Introduction to Polypropylene Resin

Thread Content

Introduction to Polypropylene Resin: The most notable property of polypropylene is its versatility, as it can be adapted to many processing methods and applications. Its value and versatility stem primarily from its excellent chemical resistance, the lowest density and highest melting point among bulk thermoplastics, and its moderate cost. Depending on the different three-dimensional structures of the polymer chains, polypropylene exists in three varieties: isotactic polypropylene (IPP), syndiotactic polypropylene (SPP), and atactic polypropylene (APP). Chemistry and properties: Polypropylene (abbreviated as PP) differs from polyethylene (PE) in that it has a methyl group at every other carbon atom, which serves to harden the chain. The polymer will not crystallize unless these methyl groups are located on the same side of the chain. Before Natta and Ziegler (independently of each other) developed stereospecific catalysts, only soft and sticky atactic polypropylene could be produced. The hardness and solvent resistance of commercial plastics stem from their crystallinity. The chain of PP is harder than that of PE; therefore, PP has a higher melting temperature and tensile strength, but a lower degree of crystallinity. The melting point of PP homopolymers is approximately 330°F, depending on the heating rate and thermal history. By inserting ethylene intermittently (random copolymerization) into the PP chain, the chain becomes less regular and more flexible, thereby reducing the polymer’s crystallinity, modulus, melting point, and sharpness of the melting point. Typical random copolymers are relatively transparent, with melting points in the range of 293–305°F. As the ethylene content increases, the crystallinity of the polymer decreases progressively, eventually turning it into ethylene-propylene rubber (EPR). Another important class of copolymers is impact-resistant heterogeneous copolymers. These products are made by polymerizing rubber (sometimes PE) in a homopolymer matrix. The rubber used is usually EPR, which creates a phase separated from the homopolymer matrix, resulting in a hazy appearance. Translucent appearance. These materials are not true block copolymers, because their rubber phase can be extracted by solvents. Similar products can be obtained by blending EPR with PP; the impact-resistant copolymer has a melting point similar to that of the homopolymer. Molecular weight and molecular weight distribution are important in the PP processing process. Melt flow at 446T and a load of 4.75 pounds is an index of melt viscosity, which is related to the weight-average molecular weight. The melt flow rate of commercial polypropylene ranges from as low as 0.25 grams/10 minutes to as high as 800 grams/10 minutes. The molecular weight distribution is expressed as the ratio of the weight-average molecular weight to the number-average molecular weight; for highly crystalline PP, this ratio can be as high as 11 ； The PP used for melt-blown fabrics can be as low as 2.1. This ratio is extremely important during fiber spinning, and it affects extrusion, extrudate swelling, internal stresses in molding, and the orientation process. Like most polymers, polypropylene oxidizes, especially during melt processing. Regarding PP, free radicals that remove attack on the secondary hydrogen are employed to protect the polymer. For PP used under high temperatures for extended periods, complex multi-component stabilizer systems are employed ； For applications where odors or tastes need to be suppressed, the stabilization system must be very simple. If used to protect against sunlight (ultraviolet rays), carbon black can be added or specialized stabilization methods can be employed. The tensile strength of ordinary PP is 34.5 MPa, and its flexural modulus is approximately 1723 MPa. There is glass-filled PP with a tensile strength of 100 MPa and a flexural modulus of 9650 MPa. The flexural modulus of mineral-filled grade PP can reach up to about 4480 MPa, but the tensile strength increases only slightly. Remains ductile at temperatures below -75°F. Impact-resistant copolymers with tensile strengths as low as 186 MPa and flexural moduli as low as 689 MPa are no longer novel products. Modern polymerization processes can produce materials that fill the gap between polypropylene and olefinic rubbers. Apart from strong oxidizing agents and non-polar solvents, PP has strong resistance to chemical erosion. For example, fuming nitric acid or hot concentrated sulfuric acid can degrade PP, but solutions with lower concentrations are harmless to PP. Liquids such as gasoline, xylene, and chlorinated hydrocarbons can swell and soften PP. The degree of swelling of the copolymer is higher than that of the homopolymer. After being removed from such solvents, PP regains its original dimensions. Due to the extreme inertness of the PP surface, it is difficult to print, paint, or bond on PP without using flame treatment or similar techniques. Polypropylene has a high heat of combustion, making it difficult to produce flame-retardant grades of it, but several grades of flame-retardant PP are available on the market. PP is also an excellent electrical insulator, with very low dielectric constant and loss factor. It has good moisture resistance, but it is not a good material for blocking oxygen. Application: Fibers represent a major market for PP. By stretching or orienting it, its tensile strength can be increased by as much as 15 times. Filament products include clothing, diapers, and non-wovens. Furniture leather, agricultural bags, ropes, floor coverings, straps. Rugs and carpet backing. PP can also be cast or orientally drawn into films. Directed films can be used as cigarettes. Packaging materials for candies and many other items ； Unoriented films are used in capacitors or packaging materials. PP sheets are used to manufacture heat-formed food containers, and their moisture-resistant as well as gas and flavor-permeable properties must meet FDA regulations. New ultra-low modulus products can be rolled using rolling mills and compete with soft vinyl resins for market share. PP can be processed into hollow products through injection blow molding, extrusion blow molding, or stretch blow molding. To improve blow molding and thermoforming properties, high melt strength grades of PP have been developed. There is rapidly processed extruded adhesive-grade PP available on the market ； The excellent electrical properties of polypropylene make it suitable as an insulating material for telephone lines and data transmission cables. Many different types of injection-molded components are made from PP or its impact-resistant copolymers. In the automotive market. The copolymer is used for interior trims and panels, as well as external components and batteries, etc ； Homopolymers and filled grades are used for engine compartment covers or as dashboards. Glass-filled grade PP is used as components for decorations or household items. All filler-free PP resins can be used for household items and medical devices, including disposable radiation-sterilized products; low-cost homopolymers and random copolymers are primarily used. Thin-walled molded containers have expanded the scope of the traditional PP packaging market, such as tamper-evident seals and dispensers. PP homopolymer: Polypropylene (PP), as a thermoplastic polymer, began to be produced commercially in 1957 and was the first among regularly structured polymers. Its historical significance lies in the fact that it has always been the fastest-growing major thermoplastic; in 1991, its global production volume reached 24 billion pounds. It has a very wide range of applications in the field of thermoplastics, especially in fibers and filaments, film extrusion, injection molding, and other areas. Chemistry and properties: PP is synthesized from propylene monomer using metal-organic stereospecific catalysts (Ziegler-Natta type) under controlled temperature and pressure conditions. Due to the differences in the catalysts used and the polymerization processes, the resulting polymers exhibit three different types of stereological structures, with varying numbers of each. These three structures refer to isotactic polymers, syndiotactic polymers, and atactic polymers. In isotactic polypropylene (the most common commercial form), the methyl groups are all on the same side of the polymer backbone, a structure that readily forms a crystalline state. The crystalline structure in the isotactic form endows it with excellent resistance to solvents and heat. The catalyst technology used during the first decade minimized the formation of non-isotactic isomers, eliminating the need to separate unwanted random components and simplifying the production process. There are mainly two processes for producing polypropylene: one is the gas-phase method ； One is the liquid propylene slurry method. In addition, there are some old-fashioned slurry process units in operation that use a liquid saturated hydrocarbon as the reaction medium. The properties of typical isotactic polypropylene homopolymers are shown in Table 1. In comparison, both high-density and low-density polyethylene have high densities, quite low melting points, and low bending moduli, that is, low stiffness. These performance differences lead to different end uses. Their stiffness and easy orientability make polypropylene homopolymers suitable for producing various fibers and elongated strips, while their high heat resistance enables them to be used in manufacturing rigid high-pressure containers and appliances as well as molded parts for automobiles. The main factors affecting the processing and physical properties of polypropylene homopolymers include: molecular weight (usually expressed in terms of flow rate) ； Molecular weight distribution (abbreviated as MWO) ； There is stereoregularity and additives. The average molecular weight of polypropylene ranges from about 200,000 to 600,000. The molecular weight distribution is usually expressed as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the polymer, namely Mw/Mn. This formula is also known as the polydispersity index. The molecular weight distribution of a polymer has a crucial impact on its processability and ultimate performance. This is because molten polypropylene is sensitive to shear, that is, its apparent viscosity decreases as the applied pressure increases. Polypropylene with a wide molecular weight distribution is more sensitive to shear than that with a narrow distribution; therefore, materials with a wide range of molecular weights are easier to process during injection molding. Certain specific applications, especially in fibers, require a narrow molecular weight distribution. The molecular weight distribution is related to both the catalyst system and the polymerization process. Common peroxides are chemically cracked during the extrusion process after the reactor, thereby narrowing the molecular weight distribution range. This process is called controlled rheology (CR) process. Compared to polyethylene, isotactic polypropylene is more susceptible to oxidative degradation caused by light and heat. Under normal processing and end-use conditions, polypropylene is subjected to random chain scission, resulting in a decrease in molecular weight and an increase in flow rate. All commercial-grade polypropylene contains stabilizers to protect the material during processing and to ensure satisfactory performance in end use. For special applications, in addition to antioxidants and UV inhibitors, other additives must also be added. For example: Lubricants and anti-adhesive agents are added to the film formula to reduce the coefficient of friction and prevent the film from sticking to itself. Antistatic properties are added to packaging materials to eliminate static charges. To improve transparency or shorten the model cycle, nucleators are required. Homopolymer resins are usually classified by flow rate and end use. The flow rate depends on both the average molecular weight and the molecular weight distribution. Certain special applications require flow rates as high as 400 grams per minute, whereas the flow rates for ordinary commercial homopolymers range from 0.5 to 50 grams per minute. Flow rate is usually the most important factor in determining processing characteristics. Processing and application: The excellent flow properties and wide range of flow rates of polypropylene, combined with other unique polymer characteristics, give it superior processing capabilities. A lower flow rate can meet the processing requirements for extruded strips, ribbon-like filaments, and single filaments; it also enables the finished products to have tensile strength and low elongation, while maintaining sufficient lateral integrity, thereby minimizing cracking and dust generation in the guiding devices of the winding machine. To counteract their inherent low lateral strength and tendency to fracture (fibrillation), film-to-fiber products with a higher degree of orientation, such as coarse-textured textiles, thin ropes, and cords, typically require a flow rate in the range of 7 to 20. Decorative strip products containing a blowing agent are extruded from polypropylene at a flow rate of approximately 10, so as to achieve an appropriate balance between melt strength and orientation ability. With moderate degree of orientation, this polymer yields a smooth, satin-like surface finish, and the product possesses sufficient lateral strength to delay fracture. The extrusion of non-woven and multi-filament products requires a material with low viscosity and free flow; therefore, polypropylene with very high flow rates is used for these purposes. Cast PP films are widely used in the field of graphic artworks. Furthermore, the film can be biaxially oriented and heat-set, resulting in excellent mechanical and thermal properties, making it suitable for use in various high-performance laminated materials and packaging materials. The tubular water-cooling quenching process can be used to process PP into co-extruded blown films as well as single-layer films. Extruded sheets for thermoforming require materials with a low-flow-rate formula to achieve sufficient melt strength. When using PP extruded profiles, better processing performance is always achieved at lower flow rates. Profile extrusion is usually limited to smaller cross-sections so that rapid water cooling can be used to ensure the product has sufficient toughness. PP can also be extruded into tubular products, such as drink straws and drinking water pipes. PP is also used in cable coatings. In terms of usage volume, injection molding, second only to extrusion, is well-suited to the properties of polypropylene. The good flow properties and strong mechanical strength of PP are utilized to produce a wide variety of products that possess inherent strong mechanical properties. Good processability and excellent resistance to stress fracture result in superior molded sealing covers. Generally, low-flow-rate formula materials are used to produce thick-walled products and those that require toughness. Materials with high flow rates are used to produce thin-walled components and products that require rapid processing. Market PP homopolymers can be processed using various techniques to produce a wide range of products. Extruded products represent the largest market for PP, with textile fibers and filaments being the biggest segment within this market. For a long time, PP has been the main raw material for manufacturing fibers, owing to its coloring ability, wear resistance, chemical resistance, and favorable economic conditions. Directed and undirected films account for the second largest share of the extruded products market, and they are areas that will continue to experience growth. Next, injection-molded products represent the second-largest market for PP homopolymers, including applications in containers, seals, the automotive industry, household items, toys, and many other consumer and industrial end uses. Many blow-molded containers are made of polypropylene due to its excellent moisture resistance and sufficient clarity. Given the new demand for plastic products in the future, PP homopolymers will continue to experience growth. Good economic conditions, excellent mechanical properties, as well as characteristics such as light weight, strong coloring ability, and ease of processing, will continue to make PP the preferred material for many applications throughout this century. Impact-resistant PP copolymers: PP possesses many useful properties, but it lacks inherent toughness, especially at temperatures below its glass transition temperature. However, its impact resistance can be improved by adding impact modifiers. The traditional modified version is an elastomer, usually EPDM. It is generally believed that the rubber particles dispersed within the semi-crystalline polypropylene matrix can create numerous stress concentration points at the interfaces, and 5 ； It induces localized deformation to prevent the propagation of fractures. Impact modifiers have always been added during blending, but recently, the in-situ synthesis of elastomeric components has gained commercial significance. Moreover, there is promotion of using a new series of impact modifiers to replace EPDM, namely Flexomer polyolefins, Exact elastomers, and Insite polymers. These are all olefin polymers that fill the gap between very low-density polyethylene and conventional EPDM. Chemistry and properties: Isotactic PP homopolymers are produced by the polymerization of propylene under the catalysis of Ziegler-Natta catalyst systems. The EPDM component is synthesized in a series of reactors, or purchased in advance and then blended with the PP homopolymer in an extruder. The produced impact-resistant polypropylene is sold after granulation. The impact-resistant PP copolymers produced on-site can have their key properties precisely controlled by selecting appropriate catalyst compositions and reactor conditions. The catalyst composition and reactor conditions determine the crystallinity of the matrix resin, the composition and amount of the rubber component, as well as the overall molecular weight distribution. Impact-resistant PP is one of the lightest thermoplastics, with a density of less than 1, and its cost per pound is lower than that of PET, PBT, high-impact polystyrene, and ABS. On a volume basis, the unit cost of impact-resistant PP is lower than that of the aforementioned resins and polyvinyl chloride (PVC). Only HDPE can compete with it in this regard. Impact-resistant PP is usually processed at moderate temperatures, ranging from 350 to 550°F. Impact-resistant polypropylene copolymers have a wide range of melt flow rates, typically ranging from less than 1 to about 30. Resins with the highest melt flow rate are usually obtained by \"viscosity-reducing cracking\" of materials with lower melt flow rates. In other words, a further reaction is carried out on the material that comes out of the reactor to reduce its average molecular weight, thereby producing a product with a higher melt flow rate. Impact-resistant polypropylene copolymers exhibit high resistance to chemical and environmental stress cracking. After treatment, the material can exhibit excellent cantilever beam impact strength and lower Gardner impact properties. The cantilever beam impact strength ranges from 0.5 to over 15 foot-pounds per inch ； At -40°F, the Garner impact strength ranges from 15 to over 300 inch-pounds. The rubber component, polypropylene, provides impact strength, but it reduces the stiffness and heat distortion temperature of impact-resistant polypropylene compared to the homopolymer. Impact-resistant polypropylene copolymers with fillers can withstand higher temperatures without deforming. The filler is generally glass fiber. Mica, talc, and calcium carbonate. End-users of these polymers should be aware that for each product specification, a trade-off must be made between different melting strengths, melt flow rates, stiffness, and heat distortion temperatures. Uses The main commercial use of impact-resistant polypropylene is in injection-molded parts for automobiles, household items, and appliances. Its impact resistance, low density, coloring ability, and processability make it an ideal material. Medium impact resin grades with higher melt flow rates possess better flow properties, a feature that is particularly useful when injection molding large components such as automotive panels. Resins with high impact resistance and a low melt flow rate (usually less than 2) can be converted into films with excellent puncture resistance; such films, owing to their impact resistance and resistance to steam sterilization, are suitable for use as bags for disposable medical waste. Extruded sheets can be processed using thermoforming to create large and thick components, such as guards in the automotive industry and linings for car trunks. The mechanism by which the elastomeric component improves the impact resistance of polypropylene is the induction of stress whitening when the material is subjected to impact. Most applications are based on the dispersion degree of the elastic components in the polypropylene matrix. Based on the opposite concept, new types of bumpers are being developed. As a result, a molecular composite structure is formed. The 5 major production processes for polypropylene PP: Currently, the production processes for polypropylene can be classified into five categories based on the type of polymerization: solution method, slurry method, bulk method, gas phase method, and a combination of bulk and gas phase methods. The specific processes include BP’s gas-phase Innovene process, Chisso’s gas-phase process, Dow’s Unipol process, the Novolene gas-phase process, the Sumitomo gas-phase process, Basell’s bulk process, the Hypol process developed by Mitsui, and Borealis’s Borstar process, among others. 1 Slurry Process The Slurry Process, also known as the slurry method or solvent method, is the earliest process technology used in the world for producing polypropylene. From the first industrial installation in 1957 to the mid-to-late 1980s, the slurry process remained the primary method for producing polypropylene for a period of 30 years. Typical processes mainly include the Montedison process in Italy, the Hercules process in the United States, the Mitsui Toyo Chemical process in Japan, the Amoco process in the United States, the Mitsui Oil Chemical process in Japan, and the Solvay process, among others. The development of these processes was based on the first-generation catalysts of that time, using vertical stirred-tank reactors; desilication and removal of random substances were required. Due to the different solvents used, the process flows and operating conditions varied. In recent years, the proportion of the traditional slurry process in production has decreased significantly; the slurry-based products that remain are mainly used in high-value applications such as specialty BOPP films, blow-molded films with high molecular weights, and high-strength pipes. In recent years, improvements have been made to this method. The improved slurry process utilizes highly active second-generation catalysts, allows the catalyst de-ashing step to be omitted, reduces the formation of random polymers, and can be used to produce homopolymers, random copolymers, impact-resistant copolymer products, and more. Currently, the production capacity of slurry-process PP worldwide accounts for about 13% of the total global PP production capacity. 2 Gas-phase process The research and development of the gas-phase polypropylene process began in the 1960s; in 1967, BASF built a pilot plant for this gas-phase polypropylene process using vertical stirred-tank reactors in Ludwigshafen. In 1969, the joint venture ROW Company, formed by BASF and Shell, built the world’s first industrial facility for the production of gas-phase polypropylene with a capacity of 25,000 tons per year in Wesseling, Germany, using vertical stirred-bed reactors; this process was named the Novolen process. In the 1970s, the American company Amoco developed a gas-phase PP production process that utilized a horizontal stirred-tank gas reactor with a flow pattern similar to that of a piston flow. In the early 1980s, UCC Company applied its mature gas-phase fluidized bed Unipol polyethylene process to polypropylene production, introducing the Unipol gas-phase polypropylene process. Japan’s Sumitomo Company also developed a gas-phase process using a gas-phase fluidized bed during the same period. Currently, the main gas-phase PP production processes in the world include BP’s Innovene process, Chisso’s process, LyondellBasell’s Unipol process, BASF’s Novolen process, and Sumitomo Chemical’s Sumitomo process. (1) Innovene process. The Innovene process is also known as the BP-Amoco process. The main feature of the process is the use of a unique horizontal stirred-tank reactor with a flow pattern close to that of piston flow. With this unique reactor, copolymer products with excellent rigidity and impact resistance can be produced, as the particle residence time distribution is very narrow. This type of reactor with **push-flow can prevent catalyst short-circuiting. In the presence of ethylene, large-particle copolymers can be formed, rather than fine particles within homopolymer particles; these fine particles would reduce the low-temperature impact strength of the copolymer and cause unnecessary gelation. Therefore, the narrow reaction residence time distribution of this process enables it to meet the requirements for high-impact copolymers that can only be produced using multiple fully mixed reactor polymerization units. Furthermore, due to this unique reactor design, the product transition time for this process is very short; theoretically, it is 2/3 shorter than that in continuous stirred-tank reactors or fluidized-bed reactors. As a result, product switching is easy, and there is little transitional product. The Innovene process uses propylene flashing to remove heat. Liquid propylene is injected into the reactor from various feed points in a manner that keeps the reactor bed dry. After vaporizing, the partial pressure of the monomer is lower than its dew point pressure, which is sufficient to remove the heat of reaction. During operation, it is necessary to strictly control the feed rate of liquid propylene and its vaporization in the reactor, in order to maintain a balance among the degree of dryness of the bed, its fluidization level, and the reaction temperature range. The air lock system is another feature of this process. When the material is transferred from the first reactor to the second reactor, the air lock system prevents cross-flow between the two reactors. Especially when producing copolymers, the gas compositions in the two reactors are different: the first reactor contains a large amount of hydrogen, while the second reactor contains ethylene along with a small amount of hydrogen. If hydrogen from the first reactor enters the second reactor, or if ethylene from the second reactor enters the first reactor, it will severely affect the quality of the product. Therefore, it is crucial to isolate the two reactors from each other. The CD catalyst used in this process exhibits excellent morphological control, high activity and selectivity; it enables the control of the formation of random polypropylene. The resulting product has a high isotactic index, a narrow particle size distribution, good flowability of the powder, low ash content, and an excellent color. The use of this catalyst allows the process flow to be simplified. All grades of products can be produced using only one type of catalyst, without the need to switch catalysts. The activity of the CD catalyst ranges from 25,000 to 55,000 kg PP/kg cat, depending on the purity of the raw materials and the number of reactors. The isotactic index of the powder products produced can reach up to 99%. Another advantage of CD catalysts is that they require no pretreatment or pre-polymerization; they can be added directly to the reactor, and this catalyst can be used to produce all polypropylene products. The MFR of the homopolymer products obtained using this process can range from 0.5 g/10 min to 100 g/10 min, and the toughness of these products is higher than that of products produced by other gas-phase polymerization processes ； The MFR of the random copolymer products is 2–35 g/10 min, and their ethylene content can reach 7%–8% (by mass) ； The MFR of the impact-resistant copolymer products ranges from 1 to 35 g/10 min, with an ethylene content of 5% to 17% (by mass). Due to the plug-flow reactor design, the catalyst residence time distribution is narrow, resulting in a more uniform distribution of the rubber phase in the impact-resistant copolymer and superior properties, especially a better balance between impact resistance and stiffness. This process can also use a single reactor to produce homopolymers and random copolymers, but it has its drawbacks: the ethylene content (or the proportion of the rubber component) in the product is not high, making it impossible to obtain PP products with high or ultra-high impact resistance. Another important feature of this process is that the polymerization reaction can be stopped quickly and smoothly by ceasing catalyst injection (in about 15–20 minutes), and it can be restarted after a few hours without affecting the conditions inside the reactor or the quality of the polymer. In the event of an outage or similar incident, the reactor can be brought to a stop within 3 minutes by releasing its pressure, either through an emergency shutdown or a gradual shutdown; it can then be restarted after the pressure is restored and catalyst is added. Due to the short process flow of the Innovene process, its unique reactor design, relatively low polymerization pressure, and the absence of large rotating equipment, its electricity consumption is among the lowest among various PP processes. Since it is a gas-phase polymerization system, there is no need to heat the liquid propylene discharged along with the polymer in the reactor using steam, as is the case in the liquid-phase method; as a result, steam consumption is very low, and the energy consumption for producing homopolymer products is the lowest among all processes. Like other gas-phase processes, the Innovene gas-phase process does not involve large amounts of liquid hydrocarbons in the polymerization system, making it inherently safer than the bulk method. The operating pressure of the Innovene gas-phase process reactor is the lowest among various process technologies. The polymerization system produces no wastewater, making it a clean production process. Currently, there are over 10 polypropylene production plants in the world that use the Innovene process technology, with a total production capacity of around 3 million tons per year, accounting for approximately 7.6% of the world’s total polypropylene production capacity ； 3 Combined bulk-vapor process: The combined bulk-vapor process mainly includes Basel’s Spheripol process, Mitsubishi Chemical Corporation’s Hypol process, and Nordic Chemicals’ Borstar process, among others. (1) Spheripol process. The Spheripol process was developed by Basell Polyolefins. Since its first industrialization in 1982, this technology has been the most successful and widely used polypropylene production process to date. The Spheripol process is a polymerization technique that combines liquid-phase pre-polymerization with homogeneous liquid-phase polymerization and gas-phase copolymerization. It uses efficient catalysts; the PP powder produced has a spherical shape, with large and uniform particles whose distribution can be adjusted to be either wide or narrow. A wide range of multi-purpose products can be manufactured. Their homopolymer and random copolymer products are characterized by high purity, excellent optical properties, and no odor. The liquid-phase loop reactor used in the Spheripol process has the following advantages: (a) it features a very high space-time yield for reactions (up to 400 kg PP/h·m3), with a small reactor volume and thus lower investment costs ； (b) The reactor has a simple structure and low material requirements; low-temperature carbon steel can be used, and its design and manufacture are straightforward. Due to the small pipe diameter (DN500 or DN600), the pipe walls remain thin even at high pressures ； (c) The straight leg section of the jacketed reactor can serve as pillars for the reactor frame, and this structural design reduces costs ； (d) Due to the small reactor volume, short residence time, fast product switching, and minimal transition material ； (e) Polymer particles are suspended in liquid propylene, with good heat transfer between the polymer and propylene. The use of a cooling jacket for removing the heat of reaction provides a large heat transfer area per unit volume, resulting in a high heat transfer coefficient; the overall heat transfer coefficient of the loop reactor can reach up to 1600 W/(m2·℃) ； (f) The slurry in the loop reactor is circulated at high speed by an axial flow pump, with a fluid flow rate of up to 7 m/s; this enables uniform mixing of the polymer slurry and even distribution of the catalyst system. The polymerization conditions can be easily controlled with high precision, resulting in uniform product quality. Hot spots are less likely to form, and sticking to the reactor walls is also less probable. Additionally, the axial flow pump has low energy consumption ； (g) The polymer slurry concentration in the reactor is high (mass fraction greater than 50%), and the reactor exhibits a high space-time yield; the space-time yield for the homopolymerization of propylene is 50%-60%. These characteristics make the loop reactor highly suitable for producing homopolymers and random copolymers. In the initial stage of the Spheripol process, high-performance catalysts such as GF-2A, FT-4S, and UCD-104 were used; the catalyst activity reached 40 kg PP/gcat, and the isotacticity of the product was 90%-99%, with no need for degreasing or removal of random components. The technology has now progressed to its second generation. Compared to the first-generation technology that uses single-loop tubular reactors, the second-generation technology employs double-loop tubular reactors, resulting in significantly higher operating pressures and temperatures, and enables the production of bimodal polypropylene. The catalyst system uses fourth-generation or fifth-generation Z-N high-efficiency catalysts; a hydrogen separation and recovery unit has been added, and the high-pressure and low-pressure degassing equipment for polymers has been improved. The vaporization, drying, and propylene emergency emission units have also been upgraded, which enhances operational flexibility and efficiency. Meanwhile, consumption of raw material monomers and various utility resources has been significantly reduced. The resulting product has a more uniform particle size, and its melt flow index ranges over a wider interval (from 0.3 to 1600.0 g/10 min), enabling the production of new PP grades with high rigidity, high crystallinity, and low heat-sealing temperatures. The impact-resistant copolymerization process in the Spheripol process is carried out using a gas-phase method, with the reactor being one or two series-connected dense-phase fluidized bed reactors. The reactor uses a gas-phase dense-bed fluidized bed. A single gas-phase reactor system can be used to produce impact copolymers with an ethylene content of 8%-12% (by mass). If it is necessary to produce special impact copolymers with a higher rubber phase content and possibly more than one dispersed phase (such as products prone to low-stress whitening), then two gas-phase reactor systems need to be designed, ensuring that the gas compositions and operating conditions in each of these systems remain independent; this allows for the production of two different copolymers to be added to the homopolymer. By using a two-step process of steaming and drying for the polymer treatment, it is possible to easily condense the vapors in the steaming exhaust gases, thereby separating out pure hydrocarbon monomers. This allows for the complete recovery of the hydrocarbons present in the exhaust gases, reducing the consumption of monomers. The closed-loop nitrogen drying system also reduced the nitrogen consumption of the device. Furthermore, the Spheripol process features a modular design that can meet the requirements of different users, allowing for phased construction (for example, starting with a homopolymer production system and adding a gas-phase reaction system at an appropriate time); the production capacity of the facility can also be easily increased. The Spheripol process features a strictly designed and comprehensive safety system, ensuring high operational stability and safety for the equipment. The new generation of Spheripol process uses pure additives in the system, resulting in more uniform and stable product quality as well as facilitating product switching. The Spheripol process technology can produce a wide range of products, including homopolymers, random copolymers, impact-resistant copolymers, and terpolymers (ethylene-propylene-butylene copolymers). The MFR range of their homopolymer products is 0.1–2000 g/10 min, while that of industrial-grade products reaches 1860 g/10 min (for special non-granulated products); the bending modulus of highly rigid products amounts to 2300 MPa. Industrially produced random copolymer products contain up to 4.5% (by mass) ethylene, and there are also ethylene-propylene-butylene tercopolymer products whose film sealing temperature is as low as 110°C, allowing them to compete with high-ethylene-content random copolymers produced by vapor-phase processes. Impact-resistant copolymer products exhibit excellent combined properties of stiffness and impact resistance. The ethylene content in these products can reach up to 25% (with 40% rubber phase), and they are also capable of having an ethylene content of 40% (with 60% rubber phase). Furthermore, the Spheripol process allows for flexible adjustment of the molecular weight distribution of the product within the range of 3.2–12 in terms of the polymer dispersion index, by adding peroxides and using a double-loop reactor; it enables the direct production in the reactor of products with an MFR as high as 1800 g/10 min, as well as large particles that do not require granulation, which makes the Spheripol process highly competitive. Another feature of the Spheripol process is advanced catalyst technology. Basell offers a variety of catalyst systems that can be used in the Spheripol process to produce different types of products. For example, the MC-GF2A catalyst is used for producing homopolymers, while MC-M1 is used to produce large-sphere impact copolymers as well as homopolymers and random copolymers. High-modulus homopolymers require the use of D-donors such as dicyclopentyldimethoxysilane (abbreviated as DCPMC), and special impact copolymers with high vinyl content also need specialized catalysts. The diether catalysts for which Basell has filed multiple patents also have commercial products available, such as MC-126 and MC-127. Diether catalysts exhibit high polymerization activity (up to 100 Tpp/kg cat) and a long service life, good control over the isotactic index, high sensitivity to hydrogen, and a narrow molecular weight distribution of the products. Currently, there are nearly 100 polypropylene plants around the world that use the Spheripol process, with a total production capacity of about 14.6 million tons per year, accounting for approximately 36.8% of the world’s total polypropylene production capacity. Of this, the production capacity in North America is 4.03 million tons per year, while in Asia it totals 4.19 million tons per year. The production capacity in Western Europe is 4.105 million tons per year, that in Central and Eastern Europe is 620,000 tons per year, and that in the Middle East and Africa is 1.315 million tons per year ； The research and development of the bulk process began in the 1960s; in 1964, the American company Dart built the world’s first industrial-scale polypropylene production facility using a batch reactor. After 1970, companies such as Sumitomo in Japan, Phillips, and EI Psao in the United States all achieved industrial production of the liquid-phase bulk polypropylene process. Compared with the slurry method that uses solvents, polymerization using the liquid-phase propylene bulk method features the absence of inert solvents, a high monomer concentration in the reaction system, a fast polymerization rate, high catalyst activity, a high conversion rate for the polymerization reaction, a greater space-time production capacity of the reactor, lower energy consumption, a simpler process flow, fewer pieces of equipment, lower production costs, and reduced amounts of waste ； It facilitates the removal of polymerization heat, simplifies heat removal control, and increases the amount of polymerization per reactor ； It has advantages such as the ability to remove low-molecular-weight random polymers and catalyst residues that have an adverse effect on the properties of the product, thereby yielding high-quality products. The drawback is that the reaction gas needs to be vaporized and condensed before it can be recycled back to the reactor. The large volume of high-pressure liquid hydrocarbon material in the reactor poses a potential risk. Furthermore, the concentration of ethylene in the reactor cannot be too high; otherwise, a separate gas phase is formed within the reactor, making it difficult to operate the reactor. As a result, the ethylene content in the resulting copolymer product is not very high. 4. Bulk process: The main difference among various bulk process routes lies in the type of reactor used. Reactors can be divided into two main categories: batch reactors and loop reactors. A kettle-type reactor utilizes the latent heat of liquid vaporization to remove reaction heat; most of the vaporized gas is returned to the reactor after being condensed in a cycle, while the uncondensed gas is pressurized by a compressor and then recycled back to the reactor. In contrast, a loop reactor uses an axial flow pump to circulate the slurry at high speed, and heat is removed through jacket cooling. Due to its large heat transfer area, it provides effective heat removal, resulting in a high yield per unit reactor volume and low energy consumption. According to the polymerization process flow, the bulk method of production can be divided into two types: batch polymerization process and continuous polymerization process. (1) Intermittent bulk method process. The intermittent bulk polymerization technology for polypropylene is a production technique that has been successfully developed independently in China. It has advantages such as reliable production process technology, low requirements for the quality of the raw material propylene, available catalysts domestically, a simple process, low investment costs and quick returns, easy operation, flexible adjustment of product grades, and minimal generation of waste; however, its drawback is that the production scale is small, making it difficult to achieve economies of scale ； The device relies heavily on manual operation, operates in a batch mode, has a low level of automated control, and results in unstable product quality ； The consumption quota for raw materials is relatively high ； There are few varieties and grades of the products, their quality is not high, and their applications are limited. At present, the polypropylene production capacity in China using this method accounts for approximately 24.0% of the country’s total production capacity ； (2) Continuous bulk method process. This process mainly includes the Rexall process from the United States, the Phillips process from the United States, and the Sumitimo process from Japan. (a) Rexall process. The Rexall bulk polymerization process is a production method that lies between the solvent-based and bulk polymerization processes; it was developed by the American company Rexall. This process uses vertical stirred-tank reactors to carry out polymerization with liquid propylene containing 10%-30% (by mass) propane. The use of an azeotropic mixture of hexane and isopropanol as a solvent during polymer degreasing simplifies the distillation process; the residual catalyst and random polypropylene are dissolved in this solvent and removed from the bottom of the solvent distillation column. Later, the company, together with the U.S.-based El Paso Company to form Joint Thermoplastic Company, developed a new production process known as the \"liquid pool process.\" By using the HY-HS high-efficiency catalyst from Montedison-MPC, the ash removal step was eliminated, further simplifying the production process. The characteristic of this process is that it uses high-purity liquid propylene as raw material and an HY-HS high-efficiency catalyst, without any steps for degreasing or removing random components. A continuous stirred-tank reactor is used; the heat of polymerization is removed through the reactor jacket and the top condenser. After flash separation of the slurry, the monomer is recycled back to the reactor ； (b) Phillips process. This process was successfully developed by the American Phillips Petroleum Company in the 1960s. Its process features the use of a unique loop reactor. This simply structured loop reactor offers advantages such as a large heat transfer area per unit volume, a high overall heat transfer coefficient, a high conversion rate in a single pass, fast flow rates, good mixing, no formation of plasticized lumps in the polymerization zone, and short turnaround times for changing product grades. This process can produce polypolymers and random polymers with a wide range of melt flow rates ； (c) Sumitimo process. This process was successfully developed in 1974 by the Japanese company Sumitomo Chemical. This process is basically similar to the Rexene bulk method, but the Sumitimo bulk method includes some measures to remove random substances and catalyst residues. Through these measures, superpolymers can be produced for certain electrical and medical applications. The Sumitimo bulk process uses an SCC complexing catalyst (titanium tetrachloride reduced with monochlorodiethylaluminum and treated with n-butyl ether), and liquid propylene is polymerized at 50–80°C and 3.0 MPa. This method yields a high reaction rate and a high degree of isotropy in the polymer. An efficient extractor is employed for de-ashing, resulting in a product with an isotropy index of 96%–97%. The product takes the form of spherical particles, possesses high rigidity, good thermal stability, as well as excellent oil resistance and electrical properties. Solution process 5 Solution process The solution process was an early production method used for manufacturing crystalline polypropylene, and it was proprietary to Eastman Company. This process uses a specially modified catalyst system – lithium compounds such as aluminum lithium hydride – to accommodate high solution polymerization temperatures. The catalyst components, monomer, and solvent are continuously fed into the polymerization reactor, while the unreacted monomer is separated and recycled by reducing the pressure of the solvent. Additional solvent is added to reduce the viscosity of the solution, and the residual catalyst is removed by filtration. The solvent is concentrated through multiple evaporators, and then a compressor capable of removing volatile substances is used to produce a solid polymer. Solid polymers are further purified by extraction with heptane or similar hydrocarbons, which also removes amorphous polypropylene; the use of ethanol and multi-step distillation is eliminated. This method is primarily used to produce special grades of products with lower modulus and higher toughness compared to those produced by the slurry method. This method has a complex process flow and high costs; it requires high polymerization temperatures. Additionally, the use of special high-temperature catalysts limits the range of applications for the resulting product, and as such it is no longer used in the production of crystalline polypropylene.