FAQ - Silicone vs Polyurethane vs Acrylic Proofing

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What are the differences?

The coating on your tent fly, parka or poncho may determine whether you stay dry in bad weather; it may also determine what your gear weighs after a storm. Top end tents these days use a silicone polymer coating (usually known as 'silnylon'), but this fabric is expensive and hard to handle in a production environment. Some mid-range tents on the market are made with a polyurethane (PU) coated fabric, while the cheaper ones often use an acrylic coating. The differences are significant for a number of reasons, given below. In general we don't see the original thin PVC coating any more: it was a bit of a disaster as it usually peeled off after limited use. It gave coated nylons a bad reputation for a while.

Acrylic coating

An acrylic coating is a thin (acrylic) layer applied to one side of the fabric. Compared to a PU coating it looks glossy. It does make the fabric waterproof when new, but the bit of thin film which bridges the gap where four threads cross can break down, either with time, use or pressure. That said, it should be pointed out that the pressure rating needed for a tent fly is not high: 10 kPa might be enough in many cases. So many tents do use an acrylic-coated fabric for the fly quite successfully. Fine, but don't use this for rainwear: it fails, quickly, in that application. It also fails as a pack-cover. Finally, no acrylic-coated fabric is suitable for use as a groundsheet: it won't take the pressure.

That said, you do have to be very careful which acrylic-coated fabric you use in a tent. The reason is because many fabrics are given a very thin acrylic coating to stabilise the weave, not to make them waterproof. The acrylic film bonds the threads together at the cross-overs and prevents large distortions (thread movement) from happening at seams. Once again, this is fair enough provided the fabric is not subsequently promoted as 'waterproof'. I have seen some such fabrics advertised as 'showerproof', which seems pretty reasonable.

Polyurethane coating

Polyurethane coated fabric has coating of PU on one side of the fabric, as with an acrylic coating. The PU coating tends to be a bit thicker than an acrylic coating and looks slightly duller. It appears that PU coatings are more expensive in practice - although this may be influenced by the obvious fact that a thicker layer needs more polymer. Normal use of a PU-coated fabric puts the coated side on the 'inside' and leaves the fabric on the outside. This protects the PU coating from UV and abrasion, but it also means that any rain can soak into the exposed fibers on the uncoated, exterior side. This makes drying a long process, as that water can only dry out of those fibers as fast as it can evaporate from the fabric surface. Since it usually rains only on cloudy days the sun won't be helping that drying process, and as a result you get to carry a water-sodden parka or tent on your back all day, or at least until you can stop hiking long enough to dry it out.

Some PU-coated nylon fabrics have a Durable Water-Repellent coating (DWR) on the fabric. In principle this should stop the fabric from absorbing any water: the water should bead up and roll off just as some pretty catalogues illustrate. But what those catalogues don't tell you is that the DWR does not last very long out in the open (a year or two), and the DIY versions for home use seem to last even less time. So the fabric gets wet. For a tent this just make it heavier; for a parka it also makes it cold.

What makes the PU-coating of value is the fact that water vapour can slowly diffuse through it, and this makes the fabric 'breathable'. There is more on this under Rainwear. Whether this makes much difference in a tent is highly debatable: when it is raining and the surface of the fly is wet, I don't think there is going to be a lot of moisture transmission. You would be much better off ensuring some good ventilation.

For a while there was a very interesting bright orange fabric on the market. Different Australian gear manufacturers tried it out, and it usually got a company name with the word 'titan' or 'titanium' in it. The base fabric is very light nylon and the coating is PU, but the coating is heavily loaded with titanium dioxide (hence the names). The presence of the TiO2 in the PU makes the PU layer very resistant to UV damage. I Believe the fabric was also available in a grey, but how boring! The history of the fabric is amusing. It was developed by Dimension Polyant in America (DP) as a fabric for making very high altitude research balloons for NASA. The PU coating is thoroughly air-tight and is meant to be sealable. (However, some of the first balloons tested near Woomera leaked. Such is life.) Then the marketing manager at DP thought to try selling the fabric to the outdoors industry, and samples were distributed. Tents were made and tested. For an example have a look at my orange winter tent in the DIY, Ultralightweight, My Tents and Winter Tent pages. It's a really waterproof material, but it didn't stay on the market long because it was soon found to have two serious disadvantages. The first was that the fabric did not have any DWR, and it got wet from condensation on the inside. This was not good. But the second was worse: the thick PU coating went quite stiff in the winter, making the tent hard to roll up. There were some fears about cracking at low temperatures as well, although I don't think this would have happened in our snow fields. Anyhow, my orange tents has lasted for many years of hard work. Fascinating stuff.

You need to look after PU-coated fabric. The PU coating itself can absorb water molecules - that's how it 'breathes' after all. The technical term for this behaviour is that the coating is 'hydrophilic' (water loving). But as a result, water can act as a very slow solvent to polyurethane. So if you pack up a wet PU tent for a very long time the PU coating may eventually stick to itself and become one big ball of goo. Actually, it will probably become a big ball of mould first, but that's another matter. I have also seen claims that PU-coated fabric doesn't age very well, and that the coating can peel off as the fabric ages. I have seen PU coating peel off the light nylon throats on packs, but it did take years for this to happen - and an awful lot of hard use too. It seems the problem is that the coating does not normally stick to the fibres in the threads all that well.

Silicone coating

Technically, the 'silnylon' fabric is 'double-coated'. That is, a coating of silicone polyer has been applied to both sides. Well, fair enough, but there is a huge difference in the result between PU and silicone coating. The PU coating sits on the surface of the fabric, but the silicone polymer goes right in. As far as I can see (with a microscope) the silicone polymer completely permeates the fabric fibers and forms a layer right through the fabric. As far as the final fabric properties are concerned, you should not think of 'silnylon' as a 'coated fabric', but rather as 'nylon-fabric-reinforced silicone polymer sheet'. This is a bit like fibreglass or glass-reinforced epoxy.

This difference translates into mechanical properties too. A key parameter is 'tear strength'. It is claimed that a PU coating focuses the stress in a tear right at the tip of the tear, and this actually makes a PU-coated fabric behave weaker than the base fabric. However, the elastic silicone polymer in silnylon fabric takes over and distributes the stress across a number of threads, and this makes the silnylon fabric significantly stronger in tear than the base fabric - reportedly up to 2.2 times stronger. Note this does not apply to EPIC fabrics.

Silicone is the stuff that is used to seal aquariums, bathtubs and sinks, and other applications where water must not be allowed to leak through. When it rains on silnylon fabric all the water stays on the surface, and a quick shake will get rid of almost all of it, and its associated weight. A few minutes in the wind or sunshine and silnylon will completely dry out. Certainly, I have found that I can shake most of the water off my silnylon tents, and the packed weight is rarely much different. In comparison with PU, silicone is 'hydrophobic' (water hating). By its nature it repels water, and water will not act as a solvent on it. Your packed-up wet tent may get mouldy if you leave it long enough, but it won't go sticky. Finally, the silicone in silnylon completely permeates the fabric fibers - it can't peel off. But remember: it does not breathe!

Cutting fabrics

There are two ways of cutting a fabric: with a sharp edge (like scissors) or with a 'hot knife'. If you are going to cut plain light nylon or polyester fabric, you will quickly discover the edge frays like mad. Use a 'hot knife' for this: it welds the fibres together along the cut line. You can buy a commercial 'hot knife', but I find a temperature-controlled soldering iron with a knife blade works just fine. Be careful whne using a hot knife: it will of course melt anything you put it down on, and it will also melt or damage what is under the fabric you are cutting. Cheap plywood works well for this. But also be careful with the melted cut edge of the fabric: it is hot, and can damage adjacent fabric if you let it touch it.

Epic fabrics must either be cut with a hot knife or hemmed as their construction does not lock the threads together. Treat them as plain uncoated fabric.

Acrylic and PU-coated fabrics don't need to be cut with a hot knife. If you are careful you can cut them with scissors, but you will need to hem them afterwards for a good job.

Should silnylon fabric be cut with a hot knife? Some people say yes for the same reasons. But my experience is that the edge of silnylon fabric which has been cut with scissors does not fray unless severely mistreated, so I don't bother hemming. It may look 'unfinished', but it works well and it is lighter. The silicone coating locks the threads together very strongly. I have been doing this with the tents I make for several years now, and have seen no problems. There might be some fraying if the edge of the fabric is severely abraded or allowed to flap like the edge of a sail for a long time. This does not happen in practice on a tent.