Repellent Finishing | Mechanism of Repellency | Fluorocarbon Based Repellents

Repellent Finish
Anju Singh
Pursuing M.Sc. in Fabric and Apparel Science
Delhi University, India
Email: anjusingh292@gmail.com





REPELLENT FINISHES:

Finishes that repel water, oil, and dry dirt are important in all parts of the textiles market for clothing, home and technical textiles. Water repellency is achieved using different products groups, but oil repellency is attained only by fluorocarbon polymers. The oldest repellent finish is to repel water. Aim of finish is that drops of water should not spread on the surface of the textiles and should not wet the fabric, wetting occur when in form of droplet is absorbed by fabric. Most finishing it is desirable that drops stay on the surface and easily drips off or can be brushed off. The drops should stay on the surface and easily drips off. Similarly, oil repellent finishes should prevent oily fluids from wetting treated textiles. In a similar manner, soil repellent finishes should protect textiles from both dry and wet soils. In all cases, the air permeability of the finished fabric should not be significantly reduced. Water repellency maintains air permeability or breath ability of fabric and is not significantly reduced. In Water proof, breathable water droplets should not penetrate into fabric but perspiration should. To create a surface with low surface energy, so that the interaction between surface and fluid is less than internal between fluids and fluids, therefore a fluid drops off. Water proofing is a finishing that would withstand the hydrostatic pressure exerted by a column of water 1 min a depth before the first drop is able to penetrate inside. Water proof extreme form of water repellency swim costume but they are extremely stiff handle and lack of air and vapor permeability and hence discomfort to the wearer. Water proof breathable and coating with micro pores. Size is too small for water droplets to penetrate but pores were large enough for perspiration vapors to escape.

In addition to the desired repellence effects, other undesirable fabric properties are often found with repellent finishes. These include problems with static electricity, poor soil removal in aqueous laundering stiffer fabric hand, graying (soil redeposition) during aqueous laundering, and increased flammability. Some fabric properties that are often improved by improved by repellent finishes include better durable press properties, more rapid drying and ironing, and increased resistance to acids, bases and other chemicals.

MECHANISM OF REPELLENCY:
Mechanism of repellency
Repellent finishes achieve their properties by reducing the free energy at fiber surfaces. If the adhesive interactions between a fiber and a drop of liquid placed on the fiber are greater than the internal cohesive interaction within the liquid, the drop will spread. If the adhesive interactions between the fiber 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 yC must be lower than the surface tension of the liquid yL(the internal cohesive interactions) that is repelled yL of water, at 73 mNm-1, is two to three times greater than yL of oils (20-35 mNm-1). 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. Low energy surfaces also provide a measure of dry soil repellency by preventing soil particles from strongly adhering to fiber surfaces. This low interaction allows the soil particles to be easily dislodged and removed by mechanical action.

FLUOROCARBON BASED REPELLENTS
Fluorocarbon (FC) provides fiber surfaces with the lowest surface energies of the repellents finishes in use. Both oil and water repellency can be achieved. FC repellents are synthesized by incorporating perfluoro alkyl groups into acrylic or urethane monomers that can then be polymerized to form fabric finishes. The final polymer, when applied to the fiber, should form fabric that presents a dense CF3 outer surface for maximum repellency. The length of the per fluorinated side chains should be about 8-10 carbons. The small spacer group, mostly ethylene, can be modified to improve emulsification and solubility of the polymer. Co monomers (X, Y, for example stearylorlauryl-methacrylate, butyl acrylateetc) affect .fabric hand, film formation and durability. In this way and by appropriate emulsifiers, FC products can be widely modified for many special performance profiles. Most FC products are padded, dried and cued. Eat treatment causes an orientation of the performance side chain almost crystalline structures.

This is crucial for optimal repellency. Washing and dry cleaning disturb this orientation and reduce finish performance. The orientation must be generated by new heat treatments (ironing; pressing, or tumble drying). Depending on the kind of blocking groups, the isocyanine is activated at different temperatures and then reacts with the functional groups of the FC, the fiber or with itself (cross linking). This fixation on the fiber surface provides durability to washing, dry cleaning and rubbing as a second important effect.

Low curing FC’s are new development. They get repellency without heat, only after drying at room temperature,. An evitable disadvantage is their low durability because of the lack of fixation 'by cross linking. It saves cost and energy.

Water proofing of the fabrics may be divided into two large classes:
  1. Processes in which interstices of the cloth as well as the, surface of the fibers, are covered with a film or skin so that the goods & not-only water shedding but impermeable to air and moisture.
  2. Processes whereby the fibres are made water repellent through coating with hydrophobic substance or by a chemical reaction, but the fabric remains porous to air, i.e. "ventile. 
 

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