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Gel Lacquer Technique for Protective Coating CARL J . -3IMIIL11.I AND HAROLD L. SMITH, .JH. E a s t m n n Korlak C o m p a n y , Rorhpster, S. F.
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?;E rccent trend in the
The gel lacquer technique is designed for t h e application ro be the cc~llulnar uut:tat+ protective coating field of unusually heavy protective coatings. This process butyrate containing approxih:ts been toward high solids depends upon the property of the lacquer to undergo gelamately 137; acetyl and 37% lacquers, using a variety of tion by temperature change alone, without the necessity butyryl, thv composition methods to obtain henvier indicated by thc cirrle in of solvent evaporation. This behavior i s obtained by coat in@ in fewer applications. Figure 213. This ester is proper selection and balance of the solvents. Film foravailable commercially a~ These t,echniques include t h e mers chosen from the group of cellulose mixed esters are use of high solvency vehicles, such or in a plastic molditip suitable for the base of these lacquers, and produce coatIotver viscosity film formers, compound, Tenite 11. ings of plastic appearance and properties. The lacquer is In brief, t h e pI'lJCCiC larger percentages of resin, applied by dipping in a viscous solution maintained at hot spray application, imrorisists of dissolving thr approximately 120' F. On cooling t o room temperature, ester or t h e molding comproved spraying apparatus the heavy coating immediately sets to a firm gel which pound in a carefully to handle more viscous will not sag or wrinkle. Because of this property of materials, arid similar methbalanced solvent comgelation, it is feasible to apply films as thick as 0.015 i n c - h bination at an ctkhvated ods. A recently developed per dip over a variety of core materials. t e m p e r a t u r e o t 110-t,echnique, kn0n.n as gel lacquer ( 1 , 6 ) ,offers a method 150" F. (60-66" C.). If the taster i i uaeti, :illy desired plmtieizer and pigment may be iiicorof obtaining a very heavy coating with e w h ;ippliration m d of poratcd at tliis point. Hon.ever, it has been found more conbuilding up a finish of true plastic: appearance and properties venient t(J tlissolvr~the Tenite 11 molding granules directly, and (Figure I). It is a means of applying :in actual "plastic venecr" thus nbttiiii film former, plasticizer, and pigment in a thoroughly hy a simple dipping process. dispersed state. When solution is complete, t h e tempcraturc T h e name "gel lacquer" is derived from tlie composition's is dropped to :ipprosim:ttely 120' F. (19" C.), bubbles are alproperty of undergoing reversible gelation merely with change in lowed t o escapr, and the lacquer is ready for we. Articles are temperature without any change in solvent content. Thc coated by a simplr dipping opcratiDn, being withdrawn slowly, composition of the solvent is so adjusted t h a t a t the temperature inverted, and set to dry. The smooth, heavy coating thereby deof application (about 120" F. or 49" C.) thp lacqurr is a smooth, posited sets ahnost immediately to n firm, nonflowing gel on being flowable, though viscous, liquid. On cooling to room temperuexposed to room temperature and thus preserves the original ture, however, the lacquer immediately sets t o a firm, nonflowcontour of the applied coating. This setting t o a tack-free, nonable gel which maintains the applied coating in its original form flowable coat occurs in a minute or so, and a t the end of 1.5 to 20 without sagging or wrinkling. This gelation occurs Ivith temminutes the film is sufficiently hard to withstand normal perature change alone and is not dependent on solvent, evaporation, alt,hough the latter does handling. Final complete curing of the lacquer takes considernormally accelerate the sett,ing. ably longer and is accompanied Thc met,hod has been sucby shrinki,ng of the gelled film ressfully used with a variety as the solvent evaporntm. of cellulose acetate-propionate and acetate-butyrate esters a:: .ADVANTAGES O F G E L the film-forming media. The L4CQUER ester compositions which work This technique has numerous satisfartorily for use in these gel lacquers are shown in Fig:idvantages o v ~ rthe customary ure 2 ( 2 ) . Since these esters methods of dip lacquering. possess rather different soluThe most obvious is the thickbilities, t,he solvent or solvent ness of coating deposited, up cmnbination used must, be t o 0.015 inch of dried lacquer v:iric,d someivhat t o produce per dip, as compared t o the proper gelation, depending on usual 0.001-0.002 inch (Figthe ester chosen. Those esters ure 1). T h e coating has all the appearance arid feel of nhich contain a lower m o u n t of t h e higher acyl group, being :t solid plastic article, since tlie Iitxivy deposit completely hides less soluble, will require a mow active solvent eombinathe core material, rcmovcs any characteristic grain, such as that tion than t h e esters with a in wood, and changes t h e heat grrator percentage of higher acyl. Figure 1. ( h n i p a r i ~ o i i of Object3 Coated conductivity of the surfacet o that Ont, \'cry useful ester for preparT\tice H ith Ordinar> Dip Lacquer ( r i g h t ) customarily :*ssociat,ed with :i irip gel lacquers has been found atid Gel Lacquer ( / e f t )
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plastic. 'l'he reaultiiig objects are comparable in appearaiice and properties to pieces made by injection molding a thermoplastic material around a core. Gel dipping, honever, requires less material (only about one t'hird as much) :ind no expensive molds. Kith this technique it is possible to clmploy a film formcv of m m -
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S U I T I B L E FOR GEL L l C Q U E R S
Vol. 38, No, 9
PHYSIC4L PKOPERIIES
The lacquer ia furidnment ;illy dependent on the ~ J ~ I ! ~ L ~ J J I I ~ ; I J , J I gelation which, in t'urn, depends on the selection of propilr $01vent,s. There are several solvents or solvent mixtures in LT-hich these esters are soluble a t elevated temperatures but not ti: room temperature, but) the ester merely precipitate,? as tht. solvent is cooled. In a limited number of solvent combinations. tio\T-ever, the Thole solution sets to a clear firm gel instead (I: precipitating upon cooling. These are the combinations used jior gel lacquers. The most satisfactory solvent combination?,. i'rom the st,andpoint both of behnrior and price, are thosc. made u p of a major port,ion of an aromatic hydrocarbon togethrr IT i t h a small amount of an aliphatic alcohol of 3 to 5 carhon atom.?. Xcaithrr of these types of solvents alone !Till dissolve these esters. e v m at elevated temperatures, but the mixture does. I n pratier, the hydrocarbon is usually a commercial grade of toluene o r .rylenc or a mixture of the tn-o, and the alcohol is 99c4 isopinot
1)31101.
Figiirr 2. Ce1luIo-e .Icetate-Propiorlatrb (.f) and $ i . e t n f e Butyrates ( B ) Suitable for I.arquer~
parable viscosity to that used in plastics. The result. is ti suri;ic.c. which has better wearing characteristics, excellent impact rejistance, and freedom from chipping. Even with f ilm formers of such high viscosity, a solids content of 20-2270 can be used. if an ester of a viscosity comparable to half-second nitrocellulose' is employed, solids contents up to 40% are feasible, though there is some loss in the physical properties of the resulting coating. Since gel lacquering is a dip process, there is a minimum amount of Taste, and recovery of the major portion of the solvent is feasible in large installations. Gel lacquer coatings have little or no real adhesion to any of the customary core materials unless a suitable primer is used. However, they shrink considerably on drying and, hence, n-heri. 'hey can shrink around the article being dipped, physical adhr4on is improved. This same shrinkage tends t o cause t,lic: ,mating to pull away from sharp interior angles. Fine dt.tui1 on the core cannot be preserved, though conversely gel lacquer v-ill obliterate minor roughness. Porous articles such as n.ood or paper must be sealed to prevent bubble formation w h w immersed in the hot gel lacquer. An earlier effort to accomplish this same result (4,5 ) was t u dip coat heavy solutions of nitrocellulose and keep them from sagging b y cooling t h e applied coat'ing to subzero temperatures in air of carefully controlled humidity. Even then only a large increase in viscosity was obtained, and articles had to be rotated throughout most of the drying process t.0 prevent sagging. With the present gel lacquer process the coating sets firmly as soon as it cools to room temperature, and there is actiially a gelation rather than merely an increase in viscosity.
GELTEMPERATCRC. The temperature at which these lacquer,< undergo gelation is a definite point. For a given cellulose ester, this gel temperature is dependent on the composition of the solvcmt mixture and the ratio of solvent to solid. Tho success of the operation of the process depends on the proper control of this gel temperature. Figure 3 s h o w the relation of gel temperaturr to solvent composition for several different hydrocarbons used with isopropanol. For these d a t a and the other ics reported in this paper, clear Tenite 11 molding granules of medium flow were used as the base of the gel lacquw unless othcrv-ise n o t d . K i t h this material in the imse of toluene-isopropanol mixtures, very small amounts of alcohol are required to obtain a gel lacquer, and slight changes in the amount cause Jyide fluctuation in gel temperature. With xylene, more isopropanol is required to reach the same gel temperature, but the tolerance is much greater in the useful range. This makes for easier control and is an important point t o CORaider i n operation, since the two solvents do not evaporate at the same rate, and minor fluctuations in solvent composition are inevitable. For best results the gel temperature should be adjusted so that it is a few degrees above the prevailing room temperature; thus 75-80" F. (21-27" C.) is an average working v;iliic T f it is too high, dipping troubles are encountered: if it i p
Toluene-isopropanol solvent mixture. 20 70 solids Commercial xylene 4 (coal tar base)-isopropanol solvent mixtures, 2070 solids 3. Cominercial xylene B (petroleum base)-iuopropanol solvent mixture, 20% solida 4. Same 8 s 3 hiit 22% polids 1.
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
below room temperature. the lacquer \\.ill iiut set rapidly enough and the advantages of gelation are lost. Gel temperature may be determined very simply by stirring a test tube of the material as it cools and taking the temperature of the lacquer when it sets up to the characteristic gel structure (Figure 4). The change is sufficiently masked SO that reproducible result,s can be obtained by this method. The same test can be run mechanically with a higher degree of accuracy by means of the instrument shown in Figure 5. The thermometer, which rotates slowly in the lacquer while i t is cooling, is driver] hy a small electric clock motor housed in the upper part of the cpparatus. The sudden iIicrclaae in viscosity as the material sets to a gel causes the suspending wire t o t x i s t and thus t o close an electrical circuit which lights a warning light. The operator then merely reads the temperature of the material in the tube, and this is taken as the gel temperature. Still another method for determining this property utilizes a stripping bar (S), Figure 6. This is a heavy rectangular brass bar, 30 inches in length and 1 inch square, with a small semicircular groove (1/3? inch in diamrterj cut along its length on one face. T h e bar is maintained under a temperature gradient by circulating 90 a F. water around one end and 60" F. water around the other. Short-stem thermometers inserted in the back of the bar a t intervals of 2 inches facilitate determination of the temperature of the bar at any point. -4strip of the viscous lacquer about 8 / 1 6 inch wide is coated in and over the groove by means of a special brass spreader. This coating is allowed t o come to temperature equilibrium with the bar; there is a sufficiently heavy layer of lacquer on the bar to prevent evaporation of any solvent from the part of the lacquer in the groove during this short period. T h e gelled lacquer is then stripped from the bar, beginning at the cold end. The temperature of the bar at. the point where t.he material first sticks in the groove is taken as the gel point. VISCOBITY.Not only the gel temperature but the viscosity of the solution is dependent on the solvent composition, as Figure 7 shows. For each hydrocarbon-alcohol mixture the viscosity drops rapidly as the alcohol is increased a t first, gradually leveling off at the higher alcohol concentrations. Tqluene-isopropanol, being the most active of t,he solvent combinations shoivn, reaches the lowest viscosity. However, Figure 3 shows that only certain ranges of alcohol content will produce satisfactory gel temperatures in the lacquers. Considering just these ranger (indicated by brackets in Figure 7), it can be seen that the viscosity is much more sensitive t o changes in alcohol content when toluene is the hydrocarbon than \Then xylene is u ~ d .Furthermore, the viscosity in the useful range of gel temperatures is much higher in the case of toluene-isopropanol mixtures. Tliuit is possible to prepare satisfactory gel lacquers of higher solid> content by using xylene-isopropanol solvent mixtures than toluene-isopropanol, though the latter will become completely dry somen-hat faster. Small amounts of water !vi11 affect both the gel temperature and the viscosity of the lacqiic~reto a certain extent. However, the behavior is not sufficic,ntly critical that any special precautions to exclude water nred be taken. Thc commercial grades of solvents mentioned are sufficiently anhydrous, and no turbidity or other deleterious effects are produced when they are used with eeter or molding granules of normal moisture content. The viscosity increase4 with decrease i n temperature in the dipping range, but the most startling change is the sharp rise in viscosity as the temperature approaches the gel point (Figuw 8 ) . This sudden rise in viscosity is the property which actually makes a gel lacquer .r?-ork. For comparison, Figure 0 shoiv> a 1 c.xnpcrnture-vi.coiity curve for the same material i n both a gc,lling and nongcllirig solvent mixture. This also include:: a curve for a nitrocellulose solution of the type described by Hamptnir ($, 5). The difference between the two types is quite marked. The viscosity varie.- n i t h the solids content (Figure 10) and rises fairly sharply if a vc~lliilosee,ster of high v i w x i t y is uwd.
Figure 1. Simple Determination of Gel Teniperature
Figure 3. iutornatic Detice for Gel Ternperature Determination
For loncr viscosity ech resistance will not be so high as t h a t for thermosettings or some baked synthetic finishes, however. Good adhesion, as mentioned before, is only obtained by the use of a suitable priming coat. By proper selection, the sealing coat applied t o wood t o prevent bubble formation can also a c t as the primer to furnish good adhesion. APPLICATIONS
Possible uses of this material are numerous. I n general, it can be used on suitably shaped articles wherever a premium finish offering excellent appearance, good protection, and permanence is desired. Specifically some possible applications might be as follons: Where wooden cores are t o be used, tool handles, kitchen utensils, baby furniture and toys, toilet seats, shoe heels, and the like suggest themselves. Any articles now finished by the nitrate sheet covering process such as the last two mentioned can be ,coated more simply by this method, as well as much more safely. I n the field of metal cores, gel lacquers can be used t o
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Nongelling lacquer prepared horn Tenite I1 -pica1 nitrocellulose aolution in active solvents
to follow minor imperfections in the surface of the core, since the lacquer layer adjacent t o the core is firmly gelled a t the start. Consequently, it is possible to obtain a smooth finish over a comparatively rough surface, such as a metal casting direct from the mold.
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Figure 10. Viscosity z's. Solids Content for Cellulose -4cetate-Butyrate Gel Lacquers at
1. Typical pel lacquer prepared from Tenite I1
2.
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*lo SOLIDS
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1. Tenite 11 in 95% toluene-590 isopropanol 2. Tenite I1 i n 82% commercial xylene A - l 8 % isoprapanol 3. Low viscosity cellulose acetate-butyrate (same composition as that umed in Tenite 11) i o 70% c o m mercial xylene B-30 90isopropanol
good advantage on tool handles; particularly for electricians' tooh the added insulating power of the coating is desirable. Any use where the strength of metal but the appearance and feel of a plastic are desired is a natural outlet, such as a n airplane control wheel. Paper can be used for a core, and the gel Lacquer imparts considerable strength as well as good appearance. It can be used n i t h glass containers t o produce bottlcs which show increased resistance to shock. If the glass does brcak, the coating makes it shatterproof and holds the contents intact. Colored lacquers add decorative effects. Gel lacquer can also be used to give a decorative finish to articles molded from cheap scrap plastic or t o l o ~ vpre>sure laminates. ACKNOWLEDGMENT
The authors wish t o thank G. J. Clarke, John Gallagher, and
W. H. Griggs for mtensive experimental work reported in this paper. LITERATURE CITED
(1) Fordyce, C. E., and Clarke, G. J. (to Eastrnan Koda?r Co.), U. S. Patents 2,350,742-4 (June 6, 1944). (2) Fordyce, C. R., and Hunter, W. F., Ibid., 2,319,051 (May 11,
1943). (3) Fordyce, C. R., and Vivian, H. F., Ibid., 2,336,238 (Dec. 7, 1943). (4) Hampton, TI'. C. (to Crawford. McGregor and Canby Co.), Ibid., 1,851,809 (March 29, 1932) and 1,899,387 (Feb. 28, 1933). (5) Hamptoxi, TT, C., and Garber, C. N.,Ibid., 1,882.574 (Oct. 11. 1932). (6) Malm. C. J., and Clarke, G. J. (to Eastmarl Kodak Co.), Ibid., 2,324,098 (July 13, 1943). PRESEXTED before the Division of Paint, Tarnish. and Plastirs Chemistry a t t h o 109th Meeting of t h e AMRRICLXCHEMICAL SOCIBTY. Atlantic City. N. J.
