How Laminated Bags Get Their Waterproof Layer

A chip bag that stays crispy for months. A fertilizer sack that survives weeks in humid warehouses. The barrier keeping moisture out is usually a thin layer of polyethylene, applied through industrial processes most consumers know nothing about.

PE film blocks water vapor because of its molecular structure—non-polar, saturated hydrocarbons with high molecular weight. The material melts at low temperatures and welds shut when cut with a hot blade. Manufacturers apply it to packaging substrates in two main ways.

Extrusion lamination melts PE resin at around 316°C and pushes it through a flat die. The molten curtain drops between two films—printed PET on one side, another plastic on the other. A pressure roller and chilled steel roll squeeze them together. The layers fuse in under a second.

Dry lamination is older. A gravure roller spreads solvent-based adhesive onto one film. The solvent bakes off. Then heat and pressure bond that film to a second substrate. Peel strength runs 1 to 5 N/15mm for plastic-to-plastic bonds.

Shreyas Pathak works for a printing ink company focused on flexible packaging. He sees PET and OPP as the most common primary films. “We are talking a combination of two or more plastics for a single product,” he says about laminates. Metalized versions show up constantly in cookie and chocolate wrappers.

Water vapor transmission rate is how the industry measures barrier performance. ASTM F1249 sets the test conditions: 38°C, 90% relative humidity. Aluminum foil laminates hit rates as low as 0.001 g/m²/day. Plain OPP is already decent. Metallizing or PVDC coating makes it better.

C-P Flexible Packaging, a U.S. converter, points out that extrusion lines cost more upfront than adhesive laminators. Fewer competitors have them. But extrusion adds bulk. Two 70-gauge films glued together make 140 gauge. The same films with seven pounds of extruded PE between them make 190 gauge.

Instant noodle bags are typically BOPP printed film extruded directly onto LDPE. Laundry detergent pouches add a middle layer: printed BOPP, extruded resin, then PE film.

Corona treatment matters for non-porous substrates. The electrical discharge raises surface energy so molten resin sticks to BOPP or aluminum foil. Skip it and the layers peel apart.

Recycling is the problem nobody has solved. “If a plastic gets laminated, there is some type of polymer or adhesive that marries the two materials,” Pathak says. “That kind of stuff is usually very hard to recycle.”

Borealis developed a Full PE Laminate using machine direction oriented film. Anton Wolfsberger, their head of marketing for consumer products, says the monomaterial approach “goes a long way to improving overall environmental performance.” The recyclate can become new PE film. But the technology requires changes across the supply chain—new equipment, new processes.

Solventless lamination has spread in recent years. No volatile organic compounds. Cleaner plants. The bond still needs three to five days to cure. Humidity has to stay in range or the adhesive fails.

The waterproof layer on a coffee pouch or a pharmaceutical blister is not complicated chemistry. PE has done this job for decades. Getting rid of it afterward is the part that still does not work.