for self-reinforced polymeric materials with a focus on processing technology has
been presented by Kmetty and Karger-Kocsis [180] and others [181, 182].
Commercial self-reinforced “all PP” composites, marketed under the trade
names Curv™, Pure™, and Armordon™, exploit lamination of extended chain
polyolefins such as PP fabrics or stretched PP tapes with different PP grades in
surface and core layers. In another approach, polyolefins are processed at
temperatures around the melting temperature. Under these processing conditions,
the polymer does not form highly entangled random coil polymer chains but
crystallizes to produce a chain-extended, virtually non-entangled conformation.
For instance, Ehrenstein and coworkers demonstrated that the elongational flow
during extrusion of HDPE melt at temperatures very close to the melting temperature produces in situ polyethylene shish–kebab-like HDPE fibers as molecular
reinforcement of the HDPE matrix [183]. The shish–kebab structure results from
oriented crystallization in elongational flow combined with epitaxial growth of
polyethylene crystals onto the extended chain polyethylene. Such self-reinforced
HDPE exhibits substantially higher stiffness and strength, paralleled by highly
reduced thermal shrinkage measured in the fiber direction. However, the narrow
processing window and the limited scope of solid state extrusion hampered commercial application of this technology. For industrial applications it is highly
desirable to enable self-reinforcement in conventional injection molding, extrusion,
and blow molding processes. Although it is well known that the presence of
UHMW polyethylene favors the formation of shish–kebab structures, most of the
model HDPE blends containing UHMWPE were prepared by solution blending
because in melt processing high amounts of UHMWPE caused a drastic increase in
melt viscosity [184, 185]. Indeed, reactor blends of polyethylene with bimodal
molar mass distribution, containing a few percent of UHMWPE, are produced on a
commercial scale using cascade reactors, multi-zone reactors, and multi-site catalyst technology. As “tie molecules,” slightly branched UHMWPE links together
polyethylene crystals, thus accounting for the melt strengthening of blow-molded
films and considerably improved fatigue resistance of HDPE pipes. Preferably, the
bimodal polyethylenes are produced in reactor cascades polymerizing ethylene in
the presence and absence of hydrogen. A massive improvement in mechanical
properties was achieved when an α-olefin comonomer was incorporated exclusively
into the UHMWPE fraction. The progress made in the development of bimodal
polyethylenes and reactor cascade technology was reviewed by Bo ¨hm [186]. Since
melt blending of UHMW polyethylene and low molecular weight polyethylene is
rather difficult, owing to the poor melting of UHMWPE, the in situ formation of
UHMWPE and HDPE reactor blends during polymerization is the synthetic method
of choice.
During the last decade, remarkable progress in catalytic olefin polymerization,
cascade and multistage- reactor technology has enabled precise control of molar
mass distributions and particle growth [187–189]. Moreover, progress in supported
single-site catalyst development has enabled unprecedented control of both molar
mass distribution and selective incorporation of 1-olefin comonomers in high
molecular weight fractions [190]. In addition to cascade and multi-staged reactor
Polyolefin Nanocomposites and Hybrid Catalysts
295
been presented by Kmetty and Karger-Kocsis [180] and others [181, 182].
Commercial self-reinforced “all PP” composites, marketed under the trade
names Curv™, Pure™, and Armordon™, exploit lamination of extended chain
polyolefins such as PP fabrics or stretched PP tapes with different PP grades in
surface and core layers. In another approach, polyolefins are processed at
temperatures around the melting temperature. Under these processing conditions,
the polymer does not form highly entangled random coil polymer chains but
crystallizes to produce a chain-extended, virtually non-entangled conformation.
For instance, Ehrenstein and coworkers demonstrated that the elongational flow
during extrusion of HDPE melt at temperatures very close to the melting temperature produces in situ polyethylene shish–kebab-like HDPE fibers as molecular
reinforcement of the HDPE matrix [183]. The shish–kebab structure results from
oriented crystallization in elongational flow combined with epitaxial growth of
polyethylene crystals onto the extended chain polyethylene. Such self-reinforced
HDPE exhibits substantially higher stiffness and strength, paralleled by highly
reduced thermal shrinkage measured in the fiber direction. However, the narrow
processing window and the limited scope of solid state extrusion hampered commercial application of this technology. For industrial applications it is highly
desirable to enable self-reinforcement in conventional injection molding, extrusion,
and blow molding processes. Although it is well known that the presence of
UHMW polyethylene favors the formation of shish–kebab structures, most of the
model HDPE blends containing UHMWPE were prepared by solution blending
because in melt processing high amounts of UHMWPE caused a drastic increase in
melt viscosity [184, 185]. Indeed, reactor blends of polyethylene with bimodal
molar mass distribution, containing a few percent of UHMWPE, are produced on a
commercial scale using cascade reactors, multi-zone reactors, and multi-site catalyst technology. As “tie molecules,” slightly branched UHMWPE links together
polyethylene crystals, thus accounting for the melt strengthening of blow-molded
films and considerably improved fatigue resistance of HDPE pipes. Preferably, the
bimodal polyethylenes are produced in reactor cascades polymerizing ethylene in
the presence and absence of hydrogen. A massive improvement in mechanical
properties was achieved when an α-olefin comonomer was incorporated exclusively
into the UHMWPE fraction. The progress made in the development of bimodal
polyethylenes and reactor cascade technology was reviewed by Bo ¨hm [186]. Since
melt blending of UHMW polyethylene and low molecular weight polyethylene is
rather difficult, owing to the poor melting of UHMWPE, the in situ formation of
UHMWPE and HDPE reactor blends during polymerization is the synthetic method
of choice.
During the last decade, remarkable progress in catalytic olefin polymerization,
cascade and multistage- reactor technology has enabled precise control of molar
mass distributions and particle growth [187–189]. Moreover, progress in supported
single-site catalyst development has enabled unprecedented control of both molar
mass distribution and selective incorporation of 1-olefin comonomers in high
molecular weight fractions [190]. In addition to cascade and multi-staged reactor
Polyolefin Nanocomposites and Hybrid Catalysts
295