PP plastic raw material, chemical name: polypropylene, features: low density, strength, stiffness, hardness and heat resistance are better than low-pressure polyethylene, can be used at about 100 degrees. It has good electrical properties and high-frequency insulation, which is not affected by humidity, but becomes brittle, not wear-resistant and easy to age at low temperatures. It is suitable for making general mechanical parts, corrosion-resistant parts and insulating parts.

Processing technology

Drying treatment

If stored properly, no drying treatment is required. Melting temperature: 220~275C, be careful not to exceed 275C.

Mold temperature

40~80C, 50C is recommended. The degree of crystallization is mainly determined by the mold temperature.

Injection pressure

Can be as high as 1800bar. Injection speed: Generally, high-speed injection molding can reduce internal pressure to a minimum. If defects appear on the surface of the product, low-speed injection molding at a higher temperature should be used.

Runners and gates

For cold runners, the typical runner diameter range is 4~7mm. It is recommended to use injection ports and runners that are round throughout. All types of gates can be used. The typical gate diameter range is 1~1.5mm, but gates as small as 0.7mm can also be used. For edge gates, the minimum gate depth should be half the wall thickness; the minimum gate width should be at least twice the wall thickness. PP materials can fully use hot runner systems.

Molding characteristics

1. Crystalline material, low hygroscopicity, prone to melt rupture, and easy to decompose in long-term contact with hot metal.

2. Good fluidity, but large shrinkage range and shrinkage value, prone to shrinkage holes, dents, and deformation.

3. Fast cooling speed, the pouring system and cooling system should slowly dissipate heat, and pay attention to controlling the molding temperature. The material is easy to be oriented at low temperature and high pressure. When the mold temperature is lower than 50 degrees, the plastic part is not smooth, and it is easy to produce poor welding and flow marks. It is easy to warp and deform above 90 degrees.

4. The wall thickness of the plastic must be uniform, avoid lack of glue and sharp corners to prevent stress concentration.

Polypropylene resin is one of the four general-purpose thermoplastic resins (polyethylene, polyvinyl chloride, polypropylene, polystyrene). It is produced by polymerization reaction with propylene as raw material and ethylene as comonomer.

The process methods used to produce polypropylene in the world are mainly divided into the following categories: solvent method, solution method, liquid phase bulk method (including liquid phase and gas phase combination) and gas phase bulk method. The characteristics of each process are briefly introduced as follows:

Solvent polymerization method

The solvent method (also known as slurry method or mud method, slurry method) is the earliest polypropylene production process, but due to the deashing and solvent recovery process, the process is long and complicated. With the advancement of catalyst research technology, since the 1980s, the solvent method has tended to stagnate and has been gradually replaced by the liquid phase bulk method.

Process characteristics:

(1) Propylene monomer is dissolved in an inert liquid phase solvent (such as hexane), and solvent polymerization is carried out under the action of a catalyst. The polymer is suspended in the solvent in a solid granular state, and a kettle stirred reactor is used;

(2) There are deashing and solvent recovery processes, the process is long and complicated, the equipment investment is large, and the energy consumption is high. However, the production is easy to control and the product quality is good;

(3) The polypropylene particles are separated by centrifugal filtration and then dried by air boiling and extruded into granules.

Solution polymerization

Process features:

(1) Use high boiling point straight chain hydrocarbon as solvent, operate at a temperature higher than the melting point of polypropylene, and the obtained polymer is completely dissolved in the solvent and distributed in a uniform phase;

(2) High temperature gas stripping method evaporates and removes the solvent to obtain molten polypropylene, and then extrude and granulate to obtain granular products;

(3) There is only one manufacturer, Kodak Company in the United States.

Liquid phase bulk method

Containing liquid phase and gas phase combination, the liquid phase bulk method polypropylene production process is a new process developed in the middle and late stages of polypropylene production. This production process came into being seven years after the industrial production of polypropylene began in 1957.

The liquid phase bulk method is used to produce polypropylene. No other solvent is added to the reaction system, and the catalyst is directly dispersed in liquid propylene to carry out propylene liquid phase bulk polymerization. The polymer is continuously precipitated from the liquid propylene and suspended in the liquid propylene in the form of fine particles. As the reaction time increases, the concentration of polymer particles in the liquid propylene increases. When the propylene conversion rate reaches a certain level, the unpolymerized propylene monomer is recovered by flash evaporation to obtain a powdered polypropylene product. This is a relatively simple and advanced industrial production method for polypropylene. The liquid phase bulk process represents the new technology and new level of international polypropylene production in the 1980s.

Process characteristics:

(1) No solvent is added to the system, and the propylene monomer is liquid-phase bulk polymerized in a kettle reactor, and ethylene and propylene are gas-phase copolymerized in a fluidized bed reactor;

(2) The process is simple, with less equipment, less investment, and low power consumption and production costs;

(3) Homopolymerization uses a kettle stirred reactor (Hypol process) or a loop reactor (Spheripol process), and random copolymerization and block copolymerization are both carried out in a stirred fluidized bed.

The typical representative of the liquid phase bulk process is the Spherizone liquid phase bulk process of BASELL. Spherizone is a gas phase circulation technology that uses Ziegler-Natta catalysts to produce polymers with high crystallinity, rigidity and more uniformity while maintaining toughness and processing properties. It can produce highly uniform multi-monomer resins or bimodal homopolymers in a single reactor. The Spherizone circulation reaction has two interconnected zones, and different zones play the role played by the gas phase and liquid phase loop reactors of other processes. The two zones can produce resins with different relative molecular weights or monomer composition distributions, expanding the performance range of polypropylene.

The core equipment of this process is the MZCR (Multi-Zone Circulation Reactor System) reactor R230 system. The reactor consists of two parts: a riser and a downcomer. In the riser, the polymer is blown upward by the reaction gas to form fluidization and sent to the upper part of the downcomer. After passing through the cyclone separator, the powder is collected in the downcomer. The reaction gas is circulated through the external pipeline by a centrifugal compressor, and the reaction heat is removed by the circulator cooler on the external circulation pipeline. The reactor product is discharged through a valve installed at the bottom of the downcomer. After high-pressure and low-pressure degassing, the discharged powder is directly steamed and dried to obtain powder products when producing homopolymers and random copolymers. When producing impact-resistant products, the powder after high-pressure degassing is discharged into the gas-phase fluidized bed reactor. The reactor still uses the Spheripol II gas-phase reactor system. The copolymerization reactor is a vertical cylindrical container with spherical heads at the top and bottom and a boiling bed at the bottom. The main material is stainless steel and the inner surface is polished.

The current maximum single-line production capacity of this process has reached 450,000 tons/year. The ethylene content of the MZCR (multi-zone circulation reactor) impact-resistant copolymer products can be as high as 22% (rubber content greater than 40%), and ternary copolymer products containing ethylene and 1-butene can also be produced.

Gas phase bulk method

Process characteristics:

(1) No solvent is introduced into the system, and propylene monomer is polymerized in the reactor in the gas phase;

(2) The process is short, the equipment is few, the production is safe, and the production cost is low;

(3) The polymerization reactors include fluidized bed, vertical stirred bed and horizontal stirred bed.

The typical representative of the gas phase bulk method is the Unipol gas phase process of DOW Chemical Company. The Unipol gas phase polypropylene process is a gas phase fluidized bed polypropylene process developed by Union Carbide Corporation (UCCP) and Shell in the 1980s. It is a fluidized bed process used in polyethylene production that was successfully transplanted to polypropylene production. The process uses a high-efficiency catalyst system, the main catalyst is a high-efficiency carrier catalyst, and the co-catalyst is triethyl aluminum and an electron donor.

The UNIPOL process is simple, flexible, economical and safe; it can produce a full range of products including homopolymers, random copolymers and impact copolymers with only a few equipment, and the operating conditions can be adjusted within a large operating range to keep the product performance uniform. Because the number of equipment used is small, the maintenance workload is small and the reliability of the device is improved. Due to the limitations of the fluidized bed reaction kinetics itself, and the low operating pressure reduces the storage of materials in the system, the process is safer to operate than other processes, and there is no danger of equipment overpressure in the event of an accident.

This process has no liquid waste discharge and very few hydrocarbons discharged into the atmosphere, so the impact on the environment is very small. Compared with other processes, this process is easier to meet various strict environmental, health and safety regulations. Another notable feature of this process is that it can be operated in conjunction with super-condensed state, the so-called super-condensed gas phase fluidized bed process (SCM). This technology can increase the existing production capacity by 200% by increasing the proportion of liquid phase in the reactor to 45%. Since the amount of liquid is not the basic factor for the instability of the fluidized bed and the formation of polymer agglomerates, the key operating variables of this technology are the density of the expanded bed and the ratio of the expanded bulk density to the settled bulk density. Since the super-condensed state operation can most effectively remove the heat of reaction, it can increase the production capacity of the reactor by more than 2 times without increasing the volume, which is very significant for the investment savings. The ethylene content of the impact copolymer product can be as high as 17% (rubber content greater than 30%).

The core equipment of this process is the gas phase fluidized bed reactor, the circulating gas compressor, the circulating gas cooler and the extrusion granulation unit. The fluidized bed reactor is a hollow container with an expansion section on the top and a distributor on the bottom. The operating pressure of the first reactor is 3.5MPaG and the temperature is 67¡æ. The operating pressure of the second reactor is 2.1MPaG and the temperature is 70¡æ; the circulating gas compressor is a single-stage, constant speed, centrifugal compressor.

Performance

Physical properties

Polypropylene is a non-toxic, odorless, milky white, highly crystalline polymer with a density of only 0.90-0.91g/rm. It is one of the lightest varieties of all plastics currently available. It is particularly stable to water, with a water absorption rate of only 0.01% in water and a molecular weight of about 80,000 to 150,000. It has good formability, but due to its large shrinkage rate (1% to 2.5%), thick-walled products are prone to dents, and it is difficult to meet the requirements for some parts with high dimensional accuracy. The product has a good surface gloss and is easy to color.

Mechanical properties

Polypropylene has a high degree of crystallinity and a regular structure, so it has excellent mechanical properties. The absolute value of the mechanical properties of polypropylene is higher than that of polyethylene, but in plastic It is still a relatively low variety among materials, and its tensile strength can only reach 30MPa or slightly higher. Polypropylene with a larger isotactic index has a higher tensile strength, but with the increase of the isotactic index, the impact strength of the material decreases, but it no longer changes after it drops to a certain value. Temperature and loading rate have a great influence on the toughness of polypropylene. When the temperature is higher than the glass transition temperature, the impact damage is a ductile fracture, and below the glass transition temperature it is a brittle fracture, and the impact strength value drops significantly. Increasing the loading rate can increase the temperature at which ductile fracture transitions to brittle fracture. Polypropylene has excellent resistance to bending fatigue, and its products can be bent 106 times at room temperature without damage. However, at room temperature and low temperatures, due to its molecular structure The regularity is high, so the impact strength is relatively poor. The most outstanding property of polypropylene is its resistance to bending fatigue, commonly known as foldable glue.

Thermal properties

Polypropylene has good heat resistance, and the products can be sterilized at temperatures above 100¡ãC. Without external forces, they will not deform at 150¡ãC. The brittle temperature is -35¡ãC. It will become brittle below -35¡ãC, and its cold resistance is not as good as polyethylene. The reported values of the glass transition temperature of polypropylene include 18qC, 0qC, 5¡ãC, etc. This is also because people use different samples, which contain different proportions of crystalline phase and amorphous phase, resulting in different chain lengths of the amorphous part in the molecular chain. The melting point of polypropylene is about 40-50% higher than that of polyethylene, about 164-170¡ãC, 100%, etc. Regular polypropylene has a melting point of 176¡ãC. Chemical stability Polypropylene has good chemical stability. In addition to being corroded by concentrated sulfuric acid and concentrated nitric acid, it is relatively stable to various other chemical reagents; but low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons and chlorinated hydrocarbons can soften and swell polypropylene. At the same time, its chemical stability increases with the increase of crystallinity, so polypropylene is suitable for making various chemical pipelines and accessories, and has good anti-corrosion effect. Electrical properties It has a high dielectric constant and can be used to make heated electrical insulation products as the temperature rises. Its breakdown voltage is also very high, suitable for use as electrical accessories. It has good voltage resistance and arc resistance, but high static electricity and is easy to age when in contact with copper. Weather resistance Polypropylene is very sensitive to ultraviolet rays. Adding zinc oxide, dilauryl thiodipropionate, carbon black or similar milky white fillers can improve its aging resistance.

Reference value of hydrophobic parameter calculation (XlogP): 3.32, number of hydrogen bond donors: 03, number of hydrogen bond acceptors: 34, number of rotatable chemical bonds: 15, number of tautomers: 6, topological molecular polar surface area (TPSA): 29.5Avoid strong oxidants, chlorine, potassium permanganate. Store in a closed, cool and dry place with good ventilation.

Introduction

Characteristics

Polypropylene, referred to as PP, is a colorless, odorless, non-toxic, translucent solid substance. Polypropylene is a thermoplastic synthetic resin with excellent performance. It is a colorless, translucent, thermoplastic, lightweight general-purpose plastic. It has chemical resistance, heat resistance, electrical insulation, high-strength mechanical properties and good high wear resistance processing properties. Since its advent, polypropylene has been rapidly developed and applied in many fields such as machinery, automobiles, electronic appliances, construction, textiles, packaging, agriculture, forestry, fishery and food industries. In recent years, with the rapid development of China's packaging, electronics, automobile and other industries, it has greatly promoted the development of China's industry. And because of its plasticity, polypropylene materials are gradually replacing wooden products, and high strength, toughness and high wear resistance have gradually replaced the mechanical functions of metal. In addition, polypropylene has good grafting and composite functions, and has huge application space in concrete, textiles, packaging, agriculture, forestry and fishery.

Mice were gavaged 1 to 5 times at a dose of 8g/kg, and no obvious symptoms of poisoning were caused. Rats inhaled the decomposition products of polypropylene heated to 210-220¡æ 30 times, each time for 2 hours, and symptoms of eye mucosa and upper respiratory tract irritation appeared. Like polyethylene, it is forbidden to use its recycled products to hold food.

Brief history of development

In 1954, G. Natta first polymerized propylene into polypropylene (using aluminum titanium chloride as a catalyst) and established the theory of directional polymerization, which attracted people's attention.

In 1957, Montecatini Company in Italy and Hecules Company in the United States established 6000t/a and 9000t/a polypropylene production facilities respectively.

From the late 1960s to the mid-1970s, polypropylene entered a period of great development.

From the 1980s to the present, polypropylene production has been at the forefront of synthetic resins, and now ranks second only to polyethylene.

China began to study polypropylene production technology in 1962. Since the 1980s, polypropylene has developed rapidly in China. China has introduced some advanced polypropylene production technologies and equipment, and has established a number of large and medium-sized polypropylene production facilities such as Yanshan, Yangzi, and Liaoyang. A large number of small-scale bulk polypropylene production facilities have also been built in various places, which have played a certain role in alleviating the contradiction between supply and demand. The substantial increase in production scale has prompted China's polypropylene resin production to enter a rapid development stage. In 2012, China's PP production capacity reached 12.967 million tons. In 2015, China's PP production capacity was 20.13 million tons/year.

Supply and demand status

Due to the large gap between supply and demand of polypropylene in China, most new large-scale oil refining, ethylene co-production projects and coal-olefin projects in recent years are equipped with polypropylene units. Therefore, China's polypropylene production capacity will increase significantly in the future. At the same time, it is also necessary to consider that those small and backward polypropylene installation technologies, especially intermittent small-body process units, will be gradually eliminated. It is estimated that by 2025, the production capacity of polypropylene in China will reach a higher level. With the rapid development of China's economy, the demand for various chemical raw materials has continued to increase, resulting in the consumption of polypropylene reaching the highest level in history. Therefore, China will become the world's largest consumer of polypropylene. In 2003, China's consumption of polypropylene had reached 5.32 million tons; in 2007, it first reached 10 million tons; in 2008, affected by the financial crisis, it slightly dropped to 10.79 million tons; in 2018, driven by infrastructure investment and China's demand, it increased to 12.32 million tons.

Polypropylene modification

In view of the poor impact resistance of polypropylene at low temperatures, poor weather resistance, poor surface decoration, and the gap between the functionality of polypropylene in electricity, magnetism, light, heat, combustion and actual needs, the modification of polypropylene has become the most active and fruitful field in the current development of plastic processing.

PP chemical modification

Through copolymerization modification, cross-linking modification, grafting modification, adding nucleating agents, etc., the polymer components and macromolecular structure or crystal configuration of polypropylene are changed to improve its mechanical properties, heat resistance, aging resistance and other properties, improve its comprehensive performance and expand its application field.

(1) Copolymerization modification

Copolymerization modification is a modification carried out in the propylene monomer synthesis stage using catalysts such as metallocene. When the monomer is polymerized, the added olefin monomer copolymerizes with it to obtain random copolymers, block copolymers and alternating copolymers, etc., and the mechanical properties, transparency and processing fluidity of homopolymer PP are improved. The complex formed by the metallocene catalyst uses the transition state with irregular shape subject to certain restrictions as a single active center to achieve precise control of the relative molecular mass and its distribution, the content of copolymer monomers, the distribution on the main chain and the crystal structure of the polymer.

(2) Graft modification

PP (polypropylene) resin molecules are non-polar crystalline linear structures with low surface activity and no polarity. They have the disadvantages of poor surface printability; poor coating adhesion; difficulty in blending with polar polymers; and difficulty in compatibility with polar reinforcing fibers and fillers. Graft modification is to introduce polar groups into its macromolecular chain to improve the blending, compatibility and adhesion of PP, thereby overcoming the disadvantages of difficult blending, difficult compatibility and difficult bonding. Under the action of the initiator, the grafting monomer undergoes a grafting reaction during melt mixing. The initiator decomposes when heated and melted to produce active free radicals. When the active free radicals encounter unsaturated carboxylic acid monomers, the unstable bonds of the unsaturated carboxylic acid monomers are opened and react with the active free radicals of PP to form grafting free radicals, which are then terminated by molecular chain transfer reactions. Common grafting modification methods for PP include: melt method, solution method, solid phase method, suspension method, etc. After grafting modification, hydrogen atoms in the PP molecular chain are replaced and show strong polarity. These polar groups enhance the compatibility of PP, and greatly improve the heat resistance and mechanical properties.

(3) Cross-linking modification

Cross-linking modification mainly modifies linear or branched polymers into network-structured polymers by cross-linking. PP (polypropylene) cross-linking modification can improve its mechanical properties, heat resistance and morphological stability, and shorten the molding cycle. The main methods of polypropylene cross-linking modification are chemical cross-linking modification and radiation cross-linking modification. The main differences between them are different cross-linking mechanisms and active sources. Chemical cross-linking modification is achieved by adding cross-linking aids. Radiation cross-linking modification is mainly achieved by strong radiation or strong light. Due to the thickness requirements of radiation cross-linking modification on PP, this method is difficult to popularize. At present, the silane grafting cross-linking method has developed rapidly because it can prepare materials with excellent performance. The PP produced by the silane grafting cross-linking method has high strength, good heat resistance, high melt strength, strong chemical stability and good corrosion resistance.

PP physical modification

Adding organic or inorganic additives to the PP (polypropylene) matrix during mixing and kneading to obtain PP composite materials with excellent performance mainly includes: filling modification, blending modification, etc.

(1) Filling modification

During the PP molding process, fillers such as silicate, calcium carbonate, silica, cellulose, and glass fiber are filled into the polymer to improve the heat resistance of PP, reduce costs, increase rigidity, and reduce molding shrinkage, but the impact strength and elongation of PP will also decrease. As an inorganic non-metallic whisker with excellent performance, glass fiber is low in price, good in insulation, strong in heat resistance, good in corrosion resistance, and high in mechanical strength. It is widely used. The performance of PP modified by glass fiber filling is significantly improved. However, the mechanical properties of the material can only be significantly improved when the glass fiber addition reaches about 30%. When the addition amount is too large, some glass fibers cannot be fully impregnated, which deteriorates the bonding performance of the interface between the polymer matrix and the glass fiber, resulting in a decrease in the mechanical strength of the composite material. In addition, as the amount of glass fiber added increases, the flowability of the composite material decreases, resulting in difficulties in the PP molding process performance.

(2) Blending modification

The modification method of blending PP (polypropylene) with polyethylene, engineering plastics, thermoplastic elastomers or rubber to improve the performance of PP. Blending modification is completed in processing equipment such as internal mixers, open mixers, and extruders. The process is easy to control, the production cycle is short, and the cost is low. It can improve the colorability, processability, antistatic properties, impact resistance and other properties of PP. Polymer blending can combine the outstanding performance of each component and make up for the shortcomings of each component. The comprehensive performance of the blend is significantly improved, but the low temperature resistance and aging resistance of the blended modified PP are still not ideal. During blending modification, shear force may cause a part of the macromolecular chain to be cut off to form free radicals and form grafted or block copolymers. These new copolymers can also effectively increase the volume of PP.

PP modification technology has doubled the mechanical properties of composite materials, greatly expanded the application field of PP, improved the cost performance of products, promoted the engineering process of PP, and also expanded PP from general plastics to engineering plastics, greatly broadening its application range. In recent years, the research and development of PP modification technology has been rapid, and more and more new technologies have been applied to PP modification. The comprehensive performance of PP has been significantly improved, the application field has been continuously expanded, and the development prospects are very broad.

(3) Reinforcement modification

Fiber-like materials added to plastics can significantly improve the strength of plastic materials, so it is called reinforcement modification. Materials with a large diameter-to-thickness ratio can significantly improve the bending modulus (rigidity) of plastic materials, which can also be called reinforcement modification.

The reinforcing materials used in the reinforcement modification of PP (polypropylene) are mainly glass fibers and their products, in addition to carbon fibers, organic fibers, boron fibers, whiskers, etc. In glass fiber reinforced PP, the most commonly used glass fibers are alkali-free glass fibers and medium-alkali glass fibers, among which alkali-free glass fibers are used in the largest amount. The diameter of the glass fiber is controlled within the range of 6 to 15 ¦Ìm, and the length of the glass fiber must be guaranteed to be within the range of 0.25 to 0.76 mm, so that the performance of the product can be guaranteed and the glass fiber can be well dispersed. It is generally believed that the modification effect can only be achieved when the length of the glass fiber in the product is greater than 0.2 mm. The glass fiber content (mass fraction) is preferably 10% to 30%, and the performance decreases when it exceeds 40%. In addition, the addition of an organic silane coupling agent can form a good interface between the glass fiber and PP, and improve the flexural modulus, hardness, load deformation temperature, and especially dimensional stability of the composite system.

Because glass fiber reinforced PP can improve mechanical strength and heat resistance, and glass fiber reinforced PP has good water vapor resistance, chemical corrosion resistance and creep resistance, it can be used as engineering plastics in many occasions, such as fan blades, heater grilles, impeller pumps, lampshades, electric furnaces and heater shells, etc.

While the production quantity of polypropylene is developing rapidly, its performance is constantly being innovated, so that the breadth and depth of its application are constantly changing. In recent years, some new varieties of polypropylene with more unique properties have been introduced, such as transparent polypropylene and high melt strength polypropylene, by improving the polymerization reaction or taking measures during granulation after polymerization.

Transparent modification

The crystallization of PP (polypropylene) is the main cause of opacity. By using the crystallization trend of PP by rapid freezing, a transparent film can be obtained. However, for products with a certain wall thickness, the core layer cannot be quickly cooled and frozen because heat conduction takes time. Therefore, for products with a certain thickness, it is not possible to expect to improve transparency by rapid cooling. It is necessary to start with the crystallization law and influencing factors of PP.

The modified PP obtained by certain technical means can have excellent transparency and surface gloss, and can even be comparable to typical transparent plastics (such as PET, PVC, PS, etc.). Transparent PP is more superior in that its heat deformation temperature is high, generally higher than 110¡æ, and some can even reach 135¡æ, while the heat deformation temperatures of the above three transparent plastics are all lower than 90¡æ. Due to its obvious performance advantages, transparent PP has been rapidly developed around the world in recent years. Its application areas range from household daily necessities to medical devices, from packaging products to heat-resistant utensils (for microwave heating).

The transparency of PP can be improved through the following three ways:

(1) Polymerize transparent PP using metallocene catalysts;

(2) Obtain transparent PP through random copolymerization;

(3) Add transparent modifiers (mainly nucleating agents) to ordinary polypropylene to improve its transparency.

High melt strength polypropylene

One of the disadvantages of polypropylene is its low melt strength and poor sag resistance. Usually, amorphous polymers (such as ABS and PS) have elast

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