Page 371 - Handbook of Adhesives and Sealants

Adhesive Composition and Formulation 327

properties. Addressing such tradeoffs is the day-to-day task of the ad-
hesive formulator.

9.3.1 Controlling flow
Controlling flow is an important part of the adhesive formulation pro-
cess. 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 or too thick of a bondline. These factors
could result in localized high stress areas within the joint and reduc-
tion 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 ad-
hesives as well as other properties. The type and amount of fillers are
chosen so that a practical bondline thickness will result after appli-
cation of the necessary pressure (usually only contact pressure, ap-
proximately 5 psi) during cure. Ordinarily, the objective is a bondline
thickness of 2–10 mils. Consideration, of course, must be given to the
curing temperature. Viscosity of the formulation could drastically be
reduced at elevated temperatures. 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.
Thixotropic adhesive pastes can be formulated which will not flow
during cure even at elevated temperatures. Thixotropic adhesives are
useful for bonding loose fitting joints. The addition of asbestos at one
time provided excellent thixotropic adhesive formulations, but health
and environmental regulations have severely limited the use of this
material. Today, fumed silica, precipitated calcium carbonate, certain
clays, and cellulose and other fibers offer thixotropic properties at rel-
atively low levels of loading. Table 9.3 shows thixotropic epoxy adhe-
sive formulations and resulting properties using fumed silica and re-
inforcing thixotropic (RT) cellulose fiber additives.
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 bondline cannot be thinner
than the thickness of these strands. Sufficient pressure need only be