LASER ADDITIVE

Abstract

The present invention relates to a laser additive comprising core/shell particles, to a process for the preparation of a laser additive of this type, and to the use thereof, in particular as laser absorber in plastics and plastic-containing coatings of articles.

Claims

1. A laser additive comprising core/shell particles, characterised in that the core/shell particles in each case have a particulate core of homogeneous composition which contains alkaline-earth metal ions and OH groups and has a surface with OH groups located thereon, where the shell consists of carbon.

2. The laser additive according to claim 1, wherein the alkaline-earth ions are at least one type of ions selected from the group consisting of Ca.sup.2+, Mg.sup.2+ and Ba.sup.2+.

3. The laser additive according to claim 1, wherein the particulate cores have a bulk density of ≤0.5 g/cm.sup.3.

4. The laser additive according to claim 1, wherein the particulate cores have a specific surface area of ≥5 m.sup.2/g (BET).

5. The laser additive according to claim 1, wherein the particulate cores have an irregular particle shape or a flake shape.

6. The laser additive according to claim 1, wherein the particulate cores have a particle size in the range from 0.1 to 100 μm.

7. The laser additive according to claim 1, wherein the particulate cores consist of Ca.sub.5(OH)(PO.sub.4).sub.3, Mg.sub.3Si.sub.4O.sub.10(OH).sub.2, or of Ba.sup.2+- or Mg.sup.2+-modified Ca.sub.5(OH)(PO.sub.4).sub.3.

8. The laser additive according to claim 1, wherein the shell consists of a mixture of nano crystalline carbon and amorphous carbon with proportions by weight in the ratio from 70:30 to 90:10.

9. The laser additive according to claim 1, wherein the shell in each case completely surrounds the particulate core in a continuous layer and is chemically bonded to the particulate core.

10. The laser additive according to claim 1, wherein the shell has a geometrical thickness in the range from 1 to 20 nm.

11. A process for the preparation of a laser additive according to claim 1, said process comprising: bringing core particles of homogeneous composition which contain alkaline-earth metal ions and OH groups and have a surface with OH groups located thereon to reaction in a reactor a) in a mixture with a solid carbon-containing precursor compound, or b) with supply of a carbon-containing precursor compound in a stream of carrier gas, where the carbon-containing precursor compound is heated under inert gas to a temperature at which carbon is deposited from the carbon-containing precursor compound onto the surface of the core particles as outermost, continuous layer and a shell forms around the respective core particle.

12. The process according to claim 11, wherein the shell has a geometrical thickness in the range from 1 to 20 nm.

13. The process according to claim 11, wherein the carbon-containing precursor compound employed is acetone, acetylene, p-xylene, toluene, 2-methyl-3-butynol-2 or saccharides.

14. The process according to claim 11, wherein the core particles are kept in motion in the reactor.

15. The process according to claim 11, wherein the reaction is carried out at a temperature in the range from 500 to 850° C. and with a time duration in the range from 20 to 480 min.

16. The process according to claim 11, wherein each core particle is completely surrounded by the shell in a continuous layer and core particle and shell are chemically bonded to one another.

17. A plastic material comprising a plastic and a laser additive according to claim 1 as a laser adsorber.

Description

[0068] FIG. 1 shows a laser-inscribed test piece with a laser additive in accordance with Example 1

[0069] FIG. 2 shows a laser-inscribed test piece with a laser additive in accordance with Example 2

[0070] FIG. 3 shows a laser-inscribed test piece with a laser additive in accordance with Example 3

[0071] FIG. 4 shows a laser-inscribed test piece with a laser additive in accordance with Comparative Example 1

[0072] FIG. 5 shows a laser-inscribed test piece with a laser additive in accordance with Comparative Example 2

[0073] FIG. 6 shows a laser-inscribed test piece with a laser additive in accordance with Comparative Example 3

[0074] FIG. 7 shows a laser-inscribed test piece with a laser additive in accordance with Comparative Example 4

[0075] FIG. 8 shows a laser-inscribed test piece with a laser additive in accordance with Comparative Example 5

[0076] FIG. 9 shows an SEM photomicrograph of the core particles in accordance with Example 1

[0077] FIG. 10 shows an SEM photomicrograph of the core particles in accordance with Example 2

[0078] FIG. 11 shows an SEM photomicrograph of the core particles in accordance with Comparative Example 1

[0079] FIG. 12 shows a TEM photomicrograph of the laser additive in accordance with Example 1 with enlarged detail

[0080] FIG. 13 shows a TEM photomicrograph of the laser additive in accordance with Example 2 with enlarged detail

[0081] The present invention is intended to be explained below with reference to examples, but is not restricted to these.

WORKING EXAMPLES

Example 1

[0082] Coating of Hydroxyapatite Core Particles in a Fluidised Bed Via Chemical Gas-Phase Deposition

[0083] 500 g of hydroxyapatite from Sigma-Aldrich (Article 21223) having a bulk density of 0.26 g/cm.sup.3 is heated to a temperature of 750° C. in an N.sub.2-flushed fluidised bed reactor having an internal diameter of 100 mm under a constantly inert N.sub.2 atmosphere. The volume flow rate of the nitrogen is set so that minimal fluidisation takes place in the reactor and optimal mixing of the starting materials and optimal energy transfer is thus ensured. When the reaction temperature has been reached, the carbon-containing precursor compound is introduced into the reactor by means of the stream of nitrogen. The carbon-containing precursor compound employed is p-xylene. The precursor compound is thermally decomposed in the reactor, and carbon layers having a total thickness in the range from 1 to 10 nm grow on the core particles over a reaction time of 120 minutes. FIG. 9 shows an SEM photomicrograph of the hydroxyapatite core particles employed.

Example 2

[0084] Coating of Core Particles Comprising Plustalc H10® from Mondo Minerals B.V. In a Fluidised Bed Via Chemical Gas-Phase Deposition

[0085] The process is carried out analogously to Example 1 using 500 g of the core particles. An SEM photomicrograph of the core particles employed is shown in FIG. 10. The product has a bulk density of 0.24 g/cm.sup.3. The thickness of the carbon layer is in the range from 2 to 10 nm.

Example 3

[0086] Coating of Hydroxyapatite Core Particles in a Rotary Tube Furnace Via a Solid-State Reaction with Very Finely Ground Sucrose in the Form of Powdered Sugar

[0087] 500 g of hydroxyapatite from Sigma-Aldrich (Article 21223) having a bulk density of 0.26 g/cm.sup.3 are mixed intimately with 188 g of powdered sugar in a Turbular T2F 3D shaking mixer from Willy A. Bachofen AG and subsequently calcined in a Nabertherm RSRC rotary tube furnace under an inert atmosphere. To this end, the stainless steel tube is heated to 700° C. and rotated at a speed of 1 rotation per second. The powder mixture is transferred into the feeder and metered uniformly into the furnace, which is flushed with 200 I/h of the nitrogen, via a conveying screw. The residence time in the furnace is 45 minutes. A carbon proportion of 8.5% by weight is achieved.

COMPARATIVE EXAMPLES

Comparative Example 1

[0088] Coating of Core Particles Comprising Boron Nitride in a Fluidised Bed Via Chemical Gas-Phase Deposition

[0089] 500 g of S1-SF® flake-form boron nitride from 3M are employed as core particles. The starting material consists of flakes having a regular shape, a smooth surface and few centres of scattering. FIG. 11 shows an SEM photomicrograph of the starting material.

[0090] The process is carried out analogously to Example 1. The carbon shell has a thickness in the range from 2 to 10 nm.

Comparative Example 2

[0091] Coating of Core Particles Comprising Calcium Phosphate Ca.sub.3(PO.sub.4).sub.2 without OH Groups Close to the Surface in a Fluidised Bed Via Chemical Gas-Phase Deposition

[0092] 500 g of calcium phosphate Ca.sub.3(PO.sub.4).sub.2 (Article 22147 from VWR) are employed as core particles. The starting material contains alkaline-earth ions, but does not have OH groups either in the core or at the surface of the particles.

[0093] The process is carried out analogously to Example 1. The carbon shell has a thickness in the range from 2 to 10 nm.

[0094] Production of Plastic Test Pieces and Laser Marking:

[0095] The pigment powders obtained from Examples 1 to 3 or Comparative Examples 1 and 2 are mixed with polyethylene granules and wetting additives in a concentration of 1% by weight, based on the total weight, in a mixer and subsequently extruded. The compound obtained is converted into test plates having a size of 6×9 cm using an injection-moulding machine.

[0096] For further comparison, the same test plates (Comparative Examples 3 and 4) are produced with known laser additives (Monarch® 280 carbon black, Printex® 60 carbon black), with these laser additives being added in an amount of 0.2% by weight of the total weight, in order to achieve similar carbon contents compared with Examples 1 and 2 and Comparative Examples 1 and 2.

[0097] In addition, the following procedure is followed for Comparative Example 5:

[0098] 18 g of hydroxyapatite from Sigma-Aldrich (Article 04238) having a bulk density of 0.26 g/cm.sup.3 are mixed with 2 g of carbon black (Printex® 60, Degussa) and homogenised by grinding in a laboratory mill (Severin KM 3868 coffee mill) for 3 minutes. The mixture is mixed with polyethylene granules and wetting additives in a concentration of 1% by weight, based on the total weight, in a mixer and subsequently extruded. The compound obtained is converted into test plates having a size of 6×9 cm using an injection-moulding machine.

[0099] The test plates are inscribed using a fibre laser. A test grid is used which covers the following performance parameters in pulse mode:

[0100] KBA fibre laser (F-9050, UHS):

[0101] Wavelength: 1062 nm

[0102] Output power: 50 W

[0103] Power in the test grid: 100% of the output power

[0104] Frequency: 20-100 kHz

[0105] Speed: 1-15 m/s

[0106] Equivalent laser marking results can also be achieved with other customary lasers for the marking of plastic parts, for example using a vanadate laser under the following conditions:

[0107] Trumpf vanadate laser (Vectormark 5):

[0108] Wavelength 1064 nm

[0109] Output power: 10.5 W

[0110] Power in the test grid: 100% of the output power

[0111] Frequency: 20-100 kHz

[0112] Speed: 500-4000 mm/s

[0113] The material properties of the specimens and details on the coating process and laser inscription are summarised in Table 1.

TABLE-US-00001 TABLE 1 BET spec. surface area CVD coating (nitrogen Nitrogen Volume-weighted sorption) Reaction flow rate percentile values - Specific Carbon temper- fluidised Malvern Mastersizer © surface area/ Carbon content/ ature/ bed/ Sample d10/μm d50/μm d90/μm (m.sup.2/g) source wt % ° C. (l/min) Hydroxyapatite 1.5 3.9 9.6 62 p-Xylene 7.5 750 1.2 Sigma Aldrich 21223 Hydroxyapatite 1.6 6.2 11.1 51 Sucrose 8.5 700 3.3 Sigma Aldrich 21223 Plustalc H1 © Mondo 3.6 8.4 16.4 10.1 p-Xylene 8.0 750 1.2 Boron nitride S1-SF © 0.8 7.8 16.7 16.7 p-Xylene 8.9 750 1.2 Ca3(PO4)2 - VWR 2.5 8.2 21.1 11.4 p-Xylene 6.0 750 1.2 Monarch © 280 carbon 0.045 45.0 black Printex © 60 carbon 0.021 115.0 black LASER marking KBA CVD coating F 9050 fibre LASER Reaction Concentration LASER Writing time in polyethylene/ power/ Frequency/ speed/ Sample t/min wt % W kHz (m/s) Hydroxyapatite 120 1.0 50 20-100 1-15 Sigma Aldrich 21223 Hydroxyapatite 45 1.0 50 20-100 1-15 Sigma Aldrich 21223 Plustalc H1 © Mondo 120 1.0 50 20-100 1-15 Boron nitride S1-SF © 120 1.0 50 20-100 1-15 Ca3(PO4)2 - VWR 120 1.0 50 20-100 1-15 Monarch © 280 carbon 0.2 50 20-100 1-15 black Printex © 60 carbon 0.2 50 20-100 1-15 black

[0114] The results of the laser inscriptions in the test grid are shown in FIGS. 1 to 8. The specimens clearly show that the inscriptions for Examples 1, 2 and 3 (FIG. 1-3) give rise to very pale markings over virtually the entire test grid, whereas Comparative Examples 1-5 (FIG. 4-8) only lead to very unclear or brownish discoloured markings. This result is notable in particular in respect of Comparative Example 5.

[0115] FIGS. 12 and 13 show TEM photomicrographs of laser additives in accordance with Example 1 (FIG. 12) and Example 2 (FIG. 13). The carbon shell is shown with enlarged detail.