9 Wood Adhesion and Adhesives Charles R. Frihart USDA, Forest Service, Forest Products Laboratory, Madison, WI CONTENTS 9.1 General..................................................................................................................................216 9.2 Wood Adhesive Uses............................................................................................................217 9.3 Terminology..........................................................................................................................219 9.4 Application of the Adhesive.................................................................................................220 9.4.1 Adhesive Application to Wood ................................................................................220 9.4.2 Theories of Adhesion ...............................................................................................221 9.4.3 Wood Adhesion ........................................................................................................225 9.4.4 Wood Surface Preparation........................................................................................225 9.4.5 Wood Bonding Surface ............................................................................................226 9.4.6 Spatial Scales of Wood for Adhesive Interaction ....................................................229 9.4.7 Wetting and Penetration in General .........................................................................230 9.4.8 Wetting, Flow, and Penetration of Wood .................................................................231 9.5 Setting of Adhesive ..............................................................................................................234 9.5.1 Loss of Solvents .......................................................................................................235 9.5.2 Polymerization ..........................................................................................................236 9.5.3 Solidification by Cooling .........................................................................................237 9.6 Performance of Bonded Products ........................................................................................238 9.6.1 Behavior under Force ...............................................................................................238 9.6.2 Effect of Variables on the Stress-Strain Behavior of Bonded Assemblies..............................................................................................241 9.6.3 Bond Strength...........................................................................................................242 9.6.4 Durability Testing .....................................................................................................245 9.7 Adhesives..............................................................................................................................246 9.7.1 Polymer Formation ...................................................................................................246 9.7.2 Self-Adhesion ...........................................................................................................248 9.7.3 Formaldehyde Adhesives..........................................................................................249 9.7.3.1 Phenol Formaldehyde Adhesives ..............................................................250 9.7.3.2 Resorcinol and Phenol-Resorcinol Formaldehyde Adhesives..................252 9.7.3.3 Urea Formaldehyde and Mixed Urea Formaldehyde Adhesives .............254 9.7.3.4 Melamine Formaldehyde Adhesives .........................................................255 9.7.4 Isocyanates in Wood Adhesives ...............................................................................257 9.7.4.1 Polymeric Diphenylmethane Diisocyanate...............................................258 9.7.4.2 Emulsion Polymer Isocyanates .................................................................260 9.7.4.3 Polyurethane Adhesives ............................................................................260 9.7.5 Epoxy Adhesives ......................................................................................................261 9.7.6 Polyvinyl and Ethylene-Vinyl Acetate Dispersion Adhesives.................................263 9.7.7 Biobased Adhesives ..................................................................................................265 0-8493-1588-3/05/$0.00+$1.50 © 2005 by CRC Press LLC 215 216 Handbook of Wood Chemistry and Wood Composites 9.7.7.1 Protein Glues.............................................................................................265 9.7.7.2 Tannin Adhesives ......................................................................................266 9.7.7.3 Lignin Adhesives.......................................................................................267 9.7.8 Miscellaneous Composite Adhesion ........................................................................267 9.7.9 Construction Adhesives ............................................................................................268 9.7.10 Hot Melts ................................................................................................................ 269 9.7.11 Pressure Sensitive Adhesives ...................................................................................269 9.7.12 Other Adhesives........................................................................................................270 9.7.13 Formulation of Adhesives ...................................................................................... 270 9.8 Environmental Aspects .........................................................................................................272 9.9 Summary...............................................................................................................................272 References ......................................................................................................................................273 9.1 GENERAL The recorded history of bonded wood dates back at least 3,000 years to the Egyptians (Skeist and Miron 1990, River 1994a), and adhesive bonding goes back to early mankind (Keimel 2003). Although wood and paper bonding are the largest applications for adhesives, some of the fundamental aspects are not fully understood. Better understanding of the critical aspects in wood adhesion should lead to improved composites. The chemistry of adhesives has been covered in detail; however, how the adhesives hold wood together when under external and internal stresses need to be better understood from the basic scientific principles. This chapter is aimed at more in-depth coverage of those items that are not covered elsewhere. It will touch briefly on topics covered by other writers and the reader should examine the recommended books and articles for more details. Many of the books on adhesives and adhesion are long and complicated, but at least one is briefer, while still being quite thorough (Pocius 2002). Adhesives are designed for specific applications, leading to thousands of products (Rice 1990). Petrie has broken adhesives into 20 groups of synthetic structural, 11 groups of elastomeric, 12 groups of thermoplastic, and six groups of natural adhesives (Petrie 2000). Brief has summarized the vast number of markets for adhesives (Brief 1990). Understanding how an adhesive works is difficult since adhesive performance is not one science of its own, but the combination of many sciences. Adhesive strength is defined mechanically as the force necessary to pull apart the substrates that are bonded together. Mechanical strength is dependent upon primary and secondary chemical bonds of the polymer chains in the adhesive, wood and adhesive-wood interphase. Thus, one needs to consider both the chemical and mechanical aspects of bond strength, and the interrelation of the two factors. Because adhesive strength is a measurement of failure, the process determines where the localized stress exceeds the bond strength under specific test conditions. One concept is the idea of the bonded assembly being a series of links representing each phase with the failure occurring in the weakest link (Marra 1980). Although the bend is actually more a continuum than discrete links. The localized stress is usually very different from applied stress due to stress distribution and concentration (Dillard 2002). It is generally preferred that the adhesive bond be stronger than the substrate so that the failure mech- anism is one of substrate fracture. There are generally three steps in the process of adhesive bonding. The first is usually the preparation of the surface to provide the best interaction of the adhesive with the substrate. Even though a separate treatment step may not be used in some cases, the knowledge of material science (surface chemistry and morphology) is important for understanding this interaction. Preparation of the surface can involve either mechanical or chemical treatment or a combination of the two. In some cases, the adhesive is modified to deal with problems in wetting of the surface or contamination on the surface. Surface analysis techniques are often more difficult on wood than other materials due to the complex chemistry and morphology of the wood. Wood Adhesion and Adhesives 217 The second step is that the adhesive needs to form a molecular-level contact with the surface; thus, it should