FLUOROPOLYMER ARTICLES HAVING IMPROVED IMPERMEABILITY TO FLUIDS

Abstract: An article molded from a composition includes a fluoropolymer and a platelet filler having a particle size between 1 and 500 micrometers and an aspect ratio of from 10 to 10,000.

Description: FLUOROPOLYMER ARTICLES HAVING IMPROVED IMPERMEABILITY TO FLUIDS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to impermeable fluoropolymer articles used to contain and transport fluids such as liquids and gases. DESCRIPTION OF THE RELATED ART Metals such as carbon steel are materials of choice to store or transport fluids because they are relatively cheap and strong and are therefore widely used in such fields. For certain applications metals are often not suitable because of either their high weight or corrosion problems. The use of plastics, either alone as materials for components and structures, or in conjunction with other materials such as metals, helps minimize the problems with metals. Plastics also have limitations: they are not compatible with all chemical substances, they are resistant to temperatures that are in general lower than the temperatures metals are resistant to, and plastics are, to various degrees, permeable to fluids. Of all types of plastics, fluoropolymers are the best suited for these applications where it is desirable to have compatibility with chemical substances, resistance to temperatures and impermeability to fluids, such as applications for fuel and harsh chemical storage and transport. Fluoropolymers such as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) and fluorinated ethylene- propylene (FEP) are resistant to attack from most of the known chemical substances under normal use, can withstand relatively high temperatures of up to 260 C continuous and have one of the lowest permeation rates of all plastics. Permeation can be a problem with plastics because gases or liquids permeating the plastic may pollute or create safety hazards in their environment, or, in the case of permeants such as acids, may corrode metal supports. liners, may cause the liner to collapse. It has been known in the art that the addition of fillers improves certain properties in fluoropolymers, such as thermal or electrical conductivity and strength. The use of such fillers for use in fluoropolymer coatings is known. Coatings are applied directly to substrates, mainly from the liquid state, and sometimes from a powder form. The coatings are subsequently dried and/or fused. Coatings form an integral part with the substrate and are not independent articles. Fluoropolymer coatings are generally thin, below 0.3 mm. In comparison to coatings, molded or extruded articles are produced by a process where first the polymer is molten and then squeezed through a narrow passage to create a backpressure, after which objects such as films, pipes and tubes are formed. It is known in the art that PTFE fluoropolymer is an exception because it is formed or extruded into these type of articles by a powder process which does not involve melting during the forming process, but only afterwards. Molded and extruded articles of fluoropolymers are usually thicker than 0.3 mm. SUMMARY OF THE INVENTION The present invention relates to a molded or extruded article made from a composition comprising a fluoropolymer and a platelet filler having a particle size between 1 and 500 micrometers and an aspect ratio of from 10 to 10,000. These molded or extruded articles exhibit reduced permeation of fluids therethrough. The articles may be used as stand alone or may be extruded or molded in combination with other materials such as metal backings and supports. DETAILED DESCRIPTION The present invention relates to a molded or extruded article made from a composition comprising a fluoropolymer and a platelet filler. The molded and extruded articles of this invention include molded or extruded article which is used to transport or contain a fluid such as a liquid or a gas. Examples of these articles include those used in the transportation of fuels, such as fuel hoses for automobiles, as well as tubes and other parts that come in which is used in the transport of a fluid where impermeability is an important criterion, including heat exchanger tubes, hoses, tubes and vessels used to transport or hold chemicals. As used herein, the term "article" means objects having a thickness of greater than 0.3 mm, and includes extruded or molded films. The term fluoropolymer is well known in the art, and includes fluoropolymers such as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), fluorinated ethylene -propylene (FEP), polyvinylidene fluoride (PVDF), ethylene- tetrafluoroethylene (ETFE) and ethylene-chlorotrifluoroethylene (ECTFE). The composition also includes a platy filler, sometimes referred to in the art as a platelet filler. Platy fillers represent a special class of fillers which, because of their structure, may be delaminated into very thin flakes that are characterized by a high ratio of their average diameter to their thickness. This ratio is known as the Aspect Ratio. Furthermore, the planar structure of the platy fillers yields a very flat and regular surface, such that the average thickness is quite uniform. As used herein, the term "platy filler" is intended to mean ultra thin particles having a high Aspect Ratio. In the present invention, the best results in lowering permeation are obtained when the average particle size of the platy filler is between 1 and 500 micrometers, or microns, preferably between 5 and 100 microns, and the Aspect Ratio of the platy filler is between 10 and 10,000, preferably between 10 and 2000. Examples of platy fillers that may be used in this invention include mica, glass flakes, talc, silicon carbide flakes, alumina flakes, graphite flakes, vermiculite, or mixtures thereof. The fluoropolymer is present in the composition in an amount of between 50 and 99.5 weight percent, preferably between 75 and 99.5 weight percent, based upon the total weight of the fluoropolymer and filler only. The platy filler is present in the composition in an amount of between 0.5 and 50 weight percent, preferably between 0.5 and 25 weight percent, based upon the total weight of the fluoropolymer and filler only. While to 50 weight percent of the filler may be used, when greater than 25 weight percent filler is used the apparent viscosity of invention. EXAMPLES COMPARATIVE EXAMPLES 1-2 AND EXAMPLES 3-4 In the following examples, a molded sheet was made by first preparing a molding composition of a fluoropolymer and a filler and then molding the composition into a flat sheet. The fluoropolymer used was perfluoroalkoxy fluoropolymer (PFA) type 5026A, available from DuPont. The fillers used were Iriodin 120 mica filler available from Merck and A125 ceramic powder available from Baikowski Chimie. The mica filler had a particle size of 5-20 micrometers and an Aspect ratio of 10 to 200. The ceramic powder had a round particle morphology of average diameter (D 50 ) of 5 micrometers. Specifically, the molding composition was molded into a flat sheet having a nominal thickness of 0.3 mm. The flat sheets were tested for the oxygen transmission rate using air according to ASTM method F 1307-90. The results are summarized in Table 1 below. TABLE 1 The results in Table 1 show that the molded sheets made from the compositions which included a mica filler had a dramatic increase in the oxygen permeation rate compared to the fluoropolymer alone. Further, the use of ceramic powder actually increased the oxygen transmission rate in the molded sheets. Molded sheets were made as in the previous Examples, except that the nominal thickness of the molded sheets was 0.4 mm. The types of fillers used are indicated below. TABLE 2 These examples show that molded sheets having a thickness of 0.4 mm made from the compositions which included a mica filler had a dramatic increase in the oxygen permeation rate compared to the fluoropolymer alone. COMPARATIVE EXAMPLE 7 AND EXAMPLE 8-10 Molded sheets were made as in the previous Examples, except that the nominal thickness of the molded sheets was 0.3 mm. The types of fillers used are indicated below. The larger thicknesses in these examples represent thicknesses commonly found in fluoropolymer liners used to protect other materials from harsh chemicals. In these examples, pure oxygen, instead of the air used in the previous examples, was used at a temperature of 90C. A Siemens DiffuPerm analytical instrument was used to measure the oxygen permeability. TABLE 3 These examples show that the use of the indicated fillers dramatically reduces the oxygen permeability of a fluoropolymer. COMPARATIVE EXAMPLE 11 AND EXAMPLES 12-14 Molded sheets were made as in the previous Examples 7-10. The types of fillers used in the molding compositions are indicated below. The permeability of the molded sheets was measured as in those examples except that helium was used as the permeant gas at a temperature of 60C. TABLE 4 These examples show that the use of the indicated fillers dramatically reduces the helium permeability of a fluoropolymer at 60C. COMPARATIVE EXAMPLE 15 AND EXAMPLES 16-18 Molded sheets were made as in the previous Examples 11-14 except that the helium permeability was measured at 90C. TABLE 5 These examples show that the use of the indicated fillers dramatically reduces the helium permeability of a fluoropolymer. COMPARATIVE EXAMPLE 19 AND EXAMPLES 20-22 Molded sheets were made as in the previous Examples 11-14 except that the helium permeability was measured at 110 C. These examples show that the use of the indicated fillers dramatically reduces the helium permeability of a fluoropolymer. EXAMPLE 23 In this example a molded plaque was made by transfer molding from a composition of 93 weight percent PFA fluoropolymer type 350, available from DuPont with 5% by weight mica type Iriodin 504, available from Merck, with an average particle size of 10 to 60 micrometers and an average Aspect ratio of 20 to 600, and 2% by weight talc type Micro-Talc IT Extra, available from Norwegian Talc, with an average particle size of approximately 5 micrometers and an average aspect ratio of approximately 50. The molded plaque, having a nominal thickness of 3mm, was subjected to a steam/cold water cycle according to ASTM test method F-423. No blistering, delamination or any other defect was present in the molded plaque after the test. EXAMPLE 24 A composition of PTFE fluoropolymer, type 669-N, available from DuPont, was dry blended with the same amount and type of mica and talc fillers as described in example 23. Subsequently the composition was lubricated with 18% Shellsol K (based on the total weight of PTFE, mica and talc) available from Shell. The lubricated composition was extruded by a process known in the art as PTFE paste extrusion to make a tube having a nominal outside diameter of 80mm and a nominal wall thickness of 2mm. These are common dimensions for fluoropolymer liners used in the chemical industry. An appropriate sample was cut from this liner and was subjected to the steam/cold water cycling test ASTM F- 423. No blistering, delamination or any defect was present in the tube sample. COMPARATIVE EXAMPLE 25 A tube was made as in Example 24, except that it was 100 weight percent PTFE, type 669-N. The tube was subjected tot the steam/cold water cycling test ASTM F-423. Small blisters and light signs of delamination were present. COMPARATIVE EXAMPLE 26 AND EXAMPLES 27-28 Fluoropolymer molded sheets are produced in the same manner as in Examples 5 and 6. Sheets that are 0.4 mm thick are tested with a carbon steel backing in an Atlas cell with dilute hydrochloric acid (0.05M) in deionized water at 98C. In Examples 27 and 28, the sheets are laid freely against the backing. In Example 28, the sheet is melt bonded in a hot press so that the sheet is strongly secured against the steel plate. TABLE 7 Examples 26-27 show that molded sheets that are produced in accordance with this invention protect a metal backing from corrosion and blistering. This occurs whether the molded sheet is laid freely against the backing or is adhered to the backing. Another beneficial aspect is that the improved permeability of the molded sheet prevents deterioration of the bonding structure or bonding agent that secures the molded sheet to the backing, and therefore there is a greatly decreased likelihood that the molded sheet will separate from the backing. to the present invention exhibit improved permeability and resistance to blistering and delamination. Other additives known in the art may be added to the inventive composition. Examples of such additives include pigments, thermally and/or conductive fillers such as carbon, reinforcements such as glass or carbon fibers, light transmittance blockers such as titanium dioxide, or mixtures thereof.

Claims: 1. An article mlded from a composition comprising a fluoropolymer and a platelet filler having a particle size between 1 and 500 micrometers and an aspect ratio of from 10 to 10,000. 2. An article extruded from a composition comprising a fluoropolymer, a platelet filler having a particle size between 1 and 500 micrometers and an aspect ratio of from 10 to 10,000. 3. The article of claim 1 or 2, wherein the fluoropolymer is polytetrafluoroethylene, perfluoroalkoxy, fluorinated propylene-ethylene, polyvinylidene fluoride, ethylene-tetrafluoroethylene, ethylene- chlorotrifluoroethylene or mixtures thereof. 4. The article of claim 1 or 2, wherein the platelet filler is mica, glass flakes, talc, silicon carbide flakes, alumina flakes, graphite flakes, or mixtures thereof. 5. The article of claim 1 or 2, wherein the platelet filler has a particle size of between 1 and 500 micrometers. 6. The article of claim 1 or 2, wherein the platelet filler has an aspect ratio offrom l0 to 2000. 7. The article of claim 1 or 2, wherein said article has been bonded to a backing or a support material.

Inventors: LEVY DANIEL N, LEVY, DANIEL, N.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1998
Main classification: C08K7
IPC codes: C08K3, C08K7, C08L23, C08L27, C08L29