What Makes Plastic Opaque?

Little girl trying to look over a wall.

How often do you take note of the types of plastic in your everyday life? It’s something we often take for granted. Some things are clear—most water bottles, for example. Other products, like phone cases, cleaning product containers, and trash cans are all opaque.

What makes certain plastics opaque, though? Anything opaque is simply anything that blocks all light from passing through it. This is opposed to materials that are translucent, meaning some light is allowed to pass through, or transparent, which allow all light to pass.

When it comes to plastics, some are opaque because of their chemical structure. Others, that are typically transparent, can be modified to become opaque with the use of an opacifier.

A common opacifier is titanium dioxide, which is a familiar ingredient in everyday products ranging from sunscreens and cosmetics to paint and pharmaceutical tablets. In its purest form, titanium dioxide is a fine powder that is a bright, white pigment.

Titanium dioxide has excellent light-scattering properties and, when incorporated into plastics, can reduce embrittlement, fading, and cracking that can occur when certain materials are exposed to light.

On its own, an opacifier doesn’t seem that complicated. But once you start to think about why certain products are opaque, things become more interesting. In some cases, an opaque plastic container can protect its contents from harm such as light exposure.

For example, some dairies are moving in the direction of using opaque materials in bottling milk. Exposure to light can degrade the quality of milk. A study at Cornell University discovered that even the fluorescent lighting in a typical grocery store display case can break down the vitamin A and B2 and will even affect the milk’s flavor.

Opaque plastics are not only used to protect products but sometimes to protect the consumer.  When Procter & Gamble first began manufacturing Tide Pods, they were sold in a clear container. However, when reports began to surface that children were being poisoned after mistaking the detergent pods for candy, the packaging was redesigned. The current, opaque orange container is among the safety measures taken to discourage kids from mistakenly seeking candy.

In truth, the importance of clarity and opacity varies based on the desired application. The Coca-Cola Company wants you to see into their 20-ounce bottles. They want you to be tempted by that sweet, brown, sugary beverage. Restaurants that hand you a clear plastic carry out box want you to open your refrigerator the next day and be reminded of the delicious leftovers that you brought home. And folks who cover their antique vehicles with an opaque plastic sheet want to protect their cars from not only wind and rain, but from the ultraviolet rays of the sun.

Next time you pick up a plastic container, consider all the implications of that plastic. When you begin to explore the world beyond the surface, our sense of curiosity is born. That sense of wonder and deep consideration is one of the things that makes science so interesting and engaging to us here at Polymer Solutions.

2 Comments


  1. This article described something I didn’t expect and didn’t touch on something I did. So let’s explore a little further below the surface.

    First, I was expecting mention of crystalline vs amorphous plastics. For example, as I understand it, polyethylene is translucent because it is partially crystalline and light scatters as it passes thru several amorphous and crystalline regions, with different refractive indices.

    It makes sense that TiO2 would be an opacifier, but I was surprised that it can be a light stabilizer. TiO2 is a semiconductor for which light absorption drops to zero just outside the visible region. When exposed to UVA, it creates positive holes that are strong oxidizing agents. Knowing that, one would expect mixing TiO2 would cause, not prevent, cracking and fading. Obviously not — why not? That might be worth a post in itself.

    Reply

    1. Hi Mark,
      This is a great idea for a blog post! Thank you for sharing with us. You are welcome to email info@polymersolutions.com to talk about your question in more detail.
      Best wishes,
      Amanda, PSI Blog Team

      Reply

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