BÜFA Thermoplastic Composites GmbH & Co. KG

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Differences in structure: The properties of plastic types

To get deeper into the world of plastics, you have to look at the molecular structure of thermoplastics, duromers and elastomers.

Molecular arrangement and cross-linking


If you look at the structure of the duromers, you can observe a three-dimensional cross-linking.

In the case of elastomers, the molecules are two-dimensional, less cross-linked and form a wide-meshed structure.

semi-crystalline thermoplastics
amorphous thermoplastics

In thermoplastics, the molecular chains are not cross-linked, the macromolecules are mainly located next to each other. They have no chemical bonds, but intermolecular forces that connect the chains. These chains shift slightly against each other at high energy or due to heat. This makes the plastic more easily deformable and decomposes when heated further.

If force is used to supply energy, the molecule chains slide against each other. For example: If a residual waste plastic bag is overloaded, this bag can be expanded until the intermediate molecular forces can no longer hold the chains together. Ultimately, the bin liner tears. Elastomers are also stretchable, but the molecular chains retreat back into their initial form.

Thermoplastics can be divided into two main groups: amorphous and semi-crystalline. They can be deformed differently. In semi-crystalline thermoplastics, the molecular chains form regular structures. On the other hand, the amorphous thermoplastics are linear, unbranched and irregular. But what exactly does this mean?

Semi-crystalline thermoplastics

In this type of thermoplast, so-called crystals are formed during cooling in areas where parallel bundles or folds of the chains exist. A dense arrangement of the molecules is formed in the crystal structure.

Semi-crystalline thermoplastics are opaque at low layer thicknesses. Looking at the mechanical properties, they are stronger, harder and tougher than amorphous plastics. In addition, they are resistant to chemicals and have a higher heat resistance.

In toolmaking, uniform cooling is required so that a uniform degree of crystallization extends over the molded part.

The best known semi-crystalline thermoplastics are:


  • Polyetheretherketone (PEEK)
  • Polyamide 66 (PA 66)
  • Polyamide 6 (PA 6)
  • Polypropylene (PP)
  • Polyethylene terephthalate (PET)
  • Polybutylene terephthalate (PBT)

These thermoplastics are used, for example, in pipes, containers and corrosion protection layers.

Amorphous thermoplastics

In amorphous thermoplastics, the molecular chains are arranged irregularly one inside the other. If the plastic lies within the temperature range in which it is solid, it has glass-like properties. That is why this area is called the glass area. The amorphous thermoplastic here is relatively brittle, transparent and hard. In addition, there is no exact melting point and the plastic is not resistant to chemicals.

The best known amorphous thermoplastics are:

  • Polycarbonate (PC)
  • Polyvinyl chloride (PVC)

These thermoplastics are used, for example, in toys, containers, sound and heat insulation.

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