New Technologies and Materials for Bonding Wood Products
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New Technologies and Materials For Bonding Wood Products By George E. Myers Research Chemist, Improved Adhesive Systems USDA Forest Products Laboratory Madison, Wisconsin 53705 he health of the wood products industry in the United States is Tcontrolled to a great extent by the rate of housing construction, which is in turn controlled by the health of the economy and by demographics. Within the wood products industry, the great est use of wood adhesives by far is for manufacturing composite panels, for example softwood plywood, particle board, waferboard, oriented strand- board (OSB), medium density fiber board (MDF), hardboard and hard wood plywood. Softwood plywood is made from Douglas fir veneer in the Northwest or from southern pine veneer in the south eastern states. Its major use is in con struction, where it is employed for structural sheathing in flooring, roofing Figure 1. U.S. composite panel production through 1995. and walls, as well as for concrete forms. Virtually all softwood plywood in the little thinner than wafers and somewhat • Waferboard and OSB use is grow United States is bonded with phenol- longer than they are wide (about 10-75 ing rapidly and at an accelerating rate. formaldehyde (PF) adhesives. On a vol × 10-75 × 0.25-0.5 mm); strands have • Softwood plywood is certainly the ume basis, softwood plywood is the an even larger length-to-width ratio predominant product and will remain so predominant wood panel product in the (about 10-75 × 5-75 × 0.25-0.5 mm). for the near future, despite the fact that United States. Oriented strandboard is usually made in the supply of large logs for making ve Particleboard is made from small three layers, alternating 90° in orienta neer has been decreasing for some time particles (about 1-10 × 0.1-1 × 0.1-1 tion. These products are becoming more and despite the growing use of wafer- mm), whereas MDF is made from wood widely used as substitutes for softwood board and OSB. This strength of the fibers and has a more uniform struc plywood in construction because of softwood plywood industry is due in ture. Almost all U.S. particleboard is their lower cost. They are mostly great part to the vigor with which it has bonded with urea-formaldehyde (UF) bonded with PF adhesive, although the developed and implemented new bond adhesive, while MDF is made with both use of isocyanate adhesive may grow ing and production methods, two of UF and PF adhesive. Particleboard is here. The PF adhesive for waferboard is which will be discussed later. used as subflooring and as the core for a solid powder, whereas for OSB it is • Both particleboard and MDF pro furniture and cabinets. UF-bonded liquid. duction continue to grow. On a per MDF is also used as the core for furni Hardwood plywood is used mostly as centage basis, the MDF growth is espe ture and cabinets, while PF-bonded an interior wall paneling and is bonded cially strong. To what extent such MDF is used for exterior siding for with UF adhesive. growth will continue if formaldehyde houses. Figure 1 (1-8) summarizes the pro regulations are tightened greatly is not Hardboard is a relatively high density duction levels for these various panel clear at this point. fiberboard made either without adhe products in the United States for the l Hardboard and hardwood ply sive or with PF adhesive. Its applica past few years and in some cases pro wood panel production are both falling. tions include furniture, cabinets and ex vides an extrapolation to 1995. In con terior siding; in the last case, the prod formance with U.S. practice, the data uct is similar to PF-bonded MDF. are given in panel surface area for Adhesive Use and Prices Waferboard particles are approxi thicknesses that differ somewhat for The reason I have emphasized wood mately square (about 25-75 × 25-75 × different products. Several interesting panel production is that those products 0.5-1 mm). Flakeboard particles are a points emerge from this figure: use the largest quantities of wood ad ADHESIVES AGE. OCTOBER 1988 31 in waferboard, usually in the core lay ers. • PVA adhesives are moderately costly and are used primarily in the fur niture industry for assembly of compo nents and in mobile homes for bonding panels to wood frames. New Adhesives What can be said about wood adhe sives in the future? New technologies and new products are continually being introduced. In addition, there is the possibility of using low quality forest- derived materials more efficiently and cost effectively. Figure 2. U.S. wood adhesive use in 1986. Data compiled by James T. White, Geor Research is under way gia Pacific, 1988. to develop new hesives, which happen to be predomi that are widely used in construction ap wood adhesives from nantly UF and PF. This predominance plications such as bonding panel skins is clearly shown in Figure 2, which plots to wood frames. These are made with a renewable resources. adhesive usage for wood bonding in variety of polymers, but the major one 1986 versus adhesive type. Sales are is probably styrene-butadiene rubber. given in metric kilotons for adhesives as • Resorcinol-formaldehyde adhesives Except for two major uncertainties, sold, i.e., for resin solids plus solvent as are quite expensive and are used in there is every reason to expect that UF appropriate. Sales ranged from nearly structural applications where durability and PF systems will continue to be the 800 kilotons for both UF and PF down may be important and where elevated dominant wood adhesives in the United to just over 1 kiloton for polyvinyl ace cure temperatures are not necessary. States. The two uncertainties are the tates (PVAs). They are often used as phenol-resorci possibility of much more stringent reg Table I provides more detailed infor nol-formaldehyde systems. Their pri ulations of formaldehyde-containing mation about 1986 sales, including total mary application is in making lami products and the possibility of limita sales value, unit prices and the major nated lumber and beams. tions or interruptions in the supply of wood bonding applications of each ad • Melamines are relatively expensive petrochemicals. One result of these two hesive type. Points to be noted here are: in the United States compared to UFs uncertainties is that considerable re • UF adhesives are clearly the least and PFs, and their use is relatively low. search has been conducted in the area of expensive, followed by PFs; as noted Their application is in laminating and in developing new wood adhesive systems earlier, these two are by far the major finger jointing, and they are often cured from renewable resources. types used. by radio frequency (RF). Adhesives From Renewable Re • Next in order of use are the mastics • Isocyanates are also somewhat ex sources. After the second world war, the - the elastomeric, gap-filling adhesives pensive and are used to a limited extent U.S. wood products industry began a shift from the so-called “natural” ad hesives based on animal and vegetable Table I - 1986 U.S. Adhesive Sales and Price*1 proteins and on casein to synthetic ad hesives. In recent years, however, in Sales dustry has been strongly supportive of Major Sales value Price research aimed at developing adhesives Adhesive application (metric kton) (Million $) ($/lb) from renewable resources for the rea Urea-formaldehyde Particleboard 700 142 0.09 sons outlined previously. Urea-formaldehyde Hardwood plywood 45.4 11 0.11 U.S. industry ROW uses synthetics al Phenol-formaldehyde Softwood plywood 681 195 0.13 most entirely and has, therefore, be Phenol-formaldehyde OSB 82 33 0.18 come highly dependent on a continuous Phenol-formaldehyde Waferboard 30 33 0.50 oil supply at reasonable prices. But, as (powder) the 1973 oil embargo clearly empha Resorcinol- Laminated lumber 3.6 17 2.10 sized, the possibility of supply interrup formaldehyde, tions exists, and the certainty of an phenol-resorcinol eventual supply depletion must be formaldehyde faced. Additionally, of course, the Melamine-formaldehyde Laminated lumber 1.4 3 1.00 formaldehyde emission problem exists. and finger joints Three general areas of research are Isocyanate Waferboard 1.4 2 0.67 being followed in the renewable re Polyvinyl acetate Furniture, doors, 1.1 2 0.80 source area (9). First is the effort to de mobile homes rive the current resin synthesis precur Mastic Construction 12 14 0.54 sors from renewable sources, for exam ple, the extraction or pyrolysis of * Includes resin solids and solvents. pulping liquors to produce phenols and 1 Data compiled by James T. White, Georgia Pacific, 1988. resorcinols or the oxidation of naturally produced methane to formaldehyde. 32 ADHESIVES AGE, OCTOBER 1988 Second is the attempt to replace syn thetic precursors partially or totally with functional compounds derived from re newable sources. Obvious examples of those functional compounds include lignins, tannins, modified carbohy drates and furans. The third area of re search is the effort to develop durable adhesives from the old “natural” sys tems - blood, soybean and casein. Although research results have indi cated that a number of new systems have promise, their commercial use is very limited as yet. Research and devel opment continues, however, because of the importance of establishing “backup” systems to be placed in use if needed. “Natural” adhesive systems still have specialized applications; for example, casein-soybean adhesives are used in assembling some types of doors, and casein adhesives are used in certain Figure 3. Relative cost of softwood plywood and OSB. lumber laminating operations. Isocyanates. These materials are hardly new and have been investigated Table II - Panel Bending Properties (MPa)* as wood-bonding adhesives for several years in numerous countries.