Wood, density

Wood FIa.ke Boards. This category covers a range of products depending on the size and orientation of the wood flakes used. The earliest product was made in the 1950s using low density wood species, such as aspen and pine, with the flakes bonded together with phenoHc resins. Today there are two types of flake board, waferboard and oriented strand board (OSB). 

Medium density wood fiberboards (MDF) are semistmctural products used mainly in furniture appHcations (95% of the market). The total shipment of MDF in 1989 was approximately 302 million. 

Fiberboard -for food packaging [FOOD PACKAGING] (Vol 11) -medium-density [WOOD-BASED COMPOSITES AND LAMINATES] (Supplement) -phenolic adhesive [PHENOLIC RESINS] (Vol 18) -sampling standards for [SAMPLING] (Vol 21) -structural [WOOD-BASED COMPOSITES AND LAMINATES] (Supplement)... 

Because of the radiation resistance of polyurethanes, they find use in the nuclear industry. Polyurethanes find an important use in short-run production where parts can be made accurately until more costly molds can be made, without the need for potentially expensive change. Polyurethanes can be used to make molds for casting polyurethane parts. A coat of mold release is required for easy part removal. These molds can be used for short-to-medium production runs. Polyurethane molds can be made from a variety of different materials. Only very low-density wood and high-moisture-containing materials such as plaster of Paris prove to be too difficult to prepare a mold. 

Connor M. A., Viljoen, M. H. Ilic, J. (1996) Relationships between wood density, wood permeability and charcoal yield, In Developments in Thcrmochemical Biomass Conversion (Ed. by A. V. Bridgwater D. G. B. Boocock), pp, 82-96. Blackie, London. 

The density and the porosity of wood seem also to play a role in the pyrolysis process. But the effects observed are rather qualitative than quantitative. Wood with a high density and a low porosity tends to produce more charcoal [23, 29, 33]. The density influences also the productivity of pyrolysis reactors through the bulk volume the same volume of a reactor will produce more charcoal if a high density wood is pyrolysed compared to a light wood. 

Connor M.A. Viljoen M.H., Ilic J. (1997), Relationships between Wood Density, Wood Permeability and Charcoal Yield. In Proc. Developments in Thermochemical Biomass Conversion , BRIDGWATER A. V. BOOCOCK D.G.B. ed, Blackie Academic Professional, Glasgow, UK, 82-96. 

Fibres make up a high proportion of the volume of most hardwoods. Fibres are imperforate, axially-elongated cells, with small lumens and ends that taper into pointed tips. The density of a hardwood is largely determined by the proportion of fibres to other cell types present in the wood. In a low density wood, the vessels occupy a major proportion of the wood volume, whereas denser woods have a larger proportion of thick-walled fibres. The secondary walls of fibres are usually sparsely pitted and the cells lack cell contents at functional maturity. 

The shrinkage of wood is different in the three principal directions longitudinal, radial and tangential (Table 4.1). Typical oven-dry shrinkage values for medium density woods would be ... 

Two terms, permeable and porous, can be distinguished. Porosity relates to the proportion of free space in a material. A low-density wood or cellular foam is porous in that it contains a large void volume. A permeable material, on the other hand, is defined in terms of the ease of fluid flow. If the cells are interconnected then air/water can escape when compressed and the material is porous and permeable (a sponge). A material is porous and impermeable where the cells are closed and the... 

Hygroscopic Forces. Hygroscopic forces can be significant factors, particularly in high-density wood. They are directly related to specific gravity. There is no indication that hygroscopicity diminishes as wood disintegrates until the point of total loss. 

If a wine has a rich tannic structure, but is lacking in body and roundness, lower density wood (e.g. Vosges and Fimousin) tends to reinforce the wine s astringency, but this effect may be partially alleviated by heavy toasting. High-density wood (such as Allier) is better suited to this type of wine, as it releases smaller quantities of phenols. A medium or heavy toast produces aromatic qualities that counteract herbaceous tendencies and attenuate bitterness. 

Pore size distributions in most wood fiber networks show F-distributions. Such behavior was also found for paper materials [23] and for low-density wood fiber insulation materials [7, 8]. The maximum likelihood estimation using the gamfit function in MATLAB [17] was used in order to prove that the assumption of a F-type pore size distribution was correct. 

Cherry n. An even-textured, moderately high-density wood of the eastern USA,... 

Figure 34 shows the relationship between apparent density, wood powder content and LSD/MLS content of PU composites. The maximum values are observed at around wood powder content 70%. This corresponds to the closed packed conditions of fillers. The sfrengfh obtained by bending test using three point supporting method shows similar variation as shown in Fig. 35. The maximum values are found from 20 to 25 MPa which depends slightly on LSD content.

Fig. 34 Relationship between apparent density, wood powder content and LSD/MLS content of PU composites...

Common core types are rigid polymeric foams, low-density woods, and honeycomb structures. 

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