Ah, the Trigger’s Broom theory!

hopefully the following will be of interest and explain some way the differences between Casein and Aerodux glues.

Casein glue is a type of adhesive made from milk protein. The glue is known to be very strong over a long period of time and is highly resistant to water. It has a drying time that allows enough time for pieces to be positioned accurately. The process of creating this glue is nearly identical to that of making cheese and can easily be performed at home. Although most adhesives are based on synthetic chemicals, casein glue still enjoys use as a natural binder.

There have been many uses for casein glue throughout history. There are records that show it was created and used by the ancient Egyptians. It was employed in the Middle Ages to bind together thin panels into thicker plates on which artists could paint. It also is believed to be used by makers of famous musical instruments that have lasted for a century or more. It was used extensively in woodworking, furniture making and to assemble wooden aircraft as late as the 1940’s

While casein glue can be purchased commercially, it also can be made at home with similar results. The process involves heating milk and then adding an acid such as vinegar to separate the curds from the whey. The curds are taken out of the excreted liquid and mixed with baking soda to neutralize the acid. The remaining liquid from the curds is removed by pressing them under a heavy weight. The final result is a white paste-like substance that will become a powerful adhesive when dry.

If one is making casein glue at home, it should be noted that the fat content of the milk plays a part in the strength of the glue. Milk that has a high fat content, such as whole milk, will produce weaker glue than non-fat and low-fat milks. This is because the fat in the milk prevents the casein from polymerizing.

In addition to being very strong and having a long lifespan, casein glue is also very water resistant. Unlike some adhesives, water will not weaken or dissolve casein. In some cases, however, water that comes into contact with the adhesive can pass on microorganisms that will eat the glue and eventually destroy it, but this is not a function of the water itself. Here is the problem with wooden aircraft from D H, Miles and Percival that have sat outside allowing water ingress into their structures. Glue failures were widespread in the late 1950’s early 1960’s causing many countries aviation authorities to ground the types affected. I know of a Miles Gemini that has never spent a night outside in its life and is still perfectly bonded with the original casein glue.

The drying time of casein glue can be a benefit in many situations. It takes a long time for the glue to properly cure and harden. This can allow the assembly of multiple complex parts that might need to be moved during assembly before securing them to allow the glue to set. This also might be a disadvantage for projects that require something that sets more immediately.

Aerodux of Phenole Rescorscine type, the Aerodux is a glue that certainly has come of age. It belongs to one of the oldest glue (and plastic, in fact) families around. Like all glue of aero quality it should arrive with a data sheet.
The glue is two component, the glue itself a reddish brown liquid that often comes in 1 litre cans. As filler is cheaper than glue it sometimes contains filler in the form of ground nut shell.

Hardener is a whitish powder packed in tin cans or plastic bags.

Phenols are poison to fish, that means if you pour your surplus Aerodux into a pond the fish might die. It contains very little solvents, and in the hardened state is chemically neutral. Apart from the mentioned effect on fish I know of no health hazhard connected to this glue. But I certainly would not drink it.

I used to mix my glue in a paper cup, and use a letter weight to assure mixing ratio. The ratio is 100 to 20 (100g of glue and 20g of hardener). The ratio is not critical. The glue also can be mixed by the volume, ratio here is 100 to 55.

After weighing the glue should be stirred properly to dissolve all hardener. Stirring should be continued till no lumps remain. The mix then should rest for about 10 minutes before being used.

The following times are relevant to Aerodux 185:

Shelf life: Here the literature seems to give different information. Some batches I have used made by Dyno Chemicals, UK, state a shelf life of 1 year. Others say only 1/2 year. This is true for the glue component, shelf life for the hardener is indefinite. It should be noted that once the can is opened, the glue starts deteriorating. It becomes darker in colour and later lumps appear. So when a can is opened the shelf life is reduced. Close the lid as quickly as possible! It is up to your judgement when to throw it away, but as a rule the glue should be liquid. No lumps are allowed. Aerodux should be stored in temperatures between +5C and +20C.

Pot life: The time from mixing till application is done. Varies with temperature of course, but a normal value is 2,5h at 20C. Temperature should not be lower than +15C (by the way this goes for the materials to be glued as well) and higher temperatures than +25C are inconvenient (I understand you aussies). High temperatures make application a real rat race – more on this later. Pot life does not increase very much on lowering temperature.

Open time: Now here is the usual criticism against this stuff coming up. Open time is the time from when spreading the glue is commenced till the two surfaces are put together. The rule here is that on putting together the first glue spread should still be clearly liquid. Open time varies, shortening with increased temperature and increasing slightly wit increased air humidity. A slightly cool (+15C) and moist atmosphere is favorable. Still the open time rarely exceeds 15min, so application of the glue should be swift – certainly an unexpected problem for the epoxy freaks. On suspicion that the open time has been exceeded the parts should be scraped clean and possibly sanded before a new (and swifter) application is done. Friends with brushes is a big asset when big areas are to be glued! To add to the problem it seems that open time is reduced gradually with time after mixing.

Closed time: The time between closing of a joint till glue pressure is applied. Now here things can move more leisurely. Again varying with temperature, we can have more than 1/2h here. But check that no part of the joint remains open in this phase – it can ruin your whole day.

Glue pressure time or clamped time: A normal value is min. 8 hours at +20C. The glue demands medium high glue pressure to produce a sound joint, to attain this often can be difficult. I use to leave the glued parts for about 24H with clamps attached. Aerodux can be express hardened in high temperatures without any side effects. The joint has full strength after about a week.

Brushes, paper cups and other equipment can be rinsed in cold water as long as the glue is not hardened. After hardening a hammer is a possible cleaning tool. Once hardened the glue is not soluble by any solvent, but have some thermoplastic properties. In fact, the ply we use is also Phenole Rescorcine glued. Here glue comes in foil that is put between the thin wood layers, and fused by high temperature and high pressure in big presses.

Using Aerodux takes a little while to get used to, especially the old epoxy users will find the short open time and need for relatively high glue pressure inconvenient. Personally I find that reduced (if any) health risk and unsurpassed aging qualities more than outweigh the disadvantages. In the end the choice will be that of your controller’s as well as your own. You have to comply to his demands.

Likewise I haste to add that in case of conflict between information found in my writing and the data sheet, the last mentioned is always correct!

The airplane business surprisingly (to some) is quite conservative. While this may be frustrating I find comfort in using tested and tried materials and solutions. New materials and methods certainly can be exiting, convenient and have other qualities. But for me there is a big difference between believing and knowing. So I guess I belong to a somewhat conservative fraternity. At least glue-vide. The oldest Aerodux glued aircraft still flying is now over 61 years old. On its last annual the glued structure was found to be perfectly solid with no sign whatsoever of glue failure anywhere. This and other Aerodux glued aircraft will fly on for decades to come.