Thursday 1 December 2011
Surface adaptation of Polyester – A apparatus to Study Polyester
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Surface adaptation of Polyester – A apparatus to Study Polyester
INTRODUCTION
Polyester is one of the resourceful manmade fibres finding it’s carry into play from apparel to industrial field. Polyester apparels are becoming more popular in these days. Poly (ethylene terephthalate) (PET) fibre is today the most widely contrived fibre having overtaken polyamide fibres (Koslowski, 1993). Its fifty years by existence has recently reviewed in a remembrance book (Brunschweiler and Hearle, 1993). In India
There are three ways of modifying the polyester to impart silky feeling: (a) modification of the cross-section of the fibres (b) production of the fine denier filament; they are more effective in producing elegant gloss and soft hand, and (c) after-treatment of polyester textiles.
It was in the year 1952; ICI came out with a copyright disclosing the action of sodium hydroxide on polyester giving silky feeling, and was supported by other patent DuPont Co. in 1958. As was disclosed in the patent, by treating polyester with sodium hydroxide, polyester loses its weight progressively as the regular monofilament is reduced to fine denier and leaves scars on the filament surface.
Among the three stated ways of producing silk-like polyester, the first two, which modify the fibre and yarn forms respectively, are found to end up with
high costs. On the contrary, the weight reduction of polyester, as disclosed by the ICI patent, is not only less expensive, but also results in enhanced comfort properties. The weight reduction of polyester is nothing but saponification of terephthalic ester, caused by sodium hydroxide. Under predetermined conditions, alkaline hydrolysis involves the progressive peeling of polyester surface resulting in loss of weight, yet the favourable properties of it remain unaltered. The main feature of this treatment is that no special equipment is required for application.
Weight reduction of polyester has been viewed from both industrial and research point of view by various research workers as detailed in review of literature. It was in 1989 that an extensive review on the surface modification was presented by Zeronian and Collins. A successful attempt highlighting the potential of alkaline hydrolysis as a tool to investigate the structural aspects of polyester was a major smash through in the polyester research. On the other hand, an objective evaluation method was developed by Matsudaira et al., (1992) for silk from the basic mechanical properties. Fabric handle, using KES-F developed by Kawabata and Niwa (1975) has been in existence for the past twenty five years.
A considerable amount of work has been carried out on the surface modification of polyester fibre (Hayavadana, 1998) to improve the characteristics, and to explore the possibility of it as a tool to appreciate structural aspects.
REVIEW OF LITERATURE
Among all the synthetic fibres, polyester fibre has have a extraordinary growth from the time of its invention. Even though other fibres have contributed to its popularity, the nature of the molecule cannot be lined out in polyester fibre (Gupta, 1993). This is because the basic repeat unit of PET confers on the resulting semicrystalline oriented fibre a number of desirable features. Due to its inherent structure (Subash Chand, 1995) polyester has high glass transition temperature, high melting point, high modulus, high elastic recovery, good impact strength, high impermeability to vapours and gases, good resistance to the action of chemicals, high clearness, quick dry, easy care and aesthetic properties equal to the natural fibres. As a result, fibres can be produced with a combination of properties which make them appropriate for a wide spectrum of
applications in our day-to-day life. Further introduction of microfibres exploits a new aesthetic with fibres finer than natural fibres.
Eventhough polyester cannot be replaced by any other fibre from apparel to industrial ground, still efforts are on the way to the scientific study of its modification. In fact, no other fibre has been able to give us such a variety of fabrics, fabrics with an aesthetic appeal as well as pleasant feel. However, polyester fabrics suffer from undesirable lack of qualities like,
a. Static charge accumulation
b. Hydrophobicity
c. Tendency to pill
d. Low moisture regains
Hence the fabrics are not comfortable to wear in tropical conditions. In comparison, silk fabrics are more comfortable and provide special feel to the wearer. Also these are soft and supple and exhibit good drape. But the silk fabrics are very expensive, and therefore their use is limited to a class of people only. freshly, a novel technique has been made available to the wet processors of polyester fabrics to make them to have silky feel.
There exist two approaches :
a. To produce polyester fibre such that it is as fine and looks like silk.
b. "Deweighting" or "Weight reduction" treatment by alkaline hydrolysis.
Substantial amount of research has been carried out, and above are some of the ways of producing silk like effects in polyester. But alkaline hydrolysis is one of the chief methods of making polyester fabrics silk-like, which, due to its practicability and inexpensiveness, has now become the talk of the industry. In essence, the alkaline hydrolysis process consists of a behavior of the polyester fabric with a dilute caustic soda solution, so as to dissolve the surface polyester from the fibres.
Polyester fabrics processed in this way acquire characteristics that make them suitable for these "sophisticated" market segments conventionally conquered by silk. By giving this finishing treatment, polyester is imparted with
i. Greater comfort
ii. A more natural and silk-like handle
iii. Improved draping aesthetics and less wrinkling
iv. Improved antistatic properties
v. Greater absorbency
vi. Ease of care
vii. Better dye uptake and printability.
An extensive text review related to the various investigations on modifications of polyester fibres by alkaline hydrolysis and its influence on various properties of polyester, carried out by various research workers, and also a bird's eye view on various ways to produce silky polyester are given. Firstly, a general account on polyester is given, followed by the treatments specified to it.
POLYESTER
Polyester is very much familiar to a layman also. It was in 1950, interest stemmed up as the result of research carried out by two English chemists namely M/s.John R. Whinfield and James T.Dickson in 1941. The profitable production started in 1950 only.
Synthesis, Structure and Properties
Polyester" is the generic term for a contrived fibre in which the fibre-forming substance is any long-chain synthetic polymer self-possessed of atleast 85% by weight of an ester of dihydric alcohol and terephthalic acid (TPA). The major monomers are dimethyl terephthalate (DMT) or (TPA), and ethylene glycol. The term polyester defines polymers formed from the reaction of a dicarboxylic acid and a diol or trifunctional alcohol.
Poly (ethylene terephthalate) (PET), the most common polyester in use throughout the world, is produced from EG and DMT or TPA using transesterification and polycondensation catalysts.
PET is a linear polymer with a long repeat unit which consists of an altering unit of flexible aliphatic segments and stiff interactive benzene rings. The chain repeat unit is slightly less than the fully extended length of the chemical unit (Meares 1965; Pajgrt and Reichstadter, 1979). An important structural feature of this material is that the quenched fibre is noncrystalline owing to the weak Vander Waal's forces between the greatly extended macromolecules. However, crystallisation occurs during drawing of the fibre, as the chains are pulled into alignment. The crystal unit cell for PET is triclinic, and contains one repeating unit (Morton and Hearle, 1975).
A number of different structural models have been suggested for the fibre. It appears that the melt spun and drawn PET fibre consist of atleast 3 different phases : amorphous and crystalline domains of the micro fibrils and intermicrofibrillar regions (Intermediate phase or oriented amorphous phase. Linder (1973) has also suggested the presence of an intermediate state of order between amorphous and crystalline phases. Valk et al., (1980) have reported an isotropic and anisotropic amorphous part with relaxation at two temperatures 35oC and 80oC.
Among the commercial melt spun fibres, PET fibre is one of the strongest and stiffest. By changing the various material and dispensation parameters, a wide range of physical and thermo mechanical properties have been introduced (Ward and Wilding, 1976). Another important property of the polymer is its low moisture regain [0.4% at 70oF and 65% RH]. Differential thermal analysis (DTA) shows a second order transition at 78-80oC, a crystallisation endotherm ranging between 125 and 180oC, and a melting point (DTA) of 255oC. Melting points are in the neighbourhood of 250-265oC.
Melt spinning aspects of polyester
In the case of PET, though an oriented, ordered, mesophase appears at spinning speeds of 1000 mpm and above, it is generally agreed (Gupta, 1995; Satton, 1978) that upto speeds of 4000 mpm as spun PET fibre is predominantly amorphous, and is thought to be in the form of an entangled molecular network with an entanglement density of around 2.4 x 1026 per cubic metre, which
means that there are about 20 monomer units between successive entanglements (Radhakrishnan and Gupta, 1978). However some authors believe that the structure may not be homogeneous as in addition to the rubberlike network crystal nuclei, (Desai, Abhiraman, 1986; Napalitano and Moet, 1986) extended chain molecules may also be present (Hristov and Schultz, 1990).
PET melt spinning (Talele and Gandhi, 1994; McIntyre, 1985) processes can be one in which relatively low winding speeds have been used to produce a filament yarn processing a little or no orientation and second in which relatively high wind-up speeds are used to produce a partially oriented yarn.
PET fibre products are classified into four groups on the basis of the level of molecular orientation developed in the fibre; these are given below (Daubeny, 1954).
a. Low spinning speed in the range of 500 mpm to 1500 mpm, the product is called Low-Oriented Yarn (LOY).
b. Medium speed in the range of 1500 mpm to 4000 mpm, the product is called Partially Oriented Yarn (POY).
c. High speed in the range of 4000 mpm to 6000 mpm, the product is called Highly Oriented Yarn (HOY).
d. Very high speed in the range of above 6000 mpm, the product is called Fully Oriented Yarn (FOY).
Among these yarns, POY became a profitable reality in the early
1970s because of the availability of commercial winders around that time and also because of the introduction of simultaneous draw texturing in 1970.
POY allows crystallisation to occur in draw texturing at a significantly lower temperature. This partially crystallisation, that occurs when PDY contacts the heater plates, greatly reduces the sticking that normally occurs above the glass transition temperature (Tg).
In addition, the high level of spin orientation and reduced crystallisation temperature result in a very rapid crystallisation in draw texturing process which increases the bulk and a very stable texturing process (Gupta, 1988).
PRODUCTION OF SILK LIKE POLYESTER
It is the trend nowadays for the fabric manufacturers to adopt newer ways of improving aesthetics and quality of the products which keep them to remain them in competition. As it is the recent trend is to produce silk like polyester whose availability seems almost endless. Silk, like polyester fabrics, is particularly used for apparels, dresses, scarves, lining materials, and ties.
Fibre modification
With respect to silk like polyester fibre, modifications in the following are highlighted.
a. Cross-section
b. Surface properties
c. Polymer structure and
d. Denier
Cross Section
Surface properties of the yarn and fabric are distorted when the fibre cross section is subjected to chemical modification. Production of silky yarns is also not
an exception. Needless to mention, the secret of mulberry, natural silk only lies in almost triangular cross section. This has been considered in producing silky yarns (Anon, 1976). The cross-section of fibres may be circular, trilobal, pentalobal, octalobal, hollow, hexagonal and other irregular shapes. But for the production of polyester silky fibre circular, trilobal, tetralobal, C-shape, V-shape and hollow cross section have been used (Datye and Vaidya, 1984). However, of these, triangular shapes having comparatively good productivity are still occupying main current (Anon, 1976; Anon, 1980).
Surface properties
By modifying the surface of the polyester by creation of cavities ridges etc., throughout spinning itself, a silk like polyester with better handle and comfort properties is obtained as reported by many research workers.
Toray Co. Japan, has claimed the development of microcrater polyester filament, which has one billion or more crater like holes per sq.cm. on its surface and it possesses excellent characteristics in colouring, handle, and absorbency etc. (Anon, 1984).Teijin industries, Japan (1986) have recently developed a novel method of creating ridges per grooves on the fibre surface.
Polymer structure
By copolymerising polyester and benzoate, a new silky material can be produced. This fibre exhibits characteristics like good glass transition temperature, dyeability, soft feel and high water absorption. In addition to these, it also exhibits an improvement in strength and heat resistance and can be woven or knitted. In brief, it nears to an ideal silky material (Anon, 1976).
Denier
By producing the filament denier in the range 1.2 to 1.3 dtex, a synthetic fibre possesses almost silky feel. In general, it can be said that finer the denier, the softer the feel of the material. Finer denier filaments are also more elegant, gloss in look and soft hand than modified cross-section yarn (Anon, 1976).
Yarn modification
Methods available for the modification, to provide silk like feel can be classified as follows:
a. Combining polyester fibres with different characteristics.
b. Blending polyester with other fibres.
c. Applying silk like twist.
Combining polyester fibres having different characteristics
Of late, combined yarns have suddenly come to light. This kind of yarn can be obtained by mainly combining two kinds polyester filaments with different monofilament denier, cross-section, shrinkage to heat substrate and thickness (Anon, 1976). Thus, by combining two or more filaments, together, the space between the filaments closely resembles that of silk, producing a comfortable feel (Anon, 1985; Ashish Cama, 1987, Anon, 1967).
The most popular combined yarns which possess different heat shrinkage rate are known as "Differentially Contractible Combined Filament yarn" (DCCF). These yarns can be divided into (a) first category of yarns in which filaments are united in the spinning stage (b) second category of yarns where the drawing phase involves the unification (c) and the 3rd category of yarns in which the filaments are bundled and then processed in the texturing stage to attain a combined filament construction (Anon, 1984). Subsequently, a filament yarn produced by this way shrinks irregularly through texturing and heat setting, and forms a complicated textured yarn consisting of thick denier filaments in the centre and fine denier filaments as sheath. Fabrics using these yarns have both soft hand and resilience and yet the appearance and indiscretion are like those of natural silk (Anon, 1976).
Blending polyester fibres with other fibres
By blending polyester with acetate, triacetate and nylon, silk like polyesters can be produced which combines the comfort and colouring of cellulosics with the strength of polyester (Anon, 1977).
Use of softening agents
In producing the hand and lustre of silk like materials, use of softening agents cannot be neglected. Wide range of heterogeneous chemical agents are available. For example, many softeners of anionic, cationic, and non-ionic nature etc. have been applied as polymer coating to fabrics by routine curing methods.
Some of the fast softening agents are silicones, aminoplasts, and pyridine compounds with long alkyl groups (Velan PF). Also included among softening agents are the so-called handle modifying agents, such as the polyethylene emulsion Perapet PE
Weight reduction by Sodium Hydroxide Treatment
As mentioned in the introduction part, alkaline treatment is becoming more popular for achieving silk like feeling. By treating with NaOH (sodium hydroxide), polyester loses its weight progressively; the regular monofilament reduces to fine denier and leaves scars on the surface of the filament. Fabrics, subjected to alkaline hydrolysis like this, are characterised by irregularities like that of natural silk, and thus silk like hand is imparted. Eventhough the other methods discussed in the previous sections produce silk like feeling similar to alkaline hydrolysis, the cost of the former methods seems to be very high. In view of this, the weight reduction of polyester to give silk like hand is attracting the attention, and will remain almost as a powerful method in future also.
REVIEW OF LITERATURE ON ALKALINE HYDROLYSIS
Weight reduction of polyester has been viewed from both industrial and research point of view by a number of workers . In 1989, an extensive review on the outside modification has been presented by Zeronian and Collins (1989).
Alkaline hydrolysis of polyester fibres
Generally polyester fibres are resistant to most of the chemicals employed in textile processing due to its chemical structure. But as polyester contains an ester group, it is liable to be attacked by alkali solutions at high temp (Pfeifer,
1964). The effect of alkali in polyester was primary disclosed by I.C.I. in a 1952 patent (Hall and Whinfield, 1952). Namboori and Haith (1968) have found that the loss in weight of PET fibre in the various anions was in the order of Hydroxide < tert-butoxide < sec-propoxide < methoxide < ethoxide. Different aspects of the alkali treatment of polyester fibres are fully described in a 1958 patent assigned to the Du Pont Co. (Gujjar, 1958). Then Harshe and Mandloi (1982) have made attempts to investigate the action of sodium hydroxide solution on the different fibres and their blends of the cotton, viscose, polynosic and polyester.
The effect of concentration of NaOH, time of treatment and the temperature on the alkaline hydrolysis of polyester fabrics have been studied by a number of workers. Polyester fabric was also treated (Dave et al., 1987) with medium ethylene glycolate (SEG), sodium diethylene glycolate (SDEG) and sodium triethylene glycolate (STEG) and it was found that the weight loss increased in the order SEG<SDEG/STEG.
Optimised percentage loss in weight for the satisfactory improvement in feel to a silk like level, static charge development and hydrophilicity, were observed as 15%, 10% and 5% respectively (Achwal, 1984). The effect of an excess of a concentrated solution of sodium hydroxide was studied (Gawish and Ambroise, 1986) on two different polyester fabrics (Crepe and Twill) and it was found that there was an appreciable difference in their rate pattern.
A different method to measure the weight defeat after hydrolysis has been formulated (De Maria, 1979) by titrating aliquots of each bath with standard hydrochloric acid solutions before and after treatment.
The reaction of PET, cationic dyeable polyester (CDP) and easily dyeable polyester (EDP) fibre materials with aqueous alkali containing quaternary ammonium salt (catalyst) was studied by Datye et al., 1990) and the order of loss in weight was given as PET<CDP<EDP. Sneha (1992) has presented an estimation of concentration of sodium hydroxide and temperature to optimise the weight loss to 8-12%. This calibration could be used to determine the NaOH concentration and temperature to obtain the required fineness of the yarn through a polynomial equation derived.
The topochemical degradation by hydrolysis with aqueous NaOH and the permanent degradation by aminolysis with aqueous ethylamine have been investigated (Ellison et al., 1982; Olson and Wentz, 1984) through their physical properties. Studies have been conducted by Latta (1984) to indicate that the hydrolysis of polyester occurs both by first and second order cleavage of the polyester chain by the hydroxide ion and by elimination of end groups in first-order saponification reactions. Based on this model, a rate expression for the hydrolysis has been derived.
Hydrolysis of PET fibres using methanolic sodium methoxide has been compared to that using aqueous sodium hydroxide (Holmes et al., 1993) and it was found that the fibre density increased after the reaction with methanolic sodium hydroxide (NaOCH3). The possibility of utilizing alkaline hydrolysis for studying changes in the properties of thermotropic liquid crystalline copolyester fibres was evaluated by Zeronian et al., (1994).
Recently the specific surface area (SSA) of delustered undrawn and drawn high speed spun PET fibres hydrolysed in aqueous NaOH was deliberate by Holmes and Zeronian (1995) using three methods : a. Geometric, based on fibre diameter; b. Gas adsorption using nitrogen (N2) and the BET equation; and c. Adsorption of a non-ionic surfactant.
Hydrolysis reaction mechanism
Hydrolysis, as referred to generally, is chemical decomposition process in which water molecules split the compound as shown by the following equation :
R.X + H.OH ---> R.H + X.OH ... (1)
Water, in the form of its hydrogen and hydroxyl ions is added to the cleared compound. The addition of water is generally catalysed by ions and without these, it may be a slow process. Consequently by adding acid or base, the concentration of hydrogen or hydroxyl ion increases with a corresponding increase in the rate of hydrolysis. Hydrolysis of an ester can take place in the
presence of either an acidic or a basic catalyst.This process is generally referred to as saponification as it is similar to the one employed in soap manufacture. The base catalysed reaction goes to completion because the acid formed during hydrolysis reacts irreversibly with alkaline catalyst. Atleast one equivalent of alkali is required for each equivalent of ester which is hydrolysed.
Nature of action of sodium hydroxide on polyester
The saponification reaction of NaOH with PET is initiated by the hydroxylation attack on the electron deficient carboxyl carbon atom of the ester linkages as shown in the equation (2.2). The carboxyl group formed is immediately converted into carboxylate anion and the reaction goes to completion in the direction of hydrolysis (Namboori and Haith, 1969). It is assumed that the alkali erratically attacks the carboxyl groups of the polymer molecule present on the surface of the fibre and removes them as short chains. Finally, the short chains are hydrolysed to disodium terephthalate (Namboori and Haith, 1968). Owing to the removal of fibre material in the form of short chains, the fibre surface suffers a loss of weight. Thus an overall balanced equation for the reaction of NaOH with PET can be represented as follows :
O O
. .
H ( O - C - - C - O - CH2 - CH2 ) OH + 2n NaOH ---->
(2GT = 192) (NaOH = 40)
O O
. .
n Nao C - - C - ONa + n HOCH2CH2OH ..(2)
(Na2T = 210) (2G = 62)
Kinetics of the reaction
Latta (1984) has studied the kinetics of the reaction between polyester and aqueous NaOH, and derived a rate expression for the hydrolysis. Wu et al., (1988) have also derived a kinetic equation for polyester alkaline hydrolysis by polyphase chemical kinetic analysis and have supplied a number of formulae for reduction, stripping and alkali consumption derived from various conditions.
Weigmann et al., (1979) have established kinetic equations to describe the course of hydrolysis and have calculated the rate steady for homopolyester, copolyester and ion-modified polyester fibres at 95oC. A theoretical model was developed by Kallay et al., (1991 and 1990) to describe the kinetics of polyester fibre dissolution in alkaline solutions based on the surface reaction concept.
The kinetics of effect of aqueous solution of soda on the texturised polyester fabric has been studied by Radillo et al., (1993), who has derived a kinetic model to study the hydrolytic degradation of PET at high temperatures. Model developed was found to fit the experimental data, and to provide satisfactory predictions of the balance concentrations of carboxyl groups for three different initial charge ratios.
The kinetics of surface area of aqueous sodium hydroxide hydrolysed high-speed spun PET fibres have been studied by Holmes and Zeronian (1995).
PARAMETERS OF ALKALINE HYDROLYSIS
It has been reported in section 2.3 that alkaline hydrolysis results in a progressive loss of fibre material and is determined by the rate of hydrolysis. But the rate of hydrolysis inturn is influenced by following parameters:
i) Prime treatment conditions
ii) Type and amount of the catalyst present
iii) Alkali medium
iv) Polyester substrates
v) Type and amount of pretreated solvents
vi) Heat pretreatments and
vii) Denier per filament and fabric construction.
Prime treatment conditions
Various research workers have extensively dealt with the prime treatments conditions of alkaline hydrolysis, viz.,
a) Concentration
b) Temperature
c) Time interval of the treatment
d) Material to liquor ratio
In an earlier paper, Brenneck and Richter (1959) have observed that the weight loss is a linear function of the time of treatment. Namboori (1969) has also studied the effect of NaOH on PET fibres at various temperatures with different alkali concentrations. Several other investigations have also concluded the same.
Dave et al., (1987) have studied the alkaline hydrolysis on 80/20 carbonised P/C blend of different known intervals of time, temperature and attentiveness, with a large material-to-liquor ratio (1:150) and have arrived at a linear relationship between fibre weight loss (w) and time of treatment (t).
W = bt ... (3)
where, b is a constant on time of treatment pertinent to a specific sample. They have observed that the weight loss proceeds non-linearly with respect to concentration of alkali (c).
W = bc2 + ac ... (4)
where, a and b are constants on treatment time and temperature respectively, pertaining to the particular samples. Earlier, Valk and Stein (1977) have also obtained a rectilinear relation between the square root of the residual fibre weight and NaOH concentration at constant time.
Houser (1983) in his observations, reports that the fabric weight loss increases with treatment time with high MLR, in the case of both untexturised and texturised fabric.
As long as the concentration of alkali second-hand is constant, there exists a linear relation between weight loss and time. On the contrary, if only limited alkali is used, the concentration of alkali decreases as the reaction time continued. This results in an exponential relation between time and weight loss. This phenomenon has been observed by many (Gawish, 1986; De Maria 1979; Goraffa, 1980). Gawish and Ambroise (1986) have carried out alkaline hydrolysis on PET fabrics with a stoichiometric quantity of NaOH and have reported that with a limited quantity of NaOH, the hydrolysis rate is exponential.
As reported in section 2.4, Radillo et al., (1993) and Sett et al., (1994) also have studied the relationship between weight loss and process parameters like alkali concentration, bath temperature, processing time and MLR.
Type and amount of catalyst present
The rate of hydrolysis of polyester textiles can be noticeably enhanced by addition of certain quaternary ammonium compounds or cationic surfactants to NaOH solution. These act as catalysts for the reaction and enable the amount of NaOH required and treatment time to be halved under specific conditions. Two earlier patents by Anon (1984) and Shenai (1982) have claimed that the weight loss of the polyester could be increased in much short time than that required with NaOH only by addition of these compounds which are widely used in a variety of wet processing operations in textile industry.
Use and effect of accelerators in alkaline hydrolysis was studied earlier by Hall (1963). According to Dorset (1983), the available quaternary ammonium compounds differ widely as regards their catalytic effect which are obtained by treating polyester fabrics under comparable conditions (Hall, 1963).The role of these compounds in alkaline hydrolysis has been defined differently by different authors.
The effect of different quaternary ammonium salts and the effect of varying amounts of quaternary with bath ratio, treatment time etc. on the weight reduction has been studied by De Maria (1979). Achwal (1984) has reported that by varying the treatment time from 5 min to 90 min with catalyst, weight loss %, at the beginning was same as that without catalyst and it became later on almost doubled (1984). Chaptwala (1986) also obtained similar trend.Gawaish et al., (1984, 1985, 1986, 1988) have investigated the catalytic action of three quaternary alkyl ammonium halides in alkaline hydrolysis in presence of quaternary compounds.
McIntyre (1985) has suggested that the increase in response rate is due to the ability of long-chain quaternary ammonium compounds to transfer hydroxide anions into the polyester phase. It is also feasible that inductive effects may be a
contributory factor. This may take place by the cationic surfactant associating with the electron-rich carboxyl oxygens of the polymer molecules. The protonation that occurs would increase the electron deficiency of the carboxyl carbon and thus increase the susceptibility of the polymer to hydrolysis.
Goraffa (1980), while proposing their industrial application, has reported that though these compounds aid in yielding much higher weight losses at a faster rate, it is counter balanced by its detrimental effect on strength degradation on the textile materials.
The effect of addition of different cationic accelerators on alkaline hydrolysis PET fabric had been studied by Ibrahim (1991).
Changes in weight reduction on different varieties of fabrics in different treatment conditions namely grey and scoured at 100oC and 120oC by varying treatment time in unset and heatset conditions with activators have been reported by Teli and Purkayashta (1992, 1993).
Achwal (1993) has suggested conventional and polymeric cationic activators for deweighting of polyester by alkalies. The effect of dissimilar additives on the catalyzed hydrolysis of poly(ethylene) terephthalate)melts has been discussed by Campanelli et al., (1994).
Alkali medium
The action of NaOH differs in different media that have been used (Namboori, 1968; Zeronian, 1984; Achwal, 1984, Shenai, 1982). Namboori and Haith (1968) have carried out a comparative study by treating the polyester fibres with alkalies and various alkoxides (e.g. NaOH in water, sodium methoxide in methanol, Sodium ethoxide in ethanol, Sodium isopropanol, and at different concentrations. They have found that the loss in weight of the polyester fibres is in the order of sodium hydroxide < Tert-butoxide < Sec-propoxide < methoxide < ethoxide, and have suggested that the observed order follows the nucleophilicity of the bases; they have attributed the relatively lower reactivity of
the Sec-propoxide and tert-butoxide to the steric retardations during the equilibrium reactions.
The morphology of unoriented poly(ethylene terephthalate) (PET) films and the selective character of the aminolysis of PET, degraded with 40% aqueous methylamine at room temperature, have been studied by Doung and Bell
Polyester substrates
The rate of hydrolysis and resulting weight loss are also strong-minded by the type of polyester substrate used viz. type of fibre. All the factors being equal, a round section flat polyester yarn will show less noticeable etching than a flat yarn with a modified cross-section (Anon, 1984). This may be due to the larger surface area of the modified cross-section than a fibre for a given denier (Houser, 1983).
Bright fibres lose weight more slowly than delustered types (Houser, 1983). In this context, Goraffa (1980) has reported that the higher rate of weight loss and noticeable pitting in semi-dull fibres reflect the removal of titanium dioxide (TiO2) particles which are dislodged from fibre surface.
Mittal and Bhatt (1983 and 1985) have investigated the rate of hydrolysis of flat and texturised filament yarn fabrics, and have observed that the fabrics with texturised filament register a higher weight loss than the flat filament at similar conditions. In texturised polyester again, the draw texturised, normal texturised, the magnetic spindle and friction have been found to behave differently towards this treatment (1986).
A few investigators have studied the hydrolytic mechanism of co-polyester fibres (1987). Datye and Palan (1989) have studied the action of NaOH on PET and two copolyesters, namely CDP (Cationic Dyeable Polyester) and EDP (Easy Dyeable Polyester) under a variety of conditions of alkali concentration in aqueous bath, additives, time and temperature and they have reported that the rate of hydrolysis increased in the order PET < EDP < CDP.
As discussed earlier, the exact surface are (SSA) of delustered undrawn and drawn high-speed spun PET fibres hydrolysed in aqueous NaOH was measured by Holmes and Zeronian (1995). Also the possibility of utilizing alkaline hydrolysis for studying changes in the properties of thermotropic liquid crystalline co-polyester fibres was evaluated by Zeronian et al., (1994).
Type and amount of pretreated solvents
The solvent pretreatment has a significant result on hydrolysis of PET fibres. Some of the workers have reported their effect on hydrolysis with aqueous NaOH.
Teli and Purkayastha (1991) pretreated the cationic dyeable (CFDP) polyesters with different solvents at different temperatures and their samples were then subjected to hydrolysis with aqueous NaOH for different span of time. The extent of weight reduction of these polyesters in terms of efficiency of solvents was determined.
In their preceding work (1991), the dye uptake of solvent pre-treated and weight reduced normal, cationic dyeable (CDP), and carrier-free dyeable (CDFP) polyesters was determined. Also the effect of dyeability of prior solvent treatment as well as that of solvent treatment after hydrolysis has been discussed.
Hye Won Shin and Hyo Seon Ryu's (1995) current research work investigated the roles of solvent treatment, for both solvent mixture systems on the micro structure and tensile properties of PET POYs using the solubility parameter concept at different temperatures. Strong interactions were observed between PET POY solvents, when three dimensional solubility parameters of those are similar. Changes in micro structures by the solvent treatment were : an increase in crystallinity, change in birefringence, a shrinkage in length and a change in DSC curves.
Heat pretreatments
The effect of heat treatment on the hydrolytic behaviour of PET fibres was extensively studied by Shouhua Niu and Tomiji Wakida (1992, 1993 and 1995). They have investigated the effect of heatsetting temperature on the hydrazine action of the POY-PET yarn. Hence although crystallinity of the heat-set POY-PET increases with increase heat-setting temperature, weight loss from the hydrazine treatment initially decreases at a temperature upto 120oC and subsequently increases above 120oC.In their continued work to find the effect of heat setting temperature on the alkaline hydrolysis of PET fibres (1993), they found that the weight loss from the alkaline hydrolysis decreased with increase in heat-setting temperature only upto 140oC. Here the changes in the fibre structure have been analysed on the basis of weight loss, crystallinity, dyeing properties, X-ray diffraction measurements and scanning electron microscopy (SEM).The hydrolytic behaviour of heat-set PET fibres was compared in the presence and absence of a quaternary ammonium, dodecylbenzyl dimethyl ammonium chloride (DBDMAC) in their recent work.They found that the rate enhancement of alkaline hydrolysis by DBDMAC decreases with increase in heat-setting temperature or degree of crystallinity of the fibre, and may preferentially occur in the amorphous of the fibre.
Denier per filament and fabric construction
Very few research workers have concentrated on the investigation of the effect of linear variation of PET filament fibre and fabric construction on the hydrolysis process.Gawish and Ambroise (1986) have studied the alkaline hydrolysis of two different polyester fabrics (crepe and twill) in excess NaOH at 100oC. The rate of hydrolysis with time was found to be linear and was compared to another investigation where the rate was exponential. The mechanical properties of the distort and filling fibres of the two fabrics were also discussed.
CHANGES AFTER HYDROLYSIS
After the alkaline hydrolysis, the changes in the mechanical, comfort, physico-chemical, morphological, physical and chemical properties have been studied by
many number of workers. Achwal (1984) has made a comparison of changes in physical properties of the different alkaline hydrolysed polyester fabric samples which are prepared with comparable loss in weight by different methods such as (a) alcoholic NaOH in long liquor (NaOH in ethanol), (b) aqueous NaOH in long liquor and (c) aqueous NaOH by pad-batch method. He has compared the samples for the changes in the physical properties like grip and appearance, hydrophilicity, static charge and degradation of polyester material.With a view to imparting comfort-related properties, polyester fabric was treated with aqueous sodium hydroxide under varying conditions of temperature, duration and concentration of the alkali solution by Dave et al., (1985, 1987). The relevant comfort, mechanical and physio-chemical properties the treated fabric were evaluated.Zeronian and Collins (1988) have compared the fabric weight loss, fibre intrinsic viscosity and fibre density of the regular PET, regular PET with catalyst at reaction and anionically modified PET fabrics. Also their tenacity losses with respect to weight loss were investigated. The fine structure and physical properties of bright and semi-dull PET fibres were investigated by Collins et al., (1991). Alteration in fine structure was also observed by thermal analysis and strength loss.The basic mechanical properties and fabric handle of polyester fibre fabric were pursued (1992) through the finishing stages by an objective assessment method developed by Kawabata and Niwa using the KES-FB system. Jan et al., (1992) have investigated the effect of the presence of organic solvents in the finishing bath on the properties (including some morphological and crystalline structure) and the dyeing property. Sett et al., (1994) have studied the effect of alkali-hydrolysis (with NaOH) and aminolysis (with methyl amine) on tensile properties, dyeing behaviour and comfort related properties of PET fabric.
Melissa Davies and Amirbayat (1994) have investigated the effect of the weight reduction level on the in-plane and out-of-plane properties (bending, shear, drape and drop) of a polyester-fibre satin fabric and have provided some interesting results.Polyester yarns containing four levels of titanium dioxide (TiO2) delustrant were hydrolysed by Solbrig and Obendorf (1991) with NaOH and then examined for physical and morphological changes. The samples were subjected to the physical, mechanical and morphological changes. The effect of hydrolytic and aminolytics degradation on the structure of PET films has been
investigated to Padhye and Nadaf (1979). Also changes in IR crystallinity, number of OH and COOH end groups and development of opacity have been taken as measures of the morphological changing occurring on hydrolysis. The topochemical squalor by hydrolysis with aqueous NaOH and the permanent degradation by aminolysis with aqueous ethyl amine have been investigated through their physical properties by Ellison et al., (1982) and Olson et al., (1984).
The moisture regain and water retention values of the alkali hydrolysed samples were determined by Erin Murphy et al., (1982) and it was found that such tests are not sufficiently sensitive to distinguish between the hydrophilicity of non-treated PET fabrics and that of PET fabrics modified either by application of a topical finish or by NaOH treatment.
It was observed that (Seiji Maekawa, 1979; Padhye and Singhi, 1981) the PET, PET/COTTON and PET/VISCOSE blended fabrics when dyed with or without previous alkali treatment gave different depths of shades. Also a detailed study of the mechanism of increased dye adsorption due to alkali pre-treatment in such blends was reported.
The alkali treated and untreated polyester fabrics were dyed (Needles et al., 1990) with a series of six disperse dyes of different substance structures. The alkali treated polyester samples adsorbed more dye and were dyed to deeper depths of shades and slightly different shades than was the untreated polyester. It was noticed that when progressive amounts of the fibre surface were removed, the fibre interior became more dyeable than the fibre as a whole, suggesting that there are differences in morphology across the fibre cross-section. The dyeing characteristics of the alkaline treated polyester fibres and polyester micro fibres were also studied by Richter (1991).
IMPORTANCE OF THE MECHANICAL PROPERTIES
The main purpose of investigation into the mechanics of textiles is to help understand, and hopefully develop analytical tools to predict their mechanical behaviour.
Studies on bending behaviour of fabrics
Fabric bending properties are important both directly in that they contribute towards the impression made in "drape" and "handle" (Steele, 1957, Howorth, 1964, 1965) and indirectly in the light they shed upon the mechanisms of fabric deformation (Shishoo, 1983, Cusick, 1965).
The statistical regression analysis theory was used to correlate bending and shearing properties on the hand (Dawes and Owen 1971, Livesey and Owen, 1964, Owen 1968 and the results of the subjective stiffness and liveliness assessments (measures of handle) on the other.
Buckling
Buckling is a property intimately related to bending. Buckling occurs when the conditions of the ends of the specimen are constrained to remain collinear (Chapman, 1972). For buckling, both equal and opposite forces and equal and opposite movements on both ends of the specimen are required. This deformation is very common in textile materials.
Tensile Behaviour
Strip biaxial tests can be regarded as an intermediate form of testing between the two extremes of a true biaxial test and an uniaxial test.
Shear Behaviour
The shear behaviour of fabrics influences a number of important fabric properties such as drape, handle, surface, fitting, tailorability and shape retention (Dhingra and Postle 1979).
Compression Properties
Compressional deformation is the result of the opposing forces applied from two sides of the fabric in a direction normal to the plane of the fabric. The surfaces of
the fabric are typically highly compressible (Mahar, 1988). Hearle (1967) has defined compression as the decrease in intrinsic thickness with an appropriate increase in pressure and compressibility, as the ratio of density to intrinsic thickness expressed as a percentage. The compressional properties of textile material have direct relevance to the handle of fabrics.
Surface Properties
It is widely recognised that the nature of the surfaces of fibres exerts a major influence on the behaviour in textile processing, and on the properties of textile products in service. The most extensively studied surface characteristic of fibres and fibre assemblies has been friction.
Formability
The main task for the cloth manufacturer is to create shell structures out of flat fabrics to match the shape of a human body. In all shape producing methods, there will be an interaction between particular method use, and various physical properties of the fabric.
FAST developed by CSIRO (1989), Division of Wool Technology, predicts whether the fabric will tailor well before production begins. Seam pucker is most often caused by low formability of the fabric [this is related to its extensibility (stretchiness and bending rigidity (stiffness) or the inclination of the fabric to be dimensionally unstable (a tendency to get smaller or expand under different conditions)]. FAST measures these parameters, allowing remedial steps to be taken.
FAST gives the finisher and the tailor a widespread language when it comes to discussing tailoring performance of fabrics.
The authors of the article are associated with Textile Technology, University College of Technology, Osmania University , Hyderabad
