Abstract:
The present invention relates to an improved method of grinding polyaryletherketones, providing very good yields and the production of powders of polyaryletherketones with an average diameter below 100 μm having a narrow size distribution with few fine particles (Dv10&gt;15 μm).

Description:
[0001]    The present invention relates to an improved method of grinding polyaryletherketones, providing very good yields and the production of powders of polyaryletherketones with average diameter below 100 μm having a narrow size distribution. 
         [0002]    The polyaryletherketones are materials with high performance, notably in terms of thermal stability, and their use in the coating of engineering components is desirable in many applications. The methods of coating with this type of polymer generally use the polymer in the form of powder. 
         [0003]    Moreover, component manufacturing techniques of the laser sintering type also use powders. 
         [0004]    Methods are therefore required for producing powders, notably in economically viable conditions. 
         [0005]    There are numerous methods for grinding polymers. We may in particular mention the equipment used, such as ball mills, impact grinding mills using various types of impactor (hammers, needles, discs), air jet mills, and the operating conditions, typically cryogenic or atmospheric. These methods lead to variable yields and particle sizes sometimes necessitating selection, for example by sieving, of the powder obtained even if a selector is often integrated in the grinding mill, which only allows particles that have been ground sufficiently to pass through. Moreover, these methods result in powders containing a large amount of fine particles that are detrimental to certain applications such as laser sintering. 
       PRIOR ART 
       [0006]    The grinding of powders of polyaryletherketones is described extensively in the literature. 
         [0007]    U.S. Pat. No. 5,247,052 describes a method of grinding of polyaryletherketones with a fluid-bed opposed air jet. 
         [0008]    This method is carried out at very cold temperatures and therefore requires the supply of cooling power, at substantial cost. 
         [0009]    US20050207931 describes several methods for obtaining powder, including grinding. Once again, grinding is carried out at low temperature. 
         [0010]    US20090280263 describes a method for obtaining powder of polyaryletherketones by grinding using polyaryletherketones having an apparent specific surface area measured by BET above 1 m 2 /g. 
         [0011]    In this method, cooling of the polyaryletherketones to be ground is also preferred. In the examples, cooling is effected with liquid nitrogen, i.e. conditions that are disadvantageous for the manufacturing costs. Moreover, in this invention it is necessary to sieve the powder after grinding, which is not the case in the present invention. 
         [0012]    Moreover, these methods lead to large amounts of fines, which poses a problem, notably in laser sintering applications. 
         [0013]    The applicant found, against all expectations, that the methods of grinding of polyaryletherketones could be carried out at ambient temperature, typically above 0° C. with yields close to 100% to obtain powders having a particle size distribution (diameters by volume) of d10&gt;15 μm, 50&lt;d50&lt;70 μm, 120&lt;d90&lt;180 μm, without additional sieving. A d10&gt;15 μm is considered necessary in the context of fine particles in applications such as laser sintering. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention relates to a method of grinding of polyaryletherketones of apparent density below 0.9 carried out in a temperature range between 0° C. and the glass transition temperature of the polymer measured by DSC. 
     
    
     DETAILED DESCRIPTION 
       [0015]    The polyaryletherketones, also called PAEK, used in the invention comprise units with the following formulae: 
         [0000]      (—Ar—X—) and (—Ar 1 —Y—)
 
         [0000]    in which: 
         [0016]    Ar and Ar 1  each denote a divalent aromatic radical; 
         [0017]    Ar and Ar 1  can preferably be selected from 1,3-phenylene, 1,4-phenylene, 4,4′-biphenylene, 1,4-naphthylene, 1,5-naphthylene and 2,6-naphthylene; 
         [0018]    X denotes an electron-accepting group; it can preferably be selected from the carbonyl group and the sulphonyl group, 
         [0019]    Y denotes a group selected from an oxygen atom, a sulphur atom, an alkylene group, such as —CH 2 — and isopropylidene. 
         [0020]    In these units, at least 50%, preferably at least 70% and more particularly at least 80% of the groups X are a carbonyl group, and at least 50%, preferably at least 70% and more particularly at least 80% of the groups Y represent an oxygen atom. 
         [0021]    According to a preferred embodiment, 100% of the groups X denote a carbonyl group and 100% of the groups Y represent an oxygen atom. 
         [0022]    More preferably, the polyaryletherketone (PAEK) can be selected from:
       a polyetheretherketone also called PEEK comprising units of formula I:       
 
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             a polyetherketone, also called PEK, comprising units of formula II: 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             a polyetherketoneketone also called PEKK, comprising units of formula IIIA, of formula IIIB and a mixture thereof: 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             and a polyetheretherketoneketone also called PEEKK, comprising units of formula IV: 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    but other arrangements of the carbonyl group and of the oxygen atom are also possible. 
         [0027]    The polyaryletherketone usable according to the invention can be crystalline, semi-crystalline or amorphous. 
         [0028]    Preferably, the polyaryletherketones used are polyetherketoneketones also called PEKK, comprising units of formula IIIA, of formula IIIB and a mixture thereof. 
         [0029]    The polyaryletherketones used in the method of the invention can be in the form of porous or non-porous granules, porous or non-porous scales with average size between 0.5 and 5 mm, and porous or non-porous coarse powders. 
         [0030]    Preferably, the polyaryletherketones used in the method are in the form of scales or coarse powders and have a porosity above 2 m 2 /g measured with a Coulter SA3100 from the company Beckman Coulter (measurement by adsorption of nitrogen at 105° C. according to the BET method) and an apparent density below 0.9 kg/l, preferably below 0.4 kg/l, and even more preferably below 0.25 kg/l (density of tamped scales measured on a STAV 2003 jolting volumeter equipped with a 250 ml test specimen after 2500 impulses). 
         [0031]    The grinding mills used in the method of the invention can be of any type, but preferably they are impact grinding mills, in which the impactors can be hammers, needles, or discs. According to a second embodiment of the invention the grinding mills used are of the air jet type. 
         [0032]    With the aim of optimizing the grinding process, a combination of different types of grinding mills can be used, for example grinding can be carried out first with an impact mill, and then the product is transferred to an air jet mill. 
         [0033]    In all cases, the grinding temperature is between 0° C. and the glass transition temperature of the polymer measured by DSC, preferably between 0 and 50° C., even more preferably between 10 and 30° C. 
         [0034]    With the method of the invention, it is possible to obtain directly, without subsequent selection of the powder leaving the mill, powders having particle size distributions suitable for application by laser sintering or for coating of articles (d10&gt;15 μm, 50&lt;d50&lt;80 μm, 120&lt;d90&lt;180 μm), with a yield approaching 100%. 
         [0035]    The powders obtained are advantageously used in processes for coating articles or laser sintering processes. The powders can have additions of fillers such as alumina Al 2 O 3  or silica such as Aerosil to facilitate their flow. 
       EXAMPLE 1 
       [0036]    A polymer in the form of scales of PEKK (OXPEKK SP), of viscosity 0.95 dl/g (viscosity in solution at 25° C. in 96% sulphuric acid according to standard ISO 307) is micronized in a Neuman ICM 7.6 impact grinder-selector at a temperature of 25° C., grinding mill speed 12 000 rev/min, selector speed 4500 rev/min. Three successive grindings give the following granulometry measured on the Insitec T granulometer from Malvern with a focal length of 300 mm (measurement by laser diffraction on dry powder, diameters expressed by volume Dv): 
         [0037]    Dv10=27 μm, Dv50=76 μm, Dv90=180 μm. 
         [0038]    The yield is 99%. 
       EXAMPLE 2 
       [0039]    A polymer in the form of scales of PEKK (OXPEKK SP), of viscosity 0.85 dl/g (viscosity in solution at 25° C. in 96% sulphuric acid according to standard ISO 307) is micronized in a Neuman ICM 7.6 impact grinder-selector at a temperature of 25° C., grinding mill speed 12 000 rev/min, selector speed 4500 rev/min. Two successive grindings give the following granulometry measured on the Insitec T granulometer from Malvern with a focal length of 300 mm (measurement by laser diffraction on dry powder, diameters expressed by volume Dv): 
         [0040]    Dv10=29 μm, Dv50=81 μm, Dv90=184 μm. 
         [0041]    The yield is 98%. 
       EXAMPLE 3 
     Comparative 
       [0042]    A polymer in the form of scales of PEKK (OXPEKK SP), of viscosity 0.87 dl/g (viscosity in solution at 25° C. in 96% sulphuric acid according to standard ISO 307) is micronized in a Mikropull 2DH hammer mill equipped with a grating with round holes of 500 μm at a temperature of −40° C. Grinding gives the following granulometry measured on the Insitec T granulometer from Malvern with a focal length of 300 mm (measurement by laser diffraction on dry powder, diameters expressed by volume Dv): 
         [0043]    Dv10=64 μm, Dv50=155 μm, Dv90=322 μm. 
         [0044]    Sieving at 145 μm on a Finex 22 sieve made by Russel gives the following granulometry: 
         [0045]    Dv10=47 μm, Dv50=95 μm, Dv90=148 μm at a yield of 48%. 
         [0046]    It can be seen that at low temperature, the yield is far lower than was obtained by grinding the scales at 25° C.