Hybrid silane-crosslinking polymers: a potential alternative to polyurethane wood adhesives? A preliminary investigation by TMA
In the last years, the research on wood adhesives has focused on matching technical performance with low health and environmental impacts. This study presents the first development of a new adhesive technology based on hybrid silane-crosslinking polymers that can combine both silicone’s and polyurethane’s adhesive properties. The advantage of hybrid silane-crosslinking polymers is the absence of potential harmful chemicals, such as isocyanates and solvents. Two hybrid formulations were characterized by thermo-mechanical analysis (TMA) and glued wood product elasticity is discussed in this brief original.
Thermal stability of glued wood joints measured by shear tests
The thermal stability of glued wood joints is an important criterion to determine the suitability of adhesives in the field of engineered wood. During their product life, glued wood joints can be exposed to high temperatures in various ways (direct exposure to the sun, fire, etc.). Thereby the cohesiveness of the adhesive must not degrade. This raises the question of how the strength of bonding changes under thermal load. The current investigation covers the influence of temperature (T=20 to 220°C) on the shear strength of glued wood joints. Different adhesive systems were investigated. With increasing temperature, the shear strength of solid wood and also of glued wood joints decreased. There were big differences in thermal stability and failure behaviour between the adhesive systems as well as within the polyurethane group. The thermal stability of one-component polyurethane systems can be greatly varied by modifying their chemical structure. Well adapted one-component polyurethane adhesives reach a strength similar to that of phenol resorcinol resin.ZusammenfassungDie Temperaturbeständigkeit von Klebstoffverbindungen ist ein wichtiges Kriterium um die Eignung eines Klebstoffes für den Holzleimbau zu beurteilen. Klebstoffe können hohen Temperaturen in vielfältiger Weise ausgesetzt sein (direkte Sonneneinstrahlung, Feuer, etc.), jedoch sollte sich die Festigkeit der Verbindungen dabei möglichst wenig verringern. Daraus resultiert die Frage, inwiefern sich verschiedene Klebstoffsysteme unter thermischer Belastung bezüglich ihrer Festigkeit unterscheiden. In der vorliegenden Untersuchung wurden Klebstoffverbindungen im Temperaturbereich von 20 bis 220°C hinsichtlich ihrer Zugscherfestigkeit untersucht. Mit zunehmender Temperatur nahm die Festigkeit von Vollholz und auch von Holz-Klebstoffverbindungen ab. Dabei waren deutliche Unterschiede sowohl zwischen den Klebstoffsystemen als auch innerhalb der Polyurethane zu verzeichnen. Die Temperaturbeständigkeit von Polyurethanen kann durch Variation der chemischen Komponenten an den jeweiligen Temperaturbereich angepasst werden, sodass Festigkeiten wie bei Phenol-Resorcin-Formaldehyd-Klebstoff erreicht werden.
Crosslinked and entanglement networks in thermomechanical analysis of polycondensation resins
Thermomechanical analysis (TMA) tests on joints bonded with synthetic phenol–formaldehyde (PF) resins have shown that, frequently, the joint increase in modulus does not proceed in a single step but in 2 steps, yielding an increase of the modulus first-derivate curve presenting 2 peaks rather than a single peak. This behavior has been found to be due to the initial growth of the polycondensation polymer, leading first to linear polymers of critical length for the formation of entanglement networks. Two modulus steps and 2 first-derivate peaks then occur, with the first due to the formation of linear polymers entanglement networks, and the second due to covalent crosslinked networks. The faster the reaction of phenolic monomers with formaldehyde, or the higher the reactivity of a PF resin, the earlier and at lower temperature the entanglement network occurs; more important is its modulus value in relation to the final, crosslinked resin modulus. The accepted methods of calculating the gel point and gel temperature of a polycondensation resin or from the single peak of the first derivate of the modulus increase curve or from the start of the uprise of the modulus increase curve is still acceptable in resins in which the entanglement effect is small or it is not present. In resin systems in which the entanglement effect, instead, is of importance, the question of what is the gel point in such systems had to be addressed, and gel temperature and gel point must be obtained from the modulus and its first-derivate curve in a different manner, which is presented. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1111–1119, 1998