An Overview of Polyester and Polyester Dyeing

Last Updated on 10/04/2021

An Overview of Polyester and Polyester Dyeing

Ardhendu Shekhar Paul
B.Sc. in Textile Engineering.
Specialization in Wet Processing Technology
Prime Asia University, Banani, Dhaka.
E-mail: ardho.txe@gmail.com

 

History of Polyester:
Polyester began as a group of polymers in W.H. Carothers’ laboratory. Carothers was working for duPont at the time when he discovered that alcohols and carboxyl acids could be successfully combined to form fibers. Polyester was put on the back burner, however, once Carothers discovered nylon. A group of British scientists–J.R. Whinfield, J.T. Dickson, W.K. Birtwhistle, and C.G. Ritchie–took up Carothers’ work in 1939. In 1941 they created the first polyester fiber called Terylene. In 1946 duPont bought all legal rights from the Brits and came up with another polyester fiber which they named Dacron.

Polyester was first introduced to the American public in 1951. It was advertised as a miracle fiber that could be worn for 68 days straight without ironing and still look presentable.

In 1958 another polyester fiber called Kodel was developed by Eastman Chemical Products, Inc. The polyester market kept expanding. Since it was such an inexpensive and durable fiber, amny small textile mills emerged all over the country–many located in old gas stations–to produce cheap polyester apparel items. Polyester experienced a constant growth until the 1970s when sales drastically declined due to the negative public image that emerged in the late 60’s as a result of the infamous polyester double-knit fabric!

Polyester:
A polyester is a polymer (a chain of repeating units) where the individual units are held together by ester linkages.

Polyester

The diagram shows a very small bit of the polymer chain and looks pretty complicated. But it isn’t very difficult to work out – and that’s the best thing to do: work it out, not try to remember it. You will see how to do that in a moment.

The usual name of this common polyester is poly (ethylene terephthalate). The everyday name depends on whether it is being used as a fiber or as a material for making things like bottles for soft drinks.

When it is being used as a fiber to make clothes, it is often just called polyester. It may sometimes be known by a brand name like Terylene.

When it is being used to make bottles, for example, it is usually called PET.

Types:
Polyesters as thermoplastics may change shape after the application of heat. While combustible at high temperatures, polyesters tend to shrink away from flames and self-extinguish upon ignition. Polyester fibers have high tenacity and E-modulus as well as low water absorption and minimal shrinkage in comparison with other industrial fibers. Unsaturated polyesters (UPR) are thermosetting resins. They are used as casting materials, fiberglass laminating resins and non-metallic auto-body fillers. Fiberglass-reinforced unsaturated polyesters find wide application in bodies of yachts and as body parts of cars.

Monomers: Functional Groups
The monomers that are involved in condensation polymerization are not the same as those in addition polymerization. The monomers for condensation polymerization have two main characteristics:

  • Instead of double bonds, these monomers have functional groups (like alcohol, amine, or carboxylic acid groups).
  • Each monomer has at least two reactive sites, which usually means two functional groups.

Some monomers have more than two reactive sites, allowing for branching between chains, as well as increasing the molecular mass of the polymer. Four examples of these difunctional monomers were introduced in Part 2 of this tutorial. Here they are again:

structure

Chemical name

Guess the names of each of these monomers. Give the letter that corresponds to the correct name of the structure (use each letter only once). Hints: Glycol means that a molecule has more than one alcohol (-OH) group. Amine means that a molecule has an amino (-NH2) group. Diamine (or diamino) means that a molecule contains two amino groups. Acid means that a molecule contains a carboxylic acid group (-COOH). Click the button when done.

Let’s look again at the functional groups on these monomers. We’ve seen three:

  1. The carboxylic acid groups
  2. The amino group (R-NH2)
  3. The alcohol group (R-OH)

The Amide Linkage:
When a carboxylic acid and an amine react, a water molecule is removed, and an amide molecule is formed.

amide molecule

Because of this amide formation, this bond is known as an amide linkage.

The Ester Linkage:
When a carboxylic acid and an alcohol react, a water molecule is removed, and an ester molecule is formed.

Ester molecule

Because of this ester formation, this bond is known as an ester linkage.

In Summary:
Monomers involved in condensation polymerization have functional groups. These functional groups combine to form amide and ester linkages. When this occurs, a water molecule in removed. Since water is removed, we call these reactions condensation reactions (water condenses out). When a condensation reaction involves polymerization, we call it condensation polimarization.

Let’s look at a few common examples of condensation polymers.

The Mechanism of Condensation Polymerization:
We know that monomers that are joined by condensation polymerization have two functional groups. We also know that a carboxylic acid and an amine can form an amide linkage, jand a carboxylic acid and an alcohol can form an ester linkage. Since each monomer has two reactive sites, they can form long-chain polymers by making many amide or ester links. Let’s look at two examples of common polymers made from the monomers we have studied.

Example 1:
A carboxylic acid monomer and an amine monomer can join in an amide linkage.

Carboxylic acid monomer

As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer.

Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide. The polymer made from these two six-carbon monomers is known as nylon-6,6. (Nylon products include hosiery, parachutes, and ropes.)

Example 2:
A carboxylic acid monomer and an alcohol monomer can join in an ester linkage.

Alcohol monomer

A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding. ( )

water molecule

Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly (ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)

Let’s summarize:
As difunctional monomers join with amide and ester linkages, polyamides and polyesters are formed, respectively. We have seen the formation of the polyamide nylon-6,6 and the polyester PET. There are numerous other examples.

Remember: 
The above process is called condensation polymerization because a molecule is removed during the joining of the monomers. This molecule is frequently water.

A Simulation of Condensation Polymerization:
During the polymerization process, the monomers tend to form dimers (two linked monomers) and trimers (three linked monomers) first. Then, these very short chains react with each other and with monomers. The overall result is that, at the beginning of polymerization, there are many relatively short chains. It is only near the end of polymerization that very long chains are formed.

Polymerize into dimers, trimers, and so on, the monomers will turn black. Polymerization will continue for a few seconds. Then the display will change into a bar graph entitled “Distribution” and show the progression of the polymerization over time. The x-axis is the number of units in the polymer (the “n” in the formula of a polymer). This is suggested graphically with the series of polymers projected into the screen. As we move to the left, the polymers are longer. The y-axis is the number of polymers. The higher the bar, the more numerous are the polymers. The graph shows dynamically the distribution of polymers in the polymerization as the reaction progresses. Notice that at the beginning of the polymerization, the distribution lies farther to the right, meaning that there are a lot of monomers, dimers, trimers, and other short chains but few long chains. As the polymerization progresses, the distribution shifts to the left, indicating that there are fewer short chains and more of the longer ones.

Some Assumptions:
First, we assume that there is only one type of difunctional monomer, as opposed to two types, as we saw in the two examples above. If we imagine that the polymers in the simulation are polyamides (like nylon-6,6), then the monomer has one carboxylic acid group and one alcohol group (picture the dimer you saw in Example 1 in the previous section). Second, we assume that there are only 90,000 monomers in the polymerization. In real life, the number of monomers is on the order of 1023. Despite the low number of monomers in the simulation, it does show the correct, real-life distribution of polymer chains over time.

Making polyesters as an example of condensation polymerisation:
In condensation polymerisation, when the monomers join together a small molecule gets lost. That’s different from addition polymerisation which produces polymers like poly(ethene) – in that case, nothing is lost when the monomers join together.

A polyester is made by a reaction involving an acid with two -COOH groups, and an alcohol with two -OH groups.

In the common polyester drawn above:

The acid is benzene-1,4-dicarboxylic acid (old name: terephthalic acid).

The alcohol is ethane-1,2-diol (old name: ethylene glycol).

polyester

Now imagine lining these up alternately and making esters with each acid group and each alcohol group, losing a molecule of water every time an ester linkage is made.

ester linkage

That would produce the chain shown above (although this time written without separating out the carbon-oxygen double bond – write it whichever way you like).

carbon-oxygen double bond

Copolymers
In addition to pure (homopolymer) PET, PET modified by copolymerization is also available. In some cases, the modified properties of copolymer are more desirable for a particular application. For example, cyclohexane dimethanol (CHDM) can be added to the polymer backbone in place of ethylene glycol. Since this building block is much larger (6 additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer’s melting temperature. In general, such PET is known as PETG or PET-G (Polyethylene terephthalate glycol-modified; Eastman Chemical, SK Chemicals, and Artenius Italia are some PETG manufacturers). PETG is a clear amorphous thermoplastic that can be injection molded or sheet extruded. It can be colored during processing.

Phthalic_acid isomers

Replacing terephthalic acid (right) with isophthalic acid (center) creates a kink in the PET chain, interfering with crystallization and lowering the polymer’s melting point.

Another common modifier is isophthalic acid, replacing some of the 1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or 1,3-(meta-) linkage produces an angle in the chain, which also disturbs crystallinity.

Such copolymers are advantageous for certain molding applications, such as thermoforming, which is used for example to make tray or blister packaging from co-PET film, or amorphous PET sheet (A-PET) or PETG sheet. On the other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of isophthalic acid, CHDM, DEG or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding (“SBM”), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages.

Manufacturing of polyethylene terephthalate
The reaction takes place in two main stages: a pre-polymerisation stage and the actual polymerisation.

In the first stage, before polymerisation happens, you get a fairly simple ester formed between the acid and two molecules of ethane-1,2-diol.

Manufacturing of polyethylene terephthalate

In the polymerisation stage, this is heated to a temperature of about 260°C and at a low pressure. A catalyst is needed – there are several possibilities including antimony compounds like antimony (III) oxide.

The polyester forms and half of the ethane-1,2-diol is regenerated. This is removed and recycled.

polyester forms and half of the ethane

Recycling to the monomers:
Polyethylene terephthalate can be depolymerized to yield the constituent monomers. After purification, the monomers can be used to prepare new polyethylene terephthalate. The ester bonds in polyethylene terephthalate may be cleaved by hydrolysis, or by transesterification. The reactions are simply the reverse of those used in production.

Partial glycolysis:
Partial glycolysis (transesterification with ethylene glycol) converts the rigid polymer into short-chained oligomers that can be melt-filtered at low temperature. Once freed of the impurities, the oligomers can be fed back into the production process for polymerization.

The task consists in feeding 10–25% bottle flakes while maintaining the quality of the bottle pellets that are manufactured on the line. This aim is solved by degrading the PET bottle flakes — already during their first plasticization, which can be carried out in a single- or multi-screw extruder — to an intrinsic viscosity of about 0.30 dℓ/g by adding small quantities of ethylene glycol and by subjecting the low-viscosity melt stream to an efficient filtration directly after plasticization. Furthermore, temperature is brought to the lowest possible limit. In addition, with this way of processing, the possibility of a chemical decomposition of the hydro peroxides is possible by adding a corresponding P-stabilizer directly when plasticizing. The destruction of the hydro peroxide groups is, with other processes, already carried out during the last step of flake treatment for instance by adding H3PO3.]The partially glycolyzed and finely filtered recycled material is continuously fed to the esterification or prepolycondensation reactor, the dosing quantities of the raw materials are being adjusted accordingly. Total glycolysis, methanolysis, and hydrolysis

The treatment of polyester waste through total glycolysis to fully convert the polyester to bis(2-hydroxyethyl) terephthalate (C6H4(CO2CH2CH2OH)2). This compound is purified by vacuum distillation, and is one of the intermediates used in polyester manufacture. The reaction involved is as follows:

[(CO)C6H4(CO2CH2CH2O)]n + n HOCH2CH2OH → n C6H4(CO2CH2CH2OH)2

This recycling route has been executed on an industrial scale in Japan as experimental production.

Similar to total glycolysis, methanolysis converts the polyester to dimethyl terephthalate, which can be filtered and vacuum distilled:

[(CO)C6H4(CO2CH2CH2O)] n + 2nCH3OH → nC6H4(CO2CH3)2

Methanolysis is only rarely carried out in industry today because polyester production based on dimethyl terephthalate has shrunk tremendously, and many dimethyl terephthalate producers have disappeared.

Also, as above, polyethylene terephthalate can be hydrolyzed to terephthalic acid and ethylene glycol under high temperature and pressure. The resultant crude terephthalic acid can be purified by recrystallization to yield material suitable for re-polymerization:

[(CO)C6H4(CO2CH2CH2O)] n + 2nH2O → n C6H4(CO2H)2 + n HOCH2CH2OH

This method does not appear to have been commercialized yet.

Hydrolysis of polyesters
Simple esters are easily hydrolysed by reaction with dilute acids or alkalis. Polyesters are attacked readily by alkalis, but much more slowly by dilute acids. Hydrolysis by water alone is so slow as to be completely unimportant. (You wouldn’t expect your polyester fleece to fall to pieces if you went out in the rain!)

If you spill dilute alkali on a fabric made from polyester, the ester linkages are broken. Ethane-1,2-diol is formed together with the salt of the carboxylic acid.

Because you produce small molecules rather than the original polymer, the fibres are destroyed, and you end up with a hole!

For example, if you react the polyester with sodium hydroxide solution:

Hydrolysis of polyesters

Polyester Fiber Manufacturing Process:
Today over 70 to 75% of polyester is produced by CP (continuous polymerisation) process using PTA (purified Terephthalic Acid) and MEG. The old process is called Batch process using DMT (Dimethyl Terephthalate) and MEG (Mono Ethylene Glycol). Catalysts like 5b3O3 (ANTIMONY TRIOXIDE) are used to start and control the reaction. TiO2 (Titanium di oxide) is added to make the polyester fiber / filament dull. Spin finishes are added at melt spinning and draw machine to provide static protection and have cohesion and certain frictional properties to enable fiber get processed through textile spinning machinery without any problem. PTA which is a white powder is fed by a screw conveyor into hot MEG to dissolve it. Then catalysts and TiO2 are added. After that Esterification takes place at high temperature. Then monomer is formed.

Polymerisation is carried out at high temperature (290 to 300 degree centigrade) and in almost total vacuum. Monomer gets polymerised into the final product, PET (Poly ethylene Terephthalate). This is in the form of thick viscous liquid. This liquid is them pumped to melt spinning machines. These machines may be single sided or double sided and can have 36/48/64 spinning positions. At each position , the polymer is pumped by a metering pump-which discharges an accurate quantity of polymer per revolution (to control the denier of the fiber) through a pack which has sand or stainless steel particles as filter media and a spinneret which could be circular or rectangular and will have a specific number of holes depending on the technology used and the final denier being produced. Polymer comes out of each hole of the spinneret and is instantly solidified by the flow of cool dry air. This process is called quenching. The filaments from each spinneret are collected together to form a small ribbon, passed over a wheel which rotates in a bath of spin finish: and this ribbon is then mixed with ribbon coming from other spinning positions, this combined ribbon is a tow and is coiled in cans.

Polyester fiber manufacturing process
Figure: Polyester fiber manufacturing process

The material is called undrawn TOW and has no textile properties. At the next machine ( the draw machine), undrawn tows from several cans are collected in the form of a sheet and passed through a trough of hot water to raise the temperature of polymer to 70 degrees C which is the glass transition temperature of this polymer so that the polymer can be drawn. In the next two zones, the polymer is drawn approximately 4 times and the actual draw or the pull takes place either in a steam chamber or in a hot water trough.

After the drawing is complete, each filament has the required denier, and has all its sub microscopic chains aligned parallel to the fibre axis, thereby improving the crystallinity of the fibre structure and imparting certain strength. Next step is to set the strength by annealing the filaments by passing them under tension on several steam heated cylinders at temperatures 180 to 220 degrees C. Also, the filaments may be shrunk on the first zone of annealer by over feeding and imparting higher strength by stretching 2% or so on the final zone of the annealer. Next the fibre is quenched in a hot water bath, then passed through a steam chest to again heat up the tow to 100 degree C so that the crimping process which takes place in the stuffer box proceeds smoothly and the crimps have a good stability. Textile spin finish is applied either before crimping by kiss roll technique or after crimping by a bank of hollow cone sprays mounted on both sides of the tow. The next step is to set the crimps and dry the tow fully which is carried out by laying the tow on a lattice which passes through a hot air chamber at 85degree C or so. The two is guided to a cutter and the cut fibres are baled for dispatch.

The cutter is a reel having slots at intervals equal to the cut length desired 32 or 38 or 44 or 51mm. Each slot has a sharp stainless steel or tungsten carbide blade placed in it. The tow is wound on a cutter reel, at one side of the reel is a presser wheel which presses the tow on to the blades and the tow is cut. The cut fibre falls down by gravity and is usually partially opened by several air jets and finally the fibre is baled. Some, balers have a preweighting arrangement which enables the baler to produce all bales of a pre-determined weight. The bale is transported to a ware house where it is “matured” for a minimum of 8/10 days before it is permitted to be despatched to the spinning mill.

FIBER SPECIFICATION:

DENIER:
Usually the actual denier is a little on the finer side i.e for 1.2 D, it will be 1.16 and for 1.4, it could be 1.35. The tolerance normally is +- 0.05 and C.V% of denier should be 4 to 5%. Denier specifies the fineness of fibre and in a way controls the spinning limit. Theory tells us that in order to form yarn on ring spinning (and also in air jet) there must be minimum of 60 to 62 ifbres in the yarn cross section. Therefor the safe upper spinning limit with different denier is

DENIER

The limit is for 38 mm fibre. The limit rises for a longer fibres. When spinning on open end system, the minimum no of fibres in the yarn cross section is 110. So, all the fibre producers recommend finer denier fibres for OE spinning. Here the safe upper spinning limit is

DENIER

However, in actual practice, 30s is an upper limit with OE AND 1.2 Denier is being used, in USA and other countries, even for 10s count in OE. Deniers finer than 1.0 are called micro-denier and commercially the finest polyester staple fibre that can be worked in a mill is 0.7 D.

CUT LENGTH:
Cut lengths available are 32, 38, 44, 51 and 64mm for cotton type spinning and a blend of 76, 88 and 102 mm – average cut length of 88m for worsted spinning. The most common cut length is 38 mm. For blending with other manmade fibers, spinners preferred 51mm to get higher productivity, because T.M. will be as low as 2.7 to 2.8 as against 3.4 to 3.5 for 38mm fibre. If the fibre legnth is more, the nepping tendency is also more, so a compromise cut length is 44 mm. With this cut length the T.M. will be around 2.9 to 3.0 and yarns with 35 to 40% lower imprfections can be achieved compared a to similar yarn with 51 mm fibre. In the future spinners will standardize for 38 mm fibre when the ring spinning speed reaches 25000 rpm for synthetic yarns. For OE spinning, 32 mm fibre is preferred as it enables smaller dia rotor (of 38mm) to be used which can be run at 80000 to 100000 rpm. Air jet system uses 38 mm fibre.

TENSILE PROPERTIES:
Polyester fibres are available in 4 tenacity levels. Low pill fibres- usuall in 2.0 / 3.0 D for suiting enduse with tenacities of 3.0 to 3.5 gpd (grams per denier). These fibres are generally used on worsted system and 1.4D for knitting

  • Medium Tenacity – 4.8 to 5.0 gpd
  • High tenacity 6.0 to 6.4 gpd range and
  • Super high tenacity 7.0 gpd and above

Both medium and high tenacity fibres are used for apparel enduse. Currently most fibre producers offer only high tenacity fibres. Spinners prefer them since their use enables ring frames to run at high speeds, but then the dyeablity of these fibres is 20 to 25% poorer, also have lower yield on wet processing, have tendency to form pills and generally give harsher feel. The super high tenacity fibres are used essentially for spinning 100% polyester sewing threads and other industrial yarns. The higher tenacities are obtained by using higher draw ratios and higher annealer temperatures upto 225 to 230 degree C and a slight additional pull of 2% or so at the last zone in annealing. Elongation is inversely proportional to tenacity e.g.

TENSILE PROPERTIES

All the above values of single fibre. Testing polyester fiber on Stelometer @ 3mm guage is not recommended.

The T10 or tenacity @ 10% elongation is important in blend spinning and is directly related to blend yarn strength. While spinning 100% polyester yarns it has no significance. Tenacity at break is the deciding factor.

CRIMP PROPERTIES:
Crimps are introduced to give cohesion to the fibre assembly and apart from crimps/cm. Crimp stability is more important criterion and this value should be above 80% to provide trouble free working. A simple check of crimp stability is crimps/inch in finisher drawing sliver. This value should be around 10 to 11, if lower, the fibre will give high fly leading to lappings and higher breaks at winding. Spin finish also gives cohesion, but cohesion due to crimp is far superior to the one obtained by finish. To give a concrete example, one fibre producer was having a serious problem of fly with mill dyed trilobal fibre. Trilobal fibre is difficult to crimp as such, so it was with great difficulty that the plant could put in crimps per inch of 10 to 11. Dyeing at 130 degrees C in HTHP dying machine reduced the cpi to 6 to 8. Mills over sprayed upto 0.8% did not help. Card loading took place yet fly was uncontrolled, ultimately the fibre producer added a steam chest to take the two temperature to 100degrees plus before crimping and then could put in normal cpcm and good crimp stability. Then the dyed fibre ran well with normal 0.15 to 0.18 % added spin finish.

SPIN FINISH:
Several types of spin finishes are available. There are only few spin finish manufacturers – Takemoto, Matsumoto, Kao from Japan, Henkel, Schill & Scheilacher, Zimmer & Schwarz and Hoechst from Germany and George A. Goulston from USA. It is only by a mill trial that the effectiveness of a spin finish can be established. A spin finish is supposed to give high fibre to fibre friction of 0.4 to 0.45, so as to control fibre movement particularly at selvedges , low fibre-metal friction of 0.2 to 0.15 to enable lower tensions in ring spinning and provide adequate static protection at whatever speed the textile machine are running and provide enough cohesion to control fly and lapping tendencies and lubrication to enable smoother drafting.

Spin finish as used normally consists of 2 components – one that gives lubrication / cohesion and other that gives static protection. Each of these components have upto 18 different components to give desired properties plus anti fungus, antibacterial anti foaming and stabilizers. Most fibre producers offer 2 levels of spin finishes. Lower level finish for cotton blends and 100% polyester processing and the higher-level finish for viscose blend. The reason being that viscose has a tendecy to rob polyester of its finish. However, in most of the mills even lower spin finish works better for low production levels and if the production level is high, high level spin finish is required if it is mixed with viscose. For open-end spinning where rotor speeds are around 55000 to 60000 rpm standard spin finish is ok, but if a mill has new OE spinning machines having rotors running @80000 rpm, then a totally different spin finish which has a significantly lower fibre – fibre and fibre – metal friction gave very good results. The need to clean rotors was extended from 8 hours to 24 hours and breaks dropped to 1/3rd. In conclusion it must be stated that though the amount of spin finish on the fibre is only in the range 0.105 to 0.160, it decides the fate of the fibre as the runnability of the fibre is controlled by spin finish, so it is the most important component of the fibre. Effectiveness of spin finish is not easy to measure in a fibre plant. Dupont uses an instrument to measure static behavior and measures Log R which gives a good idea of static cover. Also, there is s Japanese instrument Honest Staticmeter, where a bundle of well-conditioned fibre is rotated at high speed in a static field of 10000 volts. The instrument measures the charge picked up by the fibre sample, when the charge reaches its maximum value, same is recorded and machine switched off. Then the time required for the charge to leak to half of its maximum value is noted. In general, with this instrument, for fibre to work well, maximum charge should be around 2000 volts and half-life decay time less than 40 sec. If the maximum charge of 5000 and half-life decay time of 3 min is used, it would be difficult to card the fiber, especially on a high production card.

DRY HEAT SHRINKAGE:
Normally measured at 180 degree C for 30 min. Values range from 5 to 8 %. With DHS around 5%, finished fabric realization will be around 97% of grey fabric fed and with DHS around 8% this value goes down to 95%. Therefore, it makes commercial sense to hold DHS around 5%. L and B colour: L colour for most fibres record values between 88 to 92. “b” colour is a measure of yellowness/blueness. B colour for semidull fibre fluctuates between 1 to 2.8 with different fibre producers. Lower the value, less is the chemicals degradation of the polymer. Optically brightened fibres give b colour values around 3 to 3.5. This with 180 ppm of optical brightener.

DYE TAKE UP:
Each fibre producer has limits of 100 +- 3 to 100+-8. Even with 100+-3 dye limits streaks do occur in knitted fabrics. The only remedy is to blend bales from different days in a despatch and insist on spinning mills taking bales from more than one truck load.

FUSED FIBRES:
The right way to measure is to card 10 kgs of fibre. Collect all the flat strips (95% of fused fibres get collected in flat strips). Spread it out on a dark plush, pick up fused and undrawn fibres and weigh them. The upper acceptable limit is 30mgm /10kgs. The ideal limit should be around 15mgm/10kgs. DUpont calls fused/undrawn fibres as DDD or Deep Dyeing Defect.

LUSTRE:
Polyester fibres are available in

  • bright: 0.05 to 0.10 % TiO2
  • Semil dull: 0.2 to 0.3 % TiO2
  • dull: 0.5 % TiO2
  • extra dull: 0.7% TiO2 and

In optically brightened with normally 180 ppm of OB, OB is available in reddish, greenish and bluish shades. Semi dull is the most popular lustre followed by OB (100 % in USA) and bright.

PHYSICAL AND CHEMICAL PROPERTIES OF POLYESTER FIBRE:

  1. DENIER: 0.5 – 15
  2. TENACITY: dry 3.5 – 7.0: wet 3.5 – 7.0
  3. %ELONGATION at break: dry 15 – 45: wet 15 45
  4. %MOISTURE REGAIN: 0.4
  5. SHRINKAGE IN BOILING WATER: 0 – 3
  6. CRIMPS PER INCH: 12 -14
  7. %DRY HEAT SHRINKAGE: 5 – 8 (at 180 C for 20 min)
  8. SPECIFI GRAVITY: 1.36 – 1.41
  9. % ELASTIC RECOVERY; @2% =98: @5% = 65
  10. GLASS TRANSITION TEMP: 80 degree C
  11. Softening temp: 230 – 240 degree C
  12. Melting point: 260 – 270 degree C
  13. Effect of Sunlight: turns yellow, retains 70 – 80 % tenacity at long exposure
  14. RESISTANCE TO WEATHERING: good
  15. ROT RESISTENCE: high
  16. ALKALI RESISTENCE: damaged by CON alkali
  17. ACID RESISTENCE: excellent
  18. ORGANIC CHEMICAL RESISTENCE: good

PROBLEMS WHICH OCCUR DURING MANUFACTURE OF POLYESTER STAPLE FIBER:
The manufacture of polyester fiber consists of 4 stps:

  1. Polymerisation: Using PTA/DMT and MEG on either batch or continuous polymerisation (cp_ – forming final polymer
  2. Melt spinning: Here molten polymer is forced thorough spinnerette holes to form undrawn filaments, to which spin finish is applied.
  3. Drawings: In which several million undrawn filaments are drawn or pulled approximately 4 times in 2 steps, annealed, quenched, crimped and crimp set and final textile spin finish applied and
  4. Cutting: In which the drawn crimped tow is cut to a desired 32/38/44/51 mm length and then baled to be transported to a blend spinning mill.

PROBLEMS FACED IN POLYMERIZATION:
Properties of Polymer:
The polymer formed is tested mainly for intrinsic viscosity (i.v), DEG content, % oligeomers and L and b colours. Intrinsic viscosity is an indirect measure of degree of polymerisation and this value is around 0.63 for polymer meant for apparel fibres. DEG or Di Ethylene Glycol gets formed during polymerisation and varies from 1.2 to 1.8%. Oligomers are polymers of lower molecular weight and vary in quantity from 1.2 to 1.8 %. L and b are measures of colour. L colour signifies whiteness as a value of 100 for L is a perfect value. Most fibres have L colour values around 88 to 92. b colour denotes yellowness/blueness of polymer. the positive sign for b colour indicates yellowness whilst negative sign shows blueness, only polymer which contain optical brightener has b of 3 – 3.5 whilst all semil dull polymers show b values of 1.0 to 2.4. Higher values indicate more yellowness, which indirectly shows chemical degradation of the polymer.

Running a CP at lower / higher throughput:
Every CP is designed for a certain throughput per day. Like say 180 tons/day or 240 tons/day. Sometimes due to commercial constraints like high buildup of fibre stocks etc., the CP may have to be operated at lower capacities. In that case the polymer that is produced has a higher “b” colour and a lower DEG content. Higher “b” colour of say 1.5 against normal value of 1.0 will show fibre to be yellowish and has a little more chemical degradation; which gives higher fluorescence under UV light. Most spinning mills have a practice of checking every cone wound under UV lamp to find out whether there has been any mixup . However if a mill is consistently receiving fibre with a “b”colour of say 1.0 and then if one despatch comes of “b”colour of say 1.5 then in winding, ring bobbins of both “b” colours will be received, and when cones are wound and checked under UV lamp, then higher “b” colour material will give higher fluorescence compared to that of lower “b” colour materials, and will cause rings under UV lamp. Fortunately, a minor difference in “b” colour of 0.4 to 0.5 does not give variation in dyeability.

What can spinning mills do to overcome this problem:
One way is to use a Uster Glow meter which measures the reflectance of fibre samples under UV light. We understand that these values lie between 80 and 120 for samples from different bales. so then divide bales with reflectance values of say 80 to 90, another 91 to 100, third 101 to 110 and fourth 111 to 120. Then while issuing bales to blow room, issue first group say 80 to 90 then issue the enxt group and so on. Bales from different groups should not be mixed. Second is to use bales from each truck separately. Third is to mix up bales from 4/5 trucks to do blending.

Changes in DEG:
The amount of DEG in fibre is directly proportional to dye pick up or dye ability of the fibre. Higher the DEG, higher is the dye ability, so much so that some filament producers add DEG, but then higher DEG will lower tensile properties. So, this practice is not followed for fibre, where tensile properties are critical. So, if the CP is run at lower throughout, DEG drops down, so the dyeablity of the fibre goes down. Since fibre production group is keen on maintaining merge, they resort to lowering of annealer temperatures to maintain dye ability but in the process tensile properties suffer, and mills will notice thread strength falling by 5-7% if annealer temperature is lowered from say 210 degree C to 180 Degree C. If fibre production group does not do this, then they will produce fibre with a different merge – which normally accumulates in the warehouse and so is not appreciated by both marketing and top management. Also when CP is run at higher than rated, then higher temperatures have to be used to compensate lower residence time, here “b” colour actually improves It must be emphasized that the “b”colour changes occur not only due to higher / lower thorugh put but there are several other factors such as air leakages in valves / polymer lines, failure of pumps to remove product from one reaction vessel to another etc. There is yet one more problem in CP. It is a sudden increase in oligomer content. When the number of oligomers increase, it manifests itself in excessive white powder formation on rings and ring rail. Oligomers cause problems in spinning of dyed fibres. The surface oligomer content almost doubles on dying dark and extra dark shades. The only way to control oligomers is to use LEOMIN OR in 1 – 1.5 gms/litre in reduction clearing bath. All oligomers will go into suspension in reduction clearing liquor and get removed when the liquor is drained. Higher annealer temperature also cause higher surface oligomers.

Spinning Process:

Melt Spinning:
In melt spinning process, the fiber forming material is melted and subsequently passed through the holes of a spinneret. Example; Polyester, Nylon, Polypropylene etc.

Requirements for melt Spinning:

  • The polymer should not be volatile
  • The polymer should not decompose in the molten state and the melting point.
  • Polymer should be 30 degree centigrade less than its decomposition temp.

Process of Melt Spinning:

Melt Spinning Process
Figure : Melt Spinning Process.

Special Feature Of Melt Spinning:

  • High production (SPG. Speed 1000-2000 m/min)
  • Hazard, Non-Toxic
  • No environment pollution.
  • No solvent required.
  • Heat of spg high.

Melt Spinning Polymer:

Polymer Melting Point
Nylon 6.6 264 degree Celsius
Nylon 6 220 degree Celsius
Polypropylene 167 degree Celsius
PET 264 degree Celsius

Dry Spinning:
In Dry Spinning, Polymer dissolved in a volatile solvent is introduced into a heated drying chamber, where the solvent is evaporating and solid fiber is obtained. This process may be used for the production of Acetate, Tri-acetate, Acrylic, Modacrylic, PBI, Spandax and Vinyan.

What are Synthetic Fibers:
The fibers which are obtained by chemical synthesis called synthetic fibers. Polyester, Nylon, Acrylic, PBI etc are the Synthetic Fibers.

Dry Spinning Process:

Dry Spinning Process
Figure : Dry Spinning Process

2. PROBLEMS FACED IN MELT SPINNING:
Control of C.V% of Denier:
A good international value of C.V.% of denier is 4 to 5. However, some fibre manufacturers get value as high as 10 to 12. Denier is controlled by having uniform flow of polymer through each spinnerets hole. However, if a hole is dirty or has polymer sticking to it, its effective diameter is reduced; and the filament that comes out becomes finer. IF the spinneretters have been used for more than say 6 to 7 years , then some of the holes would be worn out more than others and filament emerging out would be coarser Currently sophisticated instruments are available to check the cleanliness and actual hole diameters of each and every hole automatically, but few producers have them.

Fused Fibres:
These are caused mainly at melt spinning either due to breaks of individual filaments or breakages of all the filaments (ribbon break) and polymer and block temperatures are too high. Tying of broken position in the running thread line should be as near to the broken position as possible, failure to do this will result in trailing end leading to fused fibres. Other reasons could be impurities, choking of polymer filters and non-uniform quenching or cooling of filaments. The only way to control is to ensure that breaks at melt spinning are held at the minimum.

3. PROBLEMS FACED AT DRAW LINE:
Draw line is the place where the fibre is born. All its major properties denier, tenacity, elongation at break, crimp properties, spin finish, shrinkage and dye ability are all imparted here. For obtaining excellent runnability of the fibre in a blend spinning mill, the two most important properties are – spin finish and crimp. Spin finish: Finish is applied to the undrawn tow at melt spinning stage essentially to provide cohesion and static protection.

On the draw line, a major portion of this finish is washed away, and a textile spin finish is put on the tow by either kiss roll or a spray station. This textile finish consists of two components, one that gives cohesion and lubrication and the other confers static protection, usually these 2 components are used in 70/30 ratio. These spin finishes are complex and each may contain some 18 chemicals to not only control inter fibre friction (should be high at 0.35 to 0.40), fibre metal friction (should be low at 0.15-0.20), anti-bacterial components, anti-foaming compound etc. Finish is made in hot demineralize water and is sprayed on to tow after the crimper by a series of spray nozzles mounted on both sides of the tow.

The finish is pumped to the spray unit by a motor driven metering pump, which is linked to the draw machine such that when the machine stops, the pump motor stops. The percentage of finish on the fibre is based on spin finish manufacturers recommendations and fine-tuned by tech service. Once set, the finish and its percentages are normally not changed. The percentage spin finish is decided by the end use of the fibre. Mills blending polyester with viscose need higher amount of spin finish and also mills running their equipment at high speeds. 60 to 65% of problems faced in mills are due to uneven % of spin finish on the fibre. IF a fibre producer desires to put say 0.120% spin finish on fibre, then ideally the %finish should be maintained @ 0.120 +- 0.005 i.e from 0.115 to 0.125 only; then the fibre will run smoothly. If the finish is on the lower side, card web will show high static, web will lap around doffing rolls, sliver will not pass smoothly through coiler tube – causing coiler choking. Sliver could be bulky and will cause high fly generation during drafting.

On the other hand, if spin finish is on the higher side, fibres will become sticky and lap around the top rollers, slivers will become very compact and could cause undrafted. Thus, it is extremely important to hold finish level absolutely constant. The reasons for non-uniformity is concentration of spin finish varies; sprayer holes are choked; the tow path has altered and so the spray does not reach it. Normally fibre producers check spin finish% on the fibre quite frequently- even then in actual practice considerable variations occur.

Properties of Polyester Yarn:

Weight loss:
Ture and heat of fusion of the samples were measured with a differential scanning calorimeter (Perkin-Elmer DSC-7) at heating rate of 10oC/min and in a temperature range from 30 to 350oC under a nitrogen atmosphere. A 5% alkali aqueous solution was heated to its boiling point, and then a round sample was added to this solution. The solution was stirred, washed with distilled water, and finally dried in a vacuum oven. The weight loss of the sample was calculated by the following equation.

Properties of Polyester Yarn

Shrinkage:
Where W0 and W1 are the weight of the sample before and after alkali treatment, respectively. The fibers were shaped into a hoop form and hung on a 0.7 g clip, and then the length (L1) of the hoop was measured. The hoop was placed in a glass tube in a damp contraction percentage measurement device and dipped at 100 ± 1oC for 30 min. The hoop was taken out of the tube, and after 2h, the length (L2) of the hoop was measured. The shrinkage was calculated according to the following equation.

Wet shrinkage ratio

Birefringence:
A tensile test was conducted using an Instron mechanical tester (Fafegraph-M and Textecho Co., Germany) at a temperature of 25 oC and a relative humidity of 65%. The specimen length was 100 mm and the tensile speed was 200mm/min. All mechanical property values were obtained by averaging ten experimental values. The birefringence was measured using a polarization microscope (Zeiss, Germany). The angle between the fiber and the polarized light ruler was 45o, and the light wavelength was 546mm. The birefringence was calculated using the following equations:

Birefringence equation

Where R is the natural value, λ is the wavelength, θ is the rotation angle of the analyzer, and d is the thickness of the specimen. The density of the specimen was measured using a density grade tube (Shibayama density gradient column, Japan) using carbon tetrachloride (density: 1.59)/hexane (density: 0.68) as a mixture solvent at 23oC.To evaluate the dyeing quality, Lumacron Black SE-3G was used as a dye. A 2% dye (fiber weight was used as a base) was diluted to a liquid ratio of 1:20. The samples were dyed at 100oC for 30 min and then washed with distilled water at 80oC for 10min. The surface color difference (K/S) of the samples was calculate dusing equation (6).

surface color difference
Figure: Tenacity and elongation (a), birefringence (b), wet shrinkage ratio (c), density and crystallinity (d), and thermal properties of polyester FDY as a function of spinning speed.

Crimp:
It is the most important to spin finish for smooth running of fibre. There are 3 aspects of crimp. no of crimps per inch or per cm – usually 12 – 14 crimps per inch crimp stability – be 80% plus and crimp take up – be 27% on tow crimps per inch can be measured by keeping a fibre in relaxed state next to a foot ruler and counting the no of crimps or arcs.

Crimp stability refers to % retention of crimps after subjecting fibre to oscillating straightening and relaxing. We can get an indication on how good crimp stability is in a spinning mill by measuring crimps per inch in fibre from finisher drawing sliver. The crimps per inch of drawing sliver should be at least 10 to 11, if below this, then the crimps stability is poor, so to compensate may be a cohesive compound like Nopcostatt2151 P or Leomin CH be used in the over spary. Fibres like trilobal and super high tenacity fibres are difficult to crimp. Trilobal because of its shape and super high tenacity due to very high annealer temperature (220 degree C) used which makes the fibre difficult to bend. Also, fibre dyeing particularly dark and extra dark shades reduces crimps per inch from 14 to 10 – 11 and in trilobal, as it is crimps per inch in fibre is 11 to 12, after dyeing it goes further down to 8 to 9. In dyed trilobal fibre, crimps per inch in fibre at finisher drawing may be around 6 to 7 so necessitating using almost 50% of cohesive compound in the over spray.

Crimp take up is % difference between relaxed length and straightened length of fibre in fibre stage. Normally this difference is around 18 to 20%. If the difference is much smaller, then it means the crimps are shallow and would have lower cohesion. After the tow is crimped, the crimps are set by passing tow through a hot air chamber. If crimp per inch is low, then that could be due to lower stuffer box pressure, but if crimp stability and/or crimp take up is low, it means the steam supply to crimper steam box is low.

Undrawn fiber:
As the draw line, 1.6 to 3.0 million filaments are drawn or pulled, if a filament had a break at spinning and this is fed as a trailing end to the drawing, then that end cannot be drawn fully, and causes plasticises and fused fibres. Undrawn fibres are generated if the draw point is not uniform i.e not in a straight line.

Plasticized fiber:
When drawline is running and if some filaments break then these broken filaments wrap themselves around a rotating cylinder, since most of these cylinders are steam heated, the wrapped portion solidifies. The operator then cuts out the solid sheet and throws it away as waste but then usually picks up the plastic end and uses it to thread the material and so a small piece of plastic material goes into the cutter and falls into the baler.

Tenacity / Dye ability:
Both these properties are controlled by acutal draw ratio and annealer temperature. Draw ratio does not change in running, but annealer temperature can fall due to problem of condensate water removal. Most drawlines have temperature indicators – but then some buttons have to be pressed to see the temperatures so if the annealer temperature falls, tenacity will fall and dye ability will increase which could lead to a change in merge.

PROBLEMS FACED IN CUTTING / BALING:

Nail Head / Tip Fusion:
In the cutting process, a highly tensioned tow is first laid over sharp blades and the pressed down by a Pressure Roll, resulting in filaments being cut. However, if some blades become blunt, then the pressing of tow on to those blades creates high temperature and so tips of neighbouring fibres stick to each other and so separating this cluster becomes impossible. If it is not getting removed in Lickerin it will go into the yarn and cause a yarn fault. The tip fusion occurs when the blade is fully blunt. If the blade is not very sharp, it does not give a straight edge, there could be some rounding at the cut edge. Such fibres are called nail heads.

Tungsten carbide blades give sharp cut Opening of fibre cluster after opening:
When fibres are cut, they fall down by gravity into the baler. Because of crimping clusters get formed; and so those need to be opened out; otherwise these can cause choking either in blow room pipes or in chute feed. This opening is obtained by having a ring of nozzles below the cutter through which high pressure air jets are pointed up; and these jets open up fibre clusters.

Over length / Multilength:
Over length fibres are those whose length is greater than the cut length plus 10mm and are caused by broken filaments which being broken cannot be straightened by tensioning at the cutter. Multilength are fibre whose length is exactly 2 or 3 times the cut length and are caused by nicks in neighboring blades.

SPECIALTY FIBERS IN POLYESTER:

HIGH/LOW SHRINK FIBERS:
The high shrink fibre shrinks upto 50% at 100 degree C while that of low shrinkage is 1%. The high shrink fibre enable fabrics with high density to be produced and is particularly used in artificial leather and high density felt. Low shrinkage fibre is recommended for air filters used in hot air, furniture, shoes etc.

MICRO DENIER:
Available in 0.5/0.7/0.8 deniers in cut lengths 32/38 mm. Ideal for high class shirts, suiting, ladies dress material because of its exceptional soft feel. It is also available in siliconized finish for pillows. To get the best results, it is suggested that the blend be polyester rich and the reed/pick of the fabric be heavy.

FLAME RETARDANT:
Has to be used by law in furnishings / curtains, etc where a large number of people gather – like in cinema theaters, buses, cars etc in Europe and USA. It is recommended for curtains, seat covers, car mats, automotive interior, aircraft interiors etc.

CATIONIC DYEABLE:
Gives very brilliant shades with acid colours in dyeing / printing. Ideal for ladies’ wear.

EASY DYEABLE:
Can be dyed with disperse Dyes @98 degrees C without the need for HTHP equipment. Ideal for village handicrafts etc.

LOW PILL:
In 2 and 3 deniers, for suiting end use and knitwear fibre with low tenacity of 3 to 3.5 gm/denier, so that pills which forms during use fall away easily.

ANTIBACTERIAL:
It is antibacterial throughout the wear life of the garment inspite repeated washing. Suggested uses are underwear’s, socks, sports, blankets and air conditioning filters

SUPER HIGH TENACITY:
It is above 7 g/denier and it is mainly used for sewing threads. Low dry heat shrinkage is also recommended for this purpose. Standard denier recommended is 1.2 and today 0.8 is also available.

MODIFIED CROSS SECTION:
In this there are TRILOBAL, TRIANGULAR, FLAT, DOG BONE and HOLLOW FIBRES with single and multiple hollows. Trilobal fibre gives good feel. Triangular fibre gives excellent lustre. Flat and dog bone fibres are recommended for furnishings, while hollow fibres are used as filling fibres in pillows, quilts, beddings and padding. For pillows silicoised fibres is required. Some fibre producers offer hollow fibre with built in perfumes.

CONDUCTING FIBER:
This fibre has fine powder of stainless steel in it to make fibre conductive. Recommended as carpets for computer rooms.

LOW MELT FIBER:
It is a bi-component fibre with a modified polyester on the surface which softens at low temperature like 110 degree C while the core is standard polyester polymer. This fibre is used for binding non-woven webs

Uses of polyester in garments:
Polyester is used in the manufacturing of all kinds of clothes and home furnishings like bedspreads, sheets, pillows, furniture, carpets and even curtains. The disco clothing of the 70s with all its jazz and flash was made of polyester.

Polyester product
Figure: Polyester product

Hydrophobic nature:
High tenacity and good durability makes polyester the choice of fabric for high stress outdoors use. Polyester is also a strong fiber that is hydrophobic in nature. It is thus ideal for clothing to be used in wet and damp environments. The fabric is also coated with a water-resistant finish and further intensifies the hydrophobic nature.

Being the most heavily recycled polymer worldwide, it is also used by climbers. Climbing suits, parkas, sleeping bags and other outdoor gear are using the new insulating polyester fiberfill products. One can also do winter windsurfing wearing dry suits lined with polyester fleece.

Creating insulation:
By creating hollow fibers it is also possible to build insulation into the polyester fiber. Air is trapped inside the fiber, which is then warmed by the heat of the body. This keeps the body warm in cold weather. Another method to build insulation is to use crimped polyester in a fiberfill. The crimp helps keep the warm air in. Polyester is an ideal fabric for this kind of insulation because it retains its shape. Cotton and wool tend to flatten over a period of time and loose the warming effect.

Wrinkle resistant:
Polyester is also wrinkle resistant and is used very often in everyday clothing like pants, shirts, tops, skirts and suits. Used either by itself or as a blend, it is also stain resistant and hence very popular.

Advantages of polyester:
Polyester clothing has a lot of advantages: It lasts a long time and wears very well. It is very hard to stain, holds its shape, and does not wrinkle, it can be washed or dry cleaned. It does not yellow. It takes a smaller amount of laundry soap to clean it. Contrary to popular belief, it generally will not sustain a flame, meaning that if, for example, a cigarette is held to it, the fabric will burn and melt where the cigarette is but will not catch on fire itself. Mold, mildew, fungi, etc. do not live on it. While it does not absorb moisture, it does wick it away for evaporation, so in many cases it is cooler than cotton. Many premium clothes are made of a mixture of polyester and cotton to take advantage of these good qualities.

Disadvantages of polyester:

  • If torn it’s hard to stitch it seamlessly.
  • Colour fades after so many washes.
  • Uses more supplies to make.
  • Is not heat resistant.
  • Sticks to other fabrics.
  • If torn it’s hard to stitch it seamlessly.
  • Colour fades after so many washes.
  • Uses more supplies to make.
  • Is not heat resistant.
  • Sticks to other fabrics.

Industrial uses of polyester:
While clothing used to be the most popular use of polyester and which made it a household name worldwide, there are many other uses polyester is put to.

PET:
The most common use of polyester today is to make the plastic bottles that store our much beloved beverages. Shatterproof and cheap these bottles are an absolute boon to the beverages industry.

Mylar:
An unusual and little-known use of polyester is in the manufacturing of balloons. Not the rubber kind that you use for water balloons but the really pretty decorated ones that are gifted on special occasions. These are made of Mylar – a kind of polyester film manufactured by DuPont. The balloons are made of a composite of Mylar and aluminum foil.

Miscellaneous:
Polyester is also used to manufacture high strength ropes, thread, hoses, sails, floppy disk liners, power belting and much more in industries.

Thus, polyester has many uses for homes and industries as well.

What is Disperse Dye:
A dye that is almost totally insoluble in water. Disperse dye exist in the dye bath as a suspension or dispersion of microscopic particles, with only a tiny amount in true solution at any time. They are the only dyes that are effective for “Normal” polyester. Some types are used for Nylon and Acetate. Polyester is dyed with disperse dyes by boiling with carrier chemicals or by heating the liquor to about 130°C which requires elevated pressure (Like a pressure cooker).

Where the fabric is padded with dye liquor then dried and heated to about 200°C for about 90 seconds, is also used for polyester and for coloring the polyester component of polycotton blends. Disperse dyes are also used for sublimation printing of synthetic fibres and are the colorant used in crayons and inks sold for making “Iron-ON” transfers.

The first dyes for cellulose acetate fibres were water soluble. The dye molecules contained a methylamino sulphonate group (-NHCH2SO3Na) introduced by reaction of a primary amino group with formaldehyde and sodium bisulphate (Ionamine dyes, 1922). During dyeing, this group hydrolysed to the less soluble parent amine.

Dye-NH-CH2SO3Na (aq) + H2O → Dye-NH2(s) + CH2O(aq) + NaHSO3(aq)

It was soon recognized that it was this compound that the cellulose acetate absorbed. The first true disperse dyes were simple, relatively insoluble azo and anthraquinone compounds dispersed in water using the sodium salt of sulphated ricinoleic acid.

Dye(s) ↔ Dye (aq) ↔ Dye(fiber)

Many of these dyes are obsolete but their development provided the technology for preparing fine aqueous dispersions by grinding the dye with dispersing agents. A fine dispersion is essential for rapid dyeing and avoids deposition of larger dye particles on the material.

Classification of Disperse Dye for Polyester:
Disperse dyes for a compound shade on polyester can have quite incompatible dyeing properties. The SDC classification of disperse dyes is based on migration ability during exhaust dyeing, colour build-up, sensitivity to changes in temperature and the rate of dyeing.

This type of dye is often classified on the basis of dyeing rate and sublimation fastness, particularly for polyester dyeing. These two properties are a function of molecular weight and the number of polar groups in the dye molecule. The most common classifying is given bellow:

  1. Low energy.
  2. Medium energy.
  3. High energy.

1. Low Energy Disperse Dye:
Most dyeing and fastness properties change gradually with increase in molecular size. Small dye molecules with low polarity are leveling, rapid dyeing dyes with poor heat resistance. These are called low energy disperse dye.

2. Medium Energy Disperse Dye:
Most of the dyeing and fastness properties change gradually with increase in molecular size. Moderate dye molecules with moderate polarity are leveling, rapid dyeing dyes with moderate heat resistance. These are called medium energy disperse dye.

3. High Energy Disperse Dye:
More polar, higher molecular weight dye has low dyeing rates, poor migration during dyeing but good heat and sublimation fastness. These constitute the high energy disperse dye.

Selection Properties:
Disperse dyes have some general properties which are given bellow –

  1. Solubility: Disperse dyes are insoluble in water or slightly soluble in water. It makes fine dispersion with water with water with dispersing agent. Dissolves in organic solvents like benzene, toluene etc.
  2. Fastness to washing: The fabric dyes with disperse dyes shows moderate to good washing fastness.
  3. Light Fastness: Most of the disperse are very fast to washing. The minimum light fastness rating is 4-5.
  4. Sublime ability: Due to stable electronic arrangement disperse dyes have good sublime ability.
  5. Gas Fading: Fabrics dyed with certn blue & violet disperse dyes containing anthraquinone structure become fade in presence of nitrous oxide. This nitrous oxide may be made in nature from various sources such as open gas fire, electric heating arrangement.

Polyester

Commercial (Trade name) Name of Disperse Dyes:

  • Terasil.
  • Foron.
  • Palanil.
  • Resolin.
  • Samaron.
  • Dispersal.

Dispersing Agent:
The actual disperse dye is formed as relatively large particles and, in this form, it is unsuitable for application on hydrophobic fibers. If these big particles are used in dyeing as such, they produce uneven and specky dyeing and their full colour value is not realized. In order to ensure uniform dyeing, the dye should be present in the dye bath in a uniform and very fine form, which should be stable under dyeing condition. This requires a large amount of suitable dispersing agents followed by grinding. The dispersing agent should be effective under the dyeing conditions and should be stable to hard water, high temperature and other dyeing assistants.

Soap powder, Turkey Red Oil, Alkylsulphates, Alkylarylsulphonates, Fatty Alcholethylene Oxide condensates, Naphthalene-β-sulphonate and formaldehyte etc are the recommended dispersing agent performs many functions. It assists the process of particle size reduction of the dye. It also enables the dye to be formed in the powder form. When the powder is added to the dye bath, it facilitates the recon version of the powder in to a dispersion, it is required for carrying out the dyeing. Finally, it maintains the dispersion in a fine form in the dye bath throughout the dyeing process. Dispersing agents increase the solubility of the disperse dye in water. It is seen that solubility of the dye in water is considerably increased by the dispersing agent and that different dispersing agents affect the solubility to different extents. It can be noted that the dyeing rate increase with increasing solubility the dyeing rate actually decreases. Where the solubility is very high as in the case of direct dyes, practically no dyeing takes place.

Commercial (Trade name) Name of Dispersing agent:

  • Setamol -BASF.
  • Edalon -Sandoz.
  • Calsolene Oil HS –A.C.I.
  • Hipogal –Hoechst.

Point consideration for Textile Coloration:

  • Textile Materials (Fabric/Yarns/Fibres/Garments)
  • Dyes/Pigment
  • Chemicals (Common salt, Caustic, Soda ash etc.)
  • Auxiliaries (Leveling agent, wetting agent, sequestering agent etc.)
  • Machinery

Utilities:

  1. Electricity
  2. Water
  3. Steam
  4. Compressed air
  5. Gas

Controlling Parameters:

  1. Temperature
  2. Time
  3. Concentration of dyes and chemical
  4. pH
  5. M:L ratio
  6. Pressure
  7. Man power

Dyeing Mechanism of Disperse Dye:
The dyeing of hydrophobic fibres like polyester fibres with disperse dyes may be considered as a process of dye transfer from liquid solvent (water) to a solid organic solvent (fibre). Disperse dyes are added to water with a surface-active agent to form an aqueous dispersion. The insolubility of disperse dyes enables them to leave the dye liquor as they are more substantive to the organic fibre than to the inorganic dye liquor. The application of heat to the dye liquor increases the energy of dye molecules and accelerates the dyeing of textile fibres.

Heating of dye liquor swells the fibre to some extent and assists the dye to penetrate the fibre polymer system. Thus, the dye molecule takes its place in the amorphous regions of the fibre. Once taking place within the fibre polymer system, the dye molecules are held by hydrogen bonds and Van Der Waals’ force.

The dyeing is considered to take place in the following simultaneous steps:

Diffusion of dye in solid phase into water by breaking up into individual molecules. This diffusion depends on dispersibility and solubility of dyestuff and is aided by the presence of dispersing agents and increasing temperature.

Adsorption of the dissolved dye from the solution onto the fibre surface. This dyestuff adsorption by fibre surface is influenced by the solubility of the dye in the dye bath and that in the fibre.

Diffusion of the adsorbed dye from the fibre surface into the interior of the fibre substance towards the center. In normal condition, the adsorption rate is always higher than the diffusion rate. And this is the governing step of dyeing.

When equilibrium dyeing is reached, the following equilibria are also established:

  • Dye dispersed in the bath
  • Dye dissolved in the bath
  • Dye adsorbed on the fibre
  • Dye diffused in the fibre

Effect of Various Conditions on Disperse Dyeing:
Effect of Temperature:
In case of dyeing with disperse dye, temperature plays an important role. For the swelling of fibre, temperature above 100°C is required if high temperature dyeing method is applied. Again, in case of carrier dyeing method, this swelling occurs at 85-90°C. If it is kept for more time, then dye sublimation and loss of fabric strength may occur.

Effect of pH:
For disperse dyeing the dye bath should be acidic and pH should be in between 4.5-5.5. For maintaining this pH, generally acetic acid is used at this pH dye exhaustion is satisfactory. During colour development, correct pH should be maintained otherwise fastness will be inferior and colour will be unstable.

What is Heat Setting?
Heat setting of synthetic fabrics eliminates the internal tensions within the fiber generated during manufacture and the new state can be fixed by rapid cooling. This heat setting fixes the fabrics in the relaxed state and thus avoids subsequent shrinkage or creasing of fabric. Presetting of goods make it possible to use higher temperature for setting without considering the sublimation properties of dyes and also has a favorable effect on dyeing behavior and running properties of goods. On the other hand, post setting can be combined with some other operations such as thermosol dyeing or optical brightening of polyester, post setting as a final finish is useful to get a high dimensional stability along with desired handle.

The application of heat in heat setting can be done by hot air, on a pin stenter at 220c for 20-30 seconds for polyester goods and at a lower temperature range of 190-225C for 15 -20 seconds for polyamides. Acrylics may be heat set partially at 170-190 c for 15-60 seconds to reduce formation of running creases. but higher temperature should be avoided to prevent yellowing.

Hydro setting is so rarely used particularly to get fuller and softer handle on polyamides at 125-135c in autoclaves for 20-30 minutes. It can be combined with dyeing or optical brightening.

Steam setting can be done by saturated or super-heated steam. During steaming, uniform treatment can be ensured by initial sequence of alternate short steaming and vacuum application for 20-30 min at 130C under pressure. Super-heated steam can be used in stenters and setting time is 25% shorter than for hot air on account of quicker heating up rate. Acrylic fibers have to be protested as some may undergo excessive shrinkage or loss of handle. Before the material is heat set, it should be thoroughly washed to remove spin preparations, lubricants, sizing agents and impurities as these are likely to be burned in drying heat setting making their removal difficult.

Method of the Dyeing Synthetic fibres with Disperse Dyes:

There are three common method of dyeing with disperse dyes which are as follows:-

  1. Carrier method of dyeing.
  2. High temperature dyeing.
  3. The thermosol process of dyeing.

Carrier Dyeing Method:
Commercial (Trade name) Name of Carrier:

  • Tumescal –A.C.I.
  • Matexil –A.C.I.
  • Levagol –Bayer.
  • Dilatin –Sandoz.
  • Invalon –Ciba.
  • Hisogal –Hoechst.

Recipe:
Dye:

-For light shade<0.5%
-For medium shade 0.5-1.5%
-For deep shade >1.5%

  • Carrier (Phenol): 3 gm/lit
  • Acetic Acid: 1 gm/lit
  • Dispersing Agent: 2gm/lit
  • Salt: 1-2 gm/lit
  • PH: 4-4.5
  • M: L: 1:10
  • Time: 60 min
  • Temperature: 90°C

Dyeing sequence of polyester is given below:

Carrier and vessel washed by hydrose and caustic at 1000C for 20 min

Load the package in the carrier and feed in the vessel

Add washing agent and run at 800C for 20 min

Dyeing period

Add leveling agent and acid, run at 600C for 10 min

pH check

Color mixing at 700C for 40 min

Color dosing at 600C for 20 min

Polyester dyeing at 1350C at 20 gradient for 50 min

Sample check

Ok

Temperature cool down at 780C and drain

Hot wash for 10 min

Rinse for 15 min for light shade and 25 min for dark shade

Add hydrose, soda ash, run at 800C for 20 min

Drain

Rinse for 10 min

Neutralization by acetic acid at 500C 20 min


Finishing

Procedure:

  1. At first, a paste of dye and dispersing agent is prepared and then water is added to it.
  2. Dye bath is kept at 60°C temperature and all the chemicals along with the material are added to it. Then the bath is kept for 15 min without raising the temperature.
  3. pH of bath is controlled by acetic acid at 4-5.5.
  4. Now temperature of dye bath is raised to 90°C and at that temperature the bath is kept for 60 min.
  5. Then temperature is lowered to 60°C and resist and reduction cleaning is done if required. Reduction cleaning is done only to improve the wash fastness.
  6. Material is again rinsed well after reduction cleaning and then dried.

Dyeing Curve:

Dyeing Curve
Figure: Dyeing Curve

High Temperature Dyeing Method:
Pretreatment:
Pretreatment of polyester fabric is a must before starting the dyeing operation. The pretreatment is essential to remove the lubrication oils and other auxiliaries used during spinning and weaving or knitting operation. This following simple treatment is enough to remove those impurities.

Recipe:

  • Lissopal D paste: 2 gm/lit
  • Soda Ash: 2 gm/lit
  • Treat with the above recipe at 90~95°C for 20 minutes.
  • Drain-Hot wash @ 70°C for 10 minutes> cold wash > Neutralize with 1cc/lit Acetic Acid.
  • Cheak the PH- 5.5-6.0

Dyeing Process:
Polyester Texties require a Heat Setting operation before dyeing. Heat settings eliminates the internal tensions within the fibre generated during manufacture and the new state can be fixed by rapid cooling. This heat settings fixed the fabrics in the relaxed state and thus avoids subsequent shrinkage or creasing of fabric.

Dye bath settings & Dyeing:

Recipe:

  • Lyogen DFT: 0.5 gm/lit
  • Sandozen PES: 1.0 gm/lit
  • Acetic Acid: 1 gm/lit
  • PH: 5.5-6.0
  • Temperature: 130°C
  • Time: 1 hr

Procedure:

  1. At first a paste of dye and dispersing agent is prepared and water is added to it.
  2. PH is controlled by adding acetic acid.
  3. This condition is kept for 15 minutes at temperature 60°C.
  4. Then the dye bath temperature is raised to 130°C and this temperature is maintained for 1 hour. Within this time, dye is diffused in dye bath, adsorbed by the fibre and thus required shade is obtained.
  5. The dye bath is cooled as early as possible after dyeing at 60°C.
  6. The fabric is hot rinsed and reduction cleaning is done if required.
  7. Then the fabric is finally rinsed and dried.

Dyeing Curve:

Dyeing Curve
Figure: Dyeing Curve

Dyeing of Polyester Fabric in Thermasol Dyeing Method:
Thermasol dyeing method is continuous methods of dyeing with disperse dye. Here dyeing is performed at high temperature like 180-220°C in a close vessel. Here time of dyeing should be maintained very carefully to get required shade and to retain required fabric strength.

Recipe:

  • Dye: X gm/lit
  • Dispursing Agent: 2 gm/lit
  • Sodium Alginate Thickener: 5-10 gm/lit
  • Citric Acid to get PH: 4-5

Sequence:
Pading- Drying- Thermofixing- Aftertreatment

Procedure:

  1. At first the fabric is padded with dye solution using above recipe in a three-bowl padding mangle.
  2. Then the fabric is dried at 100°C temperature in dryer. For dyeing, infra-red drying method is an ideal method by which water is evaporated from fabric in vapor form. This eliminates the migration of dye particles.
  3. Then the fabric is passed through thermasol unit where thermo fixing is done at about 205°C temp for 60-90 seconds depending on type of fibre, dye and depth of shade. In thermasol process about 75-90% dye is fixed on fabric.
  4. After thermo fixing the unfixed dyes are washed off along with thickener and other chemicals by warm water.
  5. Then soap wash or reduction cleaning is done if required. And finally the fabric is washed.

Textile Finishing
Textile finishing process is a separate subject a processor should be well versed. This is the end process that adds up value, quality and appearance to the final product.

Each substrate according the end use would finish differently.

Finishing operations can be widely divided into 2 classes;

1) Mechanical means of finishing or mechanical finishes or physical transformation of subtrate due to mechanical processes,

2) Chemical finishes.

Functional Finishes:
The properties of synthetic fibers, most important among them being polyamide, polyester and polyacrilonitrile, are essentially different from those of natural cellulosic and wool fibers. Hence the sequence of finishing operations is likely to be different. While cellulosic’s require a resin finishing treatment to impart easy-care properties, synthetic fibers already have these easy-care criteria and require only a heat setting operation. The use of 100% synthetic textiles has increased considerably since the arrival texurised yarns consisting of filaments and the growing production of knit goods. The use of open weave has enabled production of lighter, air permeable, fabrics to ensure better wearing comfort.

Filling and Stiffening finishes:
A stiffening effect is desirable in certain polyamides and polyester materials for petticoats, collar interlinings, etc., which can be done by reducing the mutual independence of structural element of fabric by polymer deposition on coating as a fine film. Some special Urea-formaldehyde pre-condensates have been found to be useful. Application of film-forming acrylates dispersions as well as latex rubber emulsions gives a fuller effect with sufficient stiffness.

When softening is desired it can be achieved by reducing the frictional coefficient between structural elements of fabrics, cationic long chain fatty derivatives and silicones may be used in conjunction with polymer forming agents. Recently some cationic softeners having reactive functional groups have been developed to get better fastness of finish.

Hydrophilic finishes:
On account of lower moisture and water absorption capacity synthetic fiber materials become uncomfortable in contact with skin. Certain products based on modified (oxy-ethylated) polyamides makes the wearing more pleasant by reducing the cohesion of water so that it spreads over a larger area and thus evaporates more rapidly.

Anti-pilling finishes:
Pilling is an unpleasant phenomenon associated with spun yarn fabrics especially when they contain synthetics. Synthetic fibers are more readily brought to the surface of fabric due to their smooth surface and circular cross section and due to their higher tensile strength and abrasion resistance, the pills formed take a long time to be abraded by wear. With knit fabric, two more problems occur, viz., “picking” where the abrasion individual fibers work themselves out of yarn loops onto the surface when garment catches a pointed or rough object. These two effects are more predominant in the weave, is more open and yarn is bulkier.

The finish has to cement the fibers within the yearn so that their dragging becomes more difficult, without affecting the handle adversely. Special polymer formers of acrylate type or latex type are useful but should form a film of good cohesion, should hydrophilic and should not form a tacky surface. padding in polymer dispersion or emulsion followed by drying at moderate temperature gives the desired effect.

Permanent Anti-static effects:
Anti-static effective chemicals are largely chemically inert and require Thermosol or heat treatment for fixing on polyester goods. Agents of polyether type are found to be useful but should not effect the dye-equilibrium on fiber otherwise the rubbing fastness is impaired. In general, Thermsolable anti-static agents also have a good soil release action which is as permanent as the anti-static effect. Anti-static finishes may also be of polyamide type being curable at moderate temperatures.

Non-Slip Finishes:
Synthetic warp and weft threads in loosely woven fabrics are particularly prone to slip because of their surface smoothness when the structure of fabric is disturbed and appearance is no loner attractive. To avoid these attempts are made to give the filaments a rougher surface. Silica-gel dispersions or silicic acid colloidal solutions are quite useful and they are used with advantage in combination with latex polymer or acrylates dispersions to get more permanent effect along with simultaneous improvement in resistance to pilling or snagging. These polymer finishes are also capable of imparting a soft and smooth handle to synthetic fabric without imparting water repellency.

Fire Resistant Finishes:
With synthetic fiber which melt on igniting by a flame, the molten moss is itself quite dangerous and a fire-resistant treatment is desirable for certain end uses. Polyester fabrics can be made flame resistant by treatment with an aqueous emulsion of xylene soluble 2,3-dibromopropyl phosphate in a pad-cure sequence. A semi-permanent effect can be produced by treating with a mixture of ammonium bromide and brominated phosphoric acid esters.

Fire retardant fabric
Figure: Fire retardant fabric

Polyamides can be made flame resistant by applying phosphorous tri-chloride ammonia reaction products or ammonium bromide with amino-triazine condensation products. For acrylics tris-dibromopropyl-phosphate as well as 2-cyanoethyl-tetramethyl-di-amino-phosphate is quite effective.

Anti-microbial Finishes:
With the increasing use synthetic fibers for carpets and other materials in public places, anti-microbial finishes have assumed importance. A reduction in soiling tendency will along way in keeping textiles free from germs and usual soil repellant as well as soil release finishes are effective in some way. products which are commonly applied are brominated phenols, quaternary ammonium compounds, organo-silver and tin compounds which can be applied as solutions or dispersions. They can also be incorporated in a polymeric film deposited on the surface to get controlled release. Some reactive systems similar to those for reactive dyes have been recently tried to incorporate anti-microbial structural features.

Finishing of Elastomeric Textiles:
The heat sensitivity of electrometric fibers limits the choice of products and finishing process that can be employed. In order to eliminate the latent tensions, these electrometric textiles are simply steamed or treated with hot water. Dry curing or heat treatment is restricted to temperature below 140C, these fabrics have e to be dried and curd with minimum tension with over feed stenter. To groups of materials, viz., foundation fabric and knitted fabric for bathing suits are resin finished. Water proofing can be imparted by using Zirconium salts containing wax emulsions as it does not require a high temperature treatment. A filling treatment can be obtained with modified methylol-urea type products.

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