Mark, I bet you have been either in the adhesive business or around it for a day or two. What you say is probably spot on. The problem lays with the dumb consumer, that would be me. We do not understand all of the properties of adhesive and like a lot of products on the market they're over inflated in price because of the name or who sells it.
Yes and yes and what you said about inflated prices for nothing more than name/sales-hype is the point I was making. The product I linked to is just a cheap imported generic epoxy resin ... the same stuff that is often repackaged, relabeled and sold at very health profit with a fancy name and/or good sales-hype. The profit margin is also increased according to the amount and type fillers that are added ... but ... in some cases specific fillers are added to "specific resin formulations" that actually improve the properties of the cured resin for "specific applications".
There are common testing standards for resins depending upon their intended application which is why there's no magic bullet one-size-fits-all resin blend. When using an epoxy resin on wood, the primary concerns are how deep the bond is and does the resin have enough give to allow for the normal dimensional changes of the wood. When it comes to bonding, the more body the mixture has, the less it's going to penetrate the wood fibers which means the bond is limited to the surface only. Combine the limited to no surface penetration with the fact that wood undergoes considerable dimensional changes while the epoxy is relatively stable and it's easy to see why there's so many bond failures over time. Thus is why an epoxy intended for wood applications should have sufficient wetting properties for maximum penetration depth without suffering failure caused by a resin starved joint. Common retail grade generic epoxy resins are all pretty much the same where even the low-viscosity versions have too much body to allow for a good penetration depth resulting in a resin-starved joint failure because the fillers create a barrier zone adjacent to the porous surface(s). Most of the instructions you'll find for the filler-bodied epoxies will say to put a thin coat on end-grain and other porous surfaces to act as a primer coat, that's all well and good but the shear strength is still going to be isolated and therefore limited to the bond between the primer coat and surface of the substrate. It's also worth noting that any re-coating be done in accordance with the parameters of the resin, most rigid-setting resins must be re-coated within a certain window of time which is typically just after the potlife window before it reaches its ultimate cure temperature; others can only be re-coated after it is fully cured and prepped with mechanical action.
Adding reinforcement to resins … The most common failure of resin-metal mixtures is caused by the residual stresses created by the thermal contraction differential between resin and metal particles upon cooling from the cure temperature and thermal cycling over time keeps eating away at the integrity of the matrix. Metal reinforcement can be utilized effectively but mixing resin and metals properly is beyond the capabilities of most DIY'ers, if the application calls for metal reinforcement it is best to purchase quality pre-mixed compound. Another issue with DIY'er metal reinforcement is the likelihood of degradation caused by galvanic corrosion especially in those resins that are hygroscopic and/or prone to out-gassing as are most generic resins. Various fiber options are available but the use of such must be correctly matched to the application which is rarely done for simply because of ignorance. I've seen numerous comments of people using common fiberglass insulation thinking they're getting the same reinforcement properties as would be seen with woven fiberglass cloth but they're not. The second thing is that long-fiber reinforcement materials must also be matched to the resin which must remain bend/twist flexible when cured just as the styrene resins and a low out-gassing resin must be utilized. Vinyl and silica are used as fillers in most generic resins because they're cheap, neither benefits long-span reinforcement and both commonly make the resin more hygroscopic. Balloon type fillers (includes all hollow and closed-cell materials) are intended only to reduce the mass of the resin and they greatly reduce the structural strength of the resin to the point where it's no longer suitable for structural applications. Ceramics and oxides are the premium short-span reinforcing materials for rigid-set compounds because they don't create the thermal dimensional disparity and subsequent internal stresses and are immune to galvanic corrosion. Short or chopped fibers are best in semi-rigid compounds. Flocked cotton fiber is still one of the best choices especially for edge & corner reinforcement in rigid or flexible resins with a 2:1 fiber/compound ratio being the standard.
Rigid setting resins are the poorest choice for most wood applications and an even poorer choice for applications comprising dissimilar substrates such wood to metal where there is disparity in dimensional cycling. For example, quality all-weather wood glues are not rigid when cured, they're formulated to remain flexible so as to maintain joint strength despite the common dimensional cycling associated with variations in the ambient temperature and humidity. Highly flexible Ultra-RVC epoxy bedding compound allows for dissimilar substrate disparity whereas rigid and semi-rigid compounds cannot with the results being seen as occasional fliers and POI shifting corresponding to the ambient and operational parameters. The more rigid an epoxy compound used, the greater the stress created at the bond when the substrate undergoes dimensional changes. Shrinkage is another consideration with rigid and semi-rigid compounds as the composition, film thickness and gelation temperature have a direct effect on the amount of shrinkage. Many mfg's/sellers
make seemingly impressive claims as to the amount of shrinkage but one must keep in mind that such claims are often based on test samples utilizing a bond-line thickness of just 0.002” so if you're filling a gap greater than 0.002”, the amount of shrinkage is increased accordingly. Yet another issue is that of hidden air/gas pockets especially with heavy-body, paste and gel resins.
Colorants can be positive or negative depending upon how the colorant and compound interact, it never ceases to amaze me at what some people will use and adamantly claim “no ill effect” … I appreciate those claims because that's job security for me. Someone gets a bad body & paint job on their car, they take it to a different shop to get it done right. Few listen but I keep saying it, you want to color epoxy, do the surface only after it's fully cured and you'll save yourself a lot of problems.
A word of caution about misleading names, typically something along the lines of xxxxx-Poxy. These are low-viscosity (thin/runny) resins advertised as “deep penetrating for lay-up and wood adhesion/lamination” - check the ingredients because more often than not these are styrene resins that are suitable only for non-structural applications because while they can be highly flexible to bending/twisting stress, they're brittle to shock loads. These are the resins commonly used for fiberglass and foam fabrication but they can be utilized to stabilize wood if special pressure/vacuum application methods are utilized.
I know this got a little long-winded but there's a lot to the subject and what's here just barely scratches the surface. My experience with epoxies started many years ago with industrial & commercial applications, the sales-hype does not a bit of good when you're wasting money, manpower and materials on re-work.
Mark
