Popular Woodworking 2009-08 № 177, страница 51

surrounding air often plays a larger part in the dissipation ofthe solvent or water. The molecules of the glue during this process are drawn together and attach to one another

Chemical reaction. Epoxies, urea formaldehyde, cyanoacrylate, polyurethane glues and cross-linkingyel low glucsallcure chemically. This is a process where a combination ormaterialsare added together tocause the curing process to start. As long as these materials are independent, they remain in an unchanged state.

Reactions can be initiated by mi xi ng resin and hardener together such as with epoxy, applying water as w ith poly uret hane glue, by adding a catalyst such as with urea formaldehyde, or by merely dryingas with contact adhesive or a cross-linking PVA. One by-product of this chemical reaction is heat. All chemically reactive glues, once mixed, gaveaspecific "pot Hie," which means thatyouhavejustsolonguntilthegluetums intoabowling ball.

Thermal conversion. Some glues work by the presence or absence of heat. Heat is the most common cat al yst for contact ad he-sives, urea lor maldehyde anc epoxies, and is required for preparing hide glue foruse.

Hide glue works best at a specific temperature (around M5°Farenheit) and starts to gel ai temperatures of less than 110" F. I've also been told that if you apply heat to contact adhesive during the evaporation

Thermal transformation. Many glues react to heat. Hide glue must be mixed with water, then properly heated before use. Below 110" F, a gelled dump can be pulled from the pot.

processthat it helps to vulcanize the rubber and increase its strength and elasticity. Urea formaldehyde and epoxy set much faster withthe introductionofappliedheatduring the curing phase.

Classification of Rigidity

It is important to understand that the ultimate strengLhofany glue jointis a function ofthe glue surface area. The more surface area, the stronger the joint. However some glues, by the way they are formulated, have different degrees of rigidity and havediffer-ent characteristics when it comes to heat, moisture and solvents.

Thermoplastic or semi-rigid. Thermoplastic glues are the most widely used adhesivesin woodworking. Theyare generally resistant to moisture, but are not waterproof. All thermoplastic glues soften with heat and get hard when cooled. It's possible to reactivate cured thermoplastic glues by re-heating the bond tosoften it, then applying pressure during the cooling phase to reactivate the bond. Thermoplastics typically are not as strong and stiff as the wood itself and they can allow thejoint lo creep duringdimensional change. White and yellow glues, hide glue and instant glue fall into this category.

Thermosel or rigid. Thermoset glues are gene rally more costly lha n lhe rmoplaslic glues, but would be lhe glue ofchoice if cree p resistance or hardness is lhe main consideration. Asa matter of raci.ihermosetglues are rigid enough to support high, long term sialic loads wit houi deforming. They become permanently hard and rigid when heat is applied and are very water-resislanl.

Thermoset glues cure through a chemical process. Urea formaldehyde and epoxy are both examples of thermoset glues. Quite surprisingly I've been told that in the contexl of wood gluing, bonds made with thermosetting glues are no stronger than those made with most PVA glues.

Elastomeric or flexible. Elastomers are substances that have rubber-like properties. They can stretch and return to their original shape. Elastomeric glues can be waterproof and may benefit fromexposure to heat in the curing process, but they are also fairly weak and can fail in response to excessive heat.

Contact adhesive falls into this family, as do cert air. plasticized PVA glues such as Tilebond's Mold ing&Trimglue. These glues

Right glue for the job. Rigidity is most important when working with bent laminations. Avoid glue "creep" by usinga thermoset glue.

are somewhat flexible and can be sensitive to loss or strength wiihelevated temperatures - more than normal PVA.

Water and How it Affects Both Wood and Ciue

We already know that water is present in living trees. Fresh-cut wood can be totally waterlogged and no glue, including poly-urethane, can effectively bond to the saturated fibers. For the best glue bonds, wood should be at least thoroughly air dried or, better yet, kiln dried to lessthan lOpercent (7 percent is ideal).

If the moisture in the wood is too high, glue will not absorb fully and could be easily squeezed out during clamping, which would create an unusually thin glue line making for a very weakjoinl. On lhe other hand, ifthe moislure content is too low, the wood will absorb glue loo rapidly and that decreases the working lime.

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