Linear low density polyethylene (LLDPE) is a molecular structure formed by copolymerization of ethylene and a small amount of α - olefins on the main chain of linear ethylene, with very short comonomer branch chains.
Linear low-density polyethylene is non-toxic, tasteless and odorless milky white particles with a density of 0.918~0.935g/cmthree。Compared with LDPE, it has higher softening temperature and melting temperature, high strength, good toughness, high rigidity, heat resistance, cold resistance and other advantages. It also has good resistance to environmental stress cracking, impact strength, tear strength and other properties, and can withstand acid, alkali, organic solvents and other fields, and is widely used in industry, agriculture, medicine, health and daily necessities.
Chinese name
Linear low density polyethylene
Foreign name
Linear low density polyethylene （LLDPE）
Purpose
Produce agricultural film, packaging film, molding, pipe
Shape
Non toxic, tasteless and odorless milky white particles
Linear low density polyethylene (LLDPE) resin is called the third generation polyethylene. In addition to the performance of general polyolefin resin, its tensile strength, tear strength, environmental stress cracking resistance, low temperature resistance, heat resistance and puncture resistance are particularly superior.
Although LLDPE and past LDPE belong to the same density range, their basic physical properties and molding processing characteristics are different due to their different molecular structures and melting rheological behaviors.[1]
LLDPE
LDPE
molecular weight distribution
narrow
wide
Melting point (℃)
110~125
105~115
Relative tensile strength
1.50~1.75
one
Relative elasticity
1.40~1.80
one
Environmental stress cracking resistance
good
difference
Heat resistance
good
Slightly poor
Oil resistance
good
Slightly poor
In structure, LLDPE is different from HDPE only in the number of short branches.The number of short branch chains of HDPE is relatively small, which in the final analysis depends on its use.The density of LLDPE and LDPE is 0.91~0.925g/cmthreebetween.This higher crystallinity also increases the melting point of LLDPE by 10-15 compared with LDPE.Higher tensile strength, penetration resistance, tear resistance and elongation increase are the characteristics of LLDPE, making it particularly suitable for film making.If hexene or octene is used as comonomer instead of butene, even the impact resistance and tear resistance can be greatly improved.The longer side chains of hexene and octene resins act as "knots" between chains, improving the toughness of compounds.Using cycloolefin metal derivative catalyst to produce resin will have unique performance.These are similar to LLDPE produced with Ziegler catalyst.[2]
In terms of transparency, LLDPE has the same disadvantage as LDPE: the turbidity and gloss of LLDPE film are not good, mainly because its higher crystallinity results in the film surface roughness.The transparency of LLDPE resin can be improved by blending with a small amount of LDPE.
production
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Linear low density polyethylene (LLDPE) uses ethylene as the main raw material, and a small amount of α - olefins (such as 1-butene-1, 1-octene, etc.)catalyzerGas phase fluidized bed polymerization is carried out under the action of high pressure or low pressure, and the structural formula is - [CHtwo-CHtwo]n-The reacted materials are granulated, dried and sent to packaging.[1]
LLDPE production starts with transition metal catalysts, especially Ziegler or Philips types.The new process based on cycloolefin metal derivative catalyst is another option for LLDPE production.The actual polymerization can be carried out in solution and gas phase reactors.Usually, octene and ethylene co polymerize butylene in a solution phase reactor.Hexene and ethylene are polymerized in a gas phase reactor.LLDPE resin generated in the gas phase reactor is in the form of particles, and can be sold as powder or further processed into particles.Linear low density polyethylene is usually characterized by melt index and density.Melt index can reflect the average molecular weight of resin and is mainly controlled by reaction temperature.The average molecular weight is independent of the molecular weight distribution (MWD).Catalyst selection affects MWD.The density is determined by the concentration of monomer for copolymerization in the polyethylene chain.The concentration of monomer used for copolymerization controls the number of short branched chains (its length depends on the type of monomer used for copolymerization) to control the resin density.High pressure LDPE has long branched chains, while linear LDPE only has short branched chains.[2]
machining
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Both LDPE and LLDPE have excellent rheological or melt fluidity.LLDPE has less shear sensitivity because of its narrow molecular weight distribution and short branched chains.LLDPE maintains a higher viscosity during shearing, so it is more difficult to process than LDPE with the same melt index.In extrusion, the lower shear sensitivity of LLDPE makes the stress relaxation of polymer molecular chain faster, and thus the sensitivity of physical properties to the change of blow up ratio decreases.In melt elongation, LLDPE usually has lower viscosity at various strain rates.That is to say, it will not produce strain hardening like LDPE during tension.[3]
With the increase of the deformation rate of polyethylene, LDPE shows an amazing increase in viscosity, which is caused by molecular chain entanglement.This phenomenon cannot be observed in LLDPE, because the lack of long branched chains in LLDPE makes the polymer not entangled.This property is very important for thin film applications.LLDPE films are easier to make thinner films while maintaining high strength and toughness.
The rheological properties of LLDPE can be summarized as "rigid in shear" and "soft in extension".When LLDPE is used to replace LDPE, the film extrusion equipment and conditions must be modified.The high viscosity of LLDPE requires the extruder to have higher power and provide higher melt temperature and pressure.The die gap must be widened to avoid production reduction due to high back pressure and melt fracture.The general die clearance dimensions of LDPE and LLDPE are 0.024~0.040 and 0.060~0.10 respectively.
LLDPE's "soft during extension" feature is a disadvantage during film blowing.The blown film bubbles of LLDPE are not as stable as LDPE.The general single lip wind ring is stable enough for LDPE.The unique membrane bubble of LLDPE requires more perfect double lip air ring to stabilize.Cooling the internal film bubble with a double lip air ring can increase the stability of the film bubble and improve the film production capacity at high productivity.When LLDPE or LLDPE rich blending materials with LDPE are used, the general LDPE extruder must be improved.According to the life of the extruder, the improvement may be to widen the die clearance, but the processing conditions need to be optimized.The rotational molding process requires LLDPE to be ground into uniform particles (35 mesh).The processing process includes filling the mold with powder LLDPE, heating and rotating the mold in two axial directions to make LLDPE evenly distributed.After cooling, the product is removed from the mold.[2]
Packaging, storage and transportation
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The products are packed in polyethylene heavy film bags, and the outer packaging is polypropylene woven bags, each bag has a net weight of 25kg.The storage warehouse shall be kept clean, dry, cool and well ventilated.It can be transported by train, car, ship, aircraft, etc.It can be transported as non dangerous goods.The transportation tools shall be kept clean and dry, and shall not have sharp objects such as iron nails.During storage and transportation, attention shall be paid to fire prevention, waterproof, sun protection, dust prevention and pollution prevention.Iron hooks shall not be used during loading and unloading.
application
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LLDPE has penetrated most traditional markets of polyethylene, including film, molding, pipe and wire and cable.Anti leakage plastic film is a newly developed LLDPE market.Geomembrane, a large extruded sheet, is used as a liner for waste landfill and waste pool to prevent leakage or pollution of surrounding areas.
Some film markets of LLDPE, such as production bags, garbage bags, elastic packaging, industrial bushings, towel bushings and shopping bags, all take advantage of the advantages of this resin after improving strength and toughness.Transparent film.The penetration resistance and the rigidity of LDPE film do not significantly affect the transparency of the film.Injection molding and rotational molding are the two largest molding applications of LLDPE.The superior toughness, low temperature and impact strength of this resin are theoretically suitable for waste containers, toys and refrigeration appliances.In addition, LLDPE's high resistance to environmental stress cracking makes it suitable for injection molding of molded lids in contact with oil food, roto plastic waste containers, fuel tanks and chemical tanks.The market for application in pipe materials and wire and cable coating is small, where the high rupture strength and environmental stress cracking resistance provided by LLDPE can meet the requirements.65%~70% of LLDPE is used to make films.[1]
LLDPE polymer produced during copolymerization has a narrower molecular weight distribution than ordinary LDPE, and its linear structure makes it have different rheological properties.The melting flow characteristics of LLDPE meet the requirements of new processes, especially the film extrusion process, which can produce high-quality LLDPE products.LLDPE is used in all traditional markets of polyethylene, which enhances the properties of tensile resistance, penetration resistance, impact resistance and tear resistance.Its excellent resistance to environmental stress cracking, low temperature impact resistance and warpage resistance make LLDPE attractive for pipe, sheet extrusion and all molding applications.The latest application of LLDPE is to use it as the liner of waste residue landfill and waste liquid pool.
