A method for enhancing the strength of thermoplastic laser transmission welding

Abstract

The present application discloses a method for enhancing the strength of thermoplastic laser transmission welding, which relates to the field of plastic laser welding technology. The method for enhancing the strength of thermoplastic laser transmission welding is as follows: first, degreasing and decontaminating the upper and lower parts to be welded; then, ultraviolet light activation treatment is performed on the degreased and decontaminated upper and lower parts to be welded; then, a rigid-flexible interface layer is constructed by applying a coating on the surface of the lower part to be welded, 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; then, the upper and lower parts to be welded are stacked together and laser transmission welded under inert gas protection; after welding is completed, cooling and forming are performed. When constructing the rigid-flexible interface layer, the present invention can improve the welding strength and oxidation resistance of the plastic weldment through the synergistic effect of oxidized carbon nanotubes and mussel adhesive protein, thereby extending the life of the weldment.

Description

Technical Field

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

[0002] In modern industrial manufacturing, thermoplastics are widely used in the production of numerous products due to their significant advantages, such as light weight, low cost, and simple molding processes. Thermoplastic laser welding technology, a key method for joining thermoplastic components, leverages the excellent directionality and high power density of the laser beam. Through an optical system, the laser beam is precisely focused on a small area of the thermoplastic part to be welded, rapidly forming a heat source zone with highly concentrated energy at the weld. Within this zone, the thermoplastic polymer chain segments transition from a glassy state to a viscous flow state in a very short period of time, achieving melting of the thermoplastic. Subsequently, a combination of cooling and pressure creates a strong weld point or seam.

[0003] As the application of thermoplastics in various key fields continues to deepen, the requirements for thermoplastic welding quality are becoming increasingly stringent. In particular, in terms of weld 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 casings 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 cannot 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 the action of oxidation, which not only reduces the reliability of the product, but also significantly shortens the product's service life.

[0005] In the aerospace industry, thermoplastic components are commonly used in aircraft interiors and electronic equipment housings. The specialized environments in this field, such as high radiation, high vacuum, and drastic temperature fluctuations, place extremely high demands on the strength and oxidation resistance of thermoplastic weld areas. However, traditional laser welding technology struggles to meet the stringent weld quality standards for thermoplastics under these extreme conditions, posing a significant safety risk 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 expanding the application scope of thermoplastic laser welding technology and improving the quality and reliability of industrial products. Summary of the Invention

[0007] In order to address the deficiencies in the prior art, the present invention provides a method for enhancing the strength of thermoplastic plastic laser transmission welding. Through the method of the present invention, the strength of thermoplastic plastic laser transmission welding can be significantly enhanced, while the oxidation resistance of the welding area is improved to meet the strict requirements of modern industry for the quality of thermoplastic plastic welding.

[0008] In order to achieve the above object, the specific scheme adopted by the present invention is:

[0009] A method for enhancing the laser transmission welding strength of thermoplastic plastics mainly comprises the following steps:

[0010] Step 1: Degrease and clean the upper and lower parts to be welded; the light transmittance of the upper part to be welded is ≥50%;

[0011] Step 2: Perform ultraviolet light activation treatment on the upper and lower parts to be welded after degreasing and cleaning;

[0012] 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;

[0013] Step 4: stack the upper and lower parts to be welded together, so that the upper surface of the rigid-flexible interface layer contacts the upper part to be welded, and perform laser transmission welding under inert gas protection;

[0014] Step 5: After welding is completed, cool and form.

[0015] Furthermore, in step 1, the specific method of the degreasing and decontamination treatment is: first placing it in an organic solvent for ultrasonic treatment, and then washing it with deionized water and drying it.

[0016] Furthermore, in step three, the coating is prepared by pouring 10 g of mussel adhesive protein and 100 g of deionized water into a beaker, stirring at room temperature for 15 minutes, adding 0.1-5 g of oxidized carbon nanotubes, and placing the beaker in an ultrasonic cell disruptor for 20 minutes.

[0017] Furthermore, in step three, the aspect ratio of the oxidized carbon nanotubes is 25-200, and they are obtained by mixed acid oxidation treatment.

[0018] Furthermore, 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.

[0019] Beneficial effects:

[0020] (1) The method for laser transmission welding of thermoplastic plastics provided by the present invention pre-constructs a rigid-flexible interface layer on the surface of the lower part to be welded, and then welds the upper and lower parts to be welded, which can enhance the laser welding strength. Mussel adhesive protein serves as the flexible part and oxidized carbon nanotubes serve as the rigid part, wherein the mussel adhesive protein plays the role of a "bridge". The catechol group of dopa in the mussel adhesive protein is used to form a covalent bond with the functional group on the surface of the part to be welded, and the hydroxyl group of dopa is used to form a hydrogen bond with the oxygen-containing polar group on the surface of the oxidized carbon nanotube, thereby improving the interfacial bonding strength between the oxidized carbon nanotube and the material of the part to be welded, thereby improving the laser welding interface strength. In addition, the mussel adhesive protein has good wettability and can effectively wet the surface of the part to be welded, and can penetrate into the micropores or uneven rough areas on the surface of the part to be welded, forming an effective mechanical interlocking, enhancing adhesion, and further improving welding performance.

[0021] (2) The present invention introduces oxidized carbon nanotubes with a certain aspect ratio into the thermoplastic plastic laser welding interface. The oxidized carbon nanotubes act as "rivets" and can penetrate the thermoplastic plastic laser welding interface layer to "rivet" the upper and lower parts to be welded, thereby enhancing the mechanical riveting force of the upper and lower parts to be welded and further enhancing the welding strength.

[0022] (3) In the process of laser welding of thermoplastic plastics, the present invention adopts inert gas to protect the welding area, which can effectively prevent the oxidation of thermoplastic plastics caused by high temperature during the welding process and reduce the formation of welding defects.

[0023] (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 implement in a general industrial production environment. At the same time, the costs of the materials used, such as the solvent, mussel adhesive protein, and oxidized carbon nanotubes, are relatively controllable, making it suitable for large-scale industrial production applications, helping to reduce production costs and improve production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of laser welding joint and interface enhancement.

[0025] Figure 2 Schematic diagram of laser welding strength test.

[0026] Graphic markings: 1. Lower part to be welded, 2. Rigid-flexible interface layer, 3. Upper part to be welded, 4. Laser. DETAILED DESCRIPTION

[0027] In order to make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0028] The present invention provides a method for enhancing the strength of thermoplastic laser transmission welding. This method can significantly enhance the strength of thermoplastic laser transmission welding and improve the oxidation resistance of the welding area. The specific method is as follows:

[0029] Step 1: Degreasing and decontamination: Place the upper and lower welded parts 3 and 1 in an organic solvent such as acetone, trichloroethylene, or ethanol and perform ultrasonic treatment for 1-30 minutes. Utilizing the ultrasonic cavitation effect, the oil, impurities, and other surface residues on the welded parts are effectively removed. Rinse thoroughly with deionized water to remove any residual organic solvent and impurities. Air dry the parts naturally or dry them in a forced-air oven at 60°C to ensure the surfaces are clean and dry, providing a good foundation for subsequent processing.

[0030] Step 2: UV activation: The degreased and cleaned upper and lower welded components 3 and 1 are subjected to UV activation. Specifically, at room temperature, the components are irradiated with UV light having a wavelength of 100-400 nm and a power of 0.5-3 kW for 10 minutes. UV activation changes the chemical structure and activity of the surfaces of the components to be welded, increasing the number of functional groups on the surfaces and raising the surface energy, thereby enhancing the bonding between the materials in subsequent processing steps.

[0031] Step 3: Constructing a rigid-flexible interface layer 2: Constructing a rigid-flexible interface layer 2 by applying a coating on the surface of the lower workpiece 1 to be welded;

[0032] Step 4: Inert gas shielded welding: stack the upper workpiece 3 and the lower workpiece 1 together, so that the upper surface of the rigid-flexible interface layer 2 contacts the upper workpiece 3. In an inert gas atmosphere, use a welding fixture to apply a certain pressure to press tightly, and then turn on the laser 4, as shown in the following figure: Figure 1 As shown, the laser penetrates the light-transmitting upper part to be welded 3 and concentrates its energy on the lower part to be welded 1. As the energy is concentrated, the temperature at the weld seam increases, causing the materials of the upper and lower parts to be welded to melt at the weld seam. Under the pressure of the welding fixture, the polymer chains of the upper and lower parts to be welded migrate and fuse together.

[0033] Step 5: Cooling and forming: After welding is completed, turn off the laser 4, maintain the tightening pressure of the welding fixture, and wait for the welded parts to cool for 10-60 seconds. During the cooling process, the molten plastic gradually solidifies to form a strong weld joint, completing the welding process.

[0034] It should be noted that the welding method of the present invention is widely adaptable to various materials of the welded parts. The upper welded part 3, as long as its transmittance is ≥50%, can be made of a variety of materials, including but not limited to LDPE, HDPE, UHMWPE, PVC, PS, PP, PC, PMMA, PEI, PET, PETG thermoplastics, and their glass fiber or mica composites. The lower welded part 1 can have light-absorbing properties, such as a composite material composed of a light-absorbing filler (such as one or more of carbon black, graphite, graphene, carbon nanotubes, carbon fibers, or carbon nanofibers) and a thermoplastic matrix (including but not limited to LDPE, HDPE, UHMWPE, PVC, PS, PP, PC, PMMA, PEI, PET, PETG, PEEK, PPS, and PI), or it can be non-light-absorbing. This broad material adaptability enables the welding method of the present invention to meet the diverse demands for thermoplastic welding in various industrial fields.

[0035] The coating used in step 3 is a mixture of oxidized carbon nanotubes, mussel adhesive protein, and deionized water in a mass ratio of (0.1-5):10:100. The specific preparation method is to take 10g of mussel adhesive protein and 100g of deionized water, pour them into a 500ml beaker, stir at room temperature for 15 minutes, and fully dissolve the mussel adhesive protein in the water; then add 0.1g-5g of oxidized carbon nanotubes. To reduce the agglomeration of the oxidized carbon nanotubes, place the beaker in an ultrasonic cell disruptor for 20 minutes. The oxidized carbon nanotubes have an aspect ratio of 25-200 and are prepared by oxidation in a mixed acid containing nitric acid and sulfuric acid for 6 hours. The oxidized carbon nanotubes can be a mixture of one or more 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, wherein the concentrated nitric acid concentration is 68% and the concentrated sulfuric acid concentration is 98%. The mussel adhesive protein, as the flexible component, acts as a "bridge," utilizing the catechol group of its dopa to form a covalent bond with the surface functional groups of the parts to be welded, and utilizing the hydroxyl groups of dopa to form hydrogen bonds with the oxygen-containing polar groups on the surface of the oxidized carbon nanotubes, thereby enhancing the interfacial adhesion strength between the oxidized carbon nanotubes and the materials to be welded. At the same time, the excellent wettability of the mussel adhesive protein enables it to effectively infiltrate the surface of the parts to be welded and penetrate into the micropores or uneven rough areas on the surface of the parts to be welded, forming an effective mechanical interlock and enhancing adhesion. The oxidized carbon nanotubes, as the rigid component, act as a "rivet," penetrating the thermoplastic laser welding interface layer to "rivet" the upper and lower parts to be welded, thereby enhancing the mechanical riveting force of the upper and lower parts to be welded.

[0036] In step 4, an inert gas is used for protection during the welding process. The inert gas can isolate oxygen, prevent oxidation of thermoplastics caused by high temperatures during welding, and reduce the formation of welding defects. The inert shielding 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 10L / min). Laser 4 can be selected from one of a carbon dioxide laser, a fiber laser, a diode laser, and a semiconductor laser, preferably a semiconductor laser, with a wavelength range of 800-1100nm, a power of 1-80W, and a welding rate of 1-10mm / s.

[0037] The technical solution of the present invention is further described below with reference to specific embodiments and comparative examples.

[0038] Example 1

[0039] A method for enhancing the laser transmission welding strength of thermoplastic plastics mainly comprises the following steps:

[0040] Step 1: Select an upper part 3 and a lower part 1 to be welded. The upper part 3 and the lower part 1 to be welded are made of UHMWPE. Place the upper part 3 and the lower part 1 to be welded in an organic solvent (trichloroethylene) and ultrasonically treat them for 30 minutes. Then rinse them with deionized water and dry them in a blast oven at 60°C.

[0041] Step 2: The upper welded part 3 and the lower welded part 1 processed in step 1 are subjected to ultraviolet light activation treatment for 10 minutes;

[0042] Step 3: spraying a layer of coating on the surface of the lower welded part 1 after the surface treatment in step 2 to form a rigid-flexible interface layer 2, and allowing it to dry naturally before use, wherein the coating is a mixture of oxidized carbon nanotubes, mussel adhesive protein, and deionized water in a mass ratio of 2:10:100; the oxidized carbon nanotubes are multi-walled carbon nanotubes with an aspect ratio of 100;

[0043] Step 4: Superimpose the upper part 3 and the lower part 1 to be welded together, apply a certain pressure using a welding fixture, place them in a nitrogen-filled environment with a nitrogen flow rate of 10 L / min, and then turn on the laser 4 to connect the parts to be welded together;

[0044] Step 5: Turn off the laser 4, keep the welding fixture tight and wait for 30 seconds to cool down, and then obtain the weldment.

[0045] Example 2

[0046] The difference between Example 2 and Example 1 is that in step 3, the coating is a mixture of oxidized carbon nanotubes, mussel adhesive protein and deionized water in a mass ratio of 5:10:100.

[0047] Example 3

[0048] The difference between Example 3 and Example 1 is that in step 3, the coating is a mixture of oxidized carbon nanotubes, mussel adhesive protein and deionized water in a mass ratio of 0.1:10:100.

[0049] Example 4

[0050] The difference between Example 4 and Example 1 is that the aspect ratio of the oxidized carbon nanotubes is 25.

[0051] Example 5

[0052] The difference between Example 5 and Example 1 is that the aspect ratio of the oxidized carbon nanotubes is 200.

[0053] Comparative Example 1

[0054] The difference between Comparative Example 1 and Example 1 is that: in step 4, there is no nitrogen protection during the welding process.

[0055] Comparative Example 2

[0056] The difference between Comparative Example 2 and Example 1 is that in step 3, the coating is a mixture of mussel adhesive protein and deionized water in a mass ratio of 10:100.

[0057] Comparative Example 3

[0058] The difference between Comparative Example 3 and Example 1 is that in step 3, the coating is a mixture of oxidized carbon nanotubes and deionized water in a mass ratio of 2:100.

[0059] Comparative Example 4

[0060] The difference between Comparative Example 4 and Example 1 is that step 3 is not included.

[0061] The performance tests of the weldments obtained in Examples 1-5 and Comparative Examples 1-4 are now carried out.

[0062] (1) Welding strength

[0063] To characterize the welding strength, single lap shear strength test (LSS) was carried out according to ASTM D5868-2001. Figure 2 During the test, a longitudinal tensile force is applied at both ends of the weldment at a rate of 10 mm / min. The maximum load F when the weldment joint is damaged or broken is tested. max, the effective welding area A is obtained by observing the fracture welding area when the specimen fails, and the shear strength of the laser welded joint is calculated by the following formula:

[0064]

[0065] Where, σ is the shear strength, unit is MPa; F max is the maximum force during the tensile fracture process, in N; A is the welding area, in mm 2 .

[0066] The shear strengths of the welds obtained in Examples 1-5 and Comparative Examples 1-4 are shown in Table 1.

[0067] Table 1 Shear strength of welds obtained in Examples 1-5 and Comparative Examples 1-4

[0068]

[0069] Referring to Table 1, the shear strength of the welds obtained in Examples 1-5 was higher than that obtained in Comparative Examples 1-4, demonstrating that the method of the present invention can effectively enhance laser welding strength. Analysis of Example 1 and Comparative Example 1 demonstrates that applying an inert gas shield during laser welding effectively protects the welded parts and enhances laser welding strength. Analysis of Example 1 and Comparative Examples 2-4 reveals that, regardless of whether the rigid-flexible interface layer 2 is omitted (Comparative Example 4), or whether only oxidized carbon nanotubes (Comparative Example 3) or mussel adhesive protein (Comparative Example 2) is added during the rigid-flexible interface layer 2, the shear strength of the welds obtained is inferior to that of Example 1. This demonstrates that the mussel adhesive protein and oxidized carbon nanotubes in the rigid-flexible interface layer 2 constructed in the present invention synergize, with the oxidized carbon nanotubes penetrating the interface layer to rivet the upper and lower welded parts, and the mussel adhesive protein acting as a "bridge" to bond the oxidized carbon nanotubes to the welded parts, thereby enhancing laser welding strength.

[0070] (2) Antioxidant properties

[0071] The weldments obtained in Example 1 and Comparative Example 4 were subjected to an oxidation resistance test, referring to the standard NB / T4216-2018. Specifically, the weldment was placed in a beaker filled with saturated bromine water to ensure that the weld joint was completely immersed; the beaker was then heated to 60°C in a water bath and kept at a constant temperature for 3 hours. The weld joint was taken out and rinsed with deionized water. Then, an LSS test was performed and the shear strength was recorded. The test results are recorded in Table 2 below.

[0072] Table 2 Antioxidation test results of weldments obtained in Example 1 and Comparative Example 4

[0073]

[0074] As can be seen from Table 2, the weld strength of the welds obtained in Example 1 and Comparative Example 4 decreased basically steadily after 5 weeks, and 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%, which shows that the weld obtained by the method of the present invention has good oxidation resistance and can extend the life of the weld.

[0075] 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 achieve good anti-oxidation performance. The method of the present invention is simple to operate, cost-controlled, and has strong process adaptability, and has good industrial application prospects.

[0076] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any equivalent changes or modifications made based on the essence of the present invention should be included in the scope of protection 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 clean the upper and lower parts to be welded; the light transmittance of the upper part to be welded is ≥50%; Step 2: Perform ultraviolet light activation treatment on the upper and lower parts to be welded after degreasing and cleaning; 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 and lower parts to be welded together, so that the upper surface of the rigid-flexible interface layer contacts the upper part to be welded, and perform laser transmission welding under inert gas protection; Step 5: After welding is completed, cool and form; In step 3, the coating is prepared by: taking 10g of mussel adhesive protein and 100g of deionized water into a beaker, respectively, stirring at room temperature for 15 minutes, adding 0.1-5g of oxidized carbon nanotubes, and placing the beaker in an ultrasonic cell disruptor for 20 minutes. The oxidized carbon nanotubes have an aspect ratio of 25-200 and are obtained through mixed acid oxidation treatment.

2. The 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: first placing it in an organic solvent for ultrasonic treatment, then washing it with deionized water and drying it.

3. The method for enhancing the laser transmission welding strength of thermoplastics according to claim 1, 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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