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A Sticky Subject - warning, LONG!

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Chris Knight

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I have recently become a bit paranoid about glues - blame the chair, I did not want it to fall apart!

I have conducted many tests now, generally crude in just about every which way but the pattern of results that emerges might be useful info for somebody. I should say however, that I am no expert and make no claims as to any real knowledge of the physics or chemistry of glues, so feel free to disregard everything I say!

My tests basically involved joining two planar faces of timber cut at various angles to the grain (eg 90 degrees- end grain, 0 degrees - long grain, mitred cuts 45 degrees for end/long grain) and joining these to other similarly cut or differently cut pieces, then after the glue had cured, breaking the joint apart to see whether the glue or the wood failed. Apart from the joining method (eg end grain to long grain), the tests also varied as to whether the joints had been specially prepared for gluing. For example, were they sanded, or planed in any particular way, were they wiped with a solvent before gluing and so forth.

In nearly all cases there was some wood failure. I guesstimated the failure I saw as a percentage of the joint area. Wood failure so observed ranged from 5% to 100% among the tests as a whole. I took wood failure to mean that I had achieved a good bond inasmuch it kind of represents a situation where I had solid wood rather than a joint - albeit solid wood of short or long grain with all that implies for strength.

So, wood failure was "good" and the oft-claimed benefit of XYZ glue "stronger than wood!" turns out to be true - WHERE THE GLUE STICKS!

Where the glue sticks turns out for me to be the crux of the issue. I found that any joint configuration including end grain to end grain - often said to be a bad configuration, can stick very well. As an aside, I should say immediately, I make no claims for longevity here, an end grain to long grain joint can be made to stick very well today but a couple of seasons of moisture exchange and the accompanying dimensional changes of the timber (which are impossible to stop completely) will render a good joint quite useless in time. This is why some writers say "EVERY GLUE JOINT WILL FAIL IN TIME" I do belive this but I also believe that the time involved can be very long if one respects the usual strictures on grain direction in one's joinery - hence my comments below on biscuits and dowels.

What I found was - the single most important factor in a good glue joint was to rough up the surfaces with coarse (100 grit) sandpaper just before gluing. This in no way creates any meaningful mechanical bond but it does two important things. First, it removes a good deal of grease and dirt that hinder gluing. Just handling your pieces after cutting them is enough to transfer glue-impairing oils or other contaminants from your fingers to the surfaces to be glued. It also helps remove natural oils as in Iroko that bleed to a fesh cut surface quite quickly. Second, it creates "hiding" places for glue that ensures not all glue is squeezed out if clamping pressures are excessive. The glue can still form its molecular bonds.

Wiping with acetone helps to glue difficult woods like Iroko and ebony but not nearly as much as sanding. Joint configuration is not very important to achieving a good glue joint if sanded first.

The most forgiving glue - in terms of bonding when conditions were less than perfect, was the set of PVA glues (eg Evostick, Titebond etc.) I think the fact that these glues contain water, which is an almost universal wetting agent, helps. Of the Polyurethanes, I found Balcotan to be far the best, Gorilla OK and Titebond to be useless. Of epoxies - to be used in the sort of quantity that say - making a table would require, Devcon 2 ton is the best. If needing more, then use the West System with suitable fillers (that ARE needed).

In short, I found that for 99% of gluing that Titebond in its various forms was best and they were all good but for indoor work where moisture is not a problem, I prefer the original. If slipperiness was needed for tight joints - as it was on my chair, then Balcotan Poly was best by a million miles.

All of the above opinionated findings are however based on "100 GRIT SANDED JOINTS" and this done just before gluing.


As to biscuits and dowels.

I see many folk using these for joining eg table tops. For reasons of strength, this is quite unecessary in that particular application - in my view and based on my tests. For alignment - well they can help and I won't argue, although I rarely use them for that. For joints specifically designed around their use as structural components ( and I use both biscuits and dowels for this at times) they are great.
 

Pete W

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Interesting info, Chris. Thanks for sharing (and for doing the research in the first place :)).

One bit of confusion to a newbie like me... could you clarify what you said in various places about the Titebonds. Although much of the confusion arises from Titebond producing so many variants - anything you can add to clarify would be great.

Pete
 

Alf

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Chris,

Hmm, interesting stuff. I'll adhere to your advice and stick close by the 100 grit in future. :D (I'm sorry, but did you really think I could resist? :oops: )

Cheers, Alf

I pun, therefore I am.
 

Chris Knight

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Pete,

Titebond does indeed come in various forms. The original Titebond dries fast and hard. Any squeeze out is rock hard and liable to chip planes or chisels used carelessly on it - very different from the rather flexible exudate of say Evostick Resin W glue.

Titebond II is designed to be water resistant (please note this isn't the same thing as waterproof!) has similar strength characteristices when cured. according to the Franklin(the mfrs.) data sheet - but the squeeze-out is relatively rubbery, exspecially so until 48 hours old.

Titebond Extend is supposed to be like the original Titebond with extended open time - it isn't available in this country (but I imported some). It seems to have a squeeze-out about half way between Titebond II and the original - it makes no claims of water resistance

Titebond III isn't yet used much anywhere - it has just become available in the USA and claims both water resistance and extended open time. I have not tried it yet.

All in all, I prefer the original. It is very predictable and although I would like a longer open time, Titebond Extend is so "grabby" when wet, that this benefit is largely theoretical in my view. A tight joint will bind very quickly.

Alf, I knew I was on a sticky wicket with you around!
 
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Anonymous

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Thanks for the research in my least favouirite subject Chris (Glue ups-shudder). Interesting reading.

I've just completed a couple of steps for the garden using titebond poly having been a fan of titebond original for some time. I now look farward to them falling apart.

Roy
 

Dog

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I echo the thanks for doing this research Chris, very interesting indeed. I've had similar experiences with Titebond II glue and wasn't that impressed for outdoor projects use where I need a waterproof glue, not just water resistant. I've tried Wudcare 'Fast Grab' PU but it's strength is very disappointing compared to the PU glue from Screwfix under the Screwfix name (in a 310ml tube) which was very quick to dry and gives a solid waterproof bond once cured but stays flexible allowing the work piece to be planned or sanded etc without damage to tools.
 

Dewy

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I have suffered a few times with side tables breaking.
I have diabetes and often suffer from 'hypos' after falling asleep in my chair and missing my eating time. During these hypos I am not aware what happens and fall on the floor often on the table at the side of my chair. I then, apparently, kick and hit out uncontrollably. 3 times the tenon joints have come apart after using either PU or PVA glues. On one occasion the tenon split leaving half in the mortice. I got my son to repair one with PU glue then pin with brads. The next time it happened the brads held & the non pinned joints failed. A body falling on a small table knocking it over then pressing hard on the legs sideways is a lot of pressure. The next time I repair the table I will use hard wood dowels to pin the joints. hopefully these will strengthen them. One thing I found was that the PU glue although it foamed out of the joints, was easily cut way as it couldn't stick to the waxed, oiled finish. the repairs didn't show as a result. The tables were made from pine. It would be handy to see how the joints hold up if hardwood is used. I pray it doesn't happen again but know it is likely to. At least I will have plenty of practice making small tables and repairing them. ;) :(
 
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Anonymous

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Chris,

A problem with some glues that you did not mention is their tendency to creep if they are under stress. I have found that sometimes, after glueing up a panel with PVA glue, the glue tends, after a few weeks or months to squeeze out of the joint and form a noticeable ridge. I think this may have been due to the fact that the jointing of the edges of the boards was not perfect.

I am also curious to know what glue you used to glue up the laminations for the rockers for your rocking chair. A quick look at your description did not reveal what type you used, but I may not have searched thoroughly enough. Having heard horror tales of Maloof-style chairs whose rockers de-laminated after a few months, I played safe with my rocking chair, and used urea formaldehyde glue for the rocker and back-slat laminations. UF glue cures rock-hard and is reputedly the glue of choice for bent laminations.

Rockerau
 

Philly

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Rock,
I have sometimes had that problem with the glueline becoming proud of the surface. The only thing I can think it could be is this-the water content of the glue soaks into the wood, bloating the fibres some. I have had the habit of planing/scraping down the joint lines pretty soon after the glue has gone off, leaving the surface smoothed and flat. But over a couple of days the glue line becomes noticable when you run you hand over the panel.
So, I now leave smoothing the panel for a couple of days after glue-up to allow any moisture to evaporate. This has seemed to work. I don't know if this is any help, but give it a try.
regards,
Philly :D
 

Chris Knight

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Rockerau,

Philly's point is well documented and I have had problems with that in the past although I am slowly learning patience in my old age and leaving panels for a while before I plane/sand them smooth. As you say, creep can be a problem and I have experienced that too in flat panels with Resin W PVA but not with Titebond.

I was principally concerned with the strength of stressed glued joints in my tests rather than other issues and I certainly was not trying to write about all the problems that can afflict our gluing jobs!

For the rockers, following Hal Taylor's strong recommendation, I used Titebond II. He has made over 200 chairs and I think he has about every angle covered now. My big problem was finding a substitute glue for some joints which were very close fitting with large gluing areas where I found that Hal's recommended PVA was too grabby for me - he acknowledges the difficulty but I think his experience allows him to work a lot faster, reducing the problem to manageable proportions.

I don't care for UF glues, I have had problems in the past and I don't like mixing glue if I don't have to. As you say, they have a reputation for being non-creep but that has not been much of an issue for me. In this case, for me, the fact that they are also made up with water would have proven problematic.

If grab and some swelling are not problems - which for many joining operations they are not, then I like hide glue. It does not creep, will take stain well and it doesn't matter if I get some on the face of things, it is easy to clean off with a scraper. It is also extremely strong.
 

Chris Knight

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Dewy,
Sorry to hear of you problems - it sounds horrible and scary. I think it would encourage me to make sure all the corners were well rounded!
 
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Anonymous

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Chris,
I think there are different types of UF glues; some, like cascamite, are a powder mixed with water, but the one I use is a mixture of two liquids (a resin and an acid, I believe). It has the advantage of a fairly long open time (about 20 minutes), so there is plenty of time to apply it with a rubber roller to the six laminations that I used for each of my rockers.

Hide glue is probably fine in the UK, but I found that a dovetailed cabinet that I used it on had the problem of the glue squeezing out of the joints. I think this happened during very hot humid weather (> 35 C) before I installed air-conditioning in my house.

Rockerau
 

johnjin

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Hi Chris

As usual there is a lot of interesting information in your post. So many of us are using various glues all of the time, and don't take the trouble to write down just what was best for a particular joint, and therefore this knowledge is lost. So once again along with the knowledge we have gained from your series on the chair, and this particular post I for one am a wiser individual in the mysteries of gluing.
Thanks again for taking the trouble.

John
 

Aragorn

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Chris
Many thanks for this review. Very useful information. And info based on a real woodworker's experience beats the scientific/technical stuff every time in my book!
I don't use titebond - just an off-the-shelf PVA glue like Resin W. Is titebond original a basic PVA or does it have properties that make it better?
 
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