MIL-HDBK-1110
overmix waterborne paints since excessive foam can be created.
Constant mixing may be required during application for paints
with heavy pigments, such as inorganic zincs.
Mixing can be done either manually or mechanically.
Two types of mechanical mixers are commonly used: ones which
vibrate and ones which stir with a propeller. Since manual
mixing is usually less efficient than mechanical mixing, paints
should only be manually mixed when little mixing is needed
because there is limited pigment settling or when mechanical
mixing is not possible. Vibrator-type mixers should not be used
with partly full cans of paint. This can cause air to become
entrained in the paint which, if applied, may lead to pinholes in
the dry film.
When pigments form a rather hard layer on the bottom of
the can, the upper portion of the settled paint can be poured
into a clean container (Figure 7), so that the settled pigment
can more easily be broken up and redispersed to form a smooth
uniform thin paste. When mixing manually, lumps may be broken up
by pressing them against the wall of the can. It is essential
that settled pigments be lifted from the bottom of the can and
redispersed into the liquid. Once the material is uniform, the
thin upper portion of the container is slowly poured into the
uniform paste while the paint is stirred. Stirring is continued
until the entire contents is uniform in appearance. No more
paint should be mixed than can be applied in the same day. Paint
should not be allowed to remain in open containers overnight.
7.3.1.1
Mixing Two-Component Coatings. Epoxies and
polyurethanes are commonly used two-component coatings. The base
component, A, contains the pigment, if any. The B component
contains the curing agent. The two components must be mixed in
the ratio specified by the coating manufacturer on the technical
data sheet, unless the coating is being applied using a plural
component gun (refer to par. 7.5.4.5). Usually the materials are
supplied so that the contents of one can of component A is mixed
with the contents of one can of component B. Failure to mix the
components in the proper ratio will likely result in poor film
formation. Binder molecules are cross-linked in a chemical
reaction upon mixing of the two components. Unless the two
components are mixed together, there will be no chemical reaction
and no curing of the paint.
a) Mixing. Two-component coatings are preferably
mixed with a mechanical stirrer as follows:
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