Polymer Testing 89 (2020) 106623 Available online 19 May 2020 0142-9418/© 2020 Elsevier Ltd. All rights reserved. Material Properties Infuence of alkaline treatment and acetone extraction of natural rubber matrix on properties of carbon black flled natural rubber vulcanizates Phattarawadee Nun-anan a , Suwaluk Wisunthorn a, * , Skulrat Pichaiyut a , Norbert Vennemann b , Claudia Kummerl€ owe b , Charoen Nakason a, ** a Faculty of Science and Industrial Technology, Prince of Songkla University, Surat Thani Campus, Surat Thani, 84000, Thailand b Faculty of Engineering and Computer Science, University of Applied Sciences Osnabrück, Osnabrück, 49076, Germany A R T I C L E INFO Keywords: Alkaline treatment Acetone extraction Natural rubber vulcanizates Carbon black Network structures ABSTRACT In this study, air dried sheet (ADS) showed higher molecular weight, proteins, lipids and more gel content than the low protein natural rubber (LPNR) from alkaline treatment or acetone-extracted natural rubber (AENR). After removal of proteins and lipids, LPNR and AENR had shorter scorch and cure times among the rubber compounds observed. This is due to higher content of free fatty acids, glycerides and sodium salts of fatty acid that might act as cure activators with sulfur curing. Furthermore, a fner dispersion of CB was found in ADS, due to chemical interactions of CB with proteins and lipids at terminal ends of NR molecules. Also, high molecular weight and gel in ADS could induce a higher reinforcing index (α) resulting to superior mechanical, dynamic, thermo- mechanical, hardness and crosslink density. In contrast, CB agglomerates occurred in LPNR and AENR with low molecular weight and reduced proteins and lipids, causing inferior mechanical, dynamic, thermo- mechanical, along with loss of stiffness and crosslink density. 1. Introduction Carbon black (CB) is a reinforcing fller that is widely used in rubber compounds. Addition of CB into a rubber matrix tends to favorably modify various properties, including tensile modulus and strength, hardness, abrasion resistance, and compression resistance [1,2]. Natural rubber (NR) is the main type of rubber matrix used together with CB in various industrial applications. This is because NR has excellent green strength in un-vulcanized state. Also, NR vulcanizates exhibit superior modulus, tensile strength, tear strength, and low heat build-up [3–5]. It has been claimed that the difference in mechanical properties between synthetic and natural rubber is mainly due to different molecular structures, and to the non-rubber components in NR [5]. It has been well established that NR has high molecular weight because it consists mostly of long-chain hydrocarbons with numerous repeating units of cis-1, 4-polyisoprene [6]. Also, the non-rubber components including pro- teins, lipids, carbohydrates, and minerals, are other components in NR [7]. It was also found that properties and molecular characteristics of NR together with types and levels of non-rubber components are mainly dependent upon weather, landscape, soil and natural rubber genotype [8]. Furthermore, the non-rubber components (especially proteins and lipids) cause inconsistencies in the properties of natural rubber, partic- ularly in viscosity and elasticity, which affect NR applications [9]. In addition, it was claimed that proteins and lipids may be the main components that control various properties of NR including plasticity retention index (PRI) and the mechanical properties modulus and green strength in the un-vulcanized state [5,10]. Furthermore, proteins and lipids infuence mechanical properties, processability and cure charac- teristics (i.e., cure rate) of vulcanized NR [3,11]. It is noted that the fundamental rubber structures consist of long and branched chains with the proteins and lipids at the two types of terminals: ω-terminal and α-terminal chain ends, respectively [12–14]. The non-rubber compo- nents (i.e., proteins and lipids) at the chain ends are origins of the branching network (i.e, aggregation) based on interactions among pro- teins and lipids via hydrogen bonding together with ionic linkages to form microgels [14]. Furthermore, macrogels are also formed by in- teractions of the microgels during prolonged storage of NR, which is known as storage hardening [14]. This affects various properties of un-flled and flled NR composites including tensile strength, modulus, and other related properties [15]. * Corresponding author. ** Corresponding author. E-mail addresses: suwaluk.w@psu.ac.th (S. Wisunthorn), charoen.nakason@gmail.com (C. Nakason). Contents lists available at ScienceDirect Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest https://doi.org/10.1016/j.polymertesting.2020.106623 Received 17 November 2019; Received in revised form 27 April 2020; Accepted 15 May 2020