Wood Adherends

There are two main types of wood in commercial use— the softwoods, which come from evergreens such as the pines, firs, hemlocks, and larch, and the hardwoods, which come from deciduous varieties such as maple and mahogany. It should be noted that the terms softwood and hardwood refer to botanical characteristics of the wood and are not related to the surface hardness. 

Wood has a well-defined cellular structure. In softwoods and ring-porous hardwoods the cells produced in the early part of the growing season, the earlywood, are larger than those produced later in the season, the latewood. In diffuse-porous hardwoods the cells are more uniform in size. These differences in cell structure are responsible for the variation in density observed among wood of different species and even in wood of the same species, depending on local growth conditions.  

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The wood adherends were clear, flat-grained, rotary-peeled yellow birch veneer with moisture content equilibrated at 27 °C and 30% relative humidity (RH). The pieces were 3.2 mm thick and 150 mm by 150 mm in area. 

Second, shear forces that develop in the vicinity of the crack tip direct it toward one or the other adherend [52]. Shear forces arise in a cleavage specimen from unequal moduli of the two adherends and the adhesive. Unequal moduli of two wood adherends is virtually certain as a result of the variable morphology and density of any two pieces of wood. When a load is applied to the cracked joint, this inequality induces shear stress around the crack tip and thereby directs it toward one adherend or the other. 

Brewis, Comyn, and Phanopoulos i) have very recently made joint durability evaluations of nonextracted, water-extracted, and solvent-extracted wood adherends their results indicate that the level of surface extractions have a major influence on both strength and durability of the joints. 

Notes Tensile DIN 53455, Shear DIN 53283 (failure in wood adherend). Cured Notes Tensile DIN 53455, Shear DIN 53283. Cured 24hr/RT + 3hr/105 C. ... 

Notes Tensile DIN 53455, Shear DIN 53283 (failure in wood adherend). Cured ... 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

Various commercial adhesives have been used to provide bond strength with nylon on the order of 250 to 1000 psi. Priming of nylon adherends with a composition based on resorcinol formaldehyde, isocyanate modified rubber, and cationic surfactants has been reported to provide improved joint strength. Some epoxy, resorcinol formaldehyde, phenol-resorcinol, and rubber-based adhesives have been found to produce satisfactory joints between nylon and metal, wood, glass, and leather. Exposure of nylon 6 to oxygen and helium plasmas for 30 s to 1 min improved the adhesion of two-part epoxy adhesives.66... 

In the case of adherends such as wood, the possibility exists that the carbohydrate polymers or lignin, or both, can be modified in situ to give activated surfaces 65,66) or surfaces containing reactive groups. Then, the surfaces are bonded by direct reaction or by reaction with a reactive, gap-filling compound. 

Experimental load deflection curves (Fig. 3.) illustrate the large difference in crack propagation observed in each case. A difference in stiffness between both bonded specimens is observed and results from either a difference in the bond line quality or from interfacial conditions. For both specimens, adherends were made from the same sample of wood. Both wood substrates contained no apparent defects and had the same longitudinal Young s modulus (14500 MPa). Both also had the same growth characteristics (oven dry specific density, annual growth rings), and as a consequence very close values of transverse and shear modulus adjacent to the bond line. Thus, any difference in stiffness is likely to be due to... 

Adhesives wet, flow, and set to a solid during bond formation. The transformation from liquid adhesive to solid bond can be achieved in a number of ways. Where the adhesive is a polymer, the initial starting material is a liquid monomer or prepolymer that, under the conditions of bonding with heat, pressure, and/or catalyst, polymerizes to the solid polymer in the glue line. It is also usual to apply solutions of preformed polymers in suitable solvents to the faces of adherends, and allow bond formation to take place with evaporation of solvent. Alternatively, polymers that can be melted or softened to flow at elevated temperatures can be applied as hot melt adhesives to form the bond on cooling. With porous adherends like wood, penetration of the pores by liquid or molten adhesives is an important factor in bond formation. 

Due to the considerably higher heat conductivity of metals in contrast to wood or plastics, it is advisable to preheat the adherends to be bonded to the temperature of the melt in order to achieve good adhesive strengths. (Hot-air gun, where appropriate, hair-dryer at highest heat level because of its electric conductivity never use a microwave oven )... 

Solvent-based adhesives are adhesives with polymers dissolved or pasted in organic solvents. The solvents or solvent mixtures are only processing aids and have to be removed, either partly or completely, from the applied liquid adhesive layer through evaporation or penetration prior to the fixing of the adherends. The first case is necessary for solvent-impermeable materials (metals, glass, thermosetting plastics), the second case concerns porous and solvent-permeable materials (paper, cardboard, wood, leather). This process can be accelerated by heat supply. Solvents are mainly esters, ketones, if applicable, portions of different alcohols. The total solvent portion ranges between 75-85%. 

In the case of permeable, respectively, porous materials (papers, cardboards, wood, etc.) the fixing of the adherends is already possible when the minimum... 

One-sided bonding (wet bonding) In this procedure, the adhesive is applied to one adherend only. This is recommendable for bonding of solvent-permeable, respectively, absorbent materials (leather, textiles, wood products). In this case, complete solvent evaporation is not required. 

Since the water content of dispersion adhesives must be absorbed by the adherends during setting, the setting time depends mainly on the moisture content, above all in the case of wood (favorable range 8-10%) (Section 9.5). 

This positive or mechanical interlocking (it is also talked of mechanical adhesion ) is indeed a possibility of joining adhesive layer and adherend. It occurs preferably in the case of very rough and/or porous surfaces, for example, of papers, cardboards, wood, ceramics or plastic foams. This concept fails in the case of smooth surfaces, however, surfaces we call smooth can actually show a mountainous structure under the microscope. But such fine roughnesses can hardly contribute to sufficient mechanical interlocking. Thus another possibility must exist which enables the adhesive layer and the adherend to be j oined firmly and permanently. 

In practice, the temperature-time curve schematically shown in Figure 7.16 does not only depend on the adhesive-related parameters, but also on the properties of the adherends, especially on their thermal conductivity. High thermal conductivity (e.g., metals) leads to shorter heating times than low thermal conductivity, as is to be found, for example, with plastics, glasses, wood. Even the dimensions of the adherends play a role. 

Setting time increases according to the moisture content of the adherends, since increasing humidity of the adherend (e.g., wood) delays the release of water from the dispersion. 

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