Porosity promoter

Shaping additives may act as lubricants, plasticizers, cements, porosity promoters (porogenic additives), etc. [9]. The powdered starting material mixed with the additives may be dry, or converted to a plastic pulp by the addition of a suitable liquid. One sometimes distinguishes ... 

Table 7.1 shows the trade-offs that can be made with different methods of achieving increased dry strength. The papermaker must assess his needs versus the disadvantages of alternative approaches. The use of a chemical system can offer other collateral benefits as well as disadvantages. Examples are improved retention and drainage, increased porosity, promotion or interference with other chemical additives, and increased BOD and COD in the water system. 

Microstructurc. Crystal size, porosity, and impurity phases play a major role in fixing the fracture characteristics and toughness of an abrasive grain. As an example, rapidly cooled fused aluminum oxide has a microcrystalline stmcture promoting toughness for heavy-duty grinding appHcations, whereas the same composition cooled slowly has a macrocrystalline stmcture more suitable for medium-duty grinding. 

Porosity ranks next to thickness in importance, especially when the finishes must serve in polluted and/or humid environments which promote tarnish and corrosion. Pores, openings in the surface that extend to the underplate or substrate, can be intrinsic in the coating (14), or can be produced by mechanical wear or by forming operations involved in manufacturing. In some environments the substrate can tarnish or corrode at pore sites and can produce localized areas of insulating films which cause contact resistance to increase. Porosity is less important for connectors that operate indoors at moderate to low relative humidities and in the absence of corrosive pollutants (15). 

The simplest apparatus used for filtration is the filter funnel fitted with a filter paper. The funnel should have an angle as close to 60° as possible, and a long stem (15 cm) to promote rapid filtration. Filter papers are made in varying grades of porosity, and one appropriate to the type of material to be filtered must be chosen (see Section 3.34). 

Filter aids are widely used in die fermentation industry to improve the efficiency of filtration. It is a pre-coated filter medium to prevent blockage or blinding of the filter by solids, which would otherwise wedge diemselves into the pores of the cloth. Filter aid can be added to the fermentation broth to increase the porosity of the cake as it formed. This is only recommended when fermentation product is extracellular. Filter aid adds to the cost of filtration. The minimum quantity needed to achieve the desired result must be established experimentally. Fermentation broths can be pretreated to improve filtration characteristics. Heating to denature proteins enhances the filterability of mycelial broths such as in penicillin production. Alternatively, electrolytes may be added to promote coagulation of colloids into larger, denser particles, which are easier to filter. The filtration process is affected by the viscosity and composition of the broth, and the cell cake.5... 

The solid electrolyte is always visible to the XPS through microcracks of the metal films. As already discussed, some porosity of the metal film is necessary to guarantee enough tpb and thus the ability to induce electrochemical promotion. In order, however, to have sufficient signal from species adsorbed on the metal it is recommended to use films with relatively small porosity (crack surface area 10-25% of the superficial film surface area). 

Catalyst films for electrochemical promotion studies should be thin and porous enough so that the catalytic reaction under study is not subject to internal mass-transfer limitations within the desired operating temperature. Thickness below 10 pm and porosity larger than 30% are usually sufficient to ensure the absence of internal mass-transfer limitations. Several SEM images of such catalyst films have been presented in this book. SEM characterization is very important in assessing the morphological suitability of catalyst films for electrochemical promotion studies and in optimizing the calcination procedure. 

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

Indeed, the mobility of the entrapped dopant is crucial in promoting the reactivity of the final materials. Thus, provided that the dopant molecules are at the surface and enjoy enough freedom, high porosity will certainly promote reactivity by limiting intraparticle diffusion but that will not be the case if microporous xerogels of different HLB are compared (c/. entrapped lipase and tetra-//-propy 1 am monium perruthe-nate (TPAP) where ORMOSIL with the smaller pores are more reactive). 

Along with hydrophobicity, large amounts of both water (to promote hydrolysis) and methanol employed as co-solvent in the catalyst preparation (to promote homogeneity) are needed to ensure optimal reactivity, showing the number of experimental parameters of the sol-gel synthesis which can be controlled independently to optimize the performance of the resulting catalyst. Finally, in contrast to zeolites and other crystalline porous materials, amorphous sol-gel materials show a distribution of porosity which does not restrict the scope of application of sol gel catalysts to substrates under a threshold molecular size. 

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