Designing green plasticizers

Plasticizers are additives in poly (vinyl chloride) (PVC) formulations that render the material flexible. This is important for many applications. Because these plasticizers are not bound to the polymer chemically, they will eventually leach out upon disposal. Considering also the widespread use of flexible PVC, it is not surprising that some plasticizers, such as di(2-ethylhexyl) phthalate (DEHP), are considered ubiquitous contaminants in the environment. Previous studies have shown that DEHP, upon degradation, forms stable, toxic metabolites. Because of this and other concerns, DEHP and other phthalates have already been banned in certain products in Canada and other countries. Hence, there is a strong incentive to develop new, green plasticizers.;The results demonstrated that esters based on succinic and maleic acids performed at least as well as or were even superior to DEHP as plasticizers. In particular, the esters with the longer alcohols were very good plasticizers. There was little effect due to branching on the plasticizer properties. The experiments with Rhodococcus rhodocrous showed how important the structure of the central diacid is for the rate of biodegradation. In particular, the maleates, which have an orientation of the two ester groups very similar to that in DEHP, showed little to no susceptibility to biodegradation over the course of 30 days. The fumarates exhibited some degradation and the succinates were degraded very quickly. These results indicate that the orientation of the esters in DEHP, is responsible for the stability of this compound in the environment. The other factor in the rate of biodegradation was the length of the alcohol and the longest chains had the slowest rates. However, all straight-chained alcohols were biodegraded without the build-up of stable metabolites. The compounds made with the branched 2-ethyl hexanol did result in the formation of stable metabolites.;Consequently, several of the tested diesters could be considered as "green". Yet, in terms of a middle molecule, the succinates should be considered as the best choice. As for side chain length, plasticizer properties improve with increasing alcohol length, and biodegradation properties improve with decreasing alcohol length. A potential candidate for a compromise would thus be dihexyl succinate.;A series of diesters based on maleic acid, which resembles a part of the phthalate chemical structure, was tested, along with other series based on the structural isomer of maleic acid, fumaric acid, and the saturated analogue, succinic acid. The alcohols used to form the ester bonds varied in length from ethanol to octanol and, also, included the branched 2-ethyl hexanol. Each of these diesters was incorporated into unplasticized PVC at about 30 weight-percent and then evaluated for plasticizer properties such as glass transition temperature Tg and tensile strength. These data were compared to each other and to results with DEHP. Pure samples of the diesters were tested for their biodegradability by the common soil bacterium Rhodococcus rhodocrous (ATCC 13808) while it was growing on hexadecane as a primary carbon source.