Use of By-product Sulfur from Petroleum Refinery as Vulcanizing Agent in Natural Rubber

Transcription

1 Chiang Mai J. Sci. 206; 43(3) 569 Chiang Mai J. Sci. 206; 43(3) : Contributed Paper Use of Byproduct Sulfur from Petroleum Refinery as Vulcanizing Agent in Natural Rubber Pathompong Pangamol [a], Pongdhorn Saeoui [b], Chakrit Sirisinha*[a,c] [a] Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok 0400, Thailand. [b] National Metal and Materials Technology Center, Thailand Science Park, Pathumthani, 220, Thailand. [c] Rubber Technology Research Centre (RTEC), Faculty of Science, Mahidol University, Nakhon Pathom, 7370, Thailand. *Author for correspondence; chakrit.sir@mahidol.ac.th Received: 28 October 203 Accepted: 9 September 204 ABSTRACT Sulfur is typically required for rubber vulcanization in order to achieve enhanced mechanical strength and elastic behavior. Basically, the sulfur used in rubber industry originates from natural resource and petroleumbased refinery. The petroleumbased sulfur (PS) generally treated as byproduct from refinery process was investigated in this work. Its influence on cure characteristics, viscoelastic properties as well as crosslink density was compared with conventional rhombic sulfur. Results obtained reveal inferiority in mechanical properties of the system with asreceived PS. By reducing average particle size of asreceived PS via a ballmilling process under optimal milling conditions, the ground PS (GPS) exhibits significant improvement in mechanical properties of rubber vulcanizates to be comparable to the conventional rhombic sulfur. Keywords: rubber, sulfur, vulcanization, mechanical properties, cure behavior. INTRODUCTION Vulcanization plays an important role on rubber industry by offering the rubber products containing threedimensional network of rubber molecules. By this mean, the significant improvement in numerous properties including tensile and tear properties, set, resilience and abrasion of rubber vulcanizates is resulted. The vulcanization could generally be divided into 3 main systems, i.e., sulfur, peroxide, and metal oxide systems [5]. The sulfur vulcanization system is generally preferential because of its superiority in mechanical properties and ease of cure behavior adjustment [60]. Typically, the sulfur used in rubber industry originates from 2 main resources, i.e., natural resource and petroleum refinery []. Basically, the sulfur from natural resource is more preferable because of its certain chemical structure in conjunction with its high sulfur content (99%) []. The petroleumbased sulfur is taken as a byproduct from petroleum refinery. The sulfurcontaining organic

2 570 Chiang Mai J. Sci. 206; 43(3) compounds are transformed to hydrogen disulfide (H 2 S) gas through a hydrodesulfurization process [23]. Such gas is toxic, and causes environmental pollution and metal corrosion in machines [25]. In practice, the H 2 S gas could be eliminated and transformed into element sulfur via Claus process [68]. In Thailand, there are several petroleum refineries still treating the sulfur gained from the refinery process as the byproduct, and usually sold out with low price. Our main goal of this work is to make full use of such sulfur by investigating the modification technique for enhancing the petroleumbased sulfur to match the requirement of rubber industry. The conventional rhombic sulfur widely used in rubber industry is taken as a reference for a comparison in properties of rubber compounds and vulcanizates. 2. MATERIALS AND METHODS 2. Materials Natural rubber (STR 5L) was purchased from Union Rubber Product Co., Ltd. Conventional rhombic sulfur used (denoted as RS) was supplied by Chemmin Corporation Ltd. while a petroleumbased sulfur (denoted as PS) was kindly supplied by IRPC Plc. Co., Ltd. Other chemical ingredients, namely, stearic acid and zinc oxide (ZnO) were supplied by Chemmin Corporation Ltd., and Ntertbutyl2benzothiazole sulfonamide (SantocureTBBS) was purchased from Reliance Technochem (Flexsys) Co., Ltd. Except for PS, all ingredients were used asreceived. 2.2 Characterization of Sulfur Characteristics Chemical structures of sulfurs used in this work were characterized using xray diffractometer (XRD) (Bruker AXS Model D8 Discover, US). Chemical compositions of sulfurs were examined by the use of scanning electron microscope equipped with energydispersive xray spectroscopy (SEMEDS) (JEOL JSM5800LV, US and Link ISIS Series 300, US). A particle size reduction of the asreceived PS was performed by the use of ballmilling device (Retsch, Germany). The milling time was 30 minutes with the use of 00 stainless balls with diameter of 0.5 cm. Particle size and its distribution of sulfurs were measured using a particle size analyzer (Malvern model SCIROCCO 2000, UK). 2.3 Preparation and Testing of Rubber Compounds Mixing was conducted using a tworoll mill (LabTech, model LRM 50, Thailand) at set temperature of 50 C for 0 minutes. Compound recipe was tabulated in Table in which no reinforcing filler was incorporated in order to exclude the filler reinforcement effect on properties of rubber compounds and vulcanizates. Cure characteristics (both scorch time and cure time) were evaluated using a moving die rheometer (TechPro MD+, US) at 50 C. Viscoelastic properties of rubber compounds were investigated using rubber process analyzer (RPA2000, Alpha Technologies, US). Storage modulus (G ) of the rubber compounds was measured as a function of strain in the range of 0.56 to 200% at test frequency and temperature of.0 Hz and 00 C, respectively.

3 Chiang Mai J. Sci. 206; 43(3) 57 Table. Compounding recipes used for preparing the rubber compounds. Ingredients Loading (phr a ) RS b PS c NR (STR 5L) Zinc oxide (ZnO) Stearic acid Ntertbutyl2benzothiazole sulfenamide (TBBS) RS PS GPS a phr: parts per hundred of rubber b RS: rhombic sulfur c PS: petroleumbased sulfur d GPS: ground petroleumbased sulfur GPS d Mechanical Testing of Rubber Vulcanizates According to ASTM D42 Die C, tensile properties of cured specimens (vulcanizates) were determined using a universal mechanical tester (Instron 5566, US) at a crosshead speed and load cell of 500 mm/min and kn, respectively. Hardness was measured using a Shore A Durometer (Wallace model COGENIX, UK) as per ASTM D Determination of Crosslink Density Crosslink density in this work was determined by the use of swelling technique with toluene as a solvent. Swelling ratio (S) of cured specimens was calculated from Eq. (). (w S = 2 w ) w () Where w is the mass of rubber before swelling, and w 2 is the mass of swollen rubber after being soaked in solvent for 72 hours to ensure swelling equilibrium [92]. Volume fraction of rubber in the swollen network (υ r ) was calculated using Eq. (2) [2]. (2) Where ρ d is the rubber density before swelling, and ρ s is the toluene density ( g.ml ) [2]. Furthermore, the crosslink density was calculated as referred to FloryRehner equation, as expressed in Eq. (3) [2]. (3) where υ e is the network chain density (mol.cm 3 ), υ is the molar volume of toluene (06.3 cm 3.mol ), and χ is the Flory/Huggins interaction parameter between toluene and rubber (0.39) [2]. 3. RESULTS AND DISCUSSION 3. Chemical Structure of PS Table 2 demonstrates the chemical

4 572 Chiang Mai J. Sci. 206; 43(3) composition of petroleumbased sulfur (PS), as determined by SEMEDS. Evidently, the asreceived PS possesses sulfur content up to 97% which is comparable to that of conventional rhombic sulfur (RS) as shown in Table 2. Table 2. Chemical compositions of asreceived PS and RS as determined from EDS spectra. Type of sulfur PS RS Sulfur content (%) 97.07± ±0.34 Oxygen content (%) 2.93± ±0.34 when the RS is replaced by the asreceived PS. Also, there is no significant difference in crosslink density implying the comparable cure reactivity of these 2 sulfurs. Consequently, these 2 sulfurs should offer the cured rubber specimens (vulcanizates) with comparable mechanical properties. Figure. XRD patterns of petroleumbased sulfur (PS) and rhombic sulfur (RS). The XRD results as exhibited in Figure reveal similar peak patterns of both asreceived PS and RS with strong peak at the similar diffraction angle (2θ = 23. ), implying a similarity in orthorhombic structure of asreceived PS and RS [22]. 3.2 Cure Characteristics Evidently, the asreceived PS possesses similar chemical composition and crystalline structure to the RS. It is therefore anticipated that the asreceived PS might be capable of functioning as a curative in rubber compounds in similar manner to the conventional RS. Table 3 exhibits a comparison of cure characteristics of rubber compounds incorporated with either asreceived PS or conventional RS. Clearly, there are no significant discrepancies in cure characteristics of rubber compounds regardless of sulfur origin. Similar scorch and cure times indicate no necessity for adjusting the curing process 3.3 Viscoelastic Properties Figure 2 shows straindependent viscoelastic properties of rubber compounds incorporated with different sulfur types. It is clearly seen that the storage modulus (G ) of all compounds is superimposable. This means the difference in sulfur types gives no profound effect on rheological behavior of rubber compounds. In other words, the processability of rubber compounds incorporated with asreceived PS and RS is comparable, and therefore no adjustment in processing conditions is required. A nonlinear behavior as evidenced by a decrease in at high strain is known to be the results of molecular slippage and/or filler network disruption and reformation (or the socalled Payne effect) [2326]. Since no reinforcing filler is incorporated in compounds, the effect of filler network could be disregarded. Replacement of RS by asreceived PS in rubber compounds shows no significant change in the magnitude of Payne effect or the onset of nonlinearity regardless of sulfurs used. This demonstrates no significant influence of

5 Chiang Mai J. Sci. 206; 43(3) 573 sulfur types on magnitude of molecular slippage. 3.4 Mechanical Properties Figure 3 shows hardness results of rubber vulcanizates prepared with different sulfurs. Compared with RS, the asreceived PS offers the vulcanizates with similar hardness range which is in good agreement with the results of crosslink density as discussed previously in Table 3. It has been reported that the hardness of rubber vulcanizates is governed by crosslink density at any given filler loading [5]. Table 3. Cure characteristics, swelling ratio and crosslink density of rubber compounds incorporated with different types of sulfurs. Cure characteristics Scorch time (min) Cure time(min) Sulfur RS PS GPS Swelling ratio Crosslink density (x0 4 mol.cm 3 ) Figure 2. Storage modulus as a function of strain in rubber compounds incorporated with different types of sulfurs. Figure 4. Tensile strength of vulcanizates cured with different types of sulfurs: rhombic sulfur (RS), asreceived petroleumbased sulfur (PS), and ground PS (GPS). Figure 3. Hardness of vulcanizates cured with different types of sulfurs. Figure 5. Elongation at break of vulcanizates cured with different types of sulfurs: rhombic sulfur (RS), asreceived petroleumbased sulfur (PS), and ground PS (GPS).

6 574 Chiang Mai J. Sci. 206; 43(3) Figure 6. Photographs of vulcanizates cured with different types of sulfurs: rhombic sulfur (RS), asreceived petroleumbased sulfur (PS), and ground PS (GPS). It is evident from Table 4 and Figure 7 Unexpectedly, in the cases of tensile strength and elongation at break, Figures 4 and 5 reveal that the rubber vulcanizates cured with asreceived PS possesses much poorer tensile strength and elongation at break than those with RS. Referred to results of cure characteristics, the crosslink density is independent of sulfur type used that means the crosslink density effect on tensile strength and elongation at break is negligible. It is then believed that the particle size of sulfur might be responsible for the discrepancies in results of strength and elongation at break. As evidenced in photographs of cured sheets in Figure 6, the large dark spots could be observed only in the cured specimen prepared with asreceived PS. The results suggest the presence of large undispersed particles of asreceived PS in rubber matrix, probably acting as flaws or stress concentrator in cured specimens, which then lead to the remarkable decreases in tensile strength and elongation at break. Furthermore, the poor dispersion of PS might cause the uneven distribution of crosslink density in cured specimens. To further investigate the root cause of such discrepancies in strength and elongation at break, the asreceived PS was ballmilled in order to reduce its particle size, and thereafter mixed with rubber according to the compounding recipe illustrated earlier in Table. Table 4. Particle size of sulfurs; rhombic sulfur (RS), asreceived sulfur (PS), and ground PS (GPS). Types of sulfur RS PS GPS Average Particle size (μm) that the average particles size of PS ground by ball milling process (denoted as GPS) significantly decreases, i.e., approximately 0 folds smaller than the asreceived PS. Also, the distribution of particle size appears to be narrower than the asreceived PS as shown in Figure 7. By substituting the asreceived PS with ground PS (GPS) in rubber compounds, the viscoelastic behavior results of rubber compounds as exhibited in Figure 2 remains unchanged demonstrating that particle size of sulfur plays no strong role on uncured compounds which might be due to the small quantity used in the compounding recipe (see Table ). Regarding the cure characteristics as illustrated in Table 3, the compounds incorporated with GPS exhibit comparable characteristics including crosslink density. This supports the previous proposed statement that the particle size of sulfur gives no significant effect on cure characteristics which is in line with the hardness results (see Figure 3). In addition, it is noticeable from Figures 4 and 5 that tensile strength and elongation at break of cured specimens with GPS are substantially enhanced by the use of GPS as a replacement of asreceived PS. Such improvement could be explained by the enhancement in degree of sulfur dispersion

7 Chiang Mai J. Sci. 206; 43(3) 575 as evidenced by Figure 6 where there is no dark spot of undispersed sulfur particles observed in the rubber matrix. Figure 7. Particle and particle size distribution of rhombic sulfur (RS), asreceived petroleumbased sulfur (PS) and ground petroleumbased sulfur (GPS). 4. CONCLUSIONS Petroleumbased sulfur (PS) as a byproduct from petroleum refinery was used as vulcanizing agent, and compared with conventional rhombic sulfur (RS). The chemical composition and chemical structure of PS are similar to RS. Both sulfurs offer rubber compounds with comparable cure characteristics and viscoelastic behavior. By contrast, tensile properties of cured specimens prepared with asreceived PS are much inferior to those with RS. Such inferiority could remarkably be alleviated by reducing particle size of asreceived PS using ballmilling technique. The results suggest the applicability of ballmilled PS as vulcanizing agent to the rubber industry. ACKNOWLEDGEMENTS The authors would like to express our profound gratitude toward IRPC Plc. Co., Ltd. for supporting petroleumbased sulfur. Financial support from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/06/2553) is also acknowledged. REFERENCES [] Hofmann W., Rubber Technology Handbook, Hanser Publishers, New York, 980. [2] Nagdi K., Rubber as an Engineering Material: Guideline for Users, Hanser Publishers, New York, 993. [3] Blow C.M. and Hepburn C., Rubber Technology and Manufacture, 2 nd Edn., Butterworth Scientific, London, 982. [4] Coran A.Y., Vulcanization; in Mark J.E., Erman B. and Eirich F.R., eds., The Science and Technology of Rubber, 3 rd Edn., Elsevier Academic Press, New York, 2005; [5] Hofmann W., Vulcanization and Vulcanizing Agents, Palmerton Publishing, New York 967. [6] Susamma A.P., Elizabeth Mini V.T. and Kuriakose A.P., J. Appl. Polym. Sci., 20; 79: 8. DOI 0.002/app [7] Debnath S.C., Mandal S.K. and Basu D.K., J. Appl. Polym. Sci., 995; 57: DOI 0.002/app [8] Aprem A.S., Joseph K., Mathew T., Altstaedt V. and Thomas S., Eur. Polym. J., 2003; 39: DOI 0.06/ S (02) [9] Vergnaud J.M. and Rosca L.D., Rubber Curing and Properties, CRC Press, New York, [0] Coran A.Y., J. Appl. Polym. Sci., 2003; 87: DOI 0.002/app.659. [] Nehb W., Vydra K.,Wiley Online Library, accessed: May, 202. [2] Simanzhenkov V. and Idem R., Crude Oil Chemistry, Marcel Dekker, Inc., New York, [3] Speight J.G., Handbook of Petroleum Analysis, John Wiley and Sons, Inc., New York, 200.

8 576 Chiang Mai J. Sci. 206; 43(3) [4] Speight J.G., The Chemistry and Technology of Petroleum, 4 th Edn., CRC Press, New York, [5] Speight J.G., Handbook of Petroleum Product Analysis, John Wiley and Sons, Inc., New York, [6] Mcketta J.J. and Kobe K.A., Advances in Petroleum Chemistry and Refinery Vol. 6, Interscience Publishers, Inc., New York, 962. [7] Mcintyre G., Technical Papers Claus Sulphur Recovery Options, Bryan Research and Engineering Inc., [8] Clark P.D., Fundamental and Practical Aspects of the Claus Sulfur Recovery Process. sitecore/shell/applications/~/media/ PDF%20files/Topsoe_ Catalysis_Forum/2007/Clark.ashx, accessed: July 202. [9] Pojanavaraphan T., Eur. Polym. J., 2008; 44: DOI 0.06/ j.eurpolymj [20] Stephen R., Jose S., Joseph K., Thomas S. and Oommen Z., Polym. Degrad. Stabil., 2006; 9: DOI 0.06/j. polymdegradstab [2] Zhan Y., Wu J., Xia H., Yan N., Fei G. and Yuan G., Macromol. Mater. Eng., 20; 296: DOI 0.002/mame [22] Li K., Wang B., Su D., Park J., Ahn H. and Wang G., J. Power Sources, 202; 202: DOI 0.06/j.jpowsour [23] Payne A.R., J. Appl. Polym. Sci., 962; 6: DOI 0.002/app [24] Dick J.S., Rubber Technology: Compounding and Testing for Performance, Hanser Publishers, New York, 200. [25] Luo M., Liao X., Liao S. and Zhao Y., J. Appl. Polym. Sci., 203; 29: DOI 0.002/app [26] Randall A.M. and Robertson C.G., J. Appl. Polym. Sci., 204; 3: DOI 0.002/app.4088.