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The effect of thermal history on flow simulation of LCPs.


Liquid crystal polymers (LCPS) are becoming the resins of preference for high performance applications such as those in the electrical/electronics market. Low viscosities and high flow length of LCPs in conjunction with post-molding properties make these materials exceptionally suitable for injection molding of intricate parts. In order to ensure proper filling of injection molded parts, design engineers utilize flow analysis software. With respect to LCPs, the predicted flow patterns and the last point to be filled are qualitatively correct, but the pressure required to fill the cavity is often grossly over-predicted.

Many assumptions are incorporated into flow analysis software in order to reduce the degree of complexity of problems associated with flow. The common assumptions are that melt is inelastic and incompressible, and that its properties are time-independent. These assumptions in conjuction with the reduction of the actual geometry into finite elements and the accuracy of the data on the material's properties determine the validity of the simulation. Dominant Factor The most important material property affecting flow is melt viscosity, which is a function of shear rate and temperature. In a flow simulation of polymers, the appropriate values for the viscosities of the fluid are usually calculated from either a mathematical model that has an equation including shear rate and temperature as the dependent variables or a matrix of viscosities measured at different shear rates and temperatures. The pressure-dependence of viscosity, however, is generally overlooked.

Highly crystalline polymers are normally processed at temperatures above the range over which their crystals melt. This processing temperature is used as the midpoint for the temperature range over which the viscosity is measured. Conventional polymers have a constant viscosity at low shear rates but have a decreasing viscosity at moderate to high shear rates. Typically, the logarithm of the viscosity of these polymers is proportional to the reciprocal of the temperature over their processing range. Wissbrun, et al (1987), and many other researchers have shown that LCPs behave differently. For example, the shear-thinning region of the viscosities of LCPs extends to very low shear rates. The existence of this extended shear-thinning region allows the viscosity, as a function of shear rate, to be modeled quite accurately by the popular power-law viscosity model.

Another observation reported by Wissbrun, et al, is that the thermotropic nature of LCPs allows them to flow at temperatures below their DSC endotherm peak. From an engineering perspective, this means that they can be injection molded below their DSC endotherm peak, and that melt viscosity can be measured at these low processing temperatures.

For example, a 30% glass-filled phydroxybenzoic acid (HBA)/6-hydroxynaphthoic 2-acid (HNA) copolyester LCP has a DSC endotherm from 263[deg.]C to 285[deg.]C, with a 277[deg.]C peak Fig. 1). Yet, viscosity measurements can be obtained for this material over a temperature range of 240[deg.]C to 340[deg.]C (Fig. 2).

The only problem is that the viscosity-temperature relationship of these materials shows a lack of continuity below the high end of the DSC peak, making it difficult to be modeled with one powerlaw/Arrhenius model over the processing-temperature range. The discontinuity in viscosity as a function of temperature can be seen in the temperature range of 280[deg.]C to 300[deg.]C.

Attempts were made by Hoechst Celanese in 1987 and Hoechst AG in 1990 to use empirical evidence to describe the rheological behavior of 30% glass-filled HBA/HNA copolyester LCP by normalizing the power-law/Arrhenius model. Partial success was obtained in both cases, but no generalization could be made. Also, software packages employing a matrix of viscosity data were used by Polyplastics in 1989 to overcome this problem by an interpolation technique. Yet, in many cases, we found that pressure was still being over-predicted. New Considerations Viscosity is usually measured after the material has been heated in the rheometer from room temperature to a particular target temperature. This is provided that the material has not been recently subjected to temperatures higher than the target temperature. Likewise, the recent shear history of the material must have been fairly uniform. However, during the filling stage of the injection molding process, the material undergoes a complex heat and shear history.

After the material is melted it is fed into the mold at a higher temperature than its melting range and is subjected to relatively high shear rates. As the material travels into the mold, it begins to cool, and the shear rate decreases. Occasionally, the core Melt-temperature increases as a result of induced shear heating. Hence, flow simulations that use currently available software in conjunction with viscosity data that are obtained through the above technique will prove to be accurate provided that the material's viscosity is not a function of temperature and shear-rate histories.

Unfortunately, the viscosities of LCP materials are functions of both temperature (Wissbrun, 1980) and shear rate histories (Cogswell, 1980). There have been some conflicting data on the influence of thermal history on the viscosity of LCPs in their DSC endotherm range (Done and Baird, 1987; Lin and Winter, 1988). Recently, Done and Baird reported on the time-dependence of temperature history on the rheological properties of HBA/HNA copolyester LCP, the base resin of our previous example.

Further investigation on the effect of temperature history on 30% glass-filled HBA/HNA copolyester LCP has been conducted by Hoechst Celanese. The results of this investigation were then reproduced with different lots of this material by Polyplastics (Japan) and Hoechst AG. The viscosity of the material was measured by heating it from room temperature to 280[deg.]C and 290[deg.]C. In sequence, the viscosity of the same material was measured after cooling the material from 300[deg.]C to 290[deg.]C and 280[deg.]C. The measured viscosities are shown in Fig. 3. The viscosity of the material that was exposed to a temperature of 300[deg.]C and then cooled has a much lower viscosity than the material that was heated directly to the measurement temperature. The results from a similar procedure that used a measurement temperature of 270[deg.]C and a high temperature of 310[deg.]C are shown in Fig. 4.

Also determined was the thermal stability of the melt viscosity of various lots of this material in order to eliminate degradation as the reason for the decrease in viscosity. The viscosities at 270[deg.]C and 295[deg.]C were measured after the material was maintained at these temperatures for 25 to 30 minutes, respectively, which was the time required to change temperature in the experiments discussed above. The results for stability are presented in Figs. 5 and 6. The change in viscosities over the 25- and 30-min intervals is insignificant in comparison with the changes brought about by varying the material's thermal history. Recommendations The shift in viscosity due to the thermal histories experienced by LCPs is a contributing factor to the overprediction of filling pressure by the CAE analysis software. Other contributing factors, such as shear histories, are still being investigated.

The CAE engineer should be aware of the hysteresis of the rheological properties of LCPs when designing for different processes. Until now a general model has not been developed that includes all the contributing factors affecting these properties. In light of this we can only recommend, in mold filling analysis, the use of viscosity data that have been obtained from cooling methods as described is this study. Acknowledgment The authors gratefully acknowledge the technical assistance of R.E. Hooper and J. Lawler in reviewing and commenting on this technical treatise.
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Title Annotation:Rheology; liquid crystal polymers
Author:Mekkaoui, Abdallah M.; Kobayashi, Hiroyuki; Eiden, Gunter
Publication:Plastics Engineering
Date:Mar 1, 1991
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