Polyester dyeing in HTHP dyeing machines

Important factors: polyester dyeing in HTHP dyeing machines.
1.Water quality

Traces of soluble copper and iron salts can affect the shade of some disperse dyes quite markedly due to the formation of coordination complexes.The presence of calcium and magnesium cations can interfere with the anionic dye dispersing agents/leveling agents and wetting agents present in the dyebath. To overcome these problems a small amount of chelating agents might be added to the dyebath. 

2.pH of the dyebath

Ideally the pH of the dyebath should be buffered by use of a quality pH regulator, in the range of 4.5-5.5 to minimize the possibility of dye hydrolysis.

3.Use of Surfactants in dyeing

There are a multitude of purposes for which anionic surface active agents are used in dyebaths, such as wetting, penetrating and deareation etc. anionic surface active are also used to stabilize the diluted dispersions of the dye.This is necessary particularly when dyeing paler shades where dyebath concentration of surfactant introduced with the dye itself is small.

The non ionic surfactants are generally used to control the rate of exchange of dyebath to the fiber.

4.Temperature
For most cost effective dyeing of polyester depends on factors such as starting temperature ,rate of heating ,dyeing temperature and duration of dyeing which should result into well exhausted bath ,satisfactory color uniformity and batch to batch shade consistency.
A general rule of thumb , the starting temperature of dyeing is 70-80 degrees , the rate of rise of temperature is 1.5-2 deg.Celsius /min and the dyeing temperature is between 115-135 deg.Celsius.
5.Time

For an ideal dyeing time selection , one must take into account the exhaustion of dye bath , uniformity of the dye , satisfactory color penetration , for good color fastness and reproducibility of the dyeing results. Generally 15-60 minutes of dyeing time is followed and it is not necessary to prolong the dyeing long beyond the time of maximum exhaustion.

6.Dyes

The dyes with similar dyeing characteristics must be selected for particular formulation, it is important to identify the actual dyeing rate of each component will be dependent on the concentration present in the formulation.Care should be taken that the dyeing conditions allow the slowest dyeing component to reach the effective equilibrium.

7.Oligomers

Typically polyester fibres contain between 1.5 and 3.5% by mass of low molecular esters, the principal oligomer being cyclic tris(ethylene terephthalate) with smaller quantities of a dimer, pentamer as well as traces of other compounds.

when oligomers are released from the polyester fibre, after some time
at 130° C, if an efficient dispersing agent not present in the bath to prevent crystallization of the oligomers, and to prevent small particles of oligomers coming together to form larger agglomerates, oligomers will deposit on the yarn and the machine.

Dispersing will maintain the oligomers in a fine dispersion so that when the machine is drained more oligomer goes down the drain. Discharging the dyebath at high temperature if this is possible can also reduce oligomer deposition because any of the material soluble during dyeing at a high temperature can precipitate during cooling.
8.Reduction Clearing
Reduction-clearing treatment is necessary to remove any dye remaining on the fibre surfaces, If not removed, this surface contamination can undermine the brightness of shade as well as the wash, sublimation and crock fastness results.Commonly, the dyed polyester is cleared of surface-deposited dye as well as auxiliaries (e.g. carriers, surfactants) by means of treatment with detergent or reductive or oxidative treatments, in order to secure optimum fastness of the dyeing and also to improve the brightness of shade.
The usual treatment carried out, especially in heavy depth, is reduction- clearing, where the dyed fibre is treated in a strong reducing bath, usually made up of sodium dithionite and caustic soda. A treatment for 20 minutes at approximately 70-80 deg Cel, is often sufficient to clear
the fibre surface, but the ease of removal varies from chromophore to chromophore and dye to dye. This treatment acts to destroy loose azo disperse dye through chemical reduction of the azo link . Anthraquinone disperse dyes are not fully destroyed by such a treatment but a degree of removal of surface dye is achieved through temporary solubilisation of the disperse dye to the alkali-leuco form.

9.Stripping 
Stripping of disperse dyed material can be accomplished by treating goods
a. In a blank bath containing non ionic leveling agent at 130 deg.cel.
b.For chemical destruction of dye use 1-2 gpl NaOH and a reducing agent
or mild acidic treatment with zinc sulfoxlate-formaldehyde or sodium chlorite can reduce the shade to almost a off white back ground.

A New Study on Stripping for Polyester Fabrics

10.Prolonged treatment of polyester in alkaline solutions

Prolonged treatment of polyester goods in alkaline solutions will cause weight loss due to hydrolysis of polyester at the fiber surfaces, since the fiber surface has been eroded considerably  after a long alkaline treatment it is very difficult to rework the goods to give them originally anticipated appearance.

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