THE DYEING OF POEYACRYLONITRIEE FIBRES WITH ANIONIC DYES BY R. H. BLAKER, S. M. KATZ, J. F. LAUCIUS, W. R. REMINGTON AND H. E. SCHROEDER Contribution No. 145 from Jackson Laboratory, E. I. duPont de Nemours and Company, Wilmington, Delaware, U.S.A. Received 15th June, 1953 The mechanism of dyeing polyacrylonitrile fibres is discussed for two cases in which the fibre, through its affinity for H+ and Cu+ respectively, acquires cationic centres capable of permitting absorption of dye anions. In the first, affinity for H+ is created by basic sites introduced through copolymerization and the dyeing reactions are best described by a mechanism involving sites differing in basicity. In the other, every cyan0 group in polyacrylonitrile appears inherently capable of absorbing cuprous copper through formation of a complex of the type (RCN)Cu+, which acts as a site for fixation of dye anions.The new acrylic fibres made wholly from polyacrylonitrile are difficult to dye by conventional procedures. Until recently they were considered undyeable with anionic (acid or direct) dyes 1 and in early work showed only limited affinity for dispersed, basic and vat dyes. Two new methods for producing dyeing sites in polyacrylonitrile fibres have made them easily dyeable with typical anionic dyes. The fist involves the incorporation of a limited number of basic groups into- the polymer structure through copolymerization of acrylonitrile with basic monomers, such as vinylpyridine.2 The other has resulted from the discovery that cuprous copper is readily absorbed by acrylonitrile copolymers which then acquire almost unlimited affiiiity for many anionic dyes.3 This discovery has been extended to homopolymers and has led to practical dyeing procedures which are widely used.4 This paper covers initial phases of a study of the absorption of anionic dyes by a copolymer of acrylonitrile and vinylpyridine in the presence of hydrogen ions and by polyacrylonitrile in the presence of cuprous ions.The two processes have several features in common. In each case the polymer shows substantially no affinity for anions until the appropriate cation is added. Thus the basified copolymer will absorb anionic dyes only from solutions having a relatively low pH and the homopolymer absorbs dye only in the presence of Cu+.* Adsorption of dye may therefore be represented by a simplified general expression : We have assumed that each polymer contains specific chemical features, i.e.sites, which possess affinity for the cations (Hf or Cu+) and cause their absorption together with associated anions. The data were then treated using concepts similar to those developed by Gilbert and Rideal for the absorption of acids by ~ 0 0 1 . 5 For the basic copolymer, reasonable agreement with the theory of equivalent sites was obtained under selected conditions, but not all of the data can be inter- preted satisfactorily on this basis. Accordingly, we have been forced to postulate the existence of sites differing in activity, i.e., basicity. Satisfactory explanation of the experimental results has been achieved on the basis of two kinds of sites.* Copolymer will also absorb dye anions in the presence of Cu+. This case is not considered in the present study. dye- + cation+ + fibre + dyed fibre. (1) 210BLAKEII, KATZ, LAUCIUS, REMINGTON AND SCHROEDER 21 1 The more affinitive possess an equilibrium constant 100 times as great as the others and represent 6 % of the total. The total number of sites approximates to the pyridine content of the copolymer. With the homopolymer, the situation appears relatively simple. The basic reaction appears to be one of complex formation between cuprous ion and nitrile groups of the polymer with every nitrile group a potential site for Cu+. A plot of copper in the fibre * (CLIJ)~ against copper in the bath (Cu,) derived on the assumption that copper is absorbed through a reaction of the type +Cu22+ + N ES C-polymer + Cu+, N = C polymer (2) is almost a straight line as predicted.A very high concentration of sites of only one type is indicated. Once either of the above fibres has accepted its cation and the accompanying anion, dyeing can occur by a simple anion exchange reaction : fibre. cation+. anion- + dye- + fibre . cation'. dye- + anion-. (3) This reaction is the same for both polymers, with all cationic centres equivalent in either case. The total amount of dye absorbable (by extrapolation) is equal to the number of cations present in the fibre. T. STUDIES OF THE ACRYLONITRILE -l- VINYL PYRIDINE COPOLYMER EXPERIMENTAL MA'I.miALs.-The basified fibre studied in this investigation was prepared from a copolymer containing approximately 94 % acrylonitrile and 6 % 2-vinylpyridine.A single lot of 4.0 denier bright rawstock was used in all the experiments. The fibre was scoured at the boiI with 0.5 % soap solution, rinsed, dried and thoroughly blended. The dye, 1 -phenyl-3-methyl-4-(4-sulphophenylazo)-pyrazolone-5, was selected because it is easily prepared and purified, has low neutral dyeing affinity, is not affected by pH, and had adequate stability in boiling solutions. The dye as prepared was shown to be pure by chromatographic analysis and from the absorption spectra of successive fractions recrystallized from and extracted with a variety of solvents. It was recrystallized several times from a mixture of 2-ethoxyethanol and water before use.DYEING ExPERnmNTs.-Each series of dyeings was made at constant pH with the aid of a buffer system of hydrochloric acid and sodium chloride. Experiments were made at three pH values, 1.0, 1.5 and 2.3 with a constant ionic strength of 0.1. The fibre was equilibrated with the dye solution in cylindrical glass tubes partially immersed in a constant temperature bath. Each tube was fitted with a thermometer and two ground- glass joints for insertion of electrodes for pH measurement. The solution temperature was 99.9 rt 0.1" C. The tubes containing 135 to 150 ml of buffered dye solution and 50 to 500 mg of fibre were closed during the dyeing with ground-glass stoppers held in place with springs. After the solution and fibre were equilibrated, the pH of the solution was measured 0.2" C below the dyeing temperature.The fibre was then removed from the solution, rinsed, and stored in a desiccator over a saturated solution of sodium dichromate (relative humidity approximately 50 %). Both the fibre and the solution were then analyzed for dye and chloride. The equilibrium absorption of the dye at the 3 pH's studied is presented in fig. 1. ANALYTICAL.-Dye concentrations were determined with the aid of a spectrophoto- meter. The concentration of dye on the fibre was determined by dissolving the dyed fibre in butyrolactone and measuring the optical density of this solution, The concentra- tion of the dye in the aqueous solution was determined by spectrophotometry of the solution (Amax = 3900& molar absorbancy index = 2.78 x 104 1. mole-1 cm-1).Chloride analyses were made by a potentiometric titration with silver nitrate. For the fibre analysis a sample was burned with oxygen in a bomb. * Throughout this paper subscripts f and s indicate fibre and solution respectively.212 DYEING OF POLYACRYLONITRILE FIBRES ACCURACY OF EXPERIMENTAL RESULTS.-Three dyeing experiments were made for each set of conditions and all the experimental data are shown on the accompanying plots. Many of these repetitive groups of three were duplicated for longer time intervals to determine whether equilibrium had been reached. Sufficient dye solution was pre- pared in one lot for all repetitions so that the initial conditions could be considered to be the same in all three (or six) dye vessels. In all cases the chloride ions in the solution (for the same period of dyeing) agreed within 10 % among the repetitive samples, and in most cases within 5 %, but wider variations were observed in chloride determinations from one set of runs to another at the same pH, although the solutions were always made up from the same stock buffer at each pH.5 10 15 20 25 FIG. 1 .-Experimental data for absorption of dye by copolymer. Duplicate experiments were usually run for both 96 or 168 h. A somewhat lower dye concentration in both solution and fibre was observed for the longer dyeing time. This loss of dye may be attributable to thermal decomposition. When the results were corrected for this loss of dye, the results for 96 and 168 h were in excellent agreement and indicated that equilibrium had been reached.Two attempts to confirm the attain- ment of equilibrium by desorption from material dyed to saturation indicated that more colour remained on the fibre than would have been predicted from absorption experiments. Presumably the excess of dye in the fibre after desorption must be attributed to a hys- TABLE ~.-DESORPTION EXPERIMENTS desorption relative amount teresis effect, unless the fibre suffered a per- time, ofdyein manent change through being dyed to h fibre * saturation. This point was not investigated PH 1.0 96 1.39 further. 240 Although the pH of the dyebath is an im- 2.3 96 2.43 portant factor in the postulated dyeingmechan- 240 2-4 1 ism, pH at 100" C is not well defined. Our measurements were made with a Beckman model G pH meter using high-temperature calomel (8970-90) and glass (8990-90) elec- trodes.The reference was a standard buffer solution at the same temperature as the dyebath. The pH of the standard buffer is not known at this temperature but was estimated by extrapolating from lower temperatures. Consequently, all pH deter- minations on the solutions are uncertain by an amount equal to the uncertainty in this extrapolation. This uncertainty is a constant additive term to all the pH values in this section and accordingly affects the absolute magnitudes of the dyeing equilibrium constants. The random errors involved in pH measurement amounted to about 0.004 pH units. 1.35 *This factor represents the observed amount of dye divided by the amount of dye which would have been predicted from absorption experiments.BLAKER, KATZ, LAUCIUS, REMINGTON AND SCHROEDER 213 DISCUSSION We propose that anionic dyes are absorbed by the copolymer as indicated in the following equation : The equilibrium constant of this reaction is where Of = concentration of dye in fibre (moles/kg), Hs = hydrogen ion activity in solution, Ds = activity of dye anion in solution, Ds- + Hsf + site + (site Hf D-) = Df(dyed fibre).(4) Kd = Df/HsDsC, (5) C = concentration of unoccupied sites in the fibre (mmoles/kg). Since the buffer anion is absorbed also and competes with the dye for protonated sites, the magnitude of C is given by C = S - D f - AS, (6) FIG. 2.-Determination of A f . where S = total number of basic sites in the fibre (mmoleslkg), Af = concentration of buffer anion in the fibre (mmoles/kg) and, just as for the dye, the absorption of buffer anion is given by Af]C = Aj*/(S - Df - A ) = KaHsAs, where A, = buffer anion activity in solution, Ka = equilibrium constant for the absorption of buffer anion. By substituting for C, and rearranging, eqn.(5) becomes (7) Df/HsDs == KdS - G(Df Af). (8) According to eqn. (8), then, a plot of Df/HsDs against ( O f + Af) should give a straight line with slope - Kd and intercept S, on the abscissa, and KdS on the ordinate. Since the experimental determinations of Af gave a large variation in this quantity, eqn. (7) was rearranged so that average values of Af could be determined : where a -- Sb and b = KaHsAs/(l + KaHsAs). The values of a and b were determined from plots of measured values of Af against Of for each pH, as shown in fig.2. The straight lines of this plot are least Af = a - bDf, (9)214 DYEING OF POLYACRYLONITRILE FIBRES square curves, from which the values given in table 2 are taken. The constants, a and b, were then used to calculate smoothed values of Af. A plot according to eqn. (S), using the calculated values for Af, together with the experimental values of Df, Ds and Hs, is shown in fig. 3. It can be seen that the data for pH 1.036 agree very well with the proposed equation, since a straight line can be drawn through the experimental points. There is less agreement at pH 1.513 and no agreement at all at pH 2,305. Clearly, if dyeing does proceed according to eqn. (3), different types of sites, having different equilibrium constants, must be involved.Thus, if there were a relatively small number of highly affinitive basic sites, TABLE 2s-VALUES OF a ANDb essentially all of them might be dyed in 1.036 0.1 178 0.2079 apparently straight line would result. At 1.513 0.0496 0.0823 the higher pH of 2.305, with a hydrogen 2.305 0.0262 0.0674 ion activity one-twentieth as great, dis- crimination between sites of different basicity would be possible and the observed curve could result. Quite possibly, then, the basic sites in the fibre exhibit a range of basicity but we have found the assumption of only two types of sites adequate. On the assumption that two types of site, S1 and S2, exist in the fibre, differing from each other in their affinities for protons, a new equation for Of can be written : A similar equation can be written for Af, and by combining the two, the following expression for Df can be obtained: PH a b all the experiments at pH 1.036, and the of = Dfl + Dh = Kd,-f&Ds(sI - Dfl - Afl) -k Kd2HsDs(S2 - Df2Af2)- (10) 0.1 0.2 0.3 0.4 0.5 FIG.3.-Absorption of dye by copolymer. Afcan be expressed by a similar equation. It is then possible to plot Df/HsDs as a function of Df -k &in terms of the parameters S1, 5'2, Kdl, Kd2, K,, and Ka2. The values of these parameters are given in table 3 and the plot is shown as the solid line in fig. 3. As a first approximation the values for one class of sites S1, Kdl and K,, were calculated from the experimental data at pH 1'036, since these data, as shown by fig. 3, agree with a single site mechanism.These values were then used in eqn. (8) to calculate Dfl. Dh was estimated as the difference between Of and Dfl. The values of the parameters given in table 3 were estimated as follows.BLAKER, KATZ, LAUCIUS, REMINGTON A N D SCHROEDER 21 5 Ah, S2, Kd2 and Kaz were then obtained from the following equations employing the data at pH 2.305 : Afi DfiKa,AS/Kd,Ds, (12) (1 3) Ka2 = KdZKa1/Kdl. (14) Eqn. (14) constitutes the reasonable assumption that the differences among sites are solely in their affinity for protons. In other words, all cationic centres in the polymer are regarded as having equivalent affinity for any given anion. These values of Kd2, Ka2, S2 and Af2 were then used to recalculate the values of Kd, Ka, S1 and Affrom the data at pH 1.036.This process was then repeated until no further changes in Kd,, Kd2, S1 and S2 were observed. It has been shown that it is necessary to postulate the presence in the fibre of dyeing sites of different affinities in order to account for experimental data. It is not unreasonable to assume that the basicity of the pyridine centres in the copolymer can be affected by interactions with neighbouring groups in the individual molecules of polyacrylonitrile or interactions between adjacent molecules of the polymer. Thus the crystallinity of the fibre may be involved as well as the dis- tribution of pyridine centres along the polymer chain, the number of pyridine end- groups, and the effect of sulphonic acid end-groups on the basicity of neighbouring pyridine centres.These sulphonic acid sulphate-catalyst. determined by the analysis of the experi- si 0.558 equiv./kg mental data (0.589 equiv./kg) agrees with s 2 0.03 1 equiv./kg 7.94 x 103 1.2/equiv.2 7.66 x 1 0 5 1.2/equiv.2 the pyridine content of the fibre (0.570 Kdl 2.85 x 101 1.2/equiv.2 equiv./kg). This agreement supports the Kdz 2.75 x l o 3 1.2/equiv.2. hypothesis that the anionic dye is absorbed on the pyridine centres in the copolymer. If the sulphonic end-groups are ionized, a dyeing mechanism based only on two types of basic sites would not be valid. However, the data have not been analyzed in terms of a zwitter-ionic fibre because the resulting equations would be much more complicated and the simpler mechanism adequately describes the experimental results. Df2 4- A, = s2 - Af2/HSDSK&, end-groups have their origin in the per- TABLE 3*-EST1MATED VALUES OF PARA- METERS FOR EQN.(11) estimated value The total number of sites, S1 + 5'2, as parameter Kal KQz TI. STUDIES OF ACRYLONITRILE HOMOPOLYMER EXPERIMENTAL MATERIALS.-A single lot of 3 denier polyacrylonitrile staple fabric containing 0.4 % titanium dioxide as a delusterant was used in all experiments. The fabric, a 2 x 2 twill weave of singles yarn, was scoured at 70" C with 0.3 % lauryl sulphate, rinsed and dried in a pin tenter at 130" C. The anionic dye, the pure sodium salt of l-amino-4-anilino-anthraquinone-2-sulphonic acid, was prepared by four recrystallizations of the commercial dye from distilled water. A paper chromatogram indicated that the recrystallized material consisted of a single component.ANALYTICAL PRoCEDuRES.-Copper in the fibre was determined electrolytically sub- sequent to solution of the fibre in hot nitric and sulphuric acids. Copper in solution was determined colorimetrically using zinc dibenzyl-dithiocarbamate. The analysis of solutions for dye was made by measuring the transmission of a sample with a Cenco photelometer and comparing the transmission measurements with a standard curve of transmission against concentration for the purified dye. Analysis of dye on dyed fibre was made as follows. The dye was stripped from the fabric by boiling with ethylene glycol for 20 min. The analysis of dye in solution in ethylene glycol was made by optical means as above. Preliminary experiments indicated that with this particular216 DYEING OF POLYACRYLONITRILE FIBRES dye and fibre combination, no dye was lost by this procedure for recovering the dye fsom the fibre.VALENCE OF THE CoPPrx-The fibre does not absorb copper readily from solutions of cupric salts unless reducing agents are present. This suggests that copper must be in the cuprous valence state to be absorbed by polyacrylonitrile. A large amount of indirect evidence supports this conclusion ; for example, if glutaronitrile was shaken with an aqueous solution of copper sulphate and a blue anionic dye, 1-amino-4-anilino- anthraquinone-2-sodium sulphonate, only the water layer was coloured. If then a reducing agent, such as sodium bisulphite, was added and the solution shaken, the dye immediately transferred to the glutaronitrile layer.The relative amounts of copper and chloride found in the fibre after the following experiments also indicated that the copper entered the fibre as the cuprous ion. Two samples of polyacrylonitrile fibre were boiled in aqueous solutions to which were added different amounts of cupric chloride, cuprous chloride, and hydrochloric acid. The fibre was then analyzed for copper and chloride. expt. TABLE 4.-cHLORIDE/COPPER RATIO analysis, % ~- copper chloride atomic ratio, Cl/CU 1 4 .O 2.06 0.93 2 5.7 2.98 0.94 These results suggest that a single chloride ion accompanied each copper ion which entered the fibre. NATURE OF ATTACHMENT OF THE COPPER.-The fact that copper is readily absorbed by polyacrylonitrile only when it is in the cuprous valence state suggests that the nitrile groups of the polymer are probably involved in the formation of complexes analogous to those described by Morgan for simple nitriles.6 These complexes are probably of the 7~ type since those metals, Cu+, Ag+, and Pt+, which are unique in their ability to accept 7~ electrons are also unique in exhibiting affinity for polyacrylonitrile and in pro- moting the dyeability of these fibres with anionic dyes.The strength of these co-ordinate bonds between the cuprous ion and the nitrile groups would be expected to be small. The apparently slight temperature dependence of the equilibrium copper absorption, described below, supports the belief. Further confirma- tion was obtained from the observation that the cyanide ion, which forms strong complexes with cuprous ion, completely inhibited the absorption of copper.The weaker-complexing citrate ion had less effect on the absorption of copper by the fibre. METHODS FOR USE OF CoPPER.-The cuprous copper is best produced in solution from cupric salts and reducing agents. Some of the first reducing agents used were ferrous sulphate, sodium hydrosulphite, and sodium formaldehyde sulphoxylate. Extreme care had to be used with these reducing agents since, if the concentration was too high or if the solution was heated too rapidly, metallic copper was formed. To avoid this difficulty it is necessary to generate the cuprous ion only as fast as it can be absorbed by polyacrylonitrile or to form a weak complex of cuprous ion in solution. This complex must be stable enough to prevent the disproportionation but weak enough for the cuprous ion to be available for absorption by the polyacrylonitrile.Hydroxylamine sulphate has been found to be particularly effective in reducing the cupric ion and at the same time maintaining the free cuprous ion at a sufficiently low con- centration to avoid the disproportionation. This stabilization is apparently due to the formation of a weak complex between hydroxylamine and the cuprous ion. The complex ion probably involves hydroxylamine itself, rather than its decomposition products, since the cuprous ion is not stabilized unless an excess of hydroxylamine is present in solution. Efforts to identify this complex from spectral and polarographic data have not been entirely successful.An alternative method of reducing the cupric ion and still avoiding the danger of disproportionation of the cuprous ion is to employ the common reducing agents, such as sodium or zinc sulphoxylate, under controlled redox potential conditions.7 EFFECT OF TEMPERATURE ON ABSORPTION OF COPPER.-Preliminary experiments indicated that polyacrylonitrile fibres absorb copper rapidly from a boiling solution containing copper sulphate and hydroxylamhe sulphate. The effect of temperature on the rate of copper absorption is shown in fig. 4. Also shown on the figure is the rate of copper absorption from a solution containing only cupric sulphate.BLAKER, KATZ, LAUCIUS, REMINGTON A N D SCHROEDER 21 7 The data presented in the figure indicate that the rate of absorption is strongly dependent on the temperature and is very slow below 93" C.Other experiments not shown on the figure indicated that, at 93" C, 17 h were required for the same amount of copper to be absorbed as in 1 h at 100' C. EFFECT OF pH ON COPPER AssoRPTroN.-Preliminary experiments indicated that pH of the solution has a large effect on the amount of copper which is absorbed by poly- acrylonitrile. The effect of pH on copper absorption is shown by fig. 5. These data 9 8 7 6 1 5 5 g4 2.5% Copper Sulphate J 2.5 Hydroxylomino Sul p hate (v u- u 3 2 I 0 B 2 3 4 FIG. 4.-Effect of temperature on rate of copper absorption. - r I I I I 1 0- - 0.5 - - 4Y0 Copper Sulp hote 4% Hydroxyiamine Sulphote 2.5gFibre in 100ml. 2 Hours at 100°C 0.4 - 0.3 - - - 0.2 - - 0.1 - - I I I I show that the absorption of copper was decreased rapidly as the pH of the solution was decreased and that relativeJy little copper was absorbed if the solution had a pH of less than 1.0.EQUILIBRIUM ABSORPTION OF CoPPER.-The equilibrium absorption of copper by POlY- acrylonitrile is affected by copper concentration as is shown by fig. 6. The experiments at 100" C were run for 15 h. In each case 2.5 g of fibre was dyed in a 100 ml dyebath FIG. 5.-Effect of pH on copper absorption. containing equal weights of CuSO4. 5)3[20 and (NH20H)2 . H2SO4. In addition the equilibrium absorption of copper up to a concentration of 150 mmoles/kg fibre was measured at 93" and 97" C. The results were nearly identical with those obtained at 100" C and therefore indicated a low heat of absorption.ROLE OF COPPER IN THE DYEING REACTION.-^ the actual dyeing of the polyacrylo- nitrile fibres, cuprous ion is absorbed with an accompanying anion. This may be an218 DYEING OF POLYACRYLONITRILE FIBRES anion from the solution or a dye anion. Fig. 7 shows the changes which take place during dyeing in a dyebath which originally contained cupric sulphate, hydroxylamine sulphate, pure sodium salt of the anionic dye, and a sample of polyacrylonitrile fibre. Cuprous ions are first absorbed by the fibre, presumably through formation of a complex with nitrile groups of the polymer. To maintain electrostatic neutrality, the cuprous ions FIG. 6.-Experimental data for equilibrium absorption of copper. are accompanied by sulphate ions and dye anions. Sulphate ions are absorbed more rapidly in the initial stages of the dyeing.Eventually the dye anion, because of its intrinsically greater affinity for the copper containing fibre begins to displace the sulphate ion from the fibre and the concentration of sulphate in the bath increases. In the case where polyacrylonitrile is treated with Cu+ and sulphate ions, dye ions can later be ex- changed for the sulphate ions. A 5 g sample of fibre was boiled for 2 h in 100 ml of water which contained 0.56 g cuprous oxide and 10 m.equiv. of sulphuric acid. The coppered fibre was then removed, FIG. 7.-Interchange of copper, dye and sulphate with fibre during dyeing. rinsed thoroughly, and added to 100 ml of water containing 100 mg of pure monovalent anionic dye. The solution was heated at the boil until all of the dye was absorbed by the fibre, The solution was then analyzed for copper and sulphate ion.The analytical data iiidicate that 2.2 X 10-4mole of dye was absorbed by the fibre and that 5.2 x 10-4 mole of copper and 7.1 X 19-4 mole of sulphate ion appeared inBLAKER, KATZ, LAUCIUS, REMINGTON AND SCHRODER 219 the bath. These data are consistent with the hypothesis that a univalent dye ion replaces a univalent bisulphate ion in the fibre. ments were made with 2-5 g samples of polyacrylonitrile fibre which had been boiled for 111 in 100 ml of aqueous solution containing 0.063 g CuSO4. 5H20 and 0.063 g (NH2OH)2H2SO4. This fibre was rinsed thoroughly with hot water and was then used for the dyeing experiments. The dyeings were made in the glass tubes described for the copolymer.The closed tubes were placed in an oil bath maintained at 100” C 4 0.1”C. The dyeings were continued for 178 h since preliminary experiments indicated that this was sufficient time for equilibrium to be reached. DYEING EXPERIMENTS WITH COPPER-TREATED POLYACRYLONITRILE.-Dy&g experi- TABLE 5.-EQUILIBRIUM ABSORPTION OF DYE BY COPPER TREATED FIBRE dye in bath (equiv./l. x 105) 0.5 4.6 18.0 5-8 67.0 175.0 5-6 11.0 26 -0 11.0 27.0 108.0 dye on fibre (equiv./l. x 103) 10 23 30 5 23 30 9 15 24 18 27 36 sulphate in bath (mole/I.) none added ” 0.07” 0.07 0.07 0.10 0.10 0.10 0.24 0.24 0.24 A series of dyeings were made with the copper treated fabric and solutions of dye, sodium sulphate, and sufficient sulphuric acid to give an initial pH of 3.0.After the dyeings were completed the dyebath and the fibre were analyzed for dye and copper. The samples had an average copper content of 4.2 x 10-2 molelkg before equilibration. The dye analyses are tabulated in table 5. DISCUSSION THE REACTION OF COPPER WITH POLYACRYLONITRILE.-~t iS Clear from the evidence presented above that cuprous ions are absorbed by polyacrylonitrile because of the presence in the polymer of nitrile groups. The large number of groups, one for each monomer unit in the polymer, which could combine with the cuprous ion might be expected to lead to solution of the copper in the polymer. The experimental data indicate that a simple solution mechanism is not obeyed since, as shown by fig. 6, a linear plot is not obtained when Cufis plotted against Cu,.This, together with the other experiments which have been described, suggests a chemical reaction wherein Cu+ and CN groups interact to form a moder- ately strong co-ordination complex. The interpretation of the data is greatly complicated by the fact that most of the cuprous copper in solution exists in the form of a cuprous ion + hydroxyl- amine complex. The exact composition of this complex is not known but there is some indication (from polarography) that it may contain two atoms of copper.8 On this basis, the reaction for copper absorption might be written : (1 5 ) 2 sites + (Cu2. hydroxylamine)x + 2HS04- + 2 (CuHS04 . . . site) + x hydroxylamine. If variations in concentration of the bisulphate ion and hydroxylamine are neglected, then the reaction may be described by the simplified expression, (Cu22+)s s 2CUff.(1 6) Then if the cuprous ion + hydroxylamine complex contains two cuprous ions, a plot of (CUJ)~ against Cus should give a straight line. Fig. 8 shows that220 DYEING OF POLYACRYLONITRILE FIBRES this method of plotting the data does approach a straight line as does a similar plot or (Cuf)3 against Cu,. Better agreement with a straight line, however, is given by a plot of (Cuf)2*5 against Cus. ]In the absence of detailed knowledge about the structure of the cuprous ion+ hydroxylamine complex but with the realization that its composition may be influenced by relative concentration of hydroxylamine in solution and the pH of the solution, it is plausible that the absorption of copper over a wide range of concentrations can be interpreted in terms of a single complex for Cus involving an average ncmber of cuprous ions.If this mechanism is accepted it follows that the absorption of copper is not dependent on the existence of a limited number of specific configurations, or sites, and in all probability that each of the nitrile groups of the homopolymer is a potential site for absorption. 0.6 0.5 0.4 0.3 0.2 0.1 FIG. 8.-Equilibrium absorption of copper. THE ION EXCHANGE REACTION.-The experiments described above suggest that once cuprous ions are absorbed by polyacrylonitrile, they will function as cationic centres which are capable of exchanging a simple a dye anion from a dyebath. For this exchange, the expression may be written : K = AsDjIDsAf, where A, = activity of simple anion in bath (moles/l.), Ds = activity of dye in bath (moles/l.), neutralizing anion for following equi I i brium Df = concentration of dye in fibre (mmoles/kg), AJ = concentration of simple anion in fibre (mmoleslkg). If we represent the total (assumed constant) concentration of cationic sites in the fibre by S, it follows that (1 8) Thus, if the quantity (hereafter referred to as A) on the left side of this equation is plotted against Df a straight line should result.However, when the data from the section on dyeing copper-treated poly- acrylonitrile are plotted (fig. 9) according to eqn. (18), they define a separate straight line for each value of As, if activity coefficients are set equal to unity. Allowance for the effect of sodium sulphate concentration on activities may be made by writing DfASf Ds = KS - KDf. R[y = Ks - KDJ. (19)2 0.07 Molesll. NatSo+ 3. 0.10 Molesll. No2504 4.0.24 MolesII. NagS04 FIG. 9.-Absorption of dye by homopol!7mer. 1 I 1 . 1 I 1 I I I 0 0.24 molesll. Nap304 7=600 4 0.10 moles/l.No2S04 1-240 n 0.07 moleslL Na2S04 7.60 6- - number of dyeing sites is approximately the same for all samples of fibre and is independent of the amount of salt. The same is true for fig. 10. The number of dyeing sites, approximately 4 X 10-2 equiv./kg, as determined from fig, 9 is in excellent agreement with the analyzed amount of copper on the fibre at the beginning of the dyeing experiments (4.2 x 10-2 mole/kg). This is added support222 DYEING OF POLYPEPTlDES for the hypothesis that at equilibrium each cuprous ion in the homopolymer is a potential dyeing site for a monovalent dye anion. A better understanding of the reactions involved will undoubtedly result as further studies show more precisely the effect of pH and anion concentration and the nature of the cuprous copper complex in the aqueous dyebath. The authors wish to acknowledge the particularly helpful assistance of DT. J . H. Trepagnier, A. W. Bauer, C. A. Young, J. Hyde and E. K. Gladding. 1 Thomas and Meunier, Amer. Dyest. Report, 1949, 38, 925. 2 Arnold, U.S. Pat. no. 2,491,471 (1949). 3 Feild and Fremon, Text Res. J., 1951, 26, 531. 4 BIaker and Laucius, Amer. Dyest. Report, 1952, 41, 39. 5 Gilbert and Rideal, Proc. Roy. SOC. A, 1944, 182, 335. Remington and Gladding, 6 Morgan, J. Chem. SOC., 1923, 123,2901. 7 Blaker, Amer. Dyest. Report, 1953, 42, 76. 8 Marcali, private communication. J . Amer. Chem. SOC., 1950, '72, 2553.