Crazing, Chalking, and UV: A Guide to Addressing These Hot Topics in Plastics
UV damage for plastics
Have you seen plastic chairs that were left out in the field for a long time with broken legs, faded color, and “look old”? It may be a result of degradation from UV rays. UV rays have high-energy photons, which can cause polymer bonds to break. The cleaved bonds are known as “radical species.” These radical species contain high energy unpaired electrons and are very reactive. They can start chain reactions leading to degradation of neighboring molecules and result in undesirable effects to a plastic part such as embrittlement.
In some cases small cracks develop on the plastic surface. This is termed “crazing.” Discoloration and “chalking” may also be caused by UV degradation. When UV rays degrade organic molecules on the surface layer of plastics, they can cause plastics to appear yellow. Plastic fillers or pigments may also be exposed, leaving a thin layer of powdery material on the plastic surface (Figure 1). When UV rays interact with pigments, they may also degrade, leading to color fade.
Figure 1: a-b) a new plastic bench showing little apparent sign of UV degradation versus an older bench, c-d), showing crazing and small cracks. e) When wiping a finger across the crazed part, white powder, possibly containing white pigments and fillers, can be seen.
Because of the known harmful effects of UV rays many plastic part manufacturers, including outdoor furniture manufacturers, rely on stabilization additive packages for plastic stabilizations. Essential components of such stabilization packages are hindered amine light stabilizers (HALS). HALS are molecules that can convert radical species into non-radical ones, breaking that chain reaction degradation described above. Based on your specific requirements, masterbatch companies can formulate with a wide range of HALS and other stabilization packages. Even at very low loadings, HALS are highly effective as they can stabilize radical species using a chemical mechanism that regenerates the stabilizer molecule. Higher molecular weight HALS can also help meet specific regulatory requirements relating to migration and extraction limitations.
A potential concern with HALS relates to exposure to pesticides, herbicides, some flame retardants and a few other chemicals: these substances can deactivate HALS. Luckily, UV packages are available with resistance to these chemistries. Product lifespan requirements, applications, and geographic locations should also be considered when choosing the proper HALS additives. Remember: UV rays are much more intense in Miami than in Anchorage! Colortech has weathering instruments that mimic the outdoor UV exposure of parts with day, night, and condensation (simulating dew) cycles at elevated temperatures simulating the effects of long term outdoor exposure.
Indoor food packaging
Although outdoor applications can lead to concern about UV exposure, it is worth noting that indoor lighting contains a UV component and can cause unwelcomed damage to materials and package contents as well! This is of particular concern for food companies and food packaging manufacturers. For instance, UV rays are known to degrade vitamins such as riboflavin in milk. To address this, UV absorbers (UVA) can be added to the container to protect the milk. Unlike HALS, UVA directly absorbs and dissipates harmful UV rays into a very small amount of heat. Because UVA act as a blocker, it is essential to ensure enough UVA is added into food packaging when the packaging is very thin. Similar to HALS, there is a host of UVA types to meet your specific product requirements including cost, packaging clarity, regulatory restrictions and degree of required protection.
Are UV rays a concern for your plastic products? Contact a Colortech sales representative about how different stabilization packages can help!
Hoi Chun Ho, PhD
Technical Services Representative