Exterior-grade

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. 

Classification—a review of exposure durabiHty in terms of interior and exterior grades of plywood... 

The manufacture of MDF, with a few exceptions, dupHcates the manufacture methods for dry-process hardboard, described at length hereia. One exception to it is that most MDF is made ia the medium-density range, 640—800 kg/m although small amounts are made at lower or higher densities. Second, the vast majority of MDF is made with UF resia adhesives with resia requhemeats ia the 7—11% range, and wax is usually added at the 0.50—0.75% level. A small amount of exterior-grade MDF is made with isocyanate resia. 

AH-acryHc (100%) latex emulsions are commonly recognized as the most durable paints for exterior use. Exterior grades are usuaHy copolymers of methyl methacrylate with butyl acrylate or 2-ethyIhexyl acrylate (see Acrylic ester polymers). Interior grades are based on methyl methacrylate copolymerized with butyl acrylate or ethyl acrylate. AcryHc latex emulsions are not commonly used in interior flat paints because these paints typicaHy do not require the kind of performance characteristics that acryHcs offer. However, for interior semigloss or gloss paints, aH-acryHc polymers and acryHc copolymers are used almost exclusively due to their exceUent gloss potential, adhesion characteristics, as weU as block and print resistance. 

Parallam, or laminated strand lumber (LSL) is a beam made by a continuous manufacturing process composed of bigger-size wood needles (very elongated wood particles) reassembled with a structural exterior grade adhesive, the favorite adhesive being isocyanates (pMDI) when heat-curing and PRFs when cold-curing. 

Other special engineered composites. Today PMDI is quite extensively used to produce exterior-grade OSB and particleboard. 

Lower formaldehyde emissions were found for the plywoods (all exterior grades) than for particleboard and MDF, but organic acid emissions still occurred, especially for the hardwood-based plywood (black butt) since such woods are... 

In the hot pressing of exterior grade plywood, phenolic glues behave quite differently. Veneers must be dried to less than 5% moisture content and glueline temperatures must be 100 to 150°C. 

Thick compact exterior grade laminates are self-supporting and suitable for applications such as infill panels or the decorative cladding of exterior walls. They are available with surfaces of anodized aluminium and coloured lacquer-coated foil as well as normal melamine. 

The addition of the nitrogen-containing crosslinking agents, melamine, urea, and urea-formaldehyde (UF) resin, resulted in a substantial increase in strength (Table III, entries 7-12). The best results were obtained by the melamine-crosslinked adhesives. The addition of 33% melamine resulted in an adhesive with strength values well within exterior-grade adhesive specifications. 

Attempts to make adhesive formulations by direct reaction of formaldehyde or its equivalent resulted in products that were excessively viscous, and the working time was too short for commercial application (57). It was concluded that formaldehyde, although readily reactive with the tannin molecule, provided much too short linkages to connect the bulky tannin molecules. This problem was circumvented by the preparation of a polymethylolphenol reagent that, when put in solution with the bark extract, formed a combination that was stable for several weeks at room temperature. When heated, the polymethylolphenol and bark extract reacted rapidly to form an infusible resin. Commercial trials were made to produce exterior-grade Douglas-fir plywood. Widespread use of the extracts for this purpose, however, was inhibited by a drop in the price of phenol below what the bark extracts could be manufactured for. (The best extract for adhesive purposes was an ammonia extract of hemlock bark converted to a sodium derivative prior to spray drying, a more costly extraction procedure than simple sodium hydroxide extraction of bark.)... 

Wattle tannin resins are also used to manufacture other resins, such as foams comparable to phenolics, as waterproofing additives, and binders for corrugated cardboard or charcoal briquettes. This discussion, however, deals only with particleboard, plywood, glulam, and finger-jointing exterior-grade wood adhesives. Formulations of the adhesives will be mentioned ad hoc, if at all necessary, as they have already been extensively discussed in articles and reviews in the relevant literature.(7)... 

Longterm Performance of Exterior-Grade Tannin-Bonded Particleboard... 

In conclusion, it is worthwhile to note that in 1970-71, when I started to be involved with tannin adhesives, there were no more than three laboratories, academic or industrial, conducting research on tannin adhesives. By 1986, I have had direct or indirect contact with 42 different laboratories in many countries that have carried out at some time, or are now carrying out, research on these bonding materials. The increase in industrial usage has also been fast but for various reasons has lagged behind the technical and scientific interest. However, after practically zero consumption in 1970, indications are that about 12,000 tons of exterior-grade tannin resin solids per year are now produced and consumed in the timber industry based on reliable information from several... 

Figure 4. Photographic comparison of 12 mm thick exterior grade commercial particleboards manufactured in the same industrial plant and bonded with MUF, wattle tannin-formadehyde, and PF resins The panels were exposed at the CSIR testing site (4,500 feet above sea level) at Pretoria, South Africa for 15 years. Note the extensive degeneration of MUF board (commerically imported MUF resins from West Germany). Also note the more marked edge and surface degeneration of the PF in comparison to the TF-bonded boards.

Melurac [Cytec]. TM for urea-melamine-formaldehyde condensation products used mainly as adhesives for bonding of veneers for the production of exterior-grade plywood. 

Furfural has replaced formaldehyde in phenol-resorcinol-formaldehyde resins.These formaldehyde-containing resins have been used as cold-set, exterior grade structural wood adhesives for almost half a century. The use of furfural to prepare these resins has several advantages. In addition to longer resin shelf life, when furfural is used, the emission of formaldehyde is lowered. To enhance the reaction rate when furfural is used, a small amount of formaldehyde is added to the furfural system. Hence, the furfural system is not formaldehyde-free, but nearly so. Finally, furfural has a higher molecular weight than formaldehyde. This results in a smaller amount of the expensive resorcinol being required in the system. 

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