What is a thermoplastic?

A thermoplastic, sometimes written as thermoplastic, is a type of plastic made with a polymer that becomes a homogeneous liquid when heated at relatively high temperatures and when it cools is a material hard in a glass transition state. When it freezes it is a brittle material. All of these characteristics are reversible, making it possible for thermoplastics to be repeatedly heated and cooled without losing these qualities and making thermoplastics a easily recyclable material.

Most outstanding properties

The main properties of thermoplastic materials What have made them so widely used in all kinds of industries and factories are:

  1. pass by liquid state when heated before passing into a gaseous state, that is, they can melt.
  2. Have good plasticity with application of heat, which allows them to be easily molded
  3. can be dissolved with some solvents
  4. They can absorb some solvents and when they do, they swell
  5. offer good resistance to creep deformation (unrecoverable deformation)

chemical structure

The polymer molecules that form a thermoplastic are joined together by Van der Waals forces that keep them in linear structures or with a certain branching. We could compare its structure to a set of strings in which each string is a polymer. The strings can be more or less intertwined, the force needed to separate each polymeric molecule being greater at greater entanglement. In the following image we can see a schematic representation of a thermoplastic, an elastomer and a thermosetting polymer.

The Van der Waals forces between the polymeric molecules that make up a thermoplastic material can be of different degrees depending on the chemical composition of the molecule itself and the spatial arrangement it adopts. Depending on this, the adopted structure can be amorphous or crystalline and both can exist in the same material.

The amorphous structure is characterized by a disorderly arrangement of polymer chains and is responsible for the elastic properties of plastics. The greater the number of amorphous structures, the greater the elasticity. of the thermoplastic, but will have lower resistance.

In the crystalline structure, the polymer molecules are arranged in an orderly manner and are much more compact than in the amorphous structure. The intermolecular forces are stronger and therefore the crystalline structures confer mechanical strength properties thermoplastic materials making them resistant to loads, traction and temperature. But greater number of crystalline structures decreases the elasticity appearing brittleness.

Types of thermoplastics

There are dozens of types of thermoplastics, each varying in crystalline/amorphous organization and density. The most widely used thermoplastics today are polyurethane, polypropylene, polycarbonate and acrylics.

The Celluloid (cellulose nitrate) is considered the first thermoplastic material made in history. It made its star appearance in the mid-19th century and for more than 100 years it was the most widely used thermoplastic material. Although it is still used today, for example in the manufacture of position selectors on guitars, it has been largely displaced by other thermoplastic materials with better properties.

Differences with thermosets and elastomers

Thermoplastics are often confused with thermosetting plastics. Although etymologically they may sound the same, the reality is that they have quite different properties. While thermoplastics can be heated to a liquid state and then cooled to a solid state again, thermosetting materials chemically deteriorate when heated. Despite this deterioration, thermoset materials tend to be more durable than thermoplastics.

Thermoplastics are also different from elastomers although some polymers can be considered both types. Many thermoplastics can be stretched to a certain point, but most tend to be resistant to elasticity and remain in the form in which they are stretched (they do not return to their original shape). On the other hand, the main characteristic of elastomers is their high elasticity being able to be stretched considerably and return to its original shape when the stretching force ceases.

To make a thermoplastic material more flexible and elastic, you can use plasticizing substances that mix when the thermoplastic is molten. These plasticizing substances lower the crystalline transition temperature (Tc) of the thermoplastic, a temperature that depends directly on its crystalline structure. The Tc can also be adjusted by entering a copolymer, that is, mixing another polymer with the polymer that forms the thermoplastic material to take advantage of some properties of this second. The most widely used copolymer is polystyrene. Until plasticizers began to be used, some thermoplastic moldings cracked easily just by immersing them in cold water.

Examples of use of thermoplastics

Thermoplastic materials have been around us for a long time and today they are part of countless products. Let's look at some prominent examples:

  • Acrylonitrile Butadiene Styrene (ABS) is a type of thermoplastic used in the automotive industry, toys (such as the famous LEGO® blocks or BANDAI® dolls), sports equipment, electronic component housings (computers, televisions, video game consoles) or office supplies (folders, staplers).
  • The polycarbonate It is a material used in the manufacture of optical discs (CDs, Blu-ray, DVDs), soft drink bottles or in the lenses of glasses.
  • The polyethylene It may be the easiest thermoplastic material to find around you and in the largest quantity. There are two types, high and low density, the first being a rigid and resistant material and the second a very elastic material. You can see them in bottles of cleaning and cosmetic products, in supermarket bags, covering electrical cables, tubes of all kinds and even in bulletproof vests.
  • Other well-known and common thermoplastics are polystyrene, polyamide, PVC (polyvinyl chloride), etc.
  • Thermoplastics are also used in the manufacture of adhesives, for example acrylates and cyanoacrylates.
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