Bent plastic

[Ankit Gupta]

Why do soft plastic (like of chair or toy) become white on bending it ,irrespective of its original color?

[StringJunky]

In the areas that are white the polymer chains are no longer ordered in the same formation which reflect light to a certain wavelength(s) and normally give you a uniform colour. Instead, the chains in the stressed areas are stretched and mixed in different directions, resulting in light being scattered as different wavelength; this results in the white areas.

Edited July 27, 2016 by StringJunky

[DrP]

SJ is probably right... I have always thought it was to do with uneven pigment or dye distribution though. The pigments in the polymer become less concentrated in the areas that have been stretched... the polymer stretches to a new longer length, but, the pigment/dye amount is now the same but spread over the new stretched area of plastic, so the colour fades. I could be wrong and I do not know what made me think this as the case, I just assumed it. SJ is probably right though and it does sound feasible. Could be that there are different reasons depending on the type of plastic and the reason it is coloured in the first place.

[StringJunky]

SJ is probably right... I have always thought it was to do with uneven pigment or dye distribution though. The pigments in the polymer become less concentrated in the areas that have been stretched... the polymer stretches to a new longer length, but, the pigment/dye amount is now the same but spread over the new stretched area of plastic, so the colour fades. I could be wrong and I do not know what made me think this as the case, I just assumed it. SJ is probably right though and it does sound feasible. Could be that there are different reasons depending on the type of plastic and the reason it is coloured in the first place.

This is from a materials site. This article adds that bending causes heating, which leads to the polymers crystallising, resulting in the light-scattering white areas.

 

Most polymers contain both crystalline (neatly ordered) and amorphous (unordered) areas on their molecular chains. Heating a polymer can cause the amorphous sections of the chain to crystalize. When that happens, the way the molecules scatter light changes and the plastic turns white.

 

Simply bending something out of shape, or making the repeated bending motions needed to break it entirely (think about that Wiffle bat), can also cause molecular changes that lead to whitening.

 

https://www.polymersolutions.com/blog/why-does-plastic-turn-white-stress/

If you think about it, and your thought is entirely reasonably at first sight, but if we take some dyeable material, like plasticine, and dye it then stretch it there are no white areas. It seems reasonable to deduce from this that the white areas in the bent plastic must be an optical effect rather than dye dilution.

[Phi for All]

In the areas that are white the polymer chains are no longer ordered in the same formation which reflect light to a certain wavelength(s) and normally give you a uniform colour. Instead, the chains in the stressed areas are stretched and mixed in different directions, resulting in light being scattered as different wavelength; this results in the white areas.

 

The work from bending the plastic causes heat, right? Is this what causes the disorder in the polymer chains? I don't think it's the heat distorting light, since the plastic still looks white after it cools.

[StringJunky]

The work from bending the plastic causes heat, right? Is this what causes the disorder in the polymer chains? I don't think it's the heat distorting light, since the plastic still looks white after it cools.

That site I linked mentions heating causing the effect. The heat causes the changes in material colour reflection properties. Yeah, it's a permanent effect even when it's cooled down.

Edited July 27, 2016 by StringJunky

[Ankit Gupta]

What if I heat the plastic enough that it don't melt and then bend it? Will it cause the same effect as before or it won't get white?

[StringJunky]

What if I heat the plastic enough that it don't melt and then bend it? Will it cause the same effect as before or it won't get white?

If it's soft enough there should be less stress and breaking less bonds and, I would have thought, leading to less whitening but I don't know really

Edited July 27, 2016 by StringJunky

[Phi for All]

If it's soft enough there should be less stress and breaking less bonds and, I would have thought, leading to less whitening but I don't know really

 

This seems reasonable, since you wouldn't get the crazing effect from aligning the polymer chains along the axis of stress, because there wouldn't be as much stress.

[Enthalpy]

Plastics are usually heated during the manufacturing process, and their pigments designed to keep their colour then. Also, bending a plastic part once produces no big heat as felt by our fingers, certainly less than 100°C or 150°C at injection. So I exclude any effect of heat on the pigments when a plastic part is bent.

 

The density of the pigments in the polymer isn't an explanation, because the polymer's density doesn't change. If bending stretches the polymer, if makes it thinner and if possible narrower too, keeping the polymer volume hence the pigment density.

 

What I could imagine (notice the doubt) is that the polymer needs some transparency so the light can reach the pigments at depth and emerge filtered, and that strectching the polymer increases light diffusion by the polymer so much that the pigments have little chance to receive and scatter light. This is consistent with transparent plastic films that become white and opaque upon stretching.

 

Or possibly that the rearranged polymer molecules create trap states for the electrons which disrupt the electronic equilibrium of the dye. Organic dyes are quite sensitive to lost or gained electrons. Many pH indicators depend on it.

[Endy0816]

Could try with clear plastic.

[Enthalpy]

The ones I tried to bend became white and opaque.

[Endy0816]

The ones I tried to bend became white and opaque.

 

Sorry, didn't catch that earlier.

 

Saw something similar when seeing how plants fared under plastic spheres. When there were only a few together they would look clear, but a larger group would appear white-ish. Put it down to scattering, but didn't investigate further.