Strength of modern epoxy

[Natlek]

I've never made a knife with wood scales, but I was an adhesives / coatings chemist, and my superhero power is gluing to dissimilar materials so here is my 2 cents (agreeing with nearly all the comments so far). Epoxy is awesome when used correctly, especially more elastic ones like G Flex, because the wood will move tangentially when the steel will not, and this is important for longevity of the joint (why you don't want to over clamp the joint). It will likely hold for a long time, but its the constant months of expansion / contraction due to thermal and moisture forces that will cause it to eventually fail, even with the ingenious ideas of multiple holes bonding though, etc because once loose, then the epoxy "pins" will break.
My favorite adhesive for wood to steel is not epoxy, though. By far superior is simple PL Premium polyurethane construction adhesive because it stays more elastic than epoxy, soaks into wood like epoxy, yet has insane bond to steel. I have never seen joint failure on high torque loads using PL for wood to steel. Its similar to Gorilla glue, as many know, but has higher cohesion due to less foaming, and is cheaper (7 bucks for 10oz tube - goes in a caulk gun). I made half my boat with it (other half epoxy).

The trick with any adhesive (particularly epoxies and urethanes) is to heat the wood prior to application - it off gasses air and moisture, then the heat lowers the viscosity allowing deeper soak in, then as the wood cools it literally sucks adhesive deeper into its pores as it returns to ambient pressure. Then the heat kicks off the epoxy faster. All around underutilized great tip if you have a heat gun.

This is knife I make for my son .Wood is black ebony . Spine is 5mm thick . I cut the thread on the tang and used this /M4 thread/ screw .Instead of epoxy I used polyurethane seal I use in service for engine oil pan .If that thing can last 30 years on oil pan...................

Now , this is Parang I make this summer .I could not find then the right rubber for handle , so I temporary made one from walnut and glued with epoxy .I did not used pins then , easier to peel off when I find right rubber to make scale . After three fishing trip and heavy used , right scale has fallen .... I have no picture from then ..Then I make scale from rubber and used polyurethane . ......and rubber pins Since then after many multiday fishing trip and cut a lot wood for fire ...No problemo

 

[jdm61]

Ken, I have been told that unmixed resin and catalyst have a recommended “shelf life” perhaps as long as ten years, but I, like you, have seen cold molded wood/epoxy boats form the 60’s and 70’s and had one guy tell me that he cut a plug out of a 1970’s hull to put in a new sonar transducer and the laminate looked just like a new boat.

I've heard that from another maker, that epoxy has a limited life, and some have even said it's designed to fail. Epoxy is going to last a lifetime and more without a chemical failure. Just think of the boats that are around. My 40 ft sailboat was built in 1964 and I'll assure you the hull was just as strong when I sold her in 2009 as the day she was launched. I've posted emails and reports from at least 3 different manufactures of epoxy (West System who makes gflex was one), who'd never even heard that epoxy had a limited "hold life" - other than fail due to UV light or being exposed to excess heat. Seems like the tech engineer from West Systems said Lloyd's of London rated their epoxy for 100 yrs in boat building - don't quote me on that, but it was a LONG time.

Ken H>

 

[Justin Schmidt]

I've never made a knife with wood scales, but I was an adhesives / coatings chemist, and my superhero power is gluing to dissimilar materials so here is my 2 cents (agreeing with nearly all the comments so far). Epoxy is awesome when used correctly, especially more elastic ones like G Flex, because the wood will move tangentially when the steel will not, and this is important for longevity of the joint (why you don't want to over clamp the joint). It will likely hold for a long time, but its the constant months of expansion / contraction due to thermal and moisture forces that will cause it to eventually fail, even with the ingenious ideas of multiple holes bonding though, etc because once loose, then the epoxy "pins" will break.
My favorite adhesive for wood to steel is not epoxy, though. By far superior is simple PL Premium polyurethane construction adhesive because it stays more elastic than epoxy, soaks into wood like epoxy, yet has insane bond to steel. I have never seen joint failure on high torque loads using PL for wood to steel. Its similar to Gorilla glue, as many know, but has higher cohesion due to less foaming, and is cheaper (7 bucks for 10oz tube - goes in a caulk gun). I made half my boat with it (other half epoxy).

The trick with any adhesive (particularly epoxies and urethanes) is to heat the wood prior to application - it off gasses air and moisture, then the heat lowers the viscosity allowing deeper soak in, then as the wood cools it literally sucks adhesive deeper into its pores as it returns to ambient pressure. Then the heat kicks off the epoxy faster. All around underutilized great tip if you have a heat gun.

I'll have to check out PL. What exactly is it?

 

[John Blazy]

I'll have to check out PL. What exactly is it?

Its construction adhesive in tubes used by carpenters, so you wouldn't really think its for other uses. Wrong. I've actually veneered high end cabinet doors with it over melamine because it virtually welds to melamine. My number one adhesive of choice for porous plywood edges, composites, phenolic, etc. You can glue wood to PVC, ABS, acrylic or Lexan by doing a wet-on-wet application by applying the wet PL onto the wood (or whatever, steel, glass FRP) then applying PVC cement or acrylic solvent cement to the plastic, and while wet, lightly clamp both together. Its such a great bond that I have glued plywood to the inside of veneered PVC pipe and my 100 lb hall mirrors still hang on them to this day (I used to be a high-end furniture builder). When I did break tests, the wood failed, not the joint. I've been using it since 1989, when it was called PL PRemium, but I think it just called PL Polyurethane in big box stores. Anyone that uses Liquid Nails anymore is living in the stone age, because its solvent based, and shrinks upon cure, therefore less adhesion. PL expands on cure, has no solvents and is in the class of moisture cure PUR's like Gorilla glue, which has been a revolution in adhesive technology.

 

[Bill DeShivs]

Whether epoxy hold up on boats made of wood is of no matter here.
We are talking about gluing handles to a piece of steel, not wood to wood, not construction.
Knives get dropped, stuff-including rust, gets under the handles, steel and handle materials don't move at the same rate.
Mechanical fasteners always work.

 

[Justin Schmidt]

If

Whether epoxy hold up on boats made of wood is of no matter here.
We are talking about gluing handles to a piece of steel, not wood to wood, not construction.
Knives get dropped, stuff-including rust, gets under the handles, steel and handle materials don't move at the same rate.
Mechanical fasteners always work.

No one is disputing that. If your using a knife in such a way that gflex epoxy and a unpeened pin can't keep the scales on your using the wrong tool for the job. Or abusing the knife. And with gflex it us water proof so there shouldn't be any issues with rusting

 

[jdm61]

The comments about epoxy having issues with metal are interesting considering that epoxy has been used to bond metal in the aircraft and automotive industry for perhaps 50 years.

 

[Bill DeShivs]

Aircraft aluminum is not steel. And I doubt any structural bonding is being done with epoxy-maybe it is.
Steel and handle materials expand/contract at different rates, as does epoxy. This breaks bonds.
I use epoxy for lots of things-handles included, but if I want to make sure the bond isn't broken you can bet your arse I use peened pins, screws, cutler's rivets, Corby bolts, or similar. I don't see how anyone can even argue this.

 

[Brock Cutlery]

So I'm working on just my 2nd knife and I've ruined 3 or 4 knife shaped pieces of steel and more pieces of walnut scales. I don't have a drill press so I have a VERY difficult time lining the holes up on the scales properly. ...

Getting a drill press shouldn't really be a problem. I go to garage sales about every saturday, and I see these things at least once a month.
I've bought and sold several this year (paid $100, got over $200) each time for floor models.
But even the little cheapo benchtops would work for your needs, and I can pick these up for 25-30 bucks pretty regularly.
I also find handle materials and other tools, so it's really quite a fun way to spend an hour or two on Saturday.

I love a good technical test, but just saying, maybe you are overthinking the problem?

 

[john april]

Why not? Traditional Katana handles are held on with one bamboo peg and friction.

and the wood tsuka is fastened with rice glue, which is just mushed up rice ! i guess rice has a longer holding time than gflex lol

 

[Ken H>]

Ken, I have been told that unmixed resin and catalyst have a recommended “shelf life” perhaps as long as ten years,

I should have been more clear - talking about "cured" epoxy, NOT uncured epoxy. Uncured stored in separate containers it certainly does have a shelf life, especially depending on temperature stored at.

 

[Ken H>]

Whether epoxy hold up on boats made of wood is of no matter here.
We are talking about gluing handles to a piece of steel, not wood to wood, not construction.

Same difference, if cured epoxy had a life before chemical breakdown, it would be breaking down in boat construction as well as when used on a handle. For the record, I use corby screws as well as epoxy on knives - well, except for folders. Don't use corby screws on slipjoints. While I think a pin would provide sufficient holding, I like the ease of corby screws.

 

[DanF]

Interesting debate, I don't disagree with anything being said, but I keep thinking about, "an ounce of prevention is worth a pound of cure". In this case, "a two buck Corby is worth saving a $75-$100 scale repair job", at least for something I'm selling and never want to see returned to the shop.
For personal use, I'll give the PL a try though.

 

[Horsewright]

Lost in the corrals. Two months buried in cow manure:

Still took a bandsaw, chisel and grinder to get the handle off even after that:

Reground, retched, rehandled and shipped er back home:

There was an epoxy in common knife making usage some years back that did seem to have a built in failure life of a couple of years. The company "off the record" admitted that they did it to sell more epoxy. I can remember Ed Cafferty's forum having long discussions about it back then. It happened to me, had several failures right at two years. Switched epoxy and have not had a failure since and its been many, many knives, even when they get buried in cow manure for a couple of months.

 

[Busto]

If

No one is disputing that. If your using a knife in such a way that gflex epoxy and a unpeened pin can't keep the scales on your using the wrong tool for the job. Or abusing the knife. And with gflex it us water proof so there shouldn't be any issues with rusting

As a knife maker it's not up to me as to how the customer will use his knife however it is my responsibility to provide a "TOOL" that he can use with confidence. If you are in a situation where the only tool available is that knife and it is going to be abused then so be it kill that sucker and send it back I will gladly fix it knowing you were able to perform a life saving task. If you are pinning your hopes and reputation on just using epoxy for handles you better do a lot of testing because there are some Knife Destroyers out in the Wild that enjoy a good Challenge...

 

[12345678910]

Aircraft aluminum is not steel. And I doubt any structural bonding is being done with epoxy-maybe it is.
Steel and handle materials expand/contract at different rates, as does epoxy. This breaks bonds.
I use epoxy for lots of things-handles included, but if I want to make sure the bond isn't broken you can bet your arse I use peened pins, screws, cutler's rivets, Corby bolts, or similar. I don't see how anyone can even argue this.

The newest jets are all glued together.

However on knife builds, I still like a minimum of a straight pin epoxied in for shear strength.

Corby's are better, but only if you've got the skills to not burn them and the epoxy around it.

 

[daizee]

Yeah, a used drill press costs about as much as a batch of G-flex. It's an invaluable tool in my shop, and 98% of the time I use the 1/5HP benchtop craftsman that cost $25 at a yard sale.

I've pulled apart handles done with 2-ton Devcon. I heat 'em in the toaster oven to ~250-300F and then knock the pins out with a punch and peel the handles off. I suspect the G-Flex (that I've now used for several years) would be substantially tougher, but I'd go the same route.

I've also done quite a few stick tangs, mostly scrap knives, without pins. I've always notched the tang with a file to give the epoxy purchase against pull-out, but my new pattern is to use a drill (could be hand-drill) to put shallow divots in the flats of the stick tang instead. I feel this may reduce stress on the tang and serve the same purpose.

So if I wanted to do epoxy-only, I'd go the stick tang route. You could probably drill your handle starter hole with a hand-drill, and then fit the rest (plus bolster).

But in all of these cases, I wouldn't build a hard-use knife with only epoxy. If you can't have the tools to build to that spec for some reason, than design different types of knives for different uses. If you're set on hard-use knives, that's great - but it's probably time to invest in a drill press.

 

[golfer1]

Love the idea of the poly u. I have used that caulk a lot on construction projects and it is tough as nails. It bonds to everything and you can not pull it away, it has to be cut. It is not bothered by weather or sunlight. I'm going to give it a try. Thanks

 

[Justin Schmidt]

Interesting responses from everyone. Well the test is done. And I'm kind of torn.
So I started this test off by throwing the knife on the garage floor like I just scored a game winning touch down. Nothing. Did that about 7 or 8 times. Again, nothing. Alright time to step it up. Let's "baton" with it. By baton I mean place half the scales on the anvil leaving just one side hanging off. (Forgot to take a picture) and beat the heck out of it with a ball peen hammer. Again, nothing. Alright I'm bringing out the big guns.

Enter my 36in snap on prybar. I wedged it between the to scales and twisted in an attempt to pop them apart. Nada. So I decided to start beating the wedged prybar with a hammer. And finally the walnut scale cracked and I was able to pull one scale off.

Here's some of the spine damage from hammer blows and football spikes

This is where things got interesting

 

[Justin Schmidt]

With one scale off, the single unpeened pin became completely useless. I could have easily pulled it out. One scale off and one to go. I took this picture to show the epoxy formed a type of weld on the scale and tang.

so I took the loose pin and banged it into the stump my anvil sits on then hit the spine of the knife with the hammer. I was thinking the shearing force would separate it. I was wrong.
After about a dozen more bouts of me trying to throw this thing through concrete I noticed a small separation between scale and tang. So I grabbed the prybar again and finished it off .

All in all it took a good 45 min of non stop abuse to get both scales apart.
But when the 2nd one popped off the glue came off "clean"

The epoxy and tang were both very smooth. And you can see where the tang was in the scale and the epoxy "weld" .