Oxidative dyeing of keratin fibers

Transcription

1 j. Soc. Cosmet. Chem., 36, (January/February 1985) Oxidative dyeing of keratin fibers KEITH C. BROWN, STANLEY POHL, ANNE E. KEZER, and DAVID COHEN, C/alto/Research & Development Laboratories, 2 B/ach/ey Road, Stamford, CT Received October 31, Presented at the Annual Meeting of the Society of Cosmetic Chemists, New York, December 6-7, Synopsis Dye uptake by hair and wool cloth has been measured by a reflectance technique. Using a statistical experimental design, color formation from a single pair of reactants has been used to define and quantify many of the factors that control the process. Although peroxide concentration has little effect, color intensity depends on the concentration of reactants and added surfactants and on the dyeing time. These results can, in some cases, be predicted from the solution kinetics of the coupling reaction where the ratecontrolling step is formation of a reactive intermediate from hydrogen peroxide, followed by a series of faster oxidative and coupling steps to give the dye. However, there are significant differences between the dyeouts and solution chemistry reflecting the important role of the substrate on the dyeing process. INTRODUCTION Previoustudies on the chemistry of oxidative dyeing of keratin fibers have concentrated largely on the reactions occurring in dye solutions (1-3). While these results appear to correlate with actual fiber dyeouts in most cases, there are significant differences reflecting both chemical and physical effects of the substrate. In addition, the oxidant used for the solution studies, potassium hexacyanoferrate, is not commonly used in dyeing practice. We now report results on dye uptake by hair and wool and on the factors that control the process when hydrogen peroxide is used as oxidant. In particular we have evaluated the amount of color deposited on hair and wool from two oxidative dye couples in use in current hair dye formulations: p-phenylenediamine (PPD)/4- amino-2-hydroxytoluene (AHT) and N, N-bis-(2-hydroxyethyl)-p-phenylenediamine (BHP)/1-naphthol (NAP). This paper describes the variation of dye uptake with factors such as dye and H202 concentration, solution ph, dyeing time and concentrations of ammonia and surfactant in the dye solution and compares the results with those predicted from the kinetics and mechanism of the solution reaction. EXPERIMENTAL All dyeing solution components were commercial materials used without further puriffcation. Samples of indo dyes were isolated from coupling reactions using atmospheric 31

2 32 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS oxygen as oxidant. The Piedmont hair and wool gabardine were commercial samples from Arkinco Inc., New York, and Test Fabrics Inc., Middlesex, NJ, respectively. Hair was fabricated into 2.5 g swatchesecured by wax, and the wool cloth was cut into 2" X 2" squares weighing g. After soaking in the formulation, the dyed material was rinsed with deionized water for 5 sec and then dried. The amount of dye deposited on the fiber was determined from the reflectance spectrum measured by a Hunter LabScan Spectrocolorimeter, Model LS In general, the reflectance spectra were similar to the transmission spectra of the dyes in solution but with broader, less well-defined peaks. Spectra from wool were sharper than those from hair. RESULTS EXPERIMENTAL DESIGN We have used a standard response surfacexperimental design. Of the variety of available designs, the 5-variable, central composite rotatable second-order design (4) appeared to offer the most information from the least number of experiments in our system. The specific factors studied were: dye concentration--[ppd] or [BHP] varied from mm/l, with equimolar coupler added ph--9 to 11, adjusted with NH4OH/NH4C1 solution ß H20 2 concentration--1.0 to 11.0% (0.3 to 3.2 M) surfactant concentration--nonoxynol-9 varied from 0.5 to 4.5 wt % dyeing time--5 to 25 minutes However, we also evaluated ammonia concentration over the range % at fixed ph, but it had negligible effect on dye deposition, and sodium chloride concentration, where a significant salt effect was observed. All experiments were, therefore, carried out at constant ionic strength (1.1) by adding NaC1. The design required 32 dyeings for each substrate using the values of each of the variablespecified in Table I. After dyeing, the reflectance of each sample was measured and these values were converted into a response surface using the method described in reference 4. The specifics of the design and the data manipulation will not be described here, since they were carried out precisely as described. Table Values of Variables Used in Experimental Design [Dye] ph Time [8202] [Surfactants] 1.0 mm/l min. 1.0% 0.5 wt % I

3 OXIDATIVE DYEING OF KERATIN 33 REFLECTANCE MEASUREMENTS There is no absolute mathematical relationship between the measured reflectance (R) of a dyed fiber and the concentration of adsorbedye (C). However, the Kubelka-Munk Equation [ 1] is an approximation which is commonly used because of its ease of practical application (5). (1 - R)2/2R = KC... [1] In this equation, K is a constant for each dye, and its value was determined using fibers dyed with pre-formed indo dyes. Hair and cloth swatches were separately dyed with solutions of various concentrations of the indo dyes, and the amount of dye removed from solution by the fibers was measured spectrophotometrically. By measurements of R at various solution concentration differences (i.e., absorbe dye intensities), K can be evaluated graphically (Figure 1). The observed linearity shows that K is generally invariant with concentration as predicted by the equation and has the values for the PPD/AHT dye on wool 101, on hair 571 and for the BHP/NAP dye on wool 107 and hair 333. These values were then used to evaluate adsorbedye concentrations from the reflectance readings of dyeouts from reactions of the individual couples. DISCUSSION Dye can be deposited in a fiber by any of the following means: I :,7 I 61 n :- 4,4, "-' 0 9 r-pt [v!,3 le: 5' F:',,.,'*,.'{ :O r" E:: ::.'J,' C-r'r-, ':,:':/:-':,- l _. _.,.,_. _...._.._. Figure 1. Variation of reflectance function with concentration of adsorbedye for dyeing wool fabric with 2-amino-5-methylindoaniline (PPD-AHT dye). Slope of line is Kubelka-Munk constant K.

4 34 JOURNAL OF THE SOCIETY OF COSMETI CHEMISTS unreacted dye intermediates diffuse in and then undergo coupling indo dye is formed in solution and then diffuses in dye precursors are formed in solution, they diffuse in and undergo further oxidation to the dye Qualitative observationseem to support all three modes of coloration. Certainly color will slowly develop in the colorless hair removed from a dye bath after a short soaking time. In addition, hair can be dyed very effectively in the highly colored dyebath that results after 45 minutes of reaction with hydrogen peroxide, or from solutions of preformed indo dyes. The experimental design used in the current work assumes that the amount of dye deposited in a fiber can be expressed as a polynomial involving firstorder, square, and product terms of all the 5 individual variables, i.e., [Dye]^b = B0 + B [Dye] + B [Dye] 2 + B 3 (time) + B33 (time) B 3 [Dye][time]... etc. The design evaluates the coefficients (B., B2, B12, etc.) of these terms and determines their significance in relation to the calculated experimental error. Coefficientsmaller than the error are reduced to zero, and the associated term has no significant effect on dye deposition. The calculated values of all significant coefficients are shown in Table II. Since these values are coefficients of a response surface equation, no entry in a column means that that term does not contribute to dye deposition. Table Coefficients (B) of Response Surface Equations II PPD/AHT BHP/NAP Coefficients* Cloth Hair Cloth Hair B o (1.36) (4.68) (1.92) (1.36) B [Dye] B 2 (ph) B 3 (time) B4 [H202] B 5 [Nonoxynol-9] B B B33 -- B44 -- B B B B B B23... B B25... B B B * B t2 represents the coefficient for interaction of dye concentration with time. B represents the coefficient for a dye concentration square term.

5 OXIDATIVE DYEING OF KERATIN 35 It seems clear from these results that the reactions are more complex on hair than on wool, since many more factors are significant. However, generally dye deposition increases with dye concentration, dyeing time, and solution ph, and decreases with added surfactant. It was generally independent of the amount of hydrogen peroxide. The most important factor in determining dye uptake was the concentration of dye, and it appeared to vary largely as a first-order function. However, in the case of PPD/ AHT on hair, there was also a second-order dependence (square term), which probably reflects simultaneous deposition of finished dye. This effect is absent with the BHP/ NAP couple, probably since the indo dye from this couple is unstable to the peroxide and, therefore, it does not accumulate in solution. It is also absent in the wool cloth dyeings, but in this case, the rate of dye uptake is so fast that color saturation occurs before indo dye diffusion becomes important. Time is also a significant variable for the PPD/AHT couple, but much less so for the BHP/NAP reaction, where again the instability of the indo dye to peroxide does not permit any dye deposition once the intermediates are exhausted. In all cases, Nonoxynol-9 (an ethoxylated nonylphenol) had a negative effect on dye deposition. This was also tested with sodium lauryl sulfate with similar results. The effect of surfactant was greatest in cases where diffusion of the indo dye was a key factor. Surfactant, due to its cleansing ability, may prevent accumulation of dye (or dye precursors) at the hair surface or it may tend to hold the dyes in solution and make them less available to the substrate. In any event, there is probably lower apparent rate of diffusion of dye to the substrate in the presence of surfactant, thus reducing the overall rate of coloration. The absence of any effect from peroxide concentration is not easy to explain. It may be due to the large molar excesses over dye employed in these experiments, but is more likely due to a balance between dye formation and dye loss by excess peroxide. The effect of ph, while not large, is in the direction expected for an increase in the oxidation rate by H202 on the primary intermediate. This effect is moderated by a faster rate of indo dye decomposition at higher ph. Most of the secondary effects are absent on cloth with the PPD/AHT couple. However, there are a significant number of interactions in the hair dyeings. Particularly noticeable are the interactions of dye concentration with time and ph, and the negative effect with surfactant. These are probably related to the longer time needed for diffusion of indo dye formed at higher concentrations and the additional swelling of the hair. These interactions are much less evident with the BHP/NAP couple. The differences between wool and hair seem to be related to the greater porosity of the wool, resulting in a faster and more intense coloration. CORRELATION WITH MECHANISM There has been no thorough mechanistic study of oxidation dye chemistry using hydrogen peroxide as oxidant, partially because of the slowness of color formation and the sensitivity of many of the formed dyes to the high concentrations of H202 necessary to obtain reasonable reaction rates. It was shown (1), using hexacyanoferrate as oxidant, that color formation proceeded in three steps: Initial oxidation of the primary intermediate to a reactive imine

6 36 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß Reaction of the imine with the coupler to give a diphenylamine ß Oxidation of the diphenylamine to the dye. With hexacyanoferrate, the first and last steps are fast and the kinetics of the slower coupling reaction can be evaluated. For all phenolic couplers studied, the reaction mechanisms followed a similar pathway. With H202 as oxidant, color formation is many times slower than the coupling reaction, suggesting that the initial oxidation is rate-controlling. However, indo dye decomposition by H20 is a competitive process, especially in ot-naphthol and 4-amino-2-hydroxytoluene reactions, and thus accurate coupling rate constants are difficult to determine. Much slower dye decomposition is observed for the couple PPD/2,6-dimethylphenol, and some rate data have been obtained. Even so, the overall scheme is not simple since the reaction rate is relatively independent of primary intermediate concentration and proportional to [H O ] ø'5 (6). A possible mechanism involves rate-determining formation of reactive intermediate from peroxide followed by a series of faster oxidative and coupling steps, i.e., slow H202 k 2X X + PPD,Imine Imine + Coupler Leuco dye -Dye., Leuco dye Analysis of this scheme gives the rate equation d [Dye]/dt = k [H O2] ø'5, which satisfies the experimental data. Thus, although this mechanism predicts an increase in reaction rate with increased peroxide concentration, in dyeing practice this effect must be moderated by the increasedye decomposition rate at higher peroxide concentration. Also, this mechanism predicts no effect of dye concentration, whereas this is the main factor observed in the dyeouts. Presumably, this may reflect a change in mechanism in, or at the surface of the substrate, or may reflect the importance of diffusion processes during dyeouts. The mechanistichange could involve an alternate pathway for H O decomposition, or a change in rate-determining step so that imine formation becomes rate-controlling. The nature of intermediate X would aid in distinguishing between the possibilities. However, as yet we have no identification. Finally, the rate of reaction shows a moderate increase with solution ph as is observe during the dyeouts. This clearly points to a rate-determining step that is not coupling since the coupling rate decreases rapidly with ph over the range 9-11 for these couplers (1). CONCLUSIONS Using reflectance measurements to quantitate the amount of adsorbe dye in hair or wool appears to be a valid technique. It is clear that the major factor controlling dye deposition from oxidation dye solutions is the concentration of the dye precursors. This presumably has a strong effect on both dye diffusion and on its rate of formation. In addition, the indo dye formed in sobation does contribute to coloration if it is sufficiently stable to the excess H O. Other factors having some effect are iolution ph and dyeing time. Surfactants reduce dye deposition probably by slowing diffusion. Peroxide concentration over the range 1-11% has little effect, but this is a large molar excess

7 OXIDATIVE DYEING OF KERATIN 37 over dye concentration and, therefore, any real effects may be masked. Alternatively, there may be a balance between dye formation and dye loss by excess H20 2. Solution kinetics and mechanisms predict a dependence on peroxide and not on dye concentration. These differences may reflect the importance of diffusion processes with substrates or a change in mechanism at the substrate surface. REFERENCES (1) J. F.Corbett, The role of meta difunctional benzene derivatives in oxidation hair dyeing. I. Reaction with p-diamines, J. Soc. Cosmet. Chem., 24, (1973). (2) K. C. Brown and J. F. Corbett, The role of meta difunctional benzene derivatives in oxidative hair dyeing. II. Reactions with p-aminophenols, J.Soc. Cosmet. Chem., 30, (1979). (3) K. C. Brown, Hair colorants, J. Soc. Cosmet. Chem., 33, (1982). (4) W. G. Cochran and G. M. Cox, Experimental Designs, 2nd Ed., 0ohn Wiley, 1957), pp (5) P. Kubelka and F. Munk, Ein Beltrag zur Optik der Farbanstriche, Z. Tech. Physik, 12, (1931). F. W. Billmeyer and M. Saltzman, Principles of Color Technology, 2nd Ed., (John Wiley, 1981), p 140. (6) K. C. Brown and A. Chan, unpublishe data.