Fillers matrices

Jamet, J, Spann, J.R., Rice, R.W., Ldwis, D. and Osbienz, W.S., Ceramic-Fiber Composite Processing Via Polymer-Filler Matrices. Ceram. Eng. and Sci. Proc. 5 [7-8] 443-474 (1984). 

Free Moisture. The free moisture of a filler is the water present on the surface of the particles. This weakly bound water can sometimes contribute to iaterparticle bonding (reinforcing) or filler—matrix iateraction, ie, biader adsorption or catalysis. A determination of free moisture is usually made by measuriag the percent loss on drying the sample at either 100 or 110°C. 

The behavior of the physico-mechanical characteristics of polymeric composites is easily traceable in the table given in [144] which presents the results of experiments with polyamide matrices filled with resite particles of different shape. The filler concentrations were adjusted so that the integral contact surface area in the filler-matrix system remained the same. 

In other words, the critical fiber length is a function of its strength, diameter and the shear strength at the filler-matrix interface. 

Similar results were also reported in [100], As noted earlier, the authors measured the strength of the filler-matrix interaction by the magnitude of the thermal effect the more negative the enthalpy, the greater the interaction. Pure A-175 Aerosil and Aerosils modified with aminophenyl methylene diethoxysilane (AM-2) and butyl alcohol ( Butasil ). The base polymer was PVC plasticized with dioctyl sebacinate. The results are presented in Table 8 below. 

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

From the table it is seen that the enhanced energy interactions between the polymeric coat of the filler and the matrix does not always entail an upgraded complex of physico-mechanical characteristics of the composite. The authors of [210] have advanced an opinion that the enhanced filler-matrix interaction prevents formation of labile bonds between the two and hinders the relaxation processes at the interphases. 

Fig. 10. The variation of the mesophase moduli Ej(r) for the various filler volume contents of iron-epoxy particulates, versus the polar distance from the filler-matrix boundary...

The filler-matrix interface The interface between filler and matrix is also crucial in terms of composite performance. The interface serves to transfer externally applied loads to the reinforcement via shear stresses over the interface. Controlling the strength of the interface is very important. Clearly, good bonding is essential if stresses are to be adequately transferred to the reinforcement and hence provide a true reinforcing function [1]. 

Biopolymers have diverse roles to play in the advancement of green nanotechnology. Nanosized derivatives of polysaccharides like starch and cellulose can be synthesized in bulk and can be used for the development of bionanocomposites. They can be promising substitutes of environment pollutant carbon black for reinforcement of rubbers even at higher loadings (upto SOphr) via commercially viable process. The combined effect of size reduction and organic modification improves filler-matrix adhesion and in turn the performance of polysaccharides. The study opens up a new and green alternative for reinforcement of rubbers. 

Interfacial structure is known to be different from bulk structure, and in polymers filled with nanofillers possessing extremely high specific surface areas, most of the polymers is present near the interface, in spite of the small weight fraction of filler. This is one of the reasons why the nature of the reinforcement is different in nanocomposites and is manifested even at very low filler loadings (<10 wt%). Crucial parameters in determining the effect of fillers on the properties of composites are filler size, shape, aspect ratio, and filler-matrix interactions [2-5]. In the case of nanocomposites, the properties of the material are more tied to the interface. Thus, the control and manipulation of microstructural evolution is essential for the growth of a strong polymer-filler interface in such nanocomposites. 

Recent research has explored a wide variety of filler-matrix combinations for ceramic composites. For example, scientists at the Japan Atomic Energy Research Institute have been studying a composite made of silicon carbide fibers embedded in a silicon carbide matrix for use in high-temperature applications, such as spacecraft components and nuclear fusion facilities. Other composites that have been tested include silicon nitride reinforcements embedded in silicon carbide matrix, carbon fibers in boron nitride matrix, silicon nitride in boron nitride, and silicon nitride in titanium nitride. Researchers are also testing other, less common filler and matrix materials in the development of new composites. These include titanium carbide (TiC), titanium boride (TiB2), chromium boride (CrB), zirconium oxide (Zr02), and lanthanum phosphate (LaP04). 

Figure 3.14. CNT/polymer nanocomposites observed in SEM (a) and (b) P(S-ABu)/MW CNT films surface respectively prepared by evaporation and film formation or freeze-drying and hot-pressing but showing similar fillers distribution (c) and (d) PS matrix containing ungrafted or PS-grafted N-doped CNT a fracture performed at ambient temperature highlights the difference in fillers/matrix interface strength. Scale bars 1 pm.

The electrical conduction process depends on several parameters, mainly on filler concentration. But filler morphology such as particle size and structure as well as filler-filler and filler-matrix interactions which determine the state of dispersion and filler orientation are key factors in determining the electrical properties. On the other hand, processing techniques also influence the electrical conductivity of... 

In any preparation of polymer-filler composites, there is concern about the quality of adhesion at the filler/matrix interface, and consequently over the interaction between filler and molten polymer at the compounding stage. Various technologies have been proposed to enhance adhesion in our laboratories, we have developed surface treatment (encapsulation) techniques in which mica is exposed to a "cold" microwave plasma (l.e. Tgiectron Tgas "Large Volume Microwave Plasma Generator"(LMP)... 

The effect of filler density on the density of filled product can be closely approximated by the additivity rule. If a more precise method of density estimation is required or filler/matrix mixtures are far Ifom being perfect, several corrections are necessaiy. System density becomes nonlinear close to the critical volume concentration (CVC). The critical volume concentration determines the amount of conductive filler which rapidly increases the conductivity of the composite. Figure 5.1 shows that at, or close to the critical volume concentration, density decreases. This density difference can be detected either after the CVC (polyethylene), before (polystyrene) or the two depressions are observed - one before and one after the CVC (polymethylmethacrylate) is reached. This density depression is due to filler-matrix interaction. 

We have outlined factors which affect particle distribution in a matrix. This distribution depends partly on filler properties but predominantly on the combination of properties of the pair filler-matrix. Filler distribution in a matrix depends on intended application. Some, such as applications which use fillers for reinforcement, require a homogeneous distribution of particles. In others, such as mentioned above electrical conductive materials, adhesives), a uniform distribution of filler particles may decrease their effectiveness. 

Filler-matrix interaction affects chain mobility... 

The amount of carbon black, its particle size and structure, the filler-matrix interaction, and the processing technique determine the electrical properties of a product. At a certain concentration of filler, the conductivity of the material increases dramatically. This concentration is known as the percolation threshold and the conductivity of the material is expressed by equation ... 

Figure 3.38 shows that reaction between Al(0H)3 and dicarboxylic acid anhydride affects the sedimentation volume of filler.The limiting value of sedimentation was obtained by modifying the filler surface with a monolayer of a suitable modifier. A similar modification affects the performance of this filler in polymer-filler composites. Thus, different properties were affected by the surface coverage of filler and by the filler-matrix interactions. 

Proper choice of pair filler-matrix (there should be interaction between the filler and the matrix some combinations produce adverse results there are cases (see alumino-silicate with PVAc) where an increased interaction reduces tensile strength due to increasing material stiffiiess)... 

Tensile yield stress gives additional infonnation on filler-matrix interactions and consequently it is one of the preferred methods of composite testing. Figure 8.3 shows that the particle size affects yield stress of PP composites. Only when filler particles become very small does the yield stress value increase as the concentration increases. The smaller the particle size the higher the value of tensile yield stress. The three largest particles are CaCOs and the smallest one is silica. Thus, yield stress behavior not only depends on particle size but also on the interaction with the matrix. If the matrix is deficient in the smallest particles of CaCOs the yield stress decreases. The stress which initiates yielding can be expressed by the equation ... 

Several characteristics of the matrix and filler-matrix interphase are involved in material toughening. These include the particle size of filler, interfacial adhesion, filler concentration (already discussed), filler surface composition, the crystallization of the matrix, shell thickness, stress whitening, and strain hardening. 

The creep resistance of materials depends on filler-matrix interaction and, therefore, is very much related to fillers use. A simple equation shows creep strain ... 

Special considerations dewetting angles can be calculated which represent filler-matrix adhesion carbon black is a compatibilizer of PVDF/PS blends " the thickness of a silane layer coating was estimated to be -16 nm ... 

Polymer blends and alloys have more complex behavior in the presence of fillers than the binary mixtures of polymer and filler. The same factors, such as filler distribution, filler-matrix interaction, filler-matrix adhesion, particle orientation, nucleation, chemical reactivity, etc. have influence on properties, but this influence is complicated by the fact that there are two or more polymers present which compete for the same filler particles. These complex interactions result in many interesting phenomena discussed below. 

Adhesion, filler/matrix adhesion, dimensional stability, reinforcement, and wear resistance are the most important concerns in the development of dental compos-ites. These requirements are shared with composites used for many other purposes. So much as the methods of testing, mathematical models, methods of interpretation, and remedies developed in other applications may be applied to dental composites. 

---