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 a fabric’s look, feel, and behavior depends on the finishing steps applied. Manufacturers can use mechanical methods, such as calendaring, or chemical treatments—or both. During finishing, they apply special treatments that improve performance, reduce shrinkage, increase flame resistance, and enhance water repellency.
Objectives of Finishing:
-
Firstly, to enhance the fabric’s suitability for its intended end use.
-
Secondly, to increase its market appeal by improving comfort and utility.
-
Thirdly, to refine the texture and feel of the fabric.
-
Additionally, to impart specialized properties such as waterproofing, flame resistance, and more for specific applications.
-
Finally, to reinforce the fabric through processes like 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. A water-repellent fabric typically has its fibers coated with a hydrophobic compound. Unlike waterproof fabrics, its pores remain unfilled during the treatment, allowing air to pass through while resisting water penetration.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. Specific substances fill the pores—open spaces between the warp and filling yarns and between the fibers—of a waterproof fabric.”This treatment creates a continuous surface with very low air permeability. The term typically refers to fabrics coated or impregnated with materials such as rubber or plastic, forming an unbroken, protective layer.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. Specific substances fill the pores—open spaces between the warp and filling yarns and between the fibers—of a waterproof fabric.”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.
Two key methods measure a fabric’s water resistance:
A. Hydrostatic Pressure Test:
This method measures either the amount of water that penetrates the fabric over a specified time or the pressure required to force water through it. The test results mainly depend on the fabric’s pore size and contact angle.
B. Spray Test (Surface Wetting and Penetration):
In this method, water drops fall onto the fabric, and the test evaluates how much wetting and penetration occurs on the surface.
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-
C. Absorption Test:
This method measures the amount of water absorbed by the fabric. After exposure to water and partial drying, the test specimen is weighed to determine how much moisture it has retained.
D. Contact Angle Method:
This technique evaluates the fabric’s surface wettability by measuring the angle of contact between water and the fabric. In this method, a circular piece of fabric is forced through the surface of the water. The pressure required to push the fabric through is related to the contact angle, providing insight into the water-repellent properties of the surface.
These methods are commonly used to assess water repellency. However, to accurately evaluate a fabric’s performance, it is essential to use a combination of tests.
- 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 as a water-based emulsion. When the emulsion contains cationic emulsifiers, the finish can be applied by exhaustion, as the positively charged particles are attracted to the negatively charged fiber surfaces.
Typically, manufacturers co-apply silicone water repellents with durable press finishes, which improve the longevity of the water-repellent treatment. In addition, silicone repellents are used to make upholstered furniture stain-resistant. Retailers often spray chlorinated solvent solutions onto upholstery as an optional treatment for customers.
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. Manufacturers must either use non-rewetting emulsifiers or apply a method to deactivate the hydrophilic group after impregnating the fabric with the finish. When the fabric is heated, paraffin wax melts and wicks into it, coating most of the fibers—especially those exposed to water—with a thin layer of wax.
This coating gives the fabric excellent water-repellent properties. However, wax water repellents have a major drawback: poor durability. Mechanical abrasion easily removes the wax, and it dissolves in dry cleaning fluids. Laundry processes can also wash the wax away.
A typical wax emulsion includes paraffin wax as the hydrophobe, an emulsifying agent, an emulsion stabilizer (protective colloid), and an aluminum or zirconium salt that deactivates the emulsifier during heating.
- 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.
-
- 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.”
related link