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Bis-PEG vs. PEG dimethicone
Posted: June 5, 2007
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More recently a new nomenclature was introduced that tells the user if the molecule is terminal or internal.
Hence:

Even under the current system the “a” to “b” value for the internal product or the number of “a” units in the terminal structure are present. The reason that this is critical to a formula's performance is that as the ratio of “a“ to “b” groups increases--or the number of “a” units in terminal groups increases--the silicone becomes less and less water soluble.
