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Backpack Fabrics and Design

This guide will help you understand the materials used to construct modern backpacks and the main issues concerning carrying loads on your back. It is a long story with lots of interesting information.

The textile content on this page is limited to stuff directly related to backpacks. There is a lot more additional and complimentary information in the page on light-weight tent fabrics. You can get to this via one of the tent product pages. It covers a wide range of general textle physical properties which have not been included here.

I hope to complete the content for this page soon. In the meantime, here's a start, beginning with the description of the outstanding canvas used in the WE backpack range. Cheers, Ian Maley

WE Backpack Canvas

Since the very first packs that we made in 1977 we have used a unique standard of canvas unobtainable anywhere else in the world but Australia. Fabrics that fall under the general heading of 'CANVAS' range from "cotton duck" to the high-tech, core-spun canvas used in WE packs. Cotton duck is a coarse weave of pure cotton yarn. Modern canvas is woven from yarns containing both cotton and polyester fibres. The percentages of each fibre type spun to make the yarn are usually either around 50/50 for the stronger and heavier fabrics used for tarpaulins, packs and bags, or 65 polyester/35 cotton for the lighter, cheaper fabrics used in awnings, vehicle camping tents, chair slings and the like. There are also two, quite different yarn constructions.

BLENDED YARN. Cotton fibres are naturally short. Any fibre of limited length is referred to as a staple fibre.  With cotton, the longer the staple the higher the quality. Polyester filaments are produced in continuous lengths, as long as you like. In 'blended' yarns standard grade polyester filaments are used and these are chopped into staples, mixed with the cotton and then the mix is spun together into a yarn. This is the most common and cheapest canvas yarn construction.

CORE-SPUN YARN is constructed by spinning a sheath of cotton fibres around a continuous core of high-tenacity polyester filaments. In terms of mass the cotton and polyester content of a core-spun yarn is also about equal. It is obvious that a corespun yarn, with its continuous polyester filament core aligned perfectly along the yarn, will have a much higher tensile strength than an equivalent denier blended yarn. You will see canvas specified as so many "ounces", usually 6, 8, 10, 12 and 18. This refers to the mass of the loomstate fabric per square yard, after weaving and before finishing. As a guide 8oz / 275gsm core-spun canvas has a similar strength to 12oz / 400gsm blended canvas. (The units and conversion: 1ounce per square yard = 34 grams per square metre).

Except for the entry-level Breakout pack, classic WE canvas packs all use first quality 275gsm core-spun canvas.

WEAVING. If you could hold up to back-lighting a piece finished pack canvas side-by-side with a piece of 1000 denier nylon Cordura or Kodra you would be immediately struck by the fact that the canvas is so tightly woven that no light pin-points are visible through the fabric. In contrast, the bulked nylon fabrics, like almost all synthetics used in pack manufacture, are riddled with pin-pricks of light. Compared to the canvas they are relatively loosly woven and in fact depend on a back coating, usually polyurethane (PU), for stabilisation as well as waterproofing. Without this coating the yarns at an edge loosen and unravel easily. Achieving the extremely dense weave in canvas is time consuming and expensive. Square metre for square metre core-spun canvas is over twice the price of Cordura (see Bulked Filament Nylons, below).

DYEING AND 'PROOFING'. Giving colour to canvas fabric is done in two ways. The first, used for our backapck canvas, is the 'jig dyed' process. The fabric is passed back and forth through a dye bath, many times. The dye deeply penetrates into the yarns. After dyeing the fabric is then treated with a clear proofing solution. This is a brew of waxy acrylic resins and mildew inhibitors that are soaked into the fabric and then cooked by passing it between giant radiators. The heat causes a polymerisation or 'setting' reaction. Finally the fabric is dried completely and rolled. The less expensive way to colour canvas, used for tarpaulin canvas for example, is to combine a suitable pigment with the proofing liquid and apply and set this, as above. If you rub a piece of white paper on canvas finished with pigmented proofing some pigment will transfer to the paper. For this reason pigmented canvas is not suitable for backpacks, certainly not any parts that will contact clothing. Finally, because the proofing is in the form of waxy resins, soaps, detergents and solvents like mineral turpentine and dry-cleaning fluid, will partially or fully remove it. (Refer to the Backpack Care Advice page).

CANVAS WATERPROOFNESS. The standard test for fabric waterproofness is clamp the test piece over a bowl filled with water and slowly increase the water pressure. This is usualy done by having a clear pipe column connected to the bowl, extending upwards from it and filling this column progressively. When the fabric starts to leak the water level in the column, in millimetres vertically above the fabric level, is recorded. This is the 'hydrostatic waterhead' measurement of 'water entry pressure'. Typical coated synthetic fabrics used for backpacks are usually specified at something over 1000mm. Canvas will often leak well below this and yet good quality canvas backpacks have a reputation for excellent wet-weather performance. The reason is that the hydrostatic head test does not allow the process by which canvas achieves its excellent performance to occur. In the field, the cotton fibes in the fabric yarns slowly absorb water, swelling and tightening as they do so. (Tie a length of cotton string between two posts, wet it and watch it tighten). Combined with the repellent effect of the waxy proofing this tightening increases the pressure required to force water through the weave. To prevent water wicking through canvas to an absorbent object in contact with the other side it is important that the fabric have a reasonabe level of proofing. This is the reason the proofing on backpack canvas needs to be cared for (and why touching a traditional canvas tent was to be avoided). Re-treatment simply involves soaking the canvas with a wax solution and allowing the solvent to evaporate. (Once the coating on a synthetic fabric is deteriorated there is no effective way of re-coating it to the original performance. The original industrial process can not be reproduced on a finished backpack or tent). Finally, our canvas pack bags are stitched with a compatible core-spun cotton-polyester thread, although for strength reasons the polyester content is much higher, at 85%, than in the canvas yarns. In combination with the fabric this thread produces seams with good water-resistance and which also readily and very effectively accept re-proofing treatment. More detail on seam waterproofness is provided below.

Bulked Filament Yarn Nylon Fabrics

The common brand names for these high-abrasion-resistance fabrics used for wear areas on backpacks are Cordura® and Kodra®, although you will see any number of other variations on this phonetic theme. You will also see them simply called 1000 denier nylon (although 500d and 600d bulked filament yarn fabrics are also commonly used). These fabrics use yarns in which the nylon filaments have been 'bulked, disarranged and tangled' so that they loop to the surface and increase the cross-section of the yarn. In contrast to normal synthetic yarns where all the filaments run more or less parallel together along the yarn bundle (and contribute equally to the tensile strength) the bulked yarn has an expanded cross-section with built-in compressive resilience that resists wear. No individual filament looping to the surface of the bulked yarn plays a crucial role in the yarn's integrity at that point.

Although these abrasion-resistant bulked yarn fabrics are mainly used to reinforce wear areas on packs and bags, some products do use them for the entire pack bag. Cordura® is constructed from high-tenacity nylon 66 filaments and this premium product is applicable where the highest tensile and tear strengths are required, such as for military backpacks. Where abrasion resistance only is required and the fabric will be used as a second, external layer over another fabric like canvas, then standard nylon 6 filament yarn is more than adequate and considerably more economical. Kodra® falls in this category.

Other Synthetic Fabrics and Weaves

Plain-weave, plain dyed, 420d and 210d  'oxford' nylons are commonly used for backpack lining and throat fabrics. Polyester fabrics have two significant advantages over nylon ones: the yarns absorb little water and they are naturally much more resistant to UV degradation. On the down side they are not quite as strong and they lack nylon's elasticity. Fancy patterned 'dobby' weaves of both nylon and polyester, and weaves incorporating a Kevlar® or other dye-resistant yarns can be seen on backpacks and daypacks where a 'fashion look' is a design component. For a clear explanation of ripstop construction and tear strength please see the page on light-weight tent fabrics, where it is most relevant. They are discussed in detail there.

Synthetic Fabric Waterproofness

However tightly woven, all synthetic fabrics require a coating or film lamination on at least one face to achieve a practical level of waterproof performance. Coatings are the most common but more expensive laminations offer some distinct advantages. These coatings and laminated films are most frequently made of polyurethane (PU) because of its light-weight and its flexibility over a wide temperature range. PVC coatings are common on cheaper fabrics used in mass market products (like school bags and many surf-label daypacks and bags) - despite well known and extreme chemical hazards associated with its manufacture, product life and disposal. Besides the environmental hazards, PVC is heavy, its flexibility varies markedly over everyday temperature ranges and it becomes brittle with age as plasticisers leach out.

A PU coating is applied to a fabric by 'screeding' the liquid coating mixture onto the fabric surface. Even with precision ground straight-edges and rollers it is impossible to achieve a perfectly even coating thickness. Slight, unavoidable variations in the fabric thickness mean coating thickness will also vary. A high and consistent level of waterproofness is best achieved by first making the 'coating' as a separate membrane or film and then gluing or 'laminating' this film to the fabric. Besides the consistent and high level of waterproofness this process achieves there is another significant advantage, that of greatly increased tear strength.

Fabric Strength

TENSILE STRENGTH. Excepting the current trend to make increasingly lighter and lighter packs (mainly daypacks), conventional backpacks and travel luggage are constructed with the relatively heavy duty fabrics outlined above. They all have adequate tensile strength for the job. Of more importance are the detailed matters of seam construction (and coating durability in relation to that). These play a far more crucial role in the overall quality of the finished product. If you were to name the single most important physical property of a fabric to be used for a good quality backpack it would be abrasion resistance. Abrasion is the most common pack fabric failure. Refer to design notes to follow below.

TEAR STRENGTH. Again, the comments on tensile strength above also apply to tear strength. Only after a pack has been slashed will tear strength play an important role. To make a real difference in the tear strength department requires a quantum leap in fabric construction, to laminated technologies and the use of ultra-high-tensile yarns. A simple ripstop pattern is inadequate. Either way, a repair will be needed sooner rather than later. A more detailed explanation of tear strength is provided on the lightweight tent fabrics page but, briefly, applying a coating to a fabric effectively glues all the yarns together. When the coating chemical is forced down into the weave structure and set the coating can no longer easily stretch. Yarns have a greatly reduced ability to shuffle around when they come under stress. At the end of a tear in a coated fabric the next unbroken yarn takes the greatest share of the stress. By contrast, in an uncoated fabric the weave can distort at the end of the tear and a number of unbroken yarns share the stress. When a fabric is laminated with a film, the film 'skims' the fabric surface, glued only at dot contact points. It retains more of its natural elasticity, allowing some yarn shuffling to occur at the end of the tear. Here's an interesting observation: A polyester fabric (somewhat weaker than an identical construction nylon fabric) laminated with a TPU (thermo-plastic urethane) membrane can turn out to have a higher tear strength than the nylon fabric when the nylon is given a normal PU coating. This is a nice illustration of one advantage of laminated menbranes over coatings. 

To be continued... Jan 2009


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