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At the present time, economic considerations along with certain performance limitations are preventing metallocene polyolefins from usurping PVC's role in the disposable medical device industry.
Polyvinyl chloride (PVC) has been the plastic material of choice for disposable medical devices since the early 1940s, even though during the past 50 years a myriad of other plastics have also become available, many of which have supplemented PVC for use in medical applications. Because of its unique characteristics and its economics as a commodity polymer, no other plastic material has, until recently, seriously challenged PVCs dominance in these demanding products. However, during the past two to three years, a new class of polymers has emerged that is said to pose a threat to PVC's rank in the medical field. Known as metallocene-catalyzed polyolefins (mPOs), these new materials offer some interesting properties compared with their traditional polyolefin cousins. This article attempts to offer a balanced comparison of PVC and the new class of materials that hopes to challenge its position in medical applications.
PVC
Commercial PVC, which has a number average molecular weight (Mn) of from 30,000 to 80,000, occurs as white powder with a consistency ranging from that of sugar to that of flour. The neat polymer, without compounding, is of no value and will decompose rapidly when exposed to typical plastics processing temperatures. At the very least, PVC must be stabilized, usually with a metal salt or complex of Ca, Zn, Pb, Cd, Sn, or Ba to prevent catastrophic degradation.
In the medical device field, PVC is primarily used in flexible form, a condition achieved by compounding it with a liquid plasticizer typically of the phthalate ester family. The plasticizer used at levels of 5% to 50%, acts as a solubilizer for the PVC resin and as a medium for the polymer chain motion that gives PVC its flexibility. Other compounding ingredients may include fillers, colorants, flame retardants, and UV absorbers.
PVC can be processed in liquid formsplastisols (pastes), emulsions, or solutionsor in the dry form of powder blends or pellets. Most processing methods used in the plastics industry are adaptable to PVC. These include extrusion, injection molding, calendering, rotational molding, dipping, casting, blow molding, spraying, spinning, and slush molding.
What Are Metallocene Catalyzed Polyolefins?
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