30 Water repellent and water proof finishes

P. Mageshkumar

   1. INTRODUCTION

Many terms have been used to describe the water-repellency of textile materials, particularly fabrics, but these terms are often imprecise and do not provide the basis for a clear definition.

1.2 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. Oil repellency requires that the fiber surface be lowered to 13 dynes /cm. Only fluorochemicals are able to function as oil repellents so whatever is mixed with them must not interfere with how they are deposited.

The term ‘waterproof’ is normally taken to represent the conditions where a textile material (treated or untreated) can prevent the absorption of water and also the penetration of water into its structure. Thus, a waterproof surface provides a barrier to water under all practical end-use conditions. In absolute terms, a waterproof fabric is fully resistant to penetration by water, implying that the fabric water penetration resistance is equivalent to its hydraulic bursting strength. However, in practice a minimum performance of no penetration by water below a hydrostatic pressure of 100 cm (10 kPa) is taken to represent a waterproof fabric. The major differences between water-repellent and waterproof fabrics are illustrated in Table.

   The most widely used method of producing a waterproof fabric is by coating the fabric with a solid polymeric coating – for example, neoprene (synthetic rubber), polyvinyl chloride or polyurethanes. Such coatings are not porous and therefore provide a continuous solid barrier to liquid water and other liquids, such as oils. However, such solid non-porous continuous coatings are impermeable both to the passage of air and of water vapour. Such coated fabrics are therefore classed as waterproof but are not water vapour permeable – that is, they are not ‘breathable’.

For many non-apparel end-uses – such as technical textiles, industrial fabrics, and textiles destined for outdoor use, such as tarpaulins or awnings – this does not create any problem.

However, for high levels of comfort in apparel, a fabric in garment form must allow air, and especially the moisture (water vapour) generated by the exudation of perspiration from the skin during physical activity, to pass through the fabric. This maintains the wearer of the garment in warm, dry conditions with high levels of thermophysical and thermophysiological comfort.

2.1 FLUOROCHEMICAL REPELLENT FINISHES

Fluoro chemicals (fluorocarbons) are a class of synthetically produced organic chemicals that contain a perfluoroalkyl residue in which all the hydrogen atoms have been replaced by fluorine. Fluoro chemicals exhibit outstanding chemical and thermal stability, low reactivity through their incompatibility with water and oil, and considerable reduction in surface tension. It is this latter property which is of particular importance in the context of water- and oil-repellency, while their chemical and thermal stability contribute towards the durability of the surface finish to fabric care treatments such as laundering, drycleaning and tumble-drying.

Fluoro chemicals are used in many fields, for example, as durable lubricants, corrosion protection coatings for metals, flame-retardant polymers, fluorine elastomers used in the rubber industry and heat transfer fluids in refrigeration technology. They are widely used as protective agents against water and oil, stains and soiling in the textile, paper and leather industries. Some fluoro chemicals are used for specialist wetting applications in the textile field; others are used in firefighting, and in the electroplating and electronics industries.

2.1.1 Fluorochemical finish formulations

The main components that may be present in typical fluorochemical dispersion are illustrated in Figure. Oil-repellency and water-repellency depend crucially upon the perfluoroacrylate component but the presence of long-chain fatty alcohol acrylates, such as lauryl and stearyl acrylate, has been shown to produce synergistic improvement in water repellency with the perfluoroacrylate without adverse impairment of the oil-repellency. Vinyl chloride, vinylidene chloride, methyl methacrylate and acrylonitrile are also frequently incorporated as comonomers where special performance characteristics are required, such as soil-repellency or resistance to organic solvents and white spirit.

             Figure Major components of a fluorocarbon dispersion

Fastness of the fluorochemical finish to domestic washing, laundering and dry-cleaning can be achieved using reactive monomers such as acrylates containing methylol- or epoxy- functional groups. Such reactive functional groups may self-crosslink or alternatively react with functional groups such as hydroxyl, amino or carboxyl groups in the fibre surface. The net result is that the fluorochemical polymer becomes crosslinked and is covalently bound to the fibre surface, greatly increasing the durability to domestic washing, laundering and dry-cleaning.

A softer handle can be attained by the incorporation of high molecular weight polydimethylsiloxanes. The oil-repellency can be enhanced by the use of fluorinated urethane structures, and the precise structure of the urethane components used can be varied within wide limits in order to optimise the performance capabilities of the fluorochemical finish to suit the end-use requirements.

The aqueous fluorochemical emulsion or dispersion must be stabilised using an appropriate emulsifier. Fatty alcohol ethoxylates are used as non-ionic emulsifiers, and quaternary ammonium salts used as cationic emulsifiers. Anionic surfactants are also used but in practice it has been found that combinations of several surfactants provide more stable emulsions, and hence this approach is widely utilised. The choice of emulsifier is critical because it has a considerable impact on the overall performance of the fluorochemical formulation. In processing, the emulsifier system used must provide a formulation that exhibits acceptable manufacturing characteristics allied with satisfactory shelf-life and transportation properties. In terms of the application of fluorochemical formulations to textile materials, the diluted emulsion must be stable under the application conditions. This means that the diluted emulsion must be robust to changes in pH, temperature, pad speed, shear resistance and pumping conditions.

Other auxiliaries used along with fluorochemical finish formulations include solvents, regulators (or chain transfer agents) and initiators. The solvent is essential for synthesizing the emulsion and also assists in the film-forming process. Regulators are used to decrease the molecular weight obtained from free-radical acrylate polymerisation, which itself is catalysed by initiators based upon azo or peroxide compounds.

2.2 WATER-REPELLENT FINISHES OTHER THAN FLUOROCHEMICALS

2.2.1 Metal salt finishes

In the period from 1880 onwards, when water-repellent finishing became more important, it was normal practice to impart water-repellency to tightly woven cotton canvas by lengthy impregnation (24–48 h) in an aluminium acetate solution (5–9 °Tw.). (Degrees Twaddell, °Tw is a measure of the specific gravity, SG, of the liquor and is defined from °Tw. = 200(SG – 1).)

Alternatively, water-repellency was achieved by jig or pad application of such an aluminium acetate solution followed by careful drying in festoon form or on a stenter. To avoid precipitation of insoluble basic acetates, the impregnation liquor temperature was maintained below 38 °C. During drying, the removal of water was accompanied by removal of some acetic acid, which then converted the water-soluble monobasic salt into an insoluble dibasic compound. The latter was precipitated onto the fibres to give a water-repellent finish with a harsh handle, and with very limited durability to washing.

2.2.2 Soap/metal salt finishes

One method of improving the water-repellency obtained through the use of aluminium salts is by combining the aluminium salt treatment with treatment in soap in order to deposit insoluble hydrophobic aluminium soap within the fabric. Alternative procedures were developed to achieve satisfactory results. In one treatment, cotton fabric was first impregnated in 5–9 °Tw. aluminium acetate, squeezed and dried, or partially dried. It was then passed through a weak aluminium sulphate solution. This latter stage ensured that the maximum water-repellency was developed because any soluble sodium soap remaining in the fabric was then converted into the water-insoluble aluminium soap. The fabric could then be rinsed in soft or hard water and dried. Alternatively, 2–5% soap solution at 60 °C was padded onto the fabric, which was then partially dried

3C17H35COONa + (CH3COO)3Al    à (C17H35COO)3Al + 3CH3COONa

2.2.3 Wax finishes

A popular form of water-repellent treatment over the twentieth century, and still used today, is the application of wax treatments to fabrics at open-width. Many patents have been issued on the application of fats and waxes; typically wax formulations contain paraffin wax (melting point 52–56 °C) either by itself or in combination with one or more waxes based upon esters of higher fatty acids and higher monohydric alcohols. Beeswax (mainly myricyl palmitate, C15H31COOC30H61, m.p. up to 62–65 °C), carnauba wax (myricyl cerotate) C25H51COOC30H61, m.p. 83–86 °C) and Vaseline (C18–C22 alkane, m.p. up to 60 °C) have also been used according to the finisher’s requirements.

2.2.4 Pyridinium-based water-repellent finishes

In 1931, research work by Deutsche Hydrierwerke led to patents on the manufacture of quaternary ammonium salts with the objective of giving an increased dyeing effect on cotton and cellulosic fibres. Typically halogen ethers of the general formula shown in Structure were reacted with tertiary bases to yield water-soluble quaternary ammonium compounds. (R = alkyl radical (at least C8), X = halogen radical, R′ = hydrogen or a hydrocarbon radical (at least C8).)

                                           R-O-CH(X)-R′

Thus the reaction of octadecanol, formaldehyde and dry HCl could yield a substituted methyl chloride via the reaction in following formula. This could then be further reacted with tertiary bases such as triethylamine or pyridine to generate quaternary ammonium compounds with good aqueous solubility, and strong wetting out and foaming properties.

C18H37OH + CH2O + HCl  à C18H37OCH2Cl + H2O

2.2.5 Organo-metallic complexes

Both chromium and aluminium organo-metallic complexes have been used to impart a semi-durable water-repellency to textiles composed of either natural or synthetic fibres.

Quilon (Du Pont) was marketed as a concentrated solution of a Werner type chromium compound in isopropanol and applied by a pad–dry–cure method. The Quilon chrome complexes produced fabrics with a soft handle and with a high initial water-repellency, but their more widespread application was greatly restricted because of the blue-green coloration that was imparted to the fabric. The main textile end-uses were therefore restricted to tents, awnings, boat covers, and surgical drapes and gowns made from non-woven materials. Similar products were marketed by a number of manufacturers – for example, Phobotex CR (Ciba), Quintolan W (ICI) and Ombrophob C (Sandoz). While Quilon chrome complexes are still manufactured, their main applications now lie in materials other than textiles, such as paper, packaging, release films and leather.

2.2.7 N-Methylol derivatives

The introduction of N-methylol (N-hydroxymethyl) compounds as self-crosslinking resins, and of cyclic reactants that can form crosslinks with functional groups in fibres, gave rise to a new method of water-repellent finishing, namely the use of resin-wax emulsions or dispersion. A large number of patents have been issued in this field since 1930s largely because this route opened up the possibility of producing wax-based water-repellent finishes that were durable to high temperature washing. As a result, a large number of waxy and long-chain fatty acid type compounds have been reacted with various N-methylol-based derivatives in order to produce water-repellent finishes with greater durability that may also be incorporated as extenders for fluorochemical finishing.

2.2.8 Silicone finishes

It was fifty years after the discovery of tetra-substituted asymmetric silicon compounds by Kipping and his coworkers in 1901 that silicones, based upon polysiloxanes, were first used as textile water-repellents in the UK. The hydrophobicity of silicones was first discovered by Patnode who noted that paper treated with chloromethylsilanes was water-repellent after exposure to moist air. Water hydrolyses chloromethylsilanes to silanols, which then condense spontaneously to form siloxanes.

Polysiloxanes are based upon, with an –O–Si–O–Si– backbone, where R may be a hydrogen, hydroxyl, alkyl, aryl or alkoxyl group. Polysiloxanes for use as waterrepellent agents for textiles are usually mixtures of linear polydimethylsiloxanes (Structure) and polymethylhydrogen siloxanes (Structure).

3. NOVEL TREATMENTS WITH POTENTIAL FOR WATER-, OIL-, SOILAND STAIN-REPELLENCY

3.1 The Lotus Effect: biomimetic ultraphobic surfaces

Biomimetics mimics naturally occurring biological mechanisms with modification, to produce useful imitative synthetic items using conventional methods available to science and technology. The Lotus Effect has been named after the unusual properties of the leaf surfaces of the lotus plant, which are remarkably water-repellent and soil-repellent. The surface of the lotus leaf is covered by a thin extracuticellular membrane termed the cuticle, which is covered by waxes forming characteristic microstructures due to self-organisation. On smooth wax layers (surface area contact 10%) the contact angle of water may reach 110°, but because of the surface roughness of the wax layer, whose dimensions can be measured in micrometres, a very pronounced superhydrophobicity is generated with contact angles up to 170° and surface area contact as low as 7%. (As an analogy, imagine a mercury droplet lying on a bed of nails or a pimple rubber mat.) As a result, the area for adhesion of water is markedly diminished and air is enclosed between the droplets and the wax crystals.

3.2 Pulsed plasma polymerisation of monomers

Conventional fluorochemical-based water-repellents are produced using combinations of resins, catalysts, homo- and copolymers, surfactants, pH adjusters, cross linking agents, heat and solvents. It has been pointed out that such processes can consume large amounts of solvent, are costly, produce large amounts of waste and are highly substrate-dependen.

The method of pulsed plasma polymerisation of monomers containing long perfluoroalkyl chains linked to a polymerisable carbon–carbon double bond avoids the need for using solvents. Deposited films can exhibit ‘super-repellency’ whereby liquid droplets just roll off the surface. This phenomenon of super-repellency is, in part, dependent upon the length of the perfluoroalkyl chains.

3.3 C2F6/C2H4 plasma treatments

Plasma treatment of textile fibres and other forms of textile material in fluorocarbon gases or gas mixtures have been studied in the research laboratory and by suitable adjustment of the process parameters non-wettable highly hydrophobic surfaces can be achieved on polyester textiles. The chemical surface modification and the surface roughness have been demonstrated to be responsible for the non-wetting properties of the treated fibre surfaces, and the low-energy polymer surface treatment also acts as a diffusion barrier for protecting the bulk material against chemical attack.

 3.4 Treatment with reactive fluorinated compounds in supercritical fluid carbon dioxide (SCFCO2)

Research studies have shown that low molecular weight reactive fluorinated compounds can be made to react with fibres in a polymer-analogous way. Thus it is possible to create a well oriented very thin layer composed of –CF3 groups. The close-packing of the –CF3 groups ensures that there is a decreased tendency of re-orientation in the presence of polar interfaces. Therefore, the repellent effect persists under water

3.5 Coating textiles with organically modified ceramics (ormocers)

At present this technique is still in the development stage, but it is conceivable that it could prove to have potential within the textile industry. In this novel approach, the precursors (metal alkoxides and/or organically modified metal alkoxides) are converted into stable colloidal solutions (sols) by partial hydrolysi]. The coating of this colloidal solution onto the textile may be carried out by conventional techniques, such as dipping, spraying or padding. Treatment of the sol is then carried out using UV radiation, thermal treatment or by evaporation of the solvent, which leads to condensation reactions and agglomerations causing the sol to gelate and form transparent layers.

4.  TEST METHODS FOR WATER-REPELLENCY 4.1 Test methods for water-repellency

There are three main types of test method used to evaluate the water-repellency of fabrics, which should be suitably preconditioned prior to testing under standardised conditions:

• Class I spray tests to simulate exposure to rain;
• Class II hydrostatic pressure tests – these measure the water penetration as a function of pressure exerted by water standing on the fabric;
• Class III sorption of water by the fabric immersed in water.

The most widely used test methods will now be briefly discussed. The precise details of the test methods are given elsewhere.

In the test, a test specimen (preconditioned at 65 ± 2% relative humidity and 21 ± 1 °C for at least 4 hours prior to testing) is backed by a weighed blotter and is sprayed with water in the AATCC Rain Tester for 5 minutes under controlled conditions. The severity of the simulated rain is altered by changing the height of the water column to give pressures of 60–240 cm water gauge. The blotter is then reweighed to determine the amount of water that leaked through the specimen during the test. The fabric performance is given by determining the maximum pressure at which no fabric penetration occurs, the change in fabric penetration as pressure is increased, and ‘breakdown’, the minimum pressure required to cause a penetration of more than 5 g of water.

Class II – Hydrostatic pressure tests

For many high-performance fabrics that are rendered waterproof, a hydrostatic pressure test may be conducted in one of two ways:

(1)By subjecting the fabric to an increasing hydrostatic pressure and measuring the pressure required to cause penetration;

(2)By subjecting the fabric to a constant hydrostatic pressure for a long period of time and noting whether any penetration occurs.

Class III – Sorption of water by the fabric immersed in water

‘AATCC Test Method 70 – 1997, Water-repellency: tumble jar dynamic absorption test’. This test method measures the absorption of water into fabrics, which may or may not have been given a water-resistant or water-repellent finish, under dynamic conditions similar to those often encountered during actual use. Preconditioned and preweighed specimens are tumbled in water for a fixed period of time and then are reweighed after the excess water has been removed by a wringer method. The percentage weight increase of the specimen is taken as a measure of the absorption or resistance to internal wetting.

5. Summary

Water proof and water repellent finishes are most widely used one. It makes the fabric useable in particular end use.

REFERENCES and URLs

1. Charles tomasino , “Chemistry & Technology of Fabric Preparation & Finishing”, , Department of textile engineering, chemistry and science College of textiles North carolina state university, 1992
2. Derek Heywood, “Textile Finishing”, Society of Dyers and Colourists, 2003.
3. Fischer, K., et all, “Ullmann’s Encyclopedia of Industrial Chemistry”, 2000
4. Schindler.W.D., and P.J.Hauser, “Chemical finishing of textiles”, Wood head PublishingLtd, 2004.
5. www.textileschool.com/articles/426/water-proof-and-water-repellent-finishes
6. nvlpubs.nist.gov/nistpubs/jres/38/jresv38n1p103_A1b.pdf
7. https://www.scribd.com/document/32213225/Water-Repellent-Finsihing
8. https://textilelearner.blogspot.com/2013/01/water-proofing-finishing-of-textile.html