AND FIBERS

FIELD

[0001] The present disclosure relates to partially fluorinated thermoplastic polymers comprising repeating units derived from tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride. The polymers have a melting temperature of at least 170 °C, and an MFI (265/5) of at least 35 grams/10 minutes. The disclosure also relates to products, including fibers, containing such partially fluorinated thermoplastic polymers.

SUMMARY

[0002] Briefly, in one aspect, the present disclosure provides a THV-based thermoplastic polymer according to claim 1 to 7.

[0003] In another aspect, the present disclosure provides fibers comprising THV-based thermoplastic polymers, including core/sheath fibers, according to claims 8 to 15.

[0004] The details of one or more embodiments of the invention are also set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

DETAILED DESCRIPTION

[0005] Fluoropolymers are used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability, and optical properties including transparency and low refractive index. Tetrafhioroethylene (TFE) homopolymers (PTFE) are highly resistant materials with a very high service temperature. However, PTFE is not melt-processable by standard melt extrusion equipment because of its extremely high melt viscosity. Therefore, various TFE copolymers have been developed that are thermoplastic and have a reduced melt viscosity making them melt-processable. Even then, specialized melt-processing equipment is required to address the high viscosity and high processing temperatures required by fluoropolymers, inhibiting their use in some applications.

[0006] Perfluorinated polymers include, for example, co-polymers of TFE with perfluorinated monomers such as hexafluoropropylene (HFP), perfluorovinyl ethers (PVE) and perfluoroallyl ethers (PAE). Examples of such fluoropolymers include copolymers of TFE and perfluorinated vinyl ethers (PFA) and copolymers of TFE and HFP (FEP). Such fully fluorinated polymers have high melting points and are used in extrusion coatings, for example in the cable and wire industry. However, such polymers still must have relatively high molecular weights in order to obtain acceptable mechanical properties such as crack resistance, resulting in high melt flow indices.

[0007] Partially fluorinated polymers such as THV, which is a copolymer of TFE, HFP and vinylidene fluoride (VDF), are also known. Some THV polymers are elastomeric, making them poor materials for high-speed forming processes. Thermoplastic THV polymers are also available, such as THV 500GZ, THV 610GZ, and THV 815GZ, available from 3M Company, St. Paul, MN. While these thermoplastic polymers have high melting points (e.g., 165 to 225 °C), like FEP and PFA, they also have high molecular weights and correspondingly low MFI, e.g., MFI (265/5) of less than 25, or even less than 15 g/10 minutes. Thus, there remains a need for melt-processable thermoplastic fluoropolymers having higher melt flow indices, e.g., MFI (265/5) of at least 35 g/10 minutes, while retaining high melting points, for example at least 170 °C.

[0008] The partially fluorinated thermoplastic polymers of the present disclosure are THV-based copolymers. This means they comprise a copolymer of units derived from tetrafluoroethene (TFE), hexafluoropropylene (HFP), and vinylidene fluoride (VDF). In some embodiments, the THV-based copolymers comprise at least 90, e.g., at least 95, or even at least 99 wt.% of units derived from TFE, HFP, and VDF.

[0009] The present inventors discovered that, by controlling the relative amounts of the TFE, HFP, and VDF monomeric repeat units; and the molecular weight of the copolymer, fluoroplastic polymers having the desired balance of melting point and melt flow index could be produced. Surprisingly, the resulting fluoroplastic polymers had sufficient strength to be processed at high speeds to form small diameter fibers and thin fiber sheaths, while remaining flexible enough to avoid brittle fracture when handled and bent. In some embodiments, the polymers also have small enough crystallite sizes to retain desired optical properties.

[0010] The THV-based copolymers of the present disclosure comprise 55 to 65 mole percent of tetrafluoroethylene (TFE), 7 to 17 mole percent of hexafluoropropylene (HFP), and 25 to 35 mole percent of vinylidene fluoride (VDF), based on the total moles of monomeric repeating units in the polymer. In some embodiments, the copolymers comprise 58 to 63, or even 58 to 61 mol% TFE. In some embodiments, the copolymers comprise 10 to 14, or even 11 to 13 mol% HFP. In some embodiments, the copolymers comprise 27 to 31, or even 28 to 30 mol% VDF. For example, in some embodiments, the copolymer comprises 58 to 63 mol% TFE, 10 to 14 mol% HFP, and 27 to 31 mol% VDF; or 58 to 61 mol% TFE, 11 to 13 mol% HFP, and 28 to 30 mol% VDF.

[0011] The THV-based copolymers according to the present description are melt-processable. A fluoropolymer is melt-processable if the melt viscosity of the polymer is low enough such that the polymer can be processed in conventional extrusion equipment used to extrude polymers. Typically, a fluoropolymer is considered melt-processable if it has a melt flow index (MFI) at 265 °C and a 5 kg load (referred to as “MFI (265/5)”) of at least 1 gram/10 minutes.

[0012] However, such a low MFI is not suitable for some applications such as high-speed forming applications (e.g., high-speed fiber forming) or processes requiring high elongational strength. Therefore, the THV-based polymers according to the present disclosure have a melt flow index at 265 °C and a 5 kg load of at least 35 g/10 min, as determined according to ASTM D-1238. In some embodiments, the MFI (265/5) is at least 40, e.g., at least 60 or even at least 80 g/10 min. In some applications, very high MFI polymers may be difficult to process, and these materials may have insufficient mechanical properties for the end-use application. Therefore, in some embodiments, the MFI (265/5) is no greater than 500, e.g., no greater than 300, no greater than 200, or even no greater than 150 g/10 min. For example, in some embodiments, the THV-based polymers have an MFI (265/5) of 35 to 500, e.g., 40 to 300, 60 to 200, or even 80 to 150 g/10 min.

[0013] Long chain branching and the use of bulky side chains, introduced by, e.g., PVEs and PAEs have been used to retain desired mechanical properties at higher MFIs. However, such approaches require the use of expensive monomers, increase the complexity and cost of polymerization, and can result in undesirable gels. Surprising the higher MFI materials of the present disclosure can be obtained without the use of such approaches, resulting in materials having the desirable mechanical and optical properties. [0014] The melting behavior of the polymers can be measured by differential scanning calorimetry

(DSC). Generally, the THV-based copolymers have a broad melting range. As the materials melt over a range of temperatures, the melting temperature of the polymer (Tm) may be reported as Tp, which corresponds to the temperature at the peak of the heat flow versus temperature curve. The THV-based copolymers according to the present disclosure have a peak melting point (Tp) of at least 170 °C, e.g., from 170 to 230 °C. In some embodiments, Tp is at least 180, or even at least 190 °C. In some embodiments, Tp is no greater than 215, or even no greater than 200 °C.

[0015] In some embodiments, a distinct melting peak may be hard to discern. In some embodiments, the melting temperature may be reported as T50, which corresponds to the temperature at which 50% by mass of the crystalline portion of the THV-based copolymer has melted.

[0016] Regardless of whether it is reported at Tp or T50, the THV-based copolymers according to the present disclosure have a melting point of at least 170 °C, e.g., from 170 to 230 °C. In some embodiments, the melting point is at least 180, or even at least 190 °C. In some embodiments, the melting point is no greaterthan 215, or even no greater than 200 °C.

[0017] A broadened melting transition results in a material whose modulus also transitions over a broader temperature range. This attribute may be desirable in post-processing techniques such as drawing and thermo-forming. This broader modulus transition is also advantageous when co-processing with dissimilar materials. Therefore, in some embodiments, it may be desirable to broaden the temperature range of the melting transition. In some embodiments, the composition may be a blend of different THV- based copolymers that results in a composition with a very broad melt transition. Such a transition may be achieved by either blending different copolymers or by changing the monomer feed ratio during the polymerization. In other instances, small amounts of other fluoropolymers such as FEP may be blended with the THV-based co-polymer to raise and or broaden the melting point transition. The resulting material will have a broader melting distribution.

[0018] In some embodiments, the THV-based copolymers do not include bulky side chains introduced by copolymerization with monomers such as perfluorovinyl ethers (PVE) or perfluoroallyl ethers (PAE). For example, in some embodiments, at least 99, at least 99.5, or even at least 99.9 mol% of the THV- based copolymer consists of monomeric repeating units selected from TFE, HFP, and VDF. However, in some embodiments, the copolymers may contain small amounts, e.g., 0.1 to 5 mol% of monomeric repeating units selected from PVE and PAE. Generally, the perfluorovinyl and perfluoroallyl ethers correspond to formula (I):

CF ₂ =CF-(CF ₂ )n-0-Rf wherein n is 0 (vinyl ether) or 1 (allyl ether); and Rf represents a perfluorinated aliphatic group that may contain one or more oxygen atoms. In some embodiments, the THV-based copolymers comprise at least 0.1, e.g., at least 0.2, or even at least 0.5 mol% of repeating units corresponding to such monomers. In some embodiments, the THV-based copolymers comprise 0.2 to 3, or even 0.5 to 2 mol% of monomeric repeating units corresponding to the monomers of formula (I).

[0019] Generally, the THV-based fluoropolymers do not have so-called long chain branches or only insignificantly amounts thereof. That is, the polymers are linear or substantially linear, in that the only branches that are present are introduced to the polymer backbone by the TFE, HFP, VDF, and optional PVE and PAE monomers used.

[0020] In some embodiments, long chain branches may be introduced into the backbone by using specific branching modifiers in the polymerization. Such modifiers are either bisolefms and/or monoolefms containing iodine and/or bromine atoms. Without intending to be bound by theory, it is believed that long chain branches result from abstraction of the bromine or iodine atom from the modifier once it is polymerized into the backbone of the fluoropolymer. The so-produced radical on the backbone may then cause further polymerization with the result that a polymeric chain is formed as a branch on the backbone. Such branches are known in the art as long chain branches or “LCB’s”.

[0021] Methods of preparing THV-based polymers are well-known. Suitable methods are described in, e.g., EP 2170990B1 (“Methods for Melt-Processing Thermoplastic Fluoropolymers,” Kaspar, et ak, granted 5 October 2016) and WO 2014/031252 (“Semi-Fluorinated Thermoplastic Resins with Low Gel Content,” Kaspar, et ak, published 27 February 2014).

[0022] In some embodiments, aqueous emulsion polymerization may be used. The reactor vessel for use in the aqueous emulsion polymerization process is typically a pressurizable vessel capable of withstanding the internal pressures during the polymerization reaction, Typically, the reaction vessel will include at least one mechanical agitator, which will produce thorough mixing of the reactor contents and heat exchange system. Any quantity of the fluoromonomer(s) may be charged to the reactor vessel. The monomers may be charged batch-wise or in a continuous or semi-continuous manner. The independent rate at which the monomers are added to the kettle will depend on the consumption rate of the particular monomer with time. Preferably, the rate of addition of monomer will equal the rate of consumption of monomer, i.e. conversion of monomer into polymer.

[0023] The aqueous emulsion polymerization reaction kettle may be charged with water, the amounts of which are not critical, to provide an aqueous phase. To the aqueous phase is generally also added a surfactant, e.g., a fluorinated polyether surfactant described below. At least a part of the fluorinated polyether surfactants is added to the reaction mixture as an aqueous mixture with at least one fluorinated liquid as described below. The fluorinated surfactant is typically used in amounts of about 0.01% by weight to about 1% by weight based on the aqueous phase of the polymerization system.

[0024] Fluorinated polyether surfactants include those according to general formula (II):

Rf-0-L-C0 ₂ - X ⁺ ; wherein Rf is selected from a partially fluorinated alkyl group, a fully fluorinated alkyl group, a partially fluorinated alkyl group that is interrupted with one or more oxygen atoms, and a fully fluorinated alkyl group that is interrupted with one or more oxygen atoms, wherein Rf may be linear or branched; L is selected from a partially fluorinated alkylene group, a fully fluorinated alkylene group, a partially fluorinated alkylene group that is interrupted with one or more oxygen atoms, and a fully fluorinated alkylene group that is interrupted with one or more oxygen atoms; X ⁺ represents a cation or H ⁺ . L may be branched but preferably L is linear. More preferably, both L and Rf are linear. Preferably either Rf or L or both contain a partially fluorinated group.

[0025] The polymerization is usually initiated after an initial charge of monomer by adding a radical initiator or initiator system to the aqueous phase. For example, peroxides can be used as free radical initiators. Specific examples of peroxide initiators include, hydrogen peroxide, diacylperoxides such as diacetylperoxide, dipropionylperoxide, dibutyrylperoxide, dibenzoyiperoxide, benzoyl acetylperoxide, diglutaric acid peroxide and dilaurylperoxide, and further water soluble per-acids and water soluble salts thereof such as e.g. ammonium, sodium or potassium salts. Examples of per-acids include peracetic acid. Esters of the peracid can be used as well and examples thereof include tertiary- butyl peroxyacetate and tertiary-butylperoxypivalate. A further class of initiators that can be used are water soluble azo compounds. Suitable redox systems for use as initiators include for example a combination of peroxodisulphate and hydrogen sulphite or disuiphite, a combination of thiosulphate and peroxodisulphate or a combination of peroxodisulphate and hydrazine. Further initiators that can be used are ammonium- alkali- or earth alkali salts of persulfates, permanganic or manganic acid or manganic acids. The amount of initiator employed is typically between 0.003 and 2 % by weight, preferably between 0.005 and 1 % by weight based on the total weight of the polymerization mixture. The full amount of initiator may be added at the start of the polymerization or the initiator can be added to the polymerization in a continuous way during the polymerization. One can also add part of the initiator at the start and the remainder in one or separate additional portions during the polymerization.

[0026] During the initiation of the polymerization reaction, the sealed reactor kettle and its contents are conveniently pre-heated to the reaction temperature. Exemplary temperatures may be from 20 °C, from 30 °C, or even from 40 °C and may further be up to 100 °C, up to 120 °C, or even up to 150 °C. The polymerization pressure may range, for instance, from 400 to 3000 kPa (4 to 30 bar), e.g., from 800 to 2000 kPa (8 to 20 bar).

[0027] The THV-based thermoplastic polymers of the present disclosure are prepared using a chain transfer agent to control the molecular weight distribution. The chain transfer agent may be added, for example, prior to the initiation of the polymerization. Useful chain transfer agents include C2 to CIO hydrocarbons such as ethane, alcohols, ethers, esters including aliphatic carboxylic acid esters and malonic esters, and ketones. Particularly useful chain transfer agents are hydrocarbons. Further additions of chain transfer agent in a continuous or semi-continuous way during the polymerization may also be carried out.

[0028] Following polymerization, the resulting dispersion may be purified to remove fluorinated surfactants by known methods such as the use of ion exchange. The polymers can be isolated by coagulation, for example by physical coagulation (freezing), mechanical coagulation (increased shear force) or salt-induced coagulation, as known in the art.

[0029] The TFE-based polymers of the present disclosure may be used in a wide variety of applications, including extruded sheets and tubes, and molded articles. However, the balance of properties obtained with these polymers allows them to be used in high-speed fiber-forming processes. In some embodiments, the THV-based polymers may be formed into fibers, e.g., fine fibers having a diameter of no greater than 50 microns. In some embodiments, the fibers have a diameter of no greater than 25 microns, or even no greater than 15 microns. In some embodiments, the fiber diameter is from 1 to 50 microns, e.g., 5 to 25 microns, or even 10 to 25 microns.

[0030] In some embodiments, the THV-based polymers may be formed into a sheath surrounding a fiber. In some embodiments, the core may be an inorganic material such as a glass or metal. In some embodiments, the core may be polymeric materials, preferably a thermoplastic polymer that can be co processed with the THV-based sheath material. Suitable thermoplastic polymers include, e.g., polyolefin (e.g., polypropylenes and polyethylenes), polyesters (e.g., PET, PBT, and PEN, as well co-polyesters), polycarbonates, or polystyrenes.

[0031] In some embodiments, the sheath/core fibers can be spun as individual fibers. In some embodiments, multiple sheath/core fibers can be spun together to form a multifilament. In some embodiments, an “islands in the sea” configuration can be prepared by spinning multiple sheath/core fibers with a matrix material to surround and fill in gaps between the individual sheath/core fibers. Methods of forming such structures are known in the art.

[0032] In some embodiments, the diameter of the sheath/core fiber (including both the core and the sheath) is no greater than 50 microns. In some embodiments, such fibers have a diameter of no greater than 25 microns, or even no greater than 15 microns. In some embodiments, the fiber diameter is from 5 to 50 microns, e.g., 10 to 50 microns, or even from 10 to 25 microns.

[0033] In some embodiments, the thickness of the THV-based sheath is no greater than 2 microns, e.g., no greater, than 1 micron, or even no greater than 0.7 microns. In some embodiments, the thickness of the THV-based sheath is from 0.3 to 2 microns, e.g., 0.4 to 1, or even 0.4 to 0.7 microns.

[0034] Examples

[0035] The melt flow index was measured according to ASTM D- 1238 at a support weight of 5.0 kg and a temperature of 265 °C (i.e., “MFI(265/5)”). An extrusion die having a diameter of 2.1 mm and a length of 8.0 mm was used. Results are reported in grams per 10 minutes. [0036] The melting temperature was measured using differential scanning calorimetry (DSC) according to ASTM 4591 using a Perkin-Elmer DSC 7.0 (PerkinElmer Inc., Wellesley, MA) under nitrogen flow at a heating rate of 10 °C per minute. As the materials melt over a range of temperatures, the reported value, Tp, corresponds to the temperature at the peak of the heat flow versus temperature curve. The data generated were also used to calculate T50, which corresponds to the temperature at which

50% by mass of the crystalline portion of the THV-based copolymer has melted.

[0037] As used herein, references to standards and test methods refer to the most recent active standard or test method as of the earliest priority date of the present application.

[0038] Example EX-1. A terpolymer of 59.5 mole % TFE, 11.8 mole % HFP and 28.7 mole % VDF with a straight linear chain topography was prepared in a polymerization kettle with a total volume of 990 liters equipped with an anchor blade agitator system according to the following procedure. The kettle was charged with 560 liters of deionized water; 40 grams oxalic acid, 250 grams ammonium oxalate and 5600 grams of a 30 weight % aqueous solution of ADONA™ emulsifier (from 3M Company, St. Paul, MN). The oxygen free kettle was then heated up to 60 °C and the agitation system was set to 90 rpm. The kettle was charged with ethane to a pressure of 1200 kPa (1.2 bar) absolute; with hexafluoropropylene (HFP) to a pressure of 1020 kPa (10.2 bar) absolute; with vinylidene fluoride (VDF) to 1240 kPa (12.4 bar) absolute; and with tetrafluoroethylene (TFE) to 1700 kPa (17.0 bar) absolute reaction pressure.

[0039] The polymerization was initiated by the addition of 500 milliliters of a 1.0 wt.% aqueous potassium permanganate (KIVInOq) solution and a continuous feed of the KIVInOq solution was maintained with a feed rate of 1500 milliliters per hour. After the reaction started, the reaction temperature of 60 °C and the reaction pressure of 1700 kPa (17.0 bar) absolute was maintained by feeding TFE, VDF and HFP into the gas phase with a HFP (kg)/TFE (kg) feeding ratio of 0.296 and a VDF (kg)/TFE (kg) feeding ratio of 0.308. When a total feed of 230.6 kg TFE was reached within 330 minutes, the monomer feed was interrupted by closing the monomer valves. Within 10 minutes, the monomer gas phase was reacted down to a kettle pressure of 920 kPa (9.2 bar). Then the reactor was vented and flushed with nitrogen in three cycles.

[0040] Example EX-2. A terpolymer of 59.5 mole % TFE, 11.8 mole % HFP and 28.7 mole % VDF with a straight linear chain topography was prepared in the same manner as Example Ex-1, except that the amount of chain transfer agent was increased by charging the reaction kettle with ethane to a pressure of 1400 kPa (1.4 bar) absolute, rather than 1200 kPa as was done in EX-1.

[0041] The properties of EX-1 and EX-2 are summarized in Table 1.

Table 1: Properties of THY polymers.