SAMPE Journal - March/April 2022 - 11

INTRODUCTION
As composite materials have become more widely used, methods of joining
parts have become increasingly important. Joining with mechanical fasteners
requires machining processes that could cause delamination, as well as
create localized stress concentrations within the part. Adhesive bonding of
composites offers advantages over mechanical joining methods in a variety of
ways.
It offers more uniform stress distribution, better fatigue resistance and
damage tolerance, and may require less time and expense to assemble than
other joining methods1,2
.
This paper is a continuation of a previous paper3
in which the strength
of adhesively bonded joints was evaluated on composites pultruded with
a standard resin, using a variety of surface preparations. The adhesive bond
strength derived from this standard resin is then compared to that of a
composite pultruded with a resin formulation that has been modified with
a " primer " to improve bond strength with minimal surface preparation.
While surface preparation requires additional time and expense, it is often
necessary to prevent premature failure of the adhesive bond in the field. A
resin formulated to increase the surface energy would reduce the amount of
surface preparation required, with no loss of bond strength. This paper studies
the effect of the environmental storage conditions, specifically temperature,
humidity, and time, on the inherent ability of the surface to form strong
adhesive bonds with minimal preparation.
Most adhesives form chemical bonds with the surface of the substrate.
The quality and quantity of bond sites determine the ability of the adhesive
to transfer the load through the bonded joint. Adhesives bond more readily
to surfaces that have high surface energies. It is a challenge bonding to
composites because they generally have lower surface energies. When bonding
to composites, special techniques are often employed to increase the surface
energy. The ideal composite surface on which to bond will be clean/dust free,
dry, polar, and have a high surface energy to promote wettability2
.
Common forms of surface preparation for pultruded composites include
manufacturing the part with a layer of peel ply that will be removed just before
bonding, abrading the surface using sandpaper, or grit-blasting the surface.
Peel ply acts to keep the composite surface clean, and creates a roughened
surface that promotes bonding when removed. Surface abrasion and gritblasting
removes any residual mold release from the fabrication process, as
well as other surface contaminants. All of these methods provide a roughened
surface which creates more surface area with which to bond. The surface must
always be cleaned thoroughly from particles prior to bonding. Bond failures are
often a result of improper surface preparation during specimen fabrication4
Surface preparation improves bond quality on most composites, but adds
.
expense by way of labor and consumables. It often requires that the joint
be bonded within a specific amount of time to achieve maximum adhesion
properties. Once the surface is prepared for bonding, it immediately begins
to return to a state of lower surface energy. The rate at which the prepared
surface devolves is dependent on the material itself. Dillingham showed that
contact angle measurements performed on graphite / epoxy surfaces showed
an increase in contact angle (loss of bondability) of almost 10% after a 24-hour
period5
.
It is well known that the environment at which composite parts are stored
prior to bonding can have a detrimental effect on the adhesion properties of
the bonding surface. Temperature and humidity are two major environmental
factors that can have an adverse impact on bond quality. Storage of parts in
high temperatures can lead to polymer degradation which results in property
loss. Moisture can be introduced into a composite during the manufacturing
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SAMPE Journal - March/April 2022

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