Recent trends in easy care finishing - part 2

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Offline nawshin farzana

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Recent trends in easy care finishing - part 2
« on: November 22, 2018, 11:50:44 AM »
Another compound 1, 3-dimethylo-4, 5-hydroxy-2-imidazolidone, commonly known as dimethylol dihydroxyethyleneurea (DMDHEU) was introduced for cotton fabric for easy care finishing. The product is now the most widely used reactant and it cross-links cellulose in the presence of an acid catalyst. DMDHEU has no effect on light fastness of reactive dyes and there is less chance of discoloration of fabric dyed with reactive dyes. The formula below shows in simplified form, how cellulose is thought to become cross linked with DMDHEU:

2Process of cross linking:

(a) Pre-cure process:

In this process, the fabric is generally impregnated with resin precondensate and /or cross-linking reactants and catalysts. The impregnated fabric is then dried and cured at high temperature to obtain the cross-linking effect. The curing temperature and time are dependent on the types of cross-linking agents and fabrics. The sequence of pre-cure processes is as follows (2):
(i)  Impregnation.
(ii)  Drying at 80°c – 100°c temperature.
(iii) Curing at 150°c temperature for 4 – 5 minutes.
(iv) Cutting, sewing and making-up.
(v) The garments are steamed for 3 – 5 seconds, then pressed for 15 – 25 seconds at 205 – 230°c temperature at a pressure of 15 lbs/ inch.
Curing the day after drying is claimed to have a favorable influence on the fabric tensile strength and abrasion resistance properties. If the goods are to be calendered to impart luster this must be done before curing, otherwise the tensile strength will be very much impaired.

(b) Post-cure Process:
The post-cure process also known as deferred curing process is basically different from conventional finishing techniques. In this process the fabrics is impregnated with appropriate reactant, catalyst and other finishing agents and dried carefully so that no curing occurs. This uncured fabric is referred to as sensitized cloth, and after pressing to shape, they may be cured in an oven. The following temperatures are generally employed:
Pressing: 5 – 15 seconds at 120 – 165°c
Curing: 10 – 18 minutes at 170 – 190°c

The post cure process produces garments with outstanding easy care finishing properties, but several important points should be considered:

The dyed fabric, which is to be finished, should be free of size, alkali, and other impurities.
Impregnation may be preferred on a two-roll pad with a good dip box so as to provide a long dip with one nip; or on a three-roll pad to provide two dips and two nips.
The fabric must be dried under controlled conditions. The speed of the dryer should be adjusted so that the fabric leaves the frame with approximately 10% moisture based on the weight of the cotton present.
The fabric leaves the finishing plant in its uncured state and precaution must be taken to avoid premature curing. This means that the combination of resin, catalyst and other finishing agents must be very carefully selected.
Dimethylol-dihydroxy-ethyleneurea (DMDHEU) is the most suitable reagent for the post cure process due to its outstanding resistance to odor development and excellent chemical stability to prolong storage. Very reactive catalysts such as Magnesium chloride and Zinc nitrate are suitable for the post-cure process.

(c) Re-cure or Double cure Process:

This is another method of producing easy care finished fabric. This process involves the following steps:

The fabric is impregnated in the conventional way, except that the formulation contains a high boiling point non-reactive additive.
The fabric is given a first cure at a temperature below that at which the inert additive would volatilize.
The one-cured fabric is cut and sewn to produce a garment.
The second cure is given at a high temperature, during which the inert additive volatilizes.
After the first cure the cross-linking chemical is fixed within the fabric and cannot be washed out. At this stage the fabric had high wet wrinkle recovery but low dry wrinkle recovery. An interesting characteristic is that, after the second cure, the dry wrinkle recovery increases substantially while the wet wrinkle recovery does not decrease. The major draw-back of this process is the increased cost of the additive. Generally tetramethylene sulphone or the dimethyl ether of tetramethyl glycol is used for the purpose and these are very expensive.

The most important reactants for easy care finishing are the N-methylol compounds and their derivatives. They are easily applicable and low in price. Probably dimethylol dihydroxy ethylene urea (DMDHEU) is the most important reactant and is widely used because it does not produce yellowing on finished fabric (8). However, recently there has been a growing concern about the problems that may be generated as a result of formaldehyde release form N-mehtylol containing compound treated cotton fabrics.

Formaldehyde and its impact:

Formaldehyde is a toxic chemical being a sever eye irritant, a mucous membrane irritant, a skin irritant and toxic if ingested . In easy care finished fabrics there are several sources capable of releasing formaldehyde. The cellulose substrate may retain some free formaldehyde reactant during the finishing process. This formaldehyde will be released during storage of the finished fabrics especially under worm and humid conditions. This would couse “formaldehyde odour” problem during the garment processing of finished fabrics which have been stored for a period of time. In addition formaldehyde may be formed via the hydrolysis of the N-hydroxymethyl groups from untreated cross linking agent or from single end reacted DMDHEU molecules. Cleavage of the C-O bond of any cross links between the cellulose and the poly-functional cross linking agent will provide additional N-hydroxymethyl groups which can hydrolyze to release formaldehyde.
From a review of several hundred references it can be summarized the effect of exposure to formaldehyde; the following table shows the summary of human inhalation data of formaldehyde:

Concentration (ppm)                   Exposure                                                 Effects
20                                        Chamber (1min)                           Discomfort, lacrimation.
13.8                                        Chamber (30 min)                    Eye and nose irritation
0.5-10                                Indoor residential air                   Eye irritation, headaches, GI tract
                                                                                                symptoms, skin roblems, respiratory
4-5                                       Occupational (10-30 min)           Irritation, discomfort, lacrimation
0.67-4.82                               Indoor residential air (infants)           Vomiting, diarrhea, lacrimation.