Studies on Metallocene Polyolefin and Polyvinyl Chloride for Blood ...

Chapter 1 24
demands higher energy requirements for processing and the low level of shear-thinning results
in processing difficulties, such as sharkskin in extrusion. These processing difficulties can be
overcome by the incorporation of short and long chain branching (185). In an effort to improve
on the processing performance of m-PE's, researchers at Dow Chemical Company (186) have
indicated that branching can be incorporated into m-PE's by forming copolymers containing an
alpha-olefin such as octene. This type of branching can be obtained by using a special type
constrained geometry metallocene catalyst. Dow Chemical Company has commercialized the
catalyst and the process technology under the tradename INSlTE. The long chain branging in
the Dew's metallocene PE (Affinity) increase the amount of shear thinning, increase melt
strength and reduce susceptibility to melt fracture and draw resonance (187) and therefore
improve their processabiliry. In constrained geometry catalyst one cyclopentadienyl ring has
been replaced by a heteroatom (often N) attached to a bridging atom as in the figure 1.7. M ""
transition metal. usually goup 4b (Zr, Ti. HO; A = optional bridging atom, generally Si or C
atom with R"'CH3; R = H, alkyl, or other hydrocarbon groups, which need not all be identical;
X=halogen atom (generally Cl) or alkyl group. The characteristic feature of the structure of the
constrained geometry catalyst is the short angle (less than l iS") between Cp. M and N atoms
instead of 120"_140" between the two Cp rings as in the case of ordinary metallocene catalyst
(188).
Figure l .7 StrueMe " f Cons traIned GeOme1ry Mlltalloeene CllUlyst
The single site constrained geometry catalysts have unusually high activities and thus can be
used in small quantities. They also have high activity at higher temperature than ordinary
metallocene. It is different from usual metallocene because of its ability to incorporate chain
branging into the polymer structure as a result ofgreater exposure of active metal site.
The polyolefins, especially those based on metallocene technology, are most likely to compete
with PVC in the long run because they meet all the criteria critical :0 PVC (189). The combinetioo
of a film density approximately 30% lower than that of PVC and improved properties
results in a thinner. lighter-weight product that meets performance needs while reducing the
volume of material required-making the product lighter to ship and use and creating a lower
volume of waste material for disposable devices. The higher yield can also allow mPO films to
be very cost-competitive: although PVC resins cost less per pound than mPO resins, the price
per unit area of film or per finished device can be comparable. Metallocene polyolefins also
have a lower melt processing temperature, much lower injection molding cycle times (up to 25%
less than PVC) and offer potential price savings of 25%-50% (190). Downgauging also reduces
disposal costs for hospitals by reducing the tonnage of waste. Finally, metalloccne polyolefms
can likely be produced using the same equipment as PVC.