Leather-Like Material Based on Natural Rubber Composites

Abstract

In-house leather like material based on filled natural rubber (NR) with various filler types (i.e., clay, SiO2 and CaCO^) was prepared by calendaring method. NR/SiO2 exhibited the highest tensile strength, 100% modulus, elongation at break, and tear strength. The lower wear index and % weight loss in case of NR/SiO2 indicated the best abrasion resistance among the other fillers due to the fine dispersion of SiO2 in the NR matrix. Also, the uniform dispersion of SiO2 together with high surface area of SiO2 enhanced the interaction with NR matrix led to the higher elasticity of NR/SiO2 vulcanisate. Therefore, NR/SiO2-based leather was prepared and exhibited no significant difference in the abrasion resistance compared to the commercial leather. Furthermore, it was observed that NR/SiO2-based leather exhibited higher skid resistance than the commercial one. Therefore, NR/SiO2-based leather has been proposed to use as commercial artificial leather.

Headnote

ABSTRACT: In-house leather like material based on filled natural rubber (NR) with various filler types (i.e., clay, SiO2 and CaCO^) was prepared by calendaring method. NR/SiO2 exhibited the highest tensile strength, 100% modulus, elongation at break, and tear strength. The lower wear index and % weight loss in case of NR/SiO2 indicated the best abrasion resistance among the other fillers due to the fine dispersion of SiO2 in the NR matrix. Also, the uniform dispersion of SiO2 together with high surface area of SiO2 enhanced the interaction with NR matrix led to the higher elasticity of NR/SiO2 vulcanisate. Therefore, NR/SiO2-based leather was prepared and exhibited no significant difference in the abrasion resistance compared to the commercial leather. Furthermore, it was observed that NR/SiO2-based leather exhibited higher skid resistance than the commercial one. Therefore, NR/SiO2-based leather has been proposed to use as commercial artificial leather.

Keywords: natural rubber, leather-like material, abrasion resistance, filler

(ProQuest: ... denotes formulae omitted.)

1. INTRODUCTION

Artificial leather is an alternative material for leather used in several products including briefcases, stationery items, garments, belts, sports items footwear, etc. Artificial leather industry plays a major role on developing the economy in terms of export earnings and employment opportunities.1

In general, leather has been prepared by impregnating a nonwoven fabric/ knitted goods with a polyurethane (PU) dissolved in an organic solvent using dimethylformamide (DMF), and also PU with microfiber polycaprolactam (PCL) in the presence of DMF and formic acid.2,3 Such type of leather exhibits sufficient properties due to the excellent abrasion resistance, flexibility, durability, and toughness of original PU matrix. Polyvinylchlorides (PVC) can be used as a leathered matrix, which shows superior thermal releasing properties.4 However, it was reported that PU and PVC based leathers have chemical contamination which attacks the environment due to the tremendous harmful solvents used during the preparation and adhesion processes.5 Attempts have been made to replace or minimise the use of toxic chemicals and the leather-like composites based on natural rubber (NR) is introduced.

As a biosynthesised polymer with high elasticity, flexibility and abrasion resistance, NR is suitable for leather materials. It was reported that, the properties of NR can be varied by the addition of fillers, such as silica (SiO2), clay and calcium carbonate (CaCO3). This causes effectively to increase the mechanical properties of NR by incorporating fillers at different concentrations.6-8 The major change occurs not only in case of sheet form, but also in the form of NR foam which performs well by using blowing agent. This generates an open-cell structure of NR matrix owing to the release of carbon dioxide gas at approximately 150°C.6 However, it was reported that the blowing mechanism interrupted effectively the crosslink density of NR and the mechanical properties.7 The cell size of the foam can be adjusted by varying the concentration of the gases. It was also noted that, increasing the blowing agent leads to rise the gas decomposition, reduces the average cell size, and increases the number of cells.7

Therefore, the present work is aimed to study the suitable rubber formulation for preparing NR based leather-like material with various fillers (i.e., clay, SiO2 and CaCO3). The NR compound was fabricated by following the textile calendaring procedures in order to evaluate the optimised strength of leather. In addition, mechanical properties, cure characteristic together with morphologies were examined. Also, leather-like material based on NR from a proper NR formulation was developed and compared with the commercial leathers, i.e., PVC and PU-based leathers in terms of abrasion resistance, skid, and puncture resistances.

2. EXPERIMENTAL

2.1 Material

NR (Standard Thai Rubber 5L grade) with density 0.92 g/cm3 was manufactured by Chalong Latex Industry Co. Ltd. Songkhla, Thailand. Other chemicals used in the rubber formulation such as zinc oxide (ZnO), stearic acid, CaCO3, china clay, SiO2, sulphur, tetramethyl thiuram disulphide (TMTD), and 2-2'-Dithiobis (benzothiazole) (MBTS, were purchased from Lanxess Co. Ltd., Bangkok, Thailand. The blowing agent, Dinitrosopentamethylene tetramine (DPT) was purchased from Kij Paiboon Chemical Ltd., Bangkok, Thailand.

2.2 Preparation of Natural Rubber Compound

NR compounds were prepared by following the formulation as shown in Table 1. The mixing was performed on an open two-roll mill at room temperature with the rotor friction ratio of 1:1.4. Initially, NR was masticated for 5 min and mixed with the activators (i.e., ZnO and stearic acid) for 2 min. Then, the fillers (CaCO3, clay, or SiO2) were added individually into the compound. Further, the blowing agent DPT was incorporated before the addition of accelerators (i.e., MBTs and TMTD) and sulphur. Compounding was continued for a total mixing time of 30 min and the cure characteristics of rubber compounds were then determined by a moving die rheometer (MDR, Alpha Technologies, USA) at 150°C. NR vulcanisates were eventually prepared using compression moulding at 150°C following the respective cure time form MDR.For the preparation of leather-coated textile vulcanisates, the compound was fabricated on textile using calendaring processes. Here, approximately 2 mm of the calendaring nip was used under the controlled rolling condition of 60°C with arolling speed of 15 rpm. Subsequently, the NR leather-coated textile was then heated in a hot air oven at 150°C for controlled time as performed in the case of un-coated samples.

2.3 Characterisation

2.3.1 Cure characteristics

The cure characteristics of gum NR and its compounds filled with CaCO3, clay and SiO2 were investigated using a MDR (Alpha Technologies, USA) according to ASTM D224. In each case, the sample was tested by fixing the oscillation frequency of 1.66 Hz and 1 arc amplitude at 150°C for 20 min.

2.3.2 Mechanical properties

Mechanical properties were investigated using an Instron machine (Model 3365) according to ASTM D412. The specimens were cut into dumbbell shapes and the test was conducted through a 1000 N load cell at a cross head speed of 500 mm/min-1. The tear strength was estimated following ASTM D624 using unnicked 90° angle test pieces. The crosshead speed used was 500 mm/min-1. Hardness testing was performed using a Shore A durometer (Frank GmbH, Hamburg, Germany), according to ASTM D2240.

2.3.3 Abrasion resistance

The abrasion resistance of the samples was studied with a Taber abraser following ASTM D4060-10. Thus, wear index (I) is realised to be abrasion ability relating to the observed % weight loss (L) of the samples, as seen in Equation (1)-(2):

... (1)

... (2)

where: A = weight of test specimen before abrasion (g)

B = weight of test specimen after abrasion (g)

C = number of cycles of abrasion recorded

2.3.4 Puncture resistance

Puncture test was carried out on tensile testing machine Z005 (Zwick Roell, Germany). The puncture probe was held by a pin chuck mounted on the load cell. NR vulcanisate was clamped between two steel plates under air pressure. The hole in lower plate was chamfered to avoid stress concentration under 100 mm/min of probe speed.

2.3.5 Skid resistance

In-house leather based on NR, real leather, PVC, and PU were tested using skid resistance from a British pendulum skid tester (BPST) (S885, Wessex, United Kingdom), at 25°C. This value was essentially measured by means of the energy loss of rubber pad sliding over a flat surface.

2.3.6 Morphologies

Microstructural characterisation of gum NR and its vulcanisates filled with CaCO3, clay, and SiO2 at the loading level of 30 phr were investigated using optical microscope (Phenom Prox Scanning Electron Microscope, Brno, Czech Republic) at accelerated voltage of 15 kv, 80 qm resolution and x1000 magnification. In order to evaluate the dispersion of fillers in NR matrix, scanning electron microscope (SEM, model JEOL JSM-5800LV, Tokyo, Japan) was also employed. Specimens were cryogenically fractured in liquid nitrogen to create a fresh cross-sectional surface. Consequently, the samples were sputter-coated with a thin layer of gold under vacuum before scanning.

3.RESULTS AND DISCUSSION

3.1 Specific Properties of Leather-Like NR Product

3.1.1 Cure characteristics

Figure 1 shows the cure characteristic curves of gum NR (without filler) and the compounds filled with SiO2, clay and CaCO3. Also, Table 2 summarises the respective curing characteristics of the samples in terms of scorch time ?2), cure time (t90) and torque difference (MH-ML). All the compounds showed reversion phenomena due to thermo-oxidative degradation of weak linkages based on the existed polysulfidic -S-S- bonds.8 Here, the NR/SiO2 compound exhibits the largest reversion degree due to the breakage of silica agglomeration. The strong hydroxyl polarity of SiO2 particles develops high tendency of filler-filler interaction in the NR matrix rather than filler-rubber attraction.9 Considering ts2 and t90 values, clay exhibited an increasing trend due to the china clay forms zinc silicate with ZnO. These reactions may reduce the efficiency of ZnO in curing system which slowly resulted in the occurrence of vulcanisation reaction.10 Meanwhile, the values of Tc90 in case of SiO2 and CaCO3 filled NR compounds are found to be decreased, might be due to the effect of moisture-treated silanol groups on the SiO2 surfaces accelerate crosslinking.11 In addition, the interaction between NR with SiO2 caused to reduce retarding reaction between silica and zinc oxide accelerator. Moreover, CaCO3 filled NR compound shorten the Tc90 value due to the fact that CaCO3 particles are isotropic in nature which leads to an increased thermal conductivity of NR matrix, consequently, faster vulcanisation has been observed.12

This means that the clay retards vulcanisation of NR, whereas SiO2 and CaCO3 accelerate the curing reaction and vulcanises earlier. The result effectively affects the generation of crosslinking among NR molecules. It clearly clarifies from the MH-ML values, which are directly related to the degree of chemical linkages originating in the matrix.13 In Table 2, it is clearly seen that the NR/SiO2 showed the highest MH-ML value, even though it has a strong reversion behaviour. This can be explained by the characteristics of SiO2 which is having a small particle size with strong polarity compared to clay and CaCO3. In the NR matrix, SiO2 forms strong agglomeration but also generates high bound rubber absorption owing to it surface area. It means that, SiO2 can reinforce well in the NR matrix and provides higher crosslink density. On the other hand, the MH-ML values of NR/clay and NR/CaCO3 exhibited higher values than the pure NR, and still lower than the case of NR/SiO2. This can be examined by the specific structure of the filler. According to the layered structure of clay, a strong agglomeration may also occur, whereas the smooth surface of clay due to the packing of aluminium silicate found difficult in the rubber absorption on its surface. This lowers the bound rubber absorption, which is the reinforcement mechanism of filler inside the rubber matrix, indicating poor MH-ML. However, the addition of CaCO3 into NR lowers the value of MH-ML. It was reported that, the smaller particle size with larger surface area leads to enhance the interaction between filler and rubber matrix.14 Therefore, the large particle size of CaCO3 provided low specific surface area of filler lead to poor interaction between CaCO3 and rubber matrix, hence it shows lower rubber absorption on the CaCO3 surface.

3.1.2 Mechanical properties

Figure 2 shows the stress-strain curves of the in-house leather based on NR vulcanisates filled with CaCO3, clay and SiO2 fillers. The details of modulus at 100% elongation, tensile strength, and elongation at break from the stress-strain plots together with tear strength and hardness are summarised in Table 3.

In Figure 3, it is clearly seen that the addition of clay and CaCO3 in the NR matrix reduces the modulus, tensile strength, and elongation at break of the vulcanisates compared to the gum NR. This correlates well with the observed values of crosslink density calculated from MH-ML. It indicates that the large particle size of clay and CaCO3 causes poor rubber absorption on the filler surfaces reduces the reinforcement efficiency and it reflects in the weak mechanical properties. On the other hand, the highest properties are observed in case of NR/SiO2 vulcanisates due to the better reinforcement. Here, existence of SiO2 particles effectively causes the physical absorption of rubber within the glassy levels. This restricts the deformation of NR matrix under an applied force and therefore the breakages of NR chains have been retarded. This can be explained by the proposed model as shown in Figure 3. It is clearly seen that; all the fillers have significant shape and particle size differences. CaCO3 and clay exhibited the larger particle sizes than that of SiO2. It clarifies the reason to achieve the highest mechanical properties than the other fillers (Figures 3(A) - (D)). Higher value of elongation at break is noticed in case of NR/SiO2 than that of NR/clay and NR/CaCO3. This might be due to the fine dispersion of SiO2 together with the higher surface area of SiO2 supported better interaction with NR matrix enhanced the elasticity of NR/SiO2 vulcanisate.

On the other hand, larger particles of CaCO3 and clay resulted in poor dispersion and consequently agglomeration occurs, which becomes the weak point in the matrix. Variation of tear strength and hardness of the samples are as shown in Figure 4 and Table 3. Similar behaviour is observed in the tear strength and hardness as seen in the tensile properties. Higher tear strength and hardness are also achieved in the NR/SiO2 vulcanisate due to the small particle size of the filler, which affects effectively the reinforcement efficiency in the NR matrix. The filler-rubber interaction can also be extrapolated with the cracking extension of the samples under the applied force. With the highest value of tear strength, it is seen in the glassy bound rubber, attraction between the rubber molecules, and SiO2 surfaces retards significantly the crack propagation of the NR phases during the breakage of samples.

3.2Durability of NR-based Leather

Durability of leather in terms of tear resistance, hardness, and the abrasion ability together with puncture force and skid resistance are examined. Figure 4 displays tear strength and hardness of the leather filled with CaCO3, clay, and SiO2. It is seen that the addition of SiO2 has significantly affected on both the properties as noticed in the mechanical properties (Figure 2). The physical absorption of NR molecules onto the SiO2 surfaces effectively reduces the crack propagation of NR matrix during extension. It can also be clarified by the significant increase in tear strength of the vulcanisates reinforced using SiO2 when compared to the other fillers as shown in Figure 4.

It is not only affected the strength, but also strongly affected the abrasion resistance. Figure 5 shows the wear index and %weigh loss of the vulcanisates. An increase in the hardness of vulcanisates along with abrasion resistance is also observed. It leads the NR/SiO2 with low wear index than the ones filled with CaCO3 and clay. This can be correlated to the observed tensile and tear strengths. Addition of SiO2 in the NR matrix provided higher degree of fillerrubber interaction due to its smaller particle size and higher surface area than those of CaCO3 and clay. It clearly reflects on the abrasion surfaces of the vulcanisates as displayed in Table 4. It is clearly noted that the NR-based leather filled with CaCO3 exhibited higher roughness upon damaging the leather's surface.

The phenomenon has not been distinctly demonstrated for the vulcanisates with clay and SiO2 responding to the observed wear ability. In addition, this is in good agreement with the measured puncture strength of the material as seen in Figure 6 and Table 4. It is observed that the in-house NR/CaCO3 and NR/clay leathers have a slight decrease in the puncture force compared to the gum NR. As expected, the NR/SiO2 leather showed an enhancement in the puncture force compared to the others. This is due to the higher surface area of SiO2 by attracting the NR molecules enhances effectively the reinforcement efficiency to the NR matrix and prevented the occurrence of breakages of the matrix during puncturing.

In Table 4, the skid resistance of in-house NR leather is investigated and compared with the original NR. The incorporation of fillers increased well the skid resistance of the vulcanisates. The existence of hard particles strongly retards the breakages of rubber matrix and increases the strength of the vulcanisates. This means that the high surface area of the filler particles enhances the properties of the final product. This is the reason for increasing the skid resistance upon the addition of filler. It is also seen that the NR/SiO2 showed the highest skid resistance compared to the vulcanisates of NR/CaCO3 and NR/clay. It correlates with the mechanical properties (Figure 2 and Table 3) together with tear and puncture strength (Figure 4 and Table 4) of the vulcanisates. Therefore, it can be summarised that the NR/SiO2 vulcanisate is suitable for the preparation of in-house NR leather.

3.3Comparison of NR/SiO2-based Leather with Commercial Products

In order to clarify the utility of NR/SiO2 leather, a comparison study has been done on the prepared leather with the commercial grade leather. Here, the physical appearances of real leather (Figure 6(a)), PU, and PVC leathers (Figures 6(b) and (c)) are compared with the NR based leather (Figure 6(d)). NR/SiO2-based leather showed the smoothest surface when compared to others. This smooth surface of the sample has less attraction due to effect of polishing with stone during the measurement.

Figure 7 and Table 5 showed the lowest abrasion resistance in terms of wear index and %weight loss in case of NR/SiO2 leather when compared to the real leather, PU, and PVC-based leathers having higher roughness. In addition, it is observed that PU leather exhibited a large increase in wear index, whereas no significant differences are observed in case of other leathers. The physical appearance after abrasion propagation and NR based leathers are comparable as seen in Figure 7. It can be seen that the surface of commercial leather exhibited high roughness while a smooth surface is observed in case of NR/SiO2 leather. This is attributed by the suitability in terms of mechanical properties and hardness of the vulcanisates with the higher elasticity of NR matrix.

In order to clarify the strength of the newly developed leather, puncture force, and skid resistance are examined along with the commercial leather as the reference and the results are as shown in Figure 8 and Table 5. The real leather exhibited a tremendously higher puncture force than the other artificial leathers. However, considering the artificial leather, the NR/SiO2 leather showed the higher puncture force than PU and PVC-based leathers. This is due to the superior elasticity of NR together with the reinforcement efficiency of SiO2 in NR matrix as highlighted in the previous section. Figure 8 and Table 5 showed the skid resistance of all leathers. It is found that the artificial leather (i.e., NR/SiO2 vulcanisate, PU and PVC-based leathers) has the higher skid resistance than the real leather. Furthermore, the highest skid resistance is also obtained for the one with NR/SiO2 vulcanisate. This is due to the better damping property of NR and excellent reinforcement efficiency of SiO2 in the NR matrix.15-17 Therefore, based on the observed abrasion resistance, optimal puncture force and skid resistance of the NR/SiO2 leather, it can be summarised that the in-house NR/SiO2 leather has the ability to use as artificial leather by replacing commercial leather.

4. CONCLUSION

Preparation of artificial leather based on NR with different types of fillers (i.e., clay, SiO2, and CaCO3) suggested that the filler type plays an important role on the properties of leather-like material in terms of particle size and their shapes. SiO2 can reinforce well into the NR matrix and provides higher crosslink density. Therefore, NR/SiO2 showed the higher MH-ML than those of NR/clay and NR/CaCO3, and it depends on the specific structure of the filler. Moreover, NR/SiO2 exhibited the highest tensile strength, 100% modulus, elongation at break, and tear strength. The existence of SiO2 particles attracted the physical rubber absorption within the glassy levels effectively. This restricts the deformation of NR matrix under an applied force and hence the breakages of NR chain have been retarded. Durability of the leather in terms of tear resistance, hardness, and abrasion ability together with puncture force and skid resistance were showed the optimum value in the leather filled with SiO2. This means that the physical absorption of NR molecules onto the SiO2 surfaces effectively reduces the crack propagation of NR matrix during extension. It can be clearly confirmed from the significant increase in tear strength of the vulcanisates reinforced with SiO2 compared to the other fillers. Furthermore, NR/SiO2-based leather was compared to the commercially available product. It was observed that NR/SiO2 leather showed a smoother surface than the real leather, PU and PVC leathers. In addition, no significant difference was observed in the wear index when compared to the real leather. It indicates the applicability of NR/SiO2-based leather as a commercial product with better abrasion resistance. In addition, NR/SiO2-based leather exhibited lower maximum puncture force than the real leather, while it has higher maximum puncture force than PU and PVC leathers. The highest skid resistance was observed in case of NR/SiO2-based leather. Therefore, in terms of abrasion resistance, maximum puncture force, and skid resistance, it can be confirmed that NR/SiO2 leather is the best choice for the development of artificial leather.

5.ACKNOWLEDGEMENTS

This research was supported by Faculty of Science, Prince of Songkla University under the Research Fellowship Fund (Contract no.64001) and the authors also would like to thank Rubber Product and Innovation Development Research Unit, SCIRU63002, Faculty of Science, Prince of Songkhla University for providing the facilities. Discussion with the company (Don Prom Footwear Co., Ltd.) and research team on different concepts about the background was also beneficial to execute this work.

Sidebar

Published online: 25 April 2022

To cite this article: Sourprae, K. et al. (2022). Development and characterisation of in-house leather-like material based on natural rubber vulcanisate. J. Phys. Sci., 33(1), 65-82. https://doi.org/10.21315/jps2022.33.L5

To link to this article: https://doi.org/10.21315/jps2022.33.L5

References

6.REFERENCES

1. Sathish, M., Madhan, B. & Rao, J. R. (2019). Leather solid waste: An eco-benign raw material for leather chemical preparation-a circular economy example. Waste Manage., 87, 357-367. https://doi.org/10.1016/j.wasman.2019.02.026

2. Sur, S. H. et al. (2018). Preparation and properties of DMF-based polyurethanes for wet-type polyurethane artificial leather. Int. J. Polym. Sci., 2018, 1-9. https://doi.org/10.1155/2018/7370852

3. Chen, M. et al. (2007). Analyses of structures for a synthetic leather made of polyurethane and microfiber. J. Appl. Polym. Sci., 103(2), 903-908. https://doi.org/ 10.1002/app.25233

4. Maia, I. et al. (2017). PVC-based synthetic leather to provide more comfortable and sustainable vehicles. IOP Conf. Ser.: Mater. Sci. Eng., 254(12), 122006. https://doi.org/10.1088/1757-899X/254/12/122006

5. Jayanthi, D. et al. (2019). Green processing: Minimising harmful substances in leather making. Environ. Sci. Pollut. Res., 26(7), 6782-6790. https://doi.org/ 10.1007/s11356-018-04111-z

6. Najib, N. N. et al. (2009). Effect of blowing agent concentration on cell morphology and impact properties of natural rubber foam. J. Phys. Sci., 20(1), 13-25. Retrieved 27 July 2021 from https://citeseerx.ist.psu.edu/viewdoc/ download?doi=10.1.1.516.4653&rep=rep1&type=pdf

7. Pechurai, W., Muansupan, T. & Seawlee, P. (2014). Effect of foaming temperature and blowing agent content on cure characteristics, mechanical and morphological properties of natural rubber foams. Adv. Mater. Res., 844, 454-457. https://doi.org/10.4028/www.scientific.net/AMR.844.454

8. Sookyung, U., Thaijaroen, W. & Nakason, C. (2014). Influence of modifying organoclay on the properties of natural rubber/organoclay nanocomposites. J. Compos. Mater., 48(16), 1959-1970. https://doi.org/10.1177/002199831349 3309

9. Arayapranee, W. & Rempel, G. L. (2013). Effects of polarity on the filler-rubber interaction and properties of silica filled grafted natural rubber composites. J. Polym., 1-9. https://doi.org/10.1155/2013/279529

10. Kok, C. (1987). The effects of compounding variables on the reversion process in the sulphur vulcanization of natural rubber. Eur. Polym. J., 23(8), 611-615. https://doi.org/10.1016/0014-3057(87)90006-1

11. Prasertsri, S. & Rattanasom, N. (2012). Fumed and precipitated silica reinforced natural rubber composites prepared from latex system: Mechanical and dynamic properties. Polym. Test., 31(5), 593-605. https://doi.org/10.1016/j. polymertesting.2012.03.003

12. Da Silva, A. L. N. et al. (2002). Mechanical and rheological properties of composites based on polyolefin and mineral additives. Polym. Test., 21(1), 57-60. https://doi.org/10.1016/S0142-9418(01)00047-2

13. Yangthong, H. et al. (2018). Novel natural rubber composites with geopolymer filler. Adv. Polym. Technol, 37(7), 2651-2662. https://doi.org/10.1002/adv.21940

14. Fernandez, S. & Kunchandy, S. (2013). Comparative study of the cure and mechanical properties of natural rubber/expandable graphite vulcanízate filled with nano and precipitated calcium carbonate. Asian J. Chem., 25(15), 8638-8642. https://doi.org/10.14233/ajchem.2013.14875

15. Wang, W., Zhao, D., Yang, J., Nishi, T., Ito, K., Zhoa, X. & Zhang, L. (2016). Novel slide-ring material/natural rubber composites with high damping property. Sci. Rep., 6(1), 1-13. https://doi.org/10.1038/srep22810

16. Prasertsri, S. & Rattanasom, N. (2011). Mechanical and damping properties of silica/natural rubber composites prepared from latex system. Polym. Test., 30(5), 515-526. https://doi.org/10.1016/j.polymertesting.2011.04.001

17. Shi, J. et al. (2018). Viscoelastic damping properties of natural rubberepoxidized natural rubber composites. Fuhe Cailiao Xuebao., 35(3), 537-544. https://doi.org/10.13801/j.cnki.fhclxb.20170523.001