Dimensional stability determination

Dimensional stability is an important thermal property for the majority of plastics. It is the temperature above which plastics lose their dimensional stability. For most plastics the main determinant of dimensional stability is their Tg. Only with highly crystalline plastics is Tg not a limitation. 

For applications having only moderate thermal requirements, thermal decomposition may not be an important consideration. However, if the product requires dimensional stability at high temperatures, it is possible that its service temperature or processing temperature may approach its temperature of decomposition (Tj) (Table 7-12). A plastic s decomposition temperature is largely determined by the elements and their bonding within the molecular structures as well as the characteristics of additives, fillers, and reinforcements that may be in them. 

The use of a water-soaking, oven-drying series of cycles for the determination of dimensional stability of wood is a severe test (although it may produce useful data) and it does not necessarily reflect the conditions that wood will encounter in service conditions. As a result, some workers determine dimensional stability by subjecting samples to different relative humidities. It can be readily understood that dimensional stability values determined using different methods will not be comparable and it therefore needs to be explicitly stated how these values were obtained. 

Wood modification can improve the dimensional stabilization of wood by two mechanisms. Where the cell wall is filled in some way by the reagent (whether covalently bound or not), the cell wall is swollen. When the dimensional stability of the modified wood is subsequently determined, the wood can then only swell by an additional amount, which is dependent upon the bulking of the cell wall due to the volume occupied by the modifying agent. This is illustrated in Figures 2.8 and 2.9b. 

As noted previously in Chapter 3, the improved dimensional stability of wood as a result of anhydride modification has been found to be a function of WPG only, irrespective of the anhydride used for modification (Stamm and Tarkow, 1947 Hill and Jones, 1996b Li etal., 2000b). This shows that improved dimensional stability arises due to a bulking phenomenon, caused by the volume occupied by the bonded acyl adducts in the cell wall. The dimensional stabilization of wood modified with crotonic anhydride has also been reported ( etin and Ozmen, 2001). Reductions in the EMC have also been attributed to a bulking phenomenon (Papadopoulos and Hill, 2003), and in at least one example of decay protection (Papadopoulos and Hill, 2002). Further research is needed to determine if this is the case for resistance to other decay organisms. Only with the phenomenon of surface wettability does the relationship between a physical property of chemically modified wood and WPG seem to be broken (Hill and Jones, 1996c) (Table 4.1). 

Ahmed Kabir etal. (1992) treated wood with DMDHEU as well as DMDHEU combined with a vinyl polymer, and determined the dimensional stability of the wood. Methane sulphonic acid was used as a catalyst in both cases. DMDHEU treatment resulted in a 50 % reduction in radial swelling following immersion in water for 100 minutes, with the combined treatment being snperior. However, the ASE (one cycle) of DMDHEU treated wood (30%) was snperior to that fonnd for the combined treatment (17%). DMDHEU appeared to be stable to hydrolysis over a number of wetting cycles. When DMDHEU-treated samples were exposed in ontdoor weathering trials, they exhibited considerable variation in moistnre content and developed severe surface checks, whereas the combined treatment showed snperior performance. 

Schneider and Brebner (1985) impregnated wood samples with the organo-silane y-methacryloxypropyltrimethoxysilane (TMPS) (Figure 7.9a) and determined the dimensional stability as a resnlt of this treatment. Wood samples were conditioned to 6-8 % MC before being impregnated with a methanolic solution of the silane. The methanol was allowed to evaporate from the treated samples and they were then submerged in water to... 

Rowell, R.M. and Ellis, W.D. (1978). Determination of the dimensional stabilization of wood using the water-soak method. Wood and Fiber, 10(4), 104—111. 

Eoams were extruded from low density polyethylene (LDPE) and blends of LDPE with syndiotactic polypropylene (sPP), using isobutane as the blowing agent. The extruded materials were characterised by measurement of dimensional stability at room temperature, density, tensile properties, dynamic stiffness, and crystallinity determined by differential scanning calorimetry. The sPP, with a slow crystallisation rate, did not interfere with the expansion of the LDPE, and enhanced the temperature resistance by in-situ crystallisation. The blends were flexible, dimensionally... 

The factors determining the appearance of ordered cell-like motions were first investigated by Sternling and Scriven (S33) who considered the two-dimensional stability of a plane interface separating two immiscible semi-infinite fluid phases with mass transfer occurring between the phases. This system was shown to be unstable for mass transfer in one direction, but stable for transfer in the opposite direction. For an interfacial tension-lowering solute, instability... 

Although the chemistry of the initial thermal transformation is obviously quite complex, it was determined that considerable carbon insertion into the Si-Si bonds occurs, resulting in an intermediate carbosilane which can be drawn into fibers. At this point, brief oxidation results in the formation of a surface oxide which imparts dimensional stability, and subsequent heating to 1300°C produces silicon carbide fibers (3,5). 

Lysozyme is a natural antibacterial agent found in tears and egg whites. The hen egg white lysozyme (Mr 14,296) is a monomer with 129 amino acid residues. This was the first enzyme to have its three-dimensional structure determined, by David Phillips and colleagues in 1965. The structure revealed four stabilizing disulfide bonds and a cleft containing the active site (Fig. 6-24a see also Fig. 4-18). More than five decades of lysozyme investigations have provided a detailed picture of the structure and activity of the enzyme, and an interesting story of how biochemical science progresses. 

The choice between materials for a particular application may be determined by a combination of technical economic and aesthetic considerations. Ebonites have a great dimensional stability in moist conditions with freedom from deterioration caused by oxidation and excellent resistance to various chemicals. The temperature of use is limited where the material is under mechanical load, owing to its comparatively low softening point. This can be overcome by suitable compounding to some extent. A further weakness in appearance is the tendency of ebonite to develop an acid surface and to... 

As shown in Table 2.1, the 512 cavity is almost spherical (showing a low percentage variation in radius, that is, a low variation in oxygen atom distances from the cavity center) with aradius of 3.95 and 3.91 A in structures I and II, respectively. This small dimensional difference determines the size of the occupant. Until recently, it was thought (Davidson, 1973) that the smallest hydrate guest molecules stabilized the 512 cavity of structure I. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

Finding a PODS is a very easy task. Because the system has only two DOFs, the V = E manifolds are simply one-dimensional lines in configuration space, V(q, q2) = E. Momentum is zero on those points. Finding the self-retracing p.o. amounts to a very easy one-dimensional search. Once a p.o. is found, a linear stability determination is enough to determine the PODS character of a particular p.o. These properties have been used many times in the literature, in a classical or semiclassical, even quantum, context [6,39,43 5]. The reader is referred to the rich literature for many actual examples. The series of articles by Gaspard and Rice are particularly detailed [46]. 

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