Filler Materials for Polyphenylenesulphide Composite Coatings: Preprint Page: 4 of 16
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surface hardness and smoothness. However, when coatings are exposed to brine at 200 C, the
surface hardness of hydrothermally oxidized PPS is not high enough to sustain the impact of
hydroblasting. In fact, when such surfaces with heavy scale deposits were repeatedly cleaned by
hydroblasting, they showed severe wear and tear. The damage was due to the bombardment of
hard mineral particles present in the scales during hydroblasting. In the worst case, the coating
film was completely worn out, and the underlying steel was corroded. In addition, the PPS
coating may not be tough and resilient enough to withstand the bending and impact stresses
imposed on the tubes during transportation and installation. Finally, enhancement of the thermal
conductivity of the coating would increase heat transfer from the brine and improve plant
efficiency.
Ceramics might prove to be good fillers for PPS because of their hardness, excellent wear
resistance, and high-temperature stability. Aluminum oxide-rich calcium aluminate (ACA)
fillers are of particular interest for coatings in geothermal plants with acidified fluid because they
are less susceptible than many other ceramic fillers to reacting unfavorably with hot acid. We
investigated the wear rate of ACA-filled and plain PPS, the changes in chemical composition and
state of the ACA-filled PPS surfaces after exposure to hot acid brine, the magnitude of the
coatings' susceptibility to moisture, and the extent of their uptake of corrosive ionic electrolytes.
Adding carbon fiber to PPS improves both the thermal conductivity of the coating system and its
mechanical properties. Barkyoumb et al. (1993) reported that carbon fibers 10-11 m in
diameter have a thermal conductivity ranging from 473 to 809 kcal/hr-m- C, which is more than
six times higher than that of the silicon carbide fillers previously used to improve thermal
performance. In addition, carbon fibers have superior tensile strength and elastic modulus. Thus,
PPS coatings reinforced with carbon fibers could not only enhance thermal conductivity but also
improve mechanical properties.
In field tests at the Mammoth Lakes binary plant, operated by Mammoth Pacific LP, tubes and
test coupons coated with ACA-filled PPS are being exposed to production and injection brines.
Visual evaluations at the six- and nine-month exposure points have shown that the coatings are
completely unaffected by the geothermal fluid. The articles will remain under test for at least a
year, when the exposed coatings will be analyzed more comprehensively. FPL Energy has
offered to use the ACA-filled PPS coatings in some of their brine/working fluid heat exchangers;
preparations for that test are under way.
Experimental Procedures
Test coupons (60 mm x 60 mm) coated with ACA-filled PPS were made in the following
manner. Aluminum oxide-rich calcium aluminate was supplied by Lafarge Aluminates. The
ACA contained two major chemical constituents: 69.8-72.2% aluminum oxide and 26.8-29.2%
calcium oxide. The surface area was 3800-4400 cm2/g, and the particle size was less than 90
m. An X-ray powder diffraction (XRD) survey of the ACA revealed that its main mineralogical
composition consisted of crystalline calcium dialuminate (CaO.2A203) and calcium
monoaluminate (CaO.A203). The metallic substrate used was commercial AISI 1008 carbon
steel. A thermoplastic PPS powder with a particle size of less than 60 m was obtained from
Ticona. It had a high melt flow at its melting point, around 240 C. A 45 wt% PPS powder was2
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Sugama, T. & Gawlik, K. Filler Materials for Polyphenylenesulphide Composite Coatings: Preprint, article, July 17, 2001; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc724074/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.