Constant viscosity rubbers

Over the years, natural rubber has been modified in many ways. One of these was constant viscosity rubber, described earlier. Some of these modifications, such as chlorinated rubber, have been successful for a while but were then superseded by other materials. Others are produced in small volumes and, at times, stocks may be non-existent. A number of modified natural rubbers have been produced in the last decade, and the presumption is they may still be obtained. 

A method to overcome this is Benoit s universal calibration plot (63) of log [ti] M against V, where [ti] is intrinsic viscosity. However, this method needs the constants from the Mark-Howink [ti]M relationships for the rubber samples to be analyzed in the SEC solvents before the SEC analyses. However, a literature survey showed that few constants for rubbers are available, as shown in Table 5. Another method is to use the Q factor (64), which is defined as the ratio of the extended chain length between polystyrene and rubber samples. This method is valid only for vinyl polymers and is empirically crude (6). 

The rate of mastication, as measured by changes in plasticity or viscosity, is a complex function of temperature (Figure 11.16) with the rate going through a minimum at about 105°C. Below this temperature the increasing viscosity of the rubber causes increased shearing stresses at constant shearing rates and this... 

The elastic stress curve in figure perfectly follows elastic strain [2]. This constant is the elastic modulus of the material. In this idealized example, this would be equal to Young s modulus. Here at this point of maximum stretch, the viscous stress is not a maximum, it is zero. This state is called Newton s law of viscosity, which states that, viscous stress is proportional to strain rate. Rubber has some properties of a liquid. At the point when the elastic band is fully stretched and is about to return, its velocity or strain rate is zero, and therefore its viscous stress is also zero. 

Mechanical synthesis by cold mastication of rubber and monomers depends on the reaction condition (monomer concentration, temperature, solvent concentration, atmosphere, presence of transfer agents, or catalyst) and on the physical and chemical properties of the rubbers, the monomers and the product interpolymers. A critical factor is the shear stress developed in the system rather than instrumentally-defined shear rates. The degree of reaction of polymer and consequently also the concentration of free macroradicals depends on stress. As a consequence, the influence of the above parameters may be connected to their influence on the viscosity of the reaction medium since an increase in viscosity causes an increase in stress at constant shear rate. 

Dipping method for adhesive coating of the metal discs is the best method for moulded rubber products manufacture while the quality control required is frequent checks on viscosity and solid content of the dipping bath. The later will be maintained by slow and constant agitation both vertically and horizontally in the bath. 

Type 104 oils are subclassified into types 104A and 104B for styrene-butadiene rubber viscosity-gravity constant (VGC) greater than 0.820 (ASTM D 2501). and are naphthenic Type 104B oils have a VGC of 0.820 max., and are paraffinic. See aromatic, naphthene, paraffin. 

The application process is also dependent on the type of project. Asphalt-rubber used as a seal coat is sprayed on the surface with equipment designed for asphalt-rubber s high viscosity and need for constant stirring to suspend the rubber. Hot mix projects require little special equipment as the asphalt-rubber is premixed with the aggregate and applied in the same manner as a standard overlay (29). 

According to the change of strain rate versus stress the response of the material can be categorized as linear, non-linear, or plastic. When linear response take place the material is categorized as a Newtonian. When the material is considered as Newtonian, the stress is linearly proportional to the strain rate. Then the material exhibits a non-linear response to the strain rate, it is categorized as Non Newtonian material. There is also an interesting case where the viscosity decreases as the shear/strain rate remains constant. This kind of materials are known as thixotropic deformation is observed when the stress is independent of the strain rate [2,3], In some cases viscoelastic materials behave as rubbers. In fact, in the case of many polymers specially those with crosslinking, rubber elasticity is observed. In these systems hysteresis, stress relaxation and creep take place. 

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