Material selection
Acrylic keeps its clarity outdoors better than any other clear plastic. It also becomes more brittle every year, and the fixing detail decides whether that matters.
Outdoor work is where acrylic and polycarbonate split most decisively. Acrylic holds its clarity and colour for a decade or more; polycarbonate yellows within three to five years. Acrylic also becomes brittle with UV exposure, so a panel that flexed when new may crack after several summers.
| Age | What changes | What to watch |
|---|---|---|
| 0–2 years | Very little; slight surface chalking on the sun-facing side | Fixing detail, where the first cracks appear if it is wrong |
| 2–5 years | Clarity essentially unchanged; surface micro-scratches accumulate | Cleaning marks become visible in raking light |
| 5–8 years | Impact strength measurably reduced; edges may show slight chalking | A knock that would have been harmless may now crack the panel |
| 8–12 years | Embrittlement noticeable; colour stability still good in quality grades | Plan replacement if the panel could be struck |
| 12 years and beyond | Clarity still acceptable in many installations; strength substantially reduced | Replacement is a maintenance decision, not a failure |
This is why acrylic is specified for visible outdoor surfaces and polycarbonate where impact is the dominant risk, and why both need a planned replacement interval rather than being assumed permanent.
Acrylic expands about 0.07 mm per metre per degree Celsius. A 2,000 mm sign that sees minus 5 degrees in winter and 55 degrees of surface temperature in summer moves nearly 8 mm. If the panel is rigidly bolted at four corners, that movement goes into the holes and the panel cracks, usually at a corner, usually in the first two years.
Wind load on a sign is a function of the design pressure, the panel area and the fixing spacing. It is not a single number per thickness. A 5 mm panel at 1,000 mm span with fixings at 400 mm centres is comfortable to roughly 0.6 kPa; beyond that the panel deflects and the fixings tear.
For an exposed site, ask for the design wind pressure and specify thickness and fixing spacing against it. Where the panel could be struck, polycarbonate deforms rather than cracking, at the cost of clarity and UV life.
A dark or backed panel in direct sun gets much hotter than the air temperature: 70 to 80 degrees is realistic for a dark surface on a still summer day. Acrylic at that temperature is close to its practical limit, softens slightly and deflects under its own weight if the span is long.
Mitigations: lighter colours, a ventilated mounting so air moves behind the panel, an increased span-to-thickness ratio, and keeping the panel out of a heat trap between two surfaces.
CAST PMMA, UV-STABILISED GRADE, not extruded and not an unstated grade.THICKNESS: calculated for span, wind load and fixing spacing.FIXING: SLOTTED ±3 mm PER METRE ON THE LONG AXIS, SHOULDER WASHERS.EDGE: routed or polished, no sharp internal corners, minimum radius 3 mm.PRINT: REVERSE, WITH UV-RESISTANT INK AND WHITE BACKING.REPLACEMENT INTERVAL: n YEARS, stated, so the panel is maintained rather than forgotten.We will not warrant an outdoor acrylic installation for an indefinite life. We will state a realistic service life for the grade and the site, specify the fixing so the panel is not loaded by its own movement, and supply the sheet certificate. That is a defensible position; a promise that it will not degrade is not.
Far less than polycarbonate. Quality cast acrylic holds its colour and clarity for a decade or more. Very cheap or regrind-containing sheet can discolour sooner, which is one reason to ask for the mill certificate.
Acrylic unless impact is a real risk. It stays clear far longer. If the sign is at a height where it could be struck or hit by debris, polycarbonate is the safer choice despite the yellowing.
Only where it is not a fall-protection barrier. Acrylic is not a structural barrier material; a balustrade infill that could be leaned on or struck requires laminated glass or a rated panel, and that specification is a structural engineer responsibility, not a fabricator one.
About 3 mm per metre of panel length, and more in a climate with a wide temperature range. The gap allows the panel to move without loading the frame or the fixings.
Thermal movement against a rigid fixing, almost always. Oversize holes, no allowance for expansion, or a bonded perimeter joint will all cause a corner crack in the first couple of years.
Outdoor life of acrylic is decided by three variables that can be estimated before a part is made: UV dose, temperature cycling, and how often the surface is cleaned. They combine into four exposure classes, and each class has a different service expectation for standard cast sheet. The figures below assume unfiltered cast PMMA with UV stabiliser, not a reprocessed or extruded sheet.
| Class | Typical site | UV dose | Expected clarity | Cleaning interval |
|---|---|---|---|---|
| 1 — Benign | Northern Europe, shaded facade, north facing | Moderate | 10–15 years with slight yellowing | Every 6 months |
| 2 — Standard | Continental Europe, US north-east, shaded retail frontage | Moderate–high | 8–12 years, ΔYI under 4 | Every 3–4 months |
| 3 — Severe | Middle East, US south-west, south-facing glass | High | 5–8 years, visible tint shift after 4 | Monthly, dust is abrasive |
| 4 — Coastal or industrial | Salt spray, or airborne solvent and grit | High plus chemical | 3–6 years; surface attack dominates | Monthly with DI water rinse |
Two of these numbers are consequences rather than properties. In class 3a, dust is the real enemy: in a dusty climate, wiping a dry panel with a cloth grinds silica across the surface, and a sign that should last eight years looks sandblasted in three. The answer is a rinse first, then a wipe, and it costs a hose. In class 4, chlorides and airborne solvent attack the surface chemically, and no thickness of sheet resists that — polycarbonate does not either, and neither does glass with a coating, so the design answer is a sacrificial panel or a different location.
What acrylic does not do outdoors is yellow catastrophically. A stabilised cast sheet loses perhaps 2–4 units of yellowness index over ten years in class 2, which reads as a faint warm tint against white paper and is invisible against a coloured background. Extruded sheet with lower stabiliser loading is a different case and can shift visibly in three to four years; the cost saving at purchase is small compared with replacing a sign, which is why we do not offer extruded sheet for outdoor work.
The one detail that shortens outdoor life more than sunlight is the fixing. A panel bolted rigidly into a steel frame has nowhere to expand, and a 1.2 m panel gains about 1.6 mm between a cold night and a hot afternoon. Over a few seasons that shows up as crazing from the holes and a bow in the middle of the panel — not a material failure, an installation one. Slotted holes, nylon washers and 12 mm of edge distance are the whole fix, and they are the first thing to write into the drawing for anything that lives outside.
Design wind pressure, temperature range and how the panel is held decide whether it lasts. We will size the panel and specify the fixing to match.