I
Controlled Thermal Depolymerization of Dextran IVAN A. WOLFF, PAUL R. WATSON,, JOHN W. SLOAN, AND CARL E. RIST Northern Regional Research Laboratory, Peoria, I l l .
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.
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N THE recently intensified search for blood volume expanders (S), partially depolymerized dextran has been considered as one of the leading possibilities. The dextrans, known for many years because of their occurrence as slimes in the wine and sugar industries, are polysaccharides of high molecular weight composed entirely of anhydroglucose residues united primarily through the 1- and 6-carbon atoms by alpha glucosidic linkages (6). The difference between the linkages of dextrans and those of starch and glycogen (predominantly a-1’4linkages) is presumably responsible for the slower action of body enzymes on dextran and thus for its usefulness in “blood substitutes.” To convert native dextran t o a form suitable for injection it is necessary t h a t its molecular size be reduced to a n intermediate range, above t h a t at which excessively rapid excretion occurs (19) but below t h a t at which undesirable interactions occur with other constituents of the blood ($1). This depolymerization has usually been carried out by controlled hydrolysis in acid solution, although mention has been made of the use of ultrasonic vibrations (14),alkali (IC), and enzyme preparations ($8) to accomplish this purpose. The thermal degradation of starch is a well-known and commercially applied process for obtaining water-soluble pyrodextrins. known in the trade as British gums, canary dextrins, or white dextrins ( 1 2 ) . I n 1949 158,600,000 pounds of such corn dextrins were produced in the United States (6). It appeared reasonable t o apply the known methods for heat degradation of starch t o dextran. The authors’ experiments on the thermal depolymerization of the dextran by Leuconostoc mesenteroides N R R L B-512 were carried out not only for their practical aspects in application to the production of blood volume extenders, but also because of structural deductions t h a t may help generally t o elucidate the mechanism of the thermal depolymerization of polysaccharides.
of the viscosities of the products in aqueous solution in OstwaldCannon-Fenske capillary tube viscometers at 25’ C. The inherent viscosity
. .
*
*
‘
*-
,
designated
{?I)
of these degraded
dextrans was found to be practically independent of concentration a t the concentrations used. For example, inherent viscosities of a particular product a t concentrations of 0.51, 0.37, 0.26, 0.20, and 0.16 gram per 100 ml. of solution were found to be 0.19 in all instances. It is therefore apparent that the inherent viscosity a t one concentration of these particular dextrans can be used to approximate the intrinsic viscosity. For speed and convenience, inherent viscosities were measured a t concentrations of 0.2 to 0.3%.
TABLE I. CHANGESIN VISCOSITY~ OF DEXTRAN DEGRADED AT VARIOUSTEMPERATURES FOR DIFFERENTTIMEINTERVALS Time, Hours Tgmp., 1 2 3 4 5 6 7 7 C. 150 1.20 160 1.27 170 1.30 180 ’ 1 . 1 9 1 . 2 7 1 . 2 9 1 . 2 0 1.07 0.99 0.92 0.89 190 1 . 2 5 1 08 0.99 0.80 0 . 5 8 0.53 0 . 4 5 0 . 3 9 200 1 . 1 3 0 . 7 8 0.45 0 . 3 2 0.28 0.22 0.21 0.19 210 0 . 5 4 0 . 2 5 0 . 1 5 0.15 0.14 0 . 0 5 0 . 0 5 0 . 0 6 a Inherent viscosities a t near 0.2% concentration except in 210’ C. series where concentration was lower owing to insolubility. Original undegraded dextran had aninherent viscosity of 1.19.
I n Table 1 is summarized a series of experiments relating the extent of as n-~easuredby viscosity, t o the times and temperatures employed. Little if any degradation of the dextran occurred at temperatures below 180’ C. for time periods u p t o 8 hours. At temperatures of 180’ and above, a progressive decrease in viscosity occurred with increase of time. However, it appeared (Figure 1) 8s though in each Case some limiting viscosity might be approached which would be cht3~acteri5ticof the temperature employed. Reproducible results were obtainable on the same batch of dextran when the original moisture content and heating conditions were kept constant. The data also show (Table 1and Figure 1)t h a t a rise in viscosity occurs in the initial stage of breakdown. This rise shifts t o shorter heating times at the higher temperatures. This initial viscosity increase could not be observed at 200°, in the experiments cited, because the dextrinization was proceeding SO rapidly t h a t at the time the first sample was removed this phase had already passed. T h a t it also occurs a t the higher temperatures was established by the following set of experiments, in which samples were withdrawn every 10 minutes during the first hour:
THERMAL DEGRADATION OF DEXTRAN
-
( In?
T h e dextran used for these studies was a high viscosity product prepared b y the action of Leuconostoc mesenteroides N R R L B-512 on sucrose according t o the procedure of Jeanes, Wilham, and R / I (10). ~ ~ ~ The ~ dextran contained 0.02% nitrogen, 0.012% phosphorus, and 0.07% ash, and had a light scattering weight average molecular weight of 33,000,000. The white, fluffy original material was passed through a 60-mesh Screen prior t o heating. For the small scale runs (0.5 gram) the dextran used contained approximately 6% moisture; for larger scale runs (20 to 25 grams) the sample was dried in vacuo overnight at 700 C. All heat treatments of the dextran RTeTe carried out without added catalyst in glass containers, either test tubes or, for the larger runs, T-shaped tubes, immersed in an electrically heated oil bath maintained within f1’ C. of the stated temperatures. A gentle stream of air was passed over the surface of the powder during the heating t o avoid condensation of moisture on the product. After the desired heating period had been completed, the contents of the vessel were emptied and allowed to come t o moisture equilibrium in the laboratory. The moisture contents of the samples approximated those of undegraded dextrans, which held about 15y0water at 65y0 relative humidity and 70” F. T h e course of the degradation was followed by measurement
Time a t ZOO’, Min. 0 10 20
30
{VI 1.16 1.17 1.18 1.20
Time a t ZOOo, Min. 40 50 60 120
{VI
1 23 1.23 1.19 0.91
The reason for this viscosity rise is not known. It could represent a molecular weight increase due t o condensations between molecules or t o the occurrence of some selective cleavages, leading t o formation of more nearly linear molecules.
:7ss
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INDUSTRIAL AND ENGINEERING CHEMISTRY
156
PROPERTIES OF THERMALLY DEGRADED DEXTRAN
COLOR. The dextran showed little tendency to darken in color during thermal treatment below 180" C.; even the product prepared in a n 8-hour heating period a t 200" had only a slight tan tint. At 210°, extensive decomposition began after 3 to 4 hours. Samples heated longer than this were shriveled in appearance and brown in color. Aqueous solutions of these materials had a deeper tan color than the solids, and this color was intensified a t alkaline pH. Samples having the same viscosity seemed t o have identical color, even though they were produced under different time-temperature conditions. The color can therefore serve as a rough guide of the extent of degradation.
190'C
200'C 210'C
I I
2
3
4
5
6
7
8
HEATING TIME IN HOURS
Figure 1. Variation in Dextran Viscosity with Time of Heating at Various Temperatures
Partial decolorization of the degraded dextran solutions could be effected by treatment with activated carbon. For example, a 3.5970 solution heated for 0.5 hour a t the boiling point with 5 % (based on the dextran) of activated carbon (Darco 5-51) underKent a decrease in optical density from 0.34 t o 0.17 (measured at 436 nip against water; 16-mm. path length). Approximately 1% loss of dextran occurred during this decolorization. The presence of traces of furfural-type compounds in a solution of thermally degraded dextran u-as indicated by the ultraviolet absorption spectrum. These compounds may contribute to the color of the degraded dextrans. SOLUBILITY.The degraded dextran samples described dissolved easily and rather rapidly in a-ater a t room temperature, to give somewhat colored but clear solutions. When the inherent viscosity was below about 0.5, occasional insoluble particles were noted; these represented, hoa ever, a negligible proportion of the total material. The overdegraded 210" samples became less water-soluble, the solubilities of 0.2-gram samples in lo0 ml. of water being 61, 38, 28, and 2170 for the 5 - , 6-, 7-, and 8-hour samples, respectively. -~CIDITY. Traces of unidentified acidic materials were formed in the course of the degradation. In contrast with a 2% aqueous solution of the original dextran a hich had a p H of 7 , the pH of degraded dextran solutions decreased progressively with increased severity of treatment (Table 11). The amount of acidic material produced was small, as indicated by the titration figures. The titrations were carried out in a carbon dioxide-free atmosphere, using boiled distilled water and 0.01 S barium hydroxide solution. The specific optical rotations of the therOPTICALROTATION. mally degraded dextrans (Table 11) measured in 2% aqueous solution a t 25' C. were lower than t h a t of the original dextran, and decreased with increasing extent of degradation.
TABLE11.
PROPERTIES OF
Heating Time at 200' C., Hours
pH in 2 % Aqueous Solution 7 0 6 5
zg 4 7
THERMALLY DEGR.4DED DEXTRAN
Acid Valuea 0 0.04 0.06
0.09 0.17 0.24 0.32 0.39
Specific Optical Rotation $1990 197 197 196
193
Reduciag Powerb
Apparent DPC
...
0
+ 0.68 +d
+d
... .
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I
330 180 125
1.27 4 4 187 1.78 4 2 1,98 183 110 4 0 2.35 177 94 3 8 0.62 159 3.38 66 a 111. of 0.1 N alhali required t o raise p H of aqueous solution containing 1 gram (dry basis) of dextran t o 8.0. b -4s %isomaltose hydrate. C Degree of polymerization, based on reducing p o w r d Both saiiiples gave measurable readings in s ectrophotometer. b u t these represented reducing powers below t h a t requirezfor accurate determination.
REDWINGPOWER.;ilkaline 3,5-dinitrosalicylic acid 'I\ as used for the colorimetric estimation (16) of the reducing power of the degraded dextrans. The method was used as given by Rleyer and coworkers ( I C ) , except that the color x-as measured at a n a v e length of 500 nip in a spectrophotometer and values were based on the use of 6- [ci-D-glucopyranosj I ] -u-glucose (isomaltose) hydrate ( I ? ) as a standard. It has become customary to base the reducing pol! er of a polysaccharide on its chief constituent disaccharide unit ( 2 , 4,16). As the authors' degraded dextrans had some yellon- color, the spectrophotometer readings were corrected by appropriate blanks in which the sample had been heated with the alkali and to M hich the dinitrosalicylic acid had been added after cooling. Calculation of the number average degree of polymerization of the materials (Table 11) was made from the reducing power b\- the relation DP =
d r y weight polysaccharide X 2 weight isomaltose hydrate X 0.9
These values may not have absolute significance, either because of inherent limitations of the method (15) or the fact that some molecules may not have a terminal reducing end group; they are, however, of interest as approximations.
S = Supernoionl R = Resldue 'Sss= Soluble in 9 0 X E l O H used lo recover Ss
Figure 2.
Scheme for Fractionation of Thermally Degraded Dextran
PER CEST DIALYZABLE.Two per cent solutions of the substances described in Table I1 were dialyzed in cellulose sausage casings against distilled water for 112 hours. The total amounts of carbohydrate material t h a t diffused through the membrane, determined by anthrone analysis ( 1 5 ) of the outer solutions, were: less than 1% for the unheated dextran and for the 1-, 2-, and 3hour samples, and 1.4, 2.1, 2.8, 3.9, and 6.8% for the 4- t o 8-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
hour samples, respectively. The outer solutions were evaporated in vacuo t o small volumes and then spotted heavily on p q e r chromatograms, which were developed with butanol-pyridinewater (3:2: 1.5) and sprayed with ammoniacal silver nitrate solution. Although no evidence of glucose or other sugars was noted on these chromatograms, the thermally degraded dextrans do contain very small amounts of sugars, as is shown below. FLUORESCENCE. The heat-degraded dextrans fluoresced in ultraviolet light either in aqueous solution or in the solid state. A similar phenomenon has been observed in the case of starch pyrodextrins (11 ). IMMUNOLOGICAL ACTIVITY. According to E. J. Hehre, Cornell University Medical School (8), one sample of thermally degraded dextran which had been heated for 5 hours a t 200’ showed “Type B serological activity of ‘degraded dextran’ type.” Type B refers to those dextrans which interact with Types 2 and 20 pneumococcal antisera and slightly or not a t all with type 12. Degraded dextrans generally “precipitate only with the more potent sera.”
757
any are present in the original material, was freed from salt by ion exchange and then analyzed by paper chromatography in the manner described above for the dialyzed materials. By this technique definite spots were obtained which had €21 values corresponding to those of glucose and isomaltose. T h e presence of other oligosaccharides was also indicated. If the spot designated as glucose actually is this sugar (no chemical characterization was made), the amount would correspond t o less than 0.1% of the starting material. In Table 111are listed other properties of the subfractions.
STRUCTURE OF THERMALLY DEGRADED DEXTRAN
I n order to characterize the heat-degraded dextrans more completely, several selected products were fractionated by solvent precipitation. SOLVENT FRACTIONATION. T o get some measure of the extent of polydispersity of the degraded dextran, a careful fractionation was performed by the graded addition of ethyl alcohol to an aqueous solution containing 95 grams (dry basis) of a dextran sample heated for 5 hours at 200’ C. The fractionation was carried out according t o the scheme shown in Figure 2. Before each main separation a small scale test run wap carried out to determine the proper alcohol concentration to be used at that particular step, and the actual separation was carried out at 2% carbohydrate coricentration (in the aqueous phase) and a t 25” C. T o assure equilibration the solutions were warmed and allowed to cool slowly t o t h a t temperature. The addition of 1% sodium chloride (based on the polysaccharide) was needed to facilitate settling of the colloidal precipitate (which was also a liquid phase) and t o get a clear supernatant layer. Fractions were recovered a t the last stage by precipitation in 85 to 95% ethyl alcohol. The total recovery of solids was 96y0. Some of the color of the original material was retained by each of the fractions. Portion RRR gave a turbid aqueous solution, while the others were clear. The distribution according t o alcohol precipitability may be more readily visualized by reference t o Figure 3, in which the cumulative yield of product is plotted against the alcohol concentration. The thermally degraded products are polydisperse; this sample contained a proportion of higher molecular weight material insoluble in 41.3y0ethyl alcohol, a major portion of several intermediate fractions precipitating a t alcohol concend trations u p t o 55%, and a second end fraction of lower molecular weight, which was soluble at ethyl alcohol concentrations of 55% or higher. Fraction Sss, which should contain sugars if
0
40
f
r
I
55
70
85
I 100
PERCENT ETHANOL
Figure 3.
Precipitation Curve of Thermally Degraded Dextran with Ethyl Alcohol
The ratios of 1,G- to non-1,6 linkages in the fractions as determined by measuring the amount of formic acid produced on periodate oxidation (9) were all very nearly the same, indicating that the solvent fractionation functioned largely on the basis of molecular size rather than type. However, the ratios for the fractions are definitely lower than t h a t in the original dextran, which had a ratio of 16 to 1. This may be indicative of a shift in a proportion of the glucosidic bonds from the 6-position to another of the free hydroxyl groups-Le., a transglucosidation occurring simultaneously with degradation, possibly t o give a more highly branched molecule. There are no data now available for deciding whether the 2-, 3-, or Ppositions may be involved. This mechanism is generally similar to t h a t proposed by Brimhall ( I ) for the formation of starch pyrodextrins, except t h a t in this case redistribution of 1,6-glucosidic linkages t o other types occurs. Instead of, or in addition to, transglucosidation reactions, some cross linking by etherification involving hydroxyl groups on carbons 2 ?r 3 may take place during the thermal degradation. Such cross linking would also account for the observed periodate oxidation results. The lowered optical rotation of these subfractions as compared with that of the original TABLE111. PROPERTIES OF SUBFRACTIONS OF THERMALLY DEGRADEDDEXTRAN material is much greater than Ratio 1,6 pH in t h a t expected as a result of to 2% Reducing Power as Non-1 6 [alaD6° A ueous Isomaltose Hydrate, 1 s Molecular Weight by chain shortening only. Using Sample Linkaies ( c = 2;HxO) Shtion % C Light Scatteringb Freudenberg’s equation ( I d ) , RRR 10 $186 5.7 1.3 0.42 RRS 184 5.5 1.4 0.28 and values of f155’ and ’ 178,000 RSR 914 187 54 1.4 0.24 115,000 +199” for the specific rotaRSS 10 181 5.2 1.4 0.16 SRR 11 184 5.5 1.5 0.16 71,900 tions of anhydrous a-isomalSRS 11 186 5.5 1.8 0.13 SSR 11 185 5.5 2.2 0.09 23,700 tose and undegraded dextran, sss 12 174 5.2 2.9 0.07 respectively, and the degree Sr .. 170 4.9c 5.8 0.05 Sss ... ... 2.9 ... .. of polymerization found by a Measured inO.3% aqueous solution. reducing power measurement, b Measurement8 at 5460 A. on autoclaved solutiona. Acid value of this fraction (defined in Table 11) = 0.60. the rotation of subfraction SSR, for example, would be: C
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Lllio~= 155 X 348 Molecular rotation
+ 99 X 162 X 199
SSR
MIOL = 32.5 X lo5 32.5 X lo6 = 1980
=
Calculated [ a ] gSSR =
162 X 101
The extensive alcohol fractionation and absence of substantial amounts of sugars make it improbable that the lowered observed rotation of the degraded product is due to presence of materials of low molecular weight. Hence, the optical rotation data, in agreement with periodate results, indicate the occurrence of changes in molecular structure. Because starch, with mostly a-1,4 linkages, has approximately the same rotation as dextran, if transglucosidation occurs the newly established linkages must be partly 1,2 or 1,3 or alternatively some conversion t o &type linkages, nhich commonly lead to lower positive rotations, may have occurred. There would appear t o be no a priori reason for assuming that shifts in glucosidic linkages should occur without Walden inversion, Indeed, if some isomahation to &type linkages occurs in starch dextrinization, this might be an alternative explanation to that already given in the literature for the increased resistance of the pyrodextrins t o the action of @-amylase (1).
PERCENT ETHANOL
Figure 4. Variation of Precipitability by Ethyl AIcohol with Temperature of Dextran Degradation
The acidic material in the degraded dextran is probably of low molecular n-eight, as it appears largely in the fractions soluble at higher ethyl alcohol concentrations. Further examination of this acidic substance would be desirable, as it may clarify some phases of the depolymerization mechanism. As the dextran was heat treated in an air stream, oxidation reactions are no doubt involved in the over-all degradation mechanism. The light scattering weight average molecular weights of several of the fractions TTere determined (Table 111) by N. N. Hellman of this laboratory. These values indicate that fractions SRS, SRR, RSS, and RSR, representing a total of 47% of the original material, are within the general molecular R-eight range ( 3 ) which has been considered desirable for blood volume expanders. Further study of the distribution of molecular sizes within these subfractions is needed. VARIATION IN MOLECULAR SIZE DISTRIBUTION WITH HEATING TIME
Aqueous solutions of dextran samples, degraded at 200' for time intervals of 4 to 8 hours, were fractionated with ethyl alcohol by consecutively removing each supernatant liquid and making the liquid up to the next higher alcohol level (Figure 4). This procedure differed from the more time-consuming fractionation described above in t h a t each precipitation vias not carried
Vol. 45, No. 4
out a t the same polysaccharide concentration, and the amounts of precipitated material were estimated by anthrone analysis, and no isolations of dried material were made. Results of three such fractionations are shown in Figure 4. The 5- and 7-hour samples gave distribution curves similar to that of the 6-hour sample. As might be expected, increased time of heating produced more materials of low molecular weight with consequent shift of distribution in the direction of greater alcohol solubility. The portions of the degraded dextran which precipitated between approximately 43 and 49y0 ethyl alcohol were found to be approximately in the molecular weight range desired for use in blood substitutes. Reference to Figure 4 indicates that the change in yield of these fractions of similar solubilities is not great between heating periods of 4 to 8 hours. CONCLUSIONS
The meager amount of published information that is available t o clarify the mechanism of partial thermal degradation of polysaccharides or the structure of the products has been limited for the most part to studies on amylaceous raw materials ( 1 , 7.11, 12). The starch pyrodextrins have been represented as containing levoglucosan-type end groups, containing true ether linkages between the number 6 carbon atoms of units in two different chains, and containing an increased number of 1,6- branch points. A11 these postulates are impossible or much less probable in a primarily 1,6-linked polyglucosan such as the dextran investigated in this work. Yet degradation of the dextran occurs readily in the temperature range 180" to 200" C. The mechanism of this degradation is definitely different from that involved in partial acid hydrolysis of this particular dextran, as fractions of acid-hydrolyzed dextran, in contrast with the thermally degraded materials, have higher 1,6 to non-l,6 linkage ratios than the original material and have high optical rotations, approaching that of the undegraded dextran (18). The more branched nature of heat-degraded dextran is also indicated by the fact that their molecular weights (Table 111) are higher than would have been expected from the observed viscosities (20) if comparison is made with fractions of acid-hydrolyzed dextrans. The thermal degradation has been rationalized here as simultaneous depolymerization and redistribution in position of a proportion of t h e glucosidic linkages, some of which may appear as beta linkages in the final product with the possibility of other reactions, such as cross linking or oxidation, occurring simultaneously. The results reported are valid only for the particular dextran type used. It will be of interest t o extend these investigations to other dextrans and possibly t o carbohydrates, such as laminarin, with other types of gIucosidic linkages. A communication by Stacey and Pautard ( W I ) , published while this paper was being reviewed, describes the thermal degradation of a dextran which is rapidly insolubilized when heated in the absence of antioxidants. This behavior differs from that of the dextran described in the present manuscript but parallels the authors' experience on the dextran of different chemical structure synthesized by Leuconostoc mesentemides K R R L B-742. ACKNOWLEDGMENT
This research program on dextran has been carried out with the cooperation of many groups of workers a t the Korthern Regional Research Laboratory, The authors wish particularly to thank R. J. Dimler, Allene Jeanes, and various members of the Fermentation and Engineering and Development Divisions for supplying the raw dextran, J. C. Rankin and B. H. Alexander for carrying out periodate oxidations, Edna RI. Montgomery for supplying pure samples of 6- [ a-D-glucopyranosyl]-D-glucose and the specific optical rotation of its a-form; N. N. Hellman for valuable suggestions during the course of fractionation, and f o r light scattering studies; and E. H. Melvin for ultraviolet absorption s1,udies.
April 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
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(1) Brimhall, B., IND. ENG.CHEM.,36, 72-5 (1944). (2) Chanda, S. K., Hirst, E. L., Jones, J. K. N., and Percival, E. G. V., J . Chem. Soc., 1950,1289-97. (3) Chem. Eng. News, 29,650-4 (1951). (4) Connell, J. J., Hirst, E. L., and Percival, E. G. V., J . Chem. Soc., 1950,3494-500. (5) Corn, 6, 4 (1950). (6) Evans, T. H., and Hibbert. H., “Bacterial Polysaccharides.” Scientific Report Series No. 6, New York, Sugar Research Foundation, April 1947. (7) Graefe, G..Die Stdrke, 3,3-9 (1951). (8) Hehre, E.J., private communication. (9) Jeanes, A., and Wilham. C. A., J. Am. Chem. SOC.,72, 26557 (1950). (IO) Jeanes, A., Wilham, C. A., and Miers, J. C., J . Bid. Chem., 176, 603-15 (1948). (11) Katz, J. R., and Weidinger, A., 2. physik. Chem., A184, 10022 (1939). ed., “Chemistry and Industry of Starch,” 2nd ed., (12) Kerr, R. W., pp. 174,345-55, New York, Academic Press, 1950. (13) Lansky, S.,Kooi, M., and Schoch, T. J., J. Am. Chem. SOC.,71, 4066-75 (1949).
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(14) Lockwood, A. R., Chemistry and Industry, 1951, 46-7; i l l f g . Chemist. 23. 49-55 (1952). Reoort of talk. (15) McCready, R. M.,Guggolz, J., Siiviera, V., and Owens, H. S., Anal. Chem., 22,1156-8 (1950). (16) Meyer, K.H., Noelting, G., and Bernfeld, P., Helv. Chim. Acta, 31,103-5 (1948). (17) Montgomery, E. M., Weakley, F. B., and Hilbert, G. E.,J . Am. Chem. Soc., 71,1682-7 (1949). (18) Northern Regional Laboratory, unpublished results. (19) Ricketts, C. R.,Lorenz, L., and Maycock, W. d’A., Nature, 165, 770 (1950). (20) Senti, F. R., and Hellman, N. N., Abstracts of Papers, 121st Meeting, AM. CHEM.SOC., Milwaukee, Wis., March 30 to April 3,1952. (21) Stacey, M., and Pautard, F. G., Chemistry & Industru, 1952, 1058-9. (22) Thorsen, G.,and Hint, H., Acta Chir. Scand., Suppl. 154 (1950). (23) Whiteside-Carlson, V.,and Carlson, W. W., Science, 115, 43 (1952). RECEIVED for review September 8, 1952. ACCEPTED December 15, 1952. Presented before the Division of Sugar Chemistry at the 122nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Improved Portland Cement Mortars with Polyvinyl Acetate Emulsions JACOB M. GEISTI, SERVO V. AMAGNA2, AND BRIAN B. MELLOR Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge 39, Mass.
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ORTLAND cementmortarsandconcretesareusedasbuilding materials because among other properties they have high compressive strengths, good bonding strengths, and resistance to weathering. Their principal weakness lies in a normally low tensile strength, a quality which has been corrected by various means. Most improvements of the properties of concretes and mortars have been made without admixtures, by controlling the mix proportion of the ingredients-the water to cement ratio being the most important factor ( 6 , 7 , 8, 13, 17, 19). Additives such as rubber latex have been incorporated whh portland cement t o produce floorings which are claimed t o have high resiliency (11, I d , 16, 20-25). Other additives such as resins, metallic oxides and salts, sugar, powdered metals and nonmetals, organic fibers, and gelatin have been used in an attempt to improve one or more properties of concrete mixes ( 5 , 1 5 ) . Polyvinyl acetate (PVA) emulsions are now commonly used in numerous industrial applications. Recently, polyvinyl acetate mixtures with portland cement were reported for surfacing concrete floors, for making joints between concrete blocks, and for mending broken cement surfaces (2, 9,10, 11), b u t apparently no work had been done to determine the exact nature of the new combination. The work described in this paper was initiated t o study the effects of using polyvinyl acetate emulsions as admixtures with portland cement mortars and concretes. GENERAL PROCEDURE
TESTSA N D COMPOSITIONS.Throughout the tests, A.S.T.M. (3) standard procedures were used wherever applicable. The procedures were modified when necessary, as dictated by the nature of the mortars and the tests. Tensile and compressive mortar specimens were prepared fol1 2
Present address, Hebrew Institute of Technology, Haifa, Israel. Present address, California-Texas Oil Co., Ltd., New York, N. Y.
lowing A.S.T.M. specifications and with ratios of sand to cement of 3 t o 1 for tensile briquets and 2.75 to 1 for compressive cubes and varying:
1. T h e ratio of polyvinyl acetate to cement from 0 t o 0.56 2. The ratio of dibutyl phthalate plasticizer t o polyvinyl acetate from 0 t o 0.2 3. T h e ratio of water t o cement for some tensile specimens from 0.32 t o 0.56 4. The particle size of the polyvinyl acetate (These ratios are weight ratios-e.g., 0.2 PVA t o cement would be 1 part by weight PVA to 5 parts by weight cement.) Selected compositions were further tested for impact strength, abrasion resistance, bond strengths of mortar t o steel and mortar t o concrete, corrosion resistance, air entrainment, water adsorption, and coefficient of expansion. Tensile and compressive specimens were also examined microscopically. CURINQCONDITIONS.The test specimens were cured a t different conditions and for different lengths of time. A.S.T.M. test methods for tensile and compressive tests (5)specify t h a t curing of mortar test specimens shall be under water after removal from the molds. Curing in a fog room a t 70 o F. with 100% relative humidity was originally selected for this work, since this represents the best possible curing condition, for plain cement mortars, which can be obtained in the field. It was observed, as was expected, t h a t the 28-day tensile strengths of plain mortars cured in the fog room were less (by 30%) than the tensile strengths of plain mortars under water. I n marked contrast t o the normal behavior of plain mortars, the 28-day tensile strengths of mortars containing polyvinyl acetate were equal or greater when cured in the fog room, than when cured under water. T o investigate this behavior further, specimens were cured in a room a t 70” F. with 50% relative humidity. Under these new curing conditions, although plain cement mortars were weaker, the mortars containing polyvinyl acetate were stronger. A few additional tests were made in which specimens
