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How durable is "durable"? When there are several classes of finishing agents, which would offer a durable finish? M. R. Sampath provides the answer to this question and others

1. Which Finishes Are Durable?

For a finishing chemical (irrespective of the brand) to be durable, it must have good substantivity and exhaustion properties vis-a-vis the substrate. If the finishing chemical is reactive to the substrate and forms a chemical bond, like a covalent bond in a cotton fiber, the durability is further enhanced. If the finish can also form a polymerized film on the surface besides being reactive, it shall be more durable as in the case of reactive cationic silicone softeners. Non-reactive finishes can be made relatively more durable by application as a chemical that can polymerise to form a film on the surface of the substrate. This renders it difficult to be washed off, like in normal silicone softeners.

"How durable is 'durable'?" This question does not have a universal answer. When we say durable finishes, what we mean is that the finish imparted is relatively more resistant to being rendered non-functional during normal routine maintenance operations. The fabric maintenance will differ with different end use requirements. The maintenance will include not only the washing/dry cleaning parameters it is subjected to, but also the processes of drying, ironing, storing, etc. These routine maintenance operations will vary for fabric substrates and for colours, finishes, optical brighteners, etc. Substrates like wool or silk can get damaged in alkaline soaping; some substrates cannot tolerate high temperature during ironing; some colours and finishes can get affected by bleaching or washing at higher temperatures/dry cleaning, etc.

Therefore 'durable' or 'fast' are terms that hold good only under stipulated conditions of maintenance. While testing methods and ratings assess the functional characteristics of a finish, be it crease resistance, flame retardancy, antimicrobial, or softness, care labels indicate the limitations of the finishes to sustain conditions other than the ones stipulated. Care labels take into consideration not only the finish but also the other features of the fabric like colour, drape, shape, etc.

2. How do performance finishes affect other fabric properties vis-a-vis shade, handle and odour?

Finishes can affect various other features required in a textile material due to their own inherent properties/characteristics and/or the conditions required for imparting the finish.

It is a known phenomenon that some of the cationic finishes (softeners) and cross-linking resins tend to change shade. These finishing agents may act as mild solvents for the dye (however sparingly) and hence shift the wavelength of absorption of light even marginally to alter the tonal colour perception. The pH could also play a subtle role in altering the conjugated double-bond hybrid resonance in certain colours, contributing to a change in colour perception. Cationic fixing agents generally change the tone of certain colours. Protonation could be one of the reasons.

Finishes requiring higher temperatures for fixation (curing) can cause certain changes in shades. Thermo- migration and sublimation can occur depending on the nature of the finishing chemical and/or high temperatures.

Thermo-migration is sometimes attributed to the dissolution of the colour in the finishing agent and due to its consequent mobility. Migration is caused by possible existence of temperature difference on either face of the fabric and/or across the width during curing at higher temperatures. Non-ionic finishes possess greater propensity to cause such problems. It is desirable to completely fix the colour, as unfixed colour will be more vulnerable to the phenomenon discussed above.

Sublimation is the main cause for loss of depth (in self shades) and change in tone due to one or more colours of the recipe having low sublimation fastness getting lost during heat setting/curing. Sublimation occurs when matter changes from the solid to the vapour phase without going through the normal liquid phase, when heated above a specific temperature that is related to the molecular size and configuration of the colour. This phenomenon is generally confined to disperse colours. To avoid problems of poor sublimation fastness, colours with high sublimation rating should be used. However, there could be limitations where use of dyestuffs having lower sublimation fastness is unavoidable. In such circumstances, choice of finishing chemicals, method of application and curing temperatures would be critical.

Selection of right dyestuffs, finishing chemicals and curing temperatures would be the answer to avoiding the adverse effects of finishing on other aspects of fabric performance.

The flame retardant finishes and some of the crease resistant finishes tend to make the fabric harsh and boardy. Both resin and flame retardant finishes affect the strength of the fabric. Due to the cross-linking of hydroxyl groups of cellulose, the reactive resins tend to make the fabric brittle. The measures taken to mitigate these undesirable side-effects are incorporation of softeners like polyethylene emulsions and other silicone-based softeners to restore or improve handle.

Crease-resistant finishes with a formaldehyde base like urea formaldehyde tend to release high levels of free formaldehyde, causing an offensive odour. DHDMEU releases far less formaldehyde, yet sufficient enough to fail to meet certain stringent end uses like children's wear and next-to-skin wears. Alternate non-formaldehyde resins based on polycarboxylic acids have come in the market not only to counter odour but also the ill effects of formaldehyde on skin.

3. Will anti-ozonate softener work as non-yellowing softener?

The chemistry and mechanism of 'yellowing' of indigo-dyed denim is different from 'yellowing' by cationic softener.

The yellowing of indigo- dyed denim is due to exposure to ozone, which oxidizes indigo to oxidation products of isatin and anthranilic acid. They have a colour range of dull yellow to gray. Anti-ozonate softeners, besides being employed as softeners, are also select cationic, amine-based chemicals with a high degree of unsaturation in their molecule to bind ozone and arrest oxidation of the indigo.

The mechanism in the case of yellowing, particularly of 'whites', by cationic softeners is associated with the quaternary ammonium compounds with free hydrogen attached to the nitrogen atom. They can readily bind with chlorine to form 'chloramines', the main cause for yellowing. Non-yellowing cationic softeners have 'free hydrogen' substituted by groups that do not bind chlorine.

Therefore, the anti-ozonate softeners per se are not likely to perform as 'non-yellowing softeners' unless they also possess the chemistry of non-yellowing cationic softeners as explained above.

4. Will the performance of a softener differ in hard and soft water?

Where hard water is used, it is necessary to determine that there is no bicarbonate or carbonate in water, as this will neutralize the slight acidity, where acidic conditions are required and thus impair the application of the softener. The bicarbonate hardness will continue to be present even after normal ion exchange, unless a properly designed hydrogen cycle and blending is
included in the ion-exchange treatment system. The hardness caused by calcium and magnesium is relatively less interfering in the case of cationic softeners, except that they are likely to cause a harsh feel to the fabric and thus reduce the effectiveness of the softener to some extent.

If hard water is used for washing at the customer's end, the hardness will progressively reduce the effectiveness of the softener.

Magnesium and calcium hardness will generally precipitate anionic softeners. Non-ionic softeners are not affected chemically, but hardness can contribute an undesirable feel to the fabric. yet sufficient enough to fail to meet certain stringent end uses like children's wear and next-to-skin wears. Alternate non-formaldehyde resins based on polycarboxylic acids have come in the market not only to counter odour but also the ill effects of formaldehyde on skin.