Polypropylene for Thermoforming
“Although the polypropylene of commerce has many desirable and beneficial properties, it is deficient in melt strength. When molten, it exhibits no strain hardening (an increase in resistance to stretching during elongation of the molten material). Thus it has a variety of melt processing shortcomings, including the onset of edge weave during high speed extrusion coating of paper or other substrates, sheet sag and local thinning in melt thermoforming, and flow instabilities in coextrusion of laminate structures. As a result, its use has been limited in such potential applications as, for example, extrusion coating, blow molding, profile extrusion, and thermoforming.” (Morhard et al., US Patent 7,855,263, 12/21/2010)
Recent US Patents
12/21/2010
7,855,263
Process for producing thermoformed articles
Morhard et al of Basell, Germany, have developed a polypropylene sheet for thermoforming. “Without wishing to be bound by any particular theory it is believed that the high performance of the propylene resin used in the process of the present invention are due to the high number of long chain branching in the polymer chain. This structural feature is reflected by the correlation between eta* and the frequency .omega.. This high number of long chain branching in the polymer chain is due to the particular metallocene-based catalyst system used for the production of said polypropylene resin.” (RDC 6/22/2011)