Improving the Barrier Properties of the Biodegradable Polymers

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1		Yuezhong Meng	--	4770	2024-02-21 03:26:30	\|
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This entry is adapted from the peer-reviewed paper 10.3390/nano14040338

Biodegradable polymers have become a topic of great scientific and industrial interest due to their environmentally friendly nature. For the benefit of the market economy and environment, biodegradable materials should play a more critical role in packaging materials.

surface-nanotechnology biodegradable polymers packaging materials

1. Cellulose-Based Biodegradable Polymers

The most common biopolymer utilized in load-bearing applications is cellulose, which may be found in plant fibers, wood products, and chemically derived films and fibers ^[1]. Cellulose has a respectable ability to form films, making it suitable for use in the packaging industry as large-scale films ^[2], nanopaper ^[3], or nanocomposite ^[4]. The high modulus, high tensile strength, and low oxygen permeability of cellulose films are its main benefits. However, cellulose films’ inadequate water vapor barrier characteristics are a result of the abundance of hydroxyl groups on the surface of the material ^[5]. Chemical modification, the addition of inorganic flake nano-filler, treating the surfaces of cellulose substrates with a synthetic polymer such as PVDC or a biopolymer (e.g., whey proteins, polycaprolactone, poly(lactic acid), beeswax) ^[6]^[7], or inorganic impermeable particles such as mica ^[8], graphene oxide ^[9], or montmorrilonite ^[10] through layer-by-layer lamination are common methods to increase the moisture barrier capacity of cellulose.

Chemical Modification

Chemical modification is the process of modifying the physical and chemical characteristics of cellulose particles or fibers using one or more chemical agents. There are two secondary (C2 and C3) and one primary alcohol group in each of the cellobiose moieties. These groups can undergo a variety of chemical processes, such as oxidation and substitution.

Although cellulose nanofibrils (CNF) can provide ideal barrier layers, their usage is constrained by weak rheological capabilities, brittleness, and moisture sensitivity. Kwon et al. ^[11] created all-cellulose nanocomposite films using cellulose nanocrystals (CNCs) and 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized cellulose nanofibers (TEMPO-CNFs) via a straightforward vacuum filtration technique in order to address these difficulties. A transparent, free-standing substrate was built using TEMPO-CNFs, and the CNCs were employed as a coating material to enhance the mechanical and water vapor barrier capabilities of the finished product. The final CNC/TEMPO-CNF/CNC films demonstrated an exceptional tensile strength of 114 MPa and a comparatively low water vapor transmission rate (SWVTR) of 19 g·mm/(m²·day). The CNC/TEMPO-CNF/CNC films were also resistant to acetone, ethanol, tetrahydrofuran (THF), water, and other solvents. Balasubramaniam et al. ^[12] examined the modification of precast CNF films with saturated fatty acids lauric, palmitic, and stearic acids utilizing various modification procedures in order to increase the hydrophobicity of CNF films and preserve outstanding mechanical qualities. Surface modification and bulk modification were the two alteration methods used. The membrane created by bulk modification of CNF greatly enhances its water vapor barrier characteristics. Both modification approaches produced films with improved surface hydrophobicity. However, surface modification methods produced changed films with greater mechanical property preservation.

Addition of Inorganic Flake Nano-Filler

Over the past few decades, a different method for creating high gas barrier polymer films has been developed: incorporating nanoplatelets into polymers ^[13]^[14]. As multiple physical barriers to molecule diffusion, nanoplatelets significantly improve the barrier properties of polymer-based nanocomposites. Tayeb et al. proposed a novel nanocomposite film consisting of cellulose nanofibrils (CNFs)/colloidal montmorillonite nanoclay (MMT) with two cross-linking agents, namely, polyamidoamine epichlorohydrin (PAE) and acrodur thermoset acrylic resin (ACR). The combination of clay nanoplatelets and crosslinkers contributes to a denser membrane structure and restricted water passage. As a result, the water vapor transmission rates of the hybrid film were significantly reduced to 160 g/(m²·24·h) ^[15]. The morphology of the nanosheets (such as their exfoliation, dispersion, and orientation) in the polymer matrix, their intrinsic characteristics (such as high surface area and high aspect ratio), and the interfacial adhesion between the nanosheets and the polymer matrix all have a significant role in the enhancement of efficiency. Nanosheets, in particular, can increase the tortuosity of the diffusion molecules’ penetration path when placed at a height perpendicular to the diffusion direction. Layer-by-layer coating and hot pressing were used by Ren et al. to create graphene oxide nanosheet/cellulose nanofiber (GONS/CNF) nanocomposite films, and the exfoliated GONS and CNF were strongly orientated along the film direction under a powerful external shear flow field. Due to their unusual structure, GONS/CNF nanocomposites function very well as water vapor barriers and have a high oxygen content. With only 3.66 vol% GONS, the oxygen permeability coefficient of CNF film decreased by about 4 × 10⁴ times, from 5.5 × 10⁻¹³ to 1.4 × 10⁻¹⁷ cm³·cm/(cm²·s·Pa), and the permeability coefficient of water vapor (P_H2O) decreased from 1.61 × 10⁻¹² to 1.10 × 10⁻¹² g·cm/(cm²·s·Pa) ^[16].

Surface Coating

The enhancement of the barrier qualities of cellulose-based films in high-humidity conditions has been the subject of certain research projects. One tactic is the creation of multilayer systems containing hydrophobic polymers, such as PP and PET, as exterior layers that can shield the cellulose layers ^[17]. Kim et al. ^[18] introduced a transparent, water-stable, high-oxygen barrier packaging film made from a combination of succinylated cellulose nanofibers (SCNF) and a fluoropolymer (FP) coating. Introducing the FP topcoat on SCNF enabled a synergistic enhancement of both oxygen barrier performance (0.1–0.3 cc/(m²·day·atm) at 0% RH) and stability against water-swelling. Wang et al. ^[19] prepared PP/CNMs/PP multilayer packaging films. CNMs maintain their high oxygen resistance at 80% RH after being laminated with PP; the water vapor transmission rate of CNC film dropped from 516 to 1.0 g/(m²·day). The oxygen transmission rate of CNC film at 80% RH decreased from 126 to 6.1 cm³/(m²·day). A perhydropolysilazane-derived-SiO_x coating layer was applied to the cellulose films by a facile dip-coating followed by a UV curing method, resulting in a significant increase in their oxygen and water vapor barrier properties. The cellulose film (35 µm in thickness) with 450 nm thick SiO_x coating exhibits a low oxygen transmission rate (OTR) value of 0.82 cm³/(m²·day) and water vapor transmission rate (WVTR) value of 1.28 g/(m²·day) ^[20].

Layer-by-Layer Assembly

Layer-by-layer (LBL) assembly was first suggested by Decher and colleagues to build ultrathin films by alternating the deposition of components having complimentary chemical interactions ^[21]. The air diffusion channel was greatly extended upon the deposition of nanoplatelets in a highly orientated orientation during LBL assembly, which has been shown to be an incredibly successful strategy for enhancing the barrier qualities of the substrate films. Using quadlayers of carrageenan (CR), chitosan (CS), montmorillonite (MMT), and CS, Li et al. effectively created superhydrophobic LBL polymer-nanoclay hybrid multilayers. A paper sample modified with a wax-treated (CR/CS/MMT/CS)₂ multilayer had a water contact angle of 151.4°. This sample of superhydrophobic paper has comparable tensile strength and strong barriers to air and water vapor compared with the original paper ^[22]. Zhou et al. prepared a novel nanocomposite membrane consisting of cellulose acetate (CA)/polyethyleneimine (PEI)/reduced graphene oxide (rGO)-NiCoFeO_x) by using “molecular glue” and “nano-patching” strategies, which has excellent gas barrier properties ^[23]. All these studies indicated that LBL assembly is a suitable technique to improve some application properties of cellulose paper.

2. Starch Based Biodegradable Polymers

Starches are inexpensive and widely accessible polysaccharides. It is made up of the branching, amorphous polymers amylopectin (poly-1,4-d-glucopyranoside and -1,6-d-glucopyranoside) and amylose (poly-1,4-d-glucopyranoside), which is a linear crystalline polymer. Depending on the source, the levels of amylose and amylopectin in starch can range from 10 to 20 percent and 80 to 90 percent, respectively. Biodegradable polymers are made using a variety of starches, including potato, cassava, rice, corn, and tapioca. However, starch has numerous significant flaws that prevent the creation of goods based on it, such as poor mechanical behavior and moisture resistance ^[24]. Moreover, plasticized starch suffers from recrystallization and retrograde phenomena that affect the stability of the mechanical properties over time ^[25]. Therefore, several strategies have been proposed to improve the properties of starch-based biodegradable plastics, such as chemical modification, cross-linking, blending with various biopolymers and certain additives, the use of different plasticizers, and the development of nanocomposites ^[26].

In a recent study, Dai et al. ^[27] assessed the effects of various plant-derived starches (waxy corn, cassava, sweet potato, potato, wheat, and corn) and various modified cassava starches (esterified cassava starch, cross-linked cassava starch, and oxidized cassava starch) on the physical and chemical characteristics of starch-based films used in food packaging. Adipic acid cross-linked cassava starch films were found to have better water vapor barrier qualities than other modified starch films and unmodified starch films. Cross-linked cassava starch film has a much lower water vapor permeability than other native starch films and oxidized cassava starch film.

Other biodegradable polymers, such as polyvinyl alcohol and chitosan, are mixed with the starch to improve its mechanical and gas barrier properties. Glycerol-plasticized acetylated corn starch films were developed by Jiménez-Regalado et al. ^[28] using a casting method, and the impact of incorporating chitosan (TPS:CH) in various proportions was studied. Chitosan-protonated amino groups promoted the formation of intermolecular bonds and improved the gas barrier properties of starch films.

The kind and amount of the plasticizer are also important determinants of how starch-based materials behave as barriers. González et al. ^[29] created thermoplastic starch (TPS) films using a typical maize starch matrix utilizing the extrusion/compression process and plasticizers glycerol, 1,3-propanediolane, and D-isosorbide. They discovered that the WVP did not differ significantly when different plasticizers were used but that the OP was significantly influenced by the plasticizer, and the lowest OP value of 6 ± 3 cm³·μm/(m²·day·kPa) was obtained for TPS-I (sample plasticized with D-isosorbide), which was twenty times lower compared with that obtained for TPS-G (sample plasticized glycerol). In addition, TPS nanocomposite films were prepared incorporating waxy starch nanocrystals (WSNC) and cellulose nanocrystals (CNC) into TPS-G; the WSNC showed greater effectiveness in decreasing the permeability against O₂ molecules due to their platelet-shape morphology. The OP value reduced from 108 ± 35 cm³·μm/(m²·day·kPa) for TPS-G to 20 ± 3 (TPS-G5) cm³·μm/(m²·day·kPa) for the TPS nanocomposite with 5 wt% WSNC.

To tailor the properties of thermoplastic starch, with a particular focus on improving their barrier properties without compromising their transparency and biodegradability, Fabra et al. ^[30] produced thermoplastic corn starch (TPCS) nanobiocomposites containing bacterial cellulose nanowhiskers (BCNW) by direct melt-mixing. The addition of BCNW to TPCS-based films resulted in a great improvement in the mechanical properties as well as barrier properties due to the strong nanofiller-matrix adhesion by hydrogen bonding. Subsequently, the TPCS nanobiocomposites were successfully hydrophobized by coating them with electrospun poly(3-hydroxybutyrate) (PHB) and homogenizing by annealing, obtaining multilayer structures with further improved water vapor barrier properties and helping keep good oxygen barrier properties of TPCS at high humidity. Ruamcharoen et al. ^[31] incorporated 10% natural rubber (NR) to toughen cassava starch (CS), and nanoclays were introduced into the composites as the reinforcing or compatibilizing agent as well as barrier-enhancing agent. Three kinds of nanoclays, that is, montmorillonite (MMT), kaolinite (KAO), and intercalated kaolinite (DKAO), were used at contents of 2, 4, 6, and 8 wt%, respectively. It was found that the introduction of clays significantly improved both the mechanical and water vapor barrier properties of the pristine starch and CS/NR film. The best improvement was achieved for 4 wt% of MMT addition due to well-dispersed MMT nanosheets, the strong interaction of MMT with starch, and the formation of a tortuous path. Reductions of 53 and 68% in the WVT of the CS and CS/NR films with the loading of 4 wt% of MMT were obtained.

3. Protein Based Biodegradable Polymers

Protein film-forming components are isolated from a variety of animal and plant sources, such as animal muscles and tissues, oilseeds, milk, soybeans, wheat, corn, and grains, where each contains a different composition, structure, and functionality. For this reason, protein-based films have been studied extensively. The mechanical, barrier, and thermal properties of protein-based packaging and their capabilities in preserving foodstuffs have been comprehensively investigated over the past decade ^[32]. In the food packaging industry, films made from protein polymers are used as edible films so that they can be consumed along with the food. In non-food packaging, polymers of keratin, casein, zein, gelatin, and soy–protein, etc., may play a crucial role in the development of various commercial products such as shopping bags, mulch films, and flushable hygiene products, etc. ^[33]^[34].

However, the barrier properties of proteins against gases such as oxygen, carbon dioxide, and water vapor are not effective enough to be competitive with the respective petroleum-based barrier plastics. This can be attributed, in particular, to their sensitivity to moisture. In addition, films made from whey protein showed relatively poor mechanical properties (tensile strength 4.38 MPa ^[35]) compared with synthetic films.

The mechanical properties of protein polymer can be further enhanced by cross-linking with “cross-linking agents”, those cross-linking agents including glutaraldehyde, glyoxal, phenolic components such as gallic acid, tannic acid, ferulic acid, and enzymes, etc. ^[36], or blending them with chitosan ^[37], starch ^[38], and pectin ^[39]. Parsaei et al. ^[40] evaluated the effects of caffeic acid (CA) and tannic acid (TA) as cross-linking agents on the mechanical and physicochemical properties of cold-water fish gelatin films. The incorporation of phenolic compounds into the gelatin films resulted in a hydrogen bond between the gelatine polypeptide chain and the phenolic compound. In this way, the structure of the gelatine film becomes compacter, and WVP and OP are reduced by 32% and 44%, respectively, for the 5% TA cross-linked film as compared with uncross-linked film. Meanwhile, the incorporation of TA and CA into gelatin films also increased mechanical strength from ~28 to ~50 MPa. Lee et al. ^[41] prepared active bio-active gelatin/chitosan nanoparticles (CSNPs) composite films with chicken skin gelatin; due to the intermolecular interactions between the two biopolymers, the mechanical and barrier properties of protein-based membranes can be improved, WVP value for the film with CSNPs concentration of 6% was significantly decreased (from 2.72 to 1.31 g·mm/(h·m²·kPa)) compared with films without CSNPs; meanwhile, the tensile strength increased from 2.29 to 4.22 MPa.

Incorporation of nanofillers or nano-reinforcements into protein films is also an effective strategy to improve their water vapor barrier and mechanical properties. The improvement effect of nanofillers on the physical properties of films is attributed to the high surface area/volume ratio of the nanofillers and the creation of strong interfacial interactions between the polymer matrix and the nanofillers. Amjadi et al. ^[42] investigated the effects of the dimensions and morphology of nanofillers (sodium montmorillonite (MMT), cellulose nanofibers (CNF), and titanium dioxide nanoparticles (TiO₂NPs)) on the mechanical properties and release profile of the cinnamon essential oil (CEO) activated gelatin-based films. The WVP of the films exhibited a significant decrease upon incorporation of nanofillers. In particular, MMT-incorporated active film exhibited the lowest WVP (1.5 × 10⁻¹⁰ g/(m·s·Pa)).

In addition, by combining proteins with other biopolymers that contain eligible barrier properties such as polysaccharides, lipids, and/or other proteins, it is possible to capitalize on the specific functional characteristics of each component ^[43]. For instance, with the addition of hydrophobic lipids such as oils ^[44] and waxes ^[45] to the formulation of protein-derived films, the moisture permeability of the films declined significantly.

4. Poly(Lactic Acid)-Based Biodegradable Polymers

Poly(lactic acid) (PLA) is a synthetic biodegradable polymer derived from renewable resources such as sugar beet, corn, sugar cane, potato, cassava, wheat straw, bagasse, or wood chips that has gained intensive attention in recent years ^[46]. Although high molecular weight, good processability, and biodegradability make PLA a potential green packaging material ^[47], it still has some limitations for food packaging usage, such as poor thermal and mechanical properties, low crystallization rate, poor melt strength, moderate water vapor barrier properties, and poor oxygen barrier properties. Various studies have reported on blending PLA with immiscible biopolymers such as nanocellulose fibrils, polyhydroxybutyrate (PHB), and polybutylene succinate (PBS) to improve its crystallization and mechanical properties ^[48]^[49]. Jung et al. ^[50] improved the tensile strength and flexibility as well as barrier properties of the PLA by the incorporation of cellulose nanofibers (CNF) into the PLA matrix. CNF was first homogenized in a triethyl citrate/ethanol mixture and then blended with PLA to allow uniform dispersal. Specifically, the oxygen permeability was increased up to 47.3% (16.99 cm³·mm/(m²·day·atm)) with a loading of 1 wt% of CNF in the PLA matrix.

Other attempts have been made to improve the properties of PLA significantly through the introduction of different kinds of inorganic nanoclays, carbon-based nanomaterials, metallic nanoparticles, etc. ^[51]. Recently, Yang et al. ^[52] proposed a method for enhancing the barrier properties of PLA by incorporating a 3-aminopropyltriethoxysilane (APTES)-modified MgAl-layered double hydroxide (LDH) nanosheet into the PLA matrix through scraping the coating solution composed of APTES@LDH and PLA. The OTR of APTES@LDH (5%)/PLA and APTES@LDH(10%)/PLA hybrid films with thickness of around 60 μm are lower than the testing limit of the instrument (<0.005 cm³/(m²·day·atm)) and the WVTR of the APTES@LDH (10%)/PLA hybrid film is only 0.026 g/(m²·day·atm), 94.1% lower than that of the pure PLA film.

Recently, Prof. Sun’s research group developed a facile flow-induced coassembly technique to fabricate organic/inorganic hybrid nanocoatings containing a high concentration of well-aligned nanosheet, which mimics the structure of nacre ^[53]. They prepared a poly(vinyl alcohol) (PVA)/α-zirconium phosphate (ZrP) aqueous dispersion where the ZrP was exfoliated into single-layer nanosheets. Then, the PVA/ZrP was dip-coated on PLA films, and the coated substrate films were vertically hanged, followed by chemical crosslinking with glutaraldehyde (GA), during which a high-level orientation of the ZrP nanosheets was induced. As a result, the OTR and WVTR of the coated PLA film (20 μm in thickness) decreased from 845.6 cm³/(m²·day·atm) and 107.0 g/(m²·day) of the uncoated PLA film to 2.0 cm³/(m²·day·atm) and 22.5 g/(m²·day), respectively ^[54].

As PLA is a semicrystalline polymer, barrier properties could also be adjusted by tailoring their super molecular microstructure of the crystalline and amorphous phases. Bai et al. ^[55] reported a novel and simple strategy to greatly increase the oxygen barrier property of PLA by constructing parallel aligned shish-kebab-like crystals with the aid of a fibrillar nucleating agent. They demonstrate that the fibrillar nucleating agent can change the crystallization behavior of PLA from isotopic spherulitic crystals to unique shish-kebab-like crystals, and the shear flow during the compression molding can induce the parallel alignment of the shish-kebab-like crystals along the surface direction. More importantly, at the later stage of the crystallization, the growing lamellae are found to interpenetrate and tightly interlock with each other at the boundary regions of the shish-kebab-like crystals, forming a densely packed nanobrick wall structure and thus endow the PLA with an unprecedentedly low oxygen permeability. In order to explore the influence of the structure of the amorphous phase on the barrier properties of PLA, Marta Safandowska et al. ^[56] modified PLA with low molecular weight compounds, such as glycerol (Gly), triethyl citrate (TEC), and polyethylene glycol (PEG) by melt blending. It was found that the incorporation of small amounts of modifier (0.5–1.5 wt%) in the PLA matrix did not affect the degree of crystallinity but densified the packing of polymer chains, resulting in an improved oxygen barrier performance.

5. Polyhydroxyalkanoates (PHAs)-Based Biodegradable Polymers

Nowadays, the applications of PHAs in biodegradable packaging fields include containers, bottles, sheets, films, fibers, and coatings. Polyhydroxybutyrate (PHB) is one of the popular PHAs; however, the application of PHB has been limited due to its high production cost, brittle nature, and low thermal stability. To improve these physical adversities, copolymers known as PHBV or PHBH were obtained by insertion of 3-hydroxyvalerate or 3-hydroxyhexanoate units into PHB main chain, which are widely used due to their greater flexibility and wider processing temperature window ^[57]^[58]. Moreover, for PHBV copolymers, an extremely good balance of barrier properties towards oxygen and water vapor molecules is reported ^[59].

The blending of PHAs with other biopolymers is another effort to improve the overall properties of PHAs ^[60]^[61]. For example, poly(butylene succinate-co-adipate) (PBSA) and PHB blends have been comparatively evaluated for their mechanical and gas barrier properties, and the results showed that both OP and WVP decreased as a function of PHB content ^[62]. Unfortunately, PHAs are generally not miscible with polysaccharides and proteins due to the poor interfacial adhesion between them. To address this aspect, multilayer films can be prepared, which have fewer compatibility problems than those experienced in the fabrication of blend films. Eslami et al. ^[63] prepared a novel biodegradable sandwiching assembly of two outer layers of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and a core layer of thermoplastic starch (TPS) or maleated TPS or their blends with PHBV (80/20). The tensile test revealed that samples with a core composed of a mixture of maleated TPS and PHBV were the strongest, with a modulus as high as 178 MPa. The WVPR of the tri-layer structures was as low as 20.2 g/(m²·d), and the OP was below the detection limit of a bubble flow meter.

Recently, the addition of small amounts of inorganic fillers into the PHAs’ matrixes is a promising method to increase the mechanical and thermal performances of the PHAs ^[64]. These inorganic fillers act not only as reinforcing agents but also as nucleating agents, providing heterogeneous nucleation of the polymer and allowing for more rapid crystallization. Many inorganic nucleating agents have been studied so far, including multi-walled carbon nanotubes (MWCNT), terbium oxide, nitride, and so on. Yan et al. ^[65] used montmorillonite (MMT) as the nucleating agent for poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) and found that MMT played a heterogeneous nucleation role in the crystallization of PHBV, not only increasing the crystallization rate but also narrowing the crystallization peak width of PHBH. Compared with PHBH, the tensile strength and elastic modulus of MMT/PHBH biocomposite with 1 wt% MMT increased by 111.2% and 182.5%, respectively. In addition, the OP and WVP of the composite decreased by 43.9% and 6.9%, respectively.

6. Poly(Propylene Carbonate) (PPC)-Based Biodegradable Polymers

Poly(propylene carbonate) (PPC), a biodegradable polymer synthesized by alternating copolymerization of carbon dioxide (CO₂) and propylene oxide (PO), has attracted considerable interest in both academia and industry ^[66]. The polar carbonate linkages contained in the PPC backbone create a considerably closer arrangement of molecular chains and a greatly smaller free volume; thus, PPC has a relatively high density of 1.3 g/cm³. Pure PPC has good barrier performance, the OP (23 °C, 0% RH) and WVP (23 °C, 85% RH) of pure PPC is ~2.25 cm³·mm/(m²·24 h) and ~1.05 g·mm/(m²·24 h), respectively ^[67]. Therefore, PPC is a promising biodegradable polymer with potential applications in the packaging sector. Nevertheless, PPC is still not competitive enough in barrier properties compared with high-barrier polymers such as EVOH, PVA, and PVDC ^[68].

In recent years, in order to extend the application range of PPC, great efforts have been made to improve the mechanical, thermal as well as barrier properties of PPC, including chemically modifying its end group, blending it with other polymers (starch, PLA, PHBH, and PHB) or introducing inorganic fillers. A series of composite films of PPC/aluminum flake (ALF) with different ALF contents were prepared by Meng’s group ^[67]. The WVP and OP of the composite films decrease continuously, with ALF contents increasing up to 5 wt %, which are 75.2% and 32.4% of the pure PPC, respectively. In addition, the tensile strength and thermal properties of the PPC are also improved by the incorporation of ALF particles. Li et al. ^[69] prepared a series of PPC/organic modified layered double hydroxide (OLDH) composite films with different contents of OLDH via melt blending. The tensile strength and gas barrier properties of PPC are obviously enhanced with the incorporation of OLDH. Compared with pure PPC, the OP and WVP were reduced by 54% and 17%, respectively, while tensile strength increased by 83% with 2% OLDH addition.

7. Polybutylene Adipate Terephthalate (PBAT)-Based Biodegradable Polymers

Poly(butylene adipate-co-terephthalate) (PBAT) is a semicrystalline copolyester obtained from the copolymerization of 1,4-butanediol, adipic acid, and dimethyl terephthalate. Although derived from oil-based resources, it can be completely degraded with the help of natural enzymes. It possesses similar thermal and mechanical properties to polyethylene (PE) and is a flexible plastic suitable for film blowing ^[70]. However, the inferior gas barrier properties of pure PBAT limit its application as a barrier packaging material.

Many methods have been reported for PBAT modification, including blending with other low-cost biodegradable polymers and multi-layer coextrusion. As PPC is a biodegradable polymer with good barrier properties and is cheaper than PBAT, it is extensively used to blend with PBAT to tailor the properties of the polymer blends for diverse applications. To maximize the advantages of blends, interfacial modification of PBAT/PPC blends is needed to increase the interfacial interaction between PBAT and PPC. Zhao et al. ^[71] studied the properties of PBAT/PPC films with different component ratios and the addition of an epoxy compound ADR4488 as a chain extender on the properties of PBAT70/PPC30 blends. A reduction in the melt viscosity and a significant improvement in the water vapor barrier performance was observed with the introduction of PPC. The addition of ADR improved the compatibility between PBAT and PPC and thus increased the thermal stability and mechanical strength of the blends without affecting the water barrier property. Xie et al. ^[72] also investigated the effect of triglycidyl isocyanurate (TGIC) bearing three epoxy groups as a reactive compatibilizer for PBAT/PPC (50/50) blends. The PBAT/PPC/TGIC films with 1 phr TGIC prepared via blowing process exhibited a tensile strength of 30.8 MPa and an elongation at break of 860%, which were 136% and 142% of those of PBAT/PPC, respectively. Meanwhile, the water vapor transmission rate was reduced by 33% at this content, reaching 1.32 g·mm/(m²·24 h). Xu et al. ^[73] prepared PBAT/PPC/PBAT tri-layer films using the multi-layer coextrusion method. The tri-layer films showed good interface adhesion between PPC layer and PBAT layer, and the lowest oxygen permeation of 9.5 × 10⁻¹⁵ cm³·cm/(cm²·s·Pa) was obtained at a maximal PPC layer thickness of about 12 μm.

Incorporating inorganic nanosheets into the PBAT matrix is also employed to improve its barrier properties. Debeli et al. ^[74] reported PBAT/montmorillonite (MMT) nanocomposite films with excellent oxygen barrier properties, which were prepared by exfoliating the MMT in a water-dispersible sulfonated PBAT matrix. At higher MMT loadings of 28–32 vol%, a network-like MMT morphology was formed, and nanocomposite film with 32 vol% MMT exhibited an OP of 7.5 × 10⁻¹⁶ cm³·cm/(cm²·s·Pa) and a WVP of 1.57 × 10⁻¹¹ g·m/(m²·s·Pa). Compared with based PBAT, the oxygen barrier property of the PBAT/MMT nanocomposite was improved by two orders of magnitude.

8. Polycaprolactone (PCL)-Based Biodegradable Polymers

Polycaprolactone (PCL) is a flexible biodegradable polymer, which is obtained by ring-opening polymerization through a petrochemical pathway. Apart from being biodegradable, PCL has many advantages over other biodegradable polymers, such as low viscosity, easy processing, good solvent resistance, and good mechanical performance ^[75]. However, it is costly and also has a major drawback: it has a high permeability to gases and water vapor, making this polymer unsuitable for high-barrier packaging applications ^[76].

One strategy to reduce the permeability of PCL without sacrificing biodegradability is to blend it with another biodegradable polymer with good barrier character using classical routes (solution or extrusion) ^[77]. PPC is undoubtedly one of the best choices. Blend films of PCL and PPC were prepared using a uniaxial-stretching extrusion process by Cheng et al. ^[78]. The OPs of the blended film decreased by 42.7% and 64.6% compared with neat PCL when the PPC blending ratio was 20% and 50%, respectively, and the WVP decreased from 1.83 × 10⁻⁵ to 1.03 × 10⁻⁵ (g·m/(m²·d·Pa)) with an increase in PPC content from 0 to 20 wt%. In addition, the shelf life of button mushrooms packaged with PCL/PPC blending film is longer than that packaged with neat PCL films.

The properties of PCL and their blends can also be modulated by the addition of various nanofillers, such as clays, graphene, carbon nanotubes, and metallic oxide nanoparticles ^[79]. Recently, Bujok et al. synthesized trihexyl(tetradecyl)phosphonium decanoate ionic liquid (IL) functionalized ZnO nanoparticles (IL-ZnONPs) and layered double hydroxides (IL-LDH), and then microwave-assisted in situ ring-opening polymerization of ε-caprolactone (εCL) in the presence of dispersed IL-ZnONPs or IL-LDH was used for preparation of PCL/ZnONPs and PCL/LDH nanocomposites, which leads to the homogenous dispersion of nanofiller in the PCL matrix and the formation of large PCL crystallites, resulting in improved thermal, mechanical and barrier properties of the nanocomposite ^[80]^[81].

Glycerol tristearate (C18) has good gas barrier properties due to its hydrophobic nature and closely stacked lattice arrangement; hence, it is used as an additive to increase the barrier properties of PCL. The water vapor and oxygen barrier properties of PCL/C18 composites with 30% C18 content were reported to have increased by 81.4% and 39.2%, respectively ^[82]. However, due to the zero-dimensional dispersion morphology, the barrier performance was enhanced only at high C18 content with the sacrifice of its mechanical properties. Recently, Ding’s team reported an effective strategy for preparing PCL/C18 composites with a two-dimensional sheet-like C18 dispersion phase by multistage biaxial-stretching extrusion. Compared with conventional PCL/C18 blends with the same content of C18 (10%), the penetration path of the small molecules was significantly increased, resulting in the improvement of water vapor and oxygen barrier properties by 39.6% and 63.7%, respectively. Furthermore, when the relative humidity increased from 50% to 90%, the WVP of the sample only increased slightly from 6.94 × 10⁻¹⁴ to 7.66 × 10⁻¹⁴ g·cm/(cm²·s·Pa), exhibiting excellent stability to high humidity ^[83].

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