Application of Technical Textiles in Everyday Life

Waterproof Fabrics

Waterproof breathable fabrics are designed for use in garments that provide protection from the weather, that is from wind, rain and loss of body heat. Clothing that provides protection from the weather has been used for thousands of years. The first material used for this purpose was probably leather but textile fabrics have also been used for a very long time. The waterproof fabric completely prevents the penetration and absorption of liquid water, in contrast to water-repellent (or, shower-resistant) fabric, which only delays the penetration of water.

Traditionally, the fabric was made waterproof by coating it with a continuous layer of impervious flexible material. The first coating materials used were animal fat, wax and hardened vegetable oils. Nowadays synthetic polymers such as polyvinylchloride (PVC) and polyurethane are used. Coated fabrics are considered to be more uncomfortable to wear than water-repellent fabric, as they are relatively stiff and do not allow the escape of perspiration vapour. Consequently, they are now used for ‘emergency’ rainwear. Water-repellent fabric is more comfortable to wear but its water-resistant properties are short-lived.

The term ‘breathable’ implies that the fabric is actively ventilated. This is not the case. Breathable fabrics passively allow water vapour to diffuse through them yet still prevent the penetration of liquid water.1 Production of water vapour by the skin is essential for the maintenance of body temperature. The normal body core temperature is 37°C, and skin temperature is between 33 and 35°C, depending on the conditions. If the core temperature goes beyond critical limits of about 24 °C and 45°C then death results. The narrower limits of 34 °C and 42 °C can cause adverse effects such as disorientation and convulsions. If the sufferer is engaged in a hazardous pastime or occupation then this could have disastrous consequences.

During physical activity, the body provides cooling partly by producing insensible perspiration. If the water vapour cannot escape to the surrounding atmosphere the relative humidity of the microclimate inside the clothing increases causing a corresponding increased thermal conductivity of the insulating air, and the clothing becomes uncomfortable. In extreme cases hypothermia can result if the body loses heat more rapidly than it is able to produce it, for example when physical activity has stopped, causing a decrease in core temperature. If perspiration cannot evaporate and liquid sweat (sensible perspiration) is produced, the body is prevented from cooling at the same rate as heat is produced, for example during physical activity, and hyperthermia can result as the body core temperature increases.

If the body is to remain at the physiologically required temperature, clothing has to permit the passage of water vapour from perspiration at the rates under the activity conditions. The ability of a fabric to allow water vapour to penetrate is commonly known as breathability. This property should more scientifically be referred to as water vapour permeability. Although perspiration rates and water vapour permeability are usually quoted in units of grams per day and grams per square metre per day, respectively, the maximum work rate can only be endured for a very short time.

During rest, most surplus body heat is lost by conduction and radiation, whereas during physical activity, the dominant means of losing excess body heat is by evaporation of perspiration.

Thus, waterproof breathable fabrics prevent the penetration of liquid water from outside to inside the clothing yet permit the penetration of water vapour from inside the clothing to the outside atmosphere.

Types of waterproof breathable fabric

There are several methods that can be used to obtain fabrics that are both breathable and waterproof. These can be divided into three groups:

• Densely woven fabrics
• Membranes

Densely woven fabrics 

Probably the first effective waterproof breathable fabric was developed in the 1940s for military purposes and is known as Ventile. The finest types of long-staple cotton are selected so that there are very small spaces between the fibres. The cotton is processed into combed yarn, which is then plied. This improves regularity and ensures that the fibres are as parallel as possible to the yarn axis and that there are no large pores where water can penetrate. The yarn is woven using an Oxford weave, which is a plain weave with two threads acting together in the warp. This gives minimum crimp in the weft, again ensuring that the fibres are as parallel as possible to the surface of the fabric.

When the fabric surface is wetted by water, the cotton fibres swell transversely reducing the size of the pores in the fabric and requiring very high pressure to cause penetration. The fabric is thus rendered waterproof without the need for any water-repellent finishing treatment. It was first made for military applications but the manufacturers are now producing a range of variants to widen the market appeal. The military variants use thread densities as high as 98 per cm. Fabric for other applications uses much lower thread densities, necessitating a water-repellent finish to achieve waterproof properties.

Densely woven fabric can also be made from synthetic microfilament yarns. The individual filaments are less than 10mm in diameter so that fibres with very small pores can be engineered. Microfilaments are usually made from polyamide or polyester. The latter is particularly useful as it has inherent water-repellent properties. The water penetration resistance of the fabric is improved by the application of silicone or fluorocarbon finish.

Membranes

Membranes are extremely thin films made from a polymeric material and engineered in such a way that they have a very high resistance to liquid water penetration, yet allow the passage of water vapour. A typical membrane is only about 10mm thick and, therefore, is laminated to a conventional textile fabric to provide the necessary mechanical strength. They are of two types, microporous and hydrophilic.

Microporous membranes

The first and probably the best known microporous membrane, developed and introduced in 1976 by W Gore, is known as Gore-Tex. This is a thin film of expanded polytetrafluoroethylene (PTFE) polymer claimed to contain 1.4 billion tiny holes per square centimetre. These holes are much smaller than the smallest raindrops (2–3mm compared with 100mm),10 yet very much larger than a water vapour molecule (40 ¥ 10-6mm).

The hydrophobic nature of the polymer and small pore size requires very high pressure to cause water penetration. Contamination of the membrane by various materials including body oils, particulate dirt, pesticide residues, insect repellents, suntan lotion, salt and residual detergent and surfactants used in cleaning have been suspected of reducing the waterproofing and permeability to water vapour of the membrane. For this reason, microporous membranes usually have a layer of hydrophilic polyurethane to reduce the effects of contamination. Figure 3.1 is a schematic diagram of a fabric incorporating a microporous membrane.

 

 

Hydrophilic membranes

 Hydrophilic membranes are very thin films of chemically modified polyester or polyurethane containing no holes which, therefore, are sometimes referred to as non-poromeric. Water vapour from perspiration is able to diffuse through the membrane in relatively large quantities. The polyester or polyurethane polymer is modified by incorporating up to 40% by weight of poly(ethylene oxide). The poly(ethylene oxide) constitutes the hydrophilic part of the membrane by forming part of the amorphous regions of the polyurethane polymer system. It has a low energy affinity for water molecules which is essential for the rapid diffusion of water vapour. These amorphous regions are described as acting like intermolecular ‘pores’ allowing water vapour molecules to pass through but preventing the penetration of liquid water owing to the solid nature of the membrane

 

Methods of incorporation

Membranes have to be incorporated into textile products in such a way as to maximize the high-tech function without adversely affecting the classical textile properties of handle, drape and visual impression. There are four main methods of incorporating membranes into textile articles. The method employed depends on cost, required function and processing conditions:

1. Laminate of membrane and outer fabric– The membrane is laminated to the underside of the outer fabric to produce a two-layer system. This method has the disadvantage of producing a rustling, paper-like handle with the reduced aesthetic appeal but has the advantage of having very effective protective properties of wind resistance and waterproofing. This method is mainly used for making protective (Fig – 3.2)
2. Liner or insert processing – The membrane is laminated to a lightweight knitted material The pieces are cut to shape from this material, sewn together and the seams are rendered waterproof with special sealing tape. This structure is then loosely inserted between the outer fabric and the liner. The three materials (outer, laminate and lining) are joined together by concealed stitch seams. If high thermal insulation is required then the lightweight support for the membrane is replaced by a cotton, wool or wadding fabric. This method has the advantage of giving a soft handle and good drape. The outer fabric can also be modified to suit fashion demands.
3. Laminate of membrane and lining fabric– The laminate is attached to the right side of the lining material. The functional layer is incorporated into the garment as a separate layer independent of the outer fabric. This method has the advantage that the fashion aspects can be maximized
4. Laminate of outer fabric, membrane and lining– This produces a three-layer system, which gives a less attractive handle and drape than the other methods and, therefore, is not commonly used.

Coatings

These consist of a layer of polymeric material applied to one surface of the fabric. Polyurethane is used as the coating material. Like membranes, the coatings are of two types; microporous and hydrophilic. These coatings are much thicker than membranes.

Microporous coatings

Microporous coatings have a similar structure to the microporous membranes. The coating contains very fine interconnected channels, much smaller than the finest raindrop but much larger than a water vapour molecule. (Fig- 3.3)

• Wet coagulation: Polyurethane polymer is dissolved in the organic solvent dimethyl formamide to produce a solution insoluble in water. This is then coated onto the fabric. The coated fabric is passed through a conditioning chamber containing water As the organic solvent is miscible with water, it is diluted and solid polyurethane precipitates. The fabric is then washed to remove the solvent, which leaves behind pores in the coating. Finally, the coated fabric is mangled and dried. This method is not very popular as it requires high capital cost for machines and solvent recovery is expensive.
• Thermocoagulation: Polyurethane is dissolved in an organic solvent and the resulting solution is mixed with water to produce an emulsion. The emulsion ‘paste’ is coated onto one side of the fabric. The coated fabric then goes through a two-stage drying The first stage employs a low temperature to remove the organic solvent, precipitating the polyurethane. The coating is now a mixture of solid polyurethane and water. The second stage employs a higher temperature to evaporate the water leaving behind pores in the coating.
• Foam coating: A mixture of polyurethane and polyurethane/polyacrylic acid esters are dispersed in water and then foamed. The foam is stabilised with the aid of additives. The foam is then coated onto one side of the fabric. The coated fabric is dried to form a microporous coating. It is important that the foam is open cell to allow penetration of water vapour but with small enough cells to prevent liquid water penetration. The fabric is finally calendered under low pressure to compress the coating. As the foam cells are relatively large, a fluorocarbon polymer water-repellent finish is applied to improve the water-resistant

Hydrophilic coatings

Hydrophilic coatings use the same basic water vapour permeability mechanism as the hydrophilic membranes. The difference between microporous materials and hydrophilic materials is that with the former, water vapour passes through the permanent air-permeable structure whereas the latter transmit vapour by a molecular mechanism involving adsorption–diffusion and desorption. These coatings are all based on polyurethane, which has been chemically modified by incorporating polyvinyl alcohols and polyethylene oxides. These have a chemical affinity for water vapour allowing the diffusion of water vapour through the amorphous regions of the polymer. The balance between hydrophilic and hydrophobic components of the polymer system has to be optimised to give acceptable vapour permeability, flexibility, durability and insolubility in water and dry cleaning solvents. Swelling of the membrane is encouraged to assist water vapour diffusion yet it also has to be restricted to prevent dissolution or breakdown in water or in the other solvents with which the polymer is likely to come into contact. Poly(ether–urethane) coatings and membranes have excellent integrity. This can be conferred in two ways:

1. by a high degree of hydrogen bonding, principally between polar groups in the hydrophobic segments of adjacent polymer chains
2. by forming covalent crosslinks between adjacent polymer chains. The effective length and density of the crosslinks are variables affecting polymer swelling and thus vapour permeability

 

Methods of applying coatings

The conventional method of applying coatings to fabric is to use the direct application using the knife over roller technique. The fabric is passed over a roller and liquid coating is poured over it. Excess liquid is held back by a ‘doctor blade’ set close to the surface of the fabric. The thickness of the coating is determined by the size of the gap between the blade and the surface of the fabric. The coated fabric is passed through a dryer to solidify the coating. Sometimes the coating is built up in several layers by a number of applications. In order to achieve thinner coatings and, therefore, more flexible fabric and to apply a coating to warp knitted, nonwoven, open weave and elastic fabric, transfer coating is used. The liquid coating is first applied to a silicone release paper using the knife over roller technique. This is then passed through an oven to solidify the coating. A second coating is then applied and the textile fabric is immediately applied to this. The second coating, therefore, acts as an adhesive. This assembly is passed through an oven to solidify the adhesive layer. The coated fabric is stripped from the release paper, which can be reused.

Applications of Water Proof fabrics

(Table 3.1) Applications of waterproof breathable fabrics