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Cardanol Polysulfide As a Vulcanizing Agent for Natural Rubber

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Cardanol Polysulfide As a Vulcanizing Agent for Natural Rubber Cardanol Polysulfide as a Vulcanizing Agent for Natural Rubber Leerawan Khaokhum1, Narumon Sawasdipuksa2, Nanthita Kumthong2, Jumreang Tummatorn2 and Sophon Roengsumran2* Cardanol polysulfide (CPS), a sulfur donor type vulcanizing agent, was synthesized from a reaction of element sulfur and cardanol which separated from decarboxylated cashew nut shell liquid. The effect of CPS as a vulcanizing agent on the properties of rubber compounds was remarkable. The optimum cure time of rubber containing CPS was reduced and the mechanical properties were improved. Moreover, the CPS rubber was found to have lower reversion. Key words: vulcanizing agent, cardanol and natural rubber. 1Program of Petrochemisty Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. 2Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. *Correspondence to: e-mail: [email protected] J. Sci. Res. Chula. Univ., Vol. 30, No.1 (2005) 23 Leerawan Khaokhum, Narumon Sawasdipuksa, Nanthita Kumthong, Jumreang Tummatorn and …………………………………………………………………………………………Sophon Roengsumran คารดานอลพอลิซัลไฟดสํ าหรับเปนสารกอวัลคะไนซ ในยางธรรมชาติ ลีลาวัลย ขาวขํ า นฤมล สวัสดิ์พฤกษา นันทิตา คํ าทอง จํ าเรียง ธรรมธร และ โสภณ เริงสํ าราญ (2548) วารสารวิจัยวิทยาศาสตร จุฬาลงกรณมหาวิทยาลัย 30(1) คารดานอลพอลิซัลไฟด (CPS) เปนสารวัลคะไนซประเภทที่ใหซลเฟอรั สังเคราะหได จากปฏิกิริยาของธาตุซัลเฟอรกับคารดานอลที่แยกไดจากดีคารบอกซิเลต ของเหลวจากเปลือก มะมวงหิมพานต ผลของการใช CPS เปนสารวัลคะไนซกับสารประกอบของยาง เปนที่นา ประทับใจ ยางที่มี CPS ผสมอยูจะใชเวลาบมลดลงและสมบัติทางกายภาพดีขึ้น ยิ่งกวาน ั้นพบวา ยางดังกลาวมีคารีเวอรชันลดลงดวย คํ าสํ าคัญ สารกอวัลคะไนซ คารดานอล ยางธรรมชาติ 24 J. Sci. Res. Chula. Univ., Vol. 30, No.1 (2005) Cardanol Polysulfide as a Vulcanizing Agent for Natural Rubber…...……………………………………… J. Sci. Res. Chula. Univ. Vol. 30, No.1 (2005) 25 Leerawan Khaokhum, Narumon Sawasdipuksa, Nanthita Kumthong, Jumreang Tummatorn and …………………………………………………………………………………………Sophon Roengsumran INTRODUCTION There are many ingredients in rubber Vulcanizing agents are substances which bring compounds, and each ingredient has its about the actual crosslinking process. The specific function and impact on both properties most important ones are sulfur, sulfur donors, and price. In order to reduce the cost of selenium, tellurium, peroxides and metallic investment in compounds that are suitable for oxides.(1) certain applications, knowledge of the function Cashew (Anacardium occidentale) is a and effectiveness of compounding ingredients member of the Anacardiaceae family. The is indispensable. cashew nut shell liquid (CNSL) is a by-product Vulcanization is a chemical process by of the cashew industry. CNSL,(2) extracted which the physical properties of natural or with low boiling petroleum ether, contains synthetic rubber are improved; finished rubber about 90% anacardic acid and about 10% has higher tensile strength, increased resistance cardol. CNSL, on distillation, gives the pale to swelling and abrasion and is elastic over a yellow phenolic derivative cardanol. Natural greater range of temperatures. In its simplest CNSL contains 80.9% anacardic acid, 10-15% form, vulcanization is brought about by cardol and small amounts of other materials, heating rubber with sulfur. One of the notably the 2-methyl derivative of cardol. The disadvantages of the process is reversion of the chemical structures of major compounds in sulfur. The reversion increases with an natural CNSL are shown in Figure 1. The side increase in the amount of sulfur. Furthermore, chains exist in saturated (n = 0), monoene it is easy for blooming to occur because sulfur (n = 2), diene (n = 4) and triene (n = 6) forms has low solubility in rubber at room with cis configuration.(3-6) temperature. A sulfur donor is used to replace sulfur in order to solve this problem. OH OH COOH C15H31-n C15H31-n anacardic acid cardanol (6-pentadecenyl salicylic acid) (3-pentadecenyl phenol) OH OH CH3 HO C15H31-n HO C15H31-n 2-methyl cardol cardol (2-methyl, 5-pentadecenyl resorcinol) (5-pentadecenyl resorcinol) Where n = 0, 2, 4 or 6 Figure 1. Chemical structures of major compounds in natural CNSL. 26 J. Sci. Res. Chula. Univ., Vol. 30, No.1 (2005) Cardanol Polysulfide as a Vulcanizing Agent for Natural Rubber…...……………………………………… In the roasting process, some Preparation of CPS polymerization as well as decarboxylation Preparation of cardanol from CNSL(10-12) occurs because of the highly unsaturated All Infrared (IR) spectra were recorded nature of CNSL. Thus, a typical product would using a Nicolet Impact 410 spectrometer. contain polymer (20-25%), cardanol (60-65%), 1H NMR (200 MHz) and 13C NMR (50 MHz) cardol (10-12%) and other minor components. spectra were recorded on a Bruker ACF-200 CNSL, cashew nut shell liquid- spectrometer and the solvent used was CDCl . formaldehyde resin(7) and phosphorylated 3 Sulfur content was determined using a LECO derivative(8,9) have been used as vulcanizing SC-132 sulfur analyzer. Non-productive agents for natural rubber. The results suggested compounds were mixed by a Kneader that the presence of 10 ph of CNSL will Machinery internal mixer. Vulcanization decrease the state of cure time and increase the ingredients were added to the compounds on a cure rate index and keep constant the rate of Lab Tech Engineering two-roll mill. Cure vulcanization. With the addition of cashew nut characteristics at 150oC for 30 min and arc 0.5o shell liquid-formaldehyde resin (5-15 ph), the were determined using a Monsanto rheometer physiomechanical properties of natural rubber MDR2000. Sheets and test specimens were lead to the improvement in processability of vulcanized by Lab Tech Engineering TP-400 the mixes. Researchers claim this is because compression molding. Tensile measurements of plasticization of the mix. were carried out using an Instron Calibration This study involved synthesizing a Laboratory 1445 tensile tester. Hardness was vulcanizing agent from cardanol for use as a determined using a Zwick hardness testing sulfur donor and studying the mechanical machine. Rebound resilience was determined properties of the rubber which was prepared using a Zwick rebound tester. from the vulcanizing agent. The investigation CNSL was decarboxylated to give is concerned with the preparation of cardanol cardanol in the presence of calcium hydroxide polysulfide (CPS) from cardanol. The process as a catalyst. Calcium hydroxide (4 g) with was carried out under varying conditions such hexane (200 ml) as a solvent was charged into as reaction temperature and reaction time. CPS 200 g of natural CNSL in a 1000 ml 2-neck was used as a vulcanizing agent by mixing it round bottom flask equipped with a condenser with natural rubber and other additives. and a thermometer. The mixture was refluxed Finally, the vulcanization characteristics, at a temperature of 120oC and stirred for 3 h. mechanical properties and accelerated aging The reaction was stopped and the mixture was were determined. cooled down. Calcium hydroxide was filtrated. After evaporation of the solvent, decarboxylated MATERIALS AND METHODS CNSL was obtained. IR and NMR spectra of Chemicals the resulting product shown in Figures 3b, 4b Cashew nut shell liquid (CNSL) was a and 5b indicate that the carboxylic acid group gift from the 25 Industrial Products Co., Ltd. was removed completely. Natural rubber (grade STR20) was purchased Decarboxylated CNSL (60 g), formaldehyde from Thai Hua Co., Ltd. The chemicals for (19.4 g) and diethylenetriamine (2.57 g) were vulcanization such as N-cyclohexyl- mixed in methanol (200 ml). An exothermic benzothiazole-2-sulfenamide (CBS), reaction took place after mixing the reactants. naphthenic oil, stearic acid and carbon black After 30 minutes, two phases were formed. were obtained from the Rubber Research The upper phase was a slightly red solution. Institute. Formaldehyde was purchased from The lower phase was solidified and dark in Lab Scan. Resorcinol was purchased from color. The upper phase was decanted and Sigma. Elemental sulfur and zinc oxide were treated with water (40 ml) followed by purchased from Fisher. Diethylenetriamine and extraction with petroleum ether. The petroleum ether was purchased from Fluka. petroleum ether extract was distilled to recover Calcium hydroxide and deuterated chloroform hydrocarbon solvent. A reddish residue of were purchased from Merck. Ethyl acetate, cardanol (45 g, 80% w/w) was obtained. hexane, methanol, toluene and propylene glycol were bought from local agents as commercial grade. J. Sci. Res. Chula. Univ. Vol. 30, No.1 (2005) 27 Leerawan Khaokhum, Narumon Sawasdipuksa, Nanthita Kumthong, Jumreang Tummatorn and …………………………………………………………………………………………Sophon Roengsumran Synthesis of CPS Cardanol (70 g) and excess elemental sulfur. The diluted mixture was evaporated. sulfur (40 g) were added to a 250 ml beaker. Dry weight of the dilution was calculated. Toluene or propylene glycol was used as a Sulfur content in CPS was determined using a solvent at a reaction temperature of 110 or sulfur analyzer. In order to determine the 185oC, respectively, according to the boiling suitable reaction time to give the highest sulfur point of each solvent. At other temperatures, content in CPS, the reaction times were varied no solvent was used. The mixture was heated among 3, 4 and 5 h. To calculate the weight of to the desired temperature in a range of CPS,(13) approximately 1.5 g of a sample was 110-185oC and stirred for 2 h. The reaction poured into
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