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United States Patent (19) 11 Patent Number: 4,507,439 Stewart 45) Date of Patent: Mar

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United States Patent (19) 11 Patent Number: 4,507,439 Stewart 45) Date of Patent: Mar United States Patent (19) 11 Patent Number: 4,507,439 Stewart 45) Date of Patent: Mar. 26, 1985 54 ELASTOMERIC COMPOSITIONS CONTAINING TREATED PTFE OTHER PUBLICATIONS A. A. Benderly, Treatment of Teflon to Promote Bond 75) Inventor: Charles W. Stewart, Newark, Del. ability, J. Appl. Polymer Science VI, 20, 221-225, 73) Assignee: E. I. Du Pont de Nemours and Company, Wilmington, Del. (1962). Primary Examiner-Carman J. Seccuro 21 Appl. No.: 555,488 Attorney, Agent, or Firm-Paul R. Steyermark 22 Filed: Nov. 28, 1983 57 ABSTRACT 51 Int. Cl. ...................... C08L 27/22; C08L 27/16; An elastomeric composition comprising an elastomer C08L 27/18 matrix having dispersed therein about 3-30 parts per 52 U.S. Cl. .................................... 525/199; 525/200; 100 parts by weight of the elastomer of powdered poly 525/195; 525/196; 525/326.2, 525/366; tetrafluoroethylene having a number average molecular 525/367 weight of at least about 250,000 and a surface area of at 58 Field of Search ........................ 525/199, 195, 196 least 1 m2/g which has been treated with about 56) . References Cited 50-120% of sodium-naphthalene addition compound or U.S. PATENT DOCUMENTS another addition compound of an alkali metal required for the alkali metal to react with all the fluorine atoms 2,710,290 6/1955 Safford et al. ........................ 260/41 2,871,144 1/1959 Doban ........... ... 117/138.8 on the surface of the polytetrafluoroethylene powder. 3,019,206 1/1962 Robb ................................... 525/199 By dispersing such treated polytetrafluoroethylene 3,940,455 2/1976 Kaufman ............................. 260/888 powder in elastomer matrix, one increases significantly 4,326,046 4/1982 Miyaka et al. ...................... 525/276 the tear strength of the elastomer, while increasing its FOREIGN PATENT DOCUMENTS modulus only moderately. In this way, the processabil 54-013467 7/1979 Japan. ity of the elastomeric compositions is not adversely 57-10895 3/1982 Japan. affected. 1452429 10/1976 United Kingdom . 2084590 4/1982 United Kingdom . 14 Claims, No Drawings 4,507,439 1. 2 (Nelmor Co., N. Uxbrigde, MA 01538). PTFE powder ELASTOMERIC COMPOSITIONS CONTAINING can also be obtained commercially from E. I. duPont de TREATED PTFE Nemours and Company, Wilmington, Del. The powder surface area is determined by the well known BET BACKGROUND OF THE INVENTION 5 technique, ASTM No. C-819-77, and normally is about This invention relates to elastomeric compositions 1 to 5 m2/g. The usual particle size in its largest dimen having improved tear strength and good processability. sion is about 0.2 to 1 um. Many industrially important elastomers have low tear Alkali metal addition compounds include the addition strength and/or low cut growth resistance, which compounds of sodium, potassium, lithium, rubidium, or shortcomings decrease their utility in applications such 10 cesium, particularly with aromatic hydrocarbons, espe as, for example, belts and tubing for automotive and cially polynuclear hydrocarbons such as naphthalene or other applications. It is known to increase the tensile anthracene, and with ammonia and organic amines. The stength and to improve the tear strength of various preferred alkali metal addition compounds are sodium elastomers by the addition of polytetrafluoroethylene compounds. Sodium-naphthalene is the most conve (PTFE) powder; see, for example, U.S. Pat. No. 5 nient alkali metal addition compound which can be used 3,940,455 to Kaufman and Japanese Patent Publication in treating the PTFE powder. It is commercially avail Kokai No. 57-10895 (1982) of Daikin Kogyo K.K. able from W. L. Gore & Associates, Inc., Wilmington, Since, however, PTFE powder does not have good Del., under the tradename "Tetra-Etch'. Other suitable compatibility with most elastomers, including fluoro alkali metal addition compounds can be any of those elastomers, it cannot be readily dispersed in an elasto 20 mer to give a uniform composition. Those prior art which are suitable in the process of U.S. Pat. No. PTFE powder-containing elastomeric compositions 2,871,144, including any of those described in the refer usually had a non-uniform, blotchy appearance. In addi ences listed in column 1 of that patent: Scott et al., J. tion, their modulus was considerably increased, some Am. Chem. Soc. 58,2442 (1936); Walker et al., ibid. 60, times to the point where they were no longer process 25 951 (1938); Paul et al., ibid. 78, 116 (1956); and Yu et al., ible in conventional equipment by normal techniques. ibid. 76, 3367 (1954). It is known from U.S. Pat. No. 2,871,144 to Doban to The active portion of the addition compounds useful modify the surfaces of fluorocarbon polymer articles to in the present invention is the alkali metal, and they improve their bondability by treating those surfaces react like the alkali metal itself, but they have an advan with solutions of alkali metal additives with aromatic 30 tage over the alkali metal in that they can be handled hydrocarbons. It also is known from Japanese Pat. No. more easily, are soluble in various organic solvents, and 79/013467 to Sumitomo Electric Ind. K.K. to treat can be decomposed with either water or an alcohol PTFE powder with sodium-naphthalene, to mix such without a violent reaction. When an alkali metal or its treated powder with a lubricating liquid, and to extrude addition compound reacts with PTFE, it removes fluo it into a sheet. 35 rine atoms from the polymer surface, leaving what is There has been no disclosure or suggestion, however, believed to be essentially a hydrocarbon surface having that treated PTFE powder could be dispersed in a vari a number of active sites. Various atoms or radicals can ety of elastomeric materials to give uniform elastomeric attach themselves to those active sites, including aro compositions having improved tear strength and good matic hydrocarbon groups, oxygen, hydroxyl groups, processability. 40 and hydrogen atoms. Ideally, this modified surface layer should be of uni SUMMARY OF THE INVENTION form monomolecular thickness; if it is incomplete, that According to the present invention, there is provided is, involving less than the total surface area, less im an elastomeric composition comprising an elastomer provement or no improvement may be obtained; if it is matrix having dispersed therein about 3-30 parts per 45 too thick, a weak interlayer between the treated PTFE 100 parts by weight, based on the elastomer, of pow particles and the elastomer in the final elastomeric com dered polytetrafluoroethylene having a number average position may form, resulting in lower tensile properties molecular weight of at least about 250,000 and a surface and tear strength of the composition. However, when area of at least 1 m2/g, as determined by the BET an amount of alkali metal additive within the recited (Brunauer, Emmett, Teller) technique, which has been 50 range is used, an effective area of modified surface layer treated with an amount of an addition compound of an of correct thickness is obtained. When an amount of alkali metal of about 50-120% of the theoretical amount alkali metal additive in excess of the optimum range is required for the alkali metal to react with all the fluo used, the excess surface reaction products following the rine atoms on the surface of the polytetrafluoroethylene treatment can be removed by washing with a hypochlo powder. Preferably, the amount of the alkali metal addi 55 rite solution. tion compound is not over the theoretical amount. If desired, the thickness of the modified PTFE layer For the purpose of the present disclosure, the term can be determined by ESCA (Electron Spectroscopy "elastomer' means that the material, when stretched to for Chemical Analysis), for example, as described by twice its original length and released, will return with Dwight et al. in J. Colloid and Interface Science, 47, force to substantially its original length. 60 650 (1974). The treatment of PTFE powder with sodium-naph DETAILED DESCRIPTION OF THE thalene or other alkali metal additive can take place in a INVENTION number of ways; for example, by stirring the polymer PTFE powder suitable in the process of the present powder with a solution of the agent in an appropriate invention can be obtained by grinding commercially 65 solvent, usually tetrahydrofuran, or by percolating the available PTFE in conventional grinding equipment solution through the polymer powder, or by rolling the such as, for example, Taylor-Stiles rotary mill (Day powder with the solution on a rubber mill. The opera Mixing, Cinncinati, OH 45212) or Nelmor granulator tion preferably should be conducted in a sealed vessel, 4,507,439 3 4. in the absence of moisture, and in the absence of air or crystallization from the melt in cal/g, and a BET sur under nitrogen. The contact time can vary from about 1 face area of about 2 m2/g. The theoretical amount of minute to about 12 hours. The treated powder is filtered sodium-naphthalene required to react with all the fluo off or otherwise separated from the liquid, washed with rine atoms on the PTFE powder surface was about 2.5 tetrahydrofuran, then with water, and dried. 5 mmoles/100 m2 of surface. While untreated polytetrafluoroethylene powder tends to agglomerate, the treated powder is free-flow EXAMPLE 1 ing. It can be kept for extended periods, probably indefi To each of three dispersions of 100 g of PTFE pow nitely, without losing its physical characteristics and der in 600 mL of tetrahydrofuran prepared using a desirable surface properties. 10 homomixer there were added under nitrogen varying The improved elastomeric compositions of the pres amounts of the sodium-naphthalene solution. No sodi ent invention are made by blending together an elasto um-naphthalene solution was added to a fourth disper mer and the treated PTFE powder in the specified sion.
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