Compatibility of nonacidic waterborne preservatives with phenol-formaldehyde adhesive

Thirteen nonacidic waterborne preservatives were in­ vestigated for their compatibilitywith phenol-formaldehyde adhesive on aspen veneer treated with the preservatives to retentions of 0.2, 0.4, and 0.6 pcf. Of these preserva­ tives, didecyl dimethyl ammonium chloride (DDAC),DDAC with copper, DDAC with carbamate (DDACCA),sodium fluoride,and ammonium hydrogen difluoridedid not inter­ fere with adhesion at any retention level. Carbamate in the DDACCA preservative apparently reinforced inter­ facial regions of the bondline to produce bond strengths much higher than those of untreated controls, particularly at the higher retentions. Ammoniacal copper zinc arsen­ ate at 0.2 pcf retention did not interfere with adhesion, but at higher retention levels bond integrity was below standards. All three borated preservatives that were stu­ died caused poor bonding, even at the lowest retentions. Emulsions of copper naphthenate, copper octoate, and zinc naphthenate also caused poor bonding. Determination of wet wood failure after vacuum-pressure soaking was more effective in detecting adhesion incompatibility than de­ termination of dry shear strength and wood failure after multiple cycles of vacuum-pressure soaking and drying. Sales oforiented strandboard and waferboard are grow­ ing rapidly in residential and commercial sheathing mar­ kets. Nonveneered panels already hold at least a 10 percent share of the panel market-a market once dominated by plywood. By the late 1990s, nonveneered panels are pro­ jected to account for 50 percent of the panel market (10). With this expansion of the nonveneered panel market, a tremendous economic potential may exist for preservativetreated panels. Southern pine lumber production increased 30 percent over the past decade, primarily through sales of treated wood. In 1986, the treated-wood market was more than 5 billion board feet, or three times that in 1975 (10). A similar potential might exist for the composite panel mar­ ket. By treating composite panels to resist fungal and in­ sect attack in aboveground or ground-contact uses, higher 16 levels of performance and more practical uses could be found for composite panels. The conventional approach to preservative treatment is to impregnate whole pieces of lumber or plywood with chemicalsunder high vacuum and pressure. This approach has two major disadvantages for composite panels. First, saturation with water leads to excessive and nonrecover­ able thickness swelling, with accompanying reductions in strength. Second, equipment needed to pressure-treat fullsized manufactured products requires high capital invest­ ment. In contrast, an in-line process to treat green flakes offers potential for producing dimensionally stable and durable composite panels with lower capital investment. A major hurdle in developing an in-line process, however, is finding preservatives that will not interfere with adhe­ sion of phenolic resin to preservative-treated flakes. This problem has not been resolved satisfactorily for most com­ mercial preservatives and adhesives, nor have causes for poor compatibility between various adhesives and preserv­ atives been clearly identified. The primary objective of this study was to find one or more nonacidic, waterborne preservatives that might be chemically compatible with phenol-formaldehyde (PF) resin. A secondary objective was to compare the effective­ ness of two accelerated-aging test methods in detecting differences in compatibility of preservatives and phenolic adhesive. Specific objectives were to 1) identify those pre­ servatives in respective concentrations that showed the greatest compatibility with phenolic adhesive; 2) deter­ mine if the most compatible preservatives met accepted performance requirements for strength and durability of The authors are, respectively, Research Forest Products Tech­ nologist, Research Plant Pathologist, and Supervisory Research General Engineer, USDA Forest Serv., Forest Prod. Lab., One Gifford Pinchot Dr., Madison, WI 53705-2398. The authors wish to express their gratitude to Jill Wennesheimer for preparing and testing specimens and to Lisa Meiers for her technical sup port. This paper was received for publication in April 1989. Forest Products Research Society 1990. Forest Prod. J. 40(2):16-22.