Test data from the US Navy show the dry and wet Glass Transition Temperatures (Tg) of PEEK-based composites are the same, due to low moisture uptake of PEEK at less than 0.2%. The Naval Air Systems Command (NAVAIR) recently publicized these results based on continuous fiber reinforced PEEK composite structures made by Automated Dynamics. PEEK’s stability in wet environments is critical in applications where moisture may hinder a structure’s mechanical, electrical, or geometric properties over time. Test summary is available here PEEK Wet Tg Testing Brief.
This benefit of PEEK’s Tg is not news, but it is underutilized in various markets.The Navy has been evaluating and using PEEK composites in various applications for decades, and the recent publication of a multi-year development program ending in 2012 testifies to the conservative nature of a group who ensures our war planes don’t fall from the sky. Likewise, the Oil & Gas industry tends to be cautious of all things non-metal that are critical in a system’s reliability.
For the areas where non-metallics are considered, there’s a broad performance range with both thermoplastics and thermosets. Commodity-grade thermoplastics are fit for applications where considerable moisture infiltration is not an issue, and these applications tend to be exclusive of those suited for PEEK. Alternatively, thermosets composites are used widely where the economics call for low upfront costs. In contrast with PEEK, these epoxy resins degrade substantially in saturated wet environments over time, and the tangible effects like cracking and moisture-induced geometry changes are exacerbated when coupled with a temperature rise. Here, the polymer breaks down further and prevents effective load transfer between the fiber and matrix, which can ultimately cause macroscopic structural failure.
A composite’s quality and reliability hinge on this load sharing ability, and since the oilfield demands are usually wet, designers employ PEEK when part stability is required. Part stability is further realized when continuous fiber reinforced PEEK composites are chosen, and laminates can be tailored around the thermal expansion rates or stiffness requirements of adjoining metal components.
While PEEK bodes well with the alternatives in hot-wet conditions, environmental effects on a composite can be further complicated if the fiber uses an epoxy sizing (thin coating), which protects the fiber before and during composite tape manufacturing. New non-epoxy-based sizing is now commercially available and looks promising for several Oil & Gas applications. Automated Dynamics is evaluating these new composites and will publish test results soon…