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polypropylene blends
Polymer Testing 25 (2006) 34–41 www.elsevier.com/locate/polytest
Material Properties
Effect of vulcanization system on properties of thermoplastic vulcanizates based on epoxidized natural rubber/polypropylene blends C. Nakason *, P. Wannavilai, A. Kaesaman Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand Received 1 August 2005; accepted 11 September 2005
Abstract Epoxidized natural rubber with mole percentage epoxide of 30 was synthesized and used to prepare thermoplastic vulcanizates based on 75/25 ENR/PP blends with Ph-PP compatibilizer. Influences of various curing systems (i.e., sulfur, peroxide and a mixture of sulfur and peroxide-cured systems) were investigated. We found that the mixing torque, shear stress, shear viscosity, tensile strength and elongation at break of the TPVs using the mixed-cure system exhibited higher values than those of the sulfur and peroxide-cured systems, respectively. This may be attributed to a formation of S–S, C–S combination with C–C linkages in the ENR phase. In the sulfur-cured system only S–S linkages are formed, whilst in the peroxide curing system more stable C–C linkages are formed. However, during shearing at high temperature of the peroxide and mixed-cure systems, the peroxide caused degradation of the polypropylene molecules. Higher level of DCP was used in the peroxide-cured system and caused greater influence on properties. In the mixed-cure system, lower influence of PP degradation and influence of formation of more stable C–C linkages overcomes the drawback. Therefore, we observed the highest values of those properties using the mixed-cure system. The curing systems did not affect the hardness properties and solvent resistance of the TPVs. We also found that the dispersed vulcanized rubber domains of TPV with the peroxide-cured system were smaller rubber particles than those of the mixed and sulfur-cure systems. q 2005 Elsevier Ltd. All rights reserved. Keywords: Epoxidized natural rubber; Thermoplastic vulcanizate; Rheological properties; Mechanical properties; Polypropylene
1. Introduction Thermoplastic elastomers (TPEs) play a more and more important role in the polymer industry due to their good processability and their elastomeric properties [1]. Thermoplastic elastomers based on natural rubber and thermoplastic blends are classified as ‘thermoplastic natural rubber (TPNR))’ blends. There are two types of * Corresponding author. Tel.: C66 73 312 930; fax: C66 73 331 099. E-mail address: [email protected] (C. Nakason).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.09.007
TPNRs. One prepared by blending NR with thermoplastic (i.e., polyolefins) to get co-continuous phase morphology is technologically classified as ‘thermoplastic polyolefin (TPO)’. The other class is known as ‘thermoplastic vulcanizate (TPV)’, which is prepared by blending NR with polyolefins. The rubber phase is vulcanized during the mixing process at high temperature, and the process is known as ‘dynamic vulcanization (DV)’. Polypropylene is considered to be the best choice for blending with NR due to its high softening temperature of about 150 8C and the low glass transition
C. Nakason et al. / Polymer Testing 25 (2006) 34–41
temperature of the blend [2]. Modified NR is also used to prepare TPNRs by blending with polar thermoplastics. The modified forms of NR include epoxidized natural rubber [3–5], natural rubber-g-poly(methyl methacrylate) [6–8] and maleated natural rubber [9,10]. The main aim of chemical modification of NR molecules is to enhance blend compatibilization and to improve some useful properties of the blend. In this work, TPVs based on 75/25 ENR/PP with Ph-PP compatibilizer and various types of curing system were studied. The influence of vulcanization system on rheological, mechanical, morphological properties and swelling behavior and morphological studies of the TPVs was investigated. 2. Experimental 2.1. Materials High ammonia (HA) concentrated natural rubber latex was used as a raw material for the preparation of ENR, manufactured by Yala Latex Co., Ltd, Yala, Thailand. The non-ionic surfactant used to stabilize the latex during epoxidation was Teric N30 (alkylphenol ethoxylate) which was manufactured by Huntsman Corp. Australia Pty Ltd, (Ascot Vale Vic, Australia). The formic acid used as a reactant for the preparation of ENRs was manufactured by Fluka Chemie (Buchs, Switzerland). The hydrogen peroxide used as a coreactant for the preparation of the ENRs was manufactured by Riedel De Hae¨n (Seelze, Germany). Polypropylene used in the present study as a blend component was of injection molding grade with an MFI value of 12 g/10 min at 230 8C, manufactured by Thai Polypropylene Co., Ltd, Rayong, Thailand. The dimethylol phenolic resin (SP-1045) used to prepare phenolic modified polypropylene (Ph-PP) was manufactured by Schenectady International Inc, Freeport, USA. dicumyl peroxide used as a vulcanizing agent was manufactured by Akzo Nobel Polymer Chemicals BV, Amersfoort, The Netherlands. Triallyl cyanurate (TAC) used as co-agent for peroxide cured system was manufactured by Fluka Chemie (Buchs, Switzerland). The zinc oxide used as an activator was manufactured by Global Chemical Co., Ltd, Samutprakarn, Thailand. The stearic acid used as an activator was manufactured by Imperial Chemical Co., Ltd, Pathumthani, Thailand. The sulfur used as a vulcanizing agent was manufactured by Ajax Chemical Co., Ltd, Samutprakarn, Thailand. The N-tert-butyl-2-benzothiazolesulphenamide (Santocure TBBS) used as an accelerator was manufactured by Flexsys (USA). The polyphenolic
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additive, Wingstayw L, used as an antioxidant was manufactured by Eliokem Inc. (Ohio, USA). 2.2. Preparation of epoxidized natural rubber Epoxidized natural rubber was prepared using highammonia concentrated NR latex with a dry rubber content of approximately 60%. Details of the preparation process of the ENRs are described elsewhere [11]. The ENR latex was coagulated in methanol and washed thoroughly with distilled water and dried in a vacuum oven at 40 8C for 24 h. Infrared spectroscopy was used to analyze the molecular structure of the ENR. The reaction time of the epoxidation was set according to the required level of epoxide groups in the ENR products, as described in our previous work [11]. The level of the epoxide contents in the ENR products was later confirmed by infrared spectroscopy via a calibration curve. In this work, ENR with a level of epoxide groups at approximately 30 mol % epoxide, corresponding to ENR-30, was used. 2.3. Preparation of phenolic modified polypropylene A blend compatabilizer, phenolic modified polypropylene (Ph-PP), was prepared by a melt mixing process using a Brabender Plasticorder PLE 331 (Duisberg, Germany). Details of the preparation process are described elsewhere. [12,13] Polypropylene (100 parts) and dimethylol phenolic resin (SP-1045) (4 parts) were first mixed at 180 8C and a rotor speed of 60 rpm for 3 min. Stannous dichloride (SnCl2) (0.8 parts) was later incorporated and mixing was continued for 2 min. The blend product was later cut into small pellets using a Bosco plastics grinder (Samutprakarn, Thailand). The sample was purified by extracting with acetone and characterized using infrared spectroscopy. The Ph-PP was later used as a blend compatibilizer for ENR/PP blends without purification. 2.4. Preparation of thermoplastic vulcanizates Three types of vulcanization system, namely sulfur, peroxide and a mixture of sulfur and peroxide, were used to cure the ENR phase of TPVs based on 75/25 ENR/PP blends. The dynamic vulcanization was carried out at 180 8C in a mixing chamber of an internal mixer, Brabender Plasticorder. The various chemicals used in each formulation are listed in Table 1. ENR with mole percentage epoxide of 30 was used to study the effect of vulcanization system on properties of the TPVs prepared from 75/25 ENR-30/PP blends. This is
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C. Nakason et al. / Polymer Testing 25 (2006) 34–41
Table 1 Chemicals used for a preparation of TPVs based on 75/25 ENR/PP blends using Ph-PP compatibilizer and various types of vulcanization systems Chemicals ENR-30 PP Ph-PPa ZnO Stearic acid Wingstay L TBBS Sulphur DCP TAC a
Quantities (part by weight of rubber) Sulfur
Mixed system
Peroxide
75.00 25.00 1.25 3.75 1.88 0.75 0.38 1.50 – –
75.00 25.00 1.25 3.75 – 0.75 0.75 0.75 1.50 0.75
75.00 25.00 1.25 3.75 – – – – 3.00 1.50
5% By weight of polypropylene.
because this type of epoxidized natural rubber (i.e. ENR-30) provided high mechanical strength and moderate processability using a plastic injection molding machine [12]. Also, Ph-PP was chosen to use as a blend compatibilizer due to its proven high compatibilizing effect [12]. The mixing torque of the Brabender Plasticorder was captured using an external data acquisition system at a frequency of 1 Hz, and the relationship between mixing torque and time was then plotted for each type of blend. Rheological, mechanical and morphological properties and swelling behavior of the TPVs were also investigated.
2.5. Rheological characterization A Rosand single bore capillary rheometer (model RH7, Rosand Precision Ltd, Gloucestershire, UK) was used to follow shear flow properties of the TPVs. The experiment was carried out using a wide range of shear rates from 5 to 1800 sK1 at 180 8C. Dimension of the capillary die used were 2-mm diameter, 32-mm length and 908 entry angle with aspect ratio (L/D) of 16/1. The TPVs was incorporated into the barrel of the rheometer and preheated for 5 min under a pressure of approximately 3–5 MPa to get a compact mass. The piston was programmed to travel downward automatically at a speed of 5 mm/min for 30 s to purge the excess molten material and eliminate bubbles present in the melt. The test was then carried out at a set of shear rates in a program via a microprocessor. The pressure drop across a capillary channel and melt temperature were captured via a data acquisition system during the test. The apparent shear stress, shear rate and shear viscosity were calculated using
the derivation of the Poiseuille law. Equations used to calculate the shear flow properties were described elsewhere [14]. 2.6. Mechanical testing The dumbbell shaped specimens of the TPVs used for tensile testing were prepared by thermoplastic injection molding machine (Welltec Machinery Ltd, Hong Kong). Tensile testing was performed at 25G 2 8C at a crosshead speed of 500 mm/min according to ISO 37. The instrument used was Hounsfield Tensometer, model H 10 KS (Hounsfield Test Equipment Co., Ltd, UK). Hardness of the samples was also measured using indentation shore A, according to ISO 7619. 2.7. Morphological studies Morphological characterization was performed using a Leo scanning electron microscope, model VP 1450 (Leo Co., Ltd, UK). Injection molded samples of the TPVs were cryogenic cracked in liquid nitrogen to avoid any possibility of phase deformation during the cracking process. The PP phase was preferentially extracted by immersing the fractured surface into hot xylene for 10 min. The samples were later dried in vacuum oven at 40 8C for 3 h to eliminate the contamination of the solvent. The dried surfaces were gold-coated and examined by scanning electron microscope. 2.8. Swelling behavior The TPVs were weighed and inserted into test tubes containing test liquids and placed in an oven at 30 8C.
C. Nakason et al. / Polymer Testing 25 (2006) 34–41 12 Sulfur system Torque (dN-.m)
10
Mixed system Peroxide system
8 6 4 2 0 0
60
120
180 240 Mixing time (sec)
300
360
420
Fig. 1. Mixing torque-time of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
In this work, diesel oil, a mixture of toluene/isooctane (50% v/v) and engine oil were used as the test liquids. The test specimens were immersed in the test liquids for 72 h. The samples were removed from the solvents and blotted with filter paper to remove excess solvent from the surface of the sample before being weighed to an accuracy of 0.1 mg at a fixed temperature of 30 8C. The degree of swelling (weight increase) was calculated as follows: Degree of swelling ð%Þ Z
ðWs KW0 Þ !100 W0
(1)
where W0 and Ws are weights of the specimen before and after immersing in the test liquid, respectively.
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sulfur-cured system S–S linkages are formed, whilst in the peroxide curing system more stable C–C linkages are formed. In the mixed cure system both S–S, C–S and C–C linkages are formed in the ENR phase. Among the three different vulcanizing systems used, the mixed cure system showed the highest final mixing torque and the peroxide showed the lowest final mixing torque, with the sulfur-cured system being intermediate. In the peroxide and mixed-cure systems, during shearing at high temperature (i.e. 180 8C), the peroxide (i.e. DCP) degrades the polypropylene molecules particularly by the b-scission mechanism, as shown in reaction Scheme 1. In these systems, the degradation of PP overshadows the effect of dynamic cross-linking of the ENR phase. However, a higher level of DCP was used in the peroxide-cured system (Table 1) which causes lower viscosity of the TPV. As a result, the TPV with a peroxide-cured system exhibited the lowest value of the final mixing torque. In the mixed-cured system, a half quantity of DCP was used compared with the DCP used in the peroxide-cured system. Therefore, lower influence of PP degradation on rheological properties and, hence, mixing torque of the TPV melts was observed. Also, the influence of the formation of more stable C–C linkages overcomes the drawback of the degradation of PP molecules. Therefore, we observed the highest values of the final mixing torque for the mixed-cure system. 3.2. Rheological properties
3. Results and discussion 3.1. Mixing torques Effect of dynamic vulcanization on the mixing torque-time curves of the TPVs prepared from 75/25 ENR-30/PP blends with Ph-PP compatibilizer and various curing systems is shown in Fig. 1. In the CH3
CH3
C CH2
C
H
H
CH2
Plots of apparent shear stress versus shear rate (i.e. flow curves) and apparent shear viscosity versus shear rate (i.e. viscosity curves) of the thermoplastic vulcanizates are shown in Figs. 2 and 3, respectively. It is seen that the TPVs with peroxide-cured system exhibited the lowest shear stress and shear viscosity, whilst the TPV with the mixed-cure system showed the highest values.
Direct Chain scission
CH3 CH
·
CH3
·CH
+
2
C
CH2
Transfer H Scission
Tertiary C-H
CH2 CH3
CH3
C CH2
C
·
H
CH3
CH3 +
beta-C-C Scission
C CH2
C
CH2
CH3
CH3
CH2
CH2
+
CH CH2
·
Scheme 1. Possible types of chain scissions of PP under high shearing action and high temperature in a presence of DCP [13].
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C. Nakason et al. / Polymer Testing 25 (2006) 34–41 10
8 1.00E+05
1.00E+04
Sulfur system Mixed system Peroxide system
1.00E+03 1.00E+00
Tensile strength (MPa)
Apparent shear stress (Pa)
1.00E+06
6
4
2 1.00E+01 1.00E+02 1.00E+03 Apparent shear rate (1/s)
1.00E+04 0
Fig. 2. Relationship between apparent shear stress and shear rate of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
The TPVs with the sulfur-cured system showed intermediate values. Therefore, trends of shear stress and shear viscosity at a given shear rate correspond to the trends of mixing torque (in Fig. 1). Chain scission of the polypropylene molecules also caused the lowest shear stress and viscosity of the TPVs with the peroxidecured system. In the mixed-cure system, a lower degree of chain scission was experienced due to lower DCP content. However, influence of stable C–C linkages in combination with S–S and C–S linkages caused stronger network formation in the TPV with the mixed-cure system. Therefore, this type of TPV exhibited the highest mixing torque, apparent shear stress and shear viscosity. 3.3. Mechanical properties of TPVs Mechanical properties in terms of tensile strength and elongation at break of the TPVs are shown in Figs. 4 and 5, respectively. It is seen that the mixed 1.00E+05
Peroxide system
cured-system exhibited the highest tensile strength and elongation at break, whilst the peroxide cured-system showed the lowest values. The TPV with sulfur-cured system exhibited intermediate values. This may be attributed to influence of types of chemical bond in the vulcanizates and influence of chain scission of the PP molecules. The mixed curing system produced a rubber network consisting of C–C, C–S and S–S linkages between the rubber molecules, while the sulfur-cured system generated weaker and less stable S–S and C–S linkages. This resulted in higher strength and extensibility for the TPVs based on the mixed curing system. The peroxide-cured system generated more stable C–C linkages. However, the PP degradation occurred simultaneously during dynamic vulcanization. Therefore, overall strength and extensibility of the TPVs were inferior. The peroxide cured system also exhibited poorer permanent set than that of the sulfur and mixed curing systems, as shown in Fig. 6. However, the tension sets for all types of the TPVs were lower than 50% with the elongation at break higher than 200%. Therefore, we concluded that the TPVs prepared in this
Mixed system 1.00E+04
Mixed system
Fig. 4. Tensile strength of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
350
Peroxide system 300 Elongation at break (%)
Apparent shear viscosity (Pa.s)
Sulfur system
Sulfur system
1.00E+03
1.00E+02
1.00E+01 1.00E+00
1.00E+01 1.00E+02 1.00E+03 Apparent shear rate (1/s)
1.00E+04
Fig. 3. Relationship between apparent shear viscosity and shear rate of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
250 200 150 100 50 0
Sulfur system
Mixed system
Peroxide system
Fig. 5. Elongation at break of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
C. Nakason et al. / Polymer Testing 25 (2006) 34–41
39
25
25
Isoctane + Toluene Diesel oil
20
Engine oil 15
Swelling (%)
Tension set (%)
20
10
15
10
5 5 0
Sulfur system
Mixed system
Peroxide system
0 Sulfur system
Fig. 6. Tension set of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
work exhibited reasonable elastomeric properties and are suitable for various industrial applications. Fig. 7 shows hardness of the TPVs. It is seen that the TPVs prepared with various vulcanization systems exhibited similar values of indentation hardness (Shore A), hence, we concluded that the type of vulcanization system did not have a significant effect on hardness properties of the TPVs.
Peroxide system
Fig. 8. Swelling behavior of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
3.5. Morphological studies
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The xylene etched cryogenic fracture surfaces of the TPVs are shown in Fig. 9. The PP phase was preferentially dissolved in xylene at an elevated temperature which left un-dissolved small spheres of vulcanized rubber particles adhered to the surface. Also, the previous location of the PP phase was etched and transformed to give cavitation. The TPV with peroxide-cure system showed the smallest vulcanized rubber particles dispersed in the PP matrix, whilst the sulfur-cured system exhibited the largest particles. Therefore, a TPV with peroxide-cured system should provide the highest mechanical strength. As stated previously, a higher level of degradation of PP molecules occurred in this system and, therefore, the TPV lost mechanical strength despite containing the finest vulcanized rubber particles with high adhesion forces between the ENR and PP phases. Size of dispersed vulcanized rubber particles of the mixedcured system was marginally larger than that of peroxide-cured but smaller than that of the sulfurcured system. This gives a reason for the higher mechanical strength (i.e. tensile strength and elongation at break) and rheological properties (i.e. flow and viscosity curves) of the TPV with mixed-cure system. That is, it exhibited high interfacial force between the phases together with moderate degradation of the PP molecules because of low DCP content.
20
4. Conclusion
3.4. Swelling behavior The degree of swelling of the TPVs is shown in Fig. 8. It is seen that the TPVs with various vulcanization systems exhibited the same trend; that is, a mixture of isooctane and toluene caused the highest degree of swelling while the engine oil showed the lowest values. The diesel oil exhibited intermediate swelling. Also, the TPVs had a similar degree of swelling in each liquid and, hence, the type of vulcanization system did not have a significant effect on degree of swelling of these types of TPV. 100
80 Hardness (Shore A)
Mixed system
60
0
Sulfur system
Mixed system
Peroxide system
Fig. 7. Hardness of TPVs based on 75/25 ENR-30/PP blends using PhPP compatibilizer and various vulcanization systems.
ENR-30 was synthesized via a performic epoxidation method and was used to prepare thermoplastic vulcanizates based on ENR/PP blends at a blend ratio of 75/25 with Ph-PP (5% wt of PP) compatibilizer. The
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C. Nakason et al. / Polymer Testing 25 (2006) 34–41
Fig. 9. SEM micrigraphs of TPVs based on 75/25 ENR/PP blends using Ph-PP compatibilizer and various vulcanization systems.
influence of various curing systems was investigated. It was found that the final mixing torques, shear stress, shear viscosity, tensile strength and elongation at break of the TPVs with the mixed-cure system were higher that those of the sulfur and peroxide-cured systems. In the sulfur-cured system, S–S linkages are formed while in the peroxide cured system more stable C–C linkages are formed. In the mixed cure system, S–S, C–S and C–C linkages are formed in the ENR phase. Furthermore, in the peroxide and mixed-cured systems, the peroxide degrades the polypropylene molecules particularly by a b-scission mechanism during shearing at high temperature. Therefore, the degradation of PP overshadows the effect of dynamic cross-linking of the ENR phase. A higher level of DCP was used in the peroxide-cured system. Therefore, the TPV with a peroxide-cured system exhibited the lowest values of the final mixing torque, tensile strength, elongation at break and shear flow properties. In the mixed-cure system, lower influence of PP degradation and influence of formation of more stable C–C linkages overcomes the drawback of the degradation of PP molecules. Therefore, we observed the highest values of those properties using this curing system. We also found that the curing systems did not affect hardness properties and solvent resistance of the TPVs. In morphological studies, we found that the size of dispersed vulcanized rubber domains of TPV with the
peroxide-cured system was smaller than those of the mixed and sulfur-cured systems. Hence, the TPV with the peroxide-cured system should provide the highest mechanical strength, but the higher level of degradation of PP molecules caused lowering of flow properties and mechanical strength. Dispersed vulcanized rubber particles of the mixed-cured system were marginally larger than those of the peroxide-cured system but smaller than those of the sulfur-cured system. Therefore, it provided the highest mechanical strength (i.e. tensile strength and elongation at break) and rheological properties (i.e. flow and viscosity curves). Acknowledgements The authors gratefully acknowledge financial support from the National Metal and Materials Technology Center (MTEC), NSTDA contract no MT-B-46-POL-18-199-G. Also special scholarship granted by Prince of Songkla University, Thailand to one of us (Mr Puripong Wannavilai) is gratefully acknowledged. References [1] N.R. Legge, G. Holden, H.E. Schroeder (Eds.), Thermoplastic Elastomer: A Comprehensive Review, Hanser Publishers, New York, 1987.
C. Nakason et al. / Polymer Testing 25 (2006) 34–41 [2] A. Ibrahim, M. Dahlan, Prog. Polym. Sci. 23 (1998) 665. [3] S. Mohanty, G.B. Nando, K. Vijayan, N.R. Neelakanthan, Polymer 37 (1996) 387. [4] C. Nakason, Y. Panklieng, A. Kaesaman, J. Appl. Polym. Sci. 92 (2004) 3561. [5] P. Ramesh, S.K. De, J. Appl. Polym. Sci. 50 (1993) 1369. [6] Z. Oommen, S. Thomas, C.K. Premalatha, B. Kuriakose, Polymer 38 (1997) 5611. [7] L. Thiraphattaraphun, S. Kiatkamjornwong, P. Prasassarakich, S. Damronglerd, J. Appl. Polym. Sci. 81 (2001) 428. [8] C. Nakason, W. Pechurai, K. Sahakaro, A. Kaesaman, Polym. Adv. Technol. 16 (2005) 592.
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[9] E. Carone Jr., U. Kopcak, M.C. Gonc¸alves, S.P. Nunes, Polymer 41 (2000) 5929. [10] C. Saivaree, Thermoplastic elastomer based on MNR and PP, Master Thesis, Prince of Songkla University, Pattani, Thailand, 2005. [11] C. Nakason, A. Kaesaman, P. Klinpituksa, Songkla. J. Sci. Technol. 23 (2001) 415. [12] C. Nakason, P. Wannavilai, A. Kaesaman, Submitted for publication. [13] S. George, K.T. Varughese, S. Thomas, Polymer 41 (2000) 5485. [14] C. Nakason, A. Kaesaman, Z. Samoh, S. Homsin, S. Kiatkamjonwong, Polym. Test. 21 (2002) 449.
Material Properties
Effect of vulcanization system on properties of thermoplastic vulcanizates based on epoxidized natural rubber/polypropylene blends C. Nakason *, P. Wannavilai, A. Kaesaman Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand Received 1 August 2005; accepted 11 September 2005
Abstract Epoxidized natural rubber with mole percentage epoxide of 30 was synthesized and used to prepare thermoplastic vulcanizates based on 75/25 ENR/PP blends with Ph-PP compatibilizer. Influences of various curing systems (i.e., sulfur, peroxide and a mixture of sulfur and peroxide-cured systems) were investigated. We found that the mixing torque, shear stress, shear viscosity, tensile strength and elongation at break of the TPVs using the mixed-cure system exhibited higher values than those of the sulfur and peroxide-cured systems, respectively. This may be attributed to a formation of S–S, C–S combination with C–C linkages in the ENR phase. In the sulfur-cured system only S–S linkages are formed, whilst in the peroxide curing system more stable C–C linkages are formed. However, during shearing at high temperature of the peroxide and mixed-cure systems, the peroxide caused degradation of the polypropylene molecules. Higher level of DCP was used in the peroxide-cured system and caused greater influence on properties. In the mixed-cure system, lower influence of PP degradation and influence of formation of more stable C–C linkages overcomes the drawback. Therefore, we observed the highest values of those properties using the mixed-cure system. The curing systems did not affect the hardness properties and solvent resistance of the TPVs. We also found that the dispersed vulcanized rubber domains of TPV with the peroxide-cured system were smaller rubber particles than those of the mixed and sulfur-cure systems. q 2005 Elsevier Ltd. All rights reserved. Keywords: Epoxidized natural rubber; Thermoplastic vulcanizate; Rheological properties; Mechanical properties; Polypropylene
1. Introduction Thermoplastic elastomers (TPEs) play a more and more important role in the polymer industry due to their good processability and their elastomeric properties [1]. Thermoplastic elastomers based on natural rubber and thermoplastic blends are classified as ‘thermoplastic natural rubber (TPNR))’ blends. There are two types of * Corresponding author. Tel.: C66 73 312 930; fax: C66 73 331 099. E-mail address: [email protected] (C. Nakason).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.09.007
TPNRs. One prepared by blending NR with thermoplastic (i.e., polyolefins) to get co-continuous phase morphology is technologically classified as ‘thermoplastic polyolefin (TPO)’. The other class is known as ‘thermoplastic vulcanizate (TPV)’, which is prepared by blending NR with polyolefins. The rubber phase is vulcanized during the mixing process at high temperature, and the process is known as ‘dynamic vulcanization (DV)’. Polypropylene is considered to be the best choice for blending with NR due to its high softening temperature of about 150 8C and the low glass transition
C. Nakason et al. / Polymer Testing 25 (2006) 34–41
temperature of the blend [2]. Modified NR is also used to prepare TPNRs by blending with polar thermoplastics. The modified forms of NR include epoxidized natural rubber [3–5], natural rubber-g-poly(methyl methacrylate) [6–8] and maleated natural rubber [9,10]. The main aim of chemical modification of NR molecules is to enhance blend compatibilization and to improve some useful properties of the blend. In this work, TPVs based on 75/25 ENR/PP with Ph-PP compatibilizer and various types of curing system were studied. The influence of vulcanization system on rheological, mechanical, morphological properties and swelling behavior and morphological studies of the TPVs was investigated. 2. Experimental 2.1. Materials High ammonia (HA) concentrated natural rubber latex was used as a raw material for the preparation of ENR, manufactured by Yala Latex Co., Ltd, Yala, Thailand. The non-ionic surfactant used to stabilize the latex during epoxidation was Teric N30 (alkylphenol ethoxylate) which was manufactured by Huntsman Corp. Australia Pty Ltd, (Ascot Vale Vic, Australia). The formic acid used as a reactant for the preparation of ENRs was manufactured by Fluka Chemie (Buchs, Switzerland). The hydrogen peroxide used as a coreactant for the preparation of the ENRs was manufactured by Riedel De Hae¨n (Seelze, Germany). Polypropylene used in the present study as a blend component was of injection molding grade with an MFI value of 12 g/10 min at 230 8C, manufactured by Thai Polypropylene Co., Ltd, Rayong, Thailand. The dimethylol phenolic resin (SP-1045) used to prepare phenolic modified polypropylene (Ph-PP) was manufactured by Schenectady International Inc, Freeport, USA. dicumyl peroxide used as a vulcanizing agent was manufactured by Akzo Nobel Polymer Chemicals BV, Amersfoort, The Netherlands. Triallyl cyanurate (TAC) used as co-agent for peroxide cured system was manufactured by Fluka Chemie (Buchs, Switzerland). The zinc oxide used as an activator was manufactured by Global Chemical Co., Ltd, Samutprakarn, Thailand. The stearic acid used as an activator was manufactured by Imperial Chemical Co., Ltd, Pathumthani, Thailand. The sulfur used as a vulcanizing agent was manufactured by Ajax Chemical Co., Ltd, Samutprakarn, Thailand. The N-tert-butyl-2-benzothiazolesulphenamide (Santocure TBBS) used as an accelerator was manufactured by Flexsys (USA). The polyphenolic
35
additive, Wingstayw L, used as an antioxidant was manufactured by Eliokem Inc. (Ohio, USA). 2.2. Preparation of epoxidized natural rubber Epoxidized natural rubber was prepared using highammonia concentrated NR latex with a dry rubber content of approximately 60%. Details of the preparation process of the ENRs are described elsewhere [11]. The ENR latex was coagulated in methanol and washed thoroughly with distilled water and dried in a vacuum oven at 40 8C for 24 h. Infrared spectroscopy was used to analyze the molecular structure of the ENR. The reaction time of the epoxidation was set according to the required level of epoxide groups in the ENR products, as described in our previous work [11]. The level of the epoxide contents in the ENR products was later confirmed by infrared spectroscopy via a calibration curve. In this work, ENR with a level of epoxide groups at approximately 30 mol % epoxide, corresponding to ENR-30, was used. 2.3. Preparation of phenolic modified polypropylene A blend compatabilizer, phenolic modified polypropylene (Ph-PP), was prepared by a melt mixing process using a Brabender Plasticorder PLE 331 (Duisberg, Germany). Details of the preparation process are described elsewhere. [12,13] Polypropylene (100 parts) and dimethylol phenolic resin (SP-1045) (4 parts) were first mixed at 180 8C and a rotor speed of 60 rpm for 3 min. Stannous dichloride (SnCl2) (0.8 parts) was later incorporated and mixing was continued for 2 min. The blend product was later cut into small pellets using a Bosco plastics grinder (Samutprakarn, Thailand). The sample was purified by extracting with acetone and characterized using infrared spectroscopy. The Ph-PP was later used as a blend compatibilizer for ENR/PP blends without purification. 2.4. Preparation of thermoplastic vulcanizates Three types of vulcanization system, namely sulfur, peroxide and a mixture of sulfur and peroxide, were used to cure the ENR phase of TPVs based on 75/25 ENR/PP blends. The dynamic vulcanization was carried out at 180 8C in a mixing chamber of an internal mixer, Brabender Plasticorder. The various chemicals used in each formulation are listed in Table 1. ENR with mole percentage epoxide of 30 was used to study the effect of vulcanization system on properties of the TPVs prepared from 75/25 ENR-30/PP blends. This is
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C. Nakason et al. / Polymer Testing 25 (2006) 34–41
Table 1 Chemicals used for a preparation of TPVs based on 75/25 ENR/PP blends using Ph-PP compatibilizer and various types of vulcanization systems Chemicals ENR-30 PP Ph-PPa ZnO Stearic acid Wingstay L TBBS Sulphur DCP TAC a
Quantities (part by weight of rubber) Sulfur
Mixed system
Peroxide
75.00 25.00 1.25 3.75 1.88 0.75 0.38 1.50 – –
75.00 25.00 1.25 3.75 – 0.75 0.75 0.75 1.50 0.75
75.00 25.00 1.25 3.75 – – – – 3.00 1.50
5% By weight of polypropylene.
because this type of epoxidized natural rubber (i.e. ENR-30) provided high mechanical strength and moderate processability using a plastic injection molding machine [12]. Also, Ph-PP was chosen to use as a blend compatibilizer due to its proven high compatibilizing effect [12]. The mixing torque of the Brabender Plasticorder was captured using an external data acquisition system at a frequency of 1 Hz, and the relationship between mixing torque and time was then plotted for each type of blend. Rheological, mechanical and morphological properties and swelling behavior of the TPVs were also investigated.
2.5. Rheological characterization A Rosand single bore capillary rheometer (model RH7, Rosand Precision Ltd, Gloucestershire, UK) was used to follow shear flow properties of the TPVs. The experiment was carried out using a wide range of shear rates from 5 to 1800 sK1 at 180 8C. Dimension of the capillary die used were 2-mm diameter, 32-mm length and 908 entry angle with aspect ratio (L/D) of 16/1. The TPVs was incorporated into the barrel of the rheometer and preheated for 5 min under a pressure of approximately 3–5 MPa to get a compact mass. The piston was programmed to travel downward automatically at a speed of 5 mm/min for 30 s to purge the excess molten material and eliminate bubbles present in the melt. The test was then carried out at a set of shear rates in a program via a microprocessor. The pressure drop across a capillary channel and melt temperature were captured via a data acquisition system during the test. The apparent shear stress, shear rate and shear viscosity were calculated using
the derivation of the Poiseuille law. Equations used to calculate the shear flow properties were described elsewhere [14]. 2.6. Mechanical testing The dumbbell shaped specimens of the TPVs used for tensile testing were prepared by thermoplastic injection molding machine (Welltec Machinery Ltd, Hong Kong). Tensile testing was performed at 25G 2 8C at a crosshead speed of 500 mm/min according to ISO 37. The instrument used was Hounsfield Tensometer, model H 10 KS (Hounsfield Test Equipment Co., Ltd, UK). Hardness of the samples was also measured using indentation shore A, according to ISO 7619. 2.7. Morphological studies Morphological characterization was performed using a Leo scanning electron microscope, model VP 1450 (Leo Co., Ltd, UK). Injection molded samples of the TPVs were cryogenic cracked in liquid nitrogen to avoid any possibility of phase deformation during the cracking process. The PP phase was preferentially extracted by immersing the fractured surface into hot xylene for 10 min. The samples were later dried in vacuum oven at 40 8C for 3 h to eliminate the contamination of the solvent. The dried surfaces were gold-coated and examined by scanning electron microscope. 2.8. Swelling behavior The TPVs were weighed and inserted into test tubes containing test liquids and placed in an oven at 30 8C.
C. Nakason et al. / Polymer Testing 25 (2006) 34–41 12 Sulfur system Torque (dN-.m)
10
Mixed system Peroxide system
8 6 4 2 0 0
60
120
180 240 Mixing time (sec)
300
360
420
Fig. 1. Mixing torque-time of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
In this work, diesel oil, a mixture of toluene/isooctane (50% v/v) and engine oil were used as the test liquids. The test specimens were immersed in the test liquids for 72 h. The samples were removed from the solvents and blotted with filter paper to remove excess solvent from the surface of the sample before being weighed to an accuracy of 0.1 mg at a fixed temperature of 30 8C. The degree of swelling (weight increase) was calculated as follows: Degree of swelling ð%Þ Z
ðWs KW0 Þ !100 W0
(1)
where W0 and Ws are weights of the specimen before and after immersing in the test liquid, respectively.
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sulfur-cured system S–S linkages are formed, whilst in the peroxide curing system more stable C–C linkages are formed. In the mixed cure system both S–S, C–S and C–C linkages are formed in the ENR phase. Among the three different vulcanizing systems used, the mixed cure system showed the highest final mixing torque and the peroxide showed the lowest final mixing torque, with the sulfur-cured system being intermediate. In the peroxide and mixed-cure systems, during shearing at high temperature (i.e. 180 8C), the peroxide (i.e. DCP) degrades the polypropylene molecules particularly by the b-scission mechanism, as shown in reaction Scheme 1. In these systems, the degradation of PP overshadows the effect of dynamic cross-linking of the ENR phase. However, a higher level of DCP was used in the peroxide-cured system (Table 1) which causes lower viscosity of the TPV. As a result, the TPV with a peroxide-cured system exhibited the lowest value of the final mixing torque. In the mixed-cured system, a half quantity of DCP was used compared with the DCP used in the peroxide-cured system. Therefore, lower influence of PP degradation on rheological properties and, hence, mixing torque of the TPV melts was observed. Also, the influence of the formation of more stable C–C linkages overcomes the drawback of the degradation of PP molecules. Therefore, we observed the highest values of the final mixing torque for the mixed-cure system. 3.2. Rheological properties
3. Results and discussion 3.1. Mixing torques Effect of dynamic vulcanization on the mixing torque-time curves of the TPVs prepared from 75/25 ENR-30/PP blends with Ph-PP compatibilizer and various curing systems is shown in Fig. 1. In the CH3
CH3
C CH2
C
H
H
CH2
Plots of apparent shear stress versus shear rate (i.e. flow curves) and apparent shear viscosity versus shear rate (i.e. viscosity curves) of the thermoplastic vulcanizates are shown in Figs. 2 and 3, respectively. It is seen that the TPVs with peroxide-cured system exhibited the lowest shear stress and shear viscosity, whilst the TPV with the mixed-cure system showed the highest values.
Direct Chain scission
CH3 CH
·
CH3
·CH
+
2
C
CH2
Transfer H Scission
Tertiary C-H
CH2 CH3
CH3
C CH2
C
·
H
CH3
CH3 +
beta-C-C Scission
C CH2
C
CH2
CH3
CH3
CH2
CH2
+
CH CH2
·
Scheme 1. Possible types of chain scissions of PP under high shearing action and high temperature in a presence of DCP [13].
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C. Nakason et al. / Polymer Testing 25 (2006) 34–41 10
8 1.00E+05
1.00E+04
Sulfur system Mixed system Peroxide system
1.00E+03 1.00E+00
Tensile strength (MPa)
Apparent shear stress (Pa)
1.00E+06
6
4
2 1.00E+01 1.00E+02 1.00E+03 Apparent shear rate (1/s)
1.00E+04 0
Fig. 2. Relationship between apparent shear stress and shear rate of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
The TPVs with the sulfur-cured system showed intermediate values. Therefore, trends of shear stress and shear viscosity at a given shear rate correspond to the trends of mixing torque (in Fig. 1). Chain scission of the polypropylene molecules also caused the lowest shear stress and viscosity of the TPVs with the peroxidecured system. In the mixed-cure system, a lower degree of chain scission was experienced due to lower DCP content. However, influence of stable C–C linkages in combination with S–S and C–S linkages caused stronger network formation in the TPV with the mixed-cure system. Therefore, this type of TPV exhibited the highest mixing torque, apparent shear stress and shear viscosity. 3.3. Mechanical properties of TPVs Mechanical properties in terms of tensile strength and elongation at break of the TPVs are shown in Figs. 4 and 5, respectively. It is seen that the mixed 1.00E+05
Peroxide system
cured-system exhibited the highest tensile strength and elongation at break, whilst the peroxide cured-system showed the lowest values. The TPV with sulfur-cured system exhibited intermediate values. This may be attributed to influence of types of chemical bond in the vulcanizates and influence of chain scission of the PP molecules. The mixed curing system produced a rubber network consisting of C–C, C–S and S–S linkages between the rubber molecules, while the sulfur-cured system generated weaker and less stable S–S and C–S linkages. This resulted in higher strength and extensibility for the TPVs based on the mixed curing system. The peroxide-cured system generated more stable C–C linkages. However, the PP degradation occurred simultaneously during dynamic vulcanization. Therefore, overall strength and extensibility of the TPVs were inferior. The peroxide cured system also exhibited poorer permanent set than that of the sulfur and mixed curing systems, as shown in Fig. 6. However, the tension sets for all types of the TPVs were lower than 50% with the elongation at break higher than 200%. Therefore, we concluded that the TPVs prepared in this
Mixed system 1.00E+04
Mixed system
Fig. 4. Tensile strength of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
350
Peroxide system 300 Elongation at break (%)
Apparent shear viscosity (Pa.s)
Sulfur system
Sulfur system
1.00E+03
1.00E+02
1.00E+01 1.00E+00
1.00E+01 1.00E+02 1.00E+03 Apparent shear rate (1/s)
1.00E+04
Fig. 3. Relationship between apparent shear viscosity and shear rate of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
250 200 150 100 50 0
Sulfur system
Mixed system
Peroxide system
Fig. 5. Elongation at break of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
C. Nakason et al. / Polymer Testing 25 (2006) 34–41
39
25
25
Isoctane + Toluene Diesel oil
20
Engine oil 15
Swelling (%)
Tension set (%)
20
10
15
10
5 5 0
Sulfur system
Mixed system
Peroxide system
0 Sulfur system
Fig. 6. Tension set of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
work exhibited reasonable elastomeric properties and are suitable for various industrial applications. Fig. 7 shows hardness of the TPVs. It is seen that the TPVs prepared with various vulcanization systems exhibited similar values of indentation hardness (Shore A), hence, we concluded that the type of vulcanization system did not have a significant effect on hardness properties of the TPVs.
Peroxide system
Fig. 8. Swelling behavior of TPVs based on 75/25 ENR-30/PP blends using Ph-PP compatibilizer and various vulcanization systems.
3.5. Morphological studies
40
The xylene etched cryogenic fracture surfaces of the TPVs are shown in Fig. 9. The PP phase was preferentially dissolved in xylene at an elevated temperature which left un-dissolved small spheres of vulcanized rubber particles adhered to the surface. Also, the previous location of the PP phase was etched and transformed to give cavitation. The TPV with peroxide-cure system showed the smallest vulcanized rubber particles dispersed in the PP matrix, whilst the sulfur-cured system exhibited the largest particles. Therefore, a TPV with peroxide-cured system should provide the highest mechanical strength. As stated previously, a higher level of degradation of PP molecules occurred in this system and, therefore, the TPV lost mechanical strength despite containing the finest vulcanized rubber particles with high adhesion forces between the ENR and PP phases. Size of dispersed vulcanized rubber particles of the mixedcured system was marginally larger than that of peroxide-cured but smaller than that of the sulfurcured system. This gives a reason for the higher mechanical strength (i.e. tensile strength and elongation at break) and rheological properties (i.e. flow and viscosity curves) of the TPV with mixed-cure system. That is, it exhibited high interfacial force between the phases together with moderate degradation of the PP molecules because of low DCP content.
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4. Conclusion
3.4. Swelling behavior The degree of swelling of the TPVs is shown in Fig. 8. It is seen that the TPVs with various vulcanization systems exhibited the same trend; that is, a mixture of isooctane and toluene caused the highest degree of swelling while the engine oil showed the lowest values. The diesel oil exhibited intermediate swelling. Also, the TPVs had a similar degree of swelling in each liquid and, hence, the type of vulcanization system did not have a significant effect on degree of swelling of these types of TPV. 100
80 Hardness (Shore A)
Mixed system
60
0
Sulfur system
Mixed system
Peroxide system
Fig. 7. Hardness of TPVs based on 75/25 ENR-30/PP blends using PhPP compatibilizer and various vulcanization systems.
ENR-30 was synthesized via a performic epoxidation method and was used to prepare thermoplastic vulcanizates based on ENR/PP blends at a blend ratio of 75/25 with Ph-PP (5% wt of PP) compatibilizer. The
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C. Nakason et al. / Polymer Testing 25 (2006) 34–41
Fig. 9. SEM micrigraphs of TPVs based on 75/25 ENR/PP blends using Ph-PP compatibilizer and various vulcanization systems.
influence of various curing systems was investigated. It was found that the final mixing torques, shear stress, shear viscosity, tensile strength and elongation at break of the TPVs with the mixed-cure system were higher that those of the sulfur and peroxide-cured systems. In the sulfur-cured system, S–S linkages are formed while in the peroxide cured system more stable C–C linkages are formed. In the mixed cure system, S–S, C–S and C–C linkages are formed in the ENR phase. Furthermore, in the peroxide and mixed-cured systems, the peroxide degrades the polypropylene molecules particularly by a b-scission mechanism during shearing at high temperature. Therefore, the degradation of PP overshadows the effect of dynamic cross-linking of the ENR phase. A higher level of DCP was used in the peroxide-cured system. Therefore, the TPV with a peroxide-cured system exhibited the lowest values of the final mixing torque, tensile strength, elongation at break and shear flow properties. In the mixed-cure system, lower influence of PP degradation and influence of formation of more stable C–C linkages overcomes the drawback of the degradation of PP molecules. Therefore, we observed the highest values of those properties using this curing system. We also found that the curing systems did not affect hardness properties and solvent resistance of the TPVs. In morphological studies, we found that the size of dispersed vulcanized rubber domains of TPV with the
peroxide-cured system was smaller than those of the mixed and sulfur-cured systems. Hence, the TPV with the peroxide-cured system should provide the highest mechanical strength, but the higher level of degradation of PP molecules caused lowering of flow properties and mechanical strength. Dispersed vulcanized rubber particles of the mixed-cured system were marginally larger than those of the peroxide-cured system but smaller than those of the sulfur-cured system. Therefore, it provided the highest mechanical strength (i.e. tensile strength and elongation at break) and rheological properties (i.e. flow and viscosity curves). Acknowledgements The authors gratefully acknowledge financial support from the National Metal and Materials Technology Center (MTEC), NSTDA contract no MT-B-46-POL-18-199-G. Also special scholarship granted by Prince of Songkla University, Thailand to one of us (Mr Puripong Wannavilai) is gratefully acknowledged. References [1] N.R. Legge, G. Holden, H.E. Schroeder (Eds.), Thermoplastic Elastomer: A Comprehensive Review, Hanser Publishers, New York, 1987.
C. Nakason et al. / Polymer Testing 25 (2006) 34–41 [2] A. Ibrahim, M. Dahlan, Prog. Polym. Sci. 23 (1998) 665. [3] S. Mohanty, G.B. Nando, K. Vijayan, N.R. Neelakanthan, Polymer 37 (1996) 387. [4] C. Nakason, Y. Panklieng, A. Kaesaman, J. Appl. Polym. Sci. 92 (2004) 3561. [5] P. Ramesh, S.K. De, J. Appl. Polym. Sci. 50 (1993) 1369. [6] Z. Oommen, S. Thomas, C.K. Premalatha, B. Kuriakose, Polymer 38 (1997) 5611. [7] L. Thiraphattaraphun, S. Kiatkamjornwong, P. Prasassarakich, S. Damronglerd, J. Appl. Polym. Sci. 81 (2001) 428. [8] C. Nakason, W. Pechurai, K. Sahakaro, A. Kaesaman, Polym. Adv. Technol. 16 (2005) 592.
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