Use of mixtures of polymethyl methacrylate and styrene-acrylonitrile copolymers for the production of laser-inscribed moldings

Transparent molded articles having a high-contrast laser inscription thereon are produced from a polymer mixture consisting of PA0 A) from 40 to 99% by weight of a polymer of an alkyl ester of (meth)acrylic acid, PA0 B) from 1 to 50% by weight of a copolymer of styrene and acrylonitrile having an acrylonitrile content of from 8 to 30% by weight, PA0 C) from 0 to 50% by weight of rubber particles, and PA0 D) from 0 to 20% by weight of additives and processing auxiliaries, by preparing a transparent molded article from the polymer mixture, and exposing the transparent molded article to laser radiation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the use of polymer mixtures containing, as 
essential components, 
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A) from 40 to 99% by weight 
of a polymer of an alkyl ester 
of (meth)acrylic acid, 
B) from 1 to 50% by weight 
of a copolymer of styrene and 
acrylonitrile having an 
acrylonitrile content of from 8 
to 30% by weight, 
C) from 0 to 50% by weight 
of a rubber, and 
D) from 0 to 20% by weight 
of additives and processing 
auxiliaries, 
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for the production of moldings provided with an inscription with the aid of 
high-energy radiation. 
2. Description of the Related Art 
The production of laser-inscribed moldings from polymethyl methacrylate 
(PMMA) has hitherto been impossible in satisfactory quality without the 
addition of additives to increase the absorption coefficients of the laser 
radiation employed. 
In particular for inscription with the Nd:YAG laser which is the most 
frequently used, pigments are added, but these adversely affect the 
transparency of the PMMA, which is frequently a prerequisite for the use 
of PMMA in the application. In the area of measuring beakers, scales and 
measuring rules, this is unacceptable; on the other hand, a durable 
inscription which is highly legible both in transmitted light and in 
incident light is desirable in these very applications. 
The addition of carbon black to improve the laser inscribability of molding 
compositions comprising thermoplastics is described in DE-C 30 44 722, but 
this does not give satisfactory results in the case of PMMA. 
Mixtures of PMMA and styrene-acrylonitrile copolymers (PSAN) are known and 
described, for example, in Polymer 28 (1987), 1177; at an acrylonitrile 
content of the PSAN in the range from 18 to 23% by weight, the 
transparency of moldings produced from the molding compositions is 
retained extremely well. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to modify PMMA in such a way that 
very high-contrast inscription with sharp edges is possible using Nd:YAG 
lasers without there necessarily being any adverse effect on the 
transparency of the inscribed moldings. 
We have found that this object is achieved by the novel use of the molding 
compositions described at the outset. 
Preferred embodiments of the novel use are given in the subclaims. 
DESCRIPTION OF THE REFERRED EMBODIMENT 
The polymers A) of alkyl esters of (meth)acrylic acid are preferably those 
comprising 
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from 50 to 100% by weight of 
a C.sub.1 --C.sub.18 -alkyl ester of 
(meth)acrylic acid, and 
from 0 to 50% by weight of 
further 
free-radical-polymerizable 
monomers. 
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In general, the acrylic acid esters employed are the C.sub.1 -C.sub.18 
-alkyl esters, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, 
i-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, 
2-ethylhexyl, nonyl, decyl, lauryl or stearyl acrylate, preferably methyl 
acrylate, n-butyl acrylate or 2-ethylhexyl acrylate, or mixtures of these 
monomers. 
In general, the methacrylic acid esters employed are the C.sub.1 -C.sub.18 
-alkyl esters, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, 
i-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, 
2-ethylhexyl, nonyl, decyl, lauryl or stearyl meth acrylate, preferably 
methyl methacrylate, or mixtures of these monomers. 
The use of hydroxyl-, epoxy- and amino-functional methacrylates and 
acrylates is also possible. 
The acrylates and methacrylates and mixtures thereof are generally employed 
in amounts in the range from 50 to 100% by weight, preferably from 80 to 
100% by weight, based on the total amount of monomers of component A). 
As further comonomers, up to 50% by weight, preferably up to 20% by weight, 
of the following monomers, which are listed by way of example, can be 
employed: 
vinylaromatic compounds, such as styrene, alpha-methyostyrene, vinyltoluene 
or p-(tert-butyl) styrene; 
acrylic and methacrylic acid; 
acrylamide and methacrylamide; 
maleic acid and the imides and C.sub.1 -C.sub.10 -alkyl esters thereof; 
fumaric acid and the imides and C.sub.1 -C.sub.10 -alkyl esters thereof; 
itaconic acid and the imides and C.sub.1 -C.sub.10 -alkyl esters thereof; 
acrylonitrile and methacrylonitrile. 
The monomers are expediently selected so that the polymer formed has a 
glass transition temperature of at least 30.degree. C. in order to avoid 
the polymer beads formed from sticking. 
Observations hitherto suggest that particularly advantageous polymers are 
those comprising from 80 to 100% by weight of methyl methacrylate and from 
0 to 20% by weight of methyl acrylate and/or n-butyl acrylate and having a 
molecular weight M.sub.n (number average) in the range from 20,000 to 
300,000 g/mol. 
Corresponding products and also processes for their production are known 
per se and are described in the literature. An example of a commercial 
product is Lucryl.RTM. methacrylate copolymer from BASF 
Aktiengesellschaft. 
Component B) in the molding compositions used in accordance with the 
invention comprises from 1 to 50% by weight, preferably from 3 to 40% by 
weight, in particular from 5 to 35% by weight, of a copolymer of styrene 
and acrylonitrile having an acrylonitrile content of from 8 to 30% by 
weight, preferably from 15 to 25% by weight, in particular from 18 to 23% 
by weight. 
The viscosity number of such polymers is generally in the range from 40 to 
100 ml/g, preferably from 50 to 90 ml/g (measured in 0.5% strength by 
weight solution in dimethylformamide at 25.degree. C.). 
The molding compositions used in accordance with the invention can contain 
a rubber as further component C. If the transparency of the products is an 
important criterion, it should be ensured that the difference in 
refractive index between the matrix comprising A) + B) and the rubber is 
not greater than 0.007, preferably not greater than 0.005, in particular 
not greater than 0.002. 
The size of the rubber particles in the matrix is generally in the range 
from 0.05 to 2 .mu.m, preferably from 0.05 to 0.5 .mu.m, particularly 
preferably less than 0.25 .mu.m. 
The rubber should furthermore preferably have a glass transition 
temperature of below 0.degree. C., preferably below -10.degree. C. 
In principle, all types of commercially available rubbers, including those 
having an inhomogeneous (core/shell) structure, are suitable. 
The morphology of the rubbers can very generally be modified to a 
considerable extent by varying the type of monomers and the sequence of 
their addition. 
The following are mentioned here merely by way of example as monomers for 
the preparation of rubber: butadiene, isoprene, n-butyl acrylate and 
2-ethylhexyl acrylate. These monomers can be polymerized with further 
monomers, for example styrene, acrylonitrile and further acrylates or 
methacrylates or vinyl alkyl ethers. 
Preferred rubbers which may be mentioned here are graft copolymers of 
alkyl acrylates, butadiene and vinyl alkyl ethers, or mixtures thereof, as 
the graft base, and 
graft shells of styrene or substituted styrenes and 
acrylonitrile and/or methacrylonitrile and, if desired, (meth)acrylates. 
Such products are described, for example, in U.S. Pat. No. 1,260,135, DE-A 
32 27 555, DE-A 28 26 935, DE-A 31 49 357, DE-A 31 49 358 and DE-A 34 14 
118. 
In some cases, it has proven advantageous to add small amounts, in general 
not more than 200 ppm, preferably from 1 to 50 ppm, in particular from 2 
to 20 ppm, of carbon black, graphite or animal charcoal to the molding 
compositions used in accordance with the invention, since this can in some 
cases improve the legibility of the laser inscription. 
In addition to the components described above, the molding compositions can 
also contain other conventional additives and processing auxiliaries. 
Mention is made here, merely by way of example, of plasticizers, 
lubricants, antioxidants, adhesion promoters, light stabilizers and 
pigments. The proportion of such additives is generally in the range from 
0 to 20% by weight, and preferably from 0.001 to 5% by weight. 
Corresponding compounds are known to the person skilled in the art and are 
described, for example, in EP-A 327 384. 
The molding compositions described above are used in accordance with the 
invention for the production of moldings provided with an inscription with 
the aid of high-energy radiation. 
The production of the moldings to be inscribed is not a subject-matter of 
the invention; corresponding processes are known to the person skilled in 
the art from the literature. A preferred process which is mentioned here 
merely by way of example is mixing of the components in the melt in an 
extruder, and subsequently processing the resultant granules by injection 
molding. 
High-energy radiation which can be used in the novel use is, for example, 
electron or laser radiation, preference being given to laser radiation. 
Particular preference is given to Nd:YAG solid-state lasers. The moldings 
to be inscribed can be exposed to the radiation, for example, via an 
appropriate mask which corresponds to the desired inscription, although 
the electron or laser beam can also be controlled by means of a computer. 
The intensity and duration of the exposure affect the contrast, penetration 
depth and surface structure of the molding. Details in this respect are 
given, for example, in two relevant publications in Kunststoffe 81 (1991), 
Issue 4, pages 341ff., and in Kunststoffe 78 (1988), Issue 8, pages 
688ff., which concern the laser inscription of moldings made from 
thermoplastics. Mention may furthermore be made of DE-A 39 36 926, DE 36 
19 670, DE 30 44 722 and EP-A 190 997. 
The inscribed moldings obtainable by the novel use are distinguished by 
good contrast between the inscription and the background and 
simultaneously a good surface structure. 
Particularly good results are achieved if moldings produced from the 
compositions described herein are provided with an at least two-coat 
finish, preferably with a first pale coat and a further darker coat which 
is applied on top of the first and has sufficient color contrast to the 
first coat to ensure good legibility. The color difference .DELTA.E (in 
accordance with DIN 6174) between the colors of the finish coats is 
advantageously at least 5, preferably at least 7, particularly preferably 
at least 10. 
Particular preference is given to the combination of black as a first 
finish coat and white as a second finish coat. 
If moldings produced in this way are then inscribed with a laser in a 
manner known per se, the dark finish coat is evaporated or burnt off, 
giving a very high-contrast inscription (pale marking on a dark 
background), which is highly legible, in particular, under background 
illumination with a visible-light source. Moldings of this type are 
therefore particularly suitable for the production of displays or display 
scales, for example for consumer electronic products, such as radio sets. 
Particularly good results are achieved for such uses if the molding 
composition from which the corresponding moldings are produced comprises 
polymer mixtures as described in EP 62 223, ie. mixtures of 
styrene-acrylonitrile copolymers, methyl methacrylate polymers and graft 
polymers based on styrene-acrylonitrile copolymers which have been 
impact-modified by means of acrylate rubber and/or butadiene rubber.

EXAMPLES 
In the examples below, the inscriptions were carried out using an Nd:YAG 
laser which emits in the IR region at a wavelength of 1,064 .mu.m. 
The laser beam was, as described in EP-A 330 869 which corresponds to U.S. 
Pat. No. 4,959,406, guided over the surface which was at the point of 
focus, in accordance with the shape to be written by two 
computer-controlled rotatable mirrors. The inscription was optimized for 
the mixtures used by adjusting the lamp current, diaphragm, pulse 
frequency and speed of the laser beam. 
The contrast, which is also given in the Table, was deterined by measuring 
the ratio between the background and marking luminance under a uniform 
illuminance of 500 cd/m.sup.2. 
The light transmission X was measured in accordance with DIN 5036 on 
circular disks with a thickness of 2 mm. 
The following components were used: 
A: Copolymer comprising 99% by weight of methyl methacrylate and 1% by 
weight of methyl acrylate and having a molecular weight (M.sub.n) of 110 
000 g/mol (Lucryl.RTM. methacrylate copolymer G88 from BASF 
Aktiengesellschaft) 
B: Copolymer comprising 81% by weight of styrene and 19% by weight of 
acrylonitrile and having a viscosity number of 70 ml/g (measured in 0.5% 
strength by weight solution in dimethylformamide at 25.degree. C.). 
All mixtures also contained 10 ppm of carbon black (Printex.RTM. carbon 
block 90 from Degussa). 
The mixtures were mixed, homogenized and extruded in a twin-screw extruder 
from Werner & Pfleiderer. The quality was determined on tiles with a 
thickness of 2 mm produced by injection molding. 
The results are shown in the Table. 
TABLE 
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visual assessment of the 
Composition Contrast inscription quality 
A (% by 
B (% by 
pale dark pale dark Transmission 
Ex. wt.) wt.) 
inscription 
inscription 
inscription 
inscription 
.tau. in % 
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1 C *) 
100 -- 1.1 12.9 unsatisfactory 
poor 91 
2 95 5 1.2 12.6 satisfactory 
moderate 
91 
3 90 10 1.2 12.4 satisfactory 
satisfactory 
91 
4 85 15 1.3 10.5 good satisfactory 
91 
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*) Comparative Example