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Backpack Fabrics and Construction
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Backpack fabrics have come a long ways since the days of canvas packs and thing packcloth. The material used, type of fabric weaves, overall design, stitching, thread and weight of material used all contribute to the packs durability, weight and other features (such as water absorbability, UV resistance, elasticity, etc).
Fabrics come in a variety of weights which have a large impact on pack weight, durability and strength.
One way to measure fabric weight is by denier. A denier is a unit of fineness of yarn equal to one gram per 9,000 metres of yarn.
1 denier
= 1 gram per 9,000 meters
= 0.05 grams per 450 meters
A fiber is generally considered a microfiber if it is 1 denier or less.
A 1-denier polyester fiber has a diameter of about 10 micrometers.
9000 meters of 400 denier fabric would weigh 400 grams.
An important consideration in pack construction is the denier of each fabric used. The higher the denier, the greater the strength, durability and weight of the material. For example, 210 denier silnylon is made up of very thin yarn and the fabric itself is quite thin and incredibly light, where 1000 denier Cordura is made up much thicker yarns, is incredibly durable and weighs significantly more.
Tenacity is a measure of the fabric's ability to resist additional tearing once a tear has started. It is calculated by dividing the tensile strength by the thickness of the thread. This resistance to tearing differs between fabrics and even between fabrics made of the same materials. Nylon can be found in type 6 (tenacity of 3.0 to 6.0 grams per denier) and type 66 Nylon (tenacity of 6.0 to 9.5 grams per denier) which is much stronger.
This varies greatly between fabrics. Denier, weave, special laminates/treatments and material used all play a part in the tensile strength of fabrics used in pack construction. A lower denier ripstop nylon may be stronger than a higher denier polyester.
Coatings, such as urethane and treatments such as silicone impregnation play a big part in water resistance of fabrics, as well as the weave and material used.
Older packs were were often made of cotton canvas and waterproofed with heavy wax. These fabrics are generally heavy and prone to rot. Newer canvas bags may incorporate new canvas materials that are stronger and more water resistant but are generally not waterproof. Inexpensive canvas pack can still be purchased may have a nice soft and quite feel to them. Since cotton canvas fabrics heavy, soak up water and tend to rot, they are not recommended for backpack construction or repair.
Cordura is Dupont's, "air treated" nylon fabric and Kodra is a similar fabric made by Kolon Intl of Korea. They have a rough fuzzy texture and have great abrasion resistance. Neither is as puncture resistant as Oxford Weave Nylon and does not waterproof as well as Nylon. Polyurethane coated Cordura and Kodra are considered waterproof.
Cordura Military Ruck
These fabrics are often used in heavy duty packs and luggage and often used on the bottom of packs. Pound for pound, it is not as strong as nylon rip-stop, but is generally made in a heavier weight and is more durable by the square foot/meter.
DSM Dyneema is possibly the strongest fabric in the world and is similar to Spectra. It's used in fishing nets, ropes, cables, bullet-resistant vests and safety gloves.
Gregory Makalu Pro with Dyneema Rip-Stop
It has only a small presence in the backpacking world due to the great cost of this super strong fiber but is used as gridstops/ripstops in nylon fabrics and exclusively in very high end custom packs such as those made by McHale & Co.
There is an old debate as to whether nylon used in backpacks should be made waterproof. Some felt that a pack that doesn't breath will lead to wet and mildewed clothing, while others like the potential of waterproofing their packs.
Today most packs are made as water resistant as reasonably possible. Modern made packs made of untreated nylon are less common to find than treated ones. Therefore, most packs made of nylon either have a polyurethane coating or have been treated with silicone. Both make the fabric more waterproof, much stronger and a bit heavier.
Rip-stop nylon has a regular grid pattern of heavy threads sewn in the fabric at regular close intervals. These heavier threads help prevent the progression of a tear once started. Since light weight nylons are more likely to where than pack cloth or heavier weight fabrics, the rip-stop feature is a must. Nylon is stronger than polyester by weight but is more susceptible to UV light. Nylon rip-stops are being used by many US and European manufactures.
Polyester pack cloth has a higher resistance to ultra-violet (UV) degradation but are not as strong as nylon or ripstop nylon materials. This material is use in many of the packs made in Europe and Australia.
A is an ultra-high molecular weight polyethylene fiber made by Honeywell with a very high strength-to-weight ratio. It is highly resistant to abrasion and does not absorb very much water. It has resistance to UV light similar to polyester. This fabric is very expensive and comes in white (but can be tinted).
Kelty Cloud
This material is mostly used in ropes and at the ripstop material in nylons (identified by the white grid pattern). Since it is an expensive material, few manufactures have used full Spectra fabric for pack making.
Most backpacks use 6 to 10 stitches per inch. Many feel that with the new tightly woven fabrics used today, using more stitches than 10 per inch will damage the fabric thread while anything less than 6 stitches per inch isn't strong enough for heavy duty use.
Twin stitching is of course stronger than single stitching adding will increase the life of zipper attachment to your pack.
Another important factor in pack construction and repair is thread use. Inexpensive packs may actually use good quality fabrics but will fall apart under loads due to use of poor quality or inappropriate thread.
See Backpack Sewing for more information about backpacking gear sewing/repairs and Backpack Sewing Thread Size for thread size information.
Zippers are an important part of most packs can have a dramatic affect on pack usability and longevity. See Backpack Zippers for more information.
Foam is often used in pack construction for padding for the wear's back, shoulder and hips. It may also be used to protect contents of the pack and add a little support.
Open cell foam is made up of interconnected air chambers throughout the material, which allows it to be extremely soft and highly compressible. It is used in self-inflating mattresses since it is light and highly compressible. These may not be ideal for pack use and they will collapse under minimum loads. You can determine if padding in your pack is open cell foam if you can compress it to less than half its size with your fingers.
Closed cell foam such as Ensolite is made up of open air chambers encapsulated by foam that are not interconnected. This foam does not compress easily yet provides good padding. These are used in closed cell foam mats and provides excellent insulation and absorbs very little water. Because this foam is dense yet comfortable and doesn't absorb much perspiration, it is a great choice for backpack padding.
As the name implies, it has both closed cell and open cell foam are combined together. This allow for the soft feeling of open cell foam against your pressure points without losing the better load bearing qualities of closed cell foam. High quality dual density foam is more comfortable than either open or closed cell foam alone, supporting the load well and providing comfort for body.
Compression Molded Foam is a different manufacturing technique for closed cell foam. Through a special process that incorporates heat, foam is custom molded into varous shapes and often combined with different densities of foam to provide flexibility to the final shape.
Side Release Buckle
Acetal (aka polyoxymethylene, POM, acetal resin, polytrioxane and polyformaldehyde)
Acetal has great stiffness and tensile strength and is often used in the male half of side releases buckles. It is able to resist moisture absorption, and performs very well under freezing or arid conditions. This resin also has excellent flexural memory and property retention when subjected to repeated loading. Compared to Nylon, acetal has better fatigue resistance, creep resistance, stiffness, and water resistance, but lower impact strength and abrasion resistance.
Delrin is brand name for a DuPont acetal resin engineering plastic.
Nylon (Polyamide)
Nylon is a thermoplastic polyamide resin that offers great impact resistance under normal operating temperatures and has great resistance to repeated impact loads. It is light, flame retardant and has a softer feel for side release buckles. It can obtain a 20-30% increase in tensile strength over acetal and is often used in the female part of side release buckles. Since nylon is a hydroscopic resin, its physical properties will vary as the relative humidity changes. When dry, tensile strength is higher and impact strength is lower. When wet, tensile strength is lower and impact strength is higher. And under freezing conditions, nylon can become embrittled.
Polypropylene is cheap and is used where load bearing and impact are minimal. It also has a very soft feel for side release buckles.
Steel is still used for hardware that requires a great deal of strength even in sub freezing temperatures. Stainless steel is preferred over regular steel/iron since is does not corrode.
Although rare, there are a few plastic/metal hardware applications.
Aluminum is not very strong compared to steel and plastic and corrodes. It is used in backpack frames (internal and external).
Brass is still used in grommets and snaps. It is soft, malleable, relatively resistant to significant corrosion and easy to work with.
ITW-Nexus-Fastex made the original "hammer proof" plastic hardware and is still one of the better durable plastic hardware companies out there. Most of their side release buckles are now constructed with acetal, but most of their hardware is still available in nylon for special needs.
National Moldings has a selection of Duraflex acetal quick release buckles that are durable with a nice texture and are smooth locking. National Molding buckles are often compatible with Nexus Fastex buckles.
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