Colloidal aggregation of aminoplastic polycondensation resins: Urea–formaldehyde versus melamine–formaldehyde and melamine–urea–formaldehyde resins

Abstract

Colloidal particles formation followed by their clustering have been shown to be the normal way of ageing of aminoplastic resins, namely urea-formaldehyde (UF) resins, melamine-formaldehyde (MF) resins, and melamine-urea-formaldehyde (MUF) resins. Ageing or further advancement of the resin by other means such as longer condensation times causes whitening of the resin. This is a macroscopic indication of both the formation of colloidal particles and of their clustering. It eventually progresses to resins, which are mostly in colloidal, clustered state, followed much later on by a supercluster formation starting to involve the whole resin. The initial, filament-like colloidal aggregates formed by UF resins have different appearance than the globular ones formed by MF resins. MUF resins present a short rod-like appearance hybrid between the two. GPC has been shown to detect the existence of colloidal superaggregates in a UF resin, while smaller aggregates might not be detected at all. The star-like structures visible in the colloidal globules of MF resins are likely to be light interference patterns of the early colloidal structures in the resins. These star-like interference patterns become more complex with resin ageing or advancement due to the advancement of the resin to more complex aggregates, to eventually reach the stage in which filament-like and rod-like structures start to appear. The next step is formation of globular masses that are representative of the true start of physical gelation.

Influence of Hydrolytic Degradation on the Morphology of Cured Urea-Formaldehyde Resins of Different Formaldehyde/Urea Mole Ratios

“…After the acid etching, the spherical structures were dominant on the surface. As mentioned earlier, the spherical structures usually occur in UF resins with F/U mole ratios lower than 1.2 (Stuligross and Kousky, 1985;Despres and Pizzi, 2006). In fact, it was reported that filament-like colloidal aggregates were initially formed in UF resin, and then eventually changed to super-clusters by coalescence during the aging process (Celzard et al, 2008).…”

“…3(e)). The spherical structures of UF resins have been reported for F/U mole ratio lower than 1.2 (Despres and Pizzi, 2006;Stuligross and Kousky, 1985). In other words, lower F/U mole ratio UF resins have colloid particles, which are coalesced into clusters during the aging process.…”

“…A recent study exhibited that filament-like colloidal aggregates were initially formed in UF resin, which eventually changed to super-clusters by coalescence during its ageing (Celzard et al, 2008). Moreover, the colloidal character, or spherical structure was known to delay the gel point of amino resins significantly (Despres and Pizzi, 2006).…”

Variation of MUF and PMUF resins mass fractions during preparation

“…In other formaldehyde resins, these peaks have been shown to be due to amorphous, labile supramolecular colloidal interactions of the higher oligomers of the resin. 4,19,20 The second preparation phase of the PMUF resin is similar to the preparation procedure of sequential MUF resins such as MUF1. In Figure 10(a,b) is shown the molecular mass distribution for the PMUF resin at the beginning and at the end of the aminoplastic preparation.…”

Microstructure of Cured Urea-Formaldehyde Resins Modified by Rubber Latex Emulsion after Hydrolytic Degradation

“…Indeed, the existence of the heterogeneity of thermosetting resin as nodular structures with particle sizes ranging from tens to hundreds nanometers on free and etched fracture surfaces of cured resin has been reported (Spurr et al 1957;Racich and Koutsky 1976;Gupta et al 1985;Dušek 1996). In the case of cured UF resins, several authors concluded that the presence of nodular structures or spherical particles was a result of colloid formation (Pratt et al 1985;Stuligross and Koutsky 1985;Depres and Pizzi 2006;Ferra et al 2010). Moreover, John and Dunker (1986) and Motter (1990) stated that the UF colloidal dispersion has been shown to be partly crystalline.…”