Molecular recycling, also called chemical recycling, is the breaking down of plastic waste into its original molecular building blocks, mainly monomers, that can then be used to synthesize new, high-quality polymers (i.e., plastics) with properties comparable to virgin plastics and which can be used in the creation of new products and fuels.
This contrast with mechanical recycling, which consists of sorting, cleaning, and melting plastic waste without altering its chemical structure, and then forming it into new products, such as building materials, packaging materials and automotive parts. The number of such recycling cycles is limited because repeated melting can degrade the properties of the plastic. Mechanical recycling is best suited for widely collected plastics such as PET and HDPE, which are marked with resin identification codes 1 and 2, and it is not well suited for plastics such as fast food containers and colored bottles.
The various processes that have been developed for molecular recycling can be classified into three main categories, each with its advantages and disadvantages, including with regard to cost, health effects and environmental effects. One is purification, in which solvents are used to dissolve the plastic waste in order to separate the polymers from its additives and contaminants. A second is depolymerization, which uses heat, solvents and or catalysts to break down polymers into their monomer building blocks. The third is pyrolysis and gasification, which includes heating the plastics in a low oxygen environment so that they thermally decompose but do not combust.
Molecular recycling should be viewed as being complementary to mechanical recycling rather than as a competitor. It expands the range of plastics that can be recycled to include those that are difficult to recycle mechanically and is particularly useful for dealing with mixed and contaminated plastics, which would otherwise be discarded into the environment, burned, or buried in landfills. Thus, it can help reduce the greenhouse gas emissions and the health risks associated with virgin plastic production.
Another advantage of molecular recycling is that the resulting virgin plastic-like materials can be used in any application, including food packaging and medical equipment, due to their consistent properties and high purity. This contrasts with mechanically recycled plastics, which typically are not suitable for these applications because of potential contaminants.
Despite its potential, molecular recycling has not yet been widely adopted because its cost is still substantially higher than that of mechanical recycling. This is due largely to the high cost and other issues of scaling up from pilot plants to large volume recycling facilities. However, the cost is declining as a result of technological advances, economies of scale, and the development of improved supply chains for both the plastic waste and the recycled output.
A pioneer in molecular recycling is Eastman Chemical Company, whose recently completed molecular recycling plant in Longview, Texas in the U.S. is expected achieve an annual capacity of 110,000 metric tons.