All issues / Volume 4 (2010) / Issue 2 (February)
Editorial corner - a personal view
In this study rubber-toughened polyester-kenaf fibre composites were prepared by adding various percentages of kenaf fibre in unsaturated polyester resin and subsequently cross linked using a mixture of organic peroxide methyl ethyl ketone and cobalt octanoate. Three percent (3%) of liquid natural rubber (LNR) were added as a toughening agent. The mechanical properties of the composites were evaluated by impact and flexural testing. Environmental stress cracking resistance (ESCR) of polyester-kenaf composite in acid and base medium was also studied. It was found that the addition of LNR increased impact strength by about 66% and flexural strength by 70%. Measurement of ESCR shows that the composite has the fastest diffusion rate in acid medium, followed by that in base medium and then without medium. Bonding mechanisms were assessed by scanning electron microscope and FTIR analysis.
In this work, basic mathematical models and response surface graphs have been used to illustrate the relationship between mixing parameters in internal mixer and properties of the SBR (styrene butadiene rubber)/organoclay composites. Using a Box-Behnken statistical design experiment methodology, the effects of mixing temperature (80–140°C), mixing time (4–12 min) and nano filler amount (3–9 phr) in SBR nanocomposites on the properties (tensile properties, scorch time and Mooney viscosity) were evaluated. It was found that the mixing parameters (time and temperature) have the predominant role in properties and morphology of nanocomposite. The R2 values (the R2 values indicate the degree of agreement between the experimental results with those predicted by model) of all responses were above 0.85. Increasing temperature and mixing time facilitated a better organoclay dispersion which resulted in a better tensile property. With increase in nanoclay amount in composite the scorch time and Mooney viscosity decreased. The morphology of nanocomposite was studied by XRD (X-ray diffraction) and TEM (Transmission electron microscope). Intercalation and exfoliation of the nanoclay were observed for samples with higher temperature and longer mixing time. Due to thermal degradation of the rubber matrix at 140°C, tensile properties of the nanocomposite were decreased.
Silver-polyacrylonitrile (Ag-PAN) nanocomposites were in situ synthesized by simultaneous polymerization of acrylonitrile and reduction of silver ions, starting from mixtures of silver nitrate (AgNO3), acrylonitrile (AN), and UV photoinitiator (IN). The films obtained proved to be transparent and were characterized by a homogeneous dispersion of Ag nanoparticles within the PAN matrix without any macroscopic agglomeration. The particle size and number density were found to depend on both precursor salt and UV photoinitiator weight percentages. Optical and electrical properties were investigated as a function of both AgNO3 and IN amounts, too. We found that it is possible to finely tailor the metal nanoparticle size and number density and, consequently, the film optical and electrical response by adjusting the amounts of precursor salt and UV photoinitiator in the initial mixtures.
Fire-retardant ceramifying poly(vinyl acetate) (PVAc) sealants have been prepared. The degradation of PVA was integrated with the action of the fire retardants to reduce flammable gases, produce carbonaceous char and convert the fillers into a self-supporting ceramic barrier. PVA is readily degraded by elimination of acetic acid, yielding a char that provides a transitory phase as the filler particles fuse into a ceramic mass. Acetic acid is eliminated at similar temperature to the release of water from magnesium hydroxide fire-retardant, thereby diluting flammable acetic acid. The residual oxide from the fire-retardant filler and structural filler are fused by a flux, zinc borate. The degradative and ceramifying processes were characterised using thermogravimetry, infrared spectroscopy, scanning electron microscopy and ceramic strength. Thermogravimetry of the composites was compared with additive mass loss curves calculated from the components. Deviations between the experimental and additive curves revealed interactions between the components in the composites. The modulus of the PVAc composites and the strength of their ceramic residues after combustion were determined.
The copolymers of ethylene and 1-hexene were prepared with half-metallocene titanium complex ([t-BuNSiMe2Flu]TiMe2) and modified methylaluminoxane (MMAO). The initial concentrations of 1-hexene were varied to investigate how the different amounts of comonomer affect on the catalytic activity of copolymerization system and microstructure of the copolymers. It has been found that this catalytic system was not active for hexene polymerization, however, it can be active when ethylene was introduced to perform ethylene-hexene copolymerization. As comonomer, 1-hexene provides positive comonomer effect on the system although very high concentration of 1-hexene was introduced. However, the microstructures of the obtained copolymers, which were examined by 13C-NMR need to be improved because with highly alternating sequence distribution of comonomer causing them losing some essential specific thermal properties.
This study is devoted to the investigation of the effect of molecular mass on the α-, β- and γ-crystallization tendency of isotactic polypropylene (iPP). The crystalline structure was studied by wide angle X-ray scattering (WAXS) and by polarised light microscopy (PLM). The melting and crystallization characteristics were determined by differential scanning calorimetry (DSC). The results indicate clearly that iPP with low molecular mass crystallizes essentially in α-modification. However, it crystallizes in β-form in the presence of a highly efficient and selective β-nucleating agent. The α- and β-modifications form in wide molecular mass range. The decreasing molecular mass results in increased structural instability in both α- and β-modifications and consequently enhanced inclination to recrystallization during heating. The formation of γ-modification could not be observed, although some literature sources report that γ-form develops in iPP with low molecular mass.
Linear low-density polyethylene (LLDPE) based composites were prepared by melt compounding with 1, 2, 3 and 4 vol% of various kinds of amorphous silicon dioxide (SiO2) micro- and nanoparticles. Dynamic rheological tests in parallel plate configuration were conducted in order to detect the role of the filler morphology on the rheological behaviour of the resulting micro- and nanocomposites. A strong dependence of the rheological parameters from the filler surface area was highlighted, with a remarkable enhancement of the storage shear modulus (G′) and of the viscosity (η) in fumed silica nanocomposites and in precipitated silica microcomposites, while glass microbeads only marginally affected the rheological properties of the LLDPE matrix. This result was explained considering the formation of a network structure arising from particle-particle interactions due to hydrogen bonding between silanol groups. A detailed analysis of the solid like behaviour for the filled samples at low frequencies was conducted by fitting viscosity data with a new model, based on a modification of the original De Kee-Turcotte expression performed in order to reach a better modelling of the high-frequency region.