They are characterized by the fact that they soften when heated and can be molded into different shapes, knowing that when they cool, they will harden again while retaining their original properties. When heated, the molecules receive the energy needed to separate, which gives them the freedom to change their relative positions and take on a new shape when under pressure. This process of softening and hardening can be repeated over and over again without the material changing its appearance or properties.Some examples of thermoplastics:
POLYETHYLENE
Polyethylenes come in two types: high-density and low-density.
HIGH-DENSITY POLYETHYLENES (HDPE) are manufactured so that the polymer chains are straight, allowing them to pack tightly together and producing a high-density material. Because the chains are very close together, the attractive forces between them are very strong, and they have less freedom to move.
The result is a fairly rigid, strong, and durable plastic. It softens at a fairly high temperature (120–130 °C) and is resistant to chemical attack. Applications: Boxes, toys, pipes, bottles…
LOW-DENSITY POLYETHYLENES are manufactured using a process that creates lateral branches in the polymer chains. These branches prevent the chains from packing tightly 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 it softens at a lower temperature (85°C). This polymer can be transparent or opaque and is a very good insulator. It is probably the plastic we “use” the most. Applications: bags, sleeping bags, greenhouses…
POLYPROPYLENE
It belongs to the same family of plastics as polyethylenes. However, it is stronger and stiffer than high-density polyethylene. It also has greater heat resistance, softening at approximately 150 °C. It is the thermoplastic with the lowest density, yet it has very high impact resistance.
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 fittings, stackable chairs, children’s toys, etc., are made from polypropylene.
PVC (POLYVINYL CHLORIDE)
It comes in rigid or flexible forms. Rigid PVC is very durable and is used to make gutters and pipes. Flexible PVC is produced by adding a plasticizer to the PVC. The plasticizer contains small molecules that separate the polymer chains, causing them to attract each other with less force. As a result of this reduced attraction, the polymer becomes softer and more flexible.
Applications: In its soft form, PVC is used as insulation for electrical cables and in the manufacture of some waterproof clothing.
If we add a large proportion of plasticizer to PVC, we can use it to coat fabrics, seats, bags, some furniture, etc.
ACRYLICS
Probably the best-known acrylic is methacrylate, also known as Plexiglas. Methacrylate can be as transparent as glass or opaque. Both forms can be tinted with color pigments. However, it can crack and scratch easily. It can be shaped, bent, and twisted when heated to temperatures between 165 and 175 °C. When cold, it is very brittle, and care must be taken to prevent it from cracking when cut or drilled.
Methacrylate is also available in granule form for use in injection molding machines—which we’ll discuss later—and in this form, it is known as ACRYLIC PLASTIC POWDER.
NYLON
Many different types of nylon are produced, identified by a number, such as nylon 6.6 or nylon 6.10.
Probably the best-known form of nylon is fibers, which are used to make carpets, clothing, brushes, stockings, etc. It is a fairly hard and wear-resistant material.
In addition to fibers, nylon is used in larger components to manufacture curtain rods and accessories, housings for plugs and sockets, combs, etc., and in engineering to manufacture moving parts for gears and bearings, due to its durability, low coefficient of friction, and relatively high melting point.
POLYSTYRENE
The two most common forms in which we encounter polystyrene are as a transparent crystalline solid and as a spongy plastic known as expanded polystyrene (Porexpan).
In its “solid” form, it is very brittle and can be identified by the metallic sound it makes when dropped.
Expanded polystyrene is soft and spongy. During its production, a gas is generated that becomes trapped within its structure. It has good thermal and acoustic insulation properties, which is why it is used in the construction industry. In addition, its low density makes it very lightweight, and its spongy nature provides good shock absorption, so it is used for packaging.