Method for manufacturing low molecular weight polytetrafluoroethylene powders having high specific surface area

Abstract

The invention relates to a method for manufacturing low molecular weight polytetrafluoroethylene (PTFE) powders having high BET specific surface area, in an aqueous reaction medium comprising: • in a reactor pressurized at a pressure P of from 4 to 30 bar and a temperature T of from 10°C to 100°C forming an aqueous mixture comprising • (i) one or more radical initiator, • (ii) one or more chain transfer agent, • (iii) tetrafluoroethylene (TEE) monomers and • (iv) one or more acid functionalized monomer or salt thereof, thereby forming a polymer from said monomers, wherein said polymer is in powder form • wherein said polymer comprises at least 95% by weight of recurring units derived from TFE and at least 99% by weight of recurring units derived from perhalogenated monomers, and to the thus obtained low molecular weight PTFE powders.

Description

DescriptionMethod for manufacturing low molecular weight polytetrafluoroethylene powders having high specific surface areaTechnical Field

[0001] This application claims priority from the European Patent Application 24185144.3 filed on 2024-06-27, the whole content of this application being incorporated herein by reference for all purposes.

[0002] The present invention relates to a method for producing polytetrafluoroethylene (PTFE) powders, having low molecular weight and a high specific surface area. The method does not require irradiation or pyrolysis nor requires the use of fluorinated surfactants.Background Art

[0003] Low-molecular-weight polytetrafluoroethylene in powder form, also called commercially PTFE wax or PTFE micropowder, has a molecular weight of several thousands to several hundred thousand grams per mole, much lower than that of high-molecular-weight PTFE, also referred to as regular PTFE, which is in the order of 106to 107grams per mole.

[0004] Besides retaining the functions of regular PTFE, low-molecular-weight PTFE in powder form is provided with high dispersibility, meaning that it can be used as additive especially in rubbers and plastics and in particular for improving sliding properties and texture of coating film surfaces, inks and greases.

[0005] Low-molecular-weight PTFE powders are commonly manufactured by a method wherein scraps of regular PTFE are subjected to irradiation with ionizing radiations such as gamma rays, x rays and the like in order to effect a decomposition reaction. Known manufacturing methods employ irradiation in the presence of oxygen, which is particularly effective in rapidly increasing the degradation speed of PTFE and, hence, rapidly decreasing its melt viscosity.

[0006] However, oxygen is responsible for the generation of certain amounts of short-chain perfluorocarboxylic acids (PFCAs), in particular in the range of C6 to C14, which are presently under significant environmental concerns. Among them, for example perfluorooctanoic acid (PFOA) is regulated as a persistent organic pollutant (POP).

[0007] An alternative method to produce low molecular weight PTFE powders is pyrolysis (also known as thermal cracking) of high molecular weight PTFE, however pyrolysis presents similar concerns as irradiation with regard to the formation of PFCAs.

[0008] Therefore, efforts have been devoted so far to provide methods for the manufacture of low-molecular-weight PTFE which are less likely to generate PFCAs, and accordingly to provide low molecular weight PTFE powders which are free or substantially free from such PFCAs.

[0009] Methods have been developed wherein the irradiation is performed substantially in the absence of oxygen, but in the presence of certain additives able to effect the degradation of PTFE and, therefore, the molecular weight decrease of PTFE at an acceptable rate. For instance, US 2019 / 0023818 discloses a method for manufacturing low-molecular- weight PTFE wherein additives are selected among hydrocarbons, chlorinated hydrocarbons, alcohols and carboxylic acids.

[0010] However the current technologies are still far from optimized, in particular when low molecular weight PTFE powders having high specific surface area are desired. Low molecular weight PTFE powders having a relatively large specific surface area are generally easier to incorporate when used as additives and also more effective in providing lubricant properties so that PTFE powders having a larger specific surface area can be used in smaller amount.

[0011] According to the prior art the irradiation or pyrolysis of PTFE manufactured via suspension polymerization (in the absence of surfactants) results in PTFE powders having a relatively low specific surface area (< 5m2 / g BET). According to the prior art, in order to obtain PTFE micropowders having a large specific surface area (above 5 m2 / g) it is necessary to manufacture the PTFE via emulsion polymerization using a fluorinated surfactant andthen subject such PTFE (typically after coagulation from the latex) to irradiation. The fluorinated surfactants used in the prior art is typically PFOA (perfluorooctanoic acid or its salts), i.e. one of the environmentally undesirable PFCAs which could also form during irradiation. Also other perfluorinated surfactants which may be used as alternatives to PFOA are in generally undesirable from the environmental standpoint.

[0012] There is therefore a need for a process for making low molecular weight PTFE powders having a large specific surface area which does not include irradiation, pyrolysis nor the use of fluorinated surfactants.

[0013] The process of the present invention responds to this need. PTFE powders produced with the method of the invention have naturally low molecular weight (no need of irradiation or pyrolysis), have high specific surface area and are essentially free from C6 to C14 perfluorocarboxylic acids and from any other added fluorinated surfactants. Also all processing steps of the method do not require C6 to C14 perfluorocarboxylic acids or any other added fluorinated surfactant so that the process of the invention is environmentally friendly.

[0014] An additional advantage of the method of the invention is that it is simpler than the methods of the prior art, in fact the low molecular weight PTFE powder having high specific surface area is obtained directly upon polymerization of the TFE monomers and does not require additional treatment steps except washing and drying. Such particles can be used directly in all the applications mentioned above wherein low molecular weight PTFE powders are used.Summary of invention

[0015] The present invention relates to a method for manufacturing low- molecular-weight polytetrafluoroethylene (PTFE) powders having high BET specific surface area, in an aqueous reaction medium comprising: - in a reactor pressurized at a pressure P of from 4 to 30 bar and a temperature T of from 10°C to 100°C forming an aqueous mixture comprising(i) one or more radical initiator,(ii) one or more chain transfer agent,(iii) tetrafluoroethylene (TFE) monomers and(iv) one or more acid functionalized monomer or salt thereof, thereby forming a polymer from said monomers, wherein said polymer is in powder form- wherein said polymer comprises at least 95% by weight of recurring units derived from TFE and at least 99% by weight of recurring units derived from perhalogenated monomers.DEFNITIONS

[0016] In the present description, unless otherwise indicated, the following terms are to be meant as follows.

[0017] The term “alkyl”, as well as derivative terms such as “alkoxy”, include within their scope straight chains and branched chains. Examples of alkyl groups are methyl, ethyl, propyl, isopropyl, butyl and isobutyl. Unless specifically stated otherwise, each alkyl group may be unsubstituted or substituted with one or more substituents selected from but not limited to hydroxy, sulfo, C1-C6 alkoxy, C1 -C6 alkylthio, provided that the substituents are sterically compatible and the rules of chemical bonding and strain energy are satisfied.

[0018] The term “halogen” includes fluorine, chlorine, bromine and iodine.

[0019] The expression “low-molecular-weight PTFE” refers to PTFE having a melt flow index of at least 0.05 g / 10 min determined in conformity with ASTM D 1238 by applying a weight of 5 Kg at 372°C.

[0020] The term “PTFE” in the context of the present invention, indicates a polymer which contains at least 95% by moles of recurring units derived from tetrafluoroethylene (TFE).Detailed description of the invention

[0021] The present invention relates to a method for manufacturing low- molecular-weight polytetrafluoroethylene (PTFE) powders having highspecific surface area, in an aqueous reaction medium. The method comprises forming a mixture within a sealed reactor comprising- water- one or more radical initiator- one or more chain transfer agent,- tetrafluoroethylene (TFE) monomers in gas form- one or more acid functionalized monomer or salt thereof.

[0022] Other additional monomers may be present in the liquid phase or in gas form in addition to TFE and the acid functionalized monomer however the resulting polymer must comprise at least 95%, preferably at least 97%, more preferably at least 98% even more preferably at least 99% by weight of recurring units derived from TFE. Other monomers, if present, are preferably perhalogenated monomers such as hexafluoropropylene, chlorotrifluoroethylene and perfluoroalkylvinylethers (PAVE). Preferably additional monomers, if present, are selected from PAVE monomers. Preferably the resulting polymer comprises at least 99%, preferably 99.5% more preferably 99.8% by weight of total recurring units derived from perhalogenated monomers (95%w or more derived from TFE, 0-4.99%w derived from other optional comonomers).

[0023] The acid functionalized monomer (or its salt) is thus present in a small amount, in a preferred embodiment the total amount of acid functionalized monomer (or its salt) which is introduced during the entire polymerization process is from 0.01 to 1 % by weight percent, preferably from 0.03 to 0.5% by weight, more preferably from 0.05 to 0.3% by weight based on the total amount of monomer introduced.

[0024] Suitable acid functionalized monomers are compounds having a double C=C bond and an ionic group, preferably selected from sulfonic, carboxylic and phosphonic. The ionic group can be in acid form (sulfonic, carboxylic or phosphonic) or in salt form (sulfate, carboxylate or phosphate).

[0025] Preferably the acid functionalized monomers are selected from compounds containing at least one vinyl group and at least one sulfonic acid group or salt thereof.

[0026] More preferably said acid functionalized monomer complies with the following formula (I):RO-S(=O)2-R1-CH=CH2(I) wherein R is a hydrogen atom, an ammonium ion or an alkaline metal ion; R1 is a sigma bond or an alkyl chain comprising from 1 to 3 carbon atoms.

[0027] Most preferably said acid functionalized monomer is vinylsulfonic acid or its alkaline metal or ammonium salt.

[0028] In order to initiate the polymerization, the sealed reactor is pressurized using the monomers in gas form and optionally inert gases at a pressure P of from 4 to 30 bar, preferably from 7 to 20 bar, even more preferably from 10 to 19 bar. Preferably the reaction medium is kept under stirring to facilitate the contact between the reagents is gas and liquid phase. The temperature of the reactor is raised up to a temperature T wherein the chosen initiator will form radicals and effectively start up the polymerization reaction. In general temperature T will be comprised between 10°C and 100°C, when using peroxide based initiators the temperature T will be preferably from 50 to 90°C, while when using redox based initiators a temperature of from 10 to 50°C will normally be sufficient.

[0029] After startup, the polymerization reaction is continued until completion thereby forming a polymer from the monomers introduced into the reactor.

[0030] As common practice in radical polymerization reactions, after an initial charge of monomers, the polymerization is started and, while the reaction progresses, additional monomers, initiators and chain transfer agents may be introduced in the reactor to maintain the pressure and sustain the reaction until completion.

[0031] In some embodiment the total amount of acid functionalized monomer is present at the initiation of the polymerization, alternatively it can be in part present at the initiation of the polymerizations and then added gradually during the progress of the polymerization reaction. Also alternatively the acid functionalized monomer can be introduced into the reactor only after the polymerization has started. Preferably the entire amount of acidfunctionalized monomer is present in the aqueous phase at the start of the polymerization process.

[0032] Suitable radical initiators which can be used in the present invention are those known for initiating a free radical polymerization of fluorinated monomers. These include peroxides, peracids, azo compounds and redox based initiators. Specific examples of peroxide initiators include, hydrogen peroxide, sodium or barium peroxide, diacylperoxides such as diacetylperoxide, disuccinyl peroxide, dipropionylperoxide, dibutyrylperoxide, dibenzoylperoxide, di-ter-butyl-peroxide, benzoylacetylperoxide, diglutaric acid peroxide and dilaurylperoxide, and further per-acids and 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 tert. - butylperoxyacetate and tert. -butylperoxypivalate. Examples of inorganic initiators include for example ammonium-alkali- or earth alkali salts of persulfates, permanganic or manganic acid or manganic acids. A persulfate initiator, e.g. ammonium persulfate (APS), can be used on its own or may be used in combination with a reducing agent. Suitable reducing agents include bisulfites such as for example ammonium bisulfite or sodium metabisulfite, thiosulfates such as for example ammonium, potassium or sodium thiosulfate, hydrazines, azodicarboxylates and azodicarboxyldiamide (ADA). Further reducing agents that may be used include sodium formaldehyde sulfoxylate (Rongalite ) or fluoroalkyl sulfinates as disclosed in U.S. Pat. No. 5,285,002. The reducing agent typically reduces the half-life time of the persulfate initiator. Additionally, a metal salt catalyst such as for example copper, iron or silver salts may be added. The amount of initiator may be between 0.01 % by weight (based on the fluoropolymer solids to be produced) and 1 % by weight. In one embodiment, the amount of initiator is between 0.05 and 0.6% by weight. In another embodiment, the amount may be between 0.05 and 0.3% by weight.

[0033] Any chain transfer agent commonly used in fluoropolymers polymerization can be used herein. Examples of chain transfer agents that can be used include dimethyl ether, methyl t-butyl ether, alkanes having 1 to 5 carbon atoms such as ethane, propane and n-pentane, halogenated hydrocarbons such as CCk , CHCI3 and CH2CI2 and hydrofluorocarbon compounds such as CH2F-CF3(R134a). Additionally esters like ethyl acetate, malonic esters can be effective as chain transfer agent in the process of the invention.

[0034] The polymerization method of the present invention can be advantageously performed without the addition of fluorinated surfactants, preferably the method of the invention can be performed without the addition of any surfactant.

[0035] Once the polymerization reaction is complete, the reaction medium can be discharged from the reactor and the polymer can be isolated by draining the water phase from the reaction medium. After draining, the polymer is in the form of wet powder which can be washed with water (typically warm water at 60°-90°C), and dried (preferably at a drying T of 160°C-300°C).

[0036] The average particle size (D50 measured with the method indicated in the experimental section) of the low molecular weight PTFE produced with the method of the invention is typically from 0.5 to 300 pm, preferably from 0.5 to 250 pm even more preferably from 0.5 to 150 pm.

[0037] The particle size is not particularly critical because the particles obtained by the method of the invention are typically agglomerates of primary particles having a size of about 1 pm or even smaller below . The key objective of the invention is to obtain low molecular weight PTFE powder having an high specific surface area. PTFE powders directly obtained from the process have a high specific surface area irrespectively to their D50 average particle size because, as mentioned above, the particles forming the powder are highly porous agglomerates of smaller primary particles wherein the agglomeration is such that the surface of the smaller primaryparticles remains for the most part available even in the agglomerated state.

[0038] The low molecular weight PTFE powder of the invention can therefore be used directly as it is obtained from the described process for its final applications as additive to rubber, plastics inks and greases. Optionally such low molecular weight PTFE powder can be subject to milling to achieve a desired value for the average particle size D50 prior to be used for their final application.

[0039] Preferably, in case the particles are milled, this is done with jet mills or cutter mills. In all cases it is preferred that milling is performed mildly so to break the agglomerates of primary particles but without alter the shape of the primary particles.

[0040] Preferably, the low-molecular-weight PTFE obtained in the method of the invention has a melt flow index of at least 0.1 g / 10 min, more preferably of at least 0.15 g / 10 min, even more preferably of at least 0.25 g / 10 min, still more preferably of at least 0.5 g / 10 min, as determined in conformity with ASTM D 1238 by applying a weight of 5 Kg at 372°C. The melt flow index is used as an indicator of the molecular weight of the low-molecular-weight PTFE.

[0041] The low-molecular-weight PTFE has a melting temperature preferably from 324°C to 337°C, more preferably from 324°C to 335°C, even more preferably from 325°C to 331 °C, as determined according to the method described in the experimental section below.

[0042] As mentioned above, the low molecular weight PTFE powder resulting from the process of the invention has high specific surface area. This is measured with the BET method described in the experimental section, and in some embodiments the low molecular weight PTFE powder of the invention has a specific surface area BET of 6 m2 / g or above, preferably 7 m2 / g or above, more preferably 7.5 m2 / g or above and of 40 m2 / g or below, preferably of 30 m2 / g or below.

[0043] As mentioned above, prior art low molecular weight PTFE powders having high specific surface area were previously manufactured via irradiation or pyrolysis of high molecular weight PTFE obtained from emulsion polymerization using fluorinated surfactants. In this case irradiation and / or pyrolysis are not used , the method of the invention does not include any step wherein ionizing radiations or thermal treatments above 400°C are used.

[0044] The low molecular weight PTFE powders produced with the method of the present invention instead, not using irradiation, pyrolysis nor fluorinated surfactants, are in general essentially free from C6-C14 perfluorocarboxylic acids (PFCAs) and / or salts thereof and from fluorinated surfactants.

[0045] For “essentially free from C6-C14 perfluorocarboxylic acids (PFCAs) and / or salts thereof” it is intended that the total amount of each of C6-C14 perfluorocarboxylic acids (PFCAs) and / or salts thereof contained in the PTFE powders of the invention not more than 25 ppb, preferably not more than 20 ppb, even more preferably not more than 15 ppb, still more preferably not more than 10 ppb, most preferably not more than 5 ppb (weight calculated for each PFCA as amount of PFCA in acid form in accordance with the method described below).

[0046] In particular in some embodiment of the PTFE powders of the invention the total amount of perfluorooctanoic acid (C8 PFCA) and of its salts is less than 3 ppb, preferably less than 2 ppb, even more preferably less than 1 ppb.

[0047] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0048] The invention is described in greater detail in the following experimental section by means of non-limiting examples.Experimental section

[0049] MaterialsTFE monomers from Solvay Specialty Polymer Italy S.p.a.Ethane from Sigma AldrichVSA (vinyl sulfonic acid) sodium salt 25% water solution -from Sigma AldrichAmmonium Persulfate (APS) solution 0,131 M from Sigma Aldrich

[0050] Measurement of melting temperature:Melting temperature was measured via DSC using a Mettler Toledo DSC1 Star instrument according to ASTM D3418. A sample of about 10 mg of dried low-molecular-weight PTFE powder was heated from 150°C to 370°C at a rate of 10°C / min. The melting temperature (Tm) referred to below is the endothermic peak observed during the second heating cycle.

[0051] Determination of the amount of C6-C14 perfluorocarboxilic acids (PFCAs) and salts thereof.C6-C14 perfluorocarboxylic acids and their salts were measured using a liquid chromatography mass spectrometer (Agilent technologies Infinity 1290 and Triple Quad 6470). A dried powder sample of about 1g was mixed with 3m I of a NovecTM7100 / Ethanol mixture in a ratio of 1 :1 by volume and 0.4% by volume of an ammonium hydroxide solution at 25%by volume. The following surrogate recovery standards (SRS) were added to the mix:M5PFHxA (Perfluoro-n-[1 ,2, 3, 4, 6-13C5]hexanoic acid);M4PFHpA (Perfluoro-n-[1 ,2, 3, 4-13C4]heptanoic acid);M8PFOA (Perfluoro-n-[13C8]octanoic acid);M9PFNA (Perfluoro-n-[13C9]nonanoic acid);M6PFDA (Perfluoro-n-[1 ,2, 3, 4, 5, 6-13C6]decanoic acid);M7PFUdA (Perfluoro-n-[1 ,2, 3, 4, 5, 6, 7-13C7]undecanoic acid);MPFDoA (Perfluoro-n-[1 ,2-13C2]dodecanoic acid);M2PFTeDA (Perfluoro-n-[1 ,2-13C2]tetradecanoic acid). These SRS were provided by Wellington Lab. Inc.The mixture was shaken at 50 °C for 16 hours and then centrifuged. 1 ml of supernatant solution was then transferred in a 2 ml vial with screw cap, evaporated to dryness under N2 flow and then reconstituted with 1 ml of methanol. The following Marked Internal Standards (ISD) were then added to the solution:MPFHxA (Perfluoro-n-[1 ,2-13C2]hexanoic acid);MPFOA (Perfluoro-n-[1 >2,3,413C4]octanoic acid).(ISD were provided by Wellington Lab. Inc.).The solution was shaken by hand for few seconds and analyzed using an Agilent 1290 LC working at 50°C using a ACQUITY LIPLC Peptide BEH C18 column run in gradient mode with milli-q water modified with 10Mm Ammonium Acetate and Acetonitrile. The mass spectrometer is Agilent 6470 Triple Quad LC / MS using analyte typical transitions.The analytes were quantified using equivalent or similar isotope labelled internal standards: 13C-MPFHxA (as internal standard for C6, C7, and MPFOA for C8, C9, C10,C11 ,C12,C13, C14 as perfluorocarboxylic acid).The method has been validated using the reference guidelines Eurachem / CITAC 3rd Edition . The range of applicability of the method is from 1 to 30 ppb and the lower limit of quantitation (LLOQ) is 1 ppb for each perfluorocarboxylic acid and the upper limit of quantitation IILOQ is 30 ppb for each perfluorinated carboxylic acid.

[0052] Measurement of the melt flow index (MFI)The melt flow index (MFI) was measured according to ASTM D 1238 standard method by applying a weight of 5 Kg at 372°C.

[0053] Measurement of the BET specific surface areaBET specific surface area is measured according to ISO 9277: 2022, using nitrogen gas.

[0054] Measurement of the average particle sizeAverage particle size D50 was measured according to ISO 13320 using laser diffraction in dry conditions (analyzer Beckman Coulter LS 13 320).

[0055] Synthesis examples

[0056] Example 1 C (Comparative )Preparation of a low Molecular weight PTFE powder.In a 5 It (AISI 316 steel) vertical reactor equipped with stirrer working at 650 rpm 3 It of demineralized water was introduced, then the reactor sealed, air was removed and then was heated until 75 °C. At this point 0,25 abs bar of Ethane and 16 abs bar of TFE were introduced. Using a metering pump 50 ml of Ammonium Persulfate (APS) solution 0,131 M were also is fed thus starting the polymerization process. When 10% conversion was reached additional 35 ml of APS solution were added. During the progress of the polymerization reaction the pressure was maintained constant by feeding the TFE monomer. Feeding was interrupted after feeding 500g of TFE. The reaction time was 29 minutes. The reactor was cooled at room temperature and the stirring was reduced down to 50 rpm, residual monomers in gas phase where stripped. The powders were discharged and separated from the water phase and then washed at 60 °C with deionized water in a ratio 1 :4 for 4 times, then the powders were dried in an oven at 160 °C for 24 hrs.

[0057] Example 2 C (Comparative )The polymerization conditions are the same described in example 1 C except for the partial pressure of TFE which was 14 bar abs.

[0058] Example 3C (Comparative )The polymerization conditions are the same described in example 1 C except for the Ethane quantity introduced which was 0,8 bar abs.

[0059] Example 1 (according to the invention)The polymerization conditions are the same described in example 1 C except for the introduction after the vacuum step, before heating, of 2,2 grof a 25%w VSA sodium salt solution and for the ethane quantity which was of 0,41 bar abs.

[0060] Example 2 according to the inventionThe polymerization conditions are the same described in Example 1 except forVSA amount: 1 ,8 gr of a 25%w VSA solution Ethane: 0,53 bar abs.

[0061] Example 3 according to the inventionThe polymerization conditions are the same described in example 1 except for :VSA amount: 1 ,4 gr of a 25%w VSA solution Ethane: 0,55 bar abs.

[0062] Example 4 according to the inventionThe polymerization conditions are the same described in example 1 except for :VSA amount: 1 ,53 gr of a 25%w VSA solutionEthane: 0,55 bar abs.Total TFE fed: 700g

[0063] Example 5 according to the inventionThe polymerization conditions are the same described in example 1 except for :VSA amount: 1 ,8 gr of a 25%w VSA solutionEthane: 0,54 bar abs.APS 45ml is charged all at the beginning in a single step

[0064] Example 6 according to the inventionThe polymerization conditions are the same described in example 1 except for :VSA amount: 1 ,8 gr of a 25%w VSA solutionEthane: 0,56 bar abs.

[0065] Example 7 according to the inventionThe polymerization conditions are the same described in example 1 except for :VSA amount: 4 gr of a 25%w VSA solutionEthane: 0,53 bar abs.The resulting polymer powders from each Example were characterized results are shown below in Tables 1 and 2.

[0066] Summary of resultsTable 1(*) % by weight on total polymerTable 2Amount of C6-C14 perfluorocarboxilic acids

Claims

ClaimsClaim 1A method for manufacturing low-molecular-weight polytetrafluoroethylene (PTFE) powders having high BET specific surface area, in an aqueous reaction medium comprising:- in a reactor pressurized at a pressure P of from 4 to 30 bar and a temperature T of from 10°C to 100°C forming an aqueous mixture comprising(i) one or more radical initiator,(ii) one or more chain transfer agent,(iii) tetrafluoroethylene (TFE) monomers and(iv) one or more acid functionalized monomer or salt thereof, thereby forming a polymer from said monomers, wherein said polymer is in powder form- wherein said polymer comprises at least 95% by weight of recurring units derived from TFE and at least 99% by weight of recurring units derived from perhalogenated monomers.Claim 2The method of Claim 1 wherein said polymer powder has a BET specific surface area of 6 m2 / g or above, preferably 7 m2 / g or above, more preferably 7.5 m2 / g or above and of 40 m2 / g or below, preferably of 30 m2 / g or below.Claim 3The method of Claim 1 or 2 wherein said polymer has a MFI of at least 0.1 g / 10 min, more preferably of at least 0.15 g / 10 min, even more preferably of at least 0.25 g / 10 min, still more preferably of at least 0.5 g / 10 min, as determined in conformity with ASTM D 1238 by applying a weight of 5 Kg at 372°C.Claim 4The method of any preceding Claim wherein said one or more chain transfer agent is selected from dimethyl ether, methyl t-butyl ether, alkanes having 1 to 5 carbonatoms (e.g. methane, ethane, propane butane and pentane), halogenated hydrocarbons (e.g. CCk, CHCI3 and CH2CI2), hydrofluorocarbon compounds (e.g. CH2F-CF3 (R134a), esters (e.g. ethylacetate, malonic).Claim 5The method of any preceding Claim wherein said one or more radical initiator is selected from peroxides, peracids, azo compounds and redox based initiators.Claim 6The method of any preceding Claim wherein said acid functionalized monomer is used in an amount of 0.01 to 1 % by weight percent, preferably from 0.03 to 0.5% by weight, more preferably from 0.05 to 0.3% by weight based on the total amount of monomer.Claim 7The method of any preceding Claim wherein said acid functionalized monomer is selected from compounds containing at least one vinyl group and at least one sulfonic acid group or salt thereof.Claim 8The method of any preceding Claim wherein said acid functionalized monomer complies with the following formula (I):RO-S(=O)2-R1-CH=CH2(I) whereinR is a hydrogen atom, an ammonium ion or an alkaline metal ion;R1 is a sigma bond or an alkyl chain comprising from 1 to 3 carbon atoms.Claim 9The method of any preceding Claim wherein said method is performed in the absence of fluorinated surfactantsClaim 10The method of any preceding Claim wherein said method is performed in the absence of surfactantsClaim 11The method of any preceding Claim wherein said method does not include a step using irradiation with ionizing radiation or a pyrolysis step.Claim 12Low-molecular-weight polytetrafluoroethylene (PTFE) polymer wherein- said polymer has a MFI of at least 0.1 g / 10 min, as determined in conformity with ASTM D 1238 by applying a weight of 5 Kg at 372°C.- said polymer is essentially free from C6-C14 perfluorocarboxylic acids (PFCAs) and / or salts thereof,- said polymer comprises less than 3 ppb in total of perfluorooctanoic acid and of its salts- said polymer is in the form of a powder having a specific surface area BET of 6 m2 / g or above, preferably 7 m2 / g or above, more preferably 7.5 m2 / g or above and of 40 m2 / g or below, preferably of 30 m2 / g or below.Claim 13Low-molecular-weight polytetrafluoroethylene (PTFE) powders according to claim 12 which have been manufactured with the process of claims 1 -11Claim 14The use of acid functionalized monomers in the manufacture of PTFE powders reduce their content in PFCAs thereby improving their environmental profile.

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