Particle-surface adhesion

Inks. Refined kaolin is a common ingredient in a large variety of printing inks (qv). In addition to extending the more expensive polymers present, ka olin also contributes to improved color strength, limits the penetration of the ink into the paper, controls rheology, and improves adhesion. Kaolin for this appHcation must usually be as white as possible and free from oversize particles. Surface treated clays are used to improve compatibiHty with oil-based ink. Clays can also be an ingredient in the newer water-based or uv-cured inks. 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Transparency is often required. This is achieved by arranging that the particle size of the modifier to be below that of the wavelength of visible light (0.4-0.8 pm). This can normally be achieved by emulsion polymerisation, e.g., polybutadiene, polystyrene. Adhesion and surface compatibility between the polymer and modifier can be achieved by surface grafting of polar groups, e.g., acrylonitrile, various acrylates, onto the impact modifier surface before blending. 

A combination of adhesion and surface tension gives rise (pardon the pun) to capillary action. By its adhesion to the solid surface of the soil particles, the water wants to cover as much solid surface as possible. However, by the effect of surface tension, the water molecules adhering to the solid surface are connected with a surface him in which the stresses cannot exceed the surface tension. As water is attracted to the soil particles by adhesion, it will rise upward until attractive forces balance the pull of gravity (Figure 3.28). Smaller-diameter tubes force the air-water surface into a smaller radius, with a lower solid-surface-to-volume ratio, which results in a greater capillary force. Typical heights of capillary rise for several soil types are presented in Table 3.9. The practical relationship between normal subsurface water and capillary rise is presented in the following equation. 

The Attachment of Air Bubbles. Particles are carried upward and are held in the froth if air bubbles can be attached to them. The adhesion of bubbles is only possible if the particle surface is sufficiently hydrophobic. The contact angle (see Appendix Chapter 4) is an important variable related to the adhesion of bubbles to solid surfaces. As shown in Fig. A.4.1, Chapter 4, and by Youngs Equation (Eq. A.4.3, Chapter 4)... 

Once we have established reasonable values for the Hamaker constants we shonld be able to calculate, for example, adhesion and surface energies, as well as the interaction between macroscopic bodies and colloidal particles. Clearly, this is possible if the only forces involved are van der Waals forces. That this is the case for non-polar liquids such as hydrocarbons can be illustrated by calculating the surface energy of these liqnids, which can be directly measured. When we separate a liquid in air we mnst do work Wc (per unit area) to create new surface, thus ... 

Micromechanical force measurement apparatus (Taylor, 2006 Taylor et al., 2007) Particle adhesive forces Yes Adhesive forces vs. time (min) 15 psi >5 pm Adhesive forces between hydrate—hydrate particles, hydrate particle-surface... 

When conventional surfactants are used in emulsion polymerization, difficulties are encountered which are inherent in their use. Conventional surfactants are held on the particle surface by physical forces thus adsorption/des-orption equilibria always exist, which may not be desirable. They can interfere with adhesion to a substrate and may be leached out upon contact with water. Surfactant migration affects film formation and their lateral motion during particle-particle interactions can cause destabilization of the colloidal dispersion. 

Shu, S., Husain, S. and Koros, W.J. (2007) A general strategy for adhesion enhancement in polymeric composites by formation of nanostructured particle surfaces. Journal of Physical Chemistry C, 111 (2), 652-657. 

Pressing Operation. The consolidation of the particle mat and polymerization of the adhesive to produce a particleboard panel is accomplished in a hot press. The mat is compressed and held at the desired thickness until the adhesive on the particle surfaces has polymerized and established adequate bridges between particles. The panel is then removed from the press, cooled, and sent to the finishing phase. 

Understanding particle adhesion to a surface has applications in tissue engineering and particle processing. Experimental techniques for charactering particle adhesion to surfaces include laser trapping, AFM and microscopy with force measurement. 

It is made by dimerizing cyanamide in basic aqueous solution, and is a colorless solid melting at 208°C. Dicyandiamide is soluble in polar solvents, but at room temperature is insoluble in bisphenol A epoxy resins. It can be made into a very fine powder and milled into epoxy resins to form stable dispersions. Because the dicy is insoluble in the epoxy, the only possible reaction sites are at the particle surfaces. Although some reaction certainly occurs over a short time, the adhesives easily can have a useful shelf life of six months. On heating to about 150°C, the dicyandiamide becomes soluble in the epoxy resin, and the adhesive polymerizes rapidly. Cure can be accelerated by incorporation of tertiary aromatic amines or substituted ureas. 

FIG. 13 Free energy changes during particle-surface adhesion and particle cngulf-ment processes. 

Zhang F, Busnaina A. The role of particle adhesion and surface deformation in chemical mechanical polishing processes. Electrochem Solid-State Lett 1998 1 184-187. 

Particulate soil is removed from fibres by a two-step process. First, a thin layer of wash liquid penetrates between the particle and the fibre surface, enabling surfactants to adsorb onto the particle surface (Fig. 7.1). Then, the particle becomes solvated and is transported away from the fibre and into the bulk of the wash liquid by mechanical action. Finishes that are hydrophilic (enhancing penetration of the fibre-soil interface) with low adhesion to soil under washing conditions should improve particulate soil release. Ablative or sacrificial finishes... 

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