HARD RUBBER HENRY PETERS, Bell TeEephone Laboratories, Inc., Murray Hill, AT. J .
I n spite of its heritage of almost 100 years, hard rubber has progressed rather slowly but still remains as a thermosetting material of special interest. The effect that compounding, processing, and curing has on plastic yield, impact strength, and dielectric properties continues to be a subject of investigation. However, results indicate nothing of an unusual nature has been uncovered. Renewed studies are being made to reduce the frictional coefficient as well as increase the flame resistance of ebonite. Use of synthetic elastomers for ebonites is slowly gaining momentum. Patents covering the use of hard rubber are numerous and interesting for this year.
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HIS annual review of 1950, like the previous one (8@,deals with some fundamental studies on natural and synthetic hard rubbers. Whorlow ( 4 9 ) studied the mechanism of surface deterioration of ebonite and means of preventing deterioration. Continuing with the early investigations on this subject, experiments are reported showing that the deterioration of the surface resistivity of ebonite, caused by a particular quantity of light, increases with decreasing intensity oi illumination. From the results of exposures to diflerent types of radiation, it is concluded that the surface temperature of the hoenite during exposure is the most important variable causing this difference. The observed results are accounted for by assuming that sulfurous acid is produced as an intermediate product in the deterioration, and this may decompose to sulfur dioxide and water or oxidize to sulfuric acid. To resist this deterioration, it is recommended that the surface of the ebonite be coated with an opaque material which may not a l ~ a y be s possible. Incorporation of antioxidants such as sym-di-P-naphthyl-p-phenylenediamine reduces the deterioration. Xorman, Westbrook, and Scott ( 2 4 ) investigated the effect of various plasticizers on certain physical properties of unfilled natural rubber and GR-S ebonites and the plastic properties of unvulcanized compound. As a rule, the same plasticizer did not give the best result for all properties, but certain relationships did exist. Thus, in natural rubber ebonites both impact strength and flexibility tended to be low, and dielectric power loss was high when plastic yield temperature was high. Practically all the plasticizers lower the plastic yield temperature and the cross breaking strength of natural rubber ebonite but improve notched impact strength. In GR-S, unvulcanized ebonite mix none of t,he plasticizers gives sufficient tack vhich is so prevalent in a similar mix of natural rubber. The effect on plastic yield varies considerably from one plasticizer t o another; some have such a bad effect that the advantage of GR-S over natural rubber, as regards plastic yield resistance, is lost. Some of the plasticizers increase the dielectric power loss of GR-S and nat'ural rubber ebonites. Scott ($7) has also studied the plastic yield of butadiene-styrene and isoprene-styrene ebonites in n-hich he found t'he butadiene-styrene copolymer in unloaded stocks to have better resistance to plastic deformations a t elevated temperatures as the styrene content is increased, a t least up to 46%. The plastic yield resistance of an isoprene-styrene copolymer is poorer than a corresponding butadienedyrene ebonite. However, all styrene containing copolymers give ebonites more heat resistance than natural rubber ebonite; the best give yield temperatures 30' C. above the latter. The best plastic yield resistance in butadienestyrene ebonites can be obtained with sulfur exceeding one atomfor example, 1.2 or 1.4 atoms-per butadiene molecule. Electrical, physical, and chemical properties and methods of
fabrication of two commercial grades of ebonites containing 25 to 35% and 35 to 50% sulfur, respectively, are discussed by McKay (81). The incorporation of 1% molybdenum disulfide into ebonite was found by Bowden and Shooter ( 5 ) to reduce the coefficient of friction. With disulfide, frictional coefficient is 0.26 and without disulfide the value is 0.4. Other mechanical properties are unaffected. Hardman and Lang ( 1 6 ) claim to have produced hard rubbers from liquid depolymerized rubber that have properties similar to those ebonites made by conventional methods. Rotgans (81) has shown conclusively that it is possible to make hard rubber from a heatsensitive latex. Kambara, Ogi, and Yoshimoto ( 1 7 ) made some interesting attempts to obtain hard rubber sponge having an uniisual low density of 0.2. CHEMICAL AND PHYSIC-IL TESTING
Piganiol (26) has described an apparatus that is used t o measure the mechanical properties of plastics over a wide range of frequency and temperature. Graphs show the internal friction of ebonite as a, function of frequency. In a paper by Lethersich (19),an apparatus is described for the study of creep of dielectric polymers based on their dynamic rheological properties. Essentially it involves the application of a shear stress to solid materials and measuring the strain a few milliseconds later. Apparatus can also measure the dynamic modulus and internal friction coefficient over a frequency range of 30 to 1000 cycles per second. Modification of apparatus permits the frequency to be lowered to 10-4 cycles per second. Shear modulus and coefficient of internal friction values for ebonite and plastics are recorded in a table. Schnadt ($4) devised a method for testing the brittleness of welded seams, thin sheets, and filaments which can be accomplished with unnotched samples. h machine for testing the mechanical properties of structural sandwich constructions has been devised by Kuenzi (18). The sandwich consists of a center layer of cellular ebonite bonded on both sides with sheet metal. Determination of plastic yield temperature of ebonite by an indentation hardness test is the subject of an investigation by Scott (36). An indentation test a t elevated temperatures measures practically the same physical characteristics as the torsion plastic yield test. The test can be carried out conveniently with an ordinary dead-load hardness gage, using a very small specimen-for example, 0.5 inch square bg 0.2 inch thick. BOOKS AND REVIEWS
Several books and review articles have been published which deal directly or indirectly with hard rubber. The American Society of Testing Materials ( 1 ) has described in great detail their testing procedures for synthetic and natural hard rubbers. Barron ($) has published some interesting data concerning physical constants of natural hard rubber as well as data dealing with electrical and solvent resistance of Buna S and Perbunan hard rubbers. A useful table (?) containing the principal physical properties of twelve insulating materials, including hard rubber, has been assembled for reference purpose. A brochure (18) dis2256
October 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
cussing the merits of expanded hard rubber in addition to its many applications has been fully described. A periodical, patent review of hard rubber ( 1 1 ) and cellular rubber (16)lists 16 and 23 references, respectively. To a lesser extent, the vulcanization of hard rubber is referred to in a book by Riegel (29), and Stevens and Donald (39) discuss the same subject in greater detail in addition to its engineering applications. 1NDUSTRIAL PRODUCTS AND USES
Expanded ebonite continues to gain usage in the field of thermal insulation (8, 82). I n the construction of the walls of prefabricated houses, cellular ebonite (20) is sandwiched between light metal alloys to give good heat and sound insulating properties. Cellular ebonite (58)has been iound to be superior to phenolic foam and balsa wood in retention of buoyancy after immersion in water for various periods of time, making it suitable for floating gear. Patramold, a new use for ebonite, is shown by Scott (36) to have a low yield temperature of about 45’ C. and can be pressed against set up type to form a mold from which a printing surface may be obtained by the usual electroplating method. Ebonite bobbins (41 ) have been shown to be superior t o wood inasmuch as they do not snag or splinter and have gieater resistance to shock. Hard rubber steering wheels (4)are once again finding their way in the everyday market. Six patents (W,6,15, 14,55,42)were granted for the use of hard rubber in storage batteries: the first of these involves the substitution of lead with an ebonite having an electrodeposited surface; the second makes use of ebonite as a bonding agent, and the third, fourth, and fifth deal with battery separators made from vulcanized microporous hard rubber; the last discusses the use of hard rubber in the lid and cell container. Fireproofing ebonite with a gel obtained by the hjdrolysis of a siliceous ester appears to be a patent (a?) of unusual merit. Golf club heads, made from a hard rubber composition containing a large proportion of feathers from which the quills have been wholly or partly removed, are the subject of an interesting patent (IO). A patent (9), involving a procedure for the preparation of an unusually low density cellular hard rubber, is worthy of consideration, Hard rubber composition, involving the use of paraffin oil, loading agents, and sulfur, is the subject of an interesting patent (40). Well broken down rubber coupled with blowing agents and heated carefully a t a temperature not exceeding 140’ C . is the subject of another patent ( 2 5 ) . Miscellaneous patents cover the use of hard rubber in such items as insulating spacers in electrical cables for telecommunication (SO), hard rubber molds for molding plastics (%), closure disks for cartridges made from microporous rubber (28),ball pens (44),and flared tubes (46). LITERATURE CITED
(1) Am. SOC.Testing Materials, Philadelphia, Pa., “Standards on Rubber Products.” D11. pp. 962-8, 1950.
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(2) Barak, M., Brit. Patent 641,531 (July 1950). (3) Barron, H., “Modern Synthetic Rubbers,” 3rd ed., pp, 395-7, London, Chapman and Hall, 1949. (4) Bennett, H. R., Rubber A g e , 68, 69 (1950). (5) Bowden, F. P., and Shooter, K. V., Research, 3, 384-5 (1950). (6) Callendar,L. H., Brit. Patent 632,061 (November 1949). (7) Child, C. L., JourneBs Int. des Plastiques, 249-51 (1950). (8) Cooper, A., Inst. RubberInd. News, 1, No. 14, 4-5 (1950). (9) Cooper, L., and Pfleumer H., U. S.Patent 2,500,603 (March 14, 1950). (10) Craven, G. W. E., Catte, W. A , and Slazengers, Brit. Patent 633,662 (December 1949). (11) Davies, B. L., Ann. Rept. Progress Rubber Technol., 12, 92-4 (1948). (12) Garner, T. L., and Powell, J. F.,“Rubber in Aircraft,” Market Buildings, Mark Lane, London, E. C. 3, March 1950. (13) Gerke, R. H., U. S.Patent 2,487,233 (Nov. 18, 1949). (14) Grant, J. A., Ibid.,2,484,787 (Oct. 11, 1949). (15) Guppy, W. D., Ann. Rept. Progress Rubber Techml., 12, 18-91 (1948). (16) Hardman, K. V., and Lang, A. J., Rubber Age, 66, 419-422 (1950). (17) Kambara, S., Ogi, H., and Yoshimoto, K., Bull. Rubber Research Inst. Japan, No. 1 , 4 (1948). (18) Kuenei, E. W., ASTM Bull., No. 164, 21-8 (1950). (19) Lethersich, W., J. Sei. Instruments, 27, 303-6 (1950). (20) Loeffler,H., Rev. Gbn. Caoutchouc, 27,lO-11 (1950). (21) McKay, J. R., Materials & Methods, 31, 54-6 (1950). (22) Mitchell, H., Rubber Developments, 3, No. 1,9-€ (1950). (23) Naganuma, H., and Kawaguchi, S., Japan. Patent 175,569. (24) Norman, R. H., Westbrook, M. K., and Scott, J. R., J . Rubber Research, 19, 89-100 (1950). (25) Peters, H., IND.ENG.CHEM., 42,2007-8 (1950). (26) Piganiol, JourneBs Int. des Plastiques, 113-4 (1950). (27) Rev. Gen. Caoutchouc, 26,761 (1949) (French Patent 945,308). (28) Ibid., 27, 120 (1950) (French Patent 943,799). (29) Riegel, E., “Industrial Chemistry,” 5th ed., New York, Reinhold Publishing Corp., 1950. (30) Rosen, A., Brit. Patent 635,434 (April 4, 1950). (31) Rotgans, G. E., Rubber, 6,36 (1950). (32) Ruymbeeke de, G., Rev. Gen. Caoutchouc, 26, 823 (1949) (French Patent 943,013. (33) Schelhammer, H. J., and Hunt, K. E., Brit. Patent 634,127 (January 1950). (34) Schnadt, H., Ibid., 631,449 (November 1949). (35) Scott, J. R., India Rubber J . , 119, 10 (1950). (36) Scott, J. R., J . Rubber Research, 19, 125-8 (1950). (37) Ibid., 128-30. (38) Stark, H. J., Alpert, J., and Shoemaker, T. L., J . Am. SO& Naval Engrs., 62, 139 (1950). (39) Stevens, H. P., and Donald, M. B., “Rubber in Chemical Engineering, London, E.C. 3, Market Buildings, Mark Lane, 1949. (40) Tajima, T., Japan. Patent 175,865. (41) United Ebonite and Lorviali Plastics (London), 15, 46 (1950). (42) Varta, N. V., Brit. Patent 638,850 (April 1950). (43) Whorlow, R. W., J . Rubber Research, 19,115-23 (1950). (44) Wicks, E. A., and Fehling, H. R., Brit. Patent 630,609 (October 1949). (45) Zade, H. P., Ibid., 639,997 (May 1950). RECEIWDJuly 12, 1951.
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Weathering Fence at Northern Regional Research Laboratory, Peoria,
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