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In this article, the effects of atmospheric plasma treatment on the microstructural, chemical, and mechanical behavior of epoxy-bonded polycyanurate composites are investigated. Adhesive bond strength of plasma-treated specimens exhibited strength increases of over 35% to that of peel-ply and solvent-wiped surface preparation techniques. The improvements were as much as 50% greater than those obtained using abrasive surface preparation techniques. X-ray photoelectron spectroscopy analysis showed an increase in the surface concentration of oxygen as a function of plasma treatment passes. However, the levels were substantially lower than that of epoxy composites treated under identical conditions. In addition, the concentration of carboxyl groups (OAC¼ ¼O), which have been associated with improved adhesive strength in epoxy-based composites, was shown to saturate in cyanate ester composites after a much lower exposure period than what was observed when treating epoxies. The effect of plasma surface treatment on the surface morphology of the cyanate ester composite was also studied using scanning electron microscopy and atomic force microscopy. Atomic force microscopy analysis showed a progressive increase in surface roughness with treatment; however, this increase only translated into a marginal increase in surface area and is not believed to contribute significantly to adhesive strength.

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A study was undertaken to address the effect of using different active gases during the atmospheric plasma treatment of composite specimens for adhesive bonding. The effect of using oxygen, carbon dioxide, or carbon monoxide on the surface chemistry, morphology, and mechanical properties of cyanate ester composites was investigated. CO treatment resulted in a surface profile that could be tailored to create an oxygen/carbon ratio as high as 0.71 with a negligible degree of polymer degradation as verified by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy. On the other hand, CO 2 and O 2 treatments resulted in a fairly high degree of chain scission and degradation using otherwise similar treatment conditions. However, significant bond strength improvement (>75%) over conventional ab-rasion surface preparation techniques was achieved for all three types of gases. XPS of CO-treated specimens showed a large increase in carbonyl species formation in comparison with the weakly bonded carbonates (ash) formed when treating the same composites with CO 2 and O 2 gas suggesting a different mechanism. These results present a method by which sensitive carbon-based, hydrophobic surfaces can be modified without damaging the underlying substrate as well as improving bond performance over conventional surface preparation methods.