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Light-weight Tent Fabrics

This page describes the common fabric constructions, finishes and the physical properties of modern, lightweight backpacking tent fabrics. You will see references to many of these fabric specifications and properties in product information and advertisements. As usual, it’s not a simple story. Specifications quoted in isolation can mislead. How they relate to product performance in practice is much more valuable information.

Even just discovering whether a fabric is say nylon or polyester, the most common tent fabric yarn filament types, is complicated by the fact that the yarn manufacturers, mostly global chemical companies, have registered trade names for their products. Dacron®, Terylene® and Trevira® for example are all polyesters. Tactel®, Cordura®, Kodra® and Perlon® are all nylons. It is also increasingly common for brands to give house names to the fabrics they use. Matters are further complicated by the fact that endless variations are possible in polymer chemistry. One nylon or polyester is not the same as the next. And all that comes before the multitude of variations possible in the filament extrusion process, yarn spinning, fabric weaving and the dying and finishing choices.

But..if you are interested, here is a summary, a rough guide, starting at the beginning. Along the way the practical implications for your tent fabrics are noted. If you do read on it would also pay to look at the page on Staying Comfortable in the Outdoors, under Other Stuff in the head menu. The information there ties in with some of the sections below.

Fabric Construction

Filament. These are the solid, continuous, individual fibre strands that are spun together to produce a yarn. Nylon (polyamide) and polyester (polyethylene terephthalate) are the main filament types used for lightweight tent fabrics. Filaments made from “high-tenacity" polymer grades (for example nylon 66, as against nylon 6) or processed to optimize tensile strength (such a drawing to align the long polymer molecules along the filament) are more expensive but result in a stronger, higher quality product. Here are the important comparisons between the best high-tenacity nylons and the best high-tenacity polyesters. Nylon is about 15% stronger, it is far more elastic (stretches more than 30% at break-point), it absorbs and chemically bonds a lot of water - as much as 30% of its driest mass, it elongates as it takes up water, and it is highly prone to UV degradation. Polyester is not quite as strong as nylon, has minimal stretch at break-point (only a few percent), absorbs little water (again, only a few percent - and does not change dimension), and it has a far better natural resistance to UV light (but does still degrade). Denier is a linear measure of the mass of a filament or yarn. Its definition is given in the next section.

Yarns. These are the threads used in the fabric's weaving or knitting process. A yarn is made by combining filaments into a bundle. Heavier yarns may constructed by twisting smaller ‘yarn bundles’ together. A ‘twisted pair’ is a common example. For tent fabrics where strength is paramount the filaments are combined in continuous length form. (When tensile strength is not the over-riding objective, filaments may be cut into shorter lengths, known as staples or staple fibres, and then spun together, in the same way that natural fibres like cotton are spun into a yarn. More than one fibre type may also be included). Although there are other systems, the ‘size’ or ‘weight’ of a synthetic yarn is commonly denoted by its denier, the mass in grams of 9,000 metres of the yarn. The denier of yarns used to weave mainstream light-weight tent fabrics fall in the 30d to 75d range. It is common to use the yarn denier to describe the fabric, for example, "70d nylon". This simply means that the fabric is woven from 70 denier nylon yarns and, alone, is not a reliable comparison of fabric mass.

Fabrics. Woven fabrics consist of warp yarns running along the fabric, and weft or fill yarns running across the fabric. Different choices of warp and weft yarn polymers (or natural fibres), yarn constructions (maybe a 'twisted pair') and deniers combined with a choice from any number of weave patterns result in a huge range of fabrics with an equally huge range of both physical and aesthetic properties. Fortunately, since tent fabrics need multi-directional strength, simple yarns and plain weaves are the norm. Taffetas, with or without rip-stop yarns, account for nearly all woven tent fabrics (insect mesh is usually knitted). An explanation of rip-stop construction follows in the tear strength section. Is a 30d fabric half the weight of a 60d one in the same type of fibre? The answer is no because it is also a matter of weave density, stated as ‘thread count’ or ‘yarn count’. A 30d yarn will be about 70% the diameter of a 60d yarn or the same fibre (half the cross-sectional area). Fabrics made from finer yarns are naturally more finely-woven, otherwise the fine yarns would be widely spaced and the fabric loose and unstable. Weave density is measured by counting the number of warp yarns or 'ends' there are across one inch of the fabric and adding to this the number of weft yarns or 'picks' there are down one inch of the fabric. 75d 190T taffeta, a common tent weave, has a total of 190 yarns (maybe 90 ends per inch plus 100 picks per inch), and all the yarns or threads are 75 denier. A typical thread count for a stable 30d fabric is 240T. In its ‘loom-state’ (before any finishes are applied) this 30d fabric has a mass per unit area over half that of the 75d fabric but the denier ratio is well under half (at 0.4). After a coating and other finishes are applied the relative difference in finished mass between the two fabrics may be as little as 20%, a far cry from 30d vs 75d. (For your reference, 1 ounce per square yard = 34 grams per square metre).

Fabric Finishes

Dying. Filaments and yarns made from certain polymers can not be dyed after they have been made. The chemistry simply doesn't work. This is the case with polypropylenes and acrylics for example. Pigment must be incorporated into the 'melt' prior to the filament extrusion. Some yarns are better dyed before weaving but in most cases fabrics can be "piece-dyed" after weaving. This is the case with the nylons and polyesters used for lightweight tents. The actual choice of dye colour and the level of its saturation both critically affect illumination within the tent (as well as night time heat loss and daylight heat gain). Aiming for a medium level of neutral lighting is a good compromise. This can be achieved by using low saturations of colours that will also blend into the local surroundings. Unfortunately good light transmission is also accompanied by increased UV penetration and hastened degradation of the fabric.

Waterproofing Synthetic Fabrics. To achieve practical levels of waterproofness even the most tightly woven synthetic fabrics must be coated or laminated with a continuous waterproof film.

In the coating process the liquid coating chemical is 'scraped' or screeded onto the fabric as it passes over a precision roller under a precision straight-edge or screed. The most common fabric coating polymers are polyurethane (PU), polyvinylchloride (PVC), and acrylic (polyacrylonitrile PAN). The second two are highly UV resistant but insufficently flexible, particularly at low temperatures. PVC is an excellent barrier to water vapour but it is heavy, is particularly inflexible at low temperatures and its production and disposal are extremely bad news for the environment. So polyurethane (PU) is the coating of choice for nearly all backpacking tent fabrics. It has good resistance to wet-flexing and cold-cracking, is relatively light-weight but, like all polymers, is available in a whole range of formulations, qualities and prices with corresponding performance and durability. For a PU coating to have reasonable durability in prolonged tropical humidity or environmental dampness it must have good resistance to hydrolysis. Good quality but normal formulation PU's, common on most lightweight backpacking gear, have a polymer structure with a polyester 'backbone'. This is structure is relatively easily attacked by water molecules and broken up into shorter segments, the cause of the sticky coating problem that eventually comes with old age in any climate. In warmer climates this decomposition proceeds much more quickly. Other PU formulations with polyether or polycarbonate type backbone chains are highly resistant to degradation by water molecules. If you intend to put your PU coated tent to extensive use in the tropics (or live there) these high quality (and more expensive) coatings are necessary if a reasonable coating life is to be expected. Properly specified, well-cared-for, high-quality PU coatings can last many years.

The other increasingly common waterproof fabric finish is silicone elastomer. It may be applied to both faces of the fabric, or just on one face (which becomes the outside). Silicone rubber is widely used in the building industry and for fluid seals. It is highly water-repellent, elastic, UV and temperature stable. At first glance it is the perfect coating except it is slow to completely cure so production time and cost are high. It also has poor fire-retardant properties and there are no thermo-plastic (hot-melt) or contact adhesives that stick to it. This means that, for factory tape sealing of seams, one face of the fabric must be PU coated (invariably the back). Lapfeld seams in double-sided silicone elastomer coated fabric have quite good water resistance because of the silicone's high water repellency and elasticity, but for completely reliable resistance seams must be hand sealed with liquid sealant. A final advantage of silicone elastomer coating is that, provided it is the only coating on the fabric, it does not reduce the tear strength of the uncoated fabric like other coatings do. (See below). However, a serious disadvantage for fabrics that are coated both sides with silicone elastomer is that accidental damage can not be repaired with adhesive tapes. Tears and nicks must be either carefully sew-patched or lap-bonded with the thick application of silicone rubber, neither easily done in the field.

A laminated fabric has a polymer film or 'membrane' glued to it using an appropriate adhesive and pattern of glue dots. Membranes can be made from a wide range of polymers. The most common membranes used for tent laminations are PU and ePTFE. Membrane film thickness can be very accurately controlled and very high levels of waterproofness achieved with quite thin (and light weight) membranes. This is not possible in the coating process because the thickness variations inherent in the weaving process result in corresponding coating thickness variations. (The common way to minimise coating thickness variation is to scrape on multiple very thin coats, with curing time between them. Once the first coat is applied the initial fabric thickness variations are partly 'smoothed out'. In any case, multiple coating layers have better adhesion and durability than single-application, thicker coatings). A final comment about laminated fabrics: They also tend to have better tear strengths than polymer coated ones.

How waterproof tent fabrics need to be and how this is measured are discussed below, as are the implications of coatings and laminations on tear strength.

Water-Repellent and Other Treatments. Keeping water from soaking into fabrics used for outdoor clothing, backpacks and tents is a worthwhile objective. Saturated fabrics weigh a lot more (especially nylons) and evaporation from the wet fabric surface can rob the body of precious heat. In addition, nylon fabrics sag when water penetrates into the yarn fibres, compromising a taut tent pitch. As noted in the previous section a silicone elastomer coating is an excellent water repellent. Here we are looking at ‘DWR’ – durable-water-repellent fibre surface treatments and today these are mainly fluorocarbon based (but not CFC chloro-fluoro-carbon). Original applications at fabric manufacture can, using the garment example, last up to 10 domestic wash cycles. Heat from tumble drying also works to re-activate the fluorocarbon molecules remaining on the fibre surfaces. Sooner rather than later, DWR treatments need replenishing. Simple spray-on applications that are not followed by heat-setting (in a tumble dryer for example) tend to be leached from fibres very quickly. With backpacking tents the greatest advantage comes by applying DWR to zips since it also serves as a dry lubricant. This is worth doing regularly. Periodic application of DWR to the whole surface of the outer tent is time consuming, costly and not, as noted, a long-lived treatment. Choosing a tent with a good quality silicone elastomer face coating on the outer fabric is a more practical option.

Physical Properties

In this section we will look at the main physical properties of importance to lightweight tents. These are tensile strength, tear strength, water-entry pressure, moisture-vapour permeability, impedance to air movement and resistance to UV degradation. Other properties like coating adhesion, resistance to hydrolysis, resistance to blocking, wet-flex durability, lightfastness of dyes and spray ratings for DWR treatments will be mentioned. Infra-red reflectance is another interesting one but more applicable in the military arena.

Tensile (or 'Grab') Strength. For the same fibre type, heavier fabrics are stronger than lighter fabrics. Simple. Take a 50mm wide strip of fabric and pull it end for end. The force at which it breaks is the tensile strength of the fabric in the direction (warp or weft - you need to look at both) that the strip was cut along. The standard unit of force is quite small. One Newton (N) is approximately 0.1 kg-force or 0.22 lb-force. While fabric tensile strength is just a set of two numbers, as usual, this is not the full story. If you also track the amount of stretch in the test piece as the force applied to it increases this gives a measure of the fabric's ability to absorb energy, important in dissipating the energy of a violent wind gust. Nylon yarns are very elastic and may stretch over one-and-a-half times their original length before breaking. Polyester, although only about 15% weaker than a similar nylon, does not have this energy absorbing range and other strategies need to be employed in the tent design to resist or spill the wind energy. Coatings have little effect on tensile strength (compared to uncoated fabric). For a given yarn type the tensile strength is directly related to the total 'cross-section' of fibre running along the strip. Multiply the yarn denier by the thread count to get the measure.

Tear Strength. Unlike tensile strength, tear strength is not a simple function of the weight of the fabric. Weave construction details and coatings play crucial roles. The simple wing-rip test begins with cutting a slit partly in from one edge of a fabric test piece. Both the warp and weft axes are tested. The fabric edges each side of the cut are gripped and then pulled in opposite directions. The force needed to smoothly tear and extend the slit in the fabric is the tear strength. Same units as above.

To understand tear strength it helps to focus on what's going on at the very end of the rip, where the first unbroken yarn is now at the front-line. With an uncoated fabric yarns are relatively free to shuffle around. As the stress increases the weave distorts, resulting in the load being shared over a number of yarns immediately behind the first unbroken yarn. The more the weave is 'glued-up' with a low-elasticity, deeply-embedded coating the less the weave can distort and the more the stress concentrates on that single, front-line yarn. The result is a substantially reduced tear strength. You can think of the coating as making the fabric more 'paper-like' and less pure 'textile-like'. If the coating is very elastic - as is the case with silicone elastomer, or the fabric has a relatively elastic, laminated membrane which sits up more on its surface, then the yarns are not so constrained. The sharing of the load can still be taken by a number of unbroken yarns because the silicone coating or TPU membrane can stretch as the yarns reposition themselves. In this case the coating material's own strength may actually add a little to the tear strength. Some tent manufacturers (particularly those using nylon fabrics that are coated both sides with silicone elastomer) make a feature of the high tear strength of their outer tent fabric. Finally, it is easy to appreciate that adding a grid pattern of stronger yarns (such as a twisted pair of the standard yarn or a different yarn of a very high-strength polymer) will present a frequent, high barrier to tearing in a lightweight fabric without adding significant overall weight. This is the ripstop principle and it is very important to achieving acceptable tear strengths in lightweight fabrics woven from low-denier yarns.

What are the practical implications of tear strength? The fact is, tear strength really only becomes important after the initial damage has occurred to a fabric. Then it is essential to prevent elongation of the tear. If the tent is in good condition and not generally weakened from too much UV, such damage is only likely to occur by accident. Some that we have seen are a crampon or shovel slice when digging out a snow-bound tent, a falling branch puncturing the fabric or a burning ember melting a hole. All lightweight tent fabrics will inevitably suffer damage in these circumstances. If the damage is significant, repair will be urgent if the tent is to be relied upon, whatever the fabric. In the field the strongest, most effective solution is to tape over the split or hole with a strong, cloth-based adhesive repair tape. (Fabrics with silicone finishes both sides just can not be repaired like this - nothing will stick. A sewn patch or a bonding operation using a thick layer of silicone sealant molded across the tear or hole are the only two remedies, neither very practical in the field. Even if you have spare fabric and back-up shelter, a hand-stitched patch can be time consuming to do properly.

Water Entry Pressure (or Hydrostatic Head - HH). The most common method for measuring the waterproofness of a (coated) fabric is the hydrostatic head test. Here is a simple description of this test. A sample piece of fabric is clamped over a 100mm diameter 'bowl' of water so the fabric is in complete contact with the water surface, no air-bubbles. Connected to the bowl is a (clear) vertical pipe and a means to fill the pipe with water so the level rises at a specified rate. The vertical height of the water level in the pipe above the fabric level is a direct measure of the water pressure at the fabric. The fabric surface is watched constantly as the pipe water level steadily increases from zero height. When three growing beads of water appear on the fabric surface it is said to be leaking and the water level in the pipe noted, usually in mm. Simple. Well, not really! The pre-conditioning of the fabric and coating affect the result. Many coatings rapidly decrease in waterproofness with even a low level of wet flexing (see below). The level of 'durable' waterproofness needed for reliable performance depends on exactly where the fabric will be used: Rainshell shoulders under a backpack, overpants knees, tent outer canopies and tent floors are four examples. Beloww are some guidelines for what is required. In the meantime note the following (approximate) equivalences in the many ways/units used to express pressure: 2,000mm HH = 2.8 psi (pounds per square inch) = 19.6 kPa = 0.19 bar or atmospheres (very roughly).

Practical notes on the waterproofness of tent fabrics. A good simulation of gale force, wind-driven rain is water squirting at close quarters from a domestic hose supplied with a good delivery water pressure. An outer tent PU coated fabric that tests at 1100mm HH will not leak in this test. Compared to the HH specs many tent manufacturers quote for their outer tent / fly fabrics 1100mm is a very low number. Nevertheless it will be reliably waterproof in the field provided the coating retains this rating. One problem not widely publicised is that many apparently good quality coatings lose their initial waterproofness as they are 'wet-flexed', as happens when a wet tent is rolled or stuffed into its bag, or a PU coated rainshell flexes with the walking motion or wearing a backpack over it. One way to check the wet-flex performance of a fabric coating it to run a sample piece through a normal domestic wash cycle (performed without detergent), allow it to dry and re-test the HH. Many coated fabrics with impressive initial or 'as-received' HH specs may lose more than 50% after this wet-flexing treatment. Membrane laminations are not completely imune to this deterioration although their initial HH is typically very high. Tent manufacturers (including us) quote the as-received fabric waterproofness specification, the minimum new fabric should test at. As you will now understand, this may not be a reliable guide. For tent floors the level of fabric waterproofness for reliable performance in all conditions is debatable. At the ridiculous end of the commentary is the suggestion that the water pressure resulting from kneeling on a tent floor (or overpants), on wet rock can be as high as 40 psi or about 30,000mm HH. Our experience is that 5,000mm HH is a good practical tent floor water resistance, especially when you consider the other ways that water and dampness can accumulate in your tent, besides liquid leakage directly through the floor fabric. An inescapable amount of moisture vapour transmission occurs through PU floor coatings. This, combined with the general condensation occuring within the tent, can quickly out-do a small amount of liquid leakage through the floor fabric. (For many years we successfully used a high quality, 2,000mm HH PU coated lightweight floor fabric option in our tents with no complaints). Also, keep in mind the need to protect the floor fabric and its coating from scuffing and puncturing, which can quickly become the source of significant leaks. This is the reason we add a pigment in the floor coating on our tents. The light grey film on black yarns makes coating damage very easy to identify and repair. Another important consideration in specifying the coating performance is the matter of tape seam sealing. Some brands claim that tape sealing is the weakest link in the durability of a tent's long-term waterproofness and use this as a reason to not factory tape seal. The truth is that to effectively tape seal an outer tent requires a high level of attention to construction detail. Some simple, commonly used constructions must be ruled out. Our experience is that the long term success of tape sealing depends on having a sufficient (minimum) coating thickness and, obviously, very good coating adhesion to the base fabric. In this way the 'steps' in thickness (coating plus tape) and flexibility across the edge of the relatively thick tape band does not cause excessive stress on the coating at that edge. The alternatives to not tape sealing a tent are either accepting some degree of constant leakage or a messy application of liquid sealants that have a lower effective life than even an average tape sealing job. Durable tape sealing can be achieved on high quality coatings with HH waterproofness from 1500mm upwards. The most important precaution to observe with seam-sealed tents is to not expose them to high temperatures such as can easily develop in vehicles parked in the mid-summer sun. Above 60 degrees Celcius tape adhesives soften enough for seam tape to lift off the coating at folds and creases.


To be continued... Ian Maley, May 2012.

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