Why is Plastic Clear?

Light shining through a plastic bottleMany plastics are very safe, durable, and inexpensive to produce. That makes it an amazing material in itself. When it’s clear, though, that’s when plastic really shines as a material for many different applications. Sure, there are a few other substances that are clear–like glass–or at least translucent–like wax paper. Plastic is different though and the science behind clarity of plastics is rooted in the littlest details, the monomers.

Remember, polymers are monomers (small molecules) that join to form long, and sometimes complex, chains. Most polymers contain both crystalline (neatly ordered) and amorphous (unordered) molecular chains. The crystallinity of a polymer can impact the clarity of the plastic material.

It might seem obvious to you what is clear and what isn’t: if you can see through something, then it’s a clear substance, right? But as scientists, we’re often faced with a way to quantify these sorts of things. Just how clear is it?

That’s where some measurements come into play. According to Craftech Industries, there are two ways that we can determine the clarity of a material. First is the ‘refractive index’, which is a measure of how much light is refracted, or bent, through a substance. When light passes through a clear substance, the light isn’t bent very much at all. The less light has to bend through a substance, the clearer it is.

The other test of clarity is transparency. How easily can light penetrate a substance? That’s determined by something called the “Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics”. This ASTM test measures the amount of light that can make its way through a substance. The higher the value, the more light can get through and the clearer a substance is.

It is one thing to determine clarity–but what do you do when a plastic that is supposed to be clear becomes cloudy or white? While discoloration or whitening can have a number of causes, we have multiple analytical approaches that can help determine the root cause of the clarity issue.

One analytical option for assessing lack of clarity is optical birefringence. Remember how quantifying plastic clarity relates to refractive index? Optical birefringence testing looks at how a material refracts light, and it can help us determine whether stress factors have caused a plastic sample to experience whitening–and lose its clarity. If stress factors were a cause for the whitening it can also indicate from where the stress originated.

We’ve explored how to quantify clarity, an option for determining why clarity was compromised but we have yet to answer the fundamental question, why do clear plastics matter?

Clear plastic is important for a number of reasons. Perhaps it’s just simple marketing: companies want to make sure you can see their products through the container. In other applications, though, clarity can be much more important. Many materials in medicine and laboratory science are clear so that professionals can accurately measure and make decisions without doubt–with the information before their very eyes. In many applications, that’s vital.

When you’re designing a product that needs to be clear, though, one type of plastic reigns supreme: polycarbonate. PC is a strong, clear, and recyclable plastic that’s used in everything from water bottles to lab equipment. PC is recyclable because it belongs to a family of thermoplastic materials, meaning that it will return to a liquid state when heated. This is unlike thermoset plastics, which only decompose when reheated to high temperatures. From a renewability standpoint, a thermoplastic material is preferred.

With continued innovation in the world of plastics every day, plastic materials are just going to get better thanks to curious minds and the use of great science. From higher durability to better clarity, we expect to see the applications where clear plastics are used to evolve and improve.

What are some other ways that you use plastics and their unique ability for clarity every day?


  1. But why is plastic clear? The question is never answered in this article.


    1. Hi Larry,
      Some plastics are clear because they bend less light and more light can makes its way through. Hope this helps!
      Best wishes,
      Amanda, Polymer Solutions Incorporated


  2. I think, clear plastics have molecules which are smaller than wavelength of light. Hence it allows the light to pass thorough to make the plastic clear or transparent.


  3. In the end matter is about ‘wave-particles’ that can interact with waves, as I see it.
    We know now that molecular wave-particles that are not combined in a structure (like a crystal) often do not scatter and/or absorb waves in the light-frequencies. But why that is, seen on a ‘wave-scale, is not completely clear to me. Supposedly it has to do with energy-levels of the molecules which change when they are in an orderly vs unonderly state for example.


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