Green Packaging Material - Biodegradable Polymers

In recent years, there was growing interest in worldwide environmental protection. In this sense, green packaging is an aspect of great importance in order to reduce the impact of waste and pollution, and to promote sustainable development. Green packaging—also known as ‘eco-green packaging’, ‘eco-friendly packaging’, ‘sustainable packaging’ or ‘recyclable packaging’. Among most material for green packaging, much attention has been paid on the biodegradable polymers from renewable resources in recent year. 

Biopolymers can be used to substitute non-biodegradable plastics to reduce the environmental impact and petro-dependence. As alternative bio-packaging materials, biopolymers allow the packaging materials to be biodegradable or compostable completely. Biodegradation of biopolymers involves the hydrolytic or enzymatic cleavage of bonds in the polymer. Biodegradation is often defined as an event which occurs via the action of enzymes and/or chemical decomposition associated with living organisms (bacteria, fungi, etc.). It is worth noting that the other processes like photodegradation, oxidation and hydrolysis may also have impact on the structure and chains of polymers prior to or during biodegradation. There are several types of biodegradable polymers in the market at the moment. The characteristics of each polymer is detailed as follows.

Starch is the most abundant and renewable polysaccharides in plants. Native starch consists of two types of glucose polymers, namely amylose and amylopectin. While amylose is a chain of d-glucose unit that is connected together by α-1,-4 bonds, the amylopectin contains short chains of α-1,-4 linked d-glucose units that are branched by α-1,-6 bonds. Although starch is a biodegradable polymer that can be manufactured in large quantities at relatively low cost, handled easily and form film products with low oxygen permeability, the major challenge with native starch is that, it is brittle and hydrophilic. These limit its various applications such as its use for the manufacturing of plastic bags and food packaging. To enhance its flexibility and improve the easiness for processing or plasticizing starch, various plasticizers (glycerol, glycol, sorbitol) are employed to convert the starch into thermoplastic starch (TPS) via the application of heat and shear over extrusion processes.

PLA is a biodegradable polyester obtained from lactic acid during the fermentation of renewable crops such as sugar beets and corn. This polyester has attracted attention because it is readily available and cheap. Lactic acid is usually synthesized from either bacterial fermentation or from synthesis of petrochemicals. PLA is prepared from the condensation polymerization of D- or l-lactic acid or from the ring-opening polymerization of lactide monomer from lactic acid. The PLA with low molecular weight is manufactured by the direct polycondensation of lactic acid. On the other hand, high molecular weight PLA is produced from the ring-opening polymerization and it exhibits better mechanical properties. Moreover, the high molecular weight PLA has also been generated via the azeotropic condensation polymerization of lactic acid. PLA has been widely accepted as biodegradable polymer for packaging materials owing to its stiffness, transparency, processability and biocompatibility. When compared with other biopolymers such as PHA, PEG and PCL, PLA exhibits a better thermal processability, which allows for the various processing methods of PLA such as injection molding, blow filming, cast filming, fiber spinning, thermoforming, etc.. Furthermore, PLA presents a medium level of water and oxygen permeability that is comparable to that of polystyrene. However, PLA has a low resistance to oxygen permeation, and it is also brittle with less than 10% elongation at break. These properties limit its applications that require plastic deformation at higher stress levels. To address this challenge, blending PLA with highly exfoliated clay in conjunction with thermoplastic starch has proven to satisfactorily produce materials with improved mechanical strength. The bending of PLA with PHB by melt blending has been considered to be a facile way to improve the properties of PLA. When PLA was blended with PHB, there was an improved oxygen barrier, water resistant and mechanical properties of the polymer as compared with pure PLA. This demonstrated a valuable option for food packaging application.

PCL is a thermoplastic biodegradable polyester with good thermal processability, low melting point and low viscosity. It is synthesized by the polymerization of ε-caprolactone. Because of the weak barrier properties and poor mechanical properties of PCL which are attributed to its low melting point, PCL application as a biodegradable polymer in the packaging industry is limited. To increase the scope of its applications, PCL is usually blended with other polymers (e.g., cellulose propionate, polylactic acid, and cellulose acetate butyrate) to improve stress crack resistance, dyeability and adhesion.

Another promising material applied in packaging, medical and agriculture sector is PHA, which is a polyester of various hydroxyalkanoates that is synthesized from microbial fermentation. PHAs are non-toxic and crystalline thermoplastic elastomers with lower melting point. They are biocompatible with good UV resistance, physical and chemical properties. These properties depend on the PHA monomer compositions. PHAs application is constrained due to its poor mechanical properties, incompatibility with conventional thermal processing techniques, as well as their susceptibility to thermal degradation.

PHB is the most common representative of PHA with a high degree of crystallinity. It has been equally considered for short-term food packaging applications. It has the advantage of biodegradability by the action of PHA hydrolases and PHA depolymerases forming (R)- and (S)-hydroxybutyrates and nontoxic compounds under aerobic and anaerobic conditions. PHB with crystallinity up to 70%, exhibits remarkable mechanical properties like polyethylene. Furthermore, PHB is suitable for food packaging applications owing to its lamellar structure which contributes to its superior aroma barrier properties with water vapor permeability. PHB has a comparable melting point temperature with PLA, and thus allows for the blending of both polymers in their melted state. However, the poor mechanical performance and melt processing behavior of PHB, i.e., high brittleness, low thermal stability and difficult processing, along with insufficient barrier properties, limit its widespread use. Many attempts are being made to improve its properties for packaging application. Plasticized PLA-PHB blends incorporated with catechin were prepared by melt blending. PHB-added materials improved the mechanical properties of the plasticized PLA-PHB blends, which showed potential as biobased active packaging for fatty food.

Through the widespread use of biodegradable materials, we could reduce the amount of waste, lower the greenhouse gas emissions and ensure the sustainable use of environmental resources.