(423b) Designing Greener Plasticizers: Influence of Geometry of Central Group and Side Chains

Plasticizers are important additives to brittle polymers such as poly(vinyl chloride) (PVC). They are mixed with the resin to lower the glass transition temperature (Tg) and their blends result in soft and flexible materials. Yet plasticizers are not bound to the polymer matrix and will slowly leach out of the blend into the environment. Many studies have shown di (2-ethylhexyl phthalate) (DEHP) and some of its stable metabolites such as its monoester mono (2-ethylhexyl) phthalate (MEHP), 2-ethyl hexanol and 2-ethyl hexanoic acid to be toxic. MEHP is under further scrutiny as it is believed to be an endocrine disruptor. The above-mentioned compounds are generally considered ubiquitous environmental contaminants due to their slow breakdown by soil bacteria. This has led to efforts to design new, “green” plasticizers.

Small diester molecules were prepared synthetically by Dean-Stark esterification due to their resemblance to DEHP. These molecules were based on maleic acid (cis-isomer), fumaric acid (trans-isomer) and succinic acid (saturated analogue), and were esterified with alcohols ranging from ethanol (C2) to n-octanol (C8) as wells as 2-ethyl hexanol. Using a twin screw-extruder, these compounds were incorporated into unplasticized PVC at various concentrations (16% - 33% by weight) and subsequently tested for their plasticizer properties. The Tg reduction was measured using differential scanning calorimetry (DSC). Tensile test bars according to ASTM standards were prepared using a hot press, which were then tested for elongation at break and the secant modulus at a constant strain rate of 5 mm/s. Biodegradation tests were carried out with the pure compounds using the common soil bacterium Rhodococcus rhodocrous in shake flasks at 30°C. The medium consisted of minimum mineral salt medium, yeast extract, an auxiliary carbon source hexadecane and the plasticizer. The flasks were inoculated with 1ml of pre-grown culture, and whole shake-flask extractions were performed on a regular basis. The flasks were extracted using chloroform with added internal standard for further analysis by gas chromatography (GC). Using calibration curves for known compounds, plasticizer and metabolite concentrations were calculated. Unknown metabolites such as the respective monoesters were identified by analyzing synthesized standards.

The influence of the central structure was evaluated by comparing 2-ethylhexyl diesters of the proposed molecules to DEHP and another commercial plasticizer: di (2-ethylhexyl) adipate (DEHA). From Tg and tensile strength data, the fumarate compound was eliminated from the list of alternative “green” plasticizers due to poor Tg reduction and poor tensile strength data compared to DEHP. Both di (2-ethylhexyl) maleate and succinate did perform better than DEHP in terms of Tg reduction, and not statistically different in terms of tensile strength.

In the biodegradation experiments a strong influence of the central structure was found: the maleate compound showed almost no hydrolysis of either of the two ester bonds of the molecule over the course of 30 days, as observed similarly with DEHP. In both DEHP and di (2-ethylhexyl) maleate the esters are situated in cis-position towards another. The fumarate, in which the two ester bonds are in trans-position to one another, an increase in hydrolysis rate was observed, and assuming first-order kinetics, a half-life of 13 days was calculated. For both saturated molecules, the half-life dropped further, to about 6 days in the case of DEHA, and about 2.5 days in the case of the di (2-ethylhexyl) succinate. As expected, there was a buildup of 2-ethanol and 2-ethyl hexanoic acid, however the corresponding monoester was not observed for any of the compounds. In the case of the slow-degrading maleate this is explained by the fact that little of the diester was degraded. It was expected to see some monoester produced by biodegradation of the fumarate and succinate, however, this was not the case. It was concluded that the central structure played a crucial role in degradability of these compounds, and that a saturated molecule that is free to rotate about the central bond is more easily hydrolysed.

Using these results, the succinate compounds were further studied. Instead of the 2-ethylhexyl side chain, even-numbered unbranched alcohols were used to create the ester bond, from ethanol to n-octanol. All four compounds were better at lowering the Tg than DEHP at 29 weight-% of plasticizer in the blend. There was a trend to a lower Tg with increased side chain length, which in turn also meant increased overall molecule length. When the results for dihexyl succinate were compared to the branched di (2-ethylhexyl) succinate, it was found that their Tg at 29 weight-% in PVC were similar, suggesting that the ethyl branch did not play a role in Tg lowering.

Similar to the di (2-ethylhexyl) biodegradation experiments, a rough first-order kinetics approach was chosen to compare relative hydrolysis rates. A trend towards larger half-lives with increasing side chains (less than 1 day for diethyl succinate and about 3 days for di-n-octyl succinate) was observed. When comparing dihexyl succinate to di (2-ethylhexyl) succinate, a difference of about 1 day in half-life was observed, indicating a small steric effect of the ethyl-branch on hydrolysis, however this could also result from a lower solubility of the compound in the aqueous biodegradation broth. None of the succinate diesters with unbranched side chains showed a buildup of any kind of metabolite; in fact it could be shown that the bacteria preferentially grew on the plasticizer or respectively on its metabolites, such as the liberated alcohols. This was shown by running parallel experiments without auxiliary carbon source.

We have demonstrated the potential of small diester molecules to replace DEHP for plasticizing PVC.  Given these results, a compromise will have to be found between longer side chains needed to obtain good plasticizer properties, and short side chains for better biodegradability.  More mechanical testing and toxicity testing are needed to fully characterize these green plasticizers.

Erythropel, H.C., M. Maric, and D.G. Cooper, Designing green plasticizers: influence of molecular geometry on biodegradation and plasticization properties. Chemosphere, 2012. 86(8): p. 759-66.

Erythropel, H.C., et al., Designing green plasticizers: Influence of alkyl chain length on biodegradation and plasticization properties of succinate based plasticizers. Chemosphere, 2013. 91(3): p. 358-365.