Latex flows into a cup attached to the bottom of the cut in the tree. The latex material from many trees is accumulated in large tanks. The most common method of extracting the rubber from latex uses coagulation, a process that curdles or thickens the polyisoprene into a mass. This process is accomplished by adding an acid such as formic acid to the latex. The coagulation process takes about 12 hours.
Water is squeezed out of the coagulum of rubber using a series of rollers. The drying process generally requires several days. The resulting dark-brown rubber, now called ribbed smoke sheet, is folded into bales for shipping to the processor. Not all rubber is smoked, however. Rubber dried using hot air rather than smoking is called an air-dried sheet. This process results in a better grade of rubber. An even higher quality rubber called pale crepe rubber requires two coagulation steps followed by air-drying.
Several different types of synthetic rubber have been developed over the years. All result from polymerization linking of molecules. A process called addition polymerization strings together molecules into long chains. Another process, called condensation polymerization, eliminates a portion of the molecule as molecules are linked together. Examples of addition polymers include synthetic rubbers made from polychloroprene neoprene rubber , an oil- and gasoline-resistant rubber, and styrene butadiene rubber SBR , used for the non-bounce rubber in tires.
The first serious search for synthetic rubber began in Germany during World War I. British blockades prevented Germany from receiving natural rubber. Although this substitute, methyl rubber, was inferior to natural rubber, Germany manufactured 15 tons per month by the end of WWI. Continued research led to better-quality synthetic rubbers. The most common type of synthetic rubber currently in use, Buna S styrene butadiene rubber or SBR , was developed in by the German company I. This substance combined with SBR has been used for tires since Rubber, whether natural or synthetic, arrives at processor fabricator plants in large bales.
Once the rubber arrives at the factory, processing goes through four steps: compounding, mixing, shaping and vulcanizing. The rubber compounding formulation and method depends on the intended outcome of the rubber fabrication process. Compounding adds chemicals and other additives to customize the rubber for the intended use.
Natural rubber changes with temperature, becoming brittle with cold and a sticky, gooey mess with heat. Chemicals added during compounding react with the rubber during the vulcanizing process to stabilize the rubber polymers. Additional additives may include reinforcing fillers to enhance the properties of the rubber or non-reinforcing fillers to extend the rubber, which reduces the cost. The kind of filler used depends on the final product. The most commonly used reinforcing filler is carbon black, derived from soot.
Carbon black increases rubber's tensile strength and resistance to abrasion and tearing. Carbon black also improves rubber's resistance to ultraviolet degradation. Most rubber products are black because of the carbon black filler. Depending on the planned use of the rubber, other additives used could include anhydrous aluminum silicates as reinforcing fillers, other polymers, recycled rubber usually less than 10 percent , fatigue-reducing compounds, antioxidants, ozone-resisting chemicals, coloring pigments, plasticizers, softening oils and mold-release compounds.
The additives must be thoroughly mixed into the rubber. The high viscosity resistance to flow of the rubber makes mixing difficult to accomplish without raising the temperature of the rubber high enough up to degrees Fahrenheit to cause vulcanization. To prevent premature vulcanization, the mixing usually takes place in two stages. During the first stage, additives like carbon black are mixed into the rubber.
This mixture is referred to as a masterbatch. Once the rubber has cooled, the chemicals for vulcanization are added and mixed into the rubber. As an example, pounds of additive material was obtained with fifteen minutes of agitation and mixing. The adherence of the molybdenum disulfide particles to the particulate polytetrafluoroethylene is believed to be enhanced by an electrostatic charge differential that may be developed between the polytetrafluoroethylene particles and the molybdenum disulfide particles while they are being mixed together.
In order to permit better formulation and mixing of many varied elastomer compositions and more effective use of the additives, it is preferred to produce two additives: one with fibrillatable polytetrafluoroethylene and one with particulate, or granular, polytetrafluoroethylene. In preparing the additives with fibrillatable polytetrafluoroethylene, preferably about one-third part by weight of particulate molybdenum disulfide per part of fibrillatable polytetrafluoroethylene is mixed with one part by weight of fibrillatable polytetrafluoroethylene.
In preparing the additives with granular polytetrafluoroethylene, preferably about one-third part of molybdenum disulfide per part by weight of granular polytetrafluoroethylene by weight is mixed with one part by weight of granular polytetrafluoroethylene for use in many applications. Of course, additives can be made with other ratios of molybdenum disulfide to PTFE.
In additives for elastomer compositions to be used in injection molding, only one-sixth part by weight of molybdenum disulfide per part of PTFE may be preferred. The resulting additives thus preferably contain polytetrafluoroethylene particles with adherent particles of molybdenum disulfide. Where the PTFE particles are of a size larger than 10 to 20 micron average particle size, the PTFE particles are preferably coated with many molybdenum disulfide particles. It is believed that technical grade molybdenum disulfide with particles having an average particle size on the order of one micron and smaller will be particularly advantageous where the particulate polytetrafluoroethylene includes particles at the lower end of the 2 to 45 micron range.
The use of such polytetrafluoroethylenemolybdenum disulfide additives is preferred in making the compositions of this invention. The prior association of molybdenum disulfide particles with polytetrafluoroethylene particles greatly assists the uniform mixing of the particulate polytetrafluoroethylene into the elastomer, particularly where the elastomer composition includes fibrillated polytetrafluoroethylene, which is more difficult to mix into the elastomer. The molybdenum disulfide associated with the surface of the polytetrafluoroethylene particles is believed to enhance the entry of the polytetrafluoroethylene particles into the elastomer and deter adherent association of the polytetrafluoroethylene particles with themselves.
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Furthermore, it is believed that the coating of molybdenum disulfide particles on the fibrillatable polytetrafluoroethylene particles interacts with the surrounding elastomer upon mixing and assists in the fibrillation of the polytetrafluoroethylene. The preferred use of the invention involves the addition of one or both of two premixed additives. The fibrillatable additives are generally preferred in millable elastomers such as fluorocarbons, fluorosilicone rubber, and silicone elastomers. The fibrillation of the fibrillatable additive generally raises the temperature of the "green" elastomers; and it is, therefore, preferable to add accelerators, curing agents, and other fillers after the fibrillatable additive has been mixed into the base elastomer.
The adherent molybdenum disulfide particulates on the fibrillatable PTFE particles assist the fibrillation and uniform incorporation of the PTFE into the base elastomer. Because the fibrillated PTFE additive increases the green strength of the elastomer mix, it is preferably added to the mix in small amounts during preparation of test batches to permit an assessment of its effect during processing. In production, the requisite amount of additive may normally be added to the mix first and at one time.
The granular PTFE additives are generally preferred in millable elastomers that will benefit from improved surface lubricity and wear in applications where abrasion is a problem or surface lubricity and hardness are desired. The use of granular PTFE additives is particularly beneficial in combination with the fibrillatable additive because the fibrillatable additive provides a fibrillated PTFE matrix within the base elastomer to support high percentages of granular PTFE in the base elastomer.
Use of additives of this invention can also substantially reduce the cost of products made from expensive elastomers such as fluorocarbon and fluorosilicone elastomers while improving their manufacturability, for example, by improving their hot tear strength as well as their durability in service. In addition, additives of this invention can frequently reduce curing times and temperatures of products including the additives. Desirably, additives of the invention are kept as free of moisture as possible and in elastomers sensitive to moisture, such as fluorocarbon elastomers, may be used with small amounts of calcium oxide.
In the generally preferred procedure for preparing elastomer compositions of this invention with elastomers such as fluoroelastomers, natural rubber buna N, SBR, EPDM, and the like, the elastomer composition and carbon black, where added, are first mixed, for example, on a Banbury mixer. The additive with fibrillated polytetrafluoroethylene is then added on the mixer and mixing continues until the temperature of the blend begins to rise sharply.
The additive with granular polytetrafluoroethylene, where used, is then added to the mixer; and mixing continues until the temperature, rheology and appearance indicate that a uniform mixture has been achieved. It is sometimes desirable to "sweeten" the mixture by the addition of molybdenum disulfide. The fibrillated polytetrafluoroethylene is best uniformly mixed and fibrillated in the composition by this procedure, where it is added to the elastomer first on the mixer and before any curing agents or other fillers are added.
The mixed composition is then sheeted by running it through a mill in several passes, turning ninety degrees between each pass as commonly practiced in the art. The sheeted mixture should have a uniform corrugated surface appearance and should show only a dull finish. The sheeted mixture is then chopped for a second pass through the mixer and the addition of curing agents and all other fillers.
After the curing agents and other fillers are uniformly mixed into the elastomer-polytetrafluoroethylene composition, the entire composition is milled, sheeted, and cooled in a manner common to elastomer preparation and is ready to use. Where elastomers, like silicone, with no integrity in the base elastomer, are formulated with this invention, mixing of the fibrillated polytetrafluoroethylene additive is preferably first accomplished on an open mill.
Such base elastomers are given integrity by the fibrillated polytetrafluoroethylene and the initial mixing continues on the mill until the elastomer begins to sheet properly. Further processing of such elastomer compositions can then continue as with other processible elastomers. It is possible, however, that sufficient integrity may be obtained that mixing of the elastomer composition may be accomplished on an internal mixer.
As set forth above, to avoid loss of scortch resistance, any curing agents should be added after complete fibrillation and dispersion of the particulate polytetrafluoroethylene. Compositions of this invention, made with additives and methods of this invention, and their advantages are believed to be derived in part from the matrix-like structure including an intimate, mechanical, interengagement of elastomer with particulate polytetrafluoroethylene and molybdenum disulfide and, where fibrillated polytetrafluoroethylene is incorporated, in web-like structures of polytetrafluoroethylene within the matrix.
It is believed that throughout processing, the particles of molybdenum disulfide remain adherent to the fibrillatable PTFE as it is fibrillated, thus providing an effective interface between the fibrillatable PTFE and the elastomer that is believed to assist in the fibrillation of the polytetrafluoroethylene and in the adherence between the fibrillated polytetrafluoroethylene and the elastomer in the resulting compositions.
It is likewise believed the adherence of molybdenum disulfide particles on the granular polytetrafluoroethylene assists in its uniform dispersion throughout the elastomer during mixing and adherence between the polytetrafluoroethylene particles and elastomer in the resulting compositions.
It is also possible to mix polytetrafluoroethylene-molybdenum disulfide additives with base elastomers that are in emulsion form. The additives can be uniformly mixed with the fluid elastomer emulsion; and where fibrillated polytetrafluoroethylene particles are used, they can be fibrillated later during manufacture of elastomer compositions. For the purpose of promoting a better understanding of the invention, the following examples are given of specific compositions of the invention and their methods of preparation. A g amount of applicant's composition was compounded by combining 50 percent by weight TLA polytetrafluoroethylene powder with 20 percent by weight molybdenum disulfide crystals and with 30 percent by weight NORDEL ethylene-propylene terpolymer resin in a container.
The dried ingredients were thoroughly blended to arrive at a substantially homogeneous mixture, and a standard peroxide curing agent was added. The resulting vulcanized sheet material was dark grey in color, was 0. A g amount of applicant's composition was compounded by combining 25 percent by weight TLA polytetrafluoroethylene powder with three percent by weight molybdenum disulfide and 72 percent by weight NORDEL ethylene-propylene terpolymer resin. The ingredients were blended and cured as in Example 1. Several ring-shaped pieces were molded from this material and were found to work effectively as O-rings in various mechanical applications.
Example 3 sets forth the test of this composition for comparison with the invention. The physical properties, tensile strength, elongation, modulus, and hardness of the compositions of Examples are presented in the table below. Elongation of the compositions of Examples is also substantially improved. The effects of the invention in silicone elastomer compositions are shown in comparisons of Example 9 with Examples 10A and 10B and Example 11 with Examples 12A and 12B and the results of their testing.
Example 9 is one silicone rubber composition including its additives, and Example 11 is another and different silicone rubber composition including its additives. Examples 10A and 10B demonstrate the invention with the addition of a single fibrillatable PTFE additive of this invention.
Example 10A includes the silicone rubber composition of Example 9 with 1 part by weight of fibrillatable PTFE additive, including an effective amount of molybdenum disulfide, per parts by weight of the silicone rubber composition. Example 10B includes the silicone rubber composition of Example 9 with 2 parts by weight of the fibrillatable PTFE additive of Example 10A, including an effective amount of molybdenum disulfide, per parts by weight of the silicone rubber composition.
Examples 12A and 12B demonstrate the invention with the addition of one and two additives of this invention. Example 12A includes the second silicone rubber composition of Example 11 with 6 parts by weight of fibrillatable PTFE additive, including an effective amount of molybdenum disulfide, per parts by weight of the second silicone rubber composition.
Example 12B is the composition of Example 12A with an additional 25 parts by weight of granular PTFE additive, including an effective amount of molybdenum disulfide, per parts by weight of the base second silicone rubber composition. The physical properties of the compositions of Examples 9, 10A, 10B, 11, 12A, and 12B are presented below. These compositions demonstrated chemical inertness, stability, heat resistance, surface lubricity, and abrasion resistance. Silicone elastomer compositions of this invention can be formulated having tensile strengths and tear strengths many times greater than the original elastomer.
Such elastomers can provide tensile strengths and tear strengths that are two to three times greater than that of the silicone elastomer itself without loss of the chemical inertness, stability, heat resistance, and electrical insulating properties of the silicone elastomer, and with improved abrasion resistance. Silicone compositions of the invention can provide substantially improved electrical insulation, particularly in applications such as insulation for spark plug wires.
In such applications, the improved silicone elastomer of this invention surrounds in a generally concentric fashion the conductor carrying the high voltage necessary to produce a spark in the spark plugs of an automobile. The spark plug wires must operate adjacent to the engine block of the automobile, which now runs much hotter because of the emission limitation and gasoline efficiency requirements of the U.
The electrical conductivity of electrical insulating materials, in most cases, increases substantially with temperature, and this is generally true of electrically insulating elastomers.
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Because of the higher temperatures at which automobile engines now operate, many elastomers previously used as insulation for spark plug wires are no longer reliable insulators in that application; and silicone elastomers, which are generally more expensive than the prior elastomers in use, are used because of their more reliable electrical insulating properties at higher temperatures. Silicone spark plug wire insulation is, however, subject to mechanical failure due to pulling, tearing, and abrasion because of its generally poor physical properties.
With this invention, the physical properties of the silicone elastomer can be substantially improved in this application. Siliconepolytetrafluoroethylene elastomers of this invention can double or triple the tensile strength, tear strength, and abrasion resistance of silicone elastomer spark plug wire insulation, with no significant loss in heat resistance or electrical insulating properties, and can substantially extend the reliability and, it is believed, the life of silicone elastomer electrical insulation.
The improved silicone elastomer compositions of this invention have extended applicability to products in which substantially more expensive elastomers are now used because of the unique properties of the silicone elastomer and polytetrafluoroethylene and may permit a substantial reduction in the cost of such products.
In addition, the invention can provide similar improvements in other electrically insulating elastomers because of the heat resistance, chemical inertness, and electrical insulating properties of the polytetrafluoroethylene. Elastomers of this invention can provide excellent cable and wire insulation and coverings. In addition to the aforementioned properties, the invention can provide a flexible and tough cable or wire insulation and cable covering with improved surface lubricity which will permit a cable to be pulled through conduit with greater ease and reliability.
With the invention, elastomers less expensive than silicone elastomers may once again be usable for automotive spark plug wire insulation. Example 13 is a standard nitrile rubber mixture including parts of nitrile rubber. The physical properties of the compositions of Examples are compared in the table below.
Comparisons of the composition properties demonstrate the increased modulus of elasticity that is obtained with the addition of only four parts of fibrillated PTFE in nitrile rubber compositions of the invention. Examples permit a comparison of the effect of the invention in butadiene acrylonitrile elastomers.
Example 19 is an elastomer mixture including parts of butadiene acrylonitrile elastomer. The compositions of Examples are compared below. The ingredients were mixed together and cured and formed into test samples and tested in accordance with ASTM standards. Compositions according to the invention were made and tested with four base elastomers including Viton, an elastomer manufactured and sold by E.
Each of the base elastomer compositions of Examples 27, 29, 31, and 33 included elastomer and carbon black, curing agents, fillers, oxidants, and the like which the manufacturer believed advisable to lend desirable physical properties to the elastomer composition after it was cured. In Examples 28, 30, 32, and 34, respectively, particulate polytetrafluoroethylene and molybdenum disulfide were added to the elastomer of Examples 27, 29, 31, and 33 in such amount that the particulate polytetrafluoroethylene and molybdenum disulfide formed about 30 percent of the combined weight of the elastomer, carbon black, particulate PTFE and MOS 2 components i.
Each of these compositions included about 6 percent by weight of fibrillated polytetrafluoroethylene and about 18 percent of particulate polytetrafluoroethylene to comprise approximately 24 percent total of polytetrafluoroethylene in the composition.
The physical properties of the base elastomer compositions are compared with the physical properties of the elastomer compositions of the invention below. The test data indicate elastomer compositions of the invention including Viton and Nitrile elastomers demonstrated substantial improvements in tensile strength, 50 percent modulus, percent modulus, tear resistance and abrasion resistance. Although the elastomer compositions of the invention including acrylic and EPDM elastomers did not demonstrate an increase in tensile strength, they also demonstrated a substantial increase in their 50 percent and percent moduli, almost double the die tear resistance, and at least double or triple the abrasion resistance, respectively.
Such compositions can be used, for example, to make improved rotating lip seals. Rotating lip seals are subject to abrasion wear and tearing, and the substantial improvements in tear and abrasion resistance demonstrated with the invention will contribute a substantial improvement to the life of rotating lip seals manufactured with such elastomers with no sacrifice in the physical characteristics of the lip seal structure and, in many cases, a significant improvement in the strength of the lip seal. The composition of Example 35 contained no additives of this invention. The composition of Example 36 contained eight parts by weight of an additive of this invention including fibrillatable polytetrafluoroethylene and an effective amount of molybdenum disulfide.
The composition of Example 37 included, in addition to eight parts by weight of the additive of this invention, including fibrillatable polytetrafluoroethylene that was incorporated into Example 36, thirty-two parts by weight of an additive of this invention including granular polytetrafluoroethylene. Thus, the compositions of Examples 35, 36, and 37 differ only in that the composition of Example 36 included eight parts by weight of a fibrillatable polytetrafluoroethylene additive of this invention which includes approximately three parts by weight of fibrillatable polytetrafluoroethylene to one part by weight of molybdenum disulfide, and the composition of Example 37 includes not only the eight parts of fibrillatable polytetrafluoroethylene additive but also thirty-two parts of non-fibrillatable polytetrafluoroethylene additive.
The non-fibrillatable polytetrafluoroethylene additive of Example 37 included a slightly higher percentage of particulate polytetrafluoroethylene by weight than the weight percentage of molybdenum disulfide. The physical properties of the base elastomer composition of Example 35 are compared with the physical properties of the elastomer composition of Examples 36 and 37 below. The elastomers of Examples 36 and 37, and particularly the elastomer of Example 37, can be used in the sole of an athletic running shoe to substantially reduce the thickness of the material forming the sole and provide an almost percent increase in tear resistance and toughness of the sole material, substantially extending the life of the running shoe in addition to reducing its weight.
Thus, elastomers usable with this invention include the polymers known generally as rubbers, including natural rubber and synthetic rubber elastomers, and other polymers capable of forming elastic solids with similar properties. The compositions of the invention can also include plasticizers and softeners, extenders, reclaimed rubber, inert fillers, reinforcing fillers, coloring materials, anti-oxidants, accelerators, and vulcanization actuators.
The particulate polytetrafluoroethylene used can be granular polytetrafluoroethylene, dispersion-type polytetrafluoroethylene capable of fibrillation or a blend of both granular and fibrillatable PTFE. Such PTFE materials are sold by the following companies under their respective trademarks:. Compositions of the invention can include both granular and fibrillated polytetrafluoroethylene.
Fibrillated polytetrafluoroethylene improves significantly the modulus of elasticity and tear strength of most elastomers and can improve the tensile strength of elastomers with low tensile strengths. Generally, with about 4 percent by weight of fibrillated PTFE, about 85 percent of its benefits can be obtained.
It is believed that the more extended and complex surface of the fibrillated PTFE may provide additional mechanical entanglement and engagement with the elastomer. The improved tensile strength and modulus can be obtained in many cases with only a modest increase in the hardness and a modest reduction in the elasticity.
Fibrillated PTFE is the result of its manufacture, usually as a coagulated aqueous disperson, and in its fibrous form in the compositions of this invention can resemble small twisted and deformed webs of entangled fibers. Where it is desirable to maintain flexibility of the elastomer compositions of this invention, the fibrillated polytetrafluoroethylene desirably fibrillates to provide a high ratio between fiber length and fiber diameter.
It has been found that FLUON CD1 fibrillates with a greater length-to-diameter ratio than other fibrillatable polytetrafluoroethylene and is preferable in elastomers where flexibility is a desirable characteristic. As indicated above, it is believed that the manner in which the coagulated dispersion polymers that comprise fibrillated polytetrafluoroethylene are processed during their manufacture affects the structure of the fibrillatable PTFE particles and the ease with which they may be fibrillated into fibers having a high ratio of fiber length to fiber diameter.
Although it is not clearly understood, it is further believed that altering manufacturing processes to reduce dense or hard layers on the outside of the PTFE polymer particles permits the particles to be drawn into longer and thinner fibers. Furthermore, it is believed that the fibrillated PTFE may undergo a volume reduction upon curing of the elastomer that may provide "prestressing effect" in the cured polymer composition in cooperation with its adherent molybdenum disulfide.
Among the factors used in manufacturing coagulated dispersions that may affect the surface hardness of the coagulated dispersion particles are the processing steps used to avoid further agglomeration of the coagulated dispersion particles and to remove anti-agglomeration agents and water from, and dry, the coagulated dispersion particles.
Use of high temperatures, for example, to remove lubricants and water and dry the coagulated disperson particles may tend to make the surface of coagulated disperson particles harder or more dense and render them more difficult to fibrillate. Particulate, ground, or granular PTFE is generally finely ground and is the result of fracturing. Granular PTFE can effectively contribute the physical properties of polytetrafluoroethylene in products that do not require good tensile strength and modulus. The outer hull covering, for example, does not require these properties.
Where tensile properties are important, small amounts of fibrillated PTFE are desirably included to improve the tear strength, tensile strength, and modulus of most elastic products made with compositions of this invention. Products of the invention with increased tensile strength and modulus of elasticity generally have greater abrasion resistance, lower flexural hysteresis, or heat build-up in use, and increased durability where subjected to mechanical flexing and abrasion.
The procedure for determining the quantity of PTFE needed in many applications is first to determine the total quantity of PTFE that is desirable to achieve the PTFE properties of the composition that are desired, such as lubricity and abrasion resistance, solvent resistance and chemical inertness, heat resistance, tear strength, and the like. In compositions where lubricity is disadvantageous, such as the treads of vehicle tires and military tank pads, quantities of fibrillated PTFE on the order of only about 4 percent by weight within the composition can achieve 85 percent of the improvement possible with the fibrillated PTFE.
In tire compositions, for example, about 2 percent of fibrillated PTFE in natural rubber with up to no more than 10 percent ground PTFE can provide substantially increased life to vehicle tires without loss of their tractability. With elastomers having less strength such as butyl rubber up to about 6 percent of fibrillated PTFE with no more than 6 percent ground PTFE can provide increased life and durability.
It is generally desirable to keep the quantity of fibrillated PTFE in such compositions as low as possible consistent with obtaining the needed properties. Fibrillatable PTFE is more difficult to mix with elastomers and generates more heat as it is mixed with the elastomers. The increased heat generated in mixing the components of a composition and fibrillating the PTFE component, for example, in a Banbury mixer tends to partially cure the elastomer during mixing and reduces the scorch resistance of the resulting composition on molding.
Scorch resistance is a measure of the ability of an elastomeric composition to be uniformly curable and to resist a preferential curing at the surfaces of a mold into which heat is transferred. Such preferential curing generally increases the resistance of the cured portion of the product to heat transfer and inhibits uniform curing of the product interior without over heating adjacent the product surface. In addition, particulate PTFE having an average particle size of less than 40 microns is more readily dispersed.
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The molybdenum disulfide used may be that sold, for example, by Amax, Inc. While the effective amount of molybdenum disulfide may vary from composition to composition, the amount needed to effect uniform dispersion of particulate matter such as PTFE into the elastomer may be easily determined by adding the molybdenum disulfide and the polytetrafluoroethylene to the elastomer in the Banbury mixer until the PTFE becomes uniformly mixed with the elastomer.
The molybdenum disulfide can be incorporated into compositions of this invention in many cases with only a minor effect on most of their physical properties. In addition to permitting the uniform dispersion and fibrillation of significant amounts of particulate polytetrafluoroethylene material in elastomer materials, the molybdenum disulfide can provide a significant filler for the elastomer and can be used to contribute lubricity to the surface properties of a resulting elastic product. Furthermore, the molybdenum disulfide will reduce the heat buildup and partial curing of the elastomer during mixing of the composition and increase its scorch resistance.
Where elastic products of such compositions are subject to surface abrasion, for example, in applications such as lip seals for hydraulic and shaft seals, vane pump seals, valve seals, and the like, the elastomer at the surface may be abraded; but the surface exposed to abrasion can then become predominantly PTFE which is lubricious and highly abrasion resistant. To the extent PTFE is abraded from such elastic products by the roughness of opposed surfaces, such as the inner steel surfaces of hydraulic and compressed air cylinders, pumps, valves, etc.
One application of the invention is to watercraft which are required to move through the water as fast as possible or with the greatest efficiency possible, and more particularly to an improved outer hull covering for such watercraft which provides, among other advantages, reduced drag caused by water resistance to such movements. In connection with the movement of any object underneath or across the surface of the water, there is a continuous force exerted against such movement which is measurable and is composed of several factors, one of which is friction.
This resistance to movement through water is commonly termed "drag". With any "watercraft," which term is used in this application to mean any craft or other structure that can carry people or cargo underneath or over the surface of water, drag is a major concern because it is a significant factor in determining the maximum speed at which movement is possible as well as the efficiency of such movement in terms of cost, energy expended and the like.
In an effort to minimize drag and maximize our ability to efficiently move through water, much research has been done and continues to be done both in private industry and in civilian and military branches of government. This research involves not only variations in the design, weight and other characteristics of the watercraft themselves, but also work with paints and other coatings, solutions, and various methods attempting to decrease this water resistance to movement and thereby increase the efficiency of water travel.
Examples of such ongoing work are shown, for example, in U. Other problems with the watercraft besides drag are encountered which impair efficiency and inconvenience or endanger travelers or cargo on board such craft. One example is fouling, which refers to the buildup of foreign matter including grass or marine organisms such as algae, barnacles, and various shells which become attached to the underwater portion of the hull or other structure.
A second problem is that sound travels readily through metal hulls of watercraft creating not only a nuisance, i. In addition to the examples of the patents above, applicant is generally aware that research has been conducted at military and civilian facilities for many years in an effort to find an effective solution to efficient movement through water by watercraft, having lessened drag and eliminating problems such as fouling, sound transfer, repeated maintenance, and difficulties of repair and others. Compositions, such as those in Example 1, can be prepared in the form of flattened layers or sheets and used as panels.
These panels can correspond to patterns taken from the outer hull or surface of the watercraft, such that the sheet material is later assembled and adhered to a particular hull design or shape, much as a jigsaw puzzle.
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The possible thickness of applicant's covering layer is limited only by consideration such as cost, added weight, and the like. Applicant's preferred thickness range is about 0. The preferred method of attachment is to adhere the covering layer to the outer surface of the craft by an adhesive such as, for example, those commonly used for bonding natural rubber and synthetic elastomers to metal and various other substrates. The bonding agent used with applicant's preferred ethylenepropylene terpolymer base material was a CA brand bonding agent marketed by the 3M Company, which satisfies government specification MIL A C TYPE 1 CLASS 1 for military use, and is known to bond ethylenepropylene polymers to metal substrates and themselves.
Other adhesives and methods of attachment, of course, can be substituted with this or other base elastomers and are within the scope and contemplation of applicant's invention. The preferred covering layer was attached directly to the metal substrate being covered such as the hull or other surface of a craft. An alternate embodiment is to use the covering layer to form the outer layer of a laminate structure. The object of such a laminate can be to achieve improved properties such as adhesion, specific gravity, durability or impact tolerance, heat insulation, sound attenuation or absorption, and others.
Differences in construction such as, for example, a honeycomb inner layer can also be used if desired for a specific application. The invention permits the compounding of compositions having improved properties in many other applications. For a simple example, if the object were to make a bumper to cushion the automobile door when it slams and to keep it from rattling when it is closed, the bumper must be hard enough to stay in its slot when the door is slammed; it must be soft enough to cushion the door, yet must have a sufficient modulus of elasticity to keep the door from rattling when it is closed; it must preferably last for the life of the car which can be expected to be on the order of five to ten years; and it must be inexpensive.
The first step in deciding upon the composition of such a rubber product would be to decide upon the physical properties, such as hardness, permanent set, resilience, tensile strength, and the like. One of the advantages of compositions of the invention is their greater tolerance to aging. The method of manufacture of the product must also be considered. Since such bumpers can have a simple shape, such as a polyhedron, it may be manufactured in simple molds from an extruded preparation of mixed composition cut into short blanks with a size sufficient to fill the mold under pressure.
Thus, the composition may be extruded in the form of a uniform strip and should have good extruding characteristics.
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A quick cure is desirable for it is more economical than an extended cure; but it is desirable to avoid scorching and to obtain a composition that will not partially cure if it is not immediately molded. In addition, such bumpers will generally be rather thick; so the mixed rubber should be cured slowly enough that the outside does not cure long before the inside.
Generally, determining a composition for any application requires trial and error and several mixes are formulated for testing in such applications. Using the invention for such bumpers may, for example, lead to a composition including reclaimed tire rubber in parts by weight, particulate PTFE in 50 to parts by weight, an effective amount of molybdenum disulfide, and 10 parts by weight of antioxidants, sulfur, accelerator and fillers.
Another formulation of the invention usable in such application may include parts by weight oil extended, styrene-butadiene rubber, 50 to parts by weight particulate PTFE, an effective amount of molybdenum disulfide, and 10 parts by weight antioxidant, sulfur, accelerators, and other fillers. With this invention, the resulting automobile door bumpers will have a substantially reduced tendency to squeak because of their surface lubricity. This invention also permits the manufacture of elastomer products having different desirable physical characteristics in different portions of the product.
The first portion 10a can be formed from a cured elastomer composition to provide an article body portion with the first desirable physical properties of the cured elastomer composition. The second article portion 10b can be formed from the cured elastomer of said article body portion uniformly mixed with particulate polytetrafluoroethylene and particulate molybdenum disulfide and integrally joined with the article body portion 10a by cured elastomer.
Natural Rubber Manufacturing Process
The specific example of such an article shown in FIGS. As shown in FIGS. One portion 10a is comprised of a base elastomer which has been selected to provide freedom from compression set and durability as a window molding, to provide resiliency to permit compression by the automobile window glass as it is closed, and to provide sufficient force in its resistance to compression to maintain a seal between the window glass and the molding.
Ethylenepropylene EPDM is one elastomer that can be used to provide such characteristics in this application, but other elastomers may also be acceptable in such applications. The other portion 10b of the composite product is comprised of the base elastomer used to make portion 10a; but it includes, in addition to the base elastomers, for example, about one part by weight of a PTFE additive of this invention, including an effective amount of molybdenum disulfide per each weight part of elastomer.
The portion 10b forms the surface 10c of the window molding that engages the window glass when the window is closed. Because the portion 10b is substantially half elastomer and half PTFE, it is relatively hard; and it provides a lubricious surface 10c. The operation of such a composite automobile door window molding is illustrated in FIG. When the window glass 20 is closed, it is advanced into engagement with the window molding The window glass 20 engages the extending portion 10b; and because it is relatively hard, portion 10b is compressed without significant deformation into the softer, more deformable body portion 10a of the window molding and provides a line contact seal with the window glass by its extending lubricious surface portion 10c.
The force imposed on portion 10b by the resilience of body portion 10a maintains the lubricious surface 10c in contact with the window glass Because the composite window molding 10 provides a relatively hard, lubricious surface 10c, it does not provide a deformable surface that tends to adhere to the window glass; and the automobile window may be more easily opened. In the manufacture of such articles with the invention, an elastomer is mixed to form a first elastomer composition to provide first physical properties desirable in the first portion of the product; and the elastomer is mixed to provide a second elastomer composition with second desired physical properties.
For example, a first elastomer composition is mixed using a selected elastomer to provide the first physical properties desirable in a first portion of the product. The selected elastomer of the first elastomer composition is mixed with an additive of the invention comprising particulate polytetrafluoroethylene and particulate molybdenum disulfide to form a second elastomer composition, the particulate molybdenum disulfide having a majority of particles with sizes substantially smaller than the majority of particles of said particulate polytetrafluoroethylene and being in substantial part adherent to said polytetrafluoroethylene particles before addition to and mixing with the selected elastomer.
The additive is selected to alter one or more physical properties of the selected elastomer to provide second physical properties that are desirable in a second portion of the product. The first elastomer composition is formed or shaped for manufacture of the first portion of the product see 10a of FIGS.
The formed first elastomer portion of the product is placed together, with the formed second portion of the product, and the formed first elastomer portion and the second elastomer portion of the product are cured together to form an integral elastomer product having a first portion with first physical properties and a second portion with second physical properties. As illustrated in FIGS. The portions 10a and 10b are placed together by joining their surfaces 10d and 10e, respectively.
If necessary, liquid elastomer can be added to surface 10d or 10e or both 10d and 10e to provide "tack" where one or both of the product portions are highly loaded with additives.