SECTION VII D - BONDING WITH EPOXY
The mixed viscosity of coating and fiberglassing epoxies is not high enough to make good gap filling adhesives. Thixotropic agents like silica thickener (Cab-O-Sil, Aerosil), plastic minifibers, and wood flour are used to thicken the epoxy and change the flow characteristics. These fillers will turn the epoxy from translucent to opaque depending on the type and amount used. Silica thickener and plastic minifibers make the epoxy whitish while wood flour turns it reddish-brown. Silica thickener makes a smooth material while epoxy thickened with plastic minifibers or wood flour will be coarse. Microballoons and microspheres should not be used in adhesive formulations as they reduce tensile strength.
Making an epoxy glue joint is quite simple. It is even simpler if SilverTip GelMagic is being used since SilverTip GelMagic "self-thickens" to become a gap filling adhesive. T-88 is thick enough straight out of the bottle to be used as a gap filling glue. Assuming that our General Purpose epoxy is being used first properly measure and mix the resin and hardener, then coat both mating surfaces with this unfilled epoxy to wet them out. It is not necessary to let this coat cure. Next, add the thixotropic agent to the balance of the mixed resin/hardener blend and spread this thickened resin on either of the two surfaces. When using SilverTip GelMagic simply measure and mix the two parts and spread on both surfaces. Then close up the joint. There are some tricks and things to keep in mind.
First, remember that the ultimate strength of any glue joint is a function of the glue surface area. The more surface area, the stronger the joint. This is the reason that scarf joints are made at a minimum 8:1 slope. Fillets increase glue surface area and are used to relieve stress concentrations that build at right angle corners. Stringers, for example, should have fillets where they butt onto the boat hull planking.
Second, make sure that the surfaces being glued are clean, free of grease, oil, wax, and other contaminants that could act as release agents. If the surface is coated with cured epoxy, sand before gluing and wipe the dust off. Prior to sanding wipe away any oil or grease with a clean rag and suitable solvent. Remove paint rather than trying to glue onto a painted surface. Epoxy resins stick well to sanded paint but the overall bond strength will be no better than the paint to substrate bond.
Third, do not over-clamp. Epoxy resins require only contact pressure. Over-clamping can squeeze most of the adhesive out of the glue joint and the epoxy that is left is absorbed into the wood starving the joint. A glue-starved joint is very weak. Use only enough pressure to hold the joint immobile and keep the two surfaces in contact until the epoxy has set overnight at normal temperatures. Nails, screws, clamps, rubber bands, or staples can all be utilized. Clamp just hard enough to close up the joint.
Fourth, remember that epoxy resins continue to cure and build strength for several days after they solidify. Joints that will be under immediate stress once they are unclamped need more cure time before the clamps are removed. Overnight cures are usually sufficient for most non-stressed joints. In cold weather the time could stretch out to several days. A common cause of epoxy joint failures is excessive stress before the epoxy has reached sufficient strength. This might occur when a scarf joint is bent too soon.
Fifth, protect the finished glue joint from weather degradation. Wood that is allowed to weather will cycle through moisture content extremes. Wood expands as the moisture content increases. This expansion can set up enormous stress concentrations across a glue joint due to uneven rates of expansion on either side of the glue line. These stress concentrations can exceed the strength of any glue, including epoxy resins, causing failure. Protect the joint by epoxy coating all surfaces of the glued wood. This will stop moisture cycling and prevent failure because of weathering. This is not a problem for wood glued with epoxy that will not be subject to deep moisture cycling - indoors, for example.
Most woods can be successfully bonded with epoxy. Teak is not difficult to bond but the bond may fail if allowed to cycle moisture. When epoxy gluing a teak on plywood boat deck, the teak should be less than 3/16" thick. The expansion joints should be of a flexible material like the two part polysulfide rubber mastics. Don't use black-pigmented epoxy between teak boards that will be subjected to strong sunlight or weathering - the epoxy will crack.
SilverTip GelMagic and our general-purpose epoxy are specifically designed for use as an adhesive for wood-to-wood bonds. Both will bond well to sanded polyester and vinyl ester resins. For metal bonding we recommend using SilverTip MetlWeld, our specially designed adhesive for bonding metal and other difficult to bond materials.
Metal to metal bonding success depends upon the type of metals bonded, the surface preparation, and the intended service temperature. Bonds between different metals may degrade over time due to differential thermal expansion, which sets up shear stress that leads to interfacial failure. Potential structural bonding of metals should be thoroughly evaluated before proceeding. We recommend the use of mechanical fasteners for critical applications.
Metal to wood bonding for non-structural applications may be done successfully with epoxy providing that the metal is clean and bright. Don't pot stainless steel bolts in any epoxy resin if the application will be around water. Stainless steel works only in the presence of sufficient oxygen. The epoxy will deprive it of oxygen causing crevice corrosion in the presence of an electrolyte like seawater. Stainless steel fastener failure occurs where the bolt emerges from the epoxy resin.
Bonding to metal alone such as fairings on lead keels will work well with epoxy so long as the lead is bright and free of oxidation. Since lead readily tarnishes there may be benefit from immediately coating the bright lead with SilverTip MetlWeld before fairing with SilverTip QuikFair to fair the keel. The SilverTip MetlWeld provides an almost absolute bond to the lead and SilverTip QuikFair easily bonds to tacky or cured and scuffed SilverTip MetlWeld.
Thermoplastic materials like vinyl plastics or ABS bond reasonably well with epoxy resins. You will get the best results if you first sand the plastic with coarse paper. Before bonding flame treat these plastics by passing the flame of a propane torch across the surface without scorching or melting the surface.
Epoxy will not bond to polyethylene, polypropylene, or Teflon. It bonds well to neoprene and polyurethane rubbers.
There are too many materials and combinations to cover every possibility. Model any questionable materials that you want to bond. Glue some scraps and test them. Try accelerated aging and retest them. If they survive an hour in 160°F water they will probably last for quite a while.
Quick Cure is our 1:1 "five-minute" epoxy. Items glued with QuickCure can be stressed in as little as 15 minutes. It is very handy to have in the shop simply for this reason. Builders often find that "missed screw hole" when ready to lay down the fiberglass cloth. Mix a little Quick Cure; add some wood flour and you've got an instant putty to fill the hole. Quick Cure can also be used in combination with slower epoxies as a "spot welder" where clamping is all but impossible.
Coat the pieces to be bonded with SilverTip GelMagic, except leave several silver dollar size bare areas. Mix some Quick Cure and apply to the bare areas. Push the pieces to be bonded together with enough pressure to cause some "ooze out". Hold in place for about five minutes until the Quick Cure hardens. Now the Quick Cure will hold the pieces together while the SilverTip GelMagic sets.
Unlike our other epoxy systems Quick Cure (like all similar epoxy products) is water resistant, not water proof. It is fine for intermittent water contact but should not be exposed below the waterline on a boat, for example.
Materials Required for Bonding:
- Epoxy Resin and Hardener
- Brushes, spreading tools
- Silica thickener
- Tags: The Epoxy Book
- STR Staff