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Development and characterisation of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) for engineering applications
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European Polymer Journal 38 (2002) 2023–2031 www.elsevier.com/locate/europolj
Development and characterisation of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) for engineering applications M.S.C. Kumar a, M. Alagar
b,*
a b
Department of Chemistry, Scott Christian College, Nagercoil 629 003, India Department of Chemical Engineering, Anna University, Chennai 600 025, India Received 23 October 2001; accepted 11 January 2002
Abstract A novel and new copolymer of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) has been synthesised in toluene using dicumyl peroxide as initiator. The grafting efficiency of vinyloxyaminosilane (VOS) onto ethylene-propylene-diene terpolymer (EPDM) has been studied as a function of EPDM content, reaction time, reaction temperature and initiator concentration. Using the optimum grafting efficiency conditions, EPDM-gVOS has also been developed in a Haake Rheocord-90, torque rheometer. The grafting of vinyloxyaminosilane onto ethylene-propylene-diene terpolymer (EPDM-g-VOS) has been confirmed by Fourier-transform infrared spectroscopy. The mechanical, thermal, and electrical, properties of hot water cured EPDM-g-VOS and peroxide cured EPDM are compared. The mechanical properties of EPDM-g-VOS are decreased due to the flexibility imparted by VOS. However thermal and dielectric properties are increased due to the introduction of VOS onto EPDM as well as the formation of thermally stable three dimensional network through Si–O–Si– linkage. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Grafting; EPDM-g-VOS; Mechanical, electrical and thermal properties
1. Introduction Crosslinked ethylene-propylene-diene terpolymer (EPDM) is one of the most commonly used industrial polymer because of its outstanding resistance to heat, light, oxygen and ozone [1–4]. Most of the curing processes adopted earlier for EPDM involve chemical crosslinking using peroxides. The advent of a newer technique, involving grafting of organofunctional silane onto polymer chain followed by its subsequent condensation reaction in presence of moisture is the most attractive and useful method due to number of advantages [5]. The technology of silane grafting on polymers
*
Corresponding author. E-mail address: [email protected] (M. Alagar).
is being successfully exploited on a large commercial scale production of crosslinked polyethylene for cable insulation and hot water piping systems [5]. Duck et al. [6] prepared vinyloxyaminosilane (VTMO) and styrene grafted EPDM using benzoyl peroxide as initiator in toluene. The heat resistance, light resistance and weather ability of VTMO-EPDM-Styrene grafted copolymer are found better than those of ABS. Gartasegna [7] reported the effects on the molecular and structural parameters of vinyloxyaminosilane (VTMO) grafting and moisture crosslinking on EPDM. The process of crosslinking is found to be limited due to the diene content of the polymer through probable condensation reactions of grafted silane. Tanida and Sato [8] prepared heat shrinkable tubes by grafting VTMO with EPDM. The process of crosslinking was done by using moisture and observed that the product exhibits high stretching ratios
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ene/5-ethylidene-2-norbornene ¼ 71/20/9 by wt.% with Mooney viscosity, MLð1þ4Þ , of 50 at 125 °C and specific gravity of 0.88) of Du Pont’Dow elastomers, USA. The vinyloxyaminosilane (VOS) (Mw ¼ 313, specific gravity of 0.988, viscosity ¼ 11:5 cP and b:p: ¼ 300 °C) was procured from Wacker-Chemie, Germany. DCP (90% assay with m:p: ¼ 30 °C) was obtained from Concord Chemical Industrial Co., Taiwan. Laboratory grade toluene, n-hexane and DMF were used as received.
above its softening point. Bastin et al. [9] synthesised VTMO grafted EPDM using Brabender mixer and measured the insoluble fractions. It was observed that VTMO grafted EPDM has about 20% higher insoluble fraction than pure EPDM. Kawada et al. [10] prepared composites using silica filler with silane grafted EPDM and EPDM. They found that the heat resistance of silane grafted EPDM was considerably higher than that of ungrafted EPDM. Umeda et al. [11] studied the processability and thermal stability of silane grafted EPDM and EPDM. They concluded that the silane grafted EPDM exhibits better processability and thermal stability than those of ungrafted EPDM. King and Petty [12] reported a new method of preparation of crosslinked EPDM using alkyl terminated siloxane in presence of rhodium hydrosilylation catalyst instead of hexachloroplatinic acid catalyst. Kumar Sen et al. [13] studied the kinetics of grafting of vinyloxyaminosilane (VTMO) and vinyltriethoxsilane (VTEO) onto EPDM using DSC. In the present study a new copolymer of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) has been synthesised in toluene using dicumyl peroxide (DCP) initiator. The effect of EPDM content, VOS concentration, reaction time, reaction temperature and initiator concentration were investigated in the graft copolymerisation. Using optimum grafting efficiency conditions obtained from solution grafting technique, EPDM-g-VOS has been developed in a Haake Rheocord-90, torque rheometer. The structure of EPDM-g-VOS has been ascertained by Fouriertransform infrared (FTIR) spectroscopy. The mechanical, thermal and electrical properties of hot water cured EPDM-g-VOS and peroxide cured EPDM are determined and compared in order to utilise them for some engineering applications.
2.2. Preparation of EPDM-g-VOS The grafting reactions were carried out in 500 ml three necked, round bottom flask equipped with a reflux condenser, a Teflon-coated magnetic stirring bar and a nitrogen inlet. The details of experimental conditions are presented in Table 1. A stoichiometric amount of EPDM was dissolved in 200 ml toluene and refluxed until complete dissolution of EPDM. Further, a specified concentration of VOS dissolved in 50 ml of toluene and varying concentrations of DCP were added to EPDM. After the completion of reaction, the products were precipitated with methanol, filtered and dried in vacuum. 2.3. Isolation of graft terpolymer During the synthesis, the products obtained consists of ungrafted EPDM, poly(vinyloxyaminosilane) and EPDM-g-VOS. The ungrafted EPDM was separated from rest of the products by extraction using n-hexane. The vinyloxyaminosilane grafted EPDM (EPDM-gVOS) was separated from poly(vinyloxyaminosilane) using DMF where the former is soluble and latter is insoluble. The total conversion was calculated from the ratio of the total weight of crude products to the weight of EPDM charged. The grafting ratio and grafting efficiency were determined on the basis of changes in polymer weight during the reaction process and the total amount of EPDM-g-VOS formed respectively. The grafting ratio and grafting efficiency were calculated from the following equations:
2. Experimental 2.1. Materials The EPDM (ENB) elastomer used in this study was a commercial grade (Nordel IP 5750R) (ethylene/propyl-
Table 1 Conditions of graft copolymerisation Condition
Description
EPDM concentration (mol%) VOS concentration (mol%) Reaction time (h) Reaction temperature (°C) Initiator concentration (mol%) (based on the vinyl monomer)
10 1.0 3 150 0.1
15 1.5 5 160 0.2
20 2.0 7 170 0.3
25 2.5 10 180 0.5
30 3.0 – – –
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Total conversion ð%Þ ¼ Total weight of polymers formed 100 Weight of EPDM charged Grafting ratio ð%Þ ¼
Weight of EPDM-g-VOS 100 Weight of EPDM charged
Grafting efficiency ð%Þ ¼ Weight of EPDM-g-VOS 100 Total weight of polymers formed By using the optimum grafting efficiency of VOS onto EPDM obtained from solution (grafting) polymerisation technique, the EPDM-g-VOS was also prepared by melt mixing method using torque rheometer. The semicrystalline EPDM granules were coated with 0.2 wt.% DCP initiator dissolved in 2 wt.% vinyloxyaminosilane (VOS) and 0.1 wt.% dibutyltindilaurate catalyst. The treated polymer was processed in a Haake Rheocord-90, torque rheometer at 160 °C for 6 min in a mixer with rotation speed of 30 rpm. The pure elastomer (EPDM) test specimen required for characterisation was also processed in the same torque rheometer at 80 °C with 2 wt.% DCP.
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Japan) in nitrogen atmosphere over a temperature range between 20 and 600 °C at a heating rate of 20 °C per min. 2.8. Electrical properties Surface and volume resistivities of the samples were measured according to ASTM D-257-83 using spherical shaped test pieces (SIGMA MM87, Million Megaohm Meter) at 500 V for 60 s at 25 2 °C.
3. Results and discussion 3.1. The silane-grafting and hot water curing mechanism The overall crosslinking process involves two steps and are (i) grafting of VOS with EPDM and (ii) formation of stable Si–O–Si linkage through condensation of silane side chains in presence of dibutyltindilaurate and hot water. The tentative grafting mechanism of VOS onto EPDM using DCP initiator is presented in Fig. 1. It may
2.4. Moulding and crosslinking Both EPDM and EPDM-g-VOS were roll milled at 80 °C for 3 min. The milled samples were made into thin sheets by compression moulding with clamping pressure of 10 kg/cm2 at 180 °C for 10 min. Crosslinking of EPDM-g-VOS was carried out by immersing the compression moulded sheets in hot water at 100 °C for 2 h. 2.5. Spectral measurements The grafting reaction was confirmed by FTIR (Nicolet, IMPACT-400) spectroscopy. Spectra for EPDM and EPDM-g-VOS were obtained using compression moulded thin film samples and that of VOS was recorded using the liquid sample. 2.6. Mechanical properties Tensile strength of the samples was done at 25 2 °C according to ASTM D 412-87 method using dumb bell shaped test pieces at a cross head speed of 500 mm/min using an Universal Testing Machine (ZWICK-1484). The hardness of the samples was measured according to ASTM D 2240-86 and the results are expressed in Shore A units. 2.7. Thermal properties Thermogravimetric analysis was carried out in a thermogravimetric analyser (Seiko Instruments Inc.,
Fig. 1. Schematic representation of the graft modification reaction for EPDM.
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Fig. 2. Schematic representation of crosslinking mechanism of EPDM-g-VOS.
be explained that the grafting mechanism proceeds with the formation of radical centres in the EPDM skeleton through breaking the double bond in the third monomer (ENB in EPDM) by DCP and in ethylene by the abstraction of secondary hydrogen. Similarly the radical centre is generated by DCP in VOS by breaking C@C bond. Thus, the grafting of VOS onto EPDM occurs by the addition mechanism through coupling of these active radical centres. The tentative crosslinking mechanism of EPDM-g-VOS is presented in Fig. 2. The formation of network structure of EPDM-g-VOS in presence of hot water was accelerated by the addition of trace amount of dibutyltindilaurate catalyst. During the process of crosslinking the mixture of by-products methyl ethyl ketone and ammonium hydroxide are recovered. 3.2. Fourier-transform infrared spectroscopy Fig. 3 shows the FTIR spectra of EPDM, VOS and EPDM-g-VOS. The IR spectrum of EPDM (Fig. 3(a)) shows the C–H stretching vibration (aliphatic) at 2926 cm1 , –CH2 rocking vibration at 1457 cm1 , CH3 symmetric bending vibration at 1370 cm1 due to the presence of propylene group, –(CH2 )n – wagging vibration at 730 cm1 due to the presence of polyethylene chain, C–C stretching vibration at 2854 cm1 , and the unsaturation band (>C@CH–) at 815 cm1 due to the presence of ENB content. Fig. 3(b) presents the IR spectrum of
Fig. 3. IR spectra of (a) EPDM (b) VOS and (c) EPDM-gVOS.
VOS: C–H stretching vibration (vinyl) at 3265 cm1 , C@C stretching vibration at 1663 cm1 , Si–O stretching vibration at 1094 cm1 , >N@C stretching vibration at 1600 cm1 and –CH2 wagging vibration of Si–CH@CH2 – at 930 cm1 . Fig. 3(c) illustrates the IR spectrum of EPDM-g-VOS which reveals: C–H stretching vibration (aliphatic) at 2926 cm1 , –CH2 – rocking vibration at 1460 cm1 and –CH3 symmetric bending vibration at 1365 cm1 . However C–H stretching vibration (vinyl) at 3265 cm1 , C@C stretching vibration at 1663 cm1 and –CH2 – wagging vibration of Si–CH@CH2 at 930 cm1 are disappeared due to grafting of VOS with unsaturation or allylic positions present in the side chain of ENB termonomer at 815 cm1 without affecting the Si–O stretching vibration at 1094 cm1 . The above results clearly indicate that >C@CH– is utilised for new chemical bond formation with VOS.
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4. Effects of reaction conditions on grafting
4.3. Effect of reaction time
4.1. Effect of EPDM content
The effect of reaction time on the graft copolymerisation is shown in Fig. 6. The graft copolymerisation was carried out at 160 °C using 2 mol% of VOS and 10 mol% of EPDM. The grafting efficiency was not changed up to 10 h. However, the total conversion was increased with increasing time. This may be explained due to the increase in the formation of homopolymers along the reaction path.
The effect of EPDM content on the graft copolymerisation is shown in Fig. 4. The grafting reaction was carried out at 160 °C for 3 h with 2 mol% of VOS. The grafting efficiency increases linearly with increase in concentration of EPDM. For example, the grafting efficiency for 10 mol% EPDM is 19% and that of 30 mol% is 27%. At higher concentrations of EPDM, more active centers are generated and thus increases the grafting efficiency. On the other hand, fewer active centers [14] are created in the case of lower concentration of EPDM resulting in decreased grafting efficiency, which influences the enhanced homopolymerisation of VOS. 4.2. Effect of VOS concentration Fig. 5 shows the effect of concentration of VOS on graft copolymerisation onto EPDM. The grafting reaction was carried out at 160 °C for 3 h with 10 mol% of EPDM. The grafting efficiency increases with increase in concentration of VOS and attains the maximum value and then decreases. For example, the grafting efficiency observed using 1 mol% VOS is 20% while the highest grafting efficiency 27% is obtained when the reaction is carried out using 2 mol% VOS. On the other hand when the grafting reaction is carried out using 3 mol% VOS, the grafting efficiency decreases to 20.5%. The reason for decrease in grafting efficiency with increase in concentration of VOS after attaining the optimum value is due to the formation of more active centers in the vinyl part of VOS which favours the homopolymerisation of VOS rather than grafting.
4.4. Effect of reaction temperature The effect of reaction temperature on graft copolymerisation is shown in Fig. 7. The grafting reaction was carried out for 3 h with 10 mol% of EPDM, and 2 mol% VOS. The grafting reaction was carried out at 150, 160, 170 and 180 °C. The grafting efficiency increases with increase of reaction temperature due to the increase in the rate of decomposition of DCP. 4.5. Effect of initiator concentration Fig. 8 shows the effect of the initiator concentration on the graft copolymerisation. The grafting was carried out with various concentrations of initiator (DCP) at 160 °C for 3 h. The mol% of VOS was fixed at 2. The grafting efficiency increases with increase in initiator concentration and attains the maximum at 0.2 mol% and then decreases. The increase in initiator concentration favours the formation of more poly(vinyloxyaminosilane) homopolymer than the graft copolymer, and consequently there is a decrease in grafting efficiency.
Fig. 4. Plot of grafting efficiency against EPDM concentration (VOS: 2.00 mol%; reaction temperature: 160 °C; reaction time: 3 h; DCP: 0.2 mol%).
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Fig. 5. Plot of grafting efficiency against VOS concentration (reaction temperature: 160 °C; time: 3 h; DCP: 0.2 mol%).
Fig. 6. Plot of grafting efficiency against reaction time (VOS: 2 mol%; reaction temperature: 160 °C; DCP: 0.2 mol%).
Fig. 7. Plot of grafting efficiency against reaction temperature (VOS: 2 mol%; reaction time: 3 h; DCP: 0.2 mol%).
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Fig. 8. Plot of grafting efficiency against initiator concentration (VOS: 2 mol%; reaction temperature: 160 °C; reaction time: 3 h).
4.6. Torque curve Fig. 9 shows the torque curve obtained for EPDM-gVOS at 160 °C using Haake Rheocord-90, torque rheometer. Since at lower concentration of DCP, the grafting of VOS is dominating reaction than the cocrosslinking of EPDM in the presence of VOS and DCP [15]. Further, the increase in torque (viscosity) may be explained due to grafting of vinyloxyaminosilane to EPDM. 4.7. Mechanical properties The mechanical properties of DCP crosslinked EPDM and hot water cured EPDM-g-VOS are presented in Table 2. It is observed that tensile strength, Young’s modulus and hardness are decreased when VOS is grafted onto EPDM due to chain branching. The
Fig. 9. Torque curve of (a) EPDM and (b) EPDM-g-VOS.
Table 2 Mechanical properties of crosslinked EPDM and EPDM-gVOS Mechanical properties
EPDM
EPDM-g-VOS
Tensile strength (MPa) Elongation at break (%) Young’s modulus (MPa) Hardness (Shore A)
18.50 264 24 73
14.00 280 18.64 65
reduction in tensile strength, Young’s modulus and hardness are 24.1%, 22% and 11% respectively. However the elongation at break is increased to 6% due to high flexibility and free rotation imparted by Si–O linkages.
4.8. Thermal properties The effect of VOS grafting on thermal properties was determined from thermogravimetric analyser. Thermograms of peroxide cured EPDM and hot water cured EPDM-g-VOS are given in Figs. 10 and 11 respectively. The peroxide cured EPDM and hot water cured EPDMg-VOS have inception decomposition temperature of 415 and 429 °C respectively. The inception decomposition temperature of hot water cured EPDM-g-VOS is 3% higher than peroxide cured EPDM. Similarly the final decomposition temperatures of peroxide cured EPDM and hot water cured EPDM-g-VOS are 482 and 500 °C respectively. The final decomposition temperature of hot water cured EPDM-g-VOS is 4% higher than peroxide cured EPDM. The grafting of VOS onto EPDM increases both inception and final decomposition temperatures due to the formation of stable three dimensional network structure and the high bond energy of Si–O–Si linkages.
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resistivity to 14%, volume resistivity to 42% and arc resistance to 8% than ungrafted EPDM. However, the values of dielectric constant and loss factor are decreased to 25% and 37% respectively due to the inclusion of silane moiety.
5. Conclusion
Fig. 10. TG and DTG curves of peroxide cured EPDM.
EPDM-g-VOS graft terpolymer was synthesised by both solution polymerisation and melt mixing techniques. The following conclusions are made based on the data resulted from different experimental studies. The grafting of VOS onto EPDM and condensation of the silane side chains with the formation of stable Si–O– Si crosslinking in the presence of hot water have been confirmed from FTIR spectra. It was ascertained that the threshold grafting efficiency was maximum at 2 mol% of VOS. The grafting efficiency increases with increase of temperature and EPDM content and decreases with increase in initiator concentration. The grafting of VOS onto EPDM lowers mechanical properties and improves dielectric characteristics according to its percentage concentration. The data resulted from thermo-mechanical and dielectric studies suggest that the EPDM-gVOS elastomers could be used as low and medium voltage cable insulation for better performance than pure EPDM.
Acknowledgements
Fig. 11. TG and DTG curves of hot water cured EPDM-gVOS.
The authors thank Dr. V. Mohan, Dean of Technology, Dr. M. Rajendran, Professor & Head and Dr. A. Ashok kumar, Department of Chemical Engineering, Anna University, Chennai 600 025, India, for their help.
4.9. Electrical properties The electrical properties of peroxide cured EPDM and hot water cured EPDM-g-VOS are presented in Table 3. The grafting of VOS onto EPDM increases the surface
Table 3 Electrical properties of crosslinked EPDM and EPDM-g-VOS Electrical properties
EPDM
EPDM-g-VOS
Volume resistivity (Ohm cm) Surface resistivity (Ohm) Dielectric strength (kV/cm) Dielectric constant Loss factor Arc resistance (s)
2:8 1012
4:0 1012
4:5 1012
5:15 1012
620
760
2.7 0.0035 120
2.0 0.0022 130
References [1] Meredith CL, Barret RE, Bishop WH. US Patent 3,538,190. 1970. [2] Morimoto M, Sanijiki T, Horiiki H, Furuta M. US patent 3,876,730. 1975. [3] Whelan A, Lee KS. Rubber technology and rubber composites. London: Applied Science Publishers; 1985. [4] Mark HF, Bikales NM, Overberger CG, Menges G. Encyclopedia of polymer science and engineering. New York: Wiley; 1986. [5] Midland Silanes Ltd. US Patent 3,646,155. 1972. [6] Park DJ, Ka CS, Cho WJ. Synthesis and properties of vinyltrimethoxysilane–EPDM–styrene graft terpolymer. J Appl Polym Sci 1998;67:1345–52. [7] Gartasegna S. Silane grafted/moisture curable ethylene– propylene elastomers for the cable industry. Rubber Chem Technol 1986;59(5):722–39. [8] Tanida M, Sato Y. Japanese Patent JP 1,763,345. 1986.
ARTICLE IN PRESS M.S.C. Kumar, M. Alagar / European Polymer Journal 38 (2002) 2023–2031 [9] Bastin P, Delaunis G, Jules V, Jean J. Belgium Patent BE 905048. 1986. [10] Kawada T, Hikita M, Makino K. Japanese Patent JP 6,325,610. 1998. [11] Umeda I, Takemura Y, Watanabe J, Funahashi Y. Japanese patent JP 6,372,745. 1988. [12] King RE, Petty HE. European Patent EP 310129. 1989. [13] Kumar Sen A, Mukherjee S, Battacharya AS, De PP, Bhowmick AK. Kinetics of silane grafting and moisture
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crosslinking of polyethylene and ethylene propylene rubber. J Appl Polym Sci 1992;44(7):1153–64. [14] Seo B-D, Park D-J, Ha C-S, Cho W-J. Synthesis and properties of acrylonitrile–EPDM–N–vinylcarbazole graft terpolymer. J Appl Polym Sci 1995;58:1255–62. [15] Francis J, George KE, Joseph R. Chemical modification of blends of polyvinyl chloride with linear low density polyethylene. Eur Polym J 1992;28(10):1289– 93.
European Polymer Journal 38 (2002) 2023–2031 www.elsevier.com/locate/europolj
Development and characterisation of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) for engineering applications M.S.C. Kumar a, M. Alagar
b,*
a b
Department of Chemistry, Scott Christian College, Nagercoil 629 003, India Department of Chemical Engineering, Anna University, Chennai 600 025, India Received 23 October 2001; accepted 11 January 2002
Abstract A novel and new copolymer of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) has been synthesised in toluene using dicumyl peroxide as initiator. The grafting efficiency of vinyloxyaminosilane (VOS) onto ethylene-propylene-diene terpolymer (EPDM) has been studied as a function of EPDM content, reaction time, reaction temperature and initiator concentration. Using the optimum grafting efficiency conditions, EPDM-gVOS has also been developed in a Haake Rheocord-90, torque rheometer. The grafting of vinyloxyaminosilane onto ethylene-propylene-diene terpolymer (EPDM-g-VOS) has been confirmed by Fourier-transform infrared spectroscopy. The mechanical, thermal, and electrical, properties of hot water cured EPDM-g-VOS and peroxide cured EPDM are compared. The mechanical properties of EPDM-g-VOS are decreased due to the flexibility imparted by VOS. However thermal and dielectric properties are increased due to the introduction of VOS onto EPDM as well as the formation of thermally stable three dimensional network through Si–O–Si– linkage. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Grafting; EPDM-g-VOS; Mechanical, electrical and thermal properties
1. Introduction Crosslinked ethylene-propylene-diene terpolymer (EPDM) is one of the most commonly used industrial polymer because of its outstanding resistance to heat, light, oxygen and ozone [1–4]. Most of the curing processes adopted earlier for EPDM involve chemical crosslinking using peroxides. The advent of a newer technique, involving grafting of organofunctional silane onto polymer chain followed by its subsequent condensation reaction in presence of moisture is the most attractive and useful method due to number of advantages [5]. The technology of silane grafting on polymers
*
Corresponding author. E-mail address: [email protected] (M. Alagar).
is being successfully exploited on a large commercial scale production of crosslinked polyethylene for cable insulation and hot water piping systems [5]. Duck et al. [6] prepared vinyloxyaminosilane (VTMO) and styrene grafted EPDM using benzoyl peroxide as initiator in toluene. The heat resistance, light resistance and weather ability of VTMO-EPDM-Styrene grafted copolymer are found better than those of ABS. Gartasegna [7] reported the effects on the molecular and structural parameters of vinyloxyaminosilane (VTMO) grafting and moisture crosslinking on EPDM. The process of crosslinking is found to be limited due to the diene content of the polymer through probable condensation reactions of grafted silane. Tanida and Sato [8] prepared heat shrinkable tubes by grafting VTMO with EPDM. The process of crosslinking was done by using moisture and observed that the product exhibits high stretching ratios
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ene/5-ethylidene-2-norbornene ¼ 71/20/9 by wt.% with Mooney viscosity, MLð1þ4Þ , of 50 at 125 °C and specific gravity of 0.88) of Du Pont’Dow elastomers, USA. The vinyloxyaminosilane (VOS) (Mw ¼ 313, specific gravity of 0.988, viscosity ¼ 11:5 cP and b:p: ¼ 300 °C) was procured from Wacker-Chemie, Germany. DCP (90% assay with m:p: ¼ 30 °C) was obtained from Concord Chemical Industrial Co., Taiwan. Laboratory grade toluene, n-hexane and DMF were used as received.
above its softening point. Bastin et al. [9] synthesised VTMO grafted EPDM using Brabender mixer and measured the insoluble fractions. It was observed that VTMO grafted EPDM has about 20% higher insoluble fraction than pure EPDM. Kawada et al. [10] prepared composites using silica filler with silane grafted EPDM and EPDM. They found that the heat resistance of silane grafted EPDM was considerably higher than that of ungrafted EPDM. Umeda et al. [11] studied the processability and thermal stability of silane grafted EPDM and EPDM. They concluded that the silane grafted EPDM exhibits better processability and thermal stability than those of ungrafted EPDM. King and Petty [12] reported a new method of preparation of crosslinked EPDM using alkyl terminated siloxane in presence of rhodium hydrosilylation catalyst instead of hexachloroplatinic acid catalyst. Kumar Sen et al. [13] studied the kinetics of grafting of vinyloxyaminosilane (VTMO) and vinyltriethoxsilane (VTEO) onto EPDM using DSC. In the present study a new copolymer of vinyloxyaminosilane grafted ethylene-propylene-diene terpolymer (EPDM-g-VOS) has been synthesised in toluene using dicumyl peroxide (DCP) initiator. The effect of EPDM content, VOS concentration, reaction time, reaction temperature and initiator concentration were investigated in the graft copolymerisation. Using optimum grafting efficiency conditions obtained from solution grafting technique, EPDM-g-VOS has been developed in a Haake Rheocord-90, torque rheometer. The structure of EPDM-g-VOS has been ascertained by Fouriertransform infrared (FTIR) spectroscopy. The mechanical, thermal and electrical properties of hot water cured EPDM-g-VOS and peroxide cured EPDM are determined and compared in order to utilise them for some engineering applications.
2.2. Preparation of EPDM-g-VOS The grafting reactions were carried out in 500 ml three necked, round bottom flask equipped with a reflux condenser, a Teflon-coated magnetic stirring bar and a nitrogen inlet. The details of experimental conditions are presented in Table 1. A stoichiometric amount of EPDM was dissolved in 200 ml toluene and refluxed until complete dissolution of EPDM. Further, a specified concentration of VOS dissolved in 50 ml of toluene and varying concentrations of DCP were added to EPDM. After the completion of reaction, the products were precipitated with methanol, filtered and dried in vacuum. 2.3. Isolation of graft terpolymer During the synthesis, the products obtained consists of ungrafted EPDM, poly(vinyloxyaminosilane) and EPDM-g-VOS. The ungrafted EPDM was separated from rest of the products by extraction using n-hexane. The vinyloxyaminosilane grafted EPDM (EPDM-gVOS) was separated from poly(vinyloxyaminosilane) using DMF where the former is soluble and latter is insoluble. The total conversion was calculated from the ratio of the total weight of crude products to the weight of EPDM charged. The grafting ratio and grafting efficiency were determined on the basis of changes in polymer weight during the reaction process and the total amount of EPDM-g-VOS formed respectively. The grafting ratio and grafting efficiency were calculated from the following equations:
2. Experimental 2.1. Materials The EPDM (ENB) elastomer used in this study was a commercial grade (Nordel IP 5750R) (ethylene/propyl-
Table 1 Conditions of graft copolymerisation Condition
Description
EPDM concentration (mol%) VOS concentration (mol%) Reaction time (h) Reaction temperature (°C) Initiator concentration (mol%) (based on the vinyl monomer)
10 1.0 3 150 0.1
15 1.5 5 160 0.2
20 2.0 7 170 0.3
25 2.5 10 180 0.5
30 3.0 – – –
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Total conversion ð%Þ ¼ Total weight of polymers formed 100 Weight of EPDM charged Grafting ratio ð%Þ ¼
Weight of EPDM-g-VOS 100 Weight of EPDM charged
Grafting efficiency ð%Þ ¼ Weight of EPDM-g-VOS 100 Total weight of polymers formed By using the optimum grafting efficiency of VOS onto EPDM obtained from solution (grafting) polymerisation technique, the EPDM-g-VOS was also prepared by melt mixing method using torque rheometer. The semicrystalline EPDM granules were coated with 0.2 wt.% DCP initiator dissolved in 2 wt.% vinyloxyaminosilane (VOS) and 0.1 wt.% dibutyltindilaurate catalyst. The treated polymer was processed in a Haake Rheocord-90, torque rheometer at 160 °C for 6 min in a mixer with rotation speed of 30 rpm. The pure elastomer (EPDM) test specimen required for characterisation was also processed in the same torque rheometer at 80 °C with 2 wt.% DCP.
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Japan) in nitrogen atmosphere over a temperature range between 20 and 600 °C at a heating rate of 20 °C per min. 2.8. Electrical properties Surface and volume resistivities of the samples were measured according to ASTM D-257-83 using spherical shaped test pieces (SIGMA MM87, Million Megaohm Meter) at 500 V for 60 s at 25 2 °C.
3. Results and discussion 3.1. The silane-grafting and hot water curing mechanism The overall crosslinking process involves two steps and are (i) grafting of VOS with EPDM and (ii) formation of stable Si–O–Si linkage through condensation of silane side chains in presence of dibutyltindilaurate and hot water. The tentative grafting mechanism of VOS onto EPDM using DCP initiator is presented in Fig. 1. It may
2.4. Moulding and crosslinking Both EPDM and EPDM-g-VOS were roll milled at 80 °C for 3 min. The milled samples were made into thin sheets by compression moulding with clamping pressure of 10 kg/cm2 at 180 °C for 10 min. Crosslinking of EPDM-g-VOS was carried out by immersing the compression moulded sheets in hot water at 100 °C for 2 h. 2.5. Spectral measurements The grafting reaction was confirmed by FTIR (Nicolet, IMPACT-400) spectroscopy. Spectra for EPDM and EPDM-g-VOS were obtained using compression moulded thin film samples and that of VOS was recorded using the liquid sample. 2.6. Mechanical properties Tensile strength of the samples was done at 25 2 °C according to ASTM D 412-87 method using dumb bell shaped test pieces at a cross head speed of 500 mm/min using an Universal Testing Machine (ZWICK-1484). The hardness of the samples was measured according to ASTM D 2240-86 and the results are expressed in Shore A units. 2.7. Thermal properties Thermogravimetric analysis was carried out in a thermogravimetric analyser (Seiko Instruments Inc.,
Fig. 1. Schematic representation of the graft modification reaction for EPDM.
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Fig. 2. Schematic representation of crosslinking mechanism of EPDM-g-VOS.
be explained that the grafting mechanism proceeds with the formation of radical centres in the EPDM skeleton through breaking the double bond in the third monomer (ENB in EPDM) by DCP and in ethylene by the abstraction of secondary hydrogen. Similarly the radical centre is generated by DCP in VOS by breaking C@C bond. Thus, the grafting of VOS onto EPDM occurs by the addition mechanism through coupling of these active radical centres. The tentative crosslinking mechanism of EPDM-g-VOS is presented in Fig. 2. The formation of network structure of EPDM-g-VOS in presence of hot water was accelerated by the addition of trace amount of dibutyltindilaurate catalyst. During the process of crosslinking the mixture of by-products methyl ethyl ketone and ammonium hydroxide are recovered. 3.2. Fourier-transform infrared spectroscopy Fig. 3 shows the FTIR spectra of EPDM, VOS and EPDM-g-VOS. The IR spectrum of EPDM (Fig. 3(a)) shows the C–H stretching vibration (aliphatic) at 2926 cm1 , –CH2 rocking vibration at 1457 cm1 , CH3 symmetric bending vibration at 1370 cm1 due to the presence of propylene group, –(CH2 )n – wagging vibration at 730 cm1 due to the presence of polyethylene chain, C–C stretching vibration at 2854 cm1 , and the unsaturation band (>C@CH–) at 815 cm1 due to the presence of ENB content. Fig. 3(b) presents the IR spectrum of
Fig. 3. IR spectra of (a) EPDM (b) VOS and (c) EPDM-gVOS.
VOS: C–H stretching vibration (vinyl) at 3265 cm1 , C@C stretching vibration at 1663 cm1 , Si–O stretching vibration at 1094 cm1 , >N@C stretching vibration at 1600 cm1 and –CH2 wagging vibration of Si–CH@CH2 – at 930 cm1 . Fig. 3(c) illustrates the IR spectrum of EPDM-g-VOS which reveals: C–H stretching vibration (aliphatic) at 2926 cm1 , –CH2 – rocking vibration at 1460 cm1 and –CH3 symmetric bending vibration at 1365 cm1 . However C–H stretching vibration (vinyl) at 3265 cm1 , C@C stretching vibration at 1663 cm1 and –CH2 – wagging vibration of Si–CH@CH2 at 930 cm1 are disappeared due to grafting of VOS with unsaturation or allylic positions present in the side chain of ENB termonomer at 815 cm1 without affecting the Si–O stretching vibration at 1094 cm1 . The above results clearly indicate that >C@CH– is utilised for new chemical bond formation with VOS.
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4. Effects of reaction conditions on grafting
4.3. Effect of reaction time
4.1. Effect of EPDM content
The effect of reaction time on the graft copolymerisation is shown in Fig. 6. The graft copolymerisation was carried out at 160 °C using 2 mol% of VOS and 10 mol% of EPDM. The grafting efficiency was not changed up to 10 h. However, the total conversion was increased with increasing time. This may be explained due to the increase in the formation of homopolymers along the reaction path.
The effect of EPDM content on the graft copolymerisation is shown in Fig. 4. The grafting reaction was carried out at 160 °C for 3 h with 2 mol% of VOS. The grafting efficiency increases linearly with increase in concentration of EPDM. For example, the grafting efficiency for 10 mol% EPDM is 19% and that of 30 mol% is 27%. At higher concentrations of EPDM, more active centers are generated and thus increases the grafting efficiency. On the other hand, fewer active centers [14] are created in the case of lower concentration of EPDM resulting in decreased grafting efficiency, which influences the enhanced homopolymerisation of VOS. 4.2. Effect of VOS concentration Fig. 5 shows the effect of concentration of VOS on graft copolymerisation onto EPDM. The grafting reaction was carried out at 160 °C for 3 h with 10 mol% of EPDM. The grafting efficiency increases with increase in concentration of VOS and attains the maximum value and then decreases. For example, the grafting efficiency observed using 1 mol% VOS is 20% while the highest grafting efficiency 27% is obtained when the reaction is carried out using 2 mol% VOS. On the other hand when the grafting reaction is carried out using 3 mol% VOS, the grafting efficiency decreases to 20.5%. The reason for decrease in grafting efficiency with increase in concentration of VOS after attaining the optimum value is due to the formation of more active centers in the vinyl part of VOS which favours the homopolymerisation of VOS rather than grafting.
4.4. Effect of reaction temperature The effect of reaction temperature on graft copolymerisation is shown in Fig. 7. The grafting reaction was carried out for 3 h with 10 mol% of EPDM, and 2 mol% VOS. The grafting reaction was carried out at 150, 160, 170 and 180 °C. The grafting efficiency increases with increase of reaction temperature due to the increase in the rate of decomposition of DCP. 4.5. Effect of initiator concentration Fig. 8 shows the effect of the initiator concentration on the graft copolymerisation. The grafting was carried out with various concentrations of initiator (DCP) at 160 °C for 3 h. The mol% of VOS was fixed at 2. The grafting efficiency increases with increase in initiator concentration and attains the maximum at 0.2 mol% and then decreases. The increase in initiator concentration favours the formation of more poly(vinyloxyaminosilane) homopolymer than the graft copolymer, and consequently there is a decrease in grafting efficiency.
Fig. 4. Plot of grafting efficiency against EPDM concentration (VOS: 2.00 mol%; reaction temperature: 160 °C; reaction time: 3 h; DCP: 0.2 mol%).
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Fig. 5. Plot of grafting efficiency against VOS concentration (reaction temperature: 160 °C; time: 3 h; DCP: 0.2 mol%).
Fig. 6. Plot of grafting efficiency against reaction time (VOS: 2 mol%; reaction temperature: 160 °C; DCP: 0.2 mol%).
Fig. 7. Plot of grafting efficiency against reaction temperature (VOS: 2 mol%; reaction time: 3 h; DCP: 0.2 mol%).
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Fig. 8. Plot of grafting efficiency against initiator concentration (VOS: 2 mol%; reaction temperature: 160 °C; reaction time: 3 h).
4.6. Torque curve Fig. 9 shows the torque curve obtained for EPDM-gVOS at 160 °C using Haake Rheocord-90, torque rheometer. Since at lower concentration of DCP, the grafting of VOS is dominating reaction than the cocrosslinking of EPDM in the presence of VOS and DCP [15]. Further, the increase in torque (viscosity) may be explained due to grafting of vinyloxyaminosilane to EPDM. 4.7. Mechanical properties The mechanical properties of DCP crosslinked EPDM and hot water cured EPDM-g-VOS are presented in Table 2. It is observed that tensile strength, Young’s modulus and hardness are decreased when VOS is grafted onto EPDM due to chain branching. The
Fig. 9. Torque curve of (a) EPDM and (b) EPDM-g-VOS.
Table 2 Mechanical properties of crosslinked EPDM and EPDM-gVOS Mechanical properties
EPDM
EPDM-g-VOS
Tensile strength (MPa) Elongation at break (%) Young’s modulus (MPa) Hardness (Shore A)
18.50 264 24 73
14.00 280 18.64 65
reduction in tensile strength, Young’s modulus and hardness are 24.1%, 22% and 11% respectively. However the elongation at break is increased to 6% due to high flexibility and free rotation imparted by Si–O linkages.
4.8. Thermal properties The effect of VOS grafting on thermal properties was determined from thermogravimetric analyser. Thermograms of peroxide cured EPDM and hot water cured EPDM-g-VOS are given in Figs. 10 and 11 respectively. The peroxide cured EPDM and hot water cured EPDMg-VOS have inception decomposition temperature of 415 and 429 °C respectively. The inception decomposition temperature of hot water cured EPDM-g-VOS is 3% higher than peroxide cured EPDM. Similarly the final decomposition temperatures of peroxide cured EPDM and hot water cured EPDM-g-VOS are 482 and 500 °C respectively. The final decomposition temperature of hot water cured EPDM-g-VOS is 4% higher than peroxide cured EPDM. The grafting of VOS onto EPDM increases both inception and final decomposition temperatures due to the formation of stable three dimensional network structure and the high bond energy of Si–O–Si linkages.
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resistivity to 14%, volume resistivity to 42% and arc resistance to 8% than ungrafted EPDM. However, the values of dielectric constant and loss factor are decreased to 25% and 37% respectively due to the inclusion of silane moiety.
5. Conclusion
Fig. 10. TG and DTG curves of peroxide cured EPDM.
EPDM-g-VOS graft terpolymer was synthesised by both solution polymerisation and melt mixing techniques. The following conclusions are made based on the data resulted from different experimental studies. The grafting of VOS onto EPDM and condensation of the silane side chains with the formation of stable Si–O– Si crosslinking in the presence of hot water have been confirmed from FTIR spectra. It was ascertained that the threshold grafting efficiency was maximum at 2 mol% of VOS. The grafting efficiency increases with increase of temperature and EPDM content and decreases with increase in initiator concentration. The grafting of VOS onto EPDM lowers mechanical properties and improves dielectric characteristics according to its percentage concentration. The data resulted from thermo-mechanical and dielectric studies suggest that the EPDM-gVOS elastomers could be used as low and medium voltage cable insulation for better performance than pure EPDM.
Acknowledgements
Fig. 11. TG and DTG curves of hot water cured EPDM-gVOS.
The authors thank Dr. V. Mohan, Dean of Technology, Dr. M. Rajendran, Professor & Head and Dr. A. Ashok kumar, Department of Chemical Engineering, Anna University, Chennai 600 025, India, for their help.
4.9. Electrical properties The electrical properties of peroxide cured EPDM and hot water cured EPDM-g-VOS are presented in Table 3. The grafting of VOS onto EPDM increases the surface
Table 3 Electrical properties of crosslinked EPDM and EPDM-g-VOS Electrical properties
EPDM
EPDM-g-VOS
Volume resistivity (Ohm cm) Surface resistivity (Ohm) Dielectric strength (kV/cm) Dielectric constant Loss factor Arc resistance (s)
2:8 1012
4:0 1012
4:5 1012
5:15 1012
620
760
2.7 0.0035 120
2.0 0.0022 130
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