Introduction

Polymeric materials provide advantages such as lightweight, high durability, ease of processing, corrosion resistance, ductility, and low cost. Polymers have inferior thermal, electrical, and mechanical qualities when compared to other materials such as ceramics and metals [1]. Adding nanoparticles with a large surface area, anisotropic shape, and high surface energy reduces the distance between interparticles in the polymer matrix. Furthermore, the nanoscale additions improve the strength of the polymer matrix connection. Because of this, polymer nanocomposites with totally new characteristics are suitable for new application areas such as aviation, electronics, construction, and packaging [2,3,4,5]. The interaction between the polymer matrix and the nanofiller results in a molecular attraction effect between nanocomposites. Despite numerous studies on the fabrication of nanocomposites that have been published, attaining homogeneous dispersion of nanoparticles in polymeric matrix remains problematic. The van der Waals attraction between nanoparticles favors the development of clusters and agglomerates in many circumstances. Furthermore, hydrophilic nanoparticles and hydrophobic polymers are incoherent, resulting in weak interfacial adhesion, poor dispersion, and poor characteristics. As a result, nanocomposites occasionally exhibited inferior characteristics to regular polymers, limiting their application range [6,7,8,9].

Nanoscale clay additions induce novel phenomena that affect material characteristics. Halloysite nanotubes (HNs) are naturally occurring, harmless, and biocompatible tubular substances. Halloysite nanotubes have a high aspect ratio, large pores, a non-swelling nature, and the capacity to regenerate. The chemical formula of HNs can be defined as Al2Si2O5(OH)4. nH2O. HN clays are naturally formed in the earth's crust over millions of years and are constituted of a double layer of aluminum, silicon, hydrogen, and oxygen [10, 11]. HNs are naturally found in many different parts of the world, including Türkiye, Australia, the United States, New Zealand, Brazil, France, and China. Halloysite is most commonly found in weathered or hydrothermally altered rocks, saprolites, and soils [12,13,14]. Because of their distinctive hollow architecture and huge cavities, HNs are a suitable natural nano-carrier. The desirable interfacial affinity between the polymer and the halloysite clay, as well as adequate halloysite dispersion in the polymer matrix, is two important parameters for determining the performance of halloysite-polymer nanocomposites [15,16,17]. When compared to other materials, HNs offer several desirable properties, including a distinct micro-spatial structure, a large length-diameter ratio, a high lumen capacity, and chemical inertness. HNs have a similar structure to carbon nanotubes (CNTs), but they offer several compelling advantages, including broadly spreadable performance, enhanced water solubility, stronger biocompatibility, and environmental friendliness [18, 19]. HN, on the other hand, cannot be distributed in a polymer matrix due to the abundance of Al–OH groups, and their comparatively substantial surface energy associated with its large surface area induces agglomeration due to the existence of van der Waals interactions between tubes [20]. Furthermore, significant shear stresses induced by techniques such as ball mill homogenization or melt extrusion to obtain fine and homogeneous dispersion of HNs in a polymeric matrix may avoid agglomeration of HNs [21,22,23,24]. In addition, to use several types of silane-coupling agents, a range of chemical groups such as epoxy, methacrylate, urea, and amine may be grafted on the surface of HNs. Halloysite's electrophilic nature can be used to improve interfacial interactions in polymer composites [25,26,27].

Polybutylene terephthalate (PBT) is a substantial, well-known commercially accessible semicrystalline engineering thermoplastic of the polyester family. It is produced by the polycondensation reaction of 1,4-butanediol with terephthalic acid [28]. PBT has a structure that is comparable to that of other aromatic polyester like polyethylene terephthalate (PET) and polytrimethylene terephthalate (PTT) [29]. PBT's overall behavior is similar to that of polyamides and PET. PBT is often recognized as polyamides' main competitor due to its superior physical properties, lower production costs, and great chemical resistance. On the other hand, it is widely acknowledged that PBT's high processability and exceptional properties make it simple to use in production lines [30,31,32,33]. PBT outperforms other polymeric materials in terms of water resistance, abrasion resistance, and solvent resistance, as well as mechanical strength, stiffness, and dimensional stability. PBT has a wide range of applications due to its remarkable qualities, including electric and electronic components, packaging, the automobile sector, notably vehicle parts in both under-the-hood and external applications, biomedical devices, and construction equipment [34,35,36,37].

Because of their nanoscale dimension and high aspect ratio, halloysite nanotubes are an excellent filler for a variety of polymers. Furthermore, halloysite nanotubes can improve a variety of polymer characteristics such as modulus, strength, stiffness, impact resistance, and mechanical performance of polymers at higher temperatures under both static and dynamic situations. In the high-temperature range of 900–1000 °C, halloysite shows exothermic action. This thermal event is thought to be caused by the creation of a separate alumina-rich phase and amorphous silica. Some studies show that traces of iron in halloysite improve composite inflammation resistance, and it may also be used as a flame-resistant material in varied polymers [38,39,40,41,42]. HNs can be intercalated with salts and organic compounds; this behavior has been thoroughly reviewed by Joussein et al. In contrast to organoclays, HNs did not need to be intercalated before their incorporation into polymers because HNs have a tubular microstructure and good dispersion [43]. Therefore, few works have focused on improving the effects of HNs on polymers using intercalation [44]. In a recent study, Akar et al. fabricated polyamide (PA) nanocomposites involving amino-silane-modified HN [45]. According to the results they obtained, low content (1% by weight) of HN additions yielded improvements in dynamic mechanical, thermal, tensile, and impact performances of composites. In their other publication, they conducted comprehensive research in which polyhedral oligomeric silsesquioxane (POSS) and HN nano-additives were incorporated with thermoplastic polyurethane matrix both separately and in hybrid forms [46]. Their findings revealed that polyurethane nano-composite samples containing HN with a concentration of 1% by weight exhibited optimum performance among prepared formulations. Accordingly, the relationship between adding the amount of HN and property enhancements for its unsaturated polyester-based composites was examined by Kim et al. They reported that 1% wt content of HN displayed superior performances in terms of mechanical properties [47]. Moreover, the properties of PBT can be optimized by the integration of cross-linking agents, vitrimers, and etheric compounds [48,49,50].

Biodegradable poly (lactic acid) was compounded with HN, and produced composites were investigated by several research groups. For HN-containing PLA-based composites, it was found that strength, modulus, and toughness values were improved with the inclusion of HN clay [51, 52]. Adding a small amount of HNs to the PLA matrix led to an increase in crystallization rate, shortened the processing time, and improved the heat resistance of the fabricated nanocomposites [53]. During the melt-mixing of the HNs-polymer nanocomposite, the polymer chains can interact with the HNs due to the strong shear force, leading to interfacial compatibility. The surface treatment of HN can improve the interfacial interaction with polypropylene [54] and polyamide matrices during melt processing [55]. Jia et al. reported the effects of HN on flame resistance of low-density polyethylene (LDPE). According to their cone calorimetric data, HN inclusions showed a significant decrease in the flammability of LDPE [56]. Halloysite inclusions donated varied property improvements including antioxidant resistance and oxygen barrier behaviors in packaging, film, and bottling application fields in which polyethylene [57], polyvinyl alcohol [58], polylactide [59, 60], and oxidized starch [61, 62] were loaded with HN clay.

According to the literature survey, only one study was published related to PBT composites filled with HN clay performed by Oburoğlu et al. in which the influence of HN additions to the crystallization kinetics of PBT chains was examined by differential scanning calorimeter (DSC) analysis data [63]. This current study is different from that research work based on the characterization steps including mechanical, physical, thermo-mechanical, and morphological analyses in addition to comprehensive discussions based on the pristine and silane-modified grades and adding ratios of both HN clays. Accordingly, the investigation of silane modification of HN on the basic properties of the PBT/HN nanocomposite system makes this research a novel experimental study. This work is primarily focused on the enhancement of the basic properties of PBT with the help of the incorporation of HN clay. The effects of modification of HN were also examined to observe the interactions between HN and PBT matrix through the mechanical, thermal, thermo-mechanical, melt-flow, and morphological performance of composite samples. Research data of composites containing neat HN clay and silane-modified HN were compared according to characterized behaviors. Additionally, inclusions of pristine and modified HN into the PBT matrix were investigated individually for different concentrations.

Results and discussion

FTIR analysis of HN powders

FTIR spectra of HN and HN-S surfaces are visualized in Fig. 1. The main difference between the two spectra was obtained as a broad band at 1430 cm−1 which was attributed to the presence of zeolitic water on the HN surface. This water-related peak was found to disappear after modification [64]. The characteristic silicon-related strong peaks observed at 1008, 910, and 750 cm−1 wavenumbers were assigned to Si–O stretching, Si–H bending, and Si–O bending vibrations, respectively [65, 66]. The absorption peak located nearly at 3640 ascribed to stretching vibrations of the hydroxyl group on the HN-S surface.

Fig. 1
figure 1

FTIR spectra of HN and HN-S powders

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Tensile properties of composites

Mechanical test data of PBT and its composites are listed in Table 1, and the stress versus percentage strain curves are given in Fig. 2. Based on the tensile test data in Table 1, the tensile strength of neat PBT was found to be increased by HN additions. However, silane-modified HN inclusions yielded remarkably higher tensile strength values compared to unmodified HN. The amount of increment in tensile strength of PBT with the addition of pristine HN was obtained at a very low level. For example, PBT/3 HN gave a 5% increase in the tensile strength of PBT. On the other hand, the degree of improvement in tensile strength was found to be relatively higher for HN-S containing composites in which approximately a 30% increase in strength value was obtained for PBT/3 HN-S candidate concerning unfilled PBT. In the case of adding amounts of HN and HN-S, tensile strength exhibited an increase of up to 3% content by the inclusion of both additives. Their further loadings caused dramatic reductions which indicated that more than 3% adding ratios of halloysite showed a negative effect on tensile strength of composites. Composites reinforced with 3% of HN and HN-S displayed the greatest strength values.

Table 1 Tensile and hardness test results of PBT and composites
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Fig. 2
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Stress versus strain curves of PBT and composites

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The difference between the tensile behaviors of composites loaded with modified and unmodified HN can easily be observed from their tensile curves in Fig. 2. The characteristic stress versus strain curve of PBT started to give a necking behavior with the addition of HN-S. The necking observation in stress versus strain curves is linked to the tendency of a brittle polymer to turn into a ductile characteristic. In other words, HN-S promoted ductility by reducing the brittleness of the PBT phase. The absence of necking in tensile curves of PBT and unmodified HN-filled PBT composites proposed that HN did not affect the brittle structure of the PBT matrix.

Strain at break values of neat PBT gave completely different trends by the additions of modified and unmodified HN as can be observed in Table 1. HN-S inclusions led to an increase in strain of PBT and the improvement carried on up to 5% concentration of HN-S. On the contrary, composites involving unmodified HN yielded a drop-down in the percentage strain of PBT. Strain values of composites reduced as the HN content increased. Relatively weak interactions between inert HN surface and PBT than that of silane-modified HN and PBT may be the main reason for the lowering of strain at break values. According to the tensile modulus data listed in Table 1, both additions of HN and HN-S increased the modulus of PBT. PBT/HN-S samples were found to be higher in terms of tensile modulus compared to PBT/HN for lower adding ratios, whereas nearly identical modulus values were reached for their higher contents. Similar results were reported in which modulus values of composites involving nanofillers [67,68,69]. According to the literature, in the case of HN clay inclusion with the 1% content, the highest level of mechanical behaviors was achieved [48, 50, 70] similar to mechanical test data previously reported in this study.

Hardness behaviors of composites

Shore hardness test data of PBT and its composites are listed in the last column of Table 1. The hardness of unfilled PBT was improved with halloysite inclusions. Silane-modified HN exhibited slightly higher hardness values compared to unmodified HN. Shore D hardness of composites was enhanced as the filling amounts of HN and HN-S increased. The overall change in the Shore D hardness parameter of PBT was found to be in a narrow range after HN additions regardless of the types of HN grades.

Impact performances of composites

Impact strength results of PBT/HN and PBT/HN-S composite samples are demonstrated as bar graphs in Fig. 3. Additionally, the impact strength values of each sample were indicated above its bar graph. According to the obtained impact values, HN and HN-S inclusions with low contents (1% and 3%) resulted in enhancements for the impact performance of neat PBT. However, high loading amounts of both HN grades reduced the impact energy of composites. Silane-modified HN-reinforced PBT yielded slightly higher impact energy values compared to composites containing unmodified HN clay which may be attributed to stronger interfacial adhesion of HN-S to PBT with respect to neat HN. The related weak bonding through the HN-PBT interphase resulted in a decrease in resistance against impact deformation. Based on these findings, it can be said that halloysite clay can act as an impact modifier for PBT with its low loading [71,72,73].

Fig. 3
figure 3

Impact test results of PBT and composites

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Thermo-mechanical analysis

Storage modulus curves that were derived from DMA data are illustrated in Fig. 4a. The dropping down of storage modulus curve of PBT in the temperature range of 50–100 °C is attributed to the relaxation of polymer chains due to the characteristic glass transition [74, 75]. Initially, composites displayed lower storage modulus values concerning PBT before this transition temperature. After that point, most of the composite samples reached an identical level of PBT in terms of storage modulus. As a different behavior, storage modulus values of composites involving HN with the highest content (10%) were found to be lower than that of unfilled PBT. Silane-modified HN addition with the adding amount of 1% gave a higher storage modulus than PBT beyond the transition temperature. The highest storage modulus value was obtained for the PBT/1 HN-S sample, as well.

Fig. 4
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DMA curves of PBT and composites

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Loss modulus curves represented in Fig. 4b implied that HN inclusions with low filling amounts exhibited a positive influence on the loss modulus of PBT. Loss modulus values of composites reduced as HN composition increased. Similar to storage modulus values, 1% content of HN-S compounded composite yielded a much higher loss modulus than other composites, and the highest result was achieved for the PBT/1 HN-S sample. Tan delta curves of PBT/HN and PBT/HN-S composites as the function of temperature are represented in Fig. 4c. According to Tan delta curves, the height of tan delta peak of composites filled with HN-S was found to be lower compared to unmodified HN-containing composites. Since the lowering in height of tan delta peak was linked to enhancement in interfacial adhesion of additive to polymer matrix in composite systems [76,77,78], silane-modified HN loadings resulted in a reduction in Tan delta peak height. Similarly, 1% HN-S incorporated PBT yielded the best result thanks to the compatibilizer effect of the silane layer on the polymer-additive interface adhesion [79,80,81]

Thermal stability of composites

The weight loss curves and derivative weight loss curves of PBT/HN and PBT/HN-S composites are shared in Fig. 5. Accordingly, thermal parameters of composites obtained by TGA analysis including temperature at 5% weight loss occur (T5%), the temperature in which 10% weight loss occurs (T10%), a temperature that maxima on weight loss achieved (Tmax%), and the ratio of char residue over the total amount of samples are listed in Table 2.

Fig. 5
figure 5

TGA and DTG curves of PBT and composites

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Table 2 TGA data of PBT and relevant composites
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The weight loss of PBT occurs in a single step at nearly 400 °C. An ionic breakdown mechanism results in the development of tetrahydrofuran during the first degradation period of PBT. This is followed by concerted ester pyrolysis reactions that produce 1,3-butadiene via an intermediate cyclic transition state. In both decomposition regimes, simultaneous decarboxylation reactions occur. Finally, the development of CO and complex aromatic compounds such as toluene, benzoic acid, and terephthalic acid was seen in the latter phases of the decomposition of the PBT [82,83,84,85,86,87].

TGA thermographs in Fig. 5 revealed that the thermal stability of PBT was not affected by HN inclusions. However, the difference in thermal behaviors between PBT and composites was found to be more significant for DTGA curves relative to TGA curves. The peak temperature of the DTGA curve indicates the maximum mass loss rate achieved (Tmax) which corresponds to the thermal decomposition of the polymer. According to TGA parameters in Table 3, initial decomposition temperatures (T5% and T10%) of unfilled PBT increased with halloysite additions. Similarly, composites gave higher Tmax values than Tmax of PBT. The highest results were reached for 1% additive-containing composites as can be obtained from DTGA curves in Fig. 5. The shifting of these thermal parameters to higher temperatures corresponds to a reduction in the decomposition rate of the polymer as well as an improvement in thermal stability of macromolecules [88,89,90]. PBT/HN-S composites exhibited higher thermal stability compared to composite samples involving unmodified HN. In other words, the silane layer on the HN surface yielded enhancement in the thermal resistance of PBT. The decomposition of PBT was finalized at the end of the analysis by leaving a char with 5.8% residue content composed of organic compounds. The char yield value was found to be increased as the loading ratio of HN increased due to the thermally stable inorganic nature of halloysite clay.

Table 3 Density and MFR test results of PBT and composites
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MFR parameters of composites

MFR values of PBT and relevant composites are indicated in bar chart format in Fig. 6 and listed in Table 3. The inclusion of HN to PBT resulted in increments for MFR values regardless of HN grades. MFR of composites was extended as the amount of HN increased. Similarly, Mohamed et al. reported that HN additions caused improvement in the MFR value of the polyurethane matrix [91]. They postulated that the high aspect ratio of halloysite tubes was the main reason for that finding. Relatively higher MFR parameters were achieved with silane-modified HN-incorporated composites than the MFR of PBT/HN composites at their identical loading levels. Improved interface adhesion of HN clay to the PBT matrix displayed a positive effect on the melt-flow behavior of PBT.

Fig. 6
figure 6

MFR test results of PBT and composites

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Density measurements

The measured densities of PBT and its composites are represented in Table 3. The density of unfilled PBT increased slightly with 1% addition of halloysite clay. Prominent improvements were observed for further inclusions of HN and HN-S. Silane-modified HN grade exhibited higher density values than unmodified HN for their PBT-based composites.

Morphological studies of composites

SEM micro-images of composites based on HN and HN-S compositions of 1%, 3%, 5%, and 10% are displayed in Fig. 7. According to SEM micro-images of composites involving 1% and 3% HN, dispersions of HN clays were found to be homogeneous. Silane-modified layer of HN-S promoted dispersion uniformity of HN particles into the PBT phase stem from the enhanced interface interactions on the PBT-HN system.

Fig. 7
figure 7

SEM micro-images of composites

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SEM micro-images of composites containing highly loaded HN (5% and 10%) visualized that HN particles started to stick together and form bundles into the PBT matrix as their adding amounts increased. In contrast to composites involving a low amount of HN, the dispersion of clays was restricted as a result of the reduction in uniformity of mixing into the PBT phase. The resulting bundle regions of HN were observed as highly indicative compared to HN-S.

Conclusions

In this study, melt-blending method was applied to produce HN-reinforced PBT composites. Two grades of commercial halloysite clay were used in the forms of pristine and silane-modified. Compounding of both HN grades was taken place via four different contents using a lab-scale extruder. Tensile test results indicated that tensile strength, elongation, and tensile modulus values were improved with the low loading amounts of HN inclusions. Silane-modified grade displayed better performance compared to pristine HN based on the tensile parameters. Shore hardness of unfilled PBT was also enhanced after HN additions regardless of their grades. An increasing trend of hardness values was obtained as the amount of HN increased. Halloysite clay additions led to an increase in the impact strength of PBT. Similar to tensile properties, better impact performances were achieved for low concentrations of HN. Silane-modified HN gave slightly higher impact strength compared to unmodified HNT grade. DMA study revealed that the lowest adding ratio of HN displayed an increase in storage modulus as well as loss modulus. The relationship between the compatible silane layer of the HN surface and the PBT matrix was confirmed by lowering the height of Tan delta curves of PBT/HN-S composite samples. According to TGA test findings, HN inclusions yielded a positive impact on the thermal resistance of PBT by causing a reduction in the decomposition rate of neat PBT. Composites filled with pristine HN gave lower thermal performance compared to silane-modified HN clay containing PBT. A linear improvement in the MFR parameter of PBT was obtained stem from the high aspect ratio of tubular halloysite nanotube clay. Relatively higher MFR values were found for silane-modified HN than unmodified HN compounded composites. The density of PBT exhibited no dramatic change to the low amount of HN additions. However, highly filled composites showed a remarkable increase in the density of PBT. The uniform dispersion of HN particles into the PBT phase was observed. On the other hand, the dispersion homogeneity of HN clays reduced as their composition increased according to SEM images of highly filled composite samples. The use of PBT in the cable and wire sector can be extended by HN inclusions due to thermally stable nanocomposites being reported. Low loading amounts of HN had negligible impact on density and melt-flow parameters which are related to weight saving and ease of processing of PBT-based composites, respectively. The development of composite parts used in automobile production would be considered because of the low-cost, low-weight, and mechanically strong characteristics of HN-filled PBT composites.

Experimental methods

Materials

The commercial PBT thermoplastic polymer was purchased from Sasa Poliester Sanayi A.S. (Adana, Türkiye) in pellet form. The trade name of PBT is Advanite. It has a viscosity of 0.90 dL/g according to the producer. HN powders used in this study were supplied from Esan Eczacıbaşı Endüstriyel Hammaddeler A.Ş., Istanbul, Türkiye. Two kinds of HN grades were used with the commercial names of ESH HNT (HN) and ESH HNT S (HN-S) which refer to unmodified and silane-modified grades, respectively.

Preparation of composites

PBT pellets and HN powders were subjected to the drying stage at 100 °C in an oven for 2 h to avoid moisture contamination before processing steps. Polymer and powders were compounded by a lab-scale micro-compounder (MC 15 HT, Xplore Instrument Holland). Filling amounts of NC 130 and NC 140 into the PBT matrix were set as 1, 3, 5, and 10 by weight %. During the compounding process, an extrusion temperature of 240 °C, a mixing rate of 100 rpm, and a total compounding time of 5 min were applied and these parameters were kept constant for each composite sample. Composite samples were cut to pellet form after the melt-compounding step. Samples were injection molded and dogbone specimens were obtained using a lab-scale micro-injection molding device (Micro-injector, Daca Instruments). Processing parameters of injection molding were applied as a barrel temperature of 245 °C, an injection pressure of 800 kPa, a holding time of 4 min, and a mold temperature of 80 °C.

Characterization techniques

The characterization of surface properties of pristine and modified HN samples was performed by Shimadzu IRAffinity-1S FTIR Spectrometer. Analysis was carried out using attenuated total reflection (ATR) mode in the wavelength range of 4000–500 cm−1. Lloyd LR30K universal tensile testing equipment was conducted to perform tensile tests. 5 kN load cell and 10 mm/min were applied as crosshead speed. Tensile strength, percentage strain, and tensile modulus values were recorded by a minimum of three samples with standard deviations. Impact energy values of PBT and its composite samples were recorded by the Coesfeld material impact tester via the 4 J pendulum. The impact strength values in the unit of kJ/m2 were expressed from measured impact energy data in terms of Joules. Shore D type hardness values of test samples with the dimensions of 20 × 20 × 2.5 mm3 were recorded using a digital hardness device with the trade name R5LB041, Zwick Roell. TGA analysis was carried out using a PerkinElmer Diamond TG/DTA device with a heating rate of 10 °C/min and under the inert nitrogen atmosphere with a flow rate of 50 ml/min in a temperature range of 25–650 °C. Thermo-mechanical behavior of PBT-based nano-composites was examined by dynamic mechanical analysis in dual cantilever bending mode. DMA tests of PBT and composite samples with the dimensions of 50 × 7.5 × 2.5 mm3 were studied using Hitachi High-Technologies, Dynamic Mechanical Analyzer DMA7100. The temperature range of analysis was applied between 25 °C and 200 °C at the constant heating rate of 10 °C/min under the inert nitrogen atmosphere. MFR values of PBT and composite samples were evaluated using Meltfixer LT (Coesfeld Material Test). The measurements were conducted with the standard load of 2.16 kg at the processing temperature of 240 °C. The MFR parameters were calculated and expressed in the unit of g/10 min. The digital density meter (Easy D30, Mettler Toledo) was utilized to obtain density values of unfilled PBT and relevant composite samples. Density data were reported as an average of a minimum of three measurements for each sample. The fractured surfaces of composite samples obtained from the impact test were examined after coating a thin layer of gold. FEI Quanta 400F FESEM (Tokyo, Japan) device was used to capture high-resolution SEM micro-images.