Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1997-10-17
2003-02-18
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S440000, C524S408000, C524S409000, C524S449000, C428S403000, C523S137000
Reexamination Certificate
active
06521688
ABSTRACT:
The present invention relates to laser-markable plastics which are distinguished by the fact that they contain pigments having a coating of doped tin dioxide.
The labelling of products is becoming of increasing importance in virtually all sectors of industry. Thus, for example, production dates, use-by dates, bar codes, company logos, serial numbers, etc., must frequently be applied. At present, these marks are predominantly made using conventional techniques such as printing, embossing, stamping and labelling. However, the importance of non-contact, very rapid and flexible marking using lasers, in particular in the case of plastics, is increasing. This technique makes it possible to apply graphic inscriptions, for example bar codes, at high speed even on a non-planar surface. Since the inscription is in the plastic article itself, it is durable and abrasion-resistant.
Many plastics, for example polyolefins and polystyrenes, have hitherto proven to be very difficult or even impossible to mark by means of lasers. A CO
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laser which emits light in the infrared region at 10.6 &mgr;m produces only a weak, virtually illegible mark in the case of polyolefins and polystyrenes, even at very high output. In the case of the elastomers polyurethane and polyether-esters, Nd-YAG lasers produce no interaction, and CO
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lasers produce an engraving. The plastic must not fully reflect or transmit the laser light, since no interaction then occurs. Neither, however, can there be strong absorption, since in this case the plastic evaporates and only an engraving remains. The absorption of the laser beams and thus the interaction with the material depends on the chemical structure of the plastic and the laser wavelengths used. It is in many cases necessary to add appropriate additives, for example absorbers, to make plastics laser-inscribable.
The laser labelling of plastics is increasingly being carried out using Nd-YAG lasers in addition to CO
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lasers. The YAG lasers usually used emit a pulsed energy beam having a characteristic wavelength of 1064 nm or 532 nm. The absorber material must exhibit pronounced absorption in this specific NIR region in order to exhibit an adequate reaction during the rapid inscription operations.
DE-A 29 36 926 discloses that the inscription of a polymeric material by means of laser light can be achieved by admixing the plastic with a filler which discolours on exposure to energy radiation, such as carbon black or graphite.
EP 0 400 305 A2 describes highly polymeric materials which can be inscribed by means of laser light and which contain copper(II) hydroxide phosphate or molybdenum(VI) oxide as discolouring additive.
A plastic moulding composition based on an organic thermoplastic polymer and containing a black pigment and which can be provided with characters by exposure to laser radiation is disclosed in EP 0 522 370 A1.
However, all the fillers known from the prior art have the disadvantage that they durably colour the plastic to be inscribed and consequently the laser inscription, which is usually a dark script on a paler background, is then no longer sufficiently high in contrast.
The filler or the successful absorber should therefore have a very pale inherent colour or need only be employed in very small amounts. The contrasting agent antimony trioxide satisfies such criteria. U.S. Pat. No. 4,816,374 employs antimony trioxide for laser inscription in thermoplastic elastomers by means of Nd-YAG lasers. It is employed in a concentration of from 3 to 8%, depending on the matrix material and the writing speed of the laser. Laser marking is possible using cadmium and arsenic compounds, but such substances are no longer used owing to their toxicity.
The object of the present invention was therefore to find laser-markable plastics which enable high-contrast marking on exposure to laser light and contain only small amounts of heavy metals.
Surprisingly, it has been found that thermoplastics containing pigments which have a coating of, for example, antimony-doped tin dioxide enable high-contrast marking with sharp edges.
The invention therefore relates to laser-markable plastics which are characterized in that thermoplastics contain pigments having a doped tin dioxide coating.
The addition of the pigments in concentrations of from 0.1 to 4% by weight, preferably from 0.5 to 2.5% by weight, in particular from 0.3 to 2% by weight, based on the plastic system, achieves a contrast on laser marking which corresponds to or is even superior to that of a plastic containing significantly more antimony trioxide in terms of concentration. However, the concentration of the pigments in the plastic depends on the plastic system employed. The small proportion of pigment changes the plastic system insignificantly and does not affect its processing properties.
The tin dioxide coating of the pigments is preferably doped with antimony, arsenic, bismuth, copper, gallium or germanium, in particular with antimony, or the corresponding oxides. The doping can amount to 0.5-50% by weight, preferably 0.5-40% by weight, in particular 0.5-20% by weight, based on the tin dioxide. The tin dioxide coating may be a conductive or non conductive coating.
Transparent thermoplastics doped with such pigments in a pure colouration exhibit slight metallic shimmering, but retain their transparency. The addition of from 0.2 to 10% by weight, preferably from 0.5 to 3% by weight, of opaque pigments, for example titanium dioxide, can, if required, completely hide this metallic sheen, in particular in the case of thermoplastic polyurethane. Furthermore, coloured pigments which allow colour variations of all types and simultaneously ensure retention of the laser marking, can be added to the plastics.
The pigments suitable for the marking and their preparation processes are described, for example, in DE-A 38 42 330 and EP 0 139 557. The pigments are preferably based on platelet-shaped, preferably transparent or semi-transparent substrates of, for example, phyllosilicates, such as, for example, mica, talc, kaolin, glass, SiO
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flakes, synthetic or ceramic flakes or synthetic support-free platelets. Also suitable are metal platelets, for example aluminium platelets, or platelet-shaped metal oxides, for example iron oxide or bismuth oxychloride. The particularly preferred substrate comprises mica flakes coated with one or more metal oxides. The metal oxides used here are either colourless, high-refraction metal oxides, such as, in particular, titanium dioxide and/or zirconium dioxide, or coloured metal oxides, for example chromium oxide, nickel oxide, copper oxide, cobalt oxide and in particular iron oxides.
The tin dioxide coating is applied to the substrate in a manner known per se, for example by the method described in EP 0 139 557. The coating of tin dioxide doped with antimony, arsenic, bismuth, copper, gallium or germanium is applied to the platelet-shaped substrate in an amount of about 25-100%, in particular in an amount of about 50-75%.
Pigments which are particularly suitable for laser marking are those based on platelet-shaped metal oxides or platelet-shaped substrates, preferably mica, coated with one or more metal oxides. Particularly suitable pigments are those which are distinguished by the fact that the base substrate is first coated with an optionally hydrated silicon dioxide coating before the doped tin dioxide coating is applied. Such pigments are described in DE 38 42 330. In this case, the substrate is suspended in water and the solution of a soluble silicate is added at a suitable pH; if necessary, the pH is kept in the suitable range by simultaneous addition of acid. The silicic acid-coated substrate can be separated off from the suspension before the subsequent coating with the tin dioxide coating and worked up or coated directly with the doped tin dioxide coating.
All known thermoplastics, as described, for example, in Ullmann, Vol. 15, pp. 457 ff., published by VCH, can be used for laser marking. Examples of suitable plastics are polyethylene, polypropylene, polyamides, polyesters, pol
Krietsch Burkhard
Linzmeier Rainer
Prissok Frank
Asinovsky Olga
Merck Patent Gesellschaft mit beschrankter Haftung
Millen White Zelano & Branigan P.C.
Seidleck James J.
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