Friday, 19 June 2020 20:36

UV-Degradation in Plastics

Some plastics can be susceptible to damage when they come into contact with Ultraviolet (UV) rays for a prolonged time, leading to cracks or color fading and even the disintegration of the plastic. This raises the question; Which plastics are UV stable or can withstand UV exposure better?

Ultraviolet radiation consists of photons with high energy relative to visible light, this radiation is split into three types, these are UV-A, UV-B and UV-C.

UV-A has a wavelength range of 400-320 nm, while UV-B has a range of 320-280 nm. Meanwhile, UV-C’s range stands at around 280-200 nm.

UV-C is not present in our perceived sunlight, because wavelengths lower than 300 nm are absorbed by the ozone layer in the upper atmosphere, the general effect of UV-C is killing bacteria and microorganisms by disrupting their DNA, so we can focus on UV-A and UV-B.

UV effects on different types of plastics

UV energy can excite photons, when absorbed by plastics, in turn, can create free radicals. This is the beginning of degradation. As a matter of fact, several pure plastics simply cannot absorb UV radiation.

The most common ways you can detect if a plastic is being affected BY UV are:

  • You may notice a pale appearance or color fading on the surface of the material
  • The plastic’s surface becomes brittle or cracked

  • UV-Degradation-in-Plastics

    Notable color fading in exposed to sunlight (UV radiation) Nylon rope. (quora.com)

    The most widespread UV damage mechanism in plastics is called chain scission by photolysis, this is the breaking of long chain polymers into shorter ones. This almost always results in a degradation of physical properties such as strength or degradation of visual properties such as color and texture.

    UV resistant plastics

    Some polymers are more stable to UV exposure than others, there is a class of high-performance polymers called fluoropolymers that exhibit excellent UV resistance. Teflon has become a genericized name for all fluoropolymers. fluoropolymers are exceptionally resistant to UV degradation. Accordingly, PTFE or FEP are almost always used for wire insulation on UV lamps or in UV equipment.

    Here is a short list of them:

  • Polyethylene
  • Polytetrafluoroethylene (PTFE)
  • Fluorinated ethylene-propylene (FEP)
  • Polyvinylidene fluoride (PVDF)

  • UV-Degradation-in-Plastics

    With their high performance against UV come high prices, fluoropolymers are among the most expensive polymers.

    UV coatings and other protection

    UV protective curable coatings are composed of acrylate functionalized resins, which polymerize or cure instantaneously upon exposure to UV light, thereby being easy to apply and efficient. Energy-curable resins can be 100% solids materials or can be diluted with solvents for ease of application, since coatings on plastic parts are mostly spray-applied, solvents are used to reduce viscosity.

    In terms of the components more likely to be at risk of UV damage, automotive parts are high on the list. The effects will predominantly result in a change of the material’s surface layer – and some plastics, if damaged by UV, will ultimately lead to the component failing altogether.

    The best coatings are multi-layer systems. The coating of plastics begin with the application of a primer, which has a binding effect on the basecoat, that is then applied over the primer. Finally, a clear-coat containing the light stabilizers for UV protection seals the coating system protecting the plastic.

    Source: UV solutions Mag

    Published in UV-Curing

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