I don't know why polythen bags shrink on burning. But one of my friend informed me that it shrinks because at the time of its processing, the polymer is stretched and when burnt the internal stress cause shrinkage.

But if that's true why don't we get the same result when we cut the polythene bags ?Shouldn't the internal stress play a role there too ?


Your friend is correct.

Plastic bags (polyethylene or polypropylene) are produced using blow moulding. This causes orientation of the polymer molecules during the blowing. On cooling that orientation becomes 'frozen' (maintained) and an anisotropic material results from this.

Then, when heating the bags above the polymer's melting point, the molten polymer molecules regain their 'natural' random configurations, which results in shrinkage of the bags, more or less in one direction only.

Merely cutting the bags would not reverse this polymer orientation at all.

  • $\begingroup$ so they are not in stretched state like springs rather frozen ? And this is why cutting doesn't reverse the process ? But being frozen they are still flexible !!! How ? $\endgroup$ – Ankit Jan 26 at 17:17
  • $\begingroup$ They are stretched, THEN frozen after the blowing. So why are the bags still flexible? The freezing is not like e.g. water. Freezing a polymer is not a very good term to use, TBH. When you 'freeze' a polymer from 'molten' you create so-called micro-crystallites and only quite a limited amount. But these MCs act like crosslinks and hold the structure (including the orientation) together. They also cause the polymer to be solid but still flexible. Then, reheating the bag above the MP of the polymer causes the MCs to disappear and the orientation of the polymer molecules randomises again. $\endgroup$ – Gert Jan 26 at 17:28
  • $\begingroup$ So we are really talking about "melting" rather than "burning" everywhere in this question, right? I am not to imagine that chemical bonds are breaking, or if it is literally on fire this isn't the dominant effect. It's a thermoplast, not a thermoset. $\endgroup$ – coldwave Feb 28 at 18:39
  • $\begingroup$ @coldwave The shrinkage is due melting. But the burning provides the heat needed to melt the plastic. $\endgroup$ – Gert Feb 28 at 19:09

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