The Effect of N, N-m- Phenylenebismaleimide (HVA-2) Addition on Properties of Polypropylene (PP)/Ethylene-Propylene Diene Terpolymer (EPDM)/Natural Rubber (NR) Vulcanized Blends
Jurnal Teknologi Proses
Media Publikasi Karya Ilmiah Teknik Kimia
6(1) Januari 2007: 52 – 58 ISSN 1412-7814
The Effect of N, N-m- Phenylenebismaleimide (HVA-2) Addition on Properties of Polypropylene (PP)/Ethylene-Propylene Diene Terpolymer
(EPDM)/Natural Rubber (NR) Vulcanized Blends
Halimatuddahliana
Department of Chemical Engineering, Faculty of Engineering, University of Sumatera Utara
Abstract This paper discusses process development, morphology, oil resistance and gel content of polypropylene (PP)/ethylene-propylene diene terpolymer (EPDM)/natural rubber (NR) vulcanized blends with the addition of N,N-m- phenylenebismaleimide (HVA-2) as a compatibilizer. Blends were prepared in several blend ratios in Haake Polydrive with temperature and rotor speed of 180 oC and 50 rpm respectively. Results indicated that the combination of dicumyl peroxide (Dicup) with HVA-2 show high torque development and stabilization torque as compared to vulcanized blend alone. The combination of Dicup with HVA-2 have also showed higher oil resistance and gel content in all blend ratio compared to similar vulcanized blends without HVA-2 addition. SEM micrographs from the surfaces extraction of the blends support that the cross-linking and compatibilization have occurred during process of vulcanized blend containing HVA-2.
Keywords: polypropylene, EPDM, natural rubber, Dicup, HVA-2, oil resistance, gel content.
Introduction
There are large combinations of thermoplastic and elastomers that are commercially available. PP/EPDM blends are one of the most studies of thermoplastic elastomers. However, replacement of EPDM with NR in PP/EPDM blends has been considered due to the reduction of the cost. It has also been observed that the partial replacement of EPDM by NR decreased the properties of PP/EPDM blends (Halimatuddahliana & Ismail, 2004). Therefore, dynamic vulcanization has been introduced in the PP/EPDM/NR blend to improve the properties of such blends. Some investigations have been reported regarding the dynamic vulcanization of TPE especially on the PP/EPDM blends and most of the investigations focused on the
processing and the improvements on mechanical and physical properties of the blends (Coran & Patel, 1980; Lopez-Manchado & Arroyo, 2001; Sabet et al., 1996; Ellul, 2003; Xiao et al., 2002; Sariatpanahi et al., 2002). The dynamic vulcanization has also been introduced in the PP/EPDM/NR blend using sulfur (Ismail & Halimatuddahliana, 2004) and dicumyl peroxide (Dicup) (Halimatuddahliana et al., b 2005) to improve the properties of such blends.
The presence of Dicup in the blends is to produce reactive radicals upon decomposition at elevated temperature via exothermic reaction that is beneficial in a rubber compound. It is generally agreed that for the peroxide vulcanization of polyolefin and rubber, crosslinks or chain scission may occur simultaneously. According to Ho et al. (1993)
Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
53
the polymer free radicals in PP induced by the peroxide decomposition lead to predominantly scission reaction, while in rubber, vulcanized properties of blends are determined by crosslink structures that formed (Coran, 1989; Dluzneski, 2001).
Coagents, in the other hand, can be used to compound elastomers, thermoplastics and thermoplastic/elastomer blend with and without the presence of accelerators or peroxide. Several investigatons have been reported regarding the use of coagent especially HVA-2 and accelerators (Inoue a & b, 1994; Ishikawa et al., 1996). The addition of HVA-2 for peroxide cure is to eliminate practically all of the problems once considered to be associated with peroxide cure. According to Dikland et al., (1992) and McElwee & Lohr (2001) peroxidecoagent system can be visualized in a way similar to that which was postulated for the sulfurization of the rubber molecules by the action of accelerators. After decomposition of peroxide molecules, macroradicals are formed and subsequently the coagent incorporation process is expected to initiate by the vulcanization system. The polyfunctional molecules in coagent can also be incorporated into the crosslink structures during vulcanization forming so called ‘coagent bridge’.
Keller (1988) has suggested briefly, the process initiates via thermal decompostition of the peroxide and how the coagent affected the reaction. A generalized scheme is shown as follows:
heat (1) Peroxide (2) R* + P (3) 2P* (4) P* + R* (5) P*
2R* P* + RH P–P P–R P’* + P=
Peroxide decomposition H abstraction Crosslink No crosslink scission
The crosslink is completed when the polymer radicals combine as in Reaction (3) above. If a coagent is present in the compound, Reactions (4) and (5) are repressed because the coagent rapidly combine with P* to produce stable radicals. The polyfunctional nature of the
coagent also enhances Reaction (2). In addition, the presence of a reactive olefinic site as in the terpolymer shifts Reaction (2) from the polymer backbone to the pendant unsaturation, thus intensifying Reaction (3) and suppressing Reaction (5).
Previous paper (Halimatuddahliana et al., b 2005) reported that in terms of tensile properties and degree of crosslinking, the presence of HVA-2 in Dicup vulcanization does significantly affect the extent of recombination of macroradicals (from chain scission) and improve the crosslining efficiency which can be attributed to the formation of ‘coagent bridge’ in PP/EPDM/NR blends. It was concluded that the suppression of chain scission and the formation of ‘coagent bridges’ contributes to an improvement of the crosslinking efficiency.
This paper discusses the role of HVA-2 in Dicup vulcanization of PP/EPDM/NR blends and how it can influence the process development, morphology, oil resistance and gel content of PP/EPDM/NR blends.
Experimental
Materials
Materials have been used in this study were polypropylene (PP), ethylene-propylene diene terpolymer (EPDM - EPT 3072E), natural rubber (NR - HSL), dicumyl peroxide (Dicup) and N,N-m-phenylene bismaleimide (HVA-2).
Preparation and Processing
Studies were conducted on PP/EPDM/NR blends consist of two systems viz. blend with Dicup vulcanization and blend with the combination of Dicup vulcanization and HVA2 where each blend covering different blend compositions viz. 50/40/10, 50/30/20, 50/20/30, 50/10/40 and 50/0/50. Blends were prepared by melt mixing in an internal mixer, Haake Polydrive with Rheomix R600/610 at temperature and rotor speed of 180 oC and 50 rpm respectively. Table 1 shows the mixing sequence of components in preparation of the PP/EPDM/NR blends.
54 Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
TABLE 1: Mixing sequence of components in preparation of the PP/EPDM/NR blends
Time
Operation
(min)
Vulcanized with DCP (1 phrb )
Blend with combination of DCP and HVA-2 (3 phrc)
0 PP loading
PP loading
2 Rubbers addition
Rubbers addition
5 DCP addition + antioxidanta
DCP and HVA-2 addition + antioxidanta
-
6-
-
7-
-
8 Dump
-
9-
10 Dump
a irganox 1010 (0.4 phr) b based on the rubbers content (EPDM and NR) c based on the NR content
During blending, thermoplastic (PP) were first loaded into the internal mixer and premixing for two minutes and followed by the rubbers (EPDM and NR). For dynamic vulcanization process, Dicup was added at 5 minutes of mixing and the mixing time was set for 8 minutes. The corresponding blends with combination of Dicup and HVA-2, the mixing time was further increased to 10 minutes. The samples were then sheeted by passing through 2 roll-mill and allowed to cool at room temperature. Specimens for test were compression molded using electrically heated hydraulic press machine. The machine was preheated at 180oC for six minutes prior to moulding and followed by another four minutes of compression time under the same temperature. The specimen was allowed to cool under pressure for further four minutes
Morphology Studies
Morphological evaluations of PP/EPDM/NR surfaces were done using a scanning electron microscope (SEM), model Stereoscan 200 Cambridge. The vulcanized samples were etched with nitric acid for two days, washed with water, and then dried. All the samples were mounted on aluminum stubs and sputtercoated with a thin layer of gold to avoid electrostatic charging during examination. The examinations were done within 24 hrs of preparation.
Swelling Test
Determination of the swelling index was carried out according to ASTM D471. The specimen with a dimension of 30mm x 5mm x 2mm were cut and weighed using an electrical balance. The test pieces were then immersed in oil (IRM 903) for 48 hrs at room temperature. The samples were then removed from oil, wiped with tissue paper to remove excess liquid from the surface and weighted. The swelling index of the blends was then calculated as follow:
Swelling
index
=
W2 W1
× 100%
… (1)
Where W1 and W2 are weight of specimen before and after immersion respectively.
Gel Content
The degree of cross-linking in the rubber
was measured after extraction in boiling
cyclohexane for 8 hrs. The samples were dried at 80 oC for 30 minutes and subsequently
weighed. The percentage of gel content of the
blends was then calculated as follow:
%
gel
content
=
Wg Wo
x100%
… (2)
Where Wg and Wo are sample weights after and before extraction respectively.
Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
55
Results and Discussion
Process Development
The torque developments of selected Dicup vulcanization of PP/EPDM/NR blends with and without HVA-2 addition are shown in Figure 1.
A sharp increase in the torque value was observed immediately after the addition of Dicup and HVA-2 at five minutes of mixing. Upon the completion of mixing, the torque starts to decrease gradually due to decrease in the viscosity to more stable values. For the Dicup vulcanized blends containing HVA-2, the torques stabilizes at higher value than Dicup vulcanized blend without HVA-2. The increase of torque is associated with the interruption exerted on the rotor by the extended crosslinked on deformability of macromolecules. This indicates that the viscosity of the blend increases as crosslinking was occurred. Besides crosslinks formation, the presence of HVA-2 in Dicup vulcanization blend is also related to an
increasing adhesion between the dispersed phase and the matrix. According to Dahlan et al., (Dahlan et al., 2000), the highest torque indicates the strongest interaction between the phases of the blend. In this case, the increase in adhesion can be explained by a radical-induced crosslinking by HVA-2. As mentioned before, in the presence of peroxide in the blends, chain scissions become the dominant reaction for PP. However, when a multifunctional monomer (such as HVA-2) is used as a coagent, the rate of chain scission reactions will be reduced because of stabilization of macroradicals (Dikland et al., 1992; Keller, 1988). Therefore, crosslink reactions are favored and also leading to the formation of ‘coagent bridges’ between rubber and plastic.
The comparison of stabilization torque values between Dicup vulcanized blend combined HVA-2 with Dicup vulcanized blend alone are presented as a function of blend compositions in Figure 2.
Torque (N m)
16 14 12 10
8 6 4 2 0
0 2 4 6 8 10 12 Time (min.)
Vulcanized 50/30/20 blend Vulcanized 50/0/50 blend
Vulcanized 50/30/20 + HVA-2 Vulcanized 50/0/50 + HVA-2
FIGURE 1: The torque development of Dicup vulcanization of PP/EPDM/NR blends with and without HVA-2 addition
56 Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
Stabilization torque (N m)
5.5
4.5
3.5
2.5
1.5
0.5 50/40/10 50/30/20 50/20/30 50/10/40
Blend ratio (PP/EPDM/NR)
50/0/50
Vulcanized with DCP
Vulcanized with DCP + HVA-2
FIGURE 2: The comparison of stabilization torque between Dicup vulcanization with and without HVA-2 addition
It shows that Dicup vulcanized blend with HVA-2 exhibit higher stabilization torque compared with Dicup vulcanized blend only. In this case, the extended crosslinking and the inhibition of chain scission by HVA-2 lead to an increase the viscosity of the blend and hence the torque. With the addition of HVA-2 in Dicup vulcanization, even though the radicals are formed by peroxide, the coagent is sufficient to bridge and stabilize the PP macroradicals. As the coagent was added, a dramatic increase of melt viscosity obtained. This implies that macroradicals form by peroxide is mostly reacted with HVA-2. Furthermore, the stabilization torque increase with the increasing of NR content. This indicates that the effect of HVA-2 on Dicup vulcanized blend was more significant on NR richer blend.
Morphology Study
Figures 3 (a-d) compares the extracted surface morphologies of Dicup vulcanized PP/EPDM/NR blends with and without HVA-2 addition. Through the addition of HVA-2 in vulcanized blends (Fig. 3b and d), the particles are smaller, better dispersed and was more evenly distributed in the PP matrix as compared to Dicup vulcanized blends alone (Fig. 3a and c). It is apparent from the micrographs that the numbers of holes due to the extraction of rubber
particles are reduced significantly. This observation indicates that the combination of Dicup vulcanization with HVA-2 has crosslinked the rubber phase and increased interfacial adhesion resulting from the formation of ‘coagent bridge’. After the crosslinking of the rubber particles, the tendency of the rubber particles to dissolve by the solvent decrease.
Although a detailed distribution analysis on the dispersed particles was not carried out in this work, the decrease of the dimension and number of voids could be explained by the production of ‘coagent bridge’ between PP and rubber in PP/EPDM/NR blends under dynamic crosslinking conditions. The same observation was obtained by Wu (1987) who indicated that if the graft copolymer of plastic and rubber was produced predominantly at an early stage in the dynamic crosslinked process, it would play a role of compatibilizer to reduce the dimension and number of the particles dramatically.
Swelling Index
The result of oil resistance of Dicup vulcanized blend with and without HVA-2 addition based on the swelling index after oil immersion at room temperature for 48 hours is presented in Table 2.
Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
57
FIGURE 3: SEM micrographs of extracted surface PP/EPDM/NR blends: a. Dicup vulcanized 50/30/20; b. Dicup vulcanized 50/30/20 + HVA-2; c. Dicup vulcanized 50/0/50; d. dicup vulcanized 50/0/50 + HVA-2
TABLE 2: Comparison of the swelling index between Dicup vulcanized PP/EPDM/NR blends containing HVA2 with Dicup vulcanized blend alone
Blend Ratio (PP/EPDM/NR)
50/40/10 50/30/20 50/20/30 50/10/40 50/0/50
Dicup vulcanization
1.28 1.31 1.42 1.45 1.46
Swelling index Dicup vulcanization + HVA-2
1.25 1.28 1.38 1.36 1.31
TABLE 3: Gel content of selected Dicup vulcanized PP/EPDM/NR blend with and without HVA-2 addiiton
Blend Ratio (PP/EPDM/NR)
50/30/20 50/0/50
Gel content (%)
Dicup vulcanization
Dicup vulcanization + HVA-2
91.1 95.5 92.6 96.5
It is obvious that the oil resistances is increased for Dicup vulcanized blend containing HVA-2 as indicating by the low values of swelling index compared to Dicup vulcanized blend alone. The magnitude of decrease in swelling index of the blend increases with increasing NR content. The higher magnitude changes are observed in the Dicup vulcanized 50/0/50 PP/EPDM/NR blend as shown in Table 2. This indicates that crosslink formation in NR richer content is more predominant. These values represent tight crosslinking of the rubber chains, therefore less oil was penetrated into rubber particles.
Gel Content
The effect of HVA-2 addition on the Dicup vulcanization on the percentage of gel content of PP/EPDM/NR blend is depicted in Table 3.
The amount of gel content determined after soxhlet extraction of uncrosslinked rubber phase is in agreement with those determined from swelling index. Gel is formed when a strong interaction between components of the blend exists. It is an indicator of crosslinks and compatibilization formed in the blend. Upon addition of HVA-2 in Dicup vulcanized blend, there is an increase in percentage of gel. This is again associated with the increase of crosslink efficiency due to the formation of ‘coagent bridge’ by HVA-2.
Conclusion
The formation of ‘coagent bridge’ and the inhibition of chain scission by HVA-2 in Dicup vulcanization lead to an increase in the viscosity of the PP/EPDM/NR blend subsequently increased the stabilization torque of the blend. The incorporation of HVA-2 in Dicup
58 Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
vulcanized blends has increased the properties such as oil resistance and gel content. SEM micrographs from the surfaces extraction of the blends have also support that the ‘coagent bridge’ have occurred during Dicup vulcanization of PP/EPDM/NR containing HVA-2.
References
Coran, A.Y.; Patel R. Rubb. Chem. Technol 1980, 53, 141.
Coran, A.Y., In Encyclopedia of Polyme Science and Engineering 2nd ed. (H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges and J. I. Kroschwitz, eds.), 1989, 17, 666 .
Dahlan, H.M.; Khairul Zaman, M.D.; Ibrahim, A. J. Appl. Polym. Sci 2000, 78, 1776.
Dikland, H.G.; Does, V.D.; Bantjes, A. Rubb. Chem. Technol 1992, 66, 196.
Dluzneski, P.R.; Rubb. Chem. Techno. 2001, 74, 451. Ellul, M.D. Rubb. Chem. Technol 2003, 76, 202. Halimatuddahliana; Ismail H. Polym. Plast. Tech.
Eng. 2004, 43, 357.
Halimatuddahliana.; Ismail, H.; Akil, H. Md. (a) Inter. J. Polym. Mater 2005, 54, 1169.
Halimatuddahliana; Ismail, H,; Akil, H. Md. (b) Polym. Plast. Tech. Eng. 2005, 44, 1217.
Ho, R.M.; Su, A.C.; Wu, C.H.; Chen, S.I. Polymer 1993, 34, 3264.
Inoue, T.J (a). Appl. Polym. Sci 1994, 54, 709. Inoue, T.J (b). Appl. Polym. Sci 1994, 54, 723. Ishikawa, M.; Sugimoto. M, Inoue, T.J. Appl. Polym.
Sci. 1996, 62, 1495. Ismail, H.; Halimatuddahliana; Akil, H. Md. Journal
of Solid State science & Technology Letters, 11(2), 2004 Keller, R.C. Rubb. Chem. Technol 1988; 61, 238. Lopez-Manchado, M.A.; Arroyo M. Rubb. Chem. Technol 2001, 74, 211. McElwee, C.B.; Lohr, J.E. Rubber World 2001, 225, 41. Sabet, S.A.; Puydak, R.C., Rader CP. Rubb. Chem. Technol. 1996, 69, 476. Sariatpanahi, H.; Nazokdast, H.; Dabir, B.; Sadaghiani, K.; Hemmati, M. J. Appl. Polym. Sci. 2002, 86, 3148. Xiao, H.; Huang, S.; Jiang, T.; Cheng S. J. Appl. Polym. Sci. 2002, 83, 315. Wu, S. Polym. Eng. Sci. 1987, 27, 334.
Media Publikasi Karya Ilmiah Teknik Kimia
6(1) Januari 2007: 52 – 58 ISSN 1412-7814
The Effect of N, N-m- Phenylenebismaleimide (HVA-2) Addition on Properties of Polypropylene (PP)/Ethylene-Propylene Diene Terpolymer
(EPDM)/Natural Rubber (NR) Vulcanized Blends
Halimatuddahliana
Department of Chemical Engineering, Faculty of Engineering, University of Sumatera Utara
Abstract This paper discusses process development, morphology, oil resistance and gel content of polypropylene (PP)/ethylene-propylene diene terpolymer (EPDM)/natural rubber (NR) vulcanized blends with the addition of N,N-m- phenylenebismaleimide (HVA-2) as a compatibilizer. Blends were prepared in several blend ratios in Haake Polydrive with temperature and rotor speed of 180 oC and 50 rpm respectively. Results indicated that the combination of dicumyl peroxide (Dicup) with HVA-2 show high torque development and stabilization torque as compared to vulcanized blend alone. The combination of Dicup with HVA-2 have also showed higher oil resistance and gel content in all blend ratio compared to similar vulcanized blends without HVA-2 addition. SEM micrographs from the surfaces extraction of the blends support that the cross-linking and compatibilization have occurred during process of vulcanized blend containing HVA-2.
Keywords: polypropylene, EPDM, natural rubber, Dicup, HVA-2, oil resistance, gel content.
Introduction
There are large combinations of thermoplastic and elastomers that are commercially available. PP/EPDM blends are one of the most studies of thermoplastic elastomers. However, replacement of EPDM with NR in PP/EPDM blends has been considered due to the reduction of the cost. It has also been observed that the partial replacement of EPDM by NR decreased the properties of PP/EPDM blends (Halimatuddahliana & Ismail, 2004). Therefore, dynamic vulcanization has been introduced in the PP/EPDM/NR blend to improve the properties of such blends. Some investigations have been reported regarding the dynamic vulcanization of TPE especially on the PP/EPDM blends and most of the investigations focused on the
processing and the improvements on mechanical and physical properties of the blends (Coran & Patel, 1980; Lopez-Manchado & Arroyo, 2001; Sabet et al., 1996; Ellul, 2003; Xiao et al., 2002; Sariatpanahi et al., 2002). The dynamic vulcanization has also been introduced in the PP/EPDM/NR blend using sulfur (Ismail & Halimatuddahliana, 2004) and dicumyl peroxide (Dicup) (Halimatuddahliana et al., b 2005) to improve the properties of such blends.
The presence of Dicup in the blends is to produce reactive radicals upon decomposition at elevated temperature via exothermic reaction that is beneficial in a rubber compound. It is generally agreed that for the peroxide vulcanization of polyolefin and rubber, crosslinks or chain scission may occur simultaneously. According to Ho et al. (1993)
Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
53
the polymer free radicals in PP induced by the peroxide decomposition lead to predominantly scission reaction, while in rubber, vulcanized properties of blends are determined by crosslink structures that formed (Coran, 1989; Dluzneski, 2001).
Coagents, in the other hand, can be used to compound elastomers, thermoplastics and thermoplastic/elastomer blend with and without the presence of accelerators or peroxide. Several investigatons have been reported regarding the use of coagent especially HVA-2 and accelerators (Inoue a & b, 1994; Ishikawa et al., 1996). The addition of HVA-2 for peroxide cure is to eliminate practically all of the problems once considered to be associated with peroxide cure. According to Dikland et al., (1992) and McElwee & Lohr (2001) peroxidecoagent system can be visualized in a way similar to that which was postulated for the sulfurization of the rubber molecules by the action of accelerators. After decomposition of peroxide molecules, macroradicals are formed and subsequently the coagent incorporation process is expected to initiate by the vulcanization system. The polyfunctional molecules in coagent can also be incorporated into the crosslink structures during vulcanization forming so called ‘coagent bridge’.
Keller (1988) has suggested briefly, the process initiates via thermal decompostition of the peroxide and how the coagent affected the reaction. A generalized scheme is shown as follows:
heat (1) Peroxide (2) R* + P (3) 2P* (4) P* + R* (5) P*
2R* P* + RH P–P P–R P’* + P=
Peroxide decomposition H abstraction Crosslink No crosslink scission
The crosslink is completed when the polymer radicals combine as in Reaction (3) above. If a coagent is present in the compound, Reactions (4) and (5) are repressed because the coagent rapidly combine with P* to produce stable radicals. The polyfunctional nature of the
coagent also enhances Reaction (2). In addition, the presence of a reactive olefinic site as in the terpolymer shifts Reaction (2) from the polymer backbone to the pendant unsaturation, thus intensifying Reaction (3) and suppressing Reaction (5).
Previous paper (Halimatuddahliana et al., b 2005) reported that in terms of tensile properties and degree of crosslinking, the presence of HVA-2 in Dicup vulcanization does significantly affect the extent of recombination of macroradicals (from chain scission) and improve the crosslining efficiency which can be attributed to the formation of ‘coagent bridge’ in PP/EPDM/NR blends. It was concluded that the suppression of chain scission and the formation of ‘coagent bridges’ contributes to an improvement of the crosslinking efficiency.
This paper discusses the role of HVA-2 in Dicup vulcanization of PP/EPDM/NR blends and how it can influence the process development, morphology, oil resistance and gel content of PP/EPDM/NR blends.
Experimental
Materials
Materials have been used in this study were polypropylene (PP), ethylene-propylene diene terpolymer (EPDM - EPT 3072E), natural rubber (NR - HSL), dicumyl peroxide (Dicup) and N,N-m-phenylene bismaleimide (HVA-2).
Preparation and Processing
Studies were conducted on PP/EPDM/NR blends consist of two systems viz. blend with Dicup vulcanization and blend with the combination of Dicup vulcanization and HVA2 where each blend covering different blend compositions viz. 50/40/10, 50/30/20, 50/20/30, 50/10/40 and 50/0/50. Blends were prepared by melt mixing in an internal mixer, Haake Polydrive with Rheomix R600/610 at temperature and rotor speed of 180 oC and 50 rpm respectively. Table 1 shows the mixing sequence of components in preparation of the PP/EPDM/NR blends.
54 Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
TABLE 1: Mixing sequence of components in preparation of the PP/EPDM/NR blends
Time
Operation
(min)
Vulcanized with DCP (1 phrb )
Blend with combination of DCP and HVA-2 (3 phrc)
0 PP loading
PP loading
2 Rubbers addition
Rubbers addition
5 DCP addition + antioxidanta
DCP and HVA-2 addition + antioxidanta
-
6-
-
7-
-
8 Dump
-
9-
10 Dump
a irganox 1010 (0.4 phr) b based on the rubbers content (EPDM and NR) c based on the NR content
During blending, thermoplastic (PP) were first loaded into the internal mixer and premixing for two minutes and followed by the rubbers (EPDM and NR). For dynamic vulcanization process, Dicup was added at 5 minutes of mixing and the mixing time was set for 8 minutes. The corresponding blends with combination of Dicup and HVA-2, the mixing time was further increased to 10 minutes. The samples were then sheeted by passing through 2 roll-mill and allowed to cool at room temperature. Specimens for test were compression molded using electrically heated hydraulic press machine. The machine was preheated at 180oC for six minutes prior to moulding and followed by another four minutes of compression time under the same temperature. The specimen was allowed to cool under pressure for further four minutes
Morphology Studies
Morphological evaluations of PP/EPDM/NR surfaces were done using a scanning electron microscope (SEM), model Stereoscan 200 Cambridge. The vulcanized samples were etched with nitric acid for two days, washed with water, and then dried. All the samples were mounted on aluminum stubs and sputtercoated with a thin layer of gold to avoid electrostatic charging during examination. The examinations were done within 24 hrs of preparation.
Swelling Test
Determination of the swelling index was carried out according to ASTM D471. The specimen with a dimension of 30mm x 5mm x 2mm were cut and weighed using an electrical balance. The test pieces were then immersed in oil (IRM 903) for 48 hrs at room temperature. The samples were then removed from oil, wiped with tissue paper to remove excess liquid from the surface and weighted. The swelling index of the blends was then calculated as follow:
Swelling
index
=
W2 W1
× 100%
… (1)
Where W1 and W2 are weight of specimen before and after immersion respectively.
Gel Content
The degree of cross-linking in the rubber
was measured after extraction in boiling
cyclohexane for 8 hrs. The samples were dried at 80 oC for 30 minutes and subsequently
weighed. The percentage of gel content of the
blends was then calculated as follow:
%
gel
content
=
Wg Wo
x100%
… (2)
Where Wg and Wo are sample weights after and before extraction respectively.
Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
55
Results and Discussion
Process Development
The torque developments of selected Dicup vulcanization of PP/EPDM/NR blends with and without HVA-2 addition are shown in Figure 1.
A sharp increase in the torque value was observed immediately after the addition of Dicup and HVA-2 at five minutes of mixing. Upon the completion of mixing, the torque starts to decrease gradually due to decrease in the viscosity to more stable values. For the Dicup vulcanized blends containing HVA-2, the torques stabilizes at higher value than Dicup vulcanized blend without HVA-2. The increase of torque is associated with the interruption exerted on the rotor by the extended crosslinked on deformability of macromolecules. This indicates that the viscosity of the blend increases as crosslinking was occurred. Besides crosslinks formation, the presence of HVA-2 in Dicup vulcanization blend is also related to an
increasing adhesion between the dispersed phase and the matrix. According to Dahlan et al., (Dahlan et al., 2000), the highest torque indicates the strongest interaction between the phases of the blend. In this case, the increase in adhesion can be explained by a radical-induced crosslinking by HVA-2. As mentioned before, in the presence of peroxide in the blends, chain scissions become the dominant reaction for PP. However, when a multifunctional monomer (such as HVA-2) is used as a coagent, the rate of chain scission reactions will be reduced because of stabilization of macroradicals (Dikland et al., 1992; Keller, 1988). Therefore, crosslink reactions are favored and also leading to the formation of ‘coagent bridges’ between rubber and plastic.
The comparison of stabilization torque values between Dicup vulcanized blend combined HVA-2 with Dicup vulcanized blend alone are presented as a function of blend compositions in Figure 2.
Torque (N m)
16 14 12 10
8 6 4 2 0
0 2 4 6 8 10 12 Time (min.)
Vulcanized 50/30/20 blend Vulcanized 50/0/50 blend
Vulcanized 50/30/20 + HVA-2 Vulcanized 50/0/50 + HVA-2
FIGURE 1: The torque development of Dicup vulcanization of PP/EPDM/NR blends with and without HVA-2 addition
56 Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
Stabilization torque (N m)
5.5
4.5
3.5
2.5
1.5
0.5 50/40/10 50/30/20 50/20/30 50/10/40
Blend ratio (PP/EPDM/NR)
50/0/50
Vulcanized with DCP
Vulcanized with DCP + HVA-2
FIGURE 2: The comparison of stabilization torque between Dicup vulcanization with and without HVA-2 addition
It shows that Dicup vulcanized blend with HVA-2 exhibit higher stabilization torque compared with Dicup vulcanized blend only. In this case, the extended crosslinking and the inhibition of chain scission by HVA-2 lead to an increase the viscosity of the blend and hence the torque. With the addition of HVA-2 in Dicup vulcanization, even though the radicals are formed by peroxide, the coagent is sufficient to bridge and stabilize the PP macroradicals. As the coagent was added, a dramatic increase of melt viscosity obtained. This implies that macroradicals form by peroxide is mostly reacted with HVA-2. Furthermore, the stabilization torque increase with the increasing of NR content. This indicates that the effect of HVA-2 on Dicup vulcanized blend was more significant on NR richer blend.
Morphology Study
Figures 3 (a-d) compares the extracted surface morphologies of Dicup vulcanized PP/EPDM/NR blends with and without HVA-2 addition. Through the addition of HVA-2 in vulcanized blends (Fig. 3b and d), the particles are smaller, better dispersed and was more evenly distributed in the PP matrix as compared to Dicup vulcanized blends alone (Fig. 3a and c). It is apparent from the micrographs that the numbers of holes due to the extraction of rubber
particles are reduced significantly. This observation indicates that the combination of Dicup vulcanization with HVA-2 has crosslinked the rubber phase and increased interfacial adhesion resulting from the formation of ‘coagent bridge’. After the crosslinking of the rubber particles, the tendency of the rubber particles to dissolve by the solvent decrease.
Although a detailed distribution analysis on the dispersed particles was not carried out in this work, the decrease of the dimension and number of voids could be explained by the production of ‘coagent bridge’ between PP and rubber in PP/EPDM/NR blends under dynamic crosslinking conditions. The same observation was obtained by Wu (1987) who indicated that if the graft copolymer of plastic and rubber was produced predominantly at an early stage in the dynamic crosslinked process, it would play a role of compatibilizer to reduce the dimension and number of the particles dramatically.
Swelling Index
The result of oil resistance of Dicup vulcanized blend with and without HVA-2 addition based on the swelling index after oil immersion at room temperature for 48 hours is presented in Table 2.
Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
57
FIGURE 3: SEM micrographs of extracted surface PP/EPDM/NR blends: a. Dicup vulcanized 50/30/20; b. Dicup vulcanized 50/30/20 + HVA-2; c. Dicup vulcanized 50/0/50; d. dicup vulcanized 50/0/50 + HVA-2
TABLE 2: Comparison of the swelling index between Dicup vulcanized PP/EPDM/NR blends containing HVA2 with Dicup vulcanized blend alone
Blend Ratio (PP/EPDM/NR)
50/40/10 50/30/20 50/20/30 50/10/40 50/0/50
Dicup vulcanization
1.28 1.31 1.42 1.45 1.46
Swelling index Dicup vulcanization + HVA-2
1.25 1.28 1.38 1.36 1.31
TABLE 3: Gel content of selected Dicup vulcanized PP/EPDM/NR blend with and without HVA-2 addiiton
Blend Ratio (PP/EPDM/NR)
50/30/20 50/0/50
Gel content (%)
Dicup vulcanization
Dicup vulcanization + HVA-2
91.1 95.5 92.6 96.5
It is obvious that the oil resistances is increased for Dicup vulcanized blend containing HVA-2 as indicating by the low values of swelling index compared to Dicup vulcanized blend alone. The magnitude of decrease in swelling index of the blend increases with increasing NR content. The higher magnitude changes are observed in the Dicup vulcanized 50/0/50 PP/EPDM/NR blend as shown in Table 2. This indicates that crosslink formation in NR richer content is more predominant. These values represent tight crosslinking of the rubber chains, therefore less oil was penetrated into rubber particles.
Gel Content
The effect of HVA-2 addition on the Dicup vulcanization on the percentage of gel content of PP/EPDM/NR blend is depicted in Table 3.
The amount of gel content determined after soxhlet extraction of uncrosslinked rubber phase is in agreement with those determined from swelling index. Gel is formed when a strong interaction between components of the blend exists. It is an indicator of crosslinks and compatibilization formed in the blend. Upon addition of HVA-2 in Dicup vulcanized blend, there is an increase in percentage of gel. This is again associated with the increase of crosslink efficiency due to the formation of ‘coagent bridge’ by HVA-2.
Conclusion
The formation of ‘coagent bridge’ and the inhibition of chain scission by HVA-2 in Dicup vulcanization lead to an increase in the viscosity of the PP/EPDM/NR blend subsequently increased the stabilization torque of the blend. The incorporation of HVA-2 in Dicup
58 Halimatuddahliana / Jurnal Teknologi Proses 6(1) Januari 2007: 52 – 58
vulcanized blends has increased the properties such as oil resistance and gel content. SEM micrographs from the surfaces extraction of the blends have also support that the ‘coagent bridge’ have occurred during Dicup vulcanization of PP/EPDM/NR containing HVA-2.
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