POLYMER-POLYMER COMPOSITES: MECHANICAL PROPERTIES OF INTER-REINFORCED THERMOPLASTICS

Open Access
Author:
Saadawi, Majed Nabil
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
February 28, 2019
Committee Members:
  • Michael Anthony Hickner, Thesis Advisor
  • Evangelos Manias, Committee Member
  • Robert John Hickey, III, Committee Member
Keywords:
  • Polymer-Polymer Composites
  • thermoplastic composite
  • 3D printing
  • additive manufacturing
  • dual extrusion
Abstract:
Composites have become a necessity in many applications that require specific properties not attainable with pure polymers. Polymer composites with high strength are often processed with glass fibers or carbon fibers. However, the choice of reinforcement usually renders the composite inconvenient for recycling either due to the choice of matrix material such as thermosetting epoxy or simply the difficulty of separating thermoplastic matrices from their respective reinforcement material. Therefore, a more sustainable solution through recyclable thermoplastic composites is needed. This thesis explores the mechanical properties of thermoplastics reinforced by other thermoplastics with varying degrees of miscibility. Polar polymers such as polycarbonate and polyethylene terephthalate were expected to have the advantage of polar interaction forces that may aid in miscibility. Self-reinforced polyethylene terephthalate (PET) had been explored in literature to yield better mechanical properties than molded samples of low-melt polyesters. It was observed from experiments in this thesis that using nonwoven PET for self-reinforcement marginally enhances the tensile properties of PET composites. However, the commodity nonwoven reinforcement used is not the optimum choice for reinforcement in composites since the felts were not especially made for this purpose. Polycarbonate (PC) and polylactic acid thermoplastic resins have shown melt-impregnation through nonwoven polyethylene terephthalate fiber, yet their mechanical properties declined. As such, an immiscible or partially miscible polymer matrix is bound to have lower interfacial adhesion at the matrix-fiber interface. Using polycarbonate as the matrix, an embrittlement effect was induced in the composite. Furthermore, this thesis also expands on composites by way of polymer-polymer composite layered fused filament fabrication. As opposed to the former where processing occurred below the reinforcement’s melting temperature, extrusion deposition involves a mutual-melt interface. Dual-extruded filaments of varying miscibility were investigated due to the limited interlayer bonding single filament extrusion can attain. Interlayer adhesion is influenced by the rapidly changing temperature history profile of extrusion-based additive manufacturing where road-to-road welding does not reach virgin strength of the deposited polymer. Glycol-modified polyethylene terephthalate (PETG) was dual-extruded with polycarbonate to assess their composite’s mechanical property against pure PETG. Although good adhesion between PC and PETG is achievable, the composite iv mechanical strength did not improve above the performance of either of its constituents. PC and PETG are only partially miscible thus interaction forces between their composite are not as strong as forces of adhesion in the pure single-extruded polymers. Moreover, partial miscibility also implicates that diffusion would not occur over all molecular weights. It follows that tensile results of dual-extruded PC-PETG composites have lower strength than both PC and PETG in single extrusion processing. On the other hand, dual 3D printing of completely immiscible polymers such as polylactic acid (PLA) with glycol-modified polyethylene terephthalate will fail catastrophically in tensile testing due to depleted interfacial strength and inefficient stress transfer. Flexural testing clearly points towards multiple interfacial failures of dual extruded PLA-PETG composites. Conversely, single-extruded samples of PLA only had a singular brittle failure while PETG had no break in bending. Therefore, immiscibility has detrimental effects on interfacial strength of additively manufactured dual-extruded composites.