WEB EXCLUSIVE: Polypropylene has the edge over PET and PS for clear thermoformed drink cups because of its heat resistance, allowing it to be used for hot beverages. However, PP homopolymer suffers from lack of flexibility, which can result in unsatisfactory detail, incomplete forming, and cracking during production. Its low impact resistance can lead to breakage during transportation and it can be brittle at low temperatures. PP random copolymer costs more but provides good clarity and flexibility, though its impact resistance remains inadequate at low temperatures.
A solution was developed by ExxonMobil Chemical Co., Houston, together with Hongua Machinery Plastic & Rubber Co. Ltd. in China, a producer of extrusion, thermoforming, and molding machines for food and beverage containers. They jointly developed blends of PP and ExxonMobil’s Vistamaxx propylene-ethylene thermoplastic elastomer. For applications requiring a balance of impact resistance, clarity, and stiffness, a dryblend of 5-10% Vistamaxx 3020 in PP reportedly reduces cup breakage during production and transport. An alternative is a low dosage of Vistamaxx 6102 in PP, which boosts impact resistance and maintains clarity and low cost, while avoiding die-cutting problems. Vistamaxx blends easily with PP homopolymer and random copolymer, allowing users a broader range of options to target the flexibility they need.
Improved flexibility and elasticity of PP blended with Vistamaxx are said to allow higher draw ratios with fewer flow marks, better part quality, and lower scrap rates. In addition, the blend reduces shrinkage, making die-cutting more accurate for improved cup and lit matching. Lower melt temperature and higher flow of the elastomer allows processing temperatures to be reduced, saving energy and increasing throughput. ExxonMobil says Vistamaxx also reduces or eliminates stress whitening, lowers seal-initiation temperatures, and boosts impact strength of PP even at low temperatures. Higher impact strength provides downgauging opportunities, as well. And for applications requiring added slip resistance, the elastomer can be used to increase the coefficient of friction. Vistamaxx complies with U.S. and EU food-contact regulations.