Tensile and Flexural Properties of MMT-Clay/ Unsaturated Polyester Using Robust Design Concept

The effect of nanoclay on the mechanical properties of Isophthalic unsaturated polyester was studied with the help of robust design Concept. Organo modified MMT nanoclay (Nanomer 1.31PS) was used as reinforcement. The weight percentage of nanoclay, impeller blade design, mixing hours and mixing speed were taken as control factors. In Taguchi design of experiments, L9 orthogonal array was employed to investigate the effect of control factors on mechanical properties such as tensile and flexural strength. X-ray diffraction (XRD) and Atomic force microscopy (AFM) results show the intercalation /exfoliation of clays in the polyester matrix.

Nanocomposite preparation. Nanocomposites were prepared by varying the organo-modified clay content, impeller blade design, mixing hours and impeller speed. In this work, mechanical high shear mixer was used to distribute and exfoliate the clay nano-particles in the polyester matrix. 300 ml of polyester resin was taken in a glass beaker. Calculated amount of organic clay was added to polyester in room temperature to produce nanocomposite. The final product was collected from the high shear mixer and was allowed to degass in a vacuum desiccator at negative pressure of 20 Hg/mm for one hour. In the 500 ml glass beaker, the burette controller was fused at the bottom and this apparatus was used to separate the bubble layer, since the pure layer of resin is settled down ( Fig. 1-a and 1-b). The specimen with the mold was allowed to cure at room temperature for next 24 hours. The post curing was also done at 60 °C for 3 h in hot air oven. according to the ASTM -D638 (Type I) and D790 respectively. In flexural testing, three-point bending at room temperature was performed. In three-point bending, the load was placed centrally between the supports. Five specimens were tested in each case and the average value was reported.

Nano Hybrids Vol. 2 89
Dispersion Studies. Dispersion of nanoclay with matrix was studied and ensured with the help of Xray diffraction (XRD) and Atomic Force Microscope (AFM). In XRD step scanning was employed with a scanning rate of 2°/min with Cu-Kα radiation using SHIMADZU, XD-DI X-ray diffractometer. The scanning angle (2θ) was taken from 3º to 30º. AFM with Multimode Scanning Probe Microscope (NTEGRA, NT-MDT, Moscow) equipped with a DS 95-50-E scanner and an AC probe were used.

Results & discussions
Robust Design Concepts. Based on the literature and preliminary works, the following control factors were identified as key factors which affect the dispersion of nanocomposite. Mixing time, mixing speed, the weight percentage of nanoclay and blade design has been selected as control factors. Fig. 2 shows the photograph of blades used for this study.

Fig. 2. Different designs of Blade
After deciding the independent variables, the numbers of levels for each variable were decided. The selection of the level numbers depends on the trend in which the parameter influences the output responses (Table 1). Array selection was based on the number of parameters and the number of levels. In this investigation, 4 factors and 3 levels have been identified. Hence L 9 orthogonal array was selected.

Ex. No
After the experiments have been conducted, the optimal test parameter configuration within the experimental design must be determined. To analyze the results, the Taguchi method uses a statistical measure of performance called signal to-noise (S/N) ratio. The S/N ratio which was developed by Dr. Taguchi, is a performance measure to choose control levels that best cope with noise [12,13]. In its simplest form, the S/N ratio is the ratio of the mean (signal) to the standard deviation (noise). The S/N equation depends on the criterion for the quality characteristic to be Nano Hybrids Vol. 2 91 optimized [12]. While there are many different possible S/N ratios, in this investigation 'Larger the better characteristic' was selected based on the requirement. Equation (1) represents larger the better quality characteristic, Where n is the number of observations for every experiment and y is the observed data from the experiment.
Tensile Strength. Experiments were conducted as per the Taguchi L 9 orthogonal array, however, the test runs were selected at random, to avoid systematic error creeping into the experimental procedure.    Table 3.  (60 min), blade design B2 (three blade) and weight percentage % of clay C1 (1 wt %) were identified as optimal values. The contribution of each parameter on flexural strength is given in Table 4. From this table, blade design, % of Clay and mixing speeds are identified as significant factors affecting flexural strength. Flexural strength for the optimal parameters was found to be 70 MPa.  Fig. 7 show the diffractograms of pure nano-clay and nano-clay reinforced polyester X-ray (nine samples which are produced based on Taguchi experimentation). From the diffractogram for pure nanoclay, a definite sharp peak at 4.5 0 (2θ) was observed as crystallized nanoclay. During hybridization, in all the experiments good dispersion of nano-clay with matrix and this could be observed from the suppressed peak which was seen in the pure nano-clay in (001) plane. However

Nano Hybrids Vol. 2 93
Taguchi experiments showed better dispersion thereby improved tensile and flexural performance.
This is indicating the presence of amorphous phase and exfoliation of the clay galleries in the matrix system [15][16][17][18]. This shows that the interlayer distance of organo modified clay layers is randomly dispersed in the polymer matrix. Measurements were carried out in air, at ambient conditions. Phase and height images were recorded simultaneously. From the Fig. 8 (a, b), three different appearances of shading was found: One in dark black; it is expected to be pure matrix and the second shadow is light black which could be the interface between nanoclay and the matrix and pure white is speculated to the presence of MMT nanoclay [19,20]. This AFM image clearly shows that the uniform distribution of nanoclay in most of the region of the matrix. Fine clays can be seen in the AFM three-dimensional phase image of the optimized nanocomposite sample Fig. 8 (b).

Conclusions
From the experimentation, the optimum process parameters such as weight percentage of nano-clay, impeller blade design, mixing hours and impeller speed on tensile and flexural strengths were identified. Speed of 500 rpm, time -60 min, three blades and 1wt % of clay were found to be better in order to improve the properties. The blade design was identified as the main control factor to improve the properties.