Issue 46

K.I. Tserpes et alii, Frattura ed Integrità Strutturale, 46 (2018) 73-83; DOI: 10.3221/IGF-ESIS.46.08 74 I NTRODUCTION he extraordinary properties of carbon nanotubes (CNTs) offer the ability of manufacturing polymers with enhanced mechanical properties at low concentrations of the nanofiller in comparison with the use of other additives, such as carbon black, which requires higher concentrations for an effective reinforcement. Indeed, if the desired performance can be achieved at low concentrations, novel lightweight materials can be fabricated, to be used in a range of advanced applications such as aerospace structures. To obtain the desired properties in the nanocomposite material, it is important to achieve a homogeneous and uniform dispersion of CNTs in the matrix, having in parallel a strong interfacial bonding between the two phases (organic and inorganic) [1,2]. However, the chaotic entanglement of CNTs can lead to the formation of agglomerates in the scale of micrometers [3-5]. Generally, multi-wall CNTs (MWCNTs) are easier to disperse in polymer matrices, as they have larger diameters, in comparison with single-wall CNTs (SWCNTs). Carbon nanotubes have been distinguished from other nanomaterials, due to their extraordinary electrical properties. Several studies have investigated also the mechanical properties of such composites, focusing on the elasticity [6], tribology [7], toughness [8] and damage [9] of the specimens. Nanomechanical measurements, through nanoindentation, have also been used for the assessment of the mechanical properties of nanocomposites based on epoxy resin, reinforced with SWCNTs [10]. Among thermosets, epoxy resins have been very often studied as a potential matrix for nanocomposites with CNTs. Small quantity of CNTs, often between 0.1 and 5.0% (w/w), is added to the polymeric matrix aiming to improve mechanical and thermal properties [11]. Li et al. [12] studied the nanomechanical properties of SWCNTs reinforced epoxy composites with varying weight percentage (0, 1, 3, and 5 wt%) via the nanoindentation and nanoscratch techniques; the addition of 5 wt% SWCNTs increased the elastic modulus by 75% and the hardness by 30% when compared to the pure epoxy. An improvement by 80% in tensile modulus was obtained when thermoplastic poly(vinyl alcohol) (PVA) was mixed with only 1wt.% CNTs [13]. An increase of 28% in tensile Young’s modulus was observed in the rubbery system using 1 wt% functionalized nanotubes, compared to the unreinforced rubbery epoxy [14]. Based on nanoindentation tests, improvements of hardness by 18% and 36% for polypropylene/CNTs composite (1 and 3%, respectively) are reported [15], compared to that of neat PP; additionally, the stiffness increased from 1.22 GPa (neat PP) to 1.47 GPa (20.5%) and 1.64 GPa (34.4%) for 1 and 3% w/w respectively, while creep rate and displacement of the nanocomposites were lower than the neat PP [15]. Despite the large number of published experimental works, there is still the necessity for detailed characterization of the morphology, the mechanical and nanomechanical behavior of CNT/polymer nanocomposites in order to clearly understand the load transfer mechanisms between MWCNTs and polymers (thermosets and thermoplastics) and correlate the material structure with the mechanical properties of the nanocomposites. In the present work, the mechanical and nanomechanical properties of multi-walled carbon nanotube-reinforced polypropylene (MWCNT/PP) nanocomposite were investigated through tension tests (conducted on 2 wt% and 5 wt% specimens) and nanoindentation tests (conducted on 2 wt% specimens). E XPERIMENTAL Materials or the study, the Plasticyl™ PP2001 [16] material was used. Because of its low viscosity and high flow formulation, the Plasticyl PP2001 material it is ideal for standard injection molding and extrusion processes. Additional benefits of the specific materials are the enhanced electrical conductivity at low loading, retention and/or improvement of key mechanical properties and easier processing. The Young’s modulus of the neat Polypropylene (PP) material is reported to be 1280 MPa [17] and the tensile strength at break 28.2 MPa [17]. Tension tests The tension tests on the reference PP and MWCNT/PP specimens were conducted according to the ASTM D638 standard [17] at room temperature using the electro-mechanical testing machine Tinius Olsen H5KT which incorporated a tension/compression transducer with a load-cell of 1 kN. The tests were conducted at a crosshead speed of 1 mm/min. For each test set, 6 specimens were tested. SEM tests Scanning electron microscopy (SEM) tests were conducted using a LaB6 electron gun integrated with an e-beam lithography system to characterize the morphology of the MWCNT/PP nanocomposite. The technical data of the microscopy are: 30 Τ F