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".
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