High density polyethylene (HDPE) is a white powder or granular product.It is non-toxic and tasteless, with crystallinity of 80%~90%, softening point of 125~135 ℃, and service temperature of 100 ℃;Hardness, tensile strength and creep better thanlow density polyethylene；Good wear resistance, electrical insulation, toughness and cold resistance;chemical stabilityGood, insoluble in any organic solvent at room temperature, resistant to acid, alkali and various salts;The membrane has low permeability to water vapor and air, and low water absorption;The aging resistance is poor, and the resistance to environmental stress cracking is not as good as that of low-density polyethylene, especially the thermal oxidation will reduce its performance, so antioxidant andUV absorberAnd so on.High density polyethylene film under stressHot deformation temperatureLow, pay attention when applying.
In this century, a revolutionary progress has taken place in the pipeline field, that is, "replacing steel with plastic".With the rapid progress of polymer materials science and technology, the deepening of the development and utilization of plastic pipes, and the continuous improvement of production processes, plastic pipes have shown their outstanding performance vividly.Today, plastic pipes are no longer mistaken as "cheap substitutes" for metal pipes.In this revolution, polyethylene pipes have become more and more popular and increasingly give off a dazzling glory. They are widely used in the fields of gas transmission, water supply, sewage, agricultural irrigation, fine particle solid transport in mines, as well as oil fields, chemicals, posts and telecommunications, especially in gas transmission.
HDPE is a kind of ethylene copolymerizationThermoplastic polyolefin。Although HDPE was launched in 1956, this kind of plastic has not yet reached the mature level.This general material is still developing its new use and market.
The domestic producers of high-density polyethylene (here the high-density polyethylene does not include the high-density polyethylene produced by the full density polyethylene unit) include PetroChina, Sinopec and CNOOC. By the end of 2006, there were four sets of high-density polyethylene units belonging to PetroChina, namely, Lanzhou Petrochemical High Density Polyethylene Unit, Daqing Petrochemical High Density Polyethylene UnitLiaoyang Petrochemical High Density Polyethylene Plant, Jilin Petrochemical High Density Polyethylene Plant.
High density polyethylene is usually produced by Ziegler Natta polymerization, which is characterized by no branched chain on the molecular chain, so the molecular chain is arranged orderly and has high density.In this process, ethylene is used as raw material in tubular or kettle low-pressure reactor, and oxygen or organic peroxide is used as raw materialInitiatorInitiate polymerization.
HDPE molecule
High density ethylene is environmentally friendly material. When heated to the melting point, it can be recycled.Note that plastic raw materials can be divided into two categories: "Thermoplastic" and "Thermosetting". "Thermosetting" means that it becomes solidified after being heated to a certain temperature, and even if it continues to be heated, its state cannot be changed. Therefore, products with environmental problems are "thermosetting" products (such as tires),Not "thermoplastic" products (such as plastic pallets Note: pallets are called "plywood" in Hong Kong and Macao), so not all "plastics" are environmentally friendly.
[Basic characteristics] High density polyethylene is an opaque white wax like material with a specific gravity of 0.941~0.960, which is lighter than water. It is soft and tough, but slightly harder than LDPE. It can also stretch slightly. It is non-toxic and tasteless.
[Combustion characteristics] It is flammable and can continue to burn after leaving the fire. The upper end of the flame is yellow and the lower end is blue. It will melt when burning, with liquid dripping and no black smoke emitting. At the same time, it will emit the smell of paraffin burning.
[Main advantages] Acid and alkali resistance, organic solvent resistance, excellent electrical insulation, and certain toughness at low temperature.The surface hardness, tensile strength, rigidity and other mechanical strength are higher than LDPE, close toPPIt is tougher than PP, but its surface finish is not as good as PP.
[Main disadvantages] Poor mechanical performance, poor air permeability, easy deformation, easy aging, easy brittleness, brittleness lower than PP, easy stress cracking, low surface hardness, easy scratching.It is difficult to print. When printing, it needs surface discharge treatment. It cannot be electroplated, and the surface is dull.
[Application] It is used to extrude packaging films, ropes, woven bags, fishing nets, and water pipes;Injection molding low-grade daily necessities and shells, non load bearing components, plastic boxes, turnover boxes;Extrusion blow molding containers, hollow products, bottles.
[Injection molding process] HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, and consumes 1/5 of domestic HDPE.The injection grade generally has a melt index of 5~10, and has a lower toughness fluidity grade and a higher workability fluidity grade.Uses include daily necessities and thin wall food packaging;Tough and durable food and paint cans;High resistance to environmental stress cracking applications, such as small engine fuel tanks and 90 gal garbage cans.
Generally, the melting point of HDPE is 142 ℃, and the decomposition temperature is 300 ℃;The adjustable range of injection temperature is large.During injection molding, the general use temperature is 180 ℃ - 230 ℃;Because it is an olefin plastic, it does not absorb water. It does not need to be dried during production, but for product quality, it can be dried at 60 ℃ for 1hr to discharge floating water;Polyethylene has high melt viscosity and low flow length ratio. Thin wall products may lack glue, so the gate and runner are relatively large;The product is easy to carry static electricity and the surface is easy to absorb.The shrinkage rate is 16 ‰, and the overflow value is 0.05mm.
features
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HDPE has good heat resistance and cold resistance, good chemical stability, high rigidity and toughness, and good mechanical strength.The dielectric property and environmental stress cracking resistance are also good.The hardness, tensile strength and creep property of LDPE are better than those of LDPE;Wear resistance, electrical insulation, toughness and cold resistance are good, but slightly worse than low-density insulation;Good chemical stability, insoluble in any organic solvent at room temperature, and resistant to acid, alkali and various salts;The membrane has low permeability to water vapor and air and low water absorption;The aging resistance is poor, and the environmental cracking resistance is not as good as that of low-density polyethylene. Especially, the thermal oxidation will reduce its performance. Therefore, antioxidants and ultraviolet absorbers need to be added to the resin to improve this aspect.High density polyethylene film has low thermal deformation temperature under stress, which should be noted in application[1]。
Production process
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The most common production method of PE is slurry or gas processing, and a few are produced by solution processing.All these processes are exothermic reactions involving ethylene monomer, a-olefin monomer, catalyst system (possibly more than one compound) and various types of hydrocarbon diluents.Hydrogen and some catalysts are used to control molecular weight.The slurry reactor is generally a stirred tank or a more commonly used large annular reactor, in which the slurry can be circularly stirred.When ethylene and comonomer (as required) contact with the catalyst, polyethylene particles will be formed.After the diluent is removed, polyethylene particles or powder particles are dried and additives are added according to the dosage to produce granular materials.The modern production line with a large reactor with twin screw extruder can produce more than 40000 pounds of PE per hour.The development of new catalysts contributes to improving the performance of new grade HDPE.The two most commonly used catalysts are Philip's chromium oxide based catalyst and titanium compound alkyl aluminum catalyst.HDPE produced by Philips catalyst has medium width molecular weight distribution;The molecular weight distribution of titanium alkyl aluminum catalyst is narrow.The catalyst used in the production of narrow MDW polymer with the compound reactor can also be used to produce wide MDW grade.For example, two tandem reactors producing products with significantly different molecular weights can produce bimodal molecular weight polymers with a full range of molecular weight distributions.
molecular weight
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Higher molecular weight results in higher polymerviscosityHowever, the viscosity is also related to the temperature and shear rate used in the test.The molecular weight of the material was characterized by rheology or molecular weight measurement.HDPE grades generally have a molecular weight range of 40 000~300 000, and the weight average molecular weight roughly corresponds to the melt index range, that is, from 100~0.029g/10min (230 ℃, 2.16kg).Generally, higher Mw (lower MI) increases melt strengthtoughnessAnd ESCR, but higher Mw enables machining
The process is more difficult or requires higher pressure or temperature.
Molecular weight distribution (MWD): The WD of PE varies from narrow to wide depending on the catalyst used and the processing process.
The most commonly used MWD measurement index is the irregularity index (HI), which is equal toWeight average molecular weight(Mw) divided byNumber average molecular weight（Mn）。The index range of all HDPE grades is 4-30.Narrow MWD providesmolding Low warpage and high impact in process.Medium to wide MWD provides workability for most extrusion processes.Wide MWD also improves melt strength and creep resistance.
additive
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AntioxidantThe addition of can prevent the polymer from degradation during processing and prevent the finished product from oxidation in use.Antistatic additives are used in many packaging grades to reduce the adhesion of bottles or packaging materials to dust and dirt.Specific applications require special additive formulations, such as copper inhibitors related to wire and cable applications.Good weather resistance and UV resistance (or sunlight) can be achieved by adding anti UV additives.PE with no UV resistance or carbon black added is not recommended for continuous outdoor use.High grade carbon black pigment provides excellent UV resistance and can often be used outdoors, such as wires, cables, tank floors or pipes.
Processing method
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PE can be manufactured by a wide range of different processing methods.With ethylene as the main raw material,propylene、1-butene 、HexeneIt is a copolymer. Under the action of catalystSlurry polymerizationorGas phase polymerizationProcess, the polymer obtained is subjected to flash evaporation, separation, drying, granulation and other processes to obtain a finished product with uniform particles.Including sheet extrusion, film extrusion, pipe or profileExtrusion molding，Blow molding、Injection moldingandRotational molding。HDPE is suitable for various molding processes of thermoplastic molding, with good molding processability, such as injection molding, extrusion, blow molding, rotary molding, coating, foaming process, thermoforming, heat sealing welding, thermal welding, etc.
Extrusion molding: The grades used for extrusion production generally have melt index less than 1 and MWD of medium width to wide width.During processing, low MI can obtain appropriate melt strength.Wider MWD grades are more suitable for extrusion because they have higher production speeds, lower die pressures, and reduced melt fracture trends.
PE has many extrusion applications, such as wires, cables, hoses, pipes and profiles.The application range of pipes is from small section yellow pipes for natural gas to thick wall black pipes for industrial and urban pipelines.The use of large-diameter hollow wall pipes as alternatives to rainwater drainage pipes made of concrete and other sewer pipelines is growing rapidly.
Sheet andThermoforming: The thermoforming lining of many large picnic type refrigerators is made of PE, which has toughness, light weight and durability.Other sheet and thermoformed products include mudguards, tank liners, pan shields, transport cases, and cans.A large number of sheet materials with rapid growth are applied in plastic film or pool bottom village, which is based on the toughness, chemical resistance and impermeability of MDPE.
Blow molding:More than 1/3 of the HDPE sold in the United States is used for blow molding.These ranges are frombleaching agents, engine oilDetergent, milk and distilled water bottles to large refrigeratorsAutomotive fuelBoxes and cans.The characteristics of blow molding grades, such as melt strength, ES-CR and toughness, are similar to those used for sheet and hot forming applications, so similar grades can be used.
Injection blow molding is usually used to make smaller containers (less than 16oz) for packaging drugs, shampoo and cosmetics.One of the advantages of this process is that the production bottle can be automatically de edged, without the need for post dressing steps like the general blow molding process.Although some narrow MWD grades are used to improve surface finish, medium to wide MWD grades are generally used.
Injection molding:See aboveMaterial Properties。
Rotational molding:The materials processed by this method are generally crushed into powder, which is melted and flows in the thermal cycle.Two types of PE are used for rotational molding: general purpose and flexiblecross-linkingClass.The general grade MDPE/HDPE usually has a density range from 0.935 to 0.945g/CC, with a narrow MWD, which makes the product have high impact and minimum warpage, and its melt index range is generally 3-8.Higher MI grades are generally not applicable because they do not have the impact and environmental stress cracking resistance desired for roll molded products.
film:PE film is generally processed by ordinary film blowing or flat extrusion.Most PE is used for film, and general low density PE (LDPE) or linear low density PE (LLDPE) are available.HDPE film grade is generally used where superior tensile property and excellent impermeability are required.For example, HDPE film is commonly used in commodity bags, grocery bags and food packaging.
Main purpose
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High density polyethylene resin can be used to form plastic products by injection, extrusion, blow molding and rotary molding.Various types of containers, industrial accessories, medical supplies, toys, shells, bottle stoppers, shields and other products can be molded by injection molding.Blow molding can be used to form various hollow containers, ultra-thin films, etc.Extrusion molding can be used to form pipes, tensile strips, strapping belts, monofilaments, wire and cable sheaths, etc.
In addition, it can also form decorative panels, louvers, synthetic wood, synthetic paper, synthetic films and molded calcium plastic products[1]。
Packaging, storage and transportation
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During storage, it shall be kept away from fire sources and insulated. The warehouse shall be kept dry and tidy. It is strictly prohibited to mix any impurities, sun and rain.During transportation, they shall be stored in clean, dry and covered carriages or cabins, without nails and other sharp objects.Flammable aromatic hydrocarbon and halogenated hydrocarbon are strictly prohibitedOrganic solventMixed transportation.For example, the four liter bucket of mineral water from Nongfu Mountain Spring is made of this material.
recycling
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HDPE is the fastest growing part of the plastic raw material warehouse recycling market.This is mainly because it is easy to reprocess, has minimal degradation characteristics and is widely used in packaging.The main recycling is to reprocess 25% of recycled materials, such as post consumer recyclables (PCR), with pure HDPE to make bottles that are not in contact with food.
In this process, the polymerization solvent is n-hexane and the catalyst is highly active z-N catalyst,ethyleneAfter mixing with hydrogen, it enters the first reactor, where it is mixed with catalyst to produce polymerization reaction. The polymer in the reactor is suspended in hexane in the form of slurry. The polymerization temperature is about 80 ℃, and the polymerization pressure is less than 10 bar. This process can produce product density ranging from 0.942 to 0.965 g/cmthree, the melt index ranges from 0.2 to 80, and the comonomer ispropyleneand1-butene Production of traditional HDPE and bimodal HDPE, high density pipes with excellent performance, suitable for making pressure pipes, up to PE100+.slurry Kettle reactorThe characteristics of continuous polymerization process are: low operating pressure and temperature;The double kettle reactor can produce single peak and double peak products by adopting different forms of parallel and series connection;High flexibility in process operation, fast product grade conversion, and low requirements for raw material purity;The comonomer is propylene, 1-butene;Using hexane as solvent, the recovery unit is simple.The process is characterized by polymerization in inert hydrocarbon diluent.The process flow is as follows: send the polymerized ethylene (99.9% ethylene, 0.1% ethane) into the dryer, and then enter the kettle reactor together with the circulating diluent composed of n-hexane. The catalyst carrier contains titanium, manganeseTriethyl aluminumIt is the brand of cocatalyst.A small amount of hydrogen is added to control the molecular weight. Polyethylene particles are formed in the polymerization reaction. The reaction temperature is 90 ℃ and the pressure is 1.8MPa. The reaction can be carried out in two polymerization kettles in two steps. The slurry solid with a concentration of 34% (mass fraction) is produced, and the monomer conversion rate can reach 97%.The polymer flows out of the second reactor and is flashed to a pressure of 0.14MPa. The unreacted ethylene, ethane in the effluent and 2% cyclohexane diluent from the flash are compressed twice and cooled to 2.5MPa, and then go to the stripping tower to recycle the ethane.The slurry left after flashing is centrifuged to recover most of the diluent, and the solid filter cake is sent to the dryer to reduce the volatile component content to about 5% (mass fraction).The dryer is operated in a closed cycle with nitrogen protection.The dried polymer powder is sent to the fluidized bed for drying to remove all hydrocarbon diluents. After drying, the polymer particles are sent to the mixing section to add various additives, and then granulated.(2) Loop reactor process Typical representatives of loop reactor process are Phillips process of Phillips Company and Innovene S process of INEOS Company.The Phillips process uses isobutane as diluent and chromium catalyst. The catalyst should be activated before use. The activated catalyst powder forms catalyst slurry with high-purity isobutane under the protection of nitrogen, and then enters the loop reactor. After refined, the raw ethylene monomer is premixed with hydrogen and comonomer hexene-1, and then injected into the loop reactor,Ethylene generates polyethylene under the action of catalyst.The axial flow pump maintains the high-speed flow and very uniform mixing of materials in the reactor, and the reaction heat is evenly withdrawn from the jacket cooling water.The production MI range of this process is 0.15-100, and the density is 0.936-0.972 g/cmthree。The characteristics of the loop reactor process are: less equipment, short process and low investment cost;No wax, oligomer and wall sticking;Good powder shape, easy to transport;The reaction heat is taken out by the jacket cooling water of the reactor, which is easy to remove heat and adjust;High requirements for raw materials, requiring purification;The comonomer is hexene;useIsobutaneAs solvent, it is easy to remove residual solvent.The process flow is as follows: fresh polymerized ethylene is dried, and molecular weight regulator hydrogen, antifreeze and circulating diluent isobutane are mixed, and then sent to the multi loop continuous process reactor, and catalyst isobutane is added into the reactor.The reaction temperature is 106.7 ℃ and the pressure is 3.9MPa.The residual slurry of polymer and diluent passes through the loop reactor at the speed of 6m/s with the help of axial flow pump.The water cooling in the reactor jacket controls the reaction temperature, and the polymer solids are discharged from the vertical sedimentation port in the loop reactor.So the slurry concentration can reach 55%, and the conversion rate is 98% - 99%.After the polymer is discharged, go to flash evaporation to discharge isobutane and residual monomer into the diluent recovery unit.Other solid polymers are mixed with additives and granulated.2. Gas phase polymerization method: gas phase polymerization method (gas-phase fluidized bed method) process is typically represented by Dow Chemical Company's innovation technology and INNOS Company's innovation technology. The process uses a low-pressure gas-phase fluidized bed reactor, Z-N catalyst and chromium catalyst. Purified raw materials are injected into the reactor to generate polymerization reaction under the effect of catalyst storage. The reaction temperature is 85~110 ℃,Under the condition of 2.41 MPa, the single conversion rate of ethylene is about 1%~2%. The reaction heat is removed mainly through the cooling of circulating material. The MI range of the product is 0.01~150, and the density range is 0.915~0.970 g/cmthree。The characteristics of gas-phase fluidized bed polymerization process are: low operating pressure and low temperature;Full density polyethylene can be produced;The catalyst system includes titanium system and chromium system;Metallocene catalyst;High requirements for raw material purity, all raw materials should be refined;No solvent, low energy consumption, low maintenance and operation costs.The production process is as follows: the dried monomer is added to the reactor system together with hydrogen, the raw material is added to a large circulation steam flow loop, and enters the bottom of the large fluidized bed reactor through gas distribution. According to the design, the reactor raw materials include 69.57% ethylene (99.9% ethylene content, 0.1% ethane), 10.43% hydrogen, 7.56% ethane, and 12.44% nitrogen.The product produced from this feed gas composition has a melt index of 8g/10min and 0.964g/cmthreeDensity of.The catalyst is a mixture of magnesium dioxide with titanium trichloride and tetrahydrofuran as promoters, and the cocatalyst is triethylaluminum.The catalyst enters the reactor together with nitrogen in solid form from the reactor without pain.The operating temperature is 105 ℃, and the specific temperature is determined according to the brand number.The operating pressure of the reactor is 2.0MPa. The reaction gas comes out from the top of the reaction and is separated from the catalyst containing solid particles by the cyclone separator and sent back to the reactor.Then, the gas from the cyclone separator is compressed and circulated to the bottom of the reactor through the circulation cooler.The reactor discharge sends the product particles to the charging tank intermittently through an air lock system.Some gas entering the discharge tank enters the compressor circulation system through the upper buffer tank, filter, gas cooler and separation tank.The polymer flows from the lower part of the discharge tank to the purging tank and the post-treatment system.The post-treatment system includes adding various additives to the polymer, melting, granulation and packaging.
