Weather and UV-resistance: Exploring the Resilience of Paints and Coatings

UV and Weather Resistance in Paints and Coatigs

In paints and coatings, weathering and ultraviolet (UV) resistance go hand in hand. These weather-resistant coatings generally come under the section of exterior paints and coatings.

A paint can be called weather resistant when UV resistance is an essential aspect of that paint. Several factors in the surroundings and the coating formulation challenge the performance of most coatings. This includes the exposure of coatings to:

solar radiation (UV),
temperature fluctuations,
heavy rainfall and snow, and
environmental fall-out (acid rain).

Examples of painted objects that are exposed to such environments include automobiles, aircraft, infrastructure (bridges and roads), houses, and buildings. There is no doubt that UV degrades certain paints and coatings. Thus, the exterior coatings must be UV-stable/resistant and weather-resistant. Some of the benefits of these coatings are listed as follows:

Longer protection and coating service life.
Shade and color retention of the coating or paint.
Superior resistance to multiple coating failures. For example, chalking, antimicrobial attack, cracking, peeling, flaking, and overall coating integrity.

Delve deep into the intricacies of weather-resistant paints and coatings:

What are weather-resistant coatings?
How does weathering or UV degradation occur in paints and coatings?
What factors affect the weathering resistance of paints and coatings?
Which test methods evaluate the weather resistance of paints and coatings?

What are weather-resistant coatings?

A weather-resistant paint or coating is one that can withstand extreme conditions of weather. For example:

rain,
microbial resistance,
UV or sun resistance, and
multiple thermal cycles (day-night, hot-cold).

The weathering cycles are mostly governed by the sun. It supplies light radiation (visible light and ultraviolet light) and a wide range of energy types (X-rays, gamma rays, and cosmic rays). There are benefits and also some problems which sun or solar radiation causes.

UV light is a type of electromagnetic radiation with wavelength ranging from 290 nm to 400 nm. Most polymers are very sensitive to UV light and weathering. For example, styrene, acrylic, or epoxy turn yellowish due to UV degradation. However, some polymers like acrylic polymers are stable in UV and exhibit good weathering properties. The following image depicts the spectrum of light and energy that comes from the sun.

Light Spectrum and UV Region in Sunlight

How does weathering or UV degradation occur in paints and coatings?

STEP 1: When being exposed to UV lights, certain components in a polymer absorb UV radiation. These components are known as chromophores. Chromophores can be a pigment particle, the main chain, or an end group of the polymeric binder, impurities, residual solvent, or an additive.

STEP 2: As the chromophores absorb photons (UV), they release energy. This later results in the cleavage of bonds in paint film along with free radicals.

STEP 3: The free radicals react with polymer chains and break polymer bonds. Initial pigment fading starts with the most sensitive pigments in the coatings.

STEP 4: Film embrittlement also starts parallelly due to polymer changes. This occurs due to thermal cycles of day and night. This long-term process eventually breaks down the polymer chains. Thus, it leads to the deterioration of physical properties.

STEP 5: The direct effects of this process include the loss of impact strength, changes in color, cracking, loss of elongation, tensile strength, or chalking of the surface. 

Stepwise Film and Coating Failure Due to UV and Weathering

Effect of UV/weathering degradation

Color fading and color shifting (yellowing)
Peeling or flaking of paint film by polymer disintegration or degradation
Complete paint failure of a disintegrated coating system
High susceptibility to corrosion and microbial attack

What factors affect the weathering resistance of paints and coatings?

Role of environmental conditions

Environmental condition is the most important factor for the weathering and UV performance of a paint or coating. For example:

In a region with heavy rainfall, the paint or coating may exhibit inferior weathering resistance. It may also get contaminated by fungal or algal growth.
In a region with extreme sunlight, the paint or coating may show inferior UV resistance. This region receives extreme UV radiation as compared to standard conditions.

Presence of UV stabilizers/absorbers

The exponential effect on the performance improvement of the coating, in terms of UV protection, depends on the presence of different types of:

UV stabilizers (HALS) — They capture the free radicals. These result from the UV excitation of different chemical entities in the coatings. This stops the chain reactions or bond cleavage of polymers.
UV absorbers — They absorb UV rays in competition with chromophores and dissipate it as thermal energy.

Different types of UV stabilizers and absorbers are available in the market that are used as additives. Some of their basic chemistries are hydroxyphenyl benzotriazole, hydroxyphenyl triazine, benzophenone, hindered amine light stabilizers (HALS), etc.

Polymer selection

The selection of polymer or binder is important for UV and weather-resistant coatings. Different types of polymers or binders that are available in the market are suitable for exterior paints. The most common and used polymers are:

acrylics and
polyesters

Both of these resins/emulsions/polymers give excellent UV and weathering resistance.

Pigment selection

There are stable pigments that are highly UV resistant and show excellent weathering resistance. For example, the UV-stable and resistant inorganic pigments like:

iron oxide (yellow and red),
titanium dioxide,
zinc oxide, and
bismuth vanadate.

The organic pigments are highly prone to fading or degradation by UV rays. Similarly, some other factors that can also account for the weathering and UV performance and resistance of a coating are film thickness and extender package selection.

Approaches to resist weathering

Selection of the right polymer system for paints and coatings. This involves considering the weather conditions.
Formulating paints and coatings with UV-stable and durable pigments. This mainly involves the selection of the inorganic pigments.
Incorporating the UV stabilizers or absorbers for exceptional UV resistance.

Chemical Class	Absorbed wavelength (nm)	Polymer Types
Chemical Class	Absorbed wavelength (nm)	PO	PVC	PS	PEST	PA	PC	PMMA
Hydroxyphenyl benzotriazole	300-400	Y	Y	Y	Y	-	Y	Y
Hydroxyphenyl triazine	250-360	-	-	-	Y	Y	Y	-
Benzotriazole	240, 280-360	Y	-	-	Y	Y	Y	Y
Cyanoacrylate	240-320	-	Y	-	Y	Y	-	-
HALS	200-250	Y	Y	-	-	-	-	-
Benzophenone + HALS	280-350	Y	Y	-	-	-	-	-
PO - Polyolefin; PVC - Polyvinylchloride; PS - Polystyrene; PEST - Polyester; PA - Polyamide; PC - Polycarbonate; PMMA - Polymethyl methacrylate; HALS - Hindered amine light stabilizers

UV Stabilizers/Absorbers and Their Application in Different Polymer Systems

Which test methods evaluate the weather resistance of paints and coatings?

Natural exposure tests

In this method, the painted samples are naturally exposed at selected natural sites. This is the most basic yet impactful testing for weathering. It provides very accurate data and results.

These sites are selected based on their extreme or diverse weather conditions like rainfall, heat, snowfall, etc.

Natural Exposure and Weathering Test of Paints

Accelerated weathering/UV test methods

ISO 16474 — Paints and varnishes — Methods of exposure to laboratory light sources — Part 2: Xenon-arc lamps.

ISO 11507 — Paints and varnishes — Exposure of coatings to artificial weathering — Exposure to fluorescent UV lamps and water.

ISO 4892 — Plastics — Methods of exposure to laboratory light sources — Part 2: Xenon-arc lamps.

SAE J2527 — It determines the accelerated exposure of automotive exterior materials. This is done by using a controlled irradiance xenon-arc apparatus.

ASTM G 154 — It is a standard practice for operating fluorescent ultraviolet (UV) lamp apparatus for exposure of materials.

ASTM G155 — It is a standard practice for operating xenon-arc lamp apparatus for exposure of materials.

ASTM D7869 — It is a standard practice for xenon-arc exposure tests. This is performed along with enhanced light and water exposure for transportation coatings.

ASTM D6695 — It is a standard practice for xenon-arc exposures of paints and related coatings.

Paint Light-fastness ASTM D6695 (Source: Spektrochem Paint Technical Center)

References