By far, the largest amouts of adhesives are used to manufacture building materials, such as plywood, structural flakeboard, particaleboards, fiberboard
, structural framing and timbers, architectural doors, windows and frames, factory-laminated wood products and glass fiber insulation. Adhesives can effectively trasfer and distribute stresses, thereby increasing the strength and diffness of the composite. Effective transfer of stress from one member to another depends on the strength of the links in an imaginary chain of an adhesive-bonded joint. Thus, performance of the bonded joint depends on how well we understan and control the complexity of the factors that constitue the individual links-wood, adhesive, and the interphasing regions between-which ultimately determine the strength of the chain. The american Society for Testing and Materials (ASTM) defines an adhesive as a substance capable of holding material together by surface attachment. An adherend is a substrate held to another substrate by an adhesive. adherend [?d'hi?r?nd] n. 被粘物；黏剂 网络释义 adherend:粘附体|被粘物|涂胶体，粘附体 Adhesion is the state in which two surfaces are held togther by interfacial forces, which may be valence forces, intelocking action, or both. valence ['veil?ns] n. [化]价；原子价；化合价；[生]效价 网络释义 Valence:瓦朗斯|价数|原子价 valence band:价带|价电子带|价电带 bond valence:键价|释义：键价 valence force 原子价力 网络释义 valence force:原子价力 valence force field:价力场 Organic polymers of either natural or synthetic origin are major chemical ingredients in all formulations of wood adhesives.
6 Adhesives Polysaccharides and proteins are high molecular weight natural polymers derived from plants and animals. Animal, blood, hide, casein, starch, soybean, dextrin, and celluloseic adhesives are all derived from the natural polymers found in these indicated souces. They have been used as adhesives for centries and are still in use today, although they have been replaced mostly by adhesives made with synthetic polymers. The first wood adhesives based on synthetic polymers were produced commercially during the 1930s. This marked the beginning of fundamental changes in composition of adhesives from natural to synthesized polymers. These adhesives could not only be stronger, more rigid, and more durable than wood, but also have much greater resistance to water than adhesives from natural polymers. Synthetic polymers are chemically designed and formulated into adhesives to perform a great variety of bonding functions. Whether the baes polymer is thermoplastic or thermosetting has a major influence on how an adhesive will perform in service. Thermoplastics are long chain polymers that soften and flow on heating, and then harden again by cooling. They generally have less resistance to heat, moisture, and long-term static loading than do thermosetting polymers. Common wood adhesives that are based on thermoplastic polymers include polyvinyl acetate emulsions, elastomeric, contacts, and holt-melts. Thermosetting polymers make excellent structural adhesives because they undergo irreversible chemical change, and on reheating, they do not soften and flow again. They form cross-linked polymers that have high strength, have resistance to moisture and other chemicals, and are rigid enough to suport high, long-term static loads without deforming. Phenolic, resocinolic, melamine, isocyanate, urea, and epoxy are examples of types of wood adhesives that are based on thermosetting polymers. A formulation of wood adhesive consists of a mixture of several chemically active and inert materials that vary in proportion with the basic adhesive polymer, which enhances performance,
6 Adhesives whether it be working characteristics, strength properties, shelf life, or durability. Solvents disperse or dissolve adhesive polymers, act as carries of polymer and additives, aid wetting, and control flow and penetration of the adhesive. Water is used as the carrier for most wood adhesives, primarily because water readily absorbs into wood, is inexpensive, and is free of toxicity problems. Fillers of both organic and inorganic origins contribute to rheological control of the fluid system, particularly in reducing the spreading and penetrating of the adhesive into wood. Reinforcing fibers, mosty inert and of organic origins, can enhance an adhesive film’s mechanical properties, especially toughness, impact resistance, and shrinkage. Extenders are filler-like organic materials that may have sufficient chemical activity to improve adhesion to a small degree, but they are used primarily to control flow and other working characteristics, without exess sacrifice of adhesion capability, as is the case with most fillers. Catalysts are chemicals used to accelerate the rate of chemical reaction of polymeric components. Acids, bases, salts, peroxides, and sulfur compounds are a few examples of catalysts. Catalysts do not become a part of the reacted compound; they simply increase the rate of reaction. Table 6.1 Wood adhesives catagorized according to their expected structural performance at varing levels of environmental exposure a.b
Structural inegrity Structural Service environment Fully exterior (withstands long-term water soaking and drying) Adhesive type Phenol-formaldehyde Resorcinol- formaldehyde Phenol-resocinol- formaldehyde Emulsion polymer/isosyanate Melamine- formaldehyde
Limited exterior (withstands short-term water soaking) Interior (withstands short-term high humidity)
Melamine-urea- formaldehyde Isosyanate Epoxy Urea- formaldehyde casein
Cross-linked polyvinyl acetate Polyurethane
Nonstructural Interior Polyvinyl acetate Animal Soybean Elastomeric construction Elastomeric contact Hot-melt Starch
Assignment of an adhesive type to only structural/service environment category does not exclude certain
adhesive formulations from falling into the next higher or lower category. b Priming wood surfaces with hydroxymethylated resocinol coupling agent improves resistance to
delamination of epoxy, isocyanate, emulsion polymer/isocyanate, melamine and urea, phenolic, and resocinolic adhesives in exterior service environment, particularly bonds to CCA-treated lumber.
A broad array of adhesive types, with significant variations within each type, are available for bonding wood materials, even for bonding wood to nonwood materials. The selection process begins with determing which adhesives are compatible with the physical and chemical properties of the adherends, particularly their surface properties. The polar, aqueous wood adhesive must be capable of wetting the usually polar wood surface, within its normal variations in hydrophilicity. As the adhesive wets, it must have flow properties that enable it to spread over surfaces of variable roughness and to penetrate wood structures that differ in porosity, with respect to grain orientation at the bondline. The adhesive must make molecular contact with the lignocellulosics of wood and penetrate deeply enough to mechanically interlock with the wood’s cell structure. Metal and plastics cannot be penetrated, so these materials generally cannot be bonded with aqueous wood adhesives. However, nonaqueous, 100% solids adhesive, including epoxy, isocyanate, and polyurethane, are capable of sound bonds to nonwood and wood materials. When a group of adhesives with suitable performance capabilities for a particular bonded assembly has been determined, the user also must choose within that group an adhesive that can be mixed, applied, and cured with available equipment or consider the cost of purchasing equipment to meet specific working properties of another adhesive. Important working properties must be considered when making cost decisions. The working life of an adhesive is the time between mixing and the end of its useful life when it
6 Adhesives becomes too viscous to properly wet and flow over a surface. If an adhesive requirs mixing with a hardener or catalyst, then mixing and application equipment appropriate for the working life must be considered. Wood adhesives, including phenolic, melamine, urea, and isocyanate adhesive, must be cured at high temperatures and require expesive, heated press. Some of these can be cured within minutes in expesive high frequency heated presses. Cold presses or clamps are satisfactory for room-temperature-curing adhesives, although the long curing time in production can be a constranint. Even after hot or cold pressing, adhesive bonds must remain undisturbed until most of the curing has occurred. There are other important considerations, particularly in furniture and interior millwork, where appearance is all important. Adhesive color, ability to absorb stains and finishes, and freedom from bleeding and staining are critical factors. The urea-formaldehyde and polyvinyl acetate adhesives used in the furniture industry are formulated to give a colorless joint with good acceptance of stain, Ease and simplicity of use can also be important factors.