Random copolymers blend two different molecular chains to create unique properties in plastics. Polypropylene, a common polymer, can be made transparent and have a lower melting point by creating a random copolymer with polyethylene. Regular copolymers have a crystalline structure, making them inflexible and brittle when cold. Random copolymers are more resistant to cracking at lower temperatures, making them ideal for frozen food packaging. The ability to recycle random copolymer products has become important as petroleum-based raw material prices rise.
A random copolymer is a blend of two different molecular chains used in plastics to create different properties than a single molecular type. Polymers are chains of molecules called monomers which are chemically bonded into solid structures used for product packaging. Combinations of two monomers are a random copolymer if there is no consistent or regular structure of the two monomers, which will occur under specific manufacturing conditions.
Polypropylene is a common polymer used in several plastic applications. As a pure polymer, it can often be cloudy or milky in color, which may not be ideal for food containers or other applications. It can also have a higher melting point than desired for extrusion, which produces products by melting polymer and forcing it into molds under pressure.
Creating a random copolymer of polypropylene with polyethylene changes the molecular structure of the resulting plastic. The right operating conditions will create transparent polymers, ideal for food packaging. Formulations can be prepared with lower melt temperatures than pure polypropylene, which can aid in manufacturing.
Two monomers must be mixed in an irregular or random pattern to create a random copolymer. Regular copolymer structures tend to be harder, with what is called a crystalline structure. This regular molecular pattern can provide poor shelf life in cold conditions or create an inflexible container.
Plastic food packaging has become popular since the mid-20th century as consumer demand for frozen and ready-to-eat meals has grown rapidly. Early packaging was often aluminum trays, but the invention of microwave ovens made aluminum a problem, as they are not compatible because metals can arc and cause microwave fires. Plastic has become more common for food packaging of all types and has gradually replaced aluminum as the packaging material of choice.
Packaging frozen foods was a problem for early polymers because the regular crystalline structure of the molecules made them brittle when cold. Increasing demand for frozen food trays and storage containers has resulted in random copolymers, which remain flexible and are more resistant to cracking at lower temperatures. The ability to produce transparent packaging allowed manufacturers to develop entire lines of frozen foods that could be microwaved and served. These packets could be taken from the freezer to the microwave, were transparent so that the food could be seen inside the packet while heating, and could withstand high food temperatures.
As the recycling of metals and plastics became more common in the late 20th century, the need for plastics that could be reused became more prominent. Many random copolymer products were melted and extruded into the original package shapes and could be remelted and recycled with relative ease. This has become important as the prices of petroleum-based raw materials have risen, allowing manufacturers to recycle more plastic and reduce costs.
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