Natural rubber force-temperature

Fig. 7.4. Observation of a thermoelastic inversion point for natural rubber The temperature dependence of the force at constant extension exhibits a reversal in slope. Measurements by Anthony et al.[74]...

Fig. 85.—Force-temperature curves at constant length obtained by Anthony, Gaston, and Guth for natural rubber vulcanized with sulfur for elongations from 3 percent to 38 percent (at 20°C), as indicated.

Fig. 89.—The total force of retraction at 25°C and dE/dL)T,v obtained from the force-temperature intercepts at constant elongation for natural rubber gum-vulcanized using an accelerator. (Wood and Roth. )...

Both natural and forced-convection oven types can be employed they have been described in the section on drying. The forced-convection oven offers the advantages of uniformity of heat distribution and reduction in lag time in comparison with the natural-convection system. The dry-heat method is reserved almost exclusively for glass or metal as other materials char (cellulose), oxidize (rubber), or melt (plastic) at these temperatures. 

It has been found experimentally that the force F is directly proportional to the temperature T in the case of weakly cross-linked natural rubber extended by less than 300%. From this it follows that F = const X TovdFjdT— const, or... 

In natural rubber the attractive forces between neighboring polymer chains are relatively weak, and there is little overall structural order. The chains slide easily past one another when stretched and return, in time, to their disordered state when the distorting force is removed. The ability of a substance to recover its original shape after distortion is its elasticity. The elasticity of natural rubber is satisfactory only within a limited temperature range it is too rigid when cold and too sticky when warm to be very useful. Rubber s elasticity is improved by vulcanization, a process discovered by Charles Goodyear in 1839. When natural rubber is heated with sulfur, a chemical reaction occurs in which... 

Mechanical properties are the parameters used to measure the forces able to deform the natural rubber blended materials such as elongation, compression, twist and breakage as a function of an applied load, time, temperature or other conditions by testing materials. Results of these tests depend on the size and shape of the specimens of the tested materials. Generally, the specimens are cut into a specific shape and their mechanical properties tested with an accurate load cell capacity and crosshead speed by a tensile machine such as an Instron testing machine or universal testing machine until they deform. ... 

Viscoelasticity is one of the important mechanical properties of blended materials. Viscoelastic behaviour is the intermediate character between liquid and solid states that combines the viscous and elastic responses under mechanical stress. When a force is applied to blended materials, they can flow in the same as being liquids. The natural rubber blended materials do not stretch, but they will only gradually return to their original shapes when the force is released. This property depends on temperature, pressure, time, chemical composition, molecular weight, distribution, branching, crystallinity, and the composite of blending conditions and systems. ... 

Natural rubber based-blends and IPNs have been developed to improve the physical and chemical properties of conventional natural rubber for applications in many industrial products. They can provide different materials that express various improved properties by blending with several types of polymer such as thermoplastics, thermosets, synthetic rubbers, and biopolymers, and may also adding some compatibilizers. However, the level of these blends also directly affects their mechanical and viscoelastic properties. The mechanical properties of these polymer blended materials can be determined by several mechanical instruments such as tensile machine and Shore durometer. In addition, the viscoelastic properties can mostly be determined by some thermal analyser such as dynamic mechanical thermal analysis and dynamic mechanical analysis to provide the glass transition temperature values of polymer blends. For most of these natural rubber blends and IPNs, increasing the level of polymer and compatibilizer blends resulted in an increase of the mechanical properties until reached an optimum level, and then their values decreased. On the other hand, the viscoelastic behaviours mainly depended on the intermolecular forces of each material blend that can be used to investigate the miscibility of them. Therefore, the natural rubber blends and IPNs with different components should be specifically investigated in their mechanical and viscoelastic properties to obtain the optimum blended materials for use in several applications. 

Rgure 9.27 Force-temperature relationships for natural rubber. Extension ratios, a, are indicated by the numbers associated with the lines (110). 

Fig. 39. Force for equilibrium plotted against the absolute temperature for fibrous natural rubber. Tm is the melting point at zero force (0th and Flory, 1958).

A stress-strain isotherm for the uniaxial deformation of natural rubber, at ambient temperature, that was cross-linked in the liquid state is shown in Fig. 8.1.(5) Here f is the nominal stress defined as the tensile force,/, in the stretching direction divided by the initial cross-section, and a is the extension ratio. Using the most rudimentary form of molecular rubber elasticity theory f can be expressed as (6-9)... 

The experimental investigations of 0th and Rory (16) substantiate the major conclusions of the theory outlined above. Their studies of the force-length-temperature relations for fibrous natural rubber, that was cross-linked in the oriented state, give... 

Fig. 8.3 Plot of force required for phase equilibrium against the temperature, for cross-linked fibrous natural rubber, p = 1.56 x 10 and = 302 K. (From Oth and Flory (16))...

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