Pot life and exotherm

Optimum cures with AEP catalyzed DGEB A are obtained using 20 to 22 pph. However, the crosslink density is not great, and the cured product does not attain a high degree of thermal or chemical resistance. The pot life and exotherm are similar to those of DETA and TETA, but a postcure (2 h or more at 100°C) is required to develop properties fully. 

Certain types of fillers, even though unreactive, will affect the pot life and exotherm of the adhesive system. Generally, modifications of cure or reactivity are not the prime functions of... 

Pot life and exotherm Pot life can be increased and exotherm reduced, because fillers reduce the concentration of reactants and function as better heat conductors than the resin matrix. Commonly-used fillers for this are silica, calcium carbonate, alumina, lithium aluminum silicate, and powdered metals. 

When selecting an additive, it is important to take into account also the potential side-effects it may have on other properties. In most cases, the pot life and exotherm of the resin system will be reduced by addition of fillers, because they reduce the concentration of reactants and act as better heat conductors than the resin. In some cases, the cost of the system will be reduced (but at a penalty in other directions such as mechanical properties that can influence performance of the fabricated product). 

The greater the degree of cyanoethylation the higher the viscosity of the adduct, the larger the pot life and the lower the peak exotherm. The products are skin sensitive. 

Hexahydrophthalic anhydride (HHPA) is a low-melting-point (36°C) solid. It is liquefiable at temperatures of 50 to 60°C and can be mixed easily with hot epoxy resins. The mixed resins are characterized by low viscosity, long pot life, and low exotherm. Because of its low reactivity HHPA is generally used with an accelerator, usually BDMA or DMP-30. 

A routine measurement of diisocyanate content is usually desirable because, as most of the isocyanates are prepared by the phosgenation process, free phosgene may be present if it has not been removed in the final purification step. This will be included in the hydrolyzable chloride test, and partly included in the acidity test, if hydrolysis takes place. Also, if the diisocyanate has been stored under adverse conditions, some isocyanate could be converted by moisture to insoluble urea. Alternatively some dimerization may occur. In both cases isocyanate is consumed and the result is a decrease in active content. Additionally, diisocyanates do show some batch to batch variations in activity hence it is convenient to measure their reactivity against a control, a polyester (or polyether) of previously known activity, by making a small quantity of the polyurethane and measuring its final reaction exotherm temperature, rate at which its exotherm is achieved, pot life and time to achieve gelation. If a more... 

Pre-accelerated, low viscosity, efficient and rapid wet-out, thixotropic, LSE, orthophthalic resin offering a long pot-life and a low exotherm in thick sections, with good interlaminate adhesion and improved water resistance colour change on catalyst addition and cure. Approved by Lloyd s and Det Norske Veritas. 

Pre-accelerated, very low viscosity, thixotropic, orthophthalic, general purpose resin suitable for hand-lay and spray/projection lamination, with long pot-life and very rapid cure at low temperature offering rapid and efficient wet-out with low exotherm in thick sections, plus colour change indicating cure and approved by Lloyd s. 

The limitations on the material concerned, e.g. pot life, curing exotherm, viscosity and toxicity. 

Formulation Requirements. In order to penetrate the mass of fiber at one end of the bundle, the formulation must have sufficiently low viscosity to move easily through the bundle completely wetting all fiber surface area. Typically, formulations of viscosity less than 8000 poises have been successful. Too low viscosity or too rapid delivery of the formulation can result in the occlusion of air and the ultimate development of voids with loss of mechanical integrity. Our process demands that formulation be delivered and partially cured to an intermediate plateau termed green state. This requires a minimum pot life of 30 minutes after blending of resin and curative. The physical chemistry of the composite membrane requires that the initial exotherm not exceed approximately 150 C. 

Reactivity is reduced. Thus, pot life is increased, and the exothermic temperature is reduced. Cure may be required at elevated temperatures. 

Thermal conductivity also helps to improve heat transfer during cure. This reduces exothermic temperatures and extends pot life, particularly at high filler loadings. Shrinkage during cure is also reduced, as explained in the sections above. 

Many different test methods are used to measure the reactivity or cure rate of the epoxy adhesive. Some of these, such as working life or pot life, are very practical and are used to plan the production process. Others, such as exotherm, are used to determine reaction kinetics. Still others are used to characterize the epoxy network as it cures for the purposes of determining the degree of crosslinking and the rheological properties of the curing adhesive. 

Fillers offer a variety of benefits increased strength and stiffness, reduced cost, shrinkage reduction, exothermic heat reduction, thermal expansion coefficient reduction, improved heat resistance, slightly improved heat conductivity, improved surface appearance, reduced porosity, improved wet strength, reduced crazing, improved fabrication mobility, increased viscosity, improved abrasion resistance, and/or impact strength. Fillers also can have disadvantages. They may limit the method of fabrication, inhibit cure of certain resins, and shorten pot life of the resin. 

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