POLYOXYMETHYLENE POLYMER COMPOSITION HAVING LOW FRICTION
A tribologically modified polyoxymethylene polymer composition is disclosed. The polyoxymethylene polymer composition is comprised of a polyoxymethylene polymer, reinforcing fibers, and at least one tribological modifier. The tribological modifier may comprise at least one tribological modifier comprising an ultra-high molecular weight silicone having a kinematic viscosity of greater than about 100,000 mm 2 s −1 . The composition may exhibit a dynamic coefficient of friction against a counter-material of from about 0.001 to about 0.2. The polyoxymethylene polymer compositions provide polymer articles with improved tribological properties and mechanical properties. In one embodiment, the composition contains a polyoxymethylene polymer having functional groups, a coupling agent, reinforcing fibers, and the above silicone.
1 . A polymer composition comprising:
a polyoxymethylene polymer, the polyoxymethylene polymer having reactive groups at terminal positions on the polymer;
reinforcing fibers present in the polymer composition in an amount from about 5% to about 55% by weight;
a coupling agent that couples the polyoxymethylene polymer to the reinforcing fibers; and
at least one tribological modifier comprising an ultra-high molecular weight silicone having a kinematic viscosity of greater than about 100,000 mm 2 S −1 .
2 . A polymer composition as defined in claim 1 , wherein the ultra-high molecular weight silicone is present in the polymer composition in an amount from about 0.1% to about 10% by weight.
3 . A polymer composition as defined in claim 1 , wherein the ultra-high molecular weight silicone is a polydimethylsiloxane.
4 . A polymer composition as defined in claim 1 , wherein the composition contains a second tribological modifier.
5 . A polymer composition as defined in claim 4 , wherein the second tribological modifier comprises polytetrafluoroethylene.
6 . A polymer composition as defined in claim 4 , wherein the second tribological modifier comprises ultra-high molecular weight polyethylene particles.
7 . A polymer composition as defined in claim 1 , wherein the composition further comprises calcium carbonate particles, ultrahigh molecular weight polyethylene, stearyl stearate, an aliphatic ester wax composed of a fatty acid and of a monohydric alcohol, a graft copolymer comprising a polyethylene or polypropylene graft base grafted with an acrylonitrile or with a styrene/acrylonitrile copolymer, or a combination thereof.
8 . A polymer composition as defined in claim 1 , wherein the reactive groups on the polyoxymethylene polymer comprise hydroxyl groups.
9 . A polymer composition of claim 8 , wherein at least 50% of all terminal groups present on the polyoxymethylene polymer comprise hydroxyl groups.
10 . A polymer composition as defined in claim 8 , wherein hydroxyl groups are present on the polyoxymethylene polymer in an amount greater than 15 mmol/kg.
11 . A polymer composition as defined in claim 1 , wherein the polyoxymethylene polymer has a melt volume flow rate of from 6 cm 3 /10 min. to about 15 cm 3 /10 min.
12 . A polymer composition as defined in claim 1 , wherein the polyoxymethylene polymer is present in the composition in an amount from about 50% to about 90% by weight, the reinforcing fibers comprising glass fibers and being present in the polymer composition in an amount from about 5% to about 30% by weight, the ultra-high molecular weight silicone being present in the composition in an amount from about 0.5% to about 4% by weight, and the coupling agent being present in the polymer composition in an amount from about 0.1% to about 2% by weight.
13 . A polymer composition as defined in claim 1 , wherein the coupling agent comprises an isocyanate.
14 . A polymer composition as defined in claim 1 , wherein the coupling agent is present in the composition in an amount from about 0.2% to about 3% by weight.
15 . A polymer composition as defined in claim 1 , wherein the reinforcing fibers are coated with a sizing agent, the coupling agent coupling the reactive groups on the polyoxymethylene polymer to the sizing agent.
16 . A polymer composition as defined in claim 1 , wherein the composition includes a second polyoxymethylene polymer that is different than the first polyoxymethylene polymer.
17 . A polymer composition as defined in claim 1 , wherein the polymer composition exhibits a wear track according to VDA 230-206 of less than 1 mm.
18 . A polymer composition as defined in claim 1 , wherein the polymer composition exhibits a dynamic coefficient of friction according to VDA 230-206 of less than about 0.07.
19 . A molded article made from the polymer composition as defined in claim 1 .
20 . A polymer article made from the polymer composition as defined in claim 1 , the polymer article comprising a gear, a lever, a cam, a roller, a sliding element, a pulley, a latch, a claw, a wiper arm, a conveyor component, a medical inhaler, or a medical injector.