Method of making vinylidene fluoride polymer

Abstract: A method of manufacturing a vinylidene fluoride polymer wherein a vinylidene fluoride monomer and 1 to 5 wt.%, with respect to the total weight of the monomers, of hexafluoropropylene and/or tetrafluoroethylene comonomers are suspension-polymerized in an aqueous medium in the presence of an oil-soluble polymerization initiator, wherein: the logarithmic viscosity [η₁] of the polymer is from 1.3 to 1.9 dl/g from the start of the polymerization until the polymerization conversion rate of the vinylidene fluoride monomer is from 10 to 50%; and when the polymerization conversion rate is from 10 to 50%, a chain transfer agent is added to the polymerization system such that the logarithmic viscosity [η₂] of the polymer finally obtained is from 0.9 to 1.3 dl/g and also from 0.3 to 0.7 dl/g lower than [η₁].

Description: This invention relates to a method of manufacturing a vinylidene fluoride polymer (hereinafter referred to as "PVDF"), more particularly to a method of manufacturing PVDF by suspension polymerization. PVDF is now widely used as a material for pipes and valves in chemical plants, and as a lining and coating material for storage tanks and reaction vessels due to its excellent mechanical strength and chemical resistance. PVDF has such good melt-processability and thermal stability that it can be processed without recourse to, for example, thermal stabilizers and processing aids. Due to this unique feature, PVDF moldings have high purity. Its high purity, together with its excellent chemical resistance, often makes PVDF particularly suitable for materials for facilities for the manufacture and storage of ultra-pure water and ultra-pure chemicals used in semiconductor manufacture. PVDF is commercially manufactured by suspension polymerization and emulsion polymerization processes. In the suspension polymerization process monomers are dispersed in droplets in water as a suspension medium using a dispersant, and are polymerized using organic peroxides dissolved in the monomers to obtain granular polymers of sizes of from 100 to 300 microns. Suspension polymerization products are manufactured with a simpler process, and are easier to handle because they are granular, compared with emulsion polymerization products. In addition, suspension polymerization products are of higher purity than emulsion polymerization products because they do not contain emulsifiers or salting agents. Formation of large spherulites in moldings of suspension polymerization products, on the other hand, may be regarded as a drawback in some applications. PVDF is a crystalline resin which tends to form spherulites during cooling and solidification after melt processing, and it is generally known that the surface smoothness of PVDF moldings largely depends on the size of the spherulites. When PVDF having lowered surface smoothness due to the presence of large spherulites is used to manufacture pipes for ultra-pure water manufacturing lines, minute dents between the spherulites on the inside surface of the PVDF piping act as a source for the propagation of microorganisms, lowering the purity of the pure water. When used as coatings for reaction vessels and storage tanks, PVDF paint films having large spherulites are susceptible to stress cracking, reducing their durability. It is generally believed that the size of spherulites depends on the cooling rate of PVDF moldings after melt processing; the faster PVDF moldings are cooled, the finer spherulites are formed. In some molding methods, quenching may be impossible. During the extrusion of a thick-wall pipe, for example, when the extruded pipe is cooled from the external surface, large spherulites are formed because the inside of the pipe is not cooled so quickly, deteriorating the smoothness of the pipe inside. To overcome this problem, therefore, polymers having crystallization characteristics such that fine spherulites are easily formed even at a relatively low cooling rate have long been needed. Means for reducing the size of PVDF spherulites have previously been proposed as follows: (1) The size of spherulites of PVDF moldings can be reduced to some extent by increasing the molecular weight of the polymers. With this method, however, satisfactory effects cannot necessarily be accomplished. Conversely, as the molecular weight is increased, some undesirable phenomena, such as difficulty in processing due to increased melt viscosity, discolouring caused by the decomposition of polymers due to the increased processing temperature, and formation of hydrogen fluoride due to increased processing temperature, are encountered. (2) As proposed in US-A-3,701,749 and US-A-3,719,644, the spherulites of PVDF moldings can be reduced in size by adding a nucleating agent, such as flavanthrone and salt. Addition of these nucleating agents is not favourable because it could lower the thermal stability of PVDF, and lead to discolouring by the decomposition of polymers, as well as to the formation of hydrogen fluoride. Furthermore, these nucleating agents are impurities in polymers, making the polymers unsuitable for applications requiring high purity, such as ultra-pure water pipes. (3) It is known that the size of spherulites can be reduced by copolymerizing vinylidene fluoride monomers with, for example, tetrafluoroethylene, trifluoromonochloroethylene, hexafluoropropylene or vinyl fluoride. To sufficiently reduce the size of spherulites with this method, however, more then 10 wt.% of comonomers are required. As a result, the degree of crystallization of the polymers is lowered, and the crystalline melting point is also lowered remarkably. This leads to deterioration of the heat resistance, chemical resistance and mechanical strength, normally considered to be beneficial properties of PVDF. (4) US-A-3,798,287 has proposed a method of adding fluorine monomers which give a polymer having a higher crystallization temperature than PVDF after the polymerization of vinylidene fluoride monomers has been completed, so as to facilitate polymerization within the PVDF particles. With this process, however, vinyl fluoride severely reduces the thermal stability of the resulting PVDF. Post-addition of trifluoromonochloroethylene has less effect of reducing the size of PVDF spherulites, and also lowers the thermal stability of PVDF. Post-addition of tetrafluoroethylene, on the other hand, remarkably reduces the size of spherulites and improves the thermal stability of PVDF, but polytetrafluoroethylene does not disperse uniformly in PVDF, making the size of the spherulites of PVDF moldings uneven, and posing some problems in the surface smoothness of moldings. It is desired to provide a vinylidene fluoride polymer (PVDF) having such crystallization characteristics as to form fine spherulite sizes without sacrifices of, for example, physical properties, processability, and purity, and also to provide a suspension polymerization method to obtain the same. After years of research efforts to achieve these objectives, the present inventors discovered that the size of the resulting PVDF spherulites can be reduced materially by copolymerizing small amounts of comonomers, and yet controlling the molecular weight distribution to certain values during the suspension polymerization of vinylidene fluoride monomers. The present invention provides a method of manufacturing PVDF wherein a vinylidene fluoride monomer and 1 to 5 wt.%, with respect to the total weight of the monomers, of hexafluoropropylene and/or tetrafluoroethylene comonomers are suspension polymerized in an aqueous medium in the presence of an oil-soluble polymerization initiator (hereinafter referred to as "polymerization initiator"), wherein: the logarithmic viscosity [η₁] of the polymer is from 1.3 to 1.9 dl/g from the start of the polymerization until the polymerization conversion rate of the vinylidene fluoride monomer is from 10 to 50%; and when the polymerization conversion rate is from 10 to 50%, a chain transfer agent is added to the polymerization system such that the logarithmic viscosity [η₂] of the polymer finally obtained is from 0.9 to 1.3 dl/g and also from 0.3 to 0.7 dl/g lower than [η₁]. The vinylidene fluoride polymer produced may be further molded into a molded article. The PVDF produced has such fine spherulites that hitherto could not be obtained by the prior art in a suspension polymerization process. As a result, PVDF moldings having excellent surface smoothness can be obtained without sacrifices of physical properties, processability and purity. The amount of addition of hexafluoropropylene and/or tetrafluoroethylene in this invention is 1 to 5 wt.%, preferably 2 to 4 wt.%, of the total monomers. By using them in an amount of more than 1 wt.%, the spherulites in the PVDF moldings can be reduced to a sufficient degree, and by using less than 5 wt.%, the resulting PVDF has small spherulites, the crystalline melting point and crystallinity of the PVDF do not fall appreciably, and PVDF moldings produced therefrom have excellent heat resistance, mechanical strength and resistance to organic solvents. The hexafluoropropylene and/or tetrafluoroethylene may be added en bloc to a polymerization reactor at the time of initial charging, or a part or total amount of them may be added in separate doses or continuously during polymerization. In this invention, polymerization is effected so that the logarithmic viscosity [η₁] of polymers is 1.3 - 1.9 dl/g from the start of the polymerization until the polymerization conversion rate reaches 10 to 50%, preferably 20 - 45%. The term logarithmic viscosity used herein is a characteristic value correlated with molecular weight obtained by a measuring method, which will be described later. If [η₁] is less than 1.3 dl/g, the size of the spherulites in the resulting PVDF moldings is not sufficiently small, while if [η₁] exceeds 1.9 dl/g, unmelted PVDF remains at the time of melt molding, resulting in fish eyes, deteriorating the external appearance and surface smoothness of the moldings. The timing suitable for reducing logarithmic viscosity by adding a chain transfer agent in the course of polymerization is the time at which the polymerization conversion rate reaches 10 to 50%. If the chain transfer agent is added at a polymerization conversion rate of less than 10%, the spherulites of the eventual PVDF are not sufficiently small, while at a polymerization conversion rate of over 50%, a large amount of chain transfer agent is required to limit [η₂] to be 0.3 to 0.7 dl/g lower than [η₁], and to 0.9 to 1.3 dl/g, and yet the spherulites are still not necessarily small. The timing for adding the chain transfer agent in the manufacturing method of this invention, that is, the time at which the polymerization conversion rate is 10 to 50% and [η₁] is 1.3 to 1.9 dl/g, can be determined by the change in pressure or polymerizing time at which a predetermined polymerization conversion rate is reached in the following manner; by obtaining in advance through preliminary experiments the change in pressure and polymerization conversion rate when a relatively large amount of hexafluoropropylene is charged at the initial stage of polymerization, for example; or by obtaining the polymerization time and polymerization conversion rate for such a polymerization involving less pressure changes. In this invention, the logarithmic viscosity [η₂] of the eventually obtained PVDF must be kept at 0.9 to 1.3 dl/g. If this value is less than 0.9 dl/g, the size of the spherulites in moldings is not sufficiently small, resulting in poor surface smoothness of the PVDF moldings and lowered impact strength. In some cases, this tends to cause stress cracks. If this value exceeds 1.3 dl/g, the melt viscosity of the PVDF increases, making melt molding difficult. Furthermore, [η₂] is preferably lower than [η₁] by 0.3 to 0.7 dl/g, preferably by 0.4 to 0.5 dl/g. The difference between [η₂] and [η₁] is related to the molecular-weight distribution of the resulting PVDF. The molecular-weight distribution can be expressed by the ratio (Mw/Mn) of weight-average molecular weight (Mw) to number-average molecular weight (Mn). If [η₂] is over 0.7 dl/g smaller than [η₁]. the molecular-weight distribution of the resulting PVDF exceeds 2.8. This results in lowered impact resistance and the loss of melting uniformness, adversely affecting the processability of the PVDF and the molding characteristics, such as increasing the fish eyes in the moldings. In addition, if the difference between both is less than 0.3 dl/g, the molecular-weight distribution of the resulting PVDF is less than 2.2, and the effect of sufficiently reducing the size of the spherulites cannot be accomplished. The logarithmic viscosity of the PVDF is determined by the polymerization temperature, the type and amount of polymerization initiator and the type and amount of chain transfer agent. That is, with the same types of polymerization initiator and chain transfer agent (polymerization aid) used, an increased polymerization temperature lowers the logarithmic viscosity, while a lowered polymerization temperature increases the logarithmic viscosity. With polymerization conditions kept constant, except for polymerization initiator or chain transfer agent, increasing the amounts of these polymerization aids lowers the logarithmic viscosity, whereas decreasing the amounts of these polymerization aids increases the logarithmic viscosity. By obtaining in advance through preliminary experiments the correlationship among polymerization temperature, the formulations of polymerization initiator and chain transfer agent, and logarithmic viscosity, the desired logarithmic viscosity can be set. Furthermore, the amount of addition of chain transfer agent at a polymerization conversion rate of 10 to 50% can also be determined in the same manner. By determining the types of polymerization initiator and chain transfer agent in this way, the desired logarithmic viscosity can be easily determined by adjusting the amounts. Chain transfer agents usable in this invention include acetone, isopropyl acetate, ethyl acetate, diethyl carbonate, dimethyl carbonate, pyroethyl carbonate, propionic acid, trifluoroacetate, trifluoroethyl alcohol, formaldehyde dimethylacetal, 1,3-butadiene epoxide, 1,4-dioxane, β-butyllactone, ethylene carbonate and vinylene carbonate. When taking into consideration the capability of effectively lowering the logarithmic viscosity, the maintenance of the thermal stability of PVDF, the ease of availability, and the ease of handling, acetone and ethyl acetate, particularly ethyl acetate, are more desirable. Polymerization initiators usable in this invention include di-n-propyl peroxydicarbonate (NPP), and diisopropyl peroxydicarbonate. The types and amounts of these polymerization initiators and chain transfer agents are selected so as to obtain a predetermined logarithmic viscosity, and one or more than two of them can be used in combination. Typical dispersants usable in this invention include partially saponified polyvinylacetate, water-soluble cellulose ethers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose and water-soluble polymers such as polyacrylate and gelatin. The ratio of water/monomers is usually 1.5/1 to 3/1 in terms of weight ratio, and 0.01 to 0.1 parts by weight of dispersant are used for 100 parts by weight of monomers. In addition, this invention can use pH buffers, such as polyphosphates. As the method of charging water, monomers, dispersant, polymerization initiator, and other polymerisation aids in this invention, any methods that can be used for normal suspension polymerization may be employed. For example, as water, dispersant, polymerization initiator, chain transfer agent and other polymerization aids are charged, deaeration is carried out by decompression, monomers are subsequently charged, and stirring is started. After heating to a predetermined temperature, polymerization is started at that temperature, and the chain transfer agent is injected when the polymerization conversion rate reaches 10 to 50%, and the polymerization is further continued. As the polymerization proceeds to such an extent that the pressure in the polymerization reactor drops by more than 10 kg/cm² from the equilibrium pressure of the initial monomer mixture (that is, to the extent that the polymerization conversion rate reaches over 80%), unreacted monomers are recovered, and the resulting polymers are dewatered, flushed with water and dried. The PVDF obtained with the above manufacturing method consists of a monomer unit content of vinylidene fluoride of 99.5 to 96 wt.%, preferably 99.0 - 97.0 wt.%, and a monomer unit content of the comonomers of 0.5 to 4 wt.%, preferably 1.0 to 3.0 wt.%. The PVDF is a random copolymer. The logarithmic viscosity [η₂] is 0.9 to 1.3 dl/g, as noted earlier, and the molecular-weight distribution (Mw/Mn) is 2.2 to 2.8, preferably 2.3 to 2.6. The size of PVDF spherulites, as measured by the method which will be described later, is 1 to 30 µm, preferably 5 to 25 µm, and the crystalline melting point is 163 to 176°C, preferably 168 to 173°C. The present invention is now further described in the following Examples and Comparative Examples. Percentages and part numbers used in these Examples and Comparative Examples are expressed in terms of weight, unless otherwise specified. The properties of PVDF in the Examples and Comparative Examples were measured by the following methods: (1) Molecular-weight distribution (Mw/Mn) A dimethylacetamide solution in which the polymer powder was dissolved at a concentration of 0.2 wt.% was subjected to a gel-permeation chromatograph (manufactured by Tosoh Corp.; 8010 Series, Column TSK-GEL GMHXL, dia. 7.8mm, length 300mm, 2 columns in series; temperature 40°C, flow rate 0.8ml/min.) to measure the ratio (Mw/Mn) of weight-average molecular weight (Mw) to number-average molecular weight (Mn) . (2) Composition analysis of polymers Measurements were made using ¹⁹F NMR. (3) Crystalline melting point The endothermic peak was measured with a differential scanning calorimeter when the PVDF was heated at a rate of 10°C/min, and the temperature at that time was taken to be the crystalline melting point. (4) Logarithmic viscosity The logarithmic viscosity was calculated using the following equation on the basis of the dropping time of a 30°C solution obtained by dissolving the polymer powder into dimethylformamide at a concentration of 4 g/l on an Ubbelohde's viscometer. Logarithmic viscosity [η] = 1n(η rel )/C dl/g where η rel = dropping time (s) of sample solution/dropping time (s) of solvent C = Concentration of sample solution (0.4 g/dl). (5) Spherulite size A polymer powder was mixed for three minutes in a roll having a roll surface temperature of 165°C, then 0.1 g of the mixture was pressed by applying a pressure of 100 kg/cm² at a temperature of 240°C for three minutes. The pressed mixture, while kept in the pressurized state, was cooled at a constant rate to 100°C in 1 hour. The spherulites in the resulting approx. 20 µm-thick film were photographed with a polarized microscope to measure the average value of the spherullte size. (6) Yield-point strength As in the case of (5) above, a 1mm-thick pressed sheet was prepared, and Type-3 dumbbell specimens were taken from the sheet and subjected to measurement of yield point strength at a tension rate of 10 mm/min. and a temperature of 23°C according to ASTM-D638. (7) Izod impact strength The roll-mixed sheet obtained in (5) above was pressed for six minutes at a temperature of 240°C and a pressure of 100 kg/cm² to obtain a 6 mm-thick pressed sheet. In accordance with ASTM-D256, the impact strength of V-notched (R = 0.25 mm) specimens was measured. (8) Melt viscosity The roll-mixed sheet obtained in (5) above was cut, and the melt viscosity of the sheet at a temperature of 240°C and a shear rate of 50 sec⁻¹ was measured using a capillograph (manufactured by Toyo Seiki Co., Ltd.). (9) Fish eyes The roll-mixed sheet obtained in (5) above was pressure-formed into a 1 mm-thick sheet at a temperature of 240°C and a pressure of 100 kg/cm². This sheet is placed between an iron plate having an air-blow hole, and an iron plate having a 12cm-dia. circular opening via a rubber packing, and held in place with a clamp. While heating the sheet on the opening side to a surface temperature of approximately 150°C, the sheet was inflated to a thickness of approximately 300 µm by injecting the air from the air-blow hole. Then, the number of fish eyes (unmelted material) contained in a 5cmx5cm area of the resulting film was counted. (10) Smoothness of pipe inside In accordance with JIS B0601, the lengthwise smoothness of the inside of a pipe was measured with a surface roughness tester (manufactured by Tokyo Seimitsu Co., Ltd.: Surfcom 550A Type) to obtain the average roughness (Ra) for a reference length of 2.5 mm. Example 1 The following monomers and polymerization aids, etc. were charged in a stainless steel autoclave having an inner capacity of 14 liters, and a polymerization reaction was started at 25°C. Vinylidene fluoride 97.5 parts (3,000g) Hexafluoropropylene 2.5 parts Pure water 300.0 parts Methyl cellulose 0.1 parts Sodium pyrophosphate 0.2 parts NPP 0.61 parts After 3 hours from the start of polymerization (polymerization conversion rate:35%), 3.0 parts of ethyl acetate was added and the polymerization reaction was continued. When the inside pressure of the polymerization reactor dropped to 25 kg/cm² below the equilibrium pressure (39 kg/cm²) after the start of polymerization, unreacted monomers were recovered, and the resulting polymer slurry was dewatered, flushed with water and dried. The chemical and physical properties of the resulting polymer are shown in Table 1. Example 2 - 5 The polymerization was conducted in the same manner as in Example 1 except that the amounts of NPP and ethyl acetate and the timing of their additions, [η₁] of the polymer at the addition of ethyl acetate, and [η₂] of the polymer obtained eventually were changed as shown in Table 1. The chemical and physical properties of the resulting polymer are shown in Table 1. Example 6 The polymerization was conducted in the same manner as in Example 1, except that the amount of hexafluoropropylene was changed to 4%. The chemical and physical properties are shown in Table 1. Example 7 The polymerization was conducted in the same manner as in Example 1, except that the amounts of NPP and ethyl acetate, and polymerization temperature were changed as shown in Table 1. The chemical and physical properties of the resulting polymer are shown in Table 1. Example 8 The polymerization was conducted in the same manner as in Example 1, except that the hexafluoropropylene was replaced with tetrafluoroethylene. The chemical and physical properties are shown in Table 1. Comparative Examples 1 - 5 The polymerization was conducted in the same manner as in Example 1, except that the amounts of NPP and ethyl acetate, and the timing of their additions, [η₁] of the polymer at the addition of ethyl acetate, and [η₂] of the polymer obtained eventually were changed as shown in Table 2. The chemical and physical properties of the resulting polymer are shown in Table 2. Comparative Example 6 The polymerization was conducted in the same manner as in Example 1, except that the amount of hexafluoropropylene was changed to 6%. The chemical and physical properties of the resulting polymer are shown in Table 2. Comparative Example 7 The polymerization was conducted in the same manner as in Example 1, except that the amount of NPP was increased and ethyl acetate was not added in the course of reaction. The chemical and physical properties are shown in Table 2. Comparative Example 8 The polymerization was conducted in the same manner as in Example 1 except that hexafluoropropylene was not used. The chemical and physical properties of the resulting polymer are shown in Table 2. Comparative Example 9 The polymerization was conducted in the same manner as in Comparative Example 7, except that hexafluoropropylene was not used. The chemical and physical properties of the resulting polymer are shown in Table 2. Example 9, Comparative Examples 10 and 11 The polymerization was conducted in a stainless steel autoclave having an inner capacity of 6,300 liter with the same polymerization formulations as with Example 1 and Comparative Examples 8 and 9, in which 1,200 kg of vinylidene fluoride was used (Example 9, Comparative Examples 10 and 11). The resulting polymer was molded into a pipe having an outside diameter of 100 mm and a thickness of 6 mm using a 90 mm-dia. single-screw extruder having a cylinder temperature of 180°C under the hopper and 240°C at the tip thereof, and a die temperature of 230°C through a vacuum sizing die equipped with a water-cooled jacket. The average surface roughness (Ra) of the inside of the resulting pipe was as good as 0.3 µm (Example 9) with the polymer produced from the formulation of Example 1, compared with 0.9 µm (Comparative Example 10) with the polymer produced from the formulation of Comparative Example 8, and 2.2 µm (Comparative Example 11) with the polymer produced from the formulation of Comparative Example 9.

Claims: A method of manufacturing a vinylidene fluoride polymer wherein a vinylidene fluoride monomer and 1 to 5 wt.%, with respect to the total weight of the monomers, of hexafluoropropylene and/or tetrafluoroethylene comonomers are suspension-polymerized in an aqueous medium in the presence of an oil-soluble polymerization initiator, wherein: the logarithmic viscosity [η₁] of the polymer is from 1.3 to 1.9 dl/g from the start of the polymerization until the polymerization conversion rate of the vinylidene fluoride monomer is from 10 to 50%; and when the polymerization conversion rate is from 10 to 50%, a chain transfer agent is added to the polymerization system such that the logarithmic viscosity [η₂] of the polymer finally obtained is from 0.9 to 1.3 dl/g and also from 0.3 to 0.7 dl/g lower than [η₁]. A method according to claim 1 wherein the logarithmic viscosity [η₁] is from 1.3 to 1.9 dl/g from the start of polymerization until the polymerization conversion rate of the vinylidene fluoride monomer is from 20 to 45%, and the chain transfer agent is added to the polymerization system when the polymerization conversion rate is from 20 to 45%. A method according to claim 1 or 2 wherein the logarithmic viscosity [η₂] is from 0.4 to 0.5 dl/g lower than [η₁]. A method according to any one of the preceding claims wherein the chain transfer agent is ethyl acetate. A method according to claim 1 wherein the vinylidene fluoride polymer comprises from 99.0 to 97 wt.% of vinylidene fluoride units and from 1.0 to 3 wt.% of hexafluoropropylene and/or tetrafluoroethylene units, the ratio (Mw/Mn) being from 2.3 to 2.6. A method according to claim 1 wherein the crystalline melting point of the vinylidene fluoride polymer is from 168 to 173°C and the spherulite size is from 5 to 25 µm. A method according to any one of the preceding claims which further comprises molding the vinylidene fluoride polymer into a molded article.

Inventors: HORIE KATSUO, SUZUKI FUJIO, WAKAMORI HIDEKI, HORIE, KATSUO, SUZUKI, FUJIO, WAKAMORI, HIDEKI
Applicants: KUREHA CHEMICAL IND CO LTD, KUREHA CHEMICAL INDUSTRY CO., LTD.
Application year: 1992
Main classification: C08F214
IPC codes: C08F214