Background Art In general, with the aim of marking desired logos, symbols and designs on resin molded articles or molded articles coated with resins, printing methods using heat-reinforced ink have been performed. However, in cases of polyacetal being a crystal resin, poor ink adhesion results upon the printing process. Also, it is impossible to perform the printing process without the pre-treatment to obtain high adhesion. Further, resistances to frost, abrasion and chemicals of the printed portions by the ink are insufficient, according to usage conditions.

Thus, to solve the problems concerned with the printing method using the heat-reinforced ink, there are proposed methods of marking a thermoplastic resin by being exposed to a laser.

The laser marking method is advantageous in terms of rapid marking on small lot runs requiring frequent changeover, no need for pre-treatment and post- treatment, and a simplified process without additional materials and processes. Also, the marking designs may be freely modified through the connection with auto cads, thus requiring relatively a low investment. However, the laser marking process is unsuitable for use in wide multi-colored articles, compared to the printing process.

The laser for use in the laser marking techniques includes an Nd : YAG (neodymium: yttrium aluminum garnet) laser. The use of the Nd : YAG laser provides the laser marking on metals, plastics, silicon, woods, paper, leather and

glass, according to a marking material. Further, the Nd : YAG laser is used to cut the material or mark the surface. Furthermore, the above laser-markable surface may be embossed or engraved according to a surface-marking manner. That is, the processing surface is dug to form an engraved surface, or the surface of the target is faded to perform the marking process, or the processing surface is embossed to obtain marking effects.

As the marking type, there are the manners of engraving for use in night designs of an electric switch and plastic marking by completely removing burrs by evaporation of the laser-irradiated portion; engraving and fading for use in plastic marking and most metals with the exception of copper and aluminum by simultaneously engraving and fading the beam-irradiated marking surface while some burrs remain on the irradiated surface; and fading and bleaching for use in plastics marking such as keyboards or fading-markings of a metal surface using a high frequency by changing only the color by inducing the instant chemical change rather than the engraving on the marking surface.

In this regard, Japanese Patent Laid-open Publication No. Sho. 58-67496 discloses a relatively simple marking process which provides a desired marking by using physical changes of a laser-processing surface, that is, a thermal processing method. Further, Japanese Patent Laid-open Publication Nos. Sho. 63-216790 and Sho. 61-41320, and Japanese Patent Laid-open Publication No. Hei. 1-306285 disclose a laser marking process by the addition of a filler capable of fading and decoloring.

The above laser marking process using such a filler is characterized in that a substrate colored with pigments is subjected to a marking process by use of a color different from that of the substrate. In particular, research on laser marking compositions to obtain white marks having high contrast on a black substrate has been vigorously performed.

For example, Japanese Patent Laid-open Publication No. Hei. 11-140271 discloses a laser marking resin composition including polyoxymethylene and carbon black having a particle size of 17-90 nm and a DBP (dibutyl phthalate) oil absorption of 70-200 ml/100 mg. In such a case, the use of the above marking resin composition leads to white marks having high contrast and better brightness on a

black substrate. However, molded articles from the above composition are decreased in thermal stability, attributed to the carbon black and pigments contained in the composition, thus resulting in decoloration and decreased mechanical properties.

In U. S. Patent No. 5,218, 041, there is disclosed a polyoxymethylene composition including polyoxymethylene, an endocopolymer, a phenol type antioxidant and carbon black so as to solve the above problems and to decrease the emission of formaldehyde upon processing of the composition at high temperatures.

The above patent is advantageous in terms of reduced emission of formaldehyde, but suffers from decreased mechanical properties of the polyoxymethylene resin by using the endocopolymer.

Disclosure of the Invention Leading to the present invention, the intensive and thorough research on laser marking polyoxymethylene compositions, carried out by the present inventors aiming to avoid the problems encountered in the related art, resulted in the finding that a dihydrazide compound as a thermal stabilizer is added to the polyoxymethylene resin composition for laser marking, whereby thermal stability of molded articles for laser marking is enhanced, and the emission of formaldehyde is minimized during the processing.

Therefore, it is an object of the present invention to provide a polyoxymethylene resin composition for laser marking, having superior thermal stability.

It is another object of the present invention to provide a polyoxymethylene resin composition for laser marking, having minimized emission of formaldehyde during the processing.

It is still another object of the present invention to provide a polyoxymethylene resin composition for laser marking, exhibiting better coloration and high brightness.

To accomplish the above objects of the present invention, there is provided a polyoxymethylene resin composition for laser marking, including 100 parts by

weight of polyoxymethylene, 0.01-3. 0 parts by weight of carbon black, and 0.01-5. 0 parts by weight of a dihydrazide compound.

Best Mode for Carrying Out the Invention Based on the present invention, a polyoxymethylene resin composition for laser marking is characterized in that a dihydrazide compound is used as a thermal stabilizer, whereby thermal stability of molded articles for laser marking is enhanced and emission of formaldehyde is minimized during processing.

The laser marking composition of the present invention includes 100 parts by weight of polyoxymethylene, 0. 01-3. 0 parts by weight of carbon black, and 0.01- 5.0 parts by weight of dihydrazide.

The polyoxymethylene resin used in the present invention is a homopolymer consisting of an oxymethylene group as a main constitutive unit represented by the following Formula 1, or a copolymer obtained by random polymerization of the oxymethylene group of Formula 1 and a monomeric group represented by the following Formula 2: Formula 1 - (-CH20-)- Formula 2 - [- (CXJX2) XO-I- Wherein Xi and X2 are the same or different, each represents hydrogen, an alkyl group or an aryl group, and x is an integer of 2-6, provided that Xi and X2 both are not hydrogen. The polyoxymethylene resin has an average molecular weight of 10,000-200, 000.

The oxymethylene homopolymer results from the polymerization of formaldehyde or cyclic oligomer thereof, that is, trioxane. In addition, the oxymethylene copolymer, comprising the oxymethylene group of Formula 1 and the monomeric group of Formula 2, is obtained by randomly copolymerizing

formaldehyde or cyclic oligomer thereof, and a cyclic ether compound represented by the following Formula 3, or a cyclic formal compound represented by the following Formula 4: Formula 3 Formula 4 Wherein X3, X4, X5 and X6 are the same or different, each represents hydrogen or an alkyl group and may be linked to the same carbon or different carbon, and n and m are an integer of 2-6, respectively.

As for the comonomer used for random copolymerization, the cyclic ether compound is exemplified by ethylene oxide, propylene oxide, butylen oxide and phenylene oxide, and examples of the cyclic formal compound include 1,3- dioxolane, diethyleneglycolformal, 1, 3-propanediolformal, 1, 4-butanediolformal, 1, 3-dioxepaneformal and 1,3, 6-trioxocane. Preference is given to using the comonomer selected from among ethylene oxide, 1,3-dioxolane, 1,4- butanediolformal, and mixtures thereof. The above comonomer is added to trioxane or formaldehyde in the presence of a Lewis acid catalyst to perform the random copolymerization, thereby obtaining the oxymethylene copolymer having two or more linked carbon atoms in the main chain, with a melting point of 150°C or higher.

In the oxymethylene copolymer, a molar ratio of the oxymethylene polymer structure to the oxymethylene repeating unit ranges from 0.05 to 50, and, preferably, from 0.1 to 20.

Examples of the polymerization catalyst used for the formation of the oxymethylene polymer include BF3 0H2, BF3 OEt2, BF3 0Bu2, BF3 CH3CO2H, BF3 PFs HF, BF3-10-hydroxyacetophenone, etc. , in which Et means an ethyl group and Bu means a butyl group. Preferably, BF3 OEt2 and BF3 0Bu2 are used. The adding amount of the catalyst is preferably in the range of 2x10-6-2x10-2 mole, based on 1 mole of trioxane.

The polymerization which is exemplified by bulk polymerization, suspension polymerization or solution polymerization is carried out at 0-100°C, and preferably, 20-80°C.

Moreover, as an inactivator for inactivating the catalyst remaining after the polymerization, there are tertiary amines, such as triethylamine, cyclic sulfur compounds, such as thiophene, phosphor compounds, such as triphenylphosphine, and alkyl-substituted melamine compounds. Such an inactivator is a Lewis base material having an unshared electron pair, and forms a complex salt with the catalyst.

Upon the polymerization of oxymethylene, a chain transfer agent is used, for example, alkyl-substituted phenols or ethers. In particular, alkylether including dimethoxymethane is preferably used.

Of the prepared polyoxymethylene resins, the most preferable compound is a polyoxymethylene homopolymer or copolymer having a melting point of about 160°C or higher, a crystallization degree of 65-85% and an average molecular weight of 10,000-200, 000.

Further, in the present invention, carbon black is used as a material which absorbs laser-light to obtain better marking results by use of the laser with a lower energy level. In the coloration of a substrate formed from the polyoxymethylene resin by the carbon black, particularly, blackness, the laser marking to the carbon black-combined resin and blackness largely depend on an average particle size and a DBP oil absorption of the combined carbon black, that is, the size of respective carbon black particles and the particle-coagulated structure, and the adding amount of carbon black. The coloration, in particular, blackness, and laser marking run counter to each other by the adding amount of carbon black. Thus, with the aim of simultaneously obtaining better coloration and laser marking results, it is vital that

the proper carbon black is selected. In the present invention, preference is given to using the carbon black having a particle size of 10-50 nm, and a DBP oil absorption of 40-100 cc/100 mg. If the particle size of carbon black is smaller than 10 nm, dispersiblity in the resin weakens. Meanwhile, if the particle size is larger than 50 nm, the coloration of the polyoxymethylene resin is decreased. In addition, when the DBP oil absorption of the carbon black is less than 40 cc/100 mg, the coloration of the polyoxymethylene resin is decreased. Whereas, when the DBP oil absorption exceeds 100 cc/100 mg, dispersibility in the resin is lowered. In particular, carbon black coated with-OH acts to increase the dispersiblity due to compatibility with the terminal group of the polyoxymethylene resin, therefore achieving desirable blackness even by the small amount of the carbon black.

According to the present invention, the carbon black to be mixed with the marking composition is employed in cases where a black or dark substrate is subjected to a white marking process. In addition, the small amount of carbon black may be used where a grey substrate is colored, or carbon black may be applied where being blended with other complicated colors. As such, the carbon black is used in the amount of 0.01-3. 0 parts by weight, based on 100 parts by weight of polyoxymethylene. The use of carbon black less than 0. 01 parts by weight results in undesirable blackness, while the use of carbon black exceeding 3.0 parts by weight leads to poor marking results.

Further, as the laser-absorbing material, in addition to carbon black, titanium dioxide (TiO2), an inorganic pigment and an organic pigment may be selectively used, to improve the laser marking results.

However, the use of the polyoxymethylene molding material including carbon black and other pigments is disadvantageous in terms of deposition of the main shape or decreased releasing properties upon molding, decoloration and decreased mechanical properties due to low thermal stability, and offensive odor based on the emission of formaldehyde upon the processing at high temperatures.

Hence, in the present invention, a dihydrazide compound is used as a thermal stabilizer to provide the polyoxymethylene molding material having better color consistency and permanent brightness while exhibiting superior thermal stability during the processing at high temperatures.

The dihydrazide compound contains a reactive hydrogen, and produces an additive product together with an inorganic acid or organic acid, or produces a polymer product through polycondensation of the derivative by the reaction with a reactive compound, that is, an organic group or a radical. Thus, the dihydrazide is utilized as an epoxy resin hardener, a crosslinking agent of acrylate ester, an improver of synthetic fiber or synthetic resin, a fiber treating agent, or a radical scavenger.

In particular, a decomposition inducing material of the polyoxymethylene resin, attributed to the addition of carbon black and other pigments, is removed, by using the dihydrazide having strong reduction-inducing properties. Thereby, thermal stability of the polyoxymethylene resin composition for laser marking is enhanced, and the emission of formaldehyde is minimized upon processing at high temperatures. The dihydrazide compound is preferably selected from the group consisting of oxalic dihydrazide, malonic dihydrazide, succinic dihydrazide, adipic dihydrazide, sebacic dihydrazide, dodecanoic dihydrazide, isophthalic dihydrazide, piperazine N, N'-dihydrazide, m-benzene-dihydrazide, and p-benzene-dihydrazide.

As such, the dihydrazide compound is used in the amount of 0.01-5. 0 parts by weight, based on 100 parts by weight of polyoxymethylene. If the above amount is smaller than 0.01 parts by weight, enhancement of thermal stability is undesirable. Meanwhile, if the amount exceeds 5.0 parts by weight, the polyoxymethylene resin composition has low thermal stability and is decreased in the properties, due to the side reactions.

To provide the white laser marking having high contrast and better brightness, titanium dioxide is selectively used. In such a case, titanium dioxide is used in the amount less than 5.0 parts by weight, based on 100 parts by weight of the laser marking resin composition.

Moreover, with the intention of realizing better white marking on the substrate colored by a predetermined color in addition to black, an inorganic pigment is used to show the predetermined color. The inorganic pigment is preferably selected from the group consisting of cobalt green, cobalt blue, chrome oxide green, cadmium pigments, nickel/chrome titanates, ultramarine blue, and mixtures thereof. As such, the inorganic pigment is used in the amount less than

10 parts by weight, based on 100 parts by weight of the polyoxymethylene resin composition for laser marking.

Further, to mark the substrate colored by the predetermined color with a certain color with the exception of white, an organic pigment is selectively used to show the certain color. Preferably, the organic pigment is selected from the group consisting of anthra quinoids, phthalocyanine blue, phthalocyanine green, chromophthal red, and mixtures thereof. In such a case, the organic pigment is used in the amount less than 10 parts by weight, based on 100 parts by weight of the polyoxymethylene resin composition for laser marking.

As necessary, known additives or fillers may be further used within the range of not decreasing the marking properties by laser irradiation. For example, for improvement of weather resistance, various stabilizers, antioxidants, lubricants, plasticizers, nucleating agents, releasing agents, antistatic agents and surfactants may be further used.

Meanwhile, a laser marking process may be easily performed by irradiating the laser to a predetermined portion of the molded article formed from the resin composition. As such, to obtain the desired marks, there are exemplified methods of irradiating the laser in the size of suitable spots on the surface of a target, and of irradiating the surface of a target with the laser of desired shapes by masking the laser.

Examples of the usable laser include carbonic acid gas laser, ruby laser, semiconductor laser, argon laser, excimer laser, YAG laser, etc. Of them, an Nd : YAG laser is preferable. As such, although a continuous wave or a normal pulse may be used, preferable is a high power pulsed Nd : YAG laser in the scanning manner using a Q-switch by the continuous wave.

As mentioned above, the laser marking resin composition of the present invention is used, thereby increasing thermal stability of the molded article.

Further, during the processing, the emission of formaldehyde is minimized, and durability problems, resulting from abrasion of molded articles subjected to conventional printing methods, may be overcome. As well, aging problems, attributed to the decrease of chemical resistance upon oil or solvent immersion, may be solved. In addition, the inventive resin composition has superior laser marking

results, and is sufficiently maintained in fundamental properties as acetal, that is, mechanical properties, chemical resistance and fatigue resistance, and thus can be applied for parts requiring abrasion resistance.

The molded article obtained from the laser marking polyoxymethylene resin composition is subjected to laser marking with the Nd : YAG laser, thereby realizing rapid printing treatment, no need for pre-and post-treatment, and free design modifications through the connection with auto cads, resulting in a desired quality of the article. Further, the multi-colored articles in addition to white articles can be displayed by white or multi-colored bar-codes, numbers, logos, designs and two- dimensional symbols.

Therefore, the laser marking resin composition of the present invention can be widely applied for fields immersing printed articles in various oils or solvents; parts requiring the display of multi-colored bar-codes, numbers, logos, designs and two-dimensional symbols; parts causing the abrasion of the printed portion of the printed articles; and parts requiring high abrasion resistance and laser marking properties. For example, there are keyboard keytop parts, mobile-phone keypad parts, automotive interior/exterior parts, automotive fuel parts, buckles, zippers, souvenirs, toys, etc.

In an example of the present invention, the polyoxymethylene resin was melted and kneaded with carbon black and titanium dioxide by use of a twin- extruder, and the melt from the die of the extruder was cooled through a cooling bath, to prepare a polyoxymethylene composition in the form of a pellet. Thusly prepared polyoxymethylene composition was injection-molded using an injection molding machine, to give an injection-molded material.

A better understanding of the present invention may be obtained through the following examples and comparative examples which are set forth to illustrate, but is not to be construed as the limit of the present invention.

Physical properties of the compositions shown in the examples and comparative examples were measured according to the following procedures.

(1) Melt Index (MI) ; Fluidity According to ASTM D1238, a sample extruded for 10 min. from an orifice having a predetermined inner diameter at 190°C under the load of 2.16 kg was

measured for weight. A high value indicates good fluidity, whereas a low value indicates poor fluidity.

(2) Tensile Strength and Tensile Elongation According to ASTM D638, tensile strength and elongation were measured.

High values indicate high tensile strength and elongation, while low values indicate low tensile strength and elongation.

(3) Izod Impact Strength According to ASTM D256, izod impact strength (notched) was measured.

A high value indicates high impact strength, while a low value indicates low impact strength.

(4) Formaldehyde Gas Emission Into a 1L polyethylene bottle, 50 mL of distilled water was introduced, and a test piece was installed so as not to reach distilled water and then allowed to stand in a UL-spec oven at 60°C for 3 hours. Thereafter, predetermined amounts of acetyl acetone and ammonium acetate were added to distilled water to form a chromophore. By use of a UV spectroscope, the concentration of the formaldehyde gas dissolved in distilled water was determined. A high value indicates large production of formaldehyde gas, while a low value indicates small production thereof.

(5) Weight Loss When residing for 1 hour in a TGA (Thermogravity Analysis) of nitrogen atmosphere at 222°C, the sample was measured for the decrease of weight. A high value indicates low thermal stability, while a low value indicates high thermal stability.

(6) Laser Marking (L-ratio) Using an Nd : YAG laser of 1064 nm, a'KEPITAL'logo was marked on a square substrate of 10x20 mm. The L-ratio, which was L-white/L-black, was determined at an angle of 10° under a light source of D65 by use of SP88 Spectrophotometer purchased from X-Rite Co. The higher the ratio, the better the results.

Example 1 100 parts by weight of a polyoxymethylene copolymerization resin (trade name: KEPITAL F20-01, MI=9. 5g/lOmin., available from Korea Engineering Plastics Co. Ltd., called'POM') was melted and kneaded with 1.0 part by weight of oxalic dihydrazide (OADH), 0.15 parts by weight of carbon black coated with an OH group, and 0.05 parts by weight of titanium dioxide, by use of a twin extruder.

A melt from the die of the extruder was cooled through a cooling bath, to prepare a polyoxymethylene composition in the form of a pellet, from which a test piece was prepared using an injection-molding machine. Such a test piece was measured for physical properties according to the above measuring methods. The results are shown in Tables 1 and 2, below.

Example 2 A test piece was prepared in the same manner as in Example 1, with the exception that 0.5 parts by weight of an inorganic pigment (ultramarine blue) was further used. Such a test piece was measured for physical properties according to the above measuring methods. The results are shown in Table 2, below.

Example 3 A test piece was prepared in the same manner as in Example 1, with the exception that 0.5 parts by weight of an organic pigment (phthalocyanine blue) was further used. Such a test piece was measured for physical properties according to the above measuring methods. The results are shown in Table 2, below.

Example 4 A test piece was prepared in the same manner as in Example 1, with the exception that titanium dioxide was not used. Such a test piece was measured for physical properties according to the above measuring methods. The results are shown in Table 2, below.

Comparative Examples 1-3 Each test piece was prepared in the same manner as in Example 1, with the

exception that oxalic dihydrazide was not used, and the amount of carbon black was changed as shown in Table 2, below. Such test pieces were measured for physical properties according to the above measuring methods. The results are shown in Table 2, below.

TABLE 1 Test Method Test Condition Unit Ex. 1 (ASTM) Physical Specific Gravity D792 23°C-1. 41 Properties Water Absorption D570 23°C, 60% RH % 0.22 Thermal MI D1238 l90°C g/lOmin 9.8 Properties Melting Point DSC - °C 165 Mechanical Tensile Strength D638 23~2°C kgf/cm2 600 Properties Tensile Elongation 50~5% RH % 60 D790 Flexural Strength 23~2°C kgf/cm2 860 Flexural Modulus 50~5% RH kgf/cm2 25000 Izod Impact Strength D256 Notched (Notched) t=3. 2mm Molding (t 5 mm, d) 100 mm) Flow Direction-% 2.0 Shrinkage'

TABLE 2 Carbon Thermal Color of POM Pigment for. Formaldehyde Black Stabilizer L- Laser Background Ex. No. (parts (parts (parts by Coloring Ratio Gas Emission Marking Color by wt.) by wt.) wt.) (parts by wt.) (ppm) Part 1 100 0.15 OADH Titanium 2.5 3.4 White Black 1. 0 Dioxide 0. 05 Titanium 2 100 0.20 OADH Dioxide 0.05, - 3.9 White Ultramarine 1. 0 Ultramarine Blue Blue 0.5 3 100 0.20 OADH Phthalocyanine - 4.3 Blue Ultramarine 1.0 Blue 0.5 Blue OADH 4 100 0.15 - 2.0 3.6 White Black 1.0 C. 1 100 0.15 Titanium 2.4 9.0 White Black Dioxide 0.05 C. 2 100 0.4 Oath 0. 52 4. 7 Grey Black C. 3 100 0.4 - - 0.47 11.3 Grey Black Industrial Applicabilit

As described above, the present invention provides a polyoxymethylene resin composition for laser marking, from which the molded articles having better laser marking results and increased abrasion resistance, durability, chemical resistance and fatigue resistance can be obtained, by using the dihydrazide as a thermal stabilizer so that thermal stability of the molded articles is enhanced and emission of formaldehyde is minimized during the processing. Further, such molded articles are subjected to laser marking with an Nd : YAG laser, whereby white or multi-colored shapes can be displayed on multi-colored articles, and as well, the laser marking process can be more rapidly performed, with no requiring pre-and post-treatment, compared to conventional printing methods. Moreover, designs can be freely modified through the connection with auto cads, and thus the molded articles for laser marking-have desired quality.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.