Factors Affecting Physical Properties

Detail reproduction and the stability of color, flexibility and dimension are the most approved specifications of a facial prosthesis made with PDMS elastomer. The detail reproduction is directly related to the esthetic performance of the prothesis since the material mimics the details of the patients skin appearance in order to provide a lifelike appearance. StabiUty of color and flexibiUty of the PDMS are also important factors for longevity of facial prostheses. The dimensional stability is an important property of a material to provide prosthesis fitting over time, protection of the bloody tissues as well as esthetics [54,61]. Nevertheless, the average time of replacing the facial prostheses wcis reported between 

6-18 months [62], The failure of facial prostheses is attributed to the hmitations in the properties of existing materials, especially flexibihty and stability [63]. Recent studies mostly objected to the factors affecting the physical properties such as time, weathering and disinfection [48, 51, 53, 55, 60-67], 

Opacifiers, disinfectants and artificial aging were found as the factors affecting dimensional stability and detail reproduction of PDMS material [53, 68]. Accelerated aging was the most affective test protocol on the dimensional changes in PDMS elastomer [68]. 

Physical weathering is usually responsible for the degradation of elastomers and the replacement of prostheses [65]. Effects of outdoor weathering on the physical properties of PDMS elastomers were investigated and the importance of the exposure duration on the hardening of the material after outdoor weathering was demonstrated [65, 67]. 

Tensile strength, modulus of elasticity, elongation at break, tear strength, hardness, and color were found significantly affected by time. Mechanical and physical properties of silicone maxillofacial elastomers can be changed with time (natural aging in the dark). Time seems to be a critical factor contributing to the overall deterioration of a sihcone maxillofacial elastomer [62]. 

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A review of literature on factors affecting physical properties of solution-cast polymeric films points to the solvent in casting solution as the most important factor [47]. This is because the nature of solvent affects chain conformation, size and asymmetry of the polymer coil in dense films [16]. In many cases, however, the effect of solvent might simply be related to the effect of residual solvent in polymeric films, because the removal of residual solvent from glassy polymers is very difficult. 

Coacervation Is a very complicated physical phenomenon. And, many factors affect the properties of the resulting microcapsules. Coacervation and phase separation from organic and aqueous media Involve many properties, materials and processes such as phase Inducing agents, stirring rates, core to wall ratios, polymer characteristics, core characteristics (wettability, solubility), cooling rates and rates of addition. 

Electrostatic forces between two ions decrease by the factor 1/cP as their separation, d, increases. But dipole-dipole forces vary as l/r/. Because of the higher power of d in the denominator, l/r/ diminishes with increasing d much more rapidly than does l/d. As a result, dipole forces are effective only over very short distances. Furthermore, for dipole-dipole forces, q+ and q represent only partial charges, so these forces are weaker than ion-ion forces. Average dipole-dipole interaction energies are approximately 4 kj per mole of bonds. They are much weaker than ionic and covalent bonds, which have typical energies of about 400 kJ per mole of bonds. Substances in which permanent dipole-dipole interactions affect physical properties include bromine fluoride, BrF, and sulfur... 

Conformational analysis is the study of how conformational factors affect the structure of a molecule and its physical chemical and biological properties... 

Aluminum hydroxide gel may be prepared by a number of methods. The products vary widely in viscosity, particle size, and rate of solution. Such factors as degree of supersaturation, pH during precipitation, temperature, and nature and concentration of by-products present affect the physical properties of the gel. 

To predict the comfort of a material, a combination of hand evaluation, eg, using the Kawabata system, as well as deterrnination of the heat and moisture transport properties, is necessary. Often, these values are correlated with a sensory evaluation of the tactile qualities of the material by a human subject panel. A thorough discussion of the many physical and psychological factors affecting comfort is available (134,135). 

Numerous factors affect the spread of luizardous substances into tlie area surrounding a leaking/buming container or contaituuent vessel. Evacuation decision-makers must carefully consider each of tliese factors in order to determine the conditions created by the release, tlie areas tluit have been or will be affected, and the heiilth affects on people. The factors tliat affect evacuation include amount of released material(s), physical and cheniical properties of the released material(s), health hazards, dispersion pattern, rate of release, and potential duration of release. Each of these factors is explained below. 

The different physical properties, the reactivity of comonomers, and the reaction medium affect copolymerization. The majority of the real processes of copolymerization of acrylamide are complicated. Therefore, copolymerization may not be characterized by the classic equations. The following are the main complicating factors in the copolymerization of acrylamide. 

These results thus show that whereas the flashpoint was only moderately influenced by the compound structure (their chemical functionality but especially their atomic composition and vapour), autoignition temperatures seem to be closely linked to the structural factors that affect the chain. So additivity rules for estimation of AIT should be sought. Every time a chemical or physical property is highly influenced by the structure, chemists tried to establish rules that enable one to reduce a molecule to characteristic groups for which the contribution to the value of this property is known. This was done for instance by Kinney for boiling points and Benson2 for thermochemical properties. 

The nomographs can be used to make a quick, rough, estimate of the column height, but are an oversimplification, as they do not take into account all the physical properties and other factors that affect mass transfer in packed columns. 

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