spectra confirms no difference between the chemical structure of the PU-based on N-Methyl-2-pyrrolidone (NMP) and Green co-solvent. There was no significant effect on the coating’s final performance, which was proven through the hardness, abrasion resistance, adhesion and gloss test. Thus, they have successfully created viable options where green co-solvent work as an excellent reaction medium for both hard and flexible PU formulation. These coatings can be considered a better alternative for the coating industry with superior quality and environmental sustainability.

Raychura et al. [67] have created a PU derived from VO for wood coating applications. This was accomplished by converting peanut oil into a fatty amide, which was then reacted with hexamethylene diisocyanate using catalyst dibutyltin dilaurate (DBTDL) to produce PU (Fig. 8). Using ATR-FT-IR and Nuclear Magnetic Resonance spectroscopy (NMR) (here, $^1$H NMR), the synthetic PU was analysed. Bands at 3358 cm$^{-1}$ for the hydroxyl group and 1614 cm$^{-1}$ for the amide carbonyl demonstrate the successful synthesis of PU. Two kinds of PU were manufactured. PU-PN was derived from peanut oil, whereas PU-PAP included a hardener. Mechanical parameters were tested, revealing that PU exhibits 100% adhesion; pencil hardness was 3 H for PU-PAP and 2 H for PU-PN; and scratch resistance was 600q, somewhat lower than commercial PU. It had a greater gloss and somewhat lesser heat stability than standard PU. Water, salt, and acid (5% v/v HCl, 5% w/v NaCl, and distilled water) did not affect it, but the alkali solution (5% w/v NaOH) caused a small dulling of the surface. The MEK double-rub test demonstrated that all PU solutions were solvent-resistant up to 100 rubs, proving their resistance to solvents. Regarding domestic applications, PU exhibited stain resistance, low wettability, and a high contact angle, which indicated minimal moisture swelling and better durability. The cigarette ignition test demonstrated that PU remained unaffected for the first ten seconds, after which it went black, and film damage was seen in the 30 s. The coatings were subjected to a zone inhibition test, but no antibacterial or antifungal action was detected. A four-step degradation curve for PU was determined using SEM for surface morphology and Thermogravimetric analysis (TGA) for thermal assessment. Thus these results indicate that Raychura et al.’s innovative synthesis of PU derived from peanut oil demonstrated superior adhesion, scratch resistance, solvent resistance, and stain resistance, suggesting its potential to outperform conventional PU wood coatings in the market.

Fig. 8

Synthesis of polyurethane from peanut oil [67]

For developing PU coatings, scientists have experimented with several unique techniques for recycling PU waste. Godinho et al. [68] depolymerised several kinds of PUF wastes (polyester, polyether, and viscoelastic) using acidolysis, and the recovered polyols (RP) were utilised to formulate PU coatings for wood coating applications. The PU waste was depolymerised in the presence of dicarboxylic acid (Fig. 9). The recovered polyols showed strikingly similar properties to that of the conventional counterparts. The RP-based PU coatings showed better hardness and gloss properties, and the acid and hydroxyl values differed for different starting PU used. The findings provide strong evidence of the appropriateness of this method for utilisation in the production of PU coatings. Developing such recycling methods would be a boon to the polymer industry in terms of sustainability as it would promise the recycling of waste and provide monomers for building different polymers, which would minimise the need to obtain more and more raw materials.

Fig. 9

General scheme of depolymerization of polyurethane foam waste [68]

Metal Coating

Metal surfaces are vulnerable to corrosion and microbial growth, and this poses a danger to the structure built on these frameworks and can cause massive economic losses to the sectors employing them. PU coatings from biological origin can prove to be a promising solution contrasting to the petrochemical-based coating with increased chemical resistance, mechanical strength, better adhesion, thermal, abrasion and water resistance.

Patil et al. [69] completed a fascinating investigation using algal oil from Chlorella microalgae as a bio-based starting material for the development of PU. Algae oil-based fatty amide (AOFA) derived from Algae oil was converted into algae-based polyetheramide polyols (AEA) using three different diols: bisphenol-A (BPA), 1,4-butanediol (BUD), and isosorbide (ISO). These AEAs and Poly(tetramethylene ether) glycol (PTMEG) on reaction with a 4,4$^\prime$-diphenylmethane diisocyanate (MDI) gave PU (Fig. 10). Panels of mild steel were coated with PU and cured at room temperature for 48 h. Fourier transform infrared spectroscopy (FT-IR) and NMR spectroscopy confirmed the formation of the intermediate and final coatings. All AEAs containing PU demonstrated increased gel content, suggesting successful crosslinking of PU networks. TGA and Differential thermal analysis (DTA) analysis with different diols showed that the thermal stability of PU’s was in order BUD, ISO, BPA which can be linked to the rigidity of aliphatic (BUD), cycloaliphatic ring (ISO), and aromatic ring (BPA) polyols. Contact angle measurements further validated the hydrophobic nature of AEAs. Due to restricted adhesion and multiplication of microbes in the absence of water, the PUs containing AEAs displayed better antibacterial properties, as confirmed by SEM analysis of trays coated with AEA-based PUs, which had clean surfaces and uncoated mild steel (MS) trays with biofilm formation by E. coli and S. aureus. Cross-cut adhesion was 100% for all PU coatings, compared to 70% for PTMEG, suggesting outstanding adhesion. Also, Atomic Force Microscopy (AFM) tests showed that AEA-based coatings had better adhesion to metal surfaces than PTMEG. All the coatings had a gloss value in the range of 115–125, which is higher than PTMEG. As fatty acid chains extend, film gloss increases due to greater chain aligning on panels, enhanced flow, and settling of dry film. PU coatings tested in water, alkaline, acidic, xylene, and NaCl aqueous solutions for hydrolysis and chemical stability revealed that they are tolerant to alkaline aqueous conditions and organic solvents because of their persistent ether bonds. PU coatings were evaluated in NaCl (3.5 wt%) for seven days for corrosion which was further quantified using electrochemical analysis. Blank MS panels exhibited considerable corrosion, whereas AEA-based PU coatings were anti-corrosive. This study shows the successful transformation of Novel bio-based resource Algae oil into functional PU coatings with enhanced performance. However, performance of the material in real environment still remains a prominent question.

Fig. 10

Synthesis of polyurethane from algae oil [69]

Mahajan and Gite [70] have created smart healing PU using Cardanol and eugenol. Cardanol was employed for the development of microcapsules through in-situ polymerisation using an oil-in-water (O/W) emulsion method. In contrast, eugenol was used to make polyester polyol (EDDA) by solventless melt condensation polymerisation. FT-IR and NMR spectroscopies validated the structure of polyol, whereas Field emission scanning electron microscopy (FE-SEM) confirmed the production and surface shape of microcapsules. TGA testing revealed that microcapsules were stable up to 232 $^{\circ }$C. The prepared microcapsules were dispersed in eugenol-based polyol mixtures and cross-linking with MDI gave self-healing smart PU coatings (DAPU) (Fig. 11). FE-SEM was used to examine the self-healing feature of the prepared coatings. Before testing, the surface of self-healing coatings were scratched and submitted to FE-SEM for 48 h. When the coated surface was scratched, microcapsules ruptured, and linseed oil (self-healing agent) seeped out and distributed over the injured area. Through oxidative polymerisation of oozing linseed oil, the injured surface’s repair process was initiated, and the self-healing property was confirmed in SEM photographs. The coatings containing 16% bio-based microcapsules exhibited the most effective anticorrosive and self-healing characteristics (Corrosion resistance increased with the increase in the amount of microcapsule), confirmed by immersion and electrochemical methods. The tests results of the MEK rub test, flexibility, pencil hardness, gloss and crosscut adhesion proved the excellent physicochemical features of the developed PU coatings. Day-to-day use and environmental stresses can lead to micro-cracks development on coating surfaces, making the metal surface susceptible to chemical attack. The conventional methods fail to provide immediate restoration of such damages, and hence the concept of smart healing PU coatings, which have proven to repair such surfaces on their own, can be an eco-friendly approach to improve the coating performance. These Eugenol-based PU coatings with linseed oil in cardanol-based microcapsule can thus be a sustainable strategy to protect metal surfaces.

Fig. 11

Synthesis of polyurethane from cardanol and eugenol [70]

Extensive research has been done on castor oil (CO) based PU. This is because CO is the only naturally occurring VO with dihydroxyl and trihydroxyl groups, facilitating a direct reaction without modification with isocyanate to obtain PU. One such research was done by Zhou et al. [71], which involved the development of CO-based transparent and omniphobic PU coatings. Such multifunctional coatings find applications in various sectors, including Construction, Automobiles, furniture and metal surface protection. Firstly, Hexamethylene diisocyanate trimer (HDIT), a crosslinking agent, was added to Monocarbinol terminated polydimethylsiloxane (PDMS-OH) (anti-fouling agent) in a sealed bottle at 72 $^{\circ }$C for 72 h to obtain a prepolymer HDIT-PDMS. This pre-polymer solution was mixed with CO as the bio-polyol to get the final PU. A series of PU formulations were prepared with formula $PU_x-PDMS_y$ where x represents the mole ratio of the isocyanate group to the hydroxyl group called as R-value and y denotes the mass percentage of PDMS-OH with respect to the total weight of PDMS-OH, HDIT and CO. These formulations were cast on different supports to test the coating characteristics. The hardness of coatings was altered using the R-value and revealed substantially improved hardness compared to ordinary PU coating. The physical characterisation in terms of Hardness, transparency, adhesion, flexibility and water absorption showed excellent results. The developed coating exhibited superior anti-corrosion and anti-smudge qualities. PDMS chains could migrate and enrich the surface of the coating due to its low surface energy; hence it is able to enhance the anti-smudge properties, and it is shown by the electrochemical methods that the PU-PDMS coatings, due to increased crosslinking, offer much better corrosion resistance to the tin plate than the regular PU material. Essentially, they produced a multipurpose, high-performance coating from eco-friendly materials, providing an efficient alternative to the traditional counterparts.

Chen et al. [72] developed a PU coating using Jatropha oil-derived polyol. The reaction mechanism is demonstrated in Fig. 12. Jatropha oil is first oxidised by formic acid to form an epoxide, which on ring opening gives PU prepolymer. Trimethylolpropane triacrylate (TMPTA) was added to the PU prepolymer to ensure UV curing. This prepolymer is then reacted with Isophorone diisocyanate (IPDI) and 2-hydroxyethyl acrylate (HEA) using dibutyl tin dilaurate (DBTDL) as a catalyst to get the final PU. To enhance its properties, varying amounts of barium sulphate (BaSO₄) and titanium dioxide (TiO₂) hybrid nanoparticles were introduced into the matrix. The combination of BaSO₄ and TiO₂ resulted in UV-cured composite coatings with 97% reflectivity in the UV range (200–380 nm). The coating also exhibited radiative cooling activity in the near-infrared range (700–1400 nm), with a maximum surface temperature differential of 20.8 $^\circ$C observed after 60 min of exposure to the infrared irradiation. In addition, the presence of TiO₂ nanoparticles in the coating provided good anti-corrosion and antibacterial properties. The coating was also found to be recyclable with repetitive processes and showed promise for providing superior surface protection for materials such as metal, plastic, wood, and glass. This study not only presents a new and improved approach to non-fossil biomass polymer manufacturing but also lays the foundation for developing protective coatings with multiple features.

Fig. 12

Synthesis of polyurethane from Jatropha oil [72]

Concrete Coating

PU coatings are known for their excellent adherence to concrete and their ability to regulate water absorption and vapour transmission, which can protect the concrete from degradation. They are also highly resistant to dilute acids. Cementitious materials have poor tensile strain capacity and are prone to failure due to tensile cracking when subjected to dynamic stresses despite their rigidity. Therefore, using elastomeric coatings to protect buildings is an innovative technique for preventing concrete failures. PU coatings are a low-cost, lightweight, soft, and abrasion-resistant upgrade for a wide range of structural elements, including masonry, metal, concrete, and other composite materials. They can be applied using simple methods such as spraying, brushing, bar coating, and pouring. Furthermore, PU coatings bond strongly to surfaces and harden quickly, making them an excellent choice for coating applications in the construction industry.

Somarathna et al. [73] conducted experiments on concrete samples to evaluate the effectiveness of PU coating derived from palm kernel oil in improving the dynamic mechanical response of concrete under quasi-static and dynamic loads. The PU coating were prepared by a pre-polymerization reaction of Palm kernel oil-based polyol (PKO-p) and MDI at room temperature using Polyethylene glycol (PEG) as the plasticiser. Acetone was added 35% w/w in each sample depending upon the total weight of the particular system. The proportion of PKO-p:MDI: PEG was 100:80:6 by weight in the mixture. The researchers conducted three-point bending tests at a strain rate of 0.067 s$^{-1}$ for dynamic loading and quasi-static loading tests at a strain rate of 0.00033 s$^{-1}$ simultaneously. The results showed that the application of PU coating on the impact face, rear face, and both faces (with equal coating thickness) increased beam strength by a factor of 1.1, 1.1, and 1.15, respectively, under impact circumstances. Moreover, similar coating configurations increased the final strain by a factor of 6.8, 5.2, and 8.9 times. This increased the strain energy density by 8.3, 5.8, and 11.3% compared to uncoated samples. Increasing the coating thickness on concrete faces improved these properties (Fig. 13). The PU coating demonstrated strong adhesive properties as it did not debond even after the final failure of test samples, indicating that significant fragmentation may be minimised despite a small coating thickness of 2.5%. The use of bio-based PU as a coating is an environmentally friendly and sustainable method for protecting concrete buildings from dynamic stresses.

Fig. 13

Comparison of coated and uncoated specimens a Un-coated Concrete specimen, b PU coated concrete specimen, Enhancement compared to uncoated specimen, c Strain energy, and d Ultimate strain energy density [73]

Although it has been shown that PU coatings extend the service life of concrete, there are challenges with PU, the most prevalent of which is bonding with concrete. Some solutions to the problems faced with concrete coating can be resolved to an extent by carrying out modifications in the backbone of oil-based PU by grafting certain groups that can improve adhesion and enhance surface properties. There has been a relatively limited study conducted in the field of concrete coatings, but the use of a PU coating to preserve a concrete structure from degradation and recurrent maintenance has enormous potential.

Applications as Sealants

Polyurethane sealants (PUS) have captured a significant portion of the current sealant industry. There are both single-component and multi-component versions of PUS, each with a different range of performance qualities. The primary benefit of a one-component sealant is that it does not need mixing. Using one-component materials for high-rise construction is substantially simpler. The utilisation of two-component sealants presents a range of advantages, including the potential for unlimited package stability and the ability to achieve rapid cure. Sealants are commonly employed in construction and transportation infrastructure. There has been growing interest towards incorporating renewable materials in the production of PU products because of their renewability, cheaper cost, and reduced environmental contamination. PUS stands out among them, attributable to its superior mechanical strength, chemical tolerance, and affordability. In addition to preventing air and water infiltration, polyurethane foam sealants provide a multitude of other benefits. These sealants offer the possibility of energy savings, enhanced comfort, weather resistance, noise reduction, and less outside noxious gas penetration. As a result, PUS has seen widespread adoption across a diverse variety of industries and applications [100].

The use of bio-based PUS is limited due to its highly combustible nature, which restricts its application in various industries, including construction, electrical, and transportation. Adding compounds containing phosphorus is an effective technique to provide flame retardancy to bio-based PUS, but it may impair the mechanical properties of PUS. Therefore, it is suggested to use reactive-type flame retardants, which are added to the backbone of PU materials to strengthen their fire resistance. Ding et al. [74] has developed a new class of flame retardant polyols (FRPE) based on ricinoleic acid (RA) and synthesised flame retardant polyurethane sealants (FR-PUS) using MDI as a curing agent. The detailed reaction for preparation of FRPE is shown in Fig. 14. The characterisation of obtained polyol and PU was carried out with the help of $^1$H NMR, Carbon-13 NMR ($^{13}$C NMR) and FT-IR. The flame retardant and thermal degradation properties of FR-PUS were determined using various techniques, including the limiting oxygen index (LOI), cone calorimeter testing (CCT), and TGA. The results showed that FRPE could improve the temperature stability and flame retardancy of PUS without adding another flame retardant. TG-IR was used to examine the gaseous degradation products of FR-PUS. Additionally, FRPE can increase PUS’s rigidity and promote char production, which protects PUS from further degradation. However, achieving a balance between the flame-retardant properties and other desired properties of the coating is challenging. Overall, this research highlights the importance of using eco-friendly coatings in different industries and the benefits of WPU coatings compared to traditional solvent-based coatings.

Fig. 14

Synthesis of polyol from ricinoleic acid [74]

Zhang et al. [38] have provided valuable insight into the properties of PU produced from different VOs and their potential as a sustainable alternative. The authors utilised a unique, solvent/catalyst-free synthetic approach to generate a variety of bio-based polyols from olive, canola, grape seed, linseed, and castor oil for various applications, including sealants. In their study, the authors first converted the bio-based triglyceride oils into epoxidised VOs using formic acid and hydrogen peroxide. They then performed a ring-opening reaction using the fatty acid obtained from the oils (Fig. 15). The chemical structures of the polyols and PUs derived from them were characterised using FT-IR, differential scanning calorimetry (DSC), and TGA. The study also investigated the effects of crosslinking density and polyol structure on the thermal, mechanical, and shape memory properties of PUs. They found that increasing the molecular weight of the polyol used in the synthesis resulted in PU samples with improved thermal stability. Moreover, the new technique utilised VOs to produce materials with a broad range of flexibility, rigidity, and forms. The successful synthesis of shape-memory PU from various VOs was thus achieved.

Fig. 15

General scheme for the preparation of polyol from vegetable oils [38]

Applications in Concrete Systems

Overview

Incorporating waste materials or polymer waste to generate concretes with enhanced physical and tensile qualities while keeping sustainability in mind is the topic of current research [101]. There are three kinds of concrete-polymer composite materials: polymer concrete (PC), polymer-impregnated concrete (PIC), and polymer-modified concrete (PMC)/polymer cement concrete (PCC) [102]. PC is made up of aggregate and polymer as a binder, but it does not contain any Portland cement or water in its composition [102]. In PIC, the polymer is infused into hardened concrete to a complete or partial depth, with further polymerisation occurring by heat and radiation. Due to a continuous polymer layer between the cement paste and aggregate, PIC improves durability, decreases water penetration, and increases freeze-thaw resilience and chemical stability. PCC and PMC are conventional Portland cement concrete with a polymer additive. PMC is the designation for such concrete with polymer concentrations of 5% or less [24]. The primary purpose of incorporating polymers into concrete is to enhance the material’s performance by enhancing its mechanical qualities, durability, lightness, chemical resistance, workability, quick hardening, and thermal insulation [22]. It has been claimed that polyurethane concrete, a novel material made by combining cement, PU, aggregate and admixtures in a certain ratio, has superior mechanical performance and longevity [5]. PU-based Polymer Concrete, consisting of a PU matrix and inorganic aggregates, has shown promise in the repair of cementitious pavements due to its superior properties. These include rapid curing time, resistance to corrosion, strong bonding strength, and high waterproof performance. Its use can lead to efficient repairs, reduced maintenance, and enhanced durability of cementitious pavements [5]. As part of a sustainable approach to building construction, PU can be used as a substitute for cement and aggregates or to improve the qualities of concretes and mortar. It can also be combined with natural fibres or biodegradable waste to replace conventionally used inorganic materials.

Polyurethane as Binder

There is a substantial body of research demonstrating that PU improves the characteristics of conventional concrete. PU exhibits superior strength in comparison to other polymers and possesses exceptional properties such as strong corrosion and chemical resistance, high elongation, resistance to impact and blast loads, and eco-friendliness, making it a versatile and durable organic polymer material suitable for a wide range of applications [103]. PU has a short curing period, is resistant to corrosive environments, and bonds strongly to cementitious materials (like aggregates) [104] besides that it’s prospective applications are made possible by the simple and straightforward reaction of polyol and isocyanates.

Haruna et al. [104] made a PU-based polymer concrete with PU as the binder and sand as the aggregates. The ratios of polymer to sand were 85:15 (PUC-15) and 90:10 (PUC-10), respectively. To evaluate the impact resistance, U-shaped specimens were created and tested against conventional concrete, which served as a standard. According to their observations, this PUC exhibited significantly enhanced average impact times and average energy absorptions, as well as outstanding dynamic qualities. This is very well depicted in the Fig. 16a(I) by the increase in an average number of blows required to cause the first crack and Fig. 16a(II) ultimate failure in specimens PUC-10, PUC-15 compared to normal strength concrete (NSC). Jinhui et al. [105] and Hussain et al. [106] conducted independent research employing PU to improve the mechanical capabilities of cement-based concretes, with favourable outcomes. Results of experiments done by Jinhui et al. and Hussian et al. are depicted in Fig. 16b and c respectively. Figure 16b(I) shows an increase in flexural and Fig. 16b(II) compressive strength of samples with PU content (PUC-0.5% and PUC-2.0%) compared to ordinary cement mortars. Average of compressive strength v/s strain curve and bending stress v/s strain curve under compression as well tension is shown in Fig. 16c(I, II) respectively. Results indicate marked increase in these values compared to ordinary cement mixtures. These experiments demonstrate the viability of PU as a cement substitute or binder.

Fig. 16

Mechanical strength of polyurethane based concrete systems. a Comparison of Impact resistance of PUC with conventional concrete in terms of an average number of blows and impact resistance [104]. b Comparison of flexural and compressive strength of PUC with conventional cement mortars [105]. c Average line of stress–strain curve of PUC [106]

Experimental evidence supports the improved mechanical properties of concrete systems with PU-based binders, yet concerns arise due to the non-renewable sourcing of PU. Moreover, the limited utilisation of PU hinders its potential as a greener alternative to cement binders. Less thermal stability, and chemical and physical degradation are potential explanations for the limited research in this area. Increasing emphasis on research is necessary to advance the field of bio-based PU for concrete systems, and to promote fully green and sustainable construction practices. One of the few works available is done by Rabello et al. [75, 76] employing PU derived from CO as a binder for composite building materials. Composite samples of PU were produced through the direct polymerisation of CO and isocyanate with a 2:1 volumetric ratio, and vermiculite was incorporated into the material. Physical indices, mechanical characteristics, and chemical, thermal, and moisture resistance were evaluated on the specimens. The binder was found to be lightweight, resistant to ultraviolet radiation and humidity, impervious to water damage, and unaffected by chemical attacks. The study found that the mechanical and thermal properties of the bio-based concrete were not up to the mark. To address this issue, the authors suggest incorporating fillers to enhance the thermal and mechanical stability of the material, making it suitable for use in construction materials.

A novel work done by Nilam et al. [77] involved the development of a sustainable and environmentally friendly polymer mortar composite using PU derived from agro-industrial waste Cashew nut shell oil (CNSO) as a binder. In this study, the polymer mortar composites were prepared using PU as a green binder (14%, 12%, 10%), palm oil fuel ash (POFA) and red mud (RM) as fillers (16%, 18%, and 20%), and silica sand as an aggregate of different sizes (zones I, II, III). POFA and RM are waste materials from power plants and alumina industries, respectively. The formulation parameters were optimised using ANOVA and Taguchi techniques. The preparation of the PU binder (Fig. 17) involved the reaction of Cardanol derived from CNSO with epichlorohydrin in the presence of anhydrous ZnCl₂ to produce mono glycidyl ether of Cardanol, which was then reacted with diethanolamine to yield a triol. The Cardanol-based triol was then used to prepare PU by reacting it with Desmodur N-95, an aliphatic polyisocyanate. The resulting PU, along with POFA, RM, and silica sand, was used to prepare the polyurethane-based mortar (PUPM). POFA and RM were oven dried before sample preparation to remove any moisture and finely grounded in a ball mill to obtain a small particle size (45 microns) with high specific gravity. Various tests were performed to characterise the starting materials as well as the PUPM samples, including FT-IR, SEM, X-ray diffraction (XRD), TGA, and mechanical strength tests, all of which were performed using available ASTM standards. The uncured PU resin demonstrated a higher gel time (45 min) and lower viscosity (0.18 Pa s) in the kinetics study, which is advantageous in enhancing wetting property and improving the diffusion of the PU resin between the aggregate particles and fillers. FT-IR was used to monitor the reaction process of PUPM and the chemical characterisation of POFA, PM and prepared PUPM samples. SEM analysis of milled POFA and RM revealed that both POFA and RM had reduced and uniform size after grounding. POFA particles showed reduced porosity and voids whereas increased packing density and affinity as filler. RM had a lower size than cement particles improving the matrix density and splitting tensile strength. XRD was used to study the crystalline structure of POFA and RM. TGA analysis revealed improved thermal properties of the samples. The strength analysis indicated that the samples containing RM filler showed better mechanical properties than those containing POFA filler. The compressive, flexural and tensile strength of the PUPM composite prepared with different formulations were in accordance with the required ASTM standards. Acid resistance was observed to be affected for samples containing 10% PU, while 12% and 14% PU samples remained unaffected. SEM micrographs of PUPM samples revealed better properties of samples with 12% and 14% PU as opposed to 10% PU samples. The impact of various parameters, including the PU resin content, the amount of POFA or RM filler, and different zones of sand used, on the performance of PUPM composites was evaluated through simulation at all levels of the test parameters with regards to compressive, tensile and flexural strength. The researchers found that the mechanical strength of PUPM composites increases with the rise in resin content (10–14%) and filler content (16–20%). They also observed a slight increase in strength when the silica sand zone was between I and II and a slight decrease when it increased from zone II to zone III. The PUPM composite with the best mechanical and chemical performance was with a resin content of 14%, RM filler content of 20%, and zone II silica sand. The study shows that the PUPM with POFA content 20% and RM content 20% has improved mechanical characteristics and chemical resistance compared to studies previously reported. The authors attribute this to the strong hydrogen bonding present between the urethane linkage of the PU resin. The resin had a major impact on the mechanical properties of the PUPM composites. The researchers suggest that the prepared PUPM is a greener sustainable alternative to conventional mortar composites and is suitable for various applications in sustainable construction, such as structural repairing, precision tools, industrial flooring, machine bases, and tanks in chemical environments.

Fig. 17

Synthesis of polyurethane from cardanol [77]

BPU have been identified as potential innovative materials that could replace cement in the construction industry. However, further research is required to address their mechanical characteristics, thermal stabilities, and other deficiencies encountered. While bio-based PUs offer several advantages over traditional building materials, including reduced carbon footprint, lower embodied energy, and improved thermal insulation properties, they still need to be optimised to meet the requirements of real-world building projects. The mechanical properties of BPUs, such as their strength, toughness, and durability, must be improved to ensure their long-term performance and suitability for structural applications. Moreover, their thermal stabilities need to be enhanced to withstand fire hazards and exposure to high temperatures. Additionally, the potential impact of moisture and other environmental factors on the long-term performance of BPUs should be studied. Hence, it is essential to conduct further research to address these issues before bio-based PUs can substitute traditional building materials. Such studies will aid in the optimisation of the manufacturing process, choice of raw materials, and formulation of bio-based PUs, resulting in enhanced performance and successful application in construction projects.

Polyurethane as Aggregate Substitute

The treatment of bio-based aggregate has gained interest in recent years as a means of generating green concrete and promoting sustainability by reducing reliance on natural resources. This approach has the potential to address environmental concerns associated with traditional building materials and promote the use of renewable and eco-friendly alternatives [107]. To lessen the detrimental effect of the construction sector on the environment and to generate more sustainable construction materials, several bio-based wastes, such as coconut shell, bamboo, oil palm shell, apricot shell, rice husks, etc., have been exploited as renewable aggregates in the production of bio-based concrete during the last few decades. In comparison to traditional concrete, bio-based concrete provides superior absorption of sound, thermal insulation, and reduced density with significant promise for conserving natural resources, decreasing energy usage, and lowering building expenses [108].

Plastic and modular construction produce enormous quantities of waste. This material must be used via recycling and innovative approaches. Insulation is the primary usage for PUFs. They do not break down into harmful substances over their lives and emit a small amount of pollutants. When utilised in prefabricated materials such as partitions or cement matrix that are coated with plasters or paints, stay embedded throughout to provide a stable matrix. Researchers are exploiting PUF wastes as aggregate replacements because this strategy offers sustainability by reusing waste and decreasing reliance on natural aggregates, which are nothing more than mineral raw materials. This would consequently cut water and transportation loads.

Mounanga et al. [78] have attempted to utilise the foam waste by adding PUR foam waste from dismantling insulating panels in the construction sector into lightweight concrete mixes, substituting aggregates. Two distinct PUR foam concrete mixes were created with and without limestone filler, each comprising varying amounts of PUR foam by altering the water-to-cement ratio as sand is progressively replaced by PUR foam to achieve similar workability to reference concrete mixtures. PUR foams exhibit strong compression and high absorption, which tends to enhance the packing density of the mixture by decreasing the pore volume of concrete. The specimens demonstrated decreased thermal conductivity by a factor of 2–7 and a mechanical strength ranging from 1.3 to 10.4 MPa. These results are the consequence of high porosity, and it has been suggested that the addition of mineral additives may boost mechanical strength. SEM image (Fig. 19a) shows that the pore size of PUR foam is 200 μm which is close to the size of cement particles and limestone filler. This ensures better absorption of cement paste and homogeneous distribution, ultimately affecting the strength and thermal properties. Regarding density and workability, the aggregate replacement has been examined with favourable results. However, impaired thermal and mechanical qualities limit the applicability of concrete mixes.

In a study by Gadea et al. [109], different amounts of PU from the automotive sector were used to substitute sand aggregates to produce lightweight concrete. Two series of concrete mixes were created, each with varying proportions of PU. The ratio of cement to aggregate (sand + PU) was maintained at 1:3, where the amount of removed sand was replaced by an equivalent volume of PU. As the quantity of PU increased, the ratio of water to cement needed decreased. This was attributed to the sand substitution, which impacted the mechanical characteristics of the hardened state. The high porosity of PU facilitated water retention, resulting in a lower tendency of mortar segregation during application. In all cases, the density of the concrete was 200 kg/m$^3$ lower in the hardened condition compared to the fresh state, with 8% more occluded air present in both cement sets. The large pore diameters of PU foams allowed for efficient transport of water through the capillary network linking pores, making them resistant to water condensation in any climate and exhibiting superior water diffusion. However, the mechanical strength of the concrete was degraded in terms of compressive and flexural strength due to the high porosity of the material (Fig. 18) and the large interfacial transition zone as seen in SEM image of the sample (Fig. 19b). This degradation was offset by the flexibility of PU, which allowed the mixture to absorb minor movements without fracturing, thus preserving an adherence to supporting structures. The thermal characterisation was conducted using TGA, which showed that PU degraded partially between 350 and 530 degrees Celsius. As the quantity of PU increased, the compressive strength of the concrete ranged from 35 to 10% but exhibited a 5% recovery prior to failure, suggesting acceptable deformability. The authors recommended this concrete mixture for non-structural uses, such as partitioning walls and shock-absorbing joints.

Fig. 18

Change in compressive and flexural strength with % replacement of sand by PU [109]

Arroyo et al. [79] employed non-ionic surfactants to improve the characteristics of structural materials. PU, a byproduct of the car industry, was crushed to a particle size of less than 4 mm and substituted for 25–100 % of the aggregate in volume. It included 2% ionic surfactant B by weight of cement. After 28 days of curing at 20 $^\circ$C, the flexural and compressive strengths indicate that the surfactant’s Hydrophilic-lipophilic balance (HLB) value (measure that helps us understand whether a substance is more attracted to water or to oil) is responsible for increasing or lowering the mechanical strength of the specimens. Mercury intrusion porosimetry (MIP), SEM, and Computerized axial tomography (CAT) were used to assess microporosity, surface characterisation, and macroporosity, respectively. SEM analysis was done for the polished sections of various specimens, which shows the effects of variation of a non-ionic surfactant, keeping the substitution rate of sand by PUR foam constant (mortars with 75% PUR foam were used). As seen in Fig. 19c, Mortar with $S_1$ surfactant (PU75-$S_1$) showed better interfacial transition zone as compared to reference mortar (REF-$S_1$) without PUR foams due to size of PUR foam aggregate and mortars with $S_2$ (PU75-$S_2$) and $S_3$ (PU75-$S_3$) surfactants due to increase in HLB value of the surfactant ($S_1, S_2$ and $S_3$ represent fully hydrolysed, partially hydrolysed and partially hydrolysed with long chains surfactants). This leads to an important conclusion that the surfactant with low HLB value can promise lightweight mortars with enhanced adhesion between PUR foam and cement paste, homogeneous distribution, and structurally robust lightweight concrete even though the PUR foam content is very high. However, with high HLB surfactants, a large interfacial transition zone is seen due to water released by the foam waste, which absorbs large amounts of water at the initial stage. Likewise, a drop in mechanical strength was seen owing to a decrease in bulk density as a result of the material’s high porosity. However, HLB values of surfactant are influenced by hydrolysing the polymer chain to varying degrees; hence, changes in terms of air entrainment (occluded air increases with polymer chain lengthening and thus decreases density), therefore, the final mortar weight rely solely on surfactant levels. This was a novel method for improving the mechanical strength of concrete, which is the most prevalent difficulty encountered. As a result, it’s worth noting that modifying the PUR foam may be a profitable route to achieving higher mechanical strengths in concrete.

Fig. 19

SEM micrographs of PUR foam waste and concrete with foam waste aggregates. a Microstructure of PUR foam waste [78]. b Interface between PUR foam waste and cement paste [109]. c Microstructure of samples with varying surfactants [79]

Mortar Systems

When mortars are part of supporting walls, they withstand compressive and flexural strengths caused by temperature fluctuations and dry stress. However, tiny cracks in the mortar matrix stimulate fatigue fracture, which can cause the constructive unit to disintegrate, whether it be masonry mortars used for horizontal and vertical rendering, masonry bricks, or concrete masonry units. In an effort to improve the mechanical properties and durability of building materials, researchers have turned to incorporating polymers to produce composite building materials. Among the various resins under investigation, PU has garnered significant attention due to its high mechanical properties and sustainability. Adding PU to traditional mortar mixes can help reduce recurring maintenance and other costs, thereby producing positive economic effects. Nevertheless, the mechanical properties of mortars containing PUF have presented issues that limit their application. As a result, integrating various components that improve the properties of PUF in concrete mixes has emerged as a popular innovation.

Junco et al. [80] tested mortars constructed by substituting sand with PUF waste in 50%, 60%, and 75% v/v to demonstrate that their fatigue behaviour is equivalent to that of commonly used reference mortars. Different percentages of sand replacement by PUF were done considering end-use applications, where larger substitution is meant for less mechanical need. 1/4 cement and aggregate ratio and 1/6 cement and aggregate ratio were made using two separate PU wastes, A from refrigeration and construction and B from the automobile market. Less cement in 1/6 sample reduced mechanical strength. A showed superior flexural and compression resistance, whilst B had better flexibility and uniformity for coating applications. Various writers employ different methodologies to investigate fatigue in terms of loading and unloading cycles and frequency. This approach utilised three tests. The test consisted of repeating the previous test (load 20–60%, 2 Hz, 300,000 cycles), plus 150,000 cycles with loadings of 20–80% and 75,000 cycles with loadings of 20–90%. Up to 50% and 60% of the substitutes behaved similarly to traditional mortars, while 75% behaved differently. CAT scans showed no substantial cracks, proving no structural collapse. There are many options for lightweight mortar on the market, but they use a lot of energy and aren’t sustainable. The mortar developed by Junco et al. can be a superior alternative as it is durable as well as suits the requirements of commonly used masonry mortars.

Jiang et al. [81] developed Polyurethane-based Polymer mortar (PUM) by utilising crumb rubber powder from waste tires to improve the mechanical properties and internal pore structure of the mortar. The incorporation of rubberised aggregate as a flexible material contributes to promoting environmentally friendly and sustainable concrete development while reducing the load on natural aggregates. The use of PU as a binder presents a promising alternative to conventional cement binder, which is energy-demanding and hazardous. This research was conducted by Jiang et al. to investigate the influence of crumb rubber powder (CRP) on the mechanical properties of mortar, as well as to evaluate its potential as a repair material. PU was composed of two constituents. A was a polyol derived from CO, and B was polyethylene polyphenylene isocyanate (PAPI) (Fig. 20). Mortars were produced with two distinct mixing ratios of PU as a binder and regular river sand as aggregate, and both mixing ratios included four distinct amounts of CRP replacing the sand at concentrations ranging from 0 to 5%. The mechanical qualities were evaluated by measuring compressive, flexural, and bond tensile strength. At 5% CRP inclusion, the compressive and flexural strengths exhibited considerable enhancements. The compressive strength remained unchanged, whereas the flexural and tensile strengths improved by 7.9% and 17.3%, respectively. The sample exhibited improved ductility, as shown by the shape of its failure. Pot life showed that CRP did not affect the working duration of mortar, although a rise in PU lengthened its working time. The microstructure of PUM was analysed using MIP, and results showed that 5% CRP decreased overall pore volume and porosity. The research showed that PUM decreases cracking and deformation and has higher robustness and adhesion than cement mortars; hence, it possesses the capability to be used as a rapid repair material.

Fig. 20

Preparation of polyurethane based polymer mortar (PUPM) [81]

Road Construction

Overview

Asphalt made from petroleum is a finite resource, and therefore alternative methods to reduce reliance on it in pavement structures are gaining interest among researchers. The use of bio-oil as a replacement for petroleum asphalt has shown to be a successful strategy for addressing resource depletion. However, compatibility and thermal performance issues with the modified asphalt remain a concern. Chemical alteration has emerged as an efficient method for enhancing functionality. As traffic volumes continue to rise and ambient temperatures vary, asphalt pavements are prone to various issues such as fatigue, thermal cracking, and rutting. To mitigate these problems and enhance the performance of asphalt, multiple polymers have been added to the asphalt matrix to improve its rheological and mechanical properties. These include Styrene-Butadiene-Styrene (SBS), Styrene-Butadiene-Rubber (SBR), Ethylene-Vinyl Acetate (EVA), Elvaloy AM, High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and crumb rubber, among others [46]. In recent years, there has been a growing interest in PU modified asphalt owing to its outstanding operating performance across a broad temperature range and high compatibility at moderate blending temperatures (120–140 $^\circ$C) [110]. PU-modified asphalt is created by adding active polyol and isocyanate to asphalt, which initiates a series of chemical processes leading to the formation of a highly compatible spatial network structure in situ. Moreover, polyols and isocyanates are often low-viscosity liquids that minimise the initial viscosity of asphalt, thereby reducing manufacturing energy usage and high-temperature ageing. PU incorporation has the potential to enhance numerous flaws associated with modified asphalt [82].

Polyurethane Modified Asphalt

The use of BPU modified asphalt binder (BPMA) as a sustainable solution for asphalt modification is gaining popularity [111]. Meng et al. [82] conducted a study on the synthesis of BPMA using castor oil, isosorbide (IS), Hexamethylene diisocyanate (HDI), and virgin asphalt binder (VB). The reaction scheme for preparing BPU using C, IS, and HDI is shown in Fig. 21. The authors explored the BPU formulation ratio by conducting mechanical tests, and the final composition was determined to be CO/SI at 1:7, exhibiting maximum tensile strength and break elongation. FT-IR was utilised to analyse BPU synthesis, and the flowability of BPMA during construction was assessed by a rotational viscosity (RV) test. The compatibility of BPMA was evaluated by storage stabilisation and fluorescence microscopy (FM), while FT-IR was used to analyse the modification pathway of BPMA. The high and low-temperature performance of BPMA was evaluated by dynamic shear rheometer (DSR) and bending beam rheometer (BBR), respectively. The BPMA demonstrated good performance at both high and low temperatures, and its microstructure was represented by three forms of phase distribution. The BPMA’s low viscosity allowed for workability at moderate mixing temperatures, minimising blending energy. The modified SCRB test showed that BPU improves VB’s permanent deformation resistance. The BPMA can withstand intense traffic with extended curing time. The authors found that IS inclusion decreased PU molecules’ crosslink density. BPU’s flexible and stiff characteristics contributed to VB’s elasticity and strength. The chemical linkage between the two phases improves compatibility and maximises BPMA’s flexibility and strength. The combination of microstructure, physical modification, and chemical modification contributes to the acceptable operating performance of BPMA.

Fig. 21

Synthesis of BPU from castor oil [82]

Xia et al. [83] conducted research to investigate the use of castor oil and liquefied MDI to produce a BPU, C-PU, for use as an asphalt modifier. The C-PU was synthesised using simple raw materials through chemical synthesis. The modified asphalt made with C-PU had advantages such as ease of production, low shearing temperature, and the absence of high-temperature development requirements. The fundamental and rheological parameters of the modified asphalt, including the modifying process and high-temperature stability at 130 $^\circ$C, were analysed. The results showed that 30 wt% was the optimal concentration of C-PU, and the thermal stability of the modified asphalt at 130 $^\circ$C was satisfactory. A frequency sweep test assessed the relationship between asphalt modulus and driving speed, representing wheel load on the asphalt pavement. The incorporation of the C-PU modifier resulted in increased resistance to deformation. Moreover, the performance of the modified asphalt improved as the modifier concentration increased. The rheological test revealed that the modified asphalt’s complex modulus was higher than that of the reference asphalt, indicating that its resistance to deformation and elastic properties had been significantly improved, and it recovered well when compressed. C-PU with terminal isocyanate functionality may react further with the active hydrogen in the asphalt matrix and water present in the air and the surface of the material, allowing for a shift from simple physical transformation to chemical transformation, which can increase the overall strength of the product. The process of chemical curing is shown in Fig. 22. As a result, the features of the modified asphalt were superior to those of the base asphalt, showing better performance at both high and low temperatures.

Fig. 22

Reaction involved in chemical curing [83]

According to research done by Liu et al. [84], the efficiency of bio-asphalt treated with PU derived from CO was greatly enhanced. The study identified the optimum percentages of PU, CO, and VO through the response surface methodology, which were 20%, 33.61%, 8%, and 38%, respectively. The physical performance of asphalt was assessed using the Box–Behnken Design approach, which examined the effects of PU, CO, and VO on penetration and softening point tests. To study the rheological behaviour of asphalt, temperature sweeps, repeated stress creep recovery tests, and low-temperature creep bending tests were conducted. Fourier transform infrared reflection and scanning electron microscopy were utilised to examine the interaction and compatibility between CO, VO, and PU on asphalt at the microscopic level. The summary of work done by Liu et al. is shown in Fig. 23. The study found that PU significantly improved the softening point of asphalt, enhancing its performance at high temperatures. Moreover, the use of CO and VO improved the low-temperature performance of asphalt, while PU and CO alone had a negative effect on the same. The research showed that the combination of CO, VO, and PU significantly improved the performance and service life of modified bio-asphalt. However, further investigation is needed before practical engineering applications. The findings suggest that modified bio-asphalt has the potential to improve pavement performance and durability.

Fig. 23

Summary of work done by Liu et al. [84]

Kok et al. [85] investigated the effects of PU modification on the rheological and chemical structure of bitumen binder by synthesising a new polyol derived from palm oil and combining it with MDI (Fig. 24). The study aimed to determine the impact of PU modification on the classical and rheological characteristics of the bitumen foundation. The softening point, rotational viscosity, dynamic shear rheometer (DSR), multi-stress creep recovery (MSCR), BBR, and FT-IR tests were conducted on the modified binders. The effect of curing time on the performance of PU-modified binder formulations was also investigated. The results showed that the rutting parameter of the PU-modified binder increased by 256% after 24 h of curing at 85 $^\circ$C. Compared to the 4% SBS modification, binders with a 4:1 NCO/OH ratio exhibited superior elasticity under long-term loads and high temperatures, as well as overall performance. The complex modulus and rutting characteristics increased with increasing PU content, while the low-temperature capabilities degraded with increasing PU content. However, the study found that the 7% PU-modified binder had similar stiffness and relaxation properties as the 4% SBS-modified binder. Therefore, the 7% PU modification with a 4:1 NCO/OH ratio was found to have comparable or superior performance to the 4% SBS modification in terms of elasticity and high- and low-temperature characteristics. In conclusion, the study suggests that PU modification can enhance the performance of bitumen binder and improve pavement durability, which could lead to more sustainable and long-lasting road infrastructure.

Fig. 24

Summary of reaction process carried out by Kok et. al. [85]

Adhesive Applications

Polyurethane-based adhesives (PUA) find widespread application across various substrates, including synthetic leather, fabrics, rubbers, wood, glass surfaces, and galvanised steel, among others. These adhesives possess a polymeric structure comprising of soft and hard portions, owing to polyol and diisocyanate, respectively, which plays a significant role in determining their adhesive properties. The desired adhesive characteristics can be achieved through a combination of physical and mechanical strength derived from the hard portions and wetting ability from the soft segments. However, despite their widespread use, many adhesives are derived from petroleum-based materials, which can result in negative environmental impacts during both production and application. Thus, it is imperative to develop sustainable adhesive materials derived from renewable sources.

Gama et al. [86] synthesised a PUA for wood, derived from CO to create an eco-friendly alternative to traditional wood adhesives. To study the impact of RNCO/OH on PUA properties, various RNCO/OH ratios were used to produce a range of adhesive formulations. The adhesive combinations were characterised using FT-IR, DMA, and TGA before curing at room temperature. The highest adhesion was seen for RNCO/OH = 2.50, which also produced final bonding strength in less time than the commercial adhesive. Further characterisation demonstrated that both PUA-2.50 and commercial adhesives had high-temperature stability but were susceptible to the chemical environment. The activation energy ($E^a$) was determined using non-isothermal DSC studies and the Kissinger and Ozawa techniques, which showed that the $E^a$ of cure for PUA-2.50 was 80.55 and 87.07 kJ/mol (using the Kissinger and Ozawa techniques, respectively) and was dependent on the degree of cure ($\alpha$). This research demonstrated that the CO-based adhesive has the potential to replace petroleum-based adhesives, offering a sustainable alternative for various applications, including wood adhesives.

Beran et al. [87] developed a novel approach involving the reverse polymerisation of waste F-PUF in order to prevent secondary waste. The regenerated FPUFs were analysed using FT-IR, Gas Chromatography/Mass Spectrometry (GC/MS), Gel Permeation Chromatography (GPC), and the detection of amine and hydroxyl numbers. The liquefaction process employed propylene carbonate (PC) as a heat transfer medium and catalyst carrier, and potassium acetate was used as the catalyst. Two products were obtained from the depolymerisation of the liquid: a separated product from vacuum distillation and a non-separated reaction product (Fig. 25), both of which were used to create various adhesives. Both depolymerisation products, polyol and resin, may be utilised as raw materials in the PU industry. The usage of polyol in three PUR adhesive formulations was demonstrated and evaluated by measuring wood-to-wood bond tensile strength. Elementary analysis showed that nitrogen originating from urethane linkages in the PUR foam is concentrated in the thermoplastic resinous phase. The potentiometric titration of amines revealed that only a small amount of free amines are produced, both of which are important for the practical applicability of the recycling process, preventing the production of hazardous amines and/or undesirable polyurea solid wastes. The authors have presented an intriguing possibility with their depolymerisation process and the masking reaction of in-situ-generated diisocyanate. If further research in this area is successful, the diisocyanate is expected to be recycled as an essential raw material in addition to polyol. Thermoplastic resinous materials may be mixed with aromatic diisocyanates for the production of rigid insulating foams, which would indicate the complete use of foam waste. This study showcases an innovative approach to prevent the generation of secondary waste and reduce the environmental impact of PUF waste.

Fig. 25

Products of depolymerisation and their uses [87]

Environmental Impact Assessment

Construction industry alone, globally accounts for 33% greenhouse emissions and 40% energy consumptions. During the production phase, cement, steel, and concrete impose the most significant environmental impact. When materials such as stone, metal, cement, and wood reach the disposal stage of their life cycle, they emit leachates containing organic acids, bacteria, heavy metal ions, and various air pollutants. Incineration procedures release heavy metals and volatile organic acids into the environment [112]. According to a study done by Huang et al. on the life cycle assessment of the construction materials, every activity in life cycle of conventionally used construction materials is resource and energy intensive and involves pollution. The subsequent Fig. 26 provides insights into life cycle assessment of conventionally utilized construction materials.

Fig. 26

Environmental impacts of building materials [11]

As seen above, Fig. 26 clearly portrays the harmful environmental consequences of traditional construction materials. Shifting our focus to PUs, with a global consumption exceeding 18 million tonnes, primarily in the form of lightweight and thermally efficient foams, concerns have surfaced about their reliance on toxic phosgene and potentially carcinogenic isocyanate monomers throughout their life cycle. A study from the Zero Emission Australia project advocates for alternative materials as a strategy to reduce emissions linked to construction materials, such as cement. The transition to BPU emerges as an environmentally advantageous choice, significantly diminishing the carbon footprint and greenhouse gas emissions associated with constructing an average house, which typically generates 50 tonnes of carbon emissions [113].

In recent decades, extensive research has been dedicated to enhancing the environmental sustainability of construction practices. Simultaneously, there have been advancements in methodologies for assessing a building’s ecological impact. Manzardo et al. [114] highlights the importance of optimizing physical characteristics, such as thermal conductivity and density, to improve the environmental performance of BPU foams for thermal insulation. Noreen et al. [115] emphasizes the lower environmental impact, easy availability, and biodegradability of BPU coatings compared to petrochemical-based coatings. Luna et al. [116] focuses on the recent trends in waterborne and BPU coatings for corrosion protection, highlighting their potential as environmentally friendly alternatives to traditional petroleum-based coatings. Wray et al. [55] have employ life cycle assessment methods to analyse the early development stages of processes that incorporate biobased elements into PU coating production. Their findings indicate that integrating biobased compounds into hybrid coatings can effectively mitigate the environmental impact of PU coatings when compared to their fossil-based counterparts. However, it is essential to continue exploring biobased cross-linkers and researching end-of-life scenarios to further enhance the environmental benefits of BPU coating production. Suttie et al. [54] have presented an extensive environmental performance analysis, comparing the utilization of bio-based materials in building envelopes to traditional synthetic construction materials. Their study reveals the promising potential of bio-based materials in improving in-use energy efficiency, along with renewable sourcing, low embodied energy, carbon neutrality or negativity, and exceptional thermal regulation properties. They have also addressed the significance of regionalized versus global approaches in the environmental assessment of bio-based materials.

Hence, research indicates that BPU has the potential to be more environmentally friendly than traditional fossil-based materials. The key question is why BPU is superior and why transitioning to it is essential. This comparison can be elucidated through the following points:

• Embodied carbon Embodied carbon is the total greenhouse gas emissions from making, using, and disposing of a product. Bio-based materials, derived from plants, can absorb CO₂ while growing and trapping it when harvested. If a biomaterial absorbs more CO₂ than is used in its production, it is considered carbon negative. This can also apply to a whole product if the absorbed CO₂ outweighs that used in all stages of its life. Petrochemical based PU have a high associated embodied carbon, and eventually has negative impact because of its recyclability at end-of-life. BPU made from plant based starting materials generally require much less energy in its production than conventional materials, such as aluminium, concrete and steel, requires less maintenance, benefits like insulating properties further cuts down energy requirements and has better recycling ability. Accordingly, BPU will typically have a low embodied carbon [117].

• Promoting well-being Protective coatings are commonly employed in construction to protect the structural components of buildings. Unfortunately, these coatings often contain detrimental substances such as Volatile Organic Compounds (VOCs) and toxic isocyanates, which have adverse effects on indoor air quality and pose health risks. These chemical compounds, are not just present in coatings but are commonly found in various construction materials like paints, varnishes, adhesives, solvents, and flame retardants. BPU and WPU adhesives and coatings offer a viable solution to address these concerns [26].

• Circular economy A circular economy is like a loop where we continually use and reuse things, instead of throwing them away, to reduce waste. When biobased construction materials are responsibly sourced, they qualify as renewable resources. Utilizing locally sourced biobased materials for BPU production can effectively reduce the carbon footprint associated with transportation in construction. This focus on local sourcing offers additional advantages, including the creation of local job opportunities, the retention of economic value within the region, and support for a more diverse economic distribution across the supply chain. Furthermore, at the end of their lifecycle, these materials can be recycled for the same or different purposes. For instance, PU waste can be depolymerized to produce polyols, crushed for use as aggregates, or transformed into innovative materials [118].

• Offsite construction and transportation While prefabricated and modular construction methods offer the advantage of reducing construction waste, the energy expended in transporting these components over long distances may counteract the benefits gained from waste reduction and greenhouse gas mitigation during construction. In the broader context of a building material’s entire life cycle, these efforts could potentially yield an overall negative environmental impact. However, BPU is derived from locally available plant-based materials, and ongoing developments in rapid BPU product curing can allow easy onsite construction which can substantially decrease the need for long-distance transportation. Additionally, the lightweight nature of BPU facilitates convenient transport if offsite construction is considered [119].

In light of this, BPU based building materials hold significant promise as sustainable and renewable alternatives to traditional construction materials. Their adoption can lead to substantial reductions in the carbon footprint of the building environment. This highlights the need for further research and development to make BPU materials widely accepted by both the industry and society. While there’s been some discussion about using partially biobased PU and repurposing PU waste in construction, there’s still limited information about their overall environmental performance. To better understand the sustainability of these materials in real-world applications, more studies, standard guidelines, and thorough research are necessary.

Future Prospects

Emerging trends of BPU in construction reflect the industry’s growing focus on sustainability, environmental responsibility, and the need to reduce the carbon footprint of buildings. Here are some of the notable present and upcoming trends regarding the use of bio-based PU in construction:

• Greener isocyanate alternatives Ongoing research and development efforts are focused on improving the properties and cost-effectiveness of bio-based PU in construction applications. Efforts are made to develop 100% bio-based products. This is evident from extensive research going on in NIPU and bio-based Isocyanates. Example: Desmodur®  CQ N 7300 developed by Covestro [59] is a bio-based aliphatic diisocyanate, STABiO™ by Mitsui Chemicals [120] is again a Bio-based 1,5-pentamethylene diisocyanate (PDI) based polyisocyanate with high bio-mass content of about 60%. Covestro and Selena Group [121], a Polant based construction chemicals producer, are collaboratively developing a more sustainable line of PUF for enhanced building thermal insulation. Utilizing Covestro’s bio-based MDI, the upgraded Ultra-Fast 70 foam by Selena incorporates plant-based feedstocks, achieving a 60% carbon footprint reduction compared to fossil-derived counterparts. The ISSC Plus certified material significantly accelerates installation times for windows and doors, boasting a 90-min curing period and increased yield, maintaining equivalent properties to fossil fuel alternatives. Selena further integrates bio-based polyols and recycled PET materials in its foam range. MCPU Polymer Engineering have patented technology of producing soy based polyols and polyisocyanurate based high temperature thermosetting PU resins through solvent free polymerization.

• Additives and reinforcements Addition of natural fibres to polymer matrix can enhance its properties and expand its applications as flooring material, concrete mixes temporary fixes and so on. The growing market for biopolymer-based rigid foams, derived from functionalized vegetable oil reactants, offer reduced environmental impact and enhanced biodegradability, supporting eco-friendly practices. Strategic incorporation of plant-based additives further refines properties, including moisture resistance, fire retardation, and ultraviolet protection. Bio-powder [122] a Spain based company specializes in conversion of agricultural waste into biodegradable materials to replace mineral based additives, offering fruit stone powders and granules that serve as versatile components in various applications. Notably, our olive stone powders reinforce composites in flooring, roofing, and structural elements, while almond and walnut shell powders contribute eco-friendly qualities and texturizing capabilities to bio-plastics. In construction materials, such as concrete and ceramics, our fibrous fillers enhance strength and porosity, while in asphalt and bitumen, cellulose additives improve resistance and recyclability. Additionally, our milled almond shells, pistachio shells, and olive stones provide low-density fibers for efficient insulation materials, and olive pit and walnut shell granules serve as natural abrasives for cleaning, sanding, and smoothing surfaces.

• Research on concrete The application of self-healing concrete presents a significant opportunity to reduce maintenance costs and improve the longevity of structures. Other research opportunities involves employing BPU binders instead of traditional cement while maintaining strength requirements, utilizing recycled aggregates without affecting the overall performance of the structure, and incorporating advanced materials like shape memory-based PU for efficient crack closure. The successful integration of highly functional composite materials could be a further enhancement in the field of structural engineering [119].

• Recycling and circular economy There are many emerging industries and extensive researches going on in construction industry exploring recycling and upcycling options to reduce waste and extend the life cycle. Example: The company named Hutsman recycles the equivalent of over 1 billion 500 ml plastic bottles to product called TEROL®  polyols every year, having recycled content of up to 60% for foam applications. For over three decades, the RAMPF Group has been a pioneer in PU waste recycling. Through advanced chemical processes like solvolysis, post-consumer PU foam scraps from diverse sources are transformed into high-quality recycled polyols. These polyols, comparable in quality and technical properties to traditional counterparts, are custom-tailored to match specific production requirements. The resulting PU systems, based on recycled materials, demonstrate versatility equivalent to those derived from new polyols.

• Government policies and environmental regulations Builders and developers are pursuing green building certifications, such as Leadership in Energy and Environmental Design (LEED), which encourage the use of sustainable materials like bio-based PU. The European Union (EU) has enacted several legislative measures aimed at promoting the adoption of BPUs and other sustainable materials, notably including the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) Regulation. In the United States, the Environmental Protection Agency’s (EPA) Green Chemistry Program actively encourages the development of environmentally friendly products through the provision of research grants and awards for innovative green inventions. Numerous countries have implemented regulations to curb emissions from conventional petrochemical-based polyols. Governments worldwide are incentivizing businesses involved in the production of eco-friendly products, offering subsidies and providing tax breaks to companies utilizing renewable resources [27].

As the construction industry continues to adapt to the demands of a more sustainable and eco-conscious future, bio-based PU is expected to play a significant role in reducing the environmental impact of buildings while meeting performance and cost-effectiveness criteria. However, when it come to the construction sector, a thorough examination of materials is crucial to identify environmentally friendly options without compromising essential properties. Although, BPU offers substantial potential as future PU products, presenting an opportunity for partial or total replacement of petroleum-based counterparts, literature studies reveal several challenges [123, 124]. The greatest hurdle lies in securing reliable raw materials with consistent quality for the scalable commercial production of these renewable-based PUs. And a prominent challenge lies in avoiding products marked for human consumption or that directly compete with food production. This constraint excludes various derivatives sourced from vegetable oils, such as those derived from soybean, peanut, olive, corn, and sunflower, among others. Additionally, an energy/cost imbalance in reaction processes poses an open difficulty. The cost competitiveness of bio-based raw materials is a concern, requiring consumer demand, governmental incentives, or the recognition of unique performance characteristics for broader adoption. There are issues encountered with the end product such as low bio-based content, limited understanding of formulations, insufficient insights into catalyst toxicity and degradation products, consistent reactivity, UV stability, thermal and mechanical properties and curing times. Avoiding isocyanates and using an alternative bio product and method may be also a quite big challenge. While bio-based replacements exist for almost every fossil-based application, questions persist about production quantity, standardized testing, costing, and performance in real-world applications. Evaluating, scrutinizing, and comparing the sustainability and environmental impact of fossil-based and bio-based materials is essential, yet challenging. Additionally, the use of renewable resources alone does not ensure sustainability; it depends on manufacturing processes, application contexts, and recyclability. Despite these challenges, technological advances suggest that BPU could propel plastic-intensive industries toward a circular economy. Through the use of suitably altered building blocks during synthesis, PU could achieve 100% biobased content and degradability, providing a practical solution. In summary, the evolution of PU synthesis progresses from hazardous to non-toxic, ending in sustainability and eco-friendly applications.

Conclusions

This comprehensive review presented recent developments in the production of sustainable PU for construction applications. The review provides insights into the sources and contemporary methods used for producing bio-based PU, potential applications within the construction industry and also addressed the challenges encountered in utilizing BPU for intended applications, highlighting potential research gaps that can lead to significant advancements in the field. While extensive research has been conducted on producing bio-based polyols and utilizing them in PU production, their application in the construction industry remains limited. Further research is needed to enhance the strength and thermal stability of PU-based products, thereby ensuring their suitability for construction applications. It is worth noting that despite originating from renewable resources, the classification of PU as completely sustainable is debatable, as the isocyanate components used (such as MDI and HDI) are still considered toxic chemicals. The exploration of viable alternatives for both polyol and isocyanate components is crucial to achieve a truly green and sustainable PU that can revolutionize the construction industry and promote eco-friendliness. Furthermore, the limited literature available necessitates further investigation into the feasibility of implementing the existing methods at an industrial scale. It is essential to establish common approaches and standard testing conditions for these materials to ensure the quality of future research.