Corrosion resistance, electricalInsulation(Especially high frequency insulation) excellent, can be chlorinated, modified by irradiation, availableglass fibreEnhanced. HDPE has high melting point, rigidity, hardness and strength,Water absorptionSmall, good electrical performance andRadiation resistance;The softness of LDPE,elongation,impact strength andPermeabilityBetter;UHMWPEHigh impact strength, fatigue resistance and abrasion resistanceHigh density polyethylene is suitable for making corrosion resistant parts and insulating parts;Low density polyethylene is suitable for making films, etc;UHMWPE is suitable for shock absorption, wear resistance and transmission parts
Molding performance
1. Crystalline material has small moisture absorption, does not need to be fully dried, has good fluidity, and is sensitive to pressure. It is suitable to use high-pressure injection when molding, with uniform material temperature, fast filling speed, and sufficient pressure holding. It is not suitable to use directGate, to prevent uneven shrinkage,internal stressIncrease. Pay attention to selecting gate location to preventShrinkage cavityAnd deformation
2. Large shrink range and shrink value,directionalObvious, easy to deform and warpCooling rateIt should be slow, and the mold should be set with cold material cavity, andCooling system.
threeheating time It should not be too long, or it will decompose and burn
4. When soft plastic parts have shallow side grooves, forced demoulding is allowed
fivePossibleMelting body is broken, which is not suitable forOrganic solventContact to prevent cracking
High pressure low density polyethylene (LDPE)High density polyethylene(HDPE) andLinear low density polyethylene（LLDPE）。Film is its main processing product, followed bysheetAnd coating, bottles, cans, barrels and other hollow containers, and other various injection molding andBlow moldingInsulation and sheath of products, pipes, wires and cables.Mainly used in packaging, agriculture, transportation and other sectors.
PE is soft and feels waxy. Compared with the same plastic, PE is light and transparent. The flame is blue when burning.
toxicity
It is non-toxic and harmless to human body.
Density classification
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high-density
The English name is "High Density Polyester", referred to as "HDPE" for short.HDPE is acrystallinityHigh, non-polarThermoplastic。The appearance of original HDPE is milky white.
HDPE is aethyleneCopolymerizedThermoplastic 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.
Main characteristics
HDPE is a kind of thermoplastic with high crystallinity and non polarity.The appearance of the original HDPE is milky white, and it is translucent to a certain extent in the thin section.PE has excellent resistance to most domestic and industrial chemicals.Certain kinds of chemicals can produceChemical corrosion, e.g. corrosive oxidant（Concentrated nitric acid），Aromatic hydrocarbon（xylene）And halogenated hydrocarbon（carbon tetrachloride）。The polymer is non hygroscopic and has good water vapor resistance, and can be used for packaging purposes.HDPE has good electrical properties, especially the high dielectric strength of insulation, which makes it suitable forWire and cable。Medium to high molecular weight grades have excellentImpact resistance, even at low temperature of - 40F.There are four unique characteristics of various grades of HDPEBasic variableAppropriate combination of: density, molecular weightmolecular weight distributionAnd additives.Different catalysts are used to produce customized polymers with special properties.These variables are combined to produce HDPE grades for different purposes;Achieve the best balance in performance.
density
This is the main variable that determines the characteristics of HDPE, although the four variables mentioned do interact.Ethylene ispolyethyleneMain raw materials, a few other comonomers, such as 1-butene, l-hexene or 1-octene, are also often used to improve polymer properties. For HDPE, the content of the above few monomers generally does not exceed 1% - 2%.The addition of comonomer slightly reduced the crystallinity of the polymer.This change is generally measured by density, and the density and crystallinity arelinear relationship。According to ASTM D1248, the density of HDPE is 0.940g/.C or above;Medium density polyethylene(MDPE) density range 0.926~0.940g/CC.Other classifications sometimes classify MDPE as HDPE or LLDPE.HomopolymerIt has the highest density, maximum stiffness, and goodimperviousPermeabilityAnd the highest melting point, but generally has very poor resistanceEnvironmental stress cracking（ESCR）。ESCR is the ability of PE to resist cracking caused by mechanical or chemical stress.Higher density generally improves mechanical strength, e.gtensile strength , stiffness and hardness;Thermal performanceassoftening pointTemperature andHot deformation temperature；Impermeability, such asAir permeabilityorsteamPenetrability。Lower density improves itsimpact strength And E-SCR.The density of polymer is mainly affected by the addition of comonomer, but to a lesser extent, it is also affected by molecular weight.high molecular weightpercentageSlightly reduce the density.For example, homopolymers have different densities in a wide range of molecular weights.
Production and catalyst
PE most commonProduction methodIt is produced by slurry or gas processing, and a few are produced by solution processing.All these processes are composed of ethylene monomer, a-olefinMonomer, catalyst system (may be more than one compound) and various types ofHydrocarbonsdiluentParticipatingExothermic reaction。Hydrogen and some catalysts are used to control molecular weight.Slurry reactorGenerallystirred tankOr a more commonly used large ringReactorIn which the slurry can be stirred circularly.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 produceGranular material。haveDouble screw extruderThe modern production line of the large-scale reactor can produce PE40000 pounds 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 chromiumoxideBased catalyst and titanium compound IAlkyl aluminiumcatalyzer.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 products with significantly different molecular weightsSeries reactorA bimodal molecular weight polymer can be produced, which has a full range of molecular weight distribution.
molecular weight
Higher molecular weight results in higherpolymerViscosity, but the viscosity is also different from the temperature andShear rateofThe 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,Weight average molecular weightRoughlyMelt indexThe range is corresponding, that is, from 100 to 0.029/10min.Generally, higherMW(lower MI) enhancedMelt Strength , better toughness and ESCR, but higher MW enables processing
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.
AntioxidantThe addition of can prevent the polymer from degradation during processing and prevent the finished product from oxidation in use.Antistatic additiveUsed 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 those related to wire and cable applicationsCopper inhibitor。ExcellentWeather resistanceAnd antiultraviolet rays(or sunlight) by adding antiUVadditive.No UV resistance orcarbon blackIt is recommended not to use it continuously outdoors.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
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 polymerizationOr gas phasePolymerization processThe obtained polymer is subjected to flash evaporation, separation, drying, granulation and other processes to obtain a finished product with uniform particles.IncludingsheetExtrusion, film extrusion, pipe or profile extrusion,Blow molding, injection molding and rotational molding.
▲Extrusion molding: The grade used for extrusion production is generally less than 1Melt indexAnd medium to wide MWD.During processing, lowMISuitable melt strength can be obtained.Wider MWD grades are more suitable for extrusion because they have higherProduction speedLower die pressure and lower melt fracture tendency.
PE has many extrusion applications, such as wires, cables, hoses, pipes and profiles.PipeScope of applicationFrom small section yellow pipeline for natural gas to 48in diameter industrial and urban pipelineThick wallBlack pipe.Large diameter hollow wall pipes are used as rainwater made of concretea drainAnd otherssewerPipeline substitutes are growing rapidly.
boardandThermoforming: Many large picnic typesRefrigeratorThe thermoforming lining ofdurability。Other sheet and thermoforming products includeMudguardTank lining and panprotection cover, transport cases and tanks.A large number of sheet materials with rapid growth are applied in plastic film or pool bottom village, which is based on MDPE's toughnessChemical resistanceAnd noPermeability。
▲ Blow molding: More than 1/3 of the HDPE sold in the United States is used for blow molding.These ranges are frombleaching agents, oil, detergent, milk anddistilled waterFrom bottles to large refrigerators, car fuel tanks and cans.Blow moldedCharacteristic index, such as melt strength, ES -CRAnd toughness, similar to that 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 drugsliquid shampooAnd 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 there are some narrow MWD grades for improvementSurface finishGenerally, MWD grade from medium width to wide width is used.
▲Injection molding: HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming 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.
▲ 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 PE and crosslinkable PE.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 minimumwarpingThe 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.
High performance roll molding applications utilize the unique properties of its chemically crosslinkable grade.These grades areMolding cycleThe first section of the resin has good fluidity, and then it is cross-linked to form its excellent resistance to environmental stress cracking and toughness.Abrasion resistanceAnd weather resistance.Crosslinkable PE is only suitable for large containers, ranging from 500 gal storage tanks for transporting various chemicals to 20000 gal agricultural storage tanks.
▲ Film:PE filmGeneral processingBlown filmProcessing or flat extrusion processing method.Most PE is used for film, and general low density PE (LDPE) or linear low density PE (LLDPE) are available.HDPE filmClass A is generally used where superior tensile property and high impermeability are required.For example,HDPE filmIt is commonly used in commodity bags, grocery bags and food packaging.
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.The transportation shall be stored in clean, dry and covered carriages or cabinsNailsAnd other sharp objects.It is forbidden to mix with flammable aromatic hydrocarbonHalohydrocarbonetc.Organic solventMixed transportation.
recycling
HDPE isPlastic recyclingThe fastest growing part of the 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.
Such as agricultural film, ground covering film, agricultural film, vegetable greenhouse film, etc;Packaging filmascandy, vegetables, frozen food and other packaging;For liquid packagingBlown film(Milk, soy sauce, juice. TofuSoymilk）；Heavy bags,Shrink packagingFilm, elastic film, lining film;Film for building, generalIndustrial packagingFilm and food bags, etc.LDPE is also used forInjection molded products, such as small containers, lids, daily productsplastic flowersInjection molding stretching blow molding container.Medical devices, medicines andFood packaging materialsExtruded pipes, plates, wires and cables,profiled bar, thermoforming and other products;Blow molded hollow products, such as food containersDairyandJamClass A, drug, cosmetics, chemical product containers, tanks, etc.
Production method
low density polyethyleneIt is also called high-pressure polyethylene.According to reactor type, it can be divided into kettle method andTubular method。Ethylene is used as raw material, sent to the reactorInitiatorUnder the action ofpolymerization, material from reactorseparatorAfter the unreacted ethylene is removed, it is fusedExtrusion granulation, dry, mix, and send to packaging.
Product performance
LDPE is milky white bead shaped particles.It is non-toxic, tasteless, odorless, and has no luster on the surface.The density is 0.916~0.930g/cm ³.It is soft and has good extensibilityElectrical insulationChemical stability, processability and low temperature resistance（Tolerable-70 ℃), but mechanical strength, moisture resistance, gas resistance andSolvent resistancePoor.molecular structureIt is not regular enough, with low crystallinity (55%~65%) and low crystallization melting point (108~126 ℃).
Packaging, storage and transportation
The product is packed in polyethylene heavy packaging film bag, which can be jacked according to user needspolypropyleneWoven bagbyOuter packaging。The products shall be stored in a clean and dry warehouse and can be transported by train, car, ship, etc.During storage and transportation, attention shall be paid to fire prevention, waterproof, sun protection, dust prevention and pollution prevention.Means of transportIt shall be kept clean, dry, free of sharp objects such as iron nails, and shall be provided with a compartment shed or tarpaulin
Linear low density
(LLDPE), ethylene and a small amount of advancedAlpha olefin(e.gButene-1. Hexene-1Octene-1. Tetramethylpentene-1, etc.) under the action of catalystLow pressure polymerizationA kind ofcopolymerThe density is between 0.915 and 0.940 g/cm3.But pressASTMAccording to D-1248-84, the density range of 0.926-0.940g/cm3 belongs to medium density polyethylene (MDPE).The new generation LLDPE expands its density toPlastic body(0.890~0.915 g/cm3) andelastomer(<0.890 g/cm3).However, the American Plastics Industry Association (SPI) and the American Plastics Industry Council (APC) only extend the scope of LLDPE to plastic bodies, excluding elastomers.In the 1980s, Union Carbide and Dow Chemical called their early plastic and elastomer products VLDPE andUltra low density polyethylene(ULDPE) resin.
The molecular structure of conventional LLDPE is characterized by its linear main chain, with little or noLong branched chain, but contains someShort branched chain。No long branched chain makes the polymerCrystallinityHigher.
In general, LLDPE resins are characterized by density and melt index.The density is determined by the concentration of comonomer in the polymer chain.The concentration of comonomer determines the amount of short branched chains in the polymer.The length of the short branched chain depends on the type of comonomer.The higher the comonomer concentration, the lower the resin density.In addition, the melt index is resinAverage molecular weightIs mainly reflected by the reaction temperature（Solution method）And joiningChain transfer agent(gas phase method).Average molecular weight and molecular weightDistribution independentThe latter is mainly affected by the type of catalyst.
LLDPE was industrialized by Union Carbide in the 1970s, which represented polyethylene catalyst andProcess technologyThe major change of polyethyleneProduct rangeSignificant expansion.LLDPE replaces free radicals with coordination catalystsInitiator, and low voltage with lower costGas phase polymerizationReplace higher costHigh pressure reactorIn a relatively short time, it has replaced LDPE in many fields with its excellent performance and low cost.LLDPE has penetrated almost all traditional polyethylene markets, including films, molding, pipes, wires and cables.
LLDPE products are non-toxic, tasteless, odorless, and milky white particles.Compared with LDPE, it has the advantages of high strength, good toughness, strong rigidity, heat resistance, cold resistance, and good resistance to environmental stress crackingtear strength It can resist acid, alkali, organic solvent, etc.
In 2005, China's LLDPE output was 1.88 million tons, accounting for about PEtotal output35.5% of;Consumption3.55 million tons, accounting for 33.8% of the total PE consumption.It is estimated that LLDPE consumption will continue to grow at a rate of about 8% in the next 2-3 years.According to the currentmarket price12000 yuan/ton, LLDPE'smarket size It has exceeded 40 billion yuan.
The main application fields of LLDPE are agricultural films, packaging films, wires and cables, pipes, coating products, etc.
Linear low density polyethyleneBecause of the higherTensile strengthGood puncture resistanceTear resistance , mainly used to make films.In 2005, the world LLDPE consumption was 16.17 million tons,Year on year growth6.4%。stayconsumption structureAmong them, film products still account for the largest proportion, with the consumption of 11.9 million tons, accounting for 73.6% of the total consumption, followed by injection molding, with the consumption of 1.148 million tons, accounting for 7.1% of the total consumption of LLDPE.
In 2005, the total consumption of LLDPE and LDPE in China was 5.98 million tons, including 3.55 million tons of LLDPE, a year-on-year increase of 25.4%, accounting for 59.4% of the total consumption of LLDPE/LDPE;LDPE consumption was 2.43 million tons, up 0.7% year on year, accounting for 40.6% of the total consumption of LLDPE/LDPE.
From the perspective of LLDPE/LDPE consumption structure, film is still the largest type of consumption, with consumption of 4.85 million tons, accounting for 77.5% of the total consumption of LLDPE/LDPE, including 3.13 million tons of packaging film, accounting for 50% of the total consumption;1.345 million tons of agricultural film, accounting for 22.5% of the total consumption;376000 tons of special packaging film, accounting for 6% of the total consumption.The second is injection molding products, with a consumption of 557000 tons, accounting for 8.9% of the total consumption.Then followed by coating products, pipes, wires and cables, with consumption of 313000 tons, 188000 tons and 157000 tons respectively, accounting for 5%, 3% and 2.5% of the total consumption;Other consumption was 188000 tons, accounting for 3% of the total consumption.
According to the consumption of LLDPE/LDPE from 2003 to 2005, the consumption proportion of film has always been around 77%, and the consumption proportion of the second largest variety of injection molding products has also been around 9%.It is predicted that in the next 2-3 years, although the absolute consumption of various varieties will continue to grow, their consumption proportion will basically maintain the trend;Due to the relatively rapid growth of demand for packaging film, the consumption proportion of agricultural film will be reduced to about 20%.As the performance of LLDPE continues to improve and its application field continues to expand, the future market demand for LLDPE will grow much faster than LDPE and HDPE.
Classification of LLDPE
According to the type of comonomer, LLDPE is mainly divided into three types of copolymers:C4(butene-1), C6 (hexene-1), and C8 (octene-1).Among them, butylene copolymer is a globaloutputThe largest LLDPE resin, while hexene copolymer is the fastest growing LLDPE variety.In LLDPE resin, the typical amount of comonomer is 5%~10% by weight, and the average amount is about 7%.MetalloceneThe LLDPE plastomer (mLLDPE) with the content of comonomer more than 3 times that of conventional LLDPE.Chart 1 shows the output of three comonomers LLDPE in the world in the past 10 years quoted from the Foreign Journal.
Table 1: Output of three comonomers LLDPE in the world during 1997~2007
At the end of 1984, the then United Carbon Company introduced the production of LLDPE for hexene copolymerization, followed by Exxon, Mobil and other companies.Dow Chemical(Dow ChemistryThe company) almost all uses octene as comonomer in its low-pressure solution process,CanadaNOVA (Nova Chemical) also mostly uses octene in its intermediate pressure solution process.Octene copolymer LLDPE resin has slightly better strength, tear resistance and processing properties, while hexene copolymer resin and octene copolymer resin have little difference in performance.ExxonMobil Chemical is the main manufacturer of hexene LLDPE resin（ExxonMobilChemical Company), Eastman Chemical（Eastman Chemical ）Equistar andChevronPhillips(ChevronPhillips Chemical Company), etc.In addition, Dow Chemical, Basell, Innovene, SamsungTotal(SamsungTotal）It also produces LLDPE of hexene.
Compared with commonly used butene comonomers, LLDPE produced with hexene and octene as comonomers has better performance.The greatest use of LLDPE resin lies in the production of films. The films and products made of LLDPE resin with long-chain alpha olefins (such as hexene and octene) as comonomers are superior to LLDPE resin produced with butylene as comonomers in many aspects, such as tensile strength, impact strength, tear strength, puncture resistance, environmental stress cracking resistance, etc.Since the 1990s, foreign PE manufacturers and users have tended to replace butene with hexene and octene.It is reported that with octene as the comonomer, the resin performance may not be further improved than that of hexene copolymerization, and the price is more expensive. Therefore, the trend of major foreign LLDPE manufacturers to use hexene instead of butene is more obvious.
present situation
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Since there is nomass production Hexene and octene, and the import price is relatively expensive. Therefore, the LLDPE resin produced in China today mainly uses butylene as the comonomer.Although some domestic enterprises used hexene as the brand of comonomer when introducing LLDPE production units, they had to give up because there was no hexene production in China, and only imported a small amount of hexene during start-up assessment.Most of the high-grade LLDPE products imported into China are of this kind.It is expected that in the future1-hexene The demand for single LLDPE will increase significantly.
