thumb|Linear low-density polyethylene (LLDPE) granules

Linear low-density polyethylene (LLDPE) is one of several varieties of polyethylene (PE).

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|+ Polyethylene (PE) densities and structure

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! PE type !! density (g/cm<sup>3</sup>)!!structure

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| LDPE || ca. 0.924|| highly branched

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| LLDPE || 0.916-0.940 The copolymerization process produces an LLDPE polymer that has a narrower molecular weight distribution than conventional LDPE and in combination with the linear structure, significantly different rheological properties.

Processing

LDPE and LLDPE have unique rheological or melt flow properties. LLDPE is less shear sensitive because of its narrower molecular weight distribution and shorter chain branching. During a shearing process, such as extrusion, LLDPE remains more viscous and, therefore, harder to process than an LDPE of equivalent melt index. The lower shear sensitivity of LLDPE allows for a faster stress relaxation of the polymer chains during extrusion, and, therefore, the physical properties are susceptible to changes in blow-up ratios. In melt extension, LLDPE has lower viscosity at all strain rates. This means it will not strain harden the way LDPE does when elongated. As the deformation rate of the polyethylene increases, LDPE demonstrates a dramatic rise in viscosity because of chain entanglement. This phenomenon is not observed with LLDPE because of the lack of long-chain branching in LLDPE allows the chains to slide by one another upon elongation without becoming entangled. This characteristic is important for film applications because LLDPE films can be downgauged easily while maintaining high strength and toughness. The rheological properties of LLDPE are summarized as "stiff in shear" and "soft in extension". LLDPE can be recycled, though into other things like trash can liners, lumber, landscaping ties, floor tiles, compost bins, and shipping envelopes.

Application

LLDPE has penetrated almost all traditional markets for polyethylene; it is used for plastic bags and sheets (where it can be thinner than LDPE of comparable strength), plastic wrap, stretch wrap, pouches, toys, covers, lids, pipes, buckets and containers, covering of cables, geomembranes, and mainly flexible tubing. It is also common to use blends of LDPE and LLDPE for optimal material properties in specific applications.

In 2013, the world market for LLDPE reached a volume of $40 billion.

See also

  • Cross-linked polyethylene (XLPE/PEX)
  • High-density polyethylene (HDPE)
  • Low-density polyethylene (LDPE)
  • Medium-density polyethylene (MDPE)
  • Plastic recycling
  • Stretch wrap
  • Ultra-high-molecular-weight polyethylene (UHMWPE)

References

  • Modern Plastic Mid-October Encyclopedia Issue, page 56 and 61
  • [http://plastics.sabic.eu/_scripts/gradeselector.pl?material=f8c66b8e-ff72-4a0e-8cf2-678b04769dc0&template=material] Example of LLDPE Physical Properties