Epoxy not adhering

[Bodgy]

Was the surface rough enough for it to key to? I have found with some tearout that it is "smooth" inside and you have roughen it a little bit.

Spot on , that now totals 4 cock-ups in a single procedure. Some kind of record?

The tearouts (this is Oregan) were quite smooth at the base. As the glue (even with my flawed technique) held in the nail holes etc., I obviously need to roughen up the tearout bases for a better key.

Learnt a lot here, thank you Gentlemen.

[brit_in_oz]

Hi

I work as a development chemist for one of the largest adhesive and selant manufacturers in the world. The problem described is virtually garaunteed to be to do with the suface and not the product.

Epoxies (be they floor sealants or glues) work by a reaction between the resin (virtually always bisphenol A or a derivative), and a hardener / catalyst. If you dont mix enough hardener in then the reaction is slowed down (also caused by lower temperatures). It will, however, ultimately harden, but it can take a lot longer. The reverse is true as well, too much hardener will make the reacton speed up. By changing the catalyst or resin or the ratio's of the two in the tube then other types of epoxy can be made such as 'quick set' types

This site has a good explanation if you interested in thh chemistry
http://www.chem.rochester.edu/~chem424/epoxy.htm

Poor adhesion to a substrate is one of the most common questions we get asked / complained aout. The majority of the time it is down to poor surface preparation. Someof the commmon problems

1. Dirty surface - eg a layer of dust or sirt is inthe hole. The adhesive sticks the the dust and then is easily displaced

2. Surface contaminent, eg a chemical coating. Epoxies dont stick too well to a lot of polymers, they will peel off as in your case This could be a varnish or a penetrating sealant. These form 'low energy' surfaces, (the concept is similar to non stick frying pans) and prevents a bond been formed. the other thing that can happen is that the surface is not wetted out, so the product cures without actually reaching the wood surface .

3. On a clean timber, free from large amounts of resins (eg some oily timbers) most adhesives will bond perfectly. This includes polyurethanes, PVA, solvent based construction adhesives, water based acrylics. In the testing we perform when properly prepared in hearly all cases we find 'fibre tear' , ie the wood surfaces fails or is ripped out, not the glue that fails.

Hope that helps a bit .

[DPB]

Brit, it seems you are qualfied to comment on adhesives.

As I read the previous responses in this thread, based on experience I found myself doubting the frequently stated "rule" that mixture proportions were critical i.e. "Incorrect ratio of the two parts. As Alex said, epoxy needs to be mixed EXACTLY correctly, not guessed. For measuring small quantities I use syringes and measure to an accuracy of about one cc."

I have to confess I have always just eye-balled my proportions and its always worked.

Your comments on this seem to confirm my doubts about the validity of a number of the responses. Have I got this right?:confused:

[brit_in_oz]

Yes your correct, approx will normally do, however, dependingon the type of glue approx in some cases will be better than others, for example a fast curingproduct will be a lot more susceptable to errors than a slow curing one. The worse that willhappen is that it goes off too fast, or that it stays tacky for a long time.Same goes for builders bog type filler, too little hardener and it will take ages to cure, too much and it will go off too quickly

[bitingmidge]

Brit,

You may indeed be chemically correct, but your "near enough is good enough" scares me!! :eek: .

I've seen too many serious problems from incorrect mixing to sit back idly! (It's not a pretty sight watching guys chip "rubber" off a biaxial glass layup on a 40' boat, because they got the mix a "bit" wrong!

It may indeed go off given enough weeks or months, but will it perform to the desired capacity? I have had chemists say no!

I am not qualified to comment on the chemistry, but agree that to my knowledge most epoxies use the same A resin base, however depending on purpose, all have different admixtures and diluents, and different catalysts.

Surely you are making a bit of an over generalisation?

(Don, my DEADLY 1cc accuracy is for small quantities say 15ml max NOT litres at a time, where accuracy is a lot easier to achieve )

I haven't found a manufacturer who will guarantee a product not mixed in correct ratios, or properly and thoroughly mixed, and in my very short "hands-on" time in the industry, the ONLY problems I encountered were as a result of either of the above, chemically correct or not!

I am not having a go at you, I would like to hear your response about the actual rate of cures, what happens to the unbonded bits if the mix is not correct, what is the impact on strength of the bond? I'm thinking if one needed a certain amount of glue, got the mix 10% wrong, then 10% of it wouldn't catalyze, leaving a substantial reduction in bond strength?

Or have the marine and aviation industries just got it wrong? (serious, not sarcastic question!)

cheers,

P


P

[brit_in_oz]

No offense taken,

I did not mean to imply you should be lazy with volumes, of course following the manufacturers instructions will always given the best results. The only thing I was trying to point out is that for a standard epoxy you do have some play with volumes, though i cannot say what those levels are.

Epoxy curing is a catalysis reaction, its not consumed in the reaction, therefore, the concentration determines the speed of reaction (in addition to the temperature). The problem can occur when different ratio's are used because the mobility of the catalyst is reduced as the reaction continues and the polymer formed,and so it is harder to get access to the next monomer to react with. This is pretty much why the ratios are carefully calculated in a epoxy glue to get the optimum collision rate.
If this was a catalysis reaction in a moving fluid, then it would all be as i stated, that a difference in the ratio would not affect the final outcome just take longer to occur.
I am happy to agree that in a thickened system this may not occur and you may get worse results, however, their is a window in which this will make little difference (though i cannot say what this is depending on product). The worse situation is too little catalyst not too little forthe above reason.

[bitingmidge]

I am happy to agree that in a thickened system this may not occur and you may get worse results, however, their is a window in which this will make little difference (though i cannot say what this is depending on product). The worse situation is too little catalyst not too little forthe above reason.

Thanks, that's what I hope I was getting at. I think while you are technically correct, unless we are chemists, practically we should strictly comply with the manufacturer's instructions!

Cheers,

P (and thanks for not taking offence!)

[stevebaby]

Did you clean the area you're filling with metho first? Also, proportions for epoxy have to be pretty well as specified - near enough is not good enough with it.

I use acetone to clean epoxy joins,particularly with resinous stuff like oregon or pine,or waxy stuff like spotted gum.If you have access to a TAFE library maybe they could get a copy of the Gougeoun brothers book on boatbuilding which has lots of info on epoxies.