Extrusion processing temperature

Miscibility can be predicted by locating the minimal viscosity point. This method can also provide the minimum hot-melt extrusion processing temperature. 

FIGURE 17.9 Effect of endothermic CFA (BIH40) content on the density of amorphous PLA 8302D foamed at 40 rpm. The extrusion processing temperatures were set at 180-180-175-170°C from the hopper to the die [86]. 

Figure 13.6 shows the influence of temperature on specific volume (reciprocal specific gravity). The exaet form of the eurve is somewhat dependent on the crystallinity and the rate of temperature change. A small transition is observed at about 19°C and a first order transition (melting) at about 327°C. Above this temperature the material does not exhibit true flow but is rubbery. A melt viseosity of 10 -10 poises has been measured at about 350°C. A slow rate of decomposition may be detected at the melting point and this increases with a further inerease in temperature. Processing temperatures, exeept possibly in the case of extrusion, are, however, rarely above 380°C. 

Additives such as antioxidants and photostabilizers of low-molecular weight face two major problems (1) they may evaporate during high temperature moulding and extrusion process or (2) they may migrate to the surface of the plastic and get extracted. There are, in general, three ways of overcoming these problems. 

Preliminary tests were made on melt mixed blends of PP and LCP to study the effect of processing temperature on the shape and size of the dispersed LCP phase. Extrusion experiments were made below as well as above the melting temperature of the LCP. Two different polypropylenes were tested to determine the effect of the viscosity of the matrix on the final morphology. 

Contamination problems act as a barrier to the recycling of PET bottle waste. The presence of impurities that generate acid compounds at the high temperatures reached during the extrusion process prior to blow molding is a major problem in the reprocessing of PET because chain cleavage reactions are acid catalyzed. EVA... 

Thermal quenching of the crosslinking reaction In an actual reactive extrusion process, the degree of crosslinking can be controlled by adjusting the residence time at elevated temperature. 

Incorporated in PP, P.R.48 4 often causes aging, which makes the plastic brittle and precludes the use of P.R.48 4 in this medium. The effect on PE is less noticeable. The shrinkage of PE extrusion products at higher processing temperatures is practically unaffected by P.R.48 4. 

Fiber orientation uniformity is also affected by small-scale or timewise variations in polymer viscosity, related to breakage of polymer chains during the extrusion process. The degradation occurs as a result of residual moisture that immediately reacts to break chains, and by thermal degradation that occurs more gradually over time. Different residence times and temperature histories within the laminar flow streamlines lead to different viscosities, and hence different average orientation levels in the different fibers. 

Performing numerical simulations of the extrusion process requires that the shear viscosity be available as a function of shear rate and temperature over the operating conditions of the process. Many models have been developed, and the best model for a particular application will depend on the rheological response of the resin and the operating conditions of the process. In other words, the model must provide an acceptable viscosity for the shear rates and temperatures of the process. The simple models presented here include the power law. Cross, and Carreau models. An excellent description of a broad range of models was presented previously by Tadmor and Gogos [4]. 

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