Light stabilizers and UV absorbers to prevent polymer degradation
Exposure to sunlight and some artificial lights can have adverse effects on the integrity and the durability of plastics articles. UV radiation can break down the chemical bonds in a polymer. This process is called photodegradation and ultimately causes cracking, chalking, color changes and the loss of physical properties such as impact strength, tensile strength, elongation and other properties.
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| Figure 1: Polymer before exposure to sunlight |
Figure 2: Polymer after exposure to sunlight |
There is one effective solution to prevent polymer from photodegradation: the use of light stabilizers.
What causes polymer degradation?
Solely the UV radiation of the solar spectrum has enough energy to degrade a polymer. According to the 1st law of the photochemistry, light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
The UV radiation on the globe varies with latitude, altitude and seasons as shown in Figure 3:
Figure 3: Annual UV radiation on the globe
Weathering of plastics includes, in addition to the effect of UV radiation, environmental factors like temperatures, precipitations, contaminants and others:
- Polyolefins absorb UV light only because of impurities present in the polymer, oxidation products formed during processing or additives and pigments used in the formulation of a finished article.
- On the other hand, other polymers like Engineering Plastics and Rubbers absorb UV-light because of their intrinsic chemical structure (e.g. Polystyrene, Polyesters, ...)
How do UV and Light stabilizers work?
To counteract these damaging effects on plastics performance, a broad range of UV stabilizers designed to solve the degradation problems associated with exposure to sunlight have been developed.
Although you can find many types of UV stabilizers, they can be categorized into two general classifications:
Ultraviolet Light Absorbers (UVA) to retard photodegradation
UVAs slow down the degradation process by preferentially absorbing harmful ultraviolet radiation and dissipating it as thermal energy.
Figure 4: UV radiation absorption
The absorption of UV Light is governed by the Law of Lambert-Beer:
Absorbance = Molar Absorptivity · Concentration · Path Length
or A = ε.c.l = log 1/T where T (transmittance) = I/Io (Io is the initial light intensity)
To obtain an optimal absorption and thus effectively retard photodegradation, it is required to have high concentrations of absorbers and sufficient polymer thickness.
UV Absorbers are therefore effective:
- At protecting the polymer bulk
- At protecting the content when used in packaging films or bottles
- At protecting other additives, more sensitive to UV light (e.g. pigments and flame retardants)
- In UV absorbing substrates, such as polystyrenes, polyesters, etc.
However, they are less effective at protecting surfaces (crazing) and very thin articles such as film/fiber.
Hindered Amine Light Stabilizers (HALS) to inhibit polymer degradation
HALS are very efficient stabilizers for polymers and especially polyolefins. They do not absorb UV radiation, but act to inhibit degradation of the polymer. Significant levels of stabilization are achieved at relatively low concentrations. HALS' high efficiency and longevity are due to a cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process.
Figure 5:
Cyclic process - HALS regeneration
HALS are therefore:
- Very effective in high surface area applications such as film and fiber
- The most effective additive for the light stabilization of polyolefins
- Also highly effective thermal stabilizers for longer polyolefin articles service life
UVAs and HALS synergy to enhance light stability
In some applications, combinations of UVAs and HALS create a synergistic interaction that may enhance light stability of a plastic article. Other additives could also have their importance in the light stability performance of a UVA - HALS system and particularly the process stabilization package.
A variety of UV Absorbers and HALS have been developed to meet polymer and specific applications requirements. High molecular weight products were commercialized to address issues like volatility and migration of light stabilizers. The selection of the optimal UV Stabilizer will depend on the polymer type, the thickness, the use of pigments and other additives and of course the expected lifetime in a specified environment.