Physics of Injection-moldable PET

Abstract

In this article it is shown that in order to render poly(ethylene terephthalate) (PET) injection moldable, the PET resin must contain together at least one of each kind of the following additives: A plasticizer, a crystallization nucleation agent, an impact modifying agent, and a chain extending agent Additional components, such as fiberglass, are usually added. Only the combined presence of the plasticizer and nucleating agent creates the conditions whereby the crystallization upon heating at the exterior surfaces of the molded parts may occur at sufficiently fast rates at the temperatures deliverable by water-cooled molds and the crystallization upon cooling at the interior of the molded parts will occur fast enough for the parts to solidify and not sag or deform upon ejection from the mold. For the surfaces of the molded parts to be acceptable, and for the parts to exhibit good mechanical properties, highly effective crystallization nucleation is required, resulting in a multitude of spheruhtic fragments filling the interior and exterior surfaces of the molded parts, instead of separately grown, large individual spherulites substantially separated from one another by amorphous PET. To counteract the brittleness of the crystalline PET, slightly compatible impact modifying additives are added, serving as rubbery inclusions at ambient temperatures. To mitigate against molecular weight degradation, small amounts of multi-functional chain extension additives are added. The above four kinds of additives are compounded together into the PET with or without reinforcements or fillers to produce acceptable grades of injection moldable PET.

Key Words:

• PET
• Poly(ethylene terephthalate)
• Crystallization temperature
• Impact test
• Nucleating agents
• Plasticizers
• Impact modifiers
• Chain-extenders
• Injection molding