Physical gelation of melamine formaldehyde resin solutions. II. A combined light‐scattering and low‐resolution relaxation proton NMR study

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.

Section: Resultssupporting

confidence: 74%

Section: Introductionmentioning

confidence: 99%

Rheometry of aging of colloidal melamine–urea–formaldehyde polycondensates

Pizzi

George

Zanetti

et al. 2005

“…It has already been confirmed that the storage stability of aqueous MF resin solutions is limited at room temperature . There have been detailed discussions about the kinetics and modeling of the synthesis of traditional melamine–formaldehyde resins .…”

Section: Introductionmentioning

confidence: 97%

Stability and spinnability of modified melamine–formaldehyde resin solution for centrifugal spinning

Xiong

Hou

2017

Melamine microfibers were first prepared by centrifugal spinning. The stability and spinnability of a melamine-formaldehyde (MF) resin solution were improved as expected by adding various modifier combinations. Considering the storage stability of solutions characterized by visual inspection, turbidity tests, and viscosity measurements and combined with the fiber morphology, the optimal modifier combination was obtained. The spun fibers manifested a good morphology and thermal stability as measured by scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Moreover, microfibers prepared by three spinning methods (centrifugal spinning, electrospinning, and centrifugal electrostatic spinning) were compared to choose the suitable spinning method for different fields in the future. This work provides systematic and scientific guidance on the synthesis of MF resin solutions and rapid mass production of melamine microfibers and also demonstrates that centrifugal spinning of melamine microfiber is a promising candidate for flame retardance and CO 2 adsorption at elevated temperature. V C 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46072.

“…Recently, colloidal particles formation followed by their clustering have been shown to be the normal way of ageing of all aminoplatic resins,8–14 including urea‐formaldehyde (UF) resins 8, 14. Ageing (or further advancement of the resin by other means such as longer condensation times) causes whitening of the resin.…”

Section: Introductionmentioning

confidence: 99%

Microcrystallinity and colloidal peculiarities of UF/isocyanate hybrid resins

Wieland

Pizzi

Grigsby

et al. 2007

Combined mixtures of polymeric diphenylmethane diisocyanates (pMDI) with Urea-formaldehyderesins (UF) adhesives for wood panels are shown (a) by X-ray diffraction analysis (XRD) of the cured adhesive to present a certain percentage of microcrystallinity, this being due exclusively to the proportion of urea-formaldehyde resin present in the mix and, (b) by polarized light optical microscopy (PLOM) to present colloidal structures in which oligomers and colloidal structures of one resin have migrated within the colloidal structures of the other resin.