HOW PLASTIC IS MADE
Plastic: Any of various complex organic compounds produced by polymerization, capable of being
molded, extruded, cast into various shapes and films, or drawn into filaments used as textile fibers.
-- Webster's Dictionary

That's an interesting definition, you may be thinking, but really -- how is plastic made? What's involved in
the plastics manufacturing process?
The Basics of Plastic Manufacturing

The term "plastics" encompasses organic materials, such as the elements carbon (C), hydrogen (H),
nitrogen (N), chlorine (Cl) and sulfur (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 syththesis from primary chemicals coming from oil, natural
gas or coal.

The plastic manufacturing 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 vinylchloride 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 molded 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

The Structure of Polymers

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.

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 sulfur 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.

The Additives

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 color; and to provide
special characteristics such as improved surface appearance or reduced friction.

Types of Additives:

antioxidants: for outside application
colorants: for colored plastic parts
foaming agents: for styrofoam cups
plasticizers: used in toys and food processing equipment

Two Characterizations Of Plastic

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,
molding 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 fulfill 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 fibers
automotive bumpers
icrowave containers
external prosthesesyes

Polyvinyl chloride (PVC):

sheathing for electrical cables
floor and wall coverings
siding
credit cards
automobile instrument panels

Processing Methods

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 molding - Since this process can produce moldings 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 molding, 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
mold. Once the plastic cools to a solid state, the mold opens and the finished product is ejected. This
process is used to make such items as butter tubs, yogurt containers, closures, fittings and razors.

Blow molding - Blow molding 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 mold 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 molded parts. This method is used
to make items such as commercial drums and bottles.

Rotational Molding - This process is relatively simple in concept since heat is used to melt and fuse a
plastic resin inside a closed mold without using pressure. Rotational molding consists of a mold mounted
on a machine capable of rotating on two axes simultaneously. Solid or liquid resin is then placed within
the mold and heat is then applied. Rotation distributes the plastic into a uniform coating on the inside of
the mold until the plastic part cools and sets. This process is used to make hollow configurations.
Common rotationally molded products include shipping drums, storage tanks and some consumer
furniture and toys.




By using plastic in packaging, product manufacturers save enough energy each year to power a city of 1
million homes for three and a half years.



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