Recycling is a crucial practice taught from childhood to reduce pollution and landfill growth. It allows us to transform plastics into different objects, giving them a new life. But have you ever wondered about the chemical journey involved in recycling? This process is largely dependent on the structure of polymers, materials composed of many monomers linked together. Polymers can be classified as thermoset or thermoplastic. Thermoplastics, like polyethylene and polypropylene, soften when heated and harden when cooled, making them ideal for recycling. On the other hand, thermoset polymers, like rubbers and epoxy, do not soften when heated due to strong covalent crosslink bonding, and are typically repurposed rather than recycled. Polymer scientists are constantly finding new ways to recycle and create more recyclable objects to preserve our planet.
For many decades, chemists, chemical engineers, and materials scientists have searched for ways to process and create something else with recycled materials. Repurposing is to give a new purpose or use, such as using a plastic grocery bag to hold other objects, or using a plastic milk jug as a planter.
When it comes to polymers whether we recycle or repurpose will depend on the structure of the polymers. The word polymer has two root words that are actually very explanatory – ‘mer’ means unit and “poly” means many. Taken together, the word polymer can be deconstructed as many units. Polymer is a material that is composed of many monomers (from 10s to 1000s) all linked together to form chains. A monomer can be composed of one to many atoms which form the base unit, which is repeated to form a polymer.
Polymers can look a lot like spaghetti noodles (linear), ladders (cross-linked), long chains with smaller chains hanging off the main chain (backbone) known as branched polymers, and 3-D structures (network). Other forms are copolymers constructed from two or more different monomers and are polymers classified as block, random, or alternating.
When it comes to recycling and repurposing polymers, one useful classification is whether they are thermoset or thermoplastic polymers. Thermoplastics soften when heated and harden when cooled analogous to liquid water and ice. We can heat a piece of ice to make water. Then you can cool the water to make ice. This process can be repeated an infinite number of times. This is totally reversible and repeatable. Most linear polymers and branched structure polymers with flexible chains are thermoplastics. These materials are ideal for recycling. Examples of thermoplastics include polyethylene, polypropylene, polystyrene, Teflon, polyvinyl chloride, acrylic, and nylon.
In contrast, thermoset polymers do not soften when heated due to strong covalent crosslink bonding between the main chains. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability. Thermosets are analogous to an egg. The soft liquid-like egg can be heated to make a hard egg. However upon cooling, the firm egg will not revert to a soft liquid-like egg. These materials typically are not recycled. Examples of thermoset polymers are rubbers, phenolic plastics, polyurethane, epoxy, and silicone. These materials can be repurposed. For example, automobile tires are crosslinked rubber that can be repurposed as a surface covering for playgrounds.