Investigations of polyamide nano-composites containing bentonite and organo-modified clays: Mechanical, thermal, structural and processing performances

2.1 Materials

The commercial polyamide 6 (Bergamid B65 W25) was supplied from PolyOne, USA. Dry ground Na-activated bentonite (EBNT SA), amino functional silane treated bentonite (EBNT) and organo-modified clays, NC 130 (EsanNANO 1–130) and NC 140 (EsanNANO 1–140), were obtained from Esan Eczacıbaşı Industrial Raw Materials Co., Istanbul, Turkey. NC 130 and NC 140 clays were obtained as modified by producer using quaternary ammonium salts involving dimethyl and dimethyl benzyl units, respectively.

2.2 Preparation of Composites

PA and additives were pre-dried at 80°C for 2 hours prior to compounding. The preparation of composites was carried out using a lab-scale twin-screw micro-extruder (MC 15 HT, DSM Xplore Instruments) with the loading ratios ranging from 1.0% to 20.0% by weight. Process temperature, screw speed and mixing time were 230°C, 100 rpm and 5 min, respectively. The unfilled PA was also mixed under the same conditions in order to provide clear comparison with its composites. PA and relevant composites were cut into chip form after extrusion process. Test samples were shaped by an injection molding instrument (Micro-injector, Daca Instruments) in which barrel temperature of 235°C and the injection pressure of 5 bar were applied. Injection-molded dog-bone shape specimens with the dimensions of 7.4×2.1×80 mm³ were obtained from the injection molding process.

2.3 Characterization Methods

Force measurements of samples were performed by DSM Xplore software during extrusion process. The screw force values in the melt were determined by the function of mixing time using rheological software of the micro-compounder. In order to investigate structural properties of samples X-ray diffraction analysis (XRD) was utilized by X’Pert PRO, PAN analytical with Ni-filtered CuKα radiation (λ = 0.154 nm) at 45 kV and 40 mA. JEOL JSM-6400 electron microscope was utilized to visualize the dispersion of BNT and NC particles into PA matrix. Surfaces of fractured samples obtained from impact test were coated with gold to create conductive surfaces. SEM micrographs were taken at x1000 and x4000 magnifications. Density measurements were conducted to composite samples using a digital density meter (Easy D30, Mettler Toledo). All the results represent an average value of three samples for thermal tests (TGA and DSC) and density measurements. The characterizations of the tensile properties were carried out using Lloyd LR 30 K universal tensile testing machine by using load cell of 5 kN at crosshead speed of 5 cm/min according to the ISO 37 standard. Tensile strength, percentage elongation at break and tensile modulus values were recorded. Impact test was performed according to ASTM D256 standard with 4J pendulum using Coesfeld material impact tester. All the results represent an average value of five samples with standard deviations. Shore hardness of composites was evaluated by TQC Sheen BV hardness tester according to ISO 868 standard. Thermal behaviors of samples were determined using Netzsch Jupiter STA 449 F3. TGA and DSC measurements were carried out in the temperature range of 25°C to 600°C with a heating rate of 10°C/min under nitrogen flow of 50 ml/min.

3.1 Structural analysis

Structural properties of BNT and NC in their bulk form and into PA matrix were studied by XRD analysis. Test results of bulk and composite samples are represented in Figure 1a and Figure 1b, respectively. The distinct band located at nearly 2θ = 6° as can be seen for NC samples in Figure 1a. The disappearance of these characteristic bands completely for PA/NC 130 1 and PA/NC 140 1 corresponds to intercalated structure of clay layers into PA matrix [18]. The characteristic Bragg peaks of nanoclay-filled composites at nearly 2θ = 10° (Figure 1b) show increase in d-spacing values associated with the enhancement dispersion with the help of quaternary ammonium salts. NC 130 and silane-modified BNT exhibit peaks at higher values confirming enlargement of interlayer spacing of the mineral. Inclusions of NC 140 with 1% and NC 130 with 3% amounts cause intercalation and exfoliation structures in PA matrix.

Figure 1

XRD results of bulk and composite samples.

The representative SEM micrographs of composites are displayed in Figure 2. Si-BNT particles show slightly higher dispersion homogeneity than BNT particles according to their SEM images. Agglomerations are observed for NC 130 and NC 140 containing composites at their highest loading ratio (20%) as indicated by circles in SEM micrographs. On the other hand, large aggregates disappear for 1% concentration of NC loaded composites which arises from the more homogeneous dispersion of clay layers into PA phase at low adding amount. SEM images of composites provide visual evidence for previously discussed XRD finding based on the separation of the clay layers in PA matrix.

Figure 2

SEM micrographs of composites.

3.2 Force measurements

Shear force values are important parameters in order to determine the production cost during processing of the polymeric materials in industrial applications where melt blending is the preferred production method. According to force values recorded during extrusion process of materials (Figure 3), bentonite additions caused increase for shear force of unfilled PA. The final shear force values of composites are improved by increase in loading ratio of bentonite. Silane modified BNT exhibits reduction for shear force by behaving like a plasticizer. NC inclusions with the highest concentration (20%) cause level up for exerted force due to the formation of agglomerates. According to these results, increase in concentration of BNT and NC led to obtain higher amount of exerted force stem from the formation of elevated shear rates during extrusion by the addition of higher levels of additives [19, 20].

Figure 3

Force values of PA and composites.

3.3 Tensile properties

The tensile test characteristics of composites are investigated by the stress–strain curves shown in Figure 4 for PA/BNT, Figure 5 for PA/NC composites and the relevant test data are listed in Table 1. It can be seen that BNT additions yield slight improvement for tensile strength values of unfilled PA. The greatest increase in tensile strength is observed for Si-BNT loaded composite thanks to enhanced surface interaction between PA and silane modified BNT. These strong chemical interactions stem from the amino group of silane coupling agent which promotes the elevation for tensile strength of composite [21, 22].

Figure 4

Stress–strain curves of BNT-filled composites.

Figure 5

Stress–strain curves of NC-filled composites.

All of the BNT-filled composites give higher tensile modulus values than that of PA. Elongation of PA also increases with the incorporation of BNT at 5% concentration. NC additions at low filling ratios exhibit increasing trend for tensile strength of unfilled PA. Reduction of tensile strength after 3% loading level of NC is attributed to stress concentrator action of organoclay agglomerates [23].

Tensile modulus of PA is not affected by NC inclusions. However, BNT additions caused slight increase in modulus of unfilled PA. These results are agreement with the theoretical predictions of tensile modulus in literature [24,25,26]. NC containing composites give increase drastically for elongation values at their lower amounts. These findings imply that layered structure of NC causes plasticizing effect for PA matrix [27, 28].

3.4 Hardness test

Shore hardness is a characteristic parameter for polymeric materials. The Shore D hardness values of PA and its composites are listed in the last column of Table 1. It can be seen from these values that NC and BNT additions lead to significant increase in Shore D hardness of PA. The highest loading levels of additives display around 10 points higher hardness values than that of unfilled PA. Silane treated BNT addition causes reduction for hardness compared to untreated BNT. Similar results are observed from literature that BNT addition yields higher Shore hardness values compared to neat polymer [29, 30].

3.5 Impact resistance

The impact energy values of composites are demonstrated in Figure 6. Impact energy value of PA exhibits reduction trend with BNT additions. Impact energy decreases with the increase in loading level of BNT. This trend may be described by the formation of stress failures between BNT and PA matrix. BNT particles cause restriction for deformation ability of PA phase which results in decrease for the total free energy of the composite system. Silane modified BNT yield slightly higher impact energy with respect to unmodified BNT thanks to formation of strong interaction between amino functional group of silane layer and PA phase [31]. Impact energy of PA is enhanced about 15 points higher level with the addition of the lowest filling ratios for NC 130 and NC 140. NC 140 loaded PA gives slightly higher impact energies compared to NC 130. PA gains toughness which means ability to absorb more energy during deformation after incorporation with 1% of NC 130 and 1% of NC 140 thanks to homogeneous dispersion of clay layers.

Figure 6

Impact energy values of PA and composites.

3.6 Density measurements

In addition to a high level of mechanical performance, lightweight composite parts are highly required by automotive manufacturers. Nanocomposites exhibit resistance to impact and tensile deformations as mentioned earlier discussions besides providing weight saving behavior in transporting applications. Density values of PA and composites are listed in Table 2. According to these results, BNT leads to an increase in density values of composites since it has a higher density relative to PA. NC containing composites display reduction trend for densities compared to unfilled PA. The inclusion of NC 130 at its lowest concentration gives the best weight reduction value which may be important for automotive applications. However, effective density of composites depends on the dispersion homogeneity of BNT and NC. The presence of agglomerated filler particles in measured part of samples may lead to obtaining higher bulk density values compared to the effective density of homogeneously dispersed composite system [32]. Accordingly, sharp improvement in density values of 20% NC loaded composites corresponds to formation of large agglomerates of nanoclay in PA phase as confirmed by SEM images of these candidates (Figure 2).

3.7 Thermal properties

DSC curves of PA/BNT, PA/NC 130 and PA/140 composites are given in Figure 7a, Figure 7b and Figure 7c, respectively. DSC data including melting temperature (T_m) of PA and composites are indicated in Table 3. BNT additions cause shifting of T_m value for unfilled PA to nearly two-point higher temperatures. However, silane-modified BNT gives slightly lower Tm compared to neat BNT. NC inclusions also lead T_m of unfilled PA to higher temperatures. The highest T_m is obtained for nano-composites reinforced with the lowest NC concentration since the segmental motion of the PA chains may be prevented by the intercalated or exfoliated clay layers at their lower loading levels [33]. NC incorporated samples for their low concentrations display relatively higher T_m values. However, melting temperature of nano-composites containing 20%wt of NC 130 and NC 140 exhibit nearly identical T_m with that of unfilled PA. The intensity and area of the endothermic peak indicate the enthalpy of melting which is attributed to the melting of PA crystals [34, 35].

Figure 7

DSC curves of PA and composites.

According to Figure 7, 1%wt of NC additions display slight increase or no change for the melting enthalpy of unfilled PA. On the other hand, reduced intensity and area were observed for 20%wt of NC filled composites with respect to PA. This result is related to the nucleation effect of NC at their lower concentrations due to the formation of intercalated clay layers into PA structure [36].

TGA curves of PA/BNT, PA/NC 130 and PA/140 composites are presented in Figure 8. TGA data including decomposition temperature (T_d) and char content of samples are provided in Table 3. According to TGA curves, the weight loss of unfilled PA occurs in a single step degradation. T_d values indicate the temperatures which maximum weight loss rate occur. Enhancement of thermal stability is highly required for high temperature durability of polymer-based composites in numerous applications. Improvement of thermal stability is confirmed by shifting of T_d values to higher levels. TGA results show that, thermal stability of PA is enhanced with Si-BNT inclusions which lead to slow down for degradation rate compared to untreated BNT. Organo-clay additions cause remarkable increase for T_d at their lowest filling ratio (1%wt). The highest loading level of NC exhibits reduction for T_d regardless of organo-clay type due to disappearance of exfoliated clay layers. The char yield of unfilled PA also increases by the amount of incorporated additives increase. Similar trends are obtained in the literature related to BNT and nanoclay loaded polymer matrices [37,38,39].

Figure 8

TGA curves of PA and composites.