29 Wash ‘n’ wear and crease recovery finishes

S. Karpagam Chinnammal

1. Introduction

The primary function of clothing is to prevent the loss of body heat and to allow the sweat to pass through it to the outside atmosphere. Cotton is good in each of these functions. These properties together with its hydrophilicity, durability, ability to withstand rough laundering, good perspiration absorption, comfort during wear, ability to take up a wide range of dyestuffs, freedom from static charge generation and pleasant natural feel make it an excellent and comfortable fibre for apparel.

However, Cotton has certain deficiencies in properties and finishing is required to improve these properties to maximize consumer usage and acceptance. One of the major shortcomings of cotton is that it is prone to creasing during wear and washing. The formation of creases is an undesirable property to apparels. So it has to be made crease resistant. A treatment with cross linking agent is given to impart wrinkle free properties to cotton. This finish imparts crease resistant properties to cotton and keeps shrinkage under control.

The object of this finish is to keep the fabric flat and smooth and free from undesirable creases. So the finish is referred as anti crease, crease resistant or wrinkle free finish. Fabrics treated with these finishes tend to smooth out when properly hung after wearing. It is also called resin finishing as resins are used for this finish. This is a chemical and permanent finish. Usually crease resistant and wash ‘n’finishes are applied to cellulose fibers ie cotton, linen, viscose and rayon .

2. Learning Objectives

To know about Crease recovery and Wash ‘n’ Wear finishes
To get an insight into the finishing agents used for these finishes
To be acquainted with the process of application of theses finishes
To be aware of the various care practices for the finished fabrics

3. Crease resistant finish

3.1. Causes for Wrinkles or crease formation

Cotton cellulose chains contains hydroxyl groups in both amorphous and crystalline region. There are hydrogen bonds between the hydroxyl groups of the molecular chains. When a load is applied on the cotton fabrics, the cellulosic chain in the fibre bends, hydrogen bonds between the molecular chains in the amorphous region break and allow the chains to slip past one another, and this bending remains permanent since cellulosic chains are in elastic. This bending of the cellulose chains of the fibre under the application of load causes the formation of crease.

The primary cause of shrinkage of cellulosic fibres is the fact that these fibres can readily absorb moisture. The absorbed moisture facilitates internal polymer chain movements in the amorphous fiber areas by lubrication. It disrupts the internal hydrogen bonding between these polymer chains. When a moisture laden cellulosic fiber is stressed, the internal polymer chains of the amorphous areas are free to move to relieve the stress. Hydrogen bonds can reform between the polymer chains in their shifted positions, With no restoring force available, a newly formed wrinkle or crease will remain until additional processes (ironing for example) apply adequate moisture and mechanical forces to overcome the internal forces. The swelling of cellulosic fibres by moisture can be reduced by application of self crosslinking urea or melamine products as well as products that crosslink with cellulose. without such a crosslinking finish, cellulose fibres can take up more than 10 % of their weight in water. As the fibber swells the fabric must crease and shrink to relieve the internal stress caused by swelling

3.2. Method to prevent creases

The method of minimizing the formation of creases involves the reaction of the hydroxyl groups of adjacent cellulose macromolecules with some cross liking agents. The introduction of cross links imparts dimensional stability and elasticity to the fibrous material and make it Crease resistant .The most commonly used cross linking agents are Resins. The resins react with the hydroxyl group in cellulose to form a bond which gives good recovery from creasing and improved dimensional stability.

The resin cross link cellulose chains and they do so in the amorphous regions of the structure where the spacing between fibre molecules is relatively open. The effect of this is to give a more crystalline character to the fibre, so that if the fibre is distorted it will have a greater tendency to spring back to its original position. If the fabric is cured with a resin in a flat configuration it will have a tendency to recover to the original flat position.

3.3. Finishing agents for imparting crease recovery

3.3.1. Resins or crosslinking agents

Resin cross links give fibers a ‘memory’ and good wrinkle recovery. Resins are of two types – deposition and crosslinking type. Cross linking type of resins are used for this finish. This type of resins chemically react with the fibre and cross link the fibre molecules. This type of resin finishing is durable and much better than deposition type of resin finishing.

These are also known as N-Methylol compounds as the Methylol groups are attached to the nitrogen. Though these compounds are commonly called resins, calling them as resin precondensates is correct. The precondensates further polymerise to form resins. The resin precondensates are used for this kind of textile finishing and not resins. The following cross linking type of resins are mostly used for crease resistant finishing.

DMU- Dimethylol urea /Urea formaldehyde
DMEU- Dimethlol ethylene urea
DMDHEU-Dimethylol dihydroxy ethylene urea
Dimethylol ethyl carbamate
DMPU- Dimethylol Propylene Urea
TMM-Trimethylol melamine/melamine formaldehyde

They may be grouped as

Highly reactive
Low stability to hydrolysis low durability to laundering
High chlorine retention
High content and release of formaldehyde
Very high elastic resilience

3.3.1.1.2. Melamine formaldehyde resins

These products are mostly three to six reactive N-methylol groups connected to one melamine ring. This leads to higher crosslinking and easy care finish with better washfastness
These pproducts provide tri to hexamethylol melamine (TMM) and their methyl ethers
TMM is preferred for easy care finish, often only as a component of product mixture to give a better performance of effects. These products produce firmer hands than HMM

Properties

Better stability to hydrolysis and better washing durability
Relatively high formaldehyde content and release
Better chlorine retention than DMU
More dimensional stability and stiffness

3.3.1.1.3. Glyoxal resins : N,N-diethylol-4,5-dihydoxyehylene urea (DMDHEU)

This chemical is the basis for about 90 % of easy care and durable press finish products in the market. It is synthesized from urea, glyoxal and formaldehyde.
It is less reactive than DMU and TMM and therefore requires more active catalyst
It is more stable than finish baths with DMU and TMM

Properties:

Low to very low reactivity
Excellent durability and laundering
Low chlorine retention
Medium to very low formaldehyde release
Most commonly used durable press products

Formaldehyde released can be reduced by

After washing of cured fabrics.
Addition of formaldehyde scavengers like carbohydrazide to the bath
Use of urea in the pad bath or application through spray
Modification of DMDHEU

3.3.1.2. Non formaldehyde containing products

DMeDHEU does not contain formaldehyde it is derived from the relatively expensive dimethyl urea and glyoxal
Like DMDHEU it can be modified by reaction with alcohols such as methanol, diethylene glycol
These products are less reactive than DMDHEU because of their hydroxyl groups. Stronger catalysts or harsher condition are needed for successful crosslinking
It costs twice as much as DMDHEU
To achieve comparable easy care effects to DMDHEU nearly twice the amount of DMeDHeu is needed

Properties

Formaldehyde free
Very low reactivity
Limited durability to laundering
Development of unpleasant odour

Commercially, it is the modified DMDHEU (glycolated or methylated) that is most used today. The product is pre buffered to prevent premature curing and also pre blended with a catalyst.

Effective cross linking reactants should be very stable, soluble in water and remain colourless during heat curing. Moreover, they should not be toxic, odourous or irritating and should be available at low cost.

3.3.2. Catalysts for easy care finish

Catalysts facilitates crosslinking reaction. Magnesium based catalysts are the most popular in use today.

The reaction of DMDHEU with cellulose requires an acid catalyst the most common catalyst are magnesium chloride and zinc nitrate that generate acid conditions during curing process, thus providing neutral liquors and good finish bath stability
Sulphuric acid and hydrochloric acids and their ammonium salts serve as excellent catalysts but also lead to undesirable fibre degradation
Often citric acid is combined with magnesium chloride to provide additional boost to reactions
Citric acid or aluminium chloride can also be blended to increase catalysis when curing times are short.

3.3..3. Polyethylene emulsion

A high density polyethylene emulsion restores some of the lost tear strength and abrasion resistance by providing lubrication and imparts hand and sewability.

3.3.4. Wetting agent

Wetting agent helps to achieve quick wetting and even distribution or penetration of resin.

3.3.5. Softner

Softner imparts soft hand to the finished product

3.4. Preparation of resin precondensate

Urea formaldehyde precondensates were successfully used to deposit the resin in the fiber. Under neutral or slightly alkaline conditions urea and formaldehyde condense to give Dimethylol urea (DMU)

These products are known as precondensates are highly reactive and can react further. Therefore , dimetylol urea is further methylated to give a stable product which is crystalline and readily soluble in water.

These precondensates are of a molecular size sufficiently low to penetrate the amorphous regions of cellulose. When heated under acid conditions, these condensates form urea formaldehyde synthetic resin.

The amount of urea and formaldehyde are to be taken in the ratio of 1:1.6 which give optimum effect.
In the first process, urea is dissolved in formaldehyde solution (40%) at room temperature for 4-6 hours with periodical stirring and with addition of ammonia and the temperature should not exceed 35 °C.
In other process, the precondensates are prepared by treating formaldehyde soultion (40%) ,adjusting its pH to 7 by adding NaOH . The pH of the mixture is raised to 9 with NaoH solution. Equal amount of water is added and the contents are gradually heated to 80°C. The temperature is maintained at 80 °C for 15 minutes and cooled rapidly.
The pre condensates are marketed in the form of a colourless liquid which can be mixed with water in any proportion.

Melamine formaldehyde precondensates also polymerise to insoluble resins at high temperature under acidic conditions and are used for making cotton fabrics crease resistant. Trimethoxy methyl melamine-TMM and hexamethoxy methyl melamine are two precondensates commercially available in liquid form, like urea formaldehyde precondensates they are used after dilution for padding the fabric followed by curing at 150°C

3.5. Finishing Process

Crosslinking agents are applied by

The application of urea formaldehyde resin consists of the following steps

3.5.1. Impregnating by padding

Before impregnation the fabric to be treated should be in a highly absorbent condition hence the material should be scoured and bleached for better penetration of liquor. Dyed and printed fabrics can be taken directly.

The fabric is padded by using either two or three bowl padding mangle with precondensate solution (100 to 300 g/l ) containing an acid catalyst like diammonium hydrogen phosphate or ammonium chloride and other additives such as softner and wetting agent with a pick up of about 80 % at room temperature.

Wrinkle free finishing can also be imparted using Dimethylodihydroxyethylene urea resin (DMDHEU) (40-120g/l) with magnesium chloride (10-25 g/l) as catalyst, non ionic silicon softner (40 g/l), polyethylene emulsion, citric acid (0.3 g/l) and wetting agent (1 g/l) and acetic acid to maintain pH bath at 4-4.5

3.5.2. Drying

After padding, the material is dried in stenter with minimum tension at 70° C-80°C. Higher temperature of drying leads to migration of finish causing loss in tensile strength and Abrasion resistance of fabric

3.5.3. Curing of impregnated material

After drying the material is cured at a high temperature of 120-150 °C for 2-5 minutes using dry heat in a stenter machine Polymerization of resin and cross linking of molecular chains take place during curing process thus crease resistance is achieved.

3.5.4. Washing and soaping

After curing the material should be washed in open width or rope form in a dilute solution of soap and 1-2 gpl of anionic wetting agent (TRO) and 2-4 gpl of soda ash at 50-60° C for 10 minutes. The purpose of this washing is to neutralise the residual acidity and also to remove the unfixed resin

3.5.5. Softening and drying

After washing, the material is rinsed in water containing softening agent. Then finally it is stentered in ordinary stenter to dry and get even width of fabric.

Major application methods are

Pre cure
Post cure
Garment dip
Spray (metered) application and
Vapour phase

These have been discussed already under durable press finish and precure is the most commonly adopted method

3.6. Characteristics of finished product

The resulting fabric from such a process would have an improved crease recovery. However, in addition to improving the crease recovery, it was also found that resin finishing improved the ease of maintenance and dimensional stability of the fabric to washing. So a fabric which had a greater wash shrinkage would have lesser shrinkage after resin treatment. Less tendency to pilling, improved wet fastness of dyes and prints

3.7. Effect of resin on fabric

Resins do how ever have several effects on the fibres

Excessive crosslinks causes brittleness, stiffness and harsh feel giving loss in tear , tensile strength (30-40%) and abrasion resistance. Tear strength can be improved by optimum selection of additives and softners but it has no improvement on tensile strength.
Absorbency is reduced and so the fabric becomes uncomfortable in warm humid weather
Resins have an affinity for oily soils
Hypochlorite bleaching causes chlorine retention ie chlorine from hypochlorite is retained by NH group of resin as NCl and cleaves the cross links and results in loss of creases resistance.
Chlorine in association with NH groups forms chloramines and results in yellowing of fabrics.
Melamine formaldehyde products do not absorb chlorine but the treated cloth becomes yellowish and their crease resistance is not satisfactory.

3.8. Fabrics for wrinkle free finish

The base fabric must have sufficient strength to withstand 40-60% loss in tensile and tear strength and still maintain sufficient strength to provide a garment of acceptable wear life and durability
It must also have excellent absorbency to allow resin to penetrate into the interior of the fibres and form crosslinks.
If fabric is dyed , the dye must be fast to acid catalyst and high temperature
Fabric must be free from size as it may react with the resin
Fabric pH should be between 6.5-7

4. Wash ‘n’ Wear

It is also called drip dry finish or easy care finish. Cotton fabrics or other cellulosics like rayon and linen crush badly when in use. When wash-n-wear finish is applied cellulosic fabrics acquire qualities similar to that of synthetic fibers. They become easy to maintain as they dry smooth, do not wrinkle and need little or no ironing after washing (wash –dry-wear and no need for ironing). The fabrics have a good, soft hand and a neat appearance. Treated fabrics become less moisture absorbent thus they dry easily and rapidly. This finish is a durable one. The fabric is saturated with resin and cured at temperatures of about 360°F. The fabric becomes stiff, less absorbent and more resistant to wrinkling. Tensile strength and abrasion resistance are reduced. This finish is also called Crease-Resistant finish or “wrinkle-free”finish.

4.1. Finishing agents

Wash ‘n’ wear finish are also given using resin precondensates

4.2. Process sequence for wash ‘n’ wear finish

This process involves two major steps such as polymerizing and then cross linking. The fabrics are cured twice in this process. Different catalysts are used for these two purposes. This method of resin application gives wash &wear finish to the garments

The fabric is padded with a composition of resin (TMM/ DMEU/DMDHEU) , polymerizing catalyst (Zirconium acetate/Zn or Cu or lead acetate) and wetting agent. The padded fabric is dried at 80°C for 8 min and cured at 160°C for 5 min .Then it is washed and dried or without drying it goes to the next step ie padding with cross linking catalyst eg MgCl 2 6 H 2 O dried at 80°C for 5 min and cured at 160°C for 3 min it is given hot rinse,cold rinse and dried.

5. Care practices for finished fabrics

General care guidelines for Crease recovery and wash ‘n’ wear items include the following

Some of these finishes cause yellowing when a chlorine bleach is used. So labels should be read before using a bleach
The garments must not be wringed or spin dried in washing machine or in a dryer
It must be Rinsed well and hanged on a wooden or plastic hanger, buttoned and straightened out and allowed to drip dry. Metal hangers may cause rust stains
Some garments tend to pucker at the seams when they dry; some require quick, light ironing with a cool iron.
Items must be washed frequently because resins have a strong affinity for oil and grease so that soil penetrates deeply and builds up
Stains, collars, and cuffs must be pretreated. Use a spot removal agent on grease spots
Wash loads must be kept small to minimize wrinkling
Items must be removed promptly when dry
Garments must be laundered inside out to lessen abrasion , since this will reduce the wear on the finish

Conclusion

Specific characteristics not present in fibres are imparted through functional finishes thereby adding value to the fabric. One such important finish is crease resisitant and wash ‘n’ wear finish.

In this module we have learnt what is a crease resistant and wash ‘n’wear finish, the chemicals and process involved and care for the finished fabrics. The durability of finish depends on its quality, the amount of fabric saturation, the care in applying and curing it, as well as the care with which the consumer follows the instruction on the garment label. To overcome the boundaries of using resin, researchers have employed TiO2 nano particles to improve the wrinkle resistance of cotton .

you can view video on Wash ‘n’ wear and crease recovery finishes

REFERENCE

Subrata Das , important aspects of wrinkle free finish, Asian dyer, june-july 2013 p.39-42
Tushar Bharat Gaikwad and A.I wasib, study on wrinkle free finish on 100 % cotton fabric, Colourage, March 2016 p 42-43