Polymers and Polymer Composites

A wrinkling criterion for non-linear elastic membrane materials such as thin polymer, fabric or rubber structures has been determined by employing finite element analysis as an alternative to the difficult experimental counterpart. The hyperelastic constitutive equation proposed by Ogden was implemented into the commercial finite element program ABAQUS with user material subroutine VUMAT. The relation between strain energy and the principal values of the stretch tensor was obtained in order to trace the wrinkle/non-wrinkle boundaries. It was shown that the wrinkling of an elastic membrane can be regarded as being initiated at the instance of abrupt slope change of the strain energy-stretch curve, which was verified by comparing the criterion obtained by finite element analysis with the existing analytical solution.

Sandwich materials with a cellular foam core are increasingly used in engineering applications. The basic advantage of these materials is their high strength-weight ratio. However, sandwich structures are notoriously sensitive to failure by the application of highly localised external lateral loads. Therefore the proper design requires an understanding of the response of these materials to localised loads. The purpose of this work was the evaluation of the low velocity localised impact behaviour of sandwich structures made of glass-fibre reinforced composite laminate face sheets and polyvinylchloride (PVC) foam core. The impact tests were performed with a drop-weight rig. Beam and panel sandwich specimens were impacted using steel cylindrical and spherical impactors, respectively. During the testing, the specimens were supported by a stiff steel continuous substrate in order to avoid global bending. The load and deceleration histories were recorded during impact. A finite element model was developed to simulate the impact response. Results obtained by this model exhibit good correlation with the experimental data. It is shown that the model can be used to successfully predict the elastic-plastic response of sandwich composite structures subjected to low velocity localised impact.

Polystyrene sulfonic acid (PSS) was prepared by sulfonating polystyrene (PS), obtained by emulsion polymerisation of styrene. Sulfonate groups were introduced on the phenyl groups of PS. Nanocomposites were prepared by dispersing nanoscale graphite (NanoG) in PSS aqueous solution with the aid of sonication, and casting the dispersion on a plastic film. The sulfonate anions on PS resulting from the ionisation of sulfonic acid groups facilitated the dispersal of NanoG carrying the positive charges. Scanning electron microscopy was used to characterise the morphologies of expanded graphite, NanoG and the nanocomposites, and the results showed that the graphite sheets varied in thickness from 40 to 70 nm. Transmission electron micrographs revealed that the PSS was intercalated into the sub-layers of nanoG homogeneously. The electrical conductivity of the nanocomposites reached 1.25 × 10⁻⁴S.cm⁻¹ at a concentration of 0.2 vol.% and the electrical conduction phenomenon could be interpreted by classic percolation theory.

New eco-composites were prepared from castor oil polyurethane reinforced with Luffa cylindrica fibres. These fibres were used without treatment as well as after mercerisation and mercerisation/benzylation surface treatments. TG curves of mercerised Luffa showed a small increase in thermal stability, associated with the removal of polyoses and soluble lignin. TG curves of benzylated fibres showed a decrease in thermal stability (T_onset and T_max.) and an increase in residual carbon due to aromatic ring insertion. FTIR spectra and elemental analysis also confirmed benzylation, and X-ray diffractograms showed a decrease in cellulose crystallinity. The eco-composite (crude fibre) absorbed more water (14% after 4 – weeks' exposure) than the resin, because of the hydrophilic character of natural fibres, but mercerisation and benzylation reduced this value by about 50%. Tensile moduli of composites reinforced with untreated and mercerised fibres were comparable, but increased by 33% with the use of mercerised/benzylated Luffa cylindrica. SEMs of the fibres and fibres-matrix interfaces are presented.

The aim of this study was to evaluate the influence of barium sulfate on the mechanical properties of glass/epoxy resin composite. Eight groups of glass fibre-reinforced resin bar and barium sulfate/epoxy resin bar specimens containing 0%, 9.7%, 13.9%, 17.7%, 21.2%, 24.4%, 27.3%, and 30.1 w.t% barium sulfate were fabricated. The three-point bending test and Charpy impact test were employed to measure the flexural strength, modulus, and impact strength of the composite. It was found that the flexural strength value increased about 30% from 0% (635.75 MPa) to 30.1% (822.82 MPa) as the amount of barium sulfate increased. The elastic modulus values of all group specimens varied from 23–27 GPa but there was no statistical significance among the groups (P = 0.524). The impact strength decreased noticeably with the addition of barium sulfate to the epoxy resin, while there was no significant difference among the filler-containing groups. In conclusion, barium sulfate reduced the impact strength of the resin; but on combining with glass fibre reinforcement, the barium sulfate improved the flexural strength of the composite.

Poly(lactic acid) (PLA)/organo-montmorillonite (OMMT) nanocomposites were prepared by using solution intercalation and melt intercalation methods. For solution intercalation method, toluene was used as solvent. For melt intercalation method, PLA/OMMT composites were prepared by using single-screw extruder. The flexural properties of PLA/OMMT composites have been studied by using three point bending tests. Comparisons between the flexural properties of PLA/OMMT nanocomposites prepared by both methods have been carried out. The microstructures of the PLA/OMMT nanocomposites were assessed by Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) analysis. The thermal properties of PLA/OMMT nanocomposites have been characterized by using Differential Scanning Calorimeter (DSC). The flexural modulus of PLA/OMMT nanocomposites prepared by both solution intercalation and melt intercalation methods increased significantly by the addition of OMMT. For both methods, it was found that the optimum flexural properties of the PLA nanocomposites was achieved at 1 wt.% OMMT loading. The dispersion of OMMT in the PLA prepared by solution intercalation is better than that of melt intercalation method. This is attributed to the ease of intercalation of PLA chain into the OMMT layered silicate during the solution intercalation process. XRD results revealed that intercalation of OMMT layered silicate occurred in the PLA matrix.

Glycidyl methacrylate (GMA) functional core-shell impact modifiers of butyl acrylate as a core and methyl methacrylate as a shell (gACR) have been prepared by a multi-step seeded emulsion polymerisation. FTIR spectroscopy results show that GMA had been grafted onto ACR and the GMA content in gACR was measured by titration of the epoxy groups. DMA was used to characterise the mechanical properties of the PBT/gACR blends. DSC results showed that the melting point of PBT shifted to lower temperature with an increase in GMA content. Scanning electron microscopy (SEM) images showed that the dispersion of gACR in PBT was uniform. The optimum recipes for gACR toughened PBT had a GMA content of 3% (wt) in gACR resins. The gACR particle size was about 350 nm and the gACR content was 20% (wt) in PBT resins. The notched impact strength of PBT/gACR blends exceeded 440 J/m. It was confirmed that functional core-shell impact modifiers are efficient in toughening PBT.

Nanocomposite materials of natural rubber (NR)/poly(ethylene-co-vinyl acetate) (EVA)/montmorillonite (MMT) were prepared by latex compounding without modification of the MMT. EVA and MMT were well mixed at room temperature in the latex state and subsequently blended with NR latex at NR/EVA dry weight ratios of 100/0, 90/10, 80/20, 70/30, 60/40 and 50/50. Where used, MMT was added at 4 parts per hundred of dry rubber and EVA (phr). The homogeneous latex mixture was then cast into sheets on a glass mold, air dried for 2 days at room temperature and then in an oven at 70 °C for 6 h. It was subsequently cured at 110 °C for 2 h. After curing, samples were examined for their structure and morphology by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), whilst tensile properties were investigated using a Universal Testing Machine. The XRD patterns and TEM images of the nanocomposites showed an exfoliated structure. The 90NR/10EVA/4MMT exhibited the highest tensile strength, with incorporation of MMT offering a substantial improvement in tensile strength. SEM micrographs revealed that the MMT particles were well dispersed in the polymer matrix. However, increasing the EVA content to 50 wt.% reduced the compatibility between NR and EVA.