# Long term integrety of Epoxy

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#### narfi

##### Well-Known Member
Log Member
I saw a headline last week they have invented a "super cool" white much like they have the venta black or whatever the name of the super black is. The super black is extremely good at absorbing color(light(heat)) and now the new white is extremely good at rejecting it.
I dont know any specifics, just saw the headline and thought "hmm"

#### ToddK

HBA Supporter
It's not a good idea to tell people where their priorities should lie as you wouldn't want others to tell you what your priorities should be. Priorities are neither right or wrong, it only matters if it serves them well. Cost will always be a big factor in airplane ownership. An airplane that is always hangered, maintained properly will always command more $than a similar one that wasn't looked after (obviously) whether it is made of wood, composite or metal. I personally have no concerns with a wooden airplane built with epoxy rather I am more concerned with how it was cared for. There are certainly priorities that are right and priorities that are wrong. The foolishness of relativism was proven long ago. A builder who is going to spend anywhere from 1200-2000 hours constructing a airplane better be doing so because its a compelling design that he plans on flying. Pretty much all experimental aircraft sell used for less or as much as the cost of materials. So yes, a person who is building from scratch and is worried about resell value because of his selected glue clearly has his proprieties wrong. The most important decisions in home built aviation construction is the selection of proven materials and techniques that the builder can confidently utilize. The old wives tails and studies that seemingly contradict decades of real aviation experience do not have a valid place in those decisions as they introduce an element of irrationality. #### joolkano ##### Member There are certainly priorities that are right and priorities that are wrong. The foolishness of relativism was proven long ago. A builder who is going to spend anywhere from 1200-2000 hours constructing a airplane better be doing so because its a compelling design that he plans on flying. Pretty much all experimental aircraft sell used for less or as much as the cost of materials. So yes, a person who is building from scratch and is worried about resell value because of his selected glue clearly has his proprieties wrong. The most important decisions in home built aviation construction is the selection of proven materials and techniques that the builder can confidently utilize. The old wives tails and studies that seemingly contradict decades of real aviation experience do not have a valid place in those decisions as they introduce an element of irrationality. The Vans RV-10 are routinely sold for a lot more than its cost to build (see here). An F8L Falco built in 1994 which was an Oshkosh 1995 Reserve Grand Champion, Kit Built (see below) was sold back in 2015 for$125k (I know because I lost out on that purchase) to someone from the UK and just heard he recently sold it within a week or two of listing it which I am sure for a good amount of money probably recouping his purchase price. So, not ALL experimental sell for much less of its build cost as you claim. In this instance you are wrong. If cost is not important to you then you're in the minority of aircraft owners who has that luxury.

But going back to the topic at hand. I do agree with you that it is about quality materials and construction. I don't believe Dave (builder of the Falco pictured below) was thinking about the resell value at the time he was building it. But, I am sure he knew if he build the airplane right when the time comes there will be people (like myself) willing to pay for his quality work. With Falco builders this topic of epoxy vs formaldehyde based glue is a big thing. Falco's made with epoxy although just as good sell for much less than its formaldehyde constructed brother; that's why there's not a lot of them. 95% of all Falco airplanes are built using resorcinol or UF because it is the glue of choice for wood aircraft.

The battle between epoxy vs formaldehyde as to which is the best glue to use is like asking who's better LeBron or Jordan (its Jordan by the way) will never end. So your desire to put this argument to bed will never come to pass. Many years from now someone will be reading this thread just like the last thread started back in 2004.

Lastly, there is no wrong or right when it comes to priorities. Your priority of being proven correct and that everyone should agree with what you're saying in this matter takes away from the rational point you're trying to make. This priority doesn't seem to serve you well.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Back on the topic of epoxy for assembling wooden airplanes. If we were to write the requirements for gluing airplanes together, they might be summed up as:
• Glue must be stronger than underlying wood, even when aged or warmed up;
• Glue joint must stay together over flight loads imposed by the entire flight envelope.
But maybe we can simplify our examination - Has anybody got an accident report indicating a wooden airplane that has crashed after epoxy failing in a joint? I suspect that we are dealing with a "tempest in a teapot" here. We have a lot of history with wooden airplanes assembled with epoxy, and no compelling failure stories. That makes me think we are discussing a problem that does not exist, at least in designs that have a history.

As for designing new airplanes using wood and epoxy, see the first two rules above, and then understand that glued joints typically have factors of safety not equal to 1.5 or 2.0 as are used in design of primary structure loads, but much higher, similar to what is done with bolted joints. FOS for glued joints exceeding 6 are common. Even with the joint losing some strength due to elevated temperatures, adequate strength still remains. For instance, a joint with a FOS of only 4 that has its strength halved by elevating temperatures still has a FOS of 2.0 over max in flight loads. In new designs, some care in the design of all joints must be taken to ensure adequately high FOS has been designed in. I suspect that good practice as exemplified in past designs will continue to be adequate.

Billski

#### Vigilant1

##### Well-Known Member
Agree with all Biilski's points. And, obviously, if there's uncertainty about the original FOS built into the joints and the rate of degradation due to various factors, it makes sense to just take any easily available steps to avoid cutting into those FOS margins.
As far as peace if mind and possibly future resale value, it probably makes good sense to include expendable test coupons inside the structure at an easily accessible spot. These will be exposed to the same heat and humidity as the structural joints. It's probably not practical to match the structural stresses, but it is better than nothing. Repeatable tests over time and tracking of trend data from testing the coupons to destruction would be useful information.

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#### ToddK

HBA Supporter
JoolKano: "Pretty much all" leaves plenty of room for exceptions to the rule. A "grand champion" selling for $125K is hardly representative of the majority of wood homebuilts. What you think about me and what I think, or if you think my comments are helping me or hurting me does not matter a bit to me or anyone else here. On the rare occasion the discussion descends into that sort of juvenile passive aggressive territory, I typically check out. Billski : I agree 100% that this is a tempest in a tea pot, my hope in starting this thread was that some of the engineer types who frequent this forum might be able to offer some insight and put an end to it possibly with some relevant empirical data that I don't know about (other then the lack of wrecked wooden airplanes cased by failed epoxy), or at least some really good ideas that might put builders/buyers minds to rest. Your comment is exactly that, and it makes a lot of sense. I appreciate it. #### joolkano ##### Member JoolKano: "Pretty much all" leaves plenty of room for exceptions to the rule. A "grand champion" selling for$125K is hardly representative of the majority of wood homebuilts.

What you think about me and what I think, or if you think my comments are helping me or hurting me does not matter a bit to me or anyone else here. On the rare occasion the discussion descends into that sort of juvenile passive aggressive territory, I typically check out.
I was responding to your statement that ALL experimental airplanes are sold for less than their build cost which clearly are not case. But, I am willing to move on. That's it for me on this topic.

#### BrianW

##### Well-Known Member
On another thread about glue for wood aircraft construction, several posted comments about epoxy softening at high temperatures, and the need for composite aircraft to be painted white. The implication being that epoxy is inferior to another glue.
///
It seems to be that concerns about epoxy are nonsense, and there are plenty of flying aircraft (including fast, and aerobatic aircraft) to prove it.
Now that a member has added an Epoxy Strength/Temp graph - I see that at Oklahoma Summer temperatures of 100 degF, epoxy strength is halved under dark/black tints. It's been my experience, flying down to the Gulf coast that Texas temperatures run a little hotter. I had better add another astounding fact for this incredulous Texas viewer: Solar UV degradation is also noted on bare epoxy composite structures.

Brian W <g>

#### ToddK

##### Well-Known Member
HBA Supporter
Now that a member has added an Epoxy Strength/Temp graph - I see that at Oklahoma Summer temperatures of 100 degF, epoxy strength is halved under dark/black tints. It's been my experience, flying down to the Gulf coast that Texas temperatures run a little hotter. I had better add another astounding fact for this incredulous Texas viewer: Solar UV degradation is also noted on bare epoxy composite structures.

Brian W <g>
Considering we are not discussing bare epoxy composite structures, I am not sure what that has to do with anything. Perhaps you have some helpful ideas about why there have been ZERO heat related epoxy failures on wood aircraft flying all over the county in all colors in the many decades epoxies have been used in aviation?

#### Vigilant1

##### Well-Known Member
... there have been ZERO heat related epoxy failures on wood aircraft flying all over the county in all colors in the many decades epoxies have been used in aviation?
Let's be fair. Nobody can accurately say there have been zero heat related epoxy failures. There have been zero heat related epoxy failures you know about. And that might be all you need to know to be comfortable. Great! But let's not pretend we'd know about planes that were discovered to have problems and didn't fly anymore. And let's not pretend that every structural failure of an E-AB gets a thorough investigation, and that knowledge is perfectly disseminated and recoverable.

Wood planes built with epoxy appear to be very safe in service.

The strength of epoxy decreases with temperature.

Certainty is the enemy of increased understanding. And the world ain't black and white.

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#### ToddK

##### Well-Known Member
HBA Supporter
Let's be fair. Nobody can accurately say there have been zero heat related epoxy failures. There have been zero heat related epoxy failures you know about. And that might be all you need to know to be comfortable. Great! But let's not pretend we'd know about planes that were discovered to have problems and didn't fly anymore. And let's not pretend that every structural failure of an E-AB gets a thorough investigation, and that knowledge is perfectly disseminated and recoverable.

Wood planes built with epoxy appear to be very safe in service.

The strength of epoxy decreases with temperature.

Certainty is the enemy of increased understanding. And the world ain't black and white.
Fair enough. I guess you could write the same about resorcinol. It might very well be the case the the failures due to poorly fit joints, or bad batches or other handling errors are not reported, although it can happen. After all you can't know everything for sure...

#### Topaz

##### Super Moderator
Staff member
Log Member
Conspiracy theories and "what everyone knows" are always really annoying. The only way to fight them is with facts and data, and even then, there are going to be people who argue with you and refuse to believe even the facts and data, or parse them in a way that supports their theory, contrary to all logic.

You can only do so much. You'll never completely win the battle. This was a good thread, especially for some of the arguments that rationally point out that there is no known evidence to support that there is a problem here, with data and rational reasons why that might be. Some people will still "fear" that there "might" be a problem here. You'll never change that. Seems to be human nature.

#### User27

##### Well-Known Member
I work with epoxy all day long repairing composite aircraft. My own view is (in general) epoxies used correctly are stronger than just about any other type of glue and so when bonding wood are more tolerant to poor working practices, as sometimes demonstrated by homebuilders. They are perhaps the easiest structural adhesive to use as they do not demand really tight fitting components or high clamping forces and their behaviour when setting up can be modified with a range of fillers. But they do have some health issues.

As with any type of chemical they are not all the same. Some (such as MGS 285) increase their properties markedly when they are "post-cured", heated to an intermediate temperature for several hours. Some (such as West 205) is not advertised to benefit much from post curing. That post curing does not have to happen in an oven under controlled conditions, but can happen on the ramp when the aircraft is first parked out in the sun.

All epoxies lose strength as they are heated, the amount depends on the particular epoxy. The T88 spec sheet says the shear strength decreases by 60% from 67F to 180F! If the temperature is too hot for too long (Tg is exceeded by any margin) and the structure is put under any load it may well fall apart. But Tg for any decent epoxy, after post curing, is above 200F, and sometimes significantly above. Does any one know what the internal temperatures in a non-white wooden structure are? Some composite aircraft manufacturers allow pretty much any colour as long as it is not black or very dark (usually when their structures have been post-cured to 170F).

I would use 285/287 to bond together a wooden aircraft (as long as I could post cure to at least 140F), and then paint any not too dark colour. The risk of any structural glue failure is likely to be very low.

#### ToddK

##### Well-Known Member
HBA Supporter
I work with epoxy all day long repairing composite aircraft. My own view is (in general) epoxies used correctly are stronger than just about any other type of glue and so when bonding wood are more tolerant to poor working practices, as sometimes demonstrated by homebuilders. They are perhaps the easiest structural adhesive to use as they do not demand really tight fitting components or high clamping forces and their behaviour when setting up can be modified with a range of fillers. But they do have some health issues.

As with any type of chemical they are not all the same. Some (such as MGS 285) increase their properties markedly when they are "post-cured", heated to an intermediate temperature for several hours. Some (such as West 205) is not advertised to benefit much from post curing. That post curing does not have to happen in an oven under controlled conditions, but can happen on the ramp when the aircraft is first parked out in the sun.

All epoxies lose strength as they are heated, the amount depends on the particular epoxy. The T88 spec sheet says the shear strength decreases by 60% from 67F to 180F! If the temperature is too hot for too long (Tg is exceeded by any margin) and the structure is put under any load it may well fall apart. But Tg for any decent epoxy, after post curing, is above 200F, and sometimes significantly above. Does any one know what the internal temperatures in a non-white wooden structure are? Some composite aircraft manufacturers allow pretty much any colour as long as it is not black or very dark (usually when their structures have been post-cured to 170F).

I would use 285/287 to bond together a wooden aircraft (as long as I could post cure to at least 140F), and then paint any not too dark colour. The risk of any structural glue failure is likely to be very low.
I have wondered about MGS 285. I know the tailwind guys have been using west 206 over T88 in and on their wings because it’s thinner and easier to work with. I guess using a laminating epoxy like 285 is ok for a structural wood joint?

#### Topaz

##### Super Moderator
Staff member
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The point is being lost that even a 60% reduction in shear strength is completely irrelevant if the FOS on the limit load of the joint is 4+.

#### pictsidhe

##### Well-Known Member
The point is being lost that even a 60% reduction in shear strength is completely irrelevant if the FOS on the limit load of the joint is 4+.
Not on me. But with other epoxies, you may lose 90% of the shear strength. Then it IS a factor on your 4x FOS joints. It is something to check. Grabbing any old el-cheapo 'poxy could be a deadly mistake.
Look up the specs of your West/T-88/Brand-X epoxy. Does it have 50% more shear strength than the wood at 180F. It does? Get with the Rolling Stones. Paint it black.
It doesn't? Hmmm, might want to be picky about paint, or choose another 'poxy.
Really, it doesn't take long to check something this. Tg varies enormously with available epoxies. Yes, I have seen stuff that is cheese at sunbaked temperatures. If you are good, stick with that epoxy and worry about the next issue.

#### Topaz

##### Super Moderator
Staff member
Log Member
It doesn't take long to check, no. But since T-88 is what most people are using, and it's fine, then the problem is really being overstated, even as a hypothetical.

#### BrianW

##### Well-Known Member
/// glued joints typically have factors of safety not equal to 1.5 or 2.0 as are used in design of primary structure loads, but much higher, similar to what is done with bolted joints. FOS for glued joints exceeding 6 are common.
///
Billski
There is a design concept sparkling here which can be briefly expressed in this way.
The behavior of metal structure is quite well understood. You would like it to fail at a stress of a 1G loading times the g multiplier to which you expect the plane to conform: -+ 6g, 3.8g etc., times a design factor of say 1.5 to account for loads that are not predicted to happen. And that leads to rather satisfactory structures: where the design force is exceeded AND the design factor is exceeded - the failure may well result in warped wing panels, droop and dog-legged bolts. (As you know an AN bolt is rather like an SAE grade 5 not grade 8 bolt because it is FAR more ductile and has an extended plastic range.)
And as Bill notes, composite structures like wood and polyester/glass, epoxy/glass and epoxy/carbon fiber do NOT have an extended yield characteristic - when overloaded, they may explode into dust at times, and so the airworthiness licensing authorities demand a much bigger design factor - say X4 - and those structures work rather well. In the case of glue and plastic substrates with relatively lower plastic temperatures, one would obviously like to NOT squander all that lovely composite design factor on cooking the plastic at high temperatures, and so people - even Texans - tend to paint their composite aircraft in white or light colors. <g>

Brian W

#### TFF

##### Well-Known Member
The people using Wests is taking advantage of being able to customize the properties of the epoxy. A good number of options. T-88 is about simplicity not sophistication. Mix and go. Both were designed for boats. T-88 is very popular for building wing ribs. Building a Tailwind means you have to glue a 4x8 piece of ply on top and bottom. As slow as T-88 is, it’s going to gel up gluing that much at once. There is no best; there is only options.