High Temperature Polyimide Coating at 400^oC
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Polyimide coated silica glass products including optical fiber and capillary tubing have been manufactured at Polymicro Technologies for over a quarter century. The products coated with polyimide offer excellent thermal properties desired in a variety of applications. Standard polyimide coating is capable of operating at a wide temperature range from cryogenic to >300^oC. It is the standard coating for capillary used in separation science and is widely used on optical fiber in the aviation and aerospace industries. Polyimide coating is ideal for high temperature environments as it has exceptional heat durability. Polymicro’s standard polyimide coating operates continuously at 300^oC with occasional excursions to 350^oC. Polymicro also offers high temperature polyimide coating that functions up to 400^oC in short excursions.

While polyimide coated fiber is usually used at temperatures <300^oC continuously, metal coated fiber is often selected for >300^oC. However, metal coated silica fiber is costly, and the coating consistency and reliability is often questionable. Some applications may only require optical fiber to cycle from room temperature to ~400^oC for a limited period of time. One example is a fiber optic temperature sensor for intermittently monitoring industrial furnaces. Here, high temperature polyimide coating can be a cost effective and reliable alternative to the metal coating.

Recently, Polymicro’s high temperature polyimide coating and standard polyimide coating on silica glass products were tested at exceptionally high temperature. The test result shows a considerable increase in heat durability over standard polyimide coating, offering the potential to replace costly metal coatings in some high temperature fiber optic sensing applications.

Experiment

Samples of both types of polyimide coated silica products each from different lots were placed inside a high temperature oven set to 400^oC. Before the test, the samples were measured for tensile strength using a two point bend technique. Every 24 hours, a fixed length was removed and tested for strength and also examined under a 40X microscope.

Results

The test results show that high temperature polyimide coating withstands several days of exposure to 400^oC without visible coating degradation. Experimental Weibull curves in Figure1 show that the high temperature polyimide coated silica glass still retains good median tensile strength >400kpsi. Standard polyimide coating that is engineered to continuously operate at 300^oC does not endure lengthy exposure to 400^oC. It is concluded that high temperature polyimide coating has improved its heat durability over the more economical standard polyimide coating.

Summary

Polymicro’s high temperature polyimide coating is capable of withstanding exceptionally high temperature ~400^oC for several days. It has greatly improved its heat durability over the more economical standard polyimide coating that was designed to operate at 300^oC continuously and 350^oC intermittently. The high temperature polyimide coating offers the potential to replace costly metal coatings in applications that require the silica glass fiber or tubing to intermittently experience ~400^oC for a limited period of time.

Figure 1: Typical Weibull curves of the glass product with high temperature polyimide coating still
retains good median tensile strength >400kpsi after baking at 400^oC for several days.