1 Introduction and global scenario

Polyurethane foams (PUF) were firstly synthesized in the 1950s and have been used in many areas due to their versatility. Rigid polyurethane foams (RPUF) play an essential role in many industries, such as construction, refrigeration, piping and automotive. For instance, in automotive industry foams are used in parts like the seats, bumpers, the inner section of the “headliner” roof, car body, spoilers and all doors and windows. In this regard, mechanical and acoustical properties play a major role for automotive and mobility sector, and the understanding of these properties mechanism is fundamental for the manufacturing of cheap and high performance foams [1,2,3]. Different types of PU foams, rigid or flexible, may be obtained, depending on the raw materials (polyol and isocyanate) and additives employed in the PUF synthesis [4]. The combination of additives may produce foams with high, medium or low density and high or low flexibility.

Briefly, surfactants are employed with the purpose of regulating the polymer apparent viscosity, aiming to produce foams with a more stable cell structure and low friability. Surfactants are generally oils, with inert characteristics and do not participate in the polymerization or kinetic reaction. Some silicone oils are commonly used for this purpose, some examples include poly (dimethylsiloxanes) and poly (phenylmethylsiloxanes), important in flexible and semi-flexible foam systems.

Catalysts are used for decreasing the energy required to start the reaction between hydroxyl and isocyanate groups, to form the urethane bonds, and then, reduce the time for the PU synthesis. Tin compounds (such as tin octoate and dibutyl tin dilaurate) are very useful in promoting the gelation reaction, but they are very sensitive with respect to hydrolysis stability. In this sense, amino-based reactants may be employed. Some examples include triethylenediamine and 4-dimethylaminopyridine (DABCO).

Usually, chain extenders (molecules with low molecular weight and functional groups) are employed for the increase on the foams’ strength and stiffness. Chain extenders are usually employed aiming to increase the polyol’s chain extension and the degree of polymerization of the formed PU. It promotes a high number of branches capable of forming bonds with NCO groups belonging to isocyanate. Some chemicals used for this purpose include glycols (e.g., polyglycols, glycerol and butanediol).

Different blowing agents may also be employed for PUF expansion. Usually, water is the most blowing agent employed for PUF expansion. However, if a high control of cell’s foams morphology is required, trichlorofluoromethane (137.4 g/mol) and n-pentane (72.2 g/mol) would be used, in addition to the polymerization conditions [4, 5].

The most common feedstock for PUF synthesis come from petrochemicals, such as polyesters or polyether polyols and isocyanates, such as methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI). On the other hand, PUF market have been increased in the last few decades, due its versatility and easiness to obtain different properties, depending the aforementioned chemicals employed for its synthesis. In this regard, aiming to obtain a higher environmental appeal, bio-based polyols have been used for PUF synthesis. Castor oil, palm oil, soy bean, among other fat oils are developed day by day aiming to replace conventional petrochemical reactants [6, 7].

Despite the aforementioned advantages in using oils from renewable resources, these feedstocks present some drawbacks. It is well known that bio-based foams present low mechanical response, thermal stability and sometimes friable characteristics. Aiming to by-pass these disadvantages, fillers may be employed, regardless the foam is flexible or rigid, aiming to produce a material with greater performance [8,9,10,11]. Bio-fillers (fillers from natural resources, without major chemical modification and/or treatment) may also be used aiming to manufacture a PUF with high performance and environmentally friendly characteristics. In addition to being abundant and renewable resources, bio-fillers are easily chemically modified, resulting in foams with enhanced mechanical properties, relatively low cost and greater thermal stability [9]. Furthermore, structural modification of polyols is usually carried out by adding flame retardants, which cause deterioration of mechanical and/or thermal response. Then, to pay-pass this concern, the addition of fillers is usually required [7].

For instance, Zhou et al. [9] produced semi-rigid PUF, using palm oil as polyol and cellulose nanocrystals (CNC) as reinforcement for the bio-polyol (BPU). The foams’ properties were compared with those produced using a petrochemical polyol (PPU). According the results, those PPU presented thicker cell walls and larger cell struts than those of the BPU foam. The compressive strength (10% deformation) of BPU foam is lower than that of PPU foam, 54 and 125 kPa, respectively. On the other hand, with the addition of 4 phr of CNC, there is a twofold increase in the compressive strength of the BPU4 foam (117 kPa).

Another vegetable oil widely studied for the production of PUF is castor oil, in which we find ricinoleic acid as the main fatty acid. This acid naturally has an enough content of hydroxyl groups, functional groups important for the synthesis of PUF. The adequate amount of these functional groups means that there is no need for chemical modification and it can be used directly as a polyol for the PUF’s synthesis. Furthermore, the uniform distribution of these groups in the castor oil chain enables the synthesis of PUs with a more uniform cross-linked structure, high mechanical performance and thermal stability. Also, the use of this oil becomes interesting for the production of PUF on an industrial scale because it is a non-food seed vegetable oil, not competing with food, and can be obtained at a lower cost [4, 9, 12].

To solve these challenging issues, many efforts have been paid to develop high performance PUF. The incorporation of fillers in polymeric matrices improves their mechanical properties. Several studies bring the incorporation of different fillers in PUF, such as micro-fibrillated cellulose [8], rice husk ash [11], cellulose nanocrystals [9], silica nanoparticles [13], kaolin clay [14], in addition to carbon structures such as graphene oxide [14,15,16]. The addition of graphene oxide gives PUFs greater thermal and mechanical response, electrical conductivity and a high surface area, when compared to carbon nanotubes, for example [17]. These foams have characteristics such as flexibility, lightness, low cost and can be used in different applications, one of which is acoustic insulation [18].

Sound absorption is influenced by several factors, in foams, morphology (associated with the cell wall structure) and pore characteristics (size, quantity, interconnectivity, tortuosity) are the most important. The airflow resistivity provided by the cell walls in foams with small interconnected pore structures provides a better sound absorption coefficient. Also, porosity, associated with low density, offers less resistance to sound waves dissipation, resulting in a low sound absorption coefficient. With the incorporation of GO, there is an increase in the compression modulus, and thus the cells are more likely to undergo cell stretching, bending and buckling without deformation [19].

Based on this, the objective of the present paper is to discuss the recent progress of the addition of graphene as reinforcement in polyurethane foams. The type of composite (if only graphene or hybrid composites) studied, as well as the methodology of adding reinforcement into PU foams will be detailed. Besides, the main differences in morphology and its reflection on the mechanical and acoustic response are presented and discussed in detail.

2 Systematic review methodology

The systematic review of recent studies of polyurethane foam reinforced with graphene oxide was carried out based on the methodological guidelines outlined by the Transparent Reporting of Systematic Reviews and Meta-Analysis (P.R.I.S.M.A.). The papers were select using the Scopus (www.scopus.com), Web of Science (www.webofknowledge.com) and Mendeley (www.mendeley.com) online databases. Figure 1 presents the evolution of publications in the three data bases used for this work with the keywords previously listen. It is noteworthy that the total number of papers in 2011 was only 2 and, in 2019, 44, having an increase in 2100%.

Fig. 1
figure 1

Evolution of publications about graphene and polyurethane foams

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The search terms used were polyurethane and graphene and foam, need to be at least on the abstract. The results were limited to English language, only research papers and published within 2017–2022. From the articles listed in the three databases, the titles and abstracts were read independently by two reviewers (Eduardo Kerche and Lídia K. Lazzari) to identify their suitability.

In agreement with both reviewers, some criteria were selected as relevant or irrelevant for the analysis of the papers, they are listed in Table 1. Some analysis and results that were evaluated and compared between the papers are listed in Table 1 as relevant criteria for paper inclusion. Besides, some exclusion criteria are listed since they don’t attend to the study scope.

Table 1 Relevant/irrelevant criteria for the papers’ analysis
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3 Results of data collection

The selection of studies (number of studies and studies included or excluded) is shown in Fig. 2 based on P.R.I.S.M.A. requirements. The search in the Web of Science, Scopus and Mendeley databases resulted in 502 articles, excluding review articles, in languages other than English, conferences, and duplicate articles, leaving 160 documents. Considering the relevant and irrelevant criteria presented in Table 1, at the end of the careful analysis of each of the 160 articles, 31 articles were considered relevant to be included in this study.

Fig. 2
figure 2

Systematic review flowchart, in accordance with PRISMA protocol

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3.1 Discussion of results

The papers were further classified based on the polyol type, the reinforcement used, and impregnation process of the reinforcement, and this is shown in Fig. 3.

Fig. 3
figure 3

Polyol type (a), type of reinforcement (b) and impregnation process (c), reported in the papers found in this systematic review

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Regarding the type of polyol (Fig. 3a), the vast majority of foams found in this review were produced with commercial non-bio-based polyol (96%). The authors focused on evaluating the effect of reinforcement on foam properties rather than to synthesize the foam.

Regarding the type of reinforcement (Fig. 3b), about 59% of the articles use graphene oxide (GO) as reinforcement for polyurethane foams, graphene nanoplatelets are also widely used, about 23% of the articles use this reinforcement, reduced graphene oxide, carbon nanotubes and graphite are also mentioned.

The addition of reinforcements occurs in two ways, the impregnation of the reinforcements in the polyols (67%) or deposition on the foams’ cell structure (33%). The impregnation of the reinforcements in the polyol occurs, mostly, by mechanical processes, with the mixing of the reagents by mechanical agitation and two roll mill or sonication of the reinforcement in the polyol solution, or chemical modification of the polyol. The second way is to deposit the reinforcement on the polyurethane foam cell structure through methods such as dip coating, layer-by-layer and solvent casting.

The 31 articles referenced in this review are summarized in Table 2. In addition to the characteristics of the type of polyol, reinforcement and reinforcement impregnation methodology, Table 2 also presents the type of foam produced, the amount of reinforcement used and the main results found by the studies.

Table 2 Synthesis of articles referenced in this review
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3.2 Polyurethane foam manufacturing techniques

Regarding the manufacturing techniques employed to PUF, free rise expansion is by far the most used technique for the fabrication of components, due to its easiness of processing and not the needing for complex apparatus or machinery for the production of components in large scale [35]. However, in current literature, not only simple techniques are employed for the manufacturing of PUF, but the expansion and reaction into closed molds are explored.

Polyurethane foams, expanded into closed molds, present a more regular cellular structure and great cell size distribution and uniformity [8, 45]. Moreover, the density’s fitting and adjustment may be due, depending on the application and final product destination. For instance, it is possible to manufacture a foam with high density to be used in structural application with the same formulation as that used in upholstery. Some models, using statistical approaches, may also be employed to predict the final product density and property, in a specific range, this is also an advantage in using a closed mold to manufacture the components using PUF [46].

Many methods have been proposed to create TPU foams, such as in situ polymerization using water as a foaming agent, gas foaming, salt leaching, phase inversion and water vapor induced phase separation. The use of supercritical fluids as blowing agent, and in particular carbon dioxide (sc–CO2), has become a promising and efficient strategy for the preparation of microcellular polymeric foams [47].

Indeed, differently from a free rise expansion, using blowing agents such as water, chlorofluorocarbons, hydrochlorofluorocarbons and methylene chloride [48] PUF produced by scCO2 present a more controllable and thinner cell structure rather those freely raised PUF. Furthermore, this technology is low cost, used in moderate conditions (T ≈ 30 °C, P ≈ 7 MPa), lowered burden, on the environment, and greater safety offered, compared to the aforementioned blowing agents. Finally, this technology also possesses the great advantage of being easily scaled-up to industrial level [42].

3.3 Polyol types

Polyols and poly isocyanates are the main sources of raw materials to produce PUF. There are several types of raw materials for the PUF production, however, those from petrochemical industry are the most diffused in industry, due to their well-known better performance, compared to bio-based ones, such as higher mechanical and thermal properties as well as fire resistance. However, with the use of PUF growth day by day, the generation of waste is an alarming concern for environmental protection, so proposals for eco-friendly PUF arise all the time [7].

One way to relieve this problem is the use of raw materials, derived from renewable sources, such as vegetable oils [8,9,10,11]. In addition to being more abundant, they are easily chemically modified, resulting in excellent properties and relatively low cost of final products [9]. This chemical modification of polyols directly affects the structural and morphological characteristics of the final PUF. These differences are usually carried out by the incorporation of additives, such as fillers, flame retardants, open cell agents and so on. However, some of these modifications bring a detrimental effect on mechanical and dynamical mechanical properties as well as in the cell structure distribution and growth [7].

Castor oil (CO) is one example of bio-based polyol. CO is composed primarily of ricinoleic acid, a fatty acid that presents, naturally, hydroxyl groups, a functional group important for the synthesis of polyurethane, which react with the isocyanate to form a urethane bond, by block polymerization. The adequate amount of these functional groups means that there is no need for chemical modification, and it can be used directly as polyols, for the PUF synthesis. Besides, the uniform distribution of these groups in the castor oil chain enables the synthesis of PUs with a more uniform cross-linked structure, which enables high mechanical performance and thermal stability, desired properties for RPUF. Furthermore, the use of this oil becomes interesting to produce bio-based PUF on an industrial scale, because it is a non-food seed oil, not competing with food, and can be obtained at a lower cost, contrasting with other oils, such as soybean [4, 12, 49].

3.4 Filled polyurethane foams with carbon structured reinforcements

As already mentioned, the addition of reinforcement in polyurethane foams brings benefits to their properties, as it modifies their morphological structure. Fillers, in a general way, also influence directly on the aforementioned foams’ expansion kinetic parameters and consequently on the final foams’ cell structure [50]. Bio-fillers, such as those from waste plays an important role on the manufacturing of PUF [51]. On the other hand, fillers with low dimensions and a high content of hydroxyls on its surface plays an important role for the higher thermal instability and PUF’s mechanical properties.

Kerche et al. [8] used micro-fibrillated cellulose to improve mechanical properties and thermal stability of a rigid, closed cell polyurethane foam. CO and glycerin were used as polyols and MDI as catalyst. Zhou et al. [9] compared the behavior of semi-flexible PU foams, produced from palm oil polyol (BPU) and CNC, as reinforcement, and a polyester from petrochemical industry (PPU). According to the study, PPU foams presented thicker cell walls and cell struts, and both aspects were larger than those of the BPU foam. The compressive strength (10% deformation) of BPU foam is lower than that of PPU foam, 54 and 125 kPa, respectively. However, with the addition of 4 phr of CNC, there was an increase ~ 50% in the compressive strength of the BPU foam with 4 phr of CNC (117 kPa).

In the same way, graphene (Gr) is a material with detrimental aspects, regarding its use as additive for improvements in thermal, mechanical, anti-fire resistance and thermal insulation of PUF [52]. There are several types of Gr and its specific applications is very related to the type, source and Gr obtention route. For instance, graphene nano platelets (GNP) generally come from powder flake graphite, obtained by exfoliation into a combination of ammonium persulfate, sulfuric acid and fuming sulfuric acid [53]. Neutralization, quenching with water, filtration and washing are also steps used to produce GNPs.

According to Fig. 3a, it is noted that 58% of the articles selected in this review use graphene oxide (GO) as a reinforcement for polyurethane foams. GO is a type of graphene with different characteristics, and consequently, properties. GO presents functional groups, such as hydroxyls and organic acids on its surface, providing characteristics such as high polarity and compatibility with many systems, such as polyurethane.

Multi-walled carbon nanotubes (MWCNTs) and GNPs can also be use as reinforcement into PUF due to their superior properties like GO. Wu et al. [26] established the synergistic effect of different GNPs (in fillers from 0.25 to 0.75% by weight) with MWCNTs in PUF. The results showed a higher synergistic effect with a low content of nanofiller (0.25% by weight), whose tensile strength was improved by about 43%, compared to pristine PU.

In the study carried out by Jia et al. [20], who produced RPUF with the addition of nano porous graphene, they obtained an improvement in morphology and mechanical/thermal properties compared to non-reinforced foam. By adding NPG, the average cell size was reduced and the mechanical properties were affected. When using 0.25 wt.% of NPG, the compressive strength and compressive modulus were increased by 10.7% and 66.5%, respectively.

3.4.1 Strategies to decrease the particles’ agglomeration

Due to the well-known trend of graphene agglomeration and formation of clusters, many authors have been developed techniques to avoid this issue. The using of nano or micro particles to increase the distance of GNP or GO is an interesting technique. In their work, Liu et al. [42] used hybrid fillers, composed of Hollow glass microspheres (HGMS) functionalized with silane and used the fillers to improve the composites’ compression properties and the lateral compression deformation. Li et al. [22] used Fe3O4 nanoparticles aiming to intercalate GO layers with high grafting ratio, resulting in complete exfoliation of GO in matrix.

From the composite foam point of view, the average cell size, cell wall thickness and apparent density gradually decreased along magnetic field direction, forming gradient cell structure in foam. The aforementioned techniques facilitate the dispersion and also the manufacturing of the PUF, since no additional step for the filler dispersion (as sonication) were required. Figure 4a presents the aforementioned strategies, for the improvement of dispersion of graphene.

Fig. 4
figure 4

Schematic diagram of GO nano or micro hybrid filler preparation and functionalization. Adapted from: [15, 23, 54, 55]

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Li et al. [55] also used a GO functionalized with Isophorone diisocyanate (IPDI), aiming to functionalize the GO surface (Fig. 4b). According the authors, these —NCO groups on the surface of modified GO could participate in the reaction of RPUFs through reacting with polyols to form polyurethane (PU) bonds, then, the mechanical and thermal properties were improved, due to the grater nanocomposite’s interfacial characteristics. Other study also used a functionalized GO, with APTES. The authors focused in the improvements of flame retardancy of RPUF with the silane treatment. Despite the improvements in these characteristics, decreases in mechanical properties, when the foams were compared to the neat RPUF were reported regardless a better interactions between GO and RPUF [55].

3.4.2 Techniques for the filled graphene PUF’ manufacturing

3.4.2.1 Mechanical stirring and sonication

One of the most used techniques for the incorporation of carbonaceous particles in PU foams is the addition of the same directly to the polyol or isocyanate, using mechanical agitation and/or sonication (see Fig. 3c). These are simple and easily manipulated techniques for the production of foams. Different formulations of PU foams are found in the literature, usually the concentration of carbonaceous structures used is from 0.25 to 5% [1, 18, 19, 21, 34, 38, 39, 43].

Another strategy is to disperse the nano particles into the blowing agents, such as Hui et al. [31]. In the study, the authors sonified GO into pyromellitic dianhydride (PMDA). Afterward, TDI was added and under vigorous stirring. Although the increase in density for all contents of GO studied, the authors obtained increases in mechanical and dynamical mechanical properties, as well as in Tg.

3.4.2.2 Dip coating and layer-by-layer impregnation of graphene

An interesting alternative, used by some authors, is to coat the foams with graphene solutions. According Li et al. [15], hierarchically nano-architectured graphene-polyurethane hybrid foams can be developed, depending the impregnation technique used for such purpose. A sound-absorbing material may be developed and different cell structures can be formed (varying from Wavy to Web foams` cells), depending the technique used to coat the foam, as presented in Fig. 5.

Fig. 5
figure 5

Cells` morphology of hierarchical porous graphene-polyurethane foams. a Web-disordered lattice graphene foam. b Wavy-ordered lattice graphene foam. Adapted from: [15, 27, 29]

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Oh et al. [15] also used an aqueous solution of GO, using different ratios of compressibility (10 and 20%), for the manufacturing. Compared with neat FPUF the foam interconnected with 2D corrugated GO shows a sound-absorbing capacity of 99.7% at a frequency of 2,236 Hz and a shock energy absorbing time of 189%, during the impact loading. Figure 5 presents the cells` morphology of such foams.

Others studies also addressed this interesting technique, however, using a silicon-based resin, with GO dispersed. Wu et al. [54] reported improvements of fire resistance of 14.7% for the pure foam to 31.5% for PU-RGO–SiR. Moreover, stable electrical conductivity and high flame retardancy, even in a variety of harsh conditions (high temperature, flame, organic solvents and external compression) were reported. Indeed, coatings, using graphene as filler is a new strategy to enhance mechanical, thermal and fire resistance of PUF. Furthermore, self-healing is an interesting characteristic achieved, when these techniques are employed [56, 57].

Meng et al. [15] obtained self-healing FPUF with polyelectrolyte complex coating for flame retardant and also enhanced mechanical properties. Using polyethyleneimine dissolved in deionized water and added to ammonium polyphosphate and subsequently adding GO into the solution, the authors reported a product with the capacity to self-repair even after the destructive mechanical tests.

According to Jia et al. [37] the principle of the layer-by-layer process is to arrange materials of opposite charge into multiple bilayers so that they are connected and assembled through a variety of attractive forces (such as van der Waals forces, electrostatic bonds and hydrogen bonds). Also, this method is a very effective and environmentally friendly technology for depositing flame-retardant coatings without sacrificing beneficial FPUF properties.

Figure 6 shows the process of impregnation of GO in PU foams carried out by Hou et al. [30]. Basically, the GO solution was poured over PU foam, after which negative pressure (–5 kPa) was applied. After complete immersion of the foam in the solution, the GO-impregnated PU foam was dried in air for one day. This step iterated up to 5 times to increase amount of GO impregnation.

Fig. 6
figure 6

Aqueous solution for the coating using the layer-by-layer self-assembly technique of graphene on the PUF surface. Adapted from [28, 30, 33, 55, 58]

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3.4.3 Sound mechanism improvement by graphene using

PU foam is widely used in acoustic materials due to its superior sound-absorbing properties, vibration damping and robustness. In this application, two main mechanisms of acoustic energy damping can be considered. In the automotive industry, there are several different frequency intervals, from those that originate at engine rotation (5–50 Hz), to vibrations generated on irregular road surfaces (500–3000 Hz). So, PU foams are widely used to absorb sounds and noise and thus provide acoustic comfort in the interior of the car [28, 33].

Basically, there is two main mechanism that govern the acoustic energy damping. The first one is related to the sound energy dissipating by the friction between the air gas molecules oscillating inside the foam cells. The air friction can convert the kinetic energy into heat that need to be dissipated by the cells’ skeleton. Then, the cell’s morphology (open or closed cells’ content), stiffness and its capacity to dissipate heat are the primarily responsible for the amount of sound energy absorbed. In the same way, the other mechanism is related to the cell wall material that absorbs sound, also called Intrinsic damping. In this mechanism, the sound waves propagate within the material, where the main factor that influences the sound absorption in flexible PU foam is flow resistance, based on the measurement of airflow resistivity [29].

As reported in this systematic review, the PUF sound absorption capacity may be greatly improved by incorporating particles into the PUF before or after manufacturing [34]. For this purpose, carbon nanotubes [59], silica [60] and graphene [15, 27, 33, 42] are the nano fillers that presented better properties for the improvement on PUF’s sound absorption capacity.

OH et al. [42] studied an antagonistic graphene oxide-polyurethane hybrid aerogel with aligned pores and graphene oxide face sheets as a novel sound absorber. The authors reported that the developed material presented a high sound absorption ability due to the effect of graphene nano layer. According the authors, the wave propagation was absorbed by the principle of thermal damping and viscoelastic frame damping, when passing through graphene oxide and polyurethane microcellular structures. Furthermore, tortuosity in wave propagation on the graphene oxide surface and microvibration of narrow GO layers with heat extraction also influences on the sound absorption at low frequencies.

Another issue explored by the authors was the influence of PUF cells’ anisotropy on the dissipation of heat that comes from the air friction, as aforementioned. Figure 7 presents the mechanism for the perpendicular (Fig. 7a) and parallel (Fig. 7b) sound energy reflection or absorption on the cell structure, for the developed Gr–PUF. The foams here represented are subjected to the tube impedance test aiming to measure the sound absorption coefficient (α).

Fig. 7
figure 7

Representation of the cells with graphene oxide incorporated and the mechanism for the sound wave propagation for the cells oriented perpendicular to the sound wave emission (a) and parallel to the sound wave emission (b). Adapted from [43]

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Perpendicular G–PUF will produce wave reflections and transmission while passing through the GO layers. In addition, as the sound waves enter, the gaps between GO walls, the fluctuation of acoustic pressure will induce micro-vibration of aligned GO layers [43]. On the other hand, when the foam is positioned with cells parallel to the wave propagation (Fig. 7b) the sound absorption principle follows the mechanism of entering of sound wave enters to the inner void. Here, multiple scattering phenomena strongly reduce the intensity of an acoustic wave passing through the highly porous GO–PUF, resulting in greatly increased sound absorption. Besides, as the sound waves approach the narrow gaps between the GO layers, the air starts to oscillate considerably, and the kinetic energy of the sound waves is lost to friction between the air and the GO layers and to the micro-vibration of the graphene layer [43].

4 Conclusions and future perspectives

This systematic review brings the newest technologies about graphene-modified polyurethane foams. Different types of foams may be manufactured by the use of graphene and its derivatives. Rigid or flexible foams can be reached, depending the chemicals used for the foams’ synthesis, additives and technique for the PUF expansion (freely expanded and using a closed cavity). In the same way, the use of graphene may be a benefit, depending the properties desired, which is related to the foam type required for each application (flexible or rigid).

As presented, the deposition of graphene, following layer-by-layer technique is an interesting alternative for foams with high damping properties. On the other hand, sonication and other dispersion techniques for graphene on the polyol are required, when a foam with a high stiffness is desired.

Bio-based polyols were used in many studies. However, new technologies to synthesize foams with polyols from renewable resources are still required, main when a high performance PUF is desired.

Finally, it was presented that the sound absorption as well as mechanical properties are high dependent on the direction of property analyzed. Due to the high anisotropy of freely expanded polyurethane foams, its cells growth influence directly on the property direction measurement. Then, its destination and product design need to take into account these issues.