- THE MAGAZINE
There is a growing demand for abrasion resistant polymer systems for coating applications in sensors, optics, textiles and numerous consumer goods. One such system, a carbon filament-wound epoxy composite sensor coating that is subjected to severe in-use wear, is processed by continuous dip-coating of the carbon fiber reinforcement prior to filament winding. During processing, the resin viscosity in which the fiber is dipped must be less than 3000 cP (viscosity) at 80°C to prevent resin degradation.
Conventional epoxy materials generally don’t provide enough wear resistance. To solve these problems, a polymer dispersion incorporating nanocrystalline alumina was developed.
GPC TechnologyThe nanocrystalline alumina is produced by a technology known as gas phase condensation (GPC). GPC technology is at the forefront of some of today’s most advanced nanophase materials (also called nanomaterials). Used for synthesizing inorganic and metallic powders, GPC is based on the evolution of a physical vapor—from the evaporation of elemental or reacted material—followed by immediate condensation and reaction of the vapor into small nanometer particles.1 To maintain nanometer size particles and weak agglomeration, the aerosol is rapidly cooled and diluted to prevent extensive sintering (to form hard agglomerates) and coalescence growth (aggregates).
Many applications require the nanosized particles to be dispersed in a fluid. Although every dispersion application is unique, the powder surface must always be rendered compatible with the dispersing fluid. If a coating is used, individual powders should be coated with a minimal layer of material without causing aggregation and agglomeration.
To meet these requirements, a coating process was recently developed and patented2 that encapsulates nanocrystalline particles with a durable coating. The particle size distribution generated by the GPC process is preserved after coating. The coating can be engineered to allow dispersion of nanoparticles in organic fluids with dielectric constants ranging from 2 to 20, as well as water. Tailored coatings enable the value of individually coated and dispersed nanoparticles to be realized. Steric stabilizers or specific chemical groups can be incorporated into the coating to affect greater dispersion stability or specific (targeted) chemical reactivity, respectively. Figure 1 shows a TEM picture of coated TiO2, with arrows pointing to the coating.
Composite ManufactureFor the carbon filament-wound epoxy composites, a nanocrystalline Al2O3 powder (57 m2/g) was rendered compatible with the composite polymer matrix, Shell 862, by coating the powder with the patented process. In addition, the coating was chemically modified to enable covalent incorporation into the Shell 862 resin. The coated Al2O3 was dispersed into the Shell 862 resin. The resin dispersion was activated by mixing it with Curing Agent W (26.2 parts of Agent W per 100 parts of Shell 862).
The composite was manufactured by continuous dip-coating of a graphite fiber into the activated dispersion followed by winding around a mandrel. The composite was cured under vacuum at 300°F to 350°F for one hour.
Material Evaluation—ViscosityFigure 2 plots viscosity (cP) as a function of shear rate for 40 wt% uncoated and coated Al2O3 in Shell 862. There is a small degree of shear thinning, but the viscosity of coated 40 wt% Al2O3 is less than 3000 cP at 70°C and meets the application requirement.
Material Evaluation—Wear ResistanceThe relative wear resistance of the Al2O3/Shell 862 composite was compared with two other material systems.
System A is an elastomer-extended epoxy filled with ceramic particles, amorphous silica, calcium silicate and TiO2. The resin and hardener contain approximately 49 wt% and 44 wt% solids, respectively. The cured sample contains 46.5 wt% solids. The viscosity of the composite mix prior to cure is not below 4000 cP at any reasonable processing temperature. This is a trowel-applied product.
System B is an 80-83 wt% filled epoxy composite. The viscosity of the composite mix prior to cure is not below 4000 cP at any reasonable processing temperature and cures faster as the temperature is increased. This is also a trowel-applied product.
The relative wear resistance of the composites and steel was quantified by pin-on-disk testing. The test is conducted by rotating three, 1⁄2-in. tungsten balls on a large disk (0.812 in. mean test radius) at the following conditions: ambient temperature, rotation speed of 50 rpm for 3000 cycles, and dry state. Samples were machined to have parallel faces and tested in a fixture that prevented sample/substrate debonding. Samples were wear-tested at 30 pounds force (30 pounds on three balls at 50 rpm for 3000 cycles) followed by 45 pounds force (45 pounds on three balls at 50 rpm for 3000 cycles). Data are reproducible.
Since tested samples have different densities (precluding the use of mass or volume loss during testing), the variable used to quantify wear resistance is the wear track depth. Materials are ranked with respect to wear resistance in Table 1 and Figure 3 for 30 and 45 pounds force. The nanocrystalline polymer coated Al2O3/Shell 862 system provides the highest degree of wear resistance for all polymer-based composite materials tested, irrespective of weight loading. Furthermore, it is also the only material that is capable of being fiber draw processed. The wear results for steel are anomalous because tungsten is transferred from the load source to the wear substrate during wear testing.
Shell 862 composites fabricated with uncoated Al2O3 powder did not exhibit improved wear resistance.
New Coating DevelopmentLaboratory wear testing at 45 pounds applied force demonstrates that nanocrystalline polymer coated Al2O3 covalently incorporated into an epoxy formulation (using targeted chemical reactivity) at 45 wt% provides nearly 4 times and 19 times more wear resistance compared to other commercial materials (80-83 wt% filled epoxy and 46.5 wt% filled elastomer-modified epoxy, respectively). The nanocrystalline polymer coated Al2O3/epoxy dispersion is also the only material than can be processed by filament winding techniques.
The ability to tailor the surfaces of nanosized particles to improve the electronic, optical and chemical performance of polymer-based materials will enable new functional, abrasion-resistant coatings to be developed in the future.