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Modifying Epoxy with Fillers
MODIFYING EPOXY WITH FILLERS
Epoxies formulated for coating and fiberglassing are too
thin to serve as gap-filling adhesives. They can be modified by the addition of
thixotropes to form non-sagging pastes very useful as gap filling glues. These
pastes can be further modified with the addition of microballoons to form
putties for fairing and hole filling.
Wood flour can be used to make filleting putty for
stitch-and-glue boat construction. All these solid dusty additives are called
fillers. Fillers change the flow and density characteristics of the epoxy
system.
Each filler changes the liquid resin and hardener in ways
that make epoxy useful for other applications besides coating or fiberglassing.You
will need to learn how to use fillers if you intend to use ourgeneral-purpose
epoxy. You can avoid using fillers if you choose to use products from the
SilverTip Series. SilverTip GelMagic, SilverTip EZ-Fillet, SilverTip QuikFair,
and SilverTip MetlWeld have already been modified and optimized for their
particular end uses.
You simply measure and mix these products and do not have to
bother with fillers. So, if you are using any of the SilverTip Series products,
skip this and move on to the next section.
Fillers fall into four general classes: thixotropic agents,
bulking agents, fibrous fillers, and pigments. There is some overlapping as to
function of certain fillers. For example, plastic minifibers are both fibrous
and act also as a thixotropic agent. Silica thickener (Cab-O-Sil or Aerosil ),
plastic minifibers and wood flour are thixotropic agents. They turn the epoxy
into a thixotropic fluid. These fluids flow under shear stress but do not
readily flow once the stress is removed. Ketchup and latex house paints are
examples of thixotropic fluids.
Adding these agents to the mixed resin and hardener produces
a fluid that will easily flow under the spreading stress of a putty knife. Once
the stress is removed the thickened epoxy retains its shape. In short, these
fillers make the epoxy non-sagging and are added specifically to make
gap-filling adhesives.
Phenolic microballoons, quartz microspheres, and wood
flour are bulking agents.They "bulk out" the epoxy making a light-weight
putty like mix. Although all these thicken the epoxy, only wood flour will also
make it thixotropic . Attempting to add sufficient
Chopped glass strands, milled glass fibers, and plastic
minifibers are fibrous materials that can be incorporated into structural
filleting putties to improve tensile strength, and are listed above in
descending order of tensile strength improvement.
White paste pigment (titanium dioxide) and graphite powder
are generally used as pigments. Graphite powder added at high loading levels
(25%) to coatings, which are then sanded, produce a "slick" racing finish due
to the lubricating qualities of the graphite. Graphite is a conductive material
and could cause electrolysis problems under the right circumstances. Since it
is the most "noble" of all conductors you should avoid direct contact with
other metals under wet conditions. Adding white paste pigment produces a white
resin coating that is useful where a light color is desired and painting is
difficult. Pigments aren't meant to serve as substitutes for paint in areas
exposed to strong sunlight .White paste pigment can be added to the final fill
coat when fiberglassing ,allowing this coat to serve as a base coat for finish
painting.
Our other pigments are pure dry colorants ground into epoxy
resin to produce an epoxy paste pigment. Since they are dispersed into epoxy
resin they may be added to the resin side of our epoxy systems to produce
stable pigmented resin. The volume of the
These fillers, pigments and additives may be used with any
of our epoxy systems except for SilverTip Laminating Epoxy, which was designed
as a coating and fiberglassing resin only. Higher filler loading levels are
possible with Clear Coat epoxy because it is
Many combinations of filler materials are possible and we
have not tested them all. If you have an idea that a certain combination might
do something special for you then check it out. Little pieces of scrap plywood
are good for this. Think up some destructive tests that will simulate the
stresses the material will see in service. Check to see where the failure
occurs. If the wood breaks then your combination should work well with wood, at
least.
The correct sequence for the addition of filler materials:
1. Correctly measure and mix resin/hardener.
2. Add fiber fillers , if any, and mix well.
3. Add bulking agents, if any, and mix well.
Reproduced from System Three Resins "The Epoxy Book":
Reproduced from System Three Resins "The Epoxy Book":