Determining Curing Properties of An Adhesive in Contact with Wood

Little is known about what occurs on a glueline during cure and how these events affect strength and durability. In these studies, the three stages of mechanical changes occurring during cure were measured with a free vibrational apparatus. When the adhesive is placed on the substrate in a liquid state, its dominant mechanical characteristics are due to the gross movement between molecular chains. In the “rubbery” state, chemical bonding and solvent loss restrict molecular movement to short range displacements of molecular segments. When the adhesive reaches the “glassy” state of cure, molecular movement is further restricted so that only short-range displacements of bonds within the molecule occur in response to stress. The times at which the gel transition–from a liquid to a rubbery state–and the glass transition–from a rubbery state to a glassy solid state–occur are measures of the rate of cure. The gel transition is the point at which there is an average of one crosslink per molecule; the glass transition is the point on the temperature scale at which the thermal expansion coefficient undergoes a discontinuity and the onset of brittleness and the hardening of amorphous polymers are observed. The test apparatus measured the changes in the free vibrational properties of the adhesive during cure. A thin veneer specimen coated on both sides with a film of adhesive was suspended from a support and vibrated manually by deflecting a weight attached to the bottom of the sample. Energy loss during deformation (damping constant) and elasticity (flexural modulus) were measured. In a liquid state, the resistance, of the adhesive to deformation was very low. As curing progressed, the adhesive contributed to the mechanical response of the specimen. In the rubbery state the damping constant rose, at the gel transition there was a discontinuity in both the damping and frequency curves, and at the glass transition the damping constant reached a maximum and there was an inflection in the frequency curve. In further studies exploring the relationship of the dynamically registered mechanical properties and actual bonding, cross-lap specimens were used, with a polyethylene tape shim placed in the glueline to create a pocket for the adhesive. In addition to these rate of strength development tests, ultimate strength and durability tests were performed to ascertain the relation between the quality of the complete cured joint and the curing properties observed. To test durability, a wet-dry cyclic test program to produce suitable degradation conditions was used. To illustrate some of the practical applications of this method, the curing conditions of a resorcinol formaldehyde resin–ambient moisture content, temperature, and molecular weight of the resin–were varied, and the changes in curing properties and the qualities of the resulting joints were observed.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member