Preparation process of low-volatile polyphenylene sulfide plastic particles

Abstract

The invention discloses a preparation process of low-volatile polyphenylene sulfide plastic particles, and relates to the technical field of preparation of polyphenylene sulfide modified particles. The method sequentially comprises the steps of anhydrous sulfide activation, ionic liquid dissolution and polycondensation, online HS trapping, multi-stage washing-vacuum micro-aerobic hardening and tempering, double-screw'two-stage vacuum + supercritical CO 'melting devolatilization with the length-diameter ratio larger than or equal to 44: 1, segmented functional modification and closed pelletizing and drying. According to the process, the reinforced-toughened-flame-retardant composite PPS particles with the total volatile matter being smaller than or equal to 0.10 wt%, the UL-94V-0 and the oxygen index being larger than or equal to 50% are obtained through source volatile sulfide cutoff, thinning-steam stripping coupling deep exhaust and closed-loop cooling, the strength retention rate is larger than or equal to 90% after heat aging is conducted for 1000 h, and the reinforced-toughened-flame-retardant composite PPS particles are suitable for harsh scenes such as automobile engine cabins and household appliance high-temperature insulation.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of polyphenylene sulfide modified particles, and specifically to a preparation process of polyphenylene sulfide plastic particles with low volatile content. Background Art

[0002] In the existing preparation process of polyphenylene sulfide (PPS) modified particles, high-boiling solvents or salt melt systems are generally used for polycondensation, and directly enter the extrusion granulation process after single-stage water washing and drying. Due to the lack of fine control over the whole process of the water content of raw materials, reaction by-product H2S and oligomeric sulfides, the residual volatile matter is usually higher than 0.5 wt%, and the finished product is prone to generate irritating odors and silver wire defects during injection molding or secondary extrusion, and the mechanical properties decay significantly after long-term thermal aging. In addition, the conventional twin-screw is only equipped with a section of medium vacuum exhaust, the melt residence time is short and the melt film thickness is large, and the deep devolatilization efficiency is limited; during the open water ring cutting granulation cooling process, the particles are easily re-moisturized and oxidized again. The above disadvantages make it difficult for the material to meet the double high standards of "low volatility, dimensional stability, and high temperature aging resistance" for automotive engine compartments and heat-resistant insulation parts of household appliances. Summary of the Invention

[0003] To overcome the defects of the above-mentioned prior art, the present invention provides the following technical solutions: A preparation process of low-volatile polyphenylene sulfide plastic particles, which is characterized by successively implementing the following steps: S1. Raw material pretreatment: Under the protection of inert gas, sodium sulfide is mixed with 0.5–5wt% NaOH, and dehydrated and activated at 140–180°C until the residual water ≤ 0.05wt%; S2. Ionic liquid dissolution and monomer feeding: After adding 1-propyl-3-methylimidazolium hexafluorophosphate ionic liquid to the system obtained in step S1 for deoxygenation and dehydration, p-dichlorobenzene is added in an amount of 1.02–1.10 molar equivalents based on sulfide; S3. Polycondensation reaction: React at 200–250°C and ≤ 0.3MPa for 4–6h, terminate the reaction when the concentration of by-product H2S is monitored to drop to ≤ 100ppm in real time, and introduce H2S into the caustic scrubber to be converted into sodium hydrosulfide for recovery; S4. Polymer separation and ionic liquid recycling: The crude PPS is separated from the by-product salt by vacuum flashing - pressure filtration - high-speed centrifugation; the ionic liquid is evaporated and purified at 40–80°C and ≤ 0.5kPa and recycled for ≥ 50 batches; S5. Multi-stage washing - thermal oxidation conditioning: The crude PPS is successively washed with warm deionized water, soaked in dilute alkali, washed with water, centrifuged and dried, and then heat-treated at 240–260°C, 0.1–10kPa and passed with ≤ 0.5vol% air for 2–4h, and the oxygen content is closed-loop at 0.05–0.30vol%; further remove oligomers, end-group sulfides and moisture, and balance the molecular weight uniformity. After centrifuging and drying and cooling, purified powder is obtained; S6. Melt devolatilization: The purified powder is fed into a co-rotating twin-screw extruder with a length-diameter ratio ≥ 44:1, plasticized at 300°C in the front section, 5–10kPa at the first vent, < 1kPa at the second vent, and set with no less than 4 groups of backpressure - dispersion - thin film screw elements, and introduce 0.1–1.0wt% supercritical CO2 microbubbles for stripping for 0.5–3min; S7. Segmented feeding: 20–40wt% of glass fibers treated with γ-APS (length 4–8mm, diameter 10–14µm), 5–10wt% of maleic anhydride-grafted EPDM toughening agent (D 90 ≤ 3µm, interfacial adhesion work ≥ 50mJ / m²), 10–15wt% of DOPO-modified aluminum phosphonate / melamine cyanurate flame retardant system (mass ratio 4–6:1–2, particle size 1–5µm and coated with 3wt% silane, optionally 0.5–2wt% zinc borate), 1–2wt% of hindered phenol / phosphite antioxidant and ≤ 1wt% of lubricant are added laterally after the second vent; S8. Extrusion granulation: The melt is underwater at 20–40°C through a wear-resistant die or cooled by closed nitrogen air to cut particles (O2 < 5ppm), and closed-loop cooling - centrifugation - hot air drying is carried out to obtain modified PPS particles with a total volatile content ≤ 0.10wt%; S9. Closed-loop cooling and drying (≤ 0.10wt% volatile matter): Centrifugal dehydration at 1500g - hot air at 80°C; final moisture ≤ 0.02wt%.

[0004] Preferably, the co-rotating twin-screw extruder is equipped with a liquid ring pump or a dry claw pump and a two-stage vacuum pump group composed of a Roots-oil diffusion combination, which are respectively connected to independent condensers; the screw elements in front of the second exhaust section form a minimum melt film thickness of ≥0.5 mm to further improve the devolatilization efficiency.

[0005] Preferably, the water temperature of underwater pelletizing is controlled at 20 - 40 °C, and circulating nitrogen air curtain is used for air-cooled pelletizing; the obtained pellets have a UL-94 rating of V-0 at a thickness of 1.6 mm, an oxygen index of ≥44%, a tensile strength of 100–150 MPa, a flexural modulus of 8–10 GPa, a notched impact of 60–80 J / m, a linear expansion coefficient of (1–2)×10⁻ 5 / K, and the viscosity attenuation is ≤5% after processing at 280 °C and 10 MPa for 30 min, the tensile strength retention rate is ≥90% after heat aging at 200 °C / 1000 h, and there are no cracks and the creep is ≤0.02 mm after the -55 °C impact -180 °C creep cycle.

[0006] A preparation system for low-volatile polyphenylene sulfide plastic pellets includes: a corrosion-resistant stirred high-pressure polymerization kettle or a continuous tubular reactor, with an online filtration and salt separation mechanism and inert purging; an ionic liquid circulation and purification unit: a precision filter + a vacuum evaporator; a multi-stage washing tower and a high-temperature vacuum heat treatment bed (online oxygen monitoring), the washing tower is three-stage alkali-resistant 316L stainless steel or PTFE-lined and equipped with a tail gas absorption tower; a co-rotating twin-screw devolatilization-modifying extruder with a length-diameter ratio of ≥44:1, equipped with a two-stage vacuum pump group, a condensation trap, an inert stripping inlet and multiple side feeding ports; an underwater / air-cooled pelletizing device and a closed-loop cooling-drying system.

[0007] Preferably, for the preparation system of low-volatile polyphenylene sulfide plastic pellets, the second exhaust section of the extruder is equipped with no less than 4 groups of thin-film screw elements and a CO2 microbubble stripping inlet; the first stage of the vacuum pump group is a liquid ring pump or a dry claw pump, and the second stage is a Roots-oil diffusion combination, and is equipped with an independent condenser.

[0008] A composition of low-volatile polyphenylene sulfide plastic pellets by mass percentage is: PPS 50–65%, glass fiber 20–40%, maleic anhydride-grafted EPDM toughening agent 5–10%, DOPO-modified aluminum phosphonate / melamine cyanurate flame retardant system 10–15%, hindered phenol / phosphite antioxidant 1–2%, lubricant ≤1%, and may contain 0.5–2 wt% zinc borate, and the total volatile matter ≤0.10 wt%, and the UL-94 rating is still V-0 at a thickness of 3.2 mm, the oxygen index ≥50%, the dielectric constant ≤3.3 (1 MHz), and the CTI ≥175 V.

[0009] Preferably, the low-volatile polyphenylene sulfide plastic particles have a dimensional change of ≤ 0.05% after 100 thermal cycles from -40°C to +200°C, and a tensile strength retention rate of ≥ 90% after thermal aging at 200°C for 1000 h.

[0010] Preferably, the low-volatile polyphenylene sulfide plastic particles are used in high-temperature structural components in the automotive engine compartment. The structural components include an ignition coil skeleton, a lighting frame, and a temperature-pressure sensor housing. After injection molding, the molded parts are tempered at 220–240°C for 2 h to improve dimensional stability.

[0011] Preferably, the low-volatile polyphenylene sulfide plastic particles are used in heat-resistant insulation components and electronic / electrical components of household appliances. The household appliance components are a microwave oven turntable bearing, a high-temperature spray arm seat of a dishwasher, and an air duct impeller of a heat pump dryer. The electronic / electrical components are connectors, relay housings, and CT coil skeletons.

[0012] The present invention has the following beneficial effects compared with the prior art: (1) The polycondensation of the present invention is carried out in an ionic liquid medium, and the reaction is terminated by online monitoring when the H2S concentration ≤ 100 ppm, and then the tail gas is introduced into an alkali washing tower for immediate neutralization and recovery, so that sulfur-containing volatile substances are cut off at the source; supplemented by ≥ 50 batches of ionic liquid circulation purification, the "carry - evaporation" path of traditional high-boiling solvents is completely eliminated, ensuring that the total sulfur gas concentration in the subsequent system is maintained at a trace level, fundamentally solving the irritating odor and corrosion hidden danger during the use of the product; (2) Aiming at the problem that it is difficult to remove oligomers and end-group sulfides, the process of the present invention adds a combined step of "multi-stage warm water - dilute alkali - water washing + vacuum - micro-oxygen heat treatment at 240 - 260 °C", and the oxygen content is controlled in a real-time closed-loop manner at 0.05 - 0.30 vol%; this continuous conditioning not only extracts residual oligomeric sulfides, but also uses micro-oxygen coupling to homogenize the molecular weight distribution, reducing the content of polar small molecules with high devolatilization difficulty by one order of magnitude compared with the conventional process, and at the same time endowing the resin with a more stable chain structure and improving the heat aging retention rate; (3) The present invention sets a "two-stage vacuum + supercritical CO2 microbubble stripping + ≥ 4 groups of backpressure - dispersion - thin film elements" coupled exhaust area in a co-rotating twin-screw with a length-diameter ratio ≥ 44:1: The first port removes most of the moisture at 5 - 10 kPa, and the second port achieves deep devolatilization at < 1 kPa. The minimum melt film thickness of the film flow state ≤ 0.5 mm significantly increases the gas - melt interface; at the same time, 0.1 - 1 wt% CO2 forms nano-stripping nuclei within 0.5 - 3 min, taking away residual difficult-to-volatilize substances, reducing the total volatiles of the particles to ≤ 0.10 wt%, achieving a low-volatility index incomparable to traditional single-stage exhaust processes; (4) The present invention adopts an integrated granulation scheme of "lateral segmented feeding + inert nitrogen curtain air cooling / underwater pelletizing at 20 - 40 °C + closed-loop pure medium drying". Glass fiber, toughening agent, halogen-free flame retardant, etc. enter the melt only in the latter section of the vacuum area, which can avoid fiber entrainment and the extraction of light auxiliaries; the closed cooling - drying process prevents environmental water and oxygen from re-invading the particles, ensuring that the final moisture ≤ 0.02 wt%, further consolidating the low-volatility characteristics and maintaining the surface smoothness, significantly reducing the defect rates of silver streaks and jet patterns in injection molding; (5) The present invention is synergistically modified by "20 - 40 wt% glass fiber + 5 - 10 wt% maleic anhydride grafted EPDM + 10 - 15 wt% DOPO - aluminum phosphonate / melamine cyanurate", and under the premise of maintaining low volatility, comprehensive properties such as UL-94 V-0, oxygen index ≥ 50%, tensile strength 100 - 150 MPa, and notched impact 60 - 80 J / m are achieved; and there are no cracks after the impact at -55 °C - creep at 180 °C cycle, and the heat aging strength retention rate at 200 °C / 1000 h ≥ 90%. The high reliability is significantly better than the traditional high-volatile PPS system, enabling the material to enter extreme service environments such as engine compartments and microwave ovens. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic structural diagram of the present invention. Detailed implementation mode

[0014] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and through specific implementation modes.

[0015] For the preparation process of the low-volatile polyphenylene sulfide plastic particles provided by the present invention, high-purity p-dichlorobenzene (PDCB) and high-purity sodium sulfide are used as the main monomer raw materials to ensure low impurities and reduce by-products. The reaction medium is selected as a specially designed ionic liquid system (such as 1-propyl-3-methylimidazolium hexafluorophosphate, etc.), which has an extremely low vapor pressure and can be recycled. In terms of additives, a small amount of alkali (metal hydroxide such as NaOH) is introduced as an alkali washing agent in the polymer post-treatment washing to change the end groups of PPS and extract oligomers. In the modification stage, glass fibers (or carbon fibers) treated with coupling agents, functionalized elastomer toughening agents, halogen-free flame retardant additives (such as organic phosphonate / melamine systems), antioxidants, and heat stabilizers, etc. will be used. All additives need to be fully dried or purified before addition to avoid introducing moisture or volatile impurities.

[0016] Process steps: Pretreatment and raw material feeding: Sodium sulfide (in the form of particles or solution) is pre-mixed with a small amount of NaOH and heated for dehydration under the protection of an inert gas to form an anhydrous sulfide active component. An ionic liquid medium is added to the reactor to dissolve the above sulfide, and at the same time, nitrogen is introduced to expel the dissolved oxygen and moisture. Subsequently, a quantitative amount of p-dichlorobenzene monomer is added. The raw material ratio is slightly excessive in p-dichlorobenzene (relative to sulfide) to ensure complete reaction and cap the chain ends with excess monomer; Polymerization reaction: The polycondensation reaction is carried out in a closed stainless steel reaction kettle. The temperature is raised to about 200–250°C for 4–6 hours of reaction (the ionic liquid medium can be used to carry out the reaction efficiently at a lower temperature than conventional ones). During the reaction process, the reaction is ensured to proceed evenly through stirring and a highly solvating environment. When the reaction tends to be completed, the system pressure is gradually reduced or nitrogen is introduced for purging to remove volatile small molecules such as H2S generated by the reaction; these volatiles are immediately introduced into an alkali scrubbing tower for neutralization and recovery to reduce the "stagnant" harmful volatile components in the system. After the reaction is completed, the polymer is separated from the ionic liquid by vacuum flashing or precipitation method to obtain crude polyphenylene sulfide powder. The ionic liquid remains in the reaction kettle or is recycled after simple treatment; Post-treatment purification: The crude PPS polymer is subjected to multiple steps of washing and heat treatment. First, it is repeatedly washed with deionized water to remove entrained salts, by-products, and residual media. Then, an alkali washing process is employed: the PPS is immersed in a dilute alkaline solution at an appropriate temperature to neutralize or saponify the residual thiol / sodium sulfide groups at the polymer ends, and further extract low molecular weight sulfides and oligomers. Subsequently, thermal oxidation conditioning is carried out: a small amount of air is introduced into a vacuum oven, and the polymer powder is heat-treated at about 250 °C for several hours. This step causes the oxidation coupling of a small amount of unreacted chain ends to form a more stable chain structure, while driving off the remaining trace volatiles. After the above treatment, the volatile content of the PPS resin has been significantly reduced. The purified PPS resin is dried to a moisture content of <0.02% for standby; Melt blending and devolatilization: The dried and purified PPS resin is fed into the feeding port of a special twin-screw compounding extruder. The front section of the extruder heats and melts the PPS (at about 300 °C to completely plasticize it). In the middle section, multiple stages of vacuum devolatilization zones are set: the first devolatilization port is under medium vacuum (such as 5 - 10 kPa) to remove most of the moisture and residual solvents; the second devolatilization port is under high vacuum (such as <1 kPa), and special screw elements are used to form a thin layer of the melt to deeply remove extremely difficult-to-volatilize components. When necessary, a small amount of inert gas is injected into the melt as a stripping agent to further remove volatiles. After multiple stages of devolatilization treatment, the content of volatile components in the melt is reduced to an extremely low level; Introduction and mixing of modifiers: In the rear section of the extruder (after the devolatilization zone), the pre-treated modifiers are gradually added through a side feeding port: first, glass fiber is added for melt dispersion to avoid fiber carry-over loss caused by premature entry of fibers into the high vacuum zone; subsequently, a toughening agent (such as functionalized rubber particles or thermoplastic elastomers) and flame retardants and other additives are added. Under the action of high-efficiency mixing elements, the fibers are uniformly dispersed and cut to a certain length, the toughening phase forms a finely dispersed modification system, and the flame retardants and others are uniformly mixed in the matrix. The entire mixing process controls appropriate shear and temperature to prevent the decomposition of additives or the generation of new volatiles due to excessive shear or overheating. The vacuum system can appropriately reduce the vacuum degree or be closed in the rear section of the feeding to avoid sucking away light modifier components, but still maintain a slight negative pressure to remove a small amount of volatile gases generated during mixing (such as residual moisture or low-boiling substances in the additives); Extrusion granulation: Finally, the molten blend is extruded through the die head of the extruder, and the melt is cut into pellets by an underwater pelletizing or air-cooled pelletizing system. Underwater pelletizing can quickly cool the pellets to prevent over-oxidation. Subsequently, the pellets are centrifugally dehydrated and hot air dried to obtain the finished PPS pellets. A closed-loop pure medium is used throughout the cooling and drying process to prevent impurities or moisture in the environment from being reintroduced into the pellets. Thus, low-volatile, high-performance modified PPS pellets are obtained.

[0017] The preparation system of low-volatile polyphenylene sulfide plastic particles includes: Polymerization reactor: An anti-corrosion high-pressure autoclave with stirring or a continuous tubular reactor is used, which can withstand at least 300 °C and a certain pressure. The reactor body is equipped with multiple seals and vent interfaces for medium circulation and volatiles discharge. An on-line filtration device is specially designed to separate the by-product salt (NaCl) generated from the reaction mixture. After the reaction, the solid salt and polymer coarse powder can be separated from the ionic liquid by pressure filtration or high-speed centrifugation. The reactor is also integrated with a rapid cooling and inert purge function to quickly cool and precipitate the polymer after polymerization is achieved; Ionic liquid circulation system: It is equipped with an ionic liquid recovery unit, including a precision filter and a vacuum evaporator. After desalting by filtration, trace organic impurities (such as trace by-product organic substances) in the ionic liquid are removed by vacuum evaporation. The purified ionic liquid is cooled and stored for reuse in the next batch, realizing clean production; Alkali washing and heat treatment device: A continuous polymer washing tower and a heat treatment bed are designed. The washing tower is divided into multiple stages: the first stage is warm deionized water washing, the second stage is alkali liquor immersion washing, the third stage is again water washing and neutralization, and finally centrifugal drying. The tower body and pipelines are made of alkali-resistant materials and are sealed to prevent the volatiles in the washing liquid from escaping. Heat treatment is carried out using a high-temperature vacuum chamber or a rotary kiln with controllable introduction of inert gas and trace air. The dried PPS powder is spread out and heated to make each particle uniformly heated and oxidized. The equipment is equipped with on-line monitoring to control the oxygen content and avoid over-oxidation causing over-crosslinking of the polymer; Twin-screw devolatilization extruder: This is the core equipment. A co-rotating twin-screw with a relatively large length-diameter ratio and modular screw elements arrangement is selected. The front section is the feeding and plasticizing area. There are two vacuum exhaust ports in the middle section: An exhaust barrier element (such as a vacuum bite thread) is configured in front of the first exhaust port to prevent the melt from escaping; Before the second exhaust port, the screw elements form an extended melt pool and a thin-layer flow area to enhance the volatilization of volatiles. Each vacuum port is connected to an independent vacuum pump group and a condenser trapping system. The first stage uses a liquid ring pump or a dry claw pump, and the second stage uses a high-vacuum pump. The condenser traps the solvent and sulfide condensates to prevent contamination of the vacuum pump. In the second vacuum section, a trace injection port for inert gas is also designed for purging the melt. Multiple side feeding ports and liquid injection ports are provided in the rear section of the extruder to add glass fiber, toughening agent and flame retardant correspondingly. High-torque dispersion elements are equipped in the rear section of the screw to ensure uniform mixing. The die head of the extruder uses a multi-strand draw bar or an underwater pelletizing die head. The material needs to be wear-resistant (due to containing glass fiber) and the temperature can be controlled to ensure stable discharge; Process control and safety: The entire system is centrally controlled by a DCS system, and key parameters such as temperature, pressure, vacuum degree, torque, and volatile concentration are monitored in real time. A volatile sulfide sensor is specially installed. Once the detected escape gas exceeds the standard, the vacuum and inert gas spraying can be automatically adjusted. The caustic scrubber and condenser are equipped with a tail gas treatment unit (activated carbon and washing liquid) to ensure zero harmful gas emissions. The equipment materials are selected as alloys resistant to sulfide corrosion, and the key seals are heat-resistant and chemical-resistant to ensure the reliability of long-term operation.

[0018] While ensuring low volatility, synergistic modification means are adopted to endow PPS resin with excellent mechanical properties, toughness, and flame retardancy to meet the usage requirements of automotive and household appliance parts. The main modification measures include: Reinforcement (improving rigidity and dimensional stability): Glass fiber (GF) is used to reinforce the PPS matrix. By adding about 20–40wt% of chopped glass fiber, the stiffness, strength, and heat distortion temperature of the material can be significantly improved. The addition of glass fiber increases the HDT from about 115°C of the pure resin to nearly 260°C, meeting the usage requirements in high-temperature environments; at the same time, it significantly reduces the linear thermal expansion coefficient of the material and improves dimensional stability. To ensure interfacial bonding, a silane coupling agent is pre-coated on the fiber surface or the fiber sizing agent suitable for PPS is used to enhance the adhesion between the fiber and the matrix. The fiber content and length are optimized to make the product have both high modulus and processable fluidity. For parts with specific high-strength requirements, carbon fiber can also be partially used for reinforcement to obtain higher modulus and lower density, but mainly glass fiber is considered due to cost; Toughening (improving impact strength): Since the pure PPS body is relatively rigid and brittle, an elastomeric toughening agent is introduced to improve the fracture toughness and impact resistance of the material. About 5–10wt% of a heat-stable toughening agent is selected, such as maleic anhydride grafted ethylene-propylene rubber (EPDM-g-MAH), functionalized styrene-butadiene rubber particles (SBR nano-rubber), or core-shell structured acrylate elastomer. The toughening agent and the PPS matrix are well compatible through chemical bonds or polar interactions, forming fine dispersed phases in the matrix to effectively passivate cracks. The content is controlled within the range of not overly sacrificing rigidity, and a shear dispersion process is used to ensure that the particle size is in the micron level, so as to greatly improve the notched impact strength while maintaining fluidity. When necessary, a compatibilizer (such as functionalized polyphenylene ether, etc.) is added to further improve the interface between the toughening agent and PPS. The optimized toughening formula can increase the impact strength of the material several times, meeting the toughness requirements of automotive impact parts or thin-walled products, and at the same time avoiding excessive reduction of the material fluidity and heat resistance through formula regulation; Flame Retardancy (Enhancing Fire Safety): The PPS resin itself has flame retardancy, and the pure resin can reach the UL94 V-0 level. However, for thick-walled parts or higher flame retardancy specification requirements, flame retardants still need to be added to increase the oxygen index and reduce combustion smoke. A halogen-free flame retardant system is used to maintain low volatility and environmental friendliness. For example, adding 10–15 wt% of organophosphorus flame retardants (such as aluminum phosphonate or DOPO derivatives), combined with a small amount of melamine cyanurate as a charring promoter, and additives such as zinc borate to synergistically inhibit smoke and improve the flame retardancy efficiency. This system promotes the formation of a carbon layer and releases non-combustible gases during combustion, preventing flame propagation. Due to the choice of halogen-free formulation, the risk of toxic volatiles being released by halogenated flame retardants during processing or combustion is avoided. Through this flame retardant modification, the oxygen index of PPS pellets can be increased to over 50%, and the products have a higher flame retardant safety margin when used in the automotive engine compartment or inside household appliances; Other Additives: Add 1–2 wt% of a high-efficiency antioxidant system (such as a combination of hindered phenol / phosphite) to prevent thermal-oxidative aging during processing and long-term use, ensuring long-term stability of the material properties. In addition, a small amount of lubricant (such as stearate) can be added to improve processing flow and demoldability, and the additives are selected based on the principles of high-temperature stability and low volatility. The combination of all modifiers has been optimized through experiments to enable the composite material to have high rigidity, high impact toughness, flame retardancy, and excellent processing performance.

Claims

1. A preparation process of low-volatile polyphenylene sulfide plastic particles, characterized in that, The following steps are implemented in sequence: S1. Pretreatment of raw materials: Under the protection of inert gas, sodium sulfide is mixed with 0.5–5 wt% NaOH and dehydrated and activated at 140–180 °C until the residual water ≤ 0.05 wt%; S2. Dissolution of ionic liquid and feeding of monomers: After adding 1-propyl-3-methylimidazolium hexafluorophosphate ionic liquid to the system obtained in step S1 to remove oxygen and water, p-dichlorobenzene is added in an amount of 1.02–1.10 molar equivalents based on sulfide; S3. Polycondensation reaction: React at 200–250 °C and ≤ 0.3 MPa for 4–6 h. Terminate the reaction after continuously monitoring that the concentration of by-product H2S drops to ≤ 100 ppm, and introduce H2S into the alkali scrubbing tower to be converted into sodium hydrosulfide for recovery; S4. Separation of polymer and recycling of ionic liquid: The crude PPS and by-products are separated by vacuum flashing - pressure filtration - high-speed centrifugation; the ionic liquid is evaporated and purified at 40–80 °C and ≤ 0.5 kPa and recycled for ≥ 50 batches; S5. Multi-stage washing - thermal oxidation conditioning: The crude PPS is washed successively with warm deionized water, dilute alkali solution, water, and centrifugally dried, and then heat-treated at 240–260 °C, 0.1–10 kPa while passing ≤ 0.5 vol% air for 2–4 h, with the oxygen content closed-loop at 0.05–0.30 vol%; further remove oligomers, terminal sulfides, and moisture, and balance the molecular weight uniformity. After centrifugal drying and cooling, purified powder is obtained; S6. Melt devolatilization: Feed the purified powder into a co-rotating twin-screw extruder with a length-diameter ratio ≥ 44:

1. Plasticize at 300 °C in the front section, 5–10 kPa at the first exhaust port, < 1 kPa at the second exhaust port. Set no less than 4 groups of backpressure - dispersion - film screw elements, and introduce 0.1–1.0 wt% supercritical CO2 microbubbles for stripping for 0.5–3 min; S7. Segmented feeding: Add 20–40 wt% of glass fiber treated with γ-APS, 5–10 wt% of maleic anhydride grafted EPDM toughening agent, 10–15 wt% of DOPO-modified aluminum phosphonate / melamine cyanurate flame retardant system, 1–2 wt% of hindered phenol / phosphite antioxidant, and ≤ 1 wt% of lubricant laterally at the rear of the second exhaust port; S8. Extrusion granulation: The melt is underwater cut into pellets at 20–40 °C or cooled by closed-loop nitrogen air through a wear-resistant die head, and closed-loop cooling - centrifugation - hot air drying is carried out to obtain modified PPS particles with a total volatile matter ≤ 0.10 wt%; S9. Closed-loop cooling and drying.

2. The preparation process of a low-volatile polyphenylene sulfide plastic particle according to claim 1, wherein: The co-rotating twin-screw extruder is equipped with a liquid ring pump or a dry claw pump and a two-stage vacuum pump group combined with a Roots - oil diffusion pump, which are respectively connected to independent condensers; the screw elements in front of the second exhaust section form a minimum melt film thickness of ≥ 0.5 mm.

3. The preparation process of a low-volatile polyphenylene sulfide plastic particle according to claim 1, characterized in that: The water temperature for underwater pelletizing is controlled at 20 - 40°C, and circulating nitrogen air curtain is used for air-cooled pelletizing; the obtained pellets have a UL-94 rating of V-0 at a thickness of 1.6 mm, an oxygen index ≥ 44%, a tensile strength of 100–150 MPa, a flexural modulus of 8–10 GPa, a notched impact of 60–80 J / m, a linear expansion coefficient of (1–2)×10⁻ 5 / K, and the viscosity attenuation is ≤ 5% after processing at 280°C and 10 MPa for 30 min, the tensile strength retention rate is ≥ 90% after heat aging at 200°C / 1000 h, and there are no cracks and the creep is ≤ 0.02 mm after the -55°C impact - 180°C creep cycle.

4. A preparation system for low-volatile polyphenylene sulfide plastic particles, characterized in that: Including: A corrosion-resistant stirred high-pressure polymerization kettle or a continuous tubular reactor, with an on-line filtration and salt separation mechanism and inert purging; Ionic liquid circulation and purification unit: precision filter + vacuum evaporator; Multi-stage washing tower and high-temperature vacuum heat treatment bed. The washing tower is three-stage alkali-resistant 316L stainless steel or PTFE-lined and equipped with a tail gas absorption tower; A co-rotating twin-screw devolatilization - modification extruder with a length-diameter ratio ≥ 44:1, equipped with a two-stage vacuum pump group, a condensation trap, an inert stripping inlet, and multiple side feeding ports; Underwater / Air-cooled Pelletizing Device and Closed-loop Cooling-Drying System.

5. The preparation system of a low-volatile polyphenylene sulfide plastic particle according to claim 4, characterized in that: The second exhaust section of the extruder is equipped with no less than 4 groups of film screw elements and CO2 microbubble stripping inlets; the first stage of the vacuum pump group is a liquid ring pump or a dry claw pump, the second stage is a Roots-oil diffusion combination, and an independent condenser is provided.

6. A low-volatile polyphenylene sulfide plastic particle, characterized in that: The composition by mass percentage is as follows: PPS 50–65%, glass fiber 20–40%, maleic anhydride grafted EPDM toughening agent 5–10%, DOPO-modified aluminum phosphonate / melamine cyanurate flame retardant system 10–15%, hindered phenol / phosphite antioxidant 1–2%, lubricant ≤1%, and may contain 0.5–2 wt% zinc borate, and the total volatile matter ≤0.10 wt%. The UL-94 rating is still V-0 at a thickness of 3.2 mm, the oxygen index ≥50%, the dielectric constant ≤3.3 (1 MHz), and the CTI ≥175 V.

7. A low-volatile polyphenylene sulfide plastic particle according to claim 6, characterized in that, The dimensional change is ≤0.05% after 100 thermal cycles at -40°C to +200°C, and the tensile strength retention rate after thermal aging at 200°C / 1000 h is ≥90%.

Images