Paper SE+PS-WeA3
Defining Atmospheric Pressure Air Plasma Treatment Parameters for Bonding Sheet Molding Compound

Wednesday, October 30, 2013, 2:40 pm, Room 203 C

Sheet molding compound (SMC) is used in the automotive industry for constructing body closure panels, typically on vehicles of low volume where tooling costs can be kept to a minimum. SMC is a reinforced composite material consisting of dispersed strands of chopped glass fibers in a polyester/polystyrene blended resin along with surface additives and inorganic fillers. The SMC body panels are typically constructed with an inner structure and an outer skin that are mated by adhesive bonding. Epoxy adhesives are used to join the panels. Although the resin chemistry should allow for a strong covalent bond with the epoxy adhesive, instead poor bonding is often noted in production. This is mostly attributed to the use of internal and external mold release agents. Physical sanding will help improve bonding, but the process creates substantial amounts of dust and debris and is prone to human error. Atmospheric pressure air plasma is therefore being evaluated as an alternative to prepare the surfaces for bonding. Plasma was shown to improve bond reliability at low to moderate levels of treatment. Interestingly, it also yielded impeccable bond durability at high to extremely high levels of treatment; conditions that would be deleterious to many plastics, especially polyolefins that can revert treatment at elevated temperatures. Measurements were conducted in an attempt to elucidate how this material interacts with the plasma to produce a surface exceedingly robust to bonding at an unexpectedly wide range of treatment conditions. Atomic force microscopy measurements gave insight into the amount of surface roughness created by the plasma treatments, while chemistry was determined by surface energy measurements and X-ray photoelectron spectroscopy (XPS). Dyne level testing was unable to accurately discern varying treatment levels since measurements reached 72 dynes/cm (the wetting point of water) even at lower treatment levels, while robust bonding occurred at still considerably higher dosage levels. Solvent wiping experiments along with XPS measurements revealed that, under the time constraints of the automotive manufacturing process, plasma treatments essentially achieve a condition of steady state that will remove material and yet still present a highly oxidized surface that is conducive to bonding. Thus, in practice, a condition of overtreatment is essentially unattainable.