PVC (polyvinyl chloride) plate for refrigerator back plate

Abstract

The invention provides a PVC (polyvinyl chloride) plate for a refrigerator back plate, which is prepared from the following raw materials: 50 to 75 parts of PVC resin, 5 to 20 parts of chlorinated polyvinyl chloride, 5 to 15 parts of phosphorus-nitrogen flame retardant, 5 to 20 parts of inorganic flame retardant, 3 to 10 parts of mica powder, 3 to 10 parts of plasticizer, 0.2 to 3 parts of heat stabilizer, 0.2 to 3 parts of lubricant, 0.2 to 3 parts of coupling agent, 0.3 to 3 parts of nucleating agent, 0.5 to 1 part of antioxidant and 2 to 10 parts of processing aid. The phosphorus-nitrogen flame retardant is one or a combination of melamine polyphosphate, ammonium polyphosphate and phosphazene flame retardants; the inorganic flame retardant is one or a combination of aluminum hydroxide, magnesium hydroxide and antimony trioxide. The composition of the flame retardant in the PVC board is regulated and controlled, and the flame retardant property of the refrigerator back board can be greatly improved by compounding the phosphorus-nitrogen intumescent flame retardant and the inorganic flame retardant, so that the flame retardant property of the refrigerator back board reaches the 5VA grade and meets the industrial standard.

Description

Technical Field

[0001] This application relates to the technical field of refrigerators, and particularly to a PVC board for the back panel of a refrigerator. Background Art

[0002] In the refrigerator manufacturing industry, the back panel and bottom panel of a refrigerator are key components, and the selection of their materials directly affects the performance, cost, and market competitiveness of the product. Currently, PVC (Polyvinyl chloride) materials, with advantages such as low cost and convenient processing, account for a relatively high proportion in the materials for refrigerator back panel / bottom panel parts. However, with the continuous upgrading of the IEC safety regulations, extremely strict requirements have been put forward for the PVC materials used in refrigerator back panel / bottom panel. The new industry regulations clearly stipulate that the PVC back panel / bottom panel must reach the flame retardant 5VA level and also perform qualified in the heat resistance test at 75°C for ball pressure. Once the PVC material fails to meet these requirements, manufacturers have to choose metal materials to replace it, which will greatly increase the product cost.

[0003] In the prior art, the PVC materials used for refrigerator back panel / bottom panel have their specific methods in terms of formula and production process. However, due to considerations such as cost and structure, the thickness of PVC parts is generally controlled at 0.6 - 0.8 mm. At this thickness, the current flame retardant grade of PVC parts is 5VB, and when improving the flame retardant performance, conventional means such as adding flame retardants are usually adopted. In terms of heat resistance performance, it mainly depends on the characteristics of PVC itself and some simple additives to maintain a certain heat resistance level.

[0004] In the prior art, there are many problems with the PVC materials used for refrigerator back panel / bottom panel. First, when conducting the 5VA flat sheet burn-through test at a thickness of 0.6 - 0.8 mm, burn-through phenomena are likely to occur, and it cannot meet the requirement of flame retardant 5VA in the new IEC regulations. Second, when trying to improve the flame retardant performance, the heat resistance performance is often sacrificed, and it is very difficult to simultaneously meet the dual requirements of flame retardant 5VA grade and heat resistance at 75°C at this thickness. In addition, due to insufficient heat resistance performance, the PVC back panel is prone to bulging problems after the box body is foamed, affecting the appearance quality of the refrigerator. These problems also limit the application of PVC materials on the refrigerator back panel / bottom panel. Summary of the Invention

[0005] This application provides a PVC board for the back panel of a refrigerator to solve the problem that the performance of the PVC material for the back panel of a refrigerator in the related art does not meet the standards and cannot balance the flame retardant performance and heat resistance performance.

[0006] In a first aspect, the present application provides a PVC sheet for a refrigerator back panel, which is composed of raw materials in the following weight ratios: 50-75 parts of PVC resin, 5-20 parts of chlorinated polyvinyl chloride, 5-15 parts of phosphorus-nitrogen flame retardant, 5-20 parts of inorganic flame retardant, 3-10 parts of mica powder, 3-10 parts of plasticizer, 0.2-3 parts of heat stabilizer, 0.2-3 parts of lubricant, 0.2-3 parts of coupling agent, 0.3-3 parts of nucleating agent, 0.5-1 part of antioxidant, and 2-10 parts of processing aid;

[0007] Among them, the phosphorus-nitrogen flame retardant is one or a combination of melamine polyphosphate, ammonium polyphosphate, and phosphazene flame retardants; the inorganic flame retardant is one or a combination of aluminum hydroxide, magnesium hydroxide, and antimony trioxide.

[0008] In some possible implementation manners, the plasticizer is one or a combination of trioctyl trimellitate, dioctyl phthalate, and dioctyl terephthalate.

[0009] In some possible implementation manners, the lubricant is one of stearic acid-based lubricants and polyethylene wax oxide.

[0010] In some possible implementation manners, the coupling agent is a silane coupling agent or a titanate coupling agent.

[0011] In some possible implementation manners, the nucleating agent is sodium benzoate.

[0012] In some possible implementation manners, the antioxidant is antioxidant 168 and antioxidant 1010.

[0013] In some possible implementation manners, the heat stabilizer is an organotin heat stabilizer.

[0014] In some possible implementation manners, the sheet further includes 0.1-3 parts of nano-montmorillonite or nano-silica, and 0.5-5 parts of boron-based flame retardant synergist; the mass ratio of the boron-based flame retardant synergist to the phosphorus-nitrogen flame retardant is 1:2-1:5.

[0015] In some possible implementation manners, the preparation method of the PVC sheet for the refrigerator back panel described in the first aspect includes:

[0016] Weigh raw materials in a preset mass ratio;

[0017] Put the inorganic flame retardant and mica powder inorganic fillers into a high-speed mixer, add the coupling agent, and mix at a speed of 300-500 r / min for 5-8 min to obtain material A;

[0018] Mix PVC resin, chlorinated polyvinyl chloride, phosphorus-nitrogen flame retardant, plasticizer, heat stabilizer, lubricant, nucleating agent, antioxidant, processing aid and Material A in a certain proportion and add them to a mixer for mixing for 8 - 15 minutes, with the mixing temperature being 135 - 145 °C;

[0019] Inject the mixed material into an extruder, extrude it through the extruder, and form, draw and cut it through a die to obtain a PVC sheet.

[0020] In some possible implementation manners, the temperature of the extruder screw is 145 - 165 °C, and the die forming temperature is 165 - 180 °C.

[0021] From the above content, it can be seen that the present application provides a PVC sheet for a refrigerator back panel, which is composed of raw materials with the following weight ratios: 50 - 75 parts of PVC resin, 5 - 20 parts of chlorinated polyvinyl chloride, 5 - 15 parts of phosphorus-nitrogen flame retardant, 5 - 20 parts of inorganic flame retardant, 3 - 10 parts of mica powder, 3 - 10 parts of plasticizer, 0.2 - 3 parts of heat stabilizer, 0.2 - 3 parts of lubricant, 0.2 - 3 parts of coupling agent, 0.3 - 3 parts of nucleating agent, 0.5 - 1 part of antioxidant, and 2 - 10 parts of processing aid; wherein, the phosphorus-nitrogen flame retardant is one or a combination of melamine polyphosphate, ammonium polyphosphate and phosphazene flame retardants; the inorganic flame retardant is one or a combination of aluminum hydroxide, magnesium hydroxide and antimony trioxide. The present application regulates the composition of the flame retardant in the PVC sheet. By compounding the phosphorus-nitrogen intumescent flame retardant and the inorganic flame retardant, the flame retardant performance of the refrigerator back panel can be greatly improved, so that its flame retardant performance reaches the 5VA level and meets the industry standard; at the same time, the reasonable addition of the inorganic flame retardant has little impact on the mechanical properties of the PVC sheet while improving the flame retardant performance, ensuring the rigid strength of the PVC material and enabling the PVC sheet to take into account both the flame retardant performance and the heat resistance performance. Detailed implementation manners

[0022] The implementation manners described in the following examples do not represent all implementation manners consistent with the present application. They are only examples of systems and methods consistent with some aspects of the present application detailed in the claims.

[0023] In the refrigerator manufacturing industry, the refrigerator back panel and bottom panel, as key components, the selection of their materials directly affects the product's performance, cost, and market competitiveness. Currently, PVC (Polyvinyl chloride) materials, with advantages such as low cost and convenient processing, account for a relatively high proportion among the materials for refrigerator back panel / bottom panel components. However, with the continuous upgrading of the International Electrotechnical Commission (IEC) safety regulations, extremely strict requirements have been put forward for the PVC materials used in refrigerator back panels / bottom panels. The new industry regulations clearly stipulate that PVC back panels / bottom panels must reach the flame retardant 5VA grade and also perform qualified in the heat resistance test at 75°C for ball pressure. Once the PVC material fails to meet these requirements, manufacturers have to choose metal materials as substitutes, which will significantly increase the product cost.

[0024] In the existing technology, the PVC materials used for refrigerator back panels / bottom panels have their specific methods in terms of formulation and production process. However, due to considerations such as cost and structure, the thickness of PVC components is generally controlled at 0.6 - 0.8 mm. At this thickness, the current flame retardant grade of PVC components is 5VB, and when improving the flame retardant performance, conventional means such as adding flame retardants are usually adopted. In terms of heat resistance performance, it mainly relies on the characteristics of PVC itself and some simple additives to maintain a certain heat resistance level.

[0025] In the existing technology, there are many problems with the PVC materials used for refrigerator back panels / bottom panels. First, when conducting the 5VA flat sheet burn-through test at a thickness of 0.6 - 0.8 mm, the burn-through phenomenon is likely to occur, and it cannot meet the requirement of flame retardant 5VA in the new IEC regulations. Second, when attempting to improve the flame retardant performance, the heat resistance performance is often sacrificed, and it is difficult to simultaneously meet the dual requirements of flame retardant 5VA grade and heat resistance at 75°C at this thickness. In addition, due to insufficient heat resistance performance, the PVC back panel is prone to bulging problems after the box body is foamed, affecting the appearance quality of the refrigerator. These problems also limit the application of PVC materials on refrigerator back panels / bottom panels.

[0026] Based on this, the present application provides a PVC sheet for refrigerator back panels, regulates the composition of the flame retardant in the PVC sheet. By compounding a phosphorus-nitrogen type intumescent flame retardant and an inorganic flame retardant, the flame retardant performance of the refrigerator back panel can be significantly improved, making its flame retardant performance reach the 5VA grade and meet the industry standards; at the same time, the reasonable addition of the inorganic flame retardant has less impact on the mechanical properties of the PVC sheet while improving the flame retardant performance, ensuring the rigid strength of the PVC material, so that the PVC sheet can take into account both flame retardant performance and heat resistance performance.

[0027] In some embodiments, the present application provides a PVC sheet for a refrigerator back panel, which is composed of raw materials in the following weight ratios: 50-75 parts of PVC resin, 5-20 parts of chlorinated polyvinyl chloride, 5-15 parts of phosphorus-nitrogen flame retardant, 5-20 parts of inorganic flame retardant, 3-10 parts of mica powder, 3-10 parts of plasticizer, 0.2-3 parts of heat stabilizer, 0.2-3 parts of lubricant, 0.2-3 parts of coupling agent, 0.3-3 parts of nucleating agent, 0.5-1 part of antioxidant, and 2-10 parts of processing aid;

[0028] Among them, the phosphorus-nitrogen flame retardant is one or a combination of melamine polyphosphate, ammonium polyphosphate, and phosphazene flame retardants; the inorganic flame retardant is one or a combination of aluminum hydroxide, magnesium hydroxide, and antimony trioxide.

[0029] In the PVC sheet provided by the present application, the flame retardants in the raw materials are a combination of a compound phosphorus-nitrogen flame retardant (such as melamine polyphosphate, ammonium polyphosphate, and phosphazene flame retardants) and an inorganic flame retardant (such as aluminum hydroxide, magnesium hydroxide, and antimony trioxide), enabling the two flame retardants to exert a synergistic effect. The phosphorus-nitrogen flame retardant forms an expanded carbon layer during combustion, which can isolate the transfer of oxygen and heat; since the inorganic flame retardant decomposes endothermically at high temperatures, the non-combustible gas released when the temperature decreases can dilute the concentration of combustible gases, thereby achieving a flame retardant effect. The synergistic effect of the phosphorus-nitrogen flame retardant and the inorganic flame retardant significantly improves the flame retardant performance of the PVC sheet, reaching the flame retardant 5VA grade, effectively preventing the refrigerator back panel from burning through during a fire, and providing higher safety protection for users.

[0030] Compared with the single use of a certain flame retardant, the compound system of the phosphorus-nitrogen flame retardant and the inorganic flame retardant improves the flame retardant efficiency of the PVC sheet, enabling it to reduce the overall addition amount of the flame retardant while meeting the flame retardant requirements, thereby reducing the raw material cost and minimizing the adverse effects of a large amount of added flame retardant on other properties of the sheet, such as avoiding excessive decline in the mechanical properties of the sheet.

[0031] Adding chlorinated polyvinyl chloride and mica powder to the raw materials of the PVC sheet helps to improve the heat resistance of the PVC sheet. Chlorinated polyvinyl chloride itself has high heat resistance and can improve the thermal stability of the sheet; mica powder has good heat insulation performance and chemical stability, and can form a barrier layer in the sheet, and delay the heat transfer through this isolation layer to achieve heat insulation. The combined action of chlorinated polyvinyl chloride and mica powder significantly increases the heat distortion temperature of the sheet. During high-temperature processes such as refrigerator cabinet foaming, the sheet is not prone to problems such as bulging and deformation, ensuring the appearance quality and dimensional stability of the refrigerator back panel.

[0032] After adjusting the components of the PVC sheet, the prepared PVC sheet can meet the heat resistance requirement of 75°C for ball pressure, ensuring that the refrigerator back panel can still maintain good performance and structural integrity under normal use and high-temperature environments, and extending the service life of the refrigerator.

[0033] By adjusting the components, the impact resistance of the PVC sheet is improved. During the handling, installation, and daily use of the refrigerator, it may be subject to various impacts and collisions. Good impact resistance can ensure that the refrigerator back panel is not easily damaged, protect the components inside the refrigerator, and improve the overall reliability of the refrigerator.

[0034] Among the components of the raw materials of the PVC sheet, there is good compatibility, enabling the PVC sheet to adapt to the existing production processes and equipment for refrigerator back panels. Under process conditions such as the specified mixing temperature, extruder screw temperature, and die forming temperature, the sheet can be smoothly processed, ensuring the stability and consistency of product quality.

[0035] Most of the phosphorus-nitrogen flame retardants and inorganic flame retardants used are environmentally friendly materials and do not produce a large amount of toxic and harmful gases and smoke during combustion, meeting environmental protection requirements. This is not only beneficial to protecting the life and health of users but also in line with the current development trend of green environmental protection in the household appliance industry.

[0036] The adjustment of the components of the PVC sheet in this application achieves a balance of multiple properties. While improving key properties such as flame retardancy and heat resistance, it also takes into account aspects such as mechanical properties, processing properties, and environmental protection properties, making the PVC sheet for refrigerator back panels have more excellent comprehensive properties and better meet market demands and user expectations.

[0037] In some embodiments, the plasticizer is one or a combination of trioctyl trimellitate, dioctyl phthalate, and dioctyl terephthalate.

[0038] Using one or a combination of trioctyl trimellitate (TOTM), dioctyl phthalate (DOP), and dioctyl terephthalate (DOTP) as plasticizers in the PVC sheet for refrigerator back panels can enhance the flexibility and plasticity of the PVC sheet. The molecules of the above plasticizers can insert between the molecular chains of the PVC resin, weaken the intermolecular forces, and increase the mobility of the molecular chains. Taking trioctyl trimellitate as an example, it can lower the glass transition temperature of the PVC sheet and maintain good flexibility even in low-temperature environments, preventing the refrigerator back panel from becoming brittle and cracking due to low temperature. During the production process, the improved plasticity makes the PVC sheet easier to process and form, and the required shape of the refrigerator back panel can be accurately shaped through processes such as extrusion and molding, improving production efficiency and product qualification rate.

[0039] The addition of plasticizers can improve the flexibility and plasticity of the board and prevent the board from cracking during processing and use; inorganic flame retardants such as aluminum hydroxide and magnesium hydroxide can enhance the rigidity of the board to a certain extent; nucleating agents help to refine the grain structure of the board and improve the tensile strength and impact toughness of the board. Under the combined effect, the tensile strength of the board can be maintained in an appropriate range, while having a certain elongation at break, and can withstand a certain external force without damage.

[0040] The use of plasticizers and processing aids improves the fluidity of PVC sheets during processing. In the processing links such as mixing and extrusion, good fluidity allows the material to be evenly dispersed and easier to shape. This helps to improve production efficiency and reduce production costs, while reducing product defects caused by poor processing performance, such as uneven surface and internal bubbles.

[0041] At the same time, plasticizers can improve the fluidity of PVC materials. In the mixing stage, it helps to evenly disperse various raw materials; during extrusion molding, it reduces the friction between the material and the inner wall of the equipment, reduces energy consumption, and makes the extrusion process smoother. For example, the addition of dioctyl phthalate can reduce the viscosity of the PVC melt and increase the melt flow rate. At 160°C, the melt flow rate of PVC materials with an appropriate amount of dioctyl phthalate can be increased by 30%-50% compared to when no dioctyl phthalate is added, ensuring a stable production process and reducing the defective rate.

[0042] Trioctyl trimellitate itself has certain flame retardancy. The ester structure contained in trioctyl trimellitate can inhibit the spread of flames during combustion, and synergistically with phosphorus-nitrogen flame retardants and inorganic flame retardants to further enhance the flame retardant effect of PVC sheets. During the combustion process, the substances produced by the decomposition of trioctyl trimellitate can dilute the concentration of combustible gases, slow down the combustion speed, and make it easier for the sheet to meet the flame retardant 5VA grade requirements.

[0043] Adding an appropriate amount of one or a combination of trioctyl trimellitate (TOTM), dioctyl phthalate (DOP), and dioctyl terephthalate (DOTP) as a plasticizer can ensure tensile strength while increasing elongation at break. For example, after adding dioctyl terephthalate, the tensile strength of the PVC sheet can be maintained at 18-20MPa, and the elongation at break is increased to 15%-18%, making the refrigerator back panel less likely to break when subjected to external force, thus extending its service life.

[0044] At the same time, adding the above plasticizers can reduce the degradation and cross-linking of PVC molecular chains under the influence of heat, light and other factors, and maintain the stability of the board performance. For example, dioctyl phthalate can effectively inhibit the dehydrochlorination reaction of PVC in a high temperature environment, slow down the yellowing and brittleness of the board, and maintain the good appearance and performance of the refrigerator back panel during long-term use.

[0045] In some embodiments, the lubricant is one of stearic acid lubricants and oxidized polyethylene waxes.

[0046] Using stearic acid lubricants or oxidized polyethylene waxes as lubricants in the PVC sheet for refrigerator back panels can improve the processing fluidity, reduce the melt mass viscosity, and enhance the surface quality of the sheet. Stearic acid lubricants and oxidized polyethylene waxes can effectively reduce the viscosity of the PVC melt. During the formation of the PVC melt, the long-chain fatty acid part of the stearic acid lubricant interacts with the PVC molecular chains, reducing the friction between the molecular chains and making the melt flow more easily. Oxidized polyethylene wax, due to its low molecular weight and good dispersibility, is evenly distributed in the PVC melt, playing a role similar to "ball bearings" and further reducing the internal resistance of the melt. During the extrusion process, after adding these two types of lubricants, the flow rate of the PVC melt can be increased by 20% - 30%, greatly improving the processing efficiency, enabling the material to pass through the extruder screw more smoothly, reducing the residence time of the material in the equipment, and reducing the degradation risk caused by overheating.

[0047] Stearic acid lubricants and oxidized polyethylene waxes can improve the surface state of PVC sheets during processing. During extrusion molding, the lubricant forms a lubricating film between the PVC melt and the die surface, reducing the friction between the melt and the die wall and avoiding surface defects such as scratches and flow marks. For PVC sheets processed with stearic acid lubricants, the surface roughness Ra can be reduced to less than 0.8 μm; when using oxidized polyethylene wax, the sheet surface is smoother and more flat, with increased gloss, improving the appearance quality of the refrigerator back panel.

[0048] In some embodiments, the coupling agent is a silane coupling agent or a titanate coupling agent.

[0049] Silane coupling agents and titanate coupling agents have two different types of groups in their molecular structures. One end can chemically react with groups such as hydroxyl groups on the surface of inorganic fillers (such as aluminum hydroxide, mica powder, etc.) to form chemical bonds; the other end can interact with PVC resin molecules and be combined through physical entanglement or chemical reactions. Taking the treatment of mica powder with a silane coupling agent as an example, during the sheet preparation process, the silane coupling agent builds a "bridge" between the mica powder and the PVC resin, enabling the filler to be better dispersed in the resin matrix and enhancing the interfacial bonding force between the two. This tight combination can effectively improve the mechanical properties of the material. After testing, after adding a silane coupling agent, the tensile strength of the PVC sheet can be increased by 15% - 20%, and the flexural strength can be increased by 10% - 15%.

[0050] Through enhanced interfacial bonding, coupling agents can significantly improve the comprehensive properties of PVC sheets. When subjected to external forces, stress can be more effectively transferred between the filler and the resin, avoiding premature material failure caused by interfacial debonding. For PVC sheets added with inorganic flame retardants, the use of coupling agents enables the flame retardants to be tightly bonded to the PVC resin, not only improving the flame retardant performance but also reducing the negative impact on the mechanical properties of the material due to the agglomeration of flame retardants, ensuring that the sheets maintain good mechanical properties while meeting the flame retardant requirements.

[0051] In some embodiments, the processing aids are preferably one or a combination of chlorinated polyethylene (CPE) and acrylate processing aids (ACR).

[0052] Using chlorinated polyethylene (CPE), acrylate processing aids (ACR) or a combination thereof as processing aids in PVC sheets for refrigerator back panels can optimize the processing performance and improve the melt fluidity.

[0053] Both CPE and ACR can significantly reduce the viscosity of the PVC melt and enhance its fluidity. The chlorine atoms in the CPE molecular structure can interact with the PVC molecules, weakening the intermolecular forces and making the melt flow more easily; ACR, through its special molecular structure, plays an internal lubrication role in the PVC melt, reducing the friction between molecular chains. During extrusion processing, after adding CPE or ACR, the melt flow rate of PVC can be increased by 25%-40%, allowing the material to pass through the extruder more smoothly, reducing the processing energy consumption, improving the production efficiency, and at the same time reducing product defects caused by poor melt flow, such as uneven surfaces and internal bubbles.

[0054] These two types of processing aids can accelerate the plasticization process of PVC and make it more uniform. ACR can promote the plasticization of PVC at a lower temperature, reduce the plasticization temperature and time, and reduce the degradation risk of PVC at high temperature; CPE can improve the melt uniformity during the PVC plasticization process and ensure consistent properties of each part of the PVC sheet. In actual production, after adding processing aids, the plasticization time of PVC can be shortened by 15%-20%, and the plasticization temperature can be reduced by 5-10°C, effectively improving the product quality stability. Both CPE and ACR have a toughening effect and can effectively improve the impact strength of PVC sheets. The rubber elastomer characteristics of CPE can absorb impact energy. When subjected to external impact, CPE particles can initiate crazes and shear bands, consuming a large amount of energy, thereby improving the impact resistance of the sheet; ACR enhances the toughness of the material by forming an interpenetrating network structure with PVC. After adding CPE or ACR, the notched impact strength of PVC sheets can be increased by 30%-50%, making the refrigerator back panel more impact-resistant during handling, installation, and use, not easily broken, and extending its service life. While improving the impact strength, CPE and ACR can balance the tensile strength of PVC sheets to a certain extent. ACR can enhance the interaction between PVC molecular chains and improve the tensile strength; while toughening, reasonable addition of CPE will not excessively reduce the tensile strength. After optimizing the formula, the tensile strength of PVC sheets using these two types of processing aids can be maintained at 18-22 MPa, meeting the mechanical property requirements of refrigerator back panels.

[0055] In some embodiments, the nucleating agent is sodium benzoate.

[0056] Sodium benzoate can act as a heterogeneous nucleation center in the PVC system, prompting PVC molecules to rapidly crystallize around it. Compared with PVC without a nucleating agent, after adding sodium benzoate, the crystallization rate of PVC increases, the number of formed crystal grains increases, and the size is significantly refined. Through scanning electron microscope observation, when sodium benzoate is not added, the average crystal grain size of PVC is about 20-30 μm; after adding an appropriate amount of sodium benzoate, the average crystal grain size can be refined to 5-10 μm. The refined crystal grain structure makes the internal structure of PVC sheets more uniform and dense, effectively improving the comprehensive properties of the material.

[0057] The refined grain structure can transfer stress more effectively and reduce stress concentration. When the PVC sheet is subjected to a tensile force, numerous small and uniform grains cooperate to bear the load, significantly enhancing the tensile strength of the material. Experimental data shows that after adding sodium benzoate as a nucleating agent, the tensile strength of the PVC sheet can be increased by 15%-20%. In some embodiments, the tensile strength can be increased from the original 16 MPa to 18-19 MPa, meeting the strength requirements of the refrigerator back panel during actual use and making it more resistant to external pulling and less likely to break. The small grains increase the grain boundary area, and the grain boundaries can absorb more energy when the material is impacted, triggering more crazes and shear bands, thus improving the impact toughness of the material. After adding sodium benzoate, the notched impact strength of the PVC sheet can be increased by 20%-30%, enhancing the anti-breakage ability of the refrigerator back panel when it is accidentally impacted, reducing the risk of cracking caused by impact, and ensuring the normal use and service life of the refrigerator.

[0058] Sodium benzoate as a nucleating agent can increase the crystallization temperature of PVC, enabling PVC to start crystallizing at a higher temperature. This means that during the processing, the PVC sheet can solidify and form faster, shortening the forming cycle. In the extrusion molding process, after adding sodium benzoate, the crystallization temperature of PVC can be increased by 5-10 °C, effectively improving production efficiency and reducing production costs. Due to the increase in crystallization temperature and grain refinement, the crystallization of the PVC sheet is more perfect, and the molecular chain arrangement is more regular. This makes the size change of the sheet smaller in different temperature environments, improving the dimensional stability. During the long-term use of the refrigerator back panel, it may experience temperature fluctuations. The PVC sheet with stable dimensions can better cooperate with other components of the refrigerator, avoiding problems such as deformation and warping caused by thermal expansion and contraction, and ensuring the overall structural integrity and sealing of the refrigerator.

[0059] In some embodiments, the antioxidant is antioxidant 168 and antioxidant 1010.

[0060] Antioxidant 168 is a phosphite antioxidant, mainly acting as a secondary antioxidant. It can effectively decompose the hydroperoxides generated during the processing and use of PVC, preventing them from further decomposing to produce free radicals, thereby interrupting the oxidation chain reaction. Antioxidant 1010 belongs to the hindered phenol primary antioxidant. It can capture the free radicals generated during the oxidation of PVC and form stable compounds, inhibiting the oxidation reaction initiated by free radicals. When the two are used in combination, they exert a synergistic effect and can significantly inhibit the oxidative degradation of the PVC sheet. In the hot air aging experiment, the tensile strength of the PVC sheet without adding antioxidants decreased by 50% after aging at 100 °C for 200 hours; while for the PVC sheet added with the mixture of antioxidant 168 and antioxidant 1010, after aging under the same conditions for 500 hours, the tensile strength can still remain above 80% of the initial value, greatly extending the service life of the refrigerator back panel.

[0061] During the processing of PVC, oxidation is accelerated due to factors such as high temperature and mechanical shear. The addition of antioxidant 168 and antioxidant 1010 can effectively improve the stability of PVC sheets during processing. Antioxidant 168 can rapidly decompose the hydroperoxides generated during processing, reducing their damage to the PVC molecular chain; while antioxidant 1010 promptly captures free radicals, preventing the breakage and cross-linking of molecular chains. During the extrusion process, after adding these two antioxidants, the fluidity of the PVC melt becomes more stable, reducing the changes in melt viscosity and the generation of degradation products caused by oxidation, lowering the processing difficulty, and improving the molding quality and production efficiency of the product.

[0062] During the use of the refrigerator back panel, it is affected by various environmental factors such as light, oxygen, and temperature changes, and is prone to aging. The compound system of antioxidant 168 and antioxidant 1010 can effectively resist the oxidative aging caused by these environmental factors. The ability of antioxidant 1010 to capture free radicals plays an important role in resisting the oxidation reaction induced by light, while the property of antioxidant 168 to decompose hydroperoxides helps to stabilize the performance of PVC under different temperature conditions. After the artificial accelerated aging test (simulating environments such as light, high temperature, and humidity), for the PVC sheet added with this compound antioxidant, after aging for 1000 hours, the surface only slightly changes color and the mechanical properties remain good; while for the sheet without antioxidant, serious yellowing and embrittlement occur, and the mechanical properties significantly decline.

[0063] In some embodiments, the heat stabilizer is an organotin heat stabilizer. The organotin heat stabilizer is one of dibutyltin dilaurate (DBTL), dibutyltin distearate (DBTS), dibutyltin maleate, and monobutyltin maleate. The above organotin heat stabilizers can all enhance the heat stability performance.

[0064] Taking dibutyltin distearate (DBTS) as an example, during the long-term use of the PVC sheet, DBTS can continuously play a stabilizing role. It can react with the unstable structures on the PVC molecular chain to make it more stable, thereby effectively delaying the degradation rate of PVC. After a long-term high-temperature aging test (such as placing at 80 °C for 1000 hours), for the PVC sheet added with DBTS, the decline in mechanical properties such as tensile strength and elongation at break is significantly less than that of the sheet without addition, which ensures the performance stability of the refrigerator back panel during long-term use.

[0065] Dibutyltin maleate has a significant effect on improving the thermal stability of PVC sheets. It can stabilize the PVC molecular chain during high-temperature processing and reduce the occurrence of thermal degradation. At the same time, the maleic acid group in its structure can absorb ultraviolet light and effectively resist photooxidation. In outdoor aging tests (simulating sunlight irradiation and temperature changes), after 500 hours of testing, the PVC sheets added with dibutyltin maleate only showed slight discoloration and powdering on the surface, while the sheets without addition showed severe fading and embrittlement. For the refrigerator back panel that may be exposed to a certain amount of light, it can effectively extend its service life and maintain its appearance quality.

[0066] In some embodiments, the nucleating agent is sodium benzoate. Since sodium benzoate provides numerous nucleation sites, the number of crystal grains formed during the crystallization of the material increases, the crystal grain size becomes smaller and the distribution is more uniform. For polypropylene (PP) materials, after adding sodium benzoate, its crystal grain size can be refined to about half of the original. The refined crystal grains can improve the mechanical properties of the material, such as increasing the strength and toughness of the material, and enhancing the comprehensive performance of the material.

[0067] In some embodiments, the sheet further comprises 0.1-3 parts of nano-montmorillonite or nano-silica, and 0.5-5 parts of boron-based flame retardant synergist; the mass ratio of the boron-based flame retardant synergist to the phosphorus-nitrogen flame retardant is 1:2-1:5.

[0068] Introducing nano-level materials such as nano-montmorillonite and nano-silica can increase the mechanical properties of PVC sheets. Nano-montmorillonite has a large specific surface area and good barrier properties. When added to PVC sheets, it can form a barrier layer at the microscopic level, delaying the transfer of heat and gas during combustion and further enhancing the flame retardant performance; nano-silica can enhance the mechanical properties of the material, increase the hardness and wear resistance of the sheets, making them more scratch-resistant during the use of the refrigerator back panel and extending the service life. Nano-montmorillonite or nano-silica has an extremely large specific surface area and strong surface activity, and can form a good interfacial bond with the sheet matrix. Taking nano-silica as an example, it can be evenly dispersed in the sheet, like "nano-scale steel bars", hindering the expansion of microcracks inside the sheet, thereby increasing the strength and toughness of the sheet. In some studies, after adding an appropriate amount of nano-silica, the tensile strength of the sheet can be increased by 20%-30%, and the impact strength can also be significantly improved.

[0069] Improving thermal stability: Nano-montmorillonite has a unique layered structure that can act as a barrier when the board is heated, delaying heat transfer and inhibiting the thermal degradation of the board. When a boron-based flame retardant synergist is used in combination with a phosphorus-nitrogen flame retardant, a synergistic flame retardant effect can be produced. Boron-based compounds can form a glassy protective film during combustion, covering the surface of the board, isolating oxygen and heat, and at the same time promoting the decomposition and carbonization reactions of the phosphorus-nitrogen flame retardant to form a denser carbon layer, preventing the transfer of heat and combustible gases. When the mass ratio of the boron-based flame retardant synergist to the phosphorus-nitrogen flame retardant is in the range of 1:2 - 1:5, the flame retardant performance of the board can be significantly improved, and the oxygen index can be increased by 5 - 10 percentage points, reaching the difficult-to-combust level. Nano-silica can increase the glass transition temperature of the board, enabling the board to maintain good dimensional stability and mechanical properties at high temperatures. For example, the thermal decomposition temperature of the board with nano-montmorillonite added can be increased by 30 - 50 °C, effectively enhancing the thermal stability of the board.

[0070] Adding a new type of flame retardant synergist, such as a boron-based compound with a special structure. Such compounds can have a synergistic effect with phosphorus-nitrogen flame retardants and inorganic flame retardants, significantly improving the flame retardant effect while reducing the total addition amount of flame retardants.

[0071] In some embodiments, the present application also provides a method for preparing the PVC board described in the above embodiments for the refrigerator back panel, including:

[0072] Weighing raw materials in a preset mass ratio;

[0073] Putting the inorganic flame retardant and mica powder inorganic filler into a high-speed mixer, adding a coupling agent, and mixing at a speed of 300 - 500 r / min for 5 - 8 min to obtain material A;

[0074] Adding PVC resin, chlorinated polyvinyl chloride, phosphorus-nitrogen flame retardant, plasticizer, heat stabilizer, lubricant, nucleating agent, antioxidant, processing aid, and material A into a mixer according to a certain ratio and mixing for 8 - 15 minutes, and the mixing temperature is 135 - 145 °C;

[0075] Injecting the mixed material into an extruder, extruding through the extruder, forming, pulling, and cutting through a mold to obtain a PVC board.

[0076] In some embodiments, the temperature of the extruder screw is 145 - 165 °C, and the temperature of the mold forming is 165 - 180 °C.

[0077] Example 1:

[0078] A method for preparing a PVC board, including the following steps:

[0079] Weighing raw materials in a preset mass ratio;

[0080] Put inorganic fillers such as aluminum hydroxide, antimony trioxide, and mica powder into a high-speed mixer, add a coupling agent, and mix at a speed of 300 - 500 r / min for 5 - 8 min to obtain material A;

[0081] According to a certain proportion, add PVC resin, chlorinated polyvinyl chloride, melamine polyphosphate, trioctyl trimellitate, heat stabilizer, lubricant, nucleating agent, antioxidant, processing aid, and material A into a kneader and knead for 8 - 15 minutes, and the kneading temperature is 135 - 145 °C;

[0082] Inject the kneaded material into an extruder, extrude it through the extruder, and form, draw, and cut it through a mold to obtain a PVC sheet.

[0083] Example 2:

[0084] A method for preparing a PVC sheet, comprising the following steps:

[0085] Weigh raw materials according to a preset mass ratio;

[0086] Put inorganic fillers such as aluminum hydroxide, magnesium hydroxide, antimony trioxide, and mica powder into a high-speed mixer, add a coupling agent, and mix at a speed of 300 - 500 r / min for 5 - 8 min to obtain material A;

[0087] According to a certain proportion, add PVC resin, chlorinated polyvinyl chloride, melamine polyphosphate, trioctyl trimellitate, dioctyl terephthalate, heat stabilizer, lubricant, nucleating agent, antioxidant, processing aid, and material A into a kneader and knead for 8 - 15 minutes, and the kneading temperature is 135 - 145 °C;

[0088] Inject the kneaded material into an extruder, extrude it through the extruder, and form, draw, and cut it through a mold to obtain a PVC sheet.

[0089] Example 3:

[0090] A method for preparing a PVC sheet, comprising the following steps:

[0091] Weigh raw materials according to a preset mass ratio;

[0092] Put inorganic fillers such as aluminum hydroxide, antimony trioxide, and mica powder into a high-speed mixer, add a coupling agent, and mix at a speed of 300 - 500 r / min for 5 - 8 min to obtain material A;

[0093] According to a certain proportion, add PVC resin, chlorinated polyvinyl chloride, melamine polyphosphate, trioctyl trimellitate, heat stabilizer, lubricant, nucleating agent, antioxidant, processing aid, and material A into a kneader and knead for 8 - 15 minutes, and the kneading temperature is 135 - 145 °C;

[0094] Inject the kneaded material into an extruder. After extrusion by the extruder, it is formed, drawn, and cut by a mold to obtain a PVC sheet.

[0095] The raw material compositions in Examples 1 to 3 are shown in Table 1.

[0096] Table 1

[0097]

[0098]

[0099] Performance Test

[0100] Take Examples 1 to 3 and conduct tensile strength, ball pressure performance, and flame retardancy performance tests on them. The test results are shown in Table 2 below.

[0101] Table 2

[0102] Performance Unit Example 1 Example 2 Example 3 Tensile strength MPa 18 16 20 Elongation at break ％ 16 15 12 Ball pressure performance at 75°C / Meet Meet Meet Flame retardancy / 5VA 5VA 5VA

[0103] From the performance test results of Examples 1 to 3, it can be seen that for the raw material components of the PVC sheet regulated in this application, the flame retardancy performance is improved through the compound system of phosphorus-nitrogen flame retardant and inorganic flame retardant. The addition of chlorinated polyvinyl chloride and mica powder helps to improve the heat resistance of the PVC sheet, enabling the PVC sheet to have both flame retardancy performance and heat resistance performance. And by adding any one of trioctyl trimellitate, dioctyl phthalate, and dioctyl terephthalate as a plasticizer in the raw materials, the PVC sheet maintains good flexibility and improves the tensile strength.

[0104] As can be seen from the above examples, this application provides a PVC sheet for a refrigerator back panel, which is composed of raw materials with the following weight ratios: 50 - 75 parts of PVC resin, 5 - 20 parts of chlorinated polyvinyl chloride, 5 - 15 parts of phosphorus-nitrogen flame retardant, 5 - 20 parts of inorganic flame retardant, 3 - 10 parts of mica powder, 3 - 10 parts of plasticizer, 0.2 - 3 parts of heat stabilizer, 0.2 - 3 parts of lubricant, 0.2 - 3 parts of coupling agent, 0.3 - 3 parts of nucleating agent, 0.5 - 1 part of antioxidant, and 2 - 10 parts of processing aid; wherein, the phosphorus-nitrogen flame retardant is one or a combination of melamine polyphosphate, ammonium polyphosphate, and phosphazene flame retardants; the inorganic flame retardant is one or a combination of aluminum hydroxide, magnesium hydroxide, and antimony trioxide. This application regulates the composition of the flame retardant in the PVC sheet. By compounding the phosphorus-nitrogen intumescent flame retardant and the inorganic flame retardant, the flame retardancy performance of the refrigerator back panel can be greatly improved, making its flame retardancy performance reach the 5VA level and meet the industry standards; at the same time, the reasonable addition of the inorganic flame retardant has little impact on the mechanical properties of the PVC sheet while improving the flame retardancy performance, ensuring the rigid strength of the PVC material and enabling the PVC sheet to have both flame retardancy performance and heat resistance performance.

[0105] For the similar parts between the embodiments provided in this application, reference can be made to each other. The specific embodiments provided above are only several examples under the general concept of this application and do not constitute a limitation on the protection scope of this application. For those skilled in the art, any other embodiments extended based on the solution of this application without creative efforts fall within the protection scope of this application.

Claims

1. A PVC sheet for refrigerator back panel, characterized in that: The invention is composed of the following raw materials in weight ratio: 50-75 parts of PVC resin, 5-20 parts of chlorinated polyvinyl chloride, 5-15 parts of phosphorus-nitrogen flame retardant, 5-20 parts of inorganic flame retardant, 3-10 parts of mica powder, 3-10 parts of plasticizer, 0.2-3 parts of heat stabilizer, 0.2-3 parts of lubricant, 0.2-3 parts of coupling agent, 0.3-3 parts of nucleating agent, 0.5-1 parts of antioxidant and 2-10 parts of processing aid; Wherein, the phosphorus-nitrogen flame retardant is one or a combination of melamine polyphosphate, ammonium polyphosphate and phosphazene flame retardants; and the inorganic flame retardant is one or a combination of aluminum hydroxide, magnesium hydroxide and antimony trioxide.

2. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The plasticizer is one or a combination of trioctyl trimellitate, dioctyl phthalate, and dioctyl terephthalate.

3. The PVC sheet for refrigerator back panel according to claim 2, characterized in that: The lubricant is one of stearic acid lubricants and oxidized polyethylene wax.

4. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The coupling agent is a silane coupling agent or a titanate coupling agent.

5. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The nucleating agent is sodium benzoate.

6. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The antioxidants are antioxidant 168 and antioxidant 1010.

7. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The heat stabilizer is an organic tin heat stabilizer.

8. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The plate also includes 0.1 to 3 parts of nano-montmorillonite or nano-silicon dioxide and 0.5 to 5 parts of a boron-based flame retardant synergist; the mass ratio of the boron-based flame retardant synergist to the phosphorus-nitrogen flame retardant is 1:2 to 1:

5.

9. The PVC sheet for refrigerator back panel according to claim 1, characterized in that: The preparation method of the PVC sheet for the refrigerator back plate comprises: Weighing raw materials with a preset mass ratio; Put the inorganic flame retardant and mica powder inorganic filler into a high-speed mixer, add a coupling agent, and mix at a speed of 300-500 r / min for 5-8 minutes to obtain material A; According to a certain proportion, PVC resin, chlorinated polyvinyl chloride, phosphorus nitrogen flame retardant, plasticizer, heat stabilizer, lubricant, nucleating agent, antioxidant, processing aid and material A are added into an internal mixer and mixed for 8 to 15 minutes at a mixing temperature of 135 to 145°C; The mixed material is injected into an extruder, extruded by the extruder, molded, pulled and cut to obtain a PVC sheet.

10. The PVC sheet for refrigerator back panel according to claim 9, characterized in that: The extruder screw temperature is 145-165°C, and the mold molding temperature is 165-180°C.

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