Polyester resin composition suitable to be marked with laser.

Abstract: A polyester resin composition; (A) which contains (a) a thermoplastic polyester resin, and (b) a carbon black and/or a graphite, and (B) in which the content of the above component (b) is 0.001 to 2 % by weight based on the total weight of the components (a) and (b). The polyester resin composition may further contain (c) a bromine-containing flame retardant, (d) an antimony-containing flame retardant aid and (e) an inorganic filler in addition to the components (a) and (b). This polyester resin composiotion permits marking of fine lines and precise print with a high contrast with a laser light.

Description: Detailed Description of the Invention The present invention relates to a polyester resin composition which permits marking with laser light. When the polyester resin composition provided by the present invention is used, not only a molded article having a desired form can be produced, but also the marking can be easily made on the molded article with a YAG laser, a carbon dioxide gas laser, etc. Most of electric and electronic parts have prints made of a variety of inks on their outer surfaces for their identification and explanation of their functions. When a printing portion is made of a plastic, it is necessary to remove a mold lubricant, etc., which have been incorporated into a molding material, in order to facilitate the adhesion of an ink. It has been therefore general practice to wash the plastic (entire parts) with freon. From the viewpoint of prevention of the ozonosphere destruction, of course, the washing with freon is undesirable, and it is desired to develop innovative means. It must, however, be noted that the technique which allows to omit the step of washing with freon is required to reduce the printing cost and technically meet with the requirement for making precise and fine prints. The marking with a laser has attracted attention as a printing technique which can overcome all of these problems at once. At present, a carbon dioxide gas laser and a YAG laser are mainly used as a laser marking apparatus. To perform the laser marking effectively, a plastic is required to be preliminarily modified into an easily photosensitive one. For example, Japanese Laid-Open Patent Publication No. 254743/1989 discloses the incorporation of either titanium oxide or a combination of titanium oxide with carbon black as means of improving the marking properties of a plastic for the marking with a YAG laser. Specification DE-C-3544385 describes the incorporation of a small amount of carbon black in PVC film. It has been accordingly attempted to apply this means to thermoplastic polyesters. See, for example, specification EP-A-0101667, which describes incorporating graphite into, for example, polyesters. However, the incorporation of titanium oxide or carbon black into polyesters has not given any satisfactory result on the improvement in the marking properties. It has been further found that the marking with a carbon dioxide gas laser is not satisfactory presumably because a carbon dioxide gas laser has a small pulse energy. Therefore, as far as polyesters are concerned, there is no expectable forecast for alternative means to replace a conventional stamping method. It is an object of the present invention to provide a polyester resin composition which permits the marking with laser light. It is another object of the present invention to provide a polyester resin composition which exhibits a high contrast in the marking with a YAG laser or a CO 2 laser and which permits the marking of fine lines or precise prints. The above and other objects and advantages of the present invention will be apparent from the following description. The above objects and advantages of the present invention will be achieved, first, by a polyester resin composition comprising: (a) a thermoplastic polyester resin, and (b) a carbon black, optionally together with a graphite having an average particle diameter of 0.1 to 150 µm, as a sole laser light absorbing agent, said carbon black having a thermal conductivity of not less than 0.2 kcal/m.hr.°C, in which the content of component (b) is from 0.001 to 2% by weight based on the total weight of the components (a) and (b). Fig. 1 is a schematic view showing the principle of irradiation of the surface of a molded article formed from a polyester resin composition of the present invention with a carbon dioxide gas laser. Fig. 2 is a schematic view showing the principle of irradiation of the surface of a molded article formed from a polyester resin composition of the present invention with a YAG laser. The thermoplastic polyester resin (a) used in the present invention refers to a substantially linear saturated polyester. For example, preferred are homopolyers or copolymers having one recurring unit or at least two different recurring units such as butylene terephthalate, ethylene terephthalate, butylene naphthalene dicarboxylate, and ethylene naphthalene dicarboxylate, or mixtures of these. Examples of the homopolymers are polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polybutylene naphthalene dicarboxylate (PBN), polyethylene naphthalene dicarboxylate (PEN), and mixtures of these. The above polyester resins are required to have an intrinsic viscosity with which the polyesters are moldable. The component (b) used in the present invention includes a carbon black and optionally also a graphite. The carbon black (b) used in the present invention has high thermal conductivity, i.e. of not less than 0.2 Kcal/m.hr.°C. These carbon blacks have characteristic features in that their graphite formation has advanced in the crystal structure or that these have a relatively bulky structure as a linear agglomerate, etc., formed in the secondary structure. The "marking" refers to a phenomenon that a resin heated under the irradiation with a laser is gasified and cured while retaining air bubbles. Therefore, an additive having high absorbtivity to laser light and excellent thermal conductivity is effective for marking. It can be therefore said that such a carbon black having high thermal conductivity is an additive having a high marking effect similarly to graphite. Further, since the carbon black plays a role as a black pigment due to its high coloring properties, the amount of the graphite can be reduced, and yet the marking having a high contrast is made possible. That is, the carbon black incorporated into a white or "colored" molded article has a great influence on its color tone (turns it blackish). It is therefore preferred to incorporate the carbon black alone or in combination with graphite when a molded article has a color of grey to black. In the present invention, the carbon black (b) is incorporated into the thermoplastic polyester resin (a) in a such a proportion that the amount of the carbon black (b) based on the total amount of these components (a) and (b) is 0.001 to 2 % by weight. When the amount of the carbon black, if incorporated alone, is less than 10 ppm (0.001 % by weight), the resultant polyester resin composition does not show any high-contrast marking properties. The upper limit of the amount of the carbon black is 2 % by weight, since the marking effect is not improved any further even if it exceeds 2 % by weight. It is also preferable to incorporate a small amount of the carbon black in order to prevent an excess amount of the carbon black from coloring a molded article. A molded article containing 0.001 to 0.05 % by weight of carbon black is colored grey, and a molded article containing more than 0.05 % by weight of carbon black is colored black. Under the irradiation with a YAG laser, the marked portion is turned white. On the other hand, when a CO 2 laser is used, the irradiated (marked) portion is also turned white, and the carbon black having high coloring properties shows a high contrast in the marking. The graphite (b) optionally also used in the present invention has a crystalline structure, and its average particle diameter is in the range of from 0.1 to 150 µm. The form of the graphite in (b) is scale-like, particulate, massive or a soil-like. Therefore, graphites, too, other than genuine graphites of hexagonal system have marking properties and so can be also used in this invention. In the present invention, the graphite (b) is incorporated into the thermoplastic polyester resin (a) in a such a proportion that the amount of the carbon black and graphite (b) based on the total amount of these components (a) and (b) is 0.001 to 2 % by weight. When the amount of (b) is less than 10 ppm (0.001 % by weight), the resultant polyester resin composition does not show any marking properties. The upper limit of the amount of (b) is 2 % by weight, since the marking effect is not improved any further even if it exceeds 2 % by weight. It is also preferable to incorporate a small amount of (b) in order to prevent an excess amount of (b) from coloring a molded article. The graphite used produces the marking effect even if used in a very small amount, since it has a very high absorptivity to laser light. Further, the graphite advantageously has low coloring properties as compared with carbon black. For example, 0.01 % by weight of carbon black is incorporated into a polyester, the resultant composition is colored (turned dark grey), whereas a polyester containing the same amount of the graphite changes its color to a slight extent. Meanwhile, the graphite produces an outstanding change (contrast) from a nonirradiated portion when irradiated with laser light. For example, a molded article formed from a composition containing 0.001 to 0.005 % by weight of graphite is as white as a composition containing no graphite. Under irradiation with a YAP laser, a portion marked by the YAG laser is turned black. That is, the portion irradiated with the laser light exhibits as high a contrast as that of stamps made of a black ink. Meanwhile, as the amount of the graphite is increased, the composition color is changed from white to dark grey or black. When the amount is as large as 1 to 2 % by weight, the composition is colored black. When this black composition (molded article) is irradiated with a laser, the irradiated (marked) portion turns white. On the other hand, when a CO 2 laser is used, the irradiated (marked) portion turns white. It is therefore preferred to further incorporate not only a very small amount of graphite but also a pigment or dye which exhibits a color in order to produce a high contrast. The polyester resin composition of the present invention may further contain (c) a bromine-containing flame retardant and (d) an antimony-containing flame retardant aid in addition to the above components (a) and (b). Examples of the bromine-containing flame retardant (c) preferred in the present invention include a brominated bisphenol A type polycarbonate flame retardant (I), a brominated bisphenol A type epoxy resin and/or a modified product obtained by blocking part or all of the terminal glycidyl groups thereof (II), a brominated diphenyl ether flame retardant (III), a brominated imide flame retardant (IV) and brominated polystyrene flame retardant (V). For example, compounds of the following formula (I) are preferred as the above brominated bisphenol A type polycarbonate flame retardant (I). wherein Y 1 is Y 2 is Xs are bromine atoms, n is an integer of 2 to 30, and each of i, j, k and h is an integer of 1 to 4. For example, compounds of the following formula (II) are preferred as the above brominated bisphenol A type epoxy resin and/or modified product obtained by blocking part or all of the terminal glycidyl groups thereof (II). wherein Xs are bromine atoms, each of i and j is an integer of 1 to 4, and m is an average polymerization degree ranging from 0 to 40. For example, compounds of the following formula (III) are preferred as the above brominated diphenyl ether flame retardant (III). wherein Xs are bromine atoms, and i' is an integer of 1 to 5. For example, compounds of the following formula (IV) are preferred as the above brominated imide flame retardant (IV). wherein Xs are bromine atoms, and i is an integer of 1 to 4. For example, compounds of the following formula (V) are preferred as the above brominated polystyrene flame retardant (V). wherein X is a bromine atom, i is an integer of 1 to 4, and ℓ is an average polymerization degree of up to 40. Examples of the antimony-containing flame retardant aid (d) preferred in the present invention include Sb 2 O 3 and/or pNa 2 O·Sb 2 O 5 ·qH 2 O in which p is 0 to 1 and q is 0 to 4. These flame retardant aids are incorporated to improve the flame retarding effect. Although not specially limited, the particle diameter thereof is preferably 0.02 to 5 µm. The flame retardant aid may be surface-treated with an epoxy compound, a silane compound, or a titanate compound as required. The above flame retardants of the formula (I) to (V) are known per se. The amount of the flame retardant based on the total amount of the components (a), (b), (c) and (d) is 2 to 25% % by weight, preferably 8 the 20 % by weight. The amount of the flame retardant aid based on the above total amount by weight. is 2 to 15 % preferably 4 to 10 % by weight. It is preferred to incorporate the flame retardant aid in an amount of 20 to 70 % based on the flame retardant (c). by weight When the amount of the flame retardant aid based on the above total amount exceeds 15 % by weight, the flame the resultant sometimes undesirably promotes resin or the additive to decrease composition of the the strength of the resultant molded article. The polyester resin composition of the present invention is preferably a composition comprising (a) a thermoplastic polyester resin, (b) a carbon black (optionally together with a graphite), as a sole laser light absorbing agent, said carbon black having a thermal conductivity of not less than 0.2 kcal/m.hr.°C and said graphite having an average particle diameter of 0.1 to 150 µm, (c) a bromine-containing flame retardant, and (d) an antimony-containing flame retardant, aid, and in which, based on the total amount of the components (a), (b), (c) and (d), the content component (a) is at least 58 % by weight, the content of the component (b) is 0.001 to 2% by weight, the content of the component (c) is 2 to 15 % by weight and the content of the component (d) is 2 to 15% by weight. The polyester resin composition of the present invention may contain not more than 70 % by weight of an inorganic filler (e) as required. It is naturally preferred to incorporate other thermal stabilizer, pigment, phosphorus-containing flame retardant, etc., in combination with the inorganic filler. The inorganic filler (e) usable in the present invention is selected, for example, from fibrous materials such as glass fibers, carbon fibers, steel fibers, ceramic fibers, potassium titanate whiskers and boron whiskers; and powdcrs, particulates or sheets of kaolin, clay, wollastonite, talc, mica, potassium carbonate, barium sulfate; glass beads, glass flakes, etc. The polyester resin composition of the present invention can be easily produced by a known method generally used for the production of conventional resin compositions by means of known equipment generally used for the production of conventional resin compositions. For example, there are available (1) a method in which all the components are mixed and the mixture is melted, kneaded and extruded with an extruder to prepare pellets; (2) a method in which pellets of some of the above components and pellets of the remainders of the above components are separately prepared at first, and then all the pellets are mixed in predetermined amounts and molded to obtain a molded article having the intended composition; and (3) a method in which one or at least two of the above components are directly charged into a molding machine. It is also preferred to mix a fine powder of some of the above components with the remaining components, since these components can be uniformly mixed. As described above, the present invention has made the following clear. Carbon black optionally together with graphites has high absorptivity to laser light so that the marking properties are improved even when these components are used in a very small amount, and that carbon black, or graphite and carbon black, has low coloring properties so that these components do not limit the color tone of the resultant molded article. The present invention will be detailed further hereinafter by reference to Examples. In the present invention (Examples), the marking was carried out with a YAG laser and CO 2 laser. (i) Marking method with carbon dioxide gas laser: The marking was carried out on a molded article formed from a polyester resin composition containing a predetermined amount of carbon black or carbon black and graphite by a batch transfer mask method in which a mask for writing characters, symbols, marks, barcodes, etc., was prepared in advance, and mask pattern images were transferred to the surface of the above molded article by adjusting an image formation lens and irradiating the surface with a laser through the mask. To explain the procedure by reference to Fig. 1, the marking is carried out according to the following principle. In Fig. 1, laser light is reflected on a reflection mirror 11, and directed through a cylindrical lens 12, and an image is formed, through a stencil (mask) 13, on a molded article surface with a meniscus lens 14. (ii) Marking method with YAG laser: The marking with a YAG laser was carried out according to the system shown in Fig. 2. Laser light emitted from a laser head 16 is guided through two deflection mirrors, and the marking is made on a molded article surface with a focus lens 18. In Fig. 2, numerals 15 and 17 indicate galvanometers for adjusting the deflection mirrors. (iii) Evaluation of marking properties: The marking properties were evaluated on a foaming state, fineness of a line, (photo)etching and a contrast ratio. The foaming state refers to a state of a phenomenon in which carbon black and graphite contained in a molded article absorbs laser light to generate heat, and polymers around it are thermally decomposed and foamed to form a marking. The foaming occurred in a micro state, and was observed with an electron microscope. The fineness of a line was evaluated on the basis of whether or not a marking with characters having a line width of 0.1 mm or less could be carried out. The etching was evaluated by measuring a marked portion for a depth with a surface roughness tester. The etching can be said to quantitatively show the degree of guarantee of the marking (degree of preservability of the marking). For the contrast ratio, a background luminance (BL) and a characteristic luminance (CL) were measured. The luminance indicated a unit of cd/cm 2 , and the diffused lighting having a luminance of 500 ± 50 luxes was considered to be that observed as a side constraint. The contrast ratio was evaluated on the basis of BL:CL. Example 1 (Samples Nos. 1 to 3) and Comparative Example 1 (Samples Nos. 4 to 7 Samples Nos. 1 to 3 containing polyesters and various amounts of graphite and carbon black were prepared, and molded into block-like articles according to a customary method. The polyester resin compositions (molded articles) were evaluated on the marking properties with a YAG laser (laser marker SL 475E, supplied by NEC Corp.). Table 1 shows the compositions and the results of evaluation on the marking properties. In addition, as Comparative Example 1, there were prepared molded articles formed from compositions (Samples Nos. 4 and 5) containing PBT and carbon black having low thermal conductivity, and compositions (Samples Nos. 6 and 7) containing no carbon black (Sample 6) and an excess amount of carbon black (Sample 7), and the compositions (molded articles) were evaluated on the marking properties. Table 1 shows also the compositions and the results of marking properties. Example 2 (Samples Nos. 8 to 13) and Comparative Example 2 (Samples Nos. 14 and 15) Table 2 shows the compositions of polyester resin molded articles prepared according to a customary method and the marking properties thereof with a CO 2 laser (Unimark 400, supplied by Ushio Inc.). Sample 16 represents a Comparative Example. The above Example and Comparative Example show that according as the amount of graphite and carbon black having high thermal conductivity is increased, the absorptivity to laser light increases and the etching is therefore clearly increased. It is also characteristically shown that when the amount of graphite and carbon black is 0.001 to 2.0 % by weight, the contrast ratio is particularly high. The compositions described in Comparative Example contained carbon black in which the graphite formation did not advance and the secondary structure -was not developed. Therefore, the capacity of the carbon black to absorb laser light was small, and the foaming occurred nonuniformly. As a result, there was found a decrease in the etching and the contrast ratio.

Claims: A polyester resin composition comprising: (a) a thermoplastic polyester resin, and (b) a carbon black, as a sole laser light absorbing agent, said carbon black having a thermal conductivity of not less than 0.2 kcal/m.hr.°C, in which the content of component (b) is from 0.001 to 2% by weight based on the total weight of the components (a) and (b). A composition according to claim 1, wherein the component (a) is polybutylene terephthalate. A composition according to claim 1, wherein the component (a) is polyethylene terephthalate. A composition according to claim 1, wherein the component (a) is polybutylene naphthalene dicarboxylate. A composition according to claim 1, wherein the component (a) is polyethylene naphthalene dicarboxylate. A composition according to claim 1, which contains (c) a bromine-containing flame retardant and (d) an antimony-containing flame retardant aid in addition to the components (a) and (b). A polyester resin composition comprising (a) a thermoplastic polyester resin, (b) a carbon black, as a sole laser light absorbing agent, said carbon black having a thermal conductivity of not less than 0.2 kcal/m.hr.°C, (c) a bromine-containing flame retardant, and (d) an antimony-containing flame retardant aid, and in which, based on the total amount of the components (a), (b), (c) and (d), the content of the component (a) is at least 58 % by weight, the content of the component (b) is 0.001 to 2% by weight, the content of the component (c) is 2 to 25% by weight, and the content of the component (d) is 2 to 15% by weight. A composition according to any of claims 1 to 7 wherein the component (b) additionally contains a graphite having an average particle diameter of 0.1 to 150 µm. A composition according to claim 8, wherein the graphite of component (b) is a particulate, scale-like, soil-like or massive graphite. A composition according to any of claims 1 to 9, which further contains not more than 70% by weight, based on the total weight of the composition, of (e) an inorganic filler. The use of a carbon black having a thermal conductivity of not less than 0.2 kcal/m.hr.°C as a laser sensitive pigment in polyester compositions which are suitable to be marked with laser light.

Inventors: SASAKI MIE, SHIMOTSUMA SAKAE, SHIOZAKI TETSUYA, SUZUOKA AKIHIRO, SASAKI, MIE, SHIMOTSUMA, SAKAE, SHIOZAKI, TETSUYA, SUZUOKA, AKIHIRO
Applicants: TEIJIN LTD, TEIJIN LIMITED
Application year: 1991
Main classification: C08K3
IPC codes: C08K3, B41M5