A method for preparing a porous polyethylene material for use as a substrate for electromagnetic wave lenses

Abstract

The present application relates to a kind of porous polyethylene material preparation method used as electromagnetic wave lens substrate, comprising the following steps: 1) by weight, foaming agent 10~60 parts, crosslinking agent 2~10 parts, activator 3~9 parts, nucleating agent 10~50 parts and resin 550~650 parts are mixed to obtain mixed powder by stirring sufficiently;2) the mixed powder obtained in step 1) is heated, and after PE resin is melted into viscous state, again homogeneous stirring is mixed to obtain softened mixed resin;3) the mixed resin obtained in step 2) is placed into foaming mold and foamed to obtain foaming solution;4) the foaming mold is opened to release pressure constraint, and foaming is completed to obtain foaming body with countless foaming holes;5) cooling and setting;6) demoulding to obtain porous polyethylene material.The present application has the advantages of small equipment investment, simple generation process, high production efficiency, low production cost, can generate bubble fine and uniform, dielectric constant meets the use requirement in high frequency communication field and the advantage of high structural strength material.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses. Background Technology

[0002] In high-frequency communication, reducing the dielectric constant (Dk) and dielectric loss (Df) is crucial for improving signal transmission speed and quality. Traditional low-dielectric materials such as polytetrafluoroethylene (PTFE) are expensive and difficult to process; polyimide (PI) is expensive and offers limited Dk reduction (typically >3.0); ceramic filler composite systems suffer from brittleness, weight, and difficulty in molding. Polyethylene (PE) itself has a low dielectric constant (approximately 2.3) and extremely low loss factor (Df < 0.0005 @ 1GHz), making it an ideal base resin. However, the Dk value of dense PE still cannot meet the stringent requirements of the millimeter-wave band (Dk < 2.0). Introducing air (dielectric constant ≈ 1) to form a porous structure is the most effective method for reducing the dielectric constant of materials. Commonly used methods include supercritical foaming, blending porous fillers, and chemical foaming.

[0003] (1) Supercritical fluid foaming: Supercritical CO2 or N2 is used as a foaming agent, which is then injected into the PE matrix under high pressure. Subsequently, the pressure is rapidly reduced and the temperature is increased to form nano- or micron-sized pores. The principle is mainly based on the dependence of the solubility of gas in polymer on pressure and temperature. The polymer mixture after supercritical fluid supersaturation enters a thermodynamically unstable state during cooling, inducing bubble nucleation and thus forming a microporous structure. The foaming process can be divided into four stages: First, supercritical fluid enters the polymer matrix and reaches a saturated state, forming a polymer / gas homogeneous system; second, the temperature rises sharply or the pressure drops sharply, causing the gas in the homogeneous system to reach a supersaturated state, that is, enter a thermodynamically unstable state, thereby initiating bubble nucleation; subsequently, the gas rapidly diffuses into the bubble nucleus, and the pores gradually grow; finally, the pore structure is finalized by rapid cooling. The microporous materials obtained by supercritical CO2 or N2 foaming technology have higher pore density and smaller pore size, and can exhibit superior performance. However, this method requires large equipment investment and has limited production efficiency, making it difficult to meet the market demand for large-scale, low-cost production.

[0004] (2) Blending porous filler method: PE is blended with porous materials, and air is introduced by utilizing the pores of the filler itself. Commonly used fillers include hollow glass microspheres / ceramic microspheres (rigid hollow structure, which can reduce both dielectric constant and density) and porous silica / zeolite (high specific surface area, but interfacial compatibility needs to be considered). The dielectric constant of these fillers is usually lower than that of PE. Physical mixing can dilute the dielectric properties of PE and reduce the overall dielectric constant. However, the degree of reduction in dielectric constant by blending porous filler method is limited. To effectively reduce the dielectric constant, a higher filler addition is often required. When the filler is increased to a certain extent, it is easy to disperse unevenly in the matrix, agglomerate, and form stress concentration points. In addition, during blending and processing, molten PE resin may penetrate into the pores of the filler under high pressure and high shear force, "blocking" or "filling" the air pores originally used to reduce the dielectric constant.

[0005] (3) Chemical foaming method: This method utilizes the decomposition of chemical foaming agents during thermal processing to generate gas. This method requires precise control of the decomposition temperature of the foaming agent and the processing temperature of PE to avoid excessive decomposition or cell merging. Although chemical foaming is currently a commonly used method for producing foamed materials, the selection of materials, the proportion of materials used, and the determination of foaming conditions are technically challenging. This can easily lead to insufficiently fine and uniform bubbles in the prepared porous polyethylene material, making it difficult to meet the dielectric constant requirements of high-frequency communication applications. Furthermore, the dielectric constant varies significantly between different regions of the same material, resulting in low structural strength, poor resistance to compressive fatigue, and poor tensile strength / toughness. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses. This method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses has low equipment investment, simple production process, high production efficiency, low production cost, and can produce materials with fine and uniform bubbles, dielectric constant that meets the requirements of high frequency communication, and high structural strength.

[0007] The technical solution of the present invention is implemented as follows: a method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses, specifically comprising the following steps: 1) Preparation of mixed powder: This includes only foaming agent, crosslinking agent, activator, nucleating agent, and resin; wherein, by weight: 10-60 parts foaming agent, 2-10 parts crosslinking agent, 3-9 parts activator, 10-50 parts nucleating agent, and 550-650 parts resin, thoroughly mixed to obtain the mixed powder; the foaming agent is ADC, the full chemical name of ADC is azodicarbonamide; the crosslinking agent is BIBP, the full chemical name of BIBP is bis(tert-butylperoxyisopropyl)benzene; the activator is ZnO, the full chemical name of ZnO is zinc oxide; the nucleating agent is talc or SiO2, talc is hydrated magnesium silicate, and SiO2 is silicon dioxide; the resin is PE resin, the chemical name of PE resin is polyethylene; 2) Heat the mixed powder obtained in step 1), and after the PE resin melts into a viscous flow state, stir and mix it evenly again to obtain a softened mixed resin. 3) Place the mixed resin obtained in step 2) into a foaming mold for foaming. The foaming temperature is 165℃~185℃. Hold the pressure at the foaming temperature for 30min~40min to obtain the foamed solution. 4) Open the foaming mold to release the pressure constraint, so that the gas in the foaming solution will be released due to saturation, and the foaming process will be completed to obtain a foam with countless foam pores. 5) Cool and shape the foam obtained in step 4) to stabilize the cell structure; 6) Demolding yields porous polyethylene material.

[0008] This method involves mixing a specific ratio of foaming agent ADC, crosslinking agent BIBP, activator ZnO, nucleating agent, and PE resin for foaming, with a foaming temperature of 165℃~185℃ and a foaming holding time of 25min~45min. PE resin has a high degree of branching and good melt strength, which is conducive to the formation of uniform and stable cells and prevents bubble coalescence and collapse. The decomposition temperature of the foaming agent ADC is high (pure ADC is about 200℃). Therefore, this solution adds the activator ZnO to lower its decomposition temperature, so as to match the processing temperature window of PE resin (usually 160~180℃). The activator does not simply lower the decomposition temperature of the foaming agent ADC, but accelerates the decomposition rate, making the gas generation process more concentrated and synchronous. The crosslinking agent BIBP can form a three-dimensional network structure between PE molecules, which greatly improves the melt strength, thereby encapsulating more gas and forming uniform and fine cells. The nucleating agent makes the cells finer and more uniform. The addition of the nucleating agent is the core of obtaining fine and uniform cells. The nucleating agent provides a large number of starting points for the formation of pores, so that the gas can nucleate uniformly, rather than agglomerating at a few defect points to form large cells. In addition, the nucleating agent can play a role similar to "physical crosslinking", which enhances the melt strength and prevents the bubbles from expanding excessively during the growth process and merging or rupturing.

[0009] The preferred particle size range for the foaming agent ADC is 1µm to 10µm.

[0010] Step 2) involves heating the mixed powder at a temperature of 135℃ to 145℃ for 35 to 45 minutes.

[0011] The preferred PE resin is low-density polyethylene (LDPE). The preferred ZnO is nano-ZnO. The preferred nucleating agent is hollow SiO2.

[0012] The preferred foaming agent ADC models are SA3000 or SA7000. Different particle sizes of foaming agent ADCs generally have different applications. Specifically: SA3000 is a fine-particle-size foaming agent ADC, with a particle size of 8µm to 10µm, mainly used for manufacturing ordinary foamed products; SA7000 is an ultra-fine-particle-size foaming agent ADC, with a particle size of 3µm to 5.5µm, mainly used in fields with strict requirements on cell structure.

[0013] In step 5), cooling involves immersing the foam along with the foaming mold in cold water. This helps stabilize the cell size and facilitates subsequent sample demolding.

[0014] The beneficial effects of this invention are: 1) Excellent and stable low dielectric properties: The prepared foamed material has a low dielectric constant. The high volume fraction (up to 75% or more) of uniform closed air bubbles can reduce the overall dielectric constant of the material to 1.3 to 1.6, and the dielectric loss is as low as 0.0007 (38 GHz).

[0015] 2) Fine and uniform microporous structure: Through nucleation control and dynamic cross-linking, a uniform and fine microporous structure is obtained, with an average pore size of about 130 μm.

[0016] 3) Excellent comprehensive mechanical properties: The uniform and dense pores and cross-linked network endow the material with high strength, good resistance to compressive fatigue and a high tensile strength / toughness ratio.

[0017] 4) Green and efficient industrialization path: No expensive equipment is required, the production process is simple and the production efficiency is high, and it achieves a combination of high performance and large-scale, low-cost manufacturing. Detailed Implementation

[0018] Example 1 This embodiment describes a method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses, including the following steps: 1) Weigh 1-6g of ADC powder (model SA3000), 0.2-1g of BIBP, 0.3-0.9g of ZnO, 1-5g of SiO2 and 60g of LDPE resin and mix thoroughly to obtain a mixed powder. 2) Add the mixed powder obtained in step 1) to an oil bath and heat it at 140°C for 40 minutes to melt the LDPE resin into a viscous flow state. After the LDPE resin melts into a viscous flow state, stir and mix it evenly again to obtain a softened mixed resin. 3) Place the mixed resin obtained in step 2) into a foaming mold for foaming. The foaming temperature is 165℃~185℃. Hold the pressure at the foaming temperature for 30min~40min to obtain the foamed solution. 4) Open the foaming mold to release the pressure constraint, so that the gas in the foaming solution will be released due to saturation, and the foaming process will be completed to obtain a foam with countless foam pores. 5) Cool and shape the foam obtained in step 4) to stabilize the cell structure; the cooling in step 5) involves immersing the foam along with the foaming mold in cold water. This prevents the cells from continuing to grow, merge, or collapse due to insufficient melt strength of LDPE. Experiments have shown that immersing the foaming mold in cold water for cooling helps stabilize the cell size and also facilitates subsequent sample demolding.

[0019] 6) Demolding yields porous polyethylene material, which has the advantages of fine and uniform bubbles, dielectric constant that meets the requirements of high-frequency communication, and high structural strength.

[0020] Example 2 To investigate the effect of foaming agent dosage on material structure, the amount of foaming agent ADC (model SA3000) added was systematically varied while maintaining all other process parameters identical to those in Example 1. By comparing the cell structures of porous polyethylene materials obtained with different dosages (e.g., 1g, 2g, 3g, 4g, 5g, 6g), it was found that a uniform and dense closed-cell structure was formed inside the material when the dosage was 3g. Insufficient dosage (e.g., 1g) resulted in insufficient foaming and sparse pores; while excessive dosage (e.g., 4-6g) easily led to cell merging and rupture, forming open cells or structural defects. Therefore, 3g was determined to be the optimal dosage of SA3000 foaming agent.

[0021] Example 3 To investigate the effect of crosslinking agent dosage on material structure, the amount of crosslinking agent BIBP was systematically varied while maintaining all other process parameters identical to those in Example 2. By comparing the cell structures of porous polyethylene materials obtained with different dosages (e.g., 0.2g, 0.6g, 1g), it was found that a uniform and dense closed-cell structure was formed inside the material when the dosage was 0.6g. Excessive BIBP dosage resulted in a high degree of crosslinking and high hardness of the LDPE resin, leading to poor uniformity of the foamed samples; insufficient BIBP dosage disrupted the stability of bubble growth, causing the cell structure to collapse. Therefore, 0.6g was determined to be the optimal amount of crosslinking agent.

[0022] Example 4 To investigate the effect of activator dosage on material structure, the amount of ZnO added was systematically varied while maintaining all other process parameters identical to those in Example 3. By comparing the cell structures of porous polyethylene materials obtained with different dosages (e.g., 0.3g, 0.4g, 0.6g, 0.8g, 0.9g), it was found that a uniform and dense closed-cell structure was formed inside the material when the dosage was 0.8g. Excessive ZnO dosage resulted in larger and less uniform pores in the foamed samples; insufficient ZnO dosage led to insufficient melt strength and unstable bubbles. Therefore, 0.8g was determined to be the optimal activator dosage.

[0023] Example 5 To investigate the effect of nucleating agent dosage on material structure, the amount of SiO2 added was systematically varied while maintaining all other process parameters identical to those in Example 4. By comparing the cell structures of porous polyethylene materials obtained with different dosages (e.g., 1g, 2.5g, 5g), it was found that a uniform and dense closed-cell structure was formed inside the material when the dosage was 2.5g. The nucleating agent SiO2 provides numerous starting points for pore formation, enabling uniform gas nucleation. Furthermore, SiO2 particles can act as a kind of "physical cross-linking," enhancing melt strength and preventing excessive expansion and merging or rupture of bubbles during growth. Insufficient dosage results in insufficient melt strength and a lack of nucleation points, leading to poor cell uniformity. Excessive dosage degrades the material's resistance to compressive fatigue. Therefore, 2.5g was determined to be the optimal dosage of SiO2.

[0024] Example 6 To investigate the effect of foaming temperature on the material structure, the temperature of the foaming system was systematically varied while maintaining all other process parameters identical to those in Example 5. By comparing the cell structures of porous polyethylene materials obtained at different temperatures (e.g., 165°C, 175°C, 180°C, and 185°C), it was found that a uniform and dense closed-cell structure was formed inside the material at a foaming temperature of 180°C. When the foaming temperature was decreased, the uniformity of the closed-cell structure inside the material deteriorated; when the foaming temperature was increased, the cell structure was destroyed. Therefore, the optimal foaming temperature was determined to be 180°C.

[0025] Example 7 The optimal process conditions for the SA3000 foaming agent ADC were determined through Examples 1-6: foaming temperature of 180℃, LDPE resin: foaming agent ADC: crosslinking agent BIBP: activator ZnO: nucleating agent SiO2 = 60:3:0.6:0.8:2.5, and foaming time of 40 min resulted in fine and relatively uniform cells.

[0026] Example 8 The difference from Example 6 is that this example uses a foaming agent with a smaller particle size, ADC (SA7000), which significantly reduces the average pore size of the resulting material and makes the pores finer.

[0027] Example 9 While maintaining all other process parameters identical to Example 8, this example modifies the amount of foaming agent ADC (model SA7000). By comparing the cell structure of porous polyethylene materials obtained with different dosages (e.g., 1g, 2g, 3g, 4g, 5g, 6g), it was found that when the dosage was 3g, a uniform and dense closed-cell structure was formed inside the material. Too little dosage (e.g., 1g) resulted in insufficient foaming and sparse pores; while too much dosage (e.g., 4-6g) easily led to cell merging and rupture, forming open cells or structural defects. Therefore, 3g was determined to be the optimal dosage of SA7000 foaming agent.

[0028] Example 10 To investigate the effect of foaming time on the structure of the foamed material, while maintaining all other process parameters identical to Example 9, the foaming time of Example 9 was shortened. The resulting material showed a significantly reduced average pore size and a denser, more uniform pore distribution. Examples 8-10 determined the optimal process conditions for SA7000 foaming agent ADC: a foaming temperature of 180℃, LDPE resin: foaming agent ADC: crosslinking agent BIBP: activator ZnO: nucleating agent SiO2 = 60:3:0.6:0.8:2.5, and a foaming time of 30 min, resulting in finer and more uniform pores.

[0029] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

[0030] Any aspects of this invention not described in detail are well-known to those skilled in the art.

Claims

1. A method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses, characterized in that: Includes the following steps: 1) Preparation of mixed powder: comprising only foaming agent, crosslinking agent, activator, nucleating agent and resin; wherein, by weight: 10-60 parts of foaming agent, 2-10 parts of crosslinking agent, 3-9 parts of activator, 10-50 parts of nucleating agent and 550-650 parts of resin, thoroughly mixed to obtain mixed powder; wherein the foaming agent is ADC, the crosslinking agent is BIBP, the activator is ZnO, the nucleating agent is talc or SiO2, and the resin is PE resin; 2) Heat the mixed powder obtained in step 1), and after the PE resin melts into a viscous flow state, stir and mix it evenly again to obtain a softened mixed resin. 3) Place the mixed resin obtained in step 2) into a foaming mold for foaming. The foaming temperature is 165℃~185℃. Hold the pressure at the foaming temperature for 30min~40min to obtain the foamed solution. 4) Open the foaming mold to release the pressure constraint, so that the gas in the foaming solution will be released due to saturation, and the foaming process will be completed to obtain a foam with countless foam pores. 5) Cool and shape the foam obtained in step 4) to stabilize the cell structure; 6) Demolding yields porous polyethylene material.

2. The method for preparing a porous polyethylene material for use as an electromagnetic wave lens substrate according to claim 1, characterized in that: The particle size range of the foaming agent ADC is 1µm to 10µm.

3. The method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses according to claim 1, characterized in that: Step 2) involves heating the mixed powder at a temperature of 135℃ to 145℃ for 35 to 45 minutes.

4. The method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses according to claim 1, characterized in that: PE resin is low-density polyethylene (LDPE).

5. The method for preparing a porous polyethylene material for use as an electromagnetic wave lens substrate according to claim 1, characterized in that: ZnO is nano ZnO.

6. The method for preparing a porous polyethylene material for use as an electromagnetic wave lens substrate according to claim 1, characterized in that: The nucleating agent is hollow SiO2.

7. The method for preparing a porous polyethylene material for use as an electromagnetic wave lens substrate according to claim 1, characterized in that: The foaming agent ADC is model SA3000 or SA7000.

8. A method for preparing porous polyethylene material for use as a substrate for electromagnetic wave lenses according to claim 1, characterized in that: The cooling step in step 5 involves immersing the foamed material along with the foaming mold in cold water.