Mechanochemical Reactions in Black-Filled SBR Vulcanizates under Large Deformation

Mechanochemical degradation of black-filled SBR's at large deformation was studied in relation to abrasion. Deformation of rubbers caused bond scission and reduced the crosslink density; however, subsequent free radical reactions reconstructed the network structure in a short time to some extents depending on the strain. The extent of crosslink recovery was larger with larger strains. As a consequence, the loss in crosslink density showed a peak when plotted against the strain. When rubber was deformed by repeatedly passing through a narrow gap of a two-roll mill, the crosslink density was raised appreciably above the original level after a small number of passes. Surprisingly, the rubber could become plastic-like while the crosslink density was maintained high. All of these results matched well with the characteristic phenomena observed in abrasion of rubbers, e.g., the varying appearance of wear with severity, the worn surface and debris prone to be oxidized, and the bimodal size distribution of debris. It was strongly suggested that mechanochemical degradation participated in abrasion in a wide range of severity.

Aging of Black Filled Rubber under Deformation

Prolonged dynamic deformation of a rubber is found to significantly accelerate oxidative crosslinking due to mechanochemical oxidative crosslinking. The extent of acceleration is larger at lower temperatures and increases with increasing strain. The activation energy of the rate of oxidative crosslinking decreases with increasing strain toward the highest value among the unit reactions of oxidative chain reaction cycle or the value of diffusion of oxygen, which is much smaller than that of static aging. On the other hand, static deformation is also found to accelerate oxidative crosslinking but to considerably lesser extents. Mechanochemical acceleration can be noticed only in the early stage of prolonged deformation. The degree of acceleration in prolonged deformation increased with increasing strain without change in the activation energy. The acceleration observed in prolonged static deformation is considered to arise from the increased area of exposure to the oxidizing atmosphere. It appears that mechanochemical acceleration is merely spent to compensate the loss in crosslink density upon deformation in this case.

Reactions of Radicals in Filled Rubber Compounds: I. Detection of Free Radicals in Rubbers by ESR

ESR measurements were made with vulcanized rubbers of a gum, three carbon black filled and a silica filled compounds. The radicals in the filled rubbers were found to have long lives unlike the gum rubber. Carbon black was found to reduce the sensitivity of ESR due to shielding effect in the magnetic field. The degree of sensitivity reduction appeared to depend on the inter-aggregate distance. Upon exposure to toluene vapor, the ESR intensity of black filled rubbers increased for a short period as swelling widened the inter-aggregate distance and then decayed steadily with further exposure. Such a variation was not found with the gum and silica filled vulcanizates. The decay was faster with higher oxygen content of the environment. In a given environment, however, the decay could be dealt by simple kinetics. Diffusion of oxygen did not seem to play any part. It was inferred that ESR observes radicals only in a thin surface layer of the black filled rubber. The kinetic zero-time radical concentration, obtained by toluene vapor exposure in a nitrogen environment, was considered to be the surface radical concentration in the absence of the shielding effect. Among the rubbers of different black loadings, it showed an exponential relation with the value obtained by direct observation due to the difference in the shielding effect arising from difference in loadings. With the rubber of the same formulation, however, the radical concentration obtained by direct observation showed a linear relation with the kinetic zero-time value, suggesting possibility of quantitative comparison by ESR among the rubbers with the same carbon black content.

Reactions of Radicals in Filled Rubber Compounds: II. Generation and Subsequent Reactions of Radicals Upon Deformation

Effect of stretching on oxidative crosslinking of a carbon black filled IR vulcanizate at a normal temperature was studied by means of ESR and crosslink density measurements. Upon deformation, ESR intensity increased and crosslink density decreased, indicating scission of load supporting bonds generating free radicals. Bond scission increased with increasing strain. Resting after deformation, the ESR intensity decayed steadily following simple combination of first order and second order kinetics. The decay was overwhelmingly caused by first order reactions, which were considered to be irrelevant to crosslinking. On the other hand, crosslink density increased during resting. The results strongly suggested that ESR observed the radicals at the surface of the carbon black filled rubber, while crosslink density evaluated the bulk property. The crosslink density increase during resting outweighed the reduction due to deformation. Deformation accompanied by radical generation apparently accelerated oxidative crosslinking.

The Chain End Distributions and Crosslink Characteristics in Black-Filled Rubbers

A black-filled rubber compound consists of two phases: the free polymer phase, where no particle exists, and the carbon black agglomerate phase, where highly concentrated particles are bound by a small amount of the polymer—so called bound rubber. The Charlesby—Pinner virtual linear number-average molecular weight Mn1 of the polymer in each phase is determined for black-filled compounds to obtain information about the chain end distribution in the compounds. The nominal crosslink density of the bound rubber is also measured by means of the swelling measurement of the “carbon gel” to characterize the crosslink variation in the vulcanizates. The results indicate that the chain end density is much higher in the agglomerate phase than in the free polymer phase due to enhanced chain scission during mixing. The enhancement of scission is considered due to the free radical crosslinking which imposes restriction to chain slippage in the flow field. This together with the previous findings suggests the features of the phase construction in the filled vulcanizates: tightly crosslinked free polymer phase with fewer chain ends, and loosely crosslinked agglomerate phase with more chain ends but, with rather suppressed chain mobility due to the dense nano-scale particles. The features match well with the energy dissipating and the toughened nature of the filled vulcanizates.

Evaluation of Chain Scission during Mixing of Filled Compounds

Whether chain scission takes place during mixing of black-filled compounds has been a debatable subject which is yet unsettled. The uncertainty originates from the material system which contains gel. A direct method to evaluate scission is to quantify the change in the number of chain ends, since two chain ends will be newly created on each event of scission. It requires determination of an average molecular weight of the linear components which constitute the gel-containing system. The degree of polymerization denned in the Charlesby-Pinner theory, derived for crosslinking study by means of sol-gel analysis, could be conveniently used for the purpose. The theory, however, cannot be directly applied for black-filled compounds because the composite structure in the compounds does not allow one to satisfy the assumptions made in the theory, i.e., an equal chance of crosslinking for every reactive site. A new technique is developed so that the assumption is satisfied and the sol-gel analysis can be carried out even with black-filled compounds. This technique is applied for numbers of compounds, with the same formulation but mixed under various conditions. A NR and an SBR formulations are used here. The results clearly indicate that chain scission takes place during mixing of both NR and SBR compounds.

Molecular Weight Defined in Sol-Gel Analysis and Its Application to Evaluate Branching

It is considered that many “linear” polymers are actually branched; however, it is difficult to show this with ordinary methods for an arbitrarily chosen polymer. Branching can be regarded as premature crosslinking below the gel point. Attention is then paid to the well-established Charlesby-Pinner Equation used for sol-gel analysis in crosslinking studies. It contains the number average degree of polymerization before crosslinking as a parameter. The molecular parameter is considered here to be that of the virtual linear polymer which would be obtained by unlinking any branch points contained in the polymer. Evidence is shown to support this. It is then possible to estimate the total number of linear components on an average molecule of a branched polymer by taking the ratio of the number average molecular weight measured by two methods, i.e., sol-gel analysis and an ordinary method like GPC. Further information about the branching structure can be obtained by additional measurements of effective crosslink density for a series of polymers obtained from similar polymerization processes.

States of Carbon Black Dispersion and Extensibility of Rubbers

Analysis is made for the origin of the mixing-induced tensile property variation of a filled rubber. Attention is paid to the hydrodynamic effect f(ϕe) of the filler, defined here as the factor to adjust the deviation of 100% modulus from the theory of rubber elasticity. For the rubbers mixed under variety of conditions, the f(ϕe)'s are calculated from the observed values of the modulus, at 25°C and 100°C, and the crosslink density. The variation of the f(ϕe) is considered to be governed by the mobility of the polymer confined in agglomerates of the filler. The mobility variation due to mixing seems to be mainly influenced by agglomerate size at 25°C, and by agglomerate size and chemical constraints at 100°C. Therefore, the f(ϕe)'s at the two temperatures are suggested to be useful measures of the state of carbon-black micro-dispersion. The extensibility of the rubbers is closely related f(ϕe). This indicates that the failure property is also governed by the mobility of the confined polymer.

Aging of Tire Parts during Service. I. Types of Aging in Heavy-Duty Tires

Three types of aging were found to exist. The factors determining the types were the temperature and the air-supply conditions irrespective of where the rubber was aged, in the tires or in the laboratory. This means that the aging characteristics of a tire part in the field can be properly predicted if these factors in a tire are taken into account in the laboratory evaluation. In the first type, Type I, the aging yields increased M100 and reduced λb closely following the reference relation, Equation (3), which holds for the rubbers crosslinked with increasing the amount of curatives. This type of aging was found at temperatures below about 80°C, under either oxidative or anaerobic conditions. The extents of the changes in λb and M100 were large under the former conditions and small under the latter. As for the aging in the heavy-duty tires, the sidewall and the belt showed this type, with the changes to considerable extents suggesting an oxidative aging. The oxidative condition for the sidewall is apparent. Although air supply to the belt seems difficult because of the interior position and the massiveness of the tires, the cords of air wicking type occupy a substantial section of the part shortening the permeation path of the inflating air to create an oxidative condition. This type of aging is governed substantially by crosslinking, so that aging yields the same effect on the λb vs.M100 relation as the one obtained in crosslinking with increasing curatives. In the second type, Type II, M100 was either changed little or even reduced, while λb was reduced. This type was observed in an anaerobic aging at temperatures higher than about 90°C. The extents of the changes in this type appeared relatively small. This type of aging in the heavy-duty tires was observed mainly in the belt-edge filler and sporadically in the belt. The belt-edge fillers are placed at the edges above the second and below the third belt layers with the thickness considerably larger than that of the belt rubber between the cords. This makes the air supply condition in the part virtually anaerobic. Furthermore, being in the midsection of the thick crown region and under a severe flexural condition, the part should operate at considerably high temperatures. The sporadic appearance of this type in the belt may be due to the thickness variation of the rubber layers including the underlying parts, together with the severe service conditions causing extensive temperature rise in the particular service. The characteristics of this type are speculated to come from extensive main-chain modifications like cyclic sulfide formations. In the third type, Type III, M100 was increased, and λb was reduced but to an extent larger than expected from the M100 value on the basis of the reference relation. This type was obtained in the oxidative aging at temperatures above about 90°C. This type of aging was not found in the tire parts of the present study. The causes of this type are considered to be from appreciable amounts of chain scission in addition to crosslinking, due to oxidation at high temperatures.

Aging of Tire Parts during Service. II. Aging of Belt-Skim Rubbers in Passenger Tires

The belt-skim rubber of a passenger-car tire has changes in its tensile properties, M100 and λb, during service in the field. The changes are larger in higher temperature fields with an equivalent duration. It can be interpreted that the changes are caused simply by oxidative crosslinking similar to that which takes place on aging rubber sheets in an air oven at relatively low temperatures, below 100°C. The belt-skim compound showing better aging resistance in the laboratory also shows better aging resistance when used in a tire. Therefore, the aging characteristics of the rubber part in a tire can be satisfactorily predicted. The Arrhenius equations with the same activation energy can be used for the aging of a rubber in the laboratory and in the tires by introducing a factor to the tire equation. The factor is added to the atmospheric temperature to correct for a difference; however, it was found to be slight in the present case.

Friction on Ice

The friction on ice is strongly dependent on temperature. At sufficiently low temperatures, the frictional resistance on ice is high comparable to those on wet or even dry solid surfaces. As temperature rises and approaches the melting point of ice, however, friction rapidly decreases. Differing from the friction of a rubber on ordinary dry or wet solid surfaces the energy loss processes in the rubber do not seem to be the direct source of the frictional resistance on ice. Although frictional melting of ice could occur at high sliding speeds, an ice surface is inherently lubricated with a persistent mobile fluid layer at relatively high temperatures, near the melting point of ice. When a rubber slides on an ice surface, the fluid layer is sheared and undergoes drag flow. The energy loss process necessary for the frictional resistance takes place primarily in the fluid layer, and not in the rubber. The frictional resistance on ice is primarily determined by the viscosity and the thickness of the lubricating fluid layer. What is required of a rubber for better traction under such a condition is that the rubber surface follows the topography of the ice surface as closely as possible, so that more patches of ice surface can be sheared. Therefore, the rubber has to be sufficiently soft to show high friction on ice. Further improvement of the friction could be obtained by making it more resilient. Thus, a rubber with high friction on ice must be compounded so that the polymer chains maintain a high level of mobility at moderately low temperatures. This can be achieved by using polymers with low glass-transition-temperatures. An increased softener loading level helps to improve friction, but to a limited extent. In order to take maximum advantage of softeners, the choice of softener system is important. A relation common to all the mixed softener systems, except the ones containing high-viscosity softeners, was found to exist between the friction on ice and the solubility parameter of the softener mixture in the rubber. The friction on ice was maximized by selecting a softener system with a solubility parameter near that of the polymers in the rubber. The solubility parameter dependence of the friction was consistent with those of softness and resilience.

Plasma monoaminergic metabolites and catecholamines in subarachnoid hemorrhage. Clinical implications

We examined plasma catecholamines and monoaminergic metabolites (3-methoxy-4-hydroxyphenylethyleneglycol [MHPG], homovanillic acid [HVA], and 5-hydroxyindoleacetic acid) in patients with stroke successively up to three weeks after the initiation of symptoms. Plasma levels of free catecholamines were significantly elevated in patients with subarachnoid hemorrhage (SAH). However, no significant differences in plasma catecholamines were found when the patients with SAH were subdivided into noncomatose and comatose groups. In contrast, plasma HVA, MHPG, and 5-hydroxyindoleacetic acid levels in comatose patients with SAH significantly increased as compared not only with normal controls but also with noncomatose patients with SAH, and the peak levels of HVA and MHPG occurred within seven days poststroke. Such trends as observed in SAH were not observed in patients with cerebral hemorrhage. Our results suggest the usefulness of plasma monitoring of possibly centrally originating monoaminergic metabolites for predicting the degree of cerebral dysfunction in patients with SAH.

Plasma 3-methoxy-4-hydroxyphenylethyleneglycol and homovanillic acid concentrations in patients with subarachnoid hemorrhage

To ask if the determination of central-nervous-system-derived catecholamine metabolites in peripheral circulation could be a useful index of brain dysfunction after subarachnoid hemorrhage, 3-methoxy-4-hydroxyphenylethyleneglycol and homovanillic acid concentrations in plasma, together with those of free catecholamines (noradrenaline, adrenaline, and dopamine), were serially measured for up to 3 weeks after the initiation of symptoms in 23 patients with aneurysmal subarachnoid hemorrhage as compared to 17 healthy and 9 patient controls. Catecholamines and their metabolites were determined by using high-performance liquid chromatography with electrochemical detection. Plasma 3-methoxy-4-hydroxyphenylethyleneglycol concentrations were markedly elevated in subarachnoid hemorrhage patients with coma compared to those without, and the maximal concentrations observed in comatose patients never occurred in normal subjects or in patients with other neurological disorders. The mean maximal plasma concentrations of free catecholamines did not differ significantly between the comatose and noncomatose groups. Combining 3-methoxy-4-hydroxyphenylethyleneglycol with homovanillic acid level data more clearly discriminated between the comatose and noncomatose subarachnoid hemorrhage groups. The results suggest that plasma concentration of 3-methoxy-4-hydroxyphenylethyleneglycol, a major metabolite of brain noradrenaline, can be a prognostic discriminator for patients with subarachnoid hemorrhage and its discriminating power can be strengthened by combining it with homovanillic acid data.

Extensibility of Black Filled Elastomers

Unique empirical relations are found to exist between the extension ratio at break λb and the crosslink density ν λb ∝ ν−0.75 in the absence of gross strain induced crystallization, and λb ∝ ν−0.2 in the presence of gross strain-induced crystallization. With black-filled elastomers, the modulus at 100% strain, M100, is proportional to the crosslink density over a wide temperature range. Thus, unique empirical relations exist between λb and M100 of black-filled elastomers; λb ∝ M100−0.75 in the absence of gross strain-induced crystallization, and λb ∝ M100−0.2 in the presence of gross strain-induced crystallization. The proportionality constants can be used as convenient measures of the extensibility in material evaluation on a constant modulus basis.

Suprasellar germinoma in association with Klinefelter's syndrome. Case report

✓ A case is presented of suprasellar germinoma in a 20-year-old man with Klinefelter's syndrome. Hypogonadism is known to be a characteristic of Klinefelter's syndrome, and has often been described in cases of suprasellar germinoma. Thus, both pathological entities may mimic one another. It is emphasized that a chromosomal karyotype should be determined in every case of intracranial germ-cell tumor associated with hypogonadism.

Histochemical changes of brain dopamine in an acute stage of cerebral ischemia in gerbils

The fluorescence histochemical method of Falck et al. was applied to 40 gerbil brains after ligation of a unilateral common carotid artery to investigate alterations of brain dopamine in the acute stage of cerebral ischemia. The distribution of dopaminergic terminals and cell bodies in gerbils is the same as in other mammals. On the ligated side after one hour of ischemia, diffuse green fluorescence of dopaminergic terminals showed only a slight decrease in intensity when compared to the nonligated side. But white matter and bundles of myelinated fibers adjacent to and in the dopamine-rich regions had an intense green fluorescence in contrast to the non-ligated side where they are normally non-fluorescent. This is considered to indicate the extraneuronal leakage and diffusion of dopamine. The intensity of extraneuronal green fluorescence was especially high in glial cells. Occasionally, there was also an unusual green fluorescence in the lumen of small vessels in dopamine-rich regions on the ligated side. Dopaminergic cell bodies in the substantia nigra on the ligated side revealed a conspicuous reduction in the fluorescence intensity in severely affected cases. After 2 or 3 hours of ischemia, there was a marked reduction or disappearance of the diffuse green fluorescence on the ligated side. This may be attributed in part to further diffusion of leaked dopamine.

Fracture Energy of Elastomers in Mode I (Cleavage) and Mode III (Lateral Shear)

Attention has been drawn here to three different reasons why measurements of fracture energy by different methods may not agree: (1) When the test involves propagation of a tear by stored strain energy, as in the method shown in Figure 1, the energy available to cause rupture will be less than that supplied, because of dissipation within the elastomer. Calculation of the fracture energy on the basis of input energy would then lead to an overestimate, by about 100 per cent or more for typical filled elastomers. (2) As shown in Figures 4a and 5, the tear path is sometimes wider than the thickness of the test sheet. In consequence, fracture energy calculated from the sheet thickness would be too large, by about 40 per cent in the cases considered here. (3) Even when allowance is made for the true width of the tear path, measurements of fracture energy in shear (Mode III) are about 50 per cent larger than in cleavage (Mode I). This is attributed to frictional work expended in sliding the rough torn surfaces past each other.