Waterproof breathable fabrics: Technologies and practices

This article is being published in two parts, this is the first part.

Abstract

Textile and Apparel are sun-riser industries in India. It is also one of the oldest and largest profits making industry. Textile contributes substantially to India economy by exporting goods to various countries in the globe. For the textile industry, it is time to take stock of the situation and aim at vertical integration or tie up with downstream manufacturers to realize greater benefits of value addition and face the future with greater confidence. As the globalization of textile industry becomes a reality, we need to develop strategies for survival and growth.

Due to technological advances and globalization business of apparel textiles is saturated. And it opens the market for technical textiles. Textiles with multiple functions are the need of the hour; this has given a positive dimension and potential for the growth of water proof breathable fabrics. In spite of several methods to produce the fabric, really it is a difficult task as well an opportunity to produce quality products in this area. In near future India will be one of the leader in this area.

In this paper, we have made an attempt to elaborate various techniques with benefits and limitations to produce the quality product so that we can compete in the global market in spite of byjentine competition.

Keyword: Air permeability, Breathability, Ventile, Water proofness.

1. Introduction

The human body continuously generates heat by its metabolic process. The heat is dissipated from the body by convection, radiation, evaporation and perspiration. The heat generated should be maintained constantly within and outside the body. 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. If the temperature rises the sweating mechanism gets activated and eliminates the heat in the form of heat waves and perspiration. The fabrics protect the human body from external heat, wind, water, and many harmful agents, and at the same time it also permits effective transmission of moist vapor from inside to outside atmosphere. If the breathability is high enough to compensate the body heat then the requirement of generating perspiration will be limited. The term breathability is usually confused with wind penetration and wick ability of the material. The term ‘breathable’ implies that the fabric is actively ventilated. This is not the case. Breathable fabrics passively allow water vapor to diffuse through them yet still prevent the penetration of liquid water. Breathability is one such factor that decides the designing of apparel wears and some specific technical products. The pores of breathable fabrics are 20000 times smaller than a drop of liquid water, but 700 times larger than a water molecule, thus they are too small to allow liquid water to pass through, but large enough to allow the passage of molecules of water vapor. For a fire fighting operation, in racer suits and other jobs were higher metabolic activities are carried the body perspires heavily and the fabric should transmit good level of water vapor while protecting the body from external heat and pressure. Water proof is another such property that retards the passage of water molecules through the structures they are specifically used in technical products. The property of waterproof and breathability can be termed as water repellent fabrics.

The market going the customer way, the comfort property has become the prime aspect. The introduction of new waterproof breathable fabrics has greatly increased the range of choice for consumers. Measurement of comfort is not possible when it comes to perspective views. The breathability and waterproof feature has been and will remain as major factor in deciding the comfort level of the apparel [1-5].

2.1 Principles of moisture vapor transfer

The Principles of moisture vapor transfer is governed by inter yarn/ inter fiber spaces. The moisture in the vapor form transmit through textile material by

• Diffusion of the water vapor through the air spaces between the fibers.
• Absorption, transmition and desorption of the water vapor by the fibers.
• Transmition of water vapor by forced convection.

Textiles made from absorbent fibres, such as natural fibres and regenerated fibres; water is absorbed by hygroscopic fibres, transported through the swollen fibres, and evaporated from the outer surface of the textile. For textiles made from synthetic fibres water is taken up into the capillary spaces between fibres and yarns [3]

2.2 The barrier for vapor diffusion is

• Evaporating fluid layer (mix of fluids)
• Confirmed air layer ( between skin and the fabric layer)
• Boundary air layer and
• Ambient air layer

2.3 Water vapor/Moisture transport mechanism

The mechanism by which moisture is transported in textiles, by water vapor diffusion and convection in void space within the textile structure and, the liquid water diffusion by wicking of liquid in capillaries. Capillary action is determined by two fundamental properties of the capillary [1,2]:

2.3.1 Capillary’s diameter and surface energy.

Smaller the diameter greater is the surface energy, greater the tendency of a liquid to move up the capillary. In textile structures, the spaces between the fibres effectively form capillaries. Hence, the narrower space between these fibres, the greater is the ability of the textile to wick moisture.

Capillary pressure and capillary raise are determined using
P = 2γLV cos q / Rc   ,                                                                                  L = Ö((Rcγ cos q/2h)* t1/2
P = capillary pressure                                                    L = liquid pressure
Rc – capillary radius                                                       t and h – time and viscosity

Fig 1(a) and (b): Water vapor transportation through breathable fabric.

2.4 Factors affecting ventilation

During no contact (wind with moisture), the variation in the air gap thickness creates a high pressure between the air gap thickness and the atmosphere. When wind speed increases the touch period also increase because of high pressure created between the outer and inner microclimate and causing higher ventilation. The effect of air motion, fabric thickness, clothing aperture, fabric permeability and the swinging action of the fabric affects the ventilation factor [3].

3.Moisture management through breathable fabrics

Highly hydrophilic polymers are totally unsuitable for permanent fabric coatings. Hydrophilic materials are too sensitive to liquid water and, if used as water proof coatings, they would either dissolve completely or become so heavily swollen by rain that they would suffer severe flex or abrasion damage. Coatings of silicone rubbers and some blends of polyurethane (PU) and acrylic results in the formulation of better water proof fabric.

Fig 2: Hydrophilic polymer mechanism

The density and the geometry of the fabric pores can be varied according to the woven fabric structure that influences on the liquid flow pattern (both intrinsic and downstream). Darcy’s law is used to describe

γSV – γSL = γ LV cos q
SV- solid/ vapor,           SL – solid/ liquid           LV – Liquid/vapor

4. Properties requirement for breathable fabrics:

The desirable properties of waterproof breathable fabrics are listed below [9].
• Optimum heat and moisture regulation (thermo-regularity effect)
• Absorption of surplus heat.
• Water proofness.
• Good air and water vapor permeability.
• Rapid moisture absorption and conveyance capacity.
• Rapid drying to prevent catching cold.
• No/Minimum water absorption of the layer of clothing just positioned to the skin.
• Dimensionally stable even when comes in contact to water
• Durable (tear, peel, abrasion resistance)
• Easy care/launderability
• Hydrostatic resistance
• Lightweight
• Soft and pleasant touch

5. Methods of producing waterproof breathable fabrics

Different methods involved to produce breathable fabric

• Closely woven fabrics
• Micro porous membranes and coatings
• Hydrophilic membranes and coating
• Combination of micro porous and hydrophilic membranes and coating
• Smart breathable fabrics

Fig 3: Different layers of breathable fabric

5.1.1 Closely woven structure

Closely woven fabrics are constructed either from absorptive and hydrophilic yarns or microfiber synthetic yarn which results in small size of pores to give maximum protection against wind and rain. The surface area and concentration of inter yarn spaces should be as high as possible to maximize water vapor transmission through woven fabrics and, the fabrics should preferably be constructed from absorptive and hydrophilic yarns. The ability of the fibers to undergo diffusion depends on the hydrophilic nature of the fibers, and then further liquid transmission is assisted by capillary transfer within the fiber bundle. These fabrics initially are not water proof, but as it comes in contact with water the cotton fibers swell to such an extent that the inter yarn pores of the fabric are significantly reduced and there by restricting the passage of water. The air permeability is also low, but the inter yarn spaces and hydrophilic nature of the fiber allows adequate water vapor permeability. One of the famous waterproof breathable fabrics “VENTILE” was manufactured by using long staple cotton with minimum spaces between the fibres. Usually oxford weave is preferred to produce breathable fabrics. When fabric surface is wetted by water the cotton fibres swell transversely reducing the size of pores in the fabric and requiring very high pressure to cause penetration by reducing the pore size to 3–4 µm. It can protect the water penetration for up to 20 minutes when the wearer is submerged. Thus the swollen fabric in combination with the repellent finish provides excellent protection against the wind, rain, seawater, and cold. The choice of the repellent treatment is critical, it should allow absorption of water by substrate yarn to swell and constrict the inter-yarn pores. Therefore waterproof is provided without the application of any water repellent finishing treatment. Densely woven fabrics can also be produced from micro-denier synthetic filament yarns. The individual filaments in these yarns are of less than 10 micron in diameter, so that fabrics with very small pores can be engineered.

5.1.2 High-density woven (micro denier fiber)

Fabrics made using micro denier man-made filaments/fibers owe their breathability to the densely woven, thin and smooth yarns that are usually made from microfibers. This type of weaving results in a wind proof fabric with an excellent water vapor permeability compared with laminates and coatings. Microfibers do not actually swell when wet. High density fabric made out of microfilament yarn fabric exhibit very small pores.

5.2 Micro porous Membranes Coating and laminating

The coated and laminated breathable fabrics are either micro porous with hundreds of open micro pores through which vapor passes or monolithic wherein the diffusion of vapor takes place molecularly through hydrophilic hydrogen groups on the polymer chain through the solid film or sometimes the combination of the two. The coating contains very fine inter connected channels, much smaller than the finest raindrop but much larger than a water vapor molecule. In Micro porous Membranes and Coatings the pore size ranges from 0.1 to 50 µm. PUs, poly-tetrafluoroethylenes, acrylics, and polyamino acids are the most widely used coating elements. Among these, PU is the most popular polymer because of toughness, flexibility of the film and capability of tailor making the property of the film to suit the end use requirement. If the maximum pore size at the outer surfaces of the barrier layer is about 2–3 µm or less, the waterproof properties of the fabric are usually adequate. The micro porous structure is air-permeable and is capable of transmitting water vapor at physiologically acceptable rates.

 
Fig 4: (a) and 3(b): pore size of coating material with water molecule’s size

In a moisture vapor permeable waterproof coated fabric, it comprising of a base fabric and a synthetic microporous layer consisting of PU, the inner wall of each cavity is covered with a water-repellent agent and at least part of the inner wall has a hole in it to communicate with adjacent cavities. The microporous layer is formed from a coating solution of an organic solvent containing PU elastomers. In case of micro porous membrane, holes are much smaller (2–3 µm) than the smallest raindrops (100 µm), yet very much larger than a water vapor molecule (40 µm) [5]. The micro porous films and coatings operate on similar principle, the water droplets cannot penetrate the microspores of the film and coatings while the moisture vapor molecules are pushed through. For a constant porosity and thickness, water vapor transmission through the surface increases as the pore size decreases and as the fabric thickness increases water vapor permeability decreases [1].

Various methods of generating micro porous membranes and coatings are as follows.

• Mechanical fibrillation (only for membrane)
• Wet coagulation process
• Thermo coagulation (only for coating)
• Foam coating (only for coating)
• Solvent extraction
• Radio frequency (RF)/ion/UV or E beam radiation
• Melt blown/hot melt technology
• Point bonding technology

5.2.1 Mechanical Fibrillation

Certain polymer films can be stretched in both directions and annealed to impart microscopic rips and tears throughout the membrane. The best-known example is the interconnecting pore structure of the very thin PTFE films

The microstructure of the uniaxial stretched film consists of nodes elongated at right angles in the direction of the stretch. These nodes are interconnected by fibrils that are oriented parallel to the direction of the stretch. Typically the size of the nodes varies from 50–400 µm. The fibrils have widths of about 0.1 µm and lengths ranging from 5 to 500 µm. The development of porosity occurs due to void formation between nodes and fibrils. When the films are biaxial stretched, the fibril formation occurs in the other direction with the production of cobweb-like or cross linked configurations with an increase in strength. Porosity increases as the voids between the nodes and fibrils become more numerous and larger in size. The factors affecting the porosity and strength of the film are as follows:

• The crystallinity of polymer should be high, preferably > 98%.
• Temperature and rate of stretching: higher temperature and higher rate of stretch leads to more homogeneous structure with smaller closely spaced nodes, interconnected with a greater number of fibrils, increasing the strength of the polymer matrix.
• The temperature and duration of heat treatment: during heat treatment above the melting point of the polymer, an increase in amorphous content of the polymer occurs. The amorphous region reinforces the crystalline region enhancing the strength without substantially altering the microstructure.

The expanded PTFE film contains a network of micro pores (82% by volume) of size ranging from 0.1 to 50 µm and is claimed to have a density of 9 billion pores per square inch [1,2]. In comparison to water molecules these pores are 20,000 times smaller, whereas 700 times larger than that of water vapor molecules. Tetratec technology allows the pore size to be cast from as little as 0.04–3.0 µm according to the customer’s engineered product requirements.

5.2.2 Wet Coagulation Process

This process produces a very fine interconnecting structure in which the micro pores are small enough to keep droplets of water out but, large enough to let the small water vapor molecules. Micro-porosity can also be created by leaching the salts on treatment of the film with water. The coating obtained on wet coagulation shows ultrafine pores of < 1µm, in addition to a honeycomb skin core structure of 1–20 µm pores. The formation of these micro pores was due to the subtle difference in the rates of coagulation at the resin particle interface. Precise control over the coating operation is required to generate a consistent, uniform pore structure, preferably below 3 µm for optimum balance of breathability and water proofness.

5.2.3 Thermo Coagulation

The coating polymer can also be applied to the fabric from a mixture of a relatively volatile solvent mixed with a proportion of a higher boiling nonsolvent. PU-based coating technique operates on thermo-coagulation technique. PU is dissolved in a solvent mixture of methyl ethyl ketone, toluene, and water, having 15–20% solids and coated on the fabric. The low boiling solvent evaporates and leads to precipitation of PU in the nonsolvent.

5.2.4 Foam Coating

In this method, water based polyurethane/ polyacrylic acid esters are used. The foam is stabilized with the aid of additives. Then the foam is coated on one side of the fabric. The coated fabric is dried to form a micro porous coating. The fabric is finally calendared under low pressure to compress the coatings. The foam cells being relatively large, a fluorocarbon (FC) polymer based water-repellent finish is applied to improve the water resistant properties. The water vapor permeability of foam-coated fabrics is higher and their water resistance was found to be lower than those of non foam-coated fabrics with the same coating thickness. The water vapor permeability and the water resistance become higher when the foaming speed is increased [1].

Polyurethane-based (PU) polymers have high flexibility even at low temperature because of their low Tg. They also have good abrasion resistance and high resistance to chemicals and water. As compared to PVC, PU coated fabrics form are soft and flexible without the aid of plasticizers. In the absence of plasticizers the coated fabrics can withstand dry-cleaning and washing.

Some applications of polyurethane coatings are

• PU-based film designed (Permatex) by J.B. Broadley coated on fabrics offers a vapor permeability of at least 70%.
• Grabotter membrane (Grabo Ltd.) used in waterproof shoes is a PU-based film.
• Micro porous PU film is being produced by Acordis (Tarka) is applied by a transfer process from the release paper and it can be applied to almost any type of substrate

5.2.5 Solvent Extraction

In Solvent Extraction the polymer is dissolved in a water-miscible solvent and is coated directly on to the fabric. The micro porous structure of the coating is developed by passing the coated fabric through a coagulation bath where the solvent is displaced by water. Whereby, finely divided water-soluble salts are incorporated into the coating formulation. The salt particles are subsequently extracted from the dried and cured coating, by passing the fabric through a water bath.

5.2.6 Radio Frequency/UV

In this process, various FC films are deposited by ion beam sputtering in the deposition chamber. Sputtering of Teflon target was performed by an Ar ion beam with energy in the range of 1.0–2.1 KeV with an ion current ranging from 20–35 mA. The moisture permeability of the coated fabrics is similar to that of uncoated fabrics since only the fiber surface has been individually covered with the coated grains while the fabric still remain original with numerous pores. It was found that an increased target-substrate distance (TSD) led to a decrease in deposition rate, while decreased TSD makes the coating more uneven. The contact angle of the coated specimens obtained at higher energy was found to be smaller than that obtained at lower energy for a given washing time.

5.2.7 Melt blown /hot melt technology

Breathable fabric produced with melt blown technology, may comprise at least one layer of course, melt-spun, thermoplastic filaments and at least one layer of fine, melt-blown thermoplastic microfibers. The layers are thermally bonded together at intermittent points.

Micro pores can become enlarged when garments are stretched at elbows and knees affecting water proofing characteristics. On the other hand hydrophilic film and coatings do not lose their properties on cleaning and stretching of garments. A hydrophilic film is sometimes applied on microporous films to upgrade the water resistance.

Breathable coatings based on polyacrylamide (highly hydrophilic polymer) on cotton fabrics have been developed. The coatings showed high water vapor permeability, while providing desirable protection against air and liquid water-penetration.

5.2.8 Combination of Micro porous and Hydrophilic Membranes and Coatings

Fabrics coated with copolymers having both hydrophilic and hydrophobic segments. The hydrophobic part provides water resistance and facilitates adherence of the coating to the substrate, while hydrophilic part allow water vapor permeability. It offers solid layer like properties of ‘wind-proofness’ and resistance to penetration by some solvents and light mineral oils. It reduces stretch, which may cause opening of pores and possible water ingress through the laminated product. But it adds to the stiffness of laminate, cost and reduces the breathability of the fabric.

5.2.9 Smart breathable fabrics

The Shape memory polymer restricts the loss of body warmth by stopping the transfer of vapor and heat at low temperature and at high temperature. It transfers more heat and water vapor from inside clothing to outside than ordinary waterproof breathable fabrics. Shape memory PU is one of the several shape memory polymers. At lower temperature the coating substance on the fabric exists in a swollen state (by absorbing water from the surroundings) which results in the closure of micro cracks. At a temperature higher than the transition temperature, the coating exists in collapsed state (due to the predominance of Hydrophobic interactions) resulting in opening of the micro cracks. Another factor which effects is the change in diffusion flux that is governed by changes in both the diffusion coefficient and diffusion path of water molecules through the swollen and collapsed coating. When a sudden change in ambient conditions occurs, the PCM fabric delays the transient response and decreases body heat loss.

 5.2.10 Polyurethane water breathable coating

Manjeet Jassal et al made studies on the Waterproof Breathable Polymeric Coatings Based on Polyurethanes and stated that the water vapor permeability values were found to increase with an increase in hydrophilic component while the water penetration resistance increased with an increase in hydrophobic component. By varying the relative proportion of hydrophilic and hydrophobic components, the WVP and water penetration resistance for different end applications can be optimized [7].

Micro porous polyurethane coatings consist of an interconnected network of minute pores usually produced by wet coagulation process. The pores are sufficient enough to allow individual molecules of water vapor (0.0004 µm dia) to pass through the coating, whereas they do not permit the passage of liquid water droplets (> 100 µm diameter).

to be continued……