ORGANIC FINISHES Effect of Film Thickness
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on Physical Properties and
Exposure Behavior A. E. SCHUH AND H. C. THEUERER Bell Telephone Laboratories,New York, N. Y.
The behavior of organic finishes is partly determined by the thickness of coatings applied. The same finishing material at two different coating thicknesses may exhibit widely different durability, and the type of failure may be completely changed. Physical measurements of the coatings during their early exposure life serve to reveal the mechanism by which subsequent visual failure occurs. Several important relations between the physical characteristics and exposure behavior of finishes are given.
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hundred panels and twelve hundred tensile strips were used in the investigation. Annealed brass was selected because of its relatively noncorroding character and high distensibility. The finishing materials included three ester-gum varnish enamels of different oil lengths, a baked black japan, several nitrocellulose lacquer enamels, an outdoor paint containing a synthetic resin, and several air-dried and baked synthetic-resin varnish enamels, All the materials were pigmented black to give a rubber-finish appearance. The outdoor exposures were conducted on the roof of the laboratories in New York and were begun in September, 1934. Panels were mounted at an angle of 45" facing south and during all but freezing weather were g i v e n three 15-minute periods of w a t e r s p r a y daily. Visual observations were made monthly. At four intervalsnamely, after 1.5, 3, 8, and 13 months of roof exposure-complete sets of panels were b r o u g h t to a constant-temperature and -humidity room and subjected to physical analysis. The tests applied were abrasion resistance, impact resistance, hardness, and distensibility. The details of these tests have already been described (3, 4, 6). An additional set of panels was aged indoors under constant conditions of temperature
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formation on the relative effects of indoor and outdoor exposure on durability. The anel stock was annealed brass sheet in& ( 0 . 8 ~ thick ~ ~ )cut to the dimensions of 3 X 6 inches (7.6 X 15.2 cm.1. The method of cleaning the panels consisted of immersion in 10 per cent hydrochloric acid, a bright acid dip, a rinse, drying, and, just prior to application of the finish, two rinses in clean acetone. The finishing materials were applied by m a n * of an automatic spraying machine developed here ( 1 ) . In the case of the air-drvinz oleoresinous materials, 2 days were az lowed for d r y i n g between coats. The s y n t h e t i c baking finishes were heated 1 hour at 300" F. (149" C.) for each successive application of 1 mi1.l The black japan was baked in a similar manner at 400" F. (204" C.). The oven used for baking was a lateral forceddraught type in which the temperature was controlled to 1 2 " F. In the case of the lacquers, the necessary applications to obtain a coating of 6 mils were performed within 24 hours. All of the materials were allowed to age one month indoors, at which time an initial set of physical determinations was made, and the outdoor exposure tests were begun. The thickness of the organic coatings was determined while they were attached to the base metal by rigidly fastening the coated specimen beneath the measuring stylus of a dial gage and noting the readings when the stylus was gently lowered, first, on the faint surface, and then on the metal surface directly beneath it, rom which the paint had been removed with a suitable solvent. The dial gage was calibrated by standard methods and used as a comparator with an accuracy of 10.02 mil.
ANY of the factors that determine the serviceability
of an organic finish have been thoroughly studied, but little has been said about the effect of the thickness of coating upon the life performance of a finish. No quantitative information, to our knowledge, has been published on this subject. Unquestionably the same finishing material in its various uses is applied over a thickness variation of several hundred per cent in practice. The present study was undertaken to learn just how significant such a variation in thickness is. I n addition, a n excellent opportunity was provided for comparing two methods of evaluating the durability of organic finishes-namely, the method of exposure testing and the periodic measurement of physical changes, Accordingly, the test results will be discussed to bring out several significant and important relations between these two methods of testing durability.
Significance of Physical Tests A preliminary discussion of the significance of the physical tests will aid in anticipating the relations found between the physical test data and attendant exposure behavior. The abrasion test measures the work of overcoming the effective cohesion of an organic finish coating. I n effect it measures the area beneath the stress-strain curve such as is obtained in the measurement of tensile strength and elongation of a free film. Thus, a film of high tensile strength and low elonga-
Test Program Thirteen different finishing materials were applied on a common base metal a t several thicknesses of coating, were then exposed outdoors and indoors for a period of almost two years, and were periodically observed as to changes both in appearance and in physical properties. The work was confined to a single base metal because of the size of the investigation. Even with this restriction, more than four
1 The term "mil" (0.001 inoh or 0.0254 mm.) serves as a oonvenient unit for expressing thickness of organic finishes and is used throughout this papet.
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tion may give the same area in a stress-strain diagram as a film of low tensile strength and high elongation. The same materials when tested as finish specimens would give equal abrasion values. Therefore, if elongation is determined, as in a distensibiiity test, then the combination of the abrasion value and the percentage elongation of the finish offers a means of estimating its tensile strength. Thus, in the case of two finishes with equal abrasion values, one of which has a high and the other a low elongation, the second material requires higher tensile strength to give an equal abrasion value. These conclusions were confirmed in a series of experiments in which measurements were made on both free and anchored a m s of the same finishing materials. To be durable, a finish most also possess and retain good adherence of the organic coating to the base material. The impact test gives the most direct evidence of the adherence of the coating. Any finish with poor adherence to the base material will yield low impact values, whether it be hard or soft, strong or weak, plastic or brittle. It will be shown a t a later point that certain finishes, which lose adherence as a consequence of chemical reactions at the coating-substrate interface, suddenly drop off in impact value, even though the other physical characteristics such as abrasion resistance and distensibility may show but little change.
Air-Drying Ester-Gum Varnish Enamels
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shown in the hardness measurements, where hardness falls off rapidly as the thickness of the finish is increased. The relation between abrasion resistance and thickness reveals an interesting phenomenon. The 0.5-mil coating of the &-gallon enamel drops off in abrasion resistance from an initial wear value of 45 grams per mil to 16 grams at the end of 13 mouths of outdoor exposure, revealing a definite loss in film strength with age. The 1-mil coating of the same enamel, however, shows this reduction of wear value to a considerably lesser extent, and the still thicker coatings do not show it at all over the period of exposure under test. Associated with this reduction in wear valne there is exhibited a simultaneous loss in distensibility, which helps to account for the loss in film strength as aging progresses. The fact that this weakening of the film shows up clearly in thin coatings and is less apparent as the thickness of coating increases, indicates that the film structure changes rapidly with age at the surface and progressively less at each element below the surface. In this manner a definite case-hardening of the finish is developed after application. The fact that in thick coatings the abrasion and distensibility values do not reveal this structure change is explained by the masking effect due to the presence of a large fraction of relatively unconverted material. This case-hardening action accounts for the exposure behavior of the long-oil enamels in which the thick coatings began to show surface checking a t an earlier date than the thin coatings (Figure 1). Whereas this structure gradient is also observable in the %-gallon enamel, it does not show up a t any point in the 10-gallon material which is too weak and brittle throughout a t all thicknesses.
I n this group three ester-gum varnish enamels of lo-, 25-, and 4O-gallou oil lengths were investigated. The results of the physical tests on these enamels are given in Table I. The abrasion values are expressed in terms of grams of Carborundum per mil of coating, and the impact values in terms of the speed of the B . 25-gnllon enamel impact hammer in revolutions per minute at which the coating broke down. The hardness of the coating was determined by the Pfund instrument (3), and the values are expressed in grams required to give a fixed diameter of impression. Distensibility was measured by the extension of tensile specimens (6),and the values in the tables give the percentage elongation a t which the coating ruptured; immediately below t h e s e v a l u e s a r e given qualitative adherence ratings as determined by scratching with the fingernail. The most striking fact of Table I is the sharp differencein physical characteristics &4 the oil length of the enamel is increased. Thus, comparison of the three freshly prepared enamels a t a common thickness, such as the 1-mil level, shows that the 10-gallon enamel has a wear resistance of 8 grams, an impact resistance of 270 r. p. m., a hardness of 380 grams, and a distensibility of 8.2 per cent. Increasing the oil length to 25 gallons has modified these values to 38 grams, >1500 r. p. m., 130 grams, and 59 per cent, respectively. A further inorease in oil length does not show a proportionate change in physics1 values, particularly in the caseof the fresh coatings. Of the physical tests, abrasion value reflects the oil length of the varnish most sensitively. The effect of changes in the thickness of the coatings is definitely reflected in all of the physical tests. It is most clearly
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The effect of thickness on distensibility is of interest. We find in the case of the 10-gallon enamel that the effect of increasing thickness is to reduce the initial distensibility. This drop in distensibility is probably due to contraction forces set up in the film during aging. These forces, coupled with poor film strength, account for the early exposure failure of the thicker coatings of this material by deep cracking and flaking (Figure 2). However, the initial distensibility of the longer oil varnishes is higher for the greater thicknesses. The thinnest coating of the 25-gallon enamel drops on aging from an initial value of more than 65 per cent elongation to 25.5 per cent on outdoor exposure for 13 months, whereas coatings thicker than 3 mils show persistent distensibilities beyond the point to which the base metal can be stretched without rupture. A similar condition prevails for the 40gallon varnish. These materials also show a thickness effect on impact resistance. Aging brings about a continuous falling off in impact resistance of the 25- and 40-gallon enamels a t the 0.5- and 1-mil thicknesses. However, a t thicknesses greater than this, we find a sudden loss in impact resistance after only 1.5 months of outdoor exposure. The entire coatings, after a mild scuffing, could be readily peeled from the panel, since the normal adhesion was virtually destroyed. Chemical analysis of the underside of the detached films revealed the presence of metallic reaction products. This loss of adherence was not shown by the same finishes when exposed indoors even after a period of 15 months, indicating that the presence of water was necessary to bring about an interfacial reaction with the base metal. Obviously, the possibility of chemical reaction, with consequent loss of adherence of the finish, is dependent as much on the chemical nature of the base material as on the composition of the finishing material. Variation of thickness also influences durability, as evidenced by visual changes on outdoor exposure. The physical evaluation of all the finishes was terminated a t the end of 13 months of outdoor exposure. However a t that period of exposure so little visual change had occurred in all but the two poorest finishes, that a quality differentiation was not possible. I n fact, another year elapsed before visual differences became discernible. On the other hand, the method of physical evaluation permitted large differences to be ascertained in the first month after application, and these differences increased rapidly with time. The importance, therefore, of relating the information obtained quickly by physical testing with subsequent visual behavior of the finishes is evident. The ester-gum varnish enamels exhibit definite relations between thickness of coating and exposure behavior. The 10-gallon varnish failed on exposure by checking and cracking, which we might expect in view of the low film strength as shown by the low wear and distensibility values. However, the thicker coatings reached visual failure much sooner than the thin coating, as shown in Figure 2. After only 3 months of roof exposure the thick coatings of this enamel had cracked and flaked so badly that further physical measurements became impossible. This same condition was not reached by the thin coatings even after 23 months of exposure, although after 8 months perceptible checking had begun. The 25- and 40-gallon enamels did not show as pronounced a thickness effect even a t the end of 2 years of exposure. The depth of checking increased somewhat with thickness and diminished slightly with increase in oil length. However, the previously mentioned loss in adherence with thick coatings took place. It is predicted that the ultimate exposure failure for these two materials a t heavy coating thickness will be in the form of peeling, and that the thin coatings will probably outlast them and ultimately fail by cracking. The series of panels exposed indoors showed no visual
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l-rnil roetinp Aged 23 Months Outdoors; MsgniCed Sectiona, X 45
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FIODRE 2. EFFECTOF T n r c w ~ s sON EXPOSUHID BEHAVIOR OF ESTER-GUM VuNIsn ENAMEL
changc on any of the finishes even after a period of 2 years. This behavior is to be expected from the relatively small change in the values of the physical test,s after 15 months of indoor exposure as compared piith t.he very large changes shown by duplicate panels exposed outdoors.
Air-Drying Synthetic-Resin Varnish Enamels The values of the physical properties of three air-dried synthetic-resin varnililies are shown in Table 11. Materials with a wide qiiality range, as revealed by tlie physical data, were purposely selected. The thickness effects on physical characterisbics were of the same character BS in the case of ester-gum varnishes. Tho wear values in grams per mil again increased with coating thickness. A similar effect is shown by the distensibilities of the finishes. The development of a weak external layer is again shown in the series by the abrasion and disterisihility valnes. Material F is of interest. I t was not possihle to obtain a satisfactory finish with it at a bhickness much over 1 mil because of solvent-lifting difficulties; consequently only two thicknesses -namely, 0.5 and 1 mil, were studied. Even though the spread in thickness was so narrow, Table I1 shows that this niaterial had a marked difference in physical behavior a t these two thicknesses, particularly with respect
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to abrasion value and distensibility. Although after 23 months of roof exposure very little visual differenceis shown between the two thicknesses, it is expected that the 1-mil coating will definitely outlast the coating one-half as thick because of its inherently better physical characteristics. A brief discussion of the relation between the physical data of Table I1 and the CORsponding exposure behavior is in order. Material D, which resembles the 10-gallon ester-gum varnish enamel in physical aspects, fails on exposure by checking and cracking after only 8 months, just as the ester-gum enamel does (Figure 3). Conversely, material E, which combines high coating strength with high distensibility and adherence. shows nracticallv no visual deeradation except sl&ht chafking even aft& almost two years of roof exposure. Material F, although showing no intrinsic fihn failure after two years, has poor adherence and will probably fail on exposure as a result of this weakness. For the sake of completeness, a modern commercial exterior paint., G, fortified by the addition of a small amount of synthetic rcsin (probably of the alkyd type), was ineluded in this series. The physical data a t thicknesses of 0.5 and one mil are given in Table 11. A thickness effect is seen on both the abrasion value and tlie distensibility, from which it would be predicted that the thicker coating, owing to its better inherent physical characteristics, will outlast the thinner application. The material gives no sign of film failnre aft.er 2 years of exterior exposure, although its adherence to brass outdoors is very poor; even a mild scratch or scuffing removes the finish. On indoor exposure, however, adherence is retained, again indicating t.hat water is necessary to pennit the chemical reactions wliich destroy adherence.
Nitrocellulose Lacquer Enamels Three oornniercial lacquer enamels were selected for comparison. The periodic physical data at several thickness levels are shown in Table 111. The relation between film thickness and physical charact.eristics is again worthy of note. In contrast to the oleoresinous fioishes, the lacquers show only a small increase in wear coefficientwith thickness of coating. Owing to the absence of an oxidative mechanism in the gelation of lacquer coatings, the 6lms might be expected not to show the conspicuous structure gradient of the oleoresinous finishes. Lacquer J , however, shows a slight gradient of abrasion value with thickness, indicating that one of its components is of an oxidative nat.ure. This is confirmed by the fact that, as the film ages, it becomes increasingly insoluble in lacquer solvents. As was expected from the physical analysis, both lacquers €I and J stand up well on exposure as far as visual film defects are concerned. Lacquer J , however, a t the 0.5- and 1-mil thicknesses showed weak adhesion when scratched by the fingernail, after only 1.5 months of roof aging. This is quantitatively confirmed by tlie values of the impact resistance which a t the thin coating level quickly fell from a high t o a low value. Lacquer I is worthy of separate discussion. At first glance, an analysis of the physical test data would indicate
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good durability. However, the distensibility of this lacquer diminishes markedly 88 the thickness of the finish is increased. This b e havior indicates the development of progressively higher contractile stresses in the film during gelation and aging. These stresses increase with thicknesses, and their magnitude is demonstrated by the fact that a 20-mil coating of this lacquer caused a copper panel '/a inch (0.8 mm.) thick to bow appreeiably. When applied as a thin coating to a copper foil, tight spirals were developed. In the free film this material showed high tensile strength and low elongation. The related exposure behavior of this n Ooatinp I-mi1 ooatina material is remarkable. After 11 months of roof exposure, the thin coating (0.5 mil) had developed a pockmarked appearance, in which a large number of sniall, round, and completely bare spots of the base metal were revealed (Figure 4). Otherwise, bhe rest of the coating was in excellent condition and showed good adherence. The coatings 1 mil thick were still in excellent condition after 2 years of exposure. When the panels were coated to a thickness of 2.5 mils, however, large areas of the finish spontaneously split off, even though the parts of the coating remaining on the panel showed little film deterioration and were extremely adherent, withstanding the highest impact shock de?-mil coatinq 6-mil ooatina livered by the testing machine. This is an example of a lacquer finish whnse betiavior FlGUnE a. EFFECT OF TIIICXNESS ON EXPOSURE REtiAYIOR OF AN AIR-DRYING SYNTHRTIC ENAMEL (MATERSAL D),ARED OaTDooRS FOR 23 M O N T R ~ resembles a thick coatingof gelatinover glass, which. on beine annlied sufficimtlv thick. ., ' ~" reproducible and properly baked coatings. The related develo'psenough internal strain to rip itself bff spontanenusly hardness data showed that the coatings were not overbaked. from a tightly gripped base surface. Thus the exposure beThe results of the physical testa are shown in Tsble IV. havior of this material represents a tug of war between contractile and adherence forces. The results in general are comparable to those obtained b might be exwith the air-dried oleoresinous enamels. i Baked Finishes perted, the coatings are considerably harder than the airdried oleoresinous materials. Again, a cohesional structure In this group are included two commercial baking synthetic-resin varnishes and a baked black japan finish of known gradient of the coatings is shown by the abrasion test, the composition. The method of stepwise application and thinner film giving a somewhat lower abrasion vdue than separate baking w&8 adopted for the production of thick coatthe thick coatings. On the relation between thickness and ings and was found to offer the best compromise to nlrtilin distensibility, two of the synthetic enamels, K and M , show
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n 5-mil coating: magnified section, x z 6 I-rnil aoating a.mo oanting The panela were *Bed 23 months outdoors; the &mil aoating failed aompletelg by sosling. similar t o that of the 3 4 1 mating sfter 13 month- of erposnre.
FIQURE 4. EFFECT OF TEICKNES~ ox EXPOSURE B ~ t i ~ v r oOF ti
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a pronounced decrease of distensibility with increase in thickness, thus indicating the presence of strains due to contractile forces, whereas L shows the opposite behavior. Evidence of internal strain is nicely revealed in the behavior of the test specimens of japan finish M during the performance of the distensibility test. The 1-mil coating, on being stretched to 24.2 per cent elongation, develops many minute cracks to the base metal but otherwise adheres well. The 6-mil coating, however, when stretched to only 4.2 per cent elongation, suddenly develops one complete crack across the specimen and immediately peels away from the entire base metal. The baked finishes exhibit good durability on exposure, as is to be expected from the physical analysis. The thin coatings of finish L , however, failed rather early on exposure. This material, when applied in a 0.5-mil coat, produced a finish which showed irregularities in color owing to a partial pigment-vehicle segregation. Evidently this segregation was attended by poor water resistance, inasmuch as coatings, u p to 3 mils in thickness, all failed by peeling as a result of water corrosion of the base metal (Figure 5). The time required for this form of failure to develop increased with coating thickness, until a t the 6-mil level it had not appeared even after 23 months of exposure. That water was necessary t o bring about this adherence failure is shown by the fact t h a t the finish exposed indoors showed no visual change. This adherence behavior is clearly revealed by the sudden drop in impact resistance after only a short exposure. It is somewhat surprising to find that the ready entrance of water was not retarded more effectively, inasmuch as it is still evident a t the 3-mil coating which required three separate sprayings and bakes to produce.
Summary It has been shown that the performance and durability of almost any type of finish is definitely influenced by thickness variation. The manner in which this influence is exerted depends largely on the kind of finish in question. Thus, in a short-oil varnish, failure is expedited if too much finish is applied. I n long-oil varnishes, the opportunity for chemical reaction of the coating material with a reactive metallic substrate is enhanced as the coating thickness is increased. I n other cases, when coatings develop a progressively dense structure on aging, a n increase in thickness may lead to the point where the progressiveIy increasing shrinkage forces may bring about a spontaneous scaling of the finish. I n certain other cases in which thin coatings offer ready access of water to a reactive base metal, the addition of more finishing material may prove beneficial in either delaying or completely preventing the adherence-destroying reaction. This investigation has also provided several interesting and important relations between physical evaluation and exposure appraisal of finish quality : 1. The measurement of abrasion resistance along with distensibility of the coating gives a measure of relative film strength which is, in turn, related to exposure durability. Thus two of the materials which gave exceedingly weak coatings began to show checking and cracking after a few months of exposure. The short life of these finishes is predictable from the very first set of physical measurements. With finishes of higher film strength, the rate of loss of such strength becomes important, the finishes showing the best retention being the most durable. If a severe gradient, in film strength is developed during aging, as appears t o be the case for most oleoresinous finishes, then a mild form of surface checking may develop fairly early in thick coatings. In the absence of such a structure gradient, the film strength must reach quite low values of abrasion resistance and distensibility before visual checking begins to appear. 2. The measurement of impact resistance reveals not only initial adherence of the finishes but also changes in adherence with time as a result of the reactions which occur at the finish-substrnte interface. Depending on the mechanism of such reactions,
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I-mil coating
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6-mil C O s 6 n K
The PBD& were exposed outdoors for 23 months: the 0~6-rnilaoating failed by peeling after 13 months of ~ ~ p o ’ l ~ u m .
FIQURE 5. EFFECTOF THICKNESS ON ESPOSUREBEHAVIOR OF
the manner in which adherence is destro ed may vary and thus sffect the character of the final visual k r e , such as flaking, waling, or peeling. Inasmuch as contact with water appears to be necessary to bring about such interfacial reactions, BS evidenced by the fact that none of the finishes exposed indoors showed adherence losses even after 2 yeare, the suggestion IS offered that instability of adhereuce might be revealed quickly by immersing a young-hiah specimen in water to accelerate interfacial reactions snd.then subjecting it to an impact test. 3. A finish, however, may also lose effective adherence to its base even though its specific adhesion may not be affected. This loss may be brought about by the development of shrinkBee forces sufficientto urd1 off a CostInE from its snchoraee swnt&eously. Evidence d such shrink-; forces has been Gbtahed by coatin thin metal foils and noting the degree of coiling of the foils devekped as aging progressed. Additions1evidence of such forces is revealed bv the reduction in distensibilitv of a finish with increase in thickness of coating. 4. The chief virtues of the physical approach t o test,ing the quality of a finish appear to be: (a) It m2tt.erially expedit,es tho
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point at which evaluation can be made, and (h) a deeper insight into the mechanism of finish degradation is provided. The results of physical test, when con led wit.h the knowledge of the eom osition and formulation o f t h e finishing material, should aeceferate further advances in paint technology.
Literature Cited (1) h i t . H. G.,Bell Lab. Record, 14 ( i )216-17 , (1936). (2) Pfund, A. H., Proc. .4m. SOC.Testin? Xalerials, 25, 3 9 2 . 4 0 2 (1929). (3) Sohuh, A. E., IND. ENU.CHESI.,23, 1846-52 (1931). (4) .~ Schuh, A. E., and Kern, E. W., Iao. Eno. CHEX.,A n d Ed.. 3, 72-6 (1931). (5) Sohuh, A. E., m d Theuerer, H. C . , ZbiL, 6,91-7 (1934). (6) Ibiil.. 9, 9 (1937). R ~ c s r v Segteniber ~o 14. 1936. Presented beioie the Division oi Paint and Vainish Chemistry at the 9zod Meeting 01 tbe Ameriaaii Chernioal Sooiety. Pittsburgh, Pa., September 7 t o 11, 1938.
Dehydroascorbic Acid Reductase E. F. ROHMAN m n N. H.SANBORN National Canners Association, Washington, D. C. EZSSOSOFF (g) in 1921, while attempting to determine whether the antiscorbutic principle of raw potatoes resides in the juice or in the insoluble portion, found so little in either that he at once recognized that it was rapidly being inactivated in his process of extracting the juice. The writers’ early studies (7) of the vitamins in canned foods brought out the point that vitamin C is lost during storage of raw produce and that such loss is arrested by canning (4) and greatly accelerated by the shredding of carrots (5). These Gndings suggested enzymic activity. Yzent-Gydrgyi (8) stat.es that in minced cabbage leavcs the hexuronic acid present is reversibly oxidized catalytically in a short time by an enzyne which he tenns “liexoxidase,” hut lie was unable to demonstrate the reduction of thc reversibly oxidized form by enzymic activity. Kertesz, Dearborn, and Mack (3) recently ascrihed the loss of ascorbir acid in raw frozen peas to the activity oi an crlaymc dt-signated by them “ascorbic acid oxidase.” These are all oxidative effects; hut, since plant life is endowed with an ~,xidation-redoctionsystem, there must 1)c an
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opposite reducing effect. In 1928 the writers pointed out (1) that raw frozen vegetables were not a commercial feasibility because of the off-flavors that developed. Subsequently (6) these off-flavors, as well as similar ones developed in other conditions involving a ruptured plant cell, were ascribed t.o anaerobic enzymic activity which involves reduction. During the p a t seaSon in working with peas, observations were made that suggested a technic to illustrate in raw pea juice tlic restoration of reducing value to 2,6-dichlorophennlindophenol after its destruction is caused hy aeration alone or is catalyzed by copper or by oxidation with the dye itself. If this dye is a ineasure of ascorbic acid, a basis on which many workers are puhlishing data, then there exists in raw pea juice a “dehydroascorbic acid reductase.” In the writers’ experiments it resultcd in tire production of the equivalent of 15 mg. ascorbic acid per 100 ec. in 5 to 6 hours. Hcated pea juice or raw cabbage juice did not yield the same results. In fact, a loss of dye-reducing effect was found instead. This is not to he construed as a coinnient on the degree of
