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There is a phenomenon with plastic in which it changes color to white in areas where stress is applied. When I bend a plastic rod the area in the centre, it turns white or loses colour. Why does this occur with plastic materials?

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This colour change effect occurs when the strands of plastic material, the polymers in the plastic, start to stretch as you twist the plastic. As they do so, this changes the way the light is reflected from the plastic. Say for example, you chew a biro top, by doing so, the Refractive Index of the plastic is altered, from its original colour, to a whitish colour. Once the light is scattered, you start to lose the original colour the plastic had.

From Why does stressed plastic turn white? gives a fuller explanation of the colour change effect

Many polymers are semi-crystalline, containing both crystalline and amorphous (non-ordered, think spaghetti) regions. When the crystalline region size is on the order of the wavelength of light, it can scatter light making the plastic opaque. For polymers that are entirely amorphous, you have no crystalline regions and thus the polymers are transparent..... You can think of the amorphous regions as something similar to spaghetti, a messed of tangled polymer chains. When you bend the plastic (i.e. stress), you are forcing those polymer chains to align in the axis of strain, inducing crystallization in that region, which can then scatter light and turn the plastic opaque or white.

Here is the formula of polyethylene, the plastic that makes up mineral water bottles, it's 2 carbon atoms and 4 hydrogen ones, linked together in long hydrocarbon chains, for the exact chemical formulas of various plastics you could look up Wikipedia or ask on ChemistryStackExchange, but I would imagine it's the same physical process that occurs to change the colour, which is the physics part of your question:

enter image description here

and this is a bit of the polymer chain, join enough of these together and you have a plastic bottle:

enter image description here

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  • $\begingroup$ Could you include some chemical and physical formulas in your answer please? $\endgroup$ – Crystal Optics Sep 13 '15 at 19:28
  • $\begingroup$ I'll certainly try , what type of plastic is it? Is it something like a biro top, or something plastic that keeps breaking on you? $\endgroup$ – user81619 Sep 13 '15 at 19:33
  • $\begingroup$ Colour and refractive index aren't the same thing. Transparent plastics (like PE) often do become opaque on stretching due to micro-crystallinity, induced by chain orienting. $\endgroup$ – Gert Sep 13 '15 at 20:18

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