Why does transparent plastic become opaque when it breaks?



My 7yo snapped the clip off of a transparent pink plastic pen. He noticed that at the place where it broke, the transparent pink plastic became opaque white. Why does that happen (instead of it remaining transparent throughout)?

This is best illustrated by the pic I took of the [broken pen](https://imgur.com/S8rasqb).

In: Physics

Plastic polymers are structures of relatively ordered chains of hydrocarbons.

When you put stress on this structure, you’re breaking apart molecular bonds in those chains, causing them to form small void spaces in the structure as they’re displaced and rearranged. These voids refract light differently than the rest of the structure, causing the opacity you observe.

The break isn’t completely clean. The plastic probably has really tiny tears and cracks caused by the break which adds to the texture and diffuses light.

Probably a combination of 2 things

1) The broken side isn’t smooth any more. Rough surfaces get hazy since they diffuse light that passes through them.

2) Plastic tends to bend before it breaks. When it bends, it’s ripping apart in a bunch of tiny cracks which traps air. These gaps and air bubbles diffuse light similar to reason #1

Source: I am a programmer and 3D artist who deals with surface materials and light transport equations for most of my day. Basically I get paid to ask the question “Why did the pen turn opaque when it broke,” and re-create it 😁

Basically, think of plastic like hair.

What do I mean? Well, plastic isn’t a solid material like glass or metal. Instead, it’s made of tons of small strands that look kind of like hair.

Now, think of a Queen’s hairdo – let’s take Elsa from Frozen as an example. Imagine her hair all nice pretty and shiny, and maybe a bit translucent. It’s all perfectly braided together, woven into a perfect bit of art. This is the clear state. It’s the state that plastic is when it’s fresh poured out of the mold.

Now give Elsa a case of bedhead. Her hair has been tussled, and bussled, and all sorts of messed up. It doesn’t look so pretty anymore. it’s lost it’s luster, and is all slept on with weird shapes and frizzle with a nightmare’s bluster, and that reflective sheen is nowhere to be seen. Plastic when you flex it, is like giving your material bedhead, and it does like that. Whatever original look it has, it gets all messy, and it changes. In both colored, and clear plastics, this “bed head plastic” tends towards white as the original color properties get all messed up as the strands of plastic rearrange out of their perfect pretty original form and start to reflect the light differently then they used to. Just like how bedhead hair isn’t shiny and light reflecting anymore.

To get the “pretty” hair back, you have to melt it down again and repour it at a foundry, which is the plastics version of going back to the hairdresser. Some plastics do this super easily, some not at all. Kinda of like how if you’re trying to get perfect straight hair to fold into a new Scandinavian royal hairdo, a woman like Elsa might be able to do it super easily, whilst a lady like Merida (From Brave) will have a nightmare getting her hair to lie straight enough to go into great braid, and it’ll be suuuper easy to get it back out of that shape.

Yay finally my time to shine!

Plastics are made of polymers, which are long molecules, all entangled together – imagine cooked spaghetti. In this state, the material is see-through. This is known as amorphous, and is the reason glass is see-through too.

When you bend the plastic, you stress these polymer chains and stretch them out. This allows them to align together, imagine raw spaghetti. In this state, the polymer chains can crystallise, and this blocks light.

Crystallisation is essentially just the process of creating an ordered structure of atoms or molecules.

To prove this, try heating the plastic up a bit, and see if it goes transparent again. The heat allows the chains to move back into their relaxed position.

Source: have a degree in Materials Science.

EDIT: Seems most of these other answers are contradictory, shows how misinformation can spread. Best is to just read up yourself: https://www.polymersolutions.com/blog/why-does-plastic-turn-white-stress/

[This has popped up on reddit before](https://www.reddit.com/r/askscience/comments/1fyf6y/why_does_plastic_turn_white_when_you_bend_it/)

[And elsewhere](https://chemistry.stackexchange.com/questions/15084/why-do-many-transparent-plastics-turn-opaque-when-deformed)

ELI5 explanation:
Plastics are like glass, they are a jumble of molecules fused into a lump. When you bend plastic you can cause tiny internal surfaces to form, like smashing a piece of glass into powder will now be white looking. You are also causing changes in density of those molecules in the plastic. You can undo it by melting the plastic like melting that powdered glass back into a single lump.

Maybe a little more ELI5 to grow understanding:

Imagine a pane of glass. Light gets through glass because the glass is one solid piece with no discontinuities. When the glass cracks, maybe up close you could still see through it, but from far away the glass becomes opaque. This is because all the light that was traveling unhindered through the glass, now has to deal with the imperfections in it. At every seam in the broken glass pane, the light gets reflected at an angle, which makes it less focused by the time it gets to your eyes.

It’s the same thing with plastic, except that plastic doesn’t completely shatter when it breaks. However, there are millions of tiny molecular-level imperfections at the place where it breaks, causing the light to get reflected in many different random directions as is passes through the plastic.

[This video](https://youtu.be/AfVjUXiRGks) has a good demonstration of glass breaking and becoming opaque. It is fundamentally a similar process for what happens in plastic, except plastic is not brittle like glass. Plastic does, however, still break on the molecular scale, in irregular ways, thus producing lots of interfaces for light to reflect and scatter off of.

Ok, here is the ELI5:

Well matter is compose by small balls (atoms), that balls are arranged in basic shapes like cubic and rectangles, OR can have a “random” arrange.

Ok so plastics can have a mix of a part that have a basic shape AND random; OR can be all random arrange.

So now any material to be transparent/translucent it have to have a high %% of ONLY one of the 2 arrangements (random or basic shape), transparent plastics have in general a high %% random one (if not 100%).

This occurs in a natural way, for them, if you cut it, it can or can’t change in the edges (depending on how you cut it). If you stretch it, the balls start rearrange, losing his natural positions so it became les translucent/More opaque.

This is due the diffraction of light into the material.

Hope it is enough ELI5. Of course this is a bit more complex.

Plastics are made up of chains of molecules, when you stretch them, the molecules align, allowing bonding between the chains, this is crystallization, and results in opaqueness. In addition, when stretching the plastic on a microscopic level, voids form (this is crazing) these voids also result in opaqueness, so what you are seeing is a combination of the two effects. Even if the plastic doesn’t seem deformed on a macro level, it has been severely traumatized on a micro level.

fun fact, architects and structural engineers back in the day would make plastic scale recreations of their designs and apply a load to them. the most concentrated points of stress would become apparent due to this sort of discoloration as well as changes in light refraction due to the stressed plastic becoming thinner. really cool stuff

Polymers are made of of many “mers”. These are chains of atoms. Normally they are randomly oriented and amorphous. When you bend or stretch plastic you are aligning these polymer chains which causes the optical properties to change.

Fun experiment. Go find a plastic shopping grocery bag. stretch the bag and watch it turn white.