How is plastic made clear?
In: Chemistry
There actually are some things that can be done to certain plastics to make them more clear. One material in particular is polypropylene (PP). 20-25 years ago, PP could only be found as a translucent material: think about older Tupperware that was thick, white, and durable. This is because standard PP is a semicrystalline material with crystals of a size that is comparable to the wavelength of visible light (400-700nm).
Chemists found that by introducing certain additives, they could make PP create smaller crystalline areas that were no longer near the same size as the wavelength of visible light, thus rendering the PP more clear. There are also more of these smaller crystals, making the PP stronger, increasing its performance and allowing it to compete in more demanding applications.
They were so successful in tweaking the clarity of PP that by 2010 it was stealing business from more expensive clear plastics like polystyrene and polycarbonate.
Other plastics can be made clearer by introducing an additional comonomer. (Think of the chemical structure of a plastic as a string of beads. If all the beads are the same, then strings next to each other can pack efficiently and form crystal areas. Now imagine that you randomly replace a few of the beads with larger beads: the strings of beads no longer can line up straight in large areas and crystallize.) As with PP above, the end result is smaller crystals and a clearer plastic.
TL;DR: adding certain stuff to certain specific plastics can make them clear, or at least clearer.
Transparent plastics are clear because they aren’t crystalline, either because that’s their natural way to be or because they are carefully made that way in manufacturing. There are two major ways solid materials exist: as crystals or as glasses. Crystalline materials have their atoms and molecules perfectly ordered in a packed structure, like stacking golf balls in a box. Glassy materials don’t have that order, their molecules make big messy piles like a plate of spaghetti. Glassy materials don’t scatter light, so they look clear. Crystalline materials do scatter light, so they look opaque.
Plastics are very versatile. Some of them tend to crystallize some or a lot in the solid state, while others remain totally glassy. Glassy plastics are naturally transparent. A good example of this is the polyester that makes plastic soda bottles. Other plastics form little crystals embedded in a mesh of glassy material, which makes them opaque. A good example of this is high-density polyethylene, a common plastic used to make things like milk jugs and molded plastic items.
Even more crystalline plastics can be made transparent, though the use of added chemicals and/or the way they are processed. Certain chemicals, called “plasticizers”, stop the plastic molecules from ordering into crystals, and can make a normally opaque plastic clear. You can also do it just by cooling melted plastic really, really fast when you are shaping it. If the plastic is cooled very quickly, it freezes the molecules in place and doesn’t give crystals a chance to form. The lack of crystals makes the plastic transparent.
Plastic that is clear is not modified to be clear. It is simply a plastic that is naturally clear that you do not add any pigment to. So lots of the nontransparent plastic that is used is transparent plastic mixed with pigment to the appropriate color.
Loss of material is naturally transparent if there they are solid without any air gaps. Glass, sugar, salt. gypsum, water etc.
If a material is transparent depends on how the atomic structure of the material. It is about if the energy levels an electron can be excited to be in the range of the energy of a photon of visible light. [https://www.youtube.com/watch?v=Omr0JNyDBI0](https://www.youtube.com/watch?v=Omr0JNyDBI0)
You don’t do something to take non-clear plastic and turn it into clear plastic. Some plastics are clear by nature; specifically, they’re transparent to the wavelengths of light that we can see (they’re definitely not transparent to all wavelengths). This happens when none of the frequencies of those particular plastic molecules absorb wavelengths that we can see. This depends on the chemistry of the particular plastic, what other things might have been mixed into it, and sometimes how it was formed & cooled (basically, how the molecules are arranged).
It’s just like why glass (mostly silica) is transparent but many other rocks aren’t.
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