What specific structural flaw caused the rubberized material to soften unduly in warmth in the original coats?

Answer

Lack of chemical cross-linking (vulcanization).

The material often softened in warmth, which was a consequence of not yet having the chemical process of vulcanization widely integrated into rainwear production.

What specific structural flaw caused the rubberized material to soften unduly in warmth in the original coats?

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The story of Charles Mackintosh - he invented rainwear! - YouTube

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