“PP does not absorb water” is the standard answer and it is correct. It is also the reason a lot of moisture problems get dismissed and then reappear later, because the water is not in the material – it is in the structure.
Where water actually goes
- Inside open cells and flutes. A cut edge on a honeycomb panel or a corrugated sheet exposes the internal voids. Water enters and stays.
- Between bonded layers. Water migrates along a bond line and into any gap at the perimeter.
- In joints and crevices. Where two panels meet, or where a panel meets a frame, capillary action pulls water in and holds it.
- Against a flat surface. A panel lying on a wet floor traps a film of water underneath with no way to evaporate.
- Behind inserts and fastenings. Any hole creates an entry point, and if a metal fixing is involved, that water now has something to corrode.
What the water then does
Adds weight. A panel that has taken on water in its flutes is heavier, and if you are handling it by hand or paying by weight, that matters.
Carries contamination. Wash water carries detergent, product residue and whatever was on the floor. Trapped inside a panel, that concentrates instead of diluting.
Corrodes fixings. The most common actual failure. A steel staple, screw or rivet in a wet PP structure rusts, stains the panel and eventually lets go.
Splits on freezing. Water trapped in a closed section expands when it freezes. In a cold store or an outdoor winter, this cracks panels that were structurally sound.
Grows biofilm. Damp internal surfaces with a food residue are exactly what microbial growth needs. This is the one that matters most in food and pharmaceutical applications.
Why a sealed edge is not automatically the answer
Sealing every edge sounds right and often is. But water that gets in anyway – through a puncture, a damaged seal, or a fastener hole – now has no way out, and a fully sealed panel holding water is worse than an open one that drains.
The better approach is usually: seal what can be sealed, drain what cannot, and make sure every enclosed section has a path to the lowest point. Then check the drain works, because surface tension can hold a film across a small hole.
Practical measures
- Seal edges that face up and where water pools; leave drainage at the base.
- Drain the lowest point of any enclosed section, and test it by filling and draining.
- Store panels flat and off the floor, with a gap underneath so both faces can dry.
- Let panels dry before stacking after a wash. Stacking damp panels traps the moisture between them.
- Choose plastic fixings in wet service, or accept that the fixing is the life-limiting part.
- Inspect after freezing weather for splitting at any section that could have held water.
Cold is the version worth planning for
Standing water in a panel is mostly an annoyance until it freezes, and then it becomes a structural problem. Water expands on freezing, and in a sealed cell it expands against the cell walls. One freeze-thaw cycle rarely does visible damage. A cold store that runs to minus twenty and then defrosts weekly does cumulative damage over a winter, and it shows as a gradual loss of skin-to-core bond that nobody attributes to water because the panels looked fine in September.
The other thing worth separating is stagnation from flow. Water that runs through a panel and out again carries little risk. Water that sits in the bottom cells of a panel stored outdoors on a slight tilt is where you get staining, odour and, eventually, growth on whatever dirt came in with the water. The plastic itself is not food for mould. The film of organic matter inside it is. That is why two identical panels can age very differently depending on whether they were stored flat or on edge.
Our honest position: if a panel will never dry out in service, plan for drainage rather than fighting to seal it perfectly. A drained panel that dries is a better long-term bet than a sealed panel whose seal eventually fails and holds water behind it.
What we can do
We build PP honeycomb panels with sealed or drained edges depending on how the panel will be used, and we will ask about washing and outdoor storage before quoting. Tell us where the water comes from and how the panel dries, and we will design the edge treatment around that.