Filled epoxy-resin systems

It should be pointed out that diluents are not the only way to lower the viscosity of filled epoxy resins systems. Surface active agents can also be added to the system. They provide better wetting of the filler by the epoxy resin matrix. This can lead to substantial viscosity reduction for systems having equivalent filler concentration. The surface active agent, in turn, could also be used to produce formulations with higher filler loading at equivalent viscosity. These surface active agents are discussed in Chap. 10. 

The power-law model is the most extensively used shear-mte model for thermosets and has been used for unfilled (Ryan and Kamal, 1976, Kascaval et al., 1993, Riccardi and Vazquez, 1989) and filled (Ryan and Kamal, 1976, Knauder et al., 1991) epoxy-resin systems. Sundstrom and Burkett (1981) showed that there was a good fit of the viscosity of diallyl phthalate to the Cross model. The viscosity of polyesters has been modelled by Yang and Suspene (1991) using a Newtonian model. The WLF model has been used by Pahl and Hesekamp (1993) for a moderately filled epoxy-resin system. Rydes (1993) showed that the viscosity of DMC polyesters followed a power-law relationship at high shear rate. 

It should be noted that the effects of fillers may be incorporated into the cure and shear-rate effects. The main forms of combined-effects model consist of WLF, power-law or Carreau shear effects, Arrhenius or WLF thermal effects and molecular, conversion or empirical cure effects. Nguyen (1993) and Peters et al. (1993) used a modified Cox-Merz relationship to propose a modified power-law model for highly filled epoxy-resin systems. Nguyen (1993) also questions the validity of the separability of thermal and cure effects in the derivation of combined models. 

The complex viscosity can be related to the steady-shear viscosity rf) via the empirical Cox-Merz rule, which notes the equivalence of steady-shear and dynamic-shear viscosities at given shearing rates ri y) = rj (co). The Cox-Merz rule has been confirmed to apply at low rates by Sundstrom and Burkett (1981) for a diallyl phthalate resin and by Pahl and Hesekamp (1993) for a filled epoxy resin. Malkin and Kulichikin (1991) state that for highly filled polymer systems the validity of the Cox-Merz rule is doubtful due to the strain dependence at very low strains and that the material may partially fracture. However, Doraiswamy et al. (1991) discussed a modified Cox-Merz rule for suspensions and yield-stress fluids that equates the steady viscosity with the complex viscosity at a modified shear rate dependent on the strain, ri(y) = rj yrap3), where y i is the maximum strain. This equation has been utilised by Nguyen (1993) and Peters et al. (1993) for the chemorheology of highly filled epoxy-resin systems. 

Kogan et al. (1988) examined the chemorheology of silica- and carbon-fibre-filled epoxyresin systems. They found unusual effects of carbon fibre on the uncured rheology and chemorheology of filled epoxy-resin systems, and related these to the anisotropic namre of the filler shape and the effect of filler surfaces on the kinetics. 

Tillie et al. (1998) examined the effect of the fibre/matrix interface on the cure of glass-fibre-filled epoxy-resin systems. They found that the introduction of a lower-Tg interphase based on hydroxylated PDMS oligomers allowed an increase in toughness without a reduction in modulus or Tg. This was due to a modification of the stress field under load due to the elastomeric interphase. 

Expansion of gas-filled beads by application of heat or expansion of these beads in a polymer mass by the heat of reaction, e.g. expansion of polystyrene beads in a polyurethane or epoxy resin system. 

Covers a two-part epoxy-resin system in the form of a bisphenol "A" epoxy resin filled with fumed silica and carbon microspheres and a separate aromatic diamine curing agent. 

The filler effects on the chemoviscosity of thermosetting resins have not been studied extensively, but are vital to understanding the rheology of filled thermosets. For example, the effects of filler concentration on viscosity can be used in process control to monitor batch-to-batch variations or to provide essential information for research into alternative filler/resin batches. Ng and Manas-Zloczower (1993) examined an epoxy-resin system with silica filler and established that the elastic modulus of the resin can be expressed in terms of... 

The effect of fillers on the gel point of thermosets has not been studied extensively. Ng and Manas-Zloczower (1993) used isothermal dynamic time tests to measure the crossover point for a silica-filled epoxy resin. They noted a decrease in gel time with increasing filler loading. Metzner (1985) also noted that the storage modulus and loss modulus increased by different amounts with filler loading. Therefore, the gel-point tests for highly filled systems must involve knowledge of the effect of filler characteristics at various levels. 

Chang et al. (1998) examined the chemorheology of a highly filled epoxy-resin moulding compound. They found that a coupled power law and the Macosko model adequately describe the chemorheology of the system. This equation is... 

He subsequently assumed, that the whole body of the carbon black filled system can be represented by one single RC circuit. He was able, using this model, to calculate accurately the AC properties as a function of the frequency for a carbon black filled PVC system. This model failed, however, in the calculation of the AC properties of the above decribed, conductive epoxy resin systems. 

These resin cement systems require only moderate capital investment, yet can yield dramatic results. For example, the many thousands of cracks in the Los Angeles City Hall produced by the 1971 earthquake were repaired by the use of 20 thousand gallons of an aluminum and ceramic filled epoxy resin mortar.— Likewise, wood whose cracks have been sealed by polyurethane mortars is suitable for continuous lathe cutting for veneer manufacture. 

Fillers. In practice most epoxy resin systems have fillers incorporated, often simply to reduce cost although they may also assist in gap filling, reduction of creep, reduction of exotherm, corrosion inhibition and fire retardation. Their incorporation will also alter the physical and mechanical properties of the adhesive. Construction resins in particular often include a large volume fraction of sand or silica. 

Sand filled epoxy resins find utility as simulated terrazzo flooring, concrete highway and bridge abutment repair kits, and in various other applications where a concrete substitute is required that is convenient to use and cures rapidly. Table 9 indicates the strength improvements that can be produced in these systems by the use of silane coupling agents, both initially and after environmental aging. 

Compounds such as 2-methylimidazole 41, 2-undecylimidazole, and 2-ethyl-4-methylimidazole 42 are highly reactive materials. It has been stated that soluble imidazoles are too reactive to permit their use in one-part adhesive systems stable at ambient temperature. Among the other compounds, the less reactive 4-phenylimidazole 43 would be a convenient curing agent at elevated temperature, although the reaction still occurs over a few days of storage at 25 °C. 4-Methyl-2-phenylimidazole is cited in different patents allowing a 3-day pot life at room temperature for silver-filled epoxy resins based on bisphenol-A, bisphenol-F, and phenol novolac hardener. Imidazole 44 mixed with bisphenol-F... 

Electrical properties of filled resin systems are also improved by filler treatment. Filler particles are naturally hydrophilic via their metal hydroxide surfaces, and the particles naturally seek to agglomerate with each other, and so transport electrical charges through resin composite. Treatment with silane-coupling agent alters the chemistry of the filler surface, allow better dispersion of the filler throughout the resin matrix, and imparts improved electrical properties to the composite. Table 15.13 indicates the improved electrical properties of a quartz-fiUed epoxy resin system with 0.3% silane admixed into the formulation. Improved insulation values, including reduced dielectric constant and reduced dissipation factor, are also denoted. 

A number of examples of reinforced or filled polymer systems are shown in Table 6. As a general rule, as the thermal conductivity or the amount of the non-polymeric phase increases, so does the thermal conductivity of the composite. Examples listed in Table 6 include glass-reinforced polycarbonate and pol3Kethylene terephthalate), and mica-filled epoxy resins. On the other hand, plasticizers... 

Chem. Descrip. 77% Salt of unsat. polyamine amides and acidic polyesters with 20% 2-butoxyethanol, 2% xylene, 0.5% ethylbenzene Uses Wetting agent, dispersant, vise, reducer for filled, unsat. polyester and epoxy resin systems, esp. those with alumina trihydrate or calcium carbonate fillers and silica-filled epoxy flooring compds. 

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