Vulcanization carbon black-filled rubber

Nitrile Rubber. Vulcanized mbber sheets of NBR and montmorillonite day intercalated with Hycar ATBN, a butadiene acrylonitrile copolymer have been synthesized (36). These mbber hybrids show enhanced reinforcement (up to four times as large) relative to both carbon black-reinforced and pure NBR. Additionally, these hybrids are more easily processed than carbon black-filled rubbers. 

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown [93] that the revulcanization process of devulcanized SBR was essentially different from that of the virgin SBR. The induction period is shorter or absent for re vulcanization of the devulcanized SBR. This is also true for other unhlled and carbon-black-filled rubbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur-containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in the case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules, resulting in crosshnking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber [93]. 

Vulcanization of rubber in the tire industry is the first industrial application of microwave irradiation for the processing of polymeric materials [139]. Improved product uniformity, reduced extrusion-line length, reduced scrap, improved cleanliness, enhanced process control and automation accomplished through the use of an appropriate combination of microwave, hot air, and/or infrared heating technologies are the main advantages of microwave carbon black-filled rubber... 

Even a modest increase in strain amplitude can greatly reduce the dynamic modulus of a carbon-black-filled rubber [58,80,88-90]. Because the effect on the modulus of unfilled rubbers is very small by comparison, the effect has largely been attributed to the carbon black aggregate-aggregate network [58,90]. The difference between a modulus measured at low strain and that masured at very high strain (or a value extrapolated to infinite strain) has been used as a measure of carbon black network per se [58]. The loss of dynamic shear modulus of filled rubber which occurs with increases in strain amplitude is greater if the rubber is not vulcanized [80]. 

The standard phenomenology of carbon black-filled rubber will be presented and the influence on the constitutive response of temperature and filler concentration will be discussed. Although the focus is on traditional vulcanized rubber, other thermoplastic elastomers show similar mechanical properties even if their chemical composition is quite different. Moreover, from a macroscopic point of view, the behavior of such materials is very close to the behavior of some biological soft tissues, such as ligaments and tendons, for what concerns both their static and dynamic responses. 

While RF and microwave-based techniques each play important roles in industrial material processing involving plasmas, the use of these radiations as a heating source for thermally initiated chemical reactions is (at present) mainly restricted to the laboratory scale. However, there is one exemption, namely the apphcation of microwave heating to the vulcanization of rubber Microwave extmsion lines are operated worldwide in the automotive and construction industries for the vulcanization of carbon black-filled rubber [120]. Carbon black is a good microwave absorber, but in the case of white and colored rubbers special sensitizers that permit the absorption of microwaves must first be apphed [121]. 

According to Mullins, carbon black-filled vulcanized rubbers consist of a rigid hard phase and an extensible soft phase. The rigid hard phase is associated with the carbon black and the soft extensible phase is the rubber. When carbon blacks are loaded into rubbers, the stiffness of the rubber increases markedly. The stiffness of the black-filled vulcanizate can be expressed quantitatively in terms of the volume fraction of the filler in the vulcanized rubber by the Guth and Gold equation ° ... 

By contrast, however, some rubber materials exhibit abnormal, strain-dependent ozone crack growth. Here, fewer T4 level ozone cracks appear because of the more rapid depth increase, i. e., the total destruction of a carbon-black filled vulcanized rubber strip specimen protected only by wax under 20% strain occurs 30 days earlier compared to material under 30% strain. When strain is increased from 30 to 200%, the growth of the numerous cracks, which were approx. 1 mm deep at first, increases rapidly. The isochrone charts of different crack depths shown in Figure 5.106 illustrate that the weathering time required to reach T3 and T4 levels in the strain range from 80 to 130% (even to 200%) decreases relatively little however, in the strain range from 30 to 80%, it decreases markedly, while it rises again between 20 and 30% strain. These differences in strain influence are seen in particular at the transition from crack level T3 to T4 [698]. 

Carbon black filled vulcanized rubbers, equipped with a high-performance protection system, develop a rather stable, highly efficient surface layer under artificial ozone aging conditions, but in outdoor test this surface layer decreases in thickness and thus in effectiveness due to atmospheric precipitation [231]. 

Figure 6.10 shows typical dynamic properties of vulcanized PDMS-silica systems, as investigated through strain sweep experiments at constant frequency and temperature. As can be seen, dynamic strain softening is observed in a qualitatively similar manner to other filled polymers. It follows that models, which successfully fit conventional filled rubbers (e.g., carbon black filled compounds), are expected to well suit such data. This is indeed the case, as shown by the curves in Figure 6.10, drawn by fitting the Kraus-Ulmer equations, i.e.. 

We prepared a strip-type specimen (100 X 50 X 2 mm) from the rubber sheet of SBR filled with HAH carbon black (50 phr), vulcanized for 30 min at 155°C. On the specimen, a slit of different lengths (si = 30 mm, S2 = 20 mm, S3= 10 mm) parallel to the extension direction and a notch of different lengths (2 or 5 mm) at the center of the side surface of the specimen perpendicular to the extension direction were made by razor-cutting (see the inserted figure in Figure 18.14). The distance 8 between slits and between the slit Si and the tip of notch was 1 and 3 mm, respectively. The no-slit specimen means that it only includes a notch, without slits. 

An elastomer filled with Aerosil, technical carbon (lamp or acetylene black), iron and titanium oxides and other ingredients including a vulcan-iser is raw rubber used to manufacture various products. The elasticity and resilience of silicone rubbers depend on the number of siloxane links in the chain and on the number of cross links. The higher the molecular weight of the elastomer and its elasticity the more the quantity of cross links (to a certain extent), the greater its mechanical strength. 

In rubber systems containing carbon black, flocculation may cause substantial changes in mechanical properties. Flocculation in these systems counteracts filler dispersion. Carbon black flocculation occurs in filled rubber stock during storage or during vulcanization in the absence of shear. " Temperature is the important kinetic factor which affects the flocculation rate (Figure 5.19). In addition to temperature and time, flocculation depends on the type of carbon black and its concentration. 

Diamine salts of fatty acids are used as multifunctional additives in natural rubber compounds filled with carbon black.They affect the elastomer-carbon black interface. With an increased concentration of multifunctional additive, the concentration of bound rubber decreases but dispersion of carbon black is improved. In silica filled rubber, multifunctional additive also improves the dispersion of silica, but in addition, it decreases the negative influence of silica filler on vulcanization rate. 

For small strains the stress-relaxation rate of vulcanized rubbers at long times is proportional to tan 8 (178). This will also be true at large strains if strain-time factorization applies. The implication of this for the results of Cotten and Boonstra (150) is that tan 8 in unswollen vulcanizates is only little affected by carbon black-polymer interactions at strain levels between 75 and 250% elongation (and at very low frequencies) and suggests that the substantial increases in tan 8 observed in filled rubbers at small strains are due primarily to the effects of secondary filler aggregation. 

Rubber is filled with carbon black or calcium silicate which act also as reinforcing agents. For example, the tensile strength of vulcanized SBR can be raised tenfold through compounding with 50% carbon black. Elastomers of... 

The particles of carbon black are not discrete but are fused clusters of individual particles. The reinforcement conferred by the black is not influenced to any extent by the size of the unit but predominantly by the size of the particles within the unit. The primary particle typically has cross-sectional dimensions" of 5-100 nm. It is well established that the most appropriate way of describing the size of the primary particles is to express it as speciflc surface area/weight Particle size of itself has relatively little effect on the modulus. But tensile and tear strengths are affected by the particle size and both properties are normally enhanced as the surface area increases (i.e. surface area increases with decreasing particle size). The high surface area enhances the ability of the filler to wet the rubber and thus enhances the interaction at the rubber filler interface. It is the enhancement of the filler-rubber interface that provides the desired reinforcement in filled vulcanized rubber. 

This compound also uses Neoprene W type polychloroprene rubber. The compound, though coloured black with a few parts of carbon black is mainly filled with silica, which is a reinforcing mineral filler. The silica also helps to bond the rubber to the steel cords other ingredients added for this purpose include cobalt naphthenate. A commonly used system for this purpose is one comprising resorcinol and hexamethylene tetramine (HMT) which acts as a formaldehyde donor to form a phenolic resin in situ, but this is not suitable for Neoprene compounds because resorcinol is a fast accelerator for Neoprene vulcanization and interferes with its processing safety. 

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