ANNUAL REVIEW
Elastomer Tech nology Elastomeric compounds continue to be developed for engineering use as composites with carbon black, plastics, and cord he design and production of the automobile tire-
Tstill-as always-dominate the rubber industry. A key new emphasis in material science and engineering is in the word “composite.” The tire technologist can easily sit back and say, see, we have developed by trial and error composites of steel, cord, and rubber. They fit nearly all requirements for a smooth, cushiony, and safe ride for tens of thousands of miles at high speeds in all kinds of weather. Additional quality improvements can come both from the development of new components and the discovery of new combinations. The science of engineering mechanics is now building a mathematical framework upon which to display the complex behavior of multicomponent systems. The skilled designer must exercise judgment as to whether he should construct and then test the proposed composite or attempt first to calculate its behavior. I t might make a difference whether he is looking for a marginal improvement in a production tire or a major advance in a more costly and
A$$roximately 900 16 of rubber and 900 16 of body material, i n c hding more than 2 miles of bead wire, are used in the manufacture of a tire of this size and ty$e
GLEN ALLIGER FRED C. WEISSERT
In this review let us look both for generalized behavior and for uniqueness in the summation of properties of the several components of a composite structure
less familiar rocket propellant. The pat answer and correct answer is the pragmatic one-use the method which works best I n this short review of some of the literature on elastomers in 1967 we shall be discussing the different elastomers and their behavior mostly as if they were isolated entities, which they are not. W e shall here make passing reference to the fact that all high-molecular-weight materials have much in common and differ mainly in their , to crystallize, and chemglass temperatures ( T o ) ability ical reactivity. A high-molecular-weight material with either a T o or melting temperature (T,) greater than 50 "C can generally be called a plastic, while a polymer with a T , below -50 "C can easily be called a rubber (unless like polyethylene it cr)-stallizes). We (7A, 4A) have discussed the processing, dynamic properties, coefficient of friction, and abrasion resistance of butadiene/ styrene copolymers in terms of molecular weight distributions, T o and T,. These few parameters are very useful in estimating the performance of any polymer and should be understood before trying to find out if there are more profound complexities. PV = nR T is a useful equation with which to describe the equation of state of any gas. T h e modulus of any rubber is also proportional to the number of cross-links mulitplied again by RT. The viscosity of any polymer u p to temperatures ( T )of 100 "C above its T , and over a wide range of frequencies can be described by the WLF equation [log a, = - 17.4 ( T T,)/(51.6 T - T o ) ] , Log a, is the shift factor to be used for different frequencies ( 5 A ) . The equations which properly apply to a composite of a tire cord network protected by a rubber which is itself reinforced by small particle-size carbon black are more difficult to develop. Engineering mechanics with the aid of the slave labor furnished by computers is making progress on this area. Thus although we find it easier to present this review in categories of individual elastomers, specific laboratory tests, and special applications, we ask the reader to attempt with us to look both for generalized behavior and also for uniqueness in the summation of properties of the several components of a composite structure. I
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General
According to D. V. Rosato (ZA), the annual average rate of growth in elastomer production has been 9-1/2 70 over the past 10 years but is expected to drop to about 67, over the next 10 years. Natural rubber usage is now about three fourths of that of other elas52
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tomers in the United States. Emulsion styrene/butadiene (SBR) accounts for 66YG of the usage of synthetic elastomers in the United States followed by chloroprene (loyo)and butyl (7YG).Rosato lists 13 other elastomers to fill the remaining 177,. He quotes Angelo Fornara, managing director of AKIC, that the present synthetic rubbers can be expected to satisfy all current technical requirements provided that consumers accept the necessity of using the various rubbers selectively and in blends. I t is hoped that this review will aid in this respect. Of equal importance to research in the development of raw materials for use by the rubber industry is research in the fabrication operations themselves. Mi. F. Watson (3A) has pointed out continuous vulcanization by fluid beds, the use of liquid polymers, injection molding, and the use of computers to optimize the sequencing and production control as some of the newer developments in the modernization of current factory processes. Steel, cord, carbon black, curatives, antioxidants, and rubber must be skillfully and economically processed into a tire. The same engineering skills are required to assemble a motor mount, a collapsible rubberized fabric dam, a heart valve, or a solid rocket propellant.
Polyisoprene, Polybutadiene, Butadiene/Styrene Copolymers
Measurement and grading of the sensitivity of natural rubber to thermal and oxidative breakdown (plasticity retention index or PRI), utilization of highly oil extended compounds, and the development of more thermally stable vulcanizates were three items of major interest. T h e mastication behavior of natural rubber under conditions of low shear was shown by Greensmith (4B) to be essentially similar to its behavior in oven aging. The maximum dynamic heat loss of natural rubber vulcanizates was reduced with the use of higher hfooney viscosity polymer and by the addition of black and oil at the beginning of the Banbury mixing cycle instead of after a 3-min breakdown of the natural rubber (ZB). Grosch (5B) has reported that oil-extended natural rubber has superior traction over that of oil-extended SBR tire treads on ice and snow. The reverse is true for skid resistance at high temperature for nonoil-extended NR and SBR. U p to 2070 polybutadiene was recommended by Grosch in oil-extended natural rubber wintertire tread compounds in order to gain increased abrasion resistance.
Skinner and Watson (72B) found that large increases in the organic accelerator/sulfur ratio increased the amount of monosulfide and hence the thermal stability of natural rubber vulcanizates. They utilized a relaxation modulus test to obtain systems as efficient as the sulfurless TMTD-zinc oxide system without the disadvantage of the low scorch time and heavy bloom which accompany the T M T D system. They have gone as high in accelerator as 5 parts per hundred of rubber with as low as 0.33 parts of sulfur. Higher curing temperatures with less reversion and more thermally stable N R vulcanizates such as used in conveyor belts are possible with these efficient vulcanization (EV) systems. Mullins (6B) lists increased productivity (800 lb/ acre to 3000 lb/acre), improved quality and presentation, and applications research as being major goals of natural rubber research. He considers natural rubber to be the best general purpose rubber with regard to processing and performance in high-quality application where high resilience and high tear strength are required. Natural rubber can crystallize upon stretching. Thus, a vulcanized natural rubber gum has a much higher tensile strength than a nonreinforced SBR compound. The milling, tack, green strength, and tear resistance of N R may also be related to this ability to crystallize upon stretching. Dunning and Pennels (3B) have shown that a finite time is required to permit the molecules to align in crystalline order. Ten sec are required at 260% elongation while about 0.1 sec is required at 350% elongation. Thus at very high rates of strain, this reinforcement effect due to crystal formation and alignment may disappear, while at low rates of strain the reinforcement may be quite strong. Although in recent years organic solution polymerization methods have been developed as special processes for polymerization of butadiene and butadiene/styrene, the basic emulsion techniques developed in the 1940’s and 1950’s for SBR are still used to produce by far the greatest volume of present-day synthetic rubber. Recently new studies have been reported to improve upon the basic emulsion process. Uraneck and Burleigh (74B) have measured the rate of depletion of Cg and Cl6 mercaptan modifiers with respect to degree of conversion. If the rate is low, modifier remains unused at the end of the polymerization. If the rate is too high, the modifier is depleted before the polymerization is carried to its desired end. Careful selection of modifier type, including the use of incremental addition of modifiers, can be used to control Mooney viscosity and molecular weight distribution of the polymer. A polymer of broad molecular weight distribution broke down on milling to a greater extent than a narrower one. Vaclavek (75B)has published three papers dealing with the regulation of the molecular weight of emulsion SBR. He selected diisopropyl xanthogen disulfide as being the most convenient of 14 different xanthogen disulfides for the regulation of molecular weight. He then studied the effect of the kind and amount of anionic emulsifier, pH, rosin type, and temperature on the regulating efficiency of diisopropyl xanthogen disulfide. tert-
Dodecyl mercaptan with negligible termination was considered the best aliphatic mercaptan for the regulation of molecular weight. The maximum intrinsic viscosity that could be obtained with this system was 6.5. Orr and Breitman (7B)have prepared emulsion copolymers from isoprene and acrylonitrile and have found the 1,4 content of the isoprene to be higher than normally expected from emulsion polymerization. X-ray and phase contrast microscopy showed a degree of lateral ordering attendant with stretching. I t is by now well known that solution polymerized polybutadienes either of the medium or high cis-l,4 type show better abrasion resistance but lower wet pavement traction than emulsion SBR in tire treads. I t would be well to know whether there are any general polymer features that encompass this behavior. As stated previously, the value of the glass temperature, crystalline melting point (if any), and molecular weight distribution are three of the most general parameters with which to characterize any polymer. Alliger, Weissert, and Johnson ( 7 4 4 4 ) have pointed to a simple generalization which indicates that in normal passenger tire service, the wear resistance generally increases and the traction decreases with elastomers containing a lower T,. With variations in the styrene and 1,2 content to achieve a desired T u ,a butyllithium copolymer of butadiene and styrene was developed having a good balance-of-wear resistance and traction. The rationale behind this correlation between To and wear and traction is related to expectedviscoelastic behavior. At operating conditions of low temperature or very high speeds (as postulated in the abrasion process), a lower T opolymer will retain its rubbery behavior to a lower temperature or higher speed than a high T o polymer. I n general, it has been observed that the wear conditions of increasing severity and/or lower ambient temperature result in higher wear ratings for the low Tu solution polybutadiene us. SBR. Werner, Gunberg, and Roach (76B) recently reviewed similar differences between low Tu emulsion polybutadiene us. emulsion SBR. Even if the above T,-wear traction correlation has some cause-and-effect reality behind it, much more needs to be done to establish the actual range of temperatures and rates of deformation in tire abrasion and traction with variable speeds, slip angles, road surface, and ambient temperature. Later in this review we shall point to Grosch and Schallamach’s tire-surface temperature and tire-wear correlations. They do not mention a possible major controlling effect of polymer Tu. Other factors especially including the removal of highly oxidized surface layers have also been suggested as a major factor in tread wear. The issue is complicated. Sarbach, Hallman, and Sudekum (70B) have reported oil-extended polybutadiene/SBR blends which have greater tread wear resistance plus greater resistance to cutting and chipping than natural rubber/SBR blends. This increased resistance to cutting and chipping was accompanied by a somewhat lower state of cure and higher running temperature on a Goodrich flexometer. Polymer-type filler, extender, and curative level must all VOL. 6 0 N O . 8
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be balanced to give the best “trade-off of desirable properties.” Stephens and Bierman ( I 3 B ) also showed that at low wear severities, polybutadiene was marginally better than SBR, while at high wear severities the relative wear rating was greatly improved by the addition of polybutadiene. They found a good compromise between traction and abrasion with the use of oil-extended polybutadiene. Alfin rubbers, copolymers of butadiene, isoprene, or styrene, made with a catalyst of the sodium salts of an alcohol, and an olefin are stated to give from 15 to 30% longer wear than general purpose SBR and to have improved cut-growth and cracking resistance. Sims ( 7 I B ) has reviewed the polymerization methods for the preparation of polybutadienes including the Ziegler, alkali metals, metal alkyl, metal oxide, Alfin, cationic, and free-radical emulsion techniques. He has outlined the syndiotactic, isotactic, cis-l,4, trans-l,4, and vinyl 1,2 microstructure of the resulting polymers. Bahary (7B) has listed the macro-as well as the microstructural differences between six commercial polybutadienes. He listed alkyllithium polybutadiene as having the most narrow molecular weight distribution and the lowest degree of long chain branching, while the emulsion polybutadiene is highest in regard to both molecular weight distribution and branching. Ring and Cantow (9B) have elucidated the cationic reaction mechanism by which the molecular weight of a high cis-1,4 polybutadiene made from a cobalt and organoaluminum compound can reproducibly be raised after polymerization by the addition of alkyl or acyl halides. Other Elastomers
Samuels and Wirth (70C) have described a new ethylene/propylene/diene terpolymer which has a rapid rate of cure, higher dynamic modulus, and is highly adaptable to oil extension. The nature of the diene was not specified. The new EPDM terpolymer was said to blend more easily with SBR without loss of physical properties because of the similarity in cure rate of the two components in the polymer blend. The ethylene propylene rubbers are being used in blends with synthetic or natural rubber to produce white sidewalls with improved ozone, sunlight, and temperature resistance. German, Vaughan, and Hank (6C)have prepared an excellent review of ethylene propylene terpolymers. The selection of the proper diene depends upon price, ease of copolymerization, steric effects, and vulcanization characteristics. Molecular weight control is a function of the type of solvent mixture used in the polymerization. The vulcanization reaction is influenced by the termonomer type and amount, the molecular weight distribution, and the sulfur/accelerator ratio. The bicycloheptene ring is attractive because in this case the rate and distribution of ethylene/propylene along the chain are satisfactory and many variations in termonomer content and molecular weight distribution are possible. Ferrari (4C) considers polyurethanes as being block copolymers, with a flexible block being derived from the 54
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polyols, and a more rigid block formed from the condensation of the diisocyanate and diamine. I n comparing polyether us. polyester based urethanes, he generally classifies the polyethers to be better than the polyester urethanes with respect to processibility, low brittle point, low heat buildup, and humidity resistance. The polyesters are preferred for toughness, abrasion, solvent, and heat-resistant properties. Dunleavy and Critchfield (3C) have presented an excellent paper on the influence of polyurethane structure on the tensile, modulus, and dynamic torsional pendulum properties at both low and high temperatures. Amine-cured systems exhibited better high-temperature strength and low-temperature performance than similar hydroxy-cured systems. Sulfur cures of unsaturated amines and hydroxyl-extended systems further increased the thermal stability but decreased the low-temperature performance. As the molecular weight of the poly01 is increased and the mole ratios of the other components are held constant, the modulus, elongation at break, and the mechanical loss are lowered over a temperature range from - 7 5 to 400 O F . Tobolsky, Johnson, and MacKnight (72C) have observed that polyurethanes are remarkable elastomers in that they undergo nonoxidative stress relaxation at elevated temperature even though they are crosslinked. This behavior has been observed with polysulfide and silicone elastomers and has been attributed by Tobolsky to bond cleavage and reformation. They saw evidence for both a strong and weak linkage. Singh and Vb-eissbein ( 7 7C),in studying the stress relaxation of welldefined polyester-toluene diisocyanate-1 ,1’,1”-trimethylol propane networks, identified the dominant slower process to be dependent upon the content of urethane groups. They were not able to identify the more transient stress degradation bonds. The high strength and abrasion resistance of cast polyurethanes prompted the development of millable polyurethane rubbers for processing by normal rubber techniques. The performance of these millable rubbers has been reviewed by Pyne (9C). I n the case of the millable polyurethanes, filler reinforcement was then necessary to develop the desired hardness and toughness observed in the cast polyurethanes. Probably the main thought to be kept uppermost is that options €or obtaining a wide range of properties are immense in the field of polyurethanes. Continued efforts toward specifically defining the effects of polymer structure on the mechanical behavior of polyurethanes will enable the polymer design engineer to produce a given polyurethane for a specific end use. Perhaps a millable polyurethane will, in the future, be developed for use as a tire tread material with a broad range of improved properties over conventional polydiene-fillersulfur compounds. Already, of course, there are some applications where a polyurethane compound fits the need better than normal rubber-filler-sulfur compounds. We shall not be discussing in this review any new current literature concerning polychloroprene, butyl, and silicone rubbers. Their important industrial fields of elastomer application are well established.
Delafield (2C) has reviewed the effect of improved manufacturing techniques including the use of cold polymerization and modifiers on the processing and properties of nitrile rubbers. The early nitrile rubbers had a high gel content and were difficult to process. Lower polymerization temperature and the use of modifiers have led to polymers which have less gel, are more linear, have better processibility, less shrinkage, and better filler acceptance. A torque rheometer was used to characterize the rheological behavior of these new nitrile rubbers. Henry and Griffis (7C) have prepared a Nitroso Rubber Handbook which describes new nitroso elastomers which have good low-temperature properties, solvent resistance, stability to corrosive environments, and flame resistance. These elastomers are characterized by a given nitrogen-oxygen-carbon sequence in combination with a highly fluorinated linear polymer chain. I n the next section of this review we shall be emphasizing blends of polymers either in simple mixtures or in block copolymer form. The current trend clearly is to find special properties by combinations of pure structures. This approach can also be considered in the placement of different atoms along a single polymer chain as did Ossefort and Veroeven (8C) in order to develop low-temperature, oil-resistant, millable polyether urethane urea elastomers. Millable, carbon black-reinforced, peroxide- and sulfur-cured vulcanizates exhibited good strength after oven aging at 250 O F , excellent low-temperature properties, and excellent oil, ozone, and fuel resistance. Fournier (5C) has described epichlorohydrin rubber produced by opening of the epoxide ring to produce a polyether main chain with good low-temperature flexibility and chlorinated side groups which confer exceptional oil and flame resistance. The elastomers may be cross-linked with diamines. Polymer Blends, Blocks, and Composites
All mixtures of high polymers will be characterized by a certain degree of heterogeneity. No such blends can be thermodynamically described by saying that the components are completely soluble one in the other, although by certain test procedures the degree of mutual solubility might be quite high. The sum of the physical properties of the individual components of a blend will be expected to be equal to the properties of the composite if the components are mutually soluble. New and unusual composite properties, not so readily apparent from the properties of the individual components, are expected with heterogeneous blends. Corish (30) has shown that in the case of the blending of two rubbers with similar solubility parameters, a high degree of mutual solubility is evidenced by the detection of one average glass temperature value, as with polybutadiene and SBR. A blend of polybutadiene and natural rubber showed two separate glass temperaturesevidence of a heterogeneous mixture. The dynamic
modulus and hysteresis losses of a polymer vary greatly over certain ranges of temperature, frequency, and amplitude of test. DeDecker and Sabatine (50)have shown how cis-1,4, trans-1,4, and vinyl groups and per cent styrene in butadiene/styrene copolymers affect the glass temperature and hence the regions of variations in dynamic response. An important method for broadening the temperature or frequency range of the damping maximum is by blending of polymers. Morris (720), by a rate of crystallization technique, has come to the same conclusion as Corish-namely, that high cis-l,4 polybutadiene is more compatible with SBR than with natural rubber. The more soluble SBR affects the crystallization rate and behavior of high cis-l,4 polybutadiene while the more incompatible natural rubber does not alter the crystallization rate of the high cis1,4 polybutadiene. Smith (730),in studying the degree of vacuole formation and stress softening (Mullin’s effect) in SBR/polybutadiene blends reinforced with carbon black, has noted that the amount of vacuole formation in the areas of carbon black agglomeration was reduced with increases in the polybutadiene portion of the blend. Hess, Scott, and Callan (80) have determined by a new electron microscopy technique that the carbon black in a polybutadiene/natural rubber heterogeneous blend tends to preferentially end up in the polymer phase with the lowest viscosity. The appropriate viscosity value is determined at the mixing rates which are used during the mixing process. The foregoing examples describe rubber blends for which a high degree of compatibility was expected. Polystyrene is not expected to be soluble in polybutadiene. I n recent years composite polymers have been prepared where segmented blocks of different polymers exist in the same chain. Styrene-butadiene-styreneblocked copolymers have melting characteristics and rubbery properties at room temperature such that they may be described as thermoplastic rubbers. The properties, compounding, and applications of these materials produced by lithium-based solution polymerization have been given by Deanin ( 4 0 ) . The dynamic moduli, glass temperatures, tensile and melt flow properties of random and block copolymers prepared from butadiene have been analyzed and interpreted by Kraus, Childers, and Gruver ( I D , 700). With block polymers there is a tendency toward association of similar segments with the observation of two different glass temperatures and points of maximum loss tangent. Polymers with equal styrene level may show different glass temperatures, depending upon the value of the molecular weight of the segments. The tendency for association persists in the melt, even at temperatures above the T,’s in the polymer. Anomalies in flow behavior were attributed to association of compatible segments. The initial high tensile modulus of SBS elastomers was reduced during stretching presumably because of the partial dissociation of the polystyrene aggregates. Additional strengthening of the SBS elastomers, which are already reinforced by the association of terminal polystyrene blocks, by VOL. 6 0
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normal carbon black loading or sulfur curing of the rubbery flexible polybutadiene segment did not improve the over-all properties of the composite. I t is not uncommon to observe that by increasing the level of any of the parameters which increase the stiffness, a curved response for energy to break is realized. There is usually a unique value of modulus, below and above which the energy to break will fall below a peakvalue. Cooper and Tobolsky ( 2 0 ) have also postulated in the case of polyurethanes that association of hard segments in the solid state is required for the occurrence of a second higher T , phase which is responsible for the self-reinforcement of the polymer. A novel thermoplastic rubber which can be remelted and reused several times has been developed by blending asphalt, oil, and polyethylene copolymer, modified with bulky side groups to improve the compatibility with the asphalt and oil. This rubber sealant which can be applied like a sealing wax can be utilized in driveways, roads, and water tanks. Noncrystalline polyethylene is preferred for use in this thermoplastic rubber blend ( 7 0 ) . Rubber reinforcement of plastics has made the composite tougher and better than the pure plastic. A great deal of effort is being devoted to study of the best compositions, not just as to the percentages of different components in the total system-but as to the microstructure of the composite. The variety of attainable differences in heterogeneous compositions increases many fold over that possible with homogeneous blends. Keskkula and Traylor (9D) have observed the particle size, shape, and structure of the rubber inclusions in rubber-reinforced polystyrene after different types of processing. Molau and Keskkula ( 7 7 0 ) have described the mechanism of particle formation in rubbermodified vinyl polymers where the rubber is first dissolved in the monomer. Phase inversion occurs below 4oYe conversion. The rubber plus styrene particulate phase is then hardened both by further monomer polymerizations and cross-linking of the rubber. Deland, Purdon, and Schoneman (6D) have reviewed the processes for making rubber-modified high-impact polystyrene. Only 5Ye rubber is requiFed to increase the notched izod value by 6-fold. The rubber type must be carefully selected to meet the customer’s process equipment, Some of the important rubber properties for use in high impact polystyrene are (1) solubility characteristics of the rubber in monomeric styrene to a given solution viscosity, (2) molecular weight distribution, (3) freedom from gel and contaminants, (4) inhibition of the polymerization, and (5) color contamination. Compounding, Fillers, Extenders, Curatives, Protective A g e n t s The rubber-fine particle size composite is one of the oldest and still most important examples of a heterogeneous mixture which is more useful than expected from consideration of the properties of the individual components. Carbon black is said to reinforce rubber in that it imparts toughness and abrasion resistance to 56
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the composite. The micromechanistic description and understanding of this behavior are still far from being understood. Chemical, physical, and molecular-slippage bonding theories between black and polymer have been proposed as major contributing factors. Perhaps the surface layer of rubber around the black is somehow different from the major portion of the rubber matrix. The formation and reformation of the aggregate nature of the carbon particles may be a significant factor. Then again, perhaps some Isaac Newton of the future, after a clear and simple analysis of the microstresses and -strains, will, in tensor notation, say that of course this is what one would expect-that carbon black should reinforce rubber! Meanwhile, Brennan and Jermyn (3E) have related the modulus of a filled rubber at low extension to reinforcement by the chain-like structure of the filler. At high extension ratios they related the modulus values to that expected from the volume of loading of the unit “filler plus bound rubber.’’ After compensation for the actual elongation of the rubber matrix, as distinct from the measured elongation for the filled specimen, they found the Mooney-Rivlin equation to fit the stressstrain data of filled stocks. Tt‘estlinning (16E) has also suggested that a highly ordered sheath of rubber is formed around the filler to change the physical properties of the filled rubber. Smit (72E) has interpreted the dynamic properties of a carbon black-filled rubber in terms of a physical adsorption of rubber on the black surface. This adsorbed layer is assumed to have different properties from the bulk rubber. Spath (73E) has attributed the static and dynamic behavior in filled vulcanizates to the effect of the statistical mechanical response of elementary bonds between filler particles and rubber. Voet and Cook (15E)have studied the dynamic properties of filled rubbers over a wide range of amplitudes in cyclic deformation. They concluded that the measured reversible stress softening up to 1007, strain is caused by a reversible thixotropic breakdown of carbon chains. This process was said to reinforce b>- an energy dissipation mechanism. At high extensions they concluded that persistent, fixed carbon chains contribute to reinforcement by their influence on the cross-link density of the network. Harwood, Mullins, and Payne (6E) have given an excellent review of the proposed micromechanical processes which may occur in filled rubber systems. These include network rearrangement, breakdown, and reformation of carbon black aggregates, polymer molecular slippage along the filler surface, strong and weak bonding of filler to the matrix, and displacement of large particles through the rubber. The phenomena of stress softening have been observed in gum as well as in filled stocks if both are deformed to the same stress as opposed to the same strain level.
Glen Alliger is Director of Physical and Chemical Laboratories and Fred C. Weissert is Senior Research Chemist, Fundamental Polymer Structure Group, Firestone Tire and Rubber Co., Akron, Ohio. AUTHORS
Harwood and Payne (7E) have suggested that the recoverable stress softening may be attributed to localized nonaffine deformation in microregions where short chains have already been elongated to their limit of extensibility. Oberth (TOE) has discussed the strength reinforcement of filled rubbers as being related to stress concentration patterns around the filler. The shape, concentration, and particle size of the filler are related to reinforcement. Adhesion between filler and matrix is assumed. Tensile reinforcement is considered to be more readily obtained with a matrix capable of a high degree of stress softening. Carbon black particles are composed of tiny crystallites. Hess (8E) has, by diffracted beam electron microscopy, resolved groupings of crystallites in the carbon black. A concentric orientation of crystallites was observed a t the surface of the particles. Graphitization of the carbon black resulted in a growth in the crystallite size. The relative effect of aromatic and paraffinic nature of the extender oils was evaluated by Stout and Eaton (74E) in the oil extension of natural rubber and SBR. Compounds containing the more aromatic oils had the higher Mooney viscosity and smoother extrusions, and they reduced die swell. The same compounds cured at a faster cure rate, had higher tear and flex resistance and a higher heat buildup than those compounds containing the more aliphatic oils. Saville and Watson (7 7E) have written an excellent review of the available methods for the determination of the microphysical network structure, including crosslink type and concentration, and the chemical nature of the cross-links. T h e density and functionality of chemical cross-links, entanglement, and chain ends are determined by the C1 term of the Mooney-Rivlin stressstrain plot, swelling, equilibrium compression modulus of a toluene-swollen sample, freezing-point depression, relaxation modulus, or sol-gel studies. Current problems concern efforts to distinguish between chemical and physical cross-links, effect of polymer branching, methods for correcting for filler content, and the determination of proper swelling of interaction parameters. The chemical nature of the cross-link is inferred from sulfur/accelerator reactions with model compounds and the use of chemical probes, including sodium sulfite, methyl iodide, lithium aluminum hydride, triphenylphosphine, and trialkyl phosphite. The chemical probes enable distinctions to be made between mono-, di-, and polysulfide network linkages. There is general agreement that as the vulcanization proceeds the initial high concentration of polysulfides decays with the gradual formation of monosulfide cross-links. Loan (923) has reviewed the mechanism of the peroxide vulcanization of elastomers. The physical properties of peroxide-cured unsaturated elastomers are generally rather inferior to those obtained with accelerated sulfur cures. tert-Butyl peroxide and cumyl peroxide are the major useful peroxides that can be used in the presence of reinforcing fillers. There is a great difference in the behavior of different rubbers toward the peroxide
with respect to both the degree of degradation and cross-linking. With butyl rubber and polypropylene, there is more degradation than cross-linking. The number of moles of cross-links formed per mole of peroxide decomposed, is somewhat less than 1 for ethylene propylene rubber and polychloroprene, about 1 for polyisoprene, but may vary from 2 to 100 with polybutadiene. A chain reaction vulcanization may occur with the polybutadiene. Diacyl peroxides are better than dialkyl peroxides in curing silicone rubbers which preferably have a small amount of vinyl groups added to the polymer backbone. Colclough (4E) recommended the use of tellurium diethyldithiocarbamate and dipentamethylene thiuram tetrasulfide for increasing the cure rate of EPDM rubbers. Curing temperatures above the usual 160 "C for EPDM can be recommended as EPDM sulfur networks are resistant to heat reversion. Amsden (7E) has devised a quantitative annulus ozone test for a rapid measurement of the threshold strain for the beginning of ozone cracking. He considers ozone cracks to initiate at flaws and grow at a rate which increases as the strain is increased provided that the stress is above a critical stress. He considers this test to have advantages over those ozone tests which measure exposure time required for cracking, visual characterization of crack severity, rate of crack growth, or degree of stress relaxation. He found bis(dimethy1pentyl p-phenylenediamine) without wax to furnish better protection than isopropyl phenyl p-phenylenediamine without wax. I n the presence of wax, the ratings of the above two antiozonants were reversed. Ethylene propylene rubbers are being blended with natural rubber for increased ozone resistance. Andrews (ZE)with the aid of an electron microscope found these two elastomers to exist in separate phases. Ozone cracks traverse the reactive natural rubber phase and occasionally jump across the inert ethylene propylene phase without severing it. The inclusion of an ozone inert phase raises the critical stored energy necessary to initiate ozone cracking by provision of physical barriers which inhibit crack propagation. For the protection of polychloroprene, diaryl-substituted p-phenylenediamines give the greatest ozone resistance plus the least tendency to induce bin scorch in the rubber. Diaryl-substituted p-phenylenediamines in which the amine function is hindered by the presence of an alkyl group adjacent to the amine are preferred (5E). Tire Cord
I n the development of a tire as a composite of rubber and cord elements, most of the laboratory tests evaluate the mechanical properties of the individual components. The composite behavior is usually ascertained from the performance of a tire. Eccher ( 2 F ) has described the similarity in the failure properties of a laboratory Mallory tube specimen of cord and rubber and a tire. The Mallory tube prepared from rubber reinforced with one ply parallel and two plies perpendicular to VOL. 6 0
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the cylinder axis was inflated with air pressure and then flexed to failure. He found the measured time for the tube to burst to be more discriminating than a measurement of: the cord properties at intervals during the test. He observed several modes of failure, such as separation of the rubber and adhesive and initial tears within the rubber, mainly where two textile cross-sections have approached each other. He considers the fatigue failure as not being directly related to the fatigue of the textile cord itself, but as a fatigue test of the whole system: rubber, adhesive, and cord. Another example where apparently test conditions are being designed to more closely approximate those of tire performance is the recent use of high speed tensile and tire plunger tests to evaluate the strength of tire cords. The relative energy to break of rayon us. nylon is increased as the temperature and rate of deformation are brought into closer correspondence to those which exist under conditions of impact breaks in tires. Howard and Williams ( 3 F ) have resolved the mechanism of the flat-spotting of nylon tires in terms of conventional principles of polymer viscoelasticity. Flatspotting is considered to be caused by a condition where strains in the cord in the footprint area are less than the strains elsewhere in the tread. The creep compliance function of Nylon 66 was useful in interpretation of flat-spotting in tires. Claxton, Forster, Robertson, and Thurman ( I F ) have described a dynamic flatspot cord-testing machine which permits close simulation of tire cord stress cycles that duplicate the flatspotting phenomena. The incorporation of molecules of bulky groups along the polymer chain achieved through blending with other polymers and plasticization were used to favorably alter the viscosity-temperature relationship of nylon tire cords. A new interest in glass fibers has developed especially for use in the belt of either a radial ply or bias ply tire. Outstanding dimensional stability is claimed with the use of glass instead of conventional polymeric organic fibers (5F). General Viscoelasticity
T h e general concepts of viscoelasticity are pertinent to all phases of rubber processing, static or dynamic deflection, and failure properties. Depending upon the glass temperature, crystalline melting point, and the macrostructure including molecular weight and degree of cross-linking, and further depending upon the temperature and frequency of the test, the polymeric material may behave as a nearly elastic solid, a viscoelastic leather, a nearly elastic liquid, or a viscous liquid. T h e purpose of this part of the review will be to alert the general reader that a great deal of information is being developed toward the understanding and formalization of the general behavior of complex viscoelastic behavior. The rubber industry will continue to operate within the complexities of non-Newtonian flow behavior, normal stresses, capillary end effects, die swell, heat buildup at constant force, amplitude or 58
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
energy over a wide range of frequencies and temperatures, complex crack growth, and abrasion processes with the current incomplete understanding of underlying viscoelastic mechanisms. Metzer, White, and Denn (6G) have presented a short review of current constitutive equations which attempt to describe completely the time-dependent properties of all materials. I t would be instructive for the general reader of this review to skim these papers to behold the mysteries of a tensor analysis of fluid-like rubbers. They possess a memory of the deformations forced upon them in times past and perform odd motions akin to cake batter (which climbs up the beater blades at right angles to the direction of centrifugal force). The apparent viscosity of polybutadiene, as an example, is dependent upon molecular weight, temperature, and shear rate or shear stress. Krishnamurthy (5G) has found that the apparent viscosity molecular/ weight relations can be represented by a set of parallel lines, each representing data obtained at a fixed shear stress. The critical molecular weight required for the onset of chain entanglement increased with increasing shear stress. Kraus and Gruver (4G), who in the past have well characterized the flow behavior of polybutadienes, have measured the steady-state apparent viscosity of several elastomers plasticized with liquid hydrocarbons. They have developed master curves of the flow behavior by plotting a function containing shear rate, volume concentration of elastomer and a shift factor on the x-axis and the apparent viscosity and the same shift factor on the y-axis. The effect of the diluent is both to loosen the polymer entanglements and to reduce the polymer segmental friction factor. The apparent viscosity of rubber as a function of the shear rate has also been measured by Smit and Van der Vegt (8G). They found that the apparent viscosity of natural rubber, synthetic polyisoprene, and SBR dropped by three orders of magnitude as the shcar rate was increased. The Braebender torque rheometer is being used to evaluate the general processibility behavior of elastomeric mixes. The usefulness of the data has been limited by the inability to convert the instrument data to absolute rheological units. Goodrich and Porter (2G) by calibrations with a Newtonian liquid and making some assumptions as to the nature of the sigma-blade mixing head have been able to make approximations which correlated well with capillary rheometer data. I n the past few years Ninomiya, Yasuda, Furuta, Kusamizu, Maekawa, Homma, and Nakao have published a very important series of papers on the nonlinear viscoelastic properties of rubberlike polymers (7G, 7G, 9G, 7OG). A partial listing of these articles include viscoelastic analysis of resilience, Mooney viscosity, hardness, and Goodrich flexometer tests. They have also measured and predicted the dependence of the molecular weight and molecular-weight distribution on the relaxation spectrum of many different elastomers either singly or in blends. I t should be pointed
out that if the distribution of relaxation times is known for a given system, then, in principle, all of the types of viscoelastic response like stress relaxation, creep, tensile, and dynamic response are calculable by approximation methods from the same single set of relaxation times. Yasuda and Ninomiya, among others, are making a fast start in this direction in the field of elastomers. Ito (3G) has measured the non-Newtonian flow of dimethyl siloxanes and found it to increase with increasing molecular weight of the siloxane. He also measured the magnitude of the capillary end effect for these polymers. Processing
Hooper ( I H ) has measured the apparent viscosity, extrudate swell and surface roughness of SBR at several carbon-black and oil loadings and at shear rates from 10 to 3000 sec-I. With an increase in oil level, the viscosity and die swell were reduced. With an increase in carbon black, the viscosity was raised and the die swell reduced. This type of information on the rheological behavior of an elastomer compound should be more useful than a single Mooney viscosity value obtained at one shear rate. The hydrosolution masterbatch technique whereby carbon black from a water slurry is transferred to polymer-solvent solution before dewatering and drying is said to provide better carbon black dispersion, reduce power consumption, and avoid polymer degradation. I t is also possible to produce complete compounds containing sulfur and accelerator. I n this latter case the problem of dewatering and drying without incipient vulcanization and the sensitivity of accelerators and antioxidants toward hydrolysis must be handled with care (SH). Perlberg (4H) has described a Transfermix which is said to move rubber more smoothly and permit better mixing than occurs in the conventional screw extruder. T h e injection-molding process of producing rubber vulcanizates continues to draw a great deal of interest. Rosenthal and Reissenger (5H)list high production rates, lower labor and lower-scrap costs, and more uniform cure profiles, since the rubber is hot when injected into the mold, as being major advantages for injection as opposed to compression molding. With the ram and screw machine, the temperature of the material in front of the screw is 120 and 170 O C after injection. The flow properties cannot be correlated with Mooney viscosity or Defo plasticity tests but do show some relation to values obtained with high-speed capillary extrusions. Formulations for injection-moldable compounds are not much different from those found suitable for normal compounds designed for compression molding. Izod and Watson (2H) have pointed out advantages for the screw ram as opposed to the plunger-type injection-molding machines. With pressures at 18,000 psi or above, there are few limitations as to the nature of the rubber compound suitable for injection-molding. T h e physical properties of the
injection-molded vulcanizates are fully equal to those obtained by compression molding. Dynamic Testing of Vulcanizates
We have at many points throughout this report pointed to several specific examples of dynamic testing of vulcanizate products. I n the general section on viscoelastic behavior we have also reviewed some of the more general considerations of the importance of knowing the whole distribution of relaxation times of a given material. Buswell, Gee, and Thornley (11) have provided an excellent review of the theory behind most of the current types of dynamic test equipment which measure such viscoelastic parameters as the elastic and loss moduli of rubber compounds. One of the experimental problems of analysis of either experimental or in-use deflections is how to describe nonlinear behavior at large amplitudes as well as the effect of the self-heating of the sample caused by the repeated large deflections at high frequencies. The above cyclic test function is usually sinusoidal in nature. However, it has been pointed out by Priss (81) that although the deformation of the tire surface is periodic, it is not sinusoidal. This factor can cause considerable difference in the interpretation of the dynamic data as applied to tire performance as especially related to fatigue failure. Moore and Larson (71) have simulated the dynamic penetration of tread rubber by the asperities of a road surface. The resulting penetration-time history of the draping of the rubber was analyzed to determine its damping and elastic properties. When selecting optimum tread rubber properties, the importance of jointly considering both draping and traction zones in the contact patch of a rolling tire was emphasized. T h e draping (or penetration) action of the rubber around the asperite is very rapid. At 60 mph, it takes place in about 3 msec. The general concepts of current viscoelastic theories are expected to apply to the regions of small deformations and not necessarily at the large deformation characteristic of failure. However, it appears that toughness or high breaking energy may be related to processes which can dissipate mechanical energy into heat instead of producing two new fracture surfaces. Grosch, Harwood, and Payne (31) have recently demonstrated that the breaking energy of a large variety of elastomers is proportional to the hysteresis losses which result from stretching the sample to its breaking strain. The above relationship holds for filled and unfilled rubbers, swollen and unswollen networks, tested from - 50 to 160 OC. Ecker (21) has presented stress-strain curves for filled and unfilled rubbers over a broad range of test temperatures and speeds. He has investigated the nonlinearity in the stress-strain curves of amorphous and crystalline rubbers, filled and unfilled, over a temperature range from -150 to 150 " C , and over a four-decade range of test speeds. I t was not possible to deduce a general nonlinear behavior pattern which represented all of the test conditions. VOL. 6 0
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The interpretation of strain rate-test temperature parameters especially at high strain rates is complicated by a rise in sample temperature. Jones and Beeson (41) have noted that although they were not able to measure the temperature increase, by referring to stressstrain properties at lower speeds over a range of test temperatures, they concluded that a 5 to 15 " C increase in sample temperature may occur due to the GoughJoule effect. Kawabata and Blatz (51) have developed a theory for creep failure of a gum rubber like SBR 1500. Equations for first-order crack growth rate were postulated with the assumption that the test specimen contains defects which may grow into a crack with a given applied load. The creep which accompanies the "growthof-defects" process is due to slow rupture of polymer chains which are locally highly stressed in the neighborhood of a growing defect. An empirical relation between break time and true stress was presented. The failure envelope, characteristic of most polymers, describes the locus of failure points relating the ultimate strength to the ultimate elongation. Different test conditions change the position of the failure point of a polymer on its failure envelope but are not expected to change the shape of the failure envelope itself. T h e degree of cross-linking and presence of filler will change this envelope if the tensile strength is plotted against degree of elongation. Smith and Frederick (91) have generalized the failure envelope by plotting the true stress at break (tensile at break X relative elongation) against the factor of relative elongation at break minus one times the equilibrium modulus. They studied hydrofluorocarbon elastomers, butyl, silicone, and SBR rubbers at different degrees of cross-linking. The tensile tests were carried over a range of test speeds and temperatures. They were also able to estimate the maximum degree of chain extensibility for a given vulcanizate. Landel and Fedors (61) were able to construct a different form of a generalized failure where the tensile stress is normalized to a unit amount of cross-linking. This normalized tensile stress is then plotted directly against elongation at break. I n conclusion, the viscoelastic character of the polymer determines the position of the failure point on a failure envelope whose shape is determined in part by the value of other parameters including the degree of cross-linking. l i r e Friction
and Wear
The interplay of all factors involved in tire traction is under active investigation in order to increase the safety of automobile performance. I t is clear that road surface, tire design, and the nature of the tread compound all play a significant role in the demands for tire traction over a range of speed and temperature. Kummer and Meyer ( 4 J ) have presented a unified theory of rubber friction which suggests that the adhesion and hysteresis component of friction are both different manifestations of the same energy dissipation process. They postulate that the adhesion component 60
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
of a friction system is due to the excitation of the molecular structure by bond rupture at the rubber-pavement interface followed by the dissipation of energy within the vibrating structure. The so-called hysteresis mechanism is derived from the periodic flexing of a finite layer of the bulk rubber due to deflection over the rough asperities of the road surface. The most significant suggestion of this theory of Kummer and Meyer is that high damping rubber increases not only the hysteresis but the adhesion coefficient as well. O n wet pavement the adhesion component is reduced and, especially at high speeds, the role of the hysteresis drag is increased. Sakey ( 6 4 has set the first requirement for good skidding resistance on wet roads as a breakthrough of the water film in order to establish areas of dry contact. Small-scale sharp edges in the road surface result in high local pressures (about 1000 lb/in.2) sufficient to penetrate the film. Drainage channels provided by coarser road texture and by tread patterns facilitate removal of the water. At 30 mph, the small-scale texture is considered to be the dominant factor. At high speeds it becomes increasingly difficult to penetrate the water film in the time available. At these speeds reliance must be placed on the use of energy losses in the rubber as it is deformed by the larger projections in the road surface. Ludema and Tabor (5.4, from results of friction measurements over a temperature range of -100 to 200 "C, showed that, for rubber, there is a close relation between sliding friction (at various speeds and temperatures) and viscoelastic properties. With polymers below their glass transition temperature, sliding friction does not correlate directly with viscoelastic properties. With rubber, the slip of chain segments over one another appears to remain the basic mechanism of shear even at the high rates involved in sliding. Savkoor ( 7 4 estimated the friction coefficients of various elements of the tire tread in the contact patch by considerations of the viscoelastic dependence of the rubber friction and heat transfer at contact and estimates of slip speeds, slide distances, and surface temperatures under various conditions of operation. Kern ( 3 J ) has suggested the use of ethyl palmitate as an additive in a tire to aid in the removal of water from the contact area. Grosch ( 2 J ) has correlated the relative abrasion resistance ratings of natural rubber, SBR, and natural rubber/polybutadiene blends over a wide range of wear severities, weather conditions, and ambient temperature with one parameter, namely the tire surface temperature. This correlation may or may not apply to different types of road surfaces. Once the relative wear rating between two tire compounds is established as a function of surface temperature, then it is possible to design specific tire compounds. One compound may be best in city driving and another best in expressway driving. I t is not apparently obvious from molecular considerations why this above correlation should exist. Rubbers with the lower glass temperature
(PBD us. SBR) generally show the better wear resistance. This effect may be included in Grosch’s observation. One might imagine the effect of high surface temperatures to be related to the formation of easily abradable oxidized layers. Grosch’s correlation of tireto-surface temperature and wear is quite useful and should be investigated further for possible greater generality in application to tire compounds and test conditions. Molecular mechanisms should be postulated to fit the correlation. Frank and Hofferberth (7J) have presented an excellent “state-of-the-art” review of the problems associated with the analyses of tire mechanics. A direct quote from this review describes a tire. “By proper selection of the anisotropy of this composite material, optimal properties can be achieved, as, for example in certain parts of the tire, high tensile modulus, and, if necessary, also high shear stiffness, with the least bending stiffness. I t shows less permanent set at elevated temperatures and permits employment of lighter structures than would be possible with corresponding isotropic materials. I n many problems related to tire mechanics, the cords are the dominating component of the composite material.’’ Frank and Hofferberth described the mechanical problems of tires from the point of view of elasticity theory. They have regarded research on the stresses and deformations in the casing as the primary problem of tire mechanics. Nontire Applications
Neal (9K) has reviewed the cost, mechanical properties, electrical resistance, and applicability to various kinds of environments of all the commercially available types of rubber for the benefit of the design engineer. He has given examples of good and bad design in the major areas of application for each type of rubber. Flexible bearings, bridge bearing pads, and parts of automobile suspension systems were listed as examples. Each rubber compound has a spectrum of mechanical and chemical properties. T o the extent that this information is determined and reported, the design engineer will be able to envision new rubber products, worthy of being evaluated in the development laboratories. Eshelman and Garwood (2K,3 K ) in separate reviews have outlined the role of rubber compounds in the 1968 automobile. With improvements in design engineering and new federal safety standards, one half of the 200 pounds of rubber products used per car are in the chassis and body parts. New, better, and more uniform materials with greater toughness, wear resistance, flex resistance, and ability to withstand thermal degradation are being used in such applications as safety padding, hoses, seals, and air springs. Cost, abrasion, flex, cut growth, stress relaxation, compression set, electrical, ozone, fluid, moisture, temperature, and dynamic parameters are measured and utilized to aid the compound selection for specific applications. The isolation and absorption of unwanted vibrations require knowledge of the spring rate, damping, and fatigue char-
acteristics of rubber compounds. Garwood ( 3 K ) lists the following problem areas for future development: improvement of wet traction of polybutadiene treads perhaps by tread design, use of EPDM in sidewalls, development of rubbers with the toughness and flex resistance of natural rubber but with a constant dynamic modulus from -20 to 120 O F , and good abrasion and high resilience over a temperature range from -65 to 325 O F . Bertouille ( I K ) has described the key role of neoprene boot seals in the evolution of “lube-forlife” guarantees in automobiles. The purpose of these seals is to keep the contaminants out and keep the lubricant in the socket joint assembly. Neoprene is used because of its oil-, solvent-, oxygen-, water-, flame-, and heat-resistant properties. Huret (4K) has considered the elastomers which might be used for joints, moldings, and sealants in supersonic aircraft where conditions of both low and especially high temperature are encountered. Fluorinated elastomers and silicones provide some of the answers, but they seem incapable by themselves of solving sealing problems at high temperature. Studies are now directed toward solutions involving the combinations of metals and elastomers. Thomas ( 7 7K) has shown that fluoroelastomers and silicones possess outstanding heat stability after conventional heat aging tests. However, the results of compression set tests at the same temperature gave a different picture of heat stability for these two elastomers. Lefebre (7K) has sought for means greatly to increase the heat stability of macromolecules. Polycyclic polymers appear to have the most promise of increasing the resistance toward melting and pyrolysis at high temperatures. Lindley (8K) has published an excellent engineering study of the compression characteristics of laterally unrestrained rubber O-rings. The load and deflection of the O-ring have been related in dimensionless parameters to the geometry and Young’s modulus of the O-ring. The stress-relaxation properties of the rubber must also be considered in the design of O-rings. The addition of 2-5% SBR to hot asphalt has resulted in rubberized road pavements which have better low-temperature ductility and flexibility, improved stability at elevated temperatures, greater impermeability, and increased adhesion and cohesion. Thin coat overlays of this rubber-modified asphalt may be used as seal coats (5K). An elastomeric matrix has been developed to accept antifouling toxicants and then to provide for their release through a diffusion and dissolution process to produce a long-term antifouling rubber. This rubber can be applied in sheet form for the longer lifetime protection of buoys against barnacle formation (72K). We shall conclude with two new applications of rubber which are designed to assist in the management of our land and water resources. Large, inflatable 4-ply nylon reinforced neoprene Fabridams up to 30 ft in height and 500 ft long are produced for water control. These structures are designed to withstand high stresses, puncture, and environmental V O L . 6 0 NO. 8
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attrition and yet be flexible enough to be inflated, deflated, or folded away at will ( 6 K ) . Finally a mixture of oil and SBR latex has been proposed for application to waste lands which require more secure stabilization of the soil ( I I K ) . Summary
Elastomeric compounds continue to be developed for engineering use as composites with carbon black, plastics, and cord. A new understanding of the microstress and micro-failure characteristics of these composites is speeding this development. Current viscoelasticity theory is providing a framework for describing the time-dependent properties of materials. Glass temperature, crystallization phenomena, and the gross macromolecular character of a polymer are considered to be the three major factors which can usefully characterize the rubbery or plastic behavior of high molecular weight materials. References Introduction (1A) Alliger, G., and Weissert, F. C., Rev. Gen. Caoutchouc, 43, 1321 (1966). (2A) Rosato, D. V., Plastics World, 25 (4), 36 (1967). ( 3 4 ) Watson, W. F., J . Inst. Rubber. Znd., 1 (21, 81 (1967). (4A) Weissert, F. C., and Johnson, B. L., Rubber Chem. Techno(., 40, 590 (1967). (5A) Williams, M. L., Landel, R. F., and Ferry, J. D., J. Amer. Chem. SOL.,77, 3701 (1955). Polyisoprene, Polybutadiene, Butadiene/Styrene Copolymers (1B) Bahary, W. S., Sapper, D. I., and Lane, J. H., Rubber Chem. Technol., 40, 1529 (1967). (2B) Barker, L. R., Payne, A. R., and Smith, J. F. J., Inst. Rubber Znd., 1 (4), 206 (1967). (3B) Dunning, D. J., and Pennels, P. J., Rubber Chem. Technol., 40, 1381 (1967). (4B) Greensmith, H. W., Trans. Inst. Rubber. Ind., 42 (15), 257 (1966). (5B) Grosch, K. A,,Rubber Age, 99 (lo), 63 (1967). (6B) Mullins, L., J . I n s f . RubberInd., 1 (21, 77 (1967). (7B) Orr, R . J., and Breitman, L., Reu. Gen. Caoutchouc Plast., 44, 891 (1967). (8B) Plastics Rubber Weekly, 161 (91, Feb. 17 (1967). (9B) Ring, W., and Cantow, H. J., Rubber Chem. Technol., 40, 895 (1967). (10B) Sarbach, D. V., Hallman, R. W., and Sudekum, J. H., Rubber World, 157 (3), 48 (1967). (11B) Sims, D., J. Inst. Rubber Ind., 1 (4), 200 (1967). (12B) Skinner, T. D., and Watson, A. A., Rubber Age, 99 (ll), 76 (1967) and 99 (12), 69 (1967). (13B) Stephens, R. W., and Bierman, H., Rubber Plastics Age, 48 (2), 160 (1967). (14B) Uraneck, C. A., and Burleigh, J. E., Kautichuk Gummi Kunstrt., 19 (9), 532 (1966). (15B) Vaclavek, V., J . Appl. PolymerSci., 11,1181,1893,1903, (1967). (16B) Werner, A. F., Gunberg, P. F., and Roach, P. G., Rubber India, 19 (51, 11 (1967). Other Elastomers (IC) Chem. Eng., 73 (231, 114 (1966). (2C) Delafield, P., Trans. Inst. Rubber Znd., 1 (5), 262 (1967), 1 (6), 319 (1967). (3C) Dunieavy, R. A., and Critchfield, F. E., Rubber World, 156 (31, 53 (1967). (4C) Ferrari, R. J., Rubber Age, 99 (2), 53 (1967). (5C) Fournier, P., Rev. Gen. Caoutchouc Plast., 43, 1469 (1966). (6C) German, R., Vaughn, G., and Hank, R., Rubber Chem. Technol., 40, 569 (1967). (7C) Henry, hl C and Grifiis C. B., U. S. Dept. Commerce, Clearinghouse for Tech. Inform.’A6632196 (1926). (8C) Ossefort Z T. and Veroeven, W. M., ACS, Div. of Rubber Chem., Paper 33, Fall Mekting (i966). (9C) Pyne, J. R., Rea. Gen. Caoutchouc Plast., 43 ( I t ) , 1429 (1966). (1OC) Saniuels, M. E., and Wirth, K. H., Rubber Age, 99 (91, 73 (1967). (11C) Singh, A,, and Weissbein, L., Rubber Chern. Technol., 40, 1230 (1967). (12C) Tobolsky, A. V., Johnson, V., and MacKnight, W. I., Rubber Chem. Technol., 40, 614 (1967).
Polymer Blends, Blocks, a n d Composites (1D) Childers, C. W., and Kraus, G., Rubber Chem. Technol., 40, 1183 (1967). (2D) Cooper, S. L., and Tobolsky, A. V., Ibid., p. 1105. (3D) Corish, P. J., Ibid., p. 324. (4D) Deanin, R . D., SOL.Plastics Eng. J.,23 ( I ) , 45 (1967). (5D) DeDecker, H. K., and Sabatine, D. J., Rubber Age, 99 (4), 73 (1967). (OD) Deland, D. J., Purdon, J. R., and Schoneman, D. P., Chem. Eng. Progr., 63 (7), 118 (1967). (7D) Hendel, F. J., Rubber Age, 99 (8), 57 (1967). (8D) Hess, W. M., Scott, C. E., and Callan, J. E., Rubber Chem. Technol., 40, 371 (1967).
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(9D) Keskkula, H., and Traylor, P. A., J. AppI. Polymer Sci., 11,2361 (1967) and Gruver, J. T., Ibid., pp. 1581, 2121. (10D) Kraus, G., Childers, C. W., ( l l D ) Molau, G. E., and Keskkula, H., Rubber Chem. Technol., 40, 909 (1967). (12D) Morris, M. C., Zbid., p. 341. (13Dj Smith, R. W., Ibid., pp. 350. Compounding, Fillers, Extenders, Curatives, a n d Protective Agents (1E) Amsden, C. S., J . Inst. Rubber Ind., 1 (4), 214 (1967). (2E) Andrews, E. H., Rubber Chem. Technol., 40, 635 (1967). (3E) Brennan, J. J., and Jermyn, T. E., Ibid., p. 817. (4E) Colclough, T. “High Temperature Vulcanization,” Stockholm, 1966; R A P R A 46, abs. 6589 (1967). (5E) Geschwind, D. H., Gruber, W.F., and Kalil, J., Rubber Age, 99 ( I l ) , 69 (1967). (6E) Harwood, J. A . C., Mullins, L., and Payne, A. R., J.Inst. Rubber Ind., 1 ( I ) , 17 (1967). (7E) Harwood, J. A . C., and Payne, A. R., Rubber Chem. Tedhnol., 40, 840 (1967). (8E) Hess, W.M., and Bdn, L. L., J.Inst. Rubber Znd., 1 (3),159 (1967). (9E) Loan, I. D., Rubber Chem. Technol., 40, 149 (1967). (10E) Oberth, A. E., Zbtd., p. 1337. (11E) Saville, B., and Watson, A. A,, Ibid., p. 100. (12E) Smit, P. P. A , , Rheol. Acta, 5 (4), 277 (1966). (13E) Spath, W.,Gummi, Asbest. Kunstst., 20 (Z), 106 (1967). (14E) Stout, W.J., and Eaton, R. L., Rubber Age, 99 (12), 82 (1967). (15E) Voet, A., and Cook, F. R., Rubber Chem. Technol., 40, 1364 (1967). (16E) Westlinning, H., Koll. Zeits., 211 (1, 2) 76, 84 (1966). Tire Cord
(1F) Claxton M’ E Forster M. J., Robertson, J. J., and Thurman, G. R., Text. Res. J . , 36 ?lOj, 96)3 (19663. (2F) Eccher, S., Rubber Chem. Technol., 40, 1014 (1967). (3F) Howard, W.H., and Williams, hl. L., Ibid., p. 1139. (4F) Lothrup, E. W., Jr., High Speed Testing VI. Rheology of Solids, p. 53, 1967, Interscience Publishers, New York. (5F) Marzocchi, A., and Gagnon, R. K., Rubber World, 156 ( 5 ) , 55 (1967). General Viscoelasticity
(IG) Furuta, I., Kusamizu, S., and Ninomiya, K., R A P R A , 46, labs. 1594 and 9266 (1967). (2G) Goodrich, J. E., and Porter, R. S., PIartics E n g . J.,7 (1), 45 (1967). (3G) Ito, Y., Rubber Chem. Technol., 40, 1483 (1967). (4G) Kraus, G., and Gruver, J. T., Ibid., 40, 734 (1967). (5G) Krishnamurthy, S., J. Poiymer Sci., B5, 69 (1967). (6G) Metzer, A . B., White, J. L., and Denn, M. M., Rubber Chem. Technol., 40, 1426 (1967). (7G) Ninomiya, K., and Yasuda, G., Zbid., p. 493. (8G) Smit, P. P. A., and Van der Vegt, A. K., Reu. Gen. CaoutchoucPlart., 44, 485 (1967). (9G) Yasuda, G., Ibid., p. 484. (10G) Yasuda, G., Maekawa, E., Homma, T., and Ninomiya, K., Ibid., p , 1470. Processing (IH) Hooper, J. R., Rubber Chem. Technol., 40, 463 (1967). (2H) Izod, I. A. W., and Watson, W.F., Zbid., p. XIII. (3Hj Mulligan, B., Rubber W o r l d , 156 (4), 77 (1967). (4H) Perlberg, S. E., Ibid., 156 (3), p. 71. (5H) Rosenthal, O., and Reissenger, S., J.Inst. Rubber Ind., 1 (6), 305 (1 967). (6H) Scott, C. E., and Eckerr, F. J., Zbid., 1 (2), 99 (1967). Dynamic Tests a n d Fracture (11) Buswell, A . G., Gee, G., and Thornley, E. R., J . Inst. Rubber Ind., 1 (l), 43 (1967). (21) Ecker, R., Kautschuk, Gummi, Kunstrt., 20 (5), 269 (1767). (31) Grosch, K., Harwood, J. A . C., and Payne, A . R., Rubber Chem. Technol., 40, 815 (1967). (41) Jones, R . E., and Beeson, M. J., J.Inst. Rubber Ind., 1 (31, 174 (1967). (51) Kawabata, S., and Blatz, P. J., Rubber Chem. Technol., 39, 923 (1966). (61) Landel, R. F., and Fedors, R. F., Ibid., p. 1049. (71) Moore, D. F., and Larson, D. B., Wear, 10 (2), 166 (1967). (81) Priss, L. S., Kautschuk, Gummi, Kunstst., 19 (IO), 639 (1966). (91) Smith, T. L., and Frederick, J. E., Rubber Chem. Techno[., 40, 544, 694 (1967). Tire Friction a n d Wear (1 J) (25) (3J) (4J) (5J) (6J) (75)
Frank, F., and Hofferberth, W.,Rubber Chem. Techno(., 40, 271 (1 967). Grosch, K., J . Inst. RubberInd., 1 ( l ) , 35 (1967). Kern, W. F., Rubber Chem. Technol., 40, 984 (1967). Kummer, H. W., and Meyer, W.E., SOL.Automobile Eng. J.,70 (1967). Ludema, K. C., and Tabor, D., Wear, 9 (5), 329 (1966). Sakey, B., Rubber Chem. Technol., 40, 684 (1967). Savkoor, A . R., RAPRA, abs 2938 (1967).
Nontire Applications ( I K ) Bertouille, R., Rubber World, 157 (2), 60 (1967). (2K) Eshelman, R., Rubber Age, 99 (91, 79 (1967). (3K) Garwood, M, F., Rubber World, 156 (31, 45 (1967). (4K) Huret, hi.,Rev. Gen. Cuoutchouc Plast., 43, 7 (1966). (5K) Kaliin, D. A , , Rubber Age, 99 ( l o ) , 75 (1967). (6K) Krill, J., Zbid., (8), 79 (1967). (7K) Lefebre, G., Rev. Gen. Caoutchouc Plast., 43, 991 (1966). (8K) Lindley, P. B., J. Init. Rubber Ind., 1 (4), 209 (1967). (9K) Neal, M., E n g . M a t . Design, 9 ( I l ) , 1794 (1966); Ibid., (12), 1956. (10K) Rubber Journal, 149 (5), 130 (1967). (11K) Thomas, D. K., Reu. Gen. Caoutchouc Plast., 43, 1001 (1966). (12K) Wuerzer, D. R., Senderling, R. L., and Cardarelli, N. R., Rubber World, 157 (2), 77 (1967).
