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From wood and silk to nylon and plexiglass

All plastics consist of polymers – large molecules that contain many small units, or monomers, joined together to form long chains, much like strings of beads. The chemical structure of the beads and the bonds that join them together determine polymers' properties. Some polymers form materials that are hard and tough, like glass and epoxies. Others, such as rubber, can bend and stretch. A monomer of Teflon, a nonstick synthetic resin (top), and a chain of monomers (bottom). Chromatos For centuries humans have made products out of polymers from natural sources, such as silk, cotton, wood and wool. After use, these natural plastics are easily degraded by microorganisms. Synthetic polymers derived from oil were developed starting in the 1930s, when new material innovations were desperately needed to support Allied troops in World War II. For example, nylon, invented in 1935, replaced silk in parachutes and other gear. And poly(methyl methacrylate), known as Plexiglas, substituted for glass in aircraft windows. At that time, there was little consideration of whether or how these materials would be reused. Modern synthetic plastics can be grouped into two main families: Thermoplastics, which soften on heating and then harden again on cooling, and thermosets, which never soften once they have been molded. Some of the most common high-volume synthetic polymers include polyethylene, used to make film wraps and plastic bags; polypropylene, used to form reusable containers and packaging; and polyethylene terephthalate, or PET, used in clothes, carpets and clear plastic beverage bottles.

Recycling challenges

Today only about 10 percent of discarded plastic in the United States is recycled. Processors need an input stream of non-contaminated or pure plastic, but waste plastic often contains impurities, such as residual food. Batches of disposed plastic products also may include multiple resin types, and often are not consistent in color, shape, transparency, weight, density or size. This makes it hard for recycling facilities to sort them by type. Melting down and reforming mixed plastic wastes creates recycled materials that are inferior in performance to virgin material. For this reason, many people refer to plastic recycling as "downcycling." As most consumers know, many plastic goods are stamped with a code that indicates the type of resin they are made from, numbered one through seven, inside a triangle formed by three arrows. These codes were developed in the 1980s by the Society of the Plastics Industry, and are intended to indicate whether and how to recycle those products. Filtre However, these logos are highly misleading, since they suggest that all of these goods can be recycled an infinite number of times. In fact, according to the Environmental Protection Agency, recycling rates in 2015 ranged from a high of 31 percent for PET (SPI code 1) to 10 percent for high-density polyethylene (SPI code 2) and a few percent at best for other groups. In my view, single-use plastics should eventually be required to be biodegradable. To make this work, households should have biowaste bins to collect food, paper and biodegradable polymer waste for composting. Germany has such a system in place, and San Francisco composts organic wastes from homes and businesses.

Designing greener polymers