Linear low-density polyethylene (LLDPE) occupies an important position in the polyethylene family. Its differences from low-density polyethylene (LDPE), high-density polyethylene (HDPE), and ultra-low-density polyethylene (ULDPE) mainly lie in molecular structure, performance characteristics, and application areas. These differences determine their respective roles and positions within the industry.
From a molecular structure perspective, LDPE is produced by high-pressure free radical polymerization, with a main chain containing numerous long and short branches, a loose molecular arrangement, and low crystallinity (approximately 50%-60%). HDPE, on the other hand, is produced through low-pressure coordination polymerization, forming a nearly linear main chain with very few branches, resulting in a well-ordered and tightly packed molecular chain and high crystallinity (80%-90%). LLDPE, made by copolymerizing ethylene with α-olefins (such as 1-butene, 1-hexene, and 1-octene), has a linear main chain structure, with short branches introduced by the comonomer and evenly distributed, resulting in crystallinity between LDPE and HDPE (approximately 55%-70%). This structural difference directly leads to different densities: LDPE has a density of 0.910-0.925 g/cm³, LLDPE 0.915-0.935 g/cm³, and HDPE between 0.941-0.965 g/cm³, exhibiting a gradient change from "low density to linear low density to high density."
In terms of performance, LLDPE exhibits significantly superior tear strength, puncture resistance, and environmental stress cracking resistance compared to LDPE. Due to its uniform distribution of short branches and tighter molecular chain entanglement, it is less prone to stress concentration under external forces. Therefore, it can replace LDPE in packaging films and heavy-duty bags to improve durability. Compared to HDPE, LLDPE has lower density and better flexibility, but its rigidity and hardness are lower. Thus, HDPE is more suitable for manufacturing rigid containers, pipes, and other products requiring high structural strength, while LLDPE excels in flexible packaging and film products. Furthermore, the melt viscosity of LLDPE is sensitive to shear rate, allowing for optimized flowability through process parameter adjustments. It also results in lower haze and better transparency in blown film compared to LDPE. HDPE, on the other hand, is not sensitive to shear rate, making it more suitable for processes requiring stable flow, such as extrusion molding.
In terms of applications, LDPE, due to its good processability and high transparency, is widely used in lightweight packaging films, agricultural films, and wire and cable insulation layers. HDPE, with its high strength and chemical resistance, dominates the markets for hollow containers, gas pipes, and landfill linings. LLDPE, with its balanced strength and flexibility, has become a core raw material for heavy-duty packaging films, stretch wrap films, agricultural greenhouse films, and rotational molding products, especially irreplaceable in scenarios requiring both impact resistance and weather resistance.
Although ultra-low density polyethylene (ULDPE) shares a linear structure with LLDPE, its higher comonomer content allows for a density as low as 0.870-0.915 g/cm³, further enhancing its flexibility and transparency. It is mainly used for extremely soft films and special elastomer products, but its strength and heat resistance are relatively limited.
In summary, LLDPE achieves a better balance between density, strength, and processability through its unique linear main chain and controllable short branch structure. It complements other polyethylene materials such as LDPE and HDPE, together forming an application matrix for polyethylene materials that meets the refined performance and cost requirements of different fields.