Rain happens. Especially in Poland.
If you're shipping anything in this country, your package is going to encounter wet conditions at some point. It sits on a wet loading dock. It rides in a van with a leaky door. It gets left on a doorstep in the rain while the courier moves on to the next delivery. If your packaging can't handle moisture, your product arrives damaged — and your customer is unhappy.
So how does a poly mailer actually block water?
It's not a special coating or treatment. It's the material itself. LDPE (Low-Density Polyethylene) is naturally water-resistant. The polymer structure is hydrophobic — meaning water molecules don't bond to it and slide right off. There are no pores or fibres for water to soak into the way there would be with paper or cardboard.
This is why poly mailers are waterproof by default. You don't need to buy a "waterproof" version — any LDPE mailer already is one. The waterproofing is built into the material, not added on top.
What's the difference between waterproof and water-resistant?
Water-resistant means the material can handle light moisture — a splash, some humidity, brief contact with water. It might eventually let some water through if exposed long enough.
Waterproof means the material blocks water completely, regardless of how long it's exposed. LDPE falls into this category. A poly mailer sitting in a puddle for an hour will still be dry on the inside.
Cardboard boxes, by contrast, are only water-resistant at best. One trip in the rain and they start breaking down. That's the fundamental reason e-commerce sellers use poly mailers for anything that doesn't need a rigid box.
What about the seal? Doesn't that let water in?
Good question. The mailer itself is waterproof, but the seal matters too. A peel-and-seal closure (the kind with the adhesive strip) creates a tight, continuous bond along the top edge. Once sealed properly, there's no gap for water to get in.
The key is sealing it fully. Press the adhesive strip down firmly across the entire length. If there's a loose corner or a section that hasn't stuck, that's where water will find a way in — not through the LDPE itself.
Does waterproofing affect recyclability?
No. The waterproof properties come from the LDPE material, not from any added chemicals or coatings. The mailer is still 100% LDPE, still plastic code 4, and still recyclable at most facilities in Poland and the EU. Waterproof and recyclable aren't a trade-off — they come together by default with this material.