Particleboard drying

After conditioning the moisture content of the boards was almost the same as the original content, with exception of the dried particleboard. The formaldehyde parameters of the treated samples appeared to have changed very much. 

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

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. 

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... 

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]... 

Basecoats are normally cured in heated ovens with high air velocity or with infrared heaters excessive panel temperatures should be avoided to prevent drying of the particleboard. A light scuff sanding of the basecoat is used to remove high spots, followed by a brush cleaning to remove the dust and debris. 

Another report came from Canada in 1959 when Bender published research results of utilizing eastern Canadian barks as furnish for wet-process insulation board and hardboard (33). Bark species included in the study were black spruce and balsam fir each contained 25-35% wood. A Sprout-Waldron disk refiner was used to prepare the bark fiber, and boards were made with lh% wax emulsion but contained no added binder. Physical tests indicated the boards met some commercial specifications the author believed that addition of more woody fiber would improve the properties. In addition, a few experimental dry-process particleboards were made with addition of some unnamed binder that was a byproduct material. 

One of today s fastest growing segments of the wood composition board industry is production of medium density fiberboard (MDF) using a dry process similar to that used for particleboard. First mention of the possibility of utilizing bark for MDF came in a presentation by Brooks in 1971 (43). He described a process in which a homogenous board with superior properties could be made from such raw materials as mixed, unbarked hardwood pulp chips unbarked pine chips, if bark content was less than 30% forest thinnings, branches, and so on and hardwood bark. Furnish was prepared by double-disk pressurized refiners. Brooks concluded a plant could be built to operate on 100% hardwood bark. 

All dry veneer waste and plywood trim waste not used for fuel is commonly chipped and sold to particleboard plants as furnish. 

One-layer particleboards were prepared from Eucalyptus chips with a resin content of 10 percent on dry chips. The boards were formed 12-mm thick on a laboratory press with the press platens at 170 °C for press times of 15 minutes. 

Table IV. Internal Bond Strength of Particleboard, 12-mm-Thick, Prepared from Soda Bagasse Lignin Adhesives (10% on Dry Chips) After Pressing at 170 °C for 15 Minutes...

Great]y improved resistance to biological attack has been achieved aLso in particleboards made from PF resin-impregnated particles of Japanese cedar [65,66]. Particles had average dimensions of 0.43 mm in thickness, 2.83 mm in width, and 29.6 mm in length. They were oven-dried until moisture content reached about 3%. A water-soluble PF resin of Mn 389 was used for the treatment of particles. Two methods were employed for incorporating the resin into the particleboards method I—pretreatment by dipping of particles... 

If wood is conditioned at 20°C and 65% relative humidity the sapwood of most timbers will equilibrate to about 12% moisture content. This corresponds to the typical moisture content of lumber in an unheated building in many temperate regions. There are exceptions. Nothofagus fusca, one of the Southern Beeches, would dry to 9.5% or so under these conditions, so drying to 12% would be inadequate (Harris, 1961). Medium density fibreboard and particleboard also equilibrate at a lower moisture content of about 8-9%. For this reason Hoadley (1979) observed that it is more logical that wood products be dried to a specified relative humidity than to a specified moisture content (Table 4.1). 

Early fibreboards were developed by taking fibre from a wet-process fibre line, drying this, mixing it with resin, and pressing it as had been done for particleboard. However, in the last three decades medium density fibreboard (MDF), a UF bonded and dry formed panel, has largely replaced the early fibreboard products. 

The first MDF was made using particleboard equipment. The fibre was dried and mixed with resin in a particleboard blender. The product had excellent properties, but there were dark areas of varying sizes in the panel that on analysis were shown to have a resin content two or three times the average. These clumps of resin-rich fibre - known as resin spots - were sometimes sufficiently hard as to break saw teeth or to form lumps under overlays. A range of techniques was adopted to break up these lumps, including attrition mills, but with limited success. 

Gran G (1982) Blowline blending in dry process fibreboard production. In Maloney T (ed) Proceedings of the 16th Washington State University Particleboard Symposium, Pullman, Washington, 261-7... 

Figure 1. Particleboard elution by dry nitrogen sample geometry effects (o shredded 1.0 NCM. A A 25x25x16 mm 0.5 NCM duplicate runs. P = Perforator value in mg/100 g dry board/measured on starting material at " 6 pet moisture content.) (ML85 5428)...

Figure 2. Particleboard elution by different dry gases. (Differences in the two curves due to different flow rates and experimental configurations. P as in Figure 1.) (M 85 5429)...

Duplicate 12 mm thick three layers particleboard 600 mm x 300 mm in dimensions were prepared in the laboratory using 7 Z UF resin solids total on oven dry pine wood chips. The glue mix used was as follows ... 

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