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Name: Sam
Status: student
Grade: 6-8
Location: IA



Question:
Why does colored plastic turn white when bent? example - Lime green pen cap


Replies:
Sam,

There are two possible reasons for this: crystal formation due to cold drawing or a process known as crazing. Essentially what is happening is that the amount of refraction (bending of light) increases either due to the increased size or organization of crystals in the plastic or an increased amount of micro-fissures in the sample.

Plastics are semi-crystalline. This means that they are often a mixture of crystals embedded in a matrix of amorphous polymers. If the crystal size grows then that increased size may interact with light more and make the sample opaque. Alternatively if the small crystals in the sample orient themselves well relative to each other, than the incident beam may scatter more as it interacts with these organized crystals and these will also make the sample look opaque as less light transmits to your eye. Crystals can grow or become more organized when you pull on a plastic sample without heating it. Heat often has the effect of allowing the polymers to move around or slip past each other so that pulling on the plastic does not cause better organization of the crystals. However, if the sample is cold, the molecules or do not have enough kinetic energy to wiggle around and so they get oriented in the direction of the pull (kind of like the way uncooked pasta will organized themselves in straight lines relative to each other, but cooked pasta will be more disorganized).

The other explanation -crazing- is the development of small cracks in the sample. Here the crystals are already big and organized so that when you pull on the plastic, the spaces between the crystals increase and result in little cracks or fissures. The difference in the density of the cracks and the crystals cause the light entering the sample to bend or scatter. We perceive this as a diminishing of the light reaching our eye and a "whitenning".

Hope this clarifies the effect for you.

Greg (Roberto Gregorius)



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