C6 WATER REPELLENT

Site: Textile Value Chain

Abstract:-

During the last few years, easy care concept has expanded rapidly to include Oil and water resistant features in many apparel applications. Easy care concept now encompasses both minimizations of ironing and easiness to clean garments that get stained in day to day use. Fabric manufacturers and chemical manufacturers have made significant advances in finishing processes to ease stain removal and prevent stain penetration Substrates that possess water and oil repellency are desirable for in many textile applications. Water repellency is increasingly becoming the focus of interest for protective clothing. This repellency can be achieved by implementing water repellent chemicals on textile fibres with minimal effects on other functional properties like strength, flexibility etc. Water and oil repellant fabrics can be used for a variety of end uses such as outerwear, where the requirement is for a high degree of water repellency and general wear such as expensive silk sarees, where the focus is more both water and oil repellency. Other common end uses for these finishes include upholstery, rugs, carpets, protective clothing Filter fleece, Uniforms, Table cloths, wallpaper, etc, etc.

Introduction

Water repellency is defined as the ability of a textile material to resist wetting. In principle, water repellent treatments are based on the deposition of hydrophobic substances on the fibre. The difference between waterproof and water repellent finishes is that in the former the interstices between warp and weft yarns are completely blocked by the continuous hydrophobic film of the substance and not only water pass through the fabric, but the fabric will become impermeable to air also. Water repellent finish, on the other hand, is one in which the material is made water repellent without filling up the interstices of fabric completed which results in fabric permeable to air but not to water. Mechanical treatment, chemical treatment and coating methods are three main methods for imparting water repellency to textiles. In these processes, generally durable repellency effects are obtained. In addition to the desired repellency effects, other undesirable fabric properties are often found with repellent finishes. These include problems with static electricity, stiffer fabric hand, greying during aqueous laundering and increased flammability. Research is going on to reduce the above-mentioned problems and obtain a durable water repellent fabric.

Quality of Water Repellent Finish

• The product should impart water repellent finish for the majority of fibre types although there is not available any universal agent for all fibres.
• The finished fabric should be durable to washing and resistant to dry cleaning
• Easy handling and storing
• Low foaming
• High resistance to yellowing tendency making it suitable for both white and colored goods.
• It should not affect adversely the light fastness of shade
• The fabric finish should not become very stiff and harsh.

Mechanism

Repellent finishes achieve their properties by reducing the free energy at fibre surfaces. If the adhesive interactions between a fibre and a drop of liquid placed on the fibre are greater than the internal cohesive interactions within the liquid, the drop will spread. If the adhesive interactions between the fibre and the liquid are less than the internal cohesive interactions within the liquid, the drop will not spread. Surfaces that exhibit low interactions with liquids are referred to as low energy surfaces. Their critical surface energy or surface tension γC must be lower than the surface tension of the liquid γL (the internal cohesive interaction) that is repelled.

For fabrics to be water repellent, the Surface free energy of the fibre’s surface must be lowered to about 24 to 30 mN/m. Pure water has a surface tension of 72 mN/m so these values are sufficient for water repellency.

Oil repellency requires that the fibre surface be lowered to 13 mN/m. Only fluorochemicals are able to function as oil repellents so whatever is mixed with them must not interfere with how they are deposited. Therefore, oil repellency finishes with fluorocarbons (yC = 10-20 mNm-1) always achieve water repellency but fluorine free products, for example silicones (yC=24-30mNm-1) will not repel oil.

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 nonfluorinated segment of the molecule, the orientation of the fluorocarbon tail and the distribution and amount of fluorocarbon moiety on fibres. Low surface energy can be described in critical surface tension terms.

APPROACHES TO IMPART WATER REPELLENCY TO FABRICS

• Spray-on and wash-in products:There are the number of temporary solutions for creating waterproof (or ‘spill-proof’ to be precise) solutions for garments and fabrics by sprays or additives in home laundries. These solutions, although effective for a one-off use, are not for long term and tend to diminish in effect in a few washing cycles.
• Fabric finishes:Finishes are the most common and widely used approach for imparting durable water repellency on fabrics and garments. These finishes (commonly called DWR finishes) are performed after the fabric is constructed. The oldest water repellent finishes for fabrics started with coatings of paraffin or wax but they used to wash out eventually. Recent finishes mainly involved fluorocarbon based chemistries. Perfluorocarbons (PFC’s) are capable of repelling water, oil and other liquids that cause stains. However, their toxic effects and bioaccumulation have been a major ecological concern.
• Yarn based solutions:Yarn based solutions have not deviated much from fabric finishes, in terms of chemistry, but they are focused more on treating the yarn instead of the fabric. This approach not only provides better protection from stains but also help in maintaining the breathability of the fabric similar to without the treatment.
• Water proof membranes:Water proof membranes are typically made of PTFE (poly tetra-fluoro ethylene) and related compounds. These are the same fluorinated compounds found in non-stick cookware and paints and coatings. The outdoor market utilizes a range of such solutions which resist water to penetrate the fabric but maintain the breathability to an acceptable level.
• Fluorocarbon based Water RepellentFluorinated water repellents are widely used on textile products due to their outstanding ability to protect against water, oil and soil. Fabrics treated with fluorinated water repellents are suitable for outdoor clothing to help provide excellent durability.Fluorocarbons (FC) provide fibre surfaces with the lowest surface energies of all the repellent finishes in use. Both oil and water repellency can be achieved. FC repellents are synthesised by incorporating perfluoro alkyl groups into acrylic or urethane monomers that can then be polymerised to form fabric finishes. Originally, the perfluoro alkyl groups were produced by electrochemical fluorination, but today they are produced by telomerisation. The telomerization process produces primarily or exclusively linear PFASs, whereas the electrochemical fluorination process produces a mixture of branched and linear isomers. The final polymer, when applied to a fibre, should form a structure that presents a dense CF₃ outer surface for maximum repellency. General advantages of fluorocarbon-repellent finishes include low active addons (< 1 % owf) and more rapid drying of treated fabrics. Special FCs allow improved soil release during household laundering or stain resistance on nylon, which is especially useful for carpets.They also have excellent chemical and thermal stability which provides treated fabrics with good durability (e.g., during laundering and dry-cleaning. Most repellents based on this chemistry are applied by padding process and then dried and cured.The presence of fluorine, the most electronegative atom, allows PFC-based DWRs and other fluorosurfactants to reduce the surface tension of the fabric to lower than that of water and oil. The unique ability to repel oils as well as water has been a major contributor to the popularity of PFC-based finishes.C8 Chemistry – The backbone of these compounds is made of a chain of 8 carbon atoms. Two methods are used to produce two slightly different products, namely “electro-fluorination” (electrolysis to replace hydrogen atoms in a molecule by fluorine atoms to create the 8 unit chain containing just carbon and fluorine) and “telomerisation” (joining single units together on to the growing polymer chain). The molecules, PFOA (perfluoro octanoic acid) and PFOS (perfluoro octanoic sulphate), so produced, have been found accumulated in various animals and have adverse effects, including carcinogenicity and toxicity. As a result of their strong carbon-fluorine bonds, PFOA and PFOS do not break down in the environment. They have been shown to be persistent in the environment and have long elimination half-life in wildlife and in humans.

The good news is there’s a way to limit the negative side effects of fluorocarbons to a minimum. The solution lies in reducing the fluorocarbon chain from C8 to C6.

C6 Chemistry – C6 based fluorocarbons were introduced to minimize the release of toxic chemicals. C6 based fluorocarbons showed decreased water repellency than C8 based fluorocarbons ones. C6 Chemistry – PFHA (perfluorohexanoic acid) with backbone of 6 carbon atoms, is supposed to be 40 times less bio-accumulative than PFOA (the 8-carbon counterpart). But it is also less effective, so more of the chemical has to be used to achieve the same result. Additionally, the recipe also involved small traces of C8 molecules. At present C6 technology is most prevalent in the textile industry although a growing number of sustainability-conscious brands are phasing out the use of such chemicals in their products.