Contact resistance, polymer thermal

Seals, o-rings, and valve membranes must be manufactured out of polymers that are resistant to thermal sterilization and to the bases and acids used in the biopharmaceutical industry. When in contact with the product, the elastomers employed must additionally present the following safety characteristics the rates of release of polymer components must be low and remain below well-established threshold levels the components released must be innocuous to human health, according to rules previously established. 

In the guarded hot-plate method, a sheet of the material is sandwiched between a metal heater plate and a heat sink plate. The temperature differential across the thickness of the specimen is measured when the system is at equilibrium. This technique is well suited for measurements of solids, particularly in cases where orientation exists within the test specimen. It is unsuitable for measurements of molten polymers. Thermal contact resistance is a problem that must be overcome with such measurements. 

There are a number of difficulties that may be encountered in measuring thermal conductivity in the solid and melt states. Problems such as thermal contact resistance and shrinkage are intrinsic to the polymer system and appear regardless of the method used for the measurements. 

The above equation should be used with caution, however, because it does not account for the quality of interfacial contact between the plastic and the filler system. Poor interfacial contact has the same effect as a thermal contact resistance and can result in a significant lowering in the ability of the highly conducting filler particles to transmit heat to the low-conductivity polymer matrix. What complicates the matter further is that these systems may possess good interfacial contact while the polymer matrix is molten but then become lower in thermal conductivity as interfacial contact resistance develops between the filler and the now-solidified polymer. This can be particularly confusing in the case of some filled semicrystalline polymers, where the appearance of the crystalline phase upon solidification should result in increased thermal conductivity, while the actual value appears to decrease. For this reason, it is considered safer to measure the thermal conductivity of filled materials. 

Other models take into consideration the effects of shape, size, and interfacial resistance on thermal conductivity. However, these models are unable to predict the effective thermal conductivity accurately if contact among filler particles exists. The Cheng and Vachon (Tavman 2003) model assumes a parabolic distribution of disperse phase (spheres or fibers) in a solid matrix. When k, > kp, thermal conductivity of the polymer composite is given by equation (11.7) ... 

Contact resistance between matrix and particles (known as Kapitza resistance) was investigated by Hasselman (1987), who developed formulas for spherical, cylindrical, and flat-plate geometry to estimate the effective thermal conductivity of polymer composites with interfacial thermal barrier resistance—equation (11.9). 

Abstract. The context of this work is the enhancement of the thermal conductivity of polymer by adding conductive particles. It will be shown how we can use effective thermal conductivity models to investigate effect of various factors such as the volume fraction of filler, matrix thermal conductivity, thermal contact resistance, and inner diameter for hollow particles. Analytical models for lower bounds and finite element models will be discussed. It is shown that one can get some insights from effective thermal conductivity models for the tailoring of conductive composite, therefore reducing the amount of experimental work. 

It is generally assumed that once the melt contacts the mold wall, it will acquire and maintain the mold temperature. However, the contact can never be perfect and heat resistance inevitably exists between the mold wall and molten polymers. According to the recent study, the thermal contact resistance between plastics and the mold metal is in the range 10 —10 m KAV. [2] Therefore, the... 

Thermal impedence was affected by crosslinker and filler amount in a statistically significant way. As the filler particles are more conductive than the polymer, the greater the amount of filler present, the higher the composite conductivity. Crosslinker amount affected the hardness of the samples. The hardness of the composite affected its conformability, which in turn affected the contact resistance between the composite and test device. Additionally, the test method for thermal impedence was responsible for some of this effect. The test fixture... 

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