Is 30 min epoxy stronger when fully cured as compared to 15 min epoxy?

I've always been told the longer the cure time the stronger it will be, is it true or not I don't know? The one rule I try to follow is room temp, if it's cold it will take longer to setup so I'll take my stuff indoors to cure if it's too cold in my hobby shop.

Yes

As already mentioned, the longer epoxy takes to cure, the longer it penetrates the wood you expect it bond or strengthen. The farther it penetrates, the greater the area/volume of the joint. The larger area strengthened, the stronger the structure.


I used to "autopsy" every crashed plane I could. You'd be amazed how easy it is to pull apart joints glued with 5-minute. 15-minute isn't half bad for strength. 30-minute seldom can be defeated. You wind up tearing away the wood near the joint.

30 Minute has more time to seep into the cracks and uneven parts of your wood, which would produce a stronger joint. 5 Minute is quicker and I have never had a joint fail on a plane and a wreck usually shows the glue joints all stayed together, it was the wood around them that gave up on impact. I usually use the 15 and 30 minute if I am putting something together and need more time to spread the glue. Good Luck, Dave

The strength of epoxy is determined by the length of the molecular chains that form as it cures. Epoxy does not get its major bonding power by physically grabbing a hold on the two pieces being fastened together but by the molecular bond of the atoms in the pieces being bonded and the epoxy atoms, very little to do with how well it soaks into the parts although that has a slight bearing on the strength of the bond. Yes, the slower cures are stronger than the faster ones as the molecular chains formed as it cures are much longer. Perhaps there is a chemical engineer reading this that can provide a more exact and accurate description of how epoxies work.

We don't need a chemical engineer because more words won't make your first sentence any clearer. The question was whether or not 30 is stronger than 15. Now that could need an engineer and he may take pages explaining all the if's, and's, and but's. That could be a waste however, since it's also simplifiable. For model airplanes, almost every blob of cured epoxy is immensely stronger in every way than what model airplane builders use epoxy to join together or reinforce.

So what is left to consider? Unless you're interested in having stronger joints or reinforcing (or lighter joints that are adequately strong), there isn't anything.

BTW Rodney, length of chains is spot on but not the only thing by far. The resins chosen have enormous influence over the strength of the cured epoxy. They choose the different components to influence the desired characteristics. What they put into the two tubes for 5 minute was chosen to give the strength and bonding (two different characteristics) they intend to market. It won't be the same components they put into 15 minute. Very often the quicker epoxies will exhibit far more flexibility when cured than the longer cures. But that still depends on what solids are in each and their rigidity etc.

As for modeling, one thing that will help save weight and insure adequate strength of whatever we epoxy, is how much wasteful overage we have after the epoxy has cured. We should always try to use as little of whatever epoxy we've chosen. Take the time to coat end grain if possible, and wipe away excess when you can.

BoiseBee, if you're looking to build faster without sacrificing strength, there isn't really much time to be gained between fast and slow epoxy. Thick CA however will save time and will often give far more joint strength as it will penetrate like slow cure to a great degree.

Some good info here even though it is quite generalized. I will have to agree with Rock here in that for most of the woods we deal with in what few kits we still have nothing beats having thin, med and thick CA. The only need I have for epoxy these days is when sheeting foam wings or when I am making/bonding to fiberglass parts. I stopped buying hobby epoxy decades ago and IMO any one to one epoxy is junk. These days I always have a gallon of quality laminating epoxy around that I can thicken up with some Cabosil if needed.

it is , but make sure you always rough up the joining surfaces

Spot on advice!

Bob

Yup