METHOD FOR PREPARING COMPOSITION SUITABLE FOR ELECTROSTATIC PAINTING

Abstract

Methods are provided for preparing a composition having a melt viscosity of between 160 Pas and 400 Pas, as determined according to ISO-11443-2014 at 270° C. and a shear rate of 265 1/s, and a volume resistivity of at most 10.sup.5 Ohm.Math.cm, measured according to ASTM D257 on an injection molded test sample of 3 mm thickness and a diameter of 50 mm and coated with a gold layer on an upper and lower surface and, as well as the composition itself and painted parts comprising the composition.

Claims

1. A method for preparing a composition having a melt viscosity of between 160 Pas and 400 Pas, as determined according to ISO-11443-2014 at 270° C. and a shear rate of 265 1/s, and a volume resistivity of at most 10.sup.5 Ohm.Math.cm, as measured according to ASTM D257 on an injection molded test sample of 3 mm thickness and a diameter of 50 mm and coated with a gold layer on an upper and lower surface, the method comprising the following steps: (i) providing a polybutylene terephthalate (PBT) having a relative solution viscosity (RSV) of between 1.6 and 2.4 as measured at 25° C. in meta-cresol according to ISO 1628-5, in an amount of between 40 and 70 wt % with respect to the total weight of the composition; (ii) providing a polyethylene terephthalate (PET) having a relative solution viscosity (RSV) of between 1.1 and 1.5 as measured at 25° C. in dichloro acetic acid according to ISO 1628-5, in an amount of between 10 and 40 wt % with respect to the total weight of the composition; (iii) providing a filler in an amount of between 10 and 30 wt % wherein the filler is chosen from the group consisting of glass fibers, talc, mica, wollastonite, glass beads and milled glass and combinations thereof, wt being with respect to the total weight of the composition; (iv) providing an electrically conductive additive in an amount between 2 and 10 wt % with respect to the total weight of the composition, wherein the electrically conductive additive is chosen from the group consisting of carbon black, carbon fibers, graphite, carbon nanotubes, and graphene and combinations thereof and wherein the electrically conductive additive exhibits a volume resistivity of at most 10.sup.5 Ohm.Math.cm, as measured according to ASTM D257 on an injection molded test sample of 3 mm thickness and a diameter of 50 mm, consisting of 8 wt % of electrically conductive additive and 92 wt % of PBT having a relative solution viscosity of 2.0 in meta cresol, wherein the wt % is with respect to the total weight of the injection molded test sample, which injection molded test sample was coated with a gold layer on an upper and lower surface; and (v) mixing at least the PBT, PET, filler and electrically conductive additive at a temperature of between 240 and 260° C. in order to obtain the composition.

2. The method according to claim 1, wherein the electrically conductive additive exhibits a volume resistivity of at most 10.sup.5 Ohm.Math.cm when measured on an injection molded test sample consisting of 7 wt % of electrically conductive additive and 93 wt % of PBT having a relative solution viscosity of 2.0 in meta cresol.

3. The method according to claim 1, wherein the electrically conductive additive exhibits a volume resistivity of at most 10.sup.5 Ohm.Math.cm when measured on an injection molded test sample consisting of 6 wt % of electrically conductive additive and 94 wt % of PBT having a relative solution viscosity of 2.0 in meta cresol.

4. The method according to claim 1, wherein at least part of the mixing is performed in an extruder.

5. The method according to claim 4, wherein the extruder is a twin-screw extruder.

6. The method according to claim 4, wherein the filler and/or the electrically conductive additive is provided to the extruder via a side feeder.

7. The method according to claim 1, wherein the filler is talc.

8. The method according to claim 1, wherein the electrically conductive additive is carbon black.

9. The method according to claim 8, wherein the carbon black has an oil absorption number of at least 300 ml/100 g, as measured according to ASTM D-2414, and wherein the carbon black is provided in an amount of between 3 and 5 wt %.

10. A method for preparing a painted part, comprising the steps of: (a) preparing the composition according to the method of claim 1; (b) preparing a part comprising the composition; (c) grounding or charging the part such that the part attracts paint particles; (d) coating the part with at least one layer of paint either by spraying the paint in such a manner that the paint is charged so as to be attracted to the part or dipping the part into a charged paint reservoir; and (e) drying and/or curing the paint after application of one or more layers of the paint thereby obtaining a painted part.

11. A painted part obtained by the method according to claim 9.

Description

EXAMPLES

[0049] RSV (relative solution viscosity) of PBT was analyzed according to ISO 1628-5. This method describes the determination of the viscosity of PBT in dilute solution in m-cresol using capillary viscometers.

[0050] The PBT samples were dissolved during 15 min at 135° C. and diluted in m-cresol; concentration was 1 gram in 100 gram m-cresol at 25° C.

[0051] The flow time of the m-cresol and the flow time of the PBT solution were measured at 25° C. The RSV was calculated from these measurements.

[0052] RSV of PET was analyzed according to ISO 1628-5. This method describes the determination of the viscosity of PET in dilute solution in DCA (dichloroacetic acid) using capillary viscometers. The PET samples were dissolved at 15 min and 90° C., and diluted in DCA; concentration is 0.5 gram in 100 ml solvent at 25° C.

[0053] The flow time of the DCA and the flow time of the PET solution were measured at 25° C.

Materials Used

[0054] PBT, polybutylene terephthalate having a relative solution viscosity (RSV) in m-cresol of 2.0, supplier LanXess
PET, polyethylene terephthalate having a relative viscosity (RSV) in DCA of 1.34, supplier DSM
Filler: talc Tital 4591, delivered by Sibelco
Conductive Carbon Black CB-1: Ketjenblack EC-600 JD, delivered by AkzoNobel; OAN of 490 ml/100 gram.
Conductive Carbon Black CB-2: Vulcan XCmax, delivered by Cabot; OAN of 320 ml/100 g.
Transesterification inhibitor: monosodium dihydrogen phosphate, NaH.sub.2PO.sub.4, supplier Budenheim.
Mold release agent: Glycolube P, delivered by Lonza
Paint system: water based primer layer (BASF SecuBloc Diamantweiss FU70-0000-002548-900), color layer (BASF Colorbrite MB9149 polar weiss FV58-9149-0025) and top coat (2K-Klarlack iGloss Harter SB81-0400-0025 and iGloss FF81-0400-0025). Top coat was prepared according to manufacturer recommendation using butyl acetate as diluent.

Determination of Volume Resistivity of Electrically Conductive Additive

[0055] In order to determine the volume resistivity of the electrically conductive additive, test sample were prepared from a composition consisting of various amounts of electrically conductive additive and the remainder of the composition consisting of PBT having a relative solution viscosity (RSV) in m-cresol of 2.0. The compositions were prepared on a ZSK 25 38D twin-screw extruder from Coperion Werner & Pfleiderer. Barrel temperature was set at 240−260° C., screw speed was 300 RPM. PBT was dosed to the hopper. The carbon black was introduced via a side-feeder into the polymer melt. Extruded strands were cooled in water and granulated. The test samples were injection molded from pre-dried (10 hours at 120° C. under vacuum with nitrogen flow) granules on an Engel EVC 110 injection molding machine, with temperature settings of between 240 and 260° C., and mold temperature of 90° C. Geometry of the test sample was a disk of 3 mm thickness and 50 mm in diameter.

[0056] The method of to determine volume resistivity is based on the ASTM D257 standard with the difference that the sample was coated with gold on the upper and lower surface using a standard sputtering technique to ensure a homogeneous electrical field and thus a homogeneous current density. The sample was placed between two 3 mm thick electrodes with a diameter of 50 mm. These two electrodes were connected to one of the measuring devices, depending on the range of the sample resistance: Keithley 617 Electrometer (High range), Fluke 179 Multimeter (Mid range), or HP 3478A Multimeter (Low Range).

[0057] The volume resistivity was calculated using the geometry of the sample. The measurements were performed at 23° C. and 50% RH. Volume resistivity of PBT materials based on CB-1 and CB-2, respectively is shown in Table 1.

TABLE-US-00001 TABLE 1 CB-1 [%] 4 5 CB-2 [%] 8 10 Volume 9 .Math. 10.sup.9 1 .Math. 10.sup.3 7 .Math. 10.sup.10 5 .Math. 10.sup.3 res. [ohm .Math. cm]

Method for Preparation of the Composition

[0058] The method for preparation of the compositions were carried out on a ZSK 25 38D twin-screw extruder from Coperion Werner and Pfleiderer. Barrel temperature was set at 240 to 260° C., screw speed was 300 RPM and yield was 18-24 kg/hour. Components such as PBT, PET, transesterification inhibitor, and mould release agent were dosed to the hopper as a pre-blend. Talc and carbon black were introduced via a side-feeder into the polymer melt. Extruded strands were cooled in water and granulated.

Melt Viscosity

[0059] Melt viscosity of the composition was measured with a capillary rheometer as described in ISO-11443-2014. As melt viscosity determination is known to be sensitive to experimental conditions, the following protocol consistent with ISO-11443-2014 A2-type test (capillary die with specified volume flow rate) was used. Experiments were performed with a Gottfert Rheograph 120 with oven diameter 12 mm, and a capillary of length 30 mm, diameter 1 mm, a ratio of length to diameter of 30/1 and entrance angle 180° (flat). The pressure transducer was a CAN-bus with maximum 2000 bar. The filling time was 45 to 60 seconds, the melting time 300s, and the piston speed changed from low to high speed in the following steps: 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5 and 10 mm/s. The measuring temperature was 270° C. Data point, take over: 1% tolerance, comparison time 10 seconds. The volume flow rate, Q, is calculated to be equal to the piston velocity in mm/s multiplied by the area of the piston in square mm. The shear rate is calculated according to ISO-11443-2014 section 8.5 equation 12. No Bagley correction is applied and the shear stress is calculated according to ISO-11443-2014 section 8.3.2 equation 5. The ratio of the shear stress and shear rate as previously defined is the melt viscosity according to ISO-11443-2014 section 3.10. The results are reported as melt viscosity at a piston velocity of 0.2 mm/s, and the extrapolated melt viscosity at a shear rate of 265 1/s. The compositions all had a moisture content below 0.01%.

Preparation of Test Samples by Injection Moulding to Measure Mechanical Properties.

[0060] Various test samples were injection moulded from pre-dried (10 hours at 120° C. under vacuum with nitrogen flow) granules on an Engel EVC 110 injection moulding machine, with temperature settings 245-265° C., and mould temperature of 90° C.

Properties Injection Moulded Test Samples

[0061] The tensile tests were carried out at 23° C. according to ISO 527 type 1A method. Tensile testing speed 5 mm/min was used. [0062] The Charpy impact strength was evaluated at 23° C. according to ISO 179/1eU method. [0063] The method of resistivity measurements were based on the ASTM D257 standard with the difference that samples having the geometry of a disk of 3 mm thickness and 50 mm diameter were coated with gold on the upper and lower surface using a standard sputtering technique to ensure a homogeneous electrical field and thus a homogeneous current density. The sample was placed between two 3 mm thick electrodes with a diameter of 50 mm. These two electrodes were connected to one of the measuring devices, depending on the range of the sample resistance: Keithley 617 Electrometer (High range), Fluke 179 Multimeter (Mid range), or HP 3478A Multimeter (Low Range). [0064] The volume resistivity was calculated using the geometry of the sample, which was a disk of 3 mm thickness and 50 mm in diameter. The measurements were performed at 23° C. and 50% RH.

[0065] Parts were spray painted with a 3 layer paint system consisting of a primer layer, a color layer, and a clear coat. Paint can be applied via electrostatic spray application (ESTA-Bell application) or pneumatic spray application. Each layer was dried and cured according to the manufacturer guidelines for that paint system.

[0066] Paint adhesion was assessed on painted parts after the paint system as fully cured. Two crossed scratches were made such that the scratch was fully through the paint coating and into the body of the part. Then pressurized water was run along the scratches. This water jet testing was done according to ISO 16925:2014, method. Paint adhesion was judged visually with little or no paint removed being rated as good and larger areas of paint removal judged as bad. Good and bad ranking corresponds to ISO 162925:2014 ratings of 0-1 and 2-5, respectively.

TABLE-US-00002 TABLE 2 Comp-A Comp-B Example-1 Comp-C Comp-D CB-1 [%] 0 3 4 CB-2 [%] 6 8 Volume res. >10.sup.12 7 .Math. 10.sup.9 4 .Math. 10.sup.3 1 .Math. 10.sup.10 2 .Math. 10.sup.3 [ohm .Math. cm] Mechanical properties E-modulus [MPa] 5210 5710 5800 6045 6510 Stress at break 59 56 55 58 58 [MPa] Strain at break [%] 2.1 1.4 1.3 1.4 1.2 Charpy [kJ/m2] 33 24 23 27 23 Capillary flow at 270° C. Shear rate [1/s] 264 263 266 265 267 Melt viscosity 162 241 248 302 418 [Pa .Math. s] Electrostatic painting Paintability bad bad good bad good Paint adhesion good good good bad bad

Comparative Experiment a

[0067] A composition was prepared and tested comprising 20 wt. % PET, 20 wt. % talc, 0.3 wt. % mould release agent, and 0.1 wt. % transesterification inhibitor, the balance being PBT. The material properties are shown in table 2. The paintablity is rather poor due to the high volume resistivity of the material.

Comparative Experiment B

[0068] As in Comparative example A, however 3% of the electrically conductive component CB-1 was used, the balance being PBT. The material is more brittle, e.g lower strain at break and Charpy impact strength when addition of the Carbon black. The paintability appears to be not sufficient due to the high volume resistance.

Example 1

[0069] As in Comparative example B, however 4% of the electrically conductive component CB-1 was used, the balance being PBT. The paintability was good due to improved volume resistivity. Moreover, the paint adhesion of the lacquer system and the conductive PBT substrate was good.

Comparative Experiment C

[0070] As in Comparative example B, however 6% of the electrically conductive component CB-2 was used, the balance being PBT. The paintability appears to be not sufficient due to the high volume resistivity. In addition, the higher melt viscosity of the composition resulted in poor paint adhesion properties.

Comparative Experiment D

[0071] As in Comparative example C, however 8% of the electrically conductive component CB-2 was used, the balance being PBT. The paintability improved with decreasing volume resistivity, but resulting paint adhesion was rather poor as the melt viscosity of the composition was above 400 Pas.