Electrodeposition of Shellac N. NARASIMHAMURTY AND M. SREENIVASAYA. Indian Institute of Science, Bangalore, India
S
HELLAC behaves anoma-
t h i s investigation i n v o l v e d a A solution of shellac in alkali carbonate has the lously toward solutions of study of the chemical composiproperties of a true colloid, and the shellac mialkali carbonates. Election of the deposit. A nickel celles which are formed carry a negative charge trometric titrations carried out cathode was employed in all the and migrate toward the anode under the influence by Gardner and Whitmore ( 1 ) experiments. of a difference of potential. This suggested the point to the conclusion that the B 10 per cent cut of Palas acid components of shellac cor(Butea frondosa) seed lac in 1 possibility of electrodeposition of lac. respond in strength to weak orper cent sodium carbonate soluA current of 0.2 ampere at 6 volts will secure ganic a c i d s s u c h a s benzoic, t i o n (free from wax) w a s the deposition of lac at the anode. ilddition of lauric, glutaric, and c i n n a m i c . placed in a platinum basin which salts such as sodium chloride increases the rate A clear aqueous solution of the served as the anode, and a rod of deposition but brings about other changes alkali soap of shellac has been of nickel dipping into the soludescribed by Gardner,Whitmore, tion acted as the cathode. Elecsuch as oxidation and polymerization. and Harris ( 2 ) which c a n b e trodeposition w a s a l l o w e d t o The accelerated elecfrodeposit consists of a n made to pass completely through proceed at 6 v o l t s , a c u r r e n t alcohol-soluble fraction, a large portion of which a membrane found to retain gold of 0.2 ampere (0.0052 ampere is also soluble in ether. It approximates in sol. The soap solution, thereper sq. cm.) flowing in the initial composition the ether-soluble portion of shellac. fore, does not possess true colstages. A s the d e p o s i t g r e w l o i d a1 characteristics. N a g el thicker, the current decreased The alcohol-insoluble portion, consisting o j about and Kornchen ( 5 ) have observed to a value as low as 0.03 ampere 70per cent of the deposit, represents a polymerized that shellac can be easily esteri(0.0008 a m p e r e p e r s q . cm.). modificafion of the “pure resin.” fied with methanol, and that an By progressively increasing the The electropolymer is similar in behauior to alcoholic solution of shellac acts voltage, t h e original s t r e n g t 11 the hydrochloric acid-polymerized lac but differs upon metals s u c h a s c o p p e r , of the current could be mainzinc, etc. On the other hand, tained, but such high voltages f r o m it as regards chemical composition. During Wolff (10) noted that s h e l l a c are not conducive to obtaining electrodeposition the resin is partially oxidized i s insoluble i n a s o l u t i o n of either a u n i f o r m d e p o s i t of in bleaching, as shown by a n and saturated as resin or an increased yield of sodium bicarbonate and does increase in the saponification value and a decrease deposit, because of the vigorous not displace t h e t h e o r e t i c a1 in the iodine value. It would be of interest to gassing a t the electrodes. By quantity of carbon dioxide when mechanically scraping out the it is dissolved in a solution study the nature of polymerizations brought deposit, however, the phenomena of sodium carbonate. This led about by accelerators such as urea, hexamdhylene could be continued. him to conclude that the main tetram ine, etc. With a g i v e n d e p o s i t , t h e function of sodium c a r b o n a t e material nearest the electrode was to peptize shellac. 1Vhate;er be the nature of dilute solutions, there is little surface mas white, porous, and crisp. The outer layer was doubt that concentrated cuts of shellac exhibit marked char- brownish. Successive deposits, after scraping off the previous acteristics of a true colloid. They show the properties of imbi- ones, became thinner, browner, and less crisp. bition, syneresis, and setting similar to those of a typical As the solution became weak, the deposition was slow and hydrophilic colloid such as gelatin. The shellac solution the current value was reduced to 0.01 ampere, The residual can, in fact, be employed as “office glue” with advantage. solution a t this stage was treated with dilute sulfuric acid and Salts coagulate the alkaline solution of shellac, throwing out the resulting precipitate of lac recovered, washed free from a jelly-like spongy mass which, on squeezing, yields a solid. acid, and dried. The sample on analysis showed that its This on treatment with distilled water can be repeptized, properties were very similar to those of the original lac. Depoyielding the original colloidal solution. sition of shellac on the abode was obtained from other While attempting to electrodialyze alkaline solutions of alkaline solutions such as ammonia, sodium silicate, etc. shellac, it was observed that shellac was deposited on the INFLUENCE OF ELECTROLYTES ON DEPOSITION.For a parchment membrane nearest the anode. Shellac micelles, study of the nature of the deposit, genuine kusum shellac and therefore, carry a negative charge similar to that carried by “pure resin” (the ether-insoluble portion) were used. In a n aqueous suspension of shellac in the experiments of Picton both cases a thin, transparent, insulating film was obtained and Linder ( 7 ) . This suggested the possibility of electro- which offered resistance to a further flow of the current a deposition of shellac from its alkaline solutions, and the pres- few minutes after the start of the experiment. By frequent ent communication deals with a study of the conditions of removal of the film, however, further deposition could be deposition and the nature of the deposit. secured, and a sufficient quantity of the substance was thus obtained to establish its identity with the original resins. EXPERIMENTAL PROCEDURE I n the case of shellac and “pure resin” solutions, the resistPreliminary trials showed that nickel, copper, and lead ance offered to the flow of current was very high as compared anodes were unsuitable since they were attacked; with an with seed lac solution, which is presumably associated with aluminum anode, no deposition took place (possibly because electrolytes, including laccaic acid. The ash content of seed of the formation of a thin insulating oxide film as in the case lac, in fact, is higher than that of shellac. It was of interest of the aluminum electrolytic rectifier). A platinum anode therefore to determine if additions of electrolytes to shellac has been used throughout as being the most suitable since solutions would improve the deposition. 882
August, 1934
1N D U S T R I h L A N D
E I\; GI N E E R I N G C H E 1cI I S T R Y
Electrodeposition experiments were carried out with solutions of shellac and “pure resin” in the presence of sodium chloride. It was found that the rate of deposition far exceeds t’hat obtained with seed lac solutions, and continues for a longer time. In the presence of high concentration of sodium chloride, the size of the shellac micelle is found to increase, giving a bluish opalescence. With still higher concentrations actual precipitation of shellac takes place. Thus, it is probable that the increased size of the ionic micelles is partly responsible for the increased rate of deposition in the presence of salt. The more easily noticeable effect of the addition of salt is the increase in conductivity of the shellac solution and the porosity of the deposit which, unlike the tough insulating deposit, helps in maintaining the current. Other salts such as sodium sulfate, sodium nitrate, sodium acetate, and ammonium sulfate were employed. Sodium chloride \vas found to be the best from t’he point of view of the rat’e of deposition. With sodium acetate, however, the deposit was dark, slimy, and gelatinous. Shellac deposition in the presence of electrolytes occurs with such great ease that a simple voltaic couple of copper and zinc is sufficient; the deposition takes place on the zinc surface.
883
facilitate the solution of polymerized lac. Saponification values were determined by a procedure described by Whitmore and Weinberger (9). Acid values were determined electrometrically with a quinhydrone electrode as described by Narasimhamurty (8).
DI~CUSSION OF RESULTS
When shellac is heated a t 150” C., the molten mass gradually turns viscous and in about 70 minutes is converted into a rubber-like mass which soon hardens into a brittle, dark brown resin. This change is accompanied by the release of water and vapors of an acid character which render the mass porous. This heat-hardened shellac is soluble in alcohol only to the extent of 25 per cent, and the acid value of the extracted fraction (113) corresponds to that (114) of the ether-soluble portion of shellac. The insoluble residue is only partially soluble in acetic acid, unlike that of electropolymerized or hydrochloric acid-polymerized shellac. The polymerizations brought about through the agency of heat differ from those brought about by hydrochloric acid. Xagel and Kornchen (5) have shown that during heat polymerization water is split off. This is probably due to the PHYS~CAL AND CHEMICAL EXAMINATION OB THE DEPOSIT. formation of lactides, the hydroxyl and carboxyl groups of The deposits obtained from solutions of seed lac, shellac, and the aleuritic and shellolic acids participating in the reaction. “pure resin,” after washing with distilled water, were sub- This involves a decrease or a disappearance of the hydroxyl jected to electrodialysis to insure a m r e complete removal and carboxyl groups, rendering the product insoluble in polar of the electrolytic impurities. The material was then dried solvents such as ethyl alcohol. The hydrochloric acid polyin Z ~ C U Oin a desiccator over sulfuric acid. Each sample merization is mainly a process of aggregation of molecules thus obtained can be mechanically separated into three por- into a more complex one, effecting a more or less complete tions: (a)a brown mass melting between 74” and 78” C.; saturation of the residual valences. This aggregation, (b) a whitish brown substance commencing to soft’ena t 70” C. characteristic of the shellac resin, is favored probably by the but not melting even a t 110’ C.; and (c) a whitish mass presence of lactones, acid anhydrides, or hydroxyl groups (8). which is infusible and insoluble in all organic solvents, sodium Harries and Nagel (3) have observed that hydrochloric acidcarbonate, and dilute alkali solutions. polymerized shellac or “pure resin” is more difficult to saThe bulk of the anode deposit was extracted with absolute ponify than the corresponding untreated substances. Nagel alcohol, yielding a dark green solution; films from this solu- and Kornchen (6) extended their observation to shellac which tion have a greenish tinge and possess a greater resistance to had become insoluble on aging, and found that it exhibits a water. The acid value of the extract ranges from 99 t o 119, decreased rate of hydrolysis. Heat polymerization of shellac which is nearer that of the ether-soluble portion than that or “pure resin,” however, does not alter its rate of saponificaof shellac. The alcohol-soluble portion of the anode deposit tion. The above conclusions regarding hydrolyzability have has a higher percentage of the ether-soluble portion than been drawn from the yields of potassium aleuritate obtained shellac. The alcohol-insoluble portion of the deposit, consti- by the hydrolysis of the samples with 5 N caustic potash. It tuting about 70 per cent of the anode deposit, appears to be was of interest to find out how the saponification value and identical with the infusible whitish mass described above. rate as determined by standard methods differ in the case of Both these samples can be rendered soluble in alcohol by the electropolymerized and of hydrochloric acid-polymerized glacial acetic acid treatment. samples of shellac. Table I shows that the rate of hydrolysis of the polymerized samples is not very different from that of TABLEI. ANALYSISOF SAMPLES the untreated control samples. The saponification value of HClthe hydrochloric acid polymer is practically the same as that POLYELECTROPOLYIn the case of the electropolymer there is a MERIZED PURE MERIZED LAC BLEACHEDof the control. SAMPLE SHELLAC SHELLAC RESIN A B LAC in the saponification value as compared with distinct increase Iodine value 15.8 16.1 9.7 7.7 6.8 8.0 the value for the untreated “pure resin.” This points to the Acid value 72.4 ... 5 9 . 2 .. . . ., 94.0 Saponification value: conclusion that electropolymerization is accompanied by an 0.5 hr. 211.0 209.0 232.0 247.0 258.0 253.0 2 . 5 hr. 217.0 212.0 237.0 255.0 261.0 258.0 oxidation of the resin, resulting in an increase of carboxyl groups, the oxidation being brought about by the nascent One of the main effects of the chlorine ion on the deposit oxygen or chlorine evolved during the deposition. Such a appears to be to polymerize or harden the “pure resin” frac- type of oxidation occurs during the bleaching of lac whose tion of shellac during electrodeposition. The polymerization saponification value should therefore be expected to be high. is favored by the nascent chlorine liberated a t the anode. It Analysis confirms this suspicion. An independent confirmawas of interest to study the nature of this change with a view tion of the oxidation theory has been obtained by a deterof determining horn far the phenomenon corresponded or dif- mination of the iodine value of the samples. Table I shows fered from the polymerization brought about by heat, on that, while there is a lowering of the iodine value both in the the one hand, anct by hydrochloric acid on the other. case of the electropolymerized and bleached lac, the iodine A comparative analytical study of the samples, with refer- value of the hydrochloric acid polymer is not altered. ence to their acid, saponification, and iodine values, was underThe insoluble portion of the electrodeposited lac consists taken and the results are given in Table I. Iodine values entirely of the modified ‘(pure resin’, and has been found to were determined by the Wijs method, the procedure being be rendered soluble by glacial acetic acid treatment. The the same as that described by Langmuir (4:1 except for the resulting product, while being soluble in ethyl alcohol, is fact that a small trace of hydrochloric acid was added to completely insoluble in ether. It is curious that both the
electropiyilrer t i i ~ ltho well-naslied hydrochloric acid pdynier of lac require for ti& solntion in acetic acid a trace oi
hydrochloric arid. EiectroiIel,osit,i(Jnof aiieliar: IELS possibilities of wide agplicatioii. 1~hperirrieiit.sare in progrcss for obt.aining insulating coatings 011 metal surfaces by siiiiultaneous electrodeposition of ruiher and shellac From a mixture of their alkalinc solutions. The possibility of rccovering *liellae froin nast,e pro& octs will also be investigated. hCKSOWIrX:UiiM
Tlie w r k eiiil)udieii in this corrinniwicatioi~WAF uii(lcrtakeii by thc aiitliors in March, 1930,in tlre Departrnont of 1%)-
clieinistry, Indim Institute oi Science, and tiicir LTatefiii t,lianks are diie T'. Subralmianym for tlic interest hr 1i:w tnlieii in the w r k .
CHEMICAL LABORATORIPARIS,1760
(1) Gmiiier, W. H . , and Whitmore, W. F.. la". Evo. C:aax..Aiisl. Eii., 1, 905 (1929). (2) Gardner. W. li.. Whitmore. W. I?.. and Harrix. H. J.. IN". Exo. C k ~ w .25. , 6% (1933). (3) EIwries and &gel. Kolloirl-2.. 33, 247 (1923). (4) Lnngmuir. A. C.. A. S. T. >I. Stsndnrds, Standard Methods of Tesking Shollae. (5j Nngel, W.. and Kornohen. M., Wirr. VeiDfentiich. SiemenaKoniorn. 6, 235 (1027). (6) Nnrasiiehilmurty. papor r e d bsiore the Indian Sci. Congr.. 1932. ( 7 ) Pieton, H., and Liiider. E., J . Chsni. Soc., Tram., 1897, 568. (8) Schoiber and Siindig. tr. by Fyleman, "hrlifiainl Reainr." pp. 55, 57. Sir IS%CLC I'itman & Sans, 1,uiiiion. 1931. (9) n'tii4amro, W. F.. and Weinbergor. If.. with Gnrilner, W.H.. lrn. I ~ G (.J ~ ~ r j i r Anal. .. Ed., 4, 48 (1932). (10) Waili. ii., Parhan-Ztn.. 27, 3130 (10!?!. 1 L e r . r . i ~Si,vrniliri ~~~ 2 0 , 191:J.
'l~hnxight h ci,ui.tes,v ~ of 1'crc.y C. Kingstitry we are enabled to bring as No. 4.1 i n the f3erolzheimer series
