Dry adhesion

Oxane bonds, M—O—Si, are hydroly2ed during prolonged exposure to water but reform when dried. Adhesion in composites is maintained by controlling conditions favorable for equiUbrium oxane formation, ie, maximum initial oxane bonding, minimum penetration of water to the interface, and optimum morphology for retention of silanols at the interface. The inclusion of a hydrophobic silane, such as phenyltrimethoxysilane [2996-92-17, with the organofunctional silane increases thermal stabiUty of the silane and make the bond more water resistant (42). 

Tackifiers are used to increase the tackiness and the setting speed of adhesives. They increase tackiness by softening the poly(vinyl acetate) polymer in the wet and the dry adhesive film. Tackifiers are usually rosin or its derivatives or phenoHc resins. Other additives frequently needed for specific apphcation and service conditions are antifoams, biocides, wetting agents, and humectants. 

One of the most common rubber adhesives are the contact adhesives. These adhesives are bonded by a diffusion process in which the adhesive is applied to both surfaces to be joined. To achieve optimum diffusion of polymer chains, two requirements are necessary (1) a high wettability of the adhesive by the smooth or rough substrate surfaces (2) adequate viscosity (in general rheological properties) of the adhesive to penetrate into the voids and roughness of the substrate surfaces. Both requirements can be easily achieved in liquid adhesives. Once the adhesive solution is applied on the surface of the substrate, spontaneous or forced evaporation of the solvent or water must be produced to obtain a dry adhesive film. In most cases, the dry-contact adhesive film contains residual solvent (about 5-10 wt%), which usually acts as a plasticizer. The time necessary... 

The dry adhesive films on the two substrates to be joined must be placed in contact to develop adequate autoadhesion, i.e. diffusion of polymer rubber chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. The application of pressure and/or temperature for a given time allows the desired level of intimate contact (coalescence) between the two adhesive film surfaces. Obviously, the rheological and mechanical properties of the rubber adhesives will determine the degree of intimacy at the interface. These properties can be optimized by selecting the adequate rubber grade, the nature and amount of tackifier and the amount of filler, among other factors. 

Knife test (KNF) The test is done by making two intersecting scratches through the paint film to the substrate with a sharp steel knife. Adhesive or cohesive failures are evaluated by peeling the coating from the intersection point and outwards. Common for the three adhesion evaluation methods are that the test is performed on immersed and non- immersed panel-half (referred to as respectively "wet" and "dry" adhesion). The type of rupture is reported, and the severity is judged on a scale from 5 (perfect) to 0 (poor). 

The use of dry adhesive films is expanding more rapidly than other forms because of their following advantages ... 

D 1382 Test Method for Susceptibility of Dry Adhesive Films to Attack by Roaches... 

D 1583 Test Method for Hydrogen Ion Concentration of Dry Adhesive Films... 

The differences among the breaking strengths of the several adhesives may be attributed, in part, to the variation in amount of adhesive present in the sample. The loading factors, the mass of dried adhesive per unit area of paper support, however, reveal no such correlation. For example,... 

The casein content of the dry adhesive varies from 30% to 70% depending on its use. Extenders and fillers such as soy and wood flours are added to obtain an economical adhesive. One or more alkalis such as sodium hydroxide, lime,... 

Quarterly data on sales in dollars and tonnage of liquid and dry adhesives are available from the Adhesives Manufacturers Association of America, located in New York. In addition, annual labor statistics are compiled. 

Data on liquid and dry adhesives sales in dollars and tonnage, quarterly labor, annually. 

The coreaction of phenolic resins with alkyds is an "old timer" in which the excellent gloss retention and durability of alkyds are combined with the water and alkali resistance of phenolics. Alkyd resin can be designed to have compatibility with hydroxyl-modified vinyl chloride-vinyl acetate copolymer resins, and this combination of fast drying, adhesion, excellent water resistance, and durability is specified in a number of coatings for military and marine use. 

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. 

The reduction of scrub resistance is the main disadvantage of HEUR/ hydrophobically modified alkali-swellable latex thickener systems 18, 19). Hydrophobically modified alkali-swellable latex-thickened paints often show increased water and alkali sensitivity, reduced wet and dry adhesion, and lower scrub resistance than similar cellulosic-thickened paints. 

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... 

Special boiling point hydrocarbons are used for quick-drying paints, dipping solutions, and quick-drying adhesives. Their flash point is below 21 C, which means that they must be used in explosion-proof areas. Petroleum ether is a special petroleum distillation fraction (hp 40-60 C). and is used as a solvent in the chemical industry. 

There are many different types of adhesives, but you re probably thinking of that white glue you had as a kid. This type of glue is known as a drying adhesive because it hardens by evaporation of a solvent. In the case of white glue, the solvent is water and the sticky stuff that gets left behind is polyvinyl acetate. 

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