In daily life, phenomena such as white marks on transparent plastic folders after bending and unexplained whitening of automotive PP plastic interiors are common. This is actually the "stress whitening" phenomenon in materials science. Stress whitening is not surface cracking or fading, but a reversible physical change where plastics develop microcracks, crazes, or micropores internally when subjected to external forces like impact or stretching, leading to reduced refractive index and increased light scattering.
    Although there are disputes over the mechanism, the core lies in internal microdefects. In terms of materials, PP copolymers and rubber-toughened materials are more prone to whitening; higher crystallinity and longer molecular chains increase the tendency. Technically, excessive holding pressure and low mold temperature exacerbate the problem. External conditions such as low temperature and slow strain rate also contribute.
    Engineers can mitigate the phenomenon by optimizing material formulas (selecting homopolymer PP, adding rigid particles), improving product design (avoiding sharp corners), and adjusting injection molding processes (reducing holding pressure). Understanding the causes and solutions of stress whitening helps better address common issues in the use of plastic products.

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