Cyclization of natural rubber (NR) latex: Synthesis, characterization and application in NR compounds and as a compatibilizer in NR/acrylonitrile butadiene rubber (NBR) blends

Abstract

Cyclized natural rubber (CNR) was synthesized through the acid-catalyzed reaction of natural rubber (NR) latex using sulfuric acid as a catalyst and stabilized with a non-ionic surfactant. Cyclization was evaluated by iodine numbers under varying reaction times, temperatures, and NR-to-acid ratios. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy ("H-NMR) confirmed the formation of cyclic structures in CNR molecules. Differential scanning calorimetry (DSC) showed that the glass transition temperature (7,) of CNR increased with cyclization, indicating greater rigidity and less chain flexibility. CNR was then blended with NR and used as a compatibilizer in NR/acrylonitrile butadiene rubber (NBR)blends. It increased blend viscosity, hardness, and dimensional stability but reduced tensile strength and elongation due to its rigid cyclic domains. In NR/NBR blends, CNR outperformed a commercial homogenizer in enhancing interfacial interactions, leading to superior shear flow properties, curing behavior, and mechanical performance. This is attributed to the polar groups in CNR, which enhance intermolecular interactions and phase compatibility, resulting in finer phase morphology. This study highlights the potential of CNR as a versatile material for enhancing the performance of rubber compounds, with promising applications in advanced industrial formulations.

Headnote

Abstract. Cyclized natural rubber (CNR) was synthesized through the acid-catalyzed reaction of natural rubber (NR) latex using sulfuric acid as a catalyst and stabilized with a non-ionic surfactant. Cyclization was evaluated by iodine numbers under varying reaction times, temperatures, and NR-to-acid ratios. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy ("H-NMR) confirmed the formation of cyclic structures in CNR molecules. Differential scanning calorimetry (DSC) showed that the glass transition temperature (7,) of CNR increased with cyclization, indicating greater rigidity and less chain flexibility. CNR was then blended with NR and used as a compatibilizer in NR/acrylonitrile butadiene rubber (NBR)blends. It increased blend viscosity, hardness, and dimensional stability but reduced tensile strength and elongation due to its rigid cyclic domains. In NR/NBR blends, CNR outperformed a commercial homogenizer in enhancing interfacial interactions, leading to superior shear flow properties, curing behavior, and mechanical performance. This is attributed to the polar groups in CNR, which enhance intermolecular interactions and phase compatibility, resulting in finer phase morphology. This study highlights the potential of CNR as a versatile material for enhancing the performance of rubber compounds, with promising applications in advanced industrial formulations.

Keywords: natural rubber latex, cyclization, sulfuric acid, acrylonitrile butadiene rubber, curing properties, mechanical properties.

1. Introduction

Natural rubber (NR), a biopolymer primarily sourced from Hevea Brasiliensis, is composed mainly of cis1,4-polyisoprene [1]. Recognized as a renewable and eco-friendly green polymer, NR is highly valued for its exceptional elasticity and resilience, making itindispensable in a wide range of industrial applications. The molecular structure of NR contains an unsaturated bond (-C=C-) in each isoprenyl unit, which not only facilitates vulcanization but also provides significant potential for enhancing its properties and expanding its applications through chemical modifications. Consequently, the chemical modification of NR molecules has been extensively explored to enhance their mechanical, thermal, and chemical properties for industrial applications. Recent advancements have focused on modifications during the latex stage to avoid solvent toxicity, including epoxidation, hydrogenation, and grafting vinyl monomers onto NR molecules, which have proven effective in tailoring NR for diverse and specialized applications [2, 3]. For example, epoxidation followed by hydrogenation of natural rubber latex (NRL) was performed continuously in a one-pot system, resulting in significant improvements in thermal and mechanical properties, as well as enhanced ozone resistance [2]. Additionally, hydrogenated natural rubber containing epoxide groups was synthesized, exhibiting enhanced thermal stability [4]. Hydrogenated natural rubber (HNR) latexes filled with expanded graphite (EG) were also developed as electromagnetic shielding materials [5]. Similarly, cardanol, a renewable resource and a byproduct of the cashew industry, was grafted onto NR, resulting in modified NR with improved compatibility and performance [6]. These advancements highlight the increasing focus on sustainable and functional modifications to NR to address diverse industrial applications.

Cyclization is a chemical process used to modify NR molecules by introducing cyclic structures along the polyisoprene backbone. This specialized NR derivative is well-known for its enhanced properties, such as increased thermal stability, improved hardness, and better compatibility with other polymers [7-9]. Cyclization of natural rubber is typically performed in the latex state [7-10], though it can also be achieved in a rubber solution under certain conditions [1 1, 12]. Cyclization of natural rubber has been achieved through the use of various catalytic systems that facilitate intramolecular reactions of polyisoprene chains. Proton donor catalysts such as sulfuric acid [7, 8, 10] and p-toluenesulfonic acid [13] have been widely employed, alongside Lewis acids like stannic chloride and tin tetrachloride [11, 12]. Additionally, alternative catalytic systems, including phosphorus pentoxide [14], trimethylsilyl triflate [15, 16], and a combination of benzotrichloride with sulfuric acid [9], have demonstrated efficacy in inducing the formation of cyclic structures. The cyclization process transforms polyisoprene chains by eliminating their unsaturation, resulting in materials that exhibit a gradual loss of the natural elasticity inherent to natural rubber (NR). This progression continues until a hard, rigid, or even powdery product is achieved, depending on the degree of cyclization [9, 12]. Despite the loss of elasticity, the cyclization process significantly enhances the material's properties, including superior thermal stability, increased rigidity, and heightened resistance to oxidative degradation [9]. These improvements greatly expand its potential applications, making cyclized natural rubber a valuable material for advanced industrial and engineering uses. Cyclized natural rubber broadens its applicability to a range of novel and specialized uses, including adhesives, printing inks, paints, coatings, and advanced composite materials [9, 17, 18]. In particular, it is highly valued in the formulation of heat- and chemical-resistant, as well as anticorrosive paints. Specific applications include primers for self-adhesive tapes, adhesive formulations, marine paints, mirror backing paints, swimming pool paints, and cold galvanizing primers [18]. These diverse applications underscore the material's versatility and enhanced performance characteristics. CNR can be further modified for high-performance applications, for example, through grafting with maleic anhydride (MA) in the melt phase. The incorporation of trimethylolpropane triacrylate (TMPTA) in this process significantly enhances the degree of grafting [19]. Owing to the cyclic structures and the presence of some polarity in cyclized natural rubber structure, this material holds potential as a blend compatibilizer for NR blends with other polar rubbers, such as acrylonitrile butadiene rubber (NBR). The primary objective of such applications is to improve key properties, including curing behavior and mechanical performance, making CNR an invaluable component in advanced material formulations. It is noteworthy that blends of natural rubber (NR) and acrylonitrile butadiene rubber (NBR) offer a balanced combination of properties. NR provides high tensile strength, while NBR imparts excellent resistance to oils and solvents. This synergy allows for the development of materials with tailored performance for a wide range of applications, including adhesives [20], seals [21], hoses, belts, and gaskets [20-22], as well as damping base isolators [23]. Additionally, these blends are widely used in rubber products such as hydraulic cylinder seals and other components that are regularly exposed to oils and solvents [24].

In the current study, cyclized natural rubber (CNR) with varying degrees of cyclization was prepared inhouse using a latex medium. The process employed sulfuric acid as a catalyst, stabilized by a non-ionic surfactant. The influence of reaction time, temperature, and the molar ratio of natural rubber (NR) to H»SO4 on the degree of cyclization, as measured by iodine number, was systematically investigated. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy !H-NMR) were utilized to analyze the molecular characteristics of the CNR products. Additionally, differential scanning calorimetry (DSC) was employed to determine the thermal properties, specifically the glass transition temperature (7). The prepared CNR was subsequently used as a blend component with NR to evaluate its impact on curing and mechanical properties. Furthermore, CNR was tested as a blend compatibilizer for NR/NBR blends, with its performance compared against a commercial homogenizer, ULTRA-BLEND™ 6000. Particular emphasis was placed on examining the curing behavior, mechanical properties, morphological characteristics, and flow properties of the resulting blends, providing valuable insights into the potential of CNR as an advanced material component.

2. Experimental

2.1. Materials

High-ammonia (HA) concentrated natural rubber latex, with a total solid content (TSC) of approximately 61.43%, was procured from Yala Latex Co., Ltd. (Yala, Thailand) for use as the raw material in the preparation of cyclized natural rubber (CNR). The latex was diluted to 25% dry rubber content (DRC) before undergoing the cyclization process. Sulfuric acid (H,SO4), with an assay purity of 99.99%, served as the catalyst (cyclizing agent) for the cyclization reaction and was obtained from Sigma-Aldrich Chemie GmbH (Taufkirchen, Germany). Terric N30 (Huntsman Corporation Australia

Pty Ltd., Melbourne, Australia), an alkylphenol ethoxylate non-ionic surfactant, was employed to stabilize the latex under acidic conditions during the cyclization process. This prevented phase separation and ensured a uniform dispersion of rubber particles and additives. Terric N30 has a pH of approximately 6.8, a viscosity of 1.5 mPa-s at 20°C, and a specific gravity of 1.066 g/cm? at 20°C. Its hydroxyl value of 33 mg KOH/g reflects the presence of hydrophilic groups, while its cloud point (above 100 °C for a 1% solution) confirms thermal stability suitable for the reaction conditions. To monitor the extent of unsaturation, which directly correlates with the degree of cyclization during the process, the iodine number of both unmodified NR and CNR products was measured using Wijs solution. This specialized solution, comprising 0.1 mol/L iodine monochloride (ICI) dissolved in glacial acetic acid, was supplied by SigmaAldrich Chemie GmbH (Taufkirchen, Germany). Natural rubber in the form of air-dried sheets (ADS), used as a blending component with CNR and nitrile rubber, was manufactured by the Ban Khuan Ban Tae Rubber Plantation Fund Cooperative Limited (Phatthalung, Thailand). Additionally, nitrile rubber (NBR), specifically Krynac® 3345F, was employed as a blending component with NR to evaluate the compatibilizing efficacy of CNR. This NBR grade, supplied by Arlanxeo (Mumbai, India), is a coldpolymerized butadiene-acrylonitrile copolymer with an acrylonitrile content of 33+1 wt% and a Mooney viscosity (ML (1+4) 100°C) of 45+5 MU. For comparison purposes, a commercial non-staining homogenizer, ULTRA-BLEND™ 6000 (UB6000), was utilized as a compatibilizing agent in the NR/NBR blend. This chemical is reported to enhance the homogeneity of polymer blends with varying viscosities or polarities. It is noted that ULTRA-BLEND™ 6000 (UB6000) is generally derived from a mixture of aliphatic hydrocarbon resins. Additional chemicals for rubber compounding were used as received. Cure activators included stearic acid (Imperial Industrial Chemical Co., Ltd., Pathum Thani, Thailand) and zinc oxide (Global Chemical Co., Ltd., Samut Prakarn, Thailand). The curing accelerator, N-tertbutyl-2-benzothiazolesulfenamide (TBBS), was supplied by Thermo Fisher Scientific Chemicals, Inc. (Ward Hill, USA), while sulfur curing agents were obtained from Mahachai Chemicals Co., Ltd. (Samut Sakorn, Thailand). Furthermore, 2,2,4-trimethyl-1,2dihydroquinoline (TMQ), used as an antioxidant to stabilize the rubber compounds, was sourced from Henan Kailun Chemical Co., Ltd. (Henan, China).

2.2. Cyclization of natural rubber in the latex state

The cyclization process of high ammonia concentrated latex began by diluting the latex from about 60% DRC to approximately 25% DRC by adding distilled water and a 20 wt% solution of the nonionic surfactant (Terric N30) into a 2.5 L reactor. The latex mixture was then stirred at a moderate speed of 100 rpm for approximately 15 min to ensure thorough dispersion of the surfactant and rubber particles within the latex medium. Subsequently, sulfuric acid (30 wt%) was slightly added dropwise to the mixture over a period of 90 min, maintaining a fixed molar ratio of NR:H>SO4 at 1.00:1.50. This controlled addition allowed for precise acid-catalyzed cyclization, ensuring uniform reaction progress and minimizing unwanted side reactions. The reaction was initiated at about a temperature of 30 °C. As the acid addition progressed, the temperature was gradually increased, reaching 80°C by the time the acid addition was completed. The reaction mixture was maintained at this elevated temperature (80 °C) for a total reaction time of 8 h, allowing for complete cyclization of the natural rubber molecules.

During this reaction period, samples were taken at regular intervals, every hour, until the reaction was complete. Furthermore, the influence of reaction temperature on cyclization was investigated by varying the temperature at 60, 70, 80, and 90 °С at a fixed reaction time of 8 h. Upon completion of the reaction, the latex product was coagulated by adding boiling water, which facilitated the separation of the rubber phase from the water medium. The coagulated rubber was then neutralized through repeated washing with a sodium hydroxide solution (10 wt%), followed by rinsing with deionized water until the pH of the wash water reached approximately 7.0, ensuring neutral conditions. The final cyclized rubber product was dried in a vacuum oven at 40°C for about 24 h to remove any residual moisture, yielding a stable product suitable for subsequent testing and characterization. It is noted that the influence of molar ratios of NR to H,SO4 on the properties of CNR was also investigated by varying the ratios to 1.00:1.00, 1.00:1.25, 1.00:1.50, 1.00:1.75, and 1.00:2.00, while maintaining the reaction at the optimum temperature and time. To facilitate understanding of the experimental process, the CNR synthesis is schematically illustrated in Figure la.

2.3. Determination of unsaturation in NR molecules

In this study, the iodine number was measured to quantify the carbon-carbon double bonds (C=C) in the rubber sample, serving as an indicator to monitor the extent of cyclization in NR molecules. The iodine number reflects the halogen content reacting with a 100 g sample, expressed as the equivalent weight of iodine in grams [25, 26]. For the analysis, approximately 0.1 g of finely cut CNR was dissolved in a mixture of 25 mL each of chloroform and p-dichlorobenzene by heating until the rubber was fully dissolved. The solution was then cooled to room temperature, and 25 mL of Wijs reagent was added. The mixture was conditioned in the dark at room temperature for approximately 24 h. Afterward, 20 mL of potassium iodide solution and 100 mL of distilled water were added, mixed thoroughly, and immediately titrated with standard 0.1 N sodium thiosulfate solution until the yellow coloration of the mixture nearly disappeared. At this stage, 1 mL of a 10 wt% starch indicator solution was added, and titration continued with 0.1 N sodium thiosulfate until the blue starch-iodine complex color disappeared, leaving the solution colorless for at least 1 min. A blank solution, prepared using the same procedure without CNR, was also titrated for comparison and calculation. The iodine number was determined using the Equation (1) [27]:

... (1)

where И, is the volume [mL] of 0.1 N sodium thiosulfate used to titrate the blank solution, V> is the volume [mL] of 0.1 N sodium thiosulfate used to titrate the CNR sample, N is the concentration of the sodium thiosulfate solution, 12.69 is a conversion factor based on the atomic weight of iodine, and Y is the weight [g] of the CNR sample.

2.4. Compounding of CNR/NR blends

Natural rubber air-dried sheets (ADS) were first dried and then masticated using an internal mixer (Model MX 500, Chareon Tut Co., Ltd., Samut Prakan, Thailand) equipped with tangential-type rotors for approximately 3 min to reduce its molecular weight and shear viscosity, thereby enhancing the efficiency of the mixing and compounding process. Subsequently, CNR was incorporated into the mixing chamber at varying concentrations of 5, 10, 20, 30, 40, 50, and 60 phr. The mixing process was then continued for an additional 3 min under controlled conditions, maintaining a temperature of 50°C, a rotor speed of 60 rpm, and a fill factor of approximately 85% of the mixer's total capacity. Following the initial blending, various compounding ingredients, as detailed in Table 1, were systematically added to the mixing chamber according to the specific timing sequence outlined in the table. The rubber blend was then subjected to a compound finishing process for approximately 3 min to ensure thorough homogenization of the rubber and chemical components. Once mixed, the compound was discharged from the mixing chamber and passed through a two-roll mill for several cycles to produce a smooth and uniform sheet with further homogenization of all ingredients. The resulting sheet was conditioned at room temperature for a minimum of 24 h prior to subsequent property characterization.

2.5. Compounding of NR/NBR blends with CNR and UB6000 compatibilizers

Natural rubber (ADS) was first masticated for 3 min following the same procedure as described in the previous section, except at a higher mixing temperature of 70 °C. Subsequently, NBR was incorporated into the mixing chamber and blended for an additional 3 min. Different contents of CNR, synthesized with a molar ratio of NR to H,SO4 = 1.00:1.75, were then added as a compatibilizing agent. For comparison, the commercial compatibilizing agent UB6000 was also introduced at the same loading level. Thereafter, various compounding chemicals were sequentially incorporated into the mixing chamber following the mixing protocol detailed in Table 2. After a compound finishing step lasting approximately 3 min, the mixture was discharged from the chamber. The compound was further processed by passing it through a two-roll mill for several cycles, ensuring simultaneous sheeting and further homogenization of the chemical ingredients. The resulting sheet was conditioned at room temperature for approximately 24 h before proceeding with characterization. The schematic representation of the compounding of NR/NBR blends and their subsequent characterization is also illustrated in Figure 1b.

2.6. Fourier transform infrared (FTIR) spectrophotometry

FTIR spectroscopy was employed to elucidate the chemical structure of cyclized natural rubber in comparison to neat natural rubber, utilizing the high sensitivity of the Bruker INVENIO® FTIR spectrophotometer (Bruker, Ettlingen, Germany). The instrument is equipped with a diamond crystal attenuated total reflection (ATR) accessory and is complemented by the advanced OPSU 8.5 software for detailed data analysis, monitoring, and reporting. Measurements were conducted over a broad spectral range of 4000 to 400 cm, with 64 scans performed at a fine resolution of 4 cm"!, ensuring precise detection of molecular vibrations.

2.7. Proton nuclear magnetic resonance spectroscopy ( 'H-NMR)

The 'H-NMR spectra of natural rubber (NR) and various types of cyclized natural rubbers (CNRs) were analyzed using a proton nuclear magnetic resonance spectrometer (600 MHz Avance III HD AscendTM, Bruker, Fállanden, Switzerland) at room temperature. Prior to analysis, the rubber samples were dissolved in deuterated chloroform (CDCIs) as a solvent.

2.8. Differential scanning calorimetry (DSC)

Differential scanning calorimetry was carried out using a DSC 8500 apparatus from PerkinElmer, Inc. (Shelton, USA). Indium was employed as the calibration standard due to its well-characterized melting point and enthalpy of fusion, ensuring accurate temperature calibration. The sample was prepared with care and enclosed in a sealed aluminum pan, while an empty aluminum pan was used as the reference. The DSC analysis spanned a temperature range of -80 to 160°C, with a constant heating rate of 10°C/min. To remove any prior thermal history and achieve consistent measurements, the sample was initially cooled from room temperature to -80 °C at the same rate. This preconditioning step ensured that the thermal transitions captured during subsequent heating accurately reflected the inherent properties of the material. After preconditioning, the sample was reheated from -80 to 160°C at 10°C/min, with continuous recording of thermal transitions. The resulting data, presented as DSC thermograms, depicted heat flow changes as a function of temperature. These thermograms offered valuable information about the material's thermal characteristics, such as the glass transition temperature (7), which was observed as a step change in the baseline heat flow. This shift indicated a change in heat capacity (ср) as the polymer transitioned from a rigid, glass-like state to a more elastic, rubber-like state. The 7, was determined from the midpoint of this baseline shift.

2.9. Rheological properties

Two approaches were employed to evaluate the rheological properties of the rubber compounds: shear flow properties and Mooney viscosity. For shear flow characterization, a capillary rheometer (Model КН", Rosand Precision Limited, Stourbridge, UK) was used. The rheometer was equipped with a capillary die of 2 mm diameter, 32 mm length, and a 180° entry angle. Tests were conducted at 160°C within a shear rate range of 10-2000 s'. The results were presented as the relationship between apparent shear stress and apparent shear rate (flow curve) as well as apparent shear viscosity and apparent shear rate (viscosity curve). Mooney viscosity, on the other hand, was measured using a Visctech+ Mooney viscometer (Techpro Co. Ltd., Cuyahoga Falls, USA), equipped with a large rotor. The temperature of the upper and lower dies was controlled at 100+0.03 °C. The testing procedure began with preheating the sample for 1 min, followed by rotating the rotor at a speed of 240.02 rpm for 4 min. The Mooney viscosity was reported as ML(1+4, 100 °C) in accordance with ASTM D1646 standards.

2.10. Curing characterization

The curing characteristics of rubber compounds were assessed using a moving die rheometer (model MD+, Tech Pro, Inc., Cuyahoga Falls, USA). Before testing, the rubber samples were preconditioned at 23+5°C for at least 3 h. Measurements were performed at 180°C with an oscillation frequency of 1.7+0.1 Hz, following the ISO 6502 standard. Key parameters obtained from the curing curves included maximum torque (My), minimum torque (Mp), torque range (My - My), scorch time (751), and cure time (790).

2.11. Mechanical properties

The tensile properties of rubber vulcanizates were assessed using a universal testing machine, specifically the Hounsfield H 10KS model (Hounsfield Test Equipment Co., Ltd, Surrey, UK). The procedure adhered to die type C specifications and complied with the ASTM D412 standard. Hardness measurements of the rubber materials were performed using a Shore A durometer (AFFRI Inc., Wood Dale, USA) in accordance with ASTM D2240 standards. For the tearing experiment, the testing procedure followed ASTM D624 using Die B to prepare crescent-shaped samples. A Hounsfield tensometer (Model H10KS), operating at a crosshead speed of 500 mm/min, was employed to measure the tear strength of the rubber samples.

2.12. Morphological properties

The morphological characteristics of rubber blends were examined using a scanning electron microscope (SEM), specifically the ZEISS LEO-1450 VP model from Carl Zeiss AG (Jena, Germany). Rubber samples, produced through compression molding, were fractured to reveal a fresh surface before undergoing gold sputtering. The gold-coated specimens were then analyzed using SEM to study their surface morphology.

3. Results and discussion

3.1. Analysis of cyclized natural rubber

3.1.1. Fourier transform infrared spectroscopy Figure 2 illustrates the FTIR spectra of cyclized natural rubber (CNR) prepared with different molar ratios of NR to H,SO4 (1.00:1.00, 1.00:1.50, and 1.00:1.75), compared with unmodified neat NR derived from the original latex used as the raw material. A decrease in the intensity of the absorption peak corresponding to the C=C stretching vibrations in NR molecules, observed in the wavenumber range of approximately 1660-1630 cm! [28], is evident when compared with the FTIR spectra of CNRs at the same peak locations. This reduction likely indicates partial or complete cyclization of the double bonds, where C=C bonds in NR are converted into cyclized structures. Furthermore, the disappearance of FTIR peaks in unmodified NR at 835 and 780 em, corresponding to trialkyl-substituted double bonds, along with the appearance of a sharp, strong absorption peak at 882 cm, suggests the formation of cyclic structures in NR molecules [11, 29]. Alterations in the out-of-plane С-Н bending vibrations, observed in the region of 690-970 cm", further signify changes in alkene substitution patterns resulting from cyclization [28]. Additionally, a new peak appears in the CNR spectra at around 1718 cm"! [30], typically associated with C=O stretching vibrations, indicating the formation of carbonyl groups. This may result from oxidative side reactions occurring during the cyclization process.

3.1.2. Proton muclear magnetic resonance spectroscopy

Figure 3 presents the proton NMR spectra of neat unmodified NR and cyclized NR prepared using different molar ratios of NR to H,SO4. The magnified spectra of NR and a representative CNR, demonstrating the allocation of proton types (i.e., hydrogen atoms) within these materials, are presented in Figure 4. This visualization provides a clearer understanding of the chemical environment and distribution of hydrogen atoms in the analyzed samples. The spectrum of neat NR shows a dominant singlet signal at a chemical shift of 5.14 ppm, corresponding to unsaturated methylene protons adjacent to C=C bonds in the NR molecules. Additionally, singlet signals at 2.10 ppm and 1.70 ppm are observed, indicating the presence of methylene and methyl protons within the NR structure [7, 15].

In Figures 3b to 3e and 4b, a triplet signal centered within the chemical shift range of 1.00 to 0.85 ppm is observed, indicating the presence of methyl protons (-CH;3) attached to saturated carbons linked to cyclic structures, which are not connected to double bonds [7-9, 15, 31]. Additionally, a central signal at 1.4 ppm corresponds to methylene protons (-CH>-) attached to saturated carbons [15]. Moreover, an extra peak at approximately 5.40 ppm is detected, which is attributed to methylene protons in the cyclized NR molecules [7]. These "H-NMR and FTIR signals collectively confirm the successful formation of cyclized natural rubber (CNR) molecules, as proposed in the molecular structures illustrated in Figure 5. The resulting CNR products comprise a combination of mono-, di-, and polycyclic ring structures within their molecular framework. This unique structure allows for further characterization and diverse applications. In this study, two potential applications of CNR are demonstrated: first, as a blending material with natural rubber, and second, as a compatibilizer in NR/NBR.

3.1.3. Thermal analysis by differential scanning calorimetry

Figure 6 illustrates the DSC thermograms of CNRs synthesized using various molar ratios of NR to H»SO4, compared to unmodified NR. The glass transition temperature (7,5) appears as a step change or a shift in the baseline of the DSC curve. This change occurs because the material transitions from a glassy state to a rubbery state, leading to a change in heat capacity (cp). In this work, the midpoint of the transition step was used to indicate the 7, value for each material analysis. In Figure 6, the 7, of unmodified natural rubber (NR) is observed at approximately -67.71 °C. This value aligns well with the typical T, range of NR (-70 to -62 °C) reported in the literature [32, 33], which is known to vary depending on factors such as purity, molecular weight, and processing conditions [34]. Notably, the 7, of all CNRs is observed to be above room temperature, indicating a transition from a rubbery state to a more glass-like, rigid state. This shift reflects significant alterations in the material's physical properties, such as increased stiffness and reduced molecular mobility, which are characteristic of the glassy phase.

Furthermore, as the molar ratio of NR to H,SO4 increases, the 7, of CNRs shifts to progressively higher temperatures. This trend suggests a reduction in polymer chain flexibility, likely caused by chemical modifications occurring during the synthesis process. A higher 7, is typically associated with decreased chain mobility and enhanced intermolecular interactions, such as hydrogen bonding or dipole-dipole interactions. These modifications are consistent with the introduction of cyclic structures onto NR molecules, as indicated by the FTIR results (Figure 2) and structural representations (Figure 5). The increasing T, with higher NR:H,SO4 ratios corresponds to the observed decrease in iodine numbers, as shown in Figure 7, which reflects a reduction in unsaturation and supports the formation of cyclic structures. The rigidity introduced by these cyclic structures, combined with the incorporation of polar functional groups as by-products (evident from the FTIR spectrum in Figure 2), enhances intermolecular interactions and contributes to the observed rise in 7, values. This evidence collectively demonstrates that sulfuric acid modification of NR induces cyclization, resulting in the formation of mono-, di-, and polycyclic structures (Figure 5), which are influenced by the molar ratio of NR to sulfuric acid. Additionally, the process introduces polar by-product groups, significantly impacting the thermal and structural properties of the material.

3.2. Influence of reaction time, reaction temperature, and molar ratio of NR to H,SO,4 on iodine number

The iodine number (or iodine value), in the context of natural rubber analysis, is a measure of the degree of unsaturation (double bonds) in the molecular structure of natural rubber, a polyisoprene consisting of repeating isoprene units with double bonds. It indicates the amount of iodine, expressed in grams, that can react with the double bonds in 100 g of natural rubber. Typically, the iodine number is determined by adding a known excess of iodine or iodine monochloride to the sample. The unreacted iodine is then titrated with sodium thiosulfate, and the amount of iodine absorbed by the sample is calculated [26]. Figure 7 shows the relationship between iodine number and cyclization time, reaction temperature, and molar ratios of NR to H,SO14. In the reaction with a NR to H,SO4 molar ratio of 1.00:1.75 at a fixed reaction temperature of 80°C (Figure 7a), the iodine value gradually decreased from the start of cyclization until a reaction time of approximately 4 h, when the iodine value approached its lowest and nearly constant value of about 220. This value remained consistent up to the maximum reaction time of 8 h. This indicates a high degree of conversion to cyclized rubber within the first 4 h of the reaction, and extending the reaction time beyond 4 h does not significantly increase the conversion of unsaturation to cyclization. In Figure 7b, with a fixed reaction time of about 5h and a molar ratio of NR/H,SO4= 1.00:1.75, the iodine value was observed to decrease gradually with increasing reaction temperature. The minimum iodine value of about 210 was observed at a reaction temperature of 90°C. This suggests that higher temperatures favor a greater degree of cyclization due to an increase in kinetic energy and a reduction in activation energy for the cyclization process.

In Figure 7c, with a fixed reaction time of 5 h at 80°C and varying molar ratios of NR to HySOy, it is evident that increasing the sulfuric acid content caused a decrease in the iodine value, indicating an increase in the degree of cyclization. The molar ratio of NR to H,SO4 = 1.00:1.75 resulted in the optimum low iodine number of about 220, corresponding to the highest degree of cyclization. Increasing the acid content beyond this level caused only insignificant decreases in the iodine value.

The estimated degree of cyclization in NR molecules can also be evaluated through thermal analysis, as evidenced by the DSC thermograms presented in Figure 6. Furthermore, the degree of cyclization is visually apparent in the physical appearance of the reaction products, as illustrated by the photographs of samples prepared with varying molar ratios of NR to H,SO4 shown in Figure 8. It is observed that the yellow raw unmodified NR material transformed into light brown and then dark brown samples in reactions with the NR to H,SO4 molar ratios of 1.00 to 1.00 (Figure 8b) and 1.00 to 1.25 (Figure 8c), respectively. However, with higher degrees of cyclization (lower iodine numbers) observed at increased sulfuric acid contents (Figure 8d to 8f), the reaction products appeared as yellowish powders. This reveals the distinct cyclization degree along with their physical characteristics of the cyclized products.

3.3. Application of CNR as a blending component with NR

3.3.1. Curing properties

In this study, the optimally cyclized natural rubber, prepared from a molar ratio of NR to H;SO4= 1.00:1.75, was compounded with natural rubber (ADS) according to the formulation and mixing schedule outlined in Table 1. Figure 9 illustrates the curing properties of CNR/NR compounds with varying CNR loadings, including scorch time, cure time, torque difference, and cure rate index (CRI). As shown in Figure 9a, the incorporation of CNR delayed the onset of vulcanization by increasing the scorch time, thereby enhancing the processing safety window for rubber compounds. However, further increasing the CNR content beyond 10 phr resulted in only minor changes to the duration of the scorch period. Conversely, the addition and increased loading of CNR prolonged the cure time required to achieve a fully crosslinked rubber network. This trend is associated with a reduction in vulcanization rate, as indicated by the decreasing CRI in Figure 9b. Additionally, increasing CNR content led to a reduction in torque difference (Figure 9b), reflecting a lower crosslink density and, consequently, reduced mechanical strength of the vulcanizate. These observations can be attributed to the structural modifications of NR during its conversion to CNR. The introduction of cyclic structure along with some polar functional groups, such as carbonyl groups, byproduct (as confirmed by FTIR in Figure 2), disrupts the elasticity of the rubber material (as supported by 7, based on DSC results displayed in Figure 6). This cyclic structure and polar groups likely engage in strong interactions with other components in the rubber formulation, such as activators and accelerators, thereby hindering their intended roles in sulfur vulcanization of NR molecules. This corresponds to the addition of a cyclic ring structure of graphene with some polarity, which notably reduced the induction period of the vulcanization process and the vulcanization rate of natural rubber compounds [35].

3.3.2. Mechanical properties and Mooney viscosity

Figure 10 illustrates the Mooney viscosity and hardness of natural rubber CNR/NR compounded with varying contents of cyclized natural rubber. It is observed that both properties exhibit an upward trend with increasing CNR content. This behavior can be attributed to the unique cyclized structure of CNR, which enhances molecular rigidity through increased chain entanglement and introduces polar groups that promote stronger intermolecular interactions within the rubber matrix. These structural changes restrict chain mobility, as evidenced by the increase in 7, to higher temperatures (Figure 6), resulting in elevated viscosity and hardness. The sharp increase observed beyond 40 phr of CNR likely indicates a transition where the cyclized domains significantly influence the physical properties, forming a more rigid and reinforced rubber structure. This suggests that the rubber matrix reaches a point of structural saturation, where additional CNR content amplifies the rigidity and stiffness. The observed increases in Mooney viscosity and hardness are advantageous for improving the dimensional stability and durability of rubber products, particularly for applications requiring high resistance to deformation, such as automotive components, seals, and gaskets. However, these benefits come at the cost of reduced flexibility and process- ability, necessitating careful optimization of the formulation. Balancing these enhancements with other mechanical and functional properties, such as tensile strength, elongation, and resilience, remains crucial to ensure the overall performance of the rubber compound meets specific application requirements.

Figure 11 illustrates the tensile strength, tear strength, and elongation at break of CNR/NR compounded with varying contents of cyclized natural rubber. As shown in Figure 11a, both tensile strength and tear strength decrease with increasing CNR content. The tensile strength exhibits a sharp decline, particularly at lower CNR levels, while the reduction in tear strength is more gradual. This decline can be attributed to the presence of rigid cyclized domains in the CNR, which disrupt the continuity and elasticity of the rubber matrix. These rigid domains hinder the material's ability to evenly distribute stress, resulting in premature failure under tensile or tearing forces. At higher CNR contents, the matrix becomes dominated by these cyclized structures, further compromising elasticity and diminishing mechanical properties critical for stress resistance. Elongation at break, depicted in Figure 11b, reflects the material's ductility and ability to deform before failure. The cyclized structure of CNR significantly reduces the flexibility of rubber chains, leading to a marked loss in ductility. As the CNR content increases, the rigidity of the matrix restricts large-scale deformation, resulting in progressively lower elongation at break values. The incorporation of CNR, therefore, enhances rigidity and dimensional stability but comes at the cost of mechanical properties such as tensile strength, tear strength, and elongation at break. These trends highlight the inherent trade-offs between rigidity and flexibility when adding CNR to NR. The observed reduction in mechanical properties aligns with the behavior of rigid fillers or additives in elastomer matrices, where increased rigidity disrupts stress distribution and introduces stress concentration points that facilitate early failure during tensile or tearing stresses. In conclusion, the formulation of CNR/NR blend requires careful balancing between the benefits of enhanced rigidity and processability and the drawbacks of reduced tensile strength, tear strength, and elongation at break.

Applications prioritizing high-dimensional stability, such as seals or gaskets, may tolerate these reductions, whereas high-performance applications like tires demand more careful optimization of the formulation to maintain a balance between mechanical strength and rigidity.

3.4. Application of CNR as a blend compatibilizer in NR/NBR blend

3.4.1. Shear flow properties

Figure 12 illustrates the shear flow behavior of NR/ NBR blends compatibilized with CNR and UB6000, presenting the relationships between shear stress and shear rate (flow curves), and shear viscosity and shear rate (viscosity curves). In Figure 12a, the flow curves demonstrate a non-linear increase in shear stress with increasing shear rate for both compatibilized NR/NBR blends, which is characteristic of nonNewtonian shear-thinning behavior. This phenomenon, as illustrated in Figure 12b, demonstrates that the viscosity of the blends decreases with increasing shear rate. Such behavior is characteristic of polymeric systems, where the alignment of polymer chains under shear flow reduces internal resistance, thereby facilitating easier deformation [36]. In Figure 12, it can be seen that the shear stress and shear viscosity of the CNR-compatibilized blend remain consistently higher than those of the UB6000-compatibilized blend across the entire shear rate range. This observation suggests that the presence of CNR leads to stronger interfacial interactions and enhanced compatibility between the NR and NBR phases. Such enhanced interactions can be attributed to the chemical modification of CNR, which introduces cyclic structures along with polar groups as byproducts and enables better adhesion between the rubber phases. Furthermore, the functional groups and structural characteristics of CNR facilitate hydrogen bonding or other polar interactions at the interface, resulting in a more cohesive material structure capable of withstanding higher stresses and viscosities under deformation. Conversely, the UB6000-compatibilized NR/NBR blend demonstrates lower shear stress and viscosity values. This difference may arise because UB6000, a commercial compatibilizer composed of a mixture of aliphatic hydrocarbon resins, lacks the polar functional groups necessary for robust interfacial bonding. The less effective interaction between the two phases in the presence of UB6000 results in weaker interfacial adhesion, thereby lowering the stress response. These findings are consistent with previous research on the use of styrene-(epoxidized butadiene)-styrene (ESBS) triblock copolymer as a compatibilizer, particularly in relation to the curing characteristics, mechanical properties, and oil resistance of styrene-butadiene rubber (SBR) and epoxidized natural rubber (ENR) blends [37]. The chemical composition and molecular architecture of the compatibilizer are crucial factors influencing the mechanical and rheological properties of such blends.

3.4.2. Curing characteristics

Figure 13 illustrates the curing characteristics of NR/NBR blends incorporating two types of compatibilizers, namely CNR (blue diamonds) and UB6000 (orange triangles), evaluated across four parameters: scorch time Figure 13a, cure time Figure 13b, torque difference Figure 13c, and cure rate index (CRI) Figure 13d. In Figure 13a, it is evident that scorch time (measured in minutes) increases slightly with the addition and increasing loadings of UB6000 but remains nearly constant with the addition of CNR compatibilizers. Additionally, the blend containing UB6000 exhibits a slightly higher scorch time at a given loading compared to the one with CNR, indicating that UB6000 more effectively delays the onset of vulcanization. This delay provides greater processing safety, making it particularly advantageous for applications requiring extended workability before curing begins. Similarly, Figure 13b shows that cure time gradually increases with the addition and increasing loading of UB6000, whereas it remains nearly constant for the blend with CNR, with only minimal divergence between their respective curves. The effect of these compatibilizers on the cure rate index (CRI) is illustrated in Figure 13d. It can be seen that the cure rate increases consistently with the addition and increasing loading of both compatibilizers, with CNR inducing a slightly higher increase than UB6000 at loadings greater than 3 phr. This indicates that CNR promotes slightly faster curing kinetics of NR/NBR blends. This behavior may be attributed to the remaining unsaturation content in the CNR structure (Figure 5), which can promote the curing reaction and facilitate a higher cure rate in the rubber compounds. Turning to Figure 13c, it is observed that the torque difference (measured in Nm) decreases as the compatibilizer content increases for both CNR and UB6000. However, the reduction is more pronounced in blends containing UB6000 compared to those with CNR. Since torque difference reflects the crosslink density of the rubber network [38], this result suggests that UB6000 induces a lower degree of crosslinking than CNR. This discrepancy may arise from structural differences between the compatibilizers: UB6000, being a mixture of aliphatic hydrocarbon resins, has a less rigid structure, whereas CNR, derived from NR, contains cyclic ring structures with some unsaturation, leading to faster curing and stronger vulcanized networks. These curing property differences indicate the varying effects of compatibilizer types on the vulcanization behavior of NR/NBR blends. The selection of either CNR or UB6000 should account for the specific trade-offs between processing safety, curing speed, and crosslink density. UB6000 provides greater processing safety by delaying the curing process more effectively, while CNR offers faster curing kinetics and higher crosslink density, as evidenced by the greater torque difference. In conclusion, the in-house synthesized CNR emerges as a promising compatibilizer for NR/NBR blends. It facilitates a higher cure rate and stronger structural networks, as indicated by the higher torque difference. However, this comes at the cost of a shorter scorch time, which sacrifices some degree of processing safety. Thus, the choice between these compatibilizers depends on the desired balance of processing requirements and final material properties.

3.4.3. Mechanical properties

Figure 14 illustrates the effect of increasing compatibilizer content on the mechanical properties of NR/ NBR blends, focusing on hardness Figure 14a, 300% modulus Figure 14b, tensile strength Figure 14c, tear strength Figure 14d, and elongation at break Figure 14e. The hardness of rubber vulcanizates (Figure 14a) increases with rising CNR content, reaching a plateau at approximately 8-10 phr. In contrast, vulcanizates with the UB6000 compatibilizer exhibit a smaller increase in hardness, with the curve leveling off at higher compatibilizer levels. A similar trend is observed for the 300% modulus (Figure 14b). The CNR compatibilizer significantly enhances the modulus at higher concentrations, whereas UB6000 shows minimal changes and a slight decrease in modulus across the compatibilizer range. This behavior may be attributed to CNR"s ability to improve compatibility and interfacial stress transfer between NR and NBR phases, resulting in increased resistance to deformation. Conversely, UB6000 provides weaker reinforcement, likely due to differences in its chemical structure or interaction mechanisms. Thus, for applications prioritizing higher hardness, modulus, and stiffness, CNR is a more effective blend compatibilizer for enhancing NR/NBR compatibility. For tensile strength (Figure 14c), the UB6000-containing blends show a slight decrease with increasing compatibilizer content. The CNR blends also exhibit a decreasing trend, but they start at a higher tensile strength (at 2.5 phr) and display a gentler decline compared to UB6000. The decrease in tensile strength is attributed to the impact of compatibilizer concentration on interfacial adhesion and crosslinking. CNR offers higher initial tensile strength due to its strong chemical interactions with both NR and NBR, which lead to better stress transfer and a more uniform phase morphology. The gradual decline with increasing CNR content reflects effective interfacial reinforcement, while UB6000's steeper decline suggests weaker interfacial bonding and potential phase softening at higher concentrations. In terms of tear strength (Figure 14d), the blend with CNR exhibits a consistent increase in this property with rising content, whereas the blend with UB6000 shows lower tear strength and a slight decline as its content increases. This difference indicates CNR"s superior performance in enhancing strength properties, making it a better candidate for applications requiring robust mechanical properties. UB6000's weaker performance may reflect its lower compatibility or reinforcement efficiency. Furthermore, the trend of elongation at break (Figure 14е) decreases with increasing CNR content, suggesting reduced flexibility due to increased rigidity and higher crosslink density, which restrict molecular mobility. In contrast, UB6000 demonstrates better retention of ductility, with a smaller decline and even a slight increase in elongation at break at higher compatibilizer levels, reflecting a better balance between reinforcement and flexibility. In summary, using CNR as a blend compatibilizer in NR/NBR blends generally results in superior hardness, modulus, and tear strength, indicative of stronger interfacial interactions and improved compatibility. On the other hand, UB6000 exhibits less pronounced effects on these properties but offers a notable advantage in maintaining elongation at break, highlighting a balance between reinforcement and flexibility.

3.4.4. Morphological properties

presents the SEM micrographs of NR/NBR blends compatibilized with CNR and UB6000 at two distinct loading levels. As illustrated in , the SEM image for the 5 phr CNR loading reveals a relatively uniform phase distribution with notable interfacial adhesion between the NR and NBR phases. The CNR compatibilizer effectively mitigates phase separation, resulting in a more homogeneous blend. Nevertheless, minor phase boundaries remain visible, suggesting that 5 phr of CNR does not fully saturate the interface. In contrast, the blend with 5 phr of UB6000 ( ) exhibits a slightly different morphology, characterized by more pronounced phase separation and weaker interfacial adhesion, with distinct phase boundaries. This indicates that UB6000 is less effective than CNR at 5 phr in compatibilizing the NR/NBR blend. At the higher loading level of 10 phr, the NR/NBR blend with CNR ( ) demonstrates a significantly refined and uniform phase structure. The interfacial adhesion is markedly enhanced, and phase boundaries become less discernible, indicating that increasing the CNR concentration improves the compatibility between NR and NBR, leading to a more homogeneous blend with superior mechanical properties (Figure 14). Conversely, the blend with 10 phr of UB6000 (Figure 15d) also shows improved morphology compared to its 5 phr counterpart, with reduced phase separation and better interfacial adhesion. However, this improvement is less pronounced than that observed with CNR, as phase boundaries remain somewhat visible. This suggests that UB6000 is less effective than CNR even at higher loading levels. These results are consistent with the higher torque difference observed in Figure 13c and the enhanced mechanical properties, including hardness, 300% modulus, tensile strength, tear strength, and elongation at break, for the NR/NBR blend with CNR compatibilizer (Figure 14). Consequently, CNR proves to be more effective than UB6000 in compatibilizing NR/NBR blends at both 5 and 10 phr loading levels. The blends with CNR exhibit superior phase dispersion and stronger interfacial adhesion, which are critical for optimizing mechanical performance.

4. Conclusions

This study successfully synthesized cyclized natural rubber (CNR) via the acid-catalyzed cyclization of natural rubber latex (NRL), employing sulfuric acid as a catalyst stabilized by a non-ionic surfactant. The degree of cyclization, evaluated by iodine number, was influenced by reaction time, temperature, and the molar ratio of NR to sulfuric acid. Structural analyses using FTIR and "H-NMR confirmed the formation of cyclic structures, while DSC thermograms revealed a significant increase in the glass transition temperature (75) with higher cyclization degrees, indicating enhanced rigidity and reduced molecular mobility. These structural and thermal modifications were accompanied by notable physical changes in the CNR products, which ranged from rubbery to rigid, powdery materials depending on the reaction conditions. CNR demonstrated its potential as a functional material in two key applications. When blended with natural rubber (NR), CNR improved viscosity, hardness, and dimensional stability, though it reduced tensile strength and elongation at break due to the rigid cyclic domains. For instance, hardness increased from 37.5 to 60.0 Shore A, and Mooney viscosity rose from 36.0 to 47.0 MU with increasing CNR content (0-60 phr), while tensile strength decreased from 25.5 to 2.2 MPa, and elongation at break declined from 787 to 125%.

As a compatibilizer in NR/NBR (nitrile butadiene rubber) blends, CNR significantly enhanced interfacial adhesion and compatibility, resulting in superior shear flow behavior, improved curing properties, and enhanced mechanical performance compared to a commercial homogenizer (ULTRA-BLEND™ 6000, U6000). Specifically, in the NR/NBR blends with 10 phr of compatibilizers (CNR and UB6000), the torque difference (My - Mp), which reflects crosslink density, was higher for CNR (16.2 dN-m) than for UB6000 (13.3 dN-m). In addition, tear strength increased from 18.6 to 32.9 N/mm, tensile strength rose from 10.3 to 13.8 MPa, and Shore A hardness improved from 28.0 to 37.5. Although the elongation at break slightly decreased from 790 to 625%, it still indicated adequate flexibility for many applications. These enhancements were attributed to the introduction of polar functional groups and cyclic structures in CNR, which promoted stronger intermolecular interactions and more effective phase dispersion, as confirmed by the finer phase structure observed in the SEM micrographs.

Overall, the findings confirm that CNR is a versatile material with advanced thermal, structural, and mechanical properties. Its dual functionality as a blending material and compatibilizer highlights its potential for a wide range of industrial applications, including high-performance rubber formulations requiring tailored rigidity, compatibility, and stability.

Acknowledgements

This work was supported by the Thailand Research Fund (TRF) contract no RDG 4850020, along with the Prince of Songkla University, Pattani Campus.

Sidebar

Received 25 March 2025; accepted in revised form 7 May 2025

References

References

[1] Nawamawat K., Sakdapipanich J. T., Но С. C., Ма Y., Song J., Vancso J. G.: Surface nanostructure of Hevea brasiliensis natural rubber latex particles. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 390, 1-3 (2011). https://doi.org/10.1016/j.colsurfa.2011.09.021

[2] Saengdee L., Phinyocheep P., Daniel P.: Chemical modification of natural rubber in latex stage for improved thermal, oil, ozone and mechanical properties. Journal of Polymer Research, 27, 275 (2020). https://doi.org/10.1007/s10965-020-02246-7

[3] Dieu T. V., Chuong B., Hung D. V., Tung N. H., Linh N. P. D., Oanh D. T. Y.: Review: Natural rubber - Improvement of properties. Vietnam Journal of Chemistry, 61, 269-283 (2023). https://doi.org/10.1002/vjch.202200225

[4] Ngudsuntear K., Limtrakul S., Arayapranee W.: Synthesis of hydrogenated natural rubber having epoxide groups using diimide. ACS Omega Journal, 7, 21483- 21491 (2022). https://doi.org/10.1021/acsomega.2c01011

[5] Nguyen T. H., Nguyen V-A., Ougizawa T., Kawahara S., Cao H. H.: Electromagnetic shielding material based on hydrogenated natural rubber/expanded graphite blend: Preparation and characterization. Polymers for Advanced Technologies, 32, 3008-3017 (2021). https://doi.org/10.1002/pat.5313

[6] Mohapatra S., Nando G. B.: Chemical modification of natural rubber in the latex stage by grafting cardanol, a waste from the cashew industry and a renewable resource. Journal of Industrial and Engineering Chemistry, 52, 5951-5957 (2013). https://doi.org/10.1021/ie400195v

[7] Jaratrotkamjorn R., Hayeemasae N., Zakaria Z., Masa A.: Influence of acid concentration on thermomechanical, tensile and thermal properties of cyclized natural rubber. Journal of Elastomers and Plastics, 55, 768-786 (2023). https://doi.org/10.1177/00952443231173827

[8] Riyajan S-A., Sakdapipanich J. T.: Thermal property and thermodynamic parameter of rubber blends containing the cyclic structure. KGK Kautschuk Gummi Kunststoffe, 16, 163-169 (2009).

[9] Riyajan S-A., Tanaka Y., Sakdapipanich J. T.: Preparation of cyclized deproteinized natural rubber in latex state via a combination of benzotrichloride and sulfuric acid system, and its properties. Rubber Chemistry and Technology, 80, 365-377 (2007). https://doi.org/10.5254/1.3539412

[10] van Veersen G. J.: Preparation of cyclized rubber from natural rubber latex. Journal of Polymer Science, 6, 29- 32 (1951). https://doi.org/10.1002/pol.1951.120060105

[11] Mirzataheri M.: The cyclization of natural rubber. Iranian Journal of Chemistry and Chemical Engineering, 19, 91-96 (2000). https://doi.org/10.30492/ijcce.2000.10189

[12] Lee D. F., Scanlan J., Watson W. F.: The cyclization of natural rubber. Rubber Chemistry and Technology, 36, 1005-1018 (1963). https://doi.org/10.5254/1.3539621

[13] Patterson D. J., Koenig J. L.: A Fourier transform infrared and nuclear magnetic resonance study of cyclized natural rubber. Die Makromolekulare Chemie, 188, 2325-2337 (1987). https://doi.org/10.1002/macp.1987.021881009

[14] Tutorskii A., Markov У. V., Fedyuk O. I., Vitsnudel' М. B., Dogadkin B. A.: The kinetics of the cyclization of natural and synthetic polysoprenes by means of phosphorus pentoxide. Polymer Science U.S.S.R., 7, 1051- 1056 (1965). https://doi.org/10.1016/0032-3950(65)90378-3

[15] Riyajan S-A., Liaw D-J., Tanaka Y., Sakdapipanich J. T.: Cationic cyclization of purified natural rubber in latex form with a trimethylsilyl triflate as a novel catalyst. Journal of Applied Polymer Science, 105, 664-672 (2007). https://doi.org/10.1002/app.25026

[16] Riyajan S-A.: Activation energy and thermodynamic parameters of cyclization in purified natural rubber latex using a trimethyl silyl triflate. Journal of Elastomers & Plastics, 41, 133-144 (2009). https://doi.org/10.1177/0095244308092437

[17] Hashim A. S., Ong $. K., Jessy R. S.: A general review of recent developments on chemical modification of natural rubber. Natuurrubber/Natural Rubber, 28, 3-9 (2002).

[18] Wirjosentono B., Tamrin, Siregar A. H., Nasution T. I., Dalimunthe K. Z., Nasution, D. A.: Compatibilitation of cyclic natural rubber (resiprene-35) with polypropylene in the presence of oleic acid and benzoyl peroxide. Journal of Physics: Conference Series, 1116, 042043 (2018). https://doi.org/10.1088/1742-6596/1116/4/042043

[19] Siregar М. S., Ardilla D., Eddiyanto, Nasution A. S.: Grafting of maleic anhydride onto cyclized natural rubber in the melt phase: The effect of trimethylol propane triacrylate. Journal of Physics: Conference Series, 1764, 012200 (2021). https://doi.org/10.1088/1742-6596/1764/1/012200

[20] Teik P. B., Wai S. K., Azahari B.: Adhesion properties of zinc-oxide filled epoxidized natural rubber/ acrylonitrile-butadiene rubber blend adhesives. Chiang Mai Journal of Science, 44, 1583-1590 (2017).

[21] Kueseng P., Sae-oui P., Rattanasom N.: Mechanical and electrical properties of natural rubber and nitrile rubber blends filled with multi-wall carbon nanotube: Effect of preparation methods. Polymer Testing, 32, 731-738 (2013). https://doi.org/10.1016/j.polymertesting.2013.03.002

[22] Han T., Nagarajan S., Zhao H., Sun C., Wen S., Zhao S., Zhang L.: Novel reinforcement behavior in nanofilled natural rubber (NR)/butadiene-acrylonitrile rubber (NBR) blends: Filling-polymer network and supernanosphere. Polymer, 186, 122005 (2020). https://doi.org/10.1016/j.polymer.2019.122005

[23] Çavdar Ó., Yildiz Z., Naghshineh A. K.: Investigation ofnew mixtures based on natural rubber and nitrile-butadiene rubber for high damping base isolation systems. Journal of Elastomers and Plastics, in press (2025). https://doi.org/10.1177/00952443251328185

[24] Lewan M. V.: NR/NBR blends - Basic problems and solutions. in "Blends of natural rubber: Novel techniques for blending with specialty polymers' (eds.: Tinker A. J., Jones К. P.) Springer, Dordrecht, 53-67 (1998). https://doi.org/10.1007/978-94-011-4922-8 5

[25] Lee T. S., Kolthoff I. M., Mairs M. A.: Determination of unsaturation of synthetic and natural rubbers by means of iodine monochloride. Journal of Polymer Science, 3, 66-84 (1948). https://doi.org/10.1002/pol.1948.120030109

[26] Gorgas A.: Determination of the iodine number of raw rubber. Rubber Chemistry and Technology, 2, 362-364 (1992). https://doi.org/10.5254/1.3535364

[27] Cifriadi A., Chalid M., Puspitasari S.: Characterization of hydrogenated natural rubber synthesized by diimide transfer hydrogenation. International Journal of Technology, 3, 448457 (2017). https://doi.org/10.14716/ijtech.v8i3.1991

[28] Smith B. C.: The infrared spectra of polymers IV: Rubbers. Spectroscopy, 37, 8-12 (2022). https://doi.org/10.56530/spectroscopy.mz6968v1

[29] Aritonang B., Tamrin T., Wirjosentono B., Eddiyanto E.: Functionalization of cyclic natural rubber (CNR) with oleic acid and divinylbenzene as compatibilizer in variation of dicumylperoxide. in "The 3" International Seminar on Chemistry: Green Chemistry and its Role for Sustainability' Vol. 2049, 020060 (2018). https://doi.org/10.1063/1.5082465

[30] Kim I-S., Cho H., Sohn K-S., Choi H-S., Kim S-U., Kim S.: Characterization on the thermal oxidation of raw natural rubber thin film using image and FT-IR analysis. Elastomers and Composites, 55, 51-58 (2020). https://doi.org/10.7473/EC.2020.55.1.51

[31] Agnihotri R. K., Falcon D., Fredericks E. C.: Cyclization of synthetic cis-1,4-polyisoprene. Journal of Polymer Science Part A-1: Polymer Chemistry, 10, 1839- 1850 (1972). https://doi.org/10.1002/pol.1972.150100621z

[32] Phattarateera S., Pattamaprom C.: The viscosity effect of masticated natural vs. synthetic isoprene rubber on toughening of polylactic acid. International Journal of Polymer Science, 2019, 5679871 (2019). https://doi.org/10.1155/2019/5679871

[33] le Ferrand H., Bacha A.: Discovery and rediscovery of gutta percha, a natural thermoplastic. MRS Bulletin, 46, 84-85 (2021). https://doi.org/10.1557/s43577-020-00004-0

[34] Loadman M. J. R.: The glass transition temperature of natural rubber. Journal of Thermal Analysis and Calorimetry, 30, 929-941 (1985). https://doi.org/10.1007/BF01913321

[35] Wu J., Xing W., Huang G., Li H., Tang M., Wu S., Liu Y.: Vulcanization kinetics of graphene/natural rubber nanocomposites. Polymer, 54, 3314-3323 (2013). https://doi.org/10.1016/j.polymer.2013.04.044

[36] Xu X., Chen J., An L.: Shear thinning behavior of linear polymer melts under shear flow via nonequilibrium molecular dynamics. The Journal of Chemical Physics, 140, 1749021 (2014). https://doi.org/10.1063/1.4873709

[37] Ismail H., Hairunezam H. M.: The effect of a compatibilizer on curing characteristics, mechanical properties and oil resistance of styrene butadiene rubber/epoxidized natural rubber blends. European Polymer Journal, 37, 39-44 (2001). https://doi.org/10.1016/S0014-3057(00)00099-9

[38] Wtoch M., Ostaszewska U., Datta J.: The effect of polyurethane glycolysate on the structure and properties of natural rubber/carbon black composites. Journal of Polymers and the Environment, 27, 1367-1378 (2019). https://doi.org/10.1007/s10924-019-01437-8