19 Finishing agents – Stiffening agents, cross linking agents – types, characteristics and method of application

S. Ariharasudhan

1. Introduction

Finishing is the integral part of the textiles, without finishing fabrics cannot be sold to the desired customer. Finishing is the sequence of operations, other than scouring, bleaching, and coloring, by which fabrics are improved in appearance and feel and provide desirable as well as essential properties. Fabric coming directly from the loom is unattractive. To make the fabric attractive and acceptable to the consumer several finishing processes are applied. Sometimes special finishes are also applied to the fabric to make it serviceable for particular operations. Following are the objectives of finishing operations.

2. Objectives of Finishing

The objectives of finishing are:

• To improve the appearance of the fabric, that is, to make it more attractive by operations like calendering, optical whitening
• To improve the feel of the fabric by softening, stiffening
• To cover faults in the original fabric
• To improve wearing qualities of cloth by making it shrink resistant, crease resistant or free from pills and soiling
• To make garments hold their shape and enable them to be worn without ironing.
• To impart special properties to the fabric for specific end uses (eg. flame retardant, water repellant).
• To set the texture of certain fabrics and make them dimensionally stable.
• To produce stronger and more durable fabrics

3.  Selection of Finishes

Selection of the finishes generally depends on the following factors:

Contains of fabrics which means type of the fibre and yarn used in the fabrics Thread count (Total no of yarns presents in the one square inches of fabric)Method of fabric construction Hand, weight, drapability qualities End use of the fabric or garment

4.  Stiffening agents

4.1. Temporary Stiffening

Fabrics, particularly cotton and linen, are given a temporary stability and stiffness by application of a firming agent which is often a solution of starch. It is commonly known as ‘starching’ or ‘temporary stiffening’. When this process is done while preparing warp for weaving, it is called ‘sizing’ and ‘dressing’. The term ‘dressing’ is generally used for the warp of wool. Other than starch, the substances used for stiffening fabrics are flour, dextrine, glue, shellac, fats, wax, and paraffin. Sometimes clay, chalk, barium sulfate, calcium sulfate or magnesium sulfate are also used for stiffening cotton fabrics. At times back starching is also done in which only the back side of the fabric is starched. Temporary stiffening is required to retain the freshness of the fabric till it is not used for making any product. Stiffening also allows the fabric to be cut more easily into patterns for the textile products.

4.2. Permanent Stiffening

The fabrics which are permanently stiffened usually need less laundering and therefore become more durable. Permanent Stiffening is done by chemical processes that change the cellular structure of the fiber. This process makes the fabric smoother and dirt resistant as the dirt tends to slide off rather than cling onto the fabric. Some of the permanent finishes are Ankord, Basco, Clearight, Kandarized, Saylerizing, Sheercroft, Staze- Right and Turbenizing. They all give the fabric such properties as tensile strength, luster, shrinkage resistance, crispness, abrasion resistance and improve the appearance of the fabrics. Turbenizing is done to avoid the need of starching fabric for its life. This can be done through three methods. The parts to be stiffened like collar, cuffs, belts are interlined with a thermoplastic fiber or with cellulose acetate or the fabric my be coated with synthetic resin. The thermoplastic fibers melt and bonds with the garment when pressed with a hot iron producing a stiffened fabric. When cellulose acetate is used, it is softened by acetone which is also heat pressed on to the fabric giving permanent stiffness. A coating of resin is also heat pressed on to the fabric.

5. Cross linking agents

In polymer chemistry, it is well known that certain types of polymers must be cross-linked before they develop elastic properties. Elastomers are materials that readily recover from deformation stresses. Since the case has been made that chain slippage under moist conditions is responsible for wrinkling, it is logical to reason that cross-linking adjacent cellulose chains should be a way of improving crease recovery. Time has shown that this theory does indeed work so the mechanism for improving cotton’s resilience is to cross-link cellulose chains with appropriate reactants.

5.1. Cross linking Cellulose

The hydroxyl groups on the anhydroglucose units of the cellulose polymer backbone are functional groups that can undergo typical reactions involving alcohols. Organic acids, isocyanates, epoxy and aldehydes all react with hydroxyl groups to form their corresponding reaction products. Over the years nearly all known reactions involving alcohols have been investigated as possible cellulose crosslinkers. Formaldehyde and non-formaldehyde derivatives were among the earliest ones discovered and have stood the test of time in terms of effectiveness and cost. They are still the main stay of today’s chemistry.

5.2. History of Crease Recovery

In the mid 1920’s, the managing director of Total, Broad hurst and Lee challenged his research chemists to make cotton fabric as wrinkle resistant as silk. It was know that formaldehyde would react with cellulose, however formaldehyde is not a pleasant material to work with as it irritates mucus membranes, causes runny noses and watery eyes. The chemists discovered that phenol/formaldehyde condensates would also cross link cotton and at the same time reduce the formaldehyde risks. Unfortunately the phenol/formaldehyde resins caused the cotton fabrics to become severely discolored and excessively boardy. Soon thereafter, they discovered that urea/formaldehyde resins also improved crease recovery and did so without discoloring the fabric. While not completely free of formaldehyde odor, these resins could be handled on commercial equipment.

From the 1930’s until 1961, cotton fabrics were cross linked with a number of N methylol compounds to give fabrics that were classified as Wash & Wear or Easy Carefabrics.

Several other names were associated with these fabrics e.g. Crease Resistant, No Iron, and CRFetc. The performances of these fabrics were pretty good. As will be made clearer in ensuing discussions, the best balance of improved ease of care properties to fabric strength loss was struck. All of the fabrics were treated at the mill in the flat state.

In 1961, The Koret Company marketed garments with permanent creases. This was revolutionary at that time because the activation of the cross-links was delayed until after the garment was made and the creases pressed in. What Koret did was to impregnate the fabric with the chemicals, carefully dry without activating the cross-links, make, press and then hang the garments in an oven to activate the cross-links. The consumer’s reaction to these garments was double edged. They complained about the fact that the cuffs fell off and holes appeared in the sharp creased areas when the garments were worn and laundered, yet turned around and wanted more. The consumer had been freed from the tedious task of ironing their garments. signalling the birth of the Permanent Press era. From the chemist’s point of view, the lack of abrasion resistance and weakened fabric was expected since the original garments were made from 100 % cotton. It was well understood at that time that creases set into the fabric prior to curing would be permanent and that excessive chemicals, curing times and temperatures would severely weaken the fabric and drastically reduce abrasion resistance. Nonetheless the industry, sensing a new marketable product, immediately launched into development programs to overcome the deficiencies and still maintain the permanent press qualities. The solution came about quickly. Serviceable garments were made from nylon blended with cotton. Soon thereafter, polyester blended with cotton also provided serviceable garments. At that point in history, polyester was relatively new, expensive and not a truly commercial item. Yarn spinners found that polyester was easier to blend with cotton than nylon, so it became the preferred fiber for permanent press fabrics. The original blend was 35% polyester, 65% cotton. This amount of polyester provided the strength without interfering with cotton’s contribution to shape retention.

It can be said that the commercial success of Permanent Press (Durable Press) is largely, responsible for today’s position enjoyed by polyester manufacturers. It provided the initial need for volume production which leads to the polymer’s more competitive pricing structures, opening up more and more markets.

    5.3. CELLULOSE CROSSLINKERS

Cellulose cross linkers can be divided into two categories, those that pre dominately crosslink cellulose, also known as cellulose reactants, and those that self-polymerize as well as crosslink cellulose (aminoplasts). Both types involve the reaction chemistry of formaldehyde so a review of pertinent formaldehyde chemistry will help in understanding how these auxiliaries work.

5.3.1. Reactions of Formaldehyde

Formaldehyde is capable of reacting with many active hydrogen compounds,

e.g -OH, -NH and activated -CH.

The hydroxyl methyl group formed in the first reaction is also capable of undergoing a second reaction. The second reactions also involve activated hydrogens described above and result in the formation of a methylene link (-CH2-) between the reacting species. Usually this reaction requires an acid catalyst and heat.

5.3.3. Reactions of Hydroxymethyl

5.4. Resin Formers (Aminoplasts)

There are two major types of formaldehyde condensates that fall into the resin former category, urea/formaldehyde and melamine/formaldehyde. These condensates are capable of self-crosslinking to form resinous, three-dimensional polymers as well as crosslinking cellulose. They find non-textile applications as plastics, adhesives and are also used to modify other polymeric systems. Because of the tendency to self-condense, the two mentioned above are often called Aminoplasts. The tendency to self-crosslink adds stiffness to fabrics. While stiffness may be undesirable on some fabrics it can be an asset as for example when Handbuildersare needed to enhance fabric properties.

5.4.1. Urea/Formaldehyde (U/F)

The reaction of an amide -N-H with HCHO to form a -NCH2OH is often termed Methylolationbecause the reaction product is called an N-methylol group. Accordingly urea can be metholylated with up to 4 moles of formaldehyde and the reaction products used as crease resistant finishes. When 2 moles of HCHO is reacted with one mole of urea, dimethylol urea is formed. Being di functional, it is capable of serving as a crosslinking agent.

1. Synthesis of Dimethylol Urea

   b. Important Features

• The condensate has an extremely short shelf life. It must be used within a few days after its been made. When formulated with catalyst, the finish bath must be used within a few hours. The solution has high free formaldehyde and will readily liberate formaldehyde into the work place.
• It is easy to cure on fabrics and imparts outstanding crease recovery. Fabric hand becomes stiffer which is usually undesirable for many cotton fabrics. Most rayon fabrics are very limp compared to cotton so the added firmness is desirable.
• Finished fabrics have poor durability to repeated laundry. Crease recovery is lost because the cross links have poor stability to hydrolysis.
• The finish adversely affect the light fastness of direct and fiber reactive dyes.
• The finish reacts with hypochlorite bleaches to form a reactionproduct which decomposes with heat to form HCl. Acid degradation of the cellulosescorches the fabric and causes it to become very much weaker (tender).
• Finished fabric is prone to liberate formaldehyde odor. Released formaldehyde is high. Overcured fabrics also develop an unpleasant fish odor.

5.4.2. Melamine/Formaldehyde

Melamine can react with up to 6 moles of formaldehyde to form a variety of products. Commercially, trimethylol and hexamethylol melamine are the more important condensates. In storage, the hydroxymethyl (N-methylol) groups tend to polymerize and liberate formaldehyde. By converting them to the methoxymethyl derivative, the shelf life is much improved. Most of the commercial products aremethylated.

1. Synthesis of Trimethylol Melamine

1. Synthesis of Trimethoxy Melamine

c. Synthesis of Hexamethoxymethyl Melamine

The corresponding hexamethylol- and hexamethoxymethyl condensates are made by increasing the formaldehyde/melamine mole ratio to 6:l.

d. Important Features

1. The tri- products produce firmer hands than the hexa derivatives and are extensively used as hand builders for fabrics other than cotton (Acrylic, nylon, polyester).
2. Durability to repeated laundering is much better than U/F. This feature is a plus for permanent hand builders.
3. Chlorine bleaches cause the fabrics to yellow; however, the reaction product does not decompose with heat to liberate HCl and tender the fabric.
4. They are used in combination with phosphorus flame retardants as a source of nitrogen. Nitrogen synergism enhances phosphorus flame retardants and melamine is an excellent source of nitrogen.
5. They are used as finishes for reducing wool shrinkage.

5.4.3. Reactants

Reactant N-methylol compounds differ from aminoplasts in that reactants do not form three-dimensional polymers by self-condensation. When applied to cellulose, they mainly crosslink adjacent polymer molecules. The commercially important ones are derived from ethylene urea, 4,5-dihydroxy- ethylene urea and hydroxyethyl carbamate. There is a wealth of literature published about these and related compounds – the reader is urged to consult these reference for greater depth.

1. Dimethylolethylene Urea (DMEU)

The starting material for making dimethylolethylene urea (DMEU) is ethylene urea. a 5 membered heterocyclic, nitrogen compound (imidazolidone-2). It is made by reacting ethylene diamine with urea. Ethylene urea contains 2 N-H groups capable of reacting with formaldehyde and forms a di functional product. Since there are no other active hydrogen sites, the N-methylolated product cannot self-condense.

1. Synthesis & Monomer

1. Methylolation Reaction

Important Features

1. It was widely used prior to 1961 as a wash and wear finish.
2. The product has moderately good shelf life, much better than the aminoplasts. Even with catalyst mixed in, the bath life is more than adequate for most commercial applications.
3. DMEU is easily cured. It will begin to cure at 90 to 1000 C.
4. It is highly efficient and gives good wrinkle recovery with nominal losses in fabric strength.
5. The product does affect lightfastness of certain direct and fiber reactive dyes
6. Chlorine resistance is poor even though there are no remaining N-H groups
7. Hydrolysis resistance is poor. Crosslinks are not durable to laundering, especially industrial laundering conditions.

2. Dimethylol-4,5-Dihydroxyethylene Urea (DMDHEU)

DMDHEU is the workhorse durable press finish. It and some of its modified versions account for over 85% of all crease resistant chemicals consumed today. DMDHEU achieved this prominent role in 1961 when delay cure processing came into being. In the trade, DMDHEU is often referred to as the Glyoxalresin. This jargon came as a way to distinguish it from DMEU, in that glyoxal was used to make the starting monomer.

The starting heterocycle is made by reacting stoichiometric amounts of urea and glyoxal. The reaction is straightforward and can be carried out in regular laboratory glassware. The methylolation step is also straightforward. While the synthesis is shown in two steps, commercially DMDHEU is made directly in one step. Urea, formaldehyde and glyoxal are all combined together and heated. The extent of reaction is followed by monitoring the free formaldehyde content. The product is sold as a 46% solution.

1. Synthesis of 4,5 – Dihydroxyethylene Urea

1. Methylolation

1. Important Features

The commercial product has low free formaldehyde which makes it easy to handle in a finishing plant. It does not liberate formaldehyde from the reverse reaction as rapidly as do other reagents. The product has extremely good shelf life and even finish baths with catalyst present are stable for prolong periods of time. Fabric temperatures exceeding 1300C are needed before the cross-linking reaction takes place.

This feature is responsible for why it has become the dominate DP finish. The reactant does not crosslink on storage so fabrics can be left in a sensitized state (uncured) for over six months before post curing. Hydrolysis resistance of the cellulose crosslinks are much better than DMEU so durability to laundering is very acceptable. Resistance to chlorine bleach is also acceptable. While this finish reduces the light fastness of direct and fiber reactive dyes, it isbetter than DMEU.

3. Carbamates

Carbamates are a family of related compounds that also react with formaldehyde to form N-methylol derivatives. A general structure of the starting compound is shown in the box below. This structure is also called a urethane so carbamates are simple urethanes. The alkyl group (R-) can be methyl, ethyl, propyl or hydroxyethyl. The methyl and ethyl carbamates are carcinogenic and no longer used. The propyl and hydroxyethyl are safe and are used today. Carbamates reactwith formaldehyde to form N-methylol derivatives. They can react with up to two moles as shown below. The reaction is difficult to drive to completion and the equilibrium is such that the best that can be done is 1.7 to 1.8/1. This leads to products that have high free formaldehyde.

1. Synthesis

1. Important Features

Carbamate finished fabrics have good DP properties. These products are harder to cure and require stronger catalyst and/or higher curing conditions. This leads to greater losses in strength and abrasion resistance. The crosslinks are extremely stable to alkaline hydrolysis and stand up well to the rigors of commercial laundries. Fabrics treated with carbamates are well suited for industrial laundry applications such as rental uniforms and hospital linens. Chlorine resistance is good. The commercial products have high free formaldehyde leading to fabric with high formaldehyde release.

REFERENCES and URLs

1. Shenai, V.A., “Technology of Textile Finishing”, Sevak Publications, Bombay, 1995.
2. Marsh, J.T., “An Introduction to Textile Finishing”, Chapman and Hall Ltd., London, 1979.
3. W.D.Schindler and P.J.Hauser, Chemical finishing of Textiles, CRC Pr LIC Publication,2004.
4. Charles Tomasino, Chemistry and Technology of Fabric Preparation and Finishing, Department of Textile Engineering, Chemistry and Science College of Textiles, North Carolina State University, 1992
5. Heywood, “Textile Finishing”, Woodhead Publishing Limited, 2003.