Thirty Percent Recycled PE: What It Does to a Gel Pack Seal Window

A UK pack converter switched to a 30 percent post-consumer recycled polyethylene film to meet a retailer’s packaging target, ran the sealing parameters that had worked for six years, and produced 4,300 packs whose seals passed a visual check and failed a 6 kg stack test. Peel strength had dropped from 34 N per 15 mm to 19, and the failure mode had changed from film tear to clean peel.

Recycled content is now a purchasing requirement in most European grocery and meal-kit tenders, and it changes the physics of sealing in ways that a line trial at one parameter set will not reveal. This article covers what recycled PE actually does at the seal bar, how to re-qualify a window, and which interactions with SAP gel matter.

Why Recyclate Behaves Differently at 150 Degrees

Virgin seal-layer resin has a narrow molecular weight distribution and consistent slip and antiblock loading. Post-consumer recyclate carries gel particles, traces of incompatible polymers such as PET or PA from caps and labels, and ash from inks. Those contaminants do not melt at the same temperature as the seal layer, so they sit in the seal interface as islands that reduce the bonded area. A seal that looks continuous at 100 percent visual inspection may be 80 percent bonded.

The practical consequence is that the seal window narrows from both ends. On a standard 90 micron coextruded structure we measured a window of 145 to 168 degrees and 0.35 to 0.55 seconds. On a 30 percent recycled version of the same structure, the usable window was 150 to 158 degrees, with pressure needing to rise from 3.0 to 3.8 bar to reach the same peel strength.

Re-Qualifying the Window in Six Seals

Cut twelve pouches from the new film and seal them in pairs at 145, 150, 155, 160 and 165 degrees, holding pressure and dwell constant. Peel each pair at 300 mm per minute and record both the peak and the mode of failure. Plot peel strength against temperature and look for the plateau: the recycled film will show a narrow one, often only 8 degrees wide, where the virgin film showed 20.

Then repeat at the top and bottom of that plateau with gel inside, because a filled pouch conducts heat away from the seal bar and shifts the effective seal temperature down by 5 to 8 degrees. Sealing an empty pouch is a laboratory result; sealing a full one is the production result.

The SAP Interaction Nobody Warns You About

Gel packs stress a seal in three ways that food pouches do not. The contents are 99 percent water, so any pinhole wicks immediately. The pack is frozen, so the seal sees a freeze-thaw cycle that embrittles the seal layer at minus 18. And the pack is stacked, so the seal carries hydrostatic pressure for days. Recycled film with lower tear resistance fails at the seal edge rather than in the body, which is why the failure looks like a clean peel.

Two mitigations work. Increase seal bar width from 8 mm to 10 mm, which buys 25 percent more bond area for the same peel strength per millimetre. And move the seal away from the pack corner radius, because the corner is where frozen gel exerts the highest local pressure.

Spec Clauses to Write, and What Re-Qualification Costs

Specify peel strength after a 24-hour freeze at minus 18, not just at 23 degrees, with a minimum of 28 N per 15 mm and a requirement that the failure mode is film tear rather than interfacial peel. Specify gel particle count and ash content for the recyclate, because a film that meets peel strength but carries visible gels will fail a leak test at 1 in 400 packs. And specify a drop test on the finished frozen pack from 1.2 m onto a hard floor at minus 18, ten packs, zero failures.

Ask the converter for the seal curve, not a single seal strength number. A converter who cannot supply a curve has not qualified the recycled structure and is passing the risk to your line.

Recycled film runs 6 to 11 percent more per kilo than virgin in most markets, and the wider seal and slower line speed add perhaps 1.4 cents per pack. That is cheap against a leak rate that doubles. The bigger cost is the re-qualification time, which is two days of line trials and a week of freeze testing, and it should be scheduled as a project rather than squeezed into a changeover.

Film and gel are a system, and neither can be qualified alone. If you are moving to recycled content this year, start with a gel-grade SAP whose retention is stable across freeze cycles, then tune the seal window around it, rather than the other way round.