Page 145 - Handbook of Adhesives and Sealants

114 Chapter Three

3.5.5 Plastic and elastomer joints
Design of joints for plastics and elastomers generally follows the same
practice as for metal. However, the designer should be aware of certain
characteristics for these materials that require special consideration.
Such characteristics include flexibility, low modulus, high thermal ex-
pansion coefficients, thin section availability, and anisotropy. These
characteristics tend to produce significant non-uniform stress distri-
bution in the joint. Thus, tough, flexible adhesives are usually rec-
ommended to bond plastic or elastomer substrates.

3.5.5.1 Flexible plastics and elastomers. Thin or flexible polymeric sub-
strates may be joined using a simple or modified lap joint. The double
strap joint is best, but also the most time-consuming to fabricate. The
strap material should be made out of the same material as the parts
to be joined, or at least have approximately equivalent strength, flex-
ibility, and thickness. The adhesive should have the same degree of
flexibility as the adherends.
If the sections to be bonded are relatively thick, a scarf joint is ac-
ceptable. The length of the scarf should be at least four times the
thickness; sometimes larger scarfs may be needed.
When bonding elastic material, forces on the elastomer during cure
of the adhesive should be carefully controlled, since excess pressure
will cause residual stresses at the bond interface. Stress concentra-
tions may also be minimized in rubber-to-metal joints by elimination
of sharp corners and using metal thick enough to prevent peel stresses
that arise with thinner-gauge metals.
As with all joint designs, flexible plastic and elastomeric joints
should avoid peel stress. Figure 3.19 illustrates methods of bonding
flexible substrates so that the adhesive will be stressed in its strongest
direction.

Figure 3.19 Joint designs for flexible substrates. 9