Exposure to UV Radiation

Over time, exposure to the UV radiation from sunlight will degrade plastics. Polymer photo-degradation occurs when non-visible UV radiation made up of short wave lengths breaks down the polymer chains in plastics. This is called the photo-degradation process.  It is caused by exposure to UV radiation and results in the deterioration of physical properties such as loss of impact strength, changes in color, cracking,  loss of elongation and tensile strength as well as chalking of the surface. For example, photo-degradation from UV radiation causes garden chairs to lose their gloss and become brittle.  Also,  the color of stadium seats appears chalky and some plastics will even become yellow and crack.

Some Characteristics of UV Radiation

UV radiation accounts for only 4.6% of the solar spectrum.  It is calculated in nanometers (nm) and ranges between 290 and 400, but the most aggressive part of the UVB range is the very short wavelengths between 280 and 315 nanometers.  The amount of uv radiation energy exposure (irradiation) depends on where you are in the world. 

What is Irradiation

Irradiation is the UV radiation energy incident over a specific area over a given period of time.  1Ly = 1 cal/cm2 = 4.184 E4Joule/m2  (Ly=Langley)  Thus the amount of energy transmitted to a plastic part in one year of continuous outdoor use is 220kcal/cm2/year in Sudan while in Sweden it is 70.

Each plastic is sensitive to certain wavelengths within the 290-400 nm UV region. Polypropylene has three maxima at 290-300, 330 and 370 nm. The range for nylon is 290-315 and PVC homopolymer is 320.

UV Wavelength Sensitivity of Polymers (nm)

Material Activation spectra maxima
Nylon 290-315
Acrylic 290-315
Styrene Acrylonitrile 290, 310-330
Polycarbonate 280-310
Polystyrene 310-325
Polyethylene 300-310, 340
Polypropylene 290-300, 330, 370
ABS 300-310, 370-385
PVC homopolymer 320
PVC copolymer 330, 370
Polyurethane (aromatic) 350-415
Polymer degradation

Polymer photodegradation occurs when UV radiation from the sun is absorbed by chemical groups in the polymer formation called chromophores. Chromophores are “an atom or group of atoms whose presence is responsible for the color of a compound”. The polymer formula may include other additives such as halogenated flame retardants, fillers and pigments.   UV stabilizers have been developed and are added to a polymer to inhibit the photoinitiation processes.   The top three types are Ultraviolet Absorbers, Quenchers and Hindered Amine Light Stabilizers (HALS).

1) Ultraviolet Absorbers

Absorbers are a type of light stabilizer that functions by competing with the chromophores to absorb UV radiation.  Absorbers change harmful UV radiation into harmless infrared radiation or heat that is dissipated through the polymer matrix. UV absorbers have the benefit of low cost but may be useful only for short-term exposure.  UV absorbers include:

Absorbers

UV absorbers have the benefit of low cost but may be useful only for short-term exposure.  UV absorbers include: 

  • Carbon black is one of the most effective and commonly used UV radiation absorbers.

  • Another UV absorber is rutile titanium oxide which is effective in the 300-400 nm range.  However it is not very useful in the very short wavelength UVB range below 315.  

  • Hydroxybenzophenone

  • Hydroxyphenylbenzotriazole are also well known UV stabilizers that have the advantage of being suitable for neutral or transparent applications.  Hydroxyphenylbenzotriazole is not very useful in thin parts below 100 microns. 

  • Benzophenones for PVC

  • Enzotriazoles and hydroxyphenyltriazines for polycarbonate

  • Oxanilides for polyamides

2) Quenchers

Quenchers return excited states of the chromophores to ground states by an energy transfer process.  The energy transfer agent functions by quenching the excited state of a carbonyl group formed during the photo-oxidation of a plastic material and through the decomposition of hydroperoxides. This prevents bond cleavage and ultimately the formation of free radicals. 

Nickel Quenchers

Nickel quenchers are a common type used in agricultural film production.  These are not widely used as they contain heavy metal and contribute tan or green colors to the final product.  However, nickel quenchers do not effectively stabilize UV radiation as the Hindered Amine Light Stabilizers discussed next.

3) Hindered Amine Light Stabilizers (HALS)

HALS are long-term thermal stabilizers that act by trapping free radicals formed during the photo-oxidation of a plastic material.  Therefore HALS limit photodegradation process. The ability of Hindered Amine Light Stabilizers to scavenge radicals created by the absorption of UV radiation is explained by the formation of nitroxly radicals through a process known as the Denisov Cycle.

Although there are wide structural differences in the HALS products commercially available, all share the 2,2,6,6-tetramethylpiperidine ring structure.  HALS are some of the most proficient stabilizers for UV radition.

UV Stabilizers for a Wide Range of Plastics

For example, HALS has enabled the growth of polypropylene in the automotive industry.  While HALS are also very effective in polyolefins, polyethylene and polyurethane they are not useful as stabilizers for UV radiation in PVC

Combinations of UV Stabilizers

As all three function by different mechanisms, they are often combined into synergistic UV radiation absorbing additives.  For example, benzotriazoles are often combined with HALS to protect pigmented systems from fading and color changes.

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29 responses to “The Top 3 Plastic Additives for UV Stabilization

  1. As a product developer It is nice to find a little quick study course such as yours to help me understand terms and applications for modern tech. Thank you for the information. will revisit your sight in future. Thanks.

  2. May you let me the commercial additives for Uv stabilization for plastics film used in agriculture or film exposed to the sun and the sources where they can be bought from. Thanks.

    1. Munacho,

      Craftech does not manufacture film. We do use UV additives in various plastic resins we injection mold, but I’d suggest you check with a film maker for accurate guidance. The answer will in part depend on what plastic you need UV protection for. Good luck!

  3. Hi, I have a question regarding certified plastic resins without UV additive.

    My question is, if we add a UV treatment to a UL certified resin with no UV treatment, do we void the certification?
    Does the addition of the UV treatment affect the resins flammability or other ratings?

    Best Regards,

    1. Jason – To the best of my knowledge, UL certification is given to products – not materials going into products. “The acceptability of a polymeric material is determined for each application. The determination shall consider all required properties (physical, electrical, etc.) for the specific application.” That comes from a UL Standard explanation. Also of interest: “The molder shall not employ colorants, flame retardants, fillers….or reinforcements – unless the additive is tested and found not to adversely affect critical properties.”

      1. Yes the UL approval is on the material also and changing the formula for the resin would require complete UL Re-Certification.

  4. I have a set of Grosfillex polypropylene chairs on my patio and they receive a south western sun every day all year long. They are never put away/stored during the winter. Grosfillex tells me that they put UV inhibitors in the poly mixture prior to pouring in the mold.

    My question is: Do the additives break down in polypropylene after a time? If so, what is the approx time?

    1. Don – I would suggest asking the manufacturer this question. Only they would know how much UV inhibitor was used – and they may have tested for longevity and have your answer.

    1. Mr Chidi,
      Thanks for reading Craftech Industries’ blogs. Yes, UV protection can be used in GPPS – just check with your material supplier for options.

  5. Thanks for this article. However, I produce transparent products which turns yellow and some which did not turn yellow cracks. What is the name of the resin will I request in the market that resist UV yellowing and cracking?
    Which of the above listed additive(s) is/are suitable for preventng yellowing and cracking of epoxy resins?
    To what ratio is/are the additive(s) will be mixed with the resin and hardner?
    How long will the additive(s) last on the product I would produce?

    1. Harry – Thanks for reading Craftech Industries’ blogs. I would suggest approaching your material supplier with these questions. Each plastic reacts a little differently to UV exposure. Normally the UV additive would be in the material when you buy it. Your supplier can make suggestions based on your application, as well as furnish pricing for the options. A different approach would be to color your product a dark color, which might prevent the “yellowing.”

  6. So, I’m seeking to roll out some thick plastic by hand, into useable thin sheets. Is there a dangerous gas produced, if using a hot air gun? Can I melt chunks in a vat outside, then seek to pour flat then roll them into sheets? Also, can I trade sorted plastic to a recycler, then receive credits on the purchase of repro sheets?

    1. David – Thanks for reading the Craftech Industries blogs. There are very few plastics that produce dangerous gas when melted. Not knowing how many of these sheets you will need, I would think the equipment needed to melt and then form plastic material would be costly compared to buying sheet material from a vendor – unless you will need high volumes. Regarding recycling, thousands of pounds of plastic is usually necessary to get a recycler interested – since they have to justify the cost of picking it up. I don’t know of any recyclers who produce sheet stock.

    1. Thanks for reading the Craftech Industries’ blogs. UV inhibitor is added at the time the plastic is formulated, and before molding it.

  7. Do you have information about environmental release of your photo-stabilizers and how they compare with the growing list of concerns that sunscreen UVFs have on our ecosystems? I am writing a comprehensive review of this topic for a dermatology related climate change article. Many thanks in advance

    1. David – Thanks for reading the Craftech Industries’ blogs. We don’t manufacture plastic resin or stabilizers, and don’t feel qualified to answer your question. We suggest you contact a couple of plastic makers – they should be more knowledgeable in the arena of stabilizers.

    1. Thanks for reading the Craftech Industries posts. I am told that the answer to your question will depend on the plastic material and process (injection, extrusion) used.

  8. WE ARE MANUFACTURER OF REGENERATED CAST ACRYLIC SHEET IN INDIA SINCE 1994 BUT THE SHEET WE MANUFACTURE ARE BECOME YELLOWISH AFTER 2-3 MONTHS SOME ONE TOLD ADDING UV STABILIZER IN THE PRODUCT IT DOES NOT BECOME YELLOWISH IN SHORT TIME SO
    LET US KNOW THAT WHICH PRODCUT IS SUITABLE FOR US

    1. Thanks for reading the Craftech Industries’ posts. Our suggestion is to contact your raw material supplier and ask what they have available for a UV stabilizer. They should be able to recommend and supply one. We’ve had good success using UV stabilizer is several of our products which are exposed to UV radiation.

  9. Excellent content. Well done. Craftech should be considered a value add to customers looking for polymer support.

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