A waterproof garment is only as weatherproof as its seams

A waterproof garment is only as weatherproof as its seams

Fabrics, and the processes of joining them together, continue to evolve. One of the key factors that can deliver successful seam sealing and thermal bonding in textiles is the process of temperature control, as Ian Collins, product manager and strategic account manager at West Controls Solutions, explains.

By most estimates, humankind had been joining pieces of fabric together the same way for around 25,000 years - perhaps longer - before a significant change came about in the early nineteenth century when the world’s first practicable sewing machine was invented.

Textiles too have a history. While the earliest use of sewing was to stitch together animal hides, weaving and spinning have been around for almost as long. For the large majority of that time, the base materials used were natural – cotton, wool and silk. However, around half a decade after the invention of the first viable sewing machine, ‘artificial silk’ – which became known as rayon – caused a sensation at the Paris Exhibition. In 1891, commercial production of the new fabric began.

Rayon, however, was still plant-based. It wasn’t better, just significantly cheaper than natural fibres such as silk. But in 1939, DuPont began production of nylon – a textile derived from petrochemicals that was in many ways the precursor of many of the man-made fabrics we know today such as polyester, Kevlar and Nomex.

Functional capabilities
The latter two are representative of the rise over the past few decades of so-called 'technical textiles': materials developed and chosen, not for their aesthetic appeal, but for their functional capabilities. Kevlar is designed to be bulletproof, while the functional property of Nomex is that it is both fire- and electricity proof.

In clothing, that functionality ranges from industrial, such as protective garments for firefighters, to personal, such as active sportswear with leggings, for instance, prized for their wicking, quick drying and anti-bacterial properties.

The advent of technical/functional textiles saw the need arise for different ways of joining fabric together – ways that would eliminate sewing disadvantages. Sewing, for example, creates raised seams. In sportswear and underwear, raised seams are deemed undesirable either because of chafing (close fitting active wear, such as running or cycling apparel, needs to feel as smooth as possible when worn next to the skin) in the former, or to create a smoother, invisible outline in the latter.

Creating visible seams is only one of the disadvantages of sewing as a method of joining pieces of fabric together. Another is that, by definition, it creates holes in the fabric – even in a supposedly watertight fabric, thus reducing its efficiency. A tent, for example, is only as weatherproof as its seams. Sewn seams are also, typically, the weak point of any structure that sees fabrics joined together.

There are differences of opinion, but there seems to be some consensus that the first appearance of thermal bonding of materials occurred in the 1940s. Since then, there has been growing interest in the approach.

Welded seams already feature in clothing from consumer brands such as Patagonia and North Face, and the US Navy was reportedly considering turning to welding to make its uniforms lighter, stronger, cheaper and waterproof.

Temperature control
For manufacturers, however, whether the application is welding/thermal bonding or seam sealing, one of the biggest challenges is the accurate control of temperature within the process. If the temperatures are too low, the materials will not adhere. But if the temperatures are too high there is the risk of either fabric damage or liquidity of the adhesive.

What’s more, the requirements of each application vary considerably. For example, it is generally recommended that, for one- and two-layer tapes, the starting position should be 400°C, while for three-layer tapes, 550°C is recommended.

In a typical hot air sealing machine, the placement of the hot air nozzle in relation to the rollers through which the seam tape and fabric pass is critical in ensuring the delivery of the correct temperature. The heat delivered to the hot melt layer of the tape is a function of the nozzle temperature and the nozzle air flow, either of which can be adjusted to achieve the desired result. Some machines, of course, are fitted with a heated upper roller.

In welding – whether hot air or hot wedge – tight control of temperature is equally essential, usually in a range between 200°C and 750°C. Speed and pressure are the other two variables that need to be controlled. There will also be differences according to whether polyvinylchloride (PVC), polyurethane (PU), polyethylene fabric (PE) or polypropylene (PP) is the fabric of choice.

Where breathable TPE (thermoplastic elastomer) films are used, the melting temperature is strongly determined by the type of hard segments present. A high melt temperature is desirable for thermal stability during high temperature processes such as seam sealing. PA12 – polyamide with 12 carbon units in its main molecular repeating unit – and most TPU hard segments provide melting temperatures of 175°C and below, whereas use of some polyester-based TPEs leads to melting points of 200°C.

Measure, then control
As with any industrial application, before you can control it you need to be able to measure it. That’s certainly the case for temperatures in seam sealing and thermal bonding.

Many temperature controllers are designed for more sophisticated applications. In the appropriate environment, there is no substitute – but by virtue of their sophistication, they can be inevitably complex and often expensive. One more cost-effective solution is the MAXVU temperature controller for applications where primary temperature control functions such as heat only or heat/cool are needed.

Seamless and stitch-free seaming technologies are still some way away from replacing sewing as mankind’s long-preferred method of joining fabrics together. Progress over the past few years has, however, been rapid. The time may not be far off when sewing is looked on in the same way we look back on writing with quill pens.