LDPE

LDPE: is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. The most common process used for LDPE is extrusion (tubes, blow and cast films, cables, ...). LDPE can also be processed by injection molding or rotomolding.

It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less.

This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.

Key Points: The crystallinity is dependent on the chain branching, highly branched PE is less crystalline and hence its density remains low and is therefore, known as low density polyethylene.

Long chain branching significantly affects the melt Rheology because of the molecular size reduction and entanglement.

Extrusion moulding :

Melt temperature : 180-240°C

Higher melt temperatures are needed for extrusion-coating (280-310°C).

A three zone screw with a L/D ratio of around 25 is recommended.

Injection moulding :

Melt temperature : 160-260°C

Post mold shrinkage lies between 1.5 and 3.5 %.

APPLICATIONS OF LDPE

Packaging: 

Bags for grocery, garbage, shopping, pouches, milk packing pouches, liners for HDPE woven sacks, garment packing, cellular foams for packaging.

Medical: 

Bottles for packing fluids, Saline, Liquid Injection,

Bottle caps and inner seals, 

Tubes for saline sets, gas, Fluids.

Agriculture: 

Pipes for irrigation, film for green houses, nursery bags,covers for crops, canal liners, mulching films 

Electrical: 

Wire coating, electrical cable sheathing and telephone cable sheathing. 

Strengths:

1. Very low cost.

2. Excellent chemical resistance.

3. Very good process-ability.

4. High impact strength at low temperature.

5. Excellent electrical insulating properties

6. Very low water absorption

7. FDA compliant

8. Can be transparent in thin film form

9. Since it is non-polar, electrical properties are almost independent of temperature and frequency

10. Tg is well below the room

temperature

Limitations:

1.  Susceptible to environmental stress cracking

2. Low strength, stiffness and maximum service temperature

3. High gas permeability, particularly carbon

dioxide

4. Poor UV resistance

5. Highly flammable

6. High-frequency welding and joining impossible

7. Poor scratch and abrasion resistance.

Properties

1. Excellent resistance to organic and inorganic liquids

2. Water repellent

3. Sterlizable with ethylene oxide

4. Free from odour and toxicity

5. Low gas permeability

6. Good resistance to micro organisms

7. Since it is non-polar, electrical properties are 8. almost independent of temperature and frequency.

9. High elongation at break

10. Low brittleness

11. Easy processability 

12. Good impact resistance

13. Sufficiently low water vapour absorption

14. Tg  is well below the room temperature

15. Susceptible to UV radiation

16. Low molecular grade is susceptible to environmental stress cracking

17. Poor abrasion resistance

18. Poor scratch resistance