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Titolo:
Storage stability of melamine-formaldehyde resin solutions, 1 - The mechanism of instability
Autore:
Jahromi, S;
Indirizzi:
DSM Res BV, NL-6160 MD Geleen, Netherlands DSM Res BV Geleen NetherlandsNL-6160 MD NL-6160 MD Geleen, Netherlands
Titolo Testata:
MACROMOLECULAR CHEMISTRY AND PHYSICS
fascicolo: 10, volume: 200, anno: 1999,
pagine: 2230 - 2239
SICI:
1022-1352(199910)200:10<2230:SSOMRS>2.0.ZU;2-J
Fonte:
ISI
Lingua:
ENG
Soggetto:
FORMING POLYMER-SOLUTIONS; GEL-POINT; VISCOELASTIC PROPERTIES; THERMOREVERSIBLE GELATION; THERMODYNAMIC THEORY; CROSS-LINKING; SPECTROSCOPY; DEPENDENCE; BEHAVIOR; SYSTEM;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
45
Recensione:
Indirizzi per estratti:
Indirizzo: Jahromi, S DSM Res BV, POB 18, NL-6160 MD Geleen, Netherlands DSM Res BV POB 18 Geleen Netherlands NL-6160 MD n, Netherlands
Citazione:
S. Jahromi, "Storage stability of melamine-formaldehyde resin solutions, 1 - The mechanism of instability", MACRO CH P, 200(10), 1999, pp. 2230-2239

Abstract

The instability mechanism of aqueous melamine-formaldehyde (MF) resin solutions is elucidated. In general, the plot of the storage stability, which is defined as the time from production of the resin till turbidity is observed, against the reaction rime consists of an ascending and a descending branch. Investigations on fresh and aged (unstable) MF resin solutions, by various analytical techniques Like HPLC and C-13 NMR, revealed that the storage stability is governed by a combination of three "different" physical processes. In aqueous MF solutions with a relatively low degree of condensation(at the start of the reaction) the precipitation of monomeric species controls the instability mechanism. As the reaction proceeds (as the degree of condensation increases) the apparent solubility of these monomers increasesand their absolute concentration decreases. As a consequence, storage stability develops. The dissolved monomeric and oligomeric species, i.e., water-soluble MF resins, can undergo cooperative secondary intermolecular interactions (hydrogen bonding) as a result of which supramolecular aggregates are formed. This process will eventually lead to the formation of a physical gel. At still, longer reaction times the solubility of higher molecular weight species decreases and, as a result, the classical process of liquid-liquid phase separation sets in. The proof for the existence of a gel was provided by frequency-dependent rheological experiments on the unstable MF samples. NMR investigation on fresh and aged MF resins indicated that the chemical structure does not change noticeably during storage at room temperature. These studies indicated further that methylene bridges have a greater tendency towards precipitation than ether bridges.

ASDD Area Sistemi Dipartimentali e Documentali, Università di Bologna, Catalogo delle riviste ed altri periodici
Documento generato il 10/07/20 alle ore 09:15:27