Method for enhancing laser transmission welding strength of thermoplastic plastic

Abstract

The invention discloses a method for enhancing the laser transmission welding strength of thermoplastic plastics, and relates to the technical field of plastic laser welding, and the method for enhancing the laser transmission welding strength of the thermoplastic plastics comprises the following steps: firstly, degreasing and decontaminating an upper to-be-welded part and a lower to-be-welded part; then the degreased and decontaminated upper to-be-welded part and the degreased and decontaminated lower to-be-welded part are subjected to ultraviolet light activation treatment; then coating the surface of a lower part to be welded with a coating to construct a rigid-flexible interface layer, wherein the coating is a mixture formed by compounding oxidized carbon nanotubes, mussel adhesion protein and deionized water according to the mass ratio of (0.1-5): 10: 100; then the upper to-be-welded part and the lower to-be-welded part are stacked together, and laser transmission welding is conducted under inert gas protection; and after welding is completed, cooling and forming are conducted. When the rigid-flexible interface layer is constructed, through the synergistic effect of the oxidized carbon nanotubes and the mussel adhesion protein, the welding strength and oxidation resistance of the plastic weldment can be improved, and the service life of the weldment can be prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of thermoplastic laser welding, in particular to a method for enhancing the laser transmission welding strength of thermoplastic. Background Art

[0002] In the field of modern industrial manufacturing, thermoplastics have been widely used in the production and manufacturing of many products due to their significant advantages such as light weight, low cost and simple molding process. As an important means of connecting thermoplastic parts, thermoplastic laser welding technology uses the excellent directionality and high power density characteristics of the laser beam to accurately focus the laser on a smaller area of ​​the thermoplastic part to be welded through an optical system, so that a heat source area with highly concentrated energy is quickly formed at the welding point. In this area, the thermoplastic polymer chain segments change from a glassy state to a viscous flow state in a very short time, realizing the melting of the thermoplastic. Subsequently, under the combined action of cooling and pressure, a firm weld or weld is formed.

[0003] As the application of thermoplastics in various key fields continues to deepen, the requirements for the welding quality of thermoplastics are becoming increasingly stringent. In particular, in terms of welding strength and oxidation resistance, traditional laser welding technology has gradually exposed obvious shortcomings.

[0004] Taking the field of electronic equipment manufacturing as an example, thermoplastic plastic components are widely used in the outer shells and internal structural parts of electronic products. In daily use, these thermoplastic plastic components not only have to withstand various external forces, but are also continuously exposed to the air and affected by environmental factors such as oxygen and temperature. Traditional laser welding methods make it difficult to ensure that the welding area has sufficient strength. Under the action of long-term external forces, the welds are prone to cracking and other problems. At the same time, due to the lack of effective measures to improve the oxidation resistance of the welding area, thermoplastics will gradually age and become brittle under oxidation, which not only reduces the reliability of the product, but also greatly shortens the service life of the product.

[0005] In the aerospace field, thermoplastic parts are often used to manufacture aircraft interiors, electronic equipment housings, etc. The special environment in this field, such as high radiation, high vacuum, and drastic temperature changes, places extremely high demands on the strength and oxidation resistance of the thermoplastic welding area. However, traditional laser welding technology is difficult to meet the strict standards for thermoplastic welding quality under these extreme conditions, which poses a greater safety hazard to aerospace products.

[0006] In summary, it is urgent to develop a high-strength, oxidation-resistant thermoplastic laser transmission welding method, which is of great significance to expand the application scope of thermoplastic laser welding technology and improve the quality and reliability of industrial products. Summary of the invention

[0007] To address the deficiencies in the prior art, the present invention provides a method for enhancing the laser transmission welding strength of thermoplastic plastics. By means of the method of the present invention, the strength of laser transmission welding of thermoplastic plastics can be significantly enhanced, and at the same time, the oxidation resistance of the welding area can be improved to meet the strict requirements of modern industry for the welding quality of thermoplastic plastics.

[0008] To achieve the above object, the specific solution adopted by the present invention is as follows: A method for enhancing the laser transmission welding strength of thermoplastic plastics mainly includes the following steps: Step 1: Degrease and decontaminate the upper and lower workpieces to be welded; wherein, the light transmittance of the upper workpiece to be welded is ≥50%. Step 2: Ultraviolet light activation treatment is performed on the degreased and decontaminated upper and lower workpieces to be welded. Step 3: A rigid-flexible interface layer is constructed by applying a coating on the surface of the lower workpiece to be welded. The coating is a mixture prepared by compounding carbon oxide nanotubes, mussel adhesive protein, and deionized water according to a mass ratio of (0.1~5):10:100. Step 4: Stack the upper and lower workpieces to be welded together, making the upper surface of the rigid-flexible interface layer contact the upper workpiece to be welded, and perform laser transmission welding under the protection of an inert gas. Step 5: After welding is completed, cool and form.

[0009] Further, in Step 1, the specific method of the degreasing and decontamination treatment is: first place it in an organic solvent for ultrasonic treatment, and then wash and dry it with deionized water.

[0010] Further, in Step 3, the preparation method of the coating is: Pour 10 g of mussel adhesive protein and 100 g of deionized water into a beaker, stir at room temperature for 15 minutes, then add 0.1~5 g of carbon oxide nanotubes, and place the beaker in an ultrasonic cell disruptor for 20 min to obtain it.

[0011] Further, in Step 3, the aspect ratio of the carbon oxide nanotubes is 25-200, and it is obtained by mixed acid oxidation treatment.

[0012] Further, the mixed acid oxidation treatment is to oxidize with concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 1:3 for 6 hours.

[0013] Beneficial effects: (1) The method for laser transmission welding of thermoplastics provided by the present invention pre - constructs a rigid - flexible interface layer on the surface of the lower workpiece to be welded, and then welds the upper and lower workpieces to be welded, which can enhance the laser welding strength. Mussel adhesive protein serves as the flexible part, and carbon nanotubes oxidized serve as the rigid part. Among them, mussel adhesive protein plays the role of a "bridge". By using the catechol group of dopa in mussel adhesive protein to form a covalent bond with the surface functional groups of the workpiece to be welded, and using the hydroxyl group of dopa to form a hydrogen bond with the oxygen - containing polar groups on the surface of carbon nanotubes oxidized, the interfacial adhesion strength between carbon nanotubes oxidized and the material of the workpiece to be welded is improved, thereby enhancing the laser welding interface strength. In addition, mussel adhesive protein has good wettability, can effectively wet the surface of the workpiece to be welded, and can penetrate into the micropores or concave - convex rough areas on the surface of the workpiece to be welded, forming an effective mechanical interlock, enhancing adhesion, and further improving the welding performance.

[0014] (2) The present invention introduces carbon nanotubes oxidized with a certain aspect ratio into the laser welding interface of thermoplastics. The carbon nanotubes oxidized play the role of "rivets", which can penetrate the laser welding interface layer of thermoplastics to "rivet" the upper and lower workpieces to be welded, enhancing the mechanical riveting force between the upper and lower workpieces to be welded, and thus enhancing the welding strength.

[0015] (3) During the laser welding process of thermoplastics in the present invention, an inert gas is used to protect the welding area, which can effectively prevent the oxidation of thermoplastics caused by the high temperature during the welding process and reduce the formation of welding defects.

[0016] (4) The processing steps and materials used in the welding method of the present invention are relatively common, and the operation process is relatively simple. Degreasing and decontamination treatment, ultraviolet light activation treatment, coating preparation, and parameter control during the welding process are all easy to achieve in a general industrial production environment. At the same time, the material costs of solvents, mussel adhesive protein, carbon nanotubes oxidized, etc. used are relatively controllable, suitable for large - scale industrial production applications, helping to reduce production costs and improve production efficiency. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of a laser - welded joint and interface enhancement.

[0018] Figure 2 It is a schematic diagram of laser welding strength test.

[0019] Illustration marks: 1, lower workpiece to be welded; 2, rigid - flexible interface layer; 3, upper workpiece to be welded; 4, laser. Detailed Embodiments

[0020] To make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] The present invention provides a method for enhancing the laser transmission welding strength of thermoplastic plastics. By this method, the laser transmission welding strength of thermoplastic plastics can be significantly enhanced, and at the same time, the oxidation resistance of the welding area can be improved. The specific method is as follows: Step 1, degreasing and decontamination treatment: Place the upper workpiece to be welded 3 and the lower workpiece to be welded 1 in an organic solvent such as acetone, trichloroethylene or ethanol for ultrasonic treatment for 1-30 minutes. Utilize the cavitation effect of ultrasonic waves to effectively remove the oil stains, impurities, etc. on the surface of the workpiece to be welded. Subsequently, rinse it with deionized water to remove the residual organic solvent and impurities, and then dry it naturally or dry it in a blast drying oven at 60 °C to ensure that the surface of the workpiece to be welded is clean and dry, providing a good basis for subsequent treatment; Step 2, ultraviolet light activation treatment: Perform ultraviolet light (UV) activation treatment on the upper workpiece to be welded 3 and the lower workpiece to be welded 1 after degreasing and decontamination; specifically, at room temperature, irradiate the workpiece to be welded with ultraviolet light having a wavelength range of 100-400 nm and a power of 0.5-3 kW for a treatment duration of 10 minutes; Ultraviolet light activation can change the chemical structure and activity of the surface of the workpiece to be welded, increase the number of surface functional groups, and improve the surface energy, thereby enhancing the bonding force between materials in subsequent treatment steps; Step 3, constructing a rigid-flexible interface layer 2: Construct a rigid-flexible interface layer 2 by applying a coating on the surface of the lower workpiece to be welded 1; Step 4, inert gas shielded welding: Stack the upper workpiece to be welded 3 and the lower workpiece to be welded 1 together so that the upper surface of the rigid-flexible interface layer 2 contacts the upper workpiece to be welded 3. In an inert gas atmosphere, use a welding fixture to apply a certain pressure to clamp, and then turn on the laser 4, as Figure 1 shown, the laser passes through the transparent upper workpiece to be welded 3 and concentrates the energy on the lower workpiece to be welded 1. The temperature at the weld position rises as the energy accumulates, causing the materials of the upper and lower workpieces to be welded to melt at the weld. Under the action of the pressure of the welding fixture, the polymer chain segments of the upper and lower workpieces to be welded migrate and merge into one; Step 5, cooling and forming: After welding is completed, turn off the laser 4, maintain the clamping pressure of the welding fixture, and wait for the workpiece to be welded to cool for 10-60 seconds. During the cooling process, the molten plastic gradually solidifies to form a firm welded joint, completing the welding process.

[0022] It should be noted that the welding method of the present invention has wide adaptability to workpieces to be welded with different materials. As long as the light transmittance of the upper workpiece to be welded 3 is ≥50%, it includes but is not limited to various materials such as LDPE, HDPE, UHMWPE, PVC, PS, PP, PC, PMMA, PEI, PET, PETG thermoplastics and their composites with glass fiber or mica. The lower workpiece to be welded 1 can either have light absorption characteristics and be a composite material composed of a light absorption filler (such as one or more of carbon black, graphite, graphene, carbon nanotubes, carbon fibers, carbon nanofibers) and a thermoplastic matrix (the material includes but is not limited to LDPE, HDPE, UHMWPE, PVC, PS, PP, PC, PMMA, PEI, PET, PETG, PEEK, PPS, PI materials), or it can not have light absorption characteristics. This wide material adaptability enables the welding method of the present invention to meet the diverse needs of thermoplastic welding in different industrial fields.

[0023] The coating used in step three is a mixture prepared by compounding carbonized nanotubes, mussel adhesive protein and deionized water according to a mass ratio of (0.1~5):10:100. The specific preparation method is as follows: Take 10 g of mussel adhesive protein and 100 g of deionized water and pour them into a 500 ml beaker, stir at room temperature for 15 minutes to fully dissolve the mussel adhesive protein in water; then add 0.1 g - 5 g of carbonized nanotubes. To reduce the agglomeration of carbonized nanotubes, place the beaker in an ultrasonic cell disruptor and process for 20 min. The aspect ratio of the carbonized nanotubes is 25 - 200, and it is prepared by oxidizing with a mixed acid containing nitric acid and sulfuric acid for 6 h. The carbonized nanotubes can be one or a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes; the volume ratio of concentrated nitric acid to concentrated sulfuric acid in the mixed acid is 1:3, where the concentration of concentrated nitric acid is 68% and the concentration of concentrated sulfuric acid is 98%. The mussel adhesive protein, as the flexible part, plays the role of a "bridge". One end of its dopa's catechol group forms a covalent bond with the surface functional groups of the workpiece to be welded, and the hydroxyl group of dopa forms a hydrogen bond with the oxygen-containing polar groups on the surface of the carbonized nanotubes, thereby being able to improve the interfacial adhesion strength between the carbonized nanotubes and the workpiece material to be welded. At the same time, the good wettability of the mussel adhesive protein enables it to effectively wet the surface of the workpiece to be welded and penetrate into the micropores or concave-convex rough areas on the surface of the workpiece to be welded, forming an effective mechanical interlock and enhancing the adhesion. The carbonized nanotubes, as the rigid part, play the role of a "rivet", penetrating the laser welding interface layer of the thermoplastic to "rivet" the upper and lower workpieces to be welded, enhancing the mechanical riveting force between the upper and lower workpieces to be welded.

[0024] In Step 4, an inert gas is used for protection during the welding process. The inert gas can isolate oxygen, prevent the oxidation of thermoplastic plastics caused by the high temperature during the welding process, and reduce the formation of welding defects. The inert protective gas used includes but is not limited to one or more of nitrogen, argon, helium, neon, and krypton. For example, in an environment filled with nitrogen, the nitrogen flow rate can be controlled within a certain range (such as 10 L / min). The laser 4 can be selected from a carbon dioxide laser, a fiber laser, a diode laser, and a semiconductor laser. Preferably, it is a semiconductor laser, with a wavelength range of 800 - 1100 nm, a power of 1 - 80 W, and a welding rate of 1 - 10 mm / s.

[0025] The technical solution of the present invention will be further elaborated below in conjunction with specific embodiments and comparative examples.

[0026] Example 1 A method for enhancing the laser transmission welding strength of thermoplastic plastics mainly includes the following steps: Step 1: Select the upper workpiece to be welded 3 and the lower workpiece to be welded 1. The materials of the upper workpiece to be welded 3 and the lower workpiece to be welded 1 are UHMWPE. Place the upper workpiece to be welded 3 and the lower workpiece to be welded 1 in an organic solvent (trichloroethylene) for ultrasonic treatment for 30 min, then rinse with deionized water, and then dry in a forced-air oven at 60 °C. Step 2: Perform ultraviolet light activation treatment on the upper workpiece to be welded 3 and the lower workpiece to be welded 1 processed in Step 1 for a treatment duration of 10 min. Step 3: Spray a layer of coating on the surface of the lower workpiece to be welded 1 after surface treatment in Step 2 to construct a rigid-flexible interface layer 2, and let it dry naturally for later use. The coating is a mixture prepared by compounding carbon nanotubes, mussel adhesion protein, and deionized water according to a mass ratio of 2:10:100; the carbon nanotubes are multi-walled carbon nanotubes with an aspect ratio of 100. Step 4: Stack the upper workpiece to be welded 3 and the lower workpiece to be welded 1 together, apply a certain pressure using a welding fixture, place it in an environment filled with nitrogen, with a nitrogen flow rate of 10 L / min, and then turn on the laser 4 to connect the workpieces to be welded together. Step 5: Turn off the laser 4, and keep the pressure of the welding fixture tightened, wait for a cooling time of 30 s, and the welded part is obtained.

[0027] Example 2 The difference between Example 2 and Example 1 is that in Step 3, the coating is a mixture prepared by compounding carbon nanotubes, mussel adhesion protein, and deionized water according to a mass ratio of 5:10:100.

[0028] Example 3 Example 3 is different from Example 1 in that: in Step 3, the coating is a mixture prepared by compounding carbon nanotubes, mussel adhesive protein, and deionized water according to a mass ratio of 0.1:10:100.

[0029] Example 4 Example 4 is different from Example 1 in that: the aspect ratio of the carbon nanotubes is 25.

[0030] Example 5 Example 5 is different from Example 1 in that: the aspect ratio of the carbon nanotubes is 200.

[0031] Comparative Example 1 Comparative Example 1 is different from Example 1 in that: in Step 4, there is no nitrogen protection during the welding process.

[0032] Comparative Example 2 Comparative Example 2 is different from Example 1 in that: in Step 3, the coating is a mixture prepared by compounding mussel adhesive protein and deionized water according to a mass ratio of 10:100.

[0033] Comparative Example 3 Comparative Example 3 is different from Example 1 in that: in Step 3, the coating is a mixture prepared by compounding carbon nanotubes and deionized water according to a mass ratio of 2:100.

[0034] Comparative Example 4 Comparative Example 4 is different from Example 1 in that: it does not include Step 3.

[0035] Next, performance tests were carried out on the welded parts obtained in Examples 1-5 and Comparative Examples 1-4.

[0036] (1) Welding strength To characterize the welding strength, a single-lap shear strength test (LSS) was carried out with reference to the standard ASTM D5868-2001. The test schematic diagram is shown in Figure 2 , and during the test, a longitudinal tensile force was applied at both ends of the welded part. The application speed of the tensile force was 10 mm / min, and the maximum load F when the joint of the welded part was damaged or fractured was measured max . The effective area A of the welding was obtained by observing the welded area at the fracture of the specimen when it failed, and the shear strength of the laser-welded joint was calculated by the following formula: where σ is the shear strength, with the unit of MPa; F max is the maximum force value during the tensile fracture process, with the unit of N; A is the welding area, with the unit of mm 2 .

[0037] The shear strengths of the welded parts obtained in Examples 1-5 and Comparative Examples 1-4 are shown in Table 1.

[0038] Table 1 Shear Strength of Welded Parts Obtained in Examples 1-5 and Comparative Examples 1-4 Referring to Table 1, the shear strength of the welded parts obtained in Examples 1-5 is higher than that of the welded parts obtained in Comparative Examples 1-4, indicating that the laser welding strength can be effectively enhanced by the method of the present invention. By analyzing Example 1 and Comparative Example 1, it can be seen that applying inert gas protection during the laser welding process can effectively protect the parts to be welded and enhance the laser welding strength. By analyzing Example 1 and Comparative Examples 2-4, it can be seen that whether the rigid-flexible interface layer 2 is not constructed (Comparative Example 4), or only carbon nanotubes (Comparative Example 3) or mussel adhesion protein (Comparative Example 2) are added during the construction of the rigid-flexible interface layer 2, the shear strength of the obtained welded parts is inferior to that of Example 1, indicating that in the rigid-flexible interface layer 2 constructed by the present invention, the mussel adhesion protein and carbon nanotubes cooperate with each other. The carbon nanotubes penetrate the interface layer to rivet the upper and lower parts to be welded, and the mussel adhesion protein acts as a "bridge" to adhere the carbon nanotubes to the parts to be welded, jointly improving the laser welding strength.

[0039] (2) Oxidation Resistance The welded parts obtained in Example 1 and Comparative Example 4 were subjected to an oxidation resistance experiment. Referring to the standard NB / T4216-2018, specifically, the welded parts were placed in a beaker containing saturated bromine water to ensure that the welded joints were completely immersed; then the beaker was heated in a water bath to 60 °C and kept at a constant temperature for 3 h. The welded joints were taken out, rinsed with deionized water, and then subjected to LSS testing, and the shear strength was recorded. The test results are recorded in Table 2 below.

[0040] Table 2 Oxidation Resistance Test Results of Welded Parts Obtained in Example 1 and Comparative Example 4 As can be seen from Table 2, for the welded parts obtained in Example 1 and Comparative Example 4, the decrease in the welding strength of the two was basically stable after 5 weeks. After 6 weeks of testing, the shear strength of Example 1 decreased by 6.2%, while the shear strength of Comparative Example 4 decreased by 18%. This indicates that the welded parts obtained by the method of the present invention have good oxidation resistance and can extend the life of the welded parts.

[0041] Through the analysis of the above examples and comparative examples, the rigid-flexible interface layer 2 constructed in this application can significantly enhance the laser transmission welding strength of thermoplastics and can obtain good oxidation resistance. The method of the present invention is simple to operate, has controllable costs, and strong process adaptability, and has good industrial application prospects.

[0042] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any equivalent transformation or modification made according to the essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for enhancing the laser transmission welding strength of thermoplastic plastics, characterized in that: The main steps are as follows: Step 1: Degrease and decontaminate the upper and lower welded parts; the light transmittance of the upper welded parts is ≥50%; Step 2: Perform ultraviolet light activation treatment on the upper and lower welded parts after degreasing and decontamination; Step 3: constructing a rigid-flexible interface layer by applying a coating on the surface of the lower welded part, wherein the coating is a mixture of oxidized carbon nanotubes, mussel adhesive protein and deionized water in a mass ratio of (0.1-5):10:100; Step 4: stack the upper workpiece to be welded and the lower workpiece to be welded together, so that the upper surface of the rigid-flexible interface layer contacts the upper workpiece to be welded, and perform laser transmission welding under the protection of inert gas; Step 5: After welding is completed, cool and shape.

2. A method for enhancing the laser transmission welding strength of thermoplastics according to claim 1, characterized in that: In step 1, the specific method of the degreasing and decontamination treatment is: firstly placing it in an organic solvent for ultrasonic treatment, and then washing it with deionized water and drying it.

3. A method for enhancing the laser transmission welding strength of thermoplastics according to claim 1, characterized in that: In step three, the coating preparation method is as follows: 10 g of mussel adhesion protein and 100 g of deionized water are poured into a beaker respectively, stirred at room temperature for 15 minutes, 0.1-5 g of oxidized carbon nanotubes are added, and the beaker is placed in an ultrasonic cell disruptor for 20 minutes.

4. A method for enhancing the laser transmission welding strength of thermoplastics according to claim 1, characterized in that: In step 3, the oxidized carbon nanotubes have an aspect ratio of 25-200 and are obtained by mixed acid oxidation treatment.

5. A method for enhancing the laser transmission welding strength of thermoplastics according to claim 4, characterized in that: The mixed acid oxidation treatment is carried out by using concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1:3 for 6 hours.

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