Лако-красочные материалы — производство

The moisture content of both the wood and the adhesive affect the fracture behavior of adhesive bonded joints. Wood joints are especially sensitive to moisture effects as a result of the porosity and permeability of wood, which allows ready access by water to both the interior of the wood member and the adhesive layer. Irle and Bolton [57] showed that the superior durability of wood-based panels bonded with an alkaline PF adhesive compared to panels bonded with a UF adhesive was due to the ability of the phenolic adhesive to absorb and be plasticized by water. In the plasticized state, the phenolic adhesive is able to reduce stress concentrations that otherwise fracture the wood or the adhesive in urea — bonded panels.

Another important effect of moisture is due to a change in the moisture level, or content, of the wood member in a dynamic service environment. In thick members, changes in moisture content and the moisture-dependent dimension in the center fall behind changes that occur at the surface of the member. The difference in dimension creates stress in the member and bonded joints in the member. Adhesive bonds also restrain the swelling and shrinking of bonded members with different swell/shrink coeffi­cients resulting from grain direction, growth-ring angle, or species. Moisture gradients and differential swelling or shrinking of the adherends are common causes of fracture of joints or materials. In this regard the size of the bonded members and the mechanical properties of the adhesive and the adherends have important roles in determining the magnitude of the stresses (and stress concentrations) that arise from moisture changes. The most severe stresses arise as both the adhesive and the wood dry because of the attendant differential increases in the adherend and adhesive moduli.

Simply changing the growth-ring orientation in adjoining laminate can alter the possibility of fracture in the vicinity of the joint caused by a change in moisture content of the laminated member. Laufenberg [60] studied the effects of growth-ring orientation in parallel Douglas-fir laminates. By finite element analysis, he showed that maximum stresses occurred at the edge of the laminate when one lamina had flat grain and the other vertical grain. He also found that a difference of growth-ring angles of only 15° was likely to produce splits or delamination as a result of moisture content cycling.

Nestic and Milner [61] also examined the effects of growth-ring orientation and found vast differences, particularly in the peak tensile stresses perpendicular to the grain, that depended on the difference between growth-ring orientation of adjoining lami­nae. The authors also found that the closer the pith was to a bondline, the greater the stress in the bondline induced by a moisture content change in the wood.

When the laminae are cross-laminated, the stresses are even more severe. Adherends thicker than roughly 5 mm will create sufficient stress to fracture the wood when bonded in a cross-laminated configuration. The most severe stresses arise as both the wood and the adhesive dry out, with an accompanying increase in strength and modulus. However, the stresses imposed by differential swelling of wood members are also severe in the case of an adhesive that is overly plasticized and weakened at high moisture contents. The effects of wood and adhesive properties and the environment on fracture behavior are complex. The effects of internal stress generated by wood on adhesives with varying sensitivity to moist­ure have been described [62]. Gillespie [62] compared the effects of medium-density, high — swelling maple (Acer saccharum) to low-density, low-swelling pine (Pinus strobus) using the same adhesives. The joints of maple bonded with moisture sensitive adhesives (PVA, catalyzed PVA, and casein) were destroyed or suffered severe and irrecoverable loss of strength from soaking. Similar joints of pine recovered all or most of their original strength upon redrying.

Internal stress may detract significantly from the apparent strength of a joint even if it is insufficient to fracture the joint. For example, if the internal tensile stress in a joint is equal to one-half the ultimate stress or strength of the weakest material, the available tensile strength of the joint is lowered by 50%.