Chapter 25 Section 3

Polymers A polymer is a class of natural or manufactured substances that is composed of molecules arranged in large chains with small, simple, repeating units called monomers. The idea of large long chains was actually adapted from spider spinning their silk. A monomer is one specific molecule that is repeated in the polymer chain. Polypropylene might have 50,000 to 200,000 monomers in its chain.

Some polymers are naturally occurring such as proteins, cellulose, and nucleic acids. However there are a lot that are synthetic or manufactured. Humans have used polymers for centuries. From ancient Egyptians soaking their burial wrappings in resin to using synthetic skin to help a burn patient heal.

Hydrocarbons Synthetic polymers are usually made from fossil fuels such as oil, coal, or natural gas. Fossil fuels are primarily composed of carbon and hydrogen and are referred to as hydrocarbons. This means that synthetic polymers are primarily composed of carbon and hydrogen.

Plastics Plastic are widely used because of its versatile properties. It is usually lightweight, strong, impact resistant, waterproof, moldable, chemical resistant, and inexpensive. The properties of plastics varies for the needs of the job. It can be clear, it can be made to melt at high temperature, it can be flexible. These are all examples of how the composition of the polymer changes the properties.

Plastics are made of the monomer ethylene Plastics are made of the monomer ethylene. It can be modified into LDPE (Low-density polyethylene), HPDE (High-density polyethylene), or even have other substances replace part of the ethylene monomer like chlorine to make polyvinyl chloride (PVC) Low-density is softer and more flexible as it has many branches off making lots of spaces between the particles. High-density is harder and more durable as it has just strait chains that can me stacked tightly together. By adding new elements to the monomer you can manipulate properties like strength and heat resistance.

Synthetic Fibers Nylon, polyester, acrylic, and polypropylene are examples of polymers that can be manufactured as fibers. Nylon is often used in wind and water resistant clothing such a lightweight jackets. Polyester is used in conjunction most often with cotton to make things like clothes and fill things like pillows and quilts. Aramids are a special type of nylon that can be used to make fireproof clothing, bullet proof vests, race car survival cells, puncture resistant gloves, and motorcycle clothing. Even though this material is lightweight it is five times stronger than steel.

Adhesives Synthetic polymers are used to make adhesives that can be modified to provide the best properties for a particular application. Contact elements are used in the manufacture of automobile parts, furniture, leather goods, and decorative laminates. They adhere instantly and get stronger after is dries. Structural adhesives are used in construction projects. An example would be silicone which is used to seal windows and doors to prevent the loss of heat. UV-cured adhesives are used by orthodontists to adhere brace brackets to teeth. There are also hot-melts and transparent, pressure sensitive tape.

Surface Coatings and Elastic Polymers Many surface coatings use synthetic polymers. Polyurethane is a popular polymer that is used to protect and enhance wood surfaces. Many paints use synthetic polymers in their composition. Synthetic rubber is an elastic polymer, it is used to manufacture tires, gaskets, belts, and hoses. The soles of some shoes are even made from this polymer.

Composites A composite is a mixture of two or more materials, one embedded in the other. Composite materials of plastic are used to construct boat and car bodies. These bodies are made of a fiberglass composite which is composed of small particles of glass embedded into plastic. The structure of the fiber glass enhances the plastic making it stronger while still being very light.

If a substance was lightweight but brittle, as most plastics are, then a flexible fiber can be embedded in the substance to make it more flexible and less brittle. Many composites are used in the construction of satellites and aircrafts. This is due to the light weight yet strong properties of the composites. Because it is lighter it reduces the cost for fuel making it cheaper over all to produce and use.

Section Check What is a polymer? Name at least three categories of polymers. What are some specific uses of adhesives? What is a composite? Why are composites used in aircrafts? Define the term fiberglass composite.