Flakeboard bonding

Flakeboards bonded with 5 percent phenol-formaldehyde adhesive. 

The most popular is the 40% solids content family, since it seems to give a little better cost/performance relationship. This experiment examined the effect of phenolic resin solids on the properties of structural flakeboards bonded with the lignin-phenolic resin system. The variables were three levels of phenolic resin solids content - 39%, 46%, and 54%. The formaldehyde/phenol ratio and NaOH/phenol ratio were 3/1 and 0.7/1, respectively. 

Lei and Wilson [35,67] developed a model for the fracture toughness K j of flakeboards bonded with PF resin adhesive. The model is based on the initial crack length a) in the specimen, the average size of the inherent flaw (S2) in the solid wood, the expected increase in crack length resulting from nonbonded interflake cracks and voids (Au), and the Kjc value of the wood used to make the flakes ... 

Rowell etal. (1987b) produced PF-bonded flakeboard from acetylated southern pine (21.6 % WPG) or aspen (17.6 % WPG) flakes. This was not completely resistant to attack by termites Reticulitermes flavipes) in a 4-week test. It was thought that acetylation was less effective in preventing termite attack than other chemical modifications because cellulose decomposition in the intestines of termites leads to acetic acid formation in any case. 

Hadi etal. (1995) examined the resistance of PF-bonded flakeboards of acetylated rubberwood to attack by dry wood (Cryptotermes cynocephalus) and subterranean... 

Vick, C.B., Krzysik, A. and Wood, J.E. (1991). Acetylated isocyanate-bonded flakeboards after accelerated aging. Dimensional stabihty and mechanical properties. Holz als Roh- und Werkstoff, 49(6), 221-228. 

Reactive organic chemicals can be bonded to cell wall hydroxyl groups on cellulose, hemicelluloses, and lignin. Much of our research has involved simple epoxides (1 3) and isocyanates (4), but most of our recent effort has focused on acetylation. Acetylation studies have been done using fiberboards (5f6), hardboards (7 11) particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2 257, liquid phase acetylation (, ), or reaction with ketene (28). 

Mycelium fully covered the surfaces of isocyanate-bonded control flakeboards within 1 week, but mycelial development was significantly slower in phenol-formaldehyde-bonded control flakeboards. Both isocyanate- and phenol-formaldehyde-bonded acetylated flakeboards showed surface mycelium colonization during the test time, but the fungus did not attack the acetylated flakes, so little strength was lost. 

After a l6-week exposure to T. palustris, the internal bond strength of control aspen flakeboards made with phenol-formaldehyde... 

Figure 6--Deflection-time curves of phenol-formaldehyde- (PF-) and isocyanate- (IS-) bonded flakeboards in bending-creep tests under progressive fungal attack by T. palustris (upper) and T. versicolor (lower). = PF control = PF acetylated = IS control O = IS acetylated.

Experiment 2. Effect of Molar Ratio of Sodium Hydroxide to Phenol of Phenolic Resin on Strength Properties of Lignin-Phenolic Resin Adhesives. Sodium hydroxide has been the predominant chemical used as a catalyst in resol resin technology. Through variation in the amounts of the catalyst and the method of catalyst addition, a wide variety of resin systems can be formulated. This experiment examined the properties of phenolic resins formulated with various sodium hydroxide/phenol ratios and their effects on the bond properties of structural flakeboards made with lignin-phenolic resin adhesive systems. Variables for resin preparation were four molar ratios of sodium hydroxide/phenol (i.e., 0.2, 0.45,0.7, and 0.95). The formaldehyde/phenol ratio and solids content were fixed at 3/1 and 42%, respectively. 

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

Interest in pine bark as a source of adhesive components began to accelerate following the oil crisis of 1973. Sodium hydroxide extracts of southern pine bark were successfully used in replacing up to 40% of the phenolic resin for bonding of particleboards, oriented strandboards, and composites with a flakeboard core and veneer facing (50f51). Similar results were obtained with extracts from patula pine (52). Encouraged by results of this type, the New Zealand Forest Products Ltd. Corporation expanded their radiata pine bark tannin pilot plant to full-scale operation in 1981 to produce an extract trademarked Tannaphen. This material was crosslinked with paraformaldehyde and used as an adhesive... 

It may also be possible to eventually extend this bonding concept to the preparation of flakeboards and other wood-base composite materials. If plastics like polystyrene are incorporated into the composite matrix, and successful bonding between the wood and plastic is developed through the graft polymers, the three-dimensional network of plastic material throughout the composite matrix may lead to enhanced physical and mechanical properties as well as improvements in dimensional stability (Figure 2). 

In three-layer flakeboards based on five softwood and hardwood species of 4, 10, and 18% moisture content, with press temperature of 177 °C, 6-min press time, and 3% isocyanate binder, the results showed that moisture content of wood was the most important variable at 18% moisture level, IB and bonding properties were lowest. Species of wood influenced strongly the bonding efficiency. In almost all cases the bending properties were the key characteristic of the panel performance. Southern pine produced the boards with lowest IB (81-116 psi), and red oak gave the highest IB values (98-213 psi) (J95). 

The West German parent of Mobay Chemical Corporation was very active, along with Deutsche Novopan, in the development of isocyanate as a particleboard binder in Europe. The work of Sachs (12, 13, lL) and of Deppe (L, 5.), along with the aforementioned commercial developments, has provided a background for interest in isocyanate binders in North America. This interest is beginning to be reflected in the literature. For example, Hse (15.) has published papers on the development of both plywood and flakeboard adhesives which combine isocyanate and phenolic resins. In the former case, he claims adequate bonding with wetter veneers than can be used with conventional phenolic adhesive. In the case of... 

Internal bond and thickness swell vs. % wood flakeboards. 

Stephens and Kutscha [28] investigated the effect of the resin molar mass on the bonding behavior of flakeboards. A commercial PF resin was separated into two molar mass fractions by dialysis, and aspen flakeboards were prepared with the separate fractions as well as with mixtures of the resin fractions. Penetration characteristics of the different fractions were determined microscopically. Results indicate that both low and high molar mass components of the resin are needed to achieve boards with acceptable properties. 

These adhesives performed equally well when used for bonding of oriented strandboards made from Douglas-fir flakes. In addition, composites made from oak or southern pine flakeboards to which southern pine veener was bonded on the face and back passed the 6-cycle durability test of the American Plywood Association when the core was bonded with the peanut hull-based adhesives, even when the 0.64-cm cores were pressed for only 75 seconds. Chen s work on these adhesives is described further in a series of four U.S. Patents (41-44). 

The copolymer resins of natural products were also evaluated for their bonding qualities in particleboards and in composite panels involving flakeboard cores with veneer faces and backs. 

In conjunction with commercial development of the copolymer resins, two such resins (one tailored for particleboard and the other for plywood) with 40% by weight of their phenol replaced by peanut hulls extracts were synthesized as previously described and submitted to an industrial laboratory for evaluation in bonding thin (1/4 thick) oak and southern pine flakeboards and in composite panels where 1/8 southern pine veneer faces and backs were laminated onto oak flakeboard cores. 

As Figure 5 indicates, the copolymer resins of peanut hulls extract cured much faster than the resorcinol catalyzed commercial phenol-formaldehyde control resin in bonding these thin flakeboards. The cure time needed to retain a thickness of 0.250" (the thickness of stops) can be estimated from the graphs. In so doing, the necessary cure time of the copol3mier resin for the 1/4" oak flakeboards was found to be approximately 104 seconds, whereas, a more than 120 seconds cure time was needed for the commercial control resin. 

Amino resins are manufactured throughout the industrialized world for a variety of applications. Adhesives represent by far the largest market. Large amounts of simple urea-formaldehyde resin adhesives are used to bond plywood and particleboard, also known as chipboard, flakeboard, or waferboard. More complex adhesive formulations are used to make laminated wood beams, parquet flooring and interior flush doors, and for furniture assembly. 

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