Page 359 - Handbook of Adhesives and Sealants

314 Chapter Eight

capability. When cured, water-based adhesives do not have the mois-
ture resistance that solvent-based contact adhesives have.

8.2.4.4 Solid forms. Common forms of solid adhesives are tape or film,
powder, or solid shape forms. The main advantages of these single
component adhesives are that metering and mixing is not required,
and they can be applied uniformly to a substrate with little or no
waste. Thus, solid adhesives are popular in many production appli-
cations. Both thermoplastic and thermosetting adhesives can be for-
mulated as solid systems.

8.2.4.4.1 Tape and film. Tape and film are terms used loosely and in-
terchangeably for adhesives in sheet form. More correctly, tapes are
supported on a web of paper, nonwoven fabric, or on open weave scrim
of glass cotton or nylon. Supporting fibers are useful in that they pro-
vide a positive stop under bonding pressure. This can be used to con-
trol bond-line thickness and often to help distribute stresses. Films,
on the other hand, are free of reinforcing fibers and consist only of the
adhesive in thin, sheet form.
Tape and film adhesives are poor gap fillers especially if the gap
between parts varies in depth across the bonding surface. However,
they offer a uniformly thick bond-line and easy dispensing. Being a
single component system there is no need for metering. Tape and film
adhesives can be used in bonding large areas such as in the aerospace
industry. For example, the joining of aluminum honeycomb structure
to flat metal sheets is best accomplished with thermosetting film ad-
hesives.
8.2.4.4.2 Powdered adhesives. Another form of solid adhesive is powder
or granules, which must be first heated or solvent-activated to be
made liquid and flowable. Being a single component system, powdered
adhesives also eliminate proportioning and mixing errors, but uniform
distribution over large assemblies is difficult.
Solid powder adhesives may also be fused or compacted into various
preforms such as sticks, gaskets and beads. They can then be easily
applied to a specifically shaped part. Certain types of thermosetting
epoxy powder can be fused into small rings that are then used in rod
and tube type joints. The ring is slipped over one of the substrates
before assembly. When exposed to high curing temperatures, the epoxy
in the ring melts, flows, and wets the substrate. Additional heating is
used to fully cure the adhesive into a thermosetting part. In this type
of application, a well or trough must be designed into the joint to hold
the liquid adhesive in place while it cures.