Facile Fabrication of High-Contrast and Light-Colored Marking on Dark Thermoplastic Polyurethane Materials.

In this work, using ferroferric oxide (Fe3O4) and zirconium oxide (ZrO2) as laser-sensitive particles and thermoplastic polyurethane (TPU) as the matrix resin, a series of TPU/Fe3O4/ZrO2 composites were prepared by melt blending, and the effect of the laser marking additive content, composition, and laser marking parameters on the laser marking properties of composites was investigated. The laser marking mechanism of Fe3O4/ZrO2 additives and the role of each component in TPU laser marking were studied by metallographic microscopy, color difference test, scanning electron microscopy, and Raman spectroscopy. Fe3O4 nanoparticles as a laser sensitizer component, on the one hand, can act as a pigment to make the TPU substrate black and, on the other hand, can absorb laser energy to contribute to the formation of laser markings on TPU composite surfaces. In addition, the introduction of ZrO2 nanoparticles can help absorb the laser energy, while the contrast can be improved to enhance the laser marking performance of the TPU composite. Through thermogravimetric analysis, the changes in the thermally stable properties of TPU composites before and after laser marking were investigated, and the results indicated that Fe3O4/ZrO2 nanoparticles can absorb the laser energy, causing melting and pyrolysis of the TPU backbone at a high temperature, to produce a gaseous product resulting in foaming. Finally, the high-contrast and light-colored markings were formed on the black TPU composite surface. This work provides a facile method for producing high-contrast and light-colored markings on the dark TPU composite surface.

Introduction

Thermoplastic polyurethane (TPU) is a commonly used engineering thermoplastic elastomer with excellent properties such as high strength, high toughness, wear resistance, and oil resistance. It has been widely used in the field of medical, food packaging, automotive, aerospace, anticorrosion coating, electronic devices, and so forth.[1−7] In general, TPU products need to be decorated or marked on their surface in industry, and markings such as barcodes, QR codes, anticounterfeiting marks, and company logo fabricated directly on the surface provide clear product information, longevity, product traceability, and anticounterfeiting capabilities. However, the widely used screen and ink printing methods for fabricating markings and patterns have many disadvantages such as the use of toxic solvents and organic volatile compounds polluting the environment, high production cost, and easy fading.[8,9] Therefore, the development of a facile method for fabricating high-contrast, long-lasting, and environmentally friendly markings on the surface of TPU products has become an issue to be studied.

Laser marking is a newly developed pattern-marking technique that uses a laser generator to generate a high-energy, high-power, continuous laser beam that focuses on the surface of the material to discolor it. The surface of a typical laser-irradiated material shows black marking by carbonization,[10] light/white marking by foaming, and color changes by the photothermal reaction.[11] In recent years, laser marking technology has been used in the fields of metals,[12−14] ceramics,[15,16] plastics,[17−19] and wet soft materials.[20,21] Compared with the conventional printing method, laser marking is a fast and flexible process technology and more environmentally friendly, and the contrast and durability are also improved. General-purpose polymer materials such as polypropylene (PP), polyethylene (PE), and TPU do not absorb laser energy well, physical or chemical changes are difficult to occur, and high-definition and contrast laser marking patterns cannot be obtained.[18,22] In order to improve the laser marking performance of such materials, it is usually a viable solution to add laser-sensitive pigments to the matrix resin. The laser-sensitive additives are capable of absorbing laser energy well and causing local overheating, leading to carbonization and formation of black markings. Laser marking sensitizers are mainly classified into inorganic,[18,19,23] organic,[24,25] and polymer/inorganic composite materials.[21,26,27] Wen et al.[19] studied the use of graphene in the field of polymer laser patterning. It was found that graphene could act as an effective 1064 nm near-infrared (NIR) pulsed laser absorber, and the addition of only 0.005 wt % (50 ppm) graphene to the polymer materials led to very excellent laser marking properties. Because of the high photothermal conversion ability of graphene, the discoloration of the polymer surface is achieved by the local carbonization of the polymer caused by graphene. Cheng et al.[26] and Zhang et al.[27] reported the preparation of polymer/inorganic laser-sensitive particles with polymers of easy carbonization as the shell and laser-sensitive inorganic oxide particles as the core by physical and chemical modification, respectively. The core–shell structure combines the excellent photothermal conversion efficiency of the inorganic oxide particles with easy carbonization and high char residue characteristics of polymers including polycarbonate and polyimide. The introduction of the laser-sensitive additive with the core–shell structure into the polymer matrix resin can produce a high-contrast and clear pattern on the surface of the polymer, which contributes to the synergistic effect of the carbonization of the polymer and the photothermal reaction of the inorganic laser-sensitive particles. The above research work provides an important reference to design and prepare high-contrast laser marking materials, which is beneficial to solve the problem that it is difficult to carry out black carbonization marking on the surface of materials such as TPU.

So far, Liu’s team investigated the use of Bi2O3,[18] Sb2O3,[23] and BiOCl[28] as laser-sensitive additives and introduced them into TPU by melt blending to achieve laser-markable properties of TPU. The inorganic laser-sensitive particles can absorb the laser energy and convert it into thermal energy, causing the surrounding TPU chains to carbonize to form a black marking. Most of the research efforts have focused on producing black carbonization marks on white or light-colored TPU surfaces, and less research has been reported on the production of light or white marks on black TPU substrates. Compared with the black carbonization mark, the generation of the light-colored marking requires the material to generate a gaseous decomposition product and foam after the laser irradiation, which is relatively complicated and difficult to control. Therefore, the study of light or white marking on TPU black substrates is of great significance for expanding the application of laser marking of TPU materials.

Fe3O4 nanoparticles are a class of magnetic particles that have been widely used in materials and biological research because of their unique magnetic response properties, nontoxicity, and excellent biocompatibility. In recent years, many studies have used the NIR laser absorption properties and photothermal effects of Fe3O4 nanoparticles to carry out cancer treatment.[29,30] In this research work, it is proposed to use Fe3O4 nanoparticles as a black pigment to make the TPU substrate black, and Fe3O4 nanoparticles can also absorb laser energy and induce a photothermal reaction for marking. ZrO2 is a white pigment particle that has attracted great interest because of its high thermal stability and chemical inertness. It is widely used in high-performance ceramics, catalysis, sensing, and fuel cell applications.[31−33] ZrO2 was used in this experiment to assist in absorbing laser energy and improving contrast to increase the laser marking property of TPU composites. In this work, Fe3O4 and ZrO2 were compounded as laser-sensitive particles, which were added to TPU by melt blending. The obtained TPU/Fe3O4/ZrO2 composite was laser-marked by a controlled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser beam at a wavelength of 1064 nm. The effects of the Fe3O4/ZrO2 feed ratio and laser process parameters on the laser marking properties of TPU/Fe3O4/ZrO2 composites were investigated. The laser marking mechanism of the Fe3O4/ZrO2 laser marking additive and the formation of high-contrast and light-colored markings on TPU composites were studied by metallographic microscopy, color difference test, scanning electron microscopy (SEM), and Raman spectroscopy.

Results and Discussion

In order to illustrate the necessity of laser marking of TPU, the NIR laser response performance of pure TPU materials was first characterized. Pure TPU samples were laser-marked with different laser current intensities, and the visual appearance of the surface of pure TPU before and after laser marking is shown in Figure . It can be seen from Figure that compared to the unmarked TPU, the laser-marked TPU did not show any changes after laser irradiation at the laser current intensity of 8 and 14 A. As the laser current intensity was increased to a very high value of 20 A, the color of the surface becomes deeper and deeper and a few black spots appear, which means that the laser marking performance of pure TPU cannot be clearly distinguished from the original color. The formation of black spots can be explained that when the laser current intensity was very high, the pyrolysis and carbonization of a small amount of TPU chains occurred. Therefore, pure TPU cannot absorb laser light energy at 1064 nm, and it is necessary to introduce a suitable laser marking additive to enhance the laser marking property of the TPU material.

Figure 1

Visual appearance of the surface of pure TPU before and after laser marking at a laser current intensity of (a) 0, (b) 8, (c) 14, and (d) 20 A.

Figure shows visual appearance of markings on the surfaces of TPU composites with different Fe3O4 contents at a laser current intensity of 12 A and the same ZrO2 content (0.2%). From Figure , it can be seen that in the absence of addition of Fe3O4, TPU/0.2% ZrO2 itself exhibits a light-colored background, and it is difficult to form a laser marking with a sharp contrast after laser irradiation. Note that the area inside the red dotted frame is the area irradiated by the laser. This is because ZrO2 itself is a white inorganic pigment that is added to the TPU to give the entire material a white background. There is no significant color change or foaming generated after laser irradiation, but ZrO2 can be used to adjust the contrast of the marking on the TPU surface. As the content of the black Fe3O4 pigment increases from 0 to 0.1%, the background color of TPU becomes black and a light-colored marking pattern is produced after laser irradiation; thus, the contrast between the light-colored mark and the black background gradually increases, and the color of the marking pattern also changes from gray to light color. When the Fe3O4 content is increased from 0.1 to 1%, under the same current intensity and ZrO2 content, the laser marking color becomes black and gradually approaches the TPU background color, resulting in a decrease in the contrast of the laser-marked pattern. In order to better characterize the laser marking property of the TPU/Fe3O4/ZrO2 composite with the change of the Fe3O4 pigment content, a colorimeter was used to quantitatively test and analyze the color difference of the marking pattern in this study.

Figure 2

Visual appearance of markings on the surfaces of TPU composites with different Fe3O4 contents at a laser current intensity of 12 A. (a) TPU/0.2% ZrO2, (b) TPU/0.05% Fe3O4/0.2% ZrO2, (c) TPU/0.1% Fe3O4/0.2% ZrO2, (d) TPU/0.2% Fe3O4/0.2% ZrO2, (e) TPU/0.5% Fe3O4/0.2% ZrO2, and (f) TPU/1% Fe3O4/0.2% ZrO2. A red dotted square indicates the area irradiated by the laser.

Figure shows the ΔE value of the laser-labeled TPU composite with increasing Fe3O4 content at a laser current intensity of 12 A. As can be seen from Figure , when the Fe3O4 content is 0, 0.05, 0.1, 0.2, 0.5, and 1%, the ΔE values of the laser marking patterns on the surface of TPU composites are 2.56, 10.28, 36.19, 28.69, 13.11, and 8.12, respectively. From Figure , the laser marking performance of TPU composites increases with the increase of Fe3O4 content from 0.05 to 0.1%. When the Fe3O4 content is 0.1%, the ΔE of the marking pattern reaches a maximum value of 36.19. After reaching the maximum value, ΔE decreases with the increase of Fe3O4 content from 0.1 to 1%. This peak appears because the formation of a light-colored marking pattern on the surface of the black substrate is related to the foam generation in the matrix during laser irradiation. Under laser irradiation, Fe3O4 particles can absorb laser energy and undergo photothermal conversion. Local overheating causes TPU to melt, carbonize, and degrade into gas products. The change of both Fe3O4 content and laser current intensity leads to the foaming of the surface materials to be different in severity, resulting in a light-colored marking pattern different from the color of the TPU substrate background. However, Fe3O4 itself is a black pigment. When the Fe3O4 content is increased, the blackness of the TPU substrate becomes also deeper. The light color mark generated by foaming itself is not obvious. Therefore, there is a peak in the curve of the ΔE value of the laser-labeled TPU composite with the increase of Fe3O4 content at the same laser current intensity.

Figure 3

ΔE value of the laser-marked TPU/Fe3O4/ZrO2 composites at a laser current intensity of 12 A as a function of Fe3O4 content.

In order to further explore the effects of the Fe3O4/ZrO2 feed ratio and laser marking process on the laser marking performance of TPU/Fe3O4/ZrO2 composites, the color difference test was used to quantitatively analyze the laser marking pattern, and the comparison and analysis of the results obtained by different conditions were carried out. Figure shows the change of the ΔE value of TPU/Fe3O4/ZrO2 composites containing different amounts of Fe3O4 and ZrO2 as a function of laser current intensity. It can be seen from Figure a that when the Fe3O4 content is 0.05% and there is no ZrO2 addition or the ZrO2 content is lower (0.01 or 0.02%), the ΔE value of the laser-marked TPU/Fe3O4/ZrO2 composites is increased with the increase of the laser marking current intensity. A higher laser marking current intensity leads to more laser energy being absorbed by the inorganic laser-sensitive particles, which is more favorable for the photothermal reaction to take place. Therefore, the chain of TPU breaks to generate gas bubbles, which is beneficial for the formation of light-colored markings. With the increase of ZrO2 content in the composite, the ΔE curve of the laser marking pattern gradually shifts upward, and when the ZrO2 content is 0.5%, ΔE reaches the maximum value of 24.94 at the laser current intensity of 18 A. When the laser current intensity is further increased to 20 A, ΔE has a downward trend, that is, the laser marking has the largest ΔE value at the laser current intensity of 18 A. When the ZrO2 content is less than 0.2%, the ΔE value is still in an upward trend at a laser current intensity of 20 A.

Figure 4

ΔE value of the laser-marked TPU/Fe3O4/ZrO2 composites with different Fe3O4 contents [(a) 0.05% Fe3O4; (b) 0.1% Fe3O4; (c) 0.2% Fe3O4; (d) 0.5% Fe3O4; (e) 1% Fe3O4] as a function of laser current intensity.

It can be seen from Figure b that when the Fe3O4 content is 0.1% and ZrO2 is not added, ΔE of the laser-marked TPU composite increases to 36.31 with the current intensity from 8 to 14 A. When the laser marking current intensity continues to increase to 16, 18, and 20 A, the ΔE values are 41.5, 41.67, and 41.43, respectively, gradually becoming stable. At the same Fe3O4 content and the ZrO2 content of 0.01%, ΔE of the laser-marked composite follows the same law as the increase of the laser current intensity. However, when the ZrO2 content is 0.02%, the ΔE curve of the composite after laser mark increases first and then decreases, and ΔE reaches the maximum value of 40.54 at a laser current intensity of 16 A. When the ZrO2 content is higher than 0.02%, the ΔE curve of the laser-marked composite also shows a trend of increasing first and then decreasing, and ΔE reaches a maximum value of about 40 at the laser current intensity of 14 A.

From Figure c, it can be found that when the Fe3O4 content is 0.2% (larger than 0.1%), ΔE of the laser-marked TPU composite with different ZrO2 contents increases first and then decreases with the increase of the laser current intensity and reaches the maximum value of around 31 at the laser current intensity of 10. As shown in Figure d,e, when the Fe3O4 content is 0.5 or 1% (larger than 0.1%), ΔE of the laser-marked TPU composite with different ZrO2 contents decreases with the increase of the laser current intensity. The largest ΔE value of the laser marking was found to be less than 26 at the smallest laser current intensity of 8 A.

Comparing the results of the laser-marked composites with different Fe3O4/ZrO2 contents and laser current intensities in Figure , it is found that the TPU/0.1% Fe3O4/ZrO2 composite shows the best laser marking performance (see Figure b). In this research work, the laser marking current intensity of 12 A was used in the field of TPU marking. On the one hand, the whole polymer degradation can be avoided without using a high laser current intensity, and on the other hand, a lower laser marking current intensity can also save energy, which requires a lower laser current intensity to achieve higher laser marking performance. In general, in order to reduce the laser current used to achieve the most excellent laser marking property, it is necessary to increase the content of laser-sensitive Fe3O4. However, as shown in Figures and 4, when the Fe3O4 content is increased, the laser marking performance is rather decreased. This requires the addition of another laser marking additive ZrO2 to reduce the laser current intensity used to achieve optimal laser marking. Using the laser marking current intensity of 12 A and the Fe3O4 content of 0.1%, the composite materials containing 0.2 and 0.5% ZrO2 both can achieve better marking property, and the corresponding ΔE values can be 36.15 and 37.43, respectively. Because the ΔE difference between two laser-marked samples is not large, the ZrO2 content of 0.2% and the Fe3O4 content of 0.1% were chosen to obtain the TPU composite with the largest ΔE of 40.34 at the laser current intensity of 14 A. It can be seen that a better laser marking property can be achieved with a lower current intensity under the conditions of a suitable TPU composite composition. It can be also concluded that Fe3O4 absorbs laser energy in the composite material and causes photothermal reaction, which enhances the absorption capacity of the composite material to the laser. In this case, increasing the content of ZrO2 at a lower Fe3O4 content results in a decrease in the laser current intensity required for the composite to achieve maximum laser marking. In this study, a reasonable selection of the content of Fe3O4 and ZrO2 and the laser marking current intensity in TPU composites can fully exhibit the ability of the inorganic oxide particles to absorb the laser light energy and undergo the photothermal conversion, and achieve the high-contrast and light-colored marking performance of TPU composites.

Figure shows the metallographic microscopy image of the marked and unmarked areas of the TPU/0.1% Fe3O4/0.2% ZrO2 composite at different laser current intensities. It can be seen from Figure a,d that when the laser current intensity is 8 A, it is difficult to find foams on the surface of the composite material, and it is almost impossible to distinguish the boundary between the laser-marked and unmarked areas. When the laser current intensity is increased to 14 A, as shown in Figure b,e, a regular strip-like bulge appears on the surface of the composite material. The laser-marked area is in contrast with the unmarked area, and the unmarked area shows a flat surface. When the laser current intensity is 20 A, as shown in Figure c,f, a large number of convex bubbles appear in the laser marking area and the marked area is obviously higher than the unmarked area, and the surface thickening phenomenon is more obvious. After the laser irradiation, bubbles appear on the surface of the TPU composite. The inorganic laser-sensitive particles generate a local high temperature by absorbing the laser energy, which causes the surface TPU to melt, degrade, and produce gaseous decomposition products, resulting in surface bulging and foaming. As the intensity of the laser marking current increases, the penetration depth of the laser into the composite is deeper. More inorganic oxide particles can absorb the laser energy and release heat so that the TPU material degrades faster and produces more gaseous decomposition products. Under the action of gas, the molten TPU is squeezed out of the surface by the gas and the bubbles on the surface of the composite material are gradually dense, and bubbles gradually agglomerate and then cool to form a rugged surface. Thus, there are two mixed phases of bubbles and substrate solids at the foaming point, and such a mixed phase has a high refractive index so that the foamed portion exhibits a very noticeable light-colored marking.

Figure 5

Metallographic microscopy image of the laser-marked area of the TPU/0.1% Fe3O4/0.2% ZrO2 composite. The laser marking current intensity is (a) 8, (b) 14, and (c) 20 A. Metallographic microscopy image of the marking boundary positions of the TPU/0.1% Fe3O4/0.2% ZrO2 composite. The laser marking current intensity is (d) 8, (e) 14, and (f) 20 A.

Figure shows transmission electron microscopy (TEM) images of the laser-sensitive particles and SEM images of the cross-section of TPU/0.1% Fe3O4/0.2% ZrO2 composites. It can be seen from Figure a,b that the ZrO2 and Fe3O4 particles both have regular morphology and uniform size, which are 18 ± 1 and 165 ± 18 nm, respectively. It can be seen from Figure c,d that the Fe3O4 and ZrO2 particles are uniformly distributed in the TPU matrix, and it is found that a pore structure appears on the cross-section of the TPU/0.1% Fe3O4/0.2% ZrO2 composite after laser marking, which is related to the foaming pore structure. The refractive index of the mixed phase changes, resulting in the formation of light-colored markings and the marked color differences between the marked and unmarked areas. From Figure c, it can also be estimated that the penetration depth by laser marking is approximately 100 μm deep from the surfaces without damaging the properties and structures of the whole TPU composites. The experimental results also verified the analysis of the metallographic microscope.

Figure 6

TEM images of the laser-sensitive particles [(a) ZrO2 and (b) Fe3O4] and SEM images of the cross-section of TPU/0.1% Fe3O4/0.2% ZrO2 composites [(c) scale bar 50 μm and (d) scale bar 10 μm].

Figure shows the water contact angle of the surface of the TPU/0.1% Fe3O4/0.2% ZrO2 composite at different laser marking current intensities. It can be seen from Figure that the water contact angle of the unmarked composite material is 75.5°. After laser marking, the water contact angle of the composite marking surface increases to 77.0° at the laser marking current intensity of 8 A. When the laser current intensity is 14 and 20 A, the water contact angle of the composite material can reach 98.0 and 95.5°, respectively. This phenomenon can be explained by the generation of foaming after laser marking the TPU/0.1% Fe3O4/0.2% ZrO2 composite material. In combination with Figure , it can be seen that many tiny bumps were formed on the surface of the composite material, which is similar to the surface of the lotus flower. The structure formed makes the surface of the material more hydrophobic, which increases the water contact angle of the Fe3O4/ZrO2/TPU composite. This hydrophobicity makes it possible to make the laser-marked areas more difficult to be obscured by water stains and thus helping to maintain the sharpness of the marking pattern during use. Figure S1 shows the infrared spectra of the TPU/0.1% Fe3O4/0.2% ZrO2 composite before and after laser marking. It can be seen from Figure S1 that there is no obvious new characteristic absorption peak appearing before and after laser marking. The laser irradiation most likely only affects the surface of TPU and will not substantially change the structure of TPU.

Figure 7

Water contact angle of the TPU/0.1% Fe3O4/0.2% ZrO2 composites before and after laser marking varies with the laser current intensity.

Figure shows the Raman spectra of the TPU/Fe3O4 and TPU/Fe3O4/ZrO2 composites before and after laser marking. It can be seen from Figure that the spectrum of the TPU/Fe3O4 composite before laser marking showed several peaks at 2059, 2921, 1611, 1440, 1305, 1176, 866, and 636 cm–1, and it is rather difficult to find the relevant peak of Fe3O4 nanoparticles because of the more intensified signals of TPU chains. Note that the characteristic peaks of Fe3O4 nanoparticles appear at 1308, 598, 494, 395, 282, and 215 cm–1, which is shown in Figure S2a of the Supporting Information. Compared with the unmarked TPU/0.1% Fe3O4 composite, the Raman spectrum of the marked TPU/0.1% Fe3O4 composite shows a broad diffusion band of amorphous carbon in the range of 1000–2000 cm–1, which is in agreement with the result reported in the literature.[28,34] After adding 0.2% of ZrO2 laser-sensitive additive to the TPU/Fe3O4 composite for laser marking, it can be found that the intensity of the broad diffusion band assigned to amorphous carbon decreases, which indicates that the addition of ZrO2 nanoparticles exacerbates the degradation of TPU and produces more gaseous products. Therefore, the degree of foaming on the TPU/Fe3O4/ZrO2 composites after laser marking is increased. In contrast, the degree of carbonization blackening caused by the Fe3O4 laser-sensitive particles is weakened so that the marked area is closer to a light color. This explains that although the laser marking current intensity is reduced after the addition of the ZrO2 powder, a better marking performance is obtained. Figure S2b (see the Supporting Information) shows the X-ray diffraction (XRD) patterns of the TPU/Fe3O4/ZrO2 composite before and after laser marking. It can be seen that the TPU composites showed a similar XRD spectra before and after laser marking, which indicates that the laser marking has not caused changes in the crystal structure of Fe3O4 and ZrO2. Both Raman and XRD results indicated that the laser marking additive plays the role of light absorption and heat transfer during the laser marking process, and its crystal structure has not been changed.

Figure 8

Raman spectra of TPU/Fe3O4 and TPU/Fe3O4/ZrO2 composites before and after laser marking.

Figure shows the TGA and derivative thermogravimetry (DTG) curves of the TPU/0.1% Fe3O4/0.2% ZrO2 composites before and after laser marking at a laser current intensity of 12 A. Table shows the TGA characteristic data of the TPU/0.1% Fe3O4/0.2% ZrO2 composite before and after laser marking. As can be seen from Figure , T2%, T5%, Tmax1, and termination temperatures of the composites before and after laser marking decreased by about 14, 7, 14, and 22 °C, respectively. This indicates that the thermal stability of TPU decreases to some extent after laser marking. This is because when irradiated with laser, Fe3O4 and ZrO2 particles inside the TPU composite can absorb the laser energy, undergo photothermal reaction, and release heat. Therefore, the TPU molecular chain breaks and decomposes to cause carbonization and foaming of the TPU composite. TPU is composed of soft segment polyol and hard segment isocyanate. It can be seen from Figure b that the DTG curve of the composite shows two peaks, the first peak appears at about 350 °C, corresponding to the decomposition of hard segment isocyanate. Petrović et al. and Pielichowski et al. also reported that the degradation of PU in the first stage takes place mainly in hard segments.[35,36] The weight loss phenomenon of the TPU caused by the second peak occurs at 400 °C, which corresponds to the thermal decomposition of the soft segment polyol to cause weight loss due to the small molecule gas or macromolecular volatile matter. After laser marking, it can be seen that the peak area corresponding to the hard segment decomposition in the DTG curve tends to decrease because in laser marking, the TPU chains on the surface of composites are decomposed into a gaseous product in advance because of high temperature. This means that there is a generation of harmful gases during laser marking, so the use of lower laser marking current intensity appears to be important in industrial applications.

Figure 9

TGA (a) and DTG (b) curves of TPU/0.1% Fe3O4/0.2% ZrO2 composites before and after laser marking.

Table 1

Thermogravimetric Analysis (TGA) Data of TPU/0.1% Fe3O4/0.2% ZrO2 Composites before and after Laser Marking

TPU/0.1% Fe3O4/0.2% ZrO2	temperature for 2% weight loss T2% (°C)	temperature for 5% weight loss T5% (°C)	temperature for the first degradation peak Tmax1 (°C)	temperature for the second degradation peak Tmax2 (°C)	termination temperature (°C)
before laser marking	287.8	315.7	343.6	400.4	460.7
after laser marking	273.7	298.6	329.7	389.5	428.7

In order to compare the effect of TPU/Fe3O4/ZrO2 composites prepared by this research work with commercial laser marking powders, the commercial Merck L8835 laser marking powder was introduced into the TPU system, and ΔE obtained by the colorimeter was used to quantify the laser marking pattern (see Figures S3, S4, and S5, Supporting Information). Figures S3 and S4 show the laser marking pattern of the TPU/L8835 composite at the laser current intensity of 14 A and the corresponding ΔE as a function of L8835 content. It can be seen from Figures S3 and S4 that when the content of L8835 is 0.01, 0.05, 0.1, 0.2, and 0.5%, the ΔE values of the laser marking pattern on the TPU composite are 8.26, 13.41, 32.94, 28.98, and 18.32, respectively. At the same time, the ΔE curve of the laser marking pattern of TPU/L8835 composites with the laser current intensity is also studied (Figure S5). Using the laser current intensity of 8, 10, 12, 14, 16, 18, and 20 A, the ΔE values of TPU/L8835 composites after laser marking are 5.25, 20.31, 27.64, 32.94, 34.59, 34.73, and 33.34, respectively. It can be seen that as the laser current intensity increases, there is a maximum ΔE value of laser marking patterns on the TPU/L8835 composite materials. Therefore, to achieve the best laser marking performance, the L8835 content of the TPU/L8835 composite material is 0.1% and the optimum laser marking current intensity is 14 A.

Figure shows the comparison of the visual appearance of laser marking QR code patterns on TPU composites made from TPU/0.1% Fe3O4/0.2% ZrO2 and TPU/L8835 composites. It can be seen from Figure that compared with TPU/L8835 composites marked at 12 and 14 A, the TPU/0.1% Fe3O4/0.2% ZrO2 composite shows a higher ΔE value and better laser marking performance. From the perspective of effectiveness and energy saving, the TPU/0.1% Fe3O4/0.2% ZrO2 composite material was selected and laser-marked at the laser current intensity of 12 A to achieve QR code patterns, which can be well recognized by the smart mobile phone app and can be quickly linked to the official web of Changzhou University. The TPU/0.1% Fe3O4/0.2% ZrO2 composite material achieved better laser marking performance while using a lower laser current intensity in laser marking.

Figure 10

Visual appearance of laser marking QR code patterns on TPU composites. (a) TPU/0.1% L8835 composite (laser current intensity of 12 A); (b) TPU/0.1% L8835 composite (laser current intensity of 14 A); (c) TPU/0.1% Fe3O4/0.2% ZrO2 composite (laser current intensity of 12 A).

Conclusions

The TPU material itself does not produce significant color change after laser marking. For the TPU material, a suitable laser marking additive must be added to achieve good laser marking performance. After adding the Fe3O4/ZrO2 laser marking additive to the TPU material, the surface of the black TPU material can be light-colored and has a significant contrast effect before and after marking. In the Fe3O4/ZrO2 laser marking additive system, the laser marking property of TPU composites increases first and then decreases with the increase of Fe3O4 content. The laser current intensity used for the optimal value of laser marking property can be decreased after the addition of the suitable amount of ZrO2 to the TPU composite, which is helpful for reducing energy consumption. The TPU/Fe3O4/ZrO2 composites achieved the best laser marking performance corresponding to a ΔE of 40.34 when the content of Fe3O4 was 0.1%, the content of ZrO2 was 0.2%, and the laser marking current intensity was 14 A. When the laser marking current intensity was 12 A, the good laser marking performance with a ΔE of 36.15 was achieved. It can be concluded that the role of the laser marking additive in the TPU composite is to convert the absorbed light energy into thermal energy so that the TPU melts, the molecular chain breaks, carbonizes, and a gaseous product is produced. However, during the marking process, pyrolysis carbonization of the TPU occurs, making the marking black, and the addition of an appropriate amount of ZrO2 nanoparticles can reduce the occurrence of such carbonization. Compared with the TPU composites incorporated with the commercial laser marking powder L8835, the TPU/0.1% Fe3O4/0.2% ZrO2 composite has a higher ΔE value, exhibit better marking performance, and is more easily recognizable. In this study, a facile method for fabricating a high-contrast and light-colored laser marking on the surface of black polymer materials has already been developed, which will be helpful for the research and development of the TPU-based laser marking materials.

Experimental Section

Chemicals and Materials

TPU (U-285AL) was purchased from Taiwan Kunzhong Co., Ltd., and the analytically pure antioxidant 1010 (Irganox 1010) was purchased from BASF, Germany. ZrO2 nanoparticles of analytical grade (particle size 18 ± 2 nm) were obtained from Nanjing Shanshan New Materials Technology Co., Ltd. Fe3O4 nanoparticles (99.5%, particle size 165 ± 18 nm) were obtained from Adamas-Beta Reagent Company (Shanghai, China).

Preparation of TPU/Fe3O4/ZrO2 Composite Samples

TPU pellets, antioxidant 1010, Fe3O4, and ZrO2 powders were dried in a vacuum oven at 70 °C for 8 h. In order to avoid the degradation of the TPU chains during thermal processing, the antioxidant 1010 should be added in TPU composites. Then, the quantitative TPU particles were weighed according to the selected formula, and then, the required mass of antioxidant 1010 (0.5 wt %), Fe3O4, and ZrO2 powders was accurately measured (see Table S1 in the Supporting Information), which were subsequently mixed well with TPU pellets in a torque rheometer at the temperature of 190 °C. A 2 mm-thick sheet sample was produced at 190 °C using a flat vulcanizer under a pressure of 10 MPa.

Table S1 (Supporting Information) shows all TPU/Fe3O4/ZrO2 composite formulations made by using 60 g of TPU as the matrix resin. In order to avoid degradation, 0.3 g of the antioxidant 1010 (0.5 wt %) was added. In this formula, a controlled variable method was used to explore the optimum ratio of Fe3O4 and ZrO2 for the best laser marking performance and the role of each component in the laser marking process. The mass of Fe3O4 added to TPU was preset as 0.03, 0.06, 0.12, 0.3, and 0.6 g, respectively, so as to study the effect of the change of the Fe3O4 content on the laser marking performance of TPU/Fe3O4/ZrO2 composites. According to the amount of Fe3O4, 35 formulas were divided into 5 groups, and each group of 7 formulations contained a ZrO2 content of 0, 0.006, 0.012, 0.03, 0.06, 0.12, 0.3 g, in order to investigate the effect of the change of the ZrO2 content on the performance of each group. For example, the final sample code “TPU/0.1% Fe3O4/0.2% ZrO2” indicates that it was prepared from 60 g of TPU, 0.06 g of Fe3O4, and 0.12 g of ZrO2.

Laser Marking of TPU/Fe3O4/ZrO2 Composite Samples

The marking on the sheet sample was carried out by a pulsed laser beam of Nd:YAG (KDD-50, Suzhou Kaitai Laser Technology Co., Ltd., China) at a wavelength of 1064 nm. The laser marking machine process parameters include the laser focal length of 219 mm, spot size of 100 μm, laser pulse repetition frequency of 4000 Hz, and laser scanning speed of 450 mm/s. The laser current intensity used is set to 8, 10, 12, 14, 16, 18, and 20 A, which are corresponding to 18.5, 23.5, 28.5, 33.2, 37, 42.3, and 47 W, respectively.

Instruments and Characterization

The X-Rite 7000A color difference spectrometer (X-Rite, USA) was used to test the surface color difference (ΔE) of TPU/Fe3O4/ZrO2 composites before and after laser marking. The effect of different laser additive contents and different laser marking currents on the laser marking performance of TPU composites was studied. ΔE is introduced using Commission Internationale de I’Eclairage (CIE) L*a*b* coordinates in this work. ΔE can be defined as the numerical comparison of a sample’s color to the standard. It indicates the differences in absolute color coordinates. Defined by the CIE, the L*a*b* color space was modeled after a color-opponent theory stating that two colors cannot be red and green at the same time or yellow and blue at the same time. As shown in the equation below, L, a, and b indicate lightness, the red/green coordinate, and the yellow/blue coordinate, respectively. Before laser marking, these values can be recorded as L0, a0, and b0. After laser marking, these values can be recorded as L1, a1, and b1. ΔL (L1 – L0), Δa (a1 – a0), and Δb (b1 – b0) may be positive (+) or negative (−). The total difference, ΔE, however, is always positive and calculated as follows

ΔE was used to determine the laser marking performance of composites before and after laser marking. The larger the value of ΔE, the more obvious is the marking contrast.

The laser-marked and unmarked areas of TPU composites were observed and compared using a metallographic microscope (ECLIPSE-LV150N, Nikon), and the marking boundary positions were observed to study the change characteristics before and after laser marking.

The water contact angle tester (JC2009D1 goniometer, Shanghai Zhongchen Digital Technology Equipment Co., Ltd, Shanghai, China) was used to determine the change of the water contact angle before and after laser marking.

The laser-marked region of the TPU composite before and after laser marking was tested using a Nicolet Avatar 370 Fourier transform infrared spectrometer with a spectral range of 4000–400 cm–1.

The samples were first frozen in liquid nitrogen and then broken off. The gold layer was sprayed on the section, and the cross-sections of the composites were observed using a JEOL (JSM-IT100, accelerating voltage 10 kV) scanning electron microscope. The morphology and size of the inorganic particle were measured by dispersing the inorganic particles in ethanol, dropping them on a copper mesh, and using a JEOL JEM-2100 transmission electron microscope (acceleration voltage 200 kV).

The XRD spectra of samples were obtained using a powder diffractometer (RINT2000, Rigaku). The TPU composites before and after laser marking were tested using a DXR laser confocal microcontrolled Raman spectrometer (Thermo Science and Technology Company). The sample was tested in different positions and the tests were repeated three times in order to reduce the error.

The surface materials of the laser-marked and unmarked TPU composite were analyzed using a NETZSCH-TG 209 F1 thermogravimetric analyzer (NETZSCH, Germany) in a nitrogen atmosphere (gas flow rate of 20 mL/min). The heating rate was 20 °C/min, and the scanning temperature ranged from 30 to 700 °C.