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Being a Glasair builder, I use the stuff alot. I am attaching an article from
the Glasair News Site which is the definitive summary of Q-cells, Milled Fiber,
and Cabosil (used to thicken the resin so it doesn't run and helpful in
smoothing out milled fiber fillets -- at the expense of some strength.) I also
reviewed all the Safety Data Sheets for everything I work with and found that
there is no major hazard with Q-cells or Milled Fiber (besides the
itchy-scratchy.) THAT surprised me...
ALL ABOUT FILLERS
by Bruce Bashforth
Stoddard Hamilton Technical Support
Every Glasair and GlaStar
® kit contains plastic bags of the white powders
(guaranteed to raise the suspicions of
any DEA agent!) that we know and love as "mill fiber," "Q-cell" and
"cabosil." But what are these materials
really for, anyway? Bruce shares his insight into the true uses of these
oft-confused ingredients.
Since beginning to answer the Tech Support phones here at S-H, I have noticed
that there seems to be some confusion
regarding the various powders included in Glasair and GlaStar® kits and what
their proper uses are in creating resin
mixtures. The three powders are mill fiber, Q-cell and cabosil. Each one is used
to achieve a particular resin mix to be
used to accomplish specific jobs. They are not interchangeable.
Let's start with what is perhaps the least understood of the three chemicals,
cabosil. Cabosil is silicon dioxide, or fumed
silica. It is a very fine white powder, very light in weight. The function of
cabosil is to act as a "thixotropic agent." This 50˘
word means?essentially?"thickening agent," but thickening of a very particular
chemical sort. After all, adding mill fiber,
Q-cell or pea gravel to your resin will also thicken it, but not
thixotropically!
Think of a thixotropic agent as a bunch of chemical S-hooks. When introduced to
certain liquids (like vinyl ester resin) in
proper proportions, it "hooks" onto the molecules of the liquid and holds them
together. When agitated (stirred), the
thixotropic hooks let go and the gelled mixture liquefies until the agitation
stops, at which time it gels again.
So, descending from the pedestal of science to the practical realities of
airplane-building, the purpose of cabosil is to get
resin to stay put. Its primary use in Glasair and GlaStar® construction is in
making a thick resin mix that will hang on the
bonding flanges when installing the acrylic windows. Similarly, it can be used
in any bonding situation when you'd rather not
have resin running down the sidewall of the airplane, down the resin brush and
down the sleeve of your new shirt (which
you should know better than to wear while working with resin anyway). Cabosil
can also be useful as a secondary
ingredient when mixing Q-cell or mill fiber mixtures, as explained below.
What cabosil is not good for?completely useless, in fact?is adding strength to
resin. That job falls to . . . mill fiber! As
the name implies, mill fiber is short, or milled, fibers. In the case of the
Glasair and GlaStar®, the fiber in question is glass.
When mixed with resin, the mill fiber adds strength just like woven glass cloth
does, forming a strong bonding or filling
mixture.
Usually enough mill fiber is added to the resin to form a thick mixture that
won't fall off its stir stick. However, such a high
ratio of fiber to resin sometimes results in a mix too dry to apply easily into
the nooks and crannies it was mixed to fill. It
can, in fact, be too dry to stick properly even if you can get it where you want
it. The solution is to add about 20% cabosil
by volume along with the mill fiber to achieve the desired mix consistency. With
just a bit of cabosil added, the mill fiber
mixture will flow well, stick well and still stay put until cured.
Q-cell is simply hollow microspheres, or tiny bubbles of glass. Its purpose when
mixed with resin is to form a filler or a
sealing agent that is lighter than resin alone and that will stay put until it
cures. To use a resin/Q-cell mix as a filler, a thicker
mix is desirable, while a thin mix is ideal for sealing foam surfaces and so on.
The same comments about adding cabosil to the mix apply to Q-cell just as they
did with mill fiber. About 20% cabosil by
volume added along with the Q-cell will help control the consistency of the mix
and make it easier to apply.
As an aside, a little mill fiber added to the resin/Q-cell mix in certain
applications will add strength. Most Q-cell mixes are
fillers or sealers only, and don't carry a load. If the application does happen
to be used in a load bearing situation, a little
mill fiber won't hurt, again maybe 20% by volume.
Note when catalyzing these mixtures to use the total weight of the mixture to
calculate the amount of catalyst to use, not
just the weight of the resin. Finally, some of the preceding suggestions are not
covered in the assembly manuals. This
information is intended to supplement the manuals, not contradict them.
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