Polyethylene (PE) is one of the most widely used members of the plastics family, and is widely used in films, pipes, injection molded parts, and other applications because of its excellent performance and relatively low price. Polyethylene can be categorized into three main groups based on their density and structure: low-density polyethylene (LDPE), high-density polyethylene (HDPE), and linear low-density polyethylene (LLDPE).
Detailed introduction to LDPE, HDPE, LLDPE
(1) Low-density polyethylene (LDPE) film
- Structural differences:
LDPE: has a large number of short and long branched chains, loose structure, low density (0.910~0.935 g/cm3). - Characteristics
a. Stable chemical properties, insoluble in – general solvents, moisture barrier, excellent resistance to medicines
b. Great permeability, poor flavor retention, poor grease resistance
c. Large elongation, impact strength, softness, toughness.
d. Excellent cold and low temperature resistance, not high temperature resistance
e. Non-toxic, odorless, tasteless, good transparency.
f. The softening temperature of the film is 80~90C, the melting point is 110~120C, and the heat sealability is excellent. - Application
LDPE applications include food packaging, fiber packaging, daily chemical packaging and pharmaceutical packaging, etc., can also be used as a composite flexible packaging materials, heat sealing layer. Such as squash packaging, the material can be designed as BOPP/VMPET/LDPE structure, of which LDPE as a non-toxic and odorless heat sealing material.
(2) High-density polyethylene (HDPE)
- Structural differences:
HDPE: Few branched chains, regular structure, high density (0.940~0.970 g/cm3). - characteristics:
a. Small elongation of the film, tensile strength, impact strength.
b. Chemical stability, moisture resistance, heat resistance, oil resistance is better than LDPE film.
c. Semi-transparent, milky white appearance, poor surface gloss.
d. Heat sealing is easy, can repeat heat sealing.
e. Heat resistance is better, and it can show better strength under low temperature.
f. Non-toxic, odorless and tasteless - Applications:
Mainly used for moisture-proof bags, convenience bags (undershirt bags) garbage bags, but also with its co-extruded film to produce dairy products and frozen food packaging, etc.. Such as frozen food packaging and its materials for HDPE / EVA, the use of HDPE non-toxic and odorless, excellent resistance to high and low temperatures as well as the advantages of low prices.
(3) linear low-density polyethylene (LLDPE) film
- structural differences:
LLDPE: with short branched chain, but no long branched chain, the structure between LDPE and HDPE, the density is also between the two (0.920 ~ 0.940 g/cm3).
2.Characteristics:
a. Good tensile strength and impact strength, soft and good toughness, oil resistance, chemical resistance is better than LDPE.
b. Melting point is 10-20C higher than LDPE, low temperature embrittlement temperature is 20-30C lower than LDPE.
c. Non-toxic, odorless, tasteless, good transparency, good gloss.
d. Very good thermal adhesion, and the heat sealing strength has little relationship with the heat sealing temperature, wide range of heat sealing temperature and high strength.
3.Application:
LLDPE can be made into garbage bags, ice bags, grocery bags, heavy bags and winding bags, etc., can also be used for composite flexible packaging materials, the inner sealing layer, compared with the LDPE so that the sealing of the bag is more reliable, the sealing heat sealing strength at the sealing is higher but the cost is higher than the LDPE. Such as liquid detergent packaging material structure: BOPA / LLDPE, LLDPE as the inner sealing material, good resistance to sealing pollution.
Comparison of LDPE, HDPE and LLDPE properties
| Properties | LDPE | HDPE | LLDPE |
| Mechanical strength | Weak | Strong | Medium |
| Rigidity | Flexible | Rigidity | Medium |
| Transparency | Transparent | Translucent | Translucent |
| Wear resistance | Poor | Good | Moderate |
| Environmental stress cracking resistance | Poor | Better | Better |
| Chemical stability | Good | Good | Good |
How to Choose LDPE, HDPE, LLDPE
When choosing polyethylene materials, its performance and price should be considered comprehensively according to the specific requirements of use.
Generally speaking, if the material is required to have high flexibility and transparency, LDPE can be selected; if the material is required to have high mechanical strength and rigidity, HDPE can be selected; if the material is required to have both advantages, LLDPE can be selected.
Conclusion
LDPE, HDPE and LLDPE are three important materials in the polyethylene family, each with its own advantages and disadvantages, and different scope of application. When choosing, comprehensive consideration should be made according to specific needs.