Proposed mechanism for the curing of epoxy resins with amine‐lewis acid complexes or salts

Abstract

SynopsisThe common assumption in the literature is that amine complexes of Lewis acids (particularly boron trifluoride) cure epoxy resins by virtue of their dissociation to liberate the Lewis acid which then functions as the "true" catalyst. Evidence is presented to indicate that this picture of aminelewis acid complex curing may be incorrect. Another mechanism is proposed which is consistent with ( a ) lack of correlation between adduct stability and reactivity, ( b ) a similarity in reactivity of BF, adducts and the corresponding fluoroborate salts, ( e ) the relationship between reactivity of BF, and PFh adducts, ( d ) a correlation between reactivity and structure of the amine in the complex, and (e) the effect of the anion on reactivity of amine salts. The relative reactivity of the catalysts has been equated with the temperature at which gelation occurs on heating at a fairly constant rate.

Control of reactions and network structures of epoxy thermosets

“…Harris and Temin [260] refuted the assumption of thermal dissociation of the complex. Indeed, they observed that BF 3 -amine complexes are thermally stable up to their sublimation temperature.…”

“…Due to their higher stability, BF 3 -amine complexes are usually preferred. Several authors demonstrated that the reactivity of such complexes increases with decreasing basicity of the amine [63,260]. For instance, Pascault and coworkers observed that BF 3 -aromatic amine complexes do not show latency (aniline, pKa=4-5).…”

Processing science: an approach for prepreg composite systems

“…The approach utilized in the above calculations is approximate, but it does account for the effect of the moving bubble-resin boundary on the concentration profile and diffusion. 4. If the prepreg is equilibrated with moisture at a relative humidity, (RH)0, then in order to prevent the potential for void growth by diffusion at all times when the temperature during the curing cycle is T(t), the pressure at all points of the prepreg P(t) must satisfy the following inequality: where (RH)0% -the relative humidity to which the prepreg was exposed.…”

The use of Tetraalkylphosphonium-Tetrafluoroborate-Tetrafluoroboric Acid in the curing of a liquid crystalline epoxy resin

“…6 They found similarity in the action of amine-boron triCuring of epoxy resin is by either step-growth refluoride complexes and the corresponding amine actions with curing agents such as amines, tetrafluoroborate salts in epoxy curing. This impolyphenols, or by chain growth reactions with plied that the predominant catalytic species are Lewis acids and bases.…”

“…4,5 They concluded that the by the BF 3 complexes are tetrafluoroborate salts. primary step in curing is the reaction between the The fact that the amine boron trifluoride complex anion and the epoxy group, while the oxonium ion and the corresponding fluoroborate salts have might convert slowly into a carbonium ion, and similar reactivity 6 suggests that tetrafluoroborate the crosslinking reaction is initiated by attack of ion is another source for catalyzing the curing rethe carbonium ion on the epoxy ring. In other action.…”