Water Repellent Finsihing
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TEXTILE FINSHING TOPIC:
WATER REPELLENT FINISHES
SYED ASIM NAJAM (BS-Textile)
Water Repellency: Water Repellency is more difficult to define because various static and dynamic tests are used to measure water repellency. Generally speaking water repellent fabrics are those which resist being wetted by water, water drops will roll off the fabric. A fabric's resistance to water will depend on the nature of the fiber surface, the porosity of the fabric and the dynamic force behind the impacting water spray. The conditions of the test must be stated when specifying water repellency. It is important to distinguish between water-repellent and water-proof fabrics. Water Repellent Fabrics have open pores and are permeable to air and water vapor. Water-repellent fabrics will permit the passage of liquid water once hydro-static pressure is high enough.
Water-Proof Fabrics are resistant to the penetration of water under much higher hydrostatic pressure than are water-repellent fabrics. These fabrics have fewer open pores and are less permeable to the passage of air and water vapor. The more waterproof a fabric, the less able it is to permit the passage of air or water vapor. Waterproof is an overstatement, a more descriptive term is impermeable to water. A fabric is made water-repellent by depositing a hydrophobic material surface; however. Waterproofing requires filling the pores as well. Fabric Construction While the chemical finishes used for both applications are very similar, the key difference between outerwear (rainwear) and general apparel (casual pants) applications is the requirement to pass the rain test (AATCC 35). Performance in the rain test depends heavily on fabric construction and not only on fabric finish. A regular cotton 3/1 twill fabric (7.5 oz/yd2), which is typically used for casual pants, does not have a tight enough construction to pass the rain test. However, a heavier cotton fabric such as a 9.0-10.0 oz/yd 2 canvas does have a very tight construction and can perform successfully in the rain test with proper treatment. Another alternative is to use a specialty fabric that has very fine yarns and a very tight weave.
PHYSICAL CHEMISTRY OF WETTING When a drop of liquid on a solid surface does not spread, the drop will assume a shape that appears constant and exhibits an angle , called the contact angle. The angle is characteristic of the particular liquid/solid interaction; therefore, the equilibrium contact angle serves as an indication of wettability of the solid by the liquid. As seen in figure 52, the interfacial forces between the liquid and vapor, liquid and solid and solid and vapor all come into play when determining whether a liquid will spread or not on a smooth solid surface. The equilibrium established between these forces determine the contact angle 0. Spreading of Liquids on Smooth Surfaces
Hydrophobic treatments and products For the above mentioned reasons, to avoid the wetting of the surface of a macromolecular and porous material like a fibre, the surface must be covered with a film formed by a substance with a surface tension that is smaller than the textile. In this way the drops of water will assemble on the surface, isolated from each other, to create the pearl effect. A waterproof effect can be also obtained by using non-filmogenic substances that form a sort of .brush. of short molecules. By limiting the formation of a hydrophobic state on the surface of the textile substrate, the basic properties of mechanic resistance and flexibility of the fibres remain unaltered. Both polymer and low chain water-repellent products should have essentially a hydrocarbon character (with groups having a lower surface tension such as =CH 2, -CH 3 or perfluorinated chains), to reduce the surface tension of the fiber until making it water-repellent. Method based on emulsions in single bath This method is recommended for producing awnings or camping tents and umbrellas but it does not stand washing in water and solvents. The textile substrate treated with this method is dried in stenters at 80-100°C. Good results can be obtained with medium-weight/light fabrics made of cotton, cottonsynthetic blends and wool, with a good cost-efficiency ratio. Better results for waterproof ability and solidity to usage and washing can be obtained by replacing Al salts with Zr salts (ammonium and zirconium dicarbonate, zirconium oxychloride, etc.), by applying them on fabrics made of cotton, cottonsynthetic blends and wool, with a technique similar to the above mentioned one. Good results can also be obtained on cotton, blends and wool.
Durable methods It is possible to bind hydrophobic chains to cellulose fibers by reacting the primary hydroxyl groups. The results can be remarkably improved when these products are combined with resins. Usually the fabric is wetted by dispersing the product; the fabric is then dried and treated at 90-120°C.
Products made of resins linked to fatty acids These products feature excellent solidity when washed in water at 60-90°C, though not when washed with solvents. They are recommended for treating raincoats, uniforms etc. It is possible to use methylol-ureastearamide derivatives, ether alcohols superior to dimethylolurea, but more frequently melamine derivatives linked in different ways with lipophile chains. These products (whatever their formulation) can be used for padding applications in the presence of acid catalysts (ammonium chloride, ammonium nitrate, aluminium chloride or zirconium oxychloride), dried and polycondensed at 140-150°C. It is important that fabrics made of synthetic fibres dyed with dispersed dyes, are dried at low temperatures after the padding process; this prevents the dyes from migrating toward the hydrophobic surface and maintains the fabrics solidity to rubbing. Silicones These products are stable to washing in water at 60°C with solvents. They can be used to treat uniforms, raincoats and sportswear. These products are very popular since they allow excellent waterproofing properties, optimum solidity (that can also be enhanced if combined with resins) and a very pleasant and soft hand (they are also used as softening agents.) They are suitable to be applied on all types of fibres. The high water-repellent effect and the soft hand are due to the orientation toward the outer surface of methyl groups
Application to Fabrics Silicone finishes are applied to fabrics either from an organic solvent or from water as an emulsion. When cationic emulsifiers are used to make an emulsion, the finish may be applied by exhaustion since the negative fiber surface charges attract positively charged particles. Generally however, silicone water repellents are co applied with a durable press finish. Durable press resins enhance the durability of the water-repellent finish. Silicone repellents are also used to make upholstered furniture stain repellent. Chlorinated solvent solutions are sprayed onto upholstery by the retailer as
a customer option. The fabric is resistant to water borne stains such as coffee and soft drinks. Advantages and Disadvantages Silicone water repellents are durable to washing and dry-cleaning. Durability 163is brought about by the formation of a sheath of finish around the fiber. If the sheath cracks, durability is lost. Adsorption of hydrophilic substances found in dry cleaning and laundry products also impair water repellency. Silicones are more durable than wax repellents but less durable than fluorochemical finishes. Silicones are more expensive than wax repellents and less expensive than fluorochemical repellents. Silicone finishes resist water borne stains but not oil borne stains. Fabric hand can be made soft and pliable. Fluorinated products They feature an excellent fastness to washing in water and solvents. They are suitable for treating uniforms, raincoats and sportswear, tablecloths and protection clothes (after washing they must be ironed to recover their original effect). These products allow excellent waterproofing results also with oilrepellency properties. The application is carried out by padding the fabric, which is then dried at 100-120°C.
REPELLENT FINISHING WITH FLUOROCHEMICALS The oil and water repellent features of fluorochemical polymers lead to finishes applicable in two consumer product areas, durable rainwear fabrics and stain/soil resistant products. For rainwear products, superior durability to repeated laundering and drycleaning is the major advantage. For stain and soil resistance, the plus features are the fluorochemical's ability to prevent oils from penetrating into the fabric or from soils sticking to the fiber surface. Most fabric stains are caused by liquids depositing coloring matter on the fabric. Water borne stains can be held out by silicone water repellents; however, oil based stains can only be repelled by the low surface energy of closely packed fluorocarbon tails. For textiles that cannot be laundered, e.g. upholstery fabrics and carpets, stain and soil repellency is an important consumer plus. For fabrics that can be laundered or dry cleaned, stain removal is more important than stain prevention. Repellent Finishes For fabrics to be water repellent, the critical surface tension of the fiber's surface must be lowered to about 24 to 30 dynes/cm. Pure water has a surface tension of 72 dynes/cm so these values are sufficient for water
repellency. This section will be devoted to describing materials that are used mainly as water repellent finishes. In a later section, it will be shown that some of these can be combined with fluorochemical finishes to enhance both water and oil repellency.
HYDROCARBON HYDROPHOBES A. Paraffin Waxes The oldest and most economical way to make a fabric water repellent is to coat it with paraffin wax. Solvent solutions, molten coatings and wax emulsions are ways of applying wax to fabrics. Of these, wax emulsions are the most convenient products for finishing fabrics. An important consideration in making water repellent wax emulsion is that the emulsifying system not detract from the hydrophobic character of paraffin. Paraffin wax melts and wicks into the fabric when the fabric is heated. This will cause most of the fibers to be covered with a thin layer of wax, especially those that are exposed to water, and the fabric will have excellent water repellent properties. The major disadvantage of wax water repellents is poor durability. Wax is easily abraded by mechanical action and wax dissolves in dry cleaning fluids. It is also removed by laundry processes. A typical wax emulsion consists of paraffin wax as the hydrophobe, an emulsifying agent, an emulsion stabilizer (protective colloid) and an aluminum or zirconium salt to deactivate the emulsifying agent when the fabric is heated. Wax Emulsion Composition
The stability of wax dispersions and the durability of wax finishes have been increased by formulating polymers such as poly(vinyl alcohol), polyethylene and copolymers of stearyl acrylate-acrylic or methacrylic acids.
Fiber Reactive Hydrocarbon Hydrophobes: N-Methylol Stearamide In an effort to improve the durability of hydrocarbon based water repellents, several approaches incorporating reactive groups have found commercial success. The simplest of these is N-methylol stearamide. Stearamide reacts with formaldehyde to form the N-methylol adduct. This adduct is water dispersible and either will react on curing with cellulose. Pyridinium Compounds A variation of N-methylol stearamide is the pyridinium type water repellents. These were once very popular and used extensively as reactive type water repellent finishes. Toxicological considerations have curtailed the use of pyridinium-type water repellents.
Pyridinium type water repellents co-applied with fluorochemical repellents resulted good, long-lasting water repellency. The finish was durable to field laundry procedures and named Quarpel by its inventors.
Resin Formers The multiple reactive sites on methylolmelamines can be utilized for making resin-forming water repellents. Rainwear A typical formulation for polyester-cotton rainwear and outerwear is shown in Table 16. The finish is applied by padding the formulation onto fabric, drying at 120°C and curing 1-3 minutes at 150-182o C. The fabric will give a 100 spray rating initially and an 80 rating after 5 home laundering-tumble drying cycles. An 80 spray rating is expected after one dry cleaning cycle. In addition, oil repellency rating of 5 initially and 4 after laundering or dry cleaning is expected.
WATERPROOF BREATHABLE FABRICS Complete waterproofing is not the best option for fabrics for garments, because perspiration moisture accumulates in a waterproof garment-particularly during periods of exertion-causing discomfort and putting the wearer at risk of developing fungal skin infection. Hence, a good weatherproof garment allows moist air to escape and fresh, dry air to enter- a process called breathing-while preventing liquid water, such as rain, from reaching the lining layers. The compromise between skin breathing and weatherproofing is achieved using woven fabrics whose pores are wide enough to allow breathing
Waterproof/moisture permeable fabrics from several synthetic fibers have been designed for skiwear, track suits, rainwear, and mountaineering clothing Lightweight fabrics coated with poly(vinyl chloride), polyurethane, and other synthetic rubbers have become increasingly popular for foulweather clothing. Although garments manufactured from these fabrics provide exceptional protection against rain and to a less extent, wind penetration, This problem arises because of the water-vapor impermeability of the polymer coating. A buildup of moisture vapor inside the clothing may therefore cause discomfort. To overcome this problem, breathable polymer coatings are now available that have made possible great improvements in the comfort of this type of clothing. Classification of Breathable Fabrics The breathable fabrics can be classified into three main categories
1. coated fabrics 2. laminated 3. high-density woven fabrics. Microporous Coating and Laminating Films The microporous barrier layer “breathes” primarily through a permanent airpermeable pore structure. Diverse techniques have been used to manufacture microporous coatings and films. Hydrophilic Coated and Laminated Fabrics Hydrophilic coated and laminated fabrics transmit water vapor selectively by a molecular process. High-Density Fabrics In open-weave fabrics, water vapors transmit mainly through interyarn spaces and transfer through individual fibers, and fiber bundles are relatively unimportant. Thus, fabrics of similar open construction, weight, and thickness are expected to show similar transmission rates, irrespective of the type of yarn or fibers used. As the size of the interyarn spaces decreases, the secondary transmission mechanisms become more important. Thus, tightly woven fabrics constructed from absorptive or hydrophilic fibers are more transmissive of water vapor than similar construction of nonabsorptive, hydrophobic fibers WATER RESISTANT FABRIC •
Boat Shrunk Canvas (Sunforger) Water repellent and mildew resistant, 100% Cotton Fabric used for boat covers and tarps
Chair Duck This water resistant heavy cotton duck is used for director's chairs, cushions and hand bags. Phifertex (Mesh) Pfifertex® by Phifer Wire Products. Durable open weave construction (looks like window screen dipped in vinyl) is self-draining. This vinyl encapsulated mesh fabric is perfect for boat cushions, outdoor furniture, umbrellas and other outdoor applications Rip Stop This lightweight (1.9 oz) water repellent fabric makes great jackets and wind-breakers Seamark The Ultimate Marine Fabric. An Exterior Grade, Waterproof Fabric, 60 inches wide. If it has to be waterproof, it has to be Seamark. •
Stamoid Stamoid Marine Fabric. Lightweight, waterproof marine fabric. Mostly used for covers.
Sunbrella Canvas Sunbrella is the most common Awning and Marine canvas. It is a solution-dyed woven acrylic canvas and is available in over 40 colors.Used for sailcovers, bimini tops, boat covers, awnings and has many other outdoor applications. This fabric is highly water-resistant. Furniture Weight Sunbrella Solid Colors and Furniture Weight Sunbrella, Stripes and Textures From Glen Raven Mills, Sunbrella ® furniture fabrics are not only beautiful, they're incredibly durable. Striking and vibrant stripes, solids, textures and linens. Sunbrella furniture fabric is made from the same acrylic fiber as our awning and marine fabrics. So it's tough. Resists sunlight, mildew, rot and atmospheric chemicals. Sunbrella is porous, but highly water repellant. This allows the fabric to breathe, for even greater comfort in hot weather. •
Team Jacket Satin Nylon - This 100 % nylon water repellent fabric is excellent for sports jackets. Its shiny finish gives you a very professional look. Available in 8 colors. Poly Cotton Canvas Water repellent and mildew resistant fabric. Used for boat and car covers.
Water Repellency Test methods: The susceptibility of a fabric to water is measured in terms of two phenomena: penetration and absorption. Penetration is the passage of water through a fabric, which can cause discomfort for the wearer of a garment or cause damage to items within a tent, for example. Absorption is the ability of a fabric to soak up water or become water logged. Penetration depends to a large extent on the construction of the fabric, whereas absorption reflects the lack of ability of the constituent fibers to shed water. An open fabric will allow water to pass through its interstices-the spaces between its fibers. On the other hand, a closely woven fabric will prevent penetration provided the water-shedding properties of the fibers are adequate.
Test Methods are as follows:
Spray Test: Measures resistance of fabric to surface wetting under mild water impact. • Does not measure water penetration through the fabric, only the fabric surface is visually rated? Widely used as a quick, easy, inexpensive screening test. Highest rating is 100; 70 is the lowest acceptable rating required for many repellent fabrics.
Rain Test: Penetration of water through the fabric is tested by placing a blotter of known weight behind the fabric sample, spraying with water for a specified time, and re-weighing the blotter. The intensity of the water impact can be adjusted by using a higher column of water. • The highest test level generally used for apparel is 3 feet for 5 minutes. • Passing the test depends heavily on fabric construction (tightness).
Bundesmann Rain/Shower Test: Subjects fabric to an artificial rain shower, rates the specimens visually for surface wetting, measures the water absorbed by the specimen, and measures the amount of water that penetrates through the specimen. • This is a severe test of water repellency.
Hydrostatic Pressure Test: Fabrics are subjected to increasing water (hydrostatic) pressure until 3 points of leakage appear. • Severe test of waterproofness used for high-tech fabrics.