How adhesives bond, set and fail (CWC and CCI)

How adhesives bond, set and fail (CWC and CCI)

The RSOS names the adhesives a cabinetmaker selects (contact cement, wood glues, resin, hot-melt glue; for laminating also epoxies, casein glue and polyurethanes; for laminate sheets PVA and contact cement) and, for laminating, the adhesive properties that matter (open time, setup time, curing time, clean-up). Two Canadian sources describe how adhesives behave:

• the Canadian Wood Council's short article Adhesives, on structural wood product adhesives; and
• the Canadian Conservation Institute's Adhesive Compendium for Conservation (Jane L. Down, 2015). This book is written for museum conservators, not cabinet shops. Only its general facts about how adhesive classes behave are summarized here; where it recommends something, it recommends it for treating historic objects, and that context is kept.

The CWC on structural wood adhesives

These points concern the adhesives used to manufacture structural wood products.

• Adhesives can also be referred to as resins.
• Engineered wood products (finger-joined lumber, plywood, OSB, glulam, CLT, wood I-joists and other structural composite lumber) need adhesives to transfer stresses between adjoining wood fibres. Waterproof and heat-resistant adhesives are commonly used in structural wood products.
• Newer adhesives must achieve the performance of the traditional structural adhesives, phenol-formaldehyde (PF) and phenol-resorcinol formaldehyde (PRF). Families named: emulsion polymer isocyanate (EPI), one-component polyurethane (PUR) and phenolic resins (PF, PRF).
• Extenders such as walnut shell flour, Douglas fir bark flour, alder bark flour and wood flour are sometimes used to reduce cost, control penetration into the wood fibre or moderate strength.
• CSA standards the CWC lists: CSA O112.6 (phenol and phenol-resorcinol adhesives, high-temperature curing), CSA O112.7 (resorcinol and phenol-resorcinol adhesives, room- and intermediate-temperature curing), CSA O112.9 (evaluation of structural wood adhesives, exterior exposure), CSA O112.10 (the same, limited moisture exposure) and CAN/CSA O160 (formaldehyde emissions from composite wood products).

How a bond works (CCI, Chapters 1–4)

• Adhesion is the attachment of adhesive and substrate at their interface (by adsorption, chemical bonding, mechanical interlocking, interdiffusion and electrostatic forces). Cohesion is the internal strength of the adhesive film. Both must be strong.
• Bond failure can be adhesion failure (at the interface), cohesion failure of the adhesive (within the film), both together (the most typical), or cohesion failure of the substrate (the bond was stronger than the material).
• Thermoplastic polymers can be melted and hardened repeatedly or dissolved; thermosetting polymers are crosslinked and cannot be softened by heat or dissolved.
• Wetting: the liquid adhesive must wet the surface; liquids wet only surfaces of higher energy than their own, and sanding or etching makes a surface more polar and wettable.
• Additives: thickeners raise viscosity and open time and reduce the risk of a starved joint on porous surfaces; fillers extend the product and reduce penetration; humectants prevent skinning and lengthen open time; plasticizers add flexibility; freeze-thaw stabilizers keep dispersions from separating if frozen in shipping.
• Good bonding needs clean surfaces (among the CCI's examples of industrial surface preparation: sanding or planing wood to remove resins, waxes, paints and oxidation products), a good fit between parts (gaps give weak bonds), good contact (a liquid that wets, plus clamping while it sets) and a thin bond line (strength falls as the glue line thickens; thick bonds hold more voids).
• Joint stresses: shear uses the joint area best and resists failure most; tensile is comparable but needs thick parts; peel occurs when a part is flexible and, unless the bond is wide and the load small, leads to failure; cleavage occurs when a tensile join deflects. Bond strength grows with width, but extra overlap length stops helping after a point.
• Set and cure: "set" is any conversion to a hardened state; "cure" is setting by chemical reaction. Three setting mechanisms: liquid solidification (melt-freeze, e.g. ethylene/vinyl acetate copolymers and waxes; animal glues gel and then lose water), evaporation (solutions and dispersions; contact adhesives are dried on the surfaces and then pressed together) and chemical cure (epoxies).
• Shrinkage on setting (typical, by volume): epoxy 4–5%, wax 13%, PVAC dispersion 50%, cellulose nitrate 60%, hot-melt animal glue 65%.
• Glass transition temperature (Tg): below it a set adhesive is hard and glassy, above it rubbery. A soft or tacky set adhesive picks up dust; a hard, brittle one can crack when stressed; and Tg should not be close to ambient temperature.
• Stiffness: an adhesive should generally mimic the flexibility and strength of its substrate, except when the substrate is rigid; wood is semi-rigid, so a semi-rigid adhesive is best. For most polymer-based adhesives, flexibility and cohesive film strength are inversely related.
• Shelf life (storage life) is the period during which the adhesive remains usable, or during which the manufacturer guarantees that it, stored at a specified temperature, will give the specified properties; refrigeration usually extends it.

Adhesive classes (CCI, Chapters 6 and 7)

Animal (hide) glue. Its parent substance is collagen (from hides, skins or bones); hide glue is generally stronger than bone glue and usually exceeds the strength of wood; high-strength bonds are obtained under dry conditions and can be structural as long as they remain indoors. Hot hide glue is soaked in cold water, heated to 60–70°C, gels as it cools and then hardens by losing water; it can be re-liquefied with heat. Cold liquid hide glue has gel suppressants (salt, urea), a working time of up to an hour, and may not harden if a urea-type glue is past its shelf life. Open time depends on gel temperature and gel strength; too much penetration starves the joint. Dried glue can be sanded and has excellent creep, solvent and shock resistance, but may become brittle with age; it does not function as a particularly cohesive material in thick applications, becomes brittle when desiccated and softens when humid. Probably the most important reason conservators use it so widely on wooden furniture is that it is almost completely reversible while performing very well under reasonable environmental conditions.

Casein glue. From milk protein; alkaline, insoluble films, cures hard and sands, good strength and creep resistance, but may dull tools, stain oak and mahogany, and biodegrade.

Poly(vinyl acetate) (PVAC; the RSOS writes PVA). Adheres to more surfaces than any other adhesive and is the most common wood glue. Dispersion (emulsion) adhesives are the most common adhesive in contemporary woodworking; their advantages are that they are ready to use, have a moderate working time, clean up with water and have a good shelf life. Limits: may creep under sustained load, low resistance to weather and moisture (interior use), some grades have a Tg near room temperature. Shelf life of dispersions 6–12 months in cool conditions; some dispersions contain vinyl acetate monomer (a suspected carcinogen), so ventilate and check the SDS. Copolymers have a lower Tg and more flexibility than homopolymers; ethylene copolymers have good heat resistance and better creep resistance than the ester copolymers.

Hot-melts. Used molten and hardened solely by cooling; sticks for a heated glue gun, or sheets and films for heated or pressed surfaces. EVA copolymers are typical hot-melt solids. In wood conservation they are valued for flexible gap-filling, and formulations easily reversible with heat or solvents are available.

Contact adhesives. Applied as solutions or dispersions to one or both surfaces, allowed to dry, then pressed together. Those found on wooden artifacts are frequently rubber-based, at least initially thermoplastic, and can often be softened with heat and/or organic solvents.

Epoxy resins. Two parts, resin and hardener, mixed in a set ratio and reacting exothermically into a crosslinked thermoset. Outstanding toughness, adhesion to many materials, chemical and heat resistance, high strength, low creep and low shrinkage, but generally low peel strength; solventless; cannot be dissolved once cured. Mix small quantities (heat), cure preferably at 40–55% RH and 20–25°C, use fresh components, keep alcohols (which speed the cure) and acetone (which retards it) in mind; wear gloves and glasses and mix with ventilation.

One-part polyurethanes (described in the compendium's chapter on plastics and rubbers). Cure by reacting with ambient moisture, so open time depends on relative humidity; they expand and foam as they cure, fill well and are flexible, but discolour and need very firm clamping to resist the expansion.

Reactive adhesives in conservation. Epoxies, urea formaldehydes, urethanes and phenolics are essentially intractable and irreversible once cured and usually much harder than the surrounding wood, so their use for routine gluing in wood conservation is generally discouraged. Because they change dimension very little on curing they may be good gap-fillers (with the gluing margins sized with an isolating barrier coating). Crosslinking reactive and thermosetting adhesives are almost never used as replacement adhesives when treating joint failure; the most widely used are hot and cold hide glues and hot-melt synthetics.

Restoring a failed glue joint (CCI, Chapter 7)

Three approaches: re-activate the old adhesive (only thermoplastics; aged hide glue is swollen with warm water by syringe and lightly clamped, rarely giving structural strength); add a new adhesive that is compatible with and bonds to the old one (the RSOS requires the same compatibility in H-22); or remove the degraded adhesive (mechanical removal and solvent, used together) and re-glue. Clamping frequently squeezes out excess glue, which must be cleaned immediately. A fill must be weaker than the adjoining artifact, so that if stresses cause damage in the future it is to the repair, not the artifact. Adhesive removal techniques used in conservation include mechanical scraping, heat, solvents, enzymes (generally only when all else fails) and microemulsions; usually, but not always, one starts with the least intrusive.