Different densities of polyethylene

High density polyethylene (HDPE)

High density polyethylene is mainly a linear polymer and typically contains no long chain branching (LCB), or very few if any.

Due to the low level of branching, there is little to hinder the crystallization.

Crystallinity levels are typically in excess of 60%.

Densities range from approximately 0.94 to 0.97 g/cm3.

HDPE is the stiffest of all the polyethylene types.

Small amounts of a comonomer, such as 1-hexene, can be incorporated to reduce the crystallinity level. This improves toughness at the expense of stiffness.

Low density polyethylene (HDPE)

Low density polyethylene is a branched polymers and have numerous short and long chain branches (LCB).

Distribution of chain lengths is relatively broad.

High levels of branching disrupt crystallization.

Crystallinity levels range from approximately 30 to 55%.

Densities range from approximately 0.91 to 0.94 g/cm3.

Broad molecular weight distribution and LCB contributes to high melt strength and good sheer thinning properties.

LDPE is frequently blended into other types of polyethylene to improve their processability.

Polar, non-olefinic monomers produce EVA(ethylene-vinyl-acetate), EAA(ethylene-acrylic acid) copolymers.

Linear low density polyethylene (LLDPE)

LLDPE is produced in a low pressure process

  • Ziegler Natta catalyst are used to make narrow MWD grades (film, IM and rotomoulding) on gas phase reactors and in solution processes, and bimodal grades (film).
  • Gas phase processes are limited to C4= and C8= comonomers; solution processes can also accommodate C8=.

LLDPE is a linear polyethylene and typically contains no LCB. SCB is minimal, and is produced by comonomer insertion - the degree of SCB determines density.

Although more difficult to process than LDPE, LLDPE is tougher and generally has better sealing properties. Its stiffness balance is governed largely by comonomer content.

For a given MI/density, higher mechanical and sealing performance is achieved for C8>C6>C4.

Linear low density polyethylene (LLDPE)

Linear low density polyethylene molecules contain numerous short alkyl branches. Typically 1 to 4 branches per 100 backbone carbon atoms.

Branches consist of:

                    Ethyl : ethylene-butene copolymer

                    Butyl : ethylene-hexene copolymer

                    Hexyl: ethylene-octene copolymer

Densities range from 0.91 to 0.94 g/cm3 depending on the comonomer content.

Similar crystallinity levels to LDPE, i.e. approximately 30 to 55%.

Ziegler-Natta (ZN) LLDPEs have moderately broad molecular weight distributions with a higher concentration of alkyl branches in the shorter chains.

Metallocene LLDPEs have narrower molecular distributions with an uniform distribution of alkyl branches.

Very low density polyethylene (VLDPE)

Very low density polyethylene molecules contain very numerous alkyl branches. Typically 4 to 7 branches per 100 backbone carbon atoms.

Densities range from 0.86 to 0.91 g/cm3 depending on the comonomer content.

Crystallinity levels range from approximately 5% to 30 %.

Crystallites are small and poorly organized.

All VLDPE resins are polymerized using metallocene catalysts.

m-VLDPE are characterized by their homogeneous composition provided by a "single site" catalyst which give relatively narrow molecular weight and composition distributions. This results in much improved sealing and mechanical performance in film applications cf. conventional ZN resins.