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?
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 expoxy.
(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...
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.
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.
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 with it.
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 heard.
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 wasted.
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 heavy.
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 mildly scorched.
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 than fiberglass-thermoplastic.
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 expert. ;
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 :-) ).
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.
Mostly, with some harder woods thrown in around the edges. One of these days I'm gonna make a hot-wire cutter and start cutting cores, but for now I'm just enjoying stick-and-rag construction.
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.
Correct.. paper mill. The pulps were bought in, a paper machine that has been shut for a couple of days doesn't smell too good on start up either. Once you start stirring again... >:-)
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