Dyeing of Polyester

Dyeing of Polyester

Polyester is hydrophobic and nonionic in nature and is dyeable with only disperse dye, although solubilised vat dyes are seldom used on polyester–cotton blends to dye both the components with light shades. Fibre structure is too compact and hinders chemicals to enter inside below its glass transition temperature (Tg ~ 85°C); alkaline hydrolysis is an example where only the outer surface is scooped out by alkali with no change at the interior. Beyond Tg, structure slowly opens up with corresponding increase in pore size at the surface and interior – extent of opening is directly proportional to increase in temperature. Dyeing temperature is not constant, rather depends on required opening at specific temperature for dyes possessing specific size – the smaller the size the lower the dyeing temperature and is related as Td = Tg + ΔTdye, where Td is dyeing temperature and ΔTdye is the rise in temperature beyond Tg to provide required opening to accommodate a dye of specific size. Under practical situations, most of possess the size to cause efficient dyeing at 120°C; however, for thorough distribution of dye through migration required to produce levelled shades, the dyeing temperature is maintained at 130 ± 2°C. Dyeing of polyester with disperse dye can be described as: (i) deposition of dye on surface at lower temperature, (ii) diffusion at the surface layers with increase in temperature and (iii) diffusion at the interior at higher temperature – the rate of diffusion at the interior is the rate determining step in dyeing. Once distribution of dye is over, the bath is cooled down to collapse fibre structure in which dye remains attached with fibre through H-bonds and Van der Waals forces. In spite of weaker nature of these bonds, wash fastness remains excellent as dye cannot comes out from i structure. Non-disperse dyes (vat, sulphur etc) possess bigger structures and are not used in dyeing of polyester as these dyes cannot penetrate fibre matrix even at higher temperature, also most of the nondisperse dyes lack good thermal stability at very high temperature. The highest activation energy required for polyester–disperse dye system clearly reflects compactness of the fbre and also reflects that why such a dyeing process cannot be conducted t or below boil satisfactorily, even in the presence of carriers.

Methods of dyeing polyester:

There are four methods for dyeing polyester, viz.

 (i) dyeing at boil without carrier,
(ii) dyeing at boil with carrier,
(iii) HTHP dyeing and
(iv) thermosol dyeing.
The first three methods are batch methods whereas the thermosol method is a continuous one. Efficient pretreatment of polyester is prerequisite to produce levelled and bright shades. Molten beads formed during singeing develop spot effect and so singeing is done after dyeing when batch dyeing is followed. In contrast, continuous dyeing or thermosol process do not cause spot dyeing of molten beads as dyeing is affected not through exhaustion but through surface deposition and diffusion and so singeing may be included in pretreatment sequence. Disperse dye manufacturers blend dispersing agent with dye during grinding as application of dispersing agent is must in dyeing with disperse dyes for any shade to avoid precipitation of dye in bath and this eliminates further use of dispersing agent in dyeing except for certain cases, like dyeing of textured polyester.

(i) dyeing at boil without carrier:

This method is used only for pale shades; fibre opening is too little at boil, necessitating use of only low molecular weight disperse dyes. Dyes do not penetrate due to very limited opening causing poor wash and sublimation fastness ratings – domestic pressing causes partial sublimation of dye too. Dye bath is prepared with dye, dispersing agent and acetic acid (pH~5.5-6.5); dyeing is started at 50-60°C, raised slowly to boil and is carried out at this temperature for 2–3 hours followed by soaping and washing. In this method, only very light shades can be produced on polyester or its blends.

(ii) dyeing at boil with carrier:

Carrier brings down Tg of polyester and swells up fbre structure for better diffusion of dye at boil. Dyebath is prepared with dye, dispersing agent and acetic acid (pH~5.5–6.5). Dyeing is started at 50–60°C with slow rise in temperature to 80°C when required amount of carrier is added; temperature is raised to boil and dyeing is continued for 2–3 hours. Dyebath is discharged and dyeings are washed with anionic or non-ionic detergent along with Na2CO3 (2–3 g/l each) to remove last trace of carrier. To enhance performance of the process, a closed jigger or winch may be swung into action to get uniform temperature throughout bath. This method is restricted to production of light shades with negative impact of carrier, e.g., irritation on skin, poor light fastness and spotting – due to volatile nature of carrier. Although no definite mechanism of carrier action has been devised, the more popular concept suggests swelling of fibre with ease in diffusion of dye inside. The swelled up structure subsequently collapses with cooling of bath followed by washing out of carrier.

(iii)High temperature with high pressure:

HTHP dyeing is the most popular commercial batch method of dyeing polyester to produce levelled shades. The process is based on opening of fibre structure at higher temperature, includes no carrier and generates lesser waste water load. Disperse dyes show feeble water solubility beyond 90°C; a part of disperse dye goes into solution with simultaneous affinity for polyester – the higher the temperature the better the solubility and the higher the affinity due to more solubility of dye as well as opening of fibre. Equilibrium is only reached when the subsidiary equilibria are achieved through a balance in all the dyeing steps, viz.
(i) dye dispersed in bath ↔ dye dissolved in bath
(ii) dye dissolved in bath ↔ dye adsorbed on fibre
(iii) dye adsorbed on fbre ↔ dye diffused in situ fibre

Strike rate is directly proportional to increase in temperature; the latter is increased from 90°C at 1°C/min to 117°C the temperature at which all existing disperse dyes complete exhaustion due to sufficient opening in structure seems to be right enough for levelled shades. However, another increase by 10–12°C is adopted in commercial application for uniform distribution and ease in migration of dye to compensate heat loss during dyeing shown
by the absorption curve in Fig. 1.
Dye uptake decreases initially with increase in heat setting temperature of polyester and remains minimum from 140–180°C beyond which remarkable increase starts again – a
heat setting temperature beyond 200°C causes sharp increase in dye uptake. If heat setting is carried out beyond 220°C, uneven dyeing may occur due to improper setting. Setting temperature must be within 180–200°C as explained in Fig. 2. Dyebath is prepared with acetic acid (pH ~ 5.5–6.5) and dispersing agent. Temperature is raised to 60°C, dye is added and temperature is again raised to 90°C beyond which rise in temperature is restricted to 1–1.5°C/min up to 117°C, and beyond that temperature is raised quickly to 130 ± 2°Cat corresponding steam pressure around 3–3.5 kg/cm2 and continued for 0.5–2 hours depending on the depth of shade. It is preferred to discharge the bath at 130± 2°C to drain out liquified low molecular weight oligomers which otherwise get precipitated on fibre surface during cooling below 120°C and causes harsh feel as well as deteriorates rubbing fastness of dyeings. Alternately the machine may be cleaned after dyeing with NaOH (2 g/l) beyond boil. Dyeing follows reduction cleaning of dyed polyester with NaOH and Na2S2O4 (3 g/l each) at 50–60°C for 15–20 min to remove superficial dye which otherwise inhibits wash fastness. HTHP dyeing is – undoubtedly – a unique method as all types of well levelled shades with superior fastness can be produced on polyester and its blends. Dyeings are generally performed either in jet dyeing machine in rope form or in open width in beam dyeing machines.
Levelling of shade crucially varies with control over temperature. Up to 100°C, as high as 10% of the bath is exhausted, 80% exhaustion occurs during 100–120°C and the rest 10% between 120–130°C. Neither the initial nor the final absorption stages affect levelling and so such control over heating of bath is necessary, but an efficient control is needed during heating in the range of 100–120°C.

Thermosol dyeing:

This is a continuous method of dyeing polyester. In this process, dye is forced to get solubilised in solid fibre phase through partial loosening of intermolecular bonds under heat treatment when the internal structure of the fibre opens up to allow dye molecules to diffuse at the interior. The process is not viable for 100% polyester due to feeble pick up of liquor during padding as the fibre is highly hydrophobic in nature. Although multiple dip and nip may improve dye solution pick up to some extent, chances of unlevelled shade because of migration of dye poses a problem. Thermosol process is unique for continuous dyeing of polyester–cotton blends, where cotton picks up dye liquor from bath and transfer it to the polyester phase at high temperature during curing. The cotton part is subsequently dyed in usual ways. The process is of non-migrating type, i.e., no further scope of levelling once cured so a careful pretreatment of fabric is must to develop good and consistent wettability. Well pretreated polyester–cotton fabric is padded with liquor containing dye, acid and dispersing agent at 70–80% expression, followed by controlled drying at 90-100°C with uniform circulation of air to avoid migration. Dried fabric passes through stenter, curingchamber or loop ager at 180–210°C for 45–60 s for complete diffusion of dye in partially or fully sublimised state or passed through superheated steam. The coating of dye dispersion on fabric is very sensitive to touch and so the padded material be instantly passed through drying zone without any contact with guide rolls or other accessories. The best way is to place infra red heaters just above the last nip for mild drying without migration of dye followed by drying in float drier at 90–100°C. Due to hydrophobic nature of polyester, dye concentration is kept on higher side. Liquid disperse dyes provide excellent result due to high rate ofdispersion and diffusion, these diffuse more rapidly into fibre structure with saving of energy and shortening the time of fixation. Dyes from B and C classes possessing moderate sublimation and wash fastness properties may be selected for levelled shades, dyes form D class produce excellent fast shades but are not recommended on grounds of having higher sublimation temperature and excess of dye is required to produce desired depth due to poor diffusion coefficient. However, a reduction clearing definitely improves wash fastness and the latter can be verified by dipping dyed sample in dimethyl formamide at room temperature – stripping of colour in solvent confirms presence of dye on surface. Introduction of a migration inhibitor in padding liquor like sodium alginate or CM arrests migration of dye and produces levelled shades. Increase in draw ratio during melt spinning promotes orientation and decreases rate of diffusion even heat setting at 180–210°C causes surface melting of the polymer with less dye uptake. Uncontrolled drawing in filaments produces barré; stains on fabric must be removed prior to dyeing to produce levelled shades. Sometimes, a ring dyeing effect is produced due to insufficient diffusion of dye which may be rectified by exposing dyeings at 180–210°C for 30–45 s. Non-ionic surfactants – when applied as dyeing accelerants – promote rate of dye dissolution in auxiliary melt, thereby increasing the overall rate of thermosol dyeing. At high surfactant concentrations, dye retention in surfactant may adversely affect dye fixation .

Dyeing of Polyester Dyeing of Polyester Reviewed by Suraj Gupta on April 25, 2020 Rating: 5

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