EP2524009A1

EP2524009A1 - Laser additive

Info

Publication number: EP2524009A1
Application number: EP10793172A
Authority: EP
Inventors: Gerhard Edler; Helge Bettina Kniess; Klaus Bernhardt
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2010-01-14
Filing date: 2010-12-17
Publication date: 2012-11-21
Also published as: DE102010004743A1; JP5752149B2; WO2011085779A1; US20120292295A1; CN102741360A; BR112012017238A8; BR112012017238A2; JP2013517335A; CN102741360B

Abstract

The invention relates to a laser additive in the form of particles comprising a white core and a casing that contains elemental carbon, to a method for the production thereof, and to the use of such a laser additive in organic polymers, in particular in plastics, paints, automobile paints, powder coatings, printing inks, paper coatings, and paper pulps to produce permanent light laser markings on a preferably dark base.

Description

laser additive

The present invention relates to a laser additive in the form of particles comprising a core of white particles and a shell containing elemental carbon, a process for its preparation and its use for the laser marking of black, gray or colored organic polymers, in particular plastics, paints , Automotive coatings, powder coatings, color printing, paper coatings and pulps.

The labeling of assets by means of laser radiation is now a common technology in almost all industries. For example, many times, e.g. Production data, batch numbers, expiry dates, barcodes, company logos, serial numbers, etc. are applied to plastics or plastic films.

The contrast required for a marking is preferably produced according to the following methods:

1. Removal of layers of different coloring

The disadvantage is that this is a very complex process, which is applicable only to a limited extent.

2. Carbonization of an organic matrix

This is currently the most commonly used method. The

Carbonation is caused either by absorption of the laser radiation in the organic matrix itself or by absorption on added absorbers. In both cases, the carbonization of the polymer material is caused by a brief heat shock which burns the surrounding matrix. The ability of the matrix to form enough carbon when burning is of crucial importance for the marking result. Thus, the polymer used or the formulation of the matrix has a strong influence on the marking result. This dependency usually leads to extensive preliminary tests Determination of a sufficient for the respective application

Marking result. In the case of changes in the composition and in many cases even fluctuations in the quality of the raw materials, the appropriate labeling parameters must always be determined again.

3. Color change of added pigments

In order to escape the above-mentioned dependence, it has long been attempted to develop pigments or additives which themselves perform a color change (mark intrinsically) during laser bombardment. Such additives produce a label near n

w regardless of the surrounding matrix. They are therefore in all

Plastics can be used. Marking is also possible in thin layers such as coatings, lacquers and prints without significantly damaging the layers.

In particular, the first two of the methods described above are suitable5

but only for the application of black or dark markings on light or light colored ground. However, laser marking applications are also known in which white or light markings on a colored, gray or black background are desired, for example for computer keyboards. Such markings ought to have a high resistance to mechanical influences, in particular to be abrasion-resistant, and to retain their bright, preferably white, coloring as unchanged as possible over a long period of time.

Dark or black additives are usually added for this purpose to dark or dark-colored plastics, which are decolorized by the action of high-energy laser radiation, ie they are marked intrinsically.

It has been found, however, that the mere removal of the color on the marked areas often leads to only a low contrast with the colored to black surroundings, since the areas exposed to the laser irradiation are generally not large and often also not complete by the laser bombardment Discoloration can be achieved.

0

Therefore, it has proven to be advantageous if, within the marking, the laser bombardment simultaneously with the discoloration of the additive tive or instead of this still a light foaming can be achieved, which significantly improves the contrast between the surface exposed to the laser radiation and the surface surrounding it. This method makes it possible to produce laser markings that are almost white.

However, the laser markings described above have the disadvantage that the foaming within the mark often leads to a raised surface and also inevitably has a certain porosity. If the marking is mechanically stressed, for example when using a computer keyboard marked in white on black, constant pressure is exerted on the marked areas over a long period of time. By this pressure the porous becomes

Foam, in particular if it is raised above the other, unmarked keyboard surface, compressed, which leads to a significant reduction in the contrast of the laser marking and at the same time to increased wear of the mark on the keyboard surface.

There is therefore still a need for laser additives, which lead in particular on colored, gray or black substrates by laser bombardment to a bright to white mark, which remains unchanged even under mechanical stress over a long period.

The object of the present invention is therefore to find an additive for the laser marking that, under the action of laser light in the polymer doped therewith, gives very good marking results, in particular high-contrast and sharp light to white markings on a dark background, leads to mechanically stable laser markings, and industrial scale can be easily produced.

It is also an object of the present invention to provide a method for producing such a laser additive. Another object of the invention is to demonstrate the use of such a laser additive. It has now been found that particles which consist of a white core and a preferably black or gray shell, which can be decolorized by laser irradiation, are outstandingly suitable as marking additives for the laser marking of plastics.

The present invention is a laser additive in the form of particles comprising a core of white particles and a shell, wherein the core consists of one or more particles, has a size of at least 100 nm and is chemically stable against the action of directed high-energy radiation , and wherein the shell contains elemental carbon.

The present invention likewise relates to a process for producing the laser additive according to the invention, wherein white particles which are present as individual particles or as agglomerates of a plurality of particles and the individual particles or agglomerates have a size of at least 100 nm are provided with a shell which is elemental Contains carbon, and wherein the white particles are chemically stable against the action of directed high-energy radiation.

The present invention also provides the use of the laser additive according to the invention as an additive for the laser marking of black, gray or colored organic polymer systems, in particular in plastics, plastic films, paints, automotive coatings, powder coatings, color prints, paper coatings and pulps.

The invention furthermore also relates to the organic polymer systems which contain the laser additive according to the invention.

Under the action of laser light, the polymer system doped with the inventive laser additive shows a bright to white marking with high contrast and pronounced edge sharpness on a black, dark or colored background. The laser additive according to the invention is in the form of finely divided particles, preferably as a powder. It can be easily incorporated into the respective polymeric application medium in this form. The shape and size of the powder particles are not particularly critical. As a rule, there are spherical, egg-shaped, lenticular or strand-like particles. The shape is determined by the shape of the white particles used as the core material and the subsequent coating process. Depending on the enveloping material and the layer thickness of the cladding, irregular but often nearly spherical particles of different sizes are obtained. The size of the individual particles can vary greatly and is generally in the range of 0.2 to 250 μηι. A narrow particle size distribution is advantageous, but not required.

A particle of the laser additive according to the invention is composed of a core and a shell surrounding the core. In this case, the cladding of the core need not be complete, it is sufficient if the vast majority of the surface of the core is surrounded by the shell. In the event that only a minor part of the surface of the core (<50%) is surrounded by the shell, the laser additive would be in the

Application medium depending on the total particle size visible, which is usually not desirable. Therefore, it is preferable if> 50% of the surface of the core is covered by the sheath.

The core of the laser additive according to the invention consists of one or more white particles and has a size of at least 100 nm. If the core consists of several white particles, they are in the form of an agglomerate having a total size of at least 100 nm. It goes without saying that the primary particle size of the particles forming the agglomerate may be less than 00 nm; it is preferably in the range of 10 to 50 nm. The size of the core is assumed to be the length of the largest axis of the core, and the primary particle size to be the length of the largest axis of the primary particles. For the purposes of the present invention, white particles are those particles which have almost no spectral absorption in the wavelength range from 380 nm to 780 nm and which reflect incident light in all directions at these wavelengths. A sample of the white particles is then considered to be white in the sense of the invention if a planar powder coating applied to a flat surface measured with a conventional color measuring instrument under daylight has such a high reflection of irradiated visible light that, measured in the L, a, b system according to Hunter, has a brightness value L of> 50 to 100, but has a, and b values in the vicinity of the non-colored point, that is to say smaller than 10, in particular smaller than 7. Such color values are usually perceived as white by the human eye. It goes without saying that the actual sample deviates from an ideal white sample. However, for the purposes of the present invention, a particle or an agglomerate with a size of at least 100 nm should be considered to be white when a viewer with normal vision perceives a pigment bed of these particles or agglomerates without a reference sample as white.

On the other hand, with individual particles or agglomerates of materials which are non-absorbing in the visible wavelength range and have sizes of less than 100 nm, it is not possible to obtain a sufficient degree of whiteness in the use envisaged according to the invention.

As the size of the core increases, the hiding power of the core particles increases. In general, the size of the core is in the range of 0.1 to 200 μm. To achieve a very high-contrast marking with a particularly good edge sharpness cores with a size of 0.2 to 100 [im preferred.

The shape of the individual particles or agglomerates which form the core of the laser additive according to the invention plays only a minor role. In principle, the cores can be present in all known particle shapes, for example as platelets, spheres, fibers, cubes, rods, cuboids or even as approximately isotropic granules of irregular shape. Preferred are isotropic shapes such as spheres or cubes, or irregular shaped granules. The agglomerates of multiple particles are often present as irregular particle clusters.

Suitable white particles in the context of the present invention are those fine-particle materials which are chemically stable under the action of directed high-energy radiation, irrespective of the medium surrounding them. For the purposes of the present invention, this means that the materials suitable for use as white particles do not change, at least optically under the action of directed high-energy radiation, irrespective of the medium surrounding them, ie retain their white color, but are preferably not subject to any chemical change. These include, for example, customary white pigments or white fillers.

It should be emphasized, however, that the white pigment most frequently used, for example, for color paints, namely titanium dioxide, is unsuitable because it can be reduced to suboxides which have a blue to gray coloration under the action of directed high-energy radiation and under certain conditions (reducing conditions) and thus are not white anymore.

For this reason, as the core particles for the inventive

Laser additive white pigments or white fillers of the appropriate magnitude used, regardless of the surrounding medium under the action of directed high-energy radiation not to colored compounds, especially colored oxides are reduced. As materials for the cores zirconia, silica, barium sulfate (baryte), kaolin or talcum are preferably suitable, each in the form of individual particles or agglomerates with a minimum size of 100 nm. Particularly preferred are kaolin, talc and barium sulfate.

Directed high-energy radiation in the context of the present invention is understood to mean the radiation of conventional lasers, as described below. The radiation generated by the laser is monochromatic, practically parallel, coherent over long distances and mostly bundled. The wavelengths of the laser radiation are usually between 157 nm and 10.6 [im.

The shell surrounding the core contains elemental carbon. This is in the form of carbon black or as a black pigment. Suitable are all forms of technical carbon blacks or color blacks with particle sizes of 1 to 100 nm, which can be used in pulverized form or in the form of aqueous carbon black dispersions, for example the known under the trade names Derussol®, Color Black FW or Color Black S carbon blacks Evonik , in particular Color Black FW 1, Derussol® A and Derussol® N 25 / L.

Although the elemental carbon can be bound to the surface of the core particles by simple mixing via adhesive forces, it is advantageous if the shell surrounding the core also contains an organic polymer in addition to the elemental carbon, with the aid of which the elemental carbon is very homogeneous to the surface of the core particles can be applied and at the same time enclosed in this enclosure.

Suitable organic polymers are those which are not carbonized by high-energy radiation. Preferably, the organic polymer is selected from the group of aminoplasts, in particular from the group melamine resins, urea resins, urea-formaldehyde resins, melamine-formaldehyde resins, urea / melamine mixtures or polyamides. These, or the starting materials for the preparation of the polymers, can be mixed in a simple process with the elemental carbon and applied to the surface of the cores in the form of polymers.

The elemental carbon is in the shell of the laser additive according to the invention in a proportion of 0.1 to 50 wt.%, Preferably from 0.1 to 20 wt.%, Based on the weight of the shell, included.

In principle, no further additives are necessary for the production of the laser additive according to the invention, apart from the materials already described. In a preferred embodiment, the inventive

Laser additive, however, in addition to the core particles, the elemental carbon and an organic polymer also additionally contain one or more protective colloids. These prevent the caking of a larger number of core particles, if their particle size is sufficient as a single particle for the production of a laser additive according to the invention already.

Suitable protective colloids are the classes of compounds known to the person skilled in the art for such purposes, in particular water-soluble polymers such as partially saponified polyvinyl acetate, polyvinyl alcohol, polyvinylpyrrolidone, cellulose ethers (Tylose) such as, for example, methylcellulose, hydroxyethylcellulose and hydroxypropylmethylcellulose, polyacrylates, starch, proteins, alginates, pectins or gelatin consideration. Particularly preferred are cellulose ethers, in particular hydroxyethyl cellulose.

The protective colloids are added in small amounts of from 0.01 to 5%, preferably from 0.1 to 2%, based on the weight of the preparation used for the preparation of the laser additive according to the invention. In this case, the size of the cores can be adjusted in a targeted manner via the amount of the protective colloids used and optionally also additionally present surfactants. Basically, the principle applies here that a larger amount of protective colloids leads to a reduced size of the cores.

For the purposes of the present invention, the size, particle size or primary particle size in each case is the length of the largest axis of the respective particles (cores, primary particles or inventive particulate laser additive). The determination of this size can in principle be carried out by any method familiar to the person skilled in the art for particle size determination. In a simple way, the particle size determination, depending on the size of the laser additives, for example, by direct observation and measurement of a number of individual particles in high-resolution light microscopes, but better in electron microscopes such as the scanning electron microscope (SEM) or the high-resolution electron microscope (HRTEM), but also in the Atomic Force Microscope (AFM), the latter in each case with appropriate image evaluation software done. Advantageously, the determination of the particle size can also be carried out with measuring devices (eg Malvern Mastersizer 2000, APA200, Malvern Instruments Ltd., UK), which operate on the principle of laser diffraction. With these measuring instruments, it is possible to determine both the particle size and the particle size distribution in the volume by means of the standard method (SOP) from a pigment suspension. According to the present invention, the latter measuring method is preferred.

The laser additive according to the invention is preferably present as a gray or black powder. Core and shell have a weight ratio of 20: 1 to 1: 1, preferably from 8: 1 to 2: 1, on.

Particular preference is given to laser additives according to the invention whose core consists of kaolin, talc or barium sulfate in the stated order of magnitude and whose shell contains a melamine resin or a urea-formaldehyde resin, in each case in combination with carbon black. If the particulate laser additive according to the invention is introduced into organic polymers or polymer systems which have a dark-colored, gray or black inherent color, and if these are subjected to the action of high-energy radiation (laser bombardment), the shell of the laser additive containing the elemental carbon is decolorized. Although this is done as with the use of particulate soot as a laser absorber to form CO2, but this arises in the surrounding the core of the laser additive shell and is there extremely fine-pored and in close proximity to the particulate core of the laser additive. Since the organic polymer preferably used in the shell does not carbonise under the conditions of the laser irradiation, the shell as a whole is decolored. At the same time, the latent through the white core

The scattering of the incident light is activated, the white core becomes visible at the marked points and forms a strong contrast to the surrounding (dark) plastic, possibly in cooperation with the shell surrounding the core. It not only high-contrast bright to white marks on dark ground, but also markings are obtained with a pronounced edge sharpness. Foaming on the surface of the marked article practically does not take place. For this reason, the marks obtained are mechanically extremely stable and lose neither contrast nor clarity with increasing mechanical stress.

Another object of the present invention is a method for producing the laser additive according to the invention in the form of particles, wherein white particles having a size of at least 100 nm, which are present as individual particles or as agglomerates and are chemically stable against the action of high-energy radiation, with a shell which contains elemental carbon.

Particularly preferably, a shell is applied to the core forming the white particles, which still contains an organic polymer in addition to the elemental carbon.

The polymer is selected in particular from the group of aminoplasts, in particular from the group of melamine resins, urea resins, urea-formaldehyde resins, melamine-formaldehyde resins, urea / melamine mixtures or polyamides.

In the selection of these polymers, the coating of the white particles forming the core of the laser additive according to the invention, with a polymer shell containing elemental carbon by wet-chemical acid-catalyzed polycondensation of an aqueous preparation, preferably solution, an aminoplast resin in the presence of dispersed core particles and elemental Carbon, preferably in the form of also dispersed carbon blacks. The condensate of the aminoplast becomes insoluble in water and precipitates together with the elemental carbon on the surface of the core.

Preferably used further materials have been previously described. These are referred to here.

In the method according to the invention, the materials mentioned in advance as cores, which are present as white particles, with elemental carbon and the aqueous preparation of an organic polymer and optionally additional additives at a temperature of at least 50 ° C with the addition of an acid intimately mixed, cooled and dried. As aqueous preparation, both dispersions and solutions of organic polymers can be used. By adjusting the pH to between 1 and 7, preferably between 2 and 5, deposition of a polymer shell containing the elemental carbon occurs on the white particles forming the core. The reaction time is usually between 10 and 60 minutes. After cooling the reaction product, the batch is dried at temperatures between 100 and 250 ° C over a period of 2 to 20 hours. The powder obtained can additionally be prepared by grinding and / or screening operations. A gray to black powder is obtained.

The present invention also relates to the use of the above-described laser additive for the laser marking of organic polymers, in particular of dark-colored, gray or black plastics.

In particular, a plastic, preferably a dark-colored, gray or black, in particular a gray or black plastic containing the laser additive according to the invention, subject of the present invention.

By adding the laser additives according to the invention, in particular in concentrations of 0.1 to 30 wt.%, Preferably 0.5 to 20 wt.%, And most preferably from 1 to 10 wt.%, Based on the organic polymer to be marked or In the polymer system, significantly higher contrast is achieved in the laser marking of polymers than with the commercially available (foaming) laser absorbers at comparable concentrations. The concentrations mentioned are not solely dependent on the desired contrast but also on the layer thickness of the feed medium. Thus, in printing and coating applications, significantly higher concentrations are required than in plastics to provide the laser beam with a sufficient number of pigment particles available. However, the concentration of the laser pigment according to the invention in polymers or in polymer systems, preferably in thermoplastics, thermosets or elastomers, is also dependent on the polymer material used. The low proportion of laser pigment changes that

Polymer system insignificant and does not affect its processability.

Furthermore, colorants can be added to the polymers which allow color variations of any kind, in particular the dark-colored, gray or black color of the polymer, and at the same time ensure the visibility of the laser marking. Suitable colorants are in particular colored metal oxide pigments as well as organic pigments and dyes which do not decompose during laser marking or do not react under laser light.

However, very particular preference is given to colorants which color the polymer gray or black, since on dark, gray or black ground the mark which can be achieved under the action of a laser beam with the laser additive according to the invention is particularly rich in contrast and has sharp edges. Although it is also possible to obtain a weak light to white laser marking when adding the laser additive according to the invention to white or light plastics, the achievable contrast is only slight. In addition, the addition of the gray or black laser additive, even if used only in minor amounts, could possibly falsify the original color of the polymer. By contrast, the use of the laser additive according to the invention in dark-colored, gray or black polymer systems is also possible in relatively large quantities and leads there to the expected high-contrast bright marking.

As polymeric materials for laser marking, in particular all known plastics, in particular thermoplastics, furthermore thermosets and elastomers, are suitable, as e.g. in Ullmann, Vol. 15, p. 457 ff., Verlag VCH. Suitable polymers are e.g. Polyethylenes, polypropylene, polyamides, polyesters, polyester esters, polyether esters,

Polyphenylene ether, polyacetal, polyurethane, polybutylene terephthalate (PBT), polymethyl methacrylate, polyvinyl acetal, polystyrene, acrylonitrile butadiene Styrene (ABS), acrylonitrile-styrene-acrylate (ASA), polycarbonate, polyethersulfones and polyether ketones and their copolymers, mixtures, and / or polymer blends, such as PC / ABS, MABS. The incorporation of the laser additive according to the invention into the polymer to be marked, preferably a plastic or a plastic film or a coating, for. As a paint, a paper or powder coating, a car paint or a color print, takes place by the polymer granules, the paint, the ink or paper raw material is mixed with the laser additive and possibly deformed under the action of heat. The further processing of paper raw materials, printing inks and paints then takes place as usual. The addition of the laser additive to the polymer may be simultaneous or sequential. When polymerizing the laser additive, it is optionally possible to add to the polymer, preferably a plastic granulate, adhesives, organic polymer-compatible solvents, stabilizers and / or surfactants which are temperature-stable under the working conditions.

The production of a plastic granulate doped with the laser additive is generally carried out in such a way that the plastic granules are placed in a suitable mixer, wetted with any additives and then the laser additive is added and mixed in. The pigmentation of the polymer is generally carried out via a color concentrate (masterbatch) or compound. The mixture thus obtained can then be processed directly in an extruder or an injection molding machine. The shaped bodies formed during processing show a very homogeneous distribution of the

Laser additive. Subsequently, the laser marking takes place with a suitable laser.

The inscription with the laser takes place in such a way that the specimen is brought into the beam path of a pulsed laser, preferably an Nd: YAG laser. Furthermore, a lettering with an excimer laser, for example via a mask technique, possible. However, the desired results are also to be achieved with other conventional laser types which have a wavelength in a range of high absorption of the additive used. The label obtained is replaced by the time (or pulse rate in pulse lasers) and irradiation power of the laser and the plastic system used. The power of the laser used depends on the particular application and can be determined in individual cases by the skilled person readily.

The laser used generally has a wavelength in the range of 157 nm to 10.6 pm, preferably in the range of 532 nm to 10.6 pm. For example, C0 ₂ lasers (10.6 pm) and Nd: YAG lasers (1064 or 532 nm) or pulsed UV lasers may be mentioned here. The excimer lasers have the following wavelengths. F ₂ excimer laser (157 nm), ArF excimer laser (93 nm), KrCl excimer laser (222 nm), KrF excimer laser (248 nm), XeCl excimer laser (308 nm), XeF excimer laser (351 nm), frequency multiplied Nd: YAG laser with wavelengths of 355 nm (frequency tripled) or 265 nm (frequency quadrupled). Particular preference is given to using Nd: YAG lasers (1064 or 532 nm) and C0 ₂ lasers. The energy densities of the lasers used are generally in the range of 0.3 mJ / cm ² to 50 J / cm ² , preferably 0.3 mJ / cm ² to 0 J / cm ² . When using pulsed lasers, the pulse frequency is generally in the range of 1 to 60 kHz. Corresponding lasers which can be used in the process according to the invention are commercially available.

The use of the polymer doped with the laser additive according to the invention can be carried out in all areas where hitherto customary printing processes for the inscription of plastics, plastic films, are used. For example, molding compounds, semi-finished products and finished parts of the polymer according to the invention can be used in the electrical, electronics and motor vehicle industries. The marking and

Labeling of e.g. Cables, wires, trim or functional parts in the heating, ventilation and cooling area or keyboards, switches, plugs, levers and handles, which are doped according to the invention

Polymers can be marked even in hard to reach places with the help of laser light. Furthermore, the polymer system according to the invention can be used in packaging in the food sector or in the toy sector. The markings on the packages are characterized by the fact that they are smudge-proof and, in particular, mechanically stable, for example scratch-resistant, stable in the event of subsequent damage Sterilization processes, and hygienically pure in the marking process can be applied. Furthermore, plastic corks can be labeled eg for wine bottles.

Complete label images can be permanently applied to the packaging for a reusable system. Furthermore, the polymer system according to the invention finds application in medical technology, for example in the labeling of petri dishes, microtiter plates, disposable syringes, ampoules, sample containers, supply hoses and medical collection bags or storage bags.

Another important field of application for laser marking are plastic brands for the individual marking of animals, so-called cattle tags or ear tags. A barcode system stores the information that is specific to the animal. These can be recalled when needed with the help of a scanner. The label must be very durable, because the mark sometimes remain on the animals for several years.

The laser marking of molding compounds, semi-finished products and finished parts which consist of polymers doped with the laser additive according to the invention is thus possible. Particularly preferred is the use of the laser additive according to the invention for laser marking gray, black or dark colored plastic parts that are exposed to high mechanical stress. Here, a permanent white or light mark on dark or black ground can be achieved. In addition, the inventive laser additive can optionally be introduced into differently colored dark plastic masses without distorting their visual appearance (no gray haze).

When the laser additive according to the invention is used in plastics, almost no foaming of the laser additive takes place on the surface of the plastic during laser bombardment, so that the resulting bright laser marking has virtually no porosity on the plastic surface and thus neither scrapes nor scrapes together under mechanical stress. can be pressed. The achievable bright marking is therefore mechanically stable and durable.

The following examples are intended to illustrate the invention without, however, limiting it. The percentages given are percent by weight.

Example I. ^¬ SO g of kaolin having a particle size of ₀ Dg = 22 pm (determined with a Mastersizer 2000 from Malvern Instruments Ltd., under standard conditions) of water are made into a paste in 50 g. 25 g of melamine-formaldehyde resin (Madurit MW 830, Ineos, 75% solution) and 1 g of carbon black (Color Black FW 1 Evonik) are stirred into the kaolin suspension. 50 g of a 0.3% tylose solution (Tylose H20 from SE Tylose GmbH & Co. KG) are added and the batch is dispersed in a bead mill (bead mill attachment for dissolver "Dispermat" from Getzmann) is then heated to 70 ° C and adjusted with stirring with about 26 ml of a 1 molar hydrochloric acid to pH 3.5 After a reaction time of 30 min, the mixture is allowed to cool.

It is then filtered through a suction filter and dried at 200 ° C overnight. The resulting material is ground and sieved (sieve with a mesh size of 40 pm). A black powder is obtained with a Mastersizer 2000 from Malvern Instruments Ltd. under standard conditions determined particle size Dgo = 31 pm.

The resulting powder is incorporated 2% in polypropylene provided with a black dye, and the resulting compound formed on an injection molding machine to test plates with a size of 6 x 9 cm.

On the test plates is applied with a Nd.YAG laser at a pulse frequency of 1-20 kHz and a speed of 300 mm / s, a white marker in the form of a lettering. If the marked test surface is under intense pressure (friction with a metal object over 120 minutes), the surface of the plastic is smoothed, whereas the white marking shows no appreciable change. Comparative Example 1

Carbon black (Color Black FW 1) is incorporated 2% in black polypropylene analogously to Example 1 and molded on an injection molding machine to test plates in the dimensions given in Example 1. A white lettering is written in the test plates using a Nd: YAG laser operating at a pulse rate and laser speed analogous to Example 1. The resulting label shows a contrast which is comparable to the label according to Example 1. Under the pressure load mentioned in Example 1, the whiteness of the marking changes within a short time (30 minutes) to a brownish yellow.

Example 2:

50 g of kaolin (product of Merck KGaA) having a particle size of Dgo = 22 pm (determined using a Mastersizer 2000 from Malvern Instruments Ltd. under standard conditions,) are mixed with 25 g of urea-formaldehyde resin (Kaurit liquid, 50% , Product from BASF), 4 g of an aqueous carbon black dispersion (Derussol® N25 / L from Evonik, carbon black content 25%) and 50 g of a 0.3% tylose solution (0.15 g Tylose H20, product of the company SE Tylose GmbH & Co., dissolved in 49.85 g of water at 60 ° C) dispersed in a dissolver. The ready-dispersed mixture is then heated to 70 ° C and adjusted with stirring by addition of a 0% strength oxalic acid, a pH of 3 to 4. After a reaction time of 60 minutes, the batch is allowed to cool. The batch is washed with demineralized water, then filtered through a suction filter and dried at 180 ° C overnight. The resulting material is ground and sieved (sieve with a mesh size of 40 pm). A black powder is obtained with a Mastersizer 2000 from Malvern Instruments Ltd. under standard conditions determined particle size D ₉₀ = 28 pm.

The resulting powder is incorporated 2% in polypropylene provided with a black dye, and the resulting compound formed on an injection molding machine to test plates with a size of 6 x 9 cm. An Nd: YAG laser with a pulse frequency of 1-20 kHz and a speed of 300 mm / s tion in the form of a lettering applied. If the marked test surface is under intense pressure (friction with a metal object over 120 minutes), the surface of the plastic is smoothed, whereas the white marking shows no appreciable change.

Claims

claims

A particle comprising a core of one or more white particles, wherein the core has a size of at least 100 nm and is chemically stable against the action of directed

high-energy radiation, and a shell containing elemental carbon.

Particles according to claim 1, characterized in that the white particles consist of a white pigment or a white filler.

Particles according to claim 1 or 2, characterized in that the white particles consist of zirconium dioxide, silicon dioxide, barium sulfate, kaolin or talc.

Particles according to one or more of claims 1 to 3, characterized in that the core has a size in the range of 0.1 to 200 pm.

Particle according to claim 4, characterized in that the core has a size in the range of 0.2 to 100 pm.

Particles according to one or more of claims 1 to 5, characterized in that the shell additionally contains an organic polymer which is not carbonized by directional high-energy radiation.

Particles according to claim 6, characterized in that the polymer is selected from the group of melamine resins, urea resins, urea-formaldehyde resins, melamine-formaldehyde resins,

Urea / melamine mixtures or polyamides. Particles according to one or more of claims 1 to 7, characterized in that the elemental carbon is present in the form of carbon black or as a black pigment.

Process for the preparation of particles according to one or more of claims 1 to 8, characterized in that white particles with a size of at least 100 nm, which are present as individual particles or as agglomerates and are chemically stable against the

Exposure to directed high-energy radiation, be provided with a shell containing elemental carbon.

10. The method according to claim 9, characterized in that the

white particles are provided with a shell which contains an organic polymer in addition to the elemental carbon.

11. The method according to claim 9 or 10, characterized in that the polymer is selected from the group melamine resins, urea resins, urea-formaldehyde resins, melamine-formaldehyde resins, urea / melamine mixtures or polyamides.

12. The method according to one or more of claims 9 to 11,

characterized in that the white particles are intimately mixed with elemental carbon and an aqueous preparation of an organic polymer and optionally additional additives at a temperature of at least 50 ° C with the addition of an acid, cooled and dried.

13. Use of particles according to one or more of claims 1 to 8 for the laser marking of plastics.

14. Use according to claim 13 for the preparation of a non-foaming on the surface of the marked plastic marking. 15. Plastic, containing particles according to one or more of claims 1 to 8.