Preparation and Laser Marking Properties of Poly(propylene)/Molybdenum Sulfide Composite Materials.

In this study, using molybdenum sulfide (MoS2) as laser-sensitive particles and poly(propylene) (PP) as the matrix resin, laser-markable PP/MoS2 composite materials with different MoS2 contents ranging from 0.005 to 0.2% were prepared by melt-blending. A comprehensive analysis of the laser marking performance of PP/MoS2 composites was carried out by controlling the content of laser additives, laser current intensity, and the scanning speed of laser marking. The color difference test shows that the best laser marking performance of the composite can be obtained at the MoS2 content of 0.02 wt %. The surface morphology of the PP/MoS2 composite material was observed after laser marking using a metallographic microscope, an optical microscope, and a scanning electron microscope (SEM). During the laser marking process, the laser energy was absorbed and converted into heat energy to cause high-temperature melting, pyrolysis, and carbonization of PP on the surface of the PP/MoS2 composite material. The black marking from carbonized materials was formed in contrast to the white matrix. Using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy, the composite materials before and after laser marking were tested and characterized. The PP/MoS2 composite material was pyrolyzed to form amorphous carbonized materials. The effect of the laser-sensitive MoS2 additive on the mechanical properties of composite materials was investigated. The results show that the PP/MoS2 composite has the best laser marking property when the MoS2 loading content is 0.02 wt %, the laser marking current intensity is 11 A, and the laser marking speed is 800 mm/s, leading to a clear and high-contrast marking pattern.

Introduction

Poly(propylene) (PP) is a typical thermoplastic. Due to excellent heat resistance, high chemical stability, resistant ability to most acid and alkali corrosions, and good processability, PP and its composite materials have been widely used in automobiles, packaging, membrane separation and pipelines, and other fields.[1−4] Generally, the surface of PP products needs to be marked with patterns and texts such as two-dimensional (2D) codes and serial numbers to indicate the production date, company logo, the product expiration date, and anticounterfeiting functions. Usually, ink printing is used to produce these marks on the surface of the material, but there is the use of toxic and harmful reagents, the process flow is complicated, and there are serious environmental pollution problems.[5,6] In recent years, laser marking technology has emerged as a new type of printing technology that uses high-energy lasers to mark the surface of materials to form patterns, text, crystalline and self-assembly structures, and morphology.[6−12] For example, Park et al.[13] reported the laser-directed supramolecular assembly of sub-5 nm columnar structures with adjustable orientation control. The high energy of the laser causes the matrix resin to be heated and carbonized, forming a clear pattern[14,15] of carbonization and blackening on the surface of the material. Because laser marking has a fast processing speed and flexibility, it is highly suitable for marking the surface of plastic products and can overcome the defects caused by ink printing. After the product is completed, no additional processing is required, and a clear, beautiful, and durable pattern appearance can be obtained by directly performing laser printing.

Laser marking is characterized by noncontact processing. The laser with a wavelength of 1064 nm is a more commonly used laser in the industry and is suitable for materials such as metal, ceramics, wood, and plastic.[16−19] Compared with the easy carbonization and high char residue features of polycarbonate[20] and polystyrene,[21] it is rather difficult to perform laser marking with PP itself due to the poor absorption of the near-infrared laser energy of the 1064 nm wavelength. Usually, the method of adding laser marking additives can effectively improve the laser marking performance of PP. General laser marking additives used in industrial production include inorganic laser marking additives, which are mostly metal oxide powders,[22] such as ferroferric oxide (Fe3O4),[23] bismuth oxide (Bi2O3),[24,25] bismuth oxychloride (BiClO),[26] antimony trioxide (Sb2O3),[27] graphene,[28,29] montmorillonite,[30] carbon nanotube,[31] and other inorganic fillers. After laser irradiation, the composite material undergoes color changes on its surface by absorbing the laser energy. At present, there are only a few organic laser additives reported, which are mainly halogen-containing organic compounds, color masterbatches, and organic dyes,[32] causing serious environmental pollution. The other type of laser additive is inorganic/organic composite laser marking particles. Most of the composite additives are formed by coating a layer of easily carbonized polymers, such as antimony-doped tin oxide@polyimide (ATO@PI),[33] polystyrene-grafted antimony trioxide (PS@Sb2O3),[34] stannic oxide/polycarbonate (SnO2/PC) microcapsules,[35] and graphene/polystyrene, on the inorganic particles.[36] The core of the inorganic particles can effectively absorb laser energy, which makes it easy to carbonize the polymer of the shell layer, enhances the laser absorption of the matrix resin, and improves the definition of laser marking. In addition, the use of polymer-encapsulated modified inorganic particles, due to the existence of the organic polymer shell, can make the core–shell structure particles more compatible with the matrix resin material so that they can obtain better mechanical properties.[37−39] Looking for new inorganic laser-sensitive particles with the near-infrared laser response performance and introducing them into PP to prepare PP/inorganic composite materials with both laser-markable performance and excellent mechanical properties have also been research hotspots recently.

Single-layer MoS2 is a typical layered metal disulfide and has been reported as a new type of near-infrared absorber.[40] The MoS2 absorber in the near-infrared region has higher absorbance than both graphene and gold nanorods.[41] Due to the advantages of wider absorption bands, better stability, excellent photoelectric performance, and a unique two-dimensional structure, MoS2 has been widely used in near-infrared photothermal therapy, electronics, catalysis,[42,43] and antibacterial applications.[44−46] In our study, the introduction of MoS2 into PP resin to prepare composite materials and the laser marking performance have not been reported yet. In this design, using MoS2 as a laser marking additive and PP as the matrix resin, PP/MoS2 composite materials were prepared by melt-blending with different contents of MoS2. Considering that the addition of MoS2 laser marking additives affects the mechanical properties of PP, the optimum ratio of PP/MoS2 is also determined. The near-infrared laser response performance of the composite material was studied by controlling the content of laser additives, laser current intensity, and laser scanning speed. The color difference tests, a metallographic microscope, an optical microscope, a SEM, X-ray diffraction (XRD), FT-IR and Raman spectroscopies, and mechanical performance measurements were used to characterize the surface morphology, marking performance and mechanism, and mechanical properties of PP/MoS2 composites. This research is of great importance for the realization of high-functionality and high-performance applications of PP-based materials and meets the high-quality marking needs of characters, patterns, and QR codes on the surface of PP products in the fields of automobiles, medical equipment, and precision instruments.

Results and Discussion

Laser-Markable Performance of Pure PP

PP was directly pressed into sheet samples for laser marking without adding MoS2. Figure shows the response performance of pure PP before and after laser marking. The laser current intensities of 8, 9, 10, 11, and 12 A were chosen to mark the pure PP sheets. Note that the marking pattern of a cartoon bird was selected for this experiment. From Figure , it can be found that at a very low laser current intensity of 8 A, there are hardly any changes between the unmarked and marked PP samples. As the laser current intensity is increased to 11 A, black particles and a blurred marking pattern appear on the PP surface. More black particles appear at a higher laser current intensity of 12 A. When the marking current intensity is low, the color contrast between the marking and the substrate is low. When the color of the printed pattern differs greatly from the color of the substrate, the surface of the material seems carbonized, the laser current intensity required is also large, and more energy is consumed. Therefore, pure PP cannot display a clear pattern under a small laser current intensity, and it is necessary to add suitable laser marking additives to improve the markability of the PP material.

Figure 1

Visual appearance of PP before and after marking at different laser marking current intensities ((a) 0 A, (b) 8 A, (c) 9 A, (d) 10 A, (e) 11 A, (f) 12 A; scale bar of 1 cm).

Effect of the Laser Marking Process and MoS2 on the Laser Marking Property of PP/MoS2 Composites

Effect of MoS2 Content on the Laser Marking Property

MoS2 with different contents (0.005, 0.01, 0.015, 0.02, and 0.2%) was added to the PP matrix by the melt-blending method to prepare the PP/MoS2 composite material for laser marking. The purpose of this research is to obtain the PP/MoS2 composites with excellent laser marking performance and mechanical properties. The MoS2 loading content is expected to be as low as possible because the addition of black MoS2 powder particles with a content higher than 0.2% influences the color of the matrix and probably decreases the mechanical property of PP. Therefore, in our case, the MoS2 loading contents of 0.005–0.2% were chosen for the experiments. Figure shows the visual appearance of the laser marking of pure PP and PP composites containing different contents of MoS2 at the same laser current intensity of 11 A and the same scanning speed of 800 mm/s. Compared with the fuzzy pattern produced on the pure PP surface, the PP composite material with MoS2 added shows sensitivity to laser absorption, and the laser marking pattern becomes much clearer. At a very low MoS2 content of 0.005%, a clear marking pattern of a cartoon bird is displayed. As the content of MoS2 increases, the pattern contrast remains unchanged. As the content increases to a higher content of 0.2%, the pattern contrast decreases and it is difficult to recognize the pattern. The main reason for this is that MoS2 is black powder particles, and the addition of the black pigment makes the color of the matrix black. It is difficult to make precise comparisons by visual observation alone; therefore, a colorimeter is used for the quantitative analysis below.

Figure 2

Visual appearance of (a) pure PP, (b) 0.005% PP/MoS2, (c) 0.01% PP/MoS2, (d) 0.015% PP/MoS2, (e) 0.02% PP/MoS2, and (f) 0.2% PP/MoS2 composite materials after laser marking at the laser current intensity of 11 A and a marking speed of 800 mm/s (scale bar of 1 cm).

Figure shows the laser marking color difference values of PP/MoS2 composites with different MoS2 loading contents at 800 mm/s and 11 A for quantitative analysis of the laser marking property of composites. As shown in Figure , the color difference values (ΔE) measured of the laser marking patterns on composites with MoS2 loadings of 0.005, 0.01, 0.015, 0.02, and 0.2% are 12.82, 12.47, 11.87, 13.47, and 2.72, respectively. This result is in agreement with the observation mentioned above. This means that the laser marking property depends on the proportion of MoS2. When the MoS2 loading content is 0.2%, the color of the matrix material becomes much darker due to the excessively high MoS2 content, resulting in a decrease in the contrast of the marking and a lower color difference. When the concentration is 0.02%, the laser marking color difference value reaches the maximum value of 13.47, and the highest color contrast between the marking area and the matrix material can be obtained, leading to the best laser marking performance.

Figure 3

Laser marking color difference values of PP/MoS2 composites with different MoS2 loading contents at 800 mm/s and 11 A.

Effect of the Laser Current Intensity on the Laser Marking Property

The laser marking current intensity is the most important parameter in the laser marking process. A suitable laser marking current intensity can save energy consumption while ensuring a favorable laser marking effect. Figure shows the visual appearance of 0.02% PP/MoS2 composite materials after laser marking at different laser current intensities of 8, 9, 10, 11, and 12 A and the marking speed of 800 mm/s. It can be seen from Figure that as the laser current intensity of the laser marking increases from 8 to 11 A, the laser marking pattern becomes much clearer and the contrast is higher obviously. After the laser marking current intensity is greater than 11 A, the pattern definition decreases. Similarly, the color difference analysis for the detailed comparison was carried out in the following discussion.

Figure 4

Visual appearance of 0.02% PP/MoS2 composite materials after laser marking at different laser current intensities of (a) 8 A, (b) 9 A, (c) 10 A, (d) 11 A, and (e) 12 A and the marking speed of 800 mm/s (scale bar of 1 cm).

Figure shows a histogram of laser marking color difference values of 0.02% PP/MoS2 composite materials at a scanning speed of 800 mm/s and different current intensities. From Figure , the color difference values of laser marking with current intensities of 8, 9, 10, 11, and 12 A are 2.01, 0.96, 12.27, 13.47, and 3.01, respectively. When the laser marking current intensity is 11 A, the color difference value is the largest, and the best laser marking performance is obtained. As the laser marking current intensity increases to 12 A, MoS2 can absorb more laser energy and produce strong photothermal conversion, resulting in the local temperature rise and overheating, which makes the PP chains surrounding the MoS2 particles decompose and bubble on the composite surface. In addition, the color difference value begins to decrease because the surface of the material begins to bubble and the color becomes lighter. To summarize the above result, it can be seen that when the laser current intensity is 11 A, the laser marking color difference value reaches the maximum value of 13.47. The carbonization degree of the material surface becomes higher, and the best laser marking performance is obtained due to the increase of the laser intensity.

Figure 5

Laser marking color difference values of 0.02% PP/MoS2 composites at different laser current intensities (8, 9, 10, 11, and 12 A) and 800 mm/s.

Effect of Different Marking Speeds on the Laser Marking Property

Figure shows the visual appearance of the laser markings of 0.02% PP/MoS2 composites at different marking speeds (200, 400, 600, 800, 1000 mm/s) under the same current intensity of 11 A. From Figure , there are no obvious differences observed among the composites, and all of the composites show very clear patterns.

Figure 6

Visual appearance of 0.02% PP/MoS2 composites after laser marking at the scanning speeds of (a) 200 mm/s, (b) 400 mm/s, (c) 600 mm/s, (d) 800 mm/s, and (e) 1000 mm/s and the laser current intensity of 11 A (scale bar of 1 cm).

Figure shows laser marking color difference values of the 0.02% PP/MoS2 composite material at different line speeds (200, 400, 600, 800, 1000 mm/s) at 11 A. As shown in Figure , it can be seen that the laser scanning speed has little effect on the color difference values of the 0.02% PP/MoS2 composite. Under the same current intensity of 11 A and at the scanning speeds of 200, 400, 600, 800, and 1000 mm/s, the color difference values of composites are 11.15, 11.02, 12.3, 13.47, and 12.47, respectively. When the laser marking speed is 800 mm/s, the composite shows better laser marking performance, but the color difference values of other marking speeds are also very high. The scanning speed higher than 1000 mm/s was also investigated (see Figure S4 in the Supporting Information) because from an industrial point of view, extending the upper limit of the laser marking speed with a relatively good definition would be helpful. From Figure S4, it can be seen that at high scanning speeds of 1200 and 1500 mm/s, the color difference values are slightly decreased, compared with the values at lower scanning speeds. This is because if the laser marking scanning speed is too high, the distance between the laser spots becomes too large and the definition of laser-induced marking is relatively low. Only when the laser spot size and distance are moderate, carbonization is more uniform and the optimum laser marking can be obtained. In addition, a high laser scanning speed leads to high energy consumption and also affects the service life of the laser. In our work, a scanning speed higher than 1000 mm/s was not considered for the following experiments and data analysis. From the perspective of laser marking performance and energy-saving, the optimum conditions for laser marking composites include a 0.02% loading content of MoS2, a laser current intensity of 11 A, and a laser marking speed of 800 mm/s, leading to the maximum color value of 13.47.

Figure 7

Laser marking color difference values of the 0.02% PP/MoS2 composite material at different line speeds (200, 400, 600, 800, 1000 mm/s) at 11 A.

Metallographic Microscopic Images of PP/MoS2 Composites

Although a metallographic microscope is also an optical microscope, there are some differences in the observed information. The magnification of the metallographic microscope observation is usually higher than that of the ordinary optical microscope. The working principle of the metallographic microscope is that it uses the reflection of light on the surface of the material and mainly observes the degree of laser ablation on the surface, which enables us to clearly see the phenomenon and visualize the appearance of the material surface. The working principle of the optical microscope is that it mainly uses the transmission of light. Through the light passing through the film material, the degree of carbonization on the surface of the material can be clearly seen. When the surface of the material is more carbonized, the light transmittance of the material is relatively poor, and black carbonized dots usually appear and can be observed. The results using both microscopes have been revealed and discussed in this study.

Figure shows the metallographic microscopic images of PP and PP/MoS2 with different MoS2 loading contents before and after laser marking. From Figure a–e, the unmarked PP and PP/MoS2 composite materials show a rather flat and smooth surface with the increase of the MoS2 content without laser marking. After laser marking (see Figure g–l) at the laser current intensity of 11 A and the scanning speed of 800 mm/s, the surface of PP remains unchanged and flat due to its poor absorption of laser energy. On the contrary, the PP/MoS2 composite show an increasing laser sensitivity with the increase of the MoS2 loading content. At a lower MoS2 content of 0.005%, the laser pattern is not clear and the surface only shows a small number of spots, indicating the slight carbonization on the composite surface. With the increase of the MoS2 content from 0.01 to 0.02%, the composite can absorb more laser light energy and undergo a severe photothermal reaction, leading to a rough surface and a clear laser marking pattern. However, at a very high MoS2 content of 0.2%, the composite absorbs too much laser energy, and the carbonization degree of the composite is increased, causing the surface to bubble. It is impossible to form a clear and high-contrast laser marking on the composite surface. Similarly, the influence of the laser current intensities of 8, 9, 10, 11, and 12 A on the laser marking performance of the composites was also investigated. The result shows that with the increase of the laser current intensity, the composite becomes more sensitive to laser energy, and carbonization can easily occur, leading to high-contrast and clear laser markings on the surface. However, a high laser current intensity leads to severe photothermal conversion and formation of foams on the surface, which has a negative influence on the laser marking performance. In summary, the optimum conditions of the 0.02% PP/MoS2 composite material at the laser current intensity of 11 A and the marking speed of 800 mm/s can lead to the formation of a clear laser marking.

Figure 8

Metallographic microscopic images of (a) PP and PP/MoS2 with different loading contents of (b) 0.005%, (c) 0.01%, (d) 0.015%, (e) 0.02%, and (f) 0.2% (scale bar: 100 μm) before laser marking and (g) PP and PP/MoS2 with different loading contents of (h) 0.005%, (i) 0.01%, (j) 0.015%, (k) 0.02%, and (l) 0.2% (magnification 200) after laser marking.

Microscopic Images of PP/MoS2 Composites

Figure shows the microscopic images of PP and PP/MoS2 composites under different conditions including the loading content, laser current intensity, and the marking speed.

Figure 9

Microscopic images of PP (a), (b) 0.01% PP/MoS2, (c) 0.015% PP/MoS2, (d) 0.02% PP/MoS2, and (e) 0.2% PP/MoS2 composite materials after laser marking at the laser current intensity of 11 A and the marking speed of 800 mm/s; images of 0.02% PP/MoS2 composite materials after laser marking at different laser current intensities of (f) 8 A, (g) 9 A, (h) 10 A, (i) 11 A, and (j) 12 A and the marking speed of 800 mm/s; images of 0.02% PP/MoS2 composites after laser marking at the scanning speed of (k) 200 mm/s, (l) 400 mm/s, (m) 600 mm/s, (n) 800 mm/s, and (o) 1000 mm/s and the laser current intensity of 11 A (scale bar of 100 μm).

The uniformity of MoS2 dispersion in PP was first investigated by analyzing the results of optical microscopy and laser pattern analysis after laser marking. Since the size of MoS2 is about 1.5 μm and MoS2 itself is black flake particles, the dispersion and aggregation of inorganic MoS2 in the composite film are observed through the transmitted light of an optical microscope. If larger aggregates are formed, black spots with a large size can generally be observed. Figure S3 shows the optical microscope photos of the 0.2% PP/MoS2 composite before laser marking. It can be seen from Figure S3 that the 0.2% PP/MoS2 composite material did not show large black spots caused by obvious aggregates before laser marking, indicating that MoS2 was uniformly dispersed in the PP matrix resin. After laser irradiation, from Figure b–e it can be clearly observed that the PP/MoS2 composite material shows an ordered dot array in the marked area, especially, the PP/MoS2 composite material itself forms ordered black carbon dots through line-by-line scanning and then forms a pattern. Comparing the marked area with the unmarked area, it can also be seen that MoS2 is evenly dispersed in PP. Conversely, if the dispersion of MoS2 in PP is poor and aggregation occurs, the resulting pattern should be uneven, and there will be differences in various places. Taking into account that the amount of MoS2 added is relatively small and the pattern definition and contrast after laser marking are relatively high, it can be concluded that the dispersion of MoS2 in PP is quite good; therefore, uniform carbonization and a blackening pattern can be formed. From Figure a–e, it can be clearly seen that compared with pure PP, the composites show a clear laser marking, and the carbonization degree increases with the increase of the MoS2 content. At a very high MoS2 content of 0.2%, because too much MoS2 content is added, the composite surface is too dark, the contrast is not obvious, and it is impossible to display clear laser markings. It is suitable for the carbonization of the composite with 0.02% MoS2 to form a relatively clear laser mark, which is consistent with the result drawn from the color difference tests. In summary, the 0.02% PP/MoS2 composite material at a laser marking speed of 800 mm/s becomes more uniformly carbonized at a laser current intensity of 11 A, and it is easier to form a clearer laser mark. From Figure f–j, with the increase of the laser current intensities, the PP/MoS2 composites become more sensitive to laser energy and adsorb more laser energy, which leads to the increased degree of carbonization and a clearer laser marking. However, the higher energy seriously damages the surface and influences the mechanical properties of composites. From Figure k–o, it can be seen that there is no large difference in the laser marking performance for composites at different marking speeds from 200 to 1000 mm/s. Normally, a high marking speed is beneficial to improve the efficiency of laser processing. In conclusion, at the laser marking speed of 800 mm/s, the 0.02% PP/MoS2 composite material has more uniform carbonization at the laser current intensity of 11 A, and it is easier to form a relatively clear laser marking.

SEM Images of PP/MoS2 Composites

Figure displays the SEM image of pure PP and PP/MoS2 composites with different MoS2 loading contents of 0.005, 0.01, 0.015, 0.02, and 0.2% after laser marking at the laser current intensity of 11 A and the marking speed of 800 mm/s. As shown in Figure , the marked PP shows a flat and smooth surface. After the addition of laser-sensitive MoS2 into PP, the obtained laser marked surface is rough and covered with carbon particles. At a lower content of 0.005%, the marked composite has a relatively smooth surface and there are a few particles and aggregation, indicating the slight changes including photothermal conversion, melting, and carbonization caused by laser irradiation. With the gradual increase of the MoS2 content, the responsiveness of the PP/MoS2 composites to the laser gets better and better, and an uneven and rough surface is observed. When the content of MoS2 increases to 0.02%, the laser ablation area expands and PP melts. This laser marked area is uniform, and the overall carbonization degree is quite high, which also indicates the good laser marking performance of composites. As shown in Figure e, the platelike particles that appear on the surface of composites with MoS2 are carbonized PP, indicating the melting, cooling, and solidification of PP induced by the severe photothermal reaction. At a high MoS2 loading of 0.2%, the platelike particles disappeared (see Figure f). In relation to the results obtained by the microscope, this phenomenon can be explained by the high MoS2 loading being able to absorb more laser energy and the temperature suddenly increasing dramatically, leading to pyrolysis and foaming on the PP surface. On the contrary, the extent of the carbonization reaction that occurs is not as much as when the loading content is lower. This is difficult to be observed by a SEM and is a possible reason why the carbonized PP disappeared in the 0.2% PP/MoS2 composite. In addition, when the content of MoS2 is 0.2%, the uniform carbonization degree of the surface of the composite material and the contrast of laser marking on the surface of the material are both reduced, resulting in a decrease in the laser marking property of the material. In combination with the visual and microscopic observations, it can be concluded that with the increase of the MoS2 content, the PP/MoS2 composite material increases its responsiveness to the near-infrared laser and undergoes a severe photothermal reaction, which causes the surface of the composite material to be uneven and rough.

Figure 10

SEM image of (a) pure PP, (b) 0.005% PP/MoS2, (c) 0.01% PP/MoS2, (d) 0.015% PP/MoS2, (e) 0.02% PP/MoS2, and (f) 0.2% PP/MoS2 composite materials after laser marking at the laser current intensity of 11 A and the marking speed of 800 mm/s (magnification 1000) (the red dotted box indicates the laser marked area).

Water Contact Angle on the Composite Surface

Figure shows the changes in the water contact angle of the surface of PP and the PP/MoS2 composite material containing different MoS2 loading contents before and after laser marking. It can be seen from Figure a–f that before laser marking, the water contact angle of the pure PP surface is about 71°. With the addition of MoS2, the roughness of the prepared composite material is slightly increased, the water contact angle increases in the range of 72–80°, and the overall change is not big. This is because the material has not been laser marked on the surface, there have been no drastic photothermal conversion and surface roughness changes, and MoS2 is incorporated in the PP matrix, which does not have much impact on the water contact angle; therefore, the water contact angle changes slightly. The water contact angle test results in Figure g–l show that after laser marking, since pure PP has a poor response to the near-infrared laser, the changes in the hydrophilic and hydrophobic properties of the PP surface before and after laser marking are small. With the increase of the MoS2 content, the PP/MoS2 composite material becomes more and more responsive to the laser. The carbonized materials are formed on the surface of the sample sheets. Compared with pure PP laser marking, the surface roughness of the PP/MoS2 composite becomes higher, resulting in a larger contact angle. When the MoS2 content is 0.2%, the water contact angle of the composite material after laser marking is 98°, which is about 20° larger than that of the unmarked surface. During the laser marking process in air, the PP chains are easily carbonized and oxidized due to the severe photothermal reaction induced by MoS2. The surface chemical composition of PP/MoS2 is changed due to the increased number of oxygen groups, which most probably improves the hydrophilic property of the PP composites. However, from Figure , it can be found that the water contact angles of PP/MoS2 with different loading contents of 0.005, 0.01, 0.015, 0.02, and 0.2% after laser marking were increased, compared with those of the unmarked samples. This indicated that the roughness change plays a greater role in the contact angle change than the surface chemical composition change. By observing the change of the water contact angle before and after the laser marking of the PP/MoS2 composite material, it can be concluded that due to the high temperature of the laser marking, the surface is carbonized, forming uneven carbonized particles and porous structures, and the surface roughness increases. The contact angle of the material after marking is relatively increased. This result is consistent with the above SEM and visual observation data.

Figure 11

Water contact angles on the surface of PP (a) and PP/MoS2 with different loading contents of (b) 0.005%, (c) 0.01%, (d) 0.015%, (e) 0.02%, and (f) 0.2% before laser marking and PP (g) and PP/MoS2 with different loading contents of (h) 0.005%, (i) 0.01%, (j) 0.015%, (k) 0.02%, and (l) 0.2% after laser marking.

XRD Patterns and Raman Spectra of PP/MoS2 Composites

To better clarify the mechanism of laser marking on the surface of composite materials, XRD and Raman spectroscopy were used to study the structural changes of pure PP and composite materials before and after laser marking. Figure shows the XRD patterns and Raman spectra of pure PP and composite materials before and after laser marking. It can be seen from Figure a that there are hardly any changes observed in the spectra of PP and PP/MoS2 composites before and after marking, which indicates that after laser marking, the crystal structures of PP and MoS2 did not change, that is, the laser-induced marking only changes the surface properties of the PP material without influencing the crystal structures. Figure S1 in the Supporting Information indicates that the XRD pattern of pure MoS2 appears at 15°. Since MoS2 dispersed in PP has a very low loading content, the XRD pattern of the composite material does not show the characteristic peak of MoS2. The carbonization of the PP/MoS2 composite material is due to the role of pure MoS2 in the matrix material, and there is no change in the crystal structure inside the material during the marking process. Figure S2 in the Supporting Information shows that the Raman spectrum of MoS2 has two characteristic peaks at around 408 and 376 cm–1. As shown in Figure b, compared with the unmarked PP and composite samples, the Raman spectra of the laser marked PP and the PP/MoS2 composite show a broad band appearing from 1000 to 2000 cm–1, which corresponds to the characteristic band of the amorphous carbon. This is because after laser marking, the surface of the PP/MoS2 composite is carbonized and the amorphous material is formed. Compared with the marked PP, the marked PP/MoS2 composite shows a higher broad band in the range of 1000–2000 cm–1. The composite material with MoS2 has a good response to the near-infrared laser and undergoes a severe photothermal reaction, and the content of amorphous carbonized material on the surface increases; therefore, the intensity of the broad band increases. This proves that the composite material can absorb the laser energy and undergo photothermal conversion, causing surface pyrolysis and carbonization to form amorphous carbonized materials.

Figure 12

XRD patterns (a) and Raman spectra (b) of PP and 0.2% PP/MoS2 composite materials before and after laser marking at 11 A.

Mechanical Properties of PP and PP/MoS2 Composites

Figure shows the elongation at break and the tensile strength of PP, 0.005% PP/MoS2, 0.01% PP/MoS2, 0.015% PP/MoS2, 0.02% PP/MoS2, and 0.2% PP/MoS2 composite materials after laser marking. From Figure a, it can be seen that with the gradual increase of the laser current intensity from 8 to 12 A, the elongation at break of the PP/MoS2 composite material gradually decreases, that is, the mechanical properties of the PP/MoS2 composite material gradually decrease. This is because the increase of the current intensity damages the laser marked matrix material. From SEM observations, there are many carbonized particles and bumps after the laser marking, and the stress concentration during the stretching process causes the material to break.

Figure 13

Elongation at break (a) and the tensile strength (b) of PP, 0.005% PP/MoS2, 0.01% PP/MoS2, 0.015% PP/MoS2, 0.02% PP/MoS2, and 0.2% PP/MoS2 composite materials after laser marking at the laser current intensity of 11 A and the scanning speed of 800 mm/s.

On observing Figure a, it can be seen that the mechanical properties of the laser marked PP/MoS2 composites gradually become lower than those of pure PP. Considering the laser marking performance, the 0.02% PP/MoS2 composite can be selected for the tensile strength properties. From Figure b, it can be found that the tensile strength of pure PP is about 26.86 MPa. Since the carbonized particles after laser marking can cause stress concentration, the tensile strength decreases gradually with the MoS2 content, resulting in poor mechanical properties at high MoS2 contents. Compared with pure PP and other composites, the 0.02% PP/MoS2 composite material can maintain a good laser marking property and mechanical performance at the laser current intensity of 11 A and the marking speed of 800 mm/s.

Conclusions

In this work, from multiple perspectives such as the laser marking performance, mechanical properties, and energy consumption during the laser marking process, the optimum formula and conditions are determined for the PP/MoS2 composite material.

The laser marking additive MoS2 has stable properties and good absorption for the near-infrared laser, which makes the PP material undergo pyrolysis and carbonization to form a relatively clear laser marking. Through visual and microscopic observations, scanning electron microscopy, Raman spectroscopy, XRD, color difference measurements, and mechanical property tests, the PP/MoS2 composite material has the best laser marking performance at the MoS2 loading content of 0.02%, the laser current intensity of 11 A, and the laser marking speed of 800 mm/s. The addition of the small amount of MoS2 has a slight influence on the tensile strength of the composite materials. During laser marking, the additive MoS2 can absorb the laser energy and undergo a photothermal reaction and the PP chains around MoS2 can be pyrolyzed and carbonized, leading to the formation of the black carbonized materials on the surface of the composites. This topic is of great significance to the realization of the high-functionality and high-performance application of PP-based materials and meets the laser marking needs of characters, patterns, and 2D codes on the surface of PP products in the fields of automobiles, medical equipment, and precision instruments.

Materials and Methods

Chemicals and Materials

PP (1600E) was purchased from Sinopec Shanghai Petrochemical Co., Ltd (Shanghai, China). MoS2 (1.5 μm, XF184–1) was commercially available at Nanjing Xianfeng Nano Co., Ltd (Nanjing, China).

Preparation of the PP/MoS2 Composite Materials

PP granules and MoS2 powders with different contents, including 0.005, 0.01, 0.015, 0.02, and 0.2%, are fully mixed and then added into an internal mixer (Su-70, Changzhou Suyan Technology Co., Ltd., Changzhou, China) for internal mixing, and the temperature is set at 180 °C. The temperature of the upper and lower templates of the flat curing machine (YF-8017, Yangzhou Yuanfeng Experimental Machinery Factory, Yangzhou, China) is set at 180 °C. After the materials are molded into sheets, they are laser marked by the laser marking machine. The composite samples are denoted as PP/MoS2. For example, 0.02% PP/MoS2 indicates the incorporation of 0.02% MoS2 in PP.

Laser Marking Process

The laser labeling of PP/MoS2 composites was performed by a Nd: YAG (KDD-50, Suzhou Kaitai Laser Technology Co., Ltd., China) pulsed laser beam at 1064 nm. Laser marking process parameters include the laser focal length of 219 mm, the laser spot of 100 μm, and the laser pulse repetition frequency of 4000 Hz. In addition, the laser scanning speeds are set to be 200, 400, 600, 800, and 1000 mms. The laser current intensities of 8, 9, 10, 11, and 12 A are adopted in the laser marking process.

Instruments and Characterization

Color Difference Tests

A spectrophotometer (7000A, X-Rite) was calibrated with a black-and-white standard sample, and then the color difference of the test samples was detected. The unmarked area was referred to as the standard, and the laser marked area (15 mm × 15 mm) on the surface of the substrate was the comparison sample. The corresponding values of ΔE, ΔL, Δa, Δb were recorded and analyzed. Among the color difference values, Δa represents yellowness, Δb represents blueness, ΔL represents brightness (black and white), and ΔE represents the color difference value. The calculation formula of the color difference value is: ΔE = [(ΔL)2 + (Δa)2 + (Δb)2]1/2.

SEM Tests

A scanning electron microscope (JSM-1T100, JEOL Ltd.) was used to observe PP and PP/MoS2 samples before and after laser marking. The observation sample was sprayed with gold and the surface morphology of the material was observed and recorded.

XRD Measurements

The PP and PP/MoS2 samples with the size of 15 mm × 15 mm before and after laser marking were scanned using an X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å) (Rigaku, D/max 2500) to characterize the crystal structure characteristics of PP composites after laser marking.

Raman Analysis

The samples of PP and PP/MoS2 before and after laser labeling were analyzed using Raman spectroscopy (DXR2-5967, Thermo Fisher Scientific).

FT-IR Tests

An FT-IR spectrometer (Nicolet IS50R, Thermo Fisher Scientific) was used to analyze the samples such as PP and PP/MoS2 sheets, and the reflection mode was selected for the detection. The wavenumber range was 400–4000 cm–1.

Water Contact Angle Measurements

The surface water contact angles of PP and PP/MoS2 before and after laser irradiation were measured using a dynamic contact angle measuring instrument (JC2009D1 goniometer, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.).

Metallographic Microscopic Observation

The PP and PP/MoS2 samples before and after laser marking were placed under a metallographic microscope (4XC-VS, Dexon Testing Equipment Co., Ltd.) to observe and record the surface morphology of the materials.

Mechanical Property Tests

The standard dumbbell-shaped tensile test samples of PP and PP/MoS2 were prepared from the sheets using a dumbbell cutting die cutter. The sheet dumbbell-shaped samples have a central part of 20 mm length, 4 mm width, and 2 mm thickness. The tensile test was carried out using a universal testing machine (WDT-30, Shenzhen Kaiqiangli Experimental Instrument Co., Ltd.). The tensile speed is 20 mm/min, and the measuring span is 20 mm.