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Thermo- migration and Sublimation
Thermo-migration
In the case of certain colours, we come across the phenomenon of thermo migration. It is some times attributed to the dissolution of the colour in the finishing agent and due 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. It is desirable to completely fix the colour, as unfixed colour will be more vulnerable to the phenomenon discussed above. Sublimation 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 is – matter changing from solid phase without going through the normal liquid phase when heated above a specific temperature. This phenomenon is generally confined to disperse colours. To avoid problems of poor sublimation fastness, colours with high sublimation rating should be used and they are available in the market. However, higher temperatures and specialty chemicals are required for dyeing / thermosoling these colours. Sublimation could be controlled by regulating the finish / curing temperatures below the sublimation temperatures of the component colours. Safe guards against failures in maintaining temperature The thermometer probes generally indicate the temperatures. However it would be prudent to counter check the temperatures with thermal papers periodically to confirm that the temperatures shown by the probes are accurate particularly where temperature parameters are critical like in curing machines. All instruments can go wrong and there must be some verification procedure to ensure that such failures do not affect performance. Similar checks on the stenter chamber temperatures need to be monitored particularly during heat setting, thermosoling. Such precautions are necessary for all finishes involving chemical bonding or thermo fixations at elevated temperatures. Shade matching is incomplete unless the finish also is included. Bitter arguments on the shade changes after finishing are quite common in process houses where the finisher and the dyer do not coordinate. In a classic case of ‘negligence – shall we call it more mildly a case of confusion’, the shade was ‘approved’ before finishing and the dyer had his sample. The finisher after finishing had forwarded the sample through ‘proper channels’ without looking at it; buyer had approved the finished sample for whatever reason. The ‘proper channel’ – the marketing department had kept a counter sample as a matter of procedure. The dyer was astonished to see the shade had significantly changed after finishing in bulk as compared to his retained sample. The dyer and finisher were at loggerheads. Some wisdom donned on them after some time, to seek for the finished counter sample from the marketing. To their great relief and delight they found the buyer’s approved sample was matching the finished sample. This is a case of comeddy of errors – a very lucky coincidence. It would have been a tragedy had the buyer approved the shade before the finish. Many times the approval is sought on the dyed samples before finishing due to urgency and the process house invariably be ‘optimistic’ to remember and ‘take care of’ in bulk invariably this aspect is forgotten (due to the long time interval between approval and bulk processing) and end up either stuck at the finishing stage / re-work or endless correspondence with the buyer explaining the reasons and pleading to accept. A systematic procedure should be laid down for sample approvals and any ‘urgency’ should not compromise the system. Individual finishes require special conditions and precautions and it is necessary to understand the chemistry of the finish and its dynamics to intelligently adopt it to the equipments and facilities available. Standard procedures and recipes need to be established and written out. The machine settings for speed mangle pressures (where padding is employed), temperature parameters as applicable to the relevant finishes need to be explicitly displayed at the site and complied with during the operation. The concerned personnel need to exercise strict discipline in carrying out the operations as laid down in the procedure.
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