Two-component adhesives

In the crosslinked state, epoxy resins are highly resistant to chemicals, temperature, and solvents and are also endowed with good electrical properties. They are therefore employed, for example, as casting resins in electro- and electronic industry as well as resistant lacquers and coatings. Moreover, they possess excellent adhesive power for many plastics, wood, and metals ( reaction adhesives two-component adhesives ). 

Second-generation solventless polyurethane adhesives Two-component second-generation adhesive systems are the most widely used of the 100% solids products. These two-part adhesives comprise an isocyanate terminated prepolymer based on the reaction product normally of MDI and polyols (polyether or polyester) in the presence of excess isocyanate, and a polyol (polyether or polyester based). 

Laminating adhesive (two-component solvent-free laminating adhesive 100 parts of MDI based prepolymer, 35-50% free monomer)... 

Pressure-sensitive adhesive, hot-melt adhesive, contact adhesive, two-component adhesive as a reference to a certain processing procedure. 

Solvent-based adhesives, two-component epoxy resin adhesives, two-component methacrylates for assembly purposes (priming, filling, smoothing, sealing, etc.) For these applications, a multitude of so-called building/ construction adhesives is commercially available. 

Uses Epoxy tor grouts, adhesives, two-component coatings, flooring Features 100% reactive resin iow coior and exc. toughness Properties Gardner 3 iiq. sp.gr. 1.13 dens. 9.4 ih/gai vise. 2600 cps EEW 185 100% solids Epotuf 95-467 [Reichhold]... 

Adhesive, two-component—An adhesive supplied in two parts which are mixed before apphcation. Such adhesives usually cixre at room temperature. 

Adhesive, two-component An adhesive supplied in two parts that are mixed before application. Such adhesives usually cure at room temperature. 

Two-Component Conductive Adhesives Two-component electrically conductive adhesives include products that provide high peel and tensile lap shear strength over a broad temperature range, and silver-filled epoxies recommended for electronic bonding and sealing applications. 

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Fig. 13. Two-component development involving competition between electrostatic and adhesive forces, where + outside and beneath the large circles represents the positively charged toner and the large circles containing negative signs represent carrier beads.

Friedrich et al. also used XPS to investigate the mechanisms responsible for adhesion between evaporated metal films and polymer substrates [28]. They suggested that the products formed at the metal/polymer interface were determined by redox reactions occurring between the metal and polymer. In particular, it was shown that carbonyl groups in polymers could react with chromium. Thus, a layer of chromium that was 0.4 nm in thickness decreased the carbonyl content on the surface of polyethylene terephthalate (PET) or polymethylmethacrylate (PMMA) by about 8% but decreased the carbonyl content on the surface of polycarbonate (PC) by 77%. The C(ls) and 0(ls) spectra of PC before and after evaporation of chromium onto the surface are shown in Fig. 22. Before evaporation of chromium, the C(ls) spectra consisted of two components near 284.6 eV that were assigned to carbon atoms in the benzene rings and in the methyl groups. Two additional... 

Variety of form. Rubber base adhesives can be supplied for assembly operations as solvent or water-borne dispersions, hot melts, precast films, extruded tapes or reinforced films. In addition solvent and water-borne dispersions can be supplied as single or two-components systems. 

Neoprene AD-G. It is a Neoprene AD grafted with methyl methacrylate. It provides longer pot life (i.c. the time after which a mixture of two components in an adhesive maintains its properties) in two-part adhesives systems. 

Urethane adhesives are classified as one-component or two-component adhesives. Each category includes several different types of adhesives. 

The first urethane reaction in Fig. 1 is used in two major ways in adhesives. In one case, a two-component adhesive usually employs a polyol and polyisocyanate with catalyst. This can react at room temperature to form the polyurethane. The second use of this reaction is to make an isocyanate-terminated prepolymer. Reacting a stoichiometric excess of isocyanate with polyol can produce an isocyanate-terminated prepolymer. A prepolymer is often made with an NCO/OH ratio of 2.0, as shown below, but the isocyanate ratio can range from 1.4 to over 8.0, depending upon the application ... 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

Delayed action catalysts are used in certain two-component adhesives systems but will not be discussed here. A review of delayed action catalysts can be found elsewhere [32]. 

The soft segments made from asymmetrical (amorphous) polyols are important for two-component structural adhesives and one-component moisture-curing adhesives. These materials are applied and usually cured at room temperature. 

Two-component urethane adhesives 7.1. Structural urethane adhesives... 

The two-component urethane structural adhesives are among the most difficult to characterize, simply because of the widely varying properties that are possible. These adhesives may be rigid plastics similar in modulus to standard epoxy adhesives, with glass transition temperatures of the cured adhesive being approximately 60°C. 

Two-component urethane adhesives are used to bond sheet molding compound (SMC) panels for automotive OEM and aftermarket applications. Two-part urethanes are used as laminating adhesives in the RV industry. 

It would not be possible to discuss the infinite variations of two-component urethane adhesives in this writing. In order to demonstrate the extremes possible with two-component urethanes, two simplified adhesives are shown below with widely differing properties ... 

Two-component waterborne urethanes are the preferred choice to replace solvent-borne urethane adhesives, especially in the packaging and shoe industries. At this time, the packaging area is the largest application of two-part waterborne urethanes. Good strides have been made by the two-part waterborne urethanes in the shoe industry, especially in the athletic shoe market. Waterborne urethanes are also replacing the solvent-borne products in the OEM door panels. 

The two-component waterborne urethanes are similar in nature to the one-component waterborne urethanes. In fact, many one-component PUD s may benefit from the addition of a crosslinker. The two-component urethanes may have higher levels of carboxylic acid salt stabilizer built into the backbone than is actually needed to stabilize the urethane in water. As a result, if these two-component urethane dispersions were to be used as one-component adhesives by themselves (without crosslinker), they would show very poor moisture resistance. When these two-component urethane dispersions are used in conjunction with the crosslinkers listed in Fig. 8, the crosslinkers will react with the carboxylic pendant groups built into the urethane, as previously shown in the one-component waterborne urethane section. This accomplishes two tasks at the same time (1) when the crosslinker reacts with the carboxylic acid salt, it eliminates much of the hydrophilicity associated with urethane dispersion, and (2) it crosslinks the dispersion, which imparts solvent and moisture resistance to the urethane adhesive (see phase V in Fig. 5). As a result of crosslinking, the physical properties may be modified. For example, the results may be an increase in tensile properties and a decrease in elongation. Depending upon the level of crosslinking, the dispersion may lose the ability to be repositionable. (Many of the one-component PUD s may... 

Once the crosslinker is added, it is important to apply the adhesive and dry off the water. Most of the commonly used crosslinkers will react with water over a period of time and lose effectiveness. In some two-component PUD s, the system may increase in viscosity and even gel, giving the user some idea of when the useful life of the crosslinker is approaching its end. In other instances, no viscosity increases or other visible indications signal that the crosslinker has reached the end of its useful life. The improvements in physical properties, solvent resistance, and water resistance normally provided by a crosslinked PUD adhesive would not be fully realized, in this case. 

Thus far, the discussion has dealt with the versatility of urethane adhesives, the wide variety of substrates urethanes can bond, and the infinite variations of one-component and two-component urethanes. Some substrates have posed particular challenges for urethanes. Many adhesion problems have been discussed, but not all can be covered in this brief synopsis. However, two of the more common problems, with possible solutions, will be mentioned. 

Depending on the formulations various grades of water resistance can be achieved according to EN 204 (D1-D4) [172], For the two-component PVAc adhesives crosslinking and hence a duroplastic behavior is effectuated by addition of hardening resins (e.g. on basis of formaldehyde), complex forming salts (based... 

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