The Boat LadyC: Bond Rating

Sorry, this is not a blog about bond ratings in the financial world.

This is much less exciting.

This is about a little experiment I did with epoxy secondary (mechanical) bonds.

In the world of epoxy there are epoxy glue bonds known as primary and secondary bonds. These glue bonds are often called chemical and mechanical bonds.

A primary bond between layers of epoxy is where the first layer of epoxy is not allowed to cure and the second layer of epoxy is slathered onto the first uncured layer. The two layers then bond to each other as they cure and the bond occurs on a molecular level. Thus the name "chemical bond".

A secondary bond between layers of epoxy is where the first layer of epoxy is allowed to cure before applying the second layer of epoxy. The second layer of epoxy is applied over the first (cured) layer. The bond between the two layers is not a chemical bond. It is mechanical adhesion, thus the name "mechanical bond".

In theory, a primary bond is stronger than a secondary bond. I'll agree to that simply because I expect the chemical bond to be stronger than the mechanical adhesion. Since primary bonds are theoretically stronger than secondary bonds, many epoxy/wood boat builders go to great lengths to achieve primary bonds and avoid secondary bonds. Unfortunately, these poor souls are unknowingly wasting their efforts. And, building an entire boat with only primary bonds is not practical and may well be impossible.

There is no reason for any bond to be stronger than the wood. If the secondary bonds are stronger than the wood then they are sufficient. There is no need for the bond to be any stronger so primary bonds are unnecessary.

Enough talk.

I glued up some plywood with epoxy secondary bonds, let them cure and broke them.

You'll never believe it. In no case did the secondary bond fail. In all cases the plywood failed. The secondary bond is stronger than the plywood. There is no need for bonds stronger than secondary bonds. Here is the bond rating part of this blog....In wooden boat building with epoxy

primary bonds are overrated.

Here is what I did...

First, this little experiment used West System 105 epoxy resin and West System 207 non-blushing epoxy hardener.

Then, I took a piece of marine plywood. It was actually a pretty darn good piece of plywood. It was BS-1088 Hydrotek Meranti plywood. About two years ago I coated one side with epoxy, just epoxy, no fiberglass. The first coat of epoxy went on the bare plywood, which at the time was lightly sanded and vacuumed before coating with epoxy. I let that epoxy cure then applied another layer of epoxy with no preparation of the first layer of epoxy, creating a secondary bond between the epoxy layers. I built a boat out of the plywood. There were some scraps of the epoxy-coated plywood left over and I tossed them in the scrap bin.

Fast forward two years...

I took one of the epoxy-coated plywood scraps out of the scrap bin. After two years the epoxy was probably as cured as it was gonna be.

I cut some little pieces of select pine and glued them to the epoxy-coated plywood using structural epoxy (epoxy, silica powder, glass fibers and wood flour). Since the epoxy coating on the plywood is two years old, the bond between the pine pieces and the plywood is a secondary bond.

I did nothing to prepare the old epoxy surface of the plywood other than vacuuming. I did not even bother to sand the epoxy surface of the plywood. I brushed a light layer of new uncured neat epoxy on the two-year-old epoxy surface of the plywood then glued the pine pieces to the plywood. I used temporary clamping screws to secure the pine pieces until the structural epoxy cured.

Two-year-old epoxy-coated plywood, clear pine pieces and temporary clamping screws

Gluing the pine pieces to the plywood. Structural epoxy was applied to the ends
of the pine. The screws were tightened and fillets were formed with the excess
structural epoxy the squished out of the joint.

Gluing complete with fillets of structural epoxy

After the structural epoxy cured for about 72 hours, I removed the temporary clamping screws.

Then, I used my high-tech fish scale to fail the joints. I broke the pine pieces off of the plywood. The fish scale recorded the pounds force required to fail the joints.

I intended to pull the pine pieces off vertically but I could not break the joints. I was not strong enough to fail the joints by simply pulling vertically. I resorted to breaking the joints by pulling the pine pieces off horizontally.

Attempting to fail the bonds by pulling vertically.

Failing the bonds by pulling horizontally.

Each of the joints was numbered 1 though 8. Joints 1 though 4 were failed by pulling with the direction of the surface grain of the plywood. Joints 5 though 8 were failed by pulling across the direction of the surface grain of the plywood. I had to start with joint number 8 and work my way down to joint number 1 due to the direction of pulling.

In all 8 joints, the epoxy secondary bond did not fail. In all 8 joints, the plywood failed.

Joint #8 plywood failure at 29 pounds of pull force

Joint #7 plywood failure at 30 pounds of pull force

Joints #5 and 6 plywood failure at 40 and 35 pounds of pull force

The along-the-grain failures (#1 through 4) required more pull force, reaching the maximum of the fish scale (50 pounds) but, still, the secondary bonds never failed. The plywood failed instead.