Laser-markable plastics

Details Laser-markable plastics Laser-markable plastics

2001-04-13

2004-04-27

Wyrozebski, Katarzyna (Department: 1714)

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...

C524S492000, C524S436000, C524S439000, C524S445000, C524S449000, C523S210000

Reexamination Certificate

active

06727308

ABSTRACT:

The present invention relates to laser-markable plastics of which a feature is that they comprise as dopant at least one metal powder and/or semimetal powder selected from the group consisting of aluminium, boron, titanium, magnesium, copper, tin, silica and zinc and one or more effect pigments based on phyllosilicates.
The identity marking of production goods is becoming increasingly important across almost all sectors of industry. For example, it is frequently necessary to apply production dates, expiry dates, bar codes, company logos, serial numbers, etc., to plastics or polymer films. At the present time, these marks are made predominantly using conventional techniques such as printing, embossing, stamping and labelling. Growing importance is being acquired, however, by contactless, high-speed and flexible marking using lasers, especially in the case of plastics. This technique makes it possible to apply graphic inscriptions, such as bar codes, at high speed even to a non-planar surface. Since the inscription is within the plastics article itself, it is durably abrasion-resistant.
Many plastics, such as polyolefins and polystyrenes, for example, have to date proved to be very difficult or even impossible to mark by means of laser. A CO
2
laser which emits light in the infrared region at 10.6 &mgr;m produces only a faint, barely legible mark on polyolefins and polystyrenes, even at very high output levels. In the case of the elastomers, polyurethane and polyether esters, there is no interaction with Nd-YAG lasers, whereas with CO
2
lasers engraving occurs. The plastic must not completely reflect or transmit the laser light, since if it did so there would be no interaction. However, there must also not be strong absorption, since in that case the plastic would evaporate to leave only an engraving. The absorption of the laser beams, and hence the interaction with the material, is dependent on the chemical structure of the plastic and on the laser wavelength used. In many cases it is necessary to add appropriate additives, such as absorbers, in order to render plastics laser-inscribable.
For the laser identity marking of plastics, use is increasingly being made, besides CO
2
lasers, of Nd:YAG lasers. The YAG lasers commonly used emit a pulsed energy beam having a characteristic wavelength of 1064 nm or 532 nm. The absorber material must exhibit pronounced absorption within this specific NIR range in order to show a sufficient reaction in the rapid inscription processes.
The dopants known from the prior art, however, all have the disadvantage that they persistently color the plastic that is to be inscribed, with the consequence that the laser inscription, which is usually a dark text on a light background, lacks sufficient contrast. Moreover, they have to be added at comparatively high concentrations, and in many cases are toxicologically unacceptable.
It was an object of the present invention, therefore, to find laser-markable plastics which permit high-contrast marking on exposure to laser light. The filler, or the successful absorber, should therefore possess a very pale intrinsic color or should need to be used only in very small amounts.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has surprisingly been found that the laser markability of plastics, and in particular the contrast of the marking, may be improved by using a mixture comprising metal or semimetal powder and one or more effect pigments based on phyllosilicates.
The laser marking of plastics with pearl lustre pigments was first described in Speciality Chemicals, Pearl Lustre Pigments—Characteristics and Functional Effects—May 1982.
Through the addition of a metal or semimetal powder in concentrations of from, for example, 0.5 to 10% by weight, preferably from 0.5 to 7% by weight, and in particular from 0.5 to 5% by weight, based on the effect pigment, a significantly higher contrast is achieved in the laser marking of thermoplastics.
The invention accordingly provides a laser-markable plastic, characterized in that the thermoplastic comprises as dopant at least one metal powder or semimetal powder, preferably selected from the group consisting of aluminum, boron, titanium, magnesium, copper, tin, silicon and zinc, and one or more effect pigments based on phyllosilicates. Besides boron and silicon, other possible semi-metals are Sb, As, Bi, Ge, Po, Se and Te.
The concentration of the dopant in the plastic is dependent, however, on the plastics system used. A too small fraction of dopant does not significantly alter the plastics system and does not influence its processability. Of the metal or semimetal powders mentioned, silicon powder is preferred. Besides the effect pigment, metal or semimetal powder mixtures may also be used as dopant. The mixing ratio of metal to semi-metal is preferably from 1:10 to 10:1, but the metal and/or semimetal powders may be mixed with one another in any ratio. Preferred metal powder mixtures are: silicon/boron, silicon/aluminium, boron/aluminium and silicon/zinc.
In certain compositions of the dopant, the addition also of small amounts of a metal halide, preferably calcium chloride, is advantageous for the contrast of the laser marking of the plastic.
Transparent thermoplastics comprising the aforementioned dopants in straight coloring exhibit a slightly metallic glimmer but retain their transparency. This metallic gleam, especially in the case of polyolefins, may be completely masked if required by the addition of from 0.2 to 10% by weight, preferably from 0.5 to 3% by weight, of hiding pigments, such as titanium dioxide, for example. Furthermore, colorants may be added to the plastics, permitting color variations of any kind and at the same time ensuring retention of the laser marking. Appropriate colorants include, in particular, colored metal oxide pigments and also organic pigments and dyes.
The effect pigments suitable for marking are based preferably on platelet-shaped substrates, preferably transparent or semitransparent substrates comprising, for example, phyllosilicates, such as synthetic or natural mica, talc, kaolin or sericite, for instance. However, the effect pigments may also comprise glass flakes, SiO
2
platelets, Al
2
O
3
platelets and/or TiO
2
platelets.
Particularly preferred substrates are mica flakes coated with one or more metal oxides. Metal oxides used in this context include both colorless metal oxides of high refractive index, such as, in particular, titanium dioxide and/or tin oxide, and colored metal oxides, such as antimony-tin oxide, iron oxide (Fe
2
O
3
, Fe
3
O
4
) and/or chromium(III) oxide, for example. In addition, it is also possible for oxide layers of low refractive index to be present in the pigment structure, such as silicon dioxide for example.
Platelet-shaped pigments are known and to a very large extent are available commercially, for example under the brand name Iriodino® from Merck KGaA, or may be prepared by standard methods known to the person skilled in the art. Effect pigments based on transparent or semitransparent, platelet-shaped substrates are described, for example, in the German patents and patent applications 14 67 468, 19 59 998, 20 09 566, 22 14 454, 22 15 191, 22 44 298, 23 13 331, 25 22 572, 31 37 808, 31 37 809, 31 51 343, 31 51 354, 31 51 355, 32 11 602, 32 35 017 and 38 42 330.
Particularly suitable effect pigments are those having the following structure:
mica+SnO
2
+TiO
2
mica+TiO
2
mica+TiO
2
+Fe
2
O
3
mica+TiO
2
+(Sn,Sb)O
2
.
All known thermoplastics, as described for example in Ullmann, vol. 15, p. 457, Verlag VCH may be employed for laser marking. Examples of suitable plastics are polyethylene, polypropylene, polyamides, polyesters, polyester esters, polyether esters, polyphenylene ethers, polyacetal, polybutylene terephthalate, polymethyl methacrylate, polyvinyl acetal, polystyrene, acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene-acrylate (

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