Improved mechanical and tribological properties of Boehmite-filled polymers
The fierce competition of the plastics market demands superior materials that exhibit ideal mechanical features yet are easy and cost-effective to manufacture. An important objective of our laboratory's research is to improve the tribological properties of polymer-based materials (PBMs). Although commonly used, low-density polyethylene (LDPE) lacks certain mechanical and tribological properties. Solid fillers can be used to improve some polymer properties,1 e.g., to enhance flame retardancy and corrosion resistance. Despite these advantages, fillers increase melt viscosity of polymers and thus impair manufacture.2 The ideal viscosity for molding such PBMs requires increased processing temperatures, which leads to increased production cost. Using Boehmite (aluminum oxyhydroxide) as a filler for LDPE, we have achieved higher tensile modulus, lower friction, and—importantly—lower melt viscosity.
Filler effectiveness is limited by its adhesion to the surrounding polymer matrix,3 and these interfaces can be decisive in property outcome.4 This is particularly true in ceramic oxide composites, where the hydrophilic fillers are unable to interact strongly to hydrophobic polymers. To improve matrix-filler adhesion, we grafted Boehmite particles with silane-coupling agents (SCAs)3 containing at least two reactive groups, for example, methacryl or vinyl. The incorporated SCAs increased Boehmite surface hydrophobicity.3
We investigated the influence of the ceramic filler on polymer tensile strength (see Figure 1).5 Addition of 20% by weight of Boehmite resulted in more than doubling the tensile modulus. When Boehmite was pre-heated (550°C, 3h) to eliminate moisture before blending with LDPE, a slightly higher modulus was achieved. Brittleness is related to impact strength6 and visco-elastic recovery in scratch testing.7 It is also inversely proportional to tensile elongation at break. We found that the latter increased with silane treatment and decreased with increased filler loading,5 indicating that SCAs probably have a plasticizing effect. Furthermore, Boehmite inclusion results in lower visco-elastic recovery from scratching: the macromolecular chains in neat polymer recovered better.8 However, presence of Boehmite significantly reduced both static and dynamic friction (see Figure 2).
We examined the effects on polymer rheology and observed several trends across all test samples.9 All specimens exhibited shear thinning, i.e., decreased viscosity with increased shear rates (see Figure 3). Inclusion of Boehmite filler (either SCA-modified or not) translated entire curves to lower viscosities with minimal changes to curve shape. Fillers often increase polymer melt viscosity. However, this material—with modified matrix-filler interactions—opposes this trend. Micrograph images have shown that addition of SCAs improved matrix-filler adhesion.9 Additionally, adhesion was also affected by coupling-reaction temperature, which in turn affected viscosity.
In summary, we have demonstrated that modified ceramic oxide fillers improve the mechanical and tribological properties of LDPE, lowering viscosity at melt and thus improving processing. These concepts can be extended to other polymer and filler types. For example, we have obtained similar results for polymer blends with thermal-shock-resistant ceramic powder. In this case, brittleness decreased upon surface modification of the ceramic—as seen with Boehmite—which is a manifestation of increased filler cooperation with the polymeric matrix. We are currently developing and analyzing new PBMs with metal micro- and nanopowders, ceramics, and carbon nanotubes as fillers (depending on the application).
- Performance of Plastics, Hanser-Gardner Publ., 2000.
- A. Shenoy, Rheology of Filled Polymer Systems, Springer, 1999.
- W. Brostow and T. Datashvili, Chemical mofication and characterization of Boehmite particles, Chem. Chem. Technol. 2, pp. 27-32, 2008.
- A. Kopczynska and G. W. Ehrenstein, Polymeric surfaces and their true surface tension in solids and melts, J. Mater. Ed. 29, pp. 325-340, 2007.
- W. Brostow, T. Datashvili, B. Huang and J. Too, Tensile properties of LDPE+Boehmite composites, Polym. Compos. 30, pp. 760-767, 2009.
- W. Brostow and H. E. Hagg Lobland, Brittleness of materials: implications for composites and a relation to impact strength, J. Mater. Sci. 45, pp. 242-250, 2010.
- W. Brostow, H. E. Hagg Lobland and M. Narkis, Sliding wear, viscoelasticity, and brittleness of polymers, J. Mater. Res. 21, pp. 2422-2428, 2006.
- W. Brostow, T. Datashvili, D. Kao and J. Too, Tribological properties of LDPE+Boehmite composites, Polym. Compos. 31, pp. 417-425, 2010.
- P. Blaszczak, W. Brostow, T. Datashvili and H. E. Hagg Lobland, Rheology of low-density polyethylene+Boehmite
composites, Polym. Compos. 31, pp. 1909-1913, 2010.