Fillers, rubber Active

In the literature it has been found that during mixing aggregate breakdown occurs for a number of carbon blacks in highly viscous rubbers [115— 118]. Recently, the aggregate breakdown was also attributed to classes of specific shapes of individual carbon blacks [108]. The opinion about the mechanical consequences of this process is quite different. On the one side, no obvious relationship to reinforcement is conjectured [116]. On the other side, improvements of the mechanical performance, due to the creation of new, active carbon surface, is assumed, which participates in formation of a strong filler-rubber coupling [118]. 

Applications. Bound rubber is a measure of filler surface activity to the matrix, and it is considered as a factor in the estimation of filler reinforcement. 

These workers also showed that the apparent energy of activation of the failure process could be calculated assuming an Arrhenius mechanism. As illustrated in Table 10.3, addition of reinforcing filler raises the apparent activation energy of the viscoelastic failure processes. Halpin and Bueche ascribe the enhanced reinforcement to those processes that spread the viscoelastic motions of the filler-rubber complex over a much wider time scale, and concluded that the lower strength observed at elevated temperatures was due to the increased rate at which viscoelastic response to deformation... 

Classical rubber processing is time and effort consuming because of the high polymer viscosities working in of vulcanization accelerators, fillers, plasticizers, activators, etc., must be carried out on roll mills, the vulcanization must take place in heated presses. In contrast, liquid rubbers have low viscosities, and so, can be more easily processed. These have already been known for a long time in the case of silicones, polyurethanes, polyesters, and polyethers, but have only been very recently developed in the case of diene rubbers. 

One way of improving the adhesion between polymer and filler is to improve the level of wetting of the filler by the polymer. One approach, which has been used for many years, is to coat the filler with an additive that may be considered to have two active parts. One part is compatible with the filler, the other with the polymer. Probably the best known example is the coating of calcium carbonate with stearic acid. Such coated or activated whitings have been used particularly with hydrocarbon rubbers. It is generally believed that the polar end attaches itself to the filler particle whilst the aliphatic hydrocarbon end is compatible with the rubbery matrix. In a similar manner clays have been treated with amines. 

Elastomer-plastic blends without vulcanization were prepared either in a two roll mill or Banbury mixer. Depending on the nature of plastic and rubber the mixing temperature was changed. Usually the plastic was fed into the two roll mill or an internal mixer after preheating the mixer to a temperature above the melting temperature of the plastic phase. The plastic phase was then added and the required melt viscosity was attained by applying a mechanical shear. The rubber phase was then added and the mixture was then melt mixed for an additional 1 to 3 min when other rubber additives, such as filler, activator, and lubricants or softeners, were added. Mixing was then carried out with controlled shear rate... 

The specific activity of a filler at its interface with a rubber. 

A rubber to fabric bonding system comprising resorcinol, a formaldehyde donor and a hydrated silica filler. Incorporated as dry ingredients in the rubber compound and activated by heat. 

An activator in rubber compounds containing organic accelerators. In polychloroprene, zinc oxide is considered to be the accelerator rather than the activator. The use of zinc oxide as a reinforcing agent and as a white colouring agent is obsolescent. Zinc oxide is manufactured by either the French (or indirect) process or by the American (or direct) process. It can be used as a filler to impart high thermal conductivity. 

Resorcinol formaldehyde latex (RFL) cord dips have only a limited application within the general rubber goods industry and for adhesion to be achieved with synthetic fibres it is necessary to use the systems developed by Bayer and Degussa. These systems are based upon a combination of resorcinol, a formaldehyde donor and a hydrated silica filler (commonly called the RFK system). This system is incorporated as dry ingredients into the rubber compound and is activated by the application of heat. 

The intensity factor - the specific activity of the filler at its interface with the rubber. This is dependant upon the filler s physical structure and the chemical nature of its surface. Different rubbers will behave differently to the same filler. 

Silica fillers also react with the rubber causing an increase in viscosity and dry and unmanageable processing behaviour. Filler activators need to be added to silica-reinforced compounds to overcome these problems. The usual filler activators used are diethylene glycol, polyethylene glycol and amines such as triethanolamine. Some of these activators not only overcome the problems of processing and accelerator absorption, but depending on the cure system used, will also act as vulcanisation activators. 

Fillers. Materials used as fillers (qv) in rubber can also be classified as acidic, basic, or neutral. Furnace blacks, ie, HAF, FEF, or SRF, are somewhat basic. As such, they can have an activating effect on sulfur cure rates. Furthermore, carbon blacks have been found to promote formation of mono/disulfide cross-links thereby helping minimize reversion and enhance aging properties. 

Nonblack fillers such as the precipitated silicas can reduce both rate and state of cure. The mechanism appears to be one of a competitive reaction between rubber and filler for the zinc oxide activator. Use of materials such as diethylene glycol or triethanolamine prevents this competition thereby maintaining the desired cure characteristics. Neutral fillers such as calcium carbonate (whiting) and clays have little or no effect on the cure properties. 

Zinc oxide (ZnO) is widely used as an active filler in rubber and as a weatherability improver in polyolefins and polyesters. Titanium dioxide (TiOj) is widely used as a white pigment and as a weatherability improver in many polymers. Ground barites (BaS04) yield x-ray-opaque plastics with controlled densities. The addition of finely divided calcined alumina or silicon carbide produces abrasive composites. Zirconia, zirconium silicate, and iron oxide, which have specific gravities greater than 4.5, are used to produce plastics with controlled high densities. 

The substances listed in Table 28 correspond to the basic structure of elastomeric closures. The other components in rubber formulations are curing or vulcanizing agents, accelerators, activators, antidegradants, plasticizers, fillers, and pigments. The most common additives used to compound rubber for the pharmaceutical industry are listed in Table 29. The amount of each component may vary from rubber to rubber, and, depending on the component, the amount can reach more than 50% of the total mass of a formulation. While accelerators are used in amounts of around 1%, fillers may make up more than 50% of the formulation mass. 

Discussion - The morphological properties of active fillers are important aspects of rubber reinforcement. The structure of the reinforcing filler is characterized by aggregates of primary particles, which form cavities for attachment and penetration of polymer molecules. The SEM pictures show that the three-dimensional morphology is basically maintained. 

The results are shown in Fig. 9. A small amount of the filler strongly increases the energy contribution which is in full contradiction to the assumed increase in the concentration of active network chains caused by the filler. Curve 2 summarizes the results for filled PDMS rubber and for PDMS block and graft copolymers. It is seen that below 20% of the filler or hard phase, the energy contribution is practically independent of the amount of hard phase, but then a considerable increase of (AU/W)v>t is observed. Although in all these cases the energy contribution is... 

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