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Long term integrity of resorcinol glue

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Scott Black

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Jul 21, 2019
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I have a Jodel built in 1969 which I bought in 2009. Resorcinol was used throughout. It has about 1200 hrs on it. It has always been hangared and I inspect the wing structure every year to make sure no moisture has entered the structure and I keep the drain holes clear.

Recently one person has been saying things like “I wouldn't trust those glue joints, I wouldn’t fly in that” etc. Of course those sorts of statements are meaningless In that the fact that a certain person would not do something has little to do with how dangerous it really is. As far as I’m concerned the airplane is as strong as it was 51 yrs ago. I have never heard anything regarding resorcinol breaking down over time like some of the other early glues. But just because I haven’t doesn’t mean it hasn’t happened. I think I need to be careful about this and find more information.

I am coming close to completing another airplane and soon it will be time for somebody else to enjoy the Jodel. How do I convince myself and a potential buyer that my 51 yr old airplane is safe? Is there any research out there on the long term strength of this glue? Are there instances of it failing? Symptoms to look for? My understanding is that there are lots of emeraudes, jodels and falcos flying around that were built with resorcinol and are continuing to operate without issues. Does anyone know where I could find some evidence one way or another?

thanks

Scott Black
Montreal
C-FSIC
 

karmarepair

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Resorcinol is approved for use in STRUCTURAL laminations for building construction, of presumably infinite life. Somewhere, in the technical literature, there are studies backing this up, but I could not find one in publicly available free literature in the 5 minutes search before suppertime tonight.

From the Aerodux literature: "Aerodux 185 liquid phenol-resorcinol resin adhesive mixed with a powder hardener provides cold-setting weatherproof and gap-filling adhesives especially suited to the manufacture of exterior high hazard structural components as defined in BS 5268 : Part 2. The adhesives are also suited to the production of heat resistant composite structures, e.g., fire-resisting doors. Aerodux 185 mixed with powder Hardener HRP.150 or HRP.155 meet the requirements of EN 301 - Type 1 and BS 1203 (Type WBP). Aerodux 185 with hardeners HRP.150 and HRP.155 has been tested according to the German Standard DIN 68 141 by the Otto-Graf-Institut (MPA) in Germany and found to suited for gluing load-bearing wooden structures for indoor and exterior use in accordance with DIN 1052. Aerodux 185 with Hardener HRP.150 and HRP.155 is also approved by Luftfahrt-Bundesamt for the production of of glued wood products to be used in aeroplanes. Aerodux 185 with HRP hardeners, when fully cured, is resistant to acids, weak alkalis, solvents and boiling water."
 
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Wanttaja

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I have a Jodel built in 1969 which I bought in 2009. Resorcinol was used throughout. It has about 1200 hrs on it. It has always been hangared and I inspect the wing structure every year to make sure no moisture has entered the structure and I keep the drain holes clear.
FAA Advisory Circular AC-43.13-1B, "ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICESAIRCRAFT INSPECTION AND REPAIR", Page 1-4:

(4) Federal Specification MMM-A-181D and Military Specification MIL-A-22397 both describe a required series of tests that verify the chemical and mechanical properties of resorcinol. Resorcinol is the only known adhesive recommended and approved for use in wooden aircraft structure and fully meets necessary strength and durability requirements.

(Emphasis added)

Ron Wanttaja
 

TFF

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All wood airplanes have this question. Resorcinol is the only approved glue right now. Epoxy is maybe better, definitely easier, but it will never be directly approved because no one will pay for the FAA proving. They would never get their money back. If your plane was before WW2 with horse glue, no; it’s held together with the nails. Resorcinol is a hard glue to use correctly; modern epoxy is stupid simple. Instead of glue, how was the plane treated? How well was it built?
 

joolkano

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The Bellanca Viking's wings are made of wood and the glue used is resorcinol. Here is an excerpt from Aviation Consumer article which said...
"For the most part the wings are held together with resorcinol glue. Resorcinol has proven to be extremely durable and long lasting. It is not affected by chemicals or moisture, plus it differs from epoxy in that it isn’t affected by heat. This allows Viking wings to be painted in any color without degrading the structural integrity of the wings. There is no service life limit on the Viking wings.
1603422663440.png
 

Derswede

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I am sure lots of us have flown planes that are 60+ years old with wood wings that still are holding together just fine. One of the Monocoupes we rebuilt was crashed on takeoff by Don Taylor. I was amazed how well the wing and how much of it survived the crash, esp. considering we received the (remnants of the) airplane stuffed into 4 crates. The postwar Chief I have been chasing, at wing recover, had no wood issues when the spar was checked. There are a few data points on resorcinal glue for consideration. "The proof of the pudding is in the tasting."

Derswede
 

Doran Jaffas

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Jun 25, 2019
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I have a Jodel built in 1969 which I bought in 2009. Resorcinol was used throughout. It has about 1200 hrs on it. It has always been hangared and I inspect the wing structure every year to make sure no moisture has entered the structure and I keep the drain holes clear.

Recently one person has been saying things like “I wouldn't trust those glue joints, I wouldn’t fly in that” etc. Of course those sorts of statements are meaningless In that the fact that a certain person would not do something has little to do with how dangerous it really is. As far as I’m concerned the airplane is as strong as it was 51 yrs ago. I have never heard anything regarding resorcinol breaking down over time like some of the other early glues. But just because I haven’t doesn’t mean it hasn’t happened. I think I need to be careful about this and find more information.

I am coming close to completing another airplane and soon it will be time for somebody else to enjoy the Jodel. How do I convince myself and a potential buyer that my 51 yr old airplane is safe? Is there any research out there on the long term strength of this glue? Are there instances of it failing? Symptoms to look for? My understanding is that there are lots of emeraudes, jodels and falcos flying around that were built with resorcinol and are continuing to operate without issues. Does anyone know where I could find some evidence one way or another?

thanks

Scott Black
Montreal
C-FSIC
If the mixture was correct and obviously it has been otherwise the airplane would not have stayed in the air as long as it has,, I would not worry about it at all. I've used resource and all when I built propellers and did not have an issue and also resource and all was used in boat building years ago period wood boats I'm talking about as in ships. If you were close to me I would say I'll take a look at it but I'm sure I would be fine flying it and though I have a lot of experience I'm still kind of a coward when it comes to structural issues. Again, I would have no problems with that at all.
 

Doran Jaffas

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If the mixture was correct and obviously it has been otherwise the airplane would not have stayed in the air as long as it has,, I would not worry about it at all. I've used resource and all when I built propellers and did not have an issue and also resource and all was used in boat building years ago period wood boats I'm talking about as in ships. If you were close to me I would say I'll take a look at it but I'm sure I would be fine flying it and though I have a lot of experience I'm still kind of a coward when it comes to structural issues. Again, I would have no problems with that at all.
Sorry about the text issues but I hope you get the gist of it.
 

BrianW

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Altus SW Oklahoma
I have a Jodel built in 1969 which I bought in 2009. Resorcinol was used throughout. It has about 1200 hrs on it. It has always been hangared and I inspect the wing structure every year to make sure no moisture has entered the structure and I keep the drain holes clear.

...
How do I convince myself and a potential buyer that my 51 yr old airplane is safe?
...

Scott Black Montreal C-FSIC
Perhaps it would help to invite comparisons with epoxy composite and epoxy bonded airframes? Composite airplanes are painted white for good reason: they go soft and weak when parked under a hot Sun. Resorcinol bonded structures do not suffer this defect.
Brian W
 

BBerson

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Is there any research out there on the long term strength of this glue? Are there instances of it failing? Symptoms to look for? My understanding is that there are lots of emeraudes, jodels and falcos flying around that were built with resorcinol and are continuing to operate without issues. Does anyone know where I could find some evidence one way or another?
Take a 51 year old sample from some place and test it and compare with new glue.
Resorcinol requires high clamp pressure.
 

TFF

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Willwood plastic resin has issues. Lots treated it like epoxy with little clamping pressure, and they would glue when too cold of conditions. Those will pop apart.

Just examining it by looking inside while someone tries to twist the wing. Depending on access you might be able to do a tap test on the spar. Just look at it close. If the glue isn’t holding, there will be some sort of evidence.
 

davidjgall

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Willwood plastic resin has issues.
Just to be clear, that is NOT the glue in question. Weldwood plastic resin glue is a UREA-formaldehyde glue and is not suitable for primary structure (I know, people do).

The glue in question is RESORCINOL-formaldehyde glue and IS suitable for primary structure.

The primary source for documentation on wood glues is the US Forest Products Laboratory. There are many scholarly articles there on Resorcinol-formaldehyde glue as well as Urea-formaldehyde and the epoxies such as FPL-16a (for which FPL stands for Forest Products Laboratory) and others. Interesting reading!
 

TFF

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Yes correct, but Weldwood was at one time considered suitable for structure. That is why I added that note with explanation. I also agreed above that resorcinol is generally not a problem. So as part of a complete answer to help give context on the type of glue that has given problems, I thought bringing up Weldwood was suitable.
 

Toobuilder

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Anybody know what wood wing Mooneys used (specifically, the 1958 M-20A)? I had one of those and was planning to restore it until I started picking at the wing and it just fell apart with the slightest peel action.

Hiperbipe suffered the same fate with the factory built wings. I was able to peel my wings apart, and after a very light pass with the sander, all wing parts were effectively re-kitted. It was bizarre.
 

Hot Wings

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I thought bringing up Weldwood was suitable.
And reasonable given the common misunderstanding due to the similarity of the names of the 2 different adhesives.

My take (AKA opinion) on this is that if the plane was built years ago using purple glue then it is going to be just fine structurally. It takes precise fit/pressure/environmental to make a good bond. If those parameters are not met the joint is as good as the wood - forever.
So in this case the older the plane the better the indication that the glue joints were done correctly.

I have 2 accident reports for the AV-36 from Canada. Both planes had low "G" inflight breakups traced to poor joints made with Urea-formaldehyde glue. At the time it was considered superior to the originally specified casein glue. Both were less than 10 years old, had less than 150 hours and 250 or less flights.
 

BrianW

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Just to be clear, that is NOT the glue in question. Weldwood plastic resin glue is a UREA-formaldehyde glue and is not suitable for primary structure (I know, people do).

The glue in question is RESORCINOL-formaldehyde glue and IS suitable for primary structure.

The primary source for documentation on wood glues is the US Forest Products Laboratory. There are many scholarly articles there on Resorcinol-formaldehyde glue as well as Urea-formaldehyde and the epoxies such as FPL-16a (for which FPL stands for Forest Products Laboratory) and others. Interesting reading!
Actually, the glue under discussion is phenol-resorcinol-formaldehyde (PRF) though resorcinol-formaldehyde (RF) may be categorized as the gold-standard.

This academic source is helpful:
Phenol–Formaldehydes
A. Pizzi, C.C. Ibeh, in Handbook of Thermoset Plastics (Third Edition), 2014
Resorcinol Adhesives
Resorcinol–formaldehyde (RF) and phenol–resorcinol–formaldehyde (PRF) cold-setting adhesives are used primarily in the manufacture of structural, exterior-grade glulam, finger joints, and other exterior timber structures. They produce bonds not only of high strength, but also of outstanding water and weather resistance when exposed to many climatic conditions [8,76,77]. PRF resins are prepared mainly by grafting resorcinol onto the active methylol groups of low-condensation resoles obtained by the reaction of phenol with formaldehyde. Resorcinol is the chemical species that gives these adhesives their characteristic cold-setting behavior. At ambient temperature and on addition of a hardener, it provides accelerated and improved cross-linking not only to resorcinol–formaldehyde resins but also to the phenol–formaldehyde resins onto which resorcinol has been grafted by chemical reaction during resin manufacture. Resorcinol is an expensive chemical, produced in very few locations around the world (to date only three commercial plants are known to be operative in the United States, Germany, and Japan)
 

joolkano

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Majority of the F8L Falco airplanes (production & kits) are made with Aerolite glue which is urea/formaldahyde and it is suitable as the primary glue for structure. Below is taken from the Falco builders letter.
1603558433755.png
But if you really want peace of mind, use Resorcinol for primary structures and maybe Aerolite or even epoxy for secondary support.
 

robertbrown

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Working on a 1941 Culver Cadet, the casein glue joints were in good condition except in spots where it was possible for water to collect. The problem is: how certain are you that you can inspect every fractional square inch of every glue joint? When wood aircraft were commonly stored outside in the 1920's-30's you might wind up with serious debonding or rot problems in a few years. You have the same inspection problem with corrosion on metal aircraft.
 
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