Cause and effect of void formation during vacuum bag curing of epoxy film adhesives

Void formation during vacuum bag curing of epoxy film adhesives arises from extraneous volatile products. Potential sources of volatiles were found to include organic solvents and/or water remaining in the adhesive from manufacture, water picked up by the adhesive prior to curing, and water retained in various forms on the prepared adherend surface. Using a novel FTIR spectroscopy technique, water and methyl ethyl ketone (MEK) were identified as the principal volatiles formed during curing, and quantitative estimations showed batch-to-batch variations and the tendency of uncured adhesive to rapidly absorb atmospheric water. An estimation of void content was obtained from video images of fractured, adhesively bonded joints using a computerized pixel-counting technique. The conclusions drawn from tensile shear, peel, and wedge durability tests suggest that a void content in excess of 25% lowers T-peel and honeycomb peel strengths and may affect bond durability. Void content can be minimized using an in situ heat/vacuum outgassing treatment.