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What Is a Random Copolymer?

By Paul Reed
Updated May 21, 2024
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A random copolymer is a mixture of two different molecular chains used in plastics to create different properties than with a single molecular type. Polymers are chains of molecules called monomers that are chemically linked into solid structures used for packaging products. 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 production conditions.

Polypropylene is a common polymer used in different plastics 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 melting point higher than desired for extruding, which makes 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 polymers that are clear, which is ideal for food packaging. Formulations can be prepared with lower melting temperatures than for pure polypropylene, which can aid in manufacturing.

Two monomers need to be mixed in a non-regular 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 durability in cold conditions, or makes a container that is not flexible.

Plastic food packaging became popular starting in the mid-20th century as consumer demand for frozen, ready-to-eat meals grew 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 create electric arcs and cause fires in microwaves. Plastics became more common for food packaging of all types, and gradually replaced aluminum as the preferred package material.

Frozen food packaging was a problem for early polymers because the regular crystal structure of the molecules made them brittle when cold. Increasing demand for frozen food trays and storage containers led to random copolymers, which stay flexible and are more resistant to breakage at lower temperatures. The ability to make a package that was transparent allowed manufacturers to develop entire lines of frozen foods that could be microwaved and served. These packages could be taken from the freezer to the microwave, were clear so food could be seen inside the package while heating, and could withstand high food temperatures.

As metal and plastic recycling became more common in the late-20th century, the need for plastics that could be re-used became more important. Many random copolymer products were melted and extruded into the original package shapes, and could be re-melted and recycled with relative ease. This became important as petroleum-based raw material prices rose, allowing manufacturers to recycle more plastics and reduce costs.

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