Difference Between HDPE and LDPE

Key Difference – HDPE vs LDPE
 

Though HDPE and LDPE are two categories of Polyethylene, some differences can be observed between them based on their mechanical properties. Polyethylene is a mixture of similar organic compounds which have the chemical formula of (C₂H₄)_n. Polyethylene is categorized into numerous different groups based mainly on its density and branching. With regard to demands and supply, the most important polyethylene grades are HDPE and LDPE. The High-density Polyethylene (HDPE) and Low-density Polyethylene (LDPE) have different mechanical properties such as the crystal structure, the magnitude and nature of branching and the molecular weight. In other words, HDPE and LDPE are considered as the contrasting ends of the plastics applications spectrum.  The key difference between HDPE and LDPE is the density or the manner in which the polymer molecules align. HDPE polymers are straighter and closely packed together whereas LDPE polymers have lots of branches, and they are not closely packed. Based on the molecular structure each type of plastic has its own physical and chemical characteristics. In this article, let’s elaborate what are the differences between HDPE and LDPE.

What is High Density Polyethylene (HDPE)?

HDPE is well-defined by a density of larger or equal to 0.941 g/cm³. It has a liner polymer and lower degree of branching. This  results in molecules to be packed closer and intermolecular bonds are stronger than in highly branched polymers such as LDPE. The absence of branching also results in a higher density and somewhat higher chemical resistance than LDPE. With a high tensile strength, HDPE is used in products like garbage containers, baby toys, water pipes, jugs and jars as well as packagings such as milk jugs, butter tubs, and detergent bottles. HDPE is also more durable and more opaque and can tolerate higher temperatures mainly compared to LDPE. Industrially, HDPE is synthesized from ethylene by a catalytic procedure.

What is Low Density Polyethylene (LDPE)?

LDPE is well-defined by a density range of 0.91–0.94g/cm³. A branched polymer, LDPE is synthesized from ethylene by a catalytic procedure. The more branching nature results in molecules packing irregularly and intermolecular bonds are weaker than in highly linear polymers such as HDPE. With a low tensile strength, LDPE is used in products like plastic bags and film wrap as well as packagings such as water bottles, food storage containers, dispensing bottles and plastic tubs. LDPE is also more flexible and more transparent and cannot tolerate higher temperatures mainly compared to HDPE.

What is the difference between HDPE and LDPE?

HDPE and LDPE may have significantly different physical and functional characteristics. These can be categorized into following subgroups,

Polymer Resin Abbreviation

HDPE: HDPE is High density polyethylene

LDPE: LDPE is Low density polyethylene

Structure (Presence of branches)

HDPE: It has a linear structure. Therefore, it can be compressed, and it is less flexible and stronger (Figure 1)

LDPE: It has lots of branches. Therefore, it is hard to compress, and it is lightweight and flexible (Figure 1)

Crystalline and Amorphous Regions

HDPE: HDPE has high crystalline and low amorphous regions (more than 90% crystalline). It contains several side chains per 200 carbon atoms in the main carbon skeleton leading to long linear chains. As a result, closely packing and high crystallinity can be observed (Figure 1).

LDPE: LDPE has low crystalline and high amorphous regions (less than 50-60% crystalline). It contains less than 1 side chain per 2-4 carbon atoms in the main carbon skeleton leading to branching. As a result, irregular packing and low crystallinity can be observed (Figure 1).

Tensile Strength and Intermolecular Forces

HDPE: HDPE has stronger intermolecular forces and tensile strength than LDPE.  Tensile strength is 4550 psi.

LDPE: LDPE has weaker intermolecular forces and tensile strength than HDPE.

Melting Point

HDPE: 135°C (higher melting point compared to LDPE)

LDPE: 115°C (Lower melting point compared to HDPE)

Plastic resin codes

HDPE: HDPE is usually recycled, and resin identification code (also known as recycling symbol) is the number 2 (See figure 2).

LDPE: LDPE is usually recycled, and resin identification code (also known as recycling symbol) is the number 4 (See figure 2).

Density

HDPE: The density can range from 0.95- .97 g/cm3. The density is higher than that of LDPE.

LDPE: The density can range from 0.91-0.94 g/cm3. The density is lower than that of HDPE.

Specific Gravity

HDPE: Specific gravity is 0.95. The specific gravity is higher than that of LDPE.

LDPE: Specific gravity is 0.92. The specific gravity is lower than that of HDPE.

Chemical Properties

HDPE: HDPE is chemically inert, and resistant ultraviolet rays compare to LDPE.

LDPE: LDPE is less chemically inert and when exposure to light and oxygen results in loss of strength.

Transparency

HDPE: HDPE is less transparent or more opaque than LDPE.

LDPE: LDPE is more transparent or less opaque than HDPE.

Strength

HDPE: It is stronger and harder than LDPE.

LDPE: It is less strong and weaker than HDPE.

Flexibility

HDPE: It is more rigid than LDPE

LDPE: It is more flexible than HDPE

General applications

HDPE: Shampoo bottles, food storage containers, laundry and house cleaning bottles, shipping containers, milk, water, and juice jugs, detergent bottles, grocery bags, recycling bins, water pipes

LDPE: Bags for dry cleaning and newspapers, shrink wrap, films, squeezable bottles (honey/mustard), bread bags, garbage bags

In conclusion, HDPE and LDPE are different grades of polyethylene and the key difference between them is the alignment of the polymer molecules. As a result, they may have substantially different physical properties and different applications.