Bond Line Thickness

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 be controlled?

Questions are often asked regarding bond line thickness when designing an adhesive joint for structural adhesives.

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.

Figure 1: Effect of bond line thickness on tensile shear strength of an epoxy-amidoamine adhesive bonding steel-to-steel. 1

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 than optimal.

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 joint strength.

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 me.

Edward M. Petrie

References:

1. Shimp, D.A., "Epoxy Adhesives", in Epoxy Resin Technology, P.F. Bruins, ed., Interscience Publishers, New York, 1968, pp. 176-177.