Effects of plasticizers on extrusion of PVC: an experimental & numerical study

Plasticizers are often interchanged with the idea that they will not affect the processing behavior of Polyvinyl Chloride (PVC). However, when the plasticizer type is changed, various complaints are made by the processors that the material no longer processes the same. This research was concerned with the effect of three different plasticizers on the plasticating extrusion behavior of PVC. Di-isodecyl pthalate (DIDP), di-hexyl pthalate (DHP) and 2-ethyl hexyl pthalate (DOP) are the three plasticizers used in this study. First some differences in the extrusion performance of the three differently plasticized PVC compounds were identified. In particular, it was observed that pressure build-up, flow rate and power requirement were affected by the plasticizer type with the DIDP plasticized materials generating higher pressures and requiring more power than the other two plasticized materials. The differences in extrusion characteristics have been observed for two different dies (1/8 and 1/16 inch diameter) attached to the extruder. The differences were most significant between the DIDP and the DHP plasticized mixes.

Factors which could influence the processing behavior of plasticized PVC include viscosity, compaction, thermal conductivity, specific heat, and friction coefficient. It was found that all other properties other than the viscosity were unaffected by the plasticizer type. On the other hand, viscosities were significantly affected by the plasticizer type with the DIDP plasticized materials displaying higher values between 160 and 190 °C. The difference in viscosity was larger between the DIDP and DHP plasticized materials than between DIDP and DOP plasticized materials. The differences in viscosity between DIDP and DOP plasticized materials tend to diminish considerably at 190 °C. Two flow regions characterized by different degrees of fusion above and below 165 °C were identified for the plasticized PVC compounds. Plasticized PVC exhibited yield stresses with the DIDP plasticized materials having higher values. The yield stresses were responsible for the significant difference in viscosity at lower shear rates. The yield stress was a more dominant feature at temperatures below 160 °C and this fact was made use of in modeling the solids conveying zone as a fluid with yield stress. Correlation was established between the viscosities and the extrusion behavior of the plasticized PVC compounds. It was observed that the DIDP plasticized mixes had higher viscosities, fused earlier in the screw channel, gave rise to higher pressures, required more power and in general exhibited higher flow rates at the same screw speed.

The finite element method was used for the numerical simulations. Based on the experimental results, the numerical modeling of the melt zone was performed in order to predict the differences in the extrusion characteristics. The melt zones were modeled as a temperature dependent power law fluid having two different viscosity expressions above and below 165 °C. The numerical predictions for pressures and flow rates in the extruder with the 1/8 inch diameter die were in good agreement with the experimental results. For the case of the 1/16 inch diameter die attached to the extruder, the numerical and experimental flow rates were in good agreement but the pressure predictions, although indicating the correct trends, were off by 15 to 20% from the experimental results. In general the differences in the physical properties, viz. viscosities, were used to predict the differences in the pressure build-ups and flow rates. Also the solid conveying zone was modeled using a Herschel Bulkley model. It was possible to match the experimental and numerical results for the solids conveying zone by using an average density value for the entire solids conveying zone, but more work needs to be done in order to establish greater validity and applicability of this model.