Principles of Textile Finishing

Asim Kumar Roy Choudhury

• Publisher: Woodhead Publishing
• Year: 2017
• ISBN:
  9780081006610
• Topic: Technology & Engineering
• Subtopic: Industrial Engineering
• Index: Technology & Engineering

Principles of Textile Finishing presents the latest information on textile finishing for industry professionals and researchers who are new to the field. As these processes are versatile and varied in their applications, the book provides information on how decisions on finishes and techniques may be made subjectively or based on experience.

In addition, the book presents the desired final properties of textile materials and how they differ widely from product to product, helping finishers who face significant challenges in delivering fabrics that meet the requirements of end-users be successful.

Written by an author who is an expert in the field, and who has with many years of experience in industry and academia, this book provides an accessible introduction to the principles, types, and applications of textile finishes.

• Provides an accessible introduction to the principles, types, and applications of textile finishes
• Assists industry professionals and researchers in selecting finishes that will result in fabric properties that meet the requirements of end-users
• Written by an author with years of experience in industry and academia and who is an expert in the field

1

Introduction to finishing

Abstract

Textile finishing decides the ultimate appearance and aesthetic properties of textile material. Additionally, it is liable to change various physical and chemical properties of textile materials per consumer needs. Textiles are made with a wide variety of fibre compositions, yarns and fabric structures. On such varied platforms, the finishers face a real challenge to impart desirable textile properties to a desirable extent. The extent of a textile's finish is measurable in some cases, while in a large number of cases, the finish is related to sensory organs and is very much subjective. Finishes are mostly applied in mixtures and are difficult to identify and to standardise.

Keywords

Functional finishes; Physical finishes; Padding mangle; Stenter; Plasma finishing.

1.1 Introduction

Any operation for improving the appearance or usefulness of a fabric after it leaves the loom or knitting machine can be considered a finishing step. Finishing is the last step in fabric manufacturing and is when the final fabric properties are developed.

The term ‘finishing’, in its widest sense, covers all processes which fabrics undergo after their manufacture in looms or knitted machines. However, in a more restricted sense, it is the third and final stage of processing after bleaching and dyeing. Even this definition does not hold well in some cases where the fabric is not bleached and/or dyed. A simple definition of finishing is the sequence of operations, other than scouring, bleaching and coloration, to which the fabrics are subjected after leaving the loom or knitting machine (Marsh, 1979). Most finishes are applied to woven, nonwoven and knit fabrics. But finishing is also done in yarn form (e.g., silicone finishing on sewing yarn) or garment form. Finishing is mostly done in fabric form rather than in yarn form. However, sewing threads made from mercerised cotton, linen and their blends with synthetic fibres as well as some silk yarns require finishing in yarn form.

A fabric's finish can be either chemicals that change the fabric's aesthetic and/or physical properties or changes in texture or surface characteristics brought about by physically manipulating the fabric with mechanical devices; it can also be a combination of the two.

Textile finishing gives a textile its final commercial character with regard to appearance, shine, handle, drape, fullness, usability, etc. Nearly all textiles are finished. When finishing takes place in a wet state, it is called wet finishing, and while finishing in a dry state, it is called dry finishing. The finishing auxiliaries are applied using finishing machines, padders or mangles with one- or two-sided action or by impregnation or exhaustion. Altering the composition, rheology and viscosity of the finish applied can vary effects.

1.2 Object of finishing

The object of finishing is to improve the attractiveness and/or serviceability of fabric. There is a wide variation of techniques among different fabrics and different production units. In fact, many of them are trade secrets; that is why many details have not been published. There are actually very few published works available except about functional finishes, for which specific chemicals serve specific functions.

The variations of finishing depend on the following factors (Marsh, 1979):

1. The type of fibre and its arrangement in yarn and fabric

2. The physical properties of fibres such as swelling capacity and behaviour when pressure or friction is applied

3. The capacity of fibres to absorb chemicals.

4. The susceptibility of the materials to chemical modification.

5. The most important factor, the desirable properties of the material during its use

If the inherent property of the material is excellent, such as lustre of silk, little finishing is necessary. The materials made of worsted yarn require less finishing than those made of woollen yarn. The materials prepared from cotton need a variety of finishing techniques, as it has diversified uses.

1.3 Classification of finishes

The finishing processes may be broadly classified into two groups:

(a) Physical or mechanical

(b) Chemical.

The physical or mechanical processes encompass simple processes like drying on a steam-heated cylinder to various type of calenders, raising for soft effects on the surface of the fabric and breaking the finishing of filled goods for comfortable feel. Most of the mechanical finishes are known from ancient times and few changes have occurred in their method of operations. Some physical properties, such as dimensional stability, can be improved with chemical finishing.

Mechanical finishing or ‘dry finishing’ uses mainly physical (especially mechanical) means to change fabric properties and usually alters the fabric's appearance as well. The mechanical finishes include calendering, emerising, compressive shrinkage, raising, brushing and shearing or cropping. The mechanical finishes for wool fabrics are milling, pressing and setting with crabbing and decatising. Mechanical finishing also encompasses thermal processes such as heat setting (i.e., thermal finishing). Mechanical finishing is considered a dry operation even though moisture and chemicals are often needed to successfully process the fabric.

Chemical finishing or ‘wet finishing’ involves the addition of chemicals to textiles to achieve a desired result. In chemical finishing, water is used as the medium for applying the chemicals. Heat is used to drive off the water and to activate the chemicals. The chemical methods have changed with time remarkably, and the newer finishes have been developed continually. Many chemical methods are combined with mechanical methods, such as calendering, to improve the effect. Typically, the appearance of the textile is unchanged after chemical finishing.

Some finishes combine mechanical processes along with the application of chemicals. Some mechanical finishes need an application of chemicals; for example, milling agents are needed for the fulling process or reductive and fixation agents for shrinkproofing wool fabrics. On the other hand, chemical finishing is impossible without mechanical assistance, such as fabric transport and product application. The assignment to mechanical or chemical finishing depends on the circumstance; that is, whether the major component of the fabric's improvement step is more mechanical or chemical. Mechanical devices are used in both categories; the major distinction between the two is what caused the desired fabric change, the chemical or the machine?

Another method of classification is to classify finishes as temporary and permanent finishes. In fact, no finish stands permanently till the material is serviceable, hence a more accurate classification would be temporary or durable.

Some of the temporary finishes are:

(a) Mechanical: calender, schreinering, embossing, glazing, breaking, stretching, etc.

(b) Filling: starch, china clay and other mineral fillers

(c) Surface application: oil, different softeners and other finishing agents.

Some of the durable finishes are:

(a) Mechanical: compressive shrinkage, milling of wool, raising and cutting processes, permanent setting, etc.

(b) Deposition: synthetic resins—both internal and external, rubber latex, laminating, etc.

(c) Chemical: mercerisation, perchmentising, cross-linking agents, water repellent finish, fire-resistant and fireproofing finishes, shrinkproofing of wool, etc.

It should be noted that any such classification is arbitrary. Accurate classification is difficult because durability depends on several factors. Durability can be varied, and it is not possible to draw any borderline between temporary and durable finishes.

Finishing processes are so varied that it is difficult to classify them. For cotton, several finishing processes are used widely, but they are so varied in technique that it is difficult to group them together. For many years, the dispersion processes, namely mercerisation and perchmentisation, were the only permanent finishes on cotton, and they still remain of great importance today. The common chemicals used in these finishes are caustic soda and sulphuric acid, respectively, in a moderately concentrated form.

1.4 Physical finishing

Physical finishing methods for textiles include optical finishing, brushing and napping, softening, shearing and compacting of the textile structure.

1.4.1 Optical finishes

Lustre may be imparted to a fabric by physical means. The techniques basically involve flattening or smoothing the surface yarns using pressure. Beating the fabric surface or passing the fabric between hard calendering rolls under pressure and with some friction will tend to flatten out the yarns and lower light scattering by the fabric surface, thereby improving reflectance and lustre. Lustre may be improved further if the calendering rolls are scribed with closely spaced lines which will be imprinted on the fabric to reinforce light striking and reflecting from the fibre surface. Similar techniques can be used to impart optical light interference patterns on the fabric. Thermoplastic fibres which can deform under heat and pressure can most readily be modified to impart lustre.

1.4.2 Brushing and napping

Physical delustring of a fabric, as well as bulking and lofting of the fabric can be achieved by treatments which roughen the fibre surface or raise fibres to the surface. Fibre raising processes, such as brushing and napping, involve the use of wires or brushes which catch yarns in the textile structure and pull individual fibres partly from the yarn structure. The resulting fabric is warmer, softer and more comfortable.

During calendering or beating of a fabric interaction between individual fibres within yarns may be lessened and the textile structure softened.

Also, when a smooth textile structure free of raised surface fibres or hairiness is desired, the fabric may be sheared by passing the fabric over sharp moving blades or by passing the fabric over a series of small gas jets which singe and burn away raised fibres.

1.4.3 Compacting

During the fabric formation processes, tremendous stresses are applied on textile materials. Such stresses can be controlled by drying the finished fabric with or without tension on a stenter frame, which controls the width of the fabric and the tension on the fabric during the drying process. A second method involves compression of the fabric structure, as in the Sanforizing process. In this process, the fabric and backing blanket (rubber or wool) is fed between a feed roller and a curved braking shoe, with the blanket kept under some tension. The tension on the blanket is released after passing the fabric and blanket between the roller and braking shoe. The net result is the compaction of the fabric. Such a simple technique permits garment making with finished textile goods to be without fear of excessive shrinkage on laundering.

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