Investigating the effect of material and printing parameters on the mechanical properties of neat and modified LDH/ABS nanocomposites for FDM 3D printing

Abstract

The aim of this research was to investigate whether layered double hydroxide (LDH)-based ABS composites would improve the mechanical properties and decrease the anisotropic nature of neat FDM 3D printed ABS parts. Rubbery styrene-ethylene-butylene-styrene (SEBS) polymer and stearic acid were chosen as additives/reinforcements due to having previously shown to improve common problems associated with ABS as a 3D printing material, such as brittleness, layer adhesion, warpage and shrinkage of FDM 3D printed parts. The intercalation of SEBS grafted with male ̈ıc anhydride (MA) as well as stearic acid (SA) into both CaAl and MgAl LDH was attempted before the modified LDHs were added to the ABS at various loadings. SEBS-g-MA only partially exfoliated the LDH layers with the lower microcompounding temperature showing a higher degree of exfoli- ation. MA was not sufficient as compatibiliser and organic modification of the LDHs is recommended before compounding. Stearic acid intercalated into both LDHs successfully in a bilayer formation, acting as a binding agent which increased the average particle size ten fold compared to the neat LDHs. A combination of DMA and TEM was used to evaluate the LDH-based ABS composites. No exfoliation of the clay layers was observed for any of the composites. DMA data showed poor repeatability throughout due to the weak distribution and dispersion of the LDH particles and presence of agglomerates in the compression moulded composite sam- ples. Trend lines in the averaged DMA data resulted in weak linear fits in most cases for the storage modulus, loss modulus and tanδ and therefore the ideal loading of LDHs in SEBS-g-MA could not be determined. A midpoint of 5 wt% was chosen. Deviations in trends of the averaged DMA data from expected trends based on literature were also observed for SEBS-g-MA-LDH and SA-LDH/ABS composites. For SEBS-g-MA-LDH, the composites showed an initial decrease in the storage and loss moduli due to agglom- eration with an eventual increase at higher loadings of LDH as the polymer-additive interaction becomes more dominant. For the SA-LDH/ABS composites, it is suspected that the large particle size due to the SA acting as a binding agent, causes the compos- i ites to become more rigid before the ”softening effect” of the SA as a lubricant takes over. FDM 3D printing of tensile bars was successful for all composites except for SEBS-g- MA/ABS and SEBS-g-MA-LDH/ABS which requires a modified print head or larger nozzle size. Due to the nature of the variables/parameters investigated in this study, experimental design was limited to D-optimal designs as recommended in JMP®. All specimens printed in the upright orientation fractured in a brittle manner due to layer separation of the 3D printed layers. The 1 wt% MgAl LDH/ABS composite printed at the -45°/45° raster layup showed a prominent increase in the ultimate tensile strength (UTS) of 35.5% compared to the neat ABS. The addition of LDH to the ABS is expected to increase heat dissipation during FDM 3D printing, thereby leaving less time for the layers to adhere upon cooling. The raster angle plays a role in the amount of material coming into contact between adjacent layers. Therefore, the modelled tensile results showed that the LDH loading (L) factor and the raster angle (R) had the biggest effect on both the UTS and elongation at break (EB) response variables for the upright printed samples. The R*S (raster angle*stearic acid presence) interaction term also influenced both the EB and the elastic modulus (EM). For specimens printed in the flat orientation, those printed with a -45°/45° raster angle fractured in a ductile manner. It is believed that the layers rotate in the direction of pull during the tensile test before fracture. Similar to the upright samples, the LDH loading and raster angle factors, especially at 90° and -45°/45°, influence the tensile properties significantly. It is clear that both the material and printing parameters have a significant overall influence on the anisotropic nature of the printed parts.