What I meant in my statement about BLO not adding anything to the CA finish was in terms of durability or hardness (which are the two items that Tung has over BLO). Some folks think that BLO somehow hybridizes the CA into a really durable finish (it's difficult to improve on the hardness of regular CA anyhow).
Below is a submission made to the Yahoo pen turners forum several years back. It is very technical, but Henk is a chemist who understands this stuff better that I ever will:
BLO / CA Chemistry
Linseed oil, a (semi-)drying oil, will polymerize under influence of oxygen/radicals. The double bonds in the linseed oil fatty acids 'connect' to each other, and because there are three fatty acids in each molecule, this creates a 3D network of polymer molecules, i.e. a resinous 'plastic'.
BLO, a linseed oil with catalysts added, polymerizes a lot faster, but yields basically the same resin. Since (B)LO is yellow to begin with, the resin also is. Due to light influence over time, LO resin will yellow some more. Can't think of anything that will turn it black, other than fire, sulfuric acid, or maybe a wrong catalyst (or way too much of it).
That being said, the CA/BLO resin is a completely different resin. In BLO, the cross-links occur between individual fatty acid residues, so the triglycerides (the BLO molecules) are attached to one another, creating a true resin. In CA, the cyanoacrylate molecules attach to one another, creating a polyacrylate linear polymer. In CA/BLO, the cyanoacrylate monomers and oligomers (short chains of CA molecules in the process of becoming a polyacrylate) basically act similarly to the catalysts in BLO, in that they accelerate the activation of the fatty acid double bonds. In contrast to the normal BLO reaction though, the CA mono/oligomers actually attach to a fatty acid, then attach another CA or BLO molecule on the other end of their chain. You basically get a resin in which CA mono/oligomers form bridges between the LO molecules, so you get a copolymer resin.
Interestingly, it should be possible to control the average CA chain length in the bridges, thereby influencing the properties of the resin. Whether that can be done by time control, temperature, additives, or choice of CA (thin, thick; methyl, ethyl, octyl, etc), I have no idea. Don't even know whether any research on CA copolymers exists. Haven't found it yet...
Cheers
Henk
====================Heisenberg was right!=========================
| Dr. Henk J.M. Verhaar | |
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====================Uncertainty happens!==========================
Basically Henk is stating that BLO will bond to the CA creating a new material with some properties of each of its components.
When we put layers of BLO or CA down and follow with another layer before the first has dryed, we get a molecular bond of the two finishes.