ReviewPolyhydroxybutyrate blends: A solution for biodegradable packaging?
Introduction
A remarkable effort was observed in the last decade to replace conventional synthetic polymers with biopolymers in the packaging sector [1], [2], [3]. The development of new and more suitable biodegradable polymers-based solutions for short-life-cycle packaging is a part of the new strategy that aims to obtain products by more sustainable processes and with less waste [4]. Biodegradable biopolymers include aliphatic polyesters such as poly (3-hydroxybutyrate) (PHB) or poly(lactic acid) (PLA), plant, animal, bacterial or fungal derived polysaccharides, proteins, nucleic acids, polyphenols and other natural or synthetic polymers [5]. A lot of aspects should be met by a good candidate material for packaging: a good heat stability and processability, flexibility and strength, lightweight properties, good barrier against water vapor, oxygen, carbon dioxide or volatile compounds, transparency or opacity depending on the application, moisture absorption, compostability or biodegradability and, eventually, other specific properties such as antimicrobial activity, inflammability or antistatic properties [6]. For most of applications, the biopolymer should be able to be processed without thermal decomposition by extrusion or co-extrusion (films or sheets), thermoforming, injection (blow) molding, 3D printing, lamination or coating forming multilayer structures [7]. Good barrier and mechanical properties are mandatory requirements for packaging materials. Mechanical stresses during handling and storage can lead to material failure in the case of poor mechanical properties and the level of barrier properties has a direct influence on the quality of the packaging content [8], [9].
A great interest was devoted toward PHB, an aliphatic polyester which belongs to the large family of polyhydroxyalkanoates (PHAs) [2], [10]. PHAs are synthesized by both Gram-negative and Gram-positive bacteria under excess of carbon sources and limited amounts of phosphorous or nitrogen compounds [11], [12]. They are accumulated intracellularly as granules and are fully biodegradable and biocompatible [12]. Different PHAs types may be obtained depending on the carbon sources, microbial cultures and biosynthesis conditions: short-chain-length PHAs with 3–5 carbon atoms, medium-chain-length PHAs (mPHAs) with 6–14 carbon atoms or long-chain-length PHAs with more than 14 carbon atoms [13], [14].
A great number of microorganisms are able to produce PHAs. Most of the industrial PHB manufacturers reported Cupriavidus necator as microbial production strain and Pseudomonas spp. are considered the microorganisms with best mPHAs yields [15], [16]. An important strategy to increase the efficiency of PHAs production is the application of mixed microbial consortia instead of pure cultures. The use of mixed cultures allows the production of PHAs from various low-cost feedstocks as substrates [14], [17]. Yields close to that of pure cell culture, up to 89 wt% of the total suspended solids, were reported for PHAs produced in mixed cultures [18]. The high cost of feedstock is an important barrier for an increased competiveness of PHAs [14], [17]. For reducing the production costs, different agricultural and industrial wastes such as sugarcane molasses, lignocellulosic or oil waste were tested as carbon source in the fermentation process [14], [15]. Another strategy to reduce the costs is the application of genetically modified strains which may increase the accumulation of PHAs inside cells [17]. The recovery and purification of PHAs are also costly processes, being large consumers of solvents and labor [15]. Therefore, the reduction of chemicals and the replacement of chlorinated solvents in the recovering process of PHAs from the reaction medium are necessary from economically and environmentally points of view.
Currently, the world production of PHAs is close to 100,000 tons per year [19]. Among the major manufacturers of PHAs for biomedical and packaging applications are Bio-On (Italy), Tianjin GreenBio (China), Kaneka (Japan), Danimer Scientific (USA) and TianAn Biologic Material Co (China) with 5000–10,000 tons PHAs per year [20], [21]. The price of different PHAs strongly depends on the required quantity and properties, ranging generally from 2.0 to 6.5 US$/kg [20]. Several PHAs have already been certified by the US Food and Drug Administration for use in contact with food [22]. Food and drink producers such as Nestle Waters and PepsiCo developed partnership with Danimer Scientific for the development of biodegradable water bottles and compostable snack bags [23]. A great fruit producer and distributor, Rivoira, and Bio-On created Zeropack company to produce 100% natural and biodegradable films, containers, fruit supports and labels based on PHAs [24].
Depending on the composition of monomers, PHAs may cover a wide range of properties, from elastomeric mPHAs to rigid PHB, which is the most studied and commercially available of all PHAs [2], [10]. This interest is determined by the properties of PHB, some of them at the same level with that of well-known commodity polymers [2]. PHB melts around 175 °C similar to polypropylene (PP), largely used in packaging, and can crystallize in two forms, α and β when cooled [25], [26]. When PHB is crystallized from melt or solution, usually the α-form is obtained. This is characterized by an orthorhombic unit cell having a = 0.576 nm, b = 1.320 nm, c = 0.596 nm and containing two antiparallel chains in a left-handed 21 helical conformation [26], [27]. When PHB is stretched as fibers or films, the β crystal form is also obtained, having planar-zigzag chains packed in a hexagonal unit cell with a = b = 0.922 nm and chain axis c = 0.466 nm [26], [28]. The PHB β crystals are formed by the recrystallization of the amorphous regions between neighboring lamellae of α-crystals due to highly tensioned tie chains [28]. The β-form of PHB is less stable and less ordered than the α-form and the phase transformation from β to α takes place after heating or annealing. The glass transition temperature (Tg) of PHB is usually around 0 °C, similar to PP. Therefore, PHB may undergo physical aging at ambient temperature which is above its glass transition temperature [25]. As a result, its Tg increases, in particular with about 9 °C after aging at 15 °C for 168 days [29].
Another characteristic of bacterial PHB is its perfect isotactic structure, all chiral centers being in R configuration [30]. Due to its remarkable perfection, PHB is characterized by a high crystallinity, usually between 55 and 70%. Analysis of infrared spectra and X-ray diffraction results showed a shorter H···O distance value than the normal van der Waals distance, indicating the presence of strong hydrogen bonding interactions in PHB [27]. These strong interactions have a stabilizing effect and an important influence on the properties of PHB, being partly responsible for its large crystallinity. The high crystallinity and stereoregularity along with the low crystallization rate result in large spherulites which are an important cause of PHB brittleness [31]. The PHB embrittlement is not completely understood and a consensus on the involved mechanisms is lacking. It is assumed that the large spherulites are formed by the cooperative twisting of lamellar crystals during their growth along the crystal axis [32]. The spherulite has a complex hierarchical structure created by the twisting, bending and branching of the crystalline lamellae starting from its center [33]. Although the mechanism of the formation of ring-banded spherulites is not fully understood, their characteristics were well studied. Thus, the size of ring-banded spherulites can vary in a large range, from a couple of microns to even one millimeter, depending on PHB purity, additives and treatments [34]; moreover, they contain cracks at the interface between the valley and ridge bands at inter-lamellae discontinuity, leading to PHB brittleness [31].
Another peculiar characteristic of PHB is the significant change in crystallinity and mechanical properties during storage at ambient temperature. During storage, the crystallinity of freshly molded or casted PHB increases and its behavior changes from a ductile to a brittle one [25], [29]. Thus, the tensile modulus increased with about 40% and the elongation at break drastically decreased after four weeks of storage of PHB at room temperature [29]. The increased brittleness during storage has been described as an effect of the secondary crystallization or the gradual rearrangement of the inter-lamellar amorphous regions, determined by the tendency of the amorphous phase toward an equilibrium state [25], [29].
Due to the above mentioned features, certain mechanical properties of PHB, such as the Young's modulus and elongation at break, are weaker than that of non-biodegradable synthetic polymers used in packaging and its brittleness is very high [35]. In addition, PHB has a narrow processing window due to the closeness of the melting and onset degradation temperatures, which leads to a poor thermal stability during melt processing [34]. However, its barrier properties are similar to that of several common thermoplastic polymers such as low-density polyethylene (LDPE) or PP [35], [36]. The above mentioned shortcomings, to which is added the big price, four to six times higher than that of common synthetic polymers [12], are real obstacles in the application of PHB in packaging. In addition to the solutions proposed for decreasing its costs, such as the use of agricultural and industrial wastes as low-cost carbon source [17], [37], the building of new high PHB capacities, correlated with market requirements [11], was also proposed. For improving its flexibility and thermal stability, PHB was copolymerized [38], plasticized [34], [39] or blended with other polymers [40], [41]. Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [38], [42], poly (3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) [44] and poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHH) [43], [44] are copolymers characterized by a higher flexibility compared with PHB. The thermal and mechanical properties of these copolymers may be adjusted to some extent by the content of the co-monomer. However, they are characterized by a low strength and Young's modulus and, for the moment, they are not obtained through efficient, easy-to-control processes [45]. The use of environmentally friendly plasticizers to increase the flexibility and reduce brittleness of PHB leads to a lower thermal stability and enhanced migration, which are detrimental for packaging application [46].
Blending PHB with other biodegradable polymers and bioelastomers emerges as the most accessible and cost-effective route to correct the properties of PHB and to obtain high performance materials for packaging and biomedical applications. Polymer blending is a largely used method to obtain new materials with improved properties. The advantages of the PHB blends with biodegradable polymers result from their ability to be processed into various products by well-established techniques, their almost 100% biodegradability and the lack of toxic products after degradation. New methods for cost-effective production of PHAs [2] along with nanocomposites from biodegradable polymers filled with layered silicate nanoclays [6] or PHBV nanocomposites with nanocellulose and inorganic nanofillers [35] were revised and their application in food packaging was discussed. Moreover, the properties of PLA-PHB blends for food packaging application were punctually discussed by Arrieta et al. [36] and the chemical functionalization of PHB by hydroxylation, aminolysis and block copolymerization with suitable biopolymers was also reviewed considering only the biomedical applications [13], [47]. Despite these remarkable reviews on bionanocomposites, the suitability of PHB-biopolymer blends for biodegradable packaging was not critically analyzed so far. This review provides a comprehensive overview of PHB blends with other biodegradable polyesters or polysaccharides for packaging applications and critically analyzes the advantages and limitations. Particular attention was paid to the miscibility of PHB with these polymers and the compatibilizing methods used to improve the dispersion and interface.
A multitude of biopolymers (Fig. 1) such as PLA [39], [48], [49], poly(ε-caprolactone) (PCL) [50], [51], PHAs [46], [52], polysaccharides [53], [54] and other biodegradable polymers have been blended with PHB to improve its properties and to obtain biomaterials with more appropriate properties for these applications. PHB was also blended with natural rubber (NR) and thermoplastic polyurethanes (PU); however these polymers are hardly degradable in the environment and their blends with PHB were not discussed here.
Section snippets
Polymer blends from PHB and polysaccharides
Several polysaccharides such as starch, chitin or chitosan together with cellulose esters and anionic polysaccharides have been blended with PHB [55], [56], [57], [58]. Polysaccharide modifiers are attractive because they are inexpensive and readily available, being extracted from plants, animals or seaweeds, but their addition in PHB have rarely led to improved properties. For example, the addition of starch in PHB failed to improve its brittleness and low thermal stability [55], [56]. The
Blends with polyhydroxyalkanoates
Blending PHB with other polyhydroxyalkanoates may improve the flexibility, processability and thermal stability of PHB, thanks to the structural similarity and the elastomeric properties shown by several members of the PHAs family. It has been shown that PHB and PHBV are miscible in PHB/PHBV blends when PHBV has a low content of hydroxyvalerate (HV) units. This was demonstrated for PHB/PHBV(6% HV) blends, which exhibited only one glass transition temperature (Tg) and a linear dependence of Tg
Morpho-structural organization in the PHB blends
The above discussed works showed that PHB is immiscible or partially miscible with most of the biodegradable polymers tested so far to improve its properties. The morphology of the PHB blends depends on a multitude of parameters, but mainly on the difference between the viscosities and the melting temperatures of the polymers along with the miscibility of the components, which in turn is influenced by the chemical structure and molecular weight (Mn) of the polymers, the applied treatments or
Thermal and mechanical properties
The few PHB blends considered as miscible exhibited a single Tg and a shift of the melting temperature of PHB (Tm) [68], [70], [99]. This was observed in PHB/PHBV blends with low content of HV, which showed miscibility in the melting state and cocrystallization [70], [71]. In the PHB/CA blends obtained by electrospinning, PHB and CA were miscible in the amorphous state because only one Tg was detected in the blends with 10…40 wt% CA [68]. The miscibility of PHB and CAB in the melt state was
Processability and applications of PHB blends
PHB blends may be obtained by various techniques that ensure a good homogenization of the components either in solution or in melt. Melt blending using kneaders or tween screw extruders is the most used processing route because it may ensure a fine dispersion of the second polymer in PHB and is easier to scale in industrial installations. The processing temperature must allow the melting of PHB and second polymer and the processing duration should ensure their perfect mixing also avoiding the
Conclusions and further perspectives
In this review the last findings on the PHB blends with biodegradable polyesters, polysaccharides and their derivatives were discussed. Special emphasis was placed on the morpho-structural aspects in the PHB blends in correlation to their miscibility and compatibility which decisively influence the thermal, mechanical, and barrier properties of PHB blends along with their biodegradability and processability. The level of these properties is decisive for the application of the PHB blends in the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI – UEFISCDI, project number TE67/2020, within PNCDI III.
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