This week's editorial considers several common questions
concerning structural adhesive joints: what is the importance of bond
line thickness, what is the correct bond line thickness, and how can it
Questions are often asked regarding bond line thickness
when designing an adhesive joint for structural adhesives.
- What is the importance of bond line thickness?
- What is the correct bond line thickness?
- How can it be controlled?
This editorial will attempt to give some guidance to the parameter of
bond line thickness.
The most important aspects of adhesive thickness, or bond-line thickness,
are its magnitude and its uniformity or homogeneity. Generally, one tries
to have as thin a bond layer as possible without any chance of bond starvation.
In practice, this translates into bond-line thicknesses from 0.002 in.
to 0.008 in.
Adhesive strength will vary in this range due to the nature of the adhesive,
the substrates, and the joint design. Many laboratory experimentalists
will try to achieve a constant bond line thickness of 0.005 in. With thicker
adhesive bond-lines, one runs the risk of incorporating higher void concentrations
into the joint. In addition, stresses at the corner of the adhesive-adherend
tend to be larger because it is difficult to keep the loads axial with
a very thick bond-line. Figure 1 shows the effect of bond line thickness
on joints made with a DGEBA epoxy adhesive cured with an amidoamine.
Effect of bond line thickness on tensile shear
strength of an epoxy-amidoamine adhesive bonding
It should also be remembered that adhesives are generally formulated
to cure in thin sections. Thicker sections could change the curing properties
and result in increased internal stresses and different physical properties
The substrates should be as parallel as possible, thus requiring uniformity
in adhesive thickness across the bonded area. If the substrates were not
parallel, the loading would not remain aligned and this condition would
translate into cleavage stresses on the adhesive.
There are several methods used for maintaining a constant, predetermined
adhesive thickness. These methods include adjusting the viscosity of the
adhesive, application of a precalculated amount of pressure during cure,
using fixturing that is specifically designed for the application, and
application of a shim or insert within the bond-line so that a uniform,
predetermined thickness can be maintained.
If the adhesive has a propensity to flow easily before and during cure,
then one risks the possibility of a final joint that is starved of adhesive
material. If the adhesive flows only with the application of great amount
of external pressure, then one risks the possibility of entrapping air
at the interface and too thick of a bond line. These factors could result
in localized high stress areas within the joint and reduction of the ultimate
Flow characteristics can be regulated by the incorporation of fillers,
by the use of scrims or woven tapes as "internal shims" within
the adhesive itself, or by the careful regulation of the cure cycle. All
of these options along with a few more are available to the adhesive formulator.
Generally, fillers are incorporated to control the viscosity of the adhesives
as well as other properties. The type and amount of fillers are chosen
so that a practical bond line thickness will result after application
of the necessary pressure (usually only contact pressure, approximately
5 psi) during cure. Consideration, of course, must be given to the curing
temperature. Viscosity of the formulation could drastically be reduced
at elevated temperatures, and unless there is a furrow designed into the
joint to contain the adhesive much of the adhesive could flow out of the
joint area before the adhesive is completely cured.
Glass, nylon, polyester, and cotton fabric or mat are often used as a
carrier in tape or film adhesive systems. In addition to reinforcement
and a way of distributing stress within the joint, the strands of the
fabric offer an "internal shim" so that the bond line cannot
be thinner than the thickness of these strands. Sufficient pressure need
only be applied to cause the adhesive to flow so that the "shims"
meet one another to provide a positive stop. Paper, mat, and other carrier
materials may also be used for this purpose.
Glass or polymeric microballoons, incorporated directly into the adhesive
formulation can provide the shimming function. Here the diameter of the
microballoons is the positive stop that will prevent too thin a bond-line.
Another option is to design mechanical shims into the joint itself. The
parts to be assembled are designed with lips or stops so that the adhesive
cannot flow out of the joint area or that a certain predetermined adhesive
thickness is always maintained.
Should you have any comments or feedback, please contact
Edward M. Petrie
1. Shimp, D.A.,
"Epoxy Adhesives", in Epoxy Resin Technology, P.F. Bruins, ed.,
Interscience Publishers, New York, 1968, pp. 176-177.