WATER REPELLENT FINISHES Textile

INTRODUCTION

What is Finishing:

Finishing is a series of processing operations applied to gray fabrics to enhance their appearance and hand, properties and possible applications. Finishing play a fundamental role for the commercial excellence of the result

of the textile

Develop the “product finishing” in all its fundamental elements such as hand and appearance; give the finished fabric some properties that grant an optimum behavior during the making up and all through the life of the textile. Much of the look, feel, and behavior of a fabric are determined by the finishing steps taken. Finishing can be mechanical (as in calendaring) or chemical, or both. Special treatments are applied to fabrics during finishing making them perform better, shrink less, resist flames, and repel water.

Object of finishing:

To enhance the suitability of the fabric for end use.

To improve sale appeal for comfort and utility.

To improve the feel of the fabric.

To import special properties to the fabric for special end uses such as waterproofing, flame-proofing etc.

To strengthen the fabric by coating or laminating.

Water-repellent Finishes

Water-repellent finishes resist wetting, 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. If the fabric becomes very wet, water will eventually pass through. Applied to fabrics found in raincoats, all-weather coats, hats, capes, umbrellas and shower curtains. The chemical treatment of a fabric to reduce its affinity for water. Pores of the fabric are open, and the degree of repellency varies. A water repellent fabric will give protection in a shower, but not in heavy rain. Water repellent fabrics will permit the passage of liquid water once hydro-static pressure is high enough. Water repellency is often created with wax or silicone resin finishes that enable the pores of the fabric to stay open so that it is more comfortable to wear than waterproof fabrics. Another name for water repellent is water resistant. It is important to distinguish between water-repellent and water-proof fabrics.

Difference Between waterproof finishes and water-repellent finishes

The term waterproof refer to the finish that block interstices of fabric by forming a hydrophubic film on its surface thereby making it impermeable to air. A waterproof fabric is one in which the pores, the open spaces between the warp and filling yarns and between the fibers, are filled with appropriate substances resulting in a fabric having a continuous surface and very small air permeability. It is used to describe a fabric which is coated or impregnated to form a continuous sheet composed of rubber, plastic,and the like. Water-Proof Fabrics are resistant to the penetration of water under much higher hydrostatic pressure than are Water repellent fabrics. Waterproof Finishes -Allows no water to penetrate, but tend to be uncomfortable because they trap moisture next to the body.

The water repellent finish is permeable to air but not to water and so garment made from such treated fabric are comfortable to wear. A water- repellent fabric is one whose fibers are usually coated with a hydrophobic-type compound, and whose pores are not filled in course of treatment. These types of fabrics are quite permeable to air and water vapor. A water repellent fabric will give protection in a shower, but not in heavy rain.

The Test Method Use for Measuring the Performance of Fabric in the Rain:

A. Method by which the hydrostatic pressure required to free water through a fabric is measured- In this class of test methods, the fabric is subjected to the action of water under pressure. Either the amount of water penetrating in a specified time or the pressure required to force water through, the fabric is measured. The hydrostatic-pressure test values are dependent mainly on the pore size and the angle contact.

B. Method by which surface wetting and penetration under the influence of falling drop is measured

– In this class of test methods, the fabric is subjected to the action of water drops. The number, size, frequency, and energy of the drops in the various tests vary considerably.

C. Method by which the absorption of water by the fabric, when immersed or manipulated underwater,Imeasured- In this class of test method, the water absorbed by the fabric is measured by weighing the test specimen after some form of partial drying.

D. Method by which the wettability of the surface of the fabric is measured by means of the angle of contact or some function of the angle of contact-

In this class of test method, the circular piece fabric is forcing through the surface of the water, by obtained a relationship between the pressure necessary to force the disk of cloth through the surface of the water and the angle of contact of the water to fabric.The above methods are used to measure the phenomenon of water repellency. It appears necessary that a combination of tests be used to evaluate the water repellency of a fabric.

Mechanical incorporation of finish in or on the fibre and fabric surface, fibre pores and in the spacing between the fibres and the yarns. e.g. Paraffin emulsions.
Chemical reaction of the repellent material with fibre surface. e.g. Fatty acid resins (stearic acid-melamine)
Formation of repellent films on the fibre surface. e.g. Silicone and fluorocarbon products.
Special fabric constructions like stretched polytetafluoroethylene films (Goretex), Films of hydrophilic polyester (Sympatex) and microporous contains (hydrophilic modified polyurethanes).

Silicon Polydimethylsiloxane products that are useful as water repellents can form a hydrophobic layer around fibres. It provides the ability to form hydrogen bonds with fibres as well as display a hydrophobic outer surface. In order to gain some measure of durability, silicones designed as water-repellent treatment usually consist of three components, a silanol, a silane and catalyst such as tin octoate. The outward oriented methyl groups generate the water repellency. During the drying step after pad application the silanol and silane components can react to form a 3-D corsslinked sheath around the fibre. This reaction is often completed after storage of about one day, then providing full repellency. The Si-H groups of the silane are the reactive links in the silicone chain generating cross-links or being oxidised by air or hydrolysed by water to hydroxyl groups. These hydroxyl groups may cause further cross-linking, but if too many of them stay unreacted, their hydrophilicity will decreasethe repellency.

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. .

Advantages

High degree of water repellency at relatively low weight of fabric concentrations.
Very soft fabric hand, improved sewability and shape retention, and improved appearance and feel of pile fabrics.
Durable to wash and dry cleaning, because there is a formation of a sheath of finish around the fiber.
More durable than wax repellent.
Less expensive than flurochemical repellent.

Disadvantage

Increased pilling and seam slippage.
Reduced repellency if excessive amounts are applied (for e.g. silicone double layer with polar.
Adsorption of hydrophilic substances found in dry cleaning and laundry products impair water repellency
No oil and soil repllency. And may enhance the attraction of hydrophobic dirt.
Waste water residual baths are toxic to fish.
Less durable than fluro chemical finish.

Fluorocarbon-based repellents

Fluorochemical repellents are unique in that they confer both oil and water repellency to fabrics. The ability of fluorochemicals to repel oils is related to their low surface energy, which depends on the structure of the fluorocarbon segment, the non-fluorinated segment of the molecule, the orientation of the fluorocarbon tail and the distribution and amount of fluorocarbon moiety on fibers. Most FC products are padded, dried and cured. Heat treatment causes an orientation of the perfluoro side chains to almost crystalline structures. This is crucial for optimal repellency. Washing and dry cleaning disturb this orientation and reduce finish performance. The orientation must be regenerated by a new heat treatment (ironing, pressing or tumble drying.) But with some new product only Launder-Air Dry is sufficient, but are not fast to laundering due to lack of fixation by crosslinking.

Another novel technique is application of FC together with dendrimers causing self organisation where the fluorocarbon chains are enriched on the surfae and co-crystallise with the dendrimers. This technique has better or equal effect at lower amounts of fluorocarbon compared to dendrimer free FC finish. The condensation temperature is also low (80-1300C).

Chemically fluorocarbon polymers, also known as Perfluroalkylacrylate copolymer and its fundamental structure resemble acrylic resins. Fluorocarbon polymers are special class of polymers and represent an indispensable part of the technology of water and oil repellant finishing and contain Carbon and Fluorine bonds. The relatively low reactivity and high polarity of the carbon- fluorine imparts unique characteristics to fluorocarbon polymers. FCP decreases the wettability but form water repellant and oil repellant polymer on its surface. A fluro polymer sheth around the fibers strongly reduces the textiles surface free energy, accompanied by the increase of the contact angle of liquids on its surface.

The non fluorinated part, in addition to being an extender by lowering the cost of the fluorochemical, serves two other useful purposes. It forms a backbone to the fluro chemical making it more durable and acts as a glue to bond the fluro chemical part to the fiber. The important feature of the polymeric back bone is that it is capable of forming a durable film on the surface of the fiber. The principle of fluorocarbon polymer finishing is similar to that of non stick frying pans. Fluorocarbon polymers act as a plastic sheet on top of the fabric and any liquid coming in contact is repelled and cannot pass through the barrier. The liquid beads up and roll off the fabric.

Advantages

Low active add on (0.5% ) and more rapid drying of treated fabrics.
Special FC allow soil release during household laundering or stain resistance on nylon. Which is especially useful for carpets.

Disadvantage

High cost
Greying during laundering.
Potentially hazardous aerosols
The need for special treatment of waste water from application processes
Cannot be applied effectively by exhaustion method.

Paraffin repellents

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 detracts from the hydrophobic character of paraffin. Either non-rewetting emulsifiers or some means of deactivating the hydrophilic group after the fabric is impregnated with the finish must be used.

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

They repel water not oil.
They are emulsions that contain aluminium or zirconium salts of fatty acids usually stearic acid.
These materials increases the finish’s adhesion to polar fibre surfaces by forming polar-non-polar junctions as shown in fig 9.
The paraffinic portion of the repellent mixture is attracted to the hydrophobic regions. While the polar ends of the fatty acid are attracted to the metal salts at the fibre surface.
These finished can be applied by both exhausting and padding. They are compatible with most kinds of finished but they increase flammability.

Although they are available at relatively low cost and generate uniform waterproof effects, the lack of durability to laundering and dry cleaning and their low air and vapour permeability limits the use of paraffin-based repellents.

Stearic acid-melamine repellents

Compounds formed by reacting stearic acid and formaldehyde with melamine constitute another class of water-repellent materials. Eg fig 10.
They hydrophobic character of the stearic acid groups provide the water repellency, while the remaining N-methylol groups can react with cellulose or with each other (cross-linking) to generate permanent effects. Advantages of the stearic acid-melamine repellents include increased durability to laundering and a full hand imparted to treated fabrics.
Some products of this type can be effectively applied by exhaustion procedures. Their use asextenders for fluorocarbon repellents is now increasingly replace by boosters as described ahead.
Disadvantages of stearic acid-melamine repellents include problems similar to durable press finished.

Advantages of Water repellent finishes

Especially durable to washing Also resistant to dry-cleaning.
Easy handling and storing because it is not inflammable.
Low foaming.
High resistant to yellowing, suitable for colored and white goods.
No influence on the light and shade.

Application of Water Repellent Finishes:

Carpet finishing.
Clothing (e.g. sportswear & rain coat.).
Umbrella cloth.
Table linen, curtains and upholstery fabric.
Tarpaulins and tents.
Technical fabrics (e.g. protective clothing).

Conclusion:

It should be emphasized that, the influence of fiber wetting is held to a minimum by treated with water repellent finishes.
If the fabric is sufficiently tight to repel water via high “jamming” of yarns, the problem of moisture vapor transfer and body comfort appears, since, because the fibers are hydrophobic, water vapor cannot transfer via the fiber itself. Presuming that a compromise can be made between tightness of structure and body comfort by means of chemicals water repellent treatment, it can be conclude that fabrics made from inherently water impermeable fibers should provide excellent “rain resistance.”
When fabric is treated with water repellent finishes, it “strong resistance to water under some pressure”, “moderate resistance to penetration by falling water” and resistance to actual wetting by water.”