V O L U M E 2 7 , NO. 4, A P R I L 1 9 5 5
589
(436) Troy, D. ,J.. Diri4on of Analytical Chemistry, Symposium on C H E x Soc., Los .kngeles, Air Pollution, 123rd Meeting, Calif., March 1953. (437) Turnbull, A. H., and Harrison, 31. F.-4.. J . Sei. I m t r . , 30, 266 (1953). (438) Chl, W.C., Petroleum Processing, 7, 1433 (1952). (439) Underwood, S . , and Doermann, -1.I%., Phys. Rev., 83, 489 (1951). (440) Unterzaucher, J.. A?agew. Chena., 61, 452 (1949). (441) Vacher, >I., and Lortie, Y., Coinpt. r m d , , 236, 1759 (1953). (442) Van den .Ikker, J. A,, and Hardacker, h.IT,, T a p p i , 35, S o . 7, 138.1 (1952). (443) Varner, J. E., and Bulen, W.h.,J . Chein. Educ., 29, 625 (1952). (444) VedeFa, 11.. Chein. L i s t y , 47, 1090 (1953). (445) Vieweg, R., and Wesp, 1., Rev. plcisticos ( M a d r i d ) , 2, 270 (1951). (446) Von Brand, E. K., U. S.Patent 2,667,779 (1954). (447) Voter, R. C., and Hare, G., A p p l . Spectroscopy, 7, KO.3, 135, 138 (1953). (448) Talker, J. K., Gifford, P., and Selson, R. H.. I n d . E ~ L Q . Chein., 46, 1400 (1954). f449) Walker. W.G.. and Baker. S. C.. J . Sei. I m t r . . 30. 328 (1953). A\,
i 4 5 O j Wall, R. F., Giusti. d.L.. Fitzpatrick, J. W.,and Ji-ood,‘C. E., I n d . E n g . Chena., 46, 1387 (1954). (451) Yalters, S.H., I b i d . , 46, 1390 (1954). (452) Washburn, H. W,, and .Lustin, R. R . , Pmc. S a t l . Air Pollution S y m p o s i u m , 1949, 1. 69 (1951). (453) Katers, J. L., and Hartz, S . W., I n s t r u m e n t s , 25, 622 (1952). (484) Watkins. J. l l . , and Gemmill, C. L., -1s.i~. CHEX.,24, 591 (1952).
(455) Wearer, F. D., I n s t r u m e n t s , 26, 1362 (1953). (456) Weaver, H. A, and Jamison, V, C.. Agronomij, 43, 602 (1951). (457) Weissler, G. L., Einarsson, .1.W.,and McClelland, J. D., Rex. Sei.Instr., 23, 209 (1952).
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Patent 167,840 (1951). (463) Wilkinson, R., I n s t r u m e n t Practice. 8 , 414 (1954). (464) Wilkinson, R., and Briggs, R., Chemistry &. I n d u s t r y , 1953, 866. Jr., and Dean, J. A, “Instru(465) Willard, H. H., Ilerritt, L. mental Methods of Anal.
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(467) Williamson, D. E., and Robinson, D. Z.,
-&SAL.
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(468) Kilner, T., A m . I n d . Hyg. dssoc. Quart. (469) Wingo, W. J., and Browning, L., -1. (1953). (470) Winteringham. F. P. W., et al., Szvdeonics. 10, 52 (1952). (471) TTise, E. S . ,AXAL.CHEX.,23, 1479 (1951). and Reynolds. C. d.,Jr., Ibid., 25, (472) Wise, E. S . , Gilles, P. W., 1344 (1953). (473) Kithrow, R. B., and Withrow, A. P.. Physiol. P l a n t a r u m , 6, 444 (1953). (474) TTohle, W., Die Technik, 8, 701 (1953). (475) Wold, J. 31., Plant Physiol., 27, 70 (1952). (476) TToodhull. E. H.. Siegler, E. H., and Sobuov. TI.. Ir‘d. Eng. Chenz., 46, 1396 (1954). (477) Wooster. IT. S., Carritt, D. E., and Isaacs. ,J. D., J . Xarine Research ( S t a r s Foun~lation).10. 194 (1951). (478) Yanikoski, F. F., U. S. Patent 2,586,899 ( 1 9 i 2 ) . (479) Young, IT. S..ct al., .AXAL. CHEX.,24, 598 (1952). (450) Zaikowsky. IT’. 3I.,U. S.Patents 2,591,759--60 (1952); 2,639,978 (1953).
Natural and Synthetic Rubbers
I
NORMAN BEKKEDAHL and MAX TRYON National Bureau of Standards, Washington,
D. C.
T
H I S is the sixth of a series of revieTT articles on physical and chemical test methods pertaining to natural and synthetic rubbers. T h e first five appeared annually, but the present one is the beginning of a biennial series. The first one (18) covered only chemical methods up through the bear 1948, but the second (14), third ( 1 6 ) , fourth (16), and fifth ( 1 7 ) included physical testing. The present review liken ise covers both physical and chemical methods, and refers t o articles appearing in the journals for the 2-year period ending October 1, 1934. Like the previous reviews, it omits test methods applied to compounding ingredients, except for their identification or quantitative determination in rubber. It also omits tests on raw materials used in the manufacture of synthetic rubbers. It does not refer to procedures which are concerned more s i t h problems of a fundamental nature than n i t h testing, unless they employ unique apparatus or techniques that seem likely to prove of value in testing procedures for rubber. Old procedures employed during this review period for either research or testing are not referred to again unless they have been modified or improved. Because of the extremely large number of articles on the subject of rubber testiiig.published during these past 2 years, it has not been possible t o make the survey exhaustively complete, but it is hoped that all the more important articles have been included.
GEKERAL ISFOR\IITION During the past 2 years several review articles and books have presented general information on rubber testing. T h e most iniportant sources of reference are the British monthly journal Rubber Abstracts and the annual repoi ts for 1952 (64)and 1953
(55)on the progress of rubber technology, which are issued by the Institution of the Rubber Industry. Fisher (65, 66) has continued his annual revien on rubber science and technology, in nhich he makes some reference to methods of testing rubber. Gehnian’s paper (69) on developments in the physics of rubber includes several ne11 types of tests, such as the use of ultrasonic naves for tire inspection, and the use of sulfmr-35 for determining the quantity of combined sulfur in a rubber compound. T h e Division of Rubber Chemistry of the .i\rERrckv CHEMICAL SOCIETY, under the editorship of Lerner (126),released the eighth volume of the “Bibliography of Rubber Literature,” which covers the 3-year period 1946-1948. The editors exbect to prepare these 3-?-ear volumes each year until the\ are current. Kreuter (115) continued his series of reviery articles, which began in early 1950, on testing methods used in the rubber industry. I n another article on recent developments Braber ( 2 5 ) discussed the advantages and limiting conditions of laboratory testing as an indication of the practical behavior of rubber products. I n a critical survey of rubber testing methods U l l a n e ( I & ) , explained the importance of conducting tests over wide ranges of temperature, time, and stress. Several papers presented a t the Third Rubber Technology Conference held in London in 1954 \?ere related to testing methods. Good reviews of this conference were given in the rubber trade journals ( 1 2 7 , 1 $9) 1 8 4 ) . Preprints of the papers have been made available, and the complete proceedings will appear later. Committee D-11 (on Rubber and Rubberlike SIaterials) of the American Society for Testing Materials issued a revised “Standards on Rubber Products’’ ( 2 ) and a supplement ( 3 ) to the “A4STi\IStandards, 1952, Part 6.l’ I n these editions many
590
ANALYTICAL CHEMISTRY
of the older tests on rubber products have been revised and some new ones adopted. The British Standards Institution produced new issues of Part 2 (“Methods of Chemical Analysis”) (69) and P a r t 4 (“Evaluation of Vulcanizing Characteristics”) (30) of its Specification 1673 on “Methods of Testing Raw Rubber and Unvulcanized Compounded Rubber.’, Brazier ( 2 7 ) and Buist ( 3 4 )reviewed the progress made in recent years in the standsrdization of rubber goods and of test methods, with special reference to the work of the British Standards Institution and similar standardizing organizations. Technical Committee T C 45 (Rubber) of the International Organization for Standardization held a meeting in June 1953, which has been reported in several places (33,51,95,195). -4 number of the specifications for GR-S and GR-I synthetic rubbers made in the United States have been revised by the Reconstruction Finance Corp. (173). Several reference books on rubber have been published during the past 2 years. The most recent is (‘Synthetic Rubber,” edited by Whitby, Davis, and Dunbrook ( @ I ) , which contains an excellent chapter by Juve (97) on “Physical Test Methods and Polymer Evaluation.” Unfortunately, this book does not contain a similar chapter on chemical test methods. However, Wood (247) in his chapter on “Physical Chemistry of Rubbers” includes various testing procedures such as the quantitative determination of bound styrene in GR-S synthetic rubber. Howland’s chapter (91) on “GR-S Latex” describes some tests used in the evaluation of the latex. Stern’s recent book (210) on “Rubber: Xatural and Synthetic” includes a chapter on the testing and analysis of rubber. Frey, in his book on methods of chemical analysis of rubber mixes (68), not only describes methods previously presented in the literature, but also adds his comments on methods with which he has had personal experience. A loose-leaf binder book entitled “Technically Classified Rubber” has been issued by the Natural Rubber Bureau (148), which reprints information from various sources. Included in this book are the News Sheets and Users’ Information’ Circulars on technically classified rubber, which are issued from time to time by the International Rubber Research Board. The American Society for Testing Materials has issued a special technical publication ( 4 ) which includes all the papers presented a t a Symposium on Recent Developments in the Evaluation of Natural Rubber, held in S e w York in June 1952. Noble revised and greatly enlarged his book, “Latex in Industry” (168), which was first issued in 1936 and has long been out of print. It contains all the procedures used in the chemical and physical testing of latex. The R. T. Vanderbilt Co., under the editorship of Winspear, has prepared a handbook on latex (246) which includes many latex-testing procedures. Flory’s “Principles of Polymer Chemistry” (67) does not pretend to be a textbook for the physical and chemical testing of rubber or latex, but it is such an excellent book that any technologist connected with the field of rubber should be familiar with it.
CHEMICAL ANALYSIS The application of absorption spectroscopy to the analysis of rubber compounds has been described by Kendall and Davison (103),including the theory and the instrumental technique of both ultraviolet and infrared spectroscopy. The ultraviolet is most useful for estimating accelerators or antioxidants in rubber or combined styrene in a copolymer, while the infrared can be used for polymer analysis, as atomic groupings can be identified in situ in the polymer when prepared either in film form or in solution. A later paper ( 4 7 )emphasizes the general identification and estimation of a wide range of elastomers by infrared techniques. However, because polymers are usually not easy to identify by conventional infrared techniques, Harms ( 7 8 ) and &use and Wallace (117, 118) found that a preliminary pyrolysis aided in the identification. Only a few drops of the pyrolyzate are necessary either for direct examination or as a solution in
carbon tetrachloride. Richardson and Sacher ( 1 7 7 ) used infrared examination to determine quantitatively the different types of addition which take place when isoprene is polymerized. Identification of polymers from their pyrolytic products has also been made by mass spectrometric analysis. Using this method, Zemany (252)requires only a few tenths of a milligram of sample for the analysis. Using methods based on the thermal decomposition of a polymer in a stream of nitrogen over superheated carbon, followed by the iodometric titration of the carbon monoxide formed, Unterzaucher (234) was able to make precise determination of the oxygen content. Kirshenbaum and Streng (108) compared the method of Unterzaucher with that of Grosse and Kirshenbaum ( 7 4 ) , which makes use of isotopic principles, and found that the latter method yielded more precise results. &leeks and coworkers ( 14 1 ) improved the existing gravimetric methods for determining rubber hydrocarbon in rubber-bearing plants. Comminution of the plant is accomplished by crushing between corrugated and smooth rolls, so that all the plant cells are ruptured and rubber is completely extracted. The bromination procedure is then followed. For determining gutta or rubber hydrocarbon in plants, Doman ( 5 3 ) developed a nephelometric micromethod which he believes to be superior to the usual gravimetric procedures. Salomon and coworkers (187) improved an old method for determining the amount of rubber in asphalt. A prolonged extraction with higher boiling solvents dissolves out t h e rubber, which is then made to react with a large quantity of sulfur. The rubber content is then determined from a sulfur analysis of the purified ebonite formed. The Rubber Research Institute of Malaya (186) states that the only accurate method for determining the dry rubber content of natural latex is to weigh a representative sample, coagulate it, dry the coagulum under specific conditions, and weigh the dried coagulum. Linnig and coworkers (132, 133) presented a scheme for the analysis of gross chemical constituents of uncompounded GR-S aynthetic rubber by complete solution procedures using a single weighed sample. The whole analytical procedure is less timeconsuming than the extraction and other specification procedures, but unfortunately cannot be used for vulcanized GR-S or for GR-S containing gel. Koide, Kubota, and Kuroi (111) compared various methods for the determination of total sulfur in vulcanized rubber. They report that the potassium hydroxide-potassium nitrate fusion method and the sodium peroxide fusion method are the quickest in manipulation but are less precise. Although the nitric acidbromine oxidation-alkali fusion method is more precise than the previous ones mentioned, it is still not so precise as the nitric acid-bromine oxidation-nonfusion method recommended by the ASTM. The latter method is also the easiest to manipulate. Khoroshaya and Kovrigina (106) recommend a rapid method for the determination of free sulfur. Its basis is the conversion of the free sulfur in the rubber (by means of sodium sulfite) into sodium thiosulfate, the combination of the surplus sodium sulfite with formaldehyde, and the titration of the acid-neutralized mixture with iodine. The whole procedure can be completed in about 30 minutes. Khoroshaya and others (105) recommend that for ash determination in rubber a crucible be used which has an enlarged bottom surface of heat-resistant stainless steel. This crucible can be brought rapidly to the combustion temperature and thus saves considerable time. The authors recommend the procedure as suitable for routine testing. A detailqd procedure is given by Kress (113) for the quantitative analysis of the ash in rubber with the Spectranal. A simultaneous determination is made of the relative concentrations of several elements normally found in the ash. An accuracy within 20% of the amount present is achieved, and the procedure is considered to be practical wherever speed and convenience are of greater importance than absolute accuracy. Stern and Hinson
V O L U M E 2 7 , NO. 4, A P R I L 1 9 5 5 (211) found that in the ashing of neoprene some of the zinc pres-
ent is volatilized, and they recommend that for the destruction of the organic matter a wet-oxidation procedure be used, similar to that which is often employed for copper and manganese in rubber. Committee D-11 of the American Society for Testing Materials ( 2 , s ) adopted two official methods for determining the “harmful” dirt in rubber, which is defined as that portion of the “insolubles” which does not pass through a 325-mesh screen. I n method A, called the hot oil method, a large sample of rubber is mixed in hot oil and filtered hot through the screen. In method B, the solvent method, a small sample is dissolved in a solvent and the solution filtered through the screen. The former method is much more rapid, but unfortunately is much more difficult t o manipulate. The methods are based on work done by a task group in the American Society for Testing Materials (217 ) and also by chemists a t the Indonesian Rubber Research Institute (84). Heinisch (82,83) later surveyed the problem of contamination of rubber by dirt and presented a critical discussion of the subject. Wyatt (249) described in detail a method for separating small amounts of copper, manganese, and iron from each other and then determining their concentration by optical absorption measurements. Several other very good procedures (12, 269, 233) relate to the determination of minerals and other nonrubber constituents found in rubber and latex. Kidder (IO?‘) developed a method for determining the degree of yellow coloring in raw rubber latex films and crepes. The absorptions of the acetone extract of the rubber at wave lengths of 360, 400, and 560 mp are combined by an empirical equation to give a measure of the coIoring matter in the sample. The test is claimed to be effective even in the presence of considerable darkening due t o the oxidation. Rostler and White (179) have developed a procedure for the analysis of rubber extender oils in terms of asphaltenes, nitrogen bases, acidaffins, and paraffins, which can serve as a basis for standardizing the oils used in the oil-extended GR-S synthetic rubber. The Sun Oil Co. (619) suggests the classification of the oils as t o viscosity index, or viscosity gravity constant, which is a useful measure of the nature of the oil in relation t o its action in the rubber. An exceptionally large amount of u-ork seems t o have been done these past 2 years on the identification and quantitative determination of rubber accelerators and antioxidants. Buscarons and Capitan ( 3 7 ) gave a series of comprehensive reviews of analytical procedures for 90 important accelerators known by their trade names, which they identified and classified according to chemical structure. Blokh, Kulberg, and Golubkova ( 2 2 ) described amethod which can becompleted in from 7 to 10 minutes for detecting thiuram and mercaptobenaothiazole in rubber mixes. Bauminger and Poulton (11 ) developed a colorimetric method for determining 2-mercaptobenzothiazole, which is based on the formation and evaluation of the red-brown color formed by the reaction of its alkaline salt with the nickel ion. The R. T . Vanderbilt Co. (235) described four reagents and the colors which they develop with the company’s products. Scheele and Gensch (189) described apparatus, procedures, and chemical reactions, and presented tables and graphs for conductometric titrations of dithiocarbamates, tetramethylthiuram disulfide and monosulfide, mercaptobenzothiazole, and diphenylguanadine. Hilton and Newel1 ( 8 7 ) developed a method for the quantitative determination of dithiocarbamates and thiuram disulfides in rubber. As the latter is converted into the former during cure, this method can measure the degree of conversion of the accelerator a t any stage of cure. Kress and Stevens (114) identified a number of curing agents in rubber products by ultraviolet absorptiometric analysis of selective solvent extracts. They claim their procedure to be more simple, mecific, and pensitive than rhromatopraphic pro-
591 cedures. Interference of the common softeners and antioxidants is usually negligible. Tanaka (222, 228, 225) made a spectrochemical study of 12 commercial organic accelerators in the ultraviolet before and after vulcanization in order to determine them and to estimate their proper distribution in rubber. The behavior of some of these accelerators was also studied (264) during various stages of vulcanization. Brock and Louth (31) separated 24 accelerators and 12 antioxidants as to types, and made subsequent identifications by physical measurements such as x-ray diffraction and ultraviolet absorption. Fink and coworkers ( 6 4 ) developed procedures for the separation of accelerators by elution chromatography. When mercaptobenzothiazole is separated from interfering substances such as antioxidants and tars, ultraviolet spectroscopy offers a sensitive and specific identification. Van Bloois ( & 3 , 2 4 )gave a popular description of the chromatographic separation of ingredients such as accelerators, antioxidants, and plasticizers from vulcanized rubber. Parker and Barriman (161, 162) presented a scheme for chromatographic identification of 32 accelerators and antioxidants. The acetoneextracted material is chromatographed on silica gel-Celite in columns using a series of different binary solvent mixtures as developers. R h e n the extruded columns have been streaked with the appropriate reagents, the positions of the zones, together with the colors of the streaks, give a reasonably good identification. Later these same authors (168) extended the work t o include the mutual separation and identification of six alkylsubstituted dithiocarbamates. Hamazaki ( 7 7 ) found it possible t o identify several accelerators and antioxidants by ultraviolet observations. H e observed the color change during development of the absorption band and the color reaction of cobalt oleate. Isayama ( 9 6 ) found the identification by copper sulfate solution more effective than by the cobalt oleate. Weber (239) developed a rapid and reliable technique for detecting accelerators by means of paper chromatography. Quantitative estimations can also be made by this method. Hively and coworkers (88) found no interference from stearic acid, sulfur, paraffin, or rubber extract when they identified antioxidants by ultraviolet spectroscopic techniques after separation from one another by elution chromatography.
AGING AND DETERIORATION Shelton, Wherley, and Cox (199) suggested that accelerated aging a t only one temperature cannot predict accurately the aging characteristics of a stock, because a t loxer temperatures a cross linking of the molecules predominates while a t the higher temperatures a chain-scission reaction is more likely to take place. Juve and Schoch (98) stated that an extrapolation of accelerated aging test data t o room temperature requires a knowledge of the temperature dependence of the deterioration for the particular material being considered. Toumans and Maassen (260) tried to correlate room-temperature shelf aging with (1) individual test tube aging at 70’ C., ( 2 ) oxygen bomb aging a t TO” C. and 300 pounds per square inch and ( 3 ) air bomb aging a t 127’ C. and 80 pounds per square inch. S o n e of these tests deteriorated all physical properties a t the same rate, but the 70’ test tube method most closely simulated natural aging. Pollack (168) believes that the oxygen-absorption test provides a more fundamental and rapid evaluation of aging than do the other types of test. The oven test gives undue weight to aftervulcanization effects. The oxygen-bomb method does not produce a constant rate of loss of physical properties during the first few days; however, its 4- to 14-day rate should provide a good evaluation of the rubber tested. Kirshenbaum, Streng, and Nellen (110) applied the isotopic method ( 7 4 ) of determining oxygen to the aging of rubber. This method would probably yield results similar to those obtained by measuring the oxygen absorption.
ANALYTICAL CHEMISTRY
592
Heiniscli’s work (SO, 81) with an improved “rubberscope” nhich heats natural rubber to 130” t o 1.1-0’ C. by infrared radiation shows that it is possible t o determine within 20 minutes the aging behavior of rubber during storage. V-heri this instrument is used in conjunction ith plasticity measurements by the Hoekstra steam plastometer, it is especially useful in determining whether or not an excess of sulfuric acid has been used as a coagulant oi an excess of copper or manganese is present in the rubber. Zuev and Kuz’niinskiI ( 2 5 3 ) found a close correlation betn een heat aging and light aging. The Institut F r a n p i s du Caoutchouc ( 9 4 ) conducted some experiments on the relative influence of different n ave lengths of radiation on the aging of rubber, and designated a specific high-pressure mercury vapor lamp to be uqed in the test appaiatus to simulate the action of sunlight. JVork fiom t n o different laboratories (99, 231) has shoJm that materials can migrate from one lubber sample to another in the same aging oven, and thus change the aging characteristics of the samples. Contamination may also take place from one set of samples to the next set in the same oven, if the air in the oven has not been exhausted in the meantime. .In appaiatus is described for converting existing air-circulating laboratory ovens for isolated aging of eight or more different compounds. Leigh-Dugmoie ( 1 2 6 ) g w e a critical historical survey of previous investigations into the causes of ozone cracking of rubber. Two distinct phenomena elist: the formation of cracks, and the subsequent growth of the cracks. €€ealso discussed the methods used for measuring the length and depth of cracks. Lleynard and Madelaine ( 1 4 3 ) described apparatus and tests for assessing the effects of ozone on rubber mixes. They used a dischaige ozonizer to produce the ozone, the concentration of which T Y ~ Sdetermined by the iodometric method. Smith and Gough ( 2 0 1 ) studied ozone cracking by cinematography. They developed a technique of time-elapse photomicrography, from vihich 11-as obtained a dynamic picture of the formation and growth of the cracks. Iialinsky and Kerkenthin (106) applied radiometric procedures to the evaluation of surface clacking of elastomers. The volume of the crack is measured radiometrically froin the volume of a radioactive poTvder included within the cracks. The use of radiations of different penetrability permits discrimination between volume and surface area of the cracks, the ratio of which is proportional to the average crack depth. Creed, Hill, and Breed ( 4 2 ) developed an apparatus for the accelerated evaluation of ozone-protective agents. Crabtree and Biggs ( 4 1 ) studied the cracking of stressed lubber and found that it was caused not only by ozone but also by free radicals TI hich could be formed by the photolysis of peroxides in the rubber. Cooperative tests from nine different laboratories shored poor agreement in results of tests of flex cracking and crack gioir t h on the DeMattia machine ( 3 5 ) . Some changes in the present ASTM procedures are therefore recommended. Hall, Conant, and Liska (76) developed a satisfactory apparatus and method for testing the long-time aging of elastomers under load. It more closely approximates service conditions than do most of the aging tests that have been reported. Schoon ( 1 9 2 ) described a method for determining the growth late of mold in raw natural rubber as a function of time. The samples are kept a t 98% relative humidity, and the area covered with mold is measured every 24 hours. Rogers and Heineman (178) reported that the conventional methods used for determining the aging characteristics of foam lubber offer little correlation and are not a true measure of the service life of the product. Hoviever, an oxrgen-absorption test and a compression change aging test a t 100” C. appear to give satisfactory evaluation of the aging of foam rubber in a ielatively short period of time.
L.4TEX Latex testing methods were the subject of an article by van den Tempel and Sinn (2501, in which they attempted to correlate
laboratory tests with the quality of the latex. The British Standards Institution (28) in 195.1-issued a revised edition of its booklet oii the cheinical and physical testing of rubber latex. The soap titration method for determining the average particle size of sjnthetic latexes has heen modified by Cockbain (381 so as t o make it applicable to natural rubber lateues. The method consists essentially of detei mining the distribution of an added deteigent betn-een the rubber and the aqueous phases a t pH 6 0, the total detergent concentration being just sufficient io1 the formation of micelles in the aqueous phase. Ogilby ( 1 5 9 ) detelmined the latex particle size of both natural and sj-nthetic rubbers by making titrations conductometrically with electrolytes a t pII levels low enough to inactivate the soap stabilizers. He also used electron microscopic techniques, as did van den Tempel (226), for measuring the particle size distribution Siqonoff, Messer, and HoiTland (157) used a method based OD Stokes’ Ian, which involves the use of an ordinary laboratorjcentrifuge. The method was shoivii t o be valid by comparing the results n-ith those obtained with light and electron microscopy. I n testing the mechanical stability of natural rubber latex by the usual procedure, the rod-dipping method for determining t h e end point has not proved veiy satisfactory. I n order t o improve on the method hleyer ( 1 4 2 ) made successive dips into the latev n ith narrow 80-mesh Monel screens, which were subsequentllremoved and washed free of adhering latex. The end point shows a rapid increase of adhering material. Because the usual mechanical stabilitr tests for natural rubber latex are not suitable for the more stable synthetic latexes, Maron and Ulevitch (131) developed a test which subjects the latex t o a shearing force exerted by a metal disk under load rotating in contact v, ith a polyethylene surface. The amount of coagulum formed in a given period of time is taken as a measure of the stability. Broese (52) described the nature of thixotropy and gave detailed instructions for measuring it, making use of a Couette rotation viscometer. Active fillers can be distinguished from inactive ones, but the ammonia content partially evens out t h e differences between latexes. Philpott and Sekar (166) described a method for measuring the volatile fatty acids in natural rubber latex. Resing ( 1 7 5 ) devised a chromatographic technique for studying the undesirable colored pigments in natural rubber latex. Medalia. Townsend, and Glover (140) devised a unique method for measuring the viet gel strength of coagulant-dipped films. It employs an absorbent paper of low \vet strength, impregnated n i t h calcium nitrate, as a base for the deposition of a latex film. After the latex dipping the film is rinsed in Eater, dried, and tested on a tensile machine designed for high elongations and low tensile strengths.
UNYULC.4NIZED RUBBER Under this general heading are discussed tests applied t o unvulcanized rubber and also some tests on vulcanized rubber. Only the tests on vulcanized rubber which are used to determine the properties of the raw product or the characteristics of the process of vulcanization are considered. , VISCOSITY AND PLASTICITY
Scott ( 1 9 4 ) gave an account of a conference held by the British Society of Rheology. KO single plastometer or viscometer can meet all the requirements of the rubber industrv. I n the rotation type the rate of shear is limited and the problems of slippage arise a t the surfaces. The parallel plate type does not seem t o be as suitable for fundaniental studies. I n another article Scott ( 1 9 6 ) compared the rotation, estrusion, and compression types of tests, and concluded that for rapid plastometry the parallel plate compression test is the moat suitable, provided that the test piece is precompresscd t o a small
V O L U M E 27, NO. 4, A P R I L 1 9 5 5 thickness for quicker heating to the test temperature before the load is applied H e also described the new IVallace modification of the Hoekstra instrument and discussed the fundamentals of all three tJpes of instruments (193). The extrusion type does not measure true plasticity becau-e of slippage between the rubher and the walls of the nozile. The rotation type is the best suited for research work because of the good control of variable‘. \I horlow ( 2 4 2 ) also compared the three types of instruments, and reported that there is no evidence of slippage in the Dillon extrusion plastometer and that this instrument may have advantages for testing carbon black-loaded stocks. The shearing-cone rotation viscometer is reported to he the best geneial-purpose instrument. Ecker ( 5 7 ) and coworkeis (58) desciibed the operations and gave references for the use of the Llarzetti, llooney, \\ illiams, and Defo plastometers. Baader (9) made an evaluatioii and comparison of test results x i t h the Defo and the 11 illiams plastometers and concluded that the former has much better discriminating pol\ er for gradations of breakdown and is faster in operation. In a study of the Mooney viscometer Decker and Roth (49) found that (1) viscosity readings decrease slightly as the depth of serrations of the rotor increases, ( 2 ) the serrations are unnecessary in testing commeicial lubbers, (3) viscosity readings increase with an increase of either the thickness or diameter of rotor, ( 4 ) the usual rotor speed of 2 r.p.m. is too high to characterize high-molecular-weight polymers, and ( 5 ) the amount of surface slippage depends on the type of rubber and the type of metal surface of rotor and die. Decker and Stiehler (60)showed further that in a >looney viscometei a n integral die and die holder transfers the heat much more rapidly than does the usual two-piece die arrangement. The integral unit theiefore lesds to a more unifoim and better control of temperature conditions if the temperature is measured in the dies. Decker (48)also described a procedure for adjusting the Mooney viscometer die closures, \% hich has led to considerable improvement in accuracy and reproducibility in dieclosure adlustments. Methods of adjusting and operating the 1Iooney viscometer were also reviewed by Taylor and Veith (627),who discussed the effects of die closures, calibration, temperature, dimensions of parts, and sample treatment. Braun (26) stated t h s t the velocitv gradient in the plate and cone viicometers is not constant, as assumed by Piper and Scott, and he gave a formula to correct for this error. Fensom (62. 63) found that the hlooney viscometer discloses much information on the structure of a high polymer. Evidence is shown of a relationship between the time interval from the start of shearing t o the primary peak and the amount of free branching in the polymer. Scott and Khorlon (197) showed that lack of precision in the preparation of plasticity test pieces for the psrallel-plate compression-type plastometers will give variable results of measurement, and they described the technique of preparing test specimens for the TT-illiams and the Defo instruments. Linhorst (229) designed a new compiession plastometer which he claims can be adapted to the testing of relatively hard or soft materials by simply changing the temperature or pressure. Bergen and Patterson (20) described a translational viscometer Jvhich can be used for measuring rubber latex. They found that latex of high solids content appears to be completely Kewtonian, while the latex of synthetic rubber of 28% total solids content appeals to he non-Sewtonian. \ ULCAYIZATIOh-
Stiehler and 110th (216) compared several methods recently proposed for the evaluation of the rate of cure of Hevea rubbers, and found essential agreement among (1) conclusions rewhed from strain arid modulus data, ( 2 ) changes in Mooney viscohity during the initial peiiod of vulcanization, and (3) measurements of stress and elongation a t failure.
593 Veith (236) measured the state of cure of three classes of technically classified natural rubber by several methods rvhich involve stress-strain testing and Nooney scorch testing. All methods placed the classes of rubber in the same order. The cure rate was the slowest for rubbers having lowest noiirubber content, and fastest for those having the highest nonrubber content. Cretin ( 4 3 )found a negative correlation between the vulcanization characteristic of a natural rubber, as measured by modulus test, and the dry rubber content of the latex; and a positive C O I relation lietween the modulus and the nitrogen content of the latex. These correlations are a t least true if th3 latev is prod w e d from the same general area and processed by a standard met hod. SVhorloi? (243)shoir ed that rotation and compression plastometers give equivalent data in evaluating cure characteiistics, provided that allowance is made for any delay in the heating of test specimens used for the compression test. Sperberg (206, 207) found that the relaxed compression set test provides data n hich are highly indicative of a specific state of cure. Honda ( 8 9 ) determined the time of scorching and the velocity coefficient of scorching of rubber by measurements obtained xith the Griffith extrusion plastometer. Taylor, Clark, and Ball (226) found that the presence of nioisture. even in very small amounts, has an effect on the rate of cure. Equivslent moisture contents may not have the same effect on all compounds, and thus the presence of moisture may make difficult the task of evaluating the curing characteristics of a rubber. GRADI\G AYD EVALU4TIOh
Sears and Miller ( 1 9 8 ) of the Rubber Manufacturers’ Association prepared a booklet which describes the RlIA-type samples of crude natural rubber, and discusses purchase of natural rubber on the basis of visual inspection only. 1-ervloet and Toothout ( 2 3 8 ) , however, recommend that the R M h types of rubber be clasqified further as to superclean, clean, and general purpose, depending on the amount of “harmful” dirt present in the rubber. LeBras (124)says that the producers know how t o produce a more homogeneous rubber, but are not now doing so because their rubber would not fit into the present visual classification scheme of the association. The more recent technical classification scheme was therefore designed and adopted as a n intermediate stage b e b e e n the appearance and pei formance criteria as a basis for marketing rubber. Several articles (Fjl 23, 250, 15f, 286, 252) in the past 2 years have summarized the methods of technically classifying crude natural rubber and the progress which has been made. The International Rubber Research Board has continued its two series of publications, the Sews Sheets (262, 153, 164) and the rse7.s’ Infornzatzon Czrculars (155, 166), in which they keep rubber technicians informed on technically classified rubber. The three classifications based on AIooney viscosity were dropped, and the rubber is now graded only by its tensile properties ( 1 5 3 ) . High-elongation modulus tests were found to be not as reproducible as lov,v-elongation tests when compared under highly different climatic conditions ( l 5 3 ) , and modulus testing has since been shifting toward the lower elongations. Because of complaints from consumers that the properties of the classified rubbers too often fall outside the specification limits, these limits viere changed so that now on the average no more than 1% of the rubber \Till be misclassified (164). T’ervloet (837) and Arentzen and Zeehuisen (6) report that interlaboratory variations in test results are very small in the tropics, even for unconditioned samples, b u t the results may be very different from those obtained in the coder laboratories of Europe and the United States. These differences may actually cause a shift in the classification of the rubber. For this reason, it is advisable to have a standard natural rubher available in all testing laboratoiies, so that adjustments can be made in testing
ANALYTICAL CHEMISTRY
594 procedures which will place all the laboratories on an equivalent basis. McCulm (134) showed that a standard uniform rubber could be made. Wood ( 2 4 8 ) found large permanent increases in Mooney viscosity of the order of 30 or 40 units when freshly prepared raw natural rubber is stored under conditions of low relative humidity, especially below 30%.
VULCANIZATE S STATIC TEhSIOK A h D COMPRESSION
Gehman and Clifford ( 7 0 ) employed the Instron tester to explore new opportunities for the evaluation of rubber on the basis of stress-strain characteristics. The instrument was found to be excellent for research work, but not so adaptable to routine testing. Cooper and Kersker (40) described a technique of preparing tension-testing samples of such high quality that there should be very little chance of low tensile results caused by faulty specimens. Labbe and Leeper ( 1 2 1 ) described a precise method for determining the width of a dumbbell test specimen used for tension testing. I t requires the measurement of the thickness of the specimen by the usual type of gage, the weight of the sample, and the specific gravity of the rubber. According to Best (21), the molded strain test specimens used in the B.R.P.R.rl. tester are likely to vary in cross-section area, and a simple method is given for correcting the resulting errors. Koide, Kubota, and Shinoda ( 1 1 2 ) recommend that in determining the “best” value for tensile strength the average of the four highest values be used when five specimens are tested, and when three specimens are tested the average of the two highest be used. -4 simple device was described by Stafford (208) in which a steel spring of known characteristics is used as a “test piece” for calibrating the tensile testing machine. I t enables a complete calibration under actual operating conditions. Stubbs (218) devised self-closing grips of the wedge type, which are used for holding rubber specimens in tension-testing instruments. Eller, Gondek, and Chatten (60) modified and improved their earlier machine for buffing vulcanized rubber specimens to uniform thickness before testing. It is claimed that rough strips of rubber can be buffed in this machine to a predetermined thickness mith an over-all tolerance of 0.001 inch. A recording stress relaxometer has been devised by McLoughlin ( I % ) , which automatically maintains the strain in the sample constant to within 0.001 inch during the complete relaxation of stress. Labbe and Phillips (122) developed an instrument for determining the rate of relaxation under compression or shear. I t can also measure the recovery after partial or complete release of the load. The instrument can be used in the temperature range from -70” to 158” F. From a mathematical examination of the influence of crosssectional dimensions of test pieces of rubber used in testing for torsional deformation Gent ( 7 3 ) recommended that the test pieces be either square or circular in cross section if precise measurements are desired for the modulus of rigidity. DYNA.MIC PROPERTIES
The Goodrich flexometer u a s studied by Labbe ( 1 2 0 ) , who found that the lack of reproducibility among laboratories is probably caused by the failure to adjust precisely the throw deflection of the test specimen. iictually, a change in load has less effect on the temperature rise and permanent set than any other single factor studied. Stechert ( 2 0 9 ) described a simple test for flexural rigidity. He also found that the test is sensitive to small differences in flexibility. Davies ( 4 5 ) described a new testing machine which records autographic hysteresis loops and has an exceptionally broad frequency range. He found it very useful in studying the effect of
frequenrj on the behavior of rubber under cyclic deformation. Davies ( 4 6 ) also pointed out that the two terms in current use for specifying hysteresis, “percentage damping’’ and “resilience,” are complementary only if the hysteresis loop starts from zero strain. When a cycle is superimposed on a statir strain, the terms are no longer complementary. Kainradl and Haendler (101) classified fatigue tests, and stated that the rating of rubber compounds for heat build-up and fatigue life depends on the conditions of dynamic stressing, whether comparisons are made at constant amplitude or constant dynamic load. Dynamic measurements at room temperature are not adequate t3 predict heat buildup because of dependence of dynamic properties on temperature and amplitude. Ecker ( 5 6 ) made a survey of instruments used for dynamic vibration testing, and described a new apparatus which enables measurements to be made of the plastic-elastic behavior of high polymers in the elastic range. Ourednik (160) designed an apparatus in which a cylindrical specimen is subjected to periodic compression, and the heat generated in the sample is measured. Gehman and Clifford ( 7 1 ) constructed a machine which they used to study the fatigue of rubber when subjected to biaxial strains of controlled amplitude and phase relationships. It wm thought that this type of test would more nearly compare with service conditions than the relatively simple extension and retraction cycle usually used in laboratory testing. A rotating drum camera u a s used by Muster and Volterra ( 1 4 7 ) for recording impact loading deformations. The camera records displacement-time data for short cylindrical specimens of rubberlike materials which are subjected to compressive impact loadings which last from 0.005 to 0.02 second. HARDNESS
Kruse ( 1 1 6 ) and Roy0 (180) discussed problems in connection u ith the hardness testing of rubberlike materials and made tentative suggestions for finding a common basis for a uniform method of test which could be applicable to all types of materials. Lichtman and Chatten (128) described a modification of the Admiralty rubber meter, which is particularly well adapted to precise measurements a t low temperatures. A new bench-type hardness meter (93, 181) has been announced in which a dead load is applied to a spherical indentor by weights, not springs. rin electric vibrator is built into the base to prevent friction and sticking in the machine. Several new meters (1, 186, 183, 191, 221) have been developed which measure the hardness of foam or sponge rubbers. ABRASION
The abrasion properties of vulcanized rubber have been the subject of considerable review and discussion (61, 100, 174, 188 190, 202, 203). I n general, the authors have dealt with the different types of testing machines; the comparison of their results with actual performance; the various factors which affect wear, such as temperature, coefficient of friction, and the hardness and elastic modulus of the rubber compound; the reproducibility of results; and the mechanism of abrasion. Kainradl ( 1 0 0 ) does not believe that a neiv type of abrasion instrument is needed, but that the present methods of test should be refined and better methods of interpreting their results found. Scheele and Hillmer ( 1 9 0 )point out the necessity of making many more than the usual two to five individual abrasion tests on a particular compound in order to obtain a reliable average of the results. Schallamach (188) developed an abrasion test in which he makes use of a needle bearing on a cylindrical test piece. The nature and direction of the abrasion patterns are examined microscopically and evaluated. Herzog and Burton (85) described a new machine which incorporates a rotating disk with an emery paper surface against which the rubber samples are abraded. Powell and Gough (169) described in detail the modification
V O L U M E 27, NO. 4, A P R I L 1 9 5 5 of a constant-slip machine so as to make it conform with the constant-power principle. Baird and Svetlik ( 1 0 ) came to the conclusion that in order to correlate laboratory data with road tests the severity of service must be known. Several investigators (52, 92, 172, 204, 240, 244) studied the wear on rubber and its relation to friction. Ratner ( 1 7 2 ) concluded that the index of wear depends on the load and is inversely proportional to the friction coefficient of rubber. He also stated that the coefficientof wear is constant for a given type of rubber. Denny ( 5 2 ) believes that a t very low loads the friction is very nearly proportional to the load, while a t very high loads the friction remains practically constant because the contact between the specimen and the track is complete. Rilkinson (244) described and illustrated a new method for evaluating the friction behavior of tire tread compounds. ADHESION
Methods and conditions of adhesion testing were outlined and discussed critically by Swire and Hardy ( $ 2 0 ) and the common faults in the preparation of test specimens were shown. Eller (59) described an inexpensive attachment to the Scott tensile tester which will measure the force required to separate a rubber strip from a metal base, the separating force being applied at right angles to the base. An autographic record can be made of the adhesive strength over the whole length of specimen. Pickup ( 1 6 7 ) developed a machine having a high natural frequency and appropriate damping in order to measure and record the fluctuations which take place in the usual constant-speed stripping tests. LOW-TEMPERATURE TESTING
The literature on low-temperature test methods has been reviewed by Laurent (125), who discusses the testing for brittleness and for stiffness. Buist and Stafford ( 3 6 ) draw attention to the effects of continued exposures of elastomers to low temperatures over long periods of time. I t must be remembered that crystallization is a function of time. Radi and Britt ( 1 7 1 ) made some low-temperature retraction studies of natural and synthetic rubbers over a wide range of temperature. They measured retraction a t constant temperature, and plotted the percentage of retraction against the temperature at which retraction occurs. A deviation or dip in the curve from the sigmoid is an indication of the rate of crystallization at a specific temperature. I t is believed to represent optimum thermal conditions for crystallization. Labbe ( 1 1 9 ) studied hardness testing of elastomers a t low temperatures using various instruments, and concluded that the spring-actuated instruments are desirable because they allow instantaneous readings. The Pusey and Jones and Olson instruments gave friction betueen moving parts at these low temperatures. It appears that the Materials Laboratory (modified British Admiralty) Indentometer is the most satisfactory of the dead-weight hardness testers for use at lox temperatures. Morris and Barrett ( 1 4 5 ) find that hardness tests made on vulcanizates after conditioning for short periods of time at low temperatures are not always a true measure of the hardness which the vulcanizates will attain in the Arctic. Sinclair and Griffis ( 2 0 0 ) describe the use of a modified Gehman torsional apparatus utilizing a liquid medium that measures accurately and rapidly the relative stiffening of rubber and rubberlike materials a t low temperatures. Conant ( 3 9 ) finds that the different tests show good agreement in the temperature region of greatest technical importance, \+here the rubber is changing from the ordinary flexible state to a hard stiff material. hlarei ( 1 3 6 ) developed a procedure for obtaining the vitrification temperature of a rubberlike material by measuring the deformation of a polymer under compression. Belfer (19) found that a quick-acting wrench compression-set apparatus was superior a t low temperatures to the screw-type apparatus for measuring
595 short-time (10 seconds) compression set with respect to duplication of results, accuracy, and ease of manipulation. TIRE TESTING
Williams and Clifton ( 2 4 5 ) gave a broad review of the newer techniques used in tire testing, in which they included indoor machine methods which attempt to simulate conditions on the road. Kern ( 1 0 4 ) stated that in order to have good abrasion properties the work of friction should be lo^, and he discussed techniques for measuring the work of friction in the evaluation of abrasion. Stiehler ( 2 1 2 , 2 1 4 )described ne- apparatus and methods used a t the Sational Bureau of Standards for the testing of automobile and truck tires. Tests are made for (1) poner loss and running temperature, (2) endurance or fatigue resistance, (3) energy of rupture (plunger test), (4) growth of tread cuts or cracks, and (5) tire life (road tests). A new machine is being developed which is designed to evaluate endurance, running temperature, cut groa th, and tread wear in a single test. Stiehler, Richey, and 3landel(216) described a method for measuring the tread wear of commercial tires, in which the tires are neighed a t intervals of wear. Reliable results are obtained from this method after 4000 to 6000 miles of operation. Their tests are designed statistically so as to compensate for inequalities of the wear of tires on different wheels. Comparisons of this weighing method are made u i t h the method which measures the depth of groove. The former method will give reliable results with a small number of miles on the tire, while the latter method will not. Prat (170) compared Stiehler's method with two other methods and stated that all methods necessitate strict discipline if reliable results are to be obtained. Morris, Stambaugh, and Gehman ( 1 4 6 ) gave a detailed description of an ultrasonic method of tire inspection. Internal flaws such aa ply separation can be detected by measuring the attenuation of high-frequency sound transmitted through the tire. Homing ( 9 0 ) described and illustrated a portable instrument which measures tire temperatures within 2 to 3 seconds. The reading is claimed to be accurate to within 0.1" F., and the operator has control over the depth which the needle thermocouple is inserted into the tire. Axe ( 8 ) described some tests and the refrigeration methods used by the Dunlop Rubber Co. for lowtemperature tire evaluation. MISCELLANEOUS
A rapid method for measuring the specific gravity of rubber compounds, described by Gundavda ( 7 5 ) , is claimed to be very good for routine measurements. It makes use of a tube of liquid with a density gradient. Such tubes are easily prepared, and the density gradient remains stable for many weeks. Linhorst ( 1 3 0 ) devised a new gravitometer which employs TTater and two springs for measuring the specific gravity of rubber compounds. I t s speed and accuracy make it useful as a routine laboratory testing instrument. Reynolds ( 1 7 6 ) constructed a special test cell using standard -4STM electrodes for measuring the dielectric strength of small specimens of elastomeric materials in air. The cell is constructed so that samples can be measured in a stretched or unstretched state. Pfestorf ( 1 6 5 ) makes use of conductivity measurements of rubber compounds to check the composition in the factory. The method is not only accurate for its purpose, but also very fast. Patton and Hatfield (79, 164) made use of ultrasonic techniques to detect air films as faults in tires. Ultrasonics can also be used for measuring rubber thickness when only one side is available. Low frequency 1% aves are used because of the high absorption of rubber and the scattering caused by the fabric. Heughan and Sproule ( 8 6 ) found that bond f l a w and voids inside a flat-bonded specimen can also be detected by ultrasonic waves as long as the specimen is strained. Spath ( 2 0 5 ) also described the value of ultrasonic techniques in determining f l a w in tires and in metal bonds.
596
ANALYTICAL CHEMISTRY
Todd and Tramutt ( 2 3 2 ) developed a flame photometric determination for the rubber solids deposited on cords and fabrics. The method is based on folloning the sodium content of the system. Czuha ( 4 4 ) described and illustrated a method for measuring the air permeability by diffusion tests, using a modified Warburg diffusion apparatus. Kirshenbaum, Streng, and Dunlap (109) described an isotopic procedure and apparatus for the rapid and accurate measurement of the permeability of gases through elastomers. They made use of radioactively labeled gases containing carbon-14. -4uerbach and Gehman ( 7 ) employed radioactive sulfur-35 isotope in a new method for measuring the solubility and diffusivity of sulfur in rubber. By this method hlooming characteristics may also be studied. Geldof ( 7 2 ) presented an illustrated review and discussion of methods used for measuring physical properties important in evaluating the qualities of rubber soles, heels, and shoes. Martin (138) studied the control of errors in physical test calculations in the rubber laboratory by introducing the statistical quality control system. The success of the method xvas shonn by the gradual reduction of the incidence of errors over a period of time and a remarkable stability in the level from xeek to week. He also showed (139) how simple statistical quality control charts may be used to control the variability in results arising from mixing, compounding, Vulcanizing, and testing. Linnig, Mandel, and Peterson (131)devised a plan for studying the accuracy and precision of an analytical procedure. They illustrated their plai, by the application of analytical tests on synthetic rubber. It involves the segregation of relative-tj-pe and constant-type systematic errors. Stiehler (213) has shon-n that statistics has become a very important tool in the testing of rubber. It should be used in (1) planning and designing the experiment, ( 2 ) assessing the validity of data, ( 3 ) interpreting the experimental results, and (4)comparing the relative merits of the methods of test. LITERATURE CITED
(1) Am. Soc. Testing Materials, A S T M Bull., 197, 81-2 (1954). ( 2 ) Am. SOC. Testing LIaterials, Committee D-11 on Rubber
(3) (4) (5) (6)
(7) (8)
(9) (10)
(11) (12) (13) (14) (15) (16) (17) (18)
(19) (20)
and Rubberlike Materials, “Standards on Rubber Products. Methods of Testing and Specifications,” 1953. Am. SOC.Testing AIaterials, Committee D-11 on Rubber and Rubberlike Materials, “1953 Supplement t o Book of ASThI Standards, Part 6,” pp. 67-70. Am. Soc. Testing Materials, “Symposium on Recent Derelopments in the Evaluation of Katural Rubber,” Spec. Tech. Pub. 136 (1953). brentzen, A. G. J., and Breejen, T. den, Arch. Rubbercult. 12’ed. Indie, 29, 1-9. 10-30 (1952). hrentzen, A. G. J., and Zeehuisen, J. J., Ibid., Extra KO, 2, 211-14 (1953). Auerbach, Irving, and Gehman, S. D., ds.4~.CHEW, 26, 685-690 (1954) ; Rubber Chem. and Technol., 27, 773-83 (1954). Axe, R., Aeroplane, 83, 694-5 (1952). Baader, T., Kautschuk u . Gummi, 6 , TT’T210-13, WT235-6, WT 258-60 (1953); 7, WT 15-17 (1954). Baird, C. C., and Svetlik, J. V., India Rubber World, 127, 363-4 (1952). Bauminger, B. B., and Poulton, F. C. J., Trans. Inst. Rubber Ind., 29, 100-13 (1953); Rubber Chem. and Technol., 26, 986-98 (1953). Beaufils, E. R., Third Rubber Technology Conf., London, June 1954, Preprint 84, Institution of Rubber Industry, London. Bekkedahl, Korman, -im. Soc. Testing Materials, Spec. Tech. Publ. 136. 1-5 (1953). Bekkedahl, korman, A i v a ~CHEX., . 22,253-64 (1950). Ibid., 23, 243-53 (1951). I b i d . , 24, 279-93 (1952). Ibid., 25, 54-64 (1953). Bekkedahl, Korman, and Stiehler, R. D., Ibid., 21, 266-78 (1949). Belfer, J. I., A S T M Bull. 190, 43-5 (1953). Bergen, J. T., and Patterson, IT., Jr., J . Appl. Phys., 24, 712-19 (1953).
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