Exterior plywood adhesives

Carbohydrate-phenolic-based resins have shown promise for partial replacement of phenol and formaldehyde in exterior plywood adhesives (7,2). Such resins are produced in a two-stage reaction sequence. First, the carbohydrate is reacted with phenol, and sometimes urea, under acid catalysis at elevated temperatures (up to 150 °C), to produce an acid-stage resin. The acid-stage resin is then made basic, formaldehyde added, and the reaction continued at lower temperatures to produce a resol-type resin. Adhesives formulated from these resins have curing speeds consistent with present-day plywood production needs in the western United States, veneers are typically dried to 0 to 7% moisture content and the adhesive cured by hot pressing the panels at approximately 140 to 150 °C and 1.2 MPa. 

Woo (1982, personal communication) has described the exterior plywood adhesive formulation based on Tannaphen and used commercially by New Zealand Forest Products Ltd. since 1981 as Tannaphen - 56 parts, a phenol-formaldehyde fortifying resin - 21 parts, olivestone flour - 18 parts, and paraformaldehyde - 5 parts on weight of solids. These adhesives are tolerant of open assembly times of 20 minutes, closed assembly times of up to 4 hours, and veneer moisture contents of up to 15%. Both shear strengths and wood failure were comparatively high even after 5 weeks in water at 25 °C. 

Melamine or melamine—ureas are used in the manufacture of tmck and railroad flooring, laminated lumber, beams, exterior doors, marine plywood, toilet seats, and school furniture. The bonds in these products meet a variety of commercial, military, and federal specifications for exterior waterproof adhesives. 

Early attempts to use mimosa tannin in particleboard adhesives involved high-temperature alkaline treatment of the extract to reduce viscosity of the 40% solids level needed (43,44) Subsequent improvements followed the same course as with plywood, namely the use of phenol-formaldehyde or phenol-resorcinol-formaldehyde as crosslinking agents (45) and the use of catalysts or mix modifications to reduce press temperature requirements and to extend pot life. Recent work (46) has shown that exterior chipboard adhesives can also be prepared by crosslinking of mimosa tannins with 4,4-diphenylmethane diisocyanate. 

Resin synthesis, adhesive formulation, and evaluation techniques are described by Clark et al. (3,4) Plywood shear specimens were prepared according to U.S. Product Standard PS 1-83 for exterior plywood (5). Glulam shear specimens were tested according to the American Institute of Timber Construction (AITC) standards AITC-T107 and AITC-T110 for dry shear and vacuum-pressure soak (3). Glulam test specimens were also subjected to a 2-hour boil treatment (not an AITC test) prior to shear (7,8). 13C-NMR spectra were recorded at 100.6 MHz on a Bruker AM-400 NMR spectrometer. Gel permeation chromatography... 

Plywood, The weathering of plywood is related directly to the quality of the veneer exposed and to the adhesives used. Because of its tendency to check, most exterior plywood is protected with a finish or with overlay material. Such plywood weathers and performs similarly to solid wood (J, 2, 71). 

The earliest wood adhesive for mass produetion of panels was phenol formaldehyde (PF). This is widely used for products designed to perform in severe weathering eonditions and dominates the exterior plywood market. Resorcinol formaldehyde is a somewhat similar resin system that cures at ambient temperatures, but the cost is much higher, limiting its use in high-volume applications. These systems, although formaldehyde-based, do not have the continuing formaldehyde emission problem that is associated with other formaldehyde-based systems. 

In general, lower resin alkali content and lower molecular weight are associated with slower cure, which explains why softwood plywood adhesive resins are both more alkaline and more condensed. Adequate exterior-grade adhesion can be attained on softwood veneer if these considerably condensed phenolic resins, which also cure more quickly, are used. Conversely, the increased functionality of hardwood adhesive resins partially compensates for their inherently slower cure rate. Hardwood phenolic resins require about 30% longer press times for adequate cure. 

Isocyanates [polymeric MDI (PMDI)] as a fortifier for phenolic resins have only been used in the past in rare cases. Deppe and Ernst [41] reported a precuring reaction between the isocyanate and the phenolic resin, even if both components had been applied separately to the particles. Hse et al. [36] also found good results with an isocyanate and a PF resin added separately to wood particles. Pizzi and Walton [191] reported on the reactions and their mechanisms of PF resins premixed in the glue mix with nonemulsifiable water-based diisocyanate adhesives for exterior plywood. Pizzi et al. [192] reported on the industrial applications of such systems (PF -I- PMDI -I- sometimes tannin accelerator UF + PMDI)... 

These adhesives are suitable for exterior use and are unaffected by water (even boiling water), molds, grease, oil, and most solvents. Their appUca-tions primarily include wood, plywood, plastics, paper, and fiberboard. Resorcinol-formaldehydes are excellent marine-plywood adhesives. Curing at room temperature normally takes 8-12 hours, while phenoUc wood adhesives require a high-temperature cure. The adhesives are also used for indoor applications because of their high reliability. ... 

Herrick F W, Bock L H 1958 Thermosetting exterior plywood type adhesives from bark extracts. For Prod J 8 (10) 269-274... 

The development of wattle tannin-based plywood adhesives occurred concurrently with the particleboard adhesives and, although quite different adhesive properties are required to bond these two types of furnish, many of the basic premises on which their development were based are the same. A series of wattle tannin-based plywood adhesives that provide exterior bond qualities have been described in detail by Pizzi (182, 186). All recent formulations use low-molecular-weight polymers, as first suggested by MacLean and Gardner (131), rather than formaldehyde for crosslinking agents. 

Research directed to the use of bark extracts from various species of pines has continued despite the marked reduction in prices for petroleum in the late 1970s and early 1980s. A number of new plywood adhesive formulations based on extracts of Pinus radiata have been described recently. Weissmann and Ayla (253) used sulfonated tannin extracts at 40% solids and fortified these extracts with a phenol-formaldehyde resin (Kauresin 260 produced by BASF) at levels to 10% to 50% by weight of solids. Both paraformaldehyde and hexamethylenetetramine were examined as aldehyde sources. Exterior grade plywood bonds were obtained at 10% and 30% fortifying levels. 

Chen (46) used sodium hydroxide extracts from southern pine Pinus spp.) bark, peanut Arachis hypogea) hulls, and pecan Carya illinoensis) pith as 20% replacements for phenol-formaldehyde resins in plywood adhesives. Bond quality was highly dependent on veneer moisture content and assembly time. Exterior quality bonds (U.S. Product Standard PS-1-74) in southern pine plywood were obtained if the veneer moisture content was 7.0% with 60-minute assembly times. Phenol-formaldehyde resin replacements could be increased to 40% by peanut hull and pecan pith extracts when the extraction solutions containing 5% of sodium hydroxide were used (45). Southern pine veneer (0.32 cm) was spread at a rate of 41.5 to 42.5 g/lOOOcm of double glue line and the layups were stored in a... 

Pizzi A, Scharfetter H O 1978 The chemistry and development of tannin-based adhesives for exterior plywood. J Appl Polym Sci 22 1745-1761... 

Exterior plywood glued between the laminations with waterproof adhesive. 

For exterior appHcations, where water exposure is expected, phenol—formaldehyde (PF) or phenol—resorcinol—formaldehyde (PRF) adhesives are used. Only small quantities of this type of hardwood plywood are made, primarily for marine use. 

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in stmctural appHcations. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

Wood Bonding. This appHcation requires large volumes of phenoHc resins (5—25% by weight) for plywood, particle board, waferboard, and fiberboard. Initially, phenoHc resins were used mainly for exterior appHcations, whereas urea—formaldehyde (UF) was used for interiors. However, the concern over formaldehyde emission has caused the replacement of UF by phenol-formaldehyde adhesives. 

Adhesives (qv) used to make plywood are classified as either the exterior adhesive or the lesser quaHty interior adhesive. The terms relate to the abUity of the adhesive to survive exposure to moisture and weather. PhenoHc resins (qv) are commonly used as adhesives to make plywood. The difference between interior and exterior phenoHc resin adhesive is the filler level. Exterior rated plywood uses higher resin content adhesives. Interior rated plywood uses either highly extended (below 24% resin soHds) or protein-based adhesives. The adhesive is appHed to the veneer by roU coating, spraying, curtain coating, or foam extmsion. 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

Other applications for phenolics are switchgears, handles, and appliance parts, such as washing machine agitators (that s why they re usually black). Phenolics are widely used to bond plywood, particularly exterior and marine grades. Although urea-formaldehyde resins are cheaper for this purpose, they were not nearly as water-resistant and have been limited to interior grades. Abrasive wheels and brake linings also are bonded with phenolic adhesives. 

The types of adhesives suitable for laminating beams are restricted by the conditions of application and by their end-use requirements. A wider choice of adhesives for plywood depends on whether softwoods or hardwoods are used, whether they are required for internal or external exposures, or whether they are to be used for ornamental or structural purposes. Thus phenol-formaldehyde types would be used for marine or exterior construction uses urea-formaldehyde types would be advantageous for cold pressing, or melamine-urea adhesives might be preferred for hardwood plywood, or lumber-core panels used in furniture production. 

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