1,721,515 research outputs found
Des adhésifs d’origine naturelle pour le collage du bois. Pourquoi formuler des colles d’origine naturelle ?
No full textDependence on the adhesive formulation of the upgrading of MUF particleboard adhesives and decrease of melamine content by buffer and additives
No full textThe type of MUF adhesive formulation has been shown to have a determinant effect in the performance of MUF adhesives for wood panels upgraded with iminoamino methylene basis of the "hexamine sulphate" type. The high performance sequential MUF formulations are upgraded little or not at all by these additives. Much weaker non-sequential MUF formulations instead are so strongly upgraded for IB boiled strength by the use of these additives to markedly outperform the normally stronger sequential type formulations and even phenolic resins. To the buffering action leading to optimization of the hardening ⇄ degradation equilibrium described previously as the main cause of the functioning of these additives must then be added the effect of the type of formulation. This is quantified and described. It is partly related to the presence of higher or lower levels of unreacted primary amine groups of the melamine and their consequent level of contribution to buffering. It is also partly related to the level of plasticization of the hardened glue line by unreacted urea and the consequent visco-elastic dissipation of energy that would derive from this
Upgrading of MUF polycondensation resins by buffering additives. II. Hexamine sulfate mechanisms and alternate buffers
No full textIminoamino methylene bases intermediates are obtained by the decomposition of hexamethylenetetra- mine (hexamine). These are stabilized by the presence of strong anions such as SO24 and HSO 4 , that is, “hexamine sulfate.” The effect of hexamine sulfate was closely linked to the strong buffering action it has on MUF resins. Its role is mainly to induce regularity of the reaction and the stability of conditions during resin networking, due to the buffer. Shifting of the polycondensation a degradation equilibrium to the left appeared to be the determinant factor. This was a consequence of maintaining a higher, constant pH during curing, due to the buffer action. The modulus of elasticity (MOE) increases the curves of hexamine sulfate-catalyzed MUF resins, confirming this trend. The resins are faster curing than when catalyzed by ammonium sulfate. The effect is valid within the narrow buffering range of pH’s used for resin hardening. Polycondensation is far too slow to occur at a much higher pH, and degradation is, instead, more predominant at much lower pH’s. The network formed is then more crosslinked and less tainted by degra- dation when curing occurs within the correct pH range. The result is a much better performance of the wood board after water attack. The effects induced by hexamine sulfate effects are of longer duration than those of other potential buffers. This is due to the hexamine sulfate heat stability under standard hot curing conditions of the resin. Alternate systems were found and shown to have a comparable effect
Structure effects of additives in performance improvement of aminoplastic adhesive resins
No full textThe structural effects of additives in performance improvement of aminoplastic adhesive resins were discussed. The comparative titration behavior of pure melamine-urea formaldehyde (MUF) resins and MUF resin with seven different additives were shown. The highest modulus of elasticity (MOE) values obtained overall by thermomechanical analysis (TMA) were reported
Colloidal aggregation of MUF polycondensation resins: formulation influence and storage stability
No full textColloidal particle formation followed by their clustering has been shown to be the normal way of ageing of aminoplastic resins, in particular 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 both of the formation of colloidal particles and of their clustering. Some clustering appears rather early in this process, even when the great majority of the resin does visually appear to be in colloidal state, being transparent. However, it eventually progresses to resins which are mostly in colloidal, clustered state, followed much later by a supercluster formation starting to involve the whole resin. There appears to be clear correspondence between molecular mass increases as obtained by gel permeation chromatography (GPC), low-angle laser light scattering (LALLS) analysis, and observation by polarizing optical microscopy. LALLS, however, appears to indicate the dimensions of the colloidal particles themselves when the level of colloidal aggregation is rather low, but it indicates the dimensions of the clusters once these are mostly aggregated. The smaller visible colloidal particles, already aggregates, were found by polarizing optical microscopy to be of a mostly elongated, rodlike shape, the length of which was shown to grow much further than their width with resin advancement and ageing. As their dimensions indicate, these are already clusters; this implies that the mainly linear increase of the polycondensate chains influences also the simpler colloidal clusters' growth direction, possibly explaining the resins' lack of tridimensional hardening while still in storage. It also explains why molecules such as free urea and acetals, by disrupting these colloidal aggregation mechanisms, allow both a much longer shelf life of the resin and its better performance in hardening. These findings explained the considerable difference in the behavior and performance of different MUF resin formulations. The ageing of the MUF resins of different preparation procedures appeared then to proceed from (1) clear resin (molecular colloidal aggregation) to (2) superclusters of a whitened, heavily thixotropic resin, which is the beginning of physical gelation to (3) liquid/cluster separation, which is the terminal stage of physical gelation
Low addition of melamine salts for improved Melamine-Urea-Formaldehyde adhesives water resistance
No full textThe addition of melamine acetate salts to an adhesive glue mix can allow the use of melamine–urea– formaldehyde (MUF) resins of lower melamine contents (rather than just urea–formaldehyde resins) and lower total amounts of melamine. Performances can be obtained that are characteristic of the top-of-the-line, generally higher mel- amine content MUF adhesive resins for the preparation of wood particleboard panels. Improvements in the panel internal-bond strength of greater than 30% can be obtained by the addition of melamine acetate salts to top-of-the-line MUF resins. The approach to the concept of increased melamine solubility with a melamine salt is compatible with the approach of increasing melamine solubility with solvents such as acetals (e.g., methylal). However, the synergy ad- vantage of using the two approaches jointly is not very marked
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