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Hydraulic PU seals resist extreme temperatures, hydrolysis

By Liz White, UT staffPlymouth, Michigan-A new polyurethane material developed by Freudenberg-NOK unit Simrit for hydraulic seals functions well in the cold as well as resisting degradation by water, the firm claims. Simrit says its new 92 AU 21100 material is a high-performance PU approved for use in high-frequency, hydraulic applications at low temperatures (down to -50°C). The material offers “no starting leakage,” since it seals reliably immediately the application is turned on, even under critical conditions, according to a Simrit statement. As well as an exceptional temperature range, it has low friction, and extreme wear and extrusion resistance, the company said. “During cold weather start-up, sealing lips have to remain in contact with the sliding surface,” Joel Johnson, vice president of technology for Simrit explained, adding that in cold conditions, previous sealing materials had insufficient flexibility and the lips would lose contact and cause leakage. With the new material, “the sealing lip always stays in contact with the sliding surface,” said Johnson.The new PU suffers significantly less stiffening at lower temperatures than conventional polyurethanes as well as less softening at between 120 and 140°C. Another proprietary Simrit material that offers outstanding temperature resistance is Disogrin 7695. Simrit said that Disogrin 7695 was developed to withstand the hydrolysis-or water degradation-that commonly occurs in polyurethane seals, causing a brown, flakey appearance. Since Disogrin 7695 is compatible with both water and vegetable-based hydraulic fluids, it covers a diverse range of uses.Fluid power seals made of the material can handle water at up to 250°F (121°C), making it an alternative to chloroprene and ethylene propylene rubbers. It is also resistant to acidic and basic solutions (pH 2-13) at up to 220°F (105°C).Use of environmentally friendly or biodegradable hydraulic fluids has been, “an increasing trend in the fluid power market,” said Johnson, in Simrit’s statement. Vegetable-based biodegradable fluids, commonly used in environmentally sensitive applications such as forestry, landscaping, agriculture and marine, absorb more water than mineral-based fluids, resulting in seal failures. Disogrin 7695’s compatibility with these fluids. eliminates this problem, the firm claims. “

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