Process for preparing polyamide granules and uses

Abstract

The subject matter of the present invention is a process for preparing polyamide granules having heat-resistance properties, and also the use of these granules, in particular in the aid of the manufacture of yarns for airbags or for tyre cords. More specifically, the invention relates to a process for preparing polyamide granules having heat-resistance properties by wet impregnation of the granules with an aqueous solution comprising at least one heat stabilizer.

Claims

1. A process for preparing polyamide granules with heat resistance properties, comprising: contacting the polyamide granules with a single aqueous solution comprising at least one heat stabilizer under a controlled atmosphere of inert gas, wherein the contacting is performed on stirred granules; wherein a total amount of an aqueous solution relative to the polyamide granules is less than or equal to 1% by weight; and wherein the heat stabilizer is a mixture of alkali metal or alkaline-earth metal halide and of copper halide; and subjecting the polyamide granules comprising the heat stabilizer to a solid-phase post-condensation step, without drying the polyamide granules before the solid-phase post-condensation step.

2. The process as claimed in claim 1, wherein the alkali metal halide is potassium or sodium iodide.

3. The process as claimed in claim 1, wherein the copper halide is copper iodide.

4. The process as claimed in claim 1, wherein the heat stabilizer is a mixture of potassium iodide and copper iodide.

5. The process as claimed in claim 1, wherein the ratio between the number of moles of halide and the number of moles of copper is greater than or equal to 7/1.

6. The process as claimed in claim 4, wherein the copper iodide is present in an amount of less than or equal to 6% by weight in the single aqueous solution of potassium iodide at 50% by weight.

7. The process as claimed in claim 1, further comprising performing a step of rinsing the polyamide granules with water after contacting the polyamide granules with the single aqueous solution.

8. The process as claimed in claim 1, wherein the total amount of the aqueous solution relative to the polyamide granules is between 0.15% and 1% by weight.

9. The process as claimed in claim 1, wherein the polyamide in the polyamide granules is polyamide 66.

10. The process as claimed in claim 1, wherein the contacting of the polyamide granules with the single aqueous solution is performed by spraying of the single aqueous solution.

11. The process as claimed in claim 7, wherein the total amount of the aqueous solution relative to the polyamide granules includes the amounts of the single aqueous solution and the water from the rinsing step.

Description

EXAMPLES

1Preparation of Aqueous Solutions Comprising at Least One Heat Stabilizer

(1) 1.1Single Solution of CuI/KI:

(2) To prepare 200 g of a single aqueous solution of CuI/KI, use is made of a stirring mixer precleaned and rinsed with demineralized water, into which are placed 80 g of demineralized water and then 94.9 g of potassium iodide and the rest of 14.9 g of demineralized water. The mixture is stirred for 1 hour. The dissolution reaction is highly exothermic. 10.2 g of copper iodide are then added and the mixture is stirred for 20 minutes. The aqueous solution thus obtained is filtered and stored in a brown glass container.

(3) The composition of the solution is checked by elemental analysis (potentiometry with AgNO.sub.3 for iodine and ICP/OES for copper and potassium).

(4) 1.2Single Solution of CuBr.sub.2/KBr:

(5) To prepare 200 g of a single aqueous solution of CuBr.sub.2/KBr, use is made of a stirring mixer precleaned and rinsed with demineralized water, into which are placed 120 g of demineralized water and then 58 g of potassium bromide and the rest of 15.5 g of demineralized water. The mixture is stirred for 1 hour. The dissolution reaction is highly exothermic. 6.5 g of copper dibromide are then added and the mixture is stirred for 20 minutes. The aqueous solution thus obtained is filtered and stored in a brown glass container. The composition of the solution is checked by elemental analysis (potentiometry with AgNO.sub.3 for bromine and ICP/OES for copper and potassium).

2Preparation of the Polyamide Granules

(6) A polyamide 6.6 is prepared from an aqueous solution of N salt at 52% by weight placed in an evaporator with external recirculation with 9 ppm of antifoam (silicone composition). The solution of N salt is concentrated by heating to 154.0 C. at a pressure of 0.24 MPa. At the end of the evaporation, the concentration of dissolved species in the solution is 85.0% by weight. This solution is then transferred into an autoclave. The autoclave is heated so as to obtain an autogenous pressure of 1.85 MPa. The polymerization phase under pressure lasts for 42 minutes, and the pressure is then gradually reduced to atmospheric pressure. The reactor is then maintained at atmospheric pressure for 20 minutes and the temperature reached by the reaction mass at the end of this step is 277 C. The reactor is then placed under a nitrogen pressure of between 0.4 and 0.5 MPa in order to allow the polymer to be extruded in the form of rods, which are cooled with water and cut so as to obtain granules.

(7) The polyamide 6.6 obtained has a viscosity index of 134 mL/g measured in 90% formic acid, at a concentration of 0.5 g/100 mL. Its moisture content is 0.3% measured by Karl-Fischer and its particle size is 25 mg per particle.

(8) The polyamide 6.6 obtained has a (GT)=GTCGTA=68.653.7=14.9 mmol/kg.

3Supplementation by Wet Impregnation

(9) The wet impregnation tests on the granules are all performed in an MLH 12L horizontal mixer from MAP (WAMGROUP) equipped with a jacket that can be heated or cooled, a stirring rotor of ploughshare type and a liquid injection system. The injection nozzle used is a Teejet nozzle with a flow rate of 0.1 L/min and a spraying angle of 50 at a pressure of 0.3 MPa.

(10) To optimize the mixing, the mixer tank is filled to 60% of its volume, i.e. about 4.7 kg, with fresh polyamide. During impregnation on hot granules, the mixer jacket is preheated to 90 C.

(11) The polymer granules are placed in the mixer tank, which is then rendered inert with nitrogen for 5 minutes, during which the stirring is started at 150 rpm. This value was chosen to optimize the mixing and to limit the abrasion of the granules. Next, the aqueous solution of heat stabilizers is injected by introducing the amount of solution required to obtain the desired amount. The liquid introduction system is set at a nitrogen pressure of 0.3 MPa and the valve connecting this lock chamber to the injection nozzle is opened. After 1 minute, the valve of the liquid introduction lock chamber is closed. The rinsing water is introduced into the lock chamber, which is then placed once again under a nitrogen pressure of 0.3 MPa so as to inject the rinsing water via the injection nozzle. At the end of this operation, the mixing time before stoppage of the stirring and emptying-out of the polyamide is counted down.

(12) The supplementation conditions: granule temperatures, the type of heat stabilizers, the amounts of heat stabilizer solution and of rinsing water, the mixing time, are specified for the comparative examples in Table 1 and for the examples according to the invention in Table 2.

4Summary Table of the Supplementation Conditions

4.1Comparative Examples

(13) 1: total weight amount of aqueous solution/weight of polyamide=2%

(14) 2: total weight amount of aqueous solution/weight of polyamide=3%

(15) TABLE-US-00001 TABLE 1 Comparative examples Additives Supplementation process Heat stabilizers Solution Rinsing T C. Stirring Mixing Comp. (ppm) Amount Water Granules Speed Time Ex. Type Cu Br I K (g) (g) ( C.) (rpm) (min) 1 a CuI/KI 60 0 1400 393 16.6 77.4 20 150 5 b 15 c 30 2 a CuI/KI 60 0 1400 393 16.6 124.4 20 150 5 b 15 c 30

4.2Examples According to the Invention: Amount of Aq. Sol.=1% Max

(16) 1: total weight amount of aqueous solution/weight of polyamide=1%

(17) 2: total weight amount of aqueous solution/weight of polyamide=1%

(18) TABLE-US-00002 TABLE 2 Examples according to the invention Additives Supplementation process Heat stabilizers T Stirring Mixing (ppm) Solution Rinsing Granules Speed Time Ex. Type Cu Br I K Amount g Water g ( C.) (rpm) (min) 1 a CuI/ 60 0 1400 393 16.6 30.4 20 150 5 b KI 15 c 30 d 90 5 e 15 f 30 2 a CuBr.sub.2/ 60 1400 0 646 30.28 16.72 20 150 5 b KBr 15 c 30 d 90 5 e 15 f 30

5Results

(19) 5.1. The Granules

(20) Description of the Tests and Measurements Performed on the Supplemented Granules

(21) Assay of the Copper and Potassium by ICP

(22) 0.5 g of polyamide granules is placed in a Parr bomb with 5 ml of nitric acid. Mineralization of the polyamide takes place in a microwave oven. At the end of mineralization, the material is recovered with water and transferred into a 50 ml flask made up with water.

(23) The copper and potassium are assayed by ICP/OES with external calibration in 10 vol % HNO.sub.3 medium.

(24) Assay of the Iodine and Bromine by Argentimetry

(25) Weigh out exactly 0.5 g of polyamide granules, 2 g of 0.06 M sodium sulfite solution and 6 g of osmosed water and then add 50 ml of formic acid at 90% by weight. Allow the mixture to dissolve with stirring for about 1 hour. When the polyamide is dissolved, add 8 g of water. The potentiometric assay of the iodine and bromine is performed with a 0.02 M silver nitrate (AgNO.sub.3) solution.

(26) Scanning Electron Microscopy Analyses

(27) For the analyses of the surface state of the granules, two granules are bonded to a carbon pellet and then metallized with platinum.

(28) To observe the interior of the granule, two granules are included in an Araldite epoxy resin (to facilitate their handling) and are then levelled off with a knife. These slices are bonded to a carbon pellet and then metallized with platinum.

(29) The various preparations are observed with a scanning electron microscope (SEM) at 15 kV with EDX analysis.

(30) Thermal Degradation by UV Spectrophotometry

(31) 0.8 g of polyamide granules is added to 20 ml of a trifluoroethanol/chloroform mixture (10/3 by volume) The mixture is allowed to dissolve with stirring for about 1 hour. The solution is then transferred into a quartz cuvette and placed in the spectrometer for acquisition of the spectrum between 600 and 240 nm. The UV indices are calculated for the peaks at about 285 and 330 nm.

(32) Comparative Table of Results

(33) TABLE-US-00003 TABLE 3 Results of the comparative examples Additives Thermal Microscopy Heat stabilizers degradation Presence of Comparative (ppm) UV UV crystals at examples Type Cu Br I K 330 285 the surface 1 a CuI/ 55 1290 360 0 0 no b KI 56 1300 360 c 56 1310 370 2 a CuI/ 48 1115 315 0 0 no b KI 50 1120 320 c 49 1120 320

(34) TABLE-US-00004 TABLE 4 Results of the examples according to the invention Additives Thermal Microscopy Heat stabilizers degradation Presence of (ppm) UV UV crystals at Examples Type Cu Br I K 330 285 the surface 1 a CuI/KI 57 1340 360 0 0 no b 57 1360 370 c 58 1370 370 d 56 1370 400 e 57 1370 390 f 58 1370 380 2 a CuBr.sub.2/KBr 55 1152 475 0 0 yes b 55 1172 530 c 55 1195 518 d 56 1135 447 e 57 1077 463 f 56 1097 441

CONCLUSIONS

(35) The results of the granule impregnation tests performed with 2% of aqueous solution relative to the weight of granules (Ex. 1) show a slight lack of additive relative to the target and a lack of homogeneity. Furthermore, slight aggregation of the granules with each other and on the wall of the mixer appears, which does not facilitate the handling.

(36) The results of the granule impregnation tests performed with 3% of aqueous solution relative to the weight of granules (Ex. 2) show a real lack of additive relative to the target (20%) and high heterogeneity. Furthermore, substantial aggregation of the granules with each other and on the wall of the mixer appears, which makes them difficult to handle.

(37) The results of the granule impregnation tests performed with the CuI/KI solution at 1% max of aqueous solution relative to the weight of granule (Ex. 1) are at the target and homogeneous. During the impregnation at room temperature, it is preferable to mix for 15 minutes in order to have the same homogeneity as after 5 minutes at 90 C.

(38) From the results of the granule impregnation tests performed with the CuBr.sub.2/KBr solution (Ex. 2), the presence of KBr crystals is observed at the surface of the granules.

(39) In all cases, the impregnation of the granules performed at 90 C. does not lead to any specific thermal degradation of the polymer and a gradient of heat-stabilizing elements below the surface of the granules is observed.

(40) 5.2. The Yarns

(41) Description of the Post-Treatments Performed

(42) Solid-State Post-Condensation

(43) In order to sufficiently increase the number-average molecular mass of the polymers, a solid-state post-condensation step is performed on each 4.7 kg batch of late-supplemented polymer. This solid-state post-condensation is performed in a 50 L rotary evaporator. The granules are heated to 195 C. under 500 L/h of nitrogen for 270 minutes.

(44) SpinningDrawing

(45) The post-condensed polymer is spun and then drawn under standard conditions for industrial yarns so as to obtain 940/136 yarns.

(46) Description of the Tests and Measurements Performed on the Yarns

(47) The yarns undergo thermal aging in an oven at 165 C. for 168 hours under forced air ventilation.

(48) The samples of yarns before and after aging are subjected to mechanical tests on a standard tensile testing machine in order to determine the losses of toughness, breaking stress and elongation at break after aging. The tests are performed at a speed of 500 mm/min with a distance between the jaws of 200 mm and a pretension of 300 g. The percentage loss for the viscosity index is also measured, the viscosity indices being measured as a 0.5% solution in 90% formic acid. The results of the percentage loss of the measured parameters are reported in Table 6 below.

(49) In order to compare the impact of the late supplementation of heat stabilizers on the thermal degradation of the granules during the solid post-condensation and of the yarns during spinning and drawing and on the mechanical properties of the yarns after aging, a polymer for industrial yarns with addition of the heat stabilizers during the polymerization step was used as reference (cf. Table 5 below). This polymer is stabilized with aqueous solutions of copper acetate, potassium bromide and potassium iodide and was produced with the same industrial process and in the same reactor as the fresh polymer used for the late supplementation.

(50) TABLE-US-00005 TABLE 5 Results of characterization of the granules after solid post-condensation Thermal Additives Viscosity degradation Heat stabilizers (ppm) IV UV UV Examples Type Cu Br I K mL/g 330 285 Com- CuBrI 59 406 924 470 180 + ++ parative 1 c CuI/KI 59 1399 380 180 + ++ d 56 1395 380 180 + ++ f 57 1408 390 178 + ++

(51) TABLE-US-00006 TABLE 6 Results of characterization of the yarns after the aging test Measurement Comparative 1 c 1 d 1 f Breaking stress 27.1% 24.4% 24.7% 24.2% (percentage loss) Elongation 18.5% 11.0% 11.3% 10.6% at break (percentage loss) Toughness 27.1% 24.4% 24.7% 24.1% (percentage loss) IVN 3.2% 1.8% 1.7% 1.9% (percentage loss)

(52) It is observed that the degradation of the mechanical properties of the yarns after aging is lesser for the yarns obtained using the process according to the invention than for the yarns obtained using the process of the comparative example.

(53) Similarly, the reduction in viscosity index of the yarns obtained using the process according to the invention is lesser than that of the yarns according to the comparative example.

CONCLUSIONS

(54) The granules supplemented according to the process of the invention behave in the same manner as the reference stabilized on polymerization during the solid post-condensation phase and do not show any different thermal degradation. No loss of heat stabilizers is observed after the solid post-condensation phase.

(55) The aging tests on yarns show less degradation of the mechanical properties of the yarns obtained using the process according to the invention.