Practical Guide to High Performance Engineering Plastics by David J. Kemmish download in ePub, pdf, iPad
These long fibre-reinforced products offer several advantages, including improved impact resistance, rigidity, creep resistance and dimensional stability. Fibres types include glass, carbon, aramid and stainless steel. The characteristics of shear thinning are also dependent upon factors such as molecularweight distribution. For example, wear face temperature is dependent upon the geometry of the specimen and the test rig. They are becoming increasingly important because of trends towards more reliable and higher performance machines and devices.
It is a simple way to characterise shear thinning, and typically results in a plot of viscosity versus shear rate. Such testing is usually essential.
Effectively it measures shear rate under a given load, so this is a variable shear rate, fixed shear stress test. Various automated techniques have been developed. This may occur on the surface of some impurity or arise as a result of random fluctuation in the melt. To some extent this is offset by the use of high melt temperatures but ultimately materials decompose before they form a reasonable viscosity melt. Crystallinity can be checked by measurement, but moulders often rely on control of thermal history to ensure adequate crystallisation.
Fibres are typically available as tows, rovings, chopped strands, milled fibres, and in various weaves. As explained above, the highest values of Tg tend to be found in amorphous polymers.
Producing nanotube compounds remains challenging, and most products contain more than the theoretically required amounts of tubes. Higher temperatures provide more energy for the rearrangement of chain entanglements and so will produce higher levels of crystallinity.
Effectively it is a fixed stress, variable shear rate test. This will facilitate conversations with both materials suppliers and end users. It is important to measure viscosity at a shear rate that is relevant to the process being used.
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