Plastics Technology

JAN 2019

Plastics Technology - Dedicated to improving Plastics Processing.

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Twenty years ago, a session at the Society of Plastics Engineer's annual technical conference (ANTEC) was devoted to reviewing standard methods used to measure plastic properties that are customarily provided on a material data sheet. The objective of the session was to scruti- nize the way we have always done things and propose alternative approaches where appropriate. The proceedings were published as a standalone work and the presentations made the rounds during that year to other organizations, including the Society of Automotive Engineers (SAE). I made a presentation in that session devoted to the property of deflection temperature under load (DTUL), also known in the industry as the heat deflec- tion temperature (HDT). In that paper I suggested that the industry needed a better way of reporting the temperature-depen- dent behavior of plastic materials. The alternative method I proposed is known as dynamic mechanical analysis (DMA). Twenty years later, questions are still being posted on various industry sites about the relative utility of HDT vs. DMA. Most of the answers to these questions suggest a lack of familiarity with DMA and a comfort level with the way we have always done things—which, given the stakes, is somewhat disturbing. The test for HDT was developed in the early years of plastic testing and it is not difficult to see how it would have been considered useful 40 Heat Deflection Temperature vs. Dynamic Mechanical Analysis or 50 years ago. If there is an interest in identifying a temperature range where a material might fail, placing a beam of that material under some type of load and then raising the temperature until the material softens or melts appears to make sense. Little was expected of plastic materials at that time. The concept of plastics as engineering materials was just beginning to enter the conversation within the industry and the general public still thought of these materials as suitable only for toys, straws, and low-end housewares, despite the fact that nylon and polyester were well established and polymers like acetal, polycarbonate and polysulfone were coming into their own. Today we make parts for medical devices, cars and trucks, and the aerospace industry. Metal replacement occurs often; and even applications that were pioneered in the 1960s, such as plastic gears, are achieving greater precision and higher power ratings than were considered achiev- able even 10 years ago. With this increased sophistication, it would be expected that engi- neers and designers would need a more complete picture of the temperature-dependent behavior of plastic materials. But almost all the information available on the effects of elevated temperatures remains limited to a measurement of HDT. The problem with this narrow view of polymer behavior is that very few engineers and designers understand what this test is measuring. I once received a part drawing that called out a particular grade of flame-retardant ABS. A note on the drawing stipulated that the part was to withstand continuous exposure to a temper- ature of 190 F (87 C). The requirement was unrealistic, particu- larly because the time frame associated with "continuous" was not defined, and the number looked suspiciously familiar. A check of the data sheet showed that this temperature was the published HDT. It took a long conversation to explain to the What possible relevance can measurements of elevated-temperature performance have when they are made at stresses that are less than 10% of those at which we plan to use our parts? Does the industry need a better way of reporting the temperature-dependent behavior of plastics? With DMA, it already has one. But it's been glacially slow to catch on. Get more insights on Materials from our expert author: Learn more at KNOW HOW MATERIALS By Mike Sepe PART 1 22 JANUARY 2019 Plastics Technology K now How MATERIALS

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