Hydrophobicity loss

Hydrophobicity loss and recovery in silicone rubbers has been extensively studied and reviewed in a few recent publications with numerous references.581,582 The ability to recover hydrophobicity after oxidation or contamination is considered as one of the important features determining the applicability of silicones in electrical and other outdoor insulation. The methods of studying of what is known as aging and recovery mechanisms are discussed. 

Diffusible low-molecular-weight components have an important influence on the hydrophobicity of silicone rubber samples. They are not only responsible for the transfer of hydrophobicity to adsorbed pollution layers but also for the hydrophobicity loss as result of water storage. Removal of... 

The MPL often suffers more severe carbon corrosion and hydrophobicity loss than the substrate layer, because it is closer to the corrosive substances produced in membrane and CL (e.g., H2O2, HF). Similar to the carbon powders in the catalyst layer, graphitization of carbon powders in MPL also enhances the corrosion resistance. The hydrophobic material (e.g., PTFE) can also protect the carbon from corrosion. Wood and Borup (Wood et al., 2009) found that no significant structure change occurred in MPL after 664 h of durability test at 1 A cm. In order to enhance the durability, Gore CARBEL CL MPL uses PTFE as substrate filled with carbon black (like the concept of reinforced membrane), and it showed good durability in a PEM fuel cell life test (Cleghorn et al., 2006 Wood et al., 2009). 

In most cases, the end of the cell life is neither related to catalysis nor to the state of the membrane, but is caused by a major hydrophobicity loss of the cathode GDL (backing). Such loss leads to catastrophic flooding of the cathode catalyst and prevents oxygen from reaching catalytic sites. 

Fig. 3 Proposed pathway of membrane electrode assembly (MEA) and GDL interaction leading to observed hydrophobicity loss of GDL constituent materials...

To determine the cause of GDL hydrophobicity loss and associated degradation mechanism(s), changes in the exposed carbon phases (graphite fibers and carbon particles), the fluoropolymer coating (PTFE, EEP, etc.) overlaying the carbon surfaces, and the bonding interface between the sintered fluoropolymer particles and carbon surfaces must be studied. All three of these aspects play at least some role in GDL and MPL hydrophobicity losses, and delineation of the relative importance of each surface or interface is a central theme of this work. The relative... 

Liquid water and dissolved O, both of which are in abundance at the cathode side of an operating PEFC, are direct causes of hydrophobicity loss of the MPL. This experiment also gives an indication that the intrinsic hydrophobicity of the... 

The importance of chain mobility and flexibility can be further realized when comparing the behavior of silicone rubber to the behavior of PTFE. As has already been mentioned, PTFE is the most hydrophobic material among the materials employed in outdoor insulation and is more hydrophobic than silicone rubber. However, it is characterized by a high glass transition temperature at 117°C [51], which indicates that at room temperature the material demonstrates a strong solid form rather than rubber as in the case of silicone rubber. As a result, in the case of a hydrophobicity loss, recovery is not possible, in contrast to silicone rubber, where recovery mechanisms are present. Consequently, although PTFE is more hydrophobic than silicone rubber and has been used for the manufacture of insulators during the 70s, today silicone rubber is the dominant material, mainly due to its capability to maintain surface behavior in held conditions. 

The cause of surface degradation is the electrical activity observed on the material surface during a period of a hydrophobicity loss. Dry band arcing can supply the material volume with energy in the form of heat, which will support the development of chemical and physical changes [58]. The addition of fillers, ATH and Sihca, aims to increase the material heat conductivity [59, 60] and further decrease the amount of energy dissipated within the material volume. In addition, in the case of Alumina Trihydrate, when a material temperature of 220°C is exceeded, water is released in the form of vapor (Reaction 1.8) enhancing the filer action [46]. 

The electrical activity observed, either during periods of hydrophobicity loss (dry band arcing) or even if the surface is hydrophobic (corona along the water droplets), suppHes energy to the material that can support oxidation Reactions 1.10 following 1.9, which finally conclude in a sihca layer near the surface [72,81,82]. 

Two forms of electrical activity may appear on the coating surface, dry band arcing during a period of hydrophobicity loss and corona. The energy supplied in both cases can support physical and chemical changes of the material surface and thus affect the amount of LMW PDMS molecules present. 

J. Kim, K. Chaudhury, Corona discharge induced hydrophobicity loss and recovery of silicones. Conference on Electrical Insulation and Dielectric Phenomena, p. 703-706,1999. 

Of the cations (counterions) associated with polar groups, sodium and potassium impart water solubiUty, whereas calcium, barium, and magnesium promote oil solubiUty. Ammonium and substituted ammonium ions provide both water and oil solubiUty. Triethanolammonium is a commercially important example. Salts (anionic surfactants) of these ions ate often used in emulsification. Higher ionic strength of the medium depresses surfactant solubihty. To compensate for the loss of solubiUty, shorter hydrophobes ate used for appHcation in high ionic-strength media. The U.S. shipment of anionic surfactants in 1993 amounted to 49% of total surfactant production. 

Sutoh and Noda154 succeeded in proving, by synthesizing block copolymers of the structure (Gly-Pro-Pro)n(Gly-Ala-Pro)m-(Gly-Pro-Pro)n, that with increasing imino add content, AS° changes to higher positive values. They do, however, not relate this to lower entropy losses of conformation but to hydrophobic interactions of the proline residues in the helical state. 

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