Rheometry of aging of colloidal melamine–urea–formaldehyde polycondensates

Abstract

Aged and whitened melamine-urea-formaldehyde (MUF) resins in a colloidal state were tested with parallel-plate rheometry to determine the extent of their viscoelastic behavior. Only in advanced colloidal states, and so only when aggregated colloidal clusters occurred, did the resins present clear indications of viscoelastic responses, as illustrated by the crossover of elastic modulus and viscous modulus curves at lower strain percentages. These colloidal clusters were labile microstructures, which, broken by applied shear, justified the known thixotropic behavior of these resins sufficiently advanced by aging or other means. MUF resins already in the colloidal state, but for which colloidal clustering had not yet occurred, behaved exclusively as viscous liquids. Two different cases of physical gelation were observed, reversible physical gelation and irreversible physical gelation, underlying which a true gel situation possibly occurred. Physical gelation due to colloidal superstructures occurred in both, but the difference in the resin average molecular masses revealed if the physical gelation was reversible or irreversible and, therefore, if the liquid/cluster separation was defined as the terminal phase of physical gelation.

Variation of MUF and PMUF resins mass fractions during preparation

“…This is confirmed both by the maintainance of its clear, unclouded appearance for a long time, characteristic of MUF2 type formulations. 4,19 It is also confirmed by the results of TMA experiments shown in Figure 7. Figure 5 shows that although MOE of the bonded joint improves when passing from the UM stages to the UMF stages after last urea has been added, the improvement is not as marked as between the different stages in MUF1 formulations.…”

“…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.…”

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

“…6 -11 For example, ageing of MUF resins was shown to proceed from clear resin (molecular colloidal aggregation) to superclusters of a whitened thixotropic resin, which is the beginning of physical gelation to, finally, liquid/clusters separation, which is the terminal stage of physical gelation. 10,11 Colloidal particles formation followed by their clustering have been shown to be the normal way of ageing of MUF resins, 10 and optical microscopy and other evidence for it has also been obtained. 10,11 Ageing, or further advancement of the resin by any other means, such as longer condensation times, causes whitening of the resin.…”

“…10,11 Colloidal particles formation followed by their clustering have been shown to be the normal way of ageing of MUF resins, 10 and optical microscopy and other evidence for it has also been obtained. 10,11 Ageing, or further advancement of the resin by any 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.…”

“…[1][2][3][4][5][6][7][8][9][10][11] Performance improvements and shelf life of these resins appear to be closely linked to their colloidal state or to their disruption. [3][4][5][6][7][8][9][10][11] Their colloidal state, however, up to now has been studied in detail only for MF and MUF resins. 6 -11 For example, ageing of MUF resins was shown to proceed from clear resin (molecular colloidal aggregation) to superclusters of a whitened thixotropic resin, which is the beginning of physical gelation to, finally, liquid/clusters separation, which is the terminal stage of physical gelation.…”