Fabric surface modification is a novel technique by coating and lamination which can improve structural performances. It provides the opportunities to manufacturer the special fabrics like water-proof resistant tarpaulins, coverings, large tents and architectural uses, back coating for upholstery including auto seats, food, medical applications, parachutes, woven curtains, for heat-sensitive fabrics, automotive fabrics, disposable hospital apparel etc. the recent developments also enhanced the lamination and coating technique into state-of-art process of the future in textile field.
Coating: Polymer or elastomer, usually in viscous form, is applied directly to the fabric and cured. A variety of techniques are used. A bond-coat (adhesive) may or may not be used.
Laminating: A pre-made or extruded film is bonded onto the substrate, generally with thermal or adhesive bonding. Curing is generally not required.
Coating and laminating can involve virtually every textile form:
- Fabrics (woven, knit or nonwoven)
- And many polymers/elastomers: Rubbers of all types (natural and synthetic), acrylic, vinyl, urethane, silicone, PTFE,… the list goes on and on.
Many techniques are used:
- Yarn coating
- Spread coating – many variants
- Extrusion coating/laminating
- Film to substrate bonding
Knife Coating (floating knife) or Direct Coating
In Knife Coating, as seen in Figure 3, the liquid coating is applied to the fabric while being run at tension under a floating knife blade, the distance between the fabric and the knife blade determines the thickness of the coating. The blade can be angled and have different profiles to affect the coverage. For this process to be effective the liquid coating must be quite viscous in order to prevent it soaking through the fabric, the coating is then dried or cured.
This technique is best used for Filament yarns as the staple fibres in spun yarns can protrude on the surface creating an uneven finish, but this is dependent upon the thickness of the applied coating. For this type of coating to be most successful the weave structure has to be quite tight and the fabric capable of being held taught.
In this process, the coating liquid is rolled onto the fabric by a roller suspended in the coating solution, often a blade is positioned close to the roller to ensure not too much coating solution is applied.
Also referred to as Padding, this technique, widely regarded as a textile finishing technique, can, in fact, be used to add a variety of coatings, but this usually refers to a fibre coating for the application of micro or Nanomaterials or chemical compositions.
As shown in figure 5, the fabric is submerged in the coating solution then the excess squeezed out of the rollers, which dictates the pick-up percentage, the fabric is then dried and cured.
Properties, process, history, and application of carbon fibres
Calendar finishing involves the fabric passing through a set of heated rollers to singe off any surface fibres and add lustre and smoothness. Calendar coating is the same principle in which the fabric passes through heated rollers, but through this process, a coating is applied as demonstrated in figure 6.
This image demonstrates the simultaneous coating of both sides of the fabric with the thickness of the coating determined by the width of the nip in-between the rollers, more rollers used can provide a thinner coating.
Hot melt extrusion coating
Hot melt extrusion coating is applied in the same process as calendaring with the coating being melted from granules fed to heated rollers which then nip the coating on the fabric. It is used to produce un-supported films and these freshly produced films are added directly to the fabric. Its uses are mainly for Thermoplastic polymers such as Polyurethane, Polyolefins and PVC.
Foam finishing was developed as a more environmentally friendly version of the pad-dry-cure system, as the chemical applied requires less product in weight, but equates to a high surface area. Foam also ensures less wetting takes place, which requires less drying; furthermore, waste is reduced in terms of residual liquor. This technique is useful in coating heavy fabrics such as carpets and can be used to effectively coat only one side.
Laminated textiles consist of one or more layers of textile and component. The Textile Institute defines a laminated or combined fabric as:
A material composed of two or more layers, at least one of which is a textile fabric, bonded closely together by means of an added adhesive, or by the adhesive properties of one or more of the component layers
This adhesive is required to bond the fabric and component layers together. Creating a strong bond, which will not deteriorate through conditions experienced in use such as movement and laundering, is not the biggest challenged faced.
Adhesives are often associated with making the fabric too rigid and thus affecting the handle, which is often a negative characteristic, particularly for applications in performance clothing where comfort is a requirement. Environmental consideration has led towards more interest in hot melt adhesives, rather than solvent based adhesive, or the use of flame adhesion.
Laminated fabrics are widely used in high-performance apparel where fabrics are required to be waterproof yet breathable. In this case, a laminate membrane Laminates often consist of a non-textile membrane sandwiched between 2 textiles, for example in the case of the microporous membrane Gore-Tex.
Usually the reverse or technical back of the fabric surface is laminated, so as to not affect the look of the fabric, and in the case of the Hydrophilic membranes, these are more effective worn close to the body. As in the gore-tex example, the membrane or laminate is often sandwiched between two fabric layers. However, this is not the case for fashion fabrics where the look is the priority over function. Lamination is carried out on the fabric surface to produce some visually interesting designs such as foil holograms or textures.
Lamination is widely used in garment manufacture where woven or non-woven fabrics are pre-prepared with thermoset adhesive. These are then cut and applied to the fabric as part of the manufacturing process to provide reinforcement, for example of a buttonhole, or to give shape and stability, for example in a collar. These fusible are applied under heat and pressure for a specified time, to set the thermos adhesive.
This technique is mainly used to attach the foam to a textile fabric, which is widely used in automotive. As displayed in figure 9 the foam is presented to the flame, which encourages melting; as it then dries it bonds the textiles. This technique has associated health and safety risks due to the release of gases when melting takes place.
There are two processes involved in hot melt lamination, the application of a thermoset adhesive, and the fusing of the fabric and the non-textile component through heat and pressure. The adhesive is applied either to the whole surface or discreet, where the adhesive is applied as a thermoset, which just attaches in patterns, such as glue dots. Discreet provides greater flexibility due to the reduced contact areas.