Panel bonding

During the late 1970s, concerns were raised about levels of airborne formaldehyde in buildings resulting primarily from constmction using composite panels bonded with urea—formaldehyde resins and combined with energy-efficient building practices which reduced air losses. 

Average physical and mechanical properties of the flakeboards are summarized in Table I. On the average, panels bonded with the lignin/phenolic resin... 

In an earlier paper (2), we determined that carbohydrates could replace a significant portion of the phenol-formaldehyde resin used for bonding plywood veneer. Carbohydrates from renewable resources such as wood can replace up to 50% of the phenol and formaldehyde in resins formulated under basic conditions without significant loss of bond quality. Two-ply, Douglas-fir-veneer panels bonded with these carbohydrate-modified resins have shear strengths approximately equivalent to those for panels bonded with unmodified phenol-formaldehyde resin. 

Figure 1 compares the dry- and wet-shear strengths of two-ply, Douglas-fir veneer panels bonded with a commercial phenol-formaldehyde resin (basic), a phenol-formaldehyde resin prepared in the laboratory under basic conditions, and an unmodified neutral resin prepared in the laboratory. The shear strengths obtained with these three resins served as control data for further experiments. The dry-shear strengths of panels bonded with the unmodified neutral resin are lower than those for panels bonded with the resins cured under basic conditions however, the wet-shear strengths of panels bonded with the three resins are all... 

Phenol-formaldehyde resins modified directly with reducing sugars successfully bond wood veneers at neutral conditions. The dry- and wet-shear strengths of two-ply panels bonded with xylose-modified resins are not adversely affected until the amount of xylose is increased to between 0.6 and 1.0 moles xylose per mole of phenol (Figure 2). However, even resin with 2 moles xylose per mole of... 

Figure 2. Variation of the dry- and wet- (hatched) shear strengths of two-ply Douglas-fir panels bonded with xylose-modified neutral resins. Moles of xylose per mole of phenol were varied as indicated. The mole ratio of phenol to formaldehyde was 1 2.3.

Color of Bond Line. Two-ply veener panels bonded with carbohydrate-modified resins formulated under neutral conditions had bond lines that are extremely light colored in contrast to the dark red-black color characteristic of resins cured under basic conditions. The color ranges from a light yellow-tan with unmodified and xylose-modified resins to a medium tan with the prehy-drolyzate modified resins. These resins would therefore be suited for bonding wood used for decorative purposes. 

This chapter reports work on two aspects of this adhesive system 1) tests on the strength of panels bonded with phenol/carbohydrate/urea/formaldehyde (P/C/U/F) adhesive compositions outside the ranges previously reported (9,10) and 2) analysis of chemical reactions in this resin system. 

Table I. Shear Strength Properties of Yellow Birch Panels Bonded with Glucose-Based Adhesives...

Figure 7. Selective bond breaking of HOD. Dissociation yields (upper panels), bond-length expectation values (middle panels), and LCT fields (lower panels) are shown for two objectives. The left-hand panels correspond to the case where the kinetic energy of the D atom is steadily increased, leading exclusively to D + OH dissociation. The results for the H + OD selective excitation and fragmentation are shown on the right-hand side of the figure.

Figure I. Wooden form containing brick panel bonded with sulfur mortar...

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. 

C. Correlation Between the Composition of a Resin and the Properties of Wood-Based Panels Bonded with the Resin... 

Figure 2. Density profile as a function of panel thickness for a sample from a panel bonded with resin A. Note the localized dip in the board core s density indicated by an arrow.

EN 13354 2008 Solid Wood Panels — Bonding quality — Test method. 

NASA-Langley Research Center. The characteristics of the samples in the two sets are given in Table I. The first set of samples were lap-joints of Pasa-Jell cleaned Ti-6-4 panels bonded with one polyimide resin adhesive. The resin adhesive was prepared from benzophenone tetracarboxylic acid dianhydride (BTDA) and m,m -diaminobenzophenone (m,m DABP). The structures of these compounds are given in Table II. The uncured adhesive was applied on the adherend in the polyamic acid stage from either diglyme or DMAC solution and then heat cured to the polyimide resin form. This condensation polymerization reaction is shown below. 

Typical Use Automotive. Body panel bonding. Adheres to oily steel. ... 

Typical Use Construction industry materials. Sanrhvich panel bonding for a wide range of core and fecesheet materials. Typical Use Marine applications - FRP, metals, plastics, etc. ... 

Typical Use Marine. Bonding primed PVC foam and balsa core to Typical Use Automotive - panel bonding. Repairs, e.g. boat hulls. ... 

Typical Use Automotive - body panel bonding. Typical Use Automotive - glass bonding. (Vehicle driveable after 30 ... 

Authors from 3M [14] described the testing of two epoxy adhesives for aluminium designed for hem flange bonding and panel bonding. These adhesives showed outstanding sustained load durability in cyclic corrosion tests even if stressed to 40% of their initial shear strengths. 

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