There are various modification methods for linear low-density polyethylene (LLDPE) materials. The following is a detailed introduction from the aspects of chemical modification, physical modification and other modification methods:
Chemical modification
Copolymerization modification
Principle: During the polymerization reaction, ethylene is copolymerized with other monomers such as α-olefins (such as 1-butene, 1-hexene, 1-octene, etc.). By changing the type, content and polymerization conditions of the comonomers, the molecular structure and properties of LLDPE can be adjusted.
Effect: The introduction of different comonomers can change the branching degree, crystallinity and segment structure of the polymer, thereby improving the mechanical properties, processing properties and environmental resistance of LLDPE. For example, LLDPE obtained by copolymerization with 1-octene has better tensile strength and tear resistance.
Graft modification
Principle: Some groups or polymer segments with special functions are grafted onto the LLDPE molecular chain through chemical reactions, thereby giving LLDPE new properties.
Effect: Graft modification can improve the polarity, compatibility, adhesion, etc. of LLDPE. For example, grafting maleic anhydride onto LLDPE can improve its compatibility with polar materials, enable it to better combine with glass fiber, inorganic fillers, etc., and improve the performance of composite materials.
Cross-linking modification
Principle: Through physical or chemical methods, a cross-linked structure is formed between LLDPE molecular chains to form a three-dimensional network structure.
Effect: Cross-linking can significantly improve the heat resistance, mechanical strength, solvent resistance and environmental stress cracking resistance of LLDPE. For example, in the field of wires and cables, cross-linked LLDPE can be used as an insulating layer material that can withstand higher temperatures and voltages.
Physical modification
Blending modification
Principle: LLDPE is blended with other polymers such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), etc., and can also be blended with rubbers such as ethylene-propylene rubber (EPR), styrene-butadiene rubber (SBR), etc.
Effect: The performance advantages of each component can be combined to improve certain properties of LLDPE. If LLDPE is blended with LDPE, the processing fluidity and transparency of LLDPE can be improved; blending with rubber can improve its toughness and impact resistance.
Filling modification
Principle: Add inorganic fillers such as calcium carbonate, talcum powder, kaolin, etc., or organic fillers such as wood powder, cellulose, etc. to LLDPE.
Effect: It can reduce costs and improve the rigidity, hardness, dimensional stability, etc. of the material. For example, adding calcium carbonate can improve the hardness and rigidity of LLDPE and reduce its shrinkage rate. It is widely used in plastic pipes, injection molding products and other fields.
Reinforcement modification
Principle: Add fibrous materials such as glass fiber, carbon fiber, aramid fiber, etc. to reinforce LLDPE.
Effect: It can significantly improve the mechanical properties of LLDPE such as tensile strength, bending strength and modulus. For example, glass fiber reinforced LLDPE can be used to manufacture automotive parts, electronic equipment housings and other products with high strength and rigidity requirements.
Other modification methods
Irradiation modification
Principle: LLDPE is irradiated with high-energy rays such as gamma rays and electron beams.
Effect: Irradiation can cause LLDPE molecular chains to undergo crosslinking, degradation and other reactions, thereby changing its properties. Moderate irradiation can improve the heat resistance, wear resistance and anti-aging properties of LLDPE.
Surface modification
Principle: The surface of LLDPE products is treated by physical or chemical methods, such as corona treatment, plasma treatment, chemical coating, etc.
Effect: The surface properties of LLDPE can be improved, and its surface tension, wettability, adhesion and printability can be improved. For example, after corona treatment on the surface of plastic film, the printability of the film and its composite properties with other materials can be improved.
Use a screw extruder to modify LLDPE materials, and the relevant technical parameters are as follows:
Temperature parameters
Barrel temperature: generally set at 170-230℃. The temperature of the front section can be slightly lower, at 170-180℃, which is convenient for material transportation and preliminary plasticization; the temperature of the middle section is higher, at 180-210℃, so that the material is fully melted and mixed; the temperature of the rear section is 200-230℃, which ensures that the melt has good fluidity and is conducive to extrusion. For example, when PP/LLDPE is blended and modified, the extrusion temperature is 190-220℃.
Head temperature: usually 5-10℃ lower than the temperature of the rear section of the barrel, at 190-220℃, it helps the material to be better formed at the extruder head, and avoids problems such as casting or bubbles due to excessive temperature.
Speed parameters
Main screw speed: generally 30-200r/min. When the speed is low, such as 30-50r/min, the material stays in the screw for a long time, the mixing and plasticization effect is good, but the output is low, which is suitable for modification experiments with high requirements for mixing uniformity and low output requirements. When the speed is higher, such as 150-200r/min, the output is increased, but it may cause uneven mixing or overheating of the material. It is suitable for situations with high output requirements and good material fluidity.
Feeding screw speed: usually 5-20r/min, matching the main screw speed to ensure that the material enters the extruder evenly and stably. If the feeding speed is too fast, it may cause material accumulation and screw overload; if the feeding speed is too slow, it will affect the output. For example, in PP/LLDPE blending modification, the feeding screw speed is 20r/min.
Pressure parameters
Back pressure: generally controlled at 0.5-3MPa. Appropriate back pressure can improve the density and mixing effect of the material and make the melt more uniform, but too high back pressure will increase the motor load, reduce output, and may also cause overheating and degradation of the material.
Head pressure: According to different molds and product requirements, the head pressure is generally 5-20MPa. The die pressure affects the extrusion speed and molding quality of the material. If the pressure is too low, the material is difficult to extrude and the product size is unstable; if the pressure is too high, the mold may be damaged or the product may have flash, deformation and other problems.
Screw parameters
Screw diameter: Common ones are 20-65mm. The larger the diameter, the higher the output of the extruder, which is suitable for large-scale production; screws with smaller diameters are suitable for laboratory research and small-batch production.
Aspect ratio: Generally between 20-40. Screws with large aspect ratios have long residence time in the screw, good mixing and plasticizing effects, which are conducive to improving the modification effect, but power consumption and equipment costs will also increase.
The melting and mixing process of LLDPE materials in the screw extruder is a complex and orderly physical process, which mainly includes the following stages:
Solid conveying stage
Material entry: LLDPE is usually added to the hopper of the screw extruder in the form of granules or powders. Under the action of gravity and the axial thrust generated by the rotation of the screw, the material enters the feeding section of the screw.
Compacting and advancing: In the feeding section, the screw groove is deeper. As the screw rotates, the material is gradually compacted and conveyed forward. At this time, there is friction between the materials and between the materials and the screw and barrel wall. This friction enables the material to move forward stably, preparing for the subsequent melting process.
Melting stage
Heat transfer: When the LLDPE material enters the compression section from the feeding section, the depth of the screw groove gradually becomes shallower, and the compression effect on the material is enhanced. At the same time, the heating device outside the barrel transfers heat to the material through heat conduction, causing the material temperature to rise.
Start melting: As the temperature rises and the pressure increases, the surface of the LLDPE particles begins to soften and melt gradually, forming a thin layer of melt. Due to the rotation of the screw and the geometry of the groove, this layer of melt forms a continuous melt film between the material and the barrel wall, and the material in the melt film is further melted under the combined action of shear force and heat conduction.
Melting is complete: At the end of the compression section, most of the LLDPE material has melted, but there may still be some incompletely melted solid particles. These unmelted particles will enter the homogenization section with the material, and will eventually melt completely into a uniform melt under the high temperature and strong shearing action of the homogenization section.
Mixing stage
Distributive mixing: After the LLDPE material is completely melted, it enters the mixing stage. The first is distributive mixing, which mainly uses the rotation of the screw to form axial flow and circumferential flow in the screw groove. The melts at different positions exchange positions with each other, so that the material can achieve preliminary mixing uniformity on a macro scale. For example, in a single-screw extruder, the circumferential flow of the melt helps to transport the material from the center area of the screw to the barrel wall, while the axial flow propels the material forward along the axis of the screw. The combination of these two flows enables the material to be more evenly distributed throughout the screw groove.
Dispersive mixing: In addition to distributive mixing, there is also dispersive mixing. Under the shearing action of the screw, the different components or additives in the melt will be further dispersed and refined. For example, when some additives such as fillers or toughening agents are added to LLDPE, these additives will be subject to shear forces between the screw and the barrel wall and between different flow rate areas inside the melt in the melt, so that the additive particles are broken, refined, and evenly dispersed in the LLDPE melt. In a twin-screw extruder, due to the mutual engagement and relative movement between the two screws, a more complex and intense shearing effect will be produced, which can better achieve the dispersion mixing effect.
Homogenization and stabilization: After distributive mixing and dispersive mixing, the LLDPE melt is further homogenized in the homogenization section, making the temperature, pressure, composition and other parameters of the melt more uniform and stable, preparing for subsequent extrusion molding. In the homogenization section, the screw groove depth and pitch are usually kept unchanged, and the melt continues to mix and homogenize in a relatively stable flow state, ensuring that the final extruded material has good uniformity and consistency.
