Introduction

Dyeing involves dye penetration into the fibre mass and its fixation by certain known mechanisms. This maximally simplified definition does not however give a clear idea of the complexity of the number of physical, physico-mechanical and chemical phenomena which occur during the dyeing process.

First of all, during the dyeing process, conditions are created for liberating the segmental mobility of the polymer molecules, in this case the polyamide molecules. It is a necessary condition for the penetration of the dye particles into the net of macromolecules which make up the amorphous areas of the fibres. Therefore dyeing is generally carried out at comparatively high temperatures, in the presence of substances which cause the swelling or at least the plasticising of the polyamide. Water is a universal medium, causing the swelling and plasticising not only of the polyamide. It is also a suitable carrier of the dye molecules.

When dyeing with acid dyes, the dye bath is slightly acidic and facilitates the swelling of the substrate. When temperatures of the dyeing process are higher than Tg (the glass transition temperature), a sharply increased mobility of the molecule segments is observed in the polymer. That is why dyeing is effected within the temperature range of 45-98°C, when the polyamide under these conditions is above its glass-transition temperature and is in a highly elastic state [1, 2].

It is well known that this is the state where the segmental mobility of the macromolecules in the amorphous areas evinces all its special features, the most evident of which is the rapid diffusion of dyes. However, this mobility is a prerequisite of a number of concurrent phenomena, which can be summed up in oneexpression œ the relaxation of the supra-molecular structure.

Figure 1. Chemical structure of the acid dyes used

Taking into consideration the molecular-kinetic nature of the relaxation, it can be expected that in the presence of active low-molecular particles such as acid dyes, the activating energy of the transformation processes of the polymer structure will decrease depending on their concentration [2œ4]. Consequently, the concentration of the acid dyes should be one of the determining factors for the direction and extent of the processes of structural transformation in the substrate during the dyeing process.

Another essential factor, to which little attention is paid, is that after the end of the dyeing process, the molecules of the acid dyes remain tightly fixed in the amorphous areas of the substrate, which are responsible to a high extent for the deformation behaviour of the polymer [5œ8]. This is due to the chemical structure of the dye particles with their intrinsic poly-functionality, allowing the formation not only of chemical bonds, but also of contacts, arising on the basis of the van der Waals interactions and additional hydrogen bonds, which are different in character [1, 9].

The concentration of the acid dyes in respect to the textile material varies within wide limits, but it generally is from 0.05 to 5%. Such an amount of the substance with its own definite volume, included in the mass of any polymer product, should appreciably influence the relaxation processes of the structure, with a natural effect upon the physico-mechanical properties of the samples.

The interrelation between the presence of acid dyes in the polyamide substrate and the modifications in the supra-molecular structure of the polymer has been studied by other authors as well [10œ12], but no investigations have been made on the influence of the dye concentration upon the course of these processes and the effect upon the physico-mechanical indices of the dyed samples.

The influence of the concentration of three acid dyes upon certain structural and physico-mechanical indices of polyamide multifilament yarn and woven fabric is studied in the present work.

Experimental

Materials Used

The investigations were carried out with:

• polyamide multifilament yarn PА MFY 8 tex (”Vidlon‘ from Vidin, Bulgaria) with the following characteristics: PA6 MFY 8.42 tex f 32 Z 310 (BDS 17281œ92); deviation from linear density 1.3% (BDS EN ISO 2060-99); linear density after doubling 8.48 tex; doubling coefficient 30; angle of doubling (β) 7°; contracting after doubling 0.6%; yarn diameter 0.142 mm with deviation from the diameter 1.1% (BDS EN ISO 2061-99);
• polyamide woven fabric with plain weave (”Filatex‘ from Kazanlŭk, Bulgaria); area mass 67.7 g.m^-2 (BDS EN 12127-00); density of cloth (sett): 35 warp yarn/cm þ 35 weft yarn/cm (BDS EN 1049-2œ02); woven from polyamide multifilament yarns with the following characteristics:
• polyamide warp yarn PA WARP Y 8.42 tex f 32 Z 230 (BDS 17281œ92); deviation from linear density 1.1% (BDS EN ISO 2060-99); linear density after doubling 8.46 tex; doubling coefficient 27; angle of doubling (β) 6°; yarn contracting after doubling 0.5%; yarn diameter 0.147 mm with deviation from the diameter 1.5% (BDS EN ISO 2061-99);
• polyamide weft yarn PA WEFT Y 8.42 tex f 32 Z 150 (BDS 17281œ92); deviation from linear density 0.8% (BDS EN ISO 2060-99); linear density after doubling 8.33 tex; coefficient of doubling

14; angle of doubling (β) 3°; contracting of the weft yarn after doubling 0.1%; yarn diameter 0.133

mm with deviation from the diameter 0.9% (BDS EN ISO 2061œ99).

Dyeing was carried out with the following three acid dyes: Erionyl rubin 5-BLF (CIBA), Acidol orange 3 RL (BASF) and Acidol brillantgelb M-3GL (BASF), in the concentration range of 0.0 to 5.0%. For brevity in some of the figures and tables, the dyes are designated with the numbers indicated in Figure 1. In accordance with the specifics of the analyses conducted, the test samples have been prepared from polyamide multifilament yarn PА MFY 8 tex and polyamide woven fabric (PAWF), dyed in the above-mentioned concentrations.

The dyeing was conducted on an Ahiba Polymat (AHIBA AG) laboratory dyeing apparatus under the following conditions: the wetted textile material was dipped into a dye bath with a module M of 1:100, containing x% dye and 20% (in relation to the mass of the material) NaCl at an initial temperature of 40°C. After a 10-minute treatment under the above-mentioned conditions, 2% (in relation to the mass of the material) of 30%-water solution of CH3COOH (pH 4.5 œ 5.5) is added to the bath, and after a 5-minute treatment, the bath temperature was increased at the rate of 6°/min until boiling point (98°C). Dyeing continued for 40 minutes at boiling point. Then the bath was cooled down to 60°C and the the dyed samples were washed in warm and cold water.

The degree of extracting the dye has been determined by colorimetry of the residual dye baths on a SPECOL spectrophotometer (Carl Zeiss Jena) and compared with previously prepared standard absorption curves for the respective dyes. The real dye concentration in the substrate has been obtained by a correlation coefficient corresponding to the estimated percentage of the extracted dye.

Methods Used

Wide-angle X-ray scattering (WAXS) has been conducted on a TUR M62 apparatus (Carl Zeiss Jena) from fixed samples of equal mass and a mobile counter with the following working characteristics: CuKαœradiation (λ = 1.542Å); Ia = 20mA,Ua = 35 kV; angle speed of the goniometer of 1°/min; Ni-filter; constant registration in the area of 2Θ 3-30° [13, 14].

The X-ray degree of crystallinity (α^waxs) has been determined according to the following formula:

α^waxs = [ Іcr / Іcr + Іam] * 100 [%], where:

Іcrœintensity of the crystal reflexes;

Іcr + Іamœtotal area of the diffractogram.

The dimensions of the crystallites have been determined from the diffractograms for reflexes 002 (Θ = 11.5œ12°) and 100 (Θ = 10.5œ11°) according to Sherer‘s method [13].

The DSC analyses have been conducted on the Perkin Elmer DSC 2C apparatus, calibrated with In according to the instructions for use for the apparatus, in a heating regime from 350 K to 600 K, at a constant rate of heating of 20 K/min in an argon atmosphere.

The degree of crystallinity (α^DSC) [14] is determined according to the following formula:

α^DSC = (∆Н / ∆Н°) * 100 [%], where:

∆Н°- melting of temperature 100% crystal polymer [15].

The birefringence (∆n) was determined with an Ernst Leitz Wetzlar polarisation microscope equipped with a Berek calcite compensator [16œ19]. The single filaments of the dyed samples have been prepared with Canadian balsam. The results presented are the average value of 10 measurements for each concentration. ∆n has been determined according to the formula:

∆n = R / d, where where: Rœdifference in the run of the waves, which are obtained when the light rays pass through the yarn; dœyarn diameter. R = C * f(i), where:

Cœa previously calculated constant, specific for each compensator;

f(i)œa function of compensation expressing the dependence of the difference in the run of the

waves on the angle of rotation of the compensator (i). f(i) is constant for each

compensator from the Berek type; its values, calculated in advance, are attached to the

compensator in the form of tables. The angle of rotation of the compensator (i) is determined according to the formula

n n

i = [ ∑аi / n + ∑ bi / n ] / 2, n = 10

i =1 i =1

where:

аiœangle of rotation of the compensator in one direction from the position of maximum

illumination of the yarn in opposite parallelism to the object and the compensator

(decreasing the interfering colours) up to complete compensation for the difference in the

run of the waves, which corresponds to the total matting of the yarn;

biœangle of rotation of the compensator in the opposite direction up to the total matting of the

yarn, i.e. up to full compensation for the difference in the run of the waves.

The tenacity and elongation at break have been determined on a WPM apparatus (Werkstoffprüfmaschinen, Leipzig) with an inter-jaw distance of 0.5 m and a stretching rate of 0.2 m/min, maintaining a constant speed of movement of the jaws.

The measurements were conducted with non-dyed and respectively dyed polyamide multifilament yarn PА MFY 8 tex, conditioned in advance for 24 hours under standard climatic conditions.

The data presented in Figures 8 and 9 are the average value of 20 measurements for each concentration of the three dyes (BDS 9894œ83) œ v œ values between 0.6% and 2.47%..

The crease resistance (BDS 9589œ89) of the samples has been determined as the angle ofrestoration after creasing (ARAC) of samples with dimensions 2 þ 4 cm, cut longitudinally along the warp and weft yarns from PA fabric (PAWF). The samples are folded perpendicularly to the above-mentioned fibres, and then loaded with a weight of 1 kg for 30 minutes. After removing the weight, the samples are left to relax for 30 minutes, and the angle between the two folded parts is measured. The ARAC is determined according to the following formula:

n n

ARAC i = [ ∑ ARAC warp / n + ∑ ARAC weft / n ] / 2 ,

i =1 i=1

where:

ARAC iœis the average angle for a definite concentration;

n = 20 experiments conducted under standard climatic conditions (v œ values between 0.91%

and 2.31%).

The critical time of dissolution was established according to Schwertassek‘s method [20]. The time necessary for the dissolution of individual elementary fibres of the polyamide multifilament yarn PA MFY 8 tex in a 65% solution of H2SO4 under a load of 0.3 g at a temperature of 20°C was measured. The values in Figure 11 are the average of 20 measurements of each separate concentration (v œ values between 1.09% and 3.40%).

Results and Discussion

Structural modification of PA samples under the influence of the acid dyes concentration

It is known that the intensity of the wide-angle diffraction of X-rays is related to the modification in the phase composition of the samples. The WAXS experiments show that the dyed samples, irrespective of the dye used and its concentration, have characteristic peaks for αœ modification (between θ = 9.5œ10° and θ = 11.5œ12°) and for γœmodification (θ = 10.5œ11°) [21œ23].

It should be stressed that a clearly expressed reflex (100) is observed for all concentrations of the three dyes, connected with the presence of the hexagonal crystal modification. Although the intensity of this reflex is not predominant over reflex (002) of the monoclinic shape, the equal participation of the γœmodification in the crystal structure of the samples is evident.

Another characteristic feature is the comparatively low intensity of reflex (200) which, although it increases considerably in the dyed samples in comparison with the untreated polyamide multifilament yarn, remains less clearly expressed than towards reflex (002) (Figure 2). This is a kind of indication that the monoclinic areas in the crystal structure are rather far from perfection.

Figure 2. Modification in the intensity of reflexes (002), (200) and

(100) depending on the concentration of the dyes used

With the increase in the concentration of the three dyes, a continuous change in the monoclinic modification is

observed: The intensity of reflexes (002) and (200)

deepens, whereas the intensity of reflex (100) remains

comparatively constant. Moreover, this process starts even in the lowest concentration interval (0.05œ0.1%), and continues up to the highest concentrations of the acid dyes. A certain fluctuation is observed in the interval of the medium concentration (0.5œ1.0 to 2.0%), which is probably due to the disorientation processes and the relaxation of the residual tension during the penetration of the acid dyes into the more solid (intra-fibrillar) amorphous regions of the structure. It is worth mentioning that in the samples dyed with Erionyl rubin 5-BLF (the WAXS diffractograms of PA6 samples dyed with Erionyl rubin 5-BLF are shown in Figure 3 as an example), the intensity of reflex (200) almost equals the intensity of reflex (002), which shows that a considerable improvement in the monoclinic structure in the crystal phase of the fibres occurs under the influence of this dye. In higher concentrations of the acid dyes, a certain decrease in the intensity of reflex (002) is observed, probably due to the general decrease in the solidity of the

crystallites, caused by the increase in their defectiveness. Taking into consideration that in the high concentration interval the degree of crystallinity of polyamide multifilament yarn increases (Figure4), the decrease in the intensity of reflex (002) means that most probably the combination of the dye presence in the substrate and the ”cross-linking effect‘, connected with the formation of links of the type PA-dye-PA, provoke the formation of new crystallites on the one hand, but on the other hand hinder their growth into perfect crystal shapes.

Figure 3. WAXS diffractograms of PA 6 samples dyed with Erionyl rubun 5-BLF

The intensive transformation of the crystal phase of the dyed samples throughout the entire interval of concentrations of acid dyes is also confirmed by the modification of the inter-planedistances (Figure 5) [22, 24].

The fact that the inter-plane distances in the hexagonal crystal lattices (d 100) remain virtually constant throughout the entire concentration area is interesting. More intensive changes in the inter-plane distances are registered in the crystal formations of the monoclinic modification (Figure 5, d 002 and d 200). This leads to the reasonable assumption that under the influence of acid dyes the existing monoclinic shape is perfected, and at the same time conditions are created for further crystallisation with a preference for the more stable αœmodification. This assumption is confirmed by the increase in the intensity and the expansion of reflexes (002) and (200), a sign of the increase of the crystal areas with a monoclinic shape of the crystal lattice. On the other hand, the expansion of the crystal reflexes is interpreted by the increase in the defective areas in the crystallites and the presence of fine-crystal structure. This means that in the highest concentrations of acid dyes, conditions are created for rapid formation and growth of the new crystallites rather than for perfecting already existing crystal aggregates.

The dimensions of the crystallites presented inTable 1. Modification in the dimensions Table 1, as determined from reflexes (002) and (100), are ^{of the crystallites, determined for reflexes} not real but imaginary. This is because when calculating

(002) and (100)

them, neither the influence of the defects in the lattice nor the instrumental error in the shape of the reflexes have been taken into consideration. Besides, the values do not allow a correct interpretation to be made, because the modifications are within the framework of the method error.

dye	C,%	dimensions of the crystallites, Å
		/002/	/100/
Erionyl rubin 5-BLF	untr.	50.1	79.9
	0.00	49.3	71.0
	0.05	57.7	68.8
	0.10	63.6	63.9
	0.50	51.4	63.9
	1.00	60.4	62.4
	2.00	47.5	67.1
	3.00	51.5	61.9
	4.00	51.5	67.7
	5.00	50.5	75.3
Acidol orange 3 RL	0.05	61.2	79.9
	0.10	59.3	66.6
	0.50	47.7	66.6
	1.00	44.5	79.9
	2.00	47.2	72.6
	3.00	50.1	79.9
	4.00	53.5	76.1
	5.00	53.5	84.0
Acidol brillantgelb M3GL	0.05	63.5	72.6
	0.10	63.2	88.7
	0.50	50.1	84.1
	1.00	47.2	84.1
	2.00	50.1	84.0
	3.00	46.6	84.1
	4.00	53.4	72.6
	5.00	47.2	79.9

From the data of X-ray degree of crystallinitypresented in Figure 4, it is apparent that the treatment in the ”blind‘ bath results in a certain increase in the crystal phase, as compared with the untreated sample. This result is not surprising because it is known that the conditions of dyeing guarantee the transition of the polymer into a highly elastic state, in which the segmental mobility of the polymer chains is released, a prerequisite for the occurrence of relaxation processes which are different in nature and in the direction of their influence [1, 5, 6œ8].

In the lowest concentrations of the acid dyes (0.05œ0.1%), the degree of crystallinity increases considerably. This effect is probably due to the fact that under these conditions, which are far from the fibre saturation with dye, the dye molecules appear to be insufficient to compensate for the high potential energy of the active centres of the polyamide macromolecules, which favours the further arrangement of the polyamide structure. In the interval of the medium concentrations of acid dyes (0.5œ1.0%), a decrease in the degree of crystallinity is observed again, which reflects the penetration of the dye molecules into the thicker intra-fibrillar amorphous areas, characterised by a higher thickness of the packing. Evidently the diffusion and fixation of the dye particles in the above-mentioned areas are connected with the breaking of a great number of inter-molecular links, whose energy cannot be compensated with the energy of the newly created links. In other words, conditions are created for increasing the segmental mobility and expanding the conformation set of PA-macro-chains, combined with a certain decrease of the existing structural tensions, increasing the ”looseness‘ of the structure and raising the disorientation of the intra-fibrillar non-crystal regions. With the increase of the dye presence in the fibres (2œ5%), the degree of crystallinity steadily rises. Probably the so-called ”cross-linking effect‘ prevails here, involving the formation of additional inter-molecular links of the type polyamide-dye-polyamide, on the basis of ionic chemical bonds and physical forces (hydrogen bonds, and the van der Waals interactions which are various in character). This effect is accounted for not only by the pronounced affinity of acid dyes to the polyamide substrate, but also by the poly-functionality, typical for the dyes [9, 25œ27], which predetermines the formation of bonds of different

character (chemical, physical) with the chain of the fibre-forming polymer. Moreover, the dye particle can interact with more than one polymer macro-chain; that is, the dye molecules appear to act like a peculiar material agent, which connects the adjacent polyamide macromolecules at the place of contact. This justifies the expectation that the ”cross-linking effect‘ should prove to be higher with the increase in the dye concentration. But this state also leads to a certain inhibition of the segmental mobility, which is eventually a prerequisite for the appearance of areas with a pronounced close order in the non-crystal sections of the structure, corresponding to more favourable conformations in respect to energy [2œ8]. The arranged units which thus emerge can form nuclei of crystallisation, which is the probable cause of the additional crystallisation in the highest concentrations of acid dyes.

Figure 4. Modification in the X-ray - graph degree of crystallinity (α^waxs) depending on the concentration of the dyes used

◊ Erionyl rubin 5-BLF∆ Acidol orange 3 RL

� Acidol brillantgelb M-3GL

From the DSC investigations, it is evident that acid dyes (and particularly their concentration) have

a

considerable influence upon the melting behaviour of the

polyamide substrate (Table 2). All the curves of melting are

characterised by one endothermic peak, which in general

becomes sharper with the increase of the dye presence in

untr.

50

0123456 the fibres, whereas its location moves towards the higher concentration,% temperatures. An example of DSC-grams of PA6 samples

dyed with Erionyl rubin 5BLF is shown in Figure 6. It is worth mentioning that in the 4œ5% concentration of the dyes used, there is a tendency towards splitting the melting peak.

It is known that the temperature of Table 2. DSK data for polyamide fibers depending on melting depends on the degree of the different concentrations of acid dyes crystallinity, as well as on the dimensions

dye	С %	Тm ºС	∆ Тm ºС	∆Н сal/g	αDSC %
Erionyl rubin 5-BLF	untr.	219.4	17.2	20.7	37.8
	0.0	220.0	18.3	20.7	38.1
	0.05	228.9	15.4	24.8	45.4
	0.1	225.9	16.2	22.9	41.9
	0.5	219.5	21.1	20.0	37.6
	1.0	224.6	18.8	22.2	40.5
	2.0	227.5	16.0	24.2	44.3
	3.0	223.2	17.9	23.4	42.7
	4.0	228.2	19.3	24.0	43.9
	5.0	231.1	17.0	25.6	46.8
Acidol orange 3RL	0.05	226.4	16.5	23.4	42.9
	0.1	229.2	15.6	24.9	46.3
	0.5	221.5	20.1	21.0	39.2
	1.0	223.0	17.1	24.0	43.9
	2.0	228.9	18.3	24.6	45.0
	3.0	230.7	18.1	25.3	46.5
	4.0	229.0	17.3	27.0	47.8
	5.0	231.3	17.3	27.2	47.9
Acidol brillantgelb M3GL	0.05	228.0	15.9	24.6	44.8
	0.1	229.2	16.7	24.7	45.2
	0.5	224.4	19.3	21.2	40.4
	1.0	222.1	17.1	23.8	43.3
	2.0	227.8	17.3	24.1	44.1
	3.0	227.7	18.1	24.7	45.3
	4.0	229.5	17.4	26.8	47.5
	5.0	229.2	17.8	27.7	48.5

of the crystallites. But since according to the WAXS data the crystallites preserve comparatively close average linear dimensions (Table 1) while at the same time having different enthalpy of melting, it means that the change in the melting heat is due to the movement of the lower limit of the melting interval, which is determined by the respective degrees of crystallinity of the samples.

The data in Table 2 re-confirm that the concentration of acid dyes plays a major role in the direction and extent of the polymer structure‘s reorganisation during the dyeing process. The highest melting temperature and degree of crystallinity are observed in the lowest and highest concentration areas. Considering that the moving force of dyeing is the difference in the dye concentrations in the solution and the textile material, it is normal for the dye particles in the low dye concentrations to penetrate the most accessible amorphous areas of the structure. However, as the polyamide transition to a highly elastic state presupposes the system‘s aspiration towards a thermo-dynamic state, which is the most stable state for the concrete conditions, whereas the dye concentration is too low to essentially influence the high residual tensions in the structure, in the

interval of the lowest dye concentrations the processes of structural transformation will obviously mainly involve enlarging the crystallites, provoked by their high free surface energy.

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crystallinity, but also the decrease of the defects in the crystal structure formed, and primarily an improvement in the monoclinic shape. This is confirmed by the considerable change in the inter-plane distances in the crystal formations with monoclinic modification of the lattice (Figure3). The view that the medium concentrations of acid dyes activate relaxation processes, connected with the decrease in the structure tensions and the increase in the disorientation of the thicker amorphous areas of the polyamide structure, is illustrated experimentally with the expansion of the interval of melting and the reduction of the enthalpy and temperature of melting. The appearance of a shoulder on the peak of melting, showing a tendency to split in the high concentration area, is most probably due to secondary crystallisation under the conditions of the high speed of nucleus-formation, resulting in a greater number of crystallites but which are smaller in size. It seems that the kinetics of crystallisation in the high concentrations of acid dyes is also influenced by the ”cross-linking effect‘ through which, on the one hand, the macromolecular segments are fixed in relatively compact and arranged areas; but on the other hand, the filling-up of a great part of the free volumes with dye particles is bound to limit the segment movement to a certain extent. This presupposes multi-centre surface nucleus-formation and the formation of small crystallites with rough surfaces, built up from chain folds [22, 24, 28, 29], which hinders the improvement of already-formed crystal formations.

It should be pointed out that the degree of crystallinity, determined from thermo-grams (α^DSC), is lower than the X-ray degree of crystallinity (α^waxs), but this is to be expected because during the heating of the samples in the calorimeter, the re-crystallisation processes overlap with the endothermic effect of melting and lead to a decrease in the area of the melting peak . Nevertheless, it is worth paying attention to the fact that the character of modification in the degree of crystallinity, as determined by both methods, is preserved.

From Figure 7, it is visible that dyeing with acid
1 3 2							dyes considerably influences the birefringence (∆n) of polyamide multifilament yarn. It is noteworthy that the change in ∆n is a function of the dye concentration. A
							similar correlation has also been ascertained by other
							authors, studying the influence of dyeing with acid, metal-
							complex and dispersed dyes upon the supra-molecular
0,04							structure of polyamide fibres [10œ12].
0,00	1,00	2,00	3,00	4,00	5,00	6,00	On the whole, the orientation of the dyed samples
		concentration, %					increases in comparison with the non-treated sample.
							This is to be expected, as ∆n is a measure of the general

1 - Erionyl fubin 5-BLF Figure 7. Modification in the birefringence depending on the

0,07

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