to further improve the balance of stiffness, strength, toughness, dimensional stability, scratch resistance, flame retardancy, electrical and thermal conductivity, as well
as processing. Traditionally, this goal is achieved by compounding polyolefins
with fillers and fibers [5]. In alternative strategies, both in situ nanostructure and
hybrid formation are exploited, aiming at producing nanostructured polyolefin
materials that can be processed in conventional injection molding, blow molding,
extrusion, and spinning. Moreover, it is an important objective to combine effective
matrix reinforcement with other functional properties such as electrical and thermal
conductivity, scratch resistance, haptics, and barrier resistance against permeation
of gases and liquids.
During the past decade, progress in nanotechnology has enabled the production
of a great variety of nanoparticles. However, owing to their high surface area and
strong interparticle interactions, many nanoparticles are rather difficult to disperse
in highly viscous polymer melts. Therefore, it is beneficial to already disperse
nanoparticles during catalytic olefin polymerization in gas phase or in low-viscosity
polymerization media. This in situ composite formation eliminates special safety
and handling procedures required for melt compounding of nanoparticles, owing to
the potential health hazards associated with inhalation. Moreover, progress in
heterogenous catalysis offers new opportunities for creating “all polyolefin”
skeleton-like superstructures and multilayer composites without requiring the addition of alien materials. This 100% hydrocarbon nature of self-reinforced polyolefins
is highly advantageous in view of combining effective recycling with the high
resource and energy effectiveness typical for catalytic polymerization. The focus of
this overview is on exploiting advanced polymerization catalysis in (nano)composite formation by means of in situ olefin polymerization on nanoparticle-supported
catalysts and by reactor blend technology using multi-site catalysts.
2 Polyolefin Nanocomposites and Catalysis
One of the characteristic features of polymer composite materials is the formation
of skeleton-like superstructures combining the rather flexible polyolefin matrix with
highly rigid dispersed or co-continuous reinforcing phases. Typically, reinforcement is achieved by adding fibers and fillers such as talcum and calcium carbonate
[6–11]. Compared with micron-sized fillers, nanometer-sized fillers with average
diameter well below 100 nm can significantly change the polymer properties at
much lower filler content. In fact, 10
9 nanoparticles are needed to substitute the
same volume fraction equivalent to one micron-sized particle. As illustrated in
Fig. 1, most of the polymer is allocated at the nanoparticle interface, whereas just a
few percent of polymer covers the micron-sized filler surface. Converting bulk
polyolefin into interfacial polyolefin can improve properties such as glass transition
temperature, stiffness, strength, scratch resistance, and surface gloss [12].
Owing to their specific high surface area and interparticle interactions,
nanofillers assemble and percolate at much lower filler content than microfillers.
Polyolefin Nanocomposites and Hybrid Catalysts
281
as processing. Traditionally, this goal is achieved by compounding polyolefins
with fillers and fibers [5]. In alternative strategies, both in situ nanostructure and
hybrid formation are exploited, aiming at producing nanostructured polyolefin
materials that can be processed in conventional injection molding, blow molding,
extrusion, and spinning. Moreover, it is an important objective to combine effective
matrix reinforcement with other functional properties such as electrical and thermal
conductivity, scratch resistance, haptics, and barrier resistance against permeation
of gases and liquids.
During the past decade, progress in nanotechnology has enabled the production
of a great variety of nanoparticles. However, owing to their high surface area and
strong interparticle interactions, many nanoparticles are rather difficult to disperse
in highly viscous polymer melts. Therefore, it is beneficial to already disperse
nanoparticles during catalytic olefin polymerization in gas phase or in low-viscosity
polymerization media. This in situ composite formation eliminates special safety
and handling procedures required for melt compounding of nanoparticles, owing to
the potential health hazards associated with inhalation. Moreover, progress in
heterogenous catalysis offers new opportunities for creating “all polyolefin”
skeleton-like superstructures and multilayer composites without requiring the addition of alien materials. This 100% hydrocarbon nature of self-reinforced polyolefins
is highly advantageous in view of combining effective recycling with the high
resource and energy effectiveness typical for catalytic polymerization. The focus of
this overview is on exploiting advanced polymerization catalysis in (nano)composite formation by means of in situ olefin polymerization on nanoparticle-supported
catalysts and by reactor blend technology using multi-site catalysts.
2 Polyolefin Nanocomposites and Catalysis
One of the characteristic features of polymer composite materials is the formation
of skeleton-like superstructures combining the rather flexible polyolefin matrix with
highly rigid dispersed or co-continuous reinforcing phases. Typically, reinforcement is achieved by adding fibers and fillers such as talcum and calcium carbonate
[6–11]. Compared with micron-sized fillers, nanometer-sized fillers with average
diameter well below 100 nm can significantly change the polymer properties at
much lower filler content. In fact, 10
9 nanoparticles are needed to substitute the
same volume fraction equivalent to one micron-sized particle. As illustrated in
Fig. 1, most of the polymer is allocated at the nanoparticle interface, whereas just a
few percent of polymer covers the micron-sized filler surface. Converting bulk
polyolefin into interfacial polyolefin can improve properties such as glass transition
temperature, stiffness, strength, scratch resistance, and surface gloss [12].
Owing to their specific high surface area and interparticle interactions,
nanofillers assemble and percolate at much lower filler content than microfillers.
Polyolefin Nanocomposites and Hybrid Catalysts
281