Fina Oil Co. and Exxon Chemical Co. are taking different approaches to using new metallocene-catalyzed isotactic polypropylene resins in biaxially oriented PP films for food packaging and other applications.
In separate papers to be presented next month at SPO '97, the specialty polyolefins business forum in Houston, Fina champions metallocene PP resins with narrow molecular-weight distribution, while Exxon backs those with broad molecular-weight distribution.
Exxon of Houston claims its broad molecular-weight approach allows for a wider range of processing temperatures than standard isotactic PP resins, creating the possibility of higher line speeds and reduced energy input.
The low level of extractables in Exxon's isotactic metallocene PP resins will produce stronger BOPP films that will not influence the flavor or content of food, staff engineer Charlie Lin said in an Aug. 19 telephone interview from Exxon's Baytown Polymers Center in Baytown, Texas.
Lin added that resins with broad molecular-weight distribution are more shear-sensitive and, as a result, require less energy to drive the polymer through the extruder.
Demands on processing speeds have played a key role in metallocene PP development, since heavy investments in BOPP machinery have led plastics processors to want to recoup their investments much earlier than they would have several years ago, Lin said.
``We're still trying to find some synergy between the narrow and the broad [isotactic metallocene PP resins],'' he said. ``But for now we think broad is better in processing.''
Exxon has not set a date for making the new resins commercially available.
Houston-based Fina claims its BOPP films made from metallocene PP are stiffer and have better moisture barriers than films made from current commercial isotactic PPs.
Fina said its resins with narrow molecular-weight distribution performed with reasonable success compared with nonmetallocene commercial isotactic PP.
Fina attributes these results to a lower correlation of yield stress with temperature, a wider melting range and a higher molecular weight compared to isotactic PP.
Chemical Market Resources, a Houston-based industry analyst, estimates the metallocene PP market at about 6 million pounds annually, mainly for film and fiber applications. The market currently is limited to Exxon, Fina and a European joint venture between German resin makers Hoechst AG and BASF AG, according to CMR.