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Learn about
Plastics |
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How plastics are made
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Plastic: Any of various complex organic compounds produced by polymerisation, capable of being moulded, extruded, cast into various shapes and
films, or drawn into filaments used as textile fibres.
-- Webster's Dictionary
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The basics
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The term "plastics" encompasses organic materials, such as the
elements carbon (C), hydrogen (H), nitrogen (N), chlorine (Cl) and sulphur (S),
which have properties similar to those naturally grown in organic materials such
as wood, horn and rosin. Organic materials are based on polymers, which are
produced by the conversion of natural products or by synthesis from primary
chemicals coming from oil, natural gas or coal.
The plastic production process begins by heating the hydrocarbons in a
"cracking process." Here, in the presence of a catalyst, larger
molecules are broken down into smaller ones such as ethylene (ethene) C2H4,
propylene (propene) C3H6, and butene C4H8 and other hydrocarbons. The yield of
ethylene is controlled by the cracking temperature and is more than 30% at 850°C
and such products as styrene and vinyl chloride can be produced in subsequent
reactions. These are then the starting materials for several other types of
plastics. Therefore, this process results in the conversion of the natural gas
or crude oil components into monomers such as ethylene, propylene, butene and
styrene.
These monomers are then chemically bonded into chains called polymers.
Different combinations of monomers yield plastic resins with different
properties and characteristics. Each monomer yields a plastic resin with
different properties and characteristics. Combinations of monomers produce
copolymers with further property variations.
The resulting resins may be moulded or formed to produce several different
kinds of plastic products with application in many major markets. The
variability of resin permits a compound to be tailored to a specific design or
performance requirement. This is why certain plastics are best suited for some
applications while others are best suited for entirely different applications.
For instance, impact strength measures the ability of a material to withstand
shock loading. Heat resistance protects the resin from exposure to excessive
temperatures. Chemical resistance protects the resin from breakdown due to
exposure to environmental chemicals.
Some examples of material properties in plastic product applications are:
- Hot-filled packaging used for products such as ketchup
- Chemical-resistant packaging used for products such as bleach
- Impact strength of car bumpers
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The Structure of Polymers
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Polymers are created by the chemical bonding of many identical or related
basic units and those produced from a single monomer type are called
homopolymers. These polymers are specifically made of small units bonded into
long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are
bonded along the carbon backbone.
Polymers that contain primarily carbon and hydrogen are classified as organic
polymers. Polypropylene, polybutylene, polystyrene, and polymethylpentene are
examples of these. Below is a diagram of polyethylene, the simplest polymer
structure.
Even though the basic makeup of many polymers is carbon and hydrogen, other
elements can also be involved. Oxygen, chlorine, fluorine, nitrogen, silicon,
phosphorous, and sulphur are other elements that are found in the molecular
makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains
nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen.
There are also some polymers that, instead of having a carbon backbone, have a
silicon or phosphorous backbone and these are considered inorganic polymers.
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The additives
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When plastics emerge from reactors, they do not have the desired properties
that make it a material of choice, that is, it is considered a raw material. In
order to achieve a commercial product, the plastic is subject to further
treatment and the inclusion of additives which are selected to give it specified
properties. Most polymers are blended with additives during raw material
processing into their finished parts. Additives are incorporated into polymers
to alter and improve their basic mechanical, physical or chemical properties.
Additives are also used to protect the polymer from the degrading effects of
light, heat, or bacteria; to change such polymer properties as flow; to provide
product colour; and to provide special characteristics such as improved surface
appearance or reduced friction.
Types of Additives:
- antioxidants: for outside application
- colorants: for coloured plastic parts
- foaming agents: for styrofoam cups
- plasticizers: used in toys and food processing equipment
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Two Characterizations Of Plastic
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A Thermoset is a polymer that solidifies or "sets" irreversibly
when heated. Similar to the relationship between a raw and a cooked egg, once
heated, a thermoset polymer can't be softened again and once cooked, the egg
cannot revert back to its original form. Thermosets are valued for their
durability and strength and are used primarily in automobiles and construction,
although applications such as adhesives, inks, and coatings are also
significant. Other examples of thermoset plastics and their product applications
are:
Polyurethanes:
- mattresses
- cushions
- insulation
- ski boots
- toys
Unsaturated Polyesters:
- lacquers
- varnishes
- boat hulls
- furniture
Epoxies:
- glues
- coating for electrical circuits
- helicopter blades
A Thermoplastic is a polymer in which the molecules are held together by weak
secondary bonding forces that soften when exposed to heat and return to its
original condition when cooled back down to room temperature. When a
thermoplastic is softened by heat, it can then be shaped by extrusion, moulding
or pressing. Ice cubes are a common household item which exemplify the
thermoplastic principle. Ice will melt when heated but readily solidifies when
cooled. Like a polymer, this process may be repeated numerous times.
Thermoplastics offer versatility and a wide range of applications. They make up
the greatest share of plastics used in food packaging because they can be
rapidly and economically formed into any shape needed to fulfil the packaging
function. Examples include milk jugs and soda bottles. Other examples of
thermoplastics are:
Polyethylene:
- packaging
- electrical insulation
- milk and water bottles
- packaging film
- house wrap
- agricultural film
Polypropylene:
- carpet fibres
- automotive bumpers
- microwave containers
- external prostheses yes
Polyvinyl chloride (PVC):
- sheathing for electrical cables
- floor and wall coverings
- siding
- credit cards
- automobile instrument panels
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Processing Methods
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There are a variety of different processing methods used to convert resins
into finished products. Some include:
Extrusion: This continuous process is used for the production of
semi-finished goods such as films, sheet profiles, tubs and pipes. They are
termed "semi-finished" because they must be further processed before
they become useful articles. Plastic material is first loaded into a hopper and
then fed into a long heated chamber through which it is moved by the action of a
continuously revolving screw. At the end of the heated chamber, the molten
plastic is forced out through a small opening called a die that is cast in the
shape of the finished product. As the plastic extrusion comes from the die, it
is fed onto a conveyor belt where it is cooled by blowers or by immersion in
water. The operation's principle is the same as that of a meat mincer but with
added heaters in the wall of the extruder. Examples of products include lawn
edging, pipe, film and window trim.
Injection moulding: Since this process can produce mouldings of high
quality and with great accuracy, it is very widespread. It is predominately used
for thermoplastics but smaller amounts of thermosets and elastomers are also
processed this way. In injection moulding, plastic material is also put into a
hopper, which feeds into a heating chamber. A plunger pushes the plastic through
the heating chamber where the material is then softened into a fluid state. At
the end of this chamber, the resin is forced into a closed mould. Once the
plastic cools to a solid state, the mould opens and the finished product is
ejected. This process is used to make such items as butter tubs, yoghurt
containers, closures, fittings and razors.
Blow Moulding: Blow moulding is a process used in conjunction with
extrusion. The die forms a molten tube of thermoplastic material. Using
compressed air, the tube is then blown to conform to the interior of a chilled mould
which clamps around the tube. Overall, the goal is to produce a uniform melt,
form it into a tube with the desired cross section and blow it into the exact
shape of the product. This process is intended for use in manufacturing hollow
plastic products and its principal advantage is its ability to produce hollow
shapes without having to join two or more separately moulded parts. This method
is used to make items such as commercial drums and bottles.
Rotational Moulding: This process is relatively simple in concept
since heat is used to melt and fuse a plastic resin inside a closed mould
without using pressure. Rotational moulding consists of a mould mounted on a
machine capable of rotating on two axes simultaneously. Solid or liquid resin is
then placed within the mould and heat is then applied. Rotation distributes the
plastic into a uniform coating on the inside of the mould until the plastic part
cools and sets. This process is used to make hollow configurations. Common
rotationally moulded products include shipping drums, storage tanks and some
consumer furniture and toys.
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