How the impact of temperatures can affect the performance values of highly crosslinked polymers

In the field of plastic polymers there are those classifiable as crosslinked and those defined as linear or branched, which express substantial differences in the distribution and connection between the points of the molecules.

A "cross-linked" polymer can therefore be defined if there are two or more lines connecting any two points of its molecule, while a "linear" or "branched" polymer can be defined if there are no side chains headed in two or more points.

The characteristic of crosslinked chains is that they are joined together by covalent bonds, having a bond energy equal to that of the atoms on the chains and are therefore not independent of each other.

For this reason, a crosslinked polymer is generally a rigid plastic, which decomposes or burns when heated, instead of softening and melting like a linear or branched polymer.

In fact, while an elastomer, subjected to a normal room temperature expresses the softening point, the crosslinked polymers remain rigid in thermal conditions environmental, but also at higher temperatures, up to reaching a thermal level which causes its degradation.

Consequently, if a crosslinked polymer is subjected to temperatures above 200 °C, it is easy to create the phenomenon of degradation which makes the polymer difficult to use, at the same time, it has been noted that the addition of fillers improves the thermal resistance of the compound.

The influence of temperature acts easily on linear polymers, but does not find a great response on crosslinked ones, this is due to the dense crosslinking which characterizes the polymeric structure which prevents any molecular movement which may involve large deformations.

At high temperatures, densely cross-linked polymers may show viscoelastic phenomena but, at the same time, chemical reactions occur which alter the structure of the material.

The reason why crosslinks are often created is that linear polymers are not strong enough for some applications that require special strength, or great elasticity. In these cases crosslinks are created between the chains to obtain stronger crosslinked polymers, but which are no longer remodelable by melting.

As regards the mechanical behavior of a densely crosslinked polymer, such as phenolic resins, these will have different and opposite reactions , for example, compared to elastomers.

The tensile stress-strain diagram of densely crosslinked polymers therefore always indicates brittle behaviour, with small elongations at failure and high tensile strengths.

In reality, it is also necessary to consider that the densely cross-linked polymers that are on the market can also contain amounts of fillers of various types, such as cellulose, cotton waste, wood flour, glass fiber and many others, for which the study of mechanical behavior is not always easy to understand.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.