A weather-resistant low-smoke halogen-free cable for photovoltaic power generation

Abstract

The application discloses a weather-resistant low-smoke and halogen-free cable for photovoltaic power generation, which comprises a tinned copper conductor, an irradiation cross-linking halogen-free polyolefin insulation layer and a low-smoke and halogen-free irradiation cross-linking sheath. The sheath is formed by double-layer co-extrusion of a high-filled halogen-free flame-retardant main material and a weather-resistant functional material containing a high-molecular-weight hindered amine light stabilizer and an ultraviolet absorber. After the co-extrusion mold, heat-state insulation is performed, so that the two types of weather-resistant components generate selective radial migration by using the migration ability difference in the compatible polyolefin continuous phase, and then, grading cooling and electron beam irradiation fixation are performed, so that a nested radial gradient sheath with a free radical capturing zone, an ultraviolet attenuation zone, a continuous transition zone and a flame-retardant main zone from outside to inside is formed, so that the low-smoke and halogen-free flame-retardant performance, ultraviolet resistance, heat aging resistance, precipitation resistance, cold and hot cycle anti-cracking and the like are considered.

Description

Technical Field

[0001] This invention relates to the field of wire and cable technology and supporting materials for photovoltaic new energy, specifically to a weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation. Background Technology

[0002] Photovoltaic cables are laid outdoors for extended periods, and their outer sheaths must withstand ultraviolet radiation, high and low temperature cycles, condensation, and mechanical bending. To ensure fire safety, existing products mostly use irradiated cross-linked low-smoke halogen-free polyolefin sheaths, with the addition of inorganic flame-retardant fillers such as magnesium hydroxide and aluminum hydroxide.

[0003] Existing weather-resistant, low-smoke, halogen-free sheaths mainly employ uniform blending of weather-resistant additives or the application of a weather-resistant layer on the outer side of the sheath. Uniform blending results in some weather-resistant additives being distributed on the inner side of the sheath, which is less exposed to ultraviolet radiation. High-filler flame-retardant systems may also increase the adsorption of additives on the surface of inorganic particles and the loss of heat history. Although conventional two-layer co-extrusion can concentrate the weather-resistant components on the outer side, it is usually still treated as a uniform weather-resistant layer.

[0004] Hindered amine light stabilizers are mainly used to suppress surface photo-oxidative free radical reactions, while UV absorbers need to absorb UV energy at a certain depth near the surface; the two also differ in molecular size and migration ability in polyolefins. Existing bilayer co-extruded sheaths are usually cooled directly after demolding, failing to fully utilize this difference to radially distribute the two types of weather-resistant components, thus making it difficult to simultaneously achieve free radical capture on the outermost surface and UV attenuation on the subsurface. Summary of the Invention

[0005] The technical problem this invention aims to solve is that existing low-smoke halogen-free photovoltaic cables, even with weather-resistant materials placed on the outer sheath, typically treat UV absorbers and hindered amine light stabilizers as common additives in the same uniform weather-resistant layer. They lack spatial allocation based on their different action sites and migration capabilities, making it difficult to simultaneously optimize the free radical capture capability of the outermost surface and the UV attenuation capability of the subsurface layer. This invention utilizes the difference in migration rates between high-molecular-weight hindered amine light stabilizers and relatively low-molecular-weight UV absorbers in a compatible polyolefin continuous phase to actively form a misaligned radial concentration gradient within the hot-state window after co-extrusion molding. This gradient state is then locked by electron beam irradiation.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention employs a two-layer co-extrusion process to form a sheath from a flame-retardant base material and a weather-resistant functional material. The flame-retardant base material is a high-filler, low-smoke, halogen-free polyolefin system; the weather-resistant functional material uses a polyolefin carrier compatible with the flame-retardant base material and contains low-volatility oligomerizing or polymerizing hindered amine light stabilizers and benzotriazole or triazine UV absorbers. The number-average molecular weight of the former is preferably 1200–4000, and the relative molecular mass of the latter is preferably 300–800.

[0008] The flame-retardant base material and the weather-resistant functional material are extruded simultaneously using a conventional double-layer co-extrusion equipment. This allows the weather-resistant functional material to form an initial functional area on the outer side of the sheath upon demolding, with an initial thickness of 6% to 18% of the total sheath thickness. This initial functional area is not an independent outer layer that needs to be retained in the end, but rather the starting area where the two types of weather-resistant components selectively migrate.

[0009] After co-extrusion, the sheath is not immediately and completely cooled. Instead, it is fed into a conventional hot air insulation zone at 100-120°C within 2-10 seconds and held for 20-120 seconds. This temperature is lower than the melt temperature of the sheath at the co-extrusion die head, preventing the sheath from flowing as a whole while maintaining sufficient local movement of the polyolefin chain segments.

[0010] Within the aforementioned thermal window, the high molecular weight hindered amine light stabilizer, due to its larger molecular size and higher degree of entanglement with the carrier polyolefin, has limited migration distance into the sheath and mainly remains on the outermost layer. In contrast, the relatively low molecular weight UV absorber can cross the initial co-extrusion interface and migrate deeper into the flame-retardant matrix layer. Using the maximum concentration of each weather-resistant component in the outermost sampling area as a benchmark, the radial depths d1 and d2 when their concentrations are reduced to 50% were determined. The ratio of d2 to d1 was controlled by hot air temperature, residence time, initial thickness of the weather-resistant functional material, and the molecular weight difference between the two weather-resistant components. When d2:d1 is less than 1.3, the effective depths of the two weather-resistant components overlap excessively, making it difficult to establish a clear division of labor between surface free radical capture and subsurface UV attenuation. When d2:d1 is greater than 3.0, the UV absorber migrates too deeply inward, reducing the UV absorption capacity of the outer and near-surface layers, while simultaneously increasing the penetration depth of the low-filled weather-resistant relative flame-retardant matrix layer. Therefore, the ratio of d2:d1 is controlled to be 1.3 to 3.0, preferably 1.5 to 2.5, in order to balance the termination of free radicals on the surface, the ultraviolet attenuation of the subsurface layer, and the continuous retention of the highly filled flame-retardant body on the inner side.

[0011] After selective migration, the material is sequentially cooled in stages using warm water at 40–55°C and cooling water at 20–30°C, followed by electron beam irradiation at 80–110 kGy. Due to the high content of irradiation sensitizer in the weather-resistant functional material, a high degree of cross-linking is formed on the outer surface of the sheath, thereby limiting the long-term precipitation of high molecular weight hindered amine light stabilizers and fixing the misaligned radial distribution of the two types of weather-resistant components.

[0012] The final sheath is formed from the outside in, consisting of a free radical capture zone, a UV attenuation zone, a continuous transition zone, and a flame-retardant main body zone. The free radical capture zone has the highest concentration of hindered amine light stabilizer, the UV attenuation zone maintains a high concentration of UV absorber and extends to deeper positions, and the flame-retardant main body zone maintains a high content of inorganic hydroxide flame-retardant filler. The three types of functions perform different roles layer by layer from the outside in.

[0013] The present invention has the following beneficial effects:

[0014] 1. By utilizing the difference in migration ability between the two types of weather-resistant components to form a misaligned radial gradient, the high molecular weight hindered amine light stabilizer mainly acts on the outermost layer, while the UV absorber extends to the subsurface layer, forming a tandem protection of surface free radical termination and subsurface UV attenuation, thereby improving the space utilization efficiency of the weather-resistant components.

[0015] 2. The flame-retardant main body maintains a high content of inorganic hydroxide flame-retardant filler, and the surface weather-resistant reinforcement does not require an overall reduction in the proportion of flame-retardant filler, which is beneficial to balance low smoke, halogen-free and flame-retardant performance.

[0016] 3. Electron beam irradiation increases the degree of cross-linking of the outer layer of the sheath, which can reduce the migration and precipitation of hindered amine light stabilizers in high temperature and high humidity environments, fix the existing radial distribution, and improve the long-term stability of the weathering gradient.

[0017] 4. Controlling the d2:d1 ratio to 1.3 to 3.0 can avoid excessive overlap in the action depth of the two types of weathering agents or excessive inward migration of UV absorbers, thus maintaining a reasonable separation between surface free radical capture, subsurface UV attenuation, and inner flame retardant effects. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall cross-sectional structure of the weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to the present invention.

[0019] Figure 2 This is a schematic diagram showing the cross-section and radial functional area distribution of the sheath of the weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to the present invention.

[0020] Figure 3 This is a schematic diagram of the formation process of the radial distribution of weather-resistant functional components in the photovoltaic power generation weather-resistant low-smoke halogen-free cable of the present invention. From left to right, it represents the initial state after double-layer co-extrusion molding, the thermal selective migration state, and the final fixed state after graded cooling and electron beam irradiation.

[0021] Explanation of reference numerals in the attached figures: 1. Tinned copper conductor; 2. Irradiated cross-linked halogen-free polyolefin insulation layer; 3. Low-smoke halogen-free irradiated cross-linked sheath; 31. Free radical capture region; 32. Ultraviolet attenuation region; 33. Continuous transition region; 34. Flame retardant main body region. Detailed Implementation

[0022] The following embodiments are used to illustrate the present invention. Unless otherwise specified, all raw materials are industrial-grade materials for wires and cables, and existing mature equipment is used for double-layer co-extrusion, hot air insulation, cooling and electron beam irradiation. The specific equipment structure is not limited.

[0023] like Figure 1 As shown, the photovoltaic power generation weather-resistant low-smoke halogen-free cable of the present invention comprises, from the inside out, a tin-plated copper conductor 1, an irradiated cross-linked halogen-free polyolefin insulation layer 2 covering the tin-plated copper conductor 1, and a low-smoke halogen-free irradiated cross-linked sheath 3 covering the irradiated cross-linked halogen-free polyolefin insulation layer 2. The tin-plated copper conductor 1 is formed by stranding multiple tin-plated annealed copper wires. The insulation layer 2 provides electrical insulation to the conductor 1. The sheath 3 is located on the outermost side of the cable and provides weather resistance, flame retardancy, smoke suppression, and mechanical protection.

[0024] like Figure 2 As shown, the low-smoke halogen-free irradiated crosslinked sheath 3 does not uniformly distribute all functional components along the thickness direction. Instead, it sequentially forms a free radical trapping region 31, an ultraviolet attenuation region 32, a continuous transition region 33, and a flame-retardant main body region 34 from the outside in. The free radical trapping region 31 is located on the outermost side of the sheath, where the relative content of high molecular weight hindered amine light stabilizer is relatively high. The ultraviolet attenuation region 32 is located radially inside the free radical trapping region 31, where the effective distribution depth of the ultraviolet absorber is greater than that of the hindered amine light stabilizer. The continuous transition region 33 is located between the ultraviolet attenuation region 32 and the flame-retardant main body region 34, where the weather-resistant functional components and the flame-retardant main material transition radially continuously. The flame-retardant main body region 34 is adjacent to the insulation layer 2 and maintains a high content of inorganic hydroxide flame-retardant filler. Regions 31 to 34 belong to the same phase-compatible polyolefin continuous phase and do not constitute an independent layered structure that can be mechanically peeled off.

[0025] like Figure 3 As shown, the left side represents the initial state after double-layer co-extrusion molding, with the two types of weather-resistant functional components mainly located on the outer side of the sheath 3; the middle section represents the thermally selective migration state, where the migration front of the UV absorber is more radially inward than that of the high molecular weight hindered amine light stabilizer; the right side represents the fixed state after staged cooling and electron beam irradiation, where the sheath 3 forms a free radical trapping region 31, a UV attenuation region 32, a continuous transition region 33, and a flame-retardant main body region 34 from the outside in. The filling texture in the figure is only used to illustrate component or regional differences and does not indicate the existence of a solid separating layer in the actual product.

[0026] EVA represents ethylene-vinyl acetate copolymer; POE represents ethylene-octene copolymer; LLDPE represents linear low-density polyethylene; MAH-g-PO represents maleic anhydride-grafted polyolefin; TAIC represents triallyl isocyanurate; Mn represents number-average molecular weight; Mr represents relative molecular mass.

[0027] The compatible polyolefin continuous phase referred to in this invention means that the flame retardant main material and the weather-resistant functional material use at least three kinds of the same type of polyolefin as the main continuous phase components. After double-layer co-extrusion and hot treatment, the two materials can form a continuous transition at the original co-extrusion interface without the appearance of continuous voids penetrating the interface, and cannot be completely mechanically peeled into two independent layers along the original co-extrusion interface.

[0028] The high molecular weight hindered amine light stabilizer referred to in this invention is preferably an oligomeric or polymeric HALS with a number average molecular weight of 1200 to 4000, and preferably has low volatility and low water extractability.

[0029] The ultraviolet absorber referred to in this invention is a benzotriazole or triazine ultraviolet absorber with a relative molecular mass of 300–800. For specific material combinations, by selecting an ultraviolet absorber that is compatible with the polyolefin carrier and has a higher effective migration ability within the hot window than the selected high molecular weight hindered amine light stabilizer, the effective enrichment depth of the ultraviolet absorber is greater than that of the hindered amine light stabilizer.

[0030] The 50% concentration decay depth referred to in this invention refers to the radial distance from the outer surface of the sheath when the measured mass concentration of a certain weather-resistant component decreases to 50% of the maximum concentration of that component in the outermost sampling area during continuous sampling from the outer surface of the sheath.

[0031] When comparing the "average mass fraction" of components within a region, this invention first forms a concentration-depth curve based on the measured values ​​of each radial slice and the linear interpolation of adjacent measuring points, and then calculates the average by integrating the corresponding radial intervals: the average value of the interval from the outer surface of the sheath to d1 is taken as the average mass fraction of the free radical capture region 31, the average value of the interval from d1 to d2 is taken as the average mass fraction of the ultraviolet attenuation region 32, and the average value of the inner region adjacent to the insulation layer 2 and whose thickness accounts for 20% of the total thickness of the sheath is taken as the average mass fraction of the flame retardant main body region 34.

[0032] The nested radial weathering gradient referred to in this invention means that both the hindered amine light stabilizer and the ultraviolet absorber exhibit overall attenuation from the outside of the sheath inward, but their attenuation rates are different. The effective enrichment depth of the ultraviolet absorber is greater than that of the hindered amine light stabilizer, thus forming a partially overlapping but primarily functional weathering region.

[0033] This invention provides four sets of embodiments, and the formulations of the flame-retardant main material and weather-resistant functional material for each set of embodiments are shown in the table below:

[0034] Components Example 1 Example 2 Example 3 Example 4 illustrate EVA 45 42 48 45 Flame retardant main material / part POE 25 28 22 25 Flame retardant main material / part LLDPE 20 20 20 18 Flame retardant main material / part MAH-g-PO 10 10 10 12 Flame retardant main material / part Magnesium hydroxide 75 80 70 85 Flame retardant main material / part Aluminum hydroxide 45 40 50 35 Flame retardant main material / part Zinc borate 6 7 6 8 Flame retardant main material / part Organosilicon char-forming aid 4 4 5 5 Flame retardant main material / part antioxidants 0.7 0.6 0.8 0.7 Flame retardant main material / part Metal ion passivating agent 0.2 0.2 0.3 0.3 Flame retardant main material / part TAIC 1.2 1.0 1.5 1.4 Flame retardant main material / part Processing aids 1.0 1.0 1.2 1.2 Flame retardant main material / part

[0035] Table 1: Flame-retardant main material formulation for each embodiment

[0036] In addition, the formulations of the weather-resistant functional materials for each group of embodiments are shown in the table below:

[0037] Components Example 1 Example 2 Example 3 Example 4 illustrate EVA 35 30 40 35 Weather-resistant functional material carrier / part POE 35 40 30 35 Weather-resistant functional material carrier / part MAH-g-PO 20 20 20 20 Weather-resistant functional material carrier / part LLDPE 10 10 10 10 Weather-resistant functional material carrier / part High molecular weight HALS 5.0 4.0 6.5 5.5 Mn 1200~4000 UV absorber 3.5 3.0 4.5 4.0 Mr 300～800 Hindered phenolic antioxidants 1.0 0.8 1.2 1.0 Weather-resistant functional components / parts Phosphite antioxidants 0.5 0.5 0.7 0.6 Weather-resistant functional components / parts TAIC 3.5 3.0 4.5 4.0 Irradiation sensitized component / part Organosilicon processing stabilizers 2.0 1.5 2.5 2.0 Processing stabilized components / parts

[0038] Table 2: Formulation of Weather-Resistant Functional Materials for Each Embodiment

[0039] In Tables 1 and 2, the amounts of EVA, POE, LLDPE, and MAH-g-PO are based on 100 parts by mass of the corresponding polyolefin matrix or carrier polyolefin; the amounts of other functional components are also relative to this 100 parts by mass of polyolefin. When adjusting each component within the stated range, the process judgment condition is that stable extrusion is achieved and the final d2:d1 meets the target range. The polymers, flame retardants, and additives selected for the flame retardant base material and weather-resistant functional material are all halogen-free; halogen-containing polymers or halogen-containing flame retardants are not used.

[0040] The flame-retardant main material is prepared using conventional twin-screw extrusion granulation equipment. Magnesium hydroxide and aluminum hydroxide are dried at 90–100°C to a moisture content of no more than 0.15%, and then mixed with EVA, POE, LLDPE, MAH-g-PO, zinc borate, organosilicon charring aid, antioxidant, metal ion passivating agent, TAIC, and processing aids before being fed into a twin-screw extruder. Dispersion, vacuum devolatilization, and extrusion granulation are completed within a melt temperature range of 145–165°C.

[0041] The weather-resistant functional material is prepared using conventional twin-screw extrusion granulation equipment. First, EVA, POE, MAH-g-PO, and LLDPE are melted. Then, high molecular weight hindered amine light stabilizers, UV absorbers, hindered phenolic antioxidants, phosphite antioxidants, TAIC, and silicone processing stabilizers are added at 125–145°C. The mixture is then extruded and granulated after low to medium shear mixing. The content of inorganic hydroxide flame-retardant fillers in the weather-resistant functional material is lower than that in the flame-retardant bulk material, ensuring that the two types of weather-resistant functional components are mainly present in the continuous polyolefin phase and can undergo controlled migration in subsequent thermal windows.

[0042] Example 1: The flame-retardant base material was prepared using the formulation in Example 1 (Table 1), and the weather-resistant functional material was prepared using the formulation in Example 1 (Table 2). Examples 1-4 all used the same oligomeric hindered amine light stabilizer with a number-average molecular weight of approximately 2500; the same hydroxyphenyltriazine UV absorber was used in all examples with a relative molecular mass of approximately 450. Different radial migration states were obtained in each example by adjusting the amounts added as shown in Tables 1 and 2 and the heat treatment conditions described later, in order to reduce the impact of changes in the chemical type of the weather-resistant agent on the comparative results.

[0043] A tinned copper conductor 1 is formed by stranding multiple tinned annealed copper wires, and a halogen-free polyolefin insulation layer 2 is extruded onto the tinned copper conductor 1 using a conventional insulation extruder.

[0044] To ensure comparability of the radial migration depth and performance results between the embodiments and the comparative examples, the average thickness of the low-smoke halogen-free irradiated crosslinked sheath 3 of each group of samples was controlled to be 1.20 ± 0.05 mm; d1 and d2 will still be expressed as percentages of the total sheath thickness T of the corresponding samples.

[0045] The insulated wire core is fed into a conventional double-sheath co-extrusion unit. The flame-retardant main material is fed by the main extruder, and the weather-resistant material is fed by the auxiliary extruder. The two materials are extruded simultaneously through a mature double-sheath co-extrusion die head. The weather-resistant material is controlled to be located on the outside of the sheath when it exits the die, with an initial thickness of about 10% of the total sheath thickness. The sheath melt temperature at the co-extrusion die head is controlled at 148℃.

[0046] Approximately 4 seconds after leaving the co-extrusion die head, the sheath enters a 105°C hot air insulation channel and remains there for 60 seconds. During this hot window, the high molecular weight hindered amine light stabilizer is primarily retained on the outermost layer, while the UV absorber migrates deeper into the flame-retardant matrix. During production debugging, d1 and d2 are measured using the following continuous slicing method: if d2:d1 is less than 1.3, the hot air temperature is appropriately increased or the dwell time is extended without causing the sheath to remelt completely; if d2:d1 is greater than 3.0, the hot air temperature is decreased or the dwell time is shortened to bring the resulting cable's d2:d1 into the range of 1.3–3.0.

[0047] After heat treatment, the sheath is passed through a 48°C warm water cooling bath and a 25°C cooling water bath in sequence to reduce the surface temperature of the sheath to below 35°C; then it enters an electron beam irradiation device and is irradiated and crosslinked using a 95kGy electron beam in an environment with an oxygen volume fraction of no more than 5%.

[0048] Example 2: The formulations corresponding to Example 2 in Tables 1 and 2 were used. The initial thickness of the weather-resistant functional material upon demolding was approximately 8% of the total sheath thickness, and the melt temperature of the sheath at the co-extrusion die head was 145°C.

[0049] After the sheath is demolded, it enters a 102°C hot air insulation channel for approximately 5 seconds and remains there for 45 seconds. It is then cooled sequentially with 45°C warm water and 25°C cooling water. The electron beam irradiation dose is 90 kGy. This embodiment uses a lower temperature and shorter dwell time to obtain the selective migration state near the lower part of the effective window; the remaining steps, as well as the determination of d1 and d2 and the process judgment methods, are the same as in Example 1.

[0050] Example 3: The formulations corresponding to Example 3 in Tables 1 and 2 were used. The initial thickness of the weather-resistant functional material upon demolding was approximately 15% of the total sheath thickness, and the melt temperature of the sheath at the co-extrusion die head was 152°C.

[0051] After the sheath is demolded, it enters a 112°C hot air insulation channel approximately 3 seconds later and remains there for 85 seconds. It is then cooled sequentially with 52°C warm water and 28°C cooling water. The electron beam irradiation dose is 103 kGy. This embodiment uses a relatively high temperature and a long dwell time to obtain a selective migration state that is close to the upper part of the effective window but does not cause excessive inward migration. The remaining steps, as well as the determination of d1 and d2 and the process judgment methods, are the same as in Example 1.

[0052] Example 4: The formulations corresponding to Example 4 in Tables 1 and 2 were used. The initial thickness of the weather-resistant functional material upon demolding was approximately 12% of the total sheath thickness, and the melt temperature of the sheath at the co-extrusion die head was 150°C.

[0053] After the sheath is demolded, it enters a 108°C hot air insulation channel approximately 4 seconds later and remains there for 70 seconds. It is then cooled sequentially with 50°C warm water and 25°C cooling water. The electron beam irradiation dose is 98 kGy. This embodiment is used to obtain a selective migration state located in the middle of the 1.3–3.0 range; the remaining steps, as well as the determination of d1 and d2 and the process judgment methods, are the same as in Example 1.

[0054] Comparative Example 1—Both weather-resistant components were uniformly added to the flame-retardant base material: The same total amount of high molecular weight hindered amine light stabilizer and UV absorber as in Example 1 was used, but instead of preparing weather-resistant functional materials separately, both types of weather-resistant components were completely added to the highly filled flame-retardant base material for mixing and granulation. Single-layer sheath extrusion, cooling, and 95 kGy electron beam irradiation were then employed. This comparative example was used to compare the differences between nested radial distribution and overall uniform distribution.

[0055] Comparative Example 2—Immediate Cooling After Conventional Two-Layer Co-extrusion: Using the same flame-retardant base material and weather-resistant functional material as Example 1, and the same two-layer co-extrusion conditions, but instead of undergoing 105°C hot air insulation after co-extrusion, the material was immediately placed in a 25°C cooling water bath for shaping, followed by 95 kGy electron beam irradiation. This comparative example was used to verify the effect of thermal selective migration on the dislocation depth distribution of the two types of weather-resistant components.

[0056] Comparative Example 3 – Using two types of weather-resistant components with similar molecular weights: The same co-extrusion, hot-state insulation, and irradiation conditions as in Example 1 were employed, but the oligomeric high-molecular-weight HALS used in Example 1 was replaced with a small-molecule hindered amine light stabilizer with a relative molecular mass of approximately 480. The amount added remained unchanged, ensuring that its molecular size was close to that of the selected UV absorber. This resulted in similar effective migration capabilities of the two types of weather-resistant components in the same polyolefin continuous phase. These conditions brought d1 and d2 closer together, serving as a control with a d2:d1 ratio less than 1.3, to verify the effect of differences in migration capabilities on the depth of dislocation formation.

[0057] Comparative Example 4 – Excessive Hot Air Insulation: Using the same materials and double-layer co-extrusion conditions as Example 1, but increasing the hot air insulation temperature to 125°C and extending the residence time to 180 s, the UV absorber continued to migrate into the flame-retardant matrix region, while the hindered amine light stabilizer also underwent significant inward migration. This condition served as a control for excessive selective migration, evaluating the impact of decreased near-surface UV absorption capacity and the expansion of the influence range of low-filler weather-resistant functional material into the flame-retardant matrix region when d2:d1 exceeded 3.0.

[0058] Comparative Example 5—Reduced Irradiation Sensitizer Content in Weather-Resistant Functional Material: Using the same material system, two-layer co-extrusion, and hot selective migration conditions as Example 1, but reducing the TAIC content in the weather-resistant functional material from 3.5 parts to 0.5 parts, while maintaining the total irradiation dose at 95 kGy. This comparative example was used to verify the effect of a higher degree of surface crosslinking on the long-term fixation and resistance to precipitation of the weather-resistant components after selective migration. The key process conditions for each example and comparative example are shown in the table below:

[0059] plan HALS / UV absorber relationship initial thickness of the outer side Hot treatment Selective migration Irradiation Example 1 High molecular weight HALS / lower Mr UV absorber Approximately 10% 105℃ / 60s obvious 95kGy Example 2 High molecular weight HALS / lower Mr UV absorber Approximately 8% 102℃ / 45s obvious 90kGy Example 3 High molecular weight HALS / lower Mr UV absorber Approximately 15% 112℃ / 85s obvious 103kGy Example 4 High molecular weight HALS / lower Mr UV absorber Approximately 12% 108℃ / 70s obvious 98kGy Comparative Example 1 Same components but overall compounding none none No radial partition 95kGy Comparative Example 2 Same as Example 1 Approximately 10% none Basically none 95kGy Comparative Example 3 The two types of components have similar molecular weights. Approximately 10% 105℃ / 60s weak 95kGy Comparative Example 4 Same as Example 1 Approximately 10% 125℃ / 180s Over-migration 95kGy Comparative Example 5 Same as Example 1 Approximately 10% 105℃ / 60s obvious 95kGy

[0060] Table 3: Key Process Conditions for Examples and Comparative Examples

[0061] The radial distribution and performance verification methods are as follows: Continuous slices are prepared radially from the outside to the inside of the sheath, with each slice's thickness controlled at 2%–5% of the total sheath thickness. External standard quantification curves are established for both hindered amine light stabilizers and UV absorbers. Hindered amine light stabilizers can be quantified using high-performance liquid chromatography-evaporative light scattering (HPLC-UV) detection, while UV absorbers can be quantified using HPLC-UV detection, or equivalent quantification methods with validated recovery rates. All slices are treated under the same extraction conditions, with method recoveries controlled at 90%–110% to minimize the impact of matrix differences at different radial locations on the measurement results.

[0062] Using the maximum concentration of each weather-resistant component in the outermost sampling area as a benchmark, the radial positions at which the concentrations of the hindered amine light stabilizer and the ultraviolet absorber decrease to 50% of their maximum concentrations are determined, yielding d1 and d2, and the d2:d1 ratio is calculated. d2:d1 is used to characterize the radial separation degree of the two types of weather-resistant components. 1.3 to 3.0 is the effective range of this invention; values ​​below or above this range serve as controls for insufficient or excessive migration separation, respectively.

[0063] The relative distribution of magnesium hydroxide and aluminum hydroxide along the thickness direction of the sheath was evaluated by thermogravimetric analysis, ash analysis, or inorganic elemental analysis to confirm that the high-filling flame-retardant main body area still exists continuously.

[0064] The gel content was determined by taking samples from the free radical trapping region and the flame-retardant host region, respectively. The initial mass m0 of the sample was obtained, and after being wrapped in a stainless steel mesh, it was refluxed in boiling xylene for 8 h. After being taken out, it was dried to a constant weight m1, and the gel content was calculated as m1 / m0×100% to evaluate the difference in the degree of crosslinking between the outer layer and the host region after electron beam irradiation.

[0065] All embodiments and comparative examples were tested using the same sampling location, sample size, and equipment procedure. At least three parallel samples were set for each test. The average results are shown in Tables 4 to 7.

[0066] plan d1 / %T d2 / %T d2∶d1 Surface gel / % Main gel / % Difference / Percentage Points Example 1 6.8 13.7 2.01 76.8 69.3 7.5 Example 2 5.4 8.9 1.65 74.6 69.0 5.6 Example 3 9.2 22.4 2.43 80.2 70.8 9.4 Example 4 7.6 15.8 2.08 78.1 69.9 8.2 Comparative Example 1 not applicable not applicable not applicable 69.9 69.2 0.7 Comparative Example 2 8.2 8.7 1.06 76.5 69.1 7.4 Comparative Example 3 11.3 12.4 1.10 76.3 69.0 7.3 Comparative Example 4 8.7 28.7 3.30 77.0 69.4 7.6 Comparative Example 5 6.9 13.9 2.01 70.2 68.9 1.3

[0067] Table 4: Verification Results of Radial Migration and Crosslinking State in Each Example and Comparative Scheme

[0068] In Table 4, T represents the total thickness of the sheath. The d2:d1 ratio in Examples 1-4 is in the range of 1.3-3.0, and the gel content in the surface layer is 5.6-9.4 percentage points higher than that in the main body. The action depths of the two types of weather-resistant components in Comparative Examples 2 and 3 are basically the same. Comparative Example 4 shows excessive inward migration of the ultraviolet absorber. Although Comparative Example 5 still forms a migration depth ratio of about 2.0, the difference in gel content between the surface layer and the main body is reduced to about 1.3 percentage points due to the decrease in the content of the irradiation sensitizer.

[0069] Based on the aforementioned method for calculating the regional average mass fraction, the radial relative contents of HALS, UV absorbers, and inorganic hydroxide flame retardant fillers were statistically analyzed, and the results are as follows.

[0070] plan Radical trapping region HALS / main body HALS UV attenuation region / UV main region Inorganic hydroxides in the main body region / Inorganic hydroxides in the free radical trapping region Example 1 3.7 2.9 1.46 Example 2 3.2 2.4 1.38 Example 3 4.8 4.1 1.61 Example 4 4.2 3.5 1.53 Comparative Example 1 1.1 1.0 1.03 Comparative Example 2 3.5 3.2 1.44 Comparative Example 3 2.7 2.2 1.43 Comparative Example 4 1.9 1.6 1.31 Comparative Example 5 3.6 2.8 1.45

[0071] Table 5: Verification Results of Relative Content of Radial Region Components in Each Example and Comparative Scheme

[0072] Table 5 shows that in Examples 1-4, the average HALS content in the free radical capturing region was 3.2-4.8 times that in the main region, the average UV absorber content in the UV attenuation region was 2.4-4.1 times that in the main region, and the average inorganic hydroxide flame-retardant filler content in the flame-retardant main region was 1.38-1.61 times that in the free radical capturing region, all falling within the radial composition relationship range defined by this invention. Comparative Example 1, due to its overall uniform mixing, did not exhibit significant radial enrichment; in Comparative Example 4, due to excessive migration, the relative enrichment of HALS and UV absorbers in the near-surface layer decreased significantly.

[0073] Ultraviolet aging was accelerated using a UVA-340 light source for 1000 hours, with cyclic conditions of 8 hours of UV irradiation at 60℃ and 4 hours of condensation at 50℃. Tensile strength and elongation at break of the sheath were tested before and after aging, and retention rate was calculated. Surface chalking was evaluated on a scale of 0 to 5, with 0 indicating no visible chalking and 5 indicating severe chalking. High temperature and high humidity testing was conducted at 85℃ and 85% relative humidity for 1000 hours. Samples were taken from the outermost 5% thickness of the sheath before and after the test, and the HALS content was measured. The surface HALS retention rate was calculated as the ratio of the post-test content to the pre-test content.

[0074] plan Tensile strength retention rate / % Elongation at break retention rate / % Surface powdering level Surface HALS retention rate / % Example 1 92.6 89.4 0 93.1 Example 2 90.8 87.1 0～1 91.7 Example 3 94.0 91.2 0 94.2 Example 4 93.2 90.1 0 93.6 Comparative Example 1 79.2 71.8 2 86.0 Comparative Example 2 87.6 82.9 1 89.0 Comparative Example 3 84.8 78.6 1～2 82.5 Comparative Example 4 81.0 74.2 2 73.8 Comparative Example 5 87.8 82.4 1 77.2

[0075] Table 6: Results of UV and High Temperature and Humidity Stability Verification for Each Example and Comparative Scheme

[0076] As shown in Table 6, with similar total weather-resistant component content, the tensile strength retention rate after UV aging of Examples 1-4 was 90.8%-94.0%, and the elongation at break retention rate was 87.1%-91.2%, which were generally higher than those of the comparative examples that were uniformly mixed, had no thermal selective migration, had overlapping migration depths, or had excessive migration. Comparative Example 5 still had a d2:d1 ratio similar to that of Example 1, but its surface HALS retention rate decreased to 77.2%, indicating that increasing the degree of surface crosslinking is beneficial to reducing the migration loss of HALS under high temperature and high humidity conditions, and works in conjunction with the gradient effect of selective migration.

[0077] The thermal cycling test involved 100 cycles between maintaining a temperature of -40℃ for 2 hours and then at 120℃ for 2 hours, with each high-low temperature transition lasting no more than 5 minutes. After each cycle, the sample was bent 20 times with a bending radius of 6 times the cable's outer diameter, and the number of visible cracks on the surface of a 1-meter sample was counted. Simultaneously, single-strand vertical burning, smoke density, light transmittance, and thermal elongation tests were performed on samples from the same batch to confirm that the weathering gradient did not come at the expense of low-smoke flame retardancy and cross-linking stability.

[0078] plan Cracks per location after cyclic bending (m⁻¹) Single vertical burning Smoke transmittance / % Thermal elongation / % Example 1 0 pass 78.5 38 Example 2 0 pass 77.8 40 Example 3 0 pass 78.9 36 Example 4 0 pass 78.2 39 Comparative Example 1 2 pass 77.5 42 Comparative Example 2 1 pass 78.1 39 Comparative Example 3 2 pass 77.9 40 Comparative Example 4 4 pass 77.6 41 Comparative Example 5 1 pass 78.0 55

[0079] Table 7: Verification Results of Thermal Cycling, Low Smoke Flame Retardancy, and Crosslinking Stability for Each Example and Comparative Scheme

[0080] Table 7 shows that no visible cracks appeared in Examples 1-4 after thermal cycling and repeated bending. All examples and comparative examples passed single-piece vertical burning tests, with smoke transmittance concentrated between 77.5% and 78.9%, indicating that when the overall flame retardant system is basically the same, the radial redistribution of weather-resistant components does not significantly weaken the original low-smoke flame retardant properties of the material. Comparative Example 4 showed more cracks after cycling due to excessive heat treatment; Comparative Example 5, due to the reduced content of radiation sensitizer in the weather-resistant functional material, saw its thermal elongation increase to 55%, corresponding to the insufficient increase in surface gel content shown in Table 4.

[0081] During production, the final sheath ratio (d2:d1) is used as the process judgment parameter. For the selected material system and sheath thickness, when d2:d1 is less than 1.3, the hot air insulation temperature can be appropriately increased or the residence time extended; when it is greater than 3.0, the insulation temperature can be decreased, the residence time shortened, or the initial thickness of the weather-resistant functional material reduced; preferably, d2:d1 is controlled between 1.5 and 2.5. Different production lines can adjust the conventional line speed, hot air temperature, and residence time without changing the basic structure of the double-layer co-extrusion, cooling, and irradiation equipment.

[0082] The above embodiments are for illustrative purposes only. Those skilled in the art can make conventional adjustments to the formula ratio and process parameters according to the specific material grade, sheath thickness, and production line conditions. As long as the obtained sheath meets the radial component distribution and functional region relationship described in this invention, this invention can be implemented.

Claims

1. A weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation, comprising, from the inside out, a tin-plated copper conductor (1), an irradiated cross-linked halogen-free polyolefin insulation layer (2), and a low-smoke, halogen-free irradiated cross-linked sheath (3), characterized in that: The low-smoke halogen-free irradiated crosslinked sheath (3) is a nested radial gradient sheath composed of a compatible polyolefin continuous phase, and has a free radical capture region (31), an ultraviolet attenuation region (32), a continuous transition region (33) and a flame retardant main body region (34) in sequence from the outer surface of the sheath (3) inward. The low-smoke halogen-free irradiated crosslinked sheath (3) contains inorganic hydroxide flame-retardant filler; the free radical capture region (31) contains a high molecular weight hindered amine light stabilizer; the ultraviolet attenuation region (32) contains an ultraviolet absorber; and the radial enrichment depth of the hindered amine light stabilizer is less than the radial enrichment depth of the ultraviolet absorber. Taking the outer surface of the sheath (3) as the radial starting point, the depth corresponding to when the concentration of the hindered amine light stabilizer is reduced to 50% of the maximum concentration in its outermost sampling area is defined as d1, and the depth corresponding to when the concentration of the ultraviolet absorber is reduced to 50% of the maximum concentration in its outermost sampling area is defined as d2, where d2:d1 is 1.3 to 3.

0. The content of the inorganic hydroxide flame retardant filler increases in general from the outer surface of the sheath (3) inward, and there is no independent interlayer interface that can be mechanically peeled between the free radical capture zone (31), the ultraviolet attenuation zone (32) and the flame retardant main body zone (34); the nested radial gradient sheath is formed by double-layer co-extrusion of flame retardant main material and weather-resistant functional material, hot selective migration after demolding and fixation by electron beam irradiation.

2. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 1, characterized in that, The hindered amine light stabilizer is a low-volatility oligomer or polymeric hindered amine light stabilizer with a number-average molecular weight of 1200–4000; the ultraviolet absorber is a benzotriazole or triazine ultraviolet absorber with a relative molecular mass of 300–800.

3. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 1, characterized in that, Based on the total thickness of the sheath (3), the 50% concentration attenuation depth d1 of the hindered amine light stabilizer is 3% to 12% of the total thickness of the sheath (3), and the 50% concentration attenuation depth d2 of the ultraviolet absorber is 8% to 30% of the total thickness of the sheath (3). The main range of the free radical capture region (31) extends from the outer surface of the sheath (3) to d1, the main range of the ultraviolet attenuation region (32) extends from d1 inward to d2, the continuous transition region (33) extends from d2 to the flame retardant main region (34), and the flame retardant main region (34) is continuously covered by the irradiated crosslinked halogen-free polyolefin insulation layer (2).

4. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 1, characterized in that, The average mass fraction of hindered amine light stabilizer in the free radical capture region (31) is 2.0 to 6.0 times that of hindered amine light stabilizer in the flame retardant main region (34); the average mass fraction of ultraviolet absorber in the ultraviolet attenuation region (32) is 1.8 to 5.0 times that of ultraviolet absorber in the flame retardant main region (34); and the average mass fraction of inorganic hydroxide flame retardant filler in the flame retardant main region (34) is 1.2 to 2.0 times that of inorganic hydroxide flame retardant filler in the free radical capture region (31).

5. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 1, characterized in that, The flame-retardant matrix material used to form the flame-retardant main body region (34) comprises, based on 100 parts by mass of the total polyolefin matrix, 35-50 parts of ethylene-vinyl acetate copolymer, 20-35 parts of ethylene-octene copolymer, 15-25 parts of linear low-density polyethylene, and 8-15 parts of maleic anhydride-grafted polyolefin, and the total amount of the above four polyolefins is 100 parts; and includes 65-90 parts of magnesium hydroxide, 30-55 parts of aluminum hydroxide, 4-10 parts of zinc borate, 2-6 parts of organosilicon char-forming aid, 0.3-0.8 parts of antioxidant, 0.1-0.4 parts of metal ion passivator, 0.8-2.0 parts of radiation sensitizer, and 0.5-1.5 parts of processing aid.

6. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 1, characterized in that, The weather-resistant functional material forming the free radical trapping region (31) and the ultraviolet attenuation region (32) comprises, based on 100 parts by mass of the total carrier polyolefin, 25-45 parts of ethylene-vinyl acetate copolymer, 25-45 parts of ethylene-octene copolymer, 15-25 parts of maleic anhydride grafted polyolefin, and 5-15 parts of linear low-density polyethylene, and the total amount of the above four carrier polyolefins is 100 parts; and includes 3-8 parts of the hindered amine light stabilizer, 2-6 parts of the ultraviolet absorber, 0.5-1.5 parts of the hindered phenol antioxidant, 0.3-1.0 parts of the phosphite antioxidant, 2-5 parts of triallyl isocyanurate, and 1-3 parts of organosilicon processing stabilizer.

7. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to any one of claims 1 to 6, characterized in that, The nested radial gradient sheath is obtained through the following forming conditions: the flame-retardant main material and the weather-resistant functional material are extruded simultaneously using a conventional double-layer co-extrusion unit, so that the weather-resistant functional material is located outside the sheath (3) when it exits the mold and its initial thickness is 6% to 18% of the total thickness of the sheath (3); after the sheath (3) exits the mold, it enters the hot air insulation zone at 100 to 120°C within 2 to 10 seconds and stays there for 20 to 120 seconds, so that the migration depth of the ultraviolet absorber into the interior of the sheath (3) is greater than the migration depth of the hindered amine light stabilizer; then, it is cooled with warm water at 40 to 55°C and then with cooling water at 20 to 30°C in sequence, and the radial distribution state is fixed by electron beam irradiation at 80 to 110 kGy.

8. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 7, characterized in that, The flame-retardant main material and the weather-resistant functional material use at least three of the same types of polyolefins, namely EVA, POE, LLDPE and maleic anhydride grafted polyolefins, as continuous phases. The content of inorganic hydroxide flame-retardant filler in the weather-resistant functional material is lower than that in the flame-retardant main material, so that the original co-extrusion interface after double-layer co-extrusion and hot selective migration forms a continuous transition interface.

9. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 7, characterized in that, After the thermal selective migration and subsequent electron beam irradiation are completed, when samples are continuously taken along the thickness direction of the sheath (3) and the contents of hindered amine light stabilizer and ultraviolet absorber are measured respectively, the concentration decay rate of hindered amine light stabilizer with increasing radial depth is greater than the concentration decay rate of ultraviolet absorber with increasing radial depth. The d2:d1 ratio is 1.5 to 2.5, and the concentration curves of both do not have abrupt changes corresponding to independent coating boundaries.

10. The weather-resistant, low-smoke, halogen-free cable for photovoltaic power generation according to claim 7, characterized in that, Both the weather-resistant functional material and the flame-retardant main material contain radiation sensitizers, and the radiation sensitizer content in the weather-resistant functional material is higher than that in the flame-retardant main material; after electron beam irradiation, the gel content of the free radical capture region (31) is 3 to 12 percentage points higher than that of the flame-retardant main region (34).

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