The recycling process converts end of life or unwanted materials into new products with new or similar functions.
Elastomers are rubbery materials with an elastic property. Elastomers can be recycled in various ways after the original product has reached end of life. An example of a product that reaches end of life and goes through a recycling process are tires. Tires become unwanted when they have become worn down to the point of no longer being safe for road travel, or after irreparable damage has occurred. There are three large markets for scrap tires, they include: tire-derived fuel, civil engineering applications, and ground rubber applications/rubberized asphalt.
Around 117 million end of life tires were utilized as Tire-derived fuel in 2015. Tire-derived fuels are a viable alternative to the use of fossil fuels, if proper regulatory controls are in place. Scrap tires have a high heating value and are sought after for this property for use in cement kilns as well as other industrial applications. Tires produce the same amount of energy as oil and 25% more energy than coal. The ash residues from Tire-derived fuels may contain a lower heavy metals content than some coals. NOx emissions are also lower compared to many US coals, particularly high-sulfur coals.
Civil engineering applications consumed 17 million old tires in 2015. These applications can replace other materials such as polystyrene insulation blocks, drainage aggregate, or other types of fill. A significant amount of materials used in civil engineering applications have come from stockpiled tires.
Ground rubber usage consumed 62 million tires in 2015. Ground rubber is used to manufacture many products, including: asphalt rubber, synthetic sports field underlay, animal bedding, and many more. The largest use of these is asphalt rubber utilizing around 12 million tires annually. California and Arizona are the largest users of asphalt rubber. Other uses for ground rubber are for groundcover under playgrounds, anti-fatigue mats, running tracks, animal bedding, underlay and infill for athletic fields, and equestrian footing.
Tires must go through a recycling process before it can be reused for other products/uses. Most tires are collected from retailers when they are changed out for new tires, they are then sold to processing plants to prepare them for their long journey ahead. The tires contain steel wire to make them stronger, this wire must be removed before the rubber is recycled. Tires are cut into small pieces and treated with chemicals to create fine granules. This process may be done either mechanically by use of a shredding machine, or cryogenically where tires are frozen with liquid nitrogen then broken into crumbs by a hammer mill. The particles are steel are removed with a huge magnet. The rubber then goes through another screening process to ensure no wire or other contaminants remain. This stage also sorts the remaining rubber pieces by size for packaging and selling to any organization that needs the reclaimed rubber.
Another Elastomer product is rubber boots, or rain boots. This product typically is not recyclable by regular recycling processes. De-vulcanizing the rubber breaks down the bonds which were formed in the vulcanizing process which poses a challenge to maintain the characteristics when recycling. Many people have found other ways to recycle old worn out rubber boots, such as repurposing them into outdoor planters.
Thermoplastic elastomers (TPE) materials have the potential to be recyclable since they can be molded, extruded and reused like plastics, but they have typical elastic properties which are not recyclable owing to their thermosetting characteristics. Thermoplastic elastomers have outstanding thermal properties and material stability when exposed to a broad range of temperatures. TPEs also consume less energy to produce and can be colored easily by dyes, and allow economical quality control. Two important manufacturing methods with TPEs are by extrusion molding and injection molding. Recently, TPEs have been used in 3D printing giving them a wide range of uses. Other ways TPEs can be processed are by blow molding, melt calendaring, thermoforming, and heat welding.
The recycling of plastics and elastomers are categorized by several numbered categories, most of which can be found printed on the product itself. These numbers have a triangular set of arrows around the number showing its recycling category. These categories are as follows:
- PETE: Polyethylene terephthalate (e.g. Soft drink bottles, fruit juice containers)
- HDPE: High-density polyethylene (e.g. Milk jugs, cleaning agents, laundry detergents, shampoo bottles, washing and show soaps)
- PVC: Polyvinyl chloride (e.g. Trays for sweets, fruit plastic packaging, food foils)
- LDPE: low-density polyethylene (e.g. Crushed bottles, shopping bags)
- PP: Polypropylene (e.g. Furniture, luggage, toys, car bumpers)
- PS: Polystyrene (e.g. Toys, hard packing, refrigerator trays, costume jewelry, CD cases, vending cups)
- OTHER: other plastics (e.g. Acrylic, polycarbonate, polyactic fibers, fiberglass)
Polyethylene Terephthalate is one of the most commonly used plastic product and is found in most water and soda bottles. PET bottles are recyclable and should be recycled, but cannot be reused. When recycled, the plastic is crushed and shredded into small flakes and reprocessed to make new PET bottles, or can be spun into polyester fiber and used to make textiles, such as carpets, stuffing for pillows, lifejackets, or coats.
High-Density Polyethylene is a stiff plastic used to make milk jugs, toys, and some plastic bags. HDPE is the most commonly recycled plastic and is also considered one of the safest forms of plastic. Processing of HDPE is simple and cost-effective to recycle for secondary use. HDPE does not break down under exposure to sunlight or extreme heating and freezing, thus it is commonly used to make picnic tables and park benches. Products made from HDPE are reusable and recyclable.
Polyvinyl Chloride is a soft flexible plastic and is used to make such things as plastic food wrapping, cooking oil bottles, children and pet toys, and plastic pipes and parts for plumbing. PVC is relatively impervious to sunlight and weather and is used to make window frames and garden hoses as well. PVC contains numerous toxins which it can leach throughout its entire life cycle, so it has been dubbed the “poison plastic”. Products made with PVC are not recyclable but some can be repurposed. PVC products should not be reused for applications with food or for children’s use.
Many products can be recycled after they have reached their end of life and reused with a new purpose. Sadly more plastics end up in landfills rather than going to recycling centers to be processed and reused. The costs for recycling these products are very cost effective compared to the manufacturing of new materials. Everyone should do their part and help recycle more materials and put less strain on our resources.