Just a quick request to the guru's really....
How do I determine which resin (Epoxy or Polyester) has been used in
laminating up a fibreglass structure? I think I'm right in saying that the
two aren't compatible - i.e. one won't stick successfully to the other when
you're trying to carry out repairs - so discovering which type of epoxy has
been used to lay up the main structure is important. I know that Polyester
resin and foam are incompatible when the resin is in liquid state (so I
could test some liquid on some scrap polystyrene), but how do I tell the two
apart when they're set off?
Not to mention toxic.
The surface of parts made with polyester resins usually don't set up
completely for years, unless a finishing resin was used or the surface
was treated with PVA. Get the thing warm and give it a good sniff -- if
it smells like a fiberglass shop then it's polyester, and warm polyester
isn't nearly so bad as burning epoxy.
AFAIK epoxy will stick to polyester just fine, but polyester won't stick
to epoxy. I'd double check with the glue manufacturer, but I think you
could safely make your repairs with epoxy. Unless you're using
polyester paint you should be fine.
I'll bet the folks who know could heat an unpainted area
and smell the difference.
I couldn't do that myself--my nose is not well-trained
to what the polyester resin smells like. I've just
read that it is "different" from epoxy.
Just passin' time while waiting for the guru to answer
Epoxy will generally have little or no odor but polyester will have a strong
chemical odor. Generally, once either is cured, you have to thoroughly
clean and rough up the surfaces to get good adhesion.
Before you go too far, it's worth understanding things a little better.
A small scrap, touched with the tip of a soldering iron or the heat from
a heat gun, will in fact have a characteristic smell if it's polyester.
If you want to know what the smell is, visit an auto body shop, or buy
a small can of Bondo. If sniffing the stuff once to learn the smell is
really bad for you, anyone who ever worked in a boatyard or body shop
would be dead by now.
But the real question is "what properties are you aiming for?"
Polyester resins are strong, set up fast, and have generally good
adhesion. They also, once set up, tend to be brittle -- impact loads
will cause spider-web cracking, etc.. Finally, many standard mixes have
an air-inhibited reaction: the polyester that's within the fiberglass
and away from air cures nicely; the part exposed to air cures slowly and
remains tacky; that's great when you want to put down another layer of
glass and resin, but not so great for the final coat. One solution is
to use a "finishing resin," which contains a bit of "wax like" stuff, as
I understand it, which "floats" to the top and makes a thin air-barrier,
allowing the top layer to cure nicely. Epoxies (there's no ONE epoxy --
it's a general name for a class of chemical reactions) have varying
characteristics, but many of them are less brittle when set up,
substantially more expensive, have longer set-up times, and their
reactions are generally not air-inhibited. Vinylester resins
have a number of other interesting properties, but they mostly
don't seem to get used outside of boatyards.
So what about those mechanical properties? Well, if you're making a
multi-layer setup, the difference in properties will generate
substantial shear loads at the interface if the material undergoes
bending. Does that matter? Only if you care about adhesion AND expect
some bending motion.
There are also adhesive properties; others have noted that polyester
resin tends to not grab cured-epoxy surfaces very well, which is
generally true. It's unfortunately also true that epoxies will sometimes
not grab a cured epoxy surface very well. To ensure a decent bond,
you probably want to
(a) wash the area with soapy water. (This removes the "amide blush" that
appears during the curing of various epoxies, assuming you're bonding to
(b) scratch the surface with some sort of abrasive (sandpaper? Steel
wool? (risky because leftover fibers can rust) Stainless-steel or bronze
wool? 3M green scrubby pads? It's your choice) until it has a matte
appearance because of multiple scratches that provide an "anchor" for
the next resin layer (this'll at least mostly remove the wax layer from
a cured polyester. If the polyester's been painted, then you know that
someone has already removed the wax...)
(c) Give a quick wipe-down with some fast-evaporating solvent like
acetone, or fairly pure ethanol or methanol. If you use ethanol or
methanol, work on a dry day and let it sit for a while when you're done
to get all the residual water off the surface.
Last but not least -- if you're hoping to strengthen your structure with
additional resin, you definitely need some material like fiberglass,
carbon fiber, whatever; neither epoxies nor polyester resin is very
strong over a long (> 1mm) distance, while the glass- and carbon-fibers
ARE. The resins for the most part just serve to provide a bond between
the strong fiber and the substrate and neighboring fibers.
By the way, I know all this from the context of boats, not model planes.
But the chemistry and physics are pretty much the same...
Top-posting because I have nothing to say about the technical
I just want to thank John for his masterful exposition of
the issues involved.
THERE ARE GURUS HERE! It's amazing. That's what
keeps me coming back. :o)
On Wed, 7 Dec 2005 13:26:02 +0000 (UTC), "John F. Hughes"
I read somewhere that with adhesive thermoplastics (like epoxy and
polyester) it's not so much the "tooth" that improves the adhesion but
the fact that you're exposing a clean surface with a bunch of
newly-broken molecular bonds. This is known as "activating" the
surface; you do the same thing with metal when you acid-etch it.
Lacquer (dope is a form of lacquer) needs sanding for the tooth that it
provides, unless you're going over a lacquer surface in which case the
top coat will dissolve the underlying coat a little bit and become one
I'm not sure this would be the best thing to do with a freshly activated
surface; most fast-evaporating solvents are very water-absorbing, and
water has a tendency to deactivate surfaces. The advice I've seen is to
go over the surface with a tack rag to pick up dust.
But I haven't done any controlled tests, so I can only repeat what I've
I believe that this is probably right...esp. when you're bonding to
the same material and hoping to get inter-layer polymerization. For
other materials, I'll bet the "tooth" matters more...but I'm really no
Interesting point; perhaps the next time I've got some WEST-system resin
mixed up I'll try to do an A/B test (or A/B/C/D/E test :-) ).
In any structure, the two primary stresses are tension and
compression. All other forms of loading (bending, torsion, shear) are
various permutations of the primary stresses. In a composite structure,
the resin takes compression loads as well as making the composite stick
together, and the fiber takes the tension loads. By themselves, neither
are much use at all.
Many hobbyists make the mistake of adding extra resin "to
make things just a little stronger." All they've done is make it heavy.
Any more resin than it takes to saturate the cloth is pretty much
That is, incidentally, the most common mistake made by most fiberglass
shops trying to make high-performance parts. This is more because they
want to use a chopper gun rather than laying things up by hand --
chopper guns are easier to use if the fibers are cut short and there's
lots of resin, but as you pointed out the resulting part is brittle and
There's also a very wide range of resin qualities available, from the
el-cheapo stuff used for shower stalls which softens in the heat of the
sun on a hot day, to stuff which will hold it's shape even as it's being
I have no idea what's commonly available for model airplane use -- even
though my dad's company makes high quality fiberglass stuff all the
airplanes I've ever built have been cellulose-lignin composite rather
I had always been under the impression that the fibers took both the
tension AND compression loads in, for example, fiberglass. The problem
is that individual fibers are so thin that Euler buckling would happen
under even tiny loads; the resin serves to provide lateral support
against this Euler buckling, but the resin itself isn't actually all
that strong in compression. At a practial level, this probably makes no
difference at all, unless you run unidirectional fiber and hope to
withstand compression loads in orthogonal directions. But it sounds
as if my understanding may have been completely wrong (sigh). Now
you've got me intrigued, and I'll have to talk to my material-science
buddies for details...thanks for the provoking discussion.
The fibers would indeed take some compression if they're
held rigid in the matrix, but under compression loads they would mostly
keep the member from buckling and keep the resin from crumbling and
being forced out. The resin, I think, should be able to support most of
the compression as long as it's confined by the fibers.
From Tim Wescott:
Any wood, but especially tree wood >:-)
I worked in an industry which was only satisfied when the cellulose and
the lignin where seperated. Production then involved a lot of water
being first added and then expensively (in energy terms) removed.
Roughly... 10 tons of water used for every ton of production.
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