01
Introduction
Friends working in the PE film industry must often talk about PE film raw materials: LDPE, LLDPE, HDPE, etc. Perhaps some friends only know that they are low-density, linear low-density, and high-density PE respectively, but they have not yet understood their respective properties and applications in depth. However, mastering the microstructure and properties of these raw materials is of great significance for formula adjustment and process condition setting. Therefore, this article will take you beyond the surface of daily applications, reveal the difference codes of LDPE, LLDPE, and HDPE from the molecular level, analyze the delicate balance in formulas, and show how they accurately match industry needs.
02
Microstructure – The DNA of Performance Differences
LDPE (Low-Density Polyethylene)
Structural characteristics: Molecular chains with highly branched structures (long and short branches) on the linear main chain, like a tangled “tree branch”.
Packing density: Low (0.910-0.925 g/cm³), with large gaps between molecular chains.
Crystallinity: Relatively low (about 40-50%), with few regularly arranged regions.
LLDPE (Linear Low-Density Polyethylene)
Structural characteristics: Linear main chain with uniformly distributed short branches (introduced by comonomers such as butene, hexene, octene), with a very regular structure, like “tree branches with a clear main trunk but short and small forks”.
Packing density: Low (0.915-0.925 g/cm³), close to LDPE but with a more ordered structure.
Crystallinity: Medium to high (about 50-60%), higher than LDPE.
HDPE (High-Density Polyethylene)
Structural characteristics: Highly linear with very few branches, molecular chains like “straight and smooth sticks”.
Packing density: High (0.941-0.965 g/cm³), with molecular chains arranged closely.
Crystallinity: High (about 70-90%), with a large proportion of regularly arranged regions.
03
Changes in Formula Content – A Single Move Affects the Whole
In PE film formulas (often blended or co-extruded), the content ratio of the three raw materials is the core lever for regulating performance. Increasing the content of one material will affect different properties of the film, which can be divided into the following three cases:
Increase in LDPE content:
Positive effects: Significantly improves melt strength (good film blowing stability), optical properties (more transparent, better gloss), heat-sealing performance (low sealing initiation temperature, good hot tack), softness, and processing fluidity.
Negative effects: Reduces rigidity, tensile strength, puncture resistance, environmental stress crack resistance (ESCR), and heat resistance (low melting point).
Increase in LLDPE content:
Positive effects: Greatly improves toughness (excellent impact resistance and puncture resistance), tensile strength (especially more balanced in longitudinal and transverse directions), ESCR, low-temperature resistance, and allows the film to be made thinner.
Negative effects: Reduces melt strength (narrow processing window), optical properties (slightly higher haze), heat-sealing performance (sealing initiation temperature is usually higher than that of LDPE), and processability (may require higher torque).
Increase in HDPE content:
Positive effects: Greatly improves rigidity, tensile strength, wear resistance, barrier properties (especially improved water resistance), heat resistance (high melting point), and dimensional stability.
Negative effects: Significantly reduces toughness (becomes brittle), impact resistance (especially at low temperatures), transparency (usually translucent or opaque), softness, and heat-sealing performance (poor).
In our actual production process, we may need to balance multiple properties. How should we adjust the formula?
If you pursue softness/easy processing/heat-sealing property: High LDPE proportion.
If you pursue strength/toughness/thinning: High LLDPE proportion.
If you pursue rigidity/stiffness/moisture resistance: Add HDPE (often in the middle layer of composite structures or as a reinforcing component).
The way to balance: Most films use LDPE/LLDPE blends, combining the advantages of both, and achieving the perfect balance between performance and cost through proportion fine-tuning. For example: Increasing LLDPE to improve strength and toughness, supplemented by an appropriate amount of LDPE to improve processing and heat-sealing properties.
04
Industry Applications – Microstructure Determines End-Use Applications
Light packaging field:
LDPE-dominated: Benefiting from excellent heat-sealing property, transparency, softness, and processability, it is widely used as the inner heat-sealing material for bread bags, candy bags, clothing bags, and ordinary snack bags. It is often used as the heat-sealing layer in co-extruded films.
LLDPE penetration: For light packaging with high requirements for puncture and tear resistance (such as containing sharp contents), the proportion of LLDPE is increased, or even an all-LLDPE structure is used to achieve thinning.
Heavy packaging field:
LLDPE-dominated: Its excellent strength, toughness, and puncture resistance are its core competitiveness. It is used in large-capacity rice bags, fertilizer bags, feed bags, inner liners for liquid packaging (soy sauce, detergents), and inner liners for industrial flexible intermediate bulk containers (FIBC). It achieves “thinner, stronger, and more material-saving”.
HDPE participation: For heavy packaging requiring high rigidity and excellent moisture resistance (such as multi-layer composite bags), HDPE is often used as the middle layer or outer substrate.
Agricultural film field:
Traditional LDPE application: Wide-width greenhouse films (early stage), utilizing its good melt strength to ensure the stability of wide-width production and certain light transmittance.
LLDPE/LDPE blend-dominated: The mainstream of modern agricultural films (greenhouse films, mulch films). LLDPE provides core toughness, puncture resistance, weather resistance (especially when copolymerized with hexene/octene), and longevity. LDPE improves processability and initial light transmittance. EVA is also often blended to improve heat retention. HDPE is used for specific mulch films (weed control, black films).
Special film field:
Stretch film: LLDPE (especially hexene and octene copolymers) is the absolute leader. Its extremely high stretch ratio, excellent self-adhesion, long-lasting binding force, and puncture resistance are unmatched by other materials. Metallocene LLDPE (mLLDPE) provides better performance.
Barrier packaging: HDPE, with excellent water vapor barrier property, is often used as the supporting layer or moisture barrier layer in multi-layer composite structures (such as aluminum-plastic composites, EVOH high-barrier composites).
Daily and industrial film field:
Shopping bags/garbage bags: LDPE/LLDPE blends are the mainstream. LDPE provides soft hand feel, easy opening, and good heat-sealing property; LLDPE provides strength, toughness, and thinning potential. Shopping bags pursuing high stiffness will increase the proportion of HDPE (especially in handles or specific layers of the bag body).
Industrial liners/protective films: LLDPE is widely used due to its excellent chemical resistance, strength, and toughness.
05
Finally
The three major PE raw materials, LDPE, LLDPE, and HDPE, endow them with distinct yet complementary macro-performance spectrums due to their unique microstructures (degree of branching, linearity, crystallinity). Understanding their “structure-performance-processing-application” relationship chain is the cornerstone of PE film formula design and application innovation. Skillfully adjusting their proportions in the formula is like a chef preparing ingredients, which can accurately “cook” film products that meet specific end-use needs.
Post time: Aug-02-2025