Epoxy Adhesion

“Surface preparation is the most critical factor in determining a bond’s performance.”

Frank Doerner, Vice President, Materials, Processes, and Structures Technologies, Boeing

What is the point of a super strong composite, if when it is bonded into a structure the joint is weak?

Improving Adhesion of Composites

Weak bond = Adhesive failure

Strong bond = Cohesive failure

Surface Preparation with Atmospheric Plasmas

  • Cleans and activates surfaces in seconds
  • Easily scans over flat and curved surfaces maintaining proper distance
  • Completely safe – low temperature and low voltage
  • Treats plastics, composites, metals, and glass, achieving excellent adhesion

Atomflo500

  • Argon capable
  • Multiple gas chemistries
  • Graphical user interface with state-of-the-art I/O
  • Continuous monitoring of plasma parameters (forward power, reflected power, phase)
  • Operates all plasma sources
  • Real-time data logging
  • Unlimited process recipes

Sanding vs Plasma Surface Preparation

Sanding Example for Epoxy Adhesion

Damaged resin surface

Plasma adhesion bond strength

Sanding is bad for composites – widely varying results. Excerpt from paper by Rafael Zaldivar and coworkers, Journal of Adhesion Science and Technology, vol. 26, pp. 1573-1590 (2012).

Current Method – Surface Preparation by Sanding
  • Operator dependent
  • Low reliability
  • Damages delicate parts
  • Large variation in strength
  • Poor reproducibility
  • Loose debris on surfaces
  • Potential for cross contamination
  • Lots of waste
  • Use plasma as a replacement for sanding

Water contact angle when bonding epoxy and stainless steel.

Excerpt from paper by Tom Williams and coworkers, Journal of Composite Materials, vol. 48, pp. 219-233 (2014).

Surface Preparation for Adhesive Bonding
  • Water contact angle (WCA) measures surface energy.
  • A high surface energy (low WCA) is better for bonding.
  • WCA is not a good indicator of readiness for bonding!

Bonding Epoxy Composites

Adhesion failure examples
Bond failure examples

Bond toughness

Excerpts from paper by Rafael Zaldivar and coworkers (Journal of Composite Materials, vol. 44, no. 2, pp. 137-156 (2010); ibid., no. 12, pp. 1435-1453 (2010))

Failure Modes For Epoxy Composites

Solvent: adhesive failure

Sanding / Abrasion: mixed mode due to damaged composite

Plasma: cohesive failure


Mechanism of Activating Epoxy Composites
  • Lap shear strength correlates with carboxylic acid groups on surface
  • Strength does not correlate with WCA or roughness

Epoxy composite adhesion strength

Choose the Better Bond

  • Surface preparation with Surfx’s atmospheric plasmas increase bond strength by 2 to 10 times
  • Prevent heat damage!
  • Different chemistries and recipes are required to activate different materials
  • Ultimate impact:
    1. Guesswork is taken out of the air frame assembly
    2. Structures are stronger and tougher, and do not fail during the aircraft’s lifespan.