Page 168 - Handbook of Adhesives and Sealants

138 Chapter Four

1 cps Water
400 cps #10 Motor oil
1,000 cps Castor oil
3,500 cps Karo syrup
4,500 cps #40 Motor oil
25,000 cps Hershey Chocolate Syrup

There are also a number of specialty viscosity tests that employ their
own relative units. These tests have been developed for specific in-
dustries such as paints and coatings. Generally, their viscosity units
can be directly converted to poise.
Adhesive viscosity is an indication of how easily the product can be
pumped or spread onto a surface. It reveals information, together with
the liquid’s setting rate and surface tension, that is pertinent to the
wetting characteristics of the adhesive. The viscosity also reveals in-
formation regarding the age and compounding of the adhesive.
Through the relatively easy measurement of viscosity, changes in den-
sity, stability, solvent content, and molecular weight can be noticed.
Viscosity measurements for free flowing adhesives or sealants are usu-
ally based on one of the following methods described in ASTM D 1084.
The most popular test for products ranging in viscosity from 50 to
200,000 cps is by a rotating spindle instrument such as the Brookfield
viscometer. The simple equipment used for this measurement is shown
in Fig. 4.1. The instrument measures the resistance of the fluid on a
spindle of certain size that is rotating at a predetermined rate. The
method is relatively simple and quick. It can be adapted to either the
laboratory or production floor.
Another test for determining the viscosity of liquid adhesives mea-
sures the time it takes the test liquid to flow by gravity completely
out of a cup with a certain size hole in the bottom. These consistency
cups are designed to expel 50 ml of sample in 30–100 secs under con-
trolled temperature and relative humidity conditions. The number of
seconds for complete flow-out of the sample is determined. There are
different cup volumes and hole sizes that can be used and conversions
exist for relating the viscosity measured in one cup to another. This
test is commonly used in the paint industry for adjusting the solvent
content in paint systems.
The viscosity of thixotropic materials that exhibit a shear rate de-
pendency is usually determined by the procedure described in ASTM
D-2556. The viscosity is determined at different shear rates with a
viscometer. From this plot, apparent viscosity associated with a par-
ticular rotation speed and spindle shape can be obtained. Materials
with thixotropic characteristics include Vaseline jelly and toothpaste.