Effect of Polystyrene-Modified Natural Rubber As High Molecular Weight Modifier in Polypropylene Based Binary Blends

International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 04 30

Effect of Polystyrene-Modified Natural Rubber as High Molecular Weight Modifier in Polypropylene Based Binary Blends

Ong S.K. and Azanam S. Hashim

 Abstract— Laboratory synthesized polystyrene-modified can improve PP/NR blend performance since it resulted in natural rubber (SNR) was used in polypropylene (PP) based as finely dispersed micron-sized vulcanized rubber in its modifier. PP volume was reduced while natural rubber (NR) and thermoplastic matrix [21-22]. For the applications in TPE, SNR volume were increased. The range of ratios evaluated are sulfur vulcanization system is known to give the best 70/30; 55/45, 50/50 and 45/55 by volume. PP/SNR blends resulted in better tensile properties, flexural properties, impact strength, elastomeric properties and more suitable for thermoplastic elastic energy and reprocessing retention properties but similar natural rubber (TPNR) [12]. The sulfur vulcanization system tear strength in comparison with PP/NR blends at the similar known as dynamic vulcanization was introduced by Gessler blend ratios. Those mechanical properties are in agreement with and Haslett [23] in 1962 and later developed by Fisher [24], morphological analysis. In comparing PP based blends in this Coran and Patel [25], Coran et al. [26, Abdou-Sabet and Fath study with relevant thermoplastic elastomer (TPE), results [5] and Ousa et al. [27]. In the mentioned studies curatives showed that PP/NR: 70/30 and PP/SNR: 70/30 blends have better tensile properties compare to polyolefins/ NR. While in were added in-situ during melt mixing. This method is thought comparison with Santoprene®, those blends are at the higher end to be less effective as curatives will dispersed in both of Santoprene® tensile strength and modulus 100 and lower end thermoplastic and rubber phases. This study will use the of Santoprene® elongation at break. modified dynamic vulcanization method. In this study, polystyrene-modified natural rubber (SNR) Index Term— natural rubber. polypropylene, polystyrene- was investigated as modifier in PP based blends. SNR modified natural rubber, thermoplastic elastomer prepared in the laboratory by in-situ polymerization of styrene in deprotienized natural rubber latex (DPNR) contains I. INTRODUCTION approximately 25% polystyrene (PS) and 75% NR by weight. POLYMER blends consist of two or more polymers. The PP based blends investigated will focus on the application Properties of a blend depend mainly on the matrix phase but of SNR as modifier in PP based blends on the tensile factor like amount, size, shape and interfacial adhesion of the properties, flexural properties, impact strength, tear strength dispersed phase also play a role. Basis thermodynamics, and retention properties. Selected blend ratios was compared Flory-Hüggins theory and equation-of-state theories are used with relevant TPE. to explain polymer blends behavior [1]–[4]

Many studies on PP/rubber blends have been carried out. II. MATERIALS AND METHODS Some of the rubbers studied are Ethylene Propylene Diene Monomer (EPDM) rubber [9], nitrile rubber as oil resistant A. Materials TPE [6], [7], [9] and butyl rubber in low air and moisture The PP used is grade Titanpro 6431 purchased from Titan permeability TPE [10]. PP Polymers (Malaysia), with density and melt flow index 0.9 Studies on polypropylene/natural rubber (PP/NR) blends gcm-3 and 12 g/10min respectively. The NR used is SMRL have also been carried out and mainly focusing on its grade. application as thermoplastic elastomer (TPE) and toughened The following materials are used in SNR synthesized: blend for low temperature applications [11]-[20]. For the styrene as monomer (Aldrich Chemical Company), interest of impact properties, maximum rubber content is in ammonium persulphate (N2H8O8S2) as initiator (Fluka the range of 20–25% [18]. Chemical Company), and DPNR latex supplied by Sumitomo Rubber Industries (Malaysia) Studies have showed that cross-linking of the rubber phase B. Preparation and Characterization of SNR

Ong S. K. is with Universiti Kuala Lumpur, Malaysian Institute of SNR was synthesized by in-situ polymerization of styrene Chemical and Bioengineering Technology, Section of Polymer Engineering in DPNR latex and characterized according to procedures Technology, Vendor City 78000 Taboh Naning, Alor Gajah, Melaka, previously reported [13], [29]. Styrene monomer was purified Malaysia (e-mail: [email protected]). Azanam S. Hashim is with Universiti Kuala Lumpur, Malaysian Institute using inhibitor remover-packed column before it was of Chemical and Bioengineering Technology, Section of Polymer Engineering polymerized. In-situ polymerization of styrene in DPNR latex Technology, Vendor City 78000 Taboh Naning, Alor Gajah, Melaka, was carried out at 60oC for 5 hours at styrene: dry rubber Malaysia (e-mail: [email protected]).

113204-9595 IJET-IJENS @ August 2011 IJENS I J E N S International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 04 31 ration of 25:75 by weight. Upon completion of the reaction presented in Fig. 1 showed that both blend have similar shape time, the rubber was dried at room temperature to obtain gum and PP/SNR: 70/30 blend was shifted up compare to PP/NR: SNR. 70/30 blend. As mentioned, SNR contains about 25% PS Methanol was used to determine the degree of conversion of which are grafted onto the NR backbone. The grafted PS the resultant SNR (styrene to PS). Tetrahydrofuran, petroleum contributed to the stiffness and strength of PP/SNR blend. ether and ethyl acetate were used in sol-gel analysis for the Looking at the total volume of thermoplastic/rubber ratio determination of degree of grafted PS on NR. Gum SNR cure between PP/NR: 70/30 blend and PP/SNR: 70/30 blend, the characteristic and tensile properties were evaluated prior to latter blend is having higher amount of thermoplastic; i.e. PP melt mixing. and PS from SNR. At lower rubber amount, the blend is expected to be stiffer and stronger. This is in agreement with the results observed; i.e. PP/SNR: 70/30 blend is stiffer and C. Melt Mixing of PP Based Blends stronger than PP/NR blend. Melt mixing was carried out using Brabender Plasticoder o The morphology analysis is shown in Fig. 2. It is obvious Model PLE 331 at 180 C and rotor speed 50 rpm [9]. All that from Fig. 2 PP/SNR: 70/30 blend has better homogeneity blends ratios are based on volume and blend using the method. compare to PP/NR: 70/30 blend. The results obtained from First PP was preheated in the mixing chamber for 3 minutes morphology analysis is in good agreement with the tensile followed by another 3 minutes for melt mixing at 50rpm, and properties where both showed that PP/SNR: 70/30 blend that then either NR of SNR was charged-in at 3rd minute. Mixing th exhibit better homogeneity also contributed to better tensile stopped at 8 minute. Both NR and SNR were masticated and properties as compare to PP/NR: 70/30 blend. compounded separately in two-roll mill using an efficient vulcanization (EV) system as shown in Table 1. Mastication and compounding for both NR and SNR were carried out prior B. Others Mechanical Properties to melt mixing. Table 3 shows the tear strength, elastic energy per unit deflection, flexural properties and impact strength for PP/NR: 70/30 blend and PP/SNR: 70/30 blend. Results showed that D. Measurements the tear strength for PP/NR and PP/SNR blends are The final blends were molded using compression molding 2 o comparable. From study by Nguyen Van Tho [29], it showed machine KAO Tech with 100 kg/cm force at 170 C. Tensile that SNR has lower tear strength as compare to NR as the test and tear test were carried out at room temperature allylic sites was grafted with PS. However due to higher according to ASTM D638 and ISO 43 type 3 respectively degree of compatibility of PP/SNR blend compare to PP/NR using Tensometer M 500 type with cross head speed of 50 blend; i.e. showed by the morphology analysis; the tear mm/min. A nip of 0.5 mm for tear test specimens was made at strength of both binary blends are comparable. the centre of the crescent. Three-point bending flexural test In the classical beam theory on the unnotched specimen, was carried out according to ASTM D790 using Tensometer elastic energy at critical stage, Uc is given by equation (1) [30] M 500 type with cross head speed of 5 mm/min at room temperature. Impact test was carried out using Zwick Impact 2 o   Tester according to ASTM D256 at -27 C. Blends morphology  c U   BSW (1) were analyzed using SEM model Leice Cambridge S–360. c   18E f Reprocessing retention properties were carried out by   reprocess the blends using the similar melt mixing method as mentioned above. Tensile properties for the reprocess blends where Uc is the elastic energy at critical stage (J); c is the were evaluated and percentage of retention properties was critical stress in a three point bending system (MPa); Ef is the reported. flexural modulus (GPa); B is the sample depth (mm); S is the beam span (mm) and W is the sample width. The elastic III. RESULTS AND DISCUSSION energy can be reported as elastic energy per unit deflection, Uy by normalized the sample deflected dimension. Uy represent A. Tensile Properties the inherent property of a material and theoretically correlate Tensile properties of PP/NR blends and PP/SNR blends are well with its mechanical properties such as flexural properties summarized in Table 2. Tensile properties decrease with the and impact strength. This means that at higher Uy, higher increase of rubber loading from 30% to 55% except for PP/NR flexural strength, flexural modulus and impact strength is blends where no significant change for modulus at 100% expected. The elastic energy per unit deflection, flexural elongation (M100) was observed. This is due to the increment strength, flexural modulus and impact strength for PP/NR: of rubber loading. Rubber is known to be soft and elastic. 70/30 blend and PP/SNR: 70/30 blend are summarized in Therefore blends with high rubber loading are softer and have Table 3 also. The results showed that PP/SNR: 70/30 blend higher elasticity compare to those which had lower rubber properties are better that those of PP/NR: 70/30 blend. This loading. For both PP/NR blends and PP/SNR blends, blend observation is consistent with the tensile properties and ratio at 70/30 gave the best tensile properties. The stress-strain morphology analysis reported and also in the same trend as curve of PP/NR: 70/30 blend and PP/SNR: 70/30 blend as Uy.

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[5] S. Abdou-Sabet, M.A. Fath, US Patent 4,311,628, 1982.. [6] A. Y. Coran and R. P. Patel, “Rubber-Thermoplastic Compositions: Part C. Retention of Mechanical Properties via Reprocessing III: Nitroile Rubber Polyolefin Blens with Technological Compatibilization” Rubb. Chem. Technol. vol. 53, pp.141-150. Retention properties for PP/NR: 70/30 and PP/SNR: 70/30 [7] A. Y. Coran, B. Das and R. P. Patel, US Patent 4, 130, 535, 1978 blends were summarized in Table 4. Though for both blends [8] A. van der Wal, A. J. J. Verheul, R. J. Gaymans, “Polypropylene–rubber reprocessing resulted in stiffer and stronger blends but blends: 4. The effect of the rubber particle size on the fracture behaviour at low and high test speed,” Polymer, vol. 40, no.22, pp. 6057-6065. PP/SNR: 70/30 blend is more stable towards reprocessing as [9] A. Y. Coran and R. P. Patel, “Polypropylene Structure, Blends and compare to PP/NR: 70/30 blend. The results is consistent with Composites, Vol2: Copolymers and Blends,” (J. Karger-Kocsis, ed.) Nguyen Van Tho [29] where retention of SNR is much better Great Britian: Chapman & Hall, 1995, pp. 162 [10] R. C. Puydak and D. R. Hazelton, “New Butyl-Based DVAs Act Like than NR as NR has more allylic sites compare to SNR. Due to Thermoset Rubbers,” Plast. Eng. Vol. 44,pp. 27, 1988 the grafting process in SNR, its allylic sites are lesser than NR. [11] S. Al-Malaika and E. J. Amir, “Thermoplastic elastomers I. Effect of Presence of more allylic sites resulted in poorer thermal processing variables on tensile properties of natural rubber/polypropylene blends,” J. Nat. Rub. Research, vol. 1, no.2, resistance and this subsequently lead to poorer retention pp.104–121, June 1986. properties of PP/NR: 70/30 blend as compare to PP/SNR: [12] D. J. Elliott, “natural rubber - polypropylene blends,” in Thermoplastic 70/30 blend. Elastomer from Rubber-Plastic Blends, S. K. De and A. K. Bhowmick ed, Great Britain: Ellis Horwood, 1990, pp. 102. [13] Azanam S. Hashim and S. K. Ong, “Study on polypropylene/natural D. Comparison with Commercial TPE rubber blend with polystyrene-modified natural rubber as compatibilizer,” Polym. Int., vol. 51, no. 7, pp.611–616, July 2002. Table 5 showed the tensile properties of PP/NR: 70/30 [14] J.W. Teh, L.C. Tan, C.T. Chia, K.K. Tan, T.T. Teng, Structure, blend and PP/SNR: 70/30 blend in comparison with processing and mechanical behavior of polypropylene–natural rubber ® blends, in: Proceeding of Composites Asia Pacific 89 Conference, polyolefins//NR and Santoprene . It is obvious that those Adelaida, June 1989. blends prepared in this study have higher TS and EB than [15] M. Sain, I. Simek, J. Beniska, P. Rosner, “Optimization of blend those of TPO and within the range of Santoprene® while composition in modified natural rubber and polypropylene blends,” J. ® Polym. Mater., vol.7 pp. 49-52. 1990 M100 of those blends are higher than those of Santoprene . [16] S. Cook, A.J. Tinker, J. Patel. WO Patent Application 03/054078 A1, Better TS and EB of both PP/NR and PP/SNR blends as 2003. compare to TPO. The authors believe this observation might [17] K.G. Karmika, S.L.G. Rangith, S.S. Warnapura, W.P.M. Abeysekera, Some aspects of rheological characteristics of natural rubber and be due to modified dynamic vulcanization used in this study. polypropylene blends, in: Proceedings of Conference of Natural Rubber: In typical dynamic vulcanization method curative and rubbers Current Developments in Product Manufacture and Applications, Kuala was added into the thermoplastic matrix during melt mixing. Lumpur, June 1993. In this study curatives were compounded with the rubber prior [18] A.K. Tinker, “Preparation of polypropylene/natural rubber blends having high impact strength at low temperatures,” Polym Commun., vol. to melt mixing for better homogeneity of the curatives in 25, no. 11, pp. 325-326, 1984. rubber and most probably better cross-linking efficiency. [19] S. Varghese, R. Alex, B. Kuriakose, “Natural rubber–isotactic polypropylene thermoplastic blends,” J. Appl. Polym. Sci. vol.92, no. 4, pp2063–2068, 2004. IV. CONCLUSIONS [20] A. Thitithammawonga, C. Nakasona, K. Sahakaroa, and J. Noordermeer, “Effect of different types of peroxides on rheological, mechanical,and As a conclusion, 30% of rubber loading in PP based binary morphological properties of thermoplastic vulcanizates based on natural blends resulted to the best loading according to the tensile rubber/polypropylene blends,” Polym. Testing., vol.26, no.4, pp. 537- properties. The elastic energy per unit deflection, flexural 546, 2007. [21] G. Holden, Thermoplastic Elastomers, in: K.C. Baranwal, H.L. Stephens properties and retention properties for PP/SNR blends are (Eds.), Basic Elastomer Technology, Rubber Division of the ACS, New better than PP/NR blends at 70% PP loading. The tear York, 2001. strength on the other hand is comparable for blends. Both [22] N.R. Legge, G. Holden, H.E. Schroeder, Thermoplastic Elastomer, PP/NR: 70/30 blend and PP/SNR blend have higher TS and Hanser Publishers, Munich, 1987. ® [23] A.M. Gessler, W.H. Haslett, US Patent 3,037,954, 1962. EB than those of a TPO and within the range of Santoprene [24] W.K. Fisher, US Patent 3,758,643, 1973. but those blends M100 is higher than Santoprene®. Since [25] A.Y. Coran, R.P. Patel, “Rubber–thermoplastic compositions. Part I. using modified dynamic vulcanization method in this study EPDM–polypropylene thermoplastic vulcanizates,” Rubber Chem. Technol. 53 (1) (1980) 141. showed promising results as compare to the conventional [26] A.Y. Coran, R. Patel, D. Williams-Headd, Rubber–thermoplastic method, it is suggested optimization curing recipes used. compositions. Part V. selecting polymers for thermoplastic vulcanizates, Looking at results obtained, SNR applications will be Rubber Chem. Technol. 55 (1) (1982) 116. [27] A. Ousa, U.S. Ishiaku, Z.A. Ohd-Ishak, Oil-resistance studies of extended to be use as compatibilizer in PP/NR blends or other dynamically vulcanized poly(vinyl chloride)/epoxidized natural rubber PP based blends that are incompatible. thermoplastic elastomer, J. Appl. Polym. Sci. 69 (7) (1998) 1357. [28] A. Ibrahim and M. Dahlan, “Thermoplastic natural rubber blends” Prog. REFERENCES Polym. Sci., vol. 23, no. 4, 665–706, Aug.1998. [1] H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges, [29] N. V. Tho, M. O. A. Kadir and A. S. Hashim, “A comparative study of “Encyclopedia of Polymer Science and Engineering,” 2nd ed., New styrene polymerization in deproteinized and undeproteinized natural York: John Wiley & Sons, 1998, pp. 399 – 402, 436-441. rubber latex, Rubber Chem. Technol., vol.75, no.1, pp.111–118, March [2] O. Olabisis, L. M. Robeson and M. T. Shaw, “Polymer-Polymer 2002. Miscibility,” New York: Academic Press, 1979, pp. 3, 196. [30] C. B. Bucknall, “Toughened Plastics,” London: Applied Science [3] D. R. Paul and J. W. Barlow, “Polymer blends,” J. Macromol. Sci.-Rev. Publishers, 1977, pp. 284. Macromol. Chem., vol. 18, no.1, pp. 109–168, 1980. [31] D. S. Campbell, D. J. Elliott, and M. A. Wheelans, “Thermoplastic [4] J. R. Fried, “Polymer Science and Technology,” New Jersey: Prentice natural rubber blends,” NR Tech., vol. 9, no.2, pp.21–31, 1978. Hall, 1995, pp. 265.

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[32] G. O. Shonaike “Miscibility of nylon 66/Santoprene blends” in Polymer Blends and Alloys, G. O. Shonaike and G. P. Simon ed, New York: Marcel Dekker, 1999, pp.240.

Table I Efficient vulcanization system recipe Material parts per hundred rubber (phr)

Rubber 100

Zinc oxide 5.0

Stearic acid 2.0

IPPD 2.0

MBTS 2.5

TMTD 1.5

Sulfur 0.3

Table II Tensile properties of PP/NR blends and PP/SNR blends Blend ratios, PP/rubber TS, MPa EB, % Young’s modulus (E), MPa M100, MPa

PP/NR blends

70/30 21.7 362.4 307 12.7

55/45 20.9 226.3 213 14.9

50/50 18.2 167.0 183 14.1

45/55 15.6 129.5 157 13.1

PP/SNR blends

70/30 25.3 341.9 339 17.0

55/45 23.1 172.8 295 15.3

50/50 18.8 158.3 245 14.9

45/55 16.9 149.7 211 13.6

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Table III Mechanical properties of PP/NR: 70/30 blend and PP/SNR: 70/30 blend

Blends Tear strength Elastic energy per Flexural Flexural Impact

(N/mm) unit deflection, Uy strength modulus strength (J/m2) (MPa) (MPa) (J/m)

PP/NR: 70/30 blend 171.24 0.070 27.8 743.5 140.8

PP/SNR: 70/30 blend 170.02 0.073 29.4 863.6 150.3

Table IV Retention properties of PP/NR: 70/30 blend and PP/SNR: 70/30 blend Blends TS, % EB, % E, % M100, %

PP/NR: 70/30 blend +22.95 -24.02 +12.74 +50.3

PP/SNR: 70/30 blend +4.24 -9.61 +5.28 +30.4

Table V Tensile properties of PP/NR: 70/30 blend, PP/SNR: 70/30 blend, polyolefins/NR and Santoprene®

Santoprene® PP/NR: 70/30 PP/SNR: 70/30 Polyolefin/NR (TPO) [11] [12]

TS, MPa 21.7 25.3 6 – 20 4.4 - 27.6

EB, % 362.4 341.9 200 330 - 600

M100, MPa 12.7 17.0 - 2.1 - 10

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Stress (MPa) Stress 5

0 100 200 300 400 Strain (%)

PP/NR: 70/30 PP/SNR: 70/30

Fig. 1. Stress-strain curve of PP/NR: 70/30 blend and PP/SNR: 70/30 blend

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PP/NR: 70/30 blend

PP/SNR: 70/30 blend

Fig. 2. Morphological study of PP/NR: 70/30 blend and PP/SNR: 70/30 blend

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Authors’ information

Dr. Ong Siew Kooi (Corresponding Author) Section of Polymer Engineering Technology Universiti Kuala Lumpur, Malaysian Institute of Chemical and Bioengineering Technology, Vendor City, 78000 Taboh Naning, Alor Gajah, Melaka, Malaysia Phone: +606-551 2108, Fax: +606-551 2001 e-mail : [email protected]

Prof. Dr. Azanam Shah Hashim Section of Polymer Engineering Technology Universiti Kuala Lumpur, Malaysian Institute of Chemical and Bioengineering Technology, Vendor City, 78000 Taboh Naning, Alor Gajah, Melaka, Malaysia Phone: +606-551 2003, Fax: +606-551 2001 e-mail: [email protected]

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