CONTENT OF FLUOROPOLYMER COMPOSITIONS

FIEUD

[0001] The present disclosure relates to methods of reducing the amounts of perfluoroalkanoic acids and their corresponding salts in fluoropolymers. These methods include reacting the perfluoroalkanoic acid with an iodo-compound to convert the carboxylic acid group to a carbonyl iodide or iodide group. The methods may include heat-treating the resulting composition to remove these reaction products. Methods of forming fluoropolymer articles with reduced perfluoroalkanoic acid contents and the resulting articles are also described.

SUMMARY

[0002] Briefly, in one aspect, the present disclosure provides methods of removing perfluoroalkanoic acids from a fluoropolymer as well as articles made by these methods according claims 1-19. The present disclosure also provides fluoropolymer compositions useful in these methods according to claims 20 and 21

[0003] In some embodiments, the methods include (i) providing a first composition comprising the fluoropolymer, wherein the fluoropolymer contains a first amount of perfluorinated alkanoic acid(s) and an iodo-compound; (ii) reacting the iodo-compound with the perfluoroalkanoic acids at a first temperature greater than the melting temperature of the iodo-compound for a time sufficient to convert at least 25% of the amount of perfluoroalkanoic acid(s) to perfluoro-iodo reaction products selected from the group consisting of perfluoroalkyl carbonyl iodides, perfluoroalkyl iodides, and combinations thereof to form a second composition comprising the fluoropolymer containing the perfluoro-iodo reaction products; and (iii) processing the second composition at a second temperature for a time sufficient to volatilize at least 50% of the amount of the perfluoro-iodo reaction products to form a third composition comprising the fluoropolymer.

[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.

DETAIUED DESCRIPTION

[0005] Fluoropolymers are used in a variety of applications as they have several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability, and optical properties including transparency and low refractive index. A wide variety of perfluorinated and partially fluorinated polymers are available, including both fluorothermoplastics and fluoroelastomers. [0006] Historically, short chain perfluorinated alkanoic acids such as perfluorooctanoic acid (“PFOA”) have been used during the polymerization processes used to make fluoropolymers. For a variety of reasons, there have been many efforts to reduce or eliminate the use of such perfluorinated alkanoic acids. Alternatively, or in addition to such efforts, there have been other efforts to remove perfluorinated alkanoic acids from the polymerized fluoropolymer composition.

[0007] National and international regulations are trending toward the reduction in the permitted levels of these materials as detection technology advances. In addition, the scope of perfluorinated alkanoic acids that may be desirable to remove has expanded from Cq-Cg compounds to now include Cq-C 14. of even C9-C20 compounds. Such materials may inadvertently be present due to raw material or equipment contamination, and undesired side reactions occurring during polymerization. Therefore, despite the success of prior methods, there remains a need for additional approaches for removing perfluorinated alkanoic acids from fluoropolymer compositions.

[0008] Unlike some prior approaches, the methods of the present disclosure can be used to remove perfluorinated alkanoic acids present in the bulk polymer itself, rather than from the liquid phase of a dispersion or emulsion of the polymer, or from a liquid used to wash or otherwise extract the perfluorinated alkanoic acids from the bulk polymer.

[0009] The methods of the present disclosure can be used to remove both the perfluoroalkanoic acids and their corresponding salts. For simplicity, unless otherwise indicated, as used herein, the term “perfluoroalkanoic acid” refers to both the acid form

Rf-C(0)-0-H and its corresponding salts

Rf C(0)-0 +X where Rf is the perfluoroalkyl group and ⁺ X is the counterion of the salt.

[0010] The present inventors discovered that iodo-compounds can be added to a fluoropolymer to convert higher boiling point perfluorinated alkanoic acids to lower boiling point perfluoro-iodo reaction products. In subsequent steps, these lower boiling point perfluoro-iodo reaction products can then be volatilized and removed from the fluoropolymer composition, resulting in the reduction of the residual perfluoroalkanoic acid content in the polymer itself. In addition, any residual iodo-compounds may also be volatilized to remove them from the fluoropolymer.

[0011] As used herein, the term “iodo-compound” refers to the iodine-containing compounds added to the fluoropolymer composition; while the term “perfluoro-iodo reaction product” refers to the compounds formed by the reaction of such iodo-compounds with a perfluoroalkanoic acid. [0012] Generally, the methods may be used for a wide variety of fluoropolymer compositions. For example, the compositions may include one or more of any known fluoropolymers including fluorothermoplastics and fluoroelastomers. In some embodiments, the conversion reactions and volatilization can occur at temperatures and conditions used in common fluoropolymer processing steps such as the press curing and post curing of partially- and fully-fluorinated elastomers.

[0013] Also, the methods can be beneficial for the reduction in the amount of a wide range of perfluoroalkanoic acids. However, to aid in the efficient volatilization the perfluoro-iodo reaction products, the methods may be more efficient for the removal lower molecular weight perfluorinated alkanoic acids, for example, C _| to C20, C _| to C 14. C _| to Cg, or C4-C 10 perfluorinated alkanoic acids. Often such compounds are not intentionally included in the fluoropolymer. Instead, such materials may arise due to contamination or undesired side-reactions occurring in the polymerization process.

[0014] Throughout this specification, the amount of the perfluoroalkanoic acid(s) is the amount by weight of the perfluoroalkanoic acid(s) divided by the weight of the fluoropolymer. As the perfluoroalkanoic acid(s) are generally present in trace amounts, their content is usually expressed as parts per million (ppm) or even parts per billion (ppb). For example, if a perfluoroalkanoic acid is present in amount of 50 ppb, this corresponds to 50 micrograms of perfluoroalkanoic acid per kilogram of fluoropolymer.

[0015] The iodo-compound can be directly blended into a fluoropolymer containing perfluoroalkanoic acids. The resulting composition will comprise the fluoropolymer containing a first amount of perfluoroalkanoic acid(s) and the iodo-compound. Generally, the amount of the iodo-compound in the fluoropolymer should be sufficient to achieve the desired reduction in the amount of the perfluoroalkanoic acid(s). Because of inefficiencies, this may be significantly greater than the amount of the perfluoroalkanoic acid(s). In some embodiments, the amount of the iodo-compound is at least 0.05 parts by weight per 100 parts of the fluoropolymer. In some embodiments, the iodo-compound is present at 0.07, or even 0.1 parts by weight per 100 parts of the fluoropolymer. Generally, lower amounts of iodo- compound are preferred to minimize the need to remove residual, unreacted iodo-compound. Although not particularly limited, in some embodiments, the fluoropolymer contains no greater than 0.5 or even no greater than 0.3 parts by weight of the iodo-compound per 100 parts of the fluoropolymer. If two or more iodo-compounds are used, the amounts refer to the total amount of all added iodo-compounds.

[0016] Due to challenges in consistently preparing compositions with such low levels, in some embodiments, master batches may be prepared. For example, a master batch of a fluoropolymer may be prepared containing 5, 10, 20 or even greater parts by weight of the iodo-compound(s) per 100 parts of fluoropolymer in the final blend. The master batch may then be blended with a larger batch of fluoropolymer (the same fluoropolymer or a different fluoropolymer) to achieve the desired amount of iodo-compound per 100 parts of fluoropolymer in the final blend. [0017] In some embodiments, the use of solid iodo-compounds may be preferred to assist in the initial blending of the iodo-compound with the fluoropolymer, as liquid iodo-compounds may be more difficult to incorporate uniformly due to poor compatibility with the fluoropolymer. Therefore, for ease of handling, in some embodiments, the melting temperature of the iodo-compound is at least 50 °C, e.g., at least 80 °C or even at least 100 °C; and no greater than 250 °C, e.g., no greater than 200 °C, or even no greater than 175 °C.

[0018] The iodo-compound is added to react with the carboxylic acid group of the perfluoroalkanoic acids as follows: where Rf is a perfluorinated alkyl group (e.g.. a C _| to C20 alkyl group), and M ⁺ is the counter ion of the iodo-compound. As a result, the perfluoroalkanoic acid is converted to a perfluoro-iodo reaction product. The resulting perfluoro-iodo reaction products that may be produced include both perfluoroalkyl carbonyl iodides and perfluoroalkyl iodides. This same reaction process can be used to convert the salts of perfluoroalkanoic acids (i.e., Rf C(0)0 ⁺ X, where ⁺ X is the counterion of the salt) to perfluoroalkyl carbonyl iodides and perfluoroalkyl iodides.

[0019] Generally, the conversion step may be conducted at a temperature and for a time sufficient to achieve the desired reduction in the amount of perfluoroalkanoic acid(s). Generally, the conversion reactions are more efficient when the iodo-compound is present in liquid form. Although, the iodo- compound may be a liquid when blended with the fluoropolymer, solid compounds are generally preferred in the blending step. Therefore, the conversion step should occur at temperatures greater than the melting point of the iodo-compound. In addition, the conversion reactions will be faster at higher temperatures. However, to avoid volatilization of the iodo-compound during the conversions step, the temperature should be less than the boiling point or decomposition temperature of the iodo-compound. As the melting and boiling points of iodo-compounds are known or easily determined, one of ordinary skill in the art can select the appropriate temperature ranges for the selected iodo-compounds.

[0020] Once the temperature range of the conversion step is selected, one of ordinary skill can readily determine the length of time required to achieve the desired level of conversion. In some embodiments the conversion process is performed at a temperature and for a time sufficient to convert at least 25%, e.g., at least 50%, at least 80% of the amount of the perfluoroalkanoic acids (e.g., of the amount of C4 to

C I ₍₎ . C4 to C 14. or even C _| to C20 perfluoroalkanoic acids) to a perfluoro-iodo reaction product. Although it may not be possible to convert all of the perfluoroalkanoic acids, in some embodiments, greater than 90, greater than 95, or even greater than 99% of the perfluoroalkanoic acids (e.g., of the amount of C4 to C _{| ()} . C4 to C 14. or even C _| to C ₂ o perfluoroalkanoic acids) may be converted to perfluoro-iodo reaction products.

[0021] As a result of this conversion step, a second composition is formed. The second composition comprises the fluoropolymer containing the perfluoro-iodo reaction products. In some embodiments, the fluoropolymer will also contain unreacted perfluoroalkanoic acids, but in amounts significantly reduced from the amounts present in the fluoropolymer of the original composition. Generally, the second composition will also contain residual, unreacted iodo-compound.

[0022] Given the reduced amounts of perfluoroalkanoic acids in the fluoropolymer of the second composition, in some embodiments, the second composition may be used without further processing. However, in some embodiments, it may be desirable to remove some or all of the perfluoro-iodo reaction products generated. It may also be desirable to remove some or all of the residual iodo-compounds.

[0023] Generally, the perfluoro-iodo reaction products have a significantly lower boiling temperature than the corresponding perfluoroalkanoic acids from which they were formed. For example, the boiling point of various compounds at atmospheric pressure (e.g., about 101 kPa) are summarized in Table 1, where n is one less than the total number of carbon atoms in the alkanoic acid and carbonyl iodide compounds (e.g., n = 7 for perfluorooctanoic acid) and n is the total number of carbon atoms in the iodide compound, as the reaction of alkanoic acid to form the corresponding perfluoro-iodide compound generates CO and the loss of a carbon atom.

Table 1: Boiling points (°C) of perfluoroalkyl compounds.

[0024] In some embodiments, the second composition may undergo a first heat treatment step to volatilize the lower boiling point perfluoro-iodo reaction products. In some embodiments, the second composition may be processed at a selected temperature for a time sufficient to volatilize at least 50% of the amount of the perfluoro-iodo reaction products, e.g., at least 80%, or even at least 90% of the perfluoro-iodo reaction products. Although sufficient volatilization may occur at lower temperatures, in some embodiments, the second composition is heat-treated at a temperature greater than the boiling point of the perfluoro-iodo reaction products. In some embodiments, the boiling point of the perfluoro-iodo reaction product (at 101 kilopascals) is no greater than 400 °C, e.g., no greater than 300 °C or even no greater than 250 °C. In some embodiments, the perfluoro-iodo reaction product may decompose rather than volatilize. In some embodiments, the decomposition temperature of the perfluoro-iodo reaction product (at 101 kilopascals) is no greater than 400 °C, e.g., no greater than 300 °C or even no greater than 250 °C.

[0025] In some embodiments, the boiling point of the perfluoro-iodo reaction product(s) may be low enough such that at least a portion of these products are volatilized and removed during the conversion step (e.g., less than 150 °C, or even less than 100 °C). Therefore, the first heat-treatment step to remove the perfluoro-iodo reaction products may occur simultaneous with and at the same conditions as the conversion step.

[0026] As a result of this first heat-treatment, a third composition comprising the fluoropolymer is formed, where the fluoropolymer comprises significantly reduced perfluoroalkanoic acid contents compared to the original fluoropolymer and contains reduced amounts of the perfluoro-iodo reaction products compared to the fluoropolymer in the second composition.

[0027] The iodo-compound combined with the fluoropolymer has the general formula M ⁺ I ; where

M ⁺ is a cation. In some embodiments, M may be a metal, or an onium, e.g., NH4 ⁺ . However, in some embodiments, an organic cation may be used to minimize or avoid potential contamination of the finished fluoropolymer. For example, in some applications, metal ions can be detrimental. In some embodiments, polyiodides may be preferred. In some embodiments, the iodo-compound contains two, or even three iodide groups. For ease of blending and subsequent removal of residual iodo-compound, low molecular weight iodo-compounds are preferred. In some embodiments, C _j to C20 iodo-compounds may be used. In some embodiments, to C^Q, or even C _| to iodo-compounds may be preferred.

[0028] Exemplary iodo-compounds include CHI3, CH2I2, I-

(CH2) _n -I, and combinations thereof; wherein m is 2 or 3, and n is 0 to 4, e.g., 1 to 4. In some embodiments, one or more of the hydrogen atoms may be replaced by a halogen atom other than iodine, e.g., one or more fluorine, chlorine, or bromine atoms. In some embodiments, the iodo-compound is CHI3. In some embodiments, the iodo-compound is C H3I3.

[0029] Generally, the step of reacting of the iodo-compound and the perfluoroalkanoic acid and the step of volatilizing the perfluoro-iodo reaction product step may occur independent of other processing steps. However, greater efficiency can be achieved if one or both of these steps occur during the normal processing of the fluoropolymer, e.g., during the formation of fluoroelastomer articles.

[0030] For example, in some embodiments, the composition comprises a fluoroelastomer, e.g., a perfluoroelastomer. Such a composition may also include other common additives such a curatives and fillers. Such additives may be added at any stage of the methods of the present disclosure, e.g., one or more of these additives may be incorporated into the fluoropolymer as part of the first, second, or third composition. [0031] In some embodiments, the fluoroelastomer composition may be formed into an article, e.g., a sheet, seal or O-ring at any stage in the process. For example, the first composition may be formed into an article. The article may then be processed, for example, by first press curing, then post-curing, as is typical in the formation of fluoroelastomer articles.

[0032] In some embodiments the article may be press-cured at a temperature where the iodo- compound is a liquid. Then, the fluoroelastomer may be cured while reacting the liquid iodo-compound with the perfluoroalkanoic acid(s) to convert at least a portion of the perfluoroalkanoic acid(s) to the perfluoro-iodo reaction product(s).

[0033] In some embodiments, the press-cured article may then be post cured. For example, in some embodiments, the press-cured article may be post-cured at a temperature and for a time sufficient to volatilize the perfluoro-iodo reaction products. In some embodiments, the article may be post-cured at a temperature and for a time sufficient to volatilize the iodo-compounds as well.

[0034] Examples. A perfluoroelastomer (“PFE-A”) derived from about 52.4 wt.% tetrafluoroethylene (TFE), 43.7 wt.% perfluoromethylvinyl ether (PMVE) and 3.9 wt.% CF2=CFO(CF2)5CN was prepared. The perfluoroelastomer had a fluorine content of 72.4 wt.% fluorine content, and an iodide content of less than 0.01 wt.%.

[0035] PFE-A was pulverized to a fine powder in a cryogrinder. The powder was extracted by mixing a sample of 1.0 g of the fine powder with 9 milliliters of methanol on a mechanical shaker. The samples were then spiked with a known amount of as an internal standard. The samples were analyzed using gas chromatography/mass spectrometry (GC/MS) using a model 6550 iFunnel Q-TOF LC/MS obtained from Agilent Technologies, Santa Clara, California. For the quantitation, standard solutions that included a known amount of PFOA and were prepared and measured as the same manner. The amounts of Cq to C _| Q perfluoroalkanoic acids were calculated with the calibration curve for PFOA (Cg). Two samples were run for each sample.

[0036] Example EX-1. A composition was prepared by compounding 99.9 grams of PFE-A (perfluoroelastomer spiked with 0.1 grams of CHI3. Using a 15 cm (6 inch) two-roll mill. The milled sample was then heat-treated at 180 °C for thirty minutes. The amounts of C4 to C _| Q perfluoroalkanoic acids in the milled sample were measured before and after heat-treatment using the LC/MS method described above. The results are shown in Table 2.

[0037] Comparative Example CE-1. A composition was prepared and tested using the same materials and methods as Example EX-1, except that no CHI3 was added. The results are shown in Table 2.

[0038] Comparing EX-1 and CE-1, before heat treatment the amounts of the C4 to C _| Q perfluoroalkanoic acids are similar and fall within the range of normal variability that can occur in normal production. However, after heat treatment, EX-1, which contained the iodo-compound, showed a substantial reduction in each of the Cq to C _| Q perfluoroalkanoic acids and an almost 60% reduction in the total amount of C4 to CJQ perfluoroalkanoic acids. In contrast, in the absence of an iodo-compound, sample CE-1 showed an increase in the amounts of the perfluoroalkanoic acids, with a final content more than three times greater than the original sample. Therefore, not only did the iodo-compound efficiently remove the original perfluoroalkanoic acids, but it also helped remove additional perfluoroalkanoic acids thought to be generated as undesired side reactions that can arise during heating (e.g., curing) of fluoropolymers.

Table 2: C4 to C _| Q perfluoroalkanoic acids amounts before and after heat treatment in ppb.

[0039] PFE-A containing nitrile (CN) cure site groups. The content of nitrile groups in the polymer before and after heat-treatment was determined by measuring the integrated nitrile absorbance (i.e., the area of peaks in the region 2,277 to 2,232 cm l) of thin polymer films using an FTIR spectrometer. The nitrile group ratio was calculated from the equation below. Analysis was performed using a Nicolet iNlO FTIR from ThermoFisher Scientific, Waltham, Massachusetts.

Nitrile group area [2,277 — 2,232 cm ^-1 ]

Nitrile group ratio = - ² - - - — ¹ - - - - - —

C — F over tone area + nitrile group area [2,740 — 2,220 cm *]

As shown in Table 2, the heat-treatment did not adversely affect the nitrile-group content, leaving these groups available for subsequent curing of the fluoropolymer.