The Structural Causes of Variations in Dyeing Properties of Terylene Yarn Subjected to Dry and Wet Heat

142 JSDC APRIL 1972; WARWICKER

Com m u n icat ions The Structural Causeswof Variations in Dyeing Properties of Terylene Yarn

Subjected to Dry and Wet Heat J. 0. WARWICKER

The Cotton, Silk and Man-made Fibres Research Association, Didshury, Manchester M20 8R X

The dye uptake of Terylene (TCT) subjected to dry and live-steam heat can be characterised by a constant defined as the uptake divided by the square root of time (A / [* ) , as in equations for diffusion. This constant is found to decrease to a minimum with increase in temperature of dry heating and then to increase to a value greater than that for the unheated yarn at high temperatures of preheating. Steam preheating also affects the constant and, at the higher temperatures of steam heating, shows similar tendencies to rhose found for dry heat but to a less extent. These results were obtained under conditions that allow free shrinkage during dyeing. However, if dyeing is carried nut with the material under tension, although the general tendency is the same, the dye-uptake constant is greater than that for the corresponding samples dyed in the absence of tension and is greater than that of the unheated yarn for most of the samples. Measurement of parameters indicative of structural changes in fibres, e.g. moisture regain, density, X-ray orientation and X-ray lateral order, shows that these variations in dyeing properties cannot be explained on the basis of a fringe-micellar network theory of structure. An explanation is advanced which postulates a structure with rod-like morphological units separated by narrow

voids, the size, amount and tortuosity of which govern the accessibility of the dye.

Introduction Heat treatment of Terylene (ICI) and other polyester fibres, in

yarn or fabric setting and other setting treatments such as texturing, can lead to variations in the dyeing properties of materials if the heat treatments are not carried out carefully.

Marvin (I) was the first to show that, over a range of setting temperatures, the uptake of disperse dyes by heat-set Terylene initially decreased as the temperature of preheating was raised. However, at higher temperatures the dye uptake increased with temperature and could be greater than that of the untreated control. Similar effects were noted for other polyesters and other dyes by Meunier et al. (2) and by Hallanda et al. (3), who also showed that the extent of the variation could depend on the dye itself.

Most of these observations were in terms of an uptake of dye in a given time of dyeing under standard conditions, i.e. they were a measure of the rate of uptake. Merian et al. (4), however, demon- strated that similar effects could be found when the equilibrium absorption values were considered. These authors also confirmed that other indications of the rate of dyeing, e.g. the diffusion coefficient of the dye within the fibre or the time of half-dyeing, varied correspondingly. The temperature of the dyebath was also important. On the structural side of the problem, Merian et al. determined the heat of dyeing, the heat of solution, and the entropy of dyeing for heat-set material. These quantities gave corresponding minima or maxima, so that it was suggested that the minimum tinctorial yield on heat setting was due to an increasing transformation of the fibre structure, and a preferential loosening of the unset fibre by swelling agents in a heated medium. The exact nature of these processes was not, however, defined.

Salvin (9, in attempting to explain similar effects of heat setting on dye uptake, emphasised that both the size of the dye molecule and the size of the accessible regions within the fibre were important factors. Migration of the dye within the fibre also is important in obtaining level dyeings (6, 7). Beckmann and Langheinrich (7), in surveying the general dyeing behaviour of heat-set polyesters, attempted to give a measure of the variation in dyeing properties by defining a barrC parameter b. Study of this parameter should show the migration of the dye within the fibre, the effect of the size of the dye molecule and the effect of the structure of the fibre could be modified by altering the dyeing conditions, e.g. use of carriers or of high temperatures.

Sevcral attempts have been made to explain some of the varia- lions in dyeing propertics in terms of structure. Andriessen and van Soest (8) explained the known effects by the use of a two- phase theory of structure involving a crystalline and a so-called

amorphous region. On this theory, competition between crystalli- sation (with a reduction of the rate of dyeing) and a disorientation of the amorphous regions (leading to increased dye uptiake) explained the variation of dyeing behaviour with heat setting.

Dumbleton e f at. (9) studied the effect of heat setting at different temperatures and draw ratios on the diffusion of a disperse dye in a polyester, and related this to measurements of the dynamic loss modulus of the materials. It was suggested from a consideration of these results that the diffusion is controlled by the mobility of polymer-chain segments, which could be indicated by a measurement of the glass-transition temperature, T,. Both the diffusivity and the dye-saturation values depended on the differ- ence between the dyeing temperature and Tg. This is an important observation, because T, itself depends on the orientation, crystallinity and other structural features. As a result of these studies, Dumbleton et al. concluded that dye molecules will penetrate when the chain segments move so as to produce spxes equal to the size of the dye molecule. The larger the dye molecule the higher must be the dyeing temperature to permit the formation of spaces of sufficient size by segmental motion of chains.

It is now becoming clear that, if the dyeing variations found with heat-set polyesters are to be overcome, a knowledge of the structural causes is highly important However, it must not he overlooked that industrial dyeing difficulties, even with heat-set materials, can be due to other factors than those produccd in the heat processes, and these must be distinguished from variations in dyeing properties of the type discussed here. However. those variations produced by alterations in the structure brought about by heat treatments are potentially controllable and if properly understood should be capable of elimination. I t is the purpose of this paper, therefore, to contribute towards an understanding of the structural causes of the variations in dyeing properties found in heat-set Terylene.

Experimental YARN

Terylene 75-den, 36 filaments.

HEAT TREATMENI

Dry Hear (a) The yarn was passed over the heater ( 2 ft long) of a Scr-agg-

Shirley CS 9 Minibulk machine in an ambient humidity of 66",, r.h. and under a tension of 7 g, but no false-twist was impartcd t o the yarn. The temperatures of the heater for the different samples were: 190,200,210,220 and 230°C. Dwell time: 0.05 s. Shrinkage

STRUCTURAL CAUSES OF DYEING VARIATIONS IN TERYLENE 143

of those yarns under the conditions in the dyebath was 10% for the control, 30/,, and virtually zero for all the preheated yarns.

(b) Hanks of yarn were heated in an oven with freedom to relax at 190, 200, 210, 220 and 230°C for 20 min.

Steam Heat The yarn was heated in saturated steam under pressure at 100,

110, 120, 130, 140 and 150°C out of contact with air in a Sander- son steamer.

DYEIN 3 CONDITIONS

Duranol Blue G (C.I. Disperse Blue 26) was found to be a suitable disperse dye to detect structural changes in Terylene. Owing to the difficulty of preparing stable dispersions from purified dye (4, commercial dye was used for the preparation of dyebaths. The dyebath was made up of:

0.5 g Duranol Blue G (Grains) 0.5 g Lissapol D (ICI) 10 ml of 30% acetic acid in one litre Temperature: Boiling point

Two series of experiments were carried out: (1) the yarn was wound loosely on a stainless steel frame and left to shrink during dyeing in a free condition and (2) the yarn was wound on a frame under a tension of 10 g (0.13 g/den) but with each strand separated from its neighbour. Approximately 6-10 m yarn wereused for each experiment and at most only six frames were dyed in each litre of solution; this meant that the dyebath could be considered as virtually infinite.

After dyeing for the pre-arranged time, the frames were removed and the yarns washed, given standard reduction-clear and soaping treatments, washed and then dried in the air. A portion of the dyed yarn that had free access to the dye-liquor was removed from the frame, dried at 110°C and weighed. The dye was extracted in dimethylformamide at a temperature just sufficient to enable all the dye to be extracted, and then estimated spectro- photometrically. In the preparation of the calibration curve, pure dye was prepared by repeated recrystallisation of Duranol Blue G from acetone. All results (Table 1) are therefore recorded in terms of pure dye and each value is the mean of results of duplicate experiments of which individual determinations were within 2-3% of the mean.

TABLE 1 Terylene Dyed with Duranol Blue G-Uptake of Dye

Temperature of pretreatment

(“C) (dry heat)

Dye uptake (g dye/100 g fibre) at various times of dyeing (h)

t 0 . 5 1.0 2.0 3.0 4.0 5 .0 6.0 t*0,71 1.0 1.41 1.73 2.0 2.24 2.45

Dyed free from tension Control 0.069 0.118 0.152 0.174 0.177 0.216 0.254 190 0.059 0.107 0.121 0.171 0.165 0.207 0.204 200 0.056 0.089 0.124 0.172 0.173 0.205 0.216 210 0.062 0.090 0.141 0.188 0.196 0.196 0.255 220 0.085 0.117 0.151 0.181 0.188 0.206 0.222 230 0.072 0.128 0.170 0.206 0.218 0.253 0.282

Dyed under tension Control 0.072 0.106 0.154 0.186 0.198 0.242 0.240 I90 0.072 0.101 0.133 0.151 0.192 0.241 0.259 200 0.075 0.100 0.148 0.190 0.196 0.239 0.269 210 0.078 0.110 0.166 0.229 0.229 0.259 0.300 220 0,095 0.133 0.180 0.230 0.269 0.297 0.325 230 0.089 0.148 0.220 0.254 0.294 0.342 0.391

Characterisation of the Dyeing Process Prolonged dyeing with disperse dyes leads to aggregation of the

dye in the bath and equilibrium absorption values determined under these conditions are not relevant to the dye uptake in the

initial periods of dyeing. Under these circumstances, a parameter related to the rate of dyeing is not only relevant to commercial dyeing but also less affected by dyebath changes when the dyeing experiments are not too prolonged.

The diffusion coefficient based on the application of Fick‘s law to the rate measurements is a possible rate-of-dyeing parameter. A computer program is available (10) by which Hill’s equation ( I I ) , based on Fick‘s law, can be fitted to the initial data of a rate curve. A good fit was found for dyeing experiments carried out in absence of tension, but the fit was less satisfactory for dyeings done under tension (Figures I and 2). Previously the uptake in two hours had been found to be a satisfactory parameter, but a better parameter can be derived by plotting the data of the rate curve against the square root of the time, when straight lines are given for all results. The slope of such a straight line (up to a 6-h dyeing) is called the dye-uptake constant and can be related to the diffusion process in the initial stages of dyeing.

025-

0 60 120 180 260 3W 3EO Time. min

-Experimental Calculated by Hill’s equation

Figure I-Uptake of Duranol Blue G by Terylene subjected to (dyed without tension)

heat

-Experimental Calculated by Hill’s equation

Figure 2-Uptake of Duranol Blue G by Terylene subjected to dry heat (dyed under tension)

In some work recorded here, only the dye uptake in 2 h was measured, but on the assumption that a line through the origin to this point coincides with the line based on more extensive data (true within the experimental error) the dye-uptake (DU)constant A J t f (A,=uptake for t=2 h) could be calculated. This constant will be less precise than the constants based on experiments for different times, being dependent on the accuracy of the duplicate results for a given time of dyeing, rather than on the mean line derived from data of many experiments. Nevertheless, it is precise enough to show trends due to structural causes, and so it is used in some of the experiments recorded. It should be pointed out that, since the dye-uptake constant A 2 / P derived from a 2-h

1 44 JSDC APRIL 1972; WARWICKER

experiment differs from At only by being divided by a constant factor, variations reported in other papers (10, 12) in terms of A , will still have the same significance whether recorded as A&* or as A/r* based on many experiments, provided that the straight- line relation holds,

It can be seen from the results in Table 2 that, with no tension during dyeing, the dry-heated specimens mainly give DU constants less than that for the control. However, DU increases with increase in the temperature of preheating.

TABLE 2 Terylene Dyed with Duranol Blue G-Dye-uptake (DU) Constants

Heated in the Heated in the oven texturing machine

without with without with tension tension tension tension

Control 0.103 0.104 0.100 0.110 Heated at (“C) 190 0-09 0.100 0.057 0.114

200 0.09 0.108 0.074 0.124

(a) Dry Heat

Sample A/t* * Aalt*t

210 0.095 0.115 0.091 0.161 220 0.105 0.132 0.093 0.162 230 0.115 0.151 0.145 0.217

(b) Steam Heat Heated in the oven

without with tension tension

Sample AJttt

Control 0.107 0.110 Steamed at (“C) 100 0.108 0.090

110 0.112 0.097 120 0.088 0*100 130 0.088 0.097 140 0.100 0.102 150 0.114 0,128

*Based on the complete data ?Based on the uptake after 2 11

Tension during dyeing has‘a significant*effect with-dry-heated specimens and the DU constants are greater than that of the control and increase with increase in the temperature of preheating.

Steam-heated specimens (Table 2) give similar results; the DU for those specimens dyed without tension increases as the temperature of steaming is raised, and eventually reaches a value above that of the control. Tension also has an effect, at first decreasing DU below the values found for free specimens, but, for specimens steamed at 120°C or above, DU values are again above those for corresponding free specimens; only at the highest temperature of steaming, however, is DU greater than that of the control. These effects, however, are only small and may be only marginally significant.

It is the purpose of the structural studies to try to explain these results.

Fine Structural Changes The chemical nature of the fibre plays little or no direct part in

the dyeing of Terylene, so that the cause of the variation of uptake

Dry heat Moisture regain

Sample (%I date

230 0.56

with temperature of pretreatment must be largely structural. To study these structural changes, measurements of moisture regain, density, X-ray orientation and lateral order have been made by methods previously described (10). Not all the methods were applied to all the samples, but a complete analysis has been made of the results from samples treated in the texturing machine.

The moisture-regain values (Table 3) indicate that both dry and steam heat reduce the uptake of moisture to an approximately constant value. Structurally, therefore, it would be expected that the morphological units are less accessible and in consequence the fine structure must be more compact after heating.

Density (Table 3) also increases on heating the specimens to higher temperatures, which also suggests that a more closely packed fine structure is formed. Although the DU constants for specimens dyed without tension are in general less than those for the control, they do increase as the temperature of preheating increases. The increase in density might explain the decrease in DU at lower preheating temperatures, but it does not explain its subsequent increases with increase in the temperature of preheating. It also offers no explanation of the effect of tension, especially with dry-heated specimens, where the D U increases above that of the control. The highest densities recorded are for the oven-heated specimens. When dyed under tension these give the highest DU constants, which is contrary to the behaviour expected from the density results. The results from steam-heated specimens, especially those dyed without tension, at first appear to agree approximately with those predicted from density considerations, but the two results for higher temperatures seem to be contradictory.

As previously (lo), X-ray orientation and lateral order were measured. A direct evaluation of molecular orientation by measurement around meridional arcs is not possible for poly(ethy1- ene terephthalate) (PET). Dumbleton and Bowles (13) preferred to measure a near-meridion reflection, and claimed that the measurement of the equatorial reflections favoured by other workers is less satisfactory. These arguments, however, have greater validity when all degrees of orientation have to be considered. For the small variation of orientation of highly drawn Terylene, the azimuthal scans around the equatorial reflections are still of value. Examination of the X-ray photographs of fibres showed that the equatorial reflections were well separated from other reflections for all the samples. In consequence, azimuthal scans were made around the (OIO), ( f iO) , (100) reflections. Any overlap of other peaks on these scans was easily resolved and the orientation expressed as the half-width at half- intensity (pt). This orientation is that of the normals to these planes and only indirectly that of the molecular axis.

However, for a highly drawn fibre a small variation in this orientation is relatable to a similar small variation in the molecular axis and can therefore be satisfactorily used for this purpose. As in previous work, the reciprocal of this half-angle is called the orientation parameter because its magnitude varies proportionately to the changes in the orientation of the morphological units of which the molecular chains are elements. This variation is small (Table 4), with a slight tendency to increase as

TABLE 3 Moisture Regain and Density Results*

Density (g/cc)

plate oven 1 *376 1.376 Control 1.384 1.398 Steamed at (“C) 100 1.387 1.400 110 1.389 1 4 2 120 1.393 1 *405 130 1 *394 1 *407 1 40

150

Sample

Steam heat Moisture regain Density (g/cc)

(%)

0.73 0.47 0.52 0.59 0.68 0.57 0.54

1.376 1.384 1.385 1.386 1.388 . ...

1-390 1.391

*Crystal density= 1.455 g/cc


TABLE 4

X-Ray Orientation Plate heated Oven

- heated Sample 010 110 100 mean mean

'p, 1 h 'p+ ~ I ' p i 'pi 1/'p+ ' p i 1/'p* 11% Control 9-3 0.108 8 - 6 0.116 7 . 2 0.134 8 . 4 0.119 0.114 Heated at ("C) 190 9 . 6 0.104 7 . 9 0.127 7 .6 0.132 8 . 4 0.119 0.139

200 10.0 0.100 9 . 4 0.106 9 .3 0.108 9 . 6 0.104 0.153 210 8.0 0.125 7.3 0.137 7 . 4 0.135 7 .6 0.132 0.138 220 8 .2 0.122 8 . 3 0.121 8 . 3 0.121 8 .3 0.121 0.133 230 7.5 0.133 7.1 0.141 7.1 0.141 7 . 2 0.139 0.116

the temperature of preheating increases. An increase in orientation is often associated with a decrease in the uptake of dyes (7,8, 14), so that these results could be consistent with the results from those experiments with specimens dyed without tension, which yield smaller DU constants than that for the control. However, they are inconsistent with DU results for dyeing under tension, where orientation would be expected to be increased further, or with the progressive increase in DU as the temperature of preheating increases.

The observed effects therefore are only partially explained by changes in moisture regain, density and X-ray orientation.

The variations in DU thus appear to depend largely on internal structural changes, some of which may be detected by X-ray methods. The nature of the fine structure of Terylene is still largely speculative. The main facts so far deduced do not lend support to a generalised concept of a network structure based on the fringe-micellar theory, since such a theory would postulate less capacity for dye uptake when the density increases. According to Bosley (15) and Dumbleton and Bowles (13) this is accompanied by an increase in the crystallinity indexes as defined by them. Some other type of structure must therefore be postulated to account for a progressive increase in DU above its minimum value, which is also accompanied, as the temperature of preheating increases, by an increase in density and X-ray orientation.

Although the measurements of crystallinity indexes as advanced by Bosley (15) and Dumbleton and Bowles (13) have great value in work where other parameters have to be compared, these measurements reveal little of the true nature of the fine structure of PET. An X-ray study was therefore undertaken by methods similar to those described for the work on nylon (10).

Steam heated

Sample mean 1 / T i

Control 0,120 Steamed at ("C) 100 0.127

110 0.138 120 0.106 130 0.100 140 0.134 150 0.i28

If it is postulated that the morphological units from which highly drawn PET fibres are constructed are largely crystalline in nature, but with lateral disorder in the units, a reasonable line structure explaining the known facts can be envisaged. Support for such a fine structure came from the equatorial scans from the X-ray fibre diagrams of all the samples analysed by the peak- separation computer program as previously described (10). Such an analysis not only provides evidence that the X-ray diagrams are resolvable almost completely into crystalline peaks only but also gives parameters by which the lateral order (and/or size) in different directions of the morphological units can be determined.

Figure 3 shows the analysis of the equatorial scan from the control; it can be seen that the envelope can be exactly fitted by the three crystalline peaks plus the linear background. Since the original diagram was taken with nickel-filtered CuKa radiation the background can be accounted for by the residual white radiation streak, which increases towards the beam stop, found on such diagrams, plus Compton and air scatter; little is left in fact to indicate any appreciable so-called 'amorphous' component at 28= 14" (13). The heat-treated specimens give sharper resolvable diagrams, as the analysis shown represents the sample with the poorest lateral order. The evidence therefore is consistent with morphological units that are largely crystalline, although with lateral disorder that is not large enough to be classed as non- crystalline. The increase in lateral order (and/or size) is reflected in the increase in the parameter k (Table 9, which is the reciprocal of the half-breadth at half-intensity of the particular resolved peak. Figures 4 and 5 show that kl, and k, and k, for the three- equatorial reflectance, vary linearly with density, although the fit is best for k, both for dry-heated (plate) and for steam-heated samples.

Sample 20,* kii

Control 17.6 0.791 Heatedto("C) 190 17-7 1.100

200 17 .7 0.998 210 17.7 1.178 220 17.7 1.077 230 17.9 1 . l o6

TABLE 5

X-ray FAuatorial-scan Analysis

11 : 20a* ka t I* $ (a) Dry Heat (Plate)**

37 22.5 0.699 36 44 22.7 0.760 44 35 22.6 0.796 34 49 22.8 0.916 46 42 22.7 0.907 39 54 23.0 0.954 54

25.7 0.620 25.8 0.681 25 .8 0.701 26.0 0.iSi 25.8 0.762 26.2 0.806

1 3 :

64 71 61 73 67 89

"Greatest changes in lateral order k -1.178 -0.791 A=0'387 49/, k'Z0.954 -0.699 A=O.255 136kI k:=0.806 -0.620 A=O.186 (30%)

(b) Steam Heat 1 t Control 17.6 0.791 37 22.5 0.699 36 25.7 0.620 64.0 Steamed at ("C) 100 17.6 0.996 46.2 22.6 0.754 44.9 25.8 0.669 75.8

110 17.7 1.095 51-3 22-7 0.764 50.2 26 -0 0.667 81.9 120 17.6 0.861 31.9 22.6 0.890 34.9 25.7 0.713 50.1 130 17.6 1.105 60.1 22.6 0.818 61.3 25.9 0.783 103.4 140 17.7 1.047 60.3 22.7 0.952 57.9 25.9 0.741 97.7 150 17-7 I * 173 68.7 22.8 0.870 65.2 26.0 0.750 109.0

f3 T.he Bragg Angle ttGreatest changes in lateral order kEl/half-breadth of peak k -1,173 -0.791 A=0.382 (48.9%) Arbitrary units and no correlation between samples was made kiz0 .952 -0-699 A=0.253 36 9%

k,=0.783 -0.620 A=0.163 126:3%1

146

0 0

0 7 -

-Y

0 6

JSDC APRIL 1972; WARWICKER

Figure 3-Analysis of X-ray equatorial scan

I I

Figure 4- Variation of X-ray lateral order index with density (dry-heated specimens)

, i i 131'1 180 1 3 ~ 5 , 33" 1%

Density, g/cc

1 3 -a 4

Dewty. g /cc

Figure 5 Variation of X-ray lateral order index with densify (steam- heated specimens)

If the density-crystallinity index relations of Bosley (15) and of Dumbleton and Bowles (13) are used with the density values found here, the corresponding crystallinity indexes can be found for the samples according to the assumptions of these authors (Table 6). A plot of these crystallinity indexes against k, generates two straight lines for both dry-heated and steam-heated samples (Figures 6 and 7 ) ; these indexes can be re-interpreted as measures of lateral order and not percentage crystallinity. This is a better interpretation because overall density is a complex of at least the density of the morphological units themselves (unknown) and their distribution within the fibre, i.e. the internal volume of voids, and other spaces are included. It is thus possible for the morphological units to be perfectly crystalline yet the overall density measured in a non-penetrating liquid to be less than the true crystalline density; any crystallinity index would correspondingly be less than lOO','.b.

If the ratio of the volume of the voids to the volume of the morphological units remained constant during an increase in the perfection of these units, the overall changes in density would reflect lateral-order changes and not increases in the amount of crystalline material. In practice, the situation would be more complex and the exact interpretation put on any parameter might then vary according to the conditions of the experiment.

I a B O W I ~ S

0 51 I I 0 20 LO b0 00

'Ciysralliniry', X

Figure 6-Variat)w of X-ray lateral order index with percentage crystallinity' (dry-heated specimens)

u 'I

0 '

Y

36

05 jo 'Cryrtallhnlry', ",,

Figure 7-Variation of X-ray lateral order index with percentage 'crystallinity' (steam-heated specimens)

TABLE 6 Comparison of Crystallinity Index with Lateral-order Constant k,

Crystallinity (T,) Samplo Density (Bosley) (Dumbleton k,

(a) Dry Hear (Hate) & Bowles)

Control 1.376 26 31.5 0.620 Heatedat(%) 190 1.384 35 38.5 0.681

200 1.387 38.5 41.0 0.701 210 1.389 40 .5 43.0 0.761 220 1.393 45 46.5 0.762 230 1.394 46 47.0 0.806

(b) Stenm Heat Control 1,376 26 31.5 0.620 Steamed at ("C) 100 I .384 35 38.5 0.669

110 1.385 36 3 9 3 0.667 120 1.386 37 40.0 0,713 130 1.388 39.5 42.0 0.783 140 1.390 41.5 44.0 0.741 I50 1.391 42.5 45.0 0.750

Discussion The dye-uptake constant (DU) for the dyeing of yarns in the

absence of tension is less than that for the control for samples that have had a dry-heat pretreatment, except for the sample heated at the highest temperature. This would suggest that the fine structure must have compacted in some way to leave the structurc less accessible to dye. Some other opposing mechanism must also be operative, however, since DU, although generally less than DU for the control, increases as the temperature of preheating increases. In addition, in dyeing under tension DU is generally greater than that for the control and increases with increase in the temperature of preheating. Under these conditions, the structure has become more accessible to dye as a result of the preheating. The results of steam heating also indicate that in this case two


opposing mechanisms must be operating, one reducing accessibility and the other increasing it, tension again increasing the accessibility for most of the samples. With steam heating, however, the effects are less marked than with dry heat, and tension reduces rather than increases the dye uptake for the samples steamed at the lower temperatures, i.e. here the compacting mechanism appears to be predominant.

It is difficult to reconcile these facts with a fringe-micellar network type of structure, because all the changes are accompanied by an increase in density and X-ray lateral order and in general this would be expected to accompany a decreased dye uptake if such a fine structure were present. The X-ray work has suggested that the structure is largely crystalline although with lattice imperfections, and, if this is taken as the basis, it is possible to postulate a structure that can support the known facts.

The fine structure envisaged is one made up of crystalline morphological units that arrange themselves together, leaving spaces between them which can become accessible to the dye under the right conditions. The exact nature of the morphological unit cannot be determined from the evidence given, but by the nature of the molecule of poly(ethy1ene terephthalate) its length might be expected to be much greater than either its width or its breadth, so that it could be described as either rod-like or ribbon- like. Since the values of k for the three equatorial reflections- (OlO), (TlO), (100- are not very different from one another and these are to some extent measures of size, as well as imperfection, it could be postulated that the units must be rod-like, the cross- section of which may not, however, be circular. Whether any material ties these units together cannot be readily determined from the evidence, but it seems less likely than with nylon, especially if the stiffness of the molecule of PET is taken into account.

The nature of the spaces between these units needs also to be considered. It does not follow that these spaces are empty and bounded by well-defined surfaces, but the electron density in the spaces is probably much less than that in the units themselves, so that dye can penetrate if necessary by some movement of PET molecules. Again, paracrystalline material on the outer layers of the morphological units will also contribute to the spaces between the units. The dye itself is a plate-like molecule and probably penetrates the structure edge-on, so that the width of the accessible regions need not be great (around 5 A) and might be enlarged to some extent by the dye itself. Under such circumstances the dyeing temperature may be critical in relation to the glass- transition temperature for good dyeing conditions, as suggested by Dumbleton et al. (9).

The tortuosity along the fibre axis of these narrow accessible regions will also determine whether dye molecules can penetrate, since a certain minimum space will be required to allow a dye molecule to penetrate edgeways. It is this concept that allows an understanding of the observed tension effects, because, if there is a minimum of tie material between the units, tension will tend to pull out the narrow accessible region and make it easier for dye to penetrate and, at the same time, make more accessible space for the dye, without necessarily increasing the actual total volume of space. The diagram (Figure 8) clarifies this for a region only partially accessible initially, before and after application of tension. This tortuosity in the fibre direction might also be increased during heating owing to shrinkage or annealing of the yarn during this process, leading to less dye penetration unless tension is applied during dyeing. This is the reverse of the expectations of Andriesson and van Soest (8). Heating in general may be expected to anneal the structure and reduce the space between the units, and will be accompanied by an increase in density. However, the units themselves also either increase in size or achieve better lateral order (or both), as shown by the X-ray measurements. This tendency to achieve better perfection (andlor size) will tend to increase the size of the spaces between

the units if this tendency is greater than can be accommodated in the overall density changes as put forward for nylon (10). Thus, on the one hand, there will be a reduction in accessible space owing to annealing and any increase in tortuosity, but on the other hand an increase will be brought about by the changes in the fine structure of the morphological units and any increase in their general orientation.

with tension

Figure 8-Representation of effect oftension on tortuosity of channels in Terylene

The evidence tends to suggest that heating reduces the accessible space. This reduction might be considered to result from an increase in tortuosity, since tension during dyeing can largely offset it, especially for the dry-heated specimens. The width of the spaces and, to some extent, their structural length must increase as a result of an increase in the temperature of preheating, because the accessibility, after reaching a minimum, starts to increase as the temperature of preheating increases. It would seem plausible that this effect could largely result from an increase in the perfection and orientation of the units. Tension during dyeing reduces the tortuosity factor and allows the effect of changes in fine structure to be revealed more distinctly. It also allows the mobility of chain segments (9) to increase potentially the space for the dye. Steam heating in general reveals similar tendencies, but the reduction in accessibility cannot be readily overcome by tension, which suggests that the tortuosity factor is even greater for steam heating than for relaxed dry-heating (i.e. in an oven). This effect of steam is reasonable, since water, even in small amounts, within the structure has a plasticising effect and lowers the glass-transition temperature, so that shrinkage and aggregation effects will be greater at a given temperature for steaming than for dry heating.

Thus the combined evidence from dyeing and structural measurements leads to the picture of Terylene being composed of rod-like morphological units, probably with paracrystalline material at their surfaces packed close together but with sufficient space left, or with sufficient mobility of sections of chains on the surface of units, for dye molecules to penetrate between the units. Owing to the tortuosity of the channels in the direction of the fibre axis, dye penetration is limited and can be increased by the use of tension during dyeing. Any process that alters either the tortuosity or the size of the channels will affect dyeing, particu- larly the rate of dyeing as indicated by the value of the dye-uptake constant. The total capacity of the fibre for dye could be reduced by the heating processes while allowing a greater dye-uptake constant, so that the present results cannot conclusively show the effect on the equilibrium absorption values. However, the variation in the values for the dye-uptake constant is more relevant to practical processes. (MS. received I July 1971)

References I Marvin, J.S.D.C., 70 (1954) 16. 2 Meunier, Thomas and Hoscheit, Amer. DyestuffRep., 49 (1960) 153. 3 Hallanda, Keen and Thomas, ibid., 50 (1961) 445. 4 Merian, Carbonell, Lerch and Sanahuja, J.S.D.C., 79 (1963) 505. 5 Salvin, Amer. Dyestuff Rep., 54 (1965) 272.

148 JSDC APRIL 1972; NURSTEN AND WILLIAMS

6 Freytag, Blouqin and Diemunsch, BUN. Znst. Text. France, 22 (1968)

7 Beckmann and Langheinrich, Melliand Textilber., 51 (1970) 316. 8 hdrie!Fn-aed_v_n _. Smst, Textilveredlung, 3 (1968) 618; Teintex,

11 Hill, Proc. Roy. Soc., 104B (1928) 39; 586. Vickerstaff, 'The Physical Chemistry of Dyeing';(London : Olivcr &

Boyd, 1950), p. 125. 12 Warwicker, British Polymer J., 3 (1971) 68.

NO. Z jlYbY) UY.

7401

13 Dumbleton and Bowles, J. Polymer Sci., A2 4 (1966) 951. 14 Munden and Palmer, J. Textile Znst.. 41 (1950) P609. 9 Dumbleton, Bell and Murayama, J . Appl. Polymer Sci., 12 (1968)

10 WrsLker, J.S.D.C., 86 (1970) 303. I5 Bosley, J. Appl. Polymer Sci., 8 (1964) 1521.

The Ready Desulphonation of Two Simple Azo Dye Acids H. E. NURSTEN AND K. E. WILLIAMS

Atkin-Thompson Research Laboratory, Procter Department of Food and Leather Science, The University, Lee& LS2 9JT

Under normal drying conditions, the free acids of C.T. Acid Red 88 and 27 have been shown to undergo partial desulphonation in the naphthionic acid portion of the molecule, with formation of I-naphthaleneazo-2-naphthol and 1 -naphthaleneazo-2-naphthol-3,6- disulphonic acid, respectively. This must be taken into account in preparing pure dye acids for dyeing studies. The reason for the

apparent stability towards desulphonation of the intermediate member of the series, C.I. Acid Red 13, is not obvious.

Introduction No mention is made in the literature of the decomposition of

the free acids of azo dyes on moderate heating. Lemin and Vickerstaff (I), for example, used the free acids of C.I. Acid Yellow 36 (Metanil Yellow YK), C.I. Acid Orange 7 (Naphtha- lene Orange G), C.I. Acid Blue 47 (Solway Blue R), and C.I. Acid Red 116 (Coomassie Red G ) for dyeing studies after drying to constant weight at 105°C. It has now been found (2) that for C.I. Acid Red 88 (naphthionic acid-+2-naphthol) and 27 (naphthionic acid-+2-naphthol-3,6-disulphonic acid), these conditions are too severe, considerable decomposition of both dyes occurring well below 100°C. The ease of reaction made it of interest to study these decompositions in some detail. Thermal lability may well be more general among free acids of dyes, since a carefully purified sample of the disazo dye Trypan Blue (C.I. Direct Blue 14) produced subsidiary dyes when a chromatographically pure sample of the free acid was heated at 105°C for 16 h*.

Experimental PAPER CHROMATOGRAPHY

For paper chromatography Whatman No. 1 papers (10 inx 10 in) were run by the ascending method in 12 in x 12 in x I2 in tanks (Aimer Products Ltd), four solvent systems being used: Solvent Composition Developing time (h)

1 Distilled water (3) 3 2 10 ml NH,OH (sp. gr. 0.880) 8

made up to 1 litre with isobutanol-ethanol-water (3 :2:2 by vol.) (3)

formamide-water (2:l:l by vol.) (3)

(7:3:3 by vol.) (4)

3 Benzyl alcohol-NN-dimethyl- 8

4 Butanoneacetone-water 4

DYES

For comparison, small samples of the demineralised free acids of the following dyes (of general structure I) were prepared: C.I. Acid Red 18 (R2, R3 and R4=S03Na and R1=H); C.I. Acid Red 17 (R1 and R2=S0,Na and Ra and R4=H); C.I. Acid Red 25 (R3 and R4=S0,Na and R' and R2=H). Samples of C.T. Acid Red 13 (Ra and R4=S0,Na and R1 and R3=H)

from previous work and of C.I. Acid Red 88 (R1-RS-H, R4== SOsNa) were also examined.

Results and Discussion

C.I. ACID RED 27 Preliminary Examina f ion

Paper Chromatography-A batch of the free acid of C.1. Acid Red 27 (R1, Ra and R4=S0,Na and R3-=H), prepared by evaporating the solution from ion-exchange demineralisation (2) to small bulk at 100°C, followed by exhaustively drying over PaOs under vacuum at WC, was shown by paper chromatography with solvents 2, 3, and 4 to have changed greatly. The decomposition product, which was slightly bluer than the original dye, had a higher Rf value in all three solvents than the free acid applied from a stock solution that had not been dried.

The free acids of the decomposition product, C.I. Acid Red 18, 13, 17, and 25 were chromatographed in parallel on paper in several solvents, together with a sample of C.I. Acid Red 27 from the undried stock solution. C.I. Acid Red 17 and 13 ran level with the decomposition product in solvents 3 and 4, but C.I. Acid 13 was much redder than the other two compounds, which were similar in hue. In general, the Rf values of the decomposition product in the various solvents were much closer to those of the dibasic acids than to those of the two tribasic acids, and the similarity in hue of C.I. Acid Red 17 and the decomposition product suggested they might be identical.

Since C.I. Acid Red 13 recrystallised successfully from hot butanone (2) and the decomposition product seemed to be a dibasic acid also, its recrystallisation from this solvent was attempted. This yielded the decomposition product in a sub- stantially pure state, as judged by high-load runs on paper in a number of solvents, provided that a hot saturated solution was used for recrystallisation. C.I. Acid Red 27 appeared to be too polar to be appreciably soluble in butanone and was concen- trated in the excess of solid. Four crystallisations by this proce- dure left only a trace of it in the decomposition product. L,ow- load Rf values for C.I. Acid Red 17 and 13 and the decomposition product are given in Table 1.

IDENTIFICATION OF DECOMPOSITION PRODUCT OF FREE ACID OF

TABLE I RfValues of Dye Acids (Low hading)*

Rf values in C.I. Acid Solvent 2 Solvent 3 Solvent 4

Red 13 0.33 0.58 0.42 Red 17 0.28 0.57 0.42 Decomposition product 0.29 0.56 0.42 'Reproducibility 3~0.01

Documents

The Structural Causes of Variations in Dyeing Properties of Terylene Yarn Subjected to Dry and Wet Heat