Blog about plastics, materials, processing and engineering tools


Did you notice that some plastics are hard and others soft, such as cables which are soft and plastics cups which are hard.

The main reason is that plastics have a defined point of temperature. The properties of the material change depending on the temperature, if it is about or below this point.

So, above this point all plastics are soft and pliable, and below they are hard and brittle.

This point is called the glass transition temperature, or Tg and it is different for each plastic and that only happens to polymers.

Why cables are soft?

Some plastics are above their Tg at room temperature, and they are soft. In this group there are for example, rubber elastomers like polyisoprene and polyisobutylene and they are in the rubbery state.

Rubbery state means that plastics in this state are soft and flexible.

Why plastic cups are hard?

At room temperature, some plastics are below their Tg, and they are hard, such as polystyrene or poly(methyl methacrylate). When the plastic is below its glass transition temperature it is in its glassy state.

What is plasticizer?

There is an extra way to get soft plastics. During the manufacturing process an additive called plasticizer can be added. Plasticizers make plastics softer and more pliable.

Examples of plastics and their Tg:

Material Tg (°C)
Acrylonitrile butadiene styrene (ABS) 105
Poly(carbonate) (PC) -Lexan 145
Poly(methyl methacrylate) (PMMA atactic) -Plexiglas 105
Poly(vinyl chloride) (PVC) 80
Polyamide (PA) -Nylon-6 47 – 60
Polyethylene terephthalate (PET) 70
Polylactic acid (PLA) 60 – 65
Polypropylene (PP atactic) −20
Polypropylene (PP isotactic) 0
Polystyrene (PS) 95
Polytetrafluoroethylene (PTFE) -Teflon 115
Tire rubber −70



  1. wikipedia