Sheet Metal · 2021年11月30日

Solid Waste Management

In addressing all the superior attributes of polymers, it is equally important to discuss some of the challenges associated with the materials. Most plastics deteriorate in full sunlight, but never decompose completely when buried in landfills.  However, other materials such as glass, paper, or aluminum do not readily decompose in landfills either. Some bioplastics do decompose to carbon dioxide and water, however, in specially designed food waste commercial composting facilities ONLY.  They do not biodegrade under other circumstances.  

For 20051 the EPA characterization of municipal solid waste before recycling for the United States showed plastics made up 11.8 percent of our trash by weight compared to paper that constituted 34.2 percent. Glass and metals made up 12.8 percent by weight.  And yard trimmings constituted 13.1 percent of municipal solid waste by weight.  Food waste made up 11.9 percent of municipal solid waste.  The characteristics that make polymers so attractive and useful, lightweight and almost limitless physical forms of many polymers designed to deliver specific appearance and functionality, make post-consumer recycling challenging.  When enough used plastic items can be gathered together, companies develop technology to recycle those used plastics.  The recycling rate for all plastics is not as high as any would want.  But, the recycling rate for the 1,170,000,000 pounds of polyester bottles, 23.1%, recycled in 2005 and the 953,000,000 pounds of high density polyethylene bottles, 28.8%, recycled in 2005 show that when critical mass of defined material is available, recycling can be a commercial success2.

Applications for recycled plastics are growing every day. Recycled plastics can be blended with virgin plastic (plastic that has not been processed before) without sacrificing properties in many applications.  Recycled plastics are used to make polymeric timbers for use in picnic tables, fences and outdoor playgrounds, thus providing low maintenance, no splinters products and saving natural lumber. Plastic from soft drink and water bottles can be spun into fiber for the production of carpet or made into new food bottles.  Closed loop recycling does occur, but sometimes the most valuable use for a recycled plastic is into an application different than the original use.

An option for plastics that are not recycled, especially those that are soiled, such as used food wrap or diapers, can be a waste-to-energy system (WTE).  In 2005, 13.6% of US municipal solid waste was processed in WTE systems1.  When localities decide to use waste-to-energy systems to manage solid waste, plastics can be a useful component. 

The controlled combustion of polymers produces heat energy. The heat energy produced by the burning plastic municipal waste not only can be converted to electrical energy but also helps burn the wet trash that is present. Paper also produces heat when burned, but not as much as do plastics. On the other hand, glass, aluminum and other metals do not release any energy when burned.

To better understand the incineration process, consider the smoke coming off a burning item.  If one were to ignite the smoke with a lit propane torch, one would observe that the smoke disappears. This exercise illustrates that the by-products of incomplete burning are still flammable.  Proper incineration burns the material and the by-products of the initial burning and also takes care of air and solid emissions to insure public safety.

Some plastics can be composted either because of special additives or because of the construction of the polymers.  Compostable plastics frequently require more intense conditions to decompose than are available in backyard compost piles.  Commercial composters are suggested for compostable plastics.  In 20051, composting processed 8.4% of US municipal solid waste.

Plastics can also be safely land filled, although the valuable energy resource of the plastics would then be lost for recycling or energy capture.  In 20051, 54.3% of US municipal solid waste was land filled.  Plastics are used to line landfills so that leachate is captured and groundwater is not polluted.  Non-degrading plastics help stabilize the ground so that after the landfill is closed, the land can be stable enough for useful futures.

Polymers affect every day of our life. These materials have so many varied characteristics and applications that their usefulness can only be measured by our imagination. Polymers are the materials of past, present and future generations.

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