acids with those of the unsaturated polymeric ones would lead to the erroneous conclusion that the fast acid group was primary as in stearic and pelargonic acids and the slow acid group was the 3-ethyl type. The formation of a 3-ethyl acid required that the last butadiene unit be added in a 3,4- fashion. This type of addition is rare and unlikely to occur with the frequency required to explain the relative amounts of the two acid groups. Also, the rate constants for esterification of the saturated models differed from those of the unsaturated polymeric ones by a factor of 2 or more. A sample of hydrogenated Telagen was esterified and the rate of reaction determined (Table 111). The hydrogenated polymer also contains two acid groups with different reactivities. The rate constants for esterification match those of the model acids to a high degree. The fast-reacting acid group (k = 21.4 X 10-5 sec.-l) is similar to the acid group in stearic acid (k = 20 X 10-5 sec.-l). The slow-reacting acid group ( k = 0.92 X 10-5 sec.-l) is similar to the acid group in 2-ethylhexanoic acid ( k = 0.89 X 10-5 sec.-l). That the slow-reacting group is the result of steric hindrance by an 2-ethyl group is further borne out by comparison of the data with those of Smith ( 5 ) and by application of the Taft equations (6) for the correlation of esterification rates with substituents on the acid. Smith finds that the rate of esterification of butyric acid relative to 2-ethylbutyric acid is 60.1 a t 20’ C. This rate is calculated to be 27.5 at 80’ C. by application of the Arrhenius equation and activation energies of 10.0 and 12.4 kcal. per mole for butyric and 2-ethylbutyric acid. This agrees well with our value of 23.3 for the rate of esterification of the fast acid group relative to the rate of the slow one. For acid-catalyzed esterification ( 6 ) , log (klk,) = paAu AE, where k and kE = acid-catalyzed esterification rate constants for a straight-chain acid and its a-ethyl substituted analog, p a = the reaction constant, ACT = the difference in the substituent values, and AE, = the difference in the substituent steric factors. In calculating E, values, Taft assumed that to a first approximation the relative rates of acidcatalyzed esterifications were determined by steric factors alone. Thus for the substituents n-CzH7 (E, = -0.36) and (C2HJ2CH (Es = -1.98) (these substituents are used to compare butyric and a-ethylbutyric acids), log k / k E = AE, = 1.62 utilizing values given by Hine (2). The replacement of a-H by a-Et in butyric acid should cause the rate to decrease at 25’ C. The error in the stericf actor for (C*Hb)*CH to
+
is given as A0.29 (6), so the relative rates vary from 22 to 79. Smith’s data, which have been shown to be consistent with our own, give an experimental relative rate of 60 at 25’ C. for butyric and a-ethylbutyric acids. T o establish the validity of determining separate esterification rate constants of a mixture of acids, an experiment was run using a mixture of 50% pelargonic and 50% 2-ethylhexanoic acids. The rate constants for these two acids determined from the mixture were higher (1 1 to 30%) than those determined on the separate acids. These values are close enough to allow conclusions as to the nature of the steric effect, which is of the order of 2000%. Application of methods indicated above gave the composition of the mixture as 49y0 pelargonic and 51% 2-ethylhexanoic acid. This is within experimental error of the actual values. Conclusions
For anionically polymerized polybutadiene a considerable amount of 1,2-addition, allylic rearrangement of the anion, or both occurs. The amount of 1,2-addition or of allylic rearrangement which occurs on the last added unit before carboxylation of the polymer is reflected by the relative amounts of fast- and slow-esterifying groups in the acid-terminated poIymer. The speed a t which the acid groups esterify is very likely a function of the steric hindrance induced by the 2-vinyl substituent which results from 1,2-addition or allylic rearrangement of the last added butadiene unit. The relative amounts of the fast- and slow-reacting groups in the carboxylated polymers are not necessarily the relative amounts ol‘ 1,4- and 1,2-addition of units in the polymer as a whole, because, while the propagation of the polymer may be kinetically determined, the configuration of the final or end group may be equilibrium-controlled. These results have many implications in the use of acid-terminated polymers as units for the preparation of elastomers by condensation reactions. literature Cited (1) Gilman, H., Kirby, R. H., “Organic Syntheses,” A. H. Blatt, Ed., 2nd ed., Coll. Vol. I, p. 361, Wiley, New York, 1941. ( 2 ) Hine, Jack, “Physical Organic Chemistry,” pp. 276-80,
McGraw-Hill. New York. 1956. (3) Kamm, O.,’Marvel, C.’S., “Organic Syntheses,” A. H. Blatt, Ed., 2nd ed., Coll. Vol. I, p. 25, Wiley, New York, 1951. (4) Pratt, E. F., Draper, J. D., J . A m . Chem. Soc. 71, 2846 (1949). (5) Smith, H. A., Ibid., 61, 254 (1939); 62, 1136 (1940). (6) Taft, R. W., Jr., Zbid., 74, 3120 (1952).
RECEIVED for review March 21, 1966. ACCEPTEDOctober 20, 1966.
XANTHATION OF STARCH IN LOW-CO NCENTRAT IO N PASTES E. 6. L A N C A S T E R , L. T . B L A C K ,
H. F. C O N W A Y , A N D E . L
GRIFFIN, JR.
Northern Regional Research Laboratory, U. S. Department of Agriculture, Peoria, Ill.
xanthates have been crosslinked by oxidation or with heavy metals to give water-insoluble compounds that impart useful properties if incorporated as an integral part of papers and pulpboards (2, 3 ) . The particular advantage of papers prepared with xanthates is in a sixfold or more increase in wet strength. A continuous method of preparing the xanTARCH
354
I&EC PRODUCT RESEARCH AND DEVELOPMENT
thates has been developed ( 5 ) , but it requires that a heavyduty mixer blend the starch with a relatively concentrated alkali solution and the carbon disulfide. The xanthate mixture issues from the reactor as a plastic mass, which must then be dispersed in water before the xanthate is applied to the pulp slurry where the crosslinking is carried out.
~~
~
Xanthates of starch can be formed in good yield by mixing carbon disulfide with alkaline 10% starch paste when the amount of xanthation required does not exceed 0.1 2 mole per repeating unit. The rate of xanthation increases with temperature and with alkali and starch concentration. The method is readily adaptable to a continuous low-cost process.
Other methods have also been employed for making starch xanthate (7, 3 ) . Adamek and Purves (7) added large excesses of carbon disulfide to starch in 10 to 2001, alkali solutions in which the starch concentration was about 6 to 10%. The degree of substitution (DS) as moles of xanthate per repeating unit was between 0.2 and 1.5. For incorporation in paper products, a degree of substitution of 0.1 is often satisfactory, and experiments were undertaken to see if milder alkali conditions would give a satisfactory product at low starch concentrations and thus reduce the power required for mixing. This paper reports results where xanthations to 0.1 DS were obtained in 10% alkaline starch pastes in the presence of alkali concentrations as low as 0.5%, provided that the starch was first adequately dispersed by cooking the slurry. Wet strength increases obtained on incorporation into paper of xanthate. solution prepared in this manner compared favorably with the average results obtained with xanthates prepared in the heavy-duty mixer (5) and having equal degree of substitution. Methods
The method chosen for determination of degree of substitution is one in which the benzyl bromide adduct of the xanthate is formed and its sulfur content is determined by the Schoniger method (4). The results are then converted to DS by using 162 as the molecular weight of an anhydroglucose unit (AGU). Xanthate solutions were prepared by first making an alkaline starch paste and then introducing the selected amount of carbon disulfide. The starch pastes were made by heating in a laboratory flask a slurry of 40 grams of starch in 400 grams of water, containing the chosen amount of sodium hydroxide, to a temperature sufficient to gelatinize the starch : approximately 40' C . with 0.5 mole or alkali per AGU and 10' C. additional for each 0.1 decrease in alkali (1 mole of NaOH per AGU was taken as 0.25 gram of titratable NaOH per gram of dry starch). The solution was then cooled to reaction temperature, and the carbon disulfide added. For most of the work, 0.15 mole of CS2 per AGU was used. The flask was closed and samples of the reaction mixture were taken a t intervals for analysis of the xanthate content. The times are taken as the intervals from the addition of the carbon disulfide to the time the reaction was stopprd by the addition of benzyl bromide.
conversion is influenced more by alkali level than by carbon disulfide. As in most reactions, temperature markedly influences the rate and heat can be substituted for alkali as a reaction accelerator. However, a t high temperatures, the volatility of the carbon disulfide becomes a problem, and the maximum amount of reaction is lowered. The lower maximum suggests also that side reactions are important. Figure 2 illustrates the magnitude of the effect of temperature on the reaction in pastes of low alkalinity. In comparison with the data in Figure 1, the initial rate of reaction at 38' C. and 0.2 mole of alkali per AGU equals that at 26' C. with 2.5 times as much alkali. However, the maximum degree of substitution a t the higher temperature is lower than that with higher alkali a t the lower temperature, giving a maximum conversion of only 67% as compared with about 80'%. When the logarithm of the initial rates (based on the degree of substitution of the 5-minute samples) is plotted as a function of the reciprocal of the absolute temperature, as in Figure 3, the Arrhenius lines show definite curvature. Although part of the curvature can be attributed to underestimation of the higher rates by this method, it is likely that the side reactions which lower the maximum value also materially reduce the rate of the main reaction below what might be expected.
\
10
I 30
I
I 50
40
t3
Time, minutes Figure 1.
Results
The effect of alkali a t temperatures ranging from 2' to 38' C. was investigated. There was an increase in reaction rate with both temperature and alkali without a large increase in final degree of substitution. The influence of alkali is illustrated in Figure 1. The conversion of carbon disulfide to xanthate (100 X DS/moles CS2/AGU) a t 26' C. approaches a maximum of 80%, which is reached within 30 minutes with 0.5 mole of alkali per AGU but requires several hours a t 0.2 mole per AGU. The benefit of additional alkali diminishes rapidly above 0.4 mole per AGU. Other experiments showed that increasing the carbon disulfide to 0.4 mole per AGU a t the alkali level of 0.2 mole per AGU fails to increase either the rate or the maximum conversion appreciably a t 38' C. T h e maximum conversion a t low temperatures, however, is somewhat higher with the excess carbon disulfide, the rate still being unaffected. This lack of pronounced effect is probably due to the limited solubility of carbon disulfide in the starch paste. The previous work (3) also indicated that the optimum
I
20
Xanthation of 10% starch paste at 26' C.
Rate increases with mole of alkali per repeating unit but with diminishing returns
_ _ _ _ _ _ _100% _ _ Conversion _ _ _ _ _limit ______------------------
20.12 ru
-..--
c
Time, minutes Figure 2. Xanthation of 10% starch paste at 0.2 mole alkali per repeating unit
of
Increasing temperature increases initial rate of xanthation but gives lowered maximum substitution
VOL. 5
NO. 4
DECEMBER 1 9 6 6
355
Since the xanthate solution would be used commercially within a reasonable length of time after it issues from the reactor, the degree of substitution achieved by reaction for 15 and 30 minutes is summarized in Table I for experiments in which 0.15 mole of CS? per AGU was used. These data are an extension of the data in Figure 2 and illustrate that high alkali gives no benefit at the higher temperature. The 10% alkaline starch pastes are of sufficiently low apparent viscosity that xanthation can be carried out continuously in conventional equipment. In Figure 4,a schematic diagram of a continuous system for xanthating starch is shown. When 0.5 mole or more of alkali per AGU is used, the mixer supplies sufficient energy so that the starch is dispersed at ambient temperatures and satisfactory products can be made without pretreating the starch. I n a run in which the mixer was a small centrifugal pump powered by a l/is-hp. motor, throughputs equivalent to 10 pounds of starch per hour were easily achieved. Conversion to xanthate paralleled the laboratory studies, giving 0.09 DS after 10 minutes' holding. Since
t -'---I :k L"
-'-. \
n ninC
1Figure 4. Schematic diagram of continuous plant to make starch xanthates
0.16
----------100%
Conversion l i m i t
- - I - - - - - -
a
0120
c
Reducing paste concentration reduces conversion
\
! 0.30
t Figure 3.
Arrhenius plots
Lines a t equal alkali concentration
Table 1. Degree of Substitution and Conversion of Reaction after Carbon Disulfide Addition to a 10% Starch Paste Holding Time 15 M i n-_z 30 Minutes
cs.
Alkali, Mole1 AGU
DSa, mole xanthate1 AGU
conversion,b
0.2 0.3 0.5 0.2 0.3 0.5 0.2
0.02 0.03 0.05 0.05
19
% 23
OS, mole xanthate1 AGU 0.04
36
37 56 73 57 67 69
0.08 0.11 38 0.08 0.3 0.10 0.5 0.10 a Degree o j substitution. Conversion = 100 AGU.
l&EC
x
f
Time, minutes
Male NaOH/AGU
L
356
x
Figure 5. Xanthation of starch a t 0.2 mole of alkali per repeating unit and 26" C.
p 0.010
26
Solution
f
-\
E
Tzmp., C. 14
40% NaOH CS2
0.06 0.08 0.07
cs.
conversion, b
concentrated (40y0) alkali is used as feed, the heats of dilution and of mixing give exit temperatures between 35' and 38' C. and the starch is thus dispersed. This continuous system would be particularly suitable for use in a paper plant at the point of application, since it would require only simple conventional equipment, much of which the plant might already have available. The xanthate solution as produced is of sufficiently low concentration to be readily diluted for use. Manufacture of the xanthate from an originally more dilute starch paste will result in lower substitution, as the data in Figure 5 indicate. The major expense of processing starch to xanthate by this method is the cost of chemicals, because cost of equipment would be low. For this system the chemicals cost about 0.8 cent per pound of dry starch, while preliminary estimates indicate that all other processing costs would be only about 0.5 cent per pound of dry starch in an all-new system processing 100,000 pounds of starch per day.
% 31 41
59 51 65 79 63 71
0.09 0.11 0.09 0.10 0.10 50 X DSjmoIes CS21
PRODUCT RESEARCH A N D DEVELOPMENT
literature Cited (1) Adamek, E. G., Purves, C. B., Can. J. Chem. 35, 960 (1957). ( 2 ) Naffziger, T. R., Swanson, C. L., Hofreiter, B. T., Russell, C. R., Rist, C. E., Tappi46,428 (1963). (3) Russell, C. R., Buchanan, R. A., Rist, C. E., Hofreiter, B. T., Ernst, A. J., Ibid., 45, 557 (1962). (4) Schoniger, LV., Mikrochim. Acta 1955, p. 123. (5) Swanson, C . L., Naffziger, T. R., Russell, C. R., Hofreiter, B. T., Rist, C . E., IND.ENG.CHEM.PROD.RES.DEVELOP. 3, 22
(1964). RECEIVED for review March 21, 1966 ACCEPTEDJune 27, 1966
