They are characterized by the fact that they soften when heated and can be molded into different shapes, knowing that upon cooling they will harden again while maintaining their initial characteristics.When heated, the molecules are given the necessary energy to separate, and this gives them the freedom to change their relative position and give rise to a new shape when under pressure. This softening and hardening process can be repeated over and over again without the material changing its appearance or properties.Some examples of thermoplastics:
POLYETHYLENE
Polyethylenes come in two forms, high density and low density. HIGH DENSITY POLYETHYLENE (HDPE) is made in such a way that the polymer chains are straight, which allows them to be packed together, producing a high density material. Because the chains are close together, the forces of attraction between them are very large and they have less freedom to move. The result is a fairly rigid, strong and resistant plastic. It softens at a fairly high temperature ( 120 - 130 oC) and is resistant to chemical attack. Applications: Boxes, toys, pipes, bottles, ..... LOW DENSITY POLYETHYLENES are manufactured by a process that produces lateral bifurcations in the polymer chains. These bifurcations prevent the chains from crowding together, and as a result the attraction between them is weaker. The plastic is softer and more flexible than high-density polyethylene. Less energy is required to separate the chains, which means that it softens at a lower temperature (85 oC). This polymer can be transparent or opaque and is a very good insulator. It is the plastic that we probably "consume" the most. Applications: bags, sleeping bags, greenhouses...
POLYPROPYLENE
It belongs to the same family of plastics as polyethylene. However, it is stronger and more rigid than high density polyethylene. It is also more resistant to heat, softening at approximately 150 oC. It is the lowest density thermoplastic, yet it has a very high resistance to snouting.
Another of its most valuable characteristics is its ability to be bent thousands of times without breaking.
Applications: Among other products, disposable cutlery, safety helmets, plumbing parts, stackable chairs, children's toys, etc. are made from polypropylene.
PVC (POLYVINYL CHLORIDE)
It comes in rigid or flexible form.Rigid PVC is very durable and is used to make gutters and pipes.Flexible PVC is made by adding a plasticizer to PVC. The plasticizer has small molecules that separate the polymer chains, making them less strongly attracted to each other. As a result of this less attraction the polymer becomes softer and more flexible.
Applications: In its soft form PVC is used as an insulator for electrical cables, and in the manufacture of SOME WATERPROOF CLOTHING.
If we add to PVC a large proportion of plasticizer we can use it to cover fabrics, seats, bags, some furniture, etc.
ACRYLICS
Probably the best known acrylic is methacrylate also known as Plexiglas. Methacrylate can have a glass-like transparency or be opaque. Both forms can be tinted with color pigments. However, it can crack and scratches easily. It can be shaped, bent and twisted when heated to temperatures between 165 and 175 oC. When cold it is very brittle and care must be taken to avoid cracking when cutting or drilling.
We can also find methacrylate in granular form, for use in injection molding machines which we will see later, and in this form it is known as ACRYLIC PLASTIC POWDER.
NAILON
Many different types of nylon are produced, which are identified by a number, e.g. nylon 6.6 or nylon 6.10. Probably the best known form of nylon is the fibers, which are used for the manufacture of carpets, clothing, brushes, socks, etc. It is a fairly tough and wear-resistant material.
Nylon, in addition to fibers, is used in larger parts to make curtain rails and fittings, housings for plugs and sockets, combs, ....etc., and in engineering to make moving parts for gears and bearings, due to its durability and small coefficient of friction, as well as its fairly high melting temperature.
POLYSTYRENE
The two most common forms in which we will find polystyrene are as a transparent crystalline solid and as a spongy plastic known as expanded polystyrene (polystyrene foam).
In its more "solid" form, it is very fragile, and can be identified by the metallic sound it makes when dropped.
Expanded polystyrene is soft and spongy. During its production, a gas is produced and trapped inside its structure. It has good properties as a thermal and acoustic insulator and is therefore used in the construction industry. In addition, its low density means that it weighs very little and its spongy nature means that it absorbs shocks well, which is why it is used for packaging.