Field of the Invention The present invention relates to a laser marking method for marking a urea resin product containing urea resin as a main component, and the urea resin product.

Background of the Invention

A urea resin product is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2003- 321616, in which a marking component sensitive to an active energy ray is contained in transparent resin to provide a mark of a specific color.

Meanwhile, when being irradiated with a laser with a long wavelength such as a carbon dioxide (C0 ₂ ) laser, a urea resin product expands and then the irradiated portion turns into white. Thus, when a urea resin product colored in green or the like, other than white, is marked by the laser, a highly visualized mark can be made. On the other hand, when a urea resin product colored in white is marked by the laser, the mark, which turns into white, is difficult to be distinguished from its parent urea resin product. This deteriorates the visibility of the mark to make the laser marking unavailable. Therefore, such urea resin product colored in white may be marked by stamping or the like.

In marking by stamping or the like, however, easy change of contents to be marked is difficult as compared with the laser marking. Thus, there is required a method for marking the white urea resin product by the laser to have a degree of recognizable visibility, but it has not been proposed yet .

Summary of the Invention

In view of the above, an object of the present invention is to provide a laser marking method capable of marking a urea resin product to a degree of recognizable visibility, and to provide the urea resin product having a mark formed by the laser marking method.

In accordance with a first aspect of the present invention, there is provided a laser marking method in which a urea resin product containing a titanium dioxide of 3.0 mass% or more is irradiated with a laser with a wavelength of 1064 ran or less to perform laser marking.

In accordance with a second aspect of the present invention, there is provided a laser marking method in which a urea resin product containing a titanium dioxide of 0.1 mass% or more is irradiated with a laser with a wavelength of 532 nm or less to perform laser marking.

In such laser marking method, a pulse laser with a pulse width of 20 ns or less is preferably employed.

In accordance with a third aspect of the present invention, there is provided a urea resin product including urea resin as a main component and containing a titanium dioxide of 3.0 mass% or more, wherein the urea resin product has a mark formed by using a laser with a wavelength of 1064 nm or less.

In accordance with a fourth aspect of the present invention, there is provided a urea resin product including urea resin as a main component and containing a titanium dioxide of 0.1 mass% or more, wherein the urea resin product has a mark formed by using a laser with a wavelength of 532 nm or less .

According to the present invention, there can be provided a method for marking a urea resin product to a degree of recognizable visibility and the urea resin product having a mark formed by the laser marking method.

Brief Description of the Drawings

The objects and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic view showing a test method for confirming color change of a titanium dioxide caused by a laser in accordance with an embodiment of the present invention;

Fig. 2 shows photographs indicating states of the titanium dioxide after the laser irradiation in the above embodiment ;

Fig. 3 is a graph showing absorption spectra of urea resin products in the above embodiment;

Fig. 4 is a schematic view showing diffusion paths of the laser in the above embodiment;

Fig. 5 is a graph showing a relationship between content of the titanium dioxide and brightness with respect to the urea resin product of the above embodiment, in which the content of titanium dioxide is in a range from 0 to 10 mass% ;

Fig. 6 is a graph showing a relationship between content of the titanium dioxide and brightness with respect to the urea resin product of the above embodiment, in which the content of the titanium dioxide is in a range from 0 to 1 mass%;

Fig. 7 is a graph showing a relationship between content of the titanium dioxide and brightness difference with respect to the urea resin product of the above embodiment, in which the content of the titanium dioxide is in a range from 0 to 10 mass%;

Fig. 8 is a graph showing a relationship between content of the titanium dioxide and brightness difference with respect to the urea resin product of the above embodiment, in which the content of the titanium dioxide is in a range from 0 to 1 mass%; and

Figs. 9A to 9C are schematic graphs, wherein Fig. 9A shows a relationship between an expanded amount and a sweep rate of the laser; Fig. 9B shows a relationship between a degree of color change of the titanium dioxide and the sweep rate; and Fig. 9C shows a relationship between a degree of color change of the urea resin product and the sweep rate.

Detailed Description of the Embodiment

One embodiment of the present invention will be described in detail with reference to Fig. 1.

<Urea Resin Product>

A urea resin product YP (see Fig. 4) of the present embodiment contains urea resin as a main component; pulp as filler; a parting agent for removing the urea resin product YP easily from a mold; a titanium dioxide serving as a coloring agent; and a curing agent for curing the resin.

The urea resin product YP is formed by the following process. The pulp, a stabilizing agent, the curing agent, and the titanium dioxide are added to liquid urea resin; these ingredients are mixed, while heating, to provide a compound; and the compound is dried. Subsequently, the parting agent is added to the dried compound, and then the dried compound is crushed by a ball mill to provide a powdery urea resin material. Further, the powdery urea resin material is kneaded and crushed to obtain a granular urea resin material. Then, the granular urea resin material is compression molded to form the urea resin product YP. Note that when the urea resin product YP is colored other than white, a color pigment is added thereto at the time of forming the compound. The urea resin of 80 mass% or more is included in the compound, and the filler of 1 to 10 mass% is contained.

Titanium dioxide is used as a color pigment, by which the urea resin product YP is colored in white. Besides, the titanium dioxide also functions as an additive agent for permitting a marking by laser irradiation.

The urea resin product YP expands by laser irradiation.

The expanded place is recognized as white with the naked eye This expansion occurs independent of a laser wavelength. Specifically, C0 ₂ laser having a wavelength of 10.2 μπι, or YV0 ₄ laser having a wavelength of 1064 nm will cause the expansion. Further, a second harmonic of the YV0 ₄ laser (hereinafter, referred to as "SHG laser") , or a third harmonic of the YV0 ₄ laser (hereinafter, referred to as "THG laser") will also cause the expansion. Furthermore, in either case where the urea resin product YP is colored, or colorless and transparent, the urea resin product YP expands and then turns into white . The titanium dioxide turns into black by the laser irradiation. Such color change is attributed to the reduction from tetravalent titanium to trivalent titanium, which is caused by light absorption or optical heating of the titanium dioxide as shown in the following equation (1) . hv heat i

2Ti0 ₂ > Ti ₂ 0 ₃ + - 0 ₂ .. )

Note that, it is well known that titanium dioxide can be reduced by irradiating a mixture of the titanium dioxide and a reducing agent with C0 ₂ laser to change its color. On the other hand, it is confirmed that the use of laser light with a wavelength of 1064 nm or less allows the titanium dioxide to turn into black without using the reducing agent. Therefore, in the present embodiment, the reducing agent is not used.

A degree of color change of the titanium dioxide will be described with reference to Figs . 1 to 3. The titanium dioxide has a primary color of white. This means that the more the titanium dioxide turns into black, the higher the degree of color change is.

A color change test of the titanium dioxide by using laser will be described with reference to Fig. 1. A powdery titanium dioxide is placed on a glass slide 1 and fixed with a transparent tape 2. Then, the laser is irradiated from below the glass slide 1. In the color change test of the present embodiment, THG laser with a wavelength of 355 nm, SHG laser with a wavelength of 532 nm, YV0 ₄ laser with a wavelength of 1064 nm, and C0 ₂ laser with a wavelength of 10.2 μιη are employed to perform the color change test of the titanium dioxide.

Fig. 2 shows photographs of the titanium dioxide that has been irradiated with the laser. (A) of Fig. 2 shows the photograph of the titanium dioxide that has been irradiated with the THG laser light. As shown in (A) of Fig. 2, the titanium dioxide turns from white into black by irradiating the THG laser. (B) of Fig. 2 shows the photograph of the titanium dioxide that has been irradiated with the SHG laser light. As shown in (B) of Fig. 2, the titanium dioxide turns from white into black by irradiating the SHG laser. (C) of Fig. 2C shows the photograph of the titanium dioxide that has been irradiated with the YV0 ₄ laser light. As shown in (C) of Fig. 2, the titanium dioxide turns from white into black by irradiating the YV0 ₄ laser.

Note that, the titanium dioxide will also change its color by irradiating the C0 ₂ laser light. The degree of color change of the titanium dioxide by the C0 ₂ laser is smaller than that by any one of the THG laser, the SHG laser, and the YV0 ₄ laser.

Fig. 3 shows absorption spectra of the urea resin product YP containing the titanium dioxide (hereinafter, referred to as "specimen sample"), and a urea resin product YP free from the titanium dioxide (hereinafter, referred to as "comparative sample").

The absorption spectra of the specimen sample and the comparative sample have an absorption band with absorptivities of 40% or more in a range of wavelengths below 400 nm and above 1145 nm. In a range of wavelengths from 400 to 1145 nm, they have an absorption band with absorptivities of 20% or more. When comparing the specimen sample to the comparative sample, there is a difference in absorptivity around a wavelength of 355 nm and the specimen sample has a higher absorptivity than that of the comparative sample. In other words, in this absorption band, the titanium dioxide absorbs the THG laser light efficiently.

With reference to Fig. 4, there will be described a difference between a first diffusion path along which the light of wavelength 532 nm travels in the urea resin product YP and a second diffusion path along which the light of wavelength 1064 nm travels therein. As shown in Fig. 4, the light of wavelength 1064 nm travels more deeply into the urea resin product YP than the light of 532 nm wavelength travels. For this reason, it is possible that the color change of the titanium dioxide occurs in a deeper place by irradiating the light of wavelength 1064 nm than by irradiating the light of wavelength 532 nm.

Further, when comparing path lengths of the light traveling in the urea resin product YP, the light of wavelength 1064 nm has a longer path than that of the light of wavelength 532 nm. For this reason, the expanded amount of the urea resin product YP is increased by irradiating the light of wavelength 1064 nm compared with the case of irradiating the light of wavelength 532 nm.

Meanwhile, a degree of color change of the urea resin product YP by the laser depends on content of the titanium dioxide, which is contained in the urea resin product YP. Hereinafter, this point will be described. With reference to Figs. 5 to 8, brightness and brightness difference of the portion irradiated with the laser depending on content of the titanium dioxide will be described. The closer to white the urea resin product YP turns, the larger value the brightness has, and the closer to black, the smaller value the brightness has . The brightness difference is calculated on the basis of data shown in Figs. 5 and 6. Each of the data in Figs. 5 and 6 is obtained under the following conditions. Note that the content of the titanium dioxide in the figures denotes a ratio when a mass of urea resin is supposed to be 100.

[Specimen Sample]

(Composition of the urea resin product)

* Mass ratio of urea resin: 90 mass% or less

* Mass ratio of pulp: 1 to 10 mass%

* Mass ratio of titanium dioxide: 0.09 to 10 mass% [Marking Conditions]

A laser sweep rate (mm/s) and laser intensity (W) are adjusted so as to maximize the brightness difference between an irradiated portion and a non- irradiated portion. For instance, in cases where the urea resin product contains a titanium dioxide of 0.5 mass%, the laser sweep rate and the laser intensity are adjusted to 900 mm/s and 1.5 W, respectively.

As for the brightness, values measured by a spectrophotometer (e.g., CM-2002 manufactured by Minolta Co. Ltd.) are exhibited. As for the brightness difference, differences between the brightness of the non-irradiated portion, in which the urea resin product YP is not irradiated with the laser, and the brightness of the irradiated portion, in which the urea resin product YP is irradiated with the laser, are exhibited.

As shown in Fig. 5, when the content of the titanium dioxide is not less than 1.0 mass%, the urea resin product YP has a constant brightness independent of the content of the titanium dioxide. On the other hand, as shown in Fig. 6, when the content of the titanium dioxide is less than 1.0 mass%, the brightness decreases because an amount of the light transmitting the urea resin product YP increases.

As shown in Fig . 5 , the more the content of the titanium dioxide increases, the more the brightness of the irradiated portion irradiated with the YV0 ₄ laser decreases. In regard to the brightness difference between the irradiated portion and the non-irradiated portion, when the content of the titanium dioxide is 3.0 mass% or more, the brightness difference between the irradiated portion and the non- irradiated portion increases to 1.0 or more, as shown in Fig. 7.

As shown in Fig. 6, the more the content of the titanium dioxide increases, the more the brightness of the irradiated portion irradiated with the SHG laser decreases . In regard to the brightness difference between the irradiated portion and the non- irradiated portion, when the content of the titanium dioxide is 0.1 mass% or more, the brightness difference between the irradiated portion and the non-irradiated portion increases to 1.0 or more, as shown in Fig. 8.

When the brightness difference is 1.0 or more, the difference between the irradiated portion and the non- irradiated portion can be made clear, whereby the marking by the laser is distinguishable visually. Accordingly, when it is supposed that brightness differences of 1.0 or more are defined as a visibility standard, a relationship between the composition of the urea resin product YP and the laser wavelength is summarized as follows :

(1) The urea resin product YP containing titanium dioxide of 3.0 mass% or more can be visually marked by the laser with a wavelength of 1064 nm or less. (2) The urea resin product YP containing titanium dioxide of 1.0 mass% or more can be visually marked by the laser with a wavelength of 532 nm or less.

Note that, although not shown as data, the urea resin product YP containing titanium dioxide of 0.1% or more can be visually marked to have brightness differences of 1.0 or more by the laser marking of the THG laser.

<Laser Marking Method>

As described above, the laser wavelengths capable of visually marking the urea resin product YP are varied depending on the content of the titanium dioxide. Specifically, when the content of the titanium dioxide is low, a short-wavelength laser marking is preferable. When the content of the titanium dioxide is high, the visually recognizable marking can be made even by a long-wavelength laser. For instance, the laser wavelength can be selected depending on the content of the titanium dioxide as follows:

(1) The urea resin product YP containing titanium dioxide of 3.0 mass% or more is marked by the laser with a wavelength of 1064 nm or less.

(2) The urea resin product YP containing titanium dioxide of 1.0 mass% or more is marked by the laser with a wavelength of 532 nm or less.

In the meantime, when the urea resin product YP is marked by the laser, the degree of color change of the mark varies with irradiation conditions of the laser, e.g., a pulse width, or a sweep rate. For this reason, the irradiation conditions are adjusted so as to increase the brightness difference between the irradiated portion irradiated with the laser and the non-irradiated portion on which the marking is not formed.

With reference to Figs. 9A to 9C, the degree of color change of the urea resin product YP depending on the sweep rate of the laser will be described. Fig. 9A shows a tendency to an expanded amount of the urea resin product YP by the laser.

The expanded amount increases as the sweep rate of the laser is made slower. Specifically, irradiation time at which the same place is irradiated with the laser is made longer as the sweep rate of the laser is lower, and an amount of heat applied thereto is increased. For this reason, the urea resin product YP becomes easy to expand.

Even if the content of the titanium dioxide and the laser wavelength satisfy such conditions that the visually recognizable marking is formed, when the sweep rate is lower than a specific rate, the irradiated portion does not turn into black. Since the urea resin product YP expands under the above condition, it turns into white. For instance, in cases where the urea resin product YP containing a titanium dioxide of 3.0 mass is marked by the YV0 ₄ laser, when the sweep rate is lower than the specific sweep rate, white marks may be formed. Fig. 9B shows the tendency to the degree of color change of the titanium dioxide depending on the sweep rate. The degree of color change decreases as the sweep rate of the laser is increased. Specifically, the irradiation time at which the same place is irradiated with the laser is shortened as the sweep rate of the laser is higher, so that the light and the heat applied thereto is decreased. For this reason, the titanium dioxide becomes difficult to change its color.

Even if the content of the titanium dioxide in the urea resin product YP and the laser wavelength satisfy such conditions that the visually recognizable marking is formed, when the sweep rate is higher than a specified rate, the irradiated portion does not turn into black. For instance, in cases where the urea resin product YP containing a titanium dioxide of 3.0 massl is marked by the YV0 ₄ laser, when the sweep rate is higher than the specified sweep rate, the color change does not occur.

Fig. 9C shows the tendency to the degree of color change of the mark on the urea resin product YP containing the titanium dioxide depending on the sweep rate. Note that the expansion by the laser and the degree of color change of the titanium dioxide are taken into consideration in Fig. 9C.

When the sweep rate of the laser is low, the expansion of the urea resin product YP is dominant. At this time, the mark by the laser turns into white. In other words, the color of the mark becomes similar to that of the urea resin product YP, whereby its visibility is lowered.

When the sweep rate of the laser is higher than a predetermined sweep rate A but lower than a predetermined sweep rate B, the expansion of the urea resin product YP decreases, so that the color change of the titanium dioxide is made conspicuous. At this time, the mark by the laser turns into black. In other words, the color of the marking becomes different from that of the urea resin product YP, whereby its visibility is enhanced.

When the sweep rate of the laser is higher than the predetermined sweep rate B, an amount of laser light irradiating a portion of the urea resin product YP is lowered, which decreases the expansion of the urea resin product YP and the degree of color change of the titanium dioxide. At this time, a trace of the mark by the laser becomes small in the urea resin product YP.

Next, the degree of color change of the urea resin product YP depending on a pulse width of the laser will be described. The YV0 ₄ laser has a wavelength of 1064 nm, and its pulse width is about 10 ns . The YAG laser has a wavelength of 1064 nm and its pulse width is about 100 ns . It is supposed that the YV0 ₄ laser has the same intensity as that of YAG laser. When the urea resin product YP is marked by the respective lasers, the degree of color change by the YVO ₄ laser is higher than that by the YAG laser. In other words, even if the laser marking is performed by using the lasers having the same wavelength, the laser with a shorter pulse width can form the mark with a higher visibility. The laser with a shorter pulse width can feed higher irradiation energy at a short time, thereby suppressing heat dispersion from the irradiated portion, so that an area of the expanded portion can be decreased.

In accordance with the present embodiment, the following effects will be expected.

(1) In accordance with the present embodiment, the urea resin product YP containing the titanium dioxide of 3.0 mass% or more is irradiated with the laser with a wavelength of 1064 nm or less to perform the laser marking.

When the urea resin product YP containing the titanium dioxide is irradiated with the laser whose wavelength is equal to or less than the specific value, the expansion of the urea resin product YP and reduction of the titanium dioxide will occur. The expanded portion of the urea resin product YP is visually recognized as white, and the reduced titanium dioxide is visually recognized as black. For this reason, when the expanded amount of the urea resin product YP exceeds the reduced amount of the titanium dioxide, the portion irradiated with the laser is visually recognized as white .

Specifically, when the urea resin product YP containing the titanium dioxide of less than 3.0 mass% is irradiated with the laser with a wavelength of 1064 nm, the expansion is dominant: the degree of color change of the titanium dioxide is decreased, thereby making it difficult to recognize visually the mark by the laser.

In the above method, the urea resin product YP containing the titanium dioxide of 3.0 mass% or more is irradiated with the laser with a wavelength of 1064 nm or less. Thus, the mark can be recognized easily as compared with the case where the urea resin product YP containing the titanium dioxide of less than 3.0 mass% is irradiated with the laser with a wavelength of 1064 nm or more.

(2) In accordance with the present embodiment, the urea resin product YP containing the titanium dioxide of 1.0 mass% or more is irradiated with the laser with a wavelength of 532 nm or less to perform the laser marking.

When the urea resin product YP containing the titanium dioxide of less than 1.0 mass% is irradiated with the laser with a wavelength of 532 nm, the expansion is dominant: the degree of color change of the titanium dioxide decreases, thereby making it difficult to recognize visually the mark by the laser. On the other hand, in the above method, the urea resin product YP containing the titanium dioxide of 1.0 mass% or more is irradiated with the laser with a wavelength of 532 nm or less. Thus, the mark can be recognized easily as compared with the case where the urea resin product YP containing the titanium dioxide of less than 1.0 mass% is irradiated with the laser with a wavelength of 532 nm or more .

(3) In accordance with the present embodiment, a pulse laser with pulse widths of less than 20 ns is used as a laser for printing marks on the urea resin product YP. In cases where the urea resin product YP containing the titanium dioxide turns into black, the degree of color change is increased by irradiating the laser with a short pulse width compared with the case of irradiating the laser with a long pulse width, even if its wavelength and power remain the same. In the above method, the pulse laser with pulse widths of less than 20 ns is employed, thereby enabling the irradiated portion to turn into more blackish as compared with the case where the laser irradiation is performed by using the laser with a pulse width of 20 ns or more .

(4) In accordance with the present embodiment, the urea resin product YP, including a urea resin as a main component, contains the titanium dioxide of 3.0 mass% or more and is marked by the laser with a wavelength of 1064 nm or less.

When the urea resin product YP containing the titanium dioxide of less than 3.0 mass% is irradiated with the laser with a wavelength of 1064 nm, the marking is difficult to be recognized visually. On that point, in the above configuration, the urea resin product YP containing the titanium dioxide of 3.0 mass% or more is marked by the laser with a wavelength of 1064 nm or less. For this reason, the marking is easier to be recognized visually as compared with the case where the urea resin product YP containing the titanium dioxide of less than 3.0 mass% is marked by the laser with a wavelength of more than 1064 nm.

(5) In accordance with the present embodiment, the urea resin product YP, including a urea resin as a main component, contains the titanium dioxide of 1.0 mass% or more, and is marked by the laser with a wavelength of 532 nm or less.

When the urea resin product YP containing the titanium dioxide of less than 1.0 mass% is irradiated with the laser with a wavelength of 532 nm, the marking is difficult to be recognized visually. On that point, in the above configuration, the urea resin product YP containing the titanium dioxide of 1.0 mass% or more is marked by the laser with a wavelength of 532 nm or less. For this reason, the marking is easier to be recognized visually as compared with the case where the urea resin product YP containing the titanium dioxide of less than 1.0 mass% is marked by the laser with a wavelength of more than 532 nm.

(Other Embodiments)

Note that the present invention is not limited to the above-described embodiment, but can be modified, for example, as follows. Further, each of the following modifications can be applied to the above-described embodiment, and the different modifications may be combined with each other.

In the above-described embodiment, a reducing agent for reducing the titanium dioxide is not used, but the reducing agent may be included as a component of the urea resin product YP. In the above-described embodiment, the brightness difference is adopted as a parameter of the visibility of the mark. However, in cases where the urea resin product YP is colored in other than white, the mark may be recognized even if the brightness difference is less than 1.0. Accordingly, it is possible to adopt the brightness as the parameter of the visibility of the mark.

For instance, when the recognizable standard is determined such that the brightness of the irradiated portion irradiated with the laser is set to 8.5 or less, the relationship between the composition of the urea resin product YP and the laser wavelength is summarized as follows based on the data shown in Figs . 5 and 6.

(1) The urea resin product YP containing the titanium dioxide of 4.0 mass% or more can be marked by the laser with a wavelength of 1064 nm or less.

(2) The urea resin product YP containing the titanium dioxide of 0.05 mass% or more can be marked by the laser with a wavelength of 532 nm or less.

In the above embodiment, the YV0 ₄ laser is used, but the second harmonic and the third harmonic of the YAG laser may also be used. Further, it is not limited to a solid- state laser but a liquid laser or a gas laser can also be used.

While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.