Rubber materials viscoelasticity

After a brief introduction of the basic tools of NMR in Section 14.2, the 2D techniques that have been already applied to rubbers or viscoelastic materials (Section 14.3) will be reviewed. After briefly introducing each of the techniques, a more detailed overview of the applications and a discussion of some of the highlights will be given. This structure, where all information on a given method is presented within one section enables the interested reader to decide more easily which of the techniques might be most useful to them. NMR imaging which can be considered as a special form of 2D NMR will not be discussed but the interested reader is referred to the corresponding chapter in this book. 

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. ... 

Gil-Negrete N, Vinolas J, Kari L (2009) A nonlinear rubber material model combining fractional order viscoelasticity and amplitude dependent effects. J Appl Mech 76 011009... 

Abstract Rubber materials are viscoelastic systems whose properties, broadly speaking, are complex functions of time, strain, strain rate, temperature (and composition if they are inhomogeneous). Material functions are mathematical relationships that intend to describe the behavior of a material, either a solid or a liquid, when submitted to a range of strains or strain rates, with obviously temperature effects. For viscoelastic materials, such as rubber gum and compounds, these functions obviously encompass both the linear and the nonlinear domains. Providing material functions are considered in their full complexity, in other terms with respect to a multiparametric approach, they provide information about the processing behavior and the mechanical properties of rubber systems. 

A Multiparametric Approach of the Nonlinear Viscoelasticity of Rubber Materials... 

Providing tests are performed at low strain amplitude, small enough for the complex modulus to exhibit no strain dependency, then dynamic testing yields in principle linear viscoelastic functions. This implies that, with an unknown material, a preliminary strain sweep test is performed in order to experimentally detect the maximum strain amplitude for a linear response to be observed [i.e. G lo, f(Y)]-As illustrated in Fig. 6 with data from Dick and Pawlowsky [20], such a requirement is practically never met within the available experimental window with filled rubber materials, whose linear region tends to move back to a lower and lower strain range as the filler content increases. 

To return linear viscoelasticity, it is required that g(e) approaches unity for small strain. The stress-strain data for Smith s SBR vulcanisate rubber material are plotted in Figure 11.3(a). Log stress against log time plots were obtained for fixed strains and, as shown in Figure 11.3(b), form parallel linear relationships. This suggests via Equation (11.7) that the quantity g(e)/e is independent of time. It was found that for extension ratios up to 2, g(e) = 1 provided that a is understood to denote the true stress. At higher strains, the empirical function... 

G. Nijman, J.L. Leblanc. Engineering performance and material viscoelastic analyses alonga compounding line for silica based compoundspart 1 mixing line performance analysis. DIK, 8" Fall Rubber Colloquium, Hannover, Germany, Nov. 26-28,2008. 

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

There are several ways in which the impact properties of plastics can be improved if the material selected does not have sufficient impact strength. One method is by altering the composition of the material so that it is no longer a glassy plastic at the operating temperature of the product (Chapter 6). In the case of PVC this is done by the addition of an impact modifier which can be a compatible plastic such as an acrylic or a nitrile rubber. The addition of such a material lowers the glass transition temperature and the material becomes a rubbery viscoelastic plastic with much improved impact properties. This is one of the methods in which PVC materials are made to exhibit superior impact properties. 

The specimen was prepared by the following method. After mixing HAF carbon black (50 phr) with natural rubber (NR) in a laboratory mixer, carbon gel was extracted from unvulcanized mixture as an insoluble material for toluene for 48 h at room temperamre and dried in a vacuum oven for 24 h at 70°C. We made the specimen as a thin sheet of the carbon gel (including carbon black) by pressing the extracted carbon gel at 90°C. The cured specimen was given by adding sulfur (1.5 phr) to the unvulcanized mixture and vulcanized for 30 min at 145°C. The dynamic viscoelastic measurement was performed with Rheometer under the condition of 0.1% strain and 15 Hz over temperatures. 

Viscoelasticity illustrates materials that exhibit both viscous and elastic characteristics. Viscous materials tike honey resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Viscoelasticity is the result of the diffusion of atoms or molecules inside an amorphous material. Rubber is highly elastic, but yet a viscous material. This property can be defined by the term viscoelasticity. Viscoelasticity is a combination of two separate mechanisms occurring at the same time in mbber. A spring represents the elastic portion, and a dashpot represents the viscous component (Figure 28.7). 

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