Paul Andersen, May 2020
Ever since the first polymer applications were incorporated into the automobile in the 1960’s, OEM requirements for polyolefin based automotive compounds have pushed the performance envelope with respect to, for example, improved mechanical properties such as flex modulus, tensile strength, and heat distortion temperature; aesthetic properties such as surface quality; processing characteristics such as viscosity; and as always, cost. However, density was not a critical concern since the part being replaced was most probably made of metal.
To attain required physical, esthetic and viscosity properties such as those listed above, compound formulations have become very complex. The main additives to the base polymer in early automotive applications such as a battery tray, were typically glass fiber and/or mineral filler for reinforcement. However, as manufacturers have continued to push vehicle weight reduction, they are re-evaluating specifications for current polymer-based applications/parts, i.e. bumpers, trim, etc., for future model years. In most instances, all the specified mechanical and flow properties remain the same, but density is reduced between 5 and 10%. Generally, this requires an extensive material reformulation to meet the new specifications.
As part of most light-weighting reformulations, high bulk density filler content is decreased and replaced with multiple grades of polypropylene having a wide range of viscosities. These resins need to be melted and uniformly blended to provide, for example, strength from a high MW, high crystallinity component and good flow characteristics from a low MW grade. Additionally, any IM (impact modifier) needs to be dispersed and uniformly distributed. For reinforcement to be effective, fibers need to be unbundled as well as maintain a critical length during the compounding process. Minerals, depending on their structure, need to be distributed and/or distributed and dispersed.
The co-rotating twin-screw compounder has long been the equipment of choice for such compounding functions. However, compounders still face processing challenges such as how to optimize the extruder set up to uniformly compound 1) diverse viscosity matrix polymers, 2) incorporate and disperse impact modifier, 3) unbundle and distribute fibers, and/or 4) feed, distribute and disperse a poor flowing, “sticky” mineral filler or possibly an easy to fluidize low bulk density talc while simultaneously maintaining an economically viable production rate. Additionally, the process can be challenged to maximize fiber length in high viscosity mineral filled formulations.
This paper will review requirements for compounding automotive polyolefin compounds with an emphasis on recent innovations in Co-rotating Twin-screw technology that have enhanced product quality and productivity for these complex lightweighting material formulations.
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