Preparation method of high-flame-retardant plastic and application of high-flame-retardant plastic in earphone shells

Abstract

The invention provides a preparation method of high-flame-retardant plastic and application of the high-flame-retardant plastic in an earphone shell. The preparation method comprises the following steps: S1, melting and mixing polypropylene, ethylene propylene diene monomer, high-molecular-weight silicone of which the weight-average molecular weight is greater than 500,000, a phenolic resin cross-linking agent and a Lewis acid catalyst; and S2, heating the mixture obtained in the step S1 to 200-220 DEG C under the shearing action, and carrying out dynamic vulcanization, so that the phenolic resin cross-linking agent catalyzes the ethylene propylene diene monomer to generate cross-linking. According to the method, through innovative material system design and process cooperation, effective structure-effect locking of high-molecular-weight silicone and complete protection of a polypropylene matrix are realized, so that the industrial problem of contradiction between flame retardance, precipitation and matrix degradation is solved synergistically.

Description

Technical Field

[0001] This application relates to the field of polymer material modification technology, specifically to a method for preparing a highly flame-retardant plastic and its application in headphone shells. Background Technology

[0002] Polypropylene (PP), a general-purpose thermoplastic, is widely used in automotive interiors, appliance housings, and electronic devices due to its excellent mechanical properties, processing performance, and cost-effectiveness. As electronic products become increasingly thinner and more integrated, higher requirements are being placed on the flame retardant safety levels of the plastic materials used. To meet stringent flame retardant standards such as UL-94 V-0, flame retardant modification of polypropylene is usually necessary. Among various flame retardant systems, silicone-containing flame retardants have attracted much attention due to their high efficiency, low toxicity, and smoke suppression properties; however, technical bottlenecks in their application limit their further promotion in high-end fields.

[0003] In existing technologies, low molecular weight silicone oils or common silicone additives are often added to flame-retardant polypropylene systems to improve flame retardancy efficiency and processing fluidity. However, these small-molecule silicones have poor compatibility with the polypropylene matrix and are prone to migration and blooming or oil seepage on the product surface under long-term use or high-temperature environments. This precipitation not only seriously affects the appearance quality of the product but also causes the flame retardant performance to decay over time, posing a potential safety hazard. In addition, traditional polypropylene / rubber dynamic vulcanization systems often use peroxides (such as DCP) as initiators. While crosslinking the rubber phase, these initiators inevitably induce β-fracture of the polypropylene backbone, leading to a significant decrease in the molecular weight of the matrix and a substantial deterioration in the mechanical properties of the material, especially impact strength, accompanied by the generation of an irritating odor.

[0004] Therefore, how to develop a new type of flame-retardant polypropylene material that can completely solve the problem of silicone additive precipitation while ensuring high flame retardancy and excellent impact toughness, and avoid damage to the polypropylene matrix during processing, has become an urgent technical problem to be solved in the field of polymer materials. Summary of the Invention

[0005] To address the aforementioned technical problems, this application provides a method for preparing highly flame-retardant plastic and its application in headphone shells.

[0006] The first aspect of this application provides a method for preparing a high flame retardant plastic, comprising the following steps: Step S1: melt mixing polypropylene, EPDM rubber, high molecular weight silicone with a weight average molecular weight greater than 500,000, phenolic resin crosslinking agent and Lewis acid catalyst; Step S2: heating the mixture obtained in Step S1 to 200-220°C under shear and performing dynamic vulcanization, so that the phenolic resin crosslinking agent catalyzes the crosslinking of the EPDM rubber.

[0007] This method, through innovative material system design and process coordination, achieves effective structure-property locking of high molecular weight silicone and complete protection of the polypropylene matrix, thus synergistically solving the industry problem of conflicting flame retardancy, exudation, and matrix degradation. Specifically, this invention abandons conventional peroxide initiators and instead uses a phenolic resin-Lewis acid catalytic system for the reaction. This system has high selectivity, only targeting the unsaturated double bonds on the EPDM rubber backbone for cationic crosslinking, exhibiting chemical inertness to the polypropylene matrix and silicone components. This mechanism effectively avoids β-chain scission degradation caused by free radical attack on the tertiary carbon atoms of polypropylene in traditional peroxide processes, thereby completely preserving the molecular chain length of polypropylene and ensuring a high level of basic mechanical properties of the material. Secondly, this invention utilizes ultra-high molecular weight silicone with a weight-average molecular weight greater than 500,000 as a functional additive, taking advantage of its huge molecular chain radius of rotation, which is much larger than the critical pore size formed by the EPDM crosslinking network during dynamic vulcanization, to construct a "semi-interpenetrating network" topology. In this structure, silicone macromolecular chains are physically "woven" and imprisoned within a dense cross-linked rubber phase, preventing them from escaping to the matrix surface through thermal peristalsis, thus fundamentally eliminating blooming caused by silicone precipitation. Furthermore, in materials prepared using this system, the uniform dispersion of cross-linked EPDM microparticles provides excellent toughening effects, while the silicone within the network migrates to the surface during combustion to participate in char formation, forming a dense and stable ceramicized char layer with the intumescent flame retardant. This endows the material with superior flame retardancy, high toughness, and long-term service stability.

[0008] Furthermore, step S1 also includes an intumescent flame retardant, which is added at the side feed port of the twin-screw extruder. Adding the flame retardant via side feeding avoids premature exposure to high-intensity shear and high temperatures, effectively protecting the particle size and structural integrity of the flame retardant (especially the coated flame retardant), and ensuring full utilization of its flame retardant efficiency.

[0009] Furthermore, in step S2, the dynamic vulcanization temperature is 200-210℃. This temperature range ensures the full crosslinking reaction of the phenolic resin, forming a dense EPDM rubber network, while avoiding thermal degradation of the polypropylene matrix due to excessively high temperatures, thus achieving an optimized balance between process and performance.

[0010] Furthermore, the dynamic vulcanization process in step S2 is carried out in the high-shear screw element section of the twin-screw extruder. The high-shear action can promote the rapid and uniform mixing and reaction of the phenolic resin crosslinking agent and EPDM rubber, ensuring the uniformity of the rubber phase crosslinking, thereby obtaining a final product with stable performance.

[0011] Furthermore, the melt mixing in step S1 takes place in the main feeding zone of the twin-screw extruder, and the raw materials also include a premixing step before the main feeding. Premixing can improve the initial dispersion uniformity of various powder and granular raw materials, while melt mixing in the main feeding zone is conducive to the stable conveying and initial melting of materials, laying a good foundation for the subsequent dynamic vulcanization reaction.

[0012] Furthermore, the raw materials also contain compatibilizers and / or lubricating dispersants. Adding compatibilizers, such as maleic anhydride-grafted polypropylene (PP-g-MAH), can improve the interfacial bonding between the polypropylene matrix and the EPDM rubber and inorganic flame retardant, further enhancing the mechanical properties of the material. Adding lubricating dispersants, such as vinyl bis-stearamide (EBS) or zinc stearate, helps improve the processing flowability of the material and the surface finish of the final product.

[0013] The second aspect of this application provides a highly flame-retardant plastic obtained by the above-described preparation method. Due to the innovative preparation method employed, this plastic product possesses the combined advantages of no exudation, high flame retardancy, and high toughness, making it suitable for applications with high safety and reliability requirements.

[0014] Furthermore, the high flame-retardant plastic is prepared from raw materials comprising the following parts by weight: 55-60 parts polypropylene, 12-15 parts EPDM rubber, 2-6 parts high molecular weight silicone with a weight average molecular weight greater than 500,000, 22-25 parts intumescent flame retardant, 0.8-1.2 parts phenolic resin crosslinking agent, and 0.05-0.5 parts Lewis acid catalyst. This formulation is a preferred embodiment; the components work synergistically to achieve UL-94 V-0 flame retardancy, high impact strength, and excellent surface exudation resistance. For example, high-impact copolymer PP can be selected for the polypropylene, a grade with high ENB content can be selected for the EPDM rubber, and type II polyphosphoric acid coated with melamine resin can be selected for the intumescent flame retardant.

[0015] The third aspect of this application provides the application of the aforementioned highly flame-retardant plastics in electronic device housings.

[0016] Furthermore, the electronic device housing is an earphone housing. As a component frequently touched by consumers, earphone housings have extremely high requirements for material appearance (no exudation, no defects), safety (high flame retardancy), and user experience (drop resistance, high toughness). The plastic provided in this application perfectly meets these requirements.

[0017] The present invention has the following beneficial effects:

[0018] This invention constructs a semi-interpenetrating network and utilizes a topological locking mechanism to physically confine high molecular weight silicones with a weight-average molecular weight greater than 500,000 within a cross-linked EPDM rubber network, fundamentally solving the problem of migration and precipitation of organosilicon additives and achieving long-term stability and excellent appearance of the material.

[0019] This invention employs a selective crosslinking system of phenolic resin to replace traditional peroxides, fully preserving the molecular weight of the polypropylene matrix, avoiding the decline in the mechanical properties of the material and the generation of irritating odors, and ensuring the basic performance of the product.

[0020] This invention successfully solves the contradiction between flame retardancy and toughness by using cross-linked EPDM elastomer toughening and a highly efficient halogen-free intumescent flame retardant system, enabling the material to maintain high impact strength while achieving the UL-94 V-0 flame retardant rating.

[0021] The material provided by this invention has excellent comprehensive properties, which broadens the application range of halogen-free flame-retardant polypropylene. It is particularly suitable for electronic device housings with stringent standards for safety, appearance and reliability, such as high-end headphone housings, and has significant market value and competitive advantages. Detailed Implementation

[0022] To facilitate understanding of this application, a more complete description will be provided below. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0023] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of the application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. In the description of this application, "several" means at least one, such as one, two, etc., unless otherwise explicitly specified.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0025] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0026] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0027] Unless otherwise specified, the percentage content mentioned in this application refers to mass percentage for solid-liquid mixtures and solid-phase-solid mixtures, and volume percentage for liquid-phase-liquid mixtures.

[0028] Unless otherwise specified, all percentage concentrations mentioned in this application refer to the final concentration. The final concentration refers to the proportion of the added component in the system after the addition of that component.

[0029] Unless otherwise specified, the temperature parameters in this application may be either constant temperature processing or processing within a certain temperature range. The constant temperature processing allows temperature fluctuations within the precision range controlled by the instrument.

[0030] The term "particle" as used in this application, or a substance with a defined particle size distribution, is not necessarily spherical in shape; it may be irregular and can be either primary or secondary particles. The particle size of irregular particles is calculated as the average of their maximum and minimum diameters.

[0031] In some preferred embodiments, the polypropylene (PP) can be selected from either homopolymer polypropylene or copolymer polypropylene, preferably high-impact copolymer polypropylene, and its melt index (MFI, 230°C / 2.16kg) can be arbitrarily selected between 5-20 g / 10min.

[0032] In some preferred embodiments, the ethylene propylene diene monomer (EPDM) rubber is EPDM with high ethylene content and high ENB (ethylidene norbornene) content, and its Mooney viscosity ML(1+4) at 125°C can be arbitrarily selected between 40 and 60.

[0033] In some preferred embodiments, the weight-average molecular weight of the ultra-high molecular weight silicone can be selected from any value or range of 500,000, 600,000, 800,000, 1,000,000, and 1,200,000, and is preferably added in the form of silicone masterbatch. The carrier resin in the silicone masterbatch can be any one of polypropylene, polyethylene, or EVA, and the silicone content is between 40% and 60%.

[0034] In some preferred embodiments, the intumescent flame retardant (IFR) may be ammonium polyphosphate (APP), melamine polyphosphate (MPP) or a mixture thereof, preferably type II ammonium polyphosphate with a degree of polymerization greater than 1000, and the surface is coated with melamine-formaldehyde resin or silicone resin.

[0035] In some preferred embodiments, the phenolic resin crosslinking agent may be selected from alkylphenolic resin (such as SP-1045) or octylphenolic resin (such as SP-1055), and the Lewis acid catalyst may be selected from either stannous chloride (SnCl2) or zinc chloride (ZnCl2).

[0036] Example 1:

[0037] This embodiment provides a highly flame-retardant plastic and its preparation method.

[0038] Raw materials (by weight): Polypropylene (PP, copolymer, MFI=10 g / 10min): 58 parts, Ethylene propylene diene monomer (EPDM, high ENB, Mitsui Chemicals, EPT 4045): 13 parts, Ultra-high molecular weight silicone (Mw>500,000): 5 parts (provided by 10 parts of silicone masterbatch with a content of 50%, the masterbatch carrier is PP), Intumescent flame retardant (IFR, Type II APP, melamine coated, Clariant AP462): 23 parts, Compatibilizer (PP-g-MAH, grafting rate 1%): 2.5 parts, Phenolic resin crosslinking agent (SP-1045): 1.0 part, Lewis acid catalyst (SnCl2): 0.1 part, Lubricating dispersant (EBS): 0.5 parts.

[0039] Preparation method:

[0040] Step S1: Place polypropylene, EPDM, silicone masterbatch, phenolic resin, SnCl2, PP-g-MAH, and EBS into a high-speed mixer and mix at 1000 rpm for 3 minutes to obtain a premix.

[0041] Step S2: The premixed material is fed into the twin-screw extruder via the main feeder and melt-mixed at 190°C in the melt conveying zone (Zone 1-3) to ensure uniform dispersion of the components.

[0042] Step S3: The molten mixture is transported to the dynamic vulcanization zone (Zone 4-6), heated to 220°C and subjected to enhanced shearing, so that the phenolic resin crosslinking agent initiates the crosslinking of EPDM rubber under the action of the catalyst, thereby forming a semi-interpenetrating network structure of the ultra-high molecular weight silicone that is physically topologically locked.

[0043] Step S4: Add intumescent flame retardant to the side feed port (Zone 7), adjust the temperature of this zone to 190°C, and use low shear conveying.

[0044] Step S5: After the melt is degassed under vacuum (Zone 8-9), it is extruded, stretched, water-cooled, pelletized, and dried to obtain highly flame-retardant plastic particles.

[0045] Examples 2-4: Examples 2-4 were prepared using the same method as Example 1, with the only difference being the raw material formulation or process parameters, as detailed in Table 1.

[0046] Table 1. Formulations and process parameters for Examples 1-4

[0047] Raw materials / process (portions) Example 1 Example 2 Example 3 Example 4 PP (copolymer) 58 60 55 58 EPDM 13 12 15 13 UHMW silicone (Mw>500,000) 5 6 4 5 IFR 23 22 25 23 PP-g-MAH 2.5 2 3 2.5 Phenolic resin 1 0.8 1.2 1 SnCl2 0.1 0.05 0.15 0.1 EBS 0.5 0.5 0.5 0.5 Differences in process Dynamic vulcanization temperature (°C) 200 200 200 210 IFR feeding method Side feeding Side feeding Side feeding Main feed

[0048] Comparative Example 1: No crosslinking agent (phenolic resin and SnCl2) was added in this comparative example. Other components and processes were the same as in Example 1.

[0049] Comparative Example 2: In this comparative example, peroxide (0.2 parts DCP) was used to replace the phenolic resin crosslinking agent and SnCl2 system. Other components and processes were the same as in Example 1.

[0050] Comparative Example 3: No silicone additives were added in this comparative example. To compensate for the missing amount, the amount of PP was increased (5 parts of PP were added). Other components and processes were the same as in Example 1.

[0051] Comparative Example 4:

[0052] This comparative example uses low molecular weight silicone (Mw=10,000) instead of ultra-high molecular weight silicone, and the other components and processes are the same as in Example 1.

[0053] Summary of performance test data:

[0054] The plastic particles prepared in Examples 1-4 and Comparative Examples 1-5 were injection molded into standard specimens for testing.

[0055] Test method:

[0056] Flame retardancy: UL-94 vertical burning test (1.6mm).

[0057] Exudation: Place the sample in a 100℃ oven for 168 hours and observe whether the surface is sticky, oily, or has a white bloom (visual inspection and wiping method).

[0058] Mechanical properties: Notched impact strength (23°C, ISO 180), tensile strength (ISO 527).

[0059] Melt flow index: MFI (230℃ / 2.16kg).

[0060] Table 2 Test results of the examples and comparative examples

[0061] Performance indicators Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 UL-94 (1.6mm) V-0 V-0 V-0 V-2 V-2 V-0 V-2 V-2 Precipitation (100℃ / 168h) No precipitation, surface dry No precipitation No precipitation Slight precipitation Severe precipitation No precipitation No precipitation Severe precipitation <![CDATA[Izod impact (kJ / m 2 )]]> 13.5 12.8 14.2 13.1 9.4 6.5 8.2 11.1 Tensile strength (MPa) 21.5 22.6 20.5 20.8 19.1 16.3 24.1 21.5 MFI (g / 10min) 4.2 3.8 4.5 4.0 8.9 20.1 8.4 4.9

[0062] As shown in Table 2, Examples 1-3 all achieved a UL-94 V-0 rating, superior to Comparative Examples 1 and 3 (V-2 rating). This is attributed to the silicone locked in EPDM migrating to the surface and transforming into silica during combustion, synergistically forming a dense organic-inorganic hybrid char layer with the char layer produced by IFR decomposition. Comparative Example 3, lacking silicone, could not form this efficient char layer, resulting in a lower flame retardancy rating. Although Comparative Example 4 contained silicone, its low molecular weight meant it might volatilize and dissipate in a gaseous form during combustion, failing to effectively form char, resulting in a similar V-2 flame retardancy rating. Examples 1-4 all exhibited excellent resistance to exudation, with no oil seepage on the surface. This result is significantly better than Comparative Example 1 (uncrosslinked) and Comparative Example 4 (low molecular weight silicone), demonstrating the synergistic effect of EPDM forming a crosslinked network and high molecular weight silicone.

[0063] The notched impact strengths of Examples 1-4 all remained above 12.5 kJ / m², exhibiting good toughness. This contrasts sharply with Comparative Example 2. Comparative Example 2 used peroxide (DCP) to initiate crosslinking. Although crosslinking of the rubber phase was achieved, DCP simultaneously induced β-chain scission in the PP matrix, leading to a sharp decrease in molecular weight (MFI as high as 20.1 g / 10min), which severely degraded the mechanical properties of the material, reducing the impact strength to 6.5 kJ / m². 2 In contrast, the phenolic resin crosslinking system used in this invention is highly chemically inert to PP, ensuring the complete retention of the PP molecular weight while guaranteeing EPDM crosslinking, thus ensuring the high toughness of the material.

[0064] In Example 4, the use of a main-feed method for adding IFR resulted in an excessively long residence time of the flame retardant in the extruder, leading to partial degradation or particle size breakage. This affected the quality of the char and caused the flame retardant rating to drop to V-2. This demonstrates the importance of the side-feed process in protecting the activity of IFR and ensuring the high efficiency of the flame retardant.

[0065] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for preparing a highly flame-retardant plastic, characterized in that, Includes the following steps: Step S1: Melt-mix polypropylene, EPDM rubber, high molecular weight silicone with a weight average molecular weight greater than 500,000, phenolic resin crosslinking agent and Lewis acid catalyst; Step S2: The mixture obtained in step S1 is heated to 200-220°C under shearing and dynamically vulcanized, so that the phenolic resin crosslinking agent catalyzes the crosslinking of the EPDM rubber.

2. The preparation method according to claim 1, characterized in that, Step S1 also includes an intumescent flame retardant, which is added at the side feed port of the twin-screw extruder.

3. The preparation method according to claim 1, characterized in that, In step S2, the temperature for dynamic vulcanization is 200-210℃.

4. The preparation method according to claim 1, characterized in that, The dynamic vulcanization process in step S2 is carried out in the high-shear thread element section of the twin-screw extruder.

5. The preparation method according to claim 1, characterized in that, The melt mixing in step S1 is carried out in the main feeding zone of the twin-screw extruder, and the raw materials also include a pre-mixing step before the main feeding.

6. The preparation method according to claim 1, characterized in that, The raw materials also include compatibilizers and / or lubricating dispersants.

7. A highly flame-retardant plastic obtained by the preparation method according to any one of claims 1-6.

8. The high flame-retardant plastic according to claim 7, characterized in that, It is prepared from the following raw materials in parts by weight: 55-60 parts polypropylene, 12-15 parts EPDM rubber, 2-6 parts high molecular weight silicone with a weight average molecular weight greater than 500,000, 22-25 parts intumescent flame retardant, 0.8-1.2 parts phenolic resin crosslinking agent and 0.05-0.5 parts Lewis acid catalyst.

9. The application of the high flame-retardant plastic as described in claim 7 in the housing of electronic devices.

10. The application according to claim 9, characterized in that, The outer casing of the electronic device is an earphone casing.