UV Light Resistance

Ultraviolet light is a part of the electromagnetic spectrum. It is at the higher end of energy compared to visible light. It is followed by X-rays and Gamma rays in terms of energy. UV energy absorbed by plastics can excite photons, which then create free radicals.

UV light is probably the most damaging environmental factor for plastics. Although to be fair to plastics, it attacks, to a greater or lesser extent, most other materials as well. All applications of plastics that are used outdoors are thus at risk. This includes roofing and window frames for vehicles. While many pure plastics cannot absorb UV radiation, the presence of catalyst residues and other impurities will often act as receptors, causing degradation.
 

The Electromagnetic Spectrum

The UV light breaks down the chemical bonds in a polymer by photodegradation. This ultimately causes a change in appearance and deterioration in properties. It leads to discoloration. Primarily yellowing or whitening ('chalking') is most apparent. But, underneath, there is usually the beginning of a loss of physical properties. These include impact strength, tensile strength, and elongation. UV light can cause color change and degradation of physical properties, especially in:
 

• Polyolefins
• Polyvinyl chloride (PVC)
• Polycarbonate (PC), and
• Polyurethane (PU)

Hence, any attempt to design plastic parts without a clear understanding of the degradation mechanisms induced by the environment would result in a premature failure of the product.

Learn to Minimize the Discoloration of PVC Products » 

 

1. Thermoplastics with Good UV Resistance — View All Products
2. TPEs/TPVs with Good UV Resistance — View All Products
3. Rubbers with Good UV Resistance — View All Products
4. Thermosets with Good UV Resistance — View All Products

Note: UV radiation attacks all types of polymers. But a few polymers (acrylonitriles & methyl methacrylates) show better UV resistance than most.

 

Plastics get damaged due to their oxidation caused by UV light. The counter-measures to prevent/terminate oxidation of plastics by UV light include:
 

• coating,
• introducing pigments that screen out the rays, or
• neutralizing the UV energy within the compound and dissipating it.

Also, there exist many more technologies such as polymeric stabilizers, concentrates and masterbatches, fine particle technologies, etc. These help in preventing UV radiation from reaching the polymer. Thus, it helps to avoid damage. Some of the technologies are listed below.

 

Exploring screening pigments

• Carbon black is the most effective screening pigment. However, its applications are limited to black-colored products.
• Titanium dioxide is also used. But, it is expensive.
• Calcium carbonate can also have a screening effect. But, usually at a high loading, it might impair mechanical properties.
  
   

Mechanism of UV absorbers

UV absorbers are receptive to UV radiation. But, they do not themselves degrade rapidly. They convert UV energy and dissipate it harmlessly as heat. They prevent the oxidation caused by UV radiation. However, they should not be confused with antioxidants, which are not UV deactivators as such.
 

• Benzophenones are good general-purpose UV absorbers for clear polyolefin systems. They can also be used in pigmented compounds.
• Benzotriazoles are used mainly in polystyrene and PVC. But they can also be used in acrylics and polycarbonates, and in polyurethanes and unsaturated polyesters. They also improve the light stability of styrenics, polyacetals, epoxies, urea, and melamine.
   

(A) Structure of Benzophenone; (B) Structure of Benzotriazole
Source: ResearchGate

Role of UV stabilizers and quenchers

• Ultraviolet stabilizers work, unlike UV absorbers. They inhibit bond rupture by chemical means or dissipate the energy to lower levels that do not attack the bonds.
• Quenchers reduce the UV energy by means of deactivating metal ions. In effect, they intercept the energy before it can break any molecular bonds. But, they work in a different way from absorbers.

Scavenging free radicals - HALS for enhanced polymer protection

Scavengers act by inhibiting the free radicals generated by UV light. Thus, stopping any further decomposition. The most important are hindered amine light stabilizers (HALS). They are efficient scavengers. They function by inhibiting the degradation of a polymer that has already formed free radicals.

HALS have the advantage that they bind additives to the polymer at the molecular level. This causes less antagonism towards other additives. They can be used with most polymers. Polymeric HALS offer:
 

• Superior compatibility
• Low volatility
• Excellent resistance to extraction, and
• Contribute to heat stability

A combination of two high molecular weight grades gives a good balance of properties for greenhouse film. Low density polyethylene (LDPE) film is mainly used in HALS.

 

Synergists with HALS

In conjunction with other light stabilizers, HALS can exhibit synergistic effects. These are being actively explored. For example, some cyanoacrylate-based UV absorbers offer particular benefits in:
 

• acrylonitrile butadiene styrene (ABS) and polyamide (PA)
• individual components of rigid or plasticized PVC, styrene butadiene rubber (SBR), and polyurethane foams.

There exist several test methods to predict the behavior of plastic material to UV light. These test methods characterize material performance when subjected to specific and well-defined factors. But, it is also important to note that no one test can be employed to test completely the effects of UV light on any material.
 

• ASTM D2565 — It determines the xenon-arc exposure of plastics intended for outdoor applications.

• ASTM D4459 — It determines the xenon-arc exposure of plastics intended for indoor applications.

• ASTM D4329 — It determines the fluorescent ultraviolet (UV) lamp apparatus exposure of plastics.

• ASTM G154 — It operates fluorescent ultraviolet lamp apparatus for exposure to non-metallic materials.

• ISO 4892 — It determines the methods of exposure to laboratory light sources. It is a four-part standard covering different light sources.
  • Part 1: General guidance
  • Part 2: Xenon-arc exposure
  • Part 3: Fluorescent UV exposure
  • Part 4: Carbon-arc exposure

Ratings in the table below are based on an overall qualitative assessment. 

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