Linear low density polyethylene
Encyclopedia
Linear low-density polyethylene (LLDPE) is a substantially linear polymer
(polyethylene
), with significant numbers of short branches, commonly made by copolymerization of ethylene
with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene (LDPE) 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
, or octene
. 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.
and higher impact and puncture resistance
than does 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. It has good electrical properties. However, it is not as easy to process as LDPE, has lower gloss, and narrower range for heat sealing
.
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". It is not taken in most curbside pickups in communities. LLDPE can be recycled
though into other things like trash can liners, lumber, landscaping ties, floor tiles, compost bins, and shipping envelopes.
, stretch wrap
, pouches, toys, covers, lids, pipes, buckets and containers, covering of cables, geomembranes
, and mainly flexible tubing. In 2009, the world market for LLDPE reached a volume of almost $24 billion (~ €17 billion).
LLDPE is manufactured by using metallocene
catalysts labeled mLLDPE.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
(polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
), with significant numbers of short branches, commonly made by copolymerization of ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene (LDPE) 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 C6H12. 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. There are several isomers of hexene, depending on the position of the...
, or octene
Octene
Octene is an alkene with the formula . Several isomers of octene are known, 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 either in solution phase or in gas phase reactors. Usually, octene is the comonomer in solution phase while butene and hexene are copolymerized with ethylene in a gas phase reactor. LLDPE has higher tensile strengthTensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
and higher impact and puncture resistance
Puncture resistance
Puncture resistance denotes the relative ability of a material to inhibit the progression of a tear once it has been pierced by a cut or a nick. Tests devised to measure puncture resistance are generally application-specific, covering items such as roofing and packaging materials, protective...
than does 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. 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 products, packaging, and other thermoplastic materials using heat. This can be with uniform thermoplastic monolayers or with materials having several layers, at least one being thermoplastic...
.
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 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 viscosityViscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher 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". It is not taken in most curbside pickups in communities. LLDPE can be recycled
Recycling
Recycling is processing used materials into new products to prevent waste of potentially useful materials, reduce the consumption of fresh raw materials, reduce energy usage, reduce air pollution and water pollution by reducing the need for "conventional" waste disposal, and lower greenhouse...
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 allows using lower thickness than comparable LDPE), plastic wrapPlastic wrap
Plastic wrap, cling film , cling wrap or food wrap, is a thin plastic film typically used for sealing food items in containers to keep them fresh over a longer period of time...
, 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, covers, lids, pipes, buckets and containers, covering of cables, geomembranes
Geomembranes
Geomembranes are a kind of geosynthetic material made up of an impermeable membranes. Their use includes lining canals, ponds and also waste containment.-Composition:Geomembranes are made of various materials...
, and mainly flexible tubing. In 2009, the world market for LLDPE reached a volume of almost $24 billion (~ €17 billion).
LLDPE is manufactured by using metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
catalysts 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 standard released by Underwriters Laboratories of the USA.The standard classifies plastics according to how they burn in various orientations and thicknesses... |
HB |
| Volume resistivity |  Ω·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 |
See also
- Cross-linked polyethylene (PEX)
- Low-density polyethylene (LDPE)
- High-density polyethylene (HDPE)
- Medium-density polyethylene (MDPE)
- Ultra-high-molecular-weight polyethylene (UHMWPE)
- Stretch wrapStretch wrapStretch wrap or stretch film is a highly stretchable plastic film that is wrapped around items. The elastic recovery keeps the items tightly bound...
- Plastic recyclingPlastic recyclingPlastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down soft drink bottles and then casting them as plastic chairs and tables...
External links
- http://plastics.sabic.eu/_scripts/gradeselector.pl?material=f8c66b8e-ff72-4a0e-8cf2-678b04769dc0&template=material Example of LLDPE Physical Properties