The investigation of the effects of halloysite and sepiolite nanoadditives to polystyrene

Thermoplastics are composed of two main divisions, namely general purpose plastics and engineering plastics. Polystyrene (PS) is a general purpose thermoplastic and it has a wide range of application. This study aims the preparation of nanocomposites by using "commercial PS" to improve the properties of PS. The main problem faced in the production of polymer/nanoclay composites is the difficulty of dispersing the nanoclay uniformly in the matrix. The improvement in the properties of nanocomposites can be achieved only if a better dispersion of clay in the polymer matrix is accomplished. This is especially important when polymers composed of nonpolar molecules (like polyolefins and polystyrene) are reinforced with inorganic additives. In order to have a better dispersion of inorganic additives in the nonpolar polymer matrix, the surface of the inorganic additive may be modified with a nonpolar molecule and/or with the addition of a compatibilizer. In this study, the surfaces of natural and inorganic nanofillers of halloysite nanotubes (HNT) and sepiolite (SEP) were modified with styrene-maleic anhydride copolymers (SMA) to provide compatibility and dispersibility for the nanoclays in PS matrix. Four types of commercial SMAs (SMA-1000, SMA-EF40, SMA-15170, SMA-26120) having different molecular weights and styrene/maleic anhydride molar ratios were used in the surface modification reactions. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) methods were used to characterize the modified nanoclays. PS/clay nanocomposites were prepared by loading nanoclays into PS matrix in 5, 10 and 15 wt%, by using a twin-screw extruder. Finally, the structural, thermal, mechanical and morphological properties of the PS nanocomposites were investigated by XRD, differential scanning calorimetry (DSC), TGA, tensile testing, Shore-D hardness test and SEM. Three distinct chemical reaction methods were followed in the surface modification of the pristine HNT and SEP nanoclays. In the first method, the nanoclay surfaces were functionalized with (3-aminopropyl) triethoxysilane (APTES) to cover the surfaces with primer amines and then the surfaces were modified with SMA throughout amine groups. In the second method, the nanoclays were directly modified with APTES and SMA in the chemical reaction. In the third method, the nanoclay surfaces were modified with SMA directly. The surface-functionalized nanoclays via APTES were characterized by ninhydrin reaction and the presence of primary amine on the nanoclay surface were observed. The results of these three methods were investigated by FTIR analysis. Only the nanoclay samples that were functionalized via the third method exhibited more intense absorption peak of amine groups than those that were functionalized via the first and second methods. In this study, the third method was chosen for the modification of HNT and SEP nanoclays with SMA types, namely HNT-1000, HNT-EF40, HNT-15170, HNT-26120 and SEP-1000, SEP-EF40, SEP-15170, SEP-26120, respectively. In addition to FTIR analysis, X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) characterizations were used for the third method. XRD patterns of the pristine and modified HNT with SMA copolymers revealed that the characteristic d001 diffraction peak of HNT was located at 2θ=12.0°, corresponding to a basal spacing of 7.57 Ǻ, except for modified HNT with SMA-1000. Due to its lower molecular weight, SMA-1000 shifted the diffraction peak of HNT from 2θ=12° to 8.8° and increased the basal spacing from 7.57 to 9.84 Ǻ. XRD patterns of modified SEP with SMA samples exhibited that the characteristic d110 diffraction peak of SEP remained unchanged at 2θ=7.4°. This may be attributed to the difficulty in changing the interlayer spacing of SEP because of the covalent bonds between its layers. According to the results of TGA analysis, the modified HNT samples with SMA-1000, SMA-EF40, SMA-26120 and SMA-15170 have inorganic residues 39, 15, 36 and 34 wt%, respectively while the inorganic parts of HNT was 74 wt%. On the other hand, the inorganic residue results of SEP containing SMA samples were found as 43, 24, 45 and 14 wt%, respectively. Inorganic residue of the original SEP was 76 wt%. TGA results showed that the nanoclay surfaces were efficiently modified with SMAs. SEM images of the modified HNT samples exhibited that the agglomeration occurred within the modified nanotubes and the singular form of the tubes disappeared due to organic structures on the surfaces after the chemical modification reaction. SEM micrographs of the modified SEP samples indicated that the chemical modification reaction caused the formation of needle type aggregates on the surfaces and hence the smoothness of the SEP surfaces was reduced due to the assembly of organic molecules. Polystyrene composites with modified nanoclays used by the third method were prepared by using Xplore MC15 micro compounder twin-screw extruder with screw rotational speed of 120 min-1, and a temperature profile of 200°C in 2 minutes. The modified nanoclays were added to PS matrix at 5, 10 and 15 wt% loading ratios. The structural, thermal, mechanical and morphological properties of PS nanocomposites were investigated. For comparison purposes, PS nanocomposites containing 5, 10, and 15 wt% unmodified (pristine) HNT and SEP nanoclays were also prepared and the same characterizations were made. XRD analysis method was used to study the structural properties of the nanocomposites. The results of XRD analysis of PS nanocomposites revealed that increasing loading ratio of the nanoclays results in increased intensity of characteristic diffraction peaks of HNT and SEP on the XRD patterns. The thermal properties of PS nanocomposites based on HNT and SEP were investigated by differential scanning calorimetry (DSC) and TGA methods. According to DSC results, the glass transition temperatures (Tg) of the nanocomposites were generally higher than that of pure PS matrix. TGA results of PS nanocomposites based on HNT nanoadditives showed that the thermal stability of PS nanocomposites were increased. HNT-EF40, which is the HNT additive modified with SMA in which the styrene/maleic anhydride ratio is 4/1 (and also having the lowest Tg) (SMA-EF40) showed the most salient effect among the modified additives. HNT-EF40 appeared to be the modified additive which increases the thermal stability of PS most when it is added to the PS matrix in 10 wt%, as compared to the other modified additives and weight percentages. This is because HNT-EF40 contains higher wt% of HNT than other modified additives. Investigation of the TGA analysis of SEP-based PS nanocomposites showed that thermal stability of nanocomposites increases depending on the wt% of additive. It has been noted that, for PS nanocomposite containing 15 wt% SEP-EF40 (SEP additive modified with SMA-EF40), the initial temperature of thermal decomposition (Tinitial) increased by 20°C and the final temperature of thermal decomposition (Tfinal) increased by 25°C. The mechanical properties of PS nanocomposites were studied by tensile test and Shore-D hardness analysis. Considering that tensile strengths are measured with ± 5 %, tensile strengths of PS nanocomposites containing HNT between 5 – 15 wt% were observed to decrease within measurement error limits. The tensile strengths of PS nanocomposites containing HNT-1000 additive (modified with SMA-1000 copolymer having a low molecular weight and a styrene/maleic anhydride ratio of 1/1) and HNT-EF40 additive (modified with SMA-EF40 copolymer having a high molecular weight and a styrene/maleic anhydride ratio of 4/1) decreased within measurement error limits. However, the tensile strengths of nanocomposites containing 15 wt% additive were observed to decrease by 20% as compared to that of pure PS. The tensile strengths of PS nanocomposites containing 5 wt% and 10 wt% HNT modified with high-molecular weight SMAs decreased as compared to that of pure PS within measurement error limits. A 10% decrease was observed in the tensile strengths of nanocomposites when the additive contents were raised to 15 wt%. When the tensile strengths of SEP based PS nanocomposites are investigated, it was observed that while the tensile strengths of PS nanocomposites containing 5 wt% and 10 wt% SEP did not exhibit a change with respect to that of PS, the tensile strength decreased when the SEP content is 15 wt%. On the other hand, the tensile strengths of PS nanocomposites containing 5, 10, and 15 wt% SEP-1000 additive (SEP modified with SMA-1000) remained within measurement error limits. The tensile strengths changed only within measurement error limits when SEP additives modified with other SMAs are added to the PS matrix up to 10 wt%; but a decrease was observed when the additive content was raised to 15 wt%. In the Shore-D hardness test, HNT-based additives did not cause a significant change in the hardness of the PS matrix and increased the hardness in the range of 1-5%. The SEP-EF40, SEP-26120 and SEP-15170 additives increased the Shore-D hardness of the PS nanocomposite by 4% regardless of the weight percent of the additive. When the morphological properties of HNT-based PS nanocomposites were examined, it was seen that the pristine HNT was distributed in larger aggregates compared to the PS matrix containing the modified HNT samples and the aggregate dimensions were slightly smaller as the amount of pristine HNT increases. Here, due to the pushing force between the nonpolar PS matrix and the polar HNT particles, HNT particles, which are close to each other, interact with each other to form aggregates. As the amount of HNT increases, the distance between the particles becomes shorter, so smaller size aggregates form. However, it was observed that the HNT modified with SMA-1000 (named as HNT-1000) additive had a much better distribution of PS matrix than the other additive, and the aggregation was in a much smaller. In addition, the increase in amount of HNT-1000 did not cause any change in aggregate size. Due to the fact that the HNT-1000 surface is covered with higher styrene molecules than the other modified HNT additives, the additive is homogeneously distributed in PS matrix and aggregation formation is at a minimum level. When the morphological properties of SEP-based PS nanocomposites were examined, it was seen that the pristine and modified SEP nanoadditives were distributed as layers in PS matrix. It was observed that pristine SEP formed aggregate in nonpolar PS matrix aggregate; the modified SEP samples were distributed homogeneously in the PS matrix without aggregate formation. Particularly, in PS nanocomposites containing SEP-26120, it was observed that the SEP layers were entangled, and that the layer separation was less in the sample containing 5 wt.% of the additive. It can be said that SMA-26120 copolymer, which has a very high molecular weight compared to SMA-1000 and SMA-EF40, shows a more efficient distribution in PS matrix when added by 5% by weight due to the nonpolar styrene layer formed on the SEP surface. As a conclusion, it was noted that the properties of the PS matrix was best improved by HNT and SEP nanoadditives that are modified with SMA-EF40 (HNT-EF40 and SEP-EF40). On the other hand, SMA-EF40 copolymer was bonded onto the nanoclay surfaces weakly because it has a high styrene/maleic anhydride mole ratio (4/1), minimum Tg, and relatively short chain length. Hence the organic component content of HNT-EF40 and SEP-EF40 additives modified with SMA-EF40 is lower than that of other modified nanoclays. Although it seems as a disadvantage for the surface modification of nanoclays, it contributed to improving the properties of the matrix.