CHAPTER CHAPTER NAME
Properties of plastics
Common plastics & uses
Different manufacturing processes for Plastics:
Design Rules For Plastic Parts:
Maximum wall thickness
TYPICAL NOMINAL THICKNESS FOR VARIOUS
CLASSES OF THERMOPLASTICS
DEFECTS of PLASTICS
Hot gas welding
Speed tip welding
Hot plate welding
High frequency welding
Transparent Laser Plastic Welding
PLASTICS Growing demand in the automotive sector
Plastics In Automotive Markets Today
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids
that are mouldable. Plastics are typically organic polymers of high molecular mass, but they
often contain other substances. They are usually synthetic, most commonly derived from
petrochemicals, but many are partially natural.
Early plastics were bio-derived materials such as egg and blood proteins, which are
organic polymers. Treated cattle horns were used as windows for lanterns in the Middle
Ages. Materials that mimicked the properties of horns were developed by treating milk-
proteins (casein) with lye. In the 1800s the development of plastics accelerated with Charles
Goodyear's discovery of vulcanization as a route to thermoset materials derived from natural
rubber. Many storied materials were reported as industrial chemistry was developed in the
1800s. In the early 1900s, Bakelite, the first fully synthetic thermoset was reported by
Belgian chemist Leo Baekeland. After the First World War, improvements in chemical
technology led to an explosion in new forms of plastics. Among the earliest examples in the
wave of new polymers were polystyrene (PS) and polyvinyl chloride (PVC). The
development of plastics has come from the use of natural plastic materials (e.g., chewing
gum, shellac) to the use of chemically modified natural materials (e.g., rubber,
nitrocellulose, collagen, galalite) and finally to completely synthetic molecules
(e.g., bakelite, epoxy, polyvinyl chloride).
The first so called plastic based on a synthetic polymer was made
from phenol and formaldehyde, with the first viable and cheap synthesis methods invented in
1907, by Leo Hendrik Baekeland, a Belgian-born American living in New York state.
Baekeland was looking for an insulating shellac to coat wires in electric motors and
generators. He found that combining phenol (C6H5OH) and formaldehyde (HCOH) formed a
sticky mass and later found that the material could be mixed with wood flour, asbestos, or
slate dust to create strong and fire resistant "composite" materials. The new material tended
to foam during synthesis, requiring that Baekeland build pressure vessels to force out the
bubbles and provide a smooth, uniform product, as he announced in 1909, in a meeting of the
American Chemical Society.
Bakelite was originally used for electrical and mechanical
parts, coming into widespread use in consumer goods and jewelry in the 1920s. Bakelite was
a purely synthetic material, not derived from living matter. It was also an early thermosetting
ASTM: American Standard Test Methods; is a scientific organisation defining standards on
physical and mechanical testing of materials to obtain objective characteristics used for
comparison purposes and for design of articles. The standards are partly used to formulate
ISO (International Standards Organisation) ones.
Most plastics contain organic polymers. The vast majority of these polymers are
based on chains of carbon atoms alone or with oxygen, sulphur, or nitrogen as well. The
backbone is that part of the chain on the main "path" linking a large number of repeat units
together. To customize the properties of a plastic, different molecular groups "hang" from the
backbone (usually they are "hung" as part of the monomers before linking monomers
together to form the polymer chain). The structure of these "side chains" influence the
properties of the polymer. This fine tuning of the properties of the polymer by repeating unit's
molecular structure has allowed plastics to become an indispensable part of the twenty-first
Additives: these are in general low molecular weight chemicals added to plastics and
rubbers to improve certain characteristics such as ultraviolet absorbers, antioxidants and heat
stabilisers, lubricants, plasticisers, flame retardants, cross-linking and blowing agents,
pigments and dyes. Impact modifiers are polymeric materials added to improve the impact
resistance of e.g. PVC, PP, PBT, PA. A separate class of additives are the fillers such as
talcum, wood flour, and reinforcing agents like glass and carbon fibres.
Alloys: strictly speaking, alloys refer to metals and do not exist in plastics. The term is used
interchangeably with blends for mixtures of two or more polymers. Examples are alloys or
blends of polycarbonate (PC) with ABS or with polybutylene terephthalate (PBT).
Blend" an intimate mixture of two or more polymers to obtain the good properties of each,
for example semi-crystalline polypropylene (PP) mixed with 10 to 30% rubbery EPDM
results in a blend with good heat resistance and extraordinary impact resistance. Also the mix
of polycarbonate (PC) with ABS terpolymer results in a blend with the good heat resistance
of the PC part and the low temperature impact resistance of the ABS. Instead of the term
blend, trade literature and producers also use alloy. Blends are made passing the components
in powder or pellet form in a dry blender followed by a heated twin screw extruder to obtain
an intimate blend.
Polymerization process Plastics are one group of polymers that are built from relatively simple units called
monomers (or mers) through a chemical polymerization process. This process is illustrated
below. Processing polymers into end products mainly involves physical phase change such as
melting and solidification (for Thermoplastics) or a chemical reaction (for Thermosets).
Structure of polymers The basic structure of a polymer molecule can be visualized as
a long chain of repeating units, with additional chemical groups forming pendant branches
along the primary "backbone" of the molecule. Although the term plastics has been used
loosely as a synonym for polymer and resin, plastics generally represent polymeric
compounds that are formulated with plasticizers, stabilizers, fillers, and other additives for
purposes of processability and performance. Other polymeric systems include rubbers, fibers,
adhesives, and surface coatings. A variety of processes have been employed to produce the
final plastic parts,
Polymer family, the formation of plastics, and the polymerization process
Plastics are usually classified by their chemical structure of the polymer's backbone
and side chains. Some important groups in these classifications are
the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Plastics can also
be classified by the chemical process used in their synthesis, such as condensation, poly-
addition, and cross-linking.
There are two types of plastics: thermoplastics and thermosetting polymers.
Thermoplastics are the plastics that do not undergo chemical change in their composition
when heated and can be molded again and again.
A Thermoplastic, also known as a thermosoftening plastic, is a polymer that
becomes pliable or moldable above a specific temperature, and returns to a solid state upon
cooling. Most thermoplastics have a high molecular weight, whose chains associate
through intermolecular forces; this property allows thermoplastics to be remolded because the
intermolecular interactions spontaneously reform upon cooling. In this way, thermoplastics
differ from thermosetting polymers, which form irreversible chemical bonds during the
curing process; thermoset bonds break down upon melting and do not reform upon cooling.
Thermoplastic materials can be formed into desired shapes under heat and pressure and
become solids on cooling.
If they are subjected to the same conditions of heat and pressure, they can be reprocessed
into new shapes.
Thermoplastics based on their crystallization are classified into
Some thermoplastics donot crystallise on heating and are termed as amorphous plastics
Used in applications where clarity is important.