HALS and UV Absorbers in Plastic Applications

UV Absorbers UV absorbers (UV emitters) protect polymers from radiation by absorbing harmful ultraviolet rays and converting the energy in UV light into energy carrying harmless vibrations. UV absorbers do not break down rapidly themselves, but convert UV energy into harmless heat energy levels distributed throughout the polymer matrix. UV absorbers are limited in their effectiveness due to the physical constraints of the absorption process, and their absorption capacity is governed by the need for additive and polymer thickness at high concentrations before sufficient absorption occurs to effectively delay photodegradation. Benzophenones are general-purpose UV absorbers suitable for clear polyolefin systems and can also be used in pigmented compounds. Benzotriazoles are primarily used in polystyrene. Both can also be used in polyesters. Concentrations are typically around 0.25-1.0%.

Applications for UV absorbers include light stabilization in paints and coatings, polyurethanes, PVC, engineering plastics, adhesives and elastomers. Combinations of UV absorbers with other light stabilizers such as HALS or benzoates show synergistic effects.

Benzoate light stabilizers, like HALS, stop photo-oxidation processes in polymers by reacting with free radical intermediates through chemical reactions. Benzoates are typically used in combination with HALS and/or UV absorbers rather than alone. The primary function of UV absorbers is to absorb UV radiation in the presence of a chromophore (Ch) found in the polymer. The aim is to filter harmful UV light from the polymer before it has a chance to produce Ch*. First and foremost, a UV absorber must operate in the 290 to 350 nm range. The purpose of UV absorbers is to absorb harmful UV light and convert it rapidly into harmless heat. During this process, the absorbed energy is converted into vibration and rotational energy of the molecular components. For UV absorbers to be effective, this process must occur faster than the corresponding reaction in the substrate, and neither the UV absorber nor the polymer being stabilized should be damaged during the energy conversion.

Synergistic Effect of Light Stabilizers and UV Absorbers

UV absorbers cannot absorb all UV radiation exposed to a coating. Some UV radiation will penetrate the coating surface. For this reason, HALS is incorporated into polymers. These molecules work by scavenging the free radicals that form. This differs from UV absorbers, which first prevent formation by removing radicals from the system and then regenerating themselves. Most formula developers will therefore use a combination of absorbers and HALS. Synergistic combinations of UV absorbers and HALS are ideal for polymer stabilization. UV absorbers are subject to the Beer-Lambert Law; therefore, absorbance is linearly related to UVA concentration (320 to 400 nanometers, used for photocuring), molar absorptivity (extinction coefficient) and path length (coating thickness). Free radical scavengers are not subject to Beer's Law. They work anywhere in the coating system.

Hindered Amine Light Stabilizers (HALS)

One of the most important classes of antioxidants for long-term heat protection of polymers is hindered amine light stabilizers (HALS), which are very effective inhibitors against free radical-induced degradation of polymers at low and medium temperatures. This class of amine stabilizers is based on 2,2,6,6-tetramethyl-piperidine derivatives. They are generally used as light stabilizers, particularly for olefins, which explains their name. HALS is particularly effective in surface coatings, providing better gloss retention, providing higher chalk resistance in pigmented systems while preventing crack formation in clear coatings. For pigmented systems, HALS provides the primary stabilization mechanism, since most UV radiation is prevented from passing beyond the first few microns of the coating by the pigment. The choice of appropriate UVA/HALS combinations and concentrations depends on the chemistry of the polymer application system (pigments and fillers, film thickness and exposure conditions).

How to Choose UV Absorbers for Plastics?

General Rules to Follow When Selecting Antioxidants and UV Stabilizers

 

1. Stability and Extraction

Color stability is an important consideration regarding the hydrolysis resistance of UV stabilizers and antioxidants. Additionally, the two additives should not react with other components in the system, nor should they corrode equipment or be extracted by matter on the product surface. Hindered amine light stabilizer typically exhibits low alkalinity; acidic additives should not be used together and the final product should not be applied in an acidic environment.  

2. Solubility and Compatibility

Most polymers are non-polar, while antioxidants and UV stabilizers are somewhat polar. Solubility is a concern. Antioxidants and UV stabilizers will dissolve without decomposing at the polymer processing temperature, as most UV stabilizers can meet this requirement.

3. Migration

Where possible, antioxidants and UVAs with high molecular weight and relatively high melting points will be selected, with each dosage determined according to the strictest processing and end-user conditions.

4. Processing

When the melting range of the antioxidant and UV stabilizer differs considerably from that of the resins, pre-tension flow or compression may occur. When this gap exceeds 100°C, the UV stabilizer and antioxidant will be added in masterbatch form and then mixed with the resin for processing.

5. Use and Safety

Antioxidants and UV stabilizers should be non-toxic or of low toxicity. They should be dust-free or low-dust. They should be harmless to humans during plastic processing and throughout their service life. They should not be harmful to animals or soil. They should not create pollution in air, soil or water.