Two-part adhesive formulations

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. 

For bottles, there are special bottle dispensers in use like simple hand pumps or volumetric dispenser. Adhesive dispenser controllers are used to offer a high accuracy to meet customer requirements for multivalve dosing or coating or spray operations. O Figure 37.11 below shows typical dispensing equipment for one and two-part adhesive formulations. It contains a cartridge gun, cartridges, and a nozzle. 

Mono-cartridge TAH system as supplied by Nordson with two chambers (a) and double Mixpac-cartridge systems as supplied by Sulzer for applying two-part adhesive formulations 2 1 mixing ratio, 215 ml and 15 ml volume (b)... 

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. 

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). 

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. 

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. 

With improved formulations, organic-based conductive adhesives became feasible replacements for eutectic solders. Table 4 shows typical one- and two-part conductive adhesives from the 1970s. The one-part adhesive system employed a latent catalyst for long pot life but required a high-temperature cure cycle. The two-part system, formulated... 

This is a substance added to an adhesive to promote the curing reaction by taking part through catalysis or cross-linking. Two-part adhesive systems generally have one part that is the base and a second part that is the hardener. Upon mixing, a chemical reaction takes place that causes the adhesive to solidify. A catalyst is sometimes incorporated in an adhesive formulation to speed up the reaction between the base and the hardener. Very small amounts of catalyst are required, compared to the principal components such as base and hardener. 

Polyurethane adhesives can be formulated as one- or two-part systems. These are also referred to as either one- and two-pack or one- and two-component systems. The two-part adhesives have been the more widely used of the two for structural applications, particularly for high-performance bonds. 

Nitrile-phenolics are formulated as two-part adhesives dissolved in ketonic, chlorinated or, most frequently, in blended solvents. Some one-part materials are available which do not contain vulcanizing ingredients for the nitrile rubber but rely on a novolac resin with the addition of hexamine to introduce some cross-linking into the rubber. 

Pot life. One-part adhesives have indefinite working life at room temperature, whereas the two-part adhesives have a life limited from 1-8 h at room temperature once the parts have been mixed. This pot life is greatly influenced by the pH of the formulation. In the case of an R/F adhesive, the most desirable state of the material with a pH just on the alkaline side of neutral is that which gives the shortest pot life. 

The hardening of the adhesives happens either through a chemical reaction or through simply physically binding to the surface. The chemical reaction in case of a one-part adhesive depends on the chemical nature of the adhesive (Habenicht 1990). So do polyurethane- or silicone-based adhesives cure in the presence of moisture, whereas cyan-acrylic-based adhesives are inhibited by the presence of oxygen and cure under nonaerobic conditions. Other adhesives like epoxy one-part adhesives cure with heat. Two-part adhesives contain beside the resin part, a hardener part which consists of reactive molecules or polymers which react together with the resin part. Typical examples are epoxy- or acrylic-based adhesive formulations. 

In single-component adhesives, all the raw materials are added in one component In two-part adhesives, the formulation is separated into two components. One part, usually called part A, contains the organic resins and the other part, called part B, contains the hardener and accelerator. 

One-part adhesive formulations do not require a special mixing process prior to the application, but two or multiple-part adhesives need to be mixed before the application of the adhesive. 

Depending on the chemistry of the adhesive, the adhesion to different substrates (Dettner 1991) and the tolerance for surface contamination is different. Epoxy and acrylic-based adhesives bond well to a variety of different surfaces like metals, aluminium, or composites and tolerate some degree of surface contamination like oil if cured by heat. They do not adhere well to low-energy surfaces like plastic. Two-part epoxy formulations need a cleaning of the surface to remove any contamination. Special two-part acrylic-epoxy hybrid adhesives are able... 

One-part epoxy systems in the paste (liquid) form and in film form share much the same formulation ingredients. However, in comparison to the two-part adhesives, the one-part adhesives use very different curatives. Because these systems are meant to have all of the components of the adhesive in one part, the curatives used for two-part adhesives cannot be used as these would cure the adhesive in storage. Hence, latent curatives for these adhesives have been developed. Latenc/ in this case means that the curative is dormant in the formulation until a certain temperature is reached, at which point the curative begins to cure the formulation. Therefore, such latent catalysts have to have some means to stay inactive. For epoxy systems, this means that the curative remains insoluble in the epoxy resin imtil the cure temperature is reached or the curative is blocked by some chemical means which then unblocks at the... 

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