Glue and Water Weight

In some of my more recent posts, I've done a number of informal experiments testing out various materials. In thinking about what order to present them in, I've decided to start with this one, because it's rather simple, and I may want to refer to it when I write up the others.

If you scroll to the bottom of the post, where it says "Labels," you will see the tag "testing things out." Clicking on that will take you to all the other little tests I've written up. 

Often, on the various online model rocketry forums, people will ask "What's the best glue?" We've received emails on the subject at The Rocketry Show podcast. The question of which glue is "best" is a tricky one. First, it depends on what you're using it for. Secondly, there's a lot of opinion out there, and while it must be true that certain facts exist somewhere, it can be hard to sort them from the mass of opinion masquerading as fact. Even the information that looks completely factual might be incorrect, so how do you really know? A great way to start a huge debate online is to ask "What's the best glue?"

So, I'm not going to dare to do that. I have my preferences and opinions like every rocketeer. But rather than present my ideas as the ultimate truth, I thought I'd share a little of my own testing certain properties of glues out, so people can take that information and do with it what they like.

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Everything you add to a model rocket changes the weight and the center of gravity, which in turn affects how the rocket flies. This includes not only components - engine hooks or motor retainers, shock chord mounts, etc. - but also things like primer, paint, and even the glues you use to put the whole thing together.

Around the time I was trying to figure out how much weight I added to a rocket by painting it, I got a little curious about weight and glue. White glue and yellow glue - the two most commonly used adhesives for low power model rockets - are pretty dense. A full bottle feels nice and heavy. But, of course, both glues have a lot of water in them. And you really don't need to use much yellow glue or white glue at all to get a good bond. Less is usually better with white and yellow glue.

So, assuming you're adding weight when you glue on fins and launch lugs, and then make fillets of glue at the roots of the fins (and the launch lug, if you choose to fillet it), how much of that weight will be lost as the glue dries? What percentage of the weight of the glue is just water - which will evaporate - and what percentage is actually left behind?

I decided to do a little test. As with the Weight of Paint experiment, this isn't really exhaustive or scientific. I tested one batch of one formula of one brand each of white and wood glue - the two I had lying around my Rocket Room. I didn't do multiple batches. I only tested the glues I had on hand - not all formulas and all brands. I just did enough of a test to satisfy my curiosity, and hopefully shed a little light on this subject for others.

White glue is usually made of a compound called polyvinyl acetate, or PVA. Yellow glue, often called "wood glue" or "carpenter's glue," is often made of aliphatic resin. Then again, both white and wood glues, either PVA or aliphatic resins, may be used on wood, and can be called wood glue. The yellow stuff is more commonly used in carpentry, and comes in various formulas, so often when people say "wood glue," including myself, they mean yellow.

The glue I use most for building rockets is Titebond II yellow carpenter's glue.

Why Titebond II? I have no idea. I went shopping for glue one day, saw some Titebond (original, in the red bottle), and some Titebond II, and figured well, if Titebond is good, Titebond II has to be better, right?

I'm sure there's no validity to that thinking, but I do like the nice blue label.

Anyway, I decided to find out how much of the mass Titebond II was water weight, which would be lost as the glue dried.

Once I'd done that, since I had a little Elmer's Glue All lying around, I figured I should try the same test with that, and compare the two.

I knew that Elmer's was a PVA-based glue, and assumed that the Titebond was an aliphatic resin. Not because I'm a glue expert - I'd just read somewhere that yellow glue is made of aliphatic resin, and assumed it applied across the board.

It turns out that, while the original Titebond is aliphatic resin, Titebond II is another PVA-based glue. And Titebond III is something else altogether. This is not to mention other brands of yellow carpenter's glue - Elmer's, Gorilla Glue, etc.

Despite the fact that both the Elmer's and the Titebond were PVA glues, they would turn out to be quite different.

Yellow Glue

I wanted to find what percentage of the glue's weight was water. The test would involve carefully measuring out exactly 10 grams of glue, allowing it to dry out completely, and then weighing the dried hardened glue again.

I have a stack of these little plastic sample cups which had been discarded from a Starbucks coffee shop, and you may have seen me use them for mixing epoxy on this blog. They're perfect.
 

First, I weighed the cup. Exactly 2.8 grams. Once I had the cup base weight measured, I tared the digital scale back to zero, and carefully poured exactly 10 grams of glue into the cup.

With that done, I double checked the combined mass of the glue and the cup - exactly 12.8 grams.

I always have to double check with this scale. It's so accurate, it can easily slip out of calibration and be off by a tenth of a gram. I always check the tare with a 100 gram weight, so I can be sure of my results.

A thin layer of wood glue dries very quickly. But a large mass in a cup like this takes a long time to dry fully - I mean many weeks. To prevent the glue collecting dust, I covered the cup gently with a bit of cotton cloth, stuck the glue on a shelf, and checked it from time to time.

Two months later, I was satisfied that the glue was as dry as it would get, and decided it was time to check the results.

I carefully tared my scale and weighed the cup of glue. The combined weight of cup and glue was exactly 8 grams.

Subtracting the 2.8 grams of the cup's mass, that meant the glue had gone from 10 grams to 5.2 grams. It would seem that in the Titebond II yellow carpenter's glue, the mass was 48% water weight, which would be lost as the project dried.

That's a significant loss in weight. Since glue is generally used sparingly, the weight it adds to your rocket is pretty small (though I admit I thought the final dried glue would weigh even less).

I was all set to write this up here on The Rocket N00b blog, when I realized I really should compare my favorite glue with another commonly-used adhesive, Elmer's Glue All. If I wanted this information to be of interest to more people than just myself, I needed some context.

White Glue

Again, I weighed a sample cup, and again, it came to exactly 2.8 grams (I wanted to make sure in case there were some variance).

I poured in exactly 10 grams of Elmer's Glue-All, weighed the final combined cup, and it was exactly 12.8 grams.

Interestingly, the Elmer's dried much faster than the Titebond II. It only took 2-3 weeks for me to be satisfied the glue was completely dry all the way through.

Once it had dried, the combined cup and white glue weighed exactly 7 grams.

Minus the 2.8 grams of the cup, the glue went from 10 grams to 4.2 grams! So, the Elmer's Glue All is 58% water weight!

For lightweight building, it would seem that, at least between these two choices, the white glue is the winner by a small margin, although either one looks to be pretty darn good. Both white and yellow glue are cheap and nontoxic, they form a nice strong bond between wood and paper (such as balsa fins and body tubes), and any mess can be cleaned up with warm water before the glue dries. Since both are nontoxic and easy to work with, you can control how much you apply - using fingers to wipe off excess - so you can use a small amount, and once the glue dries, it will weigh about half as much as when you put it on.

What About Epoxy?

Sometimes on the forums, a newcomer will upload a small, low-power model rocket build they've been working on, and in an attempt to do a really good job, they reach for epoxy as their main adhesive.

There are some instances where epoxy might be appropriate on a low or mid power rocket build. Attaching a plastic Estes screw-on motor retainer, for example, calls for a small dab of JB Weld steel-reinforced epoxy.

For low power model rocketry, though, use of epoxy should be sparing - if used at all. For most of the main parts of the build - attaching balsa fins and a launch lug to the paper body tube, for example, or installing the motor mount into the rocket - epoxy is overkill.

There's nothing wrong with epoxy. You certainly can use it for small model rockets if you choose. But here are a few reasons you might go with white or yellow glue instead.

First, epoxy is messy. It's a two-part adhesive made up of the epoxy resin itself and a hardener, which you have to mix together. It's toxic, so you shouldn't touch it with your skin. Because of it's toxicity and its thick, honey-like viscosity, it can be hard to get just the right amount onto the part you're trying to glue on (you can use a brush, but the brush will have to be discarded), so you'll probably use more than you need. And, going with the topic of this post, epoxy starts heavy and stays heavy.

Unlike wood glue or white glue, epoxy doesn't dry, it cures. The two parts are mixed together and a chemical reaction occurs in which the molecules in the epoxy and hardener join together to form longer, more complicated molecules, and the substance changes from a liquid to a solid. Because of that, epoxy, which is already pretty heavy, doesn't lose any mass, and you'll end up with a heavier rocket than you may want, without necessarily even benefiting from a better adhesive bond.

I think the reason some beginners reach for the epoxy for their smaller rockets is that they see high power rocketeers use epoxy on the online forums, and perhaps think "this is what the REAL rocketeers use, so it must be better!"

There are good reasons for high power rocketeers to use epoxy. Some high power rockets are made of fiberglass or carbon fiber, neither of which will stick to wood glues. And the forces encountered in high power rocketry can call for heavier duty adhesives, even on paper tubes and plywood fins (though there are times when a good wood glue will work just fine). But while some epoxies are up to the task, others are... well, they're fine for general use, but not necessarily better than wood or white glues. Higher performing epoxies are out there, but they come with a higher price tag. There are many different kinds of epoxies, and it's hard to make a general statement which applies to all of them.

Let's look at what is often a hobbyist's first epoxy, Bob Smith 30 minute. Epoxies have a set cure time, and they may be 5 minutes, 15 minutes, 30 or more - up to hours. Longer cure times generally make for stronger epoxies. The Bob Smith 30 minute is a pretty good one for a low price, and I've used it for a number of mid power builds.

Here, I've measured some out. Getting just about 10 grams was an accident. Because it's hard to work with, and you have to stir it, I was just trying to get some amount in the cup, but here you see I've nearly hit 10 grams, with the cup weighing 12.9g.

A few hours later, well after it had cured, the cup weighed 12.8 grams.

This means one of two things: either the epoxy did lose a small amount of mass during its curing process, or far more likely, I needed to re-tare my scale.

Epoxy certainly will work as an adhesive for low power model rocket building. But because it is toxic, you shouldn't handle epoxy without protective nitrile gloves. When mixing large batches, you may also need to use a respirator, to protect your lungs from toxic fumes. Since it's messy and harder to handle, you are likely to use more than you need, adding extra weight. And since epoxy cures rather than drying, none of that extra weight will be lost before you paint and fly the rocket.

There are some reasons you might want to use epoxy in low power builds, depending on what you're trying to do, and I'll talk about those in an upcoming post. I'll also attempt to test out a few properties of these adhesives, including adhesive strength and brittleness vs. flexibility. I found a few surprises.

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