Colloidal systems adhesion

The forces acting between two surfaces in contact or near - contact determine the behavior of a wide spectrum of physical properties. These can include friction, lubrication, the flow properties of particulate dispersions, and, in particular, the adsorption and adhesion phenomena, the stability of colloidal system [1,2] and the ability to form Langmuir monolayer at the air - water interface. 

In the colloidal realm, given the large surface-to-volume ratio and the relatively small range of force that can sway the disposition of a colloidal particle, it is easy to appreciate the importance of controlling surface properties. Research literature abounds with the characteristics of colloid systems and model systems that mimic colloid surfaces. Applications permeate the fields of materials processing, adhesion, coatings, food science, and medicine. 

A bacterial suspension can be interpreted as a living colloidal system, and the initial step of adhesion involves, at a first approximation, an adsorption phenomenon that takes place between the organic macromolecules that constitute the bacteria outer shell and the carbon surface. Hence, bacteria adsorption can be explained by colloid and surface chemistry theories, which is why this section... 

Although colloidal systems may be in a mctastable state in relation to adhesion between the particles, it is important to stress that, if they have any significant solubility or volatility, they arc in general thermodynamically unstable with respect to particle growth processes. 

Knowledge of the structure of barnacle cement within colloidal dimensions has recently been gained. It is shown that the barnacle adhesive presents a useful tool for understanding and investigating formation of bioadhesive structures as colloidal systems. 

Chem. Descrip. 3,5-Dimethyltetrahydro-1,3,5,2FI-thiadiazine-2-thione CAS 533-74-4 EINECS/ELINCS 208-576-7 Uses Bactericide, antimicrobial, fungicide, preservative for aq. systems, adhesives, food pkg. adhesives/paper, dispersed colors, resin emulsions, powd. joint cements, protein colloids, architectural coatings, textile processing sol ns. 

The properties of polymer mixtures depend on the method by which they are obtained and are determined by many factors by sizes of particles of the dispersed phase, by their shape and number in bulk, and by the thermodynamic affinity of the components for one another [19]. Linear polymers blend either in the course of their mutual dissolution or, in two-phase systems, under conditions of thermodynamic incompatibility of the components, when the dispersion is forced. The mixtures formed can be compatible (forming true solutions of one polymer in another), incompatible (representing a typical colloid system), quasicompatible (characterized by microscopic homogeneity at a level above heterogeneity on the molecular level), or pseudocompatible (with a strong adhesion interaction on the boundary) [106]. 

Biological sciences were not excluded from this explosion of knowledge. The study of cell structure and cell behavior, including material transport aaoss membranes, cell division, and cell adhesion, raised aspects of adhesion already familiar in physical colloid systems. Then the rise of molecular biology in the last... 

Macromolecular species have played an indispensable role in the stabilization of colloidal systems since the first prelife protein complexes came into existence. We (humans) have consciously (although usually without knowing why) been making use of their properties in that context for several thousand years. Today macromolecules play a vital role in many important industrial processes and products, including as dispersants, stabilizers, and flocculants as surface coatings for protection, lubrication, and adhesion for the modification of rheological properties and, of course, for their obvious importance to biological processes. 

Gelled polymer for adhesives Environmentally desirable for solvent-free adhesives Sensitive colloid system... 

It is most impressive to find how theoretical knowledge has led to some fascinating developments in the technology. The purpose of this handbook is also to further this development. The molecular description of liquid surfaces has been obtained from surface tension and adsorption studies. The emulsion (microemulsion) formation and stability are described by the interfacial film structures. The surfaces of solids are characterized by contact angle and adsorption studies. The ultimate in interfaces is an extensive description of chemical physics of colloid systems and interfaces. Contact angle and adhesion is described at a very fundamental level. The thermodynamics of... 

For typical lyophobic colloidal systems with a complex Hamaker constant 10" J and a particle-particle separation of 0.2-1 nm, the adhesive energy in the contact is significantly larger than kT, which indicates that thermodynamics favors the formation of coagulation contacts. The primary potential energy minimum is even deeper in systems conposed of coarser particles. At the same time, for the case of low values of the complex Hamaker constant, 10 -10 J, the adhesion between the particles in systems that are not too coarse (particles with a diameter up to a micron) is overcome by the Brownian motion, and the formation of structures with coagulation contacts is impossible. 

It is worth recalling here that a dispersion medium akin to the particles, as well as surfactant adsorption, can lower both the interfacial energy, o, and the complex Hamaker constant. A by two to three orders of magnitude. In such a lyophilized system, the adhesive energy and force are also lowered by several orders of magnitude. In a concentrated disperse system in which the dispersed particles are mechanically forced to come into contact with each other, the lyophilization manifests itself as a decrease in the resistance to strain t. This means that in concentrated colloidal systems, plasticizing takes place, while in systems with a low concentration of dispersed particles, the lyophilization results in enhanced colloid stability of the free-disperse system (see Chapter 4). 

Latex dispersions have attracted a great deal of interest as model colloid systems in addition to their industrial relevance in paints and adhesives. A latex dispersion is a colloidal sol formed by polymeric particles. They are easy to prepare by emulsion polymerization, and the result is a nearly monodisperse suspension of colloidal spheres. These particles usually comprise poly(methyl methacrylate) or poly(styrene) (Table 2.1). They can be modified in a controlled manner to produce charge-stabilized colloids or by grafting polymer chains on to the particles to create a sterically stabilized dispersion. Charge-stabiHzed latex particles obviously interact through Coulombic forces. However, sterically stabilized systems can effectively behave as hard spheres (Section 1.2). Despite its simpHcity, the hard sphere model is found to work surprisingly well for sterically stabilized latexes. 

Polyvinyl acetate can be protected with colloid systems, surfactant systems, or a combination of both.By varying the protection system, it is possible to produce adhesive bases whose relative water resistance will differ in range from water remoistenable to water resistant (24 hr) and even to boiling water resistant. The choice of a protection system used in an adhesive base will also affect the degree of solvent resistance... 

Pontoni, D., Finet, S., Narayanan, T., and Rennie, A. (2003) Interactions and kinetic arrest in an adhesive hard-sphere colloidal system. / Chem. Phys., 119,... 

Compounding is quite different for the two systems. The solvent base system is dependent on magnesium oxide and a /-butylphenoHc resin in the formulation to provide specific adhesion, tack, and added strength. Neither of these materials have proven useful in latex adhesive formulations due to colloidal incompatibihty. In addition, 2inc oxide slowly reacts with carboxylated latexes and reduces their tack. Zinc oxide is an acceptable additive to anionic latex, however. Other tackifying resins, such as rosin acids and esters, must be used with anionic latexes to provide sufficient tack and open time. 

---