Nonmetallic materials include polymers, which have a repeating molecular structure based on carbon backbones. There are different types of polymers including linear, branched, crosslinked, and network polymers. Polymers are useful because they are lightweight, corrosion resistant, easy to process at low temperatures, and inexpensive. Polymers can be categorized as thermoplastics, which soften when heated and harden when cooled, or thermosets, which become permanently hardened after an initial heating and cooling cycle. Common thermoplastics and thermosets along with their applications are discussed.

1. Nonmetallic Materials
Materials that do not have the properties of, or do not contain, metal
and that are able to combine with hydrogen to form stable
compounds, acids, acidic oxides, and anions.
Polymers has a repeating structure, usually based on a carbon
backbone. The repeating structure results in large chainlike
molecules. In each polymer molecule, the atoms are bound together
by covalent bonds.

3. Types of polymers
• Linear polymers
polyethylene, polyvinyl chhloride, polystyrene, nylon, teflon
2. Branched polymers
elastomers or polymeric rubbers
3. Crosslinked polymers
thermosetting polymers
4. Network polymers
epoxies

4. Polymers are useful because they are:
3. lightweight
4. corrosion resistant
5. easy to process at low temperatures
6. inexpensive
The mechanical properties of polymers:
10. low strength
11. high toughness
The distinct properties of polymers is they are poor conductors of
electricity and heat, which makes them good insulators.

5. Physical properties of polymers
2. Composed of very large molecules
3. Low stiffness/elasticity
4. Low tensile and compressive strengths
5. Deformation is sensitive to temperature
6. Crystalline structure
7. Low thermal and electrical conductivity – good conductor
8. Creep at room temperature
9. Low temperature makes plastics brittle
10. Plastic deformation

7. Thermoplastics
2. Soften when heated and harden when cooled
3. Varying degree of ductility
4. Can be recycled
5. Can not withstand high temperature
Thermosets
2. Become permanently hard after the initial heating-
cooling cycle
3. Brittle
4. Can not be recycled
5. Can withstand high temperature

8. EXAMPLE THERMOPLASTICS
1. Acrylic (Known also as PERSPEX) is the most common plastic
used. It is purchased usually in the form of sheets and comes in a range
of colours. It can be translucent (e.g. smoked), transparent or opaque. It
is resistant to most acids and weather conditions.
2. Polyethylene or Polythene (PE) can be moulded into almost any
form due to its excellent moulding qualities. Used for the production
of bottles, bowls, toys, tube etc... It is available in large sheets. There
are two types: High density which is rigid and hard, and low density
which is tough and flexible. Machine parts are generally made from
high density polystyrene whilst bottles are made from the low density
polystyrene.

9. 3. Polyvinyl Chloride also known as PVC. It is a tough material which
can be purchased as a hard material or alternatively a flexible form. It
can be welded or bonded with an adhesive. It has a range of uses
including water pipes, raincoats, long play records, coating on
electrical wires and many more.
4. Polypropylene (PP) is used for carpet fibers, ropes, liquid containers
(cup, buckets, tanks), pipes, etc
5. Polystyrene (PS) is used for pakaging foams, egg cartoons, light
panels, electrical appliance components, etc

10. EXAMPLE THERMOSETTING PLASTICS
1. Many adhesives (glues) are thermosetting plastics. A good example
is ‘Araldite’ which is an epoxy resin that hardens when a second
chemical is added (a catalyst). It will bond most materials including
woods and metals as well as some plastics.
2. Polyurethane. This forms the basis of many paints and varnishes
because it is very tough and has water resistant qualities.
3. Melamine Formaldehyde is used in the production of plastic
laminates because of its smooth surface and hygienic qualities. It is
also used in electrical plugs and sockets because it can be cast and it is
an excellent insulator.
4. Polyester resins. If resins are combined with a material such as fibre
glass the result is a very tough material that can resist impact. This
type of material is known as a glass reinforced plastic (GRP) and is
used in car body repairs, sailing boats, corrugated sheet because of its
lightness, toughness and resistance to water.

11. MORE USEFUL PLASTICS
Styrofoam is a light-weight dense foam. It is easy to shape with saws,
files and abrasive paper and is ideal as a model making material.
Expanded Polystyrene is an efficient insulator used in cavity walls and
as decorative tiles and covering on ceilings. It can also be used for
basic model making.
Nylon is used in engineering to make gears and bearings. It’s oily
nature means that friction is reduced between moving parts made
from nylon.