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. However, Product Modulus of elasticity Modulus of rupture their commercial adoption has been 8,600 45 quite slow in the United States, so it 8,600 48 seems reasonable to include them within Waferboard 3,400 21 the category of new systems. Their slow adoption is, of course, due to their high cost and their toxicity. They do possess * Parallel to face grain for plywood and to face strand for OSB. some definite advantages, however, in­ cluding rapid cure at a moderate tem­ perature, insensitivity of cure to mod­ availability of large stands of under-uti­ Historically, softwood plywood pro­ erately high moisture, good durability lized lower quality aspen, spruce and duction has proceeded by peeling ve­ of bonded products and absence of mixed hardwoods for use as furnish in neer from wet logs and then drying the formaldehyde emission. To my knowl­ waferboard and OSB. Many of these veneer to about 3%-4% moisture con­ edge, at the present time isocyanates are large stands are located near urban tent (MC). Drying the veneer to such being used consistently in only one plant areas, thus reducing shipping costs of low MC creates the potential for several in the United States - to make wafer- the finished panels and further lowering problems, however (12): board for siding, concrete forms and their final cost. • In many plants the drying step has flooring. I am also told that isocyanates Another important factor underlying limited overall production. are used at times in several OSB plants. the acceptance of waferboard and OSB • Excessive penetration of adhesive is the development of a system of prod­ and water can lead to poor bonding. uct standards based not on specific • Excessive drying can cause surface New Technologies products but on performance require­ deactivation and poor bonding. I want to describe briefly a few of the ments (11). Where the new, less expen­ • Veneer becomes brittle, causing ve­ more exciting adhesive bonding tech­ sive panel materials could be shown to neer breakage and loss. nologies that are coming into use in the meet those performance requirements, • Plywood out of the press only con­ U.S. wood products industry. builders have been willing to substitute tains about 4% moisture; it subse­ Composition Panels: Waferboard and them for softwood plywood. Table II quently absorbs moisture and can warp OSB. I mentioned earlier that wafer- (11) presents some representative bend­ and buckle. board and OSB production was grow­ ing properties of the three panel types. On the other hand, making plywood ing rapidly and that these panels were Parallel to the strand alignment, the with high MC veneer also has prob­ finding increasing use as substitutes for OSB properties clearly equal those of lems. New PF resins had to be devel­ plywood, primarily in construction as plywood. The lack of alignment in waf­ oped because high moisture alters resin sheathing. Although this development erboard decreases its bending proper­ flow and penetration behavior and in­ has been in process for several years, it ties, but it still meets many perfor­ hibits cure. Such resins have been found is sufficiently important to examine it a mance standards. to be more costly. With high veneer little further. High Moisture Content Veneer Bond­ moisture contents, the range of mois­ What are the reasons for the growth ing. I mentioned earlier that the U.S. ture contents encountered is broader, of waferboard and OSB? One large fac­ softwood plywood industry has shown and the resin and process must be more tor is the wood raw material - its cost considerable vigor in recent years in de­ tolerant to that spread. Finally, existing and availability. The relative costs of veloping new bonding and production instruments for monitoring veneer MC softwood plywood and OSB are com­ methods; this is due at least partly to were found to be inadequate for the pared in Figure 3 (IO), where the much the increasing competition from other higher MC ranges, and new instruments lower wood cost for OSB is obvious. structural panel products. I want to de­ are needed. This cost difference is due to two fac­ scribe two examples of new veneer Several approaches have been fol­ tors: the decreased supply and increased bonding procedures, the first of which lowed to develop PF adhesives that cost of quality logs for veneer and the is high moisture veneer bonding. would allow the use of high MC veneer,

ADHESIVES AGE, OCTOBER 1988 33 mashing step. Consequently, under the pressure of the mashing rolls, a thin uniform adhesive film results during passage between the rolls. Laminated Lumber and Beams. Laminated lumber and beams are, of course, not new products; they have been around for several decades, and they continue to be used. However, there is one general class of laminated lumber whose use is expanding rapidly - parallel laminated veneer (PLV). It is

The cost of PLV beams is high relative to that of conventional lumber, but an installed system Figure 4. Schematic of a foamed adhesive extrusion system. is priced competitively because of decreased including dropping the adhesive water there the foam is pumped to the extru­ content to compensate for the higher sion head where it is divided into sepa­ installation labor veneer MC, optimizing the extenders or rate strands - up to 160 - which are and low rejection rates. fillers to gain further control of flow laid onto the veneer passing under the and penetration, and altering the adhe- extrusion head. After the other veneer sive system to increase cure rate. One of layers are in place, the assembly passes the most effective methods found to in- between rolls, where the foam strands made by a continuous process, and the crease cure rate is the use of an external are mashed into a continuous film that grain of all the laminated veneers runs catalyst, i.e., a two-part adhesive. Us- makes intimate contact with both the parallel to the beam length. These are ing such systems, a large number of upper and lower veneers. Figure 5 made by bonding the veneer with PF plywood plants are now operating with shows an extrusion head and the sepa­ adhesive in a continuous hot-pressing veneers at 12%-13% MC and higher in- rate strands of foamed PF adhesive operation. Beams are made at high pro­ stead of the previous 3%-4%. being laid on a passing panel. duction rates and are up to 76 cm wide, Foamed Adhesive for Plywood. The The foamed adhesive contains less 6 cm thick and 24 m long. Because high second relatively new plywood bonding conventional extender, but has foaming quality softwood veneer is used, and technology is the growing use of foamed aids such as animal protein. Better con- defects are small and randomized, me­ PF adhesive in the last few years. Fig- trol can be maintained of the amounts chanical properties and reliability are ure 4 (13) is a schematic of a foamed of adhesive being applied, leading to excellent. Relative to conventional lum­ adhesive extrusion system. The PF ad- 20%-30% savings in adhesive use. Be­ ber, the cost of such beams is high, but hesive is pumped into a foamer where it cause the strands have only small con- the cost of an installed system is com­ is mixed with about five times its vol- tact with the wood, water and adhesive petitive because of decreased installa­ ume of air to form the foam. From penetration are minimized prior to the tion labor and low rejection rates. PLV beams are used in housing con­ struction as headers over windows and as main roof beams. An increasingly important application of PLV is in composite I-beams. In some applica­ tions, the flanges are made from PLV, and the I-beam web is softwood ply­ wood that is pressure-fitted into the flanges and bonded with phenol-resor­ cinol-formaldehyde adhesive at about 50°C. In other I-beam installations, OSB is used as the web material. Still another application of PLV is as flanges on trusses where the webs consist of tu­ bular steel. Both the I-beams and the tubular trusses can be designed to pro­ vide the desired strength for a particu­ lar application, and they possess the ad­ vantage of light weight. Steam Injection Pressing. Steam in­ jection pressing is an emerging technol­ ogy in the United States and some other countries for very rapid pressing of composition panels. In this process sat­ urated steam is injected from perfo­ rated platens into a particle mat during Figure 5. Separate strands of foamed PF adhesive are laid on a passing panel. and after press closure. If the steam is

34 ADHESIVES AGE, OCTOBER 1988 Figure 6 (above). Observed changes in board thickness, pressure and core tem­ perature when steam is injected shortly after initiating press closure, but before the mat is fully compressed. Figure 7 (right). Relatively flat density profiles achieved with steam injection pressing.

injected before the mat density becomes too high, it creates permanent paths around the particles and penetrates to the center and edges of the mat, result­ ing in very rapid heating of all portions of the mat. Figure 6 (14) shows the ob­ served changes in board thickness. pres­ sure and core temperature when steam is injected shortly after initiating press closure but before the mat is fully com­ pressed. In this particular case, core temperature rises very rapidly and reaches a maximum of about 280°F in about 10 sec. Thereafter, the steam can be cycled off and on, and the tempera­ ture and pressure respond appropri­ ately. In addition to the rapid heating, and consequently short press times, with steam injection pressing, another ad­ vantage is the production of a relatively times less than 2 min. Press times for particleboard and the other for making flat density profile through most of the UF or isocyanate boards could ob­ a PF-bonded MDF. cured board (Figure 7) (14). It also viously be much shorter. Moreover, seems likely that particle damage is less­ quite thick boards can be pressed by Conclusion ened by the plasticizing action of the steam injection in relatively short times. high temperature steam. Thus, boards It is my understanding that several Composite panels as a class represent with good internal bond strength can be steam injection pressing plants are now the major use of wood adhesives in the produced at high rates. Boards that are planned or in construction in various United States; softwood plywood is the 12 mm thick bonded with PF adhesive countries; two of these are in the United predominate type. Other panel products reportedly can be made using total press States - one for making UF-bonded are growing, however, most notably ADHESIVESAGE. OCTOBER 1988 35 MDF and OSB/waferboard. UF and PF are by far the dominant wood adhesives by virtue of their properties and costs. Because of the formaldehyde problem and potential petroleum supply prob­ lems, new adhesive systems based on renewable resources are actively being pursued as backup systems if needed. Meanwhile, new technologies and prod­ ucts based on current adhesives are continually being developed and imple­ mented.

References

36 ADHESIVES AGE, OCTOBER 1988