Thermoplastic resin composition having laser marking ability

A thermoplastic resin composition having laser marking ability, which comprises a thermoplastic resin and at least two kinds of metal oxides, the content of total metal oxides being 0.001 to 10 parts by weight per 100 parts by weight of said thermoplastic resin; and a keyboard having keys composed of the above composition.

DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to a thermoplastic resin composition having laser 
marking ability. More specifically, the invention relates to a 
thermoplastic resin composition which can form distinct marking with 
excellent contrast upon exposure to laser irradiation. 
As a means for marking keytops, printing with ink has heretofore mainly 
been practiced. In case of printing on surfaces of plastic products, the 
surfaces are to be cleaned by washing with Flon (chlorofluoro-carbons) for 
improving adhesion of ink thereto. It is now required, however, to abolish 
the washing process with Flon to prevent destruction of the ozonosphere, 
or to develop a marking technology to allow permanent marking. Hence, a 
means of marking with laser beams is drawing attention, because it enables 
simplified and effective printing. This technology comprises incorporating 
a light- or heat-absorbing additive in raw materials in advance, thereby 
to cause foaming, decomposition or carbonization under the laser 
irradiation to induce changes in the surface of the material or bleach of 
pigment or dye, whereby the desired marking is achieved. 
Japanese Patent Publication No. 61-11771 discloses a method in which 
distinct marking with laser beam is formed by incorporating a mount of a 
carbon black or graphite. 
Japanese Laid-open Patent Publication No. 1-254743 discloses a method for 
improving marking ability of plastics with YAG laser, which comprises 
adding titanium oxide and optionally further carbon black with the 
plastics. 
Japanese Patent Publication No. 61-41320 and Laid-open Patent Publication 
No. 61-192737 describe methods of marking, utilizing decoloration or 
discoloration of pigments, dyestuffs etc. 
Japanese Patent Publication No. 2-47314 discloses a marking method in which 
volatile component(s) in the resin to be marked, such as unpolymerized 
monomers or decomposition products, are foamed by laser beam to form 
projection on the resin surface, whereby marking is formed. 
Japanese Laid-open Patent Publication No. 4-246456 teaches that marking 
with good contrast can be obtained by adding to plastics carbon black 
and/or graphite which are highly thermoconductive. 
Furthermore, Japanese Laid-open Patent Publication No. 2-59,663 discloses a 
process for making plastic key members, using a coloring powder which 
responds to heat irradiation or a dye powder sensitive to heat radiation. 
In those conventional arts, however, the laser marking portion formed by 
foaming have a low degree of blackness. In particular, use of carbon black 
markedly enhances the foaming, which causes notable decrease in black 
color development. 
Meanwhile, colored keytops are very much in vogue, and carbon-based pigment 
material is often used for adjusting colors of keytops. Because carbon 
causes foaming under laser marking, the coloring at the laser marking 
portion is liable to be liver brown, not black. This inevitably renders 
application of carbon-based pigment materials to key tops difficult, and 
hence, it has been desired to make fine marking having more high contrast 
and black coloring. 
It is, therefore, an object of the present invention to provide a novel 
thermoplastic resin composition having laser marking ability. 
Another object of the present invention is to provide a thermoplastic resin 
composition which is capable of forming distinct marking of excellent 
contrast upon exposure to laser irradiation. 
Still another object of the present invention is to provide a thermoplastic 
resin composition which gives marking with clear black color development 
under laser irradiation. 
Other objects and advantages of the present invention will become apparent 
upon reading the following description. 
According to the present invention, above objects and advantages of the 
invention are accomplished by a thermoplastic resin composition having 
laser marking ability, which comprises a thermoplastic resin and at least 
two kinds of metal oxides, the content of total metal oxides being 0.001 
to 10 parts by weight per 100 parts by weight of said thermoplastic resin. 
Examples of thermoplastic resins used for the present invention include 
such general-purpose resins as polyethylene, polypropylene, ABS and the 
like; and such engineering plastics as aromatic saturated polyesters, 
polycarbonates, polyamides, polyacetals and the like. 
Among these, the preferred aromatic saturated polyesters are those whose 
main acid component is derived from terephthalic acid, 
2,6-naphthalenedicarboxylic acid, or their ester-forming derivatives and 
main diol component is composed of at least one aliphatic diol such as 
ethylene glycol, trimethylene glycol, tetramethylene glycol, hexamethylene 
glycol, neopentyl glycol and the like. Of the polyesters, aromatic 
polyesters of high crystallization rate, e.g., polybutylene terephthalate, 
polypropylene terephthalate, polyethylene terephthalate, 
polybutylene-2,6-naphthalenedicarboxylate are particularly preferred, the 
most preferred being polybutylene terephthalate. 
Such thermoplastic aromatic polyesters may be partially substituted with a 
copolymerizable component. Examples of the copolymerizable components 
include aromatic dicarboxylic acids, e.g., alkyl-substituted phthalic 
acids such as isophthalic acid, phthalic acid, methylterephthalic acid and 
methylisophthalic acid, naphthalenedicarboxylic acids such as 
2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid and 
1,5-naphthalenedicarboxylic acid, diphenyldicarboxylic acids such as 
4,4'-diphenyldicarboxylic acid and 3,4'-diphenyldicarboxylic acid, and 
diphenoxyethanedicarboxylic acids such as 4,4'-diphenoxyethanedicarboxylic 
acid, etc.; aliphatic or alicyclic dicarboxylic acids such as succinic 
acid, adipic acid, sebacic acid, azelaic acid, decanedicarboxylic acid, 
cyclohexanedicarboxylic acid, etc.; alicyclic diols such as 
1,4-cyclohexanedimethanol; dihydroxybenzenes such as hydroquinone, 
resorcin, etc.; bisphenols such as 2,2-bis(4-hydroxyphenyl)-propane, 
bis(4-hydroxyphenyl)-sulfone, etc.; aromatic diols such as ether diol 
obtained from bisphenols and glycols such as ethylene glycol; and 
hydroxycarboxylic acids such as .epsilon.-hydroxycaproic acid, 
hydroxybenzoic acid, hydroxyethoxybenzoic acid, etc. 
The above aromatic polyesters may further be copolymerized with not more 
than 1.0 mol %, preferably not more than 0.5 mol %, more preferably not 
more than 0.3 mol %, of a polyfunctional ester-forming acid such as 
trimesic acid, trimellitic acid or the like or a polyfunctional 
ester-forming alcohol such as glycerine, trimethylol propane, 
pentaerythritol, etc., as a branching component. 
Preferred polycarbonate resins to be used in this invention are those which 
are derived from dihydric phenols and have molecular weight ranging 10,000 
to 100,000, more preferably 15,000 to 60,000, as expressed in terms of 
viscosity-average molecular weight. Such polycarbonate resins are usually 
obtained through reaction of dihydric phenols with carbonate precursors 
according to the solution phase or melt phase method. Examples of the 
dihydric phenols include 2,2-bis(4-hydroxyphenyl)propane (bisphenol-A), 
1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxy-3-methylphenyl)propane 
and bis(4-hydroxyphenyl) sulfone, etc. Of those, bis(4-hydroxyphenyl) 
alkanes are preferred, the most preferred being bisphenol-A. 
The ABS resin used for the invention is selected from known resins formed 
by adding acrylonitrile and butadiene, in various forms, to polystyrene. 
For example, 1) a mixture of styrene-acrylonitrile copolymer resin (AS 
resin) with polybutadiene (BR); 2) BR grafted with styrene and 
acrylonitrile; 3) a molten mixture of AS resin with the product of 2) 
above, and 4) a molten mixture of butadiene-acrylonitrile copolymer with 
AS resin may be exemplified. 
As the thermoplastic resins to be used in the present invention, 
polyethylene terephthalate, polybutylene terephthalate, polybutylene 
naphthalenedicarboxylate, polycarbonate, polyamide, polyacetal, ABS, 
polyethylene and polypropylene are particularly preferred. These may be 
used either singly or as mixtures. 
The resin composition of the present invention contains at least two kinds 
of metal oxides. 
Said at least two kinds of metal oxides are selected from a group 
consisting of (a) oxides of monovalent or divalent metals, (b) oxides of 
trivalent metals and (c) oxides of tetra- to hexa-valent metals. 
Examples of the oxides of monovalent or divalent metals are K.sub.2 O, 
Na.sub.2 O, Li.sub.2 O, Cu.sub.2 O, CaO, MgO, CoO, PbO, ZnO, BaO, FeO, 
MnO, CdO, CuO, NiO and SrO. 
The oxides of trivalent metals include, for example, Al.sub.2 O.sub.3, 
B.sub.2 O.sub.3, Fe.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Cr.sub.2 O.sub.3, 
Mn.sub.2 O.sub.3 and As.sub.2 O.sub.3. 
The oxides of tetra- to hexa-valent metals include, for examples, 
SiO.sub.2, TiO.sub.2, SnO.sub.2, ZrO.sub.2, CeO.sub.2, Sb.sub.2 O.sub.5, 
V.sub.2 O.sub.5, P.sub.2 O.sub.5, UO.sub.3 and MoO.sub.3. 
Such at least two kinds of metal oxides are used in a combined amount of 
0.001 to 10 parts by weight per 100 parts by weight of the thermoplastic 
resin. 
When the amount is less than 0.001 part by weight, a clear color 
development is difficult to attain, whereas when it exceeds 10 parts by 
weight, a clear contrast between the unmarked surface of shaped articles 
and the coloring portion is difficult to attain because that homogeneous 
marking is damaged. Both cases are therefore undesirable. 
The total content of said at least two metal oxides preferably ranges 0.001 
to 5 parts by weight, more preferably 0.01 to 2 parts by weight, per 100 
parts by weight of the thermoplastic resin. 
The at least two kinds of metal oxides may each form a compound 
independently of each other, or may together form a complex. 
It is preferable that said at least two kinds of metal oxides form a 
composition represented by the formula below; 
EQU R.sub.1-2 O.xR'.sub.2 O.sub.3.yR"O.sub.2-3 
wherein R.sub.1-2 O stands for an oxide of a monovalent or divalent metal; 
R'.sub.2 O.sub.3 stands for an oxide of a trivalent metal; R"O.sub.2-3 
stands for an oxide of a tetra- to hexa-valent metal; x is a number 
ranging 0.1 to 1.2; and y is a number ranging 1 to 12. 
Upon being calcined, the at least two kinds of metal oxides form a state of 
"glaze", which has no fixed chemical structure, like glass. While such 
"glaze" can be classified in various manner, e.g., china glaze and 
porcelain glaze, etc. according to the kinds of the calcination product; 
feldspars glaze, lime glaze, etc. according to the starting material which 
is the source of main component; or frit glaze, salt glaze, etc. according 
to the type of production system. It is of little significance to which of 
these classes does the "glaze" state as referred to in this invention 
belongs. 
In the process of converting at least two kinds of metal oxides to glaze 
state under heating, various phenomena such as dehydration, chemical 
reaction(s) between or among solid phases at temperatures below melting 
point or decomposition point, decomposition of carbonates or sulfates, 
partial fusion of starting materials, formation of an eutectic mixture, 
inter-fusion of molten salts, volatilization of a part of the starting 
components, etc. are involved in complex manner. 
Hence, when such plural metal oxides are converted to a state of glaze and 
vitrified, their color developing ability can be improved. Furthermore, 
the glaze itself may promote pyrolysis of the resin and develop colors. 
The thermoplastic resin composition of the present invention may further 
contain a black pigment or black dye, in an amount of not more than 2 
parts by weight per 100 parts by weight of the thermoplastic resin. 
As the black pigments or black dyes, for example, carbon-based black 
pigments, black metal oxides and black dyes are conveniently used. Of 
these, black metal oxides are more preferred. As the black metal oxides, 
low valency titanium oxides which are expressed by the formula, Ti.sub.n 
O.sub.2n-1 (n=1 to 5) are preferred. As such low order titanium oxides, 
for example, TiO, Ti.sub.2 O.sub.3, Ti.sub.3 O.sub.5, Ti.sub.4 O.sub.7 and 
Ti.sub.5 O.sub.9 are preferred, TiO.sub.n (n=1 to 1.99) being particularly 
preferred. 
According to the present invention, furthermore, other pigments and dyes 
may be added, depending on the intended use of individual compositions. 
As such other pigments, for example, inorganic pigments, e.g., basic lead 
carbonate, basic lead sulfate, basic lead silicate, metal sulfide such as 
lithopone or zinc sulfide; and organic pigments such as azo-, azomethine-, 
methine-, indanthrone-, anthraquinone-, pyranthrone-, flavanthrone-, 
benzanthrone-, phthalocyanine-, perinone-, perylene-, dioxadine-, 
thioindigo-, isoindoline-, isoindolinone-, quinacridone- and 
quinophthalone-type pigments may be used. 
Examples of the organic dyes include anthraquinone disperse dyes, metal 
complexes of azo dyes and fluorescent dyes. 
The thermoplastic resin composition of the present invention may further 
contain, within the range not impairing the objects of the invention, 
ordinary additives such as a glass-reinforcing agent, a granular or platy 
filler, a flame-retardant, a releasing agent, a lubricant, a slip 
additive, a nucleating agent, a colorant, an antioxidant, a heat 
stabilizer, a weatherability (light) stabilizer and a modifying agent such 
as impact resistance-improving agent, etc. 
The thermoplastic resin composition of the present invention can be 
obtained by blending a thermoplastic resin with metal oxides of the 
prescribed amount by an arbitrary blending method. It is preferred to 
disperse the components to be blended more uniformly. More specifically, 
the whole or a part may be simultaneously or separately blended in a 
mixing machine such as, for example, a blender, a kneader, a Bumbury 
mixer, a roll, an extruder, etc. to be homogenized. Furthermore, it is 
also possible to granulate the composition by melt-kneading a composition 
formed by advance dry-blending, in a heated extruder to homogenize the 
composition, extruding the melt into a wire form, and thereafter cutting 
the product to any desired length. 
Molding processing of the thermoplastic resin composition of the present 
invention can be quite easily practiced by ordinary means using common 
molding machines of thermoplastic resin in general. 
Hereinafter the present invention is explained with reference to working 
Examples, it being understood that the invention is in no sense limited by 
the following Examples. 
In the Examples, marking was carried out with YAG laser (Laser Marker SL 
475 E.sub.2, manufactured by NEC Corporation). 
Marking was evaluated according to the degree of color difference 
(contrast) between a molded article surface and a marking portion at which 
a color was developed upon exposure to the laser beam, and to the foaming 
condition. 
For determining the color difference, a Color Analyzer TC-1800 MK-11 
manufactured by Tokyo Denshoku Co., Ltd was used, and the result was 
expressed by the difference in brightness, .DELTA.L*. As for the foaming 
condition, it was confirmed whether a uniform and fine foam was formed.

EXAMPLE 1-10, COMATIVE EXAMPLE 1-16 
A glaze (metal oxides) as later identified, of the amount as indicated in 
Table 1, was added to a thermoplastic resin, together melt-kneaded in an 
extruder and pelletized. The products injection-molded into a disc form 
were marked and evaluated for the marking. In Comparative Example 1, 
carbon was added. Titanium oxide was added in Comparative Example 2, and 
in Comparative Examples 3 to 6, each of the thermoplastic resins alone 
were marked. The results are as shown in Table 1 below. 
TABLE 1 
______________________________________ 
Composition (wt %) 
Brightness (L*) 
Color 
Thermo- molded Color- 
differ- 
plastic article ing ence Foaming 
resin glaze surface portion (L*) Condition 
______________________________________ 
Ex. 1 a-1)98.0 b-2)2.0 66 33 33 good 
Ex. 2 a-1)99.0 b-2)1.0 73 35 38 good 
Ex. 3 a-1)99.5 
b-2)0.5 77 31 
46 good 
Ex. 4 a-1)99.9 b-2)0.1 77 32 45 good 
Ex. 5 a-1)99.95 
b-2)0.05 78 32 
46 good 
Ex. 6 a-1)99.99 b-1)0.01 82 35 47 good 
Ex. 7 a-1)99.995 
b-1)0.005 81 35 
46 good 
Ex. 8 a-2)99.99 b-1)0.01 43 29 14 good 
Ex. 9 a-3)99.99 
b-1)0.01 58 30 
28 good 
Ex. 10 a-4)99.99 b-1)0.01 -- 20 -- good 
Comp. a-1)99.995 
Carbon 54 41 
14 no good 
Ex. 1 0.005 
Comp. a-1)98.0 Titanium 89 50 39 good 
Ex. 2 oxide 
2.0 
Comp. a-1)100 -- 
79 49 30 
no good 
Ex. 3 
Comp. a-2)100 -- 43 31 12 no good 
Ex. 4 
Comp. a-3)100 -- 59 34 25 no good 
Ex. 5 
Comp. a-4)100 -- -- 63 -- no good 
Ex. 6 
______________________________________ 
The codes in Table 1 denote the following starting materials, 
respectively. 
(a) Component 
a1) polybutylene terephthalate (PBT) 7000N, product of Teijin Limited. 
a2) polyolefin HA300, product of Tonen K.K. 
a3) ABS SANTAC ST30, product of Mitsui Toatsu Chemicals, Inc. 
a4) polycarbonate (PC) L1250, product of Teijin Chemicals, Ltd. 
(b) Component 
b1) Turkish Blue glaze (main components: Fukushima feldspar, silica, 
copper oxide, barium carbonate, Asa Kaobin, lithium carbonate), product o 
Towa K.K. 
b2) Glaze (main components: SiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, 
PbO, F) product of Ferro Enamels (Japan) Limited 
In Comparative Examples: 
Comparative Example 1) Carbon manufactured by Mitsubishi Kasei Corporatio 
Comparative Example 2) 
Titanium oxide manufactured by Ishihara Sangyo K.K. 
In Comparative Example 3 the foaming condition was poor and the coloring 
portions had high brightness and hence the marking of the product was 
evaluated "no good". In contrast thereto, in Examples 1-7 both the foaming 
condition and brightness of coloring portions were improved, and the 
products exhibited good marking. 
On the other hand, a favorable foaming condition was achieved in 
Comparative Example 2, but the coloring portion had high brightness and 
color tone differing from those in the working Examples of the present 
invention. 
In Comparative Example 1, a good contrast was not obtained because the 
molded product surface was colored and the marked portion developed a 
color of high brightness. 
In comparison with Comparative Example 4, foaming condition was improved in 
Example 8. 
In comparison with Comparative Example 5, foaming condition was improved in 
Example 9. 
While the product of Comparative Example 6 was scarcely marked, that of 
Example 10 exhibited good marking, the brightness in the coloring portion 
being very low. 
EXAMPLE 11-18 
The various starting materials as identified in Table 2 were homogeneously 
dry-blended in advance at the prescribed quantitative ratios, and the 
mixtures were each melt-kneaded in a vented twin screw extruder with the 
screw diameter 44 mm, under conditions of a cylinder temperature of 
180.degree. C. to 260.degree. C., screw rotation speed of 160 rpm and a 
discharge rate of 40 kg/hr. The molten mixture was discharged as threads 
through the die, cooled and cut to provide pellets for molding. 
Using these pellets, personal computer key tops for laser marking were 
formed with an injection molding machine with an injection capacity of 5 
ounces under such conditions as an injection pressure of 800 kg/cm.sup.2, 
cooling time 15 seconds and the whole molding cycle of 28 seconds. 
The results of these experiments are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Component a Component b 
Other component 
a-1 a-3 b-1 T-1 
T-2 
T-3 
T-4 
Black coloring 
__________________________________________________________________________ 
Ex. 11 
99.99 
-- 0.01 -- -- -- -- .circleincircle. 
Ex. 12 -- 99.99 0.01 -- -- -- -- .circlein 
circle. 
Ex. 13 99.50 -- 0.05 -- -- -- -- .circlein 
circle. 
Ex. 14 -- 99.50 0.05 -- -- -- -- .circlein 
circle. 
Ex. 15 98.97 -- 0.50 -- 0.03 0.50 -- 
.circleincircle. 
Ex. 16 99.36 -- 0.10 -- 0.01 -- 0.50 
.circleincircle. 
Ex. 17 -- -- 0.50 -- 0.03 0.50 0.50 
.circleincircle. 
Ex. 18 -- -- 0.10 -- 0.01 -- -- .circlein 
circle. 
Comp. 100.00 -- -- -- -- -- -- X 
Ex. 7 
Comp. -- 100.00 -- -- -- -- -- X 
Ex. 8 
Comp. 99.90 -- -- 0.10 -- -- -- X 
Ex. 9 
__________________________________________________________________________ 
In Table 2, significations of the codes, a1, a3 and b1 are same as those 
in Table 1. The other components T1, T2, T3 and T4 are identified below: 
T1) carbon, Ketjen Black EC 600 JD, manufacture by Lion Corporation 
T2) black titanium oxide, M1, manufactured by Ishihara Sangyo K.K. 
T3) a gray pigment, manufactured by Dainichi Seika Kogyo K.K. 
T4) an ivorycolor pigment, manufactured by Dainichi Seika Kogyo K.K. 
The evaluation marks in Table 2 have the following significations: 
.circleincircle.: excellent black colordeveloping property 
X: poor black colordeveloping property.