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Linear low-density polyethylene (LLDPE) is a substantially linear polymer (polyethylene
Polyethylene

Polyethylene or polythene is a thermoplastic commodity heavily used in consumer products . Over 60 million tons of the material are produced worldwide every year....
), with significant numbers of short branches, commonly made by copolymerization of ethylene
Ethylene

Ethylene is the chemical compound with the formula C2H4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin....
 with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene because of the absence of long chain branching. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE.






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Linear low-density polyethylene (LLDPE) is a substantially linear polymer (polyethylene
Polyethylene

Polyethylene or polythene is a thermoplastic commodity heavily used in consumer products . Over 60 million tons of the material are produced worldwide every year....
), with significant numbers of short branches, commonly made by copolymerization of ethylene
Ethylene

Ethylene is the chemical compound with the formula C2H4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin....
 with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene because of the absence of long chain branching. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE. In general, LLDPE is produced at lower temperatures and pressures by copolymerization of ethylene and such higher alpha olefins as butene, hexene
Hexene

Hexene is a higher olefin, or alkene with a formula Carbon6Hydrogen12. The "Hex" is derived from the fact that there are 6 carbon atoms in the molecule, while the "ene" suffix denotes that two carbon atoms are connected via a double bond....
, or octene
Octene

Octene is an alkene, or higher olefin, with the formula carbon8hydrogen16. There are several isomers of octene, depending on the position of the double bond and the branching of the carbon chain....
. 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.

Production and properties


The production of LLDPE is initiated by transition metal catalysts, particularly Ziegler or Philips type of catalyst. The actual polymerization process can be done in either solution phase or gas phase reactors. Usually, octene is the copolymer in solution phase while butene and hexene are copolymerized with ethylene in a gas phase reactor. The LLDPE resin produced in a gas phase reactor is in granular form and may be sold as granules or processed into pellets. LLDPE has higher tensile strength
Tensile strength

Tensile strength , or is the Stress at which a material breaks or permanently deforms. Tensile strength is an Intensive and extensive properties and, consequently, does not depend on the size of the test specimen....
 and higher impact and puncture resistance than LDPE. It is very flexible and elongates under stress. It can be used to make thinner films, with better environmental stress cracking resistance. It has good resistance to chemicals and to ultraviolet
Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 400 nanometer to 10 nm, and energies from 3 Electron volt to 124 eV....
 radiation. It has good electrical properties. However it is not as easy to process as LDPE, has lower gloss, and narrower range for heat sealing
Heat sealer

A heat sealer is a machine used to seal product s, packaging, and other thermoplastic materials using heat....
.

Processing


LDPE and LLDPE have unique theoretical 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, 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 phenomena 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

Application


LLDPE has penetrated almost all traditional markets for polyethylene, it is used for plastic bag
Plastic bag

A plastic bag or pouch is a type of flexible packaging made of thin, flexible, plastic film. Plastic bags are used for containing and transporting foods, produce, powders, ice, chemicals, waste, etc....
s and sheets (where it allows using lower thickness than comparable LDPE), plastic wrap
Plastic wrap

Plastic wrap is a thin plastic film typically used for sealing food items in containers to keep them fresh. Plastic wrap, typically sold on rolls in boxes with a cutting edge, clings to many smooth surfaces and can thus remain tight over the opening of a container with no adhesive or other devices....
, stretch wrap
Stretch wrap

Stretch wrap or stretch film is a highly stretchable plastic film that is wrapped around items. The elastic recovery keeps the items tightly bound....
, pouches, toys, lids, pipes, buckets and containers, covering of cables, geomembranes, and mainly flexible tubing.

LLDPE manufactured using metallocene
Metallocene

A metallocene is a compound with the general formula 2M consisting of two cyclopentadiene anions bound to a metal center in the oxidation state II....
 catalysts is labeled mLLDPE.

Physical Properties


Property Value
Density 0.92 g/cm³
Surface hardness SD48
Tensile strength 20 MPa
Flexural modulus 0.35 GPa
Notched izod 1.06+ kJ/m
Linear expansion 20×10−5/°C
Elongation at break 500%
Strain at yield 20%
Max. operating temp. 50 °C
Water absorption 0.01%
Oxygen index 17%
Flammability UL94
UL94

UL 94 is a plastics flammability standardization released by Underwriters Laboratories of the USA.The standard classifies plastics according to how they burn in various orientations and thicknesses....
 		HB
Volume resistivity O·cm
Dielectric strength 25 MV/m
Dissipation factor 1 kHz 909090
Dielectric constant 1 kHz 2.3
HDT @ 0.45 MPa 45 °C
HDT @ 1.80 MPa 37 °C
Material drying NA
Melting Temp. Range 120 to 160 °C
Mould Shrinkage 3%
Mould temp. range 20 to 60 °C


External links

  • Example of LLDPE Physical Properties


See also


  • Low-density polyethylene
  • High-density polyethylene
  • Ultra-high-molecular-weight polyethylene
  • Stretch wrap
    Stretch wrap

    Stretch wrap or stretch film is a highly stretchable plastic film that is wrapped around items. The elastic recovery keeps the items tightly bound....