Particleboard

Plywood furniture core panels, also about 19 mm (3/4 in.) thick, were normally made of a number of layers of relatively thick, 1.5—3.0 mm (1 /16—1 /8 in.) lower value wood veneers combined with thin surface pHes of the decorative veneer. These assembhes were laid-up from glued veneers and then pressed while the bonding occurred. Both lumber core and plywood core have been almost totally displaced in recent years by particleboard or medium-density fiberboard, both discussed herein. This change resulted from the increasing availabiHty and improved finishing characteristics of composites and from decreasing suppHes of core lumber or veneer of suitable quaHty. 

Table 2. Additives Used in Production of Particleboard as a Percentage of Dry Wood Weight...

The majority of particleboard is used in furniture and cabinetry. A significant amount is used as floor decking in manufactured homes and as undedayment in conventional homes. Particleboard is used as shelving in homes and industrial businesses, as door cores in soHd core doors, in stair stepping, door frames, and a host of uses requiring small flat parts as a starting point. 

A small amount of particleboard is made with a fire-retardant treatment for use in locations where codes require this material, as in some offices and elevators. Particleboards receive overlay and finishing treatments with ease. Wood veneers, melamine overlays, printed paper overlays, vinyl overlays, foils, and direct grain printing can all be done quite simply. A small amount of particleboard is also made in the form of shaped, molded articles such as furniture parts, paper roU plugs, bmsh bases, and even toilet seats. There is another small increment of particleboard made by the extmsion process. These products are made in small captive operations owned by furniture manufacturers which consume all of this production in their furniture. The extmsion process differs from conventional flat-pressed particleboard in that the wood furnish is forced between two stationary heated surfaces. The mats are formed from one edge and this edge is alternately formed and pushed between the heated platens, which are maintained at a distance equal to the thickness of board produced. This is an old, slow, small-scale process, but is stiU in use in at least one location. 

Production, Processing, and Shipment. Medium-density fiberboards (MDF) are panels made of fibrous raw material and used ia most of the same appHcations as particleboard. MDF products generally have more smooth surfaces and edges than particleboards and are thus preferred for some uses, even though the manufacture of MDF is more cosdy and the product is significantly more expensive. 

The other major benefit of RF heating was in reduced presstimes. A typical steam-heated MDF press was operated at about 163°C. Presstimes, not including deadtime, for 19-mm (3/4 in.) board would be about 7 min. With RF, this time could be reduced to about 5 min. It will be noted that these presstimes, even with the use of RF, are longer than those requited for particleboards and this, in addition to the more costiy base fiber and the higher resin requirements, explains much of the manufacturing cost differential between MDF and particleboard. 

After pressing, the MDF process basically dupHcates the particleboard process with the steps of cooling, sanding, trimming, cut-to-size, stacking, strapping, and shipping. 

The health and safety issues outlined herein for particleboard also apply to MDE. A special note should be made of the fact that, because the MDF raw material is of dry fiber base, there exists in MDF a large component of very small, broken, dust-like wood fibers. These contribute to the dust concerns in the manufacturing areas, requiring exceUent dust-control systems, good housekeeping, and personal protection. 

The manufacture of waferboard and OSB has many of the same process steps as particleboard, but adapted to the special needs of producing an exterior quaHty panel with large wafers or strands. This discussion focuses on OSB, because waferboard has been almost entirely replaced by OSB and most of the early waferboard mills have now been converted to production of OSB. The OSB process is outlined in Figure 8. 

From the dryer, the strands are screened to remove fines and small particles, which would detract from board quaHty and economy. These fines are burned for fuel or possibly sold to a nearby particleboard mill as raw material. The larger strands are used as surface material and the smaller strands are core material. 

Particleboard, ANSI A208.1-1993, American National Standards Institute, New York, 1993. 

National Particleboard Association, 18928 Premiere Court, Gaithersburg, MD 20879, Tel. (301) 670—0604. 

National Particleboard Association, Eafge Chamber Test (FTM2-1985) OSHH 52protocol, private communication, June 27,1988. 

Particleboard Prom Start to Finish, National Particleboard Association, Gaithersburg, Md., Chapt. 1. 

For VOCs, control options are multiple. Source reduction or removal includes product substitution or reformulation. Particleboard or pressed w ood has been developed and used extensively in building materials for cabinet bases and subflooring and in furniture manufacturing for frames. If the product is not properly manufactured and cured prior to use as a building material, VOCs can outgas into the interior of the residence or building. Other sources of VOCs may be paints, cleaning solutions, fabrics, binders, and adhesives. Proper use of household products will lower volatile emissions. 

Wood particle and fiber driers are used to dry the raw material for particleboard and similar products (20). Just as with the veneer for plywood, the parhcles must be dried before being mixed with the resins and formed into board. Drying is accomplished in a gas-fired drier, a direct wood-fired drier, or steam coil driers. Many different types of driers are used in the industry. Emissions are fine particles and condensible hydrocarbons, which produce... 

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

Geimer, R.L., Eollensbee, R.A., Christiansen, A.W., Koutsky, J.A. and Meyers, G.E. In Maloney, TM. (Ed.), Proceedings 24th International Particleboard Composite Materials Symposium, WSU. Washington State University Press, Pullman, WA, 1990, pp. 65-83. 

V313 particleboard exterior according to EN 312-5 and 7, option I [2] (cycle test according to EN 321 [4]). 

The low molar ratio of the final UF-resin is adjusted by the addition of the so-called second urea, which might also be added in several steps [16-18]. Particular care and know-how are needed during this acid condensation step in order to produce resins of good performance, especially at the very low molar ratios usually in use today in the production of particleboard and MDF. This last reaction step generally also includes the vacuum distillation of the resin solution to the usual 63-66% solid content syrup in which form the resin is delivered. The distillation is performed in the manufacturing reactor itself or in a thin layer evaporator. Industrial preparation procedures are usually proprietary and are described in the literature in only a few cases [17-19]. 

For example, a UF-resin for particleboard at the end of the 1970s would have had a F/U molar ratio of approx. 1.6-1.8. To day a UF-resin for the same application has a molar ratio of between 1.02 and 1.08, but the requirements for the boards, as given in the quality standards, are still the same. The degree of crosslinking of the cured resins as well as the reactivity of the hardening reaction depends on the availability of free formaldehyde in the system. 

El-particleboard- and El-MDF-resin for water-resistant boards (EN 312-5 and 312-7 [2]). For boards according to option 1 (V313-cycle test) MUF-resins can be used for boards according to option 2 (VI00-boiling test) MUPF with approval are necessary. In case, espeeially for the MDF-production, formaldehyde catchers are added. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

Adhesives based on isocyanate (especially PMDl, polymethylene diisocyanate, more exactly polymeric 4,4 -diphenylmethane diisocyanate) have been used for more than 25 years in the wood-based panel industry [88], but still have a low market value in the wood-working industry compared to systems based on UF-, MUF- or PF-resins. The main application is the production of waterproof panels, but also the production of panels from raw materials that are difficult to glue, like straw, bagasse, rice shells or sugar cane bagasse. They can be used as adhesives for wood-based products like particleboard, oriented strandboard (OSB), laminated strand lumber (LSL), medium-density fiberboard (MDF) or... 

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