The invention relates to an object containing a burnt-in mark and a process for manufacturing the object. An object of this type is known from EP-A-53256. That patent application describes a plastic object containing a burnt-in mark, the mark having been provided by irradiating the object with high-energy radiation in an irradiation pattern that has the shape of the mark.

In the present patent application a mark is understood to be a pattern that contains information, such as an image, an emblem, a logo, a text comprising lettering and/or numerals, a code, such as a bar code, and so forth, or a pattern with an aesthetic or decorative function. Examples of objects containing marks are containers, such as measuring cups, syringes and the reservoir of a coffee maker, which have on the inside of their walls a mark indicating the capacity or the measure. Other examples are keyboard keys, buttons, advertising signs, name plates, company sign boards and housings of electronic and electromechanical equipment. Such marks are increasingly applied by burning in the mark on a surface of the object, for example with the aid of a laser beam. An advantage of such a mark is that the mark can be provided in a predetermined place with great accuracy. Another advantage is that the objects can be series- manufactured in the known manner, without the mark being applied yet, while afterwards any mark, which may differ from one individual object to another, may be applied at any stage of the production process of the object.

A disadvantage, however, is that the mark can be applied in only a limited range of colours. On a plastic

object the mark has a very dark colour if the plastic or a component in the plastic carbonizes as a result of burning-in, or a light colour if the plastic foams as a result of burning-in. Usually, the mark is even entirely black or entirely white. Although it is possible that the mark has a colour which differs from black or white, because the mark was formed by the interaction of the high-energy radiation with a pigment or a mixture of pigments in a plastic, resulting in a colour change, as described in for instance EP-A-327508, the number of colours obtainable in this way is still limited.

The aim of the invention is to provide an object of which the mark, from the viewer's side, is observed in a colour that differs from the colour of the mark itself. This aim is achieved in that, on the viewer's side of the mark, a light-transmitting plastic layer is present on the mark, which contains a colorant and/or a pigment. The viewer 's side of the mark is the side from which the mark should be read or viewed in order for the viewer to understand the information content of the mark. In this way it is achieved that the observed colour of the mark not only depends on the colour that the mark itself possesses, but also on the colour of the transparent layer. If the mark has a light colour, the observed colour of the mark is even determined largely by the colour of the transparent plastic layer. Further, the colour in which the mark is observed on the viewer 's side is dependent on for instance the concentration of the colorant or the pigment and the thickness of the light- transmitting plastic layer. The light-transmitting plastic layer can in principle comprise any plastic with sufficient light transmittancy. If the plastic layer has a relatively low light transmittancy, the thickness of the light-transmitting layer should likewise be small. In this connection it is important that it is still possible to observe the mark through the light-transmitting plastic

layer .

The light-transmitting plastic layer preferably contains a plastic with a light transmittance coefficient of at least 70%. Further requirements to be met by the properties of the plastic depend among other things on the way in which the object is manufactured, the design of the object and the use of the object. Examples of thermoplastics that the light-transmitting plastic layer may contain are polyethylene, polypropylene, polycarbonate, polyacrylates, thermoplastic polyesters, polyvinyl chloride, polystyrene and copolymers containing monomer units of the aforementioned polymers, such as styrene-maleic-anhydride and styrene-acrylonitrile copolymers. Examples of thermosetting plastics that the transparent layer may contain are epoxy resins, urethane resins, acrylate resins, saturated and unsaturated polyester resins, melamine resins and urea-formaldehyde resins. It is also possible that the transparent layer contains a light- transmitting elastomer, per example a silicone rubber or an EPDM rubber. If the light-transmitting plastic layer contains a thermoplastic or an elastomer, the thickness of the plastic layer is for example 0.05-6.0 mm, preferably 0.3-3 mm. The pigment or colorant concentration in such a plastic layer is usually 0.01-10 wt.%, preferably 0.1-1 wt.%.

If the light-transmitting plastic layer contains a thermosetting plastic, the thickness of the plastic layer is for example 10-200 microns, preferably 20-80 microns. The pigment or colorant concentration in such a plastic layer is usually 1-30 wt.%, preferably 5-15 wt.%. The pigment is preferably a mother-of-pearl and/or an interference pigment. An advantage of these pigments is that even if the mark has a dark colour or is even entirely black, the mark can be observed in a light colour. Another advantage of these pigments is that the pigments have little or no effect on the colour of a

light-coloured substrate, while a dark-coloured or black mark burnt in the substrate is observed in a bright colour.

According to DIN 55943, mother-of-pearl pigments consist of transparent, layered pigment particles. The pigment particles reflect a portion of the incident light. The rest is transmitted. Mother-of-pearl pigments cause a mother-of-pearl gloss like that characteristic of pearls. By using mother-of-pearl pigment in the light-transmitting plastic layer, a dark substrate is observed in a dark, silver-grey hue and a light substrate in a light silver- grey hue. In this way it is possible to burn in a dark mark in a light substrate through carbonization of the substrate and then observe the mark in a dark, silver-grey hue, contrasting with a light, silver-grey hue. It is also possible to burn in a light-coloured mark in a dark- coloured substrate through foaming of the material of the substrate and then observe the mark in a light silver-grey hue, contrasting with a dark silver-grey hue. Interference pigments usually contain platelet-shaped particles, which reflect a portion of the light that falls on the plate-shaped particles, possibly absorb a portion and transmit the rest, with the reflected and transmitted light having a colour that deviates from the incident light due to interference (this will hereinafter be referred to as the interference effect). Mother-of-pearl pigments and interference pigments are described in for example "Die Bibliothek der Technik, Band 56, Verlag Moderne Industrie, Landsberg/Lech, Germany (ISBN 3-478-93075-8), pp. 20-41.

Examples of mother-of-pearl pigments that are suitable for application in the plastic layer of the object according to the invention are Natural Pearl Essence (a mixture of guanine and hypoxanthine) , lead carbonate, bismuth chloride and mica particles, whether or not entirely or partially covered with a thin layer of a metal oxide, such as titanium dioxide.

Examples of interference pigments that are suitable for application in the object according to the invention are lead carbonate, bismuth chloride and mica particles, which are entirely or partially covered with a layer of a metal oxide, such as titanium dioxide, which is thick enough to bring about the interference effect. The interference effect occurs, for example, with a mica particle covered with a titanium dioxide layer of about 100 microns. The colour of the mark observed on the viewer 's side depends on several factors. The colour depends on the colour of the mark itself, for example, which in turn depends on the intensity with which the mark is burnt in and the type of plastic in which the mark is burnt in. The best results are obtained when due to the burning-in the plastic is carbonized, because in that case the mark can be observed in brighter colours.

Further, the colour of the mark observed on the viewer's side depends on for example the type of mother-of-pearl pigment, the type of interference pigment, the concentration of the interference pigment in the transparent plastic layer and the thickness of the plastic layer.

Besides mother-of-pearl pigments and interference pigments, the light-transmitting plastic layer may contain a colorant or another pigment, so that the mark can be observed in still more colours. This makes it possible, for example, to observe the mark in a colour differing from silver-grey when a mother-of-pearl pigment is used. The object according to the invention has many embodiments. It is possible, for example, that the object comprises a plastic structural member, the light- transmitting plastic layer being present on the structural member at least at the location of the mark and the mark being located at the interface of the structural member and the light-transmitting plastic layer.

The object according to the invention preferably has a structural member which, at least at the place where the

mark is located, has a colour highly contrasting with the mark. The best results are obtained when the structural member is untransparant at least at the place where the mark is located. The structural member may comprise any conceivable material, such as a plastic, a metal or a ceramic material.

In another embodiment it is possible that, at least at the place where the mark is located, the object according to the invention comprises only the light- transmitting plastic layer, or even that the object consists largely or entirely of the light-transmitting plastic layer, with the mark being located in the layer. The mark is preferably located in the innermost part of the light-transmitting plastic layer. In this way it is achieved that on the viewer 's side and on the opposite side the mark is covered with a plastic layer, so that the mark is protected against wear, damage, fouling, etc. Furthermore, burning in the mark in the innermost part of the plastic layer makes it possible to use a structural member made of a material in which no mark can be burnt. The object of the invention, without the mark having been applied to it, can be manufactured according to the known processes and be irradiated with high-energy radiation with a radiation pattern having the shape of the mark.

It is possible, for example, to mix a granulate or powder of a light-transmitting thermoplastic with the colorant or the pigment in a tumbler or a batch mixer equipped with an agitator and subsequently hot-melt-compounding the mixture in a kneader, such as a twin-screw extruder, a single- screw extruder or a batch kneader. After the colorant or the pigment has been dispersed in the melt the mixture thus obtained can be granulated and cooled. The granulate can be remelted in any of the known manners and used for shaping the light-transmitting layer that the object according to the invention contains. It is possible, for

example, to process the granulate with the aid of an extruder or an injection moulding machine into a layer- shaped object consisting entirely of the plastic layer, such as films, sheets, tubes, measuring cups and bottles. It is subsequently possible to burn a mark in the surface of the layer-shaped object according to any of the known methods using a laser light beam. The mark is of course provided in such a manner that the light-transmitting plastic layer is present on the viewer's side of the mark. Preferably, a mark is made with the aid of two laser beams which intersect at a point in the innermost part of the transparent layer. It is important to choose the beam intensity and the duration of the treatment in such a way that at the intersection a mark is formed while outside the intersection the plastic layer is scarcely affected by the laser beams, if at all.

It is also possible to hot-melt-compound the granulate together with one or more other plastics or plastics mixtures with the aid of a coextruder or a multilayer injection moulding machine and shaping them into a multilayer object, the light-transmitting plastic layer usually being on the outside, and the structural member being under the light-transmitting plastic layer. Subsequently the mark can be provided in the structural member with the aid of a laser light beam, the beam intensity, the duration of the treatment and/or the wavelength of the laser light being chosen so that the beam penetrates the transparent plastic layer without affecting this layer, but the mark being provided in the structural member at the interface between the transparent layer and the structural member. Examples of objects that can be made in this way are keyboard keys, buttons, housings of electronic and electromechanical eguipment, company signboards and advertising signs. It is also possible to mix the colorant or the pigment with a thermosetting resin and other commonly-used additives to form a paint composition. The paint can then

be applied to a structural member in the usual manner. With the aid of a laser light beam a mark can subsequently be provided in the structural member at the interface between the transparent plastic layer and the structural member, the beam intensity, the duration of the treatment and/or the beam intensity of the laser light being chosen so that the beam penetrates the transparent plastic layer without affecting the layer. It is also possible to provide the mark in the transparent plastic layer by using two intersecting laser beams, as described earlier. This may be resorted to, for example, when the structural member cannot be influenced by the laser beam, or can be influenced only with great difficulty. Of course, it is also possible to manufacture a structural member in any of the known manners, burn in the mark in the structural member and then apply the plastic layer at least onto the mark by melting it thereon, gluing it thereto or applying it thereto in the form of a paint. The invention will be elucidated in more detail with reference to the examples, without being limited thereto.

Example I 228.8 g of saturated polyester resin (URALAC

(TM) SN 805 X 70, supplied by DSM from the Netherlands) was mixed with 47.6 g of melamine formaldehyde binder (URAMEX (TM) MF 821 B, supplied by DSM from the Netherlands) in a glass beaker. 15.0 g of Iriodin (TM) 9211 Rutil Feinrot WR, supplied by Messrs Merck from

Germany, was added to this mixture as interference pigment and dispersed by stirring.

The paint composition thus obtained was sprayed onto aluminium sheets on which a white primer was already present to form a top coat of about 50 μ , and was subsequently baked at a temperature of 160°C for 20 minutes. The sheets were subsequently irradiated across

part of their surface with a laser beam from an SHG Nd.YAG Q-switch laser of the type Haas Laser (TM) 6411 Engravity System (supplied by Haas Laser from Germany). The pulse duration was 110 ns, the wavelength was 532 nm. At a low irradiation energy density (< 2 J/cm ² ) no effect was observable. At a higher irradiation energy density (2-6 J/cm ² ) after the treatment a mark is present in the layer of white primer on the sheet, which mark is observed through the top layer in a red colour contrasting with an off-white background. The top layer is not affected. Upon a further increase of the beam intensity (6-8 J/cm ² ) the red colour becomes darker, but at the same time degradation of the top layer occurs, which manifests itself in surface roughening.

Comparative experiment A

The aluminium sheets of example I, but without the top coat having been applied to them, were irradiated as in example I with irradiation energy densities of 2 and 4 J/cm ² . A dark-coloured to black mark was present on the sheets afterwards.

Example II

In a glass beaker 320.0 g of saturated polyester resin (URACRON (TM) CR 226 XB, supplied by DSM from the Netherlands) and 47.6 g of melamine formaldehyde binder (URAMEX (TM) MF 821 B, supplied by DSM from the Netherlands) were mixed. 15.0 g of Iriodin (TM) 9235 Rutil Perlgrϋn WR, supplied by Messrs Merck from Germany, was added to this mixture and dispersed by stirring.

In the same manner as in example I a top coat was applied to the aluminium sheets and the sheets were irradiated with a laser beam. At a low irradiation energy density (< 2 J/cm ² ) no effect was observable. At a higher irradiation energy density (2- 6 J/cm ² ) a mark is present in the layer of white primer on the sheet, which is observed through the top coat in a

green colour contrasting with an off-white background. The top coat is unaffected. Upon a further increase in beam intensity (6-8 J/cm ² ) the green colour becomes darker, but at the same time degradation of the top layer occurs, which manifests itself in surface roughening.

Examples III-VII

Four dry blends of 99.9 wt.% polycarbonate granulate (Xantar (TM) 24R, supplied by DSM from the Netherlands) and 0.1 wt.% of an interference pigment

(examples III-VI) or a mother-of-pearl pigment (example VII) were injection moulded into transparent sheets measuring 65*65*3 mm using an Arburg (TM) Allrounder injection moulding machine at an injection moulding temperature of 300°C.

See Table 1 for the pigments used.

The transparent sheets were provided with a mark in mirror writing on their backs with the aid of a Nd.YAG Q-switch laser of the type Haas Laser (TM) 6211 Engravity System (supplied by Haas Laser from Germany). The wavelength was 1064 nm, the pulse duration was 150 ns. The irradiation energy density was 8 J/cm ² .

Viewed from the back, all marks are black. On the front, the mark is observed through the transparent layer in a colour depending on the pigment used, as indicated in Table 1.

T A B L E

The pigments mentioned in Table 1 were supplied by Messrs. Merck from Germany.

Example VIII

A dry blend of 99.5 wt.% polycarbonate granulate and 0.5 wt.% Iriodin 9221 Rutil Feinblau WR as used in example III was injection moulded into transparent sheets measuring 65*65*3 mm using an Arburg (TM) Allrounder injection moulding machine at an injection moulding temperature of 300°C. Subsequently a dry blend of 95 wt% of the polycarbonate granulate and 5 wt% Tiofine (TM) R41, a titanium dioxide pigment supplied by Tiofine from the USA, was injection moulded in the same manner into white sheets measuring 65*65*3 mm. A transparent sheet and a white sheet were subsequently pressed onto one another using a standard compression mould with heated press platens at a temperature of 260°C. Upon cooling a white sheet was

obtained which had a transparent surface layer. The sheet was irradiated on the side of the transparent surface layer using a laser beam as in example I. At an irradiation energy density of 4-6 J/cm ² a mark is provided in the white layer which is observed through the surface layer in a blue colour. The transparent surface layer remains unaffected.

Comparative experiment B A white sheet from example VIII is irradiated with the laser beam in the same manner as in example VIII.

Afterwards, a black mark is visible on the white sheet, which has been formed due to carbonization at the surface of the sheet.

Example IX

An aluminium sheet is provided with a top coat and irradiated with a laser beam as is described in example II, the sheet this time having been provided with a red primer. The mark is once again observed in a green colour, but this time in a red-brown background.