Two-part adhesives

One component of a two-part urethane adhesive typically contains an NCO-terminated prepolymer and possibly fillers. The second component consists of polyfunctional, NCO-reactive materials, such as polyols and/or polyamines, and may also contain fillers and catalysts. The work life and rate of cure can be adjusted by the selection of catalysts, the catalyst level, and the starting materials used to prepare the resins. Table III lists the cure times for three adhesives which have identical bond strengths when cured, but different work lives. Two-part urethane systems usually cure at room temperature, but the cure can be accelerated by the use of heat. Although 

Tensile at break (psi) Elongation at break (%) Hardness (Shore D) 

When using two-part urethane structural adhesives, priming of the substrates is often required in order to obtain optimum bond durability, particularly when bonding metals. Primers are generally developed for use with a particular combination of adhesive and adherends. There are various types of primers for polyurethane adhesives, including one-part heat cure epoxies, two-part heat and room temperature cure epoxies, and one-part urethanes. 

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Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

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. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

Isocyanates can be added to solvent-borne CR adhesive solutions as a two-part adhesive system. This two-part adhesive system is less effective with rubber substrates containing high styrene resin and for butadiene-styrene block (thermoplastic rubber) copolymers. To improve the specific adhesion to those materials, addition of a poly-alpha-methylstyrene resin to solvent-borne CR adhesives is quite effective [76]. An alternative technique is to graft a methacrylate monomer into the polychloroprene [2]. 

One of the first applications of the Redux process was in construction of structural aluminum parts for the de Havilland bomber Sea Hornet produced during World War II ([198], pp. 80-81 [202,203]). In the original fonn, it consisted of a two-part adhesive including a low viscosity liquid phenolic known... 

Some items, such as electrolytic capacitors and two-part adhesives, may deteriorate when dormant. Others, such as rubber, adhesive tape, and chemicals, deteriorate with the passage of time regardless of use. These are often referred to as Shelf Life Items or Limited Life Items . Dormant electronic assemblies can deteriorate in storage and provision should be made to retest equipment periodically or prior to release if in storage for more than one year. 

Friction of rubber is a very complicated phenomenon but is generally thought of as being composed of two parts, adhesive friction and hysteresis... 

Polymer systems suitable for two-part adhesives include the following. 

Benzoquinone tetracarboxylic acid dianhydride (BTDA) has been found to provide epoxy adhesives with excellent high-temperature properties, in both the short and long terms. The formulation described in Table 12.12 provides good resistance to 260°C. This two-part adhesive can be cured 2 h at 200°C. The disadvantage of BTDA is that relatively high cure temperatures are required that result in a high degree of internal stress within the bond line. 

ATBN tougheners are generally used in room temperature formulations (see Chap. 11). ATBN liquid polymers cannot be mixed directly into the epoxy resin component of a two-part adhesive or in a one-part adhesive, since crosslinking and shortened shelf life will result. ATBN adducts are, therefore, mixed with the curing agent component of two-component epoxy adhesives. 

FIGURE 15.19 Durability of polyamide-epoxy two-part adhesives exposed to marine atmosphere (fillers are indicated). ... 

The type of pumps used to dispense epoxy adhesives depends on whether it is a one-part or two-part adhesive. One-part epoxy adhesives are dispensed using direct metering extrusion pumps. An electric motor pushes a follower plate into a drum of adhesive, which is then extruded through a hose to the dispensing valve. This technique is used for medium-and high-volume production rates. 

Two-part adhesives are generally dispensed by volumetric pumps for semiautomated and automated medium- to high-volume assemblies. Metered dispensing systems are easily capable of processing adhesives with mix ratios of 1 1 up to 15 1. 

Epoxy adhesives command a large portion of the structural adhesives market. Many people are probably familiar with epoxy structural adhesives, as these are the typical two-part adhesives found in hardware stores and supermarkets. Epoxy adhesives owe their popularity with both the general public and industry to their ease of use, their relative safety, and their compatibility with many adherends (Fig. 14.3). The various chemical reactions involving the epoxy ring provide a fertile field for the development of a wide range of properties.29 Even a half century after the first epoxy patents were issued,30 new patents on epoxy adhesive technology continue to appear every month (Fig. 14.1.)... 

While the composition and sequence of the amino acids have been known since 1983 (2,3), methods for increased-scale extraction were not developed until 1985. This scaled production has allowed for the development of single-part adhesive systems (Cell-Tak adhesive) for the immobilization of biologically active moieties to inert substrates. It has also permitted research on two-part adhesive formulations for the bonding of tissues. This paper specifically addresses the biocompatibility issue with descriptions of the immobilization of cells to Cell-Tak protein-coated plasticware, methods for wound closure, and preliminary toxicology data. 

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). 

The choice exists between using a one-part or a two-part formulation. One-part formulations are based on polyurethanes of very high molecular weight that are still soluble in petroleum-ester mixtures. Two-part adhesives are normally used where high cohesive strength and improved heat resistance are required and these comprise with the prepolymer terminated in some active group (such as polyether or polyester polyols) and as the second part either a multifunctional isocyanate (e.g., MDl) or an isocyanate... 

Pigments and fluorescent indicators in small amounts of 0.1 to 1% by weight may be added to adhesives formulations to identiiy a particular manufacturer or for quality inspection. Also, in two-part adhesives, a pigment such as carbon black is often added to one part so that after mixing the degree of mixing can be assessed visually when the color is uniform. 

This company s new 2600-093 Micro-Kiss series mix-dispense valves are designed for the low-flow mixing and dispensing of two-component adhesives and sealants such as silicones, epoxies, urethanes, and acrylics. The valves can be used to apply two-part adhesives and sealants for bonding, gasketing, potting and filling processes in aerospace, appliance, contractor, defense, electronics, filters, marine. 

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