Particleboards finishing

The objective of our work was to determine the effect of some common surface finishes and overlays on the formaldehyde emission from particleboard. Finishes used in the building trade as well as such used in the furniture and joinery industries were studied. 

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. 

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. 

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

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). 

Many wood species, both hardwoods and softwoods, are used for particleboard however, the density of the particleboard should be higher than the density of the raw material to efficiently utilize the adhesive system. The compression of the particles, which is required for consolidation into the finished product, enhances the particle-particle contact, producing more inter-particle adhesive bonds as well as reducing the total void volume in the panel. With wood of density higher than the finished particleboard, the compression of the particles is lower and the resultant reduced interparticle contact and higher void volume adversely influence the physical and mechanical properties of the parti cleboard. 

Pressing Operation. The consolidation of the particle mat and polymerization of the adhesive to produce a particleboard panel is accomplished in a hot press. The mat is compressed and held at the desired thickness until the adhesive on the particle surfaces has polymerized and established adequate bridges between particles. The panel is then removed from the press, cooled, and sent to the finishing phase. 

Vinyl overlays are thin sheets of polyvinyl chloride, often with a simulated grain pattern, which are glued to the particleboard to obtain an inexpensive finish of relatively low durability. Particleboard panels containing a 3"dimensional design can be vacuum laminated with vinyl films, provided sharp corners are not present in the design. The thermoplastic vinyl film is heated to the softening temperature and, as the air is withdrawn from between the film and the machined panel, atmospheric pressure from above forces the film to conform to the contours of the panel. 

A great store of information has been accumulated on the effectiveness of WRPs in protecting exterior wood (J, 117-119, 121, 122, 124-129). The treatments can be applied by vacuum impregnation, by immersion (which is preferred), by brush, or by spray application. They improve the performance of many finishes applied over them and add greatly to the durability of exposed wood. Even chipboard or particleboard, which is very susceptible to moisture, can be protected quite effectively against the effects of outdoor exposure by using a WRP pretreatment followed by a diffusion-resistant coating 126, 130-133). 

The density profiles reveal density areas at both surfaces, with a lower density in the centre of the panel. The high surface density gives good bending strength and surface finishing characteristics. A low density in the core of the panel allows these surface properties to be achieved with an average panel density that is only 60-70% of that at the surface. For particleboard and OSB the profile between the peaks is a... 

MDF contains 8-10 wt% UFR. In 1984 1.1 million m was produced in the U.S. ina total of 12 plants 90% of this type of board is used for furniture and cabinet work. This product is more expensive than particleboard, but its advantage is that its edges are smooth and dense, and thus are better capable of holding screws and hinges, and this product need not be further treated or finished after manufacture. 

Product tests. Clearly, the best product test is full-scale testing of finished panels under actual use conditions. This has been done (27,38) but is expensive, because several full-sized panels of each product must be pre-conditioned at constant temperature and humidity for at least a week. The next best approach is to test product samples in air chambers under standardized conditions. A summary of such methods is contained in Table I. A very large effort has been made over the last three decades world-wide to develop quick, reliable and meaningful product tests. Wittmann (16), Zartl (20), Plath (17), Verbestel (1, Neusser (21,22), Roffael (25), HUD, the U.S. Forest Products Industry (39,40), many standaraization organizations (41-43) and others have published many viable methods, but the testing involves a combination of complex factors and there is simply no single test that fulfills everybody s specific needs. Table I list some of the currently accepted test methods for formaldehyde emission from particleboard, plywood and medium density fiberboard. 

Hardwood plywood products are decorative in nature and are designed for interior use. Over 95X of all hardwood plywood is made with urea-formaldehyde adhesives. Responding to concerns about formaldehyde and certain wood products, test methods for measuring surface emissions were developed in the early 1980 s. Emissions from most hardwood plywood and particleboard products have decreased 65% to 95% in recent years primarily by use of low emitting UF adhesives and/or scavengers. Good correlation has been demonstrated between product test methods and indoor levels of formaldehyde in experimental manufactured homes. Decorative surface finishes can act to either increase or decrease surface emissions, depending on the nature of the finish and the substrate. 

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