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Second Quarter 2019
Maximizing Crystallinity and Clarity of Plug-Formed PET Cups via Change in Forming Conditions
By Amit Dharia, PhD., Transmit Technology Group, Irving, TX
Poly(ethylene terephthalate), PET, is used in making clear, deep-drawn thermoformed cups and trays where it competes with clear PS and clarified or nucleated PP.
PET is a semi-crystalline polymer with a maximum of 40% crystallinity. Its crystallinity changes both with the type and amount of co-monomer and rate of cooling (1). The rate of crystallization of PET is so low that quenching or rapid cooling can result in totally amorphous and transparent APET while heat treating PET above its glass transition temperature (Tg) for a few hours can produce crystalline and opaque CPET. A large amount of amorphous content makes PET tougher compared to CPP, but not as stiff as GPPS. Even though the melting point of PET is close to 260°C / 500°F, due to its lower Tg, it does not have the same stiffness as PS at elevated temperatures (2). Due to its low crystallinity, it thermoforms better than CPP but not as well as PS.
When heated, PET exhibits a secondary (cold) crystallization at around 120°C / 248°F. Therefore, it needs to be thermoformed above 120°C / 248°F. It begins to crystallize rapidly around 170°C / 338°F and cannot be formed. Once it approaches 190°C / 374°F it begins to lose melt strength. So, a typical forming temperature range for PET is 120°C to 170°C (248°F to 338°F).
The density of PET changes significantly with crystallization. The density of APET is 1.335 g/cm3 as compared to 1.455 for 100% crystalized PET. If a part is quenched, crystallization continues at a slow rate resulting in shrinkage and warpage. If it is cooled slowly using a hot tool, it crystalizes and becomes opaque and brittle. So controlling the heating and cooling rates is critical in controlling drawability, clarity, crystallinity, and demolding.
Heat Resistant and Home-Compostable PLA Resins
By Gregory Coué, Technical Manager, Kompuestos (Spain)
From soft-drink cups at fast food restaurants and festivals to fresh fruit and vegetable containers: the packaging industry is no stranger to biobased polylactic acid (PLA) plastic. And according to recent studies on the biodegradable plastics market, PLA will maintain its dominance in the biodegradable plastics market through 2023. Made from natural, renewable resources such as sugar cane or corn, PLA is readily available worldwide, processed using conventional converting processes and is recyclable or compostable at the end of life. However, so far, the use of conventional PLA has largely been limited to applications such as cold food packaging, disposable plates and cutlery and shopping bags. Efforts have been made to develop heat resistant PLA grades able to withstand use temperatures in the range of 80°C to 120°C. Yet these applications all require composting in industrial facilities, while the market is increasingly indicating a preference for home composting.
Other home compostable resins, such as PHA or PBSA, have emerged as alternatives in the biodegradable market, and their market share is expected to grow at a rapid rate. However, although they offer a good fit for flexible packaging and film applications, they lack the mechanical properties required for rigid packaging, in addition to being more expensive.
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