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RESEARCH
Lowdownon N e w Synthetic Rubber P r e p a r a t i o n of p o l y i s o p r e n e involves lithium lyst, r e q u i r e s exclusions of moisture, o x y g e n PHILAE>ELPH1A.-As a follow-up to its recent announcement o>£ the development of a new synthetic polyisoprene rubber closely resembling tree rubber (C&EN, Sept. 5, p a g e 371.6), Firestone has now provided details o n techniques of manufacture. The n e w product, which Firestone calls Coral rubber, bears a striking similarilry to natural Hevea rubber, as indicate*! by infrared structure, x-ray crystallines pattern, ozonolysis, unsataxation. carbon-hydrogen ratio, molecular weight distribution, inherent viscosity, a n d othear factors. More than 4000 poinads o f Coral rubber h a v e been produced i n 50-gallon reactors by various procedures, Firestone's F . W. Stavely told t h e A C S Division of Rubber Chemdstry. A simple recipe calls for 100 parts of isoprene and 0.1 part of lithium a t a reaction temperature of 30° t o 40° C. Early investigations of f:he n e w process w e r e carried out in vapor p h a s e a t 60° t o 80° C . to minimize hazards a n d provide better temperature control. Later, b y use of agitated, liquid p h a s e techniques, it was possible t o reduce the reaction temperature t o 40° C. According to Stavely, the b>est p r o c e d u r e
Essentially the same as -natural r u b b e r is how Firestone's F . W. Stavely ( l e f t ) , L. E . Forman, a n d R. F» Dunbrook describe their new Coral synthetic r u b b e r 5026
C&EN
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for mass polymerization may b e to initiate t h e reaction at 40° to 5 0 ° C . until a decided thickening of t h e liquid o c curs, and then reduce t h e t e m p e r a t u r e . Once t h e initiation reaction takes place, the propagation reaction c a n continue at lower temperatures, w i t h considerable evolution of heat. If n o t properly controlled, t h e heat evolved may degrade the polymer sufficiently to cause charring. Thf* important factors influenc-incr structure of trie polymer are purity of components a n d exclusion of moisture, oxygen, a n d oxygen-containing materials. As monomer purity increases, it becomes much more reactive. Even with only traces of catalyst, t h e reaction may proceed with, explosive violence. > Isoprene Purifications. Isoprene of high purity is further purified just prior to u s e b y refluxing over sodium for four riours followed b y distillation, after w h i c h it is passed t h r o u g h a silica column just before use. T h e isoprene is kept o u t of contact with air o r moisture throughout t h e purification a n d polymerization process. The lithium metal catalyst is p r e pared by melting the metal immersed in Vaseline o r petroleum jelly and b y subjecting t h e molten mass t o h i g h speed agitation. T h e entire operation is carried out in a closed s y s t e m u n d e r an atmosphere of helium. T h e catalyst is generally prepared as a 359fc dispersion consisting of metal particles w i t h a mean diameter of 2 0 p. T h e Vaseline, which serves as a dispersing m e dium, also protects t h e catalyst, which is highly pyrophoric. Lithium appears to b e u n i q u e a s a catalyst i n isoprene polymerization in that it strongly favors formation of t h e cw-1,4 structure ( w i t h a t r a c e of 3 , 4 ) to t n e exclusion of trans-1,4. and 1,2 structures. Metals such as sodium, potassium, cesium, and r u b i d i u m give primarily mixtures of cis a n d trans-1,4, 1,2, and 3,4 structures. • C o m p a r e d to Nafyra!. Coral r u b ber differs from Hevea rubber, says Stavely, in that i t has slightly less cis1,4 structure a n d shghtly more 3,4 structure. T h e n e w material does n o t absorb oxygen a s readily a s natural rubber, has higher thermal stability,
a n d is more resistant to cracking in tire treads. Tread w e a r road tests o n truck a n d passenger tires show that Coral r u b b e r treads containing similar amounts a n d t y p e of carbon black as Hevea treads give 9 5 % of the wear obtained with natural rubber treads. Tires m a d e of Ameripol SN, a n o t h e r synthetic rubber similar to n a t u r a l r u b ber, h a v e proved satisfactory i n intercity b u s service, according to F. K . Schoenfeld of B. F . Goodrich. Xires of this t y p e were in good s h a p e after 350,000 miles of service at speeds as high as 7 0 miles p e r hour. Indications a r e that the tread wear i n d e x of -Ainerip o l SN tires is better tban that of t h e Hevea control tires b y a s much a s 2 0 % in some instances. From a practical standpoint, Amerip o l SN is equivalent i n properities to H e v e a rubber, says Scrioenfeld. This n e w development, he declares, h a s solved t h e long standing problem of producing synthetically a general p u r pose rmlyTner adequate as a complete replacement for Hevea r u b b e r i n heavy dutv tires.
Metal Whiskers GE makes perfect crystals o f several metals b y same process used to make first perfect crystals of iron VJTENERAX
ELECTRIC
scientists
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m a d e strong, perfect crystal "whiskers" of gold, silver, p l a t i n u m , copper, nickel, a n d cobalt by t h e same process b y w h i c h G E made t h e first perfect crystals of iron (C&EN, April 1 2 , 1954, page 1467). T h e process uses a n i g h concentration of metal i n t h e form of gas, o b t a i n e d either by chemical m e a n s or b y boiling. Reduction of a volatile salt with hydrogen was the method -used t o p r o d u c e t h e iron crystals. Eliminating imperfections in crystals enables t h e m to approach t h e strengths indicated b y theory. F o r example, perfect copper crystals can withstand tensile stress as high a s 600,000 p.s.i., while samples of ordinary annealed copper, composed of m o r e than o n e crystal, break a t 30,000 p.s.i., a n d single imperfect crystals at 5 0 0 0 . Another characteristic of the whiskers is their ability to remember their original shape. When stressed beyond a certain point, a whisker remains bent, b u t on heating it returns t o shape. GE's Sidney S. Brenner, wrao grew t h e crystals, also devised a method of measuring crystal strengths. Usually, such tests are c o n d u c t e d o n specimens with diameters of t h e order of o n e quarter inch. This i s impossible on
