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With China Plastic Machinery
With China Plastic Machinery
Linear low-density polyethylene is referred to as LLDPE. It is a copolymer of ethylene and a small amount of ¦Á-olefins (such as 1-butene, 1-octene, etc.). The density is 0.918-0.940, the melting point is 122-124¡æ, and the mechanical properties are between high-density and low-density polyethylene. In addition to the properties of general polyolefin resins, its tensile strength, tear strength, environmental stress cracking resistance, low temperature resistance, heat resistance and puncture resistance are particularly superior.
Linear low-density polyethylene has excellent rheological properties and melt fluidity. Due to its narrow molecular weight distribution and short chain branches, it has less shear sensitivity. Compared with LDPE, LLDPE is difficult to process because it has a higher melt viscosity. Processing high LLDPE content films requires the use of sufficiently powerful motors and appropriate cooling systems. When processing wide-width agricultural films, a certain amount of LDPE needs to be blended into LLDPE to improve processing performance, but it will lead to increased shear viscosity at high shear rates, increased extrusion pressure, reduced extrusion volume, and reduced melt strength, affecting the stability of the film bubble. Therefore, it is recommended to use a double-inlet air ring and an IBC bubble internal cooling system to improve the stability and transparency of the bubble.
There are four main methods for producing polyethylene: high-pressure method, gas phase method, solution method and slurry method. However, at present, the gas phase method and solution method are usually used to produce LLDPE resin in the world.
Linear low-density polyethylene uses ethylene as the main raw material and a small amount of ¦Á-olefins (such as 1-butene-1, 1-octene, etc.) under the action of a catalyst. The high-pressure or low-pressure gas phase fluidized bed polymerization has a structural formula of -[CH2-CH2]n-, and the reacted materials are granulated, dried, and sent for packaging.
The production of LLDPE starts with transition metal catalysts, especially Ziegler or Philips types. The new process based on cycloolefin metal derivative catalysts is another option for LLDPE production. The actual polymerization reaction can be carried out in solution and gas phase reactors. Usually, octene and ethylene are copolymerized with butene in a solution phase reactor. Hexene and ethylene are polymerized in a gas phase reactor. The LLDPE resin produced in the gas phase reactor is in the form of particles and can be sold as powder or further processed into pellets. Linear low density polyethylene is usually characterized by melt index and density. The melt index reflects the average molecular weight of the resin and is mainly controlled by the 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 the comonomer in the polyethylene chain. The comonomer concentration controls the number of short chain branches (whose length depends on the type of comonomer) and thus controls the resin density. High-pressure LDPE has long chain branches, while linear LDPE has only short chain branches.
Linear low density polyethylene is a non-toxic, tasteless, odorless milky white particle with a density of 0.918-0.935g/cm3. Compared with LDPE, it has a higher softening temperature and melting temperature, and has the advantages of high strength, good toughness, high rigidity, good heat resistance and cold resistance. It also has good resistance to environmental stress cracking, impact strength, tear strength and other properties. It is also resistant to acid, alkali, organic solvents, etc.
1. It has good tensile strength and impact strength, is soft and tough, and has better oil resistance and chemical resistance than LDPE
2. Its melting point is 10~20¡ãC higher than LDPE, and its low-temperature brittle temperature is 20~30¡ãC lower than LDPE
3. It is non-toxic, odorless, tasteless, with good transparency and gloss.
4. The heat adhesion is very good, and the heat seal strength has little to do with the heat seal temperature, the heat seal temperature range is wide and the strength is high
Product brand: 7042 (granules)
Product brand: 7047 (granules)
Product brand: 7042 powder
Product brand: 7047 powder
Product brand: Tianlian 9020 granules, 9085 granules, 1820 powder, 1875 powder
Packaging and storage and transportation The product is packed in polyethylene heavy-duty film bags, and the outer packaging is polypropylene woven bags, with a net weight of 25kg per bag. The storage warehouse should be kept clean, dry, cool and well ventilated. It can be transported by train, car, ship, airplane, etc. It can be transported as non-dangerous goods. The means of transportation should be kept clean and dry, and no sharp objects such as nails should be allowed. During storage and transportation, attention should be paid to fire prevention, waterproofing, sun protection, dust prevention and pollution prevention. Iron hooks shall not be used during loading and unloading.
Linear low-density polyethylene (LLDPE) resin, known as the third generation of polyethylene, has the properties of general polyolefin resins, and its tensile strength, tear strength, environmental stress cracking resistance, low temperature resistance, heat resistance and puncture resistance are particularly superior, and has achieved remarkable development.
Although LLDPE belongs to the same density range as the previous LDPE, due to their different molecular structures and different melt rheological behaviors, their basic physical properties and molding processing characteristics are also different.
In terms of structure, LLDPE differs from HDPE only in the number of short chain branches. HDPE has fewer short chain branches, which ultimately depends on its use. The density of LLDPE and LDPE is between 0.91~0.925g/cm. This higher crystallinity also increases the melting point of LLDPE by 10~15 compared with LDPE. Higher tensile strength, penetration resistance, tear resistance and increased elongation are the characteristics of LLDPE, making it particularly suitable for film production. Even impact resistance and tear resistance can be greatly improved if hexene or octene is used as a comonomer instead of butene. The longer side chains of hexene and octene resins act like "knot" molecules between the chains, improving the toughness of the compound. The resin produced with cycloolefin metal derivative catalysts will have unique properties. These are similar to LLDPE produced with Ziegler catalysts.
In the property of transparency, LLDPE has similar disadvantages to LDPE: the turbidity and gloss of LLDPE films are poor, mainly because its higher crystallinity causes film surface roughness. The transparency of LLDPE resin can be improved by blending with a small amount of LDPE.
Both LDPE and LLDPE have excellent rheological properties or melt flowability. LLDPE is less shear sensitive because of its narrow molecular weight distribution and short chain branches. During shearing (e.g. extrusion), LLDPE maintains a greater viscosity and is therefore more difficult to process than LDPE of the same melt index. In extrusion, LLDPE's lower shear sensitivity results in faster stress relaxation of the polymer chain and, as a result, reduced sensitivity of physical properties to changes in the blow ratio.
In melt extension, LLDPE generally has a lower viscosity at all strain rates. That is, it will not strain harden as LDPE does when stretched. As the deformation rate of polyethylene increases, LDPE shows a dramatic increase in viscosity, which is caused by molecular chain entanglement.
This phenomenon is not observed in LLDPE because the lack of long chain branches in LLDPE prevents the polymer from entanglement. This property is extremely important for film applications. Because LLDPE films can be made into thinner films while maintaining high strength and toughness. nLLDPE's rheological properties can be summarized as "rigid in shear" and "soft in extension".
Film extrusion equipment and conditions must be modified when LLDPE is substituted for LDPE. The high viscosity of LLDPE requires a more powerful extruder. and provide higher melt temperature and pressure. The die gap must be widened to avoid reduced production due to high back pressure and melt fracture. The general die gap sizes for LDPE and LLDPE are 0.024~0.040 and 0.060-0.10 respectively.
LLDPE's "soft when stretched" property is a disadvantage in the blown film process. The blown film bubble of LLDPE is not as stable as that of LDPE.
The general single-lip air ring is stable enough for LDPE, but the unique bubble of LLDPE requires a more complete double-lip air ring to stabilize. Cooling the internal bubble with a double-lip air ring can increase bubble stability and improve film production capacity at high production rates. In addition to better cooling of the bubble, many film manufacturers use blending with LDPE to enhance LLDPE dissolution. In principle, the extrusion of LLDPE can be completed on existing LDPE film equipment when the concentration of LLDPE in the LDPE blend reaches 50%. When processing 100% LLDPE or LLDPE-rich blends with LDPE, using a general LDPE extruder, equipment improvements are necessary.
Depending on the life of the extruder, the required improvements may be to widen the die gap, improve the air ring, modify the screw design for better extrusion, and increase the motor power and torque if necessary. For injection molding applications, equipment improvements are generally not required, but processing conditions must be optimized.
Rotational molding requires LLDPE to be ground into uniform particles (35 mesh). The process involves filling the mold with powdered LLDPE, heating and rotating the mold biaxially to evenly distribute the LLDPE. The product is removed from the mold after cooling.
LLDPE can be made into garbage bags, ice bags, grocery bags, heavy packaging bags, and wrapping bags, etc. It can also be used for the inner sealing layer of composite flexible packaging materials. Compared with LDPE, it makes the sealing of bags more reliable and the heat sealing strength at the seal is higher, but the cost is higher than LDPE
LLDPE has penetrated most of the traditional markets of polyethylene, including film, molding, pipes and wires and cables. Leak-proof ground film is a newly developed LLDPE market. Ground film, a large extruded sheet, is used as a liner for landfills and waste pools to prevent leakage or pollution of surrounding areas.
Some film markets for LLDPE, such as the production of bags, garbage bags, elastic packaging, industrial liners, towel-style liners and shopping bags, all take advantage of the advantages of this resin after improving strength and toughness. Transparent films, such as bread bags, have always been dominated by LDPE because it has better turbidity. However, blends of LLDPE with LDPE will improve strength. The penetration resistance and stiffness of LDPE film without significantly affecting the transparency of the film. Injection molding and rotational molding are the two largest molding applications of LLDPE. The superior toughness and low temperature and impact strength of this resin are theoretically suitable for waste bins, toys and refrigeration equipment.
In addition, LLDPE's high resistance to environmental stress cracking makes it suitable for injection molding of molded lids in contact with oily foods, rotational molding of waste containers, fuel tanks and chemical tanks.
The market for applications in pipes and wire and cable coatings is relatively small, where the high burst strength and environmental stress cracking resistance provided by LLDPE can meet the requirements. Currently, 65%-70% of LLDPE is used to make films.
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