Лако-красочные материалы — производство

Increasing the concentration of metal particles in an insulating adhesive matrix changes the electrical properties of the composite in a discontinuous way. Assuming a random dispersion of the metal filler, as the concentration increases no significant change occurs until a critical concentration, pc, is reached. This point, where the electrical resistivity decreases dramatically, called the percolation threshold, has been attributed to the forma­tion of a network of chains of conductive particles than span the composite. A two-di­mensional cartoon of a conductive adhesive below pc and just above pc is shown in Fig. 3. A typical plot showing the relationship between particle concentration and electrical resistivity is shown in Fig. 4.

Experimental [23] as well as theoretical [24-26] studies of percolation phenomena have been reported. In random and macroscopically homogeneous materials it has been demonstrated [27-29] that at concentrations of metal particles below the percolation threshold (p < pc) a short-range percolation coherence length, X, exists. Electrical conduc­tivity is probable for length scales less than X. Thus even if the metal-filled composite exhibits no bulk electrical conductivity, conduction can occur within domains that are smaller than X. As the concentration of metal particles approaches pc, X and the

composite becomes isotropically conductive.

The concentration of metal particles required to achieve pc has been reported over a wide range, from less than 1 to more than 40 vol %. This range of values occurs due to several factors, including processing techniques [3,30,31], particle size in relatively mono­disperse systems [32], particle size distribution [27], and particle aspect ratio. In many of the systems reported [23,25,33,34] random dispersions were assumed even when dense metal particles were employed. Recent work has demonstrated, however, that dense metal particles can settle, especially when the viscosity of the polymer matrix is low [27].

Particle settling is another factor that may influence the observed onset of percolation. Depending on how the electrical properties of the sample are measured, the observed value for pc may be either higher or lower than the value of pc in a truly random system.

The size of the metal particles relative to any structure present in the polymer matrix can also affect the value of pc. Segregated composites have been prepared by compression molding a mixture of metal and polymer particles [35]. When the radius of the polymer particle (Rp) is significantly larger than the metal particle size (Rm), the metal is confined to the regions between polymer domains [36]. Values of pc as low as 6 vol % were achieved when Rp/Rm = 16. Metal-plated polymer spheres have been prepared where the effective Rp/Rm! і [37].

Another factor that influences the value of pc is the aspect ratios of the metallic filler. Metal fibres, metal-plated glass fibers, and metal flakes can significantly lower the con­centration required to achieve isotropic conduction as compared to spherical powders [3]. Values of pc as low as 1 vol % have been reported with stainless steel fibers having an aspect ratio of 750 [37].

After mixing, some conductive epoxies do not always exhibit electrical conductivity. The electrical properties develop only after cure and the final resistance may be a function of the amount of time between mixing and cure [38-40]. This effect has been attributed to a fatty acid coating applied to the surface of the silver during manufacture of the flake. The coating is removed at elevated temperatures during cure. Solvents such as polypropylene glycol may dissolve the coating before cure, rendering the pastes conductive [39]. Growth of insulating coatings about the silver flake particles is postulated as the cause for the increase in electrical resistivity of some conductive adhesives upon standing at room temperature before cure [40].