Page 356 - Handbook of Adhesives and Sealants

Adhesive Classifications 311

strength sufficient to release holding pressure and move the product).
The time for the adhesive to reach maximum strength is usually sig-
nificantly longer than the time to reach handling strength. The work-
ing time, time to reach handling strength, and time to reach maximum
strength are dependent on the type of adhesive and the method by
which it is cured. A typical hardware store variety two component
epoxy (epoxy cured with a polyamide-amine) will have a working life
of perhaps 30 min, will reach handling strength in from several hours
to 24 hrs, and will develop full strength in 3–7 days. Figure 8.8 shows
a curing curve of a typical room temperature curing epoxy adhesive.
Two-part systems may introduce problems in production scheduling,
and some waste is unavoidable. Solventless liquid adhesives can be
employed in all bonding operations without regard to size, shape, ori-
entation, or fit of substrates. The viscosity of the adhesive generally
determines how it is to be applied. Liquid adhesives lend themselves
to easy handling via mechanical spreaders or spray and brush. Paste
adhesives have high viscosity to allow application on vertical surfaces
with little danger of sag or drip. These bodied adhesives also serve as
gap fillers between two poorly mated substrates.
Two-part systems can also be used to form non-solid tape adhesives
by knifing or brushing the mixed adhesive onto a supporting web (e.g.,
glass fabric) laid either on the substrate or on a separator sheet. When
spread on a separator sheet, the supporting web can be used as a
carrier to transfer the adhesive to the substrate surface.
8.2.4.1.2 One-part solventless. One-part solventless liquid adhesive
systems are supplied in a ready-to-apply condition. Most of these sys-
tems are heat curing or cure by reaction of moisture in the air. Some

Tensile Shear
Strength Gel and Hardening

Handling
Strength Full Strength

Curing Time

Figure 8.8 Typical curing curve for a room temperature curing epoxy adhesive.