Trelic blog 26/06/2018 – Sanna Lahokallio
Polymers and plastics, the materials that enable the way of living we have nowadays and have lots of very advantageous properties. However, they are much more complicated materials than metals and ceramics – which are the materials often replaced with lighter and cheaper plastics. It might therefore be difficult to understand plastics and their properties.
The number of commercial plastics available and the amount of variation in thermal properties is of course huge, but there are a couple of basic terms you should remember and understand while selecting a material for your product. These include thermoset, thermoplastic, crystalline and amorphous structures, and glass transition (Tg) and melting (Tm) temperatures.
Polymers are either thermoplastics or thermosets
*Thermoplastics have long polymer chains, bonded together by weak secondary bonds. These materials can be re-melted and re-solidified several times which make them useful for plastic bottles, containers, toys, etc.
*Thermosets form a cross-linked structure after curing. Because of this three-dimensional network structure, the material remains permanently solid and cannot be re-melted. They are practical materials for example for glues and coatings.
As a rule of thumb, thermosets have better chemical resistance, dimensional stability, strength and lower coefficient of thermal expansion (CTE) compared to thermoplastics.
Polymers have crystalline or amorphous structure, which affects their properties
*Thermosets are amorphous. They cannot crystallize because of their cross-linked structure.
*Thermoplastics can be amorphous or crystalline. In practise, full crystallinity is impossible to achieve, and they are often called semi-crystalline polymers.
What has all this to do with thermal limits?
Polymers have various thermal parameters, of which glass transition temperature (Tg) and melting temperature (Tm) are the most important ones to understand when considering the thermal use limits of plastics. Melting temperature is easy to understand, but glass transition is more complex:
*At Tg the amorphous parts of polymer will change from rigid to rubbery form. In other words, when amorphous polymers are heated up to their Tg, they will soften or melt.
*Note! Tg is not a single temperature, but a temperature range. Still, a single temperature is often given in the data sheets.
Whether the critical thermal limit is Tg or Tm depends on the plastic:
Is it thermoset or thermoplastic, is it amorphous or semi-crystalline (and what is the degree of crystallinity)?
*For thermosets, Tg is generally considered to be the upper use temperature limit. Above Tg, mechanical properties often weaken and coefficient of thermal expansion (CTE) rapidly increases. However, cross-linked structure supports the material, and therefore thermosets can sometimes be used at temperatures above their Tg.
*For amorphous thermoplastics, Tg is the upper temperature limit. The loss of mechanical properties at elevated temperatures is dramatic due to their internal structure.
*For semi-crystalline thermoplastics, there are two thermal limits. At Tg their amorphous parts will soften. If the plastic has high level of crystallinity, amorphous parts soften, but the crystalline part holds the part together. When the material is heated further, the crystalline regions will melt at Tm and the polymer melts completely.
So, it is important to know what kind of polymer you are dealing with when deciding at how high temperature it can be exposed to. It is also good to notice that other factors may affect the thermal limits. One example is humidity which may for example decrease glass transition temperature.