When composite components come unstuck the type of epoxy that was used sometimes gets held up as the bad guy. It may not always be the case. In fact in many cases its far more likely that the issue of secondary bonding is at the core of the problem.

Header image for article on secondary bonding

What is Secondary Bonding?

The term secondary bonding is generally used in composites to describe the process of joining a new uncured laminate to an existing cured laminate, such us taping a bulkhead the hull side or applying an additional layer of reinforcement to a cured laminate.

Technically speaking this is co-bonding. 

Secondary bonding refers to the process of joining two or more cured composite parts with an adhesive.

Since this discussion is primarily about the surface preparation for any kind of epoxy bonding with composites the distinction between co-bonding and secondary bonding is not so important.

It’s about Chemistry, not Mechanics

The term secondary bonding derives from chemistry. It refers to bonds created by the donation and acceptance of electrons between elements. When we abrade the surface of a cured laminate before applying a new laminate it’s tempting to think we are giving the new laminate something to grip onto - a mechanical bond.


In fact we are raising the surface energy to create  a chemically active surface that will be attracted to, and bond chemically to the adhesive. 


Composites have low free surface energy. That is; they don’t inherently exhibit good bonding characteristics when compared with metals and ceramics.

A mechanical bond can be helpful under some load cases but in general it is a chemical bond that is the objective of surface preparation.


What makes a good bond?

The quality of the bond you achieve is only as good the quality of the chemical reaction at the interface. That chemical reaction is a function of the amount of surface energy available to the part you are bonding, not just a function of the type of the epoxy you’re using.



“Surface roughness is a secondary outcome and not an important characteristic of a surface over surface cleanliness and surface energy. 


Surface chemical composition is more important than roughness. The potential damage that goes along with inducing roughness can be a real problem for composite bonds”.


Giles Dillingham of BTG Labs


Understanding Surface Energy

When you scratch the surface, grit blast it, tear off the peel ply, or treat it with chemicals you’re creating surface energy. The molecules on the surface of the part are suddenly exposed to a whole new range of molecules in the atmosphere and are chemically unstable. We say the part has surface energy and it is in the ideal state to create a good bond.

But this surface energy is delicate in nature. It can easily be disrupted by contaminants and it degrades over time as the molecules on the surface seek a new equilibrium with the molecules they are newly exposed to.

Careful how you breathe.

The bonding work that takes place at the interface takes place in a very thin layer of molecules, A layer about 6 molecules thick. If someone comes by and sweeps the floor before you’ve made the bond then you’ve probably lost a substantial portion of the surface energy. In fact simply breathing on the surface can leave a film of contaminants between 40 to 100 molecules thick, considerably thicker than the interface where the bond is to take place..

Why we scratch (and why we mustn’t scratch too much)

Abrading the surface prior to bonding is a common and effective method of creating surface energy. But its not about creating a rough surface for a mechanical bond. It’s about creating a surface that’s chemically reactive and ready to bond with the liquid molecules in the epoxy. 

In fact with high fibre ratio composites the fibres that are critical to the strength of the part are only covered by a very thin layer of resin. Excessive abrading will damage these fibres and reduce the mechanical properties of the part.

Image of scratching for article on secondary bonding

Measuring Surface Energy

How do you determine if a surface is active and ready for bonding? 

Measuring the contact angle of water droplets on the surface is a highly reliable and accurate means of determining the amount of surface energy available for bonding.

The shape the droplet takes on the surface is determined by the attraction forces between the droplet and the surface. Where there is weak attraction the droplet beads up to minimise interfacial area. Where there is strong attraction as you have with high surface energy then the droplet is flatter and spreads more widely.

Special instruments that accurately measure the contact angle of water droplets are readily available and are broadly deployed in aerospace and other composites manufacturing operations, But simply spraying some water droplets on the surface will give a good visual indication of the suitability of the surface for bonding.

Images of contact angles for article on secondary bonding

Time is of the essence.

When you purchase a composite panel or a part that has to be bonded into your boat, or simply prepared for painting, the supplier will usually tell you not to remove the peel ply until you're ready to bond the part. There's a good reason for that.


The surface energy won’t last forever. It degrades over time as the molecules interact with moisture and other contaminates in the atmosphere. 

If you prepare a surface late in the day and come back the next morning to make the bond a significant portion of the surface energy will have been lost.

When you peel off the peel ply you have a highly reactive surface with a contact angle of water around 23˚. An hour later the contact angle of the water droplet will b e closer 26˚-27. A day later it's about 30˚.

Plot of surface energy decay for article on secondary bonding

Consistency over strength

Achieving consistency in preparation of the surface, and consistency across parts is the greatest challenge to achieving reliability in bonding. Worker skills, knowledge and training are critical elements in being able to reliably and consistently produce parts that have properties critical to the reliability of the product. Repeatability is important. To quote Giles Dillingham again "Engineers have to know how much strength they can depend on".


We’ve only just scratched the surface of this topic. If you’re building composite parts that need to have reliable and consistant mechanical properties then its important that the people on the shop floor understand secondary bonding and are trained in the procedures that will ensure the parts that need to stick will be well stuck.

The information in this article came from a number of sources including a paper presented by Giles Dillingham Ph.D of BTG Labs in Cincinnati, Ohio.

The paper is titled "Understanding and Controlling the Bond Surface in Manufacturing for Reliable Adhesive Bonding of Composites"

Giles has carried out extensive research into secondary bonding and has developed instruments for measuring surface energy.

Giles can be contacted at

web address:


We opened the factory doors in the morning to find a series of laminating tables with parts made the day before that were still very sticky. Fact was the workers hadn’t known to mix hardener with the epoxy.

This was no small time operator. It was a major player in the world of yacht manufacture with hundreds of workers. Hopefully things like this don’t happen too often and when they do we would like to think the cause gets addressed in a proactive manner.

The point is not that someone goofed. Quite a few people who should have been better informed were involved in this particular case. 

The point is that even very elementary procedures can go wrong if the staff are not adequately trained.

Understanding the complexities of secondary bonding requires a lot more training than knowing you have to mix a catalyst with the resin


The quest to build products more cost effectively puts the pressure on manufacturers to find cheaper labour. That’s fine as long as measures are taken to ensure that the workers who supply that labour have the skills and the level of supervision that’s appropriate to ensure the product is built as intended.