Biodegradable polyesters continued

Poly(lactic acid) (PLA), a representative biodegradable polyester, has been studied intensively as shown in Table 1.1 for about 80 years. Studies on biodegradable polyesters such as PLA have increased considerably since the new millennium. Nowadays, aliphatic polyesters such as PLA, poly(caprolactone) (PCL), and poly(ethylene succinate) are commercially produced and their output continues to increase. Table 1.1 shows clearly that while the number of publications on PLA as well as PET has been on the increase, the relative ratio of publications on PLA to PET is quite high. 

A very novel study that could be of importance for the future is that of functionalized cellulose nanofibers and nanocrystals blended with biodegradable polyesters and acrylic acid polymers (Winter and Bhattacharya, 2003). The nanocrystals were found to be markedly superior reinforcing agents than wood flour, and their behavior was similar to the exfoliated clays in terms of reinforcing properties (Winter and Bhattacharya, 2003). These continued new developments bode well for the future role of plant fiber based composite materials. 

Over 250,000 metric tons of microcrystaUine cellulose have been sold siace its commercialisation ia 1962 and demand continues to iacrease. Its utihty has led to development of other coUoidal polymer microcrystals (see Colloids). For example, polyamides and polyesters from recycled materials can be biodegraded to give microcrystals having a size of 30 nm (37). 

Depending on the nature of the polyester used, we obtain different results. However, biodegradable starch-polyester blends are generally not compatible with phase separations and interfacial properties which tend to be poor. Co-continuous or gramrlar-type structures can be obtained, depending on the starch lyester ratio. 

The control of biodegradation rate is of critical importance for many applications of degradable polymers. Amorphous polyesters absorb water and hydrolyse much more rapidly than crystalline materials. Consequently, in partially crystalline polymers, hydrolysis occurs initially in the amorphous phase and continues more slowly in the crystalline phase. This selective degradation leads to an increase in crystallinity by chemicrystallisation. A very similar selective abiotic oxidation process occurs in the semi-crystalline polyolefins which fragment rapidly due to failure at the crystallite boundaries. 

Maleic acid acts as a transesterification catalyst. The composition can be produced continuously in a twin screw corotating extruder. The starch-polyester graft copolymer can be solvent cast, melt cast and blown into clear transparent film particularly for use in single use disposable applications and can be biodegradable (32). 

PHB has many physical properties in common with poly(propylene), and a PHB-PHV copolymer (BIOPOL) has recently been used to manufacture plastic shampoo bottles. PHB-PHV is of special interest because it is biodegradable. Since it is a naturally occurring polymer, it is easily degraded by enzymes produced by soil microorganisms and therefore does not persist in the environment after disposal. Other biodegradable polymers, such as polyesters derived from e-caprolactone and lactic acid, are also known and have been commercialized. Although it remains to be seen how widespread the use of biodegradable plastics will become, research and development of these materials is sure to continue as we try to deal with contemporary environmental issues. ... 

---