Page 169 - Handbook of Adhesives and Sealants

Standard Test Methods 139

Figure 4.1 Digital Brookfield vis-
cometer allows laboratory tech-
nician to measure viscosity and
torque. (Photo courtesy of
Brookfield Engineering Labora-
tories)

They are materials that tend to have high viscosity characteristics and
exhibit no flow at low shear rates. However, when pressure is applied
(higher shear rates), the material flows easily, exhibiting a character-
istic of lower viscosity. Such materials are very common in the adhe-
sive and sealant industries. Thixotropic materials can be pumped
through a nozzle, mixed, or applied to a surface with little resistance.
When applied to a vertical surface, they will not flow under their own
weight. Yet, they can be easily spread or tooled before hardening with
only slight pressure.
The viscosity of non-flowable products is determined by an extrusion
1
test. A Semco 440 nozzle or its equivalent is attached to a standard
adhesive/sealant cartridge filled with the material to be tested. The
cartridge is then placed in an air operated sealant gun set for a pres-
sure of 90-95 psi. The weight of material that is extruded in 10 sec is
measured, and the extrusion rate is reported in gms/min. Care must
be taken to clear trapped air from the cartridge and nozzle.

4.3.1.2 Shelf life. When a polymeric resin is stored for a considerable
length of time, physical and chemical changes may occur within the
material that will affect its performance as an adhesive or sealant.