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Innovative Sand-Erosion Resistant Coatings for Propellers and Helicopter Rotor Blades

High performance propeller driven vehicles and helicopters for various transport, surveillance and combat tasks have gained importance in the US military's preparations to operate reliably in a wide variety of theaters of operation and environmental conditions. Since light weight and rapid maneuverability are premium considerations, considerable use of composite structures of fibers and polymers has found prominence in aircraft constructions and, in particular, for aircraft rotor blades. Especially in areas with high air particulate loadings, such as sand or other dusts, rain or ice, and salt, highly erosive and corrosive conditions are created at the leading edges of rapidly rotating equipment. Excessive wear or roughening of those leading edges can have serous deleterious effects on the efficiency and aerodynamic performance of the blades, and hence of the whole machine. Since wear is concentrated at these leading edges, maintaining them in acceptable shape may have a real bearing on the service availability of this equipment.

Recognizing the importance of these issues, the US Navy has issued several requests for proposals for research (SBIR's) to develop new and tougher coatings for leading edges of such blades, that will provide enhanced erosion and corrosion resistance, together with improved adhesion to the composite substrates and thus longer coating life. Each succeeding RFP has sought improvement over previously available materials to meet the continually escalating requirements imposed by new aircraft designs and diversifying locales of concern.

Over the past three years, POLYMERight, Inc. of Fremont, California has been developing a proprietary technology that can be used to create new polyurethane elastomers with significantly improved abrasion resistance, higher resistance to chemicals and petroleum products (including hydraulic fluids and the Navy's DS2 decontamination liquid), strong resistance to hydrolysis (water degradation) and strong adhesive properties to a variety of substrates over a broad temperature range. Although such elastomers were originally targeted for industrial applications (see later), the combination of properties they offer seems ideal for the helicopter blade application.

POLYMERight is a privately owned company dedicated to the creation of new specialty polymeric materials that offer combinations of properties not available in currently commercial products. Such products will be manufactured and sold by POLYMERight to those markets which can advantageously use the unique properties provided, typically in the form of adhesives, sealants, coatings and elastomers, for both retail and industrial uses. One example of this currently expanding sales to large retail hardware and home repair stores is a line of POLYMERight's "underwater glue" products that provide unique bonding characteristics for many substrates, including PVC, in wet environments, even underwater. A second example is a newly invented coating to protect steel structures subject to tidal zone splashing and corrosion which is now in development for the Navy.

The innovative, highly abrasion resistant polyurethane polymer suggested for the helicopter blade coating is termed a "three block" polyurethane by POLYMERight. This term is used to highlight the fact that the new polymer has three distinct alternating chemical segments in the polymer molecule, rather than the two segments of conventional polyurethanes. This third segment is a large cyclical block, and it is this unusual structure which imparts the greater abrasion resistance and other enhanced properties of the product.

Elastomeric three block polymers provide materials with properties not heretofore available for commercial use. One category of such uses is in the mining and construction industries where abrasion severely limits the working life of many pieces of expensive equipment. Field application of tough protective coatings and linings in the coal mining industry can permit extended life for conveyor belts, sieve screens, cyclones, gearing, etc., and offers major potential savings (many millions of dollars per year). Here is a list of other such examples:

" Materials for paper-making rolls and other rolls working in abrasive environments " Materials for pneumatic transmission systems for abrasive solids (coal, cement, ores, grains, etc.) " Rollers for fork lifts, carts, bins, etc. crossing abrasive surfaces, concrete floors " Materials for protection of details subject to cavitation (hydrofoil wings, pump impellers, slurry systems, etc.)

Protecting highly valuable equipment to extend its life and to reduce down time, thus enhancing its productivity, provides a major leverage to the value of these new products in a multiplicity of end uses. Finding the means to support this development will generate handsome returns on the investment for many years to come.