Water-activated adhesives

Abstract In this chapter, classification of adhesive and sealant materials is presented. For this purpose, various categories are considered depending on the polymer base (i.e., natural or synthetic), functionality in the polymer backbone (i.e., thermoplastic or thermoset), physical forms (i.e., one or multiple components, films), chemical families (i.e., epoxy, silicon), functional types (i.e., structural, hot melt, pressure sensitive, water-base, ultraviolet/ electron beam cured, conductive, etc.), and methods of application. The classification covers high-temperature adhesives, sealants, conductive adhesives, nanocomposite adhesives, primers, solvent-activated adhesives, water-activated adhesives, and hybrid adhesives. 

Laswell et al., 1971 Arato, 1974). All were prone to excessive dissolution and only one had adequate strength and film thickness. Their working characteristics were found to be unduly sensitive to changes in temperature and humidity (Simmons, D Anton Hudson, 1968). All were inferior to conventional zinc phosphate cements. No further development of these cements has taken place, nor is it likely that interest in them will be revived. The modem water-activated glass-ionomer cement has filled this niche and has vastly superior properties including adhesion to tooth material. 

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). 

Water-based adhesive tape, where the adhesive is activated by wetting and this soaks into the outer liner of the case forming a strong closure, provided the relative humidity remains reasonably low. Not recommended for tropical markets where the high humidity makes paper taping a security risk. 

Taping (plain and reinforced)—includes self-adhesive, water activated, heat activated ... 

Adhesives can set or cure by carrier (solvent or water) evaporation, chemical reaction, or thermal activation. Chemically reactive adhesives solidify primarily by a chemical reaction of one or more components in the adhesive formulation. It should be noted that solvent welding cementing processes or solvent-borne adhesives are in disfavor due to environmental, safety, and health concerns and regulations. As a result, waterborne adhesives and heat-activated adhesives are replacing solvent-based adhesives in many applications. 

In order to eliminate harmful solvents which are flammable and sometimes noxious, adhesives manufacturers have developed water-based adhesives, based on rubber latex or polymer dispersions, which set quickly due to the addition of an activator that reacts quickly with the base polymer and induces its quick coagulation. But the mixing should be done during the spraying in order to avoid any coagulation and clogging into the gun. 

Sun [24] has investigated the compaction of microcrystaUine ceUulose equilibrated to various water activities. Suns investigation shows that compaction properties of moisture-sensitive polymers depend on water activity. The data also show that adhesion between microcrystalline cellulose particles is reduced by the presence of adsorbed moisture. Presumably the reduction in adhesion results from the corresponding reduction in direct bonding between powder particles. Some of the results contained in Suns study are shown in Figure 16.11. More recently, Garcia -Mir et al [25] have studied the compaction of both microcrystalline cellulose and chitin as a function of water activity. The tablets in this study were compressed to a constant pressure without regard to porosity so much of the power of... 

Figure 16.11 Left- tensile strength vs. porosity for microcrystalline cellulose at different water activities (%RH). Right - tensile strength versus water content for microcrystalline cellulose at different porosities. Water content is determined by water activity. Vertical dashed line represents monolayer coverage by water. Plots redrawn from data by Sun [24]. Data suggest that adsorbed water reduces adhesion between power particles.

The number and variety of commercially available adhesives are now very large and continue to increase. Adhesive manufacturers are active in developing new formulations to meet a widening demand, and in modifying current formulations to meet new environmental legislation. This is reflected in the reduction or removai of known hazardous chemicals, solvents and asbestos fillers and a general trend towards water-based adhesives. 

Specializing in the manufacture of adhesives, primers, varnishes and coatings based on water, epoxy, acrylic, and polyurethane, LORD Industrial Ltda.—a subsidiary of the North /tmerican LORD Corp.—began its activities in Sao Paulo, Brazil, in 1972. The following year, it transferred its productive unit to the industrial hub of Jundiai (Sao Paulo), where it remains today. Throughout this period, LORD has become a main producer in the Brazilian adhesive market, principally with regard to the fabrication of products with tow levels of heavy metals, primers for the treatment of metal surfaces and water-based adhesives. For further information, visit www tordta. com.br and www.lord.com. 

Dry solids are measured for quality control. DS are also useful to determine how much water or solvent should be evaporated during drying and setting of the adhesive or sealant. In some cases, low solids content is a way for some manufacturers to reduce the cost of a water-based or solvent-based adhesive. If solids are too low, there might be not enough active adhesive in the glue line, which may become starved with voids or absence of adhesive between the two parts. 

The presence of an organic solvent, however, may present fire and health hazards. Water-activated These types of adhesives are generally based on polyvinyl alcohol, starch or protein glues. 

When water-base adhesives are applied to a substrate, they simply set upon removal of water. Some water-base adhesives (rubber-based adhesives) can be vulcanized or cross-linked by the application of heat or by activation provided by catalyst. 

Microencapsulation is a method for separating an activating solvent or a reactive catalyst from the adhesive base. The materials, whether solid or liquid, are packaged in very small microscopic capsules. When adhesion is desired, the encapsulated solvent is released by breaking the capsules by heat or pressure, and a tacky adhesive with instant grab is produced. In addition to solvents, small quantities of plasticizers or tackifiers may be contained in the capsules. The capsules are made of gelatin and are insoluble in water and neutral to the solvents. Heat-activated adhesives are another form of... 

Two chemical types generally used for pipeline repair arc epoxy and polyurethane (PU) resins. PUs have the advantage of curing at ambient temperatures via water activation, but the downside is that they are also easily hydrolysed in water and they have relatively low Tg values. In contrast, epoxy resins are more moisture stable and can be formulated to provide high Tg and environmental stability using aromatic amino compounds (Klein, 1991 Varma and Gupta, 2000). However, most thermosetting systems can only be applied on dry surfaces and are adversely affected by surface moisture and contaminants. Hence, ProAssure Wrap Extreme is a modified proprietary formulation that enhances adhesion in water environments. 

Radio-frequency energy has been used to heat the adhesive between two polymers. In order to fasten a PVC skin to a polypropylene panel for an automobile interior, a water-based adhesive was used that had to be heated to 90°C to be activated [19]. Five seconds of exposure to a 10- to 100-MHz source at 0.03 kW/cm was sufficient to heat the adhesive, which has a high dielectric loss compared to the skin and panel materials. Neither the PVC nor the polypropylene was healed measurably. The radio-frequency heating was 10 times faster than the conductive heating, which would have distorted the plastic skin and panel. 

Both the hquid and cured 2-cyanoacryhc esters support combustion. These adhesives should not be used near sparks, heat, or open flame, or ia areas of acute fire ha2ard. Highly exothermic polymerization can occur from direct addition of catalytic substances such as water, alcohols, and bases such as amines, ammonia, or caustics, or from contamination with any of the available surface activator solutions. 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

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