CHAPTER 1: Polyhydroxyalkanoates (PHAs), their Blends, Composites and Nanocomposites: State of the Art, New Challenges and Opportunities

Polyhydroxyalkanoates are biopolyesters with various side chains and fatty acids with hydroxyl groups at the 4- or 5-position. They consist of (R)-3-hydroxy fatty acids. There are three types of polyhydroxyalkanoates: (a) short chain length hydroxyalkanoic acids (PHA_SCL) with an alkyl side chain, which are produced by Ralstonia eutropha and many other bacteria. PHA_SCL contain 3–5 carbon atoms, for example poly-3-hydroxybutyrate (P3HB), poly-4-hydroxybutyrate (P4HB); (b) medium chain length hydroxyalkanoic acids (PHA_MCL) with alkyl side chains that are produced by Pseudomonas oleovorans and other Pseudomonas sensu stricto. PHA_MCL contain 6–14 carbon atoms and (c) long chain length (PHA_LCL) obtained from long chain fatty acids, which contain more than 14 carbon atoms The monomer composition, macromolecular structure and physical chemical properties of PHAs vary, depending on the producer organism as well as on the carbon source used for the growth.^1–3  PHAs containing double bonds can also be produced by recombinant Methylobacterium extorquens strains when fed unsaturated fatty acids. These PHAs comprise PHA_SCL and PHA_MCL. One reason for choosing a methylotrophic microorganism for such a purpose was that an important portion of the production process would use methanol. In spite of PHB being considered an environmentally friendly polymer with similar material properties to polypropylene (PP), it has not been used on a large scale to replace conventional polymers because it presents some drawbacks in its mechanical properties. Considering polymer mechanical properties, it is important to consider three basic properties when comparing the usefulness of a polymer for a given commodity application. It is hard to process PHB due to its high melting temperature of approximately 170 °C, which is very near to its degradation temperature. Therefore, a solution to these drawbacks could be the copolymerization of 3-HB with other monomers that confer less stiffness and tougher properties (which bestow greater flexibility and lessen breakage) and to reduce the melting point.

The monomer composition of PHA has considerable effects on its physical properties.

The PHA structure can effectively be controlled by adjusting the carbon substrates to achieve desired monomer contents, by engineering metabolic pathways in the hosts or by feeding the culture with carbon substrates containing functional side chains that in a second step can suffer chemical modifications.^4–8  PHAs, such as PHB and poly(3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV), are brittle, which is related to their high crystalline degree and they may lack the superior mechanical properties required for biomedical and packaging applications. These properties are a consequence of PHA's chemical structure. Therefore, since these different types of PHA have various structural and physical chemical properties, they should be classified according to their properties and modified in order to be easy to use for target applications. Different approaches were explored for the production of PHA_SCL by the Cupriavidus genus. The main studies were: the utilization of noble versus waste carbon sources and the utilization of a limiting factor to trigger PHA production versus the operation under “nutrient-sufficient” conditions. Thus, Chen et al. (2011) observed that a smaller C–N ratio was more favorable for PHA accumulation in a culture of Gamma proteobacterium in 72 hours, whereas a higher C–N ratio was more favorable for PHA accumulation in longer cultures of up to 150 hours of cultivation.⁹  Concerning activated sludge systems, Moralejo-Garate et al. (2013) have shown that the presence of ammonia during the PHA accumulation step was not damaging for PHA production.¹⁰ 

4-Hydroxybutyrate (4-HB) was produced by Aeromonas hydrophila 4AK4, Escherichia coli S17-1, or Pseudomonas putida KT2442 harboring 1,3-propanediol dehydrogenase gene dhaT and aldehyde dehydrogenase gene aldD from P. putida KT2442, which are capable of transforming 1,4-butanediol (1,4-BD) to 4HB. 4HB containing fermentation broth was used for the production of homopolymer poly-4-hydroxybutyrate [P(4HB)] and copolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-4HB)]. In this respect, attention has been given to producing PHAs bearing terminal double bonds in their side chains. The reason behind the preference for unsaturated PHAs can be found in the possibility to establish high-yield production processes.¹¹  Alkylic substrates containing double bonds are cheaper and generally exhibit less toxicity compared to substrates with reactive functional groups. At the same time, the unsaturated side chains of PHAs are susceptible to chemical modifications. Starting from alkene function, PHAs have been modified to exhibit functionalities en route towards new PHA-based biomaterials. The mechanical properties^12,13  of PHAs are directly correlated with their structure and crystallinity. An increase in the variety of side chains within one polymer chain of PHA_MCL can modify its ability to crystallize and as a consequence there are some distinct differences in the crystallinity of PHA_MCL. Obtaining a low crystallinity is possibly done once the polymers have large and irregular pendant side groups attached. These groups inhibit the close packing of the polymeric chains in a regular three-dimensional fashion to form a crystalline array.

The physical and material properties of PHAs are greatly influenced by their monomer composition and chemical structure i.e. the length of the pendant groups that extend from the polymer backbone, the chemical nature of the pendant groups and the distance between the ester linkages in the polymer. The variety of bacterial PHA that can be directly produced by fermentation is extraordinary large with more than 150 different hydroxyalkanoic acids, and even mercaptoalkanoic acid, that are known constituents of these PHAs.¹⁴  Depending on the subunit compositions and substrate specificities of the enzymes, PHA synthases are generally classified into four groups: class I, II, III, and IV.¹⁵ 

The recovery system may affect the amount of product recovered, the convenience of the subsequent purification steps and the quality of the final product. Cell separation from the fermentation broth is the preliminary step of the recovery method. In order to recover the PHA granules, it is necessary to rupture the bacterial cell and remove the protein layer that coats the PHA granules. Alternatively, the PHA has to be selectively dissolved in a suitable solvent. Generally, two methods are usually utilized for the recovery and purification of PHAs from cell biomass, which include PHA solubilization or non-polymer cellular material (NPCM) dissolution. The majority of the PHA recovery method is performed using a solvent extraction process mainly by chloroform and methanol. Modifying the cell wall's permeability and then PHA dissolution in the solvent are the mechanisms for PHA extraction. The separation of PHA from the solvent is carried out using solvent evaporation or polymer precipitation in a non-solvent material.¹⁶  PHA is very viscous and the removal of cell debris is difficult. Without considering solvent recycling, the large amount of solvent required for PHA extraction is costly.¹⁷  Consequently, PHA solubilizing into an immiscible solvent in water was explored at high temperature (above 120 °C). Then, cold water is added to extract the PHA, although the solvent can be recycled many times before being distilled.¹⁸ 

A new PHA recovery process was developed to obtain the benefits of both digestion methods using sodium hypochlorite and chloroform in a solvent extraction. The combined method creates three separate phases, which include hypochlorite solution at the upper phase, NPCM and undisrupted cells at the middle phase and a chloroform phase containing PHA. The polymer is then recovered by precipitation in a non-solvent and filtration. The molecular weight reduction due to polymer degradation is significantly reduced using this process.¹⁹ 

A few mechanical methods have also been developed to supplement these systems or as independent systems, which are widely used to recover intracellular PHA.²⁰  This field involves either solid shear (e.g. bead milling, extrusion of frozen cells) or liquid shear (e.g. high pressure homogenization). Combinations of methods, such as chemical and physical processes, can sometimes produce acceptable results whilst one method alone fails.²¹  It has been reported that chemical pretreatments increase the sensitivity of bacteria to disruption. They allow equal disruption to be obtained at lower operating pressures or fewer passes during the physical process. The success of PHA as a viable option to petrochemical-based plastics will depend upon the design and performance of efficient and selective means of PHA production and recovery.²²  Thus, further investigations on mixed cultures, recombinant microbial strains, cheap carbon substrates and efficient fermentations has allowed the production of significant quantities of PHAs, which can significantly decrease the production cost. A commercial recovery system with a simple, efficient and economical procedure will probably focus on a non-solvent extraction-based recovery amongst a variety of PHA recovery methods. In addition, the tolerance of the final product to the conditions employed is an important criterion for the selection of a PHA recovery process and the PHA properties have to be considered for the development of downstream processes. If the PHA molecular weight is too low, the transition temperatures and the mechanical properties will usually decrease, which is not suitable for any useful commercial applications. Hence, the challenge in the recovery process should be the maintenance of the original molecular weights, while not compromising the degree of purity for various applications.²³  However, severe degradation of polymer molecular weight was reported during PHA extraction of C. necator using a sodium hypochlorite treatment.²⁴  It is necessary to characterize and compare the extracted and non-extracted polymer properties to assess the feasibility of the developed recovery methods on PHA extraction and address the possible market demand and intended applications. The selection of suitable PHA extraction methods depends on several process parameters such as concentration of chemicals, reaction time, recovery temperature and pH. Basically, the impact of process parameters on the effectiveness of PHA extraction procedures have been studied and proven, but there is a limitation of concrete data on the effect of external factors on PHA recovery.

Various PHA blends have been developed to improve the performance and to offset the high price of PHAs. The blending of PHAs will offer more scope to expand their range of applications. The P(3HB)/PLA blend is one of the most studied blends, which exhibits mechanical properties that are intermediate between the individual components. Although PLA and P(3HB) are biodegradable polymers synthesized from renewable resources, their potential applications are hampered due to brittleness and formation of very large spherulites.²⁵  Zhao and co-workers (2013) reported the preparation of a P(3HB-co-3HV)/PLA blend using a co-rotating twin-screw extruder. The melt mixing was carried out above the glass transition temperature (T_g) of amorphous polymer. The reason for using a twin-screw extruder was to ensure that all the specimens undergo the same thermal–mechanical history. Blending of PCL and P(3HB) offers a good option to improve the performance of both homopolymers.²⁶  They had prepared a blend of P(3HB)/PCL by melting the mixture in an internal mixer with compositions of PCL varying from 0 to 30 wt% to study the miscibility, morphology and physical–chemical properties of these systems.

Cellulose derivatives also have attracted much interest for their compatibility with P(3HB).^27,28  Ethyl-cellulose (EtC) is also a biomaterial like P(3HB) that is approved by the FDA (Food and Drug Administration) and is widely used as a blood coagulant, in coatings for pharmaceutical tablets and matrices for poorly soluble drugs. Zhang and co-workers (1997)²⁹  had investigated the miscibility, thermal behaviour and morphological structure of P(3HB) with ethyl cellulose (EtC) blends. A P(3HB)/starch blend was prepared either by a conventional solvent casting method or by melt processing methods, such as injection molding and compression molding after compounding. Two types of maize starch, Starch 1 (containing 70% amylose) and Starch 2 (containing 72% amylopectin), were blended with P(3HB) using a melt compounding method at a ratio of 70 : 30 wt% and characterised in terms of their morphology, structure, thermal, rheological and mechanical properties.³⁰  Ikejima and co-workers (1999)³¹  had prepared P(3HB)/chitosan blend films in order to investigate the effect of deacetylation on the crystallisation behaviour of P(3HB). Chitosan is a copolysaccharide with a high degree of deacetylation. A solvent-casting method was employed to prepare the P(3HB)/chitosan blend films. P(3HB) and chitosan were dissolved separately in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) before blending.

Zembouai et al. (2013)³²  reported that the degradation of each polymer occurred separately and it was found that PLA was more thermally stable than P(3HB-co-3HV) copolymer. They also reported that the decomposition temperature, T_d, for all the blends was between PLA and the P(3HB-co-3HV) copolymer. Therefore, the thermal stability of the P(3HB-co-3HV)/PLA blends could be improved by increasing the amount of PLA. The T_m for the blend was found to be in between these two polymers. Miscibility of any polymer can be determined by evaluating the T_g. A single T_g indicates the miscibility of the polymer.³³  A study by Nanda et al. (2011)³⁴  reported one T_g which corresponded to PLA and the T_g for P(3HB-co-3HV) copolymer was poorly observed. They also reported that when the content of P(3HB-co-3HV) copolymer was increased, a reduction in T_g from 60 °C to 45 °C was observed. The same observation was reported by Richards et al. (2008)³³  and Modi et al. (2012).³⁵  Chee and co-workers (2002)³⁶  had performed viscometric studies on polymer-blend solutions of P(3HB) with PCL because it is a powerful method to assess the miscibility of the components in an amorphous state. Viscometric analysis demonstrated that P(3HB) was immiscible with PCl. Phase behaviour and crystallisation kinetics for the binary blend P(3HB)/cellulose propionate (CP) had been studied by Maekawa and co-workers (1999).³⁷  A strong dependence of the measure T_g on composition was detected at high levels of CP. The Flory–Fox equation is one of the best equations to describe the dependence of T_g on composition in miscible blend systems. Wang and co-workers (2003)³⁸  studied the miscibility, crystallisation behaviour, tensile properties and environmental biodegradability of P(3HB)/cellulose acetate butyrate (CAB) blends. Ismail et al. (2010)³⁹  studied the effect of starch content in a P(3HB) film matrix on its degree of swelling in water. Swelling in water and degradability are the most important characteristic for biodegradable materials. Polymer films were degraded by surface absorption of moisture and microorganisms.

Ten et al.⁴⁰  produced evenly exfoliated nanocomposites of PHBV/cellulose nanowhiskers, with a well-defined distribution of nanofillers within the polymer matrix. Zhijiang et al.⁴¹  reported an improvement in the mechanical properties of a PHB nanocomposite made of bacterial cellulose nanofibrils that was prepared by a solution-casting method. In addition, they found the nanocomposite to show better biocompatibility and mechanical properties than pure PHB based on cell-adhesion analysis using Chinese hamster lung (CHL) fibroblast cells and stress strain tests, respectively. Ten et al.⁴²  studied the isothermal crystallization kinetics of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanocomposites containing cellulose nanowhiskers (CNWs). The researchers studied the effects of the CNWs concentration and temperature on the crystallization rate and crystallinity of PHBV nanocomposites. Mook Choi et al.⁴³  for the first time described the production of an intercalated PHBHV-clay nanocomposite using melt extrusion in a Brabender mixer at 165 °C, 50 rpm agitation rate for 15 min. The same melt extrusion was employed by Maiti et al.⁴⁴  to produce a well dispersed PHB/layered silicate nanocomposite. Zhang et al.⁴⁵  prepared nanocomposites of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx)/layered silicates and PHBHHx/expanded graphite. For the nanofillers used in the two nanocomposites, at a lower nanofiller content the researchers observed an exfoliated morphology with good dispersion. Gorrasi et al.⁴⁶  obtained exfoliated nanocomposites of a poly-6-hydroxyhexanaote matrix through in situ intercalative ring opening polymerization of caprolactone with modified montmorillonite in the presence of a dibutyltin dimethoxide catalyst. Reports on the application of in situ intercalative polymerization for the production of bacterial PHA nanocomposites, especially those containing a medium-chain-length poly-3-hydroxyalkanoate matrix, are virtually non-existent.⁴⁷  Zhang et al.²⁹  suggested that the dispersion of the polymeric side chain into the nanofiller's layered spaces results in exfoliation. The characterization of the state of nanoparticle dispersion allows for the interpretation of the preceding morphologies, and the structural characterization relies heavily on techniques such as X-ray diffraction (XRD), wide angle X-ray diffraction (WAXD), simultaneous small angle X-ray scattering (SAXS) and electron microscopy (transmission, TEM, or scanning, SEM). Zhijiang et al.⁴⁷  employed the use of field emission scanning electron microscopy (FESEM) to qualitatively characterize a PHA/bacterial cellulose nanocomposite. Zhang et al.^48,49  reported the use of nanocomposite particles based on a PHA/folate ligand which carries doxorubicin as an anticancer therapy. The researchers reported that the membrane showed an excellent selectivity with relatively high permeation flux towards water compared to existing commercial membranes. Shape memory nanocomposite polymers have the ability to revert back to their original shape after being deformed thus expanding their applications to such areas as dry adhesion, microfluidics, biosensors and tissue engineering.⁵⁰  Ishida et al.⁵¹  reported the synthesis of a shape memory polymer nanocomposite derived from poly(3-hydroxyoctanoate-co-3-hydroxyundecenoate) composited with nanofillers of silsesquioxane (POSS). A nanocomposite membrane based on a PHB-functionalized multi-walled carbon nanotubes/chitosan matrix was used to efficiently pervaporate a mixture of water and 1,4-dioxane without the use of an entrainer.⁵²  The researchers reported that the membrane showed an excellent selectivity with relatively high permeation flux towards water compared to existing commercial membranes.

Extrusion of PHA-based materials is in general linked with another processing step such as thermoforming, injection molding, fibre drawing, film blowing, bottle blowing or extrusion coating. PHA shows a low degradation temperature compared to its melting temperature. For instance, PHB homopolymer presents a narrow window for processing conditions. The PHB thermal^53–60  and thermo–mechanical^61,62  stabilities have been well described in the literature, demonstrating that the thermal degradation occurs in a one-step process, namely a random chain scission reaction. To improve such a drawback or to create new PHA properties, a great number of multiphase materials have been developed, mainly by mixing PHB or PHBV with others products such as plasticizers, fillers or other polymers. Many authors have noticed that PHA properties can evolve when plasticization occurs, e.g. with citrate ester (triacetin).^63–65 

Wang et al. tested different plasticizers—dioctyl phthalate, dioctyl sebacate, and acetyl tributyl citrate (ATBC)—with PHB.⁶⁶  The effects of biodegradable plasticizers on the thermal and mechanical properties of PHBV were studied by Choi and Park⁶⁷  using thermal and mechanical analyses. Soybean oil (SO), epoxidized soybean oil (ESO), dibutyl phthalate (DBP) and acetyl tributyl citrate (ATBC) were tested as plasticizing additives. Several blending studies have been performed on PHB or PHBV copolymers with a range of compounds that can help vary the crystallinity and degradation of the final blends.^68,69  The possibility of blending other polymers with PHA offers the possibility of not only overcoming the drawbacks of a small processing window and low impact resistance of PHAs but can also modify the crystallization tendency and biodegradation rates. The possibility of H-bonding or formation of donor–acceptor interactions between PHA and the other blend constituents helps improve blend miscibility and reduces the tendency of phase separation.

Applications of such PHA-based multilayers as commodities are primarily limited by PHA cost and until now by PHA availability, and thus attention is being focused on products with plastics constituting only a minor part of the product, such as paper coatings such as the plastic film moisture barrier in food or drink cartons and in sanitary napkins. The presence of cellulose fibres also increases the rate of PHBV crystallization, due to a nucleating effect, while thermal parameters, such as crystallinity content, remained unchanged. Studies on the crystallization behaviour of PHB/kenaf fibre biocomposites showed that the nucleation by kenaf fibres affected the crystallization kinetics of the PHB matrix.⁷⁰  Differences in the effect of cellulose fibres on the crystallization process have been attributed to the lignin content at the surface/interface of the cellulose fibre. The addition of cellulose fibres led to some improvement in tensile strength and stiffness, but the composites remained brittle.⁷¹  At low content, the incorporation of cellulose fibres lowered the stiffness, however, higher amounts of cellulose fibres greatly improved the mechanical properties of PHB. For biocomposites based on cellulose fibres and PHB, the effect of fibre length, surface modification on the tensile and flexural properties has been investigated. Results on PHB reinforced with straw fibres have been published.⁷²  The structure–property relationships for PHA/OMMT nano-biocomposites were established and are in good agreement with the conclusions drawn in previously reported studies on synthetic polymer-based nanocomposites. Further attention was paid to the PHAs’ degradation in nanocomposite systems since these polymers are very temperature sensitive.^73–75 

Thus, scientists were interested in other PHA-based nanocomposites filled with e.g. layered double hydroxides (LDH),^76,77  cellulose whiskers^78–80  and hydroxyapatite (HA),⁸¹  the latter being used in particular for biomedical and tissue engineering applications. LDH structures are similar to layered silicate clays.

Chemical, physical and enzymatic approaches have been explored for polymer modifications, resulting in a uniquely transformed PHA endowed with a functionalized reactive group and/or enhanced properties such as thermal stability, elasticity, improved hydrophilicity and degradability. While chemical modification processes provide a large degree of freedom in controlling and designing the modified PHA in bulk quantities to suit a particular function, most often they have to contend with the drawback of toxic impurities that require difficult downstream processing. The structure of a PHA can be altered chemically to produce a modified polymer with predictable variation in molecular weight and functionality. For example, the hydrolytic rate of PHA to give an activated macromer that can accept a reactive functional group is said to depend on several factors such as the chemical nature or reactivity of the ester linkages between the monomers.⁸²  PHA modification by carboxylation is the addition of a carboxylic functional group to the polymeric macromer. Carboxylic groups incorporated into the polymer usually serve as functional binding sites for bioactive moieties such as probes for targeting proteins and hydrophilic components.⁸³ 

Condensation reactions between carboxylic acids and amine groups were exploited to graft a modified PHA and linoleic acid onto chitosan.⁸⁴  Recently, Babinot et al.⁸⁵  used click ligation to esterify the pendant –COOH of carboxylated PHA with propargyl alcohol resulting in a clickable-alkyne group that was subsequently used to copolymerize a poly(ethylene glycol) (PEG) macromer onto the modified PHA. The properties of PHA and its copolymers have been reported to be modified by hydroxylation.^86–89  Normally, acid- or base-catalyzed reactions are used in the modification of PHA by hydroxylation in the presence of low molecular weight mono or diol compounds. Hydroxy-terminated PHA is important in block copolymerization. Methanolysis of PHA resulted in PHA methyl esters bearing monohydroxy-terminated groups. Halogenation of PHA is considered as an excellent method to diversify the polymer's functions and applications. Halogen atoms such as chlorine, bromine and fluorine were added to the olefinic bonds of unsaturated PHA through an addition reaction,⁹⁰  and to the saturated PHA via substitution reactions.

In another study, Arkin and Hazer⁹¹  modified PHA-Cl into quaternary ammonium salts, thiosulfate moieties and phenyl derivatives. In addition, they cross-linked the modified PHA-Cl with benzene by electrophilic aromatic substitution using a Friedel–Crafts reaction. Mihara et al.⁹²  and Imamura et al.⁹³  filed an embodiment that detailed the procedures for PHA chemical modification by sulfanyl halogenation and the potential application of the modified PHA as a toner electrostatic charge controller in electrophotographic imaging. Another method to modify PHA is by graft copolymerization, which results in the formation of modified segmented copolymers with improved properties such as increased wettability and thermo–mechanical strength. Grafting reactions can be induced by either chemical, radiation or plasma discharge methods.^94,95  Chemical modification methods are sometimes aggressive and lead to reduced polymer molecular weight, unwanted side reaction(s) and toxic impurities. In some instances, a mild surface modification process is required without which the polymer may fail in its intended application(s). Irradiation of polymeric materials required no addition of polymer contaminants. Irradiations such as gamma-irradiation normally result in three-dimensional network structures with improved tensile strength. Several studies have demonstrated the cross-linking of unsaturated mcl-PHAs by gamma-irradiation.^96–98  The presence of olefinic bonds in PHA side chains provides an avenue for polymer modification by several irradiation processes. A highly cross-linked modified polymer was produced by irradiating unsaturated PHA obtained from tallow-grown P. resinovorans with 25–50 kilogray (kGy) of γ-irradiation. Ion implantation is another physical method employed in polymer surface modification. Its advantage over polymer modification methods is that it only modifies the polymer surface layer, without upsetting the bulk polymer's properties. Ion implantation has been successfully applied in several polymer modifications thereby expanding its applications.^99–103  Mirmohammadi et al.¹⁰⁴  compared the biocompatibility of a PHB surface upon treatment with O₂ and CO₂ plasma at 50 W discharge for 3 min, and found that O₂ plasma treated PHB showed much improvement. Ying et al.¹⁰⁵  evaluated the biocompatibility and biosorption characteristics of an electrospun scaffold of P3HB4HB through subcutaneous implantation of the fibers in rats. The researchers found a highly increased tissue response with increasing content of 4HB monomer.

The most common PHA packaging resins are polyhydroxybutyrate (PHB) and its copolymer with polyhydroxyvalerate (P(HB-co-HV). The potential of PHAs as biodegradable replacements for conventional bulk commodity plastic packaging while promoting sustainable development has long been recognised.¹⁰⁶  The potential of PHAs for truly biodegradable packaging was recognised in the 1980s with the commercial release of Biopol^®, thermoplastic resins of P(3HB) with various copolymer loadings of (3HV), by Imperial Chemical Industries (ICI, now Zeneca). One of the prohibiting factors against the use of PHAs as a resin for packaging materials is that it is economically uncompetitive in the current market compared to fossil fuel sourced synthetic raw material (ff-polymers).^107,108  In the production of PHAs, the cost of the carbon substrate represents approximately 50% of the total production cost. In order to rival current synthetic polymers used in packaging such as polyethylene (PE), polypropylene (PP) or polystyrene (PS) for example, PHAs’ physical and chemical properties need to be comparable. The optical properties of plastic packaging, certainly in the case of food packaging, offer a convenient, lightweight and flexible adaption of packaging technologies for the food industry, reducing the reliance upon glass and metallic canning. Transparency, a variety of packaging options such as shrink wrap, a modified atmosphere and printability allow plastic packaging to be tailored to the type of food to be contained. The thermal properties are a vital consideration when selecting a polymer for packaging. Fortunately, PHAs provide (through diversity of structure and chemistry) a wide range of thermal properties for selection to suit packaging needs. Melting temperatures (T_m) from 60 to 177 °C, glass transition temperatures (T_g) from –50 to 4 °C and thermal degradation temperatures at highs of 256 to 277 °C are all within the range of the PHAs currently being produced.¹⁰⁹  Carboxyl-terminated butadiene acrylonitrile rubber (CTBA) and polyvinyl pyrrolidone (PVP) have been added to PHB in an effort to modify its thermal processing. Hong et al. report a significant modification of PHB crystallisation rate, crystallinity, melting temperature and thermal stability with the addition of only 1% (w/w) of these additives.¹¹⁰  When compared to the other typical ff-polymers of polyethylene (PE) and polystyrene (PS) used in packaging, the oxygen and water barrier properties of commercial packaging PHA resins are considered to be naturally of a superior level. The vapour pressure exerted by these small aroma bearing molecules provides an added challenge for the application of biopolymers in packaging. A packaging's vapour barrier properties relates to its ability to prevent water vapour from crossing the polymer packaging boundary. Several factors including mechanical, morphology and crystallinity can play a substantial role in determining a packaging's vapour barrier properties. One particular strategy of improving PHA packaging barrier properties would be to develop suitable nanocomposites. In particular, nanocomposites incorporating nanoclays of montmorillonite and kaolinite clays could also substantially improve the mechanical strength and thermal stability as well as the gas barrier properties. In a similar strategy as that used to improve the thermal properties of polymers, PHAs have been used as additives to improve the barrier properties of conventional, synthetic chemicals. For example, addition of P(3HB) to polyvinyl alcohol (PVOH) can lead to significant improvement in its barrier properties. The herbicide product was successful delivered over a period of time with gradual degradation of the PHA packaging and was successful in controlling the growth of creeping bentgrass, Agrostis stolonifera.¹¹¹  Notably, adjustment of the content of the PHA casing enabled a level of control over degradation rate and product release.

According to Robertson (2010),¹¹²  the main functions of packaging are to contain, to protect, to be convenient, to communicate and to sell the product. The basic functions of a package are to contain a certain amount of food, unitizing the product and facilitating its transportation, storage, sale and use. Bucci et al. (2005)¹¹³  reported that PHB can be used in injection molding processes for the manufacture of food packaging, with the same equipment used for PP packaging injection. However, the process conditions should be adjusted according to polymer characteristics. The authors found a notable difference between PHB and PP bottles in relation to their performance in dynamic compression resistance and a drop test, in that PHB is as hard a material as PP, but is less flexible. PHB performance was better at higher temperatures. The physical, dimensional, mechanical and sensory tests showed that PHB can replace PP containers for food products with high fat content (mayonnaise, margarine and cream cheese), including storage in freezers and heating in microwave ovens. Likewise, Muizniece-Brasava and Dukalska (2006)¹¹⁴  reported that PHB materials are suitable for sour cream storage. To improve flexibility for potential packaging applications, PHB is synthesized with various co-polymers, such as poly (3-hydroxyvalerate) (HV), leading to a decrease of the glass transitions and melting temperatures. In addition, the HV broadens the processing window since there is improved melt stability at lower processing temperatures (Modi et al., 2010).¹¹⁵  Fabra et al. (2013)¹¹⁶  created an innovative way to develop renewable biopolyester microbial-based multilayer structures with enhanced barrier performance, which is of significant interest for food packaging applications. The researchers developed multilayer structures based on polyhydroxybutyrate-co-valerate with a valerate content of 12% (PHBV12) containing a high barrier interlayer of zein electrospun nanofibers. The incorporation of 3-hydroxyvalerate (HV) in PHB, resulting in PHBV, has increased impact strength, elongation modulus, tensile strength and decreased Young's modulus, making the film more flexible and more resistant (Shen et al., 2009).¹¹⁷  The price is very high but PHBV degrades between five and six weeks in a microbiologically active environment, resulting in water and carbon dioxide in aerobic conditions. In an anaerobic environment, degradation is faster, producing methane (Siracusa et al., 2008).¹¹⁸ 

A mixture of PHBV with PLA had a positive effect on the elasticity modulus, elongation at break and flexural strength for different blends. However, tensile strength did not improve in any of them. In the same way, Zhang et al. (1996)¹¹⁹  reported improved mechanical properties for blends of PHB/PLA compared with the common PHB. In addition, PVA (polyvinylacetate) grafted on PIP (poly-cis-1,4-isoprene) and mixed with PHB had better tensile properties and impact strength than PHB/PIP blends, which were immiscible (Yoon et al., 1999).¹²⁰  Combined with synthetic plastics or starch, PHAs make excellent packaging films (Tharanathan, 2003).¹²¹  In another study conducted by Shen et al. (2009),¹²²  thermoplastic starch (TPS) blended with PHA had a positive effect on the barrier and hydrolytic properties and UV stability of a starch-based film. With this blend, it was possible to reduce the processing temperature, resulting in less degradation of the starch.