2.1 Materials

PBT (average molecular weight of 38,000) and trifluroacetic acid were purchased from Sigma Aldrich and were used as received. The reinforcement material Kaolin (china clay) was kindly provided by local china clay plant Swat, Pakistan.

2.2 Thermal treatment of kaolin clay

Kaolin clay was dehydrated upon heating to 350 °C in the presence of oxygen. Thermal treatment below 400 °C does not lead any structural change in the clay. However, above this temperature dehydroxylation takes place which cause structural changes in kaolin^[17]. To avoid these structural changes, clay is heated up to 350 °C, cooled and store for further use.

2.3 Preparation of samples

The neat PBT and 0.5, 1, 2 wt% clay/PBT composite samples were prepared via solution casting method. The PBT was first dissolve in trifluoroacetic acid, and then added a known quantity of kaolin clay in the polymer solution. The mixture solution was stirred well and then sonicated for about 40 min in order to get homogeneous solution. The smooth dry composite films/sheets were obtained after the removal of solvent with distilled water. Similar procedure was used for the preparation of 0.5, 1 and 2 wt % kaolin/PBT composite films.

2.4 Characterization

The gold coated cryofractured surface of neat PBT and composite samples were analyzed by SEM (model JEOL JSM-5910). The POM study was performed by using POM (modal Optika B-600 POL). The samples were melted on heater, squeeze between two glass slides and then analyzed under POM. The thermal properties were studied by using TG/DTA, Perkin Elmer instrument at heating rate of 20 °C/ min from room temperature to 800 °C under nitrogen atmosphere. The mechanical properties of neat PBT and clay/PBT composites were examined using universal testing machine (UTM), Model 100-500 KN, Iestomeric Inc. The solvent uptake capacity of the samples were studied in selected solvents (distilled water, 2M HNO₃ solution, chloroform and kerosene oil). Small dry rectangular strips of the samples were weighed and then immersed in vials containing about 10 mL solvents. Percent swelling is determined by the following equation^[20].

Where “W” is the weight of dry sample and “W_o^” is the weight of wet sample in solvents.

3.1 Morphology of clay/PBT nanocomposites

The dispersion of fillers within the matrix has greatly affected the properties (such as thermal, mechanical, electrical properties etc.) of the matrixes. Figure 1 illustrates the SEM micrograph of fractured surface (broken in liquid nitrogen) of clay (2 wt %)/PBT composites film. The micrograph presented that the clay particles were not present in agglomerated but dispersed well within the PBT polymer matrix. The Figure 1 also presented that the size of the clay particles were below 800 nm.

Figure 1 SEM micrographs of 2 wt% clay/PBT nanocomposites. 

Figure 2a, b show POM images of neat PBT and 2 wt% clay/PBT nanocomposites, respectively. Thin films of PBT and the composite were prepared by cooling both samples from the melt state. Figure 2a presented almost uniform formation of spherulites throughout the PBT matrix. The shape of neat PBT spherulites is Maltese type. The size of spherulites of neat PBT was below 25 µm. The POM micrograph (Figure 2b) of clay (2 wt %)/PBT nanocomposite showed that the size of spherulites significantly decrease as compare to neat PBT. The decrease in size of spherulites in clay/PBT nanocomposite is might be due to the nucleation effect of kaolin clay. The decrease in size of spherulites in the case of nanocomposites materials were also reported by Saeed and Park, where they incorporated multi walled carbon nanotubes into polycaprolactone^[21].

Figure 2 POM images of (a) pure PBT and (b) 2 wt% clay/ PBT. 

3.2 Thermal properties of neat PBT and clay/PBT nanocomposites

TGA study was performed for the purpose to determine the effect of kaolin on the degradation temperature and thermal stability of PBT. The TGA curves of neat PBT, 0.5, 1.5 and 2 wt% clay/PBT nanocomposites are collectively shown in Figure 3. Figure 3 illustrated that the weight of neat PBT and nanocomposite remain unchanged till 300 °C. After 300 °C, the TG curves starts dipping down from 300 to 450 °C, which shows that the weight loss occurs in this particular range of temperature. The degradation of neat PBT polymer started at 300 °C and completed at about 430 °C. While in the case of Clay/PBT nanocomposite, the degradation of compsite started at about 350 °C. It was also found that the degradation temperature of clay/PBT samples was slightly shifted to higher temperature (5 to 10 °C). The residual quantities which contribute to the kaolin clay in the clay/PBT composite, remain at higher temperature.

Figure 3 TGA curves of neat PBT, 0.5, 1.5 and 2 wt% clay/PBT nanocomposites. 

Figure 4 illustrates the DTA curves for neat PBT, 0.5, 1.5 and 2 wt % clay/PBT nanocomposite samples. All the samples have a weak endothermic peak around 200 °C and a sharp endothermic peak around 400 °C (Figure 4). The weak endothermic peak around 200 °C (225 °C-227 °C) represents the melting, while the sharp one around 400 °C may represent the degradation of neat PBT^[22]. Multiple peaks in this region are also possible which may be due to recrystalization process^[23]. As clear from Figure 4 the most noticeable difference is the height of peaks which increase gradually with increasing clay contents. The strong endothermic peaks in case of 2 wt% clay/PBT nanocomposites are due to the interaction between clay and polymer, which results more stable morphology through heterogeneous nucleation mechanism^[24].

Figure 4 DTA thermographs of (a) neat PBT, (b) 0.5 wt%, (c) 1.5 wt% and (d) 2 wt% clay/PBT nanocomposites. 

3.3 Mechanical properties of neat PBT and clay/PBT nanocomposites

The dispersion of fillers plays a key role in improving the mechanical properties of polymeric materials. The well dispersed filler can results superior mechanical properties. Table 1 shows the mechanical properties of neat PBT, 0.5, 1, 1.5 and 2 wt% clay/PBT nanocomposites. The increase in clay content cause a significant increase in tensile strength and modulus values as shown in Table 1. The tensile strength values increase from1.6 N/mm² (neat PBT) to 5.6 N/mm² (2 wt %) while tensile modulus increase from 166.6 N/mm² (neat PBT) to 1080.9 N/mm² (2 wt %). The results shows that both tensile strength and tensile modulus of PBT was enhanced significantly by incorporation of clay (up to 2 wt %) in to the polymer matrix. Similarly, Gu et al.^[25] also reported the improvement of mechanical properties of PET by the addition of modified montmorillonite clay.

3.4 Solvent uptake study of neat nylon 6,6 and clay/nylon 6,6 nanocomposites

Tables 2, 3, 4 illustrate the solvent uptake of neat PBT and clay/PBT nanocomposites. The distilled water, 2 M HNO₃ solution, and kerosene were taken as solvents during the solvent uptake study. The results show that initially the solvent uptake was rapid and then the uptake of solvents slow down after 24 h. Among the solvents, uptake of distilled water was found more rapid as compare to other kerosene and HNO₃ solution. The solvent uptake of neat PBT and clay/PBT nanocomposites is in the following order:

Neat PBT > 0.5 wt% clay/PBT > 1 wt% clay/PBT > 1.5 wt% clay/PBT > 2 wt% clay/PBT

The Tables 2, 3, 4 also presented that solvent uptake decreased as the quantity of kaolin clay increased in PBT polymer matrix. This decrease in the solvent uptake with increase in concentration is might be due to some interaction between the clay and PBT, which leads to cross-linking and tortuosity in the polymer and that inhibit its solvent uptake ability.