Characteristics of
Polymers
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.
ALTERING THE PROPERTIES OF PLASTICS
The properties of the various families of
plastics vary from one another and the polymers can be modified
to alter the properties within a family of plastics. Another
way that the properties of a given plastic are changed is the
addition of items, such as additives, colorants, fillers,
and/or reinforcement.
ADDITIVES (improve specific properties)
Additives are selected to be compatible with the material
and the process conditions for shaping the material. The
improvement of a specific property of a material by the
addition of an additive is usually at the expense of some other
property. The chemist attempts to keep all of the other
material properties as high as possible while achieving the
desired improvement in the specific property, such as improved
resistance to burning. Some of the additives that are used in
thermosets and themoplastics are antioxidants to improve high
temperature stability, antistatic agents, biocides, flame
retardants, impact modifiers, friction reducers, foaming
agents, fungicides, and ultraviolet stabilizers.
REINFORCEMENTS (improve strength)
Other additives enhance the strength of a material. Some
reinforcing materials are carbon, glass, mica, and aramids.
They may be in the form of short fibers, continuous filaments,
mats, spheres, flakes, etc. These reinforcements usually
increase the material's strength at the expense of impact
resistance. The use of reinforcements in plastics permits them
to be used at higher temperatures and loads with greater
dimensional stability. The freedom of design, high strength,
and light weight of composite materials are permitting
significant advances in technology in the aerospace and
aviation fields. Reinforcements tend to make stock shapes, such
as rods, tubes, slabs, etc., more difficult to machine because
of increased tool wear.
COLORANTS (change appearance)
Another group of additives are colorants that provide the
desired color to the material. The colorants may be organic
dyes or inorganic powder. The colorant chosen must be
compatible with the base plastic, shaping process, and the
proposed usages for the finished material. For example, a
colorant must also withstand high temperatures and be
weatherable if the material is to be extruded and then used
outdoors. The type of colorant also affects optical properties
of transparent materials, such as acrylics, polycarbonate, and
styrene. A colorant can make a clear material transparent,
transluscent, or opaque.
Properties of Plastic :
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MECHANICAL PROPERTIES OF PLASTICS
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EFFECTS OF THE ENVIRONMENT ON PLASTICS
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ELECTRICAL PROPERTIES OF PLASTICS
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OPTICAL/COLORABILITY PROPERTIES OF PLASTICS
Every polymer has very distinct
characteristics, but most polymers have the following general
attributes.
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.
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 fiberfill in winter jackets is
acrylic.
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 fiber in pantyhose or Kevlar, which is used in
bulletproof vests.
Polymers can be processed in various ways to produce thin
fibers or very intricate parts. Plastics can be molded 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 (StyrofoamTM) and urethane, to name just two
examples. Polymers are materials with a seemingly limitless
range of characteristics and colors. 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
aluminum. 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
fiber 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, aluminum 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.
Without plastics, 400 percent more material by weight and 200
percent more material by volume would be needed to make
packaging.
Without plastics, 400 percent more material by weight, and 200
percent more material by volume, would be needed to make
packaging.
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For every seven trucks needed to deliver
paper grocery bags to the store - only one truck is needed to
carry the same number of plastic grocery bags!
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Plastic lumber, made with recycled
plastic, holds nails and screws better than wood, is
virtually maintenance free and lasts for 50 years!
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Foam polystyrene containers take 30
percent less total energy to make than paperboard containers.
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The number of plastics recycling
businesses has nearly tripled over the past several years,
with more than 1,700 businesses handling and reclaiming
post-consumer plastics.
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Between 1990 and 1996 the amount of waste
going into landfills declined by more than 17 percent (by
weight).
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By using plastic in packaging, American
product manufacturers save enough energy each year to power a
city of 1 million homes for three and a half years.
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Today, over 12,000 communities provide
recycling services to 184 million people.
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The post consumer plastics recycling
industry provides jobs for more than 52,000 American workers.
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Foam polystyrene containers take 30
percent less total energy to make than paperboard containers.
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