Transparent engineering plastics outdoors: correctly assessing UV and weather exposure
Transparent plastics for outdoor use in UV and weathering comparison: PC, PMMA, PSU, PES, COP. Selection matrix, coatings, design rules, practical guide.
Transparent material selection for outdoor use
Transparent engineering plastics are in demand for outdoor use. UV radiation, temperature changes, moisture, abrasion and cleaning chemicals attack the polymer structure and the surface. In this article we assess the five relevant materials polycarbonate, acrylic glass PMMA, polysulfone PSU, polyethersulfone PES and COP cyclic olefin polymer, and their suitability for robust viewing windows, covers and lighting applications.
If you are looking for a basic overview of transparent plastics, also read the basic information under Transparent engineering plastics. For specific UV topics, the article Resistant plastics against UV radiation is also worth reading.
Relevant loads outdoors
UV radiation: causes photolysis and photooxidation, leading to chain scission, yellowing, microcracks and loss of strength. Influencing factors are spectrum, intensity and duration of exposure.
Weathering: temperature changes accelerate ageing, promote stress cracks, and act on holes and edges via thermal expansion. Moisture and rain encourage hydrolysis in certain polymers, and salt spray mist and exhaust gases also have a corrosive effect on fastenings.
Mechanical stress: hail, sand and cleaning lead to abrasion and micro-scratches that increase haze and reduce transmission. Vandalism requires high impact strength.
Chemicals: cleaning agents, solvents from anti-graffiti systems or fuels can initiate stress cracks. Suitable cleaning chemicals are part of the material decision.
Material profiles at a glance
Polycarbonate PC
Polycarbonate is the standard for impact-resistant glazing. Unstabilised, PC is UV-sensitive; UV-stabilised or coextruded UV protection layers raise weather resistance considerably. Optically, PC is clear, but without a hard coating it shows a comparatively soft surface.
- Strengths: very high impact strength, good cold bendability, high heat deflection temperature
- Limits: scratch sensitivity without coating, limited chemical resistance to solvents
- Recommendation: for outdoor use, consistently use with UV protection on both sides and preferably hard-coated
Acrylic glass PMMA
PMMA is the optical benchmark for outdoor applications. It barely yellows, stays glass-clear for a long time and is surface-hard. However, PMMA is considerably more brittle than PC and has a lower notched impact strength.
- Strengths: excellent UV and weather resistance, very high light transmission, good scratch resistance
- Limits: lower impact strength, sensitive to solvents that trigger stress cracks
- Recommendation: for long-lasting, optically demanding viewing surfaces under moderate impact stress
Polysulfone PSU
PSU is a transparent high-temperature material with good dimensional stability. UV resistance is better than that of unstabilised PC, but below PMMA. In combination with coatings, PSU is suitable for thermally more demanding outdoor viewing windows.
- Strengths: high temperature resistance, good hydrolysis resistance, adequate toughness
- Limits: higher density and costs, optically slightly yellowish tint depending on batch
- Recommendation: for warm environments and increased media exposure where PMMA is not thermally sufficient
Polyethersulfone PES
PES increases thermal performance once again compared with PSU. Optically clear, but somewhat lower transmission than PMMA. Interesting for continuous use at higher temperatures.
- Strengths: very high heat deflection temperature, good chemical and hydrolysis resistance
- Limits: costs, lower availability than PC or PMMA
- Recommendation: viewing windows in hot air, steam or with high radiant heat, preferably with hard coating
COC and COP Cyclic Olefin Polymer
COP offers extremely low water absorption, very good optical properties and excellent dimensional stability under moisture. UV resistance depends strongly on grade and additives, but is often good. COP is harder than PC, but more brittle, similar to PMMA.
- Strengths: very low moisture absorption, high optical purity, low density
- Limits: limited heat resistance, potentially brittle fracture, availability restricted depending on format
- Recommendation: precision covers and sensor windows with moisture changes where high optical purity matters more than maximum impact strength
Note: in the sulfone family there is PPSU with yet further improved hydrolysis resistance. For outdoor applications with hot water or frequent sterilisation cycles, PPSU can be an alternative, but is not the main focus of this comparison.
UV and weathering behaviour in comparison
Mechanisms: PMMA has a saturated, UV-resistant chain and therefore stays clear outdoors for a long time. PC has carbonate groups that are attacked photooxidatively. Sulfones PSU and PES are aromatically stable and show better UV resistance compared with unstabilised PC, but benefit from additives and coatings. Depending on the formulation, COP is well UV-stabilised, but in exposed continuous use it frequently also requires stabilisers.
Practical guide value: For components with 10 years of outdoor exposure, PMMA is often sufficient without additional protective layers. PC requires an effective UV protection layer and benefits greatly from hard coatings. With coating, sulfones achieve very robust service lives, particularly at higher temperature. You can find further background in the article High-performance plastics outdoors.
Surface hardness and cleaning
Uncoated PC scratches faster than PMMA; PSU and PES lie in between. Hard coatings, such as polysiloxane-based hardcoats, drastically reduce the increase in haze caused by wiping abrasion. For applications with regular glass cleaning, a hard coating in combination with UV protection is the decisive driver of service life.
Recommendation in brief: Priority on optics with frequent cleaning means PMMA or hard-coated PC. High temperature plus cleaning cycles means PSU or PES with hard coating.
Selection matrix for practice
| Criterion | PC | PMMA | PSU | PES | COP |
|---|---|---|---|---|---|
| UV resistance without additives | low to medium | very high | medium to good | good | medium to good |
| UV resistance with protective layer | high | very high | high | high | high |
| Overall weather resistance | high with UV protection | very high | high | very high | high |
| Light transmission 3 mm | approx. 88 to 90 per cent | approx. 92 per cent | approx. 88 to 90 per cent | approx. 86 to 88 per cent | approx. 92 to 94 per cent |
| Impact strength | very high | medium to high | high | medium to high | medium |
| HDT heat deflection temperature | approx. 125 to 135 degrees Celsius | approx. 90 to 105 degrees Celsius | approx. 170 to 180 degrees Celsius | approx. 190 to 205 degrees Celsius | approx. 75 to 100 degrees Celsius |
| Continuous service temperature | approx. 110 degrees Celsius | approx. 70 to 80 degrees Celsius | approx. 150 to 160 degrees Celsius | approx. 170 to 180 degrees Celsius | approx. 70 to 90 degrees Celsius |
| Water absorption 24 h | low approx. 0.15 per cent | medium approx. 0.3 per cent | medium approx. 0.3 per cent | medium to increased approx. 0.5 per cent | very low approx. 0.01 per cent |
| Stress crack tendency | medium | medium | low to medium | low to medium | medium |
| Surface hardness uncoated | low to medium | high | medium | medium | medium |
| Chemical resistance | medium | medium | good | good to very good | good depending on medium |
| Density | approx. 1.20 g/cm³ | approx. 1.18 g/cm³ | approx. 1.24 g/cm³ | approx. 1.37 g/cm³ | approx. 1.02 g/cm³ |
| Relative cost class | €€ | €€ | €€€ | €€€€ | €€€ |
| Availability of formats | high | high | medium | medium to low | medium |
| Typical applications | protective glazing, machine guards | illuminated signage, canopies | hot-air viewing windows | steam and hot-air windows | sensor and optical covers |
| Important notes | plan for UV protection and hardcoat | check mechanical loads | coating for scratch resistance | check coating and media | check temperature window |
Typical applications and material selection
- Terrace or roof glazing with hail risk: PC with UV protection and optional hard coating
- Illuminated signage and displays with highest brilliance: PMMA extruded or cast; for vandalism, alternatively hard-coated PC
- Machine viewing windows outdoors near heat sources: PSU or PES with hard coating
- Sensor windows and cover discs in damp environments: COP, provided impact energy remains moderate
- Salt spray mist and frequent cleaning: sulfone-based materials with suitable hard coating and tested cleaning chemicals
Question: Which material combines maximum impact strength and good UV resistance. Answer: Hard-coated, UV-stabilised PC is the most robust all-round solution when vandalism or hail are in the specification.
Question: Where is the sweet spot for optics and lasting transparency. Answer: PMMA, provided the impact energy is limited and suitable fastenings minimise stresses.
Question: When is a sulfone material worth it. Answer: At higher continuous temperature, hot air or steam, and when media resistance and dimensional stability have priority.
Design rules for longer service life
- Deburr edges and holes over a large area, provide edge radii, avoid constrictions
- Plan for thermal expansion, provide slotted holes and sliding support, use soft intermediate layers
- Avoid stress cracks, use moderate adhesives, minimise solvent contact
- Plan for hard coatings for scratch resistance and UV durability, check compatibility with cleaning agents
- Check the static structure for natural frequencies and wind loads, optimise ribs and support spans
Guide values for linear thermal expansion: PC approx. 65 to 70 µm m⁻¹ K⁻¹, PMMA approx. 70 to 80 µm m⁻¹ K⁻¹, PSU and PES approx. 50 to 55 µm m⁻¹ K⁻¹, COP approx. 60 µm m⁻¹ K⁻¹. These values help with the design of slotted holes and tolerances.
Select coatings in a targeted way: Polysiloxane hardcoats improve scratch resistance and reduce the increase in haze. UV absorber layers protect the polymer. Antifog or antisoil versions make maintenance easier. Always test the choice of coating against real cleaning cycles and chemicals.
Standards, tests and data interpretation
- Weathering: ISO 4892 xenon weathering, ASTM G154 UV fluorescence
- Mechanics: ISO 179 notched impact, ISO 527 tensile
- Optics: ASTM D1003 haze and transmission
- Outdoor suitability: UL 746C assessment of polymers for outdoor use
Laboratory tests give trends. What matters is the correlation with your real exposure. The combination of material, protective layer, geometry and mounting decides the service life.
Procurement, availability and sustainability notes
PC and PMMA are widely available in sheets, rods and cut pieces; PC also as multiwall structural elements. Sulfones PSU and PES are available in a smaller variety of variants, COP mainly as sheets and films in engineering grades. Check minimum order quantities for hard-coated grades and define surface requirements early.
For a structured pre-selection according to application parameters, our article Material selection for plastic components will help you. You can find material data sheets on PSU, PES and PC in our material overview. For outdoor industry solutions, we additionally recommend comparing industry-specific requirements with the respective standards.
Short consultation in five steps
- Define limit loads, in particular impact energy, temperature window and continuous UV load
- Establish the optical goal, prioritise transmission, haze and scratch resistance
- Clarify the medium and cleaning, check permissible chemicals
- Assess the need for coating, UV protection, hardcoat, antifog
- Design the construction with low stress, take account of tolerances and expansion
Short decision rule: Maximum impact resistance means PC with UV protection and hard coating. Maximum UV and optical stability means PMMA. Highest temperature reserve means PSU or PES. Lowest moisture absorption with optical precision means COP.
Frequently asked questions
Which transparent plastic stays optically clear outdoors the longest?
PMMA shows the best natural UV and weather resistance and stays clear for years. Hard coatings further improve scratch resistance, but are often not strictly necessary with PMMA.
When is polycarbonate the better choice despite the UV issue?
When impact energy, hail or vandalism dominate. With UV protection on both sides and a hard coating, PC achieves very good outdoor durability with maximum toughness.
Do I absolutely need a hard coating for outdoor applications?
For frequent cleaning, sand dust and wiping abrasion, yes, particularly with PC, PSU and PES. PMMA is harder, but also benefits from hardcoats in very demanding cleaning cycles.
Which alternative do I use at higher continuous temperatures above 120 degrees Celsius?
PSU or PES. Both offer significantly higher heat deflection temperature than PC or PMMA. PES is particularly suitable for hot air and steam, ideally with a suitable hard coating.
Is COP suitable as a sensor window outdoors?
Yes, if the impact energies are moderate. COP offers very low moisture absorption, high optical purity and good UV stability depending on the formulation. However, the heat limit is below PC, PSU and PES.
How do I avoid stress cracks outdoors?
Low-stress construction with edge radii, deburred holes, slotted holes for expansion and compatible adhesives or sealants. Minimise solvent contact and test cleaning chemicals in advance.
