It Worked Perfectly Until It Spent a Summer Outside

A housing that passed every bench test comes back from a field pilot in Phoenix chalky, faded, and brittle enough that a snap fit breaks off in the technician's hand. Nothing changed except sunlight. It is one of the most avoidable failures in outdoor hardware, and the mistake is always the same: a resin chosen for mechanical properties and cost, with UV treated as a finish issue for paint to solve later.

What Sunlight Actually Does to a Polymer

The damaging fraction of sunlight is UV-A and UV-B, roughly 290 to 400 nm. Photons at those energies exceed the dissociation energy of common carbon-carbon and carbon-hydrogen bonds, so absorbed UV breaks polymer chains directly, and with oxygen present photo-oxidation becomes autocatalytic.

The sequence is predictable: color shift and gloss loss, then chalking, then microcracking at corners and molded-in stress concentrations, then embrittlement, where impact strength collapses long before tensile strength does. That is why failure shows up as a cracked clip rather than a torn wall.

Three things accelerate it: heat, since reaction rate roughly doubles per 18 degrees F; moisture, which drives hydrolysis and wet-dry surface stress; and mechanical load, because a stressed part cracks at a degradation level an unloaded part tolerates.

Which Plastics Survive and Which Do Not

Naturally good outdoors: Acrylic (PMMA) is the benchmark and holds clarity for decades. ASA was developed as a weatherable ABS replacement and is the right default for outdoor housings needing ABS-like properties. PVDF and fluoropolymers are essentially inert, and glass-filled PBT and PPS hold up in demanding applications.

Good only when stabilized: Polycarbonate has excellent impact strength but yellows and embrittles quickly under raw UV, which is why outdoor PC nearly always carries a co-extruded UV cap layer or hard coat. Unstabilized polypropylene degrades fast; stabilized grades are fine. Nylon absorbs moisture as well as degrading, so outdoor nylon needs stabilization plus a dimensional allowance for water uptake.

Poor outdoors regardless: Standard ABS chalks within a season, unstabilized PS embrittles, and POM weathers poorly despite good chemical resistance. The comparison in ABS vs polycarbonate ignores weathering entirely, and outdoors that omission changes the answer: ASA usually beats both.

For transparent parts the choice narrows to acrylic vs polycarbonate, a tradeoff of weatherability against impact strength. Outdoors, unless you need impact resistance, acrylic wins.

Stabilizers and Pigments Decide the Real Answer

Two resins with the same name behave completely differently outdoors, because performance lives in the additive package. Specifying "polypropylene" on a drawing is meaningless for an outdoor part; specify the exact grade.

  • UV absorbers. Benzotriazoles and benzophenones absorb UV photons and shed the energy as heat. Effective, but consumed and prone to migration, so protection has a finite life.
  • HALS. Hindered amine light stabilizers scavenge free radicals and regenerate rather than being consumed, making them the backbone of modern outdoor formulations. HALS plus a UV absorber outperform either alone by a wide margin.
  • Carbon black. The cheapest and most effective UV protection available. At 2 to 2.5 percent loading of small-particle carbon black a commodity polymer becomes genuinely durable outdoors, which is the real reason so much outdoor equipment is black.
  • Titanium dioxide. Grade-dependent and opposite in effect: rutile TiO2 blocks UV, anatase TiO2 is photocatalytic and accelerates degradation. Specify rutile explicitly for white outdoor parts.
  • Organic pigments. Bright reds, oranges, and some blues fade dramatically. If the brand demands saturated color outdoors, ask for a weatherable inorganic pigment system and expect to pay.

Involve the compounder early and request weathering data for the exact grade and color, not the base resin. That is the discipline of choosing the right plastic for your product applied to one specific stressor.

Design and Finish Choices That Buy Years

Material is most of the answer, not all. Generous corner radii lower stress concentration so surface microcracks do not propagate. Thicker walls in exposed areas mean degradation of the outer few thousandths does not compromise structure. Molded-in stress from poor gating creates exactly where cracks appear first. Texture hides chalking and fading far better than high gloss, a cheap specification change.

Coatings work but add cost and a failure mode, since any coating can chip and expose bare substrate. A co-extruded cap layer of weatherable resin over a cheaper core is more robust than paint, and for metal parts the choice between powder coating and wet paint follows similar logic.

Do not forget the non-plastic parts. Elastomeric gaskets often fail first outdoors, with EPDM and silicone dramatically outperforming nitrile, and a degraded seal breaks the ingress rating the product depends on. An IP rating is only as durable as the seal material behind it.

Test Before You Cut Steel

Natural exposure testing in Florida and Arizona is the reference standard and takes one to five years, which no schedule accommodates, so accelerated weathering is what you actually run. Xenon-arc testing per ASTM G155 reproduces the full solar spectrum and is the most representative method. Fluorescent UV testing per ASTM G154, commonly called QUV, is cheaper and harsher, cycling UV-B with condensation. Both compress years into weeks, though correlation to real exposure is approximate and polymer-dependent. Typical programs run 1,000 to 3,000 hours and cost in the low thousands of dollars.

Test properties that matter, not just appearance: color shift as delta E, gloss retention, and above all notched impact strength before and after exposure, because impact loss is what causes field failure. Test finished parts rather than plaques, since molded-in stress changes the result. Fold this into the wider program in reliability testing for a new product and confirm it with a field pilot in the harshest climate you sell into.

Specify an Outdoor Material That Lasts

Projects House selects resins, stabilizer packages, and pigment systems for outdoor products, writes the weathering protocol, and reviews geometry for the stress concentrations that turn UV damage into cracks. Tell us where your product lives and how long it must last through our contact form.