Thermal properties, moisture effects

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

Aloe vera gel is a mucilaginous preparation obtained from the leaves of Aloe vera (syn. Aloe barbadensis) once the sap has been drained away. It is widely used in cosmetic products for its purported emollient and moisturizing effects. It is claimed to have antibacterial, antifungal, and anti-inflammatory properties and has been proposed as a potentially useful agent for radiation and thermal wounds (see Chapter 12). 

In Sections 24.3 and 24.5 the flammability and fire resistance of individual fiber/fabric type are discussed. However, as also discussed before, the fire resistance of a fabric not only depends upon the nature of components and the FR treatments applied, but also on fabric area density, construction, air permeability, and moisture content. Nonwovens, for example, will have superior properties to woven or knitted structure, even if all other variables are kept the same.93 The air entrapped within the interstices of any fabric structure and between layers of fabrics within a garment assembly provides the real thermal insulation. For effective thermal and fire resistance in a fabric structure, these insulating air domains need to be maintained.22 In general, for protective clothing and fire-block materials, for best performance multilayered fabric structures are employed. The assembly structures can be engineered to maximize their performance. It is beyond the scope of this chapter to go into details of these composite structures hence the reader is referred to the literature on specified applications and products available. 

As the cell walls are squeezed into polyhedra, a wall-thinning effect takes place, and liquid is drained from cell-wall faces into the lines of cell intersections to form ribs or struts, which are typically triangular in cross-section. This cell wall membrane thinning can continue to the point where the cell walls collapse and the cells open. This becomes a very important characteristic of most plastic foams, and affects properties such as thermal conductivity, moisture absorption, breathability, and load bearing. 

Other Physical Properties. In addition to its important effect on the strength of wood, moisture also affects wood s other physical properties. Moisture s effect on electrical properties was described in the section on Electrical Resistance Moisture Meters (p. 130). Other properties such as specific gravity and thermal properties are discussed here. 

Flexural modulus, effect on, 147 Flexural strength, effect on, 147 In limestones, 146 In dolomite, 146 Mohs hardness, 146 Moisture content, 146 Muscovite, 146 Oil absorption, 146 Particle size, 146 Reinforcing effects, 146 Speciflc gravity, 146 Thermal properties, effect on, 146 Water absorption... 

It has been reported that silver ion-exchanged zeolites exhibit antibacterial activity [32]. The mechanism of antibacterial action of the zeolite is initiated when moisture or liquid film comes into contact with the ion exchange material and silver ions are exchanged with sodium (Na) or other cations from the environment [33]. The released silver ions attach to the bacteria by forming chelate complexes with deoxyribonucleic acid, which blocks the transport processes in the cell [34]. The use of zeolite as a filler in polymeric materials has been reported in the literature and it has been proved that they enhance the antibacterial activity of the polymer [35]. Furthermore, the effect of zeolite content on the physical and thermal properties of the polymer was also examined [35] increasing the silver/zeolite ratio in the polymer led to an increased antimicrobial activity (due to the higher silver ion concentration), but depending upon the application the zeolite content may influence physical, thermal and/or chemical properties of the polymeric material. 

Moisture Relations, Temperature Effects, and Thermal Properties... 

Despite the extreme importance of the thermal properties on the processing and performance of textile polymers, particularly in filament processing and finishing, there are surprisingly few standard methods of test dealing with the subject. This is perhaps due to the complexity of the subject in terms of the effects of temperature on oriented chain molecules and influence of moisture on the polymer. However, a standard method that is available is ASTM D 5591-95, which is concerned with the thermal shrinkage force of yarn and cord. The instrument specified is the Testrite thermal shrinkage force tester... 

Effective thermal conductivity and effective moisture diffusivity are related to internal heat and mass transfer, respectively, while air boundary heat and mass transfer coefficients are related to external heat and mass transfer, respectively. The above transport properties are usually coefficients in the corresponding flow rate and driving force relationship. The equilibrium material moisture content, on the other hand, is usually related to the mass transfer driving force. 

Moysey, E.B., Shaw, J.T., and Lampman, W.P., The effect of temperature and moisture on the thermal properties of rape-seed, Trans. ASAE, 20 768-771 (1977). 

In summary of this experimental work, moisture content has a significant effect on the thermal properties of Redberry and Impact variety red lentils. The glass transition temperature, Tg, increased with decreasing moisture content. Single broad transitions were observed from the DSC thermograms of Redberry and Impact variety red lentils at different moisture contents. 

The analysis described in this paper can be used to make judgements and estimates of the mechanical behavior of paper materials under transient humidity conditions. Changing moisture content results in altered physical and chemical properties of sheets. One example is the surface electrical resistivity of sheets which is known to be an exponential function of the local moisture content. An analysis of the type shown here allows us to predict the electrical property variation under transient humidity conditions. Another example of the applicability of this analysis is in the compressive strength of paper board. It is known that the compressive strength of paper board decreases sharply with changing moisture contents. An analysis based on the thermal and moisture transport effects can easily identify whether thermal and moisture transients are responsible for accelerated loss in compressive strengths of paper materials. 

Abstract This chapter introduces the moisture-induced shape memory effect (SME) observed in supramolecular shape memory polymers, particularly shape memory polyurethane (SMPU), containing pyridine moieties. The moisture absorption of polyurethane networks containing pyridine moeties (PUPy) is discussed followed by an investigation into the effect of relative humidity (RH), temperature, BINA content and MDI-BDO content. The induence of moisture absorption on both the thermal properties and the dynamic mechanical properties of SMPUs is also described, along with the moisture-induced SME mechanism. 

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