Hardness or Modulus

Cause changes in properties of vulcanized mbber compounds such as reduced hardness or modulus. 

This phenomenon englobes, of course, many mechanisms8 Tables 1, 2, and 3 display the most significant ones. These may account for increased strength (Table 1), the increase of hysteretic phenomena (Table 2), and increased hardness or modulus (Table 3). 

Table 3.9. The reinforcing effects are the change in Shore A hardness per phr of filler and the change in Modulus at 100% per phr of filler. A good starting point with any filler system is to determine the amount of hardness or modulus gain per amount of filler used. High alkalinity fillers, such as Carplex 1120 and Zeolex 23, have been shown to improve heat resistance in terms of physical property losses, but they have little effect on compression set.

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. 

Much more information can be obtained by examining the mechanical properties of a viscoelastic material over an extensive temperature range. A convenient nondestmctive method is the measurement of torsional modulus. A number of instmments are available (13—18). More details on use and interpretation of these measurements may be found in references 8 and 19—25. An increase in modulus value means an increase in polymer hardness or stiffness. The various regions of elastic behavior are shown in Figure 1. Curve A of Figure 1 is that of a soft polymer, curve B of a hard polymer. To a close approximation both are transpositions of each other on the temperature scale. A copolymer curve would fall between those of the homopolymers, with the displacement depending on the amount of hard monomer in the copolymer (26—28). 

To achieve low stress embedding material, low modulus material such as siUcones (elastomers or gels) and polyurethanes are usually used. Soft-domain elastomeric particles are usually incorporated into the hard (high modulus) materials such as epoxies and polyimides to reduce the stress of embedding materials. With the addition of the perfect particle size, distribution, and loading of soft domain particles, low stress epoxy mol ding compounds have been developed as excellent embedding materials for electronic appHcations. 

The hardness of wood varies markedly from soft balsa to hard ironwood with pine, oak, and maple in between. It is measured either by determining the force needed to push a hard ball (diameter = 0.444 in) into the wood to a depth equal to half the ball s diameter (Janka hardness) or by the initial slope of the force vs. penetration-depth curve (Hardness modulus). Average values of Janka hardnesses for typical woods are listed in Table 13.1. The data are from Green et al., (2006), and are for penetration transverse to the tree axis. The values are for moisture contents of about ten percent. The first set of five items are hardwoods, while the second set are softwoods. To roughly convert Janka hardnesses to VHN multiply by 0.0045. 

Softening as a result of micro-Brownian motion occurs in amorphous and crystalline polymers, even if they are crosslinked. However, there are characteristic differences in the temperature-dependence of mechanical properties like hardness, elastic modulus, or mechanic strength when different classes of polymers change into the molten state. In amorphous, non-crosslinked polymers, raise of temperature to values above results in a decrease of viscosity until the material starts to flow. Parallel to this softening the elastic modulus and the strength decrease (see Fig. 1.9). 

It is now well established that formation of hard or stiff gels is the result of association of micelles into cubic phases. The notation hard gel follows Hvidt and co-workers (Almgren et al 1995 Hvidt et al. 1994) and refers to a micellar solution with a dynamic elastic shear modulus G > 103Pa. The correlation between the formation of a cubic phase and the onset of plastic flow (i.e. formation of a gel with a finite yield stress) was first made for PS-PI solutions in... 

The importance of comparing plasticizers at equal efficiency levels (equal hardness or 100% modulus) rather than equal phr cannot be overemphasized. This is shown in Table III for a typical plastisol compound. It can be seen that use of a less efficient plasticizer, such as DIDP, even though it costs one half cent per pound more than DOP, results in a pound volume cost savings of 0.7c. 

Other important parameters are the shear modulus or modulus or rigidity (G), which is the mount s (or other elastomeric components ) ability to resist shear when forces are applied in opposing directions. For instance, there could be engine torque applying a load in a particular direction upon hard acceleration, while simultaneously a road impact force could be applied to the frame in another direction. This quantity is represented by the shear stress (x) over shear strain (e). Finally, the bulk modulus (K) plays a role in these types of components. The bulk modulus describes how a component elastomer will behave under pressure in three dimensions. Volume is considered here and typical units are in gigapascals (GPa). Equation 2.11 describes the bulk modulus mathematically, and Figure 2.7 shows the value graphically. 

PCO/PE Block copolymers with Ethylene or Propylene and norborene blocks low melt viscosity thus good processability, high elongation at break, impact strength, toughness, hardness, and modulus Epple and Brekner, 1994... 

In order to determine the local mechanical properties such as the hardness and modulus of non-flat samples or heterogeneous multiphase materials, a DSI system was integrated in a scanning device. This system combines SFM-like topography imaging with the ability of DSI tests to be performed at selected areas of interest by using a well-defined diamond tip. 

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