Peel Strength

From Cozy Newsletter #55.

The peel strength of a cured fiberglass matrix is the amount of force required to delaminate a layer of fiberglass from the layer below or the substrate below. It is the measure of the internal strength of the cured epoxy or its adhesion or mechanical bond to the substrate. It is a very important property, because the peel strength of the epoxy is literally what is holding your airplane together.

It should be obvious that when you are making a layup over a dry, previously cured surface, the peel strength will be either the strength of the mechanical bond, or else the internal strength of the epoxy, which ever is weaker. It is important, therefore, in making a wet layup over a dry surface to properly prepare the dry surface so the mechanical bond to the surface will not be the weakest link. There are two methods of preparing a fiberglass surface for the maximum strength mechanical bond. The first is to sand a previously cured fiberglass surface dull with 36 grit sandpaper. The disadvantage of this method is that if the surface is not absolutely flat, it is nearly impossible to remove all of the shine without sanding through some of the glass filaments, weakening the substrate . The second method is to squeegee peel ply over a fiberglass substrate before it cures, and strip it off after cure. The advantage of using peel ply is that the shiny surface (which might also be waxy) is stripped off with the peel ply, and the surface remaining is flat but fractured and rough. Sanding this surface is a wise, extra precaution to maximize the mechanical bond.

It is normally assumed that maximum peel strength is obtained by making a wet-on-wet layup, because then there is no mechanical bond involved. There may be an exception, or at least a reservation to this rule. We recently received a call from a builder in the state of Washington who said that he was using Epolite 2427 resin, and he noticed after glassing the inside of his fuselage sides, that the peel strength of the second layer was poor, and he could peel off the second layer rather easily. After discussing his technique with him, we think we know why this happened. Very often, first-time builders are too meticulous, and take a long time to squeegee each layer of glass to remove all of the excess epoxy before laying down the next layer of cloth. The fuselage sides have a lot of area so this could take quite a long time. If, as has been alleged, 2427 is more susceptible to contamination by humidity (and/or carbon dioxide) in the atmosphere, and it is exposed to the atmosphere for a long period of time before the next layer of glass is applied, there could be a contaminated surface between the two layers of glass which would reduce the internal strength of the epoxy at the worst possible location -- between the glass layers.

Experienced builders have learned that it is much faster, and results in better layups, to wet out the first layer of glass, squeegee the air out, but leave an excess of epoxy and then lay down the next layer of cloth on top to soak up the excess. This wets out the second layer faster, because the excess epoxy from underneath pushes the air out ahead of it. This saves much time, results in less air in the layup, requires less squeegeeing, and, if the epoxy is susceptible to contamination from the atmosphere, there is much less exposure to the atmosphere, and any contaminated surface epoxy does not end up being between two layers of glass. The same considerations apply also and argue for the use of peel ply over the top layer.

We have tested the peel strength of Epolite 2427, and haven’t found it to be any different from the several other epoxy systems we have used. It is true that we have low humidities in Arizona most of the year (not during the monsoon season, however), but it is also true we follow the procedure for faster and better layups recommended above.

Last updated: 2021-08-02 18:00:57 -0700