By P Sudhakar and V R Sam Path
Department of Textile Technology, K S Rangasamy College of Technology,
Tiruchengode 637 209, Tamil Nadu


Breathable fabrics constitute one of the major components of individual protective clothing used in high altitude conditions. Membranes have opened up entirely new possibilities for designing functional clothing.

Woven or knitted fabric can have desired properties such as insulation and fashion. Often these properties are not enough for daily-use in bad weather conditions. The rain proofness of the fabric has to be increased. This can be done by applying PVC, acrylics, silicones, polyurethanes or fluorocarbons to the fabric. However, by doing so, the fabric is sealed: water form outer-environment (rain, snow etc.) can not pass and sweat as water-vapour can not be eliminated through the fabric. This situation will cause condensation in the garment and, thus, a general state of discomfort.

Breathable fabrics constitute one of the major components of individual protective clothing used in High altitude conditions. The technical excellence and comfort aspects of these fabrics have enhanced the combat efficiency of the army personnel working under severe conditions of extreme cold weather. Fabrics having water proofness, wind proofness and water vapour permeable characteristics are commonly referred as breathable fabrics. In this paper the technology of manufacturing breathable fabrics, garments, their suitability and their properties are discussed and reviewed.

Thermo physiological comfort is a complex phenomenon and relates to the thermodynamics, heat and moisture transfer of the human body and clothing. The heat balance of the system is a function of skin, clothing and amount of air entrapped between skin and clothing. This heat balance varies with the change in wind velocity, atmospheric temperature (external factor) or the activity of the individual (internal factor) causing change in heat or moisture production of the body. The individual will feel comfortable till the amount of heat produced is equal to the amount of heat lost.

The advent of polyurethane/PTFE coating offered to the textile technologist to design and engineer comfortable water proof clothing as an alternate to PVC. The ultra light polyurethane coated fabrics eliminate the common problems experienced in PVC clothing that include poor flex properties, abrasion resistance and low temperature ageing properties. Compared to its counterparts, polyurethane coated fabrics are flexible, abrasion resistant, light weight, possess good cold ageing properties and some of the grades are even breathable. In addition to the above, special grades of PO offer excellent drape properties, softness and suppleness of hand and could be processed to exhibit high gloss and transparent properties.

In quantitative terms a breathable fabric should have the following characteristics

. Water vapour permeability 3000 g/m2 24 hr
. Water proofness min. 130 cm, hydrostatic pressure.
. Wind proofness-less than 1. 5 m!, cm2/ sec at 1 m bar,
. The fundamental considerations that need to be considered while designing a breathable fabric for high attitude conditions include,
. Water proofness
. Mass of the fabric
. Durability/flexibility of coating/ laminating
. Comfort level
. Camouflage effect to deceive the enemy and match the UV reflectance of snow/mountains.
. Aesthetic property
. Water vapour transmission
. Effective clothing against wind chill factor.

Water resistant breathable fabrics

To avoid the condensation of perspiration in a garment, it has to be made with breathable fabrics through which water-vapour can escape. In general there are three different ways to create - water resistant but breathable fabrics.

High-density fabrics (HDF)

The fabrics can be woven so densely that no intrinsic can be seen between the yarns. For yarns one must use the so-called microfibres. Microfibres are fibres that are less than 1 decitex per fibre, they are produced by special spinning techniques or by post-treatment after spinning (cleaving). Water vapour permeability is high-a certain water resistance will be given by post-treatment with water repellant agents such as fluorocarbons and silicones.

Lamination

In this technique a functional (water-resistant/breathable) barrier is 'glued' to a fabric. The fabric itself can be of almost any desired construction. Lamination of the film on a fabric uses special adhesives, sometimes even breathable ones. If non-breathable adhesives are used, care has to be taken not to cover the whole surface in the coating process. This process can be done in rotary-screen-printing, spraying, or web-lamination.

Microprous

. To render a film breathable, holes can be made in it: Teflon renders itself perfectly to this in a biaxial-stretching process, such as: Gore-Tex (W L Gore, USA) and Microtex (NIT TO Elec Ind, Japan).
. One can produce a micorporous film, such as: Porelle film (polyurethane) made by Povair, Great Britain in a coagulation process or Repel film (Polyacrylate) made by Gelman Sciences, USA in a photopolymerisation process.

Hydrophilic

The polymer out of which a film is extruded or cast can contain chemical groups that can absorb, diffuse, and desorb water as vapour, such as:

. Sympatex film (polyester), a product of Enka Glanzstoff, Germany
. BION II film (polyurethane), a product off Toyo Cloth Co, Japan
. Excepor-U (Polyamionoaicd/PU), a product of Mitsubishi-Kasei, Japan

Coating

The fabric is coated in a direct or transfer coating process with products that will close the fabric and thus, produce water-resistancy. Breathability is given by creating a microporous structure or using a hydrophilic product.

Micro porous coating

A micro porous coating can be obtained by,
. Coating the fabric with a foam: this gives a rather course cell structure which has to be crushed. Breathability is good but water-proofness has to be improved. An example is Dicrylan system of Pfersee/Ciba Geigy.
. Phase separation: The coating material is precipitated out of its solution. This process is done in Permutex produced by Stahl (ICI), Netherlands.
. Precipitation of emulsified polyurethane is induced by salts.
. In the so-called 'Wet' coagulation technique in which the polyurethane solution in DMF is coated on a fabric that is passed in a water bath. Water and DMF are perfectly miscible, but water is a nonsolvent. This combination creates a precipitation of the polyurethane on the fabric. As examples, Toray Inc. makes its 'Entrant' and Burligton makes its 'Ultrex' this way. .
. Phase inversion: A solution of polyurethane containing a nonsolvent is coated on a fabric. In selective evaporation step, the solvent is eliminated first. The polyurethane will precipitate in a microporous way. An example is Ucecoat 2000 (S) of DCB Specialty Chemicals, Belgium.


Hydrophilic coatings

The polyurethane in solution contains hydrophilic groups. After coating, the solvent is evaporated leaving a monolithic (non-porous) layer on the fabric. Examples are:
. Uceoat NPU series of UCB Specialty Chemicals, Belgium
. Witocflex/Staycool of Baxenden/ Great Britain
. Impraperm of Bayer, Belgium

Combination

A hydrophilic finish on a microporous structure is used to upgrade the water-resistancy of micorporous coatings. Care has to be taken to select a hydrophilic finish that does not impart an unacceptable loss in breathability. Ucecoat NPU2307 finish on top of Ucecoat - 2000 '(5)' microporous coating is an example.

Structure and properties of the Sympatex membrane

Unlike the porous systems, the membrane developed by Abo is a polyester membrane of the poreless, hydrophilic type that is marketed under the Sympatex brand. The material is a modified type of polyester into which poylether groups have been incorporated to impart to the membrane hydrophilic and, although with limited, swelling properties. When the membrane comes into contact with water vapour, the interstices between the molecules will open and allow an increased number of water vapour molecules to pass.

The water vapour molecules are transported along the molecular chains of the membrane by means of sorption and de-sorption. The direction and the rate of water vapour transmission depend on the difference between the partial water vapour pressures on the two sides of the membrane. The partial pressure prevailing in the microclimate between the skin and water barrier is determined by temperature, sweat evaporation, and diffusion and convection of the water vapour. The partial pressure in the ambient atmosphere is based on temperature and relative humidity. The membrane extruded is very thin, its thickness being comparable to very fine textile fibres. It has good mechanical properties and a high permeability to water vapour.

Lamination

The membrane currently used in clothing is 10 mm thick and has a resistance to water vapour transmission of less than 20xlO-3 m2-mbar/W. Many of the textiles used in laminates are found in about the same range. Fifteen or 25 mm thick membranes are produced for applications having to meet special requirements. This extremely thin hydrophilic membrane is used alone only in a few applications including gloves and caps, for instance. As a rule, it is bonded to a textile substrate before being incorporated into a garment or shoe.

Laminating methods

. Imprinting adhesives from solvents and aqueous systems.
. Aerial application of foam adhesives
. Powder point
. Paste point
. Adhesion net
. Flame bonding

Lamination is done by well-known methods, which involve applying an adhesive in a certain pattern to the textile material or the membrane, using varying techniques, and bonding the two layers by activating the adhesive. The use performance and the handle of the laminate depend largely on the textile component selected. Outer fabric laminates, for instance, are preferably made with fabrics having a construction with longer floats in order to obtain a more pleasant handle. Insert laminates, on the other hand, are made with nonwovens or knitted, lightweight fabrics. Softness, good processability, and the preservation of the high permeability to water vapour, as well as good wear and care resistant adhesion, are the characteristic features of a good laminate. The resistance to water vapour transmission of a laminate depends largely on

. The thickness of the membrane.
. The coverage of the membrane by the laminating adhesive
. The resistances to water vapour transmission of the textile component, which is determined by the fibre type, construction, thickness, density, swelling properties, and finish.


Evaluation of laminate quality

. Laminate adhesion
. Water proofness
. Permeability to water vapour.
. Handle
. All other use properties
- Original
- After 3 washing cycles
- After 3 dry cleaning cycles

The use performance of the laminates is constantly checked by the laminators. The permeability to water vapour is determined according to the ASTM method E 96-66 (method B) with the water temperature and test atmosphere being modified. What is measured here is the loss of water from the dishes closed with the laminate. In connection with development projects, frequent comparisons are made with the skin model developed by the Hohenstein institute and the simplified cup method. Further important criteria of the SYMPATEX quality standards are the resistance to washing and dry cleaning of the laminates and the sealed seams, the dimensional stability, and the adhesion of the membrane to the fabric.

Garment manufacturing standards

Different types of material and laminating methods will result in different types of laminate to be incorporated in the garments. The membrane may be bonded directly to the outer fabric. for instance, it may be bonded to a lightweight substrate, mostly a warpknitted or a nonwoven fabric (and placed loosely between the outer fabric and the lining). or be laminated directly to the lining fabric. In a three-layer laminate, the lining and the outer fabric are bonded to either side of the membrane. In a garment, the membrane must in any case form the second layer from outside placed directly below the outer fabric.

The type of laminate to be selected depends on the intended use of the garment in which it is to be incorporated. They offer the designer maximum freedom because they are invisible, and they involve hardly any problems in production planning. A fashionable variety that is now coming to the fore is the lining laminate marketed under the trade name of 'SYMPATEX micro liner'. The laminate is made with a special type of thin, warpknitted fabric with a well-closed surface that is easy to print. Active wear- where the protective function is clearly the dominant factor is preferably made with outer fabric laminates. This applies also to work clothing. Garments that are supposed to combine a protective effect with fashion appeal are made with insert or lining laminates.

The desired properties of the garment, Le., waterproofness, windproofness and breathability, must also be kept in mind when selecting the additional materials. All fabrics must have good water-repellent properties, but must not be provided with a coating impermeable to water vapour. The lining and the seams must also be dimensionally stable and resistant to washing and dry cleaning and, if necessary, be given a hydrophobic treatment. All metal parts must be given a non-oxidizing treatment. At the design stage, it should be made certain that the garment has as few seams as possible. These seams should preferably run straight in order to enable the laminate seams to be perfectly sealed. This requirement leads to the following basic rules:

. Avoid interruptions of the membrane area, if possible.
. Protect required opening (pockets) by overlappings.
. Reduce laminate seams to a minimum and seal them.
. Conditions
- Flat, narrow seams with seam curvature not too narrow
- Suitable tapes
. Use check tightness and check for tightness
. Use water-resistant 'findings'

All openings must be protected against penetration of liquid water. In the closure of a pull-on blouson, for instance, the zip opening is covered from outside by extended labyrinth-like coverings. The Sympatex laminate layer inside the blouson is cut somewhat higher up on the zip so that the outer strips cover its upper edge like roofing tiles. A traverse seam as commonly used in such closures at the lower end of the zip, where the stitches penetrate through all layers to fix the arrangement, must be avoided in this case in order to keep the garment watertight. Designers and garment makers must pay particular attention to the edges. After prolonged, heavy rain, the moisture may migrate along the sewing yarn and, after passing through the stitch holes, enter between the layers and migrate inwards to the lining, along whose capillaries the water will then move upwards. To avoid an extended outer fabric hem should be used to which the lining is then attached.


The pockets must be protected by flaps or cover strips. The pocket flaps must be attached separately from the pocket bag in order to avoid a 'water bridge', which would moisten the inside of the pocket. Pocket bags must also the furnished with a Sympatex layer. In the area of the collar and neck opening it is particularly important to provide for an overlapping of hood and collar or a possible double collar (in the way of roofing tiles). To ensure maximum water tightness, it is necessary to have no connections between the individual layers of a given part as they would inevitably make the garment tight.

In classical types of coats, it is necessary, to provide some kind of stabilising connection in the neck opening. Here little bars may be placed in the tab area as long as they are protected by a hood. Otherwise, some small laminate strips may be sealed on, which are then stitched to the seam allowance of the outer fabric. On the other side of the laminate, the seam allowances of the lining and the laminate are joined as well. In this way, a functional airtight combination of the three material layers will be achieved. In garment making all laminate seams must be designed so as to enable them to be perfectly sealed. Seams in outer fabric and lining laminates must not be wider than 0.5 cm. Seam allowances, which have to be covered by the sealing tape as well, should be stitched flat in order to prevent them from moving about when the seam sealing tape-is being applied.

Seams insert laminates are closed on the textile side so that the type may then be sealed flat onto the membrane side. The tapes used are heat-sealing tapes specifically adapted to the SYMPATEX membrane. The tapes are composed of several layers, they are covered either by release paper, or if destined for three layer laminates by warp-knitted fabric. These tapes are applied on specialised machinery where the heat from a hot-air blower activates the adhesive layer before the tape is pressed onto the seam between two rolls.

Watertight, care-resistant joints are obtained only if the adhesive compound adheres properly direct to the membrane. This adhesion is achieved by harmonising the temperature, speed, and pressure of the pressure rollers used during application of the tape. The correct machine settings must be determined by preliminary trials before starting commercial production and be confirmed by washing and dry cleaning tests. In the case of three layer laminates it must be made certain that the adhesive compound can penetrate through the lining layer to be anchored to the membrane. The tightness of the seal must be checked on random samples throughout the production run. The water tightness of the seams is tested in the manufacturing plant with the aid of a small pressure tester (Pfaff) specially designed for this purpose. The standard value for protective work wear given in the DIN standard 61539 is 1.3 m water column.

Garment making tests according to Sympatex standards

. Seam tightness
. Front closure
. Pockets
. Collar
. Hood
. Hem
. Sleeves
. Edges
. Rain test
- Original
- After washing/dry cleaning

Hydrophobic treatment

Before being marketed, all garment models are tested for design, workmanship, and seam tightness. The whole garment is examined as to whether it is properly made up and whether it will fulfill its intended function. This examination involves testing the tightness of the seams. Checking whether the quality standards have been observed, and testing whether the garment is resistant to washing and dry cleaning. A particularly informative test as far as the quality of the finished garment is concerned is the rain shower test. This test involves placing the garment to be tested on a dummy dressed with absorptive material and exposed to a rain shower. During the test, the dummy describes a rotational. Somewhat laterally displaced, movement under a spraying device as used for the Bundesmann Test. A test period of 10 minutes corresponds to about 2 hours of heavy rain.

Conclusion

Membranes have opened up entirely new possibilities for designing functional clothing. As a result of their success in purely functional all whether clothing, such systems are now more widely and still more successfully used in the field of leisurewear. The functional properties 'waterproof, wind proof, permeable to water vapour' are the main criteria for judging the quality

of clothing made with SYMPATEX from the membrane to the laminate and the finished garment. After all, quality is the decisive yardstick in this special field of application and, in addition, a key factor for success on the market.

Acknowledgement

The authors are thankful to the management, Principal and Head of the Department of Textile and Fashion Technology for their encouragement in preparing this paper.

References

1 Water resistant breathable fabrics - Mic Van Roey et ai, Journal of Coated Fabrics, Volume 21, July 1991
2 Sun beam process microporous membranes, A high performance barrier for protective clothing - Edwak C et ai, Journal of Coated Fabrics, Volume 18, July 1988
3 Performance of breathable rainwear materials with respect to protection, physiology, durability and ecology - M Weder et ai, Journal of Coated Fabrics, Volume 27, October 1997
4 Breathable coatings and laminates Adwin Kannerens, Journal of Coated Fabrics, Volume 24, July 1992
5 Technology of breathable coatings Sundaram Krishnan, Journal of Coated Fabrics, Volume 22, July 1992
6 The design of water proof, water vapour permeable fabrics - GR Lomax, Journal of Coated Fabrics, Volume 15, July 1985
7 Breathable fabrics and comfort in clothing - J H Keighley, Journal of Coated Fabrics, Volume 15, October 1985
8 Structure and processing of Sympatex laminates, Journal of Coated Fabrics, Volume 21, January 1992
9 Water resistant breathable hydrophilic polyurethane coatings - VM Desai et ai, Journal of Coated Fabrics, Volume 25, July 1995
10 Water vapour permeable PUR membranes for weather proof laminates - L Gottwald, Journal of Coated Fabrics, Volume 25, January 1996

Source: Asian Textile Journal (ATJ) - July, 2006.