Rubbery 2-part epoxy

[Allan Bennett]

I've been having a spate of rubbery 2-part epoxy for the past few months.  I've been using the cojoined syringes, so measurement shouldn't be a problem, and I've been mixing it thoroughly for longer than the instructions state.  Anyone else experiencing this?

 

It started off with some Devcon 5-minute epoxy, which also was defective in that one of the two syringes was incompletely filled.  Then I switched to Gorilla epoxy, which was available from my local Halfords, with the same result.  At the same time as the regular Devcon I was also trying Devcon gel epoxy, also in a 2-syringe pack, and it set even more rubbery than the regular 5-minute stuff.

[PeterF]

what temperature is the location where you allow the epoxy to cure, if it is too cold this can be an issue.

[Martin_K]

Are you seeing localised soft/sticky patches? Then not thoroughly mixed.

 

Are you seeing the whole mass of glue not set hard? That would be difficult to explain other than by wrong proportion of hardener or contamination.

 

I have not experienced what you describe. I mix two part epoxy in old milk bottle caps, stirred with a small steel screwdriver blade. My preference is a product with a ~20 minute setting time.

Edited November 1, 2021 by Martin_K

[Allan Bennett]

Mixing is not the issue, for the whole mass remaining on my mixing tray (margarine tub lid) is the same.  I was wondering whether maybe the plastic was contaminating it -- I changed margarine brands a few months ago, and its tub feels less brittle than the previous one.  I must try mixing on something else.

 

My workshop is an unheated garage, which at the moment is probably about 15 degrees C.  This problem started a couple of months ago, when it was probably in the low 20s.  In fact in the past, in the dead of winter, I've only noticed much longer curing time, but not this failure to harden.

[Don Fry]

That is a mixing issue. Simple fix,get a set of scales, accurate to reading in 0.1 grams increments, and use separate bottles. The scales are not expensive on EBay, and have all sorts of uses.

Those double syringe units are useless. And good scales and two bottles will be cheaper in the long run

[Dave Garrett 1]

Hi Allan

 

The only time I have experienced rubbery epoxy sets is when using the twin syringe packs. When mixing small amounts typically used by aeromodellers, the twin pack syringes don't seem to reliably eject the correct proporsion of resin & hardener. I swapped to the larger 2 bottle packs (Z epoxy or HK) & use Mk1 eyeball to judge 2 equal size puddles of resin & hardener before mixing. 

 

Regards

Dave

[John Minchell]

I have found it is the 5 and 30 minute varieties which set rubbery.  24hr old fashioned Araldite or new Locktite Hysol 9466 are fine and set hard properly.

John M

[Allan Bennett]

Don Fry and Dave, I don't think mixing is the issue; I do sometimes have problems getting one or other of the two components to come out of its syringe even though both sides should dispense equally, but when that happens I discard whatever came out first, and rely on a visual assessment of equal puddle size before mixing it.  In colder weather I heat the resin before dispensing, to help it come out.  When using separate tubes (which I think I'll go back to) visual judgement of puddle or strip size was good enough for a solid set.

 

John M, you've experienced the same problem as me?

 

 

[Martin Dance 1]

I've often questioned my use of epoxy adhesives in the construction of model aircraft, especially on wood to wood joints. Epoxy doesn't tend to penetrate wood well especially at low temperatures. I'm not sure that the strength gain over other glues makes it worthwhile. However through mixing is essential and careful measuring out of each component is important. In that context I've never had any real issues dispensing equal quantities from double barreled dispensers unless the glue is very old or it is cold. So don't expect great results when working in your unheated workshop over the winter build season. I would suggest that a temperature of 20 degrees C (60 degrees F)  is a minimum to ensure glues set properly.

[Alan H]

I have also found 5 min quick setting epoxy to be rubbery and no longer use it. My go to epoxy is Devcon 2 ton which is (I think) nominally a 30 min epoxy and always sets good and hard. Like all epoxies it requires thorough mixing.

[Nigel R]

34 minutes ago, Martin Dance 1 said:

I'm not sure that the strength gain over other glues makes it worthwhile.

 

My experience, CA and PVA, both seem to result in failure of the wood more than failure of the joint. If the joint fits properly then most any glue works well.

 

Just to add to the experiences, I've only had rubbery soft end results from what appeared to be a defective double syringe pack from a pound shop. Most reasonably priced double syringe dispensers have worked ok for me. I'm not a big epoxy user though.

[Allan Bennett]

I've been using epoxy on a depron build because the alternative, Uhoo Por, is difficult to sand.  It creates rubbery ridges at the joints.  I agree that almost any glue is stronger than the balsa, so for balsa models I usually use cyano or PVA wood glue.

[Robin Colbourne]

I have had rubbery five minute epoxy in the past too.  now I generally use 24hour as I have more faith in it.

The problem wiht the twin syringe dispensers is that the two parts tend to have different viscosities, so its very easy to end up with an imbalance.

Secrets of a good epoxy joint:
 

1. Wear disposable gloves so there is no chance of getting fingerprints on parts to be joined
2. Clean the surfaces to be joined with IPA, or at a push, Methylated Spirit
3. Fully abrade the mating faces
4. Clean again with IPA or Meths
5. Dispense equal quantities of epoxy (or in the manufacturer's stated ratio, if not 50:50)
6. Warm the two piles gently with a hair drier just enough to get both piles runny
7. Mix thoroughly
8. Quickly apply to both surfaces, ensuring full coverage.
9. Join parts and clamp,pin  or weight them to hold them in place
10. Warm the joint and parts again with the hair drier to reduce the mixed epoxy's viscosity and ensure full penetration into the abraded surfaces,  The epoxy should be runny, but not bubbling
11. Allow to set without disturbance
12. If possible, post cure the joint, whilst is not under load, by heating in as hot an environment (60°C to 80°C) as possible for 12 hours.  This helps ensure full cross linking and increases the temperature at which the joint would go soft again in service
    
    Note:   If bonding birch plywood, this has a release agent which must be removed with solvent for a satisfactory bond.  Likewise if bonding epoxy over cured epoxy, the epoxy must be cleaned with solvent and abraded, as it too can have an oily layer on top. 
    
    I still get good results using Araldite made in the 1970s, so if you are conviced you are doing everything right and your epoxy still isn't satisfactory, contact the manufacturers and tell them, including batch numbers etc.  You never know, they may send you a replacement pack.

 

Edited November 2, 2021 by Robin Colbourne

[Don Fry]

That’s a dert great list, to glue two bits of wood. Let it set, perhaps a couple of days before stress. Stress as in fly, rather than build stress. Me, never got one finished that quick.

Take your point, abraded joint is good.

[Allan Bennett]

20 minutes ago, Robin Colbourne said:

I have had rubbery five minute epoxy in the past too.  now I generally use 24hour as I have more faith in it.

The problem wiht the twin syringe dispensers is that the two parts tend to have different viscosities, so its very easy to end up with an imbalance. . . . .

 

In theory, whatever the viscosities, both sides should dispense equally because both plungers can only move down together.  But I agree, in practice an imbalance can happen.  I find that by pressing harder on the most viscous side, and holding the pressure, rather than trying to push down further, usually results in the thicker side catching up with the thinner one.  But I'll be going back to separate tubes, maybe the 30-minute stuff instead of 5-minute.

[alan p]

Had it happen to me the other day, separate bottles equal amounts never cured.

Two years old epoxy, but mancave was cool. Took a lot of cleaning up, used 5 minute afterwards no problem. 

It will happen occasionally?

[i12fly]

Only rubbery epoxy I've experienced has been when there has been too much hardener in the mix, I ensure that the mix is equal or slightly more resin and not had a problem.

Having said that I had some 3minute pound shop syringe type, when it was hot during the summer that was extra runny when mixing and it never set even after 2 days.

If a strong joint is required I only use long setting (blue pack) Araldite.

[Cuban8]

Haven't experienced rubbery epoxy for many years - the old SP113 laminating stuff was particularly prone to it no matter how you mixed it etc. I avoid the cheap five minute stuff like the plague - if anything, it sets very brittle and is not what you want. I know it's expensive, but since I bought some Z Poxy the difference in performance is vastly superior to anything I've used before. I find the 30 minute setting type a good all round performer.

Edited November 5, 2021 by Cuban8

[Allan Bennett]

18 hours ago, Cuban8 said:

. . . .  I avoid the cheap five minute stuff like the plague - if anything, it sets very brittle and is not what you want. . . . 

 

That's an interesting comment.  How hard should cured epoxy be?  Maybe my description or my percieved issue is misleading/inaccurate -- the cured epoxy remaining on my mixing tray is flexible, maybe describing its consistency as something akin to 'hard polythene sheet' would have been better.  Are you suggesting that's how it should be, not brittle as I've experienced previously?

 

I've had plenty of experience of laminating resin, and it sets nice and hard.  Its residue in the mixing cup feels like a solid block of clear casting resin when cured ?

[alan p]

If using it in high stress areas you will need some flexability to absorb the loads (twist bend etc). To hard and brittle your component parts will go there separate ways when subjected to load, the complete opposite to what you are requiring.

Also sanding is put at a disadvantage with the surround material if the 2 extremes are used.

[Martin McIntosh]

I tend to agree with AD. Why use epoxy at all on simple wood to wood joints? Even my firewalls which are usually balsa to ply are done initially with thin Zap which really penetrates each type of wood. Some triangular balsa glued on with medium ca completes the job. I certainly do use epoxy where required but only Z Poxy, Devcon and Araldite are very old hat. Sometimes I heat 5 min stuff with a heat gun after the first mixing which ensures that it is mixed properly and sets very quickly.

In my experience epoxy only goes rubbery after time which is another good reason not to use it too much.

[Cuban8]

22 hours ago, Allan Bennett said:

 

That's an interesting comment.  How hard should cured epoxy be?  Maybe my description or my percieved issue is misleading/inaccurate -- the cured epoxy remaining on my mixing tray is flexible, maybe describing its consistency as something akin to 'hard polythene sheet' would have been better.  Are you suggesting that's how it should be, not brittle as I've experienced previously?

 

I've had plenty of experience of laminating resin, and it sets nice and hard.  Its residue in the mixing cup feels like a solid block of clear casting resin when cured ?

I've used 5 min epoxy for other non-modelling purposes and have noticed that on a number of occasions the glue fillet itself would fail like brittle plastic if flexed. Might be alright for fixing a broken vase or whatever, but maybe not where toughness is required at a firewall or wing seat for example. Just look at the performance (or lack of) of some of the adhesives used in some ARTFs so goodness knows what they've used. I've been using Bob Smith brand epoxy for years and never had an issue with it in terms of setting or strength - I only tried Z Poxy when Bob Smith's became  unavailable from Hobby King. I'll stick (sorry) with Z Poxy in future.

I've never had an epoxy joint go soft once it's set and cured correctly - so Martin's observations are interesting.

Edited November 7, 2021 by Cuban8

[leccyflyer]

Agree with Martin. I only really use Z-Poxy these days as it gives consistent results and it does not seem to be super critical in terms of exact mixing quantities -mixing by eye with as close to the same size blobs of hardener and resin works for me. It's also excellent for mixing with microballoons either as a gap filling adhesive or for fabricating parts.

[Martin McIntosh]

Whether it softens over time seems to be affected by the environmental circumstances. I was thinking of when I have glued a pan handle back together or used it for a silencer repair.

[Allan Bennett]

5 hours ago, Martin McIntosh said:

Whether it softens over time seems to be affected by the environmental circumstances. I was thinking of when I have glued a pan handle back together or used it for a silencer repair.

Doesn't epoxy melt with heat?  I certainly wouldn't think of using it for a silencer repair.