Clear wood, mechanical properties

The mechanical properties of wood tend to increase when it is cooled and to decrease when it is heated (6,18). If untreated wood heated in air is not exposed to temperatures of more than - 70° C for more than about 1 year, the decrease in properties with increasing temperature is referred to as immediate or reversible ie, the property would be lower if tested at the higher temperature but would be unchanged if heated and then tested at room temperature. The immediate effect of temperature on strength and modulus of elasticity of clear wood, based on several different loading modes, is illustrated in Figures 4—6 (6). 

Higher temperatures result in permanent degradation. The amount of this irreversible loss in mechanical properties depends upon moisture content, heating medium, temperature, exposure period, and, to some extent, species. The effects of these factors on modulus of mpture, modulus of elasticity, and work to maximum load are illustrated in Figures 6—9 (6). The effects may be less severe for commercial lumber than for clear wood heated in air (Fig. 10). The permanent property losses shown are based on tests conducted after specimens were cooled to - 24° C and conditioned to a moisture content of... 

The addition of MAPP at around 1-2% ensures perfect coverage of the wood fibres by polypropylene or polyethylene. The micrographs in Figure 5.25 clearly show the lack of adhesion between the fibres (in dark colour) and the PP matrix (in light colour). The mechanical properties are increased by at least 30% when a coupling agent is used. 

Table 1 Approximate Change in the Mechanical Properties of Clear Wood When Subjected to Change in Moisture Content...

Most mechanical properties reported in the United States are done according to standards described by the American Society of Testing and Materials (ASTM). The standard method for testing small, clear specimens of timber (ASTM D143) calls for the test to be conducted at 65% RH. Control specimens have a moisture content of approximately 9% (Table 3), whereas specimens reacted with acetic anhydride or formaldehyde have a moisture content of only about 4% and specimens reacted with butylene oxide 6%. This means that the standard test is run on specimens of different moisture contents. Because of the great effect of moisture content on mechanical properties (Table 1), it is misleading or even invalid, to compare strength properties of control and modified wood since they were tested at different moisture levels. 

All of the physical and mechanical properties of wood are affected by its moisture content. The effect on mechanical properties will be discussed first, followed by consideration of some other important physical properties. In all cases the discussion is limited to clear wood, free from defects such as knots. 

Growth Characteristics. As a fibrous product from living trees, wood is subjected to many environmental influences as it is formed and during its lifetime. These environmental influences can increase the variability of the wood material and, thus, increase the variability of the mechanical properties. To reduce the effect of this inherent variability, standardized testing procedures using small, clear specimens of wood are often used. Small, clear specimens do not have knots, checks, splits, or reaction wood. However, the wood products used and of economic importance in the real world have these defects. Strength estimates derived from small clear specimens are reported because most chemical treatment data have been generated from small clear specimens. 

Table I. Functions Relating Mechanical Properties to Specific Gravity of Clear, Straight-Grained Wood...

Table II. The Average Coefficient of Variation for Some Mechanical Properties of Clear Green Wood...

The quality of eucalypt woods for producing chemical pulps was evaluated using NIR spectra and chemometric methods [124]. NIR spectroscopy was used to predict pulp yield and cellulose content from spectra of powdered wood samples [137]. In another application, in addition to estimating lignin content, NIR spectra were used to quantify hardwood-softwood ratios in paperboard [138]. NIR spectra taken from solid European larch samples subjected to axial bending and compression tests revealed an excellent ability to model the variability of mechanical properties [139]. The study demonstrated that the model is based not only on the measurement of density, but also on surface geometry, composition, and, possibly, lignin content. The authors concluded that NIR spectroscopy shows considerable potential to become a tool for nondestructive evaluation of small clear wood specimens, e.g., increment cores. 

It should be mentioned that the relationships between average molecular weights, MWD, and the power-law index of the respective polymer melts are not clear and completely unexplored in case of wood-filled composites. For example, increasing viscosity does not always improve physical properties of products. It was found that the increase of MFI of polypropylene from 3 to 30 g/10 min did not alter the efficiency of wood fiber dispersion and did not result in an improvement of any measured property of WPC. On the contrary, a change of MFI for HDPE from 0.15 to 7.0 led to better wetting of wood fiber and superior mechanical properties of the WPC. 

Vink has produced evidence in total conflict with previous experience which is suggested to show that the photo-oxidation of polypropylene is a bulk reaction rather than a surface phenomenon. ESCA studies, however, have shown that the photo-oxidation of polypropylene is clearly a surface phenomenon.In a recent study by Kollmann and Wood the photo-oxidation of polypropylene was found to be dependent upon the intensity of the light source. Thus, for unstabilized polymer the rate was proportional to I , whereas for stabilized polymer the rate was proportional to 1O 8-0.9 Thej-e appears to be some conflict in the literature as to whether chemical changes during the photo-oxidation of polyolefins correlate with the changes in mechanical properties.This has always been a difficult... 

Ashton 117, 118) reported on studies for predicting the durability of clear finishes for wood from the basic properties of the finishes. He reviewed the relationships between composition, water absorption, water vapor permeability, tensile strength, and elongation. Single and multiple regressions were used to establish the relative importance of the different properties in determining the durability of the finishes on wood exposed outdoors. The durability of clear phenolics and alkyds could be predicted from water absorption and permeability properties. Tensile strength and mechanical properties were of less importance. 

The answer to this question would seem to be no . If we look at what is going on here and try to identify the causal mechanism at work, it is clear that the property in virtue of which the balls sort is size. If we fixed the size of a particular ball, we could vary the particular lower-level microstructure all we wanted, and as long as the size remained the same, the ball would sort to the appropriate level. We could make the ball out of wood or metal or glass, instead of plastic, and it would still sort according to its size. The particular lower-level configuration of the ball is, in a certain sense, epiphenomenal with respect to the causal mechanism at work here. But if we held the lower level as fixed as possible and varied the size of the ball, the effect would be different. So clearly the causal mechanism at work, when any ball sorts, is at the higher level of size and not at the lower-level microstructure. 

In this paper, the thermal and mechanical characteristics of balsa wood and balsa wood laminates are reviewed, and it is shown that "composite" mechanics that have been developed for the class of synthetic fiber-reinforced plastic (SFRP) materials may be useful for describing the density and direction—dependent mechanical properties of balsa wood in bulk or laminated form. It may be asked whether such advanced analytical methods, perhaps combined with specially developed methods of test, could be used effectively towards developing more applicable QA/QC procedures that will clearly qualify balsa wood as a structural material in applications where strictest code compliance is a necessity. This question has prompted the following review and discussion. 

These variations between woods reflect differences in microscopic structure and chemical organization of the material, for phase geometry can be as important as molecular structure in determining the properties of both natural and synthetic multiphase systems (31). Therefore, it is clear that the mechanical behavior of the wood-water system cannot be explained entirely at the molecular level or as interaction of macromolecules with solvent. Nevertheless, the general trends observed do follow general principles of solvent-polymer interaction and can be so explained. 

Koshijima T and Watanabe T (2003) Association between lignin and carbohydrates in wood and other plant tissues. Springer-Verlag, Berlin Kretchmann DE (2003) Velcro mechanics in wood. Nature Materials, 2 775-6 Kretschmann DE and Green DW (1996) Modeling moisture content-mechanical properly relationships for clear Southern Pine. Wood and Fiber Science, 25(3) 320-37 Kubler H (1987) Growth stresses in trees and related wood properties. Forestry Abstracts, 45(3) 131-89... 

Uses Vehicle for trade sales paints, factory finishes (wood, composition board), and in coatings and impregnation of paper and textiles Features Colloid-free microparficle size emulsion chemical and mechanical sfabilify produces films fhaf are clear, fransparent, and odorless Properties Wh. emulsion, si. char, odor 0.1 particle size dens. 9.76 lb/ gal vise. 200-800 cps pH 4.0-6.5 50-52% total solids EverGlide UV-636 [ShamrockTech.]... 

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