US20100146728A1 - Windshield wiper having reduced friction characteristics - Google Patents

Abstract

A wiper blade made of a compound having a methyl vinyl silicone polymer, a filler, and a friction-reducing additive is provided. The friction-reducing additive is present in an amount from between about 5 and 42 weight percent. The average particle size of the friction-reducing additive is preferably less than about 25 microns, thereby permitting extrusion of the compound into the shape of a wiper blade. Polytetrafluoroethylene is preferred as a friction-reducing additive, but other substances, such as boron nitride or graphite could be used.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 12/123,306, filed May 19, 2008, which is a continuation of U.S. patent application Ser. No. 11/358,525, filed Feb. 21, 2006, now U.S. Pat. No. 7,373,687, which is a continuation of U.S. patent application Ser. No. 10/313,346, filed Dec. 6, 2002, now U.S. Pat. No. 7,028,367, which claims the benefit of U.S. Provisional Application No. 60/337,928, filed Dec. 6, 2001. All of the above-mentioned applications are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This invention relates generally to windshield wipers and in particular to windshield wipers having a silicon rubber wiper blade that incorporates PTFE.
• 2. Description of Related Art
• Rain, sleet, and snow have always presented a vision problem for the driver of a moving vehicle. The windshield wiper blade has attempted to minimize the problem by clearing the windshield of the light obstructing moisture and debris. Such blades are typically formed of rubber or rubber-like materials. Over the years, wiper blades have been modified in many ways in order to enhance wipe quality and therefore visibility during precipitation. In some instances, the configuration of the blade has been changed to give a plurality of contact surfaces on the blade. Various modifications have been introduced to improve the consistency and integrity of the wiping edge.
• Wiper designers have developed silicone-rubber-based wiper blades with some success. Silicone rubber is a superior material to natural rubber for several reasons. Silicone rubber, i.e., high molecular weight, vulcanizable polydiorganosiloxane, is able to withstand wide temperature variation without an appreciable effect on its physical properties. Further, silicone rubber is virtually unaffected by ultraviolet radiation, even over long periods of time. It is also resistant to ozone, oil, salt, water and other road and automotive chemicals.
• Silicone rubber as used for wiper compositions has had one significant drawback: it has an unacceptably high coefficient of friction with respect to glass. Some of the early silicone wiper blades exhibited such a high coefficient of friction that the wiper blades could tear loose from the wiper frame when wiping the windshield. Less catastrophic effects of this high coefficient of friction include an unacceptably loud squeak or chatter as the wiper traverses the windshield, and unacceptably high loads on the windshield wiper motor. The silicone wiper blades produced today have improved significantly but wiper designers continually search for improved solutions that would reduce the friction between the wiper blade and the windshield.
• Polytetrafluoroethylene (PTFE) has been used in conjunction with wiper blades in an attempt to decrease friction between the wiper blade and the windshield. However, the wiper blades are typically coated with PTFE after the blade is cured. Coating a cured blade with PTFE is less than desirable because the PTFE will wear off over time, thereby reducing the improved frictional characteristics of the wiper blade.
• Japanese Patent Application No. Hei 5[1993]-117530, by Hiroshi Honma, (the “Honma Application”) describes compounding a fluoro resin powder from 0-10 parts by weight with a silicone rubber formulation for wiper blades. The application teaches that the formulation provides excellent climate resistance and causes no vibration or squeaking. Fluoro resin powder, such as PTFE, is added to the compound in a preferable amount of 1-10 parts by weight, and an average particle size of 40 m. As described in more detail below, the primary problem with compounding PTFE as described in the Honma Application is that the particle size of the PTFE hinders the manufacturability of the compound. Larger particle sizes of PTFE tend to increase the plasticity of the silicone rubber compounds, which reduces the ability to extrude the compound, and in some cases the ability to mold the compound.
• A need therefore exists for a windshield wiper blade made of a silicone rubber compound that provides excellent friction characteristics when wiping a windshield. The reduced friction characteristics of the wiper blade will preferably allow a significant reduction in the force required to move the wiper blade across the windshield and will reduce the amount of chatter, squeaking, jumping, and other noise inducing and performance reducing actions associated with current wiper blades. A need further exists for a windshield wiper blade having these properties that is simple and inexpensive to manufacture. Preferably, the materials used in the wiper blade compound will be readily available and inexpensive. Finally, a need exists for a wiper blade compound that has a relatively low plasticity, thereby allowing the compound to be easily formed by a variety of manufacturing methods, including extrusion.
SUMMARY
• The problems presented by existing wiper blades are solved by the systems and methods described herein. A silicone wiper blade compound according to one embodiment includes a methyl vinyl silicone polymer, a filler, and a friction-reducing additive. The methyl vinyl silicone polymer may be provided in an amount from about 22 to 55 weight percent, the filler in an amount from about 35 to 50 weight percent, and the friction-reducing additive in an amount from about 5 to 42 weight percent. A preferred friction-reducing additive is PTFE having an average particle size of less than 6 μm and being compounded in an amount of about 11 weight percent. Alternatively, boron nitride, graphite, or other friction-reducing additives could be used.
• In another embodiment, a windshield wiper includes a wiper blade of the composition described above. The wiper blade is attached to a frame which is adapted for attachment to a vehicle.
• A method for manufacturing a wiper blade according to another embodiment is further provided. The method includes compounding a mixture similar to that described above and forming the wiper blade from the mixture. The wiper blade may be formed by extrusion, molding, or any other manufacturing process to create a wiper blade having any one of a variety of cross-sectional shapes.
• Other objects, features, and advantages of the illustrative embodiments will become apparent with reference to the drawings, detailed description, and claims that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 illustrates a perspective view of a wiper blade according to an illustrative embodiment, the wiper blade being received by a spline member, which is in turn connected to a wiper frame;
• FIG. 2 depicts a perspective view of the wiper blade ofFIG. 1;
• FIG. 3 illustrates a cross-sectional front view of a wiper blade according to an illustrative embodiment;
• FIG. 4 depicts a cross-sectional front view of another embodiment of a wiper blade;
• FIG. 5 illustrates a cross-sectional front view of another embodiment of a wiper blade;
• FIG. 6 depicts a cross-sectional front view of another embodiment of a wiper blade;
• FIG. 7 illustrates a cross-sectional front view of another embodiment of a wiper blade;
• FIG. 8 depicts a side view of an extruder for manufacturing the wiper blade;
• FIG. 9 illustrates a perspective view of a die used with the extruder ofFIG. 8;
• FIG. 10 depicts a perspective view of a pair of wiper-sized segments of cured silicone elastomer according to an illustrative embodiment;
• FIG. 11 illustrates a perspective view of an alternative die used with the extruder ofFIG. 8; and
• FIG. 12 depicts a perspective view of an elastomer being extruded through the die ofFIG. 11.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical mechanical, structural, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the invention, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
• Referring toFIGS. 1 and 2 in the drawings, awindshield wiper11 according to an illustrative embodiment includes awiper frame13, aspline member15, and awiper blade17.Wiper blade17 includes aspline receiving portion21 and asqueegee member23.Spline receiving portion21 includes athin neck25, and a relativelythick retainer flange27 integrally connected to theneck25.
• Squeegee member23 varies in thickness between athick base31 and a relatively thinsqueegee blade end33. In a preferred embodiment, eachside35 of thesqueegee member23 is inwardly arcuate from the base31 to thesqueegee blade end33. Thesqueegee member23 is integrally connected to theneck25opposite retainer flange27. Theretainer flange27, theneck25, and thesqueegee member23 extend axially along alongitudinal axis37.
• Referring still toFIG. 1 in the drawings, thespline receiving portion21 ofwiper blade17 is configured to receivespline member15 along the axial length of thewiper blade17.Spline member15 is engaged by aclaw41 connected towiper frame13. Movement ofwiper frame13 relative to awindshield45, or other surface, causes thewiper blade17 to remove moisture and other debris from thewindshield45.
• Referring toFIGS. 3,4,5,6, and7 in the drawings, various cross-sections of wiper blades are illustrated. Each wiper blade includesspline receiving portion21,squeegee member23, andblade end33.
• Referring more specifically toFIG. 3, a wiper blade46 includes aretainer flange47 and aneck49 defined bylongitudinal grooves51 on either side ofneck49. Thelongitudinal grooves51 extend the length of wiper blade46 on opposite sides of theneck49. Dimensions A, B, C, D, E, F, G, and H as well as radii R₁and R₂are found in Table 1 below for the wiper blade shown inFIG. 3. Dimensions B, C, D, and F are primarily determined according to the structure of thevehicle wiper frame13 andspline15. Dimensions A, G, H, R₁, R₂, hR₁, and wR₁are chosen to give optimum design and wipe quality, and may vary according to the wiper blade composition. For example, length dimensions G and H would be made relatively longer for stiffer compositions, or for compositions having polydiorganosiloxanes with a larger proportion of vinyl side groups in them or having larger amounts of small-sized particulate fillers. The end thickness A will also vary, as will the thickness E of thebase31, according to the relative resiliency of the cured composition.

• TABLE 1
  Dimension, in.

  Blade Profile	A	B	C	D	E	F	G	H
  FIG. 3	.035	.034	.180	.045	.210	.140	.079	.231
  FIG. 4	.038	.035	.110	.040	.230	.100	.070	.275
  FIG. 5	.035	.040	.220	.050	.230	.195	.060	.230

  Blade Profile	R1	hR1	wR1	R2
  FIG. 3	.236	.420	.229	.100
  FIG. 4	.246	.364	.261	N/A
  FIG. 5	.125	.377	.142	N/A

• Referring now toFIG. 4 in the drawings, awiper blade52 includes aretainer flange53 that is substantially more narrow than theretainer flange47 illustrated inFIG. 3. Atop wall55 ofwiper blade52 downwardly slopes from asidewall57 to aneck58, instead of being at right angles toneck58 andsidewall57. Dimensions A through H, R₁, hR₁, and wR₁are listed in Table 1 for thewiper blade52.
• Awiper blade61 having a slightly different cross-section is illustrated inFIG. 5. The preferred dimensions forwiper blade61 are listed in Table 1.Wiper blade61 includes afirst neck63 and asecond neck65 of approximately the same dimension. Afirst retainer flange66 is disposed betweenfirst neck63 andsecond neck65, and asecond retainer flange67 is integrally connected tosecond neck65.Second retainer flange67 has beveledcorners69. The length of thesecond neck65 betweenfirst retainer flange66 andsecond retainer flange67 is preferably about 0.045 inches. The thickness of thesecond retainer flange67 is preferably about 0.055 inches while the thickness of the unbeveled top portion of thesecond retainer flange67 is approximately the same dimension as the thicknesses offirst neck63 andsecond neck65.
• Referring toFIG. 6 in the drawings, a cross-section ofwiper blade75 is illustrated.Wiper blade75 is adapted to be received by awiper blade holder76. Thewiper blade75 includes fiveintegral ribs77,79,81,83, and85 which extend the length of theblade75 and project generally radially relative to a longitudinal axis of an uppertubular body portion93. Thecentral rib81 is a squeegee rib, and theribs77,79 and83,85 on opposite sides of thesqueegee rib81 are scraping ribs. Thesqueegee rib81 is slightly longer than the scrapingribs77,79,83, and85.
• The dimensional relationships between an uppertubular body portion93, aneck95, a lowertubular body portion97, and ribs77-85 are important to the proper function ofwiper blade75. The preferred dimensions of the wiper blade are illustrated in Table 2. It should be noted that the angle between theribs77,79,81,83, and85 is approximately 30E and the included angle of the points on theribs77,79,83, and85 is approximately 4E. It should also be noted that thesqueegee rib81 has aconcave end face99 atblade end33 in order to present a relatively sharp edge to the surface being wiped.

• TABLE 2
  Dimension, in.

  Blade Profile	A	B	C	D	E	F	G
  FIG. 6	.300-.315	.210-.225	.160	.165	.250	.350	.095

• Referring toFIG. 7 in the drawings, a cross-sectional view of awiper blade107 is illustrated. The spline-receivingportion21 includes anupper surface115 having anentry slot117. Aspline channel119 is disposed within spline-receivingportion21 and is adjacent to and communicable withentry slot117. Preferably, bothentry slot117 andspline channel119 extend the entire length ofwiper blade107 parallel to a longitudinal axis of spline-receivingportion21. In a preferred embodiment,entry slot117 is not as wide asspline channel119, and aretention shoulder125 is disposed on each side ofentry slot117. Retention shoulders125 are flexible, and are therefore configured to bend away fromentry slot117 such that a single-rail spline (not shown) can be inserted intospline channel119. After the single-rail spline is seated withinspline channel119, bothretention shoulders125 rebound to secure the spline within thespline channel119.
• Spline-receivingportion21 also includes twoframe attachment grooves131 that extend the length ofwiper blade107.Frame attachment grooves131 are configured to slidingly receive claws similar to claw41 (seeFIG. 1). Protrusions on the claws fit intogrooves131. Although the claws used with some wiper frames are crimped around the wiper blade, withwiper blade107 it is preferred not to crimp the claws, but instead to allow thewiper blade107 to slide within the protrusions. When slidingly received by the claws, thewiper blade107 is further secured with a pair of end caps (not shown). One end cap is installed on each end ofwiper blade107 to preventwiper blade107 from sliding out of the grasp of the claws.
• A person having skill in the art will recognize that the presence ofretention shoulder125 is not absolutely necessary and that in such a scenario,entry slot117 would be at least as wide asspline channel119, and the single-rail spline would most likely be secured by a friction fit between the spline and the walls of thespline channel119. It is also conceivable that only oneretention shoulder125 is provided that extends from one side of spline-receivingportion21 and either partially or completely covers the single-rail spline. It is further possible thatentry slot117 be disposed on a surface of spline-receivingportion21 other thantop surface115. For example, theentry slot117 could be located on a side surface of spline-receivingportion21, as long as theentry slot117 is still communicable with and adjacent to splinechannel119. Finally, in some embodiments, a wiper blade having a single-rail spline similar towiper blade107 could be provided without anentry slot117. In that embodiment, the single-rail spline would be co-extruded or co-molded with the wiper blade so that the single-rail spline was permanently disposed within thespline channel119.
• The wiper blades described herein (includingwiper blades17,52,61,75, and107) are constructed from a silicone rubber formulation that incorporates PTFE powder or another friction-reducing additive directly into the compound. The preferred composition of the silicone rubber formulation of the present invention is shown in Table 3.

• TABLE 3
  Material	Weight %
  Methyl Vinyl Silicone Polymer	22-5%
  Filler (Silica, Ca, or other mineral)	35-50%
  Friction-reducing additive (PTFE, Graphite, Boron Nitride, or	5-42%
  other additive)
  OH ended Silicone Polymer	1-15%
  Cerium Stabilizer	0.1-1%
  Acid Acceptor	0.1-1%
  Pigment	0.1-1%
  Peroxide	0.5-2%

• As illustrated in Table 3, the friction-reducing additive could include PTFE, graphite, boron nitride, fluoro-polymers, or other fluorine-containing additives. When PTFE is used, a powder form of the compound is added during the compounding stage of the silicone rubber material, which is performed in a Banbury mixer. While a preferred range for the PTFE is between about 5 and 42 weight percent, it has been found that an optimum amount of PTFE is about 11 weight percent. The percentage of PTFE used in the compound, coupled with the average particle size of the PTFE, plays an important part in both the friction reducing properties of the wiper blade and the ability to easily manufacture the wiper blade. The average particle size of the PTFE could be as high as about 25 μm, but it is preferred that the average particle size be below about 6 μm.
• An example of PTFE commonly used in preparing the wiper blade compound of the present invention is Polymist F-5A, which can be obtained from Ausimont USA. Polymist F-5A contains particles of a relatively small size, typically below 6 μm. Table 4 illustrates physical properties for Polymist F-5A.

• 	TABLE 4
  	Average Particle Size, μm	<6
  	Specific Surface Area, m2/g	3
  	Specific Gravity at 23EC	2.28

• It should be understood that the correct selection of amount and particle size for the PTFE or other friction-reducing agent is based on the benefit in reduced friction characteristics and the ability to easily manufacture the resulting compound. Although certain amounts of PTFE may provide better friction-reducing qualities to the compound, the plasticity of the resulting compound is sometimes increased to an extent that extrusion and molding of the compound is difficult or impossible. Extrusion of wiper blades is often preferred over molding because the extrusion process is generally quicker and less expensive.
• Several tests were conducted using various friction-reducing additives to determine the effect the additives have on the friction characteristics of the final compound. The testing protocol is a relatively standard test in the wiper industry for testing friction coefficients. A sample of test material is placed on a slab of glass, and a 200 g weight is applied to the test material. The amount of force required to pull the material across the glass (the “pulling force”) is then measured and recorded. A coefficient of friction is then calculated by dividing the pulling force by the 200 g weight. Each material was tested five times, and an average pulling force was calculated.
• Table 5 illustrates the test results for natural rubber and Standard J-7721-1 TRPL, materials commonly used in windshield wiper blades, the latter being used in wiper blades manufactured by JAMAK Fabrication, Inc. The test results illustrated in Table 6 are for silicone compounds that incorporate the listed friction-reducing additive. The friction-reducing additives listed in Table 6 are not intended to represent an exhaustive list of additives that could be used in the compound of the present invention. Instead, these additives are merely examples of some friction-reducing additives, and the values measured during testing give an indication of the friction-reducing qualities that each additive provides.

• TABLE 5
  		Calculated Coefficient
  Material	Pulling Force (g)	of Friction
  Natural Rubber	598.7	2.99
  Standard J-7721-1 TRPL	479.4	2.40

• TABLE 6
  	Amount of	Pulling	Calculated
  	Additive	Force	Coefficient
  Friction-reducing additive	(pph)	(g)	of Friction

  ALGOFLON 203	11	221.1	1.11
  CTF5 Boron Nitride	18	140.3	0.70
  CTUF Boron Nitride	18	99.8	0.50
  R-020G Graphite	18	131.9	0.66
  R-182B Graphite	18	127.0	0.64
  Polymist F5A	6	224.8	1.12
  Polymist F5A	9	242.6	1.21
  Polymist F5A	11	187.1	0.94
  Polymist F5A	12	274.2	1.37
  Polymist F5A	15	290.3	1.45
  Polymist F5A	16	186.8	0.93
  Polymist F5A	18	264.9	1.32
  Polymist F5A	100	192.0	0.96
  Polymist F510	6	308.5	1.54
  Polymist F510	9	253.6	1.27
  Polymist F510	11	169.5	0.85
  Polymist F510	12	268.9	1.34
  Polymist F510	15	245.7	1.23
  Polymist F510	18	246.4	1.23
  Polymist XPA213	9	269.2	1.35
  Polymist XPA213	6	276.1	1.38
  Polymist XPA213	11	127.2	0.64
  Polymist XPA213	12	258.1	1.29
  Polymist XPA213	15	224.7	1.12
  Polymist XPA213	18	216.1	1.08
  Polymist F5A & F510 (2.75 & 8.25	11	141.3	0.71
  pph)
  Polymist F5A & F510 (5.5 & 5.5	11	147.3	0.74
  pph)
  Polymist F5A & F510 (8.25 & 2.75	11	153.5	0.77
  pph)
  Polyurethane	10	204.5	1.02
  Silane Silwet L7607	0.2	304.2	1.52
  Silane Silwet L7608	0.2	295.6	1.48
  Silane Silwet L77	0.2	422.6	2.11

• As illustrated in Table 6, the type and amount of friction-reducing additive used with the silicone compounds described herein significantly affects the frictional properties of the compound. As mentioned previously, the preferred silicone composition includes a PTFE additive of Polymist F5A at 11 weight percent. The small average particle size of this friction-reducing additive reduces the coefficient of friction by approximately 61 percent relative to a typical silicone wiper composition such as Standard J-7721-1 TRPL. Although some of the materials listed above exhibit even better frictional characteristics than Polymist F5A, the issue becomes one of cost and ease of manufacture. For example, the Polymist F5A at 16 weight percent provides slightly better frictional properties, but the increased cost of the additional PTFE is not worth the small gain. A larger gain is obtained by using Boron Nitride or Graphite, but the cost of these materials is much greater than Polymist F5A. Finally, some of the Polymist F510 compounds, or blended compounds containing Polymist F510 and Polymist F5A, exhibit excellent friction characteristics, but the addition of Polymist F510 sometimes makes the final silicone compound more difficult to extrude.
• The Polymist F5A additive provides exceptional manufacturing characteristics to the silicone compounds described herein. Although all of the friction-reducing additives of Table 6 could be used to improve the compound's friction characteristics over standard silicone wiper blade compounds, certain materials exhibit lower plasticity than others. Plasticity is a material property determined when a material sample is subjected to a yield force that causes the material to undergo a permanent change in shape or size (i.e. a plastic deformation). The measured plasticity values for the silicone compound incorporating different friction-reducing additives is illustrated in Table 7.

• TABLE 7
  	Average Particle Size	Test Loading	Plasticity
  PTFE Additive	(μm)	(pph)	(mm/100)

  Polymist F5A	<6	11	250
  Polymist F510	<20	11	718
  Algoflon 203	<6	11	258
  Teflon 6C	480	4	560

• The plasticity values listed in Table 7 were measured according to ASTM D531-00 Standard Test Method for Rubber Property-Pusey and Jones Indentation. It is preferable that a plasticity below 400 (mm/100) be used since values above 400 make extrusion, and even molding, of the compound more difficult. The low plasticity associated with Polymist F5A makes it one of the preferred choices as a friction-reducing additive.
• In one embodiment, the wiper blades described herein may be manufactured by extrusion. Referring toFIG. 8 in the drawings, the first step in the manufacturing process is to extrude a continuous length ofcurable silicone compound211 through anextruder213.
• Extruder213 is a conventional extruder having ahopper215 which feeds into a hot cylinder. The heat softens the elastomer, and it is forced by one or more spiral screws (not shown) through adie217 having a die orifice. The die orifice forms a continuous mass of elastomer in the shape of one of the wiper cross sections previously described (seeFIGS. 3-7). Extrusion processes of this type are well known in the art.
• Referring toFIGS. 9 and 10 in the drawings, a detailed view ofdie217 includes adie opening219 which is shaped to produce a pair of wiper blades joined at a mid-section thereof in edge-to-edge relation. Thedie217 includes an adjustable scoring mechanism, such asadjustable blades227,229.Blade tips231 disposed on eachadjustable blade227,279 are not in contact, but are spaced apart a preselected distance to score the continuous length ofelastomer211 along a top andbottom surface233,235 of the wiper blade to a depth less than the thickness of the elastomer (seeFIG. 10). Theblades227,229 can be adjusted by means ofscrews237,239 mounted on the die which are carried in vertical slots provided in theblades227,229.
• The continuous length of extrudedelastomer211 is passed to a curingstation241. In the embodiment shown inFIG. 8, curingstation241 is a continuous vulcanizer. It is readily understood by those skilled in the art that thecontinuous vulcanizer241 can employ, for instance, a liquid medium such as a eutectic salt bath having liquid salt at a temperature from about 350E to 450E F. The viscosity of the salt at these operating temperatures is similar to water.
• It will also be apparent that instead of the preferred salt bath, any continuous vulcanizing method could be used. For example, the vulcanizing step could easily be performed by a hot air vulcanizing tunnel. Also, the continuous length ofelastomer211 could be cured without a heat activated catalyst, instead using infrared radiation or gamma radiation techniques familiar to those skilled in the art. It is only necessary that the previously formed and scored curable elastomer be cured such that the material can be divided and formed as subsequently described.
• After curing, a continuous length of curedelastomer259 is separated into two separate lengths ofwiper blade243,245 by allowing onelength243 to travel over a fixednip roller247 while thesecond length245 is pulled under the same niproller247. The beginning separation can be accomplished by hand with the ends of the wiper blade being engaged byroller pairs253,255 of apuller257. Preferably, the separation of curedelastomer259 occurs at an elevated temperature above ambient. Leaving theextruder213, thecurable elastomer211 is typically at a temperature in the range from about 90°-100° F. The continuous vulcanizing step then typically raises the temperature to a higher elevation above ambient. For instance, in the case of a salt bath or hot air vulcanizing tunnel, the curedelastomer259 would be at an elevated temperature on the order of 300°-450° F. The preferred temperature for the curedelastomer259 at the separatingroller247 is in the range from about 100°-300° F., most preferably about 200° F. The decrease in temperature between thecontinuous vulcanizer241 and the separatingroller247 can be achieved by exposure to the ambient atmosphere, or by pulling curedelastomer259 through a water trough with water at ambient temperature, or by exposing curedelastomer259 to a plurality of air jets.
• Referring still toFIGS. 9 and 10, the separate continuous lengths ofwiper blade243,245 are cut transversely into individual wiper-sized segments261,263 by aconventional cutter265.FIG. 10 is a perspective view of a pair of wiper-sized segments261,263, the segments being separated by anopening267 located at the approximate mid-section which formerly represented the score line prior to separation at thenip roller247.
• Referring toFIG. 11 in the drawings, another embodiment of anextrusion die287 is illustrated.Die287 includesblades289,291, theblade tips293 of which are not in contact but are spaced apart a preselected distance. In this case, however, a preforming means, such aswire295, extends between theblades289,291 to preform amid-section298 of an extrudedelastomer297 by weakening the mid-section. Theblades289,291 are fixed on the die face by means of screw sets299,301, withwire295 being, for instance, tack welded thereon. The preforming means could also comprise, for instance, a Kevlar blade arranged between thedie blades289,291. By passing the raw extruded elastomer throughdie287 and preforming means295, the elastomer reunites, or tacks together, immediately after passing thewire295. The continuous length of uncured, extrudedelastomer297 is then passed to a curing station and cured in the manner previously discussed.
• After curing, a continuous length of cured elastomer is separated into two separate lengths of wiper blade (similar tolengths243,245 inFIG. 8) by allowing one length of wiper blade to travel over a fixednip roller247 while a second length of wiper blade is pulled under thesame roller247. The lengths can then be engaged byroller pairs253,255 of apuller257, as previously discussed. The cured elastomer separates along the preformedmid-section298 into separate lengths of wiper blade having improved edge quality. The extrusion process allows a continuous length of blade to be formed at a lower cost than most molding techniques.
• The fabrication process described in conjunction withFIGS. 11 and 12 is useful for wiper blades having a specific gravity of less than or equal to about 1.40. For blade compositions having a specific gravity of greater than 1.40 the extrusion process is modified such that no Kevlar wire orfilament295 is used to preform a weakened midsection. Instead, the blades are extruded and are passed directly to the continuous vulcanizer241 (seeFIG. 8). Thereafter, the blades are separated not by the nip rollers as shown, but by a circular blade. After separation, the blades are cut transversely by aconventional cutter265.
• The silicone rubber compositions described herein are ideally suited for extrusion into wiper blades of many different cross sections. Although the extrusion process has been described in detail with reference toFIGS. 8-12, it will be understood by those of skill in the art that any extrusion process could be used to form the wiper blades. It will be further understood that other manufacturing processes, including without limitation compression molding, injection molding, and blow molding, could be employed to form the wiper blades.
• One advantage of the silicone composition and wiper blade of the illustrative embodiments described herein is the superior friction properties imparted to the wiper blade. The reduced friction between wiper blade and wiped surface reduces chatter on the wiped surface during use and improves performance of the wiper blade. The composition also greatly reduces wiper edge wear and improves tear resistance properties, which increases the overall life of the wiper blade. In addition to these exceptional properties, the silicone rubber formulation retains the desirable properties often associated with silicone, namely resistance to UV, ozone, and extreme temperatures.
• Another advantage of incorporating PTFE or other friction-reducing additives during the compounding stage is that the compound “blooms” or migrates to the surface of the wiper blade and continues to provide reduced friction characteristics over time. This is an improvement over wiper blades that have been coated with PTFE, since PTFE coatings tend to erode over time, thereby adversely affecting the wiper blade's frictional characteristics.
• A person having ordinary skill in the art will recognize that various forms and grades of PTFE could be added during the compounding stage, including PTFE in non-powder form and grades other than the Polymist F-5A described above. Alternative friction-reducing agents could also be used, including without limitation boron nitride and graphite.
• Even though many of the examples discussed herein are applications of the illustrative embodiments in windshield wiper blades, the compounds and techniques also can be applied to other devices that need a flexible material having superior tear resistance and reduced friction characteristics. Some examples of possible further uses include but are not limited to squeegees for cleaning windows, medical tubing such as peristaltic pump tubing, and materials for various sealing applications.
• It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only a few of its forms, it is not just limited but is susceptible to various changes and modifications without departing from the spirit thereof.

Claims (1)

1. A windshield wiper comprising:

a frame adapted to be attached to a vehicle;

a wiper blade attached to the frame; and

wherein the wiper blade is made from a mixture including a methyl vinyl silicone polymer from about 22 to 55 weight percent, a filler from about 35 to 50 weight percent, and polytetrafluoroethylene in an amount of from about 5 to 42 weight percent, the polytetrafluoroethylene having an average particle size of less than about 25 μm.

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