6/15/2009

Epoxy Talk

            Recently I had a old client complement me on the durability of my bamboo fly rods so I thought I would take the time to write a little on polyepoxides. If you are interested in the stuff that actually holds bamboo fly rods together for a very very long time you may find this compelling.

My old work colleagues who were employed by GE Polymer Solutions, use to call me “Chemical Joey” (Joseph is my middle name) because my obsessive interest in epoxies. In my past life I was fortunate enough to live side by side with chemical engineers that could give me some very hard and detailed facts about epoxies also known as polyepoxides. Epoxy is a copolymer that is formed by the mixing of two parts. Unlike many traditional glues and water based glues epoxy has a very high tolerance to temperature variables, can be virtually unaffected by moisture or water, and can be subjected to such abuses as extreme flexural strength, impact strength, shear strengths, and peel strengths. Epoxy is definitely the hero in the rod makers world. It solves most if not all of the age old problems rod builders ran into using hide glues, Resorcinol, and water based glues in the past. Epoxy is a beautiful thing because it has both low and high viscosity mixtures which is critical in the construction of a bamboo fly rod and it's different components. Ultimately you want the very best adhesives that will last well over a hundred years and sustain an immeasurable amount of use. 

Like anything else there is a vast amount of variables when considering the right epoxy for the right application. The levels of flexural strength, lap shear strength, and peel ratio vary greatly. For the purposes of rod making my quest for the “majic three” as I often refer to them, are three types of epoxy that fall into three different use buckets: laminating bamboo strips, bonding hardware, and gluing cork. There are similar and dissimilar materials being bonded here such as nickel silver to wood, wood to cane, cork to cork, and cork to cane. So it can get quite complex when considering which epoxy is used for which application. None of the epoxies I use are “off the shelf” because nothing bought in a hardware store can really do the job up to my level of scrutiny. It’s simply a matter of peace of mind. I have never had a ferrule come off, nor delamination, or even loose hardware. 

The first epoxy that is very important is the epoxy that can tolerate high temperature, flexural strength, is waterproof, high peel ratio, and has a very low viscosity so as not to leave a visible glue joint, this is the epoxy used to laminate bamboo splines. It has a slow cure time and needs to eliminate any possibilities of creep, which means the bond does not shift between both surface planes. If it does you will get a set that really cannot be eliminated. Wood glues have this creep problem and really should never be used for anything to do with rod building period. It simply is an inferior glue altogether because of it’s low tolerance to heat and poor peel ratio with similar and non similar media. You would also have to have rocks in your head to think it’s bond can tolerate heat. I have fixed too many other builders rods and seen the havoc wood glue causes.

The second epoxy we use is for bonding is for ferrules and reel seat hardware, it has extreme high impact strength, no shrinkage or expansion, has a slow cure, and most importantly can be used to bond dissimilar media (alloy to wood). Zinc is a difficult property to deal with as far as bonding hardware, and as we know Nickel Silver contains around 60% zinc. Most off the shelf epoxies do not have proper peel ratios to bond properly with the zinc combination. Most off the shelf epoxy, even the really good ones, simply say “metal” which is fairly misleading. Zinc is the culprit here, it needs a slow cure epoxy with a lap sheer of about 4,000 PSI and a peel ratio of about 3.6 PLI, typically this kind of epoxy is used for attaching sledge hammer heads to fiberglass or hickory handles. Typically these epoxies are opaque and do not dry clear. 

The third epoxy is for gluing cork. It needs to have flexural strength, dries clear, waterproof, and has a low viscosity so as not to leave a glue line. Most importantly it needs an extended pot life, why?, because if you’ve ever seen grips with glue lines its because the glue sets up before it can spill out of the ring seams. Bonding strength is not as critical here, but I still use a structural epoxy adhesive. Many people use wood glue for cork, again my one concern is that it sets up too quick and leaves a visible glue line. Many years ago I worked with the glue division at Elmers here in Columbus and had riveting conversations with chemists. I also got to tour their testing lab, I was like a pig in shit. I got into a discussion with a chemist and discussed the properties of white glue compared to wood glue and it comes down to a higher ratio of resin to solvent thus making the working time longer in wood glue neither of which should be used for gluing cork. Sweat from your hands contain chlorides and some urea compounds which will compromise wood glue (PVA) over time. So wood glue really is not the best glue to use for cork even the newer Crosslinking PVA’s that are waterproof. As a side note urethane glues simply do not stack up to epoxies overall strength and desired attributes. Urethanes generally are harder to control, and chemists that I've worked with still consider them more of an “all purpose household glue for specific low risk tasks”.

I have refrained from using any recommended brand names here because like any business I have my own trade secrets but I can leave you a very informative trail of bread crumbs.
        ~Clint Joseph Bova