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Learn about
Plastics |
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Plastics ... the basics
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Plastics are polymers. What is a polymer? The most simple definition of a polymer is something made of many units. Think of a polymer as a chain. Each link of the chain is the "-mer" or basic unit that is usually made of carbon, hydrogen, oxygen, and/or silicon. To make the chain, many links or "-mers" are hooked or polymerised together. Polymerisation can be demonstrated by linking countless strips of construction paper together to make paper garlands or hooking together hundreds of paper clips to form chains, or by a string of beads.
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The Structure of Polymers
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Many common classes of
polymers are composed of hydrocarbons. These polymers are specifically made of
small units bonded into long chains. Carbon makes up the backbone of the
molecule and hydrogen atoms are bonded along the backbone. Below is a diagram of
polyethylene, the simplest polymer structure.
There are polymers that
contain only carbon and hydrogen. Polypropylene, polybutylene, polystyrene, and
polymethylpentene are examples of these.Even though the basic
makeup of many polymers is carbon and hydrogen, other elements can also be
involved. Oxygen, chorine, fluorine, nitrogen, silicon, phosphorous, and sulphur
are other elements that are found in the molecular makeup of polymers. Polyvinyl
chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains
fluorine. Polyester and polycarbonates contain oxygen. There are also some
polymers that, instead of having a carbon backbone, have a silicon or
phosphorous backbone. These are considered inorganic polymers. One of the most
famous silicon-based polymers is Silly Putty.
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Molecular Arrangement of Polymers
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Think of how spaghetti
noodles look on a plate. This is similar to how polymers can be arranged if they
lack a specific for or are amorphous. Controlling and quenching the polymerisation
process can result in amorphous organization. An amorphous arrangement of
molecules has no long-range order or form in which the polymer chains arrange
themselves. Amorphous polymers are generally transparent. This is an important
characteristic for many applications such as food wrap, plastic windows,
headlights, and contact lenses.
Obviously not all polymers
are transparent. The polymer chains in objects that are translucent and opaque
are in a crystalline arrangement. By definition a crystalline arrangement has
atoms, ions, or in this case, molecules in a distinct pattern. You generally
think of crystalline structures in salt and gemstones, but not in plastics. Just
as quenching can produce amorphous arrangements, processing can control the
degree of crystallinity. The higher the degree of crystallinity, the less light
can pass through the polymer. Therefore, the degree of translucence or
opaqueness of the polymer is directly affected by its crystallinity.
Scientists and engineers
are always producing better materials by manipulating the molecular structure
that affects the final polymer produced. Manufacturers and processors introduce
various fillers, reinforcements, and additives into the base polymers, expanding
product possibilities.
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Characteristics of Polymers
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Polymers are divided into
two distinct groups: thermoplastics and thermosets. The majority of polymers are
thermoplastic, meaning that once the polymer is formed it can be heated and
reformed over and over again. This property allows for easy processing and
facilitates recycling. The other group, the thermosets, can not be remelted.
Once these polymers are formed, reheating will cause the material to scorch.
Every polymer has very
distinct characteristics, but most polymers have the following general
attributes.
1. Polymers can be
very resistant to chemicals. Consider all the cleaning fluids in your
home that are packaged in plastic. Reading the warning labels that describe
what happens when the chemical comes in contact with skin or eyes or is
ingested will emphasize the chemical resistance of these materials.
2. Polymers can be
both thermal and electrical insulators.
A walk through your house will reinforce this concept, as you consider all
the appliances, cords, electrical outlets and wiring that are made or
covered with polymeric materials. Thermal resistance is evident in the
kitchen with pot and pan handles made of polymers, the coffee pot handles,
the foam core of refrigerators and freezers, insulated cups, coolers and
microwave cookware. The thermal underwear that many skiers wear is made of
polypropylene and the fibrefill in winter jackets is acrylic.
3. Generally, polymers
are very light in weight with varying degrees of strength. Consider the
range of applications, from toys to the frame structure of space stations,
or from delicate nylon fibre in pantyhose or Kevlar, which is used in bullet-proof
vests.
4. Polymers can be
processed in various ways to produce thin fibres or very intricate parts.
Plastics can be moulded into bottles or the bodies of a cars or be mixed
with solvents to become an adhesive or a paint. Elastomers and some plastics
stretch and are very flexible. Other polymers can be foamed like polystyrene
(Styrofoam™) and urethane, to name just two examples. Polymers
are materials with a seemingly limitless range of characteristics and colours.
Polymers have many inherent properties that can be further enhanced by a
wide range of additives to broaden their uses and applications.
In addressing all the
superior attributes of polymers, it is equally important to discuss some of the
difficulties associated with the material. Plastics deteriorate but never
decompose completely, but neither does glass, paper, or aluminium. Plastics make
up 9.5 percent of our trash by weight compared to paper, which constitutes 38.9
percent. Glass and metals make up 13.9 percent by weight.
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 toys,
thus saving natural lumber. Plastic from 2-liter bottles is even being spun into
fibre for the production of carpet.
An option for plastics
that are not recycled, especially those that are soiled, such as used microwave
food wrap or diapers, can be a waste-to-energy system (WTE).
The controlled combustion
of polymers produces heat energy. The heat energy produced by the burning
plastics not only can be converted to electrical energy but helps burn the wet
trash that is present. Paper also produces heat when burned, but not as much as
plastics. On the other hand, glass, aluminium and other metals do not release
any energy when burned.
To better understand the
incineration process, consider the smoke coming off a burning object and then
ignite the smoke with a Bunsen burner. Observe that the smoke disappears. This
is not an illusion, but illustrates that the by-products of incomplete burning
are still flammable. Incineration burns the material and then the by-products of
the initial burning.
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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