Rapid Cure

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. 

In some resole appHcations, such as foam and foundry binders, a rapid cure of a Hquid resin is obtained at RT with strong acid. The reactions proceed in the same manner as those of novolak resin formation. Methylol groups react at ortho and para phenoHc hydrogen to give diphenyknethane units (41). 

Waferboard, a more recent wood constmction product, competes more with plywood than particle board. Waferboard and strand board are bonded with soHd, rather than Hquid, phenoHc resins. Both pulverized and spray-dried, rapid-curing resins have been successfully appHed. Wafers are dried, dusted with powdered resin and wax, and formed on a caul plate. A top caul plate is added and the wafers are bonded in a press at ca 180°C for 5—10 min. Physical properties such as flexural strength, modulus, and internal bond are similar to those of a plywood of equivalent thickness. 

In magnesium casting, sulfur dioxide is employed as an inert blanketing gas. Another foundry appHcation is as a rapid curing catalyst for furfuryl resins in cores. Surprisingly, in view of the many efforts to remove sulfur dioxide from flue gases, there are situations where sulfur dioxide is deHberately introduced. In power plants burning low sulfur coal and where particulate stack emissions are a problem, a controUed amount of sulfur dioxide injection improves particulate removal. 

These aqueous emulsions can also be used to size paper for improving its water repeUency and antiadhesion properties (529). An increased level of organic titanate can be used to ensure the rapid curing (<5 s) for fast-paced paper or fiber curing operations (519). 

Too high a concentration of formaldehyde will cause rapid curing of the surface of the section, thus reducing its permeability to formaldehyde, and as a result it is extremely difficult to cure adequately the centre of the casein. Too low a concentration will unnecessarily prolong the time of cure. 

Moisture-curable urethane systems (one-pack) can be considered as two-component systems which use atmospheric moisture as the second component. One-pack urethane coatings can be produced that are similar in physical properties to the two-pack systems for almost all applications. These highly complex systems can have a great deal of flexibility. Claimed advantages are a one-pack system, rapid cure, even at low temperatures, excellent chemical and abrasion resistance and good flexibility. Although these systems have been available for some time in other countries of Europe, they are only recently beginning to be of interest in the UK. 

These materials are reviewed elsewhere in this book except RTV. The RTV (room temperature vulcanization) silicone plastic is a very popular type. It solidifies by vulcanization or curing at room temperature by chemical reaction, made up of two-part components of silicones and other elas-tomers/rubbers. RTV are used to withstand temperatures as high as 290° C (550°F) and as low as —160° C (—250°F) without losing their strength. Their rapid curing makes them... 

Commonly used monomers for UV curing include acrylates (7), styrene/unsaturated polyesters (8,9), and thiol-ene compositions (10-12). Currently, acrylate-functional systems constitute a major share of the UV curable polymers market, mainly due to their rapid curing via free radical chain polymerization. 

For the radiation rapid cure experiments, appropriate resin mixtures containing oligomers, monomers, flow additives and sensitisers (UV) were applied to the substrate as a thin coating, the material placed on a conveyor belt and then exposed to the UV and EB sources. The time taken to observe cure for each of the samples was then measured on a relative basis. The UV system used was a Primarc Minicure unit with lamps of 200W per inch. Two EB facilities were utilised namely a 500KeV Nissin machine and a 175KeV ESI unit. 

The calculation requires measurements of the extent of reaction and of the distribution of functionality on the polymer and crosslinker. The functionality of the isocyanate crosslinker is formally three although it must be recognized that all of these materials have a distribution of functionality. Fully alkylated melamines have a functionality of 6. Coatings are formulated with an excess of alkoxy to hydroxy in order to achieve rapid cure (2). Thus, not all of the melamine alkoxy groups will react (ignoring side reactions) and the effective functionality will be less than six. 

On-aircraft repairs of composite using a rapid-cure resin system of composite component with UV light irradiation based on TRI patenP have been developed by the U.S. Air Force Research Laboratory. Alternating layers of the acrylate-based resin system and woven fiberglass (the widely used wet la)mp procedure) are applied to fill the hole and form a UV curable composition. The width of the patch can be up to 2 ft (0.6 m) and the depth as much as 0.2 in. (5 mm). The cure time using a 400 W UVA lamp is reported to be 20 min. Although it is essentially a depot repair, it can be done field when necessary to return an aircraft to service. Because of the necessity to cure relatively thick repair patches, Us-acylphosphine oxide was used as a photoinitiator. An example of the patented UV curable resin system used for the repairs is in Table 11.1. 

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