Elastic Filler

Then, for a particulate composite, consisting of a polymeric matrix and an elastic filler, it is possible by the previously described method to evaluate the mechanical and thermal properties, as well as the volume fraction of the mesophase. The mesophase is also expected to exhibit a viscoelastic behaviour. The composite consists, therefore, of three phases, out of which one is elastic and two viscoelastic. 

A simplified approach to the glass-transition temperature of the composite can be based on the thermal expansion curves of Fig. 2. The elastic filler (f) exhibits a... 

Fig. 2. A schematic variation of the thermal expansion, reduced to the gauge length 1, for the components of the composite (f corresponds to the elastic filler, m to the matrix, i to the mesophase and, c to the composite). The (Al/IJ of the composite is chaneing slope twice, at Tg, and Tgm. The Tgc is found approximately by the intersection of the two external linear branches of the (Al/lc) = f(T) curve...

This is the well-known Einstein-Smallwood equation, where c is the volume fraction of the filler and Gn, is the elastic modulus of the rubber matrix. The equation is obtained based on three idealized assumptions, such as (i) freely dispersed particles, i.e. low volume fraction, (ii) a spherical shape (leading to the constant 2.5) and (iii) entirely non-elastic filler particles, i.e. their elastic modulus has to be infinitely large. The reinforcement term contains two factors one is a simple number related only to the geometry of the particles, the other is linear in the volume fraction of the filler particles. 

G.Ivan, E.Danila, V.Ionescu, and A.Toader, Fine ground vulcanzed rubber as an elastic filler in elastomer matrix, Symp. of ICPCMP Polymers Composite Materials , Tg.Mures, Romania, 1988. 

Commercially produced elastic materials have a number of additives. Fillers, such as carbon black, increase tensile strength and elasticity by forming weak cross links between chains. This also makes a material stilfer and increases toughness. Plasticizers may be added to soften the material. Determining the effect of additives is generally done experimentally, although mesoscale methods have the potential to simulate this. 

Mechanical properties depend considerably on the stmctural characteristics of the EPM/EPDM and the type and amount of fillers in the compound. A wide range of hardnesses can be obtained with EPM/EPDM vulcanisates. The elastic properties are by far superior to those of many other synthetic mbber vulcanizates, particularly of butyl mbber, but they do not reach the level obtained with NR or SBR vulcanizates. The resistance to compression set is surprisingly good, in particular for EPDM with a high ENB content. 

In summary, then, design with polymers requires special attention to time-dependent effects, large elastic deformation and the effects of temperature, even close to room temperature. Room temperature data for the generic polymers are presented in Table 21.5. As emphasised already, they are approximate, suitable only for the first step of the design project. For the next step you should consult books (see Further reading), and when the choice has narrowed to one or a few candidates, data for them should be sought from manufacturers data sheets, or from your own tests. Many polymers contain additives - plasticisers, fillers, colourants - which change the mechanical properties. Manufacturers will identify the polymers they sell, but will rarely disclose their... 

Plastics are high-molecular-weight organic compounds of natural or mostly artificial origin. In fabrication, plastics are added with fillers, plasticizers, dyestuffs and other additives, wliich are necessary to lower the price of the material, and give it the desired properties of strength, elasticity, color, point of softening, thermal conductivity, etc. 

Filler orientation is not the only consequence of geometric effects in strained systems with an anisodiametric filler. The appearance of normal stresses has been named as another such phenomenon [169,171,185] which, like the abnormal viscosity behavior of suspensions, may not be due to the elasticity of system components. 

Fibrous fillers have often been reported to increase considerably 1 in comparison with unfilled systems [171,176, 189, 197, 198]. In this case, however, the inlet loss is due not to the highly elastic properties of the melt but to other reasons, such as pushing of the binder through the package (plug) of filler in the inlet zone. Pushing of the filler package forth into the channel, etc. 

The first difference of normal stresses (tr, t) may serve as an indirect index of the highly elastic properties of polymeric systems [199]. C. D. Han [200] related (ru with the residual pressure at outlet Pt)dt. Han, who observed its reduction in polypropylene filled with calcium carbonate [201], concluded that filling decreases the normal stresses. Note that addition of fibrous fillers, vice versa, somewhat increases Pexi, [180]. 

As in the previous case, the increased filler-matrix interaction bring about an increase in both dynamic (D) and equilibrium (E) high elasticity moduli. 

Note that in case of VTES (I) there is a chemical interaction via the vinyl group between silane and the filler, which results in a sufficiently rigid bond between the matrix and filler. The agent (II) undergoes homopolymerization so that an elastic sublayer ( shell ) is formed around each filler particle, the tensile and impact strength of the composition increase as a result. 

Spherical indentor deformability tests of composites with PE synthesized on different fillers have shown [164] that the deformation of PFCM containing high percentages of filler (and polymer concentrations of less than 10% by mass) is substantially elastic and the specimen recovers completely after release of the load. As the polymer content increased to 60% by mass considerable residual deforma-... 

At fixed filling ratio, using a hybrid filler may decrease extruded material swelling, raise the critical deformation parameters at which elastic turbulence of the melt may begin [366]. 

The existence of yield stress Y at shear strains seems to be the most typical feature of rheological properties of highly filled polymers. A formal meaing of this term is quite obvious. It means that at stresses lower than Y the material behaves like a solid, i.e. it deforms only elastically, while at stresses higher than Y, like a liquid, i.e. it can flow. At a first approximation it may be assumed that the material is not deformed at all, if stresses are lower than Y. In this sense, filled polymers behave as visco-plastic media with a low-molecular and low-viscosity dispersion medium. This analogy is not random as will be stressed below when the values of the yield stress are compared for the systems with different dispersion media. The existence of yield stress in its physical meaning must be correlated with the strength of a structure formed by the interaction between the particles of a filler. 

The problem of concentration dependence of yield stress will be discussed in detail below. Here we only note that (as is shown in Figs 9 and 10) yield stress may change by a few decimal orders while elastic modulus changes only by several in the field of rubbery plateau and, moreover, mainly in the range of high concentrations of a filler. 

It seems quite obvious that introducing a solid filler into a polymer melt in the general case always leads to a growth of rigidity of a material. This is really so on the qualitative level. However, it is not at all obvious how this is reflected on the ability to high reversible (large elastic) deformations. 

A representative measure of rubbery elasticity of a material may be two quantities dimensionless ratio (ct/t) and characteristic relaxation time 9 = ct/2ty. According to the data of works [37, 38] when fibers are introduced into a melt, ct/t increases (i.e. normal stresses grow faster than stresses) and 0 also increases on a large scale, by 102-103 times. However, discussing in this relation the papers published earlier, we noted in the paper cited that the data were published according to which if fibers were used as a filler (as in work [37]), 9 indeed increased [39], but if a filler represented disperse particles of the type Ti02 or CaC03, the value of 0 decreased [40],... 

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