Polyethylene (PE) can be categorized into different types based on polymerization methods, including Low-Density Polyethylene (LDPE), High-Density Polyethylene (HDPE), and Linear Low-Density Polyethylene (LLDPE). The differences in their properties are outlined below:
Differences in PE Types and Their Properties
| Type | HDPE | LDPE | LLDPE |
|---|---|---|---|
| Density | 0.940 – 0.976 g/cm³ | 0.910 – 0.940 g/cm³ | 0.915 – 0.935 g/cm³ |
| Crystallinity | 85 – 65% | 45 – 65% | 55 – 65% |
| Softening Point | 125 – 135℃ | 90 – 100℃ | 94 – 108℃ |
| Molecular Structure | Carbon-carbon and carbon-hydrogen bonds requiring high energy to break | Small polymer molecular weight, easier to break | Linear structure with short branches, easier to break |
| Mechanical Strength | High strength, good toughness, strong rigidity | Lower strength, good flexibility | High strength, good toughness |
| Tensile Strength | High | Moderate | Moderate |
| Elongation Rate | Low | High | Moderate |
| Barrier Properties | Excellent resistance to water, steam, and air; low water absorption | Poor barrier properties | Excellent resistance to water, steam, and air; low water absorption |
| Chemical Resistance | Resistant to acids, bases, and salts; poor solvent resistance | Good resistance to acids, bases, and salts | Resistant to acids, bases, and organic solvents |
| Heat and Cold Resistance | Excellent | Good, can withstand -70℃ | Excellent |
| Brittle Temperature | Below -90℃ | Below -70℃ | Below -90℃ |
Processing Differences of PE Grades
Flowability
- LDPE: Contains more short-chain branches, resulting in loose molecular arrangements and excellent flowability. Processing temperature: 160 – 200℃.
- HDPE: High crystallinity and tight molecular arrangement reduce flowability, requiring 200 – 280℃ processing temperatures.
- LLDPE: Linear structure with evenly distributed short branches; moderate flowability. Processing temperature: 180 – 240℃.
Melt Strength
- LDPE: High melt strength, suitable for blown film processing.
- HDPE: Lower melt strength; requires careful control in extrusion blow molding.
- LLDPE: Melt strength between LDPE and HDPE, requiring adjustments in processing parameters.
Heat Stability
- LDPE: Poor heat stability; high temperatures may cause degradation.
- HDPE: Better heat stability, allowing higher temperature processing.
- LLDPE: Good heat stability, comparable to HDPE.
Molding Shrinkage Rate
- LDPE: 1.5% – 5%, high shrinkage requires mold adjustments.
- HDPE: 1.2% – 3%, lower shrinkage, suitable for precision molding.
- LLDPE: 1.5% – 3.5%, moderate shrinkage.
PE Film Standardization Across Different Countries
China
- Standards: GB/T 22000-2006, ISO 22000:2005.
- Common Grades: 6070, 9001 (used for shopping bags, agricultural films).
Europe
- Standards: EN 17033 (biodegradable agricultural film).
- Focus: Environmental sustainability, recyclability.
United States
- Standards: ASTM D2103-15 (classification and performance requirements for polyethylene films).
Other Countries
- Many follow ISO standards, with local adjustments where necessary.
Common PE Grades in China
HDPE Grades
- 5200B (Daqing): Blow molding grade for industrial containers.
- 2200J (Daqing): Injection molding grade for industrial parts.
- 5301AA (Dushanzi): Extrusion grade for ropes, nets, and pipes.
- 6070 (Dushanzi): Film grade for shopping bags.
- 2908 (Fushun): Injection grade for furniture and industrial containers.
- 7000F (Yangzi): Hollow container grade for toys, floats.
LDPE Grades
- 2102TN26 (Qilu): Film grade for agricultural applications.
- Q200 (Shanghai): Light packaging film.
- 1C7A (Yanshan): Coating grade with excellent fluidity.
LLDPE Grades
- LL0209AA: Agricultural and packaging film.
- 7042: Widely used for food packaging and liners.
How to Choose the Right PE Grade for Processing?
By Processing Method
Blow Molding
- Thin film: LDPE (e.g., Yanshan 1F7B) for high transparency and flexibility.
- High-strength film: LLDPE (e.g., 7042) for better durability.
- Large containers: HDPE (e.g., Dushanzi 5301AA) for strength and stress resistance.
Injection Molding
- Precision small parts: HDPE (e.g., 2200J) for high impact resistance.
- Flexible products: LLDPE for enhanced flexibility.
Extrusion Processing
- Pipes: HDPE (e.g., Shanghai TR480) for pressure resistance.
- Cable insulation: LDPE (e.g., Qilu 2102TN00) for good electrical insulation.
By Performance Requirements
Strength
- High-strength applications: HDPE (e.g., industrial pallets, garbage bins).
- Moderate strength and flexibility: LDPE or LLDPE.
Flexibility
- Highly flexible products: LDPE (e.g., plastic bags, cling film).
- Flexible yet strong: LLDPE (e.g., agricultural films).
Chemical Resistance
- Chemical storage containers: HDPE (resistant to acids, bases, solvents).
Transparency
- Transparent films: LDPE for better optical clarity.
By Cost and Equipment Compatibility
- Lower processing cost: LDPE or LLDPE.
- Higher performance and durability: HDPE (higher cost but better properties).
Conclusion
Selecting the right PE grade requires careful consideration of processing methods, mechanical properties, chemical resistance, and cost factors. By understanding the differences between HDPE, LDPE, and LLDPE, manufacturers can choose the most suitable material for their applications, ensuring optimal product performance and cost-effectiveness.
