Wood-polymeric composites

This yields a value for the total OH content of approximately 14.8 x 10 moles per gram of dry wood mass. This value is relatively insensitive to the polymeric composition, with... 

It is significant that in the excellent and detailed book by Hebeish and Guthrie (4) only 16 out of 342 pages are devoted to this subject. Of these, 7 are on wood-plastic composites which are mainly in-situs polymerized monomers and only partially grafted. Nevertheless, they are closely related and with worldwide industrial applications. Details of these have been discussed not only in the above references but numerous other reviews and reports (37,38). These should be consulted for further details. 

This research project represents initial studies into a new approach to blending thermoplastic materials like polystyrene with wood materials, leading to a new class of wood-plastic composites. Traditional wood-plastic composites have involved the impregnation and subsequent in situ polymerization of vinyl monomers. This procedure has been adopted for selected products for which improved physical properties justify increased production costs. While producing mixtures or blends of wood and plastics, these types of composites do not demonstrate significant chemical bonding between the wood and plastic components. 

Wood is a three-dimensional, polymeric composite made up primarily of cellulose, hemicellulose, and lignin. These polymers make up the cell wall and are responsible for most of the physical and... 

Molecular. At the molecular level the relationship of strength and chemical composition deals with the individual polymeric components that make up the cell wall. The physical and chemical properties of cellulose, hemicelluloses, and lignin play a major role in the chemistry of strength. However, our perceptions of wood polymeric properties are based on isolated polymers that have been removed from the wood system and, therefore, possibly altered. The individual polymeric components may be far more closely associated with one another than has heretofore been believed. 

Treatment of solid wood over the years for increased utility included many chemical systems that affected the cell wall and filled the void spaces in the wood. Some of these treatments found commercial applications, while some remain laboratory curiosities. A brief description of the earlier treatments is given for heat-stabilized wood, phenol-formaldehyde-treated veneers, bulking of the cell wall with polyethylene glycol, ozone gas-phase treatment, ammonia liquid- and gas-phase treatment, and p- and y-radiation. Many of these treatments led to commercial products, such as Staybwood, Staypak, Im-preg, and Compreg. This chapter is concerned primarily with wood-polymer composites using vinyl monomers. Generally, wood-polymers imply bulk polymerization of a vinyl-type monomer in the void spaces of solid wood. 

When the heat of polymerization is released quickly in a wood-monomer composite the high temperature increases the vapor pressure of the moisture in the cell walls and distills the moisture out of the wood. The change in volume of the cell wall causes changes in dimensions which are manifested by shrinkage and distortion of the original wood shape. Wood-polymer composites cured by the catalyst-heat process must be machined to the final shape after treatment. Conversely, because the heat of polymerization by y-rays is released over a longer period of time, the temperature of the wood-polymer remains low and not as much cell w l moisture is driven off. Therefore, the amount of distortion and dimensional change is somewhat less (30, 31). 

Additives Effect on the Catalyzed Monomer Solution. Soluble dyes can be added to the catalyzed monomer solution to color the final wood-polymer composite. Any color of the visible spectrum can be added, browns to simulate black walnut, red and blues for national colors. The color emphasizes the grain structure of the particular species and combines with the polymer to add a three-dimensional depth not present in surface-finished wood. A dense black wood-polymer, so desirable for musical instruments, is difficult to obtain because of wood s light color and the tendency of the microstructure to chromatographically separate a dye of several components into its separate colors. Dyes have an inhibiting effect on the polymerization of wood-monomer composites, some more so than others. Additional catalyst can be added to overcome this inhibition, but in the radiation process of a given geometry additional time must be allowed for complete curing. 

JL HE POLYMERIZATION OF VINYL MONOMERS in the void spaces of bulk wood results in wood—polymer composites of increased strength properties and dimensional stability see Chapter 6). Because the different environmental conditions expose in-service timber to attack by numerous wood-deteriorating microorganisms, it is desirable to enhance the biodegradation resistance of wood, with simultaneous improvements in mechanical behavior. This chapter summarizes the formation of bioactive wood-polymer composites (1-4). The basic approach is still in situ polymerization of vinyl monomers in wood, with the appropriate choice of a bioactive, toxic, functional group incorporated in the monomer, and with other modifications based on wood-polymer reactions. 

Dardenne, D.S. Woodtruder (Patents Pending) system for extrusion of wood fiber polymeric composites. In Wood-Plastic Conference, Conference Proceedings, Baltimore, MD, December 5-6, 2000 187-188. 

U.S. Pat, Nos. 6,122,877 [107] and 6,682,814 [108] (both by Andersen Corporation) disclose a cellulosic fiber-polymeric composite comprising 45-70% of thermoplastic polymers such as PVC, polyethylene and its copolymers, polystyrene, polyacrylate, polyester and their mixtures, and 30-65% of wood fiber, such as sawdust. 

U.S. Pat. No. 6,780,359 [109] (by Crane Plastics Company, TimbeiTech) discloses a cellulosic fiber-polymeric composite comprising mixing a cellulosic material, such as wood fiber, with a plastic material, such as HDPE, LDPE, PVC, chlorinated PVC, polypropylene, EVA, ABS, and polystyrene, to form a cellulosic reinforced plastic composite. 

IMPREGNATION OF SOFT SOUND WOOD WITH MONOMERS, which are then polymerized in situ by 7 radiation, was a method used in many laboratories during the 1960s in an effort to obtain wood-plastic composites. The process was attractive in two respects there was a large choice of consolidants, and radiation-induced polymerization had many advantages. Various vinyl monomers are cured by 7 rays. By proper selection of the polymer or copolymer, materials can be tailor made for specific applications. The radiation process presents several advantages over the chemically catalyzed polymerization of monomers in wood. 

Phenol-formaldehyde resins are among the most important polymeric adhesives used in the wood based composite panel manufacturing industries [1]. Phenolic resins are prepared by the reaction of phenol or any substituted phenol with formaldehyde or other aldehydes, in the presence of acidic or basic catalyst. The price of phenol depends on the oil price and is likely to ever increase due to shortage of fossil resources. Hence, several lignin substitute products based on renewable materials derived from annual plants such as flax [2, 3] or kenaf [4], agricultural waste such as sugar cane bagasse [5] and wheat straw [6] or by-products from the... 

Lee C-H, Hung K-C, Chen Y-L. Effects of polymeric matrix on accelerated UV weathering properties of wood-plastic composites. Holzforschung 2012 66 (8) 981-987. 

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