Melt processing of biodegradable poly(butylene succinate) (PBS)—a critical review

Abstract

This review paper presents a comprehensive analysis of the melt processing of polybutylene succinate (PBS) blends and composites. PBS, a biodegradable and eco-friendly thermoplastic polyester, has garnered significant interest in sustainable material research. The paper collates and examines a wide range of studies focusing on the processability, optimization of processing parameters, and resultant mechanical properties of PBS when processed through several extrusion techniques and by injection molding. Key parameters such as melt temperature, screw speed, and mold temperature are considered for their impact on the quality and performance of the final product. The review highlights advancements in processing technologies and material modifications that enhance PBS properties, making it a viable alternative to traditional petroleum-based plastics. Furthermore, challenges and limitations in the current processing techniques are discussed, offering insights into potential areas for future research. The synthesis of findings from various studies provides a holistic understanding of the state-of-the-art in PBS processing, aiming to guide further developments in the field of biodegradable polymers. Overall, this review underscores the importance of optimized melt processing techniques in maximizing the potential of PBS as a sustainable material in diverse applications.

Graphical abstract

Introduction

Polybutylene succinate (PBS) is a thermoplastic polymer that belongs to the family of aliphatic polyesters. The development of PBS dates back to early 90’s, when researchers began exploring biodegradable polymers as alternatives to conventional plastics. It is synthesized through a polycondensation reaction between succinic acid and 1,4-butanediol. The polymerization process can be managed to control the molecular weight and physical properties of the resulting polymer (Jacquel et al. 2014; Rafiqah et al. 2021; Savitha et al. 2022). A significant aspect of PBS is it partially derives from renewable resources. Succinic acid, one of the key monomers, can be produced via fermentation of glucose, sourced from various biomass materials. This aspect of PBS aligns with the growing emphasis on sustainable and eco-friendly manufacturing practices (Barletta et al. 2022; Gucker et al. 2023; Ilarduya and Guerra 2020; Thakur et al. 2022). One of the defining features of PBS is, therefore, its biodegradability. It can be broken down by micro-organisms into water, carbon dioxide, and biomass under composting conditions. This property is crucial in the context of reducing plastic pollution and promoting environmental sustainability (Aliotta et al. 2022; Tokiwa and Calabia 2007). In this respect, PBS is often compared with other biodegradable polymers like polylactic acid (PLA), polybutylene-co-adipate-co-terephthalate (PBAT) and the broader range of polyhydroxyalkanoates (PHAs). Each of these polymers has distinct properties and degradation behaviors, making them suitable for different applications. PBS, in particular, is noted for its balance of mechanical properties and biodegradability (Barrino et al. 2023; Genovesi et al. 2022). The market for PBS has been growing steadily in the last years, driven by increasing demand for biodegradable materials and supportive environmental policies. The global polybutylene succinate (PBS) market size was valued at USD 399.4 million in 2023 and is projected to reach USD 2.0 billion by 2032, registering a CAGR (Compound Annual Growth Rate) of 20.2% during the forecast period (2024–2032) [Source: https://straitsresearch.com/report/polybutylene-succinate-market]. This growth is mirrored in the expanding range of applications and interest from various industries, including packaging, agriculture, and biomedical sectors. The escalating environmental challenges posed by synthetic polymer waste have, in fact, catalyzed the exploration of biodegradable polymers like PBS. The adverse impacts of conventional plastics on ecosystems, particularly marine environments, have necessitated the shift toward materials that can decompose naturally without leaving harmful residues. PBS, with its biodegradability and appropriate balance of technological and functional properties, presents a viable solution to this global issue (Nabeoka et al. 2021; Tokiwa and Calabia 2007). PBS exhibits a broad range of desirable properties, such as good mechanical strength and toughness, thermal processability, and compatibility with various additives. These characteristics enable its application in diverse fields such as packaging, agriculture, automotive parts, and even in manufacturing of biomedical devices (Aliotta et al. 2022). The polymer ability to degrade under composting conditions also makes it suitable for single-use products where rapid biodegradation post-use is advantageous (Gigli et al. 2016; Siracusa et al. 2015). Melt processing, encompassing extrusion and molding, is a critical aspect of PBS fabrication. These techniques are essential for transforming the raw polymer into functional products. Extrusion involves forcing molten PBS through a die to create continuous shapes, while molding involves shaping the polymer in molds under heat and pressure. These processes significantly influence the final properties of PBS-based products. The processing parameters in extrusion and injection molding, such as temperature, pressure, and shear rate, play a pivotal role in determining the physical, mechanical, and thermal properties of PBS. Optimizing these parameters is crucial for achieving the desired product characteristics. The interplay between processing conditions and polymer properties forms a complex matrix that requires thorough understanding for effective material design (Aversa et al. 2021a; Barletta and Puopolo 2020; Calderón et al. 2019a; Muthuraj et al. 2015; Sasimowski et al. 2021; Zaverl et al. 2015). Despite the advantages, melt processing of PBS presents several challenges. These include issues related to its thermal stability, processing window, and the need for specific additives to enhance properties like stiffness, impact strength and flexibility as well as gas impermeability and UV–VIS light transparency. Addressing these challenges is vital for expanding the applications of PBS and enhancing its performance in existing ones (Georgousopoulou et al. 2016; Makhatha et al. 2008; Siracusa et al. 2015).

This is therefore the context in which this review paper aims to offer an exhaustive overview of the advancements in melt processing techniques for PBS, with a special focus on extrusion and injection molding. It intends to collate and analyze existing research, drawing insights from a broad spectrum of bibliographic sources. The paper will navigate the readers through the intricacies of these processes, the interplay of processing parameters and material properties, and the challenges and solutions proposed in the literature. The review will highlight gaps in current knowledge and propose areas for further exploration, especially in enhancing the properties of PBS through innovative processing techniques and additives.

Commercial grades of PBS

PBS is widely available on the market, entirely derived from fossil or, in smaller quantities, from partially bio-derived sources. The global PBS market size was valued at USD 399.4 million in 2023 and is projected to reach USD 2.0 billion by 2032, registering a CAGR of 20.2% during the forecast period (2024–2032) [Source: https://straitsresearch.com/report/polybutylene-succinate-market].

Table 1 shows the distinction between the main commercial grades of PBS that are, nowadays, offered on the market. Manufacturers offer a low molecular weight, high MFI grade of PBS for the scope of the injection molding process. For the extrusion process, the main manufacturers instead propose a grade with a high molecular weight and low MFI. Alongside these grades, PBSA is also on offer, which is a copolymer characterized by greater flexibility and high biodegradability in relation to the presence in it of adipic acid-based co-monomers.

Table 1 Different PBS grades that are commercially available

Processability of PBS and related blends

PBS is an extremely interesting material for industrial applications as it has properties very similar to those of polyethylene (PE), which make it a biodegradable and low environmental footprint candidate to replace PE in multiple applications. PBS is quite expensive (up to 5500 Eur/ton for material derived from fossil or biological sources, respectively), with a high volatile price in the last years. Compared to the relatively cheaper PLA (about 3100—3300 Eur/ton at the current market situation), another biodegradable aliphatic linear polyester, PBS has a glass transition temperature below ambient temperature (T_g = − 32 °C) which makes it extremely flexible and tenacious in daily use. Furthermore, the sustainability of PBS is corroborated by the simplicity with which its precursors can be produced from bio-derived raw materials such as succinic acid and 1,4-butanediol (Mincheva et al. 2013), (Wang et al. 2014). Furthermore, PBS is easily processable, most of the time without modifying the equipment used for the transformation of conventional polymeric materials based on raw materials from fossil sources. In fact, PBS has excellent stability up to about 220 °C (Aliotta et al. 2022). Furthermore, in thermal degradation tests, the mass loss rate peak is obtained at about 390 °C, a good 25 °C more than PLA (Lu et al. 2012). Thermal degradation studies (Fig. 1) of PBS in nitrogen flow have also shown that the loss in mass is essentially determined by chain splitting reactions that are triggered at about 260 °C (Lu et al. 2012). The scission reactions involve the hydrogen in position “β” of the ester groups, accompanied by the breaking of the weak O–CH₂ bond, with the formation of succinic acid and/or chains terminated with alkenyl domains and carboxylic terminal groups. The succinate terminal groups further decompose to anhydride-functional species by a dehydration mechanism. In the presence of oxygen, also thermal degradative mechanisms involving the formation of radical species have been suggested, by means, in particular, of an abstraction mechanism of hydrogen in position “α” (Rizzarelli and Carroccio 2009). This aspect assumes practical importance as it is impossible to exclude the presence of oxygen in industrial processes. The aforementioned mechanisms of hydrolytic and thermo-oxidative degradation determine the scission of the PBS chain, the reduction of the molecular weight and, therefore, the reduction of the viscosity of the melt, similarly to what happens, for example, for PLA. However, it has been shown that PBS exhibits a degradation mechanism, especially when stressed with high compressive stresses, which, contrary to expectations, produces an increase in melt viscosity (Hallstein et al. 2021). This increase is observed even after short times in which the polymer is processed, not significantly increasing in the event of a further increase in the processing time of the polymer itself (Fig. 2). Basically, following the cleavage reactions, recombination reactions are observed with restoration or increase in the molecular weight of the polymer and possible formation of branched structures, which determine a significant increase in the viscosity of the polymer melt. Such processes also occur at low temperatures (< 200 °C) as observed in several scientific papers (Kanemura et al. 2012), (Georgousopoulou et al. 2016).

Fig. 1

Degradation of PBS. a Hydrolysis; b thermal oxidation; c thermal degradation (Hallstein et al. 2021)

Fig. 2

Viscosity increase in PBS processed for 5, 10 and 30 min (Hallstein et al. 2021)

Hallstein et al. (2021) have identified as a possible cause of the occurrence of branching reactions the presence in the commercially derived PBS and PBSA of vinylene groups, which are formed by copolymerization of small levels of fumaric acid. Fumaric domains are notoriously very sensitive to the presence of oxygen and to radical species centered on carbon atoms, thus favoring the formation of PBS molecules characterized by the presence of long lateral chains (branches). These ramifications are believed to be responsible for the high increase in melt viscosity. Furthermore, the researchers have noticed that the control of these degradation mechanisms, also through the use of appropriate stabilizers (for example, phenolic- or phosphite-based anti-oxidants) becomes of fundamental importance to ensure a high level of control of all phases of the production process (Hallstein et al. 2021), thus avoiding the formation of gels.

Melt compounding with twin-screw co-rotating extruders

The preparation of compounds in bioplastic material requires considerable attention due to the particular sensitivity, especially of all the biodegradable polyesters and, therefore, also of PBS to processing conditions (Calderón et al. 2019b; Hallstein et al. 2021; Meng et al. 2019; Muthuraj et al. 2014). The high temperatures that can be reached during the extrusion process associated with the high shear stresses cause, as specified in the previous section, reactions that can affect the structure of the polymer both by reducing and, more rarely, by increasing the molecular weight (Cho et al. 2001; Li et al. 2006; Rizzarelli and Carroccio 2009; Shih and Chieh 2007). The same conditions can therefore generate scissions along the polymer chain or the formation of lateral branches on the same, influencing the rheological behavior and, therefore, the processability of the material and, correspondingly, its thermal stability (Abderrahim et al. 2015; Chrissafis et al. 2005; Georgousopoulou et al. 2016; Lu et al. 2012). The presence in the extrusion phase of additional constituents can also trigger further reactive phenomena or simply favor the formation of physical bonds that can influence the final characteristics of the compound, as well as the processability of the material (Chen and Yoon 2005; Monika et al. 2019). Extruding a bioplastic compound therefore requires the definition of a converting plant that takes into consideration multiple aspects:

• i. The need to blend two (binary blends) or, more rarely, three (ternary blends) polymers, available in the form of granules, in order to obtain an engineered bioplastic that can take advantage of the synergies that can be achieved by mixing basic materials with complementary thermo-mechanical and physical properties (Coiai et al. 2021; Eslami and Kamal 2013; Fenni et al. 2020; Ravati and Favis 2013; Safari et al. 2022; Xue et al. 2019; Zhang et al. 2012a, b, c; Zhen et al. 2011);

• ii. The need to use additives, which are available in the form of powders or, more rarely, in the form of granules, such as reinforcing agents of mineral nature and/or bio-derived, rheology modifiers, compatibilizers, chain extenders, stabilizers, lubricants, dyes (Cicogna et al. 2023; Dorez et al. 2014; Dumazert et al. 2018; Liu et al. 2016; Nanni et al. 2020; Supthanyakul et al. 2016; Tachibana et al. 2010);

• iii. The need to add plasticizers, often available only in the liquid phase (Liu et al. 2015; Phetwarotai et al. 2018; Pivsa-Art et al. 2016; Reddy et al. 2012; Shibata et al. 2006; Zhao et al. 2012).

Furthermore, these constituents must be pre-dried (dehumidified up to a residual humidity value of less than 250 ppm) in order to avoid an excessive presence of residual humidity during the extrusion process, which could favor the formation of thermal degradation reactions, i.e., the well-known thermo-hydrolytic degradation of polyesters (Cho et al. 2001; Phua et al. 2011; Wang et al. 2016). All this is associated with the need to process, on an industrial scale, massive volumes of material, with productivity which, not infrequently, can exceed 1000 kg/h and which, in general, can vary between a minimum of 500 kg/h and a maximum of 3000 kg/h. These production volumes require the implementation of processes that must be fully controlled automatically and proceed in unattended mode once started in order to ensure the stability and safety of the production process itself. An industrial scale, fully automatic plant for the production of PBS blends with PLA is shown in Figs. 3, 4, 5 and 6. The images show the executive design of the plant. The system provides for three different types of raw material withdrawal points: (i) big- bag emptying devices for raw materials that are supplied in granules inside big-bags weighing over 1000 kg; (ii) bag emptying devices for raw materials that are supplied in powders or granules inside 25 kg bags; (iii) silos for raw materials that are supplied in powders through tanks (normally, mineral fillers such as talc, calcium carbonate and titanium dioxide).

Fig. 3

Full compounding extrusion line

Fig. 4

Zoom on twin-screw extruder and on the related cutting device

Fig. 5

Side view of the compound production plant

Fig. 6

Frontal view of the compound production plant

The raw materials are simply blown by the devices referred to in the preceding paragraphs (i) and (ii) to be sent to the respective lungs of the feeding device (one for the additives in powder and one for the polymers and additives in granules).

Vice versa, the raw materials coming from the silos (minerals in powder form) are directly blown and sent to the respective suppliers located laterally to the extruder. The materials referred to in devices (i) and (ii) are subsequently sent from the lungs to individual dosing systems equipped with a load cell. After weighing the individual quantities of materials required by the specific formulation of the compounds, they are supplied inside a pre-chamber where they are carefully mixed together and subsequently fed to the extruder. The materials available in granules are fed into the main mouth of the extruder. The materials available in powder form are, on the other hand, fed laterally to the extruder using side- feeding devices.

The detail of the extrusion system with the relative cutting device is shown in Fig. 4. In this specific case, the extruder is a co-rotating tween-screw device equipped with 70 mm diameter screws, with a screw length/diameter l/d ratio of 48. The device is also equipped with 12 modules with individual temperature control, numbered from 1 (entrance area) to 12 (exit area). The granules are fed into zone 1. The powder additives and minerals are fed to the extruder in zone 4 and zone 6 by using a side feeder. Figure 5 shows the side-view of the extruder. The feeding of powdered materials takes place in zone 4 and zone 6 so that the same occurs directly inside the melted polymer, avoiding direct contact of powder additives with the screws (to avoid abrasion). The side-feeders are necessary to counteract the counter-pressure exerted on the raw materials entering the polymer melt in zone 4 and, even more, in zone 6.

The extruder has ventilation ports and is equipped in zone 10 with a vacuum pump for the extraction of volatiles and humidity generated during the process, reducing the back pressure of the polymer melt inside the extrusion cylinder. The polymer melt is extruded in zone 12 inside a multi-hole die by an immersed cutting device. The polymer melt is immediately immersed in cooling water at the exit of the die, being rapidly "frozen" and cut with a rotating blade device into small “pellets” (2 or 3 mm). The pellets thus produced are conveyed by the liquid into a centrifuge where they are carefully dried. The dry pellets are sent to a dehumidification device (devices in blue shown in Fig. 6) to eliminate residual moisture and be packaged in bags that guarantee impermeability to water vapor to prevent the material from reabsorb the atmosphere humidity. Figure 7 resumes the typical sections of a state-of-the-art compounding plant, and Fig. 8 summarizes the design criteria of biopolymer compounds.

Fig. 7

Scheme of compounding plant (https://www.coperion.com/en/industries/plastics/compounding)

Fig. 8

Design of polymer compounds by blending with additives (https://www.grida.no/resources/14864)

Farrell-Pomini continuous mixing

Figures 9 and 10 show the details of the Farrell-Pomini “continuous- mixing” technology [Source: https://www.farrel-pomini.com/]. The system uses a pre-chamber equipped with two rotors inside which the constituents of the formulation can be fed through the use of a gravimetric multiple dosing system, similarly to what happens in co-rotating twin-screw extruders.

Fig. 9

“Farrell continuous-mixing” (FCMTM, Farrell Pomini) technology

Fig. 10

Detail of “Farrell continuous-mixing” (FCMTM, Farrell Pomini) technology

The two rotors, turning around their own axis, mix the constituents until a homogeneous mixture (“layered structure”) is obtained, which is progressively discharged into a single-screw extruder placed at the bottom. The single-screw extruder conveys the material to the die where it is subjected to cutting, with technologies similar to those found for systems with a co-rotating twin-screw extruder. (Lahmann and Knowlton 2019) have shown the excellent results of continuous-mixing technology applied to the processing of biodegradable polyesters loaded with minerals. In particular, the authors showed a comparative analysis between the “continuous-mixing” technology and the co-rotating twin-screw extrusion technology for the purpose of molecular weight retention of a bioplastic. Experimental evidence has shown that the "continuous-mixing" technology produces a slight degradation of the molecular weight of biodegradable polyester (range 1–11% for talc content between 20 and 60 wt.%), as well as decidedly lower operating temperatures (178–212 °C for the same talc content). Under the same conditions, the co-rotating twin-screw extruder determined a significant degradation of the molecular weight of the biodegradable polyester (18–63% for talc content between 20 and 60 wt.%), as well as distinctly higher operating temperatures (223–235 °C for the same talc content). These results can be ascribed to the greater co-volume present in the mixing chamber of the "continuous-mixing" device (Yao and Manas-Zloczower 1998) and, consequently, to the lower frictions that reduce operating temperatures. Furthermore, the "continuous-mixing" technology provides a substantially open pre-chamber, where degassing and elimination of excess moisture is easier, resulting in less thermo-hydrolytic degradation of biodegradable polyesters. However, with the same level of productivity, a higher investment cost is necessary for continuous-mixing systems, which can boast, in general, greater energy efficiency (0.127–0.166 kWh/kg vs. 0.308–0.346 kWh/kg in laboratory scale machines and for flow rates always lower than 50 kg/h, especially for talc content between 20 and 50 wt.%).

Cast extrusion

The flat head (cast) extrusion process (Agassant et al. 2005) is used to convert bioplastic materials and, in particular, biodegradable polyesters such as PBS in order to produce films with a thickness commonly between 100 and 1000 mm. Thinner thicknesses (< 100 mm) can be obtained, although a blown extrusion process is commonly used for that purpose. Very often the flat head (cast) extrusion process is combined with a further stage of the production process, thermoforming, to manufacture complex geometry products, which will, however, be discussed independently in a subsequent section. The flat head (cast) extrusion process can allow the manufacture of single-layer or multi-layer films by virtue of the construction type of the plant.

The single-layer system (Fig. 11) envisages the use of an extruder, normally single-screw, to which granules are fed in a hungry-mouth mode. The details of the sheet forming process are summarized in Fig. 12. The granule is melted in the extruder cylinder, equipped with several modules with individual thermal control. The molten material is extruded through a flat head die. The section through which the material passes through the die can be adjusted transversally on multiple points, both in manual and automatic mode. The polymeric melt exiting the die is collected on a calender consisting of two or more cylinders. The melt leaving the die nevertheless comes into contact with a first cylinder with a large diameter, whose temperature is controlled (it can be both heated and cooled). The polymer melt stabilizes by contact with the roller and is then calendered. The newly stabilized polymeric melt passes, in fact, through the passage opening between two counter-rotating metal rollers which exert a pressure on the melt and a dragging effect in relation to the rotation speed of the rollers, with respect to the flow rate of the polymeric melt exiting the flat head. The greater the speed of the rollers and the pressure they exert on the plastic material, the greater the effect of compression and orientation of the polymer chains in the machine direction. The film thus obtained is, finally, wrapped in a coil operated by a drive system.

Fig. 11

(Source: The Japan Steel Work Ltd)

Plant for the manufacturing of cast film

Fig. 12

How the material get through the equipment in film forming process

Some kind of hauling-off devices subject the film to intense traction in the machine direction in order to carry out a mechanical stretching of the polymeric material, functional to modify its morphological structure (for example, by promoting its crystallinity or by inducing an orientation of the crystals or lamellae). There are also devices that allow the so-called bi-orientation of the film (Fig. 13) at the fronts of large investments to achieve the expansion of the film in the direction transversal to the direction of the extrusion (machine direction), thus being able to deeply modify the mechanical characteristics and physical characteristics of the finished product.

Fig. 13

(Source: The Japan Steel Work Ltd)

Plant for manufacturing of bi-oriented film

The multi-layer systems (Fig. 14) differ from the previous ones, as they are equipped with a greater number of extruders, as well as dies capable of managing multiple layers. Some films are symmetrical or are characterized by layered structures of the ABA or ABCBA type, in which the outer layers are repeated in pairs. For structures of the ABA type, two extruders are used for the manufacturing of the three layers films, one for material A and one for material B. For ABCBA films, the extruders become three, at least. In these cases, the material coming from the extruder is suitably conveyed toward the corresponding portion of the die. For asymmetric film, each layer is associated to an independent extruder, as in the multi-layer ABC, where it is needed three independent extruders to make the film in three layers. The supply chain is also different in multi-layer systems, as it must convey the individual layers and allow them to adhere correctly.

Fig. 14

(Source: Fuoshan GET plastic Equipment Co., Ltd.)

Multi-layer cast extrusion

Very often, in the manufacturing process of multi-layer films, so-called tie-layers are used, i.e., very thin intermediate layers which only have the function of compatibilizing the other layers or of increasing their mutual interfacial adhesion. For example, in an ABCBA structure, layer A and C could be made up of two bioplastics with different characteristics (for example, PLA and PBS) and the intermediate B layers could be made up of tie-layers necessary to ensure proper adhesion between the layers A and C. A typical five-layer feed-block is reported in Fig. 15. The temperature-controlled calender and the coupling system does not differ not in mono- and multi-layer cast extrusion plant, thus presenting substantially similar characteristics. However, multi-layer systems have a higher purchase cost determined by the need to have a greater number of extruders and a more complex supply chain that must manage multiple layers of material at the same time.

Fig. 15

(Source: https://www.smart-ecofilms.com/en/technology)

Feed-block

Melt processing of PBS-based compounds by cast extrusion

There are not many studies in the literature related to the cast extrusion process, especially when compared to the enormous diffusion of the technology in the industrial environment. Furthermore, the high investment costs required to purchase pilot systems limit the available experiments to laboratory-scale applications, often with flat head die with widths of or less than 200 mm. Industrial systems have die widths much larger, amply exceeding 1000 mm, this determining a limited scalability of research results available.

Table 2 reports an overview of the most relevant papers on cast extrusion of PBS-based blends.

Table 2 Overview of cast extrusion of PBS-based blends

Wang et al. (2013) studied the influence of process parameters on the structure of PBS films obtained by cast extrusion on chill roll, followed by mechanical stretching of the film with haul-off device. The results showed a substantial independence between the process parameters and the degree of crystallinity achieved by the material, probably because PBS crystallizes very quickly. However, by increasing the speed of the haul-off device and reducing the operating temperatures, a progressive orientation of the material in the direction of the machine is obtained for both the crystals and the lamellae. Shish-kebab type structures are observed for very low operating temperatures (especially 155 °C) and high hauling-off speeds (ideally starting from 0.8 m/min).

Nobile et al. (2015) demonstrated the processability of PLA/PBS blends reinforced with mineral fillers in a cast extrusion process, directly feeding the constituents inside a single-screw extruder equipped with a flat head die for the preparation of 300 ± 40 μm thick films.

Voznyak et al. (2016) studied the production of PLA blends as the main polymer phase reinforced with PBS nano-fibers. For this purpose, the researchers performed an extrusion with a single-screw system equipped with a slit-die. In essence, the researchers extruded a film of bioplastic material 800 μm thick and 12 mm wide. In order to prevent degradation phenomena, anti-oxidants have been introduced at 0.2 wt.% within the formulation. The set temperature profile is decreasing (from 170 to 135 °C) in order to obtain a material with the correct resistance of the polymeric melt at the output of the die. The die temperature was set at 130 °C, with a melt pressure set at 65 MPa. The polymer melt was then extruded onto a conveyor belt, the temperature of which was set at 25 °C. Experimental results showed that slit-die extrusion is an effective process for manufacturing PLA blends reinforced with PBS nano-fibers. The process generates the presence of deformation bands (shear bands) homogeneously distributed in the material, which can significantly increase the ductility of the material, when it is subjected to tensile stresses.

Messin et al. (2017, 2020) showed a cast extrusion process capable of generating multi-layers with a very high number of layers (over 2000), using a series of so-called layer multiplication elements. In particular, the researchers have focused their attention on a multi-film layer between PLA and PBSA, showing how the manufacture of the same in nano-layer from about 60 nm in thickness can provide numerous advantages to the mechanical and physical properties of the film (Fig. 16).

Fig. 16

Schematics of the continuous two-component co-extrusion process equipped with the multiplying-element device (Messin et al. 2017)

Jost (2018) instead, evaluated the processability of PHBV/PBS blends in a cast extrusion process, using a 30 mm single-screw extruder with a ratio of chamber length L to the diameter of the extrusion cylinder d equal to 30 and a die 300 mm wide. The cast extrusion was carried out between 150 and 165 °C (< 170 °C to prevent degradation of the PHBV), with a rotation speed of 30 rpm. The final thickness of the film obtained is about 80 mm. Although the PBS resulted in an increase in the flexibility of the PHBV/PBS film (elongation at break exceeded 700%), the gas permeability was worsened by about two orders of magnitude.

Gigante et al. studied the cast extrusion process of PLA/PBS blends (15–17 wt.%) plasticized with ATBC (20 wt.%), even in the presence of CaCO₃ (max 4 wt.%) and an additive based on an acrylic copolymer (max 2 wt.%) to adjust the strength of the polymer melt (Gigante et al. 2019). The process was carried out by connecting a co-rotating twin-screw extruder directly with a flat head die, connected to a device with cooled rollers for the stabilization of the polymer melt and to a haul-off device for winding the film. The rotation speed of the rolls has been set to obtain a stretching effect of the film, which causes a reduction in the thickness of the film with respect to the gap of the die. The temperature profile was set in a range between 150 and 180 °C and the rotation speed of the screws at 270 rpm. The role of the additives (the plasticizer and the melt strength enhancer) was crucial in increasing the processability of the material in cast extrusion, allowing to obtain a stable film. The film has good ductility characteristics compared to PLA films as is. Furthermore, the degree of crystallinity is also governed by the presence of the additives.

Salomez et al. (2019) instead, studied the biodegradability of PBSA films produced by cast extrusion. The process involved the use of a co-rotating twin-screw extruder with 16 mm diameter screws and an l/d ratio of 40 with a flat die, a heat-regulating calender and a haul-off device. The rotation speed of the screws was set at 200 rpm and the temperature profile for the PBSA was set in the range between 100 and 135 °C. The films produced have a thickness of 210 ± 25 mm. The film thus obtained showed a fast biodegradation.

Aversa et al. (2020) studied the flat head extrusion of PLA (~ 67 wt.%)/PBS (~ 7 wt.%) blends reinforced with talc (~ 22 wt.%) and modified with rheological additives (3.4 wt.%), using a device with a 100 mm wide die capable of manufacturing 180 μm thick films, using a thermoregulated two-roll calender and a haul-off device. The films showed an increase in oxygen permeability by adding a-tocopherol (0.2 wt.%) to the formulation.

Barletta et al. (2020b) on the other hand, demonstrated the influence of a chain extender with glycidyl methacrylate functionality on PLA/PBS films by cast extrusion. The researchers have shown how these blends can be reprocessed with subsequent manufacturing cycles, adding the chain extender without having to recompose the granules with a co-rotating twin-screw extruder. This result opens a broader window of possibilities regarding the recycling of these blends, offering an alternative to composting for managing their end-of-life.

Barletta et al. (2020a), Barletta and Pizzi (2021) more recently, demonstrated the effect of using multiple nucleants to control the crystallinity of films in PLA/PBS blends with talc. The study involved a phase of compounding the blends by adding talc, ethylene-bis-stearamide (EBS) and PDLA (polylactic acid consisting of 99% of type d isomers) as multi-nucleants. The granules were subsequently extruded using a single-screw cast extruder (with a diameter of 25 mm) equipped with a 200 mm wide die, a thermoregulated double roller calender and a haul-off device. Finally, the films were subjected to thermoforming in a pilot device to obtain containers with complex geometry. Modulating the different nucleating agents, it is possible to generate synergistic effects that change according to the stage of the manufacturing process, as well as the heating/cooling phases that the polymeric blends undergo during the process.

Blown extrusion

The blown extrusion process (Han and Park 1975a, 1975b, 1975c) is used to transform bioplastic materials and, in particular, biodegradable polyesters such as PBS in order to produce films with a thickness commonly between 10 and 100 μm. The process can be adapted to the manufacture of simple single-layer films or more complex multi-layer films. The technology involves the extrusion of a polymeric melt through a die, through which air is also blown to allow the melt to expand (Blow-Up Ratio (BUR) = diameter of the bubble/diameter of the die) and cool (i.e., solidifying), with the resulting solid to form a tubular (bubble). Therefore, the air has the dual purpose of forming the tubular and causing cooling of the polymer melt. The tubular is then guided through a support cage and subsequently collapsed to obtain a flat film, which is wound on a reel. The thickness of the film will depend on the tubular expansion ratio (BUR), which will cause a circumferential expansion of the material in the face of a shrinkage in thickness.

In addition, the thickness of the film will also depend on how the towing is implemented. In fact, as the winding speed of the reel increases, the material will undergo a dragging along the machine direction (i.e., along the extrusion direction), which will result in a further reduction in thickness and a simultaneous elongation of the film with the same diameter of the bubble. Therefore, the blown extrusion produces an intrinsic bi-orientation effect of the material due to the expansion in the circumferential direction in the first steps of bubble formation and due to the stretching of the material obtained with the towing device. Contrary to what happens in the cast extrusion process, where the bi-orientation is obtainable only thanks to enormous investment costs that involve a substantial modification of the plant to obtain a deformation of the material in the direction transverse to the machine direction, in the process of blown extrusion the bi-orientation effect is essentially spontaneous.

The representation of a prototype scale multi-layer blown extrusion plant is shown in Fig. 17. The system provides nine extruders distributed in a radial pattern around the die, which can manage modular multi-layer films, activating or deactivating the extruders necessary for the purpose. The extruders are very simple single-screw systems, equipped with a gravimetric dosing unit to allow the control of the thickness of the single layer inside the multi-layer film. The overall thickness of the multi-layer film thus obtained is measured with a stabilized tubular, by means of suitable readers.

Fig. 17

(Source: Labtech Co. LTD, provided by AMSE Srl)

Multi-layer blown extruder

Figure 18 shows different type of dies, where the layers of extruded polymers are brought together. After the die, the layers are stretched into the final film structure. The best configuration of the die depends on the number of layers to manage and die lip diameter. For monolayer to seven layers between 150 and 600 mm as die lip diameter, the Vertex side-fed cylindrical die is common choice (Fig. 18, left). This type of die offers high outputs as a a result of the large center opening that can allow to place an insulated internal bubble cooling device to supply air directly inside the tubular. The center-fed design of the Centrex dies (Fig. 18, right) minimizes the flow path prior to the spirals, reducing resin degradation in larger dies with many concentric cylindrical layers. This is particularly useful for biodegradable polyesters that are, as said, very sensitive to thermal and hydrolytic degradation. For lip size of 750 mm or more, the internal bubble cooling capacity is quite high. Centrex is designed for the manufacturing of large films for agricultural applications, which require die lips up to 2500 mm and with three to seven layers.

Fig. 18

(Source: https://davis-standard.com)

Multi-layer blown extruder: (left) Vertex Die; (right) Centrex die

The SCD® Streamlined Co-extrusion Die was the first pancake-style die ever designed (Fig. 19). It was firstly introduced in 1990: the polymer melt is side-fed: The system features a large opening for internal bubble cooling (like the Vertex). Yet, the layers are assembled horizontally rather than nestled concentrically. Every layer has. Therefore, the same flow path length from entry to the co-extrusion chamber. The design is modular and it is possible to add more layers by assembling additional modules. Dies up to 13 layers have been designed and successfully tested.

Fig. 19

(Source: https://davis-standard.com)

Multi-layer blown extruder: SCD die

Melt processing of PBS-based compounds by blown film extrusion

Studies about blown extrusion processes of bioplastic films abounds, as the diffusion of pilot plants at research centers is greater. Furthermore, many works focus attention on the study of material processability. Blown film extrusion requires polymeric materials characterized by low MFI (very close to the value of 1 g/10 min at the processing temperature), with such viscosity levels that are essential for the correct formation and stability of the tubular. Table 3 overviews the most relevant manuscripts on blown extrusion process of PBS-based blends.

Table 3 Overview of blown extrusion of PBS-based blends

In 2011, Pivsa-Art et al. (2011) published a first work in which PBSA blends with PLA and PBAT were extruded with a blown extrusion plant for the preparation of films for industrial applications. The researchers fixed the ratio PLA:PBSA = 4:1 and varied the percentage of PBAT from 0 to 50 wt.%. The rotation speed of the screw was set at 80 rpm, setting the processing temperature in a range between 180 and 220 °C (at the die). The MFI of the blends (calculated according to the ASTM D1238 standard) was 5–7 g/10 min at 190 °C, with minimal fluctuations depending on the PBAT content in the blend. The processability of the material was good, with the introduction of PBAT which improved the tensile strength of the film (from 20.39 to 40.71 MPa) and the impact resistance (from 2.99 to 26.97 J/m) up to 20 wt.%. This result was associated with a partial compatibilization effect of PBAT on the also completely immiscible blend between PLA and PBSA. Basically, for PBAT contents up to 20 wt.%, the phases of the various polymers, although immiscible, have domains of smaller size when observed at scanning electron microscopy, an incontrovertible sign of an increase in affinity between the different phases. Jacquel et al. (2014) instead, studied the blown extrusion of PBS reinforced with nano-silica. The results showed the role of silica in bioplastic processability. The presence of silica increases the viscosity of the polymeric material, facilitating its extrudability and, therefore, the formation of the film. The presence of silica permits, in particular, to obtain a draw ratio in the machine direction of about 16. The presence of silica also determines a significant increase of the strength at break of bioplastics (from 19 to 28 MPa), as well as a marked improvement in elongation at break (from 16 to 32%) for silica contents of about 0.5 wt.%.

Jacquel et al. (2015) also studied the blown extrusion of PBS copolymers (i.e., PBSF, PBST, PBIS) obtained starting from different types and contents of co-monomers, with particular reference to 2,5-furandicarboxylic acid (FDCA), to terephthalic acid (TA) and isosorbide (IS). The blown extrusion of PBSF, PBST and PBIS was implemented in a laboratory scale plant with a 30 mm diameter die and a 0.8 mm air gap. The extruder temperature was set at 160 °C, while varying the die temperature to optimize the processability of the different PBS copolymers. Good quality films were obtained with thicknesses between 30 and 50 μm, paying attention to obtain the solidification of the bubble in the immediate vicinity of the die in order to reduce its adhesiveness. The most complex copolymers to be processed were those belonging to the PBSF group with a high content of 2,5-furandicarboxylic acid (FDCA), which showed a slow crystallization kinetics and, consequently, a difficulty in solidifying at the exit of the die. The films obtained by blown extrusion showed a reduction of the tensile strength both in the machine and transversal direction due to the slower crystallization kinetics. At the same time, the same films show a significant increase in elongation at break (up to 1400% for the PBS copolymer obtained with a content of 15 wt.% FDCA).

Petchwattana and Naknaen (2015) studied the blown extrusion of 100 mm thick films obtained from PBS/thymol blends (2–10 wt.%). The thymol determines an effective plasticization of the PBS which is less rigid and more tenacious. The presence of thymol has also been associated with an effective anti-bacterial effect, in particular against Staphylococcus Aureus and Escherichia Coli.

Vandesteene et al. (2016) studied the role on the blown extrusion process of synthesized PBS films using some branching agents such as trimethylol propane (TMP), malic acid (MA), trimesic acid (TMA), citric acid (CA) and glycerol propoxylate (GP). Also in this study, the film was produced on a lab-scale extruder with a 30 mm diameter die and a 0.8 mm gap. The temperature profile in the extruder was set at 160 °C, calibrating the die temperature in relation to the specificities of the single copolymer. The incorporation of the side chains on the PBS has significantly improved the processability of the PBS in blown extrusion. Furthermore, an improvement in the elastic modulus (from 150 to 270 MPa) and in the strength at break (from 19 and 17 MPa to 39 and 43 MPa in the machine direction and in the transverse direction, respectively) of the resulting films was obtained.

Palai et al. (2020) more recently, resumed the study of PLA/PBSA blends (0–20 wt.%) in the presence of a chain extender (3 wt.% ESA, styrene acrylate with epoxy functionality) for the manufacture of flexible films by means of the blown extrusion process. The blown extrusion process was studied on a pilot system equipped with a single-screw extruder with a screw diameter of 32 mm and length/diameter ratio l/d = 28:1. The diameter of the die is 75 mm, the air gap is 0.8 mm. The BUR was set at 2.54. The extruder temperature was set in a range between 160 and 165 °C. The die temperature was set at about 170–175 °C. The rotation speed of the screw was set at 60 rpm and the flow rate at about 3 kg/h. The process stabilized by generating a bubble of 191 mm in diameter, with a film thickness of 50 μm.

The results of the blown extrusion process of PLA/PBSA/ESA blends show the effectiveness of the chain extender with epoxy functionality in compatibilization of polyesters. In the absence of ESA, the material fails to form a stable bubble (Fig. 20). The presence of the chain extender favors the formation of branched structures that improve material processability, increasing the stability of the polymer melt. Consequently, the mechanical, optical, gas impermeability and anti-slip properties of the obtained films are considerably improved.

Fig. 20

Blown film instability without compatibilizer (ESA al 3 wt.%) (Palai et al. 2020)

Suwanamornlert et al. (2020) have recently resumed the study of blends PLA:PBSA = 70:30/thymol (3–6 wt.%) for the preparation of flexible films obtained by blown extrusion with anti-bacterial and anti- fungal properties. The material was subsequently extruded in a system equipped with a single-screw extruder (l/d = 25:1) with a 34 mm diameter die. The temperature profile used varies from 100 to 165 °C, with a screw rotation speed of 50 rpm. 65 mm thick films were obtained. The presence of thymol is effective in reducing the T_g of PLA, making the blend more tenacious and easily processable in blown extrusion. Furthermore, the presence of thymol was found to be effective in conferring anti-bacterial and anti-fungal properties to the film.

Palai et al. (2021) comparatively evaluated the biodegradation rate on soil of PLA/PBSA (5 wt.%)/ESA (3 wt.%) and PLA/TPS (20 wt.%)/GMA (glycidyl methacrylate, 1 wt.%)/BPO (0.1 wt.% Benzoyl peroxide) films obtained by blown extrusion. The temperature profile in the extruder was set, in both cases, in a variable window between 155 and 175 °C. The BUR was set at 2.5 in the first case and at 2.1 in the second case. Films with TPS have lower mechanical properties than films with PBSA (modulus 1021 vs. 3037 MPa, tensile strength 23.5 vs. 48.7 MPa, elongation at break 6.4–7.5%, with properties evaluated in the machine direction). However, the rate of biodegradation on soil of a blend PLA/TPS is decidedly higher than the same rate for PLA/PBSA blends and as is PLA. In particular, the half-life (Fig. 21) in a soil biodegradation test of the PLA/TPS blend is about 103 days, whereas PLA/PBSA and PLA blends as such have half-lives of 559 and 835 days, respectively.

Fig. 21

Biodegradation of the thin films achieved by single-layer blown extrusion (Palai et al. 2021)

Xu et al. (2019a, b) studied the morphological, waterproofing and mechanical properties of films obtained by blown extrusion in PBS reinforced with nano-crystalline cellulose (NCC) and short nano-fibers of chitin (CHW). The nanofillers restricted the mobility of polymer chains and promoted nucleation and recrystallization of polymer as reflected by increase in crystallinity (Xc) from 65.9 to 75.6%. Addition of NCC and CHW increased the tensile strength of PBS-based films from 23.2 to 32.9 MPa and 43.6 MPa, respectively. Decrease in oxygen transmission rate of PBS films from 737.7 to 280 cc/m²/day was observed by adding 3% NCC, which further reduced to 23.8 cc/m²/day by adding compatibilizer methylene diphenyl diisocyanate (MDI, 4%).

Matos Costa et al. (2020) more recently, have studied the preparation of films by blown extrusion in PBS and PBAT, again with the use of bio-derived reinforcements. In both cases, the main result is the reduction of the environmental footprint of the film, which can have faster degradation times. The results obtained point out that the blend containing 25 wt.% PBS is a good compromise between elastic modulus (135 MPa) and deformation at break (390%). Gas permeability decreased with increasing PBS content, indicating that the barrier properties of PBS can be tuned by blending with PBAT.

Injection molding

The injection molding process (Rosato and Rosato 2012) of bioplastic materials and, in particular, of biodegradable polyesters (Adams et al. 2018; Kramschuster and Turng 2010; Mistretta et al. 2018; Oliaei et al. 2016; Rothen-Weinhold et al. 1999; Tanetrungroj and Prachayawarakorn 2015; Wu et al. 2008) such as PBS or based on PBS has undergone enormous development in recent years, especially in the food packaging sector.

For example, the manufacture of capsules for coffee, tea and infusions (i.e., coffee capsules, for example) in compostable material often uses the injection molding process of PBS-based blends. The process allows the manufacture of thin-walled products, with extremely competitive times and costs in relation to the evolution that the molding technology has undergone in recent years. The production of capsules for infusions has increased from 10 to 60 billion units in the last 10 years, resulting in ever-increasing demands in the management of the manufacturing process and capsule injection molding is no exception.

Traditionally, the injection molding process involved the use of a manufacturing system equipped with a single-screw extruder capable of injecting the polymer melt into a mold through a system consisting of a sprue and primary channels and, optionally, secondary runners of the polymeric melt. The molten material injected into the mold was allowed to cool, managing the shrinkage due to the thermal contraction of the material by means of a holding pressure exerted by the screw on the melt. At the end of the molding cycle, the solidified part was extracted from the mold with the auxiliaries for the injection of the polymer melt and, finally, separated from the auxiliaries which, at most, could be remelted to be recovered in the manufacturing cycle or destined to other uses, resulting in an intrinsic waste in the manufacturing cycle (Fig. 22).

Fig. 22

(Source: Veltkamp B.V.)

Injection molding with cold runner: process steps

Figure 23 shows the difference between a system for injection molding a capsule into a single-cavity mold with a traditional cold chamber system (Fig. 24) with two or three plates and a hot chamber system. In the first two cases, the extracted part includes the component and the auxiliaries that allowed the melt to flow into the mold cavity. The difference between the two systems is linked to the ability of the three-plate mold to be able to automatically shear the part from the auxiliaries that allow the injection of the melt, without reducing, in any case, the waste that is generated.

Fig. 23

(Source: www.plasticmoulds.net)

Difference between cold and hot runner plant

Fig. 24

Cold runner injection molding compared with other systems

The hot chamber solution, on the other hand, allows the complete elimination of the auxiliaries that allow the injection of the molten polymer into the mold cavity. There are, in fact, some specific intermediate solutions in which the sprue and the slug well are heated, instead the primary and secondary runner present in multi-cavity molds, are cold (Fig. 25). Figure 26, on the other hand, shows the waste that is produced with the different solutions that allow the management of the injection molding process. It should be noted how the system that envisages the use of a cold chamber for both the sprue and the sliding channels determines by far the greatest percentage of waste (Fig. 26a). The percentage of waste can be significantly reduced by adopting the mixed solution which involves heating the sprue (Fig. 26b) and, if necessary, rationalizing the sliding runners (Fig. 26c). The solution with integral hot chamber (Fig. 26d), both for the sprue and for the sliding runners, allows the total elimination of the waste connected to the manufacturing cycle. The construction of the hot runner has been subject to profound technological evolution in recent years. The injection of the melt into the cavity of the mold is managed by the use of nozzles with a hot point at their ends. The injection of the material inside the cavity can be managed using a thermal nozzle or a shutter nozzle, with this solution which is decidedly more complex in terms of construction, but also much more technologically effective. The two technologies that can be implemented to manage the injection of the polymer melt into the mold cavity are shown in Fig. 27. In the upper part of Fig. 27, the thermal nozzle solution (also called hot spot) is represented. In the lower part of Fig. 27, on the other hand, the solution with shutter or shutter nozzle is represented.

Fig. 25

(Source: www.moldchina.com)

Hot sprue and cold runner system

Fig. 26

Scraps produced with the different setting of the injection molding process

Fig. 27

(Source: Husky Injection Molding System, Ltd.)

Different type of nozzles

In the case of a thermal nozzle, the injection cycle is managed on the basis of the thermal behavior of the polymer melt. In particular, after filling the cavity, the molten mass present at the gate cools and solidifies. Cooling of the gate plays a key role in how quickly solidification occurs. During the next cycle, the pressure of injection forces the gate material solidified within the cavity, opening the gate the same. The small "cap" which was originally formed in the gate melts, for the heating effect due to shear stresses to which it is subjected when it is projected into the new injection cycle inside the mold cavity, leaving only a witness on the surface of the molded component at the gate. Obviously, optimizing the cooling of the gate is critical in the design of the thermal nozzle. The polymer solidified in the gate acts as an insulating barrier between the plastic present in the cavity of the mold and the molten viscous mass that resides in the nozzle of the hot runner. The opening of the mold cannot take place until the injection point is solid enough to detach clearly from the part and "hold" the polymer melt inside the hot runner, avoiding the formation of drops during the process.

In essence, it is crucial to design the gate to ensure that the cooling of the material at the gate is not slower than the material cooling time in the mold cavity. Otherwise, not being able to extract the part, the time cycle should be forcibly lengthen. At the same time, the cooling of the gate also cannot be too fast, because otherwise the polymer melt at the next injection cycle would encounter excessive resistance of the polymer material at the hot spot, which could lead to clogging of the machinery. The construction techniques of the thermal nozzle (hot tip and thermal sprue gate) are shown in Figs. 28 and 29.

Fig. 28

(Source: Husky Injection Molding System, Ltd.)

Manufacturing of thermal nozzle with gate and hot tip: (left) diverted; (middle) univerted; (right) thruflow

Fig. 29

(Source: Husky Injection Molding System, Ltd.)

Manufacturing of thermal nozzle with different types of thermal sprue gate: (left) standard; (right) extra stock

Sometimes, in the case of using a thermal nozzle, it is necessary to decompress the melt in the hot runner, in order to prevent any overpressures that are generated inside it from causing molten material to escape from the gate, even after the material is solidified at the hot spot and which, theoretically, could allow the component extraction phase to start. In the case of a shutter nozzle, the gate opening and closing cycle is controlled by a shutter. It is a mechanically controlled stem that closes once the filling and maintenance phase of the injection molding process is complete. It is important that the stem closing phase is completed before the polymer melt has completely solidified at the gate, in order to prevent the stem from stalling. The stem remains closed during part extraction, preventing any accidental spillage of molten polymer from the gate. In this case, it is therefore not necessary to decompress the polymer melt in the hot runner, since the stem in the closed position ensures in all cases an optimal seal of the melt at the gate. The shutter nozzles leave the presence of a small witness on the surface of the molded part at the injection point. Nozzles with shutter generally have a larger diameter than the corresponding thermal nozzles, causing less shear deformation of the polymer melt and, therefore, less stress on the molded component. Clearly, the use of shutter nozzles guarantees a shorter cycle time, as it is not necessary to wait for the perfect solidification of the polymer melt at the gate before activating the component extraction phase. Among other things, in the case of a mold equipped with cavities with different geometry and, therefore, with different cooling times, it is possible to individually control the actuation of the valve gate nozzles. Also, the shutter nozzles can be particularly useful for thin wall molding.

Fast filling speeds, high pressures and rapid cooling characterize thin wall molding applications. Fast fill speeds—in the range of 0.5 s or less as is the case of the injection of capsules for infusions—are necessary to fill the cavity before the layer solidified is steady in the thin wall of the mold and prevents further filling of the cavity, generating incomplete parts. Shutters are ideal for meeting these requirements. The gate valve gate are provided, as aforesaid, of large diameters that do not cause flow restrictions and enable a rapid filling of the cavity, minimizing the pressure drop and the heating effect of the presence of deformation in shear.

In many thin-wall molding applications, rapid part cooling allows the valve stem to close immediately after filling the cavity. A thermal nozzle, on the other hand, would require a longer cooling time for correct solidification, significantly increasing the cycle time necessary for the manufacturing process. Finally, the construction of the shutter nozzle cannot disregard the thermal characteristics of the injected polymeric material. Amorphous polymeric materials that are processed at modest temperatures, require thermal insulation at the gate (the part in orange at the gate is the part that ensures thermal insulation, Fig. 30-left), in order to prevent the polymeric material from solidifying too fast and prevents the stem from operating correctly. Conversely, polymeric materials that have a greater tendency to crystallization or, in any case, higher working temperatures need to be kept warm for longer, so they are built allowing the conduction of heat at the gate itself and generating a double concentric witness (the gray part surrounding the gate allows the heat conduction and is to be considered part of the chamber that remains hot, Fig. 30-right).

Fig. 30

(Source: Husky Injection Molding System, Ltd.)

Shutter nozzle for amorphous resin (or otherwise for materials that are processed at low temperature) or for semi-crystalline resin (or otherwise for materials that are processed at higher temperature)

Melt processing of PBS-based compounds by injection molding

Injection molding is a broadly explored technique in the field of scientific research. In the last ten years, a significant boost has been given to the acquisition of knowledge for this technology in the bioplastics sector and, in particular, in biodegradable polyesters such as PBS. Table 4 overviews the most interesting works reported in the scientific literature.

Table 4 Overview of injection molding of PBS-based blends

Zhang et al. (2012a, b, c) explored injection molding of basalt fiber-reinforced PBS, the surface of which was modified by coating with a silane agent. The process took place at a temperature range between 145 and 155 °C, with an injection pressure of 400 bar and a holding pressure of 350 bar. The mold temperature was set between 35 and 40 °C. Bb adding the 10 wt.% of basalt fiber it is possible to increase both the elastic modulus and the tensile strength of the PBS, the values of which increase, respectively, from about 250 to about 1100 MPa and from about 31 to about 45 MPa. The impact resistance increases from 3 to about 8 kJ/m² for a basalt fiber content of 15 wt.%. Surprisingly, basalt fiber contents amounted to only 5 wt.% cause an increase in HDT from about 83 °C of as-is PBS to about 112 °C. Similarly, the VSP goes from about 95 °C to nearly 110 °C. The adhesion that occurs at the interface between the basalt fibers and the PBS matrix appears to be excellent on a morphological analysis of the reinforced polymer.

Muhuo (2013) studied injection molding of PLA/PBS blends by exploring the entire compositional range. The process temperature was set in a range between 160 and 195 °C, the injection pressure at 1 MPa, the molding time at 9 s and the mold temperature at 40 °C. Figure 31 shows the trend of the primary module, of the secondary module and of the viscosity as the PBS content in the blend varies in an interval between 0 and 100 wt.% for injection molded products. It is possible to notice how these properties assume maximum values for PBS contents between 10 and 20 wt.%. It is also interesting to note that for high PBS contents, the viscosity of the blend, in addition to significantly decreasing, is substantially independent of the frequency of the stress, exhibiting an almost Newtonian behavior. PBS exerts a strong plasticizing action on PLA at high levels, increasing the free volume inside the blend and consequently reducing viscosity. On the contrary, blends with PBS contents below 50 wt.% exhibit a decrease in dynamic viscosity with increasing frequency (i.e., shear thinning phenomenon). This aspect is very significant as these blends exhibit in twin-screw extrusion a high melt strength (high viscosity) at the low shear speeds typical of the extrusion process. Conversely, in injection where the shear speeds can be very high, the blends exhibit a much lower viscosity which determines a lower injection pressure. PBS contents above 50 wt.% produce a drastic drop in the primary modulus, secondary modulus and viscosity of PLA/PBS blends compared to as-is PLA. Figure 32 shows the trend of the tensile strength, the elongation at break and the impact resistance as the PBS content in the blend varies in an interval between 0 and 100 wt.% for injection molded products. The polymeric blends become progressively more ductile and impact resistant as the PBS content increases, where there is a concurrent reduction in the tensile strength value.

Fig. 31

Storage modulus (a), loss modulus (b) and viscosity of the blends PLA/PBS varying the content of PBS in the range 0–100 wt.% (Muhuo 2013)

Fig. 32

Tensile strength (a), elongation at break (b) and impact resistance of blends PLA/PBS varying the content of PBS in the range 0–100 wt.% (Muhuo 2013)

Ma et al. (2014) studied the performance of injection-molded PHBV/PBS and PHB/PBS blends with a PBS content of 20 wt.% and with the addition of DCP (dicumyl peroxide) with a variable content between 0 and 1 wt.%, in order to obtain a partial crosslinking. The injection molding was carried out in an interval between 160 and 170 °C, with a holding time of 5 s and a cooling time of 15 s. The injection pressure was set at 30 MPa. Experimentation has shown that it is possible to improve the crystallization process by adding DCP, with such a result that it is extremely interesting for reducing the extraction times of the articles in the injection molding process. Li et al. (2015a, b) instead, studied the injection molding of basalt fiber-reinforced PBS composites. Molding was conducted at temperatures between 140 and 155 °C, with the hot spot temperature set at 145 °C. The introduction of basalt fiber led to a strong increase in the elastic modulus (from 185 to 1094 MPa) and in the tensile strength (from 26 to 55 MPa) of the blend against a substantial reduction in elongation at break (from 509 at 5%). However, the impact resistance is significantly improved by the introduction of the basalt fiber (from 5.6 to 9.2 kJ/m²).

Muthuraj et al. (2015) investigated the miscanthus fiber-reinforced PBS injection molding process. In particular, using a compatibilizer based on maleic anhydride, the authors noticed a significant improvement in the mechanical performance of the blende obtained by mixing the 50 wt.% miscanthus fiber within a PBS matrix.

Zaverl et al. (2015) studied the use in co-injection of a blend of PBS/PBAT and of PTT/PBT (poly-co-trimethylene-co-therphthalate–poly-co-butylene-co-therphthalate) for the external and internal part of the component, respectively. In particular, the authors demonstrated that PTT/PBT blends can be made more tenacious, by adding an outer layer made of a blend with high toughness, such as that ensured by PBS/PBAT. The process temperatures in co-injection were 160/190 °C for the PBS/PBAT blend and 240/250 °C for the PTT/PBT blend. The mold temperature was made to vary between 35 and 50 °C, with injection pressures of 500/750 psi and injection speeds varying between 10 and 20 cm³/s. The most significant result was the increase in the impact resistance of the component, which, by adding the outer layer of PBS/PBAT, went from 56 to 450 J/m.

Kim et al. (2016) studied the injection molding of PBS reinforced with silk fibroin (10–40 wt.%). The blends were injection-molded using an injection molding machine. The barrel temperatures for injection molding were set in the range of 140–170 °C. The holding pressure and the injection pressure were 30 and 100 kg_f/cm², respectively. The injection-molded silk fibroin fiber/PBS bio-composites with various silk fiber contents were used directly for tensile, flexural, impact and dynamic mechanical tests. The mechanical performance of the material (elastic modulus from about 650 to 2500 MPa, tensile strength from about 40 to 45–60 MPa depending on the silk fibroin content) are found progressively improved as the strength of the reinforcement increases. On the other hand, the impact resistance of the blend is drastically decreased (by 50%, for contents of 20–25 wt.% of silk fibroin).

Cao et al. (2017) studied the thermo-mechanical properties of injection molded samples produced from PLA/PBS/MCC blends (microcrystalline cellulose):60/30/10 with the addition of a chain extender up to a percentage of 1 wt.%. The molding was carried out in a temperature range between 160 and 180 °C. The presence of the chain extender has significantly improved the impact and tensile strength of the composite material. However, the crosslinking effect promoted by the presence of the chain extender at percentages higher than 0.5% caused a reduction in the elongation at break of the material.

Sang et al. (2017) studied the injection molding of PBS matrix bio-composites reinforced with basalt fibers (from 2 to 15 wt.%) pre-treated with a silane agent. The materials, after dehumidification, were processed in a temperature range between 140 and 155 °C and with a nozzle temperature of 145 °C. The experimental results showed an extremely significant reduction in the time required for PBS crystallization. Silane-treated basalt fiber acted as an efficient nucleating agent and provided abundant nucleating sites, accelerating the completion the crystallization process of PBS. Tensile results showed that the incorporation of silane-treated BF lead to an increase in tensile strength and modulus. Furthermore, as the content of the basalt fiber increases, the material shows an increase in the elastic modulus (from 163 to 442.3 MPa) and in the tensile strength (from 29.8 to 45.8 MPa).

Liminana et al. (2019) studied composites based on PBS and almond shell flour (30 wt.%), compatibilized by adding different functionalized vegetable oils (4.5 wt.%). The results showed that, in the absence of compatibilizer, the composite shows a drastic worsening of the mechanical characteristics compared to PBS as it is, with a significant embrittlement and reduction of impact resistance. The addition of functionalized vegetable oils, favoring the interaction between the wood-cellulosic reinforcement and the PBS (Fig. 33), on the other hand, produces a partial restoration of the ductility of the polymeric material, accompanied by a high increase in the modulus (from 414 to almost 800 MPa).

Fig. 33

Compatibilization of the cellulosic filler and the polyester resin by epoxy and maleate groups (Liminana et al. 2019)

Barletta et al. (2019a, b) studied the injection molding of PLA/PBS/Talc blends compatible with additives that have maleate and glycidyl methacrylate functionalities on a PLA skeleton (added at about 2 wt.%). The injection molding was performed at a temperature range between 198 and 207 °C. The different constituents of the bio-composites were compatibilized by reactive compounding extrusion using maleic anhydride (MAH) grafted PLA (PLA-MA). The experimental results showed the good processability of the polyester blends in the presence of the compatibilizing additives. In particular, bio-composites characterized by good strength and toughness can be obtained by injection molding, without affecting thermal stability.

Zhou et al. (2019) studied the processability of PBS/talc blends for injection molding and 3D printing. The samples obtained by injection molding (process temperature range between 130 and 140 °C) showed an increase in tensile strength (+ 6.3%) and bending (+ 68.4%) due to the addition of talc. Conversely, the samples obtained with the same material with the 3D printing technique (at a temperature of 140 °C) showed a worsening of the properties, probably due to the different thermal history that the material undergoes in the two processes.

Barletta et al. (2020a, b, c) more recently, demonstrated the possibility to fabricate tamper evident caps using highly flexible bioplastic blends based on PBS. In this work, the effect of the addition of a barrier agent, polyvinyl alcohol, on the overall behavior of the blends was also studied in the perspective of the implementation of caps featuring barrier properties against water vapor and oxygen. The flexibility of the material was found to be adequate to ensure the right ejection of the screw caps by the “torn-off” ejection mechanism from the mold cavities after their manufacturing by an injection molding technique.

Picard et al. (2020) investigated the injection molding process of PBS reinforced with apple pomace (up to 50 wt.%), using maleic anhydride as compatibilizer. The material showed good processability in injection molding. The experimental results have shown how the addition of apple pomace in the presence of a compatibilizer with maleate functionality can allow the fabrication of a bio-composite with interesting mechanical properties, similar or, in many cases, superior to those of as-is PBS.

Aliotta et al. (2021) have, finally, studied the process of injection molding of blend PLA/PBSA (15–40 wt.%) in the presence of compatibilizers (an epoxy oligomer with a content of 2 wt.%). The molding was carried out in a temperature range between 180 and 190 °C. The mold temperature was made to vary between 55 and 70 °C. The holding time has been set to 5 s. The cooling time was varied between 15 and 25 s. The injection pressure was set in a compressed range between 80 and 120 bar. The experimental results showed that blends containing a PBS content equal to 40 wt.% and which have a morphological structure of the co-continuous type have exhibited the best mechanical performance, showing a significant increase in impact resistance. On the other hand, the thermal resistance of the investigated blends does not appear to be influenced by the composition.

Thermoforming

The thermoforming process is widely used for the converting of bioplastic materials based on bio-derived polyesters, which are also biodegradable and compostable (Klein 2022). PBS, by virtue of its high processability in extrusion and also in molding and its good characteristics of ductility and low melting temperature, is among the most widely used polymers to produce compounds suitable for the thermoforming process. PBS is normally considered a material for technological and functional characteristics very similar to PE (polyethylene), which makes it easy to form at moderate temperatures. The thermoforming process can be carried out in multiple ways:

• (i) Using coils previously produced on a flat head extruder and only subsequently reprocessed by the thermoforming molding machine, where the coil is unwound, pre-heated in an oven radiative at temperatures higher than the glass transition temperature of the compound, but lower than the melting temperature and, finally, molded through the simultaneous use of compressed air, vacuum pressure and, possibly, a mechanical punch that helps the forming of the material polymeric;

• (ii) Using in-line machines, in which the film to be thermoformed is produced by means of a cast extrusion arranged in line with the subsequent thermoforming station, inside which this film is directly sent hot at a temperature below the melting temperature of the material, but definitely higher than the glass transition temperature;

• (iii) With more recent technologies, using in-line machines in which the film leaving the cast extruder is cooled (stabilized) on chill roll (cooled rolls) at a temperature close to room temperature, to be subsequently sent to the thermoforming stage, where the material is reheated to a temperature below the melting temperature, but definitely above the glass transition temperature to be subjected to the forming operation.

Technologies (ii) and (iii) also use compressed air pressure, negative vacuum pressure and the thrust of a punch to form the plastic film, with technology (iii) being particularly advantageous for bioplastic materials, which are able to stabilize only in the presence of the cooling cycle on chill roll, with the second heating which favors crystallization and the consequent increase in the thermal resistance of the thermoformed product. Figure 34 shows a representation of a station for thermoforming of plastic films for the production of disposable tableware (plates, glasses, containers,…).

Fig. 34

Thermoforming plant for the manufacturing of disposable items

Processing of PBS-based compounds by thermoforming

The thermoforming process is rarely explored in the scientific literature, probably due to the lack of demonstration plants in research centers and universities (Table 5). Recently, the industrial interest in the manufacture of disposable tableware and thermoformed containers suitable for contact with food has, however, led to the development of some studies.

Table 5 Overview of thermoforming of PBS-based blends

Barletta et al. (2020a) studied the thermoforming process of containers starting from films in bioplastic material based on PLA/PBS with the addition of a package of nucleating additives (based on talc, PDLA, EBS, TiO₂), to increase the thermal resistance of the finished product. The experimental results have shown how it is possible to customize the thermal resistance of the thermoformed product by modulating the amount and type of nucleating additive. In particular, PDLA and TiO₂ are recommended for thermoforming applications where longer crystallization time is allowed for the polymeric blends, while EBS is recommended in cast extrusion, where lower crystallization time is allowed to the polymeric material.

Barletta and Pizzi (2021) more recently, investigated the thermal resistance of thermoformed artifacts using three synergistic nucleating agents within PLA/PBS blends, namely talc, EBS and PDLA. The effect of a multi-nucleation strategy by the addition of organic nucleants to talc, conventionally used as nucleant in bioplastics, was found to favor and extent the crystallization processes of the PLLA matrix. This effect is stronger when the materials are subjected to processing steps involving a heating to temperatures above the glass transition or, even more, above the cold crystallization temperature of the polymeric matrix, as in case of pre-drying of the pellets and thermoforming of the films. The greater increase in crystallinity, following the thermal treatment that occurs during the extrusion of the pellets and the thermoforming of the films, has been particularly observed for the systems with two nucleating agents, namely talc/EBS and talc/PDLA, and, to a lesser extent, for the system with three nucleating agents talc/EBS/PDLA.

The thermoforming of containers suitable for food contact produced starting from reinforced PLA/PBS blends with calcium carbonate was instead the subject of further work (Barletta and Puopolo 2020). In the latter case, the authors showed how it is possible to use the cheaper and highly hydrophilic calcium carbonate instead of talc as a reinforcing element of PLA/PBS blends in the thermoforming molding process. The high concentration of calcium carbonate in addition to talc is found to be responsible for achieving a polymeric structure in the bioplastics characterized by a greater crystalline fraction, which has the advantage of slowing down the rate of absorption of the water in the materials. Trays characterized by a high concentration of calcium carbonate have the most uniform stretching ratios in the thermoforming process, probably due to the higher thermal diffusivity promoted by the reinforcing agent, which determines a lower quantity of thermal and mechanical energy being required during the manufacturing process of the product.

Ayu and Khalina (2021) studied the thermoforming process of PBS/TPS blends for the manufacturing of food trays, demonstrating how the use of these materials can reduce the environmental footprint of packaging for the packaging of food products. In particular, the main objective of this paper is to provide state-of-the-art knowledge about the categories of biodegradable food packaging designs, to incorporate starch and to demonstrate properties of food packaging that include the transmission of oxygen and transmission of water vapor on surfaces of food packaging and thus prolong the shelf-life or enhance food safety.

Other processing techniques of PBS blends: extrusion coating

PBS and its blends find another important application in the coating by extrusion (Gregory 2005) of paper supports. Paper is a rapidly biodegradable and compostable material. However, in the face of this indisputable advantage, it has a strong hydrophilicity which compromises its performance every time it comes into contact with humid substances. In order to extend the field of application of the paper, it is not infrequently coupled or revised with plastic materials, in order to increase the impermeability. The most common technologies are the coating by extrusion and the coupling by means of cylinders with point-to-point or continuous technology, in the absence or in the presence of glue. In this way, the paper is coated with a very thin layer of plastic, generally varying between 10 and 30 microns, obtaining the necessary impermeability to liquids. At the same time, the plastic coating also serves to prevent the migration of potentially harmful substances from the paper to the product packed in it, a very frequent possibility when waste paper is used in the paper production process. The fundamental problem of paper-with-plastic coating processes is the reduction of useful options for the disposal of end-of-life products. Plastic is not biodegradable/compostable, so composting the coupled is not a viable option. Replacing the plastic from fossil sources with a biodegradable and compostable plastic (possibly of bio-derivation as possible in the case of PBS) can allow to combine the advantages of the paper/plastic combination with the low environmental footprint of an artifact that remains compostable than biodegradable. The production process of thin films in PBS and in PBS blends has already been extensively discussed in the previous section, relating to blown extrusion. In this context, it is worth adding that the coupling between paper and thin film based on PBS can be achieved by pressure on a hot roll, taking advantage of the relatively low melting temperature of the PBS. Alternatively, the coupling can be obtained using cold or hot glues. Sometimes, it may be necessary to pre-activate the PBS film to increase its affinity with the paper support. PBS and PBS blends are also widely used in the extrusion coating process. The extrusion coating process is technologically very similar to the flat head extrusion process (Fig. 35). In fact, the same provides a flat die connected to a single-screw extruder to bring the polymeric material to the molten state. The molten polymer therefore comes out of the flat die which is arranged vertically. The process temperature is regulated in such a way as to correctly set the elongational viscosity of the melt, which must flow from top to bottom (from the die to the support it has to cover), remaining as free as possible from necking phenomena (i.e., of necking). In this way, the polymeric melt that comes out of the vertical head can coat a support (for example, paper) that travels at high speed orthogonally to the melt. The support is, in this way, hit by the molten polymers, which covers it, being dragged by the advancement of the support itself. If the melt has good resistance and flexibility, it covers the support with a homogeneous layer, without tearing or other defects occurring. Adhesion is also optimal, as the melt can infiltrate the porosity of the paper support, creating bonds that are both chemical-physical and by mechanical interlocking.

Fig. 35

(Source: Extrusion Coating 2019)

Extrusion coating plant

Coating extrusion technologies are rarely explored in the scientific literature, due to the high costs of purchasing and managing the necessary machinery and the virtual absence of small-scale prototype plants. However, recently, Thurber et al. have documented the process of extrusion of a paper support, using PBS as a polymer (Thurber and Curtzwiler 2020). The researchers have shown the benefits of using PBS to coat paper in takeaway food applications. PBS guarantees similar protection to polymers from fossil sources with regard to the migration of fats. Furthermore, by virtue of its excellent compostability at low thicknesses, it does not significantly alter the end-of-life management scenarios of paper packaging. Finally, the use of biodegradable polymers avoids the development of solutions in which the paper is protected with silicones or florinated species in order to increase its hydrophobia and oleophobia, thus compromising its ecological profile. In fact, although these layers are normally very thin, both technologies produce these agents and unaware disposal of such coated papers poses a serious risk to the environment.

Other processing techniques of PBS blends: compression molding

Compression molding (Tatara 2017), on the other hand, is rarely used on an industrial scale for the production of compostable bioplastic products (Fig. 36). Two types of applications of significant practical interest are mentioned, with particular reference to continuous compression molding technologies: (i) the production of plastic caps for bottles; (ii) the production of coffee capsules. In both cases, these processes are based on the patented technology of the Italian company Sacmi Group (Source: https://www.sacmi.com/en-US/plastics/Compression-Moulding). The traditional compression molding technologies are, instead, characterized by very high cycle times, which do not find application windows in conjunction with bioplastics. On the other hand, the continuous compression molding machine is structured with a single-screw extruder capable of preparing a calibrated dose of plastic material, which is sent into a mold, in which the figures rotate around a vertical axis with a configuration very similar to that of a carousel. The dose inserted in the cavity is squeezed onto the die by the effect of the pressure of a punch, until the polymer melt has assumed the negative image of the mold cavity, is sufficiently cold and can be easily extracted from the cavity itself. The process can be easily automated, as it can be scaled up to solutions that include 64 cavities. However, it is suitable for rather simple geometries such as, for example, plastic caps or single-portion capsules for infusions. The operation mode of the machinery is continuous and unattended, allowing to obtain very high production speeds, as well as a reduction in operating temperatures compared to injection. Compared to it, in fact, in the continuous compression molding technology, the hot chamber with its extremely narrow distribution channels and the thin-walled molds are replaced by the dose that is directly loaded inside the cavity and compressed therein, allowing to operate even at higher viscosity values. In scientific literature, there are no studies in which the continuous compression molding process is used for the transformation of PBS or its blends. However, the compression molding process is sometimes used for making artifacts and samples for studies related to the definition of new bioplastic materials.

Fig. 36

(Source: https://www.iqsdirectory.com/articles/rubber-molding/compression-molding.html)

Compression molding of polymeric materials

Pivsa‐Art et al. (2015) have, for example, studied the compression molding of PLA-PBSA blends, varying the PBSA content between 0 and 100 wt.%. The researchers have shown the morphological structure of the blends as the PBSA content varies, confirming the toughening effect of the latter on PLA, especially when homogeneously dispersed in the former in the form of small drops. The compression molding process was carried out using a simple flat and parallel plate station, after drying the polymer at 80 °C for 8 h. The mold temperature was brought to 200 °C for 7 min. As-is PBSA, having a much lower melting temperature than PLA, was instead processed by compression molding with a mold temperature of 130 °C. The process involved a pressure maintenance phase in both cases, which was finally followed by a long cooling in the mold (10 min). It is interesting to note the slight decrease in the glass transition temperature of PLA blended with PBSA as the PBSA content increases (from 52.3 to 47.1 °C). This suggests a certain mixing capacity between the two polyesters, perhaps promoted by the long melting times of the process.

Barletta et al. (2017) studied the compression molding process of polylactide-based blends, being able to observe the enormous increase in the crystallinity of PLA for this process compared to injection molding, characterized by much faster execution times. Zhao et al. (2020) have, instead, studied the manufacturing process by compression molding of ultrafine PBS fibers with waste paper (60 wt.%), obtaining good mechanical performance and high biodegradability. Compression molding was led at 150 °C for 20 min, using a pressure of 10 MPa and obtaining products with a thickness varying between 1.8 and 2.1 mm.

Other processing techniques of PBS blends: injection and extrusion blow molding

More recently, the use of blends in which there is the presence of PBS is also being evaluated in the production processes of bottles and, in general, of containers for liquids. The production of containers for liquids and bottles can be obtained with multiple technologies, even using bioplastic materials. Bioplastic materials can be processed, if properly formulated, both with injection-stretch-blow molding technologies and, even more simply, using extrusion and blowing technologies. The fundamental difference between the two technologies is that, in the first case, it passes through the injection mold of a preform that, only subsequently, is blown in mono- or bi-stage machines. In two-stage machines, the preform production process and the blow molding phase is deferred and can take place in different plants. In the extrusion and blow molding process (Belcher 2011), more favorably, the bioplastic material is extruded generating a parison which, at the same time, is confined in a mold for the hot blowing phase which takes place by introducing compressed air from the top of the parison (Figs. 37, 38). The plastic material is formed at a temperature that is lower than the melting temperature, but definitely higher than the glass transition temperature. The regulation of the rheological profile of the material is fundamental to have the appropriate elongational viscosity in the parison production phase and the right compromise between tightness and ductility of the parison during the blow molding phase for the manufacture of the container. In the formation phase the parison must have uniformity of thickness both longitudinally and transversely, the something being ensured only in the case of the absence of deformation phenomena of the parison (i.e., stringing, hooking, sagging, length inconsistency), which most of the time it is promoted by the weight of the parison itself in the absence of a correct rheological profile (i.e., the correct elongational viscosity) of the bioplastic material. The cost of PBS is still an insurmountable obstacle to its industrial employment at the moment.

Fig. 37

(Source: https://learnmech.com/blow-moulding-types-advantages-disadvantages)

Injection stretch blow molding process

Fig. 38

(Source: https://www.custompartnet.com/wu/blow-molding)

Extrusion blow molding process

In 2007, Kale et al. (2007) have documented the experimentation of a bottle produced with a bio-derived polyester, evaluating its biodegradability. In particular, the paper investigates the biodegradation performance of polylactide (PLA) bottles under simulated composting conditions according to ASTM and ISO standards, and these results are compared with a novel method of evaluating package biodegradation in real composting conditions. Two simulated composting methods were used in this study to assess biodegradability of PLA bottles: (a) a cumulative measurement respirometric (CMR) system and (b) a gravimetric measurement respirometric (GMR) system.

Pati et al. (2010) evaluated the use of PLA for the conservation of carbonic maceration wine, however finding the inadequacy of PLA even with respect to a corresponding PET bottle. The PLA bottle causes a faster degradation of the sensory and organoleptic characteristics of the wine, due to less protection from oxidative phenomena.

Gironi and Piemonte (2011) have, however, demonstrated the lower environmental footprint of bioplastic bottles. More specifically, the true advantage of the PLA bottles with respect to the PET bottles arises from the use of renewable resources, but this benefit is paid in environmental terms due to the higher impact on human health and ecosystem quality (due to the use of pesticides, consumption of land, and consumption of water for the production of raw materials.

Rodríguez-Castellanos et al. (2015) on the other hand, tried to develop a low-cost bioplastic for the extrusion and blow molding of containers with a low environmental footprint, however encountering considerable process difficulties. The work investigated the possibility of using hydrolyzed corn starch–gelatin as a base matrix and cellulose as reinforcement, to produce containers by extrusion blow molding. The addition of cellulose decreased the viscosity of the starch–gelatin polymer matrix allowing the compounds to be processed at temperatures as low as 100 °C. Then, parisons were obtained by extrusion blow molding and featured suitable processing characteristics.

Barletta et al. (2019a, b) have, however, studied the process of extrusion and blow molding of bottles in PBS/PLA self-protected from light radiation, suitable for the packaging of extend shelf-life milk. The results obtained showed the suitability of bioplastic bottles, which, from a performance point of view, equal the performance of the corresponding PET bottles. However, PLA/PBS bottles have a very high raw material cost and greater weight. However, although the high material and processing costs, the novel polymeric blends are found to be suitable for extrusion blow molding of the bottle body, also exhibiting valuable performances in terms of mechanical strength and impact resistance as well as in terms of protection against gas and light permeation. This class of biodegradable blends was found to be extremely promising in replacing oil-relying plastic in the manufacturing of blown bottles that can also be suitable for oxygen and, especially, light sensitive foodstuffs, strongly decreasing the environmental impact.

Aversa et al. (2021b) have recently demonstrated the applicability of PLA/PBS blends in the production of self-protected bottles for wine packaging by means of the extrusion and blow molding process. Similarly, Aversa et al. (2021a) also demonstrated the co-rotating twin-screw extrusion of PLA/PBS/micro-lamellar talc blends for extrusion blow molding of bio-based bottles for alcoholic beverages. In this paper, the researchers tried to focus on design criteria defined to ensure, primarily, an adequate processability of the bioplastic material in the extrusion and, especially, in the subsequent blowing process in order to optimize the rheological behavior of the bioplastic material. Second, the compound was loaded with different micro-lamellar talc content so as to achieve protection from the environmental factors, which is of paramount importance to ensure a long shelf-life to wine. Although the overall good performance of the material and the suitability of the process technology used for the fabrication of the bottle, the high cost of the raw material represents, at the moment, still the greatest obstacle to further technological developments.

Other processing techniques of PBS blends: melt spinning

Melt spinning is a widely used technique for producing fibers from polymers, and it has garnered significant interest for processing biodegradable polymers like Polybutylene Succinate (PBS). Melt spinning involves extruding the polymer melt through a spinneret, followed by rapid cooling and solidification, resulting in the formation of continuous filaments. The technique is favored for its simplicity, cost-effectiveness, and scalability in industrial applications. In the melt spinning process, PBS or PBS-based blends are first melted and then extruded through a spinneret with multiple fine holes. The extruded filaments are then cooled, either by air quenching or by using a water bath, to solidify the polymer. The filaments are subsequently drawn to orient the polymer chains, which enhances the mechanical properties of the fibers, such as tensile strength and elasticity. The final step involves winding the fibers onto spools for further processing (Fig. 39).

Fig. 39

Melt spinning process (Rudolf et al. 2020)

PBS is well-suited for melt spinning due to its thermoplastic nature and relatively low melting point (around 114–120 °C). The processability of PBS in melt spinning is further enhanced by its good thermal stability and rapid crystallization, which are critical factors for maintaining fiber integrity during spinning (Li et al. 2015a, b). The intrinsic properties of PBS, such as its biodegradability and mechanical performance, make it a promising candidate for applications in textiles, medical sutures, and agricultural films. However, to optimize its properties for specific end-use applications, PBS is often blended with other polymers or reinforced with fillers.

In this respect, (Xu et al. 2019a, b) shows how PBS can be blended with Polylactic Acid (PLA) to improve its mechanical strength and thermal resistance. PLA, with its higher melting point, contributes to the overall performance of the blend, making it more suitable for high-temperature applications.

Blending PBS with other biodegradable polymers such as Polyhydroxyalkanoates (PHAs) or incorporating nanofillers like nanocellulose or graphene oxide can further enhance the thermal stability, mechanical properties, and barrier performance of the resulting fibers. These modifications are particularly advantageous in producing high-performance fibers for technical textiles or packaging materials (Zhang et al. 2012a, b, c).

Despite the advantages, there are challenges associated with melt spinning of PBS and its blends. One primary challenge is controlling crystallization during the cooling phase, as improper crystallization can lead to defects such as fiber breakage or inconsistent mechanical properties (Cho and Yoon 2005). Moreover, the relatively low melt viscosity of PBS can cause difficulties in maintaining stable filament formation during spinning.

Recent advances in melt spinning technology, such as the use of nucleating agents and the optimization of spinning parameters, have helped to mitigate these challenges. For instance, (Zhang et al. 2012a, b, c) have shown that the addition of nucleating agents can accelerate crystallization, leading to more uniform fiber structures. Furthermore, the development of multi-component melt spinning techniques has enabled the production of composite fibers with enhanced functionalities by combining the properties of different polymers in a single filament (Li et al. 2015a, b). PBS-based fibers produced through melt spinning are therefore finding increasing applications in various sectors. In textiles, these fibers are used for producing environmentally friendly fabrics with good durability and comfort. In the medical field, PBS fibers are used in biodegradable sutures and scaffolds for tissue engineering due to their biocompatibility and controlled degradation rates. Additionally, PBS fibers are being explored for use in agricultural applications, such as biodegradable mulching films, which can help reduce plastic waste in the environment (Xu et al. 2019a, b).

Melt spinning of PBS and its related blends represents an important area of research and development in the field of biodegradable polymers. By optimizing the processing conditions and exploring novel blend compositions, the properties of PBS fibers can be tailored to meet the demands of various high-performance applications. Continued advancements in this area are expected to expand the use of PBS fibers in sustainable products, contributing to the reduction of plastic waste and promoting environmental sustainability.

Conclusions

This review has provided a comprehensive examination of the melt processing techniques of Polybutylene Succinate (PBS) and its blends, highlighting the significant progress made in optimizing its properties for various industrial applications. PBS, as a biodegradable and partially bio-based polymer, holds considerable promise as a sustainable alternative to conventional petroleum-based plastics. The review underscores the importance of understanding the influence of processing parameters, such as melt temperature, screw speed, and mold temperature, on the final properties of PBS products. The studies reviewed demonstrate that PBS can be effectively processed using various techniques, including extrusion, thermoforming, and injection molding, with each method offering distinct advantages and challenges. The incorporation of additives and the development of PBS blends have been shown to enhance the material mechanical properties, thermal stability, and processability, making it more suitable for demanding applications, such as packaging, automotive, and medical devices. However, challenges remain, particularly concerning the cost of PBS relative to conventional plastics and the need for more efficient production methods to enhance its commercial viability. Future research should focus on developing catalysts and processes that reduce energy consumption, increase yield, and improve the biodegradability and compostability of PBS under various environmental conditions. Additionally, the exploration of PBS-based composites with natural fibers or other biopolymers could lead to materials with superior properties, broadening the scope of PBS applications in new industrial domains.

In conclusion, while PBS presents a sustainable and biodegradable option for various applications, continued research and development are essential to overcome the current limitations and fully realize its potential as a versatile material in a range of industrial contexts.

Future trends

Future trends in PBS will likely focus on more efficient and sustainable synthesis methods. Researchers are working on developing catalysts and processes that can reduce energy consumption and enhance yield and quality of PBS. There is also a growing interest in producing PBS from entirely bio-based raw materials to further reduce its environmental footprint. While PBS is already biodegradable, future trends may include enhancing its biodegradability under various conditions, such as on soil or in marine environments. This could broaden its applications in areas where biodegradation is challenging. Improving compostability to meet specific industrial or, even more, garden composting standards could also be a focus, ensuring PBS products can be effectively processed in existing composting facilities or at home. There is also potential for developing blends of PBS with other biopolymers or incorporating natural fibers to create high performance composites. These blends could offer improved properties such as increased strength and thermal stability or enhanced biodegradability. This could also open up new applications for PBS in sectors like automotive, construction, and electronics, which have been so far not explored by manufacturers. In the packaging industry, for instance, there is scope for using PBS in more specialized types of packaging, like food packaging that requires specific gas barrier or light properties. In agriculture, PBS could be used for biodegradable mulch films and controlled-release fertilizers or pesticides. In the medical field, research into PBS for drug delivery systems and biodegradable implants is a very promising area. In contrast, one of the challenges facing PBS is its cost compared to conventional plastics. Future trends will likely involve efforts to make PBS more cost-competitive, either through more efficient production methods or by developing high-value applications where its biodegradability adds value. Changes in environmental policies and regulations could also impact PBS market. Stricter regulations on plastic waste and increased support for biodegradable materials could drive more research and investment into PBS. Additionally, establishing clear standards and certifications for biodegradable plastics could help in increasing consumer trust and market adoption. Moreover, as consumer awareness about environmental issues grows, demand for sustainable materials like PBS is expected to increase. This could encourage more companies to adopt PBS in their products, driving further innovation and development in the field. Future trends might also include technological innovations in melt processing of PBS, such as developing methods to improve its processability or create more complex shapes. Research into the recyclability, including chemical recycling methods to break down and reuse PBS polymers, could be another significant trend. The future of PBS polymers depends, therefore, on development in various aspects: first, a low-cost manufacturing process. Mass production and growth in customer demands can have an impact on the final cost and the availability of cheaper biopolymers; second, the application of new additive like natural polymers or fibers, with different ratios, shapes and varieties can help to improve the technological properties and functionality of the products. Lastly, future studies will open up the applications of PBS in novel industrial domains by the integration of nanomaterial to the blend and discover new functionality of the PBS-based products. There are still many potential applications that need to be discovered in different areas of medical products, food packaging, thin films, and so on. Thus, numerous applications of PBS will attract industries and people to use it more in the future.