30 minute epoxy.

[Geoff S]

As I sit here waiting for my Zap 30 minute epoxy to go off, I wondered to what the 30 minutes refers.  It certainly isn't the time to go hard because it's over 30 minutes since I glued to firewall to my replacement Wot 4 fuselage and the surplus on the mixing board (an old piece of ply) is still softish and definitely not cured (the remains of what I used yesterday, is cured, so there's nothing wrong with the product).  I'm guessing it's the time span it's still usable, but there's nothing that says as much.

 

Also, what determines the cure speed for epoxy?  I think the part labelled 'Resin' is the same for all types, and it's the 'Hardener' that determines how quickly it cures.  But it's a guess.

[Ron Gray]

4 minutes ago, Geoff S said:

Also, what determines the cure speed for epoxy?

Mix ratio and heat, the warmer it is the faster it will cure.

[john davidson 1]

Shelf life for poxy seems to infinite

[John Stainforth]

Epoxy setting times are very strongly temperature dependent. When I was living in Texas in a moderately air-conditioned house - internal temperature perhaps 28 + C in the summer - 30 minute epoxy would go off in about 10 minutes, so I was sometimes confronted with a race against time. Because epoxy setting reactions are strongly exothermic, one also has to beware of thermal runaways in any climate, if the epoxy is mixed in a small container with a small surface area: the mixed epoxy can heat up very quickly and set in a flash. This is particularly the case with finishing resins, which have to be decanted immediately after mixing, into a small tray with a large surface area to prevent this happening.

[Robin Colbourne]

31 minutes ago, john davidson 1 said:

Shelf life for poxy seems to infinite

Certainly in industry it is not unusual to make test pieces for peel and tensile strength to extend the life of epoxies reaching their use by date.  It is also common for samples of mixed epoxy to be stored for future analysis in case there is any doubt about its quality.

Getting the mix ratio right is critical.  Unlike polyester resins which have a catalytic reaction. two part epoxies rely on the adhesive mixing fully with the hardener.  If one part uses up all of the other and there is surplus, that surplus will stay in liquid or jelly form, weakening the resin.

For the best bonds:

• Weigh out the adhesive and hardener with an accurate balance.
• Warm the parts to be bonded so the mixed resin wets out (reduces in viscosity), rather than chilling on contact (If the material being bonded is porous, the cooling substrate will pull the epoxy in as it cools).
• Having applied the epoxy to the materials, gently warm it to further reduce the viscosity, which helps get any air trapped when mixing or spreading out.
• Once cured, do a post-cure for 12 hours at a higher temperature than the part will ever see in service.  If this isn't done,  there is a danger of the joint softening if warm and under load (e.g. near an exhaust or speed controller) and either failing completely or resetting in a stress-relieved form.
  
  Geoff S, in answer to your original question, the time on the faster setting epoxies (e.g. 5, 20, 30 minute) seem to refer to how long the mix can be used for, whilst 24 hour epoxy seems to refer to stuff that is set hard in that time.  Well, that is my experience anyway.

Edited March 12 by Robin Colbourne

[Dickw]

51 minutes ago, Geoff S said:

.........

Also, what determines the cure speed for epoxy?  I think the part labelled 'Resin' is the same for all types, and it's the 'Hardener' that determines how quickly it cures.  But it's a guess.

R&G in Germany provide a lot of information on their resins, and also sell different hardeners with different speeds for the same resin. Worth a look just for all the information.

 

https://www.r-g.de/en/list/Resins/Epoxy-resin/Epoxy-resin-L---Hardeners--laminating-resin-

 

Dick

[kc]

Really  thorough mixing might be the solution next time.   For this time the old trick of applying more hardener onto the joint might save the job.

[Paul De Tourtoulon]

I found that maybe you could use it for 30 minutes at 20°C, then it starts to go off.

[Don Fry]

Look on E bay, Chinese sites. Couple of quid gets you an 0.1 gram, max 500g electric balance. Then you can weigh stuff accurately. 
There is a law of chemistry, the rate a reaction proceeds doubles for every 10°C rise in temperature

[Geoff S]

Thanks, all.  It's cured OK now, as did the batch I mixed and used yesterday.  I was just wondering what the terms referring to time actually meant.  I'm aware it's all temperature critical.  About 40 years ago, I decked a racing dinghy hull over winter in the garage.  It's a concrete garage that defeated my attempts to insulate it and I wore so many clothing layers I could hardly work. I was trying to use West epoxy, but it was hopeless at times.  I can't believe my enthusiasm for the job. I don't think I could repeat the exercise now.

 

I tend to use slower curing epoxy/hardener types because I not only believe it makes a stronger, but it also gives more positioning time.

[John Lee]

12 hours ago, Geoff S said:

  I was just wondering what the terms referring to time actually meant.  I'm aware it's all temperature critical.  

The safety sheet for their 30 min finishing resin states ’6. Sets in 30 minutes at room temperature (77F or 25C) with full cure in 24 hours. Note - For cooler. temperatures, setting time may be longer.’

 

Apparently US room temperature is a bit warmer than those the UK!