Plastics Technology

DEC 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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The final aspect of processing that is designed to ensure good retention of mechanical properties involves taking good care of the additive package. All commercial polymers contain an additive package, a combination of compounds designed to provide the stability needed to allow melt processing to occur without degrading the polymer. Resins may also contain addi- tives provided to enhance end-use perfor- mance. The constituents in the additive package can vary from compound to com- pound, and may include UV stabilizers, nucleating agents, antistatic agents, antimicrobials, colorants and flame retardants. But at minimum they are likely to include some level of lubrication and some degree of stabilization against thermal and oxidative degradation. It is this stabilizer package that we are most interested in for purposes of this discussion. Antioxidants, as these compounds are generally referred to, are incorporated in a material for two purposes: to allow the material to survive the high temperatures and shear stresses of the molding process; and to provide stability that may be needed so that the part can fulfill its application requirements. Often, these functions are delegated to two different sets of chemicals, classified as primary and secondary antioxidants. The chemistry of these materials is fascinating and complex. But when all is said and done, the purpose of these substances is to protect the polymer from degradation that can occur during processing and in use. The exact amounts of these additives will depend upon the polymer being stabilized and the applications that the material is intended for. Polypropylene and polyethylene are good examples of materials that require protection from oxidation. Generally, PE is more resis- tant to oxidation than PP and therefore requires lower loadings of the additives. But all commercial PE and PP materials require some minimum level just to get through melt processing. If after the part is molded it is expected to withstand elevated temperatures while in application, a higher level of stabilization will be needed A Processor's Most Important Job to ensure that the product does not fail. For PP, these types of high-temperature applications can be anything from coffee-maker components to tanks for holding automobile engine coolant. The material supplier will incorporate a certain amount of the stabilization package in the material, making this the starting point for that material. It is now the processor's job to handle the material during processing so that the molded part contains enough stabilizer to perform to the expectations of the end user. The molding process will always consume some amount of the antioxidant in the material. This is expected. But the amount of stabilizer that is consumed will depend upon the process conditions, most notably the melt tempera- ture and the time the material spends in the molten state. Lower melt temperatures and shorter residence times will produce parts that retain a higher level of stabilization, and these parts will be more capable of handling the application environment. If melt temperatures The molding process will always consume some amount of the antioxidant in the raw material. It's the processor's job to ensure molded parts contain enough stabilizer to perform to the expectations of the end use. PART 11 By Mike Sepe Get more insights on Materials from our expert author: Learn more at KNOW HOW MATERIALS 8 6 4 2 0 -2 Using DSC, a sample of material can be exposed to a constant elevated temperature in the presence of oxygen to determine the time it takes for the material to undergo oxidation. The accompanying graph shows a result from such a test. The sudden change in the slope of the plot is associated with the onset of oxidation. This material can withstand 77 min of exposure to these conditions before the antioxidant package is consumed and the material rapidly fails. Oxidation Induction for a Typical PP Time, min 0 20 40 60 80 100 Heat Flow, W/g Oxidation Induction Time: 77 min 26 DECEMBER 2018 Plastics Technology K now How MATERIALS

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