Today's KNOWLEDGE Share: Loss of Molecular Weight

Today's KNOWLEDGE Share:

Loss of Molecular Weight

It is correct that the degradation of plastic parts can lead to a decrease in their mechanical performance due to a loss of molecular weight.

However, the explanations provided in some posts I see on this platform may not always be entirely accurate.

The resistance to cavitation, which is the initiation of brittle failure, is controlled by the molecular weight between entanglements (Me) rather than the overall molecular weight (Mw). This means that cavitation will not be greatly affected by a loss of Mw as long as it remains well above Me (which is true in most commercial plastic grades except the very high flow grades).

However, many other mechanical properties, starting already from tensile strength, will sharply decrease as Mw decreases due to progressive disentanglement of the shorter polymer chains. This is especially confirmed by testing polymers at cryogenic temperatures where reputation and plasticity are suppressed.

The above explains why molecular weight does not influence the stress-strain response (measured in compression to avoid failure) while most tensile data will start collapsing with a decrease in Mw.

Source:VITO LEO

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