Multilayer film applications such as packaging and diapers are just two areas that could benefit from spot welding (instead of gluing) polyethylene to polypropylene. Normally these two resins show poor adhesion to each other. But two years of research at the University of Minnesota in Minneapolis and at ExxonMobil Chemical in Houston show that metallocene-catalyzed polyolefins can weld to each other with bond strengths much greater than are possible with conventional Ziegler-Natta catalyzed polyolefins.
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According to Prof. Frank S. Bates, lead researcher and head of UM's Dept. of Chemical Engineering and Materials Science, the ability to weld these two plastics together will reduce production costs and create stronger, more durable goods by eliminating the use of glue. A solid sheet created by welding these metallocene polyolefins is stronger than the glued laminates of plastic films used in current diaper constructions. The same would hold true for other plastic products that are glued together, such as bumpers or car seats.
More crystallinity helps
The accompanying graph shows results of peel-force experiments on laminates of different PE and isotactic PP (iPP) resins. Each data point represents the average of at least four peel tests with an Instron tensile tester. Sheets 1 mm thick were pressed together at 375 F and 160 psi for 10 min and then cooled. Laminates were then peeled apart at 5 mm/min.
The joint between mPE and miPP was so strong that every attempt to peel them apart resulted in tearing of the PE before a crack could develop at the weld line. In contrast, a crack was propagated along the zPE/ziPP interface and ultimately led to complete film separation. Adhesion was insufficient to support a sharp 90° angle at the crack tip. An intermediate result was achieved with a series of mixed laminates formed from mPE and ziPP. Here, a 90° angle of peel was sustained as the crack progressed along the weld.
The researchers explain the remarkable weld strength displayed by the metallocene polyolefins as the result of interfacial polymer entanglements that first form in the molten stage. Cooling and subsequent crystallization then lead to the incorporation of most of the metallocene polymer chains emanating from the interfacial region into chain-folded lamellae. These stitch together the mPE and miPP films with "anchored entanglements."
This entanglement mechanism does not occur in conventional Z-N polyolefins. The researchers link this to amorphous material that accumulates at the zPE/ziPP interface. This amorphous material eliminates entanglement coupling by separating and disengaging the PE and iPP crystalline domains. Composed of rubbery material and lacking anchored entanglements, the resulting weld cannot support a load.
The researchers note that zPE contains 5.5% non-crystallizable material, while mPE contains less than 0.5%. Likewise, ziPP has 0.5% non-crystallizable material, while miPP has less than 0.2%.
Implications for blending
Earlier work by Prof. Bates' team and by ExxonMobil indicates that greater interfacial adhesion between mPE and ziPP also occurs during melt blending. Prof. Bates suggests that for applications such as car bumpers, modifying ziPP with mPE, instead of a rubber like EPR, would increase toughness while retaining higher stiffness.