Plastics Technology

DEC 2014

Plastics Technology - Dedicated to improving Plastics Processing.

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In Part 1 we established that physical aging involves a change in the free volume within the amorphous regions of a polymer matrix. Specifcally, it involves a volumetric contrac- tion of the matrix that occurs over time. This should be observable as a change in dimen- sions as well as properties. The dimensional changes are usually much more subtle than the property changes. The amount of dimensional change will depend upon several factors. These include the size of the part, the molecular weight of the polymer, and the molding conditions that were used to produce the parts. Detection of these dimensional changes will depend upon the preci- sion of the measurements. We will examine each of these factors here. Dimensional changes are typically measured as a percentage. In unflled amorphous polymers, the shrinkage exhibited by the molded part compared with the tool-steel dimensions is typically 0.4-0.8%. The exact value will depend upon the material, the part geometry, and the processing conditions. Anyone who has molded parts to a demanding tolerance knows that the ability to change a molded dimension depends to a signifcant degree on the size of the part. If a change in processing conditions can alter the shrinkage by 0.1%, then in a part with a length or diameter of 0.250 in., the change will only be one-quarter of a thousandth, a diference that may go undetected if measurements are being made with a tool that has a resolution of 0.0005-0.001 in. However, if a part is 50-in. long, it may be possible to move this part length by 0.050 in. The dimensional changes that occur due to physical aging are far smaller and occur more gradually than the shrinkage that occurs during molding. Therefore, only large parts or parts that are measured to a very high degree of The Mystery of Physical Aging precision will exhibit detectable changes due to this mechanism. Even if such changes are noted, it is unlikely that the reason for their occurrence will be understood or properly interpreted. A couple of years ago I worked with a company that made molds out of PVC. The two halves of the mold were themselves injection molded and, when assembled, contained features into which casting resins were poured to form the fnal product. The dimensions of this cast product were very critical, consequently the control over the dimensions of the PVC parts was critical. Once the process reached equilibrium and parts were being produced that met print specifcations, the written procedure for using the parts to create the cast pieces dictated that use could begin 24 hr after molding. These parts could continue to be used until they were four months old, at which point they had to be discarded. This seemed like an interesting time window, and when I asked about the origin of this guideline I was told that it had been observed over the long history of this part that after four months some of the parts had become undersized. The measurements made on these parts are in terms of microns, not thousandths of an inch, therefore the dimensional changes were more evident. They occurred as rapidly as they did because the glass-transition temper- ature of rigid PVC is relatively low at 78 C (172 F). As this example suggests, once the mechanism behind the dimensional change is understood, steps can be entertained to slow down or stop the physical aging process by storing the parts at lower temperatures, thereby extending the useful life of the product. Stopping the physical aging process in PVC requires a temperature below -50 C (-58 F), which is probably not practical. But dropping the temperature at which the parts are stored by just 10° C can slow the physical aging process by almost a factor of 10, making the parts functional for over three years instead of four months. The rate at which physical aging takes place is governed in part by the molecular weight of the polymer. The reduction in free volume associated with physical aging requires some degree of mobility at a molecular level. Lower-molecular-weight polymers consist of No commercial process designed to produce parts at a competitive price can achieve a structure that is free of internal stress and achieves the perfect equilibrium state. By Mike Sepe Dimensional and property changes depend on the size of the part, molecular weight of the resin, and processing conditions. PART 2 Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS 20 DECEMBER 2014 Plastics Technology PTonline.com K now How MATERIALS

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