Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-03-04
2004-04-20
Metzmaier, Daniel S. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S261000, C524S266000, C524S268000, C524S589000, C524S590000
Reexamination Certificate
active
06723771
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to polyurethane elastomers that are useful in industrial applications, such as railroad side-bearing pads, skate wheels, tires, track pads, friction brakes, scraper blades, and the like. More particularly, the invention relates to polyurethane elastomers having improved abrasion resistance and pressure velocity limits while maintaining their coefficients of friction.
2. Description of Related Art
Polyurethane elastomers are well known, tough materials that are in high demand for a variety of uses. The properties that are most sought after by the users of these materials include superior tear resistance and abrasion resistance.
One method of improving abrasion resistance is to lubricate the surface of the polyurethane that is to be subjected to the abrasive force. The resulting reduction in friction at the surface improves abrasion resistance, since it drastically reduces the energy dissipated there. Surface lubrication can be accomplished by means of external application of a lubricant, or through use of internal lubricants, which are well known in the art. Internal lubricants are generally incompatible materials with lubricating properties that are dispersed as a discontinuous phase in the polyurethane polymer matrix. These lubricants are intended to migrate to the surface either under static or dynamic conditions, or upon the onset of abrasion, by breaking open the tiny droplets of the discontinuous phase.
The mechanism by which these internal lubricants improve abrasion resistance is the same as that of external lubrication. The coefficient of friction is reduced, and the resulting reduction in energy dissipation at the surface has the effect of removing much of the driving force for abrasion. U.S. Pat. No. 4,011,189 discloses urethane compositions that are obtained by curing fluid polyurethane compositions containing incompatible fluids uniformly dispersed therein. Such dispersions are prepared by including in the uncured urethane composition, a specific siloxane-organic copolymer dispersing agent, along with the incompatible fluids. In this patent, a wide range of incompatible fluids are dispersed in a polyurethane elastomer by utilizing a specific siloxane-organic copolymer dispersing agent along with the incompatible fluid. Such systems are said to greatly reduce coefficients of friction and, of course, thereby improve abrasion resistance. Both the incompatible fluid and the dispersing agent formulation may be contributing to the reduction in friction.
While this approach may be beneficial for some applications, such as bearings and bushings, there are many elastomer applications for which it is not acceptable. In tire and wheel applications, for example, reduced traction is often unacceptable. In recreational wheels, such as in-line skate wheels, reduced traction is also unacceptable, since the ability of the skater to accelerate and corner are both highly dependent upon obtaining good traction. In rail car suspensions, polyurethane elastomers are used as side-bearing pads. These side-bearing pads are elastomeric springs that slide across a metal surface, thereby allowing the car to navigate curves in the track. Polyurethanes are used because of their excellent load carrying capabilities, good abrasion resistance, and frictional properties. While improvements in abrasion resistance are desired, any reduction in friction is unacceptable. Maintenance of high friction coefficient is critical to prevent “hunting” of the rail car from side to side.
Many other examples can be cited. Braking mechanisms, of course, require excellent frictional characteristics and abrasion resistance. Track pads for tracked vehicles also require both abrasion resistance and excellent frictional characteristics, to allow the heavy vehicle to climb a smooth surface, even when wet.
It is also important to note that friction reducing additives may lead to inaccurate results in lab abrasion tests by lubricating the abrasive surface. In most lab abrasion tests, such as the Taber test, the same surface comes in contact with the sample over and over again. Once the surface is lubricated, it cannot provide anywhere near the same abrasive force. However, in many actual applications, such as in a belt scraper blade, the urethane is usually in contact with fresh, unlubricated abrasive surfaces and excellent results achieved in the laboratory are generally not realized.
U.S. Pat. No. 4,057,595 discloses a method of modifying the physical properties of urethane elastomers, whereby the properties of the urethane are said to be enhanced. The method involves reacting a specific siloxane-polyoxyalkylene into a urethane elastomer. Specifically, the patentees proposed reactive, primary hydroxy difunctional siloxanes to improve elongation, flexibility, and abrasion resistance of polyurethane elastomers. Since the siloxanes would thus react into the polyurethane matrix, they could not exude to the surface. The patentees disclosed improvements in Taber abrasion resistance in their modified elastomers vs. elastomers without additive. However, in elastomer formulations subject to more aggressive abrasion, it has been found that only mild improvements in abrasion resistance are obtained. In addition, tear strength can be seriously compromised.
U.S. Pat. No. 5,605,657 discloses an industrial solid tire for heavy-load, off-road use having a cast polyurethane tire thereon produced by a batch method. The tire is a heat cured polyurethane produced from a dispersion of a prepolymer, a curative and an amount of a silicone dispersing and lubricant agent. The amount of silicone is between 1.0 and 4.0 parts per hundred of the prepolymer. A method for manufacturing the tire is included. The silicone additive used was Dow Corning Q2-3238, a reactive, dispersible silicone additive. This patent demonstrates the ability of a silicone additive to improve the abrasion resistance of a polyurethane elastomer without the loss of coefficient of friction. The reactive nature of the silicone had apparently bound it to the polyurethane during cure and prevented it from migrating to the surface. However, the viscosity of Q2-3238 is so high that it is a paste and not truly a liquid. Even at elevated temperatures the material is pasty and does not flow. This high viscosity also makes it difficult to disperse in conventional mixing equipment commonly used to make polyurethane prepolymers. In addition, the material is hygroscopic and is not available in a dry form. The presence of moisture is a major problem when working with polyurethanes, since they are moisture reactive, and removal of the moisture in Q2-3238 is difficult, again because of its high viscosity.
In addition, it can be seen from U.S. Pat. No. 5,605,657 that even low levels of Q2-3238, e.g., 1.0-2.0%, caused a loss in split tear resistance. Tear resistance is very important to a wide range of urethane elastomer applications, including industrial tires. Industrial tires carrying heavy loads often run over debris that can cut the tire and reduce its life if tear resistance is insufficient.
U.S. Pat. No. 5,849,052 discloses abrasive articles containing a polysiloxane in their bond system that is said to result in a cutting performance that is maintained or increased while wear is dramatically decreased, resulting in improved abrasive efficiency.
U.S. Pat. No. 5,952,444 discloses:
(1) a polyurethane resin derivative having a polydimethylsiloxane side chain and an isocyanate group in a main chain of a polyurethane molecule which is obtainable by reacting (A) a polydimethylsiloxane having an alkyl group on one terminal and two hydroxyl groups in the vicinity of the other terminal with (B) a polyisocyanate having two or more isocyanate groups in one molecule;
(2) a coating resin composition which is obtainable by reacting a polyurethane resin derivative having a polydimethylsiloxane side chain and an isocyanate group in a main chain of a polyurethane molecule with a hydroxyl group-containing resin; and
(3)
Palinkas Richard L.
Peter Thomas H.
Dilworth Michael P.
Grandinetti Paul
Metzmaier Daniel S.
Robertson Jeffrey B.
Uniroyal Chemical Company, Inc.
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