1 Chemical Finishes

Chemical finishing is usually applied to fabric padding following by curing and drying. This also called wet finishes. Applied to fabric in water or other liquid bath

Process involves

- Application of finish solution

- Evaporation of solvent or water (drying)

- Heating to activate chemicals (curing)

2 Types of chemical finishes

- Stiff and transparent

- Padders

- Foam finishing

- Spraying technique

- Parchmentization

- Resin finishes

- Softeners finishes

- Mercerization

- Soil release finishes

- Flame retardant

- Water repellant

2.1 Parchmentizing

- Treatment of cotton fabrics with sulfuric acid. The fabric is transparent, sherr, stiff.

- Burn-out, effects created by dissolving part of the fibres in a blend fanric, leaving the second fibre in place.

- A fibrous wed is parchmentized by drawing the web from a parchmentizing bath and passing it through successive washing positions and the feature of the invention resides in maintaining at a substantially constant level the temperature of the washing liquid at the first washing position.

Parchmentizing

- If cellulose is treated with a mixture of two parts of sulphuric acid and one part of water perfectly cold, it becomes like parchment. It should at once be washed with water, and then with ammonia and water. The swan incandescent light fibres are made of parchmentized cotton thread, which is afterward carbonized.

2.2 Resin Finishes (Permanent Processes)

- Used on cotton or rayon.

- Keeps fabric smooth after washing and drying with little or no ironing needed.

- Improves shrinkage control.

- Reduces fabric strength.

- May have odor problem.

- All resins contain formaldehyde

2.3 Softeners

- Fabric softener is product used while laundering to make clothes soft and eliminate static cling. Depending on the brand of fabric softener one purchases, it can be added at the beginning of the laundry cycle at the same time detergent is dispensed or during the rinse cycle.

- Fabric softener can also come in sheets to be placed in the dryer.

- Generally used with resins to improve the way the fabric feels.

- May cause fastness problems.

- May cause yellowing or odor problems.

2.4 Mercerization

- Mercerized is produced by the action of a strong alkali on cotton fibre rinsed under tension.

- Mercerization is a finishing process used to produce high quality fabrics, such as damasks.

- It consists essentially of impregnating stretched cotton with caustic soda (although other alkalis may be used). The treatment enhances the luster of cotton (the fibres are swelled), making it similar to silk.

- It also increases the fibre strength and affinity for dyes.

- A related process (liquid ammonia treatment) produces some of the effects of mercerization.

- Double mercerization means both the yarn and the knitted fabric are mercerized.

- The gloss of properly mercerized material will not disappear on hard water or laundering.

2.5 Coating

- Generally the process starts from a fabric or from a non-woven fabric as a backing

- All fibres can be used, from light silk to linen and hemp, from synthetic fibres to glass fibres.

- Coating polymers are bonded to the fabric backing by means of calendars, in the form of thin sheets or are mainly spread in the form of aqueous dispersions or solutions in solvents.

- The characteristics and the properties of coated fabrics depend on the chemical structure of the coating resins applied and the type of backing fabric used.

- The coating layer undoubtedly plays the most important role for appearance, hand and resistance properties.

2.5.1 The coating technique

- Coated fabrics are divided into specific categories:

o fabric based on natural, artificial and synthetic fibres;

o Coating layer of natural or synthetic resins.

- The coating layer can be obtained with the following processes:

o bonding with thin resin film;

o direct coating with resins;

o Indirect resin coating with the transfer technique

- The first process method, now scarcely used, the resin film is bonded to its fabric backing on special calenders. This process is still used today to manufacture tablecloths by applying a thin PVC film to the fabric.

- Direct coating process- the resin is spread directly on the fabric by means of a doctor knife or a cylinder with the following methods:

o air knife;

o knife on belt;

o knife on cylinder;

o "reverse-roll" cylinder

- During the air coating process and the belt coating one, the quantity of resin spread is relatively small and the desired thickness can be achieved only by applying successive layers, each one stabilised with an intermediate drying stage.

2.5.2 Air knife

Reverse-roll cylinder

Knife on steel cylinder

Knife on rubber cylinder

- Knife on cylinder - the knife can have a more or less rounded profile; the quantity of resin spread is measured in grams per square meter of dry substance and is determined by the dry content of resin, by the coating speed and by the knife profile.

- The greater the process speed & sharper the knife, the smaller the quantity of resin spread on the fabric.

- This coating method is used to produce waterproof fabrics for rainproof textiles.

- The most commonly used resins are the acrylic resins in solvent or aqueous dispersion and single or bi-component polyurethane resins.

2.6 Padders

- A padder consists of a trough and a pair of squeeze rolls (mangle).

- The fabric passes under a submerged roll in the trough filled with the treatment bath and then through squeeze rolls.

- Three factors control the amount of solution remaining on the fabric,

o Squeeze pressure (which is influenced by the composition of the rolls),

o Fabric construction (the solution resides in the capillary spaces between yarns and fibers), and

o The absorptive nature of the fiber.

- All fabrics have upper and lower wet pick-up limits. Within these limits, adjustments in wet pick-up can be made by increasing or decreasing the squeeze pressure.

- If the squeeze pressure is too low, puddles of solution will remain on the surface of the fabric. When this is dried, excess chemical will deposit in the overly wet areas resulting in non-uniform treatment.

- Wet pick-ups in the range of 75 to 100% are common by padding.

Kiss Rolls/lick roll

- A kiss roll applicator consists of a drum rotating in a trough containing the finish. A layer of liquid is picked up by the drum surface.

- This liquid is transferred onto a fabric "kissing" the exposed section of the drum. Kiss roll applicators are relatively simple devices.

- Within certain ranges, the wet pick-up can be adjusted to meet whatever is desired by controlling the variables stated above.

- However, uncontrolled variations in fabric speeds or drum rpm will cause unwanted variations in fabric properties.

- Guide rolls position the fabric to contact the wetted surface.

- This system has beta gauge moisture sensors ahead of and behind the kiss roll to monitor and control wet pick-up.

- The sensors are connected with speed controllers that adjust either the fabric or the roll speed accordingly

- The viscosity of the liquid controls the thickness of the film on the drum surface. More liquid will be delivered from a viscous liquid than from a less viscous one

Kiss rolls

2.7 Water repellent

Three main types:

1. Wax finish

2. Silicone finish

3. Fluorohefinish

- Expensive

- Confers oil repellency with soil release

- Used on durable press items

3 Conclusion

Chemical finishing is very essential to imparts functional and special effects on the fabric. Chemical finishing is generally permanent finishes which does not remove after washing. In the process of chemical finishing more chemical use to improve fabric handle properties and give special effect without any hazard affect for human body.