elastomers - ACS Publications


University of Southern California, Los Angeles 7, Calif. All bids ... 4 processability index for smoked sheet rubber based on the ... oxide) prevents ...
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ELASTOMERS H-4RRY L. FISHER University of Southern California, Los Angeles 7, Calif.

A l l bids for the 27 U. S. Government synthetic rubber plants had to be received by bray 27,1954, and these are now being considered by the Rubber Producing Facilities Disposal Commission. Judging from the preliminary report of the commission there is much interest in this disposal; the bids probably exceed expectation. Announcement has been made that synthetic rubbers will be made by three companies in England, namely, Dunlop Rubber Co., Ltd., 2000 tons annually, Imperial Chemical Industries, Ltd., 10,000 tons, and Monsanto Chemicals, Ltd., 4000 tons. The price of natural rubber is still slightly less than that of GR-S, and this year the amount of natural rubber imported will probably be not much less than the amount of GR-S manufactured. There are many new synthetic rubbers and methods of physical testing and analysis.

HEN the Office of Rubber Director was established late in 1942, a plan for research and development was devised that has been carried out a t a number of universities, institutes, and the Gwernment Laboratories in Akron. No arrangements have been made for its continuance beyond 1956, and the question has been discussed from the standpoint of both industry (97) and the universities (37, 126). The 8th Foundation Lecture of the Institution of the Rubber Industry consists of a review and discussion with emphasis on the importance of fundamental research and technological developments in the progress of the industry (IS). A "Glossary of Terms Used in the Mechanical Rubber Goods Industry" has been compiled by the Technical Committee of the Mechanical Rubber Goods Manufacturers Division of The Rubber Manufacturers Association (68, f 14). EATURAL RUBBER

Before the British decidrd not to use the Mooney viscometer in arranging the technically classified rubber, i t was suggested in this country that the Mooney viscometer be employed to classify natural rubber according to its cure rate. If this is done, it will be necessary to define the degree of accuracy desired. In this work the nonrubber content was obtained from many samples and reported (149). 4 processability index for smoked sheet rubber based on the time required to reduce the viscosity to 40 Mooney units under prescribed experimental conditions is proposed (144). On mastication in a Brabender Plastograph a t a jacket temperature of 150' C., the viscosity ( 7 ) is a function of time ( t ) , as described by the equation 17

+

?I

6c - k f

where 7 6 is the initial viscosity, and k is an empirical breakdown constant. The index is dependent on both k and 7 b . Foreign material in natural rubber can be separated quantitatively by melting the rubber in aviation oil a t 255' C. or dissolving the rubber in solvent naphtha a t 120' to 125' C. and straining through a fine screen (134). Either method removes dirt in the range of 0.01 to 1%. Gutta from chicle is trans-polyisoprene with a molecular weight near 17,000 and exists in two polymorphic forms (76). Below 66.5' C . the stable form is @-gutta with an x-ray repeating unit of 8.9 A. Between 66.5 and 72' C., a-and P-gutta exist together. Above 72", 0-gutta of 4.7 A. is stable. Experimental data indicate that the vulcanization rate of synthetic rubber a t the various stages of the process is not

affected by the molecular weight of the starting material as shown by chemically combined sulfur determined periodically during vulcanization (96). The samples were fractions of butadiene styrene rubber with average molecular weights from 100,000 to 1,170,000. The following percentages of sulfur are necessary for the initial formation of the space lattice for fractions of various molecular weights: 0.18 for 700,000; 0.31 for 500,000; 1.2 for 140,000; and 1.8 for 100,000. The stiffening of uncured natural rubber by a benzidine-type agent resembles partial vulcanization as manifested by reduced plasticity, markedly increased tensile strength, and formation of insoluble gel (103). An affinity exists between the rubber and benzidine since only 10% of the benzidine could be extracted by ethanol or alcoholic HCI from a rubber sample containing 0.3% benzidine. An improved bale marking paint for crude natural rubber, which retains its legibility under simulated service conditions during shipment, has been developed (66). It consists of a mineral turpentine solution of crude rubber and damar resin to which is added Rhodamine S and Yellow OS in oleic acid and benzene. The dye mixture gives a red color and exhibits strong fluorescence, thus having the advantage of being identified by ultraviolet light. The effect of moisture on the cure rate of natural rubber has shown that, in the presence of mercaptobenzothiazole or its disulfide, the scorch time decreased and the rate increased with increase in moisture content over the range of 0.05 to 1.6% (143). These results may not be applied to rubber compounds generally. I n 5 study of the reinforcing action of alumina the finer particle size resulted in higher tensile strength, elongation, and tear strength, and lower modulus, hardness, and rebound (186). Work on cross linking in natural rubber vulcanizates has shon-n that considerable degrada,tion occurred in the sulfur vulcanization accelerated with mercaptobenzothiazole (1). The amount of zinc sulfide produced is a measure of the degradation, and the original amount of cross linking is equal to the sum of the zinc sulfide formed and the experimentally determined cross linking. The over-all stoichiometry of the cross-linking reaction is such that one atom of zinc and two atoms of sulfur are involved for each cross link produced. Katural rubber in solution degrades in the absence of oxygen (156) and this is the main cause of the decrease in viscosity of rubber solutions on storage. The rate of degradation is proportional to the rubber concentration and is independent of the purity and molecular weight of the rubber initially. Evidence is advanced for this phenomenon being associated with f he gelling of solid rubber on storage. D P R (depolymerized natural rubber) is available in a standard low viscosity grade, a standard high viscosity grade, and a special grade for mixing with asphalt and polyethylene (99). I t is compatible with many resins. Pulverized quicklime (calcium oxide) prevents porosity during the hot air cure a t atmospheric pressure used for most D P R stocks.

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INDUSTRIAL AND ENGINEERING CHEMISTRY

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Vol. 46,No. 10

An interesting review of most types of chemical reactions involving rubber has been published (70). When carried to completion the reaction of chloiirie gas with natural rubber in solution takes place in four stages, substitutive attack accompanied by cyclization, predominantly substitutive, addition of chlorine to double bonds, and only purely substitutive (108). These results are substantiated by ozonolysis.

of the carboxjl group with zinc oxide and no sulfur (15). Thc elastomer containing mercaptoacetic acid in combination caii be press-cuied in 20 niiiiutcs at 326' F. to provide a t c n d e strength of 2100 Ib. per sq. inch and on elongation OI3T07,. Sorbic acid is copolyniciized with butadiene, and isobutj lcne and acrylyl chloride ran be copolymerized a t low temperat tirc~s; all give similar products.

NEW MONOMERS AND NEW SYNTHETIC RUBBERS

OTHER SYNTHETIC RUBBERS

The manufacture of s t p e n e involves benzene and ethylene whereas a mixture of m- arid p-methylst'yrene is prepared from toluene and ethylene (28). The product of methyl- and dimcthylstyrene by the diarylet,hane process has been described in which toluene or xylenes and acetylene and catalytic vapor phase cracking of the products are used to form the corresponding substituted styrenes. Mechanical tests show that Glt-S types with high Xooney viscosity are inherently better rubbers than those of normal Mooney viscosity because the mechanical properties such as tensile strength, rebound, compression set,, and rcsistance t'o creep improve as the Mooney viscosity of the polymer increases (54). Moreover, there is no real loss in physical properties even when the high Mooney rubber is compounded with large proportions of low cost oil and carbon black in order to keep the necessary balance bciwccn good physical properties and processabilitg. Ring seals for hydraulic and pneumatic systems require elastomers that will remain strong and evteneible while rcsisting attack by the fluids that they are sea,ling (1'7). The high temperature limits of the various commercial e!astonicrs with present compounding techniques are approximately

Thiols contaiiiiiig polar groups were introduced to polybutadiene to improve the oil resistance with the minimum deleterious effect on low temperature properties (86). The products were similar to normal polybutadiene in low teniperat,ure propertics but their improvement in oil resist'ance was less than !Todd have been obtained with polar groups introduced by copolymerization as in nitrile rubbers. A4nalysesof polybutadiene, prepared a t temperatures to 270" G. in propylene carbonate with di-tert-butyl peroxide as initiator, showed that the percentage of 1J-addition remains approximately constant a t 19 f 2% from 100" t'o 270° C., and t,hat the percryitage of cis polymer increases gradually from -20" t'o +130a C. and then levels off to a constant value of about 36% to 270' C.

Silicone Polyacryliu Xitrile ruhher Neoprene Butyl

em

Natural rubbei Thiokol

'renip., 7i5 200 lis 180 150 140 1ao 120

c

Copolymers of but'adienc v-ere preparcd with o-methyl-, a-ethyl-, and a-n-pentylacrylonitrile in the charge ratio 95 :5 , S5:15, and 7 5 : 2 5 (87). Their vulcanizates had the best oil resistance with the methyl derivative and least, n-ith the npentyl derivative. The low temperature properties were best in the reverse order. These alkylacrylonitriles do not represent improvement,s over the acrylonitrile copolymers. Copolymers of butadiene and cinnamaldehyde (90: lo), cinnamic acid, methyl cinnamate, or trans-cinnamonitrile are generally equivalent t o standard GR-8 (86). The copolymer with the nitrile s h o w no improvement' in oil resistance over GR-S. Chemigum-SL is an elastomeric polyester-urethan, which is similar chemically and when cured exhibits properties very similar t,o Vulcollan ( I W C ) . I t is prepared by chain extcnsion with a diisocj-anate of the polyester from glycols and dibasic acids and results in an elastomer storable for 6 months. Thc resistance of the vulcanized product t,o high temperatures, hot water, and steam is poor, a,nd it has a tendency to harden a t low temperatures. Oxygen and ozone absorption is very low. It, has high tensile strength, low hysteresis, and high resistance to abrasion. Chemigum-SL is now in the form of Airfoam SL, xhich is resistant to fire and to deterioration from sunlight, oil, and grease (116). A new synthetic rubber is Kel-F fluorocarbon elastomer, which is unusually resistant to corrosive chemicals, hydrocarbon fuels, oxygen, ozone, and sunlight (117). A polybutadiene can be combined chemically with maleic acid or anhydride, or with mercaptoacetic acid, in the presence of benzoyl peroxide, and t,he elastomer thus produced with only 0.30 proportioii of a carboxyl group can be cured by the reaction

(89).

Water absorption of vulcanized neoprene is much loivai 11m lead oxides are used as vulcanizing agents in place of the usual niagnesium and zinc oxides (80). Litharge is too scorchy a vulcanizing agent for general use but red lead (Pb304)v-orBs very well. Infrared absorption band assignments have been niatle for aliphatic sulfonyl chlorides and other sulfonic acid derivatives (1%). Comparisons with band assignments of chlorosulfonnted polyethj-lenc have given fairly conclusive proof for the proposed mechanism of metal sulfonate cross linking with hydrolgsir of the sulfonyl chloride group as the first st,ep. BUTYL RUBBER

The heat treatment of Butyl rubber arid carbon black improves tensile strength, tear strength, abrasion redistance, and (46). Only certain carbon blacks can be used, but si sults with other carbon blacks are obtained using such additivcs as dinitrosobenzene, quinonedioxime, and sulfur, all of which art? vulcanizing agents. The elect,rical resistance can hc increasid as much as 10,000,000 t,imes. The modulus of a standard heat trcated But,yl rubber vulcaiiizate containing 50 parts of channel carbon black incrcascs linearly with the amount' of unsaturation in she Butyl rubber ovcr the range 0.9-2.0 mole 70unsaturation (107). TVhen the Butyl rubber-channel blaclr mixtures are not heat treated, the same linear relation is observed, but the moduli are consistently IowcTr. The damping is greater for vulcanixates from the more unsilturated polymers in tlie lo^ molecular weight average, but in the high molecular weight range, da,mping is iiidcpendent' of unsaturaf ion. I n any case, heat treatmcnt invariably increases the dampirig. The report'ed enhancement of physical properties of But,yl rubber vulcanizatcs that results after heat treatment of the rubber containing channel black depends on the presc~rixof oxygen on the surface of thc black (166). -4 good review is given of Butyl rubber, it,s maniilacture, vulcanization, change in propertirs hy heat treatment, especially of mixtures with carbon , and applications (146) treatment lowers the hj-st and improves abrasion re and other properties (100). Moderately improved Butyl t'irc curing-bag formulations w n be made with Philblack A, benzothiazyl disulfide-quinoncdiosilrle acceleration, and 25 parts of zinc oxide, in addition to sulfur (11.9). The desired heat treatment effect can be produced on mixtures of Butyl rubber a,nd cha,nncl or furnace carbon blacks, even if t,he furnace blacks have not received the special oxidation twat-

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INDUSTRIAL AND ENGINEERING CHEMISTRY

ment, by introducing certain promoters into the Butyl rubbercarbon black mixtures (46). Elementary sulfur is particularly effective in this respect. Lead and antimony sulfides, mercaptobenzothiazole, benzothiazolyl disulfide, and tetramethylthiuram disulfide are also effective. The relative ease of incorporating typical pigments in Butyl rubber and their effects on processing have been discussed, and data on curing in air and steam have been presented (131). Combinations of accelerators and vulcanizing agents for specific purposes are recommended, with particular references to resistance to abrasion, permeability to gases, performance a t low temperatures, and ozone resistance. SILICONE RUBBERS

'

Silicone rubbers and lubricants appear to be nontoxic when used in tissue culture techniques (86). This is in contrast to vulcanized natural rubber tubing. Three commercial silicone rubbers have been desciibed and compounded. SE-76 is a highly linear methylsiloxane polymer with high viscosity ( lo7 centistokes) and molecular weight (500,000) and flexible above -55' C. (101). SE-51 is a siloxane polynier containing both methyl and phenyl substituents. The disorder introduced by the phenyl groups lowers the minimum temperature for flexibility to -85" C. SE-30 is a methylsilicone polymer with low compression set a t high temperatures and minimum shrinkage during cure. Of the 55 pigments tested in compounding silicone rubber a commercial grade of alumina and a calcined diatomaceous earth gave the highest tensile strengths and elongations without a t the same time undue hardening ( 4 7 ) . Electron micrographs of particle size and distiibution indicate that agglomeration of pigment is the rule rather than the exception. Data from determination of dynamic modulus over the temperature range from -80" to 25" C. and frequency range of 100 to 3500 cycles per min. for natural rubber and silicone rubber vulcanizates for various conditions of strain demonstrated that the vibration-isolating properties of silicone rubbers are maintained over the entire temperature range (98). Natural rubber lost these properties in the lower half of the temperature range. A hard, brittle resin obtained from 2-thienyl-trichlorosilane (after hydrolysis), could be heated to near red heat before its decomposition became rapid (227). POLYMERIZATION

In GR-S there is little or no branching a t low conversions, the degi ee of branching increases markedly with increasing conversion, and branching occurs to the greatest degree in the species of highest molecular weight (23). Branching in a GR-S polymer is repressed as the temperature of polymerization is decreased; the lower the temperature the higher is the molecular weight a t which branching is detectable and the lower is the degree of branching a t a given molecular weight (24). In the 15" C. conversion no branching was detectable a t any molecular weight. The use of high initiation temperatures in the holdup line, to 80' F., appears to be a commercially feasible method of increasing the capacity of continuous emulsion polymerization systems (38). This initiation for a few minutes prior to continued polymerization at 41' F. for 15 hours not only increased the conversion but resulted in polymers with properties that were equivalent to those of stock made entirely a t 41 O F. t o the same conversion and viscosity levels. In the dextrose-free recipe a t 41" F. for the copolymerization of butadiene and styrene, methyl oleate peroxide and methyl linoleate peroxide gave higher polymerization rates and conversions than cumene hydroperoxide, and as high rates of conversion as p-menthane hydroperoxide (139). With the peroxide-dextrose recipe a t 122" F., a t both low and high dextrose levels, about one

2069

half as much peroxide on a molar basis was used to accomplish the same results. Antifreeze diluents for polymerization of aqueous emulsions of GR-S below 0' C. are glycerol, ethylene glycol, propylene glycol, diethylene glycol, formamide, ethanolamine, diethanolamine, and triethanolamine, and all of them have the required solubility properties (68). Glycerol can be used as a total replacement for water. Methanol has different solubility properties, gave low conversions, and, unlike the agents mentioned, formed latices with very poor foam stability. Anhydrous animonia was used a t -18" C., and the polymers formed vulcanizates that had considerably higher tensile strengths than standaid GR-S. A good review with recent developments has been published on mercaptans in emulsion polymerization ( 7 1 ) . At present mercaptans are the most suitable substances for modifying the molecular weight of polymers; stepwise addition may be a tool for obtaining constant Mooney viscosity types of GR-8 lubber. Further work on GR-S (Mooney 150) and rosin (added to the latex) shows that the maximum tensile strength is obtained with lower proportions of rosin acid and that the maximum crackgrowth resistance and best properties with 50 parts of carbon black (Philblack 0) occur with equal parts of GR-S and rosin acid ( 187). A new type of styrene iesin, Plio-Tuf GBSC, blended with a neoprene, gives an unvulcanized compound that has high impact strength, good tensile strength, and heat distortion points of 190' F. and above (81). A symposium on resin-rubber blends was held a t a meeting of the Akron Rubber Group and covers much information on their properties (61, 111). Rigid and nonrigid compounds weie discussed. OIL-PLASTICIZED SYhTHETIC RUBBERS

Chromatographic methods of analysis afford a rapid and reproducible means of characterizing petroleum extender oils for GR-S and provide quantitative estimates of nonaroniatics, aromatics, and polar compounds (33). A fourth group, asphaltenes, may be obtained as a modification when the sample is incompletely soluble in n-heptane. An oil having a high aromatic content favors good processability and high tensile strength values; oils of low aromaticity impart the best hysteresis values. The chemical nature of extenders for GR-8 from petroleum is of considerable consequence as to their performance in rubber and can best be understood by defining petroleum products in terms of components that can be isolated and then tested in rubber (110). The definition and analysis of extenders in terms of the quantitatively determinable components-asphaltenes, nitrogen bases, acidaffins, and paraffins-complemented by viscosity and the value A,,SH (measure for compatibility of the paraffinic component) can be related to performance in rubber and made the basis for specifications for petroleum products. The analysis is simple, reproducible within narrow limits (l%), and relatively quick; the time spent on each sample to complete an analysis is 2 man-hours. Extensive studies to investigate the factors that affect the physical properties of oil-extended GR-S polymers and the molecular structure of the polymer indicated variation in most properties with the aromaticity of the oil (140). Treatment of the latex prior to preparing the oil-polymer masterbatch affects the stability of the polymer in the masterbatch. Air-blowing of GR-S latex masterbatches containing oil delays breakdown during storage or reduces the amount of breakdown in a given period of time (141). These are the results of heat aging tests conducted at 140" F. Swelling of vulcanized polymers in oils can be used as a measure of compatibility of polymers and oils (142). Swelling is a voluntary imbibition of the oils by the polymers as contrasted to mechanical mixing, which is a forced incorporation. Compati-

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bility increases linearly with aromaticity ( 100-paraffins), and decreases with increase in molecular weight of the polymrr. L4TEX

A method, based on Stokes lax-, involving the use of an ordinary laboratory centrifuge, is described for the determination of particle size distribution in GR-Slatices (95). I t was shown to be valid by tests for reproducibility and by comparison x ith the results of lighr and electron niici oscopv. The yellow coloring matte1 of natural rubber is considered to be a carotenoid closely related to, but not identical w t h , CYcarotene (68). The method of determining the yellow coloring matter consists of extracting the milled very thin sample m t h acetone and combining by nn empirical equation the absorptivities of the solution a t 360, 440, and 550 mp. The high speed agitation tests employed to determine the mechanical stability of natural rubber latev are generallj not applicable to synthetic latices (83). d ne17 mechanical stability test was developed for both types of latex, and it subjects the latex to a shearing force exerted by a metal disk undei load rotating in contact with a polvethylene surface. The coagulum formed in the operation of the machine for a definite time and under specified conditions, xhen expressed as a peicentage of the solids taken is used to define the mechanical stability. The electrophoretic mobility of fresh Hevea latex varied n i t h pH in a manner that v a s characteristic of proteins (11). There n as considerable variation in the isoelectric point from clone t o clone, a small but definite variation between trees of the same (>:,ne, and probably a small daily variation for any given tree. Ammonia Ivashing reduced the isoelectric point from 4.35 to 3.86, corresponding to the isoelectric point of Protein B isolated from Hevea latex Choline and cephalin, derived from the phospholipoids in Hevea latex, are the most important natural vulcanization accelerators, and the amino acids present are the most active natural antioxidants for raw rubber (2). Zinc soap formation is an essential factor in the gelling of natural rubber latex ( 9 4 ) . This formation is an important factor in the attainment of maximum strength of the gel. The higher the concentration of ammonium salt, the higher the temperature a t which the zinc and soap react. Katural rubber in the form of latex can be oxidized by thiols under hot conditions and the rubber softened (80). The results obtained confirm the hypothesis that the thiol is transformed quantitatively to the corresponding disulfide, with resultant oxidation of the rubber. This transformation is a function of the p H value, and the best results are obtained when the p H value is about 4 and in the absence of ammonium ions, which have an inhibitory effect. Katuial rubber in the form of 60% latex can be softened from Mooney viscosity of 130 to 20 by heating it with sodium chlorite and formaldehyde for 17 hours a t 60" C. (147). An interesting method for reducing latex dipping costs is found in the preparation of multiple phenolic castings (115) for the manufacture of latex girdles. Titer measurements are more sensitive than pH measurements for establishing the alkali level of neoprene latex ( 2 1 ) . Increasing amounts of alkali increase markedly the life of both raw and compounded latices, increasing amounts decrease the original viscosity of compounds and reduce very much the viscosity increase which takes place during aging, and thc alkali content has little effect on the physical properties of films. The polychloroprene in neoprene latex Type 571 contains 1.6% of its chlorine in a form that is especially reactive ( 3 ) . This chlorine reacts completely x-ith aniline at 90" C. In latices of butadiene-styrene, butadiene-acrylonitrile, and butadiene-styrene-acrylonitrile copolymers, the sizes of particles ranged from approximately 100 to 2100 A. with averages of about 600,800, and 1700 A, respectively (136).

Vol. 46,No. 10

PHYSICAL PROPERTIES

If natural rubber is compressed between two parallel plates serving as electrodes, its d.c. conductivity, dielectric constant, at frequency 1000, and loss angle a t the same frequency decrease with increasing pressure (48). This is true only if the rubber is free to decrease in thicknesj; otherwise pressure has no effect. I n studies on the proton magnetic resonance in natural rubber made from liquid-nit,rogen temperature to room t'emperature, two changes in line Ridth were noted, the first at about 155' K. and the second a t 225" K. (50). The change a t 15j0 K. v a s attributed to the onset of methyl group rotation and the change a t 225" K. to t,he onset of segmented motions. The line-widt,h changes in vulcanized samples at' lowrr temperatures were greater than a t the higher temperat,ures, and from this it was concluded that curing has a greater effect on the methyl group motions than on seginent,ed motions. Cross linking thus is not, the primary mechanism of vulcanization. Natural rubber becomes cross linked when subjected to high energy radiat'ion such as in atomic piles, thus offering a means of studying the change in properties of rubber as t'he degree of cross linking is varied nithout the use of chemical vulcanizing agents (18). From sm-elling measurements it is shown that the degree of cross linking is directly proportional to radiation dose, unit radiation dose producing 1 cross link per 90 isoprene units. The gas evolved during radiation was almost entirely hydrogen, and from the volume of hydrogen evolved it is &mated that the irradiated specimen lost 0.4070 of its total weight or about 2.7 hydrogen atoms per 100 isoprene monomer units. The composition of polyieoprenes prepared in bulk and in emulsion systems is, within experimental error, independent of the specific catalyst and percentage conversion of monomer to polymer (109). Increasing temperat'ure leads t,o a alight increase in 3,4-addition and a large increase in cia 1,4-addition. Polymers prepared with a cationic catalyst (BF,)are apparently not exclusively linear polyisoprenes. A method has been found for measuring t,he shear moduli and losses of elastomers in a range of frequencies of about 1 to lo6, and from 10 cpe. down (104). This method provides for large cyclic shear stresses and deformations without employing resonance methods. Only one sample is needed for the complete study, t,he frequency of stress being the only variable. The permeability of natural rubber, GR-S, and But'yl rubber, all tread compounds, t o nitrogen, air, and oxygen has been measured in a modified Warburg diffusion apparatus (26). The values ranged from lowest for Butyl, intermediate for GR-S, to high for natural rubber. Permeability decreased with increasing combined styrene in GR-S type and with a decrease in polymerization temperature for polybutadiene and low styrene copolymers. The permeability of several different vulcanized elastomers to carbon dioxide containing carbon-14 has been determined (69). The method requires 4 to 15 hours to determine the actual permeability depending on the t,emperature. X-632 (with less styrene than GR-S) is most permeable, then GR-Sand natural rubber, then Hycar OR-15, and finally Butyl, which is least permeable. The article shows that carbon dioxide is almost twice as solublc in Butyl as in natural rubber whereas an earlier article gives 76% as soluble. Measurements are described of the dynamic propert.ies of rubber loaded with various amounts and types of filler, when subjected to mechanical vibration in simple shear at amplitudes from 0 to 3% shear in the frequency range 20 to 120 cycles per second (59). The decrease of dynamic modulus with increasing amplit,ude is shown for a wide range of filler types and concentrations to be determined by the amount. of stiffening produced by the filler. Rubber compounds stiffened by mixture with or chemical combination of other polymers exhibit a smaller order

October 1954

INDUSTRIAL AND ENGINEERING CHEMISTRY

of nonlinearity than that described and also exhibit much lower hysteresis values within the amplitude range 0 t o 3% shear. In the analysis of data obtained for rubber vulcanizates (10 to 28y0 sulfur) between loa and lo4 atmospheres, the process of second-order transition is not one of thermodynamic equilibrium (153). There is no conclusive evidence for a discontinuity in compressibility a t the transition nor for an isothermal transition produced by pressure. A polysulfide rubber (Thiokol LP2) shows a deciease of electrical resistance as the material is stretched (66). The resistance declines to elongation of perhaps 20%. Beyond this it becomes fairly flat for an interval, and then increases from there to the elastic limit. The electrical resistivity of vulcanized natural rubber, GR-S 1500, or neoprene Type GN-A decreases rapidly when the loading of carbon black increases from 10 to 50 parts per 100 parts of rubber (9). Beyond 50 parts loading, the resistivity decreases slightly. Acetylene blacks have a high degree of structure and, therefore, high conductivity in natural rubber. The mixing procedure destroys this structure to some extent when acetylene blacks are used with tougher polymers, such as GR-S 1500, and the resistivities are much higher than in natural rubber. The activity of fillers can best be determined by employing a modified Defo test or by measuring the relative dielectric loss angle (72). Prolonged heating of rubber-filler mixes causes the loss angle to fall; this indicates molecular degradation. The course of the vulcanization reaction can be followed by measuring the loss angle. Bis-( o-benzamidophenyl) disuliide was found to be an effective accelerator of mastication a t 130' C. (60). Ortho derivatives excelled para derivatives in their accelerating action. Bis(aminophenyl) disulfide derivatives and thiophenol lower the viscosity of masticated natural rubber, but they do not lower the viscosity of a nitrile rubber (60). The effect of the softeners on natural rubber was markedly retarded by the presence of benzoic acid. I t is shown experimentally that the velocity of frictional gliding under constant tangential stress of rubber on glass and on silicon carbide is, in the first approximation, an exponential function of the reciprocal absolute temperature and of the tangential stress (120). It is suggested that frictional gliding of rubber is a rate process. By employing electrically conductive rubber, during the frictional gliding of rubber, a thin layer in contact with the track undergoes a deformation that considerably increases the electrical impedance of this layer (121). In the case of previously abraded rubber, the layer is permanent. A new laboratory method for evaluating the behavior of a vulcanizate of the tire-tread type has been described and comparative data on vulcanizates of different elastomers shown (155). With increase of the rate of sliding from 0 to 2 cm. per sec., the friction increases; thereafter it decreases. Static friction depends on the time of stationary contact but is of little significance. Temperature has the greatest influence. With constant conditions of testing, several variables in the samples are influential-e.g., the type of elastomer and the hardness. A new method has been devised for measuring the solubility of sulfur by employing the radioactive sulfur-35 isotope (5). The solubility is deduced from the equilibrium counting rate and an appropriate calibration curve of the counts for rubber containing known concentrations of the radiosulfur. The method should also be applicable for measuring not only the solubility but also the diffusivity and migration of compounds tagged with sulfur-35 or carbon-14. A long informative article has been published on the extrusion factors of black rubber compounds (32). Data are given with seven rubbers including smoked sheet No. 1, cold and hot GR-S, and oil masterbatch GR-S 1712, Butyl, Butyl reclaim, and neoprene Type GN, and ten types of carbon black.

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COMPOUNDING AND VULCANIZATION

In a new investigation of the vulcanization of natural rubber Ivith organic peroxides, following the work of Ostromislensky in 1915, only two were revealed with any technical potentialities (12). Ted-butyl perbenzoate and 2,2-di-(tert-butyl peroxy) butane both produced pure gum vulcanizates with a high degree of transparency and good heat aging. In the case of the first one, improvements in properties were effected by incorporation of resorcinol (0.5) or p-naphthol (0.2). These peroxides lack the ability to produce satisfactory vulcanizates with caibon blacks, although other ingredients work satisfactorily. Radioactive sulfur-35 and mercaptobenzothiazole tagged with sulfur-35 have been used for measuring the sulfur and mercaptobenzothiazole reaction rates (4). The ratio of combined sulfur to consumed mercaptobenzothiaaole is constant during the vulcanization process. When polyisobutylene is substituted for GR-S or Hevea, the zinc mercaptide of mercaptobenzothiazole is found to be the initial product. B proposed mechanism involves the formation of the zinc mercaptide which reacts with sulfur and rubber to regenerate mercaptobenzothiazole and form sulfur-rubber compounds. A standard method has been devised to calculate the time necessary for the proper cure of thick sheets of rubber (10). The increasing acceptance of reinforcing furnace blacks has intensified scorch problems for which the sulfenamides derived from mercaptobenzothiazole offer only a partial solution ( 2 6 ) . Preliminary evaluation of a nonaromatic field, namely, the soniewhat undeveloped 4-alkylthiazoles, showed that 5-carbomethoxyor 5-carboethoxy-4-methyl-2-thiazolesulfenamidesin which the amido group was morpholinyl or tert-butylamido were stable compounds, possessed more delayed action and gave generally equal properties. The triisocyanate (Desmodur-R) is not toxic and the use of hundreds of tons in different countries during 12 years has resulted in no report of toxic effects (41). Its sensitivity to moisture is not great enough to preclude its use even in open steam vulcanization. Practical applications, tests, and electron micrographs indicate that pure silicic acids are more active as reinforcing agents for rubber than calcium or aluminum silicates (36). An interesting study of white factice in cold cured natural rubber proofings has been made and several tests included (@). A new method for'preparing factices has been given ( 4 2 ) . The oil is mixed with carbon tetrachloride, magnesium chloride added, and gaseous chlorine bubbled in; solvent is distilled, and the product is reacted with sodium sulfide. Heat treatment a t 800' F. and/or the addition of glycols or amines modify the surface of silica-type pigments used in the compounding of natural and synthetic rubber (54). Such treatment results in better tensile reinforcement and improved modulus a t lower accelerator levels in the compounds. Lignin treated with about 10% its weight of hexamethylenetetramine and compounded with natural rubber as latex or on a hot mill gives vulcanizates in which tensile properties and hardness comparable with those of a carbon black-reinforced compound are combined with the resilience of a pure gum compound (146). Humic acid behaves similarly. Kieselkreide is a naturally occurring chalk flint found near Neuburg, Germany, with Si02 and A1,Oa the major constituents (49). Its physical and chemical properties make it useful as a rubber filler. The series of articles has been continued on methods employed in compounding research, including descriptions of synthetic rubbers and various processes for reclaiming scrap ruhber (SO, 32). AGING, INCLUDING OZONE CRACKING

A cracking of natural rubber, indistinguishable from that produced by ozone, occurs on exposure of stressed rubber to vapors of

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INDUSTRIAL AND ENGINEERING CHEMISTRY

volatile peroxides and ultraviolet irradiation ($a). The reaction occurs in an atmosphere of nitrogen and apparently results from the action of free radicals liberated from the peroxides. Nitrogen dioxide accelerates tBhecracking by catalyzing the peroxide photolysis. With natural rubber and GR-S vulcanizates exposed to oxygen a t 1 atmosphere and 50" to 100" C., a comparison of physical properties a t equivalent amounts of oxygen absorption s h o w the modulus to be lower and elongation higher when the absorptio:i occurs a t higher temperatures (1.28). In a series of aging tests of 34 stocks of unknown composition, supplied by various commercial manufacturers in the United States, the samples were aged in the oxygen bomb, the air oven, and by an oxygen absorption test (104). The oxygen absorption t'est' provides a more fundamental and rapid evaluation of oxygen aging of a vulcanizat'e than do the other tests. The oven t,est fails to evaluate aging resistance alone but measures the result,ant effects of after-vulcanization and degradation. In the bomb test the tot'al loss of physical properties between 4 and 14 days may be used as a subst'itute for the rat,e of 10s~. A[i apparatus and method have been developed for testing the effect of long-time aging of elast,omers under continuous shear load (56). The method includes a measure of the change in modulus as well as the total amount of creep, and the importance of evaluating both quantities is demonstrated. The method more clearly approximates service conditions than do most aging tests. Aging of materials individually in ovens contaminat.ed from previous usage does not assure freedom from contamination (64). These contaminants may cause an increase in the rate of deterioration. The test t,ube method of aging is an excellent method for studying these migration effects as well as for conducting routine aging tests in their complete absence. The formation of ozone through photochemical oxidation of alcohols, aldehydes, ket,ones, acids, and hydrocarbons, such as are present in gasoline, in the presence of small quantities of nitrogen oxides has been demonstrated ( 5 1 ) . The release of large quantities of hydrocarbons to the air and the simultaneous presence of nitrogen oxides from combustion processes explains the relatively high ozone content and consequent sewre rubber cracking in the Los Angeles area. By a cinematographic study of ozone cracking, the results show that crack formation is statist,ical in character, the rates being normally distributed nith respect to the logarithm of time, and the crack location being random (130). Rate of crack growth is constant in time except when it is modified by crack joining or stress due to the presence of neighboring cracks. The spontaneous decomposition of natural rubber ozonides shows that the ozonide prepared a t 25" C. is more stable than those prepared a t 0" C. (105). Masticated and stored rubber ozonides decompose faster than unmasticated ozonide, with reversion on storage. Acetone-extracted crepe rubber ozonide i8 more stable in vacuum than that of regular crepe ozonide. Studies are being made on the evaluation of surface cracking of elastomers (65). The use of radiations of different penetrability permits discrimination betiyeen volume and surface area of the cracks, the ratio of which is proportional to average crack depth. Natural rubber, GR-S, and neoprene vulcanizates, for which low temperature hardness had previously been determined by laboratory tests, were exposed to arctic condit,ions for one year a t Point Barrow, Alaska ( 9 1 ) . Hardness was determined monthly. The natural rubber and neoprene stocks became much harder in the arctic a t -35' F. than would have been predicted from laboratory tests a t -35' F. GR-Sis a better elastomer for compounding stocks t,o be used in t,he arctic than the other rubbere. Carbon black is known to promote the decomposition of peroxide to free radicals, but in the presence of amine antioxidants

Vol. 46, No. 10

it' also promotes the decomposition to stable products ( 1 2 7 ) . This dual effect of carbon black results in a reversal of effectiveness of amines and phenols in gum and black stocks. The phenols, which function best as chain stoppers, are more effective in the gum stocks; the amines, vhich function best by reducing peroxide initiation by directing the decomposition t o stable products, are more effective in the black stocks, GR-S films cast from benzene solutions absorbed oxygen much less rapidly, retained their intrinsic viscosity better, and formed less gel polymer on exposure to air or oxygen a t 130' to 140" C. when the filme contained 2 to 29 parts of E P C carbon black per 100 parts of elastomer (79). It is desirable to test GR-S vulcanizates containing considerable quantities of plasticizers for heat loss and water extraction, if the vulcanizates are to be exposed to weather while in service (92). PHYSICAL TESTIZTG

The Instron machine is useful for evaluating the stress-strain properties of raw or vulcanized rubber samples a t low elongations on single or repeated cycles of loading, stress relaxation at constant strain, creep a t constant stress, and hysteresis effect,s, but it is not particularly well adapted for routine testing for ultinitite elongat'ion and tensile strength (43). iln apparatus is described that subjects a cylindrical rubber pellet 0.7 inch in diameter and 0.5 inch high t o any desired load or strain while continuously measuring the height of the pellet and, by means of a strain gage, the force on the pellet ( 7 3 ) . Stress relaxation was determined over the time range 0.6 to 100 minutes for vulcanized samples of natural rubber, polybutadienc, GR-S, Hycar, neoprene, and Butyl rubbers conditioned 0, 4, or 24 hours a t the testing temperature. The temperature range explored was from -57' to 100" C. h versatile group of air-jet-operated devices registers changes in mill roll pressure during mixing giving a record of the addition of batch ingredients such as sulfur and accelerator and also measuring the thickness of coat'ed fabric and rubber sheets during processing (151). Since the aircraft and automotive industries require an elastomer that is flexible a t low temperatures and that maintains its dimensions and physical properties after prolonged contact with various fuels and oils, and since existing polymers do not meet these exacting requirements, a large number of experimental polymers were made and their compounds examined ( 7 4 ) . Copolymers were made from butadiene and various proportions of styrene, acrylonitrile, methacrylonitrile, alkyl acrylates, vinylidene chloride, diethyl fumarate, diethyl chloromaleate, and unsaturated ketones; and some tripolymers of these monomers were also prepared and evaluated. However, none of the compounds had both the desired oil and freeze resistance, but the data may offer possible leads. Detection of complete bond flaws and voids inside a flat bonded rubber specimen has been made by the transmission of ultrasonic waves ( 5 7 ) . The applied strain for the rubbers investigat'ed was no great,er than 10%. Experiments are under way on mountings with curved surfaces. A summary is given of information collected from the industry to show under what conditions a plasticity test is required to be used in factory control (124). The parallel-plate compression test is the most suitable, provided the test piece is precompressed to a small thickness so that it can be quickly heated to the test temperature before applying the test load ; t'he other factors considered are size of compression plates and of test piece, use of release medium-e.g., thin paper-degree of precompression, preheat period, test load, compression period, and temperature. A new simple compreesion plastomer measures the ability of uncured rubber to flow during molding processes (76). Rubber is reproducibly chromatographed by using cyclohexa-

October 1954

INDUSTRIAL AND ENGINEERING CHEMISTRY

none and filter paper treated with a 5-volume % of methyltrichlorosilane in benzene ( 6 ) . The partitioned rubber is stained by a 0.25% solution of oil blue NA (Calco) in aqueous ethanol. Chromatograms of guayule, Hevea, and GR-S differ according to distribution of molecular weight. The newly designed Datwyler-Schiltknecht instrument for testing abrasion resistance produces results that agree well with those obtained in actual practice (56). It consists chiefly of a phonographlike apparatus having 2 “pickup” arms, under which are attached rubber samples that abrade against a rotating disk of emery cloth. One arm holds the rubber test specimen and the other holds a standard rubber abrasion specimen. Both rubber samples move back and forth from outside to inside of the abrading disk while the disk is turning. At low temperatures, in tension, the yield effect is very marked Kith neoprene GN (16). On the other hand, a definite yield effect is shown as a result of shear deformation measurements on natural rubber stored a t -40’ C., whereas previous tension tests failed to detect this. Under suitable conditions of storage time and a t high shearing stresses a natural rubber unit might fail in service by yielding and giving too much deflection. The usual type failure of a shear unit in service arises from loss of flexibility and hence deflection properties; the oil resisting rubbers are particularly likely to suffer from this effect. Studies of the Mooney Viscometer a t the National Bureau of Standards have developed designs for the viscometer rotor and dies that provide improved dimensional stability, better heattransfer characteristics, and much greater uniformity of results (112). The new rotor has radial V-grooves. Rotors and dies constructed to the bureau’s specifications have been placed on trial in synthetic rubber plants and rubber manufacturing plants with a view toward their eventual standardization. A new method of self-closing grips for rubber is described, which easily manipulates all types of rubber dumbbell test piece (136).

An interesting account has been published of rubber test methods discussed a t the fifth meeting of Committee ISO/TC/45Rubber of the International Organization for Standardization (ISO), held in Paris, June 15-20, 1953 (123). CHEMICAL ANALYSIS

The acetone extracts of specially prepared vulcanized samples, each rontaining one of 7 kinds of accelerators or one of 4 kinds of antioxidants, were chromatographically analyzed (53). By simultaneous ultraviolet observation, the color change during development of adsorption band, and the color reaction of cobalt oleate the qualitative identification of accelerators and antioxidants in vulcanized rubber is possible. Procedures for the concentration of peroxides from autoxidized methyl oleate are generally small scale procedures that are difficult to duplicate and give low yields (19). By precipitation of the nonperoxidic portion of methyl oleate autoxidation mixtures, containing 4 to 37% peroxides, as urea complexes, concentrates containing 70 to 90% peroxides have been isolated from the filtrates in 50 to 95% yields. a-Linoleic acid (37%) is a major guayule resin component ( 7 ) . Palmitic acid (4%) and stearic acid (1.6%) were isolated as the 72.5:27.5 mole % eutectic. Linolenic acid (0.5%) was isolated only as a bromine addition product. Oleic acid, although not actually identified, may be present in minor amount. For determining rubber hydrocarbon in rubber-bearing plants samples are prepared by crushing them through corrugated rolls, extracting with benzene in the presence of pebbles, and determining the rubber hydrocarbon by brominating and weighing the rubber dibromide (90). Analysis of guayule shrubs containing 1 to 20% rubber can be completed in less than 1 day. Directions for determining the gross constituents in GR-S containing soap are given ( 7 7 ) . The organic acid and soap are

2073

determined by titrating aliquots of a toluene-ethanol solution (5: 1) of a single weighed sample. Tests for stabilizer and bound styrene made on the same solution are incorporated in a continuous scheme of analysis. In addition, with the aid of corrections for ash and for moisture and other volatile constituents, a complete determination of the gross polymer and nonpolymer constituents of GR-S may be made. A method is given for the titration of mineral and organic acids in toluene-ethanol solution from GR-S ( 7 8 ) . An automatic combustion apparatus for determining sulfur and halogen is available in which a sample is vaporized in a stream of nitrogen and combustion occurs in the high temperature zone of the combustion tube, where a stream of oxygen is injected (154). An operator can complete 15 to 25 analyses in an &hour day. If vulcanized rubber can be analyzed in this apparatus, it should be useful in the rubber industry. In the dissolution of cured rubber for iodine number, fillers, and other determinations, air should be bubbled through the heated solvent from the start of the digestion (150). For iodine number determination the solvent must be an aromatic compound having one or more electronegative ring substituents-e.g., p-dichlorobenzene. A method for determining vinylidene cyanide has been developed based on its reaction with an excess of anthracene and determination of the excess anthracene by differential colorimetry a t 359 mp (148). A neophelometric micromethod for the estimation of gutta and ol rubber in plants has been developed (29). Agreement between duplicate tests were closer than by the gravimetric method, CARBON BLACK

Electrical measurements of rubber-carbon black systems show that after effects are the most important cause of the frequency dependence of dielectric constant and loss angle ( 6 7 ) . $1~0 compressive stress causes complex variations in dielectric properties connected with the shape factor (length to thickness ratio) of the carbon black flocculates. These effects are interpreted to give information on the type of carbon black dispersion, and for this purpose dielectric constant is more significant than specific resistance. Carbon black can be readily dispersed in GR-S by being jetted into the latex with a high pressure steam jet (108). Tires thus produced from cold GR-S show a 20% improvement in tread wear over control tires of the best cold GR-S produced by other methods. There have been developed large truck tires containing 30% of GR-S that approach natural rubber tires in performance (129). Above 30% synthetic rubber, there is a linear decrease in durability of the carcass, a slight gain of efficiency of the influence of the natural rubber, a sharp decrease of efficiency of the influence of the synthetic rubber, and sharp increase of cost per tire mile. Synthetic-rubber tires are more prone to fail from heat, ply separation, and tread cracking. Philblack E is 35 to 43% superior to HAF black in GR-S 1500 tread stocks in passenger tire tests, and it is 12 to 24% superior to EPC black (138) in natural rubber truck tires. Philblack E is also superior to acetylene black in electrical conductivity a t 20 to 35 parts per 100. A panel discussion of carbon blacks, their manufacture, and applications held a t a meeting of the Akron Rubber Group, January 19, 1954 (63, 118) has been reported; 47 questions with their answers are included. RECLAIMED RUBBER

A review of modern theories of reclaiming natural rubber and GR-S vulcanizates have been considered and the more important processes of manufacture outlined ( 1 3 ) .

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INDUSTRIAL AND ENGINEERING CHEMISTRY GENERAL

Elastomer steam hose with a t least one layer of braided steel wire reinforcement is superior to hose having only nonmetallictype reinforcement, such as cotton duck, asbestos cotton, or glass cord (86). This is the recommendation of the Bureau of Ships, based on a special study using saturated steam a t 200 pounds per square inch and recommending use a t not over 365' F. Waste treatment a t the government synthetic rubber plants consists of removing separable oil and suspended solids in settling basins and neutralizing the various liquid wastes by adding caustic or acid (84). A further reduction of oil and suspended solids is gained by filtration of the wastes through hay. I n a standard routine test natural rubber insulation failed in 3 to 5 months, and ordinary GR-S insulation in 6 to 9 months (8). Stable fungitoxic GR-S compounds have been developed that have invariably maintained normal high insulation resistance during 4 years in active soil. Milled natural rubber in xylene is reacted with aqueous sodium hypochlorite and hypochlorous acid and later mixed with chlorinated rubber to form a suitable adhesive for bonding rubber to metal ( 1 4 ) . d review of methods of attaching rubber to metal includes descriptions of brass plating and four types of materials used in cements, several methods of testing, and poor and good designs

(44). Glass-lined reactors used to prepare synthetic rubbers become coated with a thin film of rubber that cannot be scraped from the surface and that reduces the heat transfer coefficient of the reactor (93). Solvents such as asymmetrical ketones in combination with oxygen rapidly degrade and dissolve the GR-S film, but best practical results are obtained with petroleum naphtha containing about 4% cumene hydroperoxide, which combination effectively cleans the reactors in 32 hours a t 170" F. For nitrile rubber films this reagent is not satisfactory, and ketone solvents with cumene hydroperoxide are recommended. I n a study of accelerators for continuous vulcanization of cable compounds a t high temperatures, the results indicate that no single accelerator is satisfactory because of low activity or tendency to induce scorching, but that binary combinations of thiazoles and thiuram compounds or dithiocarbamates with 1 to 1.5% sulfur are the most suitable (88). The u8e of air conditioning in the factory for the production of braided hose has lowered the formation of "seconds" to an unusually small number (113). LITERATURE CITED

(1) Adams, H. E., and Johnson, B. L., IND. ESG. CHEJI.,45, 153946 (1953). (2) Altman, R. F. A., Proc. XIth Intern. Congr. Pure and A p p l . Chem. (London),5 , 251-4 (1947) (Pub. 1953). (3) Andersen, D. E., and Arnold, R. G., ISD.ENG.CHEM.,45, 2727-30 (1953). (4) Auerbach, I., Ibad., 45, 1526-32 (1953). (5) Auerbach, I., and Gehman, S. D., Anal. Chem., 26, 685-90 (1954). 16) Banigan, T. F.. Jr., Science. 117, 249-50 11953). . . (7) Baniian, T. F., Jr., and nIeeks,'J. W., J . Am. Citem. SOC.,75, 3829-30 (1953). (8) Blake, J. T., Kitchin, D. W., and Pratt. 0. S., Trans. Amer. Inst. Elec. Engrs., 72, 321-8 (1953) ; Power Spparatus and Swstems. _DD. _ 321-8, Ami1 1953. (9) Boonstra, B. B. S. T., and Dannenberg, E. M., IND.EUG. CHEX.,46, 218-27 (1954). (10) Bott, E. C. B., Trans. Inst. Chem. Engrs. (London).30, KO. 4, 251-9 (1952) : Rubber Chem. and Techml., 26,674-90 (1953). ISD. ENG.CImii., 45, 1790-4 (1953). (11) Bowler, IT.W., (12) Braden, Lf., Fletcher, W.P., and bIcSv-eeney, G. P., Trans. Inst. Rubber Ind., 30, T.44-56 (1954). (13) Brazier, S. A., Ibid., 29, 1 1 M 7 (1953). (14) Brooks, L. A. (to R. T. Vanderbilt Co.), C?. S. Patent 2,637,751 (May 5, 1953). (15) Brom-n. H. P. (to B. F. Goodrich C o . ) ,Ibid., 2,662,874 (Dee. 15, 1953); 2,669,550 (Feb. 16, 1954); 2,671,074 (AIarch 2, 1954).

Vol. 46. No. 10

(16) Buist, J. M., and Stafford, R. L., Trans. Inst. Rubber Ind., 29, 238-54 (1953). (17) Carlotta, E. L.,'and Hobein, E. l f . , Rubber Age (N.Y.), 74, 8590, 134 (1953). (18) Charles, A , Atomics and Atomzc Technol., 5 , No. 1, 12-21 (1954). (19) Coleman, J. E., Knight, H. B., and Swern, D., J . Am. Chem. SOC.,74, 4886-9 (1952). (20) ContB, M., Reo. ge'n. caoutchouc, 30, 2 6 2 4 (1953). (21) Cook, G. S.,and Fitch, J. C., Rubber Chem. and Technol., 27, 277-85 (1954). (22) Crabtree, J., and Biggs, B. S., J. Polymer Sci., 11, 280-1 (1953). (23) Cragg, L. H., and Bron-n, A. T., Can. J . Chem., 30, 1033-43 (1952). (24) Cragg. L. H., and Fern, G. R. H., J . Polvmer Sci., 10, 185-99 (19.53). (25) Creed, K. E., Jr., D'dmico, J. J., Harman, A I . W., and Zerbe, R.O., f S D . E X G . CHEM., 46, 808-16 (1954). (26) Ceuha, RI.,India Rubber World, 130, 207-10 (1954). (27) DiGiorgio, Philip A. (now Philip D. George) (to General Electric Co.), U. S. Patent 2,640,818 (June 2, 1953). (28) Dixon, J. K., and Sannders, K. W., ISD.ENG.CHEM.,46, 65260 (1954). (29) Doman, N. G., Diolchim., 18, 335-9 (1953). (30) Drogin, I.. India Rubber World, 128, 627-9, 771-4 (1953). (31) Ibid., 129, 63-8 (1953). (32) Drogin, I., Bishop, H. R., and Wiseman, P., Rubber Age ( N . Y.)? 74. 707-60 (1954). (33) Dunkel, W.L.,Ford, F. P., and McAteer, J. H., IND.ESG. CHEM., 46, 578-86 (1954). (34) Earley, P. J., and Sanger, >I. J., Rubber A g e (AT. Y . ) , 75, 65-72 (1954). (35) Ecker, R., Kautschulz u. Gummi, 5 , WT 171-8 (1952). 74, 899-902 (1954). (36) Eller, 9. A,, Rubber Age ( N . Y.), (37) Faull, J. H., Jr., Rubber World, 130, 358-61 (1954). (38) Feldon, M., and hfcCann, R. F., IND.EXG.CIimi., 46, 465-7 (1954). (39) Fletcher, W. P., and Gent, A. N., Trans. Inst. Rubber Ind., 29, 266-80 (1953). (40) Flint, C. F., and Featherstone, C. B., Ibid., 29, 287-311 (1953). (41) Fromandi, G., Rubber Age and Synthetics, 33, 521 (1953). (42) Gallavresi, P., alii minerali, grassi e saponi, color{ e o e m i c i , 30, 74-5 (1953). (43) Gehman, S. D., and Clifford, R. P., Symposium on Recent Developments in the Evaluation of Natural Rubber, New York, 1952, ASTM Spec. Tech. Publ. KO. 136, 97-111 (1953). (44) Gerstenmaier, J. H., Rubber Age ( N . Y . ) ,73, 495-500 (1953). (45) Gessler, A. nl.,Chem. Eng. News, 32, 494 (1954). (46) Gessler, A. nl.,and Ford, F. P., Rubber Age (dV.Y , ) ,74, 397408 (1953). (47) Glime, A. C., Duke, S.8., and Doede, C. M., India Rubber World, 128, 76670, 774 (1953). (48) Granier, J., Compt. rend., 236, 786-8 (1953). (49) Grunfeld, O., Kautschuk u.Gummi,6, WT 193-201 (1953). (50) Gutomsky, H. S., and Ifeyer, L. H., J . Chem. Phys., 21,2122-6 (1953). (51) Haagen-Smit, A. J., and Bradley, C. E., Ixn. ESG. C m x , 45, 2086-9 (1953). (52) Hall, G. L., Conant, F. S., and Liska. J. W.,India Rubber World, 129, 611-6 (1954). (53) Hamasaki, F., J . SOC.Rubber Ind. Japan, 23, 302-6 (1950). (54) Hausch, W.R., India Rubber World, 130, 59-62 (1954). (55) Heinisch, K. F.,and Wargadiwidjaja, R. M., India Rubber World, 130,63-4 (1954). (56) Herzog, R., and Burton, R. H., Schweiz. Arch,. angeui. Wiss. u. Tech., 19, 1-6 (1953). (57) Heughan, D. hI.,and Sproule, D. O., Trans. Inst. Rubber Ind., 29, 255-65 (1953). (58) Howland, L. H., Reynolds, J. A., and Brown, R. W., IND.ENG. C H E ~ I45, . , 2738-42 (1953). (59) Imoto, AI., and Kiriyama, S.,J . Chem. Soc. Japan, Ind. Chem. Sect., 55, 450-2 (1952). (60) Imoto, >I., and Kiriyama, S., J . Inst. Polytech., Osaka Citg Unic., Ser. C. 4, No. 1, 142-7 (1953) (in English). (61) India Rubber V o r l d , 129, 497-501, 629-31, 785-8 (1954). (62) Ibid., 129, 756-72, 775 (1954). (63) Ibid., 130, 227-35 (1954). (64) Juve, 1. E., and Shearer, R.. Ibid., 128, 623-5 (1953). (65) Kalinsky, J. L., and Werkenthin, T. A., Rubber Age (.V. Y.), 75, 375-84, 441 (1954). (66) Kersta, L. G., J . Polymer 9ci., 10, 447-8 (1953). (67) Kickstein, G., Kautschuk u.Gummi, 7, WT 50-5 (1954). (68) Kidder, G . A , Anal. Chem., 26, 311-15 (1954).

October 1954

INDUSTRIAL AND ENGINEERING CHEMISTRY

Kirshenbaum, A. D., Streng, A. G., and Dunlap, W. B., Jr., Rubber Age ( N . Y.), 74, 903-8 (1954). Koldehofe, E., Kautschuk u. Gummi, 5,33-6,55-7,76-8,92-5, 107-10 (1952). Krause, A. H., Rubber Age ( N . Y.), 7 5 , 217-22 (1954). Krug, H. D., Kautschuk u. Gummi, 5, 72-6 (1952). Labbe, B. G., and Phillips, W.E., India Rubber World, 129, 489-91 (1954). Laundrie, k.JG., Feldon, M., and Rodde, A. L., IND. ENG. CHEW,46,794-803 (1954). and Schlesinger, W., J . Polymer Sci., 11,307-23 Leeper, H. I!., (1953). Linhorst, E. F., India Rubber World, 128,626 (1953). Linnig, F.J.,Peterson, J. M., and associates, Anal. Chem., 25, 1511-15 (1953). Linnig, F.J., and Schneider, A,, Ibid., 25,1515-17 (1953). Lyon, F., Burgess, K. A,, and Sweitzer, C. W., IND.ENG. CHEW., 46,596-600 (1954). McCormack, C. E., Baker, R. H., and Raff, R. S., Rubber Age ( N . Y.), 74,72-6,84 (1953). McCntcheon, R. J., and Sell, H. S.,Rubber World, 130,362-5 (1954). MacDougall, J. D. B., h'ature, 172,124-5 (1953). Maron, S. H., and Ulevitch, I. N., Anal. Chem., 25, 1087-91 (1953). Martin,' A. E., and Rostenbach, R. E., IND. ENO.CHEM.,45, 2680-6 (1953). Marvel, C. S.,Clarke, K. G., and associates, Ibid., 45,2090-3 (1953). Marvel, C. S.,McCain, G. H., and associates, Ibid., 45,2311-17 (1953). Marvel, C. S.,Stiehl, R. T., and associates, Ibid., 46,804-8 (1954). Mason, J., Trans. Inst. Rubber Ind., 29,148-59 (1953). Medalia, A. I., and Freedman, H. H., J . Am. Chem. SOC.,75, 4790-3 (1953). Meeks, J. W.,Crook, R. V., Pardo, C. E., Jr., and Clark, F. E., Anal. Chem., 25,1535-8 (1953). Morris, R. E., and Barrett, A. E., India Rubber World, 129, 773-5 (1954). Morris, R.E., and Barrett, A. E., Rubber Age ( N . Y , ) ,75,768 (1954). Nettleton, J. S.,Davidson, M. J. G., and Williams, H. L., IXD. ENG.CHEM.,45,18968 (1953). Newnham, J. L. M., Trans.Inst. RubbeTInd., 29,160-72(1953). Nisonoff, A., Messer, W. E., and Howland, L. H., Anal. Chem., 26,85661 (1954). Novikov, A. S.,Bartenev, G. AI.,and Galil-Ogly, F. A., DokZady Akad. Nauk. S.S.S.R., 94,253-6 (1954). Osterhof, H . J., India Rubber World, 130,197-202 (1954). Painter, G. W., Rubber Age ( N . Y . ) , 74,701-6 (1954). Pande, H., India Rubber World, 130,211-13 (1954). Peterson, W.H., Rubber World, 130,366-9,436 (1954). Pfeifer, C. W., Savage, R. M., and White, B. B., India Rubber World, 129,4814,488 (1954). Philippoff, W., J . Appl. Phys., 24,685-9 (1953). Philpott, M.W.,Proc. XIth Intern. Congr. Pure and A p p l . Chem. (London), 5, 355-64 (1947) (Pub. 1953). Pollack, L. R., India Rubber World, 130,53-7 (1954). Ramakrishnan, C. S., J . Sci. Ind. Research (India), 11B,245-8 (1953). Ramakrishnan, C. S.,Raghunath, D., and Pande, J. B., TTans. Inst. Rubber Ind., 29,190-201 (1953). Rehner, J., Jr., and Gessler, A. M., Rubber Age (S. Y,), 74, 561-6 (1954). Resen, F. L., Oil Gas J., 51, No.46,160 (1953). Richardson, W.S., and Sacher, il., J . Polymer Sei., 10,353-70 (1953). Rostler, F.S.,and White, R. hI., IXD. ENG.CHEnf.,46,610-20 (1954). Rubber Age ( N . Y . ) , 74,547-59 (1954). Ibid., pp. 574-5. Ibid., p. 761.

2075

(114)Ibid., pp. 915-35. (115) Ibid., 75,73-5 (1954). (116) Ibid., p. 83. (117)Ibid., p. 84. (118) Ibid., pp. 227-33. (119) Sayko, A. F., India Rubber World, 129,348-53 (1953). (120) Schallamach, 9., Proc. Phys. SOC.(London),Series B 66,386-92 (195.7). (121) Ibid., pp. 817-25. (122) Schytil, F.,and Volpers, R., Kolloid-Z., 130,110-15 (1953). (123) Scott, J. R., Proc. Inst. Rubber Ind., 1, 5G-65(1954). (124) Scott, J. R., Trans. Inst. Rubber Ind., 29, 175-89 (1953). (125) Seaman, R.G., India Rubber World, 129,345-7,357 (1953). (126) Seeger, N. V., Mastin, T. G., and associates, ISD. ENG.CHmf., 45,2538-42 (1953), (127) Shelton, J. R., and Cox, W. L., Ibid., 46,816-23 (1954). (128) Shelton, J. R., Wherley, F. J., and Cox, W. L., Ibid., 45,2080-6 (1953). (129) Sjothun, I.J.,and Greer, P. S.,Rubber Age ( N . Y . ) ,74,77-83 (1953). (130) Smith, D. M., and Gough, V. E., Trans. Inst. Rubber Ind., 29, 219-37 (1953). (131) Smith, W. C., India Rubber World, 129,55-60 (1953). (132) Smook, M.8., Pieski, E. T., and Hammer, C. F., IND.ENG. CHEM.,45,2731-7 (1953). (133) Stafford, W. E., and Wright, R. A., Proc. Inst. Rubber Ind., 1, 40-53 (1954). (134) Stock, R. P., Miserentino, C. O., McKeown, C. B., and associates, Symposium on Recent Developments in the Evaluation of Nat. Rubber, New York, 1952,ASTM Spec. Tech. Publ., 136,12-17,Discussion, 18 (1953). (135) Stromberg, R.R , Swerdlow, M., and Mandel, J., J . Research ;Vatl. Bur. Standards, 50, 299-309 (1953) (Research Paper 2419). (136) Stubbs, A. J., Trans. Inst. Rubber Ind., 29,215-18 (1953). (137) Svetlik, J. F., and Hanmer, R.S.,IND. ENG.CHEM.,45,2752-8 ( 1 9 ! 3 ) . (138) Svetlik, J. F.,Railsback, H. E., and Biard, C. C., India Rubber World, 129,617-20,622 (1954). (139) Swern, D., Coleman, J. E., Knight, H. B., Zilch, K. T., Dutton, H. J., Cowan, J. C., and Gyenge, J. M., J . Polymer Sci., 11, 487-90 (1953). (140) Taft, W.K., Duke, J., Laundrie, R. W., and associates, IPTD. ENG.CHEW,46,396412 (1954). (141) Taft, W. K., Duke, J., Snyder, A. D., and Laundrie, R. W., Rubber Age ( N . Y.), 75,61-4 (1954). (142) Taft, W. K., Laundrie, R. W.,Harrison, T. B., and Duke, J., Ibid., 75,223-6 (1954). (143) Taylor, R. H., Clark, F. E., and Ball, TV. P., India Rubber World, 129,751-5 (1954). (144) Taylor, R. H., and Veith, A. G., Symposium on Recent Developments in the Evaluation of Nat. Rubber, New Y o r k , 1952,ASTM Spec. Tech. Publ. 136,19-29,discussion, 30-2 (1953). (145) Thomas, R. AI., India Rubber World, 130,203-6,213 (1954). (146) Tibenham, F.J., and Grace, N.S.,IND. ENG.CHmr., 46,824-8 (1954). (147) Tournier, J., Rev. ge'n. caoutchouc, 31,46-8 (1954). (148) Tyler, W.P., Beesing, D. W., and Averill, S.J., Anal. Chem., 26,674-7 (1954). (149) Veith, A. G., Amer. Soc. Testing Materials, Special Publication KO.138 (1952); Rubber Chem. and Technol., 26, 655-73 (1953). (150) Wkke, k. C.,Proc. XIth Intern. Congr. Pure and Applied Chem., 5,365-71 (1947)(Pub. 1953). (151) Wannov, R.,Kautschuk u. Gummi, 7,WT 56-61 (1954). (152) Watson, W.F., Trans. Inst. Rubber Ind., 29,202--14 (1953). (153) Weir, C.E., J . Research h'atl. Bur. Standards,50,311-19(1953). (154) White, T. T., Penther, C. J., Tait, P. C., and Brooks, F. R., Anal. Chem., 25,1664-8 (1953), (155) Wilkinson, C.S.,Jr., India Rubber World, 128,475-81 (1953). (156) Zapp, R.L., and Gessler, A. M., Rubber Age (-V. Y . ) , 74,24351 (1953). \ - - - - I