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COATING 3 - The PU coating has gone sticky

This is one way that PU coatings fail. For better or worse the longevity of even the best quality PU coatings seems to have shortened since the formulations used prior to 2000. Improved regulations controlling the use of environmentally toxic and otherwise hazardous chemicals have come into effect in the past few decades. New formulations and methods of dispersal of the PU onto the fabric surface are now in use. There are many advantages in these developments but it seems a casualty has been the natural life-expectancy of the PU coating itself.

Having said that coating failure remains, most often, the result of hydrolysis, a chemical process where simple water molecules attack the PU polymer chains and break them down into shorter and shorter lengths. It commonly occurs on standard grade PU coatings in humid climates. It may also be the result of contamination by a chemical or solvent.

In some cases, and this depends on the PU formulation, it is possible to ‘re-harden’ the coating and extend its life by ironing the fabric with a domestic iron set on the lowest 'one-dot' silk setting. This is practical with something like an outer tent skin but obviously not with a pack or bag which is almost impossible to 'iron'. (See about them further below).

Use of a separation layer between the iron and the fabric is absolutely mandatory. A piece of photocopy paper may work if the coating does not become stickier with heat. (If it does ironing is not going to improve the coating and there is no point in continuing further). The perfect separation layer is a piece of non-stick Teflon-coated fibreglass. (Look to see the details).

Test a small area of the tent fabric, coating upwards, near a bottom edge. Check the iron is on its lowest setting and just use the tip of the iron to start with. If the coating hardens and loses its tackiness, work over the whole tent, patch by patch making sure never to let the iron contact the coating directly. When you have finished give the whole coated area a spray with dwr (or slip silicone – commonly used by upholsterers to help foam blocks slide into cushion covers, etc). This will help stop any ‘blocking’, the problem of the coated side of the fabric sticking to itself.

On PU-coated nylon packs and bags there is nothing much that can be done. Dis-assembling the bag and giving it a hot machine wash can accelerate the process and remove loose, smelly coating so some further steps can be taken.

Most nylon and polyester fabrics depend on the bonding provided by the PU coating for weave stability. With the coating soft or gone yarns can slide, unravel from edges, and loaded seams blow out. The simple constructions used in bags, whether they are tape bound or not, can’t resist combing (where the seam allowance gradually pulls out past the stitching as edge yarns are lost). Top-stitching does help but does not eliminate this problem. To prevent seam failure and keep your leaking nylon pack or bag at least structurally sound  you will need to turn it inside out and ‘glue up’ all the main seams with Seamgrip®. The bottom seams on external pockets are also important so don't forget them. This won’t help the general leakage problem but you will at least have a reliable sac. Refer to the page on repairing holes and tears in your pack for more about Seamgrip®.

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