Recently, ultrafine fully vulcanized powdered natural rubbers (UFPNRs) have become an alternative to traditional commodity synthetic rubber powder, due to renewable resources used for their preparation, which are environmentally friendly, reasonably priced. UFPNRs are illustrated to be suitable toughening fillers in a polymer matrix (Lin Y et al., 2021; Wongkumchai, Amornkitbamrung, Mora, Jubsilp, & Rimdusit, 2021). Ultrafine fully vulcanized powdered rubbers (UFPRs) were prepared by irradiation vulcanization followed by a spray drying process to produce a controllable spherical particle powder (Qiao J. et al., 2002). Mostly UFPRs were obtained from synthetic rubber latexes as raw materials, such as styrene-butadiene rubber (SBR) (X. Liu, Gao, Bian, & Wang, 2014), carboxylated-styrene butadiene rubber (XSBR) (Taewattana, Jubsilp, Suwanmala, & Rimdusit, 2018), acrylonitrile-butadiene rubber (NBR) (Pan & Liu, 2022; Wu, Xie, & Yang, 2010), and carboxylated-nitrile butadiene rubber (XNBR) (Huang et al., 2002; Taewattana et al., 2018). It is well known that UFPRs show predominantly reinforcement effect into other rubbers (Tian et al., 2006) or other polymer matrices and modify composite properties, such as reduction of the wear mass loss and friction coefficient of epoxy composite for friction materials (Yu, Hu, Ma, & Yin, 2008) increase the toughness or heat resistance of PVC (Q. Wang et al., 2005), epoxy resin (Huang et al., 2002), polypropylene (Y. Liu et al., 2004), and phenolic resin (Y. Liu, Fan, Ma, Tan, & Qiao, 2006; Ma et al., 2005). UFPRs posses good elasticity and are dispersed easily in polymer matrices during blending, as a result of their particular microstructure i.e. a spherical powder form with high crosslinking on the particle surface and moderate crosslinking in the inner part (Tian et al., 2006). Moreover, the advantages of UFPRs over the conventional rubber in a latex form are higher stability upon long-term storage and not harmful to the human body during blending (Qiao, 2020; J. Wang, Zhang, Jiang, & Qiao, 2019).
Despite the extensive use, unfortunately, the production of UFPNR is restricted, because of aggregation between particles (Taewattana et al., 2018). Therefore, NR has been developed by enhancing the degree of crosslinking by adding polyfunctional monomers as a crosslinking agent or coagent in the rubber latex before the irradiation process. Lin et al. (Lin Y et al., 2021) have studied the effect of acrylate coagents having different amounts of functional groups i.e. dipropylene glycol diacrylate (DPGDA), trimethylol propane trimethaacrylate (TMPTMA), and ditrimethylol propane tetraacrylate (DTMPTA), on properties of UFPNR produced by radiation vulcanization and spray-drying. They suggested that ditrimethylol propane tetraacrylate (DTMPTA), which had four functional acrylate groups, demonstrated high efficiency in enhancing the degree of crosslinking in NR that led to UFPNRs with much less agglomerated particles. However, NR which is a non-polar long chain hydrocarbon, lacks in some properties i.e. it has poor solvent resistance, and limited application due to its immiscibility when blended with polar polymers (W. Arayapranee, P. Prasassarakich, & G. L. Rempel, 2002; Kangwansupamonkon, Gilbert, & Kiatkamjornwong, 2005). Therefore, it is necessary to modify the properties of NR before processing to overcome these problems.
Chemical modification by graft copolymerization is one of the most attractive techniques. The NR molecular structure which contains cis-1,4-polyisoprene with an electron-donating methyl group attached to the carbon–carbon double bond in its main chain can facilitate the reaction with other vinyl monomers and covalently bunched onto the NR backbone. Several vinyl monomers have been used for grafting modification of NR, such as styrene (St) (T. Dung et al., 2016), acrylonitrile (AN) (Prukkaewkanjana, Kawahara, & Sakdapipanich, 2013), methyl methacrylate (MMA) (Kongparakul, Prasassarakich, & Rempel, 2008), and maleic anhydride (MA) (Pongsathit & Pattamaprom, 2018) to improve solvent resistance, thermal stability, mechanical properties, and compatibility of NR. Rimdusit et al. (Rimdusit et al., 2021) have improved thermal stability and solvent resistance of UFPNR via graft-copolymerization with polystyrene (PS) and polyacrylonitrile (PAN), respectively. The results revealed that the proper monomer content was 5 phr and proper radiation dose was 300 kGy for producing UFPNR-g-PS and UFPNR-g-PAN with maintaining rather high thermal stability. Therefore, the combination of St and AN monomer to form St/AN copolymer (SAN) by grafting on NR backbone is expected to improve the thermal stability the solvent resistance of NR and probably the compatibility with various polymer matrices having different polarity. (Angnanon, Prasassarakich, & Hinchiranan, 2011; T. A. Dung et al., 2017; Fukushima, Kawahara, & Tanaka, 1988; Indah Sari, Handaya Saputra, Bismo, & R. Maspanger, 2020; Nguyen Duy et al., 2020; Nguyen, Do, Tran, & Kawahara, 2019; Prasassarakich, Sintoorahat, & Wongwisetsirikul, 2001)
The present work is devoted on modifying UFPNR by graft-copolymerization with the combination of St and AN monomers onto deproteinized NR using tert-butyl hydroperoxide (TBHPO) and tetraethylenepentamine (TEPA) as a redox initiator. The effects of monomer contents and St/AN weight ratios of DPNR-g-(PS-co-PAN) on monomer conversion and grafting efficiency were determined. The obtained DPNR-g-(PS-co-PAN) were irradiated by an electron beam in the presence of DTMPTA as coagent followed by spray drying process to produce UFPNR. The effect of irradiation doses on the morphology and thermal properties of UFPNR was also investigated.