T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Oct., 1916
879
ORIGINAL PAPERS A STUDY OF THE EFFECT OF STORAGE ON MIXED PAINTS’ By
E. E. WAREAND R E CHRISTMAX~ Received June 23, 1916
T h e manufacture of ready mixed paints is a development of comparatively recent years. Pigments ground in oil t o be mixed a n d tinted b y t h e individual painter were much more commonly used t h a n ready mixed paints twenty-five years ago, while it was not a t all uncommon practice for t h e practical painter t o mix t h e d r y pigments with oil. I t may be possible, if proper precautions be used, a n d enough time be given, t o prepare a satisfactory paint i n this way, b u t extensive investigation has shown t h a t i t is much better t o grind a t least p a r t of t h e oil with t h e pigment. T h e painter to-day has neither t h e necessary skill nor t h e unlimited patience required for mixing t h e dry pigments with the oil, even if it were possible t o compound in this manner as satisfactory a product as t h e paint manufacturer with his years of experience a n d his intimate knowledge of materials is able t o produce by intensive grinding a n d a n intelligent proportioning of t h e different ingredients. One of t h e earliest difficulties encountered in t h e manufacture a n d use of ready mixed paints was t h e tendency of t h e heavy pigments t o settle away from the vehicle i f t h e paint were permitted t o s t a n d a n y time before use. It was soon discovered, however, t h a t t h e addition of small amount of water containing a suitable emulsifying agent helped t o prevent this settling of t h e pigment. But while eliminating one source of difficulty, t h e practice has brought on other troubles, a satisfactory remedy for which has never as yet been advanced. Some emulsified mixed paints, if left t o s t a n d for several years, will show evidence of a deterioration which manifests itself in one of t w o ways, either through t h e formation of a n amorphous soapy blanket between t h e settled pigment a n d t h e supernatant vehicle, or b y t h e gelatination of t h e entire paint t o a mass of about t h e consistency of soft p u t t y , t h e vehicle seemingly having entirely disappeared. T h e first case is usually spoken of as “skinning” a n d t h e second as “puttying.” These phenomena have given t h e paint manufacturer much trouble, a n d consequently have been t h e incentive for considerable experimental work i n a n effort t o compound a paint formula t h a t will show t h e m t o a minimum extent. While it has been quite generally acknowledged t h a t neither trouble will occur i n t h e absence of water, i t has been felt t h a t water exerted only a secondary influence, t h e principal cause of deterioration lying i n some action between t h e pigment a n d t h e oil of t h e vehicle. Most of t h e published d a t a has been compiled as a result ’ of investigation along this line. 1 Presented a t t h e 8 t h Annual Meeting, American Institute of Chemical Engineers, Baltimore, January 12-15, 1916. Holder of t h e Acme White Lead and Color Works Fellowship in Chemical Engineering a t t h e University of Michigan, 19 15-1 916.
I n 1911 Gardner,l i n studying t h e possible effect of pigments upon t h e paint vehicle, ground t h e more common ones, both t h e active a n d t h e inert, in raw linseed oil a n d compared t h e ash content of t h e oil before grinding a n d after it h a d remained in contact with t h e pigments for a month. T h e oils from t h e mixes containing white lead, zinc oxide a n d red lead showed an appreciable increase in this ash value, a n d Gardner advanced t h e opinion t h a t these pigments have t h e power of saponifying t h e oil with t h e formation of their respective soaps. He s t a t e d t h a t t h e inert pigments have little action b y themselves a n d when mixed with active pigments t e n d t o retard t h e action of t h e latter. A few months later Sabin2 published a reply in which he questioned both of these theories. I n a later publication Gardner3 described t h e effects of pigments upon linseed oil after a storage of two years. Various pigments were ground in a good raw oil without drier or thinner, using a sufficient amount of oil t o bring t h e mixture t o t h e desired consistency. T h e most marked effect on t h e oil was t h e change in its iodine number, which was lower in all cases after storage. T h e oil of t h e red lead mix showed a final iodine number of 135.4, while in t h e other cases t h e value h a d dropped t o approximately 160 from a n original value of 181. The acid cont e n t of the oil was affected t o t h e greatest extent in t h e paints containing white lead, red lead, a n d basic chromate of lead, b u t t o a considerable extent also b y both carbon black a n d graphite. He concluded t h a t t h e perfect condition of these experimental mixtures warrants t h e manufacturer in assuming t h a t a n y properly prepared combination pigment paint may be safely stored in cans for long periods without bad effects, provided t h a t t h e oil used be of normal grade. He also concluded t h a t t h e action of t h e free acids upon t h e pigments was t h e real cause of t h e hardening. T h e results of work b y Boughton4 along similar lines are somewhat a t variance with those of Gardner i n t h a t t h e y show a change in iodine value of not more t h a n t e n points in a n y case, while t h e only sample in which there was a n y marked change in t h e ash cont e n t of t h e oil is t h e one containing corroded white lead. I n a more recent publication Gardner states6 t h a t t h e oil i n paint is hydrolyzed b y a f a t splitting enzyme, lipase, liberated in seeds of t h e oil-producing type. He claims t h a t t h e seed is not always steamed before expression t o a temperature t h a t will kill all of t h e microorganisms, a n d spores which pass into t h e oil soon develop t h e enzyme. H e further states t h a t t h e f a t t y acids formed are partially neutralized b y t h e basic pigments whereby lumps of a n insoluble 1 f
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THISJOURNAL, 3 (1911), 628 I b i d , 3 (1911). 790 J . F r a n k . I n s t , 174 (1912), 415-423. THISJ O U R A A L , 5 (1913), 2 8 2 . J . Frank. I n s f . , 177 (1914), 533.
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f a t t y acid soap are formed. He recommends heating cloudy oil t o a t least 100' C. before shipping, in order t o kill all microorganisms. EXPERIMENTAL
I n considering this important problem t h e authors felt t h a t it might be possible t o get a clearer idea of t h e underlying causes of these deteriorating influences if paint specimens in which t h e action was pronounced could be procured, and t h e different products contained therein isolated and examined. Accordingly a n effort was made t o collect a number of samples of standard commercial paints which had been in storage for considerable time. Those actually used in this investigation had been on t h e shelves for from z t o 8 years. T h e blanket effect of "skinning," which was very pronounced in some of t h e samples, was studied first. T h e paints in which this had occurred were all composite-base paints which had originally contained a small amount of water, while one also contained a considerable proportion of gloss oil. The oil extracted from t h e pigment a n d with t h e thinner completely distilled off showed a n average iodine number of 1 8 1 and a n acid value of I j . T h e ash content was low, running about I. 5 per cent and consisting mainly of lead oxide, with some manganese and zinc. The recovered oil was clear and of good color. Samples of the skin were repeatedly washed with petroleum ether until all adhering oil and pigment were removed. The composition of this skin was variable, the weight of t h e oxides ranging from 13 t o 16 per cent, t h e average of a number of samples being about 14 per cent. The proportions of t h e different oxides varied within narrow limits: b u t a typical analysis, corresponding t o 1 4 per cent oxides, showed: PbO.. .. . . . 12.0 ZnO.. . . . . . 87.0 R203... . . . 1 . 0 The zinc oxide was similarly predominant in all samples analyzed, indicating t h a t it was t h e active agent in the formation of the skin. The free acids were liberated from t h e skin b y heating with a dilute solution of mineral acid without access of air, t h e f a t t y acids dissolved in ether and t h e ether solution evaporated under vacuum. In t h e first experiments hydrochloric acid was used as t h e liberating agent, which procedure gave free f a t t y acids exhibiting the following constants: Neutralization Value 166 t o 167
Saponification Value 203 t o 205
Iodine Value 170 t o 178
Such abnormal differences between t h e neutralization and saponification values clearly show. according t o Browne,' t h e presence of a n unsaturated lactone. The amount of this lactone was estimated according t o the method of Lewkowitsch2 and Sound t o be 20 per cent of t h e total acid. As t h e purified lactone was liquid i t seemed hardly probable t h a t it was formed as a product of reaction during storage of t h e paint, for in t h a t case it would have remained in solution in the oil rather t h a n have been precipitated in t h e skin. I t was no doubt formed during our manipula-
tion when t h e f a t t y acid was freed from t h e zinc soap, t h e action being accelerated b y t h e presence of t h e resulting zinc chloride which acted as a condensing agent. Zhukov and Shellakovl studied a similar reaction and came t o t h e conclusion t h a t zinc chloride has the power of throwing t h e double linking along the carbon chain until i t reaches t h e gamma position, where it is converted into t h e lactone. Concentrated sulfuric acid exerts a similar influence, although t h e dilute acid has no action. The acids of t h e skin when liberated b y dilute sulfuric acid proved t o be more nearly t h e usual linseed oil acids, as indicated b y the following constants: Neutralization Value 193.5
Saponification Value 103.0
Iodine Value 181 .0
This remaining difference between t h e neutralization and saponification values showed t h a t t h e free acids still contained some lactone, probably due t o the condensation of a small amount of gamma hydroxy acid which may have been present, and which, on liberation, changed immediately t o the lactone. The Gusserow-Varrentrapp or lead-salt-ether method for t h e separation of solid and liquid f a t t y acids showed a solid acid content in t h e skin of 10-12 per cent, a figure not inconsistent with that of 7 . 5 per cent ordinarily given for raw linseed oil. Samples of the skin were dissolved in hot alcoholether solution, and gave upon titration a n apparent neutralization value as high as 168. This a t first suggested t h e possibility t h a t t h e skin was a product of t h e coagulation of a colloidal suspension of pigment and f a t t y acid, b u t experiments t o establish this failed t o yield definite results and were abandoned. On extracting the skin with hot absolute alcohol the entire mass was dissolved. On cooling, a white flocculent material composed of metallic soaps a n d hydroxides was precipitated, while t h e residue in t h e filtered solution, upon evaporation of the alcohol, was recovered as free acids. A n extraction with ether failed t o show the presence of these free f a t t y acids and i t was decided t h a t during the extraction with alcohol t h e soaps had been hydrolyzed. This same condition undoubtedly existed when the skin was titrated with dilute KOH in determining its acid value, which would account for t h e high value obtained. Weaker hydroxides, Ba(OH)* and SH,OH, acted similarly, as did an absolute alcohol solution of KOH. To check these observations, a pure zinc soap of linseed oil was prepared. This soap exhibited t h e same tendency t o hydrolyze, giving a n apparent acid value indicating complete hydrolysis as figured f r o m the formula of zinc linoleate.2 Since in t h e skin or blanket of a mixed paint t h e metallic soaps present are mostly all those of zinc, which are completely hydrolyzed in analysis, t h e determination of t h e acid number of this material does very little toward establishing its constitution. However, i t seems reasonable t o assume t h a t t h e skin consists almost entirely of the neutral metallic soaps of zinc and lead, for any free acid present should be perfectly soluble in t h e oil which is in contact with
1 THISJOURNAL,
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J. Russ. Phys.-Chem. Soc., 40, 930-9.
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Ware and Christman, next issue of THISJOURNAL.
7 (19151, 30. "Chemical Technology of Oils, F a t s and Waxes," 1, 423.
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Oct., 1916
t h e skin. Also if t h e total weights of t h e zinc a n d lead soaps be calculated from t h e oxides contained in t h e ash as given above, using 2 7 5 . j as t h e average molecular weight of t h e f a t t y acids, t h e y are found t o check u p t o t h e original weight of t h e sample t a k e n for analysis. As a further proof of t h e identity of t h e skin as a zinc soap of linseed oil a comparison was made of t h e melting points of t h e skin a n d of t h e zinc linoleate. T h e somewhat higher melting point of t h e former, 81, as compared with 76 for t h e linoleate, may be accounted for by the presence of t h e other metallic soaps. It is not probable t h a t these precipitated metallic soaps t h a t go t o make u p t h e skin result from t h e saponification of t h e neutral glycerides b y t h e pigments alone, although a t different times writers have s t a t e d t h a t white lead has this power of saponifying t h e 1 oil t o form a lead soap. Such reactions may occur at high temperatures, b u t it is doubtful if t h e y proceed t o a n y extent a t ordinary temperatures. T h e results obtained b y Gardner a n d Boughton do not necessarily signify t h a t even t h e slight changes in t h e characteristics of t h e oil as obtained b y t h e m are due entirely t o t h e pigment, since t h e action of a n y water present must be taken into consideration. It is possible t h a t t h e accelerator necessary t o this metallic soap formation is lipase, as claimed b y Gardner, b u t it is t h e opinion of t h e writers t h a t , although t h e action of lipase may a t times be considerable, i t is of secondary importance in t h e reactions taking place in t h e ordinary mixed paint. T h e occasional presence of lipase, t h e fat-splitting enzyme, in linseed oil has long been established. Zymogen, in t h e flaxseed, under favorable conditions, is capable of forming t h e enzyme, a n d i t is of real importance i n expressing t h e oil t h a t t h e seeds be heated t o a high enough temperature t o kill a n y microorganisms present, not only t o prevent t h e formation of lipase i n t h e oil, b u t also t o insure its absence from t h e press cake. T h e enzyme has t h e ability t o act upon t h e cyanogenetic glucoside, linamarin, present i n t h e cake, a n d , with t h e aid of water, t o convert it into prussic acid, making t h e cake entirely unfit for cattle food. However, t h e authors do not consider t h a t t h e presence of a fat-splitting enzyme i n linseed oil is a frequent cause of trouble. Even when using cold-pressed oil trouble arising from t h e action of lipase would not be common, for, as shown by Tanaka,l t h e enzyme lipase is inactive in t h e presence of even dilute alkali, a material which is commonly present as a constituent of t h e commercial emulsifying agent. And if t h e action of t h e lipase be so pronounced it should show in t h e stored oils as well as in t h e paints made from these oils. AS a further evidence of t h e exceptional, rather t h a n common, occurrence of this enzyme in linseed oil, cultures were made according t o t h e method recommended b y Gardner, using several different commercial oils a n d one sample of foots. I n no case was a n y growth apparent after 7 2 hrs. T h e authors believe t h a t t h e really important hydro-
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lyzing influence is t h e emulsifying agent. T h e sodium hydroxide present in t h e solution has t h e power of saponifying linseed oil a t ordinary temperatures, while t h e sodium soap so formed, acting as a n emulsifying agent, tends t o keep t h e water a n d oil in cont a c t , giving a better opportunity for further action. T h e alkali soaps are a t least partially hydrolyzed i n water solution a n d , as t h e acid formed is soluble in t h e oil present, there may be a partial recovery of t h e alkali which furthers t h e reaction b y forming more soap; a n d we might s a y t h a t t h e governing factor in establishing t h e limit t o which hydrolysis will proceed will be t h e amount of water present. T h a t t h e same saponification takes place with other alkaline agents is evidenced b y t h e following compilation of d a t a : Water 100 g. 100 g. 100 g. 100 g. 1 g.
I g. 100 g . 100 g. 100 g.
ADDITIONSTO OIL Others NarCOa 3 g. 3.g. 0 0 0 3 g. 3 g. 3 g.
.. .. .. ..
3 ;.'Casein 3 g . ZnO 3 g. White Lead 3 g. ZnO
Time T e m p . Per cent Saponified Hrs. 2 70 1.5 70 0.5 2 70 10.0 18 1.3 18 70 18 70 2.2 70 2.3 20 5.3 70 20 30.4 22 70 70 0.8 22
c.
These results show t h a t t h e water itself has a small action upon t h e linseed oil. T h e carbonate has a marked effect, although t h e so-called active pigments are almost without effect. T h e differences in activity between t h e saponifying agents should be even more marked a t ordinary temperatures t h a n a t t h e elevated temperatures employed in this experiment. When ready mixed paints are stored, t h e first general reaction is this hydrolysis of t h e oil b y t h e water, accelerated b y t h e presence of t h e alkali a n d perhaps affected t o some extent b y t h e pigments. This hydrolysis is necessarily slow since t h e water a n d oil are immiscible, a n d before a n y large amount of acid is formed t h e pigment has settled, leaving t h e clear vehicle above. T h e acid coming i n contact with t h e t o p of t h e settled pigment reacts with t h e basic materials present, of which zinc oxide is t h e most active, a n d forms t h e corresponding metallic soaps. Originally two molecules of water had hydrolyzed t h e oil a n d formed two molecules of t h e acid, a n d upon t h e reaction between these two molecules of acid a n d t h e metallic oxide, one molecule of water is liberated. I n computing t h e amount of hydrolysis under such conditions it m a y be shown t h a t with a n amount of water such as is sometimes present in mixed paints i t is theoretically possible t o get complete hydrolysis of all of t h e oil. Although this is a condition t h a t does not occur in t h e average paint t h e speculations are not greatly overdrawn, a n d the possibility of injury t o mixed paint b y t h e use of improper emulsifying agents is clearly shown. T h e zinc soap formed on t h e neutralization of t h e acid b y t h e zinc oxide of t h e pigment is almost completely insoluble in t h e cold vehicle a n d is a t once precipitated on t o p of t h e partially settled pigment. T h e white lead also unites with t h e acid, with t h e formation of t h e more soluble lead soap, p a r t of which is carried down b y t h e zinc, a n d so helps t o build u p t h e peculiar skin over t h e pigment.. T h e reactions
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between t h e acid and t h e pigments t a k e place readily so t h a t the oil never shows a pronounced free acid value. This value, previously cited as approximately I 5, is corrected t o 8 when allowance is made for the difference due to t h e dissolved soaps hydrolyzed during analysis. Some of t h e water is probably carried along with the settling pigment resulting in a similar hydrolysis of t h e oil in t h e bottom of t h e can. T h e metallic soap formed in t h e midst of t h e pigment may be t h e cause of t h e non-caking of the pigment in stored mixed paints. The opportunities for reaction in t h e presence of t h e great excess of pigment are very good, and t h e tendency is in favor of the formation of t h e basic soap rather t h a n t h e neutral soap as present in the skin. Upon extraction of the settled pigment with either t h e presence of t h e basic soap was inferred from t h e fact of its relatively slight solubility. T h e extracted oil was clear and of good color, with a n apparent acid value of 1 6 . 8 . Upon correction for t h e ash content of I . 16 figured t o hydrolyzable metallic soap, t h e t r u e acid value would be about I O . The pigment residue from the extraction still contained some f a t t y acid compounds, t h e freed f a t t y acids checking t o 2 . 1 4 per cent. These were no doubt present as the basic soaps and would represent a weight of 2 . 7 6 per cent if figured as t h e basic zinc soap and 3 . 8 per cent if figured as t h e basic lead soap. As a further evidence of t h e plausibility of t h e above analysis of conditions it is possible t o synthesize t h e zinc oxide f a t t y acid blanket if a small amount of oxide be allowed t o stand undisturbed in contact with oil containing large amount of free linseed oil acids. If one gram of zinc oxide is placed in the bottom of a flask and covered with acid, within 24 hrs. t h e zinc oxide will swell out through conversion t o t h e soap, giving the characteristic lumpy appearance of t h e “skin.” This zinc soap is quite insoluble in oil or naphtha, though soluble t o a n appreciable extent in turpentine. T h e insolubility of t h e soap may explain why t h e constants of t h e oil in t h e zinc oxide linseed oil storage tests conducted by Gardner and b y Boughton showed no marked changes. If a small amount of white lead be treated with excess acid in t h e same manner as succeeded in developing t h e linseed zinc oxide skin, we find t h a t t h e action is much slower and several weeks’ time is necessary before there are any definite indications of reaction. The lead soap formed is of a finer structure and does not show t h e same tendency toward coagulation or lumping. PUTTIED PAIKT
T h e similarities in t h e reactions giving a puttied paint t o those just reviewed is quite apparent. Several cans of t h e worst cases of puttying were studied, in which t h e entire liquid portion had seemingly gone solid t o a dense soapy mass. These paints were of a grade inferior t o t h a t of t h e paints exhibiting skinning, in t h a t t h e y contained more water as re11 as considerable rosin in t h e form of gloss oil T h e oil extracted from t h e paint and with t h e thinner distilled off was clear b u t slightly dark in color.
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T h e analytical constants of t h e oil were as follows: Apparent Acid Value Saponification Value 33 0 177.0
Iodine Value 184.0
Ash 4 5
An analysis of t h e acids of the oil b y the Twitchell volumetric method indicated the presence of 14 per cent of rosin acids. T h e ash consisted mainly of zinc oxide with some lead and lime. These oxides were present as a mixture of linoleate and resinate, a separation of which was not feasible. Assuming the same extent of hydrolysis in t h e determination of the acid number as had been proven in t h e cases of t h e soaps previously examined, t h e free acid content was shown t o be negligible. I n these puttied paints t h e pigment had settled t o some extent and i t was found convenient t o work with t h e upper layer, which was puttied vehicle containing a comparatively small amount of pigments. This was quite variable in composition, the percentage of free pigment increasing toward t h e bottom of t h e can. The amount of free pigment and of zinc soap being t h e important determinations, these were ascertained within sufficiently close limits b y shaking t h e sample with cold ether, weighing t h e residue, liberating t h e acids in this residue and calculating t h e zinc linoleate from these acids. The zinc soap of t h e oil is almost entirely insoluble in cold ether while t h e resinate of zinc is quite soluble under t h e same conditions, and since t h e samples taken were in themselves quite variable t h e results were as reliable as could be expected. Near t h e top of t h e puttied vehicle the pigment constituted I O t o 15 per cent and t h e zinc soap 20 t o 2 j per cent of t h e total. Deeper in t h e layer t h e pigment content increased t o 2 j per cent while t h e zinc soap content decreased t o 1 5 t o 2 0 per cent. The relative amounts of oil, thinner and resinate of zinc were not determined. The pigments separated from these samples analyzed about ;j per cent zinc oxide. If a small amount of zinc oxide be allowed t o remain in contact with raw linseed oil containing rosin dissolved in petroleum thinner, in a x r y short time t h e oxide dissolves t o a clear solution. T h e only apparent change in t h e solution is a n increase in t h e viscosity. The same condition holds when free oil acids are substituted for t h e raw oil. There is no tendency toward the formation of t h e skin; in fact a sample of t h e skin previously formed dissolved completely in a naphtha solution of rosin upon standing over night. T h e assumption is t h a t t h e rosin acid displaces the oil acids from their zinc soaps with the formation of zinc resinate, which is somewhat soluble in naphtha and considerably soluble in turpentine. ether, and linseed oil. T h e paints containing gloss oil seem t o be t h e most susceptible t o livering, which may be explained as follows: illthough t h e rosin is usually limed there still remains considerable free rosin acid whose combination with basic pigments takes place in a short time, T h e resinates so formed are soluble in t h e vehicle b u t increase its viscosity t o a point t h a t delays t h e settling of t h e pigment. T h e hydrolysis of t h e oil
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proceeds as in t h e case of t h e skin formation except t h a t , as there is generally more water a n d more alkali present, t h e action is faster. T h e freed acid comes into immediate contact with t h e unsettled pigment forming t h e metallic soaps, particularly t h a t of zinc. T h e vehicle portion becomes constantly thicker until we have a condition approaching t h a t of a colloidal gel formed from t h e finely divided zinc soap precipitating from t h e viscous solution of zinc resinate. This gel has occluded or adsorbed t h e remaining oil a n d what little thinner may have been present. Since, during t h e progress of t h e reactions, t h e acids a n d t h e pigments are in intimate contact, t h e opportunity for neutralization is much better t h a n is t h e case with t h e skinned paints, which accounts for t h e low free acid content of t h e extracted oil. If t h e rosin of t h e gloss oil be highly limed or for a n y other reason t h e pigments should settle before t h e oil acids form t o a n y extent, t h e action is a p t t o be one of “skinning” rather t h a n “puttying.” An experimental mixture of oil, acids, rosin, pigments, a n d petroleum thinner, corresponding t o t h e analysis of t h e puttied vehicle, resulted in a livered paint which, although not so hard as t h e samples investigated, was i n every way comparable t o them. Another sample, similar in composition t o t h e mixt u r e showing skinning, when continually shaken t o prevent settling, livered i n a very short time. CONCLUSIONS
Although t h e use of emulsifying agents in paint grinding is quite generally recognized a s necessary, their action i n preventing t h e h a r d settling of t h e pigment has not as yet been satisfactorily explained. Certain of t h e emulsifying agents hydrolyze t h e oil of t h e paint, t h e f a t t y acids subsequently forming insoluble zinc a n d lead soaps, which, if free t o precipit a t e , form a skin on t h e surface of t h e settled pigment. If, because of t h e presence of dissolved resinates formed from added gloss oil, or for a n y like reason, t h e viscosity of t h e vehicle be increased t o t h e point where t h e pigments a n d t h e metallic soaps formed from t h e hydrolyzed oil are not free t o settle, there is formed a gel or “livered” vehicle. These conditions may be remedied b y t h e use of a protective colloid containing no water, such as alumin u m palmitate or oleate, or b y t h e use of a n emulsifier containing water, b u t i n which t h e active agent exerts no saponifying action on t h e linseed oil. As t h e probable reason why emulsified oil paints do not settle hard is t h a t there is present i n t h e partly settled pigment a lead or zinc linoleate, i t may be advantageous t o grind paints i n a n oil of fairly high acid content, or t o a d d t h e optimum amount of lead or zinc soap t o t h e pigments ground i n a neutral oil, as a means of inhibiting t h e dense settling of t h e pigment. Experimental formulas compounded in a n effort t o establish t h e relative values of t h e various emulsifying agents, while t h e y show promise of interesting a n d valuable results, have not as yet been under observation for a sufficiently long time t o warrant definite conclusions being drawn from them. UNIVERSITY O F MICHIGAN,
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THE USE OF BARK FOR PAPER SPECIALTIES’ By OTTO K R E S S ~
I n the manufacture of paper from wood pulp, if t h e bark is not carefully removed preliminary t o t h e pulping of wood b y either chemical or mechanical processes, i t appears in t h e pulp a n d produces small specks in t h e finished paper, detracting from t h e appearance a n d value of t h e sheet. Some kraft mills do not clean their wood preparatory t o pulping, depending on t h e alkaline digestion t o destroy t h e bark. This practice is followed only t o a very limited extent as t h e high consumption of chemicals in t h e pulping of bark a n d unevenness in shade a n d uniformity of t h e resulting pulp are decided drawbacks. T h e loss in barking will depend on t h e nature a n d condition of t h e wood, a n d on t h e method of barking of t h e wood, a n d will vary from I O t o 25 per cent, based on t h e weight of t h e rough wood. According t o t h e census of 1911 of t h e Department of Commerce, there were consumed i n t h e United States 4,328,052 cords of pulpwood, of which 280,534 cords were classified as slabwood a n d other mill waste. Practically all of this wood was cleaned a n d barked before pulping. T h e bark, as removed a t t h e mill, is saturated x i t h water a n d even with heavy pressing can be made only about 5 0 per cent dry, so t h a t i t is of comparatively little value as fuel. One mill t h a t brought this matter t o our attention loaded t h e daily waste from t h e d r u m barking of IOO cords of spruce wood into gondola cars, a n d disposed of i t b y filling in low places around t h e mill. T h e y experienced difficulties from t h e odors of t h e decomposing bark, from fires which are a p t t o occur a n d which are hard t o control, while t h e cost of removal was estimated at from $15 t o $ 2 0 a day. Large quantities of waste bark in t h e tanning industry are likewise awaiting successful utilization. Waste t a n bark from t h e leaches is about 35 per cent dry, a n d has a n estimated fuel value of $ 0 . 6 0 i n comparison with $ 3 . 0 0 for bituminous coaL3 The latest census figures from t h e Department of Commerce a n d Labor on t a n b a r k production for rgog showed a production in t h e United States of 698,365 tons of hemlock b a r k a n d 324,070 tons of oak bark, valued a t $9,968,710. T h e production since then, however, is reported t o have diminished steadily, because of t h e increased use of chemical tanning agents, a n d accurate d a t a as t o t h e present amount is not available. Such minor uses for t h e waste bark as t h a t in t h e white lead industry, runways for stables, etc., t a k e b u t a very small amount of t h e bark, leaving t h e balance for fuel after t h e tannin extraction. Other sources of waste bark i n t h e United States are t h e lumbering of redwood, cedar, etc., where t h e bark is a decided detriment both i n t h e lumbering operation a n d a t t h e sawmill. T h e Forest Products Laboratory i n attempting t o utilize these wastes for pulp a n d paper purposes first a t t e m p t e d t o reduce t h e bark b y chemical pulping. A decided drawback is 1 Paper presented a t meeting of the Technical Section of t h e Paper and Pulp Association, New York City, September 29, 1916. 9 I n Charge, Section of Pulp and Paper, Forest Products Laboratory, Madison, Wisconsin. a J A m . Leather Chem. Assoc., 11, 361.
