Polymer composite materials using graphite
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Graphite powders commonly used in polymer compositions, alone or in combination with reinforcing fibers, as well as various mixtures and inorganic fillers, such as mica, talc. Such composite materials include graphite dry sliding properties, seals (which are important in automotive and micromechanical replacement parts and accessories).
Graphite properties that favor its use in polymeric composite materials:
• a low coefficient of friction by scaly graphite in the composite structures especially at the outer portions of the composite solid material (to reduce friction);
• tendency to transition to the mating surface of the film (it helps to decrease wear and tear, particularly when applied in the production of graphite as a water-based dispersion);
• high thermal conductivity (reduced temperature rise by friction);
• electrical conductivity (to prevent the accumulation of static charge, which can be a problem in some cases);
• chemically inert (used in combination with the active substances and in corrosive environments);
• high thermal stability (in favor of the use at high temperatures, such as polyimide, graphite composites which may be used up to 350 ° C).
Composite materials with graphite, such as polystyrene and a polyamide.
In the case of polystyrene, the addition of coarse high purity graphite results in a reduction of friction and wear rate. The polyamide graphite addition decreases the friction coefficient, but causes some increase in wear rate, but using graphite powder particle size is smaller, a better result is obtained. In the case of low density polyethylene and polypropylene, graphite administration causes an increase in friction and wear, but these figures depend on the choice of particle size.
It is believed that an increase in wear polymer composites are associated with a force of adhesion on the graphite surface and the polymer that depends on the wettability of the powder. It may also depend on the specific properties of the polymer, the graphite powder surface area to its volume and chemical interaction surfaces. Simply put, polystyrene shows a strong connection to the graphite surface, while polyolefins show poor connection. Interfacial adhesion increases with decreasing particle size of the graphite.
For this reason, a relatively thin graphite powders (95% <15 microns) are recommended for thermoplastics. Thermosetting polymers are much less sensitive to fillers and polymer interactions. They may also be used and coarser graphite powders (typically 95% <75 microns) depending on the type of polymer material. For thermoplastics, the viscosity of the polymer melt with graphite in the extrusion / spinning will also depend on the size of the graphite particles that are to be respectively technology. The surface area of the graphite particles also strongly influences the quality of the composite material. For example, excessive size of the graphite particles can lead to the formation of voids in the final composite, resulting in desorption of vapors from the hot melt. Typically, high purity graphite is more desirable in order to minimize wear and improve the properties of composite materials, though this parameter can be a priority in the presence of abrasive fillers (glass fibers, carbon fibers, reinforcing the individual components and additives).