Chemical Finishes in Textiles: Types and Applications

Chemical Finishes in Textiles:
Textile finishing can be subdivided into two distinctly different areas: chemical finishing and mechanical finishing. In this article I will discuss only chemical finishes in textiles. Chemical finishing can be defined as the use of chemicals to achieve a desired fabric property. Chemical finishing results in change of chemical composition of 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. Applying chemical finishes to garments is usually accomplished using exhaustible finishes (softeners, antimicrobials, ultra violet (UV) absorbers, and so on), which are added to the bath of the garment processing machine after all other garment wet processing steps have been completed.

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.

In chemical finishing relatively minor amounts of chemicals (often < 5 g / m² of the fabric) are applied on both sides of the fabric through padding or impregnation. In coating, relatively high levels of chemicals (15–50 g / m² of the fabric or even more) are applied on usually one side of the fabrics (although sometimes fabrics may be coated on both sides).

There are many chemical finishes in textiles. A thorough review of all those chemical finishes in textiles is beyond the scope of this article. I only discuss only main chemical finishes in textiles. The application and formulation of chemical finishes depend on several factors such as the compatibility of different chemical finishes, the nature of the material to be treated, and the chemistry of the functional chemical. It is also important to consider the environmental and sustainable credentials of the process.

If the desired functional chemical displays a high affinity toward the substrate, then exhaust (immersion) application methods may be suitable, and thus, any of the textile dyeing machines typically employed for batchwise dyeing can thus be used for this type of textile finishing. However, should the applied functional chemical have low or limited substantivity, a continuous application method should be employed. One of the most widely used machines for the application of finishes in a uniform manner to fabrics is the pad mangle. Using the pad–dry–cure method, the fabric is first immersed in a solution containing the functional chemical(s), followed by partial drying and lastly curing to permanently fix the finish to the fabric (Figure 13.2).

Chemical finishes can be applied by a number of methods including exhaust (running batchwise in finish liquor after dyeing), padding and curing (immersion in the treatment solution followed by squeezing to remove excess and heat treatment), spraying, printing, foam application or vapour techniques. In addition, the finish can be added to the spinning bath prior to formation of manmade fibers.

The proper formulation of chemical finishes is not easy. Several important factors are to be considered before the finalization of a formulation; a few are as follows:

1. The type of textile (fiber composition of the fabric and its construction)
2. The performance requirements (extent of effect and durability)
3. The economics of the formulation
4. Availability of machinery and associated process restrictions
5. Procedure requirements
6. Environmental consideration
7. Compatibility and interactions of finishing components.

Chemical finishes should meet the following requirements:

1. Low-cost product and process
2. Stable during storage and application in terms of pH, temperature and mechanical stress
3. Compatible with other finishes
4. Adaptation to customer requirement and substrate variation
5. Suitable for all kind of fibers and all textile forms such as yarn, woven or knit fabric, garment, nonwovens, etc.
6. Satisfactory stability during washing and dry cleaning
7. Should not hamper important textile qualities
8. On application should be distributed evenly on the fabric and fiber surface
9. No yellowing of white goods or color change of dyed goods.
10. Easy correction of finishing faults
11. Nontoxic and ecofriendly
12. No release of volatile organic compounds
13. Biodegradable

Types of Chemical Finishes in Textiles:
The most common types chemical finishes applied to textiles are:

1. Water Repellent Finish
2. Flame-Retardant Finishing
3. Soil-Release Finish
4. Soil Repellency
5. Oil-Repellent Finishing
6. Ultraviolet Protection Finish
7. Antimicrobial/Antifungal Finishes
8. Insect-Repellent Finishes
9. Mothproofing Finish
10. Resin Finishing (Crease Resist, Permanent Press)
11. Antistatic Finish
12. Stiffening
13. Easy-Care Finishing
14. Coating and Laminating
15. Softening Agents

Sometimes it is necessary to apply more than one type of chemical finish to meet the performance standards required. For example, in a fabric there may be a need to apply a flame retardant as well as a softening agent. In situations like this it is important that the chemical finishes applied are compatible and the effectiveness of one finish is not compromised by the presence of another finish, or vice versa.

1. Water Repellent Finish:
This is often referred to in the literature, particularly historically, as waterproofing. Besides, a large number of terms are used to indicate the water-repellent qualities of garments, such as shower-proof, rain-proof, shower-resistant, rain-resistant, shower-repellent, rain-repellent and so on. In general, water-repellency is the relative resistance of a fabric to surface wetting, water penetration or water absorption.

However, what does waterproof mean? Waterproof in a textile sense generally means a material that prevents the penetration and absorption of water, thus providing a barrier to water. Thus, it is perhaps more appropriate to talk of water repellency, but again, this term has some shortcomings, as a liquid in contact with a solid will always experience some form of attraction, however small. The easiest method to produce a waterproof fabric is by coating it with a polymer coating of polyvinyl chloride or polyurethane. This means that such materials could be used for tarpaulins, as the coating provides a solid barrier, which prevents the penetration of water (or indeed other liquids). However, such a solid barrier also prevents the passage of both water vapor and air, meaning that if it were to be used in a garment context, the wearer would quickly become uncomfortable. In this scenario, where a wearer is likely to perspire, a garment must allow the water vapor to pass through the fabric to maintain wearer’s comfort while also maintaining its waterproof properties.

To achieve a water-repellent surface, the free energy at the fiber’s surface must be decreased. In doing this, the adhesive interactions between the drop of water on the garment and the textile surface are less than the internal cohesive interactions of the drop. As a result, the drop will not spread but will remain in a bead form. The most popular way to achieve this in recent times has been through fluorocarbon finishes, which also offer good oil-repellent properties. However, environmental concerns surrounding the prevalence of perfluoro octanoic acid (PFOA) and the classification of polyfluorinated compounds as persistent organic pollutants call into question their long-term future.

2. Flame-Retardant Finishing:
The expression of ‘flameproof’ is incorrect, since only certain fiber materials can be said to be flameproof, mainly those of inorganic origin. Polyacryl nitrile fibers and cellulose fibers are the easiest to ignite and burn. The flame-proofing procedure therefore mainly concentrates on these textiles.

Textile materials that are flame-retardant are intended to resist fire but may still catch fire. To create fire, three elements are required: fuel, oxygen, and heat (or a source of ignition). Flame retardants aim to disrupt the fire triangle by depriving or reducing one of these three essential components.

Cellulosic textiles, including regenerated fibers such as viscose, account for more than 50% of fiber consumption per annum. Owing to the flammable nature of these materials, there is a large market in flame-retardant cellulosics, which comprise more than three quarters of the total flame-retardant market.

To understand how flame retardants can offer a benefit to textile materials, we first need to understand the process by which textile combustion occurs; initially, an ignition source, such as a small flame, perhaps from a match or a cigarette, or heat, perhaps from an electric heater, provides energy to the system. This ignition source causes the material to ignite and burn, leading to pyrolysis and the release of flammable gases. In the case of some materials, they will remain in the solid phase and slowly smoulder, sometimes even selfextinguishing if a char barrier is formed—this is a carbonated barrier between the ignition source and the unburnt bulk material, which leads to a breakdown of the fire triangle. If flammable gases are released from the material, they will mix with oxygen from the air and burn; this will cause more of the textile material to burn, leading to more flammable gases being released, which will cause further propagation of the burning process.

Flame retardants must act to disrupt at least one aspect of the fire triangle, in order to retard if not inhibit fire. Three main ways to achieve this are as follows:

1. Promotion of a char layer to insulate the available fuel within the textile material, providing a physical barrier to further burning
2. Emission of water, nitrogen, or an inert gas to reduce the oxygen concentration and dilute the levels of flammable gases, inhibiting flame formation
3. Delay or inhibition of flashover, which makes it harder for occupants to escape from a burning room, by disrupting the combustion stage of the fire cycle

3. Soil-Release Finish:
Applications of soil-release finishing processes are specifically designed to ensure more efficient laundering of soil and stains. Soil-release products currently in use can be based on the following chemicals, for example: silicium compounds, carboxymethylcellulose, ethoxylated compounds, polyglycol ester of terephthalic acid, acrylic acid polymers, and fluorochemicals. These are frequently utilised in combination with resin finishing agents under the conditions specified for cross-linkers. Generally speaking, there are no conditions of application specific to soil-release finishing. The level of permanence of the soil-release effects achieved is dependent on the product used. Good soil-release effects are achieved through application of the dual-action principle, using fluorochemicals that are oleophobic and hydrophilic.

4. Soil Repellency:
Soil repellency refers to the resistance to soiling as a finishing effect, which prevents soil penetration or makes it difficult. Examples of soiling include dry soil (dust), wet soil (fruit juice, ink), and oils and fats (engine oil and skin grease). Soil repellent finish is an alternative term for antisoiling finish. It prevents dry soil deposits on synthetic fiber textiles and should not be confused with a soil-release finish.

5. Oil-Repellent Finishing:
The textile auxiliaries used for water-repellent finishing are not sufficient to protect textiles against grease and oil stains. For this, special products are used, e.g., fluorocarbon polymers, which are used in the form of emulsion, sometimes in padding and sometimes in the exhaustion method. However, these products in turn do not provide a good water-repellent effect, which is why in practice oil-repellents and water-repellents are always used together. Some of the products available on the market provide effects which are resistant to washing and dry cleaning. The synthesis technique of telomerisation provides access to perfluoropolymers. Sterically small base groups permit the arrangement of the fluoroalkyl chains close and parallel to one another, so that the result is oil repellence and water repellence.

6. Ultraviolet Protection Finish:
Applying a textile covering to the skin is going to increase the amount of time taken for the skin to burn compared with bare skin, but ultraviolet (UV) radiation is still able to penetrate the textile, and thus, it can be desirable to add UV protection finishes to apparel to increase protection for the wearer. Thus, not only the textiles that are likely to be worn in the sun, such as t-shirts, sun hats, swimwear, but also items such as tents and caravan awnings are given a UV protection finish.

UV radiation refers to light with a wavelength between 280 nm and 400 nm, but it is the radiation in the UV-B region (280–320 nm), particularly 300–310 nm, that poses the greatest danger to the skin. Thus, for a textile to be effective at protecting the wearer from UV radiation, it must offer effective protection between 300 nm and 320 nm. To quantify the protective effect offered by a garment, the solar protection factor (SPF) is determined spectroscopically; the higher the SPF, the greater the protection to UV-B offered by the fabric.

For a molecule to be suitable for application to textile apparel, it must not add color to the garment, be easy to apply, and have the ability to quickly transform the UV energy absorbed efficiently into vibrational energy. Varying the ring substituents allows the UV absorption wavelength to be carefully controlled. Such molecules can be applied at the same time that the material is dyed, and thus, they often contain chemical groups similar to those found in typical dyes to aid dissolution and fixing to the fiber. Ultraviolet absorbers, in much the same way as dyes, must be wash- and light-fast; this can be evaluated by standard laundry trials.

7. Antimicrobial / Antifungal Finishes:
There are two main reasons for antimicrobial finishes: to protect the user from pathogenic or odor-causing microorganisms and to protect the fabric from conditions under which mold, mildew, and similar organisms would proliferate. This is achieved by either inhibiting the growth of microorganisms or destroying them. While some natural fibers such as wool (bacterial attack) and cotton (fungal attack) are susceptible to attack by microorganisms, it is by no means limited to this class, with synthetic fibers such as polyurethane also being able to be damaged by microorganisms.

Antimicrobial finishes are effectively produced on textiles by:

1. Addition of microbicidal substances to the spinning solution in fiber manufacture
2. Modifications involving grafting or other chemical reactions
3. The finishing of textiles with suitable active substances

Such substances are fixed on textile materials after a thermal treatment (drying, curing) by incorporation into polymers and resin finishing agents. Antimicrobial effects, resistant to washing and dry-cleaning, are obtained, for example, by the incorporation of microbicides into spinning solutions as well as by chemical modification of the fiber itself. As a result, the textile material is protected from microbial attack and can no longer serve as a culture medium. It is, however, also necessary for the active constituent to be carried to the microorganism cells being targeted, either by water, e.g., after hydrolytic breakdown, or by leeching out of the textile material.

This is an important prerequisite for an effective antimicrobial effect. Many active substances suffer reduced effectiveness or even inactivation as a result of chemical reaction with, for example, the fiber. For this reason, finishing processes, which apply substances that can be incorporated into textile auxiliaries, and which do not cross-link with, but rather exhaust onto the fiber from where they are slowly released during use, have gained increasing importance. In this case, of course, resistance to washing and dry-cleaning is limited. This limited resistance is actually desirable in terms of effective germ resistance.

8. Insect-Repellent Finishes:
In contrast to the use of insecticide (oral or contact poisons, etc.), special odoriferous substances (repellents) are employed for the purpose of repelling insects. These substances are unpleasant or unbearable to insects and therefore have a repellent effect as far as insect bites are concerned whilst, for humans, they have only a slight or even a pleasant odour. Repellents of this kind find widespread use as skin creams, body oils, etc. Experience has revealed that such insect repellency is rather non-specific, and a relationship between chemical constitution, physical data and insect repellency appears questionable. Useable repellents are, as a rule, neutral, viscous oils of low volatility or crystals with low melting points and, almost without exception, a bitter taste. In order to maintain the longest possible activity, such substances must, in addition, not give rise to skin irritations or cause damage to textiles.

Suitable products for textile impregnations include, e.g., indalone, undecenoic acid, mandelic acid hexyl ester, N-cyclohexyl-2-(butoxyethoxy) acetamide, etc. A patent for the production of insect-repellent hosiery recommends saturation with quaternary ammonium compounds followed by subsequent drying and heating to approximately 100°C–150°C.

9. Mothproofing Finish:
Mothproof finishing is directed towards the garment to prevent damage from the fur moth (and/or its larvae), the Anthrenus and Attagenus beetles, etc., which live as textile parasites on keratin-containing substances (wool amongst other protein fibers, fur, duvet feathers, etc.). Mothproofing agents should also protect against carpet beetles amongst other harmful insects. Insecticides are more or less poisonous. This applies in particular to dieldrin, which is banned in many countries. There is an intensive search for new, non-poisonous products. The synthetic pyrethroids also seem to be problematic. They are fish poisons as well, but can at least be removed from wastewater. Pyrethroids have a satisfactory protective effect against moths, but they are effective against carpet beetles only in high concentrations.

10. Resin Finishing (Crease Resist, Permanent Press):
Resin finishing is a general finishing process that gives each fiber additional properties depending on requirements (soil release, moth repellent, antifelting, creasing resistance). This finishing method has a significant practical value and causes permanent improvement in wear resistance (wash and dry-cleaning resistant), and particularly in shrinkage stability and crease recovery, of textiles made out of cellulose or cellulose compounds, by means of intercalation and/or modification of the cellulose with certain finishing products. This is also known as permanent-press process; wash-and-wear finishing; anticrease finish; non-shrink finish; swelling-resistant finish; easy-care finish; and no-iron, non-iron, durable-press, minimum-iron, and rapidiron finishes.

There are different types of resin finishing processes:

1. Wrinkle-resistant finish (crease-resistant finishes)
2. Shrink-resistant finish: mainly to maintain the dimensional stability of the textiles
3. Wash-and-wear finish (drip-dry finish): for textiles requiring only minimal ironing or no ironing after washing
4. Permanent-press finishing (durable-press finish): to impart permanent pleats for pleated skirts

11. Antistatic Finish:
Antistatic finishing involves the treatment of textiles with special chemicals to increase surface conductivity in order to prevent the buildup of electrostatic charges (especially at relative air humidity levels below 30%) during spinning, combing, sizing, weaving, knitting and also for finished goods. These finishes cause a reduction in friction associated with increased softness and smoothness. The antistatic finishing of clothing materials for persons working in situations involving the risk of explosion is an area of increasing importance. The prescribed maximum concentration for each product must on no account be exceeded in any process application (problem of adhesion to machine parts). A permanent antistatic finish for polyamide still awaits development.

Textile auxiliaries (anionic, cationic or non-ionic) are used to prevent the development of electrostatic charges during the processing and use of synthetic fibers and yarns (which also includes some natural fibers such as wool). In the case of anionic and cationic antistatic agents, the antistatic effect becomes greater with increased chain length of the fatty acid residue. Presumably this is because a marked molecular adsorption, perpendicular to the fiber surface, becomes possible with longer chains.

Antistatic action is essentially due to the combined effects of increased ionic conductivity, increased water-absorbing capacity and, possibly, a fiber lubricating effect as well. Antistatic agents have only a very limited effect on soil repellency. Wash resistant antistatic agents are based on the principle of applying, for example, polymer compounds to fibers whose water solubility is due to the presence of hydrophilic side groups, after which the water-solubilising groups are blocked by salt formation or esterification.

12. Stiffening:
It is well known that the application of some starch during ironing makes the task easier and produces a smooth finish. This can be described as a stiffening process. The action of the stiffening agent (starch in this case) gives the cotton stiffness, smoothness, weight and strength.

As well as starch, other substances such as flour, dextrin, glue and gum can be used to stiffen fabrics. Note that all of these agents have only a temporary effect, because they are not fast to washing. A more permanent stiffening effect can be gained by the use of newer types of synthetic resin polymers, but the cost is greater. Wool and silk fabrics are not usually stiffened, as the process does not suit their natural fiber properties.

13. Easy-Care Finishing:
The apparel industry is dominated by cellulosic fibers; for example, cotton is a strong, breathable garment to be manufactured, which unfortunately has a natural tendency to crease when worn and shrink during the laundering process. Easy-care clothing is a technology designed to allow garments to be washed with minimal shrinkage or creasing, while requiring little, if any, subsequent ironing to restore the original garment appearance.

Cellulose fibers are composed of a bundle of polymer chains consisting of β-glucose molecules and contain crystalline and amorphous regions. Moisture can be readily absorbed within the amorphous regions of these polymers, breaking hydrogen bonds between the chains and creating stress within the fibers. The polymer chains subsequently reorient to a lower-energy confirmation, at which point the hydrogen bonds reform, causing a wrinkle or a crease in the garment. Ironing, by the application of heat and mechanical force, provides sufficient energy to overcome the internal forces within the fiber and remove the crease, reorienting the polymer chains to their original position.

Easy care provides a mechanism to prevent the reorientation of the polymer chains within these amorphous regions, effectively locking them in a crease-free confirmation. This is achieved by the reaction of multifunctional crosslinking agents with hydroxyl groups of the cellulose fibers. It should be noted that this crosslinking reduces the elasticity and flexibility of the cellulosic fibers.

It was the mid-1920s that the first crosslinking agent that provided dimensional stability to cellulose was disclosed, but generally, they all work on the same principle; crosslinking is achieved by using amide-formaldehyde crosslinkers in an acid-catalyzed reaction with the cellulosic substrate.

Easy-care finishes are often applied by a pad–dry–cure procedure, where the crosslinking agent, catalyst, softener, and any other components are dried onto the fabric before the curing step, which causes the crosslinking reaction to occur. It may be possible to add additional finishes such as stain repellency or fire retardancy to apparels at this stage if they are compatible. When this curing step takes place depends on the application of the final garment. For formal trousers or pleated skirts, which need to retain their creases, the curing step takes place after garment manufacture, whereas for casual trousers or sheets, the fabric is cured before garment manufacture.

Formaldehyde is intimately linked to easy-care fabrics, and concerns have been expressed regarding possible health risks associated with the usage of formaldehyde. It is known that at concentrations ~1%, formaldehyde can irritate both the eyes and respiratory tract, and in susceptible individuals, it may cause a skin irritation. This has led to the establishment of maximum permitted free formaldehyde levels to be established within textiles, with different countries adopting different standards. In addition, there has been a drive within the industry to move toward low-formaldehyde products, which lead to a reduction in free formaldehyde.

Consumers are increasingly becoming more environmentally aware, and thus, the importance of recyclability and lessening the impact on natural resources becomes more important. Research into repurposing textile waste has increased with companies looking to create closed-loop systems.

14. Coating and Laminating:
Both coating and laminating require a textile substrate to be treated. The substrate plays a major role in establishing the final properties of the finished article. In addition to the chemical and physical properties of the fibers themselves, yarn construction and fabric formation are significant factors. Yarns made from staple fibers provide rough surfaces that enhance adhesion to chemical coatings. Filament yarns generally must be pretreated with chemicals to generate a more reactive surface prior to coating or laminating. Fabric structure determines the extent of textile-finish interbonding while also influencing the final mechanical properties of the treated material. Knitted and nonwoven structures are especially useful for coating and laminating, but when strength and dimensional stability are required, wovens are preferred.

The chemicals used for coating and laminating are polymeric materials, either naturally occurring or produced synthetically. These include natural and synthetic rubbers, polyvinyl chloride, polyvinyl alcohol, acrylic, phenolic resins, polyurethanes, silicones, fluorochemicals, epoxy resins and polyesters. Coating formulations typically include auxiliaries such as plasticisers, adhesion promoters, viscosity regulators, pigments, fillers, flame retardants, catalysts and the like.

15. Softening Agents:
Softening is one of the most commonly used textile finishes. Fabric softness usually depends upon four measurable fabric characteristics, i. e. coefficient of friction, flexibility / bendability, compressibility and elasticity. Objectives for the application of chemical softeners include improvement in hand-feel, drape, tear resistance or sewability of the fabric. Softeners lubricate the fibers, decrease coefficient of friction, improve fabric smoothness and may also lower the glass transition temperature of the polymer.

An important selling attribute of textile fabrics for clothing or items such as towelling is their ‘handle’. To improve handle, chemical softening agents are often applied to fabrics during manufacture. The handle, or feel, of a textile material is an important property for the consumer but is often a difficult parameter to measure scientifically and so can be subjective. Technical textiles can often become brittle or feel rough to the consumer due to the processes required to impart their technical functionality, and thus, softening is often a key part of obtaining an acceptable finish for the end user. Softeners are widely used, and in addition to improving the handle of the material, they can enhance the perceived quality of the finished fabric. They can also counteract the harshness imparted by other finishes such as easy care. The majority of softeners used are not covalently bound to the substrate and thus are removed from the material over time or by laundering. Thus, they need replenishing; as a result, softeners are used extensively in domestic washing formulations.

Conclusion:
Chemical finishing is always an important component of textile processing because it makes textile materials marketable and user-friendly. In chemical finishing the final effect obtained on the textiles is primarily due to the chemicals used in finishing. In recent years, there has been a growing trend towards ‘high-tech’ textile products. As the use of high performance textiles has grown, the need for chemical finishes to provide the fabric properties required in these special applications has grown accordingly. The general trend in chemical finishing is toward the use of more sophisticated chemical finishes that are more environmentally friendly and are specifically formulated for ease of application on automated machinery and equipment.

References:

1. A Novel Green Treatment for Textiles: Plasma Treatment as a Sustainable Technology by Chi-wai Kan
2. Textile and Clothing Design Technology Edited by Tom Cassidy and Parikshit Goswami
3. Chemical Finishing of Textiles by D. Schindler and P. J. Hauser
4. Principles of Textile Finishing By Asim Kumar Roy Choudhury
5. The Chemistry of Textile Fibres by R. H. Wardman and R. R. Mather
6. Textile Engineering – An Introduction Edited by Yasir Nawab