Some good info on bonding with epoxies

[ondablade]

This may be of interest to some of you. I've been looking around a bit as with decent progress on the workshop refurbishment i'm starting to eye its first project using the new kit - my Schwarz type Roubo bench.

Digging on epoxies I came across some good technical information on bonding wood with them - i'm thinking of using a slow epoxy to bond together the 20 or so 4x1 1/4in beech planks (on edge) which make up the top.

Despite intending to do it in groups of say four planks at a time I'm a bit wary of the relatively short 15 min open time you get with even Titebond Extend - I've never done a big glue up like this before. (thoughts anybody?)

Gurit is a well regarded supplier of epoxies and related composite construction materials. Some of you guys might have encountered their products before labelled as being by SP Systems - SP is a brand within their boat building products business so far as I know. http://www.gurit.com/

The document 'Guidelines on Bonding with Epoxy in Wood Construction':
http://www.gurit.com/downloads.asp?sect ... res&page=2 I know it's somewhat an infomercial, but it's got some interesting things to say about urea formaldehyde and resorcinol which seem to have some definite shortcomings.

There's actually a whole library of technical literature on the site about all sorts of epoxy and composite materials uses:
http://www.gurit.com/downloads.asp?sect ... +Brochures

PS Somebody claimed on a US woodworking site recently that laminations they had made with epoxy suffered from creep, which is very much against my (other) experience with the stuff. Lots of others came in to say otherwise, but it prompted me to take a look.

Keith over on the Woodwork forum has a thread up right now on some outdoors testing he's doing on various adhesives which may be of interest too.

There's more info of one sort or another on the West epoxies site: http://westsystem.com/ss/ UK sources: http://westsystem.com/ss/indealer-area. ... mit=Search

 

[srp]

I'd go with epoxy for a laminating job like this as well. You might actually want to consider a fast hardener rather than slow, especially this time of year in the UK, as you want to be certain the cure is well under way before you leave the workshop at the end of the day.

Over the years I've picked up a few useful tips on working with the stuff, apologies if you've seen them before...

1. Get organised with a box of vinyl gloves, plenty of mixing sticks (home made on bandsaw) and lots of plastic mixing containers. Especially the gloves as contact dermatitis is a real risk.
2. Cover the workbench top with polythene sheet before you start. Apart from saving the mess, it means you can clamp the work straight down onto the top and the polythene will just peel off afterwards.
3. Use methylated spirits to clean up uncured epoxy - far cheaper and much less toxic than the 'proper' epoxy solvent.
4. For most glueing work use microfibres to thicken up the mix.
5. Use a set of digital kitchen scales to measure out the resin and hardener. This is far better than using the hand pumps (which sometimes fail to deliver the correct amount) and especially useful for small quantities.
6. Use a roller to spread the epoxy on large areas. The cheap 4" foam rollers are ok, but change them before they start to harden and/or disintegrate.
7. Don't leave mixed resin in the mixing cup - spread it onto the joint as soon as possible. For large quantities pour into a shallow tray as soon as it's mixed. The resin/hardener reaction is exothermic, and the mix can get very hot (to the point of smoking and even combusting) if left in the mixing cup.

I've never experienced creep on epoxy joints, except when the mix ratio has been incorrect and has therefore failed to cure properly, in which case you have to dismantle, clean up and start again. All epoxy resins are flexible to some degree (like most plastics or plastic composites), and that is what makes them so good for working with wood.

You're right about SP now being a brand within the parent company. They used to be independent in the old days, based in Cowes in the IOW. The other big name in epoxy is the West System. They also produce good technical information and 'how-to' guides. Look in the Projects and How To Use sections of http://www.westsystem.com

Probably not any practical difference between any of the major brands when it comes to bonding timber, I've used West, SP and Ciba-Geigy on both timber and woven glass laminates with equal success, though there is sometimes a difference in mix ratios of different products.

 

[RogerS]

.....
5. Use a set of digital kitchen scales to measure out the resin.....

How much does a typical small mix weigh ?

Reason for asking is that I found that the resolution of our digital kitchen scales was too coarse to accurately measure small quantities of low density stuff like Cascamite accuately. So I bought an El Cheapo jewellery digital scale off eBay for £8 which does the trick.

 

[srp]

My scales measure in steps of 1 gm, so for something like SP106 epoxy I weigh out 5gms of resin and add 1 gm of hardener, which would be enough for, say, the 4 m+t's on a kitchen unit door.
I must admit I didn't know there was such a thing as digital jewellers scales - obviously better than kitchen scales with very small quantities.

 

[ondablade]

Thanks SRP, sounds like you are well used to using epoxies in joinery. I might pop you a PM when i get into the project in a couple of months if that would be OK!!!!

My experience with epoxies is in laminating very lightweight mouldings for use in competition model aircraft.

I had good luck using disposable plastic medical syringes bought from a pharmacy for measuring out mix quantities. The only real disadvantage I ever found is that the pistons in them are rubber, and over a few weeks they eventually swell and jam up the syringe.

 

[devonwoody]

You mentioned the word creep.

I reckon if you put a layer of adhesive or epoxy between two pieces of timber there is going to be some evaporation or expansion of the substance, so you get movement on setting. And if it is clamped up to fix when applying, something happens when the cramps are removed?

So creep must always be there?

 

[srp]

I'm not a chemist, but my understanding is this:

Epoxy is a co-polymer, that is made up of two different polymers, basically epoxides (the resin) and amines (the hardener). During curing the two groups react, and the strength is derived from the cross-linking between the molecules. As there are loads of cross links, the cured resin is very strong.

Evaporation is not part of the cure process. There may be some expansion or contraction but is not mentioned in any of the material I've ever read on epoxy curing so I think, for all practical purposes, it can be assumed to be insignificant, and certainly far less than the expansion/contraction of timber.

In the distant past I remember doing tensile testing of cured epoxy samples - it is certainly an elastic material (as are most plastics) but very tough. That is what makes it so suitable for bonding timber - it is far more forgiving of timber movement than the urea formaldehyde based adhesives ("Cascamite" etc).

Creep tends to be a term used when talking about the effects of high temperatures on cured epoxy and refers to deformation when the applied load is constant. The temperatures involved are higher than a normal piece of furniture or joinery would ever be subjected to, so again is a factor that can safely be ignored for our purposes.

The last bit of structural laminating work I did with epoxy was an iroko mast beam for a small sailing yacht:

As this supports the mast the point load at the centre when the boat is sailing can often be as high as the displacement of the boat (around 1 tonne) and the beam is 60mm wide x 70mm deep. That was 2 seasons ago, with no sign of movement or failure. Details of construction on this page

 

[Dave D]

I have used the West system epoxy with excellent results and I would support all of srp's very useful tips.

For small amounts however the pumps are not accurate enough.
The quantities of resin and hardener need to be exact. Too much hardener as well as too little can prevent a proper reaction.

I use a 0 to 200gm digital scale from Maplin with 0.1gm resolution.

For mixing I use aluminium foil baking trays which conduct the heat away nicely and can be re-used. The left over mix from the first use sets hard and stiffens up the tray making it easier to handle next time.

Don't use plastic containers in case you are sure that the epoxy will not dissolve them. (I don't want to remember the mess that experiment caused)

I have used West's colloidal silica filler to thicken the mix for outdoor jobs like large planters but after one problem when gluing oak laminations indoors I no longer thicken the glue for finer work.

What happened was that in stirring in the filler I introduced lots of air bubbles which expanded as the glue heated up while setting and forced the joints apart.

Actually this could happen with over vigorous stirring even with no filler.

Since then I have mixed very slowly and carefully and have had no more problems.

I hope that this helps.

 

[srp]

Don't use plastic containers in case you are sure that the epoxy will not dissolve them.

Good point - I know that nos 2, 4 and 5 are ok, but I'd be a bit wary of the thin white styrene cups, which I think are 6.

 

[wobblycogs]

I used to be a chemist srp and you are pretty much spot on with your explanation of the chemistry. What happens when the glue cures is called an epoxy ring opening. The epoxide group is very strained so it will pop open easily making it useful for small scale polymerization outside a lab or factory.