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The making of clothing water repellent has been an age-old application used by man. The developments in water repellency have travelled long back and started with the use of animal skins, furs, tars and whale fats to make the clothing water repellent. Then it was the use of vulcanised rubber, which was being used as water repellent coating on fabrics. With the start of the century, chemicals began to be developed that helped to make the fabrics water repellent and more comfortable. It is very important to understand that waterproof and water repellent are two different terminologies with two different meanings. We can differentiate between the two as below: A waterproof fabric has the pores of the fabrics covered with a continuous surface i.e. it can be best understood as a coating being done on the surface, air permeability is low to non-existent, with no bonding between the fibres and the coating product; whereas water repellent fabrics do not have a continuous film and are permeable to air, formed basically by chemical reaction between the fibres and water repellent chemicals. There have been various classes of chemicals introduced, which will impart water and oil repellency to the fabrics. The main aim of the processors today is to make these finishes more permanent and comfortable to wearing i.e. durable to home laundering. Mechanism of Water and Oil Repellency Before going into the processing, its important to understand the mechanism of water and oil repellency. Let us consider a liquid put on a solid:
In the above case the dimensions of surface tensions determine whether the liquid will or will not wet the solid i.e. If the critical surface tension of solid is greater than or equal to the surface tension of liquid, the liquid will wet the fabric. If the critical surface tension of the solid is less than surface tension of the liquid, the fabric will repel the liquid. (In case of solids 'Critical surface tension' is used instead of 'surface tension') Thus water repellency can be obtained in case the critical surface tension of solid is smaller than surface tension of liquid. For example in case one drop of water is dripped on cotton fabric; it has been determined by test that surface tension of water and critical surface tension of cotton are respectively 72 dynes/cm and 200dynes/cm. Critical surface tension of cotton is greater than surface tension of water therefore water readily wets the cotton fibre. However once the cotton is treated with fluorocarbon water repellent in advance relation between them changes too. Critical surface tension of water repellent finished cotton is less than the surface tension of water. What are fluorocarbons? Fluorocarbons are organic compounds consisting of perfluorinated carbon chain. They tend to decrease the surface tension of the substrate. Fluorocarbon generally lowers surface tensions by forming a thin film coating around the fibre. They are cationic in nature but can also be non-ionic and anionics. Chemically fluorocarbons are known as perfluoroalkylacrylate copolymer and its fundamental structure resembles acrylic resins. Why fluorocarbons? Since the surface tension of Fluorocarbon water repellent agent is extremely small of 10 dynes/cm. So water repellency can be obtained because the drop remains on the cotton fiber. The industry started using water repellent based on paraffin, silicone and fluorocarbons. Comparing the three systems, it was found that paraffin types have low water repellent effects at the initial stage and no durability to washing at all. The silicone types were better than the paraffin based products but were poor oil repellent. The fluorocarbon-based products were found to produce excellent oil and water repellency. The addition of crosslinking agents along with the fluorocarbon improved the permanency of water and oil repellency. Application of Fluorocarbons Fluorocarbons can be applied in a number of ways. They can be applied by padding, Kiss Coat, spray, foam and exhaust. Padding method is one of the most commonly used in treating the fabric with fluorocarbons as the consistency and complete fabric coverage is possible when doing in pad method. In case of treatment of garments with fluorocarbons we can use exhaust or spray method. The processing with fluorocarbons has to be taken care of as the quality of the water and oil repellent properties are much dependent on the right kind of processing. Following things are needed to be taken care of during the processing the garments or fabrics 1. Hot and cold rinsing after dyeing. 2.Hot and cold rinsing after soaping has to be done without fail. If the levelling agent, detarding agent, penetrating agent or some other alkaline remains on the fabric, will tend to make the treating solution. 3. The preferable pH is between 5 to 6. Alkaline pH should be avoided. 4. The fabrics should be completely scoured. 5. Use of mercerized fabric will always tend to give better results in terms of performance. 6. The fabric should be bought to its normal temperature before treating with fluorochemicals as the higher temperature of the fabric just after drying may increase the temperature of the solution more than 400 C thus making the treating solution unstable. 7. In case of using softeners alongwith the water-repellent products the choice of softener should be made in such a way that they donot adversely effect the water and oil repellency properties. 8. In case of general water repellent crosslinking agent can be avoided but in case of when super durable water and oil repellent finish is a desired crosslinking agent are unavoidable. Crosslinking Agents: A must for Durable Water and Oil repellent finish The crosslinking agents are indispensable for improving the durability of fluorocarbon water repellent agents. Crosslinking agents prevents the water-repellent agents from dropping out of fiber by washing since it forms a three dimensional network on the fiber by reacting water repellent agent and the fiber. Generally, in case of adhering material B to the surface of material A, the more physical properties of two materials are similar, the more they adhere strongly each other. In other words, the more physical properties of two materials are different, the more they do not adhere. Generally, fluorocarbons, which has hydrophobic properties, adheres strongly to synthetic fibers, which have strong hydrophobic property. Therefore, good durability cannot be obtained in case of Cotton because of insufficient adhesion force with hydrophilic fibers.
In order to improve adhesion force with hydrophilic fibers, it is necessary to make the surface of hydrophilic fibers hydrophobic. Therefore, a crosslinking agent which is hydrophobic in nature and also that which should crosslinks cellulosic fiber is required.
Generally blocked isocynate based crosslinking are found to be very effective. They are hydrophobic in nature thus having the higher adhesion to fluorocarbons and the also have the tendency to crossslink with fibers at the more reactive sites (-OH group) of hydrophillic fibers cellulose. Curing It is possible to fix Fluorocarbons
on the fibers by treating the fabric finished with fluorocarbons at
140° C for 1 minute. However in order to get durability of the performance
it is effective to take more time at higher temperature. In case of
garments, the recommended Curing time is 10 to 15 minutes. Areas of Problems 1 Poor oil and spray rating: Poor oil and spray rating is because of basically two reasons: (a) Not using the right concentration of the product. This can be adjusted by increasing the concentration of fluorocarbons or by increasing the wet pick up of the garments. (b) The fabric are not cured for the right duration, i.e., the garments or the fabric is under cured. It can be adjusted by adjusting the temperature and the speed of the stenter in case of fabrics. In case of garments, they are needed to be cured at right temperature, i.e., 1300c to 1500c and for a right duration. 2.Good oil and poor spray rating: The basic reason for the good oil and poor spray rating is residual wetting agent in the fabrics or the garment. A thorough rinsing of the fabric tends to address this problem. 3 Poor oil and good
spray rating: The cause can be excess of softener or wrong
softeners, generally silicone softeners are needed to be taken in account
for major failures as per the test parameters. Team S&A
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