Influence of Reinforcing Efficiency of Clay on the Mechanical Properties of Poly(butylene terephthalate) Nanocomposite

Abstract

In contrast to traditional fillers, clay, in particular, natural smectite clay, represents an environmentally significant alternative to improve the properties of polymers. Compared to conventional nanofillers, smectite clay can effectively enhance the physical and mechanical properties of polymer nanocomposites with a relatively small amount of addition (<5 wt%). The present study focuses on investigating the reinforcing efficiency of different amounts (up to 5 wt%) of a natural Brazilian smectite clay modified (MBClay) on the mechanical properties of poly(butylene terephthalate) (PBT) nanocomposites and also evaluates the correlation between MBClay addition and the mechanical and thermal behaviors of the PBT/MBClay nanocomposites. Natural Brazilian clay modified by the addition of quaternary salt and sodium carbonate (MBClay) was infused into the PBT polymer by melt extrusion using a twin-screw extruder. It was found that the best properties for PBT were obtained at 3.7 wt% of modified BClay. Tensile strength at break exhibited increased by about 60%, flexural strength increased by 24%, and flexural modulus increased by 17%. In addition, an increase in the crystallinity percentage of PBT/BClay nanocomposite was confirmed by DSC and XRD analysis, and a gain of about 45% in HDT was successfully achieved due to the incorporation of 3.7 wt% of MBClay.

1. Introduction

Despite the widespread examination of polymer/clay nanocomposites for a long time, there is an increased focus on developing naturally occurring nanoparticles, such as clays, as reinforcement polymerics. The current challenge to reduce the environmental impact of polymeric nanocomposites has geared efforts to increase the application of polymeric nanocomposites with natural fillers [1,2,3,4,5,6]. With the growing demand for polymer composites in various industries, innovative approaches have become quintessential to developing sustainable and environmentally friendly nanocomposite materials. Thus the properties of polymeric nanocomposites reinforced with clay and other nanoparticles derived from renewable sources are currently the subject of extensive research [6,7,8,9,10,11,12,13,14,15,16,17,18].

Owing to their layered structure and high intercalation chemistry, smectite clays from renewable sources became an attractive substitute for conventional nanofillers in polymeric nanocomposites with desirable properties [1,2,3,6,19]. Aside from their environmental sustainability, large availability and low cost made smectite clays a viable alternative to conventional nano-reinforcements [1,2]. A number of semicrystalline polymer matrices, such as poly(butylene terephthalate) (PBT), when reinforced by clay, have demonstrated desirable characteristics for a variety of practical applications. Especially their significant improvement in heat distortion temperature, impact strength, and modulus has gained industrial interest [20,21,22,23,24,25,26], and because of its easy processability, good mechanical properties, excellent dimensional stability, high stiffness, and hardness, PBT has been a highly desirable engineering thermoplastic for injection molding. PBT has been widely utilized in various applications, such as insulators in electrical and electronic industries [20,21,22,23,24,25,26]. Its superior thermal and mechanical properties and excellent behavior in micro-molding processes, 3D printing, and additive manufacturing have increased the use of PBT for engineering materials. Predominantly, PBT is used in the automotive industry for applications such as connectors and sensors. The combination of rigidity and solvent resistance of PBT has made it a major contributor to the personal computer connector industry [23,24,25,26,27,28,29,30].

Although PBT has good mechanical properties and thermal stability, these are still insufficient for its potential applications in a wide range of industrial fields. Thus, the incorporation of different micro and nano-size fillers into PBT has been evaluated in order to develop high-performance PBT composite materials applicable to advanced industries [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]. Additionally, the incorporation of only a small amount of clay into PBT can increase the low-impact strength and heat distortion temperature and reduce the brittleness and cost, widening the field of applicability of this polyester [24,25,30]. Positive characteristics in the end product of PBT–clay nanocomposites may include several aspects: chemical resistance, surface appearance, improved fire retardancy, electrical and thermal conductivity, and mechanical properties [23,24,25]. Chang et al. prepared Poly(butylene terephthalate (PBT))/clay nanocomposites by melt intercalation employing a twin-screw extruder [35]. In their study, a low-viscosity PBT (η = 0.74 dL g⁻¹) and a high-viscosity PBT (η = 1.48 dL g⁻¹) were used. The authors observed that the PBTs might be intercalated into the organoclay, with the intercalation occurring more extensively for the PBT matrix with high viscosity. The results showed that clay addition caused a considerable increase in the mechanical and dynamic mechanical properties, raising the PBT non-isothermal crystallization rates. Oburoğlu et al. studied the melt-crystallization behavior of poly(butylene terephthalate) (PBT) composites with 5 wt% of fillers using commercial grades of calcite, halloysite, and organo-montmorillonite [36]. According to their results, the production rate of the injection molded parts could be significantly reduced when organically modified alumina-silicate layers are added to PBT-based composites.

The present study focuses on developing PBT/MBClay nanocomposites by melt extrusion using a twin-screw extruder. The contribution of this work is to provide additional insight into the reinforcing efficiency of the incorporation of different amounts (up to 5 wt%) of a natural Brazilian smectite clay modified (MBClay) on the mechanical properties of PBT nanocomposites and also evaluates the correlation between MBClay addition and the mechanical and thermal behaviors of the PBT/MBClay nanocomposites.

2. Materials and Methods

2.1. Materials

The materials used in this study were PBT resin (Celanex 1600A- commercial grade by Celanese Corporation, Dallas, TX, USA) with MFI = 6.5 g/10 min at 190 °C/2.16 Kg (ASTM D1238), specific density = 1.31 g/cm³, and natural Brazilian smectite clay denominated Brazilian chocolate clay (BClay), from Boa-Vista, PB, Brazil.

2.2. Clay Preparation

Brazilian chocolate clay (BClay) is a smectite clay (2:1 layered silicates) polycationic with calcium ions (Ca²⁺) occupying, predominantly, the interlayer space. The as-received BClay was modified by the addition of quaternary ammonium salt and sodium carbonate as a source of Na⁺ ions to replace the Ca²⁺ ions [42,43]. Firstly, the clay was dispersed in deionized water (4 wt% of clay), and Na₂CO_3, at a concentration of 100 meq/100 g of clay, was slowly added to the suspension. Then, the suspension was stirred for about 30 min at 97 °C. After that, an aqueous solution of quaternary ammonium salt was added to the suspension containing sodium smectite clay (chocolate clay-Na) at a concentration equivalent to 1.1 CEC (cation exchange capacity) of the sodium clay. After stirring for 30 min, at room temperature, the suspension was filtered and washed with deionized water. The organophilic clay (MBClay) was then dried at 60 °C for 48 h, ground, stored at room temperature, and, finally, characterized. The experimental details of the clay preparation are described in Paiva et al. and Delbem et al. [42,43]. A schematic of the clay preparation is illustrated in Scheme 1.

2.3. Moisture Content of Samples

Moisture in a plastic material can cause several undesirable effects, such as processing problems, poor mechanical properties, or visual defects in injected parts. In order to prevent moisture from causing problems in the PBT/MBClay nanocomposite, the neat PBT and MBClay were kept in a laboratory oven at 100 °C before being processed. Moisture content was measured using a Mettler Toledo HR83 moisture analyzer. The moisture content in neat PBAT and BClay samples was measured for as received samples and for the samples after 4 h of drying in the laboratory oven. Since the PBT manufacturer recommends that the moisture content of the PBT for processing be below 0.02%, samples with a moisture content of 0.01% were considered suitable for processing [44].

2.4. Nanocomposite Preparation

PBT/MBClay nanocomposites containing different amounts (up to 5 wt%) of modified Brazilian chocolate clay (MBClay) were prepared according to compositions listed in

Table 1. Nanocomposites composition.

Samples	PBT (%wt)	MBClay (%wt)
Neat PBT	100	0
PBT/MBClay 3.0%	97.0	3.0
PBT/MBClay 5.0%	95.0	5.0

using a co-rotating twin-screw extruder with L/D= 25 “ZSK 18 Megalab” made by Coperion Werner & Pfleiderer GmbH & Co., Stuttgart, Germany. The temperature profile was 200/215/220/225/230/235 °C, and the screw speed was 60 rpm. The resulting extrudates were cooled down for better dimensional stability, pelletized, dried at 100 ± 2 °C, for 4 h in a circulating air oven, and fed into an injection molding machine Sandreto 430/110 to obtain standard ASTM test specimens. A schematic of the nanocomposite preparation is illustrated in Scheme 2. For this work, the loading of 3 and 5 wt% of MBClay was selected for the preparation of PBT/MBClay nanocomposites by direct melt intercalation method based on our screening experiments. These experiments showed that MBClay contents smaller or equal to 2.3 wt% did not have any significant effects on the properties of neat PBT. The effects of MBClay addition on the properties of PBT/MBClay nanocomposites are presented in the Supplementary Materials associated with this manuscript.

2.5. Characterization of Brazilian Clay

2.5.1. Measured Clay Content

The measurement of clay content in the PBT matrix was performed using a muffle furnace according to ASTM D—5630 standards. A sample of 10g of each PBT/MBClay formulation obtained from the extrusion process was weighed, placed in crucibles, taken to the muffle furnace, and heated to 600 °C for one hour until the material was completely incinerated. To extract the clay content of each PBT/MBClay formulation, the porcelain crucibles containing the clay content were weighed again after the material was completely burned and cooled. The percentage of clay content in each PBT/MBClay formulation was calculated by Equation (1)

$$X_{MBClay}(\%)=100-\frac{W{I}_{Sample}-W{F}_{Sample}}{W{I}_{Sample}}\times 100$$

(1)

where X_MBClay (%) is the percentage of clay content in each PBT/MBClay formulation; WI_Sample is the initial weight of the sample (weight of porcelain crucibles with PBT/MBClay), WF_Sample is the final weight of the sample (weight of porcelain crucibles with burned material).

2.5.2. X-ray Diffraction (XRD) of Clay

The interlayer distance of the natural and modified BClay was determined by XRD. The XRD patterns of natural and modified (MBClay) were recorded on a Simens—D5000 diffractometer operated at 40 kV and 40 mA, with CuKα radiation (λ = 1.54 Å), with 2θ varying between 2° to 35°. Bragg’s equation (Equation (2)) was used to determine the interlayer distance in the crystal of the natural and MBClay:

$$d=\frac{\lambda }{2sin\theta }$$

(2)

where d is the distance between atomic layers in crystals; λ is the wavelength of the beam of X-Ray; θ is the characteristic diffraction peak of MBClay.

2.5.3. Transmission Electron Microscopy (TEM)

TEM analysis of natural and MBClay was performed using a JEOL-2010 TEM. Natural and modified BClays were dispersed in an ethanol solution, and then a drop of the solution was distributed on a copper grid which was analyzed in TEM with an operating voltage of 80 kV.

2.6. Characterization of Neat PBT and PBT/MBClay Nanocomposites

2.6.1. X-ray Diffraction (XRD) of Composites

The XRD analysis of neat PBT and PBT/MBClay nanocomposites was performed using a Rigaku-Denki MultiFlex diffractometer (Rigaku Denki Co. Ltd., Tokyo, Japan) with CuKα radiation (λ = 1.5406 Å) at 40 kV and 20mA, with 2θ varying between 2° to 35°.

2.6.2. Microscopy Analysis of Composites

The scanning electron microscopy (SEM) of cryofractured neat PBT and PBT/MBClay sample surfaces was performed using LX 30 (Philips) instrument. The fractured surface of the sample was coated with a fine layer of gold prior to observation.

A small sample was microtomed using a Leica EM UC6 at room temperature to obtain ultra-thin PBT/MBClay specimens for TEM. The analysis of PBT/MBClay nanocomposites was performed using a JEOL-2010 TEM.

2.6.3. Differential Scanning Calorimetry (DSC)

DSC analyses of neat PBT and PBT/MBClay nanocomposites were carried out using an SDT Q 600 (TA Instruments) on four weighed samples with 5.0 ± 0.5 mg of material. Samples were heated from 25 to 300 °C at a heating rate of 10 °C/min (in an oxygen atmosphere). The scans were taken from the second heating cycle to eliminate any thermal history of the samples. Crystallinity was calculated from melting peak areas. The percentage of crystallinity (χ_c) of nanocomposite material was calculated using Equation (3), where ΔH_m is the melting enthalpy of the PBT/MBClay nanocomposites. ΔH_m^o is the initial melting enthalpy of the PBT assuming 100% crystallinity which is 140 J/g, W_PBT is the mass fraction of the PBT in the nanocomposites; χ_c is the percentage crystallinity of PBT in PBT/MBClay nanocomposites. Statistical analyses of the samples were determined with a one-way analysis of variance (ANOVA) and Tukey’s multiple-comparison tests using the OriginPro software version 9.1. The mean values were considered to be significantly different at a 95% confidence level (p < 0.05).

$${\chi }_c=\frac{\mathsf{\Delta }{H}_m}{\mathsf{\Delta }{H}_m^o\times W_{PBT}}\times 100\%$$

(3)

2.6.4. Thermogravimetric Analyses (TGA)

TGA of neat PBT and PBT/MBClay nanocomposites was carried out using an SDT Q 600 (TA Instruments). TG analyses of the materials were performed on three weighed samples with 5.0 ± 0.5 mg of the materials. Samples were heated from 25 to 600 °C at a heating rate of 10 °C/min (in an oxygen atmosphere). The statistical approach was made employing a one-way analysis of variance (ANOVA) and Tukey’s multiple comparison tests (p < 0.05; 95% confidence) using the OriginPro software version 9.1.

2.6.5. Tensile Tests

The tensile tests of neat PBT and PBT/MBClay nanocomposites were conducted according to ASTM D 638 standards to obtain tensile strength, Young’s modulus, and elongation at break. An Instron Testing Machine model 5564 at 23 °C was used. An average value of at least five specimens for each formulation was taken and recorded. Tensile results were subjected to one-way variance analysis (ANOVA) and Tukey’s tests (p < 0.05; 95% confidence) using the OriginPro software version 9.1.

2.6.6. Flexure Tests

The fracture resistance and elasticity of materials under stress are evaluated by the determination of properties of flexural strength and flexural modulus. Flexural strength is the ability of a material to resist bending failure stress, and the flexural modulus is the measure of the stiffness of the material to bending deformation. The flexural tests (three-point bending) were determined according to ASTM D790, using 5 specimens with 3.2 mm thickness, 9.5 widths, and 165 mm length for each formulation, with an EMIC DL 2000 universal testing machine. Flexural results were subjected to one-way variance analysis (ANOVA) and Tukey’s tests (p < 0.05; 95% confidence) using the OriginPro software version 9.1.

2.6.7. Izod Impact Tests

Izod impact strength is the measure of the ability of a material to absorb energy on collision. Notched Izod impact tests of neat PBT and PBT/MBClay were performed according to ASTM D 256 with a Ceast Resil impact tester. At least five specimens were tested for each formulation, and the average values were taken. Izod impact results were subjected to one-way variance analysis (ANOVA) and Tukey’s tests (p < 0.05; 95% confidence) using the OriginPro software version 9.1.

2.6.8. Heat Distortion Temperature Tests

HDT of a polymeric material is a measure of a polymer’s resistance to distortion under a given load at elevated temperature. That is, HDT values represent the upper limit of the dimensional stability of polymers in service without significant physical deformations under a given load at elevated temperatures. HDT tests of neat PBT and PBT/MBClay were performed according to ASTM D 648. The mean values of at least five specimens were reported. HDT results were subjected to one-way variance analysis (ANOVA) and Tukey’s tests (p < 0.05; 95% confidence) using the OriginPro software version 9.1.

2.6.9. Clay’s Reinforcing Efficiency

To achieve good mechanical properties, several studies indicate that clay nanocomposites, such as other nanocomposites, need to exhibit good dispersion, exfoliation, surface compatibilization, and also good stress transfer between the clay and the polymer matrix. The existence of agglomerations significantly reduces the effective aspect ratios of clay, causing stress concentration phenomenon and preventing efficient load transfer from the reinforcing phase to the polymer matrix phase, consequently reducing the mechanical properties of clay nanocomposites. Therefore, the reinforcing efficiency of MBClay on the mechanical properties of PBT/MBClay nanocomposites depends more specifically on clay dispersion and exfoliation and the presence of agglomerations than on the MBClay content. In order to quantify the reinforcing efficiency of the MBClay on the mechanical properties of nanocomposites, the reinforcing efficiency of MBClay was defined as the normalized mechanical properties of PBT/MBClay nanocomposites (M_N) with respect to those of neat PBT (M_PBT) as shown in Equation (4), and the reinforcing efficiency graph was constructed:

$$Reinforcing efficiency (\%)=\frac{M_N-M_{PBT}}{M_{PBT}}\times 100$$

(4)

where M_N is the mechanical property of PBT/MBClay nanocomposites, and M_PBT is the mechanical property of neat PBT.

3. Results and Discussion

3.1. Brazilian Clay Characterization Results

3.1.1. Measured Clay Content

Table 2. Corrected nanocomposite compositions.

Samples	PBT (%wt)	MBClay (%wt)
Neat PBT	100	0
PBT/MBClay 3.7 wt%	96.3	3.7
PBT/MBClay 4.9 wt%	95.1	4.9

presents the corrected nanocomposite composition, according to the percentage of clay content in each PBT/MBClay formulation calculated by Equation (1).

3.1.2. XRD Analysis

The XRD patterns of natural and modified BClay are shown in Figure 1. XRD pattern of natural BClay shows a peak at 2θ = 6.58, which corresponds to an interlayer distance (d₀₀₁) of 1.34 nm. However, the pattern of modified BClay (MBClay) gives a significant peak at 2θ of 4.58 that corresponds to the d₀₀₁ distance expanded to 1.93 nm, which suggests that the interlayer distance (d₀₀₁) of MBClay increased after modification. This increase confirms the intercalation of the quaternary ammonium cation in the interlamellar spacings of the MBClay. These results are also consistent with other modified clays [36,42].

3.1.3. TEM Analysis

TEM images of natural and modified BClays are shown in Figure 2. The natural BClay shows a crystalline structure with agglomerated particles and with irregular sizes and shapes, which is shown in Figure 2a,b. Figure 2c,d shows the TEM images of the modified BClay with layered crystalline structure showing a homogeneous distribution of particles with reduced size and the absence of agglomerations.

3.2. PBT/MBClay Nanocomposite Analysis

3.2.1. XRD Analysis of Composites

Figure 3 shows the XRD pattern for the modified BClay (MBClay), neat PBT, and PBT/BClay nanocomposites for 2θ varying between 2° and 35°. The characteristic and intense peaks for the neat PBT are observed at 2θ positions of 15.75° and 17.23°, corresponding to the β-crystallite form of PBT, and at 2θ of 20.34°, 21.41°, 23.40°, and 24.96°, corresponding to the α-crystallite form of PBT as shown in Figure 3a [44,45]. These characteristic crystalline peaks were also observed for the PBT/MBClay with 3.7–4.9 wt% of MBClay loadings. However, their positions were slightly shifted to lower angles. In addition to that, a very small peak is observed at 2θ = 4.58° for PBT/MBClay of 3.7 wt%, which is shown in the highlighted area in Figure 3a as well as in the expanded view (Figure 3b).

A more intense characteristic MBClay peak was observed for 4.9% MBClay loading. It is evident that a small amount of MBClay agglomerates in the PBT matrix after adding 3.7 wt% of MBClay, with the agglomeration possibly increasing after adding 4.9 wt% of MBClay. Consequently, XRD results indicate dominant exfoliation of MBClay layers and formation of intercalated structures in PBT/MBClay nanocomposite samples containing low percentages of MBClay (3.7 wt%). However, when the amount of MBClay increased to 4.9 wt%, the dispersion of MBClay in the PBT matrix became difficult, and the agglomerates on the matrix surface prevailed under their exfoliation. The SEM images in the later sections confirm the effects of the increased MBClay wt% incorporated into the PBT matrix in the formation of agglomerates.

3.2.2. Surface Morphology of the Composites

The dispersion state of the addition of MBClay into the PBT matrix was confirmed further by using the SEM and TEM. SEM micrographs of cryo-fractured surfaces of PBT and PBT/Clay nanocomposite are compared in Figure 4. As shown in Figure 4b,c, PBT/Clay nanocomposite depicts a slightly rough cryo-fractured surface compared to neat PBT (Figure 4a). However, from Figure 4b, it is clearly evident that the clay particles are uniformly dispersed in the polymer matrix, whereas Figure 4c shows that the clay particles are agglomerated in a polymer matrix surface with the increased amount of MBClay in the PBT matrix.

TEM analysis was carried out to further analyze the homogenous dispersion of the clay particles in a polymer matrix surface with a 3.7 wt% loading, as shown in Figure 4b. TEM micrographs of PBT/Clay nanocomposite with 3.7 wt% of MBClay were shown in Figure 5. The dark lines are the intersections of thick MBClay sheets, and the spaces between the dark lines are presumed to be interlayer spaces.

Some of the MBClay layers show individual dispersion of delaminated sheets in the PBT matrix. In addition to that, a region where the regular stacking arrangements are maintained with a layer of polymer between the sheets is also shown. Although a face-to-face layer morphology is retained, the layers are irregularly separated by a polymer. From the results of XRD and TEM micrographs, it is clear that the morphology of PBT/Clay nanocomposites with 3.7 wt% of MBClay presents a mixture of intercalated and partially exfoliated structures. This result indicates that PBT chains have diffused into the gallery of the MBClay, and the MBClay has been successfully intercalated in the PBT matrix.

3.2.3. Thermal Analysis

The melting enthalpy ΔH_m, melting temperature T_m, and crystallinity, χ_c, percentage obtained from the DSC analysis for the neat PBT and PBT/MBClay nanocomposites were summarized in

Table 3. Melting enthalpy, ΔH_m, melting temperature, T_m crystallinity, χ_c (%), onset degradation temperature, and total weight loss for the neat PBT and PBT/MBClay nanocomposites.

Materials	ΔHm (J/g)	χc (%)	Tm (°C)	Onset Temp (°C)	Total Weight Loss (%)
Neat PBT	41.0 a	29.2 a	212.9 a	332.5 a	89.6 a
PBT/MBClay (3.7 wt%)	45.6 b	34.1 b	213.7 b	331.2 b	83.6 b
PBT/MBClay (4.9 wt%)	46.1 c	34.6 c	219.1 c	337.2 c	82.5 c

Different lowercase letters in the same column indicate significant differences (p < 0.05) between the samples (ANOVA and Tukey’s multiple-comparison tests).

. The results showed that melting enthalpy, melting temperature, and crystallinity percentage of PBT/MBClay nanocomposites increased significantly (p < 0.05) when compared with those of neat PBT.

As shown in Table 3, the melting enthalpy values of neat PBT increased up to 12%, and the crystallinity percentage of neat PBT increased from 29.2% to 34.6% with the MBClay infusion. This is attributed to the nucleation effects of clay and the improvement in the crystal perfection of PBT. A similar observation is reported by Chow, W. S. for the PBT/Montmorillonite [24].

The results of the thermogravimetric analyses (TGA), onset temperature, and total weight loss were also presented in Table 3. According to the results, the addition of MBclay into PBT likewise affected significantly (p < 0.05) the onset degradation temperature and total weight loss of PBT/MBClay nanocomposites compared to neat PBT. The total weight loss was up to 7% less than neat PBT.

3.2.4. Tensile Test

The tensile test results for the neat PBT and PBT/ MBClay nanocomposites are shown in

Table 4. Tensile test results for the neat PBT and PBT/MBClay nanocomposites.

Tensile Parameters	Neat PBT	PBT/MBClay 3.7 wt%	PBT/MBClay 4.9 wt%
Tensile stress at yield (MPa)	59.2 ± 4.1 a	60.6 ± 1.5 b	48.4 ± 1.8 c
Tensile strength at break (MPa)	38.0 ± 3.4 a	60.1 ± 1.9 b	47.9 ± 1.6 c
Young’s modulus (GPa)	2.5 ± 0.1 a	2.7 ± 0.1 b	2.6 ± 0.1 c
Elongation at break (%)	161.6 ± 35 a	21.1 ± 1.0 b	20.5 ± 1.3 c

Different lowercase letters in the same line indicate significant differences (p < 0.05) between the samples (ANOVA and Tukey’s multiple-comparison tests).

. There is a significant dependence of the tensile properties of PBT/MBClay nanocomposites on the MBClay content. The incorporation of a very small quantity (3.7 wt%) of MBClay into PBT can substantially improve the tensile properties of PBT/MBClay nanocomposites.

The PBT/MBClay nanocomposites with 3.7 wt% of MBClay exhibited higher tensile properties compared to neat PBT and PBT/MBClay nanocomposites with 4.9 wt% of MBClay. The tensile stress at yield, tensile strength at break, and Young’s modulus of PBT/MBClay nanocomposites with 3.7 wt% of MBClay were significantly increased by 2.4%, 58.2%, and 8.0%, respectively, compared to neat PBT. This can be attributed to the stiffness and nanoreinforcing effects of MBClay and the good interfacial interaction between the MBClay and PBT matrix. The MBClay is able to act as a nanoreinforcing filler due to its high aspect ratio, platelet structure, and uniform dispersion in the PBT matrix. A smaller increase effect was observed at higher MBClay content (4.9 wt%) compared to MBClay with 3.7 wt%. The improvement in the mechanical properties of PBT/MBClay nanocomposites was not increased at higher MBClay content as expected, in comparison to that of smaller MBClay content because of intrinsic Vander Waals attractions between individual platelets within MBClay, and it is possible that they tend to group together. As a result of the high aspect ratio and large surface area, it is difficult for their dispersion in the matrix to be uniform, causing agglomerations. The agglomerations thus formed can lead to the stress concentration phenomenon, preventing efficient load transfer to the PBT matrix. The elongation at break of PBT/MBClay nanocomposites was smaller than that of neat PBT, likely due to the presence of agglomerated and intercalated MBClay layers in the PBT matrix. Similar tensile behaviors were reported by other studies [24,26,41,46].

3.2.5. Flexural Test

The flexural test results for the neat PBT and PBT/MBClay nanocomposites are shown in

Table 5. Flexural test results for the neat PBT and PBT/MBClay nanocomposites.

Tensile Parameters	Neat PBT	PBT/MBClay 3.7 wt%	PBT/MBClay 4.9 wt%
Flexural strength (MPa)	74.2 ± 3.2 a	91.7 ± 2.8 b	78.7 ± 3.6 c
Flexural modulus (GPa)	2.4 ± 0.2 a	2.8 ± 0.1 b	2.5 ± 0.7 c

Different lowercase letters in the same line indicate significant differences (p < 0.05) between the samples (ANOVA and Tukey’s multiple-comparison tests).

. The flexural strength and flexural modulus of PBT/MBClay nanocomposites also increased with the introduction of MBClay. This enhancement of the flexural strength and modulus was attributed to the reinforcement of PBT due to the incorporation of the dispersed MBClay. The enhancing effect of the flexural properties by incorporating MBClay was more significant at low MBClay content, indicating that a low MBClay loading was more effective in improving the overall mechanical properties of PBT/MBClay nanocomposites. The high MBClay content resulted in low intercalation, poor dispersion in the PBT matrix, and the presence of agglomerations. From these results, it is apparent that due to the increased MBClay content, the stiffness of the PBT/MBClay nanocomposites decreases gradually under flexure loading, which means that smaller resistance to bending deformation as the content of MBClay increases. Similar results have been reported in the literature [16,24,47,48,49,50].

3.2.6. Izod Impact Test

Unlike the tensile and flexural test results, the notched Izod impact results of PBT presented a large decrease with increasing MBClay content, with PBT/MBClay nanocomposites having the lowest impact strength of 47.2 J/m, ca. 35% lower than neat PBT, for MBClay with 3.7 wt%. The Izod impact strength of PBT/MBClay nanocomposites with 4.9 wt% of MBClay was observed to be 37.4 J/m, ca. 48.5% lower than neat PBT. Izod impact test results of neat PBT and PBT/MBClay are presented in

Table 6. Izod impact and HDT test results for the neat PBT and PBT/MBClay nanocomposites.

Test	Neat PBT	PBT/MBClay 3.7 wt%	PBT/MBClay 4.9 wt%
Izod Impact (J/m)	72.6 ±2.1 a	47.2 ± 1.4 b	37.4 ± 1.1 c
HDT (1.82 MPa) (°C)	55.4 ± 3.2 a	80.1 ± 6.2 b	69.7 ± 5.3 c

Different lowercase letters in the same line indicate significant differences (p < 0.05) between the samples (ANOVA and Tukey’s multiple-comparison tests).

. This reduction in impact strength can be attributed to a lack of impact energy absorption, transfer, and dissipation due to poor interfacial bonding between the MBClay and PBT. Moreover, the presences of MBClay agglomerates act as strong stress concentration sites and might contribute to crack propagation with low energy dissipation. At higher MBClay content, the reduction in the mobility of matrix molecules contributes to even smaller impact strength. It is observed that the Izod impact strength has a similar trend to elongation at break, where the addition of MBClay caused the attenuation of the properties. The low elongation at break shows that the area-under-the-curve value in the stress-strain curve is small, indicating that the PBT/MBClay nanocomposites have limited capability to absorb energy. This result was consistent with other studies [3,34,41,51,52].

3.2.7. Heat Distortion Temperature

HDT test results of neat PBT and PBT/MBClay are presented in Table 6. As shown in this Table, the HDT values of PBT/MBClay nanocomposites increased with increasing MBClay content. The increasing effect of the HDT value by incorporating MBClay was more significant at low MBClay content (3.7 wt%) than at high content (4.9 wt%) because of the dispersion of clay particles, the higher degree of crystallinity, intercalation, and low presence of agglomerations.

The HDT values of PBT/MBClay nanocomposites increased with increasing MBClay content. The increasing effect of the HDT value by incorporating MBClay was more significant at low MBClay content than at high content. The results showed that neat PBT has an HDT value of about 55.4 °C (HDT, 1.82 MPa), while the nanocomposite with 3.7 wt% of MBClay has an HDT value of about 80.1 °C, and the composite with 4.9 wt% of MBClay, the HDT value was about 69.7 °C. This behavior of HDT is due to the better dispersion and intercalation of clay particles, the low presence of agglomerations, and the higher crystallinity degree of nanocomposite at low MBClay content. According to the previous studies conducted by other researchers, the variation of the HDT value is closely related to the behavior of flexural modulus and onset temperature of the degradation with good dispersion of incorporated clay [2,16,22,47,50].

3.2.8. MBClay’s Reinforcing Efficiency

The variations in the reinforcing efficiency of MBClay on mechanical properties of PBT/MBClay nanocomposites are shown in Figure 6. The reinforcement of 3.7 wt% MBClay resulted in a reinforcing efficiency of 58.2% at tensile strength at break, whereas for 4.9 wt% MBClay, the reinforcing efficiency at tensile strength at break was only 26.1%. Likewise, the reinforcing efficiency of 3.7 wt% MBClay at Young’s modulus was 8%, flexural strength was 23.6%, and flexural modulus was 16.7%. The addition of 4.9 wt% of MBClay led to the reinforcing efficiency of only 4.0% in Young’s modulus, 6.1% in flexural strength, and 4.2% in flexural modulus of PBT/MBClay nanocomposites. Therefore, as shown in Figure 6, the effect of MBClay incorporation on the mechanical properties of PBT/MBClay nanocomposites was more significant at low MBClay content than at high content, indicating that a low MBClay loading was more effective in improving the overall mechanical properties of PBT nanocomposites.

From this result, it can be concluded that the incorporation of a small amount of MBClay of about 3.7% wt% in the PBT matrix was more effective in increasing the general mechanical properties of the PBT nanocomposites compared to a high amount (4.9% by weight). This is due to the uniform dispersion and high nanoreinforcing effect of MBClay on the PBT matrix at a lower concentration, which can imply an efficient interfacial adhesion between the MBClay and the PBT matrix in order to generate effective stress transfer from the matrix to the nanofiller. Hence, it is necessary to ensure good dispersion and stress transfer, to obtain high modulus and strength in the PBT/MBClay nanocomposites. In comparison, according to our screening experiments, the reinforcement efficiency of MBClay contents smaller or equal to 2.3 wt% was very small and did not have any significant influence on the properties of neat PBT (p < 0.05; 95% confidence).

4. Conclusions

The present study investigated the influence of reinforcing efficiency of incorporation of different amounts (up to 5 wt%) of a natural Brazilian smectite clay modified (MBClay) on the mechanical properties of PBT nanocomposites prepared by a melt extrusion process and also evaluated the correlation between MBClay incorporation and the mechanical and thermal behavior of the PBT/MBClay nanocomposites. TEM and XRD results suggested the formation of intercalated and exfoliated structures of PBT/MBClay nanocomposites. The incorporation of only a few amounts of MBClay, about 3.7 wt%, represented a significant gain in tensile strength at break, Young’s modulus, flexural strength, flexural modulus, heat distortion temperature (HDT), and crystallinity of PBT. The effect of MBClay incorporation into the PBT matrix was more effective at low MBClay content (3.7 wt%) compared to a high content (4.9 wt%). Under the processing conditions applied in this study (co-rotating twin-screw extruder: L/D = 25; temperature profile: 200–235 °C; screw speed: 60 rpm), MBClay exhibited high nanoreinforcing properties and achieved uniform dispersion in the PBT matrix due to also its stiffness and high nanoreinforcing effect.