FAQ - A Discourse on Fabrics

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You might think that fabrics are boring and old hat, and that there is nothing left to be discovered or developed in this area. After all, people have been making fabrics for thousdands of years. I thought so too for a while, but I spent 27 years working as a research scientist at the CSIRO Div of Textile Physics, and discovered that this is still a very active and interesting field. The last couple of decades, since the invention of synthetics, has seen a huge boom of research and commercial development in what can be done with a 'fabric', and the newest fabrics are a source of delight. Well, to the Do It Yourself enthusiastic anyhow!

Natural Fibres

Let's deal with the natural fibres right at the start. They might be suitable for underwear and handkerchiefs, but that's about all. There are a number of very good reasons for my saying this. The first is that they simply do not have the strength or the life of the synthetics. Facts of life. The second is that they degrade over time, especially under the sun (from the UV), but also from wind and abrasion. Being weaker, the natural fibre fabrics have to be made thicker and heavier to get adequate strength, while the synthetic microfibres are getting better every year. The natural fibres have a fantastic ability to absorb water, and then they take ages to dry out. This is fine if you have a clothes line and a sunny day, but not so clever when you are bushwalking in poor weather.

Yes, I know that of recent times there has been a surge in the availability of fine merino wool thermal underwear, under several brands. A wonderful inspired marketing effort for the pseudo-street market. The facts remain: wool absorb lots of water, wool fabric take ages to dry, and wool fabric is weak. I have seen reports of people ripping their wool thermals with very gentle treatment. Granted, they may be nice to wear at home. Ah yes - there is one exception to all this: many of us use lightweight silk liners for our sleeping bags. True, but we keep them very dry, and they do wear out frequently and rip down the seams at unexpected times. You can now get synthetic micro-fibre liners, but they are a bit heavier than 8 momme silk.

Ordinary Synthetics

The good woven fabrics we will normally run into are nylon (in several versions like 'nylon 6' and 'nylon 6.6'), polyester and polypropylene. Most fabrics will be nylon, although people do sometimes use polyester for tents. There is a reason for this: nylon has the curious property that it expands when it cools. You can see this sometimes on a tent fly after rain: the fabric is slack where the water is resting. That is because the wet spots are colder. Polyester does not do this.

The well-known Cordura and other look-alikes (eg Kodra) are basically nylon, but it has had extra processing as it was made. I think the fibres are stretched a bit, which aligns the molecules inside the fibres to make the fibre stronger. It is very good stuff, albeit slightly tough and harsh.

Of course, we do see a lot of polyester used in fleece clothing. It seems to be the standard fibre here. But do remember that the insulation properties of fleece are due almost entirely to the trapped air, not the material the fibres are made from. The structure of the fibres is claimed to have some influence, but how much of this claim is just marketing spiel remains to be determined.

You will meet polypropylene in some thermal underwear. It's a good fluffy light insulator, but it does have a reputation for picking up body odour. Sad about that. In some cases acrylic fibre is mixed with polypropylene for thermals, as in Macpac Geothermals.

Another fibre you will sometimes see mentioned is Spandex. This is actually an elastic fibre used in very small quantities to give an elastic stretch to some fabrics, especially Lycra. Good stuff Lycra, especially for racing ski wear in fine weather!

Advanced Synthetics

Apart from the common or ordinary synthetics, most walkers will also be familiar with some rather exotic materials like Kevlar, Spectra, Dyneema and maybe even Vectran. The fibre forms of these chemicals are even stronger than the nylons and polyesters - by a degree similar to the way nylon is stronger than the natural fibres. In fact, they are so strong they are harder than many metals, and don't stretch. However, they can be rather heat sensitive, for structural reasons. I will give some quotes from Wikipedia about these materials, but you can look them up yourself as well. You should note that these fibres are very hard and very slippery (because they are so hard), and tying reliable knots in the strings is very difficult.

Kevlar is an aramid fibre invented by DuPont. It consists of parallel long chains of a complex organic molecule which cross-link rather well. Kevlar is wonderful stuff for body armour and things like that, but when sold at OWFINC they normally charge by the inch rather than the yard. It's a bit expensive!

Dyneema and Spectra are both, believe it or not, just ultra-high molecular weight polyethylene (UHMWPE). That means the fibres contain extremely long molecules, much longer than those in Kevlar. Not quite the famed 'single-molecule fibre' of science fiction, but closing in on it. However, while very strong it is a bit heat sensitive. That said, it does not stretch. I use Dyneema kite string for tent guys. (Spectra can be made to fray over the edges of titanium snow pegs in a gale overnight ... beware hard sharp edges.)

Vectran is a 'liquid crystal' material. I know less about it at present. As Kevlar is stronger than nylon, so Vectran is stronger than Kevlar. Unbelievable stuff. It is used to make the tethers the astronauts use for wandering around outside the Space Station and the orbiters. It does not stretch either!

Fabric Weaves

A fabric can be made by one of three different processes

Woven fabric is the simplest one. A loom takes warp and weft threads and interlaces them. The warp threads run the length of the loom or fabric, while the weft threads go across the loom and the fabric. This makes a woven fabric and is this the dominant fabric for bushwalking gear.

There are in fact a myriad of weaves used in making woven fabrics. Many are designed to produce a distinctive surface appearance, against which the simplest basic 1:1 weave looks almost dull. However, while many of the more fancy weaves have interesting appearances, none of them are as strong as the basic weave. They are usually more prone to abrasion as the exposed parts of the threads are longer and more prone to being snagged and broken. For bushwalking gear, the best weave is the plain one, with equal warp and weft densities. And the tighter the weave, the better the fabric. However, note that the tighter the weave, the more difficult it is to make, and the higher the tensions required in the loom.

Knitted fabric is usually made on a (huge) circular knitting machine. The knitting is done by making rows of interlocking loops, just like in a home-knitted jumper. However, a home knitter will normally work with just one thread at a time, while a large circular knitting machine can be simultaneously knitting with a horde of threads distributed all around the machine. Such machines can go very fast. For this reasons many knitted fabrics are cheaper than woven ones. However, knitted fabrics have several different properties from wovens: they are far more stretchy (they can be distorted), they are invariably thicker, and they are frequently less wind-resistant. They are good for clothing of course, but no use for packs and tents and sleeping bags.

'Non-Wovens' are made by the third process. Here the manufacturer takes a huge mass (tangle?) of fibres, lays them out on a flat surface, and bonds them together. The process can be done much faster than weaving or knitting, and is therefore much cheaper. Typical examples of this are the Chux cleaning cloths, synthetic Post office envelopes and the 3M Micropore adhesive (surgical) tape. This sort of fabric has yet to make significant inroads into bushwalking gear, although it has huge industrial uses where the lower cost is a benefit. You will see it in Tyvek, sometimes used for groundsheets, and in rainwear like frogg toggs and DripStoppers.

Ripstop weave is a variation on the plain weave which is supposed to be important for gear. In this design heavier threads are placed at regular intervals in both directions. This produces a grid pattern in the fabric. The idea is to make the fabric a bit stronger against tears: the heavier ripstop thread won't let the tear propagate. The idea is nice, and I must say the ripstop fabric is more interesting to look at compared to a plain weave. However, in practice it is no better than a plain weave, and when used for a coated or impregnated fabric it is actually worse. There are three reasons for saying this.

Fabric Weights - Denier

Now, what weight fabric should you be looking for? Well, that's a tricky question, not the least because we have to first define fabric weight. You might think this is obvious, but no way. Here in Australia (and in the UK) you take a square metre of finished fabric, weigh it, and you have the fabric weight in grams per square metre (gsm). But in America they do something entirely different. They take a square yard of raw fabric straight off the loom and weigh it to get the 'fabric' weight in ounces. Then they start processing the fabric, adding coatings of all sorts. They might start out with a '1.0 ounce' fabric (33.9 gsm), add 0.3 oz of PU coating on one side and 0.15 oz of DWR on the other side, to end up with fabric weighing 1.45 oz or 49 gsm when finished. But they still call it a 1.0 oz fabric! If you don't know about this it can cause a lot of confusion and grief. Add to that my experience of many cases where the quoted weight was rather optimistic in the first place, with a 1.1 oz fabric usually being closer to 1.2 oz or even higher in practice. As for the coating weights ... they can vary lots!

In addition, there are at least two ways of defining fabric weight. There is the weight per area measurement I have just explained, but there is also a 'denier' measurement, with unit abbreviation 'd'. Now the term 'denier' was originally meant to define a thread weight: actually the number of grams per nine kilometres of thread. But some classes of fabrics are often defined by the denier of the yarn used to make the fabric. Thus we have the well-known 1000 d and 500 d Cordura fabrics, the 210 d Packcloth, and the 70 d tent fly fabric. However, be warned that the denier of the fabric does not give a reliable guide to the weight. I have seen what is claimed to be '1000 d Cordura' (and similar) fabrics weighing anywhere from 325 gsm to 560 gsm. Like clothing, fabric has to be 'felt' (or better, measured) before you judge it!

State of the Art in Fabrics

Having defined how we measure fabric weight, what weights should you be looking at for your application? Well, that is entirely up to you. If you want to make a pack from 1000 d Cordura at 560 gsm to take lots of abrasion, that's fine. It should go very well on a rock face. If you want to make a pack from 38 gsm silnylon, that's fine too - but it won't handle quite as much abrasion of course! On this score, note that ripstop fabric is not always the best to use for an application. I will list some typical fabrics for consideration, but please don't take this as too much gospel. The range out there really is HUGE.

Fabric Weight gsm Treatment Use
1000 d Cordura 325 - 560 PU coated, maybe DWR Base of packs and gaiters
1000 d Kodura 325 + PU coated, maybe DWR Asian alternate to Cordura
500 d Cordura 220 - 350 PU coated, maybe DWR Base of light packs and gaiters
500 d Cordura 220 - 350 UNcoated, maybe DWR Breathable gaiter uppers
420 d Nylon 260 PU, maybe DWR Cheaper alternate to 500 d Cordura
210 d Packcloth 110 - 160 PU, maybe DWR Light pack bodies, but stretchy
Dim Poly VX range 100 - 340 Laminated, Mylar reinforced Very dimensionally stable
70 d Nylon 60 - 70 Acrylic Cheaper tent flys
70 d Nylon 60 - 70 PU coated Good tent flys, rainwear
Taslan Nylon 60 - 70 Uncoated Tough clothing, air-textured yarn
Silnylon 48 Silicone coated right through
[2010]: current sylnylon made in USA has become pathetic. While the old 'wet-look' stuff was seriously waterproof, the current stuff leaks at quite a low pressure. A move to Asian fabrics is starting to happen, as their stuff is often better.
Tent flys, rain wear, gear bags, but not for high pressure
Pertex Microlight 43 - 46 DWR Down-proof, wind-shedding!
Pertex Quantum 30 DWR Down-proof, wind-shedding!
Parachute fabric 33 - 38 DWR maybe Inner tents for snow use
Spinnaker fabrics 30 - 60+ Silicone/polycarbonate mix, among other blends Crinkly, sheds water, but leaks after creasing
Cuban Fiber 15 - 30 Dyneema-reinforced Mylar Clear, incredibly light, no stretch, tricky! However, the lighter gauges have been found to develop pin-holes after a bit of folding, and then the stuff does leak.
Mosquito netting 13 - 50 often 'bonded' Huge range with various pore sizes


What is 'Waterproof'?

Since getting wet is reputed to be one of the less desirable things in the bush, the ability of your gear to shed water is of some importance. This brings us to the fascinating question of what does 'waterproof' mean? It might surprise you to know that the question is not all that simple. At the ultimate, it means that absolutely NO water molecules will ever get through, no matter what. Well, 10 mm stainless steel plate might qualify. OK, let's be more realistic: let's ask that no liquid water gets through despite there being 100 kPa of pressure. There is a military specification like this - I think Gore love quoting it becasue some of their heavy fabrics meet it. But what is 'no water'? Realistically, if a bit of tissue paper placed on the 'dry' side gets a wet patch, the material is leaking. Such wet patches are very visible on tissue, and this is a nice simple test. A criterion mentioned in at least one Standard (there are of course many) is that 'leaking' happens when three tiny drops are visible in something like a Suter test.

By the way: 100 kPa is a very high pressure rating: the fabric absolutely bulges in the test jig at this pressure! I stand back, fearful it will literally burst. Lower ratings are usually quite sufficient, especially for the fly of a tent. My very fine Macpac Olympus tent had a pressure rating of only about 10 kPa, but that is really too low.

Does this sort of pressure specification matter? Yes, it does, for several reasons. The first deals with groundsheets, packs straps, and so on. When you kneel on your groundsheet, you put pressure on it. This pressure can force water through, from the ground to your knee. Equally, your pack straps can force water through your jacket. The second deals with lifetime. A cheap proofing mechanism will degrade over time. What happens in some cases is that the proofing layer gets holes at the cross-overs between threads: very small holes, but they can leak. Cheap acrylic proofing does this quite quickly. The third aspect concerns small holes in the proofing layer. These can arise from bad process control, or from spiky scrub poking little holes through the membrane in your jacket. Apply pressure and you may get a slow weeping leak, or even a small shower. This happens to the GoreTex membrane in the NSW scrub only too easily, as many of us have found to our cost/horror. So we have to have numbers.

While we are at it, let's also define 'breathable' as much is made of that term. If a material blocks bulk liquid water but lets individual water molecules through, it can be said to be 'breathable'. How fast the water molecules go through is open to debate - which rages between the different vendors. But remember that for a fabric to breathe like this, there has to be a distinct water vapour pressure gradient - a driving force. This does not always happen in Australia. When it doesn't, you get condensation on the inside. This is also discussed under Rainwear. Note that as defined here this term has nothing to do with allowing air to blow through. That is called 'air flow' and is completely different. Of course, if you can blow air through a fabric then water vapour is going to pour through it, much better than through any membrane in practice. However, I find some fabric vendors do use 'breathable' to mean uncoated, so you have to read the fine print.

Cotton canvas was supposed to be 'waterproof', but of course it isn't that simple. Different canvases will take different pressures: it depends a lot on how they are made. There may be a bulk proofing treatment over the canvas, but this can wear off. Canvas can be made from threads which were treated beforehand: this is dearer but the result is better. Canvas sometimes blocks a lot of water simply because the outer surface gets wet, the fibres swell, and the passage of water is physically blocked - well, impeded anyhow. But you often find that the inside of the material will be damp. A good tight weave canvas is not bad, but it rarely stays 'waterproof' for very long. Its getting to be obsolete.

A natural-fibre fabric which gets completely ignored these days in America but still gets used a bit in the UK where they know about miserable damp weather is something called Ventile. It's a tightly woven cotton fabric which retains some of the natural cotton oils. It breathes very nicely and yet can be very waterproof - due to surface tension and swelling. It was developed to make protective overalls for World War II aircrew, for when they went down in the North Sea. Yes, they could actually maker a dry-suit out of it which breathed. However, while it remains an interesting fabric for polar use, it also remains quite heavy. It is stuill available from Switzerland.

Coatings on Synthetics

Synthetics require a proofing layer or treatment, of which there are several. It should be remembered that almost any layer you put on can be worn off. The ones you are likely to meet are listed below. A detailed comparison of the most common ones: silicone, acrylic and polyurethane, is given on another page.

If you are thinking of using a silnylon, be warned: silicone-coated fabrics are very slippery and a bit hard to handle for sewing. You would need to practice a fair bit first. Also, condensation can happen inside them if there is inadequate ventilation - but then it can happen inside any tent if the conditions are 'dewy'. The author has been using silnylon for tents and (well-ventilated) ponchos, and has beenquite happy with it. It is also very hydrophobic, such that water tends runs off it, and it does not absorb water. Sadly, it has to be added that it does have a bit of an attraction for dust.

But note the past tense used above. Modern (2010) dry-look Amercian silnylon is pathetic compared to the older 'wet-look' stuff. Basically, the coating has been cheapened to the point where I will not recommend the modern stuff any more. We need to look to the Asian mills for betetr stuff.

You may also hear the phrase 'zero porosity' used for a silicone-treated fabric. This can mean a silicone coating, but I have seen it applied to other less-specified coatings. I believe the term derives from the parachute market. Be warned: they don't care about 'waterproof'.

You certainly can use 'EPIC' synthetic fabric for shell clothing for sub-zero snow conditions. The author has made a seam-sealed shell jacket out of very light synthetic Epic and uses it ski touring. It is very light, stops the wind nicely, and yet breathes quite well. In pouring rain one time it continued to function provided I wore some light fleece under it to keep it warm and dry on the inside. On the other hand I have heard reports that tents made of this fabric will let a fine mist of water through under sustained heavy rain.
I have also tried using a cotton-based EPIC fabric for clothing. It was not successful. It collected sweat and dirt, and these completely nullified the surface tension effect such that the fabric leaked badly. Also, I believe the polymer coating abraded off the cotton fibres, to allow the water to get to the underlying natural fibre. This seemed to get very wet. So while I love the synthetic EPIC fabrics in the snow, I cannot see any logic in the natural fibre ones.


Seam Sealing

Sewing is wonderful, but it leaves holes in the fabric. The older cotton canvas would swell up around the holes, but modern synthetics don't. If anything, the holes can open up under tension. Sealing the seams is a good idea, and any extra reinforcing across the seam can take a load off the stitching and the holes. Tape sealing is superb for this, especially with the ultra-lightweight fabrics.

Seam sealing on PU-proofed fabrics can be done with water-based seam sealers, but I have never had much joy from them. The ones I have tried seemed to dissolve in the rain. Some of them did not stick to nylon fabric very well either, peeling off once they had really dried. Avoid them. I would far prefer to use a polyurethane sealant, and the McNett Seam Grip is excellent. For that matter, if you can thin it down, ordinary 'Shoe Goo' (from Athlete's Foot or similar) works just as well. Do allow it to dry properly before you fold the fabric up, or dust it down with Johnsons Baby Powder.

An alternative to that sort of sealant is the use of tape. You see seam sealing tapes on some gear, especially the inside of Gore-Tex and PU-nylon jackets and such. Coghlans sell a packet of several pieces of adhesive-backed light nylon cloth: this is highly recommended for small repairs. Yachting companies sell 'seam stick tape' (just the adhesive) and adhesive fabric by the reel. If you want to be a little more couth, you can buy 'double-sided' or 'transfer' seam stick tape from the yachting companies and use your own fabric. You run out several strips of the tape on some leftover bits of your fabric and rub it down. Then use the paper backing as a cutting guide to cut out the taped fabric into long strips. Peel off the second bit of paper and apply the adhesive fabric across the seam. Rub it all down well and allow a day to bond. It may be worth your while to experiment a bit first: not all tapes stick sufficiently well to all fabrics. Warning: these tapes do not stick to the silicone fabrics!

Seam sealing silicone fabrics is done the same way in principle, but with silicone adhesives. Anything else falls off. You can use McNett Silnet Sealer - a little expensive, an ordinary clear silicone sealant commonly known as 'Silastic' or RTV from a hardware store in a cartridge, or a specialised industrial silicone. Either acid-cure or neutral cure cartridges can be used; I prefer the clear or translucent neutral cure stuff. The common silicone sealant in a cartridge is viscous (non-slump) and some people reckon it needs thinning to get it to penetrate well into the holes. You can thin it with turps: anywhere from 1:2 to 2:1 turps:sealant ratio. Mix it well: it takes a little while and a power mixer. Other solvents can also be used, even something like Shellite or toluene. I have also used a special silicone oil (OS-20) from Dow Corning, but it is hard to get. On the other hand, I have been using a clear aquarium non-acid-cure silicone in a cartridge from my local hardware store without any thinning. I just push it into the seam with my finger or a smooth steel spatula. This has worked just as well in my opinion.

If the sealant is thick you can apply it with a hypodermic syringe; if it is runny you can apply it with a small stiff glue brush or a shaped bit of foam on a stick. Either way, work it into the seam (finger tip, foam brush or spatula). Yes, this tends to be a slightly messy job! Then let it cure for at least 24 hours (48 hours is better) before you fold it up! I found it useful to dust the sealed areas down afterwards with talc or baby powder, to prevent sticking.

Caution: some common commercial solvents carry a lot of water in them. So when you mix up the solvent and the silicone, you are triggering the water-0based curing process extremely effectively. This means the pot of mix may thicken and 'go off' right before your eyes! You are far better off using a thin silicone 'adhesive' rather than the thick goo which comes in cheap caulking cartridges. Alternately, use the stuff from a cartridge, but smear it on undiluted from a very thin bead with your fingers.

You can get double sided (and single sided) tapes which have a siloxane adhesive. These will stick to the silicone fabrics; the right ones will actually bond with the silicone coating. However, many formulations in the available range are not meant to provide a permanent bond: they are meant to be removable masking tapes. Be warned: while sealent is cheap, these tapes are expensive. This is an area of ongoing R&D for the adhesive tape companies.

© Roger Caffin 16/08/2006, 27/12/2011