Kasetsart J. (Nat. Sci.) 42 : 325 - 332 (2008) Epoxidized Natural Rubber for Adhesive Applications Rangrong Yoksan ABSTRACT ENR-based adhesives were prepared under the concepts of (i) crosslinking at epoxirane rings and/or (ii) curing at α-carbon positions along the rubber chain. The crosslinking at epoxirane rings was accomplished when two crosslinking agent/catalyst systems, i.e., diamine/bisphenol A and anhydride/ imidazole were used. However, the crosslinking efficiency of the anhydride/imidazole crosslink system was higher than that of the diamine/bisphenol A crosslink system. The curing of ENR at the α-carbon positions was carried out by conventional vulcanization (CV). The mechanical testing of three formulations of ENR-toluene based adhesives, i.e., (i) ENR-PA adhesive, (ii) ENR-S adhesive, and (iii) sulfur cured ENR-PA adhesive showed that ENR-S adhesive gave the highest tensile strength. The fourth formulation of adhesive was, hence, prepared by adding tackifier (resin PX1150) to the ENR-S adhesive (ENR-S-resin PX1150 adhesive). The effect of resin PX1150 on the mechanical properties of ENR-S-resin PX1150 adhesives was studied to find that when the amount of tackifier increased the tensile strength and tensile modulus of the adhesive also increased, while the %elongation at break decreased. Key words: natural rubber, epoxidized natural rubber, phthalic anhydride, adhesive, tackifier INTRODUCTION structure (cis 1,4-polyisoprene), which contains double bonds and hydrocarbon atoms (Figure 1). Natural rubber (NR) is one of the most The chemical modification at double important economic commodities in Thailand bonds and introduction of hydrophilic groups produced from Para rubber tree, Hevea along NR backbone are alternative strategies to brasiliensis. NR exhibits unique physical and improve the stability of NR and to provide chemical properties such as elasticity, stickiness, widespread applications. Epoxidized natural resilience, etc., which are suitable for various rubber (ENR) (Figure 1) is an ideal material for products, e.g., tires, under the bonnet products, the present work since the introduction of gloves, balloons, rubber bands, condoms, epoxirane ring not only reduces the number of adhesives, etc. However, most applications of NR double bonds but also increases the hydrophilicity are limited owing to the low stability to heat, of NR. The preparation of ENR by using peracids, oxygen, sunlight, etc. and the high solubility in e.g., performic (Heping et al., 1999; Okwu et al. most hydrocarbon/hydrophobic solvents including 2001; Tanrattanakul et al., 2003) and peracetic oils. Those limitations result from NR chemical acids (Kinklai et al., 2003) was reported. The Division of Physico-Chemical Processing Technology and Department of Packaging Technology and Materials, Faculty of Agro Industry, Kasetsart University, Bangkok 10900, Thailand. e-mail: [email protected] 326 Kasetsart J. (Nat. Sci.) 42(5) degree of epoxidation is controlled by the amount Latex Co., Ltd., Thailand. Resin PX1150 was of peracid, reaction temperature, and reaction time. provided by Yasuhara Chemical Co., Ltd., Japan. Similar to epoxy resin, ENR can be crosslinked Formic acid, 30% hydrogen peroxide (H2O2), by di- or multi-functional group substances such sodium sulfite, 25% ammonia solution, methanol as amines (Hashim et al., 1994; Hong et al., 1998; (MeOH), sodium carbonate (Na2CO3), bisphenol Hong et al. 2004), carboxylic acids, anhydrides A, p-phenylenediamine, hexamethylenediamine, (Masse, 1999; Masse et al., 1999), etc. to provide phthalic anhydride (PA), and 2-ethyl-4- a network structure. methylimidazole (EMI) are analytical grade and Adhesive is one of the major applications used without further purification. Emulwin W, of NR used in many industries such as shoes, bags, Terric 16A-16, ZnO, stearic acid, titanium dioxide tires, automotives, furniture, etc. NR-based (TiO2), Wingstay-L, sulfur, and zinc- adhesives were classified into two principal types, diethyldithiocarbamate (ZDEC) are industrial i.e., latex adhesives (water-based adhesives) and grade. solution adhesives (solvent-based adhesives) (Pocius, 1997). Although latex adhesives are Preparation and characterizations of ENR cheap, handled easily, non-toxic, and little ENR was prepared by performic incendiary risk, some properties such as tackiness epoxidation (Figure 1). Concentrated NR was and water resistance including adhesive shelf-life diluted with distilled water to have 20 %DRC. should be considered. The solution adhesives, Nonionic surfactant (Emulwin W or Terric 16A- generally containing a solvent such as toluene, 16) was gradually added to maintain the stability naphtha, or trichloroethane, are inflammatory and of latex. Formic acid (0.75 mole equiv. to isoprene toxic when one uses without skills. However, unit) and 30% H2O2 (0.75 mole equiv. to isoprene solvent-based adhesives provide higher tackiness unit) were, then, slowly dropped while the mixture and longer shelf-life. was heated to 50°C. The mixture solution was The present work, thus, focused on the stirred at 50°C for 2-9 hours. After the reaction, preparation and characterizations of ENR and also the mixture was cool down to room temperature studied on its vulcanization at α-carbons and and added sodium sulfite to remove an excess crosslinking at epoxirane rings. In addition, the H2O2 following by neutralization with 25% solvent-based ENR adhesives were prepared and ammonia solution. MeOH was, then, added to tested for their mechanical properties. precipitate the product. A white solid bulk was washed with water several times, rinsed with MATERIALS AND METHODS MeOH, and dried under reduced pressure. The obtained products were, then, characterized by FT Materials IR (Vector 3.0 Bruker Spectrometer and Concentrated NR (Batch No. T2 091047, PerkinElmer Spectrum 100 FT IR spectrometer), 60 %DRC) was obtained from Num Rubber & 1H NMR (Bruker DPX300), and SEM techniques a O d CH3 CH3 CH3 HCO OH e c b CH2 CCH CH2 CH2 CCHCH2 CH2 CCH CH2 Performic epoxidation O Natural rubber, NR Epoxidized natural rubber, ENR Figure 1 Performic epoxidation of NR. Kasetsart J. (Nat. Sci.) 42(5) 327 (Jeol JSM 5600 LV). The mol %epoxidation Table 1 Amount of chemicals used for the was determined by 1H NMR technique using preparation of crosslinked ENR. Equation 1. Chemicals Formulationsa ABC Mol %epoxidation = [A2.7 / (A2.7 + A5.14)] × 100 ENR25 100 100 100 Equation 1 Na2CO3 33 Bisphenol A 10 10 When A2.7 = Peak integral at δ 2.7 p-Phenylenediamine 5 ppm representing methine proton (e) of epoxide Hexamethylenediamine 5 group PA 5 A5.14 = Peak integral at δ 5.14 ppm EMI 1 representing olefinic proton (c) of unreacted NR a Unit in phr: part per hundred dried ENR Study on crosslinking of ENR at epoxirane adhesives were tested using the modified ASTM rings D2095-2096. The adhesive was applied to a cubic The crosslinking of ENR with 25 mol wood (11×11×38 mm3) on one surface (11´11 %epoxidation (ENR25) via ring opening reaction mm2). The wood was stick with another wood was studied by using two crosslink systems, i.e., and then dried with two different drying (i) diamine/bisphenol A (formulation A and B) and conditions, i.e., (i) room temperature for 2 days (ii) anhydride/imidazole crosslink systems and (ii) 150°C for 10 min following by room (formulation C) (Table 1). ENR compounds were temperature for 2 days. The mechanical properties prepared at 50°C by two-roll mill. The curing were tested in a tension mode (EZ Graph, characteristic of compounds was studied at 160°C Shimadzu Corporation, Japan) with a loading rate for 90 minutes by Rheometer (RDA III, ARES of of 3×105 Pa/sec (0.3 N/mm2/sec). Rheometric machine). RESULTS SND DISCUSSION Preparation and characterizations of ENR-S- resin PX1150 adhesives Preparation and characterizations of ENR Tackifier used in the present work was ENR was successfully prepared by resin PX1150 (terpene polymer). The ENR-S- performic epoxidation as confirmed from a new resin PX1150 adhesives were prepared in toluene peak at 870 cm-1 (asymmetric epoxide ring with various ENR25:resin PX1150 ratios, i.e., 1:0, stretching) and the decrease in peak intensity at -1 1:0.5, 1:1, 1:1.5, 1:2, 1:3, and 1:4. Compound S 3036 (R1R2C=CHR3) and 835 cm (=CH-, was firstly prepared by mixing ENR25 (10 g) with bending) (Figure 2). The successful epoxidation all chemicals for sulfur vulcanization (ZnO (0.3 also supported by 1H NMR technique as seen from g), stearic acid (0.2 g), TiO2 (0.1 g), Wingstay-L new peaks at δ 1.2 (-CH3 (d)) and 2.7 ppm (-CH- (0.1 g), and sulfur (0.1 g)), except ZDEC (e)) (Figure 3). It should be noted that the mole (accelerator). The mixing was carried out at 70°C %epoxidation increased as the reaction time by two-roll mill. Compound S was, then, dissolved increased and was not dependent on the types of in a ZDEC-resin PX1150-toluene solution with surfactants used, i.e., Terric 16A-16 and Emulwin accelerator/sulfur (A/S ratio) of 0.4 and W (Figure 4). The changes in appearance and toluene:ENR25 of 5 mL:1 g. physical properties were obtained in the case of The mechanical properties of all ENR with high mol%epoxidation, e.g., the 328 Kasetsart J. (Nat. Sci.) 42(5) 835 870 (b) (b) 835 3036 (a) (a) 3500 2500 1500 500 10 8 6 4 2 0 Wavenumbers / cm-1 Chemical shift (δ) / ppm Figure 2 FT IR spectra of (a) NR and (b) ENR. Figure 3 1H NMR spectra of (a) NR and (b) ENR. 40 30 20 10 Mole %epoxidation 0 0 2 4 6 8 10 Reaction time / Hour(s) Figure 4 Mole %epoxidation of ENR as a Figure 5 SEM micrographs of (a) NR, (b) ENR- function of reaction time; (●) Terric Tr-5h, (c) ENR-Tr-7h, and (d) ENR-Tr- 16A-16 and (❍) Emulwin W.