1. Introduction Development of thermoplastic elastomers (TPEs) by mixing natural rubber (NR) biomaterial with crys- talline thermoplastics provides a new material called thermoplastic natural rubber (TPNR). This type of material has drawn the interest of the scientific com- munity because it is an eco-friendly renewable re- source material, which can be recycled, reused, and reprocessed while retaining the key characteristics of both thermoplastics and natural rubber, namely high mechanical strength and elasticity, respectively. Compatibility between NR and thermoplastics is one of the key factors affecting the properties of the TPNR. A high degree of compatibility results in high mechanical properties of this material [1, 2]. Beyond using NR in its normal state, chemically modified nat- ural rubber, i.e., epoxidized natural rubber (ENR) [3–5], maleated natural rubber (MNR) [6, 7], and nat- ural rubber-graft-poly(methyl methacrylate) (NR-g- PMMA) [1, 8, 9], have also been frequently utilized to prepare TPNRs. The main aim of the chemical modification was to improve the thermal resistance of the rubber phase and the compatibility between natural rubber and thermoplastics. Furthermore, 1135 The influence of pre-compounding techniques and surface modification of nano-silica on the properties of thermoplastic natural rubber M. Jarnthong 1,2 , Z. Peng 1 , N. Lopattananon 2 , C. Nakason 3* 1 Guangdong Provincial Key Laboratory of Natural Rubber Processing, Agricultural Products Processing Research Institute of Chinese Academy of Tropical Agricultural Sciences, Zhanjiang 524001 Guangdong, China 2 Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, 94000 Pattani, Thailand 3 Faculty of Science and Industrial Technology, Prince of Songkla University, 84000 Surat Thani, Thailand Received 20 May 2021; accepted in revised form 17 July 2021 Abstract. Thermoplastic natural rubber (TPNR) nanocomposites based on epoxidized natural rubber (ENR) and polypropy- lene (PP) blends filled with 2.5 wt% nano-silica (SiO 2 ) were prepared using a two-step compounding process to improve the dispersion of SiO 2 in TPNR. The effects of the compounding techniques, i.e., solid pre-compounding and latex pre-com- pounding processes of SiO 2 and ENR, and surface modification of SiO 2 on the morphological, mechanical, and thermal properties together with the crystallization behavior of PP phase in ENR/PP blends were demonstrated. Scanning electron microscope (SEM) results showed that latex pre-compounding of SiO 2 and ENR gave finer dispersion and distribution of SiO 2 particles in the TPNR matrix as compared with the solid pre-compounding technique. Also, the superior mechanical properties and thermal stability of the blends were achieved by using the latex pre-compounding technique. Moreover, surface modification of SiO 2 by organosilane was found to enhance crystallization rate and the degree of crystallinity of PP phase in the TPNR, leading to an improvement in the strength and mechanical properties of the ENR/PP blends. These results agree well with the morphological properties that presented a good dispersion and distribution of silane-modified SiO 2 particles in the TPNR matrix. Keywords: nanocomposites, nano-silica, thermoplastic natural rubber, latex pre-compounding, solid pre-compounding eXPRESS Polymer Letters Vol.15, No.12 (2021) 1135–1147 Available online at www.expresspolymlett.com https://doi.org/10.3144/expresspolymlett.2021.92 * Corresponding author, e-mail: charoen.nakason@gmail.com © BME-PT