Examining the UV Resistance Qualities of Polypropylene and Nylon

Originally published on fastradius.com on September 21, 2022

There are countless thermoplastics on the market today, from acrylic to polyetherimide. Polypropylene and nylon are two other thermoplastics commonly used in manufacturing, so it’s important to understand the qualities of each before you consider these materials for your next project.

Polypropylene is a capable, low-cost crystalline thermoplastic often used in medical equipment, food containers, packaging, and water pipes. It’s most commonly used with the injection molding or CNC machining manufacturing processes, but it’s also possible to 3D print polypropylene. However, before you decide to 3D print your parts using polypropylene, it’s important to note that it does tend to warp more easily than many other 3D printing materials, and there may be more suitable options depending on what you’re making.

Likewise, nylon is also compatible with 3D printing, injection molding, and CNC machining, and is used across industries to manufacture everything from rope to engine components. It can be shaped into several different forms, including sheets, fibers, filaments, and films, and is often used in place of low-strength metals.

As you can see, nylon and polypropylene are versatile plastics with multiple benefits. However, designing parts for outdoor use requires selecting the right UV-resistant or UV-stabilized plastic, so it’s important to consider each material’s UV qualities to ensure your end product can withstand prolonged exposure to sunlight without breaking down. In this article, we’ll go over the UV resistance qualities of nylon and polypropylene to help you make an informed material choice.

What is UV-resistant plastic?

If your part will spend a significant amount of time outside or otherwise be exposed to sustained levels of UV radiation, you may want to use a plastic resistant to UV rays. For example, you may have seen some stadium seats, automotive parts, and lawn chairs suffer from discoloration, a loss of visual clarity, and/or a reduction in mechanical strength and impact resistance; this is because they were made out of a plastic with low UV resistance. Exposure to UV light can also make plastic parts more brittle and susceptible to breakage, cracking, deformation, or disintegration.

While almost no material is entirely resistant to UV light, some can absorb UV radiation. Adding stabilizers or additives can improve UV resistance for plastics that can’t absorb UV light. These non-UV-resistant materials produce free radicals (atoms with an unpaired electron), which can break down intermolecular bonds, negatively impact mechanical properties, and cause plastics to wear down.

Polypropylene UV resistance qualities in manufacturing

Polypropylene is an extremely tough yet flexible and lightweight material. It is resistant to mold, bacteria, water, oil, rot, electricity, and chemicals. However, polypropylene isn’t the best option for products that live outside or are regularly exposed to sunlight because it’s deeply affected by exposure to UV radiation. Specifically, it’s susceptible to UV wavelengths of 290 – 300 nanometers, 330 nanometers, and 370 nanometers. Exposure to high-intensity UV radiation for as few as six days can cause polypropylene to lose up to 70% of its mechanical strength, resulting in very brittle parts.

Polypropylene products will generally break down after a few months of direct exposure to sunlight, as the radiation will excite photons and create free radicals. However, using additives and coatings can help protect polypropylene parts from sunlight. For example, zinc oxide or titanium oxide fibers can block and absorb UV light, reducing the polypropylene’s exposure to UV radiation and extending the part’s life. Similarly, designing a polypropylene part with protective seals can help it better withstand sunlight, though getting polypropylene to bond with other materials can be difficult as it has little to no absorption tendencies and is quite slippery.

Even with additives, polypropylene will degrade quickly compared to naturally UV-resistant plastics like acrylic or high-density polyethylene (HDPE). Thus, we recommend against its use for products that will receive extended exposure to direct sunlight.

Nylon UV resistance qualities in manufacturing

Similarly, nylon is susceptible to degradation when exposed to UV radiation for prolonged periods, as UV light creates free radicals that impact nylon’s properties. When exposed to sunlight for a prolonged period, nylon parts will weaken and fade in color. Nylon is sensitive to UV wavelengths of 290 – 315 nanometers.

However, there are several nylon variations which offer varying degrees of UV resistance. For example, nylon 6/6 is less resistant to UV rays than nylon 6 or nylon 12. Thus, it’s important to consider each nylon variation’s mechanical strength, temperature resistance, abrasion resistance, and water resistance in addition to your product’s intended application when deciding which type of nylon to use for your project.

Choosing between polypropylene and nylon

Neither nylon nor polypropylene is particularly resistant to UV light, but polypropylene will likely degrade faster upon consistent exposure to UV radiation. However, you can boost both polypropylene UV resistance and nylon UV resistance with:

• Stabilizers: When free radicals form, these additives trap them, preventing them from degrading the intermolecular bonds within the nylon or polypropylene. Hindered amine light stabilizers (HALS) are the most common type of stabilizers.
• Blockers: Blockers work by preventing UV rays from reaching the polypropylene or nylon in the first place. Carbon black is a common, highly effective blocker for outdoor applications, but titanium dioxide pigment can also help increase your product’s longevity.
• Absorbers: On the other hand, absorbers can absorb UV radiation and convert it into heat or other less harmful wavelengths. The most common absorbers are benzophenones and benzotriazoles, but other options exist.

It’s also worth noting that UV resistance isn’t the only factor to consider when choosing between polypropylene and nylon.

Polypropylene is stronger and more resistant to moisture than nylon, and its low melt viscosity makes it ideal for use in injection molding. Polypropylene is also resistant to electricity, oils, and bacteria, though it’s highly flammable and susceptible to chlorinated solvents and oxidation.

On the other hand, nylon has a higher melt viscosity than polypropylene. It also has good electrical resistance and is ideal for high-friction applications, as it can maintain its shape without much maintenance or lubrication. Nylon is also more malleable than polypropylene, which enables designers to create more complex shapes.

Producing polypropylene and nylon parts with SyBridge

Polypropylene and nylon are acceptable materials for parts that will receive intermittent UV exposure. However, fluoropolymers (such as polytetrafluoroethylene, polyvinylidene fluoride, and fluorinated ethylene propylene), imides (like polyimide and polyetherimide), acrylic, high-density polyethylene, or polycarbonate may be better suited for parts that will receive prolonged UV exposure. If polypropylene or nylon is otherwise the best material for your needs, adding blockers, stabilizers, or absorbers can boost your product’s UV resistance and increase its lifespan.

When you work with SyBridge, our team of experienced engineers can help you select the right material for your project. We’ll work with you from the design stage through production to ensure that your parts come out as intended. If you need help getting started, contact us to discuss our capabilities and what SyBridge can do for you.