A method has been described for the preparation of water soluble CNSL resin using formaldehyde

for use in electrodeposition. The bath composition and operating conditions to get good deposit OR

mild steel was standardised. The coated panels were subjected to physical tests, accelerated tests

and electrochemical tests for assessing the protective property of the coatings

Guruviah, S.

Selvaraj, M.

IR@CECRI

Bulletin of Electrochemistry Vo1.2 (4) July-August 1986, pp.363-365 ELECTRODEPOSITION OF MODIFIED CNSL RESIN ON MILD STEEL M SELVARAJ and S GURUVIAH Central Electrochemical Research Institute, Karaikudi423 006 ABSTRACT A method has been described for the preparation of water soluble CNSL resin using formaldehyde for use in electrodeposition. The bath composition and operating conditions to  get good deposit OR mild steel was standardised. The coated panels were subjected to physical tests, accelerated tests and electrochemical tests for assessing the protective property of the coatings. Key words: Electrodeposition. CNSL resin, protective coating INTRODUCTION C ashew Nut Shell Liquid (CNSL) has been idenlified as a suitable phenolic resin base for electrodeposition as a protective coating because of its low cost and resistance towards rungus growth I . The long chain ot ~ t s  low cost and resistance towards rungus growth I . The long chain hyd- rocarbon substituent in CNSL serves to provide internal platicisation of the cured product, resulting in excellent flexibility which is also highly resistant towards acids and alkalies, unlike other phenolic resins 2 . Preparation of water soluble resin and electrodepositable resin based on CNSL was not reported earlier. The phenolic group present in the CNSL can be modified with formaldehyde and the side chain can be modified by triethanolamine. The resin thus obtained can be emulsified and electrodeposited. In this paper, a method has been described for the preparation of water soluble CNSL resin using formaldehyde, for use in electrodeposition. The bath composition and operating conditions to get uniform deposit on mild steel substrate was standardised. The coated panels were subjected to  physical tests, accelerated tests and electrochemical tests for assessing the protective property of the coatings. EXPERIMENTAL Preparation of water soluble resin CNSL was mixed with formaldehyde using alkali as catalysts, in different rahos. From this 10:1 resin. i.e. 10 parts of CNSL mixed with 4 parts of formaldehyde was found suitable for electrodeposition. I0 ml of : h ~  res~n was mlxed nl th  7.5 n ~ l  of rriethanolamine and cool- ed; 2.5 ml ethylene glycol monoethyl ether was then added to give clean solution. This was followed by slow addition of 3 0  ml of distilled water with stirring. Thus water soluble resin was prepared. Pretreatment of mild steel panels and electrodeposition of CNSL resin The mild steel plate (5.0 cm x 7.5 cm) was cleaned by pickling using dilute hydrochloric acid, dried and sand blasled to near white surface. It was made the anode and immersed in the resin bath with the currenl 'on' with two a!uminium plates acting as cathodci. The applied voltage was controlled . - 5 e r w c n  12 and 40 io!rs and thr ,.cl.i~:i;= time , m;n!lte\. I'he coated panel was taken out, kept as such for onz minute and baked at 1E'C for about 30 minutes. The bath composition and ihe owrating corditions wer: srandardised and number of plates were coated fcr further ztudies. SLsndardisntion of various parameters to  get uniform coatings Various concentrations of the resin solution were prepared and anodicallv electrodeposited on mild steel plates by using conslant volrzge an2 ':\cci time. The resin bath was kept at different temperature< ad number of pan-I: were coated. After drying, the thickness of the cohtlng was measured. Solutions of different pH values were prepared, and electrodeposlted panels were prepared in them and thickness assessed. The efficiency of deposition at various voltages were studied. The rela- tion between (i) current density and time at constant voltage and (ii) voltage and time at c o n k t  current density were determined. The voltage at which the maximum deposition occurred also was ascertained. Physical properties of the coated panels The thickness of the coatings was ~qasutad using Elcometer. The pencll hardness and scratch hardness were determined as per specification. The adhesion was tested by using 10 x IS cm. coated thin panels. Flexibility of the coated panel was detepnind by using conical mandrel bend tester. The gloss of the coating on pollshed mild steel surface was also determined. The water absorption of the coated specimen was determined by immer- sing stair\less steel coated panel in distilled water for a period of one month. The covering power of the bath was checked by electrodeposition on mild steel surface. Accelerated testa Salt spray tests The coated w e l s  in triplicate were kept in salt spray chamber for a period of IS days. The electrolyte used for spray was 3% sodium chloride and the thickness of the coating was 3 0  microns. Immersion tests The specimens of 30/u coating thickness in duplicate were immersed in the following solutions for a period of 15 days: distilled water, 5% sodium hydroxide solution, 3% sodium chloride solution, 2% sulphuric acid solu- tions. The plates were taken out and washed in running water, dried and Lhe performance of the coated panels were examined. EIectrochernical tests !5.: :.r:a?lon of potential with time of coated panel was determined by usins printing voltmeter in 3% sodium chloride solution. " ai:mrance-resistance of the coated panel was measured in 3% - o d ! l ~  ch!13;.de solution by using Wayne Kerr Universal Bridge Instrument. Selverej and Guruvieh - Electrdpogition of modifbd CNSL resin on mild steel RESULTS AND DISCUSSION Results are presented in figurea 1 to 6. It is seen from Fig.1 that the coating thickness increased as the resin concentration increased u ~ t o  7.5% and Fig. 1. Variation of Thiiknaso with C o n c e n ~  of Rrin Fig.2 shows that the coathg thicknesrr incrawl with increasing timperature linearly up to 35°C. Fig. 2. Variation of Thickness with Tmpantwe It was further found that the optimum workhg pH rany was 8 to 9. Coulombic efficiency study showed tht at 3&35 V the dcporition hsa nach- ed a maximum level and then aftuwards no furtha deposition took place. This behaviour may be seen in F'i.3. APPLIED VOLTAGE, V Fie. 3. E f k h w  of DqmMcm wtth Mlkur Vobgm And also the coulombic efficiency increased as the pH of the solution decreased. The relation between the current density and different period (Fig.4) ahowed that the current density decreased with time. Fii. 4. Variaia, of Currant Density VS flme at m r n  Voltage But at a particular current density, the thickness of the coating reached a maximum level and thereafter there was no possibii of further deposi- tion with time. The relation between voltage and thickness of coating with timc at coostant current density showed that the voltage inaeaPed with 384 Bulletin of Electrochemistry Vd.2 (4) July-August 1986, Sehrerej and Guruviah - Ehttodepbsition of modifid CNSL resin on mild steel time (Fie.5). 'OF TIME. s :ig. 5. VoltageV6 Tlme at  Constant Current Density h e  optimum voltage at which the maximum deposition occurred was in- licated in Fig.6 0 20 40 60 80 100 120 1CO TIME, s Fig. 6. Weight of Deposit V S  Time at Different Voltages The optimum voltage was 35. Above 40 volts, gassing ofcured and deposil thickness decreased. The thickness of the coating measured by Elcometer for 2 minute deposi tion was found to be 22-27p. The observation of pencil hardness shows that when the thickness of the coating increased, the pencil hardness decreaseas ed and also when the thickness of the coating was less, the pencil hardness was good. The adhesion was also satisfactory. The coating passed flex. ibility tests. The water absorption of the coated resin was 3% after im. mersion in distilled water for one month. It was calculated that the cover. ing power of 100 ml resin was 12 square feet. The CNSL electrodeposited plate was not affected by salt spray tests up to I5 days and by immersing the plate in 3% sodium chloride solution, 2% sulphuric acid solution, distilled water for I5 days. But the coating immersed in 5% sodium hydroxide was affected within 7 days. This shows that the electrocoated CNSL does not withstand in alkali medium. The potential-time study in 3% sodium chloride solution showed an in- itial potential of -0.308V and after % hrs the potential was -0.542V. The capacitance-resistan- showed an initial capacitance of 0.080 nF and a final capacitance of 0.085 nF after one month. The initial resistance was 9.909 x 106 ohms and the final resistance after one month was 4.5455 x 106 ohms. So the capacitance was not changed with time in 3% sodium chloride solution but the resistance decreased with time. This result showed that the water uptake of the film was less and also there was no swelling taking place on the surface of the coating. Hence the performance of the coating was good. CONCLUSION It was possible to prepare a water soluble resin based on CNSL resin and the condition for electrodeposition on mild steel substrate of the resin was standardised. The operating condition for obtaining satisfactory deposit was determined aud the physical and corrosion resistance properties of the coated panel were evaluated by immersion tests, accelerated tests and elec- trochemical tests. It was observed that the performance of the coating was satisfactory. REFERENCES 1. Lalit C Anand, Paint India, 28 (1978) 13 2. S K Ranganathan and K G Tandon, Paint Technology. Vol.XI (March 1946) 94 2 Bulletin of Electrochemistry V01.2 (4) July-August 1986, 365 

Electrodeposition of modified cnsl resin on mild steel

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