Epoxy Resin vs. Vinylesters and Polyesters
Here's a different way to look at epoxy resin vs. vinylester resin as was explained to me by an R & D chemist named Mike Daniels. Mike works for a huge Minnesota company and he does know his chemistry. Plus he has 25+ years of making all kinds of cool stuff and is an expert wood crafter. His stripper canoes and kayaks are rather spectacular. You just can't help but trust a guy who is a seasoned chemist, canoe builder, and canoe paddler.
Vinylester resin forms a chain of molecules that kind of wrap themselves around a kevlar fiber. Now, I know that this is really perhaps oversimplified, but I am a simple man and definitely not a chemist. Using carabiners, the first photo on the left represents a strand of vinylester resin. Pretend that the paddle shaft is a kevlar fiber and the vinylester resin (carabiner chain) wraps around it in a fairly linear path. Those vinylester resin molecules follow each other in a chain formation and end up winding around the kevlar fibers as well. Sure, it's strong, but it doesn't make anywhere near as many contact binding points with the side of the very-nonporous, kevlar fibers as epoxy resin does.
Vinylester Resin around a kevlar fiber
Epoxy resin around a kevlar fiber
Crosslinking of Epoxy
The photo on the right represents the cross-linking that goes on with epoxy resin. The number of strands of molecules that link in from the sides of the kevlar fiber also link to each other and to other kevlar fibers fibers as well. The cross linking of molecules is virtually infinite and this results in a zillion contact points with each kevlar fiber in the canoe - and there are bazillions of those. Cross linking increases as the epoxy resin cures. In the photo above, I didn't have nearly enough carabiners (needed a bazillion) to make a really great representation of epoxy's molecular cross linking, but I think you can get the idea. The molecules of epoxy would actually link together to the fiber from all angles and directions. The number of links of epoxy molecules is to a kevlar fiber would be much harder to count than the number of links made by vinylester resin to a kevlar fiber.
The epoxy resin that Souris River Canoes uses is a very pure form of resin. It does not have the additives that are inside of the epoxy resins that us ordinary folks us can buy at the grocery store. Common epoxy has additives to allow mere mortals to use it at room temperature and actually have it cure while doing the cross-link thing. Even though the resin is still incredibly strong with additives that facilitate curing and allow us to apply it without heating it up, it is not as strong as the epoxy resin SR uses. Souris River's epoxy formula is close to solid at room temperature and uses a combination of two different hardeners. It has to be heated up just to get it out of the container.
The only way SR's blend of epoxy resin cures is through heating it in an oven. The oven curing process and the need for high room temperatures to make the resin runny enough to squeegee thru four layers of cloth, are two of the main reasons you do not see epoxy resin used in all of the various Brand X kevlar canoes out there today. Vinylester resin is just easier to use at the factory level but nowhere near as strong as epoxy resin. The cross linking is just not there in vinylester resin and the kevlar fibers can pull out of the glue more easily when stressed via paddling into a big rock or over a log. A heat-cured, epoxy resin, Souris River canoe, because of the nearly infinite number of cross-linked epoxy molecules making contact with every fiber in the canoe (plus each other due to the catalytic reaction that goes on), a Souris River Canoe essentially becomes one big honkin', continuous, plastic, molecule. Vinylester resin does not do this to the magnitude of epoxy resin and is therefore substantially weaker.
One final point - Heat-cured epoxy resin is used in the carbon fiber composite, V-22 Osprey and in Cirrus Aircraft. When it comes to flying, guess what? They don't use vinylester resin in any important parts like the wings, fuselage, etc. That says a lot to me. If it doesn't mean much to you, you haven't been paying attention to all this typing.
Thanks to Mike Daniels for 'splaining epoxy cross linking to me and subsequently, to you.
Even More Information
Here is some other technical data that I gleaned off of epoxy resin websites by Maas and West Systems. (If you think this is just a sales pitch, heck, you're sitting here looking at this web page on the internet - use Google and research it for yourself.) You may find it interesting regarding the differences between Epoxy Resin and Vinylester or Polyester Resins. In case you don't already know it, Souris River Canoes are all made with a very high grade epoxy resin. Pretty much everybody else in the canoe world uses either vinylester resin and some the weaker polyester resin.
In the marine industry, liquid plastics, namely epoxies, polyesters, and vinylesters are used to saturate (wet out) the fibers of wood, glass, kevlar amarid, or carbon to form a fiber reinforced plastic (FRP). To create a quality part, adhesion to the fibers is the most important factor. Not all resins keep their grip on fibers equally.
Epoxy resin is known in the marine industry for its incredible toughness and bonding strength. Quality epoxy resins stick to other materials with 2,000-p.s.i. vs. only 500-p.s.i. for vinylester resins and even less for polyesters. In areas that must be able to flex and strain WITH the fibers without micro-fracturing, epoxy resins offer much greater capability. Cured epoxy tends to be very resistant to moisture absorption. Epoxy resin will bond dissimilar or already-cured materials which makes repair work that is very reliable and strong. Epoxy actually bonds to all sorts of fibers very well and also offers excellent results in repair-ability when it is used to bond two different materials together. Initally, epoxy resin is much more difficult to work with and requires additional skill by the technicians who handle it.
Vinylester resins are stronger than polyester resins and cheaper than epoxy resins. Vinylester resins utilize a polyester resin type of cross-linking molecules in the bonding process. Vinylester is a hybrid form of polyester resin which has been toughened with epoxy molecules within the main moleculer structure. Vinyester resins offer better resistance to moisture absorption than polyester resins but it's downside is in the use of liquid styrene to thin it out (not good to breath that stuff) and its sensitivity to atmospheric moisture and temperature. Sometimes it won't cure if the atmospheric conditions are not right. It also has difficulty in bonding dissimilar and already-cured materials. It is not unusual for repair patches on vinylester resin canoes to delaminate or peel off. As vinylester resin ages, it becomes a different resin (due to it's continual curing as it ages) so new vinylester resin sometimes resists bonding to your older canoe, or will bond and then later peel off at a bad time. It is also known that vinylester resins bond very well to fiberglass, but offer a poor bond to kevlar and carbon fibers due to the nature of those two more exotic fibers. Due to the touchy nature of vinylester resin, careful surface preparation is necessary if reasonable adhesion is desired for any repair work.
Polyester resin is the cheapest resin available in the marine industry and offers the poorest adhesion, has the highest water absorption, highest shrinkage, and high VOC's. Polyester resin is only compatible with fiberglass fibers and is best suited to building things that are not weight sensitive. It is also not tough and fractures easily. Polyesters tend to end up with micro-cracks and are tough to re-bond and suffer from osmotic blistering when untreated by an epoxy resin barrier to water. This is really cheap stuff.
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