UV light resistance of outdoor covering materials

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UV resistance is defined as the ability of a material to resist ultra violet (UV) light or sunlight. UV light will cause non-resistant materials and surfaces to fade or discolor.

The biological and chemical effects of UV light are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.

The problem that is caused by UV is known as UV degradation. There are several ways of avoiding UV degradation in plastics by using stabilizers, absorbers or blockers. There are a number of methods to reduce this problem of degradation. The addition of black carbon to the polymer is one of them as chemical inhibitors which are available for certain plastics to improve the UV resistance.

The impact of ultraviolet radiation on human health has implications for the risks and benefits of sun exposure.

Getting too much sun exposure can be harmful, but in moderation is beneficial.

The market offers several polymers that are inherently UV resistant.

Paint and silicone coatings can also be used to completely cover exposed surfaces to sunlight (UV radiation).

Other important thing that we have to consider in connection with UV resistance is the cladding material that we have to choose for our building. In construction, cladding is used to provide a degree of thermal insulation and weather resistance, and to improve the appearance of buildings. Your choice of cladding has a significant effect on the environmental performance of your home.

There are test methods that are used to predict the UV stability of a product over a number of years.

1. What is UV?

UV or Ultraviolet light is an electromagnetic radiation, which has a wavelength from 10 nm to 400 nm. It is shorter than that of visible light but longer than sun’s x-rays. UV radiation is presented in sunlight and it is about 10% of the total light output of the Sun.

“Ultraviolet” means “beyond violet” (from Latin ultra, “beyond”), violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency than violet light.

UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself.

2. Effects of UV light.

Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.

Three benefits of UV exposure are production of vitamin D, improvement in mood, and increased energy. Ultraviolet is responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans.

This vitamin helps to regulate calcium metabolism, creation of serotonin and melanin.

Sun tanning, freckling and sunburn are familiar effects of over-exposure, along with higher risk of skin cancer. Living things on dry land would be severely damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth’s atmosphere.

3. UV degradation.

Many natural and synthetic polymers are attacked by ultraviolet radiation, and products using these materials may crack or disintegrate if they are not UV-stable. The problem is known as UV degradation, and is a common problem in products exposed to sunlight. Continuous exposure is a more serious problem than intermittent exposure, since attack is dependent on the extent and degree of exposure.

Many pigments and dyes can also be affected.

4. How to prevent UV degradation?

UV attack by sunlight can be ameliorated or prevented by adding anti-UV chemicals to the polymer when mixing the ingredients, prior to shaping the product by injection moulding for example. UV stabilizers in plastics usually act by absorbing the UV radiation preferentially, and dissipating the energy as low-level heat. The chemicals used are similar to those in sunscreen products, which protect skin from UV attack. They are used frequently in plastics, including cosmetics and films.

Different UV stabilizers are utilized depending upon the substrate, intended functional life, and sensitivity to UV degradation.

UV stabilizers, such as benzophenones, work by absorbing the UV radiation and preventing the formation of free radicals. Depending upon substitution, the UV absorption spectrum is changed to match the application. Concentrations normally range from 0.05% to 2%, with some applications up to 5%.

Frequently, glass can be a better alternative to polymers when it comes to UV degradation. Most of the commonly used glass types are highly resistant to UV radiation. Explosion protection lamps for oil rigs for example can be made either from polymer or glass. Here, the UV radiation and rough weathers belabor the polymer so much, that the material has to be replaced frequently.

The effects of UV degradation on materials that require a long service life can be measured with accelerated exposure tests. With modern solar concentrator technologies, it is possible to simulate 63 years of natural UV radiation exposure on a test device in a single year.

UV radiation is split into three different types as described in table below together with their characteristic effect.

DESCRIPTION WAVELENGTH RANGE (nm) COMMON EFFECT

UVA 320 – 400 SKIN TANNING

UVB 280 – 320 SKIN BURNING

UVC 100 – 280 GERMICIDAL

5.Testining material’s resistance

Two of the accepted UV stability tests in the coated fabrics industry include AATCC Test Method 16-2004 Colorfastness to Light, and ASTM D4329-05 Standard Practice for Fluorescent UV Exposure of Plastics. These tests simulate weathering and realistically reproduce the physical damage caused by short-wave UV radiation by exposing materials to alternating cycles of UV light (using special fluorescent UV lamps) and moisture at controlled, elevated temperatures. In a few days or weeks, the UV test machine can reproduce the damage that occurs over months or years naturally.

Samples of the textile material to be tested are exposed to a UV light source under specified conditions. The colorfastness to light of the specimen is evaluated by comparison of the color change of the exposed portion of the test specimen to the unexposed original material. The change in color can be measured by a spectrophotometer.

The upholstery industry standard for UV resistance test is at least 40 hours with no color change. High performance faux leather upholstery for hospitality, contract and healthcare applications can pass 200/+ hours with no color change.

6. Importance of UV resistance

An increasingly important performance value for coated fabrics is ultraviolet (UV) light resistance. It is known that polyurethane and vinyl materials used for both interior and exterior applications are subject to photo-degradation when exposed to ultraviolet light.

7. UV Light Transmission

Ultraviolet light transmission (UV) is traditionally thought of as a negative for greenhouse growers due to its degrading effects — not only on glazing materials — but also components within a greenhouse, such as irrigation and electrical equipment. Some growers, however, welcome UV light because it contains the spectrum that is found in natural growing conditions.

By allowing UV light to enter a greenhouse, you have the best of both worlds: a protected growing environment, and the same light spectrum found outdoors. Holubowsky says some companies in the green industry have used glazing materials that allow UV light transmission, including acrylic products specifically designed to transmit UV rays to allow natural lighting for their breeding and test plots.

8. Polymers with good UV resistance

Plastic materials are often used for outdoor applications such as architectural glazing, mass transportation interiors, and marine dock fenders. Certain plastics such as acrylic, Ultem®, PVDF, and PTFE are inherently UV stable. However, most plastic materials exhibit color change and loss of elongation when exposed to UV light.

Weathering resistance of some plastics can be improved by adding UV stabilizers or UV resistant caps to protect the base polymer.

Acetal Homopolymer, Copolymer (polyozymethylene, POM)

Unmodified acetal resin will degrade over time upon exposure to sunlight. The material can crack, embrittle, and develop a chalky appearance. Pigmented and chemically modified formulations are available including Delrin® 107 acetal and Delrin® 507. DuPont has developed 20 year exposure data on these materials.

Nylon (all types)

Unpigmented resins will degrade upon exposure to sunlight evidenced by discoloration and embrittlement. Formulations containing carbon black particles provide the best UV stability.

Rigid Polyurethane

Thermoplastic polyurethane exhibits good weathering characteristics. Upon exposure to UV radiation it does experience a color shift, however there is a minimal change in mechanical properties.

Polycarbonate

Unmodified polycarbonate resins (Hydex® 4301, Lexan®, Makrolon®) will degrade upon exposure to sunlight. Polycarbonate will yellow and become hazy after 1 year of exposure. UV resistant grades are available.

PBT

Polybutylene terephthalate (PBT) (also known as Hydex® 4101, Valox®, Celanex®, Ultraform®) is “inherently” UV resistant. Supplier data indicates that there is little degradation in mechanical properties after several years of exposure. Black pigmented resins have better property retention.

PEI

Polyetherimide (Ultem®) is inherently resistant to ultraviolet radiation. After 1000 hrs. of exposure, no measurable change with tensile strength.

ABS

ABS is not suitable for outdoor applications because of its poor UV resistance. Current UV stabilized grades refer only to color fastness not mechanical property retention.

Polysufone

Polysufone (Udel®) will experience some degradation upon exposure to sunlight. Black pigmented formulations are recommended for improved performance.

E = Excellent UV resistance

F = Fair UV resistance

U = Unacceptable UV resistance

9.Cladding systems for outdoor coverings

Cladding is a non-loadbearing skin or layer attached to the outside of a home to shed water and protect the building from the effects of weather. It is a key element in the aesthetic appeal of the home and directly influences both building cost and property value.

The primary roles of cladding are to control the infiltration of weather elements and the egress of water vapour while providing a durable, aesthetically pleasing appearance. Secondary roles can include sound and thermal insulation, fire resistance, and the capacity for cleaning in dusty, polluted or vandal prone environments.

Your choice of cladding should be based on a careful assessment and prioritisation of each of these roles for each orientation of your home. By choosing cladding materials specific to an elevation or exposure, you can often achieve the best in physical performance and aesthetics.

Performance considerations

Cladding is typically made from wood, metal, plastic (vinyl), masonry or an increasing range of composite materials. It can be attached directly to the frame or to an intermediate layer of battens or spacers to prevent condensation and allow water vapour to escape.

All forms of cladding must fulfill following functions

1. Be self-supporting between the framing members

2. Provide necessary resistance to rain penetration

3. Be capable of resisting both positive and negative wind pressures

4. Provide necessary resistance to wind penetration

5. Give required degree of thermal/sound insulation

6 Give required degree of fire resistance

7. Provide sufficient openings for admittance of natural daylight and ventilation

8. Be constructed in suitable size.

Summary

The best materials with UV resistance are polymers as vinyl, plastic, acrylic and glass. If a product is to be exposed to direct sunlight, the designer or engineer must specify suitable testing standards and make sure the plastic has appropriate formulation to maintain the desired long-term properties. Including additives to the polymer melt process may provide protection, or if volumes are sufficiently high, the additives can be pre-compounded into the resin.