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Thermoplastics
vs Thermosets
Thermoplastics and thermosetting plastics are terms that describe how a polymer reacts to heat. All plastics, whether made by addition or condensation polymerization, can be divided into two groups: thermoplastics and thermosetting plastics. Thermoplastics can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics, on the other hand, harden permanently after being heated once.
The Difference - Weak Van Der Waal Forces
The reason for the difference in response to heat between thermoplastics
and thermosetting plastics lies in the chemical structures of the plastics.
Thermoplastic molecules, which are linear or slightly branched, do not
chemically bond with each other when heated. Instead, thermoplastic chains
are held together by weak van der Waal forces (weak attractions
between the molecules) that cause the long molecular chains to clump together
like piles of entangled spaghetti. Thermoplastics can be heated and cooled,
and consequently softened and hardened, repeatedly, like candle wax. For
this reason, thermoplastics can be remolded and reused almost indefinitely.
Thermosetting Plastics
Thermosetting plastics consist of chain molecules that chemically bond,
or cross-link, with each other when heated. When thermosetting plastics
cross-link, the molecules create a permanent, three-dimensional network
that can be considered one giant molecule. Once cured, thermosetting plastics
cannot be remelted, in the same way that cured concrete cannot be reset.
Consequently, thermosetting plastics are often used to make heat-resistant
products, because these plastics can be heated to temperatures of 260°
C (500° F) without melting.
Thermoplastics
The different molecular structures of thermoplastics and thermosetting
plastics allow manufacturers to customize the properties of commercial
plastics for specific applications. Because thermoplastic materials consist
of individual molecules, properties of thermoplastics are largely influenced
by molecular weight. For instance, increasing the molecular weight of
a thermoplastic material increases its tensile strength, impact strength,
and fatigue strength (ability of a material to withstand constant stress).
Conversely, because thermosetting plastics consist of a single molecular
network, molecular weight does not significantly influence the properties
of these plastics. Instead, many properties of thermosetting plastics
are determined by adding different types and amounts of fillers and reinforcements,
such as glass fibers.
The Processes of Making Plastics
The process of forming plastic resins into plastic products is the basis
of the plastics industry. Many different processes are used to make plastic
products, and in each process, the plastic resin must be softened or sufficiently
liquefied to be shaped. Although some processes are used to manufacture
both thermoplastics and thermosetting plastics, certain processes are
specific to forming thermoplastics.
Injection
Molding
Injection molding uses a piston or screw to force plastic resin through
a heated tube into a mold, where the plastic cools and hardens to the
shape of the mold. The mold is then opened and the plastic cast removed.
Thermoplastic items made by injection molding include toys, combs, car
grills, and various containers.
Extrusion
Extrusion is a continuous process, as opposed to all other plastic production
processes, which start over at the beginning of the process after each
new part is removed from the mold. In the extrusion process, plastic pellets
are first heated in a long barrel. In a manner similar to that of a pasta-making
or sausage-stuffing machine, a rotating screw then forces the heated plastic
through a die (device used for forming material) opening of the desired
shape.
As the continuous plastic form emerges from the die opening, it is cooled and solidified, and the continuous plastic form is then cut to the desired length. Plastic products made by extrusion include garden hoses, drinking straws, pipes, and ropes. Melted thermoplastic forced through extremely fine die holes can be cooled and woven into fabrics for clothes, curtains, and carpets.
Blow Molding
Blow molding is used to form bottles and other containers from soft, hollow
thermoplastic tubes. First a mold is fitted around the outside of the
softened thermoplastic tube, and then the tube is heated. Next, air is
blown into the softened tube (similar to inflating a balloon), which forces
the outside of the softened tube to conform to the inside walls of the
mold. Once the plastic cools, the mold is opened and the newly molded
container is removed. Blow molding is used to make many plastic containers,
including soft-drink bottles, jars, detergent bottles, and storage drums.
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