Wood processing variables and adhesive joint performance

Abstract

SynopsisIn order to obtain a bonded wood product from the tree, wood must be subjected to a series of processes. These include, among others, vital processes like drying of the green wood and machining of the surfaces of the dried wood in preparation for bonding. However, when wood is machined with dull blades, thermal degradation of the wood frequently occurs. The effects of kiln drying and thermal degradation of wood on the performance of the adhesive joints subsequently formed from such wood were investigated. Tapered double cantilever beam (TDCB) test specimens obtained from hard maple and phenol-resorcinol-formaldehyde (PRF) adhesives were used in the investigations. The enhancement of fracture energy due to the reduction in the moisture content of wood, the improved performance of adhesive joints due to rejointing of wood surfaces before bonding, and the reduction of adhesive joint strength resulting from thermal degradation of wood are presented and discussed. INTRODUCTIONAdhesives are, by definition, substances capable of fastening materials together by surface attachment. Therefore, the success or failure of an adhesive in the peformance of this function depends very strongly on its ability to adhere to the materials being bonded. The success of the adhesion process itself is dictated partly but significantly by the topography of the adherends. Wood surface topography is a strong function of the processing variables. Going from the tree to the bonded wood product involves a number of processes. These include, among others, such vital factors as drying of the green wood, machining of the surfaces in preparation for bonding, and storage before bonding. We have previously reported the effects of surface roughness, the nature of surface roughness, and aging of machined surface (by storage) on adhesive joint fracture energy.Q In this paper we present results of studies on the effects of kiln drying and thermal aging of wood on adhesive joint performance. ADHESIVE JOINT STRENGTH MEASUREMENT-THE FRACTURE MECHANICS APPROACHFracture mechanics is a relatively new discipline which relates the fracturing behavior of flawed bodies to applied loads. Fracture mechanics is a direct outgrowth of the Griffith theory which postulates that the lower-* To whom all inquiries should be addressed. have developed and applied the tapered double cantilever beam (TDCB) geometry to a number of composite systems using metal adherends. We have adapted this for use in wood-based adhesive systems. Details of this have been given elsewhere. 1.2 Using the TDCB, the fracture energy is given by the equation G. G is related to K by the equations 4PZEb2 GIc = -m where P, = critial load necessary to cause a previously stationary or slowmoving crack to propagate abruptly, b = width of beam, m = a constant chosen so that compliance changes linearly with crack length [m = 0.52 cm-' (1.33 in.-1) for all specimens in this study], and E = bending modulus of adherends. At P,, the crack propagates until enough energy is released to bring the crack ...

Development of wood-products adhesives from mangrove bark

“…The observed trends in the strength properties of mangrove tannin adhesive joints are similar to those observed by Ebewele et al, 23 who used Tapered Double Cantilever Beam (TDCB) test specimens and commercial phenol-resorcinolformaldehyde (PRF) as adhesive. These authors claim that, for PRF adhesives, the fracture toughness is strongly dependent on cure temperature and cure time.…”

Compatibilization of Natural Fibers with Synthetic Polymers Using Triblock Copolymers as Coupling Agents

“…In wood science, the measurement of G c with a DCB configuration was mainly employed to evaluate wood adhesive performances, or wood properties. [33][34][35][36][37][38][39] A symmetric DCB configuration was often employed in these analyses and samples were fractured toward an opening-mode I loading test. Consequently, the mechanics at the interface and the sample geometry were first studied in this manner.…”

Quasi‐Static Fracture Tests

“…Testing methods that are commonly applied and were probably developed for other material systems have traditionally been applied for evaluating the properties of bonded wood. Data regarding fracture properties of wood and bonded wood are available in the literature; just to mention a few, Ebewele et al [107,108] showed that fracture properties in bonded wood are affected by a number of factors, such as the wood structure, the adhesive penetration into the wood, and the wood surface roughness.…”

“…Given the influence of a large number of factors and the fact that wood is a natural material, it is commonly found that fracture measurements taken on wood and bonded wood are characterized by data spread that is usually larger than the corresponding values obtained when testing more uniform adherends [107][108][109][110]. In particular, Triboulot et al [111] indicated with experimental and numerical studies that the high variability in wood properties requires researchers to deal with extensive statistical analysis when critical fracture toughness values are to be evaluated.…”