4 EXOTHERMIC REACTION DURING CURE

LIKE ALL THERMOSETTING RESINS, the cure of this epoxy formulation is exothermic. Also, the addition of large quantities of microballoons which are effective insulators tends to prevent rapid heat dissipation on cure. In earlier experiments it was noted that deep fills had a tendency to become dangerously hot. Tests were therefore carried out on blocks of filler to assess this problem. Two boxes 40mm3 and 80mm3 were constructed from 3/4″ (18mm) plywood to simulate voids in a wooden object. A mixture of the 1 to 1 ratio epoxy resin and microballoons was inserted and the temperature rise during cure measured with thermocouples. The cube with 40mm sides experienced a rise of 11�C (from 20�C to 31�C). The temperature in the centre of the 80mm cube rose from 20�C to 88�C in 28 minutes. These figures clearly indicate that a limit should be set on the thickness of fill material applied to an artifact. In practice it is wise to insert an underfill of an inert substance, such as polyethylene foam, so that the fill material occupies only the top portion. This is not only mechanically sound, but also economical. A maximum thickness of 20mm of epoxy/ microballoon filler would be adequate, even for large fills on such objects as totem poles, and at this thickness little or no heat build-up on cure would be expected.