UV Colour Loss and Degradation in Exotic Timbers, Testing Concluded...
Hello to the group,
Some of you may know that I have been testing various ways to prevent UV light degradation in exotic timbers for the last eight years. I've learned a lot, and I hope to have all of my results ready to publish by the end of the year. There was a recent post about making a home-brew UV resistant finish and I thought the group may find this information useful...
I have been experimenting with making my own home brewed/blended UV stabilized finishes for some time now. Making your own UV resistant finish is no small task! The stabilizers are very expensive and hard to obtain. UV degradation and protection is a *very* complex subject that eliminates insomnia for most folks. A few of us, myself included, find the topic interesting, but then I love reading about various fungi and their effect on wood.
The use of UV stabilizers in manufactured finishing products works well, to a point. However, it is NOT a simple process for a the average person to add a bit of stabilizer to an existing product and achieve good results. There is a ton of chemistry involved in determining the right type and amount of UV inhibitor to use. Also, to test the effectiveness of your UV stabilized home-brew finish requires the use of an accelerated UV test machine. These start at $15,000 USD, unless you know how to fabricate your own. To complicate matters further, various types of UV inhibitors are available, each with their own particular drawbacks. If you're trying to reduce UV degradation, it's generally easier to utilize an existing manufactured product that incorporates a good UV stabilizer (like a high quality marine varnish), than to try and formulate your own. The problem with marine varnish is that it leaves a very thick coating on the surface and it does not penetrate well. You can thin these marine varnishes for better penetration, but that reduces the overall protection.
High UV inhibitor content marine varnishes are very expensive. On the order of $60.00 - $75.00 USD per quart. Also, the selection of products available is limited and these products may not deliver the type of ultimate finish you want to achieve on your woodturning. If you have not yet fallen asleep, this next section will certainly have you sawing logs...
Two primary methods have been adopted to stabilize light/UV light: 1.) Competitive UV absorption by UV absorbers in the 290-350 nm wavelength range 2.) Trapping of the radicals formed during polymer degradation by radical scavengers using Hindered Amine Light Stabilizers (HALS). Two systems you should consider when making your own home brewed finish are chemical and pigmented.
Chemical: Ciba Speciality Chemicals has a HALS stabilizer, which is a liquid amine stabilizer. It consists of an almost pure mixture of Bis(1,2,2,6,6-penatamethyl-4 -piperidinyl) sebacate and Methyl (1,2,2,6,6-penatamethyl-4 -piperidinyl) sebacate. It is used in automotive coatings, wood stains and industrial coatings and is a clear chemical liquid. It is VERY expensive. My research indicates that these types of chemical stabilizers loose their effectiveness over a period of time.
Pigmented: Pigments will prevent the UV from attacking both the coating system and the under surface. The best type is the old Zinc Oxide and the newer Titanium Oxide, which forms a complete block out. Other pigments of course will provide similar protection, but will obscure the grain. The Transparent Iron Oxides and Titanium Oxides have a very fine particle size and are therefore, transparent. The Titanium Oxides are used in pharmaceutical formulations and are very expensive.
Transparent Iron Oxides are cheaper, but they are colored. However, the colour can be used to enhance the surface of the timber. Since they are nearly clear, they do not obscure the grain. All UV systems protect both the coating and the surface that the coating is applied to. However, one also must consider the properties of the coating itself. Is it a varnish film on the surface, or a penetrating finish?
Thick finishes with UV protection (like marine varnish), do not really penetrate the wood surface deeply. Therefore, a breakdown of the coating allows deterioration of the wood surface, by allowing moisture to get under the coating. This delaminates the remainder of the finish. The penetrating oil type finish has the advantage of soaking into the wood and does not form a skin to lift. The varnish finish has certain advantages however, because the film forming thickness penetrating oil finishes provide is substantially less than multiple coats of varnish.
The Oxides being inert do not lose their effectiveness over time (vs. the chemicals), but if the surface coating deteriorates, the UV factor is decreased. So, it is a bit of a trade off, if the coating breaks down the UV light will get through. Penetrating type coatings offer less breakdown of the surface coating, but do not provide as thick a surface film layer. As woodturners however, thick layers of a surface finish are generally less desirable than finishes that provide a "closer to the wood" effect, like the penetrating oils. So you can see that it is a long road to hoe, no matter which system you choose.
I have experimented quite a bit with blended finishes, in an attempt to strike a balance between the better overall coverage of the chemical systems and the superior long term performance of the pigmented systems. I have achieved excellent results with several protocols, but I think I could spend the rest of my life looking for the perfect UV resistant finish for woodturners and probably only come close to that goal. Since I fund my experiments like these out of my back pocket (I do not plan to sell any UV resistant finishes), it can be a challenge to continue experimenting when the cost is so high. I've learned a lot and achieved excellent long term results with my protocols. I'm already incorporating these results into my collector pieces for increased long term stability of the delicate colours in susceptible timbers, so in that way my experiments have been a huge success! Take care and all the best to you and yours!
Better Woodturning and Finishing Through Chemistry...
Eurowood Werks Studio
Professional Studio Woodturner