Labs Develop New Bonding Experience

by Laura Johnson
posted April 04, 2013

Aerospace scientists have developed a new method for treating the surface of composite materials, allowing for more effective adhesive bonding.

Composites, which are combinations of at least two different materials, are strong, stiff, and have a low density. This makes them ideal for use in spacecraft and launch vehicles.

Composites are often joined together to create large structures. They can be joined using a fastener of some kind, but this adds undesirable weight. The preferred method is adhesive bonding, which, in layman’s terms, is basically gluing the two pieces together.

Anyone who has ever glued something knows that it is important for the surface to be clean prior to applying the glue. The surface of composites must also be prepared prior to bonding, but there are problems with current methods.

“Over the last 30 years the reinforcements used for composites continue to improve, which result in stiffer and stronger parts,” said Dr. Rafael Zaldivar, a senior scientist in the Materials Science Department. “Many of the surface preparation techniques that were developed 30 years ago (such as abrasion) have not evolved with the changes in these material systems. These new composite materials utilize fibers that are much stiffer, however, they tend to have weaker fiber-matrix interfaces which makes their composites less damage tolerant.”

The plasma wand used to treat the composite surfaces. (Photo by Eric Hamburg)

The plasma wand used to treat the composite surfaces. (Photo by Eric Hamburg)

Abrasion, or scraping the surface, is a common manual process to remove contamination and prepare the surface for bonding, but it has the potential to damage the material.

“Over the last several years, we have observed a large number of premature failures or lower than expected strengths in bonded composite joints used in our space structures,” Zaldivar said. “Our failure investigations showed that many were a result of the current surface preparation process that was employed. The process was actually damaging the composite resulting in low strengths and higher variability.”

Zaldivar, Dr. Hyun Kim, and Dr. James Nokes have developed a new method to prepare the surface using a handheld device called an atmospheric plasma system.

The scientists flow an active gas (oxygen, for example) into the atmospheric plasma system, where it is dissociated and ionized. The oxygen comes out in a highly reactive form and reacts with the contaminates on the surface of the material, thus cleaning the surface.

Dhruv Patel performs a lap shear test to determine the mechanical strength of bonded joints that have been prepared using atmospheric plasma treatment. (Photo by Eric Hamburg)

Dhruv Patel performs a lap shear test to determine the mechanical strength of bonded joints that have been prepared using atmospheric plasma treatment. (Photo by Eric Hamburg)

“Atmospheric plasma treatment allows us to tailor or chemically modify the outermost nanometers of the composite surface being treated, so there is very little potential for damage,” Zaldivar said.

Not only does this method clean the surface without damage, but it creates chemical groups on the surface of the material that actually enhance chemical bonding.

“Our studies have identified the specific chemical groups that need to be incorporated through these treatments to maximize strength gains, while not damaging the composites themselves,” Zaldivar said.

The scientists can flow different gases through the atmospheric plasma system to achieve different results. Different composites also vary in their reaction.

“What makes the treatment of composites especially interesting is that most composites have different resin chemistries … so performing the same atmospheric plasma treatment may result in significantly different groups being formed,” Zaldivar said. “Knowing how to modify your variables to generate the chemical groups you need is the real challenge.”

Dr. Rafael Zaldivar uses a contact angle analyzer to determine the surface free energy of plasma-treated composites. This technique determines the wettability, adhesion compatibility, and surface cleanliness of the composite as a function of treatment. (Photo by Eric Hamburg)

Dr. Rafael Zaldivar uses a contact angle analyzer to determine the surface free energy of plasma-treated composites. This technique determines the wettability, adhesion compatibility, and surface cleanliness of the composite as a function of treatment. (Photo by Eric Hamburg)

The scientists were up to the challenge, however, as shown by their results. Using their new surface preparation method, the team has achieved a 75 to 180 percent increase in bond strength, and a 100 percent increase in durability over conventional methods.

Their process, which has a patent pending, could benefit a variety of programs.

“The results of our work can be applied to any program that utilizes composites, be it space or commercial application,” Zaldivar said.