Acetic acid silicone rubber

Lower alcohols, acetone, acetic acid, acetic anhydride Silicone rubber... 

Room-temperature-vulcanizing silicone rubber (General Electric and Dow Corning) is available at hardware stores and is very useful as an adhesive and sealant. Atmospheric moisture is necessary to effect the cure, so broad areas of impermeable materials should not be cemented with these preparations. The uncured material evolves acetic acid, and the cured material appears to lose some weight in high vacuum but if used with moderation, it can be considered a satisfactory vacuum sealant for most chemical vacuum systems. 

Such silicone polymers are stable upon storage in the absence of moisture, e.g. filled in a closed cartouche,. Upon contact with moisture, e.g. humidity in the air, hydroxy-groups are formed at either end of the polymer chain with the cleavage of acetic acid, which react, e.g. with one another with the cleavage of water, to form a crosslinked rubber-elastic material. The crosslinking starts at the surface and then spreads into the mass as the moisture diffuses into the polymer. 

Their main field of application are sealing compounds (silicone rubber) for which they are sold in the form of reactive one-component systems (mainly in cartridges) (RTV-l-systems, room-temperature vulcanization). Just like the one-component polyurethanes described in Section 4.2.2, they cure under the influence of moisture from the ambient air. In certain formulations, this reaction leads to the separation of acetic acid perceptible by its characteristic odor. Adhesive and sealing layers on a silicone basis show the following characteristics ... 

Water can be removed from methanol by a membrane of polyvinyl alcohol cross-linked with polyacrylic acid, with a separation factor of 465.204 A polymeric hydrazone of 2,6-pyridinedialdehyde has been used to dehydrate azeotropes of water with n- and /-propyl alcohol, s- and tort butyl alcohol, and tetrahydrofuran.205 The Clostridium acetobutylicum which is used to produce 1-butanol, is inhibited by it. Pervaporation through a poly(dimethyl-siloxane) membrane filled with cyclodextrins, zeolites, or oleyl alcohol kept the concentration in the broth lower than 1% and removed the inhibition.206 Acetic acid can be dehydrated with separation factors of 807 for poly(4-methyl-l-pentene) grafted with 4-vinylpyridine,207 150 for polyvinyl alcohol cross-linked with glutaraldehyde,208 more than 1300 for a doped polyaniline film (4.1 g/m2h),209 125 for a nylon-polyacrylic acid membrane (5400 g/m2h), and 72 for a polysulfone.210 Pyridine can be dehydrated with a membrane of a copolymer of acrylonitrile and 4-styrenesulfonic acid to give more than 99% pyridine.211 A hydrophobic silicone rubber membrane removes acetone selectively from water. A hydrophilic cross-linked polyvinyl alcohol membrane removes water selectively from acetone. Both are more selective than distillation.212... 

This glycol diether is a colorless liquid. It is completely miscible with acetone, ethyl alcohol, ethyl acetate, isopropyl ether, heptane, ethylene dichloride and castor oil. Because of its being a good solvent for metallic reagents, it is particularly suitable for the Grignard type of reaction. It is also a solvent for inorganic halides and chlorosilanes, and is therefore used in silicone rubber formulations ond in the extraction of aliphatic acids from dilute aqueous solutions. 

Cold grades, that is, one-component room temperature vulcanizable elastomer grades (RTVs) dominate in the case of liquid silicone rubbers. In these cases, the rubbers are branched poly(dimethyl siloxanes) with silanol end groups that can be cross-linked with tetrabutyl titanate or methyl triacetoxy silane. The cross-linking starts on contact with the humidity in the air, whereby in the case of methyl triacetoxy silane, for example, acetic acid is liberated and the methyl trihydroxysilane produced reacts with the silanol groups of the polymer ... 

As usual with membrane separations, the membrane is critical for success. Currently, two different classes of membranes are used commercially for pervaporation. To remove traces of organics from water a hydrophobic membrane, most commonly silicone rubber is used. To remove traces of water from organic solvents a hydrophilic membrane such as cellulose acetate, ion exchange men )rane, polyacrylic acid, polysulfone, pol5 inyl alcohol, composite membrane, and ceramic zeolite is used. Both types of membranes are nonporous and operate by a solution-diffusion mechanism Selecting a membrane that will preferentially permeate the more dilute conponent will usually reduce the membrane area required. Membrane life is typically about four years tBaker. 20041. 

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