UV-Stabilised Compounds for Outdoor Plastic Parts

An outdoor plastic part can pass every test on the day it is moulded and fail in its second summer. Sprinkler bodies, drip fittings, sprayer parts, tractor panels, mirror housings and grilles all live under strong sun, often with water, heat, fertilisers and dust on top. Whether they last depends on choosing the right polymer, the right stabiliser package and a test that reflects real exposure.

What sunlight does to plastics

Ultraviolet light carries enough energy to break chemical bonds. With oxygen present, the broken bonds start a chain reaction of free radicals that cuts polymer chains and forms carbonyl groups. Heat and moisture speed it up. Damage starts at the surface and works inward, so the first signs are cosmetic: lower gloss, chalking, yellowing or fading. Then microcracks form, and because cracks act as notches, impact strength and elongation fall long before tensile strength does.

Polymers differ widely in how much help they need:

How UV stabilisers work

• UV absorbers, such as benzotriazoles and triazines, absorb UV and release it as heat. They need path length to work, so they protect the bulk of a part better than its outermost layer, and they are essential in light and transparent colours.
• HALS, hindered amine light stabilisers, do not absorb UV. They trap the free radicals that start degradation and are regenerated in the process, which is why they protect for a long time at low loadings. In polyolefins such as PP they do most of the work.
• Pigments screen the light. Carbon black absorbs across the UV range and is the most effective single protection for black parts; how well it works depends on particle size, structure, loading and, just as much, dispersion. Poorly dispersed black protects far less than the same amount well dispersed.
• Antioxidants protect the polymer during compounding and moulding and against heat in service. A part that runs hot in the sun needs both.

Two interactions catch people out. Some HALS are deactivated by acidic chemicals, including sulphur- and halogen-containing agrochemicals, so tell the compounder what a sprayer or greenhouse part will be exposed to. And flame retardants, fillers and pigments can all change how much stabiliser a grade needs, so the package has to be designed for the whole formulation, not added from a standard recipe.

Testing: what to put in the specification

The most common mistake is to treat chamber hours as years. Q-Lab, which builds both kinds of tester, states that a single conversion number is theoretically impossible: outdoor severity varies with location, season and year, by as much as 2:1 between successive years at the same site. Accelerated results are best used to rank materials against each other and against a reference with known field history.

A usable specification names four things:

1. The method and cycle, including filters, irradiance, temperature and water spray or condensation.
2. The exposure, preferably as radiant exposure in kJ/m² or MJ/m² in a stated wavelength band, not just hours.
3. The pass criteria: colour change as delta E, gloss retention, retained tensile elongation and impact, and no cracking or chalking.
4. The baseline: unexposed specimens from the same lot, tested at the same time.

Agriculture and irrigation

Farm parts face the harshest combination: sun all day, water, fertilisers and agrochemicals, abrasion from soil, and impact from handling. Typical moulded parts are sprinkler and nozzle bodies, drip and lateral fittings, valve and filter housings, sprayer components, and tractor panels, grilles and covers. Black parts are the most robust because carbon black does so much of the work. Colour-coded parts in blue, red or green rely on organic stabiliser packages and should be tested for fade as well as embrittlement.

For these parts we start from UV-stabilised PP such as OSALENE PPTF 15 (15% talc, high UV stabilised, high impact) and, where stiffness is needed, OSALENE 20 BST (20% glass, high UV stabilised). For clips, valve parts and other sliding or snap-fit parts outdoors, OSATAL 4000 HI UV is a lubricated, UV-treated POM. More on the sector is on our agriculture page.

Exterior automotive trims

Mirror shells, grilles, cowl and pillar trims, lamp parts and two-wheeler body parts are often moulded in colour, with no paint to protect them. The OEM specification usually calls for xenon exposure to SAE J2527 or its own equivalent, with limits on delta E, gloss change and surface defects, and often a retained-impact requirement. Painted parts rely on the paint, but the substrate still needs enough stabilisation for edges and stone chips.

Our starting points include OSAALLOY CP 6080, an impact-modified, UV-stabilised PC/ABS with good flow, OSAALLOY 7030 HBK (PA/ABS, UV treated) and OSALAC 3107 (SAN with 35% glass, UV and heat stabilised). PC grades with UV stabilisation are developed to order in OSANATE.

Choosing a grade: the brief

• Colour: black, dark, light or natural, and whether colour hold matters or only mechanical life.
• Exposure: region, orientation, hours of direct sun, and design life in years.
• Function after exposure: impact, snap-fit strength, appearance, or all three.
• Wall thickness: thin walls lose a larger share of their section to surface damage.
• Chemicals: fertilisers, agrochemicals, cleaners, fuels.
• Heat: dark parts in direct sun run well above air temperature, so specify heat stabilisation as well.
• Test plan: method, cycle, exposure, pass criteria and baseline.

UV-stabilised grades sit in our PP, POM, ABS and SAN, alloy and PC families, and colour-matched versions are made to order. Our lab measures colour and delta E on a spectrophotometer, and tensile and impact on the UTM and pendulum testers, so retained properties on exposed panels can be read against an unexposed baseline from the same lot; see our quality page.

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