MULTI-LAYER POLYMERIC STRUCTURE

Abstract: The invention relates to a multi-layer polymeric structure comprising at least two adjacent fluoropolymer layers in which one of the layers comprises a perfluoroalkoxy polymer and an adjacent layer comprises a fluorinated ethylene propylene polymer.

Description: TITLE MULTI-LAYER POLYMERIC STRUCTURE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a multi-layer polymeric structure, to an apparatus comprising such a structure and to a process for manufacturing such a structure. BACKGROUND OF THE INVENTION It is known in the art to manufacture multi-layer polymeric structures comprising at least three layers i.e. two dissimilar polymeric layers forming the outer surfaces of a structure bonded together by an adhesive tie- layer. The adhesive layer, although in many cases thinner than the surface layers, alters the properties of the structure, generally requires a complicated manufacturing process for the structure and increases the cost thereof. For many applications it would be useful if relatively simple multi-layer structures which do not require an adhesive tie-layer, would be available for applications in which one surface of the structure could be subjected to significantly different conditions, compared with the other surface of the structure. A problem that arises in developing such desired structures is that significantly different conditions at two sides of the structure require, in general, chemically significantly different polymers to provide satisfactory performance under these conditions. And these chemically different polymers, when employed in layers, are not likely to adhere to each other in a sufficient manner to obtain a strong multi-layer structure without the use of one or more adhesive tie-layers between the desired polymer layers. Even in cases where compounds are miscible, they do not necessarily have adhesive properties for each other which would render them suitable for a multi-layer structure without adhesive layer Suφrisingly, it has now been found that two different types of fluoropolymer layers bond together without a tie-layer to form a structure applying described technology which provides excellent mechanical- and chemical properties. SUMMARY OF THE INVENTION The present invention relates to multi-layer polymeric structures comprising at least two adjacent fluoropolymer layers in which one of the layers comprises a perfluoroalkoxy polymer and an adjacent layer comprises a fluorinated ethylene propylene polymer DETAILED DESCRIPTION It has been known for some time that certain perfluoroalkoxy polymers and fluorinated ethylene propylene polymers can be blended in the form of aqueous- or non-aqueous dispersions, dry powders or melts, based on the rheological compatibility of these polymers However, rheological compatibility bears no relation to compatibility and bonding strength of two or more adjacent polymeric layers Surprisingly, it has now been found that multi-layer polymeric structures can be created by combining a layer comprising a perfluoroalkoxy polymer with an adjacent layer comprising a fluorinated ethylene propylene polymer and thus obtaining structures having different chemical- physical- and/or mechanical properties for both surfaces of a structure The polymer in both layers of the structure is melt fabricable. This can be a great technical- and economical advantage since for many applications different requirements apply to different surfaces of the same structure which may e.g separate a hotter medium from a cooler medium or a chemically aggressive fluid from a less aggressive fluid In most cases the more strict the requirements are for a surface layer, the more expensive the material that has to be applied to meet such requirements, whereas the other surface layer may be made of a cheaper material which meets less strict requirements. Fluoropolymers are generally materials showing a good chemical resistance and temperature stability and have been applied in many forms, e.g. as tubes, sheets and shaped objects. In many cases fluorinated ethylene propylene polymers show excellent performance in chemically aggressive environments at elevated temperature and/or pressure. However, in some instances even more chemically- and/or thermally resistant perfluoroalkoxy polymers are required to withstand the environment to which one surface of a polymer structure is exposed. In that case the entire polymer structure had to be manufactured from the, generally more expensive, perfluoroalkoxy polymer material. With the present invention it has become possible to tailor the polymeric material used in two sides of a structure to the different chemical-, physical- and/or mechanical requirements for each side without using an adhesive tie-layer. This means that chemically aggressive compounds permeating to a certain extent through a layer at one side cannot attack or dissolve a tie-layer. Moreover, in many cases the directly bonded polymer layers have an adhesive strength to one another which is in order of the cohesive strength of the component layers of the laminate as shown in some of the Examples hereinafter. Preferably, both the perfluoroalkoxy polymer and the fluorinated ethylene propylene polymer employed in the polymeric structure comprise tetrafluoroethylene as monomer which selection of common monomer increases the compatibility of the two adjacent layers described hereinbefore. Moreover the selection of similar co-monomers in both polymers increases the compatibility even further. However it is also possible to use exclusively dissimilar co- monomers in preparing adjacent layers which are directly bonded in the polymeric structure according to the invention. The fluorinated ethylene propylene polymer present in the polymer structure may be prepared by using two or more monomers. In case 0 two co-monomers are used, these are preferably perfluoro(alkyl vinyl ether), e.g., perfluoropropylvinylether, and hexafluoropropylene. Suitably, the polymer can have a wide range of hexafluoropropylene content as disclosed in the literature, and in addition to the tetrafluoroethylene, can optionally contain one or more additional fluorine- containing co-monomers. A preferred additional co-monomer is fluoroalkylvinylether. Preferably the weight percentage of hexafluoropropylene co-monomer is from 7-30%, in particular 7-15% by weight of the total amount of the different co-monomers. Typical compositions of such polymers are described in International Standard No. ISO 12086- 1 : 1995(E). In a preferred embodiment of the present invention, the fluorinated ethylene propylene polymer is a copolymer based on the co- monomers fluoroethylene, fluoropropylene and a perfluoroalkylvinylether; notably a perfluoromethylvinylether, perfluoroethylvinylether or perfluoropropylvinylether. The monomers fluoroethylene and fluoropropylene are preferably tetrafluoroethylene and hexafluoropropylene, respectively. An advantage of the application of a copolymer in a multi-layer polymeric structure according to the invention is the excellent compatibility between a layer comprising such a copolymer and at least one adjacent layer comprising a perfluoroalkoxy polymer. The alkyl-group present in the fluoroalkylvinylether comonomer may comprise 1-10 carbon atoms, and preferably 2-6 carbon atoms. In a particularly preferred embodiment of the present invention the alkyl-group is a propyl-group, and the vinylether is perfluoropropylvinylether. Preferably, such a copolymer comprises 7-30% by weight of hexafluoropropylene monomer, 92.9-60% by weight of tetrafluoroethylene monomer and 0.1-10% by weight of fluoroalkylvinylether, to total 100 wt%. More preferably, such a copolymer is comprises 7-15% by weight of hexafluoropropylene monomer, 92.5-83% by weight of the tetrafluoroethylene monomer and 0.5-2% by weight of perfluoropropyl-or- ethylvinylether. The perfluoroalkoxy polymer present in the polymer structure may also be prepared by using additional co-monomer along with the fluoroethylene monomer. In case two co-monomers are used, these are preferably hexafluoropropylene and a perfluoroalkylvinylether along with the fluoroethylene. Preferably, the perfluoroalkoxy polymer contains 99.9-90% by weight of tetrafluoroethylene monomer and 0.1-10% by weight of a perfluoroalkylvinylether co-monomer. Even more preferably, the weight of percentage of tetrafluoroethylene is from 99.5-95% and for the perfluoroalkylvinylether from 0.5-5% by weight. The alkyl-group present in the perfluoroalkylvinylether may comprise 1-10 carbon atoms, and preferably 2-6 carbon atoms. In a preferred embodiment of the present invention the alkyl-group is selected from the series, consisting of methyl-, ethyl- and propyl- groups. It is furthermore possible to apply layers in the multi-layer polymeric structure according to the present invention of which at least one individual layer comprises more than one fluoropolymer in the form of a blend or a melt compound, preferably of fluoropolymers present in at least two adjacent layers of the structure. In one embodiment of the invention at least one of the layers comprises a fluoropolymer of which a major fraction consists of a perfluoroalkoxy polymer and a minor fraction consists of a fluorinated ethylene propylene polymer. Preferably the major fraction amounts to 60-99% by weight and the minor fraction 1 -40% by weight, based on total polymer weight in that particular layer. In another embodiment of the invention, which may be combined with the embodiment as described hereinbefore, at least one other adjacent layer comprises a fluoropolymer of which a major fraction consists of a fluorinated ethylene propylene polymer and a minor fraction consists of a perfluoroalkoxy polymer. Preferably, the major fraction amounts to 60-99% by weight and the minor fraction 1-40% by weight, based on total polymer weight in that particular layer. By applying such blends of fluoropolymers in one layer without a tie-layer and directly bonding such layers which are adjacent to each other in the structure according to the present invention the compatibility of these layers, one to the other, can be improved, giving structural integrity to the total structure. /07290 In order to alter the physical- and/or mechanical properties of the present multi-layer structure and sometimes to reduce the cost of the structure, it is possible to add a particulate filler material to at least one of the polymeric layers of the structure. The filler may comprise organic and/or inorganic material and preferably comprises carbon, glass in the form of particles or fibers or silicates. Such filler may be present in an amount of 0.1-50% by weight, and preferably of 0.2-20% by weight, based on the total weight of a single layer. The use of filler material in a polymer layer has the further advantage of improving the surface smoothness of the layer, in particular if a polytetrafluoroethylene micropowder is used as filler material. In such powder generally the largest dimension of one particle is from 0.1-50 μm, preferably from 0.5-10 μm. For certain applications it is essential that at least one of the polymer layers comprising a high purity perfluoroalkoxy polymer is particularly inert for certain chemicals; in that case it is envisaged to include at least one layer in the structure which comprises a post-fluorinated polymer. In a preferred embodiment of the invention, at least the layer comprising a perfluoroalkoxy polymer has been subjected to a post-polymerization fluorination treatment; the intensity of such a treatment is defined by a low level of extractable fluorine ions in the polymer layer after the treatment. The resulting high-purity perfluoroalkoxy polymer contains relatively low quantities of alkoxyfluoride-, carboxylic-, and vinyl- end groups. In a further preferred embodiment of the present invention, the use of a post-polymerization fluorination treatment for at least one polymer layer is combined with the use of inert filler material in that same layer in order to attain particularly inert and smooth surfaces. This is desirable for applications in the micro electronics industry, such as silicon-wafer manufacturing, and other applications such as for chemical transfer tubes and in medical-, pharmaceutical- and food processing applications. The multi-layer polymeric structures according to the invention can have many shapes such as flat sheets, profiled sheets, tubes and blow molding articles with a more complex form. In a preferred embodiment of the invention, in particular for use in heat-exchangers, the multi-layer structure has a tubular shape; the perfluoroalkoxy polymer ("PFA") layer can be located either at the inside or the outside of the tube depending on the application and taking into account that for most applications the PFA is able to withstand more severe conditions than the FEP. Thus, if a heat exchanger for heating a "clean" fluid such as water with a chemically aggressive flue gas is to be constructed, the PFA layer will have its outer surface in contact with the flue gas, whereas the FEP layer will have its outer surface in contact with the "clean" fluid. The multi-layer structures according to the present invention include structures having more than two layers of polymer, depending on the application. A preferred embodiment of the invention relates to three-layer structures comprising a FEP polymer layer of which both surfaces are covered by PFA layers. For certain applications, such as in the semi-conductor industry, it may be advantageous to subject the outer surfaces of the two PFA layers to a post-polymerization fluorination treatment as described hereinbefore. In another preferred embodiment of the present invention a first PFA layer is employed as central layer, one surface thereof being adjacent to an FEP layer and the other surface being adjacent to a second PFA layer which optionally may be subjected to a post-polymerization fluorination treatment. Again such a three layer polymer structure is eminently suitable for applications in the semi-conductor industry and other sectors where very high requirements are in force for the chemical- and mechanical stability of the structures. The invention further relates to a process for manufacturing a multi-layer polymeric structure as described hereinbefore in which the fluoropolymers for at least two adjacent layers are co-extruded. In a preferred embodiment of the invention, at least two different extruders feed one common extrusion head in which the individual layers of fluoropolymers are coextruded to flow together for a period of 0.1-50 s at a pressure of 2-300 bar and a temperature of 50°-400°C, in order to allow reptation between the different US97/07290 fluoropolymer layers, resulting in excellent adhesion between the different layers. The fluoropolymers are preferably extruded at a melt flow rate of 0.8-40 g/10 min., in particular of 0.8-25 g/10 min., according to ASTM D 2116 (for FEP) and ASTM D 3307 (for PFA). In addition to previously described applications the multi-layer polymeric structures according to the invention can be advantageously applied in the semi-conductor manufacturing industry, as chemical liners for vessels, in the food processing industry and in general for self-cleaning types of equipment. EXAMPLES The invention is further illustrated by the following Examples. EXAMPLE 1 Fluorinated ethylene propylene polymer ("FEP") comprising 87% by weight of monomer tetrafluoroethylene and 13% by weight of co¬ monomer hexafluoropropylene and having a melt flow rate of 2.7 g/10 min (4.67 . IO -6 kg/s) according to ASTM D-2116 was introduced into a first single-screw extruder having a diameter of 45 mm (4.5 . 10 ~ 2 m) and a length/diameter ratio of 24, operating at the following conditions: 320°-370°C, 0.75 . 10 ~ * revolutions/s and a melt pressure rate of 56 bar, and the resultant first extrusion feed was directed to a dual extrusion head in order to produce the inner layer of a two-layer tubular structure. The outer layer of this structure consisted of perfluoroalkoxy (PFA) polymer, comprising 96% of monomer tetrafluoroethylene and 4% by weight of co-monomer perfluoropropyl vinyl ether and having a melt flow rate of 2 g/10 min (3.3 x IO '6 Kg/s) according to ASTM D-21 16. This polymer was introduced in a second single screw extrusion having a diameter of 50 mm (5 . IO *2 m) and a length/diameter ratio of 24, operating at the following conditions: 330°-390°C, 0.6. 10-1 revolution/s and a melt pressure rate of 27 bar, and the resultant second extrusion feed was directed to the second cavity of the above mentioned dual extrusion head in order to produce the outer layer of the two-layer tubular structure. The structure thus produced has an elastic modulus of 620 MPa according to ASTM D-790, a total wall thickness of 1 mm, a wall thickness of the PFA layer of 0.5 mm, and a wall thickness of the FEP layer of 0.5 mm. The outer diameter of the tube is 12 mm. However, a wide variety of tube diameters and wall thicknesses and ratio's thereof can be produced with the process according to the present invention by using different extrusion heads/toolings and extrusion processing conditions. Mechanical strength of the tube thus produced was tested by applying an internal pressure to a tube of which the ends were closed off; the cold burst pressure was 29 bar without any visible delamination of the PFA and the FEP layers, indicating the adhesive bond between the two layers making up the tube is on the order of the cohesive strength of the layers themselves. EXAMPLE 2 A flat two-layer structure comprising a PFA layer having a thickness of 2 mm and a FEP layer having a thickness of 1 mm was prepared in essentially the same manner as described in Example 1 with the exception that the extrusion head was different in order to produce a coextruded flat structure with a width of 2 cm (0.02 m). Adhesion of the FEP to the PFA layer was evaluated after carrying out "ultimate tensile "tests according to ASTM-D-638. No interlayer delamination was observed upon rupture of the ultimate tensile test specimen. The ultimate tensile strength of this specimen as tested in the main extrusion direction was 27 MPa. COMPARATIVE EXAMPLE 3 A tubular structure (not in accordance with the invention) was produced in a manner similar to that described in Example 1 by extruding polyvinylidene fluoride (PVDF) and ethylene tetrafluoroethylene (ETFE) with a dual-layer extrusion crosshead. The resulting tubular structure was completely delaminated i.e. no adhesion between the PVDF and the ETFE layer was observed.

Claims: CLAIMS What is claimed is: 1. Multi-layer polymeric structure comprising at least two adjacent fluoropolymer layers in which one of the layers comprises a perfluoroalkoxy polymer and an adjacent layer comprises a fluorinated ethylene propylene polymer. 2. Multi-layer polymeric structure according to claim 1 in which the fluorinated ethylene propylene polymer is a polymer based on fluoroethylene, fluoropropylene and fluoroalkyl vinyl ether. 3. Multi-layer polymeric structure according to claim 1 in which at least one of the layers comprises a post-fluorinated polymer. 4. Multi-layer polymeric structure according to claim 1 in which at least one of the layers comprises a fluoropolymer of which a major fraction consists of a perfluoroalkoxy polymer and a minor fraction consists of a fluorinated ethylene propylene polymer. 5. Multi-layer polymeric structure according to claim 1 in which at least one of the layers comprises a fluoropolymer of which a major fraction consists of a fluorinated ethylene propylene polymer and a minor fraction consists of a perfluoroalkoxy polymer. 6. Multi-layer polymeric structure according to claim 1 in which at least one of the polymeric layers comprises filler material. 7. Multi-layer polymeric structure according to claim 1 having a tubular shape. 8. Process for manufacturing a multi-layer polymeric structure according to claim 1 in which the fluoropolymers for at least two adjacent layers are co-extruded. 9. Process according to claim 8 in which the fluoropolymers flow together for a period of 0.1-50 s. at a pressure of 3-200 bar and a temperature 50°-400°C through one co-extrusion head.

Inventors: JENEWEIN HERMANN JOSEPH KLAUS, LEVY DANIEL NATAN, JENEWEIN, HERMANN, JOSEPH, KLAUS, LEVY, DANIEL, NATAN
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1997
Main classification: B32B27
IPC codes: B29C47, B32B27