Annual Review
Elastomer Technology Research on the relationsh$s between molecular structure and mechanical behauior is leading to continual improvements in Performance ow shear and high compression modulus, coupled L with long range elasticity-these material properties define a rubber. Sets of flexible molecules in constant thermal motion with a few rigid tie points are the cause of rubbery behavior. For a specific application, choose that rubber which retains its elastomeric properties over the requisite range of temperature, rate of deformation, and chemical environment. Measurable parameters which describe these attributes are: the precise temperature a t which, depending on the specific rate of application, the molecular chain flexibility is poised between glassy and rubbery behavior; the tendency, if any, of the molecule to order itself into a crystalline structure; the gross macromolecular features such as molecular weight distribution and branching ; the chemical and solubility properties of the elastomers. These four measurable parameters go far toward characterizing the processing, flexing, and failure properties of a rubber, including those of the raw compound and the vulcanizate. The cost of the product is, of course, a n important consideration in the final selection of any material of construction. We shall discuss the progress made in elastomer technology in 1965, relating it where possible to the above unifying concepts. The rubber industry is just beginning to think of its materials and products in terms of these parameters. The engineer still depends to a large extent on his experience plus the in-service testing of plausible compounds. Nevertheless, current intense research on the relationships between molecular structure and mechanical behavior is paying off in improved performance in numerous specific areas of application. T h e automotive tire remains the most important rubber product. I t accounts for over 60% of U. S. rubber consumption. I t demands a quality perform36
INDUSTRIAL A N D ENGINEERING CHEMISTRY
GLEN ALLIGER F. C. WEISSERT
ance, but many of its requirements are so contradictory in nature-i.e., high strength us. low hysteresis-that most careful optimization of compounding ingredients is necessary. The accelerated development of new rubbers with special mechanical and chemical characteristics is resulting in new applications-often in areas where the product is borderline in function between that of a rubber and a plastic. I n looking for the most common monomers which either as homo- or copolymers make up the polymer backbone, one finds that about 6001, of all the weight of rubber consumed is isoprene or butadiene. Polymers based on isoprene and butadiene as well as many of their copolymers have such suitably low glass temperatures that they exhibit rubbery behavior in flexural modes a t temperatures from -40' to 150' C. and at high speeds. Many additional monomers are required to build each of the elastomers to best meet all of the needs of the rubber industry. Today, 6OY0of the world consumption of raw rubber is filled by synthetic elastomers-only partially for economic reasons. The fact is that synthetic rubbers are important because they permit the adjustment of the basic parameters of the rubber product through careful blending of polymers or adjustment of monomer ratios. Much of this adjustment can be profitably considered in terms of the development of compounds possessing a glass temperature which best fits the given application. The addition of black, oil, and curatives to the rubber compound affects the viscoelastic response of the composite and, hence, is of prime importance in the development of commercial rubber compounds. In fact, before there were so many varieties of elastomers, the compounder's art consisted almost solely of the ways that he could modify the few available rubbers by the addition of nonrubber ingredients. Thus, to summarize: 6OY0 of U. S. rubber consumption is in the form of tires; 60% of all the world rubber is made up directly or indirectly of butadiene or isoprene; 60% of the world rubber usage consists of synthetic rubber. Polymer Structure
Polymeric raw materials are being analyzed by differential thermal analysis, infrared, nuclear magnetic resonance, light scattering, birefringence, x-ray, solution and bulk viscosity, and molecular weight fractionation procedures to gain insight into both their microand macrostructure. As these techniques are being developed, it becomes possible to relate the physical properties of the polymer to its structure. It is now known that rubbery behavior is a direct consequence of the disorderly thermal motion of the flexible segments of the rubber molecule. The energy of retraction of a stretched piece of rubber is quantitatively related to the entropy changes inherent in moving toward the most random configuration. The new elastoplastic polymers are one example of unique behavior obtained by tailor-making an elastomer which is flexible by virtue of the polybutadiene blocks
and yet reinforced by polystyrene blocks so that it requires neither filler nor vulcanizing ingredients. I n this way, a thermoplastic is obtained which has the properties normally associated with reinforced and vulcanized rubber compounds. The gel permeation chromatograph is a n excellent instrument for obtaining molecular weight distribution (MWD) curves with a minimum amount of effort and time. The effect of MWD is important in that broadness of MWD in polymers improves overall processing characteristics from the low rate polymer cold flow to the high tubing rates of shear. Relative absence of low molecular weight material results in better vulcanizate properties, especially those related to heat buildup. The Alfin polybutadienes have been improved for commercial processing characteristics by control of the molecular weight (5B). It is now becoming clear that both branching and molecular weight distribution vary widely in commercial polybutadienes and that these parameters strongly affect the processibility of elastomers. Van der Hoff (74B) has proposed that with two polymers having equivalent intrinsic viscosity, the one with the higher Mooney viscosity has the higher degree of branching. Kraus and co-workers (8B) have shown that polybutadienes with either branched chains or broad molecular weight distribution are non-Newtonian with respect to their bulk viscosities. Butyl lithium-catalyzed polybutadiene, a highly linear and narrow MWD polymer, is Newtonian in that its bulk viscosity is relatively independent of shear rate. Polyisoprenes
Increasing importance is being given to the production and classification of natural rubber according to its intrinsic quality, whereas in the past natural rubber has been marketed largely on the basis of its appearance. ICR (initial concentration rubber), prepared by controlled coagulation of the latex without prior dilution so that the maximum amount of protective agents is retained, is more uniform with respect to accelerator and antioxidant activity than is normal smoked sheet (ZA). High oil levels used in the compounding of synthetic rubber with substantial cost savings are now being utilized with natural rubber. Mixing methods for improved black dispersion and proper black and curative levels for oil-extended natural rubber compounds have been presented by Moore et al. ( 3 A ) . I n consideration of the applicability of rubber compounds for tread wear resistance, it is becoming increasingly clear that the selection of the best compound depends upon the tire test conditions with respect to such parameters as speed, cornering severity, and test temperature. Thus, it is reported that oil-extended natural rubber gives better wear than low oil natural rubber compounds under conditions of high severity. Natural rubber performs better than SBR in original equipment tires a t low speeds. Synthetic polyisoprene made with a Ziegler catalyst is claimed to be an economic replacement for all but VOL. 5 8
NO. 8
AUGUST 1966
37
the lowest grades of natural rubber. The amount of cis-l,4 structure in natural rubber and these synthetic polyisoprenes is nearly identical. Yet there are differences in molecular weight, gel, and nonrubber content which affect the degree of acceleration of the vulcanizing reaction. Recent advances in the development of these synthetic polyisoprenes center around the minimization of both low molecular weight and cross-linked gel material. An easily processible synthetic polyisoprene with good physical properties is the result ( 4 4 ) . Synthetic polyisoprene prepared by alkyllithium catalysts in hydrocarbon solution is also finding commercial acceptance because of its good mold flow, color, low heat buildup, and flex resistant properties. Bruzzone and co-workers ( 7 A ) have compared the properties of natural and synthetic polyisoprenes which differ slightly in their cis-l,4 content. T h e alkyl lithium-catalyzed polymers are characterized by slightly lower cis-1,4 structure and a much narrower molecular weight distribution than either natural rubber or aluminum and titanium complexcatalyzed polyisoprenes. Polybutadiene Homo- and Copolymers
Polybutadienes are the best currently produced synthetic rubber for tread wear life and crack growth resistance (7B). Four polybutadiene types are available commercially. The alkyl lithium- and Ziegler-catalyzed solution polybutadienes are especially characterized by their maintenance of rubbery properties a t low temperatures. Alliger and Weissert (7B) suggest that this property of high chain flexibility is also observed under the conditions of high rate of deformation in the tire abrasion process and, hence, that polymers with good low temperature properties are also likely to possess high abrasion resistance. The glass temperatures of emulsion and Alfin polybutadiene are somewhat higher than the alkyl lithium- and Ziegler-catalyzed polybutadienes. Hansley and Greenberg (5B) have stated that Alfin copolymers TABLE I .
1 Catalyst Polymerization %Tt?::?
7G trans-1,4 7,Vinyl
~
,1
Diene
Alkyllithium Solution 36
55
Molecular Narrow weight distribution Branching Low
POLYBUTADIENES
1 High ~
&1,4
Ziegler Solution 90-95
... ...
1
1
Emulsion
Free radical
1 Emulsion 10
Broad
70 20 Very broad
Some
More
I
1
A@na
Sodium alkenyl Solution 10 70 20 Broad
...
AUTHORS G. Alliger is Director of the Central Research Laboratories and F. C. Weissert is a chemist in the Fundamental Polymer Research Group of the Firestone Tire and Rubber Co., Akron, 0hio .
38
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
with styrene and/or isoprene had some preliminary road testing in tires. A wear and heat buildup performance intermediate between SBR and polybutadiene is observed. An important development in the Alfin system is the control of the high molecular weight fractions by the use of 1,4-dihydro derivatives of benzene and naphthalene. Similarly, the processibility of emulsion polybutadiene has been improved and the improved rubber re-evaluated in tires. De Decker (3B) has commented that the complex behavior of abrasion resistance cannot be predicted from basic physical data and that the wear resistance of tires is known to be dependent on test conditions. He found emulsion polybutadiene to be superior for wear to SBR under conditions of high severity a t all ambient temperatures. Under conditions of low severity, EBR is superior to SBR at low temperatures, while the reverse is true a t high temperatures. The polybutadienes made in hydrocarbon solution tolerate large amounts of oil because nearly all of the carbon atoms are in the main chain. This reduces cost as well as improves such properties as wet skid resistance. The alkyl lithium-catalyzed polybutadienes are particularly suited for extension with large amounts of black and oil because of the absence of low molecular weight fractions. Engle and co-workers ( 4 3 ) have proposed a molecular weight jump reaction with such catalysts as alkylaluminum halides with water to increase the molecular weight of the solution polymers after they have been polymerized. H e has suggested that this is a convenient way to produce high molecular weight polybutadiene more suitable for oil extension. There has been, in the past, some concern over the chipping resistance of polybutadiene tread compounds. Sarbach and Halliman ( 7 IB), by suitable compounding, obtained excellent resistance to wear, cracking, and chipping in road tests of truck tires in Yugoslavia and the U.S.A. with treads containing high levels of polybutadiene. Vohwinkel (75B) has reviewed the present state of tire technology involving polybutadiene developments in the area of coefficient of friction. The method of vulcanization with a n extremely small dosage of sulfur is also treated in detail. cis-Polybutadiene has been reported by Svetlik and Ross (73B) to resist changes in physical properties under high vulcanizing or tire operating temperatures in comparison to natural rubber. At the same time, tires with higher ratios of polybutadiene to natural rubber were also characterized by lower operating temperatures. Polybutadiene made in a n alkyl lithium polymerization system has been found useful in sealants, surface coatings, textiles, thermoplastics, thermosets, chemical modifications to form new polymers, and cold curing liquid rubber compounds (72B). I t has been suggested that these applications are possible because of the high purity and good color; the narrow molecular weight distribution and highly linear structure with the absence of gel; the mixed microstructure and, hence, the lack of crystallinity; the low glass temperature; and the good solubility and compatibility of the alkyl lithiumtype polybutadiene.
Emulsion polymerized 75/25 butadiene/styrene (SBR) is still the number one elastomer with respect to usage in world markets, Improved and lower cost SBR polymers are in active stages of development (7B). However, so-called solution SBR’s are likely to compete actively with SBR in the future. By polymerization in organic solution with organometallic catalysts, the chemist can control the amount of cis, trans, and vinyl microstructure ; the sequence distribution of butadiene and styrene copolymers; and the molecular weight distribution and branching. This accomplishment will result in new rubbers much superior to those which are currently available. Haws (6B) has reviewed the role of both block and random butadiene/styrene polymers, Phillips’ Solprenes, in tire, plastic, sponge, and footwear applications. The cis-l,4, trans-l,4, and vinyl contents of Solprene and emulsion SBR are 32/41 /27 and 8/74/18, respectively. A wear and low heat buildup advantage is claimed for the Solprene us. SBR. The solution SBR’s in general, however, show, a t this time, a small processing disadvantage over SBR. It is expected, however, that future modifications of the macrostructure of solution polymers will improve processibility. Barlow (2B) has reported that the solution SBR’s are faster curing, higher in resilience and abrasion resistance, and have less heat buildup than emulsion SBR. He suggested that the interaction of the black and rubber is more intense in the case of solution SBR, and the compound viscosity is increased. Firestone’s solution SBR (Duradene) has the relatively low vinyl content of about 9%. Duradene rubber tread compounds have about the same excellent wear resistance as that of either alkyl lithium- or Ziegler solution-polymerized polybutadienes but have somewhat better wet traction characteristics (7B). The Shell Chemical Co. introduced Thermolastic, a n SBR type polymer, which, without vulcanization or fillers, exhibits the resilient qualities of rubber and the ease of manufacturing and versatility of plastics (720). Initially, Thermolastic is being used in adhesives and mechanical goods. T h e introduction of Thermolastic is sure to have a n important side effect in the additional demonstration of the molecular engineer’s ability to design special polymers for a specific need. Among the new copolymers, the butadiene/acrylonitrile copolymers are offered in a range of nitrile ratios, Mooney and gel content values to develop compounds with proper processibility ( 70B). Carboxylated SBR latex can be cross-linked by heat without vulcanizing ingredients, possesses good mechanical properties, and will adhere to synthetic fibers (9B). Butyl Rubber and Ethylene Copolymers
Butyl rubber has the unique property of high hysteresis combined with low dynamic modulus which, along with its good heat and age resistance, makes it highly desirable for antivibration and shock absorption applications. Booth and coworkers have discussed (3C)the effects of
changing the level and type of both black and oil on the dynamic properties of these compounds over a broad range of frequencies and temperatures. Gas impermeability and flexibility plus high weather resistance make butyl rubber especially suitable for use in the building industry in the form of sheets, extrusions, moldings, mastics, and paints (8C). The already good heat resistance, especially a t temperatures above 300” F., of butyl inner tubes and retreaded curing bags has been improved by the use of chlorobutyl compounds cured with zinc oxide (6C). Callan (4C) has used phase and electron microscopy to observe the degree of homogeneity of butyl and ethylene propylene terpolymer (EPT) blends. Where heterogeneity did occur, it was concluded that this was the result of the preference of the carbon black for EPT, rather than of differences in vulcanization rates, cohesive energy densities, or viscosities between the two polymer components. Ethylene propylene copolymers possess the properties of amorphous elastomers when the sequence distribution is sufficiently irregular to preclude the formation of crystalline blocks. Although it is easy to observe that the EPR’s have a nearly random sequence distribution, there is strong evidence that there are stereoblock and random block, as well as random sequence distribution patterns (7C, 70C). A minimum glass temperature of about -58” C. is observed over a broad range of ethylene propylene compositions which suggests the presence of nonrandom structures (702). There is still a need for more precise analytical techniques to characterize, in detail, the sequence distribution of ethylene propylene polymers. I n a study of ethylene, propylene, dicyclopentadiene (DCP) terpolymerization it was found that the rates and relative reactivities of ethylene and propylene are influenced by the concentration of DCP in the liquid phase. The rate of vulcanization of ethylene propylene dicyclopentadiene terpolymer is slower than that of butyl rubber or ethylene propylene dicyclooctadiene-l,5terpolymer (73C). Ethylene propylene terpolymers have excellent prospects in the appliance market. About 1.5 to 3 pounds of EPT per 1965 car are used in the form of weather strips, wheel cylinder boots, and seals. High performance for EPT is apparent in heat, ozone, and steam resistance ; dynamic and electrical properties ; low temperature flexibility; and oil extendibility (9C). However, EPT has not as yet broken into the large volume tire market. EPT tires have been evaluated and are reported to have the ability to diffuse heat well a t high speeds and to have improved sidewall cracking resistance, high resilience a t low temperature, and tread wear and traction about equal to the current SBR/polybutadiene blend tread compounds. Areas in the development of EPT tires center around the solution of the problem of low building tack of EPT compounds, the acceleration of the relatively slow rates of cure, and the concern over possible in-plant contamination of E P T with the normal highly unsaturated rubbers. VOL. 5 8
NO. 8
AUGUST 1966
39
There is no general agreement as to the future share of the tire market to be held by EPT ( 7 2 2 ) . McCabe (QC) projects a growth of EPT in tires from less than 1% in 1965 to 33% in 1970. The ability of EPT vulcanizates to isolate mechanical vibrations has been calculated from measurements of the dynamic spring rate a t various temperatures (ZC). EPT sponge can be produced over a broad range of hardness values for widespread application in automotive construction, footwear, and appliance parts for applications which require ozone and heat resistance plus low temperature flexibility (5C). Ethylene vinyl acetate and ethylene ethyl acrylate are two other new flexible ethylene copolymers which are being considered for molding and extrusion applications calling for long-term flexibility and toughness down to -100" F. but not for performance above 200" F. The flexibility of these polymers is inherent in their structure and is not dependent on the addition of plasticizers ( I C , i'IC). Other Polymers
Polyurethane. The term elastoplastics has been coined to describe transition materials, including some of the polyurethanes, which span the range of rubbery to plastic behavior. A quantitative description of these properties has been developed by Damusis et al. ( 4 0 ) . They rely on swelling techniques, stress relaxation, and torsional modulus to arrive at a "mechanical spectral" analysis of the effect of alterations in one- and twocomponent formulations for sealant applications. The glass transition temperatures of the systems vary from about -50" to -15" C., and the stress relaxation times a t 100" C. vary from about 100 to 2000 minutes. The good strength and excellent wear, weathering, moisture, and chemical resistance of these polyurethanes commend them for application in sealants. The design of castable polymers, based on toluene diisocyanate and propylene glycol, has led to a series of elastoplastics having the mechanical characteristics of both elastomers and plastics. Formulation changes that increase strength and hardness have a n unfavorable effect on elasticity. These particular elastoplastics should find application where a combination of hardness, strength, impact and abrasion resistance is needed (30). Polyether-based urethanes are five to ten times more resistant than polyester-based urethanes to hydrolysis ( I D ) . By the use of a flexible high molecular weight poly-e-caprolactone diol in the prepolymer, polyurethane elastomers are prepared with good mechanical properties. The low temperature range of flexibility of these elastomers is below that of SBR 1500 and Keoprene GN and in the same range as natural rubber ( Q D ) . Satisfactory low temperature flexibility to as low as -60" C. and low oil absorption properties are obtained with polyether and urethane-urea elastomers (20). Polycarbonates, Perry and co-workers ( 700) have described the development of new elastomeric polycarbonates where bulky three-dimensional norbornane type groups serve as "tie down" points having the same 40
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
general effect as the classical chemical cross-links, hydrogen bonds, or molecular crystals previously considered necessary for good elastomer properties. A polycarbonate composition which compares favorably with commercial spandex fibers contains 6570 poly(tetramethy1ene ether)glycol. I t has an elongation of 500y0 and a tenacity of 0.6 gram per denier. The glass temperature of these elastomeric polycarbonates is about -70" C., well within the range of the glass transition temperatures of elastomers, and is slightly higher than that of the spandex type elastomers. Thus, any combination of high molecular weight materials having a low glass temperature in the zone of from -20" to -120" C. and also held in shape by relatively few tie-in points has elastomeric properties. So the elastomer structural architect really has to meet only two specifications. Acrylic Elastomers. Improved temperature performance in the range of -40" to 300" F. plus outstanding oil resistance characterize new acrylic-type elastomers. A total of four or more monomers-ethyl and butyl acrylates for low temperature and oil resistance properties, another monomer to supply cure sites, and one or more additional monomers to improve low temperature performance-has been required to achieve the desired balance of properties (5D). Nitroso Rubbers. Military and space requirements placed on rubber components include resistance to low temperature, -70" F., short impulses of high intensity heat, concentrated acids, propellants, and strong oxidizers. Nitroso rubbers, highly fluorinated structures containing repeated [N(R)O(CF*)zj, linkages where R = perfluorinated alkyl or aryl groups, meet many of these requirements (60). Neoprene. Neoprene G P is a new polymer which itself performs all the functions of neoprene GN, GNA, and GRT. The new polymer combines the vulcanizate properties of the G neoprenes, the nonstaining characteristics of GN, the crystallization resistance of GRT, and the raw polymer storage stability of neoprene GNA ( 7 7 0 ) . A study of the dependence of both the raw and vulcanizate properties of polychloroprene upon temperature in the range of -40" to 100" C. detected two stiffening temperatures which correspond to the glassy ( T o )and crystalline (T,) states. Both of these effects are dependent upon the polymer structure and are a function of the deformation rate of testing (70). Knowledge of both T , and T, gives a large slice of the information required for the understanding of the mechanical behavior of a polymer. Chlorohydrin Elastomers. Two new chlorohydrin rubbers have been developed, polyepichlorohydrin (CHR) and a 1:l copolymer of epichlorohydrin and ethylene oxide (CHC). The CHR has a brittle point of -15" F. and CHC one of -50" F. These rubbers are candidates for applications requiring oil and flame resistance, along with low temperature flexibility (740). Viton. Viton, a copolymer of hexafluoropropylene and vinylidene fluoride, is suited to applications requiring exceptional resistance to heat, fluids, and compression set (80). Stress-relaxation techniques have been used
to clarify the process of thermal degradation in Viton vulcanizates. These studies have led to improvements in the postcure treatment. A new cross-linking system based on p-phenylene diamine has resulted in superior high temperature performance (730). Compounding
Fillers. The technological importance of adding fine fillers for the reinforcement of rubber products is well known, but there is no agreement as to the fundamental physics or chemistry underlying reinforcement. It is agreed that small diameter (large surface area) particles reinforce rubber better than do the large particles and the tendency of the carbon black particles to flocculate or remain flocculated in a network structure affects both processing and vulcanizate properties. As yet there is no consensus as to the basic cause and effect relationships in rubber reinforcement. Wake (78E), in reviewing the nine papers on reinforcement presented a t the DKG Munich Conference, suggests that the problem is even more complex and the evidence more contradictory than was thought. Brennan and Jermyn (4E) have related the modulus of the rubber a t low extension to the structure of the black, but found the modulus of the vulcanizate at high extension to be related to the surface activity of the black as indicated by the amount of bound rubber. Payne (73E) also uses a model of the breaking and reforming of the carbon structure to encompass the dynamic properties of filled vulcanizates as well as those of carbon black paraffin oil mixtures. Voet (77E) has concluded that the structural phenomena in carbon blacks are connected with both persistent and transient particle to particle interaction. Boonstra and Taylor (3E) have noted that the restriction of swelling of carbon black rubber vulcanizates can be correlated with the number of reactive sites on the carbon black surface. The so-called stress-softening or Mullins effect of filled vulcanizates has (incorrectly it seems) been associated with the rubber to filler bond. Harwood, Mullins, and Payne (70E) have shown that, when compared a t the same stress, the extent of stress softening is similar for both gum and filled vulcanizates. Hence, it appears that stress softening is caused by internal changes in the rubber rather than by the breaking of filler-rubber or filler-filler bonds. Vulcanization and Aging. Much emphasis in polymer synthesis has been placed on the development of high cis-l,4 polybutadiene and polyisoprene. The vulcanization reaction, either with dicumylperoxide or sulfur ( 1 4 E ) , tends to isomerize the pure microstructure toward a lower equilibrium cis-trans mixture. Increasing temperature, reaction time, or dicumylperoxide content increases this degree of isomerization. A sulfur cure of 60 minutes at 155" C. and 75 minutes at 180' C. reduces the cis-1,4 content to 98 and 81%, respectively, of the original high cis-l,4 value. T h e reactivity of dovble bonds toward vulcanizing agents increases as we go from ethylene propylene
dicyclopentadiene terpolymer to isobutylene isoprene copolymer to ethylene propylene cyclooctadiene terpolymer ( 7 E ) . The degree of cross-linking obtained through dicumyl peroxide varies greatly depending upon the rubber used. The overall cross-linking in terms of cross-links formed per molecule of decomposed peroxide varies from 0.4/1 to 15.0/1 for EPR, natural rubber, and polybutadiene. Loan (7.223) postulated that scission results from attack on tertiary hydrogen atoms and cross-linking from attack at secondary hydrogens. I n the case of polybutadiene vulcanization, Van der Hoff (76E) points out that the polybutadienyl radical itself is capable of attacking the double bonds in the polymer, thus increasing the efficiency of the peroxide cure. Vulcanizates obtained with maleimide and its derivatives are attractive relative to conventional sulfur cures in that short time-high temperature curing cycles are more feasible, the good aging characteristic of nonsulfur vulcanizates is observed, and the vulcanizates maintain their ability to crystallize upon stretching (15E). Seldom, today, does the rubber technologist use a single rubber in a compound. A common blend consists of natural rubber and polybutadiene. Both the curing and aging characteristics of the blended polymers may vary. Thus, Bell and Tiller (2E) have demonstrated that in dicumyl peroxide cures the polybutadiene crosslinks to a greater degree than the natural rubber, while the reverse is true with accelerated sulfur vulcanizing systems. Polybutadiene vulcanizates become stiffer, while polyisoprene vulcanizates become softer on aging. Squalene and, to a lesser extent, unsaturated fatty acids accelerated the degradation of rubber thread, while the unsaturated fatty acid and not the squalene promoted similar degradation in spandex fibers ( 7 E ) . Rheology and Processing. I t has already been noted in the discussion of polybutadienes (8B) that introduction of either branching or broadness in the molecular weight distribution of a polymer system results in non-Newtonian rheology. The incorporation of black and oil further increases the degree by which the apparent viscosity of a compound decreases with increasing shear rate. Therefore, it is now required that the rubber technologist should have at hand not merely a single Mooney viscosity value but, properly, he must also know the resistance to flow of polymers and compounds a t all rates of flow from those of static storage to those of high speed tubing operations. Wolstenholme (79E) has presented shear stress us. shear rate curves at several temperatures. He noted that, at lower rates of shear, solution SBR was softer than emulsion SBR; at higher rates of shear the behavior was reversed. The development and use of laboratory instruments to measure the processibility and curing characteristics of small samples of factory compounds has continued. The CEPAR apparatus has been used to construct a process characterization diagram to evaluate both the degree of degradation and the network formation in a rubber compound (6E). The Wallace-Shawbury VOL. 5 8
NO. 8
AUGUST
1966
41
Curometer is being used for routine measurements of plasticity, scorch time, cure time, and homogeneity of rubber compounds ( I IE) . Potential economic advantages in lower mixing costs and improved dispersion of such difficultly dispersible pigments as active low structure black in butyl compounds are claimed for “hydrodispersion” solution masterbatching of carbon black and elastomers (5E). Powder technology may put rubber mixing on a more competitive basis with plastics processing. I n one approach to this, all of the rubber ingredients are mixed in a Henschel mixer (a Waring Blendor type) to form a powder with little work going into the mix (8E). New coprecipitates of synthetic rubber and active silicic acid are also usable in the form of fine dry powders. This technique permits the direct manufacture of rubber molding with conventional plastic machinery (2UE). Injection molding is expected to render more competitive the processing of rubber mechanical goods. Gregory (9E), however, suggests that there are still technical and economic difficulties which must be overcome in injection molding machinery design. Physical Testing and Product Performance
Laboratory physical tests on rubber compounds should correlate more exactly with product performance. As Timm (2UF) has suggested, the difficulties inherent in the standardization of rubber test methods arise from the complicated macromolecular structure of polymers which permits different modes of molecular motion. These relaxation processes are dependent upon both time and temperature conditions. Knowledge of the detailed mechanical spectral response of a n elastomeric product permits the formation of a solid judgment as to the range of commercial applicability of a given material system. The complementary knowledge of the time and temperature spectral characteristics of the performance of the rubber-cord composite called a tire is also required so that both the laboratory and tire test pieces are executing the same kind and degree of mechanical motion. Veith ( 2 I F ) has modified the “trousers tear specimen’’ to prevent elongation of the legs, so that the tear strength in the tearing zone is independent of the modulus of the stock. Veith, in a statistically designed experiment, presented the effects of the five variables-cross-link density, HAF black level, oil level, temperature, and jaw speed-on both natural rubber and SBR stocks. The fatigue life of both SBR and natural rubber can be accounted for in terms of cut growth from small flaws initially present in the rubber. The cut growth rate of SBR is proportional to the fourth power of the tearing energy whereas that of natural rubber is proportional to the square (IOF, I5F). Lake and Lindley ( I @ ) have investigated the characteristics of cut growth a t low tearing energies and the effects of polymer, vulcanizing system, oxygen, and fillers on the critical tearing energy and fatigue limits of rubber vulcanizates. Beatty and Juve (ZF) have developed a laboratory test which in42
INDUSTRIAL A N D ENGINEERING CHEMISTRY
volves a ring size specimen running around two pulleys to evaluate crack growth in vulcanizates under a variety of environmental conditions. Halpin and Bueche (72@ have presented a failure theory to predict the time dependence of the tensile strength and ultimate elongation of both gum and filled vulcanizates. Failure is the result of the propagation of tears and cracks within the viscoelastic body. The ultimate properties are related to the time-temperature dependence of the modulus. Collins and co-workers ( 4 F ) have deduced that the energy losses in a rolling tire may be separated into the tread-bending (20%), carcass rubber (473, sidewall (5%), and cord system (4Oy0), all of which are proportional to their loss moduli plus a fifth component, treadcompression, which is proportional to the loss modulus divided by the square of the complex modulus. Coddington and co-workers ( 3 F ) have characterized the effects of load, inflation, speed, and ambient temperature on both SBR and butyl tires. They found that tire durability correlated well with dynamic temperature measurements. Davison et al. (627) have had some success in the use of laboratory abrasion results obtained under carefully controlled conditions of transmitted power, temperature, load, and wheel angular velocity to tread vulcanizates which operate under road conditions encompassing a wide range of abrasive severities. Mulligan ( I 7 F ) observes that the radial ply tire, with its promise of greater mileage, increased safety, and fuel economy, although with some sacrifice in ride qualities, will probably not require any change in the use of SBR, natural rubber, and polybutadiene rubbers as tread components, although some of the polymer ratios may be altered. A tough rubber is needed in the sidewall where most of the flexing of the radial tire takes place. Bassi ( I F ) has found that among the various physical properties such as hardness, rebound, dynamic stiffness, loss, and damping factor it is the latter that has the best correlation with the coefficient of friction in considering various compounds of the same base elastomer. I n addition to compound damping consideration, surface phenomena such as adhesion between rubber and water also play a n important part in wet traction. The basic principles of compound design for vibration damping are being developed. Low resilience is obtained by the use of high styrene (43’%) SBR’s, aromatic oil, and high HAF black, while low brittle point features are improved by high polybutadiene loadings and the use of naphthenic oils (73F). Dunnom and de Decker ( 8 F ) have observed that a single polymer has a n optimum damping range of only about 90” F. Therefore, to secure damping in the temperature range of -20” to 200” F., which is encountered in body mount applications, a blend of styrene-butadiene elastomers rather than any single polymer is required. By varying the styrene content, these elastomers exhibit transition zones varying from -100” to 150” F. Another growing application of elastomers is related to their ability to add high impact resistance to brittle polymers. ABS resins are copolymers of styrene and acrylonitrile with butadiene grafts to form a terpolymer.
The basic function of the polybutadiene blocks is to increase the damping or shock absorption capacity of the plastic. Excellent processibility, impact resistance, heat stability, toughness, and low temperature performance are achieved with ABS resins (76F). Davey and Payne (5F)have noted that rubber is a n excellent basic material for bridge bearings because of its ability to resist large compressive forces with only a small deformation and yet allow for easy deformation under weak shear stresses. Gent (9F) has studied the critical (buckling) compressive characteristics of columns of rubber springs under a variety of loading conditions. In the past two years, a n upturn has occurred in the use of latex foam in furniture applications. Rogers (78F) has reviewed the development of the Dunlop and Talalay methods for the production of latex foam. A carboxylated SBR latex which can be crosslinked without vulcanizing ingredients possesses both good mechanical properties and adhesion to synthetic fibers. I t is used in fabric, paper, and leather coating applications (9B) Del Gatto (727) has reviewed the role that over 600 different grades of elastomeric silicones play in meeting speciality requirements. Good dynamic properties plus both low and high temperature resistance have led to the use of silicone elastomers in wire and cable, plane and missile, appliance, automotive, and construction sealant applications. A large percentage of elastomeric material is used in solid-fuel rockets. Polysulfide, polyurethane, and modified polybutadiene rubbers serve as the binder for the solid propellant, as a secondary fuel, and also to provide the necessary mechanical and physical properties required to maintain the rocket's structural integrity. Inert components of the rocket such as insulators and liners also require rubber composition able to withstand severe environmental conditions (79F). This paper has attempted to review the progress that is being made in the development of elastomeric materials and their characterization with respect to specific environmental conditions of stress, time, temperature, and chemical reactivity. Simplifying views of general rubbery behavior have been presented as a general framework to encompass the 1965 technological developments in rubber-a low shear but high compression modulus material of construction.
-
REFERENCES Polyisoprenes (1A) Bruzzone, M., Corradini, G., Amato, F., Rubber Plustics Age 46,278 (1965). (2A) Fleurot, M., Rev. Gen. Caoutchouc 42, 873 (1965). (3A) Moore, C. G Simpson, K. E., Swift, P. M., Wheelans, M . A., Rubber Age 97 (l), 61 (1965):) (4A) Saltman, W. M., Farson, F. S., Schoenberg, E., Rubber Plastics Age 46, 502 (1965). Polybutadiene Homo- and Copolymers (1B) Alliger, G., Weissert, F. C., T h e International Rubber Conference, Paris, France, May 1966. (2B) Barlow, F. W., Rubber J . 147 (91, 30 (1965). (3B) De Decker, H. K., McCall, C. A., Bahary, W. S., Rubber Plastics Age 46, 286 (1965). (4B) Engle, E. F., Schafer, J., Kiepert, K. M., Rubber Age 96,410 (1964). (5B) Hansley, V. L., Greenberg, H., Rubber Chem. Technol. 38, 103 (1965). (6B) Haws, J. R., Rubber Plastics Age 46, 1144 (1965). (7B) Hill, C. A., Rubber Age 97 (9), 75 (1965). (8B) Kraus. G., Zelinski, R. P., Wofford, C. F., Gruver, J. T., Rubbcr Chem. Techno!. 98,871, 881, 893, 907 (1965).
(9B) Iepetit, F., Rev. Gen. Caoulchouc 42, 363 (1965). (10B) Minnerly, H . E., Rubber World 152 (l), 76 (1965). (11B) Sarbach, D. V., Hallman, R. W., RubberPlastics Age46,1151,1272 (1965). (12B) Simmons, P., Ibid., 45, 1347 (1964). (13B) Svetlik, J. F., Ross, E. F., Rubber Are - 96,. 570 (1965). (14B) Van der Hoff, B. M. E., Henderson, J. F., Small, R. M. B., Rubber Plastics Aee 46.821 (1965). (156) Vihwinkel, K., Kuutschuk Gummi 18, 433 (1965).
Butyl Rubber and Ethylene Copolymers (1C) Alexander, R. L., Ans on, H. D., Brown, F. E., Clampitt, B. N., Hughes, R. H., Polymer Eng. Sd. 6 5 (1966). (2C) Baseden, G. A., Rubber Chem. Technol. 38, 967 (1965). (3C) Booth, D. A., Brown, P. P.,Mayor, L., Rubber Piaslics Age 46,173 (1965). (4C) Callan, J. E., Topcik, B.,Ford, F. P., Rubber World 151 (6), 60 (1965). (5C) Cardillo, R. M., Spenadel, L., Rubber Age 97 (2), 82 (1965). (6C) Dudley, R. H., Wallace, A. J., Rubber World 152 (21, 66 (1965).
J),
(7C) Garrett,R. R., RubberPIuslicsAge46,915 (1965). (8'2) Huot, P., Agius, P., Rev. Gen. Caoutchouc 42, 1276 (1965). (9C) McCabe, R . F., Rubber Age 96,397 (1964). (1OC) Mauer, J. J., Rubber Chem. Technol. 38, 979 (1965). (11C) Modern Plustics 43 (l), 84 (1965). (12C) Samuels, M. E., Chem. Eng. Progr. 61 (4), 15 (1965). (13'2) Sartori, G., Valvassori, A., Faina, S., Rubber Chem. Technol. 38, 620 (1965).
Other Polymers (1D) Athey, R. J., Rubber Age 96, 705 (1965). (2D) Axelrood, S. L., Lajiness, W. G., Rubber J. 147 ( l l ) , 34 (1965). (3D) Berger, S. E., Szukiewicz, W., Rubber Chem. Technol. 38, 150 (1965). (4D) Damusis, A., Ashe, W., Frisch, K. E., J . Appl. PolymerSci. 9, 2965 (1965). (5D) Del Gatto, J., Rubber World 152 (l), 95 (1965). (6D) Griffis, C. B., Henry, M . C., Rubber Plastics Age 46, 63 (1965). (7D) Houdret, C., Morin, M., Rev. Gen. Caoutchouc 42, 395 (1965). (ED) Lefrancois, J., Ibid., p. 853. (9D) Magnus, G., Rubber A@ 97 (4), 86 (1965). (10D) Perry, K. P., Jackson, W. J., Jr., Caldwell, S. R., J.Appl. PulymerSd. 9, 3451 (1965). (11D) Rubber Age 96 (31, 438 (1964). (12D) Ibid., 97 (6), 111 (1965). (13D) Thomas D. K Phillips, L. N., Atkinson, A. S., Sinnott, R., Rubber Plastics Age 46,1020'(1965)'.' (14D) Vanderberg, E. J., Ibid., p. 1139. Compounding (1E) Ball, G., Rubber J . 147 (11, 30 (1965). (2E) Bell, C. L., Tiller, R., J. Appl. Polymer Sa'. 9, 3091 (1965). (3E) Boonstra, B. B., Taylor, G. L., Rubber Chem. Technol. 38, 943 (1965). (4E) Brennan, J. J., Jermyn, T. E., J. Appl. Polymer Sci. 9, 2749 (1965). (5E) Burgess, K. A., Hirshfield, S. M., Stokes, C. A., Rubber Age 97 (9), 84 (1965). (6E) Claxton, W. E., Conant, F. S . , Ibid. (7), p. 80. (7E) Crespi, G., Arcozzi, A., Rubber Chem. Technol. 38, 590 (1965). (8E) Goshorn, T. R., Wolf, F. R., Rubber Age 97 (E), 77 (1965). (9E) Gregory. C. H., RubberJ. 147 (4), 50 (1965). (10E) Harwood, J. A. C., Mullins, L., Payne, A. R., J . Appl. Polymer Sa'. 9, 3011 (1965). (11E) Hickman, J. A., Norman, R. H., Payne, A. R., Rubber World 152 (2), 76 (1965). (12E) Loan, L. D., Rubber Chem. Technol. 38, 22 (1965). (13E) Payne, A. R., Ibid., p. 387. (14E) Reichenbach, D., Kautschuk Gummi 18, 9, 213 (1965). (15E) Tawney, P. O., Wenisch, W. J., van der Burg, S., Relyea, D. I., Rubber Chem. Technul. 38, 352 (1965). (16E) Van der Hoff, B. M. E., Ibid., p. 560. (17E) Voet, A., Ibid., p. 677. (18E) Wake, W. C., Rubber Age 97 (6), 112 (1965). (19E) Wolstenholme, W. E., Rubber Chem. Technol. 38, 769 (1965). (20E) Zeppernick, F., Kuutrchuk Gummi 18, 231 (19651 Phyaical Testing and Product Performance (1F) Bassi, A. C., Rubber Chem. Technol. 38, 112 (1965). (2F) Beatty, J. R., Juve, A. E., Ibid., p. 719. (3F) Coddington, D. M., Marsh, W. D., Hodges, H . C., Ibid., p. 741. (4F) Collins, J. M., Jackson, W. L., Oubridge, P. S.,lbid., p. 400. (5F) Davey, A. B., Payne, A. R., Rubber J . 147 (2), 24 (1965). (6F) Davison, S., Deisz, M. A., Meier, D. J., Reynolds, R. J., Cook, R. D., Rubber Chem. Technol. 38, 457, 475 (1965). (7F) Del Gatto, J., Rubber World 152 (5), 69 (1965). (8F) Dunnom, D. D., de Decker, H. K., Rubber Age 97 (81, 85 (1965). (9F) Gent, A. N., Rubber Chem. Technol. 38, 415 (1965). (10F) Gent, A. N., Lindley, P. B., Thomas, A. G., Ibid., p. 292. (11F) Giles, C. G., Sabey, B. E., Cardew, K. H. F., Ibid., p. 840. (12F) Halpin, J. C., Bueche, F., Ibid., p. 263, 278. (13F) Kastein, B., Jr., Rubber Age 96, 724 (1965). (14F) Lake, G. J., Lindley, P. B., J.Appl. PolymerSci. 9,1233 (1965). (15F) Lake, G. J., Lindley, P. B., Rubber Chem. Technol. 38, 301 (1965). (16F) Modern Plastics 43 (3), 86 (1965). (17F) Mulligan, W., Rubber World 153 (Z), 59 (1965). (18F) Rogers, T. H., Reu. Gen. Caoutchouc 42, 1265 (1965). (19F) Sayles, D. C., Rubber World 153 (Z), 89 (1965). (20F) Timm, Th., Kautschuk Gummi 18, 801 (1965). (21F) Veith, A. G., Rubbur Chem. Technol. 38, 700 (1965).
VOL. 5 % NO. 8
AUGUST 1966
43
