Application of Technical Textiles in Everyday Life

Fabrics for Heat and Flame Protection

With industrialisation, the safety of human beings has become an important issue. A growing segment of the industrial textiles industry has therefore been involved in a number of new developments in fibres, fabrics, protective clothing. Major challenges to coatings and fabrication technology for production in the flame retardant textile industry have been to produce environmentally friendly, non-toxic flame-retardant systems that complement the comfort properties of textiles. The 1990s, therefore, saw some major innovations in the development of heat-resistant fibres and flame-protective clothing for firefighters, foundry workers, military, aviation and space personnel, and for other industrial workers who are exposed to hazardous conditions.

For heat and flame protection, requirements range from clothing for situations in which the wearer may be subjected to occasional exposure to a moderate level of radiant heat as part of his/her normal working day to clothing for prolonged protection, where the wearer is subject to severe radiant and convective heat, to direct flame, for example, the firefighter’s suit. In the process of accomplishing flame protection, however, the garment may be so thermally insulative and water vapour impermeable that the wearer may begin to suffer discomfort and heat stress. Body temperature may rise and the wearer may become wet with sweat.

What constitutes flammability?

Ease of ignition, rate of burning and heat release rate are the important properties of textile materials that determine the extent of fire hazards. The other factors that influence the thermal protection level include melting and shrinkage characteristics of synthetic fibre fabrics, and emission of smoke and toxic gases during burning. So, while selecting and designing flame protective clothing, the following points should be kept in mind:

• the thermal or burning behaviour of textile fibres
• the influence of fabric structure and garment shape on the burning behaviour
• selection of non-toxic, smoke-free flame-retardant additives or finishes
• design of the protective garment, depending on its usage, with comfort properties
• the intensity of the ignition source
• the oxygen

Thermal behaviours of fibres

The effect of heat on a textile material can produce physical as well as chemical changes. In thermoplastic fibres, the physical changes occur at the second-order transition (Tg), and melting temperature (Tm), while the chemical changes take place at pyrolysis temperatures (Tp) at which thermal degradation occurs. Textile combustion is a complex process that involves heating, decomposition leading to gasification (fuel generation), ignition and flame propagation.

A self-sustaining flame requires a fuel source and a means of gasifying the fuel, after which it must be mixed with oxygen and heat.

When a textile is ignited, heat from an external source raises its temperature until it degrades. The rate of this initial rise in temperature depends on the specific heat of the fibre, its thermal conductivity and also the latent heat of fusion (for melting fibres) and the heat of pyrolysis.

In protective clothing, it is desirable to have a low propensity for ignition from a flaming source or, if the item ignites, a slow fire spread with low heat output would be ideal. In general, thermoplastic-fibre fabrics such as nylon, polyester fibre, and polypropylene fibres fulfil these requirements because they shrink away from flame and, if they burn, they do so with a small slowly spreading flame and ablate. For protective clothing, however, there are additional requirements, such as protection against heat by providing insulation, as well as high dimensional stability of the fabrics, so that, upon exposure to the heat fluxes that are expected during the course of the wearer’s work, they will neither shrink nor melt and if they then decompose, form char.

The above-mentioned requirements cannot be met by thermoplastic fibres and so recourse must be made to one of the so-called high-performance fibres such as aramid fibre (e.g. Nomex, DuPont), flame-retardant cotton or wool, partially oxidised acrylic fibres, and so on. It may also be noted that the aramid fibres, in spite of their high oxygen index and high thermal stability, have not been found suitable for preventing skin burns in molten-metal splashes because of their high thermal conductivity.

Selection of Fibers suitable for thermal and Flame Protection

Selection of fibres suitable for thermal and flame protection. The fibres could be classified into two categories:

• Inherently flame-retardant fibres, such as aramid, modacrylic, polybenzimidazole (PBI), Panox (oxidised acrylic) or semi carbon, phenolic, asbestos, ceramic etc.
• Chemically modified fibres and fabrics, for example, flame retardant cotton, wool, viscose and synthetic fibres.

Inherently flame-retardant fibres

For some 2000 years, there was only one type of naturally occurring mineral fibre, asbestos which could not be completely destroyed by fire. Asbestos has many desirable properties and is cheap as well. However, the fibres are so fine that they can be breathed into the lungs and can promote fatal cancerous growth.

Glass fibres are also heat-resistant materials. In earlier times such fibres were used for printed circuit boards. Now developments in the texturing of glass fibres have provided a material that could substitute for the asbestos fibres to some extent. Unlike asbestos fibres, glass fibres with high diameters are non-respirable. They have an upper-temperature resistance of about 450°C. They spin well, knit or braid easily and can be coated with rubber, polyacrylate or silicones.

Glass fibres have also good electrical and insulation properties. However, they cause skin irritation, which limits their application in protective clothing. Silica-based fibres have high rates of thermal conductivity, a property that may be valuable in heat dissipation in some uses but in situations like hot metal splashes, where the heat is transmitted to the person by conduction, they will cause more burn injuries instead of protecting the skin. Thus, the selection of the fibre for making thermally protective clothing should be decided on the basis of the environment to which a worker is exposed, namely, whether the heat will be transmitted to the person by conduction, convection or radiation. Despite their high-temperature resistance, ceramic fibres have poor aesthetic characteristics, high densities and are difficult to process.

Aramids

Aromatic polyamides such as poly(metaphenylene isophthalamide) char above 400 °C and may survive short exposures at temperatures up to 700°C. Nomex (DuPont), Conex (Teijin), Fenilon (Russian) and Apyeil (Unitika) meta-aramid fibres have been developed for protective clothing for fighter pilots, tank crews, astronauts and those working in certain industries. Para-aramid fibres like Kevlar (DuPont),Twaron (Akzo Nobel) and Technora (Teijin) are also being used for ballistic and flame protection. Nomex nonwovens are used for hot gas filtration and thermal insulation.

Aramids are resistant to high temperatures, for example at 250 °C for 1000 hours the breaking strength of Nomex is about 65% of that before exposure. They begin to char at about 400 °C with little or no melting. Generally, meta-aramids are used in heat protective clothing, however, in intense heat, Nomex III (a blend of Nomex and Kevlar 29 (95 : 5 by wt) is preferred, in order to provide a greater mechanical stability to the char. Teijin23 has introduced a new fabric, X-fire, a combination of Teijin Conex (meta-aramid) and Technora (para-aramid) fibres. This fabric is capable of resisting temperatures up to 1200 °C for 40–60 s.

Nomex can also be blended with FR fibres, for example, FR wool and FR viscose. Karvin (DuPont) is a blend of 30% Nomex, 65% FR viscose and 5% Kevlar. Kevlar blends were formerly used by Firotex Co. UK (now defunct) with partially carbonized viscose in fabric form. This blend was developed as a fire blocking fabric for aircraft seats but found little favour because of the poor abrasion resistance of the carbonised viscose component.

Other examples of such blends include Fortafil and Fortamid needle felt NC580, which comprise aramid and FR viscose. This material is useful for gloves and mittens in which temperatures may reach up to 350°C. The outer working surface of the aramid fibre is needled through a reinforcing polyester fibre scrim over an inner layer of FR viscose.

Another aromatic copolyamide fibre developed by Lenzing AG is P84. This fibre does not melt but becomes carbonized at temperatures in excess of 500 °C and has an LOI value of 36–38%. The basic fibre is golden yellow in colour but Lenzing AG offers it as spun material dyed in limited colours. P84 fibres have irregular crosssection, which provides a higher cover factor at lower weights of fabrics made from it. Its extensibility is >30% with a good knot and loop strength. The applications of high-performance P84 include protective clothing, as a sealing or packing material, for hot gas filtration and in aviation and space including covers for aircraft seats.

Poly(amide-imide) fibres

Rhone-Poulenc produces polyamide–imide fibre called Kermel. This is available in two forms: 234 AGF and 235 AGF. Type 234 is a staple fibre for use in both cotton and worsted spinning systems and is produced in five spun-dyed colours. Type 235 is intended for nonwovens applications. In France, Kermel is used by firefighters and military personnel where the risk of fire is higher than usual. Its LOI is 31–32%, and it resists up to 250 °C exposure for a long duration. At 250°C after 500 hours of exposure, the loss of mechanical properties is only 33%. Kermel fibre does not melt but carbonises. During its carbonization, it generates very little opacity. Blends of 25–50% Kermel with FR viscose offer resistance to ultraviolet (UV) radiation and price advantage also compared with 100% Kermal fabrics. Blending with 30–60% wool also produces more comfortable woven fabrics with enhanced drapes. In the metal industry, the 50: 50 blend gives very good results, but a 65:35 Kermel/viscose blend is preferred for such applications. Kermel-based fabrics are now used both on-shore and off-shore by leading petrochemical groups. The army, navy and airforces are also using Kermel in woven and knitted forms.

Polybenzimidazole (PBI) fibres

Celanese developed PBI,28,30,31 a non-combustible organic fibre. Its LOI is 41% and it emits little smoke on exposure to flame. PBI can withstand temperatures as high as 600 °C for short-term (3–5 s) exposures and longer-term exposure at temperatures up to 300–350°C. It provides the same protection as asbestos while weighing half as much. It also absorbs more moisture than cotton. The current area of interest in PBI is in the replacement of asbestos-reinforced rubbers used in rocket motors and boosters to control ignition. Its other applications include fire blocking fabrics in aircraft seats, firefighter suits and racing-car driver suits.

Ballyclare Special Products, UK31 has recently developed a fire-resistant garment assembly for firefighters’ safety. The outer fabric of the garment is made from Pbi Gold(R), a fire-resistant fabric from Hoechst Celanese. This fabric, which was originally developed for the US Apollo space programme, combines the comfort, thermal and chemical resistance of polybenzimidazole (PBI) with the strength of aramid fibre. Pbi Gold is stable even under simulated flash conditions at 950°C. The fabric is also resistant to puncturing, tearing and ripping.

Poly(phenylene sulphide) PPS fibres

 Ryton (Sulfar) fibres (Tm 285 °C) produced by Amoco Fabrics and Fibres are nonflammable. They do not support combustion under normal atmospheric conditions, and the LOI is 34–35%. Chemical resistance and the ability to retain physical properties under extremely adverse conditions make the fibre valuable for protective clothing.

Polyacrylate (Inidex)

Polyacrylate is a crosslinked copolymer of acrylic acid and acrylamide. Its LOI is 43%, and when subjected to flame, it neither burns nor melts. It emits virtually no smoke or toxic gases. Because of its low strength and brittleness, it can be used in nonwovens although the durability of fabrics made from this fibre may not be adequate for some apparel uses.

Semicarbon/Panox fibres

These fibres are produced by thermal treatment (thermo-oxidative stabilization) of either viscose or acrylic fibres. Asgard and Firotex are produced from viscose while Panox, Pyromex, Fortasil, Sigratex and so on are made from acrylic precursors. The acrylic fibres can be oxidised in the fibre, filament or fabric form at 220– 270 °C in air, but the viscose fibres are generally partially carbonized in the fabric form in a nitrogen atmosphere.

These semi carbon fibres have excellent heat resistance, do not burn in air, do not melt and have outstanding resistance to molten metal splashes. After exposure to flame, there is no afterglow and fabrics remain flexible. In view of their outstanding properties, the Panotex fabrics (Universal Carbon Fibres) made from Panox (RK Textiles), for example, are ideal for use in protective clothing where protection against the naked flame is required. Currently, this range of fabrics is probably the most common and versatile of oxidised acrylic-based materials.

To prevent the transfer of radiant heat, Panotex fabrics may be aluminized. An aluminized Panotex fabric is thus suitable for fire-proximity work but not for fire entry. It has been demonstrated that with a heat flux of 3Wcm-2, an aluminium coating will ignite, but a stainless steel coating can withstand such a situation for a prolonged period. Multiple layers of Panotex fabric tend to protect a polyvinyl chloride (PVC)-simulated skin against irradiance as high as 170Wcm-2 applied for 2 s.

Another advantage of Panotex outer fabric is the shedding of burning petrol, and it can even withstand several applications of napalm.

Phenolic or novoloid fibres

Kynol is a well-established novoloid heat-resistant fibre that is produced by spinning and posturing phenol formaldehyde resin precondensate. The fibre is soft and golden coloured with moisture regain of 6%. When strongly heated, Kynol fabric is slowly carbonised with little or no evolution of toxic gases or smoke. However, its poor strength and abrasion properties preclude its application for making apparel. To upgrade its mechanical properties, Kynol fibres can be blended with Nomex or FR viscose to produce flame-protective clothing.

Another phenolic fibre, Philene has also been developed, for example, Philene 206 (0.9 den) and Philene 244 (2.1 den). The moisture regains of the fibre is 7.3% and is said to be non-flammable and self-extinguishing, with an LOI of 39%. It does not show any change in tensile properties after being heated for 24 hours at 140°C (or for 6 hours at 200 °C). A charred Philene fabric is claimed to form a thermal insulating barrier that retains its initial form.

Modacrylic

Flame-retardant modacrylic under different brand names, such as Velicren FR (Montefibre, Italy) and SEF (Solutia Inc.) is a copolymer of acrylonitrile, vinyl chloride or vinylidene chloride in the ratio of 60 : 40 (w/w) along with a sulphonated vinyl monomer. It has an LOI in the range of 26–31%.

Flame retardation of conventional textile fibres

FR viscose

Inherently flame-retardant viscose fibres are produced by incorporating FR additives/ fillers in the spinning dope before extrusion. For example, Sandoflam 5060 (Sandoz), polysilicic acid or polysilicic acid and aluminium (Sater).

Flame-retardant polyester

There are three methods of rendering synthetic fibres flame retardant:

• use of FR comonomers during copolymerization,
• introduction of an FR additive during extrusion,
• application of flame retardant finishes or

The first two methods would give inherently flame-retardant polyester fibres. Trevira CS(R) and Trevira FR(R) produced by Hoechst are flame-retardant polyesters. Both are manufactured by copolymerizing a bifunctional organophosphorus compound based on phosphinic acid derivative.

Flame-retardant nylon

Nylons have a self-extinguishing property due to extensive shrinking and dripping during combustion. Problems arise in blends with natural fibres like cellulosic which will char and form a supporting structure (the so-called scaffolding effect) which will then hold the molten polymer. Introduction of flame or combustion retarders into polyamide melts before spinning appears to be an economical and feasible process if they are stable.

Flame-retardant acrylic fibres

 Like other synthetic fibres, acrylic fibres shrink when heated, which can decrease the possibility of accidental ignition. However, once ignited, they burn vigorously accompanied by black smoke. Thus, many efforts have been devoted to improving the flame resistance of acrylic fibres.60–67 Among these studies, halogen-based and particularly bromine derivatives or halogen- or phosphorus-containing comonomers, are the most effective flame retardants used in acrylic fibres.

Flame-retardant finishes for polyester

 There have been some developments in flame retardant finishes for polyester fabric and its blends. Flame-retardant finishes for synthetic fibres should either promote char formation by reducing the thermoplasticity or enhance melt dripping so that the drops can be extinguished away from the igniting flame. For protective clothing, char forming finishes would be desirable.

Flame-retardant finish for wool

 Wool is not as flammable as cotton, and the wool fabric was the traditional material for thermal protection except for the more arduous conditions where asbestos was required. However, for thermal protective clothing a Zirpro(IWS) finish, based on hexafluorotitanates and hexafluorozirconates, has been developed, which is extremely stable in acid solutions and exhausts onto wool well below the boil. The Zirpro finish produces an intumescent char, which is beneficial for protective clothing, where thermal insulation is a required property of a burning textile.

Glass-fibre fabrics

In one finishing treatment, colloidal graphite was used, together with silicone oil, to provide protection at higher temperatures. Clothes treated in this way can be used at 400 °C or higher if exposure times are in minutes rather than days or in the absence of oxygen.

Another feature of glass fibre is that it melts at around 1000°C, so that in the untreated form, it is unsuitable for applications at higher temperatures. However, it can be treated to improve its resistance to such temperatures, by incorporating finely dispersed vermiculite and another involving aluminium salts. At high temperatures, the aluminium will react with the glass fibre to raise its melting point above 1500°C.

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