How to Control Thermomigration in Textiles

Dr. N.N. Mahapatra
CText FTI (Manchester)
Business Head (Dyes)
Shree Pushkar Chemicals & Fertilisers Ltd,
301/302, 3rd Floor, Atlanta Center, Sonawala Road,
Goregaon (East) Mumbai MH 400063

What is Thermomigration?
Heat movement of disperse dyes out of synthetic fibers during surface. In other words it is the movement of dye during post–heat treatment of a polyester substrate, resulting in an accumulation of dye at the fibre surface as well as limited sublimation of the dye to the surrounding atmosphere. This is known as thermal migration of disperse dyes. In other words by thermomigration we mean the desorption of dyes from fibre and their transfer into preparation and finishing chemicals (spin finish, softeners, different types of special finishes) under the influence of heat and time. This impairs the light fastness, rubbing fastness, wet fastness, fastness to perspiration and fastness to dry cleaning. Shade changes may occur, and the adjacent material can be stained during a subsequent heat treatment such as ironing, pressing etc.

It is a very common problem in spinning mills where p/v dyed yarn is manufactured. Normally the polyester fibre is dyed in HTHP dyeing machines at 135 deg c. Then the dyed fibre is mixed (or blended) with dope dyed viscose (manufactured by Grasim Industries, Nagda, (M.P). The blends vary it may be p/v 65/35, 70/30, 48/52, 80/20 etc. Grasim Nagda supplies the maximum dope dyed viscose covering all shades like Black, Coffee, Navy, Green, Red, Maroon for p/v blends normally viscose is not dyed in dye house.

Spinning mill manufactures solid shades and frosted shades (melange) in frosted shades dyed fibre is mixed with grey fibre (undyed). Then the yarn is made. The frosted fancy yarn is supplied to the customer. He weaves the fabric and the fabric is sent to finishing to process house. But unfortunately the frosted shade fabric after finishing becomes little solid that means the dyes has migrated to the grey portion. Then the yarn is rejected and the whole blame goes to the dyed p/v spinner. The spinner blames to the fibre dye house. Till date nobody has gone into the detail of this subject. The reason is plenty. The fibre dye house does not have any role. The main role is Nagda dope dyed viscose and finisher of the fabric.

Dope dyed viscose is a mass colouration process where pigments (except black) is added at the viscous solution stage. Few of the dope dyed viscose like Red, Maroon, Coffee, Navy etc fail in cold DMF test which is conducted for determination of Thermomigration. Dope dyed viscose is dipped in DMF at room temperature. Without affecting the pigments in the fibre. Immediately the DMF solution becomes colour. But this is not the case in fibre dyed polyester fibre. In all fibre dye house this cold DMF test is done for all Dark/Heavy dark shades of polyester dyed before laying the mixing. It means when the p/v dyed fabric passes in the stenter at 180ºC. The colour from the dope dyed viscose fibre has migrated from the dyed p/v portion to the grey polyester or grey viscose portion.

The dope dyed viscose manufacturer should improve on its dope dyed viscose shades. Some of the shades should pass this cold DMF tests. As a result the thermomigration problem should be solved.

In such cases like frosted or mixer shades chances are that the finish in the grey fibre may act as a solvent for the dyes at high temperature. When the fabric is passed through the stenter. The finish applied by polyester fibre manufacturer or viscose manufacturer acts as a solvent for disperse dyes at high temperature to avoid these point few of the mills in India are not directly adding grey polyester fibre in the mixing. Along with dyed fibre. They are doing scouring or hot wash at 80ºC and removing this finish applied at the fibre manufacturing stage. Then only this scoured/washed fibre is mixed with dyed fibre in the mixing stage and melange or frosted shades yarn are made. They have got good results. It has minimised thermomigration. This practice can be started in all textile mills.

But the main reason for thermomigration is the finisher. Not the dyer. Marketing people bring complaints. They blame the dyer. But it is wrong. The fibre dyer has used the best high sublimation fastness disperse dyes, they have checked the dyed polyester in cold dmf solvent. They have done very good reduction clearing in alkaline medium or acidic medium. In some cases it is the dope dyed viscose which helps in thermomigration. This matter has to be seriously dealt with dope viscose manufacturers. Till date they have not come out with any concrete solutions.

Another area which is responsible for thermomigration is the fabric finishing. The spinning mill sales the frosted/melange yarn to the weaver. The weaver weaves the fabric and get it process on job basis in various process houses. Some of the process houses do not take care in the controlling factors. Required to process such type of special fabrics. The processing will be slightly different from normal solid shade fabrics.

For cost cutting some process house use cheap chemicals for fabric finishing. Another factor is the stenter where the temperature variation from compartment to compartment varies. The temperature indicator does not work. So it is very difficult to predict the actual temperature inside the stenter. Sometimes it shoots up. So higher temperature promotes thermomigration.

Thermomigration should not be confused with sublimation. The spin finish/ finishing chemicals composition is not known to the spinner or finisher. So what happens the fibre has a chance of coming in contact with the film of the above chemical. Similarly when the treated goods are kept in storage for sometime the dyestuff has a tendency to be desorbed from fibre and be absorbed in the layer of the spin finish/ finishing chemical particularly in the influence of heat, humidity and pressure. This is because the layer of oil or the finishing chemical exerts a stronger solvent action on the dyestuff than the fibre itself. It is advisable not to use non-ionic surfactants or softener of ethoxylated fatty alcohols in finishing.

The initiating factor for dye thermomigration is the temperature and the duration of its action. Whilst drying conditions around 100ºC show no significant effect.

Thermomigration starts at 140ºC and rises maximum at 170ºC to 180ºC and surprisingly decreases at higher temp of 200ºC to 220ºC. The potential temperature of thermomigration are between 140ºC to 180ºC. Hence higher temperatures at 200ºC for shorter times would reduce thermomigration while in resin finishing use of catalyst which lower the temp of cure below 140ºC would also reduce thermomigration. Another option is the finisher still has certain possibilities of controlling this. The most favourable compromise must be found between adequate effect at low temperatures with slightly longer dwell times and the economical occupation of machines. Temperature has a greater effect than time.

Following are the disperse dyes recommended for low thermomigration:

A test method to check whether the dyed fibre will promote thermomigration or not. The dyed polyester fibre should be tested with soap- 0.5 gpl and soda ash 0.2 gpl at 140ºC for ½ hour with a multifibre stripe attached, to see the staining on nylon. No staining should take place.

It has been seen that virtually all nonionic ethylene oxide adducts, and also all nonionic emulsifiers, vigorously promote thermomigration..we should take care that such products are not present on the dyed textiles prior to heat treatment (passing through the stenter) or to a lengthy period of storage.

Most fabrics are submitted at the end of the production process to a treatment where softeners, antistatic, water repellent or anti-crease agents, combined with dimensional stabilization are applied at temperatures around 140ºC to 180ºC disperse dyes migrate with preference into such chemicals which have a greater dissolving effect than the polyester itself. The amount of thermomigrated dye varies from an extent which is virtually only dependent on the effect of the temperature to double the amount with unfavourable nonionic softeners. Careful selection by the manufacturers and testing by the finisher can keep, this problem within bounds.

Broadly speaking, non-ionic surfactants and non-ionic finishing chemicals are damaging in character. Cationics are less harmful. Anionic chemicals are relatively harmless. Polyvinyl acetate dispersions are harmless. Among cross linking agents those based on dihydroxy dimethylol ethylene chloride as catalyst. In any case a precheck with the given dye chemical system is advisable before bulk application.

Thermomigration occurs not only after the application of a heat treatment (passing through stenter) but can also be caused by storing the dyed fabric for a lengthy period of time. Even residual detergent (particularly non –ionic products) can promote thermomigration. The detergent must be therefore also be chosen with care. It must be efficient but must also be capable of being removed completely after the wash.

Determination of thermomigration:
A Test is conducted by the dyer in dyehouse which is known as DMF test. After fibre is dyed and reduction clearing is done this test is conducted. The dyed fibre is dipped inside cold DMF liquid at room temp and taken out immediately. If the DMF liquid is clear then the reduction clearing is proper. This dyed fibre is not going to give problem in thermomigration.

A similar test is conducted by the finisher in the process house.

The fabric to be finished must be scoured properly and a sample must be treated with the relevant finishing chemicals at normal concentrations and dried for 2 mins at 100ºC. Half of the sample is then heated again for 30 secs to 180 deg C. And dipped in cold DMF liquid. At room temperature. The non heated sample serves as a blank. If both extracts (liquids) are colourless or slightly stained, the product in question has no effect on thermomigration.

ABOUT THE AUTHOR
Dr N.N. Mahapatra is a B.Sc. (Tech) in Textile Chemistry from UDCT, (now ICT) Mumbai. He also holds a M.Sc. and Doctorate in Applied Chemistry from Utkal University, Orissa. He did his M.B.A from I.M.M, Kolkata. Dr Mahapatra is having 38 years of experience working in Textile and Dyes manufacturing industries in India and abroad in various senior capacities. In the year 2007 he was also awarded C Col FSDC (U.K) and C Text FTI (Manchester). In 2018 he was awarded the Fellowship (FRSC) from Royal Society of Chemistry, UK. He has written 8 books (Textile Dyes & Dyeing, Textile Technology, Textile Processing, Textiles & Environment, Textile Dyes, Sarees of India, Textile Dyeing and Modern Textile Processing). Presently he is working as Business Head (Dyes) Shree Pushkar Chemicals & Fertilisers Ltd, Mumbai.

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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