Linear low-density polyethylene is a substantially linear polymer, 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 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 a 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 strength 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. 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". 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
Application LLDPE has penetrated almost all
traditional markets for polyethylene; it is used for plastic bags and sheets,
plastic wrap, stretch wrap, pouches, toys, covers, lids, pipes, buckets and
containers, covering of cables, geomembranes, and mainly flexible
tubing. In 2013, the world market for LLDPE reached a volume of $40 billion.
LLDPE is manufactured by using metallocene catalysts labeled mLLDPE.
Physical properties See also
Cross-linked polyethylene Low-density polyethylene
High-density polyethylene Medium-density polyethylene
Ultra-high-molecular-weight polyethylene Stretch wrap
Plastic recycling References
Modern Plastic Mid-October Encyclopedia Issue, page 56 and 61
External links [1] Example of LLDPE Physical Properties