The modification of PET fibres for improved dyeability, differential dyeing, antibacterial properties, reduced flammability, high water absorbency and mechanical properties are vital to overcome drawbacks, emphasises Aravin Prince P. Polyesters 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 fibre is a medium weight fibre with a density of 1.39 g/cm3 . Compared with nylon, polyesters are rather heavy fibres; For this reason polyester textile materials are manufactured as lightweight or thin fabrics. The most common polyester apparel filament or stable fibre is usually composed of polyethylene terepthalate (PET) polymers.Why modified polyesters are prepared?
The modified polyesters are prepared to overcome some 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 fibre will have a higher moisture regain. The garments made up of hydrophilic fibre will absorb perspiration and will be comfortable.
Other drawbacks are pilling problem and extreme difficulty in dyeing. The low pilling fibres are required to retain the elegant appearance of polyester garments for a long time. These low pilling fibres have lower tenacity than normal polyester fibres. Thus, although pills are formed in these fabrics, these pills are removed by simple brushing or washing.
Modification of polyester fibres
Polyester fibres were latecomers among manufactured fibres, and had to find their way into a market where polyamide and acrylic fibres were already established. Polyester fibres used for textile application offer tangible benefits to both processors and consumers. Low denier fibres blended with cotton gave higher strengths at lower twist levels, than found in 100% cotton yarns. The characteristic property of the polyester was immediately encashed by the designers of shirt and blouse fabrics. Polyester fibres 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 from natural fibres.
High rate of growth of polyester fibres 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 fibres. 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 polymerisation.
* 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 fibres for specific applications.
* Surface modification of normal polyester fibre for producing specific effects.
Modifications for improved dyeability
During dyeing, the dyestuffs diffuse into the fibre and are absorbed primarily by the amorphous regions. The thermal coefficient of the molecular mobility, responsible for the dye diffusion, depends largely on the Tg, which increases with increase in crystallinity and the degree of orientation of the fibre. 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.
Deep dyeable PET (DD-PET):
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 fibres 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 copolymerisation is proportional to the molar percentage of the modifier, a flexible comonomeric unit of high molecular weight is particularly useful.
Poly (ester-ether) fibres:
Block copolymers made from PET and polyalkylene glycols, ie, 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 co-polyesters containing PET and polyethylene oxide [PEO] segments syntherised 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 backbone.
Polyester co-polymer fibres made from a mixture of ethylene glycol, diethylene glycol and dimethylterephthalate showed improved dyeability and are found useful as binder fibres in fibrefill battings for sleeping bags and sky jackets.
However, the fibres made from these copolymers have the drawback 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 polyester (CFDP):
Carrier free dyeable polyesters are defined as those polyesters, whose dyeability at boil without the use of carriers is similar to that of polyester fibres dyed under HTHP conditions, or at boil in the presence of carriers. There are two approaches for producing CFDP.
v 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 fibre, including good dyeability at 100o C, physical properties and tensile strength are comparable with the normal polyester. The glass transition temperature of all these fibres is about 10o C, 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 colour yield.
* Shorter dyeing cycle.
* Reduction in dyeing costs.
* Elimination of the carrier cost.
* Energy saving.
* Environment protection, ie, ecological advantages.
* Possibilities of the dyeing of PES/wool of PES/acrylic blends.
* Reduction of the oligomer problem during dyeing.
(PET-b-PEG) based CFDP:
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 fibre in aqueous solution.
* The fibre 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 - 280o C 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) nOH. 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 localised absorption of dyes in the boundary zones between fibre surface and the dye liquor, which leads to uneven dyeing. This can be rectified by maintaining a uniform concentration gradient between fibre and the dyebath, at all points of the fibre, which can be achieved by rapid dye liquor circulation, or high fabric speed.
Light fastness of dyed fabrics: It is found that 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 stabilisers, 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 fibre structure, the dye molecules are not effectively trapped within the fibre structure. In other words, the factors that enhance diffusion into the fibre 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 50o C.
Cationic Dyeable Polyester (CD-PET)
In normal dyeable polyester, there are no sites for ionic dyes. So, it can only be 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 colours. 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.
Manufacturing of CD-PET:Co-polymerisation 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 fibre that is as shown in the Figure:
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-catalysed. 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.
Low pill PET (LP-PET):
Pilling is a serious problem, which is associated with all the synthetic fibres. In order to reduce the pilling, polyester fibres having lower than usual strength has been prepared. Although such fibres form pills due to friction, these pills can be removed by simple brushing since the fibres have lower strength.
The polyester fibre having pilling tendency can be obtained by incorporation in the polymerising 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 fibre breaks off before the formation of large pills.
Modifications for hydrophilicity
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 fibre hydrophilic. ICI have suggested the addition of 5 - 10% by weight of sodium sulphate as slurry in glycol during polymerisation. The particle size of sodium sulphate should be less than 3 microns.
Polyester filament having a moisture absorption capacity of at least 1% at 65% RH and 21° C 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 fibres can be enhanced by the addition of polyethylene glycol or tetraethyl ammonium perfluorooctane sulfonate to the melt before melt spinning.
Hollow polyester
During the last few years, considerable amount of research work has been done on producing hollow polyester fibres having micro crates (holes) on the surface. The hollow polyester fibre is produced by using specially designed spinnerets. Normally, four types of spinnerets are used for producing hollow fibres and the spinnerets are shown in the Figure.
Plug-in-orifice spinnerets Fig (A):
These spinnerets have a solid pin supported in the center 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 fibre as it emerges from the annulus and prevents collapse until the fibre solidifies.
Tube-in-orifice spinnerets Fig (B):
These 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 fibre solidifies or coagulates.
Segment arc spinnerets Fig (C):
These 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 fibre hollow is drawn in through the gap in the extruded fibre upstream from the weld point.
Teijin is marketing such hollow fibre 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 fibre 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.
Wet ability of polyester has been increased by using oxygen or nitrogen plasma. Plasma- produced polar groups increase the surface free energy of the fibre 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.
Antibacterial/deodorant polyester fibres
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 microorganisms, which can cause offensive odours, 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 fibre spinnability, main properties of the fibre as dye uptake, wear and abrasion resistance and other mechanical properties.
Features of the antibacterial fibres:
* Prevent development of microorganisms, which are responsible for bacterial contamination and unpleasant odours.
* 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 fibres:
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 fibre production and present problems when worn next to skin. So, inorganic supports such as special zeolites or ceramic substrates containing Ag or Zn ions have been proposed.
Flame retardant (FR) polyester fibres
Fire accident generally results in considerable loss of life and property. The majority of fire accidents occur due to burning of textile fibres. Polyester fibre 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-trichloro 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; Ttriphenlyphosphineoxide, 3, 5-dibromo-terephthalate, decabromodiphenyl ether, tribromodiphenyl, phosphinic acid derivative etc.
Silk like polyester
For centuries silk fabrics are considered to be most elegant and gorgeous textile materials. However, the production of silk fibre 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 fibres were first developed it was thought that these fibres will be able to substitute silk. However, soon it was realised that these fibres have metallic lustre, papery feel and poor aesthetic value. Substantial amount of research work was carried out to make silk like synthetic fibres, which has resulted in the development of silk-like polyester fabrics.
The following factors should be considered in the production of silk like polyester:
* Fibre cross section to obtain the desired luster.
* Fine denier filament to obtain the desired feel.
Role of cross-section of the fibre
Modification of cross-section of the fibre 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 fibre 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 lustre resembling that of silk. The type of cross-section can be coupled with amount of TiO2 in the fibre may result in "Milky" colours when the fabrics are dyed.
Role of average denier of the yarn
It plays a primary role in determining the stiffness of the yarn. It is easy to visualise 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 fibres are "Very fine" in the range 1.2 to 1.3 dtex, and hence necessarily the synthetic fibre 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.
Conclusion
Polyester: It is a well-known fibre in the synthetic fibre because it has certain desirable properties, the properties are high strength, wash and wear property, good dimensional property, elegant appearance and suitability for blending with cellulosic and protein fibres. But polyesters have some of certain drawbacks such as moisture regain, static electricity, soiling problem, difficult to dyeing, etc.
But now many more developments in the polyester processing, ie, hydrophilic polyester, easy dyeable and cationic dyeable polyester, low pilling, antimicrobial polyester, silk like polyester, etc. The advantages of above properties are good comfortable while in wearing, easy to dyeing, so it provides cost reduction and very good appearance of the polyester garments.
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Note: For detailed version of this article please refer the print version of The Indian Textile Journal June 2009 issue.
Aravin Prince P
Lecturer
JKK Muniraja Polytechnic, Gobi, Tamil Nadu.
Email: aravinprince@gmail.com.
Mobile: 097900 80302.
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