Fluoropolymer Alloys

This invention relates to fluoropolymer alloys made of melt fabricatable extra-high-molecular weight fluorinated ethylene-propylene copolymer (EHMW-FEP) and one or more other polymers, their preparation and application.

The fluoropolymer alloys are new materials having properties different from the original polymers of which they are made. The fluoropolymers provide an important way to prepare new materials with a great variety of properties, because it permits using available polymers, in a simple process. They have attracted interest recently in Sato Kawa, plastics (Japan), 32, 69 (1981). The fluoropolymer alloys which have been reported are limited to blending polymers based on polyvinylidene fluoride, polytetrafluoroethylene (PTFE) dispersions and fluorinated ethylene-propylene copolymer (FEP) dispersions (B.P. 935, 706, (1960); Japanese patent kokai No. 12521/62.

Polytetrafluoroethylene, known as the "King of plastics", has excellent comprehensive properties. It has excellent thermal and chemical stability, and outstanding electrical insulation and anti-stick properties. But it cannot be fabricated by melt processing; it can only be fabricated by cold compaction and then sintering. Furthermore its creep-resistance is low. Therefore, its applications are limited.

A fluorinated ethylene-propylene copolymer which has good creep-resistance is a copolymer of tetrafluoroethylene and hexafluoro-propylene, as it can be melt fabricated, due to the presence of trifluoromethyl in the copolymer chain. The chemical inertness and outstanding electrical insulating properties of the copolymer are similar to polytetrafluoroethylene, but its thermal stability is not as good as PTFE, as it can only withstand temperatures up to 200.degree. C., and its cost is higher than that of PTFE.

The purpose of the present invention is to prepare a fluorine-containing material with better properties by blending PTFE and FEP, and obtaining a polymer alloy with excellent properties and which overcomes the drawbacks of the two original polymers.

Processes for preparing polymer alloys made of polytetrafluoroethylene and fluorinated ethylenepropylene copolymers are indicated in B.P. 935, 706 (1960) and Japanese patent kokai 12521/62. Such processes use co-precipitation of these two fluoropolymer dispersions. The processes are complicated and expensive. The mechanical properties of the fluoropolymer alloy are worse than that of PTFE and FEP. For example their tensile strength is much lower than that of the two original polymers and only 142-213 Kg/cm.sup.2 at room temperature.

An important problem is properly matching the polytetrafluoroethylene with a fluorinated ethylene-propylene copolymer in order to get a good fluoropolymer alloy made of PTFE and FEP. These two raw fluoropolymers have different processing temperatures and different thermal stability. The temperature of melt compression molding of FEP is about 310.degree. C., but the sintering temperature after cold compaction of PTFE is about 380.degree. C. The decomposition rate of FEP is much higher than that of PTFE at 380.degree. C. Particularly, when FEP resins having carboxyl end groups are prepared by copolymerization using persulfates as initiators, it must be heated at 380.degree. C. in order to stabilize the end group. However, FEP resins become "bread ruck", after roasting, and cannot be blended with PTFE powder.

Another important problem resides in the blend process of preparing a polymer alloy made of PTFE and FEP. In general blending polymers can be carried out by the solution blend, melt blend or dispersion co-precipitate blend methods. As there is no solvent which works for PTFE and FEP, the process could not be carried out by solution blend. Melt blend is also not possible, because PTFE cannot be melt processed. Therefore, dispersion coprecipitating blend has been the way used in B.P. 935, 706 and Japanese patent kokai No. 12521/62, although the cost of the polymer alloy is high, as one must use expensive fluorine-containing emulsifiers in preparing the dispersion of PTFE and FEP. Besides, the process blend coprecipitating is rather complicated.

The present invention provides a fluoropolymer alloy made of extra-high-molecular weight fluorinated ethylene-propylene copolymer as its chief component. The EHMW-FEP is characterized by having its melt processing property matched with PTFE.

The present invention also provides a blend process of dried powder co-mill or wetted powder co-mill for preparing the fluoropolymer alloy.

The present invention also provides a fluoropolymer alloy which is characterized by having melt fabrication properties like FEP, and cold compression and sintering properties like PTFE, and having the excellent properties possessed by both PTFE and FEP.

The present invention also provides a series of fluoropolymer alloys made of EHMW-FEP as their chief component, and at least one other polymer (including fluorine-containing or common polymers).

According to the present invention, the EHMW-FEP is a molding powder which can be melt fabricated, with melt viscosity of 1.times.10.sup.6 poises or more, a melt flow index less than 0.8 gram/10 min., 12 to 30% by weight of hexafluoropropylene, a weight-average molecular weight of 2.times.10.sup.5 or more and a tensile strength of 270 Kg/cm.sup.2 or more at room temperature.

The process for preparing extra-high-molecular weight fluorinated ethylene-propylene copolymers which can be melt fabricated involves a solution precipitating polymerization with liquid hexafluoropropylene as a solvent, including raising the concentration of monomers in the reaction area by increasing the amount of monomers in a autoclave and increasing the amount of tetrafluoroethylene in the initial monomer mixture of tetrafluoroethylene and hexafluoropropylene, and reducing the concentration of free radicals in the polymerization area and thus reducing the termination possibility of the copolymer chain by reducing the amount of initiator (diisopropyl peroxydicarbonate) used. All these conditions are advantageous to growing the copolymer chain and to preparing very high molecular weight copolymers. The polymerization conditions are as follows: (1) charging 0.2-0.5 gram/ml of a mixture of tetrafluoroethylene and hexafluoropropylene in a autoclave, (2) the weight ratio of tetrafluroethylene in the initial monomer mixture of tetrafluoroethylene and hexafluoropropylene is 11-50%, (3) the percentage of initiator diisopropyl peroxydicarbonate (IPP) is 0.001 to 0.05% equivalents based on monomers, (4) the weight ratio of water and monomer is 3/1 to 1/1; the polymerization temperature is 40.degree.-80.degree. C.; the polymerization pressure is 20-30 Kg/cm.sup.2 ; and the polymerization time is 1-5 hours per batch.

According to the present invention, the PTFE used is a commercial resin, including resins prepared by suspension or dispersion polymerization, and have tensile strength of greater than 270 Kg/cm.sup.2 at room temperature.

According to the present invention, the other commercial polymers, such as polychlorotrifluoroethylene, polyvinylidene fluoride, tetrafluoroethyle-ethylene copolymer, polyethylene, polypropylene, polysulfone, polyimide, polycarbonate, polyphenylene oxide, poly(phenylene sulfide) et al, can be used to prepare fluoropolymer alloys together with extra-high-molecular weight fluorinated ethylene-propylene copolymers.

A given weight ratio of EHMW-FEP and one or more other polymers as described above, were ground by mill, separately, and then mixed and ground again until the two or more phases were thoroughly mixed. The mixture passes through a 40-80 mesh and ready for processing.

The mixing and grinding process of two or more polymers may be carried out either in dried powder or wet powder form. During wet powder mixing and grinding, the polymers are moistened by water, ethyl alcohol, ethyl acetate or a mixture of these solvents. When the mixing is finished, the mixture of polymers must be dried, and then screened.

In the fluoropolymer alloy made of EHMW-FEP and PTFE, the weight of EHMW-FEP is 0.1-99.9% and the weight of PTFE is 99.9-0.1%. In a preferred embodiment the alloys contain 0.1-60% of EHMW-FEP and 40-99.9% of PTFE.

The fluoropolymer alloys made of EHMW-FEP and PTFE can be melt pressed into articles at temperatures of 300.degree.-350.degree. C. and pressures of 50-200 Kg/cm.sup.2, like FEP. They can also be cold compressed at a pressure of 50-200 Kg/cm.sup.2 and then sintered at a temperature of 300.degree.-390.degree. C., like PTFE.

The temperatures of melt pressing or sintering of the fluoropolymer alloys described above depend on their PTFE content. The higher the amount of PTFE, the higher the processing temperature.

The fluoropolymer alloys made of EHMW-FEP and PTFE of the present invention not only have a creep-resistance like FEP, but also high tensile strength under high temperature like PTFE. They exhibit tensile strength of 200-400 Kg/cm.sup.2 and elongation of 300-700% at room temperature, and correspondingly 50-200 Kg/cm.sup.2 and 300-700% at 200.degree. C. The extra-high-molecular weight fluorinated ethylene-propylene copolymers is compatible with polytetrafluoroethylene in the fluoropolymer alloys described above. Fluoropolymer alloys made of e.g., 10% of EHMW-FEP and 90% of PTFE, have just about only one melting peak and one peak of crystallizing in its DSC spectrum.

All kinds of fluoropolymer alloys described above can be reinforced and improved by adding glass fibers, graphite, molybdenum disulfide, carbon and various metal powders.

The fluoropolymer alloys made of EHMW-FEP and PTFE of the present invention have excellent thermal stability, chemical inertness and outstanding electrical insulating properties. Using this kind of fluoropolymer alloy material, various diaphragm, seals, lining valves, lining pipes, lining pump, various components of pump, such as shell, wheel, wind wrap wire etc. can be fabricated. The articles made of fluoropolymer alloys have better creep-resistance than that of PTFE and consequently have a rather long operating life.

The fluoropolymer alloys described above can be used in power coating by either the electrostatic or flow-bed method, and can also be fabricted into complex articles using ram-extruding a paste consisting of fluoropolymer alloy and petroleum ether or kerosene.

The invention is more specifically described and explained by means of the following examples which, however, are not intended to be limiting. In the examples all parts and percentages are by weight except as otherwise noted .

20 Grams of EHMW-FEP with a melt flow index of 0 gram/10 min., and 180 grams PTFE have successively been moistened by a mixture of ethyl alcohol and ethyl acetate, milled, filtered, dried and passed through a 40 mesh screen. The resulting powder is cold compressed into a 2 mm thick sheet under pressure of 80 Kg/cm.sup.2, and sintered in a oven for 1 hour at 350.degree. C. After cooling, its tensile strength is 277 Kg/cm.sup.2 under room temperature.

EHMW-FEP is mixed with PTFE in the weight ratio of 1/9, 1/3, 1/1, 3/1 and 9/1 respectively. The resulting mixtures are milled to pass through a 40 mesh screen, cold compressed under 80 Kg/cm.sup.2 and sintered separately. The properties of the resulting fluoropolymer alloys are shown in Table 1.

EHMW-FEP is mixed with tetrafluoroethylene-ethylene copolymer (F40) in weight ratio shown in Table 2 by blending dried powder or wetted powder. The resulting fluoropolymer alloys are fabricated by cold compression at a pressure of 70-150 Kg/cm.sup.2 and then sintered or melt pressed.

A mixture of 40 grams of EHMW-FEP, 40 grams of PTFE and 10 grams of polysulfone, is moistened with acetone, mixed homogeneously by high speed stirring, milled, dried, 40 mesh screened, cold compressed under 80 Kg/cm.sup.2 and then sintered for one hour at 320.degree. C. The hardness (Rockwell) of the fluoropolymer alloy obtained is D50.

EHMW-FEP is mixed with polychlorotrifluoroethylene (CTFE) in the weight ratio shown in Table 3 by blending dried powder or wetted powder, the resulting fluoropolymer alloys are fabricated by cold compression at a pressure of 70-150 Kg/cm.sup.2 and then sintering, or by melt pressing.

EHMW-FEP is mixed with polyvinylidene fluoride (PVDF) in weight ratios shown in Table 4 by blending dried powder or wetted powder (using diisobutyl acetone as a moistener). The resulting fluoropolymer alloys are fabricated by cold compression at a pressure of 70-150 Kg/cm.sup.2 and then sintering, or by melt pressing.

EHMW-FEP is mixed with polysulfone (PS) in the weight ratios shown in Table 5 by blending dried powder or wetted powder (including using chloroform as a solvent), the resulting fluoropolymer alloys are fabricated by melt compression under pressure of 70-150 Kg/cm.sup.2.

EHMW-FEP is mixed with polyethylene (PE) in the weight ratios shown in Table 6 by blending dried powder or wetted powder (using xylene as a solvent). The resulting fluoropolymer alloys are fabricated by melt compression at a pressure of 70-150 Kg/cm.sup.2.

EHMW-FEP is mixed with polypropylene (pp) in the weight ratios shown in Table 7 by blending dried powder or wetted powder (using decalin as a solvent). The resulting fluoropolymer alloys are fabricated by melt compression under pressure of 70-150 Kg/cm.sup.2.

EHMW-FEP is mixed with polycarbonate (PC) in the weight ratios shown in Table 8 by blending dried powder or wetted powder (using chloroform as a solvent). The resulting fluoropolymer alloys are fabricated by melt compression at a pressure of 70-150 Kg/cm.sup.2.

EHMW-FEP is mixed with polyphenylene oxide (ppo) in the weight ratio shown in Table 9 by blending dried powder or wetted powder (using chloroform as a solvent). The resulting fluoropolymer alloys are fabricated by melt compression at a pressure of 70-150 Kg/cm.sup.2.

EHMW-FEP is mixed with polyphenylene sulfide (PPS) in the weight ratios shown in Table 10 by blending dried or wetted powder. The resulting fluoropolymer alloys are fabricated by melt compression at a pressure of 70-150 Kg/cm.sup.2.

EHMW-FEP is mixed with polyimide (PI) in the weight ratios shown in Table 11 by blending dried powder or wetted powder. The resulting fluoropolymer alloys are fabricated by melt compression at a pressure of 70-150 Kg/cm.sup.2 or cold compaction under pressure of 70-150 Kg/cm.sup.2 and sintering.

10 Grams of EHMW-FEP are mixed with 90 grams of molding or dispersion powder of PTFE. The resulting mixture was milled, and 40 mesh screened. The resulting dried powder of fluoropolymer alloy are moistened with 200E gasoline, cold compressed and sintered. This fluoropolymer alloy product can be used for lining valves and pipes.

40 Grams of fluoropolymer alloy made of EHMW-FEP are mixed with PTFE with glass fiber in the weight percentage of 10%, 20% and 30% respectively. The resulting mixture is cold compressed under pressure of 80 Kg/cm.sup.2 and sintered for 2 hours at 320.degree. C. separately. The tensile strength of these reinforced fluoropolymer alloy is 250. 220 And 150 Kg/cm.sup.2, respectively, at room temperature.

A fluoropolymer alloy made of 50 grams of EHMW-FEP and 50 grams of PTFE with glass fiber in a weight percentage of 20% and graphite in its weight percentage of 3, the resulting mixture is milled to pass through 40 mesh screen, cold compressed under pressure of 80 Kg/cm.sup.2 and then sintered for 2 hours at 320.degree. C. The tensile strength, elongation and hardness (Rockwell) of the reinforced fluoropolymers alloys are 150 Kg/cm.sup.2, 220% and 58, respectively.

Process for preparing extra-high-molecular weight fluorinated ethylene-propylene copolymer, e.g. in a stainless-steel autoclave having a capacity of 130 liters are charged 60 liters of deionized water, 45 Kg of initial monomers containing 86.6% hexafluoropropylene. The contents are heated to 55.degree.-57.degree. C. at a pressure of 22.0 Kg/cm.sup.2, then 25 ml of diisopropyl percarbonate are added. The copolymerization is carried on for three hours. 7.5 Kg of dried copolymer powder having a hexafluropropylene weight ratio of 14.5%, a melt viscosity of 1.8.times.10.sup.6 poises and a melt flow index of 0.3 g/10 min could been obtained. A sample of the copolymer powder is molded into sheet with a thickness of 2 mm at 310.degree. C., which shows the tensile strength 290 Kg/cm.sup.2 elongation 320%; and flex fatigue life more than 2.times.10.sup.5 cycle under room temperature.