Dispersion adhesive

Important polymers for contact adhesives are natural and synthetic rubber types, in particular polychloropene rubber and polyurethane polymers. 

A typical example of contact bonding is the patching up of a rubber tube with a rubber solution . After evaporation of the solvent, the rubber polymers in the rubber solution combine with themselves under pressure and with the parts of the rubber surface areas swollen by solvents. 

A distinctive feature of dispersion adhesives, in contrast to solvent-based adhesives described in Section 5.2, is that water is used as an incombustible solvent . This bears an advantage regarding possible processing risks and environmentally relevant regulations. Therefore, adhesive and raw material manufacturers made considerable research and development efforts to replace solvent-based adhesives by dispersion adhesives. 

What are dispersions From a physical point of view, they are so-called multiphase systems in which a phase describes the state of a certain substance. 

In the case of dispersions (from Latin dispergere = fine distribution) a solid phase has dispersed into the liquid phase (water as dispersing agent) owing to the polymer particles in diameter ranges of 10-4—10-5 cm (ten-thousandth to hundred-thousandth cm). The smallness of the particles and special additives (stabilizers, emulsifiers) prevent them from deposition. The solid content ranges between 40-70%. The setting mechanism for the formation of the adhesive layer is initiated by the removal of the liquid phase. This may occur by 

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Rejects and Sludge Handling. Sludge from water clarification contains water, inks and soHd pigments, dispersed adhesive particles, small plastic particles or wax, short cellulose fibers, paper filler and coating particles, and large soHd materials, eg, rocks, dirt, wire, ceramics, etc. 

Other fields Friction reduction agents, dispersants, adhesives, fiber dressers... 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

The benefits demonstrated from the preservative use of Bronopol alone and in combination with other preservatives have been realised in several other application fields, namely polymer dispersions, adhesives, paper coatings and water-based paints. By way of example, the results of a study in a water-based paint system are reported, where Bronopol is evaluated alone and in combination with 2,2-dibromodicyanobutane (DBDCB). 

Young, G. C., Modifying Latex Emulsions with Epoxy Resin Dispersions, Adhesives Age, September 1996, pp. 24—27. 

A typical characteristic of physically setting adhesives is the evaporation of solvents or heatup to a molten state. Application is then in liquid form (solvent adhesives, contact adhesives, dispersion adhesives, hotmelt adhesives) or from the solid phase (adhesive films, laminate adhesives). In all of these types there is a liquid state during processing from which emissions are always a possibility due to the volatility of the basic polymer or other adhesive components. 

Adhesives are nonmetaUic substances used to join two surfaces by means of surface adherence (adhesion) and inherent strength (cohesion), DIN 16920. This definition of adhesives does not cover water glass adhesives, adhesive ceramics, or adhesive mortars. The substances used as adhesives are polymers that go through a liquid phase at least once (reactive adhesives) or more than once (hotmelts, thermally activated adhesives). The liquid phase can also be achieved by dissolution in suitable solvents (nonreactive adhesives). In dispersion adhesives, the polymer molecules are dispersed (finely distributed) in a liquid - usually water - whereby the polymer molecules themselves are not dissolved. Fig. 6. These adhesives are also known as water-based or aqueous adhesives. It must be remembered that solvents are contained in these adhesives in addition to the water. Genuine aqueous adhesives contain less than 5% solvents in the liquid phase. The dispersions crnitain, in contrast to the solute adhesive molecules, additional substances, disposal of which requires specific additional measures. Since the dispersions represent stable systems in water, the water-resistance of such adhesives is reduced. Their thermal and water resistance can be increased by additional crosslinking (usually with isocyanates). 

Polymers that are dispersible in water or organic liquids (dispersion adhesives)... 

This transformation of nondispersible plastics into dispersion adhesives is carried out with the following polymers ... 

The percentage of solids in the dispersion adhesives is 35-55%, in some cases as much as 70%, the rest is water, which after the adhesive application must either be completely evaporated (ventilation time) or diffuse into the joined parts (wood, textiles, concrete, foams, leather). The film formation temperature in types containing plasticizers and flexibilizing additives is lower than in types that are free of plasticizers. Additives, so-called high boilers (ethyl glycol, ethyl glycol acetate, ethylhexanol), are used to bring the film formation temperature down. 

Frequently, differences in substrate absorbency form the reason for variations in adhesive strength in bonds between PVC films and wood, plastic foams, or textiles. To speed up evaporation of the water from nonabsorbent plastics, continuous feed ovens can be used or water binding substances such as ettringite can be added just before application. Coarsely dispersed adhesives should be used on highly absorbent substrates, since very small particles tend to migrate into the substrate. 

One-component reactive adhesives have to be distinguished from the physically setting adhesives described in Chapter 5, which generally occur only in the form of one component, namely, the already finished polymer, for example, in the case of hot-melt adhesives, dispersion adhesives and solvent-based adhesives. They are called one-component adhesives. 

Since the latter possibility is only given in case of porous surfaces, dispersion adhesives are mainly used for materials that are capable of binding the water of the liquid adhesive layer by absorption . The remaining polymer particles with their inherent tack fuse to an adhesive layer (film formation). The open assembly time (Section 5.2) in the processing of dispersion adhesives is strongly influenced by the moisture content of the adherends and the relative humidity. The open assembly time increases with higher humidity. 

For dispersion adhesives, the most important polymers are polyvinyl acetate, acrylate, rubbers, polyurethane and polychloroprene. 

Among the preferably bonded materials there are, inter alia, wood and wood products such as hardwood and softwood, chipboards, hardboards, plywood, veneers, tongue and groove joints, dovetails and tenon and mortise. Due to their being solvent-free, dispersion adhesives are also suitable for the bonding of polystyrenes. 

Since the water content of dispersion adhesives must be absorbed by the adherends during setting, the setting time depends mainly on the moisture content, above all in the case of wood (favorable range 8-10%) (Section 9.5). 

When choosing dispersion adhesives suitable for wood, the later stress on the bonded joint, especially moisture, has to be taken into account (Section 9.5). 

The dispersion adhesive is usually applied to one side, in the case of very rough cutting edges and hard woods also on both sides. Residues from wood processing must be previously removed. 

Protect dispersion adhesives from frost, since after thawing they are no longer usable due to the distruction of the dispersion. 

Metals, glasses and other impermeable materials are not bondable by dispersion adhesives. 

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