Tuesday, 2 October 2007

Textile Finishing

Finishing is the general term for a multitude of processes and treatments which a fabric may undergo after it has been made (woven or knitted) and coloured (dyed or printed). It is the final processing of the cloth and its purpose is to make the fabric suitable for its intended end use. That may mean. for example, making the fabric shrinkproof, softer, stiffer, water repellent, crease resistant or a combination of these properties.

CLASSIFICATION OF FINISHES

Textile finishes and finishing are classified in several ways. Persons concerned with end products (designers, merchandisers and sales personnel) usually categorize finishes as aesthetic finishes and functional finishes. The former modify the appearance and/or hand (feel) of fabrics, while the latter improve the performance of a fabric under specific end use conditions.
Persons concerned with textile processing (chemists and finishers) categorize finishes into chemical finishes and mechanical finishes. These are also called wet finishing and dry finishing, respectively.

Finishes are also categorized by their degree of permanence. These finishes are called permanent, durable, semi-durable and temporary.

Permanent finishes usually involve a chemical change in fibre structure and will not change or alter throughout the life of a fabric.

Durable finishes usually last throughout the life of the article, but effectiveness becomes diminished after each cleaning, and near the end of the normal use life of the article, the finish is nearly removed.

Semi-durable finishes last through several launderings or dry cleanings and many are renewable in home laundering or dry cleaning.

Temporary finishes are removed or substantially diminished the first time an article is laundered or dry cleaned.

PRE-TREATMENT PROCESSES

Pre-treatment processes consist of cleaning operations to rid the fabric of all soil and additives used during the weaving or knitting process. These processes are usually the first treatments a fabric undergoes after leaving the loom or knitting machine and are required before any dyeing, printing or finishing can be accomplished.

The processes consist of various types of cleaning actions, depending upon the fibre, the impurities present and the fabric construction. In cottons, cotton blend, silk and man-made fibres, the processes are, known generally as the boil-off. In woolens and worsteds, it is called a scour or scouring.


RESINS

Resins are the chemical group used in many of the finishes.

Resins are the most widely used chemicals in the textile industry. They are used for many purposes, primarily on cellulosic and cellulosic blend fabrics.

Resins have a profound effect on and cause changes in the hand (feel), drapability and physical characteristics of textiles. While many benefits are achieved through these changes, there are also some shortcomings. Resins modify fabrics in the following ways:

A. They add stiffness to fabrics and are thus used as stiffening agents or to create a firm hand.

B. Resins stabilize fabrics in the same shape or configuration as when the resin was cured. Fabrics cured m a smooth, nonwrinkled condition will return to that shape after being wrinkled in wear, while fabrics cured with creases in garments will retain these creases.

C. Yarns in fabric will be stabilized and will resist shrinkage in laundering.

D. Fabrics will become less moisture absorbent, thus drying more rapidly. They will also be less comfortable in warm, humid weather.

E. Resins combine chemically with cellulosic fibres (cotton, rayon,ete.) to cause significant reductions in abrasion resistance, breaking strength and tear strength. This reduction can be as high as 50%.

F. Most resins produce an offensive "fish-like" or formaldehyde odour in fabric. This odour eventually disappears on exposure to air and/or laundering.

G. Resins have an affinity for oily soils, creating a soiling problem. Soil release finishes help alleviate this objection.

IMPORTANT FINISHES

Anti-static Finishes

Anti-static finishes are chemical substances applied at the textile finishing mill for the purpose of reducing or eliminating static. These chemicals are actually substances which absorb small amounts of moisture from the atmosphere, thus reducing the dryness of the fabric.
Anti-static finishes are not a truly satisfactory method for coping with the problem of static in textiles because they are merely semi-durable. These finishes wash out or wear out in several launderings or dry cleanings. Permanent anti-static efrects are obtainable, however, with the man-made fibres which have been especially modified for this purpose.

Antiseptic Finishes

Antiseptic finishes are chemical agents inhibiting the bacterial growths which cause irritation and odour in shoes, luggage, underwear fabrics and similar items. These finishes are low in cost, easily applied and are durable to laundering and dry cleaning.

Calendering

Calendering is not a single type of finish. There are various types of calender machinery, each producing different types of finished fabrics.

Fundamentally, a ealender is a mechanical device consisting of two or more large rotating cylindrical rollers stacked on top of each other and usually heated. The cylindrical rollers are in contact with each other under pressure. Fabric being calendered passes around and between these cylinders. The specific type of calendered finished fabric varies with the nature of the cylinder surface, the speed of the cylinders and the nature of the fabric being finished.

The various types of calendering finishes include the following
(a) Simple calendering (b) Glazing calendering (c) Embossed ealendering (d) Moire calendering (e) Schreiner calendering

Crease Resistant Finishes

Crease resistant finishes are popularly known as CRF finishes. They are used on cotton, rayon and linen because these three fibres wrinkle easily. CKF finishes are resin finishes; the fabric is saturated with resin and then the resin is cured at temperatures of about 360°F. The fabric becomes stiffer, less absorbent and more resistant to wrinkling. Resin treatments also results in tensile strength loss and reduction of abrasion resistance in cellulosic fibres. Most CRF finishes are durable.

Flame Resistant Finishes

There are two systems to make fabrics flame resistant. The first is to use selective fibres which have characteristic flame resistant properties. The second is by the use of flame resistant finishes.

All of the many types of flame retardant finishes now available suffer from at least one of the following shortcomings : (a) they cause stiffening and loss of fabric drapability; (b) they result in significant strength loss in fabric; (c) they are easily removed in laundering (nondurable); and (d) they become ineffective when laundered in household bleach, with soaps or with water softeners.

Fulling

Fulling is a permanent finish used on wool fabrics; it is also known as milling or felting. The process is a carefully controlled scouring or laundering process to induce felting shrinkage in wool fabrics. The resultant fulled fabric is smoother, more compact and has yarns more tightly embedded than an unfulled fabric. Woolens are frequently heavily fulled.

Mercerization

Mercerization is one of the most important of all cotton finishes. This finish imparts luster to the cotton, increases its strength by nearly 25% and improves dye affinity, producing brighter shades than unmercerized cotton. It also enhances the hand as well as uses less dye to achieve the same depth of shade. The finish consists of treating the material while under tension with cold, concentrated sodium hydroxide solution. Both fabrics and yarns can be mercerized, but fibres cannot. Mercerization is a permanent finish.

Napping

Napping is a mechanical finish in which woven or knitted fabrics are passed against rotating, bristled wire-covered brushes. This action results in fibres actually being raised from the fabric. The overall effect is a fabric with raised fibre surface.

Napped fabrics have a softer hand and provide better insulation than the same materials unnapped because they can entrap more air; hence, their wide use in blankets, sleepwear and winter clothing. However, the insulating value of cotton and rayon napped fabrics is not long lasting. The low resilience of these fibres causes premature flattening of the fibre nap.The nap can partially be restored by frequent brushing.

Plisse

Plisse is the name of a finish as well as the name of a fabric treated with this finish. It is a permanent finish, produced on cotton by the action of sodium hydroxide; but unlike mercerizing, no tension is used. The sodium hydroxide is printed on the fabric in the form of a paste.The fabric shrinks only where the sodium hydroxide is applied, producing a puckered effect.

Shearing

Shearing is a process used to cut off surface fibres on fabrics. It makes uniform the surface of napped fabrics. Most cut pile fabrics are also sheared to provide uniform pile height.

Soil Release Finishes

Soil release finishes in fabrics permit relatively easy removal of soils (especially oily soils) with ordinary home laundering.

There are several types of soil release finishes. All of them accomplish the end result of making the fibre more absorbent (hydrophilic), thus permitting better “wettability" for improved soil removal.

Most soil release finishes are applied at the same time that the resins are applied to textiles. Most are durable through 40 to 50 launderings and are routinely applied to fabrics for work clothes and table cloths. They are also often applied to fabrics for slacks and skirts.

Several other benefits arise from the use of soil release finishes in durable press fabrics because of their increased absorbency. These include: improved antistatic properties, improved fabric drapability and somewhat greater comfort in hot weather.

An interesting FAQ about textile finishing can be found here. 

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