| dc.description.abstract | The migration of plasticizers from flexible PVC products has caused raising concern over the last decades. Phthalates are shown to induce certain types of cancer and damages to the reproductive system in rodents. It is still unclear whether the effects are relevant to humans. Because of the clear adverse effects seen in animal testing, several European countries have imposed restrictions on the use of phthalates in for example children's toys. Migration of plasticizers causes contamination of the medium in contact, as well as deterioration in mechanical properties of the plastic.
An overview of the mechanisms governing migration of plasticizers, and proposed solutions to minimize the problem are given.
The migration of plasticizers from plastisol based flexible PVC was investigated. The variables were plasticizer type, level and composition, PVC system, heat stabilizer level, curing conditions and conditioning. The migration characteristics were investigated using migration to paraffin oil at 23 and 70°C, migration to unplasticized PVC at 70°C and extraction with isopropanol at 23°C
The migration of plasticizers to paraffin oil provides the diffusion coefficient for the diffusion of plasticizer within sheets of plasticized PVC. The results have relevance for the determination of migration rates of plasticizers in applications that exhibit diffusion controlled migration. The diffusion coefficients show large variations with plasticizer type and plasticizer level. The temperature dependence is significant.
The diffusion rate at room temperature seems to be solely determined by the plasticizer-PVC compatibility. The diffusion coefficients have very good correlations with various compatibility predictors, such as solubility parameter, dielectric constant, gelation temperature and plasticizer efficiency. The effect of the compatibility is temperature dependent. At 70°C, the compatibility dependency is much less pronounced. The likely explanation is that the diffusion coefficient consists of a contribution from polymer segment dislocations and plasticizer-polymer interactions. The two contributions have different temperature dependency. At low temperatures, the interaction term is dominating. At higher temperatures, the interaction term becomes less significant, and the diffusion rate is determined by the size of the plasticizer molecule as well.
The migration of plasticizers from plasticized PVC (pPVC) samples to unplasticized PVC (uPVC) was investigated at 70°C. The total migration rate is determined by a combination of the diffusion rate in the pPVC and in the uPVC. The effect of the diffusion in the uPVC seems to be dominating. The extent varies with plasticizer type.
The migration rate to uPVC at 70°C is mainly determined by the molar volume of the migrating plasticizer, and is well correlated with plasticizer volatility.
The fraction of plasticizer extracted with isopropanol at 23°C shows large variations with plasticizer type. The results are very well correlated with the results from migration to paraffin oil at 23°C, and are thus also correlated with plasticizer-PVC compatibility.
Isopropanol is suggested as a food simulant for the evaluation of plasticizers under consideration for food contact applications. If results from isopropanol tests correlate with migration to foodstuff, it can be concluded that plasticizers with good compatibility with PVC will be preferred in these applications.
The effect of curing time and temperature on migration characteristics was investigated. For standard oven curing of plastisols, it was found that as long as the temperature is sufficiently high, increasing the heating time has a more pronounced effect on the migration characteristics than a further increase in temperature. Oven-fused samples obtain an almost 50% reduction in diffusion coefficient when the heating time is increased from 2 to 10 minutes. The diffusion coefficient of the diffusion of plasticizers at room temperature, as measured by migration to paraffin oil, seems to give a good picture of the degree of fusion. Comparing pPVC samples cured in an oven with roll-milled samples, it was seen that mechanical mixing accelerates the fusion of plasticizer with PVC.
The migration of plasticizer to paraffin oil is greatly affected by the plasticizer level at 23°C as well as at 70°C. The diffusion rate of plasticizer is reduced by a factor of 5-10 when the plasticizer level is reduced by 10 phr. Decreasing the plasticizer concentration causes a reduction in the fraction of extracted plasticizer with isopropanol as well. The total migration rate to uPVC at 70°C is also dependent on the plasticizer level. This lead to the conclusion that the diffusion in the plasticized PVC sheet influences the total migration rate, although the diffusion in the uPVC is more important. The application of a correction factor for the diffusion coefficient for the diffusion in uPVC could to a certain extent account for the effect of plasticizer level.
A remarkable decrease in the diffusion rate of plasticizer in pPVC at 23°C was seen when a certain fraction (5-10%) of the primary monomeric plasticizer was substituted with a polyester plasticizer. A reduction of 80-97% was observed. The effect was temperature dependent, and at 70°C even a small increase in diffusion rate was seen. A decrease in the fraction of extracted plasticizer was seen for the extraction with isopropanol as well.
The reason for the dramatic reduction in diffusion rate at room temperature is somewhat unclear. It is believed that the total compatibility of the system increases by the introduction of the polyester plasticizer. Maybe the primary plasticizer interacts more strongly with the polyester than with PVC, and that the plasticizer has enough internal energy at 70°C to overcome the plasticizerpolyester interactions. A relatively strong interaction of the monomeric plasticizer with the polyester may inhibit the diffusion of the primary plasticizer when a driving force for the diffusion is introduced.
An interaction effect was also seen when two monomeric plasticizers were mixed. Mixtures of a plasticizer with high diffusion rate and one with low diffusion rate had a total diffusion rate that was much lower than a linear mixing rule should predict.
The interaction effect was also seen when an acrylonitrile-butadiene copolymer with carboxylic acid end groups was introduced. By the addition of calcium oxide, which probably deprotonized the acid group on the copolymer, the diffusion rate was reduced by 87% when only 5% of the primary plasticizer was replaced with the reactive copolymer. The effect was observed at higher temperatures as well. This may indicate that the interaction between the primary plasticizer and the ionic copolymer is stronger than dipole-dipole interactions between a primary (monomeric) plasticizer and a polyester. | nb_NO |
