Method for the surface crosslinking of polymer particles

Abstract

The invention concerns a method for the surface crosslinking of a polymer, in particle form, having one or a plurality of labile hydrogen functions, comprising a step of implementing a crosslinking agent comprising at least two functions likely to react with the labile hydrogen functions of the polymer, the crosslinking method being carried out at a temperature lower than the melting point of the polymer. The invention also concerns a powder particle and the uses of same.

Claims

1. A process for the surface crosslinking of a polymer, in particle form, comprising at least one amide function, comprising a step of using a crosslinking agent comprising at least two functional groups which each react with a labile hydrogen function of the amide functions of the polymer thereby effecting the crosslinking process, the crosslinking process being carried out at a temperature lower than the melting point of the polymer, wherein the polymer particles have an average diameter which ranges from 1 to 200 m, and wherein the crosslinking agent is selected from the group consisting of a) compounds bearing at least two of the following functional groups: isocyanate, carbodiimide, epoxy, acyllactam, oxazoline and its isomers, oxazine and its isomers, and b) PCl.sub.3, and the crosslinking agent is in a content of from 0.1 to 15 mol %, relative to the total number of moles of the monomers constituting the polymer having at least one amide function.

2. The process as claimed in claim 1, wherein the polymer is selected from the group consisting of polyamides and polyamide-imides and blends thereof.

3. The process as claimed in claim 2, wherein the polymer is a polyamide.

4. The process as claimed in claim 3, wherein the polymer is selected from the group consisting of PA6, PA11, PA12, PA6/12, PA6.12, PA6.10, PA10.10, PA10.12, PA6.6, PA8, PA4 and PA4.6, and blends of these polymers.

5. The process as claimed in claim 4, wherein the polymer is PA12.

6. The process as claimed in claim 1, wherein the crosslinking agent is a polyisocyanate.

7. A process for the polymerization of a polymer, comprising the following successive steps: a polymerization step, a step of crosslinking, in accordance with the process as claimed in claim 1, of the polymer in powder form obtained in the previous step, and an optional step of neutralization of the reaction medium.

8. The process as claimed in claim 7, wherein the polymerization step is a step of anionic polymerization in solution in a solvent, in the presence of a catalyst and of an activator.

9. A particle of polymer powder comprising: one or more areas consisting of polymer having one or more amide function(s), and one or more area(s) comprising said polymer crosslinked with a crosslinking agent in accordance with the process as claimed in claim 1.

10. The particle as claimed in claim 9, wherein the particle comprises: a core consisting of polymer having one or more amide function(s), and a layer comprising said polymer crosslinked with a crosslinking agent in accordance with the process as claimed in claim 1.

11. A composition comprising the particle of powder as claimed in claim 9.

12. A method for producing objects by agglomeration of a powder by fusion brought about by radiation chosen from a laser beam, infrared radiation or UV radiation, wherein the powder is comprised of particles of powder as claimed in claim 9.

13. A method of making cosmetic compositions, comprising using a particle of powder as claimed in claim 9 as an additive.

14. The process as claimed in claim 1, wherein the polymer is a polyamide or blend of polyamides.

15. The process as claimed in claim 1, wherein the crosslinking agent is in a content of from 0.5 to 10 mol %, relative to the total number of moles of the monomers constituting the polymer having one or more amide function(s).

16. The process as claimed in claim 1, wherein the polymer is a polyamide and the crosslinking agent is a polyisocyanate.

17. A method of making an article selected from the group consisting of composites, structural adhesives, substrate coatings and transfer papers, comprising using particles of polymer powder as claimed in claim 9 in the article.

Description

EXAMPLES

(1) The physicochemical characteristics of the powders exemplified were evaluated according to the following methods:

(2) Measurement of the Average Diameter:

(3) The average diameter of the particles of the powders exemplified is measured according to standard ISO 13319.

(4) Measurement of the Thermal Characteristics:

(5) The analysis of the powders is carried out according to standard ISO 11357-3 Plastics: Differential scanning calorimetry (DSC) Part 3: Determination of temperature and enthalpy of melting and crystallization.

(6) Measurement of the Enthalpy of Melting:

(7) The enthalpy of melting is directly proportional to the degree of crystallinity of the polymer. A comparison of the enthalpy of melting between two products therefore makes it possible to compare their degree of crystallinity.

(8) The enthalpy of melting is measured by DSC according to standard ISO 11357-3.

(9) Solubility Test:

(10) The solubility is measured by introducing 1 g of powder per liter of hexafluoroisopropanol. These mixtures are maintained at ambient temperature for 24 hours in order to dissolve the non-crosslinked polymer chains.

(11) These solutions are then filtered in order to remove the insoluble part, and then analyzed by SEC using a Waters Alliance 2695 apparatus. A Waters 2414 RID detector is used. The refractometric response coefficient K(RI) is then measured.

(12) 1. Synthesis of a Crosslinked Polyamide 12 Powder

(13) 2800 ml of solvent and then successively 899 g of lactam 12, 14.4 g of EBS (ethylenebisstearamide) and 73.0 g of Orgasol 2002 UD NAT 1 (PA12 powder) are placed in the reactor maintained under nitrogen. After having started the stirring at 300 rpm, heating is carried out gradually up to 110 C., and then 280 ml of solvent are distilled off under vacuum in order to azeotropically entrain any trace of water that might be present.

(14) After a return to atmospheric pressure, the anionic catalyst: 2.2 g of sodium hydride at 60% purity in oil, is then rapidly introduced under nitrogen and stirring is increased to 700 rpm, under nitrogen at 105 C.

(15) This temperature is then maintained for 30 minutes. By virtue of a metering pump, the chosen activator, namely stearyl isocyanate (38.3 g made up to 153.0 g with solvent), is continuously injected into the reaction medium according to the following program: 51 g/h for three hours and then the temperature is raised to 120 C. for two hours.

(16) The step of surface crosslinking the powder obtained is carried out as follows. At 120 C. with stirring, a solution of hexamethylene diisocyanate: 5 mol % of HMDI relative to the lactam 12, is added to the reaction medium for three hours, and the mixture is then left for a further two hours in order to finish all the reactions.

(17) The powder is then filtered and dried in order to be neutralized with an aqueous H.sub.3PO.sub.4 solution. The neutralized powder is then again dried. In order to verify the efficiency of the crosslinking, the powder is immersed in m-cresol at 90 C.; insoluble particles appear, characterizing good crosslinking of the polyamide (no insoluble material in the case of Orgasol 2002 D Nat1).

(18) Evaluation of the Powder:

(19) Two types of powder were compared: a commercial powder called Orgasol 2002 D NAT 1, which is not crosslinked; this powder constitutes the comparative powder A. It corresponds to a powder prepared according to the process previously described, but without the crosslinking step, the powder according to the invention denoted B, which is obtained using the process according to example 1.
Solubility Test:

(20) The refractometric response coefficient K(RI) is measured. The K(RI) is directly proportional to the concentration of PA12 in HFIP. A coefficient of 201 is measured for the Orgasol 2002 D Nat 1, and a coefficient of 45 is measured for the powder of the invention.

(21) The results are given in table 1 below.

(22) TABLE-US-00001 TABLE 1 Comparative Powder according powder to the invention (L12) A (L12) B Average diameter (m) 19.0 19.0 Melting point ( C.) 180 183 Enthalpy of fusion (J/g) 113 108 Solubility in HFIP (%) 100 23

(23) These results demonstrate the fact that the particle size of the initial powder is preserved.

(24) The test on the enthalpy of melting shows that the crosslinking of the powder does not in any way modify the crystallinity of the material.

(25) The results relating to the solubility of the powders show that the crosslinking of the powder has indeed taken place. Furthermore, this test demonstrates the solvent-resistance of the powder B according to the invention.

(26) 2. Synthesis of a Crosslinked PA6 Powder

(27) 2452 ml of solvent and then successively 919.3 g of lactam 6 (caprolactam), 4.7 g of EBS (ethylenebisstearamide) and 10.5 g of Aerosyl R972 are placed in the reactor maintained under nitrogen. After having started the stirring at 300 rpm, heating is carried out gradually up to 110 C., and then 294 ml of solvent are distilled off under vacuum in order to azeotropically entrain any trace of water that might be present.

(28) After a return to atmospheric pressure, the anionic catalyst: 6.3 g of sodium hydride at 60% purity in oil, is then rapidly introduced under nitrogen and the stirring is increased to 900 rpm, under nitrogen at 105 C. This temperature is then maintained for 30 minutes. By virtue of a metering pump, the chosen activator, namely stearyl isocyanate (27.3 g made up to 147.1 g with solvent), is continuously injected into the reaction medium according to the following program: 49 g/h for three hours and then the reaction takes place for a further two hours at 120 C.

(29) The step of surface crosslinking the powder obtained is carried out as follows. At 120 C. with stirring, a solution of hexamethylene diisocyanate: 5 mol % of HMDI relative to the lactam 6, is added to the reaction medium for three hours, and then the resulting mixture is left for a further two hours in order to consume all the reactive functions.

(30) The powder is then filtered and dried in order to be neutralized with an aqueous H.sub.3PO.sub.4 solution. The neutralized powder is then again dried.

(31) Evaluation of the Powder:

(32) Two types of powder were compared: a commercial powder called Orgasol 1002 D Nat1, which is not crosslinked; this powder constitutes the comparative powder C. It corresponds to a powder prepared according to the process previously described, but without the crosslinking step, the powder according to the invention denoted D, which is obtained using the process according to example 2.
Solubility Test

(33) The powders are immersed in m-cresol at 90 C. Insoluble particles appear, characterizing good crosslinking of the polyamide D. There is no appearance of insoluble material in the case of the comparative powder C.

(34) The refractometric response coefficient K(RI) is measured. The K(RI) is directly proportional to the concentration of PA6 in HFIP. A coefficient of 209 is measured for the comparative powder C, and a coefficient of 82 is measured for the powder D according to the invention.

(35) The results are given in table 2 below.

(36) TABLE-US-00002 TABLE 2 Comparative Powder according powder to the invention (PA6) C (PA6) D Average diameter (m) 20.9 20.9 Melting point ( C.) 210 205 Enthalpy of melting (J/g) 116 109 Solubility in HFIP (%) 100 38
3. Synthesis of a Crosslinked Polyamide-Imide Powder
Synthesis of the Polyamide:

(37) 2800 ml of solvent and then successively 919.3 g of lactam 12 (lauryllactam), 14.7 g of EBS (ethylenebisstearamide) and 1.84 g of Sipernat 320DS are placed in the reactor maintained under nitrogen. After having started the stirring at 300 rpm, heating is carried out gradually up to 110 C., and then 280 ml of solvent are distilled off under vacuum in order to azeotropically entrain any trace of water that might be present.

(38) After a return to atmospheric pressure, the anionic catalyst: 2.36 g of sodium hydride at 60% purity in oil, is then rapidly introduced under nitrogen and the stirring is increased to 800 rpm, under nitrogen at 105 C. This temperature is then maintained for 30 minutes. By virtue of a metering pump, the chosen activator, namely stearyl isocyanate (27.3 g made up to 147.1 g with solvent), are continuously injected into the reaction medium according to the following program: 49 g/h for three hours and then the temperature is raised to 120 C. for two hours.

(39) Synthesis of the Polyamide-Imide:

(40) A polyamide-imide powder is obtained as follows. At 120 C. with stirring, a solution of methylenebis(cyclohexyl isocyanate): 0.1 mol % of H12MDI relative to the lactam 12, is added to the reaction medium for three hours, and then the resulting mixture is left for a further two hours in order to consume all the functions of the crosslinking agent. A polyamide-imide powder completely soluble in m-cresol at 90 C., which constitutes the comparative powder E, is then obtained.

(41) Crosslinking of the Polyamide-Imide:

(42) The step of surface crosslinking the powder obtained is carried out as follows. At 120 C. with stirring, a solution of hexamethylene diisocyanate: 4.9 mol % of HMDI relative to the lactam 12, is added to the reaction medium for three hours, and the resulting mixture is then left for a further two hours in order to consume all the reactive functions.

(43) The powder is then filtered and dried in order to be neutralized with an aqueous H.sub.3PO.sub.4 solution. The neutralized powder is then again dried.

(44) Evaluation of the Powder:

(45) Two types of powder were compared: a comparative powder E, resulting from the synthesis of the polyamide-imide, described above; the powder according to the invention denoted F, which is obtained using the process according to example 3.
Solubility Test

(46) The powders are immersed in m-cresol at 90 C. Insoluble particles appear, characterizing good crosslinking of the polyamide-imide F.

(47) The results are given in table 3 below.

(48) TABLE-US-00003 TABLE 3 Comparative Powder according powder to the invention (PAI) E (PAI) F Average diameter (m) 49.4 49.4 Melting point ( C.) 178 177 Enthalpy of melting (J/g) 110 106 Solubility in HFIP (%) 100 30

(49) These examples show that, whatever the type of polymer used, the particle size of the initial powder and the crystallinity of the material are preserved.