Particleboard discussion

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. 

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. 

The book opens with a paper on the structure and composition of wood to define the material under discussion and then considers molds, permeability, wood preservation, thermal deterioration and fire retard-ance, dimensional stability, adhesion, reconstituted wood boards such as fiberboard and particleboard, plywood, laminated beams, wood finishes, wood-polymer composites, and wood softening and forming. A final paper treats the common theme of wastewater management. Only one of the papers presented at the meeting is not included in this volume, and its subject of conventional wood preservation methods is adequately treated in detail elsewhere (e.g., Nicholas, D. D., Ed Wood Deterioration and Its Prevention by Preservative Treatments, 2 vols., Syracuse University Press, 1973). 

A report from West Virginia University by Koch and Hall (57) discussed the use of hardwood bark as particleboard furnish, with and without added binders. Species included red oak, soft maple, and black birch. Initial studies indicated no-binder boards had to be compressed to 70 pounds per cubic foot to maintain their integrity. Both strength and dimensional stability were enhanced by pressing boards at 400°F instead of 300°F. Longer press times (15, rather than 10 minutes) also helped. Later, boards were made with 5% added starch powder. One potential use of this product was for expandable horticultural planting containers. Both raw and composted barks were tried, with promising results. 

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)... 

Chip makers dealing with waste wood produced about 1.8 million tons per year, and about 70 % of chips were for fuel and about 30% were recycled as a raw material in the paper and board industries (4). The amount of reuse of waste wood was very small compared with the generation. Particleboard or mat-formed board is a product that can use recycled wood (5) (6). Thus, the problems of recycled wood for board making was discussed in this paper. 

Europe has over 40% of the 10 billion m global market for panel surfacing materials, with market share for low pressure melamine (51%), veneer (18%), paper foils (13%), high pressure laminates and paint (7% each) etc. (O Carroll, 2001). The prime substrate is particleboard and MDF, except for the uses discussed above, e.g. formwork and floating floors where plywood excels. The laminating paper is typically an absorbent kraft sheet that is saturated with resin. 

Roffael (15) measured formaldehyde emissions from a phenolic particleboard using the WKI-Method which involves suspending small samples over 50 cm of distilled water in tightly closed polyethylene bottles and measuring formaldehyde levels in the water after varying times. Temperatures were maintained at 42 C. This work indicated that formaldehyde release from the phenolic particleboards ceased after a relatively short reaction period (approximately 96 hours). This finding is consistent with the resin stability considerations discussed previously under theoretical considerations. 

After a discussion of mechanisms for the liberation and subsequent emission of formaldehyde from particleboard, methods to assess the extent of these processes are described. Data are presented for the formaldehyde emission from particleboard with various surface treatments. These data were obtained by a laboratory method and by large climate chamber measurements and show that some of the surface treatments studied constitute very efficient diffusion barriers and considerably reduce the formaldehyde emission rate. 

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

It must be pointed out that a TMA strength improvement of 100% on the MUF resin without methylal (this is achieved by addition of 20% methylal on resin solids) corresponds in the actual wood particleboard to an increase of IB strength of 33%. This means that of all the compounds shown above only the acetals, such as methylal and ethylal, as well as the similarly structured imine/iminomethylene bases discussed above (for which the effect on strength is more marked) are capable of marked improvements in IB strength at the actual wood panel level. 

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