Polypropylene without a UV package degrades in sunlight in a sequence that is reliable enough to inspect against. That is useful, because it means a fleet can be graded before crates start failing in service, and because the early stages look cosmetic and get dismissed.
Stage one: chalking
The surface loses its gloss and develops a powdery, whitish bloom that rubs off on the hand. This is the polymer at the surface breaking down and the pigment losing its binder. It is easy to mistake for dirt, and it is the cheapest stage to catch – a crate that chalks is telling you it has no protection, and everything after this point is mechanical.
Stage two: colour change
Colours fade and shift. Blue goes pale and greyish, red goes pink, and white goes yellow. On a fleet where colour is used for grade or crop coding this is a practical problem before it is a material one, because two grades start looking alike. Where colour coding matters, note that fading is the first visible sign and plan for re-coding rather than trusting the original shade.
Stage three: crazing
Fine surface cracks appear, usually on the faces that get the most sun and usually running along mould flow lines or across stress points. At this stage the crate still functions, and this is the point at which a fleet should be retired from load-bearing use or moved indoors. Crazing is the last warning before real cracks.
Stage four: cracking and brittleness
Cracks open at the rim, the base corners and the hand holes – the thin, stressed places. The crate now fails under impacts it used to survive, and the failure is sudden rather than gradual. This is where crates start breaking in the field, and it is also where produce damage increases because a cracked base corner deflects under load.
How fast it happens
Depends on the light dose, which depends on latitude, season and how much of the year the crate is outdoors. In a Mediterranean or subtropical growing region with crates living outside year-round, the sequence can run from chalking to cracking inside two seasons. In a northern European operation with crates stored indoors for eight months, the same crate may never show more than fading. Quote the exposure, not the years.
What does not cause it
Rain, cold and washing do not cause UV degradation, and it is worth separating them because a chalky crate is often assumed to be a dirty one and gets pressure washed, which removes the loose surface and makes the next stage harder to see. If the surface blooms again within days of washing, that is degradation rather than dirt.
What to do with a degrading fleet
Move the worst affected crates to indoor or short-cycle work before they break, and stop using them for anything where a broken fragment could end up in product. Crates at stage three are still useful for storage and transport of non-food items, and downgrading them is cheaper than replacing them. The judgement on repair versus replacement is covered in repairing reusable produce boxes.
What we can do
We supply produce crates with the stabiliser package stated, and we will specify a lower-cost indoor grade where crates are stored under cover rather than charging for protection the fleet does not need. Tell us how much of the year they spend outside.