Particle dewetting

Yamamoto and Minamizaki [159] disclose the use of a curable silicone based release agent blended with resin particles which swell or are soluble in organic solvent. Coatings made with such blends can be written on with solvent based inks. For example, an addition cure silicone network containing 20 wt% 0.1 p,m diameter PMMA particles exhibited both good writeability (no ink dewetting and smear free) and a low release force of 10 g/cm for a PSA tape. 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

The migration of clay from EPDM to CR phase can also be explained as a wetting/dewetting process between polymers and filler. Hereby, the driving force of filler particle migration is the difference of the interfacial tensions between the rubbers and clay ... 

The dewetting films above were treated as a continuum, i.e a fluid without (micro)structure. However, in most applications the coating fluids are suspensions which contain particles and often macromolecules. The fluids also behave as non-Newtonian. 

The presence of particles in the film can influence the stability of the wet coating. The non-Newtonian behaviour affects the dewetting kinetics. As far as we know, neither of these aspects has been investigated. 

The dewetting model is useful in predicting critical stress from a knowledge of tensile yield stress The results of tensile testing can be used to predict adhesion of polymer to filler particles of different sizes. The following model is useful for this purpose ... 

The observed maximum (see the illustrative result presented in Figure 5.62a) was explained as a result of two requirements that stem from the main role of the silica particles, namely, to assist the globule entry by rupturing the asynnnetric oil-water-air films (Figure 5.63). The first requirement, formulated by Garrett, is that the particles should be sufficiently hydrophobic to be dewetted by the oil-water and air-water interfaces (otherwise, the solid particles would stabilize, rather than destabilize the asynnnetric film). The other requirement is that the particles should protrude... 

Figure 32 Suggested mechanism of foam film bridging by solid particle (92—94). In part a, the hydrophobic particle bridges the film, the capillary pressure dewets the particle, and the film ruptures. In b, hydrophilic particle s capillary pressure acts in opposite direction and tends to increase the film thickness. The arrows in the film indicate the direction of capillary forces.

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