INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1926
1249
Some Developments in Controlling China Wood Oil Varnishes' By W. W. Bauer RESEARCH LABORATORIES, PAINTA N D VARNISHDIVISION,PITTSBURGH PLATEGLASS Co., MILWAUKEE, WIS.
these phenomena may lead to a better understanding of t h e t e s t proposed In this paper. Solidification
China wood oil pressed at high pressures from meats containing high percentages of inner shells has formed a solid modification out of contact with light. The crystallization of China wood oil has been produced in the absence of any light. Further evidence is which indicates that this phenomenon is caused by the rapid absorption of oxygen and the formation of a surface gel. A simple, positive, and reliable method for measuring the crystallizing tendency of China wood oil varnishes is proposed. The apparatus is described and an outline of the method of procedure is given.
The solidification of' China R'ood Oil in the presence Of sunlight is now quite generally recognized to be due to the formation Of a geometric i s o m e r . This crystalline "lid is designated as the beta isomer to distinguish it from the a h h a or ordinarv liquid modkcation. T h k change to the beta form is accelerated by such substances as iodine and sulfur. Oils which have been pressed a t rat8herhigh pressures from oil meats cont'aining high percentages of inner shells undergo this change even in the absence of light. It is believed, therefore, that certain constituent's of the inner shell of the tung nuts are capable of bringing about this change. Such constituents may resemble enzymes in their action. This light isomerization of China wood oil, although a unique property, has not been of much practical significance to the manufacturer up to the present time. Gelatinization
The process of gelatinization is still a subject of much controversy. A rather complete review of the more recent researches on this subject is found in an article by Nagel and Griisse2 For our purposes we need consider only two of the most contradictory views on the subject. Wolff regards the thickening and gelatinization of wood oil as a colloidal physical process rather than one of genuine polymerization. This conclusion is based on the fact that the decrease in iodine number is not proportional to the increase in viscosity during the various stages of heating. The form of the viscosity curve is considered typical for colloids. Furthermore, molecular weight determinatione in camphor show no increase for thickened oils or their fatty acids. The other theory considers the change which takes place during the heating of wood oil to be one of polymerization 1 Presented before the Midwest Regional Me-ting and the Meeting of the Section of Paint an.d Varnish Chemistry of the American Chemical Society, Madison, Wis., May 27 to 29, 1926. * 2. angew. Chem.. 89, 10 (1926). 3 I b i d . , 37, 729 (1924).
lecular methyl ester showed no tendency to gelatinize upon heating, but the jelling tendency did increase w i t h O t h e r esters the weight Of the alcohol increased. Flatting, Frosting, or Crystallization
Our conceptions of this phenomenon are based on the varied researches of a number of i n v e s t i g a t o r s . T h e m o s t i m D o r t a n t of these con t r i b i t ions have recently been summarized by A ~ e r . ~ According to Marcusson, the frosting of the wood oil film is due to the formation of crystals of the beta isomer of the triglyceride. Eibner, Merz, and M ~ n z e r t ,as~ well as S V ~ l f fhave , ~ followed this change under the microscope, and find no evidence of a crystal structure, but rather a system of folds and wrinkles which is caused by unequal volume changes in the film during the rapid absorption of oxygen. Furthermore, Eibner states that frosting cannot be due to the formation of the crystalline modification because the rate of formation of this beta isomer is much slower than the rate of drying or frosting. Melting point determinations on the free fatty acids obtained from the dried films showed no beta eleostearic acid present. Schmidt' contends that the frosting is caused by the presence of moisture, since clear films were obtained when drying took place in a desiccator with anhydrous calcium chloride. Auer has studied the influence of moisture by using anhydrous copper sulfate in the oil film and also with sulfuric acid in the desiccator. He found that clear films could be obtained in the presence of enough moisture to color the copper sulfate. Along with others, he noted that frosting of the film occurred when it was removed from the desiccator. The conclusion was drawn, therefore, that certain light rays which caused frosting were being absorbed by the glass of the desiccator. This view was supported further by the fact that frosting had never been obtained in the dark. Since the writer has produced the frosting of China wood oil in a dark room by the test method which is proposed in this Farben-Ztg , 81, 1625 (1926). Chem Umschau, 31, 69 (1921). 8 I h z d , 31, 99 (1924). 7 F a , hen-Zfg , 2 9 , 1261 (1924).
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INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
paper, it is believed that the rate of oxygen absorption by the film is the factor of paramount importance and that conditions of moisture and light are secondary. Importance of Test Methods in Controlling China Wood Oil Varnishes
A very important consideration, therefore, in the use of China wood oil is the selection of those conditions of heat treatment which will give to the oil the properties that insure its drying with a satisfactory gloss even under unfavorable conditions, without a t the same time imparting to it a viscosity which detracts from its good working qualities or less..ens the stability of the product upon aging. I n order t o select the proper conditions of heat-treating China wood oil for a given purpose, a simple and reliable method of measuring the flatting or crystallizing tendency of %he resulting product is essential.
VOl. 18, No. 12
A series of experiments in which various oxides of nitrogen were used in conjunction with nitrogen, carbon dioxide, or oxygen, showed very clearly that frosting or flatting depended upon the presence of oxygen. Within certain limits the rate of flatting also varied with the concentration of nitrogen oxides. I t was found that a suitable concentration of nitrogen oxides could be maintained by bubbling air through concentrated nitric acid. The test method as finally developed and now used consists merely in exposing varnish films for one hour to the influence of a uniformly distributed atmosphere containing the desired concentration of nitrogen oxides. Compressed air is split into two streams; the greater amount is required to keep the fan in motion, while a much smaller quantity is sufficient to replace the oxides of nitrogen from the nitric acid. Apparatus
The essential features of the apparatus are illustrated by Figure 1. The bell jar, K , which is 30 cm. (12 inches) in diameter and 25 cm. (10 inches) high, provides a transparent system so that any changes in the appearance of the varnish films during the test may be readily noted. The aluminum rack, I , supports the glass slides upon which films of the varnish have been poured. The fan, H , which is 9 cm. (3.5 inches) in diameter, is constructed of thin aluminum sheet. A glass rod, which works smoothly in a glass tube, is fitted with a split cork. The fan is held firmly between the two sections of this cork. The jet, G, is 2 mm. in diameter and is designed to keep the fan in motion by the introduction of a small amount of compressed air. The bell jar rests upon a plate glass 38 cm. (15 inches) square, through the center of which a hole 38 mm. (1.5 inches) in diameter Figure 1 has been drilled. This hole is fitted with a rubA-Inlet compressed air G-Tube constricted t o a jet B-Flowmeter H-Fan made from aluminum sheet ber stopper carrying the three glass tubes, which C-Manometer I-Aluminum rack for glass slides serve as inlet, outlet, and support for the fan. D-Gas washing bottle J-Outlet t o drain E-Drying tube with cotton plug K-Bell jar Bv lifting. the fan from its Dosition the aluminum F-Clamp rack maialso be removed dy lifting over the jet G. Inadequacy of Present Methods The total flow of compressed air is measured by the manometer C. That portion which is directed by means of the clamp, The draft test and gas test, which are outlined in various F, through the concentrated nitric acid contained in the government and other specifications, are not sufficiently 250-cc. wash bottle, D, is regulated by means of the flowmeter, accurate and reliable for most types of development and con- B. With a jet 2 mm. in diameter and a difference in the trol work. These tests require 5 or 6 hours’ time and it is level of the mercury columns in C of 19 mm. (0.75 inch), difficult t o interpret or duplicate the results. Any differ- the flow of air is approximately 5 liters per minute. A entiation must be based upon the extent of failure a t the difference of 38 mm. (1.5 inches) in the level of the water end of a given test period rather than upon the rate a t which columns in B allows 200 cc. of air per minute to pass through the samples fail. The size of the checks, as well as the area the nitric acid. The cotton plug E is inserted in the line affected, often varies markedly with two panels of the same to remove any foreign particles in the air supply which might varnish in the same test. give the films a pitted appearance. Figure 2 shows the apparatus as it is assembled for use. Development of a More Satisfactory Test It is inexpensive and easily constructed. With the possible The following test has therefore been devised for use in the exception of the bell jar, all parts would be available or could development and control of certain China wood oil varnishes: be readily constructed in the average laboratory. By working with varnishes which had failed in the gas oven Testing Procedure test, it was possible to demonstrate that the humidity of the atmosphere a t various temperatures was not an important The varnishes to be tested are poured on the glass slides, factor. Such reducing gases as might be expected to be present in the products of incomplete combustion-for example, which are 76 x 127 mm. (3 X 5 inches). The rack supports four slides, and four samples may be poured on each slide. carbon monoxide, hydrogen sulfide, and sulfur dioxidewere found to have no crystallizing effect upon the varnish The varnish films are allowed to drain and dry a t an angle of &s. Carbon dioxide likewise showed no effect other than a about 45 degrees for 20- and 40-minute periods before placing retarding of the drying. Hydrogen chloride was without ac- them on the rack in the bell jar. One sample of known betion, but fumes of nitric acid caused a frosting and crystal- havior-i. e., one which fails in from 5 to 10 minutes-is inlizing action more pronounced than that obtained by any cluded as a check in every test, simply to make sure that the test is being conducted properly. A measured quantity known test method.
(100 ee.) of C. 1'. concentrated nitric acid is placed in the wash bottle. When the glass slides and the hell jar are in place, the San is set in motion and the current of air is started through the nitric acid. By means OS the manometer and flowmeter, the conditions of testing are easily controlled for each and every successive test. The test is continued for one hour and the raniislies nre often allo~vedto remaiii in the bell jar oiwiiiglit t,ii note any further changes. llaw China wood oil this test in less than one minute. Very varnishes fail in from 5 to 10 minutes. Vaniislies which stand the test for one hour rimy sometimes show a slight failure under the most severe conditions obtainable in a gas oven. Over a period of nearly 3 years, however, certain varnishes which have stood the hour's test have dried with satisfactory gloss in every section of the country. As a selective test to measure varying degrees of gas resistance, this method has also been found more useful than former tests.
(n) EJcd of ozygon mnienlraiion. Whcn p w e oxygen i s uscd io place of coinpressed air, not only i s the iite of cryrOll1ization correspoodingiy increased. hut films which ordinarily stand the t e s t for one houi are found
crystriiire. ( b ) Injluencc of trilrir-acid ionrcnirolion. In the absence of lhe nilroscn oxides neither compressed air nor oxypen CIUS~S the crysialiizafion 01 China wood oil w i t h i n the hour tcrt period. With a given cOneenfisfion of oxygen ilie time of crynialiirilfion c~iibe varied by chaiigfs in t h e conccntradan of Lhr nitric acid. concentrated nitric acid criiiins more rapid cryrialiization Limn t h e dilute acid. fO
Precautions
Cleanliness of apparatus is essential to accurate testing. Before perforniing this test, the bell jar and rack are waslied with soap and water. Failure to wash the inside of the bell jar before each test allows tlie Sormation of a cloud or film on this surface, which reduces or destroys the catalytic action of the nitrogen oxides. Fresh quantities of C. P. concentrated nitric acid are used Sor each test. When a fresh bottle of acid is taken from the stock room, it is exposed to direct sunlight for one day before using. Care must be taken during the first 10 minutes to avoid tire introduction of nitrogen oxides too rapidly. If the concentration of oxides is too high, the varnish films will turn to a yellowish brown and tho frosting effect will not be obtained. Aluminum sheet is most snitahle for the construction of the rack and San. Cause and N a t u r e of Crystnliization
In addition to its usefulness in controlling Chiria u w d oil varnishes, this tmt, may also lead to a clearer understanding of the phenomenon of frosting or crystallization. Several lincs of evidence indicate that crystallization is caused by a catalytic oxidation of some sort. I n this test method it is believed that oxides of nitrogen, acting perhaps as carriers of oxygen, cause very rapid ahsorption of oxygen by the film. The volume changes attending this rapid ahsorption of oxygen are responsible for the folds and wrinkles ohserved in the frosted film when viewed under the microscope. Tlris r q i d absorption of oxygen by certain Cliina wood oil film is :ieeornpanied by the iorniatiy~of a gel on the surEdce of the film, and it is because of tliis gel st,rueture that furtlier volunie changes cause the folds and wrinkles which give the film its frosted appearance. l'he cxperimental evidence for tlie belief that erystallization dcpends on the rapid absorption of oxygen and the formation of stinie gel structure on the surfaec may be summarized its folloiss: ( I ) Oxygen is essential to crystallization. Experiments with cnibon dioxide and nitrogen demonstrated that ciystalliztiiln does not occirr in tlie absence of oxygen. (2) Crystallization occurs in the absence of light. The crystallization of China wood oil and certain China wood oil varnishes has heen produced in a dark room as readily as in dircct sunlight. Under the conditions of this test, at least, the influence oi light is not essential to the crystallization of China wood oil, and it is believed that phenomena which have heen attrihuted in the past to the action of wrtain light rays are to be explaincd in othcr ways. (3) The rate and extent of crystallization arc incrcascd by factors which permit an increase in the rate of oxygen absorption.
Pieure 2 (4) ,Films, which are subject t o crystallization, form gels oil therr surfaces. By placing samples of China wood oil and China wood oil varnishes on watch glasses or in friction-top can lids in the hell-jar system, it was found that China wood oil and those varnishes which tend to crystallize formed gel st.ructiires on their surfaces in from 2 to 3 minutes. China wood oil varnishes, which are quite gas-resistant. did not surface-gel, biit remained entirely mobile. The surface gels, which iormed in certain cases, tended to disappear after removal from the test chamher. A gel which forms under such conditions and then tends to redissolvc must be regarded as unusual. No erplanation is offered for such behavior. It does not appear t o be the type of gel which is obtained by the action of acids or by the intluence of heat. It is believed that the only action of the nitric acid is one of catalytic oxidation. The formation of such a gel may be an acceleration oi the initial stage in the process of the drying of a film or the skinning of a n exposed surface of a China wood oil varnish. Under normal coiiditioiis this process is more gradual and little or no distortion oi the surface is produced.
While it is believed that crystallization is always caused by a rapid absorption of oxygen, this increased rate of ahsorption may he produced in a variety of ways. Some are distinctly chemical in character, while others are meebanicnl methods of increasing the oxygen concentration. Crystallization in the presence of ozone may be due simply to the greater activity of oxygen in tliis form nr it may be a catalytic process in wliicli certain reaction products, perhaps oxides of nitrogen, Sormed along with the ozone are aeeelerating the oxidation. The well-knomi draft test appears to be uotliing more than a mechanical rnethod of increasing the rate oxygen absorption. In general, eryst.al1ization tends to take place whenever tire rate of oxygen absorption is suddenly clian~edfrom a lower t.o a higher value. One method oS overcoming tliis crystallizing tendency is to polyrirorize t.he oil to such an extent that tlio rate of oxygen absorption is rnuterinlly reduced. The film will tlien dry witbtint. any apparent distortion of its surface. As is generally known, the desired stage of polymeriz,zt,ion can he roached only in the presence of acidic or other eomlionents which hold the diinoleeular form OS the triglyceride in solution, or at any rate prevent the formation of a gel by retarding the eolloidal aggregation of these polymers. The writer is indebt,ed t.0 1'. R. Croll, J. 1). Jelikins, and V. K. Boyntou for valuable suggestions and assistance.
