PRODUCTION OF PLASTICS

The term “plastics” includes materials composed of
various elements such as carbon, hydrogen,
oxygen, nitrogen, chlorine, and sulfur. Plastics
typically have high molecular weight, meaning each
molecule can have thousands of atoms bound
together. Naturally occurring materials, such as
wood, horn and rosin, are also composed of
molecules of high molecular weight. The
manufactured or synthetic plastics are often
designed to mimic the properties of natural
materials. Plastics, also called polymers, are
produced by the conversion of natural products or
by the synthesis from primary chemicals generally
coming from oil, natural gas, or coal.
Most plastics are based on the carbon atom.
Silicones, which are based on the silicon atom, are
an exception. The carbon atom can link to other
atoms with up to four chemical bonds. When all of
the bonds are to other carbon atoms, diamonds or
graphite or carbon black soot may result. For
plastics the carbon atoms are also connected to
the aforementioned hydrogen, oxygen, nitrogen,
chlorine, or sulfur. When the connections of atoms
result in long chains, like pearls on a string of
pearls, the polymer is called a thermoplastic.
Thermoplastics are characterized by being
meltable. The thermoplastics all have repeat units,
the smallest section of the chain that is identical.
We call these repeat units unit cells. The vast
majority of plastics, about 92%, are
thermoplastics.

TYPES OF PLASTICS.

A Thermoset is a polymer that solidifies or “sets”
irreversibly when heated or cured. Similar to the
relationship between a raw and a cooked egg, a
cooked egg cannot revert back to its original form
once heated, and a thermoset polymer can’t be
softened once “set”. Thermosets are valued for
their durability and strength and are used
extensively in automobiles and construction
including applications such as adhesives, inks, and
coatings. The most common thermoset is the
rubber truck and automobile tire. Some examples
of thermoset plastics and their product applications
are:
Polyurethanes:
• Mattresses
• Cushions
• Insulation
Unsaturated Polyesters:
• Boat hulls
• Bath tubs and shower stalls
• Furniture
Epoxies:
• Adhesive glues
• Coating for electrical devices
• Helicopter and jet engine blades
Phenol Formaldehyde:
• Oriented strand board
• Plywood
• Electrical appliances
• Electrical circuit boards and switches
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 common household items 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 are
commonly 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 carbonated soft
drink 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
• Microwave containers
• External prostheses
Polyvinyl Chloride (PVC):
• Sheathing for electrical cables
• Floor and wall coverings
• Siding
• Automobile instrument panels

METHODS OF PRODUCING PLASTICS

Extrusion – This continuous process is used to
produce films, sheet, profiles, tubes, and pipes.
Plastic material as granules, pellets, or powder, 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. The
chamber is a cylinder and is referred to as an
extruder. Extruders can have one or two revolving
screws. The plastic is melted by the mechanical
work of the screw and the heat from the extruder
wall. At the end of the heated chamber, the molten
plastic is forced out through a small opening
called a die to form the shape of the finished
product. As the plastic is extruded from the die, it
is fed onto a conveyor belt for cooling or onto
rollers for cooling or by immersion in water for
cooling. The operation’s principle is the same as
that of a meat mincer but with added heaters in the
wall of the extruder and cooling of the product.
Examples of extruded products include lawn
edging, pipe, film, coated paper, insulation on
electrical wires, gutter and down spouting, plastic
lumber, and window trim. Thermoplastics are
processed by continuous extrusion. Thermoset
elastomer can be extruded into weatherstripping
by adding catalysts to the rubber material as it is
fed into the extruder.

Calendering – This continuous process is an
extension of film extrusion. The still warm
extrudate is chilled on polished, cold rolls to
create sheet from 0.005 inches thick to 0.500
inches thick. The thickness is well maintained and
surface made smooth by the polished rollers.
Calendering is used for high output and the ability
to deal with low melt strength. Heavy polyethylene
films used for construction vapor and liquid
barriers are calendered. High volume PVC films are
typically made using calendars.

Film Blowing – This process continuously
extrudes vertically a ring of semi-molten polymer
in an upward direction, like a fountain. A bubble of
air is maintained that stretches the plastic axially
and radially into a tube many times the diameter of
the ring. The diameter of the tube depends on the
plastic being processed and the processing
conditions. The tube is cooled by air and is nipped
and wound continuously as a flattened tube. The
tube can be processed to form saleable bags or
slit to form rolls of film with thicknesses of 0.0003
to 0.005 inches thick. Multiple layers of different
resins can be used to make the tube.

Injection Molding – This process can produce
intricate three-dimensional parts of high quality
and great reproducibility. It is predominately used
for thermoplastics but some thermosets and
elastomers are also processed by injection
molding. In injection molding plastic material is
fed into a hopper, which feeds into an extruder. An
extruder screw pushes the plastic through the
heating chamber in which the material is then
melted. At the end of the extruder the molten
plastic is forced at high pressure into a closed
cold mold. The high pressure is needed to be sure
the mold is completely filled. Once the plastic
cools to a solid, the mold opens and the finished
product is ejected. This process is used to make
such items as butter tubs, yogurt containers, bottle
caps, toys, fittings, and lawn chairs. Special
catalysts can be added to create the thermoset
plastic products during the processing, such as
cured silicone rubber parts. Injection molding is a
discontinuous process as the parts are formed in
molds and must be cooled or cured before being
removed. The economics are determined by how
many parts can be made per cycle and how short
the cycles can be.

Blow Molding – Blow molding is a process used in
conjunction with extrusion or injection molding. In
one form, extrusion blow molding, the die forms a
continuous semi-molten tube of thermoplastic
material. A chilled mold is clamped around the
tube and compressed air is then blown into the
tube to conform the tube to the interior of the mold
and to solidify the stretched 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 used
to manufacture hollow plastic products and its
principal advantage is its ability to produce hollow
shapes without having to join two or more
separately injection molded parts. This method is
used to make items such as commercial drums
and milk bottles. Another blow molding technique
is to injection mold an intermediate shape called a
preform and then to heat the preform and blow the
heat-softened plastic into the final shape in a
chilled mold. This is the process to make
carbonated soft drink bottles.

Expanded Bead Blowing – This process begins
with a measured volume of beads of plastic being
placed into a mold. The beads contain a blowing
agent or gas, usually pentane, dissolved in the
plastic. The closed mold is heated to soften the
plastic and the gas expands or blowing agent
generates gas. The result is fused closed cell
structure of foamed plastic that conforms to a
shape, such as expanded polystyrene cups.
Styrofoam™ expanded polystyrene thermal
insulation board is made in a continuous extrusion
process using expanded bead blowing.

Rotational Molding – Rotational molding consists
of a mold mounted on a machine capable of
rotating on two axes simultaneously. Solid or
liquid resin is placed within the mold and heat is
applied. Rotation distributes the plastic into a
uniform coating on the inside of the mold then the
mold is cooled until the plastic part cools and
hardens. This process is used to make hollow
configurations. Common rotationally molded
products include shipping drums, storage tanks
and some consumer furniture and toys.

Compression Molding – This process has a
prepared volume of plastic placed into a mold
cavity and then a second mold or plug is applied
to squeeze the plastic into the desired shape. The
plastic can be a semi-cured thermoset, such as an
automobile tire, or a thermoplastic or a mat of
thermoset resin and long glass fibers, such as for
a boat hull. Compression molding can be
automated or require considerable hand labor.

Transfer molding is a refinement of compression
molding. Transfer molding is used to encapsulate
parts, such as for semi-conductor manufacturing
The formation of plywood or oriented strand board
using thermoset adhesives is a variant of
compression molding. The wood veneer or strands
are coated with catalyzed thermoset phenol
formaldehyde resin and compressed and heated to
cause the thermoset plastic to form into a rigid,
non-melting adhesive.

Casting – This process is the low pressure, often
just pouring, addition of liquid resins to a mold.
Catalyzed thermoset plastics can be formed into
intricate shapes by casting. Molten polymethyl
methacrylate thermoplastic can be cast into slabs
to form windows for commercial aquariums.
Casting can make thick sheet, 0.500 inches to
many inches thick.

Thermoforming – Films of thermoplastic are
heated to soften the film, and then the soft film is
pulled by vacuum or pushed by pressure to
conform to a mold or pressed with a plug into a
mold. Parts are thermoformed either from cut
pieces for thick sheet, over 0.100 inches, or from
rolls of thin sheet. The finished parts are cut from
the sheet and the scrap sheet material recycled for
manufacture of new sheet. The process can be
automated for high volume production of clamshell
food containers or can be a simple hand labor
process to make individual craft items.

Reference: American Chemistry Council, Plastics Industry
Producer Statistics Group, 2005

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