Indian Journal of Fibre & Textile Research Vol. 27, June 2002, pp. 199-210 (? _ rl \) Review Article Solvent-induced modifications in poly (eth lene terephthalate) an dyeabihty -- --- - , LQfEh idambaram" & R} enkatraj1 I ) Department of Textile Technology, PAC Ramasamy Raja Polytechnic College, Rajapalayam 626108 India - I and P anisankar\ ' Department of Industrial Alagappa University, Karaikudi 630003, ndia Received 16 OClOber 2000; revised received and accepted 12 March 2001 Poly (ethylene terephthalate) fibre (PET) is the commonly used man-made fibre for majority of end-use applications. In thep ast few decades, the significant technological developments have taken place pertaining to the production processes of PET. Regarding the dyeing of PET, there is a constant upsurge in research and development activities to modify the structure of PET with a view to develop energy efficient processes. Among the notable developments, the low molecular weight solvent treatment results in physical modifications of polymer and fibre like PET, which include structural modifications and changes in properties. Owing to the structural modifications of PET fibre, the dyeability also improves. Some of the pioneering works on solvent-induced modifications of PET structure, properties and dyeability during the past few years have been reViewed] Keywords: Poly(ethylene terephthalate), Structural modification, Mechanical properties, Dyeability 1 Introduction The use of organic solvents for structural modifications in textile substrate has become quite popular in recent times. For example, in the solution spinning routes the solvents are used to produce highly oriented material, possessing high strength and/or high modulus. In the post-spinning operations of solution-spun fibres, the textile substrates are kept in the swollen state for the structural modifications. Because of the swollen state, the post-spinning operations are more efficient and energy saving I. Poly (ethylene terephthalate), commercially known as PET, has been widely used in industrial and commercial applications 2 because of its desirable qualities such as high strength, good handling, easy care properties, etc. However, PET has certain undesirable qualities like lack of hydrophilicity and accumulation of static charges. PET fibres generally exhibit low surface energy and limited chemical reactivity, resulting in poor moisture absorption. Usually, the dyeing of PET is carried out at I30°C under high pressure, which is energy intensive 3 • " To whom all thc correspondence should be addressed. Phone: 0091-04563-522940: E-mail: [email protected]Because of the increasing energy cost, the energy efficient processes involving interactive solvent media preferably at low temperature are gaining popularity at present. It has been found that the solvent treatments result in structural modification of PET, which opens the structure sufficiently to facilitate subsequent dyeing operations 4 . The structural modification of PET depends on the extent of interaction between PET and the solvents used. The interaction between PET and solvents is associated with their respective solubility parameters. 2 Solubility Parameter The solubility parameter is defined as the square root of cohesive energy density. The cohesive energy density (CEO) is the energy in calories/cc, necessary for an infinite separation of the molecule in Icm 3 of liquid vs the action of intermolecular attraction 5 . The value of CEO can be calculated using the following relationship: CEO = 0 2 = (I1Hv - RDIV m (I) where 0 is the solubility parameter (cal/cc) 112; I1Hv, the molar heat of vaporization (cal/mol); R, the gas constant (cal/mol deg); T, the temperature (K); and V"h the molar volume (cm 3 /mol)
Transcript
Indian Journal of Fibre & Textile Research Vol. 27, June 2002, pp. 199-210
(? ~ _ rl \) Review Article
Solvent-induced modifications in poly (eth lene terephthalate) SJ!:u~ropertit: . an dyeabihty -- ---
- ~--
, LQfEhidambaram" & R} enkatraj1 I
) Department of Textile Technology, PAC Ramasamy Raja Polytechnic College, Rajapalayam 626108 India
- I and
P a nisankar\
' Department of Industrial Chemist~, Alagappa University, Karaikudi 630003, ndia
Received 16 OClOber 2000; revised received and accepted 12 March 2001
Poly (ethylene terephthalate) fibre (PET) is the commonly used man-made fibre for majority of end-use applications. In thep ast few decades, the significant technological developments have taken place pertaining to the production processes of PET. Regarding the dyeing of PET, there is a constant upsurge in research and development activities to modify the structure of PET with a view to develop energy efficient processes. Among the notable developments, the low molecular weight solvent treatment results in physical modifications of polymer and fibre like PET, which include structural modifications and changes in properties. Owing to the structural modifications of PET fibre, the dyeability also improves. Some of the pioneering works on solvent-induced modifications of PET structure, properties and dyeability during the past few years have been reViewed ]
1 Introduction The use of organic solvents for structural
modifications in textile substrate has become quite popular in recent times. For example, in the solution spinning routes the solvents are used to produce highly oriented material, possessing high strength and/or high modulus. In the post-spinning operations of solution-spun fibres, the textile substrates are kept in the swollen state for the structural modifications. Because of the swollen state, the post-spinning operations are more efficient and energy saving I.
Poly (ethylene terephthalate), commercially known as PET, has been widely used in industrial and commercial applications2 because of its desirable qualities such as high strength, good handling, easy care properties, etc. However, PET has certain undesirable qualities like lack of hydrophilicity and accumulation of static charges. PET fibres generally exhibit low surface energy and limited chemical reactivity, resulting in poor moisture absorption. Usually, the dyeing of PET is carried out at I30°C under high pressure, which is energy intensive3
•
" To whom all thc correspondence should be addressed . Phone: 0091-04563-522940: E-mail : dchidambaram @rediffmail.com
Because of the increasing energy cost, the energy efficient processes involving interactive solvent media preferably at low temperature are gaining popularity at present. It has been found that the solvent treatments result in structural modification of PET, which opens the structure sufficiently to facilitate subsequent dyeing operations4
. The structural modification of PET depends on the extent of interaction between PET and the solvents used. The interaction between PET and solvents is associated with their respective solubility parameters.
2 Solubility Parameter The solubility parameter is defined as the square
root of cohesive energy density. The cohesive energy density (CEO) is the energy in calories/cc, necessary for an infinite separation of the molecule in Icm3 of liquid vs the action of intermolecular attraction5
. The value of CEO can be calculated using the following relationship:
CEO = 02= (I1Hv - RDIVm (I)
where 0 is the solubility parameter (cal/cc) 112 ; I1Hv, the molar heat of vaporization (cal/mol); R, the gas constant (cal/mol deg); T, the temperature (K); and V"h the molar volume (cm3/mol)
200 INDIAN J. FII3RE TEXT. RES., JUNE 2002
It is observed that the polarity and hydrogen bonding affect the heat of vaporization. Accordingly, Bondi and Simku6 proposed to divide Hv into following two terms:
Wv = f).HVd + f).Hv h (2)
The subscript d and h refer to dispersion energy and hydrogen bonding respectively. Actually, the
second term (f).HVh) contains the contribution of dipole interaction. On the basis of the above approach, Hansen 7 suggested the separation of solubility parameter into following two terms:
(3)
(4)
The subscripts d, p, h and a represent dispersion energy, dipole interaction, hydrogen bonding and association energy respectively . Originally, Sma1l8
pointed out that the dipole interaction (8,,) is
negligible. Therefore, the contribution of 8h is justifiable.
Hansen9•
11 proposed that the solubility parameter is a vector composed of the hydrogen bonding solubility parameter (8h), polar solubility parameter (81') and
dispersion solubility parameter (8d) . Using these three components, which are calculated from the properties of the pure components, he constructed a threedimensional solubility diagram in which a given liquid or polymer is represented by a point. Thus, the solubility parameter of a polymer is represented in the
diagrams by the po ints 8dP,8"p,8hP and solvents by 8ds,8ps,8hs (Fig.)). The solvents that lie within the sphere are the true solvents for the polymer. Whereas, the liquids lying outside the sphere show only partial or no miscibility with the polymer.
The solubility parameter (8) value l2. n of PET is reported as 10.7 (cal/cc)"2 and the respective values for some of the solvents '4. '5 are given in Table I. The
non-polar substances have low 8 values. The
substances with high 8 values have very great dipole moments or are capable of forming hydrogen bonds. CEO is an integra. l characteristic of intermolecu lar interaction while the solvent action depends on the presence of specific functional groups with which the interactions are exhibite~1. It includes hydrogen or donor-acceptor bonds which lead to the formation of solvated complexes. The relative solubility also depends on other factors, such as polymer-free volume and size of the penetrant molecule. It is also a function of interaction between the polymer and
bd Fig. I - Solubility parameters of polymer and solvent
Table I - Data on solubility parameter of differenl so lvents
CHIDAMBARAM el 01. : SOLVENT-INDUCED MODIFICATIONS IN PET 20 1
solvent. The degree of interaction characterized by the match between the parameters of polymer and solvent.
3 Application of Solubility Parameter for PET/Solvent Interaction
can be solubility
The application of solubility parameter principle to pred ict the PET/solvent interaction has already been reported . Moore and She ldon 16 have correlated the ability of the solvents to swell and crystallize the ori ented amorphous PET with the total solubility parameter of the solvent. It is observed that the max imum solvent inte raction with PET occurs at two regimes of solubility parameter values , viz. 8 1=9.8 and 82= 12. 1. It was later on concluded that the solvents with solubility parameter of 9.8 (approximately) interact with the aromatic groups of PET fibre while those with the solubility parameter of 12.3 (approximately) interact with the a liphatic groupsl7. It seems that the solvents such as acetone and dimethyl formamide (DMF) have better interaction with PET than other solvents. Further, acetone and dimethyl formamide have been focu sed in mllch of the reports pertaining to the structural modi fi ca t ion of PET. 18·20
In the recent times, many authors have reported the interac tion between PET and various solve nts in te rms of the ex tent of swelling, ge lation and me lting point depress ion and solubility parameter values. 2 1
-23 The
decreasing order of inte rac tion between PET and various so lvents is shown in Table 2.
The studi es24-26 conducted on solvents, such as
acetone, methylene chl oride, dioxane and nitromethane indicate that these so lvents are quite effect ive in induc ing struc tural modifications in PET. Knox e/ 0 1.27 reported that the solubility parameters of solvents with most pronounced effec t a re 10.0 and 12.1 . From Table 2, it is ev ident that the so lvents, sllch as tetrachloroethane, dichloromethane and nitrobenzene, have better inte raction than DMF.
Hence, better structural modifications can be achieved using these solvents with PET. It is interesting to observe from T able 1 that the solubility parameter of nitrobenzene is very close to that of PET. Also, the
dispersion solubility parameter of nitrobenzene (8d-
9.73) is close to the 81 regime of PET. This means that there is a better inte raction between aromatic groups of PET and nitrobenzene.
4 Solvent-Induced Crystallization (SIN C) When a highly interactive solvent diffuses into the
polymer (film/fibre), it results in the increased mobility of the polymer segments . This segmental mobility permits large scale structural rearrangements which lead to solvent induced crystallization (S INC) of an initia lly amo rphous polymer and even in the case of semi -crystalline polymers such as fibres ?8.29 The rate of formation of crystallites depends on the conditions such as the type of polymer, temperature, solvent used for crystallization, the method of crystallization induction and the stress applied. These parameters also determine the morphology of the crystall i ne structure.
The structural modifications of PET induced by SINe can be resolved by the following techniques : dens ity method, X- ray diffrac ti o n method , thermal method, infrared spectroscopic (IR) method, and scanning e lectron microscope (SE M) .
SEM is used to characterize the surface morphology of solvent-induced po lymer. The density and thermal methods are adapted to measure the crysta llinity changes due to SINe. The X- ray and IR methods are used to measure the degree of c rystallinity and the molecular orientation in SINe for PET.
5 Specific Work on Polyester
5.1 Density Measurements
The dens ity measurements are used to assess the crystallization kinetics o f SINe process. When PET
Table 2 - Interacti on betwccn PET and various solvents
Solvcnt
NB > DMF > DMSO >TMU > Acctonc > AN > Su lfol anc> HMPA
Tctrachl orocthanc> Dichlorocthanc > DMF > Tri chl oroethanc
film/fibre is treated with a strong interacting penetrant, it results in the increased mobility of polymer segments in the amorphous regions of PET. Thus, the crystallites develop and result in change in sample density. The relative change in sample density reflects a change in volume crystallinity (j'). On the basis of two-phase model for solvent crystallized PET, the volume fraction of crystalline material can be given by the following relationship:
d -d P = 0 (j x 100 (5)
d e -d a
where do is the sample's density; da, the pure amorphous polymer density; and dc, the polymer crystal density. For PET24 da =1.335g/cmJ and de =1.510 g/cmJ.
The change in r with time results from two coupled processes, i.e., penetrant transport and spheru lite growth . If the rate of crystallite growth exceeds the rate of penetrant transport into the sample, then the overall crystallization process is transport controlled. Alternatively, if the crystallite growth is slow and the penetrant transport is rapid, then the crystallite growth controls the crystallization process. In other words, if the change in f increases linearly faster than --Jt, then it is called as spherulite growth controlled crystallization process (Fig 2).
The study of Kolb and IzardJO on crystallization of PET films (0.02crn) in interactive liquids shows that the penetrant transport controls the crystallization
Sh Id L 31 3? . process. e on et a .. ' - measured the change JJ1
density of 0.02-0.04cm thick films treated in solvents, like benzene and several liquid ketones. They observed that the solvent transport controls the extent of induced crystallization as the solvent uptake and the sample density are linearly related and the density increases linearly with time l12 (--It) between the solvent and sample.
Investigations pertaining to PET/solvent system on spherulite growth have been reported in the literature. The estimation of A varami exponent provides qualitative information on the nature of nucleation and growth processes in SINe. Avarami exponent (a) can be calcu lated from the slopes of plots as g iven below:
log {-In [l-r(t)lj~]} vslogt (6)
where J and 10 are the instantaneous and final values of J. Table 3 shows the values of a for various types of nucleation and growth .3J
Zachmann and Konrad34 studied the change in volume crystallinity (f') in PET/methanol system by
swelling the films in methanol below the glass transition temperature (Tg) and then by raising the temperature above Tg. The analysis of data shows an A varami exponent (a) of 3, suggesting the development of nucleated spherulite crystallites. Durning et aL.3\eporled that the film thickness and the treatment temperature playa crucial role in the crystallization kinetics of PET/solvent system. In their work on sorption of methylene chloride by PET films of 0.086cm thickness (38°C), the linear relationship of volume crystallinity and contact time (t) suggests that the penetrant diffusion controls the crystallization process. For 0.030cm thick films, as the temperature decreases from 380 -oDe, the crystall ization process changes from penetrant transport towards spherulite growth. On 0.002cm thickness at all the temperatures, the Avarami exponents (a) of 2.5 and 3 have been observed, suggesting that the spherulitic growth controls the crystallization process.
The effect of orientation and prior crystallization on the SINe of PET was reported by Jameel et aL. 36 It has been observed that the solvent d iffusion and the extent of SINe decrease with (he increase In orientation and crysta ll i n i ty in the starti ng fi I ms. In the case of heat-set or drawn PET, the sorption of solvents into the polymer matrix is hindered by the tie points (crystallites) and hence the consequent increase in volume becomes less.
Thus, the inclusion of solvent molecules among the polymer chains greatly increases the segmental
>... ~ !'l ~ <> .,
Spherulite growth
5 Penetrant transport controlleO
~~~------------------------~ Ttme (JI)
Fig. 2 - Schematic representation of penetrant transport and spheru lite growth controlled crystallization
Table 3 - Values of Avarami exponent (a) for differe nt types of nucleation and growth
Nucleation and growth
Rod-like growth from instantaneous nuclei
Avarami
expone nt (a)
Rod-like growth from sporadic nuclei 1.5
Disc- li ke growth from instantaneous nuclei 2
Disc-like growth from sporadic nuclei 2.5
Spherulitic growth from instantaneous nuclei 3
Spherulitic growth from sporadic nuclei 3.5
CI-II DAMBARAM el al.: SOLVENT-INDUCED MODIFICATIONS IN PET 203
mobility of the chains and allows suitable juxtaposition of segments for crys talli zation to occur at temperatures well below than those required for crystalli zation of dry amorphous polymer. In SINC, each pl aner unit of polymer experiences a different time of crystalli zation because the polymer pl anes that have been penetrated by the solvent molecules can only undergo crystalli zation. The extent of crystalli zati on in SINC depends on the in teracting power of the so lvent with PET, materi al thi ckness, pre-ex isting orientati on and crys tallinity of PET, and the treatment conditions such as time and temperature.
5.2 Melting Behaviour
The interaction of the polymer with the so lvent is strongly influenced by the structural parameters. In general, the sol vent enters in to the polymer structure, replaces and weakens polymer-polymer interaction with polymer so lvent interac tion. This induces extensive segmenta l mob ility, and lowers the melting temperature and the glass transition temperature of the materi al. The degree of thi s depression depends on the amount of solvent and its interaction with the polymer. Flory37 has presented the foll owing quantitative relationship between the melting point (Till) of a semi-crys talline polymer and the amount of diluent in the amorphous phase.
R V 2 (¢I - X¢I ) 2 ---= (7)
where Tillo is the melti ng temperature without so l vent ; VI and Vz, the molar volumes of solvent and polymer repeat unit respecti vely; X, the Flory-Huggi ns interaction parameter; <PI, the volume fracti on of the solvent ; and I1 H2, the heat of fusion per mol of repeat unit of polymer
A number of equations have been reported to describe qu antitati vely the lowering of the polymer glass transiti on temperature in the presence of solvent. These are based on the assum ption that the solvent acts as a plasticizer fo r the non-crystalline domains, break ing the intermolecul ar bonding, whi ch results in the enh anced mobility of the polymer segments. Gordon and Tay lor38deduced fo llowing relati onship between Tg depression and so lvent uptake:
KWl~, + WJ.~2 =
KW, + W2 T (8)
The subscripts I and 2 represent the solvent and polymer components respecti vely; and WI and Wz, the
weight fractions of the two components. The parameter K is determined by the difference between the expansion coeffi cients ( ~) of the glass and melt of the two components.
11/32 K=-11/3,
(9)
The interaction of the so lvent with the polymer may be of two types, i.e. intercry stalline and in tracrys talline. In the case of intercrysta lline interaction, the solvent penetrates inside the amorphous regions onl y. The polymer chains wi thin the non-crystalline regions are under lower stress and when the so lvent interacts, it results in the rearrangement of the molecul ar chains. This induces crystalli za tion to take place in the swollen state27 On the other hand, in case of intracrystalline interact ion, the solvent penetrates inside the crystalline region, decrystallizes the sample and affec ts the hi gher lateral order of the fi bre35 .
In general, the PET can be quenched fro m the melt to produce a material which is amorphous at room temperature39. Under appropriate conditi ons, the polymer molecules transform from the melt to a folded chain conformati on and then undergo enhanced lamell ar thi ckening and fin all y transforms . d d h . I 40.4 I Into exten e c am crysta s .
The solvent treatment on PET results in max imum swelling of small er crys tallites and parti al swelling of bigger crysta llites. Swelling is not merely the penetrati on of solvent molecules into the polymer phase but also in volves fillin g of the cav iti es or pores of the polymer. Further, it in volves a change in polymer structure which increases the vo lume of the sa mple. If the so lvent molecules penetrate in to the interstructural space, it causes super molecu lar st ructures of polymers to pry apart (Interstructural swelling). On the other hand , if they penetrate into the structures, macromolecules are pried apal1 (I ntrastructural swelling)42. The ability of the polymers to swe ll is determined by number of factors, such as chemi cal nature of polymer and solubility parameter of solvents, fl exibility of polymer chain , pack ing density of macromolecules, treatment environments, etc. The swelling and then the segmental mobil ity of the polymer chain molecules induce crystalli zat ion in the swollen state with the formation of more stable crystallites . This could result in a shi ft of melting endotherm to a higher temperature43 .
Considerable work44.45 on PET fibres treated with
various so lvents by the DSC analys is has been
204 INDIAN J. FIBRE TEXT. RES., JUNE 2002
reported; the appearance of a premelting peak before Trn is worth mentioning. Weigmann et al.44 have shown that depending on the nature of the treatment and temperature, crystallites are formed in PET fibres with varying sizes and thermal stabilities. Initially , PET fibres were treated in presence of DMF followed by heat treatment in silicon oil at different temperatures. In DSC thermogram, the presence of crystallites was observed in the form of small premelting peak prior to Tm. Hence, DMF treatment produces crystallites, in addition to voids, with a wide distribution of sizes.
Later on, Weigmann et al. 45 studied the treatment of PET fibres with DMF after heat setting in silicon oil bath. It has been observed that the premelting peaks produced by heat treatments at temperature below 180a C are eliminated by subsequent DMF treatment. This suggests that the crystallites of low stability produced during heat treatments melt out by the solvent treatment. The stabilization of these crystallites occurs only when the applied heat treatments are carried out at higher temperature.
The change in morphology of heat-set PET fibres subjected to benzoic acid treatment was studied by Simal and De Araujo46 with the help of DSC studies. The experiments revealed the appearance of premelting peak (pm 1) at around 130aC when the heat setting was carried out for 8 h in the presence of boiling water. These premelting peaks could be associated with the melting of smaller and imperfect crystals present in the amorphous region . Also, the DSC analysis of the benzoic acid treated PET fibres revealed that the pm I is transformed towards the higher temperature at around 180a C (appears as second premelting peak; pm2). Therefore, this second premelting peak is rel ated to the morphological changes due to the benzoic acid treatments. Thi s suggests that the pm I, related to small and imperfect crystals, is being transformed to pm2, which is related to more perfect ones, by the solvent treatment.
Thus, the solvent treatment on PET induces the crystallization of non-crystalline domains, increases the relative size of the crystals by increasing the crystalline perfection and alters the crystalline region of PET towards higher melting temperature. These structural changes on PET due to the so lvent treatment depend on the interaction between PET and respective solvents, thermo-mechanical prehi story of PET, and treatment conditions, such as treatment time and temperature.
5.3 Surface Morphology
When the unoriented amorphous polyester film or fibre is treated with a highly interacting solvent, it may result in extensive cavitation of the polymer surface. This cavitation arises from a combination of anisotropic swelling forces, solvent stress cracking and sphreulite formation caused due to the crystallization process47
. The depth of cavitation depends on the SINC temperature, contact time with the solvent and the nature of the solvent used.
Desai and Wilkes24 studied the surface morphology of PET films treated with various organic solvents. Cavitated, porous surface structures were observed after only brief exposure to interactive solvent whose solubility parameter is close to that of PET. The drastic surface porosity results after very short contact time (5-15 s) and the structure is not influenced by the treatment temperature. Further, Durning et al. 35
characterized the surface morphology of PET films exposed to methylene chloride environments. They studied the internal porosity of solvent-treated samples with fracture cross-sections. The internal porosity appears in thin films (O.002cm thickness) and is reduced by increasing the temperature. It is also revealed that the PET films crystallized by vapours have only minor cavitation. Further, it is postulated that the voids are formed only when the crystallization continues after saturation with the penetrant of the amorphous polymer and the rate of cavitation depends only on the rate of crystallization.
The effect of orientation on the surface morphology of PET, resulting from the solvent treatment over a wide range of temperatures, has also been studied by lameel et al.4B
• With increasing orientation, the solvent treatment causes less and eventually no modification in the surface texture. This effect is due to the fact that the stabil ization of the film structure by the orientation and crystallization is achieved during drawing. 1m and Lee20 reported the effect of draw ratio on SINC of PET with DMF and dioxane systems. It has been found that nearly perfect crystals are produced in the surface cavitated region . A new nucleation does not take place in the internal region and the extent of interaction decreases with the increase in initial draw ratio and increases with the increase in treatment temperature.
Makarewicz and Wilkes lB reported that with PET/ methylene chloride and PET/dioxane solvent systems, extensive internal cavitation is deve loped in PET films. The cavitation was observed upto the centre of PET film with no permanent voids in the film. It is
CHIDAMBARAM el al.: SOLVENT-INDUCED MODIFICATIONS IN PET 205
suggested that the high degree of swelling and crystallization rate of PET is responsible for this cavitation effect. After the solvent molecules reach the centre of film with sufficient concentration to induce crystallization, the material in this region begins to crystallize slowly. As it crystallizes, PET continues to sorb the solvent and swells until it reaches the saturation. The combined effect of initial slow crystallization and continued swelling causes the material to meld together with the surrounding swollen and crystallized layers.
Although in oriented PET fibres the internal fracture has not been observed after treatment with various solvents29
, a cylindrical void was observed at the center of the low denier, amorphous and as-spun PET fibres due to the treatment in methylene chloride. Ito et al. 49 studied the surface morphology of PET fibres treated with DMF/water mixture using SEM technique. They reported that the fibres treated with solvent containing mixtures of lower water fractions show a significant surface roughening which increases on decreasing fraction of water. It is suggested that this surface roughening is due to the partial dissolution of the PET at the surface and not due to the molecular degradation.
These studies indicate that the variations in morphology can be produced by using different solvents. The depth of cavitation or void formation primarily depends on the closeness of the solubility parameter values. As discussed earlier, the closer the solubility parameter values, the higher will be the interaction of solvent on PET and hence the void formation will be more. Controlling the parameters, such as pre-existing orientation, crystallinity, thermal prehistory and treatment conditions, wide range of surface morphologies can be produced on PET by solvent treatment.
5.4 X- ray Analysis X-ray diffraction studies are also widely used in
determining the crystallinity changes in PET due to the solvent treatment. Sheldon and Blanke/o studied the crystalline structure of solvent (acetone) treated PET and reported that the acetone has no influence on the crystal structure, but the wide-angle X ray diffraction patterns of the samples exhibit broad reflections, suggesting a smaller crystallite size.
Ito et ai.51 studied the solvent (acetone) induced modifications in PET fibre with the help of X- ray analysis. Though the fibres treated for short immersion time show only an amorphous halo, the
crystalline reflections are observed for the fibres treated for long duration. When most of the solvent is removed from the treated fibres (long duration) , the diffraction ring becomes sharp. These results suggest that in presence of acetone, the solvent-induced crystals are less perfect and/or their sizes are very small. With decreasing amount of solvent trapped in the fibres, the solvent-induced crystals increase in their perfection and size.
The effect of orientation and crystallinity for the solvent (DMF) induced crystallization of PET was investigated by W AXS and SAXS analyses.48 It has been reported that due to thermal annealing, there is an increase in crystallinity and decrease in orientation for high draw ratios of untreated PET films . Further, DMF treatment produces a better crystalline structure than that of heat treatment. In the case of highly drawn films, both the thermal and solvent treatments at high temperature cause considerable crystallite disorientation. Solvent treatments at lower temperature cause less disorientation and also a less well-developed structure. The crystallite size increases with the increase in temperature of DMF treatment. Further, it has been observed that the void content is more for solvent crystallized PET of low draw ratios . At low draw ratios, the swelling is more and a greater void volume results than in highly drawn PET where the existing crystalline structure limits both the extent of swelling and change in crystallinity.
5.5 Mechanical Properties
The practical utility of any process that induces structural modifications in a material is largely dependent on the mechanical behaviour of the material. The effect of solvents on the mechanical properties of polyester fibres requires a structural model that quantitatively describes the relation between its structure and mechanical properties. In PET, generally the semi crystalline polymers have been considered as two-phase systems in which the crystalline domains are dispersed in amorphous matrix . Prevorsek et ai.
52.53 have postulated that the
oriented amorphous domains consisting of more or less extended polymer chains exist between the microfibrils (Fig. 3). These extended amorphous regions are considered as the essential elements of fibre structure with respect to tensile behaviour. It is these domains that are plasticized and restructured as a result of PET-solvent interaction and have a major effect on the properties of the sol vent-treated fibres.
The exten t of plasticization depends on the interacting power of the penetrant with PET closeness of the solubility parameter values and the treatment environments.
Ribnik el 0[. 54-56 studied the mechanical behaviour of so lven t treated, oriented semi crysta lline PET fibres. It has been reported that the increase in shrinkage percentage of solvent-treated PET fibres, the depression of fibre's initial modulus and yield stress are due to the increase in segmental mobility of polymer chains caused by so lvent treatment. The strong interaction between the polymer and the so lvent is responsible for the inducement of the plastici zation or increased mobility in the amorpho s regions. In most cases, the so lvent molecules do not penetrate into the compact crysta lline regions in polymers and, therefore, do not affect their strength significantl y. In some cases, these are capable of penetrat ing into the crysta lline structure of PET after attack ing the amorphous reg ion and break down the pol ymer chain s, resu lting in sign ificant st rength IOSS.57
Further, it has been observed that the st ructural modifications in PET due to the so lvent treatment led to an increase in break ing ex tension values58 The phenomenon of fibre extension is manifold, which is controlled by the confi gurati on of chai n molecules. Its energy content and the number of chemica l bonds hold them together in the amorphous reg ions of fibre. The chai n molecules in the fibre are held together by lateral forces such as cova lent bonds, hydrogen bonds
and van del' Wal ls' fo rces. The flexibility of the chain molecules depends upon the holding power of these lateral forces. Solvent treatment on PET does relax these lateral forces to render flexibility to the chain molecules, thus results in higher breaking extension.
Cuddihy59 in hi s study on biaxially oriented, se micrystalline PET films observed that the certain liquid environments, such as 2-butanone, could reduce the yield stress of the material. He determined that thi s sensitivity of the PET in certain liquids depends on the mechanical anisotropy of the PET films, attributed to environmental stress cracki ng caused by liquids.
Sheldon and Hughes60 found that in acetoneinduced crystallization of PET, the last traces of acetone are not driven off until the mel ting point of PET is reached. This hypothesis was further confirmed with the observations made by Zachmann and Eichhoft1J
/ who showed that traces of acetone penetrate and become trapped within PET crysta llites, if PET is exposed to the liquid environment for a long duration . Makerew icz et 01.62 reported that both Young's modulus (£) and yield st ress (0 ) values of solvent-treated PET fibre decrease with the increase in immers ion time of acetone and dioxane. They attributed this behaviour to the effect of the residua l liquid trapped in crysta lline domains behaving as a plasticizer and thus induces segmental mobility to decrease both £ and 0 . It is also reported that the annealing of PET at 70°C prior to solvent treatment (24°C) tends to minimize the effect of liquid on the mechanical properties. Thus, it is conc luded that the mechanical properties of solvent-treated PET not onl y depend on the resid ual liquid trapped but also on the thermal pre-hi story of the polymer.
Ito el 01.51 reported the effect of solvent treatment
on the drawing behaviour and mechanical propert ies of PET. In their work , the achievab le draw ratio by the first stage cold drawing of fibres treated wi th acetone was about 40% higher thall that of the untreated fibres. ]t is suggested that the improvement in co ld drawability of solvent-treated fibres is due to the ex istence of sma ll and/o r less perfect crysta ls induced by the so lvent treatment and by the plasti cization with the res idual liquid trapped within the amorphous regions. Further, the sma ll crysta llites induced by the solven t treatment act as network po ints fixing the ends of the plasticized amorphous segments. This results in hi gher deformability without signi ficant reduct ion in strength . By the second stage hot drawing, the draw ratio of treated fibres is 20%
CHIDAMBARAM et al.: SOLVENT-INDUCED MODIFICATIONS IN PET 207
still higher. Such drawn fibres with high crystallinity exhibit high modulus and high strength values with higher structural stability. During hot drawing of solvent-treated PET fibres, the residual solvent is removed, the solvent-induced crystals increase in their perfection and size, and the stress-induced crystallization proceeds. Thus, the solvent treatment enhances the development of crystalline phase during drawing. This results in reduction of ductility and increase in strength values with good structural stability .
5.6 IR Spectral Studies
The IR vibrational frequencies of a polymer molecule depend on the inter-atomic bond strengths and valence angle. When the bond angle is reduced, the bond angular strain increases. Hence, the frequency of the absorption band increases. If the bond angle is pushed towards the higher value, the opposite effect operates. Mocherla and Statton63
studied the stress-strain behaviour of oriented PET by IR technique to correlate the molecular bond in PET with stress behaviour. It has been reported that of all the skeletal IR bands, the IR band at 973 cm-I is more suitable for investigating the stress-strain relationship of PET. This band represents the trans isomers of PET that are associated with either crystalline links or with extended lamellar links.
The Fourier transform Infrared studies are used to evaluate the trans and gauche contents in solventtreated PET fibres. The trans conformations are related to the straight parts of the molecules in the amorphous and crystalline regions and gauche conformations are related to the distorted parts of the molecules within the amorphous phase only64. The changes in the absorption ratio between the trans and the gauche conformations (T/G) are used to quantify the conformational changes due to the solvent treatment. The bands at 1473, 1343,973 and 848cm·1
refer to vibrational modes of trans ethylene glycol segment of the polymer chain65 and the bands at 1453, 1372, 1042 and 898cm,I refer to the vibrations of gauche ethylene glycol segment of the polymer chain66
.
The morphology of heat-set and solvent-treated PET fibres has been studied by Simal and De Arauj 0 67
Llsing FTIR studies. The bands at 973 cm· 1 and 898 cm·1 were chosen to estimate the structural absorbencies of trans and gauche conformations. The dichroic function of amorphous phase (DFa) and percentage T/G ratio were calculated to investigate
the structural modifications in solvent-treated PET fibres . The heat-set PET fibres showed low DF" values and higher T/G ratio than the control samples. This suggests that there is a decrease in amorphous orientation and increase in crystalline content of PET yarns due to the heat setting conditions. When the heat-set fibres are subjected to solvent treatment, both DF" and T/G ratio decrease, suggesting the decrease in amorphous orientation of PET yarns. The decrease in T/G ratio can be related to the change in trans conformation not only in the amorphous region but also in the crystalline region as mentioned before. But, it has been observed that the crystalline parameters increase continuously on heat setting and by subsequent solvent treatment conditions using xray and DSC studies. Hence, the decrease in T/G ratio of solvent-treated PET fibres could be related to the decrease in trans conformation in the amorphous region only. Further, swelling is one of the contributing factors for disorientation of the amorphous phase of PET. Hence, the increase in swelling percentage, as explained before, would be favouring the disorientation of amorphous phase by solvent treatment.
5.7 Dyeing Behaviour
PET fibres are hydrophobic, having a compact structure and are semicrystalline. Besides this, the stiffness imparted to the chains by the phenyl residues of the terephthalate group results in higher Tg value. Normal PET fibres neither contain basic groups nor strongly acidic groups. Hence, they are not generally dyeable with ionic or cationic dyestuffs. The important class of dyestuff used for the coloration of PET is disperse dyes68
. It is very difficult to achieve satisfactory dyeing results without the use of high temperaturel pressure (HT/HP) or carrier chemicals. The HT/HP dyeing involves high energy consumption and the use of carriers is said to have lot of disadvantages. An alternative approach to achieve dyeing process at atmospheric pressure is to modify the structure of PET using solvents. The solvents can be used to improve the transport of dye molecules into the structure of PET in two ways: (i) the solvents can modify the fibre structure in a temporary way either by using it as an addition to the dye bath or as the major dyeing medium, and (ii) the solvents can be used to modi fy the fibre structure by a pretreatment process. This solvent pretreatment on PET should be carried out under the conditions that are environmentally acceptable and with the cost effective technique over the current dyeing procedures.
208 INDIAN J. FIBRE TEXT. RES., JUNE 2002
Solvent treatments result in lowering of Tg value of PET and leave a much more open polymer structure, thus resulting in improved dyeability. The solventinduced modifications in PET with respect to dyeing behaviour have been reported in the literature. An attempt is made bere to illustrate PET/solvent interactions with few examples.
According to Bhattacharya69, PET can be dyed
rapidly and cost effectively by a new technique that utilises PET which has retained 4-10% of perchloroethylene (pe E) from a continuous or batch scouring treatment. The scouring with 100% PCE was carried out at 94-120°C for 30-200 s. The residual solvent acts as a plasticizer, thus enhancing the segmental mobility of the polymer segments. This improves the dyeability. Weigmann et al.7°studied the interaction of non- aqueous solvents with textile fibres. They found that the treatment of PET fibre in DMF (140°C for 2min) produces a marked increase in the rate of dyeing, which appears to generate voids in the fibre structure. It is suggested that the dye absorption percentage in fibres with a porous structure depends on the internal surface of the fibres and/ or the ability of the dye molecules to form agglomerates within the pores. This porous structure developed by the solvent treatment has limited thermal stability. The thermal after treatments7
1. 72 can be carried out to collapse the structure after the dyestuff is introduced, thus trapping the dyestuff molecules and enhancing the fastness properties.
Basu el al. 73. studied the effec t of sol vent treatment on the dyeability of PET. They observed that the solvent pretreatment brings out the intermolecular changes, which may be exploited for achieving higher dye uptake, and also for carrying out disperse dyeing at low temperature and less time. The effect of solvent-water mixtures on dye uptake of PET has already been investigated under different pretreatment conditions. It has been observed that under appropriate conditions of pretreatment, the solventtreated samples give 15% more colour yield as compared to the control samples. The solvent pretreated samples dyed at 130°C for 30min have shown the same colour yield as that of control samples (dyed at 130°C for 60 min).
Samanta et aC4 observed that without sacrificing the tensile and other mechanical properties of PET, there is some increase in breaking extension and the disperse dye uptake reaches a desirable level at 100°C for pretreatment with m-cresol, dichloromethane, nitrobenzene and phenol at their optimum concentnl-
tions in either aqueous or non-aqueous solutions. Ratajska 75 studied the effect of unsaturated halogenated hydrocarbons, such as CC1 2CHCI, CCI4,
CH3CCh and CI3CCF3, on the supermolecular structure, sorption and dyeing properties of PET. Saturated halides do no affect the sorption properties. Treatment with CCI2CHCI and CCI4 reduces the glass transition temperature of PET fibres.
The solvent treatment of PET involves two distinct processes. Primarily, SINC occurs due to the generation of new crystallites in the amorphous region of PET. The presence of such crystallites in the amorphous region of PET not only acts as a barrier to the dye molecule movement but also reduces the accessible space for the dye molecu les to penetrate into the structure. On the other hand, the solvent treatment creates voids in PET and this can break the intermolecular bonds and relax the residual orientations, thereby reducing Tg value. The increase in dye uptake could be related to the above phenomenon.
It has been observed76 that the plasticizing effect is predominant compared to swelling effect of solvent. During plasticization of fibre, the chain molecules move past each other, enhancing the segmental mobility . Whereas during swelling only the comparatively weaker bonds between the chain molecules are broken . The increased mobility of the chain segments due to the plasticizing action forms the bas is for improving the diffusion of dye molecules inside the fibre structure, giving more dye uptake values.
Thus, the solvent treatment on PET can modify the . fl. II· 77·83 structure 111 anyone 0 t le.o oWll1g ways :
• plasticization of amorphous region results 111
loosening and packing of polyester molecules 111
the amorphous region. • rupture of crystallites causes defect between the
crystallites and increase in the mobility of the polymer chain segments in the non-crystalline regIOn.
• initial disruption of crystallites reduces the accessible spaces or voids between the crystalline units available for diffusion and thereby increases the tortuosity of voids.
• melting or dissolution of imperfect crystallites increases the total amount of non-crystalline material.
• melting and recrystallization of imperfect crystal I i tes.
CHlDAMBARAM et al.: SOLVENT-INDUCED MODIFICATIONS IN PET 209
• plasticization of ti e molecules.
• crysta llization of non-crystalline domains. Solvents induce very high nucleation rate with the formation of high concentration of small crysta llites.
• increasing the re lative s ize or tota l void content of the polymer matrix due to the Increase 111
crystalline perfection
6 Conclusions The solvent treatment of poly(ethylene
terephthalate) film / fibre produces ex tensive structural modifications. The effects include induced crysta ll izat ion, swelling, vo id formatio n and pl astici zation . The changes effected in polymer matrix depend on the closeness of the so lubility paramete r values of PET and solvents . The c loser the solubility paramete r va lues, the hi gher will be the interaction of solvent in PET. Wide range of surface morphologies can be produced by controlling the pre-ex is ting ori entation and crystallin ity in PET, and treatment conditions. Since the plasticization effect is predominant in solvent induced modifications of PET, it results in higher dye uptake and improvement in breaking ex tension without significant reduction in strength . There ex ists a wide scope for research to investigate the structural modifications in PET using solvents, such as nitrobenzene, tetrachloroethane, dichloromethane, etc ., which have better interacting power with PET. Investigations can be carri ed o ut to improve the interacting power of solvents at low temperatures to induce structural modificatio ns in heat-set PET fibres. The re tenti on and removal of solvents from fibres is an important aspec t of no naqueous fin ishing for in-depth in ves ti gati on.
In man-made fibre wet process ing industry, the use of organi c solvents has been widely accepted . The solvent ass isted dye ing vis-a-vis pre trea tment o f PET with solvents for subsequent dyeing processes wi ll ga in its popularity because of the energy sav ing aspect and minimizing water po lluti on. However, the tox ic ity of the solvcnts and the retentio n of solvent in PET are the inherent d isadvantages to be cons idered while se lec ting the so lvents for the processes . Hence, there ex ists considerable scope for the use o f solvents fo r PET fibre for commercia l utili za ti on.
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