P, D.T.P, B.Tech
Texport Syndicate (I) Ltd,
Yeshwantpur, Bengaluru - 22


This chapter deals with chemical modification polyester fiber, polyester, appearing in order of their commercial importance to the textile industry. Various approaches used in modification of polyester fibers, modifying agents used, and application of the modified products based on the properties obtained have been highlighted. The information on the various modifications is classified primarily based on the functional properties acquired by the fibers after modification.

Particulars emphasis has been given to the modified polyesters, where in a special section has been included on modifications for better polymerization and faster spinning of the polyester materials. The modification for improved dyeability, differential dyeing, antibacterial properties, reduced flammability, high water absorbency and modified mechanical properties have been discussed for the pet fibers. Several modifications influence combination of properties and hence although they have been discussed under a particular functional property, should be viewed in totality.


Antibacterial, Deep dyeable, Hollow polyester, Hydrophilic, Hydrophobic, Pilling, Soiling


Polyester are polymers made by a condensation reactions taking place between small molecules, in which the linkage of the molecules occurs through the formation of ester groups. Polyesters are commonly made by interaction of a dibasic acid with a dihydric alcohol. This fiber is a medium weight fiber with a density of 1.39g/cm3. Compared with nylon, polyester are rather heavy fibers, for this reason polyester textile materials are manufactured as light weight or thin fabrics. The most common polyester apparel filament or stable fiber is usually composed of polyethylene terepthalate (PET) polymers.


The modified polyester are prepared to overcome some of drawbacks such as low moisture regain, static electricity and soiling problems, this three drawbacks are interrelated and associated with hydrophobicity of the polyester. By making hydrophilic these drawbacks can be overcome.

Thus, a hydrophilic fiber will have a higher moisture regain. The garments made up of hydrophilic fiber will absorb perspiration and will be comfortable.

Another drawback is pilling problem and very difficulty in dyeing. The low pilling fibers are required to retain the elegant appearance of polyester garments for a long time. These low pilling fibers have lower tenacity than normal polyester fibers. Thus, although pills are formed in these fabrics, these pills are removed by simple brushing or washing.


Polyester fibers were latecomers among manufactured fibers, and had to find their way into a market where polyamide and acrylic fibers were already established. Polyester fibers used for textile application, offers langible benefits to both processors and consumers. Low denier fibers blended with cotton gave higher strengths at lower twist levels, than found in 100% cotton yarns. The characteristic property of the polyester was immediately encased by the designers of shirt and blouse fabrics. Polyester fibers provided textiles with a dimensional stability, wear resistance and easy care properties with the handle, drape and appearance being preserved for longer periods than in fabrics made form natural fibers.

High rate of growth of polyester fibers is due to their outstanding physical properties, chemical resistance, easy properties, and resistance to moth, mildew and microorganism. In spite of its outstanding performance, there are some shortcomings in PET, for example.

» Hydrophobic nature
» Ease of soiling
» Static charge build up
» Tendency to pill
» Lack of dye receptor sites in the polymer chain.

Extensive research has therefore, been carried out on PET to overcome the above mentioned drawbacks. Such changes (physical and chemical) have led the manufacture of modified polyester fibers. Modification of normal polyester has been accomplished by following routes:

» Change in the chemical composition of the PET molecule by introducing a third and /or fourth component into the polymer chain during polymerization.
» Use of certain additives (particulate fillers, pigments of polymers) in the melt phase prior to extrusion.
» Modification during melt spinning such as hollow varied profile and micro-denier fibers for specific applications.
» Surface modification of normal polyester fiber for producing specific effects.


During dyeing, the dyestuffs diffuse into the fiber and are absorbed primarily by the amorphous regions. The thermal coefficient of the molecular mobility, responsible for the dye diffusion, depends largely o the Tg which is increases with increase in crystallinity and the degree of orientation of the fiber. It has been demonstrated that drawing and heat setting cause a significant reduction in the rate of dye absorption, which, however, can be improved by introducing certain hydrophilic co monomers in the PET molecule.


Modification of the polymer to reduce the glass transition temperature (Tg) is helpful in increasing the dyeing rate. The most effective co monomers are aliphatic in character. Replacing a small proportion, usually 5-10 mo1%, of terephthaloyl units with an aliphatic dicarboxylic acid such as glutaric or adipic acid produces fibers that will dye at the boil without carriers; aromatic units, derived for instance, from isophthalic acid, act primarily through reducing crystallinity, are less effective. Since to a first approximation, the depression of melting temperature on copolymerization is proportional to the molar percentage of the modifier, a flexible comonomeric unit of high molecular weight is particularly useful..


Block copolymers made from PET and polyalkylene glycols, i.e. polyethylene or polypropylene glycols having Mn 1000-3000 molecular showed good dyeability with disperse dyes. Deep shades can be obtained in a boiling bath without carriers.

Block copolyesters containing PET and polyethylene oxide [PEO] segments syntherized in the presence of lead oxide and Mn, Sb, Sn or Mg based catalysts have been reported.Poly(ester-b-ether) by incorporating ether blocks (PEG-1000)in the PET back bone.

Polyester copolymer fibers made form a mixture of ethylene glycol, diethylene glycol and dimethyl terephthalate showed improved dyeability and are found useful as binder fibers in fiberfill battings for sleeping bags and sky jackets.

However, the fibers made from these copolymers have the draw back of being very sensitive to thermal, hydrolytic and photochemical degradation reaction.

Features of deep dyeing pet are:

» Better dyeability (for disperse dyestuffs)
» Shorter dyeing time
» Spinning throughput increased by as much as 5%
» Higher water take-up (0.8% against 0.4% in unmodified PET)
» Agreeable hand and soft feel of fabrics.


Carrier free dyeable polyesters are defined as those polyesters, whose dyeability at boil without the use of carriers is similar to that of polyester fibers dyed under HTHP conditions, or at boil in the presence of carriers. There are two approaches for producing CFDP.

Physical Modification of Fibres:

The dyeing properties of polyester are strongly influenced by many of the processing conditions to which the may be subjected during manufacturing or during subsequent textile processing. Efforts have been made to improve the dyeability of polyester, to produce CFDP by making certain change in melt spinning, drawing and heat setting operations. Air texturing and filament mixing have also been used to produce a whole variety of products. But the most importance technique at hand is the draw texturing of partially oriented yarn (POY).

Chemical Modification of Polymer:

Chemically modified CFDP is produced by adding certain additives – polyethylene glycol (PEG), adipic acid azillic acid-which form block copolymers with polyester. Several properties are claimed for the fiber, including good dyeability at 100oC, physical properties and tensile strength are comparable with the normal polyester. The glass transition temperature of all these fibres is about 10oC, lower than of normal polyester, leading to higher segmental mobility. This in turn increases the rate of dye diffusion into fibres at a lower temperature and can be dyed deep shades at boil even in the absence of carriers. These fibres offer the following advantages over normal polyester.

» Better exhaustion under atmospheric conditions,
» A higher color yield,
» Shorter dyeing cycle
» Reduction in dyeing costs,
» Elimination of the carrier cost,
» Energy saving,
» Environment protection, i.e., ecological advantages,
» Possibilities of the dyeing of PES/wool of PES/acrylic blends, and
» Reduction of the oligomer problem during dyeing.


The simplest and most common method of manufacturing modified polyester is by incorporating a modifying agent, during Tran’s esterification, poly condensation or during melt blending. In considering the nature of the block to be introduced into the molecule, the following criterion could be adopted:

» The block should contain chemical groups of a hydrophilic nature to assist in the swelling of the fiber in aqueous solution.
» The fiber intermediate forming the block must have some reactive end groups like carboxyl or hydroxyl, capable of undergoing poly condensation.
» It must be thermally stable at 275-280oC in order to withstand polymer melt spinning conditions.
» It must be chemically stable under these conditions.

The above conditions limit the choice of modifying component, but of the few available, polyesters are the most interesting. Thus, the most popular modifiers today are a range of polyethylene glycols of the general formula

H (OCH2CH2) n OH. Polyethylene glycols fulfill all the four conditions stated above and also exhibit very little scatter in the molecular weight.

Problems of CFDP:

Carrier free dyeable polyester is associated with many problems. Some of them are listed below:

» Levelness of dyeing: Due to the extremely high rate of exhaustion of dyes, there is a problem of localized absorption of dyes in the boundary zones between fiber surface and the dye liquor, which leads to uneven dyeing. This can be rectified by maintaining a uniform concentration gradient between fiber and the dye bath, at all points of the fiber which can be achieved by rapid dye liquor circulation, or high fabric speed.

» Light fastness of dyed fabrics: It is found the dyes on carrier free dyeable polyester are more photosensitive that on the normal fibres. Kuster and Herlinger have studied this problem and suggested the use of stabilizers, which make their exhaustion from the dye bath possible. These compounds quench the primary radicals.

» Wash fastness of dyeing: Wash fastness of the dyeing is also slightly low for these fibres because of the fiber structure, the dye molecules are not effectively trapped within the fiber structure. In other words, the factors that enhance diffusion into the fiber will also enhance diffusion out of it, when concentration gradients are reversed. Thus, appropriate instructions should be given to consumers to wash CFDP products at temperatures below 50oC.


In normal dyeable polyester, there are no sites for ionic dyes. So, it can only b dyed by disperse dyes. Compared to ionic dyes, disperse dyes have smaller molecular extinction coefficients and lower build up property. So these dyes cannot give bright and deep colors. Moreover, fastness to sublimation and wet treatments of disperse dyes are relatively poor compared to other classes of dyes. In order to avoid these problems, cationic dyeable polyester was developed.


Co polymerization of an isophthalic acid component containing a sulfonic acid group makes it possible to use cationic dyestuffs for polyester staple fibres and filaments. Generally, the sodium salt of 5-sulfo-isophthalic acid (Na-SIPA) is used as CD co monomer. A cationic or basic dyestuff contains amines or ammonium groups or quaternary nitrogen-heterocyclic. Dyeing CD-PET is an ion exchange process. The sodium cations (Na+) from CD-PET are substituted by the bigger dye cations, whereas the sodium ions enter into the dye bath. Thus, PET is chemically modified in a manner that cationic dyestuffs can form a chemical complex with the fiber that is as shown in fig,

The chemistry of producing CD-PET is complicated. The reason for difficulty is the acidic character of Na-SIPA, especially in connection with hydrolytic or glycolytic conversion. Therefore, after direct addition of this salt into the PET esterification stage, the diethylene glycol (DEG) would reach a high level because ether formation is acid-catalyzed. Additionally, the acidic character enhances the TiO2 agglomeration. The result is difficulty in the spinning process, and an excessively low melting point of CD-PET.


Pilling is a serious problem which is associated with all the synthetic fibers. In order to reduce the pilling, polyester fibers having lower than usual strength has been prepared. Although such fibers form pills due to friction, these pills can be removed by simple brushing since the fibers has lower strength.

The polyester fiber having pilling tendency can be obtained by incorporation in the polymerizing mass, certain substances such as terepthalate of barium, calcium or zinc or organic compound of antimony, chromium or iron.

Normally the pilling resistance has been achieved by reducing the abrasion resistance so that the fiber breaks off before the formation of large pills.


Various processes for making polyester fibres hydrophilic include special spinning, non-circular cross-section, multilayered structure, dyeing finishing and plasma treatment. Some of the important modification approaches are discussed below in this section.

A large number of additives are suggested for making polyester fiber hydrophilic. ICI have suggested the addition of 5-10% by weight of sodium sulphate as slurry in glycol during polymerization. The particle size of sodium sulphate should be less than 3microns.

Polyester filament having a moisture absorption capacity of atleast1% at 65% R.H. and 21oc and a water retention capacity of at least 15% is developed by adding a water soluble aliphatic polyamide to the polyester, spinning the mixture and washing out the added amide with water.

The soil resistance property of polyester fibers can be enhanced by the addition of polyethylene glycol or tetraethyl ammonium perfluoro octane sulfonate to the melt before melt spinning.


During the last few years considerable amount research work was done on producing hollow polyester fiber having micro crates (holes) on the surface. The hollow polyester fiber is produced by using specially designed spinnerets. Normally four types of spinnerets are used for producing hollow fibers and the spinnerets are shown in fig.

Plug-in-orifice spinnerets Fig (A):

This spinnerets have a solid pin supported in the centre of a circular orifice. The polymer is extruded through the annulus. With this spinneret design, it is generally necessary to incorporate a gas-forming additive in the polymer melt. The gas fills the core of the fiber as it emerges from the annulus and prevents collapse until the fiber solidifies.

Tube-in-orifice spinnerets Fig (B):

This spinnerets have a hollow needle or tube supported in the centre of the orifice. An inert gas or liquid is injected through the needle to maintain a tubular shape until the fiber solidifies or coagulates.

Segment arc spinnerets Fig (C :

This spinnerets have C shaped orifices. The polymer solution or melt welds into a tube after extrusion through the C shaped die. The gas required to keep the fiber hollow is drawn in through the gap in the extruded fiber upstream from the weld point.

Teijin is marketing such hollow fiber under the trade name Welkey. The mechanism of water absorption and water transport by welkey is schematically shown in the figure. The water absorbing mechanism has three steps. In first step, water attached to the side of fiber enters into the hollow section of the capillary through the penetrating holes.

In the next step, water from the penetrating holes goes to both sides of the hollow section by its capillary action. In the final step total amount of water is absorbed into the hollow section where capillary migration is stopped by balance of tension from both sides.

Special spinning:

Drawn polyester filaments are hydro fixed in water in the presence of specified surface-active agents. Hydro fixing place more quickly and a more stable pore structure is obtained. This is reflected in increased moisture uptake and a higher water retention capacity. The fibres retain their hydrophilic properties for a considerable period of time, even with repeated wearing and washing.

A salt forming compound is added to the polyester spinning composition for the manufacture of flame resistant and hydrophilic polyester fibres.

Plasma Treatment:

The application of the plasma treatment has been demo started for the surface modification of various textiles. A lot of environmental and production problems can be solved by using a non-equilibrium low temperature plasma. The plasma process are dry ones and do not require water or non-aqueous solution. Promising applications of gas discharges plasma for the activation of chemical reactions in liquids have also been reported.

Wettability of polyester has been increased by using oxygen or nitrogen plasma. Plasma- produced polar groups increase the surface free energy of the fiber and decreases the contact angle.

The contact angle for water was found to decrease for PET after plasma treatment in oxygen and nitrogen, while the contact angle for cellophane increased. Such low temperature, low pressure plasma treatment is effective in inducing the high consumption of chemical wetting agents normally required chemical processing of textiles.


Comfort and protection are two very important aspects of textiles today. The increase in the health concern of the consumer has prompted a need for fabrics that can inhibit the growth of bacteria and other micro-organisms which can cause offensive odors, skin irritation, visual spoilage and disfiguring stains making garments unusable with regards to hygiene and aesthetics. Certain allergens can cause allergic reactions and asthma in humans. These microorganisms may develop from the spills of body fluids or medical liquids.

Antibacterial protection (additives) inhibits the growth of such bacteria and allergens. At the same time, providing an antibacterial protection which must not alter fiber spinnability, main properties of the fiber as dye uptake, wear and abrasion resistance and other mechanical properties.

Features of the antibacterial fibres:

» Prevent development of micro-organisms which are responsible for bacterial contamination and unpleasant odors.
» Should maintain a high level of effectiveness throughout the life of the products.
» No reduction of antibacterial activity when subjected to dyeing and finishing process.
» Greater amount of active material exposed on the surface.
» Compatibility in blends.
» Withstand robust handling and abrasion without impaired performance.

Antibacterial effectiveness is guaranteed with improved hygiene, comfort and coolness augmented by properties of heat regulation and moisture transference which leave the wearer’s skin dry and healthy.

Production of Antibacterial Fibers:

It is a common practice to give antibacterial properties to synthetic fibres, by adding organic additives combined with fibres in several ways. However, employing organic agents to provide antibacterial activity is to some extent unsatisfactory. This is because of their toxicity, lack of durability and poor resistance to heat. Organic compounds also pose problems in fiber production and present problems when worm next to skin. So inorganic supports such as special zeolites or ceramic substrates containing Ag or Zn ions have been proposed.


Fire accident generally results in considerable loss of life and property. The majority of fire accidents occur due to burning of textile fibers. Polyester fiber is flammable and can cause considerable injuries due to melting. The blends of polyester with cotton are highly flammable.

The flame retardant effect is achieved by the addition of special chemicals. Earlier, this was done by impregnating the finished fabric or by physically mixing an agent to the polymer-for instance during melt spinning.

Previously, components containing halogen, and above all bromine, were used. The effect of these substances was based on the halogen radicals interrupting the combustion chain reaction. However, as halogen enables the formation of highly toxic dioxins, the compounds used today contain phosphorus. Bromine compounds are efficient, flame retardant additives but their fastness to light is not always satisfactory. Chlorinated arylalkyl hydrocarbons and bis (2,4,6-ttrichloro phenyl) phthalate have been suggested.

A number of FR polyester fibres commercially available include: Dacron 900F, Heim (Toyobo Co.) Tetoran Exter (Teijin), Trevia CS and Trevira FR, Toyobo GH etc. A number of flame retardant additives used during the transesterification reaction in the PET or sometimes mixed with PET chips prior to extraction. The important ones are; triphenlyphosphineoxide,3,5–dibromo-terephthalate, decabromodiphenyl ether, tribromodiphenyl, phosphinic acid derivative etc.


For centuries silk fabrics are considered to be most elegant and gorgeous textile materials. However the production of silk fiber could not keep pace with increase in human population, and hence the price of silk is now beyond the reach of most of the people. When synthetic fibers were first developed it was thought that these fibers will be able to substitute silk. However, soon it was realized that these fibers have metallic luster, papery feel and poor aesthetic value. Substantial amount of research work was carried out to make silk like synthetic fibers which has resulted in the development of silk like polyester fabrics.

The following factors should be considered in the production of silk like polyester:

» Fiber cross section to obtain the desired luster
» Fine denier filament to obtain the desired feel


Modification of cross-section of the fiber allows engineering of surface properties in yarn and fabric. Many cross-section shapes are available; circular, trilobal, pentalobal, octalobal, hollow, hexagonal, and other irregular shapes. For silk like polyester fiber circular, trilobal, tetralobal, C shape, V shape, and hollow cross-sections have been used. The most popular cross-section for silk like polyester is trilobal, which gives adequate luster resembling that of silk. The type of cross-section can be coupled with amount of TiO2 in the fiber may result in “milky” colors when the fabrics are dyed.


It plays a primary role in determining the stiffness of the yarn. It is easy to visualize its effect by an analogy, where a thin glass capillary is stiff and brittle but when it is made in the form of the filament it is pliable. Silk fibers are “very fine” in the range 1.2to1.3 dtex and hence necessarily the synthetic fiber used to be in the same range or finer to obtain a feel closer to silk. Finer the single filaments in the yarn, the softer the hand of the resultant fabric. The larger the number of fine filaments in yarn of identical over all titer, and bulkier and denser the fabric hand.


Polyester:- It is a well known fiber in the synthetic fibers because it has certain desirable properties, the properties are high strength, wash& wear property, good dimensional stability, elegant appearance and suitability for blending with cellulosic and protein fibers. But polyester have some certain drawbacks such as low moisture regain, create static electricity, soiling problem, difficult to dyeing and pilling.

Now many more developments in the polyester processing i.e. hydrophilic polyester, easy dyeable, deep dyeable and cationic dyeable polyester, low pilling polyester, antimicrobial polyester, silk like polyester, hollow Polyester and flame retardant polyester.

The advantage of above properties are good comfortable while in wearing, easy to dyeing, so it provides cost , energy ,pollution reduction and very good appearance of the polyester garments.


1. E.P.G. Gohl & L.D. Vilensky, Textile science 2nd edition CBS publishers 1999.
2. S.Jayaprakasm & R.Gopalakrishnan, Fiber science and Technology, S.S.M.I.T.T publication.
3. V.A.Shenai, Technology of Textile Processing; Textile Fibers, Sevak publication,1996.
4. S.P Mishra, Text Book of Fiber Science & Technology, New age international publishing co,
5. R.M.Mittal & S.S.Trivedi, Chemical Processing of Polyester/ Cellulosic blendes, ATIRA publication, 1983.
6. A.A.Vaidhya, Production of Synthetic Fibers, Prentice hall of India publications.
7. W.Klein, Man Made Fiber and their Processing, The textile institute publication
8. V.K. Kothari, Progress in Textiles: Science & Technology; Vol 2
9. Premamoy Ghosh, Fiber Science & Technology, Tata Mc-Graw Hill Publishing Company.
10. Bernard P. Corbman, Textile Fiber to Fabric, Mc Graw-Hill Publication
11. Vaidhya A.A, John Wiley & sons, Chemical processing of man made fibers, New York, 1984.
12. Trotman E.R & Charis, Dyeing Chemical technology of textile fiber, Graffin & Company, UK.
13. Holme I, Developments in chemical finishing of textiles and apparel, Textile outlook international, March 2001.
14. Holme I, Recent advances in chemical processing, International conference on recent advance in wet processing in textiles, BTRA Publications.
15. Yair Avny, Ludwig Rebenfeld, Chemical modification of polyester fiber, Journal of applied polymer science, Vol-32 Issue 3 Pp 4009-4025
16. Martin Bide et al, Modified fibers with medical and specialty application, http://www.sprinklink.com/content/l042vo14uh874514.
17. Easy cationic dyeable polyester, http://www.cyarn.com/products/fiber/fiber-018.html
18. V.K. Kothari et al, Journal of Applied Science, Vol 61, Issue 3, Pp 401-406 http://www3.interscience.wiley.com/cgi-bin
19. Properties of modified polyester fiber, Textile Research Journal, http://www.trj.sagepub.com/cgi/content.
20. http://www.sterlitech.com/products
21. Polyester fiber &method of production, US patent no 4526738, http://www.freepatentsonline.com/4526738.html


I’m Arvin Prince. I have started my carrier with my Diploma in Textile Processing (Sandwich) in SSM Institute of Textile Technology, Erode, Tamilnadu. After completing my diploma, I worked for MS Dyeing, Tirupur, TN as a production supervisor. Latter i had UG in Textiles from RVSCET located at dindugul.

Currently I’m working in Texport syndicate (I) Ltd., Bangalore, as a Production Executive. I have published seven articles to the websites and journals too. I have presented five technical papers in nation level symposium and I’m zestful to attend seminar and conference. This is impact recording to the publication of technical papers.


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Mail Id: aravinprince@gmail.com

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