Modern packs are usually mostly made of either nylon or canvas. Each of these has strengths and weaknesses depending on the conditions in which they are used.
As a generalisation, Australian conditions are quite tough on packs: there is a lot of harsh scrub, strong sunlight and wet conditions, all of which require a durable, water-resistant fabric.
Traditional canvas is a durable fabric of tightly woven polyester and cotton fibres. Although there are many different types and qualities of canvas used in packs, the basic idea is that the polyester content gives strength while the cotton contributes durability. The cotton also swells when wet, filling gaps between the fibres and making it hard for water to penetrate.
Canvas comes in two different yarn constructions, ‘core-spun’ and ‘blended’. Although blended canvas is cheaper and more common, it is not as strong or durable as equivalent weight core-spun canvas. Cotton fibres are naturally short, while polyester filaments are made in continuous lengths, of whatever length you like. To create blended yarns, the polyester filaments are chopped into short lengths, mixed with cotton, and then this mix is spun into a yarn. Core-spun yarn is made by using a continuous core of polyester filaments, with a sheath of cotton fibres spun around it. The core-spun yarn has a continuous polyester core that aligns along the yarn, giving it a much higher strength than an equivalent blended yarn.
A huge range of nylon fabrics are used in pack manufacture, with high-quality fabric giving a good performance. Nylon is cheaper than canvas, giving obvious price advantages. It has a much looser weave and depends on a coating or laminate (usually polyurethane) for stability and water resistance. This coating wears out over time, so the fabric will not last as long as canvas when subjected to the abrasion and harsh UV of the Australian bush, nor will it offer the same long-term water resistance. Nylon reinforcing is often used as an extra layer over canvas and acts as an extra abrasion barrier in high-wear areas.
Binding: canvas and synthetics
The seams of a pack are a potential water-entry point as they interrupt the continuous barrier of the fabric. To combat this, binding tapes are sewn over the main internal seams.
Tough fabric alone does not guarantee durability: if the stitching is weak, the pack can fall apart at the seams. Special stitches called bar tacks (also used in rockclimbing slings – they are strong!) are used in key areas subject to a lot of strain. Twin rows of stitching are obviously stronger than single rows, all else being equal.
The quality of stitching is also important – even stitching is a good sign, while the use of small needles in the seams can increase water resistance in this area.