Investigation of the Effects of Rosination in the Preparation of Lithol Red R (C.I. Pigment Red 49)

W. CZAJKOWSKI* and F. JONES

Wolfson Organic Powders Research Unit Department of Colour Chemistry and Dyeing The University Leeds LS2 9JT

It has been found that alkali metal salts of wood rosin are amphiphilic and in certain concentrations in water exhibit mesophase behaviour. me influence of sodium and barium rosinates below and above their CMC values on the mor- phology and crystal structure of Lithol Red R (CI. Pigment Red 49) has been studied by Scanning Electron Microscopy (SEM) and X-ray diffraction methods.

The results indicate that the mor- phological changes which occur on rosin- ation are probably due to the incorpor- ation of the pigment in the micellar structure and also a consequence of molecular adsorption of rosin on the pigment particles.

*Current Address: Instytut Barwnikow, Politech. nika Lodzka ul Zwirki 36 90-924 Loctz, Poland.

Introduction Lithol Red R (C.I. Pigment Red 49) is a member of the class of pigments known as toners, which are prepared by the alkaline coupling of diazotized amino- sulphonic or amino-carboxylic acids to obtain the mono-azo sodium salts. These salts, in some cases, are sufficiently insoluble in water for commercial applica- tion, but more often they are converted to less soluble salts of barium, calcium, strontium, manganese or lead. Pigments of this type are not however fast to alkali.

Metal-exchange processes in preparing these pigments (the terms ‘metallization’ or ‘development’ are often used) are usually carried out by precipitating the equivalent sodium salt in the presence of the sodium rosin soap. The latter can be used in conjunction with other non- ionic surfactants [ I ] .

Rosin, either wood rosin or gum rosin (colophony) is a natural product obtained as a residue after distilling the gum of pinewood. It is a complex mixture con- taining mainly abietic acid (I) and other terpene-type acids. Rosin modified by hydrogenation or dehydrogenation, and other natural or synthetic resins are also in use [2] . Rosination is also practised on other organic and inorganic pigments [3].

Sodium rosinate is assumed to act as a surfactant, but during the metallization it is converted into insoluble rosinate salts incorporated in the pigment. Between 15 and 30 per cent by weight of rosin soap may be incorporated in the pigment without loss of tinctorial strength. Very often the colour strength of the pigment is increased and is accompanied by a definite colour change - in the case of the red pigments the colour becomes bluer. Although the methods used and the effects of rosination on pigment

properties have been adequately reviewed [4] , little if any information is available on the mode of action of the rosin soap when present either during coupling or on metal exchange.

It is the purpose of the present work to study the influence of the rosin con- centration on the physical structures of the sodium and barium salts of C.I. Pigment Red 49 or Lithol Red R (11) by X-ray diffraction and SEM methods. It should be appreciated that rosin con- centrations beZow and above CMC values have been used and therefore may not relate directly to commercial practice. Also other factors which cannot be repro- duced in laboratory scale experiments may operate in plant production to give modifications in particle size and struc- ture. Such problems require separate and more extensive investigation and this paper should therefore be considered simply as a start in this direction.

Experimental

heparation of the Sodium Salt of 121. Pigment Red 49 The preparation of C.I. Pigment Red 49 samples was carried out as in the 1.G. Farben method [5] slightly modified by Morris [6] . 2-Naphthylamine- 1 -sulphonic acid (Tobias Acid) of 95-97% purity (Ciba-Geigy) and BDH Laboratory Reagent 2-naphthol were used.

Thus, 22.3g of Tobias acid (0.1 M) was dissolved in 220 ml of water contain- ing sodium hydroxide (5.0g) and cooled to 8°C by adding ice. 25 ml of 36.5% hydrochloric acid was then added fol- lowed by a solution of 7.0g sodium nitrite in 40 ml of water. This solution was added dropwise during 10 min. The diazonium salt was then stirred at 12-15°C. A faint excess of nitrous acid was found after 30 min and the dia- zonium salt was then added during 10

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min to a solution of 16.Og of 2-naphthol in 400 in1 of water containing sodium hydroxide (7.0g) and sodium carbonate (4.4g). The reaction mixture was stirred for 30 min, tested for completion of coupling with H-acid solution and fil- tered.

Preparation of the Barium Salt of CI. Pigiiz ent Red 4 0 One tenth by weight of the wet filter cake (0.01 M) of the sodium salt was redispersed in 150 ml of water. A solu- tion of 1.4g of BaC12.2H20 in 15 ml of water was then added, followed by 6.6 ml of a 6.0% wt/wt solution of rosin contain- ing 0.75% wt/wt NaOH in water. This is equivalent to 0.4g of wood rosin. The suspension was heated to 90°C over 30 niin and maintained at this temperature for 30 min and then allowed to stand overnight. The precipitate was filtered, washed with 200 ml of hot water and oven dried at 70°C. Each sample prepared in this way was subjected to differential thermal analysis. Weight losses at temp- eratures up to 150°C indicated that each samplc contained from 6-8% volatile matter, which was assumed to be water.

Morris [6] has also shown that the method of metal exchange used gives complete or almost complete intercon- version from sodium to barium salt.

All solutions and dispersions were made with distilled water. The unused sodium salt of the dye was kept as a wet paste stock suspension for further experi- mental work. Although the amounts of rosin soap were varied, in the metallba- tion stage, no rosin was added during preparation of the sodium salt of the pigment.

Determination of the Critical Micelle Concentration (CMC) of Rosin Salts The sodium and barium salts of rosin are aniphiphilic in nature and at higher con- centrations in water exhibit mesophase behaviour. The structures of the lyotropic mesophase are easily observed during the examination of thin layers of solutions, of 5-10% strength, under the microscope between crossed polars and show a long- range ordered structure typical of lyo- tropic systems [7]. It might be antici- pated that the nucleation and growth of the sodium and barium salts of I1 may be influenced particularly by the presence of rosin either above or below its CMC.

At present no information on the CMC values of wood rosin salt is available except for some derivatives such as the

sodium salt of pinabietic acid, CMC value, 1-2 x lo1 M [8] , or the sodium salt of dehydrogenated rosin (dresine), where its CMC value is reported [9] to be over the range 5x 10" to 2x 10- M.

The CMC values of barium and sodium rosinates have been determined by using a spectrophotometric method described by Duff and Giles [ l o ] , which is based on changes in spectra with different surfactant concentrations. With aqueous sodium rosinate solution abrupt changes in the slope relating optical den- sity to concentration at Amax at 244 nm occurred at a concentration of 3.5-3.9 x

M. The corresponding change for the barium salt occurred at 4.0-5.0 x lo-' M at A,,, at 245 nm. These values were taken to be equivalent to the CMC and were confirmed by the method of Klevens [ l l ] . In this method, optical density changes in the aqueous solutions of a cationic dye were compared in the presence of different amounts of surfact- ant. The cationic dye used was recrystal- lized pinacyanol iodide (1 ,l'-diethyl-2,2'- carbocyanine iodide) at a constant con- centration of 1.65 x lo-' M.

The changes in optical density of pinacyanol iodide for both A,,, values of 552 nm and 602 nm occurred at a concentration of 2.75-3.0 x M for the sodium rosinate and at 6.9-8.1 x lo-' M for the barium rosinate.

The comparable results obtained by the two methods can be considered as confirmation of the CMC values, particu- larly since the methods show little simi- larity. The values are however much lower than those reported for rosin deri- vatives. Since wood rosin is a mixture of terpene carboxylic acid derivatives with other compounds, it is possible that the CMC of the mixture is lower than that of any of its individual components [9]. It has also been reported by Miyamoto [ 121 that magnesium and calcium salts of anionic surfactants have lower CMC values than their corresponding sodium salts. This phenomenon is now confirmed in the case of the rosin barium salt.

Scanning Electron Microscopy and X-ray Powder Diffraction Studies Scanning electron-microscope investiga- tions were carried out on samples of C.I. Pigment Red 49 prepared with different concentrations of rosin. Samples were prepared for microscopy by subjecting aqueous dispersions of the pigment (0.1-0.5% weight) to ultrasonic vibration to reduce aggregation. Each dispersion

was filtered on 25 nm 'Millipore' cellulose acetate sheet, with simultaneous U.S.

vibration, oven dried at < 70°C and gold-coated to a thickness of 200 .A layer. Samples were examined using a Jeol JSM-15 scanning electron microscope.

X-ray powder diffraction patterns were obtained using a Siemens Kristallo- flex 4 diffraction unit fitted with dif- fractometer and scanning over 2 0 angles from 4-40" with CuK,, Ni-filtered radia- tion of wavelength 1.5418 A. Powder samples were contained in quartz capil- laries and were scanned at room tempera- ture.

Results and Discussion Electron micrographs of C.I. Pigment Red 49 obtained in the absence and in the presence of rosin sodium soap show visible differences.

Sample I prepared without rosin (Figure 1) occurs in a platey habit of

Figure I - CI. Rgment Red 49 prepared without rosin

Figure 2 - C.I. Pigment Red 49 prepared with rosin

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a

d

[ a ) Sample I prepared wi thout rosin (b) Sample II prepared wi th rosin [c ) Sample I heated

(d) Sample I I heated (e) Sample II heated w i th rosin

Figure 3 - X-ray powder diffraction line diagrams of CI. Pigment Red 49

varying width with one major axis. When the metal exchange process was carried out in the presence of rosin, Sample I1 is obtained as a mixture of plates (Figure 2) of different size, more or less aggregated, and in which there appears to be no major axis. All concentrations of sodium rosinate described in this section were equivalent to the commercial amount, i s . approximately 8 x M or above. This concentration is much greater than the observed CMC. The effects observed when sodium rosinate is applied below its CMC are described later.

The similarity in position of diffraction peaks shown in the line diagrams (Figure 3, a and b) and the differences in peak intensities indicate differences in the degree of crystallinity, but very little if any differences in crystallographic structure.

The additional peaks at 27.6 and 29.4" appearing in Sample 1 are very weak and cannot be used to verify any structural difference. Since X-ray dif- fraction patterns of barium rosinate are diffuse, the possibility that barium rosinate is present in the rosinated product (11) could not be established. The possibility that the barium rosinate is present as an amorphous surface coating

still remains. Differences in the morphology of

crystals of I and I1 become more pro- nounced when 2% wtlvol. aqueous disper- sions of these pigments are heated at 90-95°C at pH 9.0-9.5.

Sample I undergoes little surface change on heating for 2 h under these conditions (Figure 4), but Sample I1 (Figure 5) changes to a ribbon-like crystal shape. From X-ray diffraction compari-

Figure 4 - Sample I heated for 2h at 90- 95°C

Figure 5 - Sample II heated for 35 min at 90-95°C

sons (Figure 3, c and d), there is a considerable increase in sharpness and intensity of the peaks on heating Sample I and the appearance of moderate inten- sity peaks at 10.8' and 24.8". However, a considerable crystallographic structural change does occur on heating Sample 11. It can be concluded that the preparation of C.I. Pigment Red 49 in presence of sodium rosinate leads to a less stable solid which, on heating, readily undergoes transformation to a different, more stable, form. This change occurs within 35 min, whereas the form prepared with- out rosin remains unchanged after 2 h.

I t was also found that the deliberate addition of 10% of sodium or barium rosinate (prepared separately) on the weight of the pigment had no effect on the morphology of Sample I (compare Figure 6 with Figure 1). The effect of these surface active agents (deliberately

Figure 6 - Sample I heated with sodium rosinate

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Figure 7 - Sample II heated with sodium ro sinate

I I I I

Figure 8 - Sample II heated with barium rosinate

I

added) on Sample I1 was to slow down the rate of change to the more stable form. (Compare Figures 7 and 8 with Figure 5).

Figure 3e shows that this change to the structure indicated by X-ray diffrac- tion diagram Figure 3d is not complete since the original diffraction peaks (Figure 3b) can still be observed.

I t has been previously reported [6 , 131 that after heating to about 80°C the sodium salt of Dye I undergoes a morphological change from plate-like to acicular crystals, although there is no change in crystal structure. This pheno- menon is confirmed in the present work. As in the case of the barium salts, there are visible differences in morphology of the sodium salt before and after heating as shown in Figures 9 and 10, There is no change in crystal structure, but as with

Figure 9 - Sodium salt of CI. Pigment Red 49 a t room temperature

the barium salts the line diagrams (Figure 1 1 , a and b) show an increase in the degree of crystallinity on heating.

An interesting feature is the similarity in appearance between unheated sodium salt, Figure 9, and the barium salt prepared in the presence of rosin, Figure 2, and between the heated sodium salt and the barium salt prepared in the absence of rosin (Figures 1 and 10). There are however structural differences seen in X-ray diffraction patterns for the sodium and barium salts, which indicate a different crystal structure.

Therefore the replacement of sodium by barium ions in metallization probably occurs through a molecular dispersion process, i.e. through a solution or colloidal state. This then may explain the influence of surfactants which increase metal-exchange reaction rates by increas-

Figure 10 - Sodium salt of C.I. Pigment Red 49 heated

ing the solubility of the dye in the micelles and the specific influence of rosin soaps may be connected with their low CMC values.

It might be anticipated that the influence of sodium rosinate below its CMC would have little or no influence on morphology or crystallographic structure. However, the effect is apparent also below the CMC of both sodium and barium rosinates. The morphology of the pig- ment prepared with sodium rosinate at a concentration of 2 x lo-’ M (Figure 12) is similar to that obtained with rosin above its CMC (Figure 2). There is how- ever a higher degree of crystallinity with- out any change in structure. After heating the pigment prepared in presence of rosin below CMC, in the same way as previously, there is a change in the crystal shape to that observed in its preparation

1 I a

(a) Sodium salt of the dye ( 1 1 ) (b) Sodium salt of the dye ( 1 1 ) heated (c) C.I. Pigment Red 49 prepared with rosin below its CMC

Figure I I - X-ray powder diffraction line diagrams of CI. Pigment Red 49 derivatives

31 6 J SDC August 1977

Figure 12 - CI. Figment Red 49 pre- pared with rosin below CMC

in the absence of rosin (compare Figure 13 with Figure 1).

The influence of rosin below its CMC may then be the result of adsorption of rosin molecules on pigment crystals during the precipitation or even by the formation of some less stable crystal structures of the [ (Dye-)2Ba2' (Rosin-)2- Ba2'] or even [(Dye- Rosin-)BaZ'] type. At or above the CMC value it is possible

Figure 13 - CI. Figment Red 49 pre- pared with rosin below CMC heated

that the influence of rosin is to form particles of platey, friable habit by micel- lization. This could be the basis for an explanation of the well-known pheno- menon that rosination can increase the tinctorial strength in both aqueous and non-aqueous dispersions.

(MS received 20 July 19 76)

References

1. ICI, German P 2,001,505 (1970). 2. Lenoir, 'The Chemistry of Synthetic

Dyes', - Vol. V, Ed. K. Venkatara- man, (New York and London: Academic Press, 1971) p. 335.

3. ICI, BP 1,100,587 (1 968). 4. Lenoir, Ref 2, p. 31 3. 5. BIOS Report, No. 1661 p.101. 6. Morris, Ph.D. Thesis, University of

Leeds, 1964. 7. Gray and Windsor, 'Liquid Crystals

and Plastic Crystals', (Chichester: Horwood Ltd, 1974) p. 48.

8. Olavi Aarva, Acta Acad. Aboeusis, Math. et Phys., 17 No. 4 (1951) 1-30; Idem, Chem. Abs., 46 (1952) 11718d.

9. Verezhnikov, Kashlinskaya and Neiman, Kolloid Zh., 32 (1970) 493.

10. Duff and Giles, J. Colloid and Inter- face Sci., 41 (1972) 47.

11. Klevens, J. Phys. Coll. Chem., 51 (1947) 1143.

12. Miyamoto, Bull. Chem. SOC. Japan, 33 (1960) 375.

13.Stead, J. Oil Col. Chem. Assocn, 30 (1947) 337.

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