Premium
Rheology of polyflavonoid tannin–formaldehyde reactions before and after gelling. II. Hardener influence and comparison of different tannins
Author(s) -
Garnier S.,
Pizzi A.,
Vorster O. C.,
Halasz L.
Publication year - 2002
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.10990
Subject(s) - formaldehyde , tannin , reactivity (psychology) , chemistry , gel point , rheology , relaxation (psychology) , solvent , viscoelasticity , polymer chemistry , chemical engineering , nuclear chemistry , materials science , organic chemistry , polymer , composite material , medicine , psychology , social psychology , alternative medicine , food science , pathology , engineering
Good correspondence of the gel‐time values obtained by two different methods, G ′ = G ″ and 1/η 0 →0, was observed for different types of natural and modified tannin extracts. The pH presents the predominant effect on both the activation energies and the gel times observed, while the proportion of a paraformaldehyde hardener has a much lesser effect on these parameters. The rate constants of the different phases of the reaction of polycondensation with formaldehyde, both before and after the gel point, were obtained for the six commercial tannin extracts tested. The viscoelastic properties of the different tannins/formaldehyde gels were measured. The gel stiffness S , relaxation coefficient n , and relaxation time λ were determined and their dependence on the proportion of the formaldehyde hardener, on the temperature, and on the type of tannin was determined. The gel stiffness S appears to be influenced considerably by the proportion of the formaldehyde hardener. Its value decreased as the percentage of the hardener increased: This was due to early network immobilization and the resulting lower level of crosslinking resulting from it. The influence on S of the temperature is not very pronounced. The relaxation coefficient n appears to depend mainly on the reactivity of the tannin used: The faster the reactivity, the higher was the value of n . This appears to be valid exclusively in tannin extracts where the colloidal state is still present, while it is not valid in extracts where the colloidal state was eliminated by, for example, solvent extraction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 864–871, 2002