The present invention relates to a polymeric bushing liner in the form of a tube. In particular, the present invention relates to a polymeric liner in the form of a rigid tube for use in elastomeric bushings in vehicle suspension systems.

BACKGROUND OF THE INVENTION

Elastomeric bushings are used in vehicle suspension systems to provide a yieldable connection within vehicle suspension systems. Elastomeric bushings function within suspension systems by allowing a suspension link of the system, such as a stabilizer bar, to be situated within a bore in the body of the elastomeric bushing. The suspension link may rotate freely about the longitudinal axis of the bore while the bushing is securely mounted to the vehicle. Elastomeric bushings provide resiliency to the vehicle suspension system, dampen vibrations produced within the vehicle suspension system and deaden noise produced at the bushing bore suspension link interface. However, elastomeric material from which the body of elastomeric bushings are fabricated does not have the required wear and frictional characteristics to be of utility at the bushing bore/suspension link interface. Hear of the elastomeric material from the relative motion of the suspension link within the bushing bore results in premature loss of elastomeric material surrounding the bushing bore and eventual failure of the elastomeric bushing. Friction between the elastomeric material and the suspension link at the bushing bore/suspension link increases the interface resistance of rotation of the suspension link within the bushing bore thereby reducing the effectiveness of the elastomeric bushing. Bushing liners are used to increase the utility of elastomeric bushings by improving the wear characteristics of the elastomeric bushing while decreasing the friction at the bushing

bore/suspension link interface. Bushing liners are fabricated in tube form from solid low friction polymeric materials such as polyester or polytetrafluoroethylene. The bushing liner is placed within the bore in the body of the elastomeric bushing providing a low friction surface for contact with the suspension link.

However, these conventional bushing liners are difficult to bond to the bore in the body of the elastomeric bushing due to the low friction nature of the material used to fabricate the bushing liner. Elastomeric bushings containing these conventional bushing liners fail due to delamination of the bushing liner from the bore in the body of the elastomeric bushing. Elastomeric bushings containing these conventional bushing liners are difficult to insert on the suspension link due to the high resistance of the solid low friction polymeric material to expansion. To alleviate this problem, the bore in the body of the elastomeric bushing must be designed with a greater inner diameter dimension or with a split in the bushing liner to allow for the expansion necessary for assembly. The lack of a tight contiguous fit permits the ingress of foreign particles into the interface between the elastomeric bushing and the suspension link and results in increased noise in the elastomeric bushing upon rotation and premature failure of the elastomeric bushing.

Bushing liners of fabrics woven of low friction fibers are used as taught 1n United States Patent 4,916,749 to Urban, et al. Fabric of low friction fibers is constructed from a combination of low friction fibers in such a manner so that fibers of polytetrafluoroethylene, which have good frictional characteristics but are difficult to bond, are located on the inside surface of the bore in the elastomeric bushing body thereby forming the bushing bore/suspension link interface, while fibers of polyester, which have lesser frictional characteristics compared to polytetrafluoroethylene fibers but are more easily bonded, are bonded to the elastomeric material forming the bore in the elastomeric bushing body. The fabric is capable of expansion thereby reducing the force necessary for assembly of the joint and

permitting a tight contiguous fit of the elastomeric bushing around the suspension link.

However, fabric bushing liners are difficult to assemble within the bore of the body of the elastomeric bushing since fabric does not have sufficient structural integrity and will tend to collapse when elastomeric material is molded around the fabric bushing liner to form the elastomeric bushing. Mandrels or other such devices must be used during the molding process to maintain the internal diameter of the bore in the body of the elastomeric bushing when fabric bushing liners are used.

SUMMARY OF THE INVENTION

This invention overcomes disadvantages of prior bushing liners by providing a bushing liner comprising a rigid tube of low friction polymeric material having a density of 0.9 g/cc or greater wherein the bore of the tube has a smooth surface. In one embodiment, the outside of the tube has a cracked surface which provides a surface for increased penetration of elastomeric material into the bushing liner. In another embodiment, the outside of the tube has been etched to provide a surface for increased adhesion of elastomeric material into the bushing liner.

The Inside and outside profiles of the tube may be either straight or contain projected surface portions.

The bushing liner may be contained in a bore in a body of an elastomeric bushing.

BRIEF DESCRIPTION OF DRAMINGS

Figure 1 shows the inventive bushing liner _(_ having a smooth inside surface 11 and a smooth outside surface 12.

Figure 2 shows the inventive bushing liner 20. having a smooth inside surface 21 and a cracked outside surface __.

Figure 3 shows a longitudinal cross section of the invention bushing liner __ with the inside profile having projected surface

portions 3. and the outside profile a straight outside profile 32.

Figure 4 shows a longitudinal cross section of the inventive bushing liner 40. with the outside profile having projection surface portions 41 and a straight inside profile 42.. Figure 5 shows a longitudinal cross section of an elastomeric bushing 5Q having the inventive bushing liner 51 disposed in a bore 52 in the body of the elastomeric bushing. Disposed in the bore 52. is a suspension link 53..

Figures 6 and 7 are microscopic photographs of outside surfaces of an embodiment of the inventive bushing liner.

Figure 8 shows different cross sections of the bushing liner such as a circular cross section fia, a square cross section fib., an oval cross section __ and a retangular cross section fid..

DETAILED DESCRIPTION OF THE INVENTION

A bushing is produced from a sheet of low friction polymeric material carefully wrapped around a mandrel and subjected to a heat-treatment step. The sheet of low friction polymeric material must be sufficiently flexible to permit wrapping of the sheet of low friction polymeric material around the mandrel so that the sheet of low friction polymeric material is in contact with the mandrel surface. The low friction polymeric material must be able to withstand temperatures 1n excess of 327 ^* C.

The sheet of low friction polymeric material 1s preferably a sheet of expanded porous polytetrafluoroethylene which exhibits the required low friction characteristics and is thermally stable in excess of 327*C. The sheet of expanded porous polytetrafluoroethylene is produced from a tape of highly crystalline, fine powder polytetrafluoroethylene produced by paste forming techniques which has been expanded in one or more directions at rates in excess of 101 per second by the process more fully described in U.S. Patents 3,953,566, 3,962,153, and 4,187,390 all to Gore and are incorporated herein by reference. Preferably, the sheet of expanded porous polytetrafluoroethylene has not been previously subjected to the heat treatment amorphous

locking process as described in U.S. Patent 3,953,566, column 3, lines 49-65 and, therefore, has not been subjected to temperatures in excess of 327 ^β C, the lowest crystalline melt point of polytetraf1uoroeth 1ene. The sheet of expanded porous polytetrafluoroethylene may be either biaxially expanded, expanded in both the longitudinal and transverse directions or, more preferably, uniaxially expanded, expanded in only the longitudinal direction.

The sheet of low friction polymeric material is carefully wrapped around a mandrel. The outside diameter of the mandrel defines the inside diameter of the resultant bushing liner. The sheet of low friction polymeric material 1s wrapped upon the mandrel by hand or through mechanical means. The sheet of low friction polymeric material may be wrapped circumferentially or may be wrapped on the mandrel on an angle or bias to the longitudinal axis of the mandrel. The strength and elongation of the resultant bushing liner are determined by the strength and elongation of the sheet of the low friction polymeric material and how that material is oriented on the mandrel. The sheet of low friction polymeric material is wrapped around the mandrel numerous times to build up the thickness of the resultant bushing liner. The sheet of low friction polymeric material is wrapped tightly against the mandrel.

The mandrel is fabricated of a high temperature material such as metal or ceramic. The preferred metal is steel. The mandrel is formed to produce the desired internal profile of the resultant bushing Hner. The cross section of the mandrel includes circular, oval, square, rectangular and other shapes dependent upon the design of the resultant bushing liner. A mandrel with straight sides will result in a bushing liner having a bore with a straight profile. A mandrel containing indented surfaces portions, such as grooves, will result in a bushing liner having a bore with a profile containing projected surface portions. Such a liner would have utility by reducing the areas of contact between the bushing liner and a suspension link disposed within the bushing liner thereby providing space within the bushing liner for

collection of loose material and dirt and for reduction of noise produced at the bushing liner.

The outside profile of the bushing liner may be controlled through the application of clamps capable of deforming areas of the low friction polymeric material wrapped upon the mandrel or through increasing the amount of low friction polymeric material in various areas around the circumference of the mandrel. If an equal amount of wraps of low friction polymeric material are used to wrap the mandrel, the outside of the resultant bushing liner will have a straight profile. If clamps capable of deforming the low friction polymeric material wrapped upon the mandrel or additional amounts of low friction polymeric material is wound around various areas of the mandrel, the outside of the resultant bushing liner will have a profile containing projected surface portions.

A bushing liner having a profile containing projected surface portions permits the bushing liner to be secured within a bore in a body of an elastomeric bushing through molding of elastomeric material around the projected surface portions thereby preventing the bushing liner from easily sliding out of the bore in the body of the elastomeric bushing.

The mandrel wrapped with the low friction polymeric material is placed within a high temperature enclosure. The high temperature enclosure may be a radiant heat oven, a convection oven and a molten salt bath. The high temperature enclosure must be capable of attaining and maintaining temperatures in excess of 345*C, more preferably in excess of 382*C.

The mandrel wrapped with the low friction polymeric material is placed within the high temperature enclosure in excess of 345*C, more preferably in excess of 382 ^# C, for a period of time. Exposure of the mandrel wrapped with the low friction polymeric material to temperature in excess of 345'C causes the wraps of the low friction polymeric material to retract thereby densifying the low friction polymeric material. The retraction of the wraps of low friction polymeric material produces a compressive force against the mandrel aiding in the densification of the low

friction polymeric material.

When the density of low friction polymeric material is between 0.9 g/cc and 2.2 g/cc, the mandrel wrapped with the low friction polymeric material has had sufficient dwell time within the high temperature enclosure. The resultant tube will have smooth inner and outer surfaces. The resultant tube will be stiffened and rigid. A rigid tube is defined as being capable of maintaining its shape, under its own weight without additional support, in both the circumferential and longitudinal directions. More preferably, the mandrel wrapped with the low friction polymeric material is placed within the high temperature enclosure in excess of 345*C for a time sufficient to cause cracking of the outside surface of the low friction polymeric material. When expanded porous polytetrafluoroethylene is the low friction polymeric material, 1t is preferable to have the temperature of the high temperature enclosure to be 382*C or greater to hasten the crack formation of the outer surface. The resultant tube will have a density between 0.9 g/cc and 2.2 g/cc, and will have a smooth Inner surface and a cracked outer surface. The cracked outer surface is characterized by a net-like series of longitudinal fissures which penetrates Into the body of the bushing liner. The fissures contain many small fibrils that connect one side of the fissure to the other. The tops of the fissures consist of highly densified material. A bushing liner having a cracked outside surface permits the bushing liner to be adhered within a bore in a body of an elastomeric bushing by providing cracks for the penetration of elastomeric material into the body of the bushing liner.

The mandrel wrapped with the low friction polymeric material is removed from the high temperature enclosure and allowed to cool. When the mandrel wrapped with low friction polymeric material now forming a tube is cool enough to comfortably handle, the tube of low friction polymeric material is removed from the mandrel. The tube of low friction polymeric material may be dislodged from the mandrel through the use of impact to cause a differential acceleration between the mandrel and the tube thereby

releasing the tube from the mandrel intact or, if the mandrel is perforated, through the introduction of a high pressure gas to the mandrel to dislodge the tube of low friction polymeric material from the mandrel . The tube of low friction polymeric material is now cut to the desired length to form the bushing liner.

The outside surface bushing liner may be subsequently treated in an etching process to increase the surface energy of the outside surface of the bushing liner. Etching process includes the use of alkali napthlate etchants (Tetra-Etchβ etchant available from W. L. Gore and Associates, Inc., Newark, DE), sodium ammonia etchants and gas plasma processes.

An elastomeric bushing containing the bushing liner of the present invention may be produced by injection molding around the outside surface of the bushing liner an elastomeric material.

Elastomeric materials Include such materials as natural rubber and synthetic rubber. Synthetic rubber may be acrylic elastomer, chlorinated polyethylene elastomer, chlorosulfonated polyethylenes, epichlorohydrin elastomers, ethylene/acrylic elastomers, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, fluoroelastomers, isobutylene-i oprene elastomers, nitrile elastomers, polybutadiene elastomers, polychloroprenes, polyisobutylene, polyisoprene, polynorbornene, polysulfides, silicone elastomers, styrene-butadlene rubbers, styrene-isoprene rubbers, urethane elastomers or vinyl acetate/ethylene copolymers. The elastomeric bushing is comprised of a body of elastomeric material which contains an axial bore. Disposed within the axial bore is the inventive bushing liner. The bushing liner is adhered to the elastomeric material either through the adherence of the elastomeric material to the outside surface of the bushing liner or through the use of an adhesive to adhere the elastomeric material to the bushing liner. Adhesives for adhering the bushing liner to the elastomeric material may be perfluoroelasto ers, fluoroelastomers, nitrile elastomers, acrylic elastomers, butyl elastomers, silicone elastomers, or polyurethane elastomers.

TEST METHODS

Density Determination Tests

The density of samples is determined in the following tests.

If a sample has smooth, flat surfaces, the following test is performed. The sample mass is determined on an analytical balance and is recorded in grams.

The sample volume is determined by carefully measuring the sample area using a linear scale, and the sample thickness is determined using a snap gauge. Care is taken not to crush the sample thickness when obtaining sample thickness. All measurements are recorded in centimeters. The linear measurements are multiplied together to obtain sample volume in cubic centimeters.

The mass of the sample 1s divided by the volume of the sample to obtain the density of the sample 1n g/cc.

If the sample has an irregular surface, or is difficult to measure using the previous test, the following test 1s performed.

The sample mass 1s determined on an analytical balance and is recorded in grams. The sample 1s then submerged in a container of isopropyl alcohol and allowed to soak in the container until fully saturated. Full saturation of the sample is assumed when no air bubbles are observed emerging from the sample.

A vessel, such as flask or beaker, containing an overflow spout is filled with isopropyl alcohol until the level of isopropyl alcohol overflows the vessel. A means for accurately measuring volume of Isopropyl alcohol, such as a graduated cylinder, is placed under the overflow spout.

The sample Is removed from the container of isopropyl alcohol and gently patted dry to remove isopropyl alcohol from the surface of the sample.

The sample 1s carefully submerged in the vessel and the volume of isopropyl alcohol caught in the means for accurately measuring volume of Isopropyl alcohol is determined and recorded as cubic

centimeters.

The mass of the sample is divided by the volume of the displaced isopropyl alcohol which is the volume of the sample to obtain the density of the sample in units of g/cc. This test 1s to be performed quickly to minimize inaccuracies in the test due to evaporative loss of isopropyl alcohol.

Example 1

A bushing liner was made in the following manner:

A sheet of uniaxially expanded porous polytetrafluoroethylene in the form of a 15.2 cm wide tape produced by the process described in U.S. Patent 3,953,566 to Gore was obtained. The sheet had been expanded 8 to 1 in the longitudinal direction, an initial density of 0.29 g/cc, and had not been exposed to temperatures in excess of 327*C and had not been heat-treated amorphously locked according to the teachings in U.S. Patent 3,953,566 to Gore.

The sheet of expanded porous polytetrafluoroethylene was wrapped 35 times by hand around a mandrel formed by a 2.5 cm diameter stainless steel pipe. The sheet was wrapped upon the mandrel in the circumferential direction so that the direction of expansion of the expanded porous polytetrafluoroethylene was oriented circumferentially around the mandrel.

The sheet wrapped upon the mandrel was immersed in a bath of a mixture of molten sodium nitrites and nitrates at a temperature of 400'C for 3 to 3.5 minutes. The sheet wrapped upon the mandrel was allowed to air cool until it could be easily handled.

The sheet wrapped upon the mandrel was in the form of a tube, the inner diameter or bore of the tube defined by the outside diameter of the mandrel. The tube was cut off the mandrel with a razor blade. The inside surface of the tube was observed to be smooth. The outside surface of the tube was observed as having numerous cracks running longitudinally along the tube. Figure 6 is a photomicrograph of the cracked surface of the tube magnified 8X. To the left of the photomicrograph is a scale with a series

of 1 mm gradations. It can be seen in the photomicrograph that the outside surface of the cracks contains material of high density. Lower density material can be seen with the depth of the crack as well as fibrils of polytetrafluoroethylene spanning from one side of the crack to another.

The density of the material was determined to be 0.96 g/cc.

Example 2

A bushing liner was made in the following manner.

A sheet of uniaxially expanded porous polytetrafluoroethylene as described in Example 1 was wrapped as in Example 1 around a mandrel as described in Example 1.

The sheet wrapped upon the mandrel was placed in a hot air convection oven preheated to a temperature of 400*C for 25 minutes.

The sheet wrapped upon the mandrel was In the form of a tube, the inner diameter or bore of which defined by the outside diameter of the mandrel. The tube of material was cut from the mandrel longitudinally. The inside surface of the tube was observed as smooth. The outside surface of the tube was observed as having numerous cracks running longitudinally along the tube. Figure 7 is a photomicrograph of the cracked surface of the tube magnified 8X. To the left of the photomicrograph Is a scale with a series of 1 mm gradations.

The density of the material was determined to be 1.13 g/cc.

Com ^p r ive Exwple I

A comparative bushing liner was made in the following manner. A sheet of uniaxially expanded porous polytetrafluoroethylene as described in Example 1 was wrapped as in Example 1 around a mandrel as described in Example 1.

The sheet wrapped upon the mandrel was placed in a salt bath described in Example 1 preheated to a temperature of 360 ^β C for 10 minutes.

At the end of the heating cycle, the sheet of uniaxially expanded porous polytetrafluoroethylene was in the form of a tube, the inner diameter or bore of which defined by the outside diameter of the mandrel. The tube of material was cut from the mandrel longitudinally. The inside surface of the tube was observed to be smooth. The outside surface of the tube also observed to be smooth.

The density of the material was determined to be 0.72 g/cc.

Exam le

A bushing liner was made in the following manner:

A sheet of uniaxially expanded porous polytetrafluoroethylene was obtained using the process described in U.S. Patent 3,953,566 to Gore.

The sheet of uniaxially expanded porous polytetrafluoroethylene had an expansion of 8 to 1 and an initial density of 0.29 g/cc. The sheet of uniaxially expanded porous polytetrafluoroethylene had not been heat-treated and therefore had not been subjected to temperatures in excess of 327 ^# C. The sheet of uniaxially expanded porous polytetrafluoroethylene was carefully wrapped 35 times by hand around a solid steel mandrel with a 2.4 cm outside diameter. The sheet of uniaxially expanded polytetrafluoroethylene was wrapped so that the longitudinal direction of the sheet was oriented so that the direction of uniaxial expansion was directed circumferentially around the mandrel.

The wrapped mandrel was carefully placed within a hot air convection oven preheated to 400*C. The wrapped mandrel was placed on its longitudinal end so that there would be no excess pressure exerted on the side of the sheets. The wrapped mandrel remained in the hot air convection oven for 15 minutes at the end of which time the wrapped mandrel was taken out of the oven and placed into a water bath to cool the wrapped mandrel.

The surface of the sheet was noted to have a cracked surface when removed from the hot air convection oven.

The adhered sheets of expanded porous polytetrafluoroethylene were removed from the mandrel by exerting on impact force upon the end of the mandrel to dislodge the adhered sheets from the mandrel.

The adhered layers of expanded porous polytetrafluoroethylene formed a tube of low friction polymeric material. The inside surface of the bore of the tube was smooth and the outside surface was cracked.

The tube of adhered layers of expanded porous polytetrafluoroethylene was placed upon a dowel of wood to act as a support so the tube was carefully cut to desired length for bushing liners.

The bushing liner was placed on a core within a mold and elastomeric material was injected around the bushing liner to form the body of an elastomeric bushing. The bushing liner was disposed in the mold so that the bushing liner was within an axial bore in the body of the elastomeric bushing.

The result was a elastomeric bushing with a bushing liner well adhered within the axial bore in the body of the elastomeric bushing. The inside of the bushing liner having a smooth surface.

Example 4

A bushing liner was made in the following manner:

A sheet of uniaxially expanded porous polytetrafluoroethylene produced by the process described in USP 3,953,566 to Gore was obtained. The sheet had been uniaxially expanded 10 to 1 in the longitudinal direction, an initial density of 0.29 g/cc and had not previously been exposed to temperature in excess of 327 ^β C.

The sheet of expanded porous polytetrafluoroethylene was wrapped by hand circumferentially around a solid steel mandrel having a 2.4 cm outside diameter until the outside diameter of the wrapped mandrel was 3.2 cm.

The wrapped mandrel was placed in an upright position within a hot air convection oven at a temperature of 390°C for 30 minutes.

After 30 minutes, the wrapped mandrel was removed from the hot air convection oven and allowed to cool. The wrapped sheets of

expanded porous polytetrafluoroethylene were adhered to one another forming a tube. The tube was removed from the mandrel. It was observed that the inside and outside surfaces of the tube were smooth. The tube had a density of 1.03 g/cc.