Causes of Radio Frequency Burns in Edge-Glued Joints

The occurrence of radio frequency burns in urea gluelines for edge gluing lumber was studied for relationship with glue conductivity, electrode voltage, RF frequency, moisture in the glueline, and wood characteristics. Results indicate that the formation of cure lines is an initiating cause of RF burns. The likelihood of the occurrence of cure lines is proportional to the conductivity of the wet glue and to the square of the electrode voltage. The tendency for cure lines to overheat and burn is proportional to the loss factor of the glue in the cured state, to the frequency, and to the electrode voltage squared. The tendency for the uncured glueline to contribute to the formation of cure lines, but not appreciably to the tendency of the gluelines to burn, is believed to be the reason a glue can have both a good conductivity and a low tendency to burn if the loss factor of the cured glueline is low. Development of a urea glue with low dielectric loss factor in the cured state and a high conductivity in the uncured state would produce a glue with less tendency to burn and improve edge-gluing efficiency.

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