SBIR-STTR Award

Adhesive bonding of composite materials is a critical component of modern aircraft construction. Adhesive bonding of metals such as aluminum is well understood, and the processes currently used for manufacture of adhesively bonded metal aircraft components are robust and controllable. However, the factors responsible for obtaining strong and durable adhesive bonds to fiber reinforced composite materials are only poorly understood. Current practice usually involves abrasive blasting of the adherend surface. This is believed to improve adhesive bond strength through improved wetting of the substrate by the adhesive and by providing for mechanical interlocking of the adhesive and the substrate. While this process can result in strong and durable bonds, variations in bond performance can occur unexpectedly. Quality control techniques for ensuring reproducibility of the bond performance are needed. Our previous work has shown that surface energy of a prepared composite correlates well with the adhesive joint performance, and can potentially serve as the basis for a shop floor-level quality assurance tool. The goal of Phase I of the proposed research is to develop and evaluate at least two techniques for predicting wettability of a composite surface by a room-temperature paste curing adhesive

Surface energy is potentially the most critical predictor of performance for composite/composite adhesive bonds. It is straightforward to measure the surface energy of a planar composite using, for example, contact angle measurements obtained from a range of probe liquids. However, once a surface has been roughened on a microscopic scale by a process such as grit blasting, quantifying the surface energy becomes a much more difficult task. In Phase I it was demonstrated that the wetting behavior of a single carefully chosen probe liquid is an excellent predictor of subsequent adhesive bond performance, even on a highly roughened surface. It was further demonstrated that the probe liquid wetting behavior was well quantified by measuring the diameter of a small drop of known volume. This technique provides fundamental information about the relationship between contamination, surface energy and performance for adhesively bonded composites. Phase II will use this technique to evaluate a wide range of substrates, adhesives, and contaminants. A practical prototype measuring tool for evaluating surface energy using this technique will be constructed and evaluated in a manufacturing environment.

Keywords:
Adhesive Bonding, Surface Energy, Surface Tension, Surface Preparation