Synthesis of sonochemical chloroacetated natural rubber and its potential use in passenger car tire tread

Terana Senakham; Puchong Thaptong; Pongdhorn Sae-Oui; Supparoek Boopasiri; Sittipong Amnuaypanich; Chomsri Siriwong

doi:10.1016/j.arabjc.2023.105299

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Original article

12 2023

:16;

105299

doi:

10.1016/j.arabjc.2023.105299

Synthesis of sonochemical chloroacetated natural rubber and its potential use in passenger car tire tread

Terana Senakham^{^a}, Puchong Thaptong^{^b}, Pongdhorn Sae-Oui^{^b}, Supparoek Boopasiri^{^a}, Sittipong Amnuaypanich^{^a}, Chomsri Siriwong^{^a,⁎}

a Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand

b MTEC, National Science and Technology Development Agency (NSTDA), 111 Thailand Science Park, Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand

⁎Corresponding author. schoms@kku.ac.th (Chomsri Siriwong)

Received: 2023-5-22, Accepted: 2023-9-20,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Abstract

This article describes the preparation of chloroacetated natural rubber (CNR) through the epoxidation of natural rubber (NR) latex with performic acid, followed by the reaction with chloroacetic acid. Ultrasound waves were utilized to accelerate the epoxidation reaction. The CNR samples obtained at various epoxidation durations were analyzed by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, gel permeation chromatography, and differential scanning calorimetry techniques. Results showed that the chloroacetate content in CNR increased with increasing epoxidation time, i.e., the chloroacetate contents were 17.4, 18.5, and 19.7% at epoxidation times of 30, 45, and 60 min, respectively. The prepared CNR samples were then employed to replace NR in silica-filled tire tread compounds based on a 70/30 solution styrene butadiene rubber (SSBR)/NR blend. It was found that, compared with NR, CNR provided significantly higher rubber-silica interaction, leading to improved mechanical and dynamic properties. Such replacement not only reduced heat build-up and rolling resistance, but also increased the wet grip index, which is highly beneficial for the production of high-performance passenger car tires. It could be observed that the improvements in rubber properties and tread performance were more obvious when the chloroacetate content of CNR was increased, probably due to the enhanced rubber-filler interaction, as demonstrated by the increased bound rubber content.

Keywords

Ultrasound

Natural rubber

Chloroacetated natural rubber

Tread

Tire

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1 Introduction

Tires are one of the main products in the rubber industry. It is the only part of a car that is in direct contact with the road, and, hence, the good mechanical and dynamic properties of the rubber components, especially the tread, are essential to ensure good driving safety. Reinforcing carbon black (CB) has long been utilized as a primary filler in tread compounds for decades because it provides easy processing and good tire performance owing to its good compatibility with various non-polar rubbers widely employed in tire tread manufacturing, such as styrene butadiene rubber (SBR), butadiene rubber (BR), and natural rubber (NR). Generally, there are three main parameters frequently used to judge tire performance: i.e., abrasion resistance, wet grip (WG) index, and rolling resistance. A tread with good abrasion resistance is essential to achieve a sufficiently long tire service life. The WG index shows how well a tire handles on a wet road, which greatly affects driving safety. It is often indicated by the loss factor or tan δ value at 0 °C of the tread (Liu et al., 2014; Sae-oui et al., 2016; Sirisinha et al., 2019; Thaptong et al., 2022; Boopasiri et al., 2022a; Boopasiri et al., 2022b ). Due to the great concern for the environment, the rolling resistance of a tire has drawn considerable attention from tire developers around the world because it is a factor indicating fuel-saving efficiency (FSE). Many attempts have therefore been made to reduce the rolling resistance of the tread in order to make tires roll more smoothly and use less fuel (Wang et al., 2010; Maghami et al., 2016; Mazumder et al., 2021 ). Based on what has been done, the tire rolling resistance is generally represented by the tan δ at high temperature (60 °C) of the tread (Li et al., 2014). Many published works have looked into the factors that affect tire performance, including the mixing condition (Thaptong et al., 2016), the type of silane coupling agent (Siriwong et al., 2014; Siriwong et al., 2017), the filler type (Flanigan et al., 2012; Zafarmehrabian et al., 2012; Boopasiri et al., 2022a; Boopasiri et al., 2022b ), and the rubber type (Mazumder et al., 2021; Mensah et al., 2018; Sirisinha et al., 2019 ). Unlike CB, silica (SiO₂) is highly polar due to the abundance of silanol groups on its surface (Medalia, 1974; Wolff et al., 1994; Meon et al., 2004; Pattanawanidchai et al., 2019; Rombaldi et al., 2021 ), resulting in high self-agglomeration via hydrogen bonding, poor compatibility with many non-polar rubbers, and poor mechanical properties. However, with the advent of silane coupling agents, SiO₂ has been increasingly used as a reinforcing filler in various applications, including tread compound. The interaction between hydrophilic SiO₂ and hydrophobic rubbers, which are widely used in the manufacture of tire tread, is greatly enhanced in the presence of silane coupling agents. The strong chemical bonds between rubber and SiO₂ reduce the filler-filler interaction, leading to a significant improvement in filler dispersion during a mixing process and, thus, providing good mechanical properties along with the benefit of lower rolling resistance compared with its counterpart, CB. The partial substitution of CB with silane-treated SiO₂ is therefore favorable in the manufacturing of green and high-performance car tires (Rauline, 1993; Seo et al., 2010).

Typically, the surface modification of SiO₂ with a silane coupling agent is unavoidable to enhance rubber-SiO₂ interaction and reduce filler-filler interaction (Qu et al., 2013). Alternatively, increasing polymer polarity is one of the promising methods to improve the interaction between polar fillers and non-polar polymers. This can be done through various reactions such as the epoxidation reaction (Luo et al., 2011; Xu et al., 2015; Kaewsakul et al., 2013 ) or the reaction with other reactive chemicals, for instance, ozone (Utara and Boochatum, 2009), maleic anhydride (MA) (Bikiaris et al., 2005; Sahakaro and Beraheng, 2008; Sekharan et al., 2012; Samsudin et al., 2016; Pustak et al., 2015 ), and aminosilane (Sun et al., 2019), etc.

According to the literature, chloroacetated natural rubber (CNR) has been synthesized by grafting chloroacetate groups into NR (Boochathum & Rongtongaram, 2016). In this work, NR latex was initially epoxidized by performic acid and then reacted with chloroacetic acid prior to coagulation and drying. It was found that the prepared CNR could significantly enhance the rubber-SiO₂ interaction and increase the filler dispersion. The application of CNR in SiO₂-filled tire tread compounds has previously been revealed (Sirisinha et al., 2019). Compared with NR, CNR provided a remarkable improvement in tire performance. Nevertheless, in the previously published works, the grafting of chloroacetate groups in NR was made through an epoxidation reaction that took place under mechanical agitation at 40 °C for three hours, followed by the addition of choroacetate acid (25 mol% chloroacetic acid) for an hour at room temperature. Obviously, the conversion of NR to epoxidized NR (ENR) is a time-consuming process. Attempts have therefore been made to shorten the reaction time. Recently, an ultrasonication method has been applied to synthesize ENR from NR latex (Lorwanishpaisarn et al., 2023). This method offers many advantages over the conventional agitation method, i.e., lower reaction temperatures and shorter reaction times.

As disclosed in the literature, the CNR is generally prepared via a two-step method, i.e., NR latex is initially converted to ENR latex and subsequently reacted with chloroacetic acid in the second step. Both steps are carried out through a conventional method, which is a time-consuming process. This work therefore aims to shorten the reaction time by using the ultrasonication method to replace the conventional method during the ENR synthesis. In this study, the ultrasonication time was varied, and the obtained ENR samples were later converted to CNR samples prior to being characterized by various techniques, i.e., proton nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). The influence of CNR samples having different chloroacetate contents on the properties of the passenger car tire tread was also thoroughly investigated and revealed.

2 Experimental

2.1

2.1 Materials

Concentrated NR latex (60% DRC) was supplied by Thai Rubber Latex Group PCL. Oil-extended solution SBR (SSBR 3626: 37.5 phr of TDAE, 36% styrene content, and 26% vinyl content), produced by LG Chemical, was imported from South Korea. Hydrogen peroxide (H₂O₂) and tetrahydrofuran (THF) were supplied by QRëC (New Zealand) and Sigma Aldrich (USA), respectively. A surfactant (Triton-X), formic acid, as well as chloroacetic acid were obtained from Loba Chemie PVT. Ltd. (India). Deuterated chloroform (CDCl₃) was supplied by Cambridge Isotope Laboratories, Inc. (USA). The silane coupling agent, Bis-(3-(triethoxysilyl)propyl) tetrasulfide (TESPT), was manufactured by Innova (Tianjin) Chemical Co., Ltd. (China). N-tert-butyl-2-benzothiazole sulfenamide (TBBS), 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), and N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine (6PPD) were supplied by Monflex Pte., Ltd. (Singapore). The other mixing ingredients were purchased from local vendors in Thailand, i.e., stearic acid from Kij Paiboon Chemical Ltd., zinc oxide (ZnO) from Thai-Lysaght Co., Ltd., tetrabenzylthiuram disulfide (TBzTD) from Behn Meyer Chemicals (T) Co., Ltd., paraffin wax (P-wax) from Petch Thai Chemical Co., Ltd., sulfur (S₈) from Siam Chemicals Public Co., Ltd., carbon black (CB; N239 with a BET of 126 m²/g) from Thai Carbon Black PCL., silica (SiO₂; Tokusil 255 with a BET of 166 m²/g) from OSC Siam Silica Co., Ltd., and treated distillate aromatic extract (TDAE) oil from PSP Specialties Co., Ltd.

2.2

2.2 Synthesis of CNR via a sonochemical method

The preparation of CNR was roughly divided into two main steps. In the first step, the concentrated NR latex (60 g) was mixed and stabilized with 90 mL Triton-X (2.80 g) as a non-ionic surfactant before being diluted with deionized (DI) water to obtain 40% DRC. After being stirred at room temperature for 1 h, formic acid (8.11 g) and H₂O₂ (17.97 g) were added dropwise before being subjected to ultrasound irradiation under a nitrogen atmosphere for 30, 45, and 60 min using the sonochemical reactor shown in Fig. 1. The ultrasonic generator (AKHGZ, ACME ultrasonic tools, Thailand), providing an ultrasonic wave (20 kHz) at a power of 200 W, was equipped with a water bath (with a dimension of 24 × 21 × 14 cm³). Because the ultrasonic transducer was not in direct contact with the reaction mixture, the actual ultrasonic power was considerably decreased from 200 W to 0.14 W. During the reaction, the temperature of the ultrasonic bath was kept constant at 30 °C via a refrigerated cooling system. After the ultrasound irradiation, the white latex obtained, denoted as ENR, was mixed with chloroacetic acid (16.62 g) to convert it into CNR. After mixing for an hour, coagulation was done by adding methanol. To ensure full elimination of excess chloroacetic acid, the coagulum was washed with distilled water many times until the washed water was neutral. The coagulum was then dried at 50 °C until it reached a consistent weight prior to being characterized by various techniques.

Ultrasound irradiation set-up; 1-Water bath, 2-Ultrasonic generator, 3-Ultrasonic transducer, 4-Reaction media, 5-Cooling coil, 6-Thermometer, 7-Stand, and 8-Water.

2.3

2.3 Characterization of CNR

Fourier transform infrared spectroscopy (FTIR; Bruker Tensor 27) was employed to determine the functional groups. Proton nuclear magnetic resonance spectroscopy (¹H NMR; Avance Neo) was employed to examine the chemical structure of the samples. Before being tested, the samples were dissolved in CDCl₃. The epoxy content and chloroacetate content were calculated from the integrated area of the characteristic peaks of ¹H NMR, as shown in Eq. (1) and Eq. (2), respectively.

(1)

Epoxy c o n t e n t (%) = \frac{I_{2.7}}{I_{2.7} + I_{5.1}} \times 100

where I_2.7 is the integrated peak area of the proton linked to the oxirane rings of ENR at 2.7 ppm and I_5.1 is the integrated peak area of the olefinic proton at 5.1 ppm.

(2)

Chloroacetate c o n t e n t (%) = \frac{(I_{4.1}) / 2}{I_{2.7} + I_{5.1} + (I_{4.1}) / 2} \times 100

where I_4.1 is the integrated peak area of the proton in CH-Cl (Saengdee et al., 2020).

The average molecular weight of the samples was estimated by using gel permeation chromatography (GPC), equipped with high-performance liquid chromatography (HPLC; Agilent Technologies). THF was utilized as the eluent at a flow rate of 1 mL/min.

The glass transition temperature (T_g) was investigated by differential scanning calorimetry (DSC; Perkin Elmer DSC 8000). The temperature was scanned at a heating rate of 10 °C/min from –80 °C to 200 °C. The Mooney viscosity (ML1 + 4@100 °C) was determined by a Mooney viscometer (Shimazu Model 301) in accordance with ISO 289–1.

2.4

2.4 Rubber compound preparation and testing

This section focused on the potential use of CNR in SiO₂-filled tire tread compounds, with the formulations given in Table 1. An internal mixer (Haake^TM Rheomix Lab Mixer) was used to prepare the rubber compounds under the following mixing conditions: rotor speed of 40 rounds per minute (rpm), initial chamber temperature of 60 °C, fill factor of 0.70, and 15 min of mixing time. After mixing, the rubber mixtures were sheeted using a two-roll mill and kept overnight at ambient temperature prior to testing and vulcanizing.

Table 1 Rubber formulation.

Ingredients	Content (phr)
NR or CNR*	30
Oil-extended SSBR	96.3
ZnO	3
Stearic acid	2
6PPD	1.5
TMQ	1
P-wax	2
TDAE oil	8
SiO₂	48
CB	32
TESPT	4.8
TBBS	1.2
TBzTD	0.2
S₈	2.2

* CNR at different chloroacetate contents (CNR1, CNR2, and CNR3).

Measurement of bound rubber content (BRC), an indicator for rubber-filler interaction, was carried out by using toluene to remove any uninteracted rubber from the filler surface. For five days at room temperature, 1 g of unvulcanized rubber was immersed in a bottle filled with excessive toluene (100 mL). The residual gel was then vacuum filtered, air-dried at 80 °C overnight, and weighed. Eq. (3) was used to get the approximate value of BRC.

(3)

BRC (%) = \frac{W_{fg} - W F_{f}}{W F_{p}} \times 100

where W is the specimen weight before immersion, W_fg is the weight of filler-rubber gel after drying, and F_p and F_f are the polymer and filler weight fractions in the rubber compound, respectively.

Cure characteristics, including minimum torque (M_L), maximum torque (M_H), scorch time (t_s1) and optimum cure time (t_c90) were studied at 150 °C using a moving die rheometer (MDR-01, CG Engineering Co., Ltd.) as per ISO 6502–3.

2.5

2.5 Rubber vulcanizate testing

Rubber sheets (∼2 mm thickness) were vulcanized using a compression molding technique at 150 °C for a duration of t_c90. A hardness test was carried out using a Shore A scale durometer (Desik Instruments Group Co., Ltd.) based on ISO48-4. Both tensile strength (TS), modulus at 100% strain (M₁₀₀) and elongation at break (EB) were measured by a universal testing machine (UTM: Instron 5567A, Illinois Tool Works Inc.) following ISO 37 (Die 1). Tear strength was also measured using the UTM based on ISO 34–1 (angle specimens). Abrasion resistance, which is expressed as volume loss per 1,000 rotations of the abrasive wheel, was evaluated by the Akron abrasion tester (Gotech GT-7012-A) using ISO 4666–3. Heat build-up (HBU), measured with a Goodrich flexometer (BF Goodrich Model II, USA), was recorded as the temperature increase at the specimen's base. For cylindrical test specimens, the cure times were prolonged to compensate for heat transfer. The cure times were set at t_c90 plus 10 min for the abrasion resistance test and t_c90 plus 15 min for the heat build-up test. The values of tan δ were measured using a dynamic mechanical analyzer (DMA Q800, TA Instruments). The following conditions were used to determine the value of tan δ at 0 °C: 2% static strain, 0.1% dynamic strain, and 10 Hz frequency. For the measurement of tan δ at 60 °C, the indicator for rolling resistance, the dynamic strain was increased from 0.1% to 1%. The morphology of rubber vulcanizates was investigated using a field emission scanning electron microscope (FESEM FEI, Model Helios NanoLab G3 CX, Netherlands). The cryogenic fractured surface was sputtered with a thin gold layer prior to analysis.

3 Results and discussion

3.1

3.1 Characterization of NR and CNR

3.1.1

3.1.1 FTIR analysis

The FTIR was utilized to analyze the molecular structure of CNR samples synthesized by varying ultrasound irradiation times of 30, 45, and 60 min, referred to herein as CNR1, CNR2, and CNR3, respectively. The FTIR spectra of all samples (NR, CNR1, CNR2, and CNR3) are displayed in Fig. 2. For NR, several distinctive peaks of the NR molecule may be seen, i.e., the peaks at 3045, 2960, 2850, 1659, 1450, and 835 cm⁻¹ corresponding to the vibrations of C—H stretching, CH₃ asymmetric stretching, CH₂ symmetric stretching, C⚌C stretching, CH₂ deformation, and ⚌CH wagging, respectively (Ibrahim et al., 2014). The FTIR spectra of all CNR samples showed a considerable intensity drop of the C⚌C stretching peak at 1659 cm⁻¹, indicating a decrease in C⚌C bonds after the chemical modification. In addition to the typical NR peaks, many other peaks were seen in the CNR samples, including the sharp peaks at 1731, 1088, 947, and 787 cm⁻¹ that were associated with the stretching vibrations of C⚌O, C—O—C, epoxide ring, and C—Cl, respectively (De Lorenzi et al., 1999; Boochathum & Rongtongaram, 2016). The broad absorption peak of OH stretching vibration at 3427 cm⁻¹ was also observed in the CNR samples. With increasing ultrasound irradiation time, the peak intensities at 787 cm⁻¹ C—Cl bond) and 1731 cm⁻¹ (C⚌O stretching) steadily increased, while the peak intensity at 835 cm⁻¹ (C⚌C) decreased, indicating an increase in the chloroacetate content in the CNR molecules.

FTIR spectra of unmodified NR, CNR1, CNR2, and CNR3.

3.1.2

3.1.2 NMR spectroscopy

The chemical structures of the NR and CNR samples were confirmed using ¹H NMR. Fig. 3 shows examples of ¹H NMR spectra of NR, ENR (the sample taken before the addition of chloroacetic acid), and CNR3. In the NR spectrum (Fig. 3(a)), three primary signals of the cis-1,4-isoprene unit were observed at 1.6, 2.0, and 5.1 ppm, which belong to the protons of methyl (–CH₃), methylene (–CH₂), and unsaturated methine (C⚌C—H), respectively (Cuomo et al., 2007). After being subjected to performic acid under ultrasound irradiation, NR transformed into ENR, as evidenced by the additional peaks at 1.3 and 2.7 ppm, which belong to the protons attached to the oxirane ring, as shown in Fig. 3(b). After the addition of chloroacetic acid, a ring-opening reaction took place and turned most of the oxirane rings into chloroacetate groups (see Fig. 3(c)). Obviously, the existence of chloroacetate groups in CNR was supported by the ¹H NMR peaks at 1.3, 3.6, and 4.1 ppm, which belong to the protons of CH₃—C, C—OH, and CH₂—Cl, respectively.

1H NMR spectra of synthesized pathway from (a) NR to (b) ENR and (c) CNR.

A chemical structure investigation using FTIR and NMR spectroscopy reveals that the structure of CNR modified from NR latex is composed of multifunctional groups including epoxy, hydroxyl, and chloroacetate groups. Table 2 represents the calculated chloroacetate and epoxy contents in the CNR samples. As can be seen, at the lowest irradiation time (30 min), the CNR1 contained approximately 17.4% and 7.6% chloroacetate and epoxy contents, respectively. With increasing irradiation time from 30 to 60 min, the chloroacetate and epoxy contents were increased slightly, i.e., the chloroacetate content was increased from 17.4% to 19.7% mole and the epoxy content was raised from 7.6% to 9.5% mole. The findings were in line with our earlier research, which showed a rapid increase in epoxidation degree at ultrasound irradiation times up to 30 min and, beyond that, the increasing rate tended to decrease afterwards (Lorwanishpaisarn et al., 2023).

Table 2 Molecular weight and functional group content of NR and CNR samples.

Sample	Time (min)	% Chloroacetate content	% Epoxy content	${\bar{M}}_{w}$ (×10⁵ g/mol)
NR	0	–	–	9.0
CNR1	30	17.4	7.6	12.5
CNR2	45	18.5	8.8	13.6
CNR3	60	19.7	9.5	14.1

3.1.3

3.1.3 GPC results

Fig. 4 shows the GPC chromatograms of NR and CNR samples. The mean values of the weight-average molecular weight ( ${\bar{M}}_{w}$ ) (of all samples are also tabulated in Table 2. Results reveal that the ${\bar{M}}_{w}$ value of NR was 8.99 × 10⁵ g/mol. After being functionalized with peracid and followed by chloroacetic acid, the occurrence of the oxirane ring and the attachment of the chloroacetate groups to the NR molecules led to an increase in the ${\bar{M}}_{w}$ values. As expected, the ${\bar{M}}_{w}$ values of CNR tended to increase with increasing ultrasound irradiation time. Fig. 5 summarizes the proposed reactions taking place during the synthesis under ultrasound irradiation.

Proposed reactions taking place during the CNR synthesis under ultrasound irradiation.

3.2

3.2 Basic properties of the raw NR and CNR samples

Table 3 displays the basic properties of the NR and synthesized CNRs. The Mooney viscosity of the NR was 63.4 MU, while the CNR samples showed a considerably higher Mooney viscosity, i.e., 104.3 MU, 110.7 MU, and 118.6 MU, for CNR1, CNR2, and CNR3, respectively. Clearly, the increased Mooney viscosity was found after the chemical modification of NR, which is expected to be the consequence of the attachment of the appendant chloroacetate groups in CNR. In addition to the increased molecular weight (see Table 2) and molecular branching, a stronger molecular interaction induced by the polar chloroacetate and epoxy groups might be used to explain the results. The presence of epoxy and chloroacetate groups in CNR is confirmed by a higher glass transition temperature (Tg), as demonstrated in the DSC results (Table 3). The additions of polar epoxy or hydroxy groups and bulky chloroacetate groups restrict the molecular mobility of NR molecules, leading to an increase in Tg, whose magnitude depends directly on the quantity of these polar or bulky groups in the molecules (Burfield et al., 1984; Saendee and Tangboriboonrat, 2006).

Table 3 Basic properties of the rubbers.

Properties	NR	CNR1	CNR2	CNR3
Mooney viscosity^a, MU	63.4	104.3	110.7	118.6
Glass transition temperature, Tg, ^oC	−61.4	−28.9	−26.5	−25.4
Chloroacetate content, %	–	17.4	18.5	19.7
Epoxy content, %	–	7.6	8.8	9.5

a ML1 + 4@100 °C.

3.3

3.3 Rubber properties

3.3.1

3.3.1 Rubber-filler interaction

After the NR in the SSBR/NR tread compound was replaced with CNR, the magnitude of rubber-filler interaction was investigated by measuring BRC using a good solvent of both NR and CNR, i.e., toluene. Fig. 6 demonstrates the effect of rubber type on BRC. The results apparently show that the replacement of NR with CNR increased BRC and, hence, the rubber-filler interaction. In addition, the results revealed an increase in rubber-filler interaction with increasing polar functional group content. Theoretically, covalent bonding, physical adsorption of rubber molecules onto the surface of the filler, or a combination of both are responsible for the interaction between rubber and filler (Mohapatra et al., 2016). In the SSBR/NR system, the interaction between SiO₂ and NR, prior to vulcanization, takes place mostly through the Van der Waals force between rubber molecules and TESPT (which are strongly attached to the silica surface). The interaction will become stronger during vulcanization due to the formation of covalent bonds between sulfur atoms in TESPT and double bonds in NR molecules. When NR was replaced with CNR, the degree of rubber-filler interaction increased, which might be attributable to the additional interaction between the polar groups of CNR (e.g., epoxy, hydroxyl, and chloroacetate groups) and the hydroxyl groups on the SiO₂ surface as demonstrated in Fig. 7. Similar observations have also been previously reported (Siriwong et al., 2017; Boochathum & Rongtongaram, 2016). This explains why CNR3, which had the highest epoxy and chloroacetate contents, possessed the strongest rubber-filler interaction, followed by CNR2 and CNR1, respectively.

Bound rubber content (BRC) of the rubber compound.

Proposed mechanism of the interaction between CNR and silica.

3.3.2

3.3.2 Cure characteristics

Fig. 8 depicts the cure curves of the rubber compounds. The scorch time (t_s1) and optimum cure time (t_c90) of the SSBR/NR were approximately 3.2 and 8.0 min, respectively. The cure curves of the SSBR/CNR compounds slightly shifted to the right, indicating the longer scorch and cure times when NR was replaced with CNR. Although the changes in scorch and cure times of the rubber compounds were not very pronounced, the torque difference (M_H-M_L) was remarkedly reduced from 15.9 dN.m for SSBR/NR compound to 14.3, 14.7, and 14.9 dN.m for SSBR/CNR1, SSBR/CNR2, and SSBR/CNR3, respectively. The slight shift in cure time and the reduction in torque difference may arise from the fact that NR has a greater degree of unsaturation, making it more reactive to sulfur vulcanization than CNR (Hirata et al., 2014). In addition, NR contains non-rubber components like proteins or fatty acids that can act as cure activators for sulfur vulcanization (Ping-Yue et al., 2012). After the chemical modification, the number of double bonds was reduced owing to the addition of the polar chloroacetate groups. The reduction in non-rubber content may also be expected due to the repeated washing during the chemical modification.

3.3.3

3.3.3 Morphology

The degree of filler dispersion was investigated by FESEM analysis. The SEM micrographs of the SSBR/NR, SSBR/CNR1, SSBR/CNR2, and SSBR/CNR3 vulcanizates are shown in Fig. 9 (a-d), respectively. The results reveal that the SSBR/NR vulcanizate exhibited slightly poorer filler dispersion than the SSBR/CNR vulcanizates. The better filler dispersion in SSBR/CNR resulted mainly from the higher rubber viscosity after the modification (see also Table 3), causing a greater shearing force during mixing. In addition, the increased rubber-filler interaction may contribute to the enhanced filler dispersion as it provides additional drag during the mixing process.

SEM micrographs: (a) SSBR/NR, (b) SSBR/CNR1, (c) SSBR/CNR2, and (d) SSBR/CNR3.

3.3.4

3.3.4 Mechanical and dynamic properties

Table 4 lists the fundamental mechanical properties of the tread vulcanizates, i.e., hardness, modulus at 100% strain (M₁₀₀), tensile strength (TS), elongation at break (EB), and tear strength. Examples of engineering stress–strain curves of the vulcanizates are also provided in Fig. 10. The hardness of all vulcanizates was in the range of 65 ± 4 Shore A, which is the typical hardness frequently found in tire treads. Actually, the hardness values tended to increase gradually with increasing substitution group content. Similar results were also observed for modulus. As previously indicated, the increases in hardness and modulus can be attributed to the improved rubber-filler interaction. The increases in molecular weight, intermolecular forces, and molecular movement restriction induced by the attachment of polar bulky groups on the main chains of NR may somehow lead to the slightly greater stiffness of the vulcanizates. Tensile strength increased marginally as substitution group content increased, reaching a maximum of 22.7 MPa for SSBR/CNR2. The better rubber-filler interaction is assumed to be the cause of a slight improvement in tensile strength. When CNR2 was substituted with CNR3, no further improvement was seen. This is probably because there is a counterpoise between the enhanced rubber-filler interaction and the reduced capability for strain-induced crystallization when NR is attached with the bulky groups. Elongation at break tended to decrease with increasing chloroacetate content in CNR, indicating the reduction in extensibility of the rubber vulcanizates in the presence of the polar bulky groups, which restrict the molecular movement of the rubber molecules. Tear strength, on the other hand, decreased slightly with increased substitution group content. The results imply that, for tear strength, the reduced strain-induced crystallization at the crack tips plays a greater role than the improved rubber-filler interaction. However, the changes in strengths were not very obvious because the vulcanizates contained only 30% NR or CNR. In addition, it could be observed that all vulcanizates possessed relatively high strengths within a suitable hardness range, which could be used for tire tread production.

Table 4 Basic mechanical properties of the vulcanizates.

Sample	Hardness (Shore A)	M₁₀₀ (MPa)	TS (MPa)	EB (%)	Tear strength (kN/m)
SSBR/NR	64.0 ± 0.2	2.43 ± 0.3	20.2 ± 1.2	749 ± 30	100.2 ± 1.4
SSBR/CNR1	65.3 ± 0.4	2.57 ± 0.3	21.3 ± 1.9	716 ± 35	97.6 ± 2.7
SSBR/CNR2	67.0 ± 0.5	2.64 ± 0.2	22.7 ± 2.1	658 ± 45	96.3 ± 3.4
SSBR/CNR3	69.0 ± 0.3	2.72 ± 0.1	22.2 ± 0.1	665 ± 27	93.1 ± 3.2

Examples of the tensile stress–strain curves of the vulcanizates.

Table 5 shows the HBU and tire performance of the vulcanizates, i.e., abrasion resistance, tan δ at 0 °C (indicating wet grip index), and tan δ at 60 °C (indicating rolling resistance). It was found that the HBU of the SSBR/NR vulcanizate was 19^οC. When NR was replaced with CNR, the HBU gradually reduced with increasing substitution group content, indicating the reduction of hysteresis. This could be related to enhanced rubber-filler interaction, which leads to less molecular slippage at the rubber-filler interfaces, which is one of the key factors influencing rubber hysteresis during dynamic deformation.

Table 5 Tire performance and heat build-up of the vulcanizates.

Sample	HBU (°C)	Tire performance
Sample	HBU (°C)	Akron volume loss (mm³)	Tan δ at 0 °C	Tan δ at 60 °C
SSBR/NR	19	37.4 ± 0.7	0.76	0.16
SSBR/CNR1	17	36.3 ± 0.8	0.80	0.14
SSBR/CNR2	16	35.2 ± 1.3	0.82	0.13
SSBR/CNR3	15	33.9 ± 0.3	0.85	0.12

Passenger car tires are normally rated by three key characteristics: abrasion resistance, wet grip index, and rolling resistance. High abrasion-resistant tread provides a longer service life while losing only a small amount of debris on the road. In this work, the maximum volume loss (37.4 mm³) was found in the SSBR/NR vulcanizate. A slight improvement in abrasion resistance was seen when NR was replaced with CNR. It was also found that the abrasive volume loss tended to reduce gradually with increasing substitution group content in CNR. The increased rubber-filler interaction, in conjunction with the stronger molecular interaction induced by the polar groups in CNR, makes it more difficult to abrade the rubber out of the specimens. The wet grip index is generally indicated by the value of tan δ at 0 °C. The high magnitude of tan δ at 0 °C reflects good wet traction of the tread and, thus, provides high driving safety. In this study, the values of tan δ at 0 °C increased continuously as the substitution group content in CNR increased. Thus, CNR3 provided the highest driving safety on wet roads, followed by CNR2, CNR1, and NR, respectively. A thorough look at the DMA curves in Fig. 11 reveals that the increase in tan δ values at 0 °C might be the consequence of the better filler dispersion, as evidenced by the increases in peak area of the curves, particularly the peaks on the right, which belong to the transition of SSBR. It is expected that the improved filler dispersion may result from the greater initial viscosity of the raw rubbers (see also Table 3), leading to a greater shearing force during the mixing process. The area of the tan δ peak at Tg has previously been reported to be directly related to the amount of rubber molecules that can participate in the transition. (Somseemee et al., 2022; Boopasiri et al., 2022b). The improved filler dispersion, which releases trapped rubber molecules into mobilized rubber, results in a larger peak area and peak height of the tan δ peak. As a result, the values of tan δ at 0 °C slightly increased with improved filler dispersion. The same phenomenon has previously been revealed by Thaptong et al. (Thaptong et al., 2016). When fuel consumption is of great concern, the rolling resistance of a tire tread, which can be represented by the value of tan δ at 60 °C, should be kept as low as possible to diminish the anti-rolling movement of the tire tread, showing the potential for energy savings. Again, the replacement of NR with CNR resulted in a reduction in rolling resistance, as obviously seen from the continuous reduction in the values of tan δ at 60 °C as the substitution group content in CNR increased. The reduced tan δ values at 60 °C can be explained by the improved rubber-filler interaction in conjunction with the enhanced filler dispersion.

Values of tan δ at different temperatures of the rubber vulcanizates.

Fig. 12 shows the magic triangle charts, comparing the tread performance of various vulcanizates by using the SSBR/NR vulcanizate as a benchmark (assumed to be 100% in all properties). Apparently, CNR outperforms NR in all aspects, including abrasion resistance, fuel saving efficiency (FSE), and wet grip (WG). The tire performance is continuously improved by increasing the chloroacetate group content in CNR. In this study, the replacement of NR with CNR3 provided improvements in abrasion resistance, WG, and FSE of about 10%, 12%, and 25%, respectively.

The “magic triangle” of tire performance.

4 Conclusions

This research successfully prepared CNR from NR latex through the two-step synthesis process. In the first step, NR was epoxidized to form ENR by performic acid before being turned into CNR by the addition of an excessive amount of chloroacetic acid in the second step. Ultrasound irradiation was applied to speed up the reaction rate during the first preparation step. The ultrasound irradiation time was varied from 30, 45, and 60 min, which yielded CNR with approximately 17.4%, 18.5%, and 19.7% chloroacetate contents, respectively. There were also some epoxy and hydroxyl groups in the CNR structure. The addition of these polar functional groups to NR caused noticeable increases in the molecular weight, viscosity, and glass transition temperature of the rubbers. When NR was fully replaced by CNR (CNR1, CNR2, and CNR3) in the silica-filled SSBR/NR tread compounds, slight increases in modulus, hardness, and tensile strength were reported with a small sacrifice of tear strength. The changes in mechanical properties were more pronounced when the chloroacetate content of CNR was increased. The replacement of NR with CNR also showed beneficial effects not only on heat build-up, but also on overall tire performance because all three main indicators, i.e., abrasion resistance, FSE, and WG, were obviously improved with the increased chloroacetate content in CNR. Such improvements can be mainly explained by enhanced rubber-filler interaction and better filler dispersion. The results show the high potential use of CNR to replace NR in the production of green and high-performance tire treads.

CRediT authorship contribution statement

Terana Senakham: Methodology, Investigation, Writing – original draft. Puchong Thaptong: Investigation, Methodology. Pongdhorn Sae-Oui: Methodology, Formal analysis, Writing – review & editing. Supparoek Boopasiri: Writing – original draft. Sittipong Amnuaypanich: Resources. Chomsri Siriwong: Conceptualization, Methodology, Validation, Project administration, Supervision, Writing – original draft, Funding acquisition.

Acknowledgements

Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, the Materials Chemistry Research Center (MCRC), Research and Graduate Studies, Khon Kaen University, Thailand, and CG Engineering Co., Ltd., are acknowledged.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

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Introduction

Experimental

Materials
Synthesis of CNR via a sonochemical method
Characterization of CNR
Rubber compound preparation and testing
Rubber vulcanizate testing

Results and discussion

Characterization of NR and CNR
Basic properties of the raw NR and CNR samples
Rubber properties

Conclusions

CRediT authorship contribution statement

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[1] Bikiaris D.N., Vassiliou A., Pavlidou E., Karayannidis G.P., . Compatibilisation effect of PP-g-MA copolymer on iPP/SiO₂ nanocomposites prepared by melt mixing. Eur. Polym. J.. 2005;41(9):1965-1978.
[CrossRef] [Google Scholar]

[2] Boochathum P., Rongtongaram N., . Influence of chloroacetate group on physical properties of natural rubber and its interaction with fillers. J. Appl. Polym. Sci.. 2016;133(13):43076.
[CrossRef] [Google Scholar]

[3] Boopasiri S., Sae-Oui P., Siriwong C., . Fabrication of microcrystalline cellulose/zinc oxide hybrid composite by hydrothermal synthesis and its application in rubber compounding. J. Appl. Polym. Sci.. 2022;139(18):52065.
[CrossRef] [Google Scholar]

[4] Boopasiri S., Thaptong P., Sae-Oui P., Siriwong C., . Fabrication of zinc oxide-coated microcrystalline cellulose and its application in truck tire tread compounds. J. Appl. Polym. Sci.. 2022;139(31):e52701.
[Google Scholar]

[5] Burfield D.R., Lim K.L., Law K.S., . Epoxidation of natural rubber lattices: methods of preparation and properties of modified rubber. J. Appl. Polym. Sci.. 1984;29:1661-1673.
[Google Scholar]

[6] Cuomo C., Serra M.C., Maupoey M.G., Grassi A., . Copolymerization of styrene with butadiene and isoprene catalyzed by the monocyclopentadienyl titanium complex Ti(η5-C5H5)(η2-MBMP)Cl. Macromolecules. 2007;40(20):7089-7097.
[CrossRef] [Google Scholar]

[7] De Lorenzi A., Giorgianni S., Bini R., . High-resolution FTIR spectroscopy of the C-Cl stretching mode of vinyl chloride. Mol. Phys.. 1999;96(1):101-108.
[CrossRef] [Google Scholar]

[8] Flanigan C.M., Beyer L., Klekamp D., Rohweder D., . Comparative study of silica, carbon black and novel fillers in tread compounds. Rubber World. 2012;245:18-31.
[Google Scholar]

[9] Hirata, Y., Kondo, H., Ozawa, Y. 2014. In Chemistry, Manufacture and Applications of Natural Rubber; Kohjiya, S., Ikeda, Y. (Eds.) Woodhead Publishing: UK. p.341.

[10] Ibrahim S., Daik R., Abdullah I., . Functionalization of liquid natural rubber via oxidative degradation of natural rubber. Polym.. 2014;6(12):2928-2941.
[CrossRef] [Google Scholar]

[11] Kaewsakul W., Sahakaro K., Dierkes W.K., Noordermeer J.W.M., . Optimization of rubber formulation for silica-reinforced natural rubber compounds. Rubber Chem. Technol.. 2013;86(2):313-329.
[CrossRef] [Google Scholar]

[12] Li Y., Han B., Wen S., Lu Y., Yang H., Zhang L., Liu L., . Effect of the temperature on surface modification of silica and properties of modified silica filled rubber composites. Compos. -A: Appl. Sci. Manuf.. 2014;62:52-59.
[CrossRef] [Google Scholar]

[13] Liu X., Zhao S., Zhang X., Li X., Bai Y., . Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites. Polym.. 2014;55(8):1964-1976.
[CrossRef] [Google Scholar]

[14] Lorwanishpaisarn N., Sae-Oui P., Sirisinha C., Siriwong C., . A new approach to the epoxidation of natural rubber through a sonochemical method. Ind. Crop. Prod.. 2023;197:116629
[CrossRef] [Google Scholar]

[15] Luo Y.Y., Wang Y.Q., Zhong J.P., He C.Z., Li Y.Z., Peng Z., . Interaction between fumed-silica and epoxidized natural rubber. J. Inorg. Organomet. Polym Mater.. 2011;21:777-783.
[CrossRef] [Google Scholar]

[16] Maghami S., Dierkes W.K., Noordermeer J.W.M., . Functionalized SBRS in silica-reinforced tire tread compounds: evidence for interactions between silica filler and zinc oxide. Rubber Chem. Technol.. 2016;89(4):559-572.
[CrossRef] [Google Scholar]

[17] Mazumder A., Chanda J., Bhattacharyya S., Dasgupta S., Mukhopadhyay R., Bhowmick A., . Improved tire tread compounds using functionalized styrene butadiene rubber-silica filler/hybrid filler systems. J. Appl. Polym. Sci.. 2021;138(42):51236.
[CrossRef] [Google Scholar]

[18] Medalia A.I., . Filler aggregates and their effect on reinforcement. Rubber Chem. Technol.. 1974;47(2):411-433.
[CrossRef] [Google Scholar]

[19] Mensah B., Agyei-Tuffour B., Nyankson E., Bensah Y.D., Dodoo-Arhin D., Bediako J.K., Onwona-Agyeman B., Yaya A., . Preparation and characterization of rubber blends for industrial tire tread fabrication. Int. J. Polym. Sci. 20182473286
[CrossRef] [Google Scholar]

[20] Meon W., Blume A., Luginsland H.D., Uhrlandt S., . Silica and silanes, Rubber Compounding: Chemistry and Applications. NY, USA: Marcel Dekker; 2004.

[21] Mohapatra S., Alex R., Nando G.B., . Cardanol grafted natural rubber: a green substitute to natural rubber for enhancing silica filler dispersion. J. Appl. Polym. Sci.. 2016;133(8):43057.
[CrossRef] [Google Scholar]

[22] Pattanawanidchai S., Sae-Oui P., Sirisinha C., Siriwong C., . Cure retardation of peroxide-cured silica filled natural rubber influenced by organosilane. Polym. Eng. Sci.. 2019;59(1):42-48.
[CrossRef] [Google Scholar]

[23] Ping-Yue W., Young-Zhou W., Bei-Long Z., Hong-Hai H., . Effect of non-rubber substances on vulcanization kinetics of natural rubber. J. Appl. Polym. Sci.. 2012;126(4):1183-1187.
[CrossRef] [Google Scholar]

[24] Pustak A., Denac M., Leskovac M., Švab I., Musil V., Šmit I., . Morphology and mechanical properties of iPP/Silica composites modified with (Styrene-b-ethylene-co-butylene-b-styrene) grafted with maleic anhydride. Polym. Plast. Technol. Eng.. 2015;54(6):647-660.
[CrossRef] [Google Scholar]

[25] Qu L., Yu G., Xie X., Wang L., Li J., Zhao Q., . Effect of silane coupling agent on filler and rubber interaction of silica reinforced solution styrene butadiene rubber. Polym. Compos.. 2013;34(10):1575-1582.
[CrossRef] [Google Scholar]

[26] Rauline, R., 1993. U.S. Patent 5,227,425. Retrieved in 23December 2015, from http://www.google.com/patents/US5227425.

[27] Rombaldi S.H., Pierozan N.J., Brandalise R.N., . Influence on mechanical and rheometric properties through different methods of chemical modification of silica applied to natural rubber compounds. J. Elastomer Plast.. 2021;53(2):165-187.
[CrossRef] [Google Scholar]

[28] Saendee P., Tangboriboonrat P., . Latex interpenetrating polymer networks of epoxidised natural rubber/poly(methyl methacrylate): an insight into the mechanism of epoxidation. Colloid Polym. Sci.. 2006;284:634-643.
[Google Scholar]

[29] Saengdee L., Phinyocheep P., Daniel P., . Chemical modification of natural rubber in latex stage for improved thermal, oil, ozone and mechanical properties. J. Polym. Res.. 2020;27(9):275.
[CrossRef] [Google Scholar]

[30] Sae-oui P., Suchiva K., Thepsuwan U., Intiya W., Yodjun P., Sirisinha C., . Effects of blend ratio and SBR type on properties of silica-filled SBR/NR tire tread compounds. Rubber Chem. Technol.. 2016;89(2):240-250.
[CrossRef] [Google Scholar]

[31] Sahakaro K., Beraheng S., . Reinforcement of maleated natural rubber by precipitated silica. J. Appl. Polym. Sci.. 2008;109(6):3839-3848.
[CrossRef] [Google Scholar]

[32] Samsudin D., Ismail H., Othman N., Abdul Hamid Z.A., . Comparative study of glut palmitate salt and polyethylene grafted maleic anhydride compatibilizer on the properties of silica filled high-density polyethylene composites. Polym. Test.. 2016;52:104-110.
[CrossRef] [Google Scholar]

[33] Sekharan R.V., Abraham B.T., Thachil E.T., . Utilization of waste expanded polystyrene: blends with silica-filled natural rubber. Mater. Des.. 2012;40:221-228.
[CrossRef] [Google Scholar]

[34] Seo G., Park S.M., Ha K., Choi K.T., Hong C.K., Kaang S., . Effectively reinforcing roles of the networked silica prepared using 3,3′ bis(triethoxysilylpropyl)tetrasulfide in the physical properties of SBR compounds. J. Mater. Sci.. 2010;45:1897-1903.
[CrossRef] [Google Scholar]

[35] Sirisinha C., Sae-oui P., Suchiva K., Thaptong P., . Properties of tire tread compounds based on functionalized styrene butadiene rubber and functionalized natural rubber. J. Appl. Polym. Sci.. 2019;137(20):48696.
[CrossRef] [Google Scholar]

[36] Siriwong C., Sae-Oui P., Sirisinha C., . Comparison of coupling effectiveness among amino-, chloro-, and mercapto silanes in chloroprene rubber. Polym. Test.. 2014;38:64-72.
[CrossRef] [Google Scholar]

[37] Siriwong C., Sae-Oui P., Sirisinha C., . Performance comparison of various surface modifying agents on properties of silica-filled chloroprene rubber. Rubber Chem. Technol.. 2017;90(1):146-158.
[CrossRef] [Google Scholar]

[38] Somseemee O., Saeoui P., Schevenels F.T., Siriwong C., . Enhanced interfacial interaction between modified cellulose nanocrystals and epoxidized natural rubber via ultraviolet irradiation. Sci. Rep.. 2022;12:6682.
[CrossRef] [Google Scholar]

[39] Sun C., Wen S., Ma H., Li Y., Chen L., Wang Z., Yuan B., Liu L., . Improvement of silica dispersion in solution polymerized styrene-butadiene rubber via introducing amino functional groups. Ind. Eng. Chem. Res.. 2019;58(3):1454-1461.
[CrossRef] [Google Scholar]

[40] Thaptong P., Sae-Oui P., Sirisinha C., . Effects of silanization temperature and silica type on properties of silica-filled solution styrene butadiene rubber (SSBR) for passenger car tire tread compounds. J. Appl. Polym. Sci.. 2016;133(17):43342.
[CrossRef] [Google Scholar]

[41] Thaptong P., Sae-Oui P., Jittham P., Sirisinha C., . Optimization of highly dispersible silica/carbon black hybrid filler ratio for tire tread based on solution- and emulsion-styrene butadiene rubber. J. Appl. Polym. Sci.. 2022;139(29):e52608
[CrossRef] [Google Scholar]

[42] Utara, S., Boochatum, P., 2009. In The Minerals, Metals and Materials Society. In: Proceedings of the 3rd International Conference on Processing Materials for Properties, Bangkok, Thailand, Dec 7–10, Paper No. 409.

[43] Wang L., Zhao S., Li A., Zhang X., . Study on the structure and properties of SSBR with large-volume functional groups at the end of chains. Polym.. 2010;51(9):2084-2090.
[CrossRef] [Google Scholar]

[44] Wolff S., Gorl U., Wang M.J., Wolff W., . Silica-based tread compounds. Eur. Rubber J.. 1994;176(1):16-19.
[Google Scholar]

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Synthesis of sonochemical chloroacetated natural rubber and its potential use in passenger car tire tread

Abstract

Abstract

Keywords

Ultrasound

Natural rubber

Chloroacetated natural rubber

Tread

Tire

1 Introduction

2 Experimental

2.1 Materials

2.2 Synthesis of CNR via a sonochemical method

2.3 Characterization of CNR

2.4 Rubber compound preparation and testing

2.5 Rubber vulcanizate testing

3 Results and discussion

3.1 Characterization of NR and CNR

3.1.1 FTIR analysis

3.1.2 NMR spectroscopy

3.1.3 GPC results

3.2 Basic properties of the raw NR and CNR samples

3.3 Rubber properties

3.3.1 Rubber-filler interaction

3.3.2 Cure characteristics

3.3.3 Morphology

3.3.4 Mechanical and dynamic properties

4 Conclusions

CRediT authorship contribution statement

Acknowledgements

Declaration of competing interest

References

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