Figure 6 Release behaviour of DEHP from PVC film at 27°C.

 Figure 6 depicts the release profile of DEHP from PVC films at 27ºC. The cumulative amount of DEHP increased linearly, showing a constant release of plasticizer over 72 hours. This release property has an adverse effect on plastics; as the plastic loses its plasticizer, flexural performance decreases. Prolonged leaching also poses risk to human health and the environment.

Figure 7 Weight loss of PVC films after extraction testing.

Figure 7 shows the weight losses of PVC films by extraction in water. The extraction from the resin was strongly dependent on the water solubility of the plasticizer. The diblocks are made soluble by their MePEG components, while the ECTO and DEHP are highly hydrophobic. Though water solubility facilitates biodegradation, this property is not suitable for certain PVC applications such as intravenous tubing.

Figure 8 Dimension change of PVC films at the 40% plasticizer concentration.

 Figure 8 depicts dimension change with increased force for PVC plastics containing 40% by weight of the plasticizer, with unplasticized PVC as the control group. The unplasticized PVC had minimal dimension change and was quite brittle. The effect of adding either DEHP, diblocks, or ECTO was to increase the elasticity and flexibility of the films. This was most effective by adding ECTO, which showed significant superior performance to that of DEHP, the commercial product. PLA-MePEG was less effective but still showed plasticizing effects.

Figure 9 Sample graph generated by TA Universal Analysis showing dimension change for films with ECTO and DEHP at the 10% concentration.

Figure 9 shows dimension change for samples with 10% of DEHP and ECTO. At this concentration, ECTO samples were more flexible than those of DEHP by as much as 700%.

Figure 10 Glass transition temperature of PVC films.

Figure 10 shows glass transition temperature (Tg). As a general trend, an increase in the amount of plasticizer added resulted in a lower Tg.  A plastic assumes its glass phase below its Tg and flexible phase above its Tg, and thus a lower Tg is desirable. ECTO had the lowest Tg overall and was the most effective alternative in lowering Tg. PCL-MePEG performed better than DEHP in samples of 30-40%.

Figure 11 Sample graph generated by TA Universal Analysis showing calculation of glass transition temperature for unplasticized PVC.

Figure 11 shows the sample graph generated by the software program TA Universal Analysis, showing calculation of glass transition temperature for unplasticized PVC. Glass transition temperatures were derived by calculating the point at which the slope undergoes the most dramatic change.

DISCUSSION

Plastic samples were tested for extraction resistance, dimension change under force, and glass transition. For extraction resistance, it was found that samples with ECTO showed the least weight loss after immersion in water. The two diblocks showed the least resistance to extraction. These are to be expected as ECTO is hydrophobic, and the two diblocks have hydrophilic MePEG tails. The two types of plasticizers (hydrophobic and hydrophilic) can be used in different applications. For example, intravenous tubing and water transportation pipes require hydrophobic plasticizers. Hydrophilic plasticizers, however, are advantageous in that they lend themselves to biodegradation much more so than a non-water soluble additive.

Dimension change is likely to be the most significant test, as DEHP is primarily used to impart flexibility to the otherwise rigid PVC polymer. From our tests it was found that films with ECTO exhibited the greatest dimension change, even greater than those of DEHP. Figure 9 shows that at the 10% concentration at a force of 0.15N, the ECTO plasticized films exhibited a 700% greater dimension change than the DEHP plasticized films. A T-test, confirmed that at all concentrations: 10%, 20%, 30%, and 40%, ECTO was of significantly higher performance than DEHP.

In our 2006 study, we found the unepoxidized Carthamus tinctorius oil to be an effective plasticizer. Though it appeared to likely be a viable alternative, it did not reach the same levels of performance as did DEHP. This year, we opted to chemically alter the oil, in hopes of raising performance and achieving a dual function of heat stabilization. Epoxides are known to be good heat stabilizers. We did this by epoxidizing the oil via a chemo-enzymatic reaction. ECTO has a high proportion of polar versus non-polar groups and so has greater solvating power for PVC, enabling it to work as an effective plasticizer. Epoxidation is achieved by reacting ethane with a peracid and hydrogen peroxide. For industrial scale production, a peracid, formed from a short chain fatty acid and hydrogen peroxide under strong acidic conditions, is used as the oxidizing agent. The problem with this manufacturing process, however, is that the strong acid used can cause the formation of unwanted chemical products such as vicinal diols, estolides, and other dimers. In our study, a safer, more environmentally-friendly was used. The use of a chemo-enzymatic reaction for the purposes of epoxidation was first suggested by Warwel and Klass.⁸ We used a lipase to react with the unsaturated fatty acid, linoleic acid (which comprises 95% of Carthamus tinctorius oil), changing the acid into a peracid when it is in contact with hydrogen peroxide. The peracid then loses an oxygen atom which forms an epoxy with its own double bond. Since linoleic acid has two doubles bonds, it allows a maximum of two epoxy groups to be formed. The complete epoxidation of linoleic acid yields 9-10, 12-13-diepoxy stearic acid, while its incomplete epoxidation results in 9-10-monoepoxy 12-octadecenoic acid or 12-13-monoepoxy 9-octadecenoic acid. (See diagram below). Such a lipase mediated reaction is both more efficient and safe than is the conventional method. Through HPLC, we determined that the percentage of epoxidized double bonds was approximately 18%.

 
                                  Camocho, Samuel et al., Eu. J. Lipid Sci. Technol., 2005.                                                                                        

Figure 12 Reaction mechanism of the chemo-enzymatic epoxidation reaction.

Our third performance test was on the glass transition temperature of our plastics. Glass transition temperature is indicative of the phase at which the plastic loses its rigid glass properties and begins to behave as a flexible plastic. ECTO performed the best overall, while DEHP and PCL-MePEG showed similar trends. ECTO is effective probably because its high number of non-polar groups can cause reduction in polar forces between polymer chains, thus lowering Tg, and improving low temperature flexibility.

SOURCES OF ERROR

The epoxidation reaction was primarily designed for chemically altering pure linoleic acid. However, since only 95% of Carthamus tinctorius oil is comprised of unsaturated fatty acid, the chemo-enzymatic reaction we performed may have chemically altered other substances in the oil. This could potentially have hindered or improved the plasticizing effects of ECTO. To improve this, samples with epoxidized linoleic acid could be compared with those using ECTO.

One problem with diblock films is that the polymer and the plasticizer did not appear to be completely miscible by visual examination. This might have affected the dimension change and the glass transition temperature results since the films were randomly cut and different sections of the films had differing amounts of PVC to plasticizer ratios. This causes a problem because the diblock and the PVC have different physical properties.