Radio Frequency Burning in Glue Joints

Radio frequency burning in edge-glued joints has been attributed to the pH of the glue or to the formation of superheated steam in the glue. This study indicates that the initiating factor is the development of high dielectric stress, due to discontinuities or distortions in the otherwise uniform glue film which in turn are related to variation in the surface characteristics of the wood due to anatomical features or machining. Radio frequency burning is characterized by a carbonized line along the grain of the wood at right angles to the electric field, in contrast to arcing–dielectric breakdown–which is characterized by a branching carbonized line parallel to the electric field. Since the carbonized areas do not adhere, the strength of the glue joint is affected. A glue line may be considered a pure resistance, the value of which is dependent on the conductivity of the glue and the dimensions of the glue line. Voltage drops across the line will be uniform if the mix and spread are homogeneous and the edge joints are parallel. However, if any discontinuity occurs, an area of high resistance is established. A division of the impressed voltage will occur in direct proportion to the resistances in series in the glue line; thus, an exceedingly high voltage gradient will be established which will induce burning. Burning occurs along the grain line because irregularities on wood surfaces are usually more or less parallel to the grain and perpendicular to the electric field, enabling a maximum breakdown gradient to be established. The factors investigated as possible causes of burning included process variables–voltage, glue spread, edge pressure, etc.–and features of the wood–anatomical and machining features and the presence of extractives. An experiment with red oak was set up with these variables controlled. A second experiment was set up to determine the effects of surface irregularities, including smooth or ridged machined surfaces and the presence of dense fibers. The study concluded that the factors which may affect the frequency of burning are the voltage gradient, non-uniformity of the wood, and the conductivity of the glue. Factors which may affect the intensity of the burning are the glue spread, the amount of filler, the edge pressure, and the duration of heating.

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