Autohesion properties

Autohesion Properties of Some Industrial Dusts and Characteristics of Adhesion in Dust-Trapping Processes... 

Davies [62] carried out a detailed investigation into the autohesive properties of dusts, measuring the forces associated with the breakup of dust layers these results constitute an objective characteristic of the autohesion of dust particles (Table VIII.3). 

Contact-bond adhesives are applied to both of the surfaces to be bonded, allowed to dry until approximately nontacky to the touch, and then pressed together to form a bond. The elastomers in these adhesives have the property known as autohesion. This means that when the dried adhesive-coated surfaces are brought together under some pressure, the films join by a process of molecular difiusion and form a completed bond. 

A special type of organic solvent-based adhesive is the so-called contact adhesive. This makes use of the fact that certain elastomeric or rubbery solids (e.g., polychloroprene) have the property of autohesion, i.e., they can stick readily to themselves, especially if compounded with resins and containing small amounts of solvents. The bonding takes place by a diffusion process, the adhesive being applied to both surfaces to be bonded. Thus substrates may be coated with a contact adhesive, the adhesives can be allowed to dry till most of the solvent has evaporated (the dry adhesive film at this point will contain... 

In the empirical world of tack experts, many professional terms are employed, like green strength (as initial tack is sometimes called), which relate to the resistance to separation before the adhesive has had a chance to vulcanize or crosslink. This characteristic may also be called quick tack or aggressive tack . It may be one of the most important properties in determining the suitability of an adhesive, such as that placed on a pressure-sensitive tape, for a certain application. Associated with tack is dry tack , which is a property of certain adhesives to stick to one another even though they seem to be dry to the touch. Autohesive tack (or autohesion) is the dry tack between materials having similar chemical compositions. Tack range is the time that an adhesive will remain in a tacky condition. 

On the basis of the properties and sizes of the particles forming these particular adherent layers of dust, we can assume that in these studies the phenomenon being investigated was autohesive detachment of particles, i.e., an erosion process, and that the values shown for the velocities were those serving to overcome the forces of autohesion since at velocities of 3-10 m/sec there is practically no removal of a monolayer of adherent particles with diameters smaller than 100 jum (see data presented on p. 322). Air-flow velocities greater than 100 m/sec are required to detach a monolayer of adherent particles. 

In a later study, Millili et al. proposed a bonding mechanism, referred to as autohesion, to explain the differences in the properties of spheres granulated with water and ethanol. Autohesion is a term used to describe the strong bonds formed by the interdiffusion of free polymer chain ends across particle-particle interfaces (59). 

A change in the properties of the suspension, in particular the turbidity of the water flow [27], and also the rheology of cohesive dispersed systems, are due to forces of autohesion. However, other still insufficiently understood factors also affect these processes. Hence, Kurgaev s attempt to associate autohesion with the rate of compaction of the residues of certain suspensions cannot be considered successful or his calculations of the forces of autohesion reliable [29]. 

Another method (theoretically better based) has been proposed for calculating the forces of autohesion from the limiting shear stress of the suspension (Psh) taking the number of coagulation bonds per unit area into consideration [30, 31], Yakhnin and Taubman [31] related Psh to the properties of the medium... 

The character of the autohesion of argillaceous soils containing 3-8% clay particles and up to 40% sand particles is determined by the cementing properties of the clay particles enveloping the particles of larger size. Thus, for a soil consisting of 8% clay (particle diameter smaller than 0.001 mm), 2% sludge (0.005-0.01 mm), 50% slit (0.01-0.05 mm), and 40% sand (0.05-0.25 mm), the autohesive force between the particles equals 0.15 kg/cm. ... 

The operating efficiency of many gas-purifying systems depends on the nature and particle size of the dust trapped. Very often poor operation of dust traps is due to the fact that no proper allowance has been made for the characteristics of the dusts, especially their particle size and their capacity to stick to each other and to other surfaces, i.e., their autohesion and adhesion. The autohesive and adhesive properties of various dusts have not yet been studied and applied sufficiently. 

An objective indication of the autohesive (cohesive) properties of dust is providedJ)y the autohesive force, which also governs the efficiency of gas purification. 

In powder metallurgy autohesive and adhesive processes are involved in extrusion and pressing techniques and in the production of finished articles from the original powdered material they also affect the free-running properties of powders. 

Evidently the small particles merge with each other, forming strong aggregates, and this also reduces the running of the powders. Since autohesive forces may be smaller in vacuum than in air ( 16), the best running properties occur for smaller particles under vacuum conditions. 

The forces of adhesion and autohesion may be reduced by hy-drophobization of the powder surface ( 10), thus increasing the free-running properties. Hydrophobization of the surface of sand may be effected by treatment with suspensions or solutions of wood pitch [447]. 

The bond strength of the particles in the original mass, i.e., the autohesion, also determines the packing density of the powder mass and its sintering properties. The packing density is given by the shrinkage of the powder. 

Of course the erosion resistance of soils is determined by the properties of the soil itself, and particularly the autohesion of the soil particles, which is affected by the particle size, the humidity of the soil, the tackiness, etc. In addition to this, the erosion of soils is largely determined by kinematic conditions. Hence, the fight against erosion must be carried on by methods adapted to particular cases. 

Combating Caking by Reducing the Autohesion, The caking of fertilizers may be prevented or greatly reduced if we lower the autohesive forces by changing the humidity of the product, the size of the particles, the interparticle contact area, and the properties of the particle surfaces. 

In the processes considered in this chapter adhesion and autohesion are of particular importance. However, these effects are often masked, and it is not always easy to see how they operate, as in the case of other processes (see Chapters III, IV, and VI). We cannot, for example, trace the changes in adhesion and autohesion as functions of the particle size, the properties and state of the surface, the properties of the medium, etc. 

This monograph is the first systematic treatment of the subject of particle adhesion (particles to surfaces) and autohesion (particles to particles). The author first attacks the subject from the theoretical standpoint of the physical and chemical factors giving rise to adhesion under ideal conditions of particle shape and surface characteristics, electrical forces, and substrate properties. The cases of adhesion in air and liquid media are differentiated, but the common factors are very nicely discussed. Following the establishment of the theoretical bases of the subject and a discussion of widely utilized experimental methods to obtain adhesion data for particles, the author proceeds to review available data for a variety of situations of practical importance where particle adhesion is intrinsic to the phenomenon. These topics include adhesion of particles to paint and varnish coatings, adhesion and removal of particles in an air flow and in a flow of water, adhesion of particles in air-cleaning equipment, particle adhesion in electrophotography, and soil erosion and silt transport. 

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