Permeation resistance

High temperature resistance of ETEE and other fluoropolymers ia automotive fuels and their permeation resistance have been discussed (28,29). 

The main fluids of interest with plastics are oxygen and water vapour (for packaging applications) and CO2 (for carbonated drinks applications). Fig. 1.13 and Fig. 1.14 illustrate the type of behaviour exhibited by a range of plastics. In some cases it is necessary to use multiple layers of plastics because no single plastic offers the combination of price, permeation resistance, printability, etc. required for the application. When multi-layers are used, an overall permeation constant for the composite wall may be obtained from... 

Polyethylene, because of its inertness and physical properties, is nearly ideal as a packaging material for food products. It is inert to most chemicals and, except for overheating (which can oxidize it) or irradiation (which can cross link the molecules), the chemical composition of polyethylene is difficult to change. There is one important property which it does lack—permeation resistance. 

The program has been useful in searching and summarizing all of the polymers, with specific vendor models, that have published chemical permeation data on a chemical class basis. The advantage of this is the increased the probability of selecting polymers and/or vendors models that may exhibit equal or better chemical permeation resistance to similar chemicals that have not yet been tested. Thus, fewer models may have to be tested with untested chemicals to find an acceptable garment. 

Good methanol/gasoline fuel permeation resistance together with clearly defined MPAPVA and MPA laminar structures were found in containers blow-molded from PE/PMPAPVA and PE/MPA blends, respectively, with an optimum CP of about 20 wt% [302],... 

R. Hermes and K. H. Bauer. Preparation and characterization of amphiphilic membranes with lyotropic mesophases for simulation of skin permeation resistance. Pharmazie 50 481-486 (1995). 

The sidewall compound is compounded to protect the ply and must possess resistance to weathering, ozone, abrasion, and tearing while providing excellent flex fatigue resistance. The innerliner compound must provide good air-permeation resistance and resist moist/hot air aging. The inside of a hot... 

The choice of permeation barrier materials selected here for coatings is reasonable based on permeation resistance, but external coatings of ceramics on metals are difficult to perfect since many metals have high thermal expansion coefficients and most ceramics have low ones. This causes large thermal stresses in the coatings to develop, which leads to defect formation in the coatings and lowers permeation resistance. A better technique, which will be discussed in the next section, relies on the formation of intrinsic oxide films on the surface of the metals, either by direct oxidation or by alloying followed by oxidation." ... 

Table 6.6 is provided as a final aid for protective glove selection decisions. Table 6.6 identifies the preferred glove types as well as those not recommended for use with pesticide formulations containing various carrier solvent types. The glove types identified as preferred are those that consistently provide the best permeation resistance to pesticide formulations and undiluted carrier solvents within each chemical classification identified. The glove types identified as not recommended are those that consistently provide very poor permeation resistance. Also note that the recommendations in Table 6.6 are... 

Ehntholt, D. J., Bodek, 1., Doerfler, T. E., Schwope, A. D., StoUd, T. J. and J. Valentine, Permeation Resistance of Polymer Glove Materials to Various Strengths of Active Pesticide Ingredients and Carrier Solvents, Draft Internal Report, EPA Offfice of Research and Development Contract No. 68-03-3293, Cincinnati, (1989). 

Palladium and selective alloys with other metals can be fabricated into dense membrane reactors in foil or tubular form, mostly in thin layers to reduce permeation resistance. In... 

Tanny et al. [71] proposed CFMF as a potential solution to the fat removal problem in whey. Using 1.2 p,m pore size MF membrane to clarify sweet cheese whey, Merin et al. [68] observed a permeate that was totally free of fat, while containing aU the other whey components. Compared to the centrifuge-separated and nontreated whey, Merin et al. [68] reported 30% increase in permeation flux when the MF permeate was used as a feed stream in UF to produce WPC. They explained that this is probably due to the absence of small fat globules and casein fines in the MF permeate, which were suggested as major contributors to permeation resistance in the UF of sweet whey. The same group observed reduction of bacteria in the MF permeate by 1 to 2 orders of magnimde. 

Seals are those elements that create or maintain process boundaries between system components and/or subassemblies in order to ensure system integrity in validated process and utility systems. Seals must be biocompatible (able to be in contact with bacteria or mammalian cells without interfering with their metabolism or ability to live and multiply), must be corrosion and permeation resistant, their surface finishes must be free of molding imperfections and foreign matter on surfaces within the sealing area, and shall not generate particulate that may entrain the product. 

The spatial arrangement of the alternating CH2 and CF2 groups along the polymer backbone creates a high dipole moment that accounts for the unique polarity, unusually high dielectric constant, complex polymorphism, and high piezoelectric and pyroelectric activity of the polymer. Because of its polar nature, PVDF is permeation resistant to chlorine and bromine. Both chlorine and bromine are known to permeate most commonly available commercial polymers. 

FIGURE 20.6-8 Highly simplified drawing of the permeation resistance of a hollow fiber. 

Note that the plotted length, zL — Z ., does not contribute to the separation hoi instead provides extra length for parmaant pressure buildup. Clearly, it should he made as short as allowed by module integrity and leak-free considerations. The highly idealized picture in accord with assumptions t, 2, and 3 is depicted in Fig. 20,6-8. This idealization will allow an independent investigation of the effects of fiber on separator performance, free from complications dne to changes in the permeation resistance of the fiber wall. 

Other applications include PA/PE blend is used for permeation resistance and HDPE/EVAc copolymer - polybutylene blends, which are used in heat sealable film. 

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