Ambient cured products

Ambient-cured products used in the construction sector to cater for a variety of end-uses are as follows ... 

Successful attempts have been made in the last few years and now it has become practical to have ambient-curing products based on epoxypolyurethane hybrids. In this section the developments based on the work... 

Thermosetting polymers are those which set irreversibly either in the presence of heat or by chemical reaction with a curing agent or additive, usually at ambient temperature. Products based on such polymers are widely used in the construction sector, and it seems certain that their use will spread, particularly in areas where there are extensive construction activities, such as the Middle East and South East Asia. 

In polyaddition, cross-linking occurs by means of chain extension. The majority of ambient-cured construction products are based on this type of reaction, where in situ polymerisation occurs after the epoxy resin base and the curing agent are mixed. The curing agent causes the epoxide or hydroxyl groups to react (Ellis, 1994). 

Homopolymerisation. Homopolymerisation occurs in the presence of Lewis acids or tertiary aminies. It may be either cationic or anionic. This reaction does not have any significance for ambient-cured multi-component epoxy products for use in construction. 

An important part of the process is the curing of the finer with resin. Early forms of ambient curing required the liner to be left under pressure overnight however, there were risks with pressure loss and possible collapse as well as issues regarding the time factor and production rate. Hence, improved curing techniques have been developed. 

A further big advantage of acrylic polyurethane paints is that when the solvent has evaporated, the paint film is lacquer dry. In an ambient cure industrial paint finishing situation, the acrylic polyurethane appears to have cured before the crosslinking reaction is complete. A comparable polyester polyurethane would still be too soft to handle, thus extending the painting time. Rapid handling time on an industrial production line is important. 

The acrylic component can now contain reasonable amounts of styrene which reduces cost and the isocyanate component can be aromatic instead of the more expensive aliphatic variety. (Although IBM and Compaq have non - yellowing requirements that preclude the use of aromatic isocyanates.) Textured formulations with low gloss levels are popular decorative requirements in this industry, used to break up the hard lines of a computer box to make it more acceptable to the eye. Application is by conventional spray, often robotically applied in this high volume production industry. Above ambient or ambient cure and rapid handling time to permit sub assembly after the box is coated, means that acrylic is the only viable resin choice. 

The industrial value of furfuryl alcohol is a consequence of its low viscosity, high reactivity, and the outstanding chemical, mechanical, and thermal properties of its polymers, corrosion resistance, nonburning, low smoke emission, and exceUent char formation. The reactivity profile of furfuryl alcohol and resins is such that final curing can take place at ambient temperature with strong acids or at elevated temperature with latent acids. Major markets for furfuryl alcohol resins include the production of cores and molds for casting metals, corrosion-resistant fiber-reinforced plastics (FRPs), binders for refractories and corrosion-resistant cements and mortars. 

Sealants obtained by curing polysulfide liquid polymers with aryl bis(nitrile oxides) possess stmctural feature of thiohydroximic acid ester. These materials exhibit poor thermal stability when heated at 60°C they soften within days and liquefy in 3 weeks. Products obtained with excess nitrile oxide degrade faster than those produced with equimolar amounts of reagents. Spectroscopic studies demonstrate that, after an initial rapid addition between nitrile oxide and thiol, a second slower reaction occurs which consumes additional nitrile oxide. Thiohydroximic acid derivatives have been shown to react with nitrile oxides at ambient temperature to form 1,2,4-oxadiazole 4-oxides and alkyl thiol. In the case of a polysulfide sealant, the rupture of a C-S bond to form the thiol involves cleavage of the polymer backbone. Continuation of the process leads to degradation of the sealant. These observations have been supported by thermal analysis studies on the poly sulfide sealants and model polymers (511). 

Thermosetting composites are cured at either ambient or elevated temperatures to obtain a hard solid by cross-linking. The use of radiation cross-linking decreases the cure time considerably. In particular, an electron beam has been used successfully in many instances. Eor example, glass-fiber-reinforced composites cured by electron beam have been used for the production of cladding panels. ... 

All polyurethanes need the complete cure as specified to develop the properties fully. If the product is allowed to stand for a week at ambient temperature, the full properties will be developed. 

Coefficient of thermal expansion Cold cure The rate at which a material expands per degree of temperature. Casting process for the production of elastomers, in which the elastomer is mixed, dispensed, and cured at or near room (ambient)... 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

Condensation reactions are used for cross-linking at ambient temperatures. The acetoxy-functional condensation system is widely used in fluorosilicone sealants. The cross-linking occurs after exposure to atmospheric moisture.62 The limitation of this system is that it is effective for only relatively thin layers. Moreover, it often requires up to 14 days to cure and the acetic by-product may corrode certain substances. 

Furthermore, epoxy resin systems are capable of curing at either ambient or elevated temperatures, and they require only minimal pressure during the cure. Thus, epoxies can be applied and cured under many adverse conditions including outdoors. These properties provide great added value in many industries engaged in product assembly. Epoxy resins have been commercially available for almost a half-century. 

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