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Laser-effected darkening in TPEs with TiO2 additives
ARTICLE IN PRESS
Optics and Lasers in Engineering 41 (2004) 791–800
Laser-effected darkening in TPEs with TiO2 additives Hongyu Zheng*, Gnian Cher Lim Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Received 1 April 2002; received in revised form 1 November 2002; accepted 1 February 2003
Abstract Investigations were carried out for laser-effected darkening and damage-free laser marking on thermoplastic elastomers (TPEs). Titanium dioxide (TiO2) was studied as the laser-sensitive additive to the TPEs for its role in enhancing laser marking contrast and its effect on the TPE properties. Laser beam characteristics, processing variables and percentage loading of TiO2 in the TPEs were found to have significant effect on the marking contrast. Surface damage-free and high contrast marking have been achieved with short pulse UV lasers. The laser-effected darkening was found to penetrate into the material in the order of a few tens of microns. XPS analysis was carried out to understand the laser-effected darkening mechanism. Potential applications of the technique are highlighted. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Laser-effected colour change marking; Laser-sensitive additives; Laser darkening
1. Introduction CO2 and Nd:YAG lasers are most commonly used for marking on a variety of materials, including polymers, metals and ceramics. The marking contrast is achieved through materials melting and removal, leading to relief structures. The resultant marking is generally characterised with rough surfaces and low-edge resolution [1–8]. Laser marking by photochemical transformation leading to clearly visible colour changes with no apparent surface damage is the most desirable option of laser marking for many applications. Unfortunately, most plastics are transparent to the laser beams or go under only slight surface melting or vaporisation. To *Corresponding author. Tel.: +65-793-8504; fax: +65-792-2779. E-mail address: [email protected] (H. Zheng). 0143-8166/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0143-8166(03)00032-0
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produce good laser marking contrast for plastics, there is a growing interest in the development of additives (pigments) that serve as sensitisers to effect laser marking. Some work on laser-effected colour changes on metals [3] and thermoplastics [1,2,4– 8] have been reported for surface damage-free, multiple colours, high contrast and high-resolution marking applications. Applications of titaniumdioxide (TiO2) and ZnO as pigments in the manufacturing of sunscreens and UV absorbing coatings on glass for automobiles have also been reported [9,10]. As thermoplastic elastomers (TPEs) are being used widely in many products including aerospace, automotive, medical and domestic products, high-quality laser marking for product identification, functionality and decoration is needed. Lasereffected darkening on pigmented TPEs provides high resolution, durable and nondamage marks. Marking on aircraft cables, computer keyboards and mobile phone keypads are some of the current applications. As TPE costs less than silicon rubber, is economically recyclable, and can be more efficiently manufactured through injection-moulding process in comparison to the much slower compressionmoulding process for silicon rubber, TPE may replace silicon rubber in many applications. This will in turn generate more demand for damage-free laserdarkening techniques. Another potential application is in the production of optical attenuators by controlling the degree of the laser-effected darkening. In this paper, studies of laser marking on TPEs with TiO2 additives are reported. The objectives of the studies are to develop a laser-markable TPE for potential industrial applications and to understand the laser-effected darkening mechanism.
2. Methodology The study was started by identifying pigments, TPE materials and appropriate UV laser sources. Experiments were then carried out on laser irradiation of pigments and pigmented TPEs. 2.1. Materials The base material is Kraton G2755 manufactured by Shell Chemicals Ltd. of USA. It consists of three discrete polymer blocks of the A–B–A type. The end block (A) is a hard thermoplastic: polystyrene, whereas the centre blocks (B) are elastomers: poly(butadiene) or poly(ethylene/butylene). The pigment investigated is TiO2, which is commonly used as the white pigment in the plastic industry due to its high refractive index, thus good opacity and chemical inertness. It has two crystallographic forms: anatase and rutile having refractive indices of 2.5 and 2.7, respectively. In the industry, rutile is most commonly used as it provides the greatest hiding power. The pigments investigated are listed in Table 1. The particle size affects the bending of light by refraction (for large particles) and by diffraction (for small particles). Smaller particles give higher colour contrast, but are more susceptible to chemical degradation. Furthermore, small particle size may cause dispersion issue when they are incorporated into a resin mixture [11]. Larger
ARTICLE IN PRESS H. Zheng, G.C. Lim / Optics and Lasers in Engineering 41 (2004) 791–800
793
Table 1 A list of pigments investigated Name of Co.
Product
Colour
Size
KEMIRA (Finland) Cerac Inc. (USA) Cerac Inc.
UV-TITAN P160 (TiO2) T2041 T2048
White (rutile, 90% min.), surface treated White (rutile TiO2, 99.5%) White (anatase TiO2, 99.9%)
17 nm 0.73 mm 33 nm
particles on the other hand need higher concentrations to obtain a desired colouring, but normally show better durability properties. TiO2 is non-toxic and appears white in powdered form. No absorption bands are present in the visible region for crystalline TiO2. The strong absorption band lies in the near UV (below 400 nm). The selected TiO2 was blended with Kraton G2755 at the loading of 0.5%, 1%, 1.5%, 2%, 3% and 5% respectively to produce 2 mm thick flat samples. Results were reviewed based on the laser marking contrast, the degree of surface damage and sample translucency. 2.2. Lasers Two UV lasers were used for irradiating the TPE samples and marking patterns by way of beam scanning and mask marking. For the beam scanning marking, the laser is a Q-switched diode-pumped third harmonic Nd:YAG laser, which has a wavelength of 355 nm and pulse width of 25 ns. The focal length of the scanning optics is 600 mm. For the mask marking, a KrF excimer laser was used. The laser has a wavelength of 248 nm and pulse width of 20 ns. A pair of beam homogenisers consisting of 8 8 lens arrays was used to improve the uniformity of the beam intensity. Imaging lenses with the demagnification ratio of 4 and 15 respectively were used to obtain the needed energy densities.
3. Result and discussion 3.1. Laser-effected darkening in TiO2 pigment Studies of laser darkening TiO2 alone were aimed at establishing the cause of the laser-induced darkening effect, as it is generally believed that the laser-effected oxygen deficiencies in TiO2 (formation of TiO or Ti) may be the reason for the darkening. It was found that dark colours were effected in the white pigments (anatase and rutile) after the UV laser irradiation. Darker colours (grey to black) were obtained with the increase in the energy density at a given number of pulses, and with the increase in the number of laser pulses at a given energy density and the higher energy density made the conversion of TiO2 particles easier, whereas the additional pulses
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Fig. 1. KrF laser-effected darkening in a TiO2 tablet (rutile, T2041), 1.5 J/cm2, single pulse.
helped to convert TiO2 particles that had not interacted with the previous pulses. The darkness, however, saturated after about five pulses with the energy density in the range of 1–1.5 J/cm2. It is worthwhile to note that the colour change for the anatase TiO2 (T2048) is less than that of the rutile TiO2 (T2041) at a given energy density. This may be related to the different bond gap energies of the TiO2, i.e. 3.23 and 3.02 eV for anatase and rutile, respectively. Higher energy density is needed when effecting changes in the anatase form of TiO2. An example of the laser-effected darkening in TiO2 is shown in Fig. 1, where a rutile tablet (T2041) was irradiated by a single pulse KrF excimer laser beam at the energy density of 1.5 J/cm2. Theoretical explanation of the laser-effected darkening may be made based on the oxygen deficiencies in TiO2 lattice. Oxygen can be reversibly removed from the TiO2 lattice by evacuation at high temperatures or by evacuation under strong UV illumination. Loss of oxygen is associated with the generation of either TiO, which is black, or Ti, which is responsible for the dark blue-grey colour assumed by the pigment. 3.2. Laser-effected darkening in pigmented TPEs The pigmented TPE samples responded well to the UV laser beams by changing into dark colours. No apparent surface damage was observed in the laser-irradiated areas. It was found that with increasing energy density, the marking contrasts improved as shown in Fig. 2, where the TPE sample contains 1% TiO2 (KEMIRA UV-Titan P160 [12], Table 1). When the energy was low (0.2 J/cm2), faint yellowish marks appeared. Grey marks appeared at the higher energy densities (0.8–1.2 J/cm2). The best contrast was achieved at about 1.5 J/cm2. The marking contrast remained almost the same (not noticeable by naked eyes) with further increase in the energy
ARTICLE IN PRESS H. Zheng, G.C. Lim / Optics and Lasers in Engineering 41 (2004) 791–800
795
2.5mm
0.2J/cm2
0.8J/cm2
1.2J/cm2
1.5J/cm2
Fig. 2. KrF laser-effected darkening with a single pulse. TiO2 loading in the TPE was 1%.
density. Excessively high-energy density also led to some material ablation. Quantitative assessment of laser-marked samples is desirable, as it provides more accurate judgement on laser marking contrast [13]. However, a standardised instrument for assessing the laser-marking contrast is not currently available for the study. Higher marking contrast was obtained with the samples of higher pigment loading as expected when the energy density remained the same. The sample translucency was however reduced due to the increase in the refractive index. For instance, the sample containing TiO2 from CERAC has shown good laser-effected darkening due to the high TiO2 content in the pigment (above 99%, Table 1). The sample is nearly opaque. On the other hand, the sample containing TiO2 from KEMIRA (UV-Titan P160) has shown both good marking contrast and better translucency. The finer powder size of UV-Titan P160 has compensated for the reduced darkening effect due to the lower TiO2 content in the pigment (90%, Table 1). The refractive index does not cause the laser darkening; however, it does change the opacity of the pigmented sample. The marking contrast is consequently affected. The sample translucency is an important factor for applications requiring backlighting such as car radio panels and mobile phone keypads. Examination of the depth of the laser-effected darkening with TiO2 loading of 1%, 1.5% and 2% respectively was attempted by cross-sectioning the laserirradiated samples. However, as the material is soft, sharp and straight edges could not be obtained. The estimated depth of the 355 nm laser-marked sample with 1.5% TiO2 loading was 75 mm (Fig. 3). The fact that the laser-effected darkening penetrated into the material (volume marking) is significant, as it is more durable when compared to the conventional laser surface marking due to burning or material removal. Theoretically, the penetration depth of radiation follows Beer’s law [14]: It ¼ I0 10al ;
ð1Þ
where I0 and It are the intensities of the beam of light before and after transimission through a slice of materials of thickness l and a the absorptivity, is a characteristic property of the material and varies with the wavelengths. If the sample absorbs less,
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Absorption %
Fig. 3. A cross-sectional view of 355 nm Nd:YAG laser-effected darkening in the pigmented TPE with 1.5% TiO2. Optical magnification is 40 .
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 200 300 400 500 600 700 800 900 1000 1100
Wavelength (nm) Fig. 4. Absorption curve of the pigmented TPE, 1% TiO2.
the penetration is deeper. To characterise the absorption behaviour of the pigmented TPE sample (2 mm thick), a UV-VIS-IR Spectrophotometer (Shimadzu, Model UV-3101PC) was used. The absorption percentage was plotted against the wavelengths as shown in Fig. 4. It is seen that the pigmented TPE is a weak absorber particularly for wavelengths above 400 nm. At 248 and 355 nm, the beam absorption rate is 3.4% and 2.9% respectively. The relatively great depth of lasereffected darkening is qualitatively explained by the low absorption rates at both 355 and 248 nm. Examples of the UV laser-marked TPE samples are shown in Fig. 5. The substrate materials are coloured TPEs containing TiO2. For the keypad, 1.5% TiO2 (p. 160) was added and the sample was translucent. For the ‘‘Gintic logos’’, 3% TiO2 was added and the sample was opaque. The keypads were marked with the third harmonic Nd:YAG laser through the beam scanning method. The ‘‘Gintic’’ logos were produced through the mask marking by the KrF excimer laser.
ARTICLE IN PRESS H. Zheng, G.C. Lim / Optics and Lasers in Engineering 41 (2004) 791–800
797
20 mm 5 mm
Fig. 5. Marking by third harmonic Nd:YAG laser (keypads) and KrF excimer laser (logos) on the pigmented TPE samples.
3.3. Investigation of darkening mechanism in laser marking TPEs Several techniques that are sensitive to surface structure and chemical bonds were used in understanding laser-effected darkening on the TPEs. These included Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS analysis has yielded the most relevant information and the results are presented below. The XPS analysis at different take-off angles was conducted on both the lasermarked and non-marked areas of a TPE sample containing 1.5% TiO2. The Ti2p peaks were detected in laser-marked and non-marked areas, as shown in Fig. 6. The O1s peaks were detected and are shown in Fig. 7. In Fig. 8, after the charging correction, the C1s peaks (284.6 eV) were detected in both the laser-marked and nonmarked areas. The spectra for the laser-marked and non-marked areas are considered identical. Two oxygen species on the surfaces are probably due to oxygen in TiO2 (529.3 eV) and oxygen in TiCOOH or TiOH (531.4 eV). By analysing the laser-treated area at different take-off angles, the only change is the relative increase in carbon contaminants on the topmost layers (take-off angle of 20 ) with respect to those at the subsurface region (Fig. 7). This implies that the darkening mechanism may be due to the bond-breaking of the polymer matrix surrounding the fine TiO2 particles. The bond-breaking process is likely to be a combined effect of both photochemical and thermal reactions. The relative increase in black carbon at the top surface layer is distinguished as the marking contrast. This agrees with the work reported by Hofmann [8]. However, the belief that TiO2 is reduced to TiO or Ti by the intense UV laser irradiation was not detected.
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Fig. 6. XPS showing the Ti2p peaks in the TiO2-pigmented TPE.
Fig. 7. XPS showing O1s peaks in TiO2. The sample is TiO2-pigmented TPE.
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Fig. 8. XPS showing C1s peaks in the TiO2-pigmented TPE.
4. Conclusions A method to obtain high quality laser marks on TPE materials has been developed. Varying the TiO2 loading in the materials can control the marking contrast and material translucency. The XPS results revealed there was a relative increase in carbon at the top surface layer after the laser irradiation due to the bond breaking of the polymer matrix surrounding the fine TiO2 particles. Black carbon is distinguished as the marking contrast. The method may be used for product identification, functionality and decoration in many applications. Acknowledgements The authors would like to thank Dr. Chen zhenda, Dr. Zhao Jian Hong, Dr. Chen Yihong, and Ms. Tan Joo Lett for useful discussions. Thanks also go to Ms. Ng Fern Lan and Mr. Teh Kim Ming for their lab support. References [1] Tang Zheng Gui, Varahamurthy R, Zheng Hong Yu. Polymer films of laser marking effects for IC encapsulation. APACK 2001 Conference on Advances in Packaging, Singapore, December 2001. p. 162–8.
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[2] Zheng Hongyu, Lim Gnian Cher, Zhao Jian Hong. Method of preparing an integrated circuit package for laser marking. Singapore Patent Application 200000516-5, filed on 1st February 2000. [3] Zheng HY, Lim GC, Wang XC, Tan JL, Hilfiker J. Process study for laser-induced surface colouration. J Laser Appl (Laser Institute of America) 2002;14:215–20. [4] Tang Lee Yung, Zheng Hongyu, Zhao Jian Hong, Lim Gnian Cher. Laser markable material. PCT/ EP00/07304, filed on 31 July 2000. [5] Lerner EJ. Laser marking systems strive for colour and speed. Laser Focus World, October issue, 1999. p. 123–30. [6] Graff G. Additives make possible laser marking of polyolefin components. Mod Plast, April issue, 1996. p. 24–5. [7] McKee TJ. How lasers mark. Plast Technol 1996;35:11–8. [8] Hofmann M, Breitenfellner F, Buhler N, Sykes RC, Gugger H. High contrast and intact surface—a challenge in laser marking of plastics. SPIE Lasers Motion Ind Appl. SPIE, Los Angeles, 1987;744:156–80. [9] Morimoto T, Tomonaga H, Mitani A. Ultraviolet ray absorbing coatings on glass for automobiles. Thin Solid Films 1999;351(1–2):61–5. [10] Stamatakis P, Palmer BR, Salzman GC, et al. Optimum particle size of titanium-dioxide and zincoxide for attenuation of ultraviolet radiation. J Coating Technol 1990;62(789):95–8. [11] Stewart EW, Rutile titanium dioxide in plastics. Plast Compounding, Jan./Feb. issue, 1985;58–60. [12] Kemira Product Data Sheet, UV-Titan P160. [13] Ng TW, Yeo SC. Aesthetic laser marking assessment using luminance ratios. Opt Lasers Eng 2001;35:177–86. [14] Srinivasan R. Ablation of polymers and biological tissue by ultraviolet lasers. Science 1986;234: 559–65.
Optics and Lasers in Engineering 41 (2004) 791–800
Laser-effected darkening in TPEs with TiO2 additives Hongyu Zheng*, Gnian Cher Lim Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Received 1 April 2002; received in revised form 1 November 2002; accepted 1 February 2003
Abstract Investigations were carried out for laser-effected darkening and damage-free laser marking on thermoplastic elastomers (TPEs). Titanium dioxide (TiO2) was studied as the laser-sensitive additive to the TPEs for its role in enhancing laser marking contrast and its effect on the TPE properties. Laser beam characteristics, processing variables and percentage loading of TiO2 in the TPEs were found to have significant effect on the marking contrast. Surface damage-free and high contrast marking have been achieved with short pulse UV lasers. The laser-effected darkening was found to penetrate into the material in the order of a few tens of microns. XPS analysis was carried out to understand the laser-effected darkening mechanism. Potential applications of the technique are highlighted. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Laser-effected colour change marking; Laser-sensitive additives; Laser darkening
1. Introduction CO2 and Nd:YAG lasers are most commonly used for marking on a variety of materials, including polymers, metals and ceramics. The marking contrast is achieved through materials melting and removal, leading to relief structures. The resultant marking is generally characterised with rough surfaces and low-edge resolution [1–8]. Laser marking by photochemical transformation leading to clearly visible colour changes with no apparent surface damage is the most desirable option of laser marking for many applications. Unfortunately, most plastics are transparent to the laser beams or go under only slight surface melting or vaporisation. To *Corresponding author. Tel.: +65-793-8504; fax: +65-792-2779. E-mail address: [email protected] (H. Zheng). 0143-8166/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0143-8166(03)00032-0
ARTICLE IN PRESS 792
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produce good laser marking contrast for plastics, there is a growing interest in the development of additives (pigments) that serve as sensitisers to effect laser marking. Some work on laser-effected colour changes on metals [3] and thermoplastics [1,2,4– 8] have been reported for surface damage-free, multiple colours, high contrast and high-resolution marking applications. Applications of titaniumdioxide (TiO2) and ZnO as pigments in the manufacturing of sunscreens and UV absorbing coatings on glass for automobiles have also been reported [9,10]. As thermoplastic elastomers (TPEs) are being used widely in many products including aerospace, automotive, medical and domestic products, high-quality laser marking for product identification, functionality and decoration is needed. Lasereffected darkening on pigmented TPEs provides high resolution, durable and nondamage marks. Marking on aircraft cables, computer keyboards and mobile phone keypads are some of the current applications. As TPE costs less than silicon rubber, is economically recyclable, and can be more efficiently manufactured through injection-moulding process in comparison to the much slower compressionmoulding process for silicon rubber, TPE may replace silicon rubber in many applications. This will in turn generate more demand for damage-free laserdarkening techniques. Another potential application is in the production of optical attenuators by controlling the degree of the laser-effected darkening. In this paper, studies of laser marking on TPEs with TiO2 additives are reported. The objectives of the studies are to develop a laser-markable TPE for potential industrial applications and to understand the laser-effected darkening mechanism.
2. Methodology The study was started by identifying pigments, TPE materials and appropriate UV laser sources. Experiments were then carried out on laser irradiation of pigments and pigmented TPEs. 2.1. Materials The base material is Kraton G2755 manufactured by Shell Chemicals Ltd. of USA. It consists of three discrete polymer blocks of the A–B–A type. The end block (A) is a hard thermoplastic: polystyrene, whereas the centre blocks (B) are elastomers: poly(butadiene) or poly(ethylene/butylene). The pigment investigated is TiO2, which is commonly used as the white pigment in the plastic industry due to its high refractive index, thus good opacity and chemical inertness. It has two crystallographic forms: anatase and rutile having refractive indices of 2.5 and 2.7, respectively. In the industry, rutile is most commonly used as it provides the greatest hiding power. The pigments investigated are listed in Table 1. The particle size affects the bending of light by refraction (for large particles) and by diffraction (for small particles). Smaller particles give higher colour contrast, but are more susceptible to chemical degradation. Furthermore, small particle size may cause dispersion issue when they are incorporated into a resin mixture [11]. Larger
ARTICLE IN PRESS H. Zheng, G.C. Lim / Optics and Lasers in Engineering 41 (2004) 791–800
793
Table 1 A list of pigments investigated Name of Co.
Product
Colour
Size
KEMIRA (Finland) Cerac Inc. (USA) Cerac Inc.
UV-TITAN P160 (TiO2) T2041 T2048
White (rutile, 90% min.), surface treated White (rutile TiO2, 99.5%) White (anatase TiO2, 99.9%)
17 nm 0.73 mm 33 nm
particles on the other hand need higher concentrations to obtain a desired colouring, but normally show better durability properties. TiO2 is non-toxic and appears white in powdered form. No absorption bands are present in the visible region for crystalline TiO2. The strong absorption band lies in the near UV (below 400 nm). The selected TiO2 was blended with Kraton G2755 at the loading of 0.5%, 1%, 1.5%, 2%, 3% and 5% respectively to produce 2 mm thick flat samples. Results were reviewed based on the laser marking contrast, the degree of surface damage and sample translucency. 2.2. Lasers Two UV lasers were used for irradiating the TPE samples and marking patterns by way of beam scanning and mask marking. For the beam scanning marking, the laser is a Q-switched diode-pumped third harmonic Nd:YAG laser, which has a wavelength of 355 nm and pulse width of 25 ns. The focal length of the scanning optics is 600 mm. For the mask marking, a KrF excimer laser was used. The laser has a wavelength of 248 nm and pulse width of 20 ns. A pair of beam homogenisers consisting of 8 8 lens arrays was used to improve the uniformity of the beam intensity. Imaging lenses with the demagnification ratio of 4 and 15 respectively were used to obtain the needed energy densities.
3. Result and discussion 3.1. Laser-effected darkening in TiO2 pigment Studies of laser darkening TiO2 alone were aimed at establishing the cause of the laser-induced darkening effect, as it is generally believed that the laser-effected oxygen deficiencies in TiO2 (formation of TiO or Ti) may be the reason for the darkening. It was found that dark colours were effected in the white pigments (anatase and rutile) after the UV laser irradiation. Darker colours (grey to black) were obtained with the increase in the energy density at a given number of pulses, and with the increase in the number of laser pulses at a given energy density and the higher energy density made the conversion of TiO2 particles easier, whereas the additional pulses
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Fig. 1. KrF laser-effected darkening in a TiO2 tablet (rutile, T2041), 1.5 J/cm2, single pulse.
helped to convert TiO2 particles that had not interacted with the previous pulses. The darkness, however, saturated after about five pulses with the energy density in the range of 1–1.5 J/cm2. It is worthwhile to note that the colour change for the anatase TiO2 (T2048) is less than that of the rutile TiO2 (T2041) at a given energy density. This may be related to the different bond gap energies of the TiO2, i.e. 3.23 and 3.02 eV for anatase and rutile, respectively. Higher energy density is needed when effecting changes in the anatase form of TiO2. An example of the laser-effected darkening in TiO2 is shown in Fig. 1, where a rutile tablet (T2041) was irradiated by a single pulse KrF excimer laser beam at the energy density of 1.5 J/cm2. Theoretical explanation of the laser-effected darkening may be made based on the oxygen deficiencies in TiO2 lattice. Oxygen can be reversibly removed from the TiO2 lattice by evacuation at high temperatures or by evacuation under strong UV illumination. Loss of oxygen is associated with the generation of either TiO, which is black, or Ti, which is responsible for the dark blue-grey colour assumed by the pigment. 3.2. Laser-effected darkening in pigmented TPEs The pigmented TPE samples responded well to the UV laser beams by changing into dark colours. No apparent surface damage was observed in the laser-irradiated areas. It was found that with increasing energy density, the marking contrasts improved as shown in Fig. 2, where the TPE sample contains 1% TiO2 (KEMIRA UV-Titan P160 [12], Table 1). When the energy was low (0.2 J/cm2), faint yellowish marks appeared. Grey marks appeared at the higher energy densities (0.8–1.2 J/cm2). The best contrast was achieved at about 1.5 J/cm2. The marking contrast remained almost the same (not noticeable by naked eyes) with further increase in the energy
ARTICLE IN PRESS H. Zheng, G.C. Lim / Optics and Lasers in Engineering 41 (2004) 791–800
795
2.5mm
0.2J/cm2
0.8J/cm2
1.2J/cm2
1.5J/cm2
Fig. 2. KrF laser-effected darkening with a single pulse. TiO2 loading in the TPE was 1%.
density. Excessively high-energy density also led to some material ablation. Quantitative assessment of laser-marked samples is desirable, as it provides more accurate judgement on laser marking contrast [13]. However, a standardised instrument for assessing the laser-marking contrast is not currently available for the study. Higher marking contrast was obtained with the samples of higher pigment loading as expected when the energy density remained the same. The sample translucency was however reduced due to the increase in the refractive index. For instance, the sample containing TiO2 from CERAC has shown good laser-effected darkening due to the high TiO2 content in the pigment (above 99%, Table 1). The sample is nearly opaque. On the other hand, the sample containing TiO2 from KEMIRA (UV-Titan P160) has shown both good marking contrast and better translucency. The finer powder size of UV-Titan P160 has compensated for the reduced darkening effect due to the lower TiO2 content in the pigment (90%, Table 1). The refractive index does not cause the laser darkening; however, it does change the opacity of the pigmented sample. The marking contrast is consequently affected. The sample translucency is an important factor for applications requiring backlighting such as car radio panels and mobile phone keypads. Examination of the depth of the laser-effected darkening with TiO2 loading of 1%, 1.5% and 2% respectively was attempted by cross-sectioning the laserirradiated samples. However, as the material is soft, sharp and straight edges could not be obtained. The estimated depth of the 355 nm laser-marked sample with 1.5% TiO2 loading was 75 mm (Fig. 3). The fact that the laser-effected darkening penetrated into the material (volume marking) is significant, as it is more durable when compared to the conventional laser surface marking due to burning or material removal. Theoretically, the penetration depth of radiation follows Beer’s law [14]: It ¼ I0 10al ;
ð1Þ
where I0 and It are the intensities of the beam of light before and after transimission through a slice of materials of thickness l and a the absorptivity, is a characteristic property of the material and varies with the wavelengths. If the sample absorbs less,
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Absorption %
Fig. 3. A cross-sectional view of 355 nm Nd:YAG laser-effected darkening in the pigmented TPE with 1.5% TiO2. Optical magnification is 40 .
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 200 300 400 500 600 700 800 900 1000 1100
Wavelength (nm) Fig. 4. Absorption curve of the pigmented TPE, 1% TiO2.
the penetration is deeper. To characterise the absorption behaviour of the pigmented TPE sample (2 mm thick), a UV-VIS-IR Spectrophotometer (Shimadzu, Model UV-3101PC) was used. The absorption percentage was plotted against the wavelengths as shown in Fig. 4. It is seen that the pigmented TPE is a weak absorber particularly for wavelengths above 400 nm. At 248 and 355 nm, the beam absorption rate is 3.4% and 2.9% respectively. The relatively great depth of lasereffected darkening is qualitatively explained by the low absorption rates at both 355 and 248 nm. Examples of the UV laser-marked TPE samples are shown in Fig. 5. The substrate materials are coloured TPEs containing TiO2. For the keypad, 1.5% TiO2 (p. 160) was added and the sample was translucent. For the ‘‘Gintic logos’’, 3% TiO2 was added and the sample was opaque. The keypads were marked with the third harmonic Nd:YAG laser through the beam scanning method. The ‘‘Gintic’’ logos were produced through the mask marking by the KrF excimer laser.
ARTICLE IN PRESS H. Zheng, G.C. Lim / Optics and Lasers in Engineering 41 (2004) 791–800
797
20 mm 5 mm
Fig. 5. Marking by third harmonic Nd:YAG laser (keypads) and KrF excimer laser (logos) on the pigmented TPE samples.
3.3. Investigation of darkening mechanism in laser marking TPEs Several techniques that are sensitive to surface structure and chemical bonds were used in understanding laser-effected darkening on the TPEs. These included Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS analysis has yielded the most relevant information and the results are presented below. The XPS analysis at different take-off angles was conducted on both the lasermarked and non-marked areas of a TPE sample containing 1.5% TiO2. The Ti2p peaks were detected in laser-marked and non-marked areas, as shown in Fig. 6. The O1s peaks were detected and are shown in Fig. 7. In Fig. 8, after the charging correction, the C1s peaks (284.6 eV) were detected in both the laser-marked and nonmarked areas. The spectra for the laser-marked and non-marked areas are considered identical. Two oxygen species on the surfaces are probably due to oxygen in TiO2 (529.3 eV) and oxygen in TiCOOH or TiOH (531.4 eV). By analysing the laser-treated area at different take-off angles, the only change is the relative increase in carbon contaminants on the topmost layers (take-off angle of 20 ) with respect to those at the subsurface region (Fig. 7). This implies that the darkening mechanism may be due to the bond-breaking of the polymer matrix surrounding the fine TiO2 particles. The bond-breaking process is likely to be a combined effect of both photochemical and thermal reactions. The relative increase in black carbon at the top surface layer is distinguished as the marking contrast. This agrees with the work reported by Hofmann [8]. However, the belief that TiO2 is reduced to TiO or Ti by the intense UV laser irradiation was not detected.
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Fig. 6. XPS showing the Ti2p peaks in the TiO2-pigmented TPE.
Fig. 7. XPS showing O1s peaks in TiO2. The sample is TiO2-pigmented TPE.
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Fig. 8. XPS showing C1s peaks in the TiO2-pigmented TPE.
4. Conclusions A method to obtain high quality laser marks on TPE materials has been developed. Varying the TiO2 loading in the materials can control the marking contrast and material translucency. The XPS results revealed there was a relative increase in carbon at the top surface layer after the laser irradiation due to the bond breaking of the polymer matrix surrounding the fine TiO2 particles. Black carbon is distinguished as the marking contrast. The method may be used for product identification, functionality and decoration in many applications. Acknowledgements The authors would like to thank Dr. Chen zhenda, Dr. Zhao Jian Hong, Dr. Chen Yihong, and Ms. Tan Joo Lett for useful discussions. Thanks also go to Ms. Ng Fern Lan and Mr. Teh Kim Ming for their lab support. References [1] Tang Zheng Gui, Varahamurthy R, Zheng Hong Yu. Polymer films of laser marking effects for IC encapsulation. APACK 2001 Conference on Advances in Packaging, Singapore, December 2001. p. 162–8.
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[2] Zheng Hongyu, Lim Gnian Cher, Zhao Jian Hong. Method of preparing an integrated circuit package for laser marking. Singapore Patent Application 200000516-5, filed on 1st February 2000. [3] Zheng HY, Lim GC, Wang XC, Tan JL, Hilfiker J. Process study for laser-induced surface colouration. J Laser Appl (Laser Institute of America) 2002;14:215–20. [4] Tang Lee Yung, Zheng Hongyu, Zhao Jian Hong, Lim Gnian Cher. Laser markable material. PCT/ EP00/07304, filed on 31 July 2000. [5] Lerner EJ. Laser marking systems strive for colour and speed. Laser Focus World, October issue, 1999. p. 123–30. [6] Graff G. Additives make possible laser marking of polyolefin components. Mod Plast, April issue, 1996. p. 24–5. [7] McKee TJ. How lasers mark. Plast Technol 1996;35:11–8. [8] Hofmann M, Breitenfellner F, Buhler N, Sykes RC, Gugger H. High contrast and intact surface—a challenge in laser marking of plastics. SPIE Lasers Motion Ind Appl. SPIE, Los Angeles, 1987;744:156–80. [9] Morimoto T, Tomonaga H, Mitani A. Ultraviolet ray absorbing coatings on glass for automobiles. Thin Solid Films 1999;351(1–2):61–5. [10] Stamatakis P, Palmer BR, Salzman GC, et al. Optimum particle size of titanium-dioxide and zincoxide for attenuation of ultraviolet radiation. J Coating Technol 1990;62(789):95–8. [11] Stewart EW, Rutile titanium dioxide in plastics. Plast Compounding, Jan./Feb. issue, 1985;58–60. [12] Kemira Product Data Sheet, UV-Titan P160. [13] Ng TW, Yeo SC. Aesthetic laser marking assessment using luminance ratios. Opt Lasers Eng 2001;35:177–86. [14] Srinivasan R. Ablation of polymers and biological tissue by ultraviolet lasers. Science 1986;234: 559–65.