Entangling additives enhance polypropylene foam quality

Linear polypropylene (PP)—composed of long polymer chains devoid of branches or crosslinked structure—and blends of PP with olefinic elastomers are widely used in the automotive and packaging industries. What makes foams produced with these materials of growing interest is their potential to replace the prevalent thermoset polyurethane foams which are not recyclable. However, the production of closed-cell foams with linear PP is difficult because the melt does not exhibit strain hardening under extensional flow.1, 2 This is problematic as it leads to thinner cell walls during bubble expansion in the foaming process, coalescence of bubbles, and the formation of open-cell foams.3, 4 Previous attempts to improve foam quality by adding 5wt% organoclay in amorphous polystyrene have reportedly achieved a 50% reduction in the average cell size5. However, the addition of 2 to 3 wt% organoclay to linear PP—with some maleic anhydride grafted PP as a dispersion aid—has only produced a slight improvement in the cellular structure of the foam samples.6, 7 The PP-clay nanocomposites (NCs) in these previous studies did not display strain hardening in extensional flows. Strain hardening is a requirement for producing closed-cell PP foams with a fully formed cell structure.1, 2 We, therefore, focused on identifying a suitable combination of additives to produce strain-hardening properties in linear PP. We prepared several PP-clay NCs from linear PP including different grades of maleic anhydride grafted PP—also known as compatibilizer—with varying molecular weights and maleic anhydride content.8, 9 We then tested them for strain hardening in extensional flow—defined as the lift of the extensional viscosity at finite strain rates from the base envelope obtained at very low strain rate. Measurements of uniaxial extensional viscosity transients, at 180C, are presented in Figure 1 for linear PP and five NCs. We observed that the linear PP and the NC melts labeled N2 and N1 did not show any lift while the NC melts labeled S1, S2, and S6 exhibited strain-hardening properties. The weight average molecular weights of the compatibilizers in N2 and N1 were 22,000 and 180,000 respectively; the ratio of maleic anhydride to organoclay was 0.8 g-mol/kg in N2 and only 0.15 g-mol/kg in N1. The Figure 1. Uniaxial extensional viscosity transients ( E ) at several strain rates for (a) linear PP and nanocomposite (NC) melts labeled N2, N1 and (b) melts labeled S1, S2, S6.