PLASTICS AND THE ENVIRONMENT

Every year in the United States, consumers throw millions of tons of plastic away—of the estimated 190 million metric tons (420 billion pounds) of municipal waste produced annually in the United States, about 9 percent are plastics. As municipal landfills reach capacity and additional landfill space diminishes across the United States, alternative methods for reducing and disposing of wastes—including plastics—are being explored. Some of these options include reducing consumption of plastics, using biodegradable plastics, and incinerating or recycling plastic waste.

Source Reduction

Source reduction is the practice of using less material to manufacture a product. For example, the wall thickness of many plastic and metal containers has been reduced in recent years, and some European countries have proposed to eliminate packaging that cannot be easily recycled.

Biodegradable Plastics

Due to their molecular stability, plastics do not easily break down into simpler components. Plastics are therefore not considered biodegradable. However, researchers are working to develop biodegradable plastics that will disintegrate due to bacterial action or exposure to sunlight. For example, scientists are incorporating starch molecules into some plastic resins during the manufacturing process. When these plastics are discarded, bacteria eat the starch molecules. This causes the polymer molecules to break apart, allowing the plastic to decompose. Researchers are also investigating ways to make plastics more biodegradable from exposure to sunlight. Prolonged exposure to ultraviolet radiation from the sun causes many plastics molecules to become brittle and slowly break apart. Researchers are working to create plastics that will degrade faster in sunlight, but not so fast that the plastic begins to degrade while still in use.

Incineration

Some wastes, such as paper, plastics, wood, and other flammable materials can be burned in incinerators. The resulting ash requires much less space for disposal than the original waste would. Because incineration of plastics can produce hazardous air emissions and other pollutants, this process is strictly regulated.

Recycling Plastics

All plastics can be recycled. Thermoplastics can be remelted and made into new products. Thermosetting plastics can be ground, commingled (mixed), and then used as filler in moldable thermoplastic materials. Highly filled and reinforced thermosetting plastics can be pulverized and used in new composite formulations.

Chemical recycling is a depolymerization process that uses heat and chemicals to break plastic molecules down into more basic components, which can then be reused. Another process, called pyrolysis, vaporizes and condenses both thermoplastics and thermosetting plastics into hydrocarbon liquids.

Collecting and sorting used plastics is an expensive and time-consuming process. While about 35 percent of aluminum products, 40 percent of paper products, and 25 percent of glass products are recycled in the United States, only about 5 percent of plastics are currently recovered and recycled. Once plastic products are thrown away, they must be collected and then separated by plastic type. Most modern automated plastic sorting systems are not capable of differentiating between many different types of plastics. However, some advances are being made in these sorting systems to separate plastics by color, density, and chemical composition. For example, x-ray sensors can distinguish PET from PVC by sensing the presence of chlorine atoms in the polyvinyl chloride material.

If plastic types are not segregated, the recycled plastic cannot achieve high remolding performance, which results in decreased market value of the recycled plastic. Other factors can adversely affect the quality of recycled plastics. These factors include the possible degradation of the plastic during its original life cycle and the possible addition of foreign materials to the scrap recycled plastic during the recycling process. For health reasons, recycled plastics are rarely made into food containers. Instead, most recycled plastics are typically made into items such as carpet fibers, motor oil bottles, trash carts, soap packages, and textile fibers.

To promote the conservation and recycling of materials, the U.S. federal government passed the Resource Conservation and Recovery Act (RCRA) in 1976. In 1988 the Plastic Bottle Institute of the Society of the Plastics Industry established a system for identifying plastic containers by plastic type. The purpose of the "chasing arrows" symbol that appears on the bottom of many plastic containers is to promote plastics recycling. The chasing arrows enclose a number (such as a 1 indicating PET, a 2 indicating high density polyethylene (HDPE), and a 3 indicating PVC), which aids in the plastics sorting process.

By 1994, 40 states had legislative mandates for litter control and recycling. Today, a growing number of communities have collection centers for recyclable materials, and some larger municipalities have implemented curbside pickup for recyclable materials, including plastics, paper, metal, and glass.

Statistical Mechanics

Statistical Mechanics, in physics, field that seeks to predict the average properties of systems that consist of a very large number of particles. Statistical mechanics employs principles of statistics to predict and describe particle motion.

Statistical mechanics was developed in the 19th century, largely by British physicist James Clerk Maxwell, Austrian physicist Ludwig Boltzmann, and American mathematical physicist J. Willard Gibbs. These scientists believed that matter is composed of many tiny particles (atoms and molecules) in constant motion. These scientists knew that determining the motions of the particles by assuming each particle individually obeys Newtonian mechanics is unworkable, because any sample of matter contains an enormous number of particles. For example, a cubic foot of air contains about a trillion trillion (1 followed by 24 zeroes) particles. Rather than dealing with all of these microscopic particles individually, Maxwell, Boltzmann, and Gibbs developed statistical techniques to average the microscopic dynamics of individual particles and obtain their macroscopic (large-scale) thermodynamic features. Through their calculations they discovered that temperature is a measure of the average kinetic energy of microscopic particles. They also found that entropy is proportional to the logarithm of the number of ways a given macroscopic system can be microscopically arranged.

Statistical mechanics had to be extended in the 1920s to incorporate the new principles of quantum theory. The nature of particles is regarded differently in quantum theory than in classical physics, which is based on Newton's laws of motion. In particular, two classical particles are in principle distinguishable; just as two cue balls can be distinguished by placing an identifying mark on one, so in principle can classical particles. In contrast, two identical quantum particles are indistinguishable, even in principle, requiring new formulations of statistical mechanics. Furthermore, there are two quantum mechanical formulations of statistical mechanics corresponding to the two types of quantum particles—fermions and bosons. The formulation of statistical mechanics designed to describe the behavior of a group of classical particles is called Maxwell-Boltzmann (MB) statistics. The two formulations of statistical mechanics used to describe quantum particles are Fermi-Dirac (FD) statistics, which applies to fermions, and Bose-Einstein (BE) statistics, which applies to bosons.

Two formulations of quantum statistical mechanics are needed because fermions and bosons have significantly different properties. Fermions—particles that have odd half-integer spin—obey the Pauli exclusion principle, which states that two fermions cannot be in the same quantum mechanical state. Some examples of fermions are electrons, protons, and helium-3. On the other hand, bosons—particles that have integer spin—do not obey the Pauli exclusion principle. Some examples of bosons are photons and helium-4. While only one fermion at a time can be in a particular quantum mechanical state, it is possible for multiple bosons to be in a single state.

The phenomenon of superconductivity dramatically illustrates the differences between systems of quantum mechanical particles that obey Bose-Einstein statistics instead of Fermi-Dirac statistics. At room temperature, electrons, which have spin y, are distributed among their possible energy states according to FD statistics. At very low temperatures, the electrons pair up to form spin-0 Cooper electron pairs, named after the American physicist Leon Cooper. Since these electron pairs have zero spin, they behave as bosons, and promptly condense into the same ground state. A large energy gap between this ground state and the first excited state ensures that any current is “frozen in.” This causes the current to flow through without resistance, which is one of the defining properties of superconducting materials.

Date: 2015-01-02; view: 908