Up-to-date review on the development of high performance rubber composites based on halloysite nanotube

Applied Clay Science 183 (2019) 105300

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Review Article

Up-to-date review on the development of high performance rubber composites based on halloysite nanotube Kumarjyoti Roya, Subhas Chandra Debnathb, Aphiwat Pongwisuthiruchtea, Pranut Potiyaraja, a b

T ⁎

Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Department of Chemistry, University of Kalyani, Kalyani Nadia 741235, W.B, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Halloysite nanotube Rubber Filler Mechanical properties

In the recent years, halloysite (Hal) nanotube has emerged as promising naturally occurring filler for the development of commercially viable advanced rubber composites. The present review is an attempt to present the relevant information from past and recent developments in Hal nanotube based natural and synthetic rubber composites. The potential of Hal nanotube as reinforcing filler in rubber composites is critically explored in terms of mechanical, dynamic mechanical, morphological and thermal properties. The homogeneous dispersion of Hal within the rubber matrix is the key parameter for the preparation of highly reinforced Hal nanotube filled rubber composites. The last section of the review will emphasize the major challenges regarding the practical application of Hal nanotube as reinforcing filler in rubber industry. This study will be the part of huge interest of modern rubber researchers regarding the progress of non-petroleum based sustainable rubber technology by the suitable application of clay science and technology.

1. Introduction Halloysite (Hal) is a type of naturally occurring clay material which is usually found in many countries worldwide, such as China, New Zealand, France, Belgium, America and Brazil (Du et al., 2010a; Rawtani and Agrawal, 2012). In the previously published reports, Hal was described as an aluminosilicate (Al2Si2O5(OH)4·nH2O, where n equals 2 and 0) having molecular formula similar to kaolinite with water molecules (Du et al., 2010a; Liu et al., 2014; Yuan et al., 2015; Kausar, 2018). The hydrated state of Hal (when n = 2) is designated as halloysite (10 Å), where 10 Å refers to the d001 spacing value between the layers (Liu et al., 2014; Yuan et al., 2015, 2016). The hydrated form of Hal contains one layer of water in the interlayer spaces. It is converted irreversibly into dehydrated state of Hal (when n = 0) due to the loss of water molecules between the layers under mild heating (Liu et al., 2014; Yuan et al., 2015). The dehydrated state of Hal is designated as halloysite (7 Å), where 7 Å is the d001 spacing of Hal (Yuan et al., 2016). Hal is structurally different from kaolinite. Hal possesses hollow micro- and nanotubular structure with high aspect ratio, while kaolinite possesses stacked-plate structure (Liu et al., 2014; Pal et al., 2015). The layered structure of Hal nanotubes (HNT) crystal is formed by octahedrally coordinated Al3+ and tetrahedrally coordinated Si4+ with stoichiometric ratio 1:1 (Liu et al., 2014). In the tubular structure of Hal nanotube, the inner side and edges are composed of aluminol ⁎

groups, whereas the outer portions are mainly consisting of primary siloxanes (Liu et al., 2014; Erpek et al., 2017). Hal nanotube possesses unique combination of properties like small diameter (outer diameter is 40–70 nm and inner diameter is 10–40 nm), high aspect ratio (10–50), high elastic modulus (140 GPa) and moderate BET surface area (22.1 to 81.6 m2/g) (Liu et al., 2007; Guimaraes et al., 2010; Lu et al., 2011; Lecouvet et al., 2013; Pasbakhsh et al., 2013; Liu et al., 2014). As a result, Hal nanotube is always a promising reinforcing material for the development of high performance polymer composites (Liu et al., 2014). As compared to talc and calcite, Hal nanotube possesses relatively lower density due to its empty lumen structures (Handge et al., 2010; Alhuthali and Low, 2013). The low density of Hal is very important for the preparation of light-weight polymeric materials (Liu et al., 2014). The nanotubular geometry is common in Hal nanotube and carbon nanotube (CNT) (Kausar, 2018). The dispersion of CNT in polymer is really difficult due to strong tubetube van der Waals attraction in the structure of CNT (Xie et al., 2005). However, Hal nanotube has relatively weak tube-tube interactions due to the presence of lower number of surface hydroxyl groups (Liu et al., 2014). Consequently, the dispersion method of Hal in polymer matrix is easier as compared to CNT. Furthermore, as compared CNT, Hal has some additional advantages like easy availability, low cost and low toxicity level (Kausar, 2018). The lower concentration of surface hydroxyl group is also responsible for more hydrophobic character of Hal

Corresponding author. E-mail address: [email protected] (P. Potiyaraj).

https://doi.org/10.1016/j.clay.2019.105300 Received 21 February 2019; Received in revised form 6 September 2019; Accepted 9 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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as compared to other nano fillers like nanoclay and nano silica (Liu et al., 2014). Therefore, Hal can effectively interact with non-polar polymer backbone during processing of polymer nanocomposites (Jia et al., 2009; Du et al., 2010b). However, there is a chance of interfacial slippage due to weak interfacial interaction between polymer matrix and unmodified Hal, which is responsible for the poor mechanical performance of unmodified Hal filled polymer composites (Jiang et al., 2017). Nevertheless, surface modification of Hal nanotube is the mandatory step to achieve proper dispersion of Hal within the rubber matrix (Rooj et al., 2010; Conzatti et al., 2012). In this respect, surface modification of Hal nanotube with organosilanes is an interesting option (Yuan et al., 2008). Fillers are generally added to the rubber matrix to reach the mechanical properties required for the industrial application of rubber composites (Roy et al., 2019a,b). In the past 50 years, carbon black was mostly applied as reinforcing filler in rubber compounds (Roy et al., 2019a,b). However, carbon black is produced from non-renewable petroleum based resources (Chattopadhyay et al., 2010; Barrera and Cornish, 2016). Thus, there is a definite possibility of environment pollution during processing of carbon black based rubber compounds (Chattopadhyay et al., 2010; Barrera and Cornish, 2016). Additionally, the processing of carbon black is strongly connected to the emissions of greenhouse gases (Chen et al., 2014). The rise in the concentration of greenhouse gases is the major reason of climate change and global warming. Thus, it is very meaningful to reduce the use of carbon black as filler in rubber composites. From the start of 21st century, there is a growing demand for the development of non-petroleum based rubber composites by the suitable application of nanosized non-black fillers (Sookyung et al., 2014; Tohsan et al., 2015; Ohashi et al., 2017; Roy et al., 2018; Roy and Potiyaraj, 2019). Actually, nano fillers have large specific surface area for effective rubber-filler interaction (Roy et al., 2019a). Among the different non-black nano fillers, Hal nanotube has gained a lot of attention of rubber researchers for the development of commercially viable and environment friendly rubber composites. In the past 10 years, different research groups published several reports regarding the use Hal nanotube as filler to improve the performance of rubber composites. Recently, Kausar (2018) published a review paper based on Hal nanotube filled polymer composites. A sincere effort was made by author to explore the effect of Hal nanotube as filler in different types of polymer, such as polyethylene (PE), poly(vinyl chloride) (PVC), polypropylene (PP) and polystyrene (PS) (Kausar, 2018). But, only three old references have been briefly discussed in the area of Hal nanotube filled rubber composites (Kausar, 2018). Consequently, the review paper was not able to cover the wide area of Hal nanotube filled rubber composites on the basis of recent progress and future scope. Hence, a review paper is necessary to explore the total research work on Hal nanotube based rubber composites published to date. In the present review, we present the continuous progress and future prospects of Hal nanotube based rubber composites with up-to-date references.

rubber (NR) composites by solution mixing method. In this method, NR was initially dissolved in toluene by mechanical stirring process. Next, HNT were dispersed in toluene by mechanical stirring process. Then, HNT/toluene solution was added into NR/toluene solution under vigorous stirring. In the last step, the solution mixture was evaporated to obtain a constant weight of NR/HNT compound. The mixing of NR/ HNT compound with vulcanizing ingredients was performed in two-roll mixing mill. 2.3. Latex mixing method Swapna et al. (2016) reported the preparation of HNT based NR composites by latex solution mixing method. In this method, HNT/ water suspension was mixed with NR latex under vigorous stirring. Next, the latex/HNT solution mixture was coagulated by using 2% acetic acid solution. In the last step, NR latex/HNT mixture was evaporated to obtain a constant weight of the compound. Like solution mixing method, the compounding of NR/HNT sample with vulcanizing ingredients was done in two-roll mixing mill. 3. Properties of Hal nanotubes (HNT) based rubber composites 3.1. Application of unmodified HNT as filler in rubber composites 3.1.1. Unmodified HNT filled natural rubber (NR) composites Ismail et al. (2013d) reported the effect of HNT on the cure, mechanical and thermal properties of natural rubber (NR) and epoxidized natural rubber (ENR 50) compounds. Table 1 represents the cure properties of rubber composites in presence of different amount of HNT. The cure properties were evaluated in terms of scorch time (ts2), cure time (t90) and maximum torque (MH). In case of HNT filled NR composites, the value of ts2 showed increment in presence of higher loading of HNT. This might be due to the tendency of HNT to adsorb accelerator of vulcanization system. But, in HNT filled ENR 50 composites, HNT can react with both ENR 50 and accelerator. Thus, the availability of accelerator to reduce the scorch time was far better in HNT filled ENR 50 composites as compared to NR/ HNT composites. As a result, the scorch time showed reduction in HNT filled ENR 50 composites as compared to unfilled ENR 50 system. The value of t90 increased steadily with increasing HNT content in NR/ HNT composites, whereas ENR 50/ HNT composites showed reduction in the t90 value as compared to unfilled ENR 50 system. As shown in Table 1, in both NR/ HNT and ENR 50/ HNT composites, maximum torque increased gradually with increasing HNT loading due to the increase in the stiffness of the composites. ENR 50 had greater chain restriction mobility as compared to NR, which results higher stiffness in ENR 50/ HNT composites than NR/ HNT samples (Ismail et al., 2013d). In both NR/ HNT and ENR 50/ HNT composites, tensile strength increased steadily up to 20 phr loading of HNT due to interfacial and inter-tubular interaction between Table 1 Curing and thermal characteristics of HNT filled NR and ENR 50 nanocomposites (Ismail et al., 2013d).

2. Preparation of Hal nanotubes (HNT) based rubber composites 2.1. Mechanical mixing method It is the most conventional method for the preparation of HNT filled rubber composites. In this method, the compounding of rubber with HNT and other vulcanizing ingredients were performed in either laboratory-sized two-roll mill or internal mixer (Rooj et al., 2010; Ismail et al., 2013b; Raman et al., 2013; Waesateh et al., 2018). During compounding method, the compounding ingredients were generally taken in parts per hundred of rubber (phr). 2.2. Solution mixing method Ismail et al. (2013c) reported the preparation of HNT filled natural 2

Rubber

HNTs loading (phr)

Scorch time (min)

Cure time (min)

Maximum torque (dNm)

T40 (°C)

T50 (°C)

NR NR NR NR NR ENR ENR ENR ENR ENR

– 10 20 30 40 – 10 20 30 40

6.01 5.86 6.01 6.16 6.60 5.24 4.78 4.77 4.94 5.32

13.49 13.51 14.59 15.26 17.64 18.00 16.90 16.02 16.10 18.76

6.53 6.72 7.01 7.18 7.23 6.89 7.54 7.96 8.51 8.52

395 396 397 – 405 407 412 414 – 415

403 404 407 – 418 414 424 426 – 428

50 50 50 50 50

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rubber matrix and HNT. However, above 20 phr loading level, fillerfiller interaction dominated over rubber-filler interaction to reduce the tensile strength of rubber composites. NR had better strain-induced crystallization behaviour than ENR 50, which is attributed to the higher tensile strength of NR composites than ENR 50 composites at same HNT loading level. It was found that HNT offer higher tensile modulus for ENR system as compared to NR system, which implies strong interaction between ENR and HNT. Table 1 also describes the decomposition temperature for various weight losses in both HNT based composites. In both NR/ HNT and ENR 50/ HNT nanocomposites, the decomposition temperatures for 40% weight loss (T40) and 50% weight loss (T50) increased continuously due to the addition of HNT. It was reported that HNT tend to act as potential barrier to delay the thermal decomposition process, which led to the improvement in the thermal stability of both NR and ENR 50 compounds (Garcia-Garcia et al., 2018). ENR 50/ HNT composites displayed greater thermal stability as compared to NR/HNT composites, which is attributed to the ring opening of the epoxide group at the start of heating (Heping et al., 1999). In a different study, Ismail et al. (2011) suggested similar type of observations for HNT filled NR composites. Ismail et al. (2013c) prepared NR/ HNT composites based on mechanical and solution mixing methods. The cure study indicated that HNT have a clear tendency to delay the cure reaction of NR compounds. The values of maximum torque (MH) and tensile strength of solution mixing based NR/ HNT composites were higher as compared to mechanical mixing based NR/ HNT composites at same HNT loading level. This result was due to excellent rubber-filler interaction and better dispersion of HNT within the NR matrix in presence of solution mixing method. The value of elongation at break decreased continuously with increasing the amount of HNT in NR composites due to the enhancement in the stiffness of the materials. The thermal stability of NR/ HNT composites prepared by solution mixing method was higher as compared to mechanical mixing prepared NR/ HNT composites during degradation at lower weight losses, whereas opposite trends were observed during degradation at higher weight losses. This might be explained by considering more decomposition of solution mixing based NR/ HNT composites than mechanical mixing based NR/HNT composites after 400 °C. The scanning electron microscope (SEM) images of NR/ HNT composites based on mechanical and solution mixing methods are shown in Fig. 1. As shown in Fig. 1a, in the case of mechanical mixing based NR/HNT composite, HNT particles were largely agglomerated within the NR matrix due to their poor dispersion. On the other hand, HNT particles were uniformly dispersed within the NR matrix in solution mixing based NR composite (Fig. 1b). Nabil and Ismail (2015) reported the replacement of recycled poly (ethylene terephthalate) (R-PET) as fillers by the use of HNT in NR composites. Actually, crosslinking reaction was interrupted due to the tendency of silanol and aluminol groups on the surface of HNT to absorb accelerators. As a result, the values of scorch time (ts2) and curing time (tc90) increased progressively with the replacement of R-PET by HNT in NR composites. The difference between maximum and minimum torque i.e. (MH-ML) of NR/R-PET/HNT composites showed considerable increment with increasing the weight ratio of HNT, which implies the enhancement in crosslink density of NR composites in presence of HNT (Ismail and Anuar, 2000). The mechanical properties like tensile strength and elongation at break were found to improve consistently due to the addition of HNT in NR composites. This type of improvement in the mechanical properties was due to the inter-tubular interactions between NR matrix and HNT.

Fig. 1. (a) SEM image of mechanical mixing based NR/HNT composite at 40 phr filler loading level, (b) SEM image of solution mixing based NR/HNT composite at 40 phr filler loading level (Reprinted from Ismail et al., 2013c)

formed on the SBR chains due to the shear stress during compounding procedure. According to the authors, SBR macromolecular chains were grafted onto the Hal nanotube surfaces via the transfer of SBR radicals to the hydroxyl groups on the HNT surface. The interesting grafting reaction in SBR/HNT composites is shown schematically in Fig. 2. During grafting reaction, there was a chemical interaction between α-H of phenyl group with SieOH and AleOH groups of HNT. Also, the chemical bonding between α-H of phenyl group and AleOH was easier as compared to that between α-H of phenyl group and SieOH. The grafting amount of SBR chains on the HNT surface increases with increasing the amount of HNT. This grafting process might be responsible for the formation of hybrid crosslinking network between SBR and HNT. As a result, the vulcanization rate of SBR showed increment consistently due to the addition of HNT. The values of maximum torque (MH) and minimum torque (ML) of the compounds increased rapidly in presence of HNT as filler. The mechanical properties like tensile strength, tensile modulus and hardness increased consistently due to the incorporation of HNT as filler into the SBR matrix, which is related to the successive grafting of SBR macromolecular chains on the surfaces of HNT.

3.1.3. Unmodified HNT filled acrylonitrile–butadiene rubber (NBR) composites Ismail and Ahmad (2014) discussed the use of HNT as a new type of filler in NBR compounds. The value of maximum torque increased continuously due to the addition of HNT into the NBR matrix, which is attributed to the improved interfacial interaction between NBR and HNT. Further, the tensile strength of NBR composites showed regular increment with increasing HNT content up to 5 phr level due to the

3.1.2. Unmodified HNT filled styrene butadiene rubber (SBR) composites Jia et al. (2016) investigated the effect of unmodified HNT on the cure and mechanical properties of SBR composites. The value of optimum cure time (T90) decreased consistently with increasing HNT loading in SBR compounds. Thus, HNT were able to accelerate the vulcanization reaction of SBR. Actually, macromolecular radicals were 3

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Fig. 2. Schematic representation for the grafting of SBR chain onto the HNT surface (Jia et al., 2016)

inter-tubular interaction between NBR and HNT (Ismail et al., 2008). Above 5 phr loading level, the tensile strength decreased slightly owing to the agglomeration of HNT within the NBR matrix. Thus, 5 phr is the optimum loading of HNT to improve the mechanical strength of NBR composites. Yang et al. (2017) reported an interesting electron transferring interaction in HNT filled NBR composites. In this study, NBR/ HNT composites were prepared by solution casting method. Fig. 3 represents the scheme for the electron transferring interaction between NBR and HNT. The eCN groups of NBR have the ability to donate lone pair of electrons. On the other hand, the Al atoms on the edges of HNT have the ability to accept electron lone pair due to the electron-deficiency nature of the Lewis acid. Thus, there was a chance of electron transferring interaction between eCN groups of NBR and Al atoms of HNT in NBR/ HNT system. The value of storage modulus decreased slightly due to the addition of 5 wt% HNT into the NBR matrix, which implies weak interfacial interaction between NBR and HNT at low HNT loading level. However, the authors suggested the formation of rigid interfacial layer due to the electron transferring interaction between NBR and HNT at higher HNT loading level. This type of rigid interfacial layer was responsible for the enlargement of storage modulus of NBR composites in presence of HNT loading above 20 wt%. The stress-strain properties of unfilled NBR and HNT filled NBR composites are summarized in Fig. 4.

Fig. 4. The stress-strain properties of unfilled NBR and NBR/HNT composites (Yang et al., 2017)

The authors also informed that the applied stress can easily transfer to the HNT surface from NBR matrix by means of electron transferring interaction between NBR and HNT. This fact allows the NBR molecules to slip on the surface of nanotubes without discontinuity. As a result, the mechanical properties like tensile strength and tensile modulus improved sharply with increasing the amount of HNT in NBR/HNT composites. 3.1.4. Unmodified HNT filled ethylene propylene diene monomer (EPDM) composites Several studies have been reported based on the application of HNT as filler in EPDM composites (Ismail et al., 2008, 2009; Pasbakhsh et al., 2009a, 2010; Ismail and Shaari, 2010). Initially, Ismail et al. (2008) studied the effect of HNT on the mechanical properties of EPDM composites. As shown in Fig. 5, the value of tensile strength increased constantly with increasing HNT loading from 0 to 100 phr. The reinforcing effect of HNT was more predominant at higher HNT loading level. This type of improvement in the tensile strength was related to the different factors like dispersion level of HNT within the EPDM matrix, interfacial interaction between EPDM and HNT surface, inter-

Fig. 3. Probable schematic representation of electron transferring interaction between Hal nanotube and NBR (Yang et al., 2017) 4

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Fig. 5. The tensile strength of EPDM/HNT composites (Ismail et al., 2008)

Fig. 7. The tensile strength of EPDM/HNT composites with and without MAHg-EPDM (Pasbakhsh et al., 2009a)

process. The SEM images of uncompatibilized and MAH-g-EPDM compatibilized EPDM/ HNT composites are presented in Fig. 8. At same filler loading level, EPDM/ HNT/MAH-g-EPDM composite showed much better dispersion of HNT within the EPDM matrix as compared to uncompatibilized EPDM/HNT composite. 3.1.5. Unmodified HNT filled fluoroelastomers (FKM) composites FKM compounds are characterized by their elastic properties under relatively harsh conditions (Logothetis, 1989). Rooj et al. (2011) reported an article based on the unorthodox combination of FKM and HNT. The effect of HNT loading on the mechanical and dynamic mechanical properties of FKM composites are summarized in Table 2. It was found that the mechanical properties of FKM composites increased continuously with increasing HNT loading up to 20 phr level. The mechanical properties of FKM/ HNT composites decreased noticeably due to the addition of 30 phr HNT, which is attributed to the deterioration of the curing state of FKM in presence of higher amount of HNT. There was a constant reduction in the tan δ peak height of FKM composites due to the addition of HNT up to 10 phr level, which indicates strong interfacial interaction between FKM matrix and HNT surface. Furthermore, the phenomenological interaction parameter B confirms the presence of strong rubber-filler interaction in FKM/HNT composites in presence of 10 phr HNT as filler.

Fig. 6. Various types of interaction between EPDM chain and Hal nanotube (Ismail et al., 2009)

tubular interaction between EPDM and HNT walls. For better understanding, different interactions in EPDM/HNT composites are represented in Fig. 6 (Ismail et al., 2009). As shown in Fig. 6, there are two types of rubber-filler interaction in HNT filled EPDM composites. One type is the interfacial interaction between EPDM chain and surface of HNT. Another one is the inter-tubular interfacial interaction between EPDM chain and walls of HNT. At higher HNT loading level, there is a chance of strong filler-filler interaction due to the smaller distance between HNT. In next study, Pasbakhsh et al. (2009a) explored the role of maleic anhydride grafted ethylene-propylene-diene monomer (MAH-g-EPDM) as compatibilizer on the tensile properties of HNT reinforced EPDM composites. As shown in Fig. 7, compatibilized EPDM/ HNT composites showed higher tensile strength as compared to uncompatibilized EPDM/ HNT composites at same HNT loading level. This result might be attributed to the enhanced interfacial interaction between EPDM matrix and HNT in MAH-g-EPDM compatibilized EPDM/ HNT composites. This research group has reported the occurrence of a hydrogen bonding interaction between OH groups of MAH-g-EPDM with SieO and AleOH groups of HNT. The curing reaction of EPDM/ HNT composites was delayed in presence of MAH-g-EPDM as compatibilizer, which is attributed to the tendency of maleic anhydride groups of MAHg-EPDM to interact with accelerator species during vulcanization

3.1.6. Unmodified HNT filled ethylene-vinyl acetate (EVA) composites Copolymers of EVA are normally used in different industrial applications such as cable and wire, membranes, flexible packaging, footwear, hose and tube (Bidsorkhi et al., 2015). However, the industrial applications of EVA are closely dependent on the tensile strength and thermal stability. Unmodified HNT have been used to reinforce the tensile strength and thermal stability of EVA composites (Bidsorkhi et al., 2015). The tensile strength showed an increment by 40.57% for EVA sample filled with 3 wt% HNT as compared to that of unfilled EVA sample. However, the tensile strength value decreased considerably due to the addition of 5 wt% HNT into the EVA matrix. This result might be attributed to the poor dispersion of HNT within the EVA matrix above 3 wt% filler loading level. There was a significant improvement in the thermal stability of EVA sample with 3 wt% HNT as filler, which is mainly due to the hydrogen bonding interaction between vinyl acetate groups of EVA and surface functional group of HNT. As claimed by the authors, the optimum properties were achieved for EVA sample 5

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Fig. 8. (a) SEM image of uncompatibilized EPDM/HNT composite at 10 phr HNTs loading level, (b) SEM image of MAH-g-EPDM compatibilized EPDM/HNTs composite at 10 phr HNT loading level (Reprinted from Pasbakhsh et al., 2009a)

HNT-s-MB showed uniform dispersion of HNT-s-MB within the surface of NR matrix with orientation, which confirms improved interfacial interaction between NR matrix and HNT-s-MB. Thus, the compatibility between NR matrix and HNT-s-MB was far better as compared to that between NR matrix and m- HNT. This might be explained by considering the replacement of polar eCl group on the surface of m- HNT during reaction between m- HNT and MB to form HNT-s-MB. The morphological observations were found to be good agreement with the mechanical properties of NR composites. As shown in Fig. 11a, the mechanical properties like tensile strength and tear strength were higher in NR/HNT-s-MB as compared to either NR/m- HNT or NR/HNT. The value of thermo-oxidation activation energy (Ea) was measured from differential scanning calorimetry (DSC) analysis of NR samples. As shown in Fig. 11b, NR/HNT-s-MB showed higher value of Ea than NR/ m- HNT/MB, which indicates greater antioxidant efficiency of HNT-sMB as compared to the equivalent low molecular MB. This result was attributed to the lower volatility and migration tendency of HNT-s-MB at high temperature as compared to those of MB (Chen et al., 2011). Tan et al. (2016) studied the effect of octadecylamine modified HNT on the mechanical and thermal properties of ENR 50 composites. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) measurements were performed to confirm the hydrogen bonding interaction between ENR 50 and HNT. Modified HNT based ENR 50 composites showed an increment in the maximum decomposition temperature with increasing HNT loading. Additionally, octadecylamine modified HNT had the ability to increase the hardness and reduce modulus of ENR 50 composites. Recently, Chen et al. (2018) investigated the effect of halloysite nanotubes (HNT) supported sulfur monochloride (HNT-s-S) on the vulcanization kinetics and mechanical properties of NR composites. In this study, NR#1 formulation represents NR composite containing 20 phr HNT-s-S and 10 phr halloysite, NR#2 formulation represents NR composite containing 15 phr HNT-s-S and 15 phr halloysite, NR#3 formulation represents NR composite containing 10 phr HNT-s-S and 20 phr halloysite, NR#4 formulation represents NR composite containing 5 phr HNT-s-S and 25 phr halloysite, NR/sulfur formulation represents NR composite containing 28 phr halloysite and 2 phr sulfur, Neat NR formulation represents NR composite containing 2 phr sulfur. Among the various HNT-s-S cured NR samples, the value of cure time (Tc90) was found to decrease with increasing the amount of HNT -s-S. Thus, HNT-s-S was able to accelerate the vulcanization process of NR. As claimed by Chen et al. (2018), lower value of (Tc90-Tc10) indicates higher vulcanization rate. The values of (Tc90- Tc10) of NR samples in presence of both HNT-s-S and sulfur (S) curing systems are summarized in Fig. 12. For NR samples cured by HNT-s-S, the rate of vulcanization process was found to decrease with increasing HNT loading. The retardation of rubber vulcanization process in presence of HNT was already reported previously (Nabil and Ismail, 2015). As shown in Fig. 12, the activation energy of NR/ HNT composites was found to decrease with increasing the amount of HNT-s-S. Also, HNT-s-S cured

Table 2 Mechanical and dynamic mechanical properties of HNT filled FKM composites (Rooj et al., 2011). Formulations

HNT loading (phr)

Tensile strength (MPa)

100% modulus (MPa)

tan δ peak height

B value (interaction)

FKM gum FH 5 FH 10 FH 20 FH 30

– 5 10 20 30

4.5 6.2 6.8 7.3 5.6

2.5 3 3.4 3.6 2.6

1.11 1.10 0.98 0.98 1.10

– 0.222 1.47 0.726 –

containing 3 wt% HNT as filler. 3.1.7. Unmodified HNT filled Poly(dibutyl itaconate-co-isoprene-comethacrylic acid) (PDIM) Composites. PDIM is characterized as a bio-based carboxylic elastomer. Zhou et al. (2017) reported an interesting hydrogen bonding interaction between eCOOH groups in the PDIM structure and Si-O-Si groups on the surface of HNT. HNT was not able to improve the mechanical properties of PDIM sample at lower filler loading level. However, there was a successful improvement in the mechanical properties of PDIM composite due to the addition of ~20 phr HNT as filler. This type of improvement in the mechanical properties was closely related to the effective transfer of stress to the HNT from the PDIM matrix via hydrogen bonds. Surprisingly, HNT had no significant role on the thermal stability of PDIM compounds. 3.2. Application of modified HNT as filler in rubber composites 3.2.1. Modified HNT filled NR composites Zhong et al. (2015) reported the synthesis of halloysite nanotubes supported 2-mercaptobenzimidazole (HNT-s-MB) via the reaction between 2-mercaptobenzimidazole (MB) and γ-chloropropyltriethoxysilane (CTS) grafted halloysite nanotubes (m- HNT). The detailed synthesis procedure of HNT-s-MB is shown schematically in Fig. 9. The main aim of this study was to compare the effect of HNT-s-MB and HNT on the performance of NR composites. In this study, NR/ HNT formulation represents NR composite containing 29 phr HNT, NR/m-HNT formulation represents NR composite containing 29 phr m- HNT, NR/ m-HHNT/MB formulation represents NR composite containing 29 phr m- HNT and 1 phr MB, NR/HNT-s-MB formulation represents NR composite containing 30 phr HNT-s-MB. SEM analysis was performed to compare the morphological features of various HNT filled NR composites (Fig. 10). Morphological analysis indicated the presence of large amount of agglomerates in unmodified HNT filled NR composites. On the other hand, modified Hal nanotubes (m- HNT) were dispersed within the NR matrix without any orientation, which indicates weak interfacial interaction between NR matrix and m- HNT. However, NR/ 6

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Fig. 9. Synthesis procedure of HNT-s-MB (Reprinted from Zhong et al., 2015)

105.7 °C due to the melting peak of sulfur. The melting peak of sulfur was divided into two peaks at lower temperature region in HNT-s-S cured NR system. This was the indication of lower curing temperature in HNT-s-S curing system as compared to conventional sulfur curing system. Another important characteristic were observed from DSC analysis. A mild endothermic process was observed between 160 °C to 180 °C in the case of sulfur curing system, whereas HNT-s-S system showed sharp endothermic process between 140 °C to 170 °C. These results might be due to the generation of polysulfide species for the system cured by sulfur and mono- and disulfide species for HNT-s-S based cure system. As claimed by the authors, HNT-s-S has much better ability to accelerate the reaction between activator and accelerator as compared to sulfur (Wu et al., 2013).

NR samples had lower activation energies than sulfur cured NR/ HNT sample, which indicates that sulfur monochloride on the HNT surface was definitely able to accelerate the vulcanization process. This might be explained by considering the lower density of aluminol and silanol groups on the surface of uniformly dispersed HNT-s-S clusters as compared to those of agglomerated HNT in sulfur cured NR system. Furthermore, except NR4#, HNT-s-S cured NR samples showed higher crosslink densities as compared to that of either sulfur cured NR/HNT sample or unfilled NR sample. It was found that HNT-s-S curing system offers better mechanical performances for NR composites than sulfur curing system. However, the values of tensile strength and elongation at break were found to decrease with increasing HNT-s-S content in NR/ HNT-s-S composites. This result was due to the over-vulcanization of NR#1 and NR#2 systems in presence of excessive sulfur content. Differential scanning calorimeter (DSC) analysis was also performed to compare the mechanism of vulcanization in presence of HNT-s-S and sulfur curing systems. In sulfur curing system, there was a sharp peak at

3.2.2. Modified HNT filled SBR composites Raman et al. (2013) reported the effect of different silane coupling agents on the mechanical and dynamic mechanical properties of

Fig. 10. SEM images of (a) NR/HNT, (b) NR/m-HNT, (c) NR/HNT-s-MB (Reprinted from Zhong et al., 2015) 7

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modified by DMS and T- HNT represent HNT particles modified by TESPT. Some interesting results were found in the mechanical properties of SSBR/HNT composites. It was found that pristine HNT (p HNT) offer noticeable improvement in the tensile modulus and tensile strength of SSBR composites. On the other hand, the tensile strength of p HNT filled SSBR composites showed very little improvement due to the addition of TESPT. Among the various SSBR samples, DMS- HNT filled SSBR composites with TESPT showed maximum level of improvement in the mechanical properties and the mechanical properties increased consistently with increasing DMS- HNT content in the formulations. This result was due to the improved dispersion of DMS- HNT into the SSBR matrix in presence of TESPT. However, T- HNT offer less reinforcing effect for SSBR compounds as compared to DMS- HNT due to the insufficient surface modification of p HNT by TESPT. At 40 phr filler loading level, the value of storage modulus (E') was considerably higher in the case of DMS- HNT filled SSBR system as compared to unfilled SSBR system. This indicates excellent interfacial interaction between rubber and filler in DMS- HNT filled SSBR composite (Krishnaiah et al., 2017). There was no significant improvement in the E' value due to the addition of T- HNT into the SSBR compound. The height of tangent delta (tan δ) peak showed sharp reduction due to the addition of 40 phr p HNT into the SSBR compound. Moreover, the lowest tan δ peak height was observed for SSBR composite filled with DMS- HNT. This further indicates the strong rubber-filler interaction in SSBR composite filled with 40 phr DMS- HNT (Ivanoska-Dacikj et al., 2015; Ghari and Jalali-Arani, 2016). Zhong et al. (2016) reported the effects of silanized HNT (m- HNT) and N-cyclohexyl-2-benzothiazole sulfenamide (CZ) grafted HNT (HNTs-CZ) on the curing, thermal and mechanical performances of SBR composites. The vulcanization reaction was faster in the SBR/ HNT composites as compared to unfilled system. This might be due to the conversion of the hydroxyl groups on the HNT surface into thiol group which has the ability to participate in crosslinking reaction (Cech et al., 2006; Wu et al., 2013). The values of activation energy of vulcanization (Ea) were calculated from the plots of ln k versus 1000/T of various SBR composites. The Ea values are also displayed separately in Table 3. The Ea value depends strongly on the diffusion of the curatives in the rubber matrix. As shown in Table 3, unfilled SBR system had lower Ea value as compared to filled SBR systems, which is attributed to the easy diffusion of the curing agents in the rubber matrix during vulcanization process. On the other hand, the trapping of curing agents by HNT was the main reason for the higher value of Ea in different SBR/ HNT systems. The uniform dispersion of m- HNT particles within the SBR matrix was responsible for the formation of greater immobilized rubber chains in SBR/m- HNT system as compared to SBR/ HNT system. Actually, the mobility of the rubber chain is generally restricted due to the formation of immobilized rubber chains (Stockelhuber et al., 2011; Rooj et al., 2013; Tang et al., 2014). Thus, the diffusion of the curatives was more difficult in SBR/m- HNT system than SBR/ HNT system. Therefore, SBR/m- HNT system showed higher activation energy than SBR/ HNT system. However, the Ea value of SBR/ HNT -s-CZ was considerably lower as compared to either SBR/m- HNT or SBR/ HNT. This might be due to the presence of grafted CZ molecules at the rubber-filler interface, which increases the chance of collision between curatives in the

Fig. 11. (a) Mechanical properties of various NR composites, (b) thermo-oxidation activation energy (Ea) of various NR composites (Zhong et al., 2015)

Table 3 Activation energy of vulcanization and mechanical properties of unfilled and HNTs filled SBR composites (Zhong et al., 2016).

Fig. 12. Variation of (Tc90- Tc10) and activation energy (Ea) of various NR/HNT composites (Chen et al., 2018)

solution styrene butadiene rubber (SSBR) composites filled with HNT. In this study, two different silane coupling agents, namely, diethoxydimethyl silane (DMS) and bis[3-(triethoxysilyl)-propyl]tetrasulfide (TESPT) were used to improve the dispersion of HNT within the SSBR matrix. For better understanding, DMS- HNT represent HNT particles 8

Formulations

Ea (KJ mol−1)

ΔHRubber (J.g−1)

Tensile strength (MPa)

300% modulus (MPa)

Elongation at break (%)

Unfilled SBR SBR/HNTs SBR/m-HNTs SBR/HNTs-s-CZ

94.7 98.3 99.8 96.0

12.41 10.46 10.18 10.82

2.1 3.9 5.7 7.9

1.4 1.7 1.7 1.8

446 595 651 678

± ± ± ±

0.1 0.4 0.5 0.3

± ± ± ±

0.1 0.1 0.2 0.1

± ± ± ±

18 21 27 23

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between optimum cure time (t90) and scorch time (t10). The (t90-t10) value is inversely proportional to the vulcanization rate (Chen et al., 2018). The (t90-t10) values were considerably lower in SBR/HS-s-CZ compounds than those of SBR/HS compounds at different filler loading levels (Fig. 13c). Thus, grafted CZ within the structure of HS was more effective to increase the vulcanization rate of SBR as compared to free CZ. This was due to the excellent resistance of volatilization and migration of bonded CZ molecules in SBR/HS-s-CZ composites (Gao et al., 2007; Zhong et al., 2016). The mechanical properties of unfilled SBR, SBR/HS and SBR/HS-s-CZ composites are displayed in Figs.14a and b. The values of tensile strength and modulus were significantly greater in SBR/HS-s-CZ composites than those of SBR/HS composites, which indicate improved interfacial interaction between SBR chains and HS-sCZ in SBR/HS-s-CZ composites. The probable mechanism of interfacial interaction in HS-s-CZ filled SBR composites are also represented in Fig. 15. This mechanism clearly explains the grafting of HS-s-CZ to the SBR chains via the formation of active sulphating agents. Actually, grafted CZ molecules can react with ZnO and S to form active sulphating agents. Next, allylic hydrogen atoms of unsaturated rubber chains can react with active sulphating agents via the breakage of sulfur bonds. Liu and Liu (2017) reported the effect of polypyrrole-wrapped halloysite nanotubes (PPy@ HNT) on the conductive and mechanical properties of carboxylated SBR (xSBR) composites. At same PPy content, xSBR/PPy@ HNT composites showed considerably higher conductivity as compared to xSBR/PPy composites. This was due to the

immobilized SBR layers of SBR/HNT-s-CZ composites. A very similar type of result was found in the enthalpy of vulcanization per gram rubber (ΔHRubber) for different SBR samples. Differential scanning calorimetry (DSC) measurement was utilized to calculate the value of ΔHRubber (Table 3). The easy collision between the curing agents was the main factor for the higher ΔHRubber value of unfilled SBR system than different HNT filled SBR systems. The value of ΔHRubber had the tendency to decrease with increasing the amount of immobilized rubber chains in SBR composites. Thus, the value of ΔHRubber of SBR/m- HNT system was lower as compared to SBR/HNT system. The value of ΔHRubber increased slightly in the SBR/HNT-s-CZ composite due to the existence of grafted CZ molecules at the rubber-filler interface. The mechanical properties of unfilled and HNT filled SBR composites are also depicted in Table 3. Among the various SBR composites, SBR/HNTs-CZ showed highest level of reinforcement in the mechanical properties like tensile strength and modulus. This result confirms the efficiency of CZ grafted HNT as reinforcing filler in SBR compounds. In another study, Zhong et al. (2017a) reported the grafting of conventional accelerator N-cyclohexyl-2-benzothiazole sulfenamide (CZ) on the surface of hybrid nanofiller (HS) containing HNT and silica. Then, the interfacial interaction between SBR matrix and CZ functionalized HS (HS-s-CZ) was evaluated in the light of mechanical performance. The cure characteristics of SBR compounds filled with HS and HS-s-CZ are displayed in Fig. 13. As shown in Fig. 13a, the torque value of SBR composites increased due to the addition of both HS and HS-sCZ. The rate of vulcanization was explained in terms of difference

Fig. 13. The cure characteristics of SBR composites filled with HS and HS-s-CZ (Reprinted from Zhong et al., 2017a) 9

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Fig. 14. (a) Mechanical properties of various SBR/HS and SBR/HS-s-CZ composites, (b) stress-strain curves of various SBR/HS and SBR/HS-s-CZ composites (Reprinted from Zhong et al., 2017a)

Fig. 15. The probable mechanism of interfacial interaction between HS-s-CZ and SBR chain (Reprinted from Zhong et al., 2017a)

values of SBR/PRd- HNT composites were considerably lower as compared to those values of SBR/HNT composites, which indicate enhanced interfacial interaction between PRd wrapped HNT and SBR matrix. Again, the mechanical properties of SBR/PRd- HNT composites were compared with SBR composites containing same concentration of PRd and HNT. SBR/PRd- HNT showed markedly higher tensile strength and modulus at 300% elongation (M300) than those of SBR composites having similar amount of PRd and HNT. This result confirms that PRd itself had less effect on the mechanical performances of SBR/PRd- HNT composites. In another way, wrapping structure was the key factor for the strong interfacial interaction in SBR/PRd- HNT composites.

formation of continuous conductive network by PPy@ HNT within the xSBR matrix. However, poor dispersion of neat PPy within the xSBR matrix was the probable reason for the low conductivity of xSBR/PPy composites. There was an obvious effect of PPy@ HNT on the mechanical performance of xSBR composites. Above 2.5 wt%, PPy@ HNT was very effective filler to enhance the mechanical properties specially tensile strength and modulus of SBR compounds. This might be due to the hydrogen bonding interaction of polar xSBR with both PPy and HNT in xSBR/PPy@ HNT composites (Du et al., 2008). However, at same loading level, PPy had lower reinforcing ability than PPy@ HNT for xSBR compounds. Kuang et al. (2016) examined the reinforcing effect of polyrhodanine (PRd) wrapped HNT (PRd- HNT) in SBR composites. According to the authors, the ratio of volume fraction of rubber in unfilled rubber composite (Vr0) to volume fraction of rubber in presence of filler (Vrf) is very important term to measure rubber-filler interaction in presence of both PRd- HNT and HNT. The value of (Vr0/Vrf) decreased constantly due to the addition of filler in different SBR composites, which is attributed to the increase in the volume of the constrained rubber at the rubber-filler interface with increasing the filler content. The swelling ability generally decreases with increasing the volume of constrained rubber in filled rubber samples. Furthermore, the (Vr0/Vrf)

3.2.3. Modified HNT filled EPDM composites Pasbakhsh et al. (2010) compared the effect of both unmodified HNT and γ-methacryloxypropyl trimethoxysilane modified HNT (MHNT) on the tensile and dynamic mechanical properties of EPDM composites. It was found that M- HNT based EPDM composites exhibited noticeably higher tensile properties as compared to unmodified HNT based EPDM composites at same HNT loading level, indicating excellent interfacial and inter-tubular interactions between EPDM matrix and M- HNT. The value of storage modulus showed tremendous improvement due to the addition of 30 phr M- HNT into the EPDM 10

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replacement of HNT by CB on the properties of NBR composites. A specific trend was observed in the variation of cure time of NBR/HNT/ CB hybrid composites. The value of t90 showed decreasing trend with the replacement of HNT by CB in NBR composites, which indicates more active crosslinking sites of NBR composites in presence of CB (Arroyo et al., 2003; Pal et al., 2010). It was reported that maximum torque increased steadily due to the replacement of HNT by CB. Thus, the rubber-filler interfacial interaction was more predominant in case of NBR and CB as compared to NBR and HNT. As claimed by the authors, there was a steady reduction in the tensile strength of NBR composites due to the replacement of HNT with CB. This might be due to the better dispersion of HNT within the NBR matrix as compared to CB. However, the authors used only 5 phr filler loading to compare the reinforcing effect of CB and HNT in NBR composites. Krishnamurthy and Balakrishnan (2019) elaborately discussed the effect of CB/ HNT hybrid filler system on the dynamic mechanical and thermal properties of NBR composites. The variation in filler loading of different NBR composites is shown in Table 4. The dynamic mechanical properties of different NBR formulations are also illustrated in Table 4. The values of storage modulus of different NBR/CB/ HNT hybrid composites were considerably higher as compared to either conventional CB filled NBR composite or NBR gum at same temperature. NBRCB60-HNT6 system exhibited highest storage modulus at 30 °C among the various hybrid NBR systems. This result might be explained by considering higher crosslink formation and improved stiffness of NBR composites in presence of CB/ HNT hybrid filler system. Furthermore, NBR-CB60-HNT6 composite showed higher glass transition temperature (Tg) as compared to either NBR-CB60 or NBR gum. This might be due to the restriction of NBR chains in presence of uniformly dispersed CB- HNT hybrid filler network. Also, hybrid NBR composites had higher value of interfacial interaction parameter B as compared to conventional CB based NBR composite. As shown in Table 4, thermal properties of conventional and hybrid NBR composites were examined in terms of temperature corresponds to 10% weight loss (T10%) and temperature corresponds to 50% weight loss (T50%). The values of T10% and T50% were markedly higher in NBR-CB60-HNT6 than conventional NBR-CB60 composite, which indicates enhanced thermal stability of hybrid NBR composite as compared to conventional NBR/CB composite. Several factors like improved NBR-CB- HNT interfacial interaction, presence of CB- HNT hybrid filler network and formation of intercalated structure were responsible for the excellent thermal stability of CB/HNT based NBR hybrid composites.

matrix. Thus, M- HNT were able to effectively increase the stiffness of the EPDM composites. Furthermore, EPDM/M- HNT composite had lower tan δ peak height as compared to EPDM/ HNT composite at 30 phr filler loading level, which indicates strong interaction between EPDM matrix and M- HNT. This is an interesting result regarding the use of modified HNT as filler in EPDM compounds. In another study, Ismail and Shaari (2010) reported the mechanical and morphological properties palm ash (PA)/HNT based hybrid EPDM composites. HNT had better ability to increase maximum torque, tensile strength and tensile modulus of EPDM composites as compared to palm ash, which is related to the availability of large surface area of HNT for rubber-filler interaction. 3.3. Comparison of HNT with other conventional fillers in rubber composites We have already discussed the effect of unmodified and surface modified HNT as filler in rubber composites. However, a comparison of reinforcing efficiency between HNT and other conventional fillers in rubber composites is really important task from industrial viewpoint. For few decades, carbon black and silica have been utilized as reinforcing filler in rubber compounds. Some researchers tried to compare the reinforcing effect of HNT with carbon black or silica in different rubber composites. In some cases, hybrid filler systems have been utilized to reinforce rubber compounds. 3.3.1. Comparison between HNT and carbon black (CB) Ismail et al. (2012) published a paper regarding partial replacement of CB by HNT as filler in NR composites. Fig. 16 represents the cure properties of NR composites in presence of CB, HNT and CB/ HNT filler systems. The values of scorch time and cure time increased steadily with increasing HNT loading in NR/CB/HNT composites, which is attributed to the tendency of HNT to absorb curative in its surface during vulcanization process. There was a continuous decrement in the MH value of NR composites due to the replacement of CB by HNT, which indicates that HNT have lower ability to restrict the mobility of rubber chains as compared to CB. On the other hand, the values of tensile strength and modulus showed reduction with increasing HNT loading in NR composites. This might be explained by considering incompatibility between NR matrix and HNT containing silanol groups on the outer surface. It was observed that the thermal stability of NR nanocomposites remains almost unchanged due to the replacement of CB by HNT. Ismail and Ahmad (2013) evaluated the effect of partial

3.3.2. Comparison between HNT and silica Zhong et al. (2017b) compared the effect of silica and unmodified HNT on the mechanical properties of SBR compounds. Silica had better ability to improve the mechanical and dynamic mechanical properties of SBR composites as compared to HNT, which is related to the formation of greater amount of tightly immobilized rubber in SBR/silica composites than that of SBR/HNT composites. As claimed by the authors, tightly immobilized rubber can act as a bridge in the rubber-filler interface to enhance the mechanical strength of SBR composites. Pasbakhsh et al. (2009b) compared the reinforcing effect of silica with unmodified HNT for EPDM composites. In this study, silica was partially replaced by HNT in EPDM formulations. The value of t90 showed continuous reduction due to the replacement of silica by HNT in EPDM composites. Thus, silica had greater tendency to interact with the accelerators as compared to HNT. The value of ML decreased steadily with increasing the amount of HNT in EPDM/silica/HNT composites, which indicates easier processability of EPDM compounds in presence of HNT. The value of M300 showed considerable increment due to the replacement of silica by HNT in EPDM compounds. The value of M300 is proportional to the stiffness of the rubber materials. Thus, HNT had greater ability to increase the stiffness of EPDM compounds. This phenomenon was closely related to the nano-tubular shape of HNT. The value of tensile strength showed maximum value for EPDM

Fig. 16. Cure properties of NR composites in presence of CB, HNT and CB/HNT filler systems (Ismail et al., 2012) 11

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Table 4 Formulation, dynamic mechanical and thermal properties of different NBR nanocomposites (Krishnamurthy and Balakrishnan, 2019). Formulations

CB loading (phr)

HNTs loading (phr)

– 70 60 60 60 60 60

NBR gum NBR-CB70 NBR-CB60-HNT2 NBR-CB60-HNT4 NBR-CB60-HNT6 NBR-CB60-HNT8 NBR-CB60-HNT10

Storage modulus (MPa) at

– – 2 4 6 8 10

composite filled with 25 phr silica and 5 phr HNT. This type of reinforcement in the tensile strength was due to the synergistic interaction between silica and HNT in EPDM composites (Yu et al., 2007). Ismail et al. (2013a) studied the effect of partial replacement of HNT by silica on the curing and mechanical performances of NBR composites. There was a specific trend regarding the variation of cure time of NBR/HNT/silica hybrid composites. The cure time (t90) increased steadily due to the replacement of HNT by silica in NBR composites, which is connected to the greater tendency of silica to interact with the accelerators during vulcanization reaction. It was found that maximum torque increased steadily due to the replacement of HNT by silica. Actually, the value of maximum torque was closely connected to the particle size and surface area of the fillers in various NBR composites. As claimed by the authors, there was a continuous reduction in the tensile strength of NBR composites due to the replacement of HNT with silica. Thus, HNT were dispersed more uniformly within the NBR matrix as compared to silica. The variation of tensile modulus was almost similar to that of tensile strength in different NBR composites. In addition, the authors suggested that HNT has superior reinforcing effect as compared to silica in NBR composites. But, this study was performed in presence of very small amount of filler loading. Recently, Thepsuwan et al. (2018) explored the effect of silica/CB/ HNT hybrid filler on the mechanical and dynamic mechanical performances of solution styrene butadiene rubber (SSBR) composites. The main aim of this study was to partially replace CB with HNT in SSBR based tire tread compounds. The mechanical properties like tensile strength and tear strength decreased constantly due to the replacement of CB with HNT in SSBR composites. This was because of the weak SSBR- HNT interaction resulting from the lower surface area of HNT as compared to CB. The dynamic properties of various SSBR composites were compared in terms of tan δ peak area, the value of tan δ at 0 °C and the value of tan δ at 60 °C. As shown in Table 5, the value of tan δ peak area increased constantly with increasing the replacing HNT content in SSBR compounds. This result indicates the availability of greater amount of rubber molecules for transition due to the replacement of CB by HNT in hybrid SSBR composites (Qu et al., 2013). The value of tan δ at 0 °C is the measure of wet grip, while the value of tan δ at 60 °C is the measure of rolling resistance of rubber compounds (Thepsuwan et al., 2018). The authors found that there was a continuous increment in the value of tan δ at 0 °C with increasing replacing HNT content in silica/ CB/HNT filled SSBR composites, which indicates the improvement of

CB loading (phr)

HNTs loading (phr)

Relative tan δ area

tan δ at 0 °C

48 48 48 48 48

32 28 24 20 16

– 4 8 12 16

1.00 1.06 1.17 1.25 1.32

0.56 0.61 0.64 0.68 0.72

−30 °C

30 °C

4297.88 8731.51 14,793.73 18,176.74 18,402.46 13,139.51 10,589.51

3362.94 7430.76 12,848.27 14,281.76 15,517.53 10,968.86 9275.35

20.23 45.39 118.28 129.44 132.82 106.52 106.40

T50%

– 370 382 385 392 391 –

– 449 480 482 483 481 –

Fig. 17. The value of tan δ at 60 oC of silica/CB/HNT based SSBR composites (Reprinted from Thepsuwan et al., 2018)

wet grip property of SSBR based tire tread compounds due to the partial replacement of CB by HNT. Rolling resistance is an important parameter for the preparation of environmental friendly green tire compounds (Sarkawi et al., 2016). Actually, rolling resistance is the measure of fuel consumption efficiency of tire tread compounds (Sarkawi et al., 2016; Thepsuwan et al., 2018). Thus, the lowering of rolling resistance value is related to the fuel benefit of consumer (Sarkawi et al., 2016). As shown in Fig. 17, the value of tan δ at 60 °C decreased steadily with the replacement of CB by HNT in various SSBR formulations. This was the indication of improved fuel consumption efficiency due to the partial replacement of CB by HNT in SSBR based tire tread composites. Recently, Lin et al. (2019a) explored the efficiency of silane functionalized HNT -silica hybrid as reinforcing filler in SBR composites. In another study, an effort was made to introduce in situ fabricated HNT /silica nano hybrid (HNT -g-silica) as new reinforcing filler for SBR composites (Lin et al., 2019b). Interestingly, SBR/ HNT -g-silica composites showed better tensile strength and storage modulus as compared to those of SBR/ HNT /silica and SBR/ HNT composites. This result confirms excellent dispersion of filler particles and strong rubberfiller interfacial interaction in HNT -g-silica filled SBR composites. Again, SBR/HNT-g-silica composite showed lower rolling resistance as compared to either SBR/ HNT /silica or SBR/HNT. Lin et al. (2019b) suggested that HNT -g-silica could be considered as better energy saving material than HNT and HNT/silica mixture in SBR composites.

Table 5 Dynamic mechanical properties of silica/CB/HNT based SSBR composites (Thepsuwan et al., 2018). SiO2 loading (phr)

−70 °C

T10%

4. Summary of the major properties of HNT filled rubber composites 4.1. Cure properties of HNT filled rubber composites The rate of curing or vulcanization process of unmodified HNT filled rubber composites was closely related to the structure of rubber matrix. Unmodified HNT had the common tendency to interact with the 12

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industrial perspective.

accelerator during vulcanization of NR composites. As a result, the cure rate of unmodified HNT filled NR composites decreased continuously with increasing filler content. However, the variation of cure rate was little bit different in unmodified HNT filled ENR 50 composites. Actually, polar ENR 50 was able to reduce the interaction between HNT and accelerator. Thus, the availability of accelerator was higher in HNT filled ENR 50 composites than HNT filled NR composites. Thus, cure rate of ENR 50/ HNT showed continuous increment with increasing unmodified HNT content. In a similar way, unmodified HNT were able to increase the cure rate of polar NBR compounds. Also, HNT were capable to increase the cure rate of EPDM composites. On the other hand, we have already mentioned an interesting chemical interaction between α-H of phenyl group with Si–OH and Al–OH groups of HNT in unmodified HNT filled SBR composites. In fact, due to this type of chemical interaction, unmodified HNT were able to increase the cure rate of SBR composites. Optimal cure rate is the initial criterion for the industrial application of any filler in rubber compounds. Thus, except for NR/HNT composites, unmodified HNT had the ability to provide optimal cure rate for almost all the rubber composites. Even, surface modified HNT was not able to provide satisfactory vulcanization rate for NR composites. This is the basic disadvantage regarding the use of HNT as alternative filler in NR composites. Also, unmodified HNT was promising candidate to improve the maximum torque of different rubbers, such as NR, ENR, SBR, NBR, EPDM etc. Now, the value of maximum torque is the measurement of stiffness of the rubber materials. Again, there is a close relationship between stiffness of rubber materials and mechanical properties, such as tensile modulus and hardness. Thus, maximum torque gives a qualitative idea about the mechanical properties of HNT filled rubber composites.

5. Conclusions and future perspectives In the present review, the potential of Hal nanotubes (HNT) as a non-black filler in rubber technology has been elaborately discussed with proper references. The ultimate performance of HNT reinforced rubber composites is strongly related to the dispersion level of HNT within the rubber matrix. Solution mixing method is an attractive technique to attain optimum dispersion of HNT into the NR matrix. HNT supported 2-mercaptobenzimidazole (HNT -s-MB) has been designated as effective reinforcing filler for NR compounds. On the other hand, HNT supported sulfur monochloride (HNT -s-S) can act as efficient curing agent for NR composites. Besides, diethoxydimethyl silane modified HNT (DMS- HNT), N-cyclohexyl-2-benzothiazole sulfenamide (CZ) grafted HNT (HNT -s-CZ) and polyrhodanine (PRd) wrapped HNT (PRd- HNT) have been used as promising reinforcing fillers to develop high performance SBR composites. Polypyrrole-wrapped halloysite nanotubes (PPy@ HNT) are interesting filler to improve the conductive and mechanical properties of carboxylated SBR (xSBR) composites. The partial replacement of CB by HNT can help to improve the wet grip property and fuel consumption efficiency of SSBR based tire tread composites. HNT have been also successfully utilized to reinforce polar rubber like NBR. Additionally, CB/ HNT hybrid filler system can be used to improve dynamic mechanical and thermal properties of NBR composites. γ-methacryloxypropyl trimethoxysilane modified HNT has been found to be more effective filler than unmodified HNT for EPDM composites. Besides, maleic anhydride grafted ethylene-propylenediene monomer (MAH-g-EPDM) has been employed as compatibilizer to improve the rubber-filler interaction of EPDM/HNT composites. Hal nanotubes have one additional advantage over other nanoclays as filler in rubber compounds. This advantage is related to the use of Hal nanotubes as filler at higher loading level for different rubbers, specially, NR and EPDM. Other nanoclays can be used as filler up-to only 5 phr loading level, while HNT can able to increase the tensile strength of EPDM composites up-to 100 phr loading level. Nevertheless, the industrial application of Hal nanotube for the preparation of highly reinforced rubber composites is in the beginning stage. There are still several drawbacks concerning the fruitful usage of Hal nanotube as filler in rubber composites. For well perception, some of the major drawbacks are listed as follows:

4.2. Mechanical properties of HNT filled rubber composites Mechanical properties directly reflect the performance of rubber composites in presence of HNT. In HNT filled rubber composites, the mechanical properties depend on the interfacial rubber-filler interaction and dispersion of filler within the rubber matrix. Unmodified HNT was able to provide little enhancement in the mechanical properties of NR compounds. Actually, uniform dispersion of unmodified HNT within the non-polar NR matrix was really difficult task. It was found that surface modified HNT offer excellent dispersion in NR/ HNT composites. It was found that surface modified HNT like HNT -s-MB had better ability to improve the mechanical properties of NR compounds as compared to unmodified HNT. Unmodified HNT was able to improve the mechanical performance of some important rubbers, such as SBR and NBR. The electron transferring reaction between NBR and HNT was the key factor that determines the mechanical properties of NBR/ HNT composites. Unmodified HNT had the ability to increase the tensile strength and tensile modulus of EPDM composites. However, the mechanical properties of EPDM/HNT were better in presence of MAH-gEPDM as compatibilizer. It was found that unmodified HNT offer noticeable increment in the mechanical properties of PDIM, which is due to the hydrogen bonding interaction between PDIM and HNT. Simply, the mechanical properties of rubber composites in presence of HNT were closely connected to the structure and functional groups of rubber.

(a) In the last 10 years, various research groups reported different concepts for the successful development of high performance Hal nanotube based rubber compounds in laboratory scale. In most cases, small amounts (less than 50 phr) of Hal nanotube have been used as filler in different rubber composites. Thus, modern rubber researchers have extremely inadequate information about large industrial scale application of Hal nanotube as filler in rubber composites. (b) For a long period, petroleum based carbon black has been continuously used as most effective filler in rubber industry. Although various efforts have been made in developing non-petroleum based Hal nanotube as alternative filler for rubber compounds, there is a large gap of mechanical performances between conventional carbon black and Hal nanotube filled rubber composites. Also, the systematic comparative reports on the overall performances of carbon black and Hal nanotube filled rubber composites are very limited. (c) As discussed in the review, surface modification of Hal is the key step for the improvement of mechanical performances of Hal nanotube based rubber composites. In most cases, researchers have reported surface modification of Hal by different complicated and costly methods from industrial point of view. Hence, it is an urgent task for present rubber researchers to find some easy alternative techniques for the surface modification of Hal nanotube.

4.3. Thermal properties of HNT filled rubber composites Thermal stability is another important criterion for the industrial application of rubber composites. Unmodified HNT had the tendency to improve the thermal stability of different important rubbers, such as NR, NBR, EPDM, EVA. The thermal stability of HNT filled rubber composites depends on several factors, such as dispersion level of HNT within the rubber matrix, interfacial interaction between rubber and HNT, entrapment of degradation materials of rubber chains inside the lumen structure of HNT. The improved thermal stability of rubber composites in presence of HNT as filler is a positive point from 13

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