Rigid Packaging

Polypropylene is widely used for rigid packaging, such as margarine tubs, yogurt containers, trays, milk jugs, and bottles. Thermoformed polypropylene margarine tubs have replaced most PS and PVC tubs. Random copolymers are frequently used in products such as food storage containers due to their excellent clarity and good balance of impact strength and stiffness. In thin-walled parts, such as injection-molded delicatessen containers or yogurt cups with a length-to-thickness ratio of up to 400:1, use of polypropylene copolymers with high melt flow rates (35 g/10 min) can reduce container weights by 30% over containers produced with other materials. Containers are tough, rigid, and recyclable. They provide a moisture barrier and top-load strength, and the low-temperature resistance protects the product at refrigerator temperatures.

Polyethylene is the material most used in rigid packaging; however, due to its lower flexural modulus compared to polypropylene, it requires a greater wall thickness in thin-walled containers. Polypropylene is used in screw-on caps and closures, due to the resin’s stiffness, resistance to stress cracking, and living hinge properties, and it is used in tamper-evident closures for medicines, liquor, and food products. Polypropylene is replacing HDPE in applications such as crates and paint containers.Polypropylene is suitable for containers used in microwave cooking, as long as the food product does not have a high fat content and the container is designated as “microwave use only”. In these cases, the moisture in the food ensures that the food temperature does not exceed 100 C (212F), which is near the hot-fill temperature limit of polypropylene. Blends of materials are used in containers designed for dual oven use. For example, a blend of polypropylene, molded pulp, fillers, and additives can withstand temperatures from 240 C (240 F) to 200C (392 F), similar to CPET. The containers have a pearlescent finish and lack the wrinkled corners common on paperboard trays. Many microwaveable products contain several layers of different materials for shelf-stable packaging. Shelf-stable products may be stored at ambient temperatures and do not require refrigeration.

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