Determination of Peroxides in Synthetic Rubbers


RICHARD F. ROBEY and HERBERT K. WIESE. Esso Laboratories, Standard Oil Development Co., Elizabeth, N. J. The ferrousthiocyanate colorimetric method is...
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Determination of Peroxides in Synthetic Rubbers RICHARD F. ROBEY

AND

HERBERT K. WIESE

Esso Laboratories, Standard Oil Development Co., Elizabeth,

N. J.

feres with the analysis. A correction can be made for ferric iron by following the procedure with a separate portion of the solution, omitting the ferrous salt from the above reagent. ANALYSIS. By means of a pipet add one volume of olymer solution t o 15 volumes of ferrous rea ent. Compare tRe color developed with that obtained by adfing 1 ml. of a standard solution instead of sample. Standard solutions may be pre ared by dissolving small known weights of pure ferric chloride Rexahydrate in a given volume of methanol followed by dilution with 19 volumes of chloroform. A blank determination should also be made, employing 1 ml. of the solvent benzene instead of sample solution, and the results corrected accordingly. Comparisons are probably best made by colorimetric or spectrophotometric technique at brief periodic intervals until no further change is observed. A Diller hotoelectric colorimeter was employed in the present work. $he optimum spectral distrlbutlon or absorption by ferric thiocyanate solution has already been demonstrated (6). It may be necessary t o dilute the sample solution in benzene with an additional known weight of the same solvent in order to make the observation in a favorable range of optical density. This range was attained with 0 to 25 p.p.m. of active oxygen in the polymer solution.

The ferrous thiocyanate colorimetric method i s applicable to the determination of active oxygen in commercial synthetic rubbers if the reagent i s made up in a mixed solvent comprising absolute ethanol and chloroform. Antioxidants ’used in commercial synthetics do not affect the results.

PEROXIDES

are found in synthetic rubbers either as the result of attack by oxygen, usually from the air, or as a residue from polymerization operations employing peroxide catalysts. Because of possible detrimental effects of active oxygen on the properties of the rubber (S),a method of quantitative determination is needed. The concentration of peroxides in substances of lower molecular weight may be determined with ferrous thiocyanate reagent, either titrimetrically as recommended by Yule and Wilson ( 7 ) or colorimetrically as by Young, Vogt, and Nieuwland (6). Unfortunately, many highly polymeric substances are not soluble in the acetone and methanol solutions employed in these procedures. This is also the case with hydrocarbon monomers, such as butadiene, containing appreciable concentrations of soluble high molecular weight polymers (4). Bolland et al. (I) recommended benzene as a solvent for natural rubber samples and the reagent made up in methanol. However, most synthetic rubbers are not readily soluble even in this combination. The following procedure employs the ferrous thiocyanate reagent in combination with a solvent capable of maintaining considerable concentrations of synthetic rubber in solution. The solvent comprises essentially 20% ethanol in chloroform.

TEST OF T H E M E T H O D

The time required for various peroxides to give maximum color density with the reagent varies considerably. Table I indicates some relative rates a t room temperature and with a n initial concentration of 10 p.p.m. of active oxygen in the sample solution. It is evident that the hydroperoxides react the more rapidly and that the peroxides in the rubber are indicated to be largely of this type. These observations are in agreement with those of Farmer and Sutton on polyisoprene (2). The response of the ferrous reagent to benzoyl peroxide may be accelerated by gentle warming of the test mixture. Bolland et al. (I) have pointed out the severe difficulties involved in preparing polymeric mixtures containing known concentrations of peroxide by direct reaction of oxygen. As a n approximation thereto, however, known mixtures were prepared for the present investigation with commercial peroxides in benzene solution and analyzed in the absence and presence of BunaS (Table 11). Values obtained in the presence of the rubber

SCOPE AND A C C U R A C Y

The procedure has been found applicable to Perbunan, Bum-S, Butyl, and other synthetic rubbers and plastics soluble in hydrocarbon or other solvents, or to that portion which is soluble in case the sample contains appreciable “gel”. I n general, it is restricted to unvulcanized samples. Highly colored samples may give difficulty in the colorimetry. The presence of fresh oxidation inhibitors, such as [email protected], and Agerite White ([email protected]) does not affect the results, indicating that the interaction between active oxygen and inhibitors is rather slow a t ordinary temperatures. Strong oxidizing agents, of course, will interfere. By this method it is possible to detect 10 parts per million of active oxygen in the polymer. Accuracy is about 5 to 10%.

Table

1.

Time for Maximum Color Density Time for Complete Reaction, Min. 75 60 5 10 10 10 5 20

Peroxide RanEovl-

Aacaridole tert-Butyl hydroperoxide Butyl rubber, air-peroxidiied Perbunan, air-peroxidized Buns-S. air-oeroxidized Isoprene, ai;-peroxidized Dimer of butadiene, air-peroxidized

REAGENT

FERROUSSOLUTION. Dissolve 0.130 gram of potassium thiocyanate in 50 ml. of absolute ethyl alcohol, add 0.065 gram of ferrous chloride tetrahydrate, and shake the mixture until dissolved. Make 79 ml. of pure chloroform up to 100 ml. with the alcoholic ferrous thiocyanate solution and acidify with 2 drops of concentrated sulfuric acid. The resulting reagent should become practically colorless. A precipitate of potassium chloride which usually occurs is allowed to settle or is centrifuged out. The reagent is stable for several hours in the dark.

Table 11. Peroxide Uaed Benzoyl

Analysis of Known Mixtures Rubber Added None

PROCEDURE

SAMPLIICG. The polymer sample should be fairly dry and as homogeneow and representative as possible. Millin in the presence of air may add peroxides. The sample need not be thoroughly dry. Dissolve about 0.60 gram, weighed to 0.01 gram, of finely cut polymer sample in 20.0 grams of benzene in a suitably stoppered vessel of such a size as to leave but little free space. Hasten solution with the aid of continuous agitation or tumbling, and if necessary, filter to remove any foreign matter. Any soluble ferric iron present in the sample undoubtedly inter-

Active Oxygen in Solution Synthesis Found P.p.m. P.p.m. 2.5 2.3 5.0 5.5 9.6 10.0 9.8 10.2 14.7 15.0 15.5

Buna-S tert-Butyl hydroperoxide

425

None

15.8

18.7 26.7

18.7 26.7

10 20

12 23

4.8 11.2

4.1 11.3

426

INDUSTRIAL AND ENGINEERING CHEMISTRY

were corrected for the small original active oxygen content of the rubber as determined in a separate analysis. Since many synthetic rubbers contain stabilizers (antioxidants) of some kind, attempts were made to learn whether such compounds have any effect on the determination. Phenyl-p-naphthylamine and Agerite White equivalent to 0.05 and 0.20 weiKht yo,respectively, based on the polymer solution were added to Butyl and t o Bum-S dissolved in benzene. Peroxide determinations resulted as follows:

Polymer Butyl Buna S

It can be analysis.

Active Oxygen Found, Inhibitor Absent P.B.N. A.W. P.p.m. P.p.m. 142 25 45 15

Active Oxygen Found in Presence of Inhibitor P.B.N. A.W. P.p.m. P.p.m 138 24 47 15

that these inhibitors do not affect the

Vol. 17, No. 7

ACKNOWLEDGMENTS

The authors thank those among their colleagues who aided with the work and the manuscript, particularly John Rehner for helpful consultation, and the Standard Oil Development Co. for permission to publish this work. LITERATURE CITED (1) Bolland, J. L., Sundralingam, A , , Sutton, D. A,, and Tristram, G . R.,Trans. Inst. Rubber Ind., 17, 29-32 (1941). (2) Farmer, E. H., and Sutton, D. A , , J. Chem. SOC.,1942, 139-48. (3) Naylor, R. F., Trans. Inst. Rubber Id.,20, 45-53 (1944). (4) Robey, R. F., W'iese, H. K., and Morrell, C. E., IND.ENG. CHEM.,36, 3-7 (1944). (6) Willard, H. H., and Ayres, G. H., IND. EKG.CHEM.,ANAL.ED., 12, 287-91 (1940). (6) Young, C. A., Vogt, R. R., and Nieuwland, J. 1., Ibid., 8, 198-9 (1936). (7) Yule, J. A. C., and Wilson, C. P., IND.ENG.CHEM.,23, 1254-7 (1931).

Determination of Nitrate in Boiler W a t e r by Brucine Reagent C H A R L E S A. NOLL W. H. & L. D. Betz, Philadelphia, Pa.

B

KCAUSE of the widespread use of >odium nitrate and the maintenance of sodium nitrate-sodium hydroxide ratios in boiler water for controlling tendencies toward intercrystalline cracking, it has become imperati\ v that anefficient, accurate, and easily manipulated method for the determination of nitrate be provided, particularly for routine plant control. The nitrate method used by some chemists consists of the reduction of nitrate ion to ammonia which is then distilled over into standard acid and back-titrated (2). Another procedure involves distillation of the ammonia which is caught in distilled water and then directly Sesslerized ( 1 ) . The e methods are obviously rather tedious even for a well-equippeci laboratory and have not been widely adopted for plant control. The phenoldisulfonic method (I) for nitrate is also rather cumbersome and not well adapted to plant control. While this nitrate study was in progre.>s, a paper reported using brucine reapcmt for the determination of nitrate in soil and plant extracts (4). The range of nitratt. concentration investigated was considerably below the boiler water range and the brucine reagent used was in acid solution, requiring preparation just prior to using. A new method for the determination of nitrate in boiler water is herein described. The use of brucine reagent was suggested by Snell (S),who employed it for the deterniination of nitrate in meat. The procedure described adapts this method with some changes for the determination of nitrate in boiler water, employing a Klrtt-Summerson photoelectric photometer.

PROCEDURE AND STANDARDIZATION

Pipet two 5.0-ml. samples of the water to be analyzed into 50-ml. beakers; t o one beaker first add 0.2 ml. of brucine reagent and then to both beakers add 10 ml. of sulfuric acid. Add the acid to avoid s attering and mix thoroughly. (Clean dry glassw r e is preferagle in this test, although the addition of as much a s 0.5 ml. of water will not affect the result.) To the sample untreated with brucine add 10 ml. of distilled water, swirl to mix, cool, transfer a portion of the sample t o the 10-ml. test tube, and set the photometer to the zero reference point using the 470-mp filter. When the brucine-treated sample has stood a minimum of 3 minutes (not over 10 minutes), add 10 ml. of distilled water, mix, cool, transfer t o the photometer as above, and determine the dial reading. Read the nitrate equivalent to the dial reading

Table I. Time Effect after Sulfuric Acid Addition Time ,Win. 3 5 7 10 20 30

Nitrate as NO1

Found P.p.m. 25.0 25.0 25.0 25.5 33.0 37.2

Table II. Stability of Color Developed Time of Standing after Sample Preparation

.Win. 0 30 60

REAGENTS AND CONDITIONS

POTASSIGN KITRATE Reaient grade potassium nitrate is dried in an oven at 105" * 1' C. for 24 hours and 1.631 grams are accurately weighed, dissolved in approximately 20 ml. of distilled water, and made up to 1 liter with distilled water. The solution strength is then 1 ml. = 1 mg. as NO?. BRUCINEALKALOID. Five grams of pure brucine alkaloid crystals are dissolved in approximately 20 ml. of chloroform and made up to 100 ml. with chloroform (reagent grade). (Brucine is a very poisonous alkaloid and care should be taken in handling it.) SKLFURIC ACID. Sulfuric acid, reagent grade, specific gravity 1.84 and possessiiig 95 to 96% assay. ~ L E T T - S U M h l E R S O N PHOTOJIETER. .A 10-ml. test tube, 13 mm. wide, 470-mp color filter.

Present P.p.m. 25.0 25.0 25.0 25.0 25.0 25.0

90

120

Table 111. A g e of Reagent

Days 0 1

7 14 30 40 60

Present P.p.m. 25.0 25.0 25.0 25.0 25.0

A g e of

Nitrate as N0a

Found P.p.m. 25.0 25.1

25.2 24.3 23.8

Brucine Reagent

Present P.p.m. 25.0 25.0 25.0 25.0 25.0 23.0 25 0

Nitrate as NO,

Found P.p.m. 25.0 25.0 25.0 25.0 25.2 24.8 25.1