Fillers permeability reduction

Fig. 9.2 Permeability reduction by fillers with high aspect ratio (L/W)...

As mentioned, inorganic fillers with high aspect ratios are expected to lead to more effective permeability reduction in membranes [19]. According to Fig. 9.2, fillers in the form of flakes would be much more effective in reducing the diffusion and therefore the permeability of membranes than spherical particles. 

The use of polyisoprene or butadiene-styrene latex with bentonite or chalk filler and polyoxypropylene as an additive has been used in a plugging solution for oil and gas wells [1042]. The solution can be pumped but coagulates within the formation at temperatures of 100° C within 2 hours. This causes a reduction in permeability. The formulation is particularly useful in deep oil deposits. 

Manson and Chin 151) reported that the addition of filler to an epoxy binder reduces the epoxy s permeability coefficient (P), as well as the solubility of water in the resin (S) and that the reduction is stronger than expected from theory 1 2). Diffusion coefficients calculated from P and S for the unfilled resin were found to be somewhat higher than those for filled resin. The difference seems to be due to the formation of ordered layers, up to 4 pm thick, around every filler particle. The layers form because of residual stresses caused by the difference between the binder and filler coefficients of thermal expansion. The effective activation energy for water to penetrate into these materials, calculated in the 0-100 °C temperature range, is 54.3 kJ/mol151). 

Sometimes filled adhesives will show better resistance to moisture resistance than unfilled adhesives simply because incorporating inert fillers into the adhesive lowers the organic volume that can be affected by moisture. Aluminum powder seems to be particularly effective, especially on aluminum substrates. The filler can provide a reduction of shrinkage on cure, a reduction of the thermal expansion coefficient, and a reduction of the permeability to water and other penetrants. However, fillers do not always produce more durable bonds. 

An early model proposed by Prager (10) assumed random orientation of fillers of various shapes. His predictions showed that while platelets would give a greater improvement than cylinders or spheres, the relative improvement with a 20 volume % loading of randomly oriented platelets would yield only about a 40% reduction in permeability. This would be a Jo/Jn of 1.67. 

The main goal of fillers addition is, due to their physical properties arrd fineness, to improve the concrete mixtirre workabihty, the increase of compactness, tlius the reduction of permeability, primarily due to capillary pores volirme decrease, as well as the reduction of trend to microcracks formation. 

The decrease of alcohol permeability and, consequently, of alcohol crossover, even if accompanied by a reduction of the proton conductivity, open the possibility to new strategies of MEAs preparation by choosing the optimal membrane thickness and alcohol concentration, among several parameters, in order to increase DAFC performance. Other beneficial effect of incorporating fillers in Nafion-based membranes, is the chance of increasing the operation temperature of the fuel cell, due to the retention of water, avoiding the dramatic drop of proton conductivity taking place in Nafion above 100 °C. 

Paradoxically, the efforts to reduce the methanol permeabilities of Nalion with inorganic or organic fillers in most cases yield composite membranes with permeabilities similar to that obtained by optimizing the cast procedure of pure Nafion [302]. Nevertheless, the reduction of methanol permeability by itself is not a criterion for improving DMFC performance because it is usually associated to a reduction of the proton conductivity. We will analyze this property in Sect. 6.5.5 as a previous step to discuss the behavior of the proton-conducting membranes in terms of alcohol selectivity defined by Eq. 6.2. 

The reduction of permeability arises from the longer diffusive path that the penetrants must travel in the presence of the filler (layered silicate in the present case). A sheet-like morphology is particularly efficient as it maximizes the path length. The tortuosity factor (f or T depending on the symbology) is defined as the ratio of the actual distance (d ) that a penetrant must travel to the shortest distance (d) that it would have traveled in the absence of the layered silicate and is expressed in terms of the length (L), width (W)/ and volume fraction of the sheets ((j)). 

A good affinity between the polymer phase and fillers can lead to a rigidified polymer region around the nanoparticles (Case 3, Fig. 7.6). The rednced mobility of the polymeric chains and the possible formation of semicrystalline domains close to the inorganic phase determine a reduction in the permeation coefficient. In this condition, the efficiency of the nanoparticles can also be threatened becanse of the reduced flux at the interface. However, the permeability decrease typically corresponds to an... 

In addition, the presence of impermeable filler in a polymer forces the diffusant molecule to travel further around the filler particles. This physical blocking effect is known as tortuosity, because the filler forces the diffusant to take a more indirect, or tortuous, path through the material. The degree of tortuosity imposed is dependent upon the anisotropy and orientation of the filler particles with respect to the direction of diffusion. For example, platy particles oriented perpendicularly to the diffusion vector will be particularly effective in retarding diffusion. The permeability of a composite can be calculated using an equation that allows for the reduction in permeant solubility and for the tortuosity (Equation 8.3). Where P and Pp are the permeability of the composite and the unfilled polymer, respectively. The terms w and t refer to the width and thickness of the filler and (pp and (pf represent the volume fraction of polymer and filler. 

Very recently, Wu et al. reported the preparation of PBI composite membranes containing small amounts (lower than 1 %) of functimialized multi-walled CNTs (MWCNTs). The membranes were doped with potassium hydroxide and applied in an alkaline direct methanol fuel cell [77]. The main advantages obtained by the incorporation of the CNT fillers were the reduction of the methanol permeability, a higher thermal stability, and the improvement of the irniic conductivity. The maximum power density (104.7 mW cm ) was achieved for the composite PBI membrane with the lowest CNT cmitent, 0.05 %, miming at 90 °C and with 2 M methanol in 6 M KOH. This peak power density was more than three times higher than for a KOH-doped pristine PBI [78]. Nonetheless, the amount of Pt catalyst on each electrode was veiy high, around 5 mg Pt cm , which could be the actual reason for the better performance achieved by the composite PBI-based alkaline direct methanol fuel cell (DMFC). 

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