Structural causes of the dyeing variations of nylon yarns subjected to steam heat

68 Warwicker: Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Steam Heat

Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Steam Heat*

by J. 0. Warwicker Shirley Institute, Didsbury, Manchester M20 8RX (Received 25 September, 1970)

Nylon 6 and nylon 6.6 yarns were subjected to wet heat in a relaxed state and the subsequent rate of dyeing of these yarns with Durazol Blue 2R was then studied. The rate of dyeing progressively increased as the temperature of steaming increased. For nylon 6 this increase in rate of dyeing was almost ekhtfold in heating up to 140"c compared with that of unheated samples; for nylon 6.6 the increase was threefold afer heating in steam at 1 5 0 " ~ . It was confirmed that Fick's law is still valid for describing the dyeing process with nylon 6.6 but not with nylon 6. Measurements of density, birefringence, X-ray orientation and degree of lateral order by an X-ray method show that these are all increased by the steam treatments. There is little change in the moisture regain and amine end-group content. An explanation of these observations is advanced in terms of a morphology that allows an increase in the size, shape or quantity of voids on heating the polyamides in steam.

Introduction

In a recent paper1 it was shown that dry heating of nylon 6 and nylon 6.6 above a particular temperature led to an increase in the uptake of Durazol Blue 2R compared with that of unheated specimens. This work has now been extended to a study of the effect of steam heat on the same polyamides free from tension over a range of temperatures up to 150"~. As before, the variation on the dyeing behaviour of the yarns was studied in relation to structural differences detected by X-ray diffraction, microscopy, density, and other methods.

Experimental

The yarns used were nylon 6 (70/24 Celon semi-matt) and nylon 6.6 (70/20 Bri-nylon ZT100).

Heat treatment

treated with live steam at 100-150"c in steps of 10"c.

Dyeing conditions Dyeing was carried out with Durazol Blue 2R under the

conditions previously described.l Since the rate of dyeing, defined as the uptake of dye (g/100 g of yarn) in two hours, is a reliable constant' for characterising the dyeing process, most of the results are in this form. However, Fick's law was found to be valid for dry-heated nylon 6.6 yarns, so a test was made in a manner similar to that previously described to see whether Fick's law was valid for steam-heated nylon 6.6 yarns as well.

Other measurements Measurements of moisture regain, density, birefringence,

X-ray orientation, X-ray lateral order, and end-group analyses were carried out in the same way as described in the preceding paper.l

The yarns were placed loose in a Sanderson steamer and

Results and discussion Rate of uptake of dye

The data for rate of dyeing only (g dye/lW g fibre/2 h) are given for nylon 6 in Table I. Previous work' has shown that the dyeing process is not a simple one obeying Fick's law and therefore it is difficult to derive a single diffusion coefficient to characterise the dyeing process. Furthermore the determination of the equilibrium absorption is also made

more difficult because, for reasons already discussed> experimental determinations are unreliable, and without a valid basis for theory it cannot be derived theoretically.

Despite these drawbacks, however, it is possible to derive reasonable conclusions from an examination of the data for the rate of dyeing. These data were derived for yarn either dyed loose or under a small tension of 10 g, and show that a tension of this magnitude has a small but not a highly significant effect on the results (Table I, Fig. 1). The change in the

TABLE I Steamed polyamides dyed with Durazol Blue 2R : Rate of dyeing of

nylon 6

Rate, g/100 g/2 h

Free 10 g tension Sample steamed at ("c):

Untreated 100 110 120 130 140

0550 0.527 0.838 0.805 1.311 1.23 1.84 1-80 2.35 2.02 4.00 3.13

01 1 I I I I lo0 110 120 130 140 150

TEMPERATURE OF STEAMING, 9

Fig. 1 . Change in the rate of dyeing of nylon 6 dyed with Durazol Blue 2R with the temperature of steaming

0 Free; 10 g tension

*Presented at a Joint I.R.I., P.I. and S.C.I. Conference on Advances in Polymer Science and Technology. 111, held at the University of London, 29 September-lOctober, 1970

Br. Polym. J., 1971, Vol. 3, March

Warwicker: Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Steam Heat 69

rate of dyeing (10 g tension) is from 0.805 for a sample steamed at 100"c to 3.13 for that of a sample steamed at 140"c. With nylon 6 samples heated dry,' the maximum difference in the rate of dyeing was from 0.44 (heated to 165"c) to 0.66 (heated to 195"c), values that are clearly in a lower range than those found with steam-heated yarns.

The value of the dye uptake (3.13 g/100 g) for the amount taken up in 2 h by the yarn steamed at 140"c shows that the equilibrium absorption value must be at least as high as this and probably even greater. The calculated value of equilibrium absorption for a nylon 6 yarn preheated dry at 195"c was 1.039-1.319 g/100 g according to the method of calculation. These values were the highest recorded' for dry-heated nylon 6 so it can be at once inferred that steam heating alters not only the rate of dyeing but also the capacity of the nylon 6 yarn for dyestuff much more than does dry heating. Steam heating thus appears to have a more profound effect on the structure and morphology of nylon 6 yarn than does dry heating.

If a similar analysis is now carried out with the results for nylon 6.6 it becomes clear that the results for dry- and steam-heated yarns are not so significantly different. It was pointed out previously' that except for the control the values of dye uptake for loose and tension-dyed dry-heated yarns are similar. The range of values varies from 0.346 for the untreated up to 0.899 for nylon 6.6 dry heated to 25Ooc,l whereas the range for steam-heated nylon 6.6 is seen to vary (Table 11) from 0.346 (untreated) to 1.22 for a yarn steam heated at 150"~. There is, therefore, only a comparatively small increase in the rate of dyeing for a steam-heated yarn compared with dry-heated yarn. Even so, it should be remem- bered that the steaming temperatures are much lower than the dry-heat temperatures, so the effect of the moisture is still large.

Since a simple diffusion process based on Fick's law had been found to be valid for the dyeing of dry-heated nylon 6.6,

evidence for this was also sought with the steam-heated samples. Data for the kinetics curves for dye uptake on steam- heated nylon 6.6 yarns are given in Table I1 and shown in Fig. 2. Fig. 2 also shows the points calculated according to Fick's law, and the data from these calculations are given in Table 111. The calculated values for the uptake of dye for a given time of dyeing are in good agreement with the experimental, so that Fick's law is valid for the diffusion of Durazol Blue 2R into all samples of nylon 6.6 so far examined.

Fig. 3 shows that with nylon 6.6 all the parameters generally used to characterise the dyeing process, namely the diffusion

I 0 120 240 360

TIME, rnin

Fig. 2. Application of Hill's Equation to the kinetics curves of nylon 6.6 dyed with Durazol Blue 2R

0, 0, Computed values; - experimental curve

eo[ , r-- '

TABLE I1

Steamed polyamides dyed with Durazol Blue 2R : Uptake of dye on nylon 6.6

Sample Uptake (g/100 g) after: 20 1:p: d D 1;. .. d- steamed at ("c): 20 rnin 40 min 60 min 120 rnin 360 rnin . -

A

100 0-137 0.235 0.282 0-394 0593

1 20 0.238 0.366 0.430 0.596 0.753 I30 0.346 0495 0.587 0.784 0.823 140 0.650 0.825 1.05 1.15 1.19 Fig. 3. Change in the dijiusion coefficient (D), the rate of dyeing, 150 0.730 0.893 1.05 1.22 1.23 and the time of halfdyeing (ti) of nylon 6.6 dyed with DurazolBIue 2R

with the temperature of steaming A Rate of dyeing; 0 D (half curve); D (full curve); A l/i+

0.00 100 no 120 1YI 140 1W

110 0130 0.191 0.233 0361 0.556 O L TEMPERANRE OF SEAMING 9:

Untreated nylon 6.6 under 10 g tension; rate of dyeing = 0.346 d100 gi2 h (mean)

TABLE I11

Application of Hill's Equation to the kinetics curves of nylon 6.6 dyed with Durazol Blue 2R

Full-curve fit 'Half'-curve fit Mean Samole steamed at ("c): - . ,

COO, D, 10-l' c m , D,10-'0 g/100g cmz/min ti, rnin g/lW g crn2/min ti, min COO tt* l / t+

100 0.846 4.88 144 0.725 7.54 100 0.785 118 0.0085 110 1.075 2.20 330 0.714 6.63 117 0.894 190 04035 1 20 0801 15.14 53 0.771 18.75 48 0.786 52 0.019 130 0.860 27.14 31 0842 35.01 30 0.851 30 0.033 140 1.190 51.94 18 1.187 51.20 18 1.188 18 0056 150 1.249 49.78 19 1.230 62.96 18 1.239 19 0.053

~

CCO (mean) * Read from curves for not $(ti(full) + t+(& curve) ); COO = equilibrium absorption value


70 Warwicker: Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to S t e m Heat

e .

g L C O W O l

Q 10- -

11 a

I I I I I

coefficient (D), the reciprocal of the time of half dyeing (tt), and the rate of dyeing (as defined here). all vary with the steaming temperature in a similar manner. It is also interesting to note that in general these parameters hardly change for yarns steamed at 140 and 150"~.

In Table IV results for the dry-heated yarns1 are quoted for comparison with those from steam-heated yarns. The main difference between these results lies in the greater changes in the diffusion coefficient for steam-heated yams than for dry-heated yams. Fig. 4 shows the variation in the equilibrium absorption values (C,) for steam-heated yarns and it is remarkable that over most of the temperature range in which the diffusion coefficient varies, the equilibrium absorption value is constant. There is a rise in equilibrium absorption value, however, for samples steamed above 13O"c, but in the range when the diffusion coefficient is constant (Fig. 3) so also is the equilibrium absorption value constant, although at a level higher than that found at the lower range of temperatures. These results suggest that in the lower range of temperatures

(100-13O"c) the steam-heated nylon 6.6 yarns vary in the disposition of morphological units, changing the shape and possibly the distribution of voids to allow for the greater diffusion speeds but not for a higher capacity for dye. Above 130"c changes take place that lead to a stable structure for samples steam-heated at lower temperatures. This behaviour is different from that found with nylon 6 where by inference both the rate of dyeing and the capacity of the fibre for dye change to higher values throughout the range of steam temperatures studied. It is thus necessary to explain these differences in terms of the morphological and fine structural features of the two fibres.

Structural variations Before attributing the dyeing changes to physical causes it

is necessary to eliminate, as far as possible, other causes. The most important factor that can affect dyeing is any change in chemical end-group content brought about by the steam treatments. However, the results of the determination of chemical end groups given in Table V show that steaming does not

TABLE IV

Application of Hill's Equation to the kinetics curves of dry-heated nylon 6.6l dyed with Durazol Blue 2R

Mean results Sample heated at ("c): D,

cme/mln Cm tt lit,

Control 4 8 3 1.070 110 0009 270 3.86 0.999 240 OWM1 230 3.22 1.156 190 00053 240 4 0 9 1.271 166 0.006 250 7.70 1.477 80 0012

TABLE V Amine end-group analysis for steamed polyamides

Amine end content, mequiv.

Nylon 6 Nylon 6.6 Sample steamed at ("c) :

Untreated 100 110 1 20 130 140

28.9 25.3 24.7 25.9 2 4 0 24.0

41.5 43.9 43.2 42.3 4 0 6 41.6

produce a great change with either fibre, and in fact any change is towards a reduction in the chemical end-group content. Such changes, therefore, cannot in any way account for the increased dye rate or capacity actually observed.

The results in Table VI show that for both fibres there is a decrease in moisture regain values as the temperature of steaming is raised. Calculations of the number of water molecules held in the fibre for the lowest moisture regain recorded show that the number of potential dye sites (if these are identical with the sites of water molecules) far outweigh the number of dye molecules held even for the highest dye uptake. It can thus be inferred that the important factor con- trolling the dyeing process is not the number of potential sites but the accessibility of these sites to dye molecules. Thus changes in the size and shape of the voids must be the most important factor to account for the increased rates of dyeing as the temperature of steaming is raised. It is therefore important to investigate the nature of these accessible regions.

Density would give an indication of the changes in the volume of accessible regions if other things are equal. How- ever, for both nylon 6 and nylon 6.6 the density increases as the temperature of steaming increases (Fig. 5) . Furthermore, as Table VII and Fig. 5 show, there is a critical point at N 11O"c at which the relative increase per degree rise of temperature becomes greater. The significance of this change in

TABLE VI Moisture regain of steam-heated polyamides

Moisture regain, % Sample steam-heated

to ("c): Nylon 6 Nylon 6.6

Control 100 110 1 20 130 140

4.42 3.98 4 4 9 400 4.37 3.89 4.40 3.81 4.17 3.66 4-25 3.75

TABLE VII Density of steam-heated polyamides

Density, g/cmS

Nylon 6 Nylon 6.6 Sample steamed at ("c):

Untreated 100 110 120 130 140 150

1.131 1.144 1.145 1.149 1.154 1.156 -

1.135 1.141 1.142 1.144 1.145 1.149 1.150


Warwicker: Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Steam Heat 71

2 0 5 0 1w 150

TEMPERATURE OF STEAMING, OC

Fig. 5. Change in density of nylon 6 and nylon 6.6 with the temperature of steaming

Nylon 6; 0 nylon 6.6

the rate of increase of density with temperature at - 11O"c is not yet known, but it seems to apply equally well to both polyamides. An increase in density alone would pre-suppose a consolidation of the fibre and superficially might be expected to lead to a decrease in dye uptake.

Another factor that has been found to relate to dyeing has been the birefringence of the fibre, especially in the relation of the degree of drawing to the dyeing of polymers.2

Birefringence measurements (Table VIII, Figs 6, 7) show that a steady increase is observed as the temperature of steaming is raised. This increase is linearly related to the rise in density (Fig. 7); the slopes of the graphs for nylon 6 and nylon 6.6 are almost parallel suggesting that the nature of the structural changes involved is similar. Whatever the structural causes of the increases in density and birefringence are they would normally be expected to be associated with a reduction in dye uptake2 rather than an increase as is actually observed. It should be emphasised that birefringence and density are related3 by the law:

An - = constant P

which appears to be obeyed by these fibres, and therefore a proportion of the changes in birefringence is attributable to changes in density alone.

X-Ray orientation is independent of density and provides a valuable independent check on changes in orientation. The method adopted here, however, suffers from the fact that it gives the mean orientation of the normals of planes parallel to the fibre axis. Under simplifying assumptions this orientation is indicative of the axial orientation, but complications can arise owing to the possible rotation of the planes around the normal, which affects the axial orientation but not the orientation of the normal itself. For highly drawn fibres this complication is less likely, and changes in the orientation of the normals to planes parallel to the fibre axis are largely indicative of axial orientation changes. The orientation results (Table IX, Fig. 8) show that the main changes take place with samples that have been pre-steamed above 120-130"~.

A plot of X-ray orientation against birefringence is linear (Fig. 9). Since birefringence is also linearly related to density these results confirm that birefringence is a complex involving both density and orientation and cannot be used exclusively as a measure of the general orientation of the polymer. An- other inference from these results is that since the X-ray orientation does not appreciably change until the samples have been steamed above 120"c the birefringence changes found with samples steamed at temperatures up to 120% must be largely attributed to changes in density. Both X-ray

TABLE VIII

Birefringence of steam-heated polyamides ~~

Nylon 6 Nylon 6.6

An Anlp An Anlp

Control 0.0519 00459 0.0566 00499 100 00522 0.0456 0.0577 0-0506 110 0.0526 00459 0.0575 0-0504 120 0.0539 0.0469 0.0579 0.0506 130 00539 04467 00575 00502 140 00549 0.0474 0.0584 00508

0.0581 00505 150 - - Mean 00464 - 0.0504

Sample steamed at ("c):

An = Birefringence; p = density

o-060r - ---, I I I I I

" O S O t

"US (00 no 110 130 140 150


Fig. 6. Change of birefringence (An) of nylon 6 and nylon 6.6 with the temperature of steaming


oo60-

m 5 0 0 1.lM 11% 1.160

DENSITY, g/cm3

Fig. 7. Relationship of density to birefringence for nylon 6 and

Nylon 6; 0 nylon 6.6 nylon 6.6

orientation and density measurements show more pro- nounced changes for samples heated above 120"c than for samples heated below 120"c, SO that this temperature appears to mark a significant structural change in the two polyamides studied. The cause of this change is not known, but at least for nylon 6 it may be associated with the removal of oligomers, which might be free to diffuse to the surface at temperatures above 120°c, or even be readily produced by the breakdown of the polymer in this temperature range.

Investigation into the X-ray lateral order also indicates differences between the two polyamides when steamed (Tables X and XI, Fig. 10). Whereas the X-ray lateral order of the (hko) planes with nylon 6.6 steadily increases with increase in temperature, there is a slight decline in X-ray lateral order of the (hol) planes of nylon 6 up to temperatures around 120-130"c followed by a rapid rise in lateral order at


72 Warwicker: Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Steam Heal

TABLE IX

&Ray orientation of steam-heated polyamides

TABLE X

%Ray lateral order of steam-heated polyamides: Nylon 6

1/9t (mean) Sample steamed at ("c):

Nylon 6 Nylon 6.6

Untreated 100 110 1 20 130 140 150

0.101 0.093 0.100 0106 0.103 0.098 0.103 0.100 0.118 0102 0.137 0.118 - 0.127

I I I 20 50 100 150


Fig. 8. Change in X-ray orientation parameter (I/++) with the temperature of steaming for nylon 6 and nylon 6.6


0.0501 I I I 1 I Oow o.im o w 0120 0130 O U )

11+,,2

Fig. 9. Relationship of birefringence to X-ray orientation for nylon 6 and nylon 6.6


temperatures of steaming higher than this. These results are consistent with the mechanism just proposed to explain some aspects of the birefringence and X-ray orientation results. Nylon 6.6 appears to be a reasonably stable structure whose morphological units can become more ordered on steaming. However, with nylon 6, the removal of short-chain and monomeric material (part of which arises by decomposition of larger units) tends to produce, or at least be associated with, disordering of the morphological units; however, after removal of the major part of the oligomers, apparently by steaming at - 120"c, the larger morphological units then tend to become more ordered, thus leading to less decomposition and to a stabilisation of the structure.

It is pertinent now to see how these structural changes can suggest a model structure that can explain the large difference in dyeing behaviour displayed by nylon 6 and nylon 6.6.

(200) Reflection (002 + 202) Reflection Sample steamed

at ("c): k1 28, Zl kz 20, 1,

Untreated 0.919 20-3 760 0511 23.2 104.9 100 1.208 20.3 67.1 0708 23.7 88.1 110 1.220 20.2 71.3 0.735 23.7 89.0 120 1.162 202 92.9 0.712 23.9 128.7 130 1.102 20.0 95.8 0.666 23.8 121.8 140 1.490 201 100.5 0954 24.1 115.5

TABLE XI X-Ray lateral order of steam-heated polyamides : Nylon 6.6

~~

(100) Reflection (010 + 110) Reflection Sample steamed

at ("c): kl 2 4 I1 kz 28, l a

Untreated 0.850 20.4 43.6 0.519 23.0 69.1 _ _ - 100 1.061 20.4 65.3 0.687 53.4 91-0 110 1.037 20.4 75.5 0.624 23.3 107.8 120 1.093 u).3 86.9 0681 23.5 120.5 130 1.097 20.4 89.2 0705 23.4 124-6 140 1.198 20-2 81.7 0807 23.6 104.8 1 50 1.185 20.2 91.8 0.807 23.6 118.0

I i

I I I I I

I I I I I

100 110 1 20 130 140 150

TEMPERATURE OF STEAMING, OE

Fig. 10. Change of lateral order index (k) of nylon 6 and nylon 6.6 with the temperature of steaming


Any concept that envisages a simple fringemicellar type of arrangement does not appear to fit the facts. With such a structure any increase in density, orientation or lateral order would be associated with a decrease in the accessible, dis- ordered regions, and hence with a decrease in dye uptake, which is contrary to the experimental facts. The explanation already advanced in a previous paper' seems to fit the facts especially with regard to the dyeing behaviour of nylon 6.6. In this explanation the morphological units that make up the fibre are thought to be essentially crystalline but with lattice imperfections and other faults associated with imperfect crystalline bodies. It is not necessary for the purposes of this explanation to define precisely the nature of such units, but the adoption of such a structural model does imply that the surfaces that are involved in the dyeing process are the outer surfaces of these units and the dye uptake will be controlled by the accessibility to these surfaces. It has already been pointed out1 that as these units either grow in size or become more perfectly ordered then unless they can adjust position to accommodate such changes, the size and shape of voids must be altered. Under these conditions the changes in overall density do not reflect the real density changes of the internal


morphological units, and there is an increase in the accessibility of the internal units to dye penetration.

The order of magnitude of the changes might be expected to be similar for dry and wet heat, and this appears to be true for nylon 6.6 but not for nylon 6. In view of this, extra structural changes seem to be involved with the steam heating of nylon 6. The explanation already advanced that the removal of oligomers is more rapid with nylon 6 than with nylon 6.6 may well provide the reason for the difference in behaviour and structure of these two polyamides. The removal of oligomers will provide space for the entry of dye in addition to that produced by the crystallisation of the morphological units alone, and wet heat will aid this process more than dry heat (providing that the overall morphological structure does not collapse to adjust to the density changes). The difference then between nylon 6 and nylon 6.6 thus very possibly lies in the difference in rates of degradation under the given conditions.

The evidence given here and previously’ shows that the structure of nylon 6 and nylon 6.6 can be envisaged more satisfactorily in terms of an aggregate of morphological units, which themselves are highly crystalline, rather than in terms of the conventional fringe-micellar theory. The nature of these morphological units requires further study and such questions as to whether they contain folded chains or not require the use of techniques of investigation other than those employed here.

Warwicker: Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Steam Heat

Discussion A . Shurples (Inveresk Research International).-Knowing the density of the fibres, and knowing also to a first approxima- tion the density of the crystalline component, have you calculated the void content of the material?

The concept of a ‘two-phase’ structure consisting essentially of crystalline regions and voids is very controversial. Do you think it also applies to isotropic nylon and Terylene? Author’s reply.-The void content cannot be estimated by simple density calculations as suggested. The actual density of the morphological units cannot be assumed to be that of a perfect crystal, indeed the X-ray diagrams show that this is not true, so without this knowledge the void content cannot be calculated.

The model proposed is one in which imperfect crystalline morphological units pack together, probably with tie-material to form the structure of the fibre. Although imperfectly crystalline, the morphological units are not, in general, penetratable by foreign molecules, unless of course these are intracrystalline swelling agents. These units can change in perfection and density, and unless in so doing the overall packing can acconunodate the physical changes involved a different void distribution must be produced. There is evidence that this model has some reality but no model can be com- pletely proved since every model is an abstraction. From this standpoint all models are controversial : the real question is, does it help in the explanation of the observed facts? In this case the model proposed fits better than any previous model, but it is not claimed that it is the final answer.

P. L. D. Peill (British Railways Board, Research Dept., ‘Technical Centre, Derby).-It was stated that either dry heat

73

Conclusions

Steam heating nylon 6 and nylon 6.6 in the temperature range 100-15O”c leads to greater rates of dyeing of the treated materials than those for the untreated polyamides. This change in dye rate is much greater for nylon 6 than for nylon 6.6.

Steam heating causes an increase in density, birefringence, X-ray orientation and X-ray lateral order, but little change in the amine end-group content and in the moisture regain of the samples. Such changes might be expected to cause a decrease in dye uptake rather than the large increases found on the basis of the fringe-micellar theory.

An explanation is advanced in terms of a structure con- structed from imperfect crystalline morphological units, which on increasing in perfection, cause voids between them to increase in size. This increase in void size is also enhanced, especially with nylon 6, by the loss of oligomers produced by degradation. In this process steam heating is more effective than dry heating.

References

Warwicker, J. O., J . SOC. Dyers Colour., 1970, 86, 303 Munden, A. R., & Palmer, H. J., J. Text. Inst., 1950, 41, 609 Hermans, P. H., ‘Physics and chemistry of cellulose fibres’, 1949,

p. 217 (Amsterdam: Elsevier)

or live steam caused an increase in the dyeing rate, yet a reduction in end-group content had taken place. Was any evidence obtained as to any change in the nature of the end groups during heating?

An explanation for the increase in dyeing rate was put forward in terms of void size and content. Had any attempt been made to confirm this by measurements of porosity or specific surface area of the fibres? Author’s reply.-An investigation into the change in nature of the end groups was not carried out. By the chemistry of the polymer it is possible that carboxyl groups could also be produced as end groups, but if these were produced the dye rate would be expected to decrease.

Work is in progress by X-ray small angle scattering experiments to try to confirm the concepts of changes in void size and content. So far these experiments have been encouraging but much more work is required.

J. G. Foss (Iowa State University).-Is it possible that there is an increase in the amount of dye bound because there has been a change in the orientation and/or distance between dye sites which permits stronger dye-dye interaction? This could lead to a more favourable equilibria binding constant. (If this idea is correct it would suggest you would observe increased metachromasy of the dyes bound to the heated fibres.) Author’s reply.-A change in void size would seem to favour the idea put forward especially if accompanied by an increase in lateral order and orientation of the basic units. It would, therefore, seem quite pIausible that the mechanism put forward might play a part in the process, and I should like to thank Dr. Foss for the suggestion.


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Structural causes of the dyeing variations of nylon yarns subjected to steam heat