Thread: PVA or Epoxy

I haven't got any scientific knowledge about this one but generally go for the long open time when gluing. I started a recurve bow build a few years ago and used Techni-glue to do the bent laminations. It's solid and I can't see it breaking ever.

Seeing that most wood glues will create a stronger joint than the wood fibres then the weakest link in your project is the timber. Given that, a stronger glue will not make any difference.

Originally Posted by samo

Seeing that most wood glues will create a stronger joint than the wood fibres then the weakest link in your project is the timber. Given that, a stronger glue will not make any difference.

This

PVA has give or flex. Epoxy does not. Therefore, in certain applications (think chairs) epoxy can be somewhat brittle. But as said by others, the glue is only as strong as the wood.

They each have their advantages and disadvantages, but as others have said, in tension, both are stronger than the wood fibres, so strength is not one of them.

If you need a long working time or if you need to tint the glue, epoxy is the go. It also has a slight lubricating quality, which may be an advantage if the joint is really tight, but can also be a PITA.

The various types of PVA have different working times. If I want something that will go off fairly quickly, I use Titebond II. PVA's also safer and less fiddly to use, not to mention that it's cheaper. However, you can get creep with PVA.

I've had one joint fail using Techniglue. I used Casuarina, a timber that has very different rates of expansion radially, tangentially and lengthwise. Most timbers would have been OK, but not Casuarina. It broke the epoxy. (The stress was in shear.) Woul a little bit of flexibility in PVE have saved it? Who knows.

there's west system 2 part epoxy on my garage floor that I have to smash off with a ball pein hammer because sanding it down is the only other way to get it off.

I think better describing what you're actually trying to achieve may help. Just remember that not all epoxy's are the same, there's different strength ratings and you need to be pretty accurate with the measurements to get it to do its thing properly.

Originally Posted by Chris Parks

Does anyone know if PVA is any less strong than epoxy or is epoxy used strictly for its water resistant properties and long open time. OR has anyone got any hard evidence that one brand of epoxy is stronger than another. I have a very small glue up to do that is going to need a very high strength and I suspect somewhere that tests have been done but I can't find them.

I suggest describing the problem you are trying to solve. This question does not have enough context to give a useful answer.

I've used epoxy for almost everything at some point or another, but it is not the best solution to every problem. I re-glued some pool coping that had fallen off with epoxy. PVA would not have been appropriate. I've used epoxy building window frames. At that time, the waterproof PVA glues were not available but had they been, that's what I would have used.

All the above is pretty good advice, however to recommend the "ideal" glue for your small high strength glue up requires more info. Don't discount CA glue for small projects.

There is pretty much a glue formulation to suit any conceived combination of materials, purpose, application, environment & machine cutter friendliness. For a hobbyist finding the right info is a bit of a task.

One thing to consider with all glues, but especially PVA, is the pH of the glue itself, and the potential "dissimilarity" of two species of wood to be glued. Some timbers rich in tannin respond better to a neutral or base pH glue and do not develop the "black glue line" associated with them. When gluing a white wood or veneer to say Jarrah or Tas Blackwood I prefer to use Sika PVA over Titebond I or II, and Selleys PVA's.

Interesting article on adhesives. There is a table at the end which gives the relative strengths of common adhesives.

Originally Posted by Bohdan

Interesting article on adhesives. There is a table at the end which gives the relative strengths of common adhesives.

But the table doesn't mention what material is being bonded with said adhesives...

This article gives very different results http://www.oldbrownglue.com/pdf/HowS...urGlue_FWW.pdf , which I somewhat call BS on because there's no way that PU is less than 60% the strength of PVA

As others have said, the exact application will determine which glue is best

Originally Posted by elanjacobs

which I somewhat call BS on because there's no way that PU is less than 60% the strength of PVA

PU is a stronge glue provided there is no gap being filled by the foaming action of the glue as it goes off. If there is a gap the glue fills it but it is filled with foam which has minimal strength.

In the above test there was always a gap because there was no clamping, if it had been clamped all of the results may have been different.

Yes, I know. The Timbatech PU we used at work stated max gap filling of 0.4mm

Poor test, but the point I was trying to make was that a simple set of numbers doesn't always tell the full story

Originally Posted by Chris Parks

Does anyone know if PVA is any less strong than epoxy or is epoxy used strictly for its water resistant properties and long open time. OR has anyone got any hard evidence that one brand of epoxy is stronger than another. I have a very small glue up to do that is going to need a very high strength and I suspect somewhere that tests have been done but I can't find them.

You haven't said what materials do you want to glue???????

Jim

It is in a bent lamination made in Sydney blue gum that was has become unglued by my poor methods or inexperience. The laminations are about 5mm thick and the gaps are on the outside of the lamination. I can easily clamp them closed but it is a very strong timber and the unglued parts are small so not a lot of glue surface for adhesion. It was originally glued with epoxy but I haven't got any and I am tempted to re-glue the unstuck bits with PVA or even super glue if it is suitable.