Functional Finishes for Textiles

Improving Comfort, Performance and Protection

Roshan Paul

• Publisher: Woodhead Publishing
• Year: 2014
• Print ISBN: 9780857098399
• eBook ISBN: 9780857098450
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Functional finishes for textiles reviews the most important fabric finishes in the textile industry. It discusses finishes designed to improve the comfort and other properties of fabrics, as well as finishes which protect the fabric or the wearer. Each chapter reviews the role of a finish, the mechanisms and chemistry behind the finish, types of finish and their methods of application, application to particular textiles, testing and future trends. - Describes finishes to improve comfort, performance, and protection of fabric or the wearer - Examines the mechanisms and chemistry behind different types of finishes and their methods of application, testing and future trends - Considers environmental issues concerning functional finishes

1

Functional finishes for textiles: an overview

R. Paul Hohenstein Institut für Textilinnovation gGmbH, Boennigheim Germany

1.1 Introduction

The textile industry is constantly striving for innovative production techniques to improve product quality and it is also important that these products are developed in an environmentally friendly way. Besides the traditional function of dressing people, textiles now provide wear comfort and protection in dangerous environments. The most important requirements for protective wear are barrier effectiveness and thermo-physiological comfort for the wearer.

Textile finishing chemicals are used to convert a textile material into a technical textile with functional properties. In the textile industry, finishing is usually carried out in the final stage of textile processing, as a result of which the textiles gain several functional characteristics. A wide variety of finishing chemicals are now available in the market that meet or exceed the expectations of consumers. Novel finishes providing high value addition to textile fabrics are greatly appreciated by a more demanding consumer market.

It is widely perceived that the end uses for technical textiles will continue to increase every year and the modification of commodity fibre and fabric properties by innovative finishes can be a cheaper route to high performance than by using a high cost fibre with inherent performance properties. The functional finishes can also be applied to apparel fabrics, household textiles and technical textiles to increase their appeal to the consumer and to stimulate the growth in niche markets. There are different types of functional finishes and the right type of finish should be selected depending on the fibre type of the textile substrate and the desired end use.

1.2 Types of functional finishes

Broadly, the finishing can be classified into the following classes, which are used individually or in combination with each other: mechanical, chemical and biotechnological finishes. Both mechanical and chemical finishes are classical finishes and these two types are, in fact, complementary. For example, the results obtained in mechanical finishing are greatly influenced by the previous chemical history of the fabric, and to obtain maximum benefit from chemical finishing, careful selection of relevant mechanical finishing treatments is essential.

1.2.1 Mechanical finishes

Mechanical finishing involves the application of physical principles such as friction, tension, temperature, pressure, etc. There are different types of mechanical finishes and calendering is one such process in which a textile is passed between rollers or calenders, usually under controlled heat and pressure, to produce a variety of surface textures or effects in fabric such as compactness, smoothness, suppleness, flatness, glazing, etc. The process involves passing the textile through a calender in which a highly polished, usually heated, steel bowl rotates at a higher speed than the softer bowl against which it works, thus producing a glaze on the face of the fabric that is in contact with the steel bowl. The normal woven fabric surface is not flat, particularly in ordinary quality plain weave fabrics, because of the round shape of the yarns, and interlacings of warp and weft are at right angles to each other. In such fabrics it is more often seen that even when the fabric is quite regular, it is not flat. During calendering, the yarns in the fabric are squashed into a flattened elliptical shape and the intersections are made to close up between the yarns. This causes the fabric surface to become flat and compact. The improved flatness of the surface in turn improves the glaze of the fabric.

Embossing is a particular type of calendering process, which allows engraving a simple pattern on the fabric. To produce a pattern, the fabric is passed through a calender in which a heated metal bowl engraved with the pattern works against a relatively soft bowl, built up of compressed paper or cotton on a metal centre. Sueding is a process which is carried out by means of a roller coated with abrasive material. The fabric has a much softer hand and an improved insulating effect thanks to the fibre end being pulled out of the fabric surface. Compacting is another process in which a durable finish is imparted on manmade fibres and knitted fabrics by employing heat and pressure to shrink them to produce a creepy and bulky texture.

Raising or napping involves the raising of the fibre on the face of the textiles by means of rollers covered with steel wires that are about one inch in height. This action raises the protruding fibres, causing the finished fabric to provide greater warmth to the wearer, and makes the textile more compact. It also causes the fabric to become softer in hand or smoother in feel and covers the minute areas between the interlacings of the warp and the filling. Napped fabrics include blankets, flannel, and several types of coatings and some dress items. Shearing is an important preparatory stage in the processing of cotton textiles. The objective of shearing is to remove fibres and loose threads from the surface of the fabric, thus improving surface finish.

Stabilization is a term usually referring to textiles in which the dimensions have been set by a suitable preshrinking operation. Decatizing is a finishing process applied to textiles to set the material, to enhance lustre and to improve the hand. In this case, the textile fabric is wound onto a perforated roller and is immersed in hot water or has steam blown through it. Steaming and heat setting are done by using high temperatures to stabilize synthetic textiles but are not effective on cotton or viscose. These actions may be performed on textiles in fabric form or garment form, but they may cause shade variation from side to side if done prior to dyeing, and may change the shade if done after dyeing. Fulling is another mechanical finishing process applied to wool to modify the structure, bulk and shrinkage of wool by applying heat combined with friction and compression.

Sanforizing is a process whereby the textile fabric is run through a sanforizer, which is a machine that has drums filled with hot steam. This process is done to control the shrinkage of the fabric. The fabric is given an optimum dimensional stability by applying mechanical forces and water vapour. Although it has long been recognized that cotton fabrics shrink when they are washed and dried, no attempts to prevent this undesirable shrinkage were completely successful until the concept of compressive shrinkage was put forward. The use of overfeed units, now a common feature of modern stenters, can somewhat reduce this potential shrinkage and width reduction. The fabric is compressed by physical means before the final finish so that its subsequent expansion during washing will offset the shrinkage attributable to the inherent properties of woven or knitted fabrics.

1.2.2 Chemical finishes

Traditionally, many of the functional properties given to textiles are achieved at the final finishing stage, where the chemical treatment of the fibres changes their appearance and improves their functional and comfort properties. The finishing is imparted by means of chemicals of different compositions and a textile can receive new properties otherwise impossible to obtain with mechanical means. A wide variety of functional properties can be created on textiles by means of chemical finishing and it is also possible to develop multifunctional textiles. The major application methods include padding, exhaustion, coating, spraying and foam application.

With the advent of nanotechnology, a new area has developed in the realm of textile finishing. Nanotechnology is opening new avenues in chemical finishing, either resulting in improved processes or helping to achieve new functional properties like self cleaning effects, which were not possible with conventional finishes. Thus the application of nanotechnology to textiles creates an expanded array of functional properties enabling textiles to be used in novel materials and products. Unlike in conventional finishing, the nanometric size of the coating will not affect negatively the hand and feel of the finished fabric. The low temperature sol–gel techniques, as well as the new generation of polymeric resins, are offering new possibilities in textile chemical finishing.

Another important development is the plasma enhanced chemical vapour deposition (PECVD) technique. It is a finishing process which can be used to deposit thin solid polymeric films from a gas state to a solid state on a textile substrate to achieve the desired properties. The advantage of such plasma treatments is that the modification turns out to be restricted to the uppermost layers of the substrate, thus not affecting the overall bulk properties. In general, plasma treatments can be considered as ‘dry’ alternatives to the wet chemical treatments and so they are environmentally friendly.

The layer by layer (LbL) assembly method is another new finishing technique by which ultrathin composite films can be developed on solid surfaces like textiles. It involves a layer by layer adsorption of polycations and polyanions to build a multilayer ultrathin polyelectrolyte coating on a textile substrate.

1.2.3 Biotechnological finishes

The recent trends in industrial biotechnology show that social, environmental and economic benefits go hand-in-hand with applications of this technology. Enzymes are the most important biological agents that are now used in the textile industry. Today, enzymes are used to treat and modify fibres, particularly during textile pretreatments and for finishing the textiles afterwards.

Enzymatic biopolishing removes the fibre fuzz and pills from the textile surface and yields a cleaner surface and a softer hand and increases lustre. Cellulases are the enzymes which are capable of hydrolysing the microfibrils protruding from the cotton surface. The weakened microfibrils tend to break off from the main body of the fibre and leave a smoother yarn surface. After treatment, the fabric shows a much lower pilling tendency. Unlike conventional chemical softeners, which tend to be washed out and often result in the yellowing of the treated textiles, the softness enhancing effects of cellulases are resistant to washing.

For cotton fabrics, biofinishing is optional, but it is essential for lyocell, which is made from wood pulp and is characterized by a tendency to fibrillate easily. If the fibrils are not removed, the finished garments will end up covered with pills. In the case of wool, the use of transglutaminases has been shown to improve shrink resistance, tensile strength retention, handle, softness, wettability and consequently dye uptake, as well as reduction of felting tendency and protection from damage caused by the use of common detergents.

Enzymatic hydrolysis of polyester fibres with different lipases or esterases can increase the hydrophilicity and absorbance properties. The selective enzymatic hydrolysis of polyacrylonitrile fibres with nitrile hydratase and amidase could be an interesting alternative to chemical finishing. In the meantime, biopolishing of polyester/cotton blends can be used as a novel technique for developing new shades. Some of the main hurdles for the industrial implementation of enzymes are their low stability, low compatibility with other chemical agents, longer processing time, and relatively high cost. But on the positive side, enzymes show the advantage of lower energy consumption and avoid the use of harsh chemicals, when compared to chemical finishing.

1.3 Finishes for improving comfort and performance

This book on functional finishes covers almost all the important finishes and finishing techniques in the textile industry. It discusses finishes designed to improve the comfort and other performance properties of textiles, as well as finishes which protect the wearers or the textiles. Each chapter reviews the role of a finish, the mechanisms and chemistry behind the finish, types of finish and their methods of application, application to particular textiles, testing and future trends.

Part One of the book deals with improving comfort and performance. The following functional finishes are detailed: thermal regulation finishes; moisture management and soil release finishes; softening finishes; enzymatic biofinishes; shrink resistant finishes; easy care finishes; self cleaning finishes; superabsorbent finishes; medical, cosmetic and odour resistant finishes.

1.3.1 Thermal regulation finishes

In recent years, new materials and new finishing processes for thermal regulation functions of textiles have been extensively studied. Microencapsules containing the phase change materials (PCM) use chemicals such as nonadecane and other medium chain length alkanes in their core. When the ambient temperature increases above their melting point, the microencapsulated chemical melts and latent heat is absorbed thereby interrupting the increase in temperature of a garment. Once the ambient temperature falls the PCMs solidify and the latent heat is released, providing a heating effect.

PCMs can be used as heat storage materials in the design and manufacture of thermoregulated textiles. In addition to the well known PCMs, such as paraffin waxes, hydrated salts, polyethylene glycols, etc., some new PCM materials such as fatty acid derivatives, bio-based materials, etc., are found suitable for textile applications. The thermal properties of PCM incorporated textiles can be designed to obtain the desired dynamic thermal insulation effect. PCM textiles with dynamic thermal insulation are now used for a wide range of industrial applications, specifically in aerospace, automotive, agriculture, biomedical, defence, sports and casual fabrics.

1.3.2 Moisture management and soil release finishes

Moisture management finishes improve the ability of textiles to absorb humidity from the skin, transport it to their outer surface and release it into the surrounding air. In a way, moisture management finishes increase the moisture holding power of the fibres. The new generation of novel softening agents which form part of moisture management finishes are capable of improving garment performance to a great extent. Such finishes are usually applied to towels, underwear, sports shirts and other items where moisture absorption is important. The manufacturers of sportswear continuously strive to improve the moisture management properties of the garments, and so there is a great potential for growth in this field.

A soil release finish does not prevent initial soiling, but it does enable deposited dirt to be removed in laundering. Soil release finishes attract water to the surface of fibres during laundering and help in removing the soils. Thus they permit relatively easy removal of soils with ordinary laundering. In the meantime, stain and soil resistant finishes prevent soil and stains from being attracted to fabrics. Such finishes may be resistant to both oil borne and water borne soils and stains.

1.3.3 Softening finishes

Chemical softeners are now widely used on textiles to develop a softer feel on the finished textiles. Softening can be considered as an optional finish and is usually carried out only when there is a need to improve the softness characteristics of a particular fabric. The right type of softener should be selected after considering the composition and properties of the textile substrate. In general, as far as softness and substantivity are concerned, cationic and silicone softeners top the list. Polyethylene competes with silicones in offering the best lubricity. Anionic and amphoteric softeners distinguish themselves in offering hydrophilicity. Hence choosing a proper mix of a variety of sof...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• List of contributors
• Woodhead Publishing Series in Textiles
• 1: Functional finishes for textiles: an overview
• Part One: Improving comfort and performance
• Part Two: Protecting wearers and textiles
• Index