Using polyester resin instead of epoxy resin

There are million of boats built with polyester that have stringers, decks, bulkheads and transoms cored with wood. They don't have problems unless there is water ingression, in which case is either back workmanship or holes created by bad installations or physical damage.

 

There are also millions of problems. Polyester is not a glue and will delaminate in time. Not a good idea. If we have to use poly on plywood, we drill thru the plywood and let the resin acts as "rivets" for mechanical bond.

 

The millions of boats without delamination, unless there is damage, show that the adhesion is adequate.

 

I'll start by saying that epoxy is far better than polyester when used over wood.

Much of the reputation for poor adhesion with polyester is due to the sloppy practices in production and by the DIY crowd. When done correctly the adhesion is at least adequate, or a bit better.

I did some long term studies with a customer that required bonding polyester to plywood, they did bond testing on every part made, these were 10' X 40' panels, they made about 20 per day. They tried it with holes drilled, or at least punched, they made a machine to punch holes about half way through the ply on about a 4" grid pattern (might have been less, I'd need to look at my notes). After about a year of testing with and without the holes they found no advantage to having holes, but what the holes did do was leave a visible defect on the surface of the part sometimes. The pass or fail test was to peel the laminate off the wood, if it peeled off the first layer of wood it passed, peeling at the bond line was a failure. Passing the test was normal, unless something went wrong it always pulled the ply apart.

Lowering the viscosity of the resin helped, but not in the way most people would lower the visc. When lowering the viscosity of a laminating resin you need to overcome the added thixitropes that were added, this messes up the chemistry by adding more styrene, plus you still have the silica in the mix that can hinder penetration into the wood. By using an infusion resin the penetration is much better. Because these were flat panels we were able to reduce the viscosity of the resin they purchased and get a better bond.

The other issue is how most builders and people apply a laminate to wood, most builders don't pre-coat the wood with resin, they just glass over it with no effort to let resin absorb into the wood. Not only does this frequently leave both the wood and laminate starved for resin, but it also allows any expanding air from the increasing temperatures to form large bubbles between the wood and laminate. These large non contact areas go unnoticed in most production shops, and even if they were noticed nothing would be done about them. This assumes they even made much of an effort to completely cover the wood with laminate and roll the air out.

For the best bond use a low viscosity resin (infusion resin), pre coat the wood, let that at least gel, then apply the laminate, this bond will hold up fairly good.

 

There was a study published in Professional Boatbuilders magazine some 25 years back as I remember. It is a comparison between poly resin and epoxy where they studied the penetration of ep vs poly on wood. Epoxy has "more teeth" to sink into the wood than poly. The problem with sheathing wooden ships with poly is that it cannot be completely encapsulated. There are joints and bolts sticking out plus the areas subjected to localized impact. Once damaged/exposed, water seeps in causing the weakness in bond.

 

Penetration has to do with viscosity mainly. A high viscosity epoxy will not penetrate the wood much. Also, the adhesion is caused by molecular attraction. Mechanical bonding is what penetration does, and it is different than adhesion.

 

The hydrofoils are pretty thin. I’d be curious he can get by with just a 6 oz cloth.

Why would anyone recommend poly? These parts are likely going to find bottom some day.

 

Please post the result of the test and procedure so that others may be guided accordingly.

 

You are splitting hairs Gonzo.

 

Not really. One is a chemical bond, the other is mechanical.

 

Nobody is recommending using polyester, just answering the OPs question if it can work, The answer which is of course it can, but as with all bonding it comes down to surface preparation and since poly is not as good at adhesion, prep is even more critical but done right it can be adequate and that's as good as one can expect. If you have a failure in the wood with poly, epoxy would not have performed any better and this is often the case. I have certainly seen more failures in the bond line such as bulkhead tabbing with poly but it is always due to poor/no prep. I have also seen a sheathing failure on a hatch cutout in a laminated plywood cabintop that was sheathed with a layer of 6oz boat cloth with west epoxy. The cutout was left outside where it got wet but was not in the sun, for some months and the glass peeled right off, clearly a prep issue but we were a bit shocked.

 

Glass skin and foam core works best for what? Structural parts? If so, would you recommend foam core over pine (wood) for the structure?

 

The reason he advocates for foam is it would not be affected by ingress is all.

You can build with wood.

He did not mean foam over wood.

 

Resin into substrate penetration depth has nothing to do with a waterproofed surface. This has been proven over and over. The quality and coating thickness are the real keys. Assuming a coating can be applied thick enough and is sufficiently waterproof, the depth into the substrate has nothing to do with the outcome of testing. Mechanical bonds can be just as good as chemical, with proper prep. Technically a chemical bond is superior, but most all projects have both in them. The PBB piece mentioned above was in the height of the "penetration epoxy" wars, which has since been discounted. The epoxy/poly debate in this submitted PBB feature, just brought what everyone already knew about slathering liquid plastics on wooden things and seeing how it works out. They acknowledged that only true encapsulation was the answer for long term, liability (and insurance company concerns). Both poly and epoxy can be made to do this with results based on film thickness and resin types. This said, given the dramatic modulus, peel strength and elongation (working with fabrics) differences between polyester and epoxy matrix's, there's only one real or logical choice on wood. Just like most things, all the resin systems need proper prep or unpredictable result will follow.

 

While on the subject of proper prep, for those who are not aware, the West system folks did some testing preparing for secondary bonding using a Norton rapid strip brush in a battery drill vs sanding with 80 grit. The laminate prepared with the rapid strip brush yielded a 29% improvement in tensile strength when tested on their Pneumatic tensile test instrument which they claim rivals bonds prepared by bead blasting or needle scaling. It should be pointed out that this testing was done with an old mat and woven roving laminate which is difficult to prepare properly by sanding but worth a thought. Most of us do not have the equipment to put numbers to any testing we might care to do.