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  Plastics Engineering

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 :

  1. MECHANICAL PROPERTIES OF PLASTICS

  2. EFFECTS OF THE ENVIRONMENT ON PLASTICS

  3. ELECTRICAL PROPERTIES OF PLASTICS

  4. 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.

  • 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!

  • Plastic lumber, made with recycled plastic, holds nails and screws better than wood, is virtually maintenance free and lasts for 50 years!

  • Foam polystyrene containers take 30 percent less total energy to make than paperboard containers.

  • 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.

  • Between 1990 and 1996 the amount of waste going into landfills declined by more than 17 percent (by weight).

  • 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.

  • Today, over 12,000 communities provide recycling services to 184 million people.

  • The post consumer plastics recycling industry provides jobs for more than 52,000 American workers.

  • Foam polystyrene containers take 30 percent less total energy to make than paperboard containers.

 

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