Significant deviations from ideal processing conditions (especially in combination) may significantly weaken adhesive joints. If not detected by subsequent control measures, these weak joints may lead to costly esthetic or technical problems and sometimes to injuries and deaths. An evidence-based analysis of such bonding failures is required to address whether the materials used, the processing conditions, the improper use conditions, or combinations of the three factors are the root cause.
The root cause of adhesive problems is recognizable by the appearance of the failed glue line, the location of defects, and the frequency of its occurrence [1, 2]. Light microscopy and scanning electron microscopy are frequently used techniques for failure analysis of wood bonds. The type of adhesive is determined by infrared spectroscopy. Staining methods may also identify the type of adhesive (e.g., staining of PVAc with iodine) and improve the contrast of the adhesive to the wood [1, 3].
Figure 1 shows schematic microscopic illustrations of cross sections from a perfect bond line and some typical failures of wood and wood-based materials. The solidified adhesive may separate within the glue line (cohesion failure), in the interface of wood and adhesive (adhesion failure) or within the wood (wood failure). An overly low pressure or overly high viscosity of the adhesive leads to thick bond lines with low adhesive penetration in the wood pores . The advantage of a good penetration of low molecular weight thermosetting adhesives (melamine-, urea-, and phenol-formaldehyde) has been demonstrated [5, 6]. These adhesives also penetrate into the wood cell walls and stabilize the weak boundary layer of the wood surface. A positive influence of a good penetration is also observed for not cell wall penetrating adhesives like polyurethane, polyvinyl-acetate and epoxy-resins . A too high pressure or a too low viscosity of the adhesive may lead to a starved bond line with overly high penetration and almost no adhesive remaining in the glue line. There is no perfect level of penetration. For a particular mill under a particular set of conditions the penetration correlates to bond performance, but a general answer to the question of ideal level of penetration does not exist .Fig. 1
Schematics of cross sections of two hardwood blocks with different patterns of failing adhesives. The schematic wood structure shows pores (present in hardwoods) and wood rays (strips of wood cells that extend radially within a tree). Several pores in some distance of the glue line are filled by adhesive. The pores are always more or less tilted to the plane of the glue line. From where they touch the glue line (in front of or from behind the cross section documented) the adhesive penetrates the pores, and the adhesive appears in pores seemingly without contacting the glue line
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Separations that occur while the adhesive is still in a viscous, liquid or paste-like, state are regularly observed in failure analysis . The two pieces may have never contacted, or they may have contacted initially but separated before the adhesive solidified (Fig. 2). In the latter case, either cavitation (a honeycomb-like structure) or air fingers (dendritic structures) develop. The separation of the glue line may be complete. Often, the two pieces are still fixed by small remaining contact points, and final separation occurs during use.Fig. 2
Schematic cross section of adhesives failing through separation during the pressing process and real-life examples for the resulting patterns of the surface of the adhesive
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The usefulness of these air fingers and cavitation for failure analysis is easily recognizable. The observation of these structures in failing adhesive joints allows a profound statement concerning the viscosity of the adhesive at separation. However, there are limitations to the interpretation of these patterns. For thermoplastic adhesives, resoftening may be promoted by elevated temperatures or chemical influences during use. Additionally, the structures might be confused with foaming polyurethane adhesives; however, the bubbles in polyurethanes may stack, while cavitation appears in this work as a monolayer. Soft adhesives that are not commonly used in the wood industry might develop similar patters by creep.
A resoftening of thermoplastic adhesives has thus far not been identified in failure analysis by the Fraunhofer Institute for wood research (WKI); nevertheless, the possibility of its occurrence cannot be excluded. We have observed this pattern several times in blisters of dark coatings with overly temperature-sensitive base coats. These patterns have been observed in renovation coatings when linseed oil residues in the wood are present. The oil penetrates and resoftens the new coat.
Several authors have investigated the principles of the formation of fingers and cavitation in detail. Depending on the separation speed, the viscosity and the thickness characteristics of the fluid, air fingers or cavitation may occur [7,8,9]. Cavitation originates from unavoidable preexisting bubble nuclei in the range of a few micrometers (µm) that enlarge when pulling forces are applied . Further studies have concentrated on the transition from the growth of individual cavities to the collective growth of a population of cavities with elongating walls . Several studies have improved the understanding and mathematical modeling of the formation of fingers [11,12,13]. The experiments described in the literature have been performed with water-free substances, such as silicone oils, and air-impermeable substrates, such as steel and glass, however, most adhesives in the wood-based products industry have water contents between 30 and 60% and wood quickly absorbs water and thereby reduces the viscosity of an adhesive and wood contains air that might change the formation of the structures. The factors that influence the shapes of the structures and the occurrence of the finger-type vs. the cavitation-type, the research presented here investigates on the influences of the separation speed, substrate, thickness and viscosity characteristics of the adhesive.
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