What Contact Angle Should Process Engineers Target for Composite Bonding?

Key Takeaways

This is the final post in our three-part series based on the webinar Next-Generation Aerospace Surface Preparation with Plasmatreat:

1. What surface prep methods are available for aerospace composite bonding? (Part 1)
2. How do you verify that your prep worked? (Part 2)
3. What contact angle should you target for good bonding? (This Post)

The short answer 

It depends on your materials, so the right number comes from testing. But in the bonded composite data Lucas Dillingham, Brighton Science's Global Applications Director, shared in the webinar, a clear pattern showed up: 

Lucas calls these good rules of thumb, not a universal spec. The rest of this post shows where they come from and how to find the right number for your own process.

Why contact angle tracks bond strength 

Contact angle and adhesion respond to the same thing: the chemistry of the top few molecular layers of a surface. That chemistry is set by: 

• The type of surface treatment

• How much treatment the surface got

• Whether contaminants are present, and what kind 

Change any of those and both the contact angle and the bond strength move. That's why researchers have been able to line the two up across many materials, adhesives, and prep methods. Three examples from composites:

Example 1: Silicone contamination on peel-plied composite 

Under the DARPA TRUST program, Southwest Research Institute and Lockheed Martin Skunk Works contaminated peel-plied carbon fiber/epoxy surfaces with varying amounts of silicone, measured contact angle, then bonded and tested them.

• Below about 50°, fracture toughness stayed high and almost none of the joints failed at the interface.
• Right around 50°, toughness dropped sharply and interfacial failure jumped.
• By 70° and above, joints failed entirely at the interface.

Mode I fracture toughness (blue) and percent interfacial failure (purple) vs. contact angle for a unidirectional tape/epoxy composite. Data generated under the DARPA Phase II TRUST program (2015) by Southwest Research Institute and Lockheed Martin Skunk Works. Read the paper.

This is the curve behind the rules of thumb above.

Example 2: Different peel plies, different bonds 

Researchers in France compared six peel plies on carbon/epoxy and glass/epoxy laminates. The plies left surfaces between about 43° and 76°. The three plies with the lowest contact angles (43° to 44°) produced the strongest lap shear joints on both laminates. 

Lap shear strength vs. contact angle for laminates prepared with six peel plies. Data from Q. Bénard, M. Fois, M. Grisel, Composites Part A 36 (2005) 1562–1568.

It's a good reminder from Part 1: peel ply works, but not all peel plies leave the same surface.

Example 3: Where the failure mode changes

In a study of epoxy/composite adhesive joints, surfaces below about 27° failed cohesively, inside the adhesive. Above that, joints failed at the interface, and toughness fell steadily as contact angle rose.

Fracture toughness vs. contact angle for epoxy/composite adhesive joints. Data from R.G. Dillingham and B.R. Oakley, J. Adhesion 82 (4) (2006) 407–426.

Notice the switch point here (about 27°) is lower than in the DARPA data (about 50°). Same trend, different system. That's exactly why your limits need to come from your own materials.

How to find your own number 

Build a curve for your process:

1. Prepare surfaces across a range of contact angles, for example by varying sanding, plasma settings, or cleaning.
2. Bond them with your adhesive and cure.
3. Break them and record strength and failure mode.
4. Find where performance starts to drop, and set your limits inside the range that performs well.

Here's what that looks like for a sanding process on a composite. A solvent wipe alone left the surface around 75° and the joints held about 500 lbs. Sanded surfaces measured between about 30° and 37° and held roughly 1,800 to 2,350 lbs.

Single lap shear load at failure vs. water contact angle for a composite surface prep process. Data: Brighton Science.

With a curve like this, you can write a number into your spec, and contact angle becomes the control limit when the process moves from the lab to the production floor.