Natural plastic aliphatic polyesters

Solid waste discharges from chemical plants can represent very large problems, especially from paper mills, plastics plants, and food processing plants. Some materials do not decompose in the environment, and can become burdens when they accumulate. Some polymers have backbones that degrade in nature, such as aliphatic polyesters and polyvinyl alcohols others do not, such as polyethylene and polystyrene. 

Biopolymers such as aliphatic polyesters are now produced on a semi-commercial scale by numerous companies that make biodegradable plastics [7]. Aliphatic polyesters have repeating units that are bonded via ester linkages and these natural esters can be degraded by enzymes (esterases) that are ubiquitous in nature... 

Polylactic acid or polylactide (PLA) is a thermoplastic aliphatic polyester that can be derived from renewable resources, such as corn starch or sugarcanes. Although PLA has been known for more than a century, it has become of great commercial interest in recent years because of its renewability and degradability to natural metabolites. In addition, the properties of PLA can be varied over a wide range which makes it suitable to be used as a substitute to many petroleum based commodity plastics, such as polyolefins,... 

There is strong evidence that the microbial biodegradation of synthetic polyesters in nature is initially caused by hydrolases secreted by microorganisms. Besides some natural polymers [e.g., poly( 8-hydroxybutyrate) (PHB)], synthetic aliphatic polyesters were mainly used in the past to produce biodegradable plastics [e.g., poly(e-caprolactone) (PCL)]. The degradation of polyesters by microorganisms is initiated by extracellular hydrolases, which are secreted by the organisms to reduce the molar mass of the polymeric substrate and to make it bioavailable [3]. 

The requirement, that all components of a plastic-composition must be degraded is of essential relevance for copolymers and especially for aliphatic-aromatic copolyesters. Here in one polymer chain structures are combined, which differ significantly in their degradation behaviour when the monomers are located in the corresponding homopolymers (aliphatic polyesters often are easily biodegraded while aromatic polyesters such as PET are quite biologically resistant). In this particular case it has to be ensured that no domains within the copolyester chains are poorly biodegradable and can accumulate in the nature. 

Aliphatic polyester layered silicate nanocomposites based on poly(e-caprolactone) (PCL) and on plasticized poly(L-lactide) (PLA) have been prepared first by melt blending of the respective polymer matrix with different (organo-modified) montmorillonites. It has been demonstrated that melt blending with organo-modified clay such as Cloisite 20A, 25A or SOB, yields intercalated nanocomposites with the possibility of partial exfoliation. Even at low organoclay content, substantial improvement of thermal stability, gas barrier properties and physical-mechanical performances have been noticed. However, melt blending of natural montmorillonite with PCL or neat (non plasticized) PLA leads to microphase-separated compositions. 

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