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EPDM blends
European Polymer Journal 39 (2003) 2283–2290 www.elsevier.com/locate/europolj
The effect of mercapto- and thioacetate-modified EPDM on the curing parameters and mechanical properties of natural rubber/EPDM blends Alex S. Sirqueira, Bluma G. Soares
*
Centro de Tecnologia, Instituto de Macromol eculas, Universidade Federal do Rio de Janeiro, Bloco J, Ilha do Fund~ ao, P.O. Box 68525, 21945-970 Rio de Janeiro, RJ, Brazil Received 30 April 2003; received in revised form 17 July 2003; accepted 18 July 2003
Abstract The vulcanization characteristics of natural rubber (NR)/ethylene–propylene–ethylidenenorbornene (EPDM) rubber blends were studied in the presence of thioacetate-(EPDMTA) or mercapto-modified EPDM (EPDMSH), using oscillating disk rheometer. The effect of both functionalized EPDMs was investigated in unaccelerated-sulfur curing system and accelerated-sulfur curing systems containing 0.4 and 0.8 phr of MBTS. Both EPDMTA and EPDMSH act as accelerator agent in the curing process, as indicated by the higher values of cure rate index and lower values of activation energy of vulcanization. A substantial increase of the crosslink density has been also observed in EPDMSHmodified blends. Both EPDMTA and EPDMSH resulted in an increase in tensile strength, but the best performance has been achieved with EPDMSH, probably because of the increase of crosslink density associated to the reactive compatibilization promoted by the reaction between mercapto groups and rubber matrix. The best ageing resistance has been observed in EPDMTA-modified blends. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Elastomer blends; Compatibilization; Natural rubber; Curing parameters
1. Introduction Blending two or more elastomers is carried out for specific objectives such as enhancement of technical properties, improvement of ageing resistance and also processing characteristics. Natural rubber (NR) is normally blended with ethylene–propylene–diene rubber (EPDM) to improve ageing resistance of the former without loosing its good mechanical properties. Due to the difference in unsaturation level between these components, a mutual incompatibility can exist, which contributes for a decreasing of mechanical performance.
*
Corresponding author. Tel.: +55-21-256-27207; fax: +5521-227-01317. E-mail address: [email protected] (B.G. Soares).
In addition to the poor interfacial adhesion caused by the thermodynamic incompatibility, these blends usually present cure rate incompatibility because of the differences between the reactivity of the elastomers with the curing agents and/or differences in solubilities of the curatives in each elastomer phase [1,2]. In the case of NR/EPDM blends, the curing system can be consumed by the vulcanization of the NR phase, which is more rapidly vulcanizable because of the higher unsaturation level [2,3]. Several strategies have been reported to improve the compatibility of NR/EPDM blends and include the addition of a third polymeric component of low molecular weight [4], the incorporation of an accelerator moiety in the less unsaturated phase [5], and the functionalization of EPDM with maleic anhydride [6–8]. In our previous work, a new reactive compatibilizing agent based on mercapto-modified EPDM (EPDMSH)
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00163-0
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has been successfully employed in NR/EPDM blends [9]. The mercapto groups react with the double bonds of the NR phase and help to decrease the interfacial energy. Better mechanical performance has been achieved by using as low as 2.5 phr of this functionalized copolymer. In addition, the presence of EPDMSH has significantly affected the curing parameters, that is, it decreased both scorch time and optimum curing time, indicating an accelerator effect of the curing process. The catalytic effect of thiol groups on accelerated sulfur vulcanization systems in rubber has been also observed by other authors [10], using silica modified with thiol groups in NR systems and also by our group in studies involving the compatibilization of NBR/EPDM with similar mercapto-modified EPDM [11]. The preparation of mercapto-modified EPDM has been recently reported by our group [12] and involves two steps, illustrated in Fig. 1. The first one consists of a free radical addition of thioacetic acid on the double bond of EPDM. The resulting thioacetatemodified EPDM (EPDMTA) is hydrolyzed to produce EPDMSH. Both functionalized EPDMs may influence on the vulcanization process and on the mechanical properties. The objective of the present work was to study the effect of both functionalized EPDMs (EPDMTA and EPDMSH) on the curing parameters of NR/EPDM blends vulcanized with an accelerated-sulfur system. For this purpose, 2,20 -dithiobisbenzothiazole (MBTS) was employed as a conventional accelerator, whose proportion in the rubber formulations has been varied, in order to observe the performance of these functionalized copolymers, not only as a compatibilizing agent but also as a secondary accelerator. Apparent activation energy for the vulcanization of the rubber blends was calculated from experiments involving oscillating disk rheometer. The effect of EPDMTA and EPDMSH on mechanical properties, crosslink density and ageing resistance of the vulcanizates has been also investigated. A NR/EPDM weight ratio corresponding to 70:30 has been chosen because at this composition, it is possible to maintain the good mechanical properties of natural rubber while improving the ageing resistance of the rubber material.
2. Experimental 2.1. Materials Natural rubber (NR, Hervea Brasiliensis), weightaverage molecular weight (Mw ) ¼ 207,000, was kindly supplied by Michelin do Brasil S.A. Ethylene–propylene–ethylidenenorbornene (EPDM) rubber samples with two different compositions were kindly supplied by DSM Elastomeros do Brasil S. A. Table 1 summarizes the characteristics of the EPDM samples. Zinc oxide, stearic acid, sulfur, irganox 245 (primary antioxidant–– (pentaerythrityl tetrakis) (3,5-di-tert-butyl(4-hydroxyphenyl)) propionate) and MBTS were of laboratory reagent grade and kindly supplied by the local rubber industries. Thioacetic acid (TAA), analytical grade, from Sigma Chemistry, was used as received and azobisisobutyronitrile (AIBN) from Merck S. A., was recrystallized from methanol/water solution. 2.2. Functionalization of EPDM The preparation of the functionalized EPDM was carried out, according to previous report [12], using EP65 grade, which contains higher amount of the diene component. A higher insaturation level of the EPDM grade is profitable to obtain functionalized EPDM with higher amount of reactive groups. The functionalization has taken place in two steps (see Fig. 1). The first step Table 1 Characteristics of EPDM samples employed in this study Commercial grade
EP65
EP57C
Weight-average molecular weight from SECa Density (g/cm3 ) Iodine index Diene content (mmol/100 g) Ethylene/propylene weight ratio Mooney viscosity (ML1 + 4 at 100 °C)
150,000
80,000
0.837 29 114 60/40
0.813 15 58.9 73/27
77
90
a
A size exclusion chromatography.
Fig. 1. Scheme for the functionalization of EPDM.
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was performed in toluene solution at 70 °C for 48 h. The TAA/AIBN and diene/TAA molar ratios were established as 10.0 and 1.0, respectively, in order to avoid crosslink during the synthesis. The thioacetatemodified EPDM (EPDMTA) was submitted to an alkaline methanolysis using 5 wt% solution of NaOH. At these conditions, an amount of thioacetate or mercapto groups in the functionalized copolymers corresponding to 2.5 mmol/g has been achieved. 2.3. Blend preparation The EPDM grade used as the blend component was EP57C, which contains lower unsaturation level and, therefore, is indicated for ageing resistance purpose. The blends were prepared in a two roll-mill operating at 80 °C and at 20 rpm. NR was masticated for 2 min and then EPDM and the functionalized compatibilizing (EPDMTA or EPDMSH) were subsequently added. After the homogenization of the rubber blend (for about 4 min), the other ingredients were added in the following order: zinc oxide (5.0 phr), stearic acid (1.5 phr), irganox 245 (1.0 phr), sulfur (S) (2.5 phr) and 2,20 -dithiobisbenzothiazole (MBTS) (variable). The processing time after each component addition was about 2 min.
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fraction of the polymer in a swollen mass was calculated by the Eq. (1): Vr ¼
w2 ðdr Þ1 w2 ðdr Þ
1
þ ðw1 w2 Þ ðds Þ1
ð1Þ
where dr and ds are the rubber and solvent densities, respectively. The insoluble material in nonvulcanized and vulcanized blends was determined by submitting a sample of about 1 g to a heating in xylene at 120 °C for 24 h. The dynamic mechanical properties of the vulcanizates were determined using a dynamic mechanical analyzer, Rheometric Scientific MKIII, operating in a bending mode at a frequency of 1 Hz and amplitude of 0.1%. The temperature was increased at the heating rate of 2 °C/min in the range of )100 to 20 °C. The molecular weight of the polymers were determined by size exclusion chromatography (SEC) using a Water 600 liquid chromatograph equipped with refractive index detector and three ultrastyragel columns: (two porosity). The molecular weight was liner and one 500 A determined from calibration curve using polystyrene standards.
3. Results and discussion 2.4. Rheometric measurements and testing 3.1. Vulcanization characteristics The vulcanization parameters of the mixes were measured on an oscillating disk rheometer (ODR) (Tecnologia Industrial, mod T100) at 1 arc degree, according to ASTM D-2084-81 method. The kinetic parameters of the vulcanization process, such as rate constant ðkÞ and apparent activation energy (Ea ), were calculated from the torque–time curves taken from experiments performed at 150, 160 and 170 °C. For tensile testing, the blends were vulcanized up to the optimum cure time, in a hydraulic press at 160 °C and 1500 lb in2 . (The optimum cure time ðt90 Þ was established as the necessary time to reach 90% of the maximum torque.) Then, dumbbell shaped tensile test specimens (ASTM 638- testing number 5) were punched out of the compression molded sheets. Tensile testing was performed on an Instron 4204 Universal Testing Machine, at a crosshead speed of 100 mm min1 . The experiments were performed before and after ageing the specimens in an air-circulating oven at 70 °C for 3 days. Crosslink densities of the blends were obtained from swelling experiments in toluene. Cured test pieces of 20 20 2 mm dimension were accurately weighted ðwÞ and immersed in toluene in closed bottles for 7 days. Then, the surfaces were dried with filter paper and the samples were quickly weighed ðw1 Þ. The swollen samples were then dried completely at 60 °C for 48 h under reduced pressure and weighted again ðw2 Þ. The volume
The vulcanization parameters of compatibilized and noncompatibilized blends, cured with different amounts of MBTS single accelerator are listed in Table 2. The effect of MBTS content on the rheograph profiles obtained at 160 °C is illustrated in Fig. 2. The presence of higher amount of MBTS (0.8 phr) in noncompatibilized blend resulted in an increase of scorch safety, which is expected since MBTS is classified as a delayed-action semi-ultra accelerator [13]. This behavior is observed in all temperatures studied but the difference on scorch time in the two MBTS concentration becomes smaller as the cure temperature increases. Blends containing 2.5 phr of EPDMTA also presented similar behavior in experiments performed at 150 and 160 °C, but the difference on scorch safety was smaller as compared to noncompatibilized blend. At 170 °C, a better scorch safety was achieved with lower amount of MBTS. In blends compatibilized with EPDMSH, a higher scorch time with higher amount of MBTS was only observed in experiments performed at lower temperature (150 °C). As indicated in Table 2, the scorch time of blends containing lower amount of MBTS did not change significantly with the presence of EPDMTA or EPDMSH. However, at higher amount of MBTS, the effect of both functionalized EPDMs was significant, indicating an accelerated action of the
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Table 2 Vulcanization parametersa of NR/EPDM blends as a function of compatibilization and MBTS content Temperature (°C) 150 b
160
170
c
d
b
c
d
b
c
d
S/MBTS (0.4 phr) 15.7 MH (lb in1 ) ML (lb in1 ) 1.4 ts1 (min) 4.8 t90 (min) 17.8
18.8 1.5 5.0 17.9
17.9 1.7 4.8 16.0
16.2 1.3 3.0 12.8
18.8 2.3 3.0 11.4
19.6 1.3 2.8 10.1
15.2 1.3 2.2 7.3
17.7 2.0 2.0 6.5
16.4 1.5 2.0 5.7
S/MBTS (0.8 phr) 17.5 MH (lb in1 ) ML (lb in1 ) 2.1 ts1 (min) 7.0 t90 (min) 15.3
17.1 2.6 5.7 17.9
20.0 2.2 5.1 15.1
17.1 2.0 4.0 8.4
19.5 1.5 3.2 8.6
20.1 2.3 2.3 7.8
16.5 1.9 2.5 5.1
18.7 1.7 1.1 4.4
18.3 2.4 1.8 4.7
a
t90 ––optimum cure time; ts1 ––scorch time; ML ––minimum torque; MH ––maximum torque. Blend without compatibilizer. c Blends with 2.5 phr of EPDMTA. d Blends with 2.5 phr of EPDMSH. b
mercapto or thioacetate groups when combined with higher amount of MBTS. This behavior is better observed in Fig. 3, where the rheographs of compatibilized and noncompatibilized blends are compared. As stated in several papers [13–15], during the scorch delay period or induction period (first region of a curometer curve), the majority accelerator chemistry takes place. Therefore, one can assume the participation of mercapto and thioacetate groups in EPDMSH and EPDMTA, respectively, on the chemical reactions that occur in this stage. The more effective participation is observed for the mercapto groups. The presence of EPDMTA or EPDMSH also resulted in a substantial increase of maximum torque ðMH Þ, which is normally related to the crosslink density. It is interesting to observe that, for blends vulcanized at 160 °C, the MH values achieved in compatibilized blends containing 0.4 phr of MBTS were even higher than that found in noncompatibilized blend containing higher amount of MBTS, indicating a strong influence of these functionalized copolymers on the crosslink density. Concerning the rheometric experiments carried out at 160 °C, one can observe that, in addition to the accelerating action, the increase of MH values was more pronounced in blends containing EPDMSH, indicating a more effective participation of mercapto groups than thioacetate groups on the vulcanization process. The accelerating action of the functionalized EPDMs was also evaluated in blends compounded with unaccelerated-sulfur formulations, that is, without the presence of MBTS. Table 3 presents the curing parameters of these blends, measured at different temperatures. The maximum torque values in all blends were low, indicating poor crosslink density. EPDMSH resulted in a slight increase of MH . Both EPDMTA and EPDMSH
resulted in a decrease of scorch time and optimum cure time in experiments performed at higher temperatures, confirming the accelerating action of both functionalized EPDMs, even in absence of MBTS. The cure rate index of these blends vulcanized in unaccelerated-sulfur system was calculated, according to Eq. (2) [16]: CRI ¼
100 t90 ts1
ð2Þ
where t90 is the optimum cure time and ts1 is the scorch time. The effect of the functionalized compounds on the cure rate index is more pronounce at 180 °C. At this temperature, a substantial increase of the CRI values has been observed, mainly in blends containing EPDMSH, confirming the higher reactivity of this group on the vulcanization process. The effect of the functionalized EPDMs on the reversion phenomenon, which occurs at the end of the curing process in the rheometer, is summarized in Table 4, for accelerated-sulfur vulcanizing systems. The reversion tax was calculated, according to Eq. (3) [17]: Reversion tax ¼
MH MHþ30 min MH
100
ð3Þ
where MH is the maximum torque and MHþ30 min is the torque 30 min after maximum torque occurs. The reversion tendency was only observed at higher temperature (160 and 170 °C). At 160 °C, only blends formulated with higher amount of MBTS presented reversion tendency, whereas at 170 °C, one can distinguish the reversion process in the two MBTS concentration
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Fig. 3. Effect of the compatibilization on the rheograph profiles of vulcanized NR/EPDM blends: (b) noncompatibilized; compatibilized with (c) EPDMTA and (d) EPDMSH.
3.2. Kinetic parameters of vulcanization
Fig. 2. Effect of the MBTS concentration on the curing parameters of 70:30 blends, obtained at 160 °C; MBTS content ¼ (a) 0.4 phr and (b) 0.8 phr.
employed. The addition of EPDMTA or EPDMSH resulted in a considerable decrease of the reversion tax, indicating an improvement on thermal stability and a lower dissociation of sulfur crosslink. Normally, natural rubber shows rapid curing and high reversion effect because of its high content of allylic hydrogens. Because all allylic hydrogens are not used for crosslinking, the remaining allyls may provide site to be radicals, which cause dissociation of sulfur crosslinks. The good protection effect against reversion achieved with EPDMTA or EPDMSH may be attributed to the ability of the corresponding thioacetate or mercapto groups in reacting with these radicals before they have time to promote the sulfur crosslinks dissociation.
The kinetic equation that describes the vulcanization process can be obtained from the torque–time values of the rheographs [15,18–21]. If the process is considered as a first-order reaction, it would be expressed by Eq. (4), as follows: MH ML ln ¼ kt ð4Þ MH Mt where MH , ML and Mt are the maximum torque, the minimum torque and the torque at a cure time t and k is the vulcanization kinetic constant. Because the rate of the first stage, associated to the rate of conversion of the cure complex, reflects the character of the main forward reaction kinetically, M values between 25 and 45% of torque changes were chosen to estimate the rate constants in this work [21]. By plotting lnðMH Mt Þ against t, a straight line graph is obtained, indicating that the cure reaction follows first-order kinetic. Theory and practice show that the vulcanization rate depends on the reciprocal temperature. So, it is possible to use the Arrhenius equation to express this dependence, according to Eq. (5):
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Table 3 Vulcanization parametersa of NR/EPDM blends vulcanized with unaccelerated-sulfur system Temperature (°C) 150 1
MH (lb in ) ML (lb in1 ) ts1 (min) t90 (min) CRI (min1 )
160
170
180
b
c
d
b
c
d
b
c
d
b
c
d
4.8 3.0 4.8 23 5.5
4.1 2.1 4.3 21 5.8
5.3 2.9 3.6 21 5.6
6.7 2.8 2.6 21 5.4
6.3 2.0 3.0 21 5.5
7.8 2.8 2.0 21 5.3
9.2 2.8 2.6 19 6.2
9.3 1.8 2.0 17 6.7
9.6 2.5 1.8 17 6.6
9.2 2.8 2.0 13 9.1
9.8 1.8 1.8 10 12.2
9.9 2.5 1.6 9 14.3
a
t90 ––optimum cure time; ts1 ––scorch time; ML ––minimum torque; MH –maximum torque. Blend without compatibilizer. c Blends with 2.5 phr of EPDMTA. d Blends with 2.5 phr of EPDMSH. b
Table 4 Effect of the compatibilization and cure temperature on the reversion tax of NR/EPDM blends obtained from ODR experiments Compatibilizer
Temperature (°C) 150
None EPDMTA (2.5 phr) EPDMSH (2.5 phr) a b
160
170
a
b
a
b
a
b
– –
– –
– –
6.0 4.0
28.0 10.0
15.0 12.0
–
–
–
3.0
2.0
7.0
Sulfur curing system containing 0.4 phr of MBTS. Sulfur curing system containing 0.8 phr of MBTS.
k ¼ A exp
Ea RT
ð5Þ
where A is the pre-exponential factor, R represents the gas constant, T is the absolute temperature and Ea is the apparent activation energy for the curing process. The values of the rate constants and activation energy of the cure reaction of all blend formulations are given in Table 5. In all curing conditions and in both S/MBTS curing systems, the rate constant values increase in this order: Noncompatibilized blends < blends with EPDMTA < blends with EPDMSH Therefore, the Ea decreases in the same order. Also the cure rate index (CRI), obtained from Eq. (1), increases considerably with the presence of EPDMTA and EPDMSH, in this order, for all curing conditions and in both curing systems. All these results confirm the accelerating action of both functionalized EPDMs. 3.3. Mechanical properties and crosslink density
or ln k ¼ ln A
Ea 1 R T
ð6Þ
These properties were determined from samples vulcanized at 160 °C. Table 6 presents the values of
Table 5 Kinetic parameters of vulcanization of NR/EPDM blends Compatibilizer
CRIa (min1 ) 150 °C
Rate constant,b k (min1 )
Ea c (kJ/mol)
160 °C
170 °C
150 °C
160 °C
170 °C
S/MBTS (0.4 phr) None EPDMTA EPDMSH
7.73 7.73 9.00
10.24 11.90 13.53
19.72 22.00 26.95
0.204 0.261 0.299
0.289 0.365 0.502
0.526 0.536 0.658
73.63 61.52 58.74
S/MBTS (0.8 phr) None EPDMTA EPDMSH
12.05 8.19 10.0
22.73 18.52 18.45
38.91 29.76 34.50
0.263 0.266 0.329
0.534 0.517 0.659
0.643 0.752 0.887
82.10 77.52 69.06
a
Calculated from Eq. (1). Calculated from Eq. (4). c Calculated from Eq. (6). b
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Table 6 Mechanical properties and crosslink density of NR/EPDM blends as functions of compatibilization and amount of accelerator Compatibilizer
rB a (MPa)
eB a (%)
Vr a
Unaccelerated-sulfur system None EPDMTA (2.5 phr) EPDMSH (2.5 phr)
9.3 0.5 12.4 0.2 10.8 1.0
1020 24 1100 6 800 40
0.100 0.100 0.103
S/MBTS (0.4 phr) system None EPDMTA (2.5 phr) EPDMSH (2.5 phr)
10.4 0.6 12.6 0.5 15.7 0.8
990 10 980 10 780 9
0.121 0.115 0.159
S/MBTS (0.8 phr) system None EPDMTA (2.5 phr) EPDMSH (2.5 phr)
10.3 0.9 12.2 1.0 13.5 0.5
790 10 960 32 740 30
0.134 0.140 0.196
a
rB ––ultimate tensile strength; eB ––elongation at break; Vr––volume fraction of the polymer in a swollen mass.
ultimate tensile strength, elongation at break and Vr, as functions of compatibilization and curing system. The noncompatibilized blends present similar values of tensile strength whatever be the curing system employed, whereas the elongation at break decreases as the MBTS content in the formulation increases, as a consequence of an increase on crosslink density, confirmed by the increasing on Vr values. It is important to observe the increase on tensile strength with the addition of EPDMTA or EPDMSH in all unaccelerated and accelerated sulfur-curing systems employed. The presence of 2.5 phr of EPDMTA resulted in an increase on tensile strength for blend containing no accelerator, but the addition of MBTS did not change the corresponding values, in spite of the increase on crosslink density, mainly in blends vulcanized with 0.8 phr of MBTS. The effect of the EPDMSH addition on the accelerated-sulfur curing blends is significant. An important increase on tensile strength with a decrease on elongation at break was observed in blends containing 0.4 phr of MBTS. Also, a significant increase on Vr values indicates an increase on the crosslink density. At higher MBTS concentration, the tensile properties of EPDMSH-compatibilized blend decrease, in spite of an increase on crosslink density. Probably, an ageing process in these blends is favored by the presence of EPDMSH with high amount of MBTS. The better tensile strength of EPDMSH-modified blends may be attributed to the increase of the crosslink density but also to the interfacial action of the functionalized copolymer, as a consequence of the reactions between the double bonds of the rubber phase and the mercapto groups of the compatibilizing agent, which occur during blending process. Blends compatibilized with EPDMTA did not present any substantial increase on the crosslink density, when compared to noncom-
patibilized blends. Therefore, the increase on the tensile strength in these EPDMTA-modified blends may be better related to the increase on the reactive compatibilization. In order to evaluate the extent of chemical reaction between the thioacetate- or mercapto-modified EPDM and the rubber matrix, a small amount of each blend has been withdrawn from the roll-mill before the addition of curatives and submitted to extraction with hot toluene for 24 h. Noncompatibilized blend did not present any insoluble material, as expected since there is no vulcanizing system in the formulation and no reaction is expected to occur between NR and EPDM during the blend processing. The presence of EPDMTA or EPDMSH resulted in an amount of insoluble material corresponding to around 7% and 23%, respectively. These proportions are higher than the amount of the functionalized copolymer used in the blend, suggesting the formation of a network during the process. This phenomenon is more effective with EPDMSH, as expected since the reaction between mercapto groups and carbon–carbon double bond is known to occur promptly [22,23]. In the case of EPDMTA-modified blends, the formation of insoluble material may be attributed to the generation of mechano-radicals formed by the dissociation of the weak S–C bond in thioacetate pendant groups, which could react with the double bond of the rubber matrix. The tensile properties were also determined from aged specimens. Fig. 4 presents the retention of the tensile properties of the blends after treatment in an air-circulating oven at 70 °C for 72 h. The best ageing resistance has been achieved with the addition of EPDMTA, except in terms of elongation at break in blends vulcanized with higher amount of MBTS. Concerning blends compatibilized with EPDMSH, the ageing resistance decreases as the concentration of the accelerator in
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the vulcanization of some fraction of the EPDM phase, thus promoting the covulcanization in some extent and (c) the reactive compatibilization, as a consequence of chemical reactions between the mercapto groups and the rubber matrix. EPDMTA was not so effective in improving the mechanical performance or crosslink density of the blends because of the lower reactivity of the thioacetate groups. However, it was efficient in increasing the ageing resistance of the corresponding blends.
Acknowledgements We would like to acknowledge the Conselho Nacional de Desenvolvimento Cientıfico e Tecnol ogico ao de Aperfeicßoamento de Pessoal (CNPq), Coordenacß~ ao de Amparo a de Nivel Superior (CAPES), Fundacß~ Pesquisa do Estado do Rio de Janeiro (FAPERJ), and PADCT/CNPq for the financial support for this project.
References Fig. 4. Retention of the tensile properties of NR/EPDM blends with thermal aging: (a) noncompatibilized; compatibilized with (b) EPDMTA and (c) EPDMSH.
the blend increases. The ageing resistance of EPDMSHcompatibilized blends is lower than noncompatibilized blends, for accelerated-sulfur curing system.
4. Conclusion The above results show that modification of NR/ EPDM blends with 2.5 phr of EPDMTA or EPDMSH results in an increase in the cure rate index and rate constant of vulcanization in both accelerated-sulfur curing systems containing 0.4 or 0.8 phr of the MBTS accelerator. The accelerating effect of these functionalized copolymers was also confirmed by the comparatively lower values of activation energy for the crosslink process. In unaccelerated-sulfur system, this behavior has been observed at higher cure temperature. The best results have been achieved with EPDMSH. This functionalized copolymer has also increased the crosslink density, as indicated by the higher maximum torque values observed from ODR experiments and by the higher Vr values observed from swelling experiments. Both EPDMTA and EPDMSH resulted in important increase of the ultimate tensile strength of the vulcanized blends. Nevertheless, the efficiency of EPDMSH was superior, which was attributed to the combination of several factors: (a) increase of the crosslink density; (b)
[1] Andrews EH. Rubber Chem Technol 1967;40:435. [2] Hess WM, Herd CR, Vegvari PC. Rubber Chem Technol 1993;66:329. [3] Woods ME, Davidson JA. Rubber Chem Technol 1976;49:112. [4] Chang Y-W, Shin YS, Chun H, Nah C. J Appl Polym Sci 1999;73:749. [5] Baranwal KC, Son PN. Rubber Chem Technol 1974;47:88. [6] Coran AY. Rubber Chem Technol 1988;61:281. [7] Coran AY. Rubber Chem Technol 1991;64:801. [8] Suma N, Joseph R, Francis DJ. Kautsch Gummi Kunstst 1990;43:1095. [9] Sirqueira AS, Soares BG. J Appl Polym Sci 2002;83:2892. [10] Poh BT, Ng CC. Eur Polym J 1998;34:975. [11] Oliveira MG, Soares BG. J Appl Polym Sci 2001;82:38. [12] Oliveira MG, Soares BG, Santos CMF, Diniz MF, Dutra RCL. Macromol Rapid Commun 1999;20:526. [13] Akiba M, Hashim AS. Prog Polym Sci 1997;22:475. [14] Krejsa MR, Koenig JL. Rubber Chem Technol 1993; 66:376. [15] Ding R, Leonov AI, Coran AY. Rubber Chem Technol 1996;69:81. [16] Menon ARR, Pillai CKS, Nando GB. Polymer 1998; 39:4033. [17] Poh BT, Kwok CP, Lim GH. Eur Polym J 1995;31:223. [18] Konar BB. J Appl Polym Sci 1997;63:233. [19] Mathew G, Singh RP, Lakshminarayanan R, Thomas S. J Appl Polym Sci 1996;61:2035. [20] Elvira MR, Macios A, Oteo JL, Royo J, Rubio JD. Angew Makromol Chem 1995;227:43. [21] Chough S-H, Chang D-H. J Appl Polym Sci 1996;61:449. [22] Boutevin B, Fleury E, Parisi JP, Pietrasnta Y. Makromol Chem 1989;190:2363. [23] Romani F, Passaglia E, Aglietto M, Ruggeri G. Macromol Chem Phys 1999;200:524.
The effect of mercapto- and thioacetate-modified EPDM on the curing parameters and mechanical properties of natural rubber/EPDM blends Alex S. Sirqueira, Bluma G. Soares
*
Centro de Tecnologia, Instituto de Macromol eculas, Universidade Federal do Rio de Janeiro, Bloco J, Ilha do Fund~ ao, P.O. Box 68525, 21945-970 Rio de Janeiro, RJ, Brazil Received 30 April 2003; received in revised form 17 July 2003; accepted 18 July 2003
Abstract The vulcanization characteristics of natural rubber (NR)/ethylene–propylene–ethylidenenorbornene (EPDM) rubber blends were studied in the presence of thioacetate-(EPDMTA) or mercapto-modified EPDM (EPDMSH), using oscillating disk rheometer. The effect of both functionalized EPDMs was investigated in unaccelerated-sulfur curing system and accelerated-sulfur curing systems containing 0.4 and 0.8 phr of MBTS. Both EPDMTA and EPDMSH act as accelerator agent in the curing process, as indicated by the higher values of cure rate index and lower values of activation energy of vulcanization. A substantial increase of the crosslink density has been also observed in EPDMSHmodified blends. Both EPDMTA and EPDMSH resulted in an increase in tensile strength, but the best performance has been achieved with EPDMSH, probably because of the increase of crosslink density associated to the reactive compatibilization promoted by the reaction between mercapto groups and rubber matrix. The best ageing resistance has been observed in EPDMTA-modified blends. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Elastomer blends; Compatibilization; Natural rubber; Curing parameters
1. Introduction Blending two or more elastomers is carried out for specific objectives such as enhancement of technical properties, improvement of ageing resistance and also processing characteristics. Natural rubber (NR) is normally blended with ethylene–propylene–diene rubber (EPDM) to improve ageing resistance of the former without loosing its good mechanical properties. Due to the difference in unsaturation level between these components, a mutual incompatibility can exist, which contributes for a decreasing of mechanical performance.
*
Corresponding author. Tel.: +55-21-256-27207; fax: +5521-227-01317. E-mail address: [email protected] (B.G. Soares).
In addition to the poor interfacial adhesion caused by the thermodynamic incompatibility, these blends usually present cure rate incompatibility because of the differences between the reactivity of the elastomers with the curing agents and/or differences in solubilities of the curatives in each elastomer phase [1,2]. In the case of NR/EPDM blends, the curing system can be consumed by the vulcanization of the NR phase, which is more rapidly vulcanizable because of the higher unsaturation level [2,3]. Several strategies have been reported to improve the compatibility of NR/EPDM blends and include the addition of a third polymeric component of low molecular weight [4], the incorporation of an accelerator moiety in the less unsaturated phase [5], and the functionalization of EPDM with maleic anhydride [6–8]. In our previous work, a new reactive compatibilizing agent based on mercapto-modified EPDM (EPDMSH)
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00163-0
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has been successfully employed in NR/EPDM blends [9]. The mercapto groups react with the double bonds of the NR phase and help to decrease the interfacial energy. Better mechanical performance has been achieved by using as low as 2.5 phr of this functionalized copolymer. In addition, the presence of EPDMSH has significantly affected the curing parameters, that is, it decreased both scorch time and optimum curing time, indicating an accelerator effect of the curing process. The catalytic effect of thiol groups on accelerated sulfur vulcanization systems in rubber has been also observed by other authors [10], using silica modified with thiol groups in NR systems and also by our group in studies involving the compatibilization of NBR/EPDM with similar mercapto-modified EPDM [11]. The preparation of mercapto-modified EPDM has been recently reported by our group [12] and involves two steps, illustrated in Fig. 1. The first one consists of a free radical addition of thioacetic acid on the double bond of EPDM. The resulting thioacetatemodified EPDM (EPDMTA) is hydrolyzed to produce EPDMSH. Both functionalized EPDMs may influence on the vulcanization process and on the mechanical properties. The objective of the present work was to study the effect of both functionalized EPDMs (EPDMTA and EPDMSH) on the curing parameters of NR/EPDM blends vulcanized with an accelerated-sulfur system. For this purpose, 2,20 -dithiobisbenzothiazole (MBTS) was employed as a conventional accelerator, whose proportion in the rubber formulations has been varied, in order to observe the performance of these functionalized copolymers, not only as a compatibilizing agent but also as a secondary accelerator. Apparent activation energy for the vulcanization of the rubber blends was calculated from experiments involving oscillating disk rheometer. The effect of EPDMTA and EPDMSH on mechanical properties, crosslink density and ageing resistance of the vulcanizates has been also investigated. A NR/EPDM weight ratio corresponding to 70:30 has been chosen because at this composition, it is possible to maintain the good mechanical properties of natural rubber while improving the ageing resistance of the rubber material.
2. Experimental 2.1. Materials Natural rubber (NR, Hervea Brasiliensis), weightaverage molecular weight (Mw ) ¼ 207,000, was kindly supplied by Michelin do Brasil S.A. Ethylene–propylene–ethylidenenorbornene (EPDM) rubber samples with two different compositions were kindly supplied by DSM Elastomeros do Brasil S. A. Table 1 summarizes the characteristics of the EPDM samples. Zinc oxide, stearic acid, sulfur, irganox 245 (primary antioxidant–– (pentaerythrityl tetrakis) (3,5-di-tert-butyl(4-hydroxyphenyl)) propionate) and MBTS were of laboratory reagent grade and kindly supplied by the local rubber industries. Thioacetic acid (TAA), analytical grade, from Sigma Chemistry, was used as received and azobisisobutyronitrile (AIBN) from Merck S. A., was recrystallized from methanol/water solution. 2.2. Functionalization of EPDM The preparation of the functionalized EPDM was carried out, according to previous report [12], using EP65 grade, which contains higher amount of the diene component. A higher insaturation level of the EPDM grade is profitable to obtain functionalized EPDM with higher amount of reactive groups. The functionalization has taken place in two steps (see Fig. 1). The first step Table 1 Characteristics of EPDM samples employed in this study Commercial grade
EP65
EP57C
Weight-average molecular weight from SECa Density (g/cm3 ) Iodine index Diene content (mmol/100 g) Ethylene/propylene weight ratio Mooney viscosity (ML1 + 4 at 100 °C)
150,000
80,000
0.837 29 114 60/40
0.813 15 58.9 73/27
77
90
a
A size exclusion chromatography.
Fig. 1. Scheme for the functionalization of EPDM.
A.S. Sirqueira, B.G. Soares / European Polymer Journal 39 (2003) 2283–2290
was performed in toluene solution at 70 °C for 48 h. The TAA/AIBN and diene/TAA molar ratios were established as 10.0 and 1.0, respectively, in order to avoid crosslink during the synthesis. The thioacetatemodified EPDM (EPDMTA) was submitted to an alkaline methanolysis using 5 wt% solution of NaOH. At these conditions, an amount of thioacetate or mercapto groups in the functionalized copolymers corresponding to 2.5 mmol/g has been achieved. 2.3. Blend preparation The EPDM grade used as the blend component was EP57C, which contains lower unsaturation level and, therefore, is indicated for ageing resistance purpose. The blends were prepared in a two roll-mill operating at 80 °C and at 20 rpm. NR was masticated for 2 min and then EPDM and the functionalized compatibilizing (EPDMTA or EPDMSH) were subsequently added. After the homogenization of the rubber blend (for about 4 min), the other ingredients were added in the following order: zinc oxide (5.0 phr), stearic acid (1.5 phr), irganox 245 (1.0 phr), sulfur (S) (2.5 phr) and 2,20 -dithiobisbenzothiazole (MBTS) (variable). The processing time after each component addition was about 2 min.
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fraction of the polymer in a swollen mass was calculated by the Eq. (1): Vr ¼
w2 ðdr Þ1 w2 ðdr Þ
1
þ ðw1 w2 Þ ðds Þ1
ð1Þ
where dr and ds are the rubber and solvent densities, respectively. The insoluble material in nonvulcanized and vulcanized blends was determined by submitting a sample of about 1 g to a heating in xylene at 120 °C for 24 h. The dynamic mechanical properties of the vulcanizates were determined using a dynamic mechanical analyzer, Rheometric Scientific MKIII, operating in a bending mode at a frequency of 1 Hz and amplitude of 0.1%. The temperature was increased at the heating rate of 2 °C/min in the range of )100 to 20 °C. The molecular weight of the polymers were determined by size exclusion chromatography (SEC) using a Water 600 liquid chromatograph equipped with refractive index detector and three ultrastyragel columns: (two porosity). The molecular weight was liner and one 500 A determined from calibration curve using polystyrene standards.
3. Results and discussion 2.4. Rheometric measurements and testing 3.1. Vulcanization characteristics The vulcanization parameters of the mixes were measured on an oscillating disk rheometer (ODR) (Tecnologia Industrial, mod T100) at 1 arc degree, according to ASTM D-2084-81 method. The kinetic parameters of the vulcanization process, such as rate constant ðkÞ and apparent activation energy (Ea ), were calculated from the torque–time curves taken from experiments performed at 150, 160 and 170 °C. For tensile testing, the blends were vulcanized up to the optimum cure time, in a hydraulic press at 160 °C and 1500 lb in2 . (The optimum cure time ðt90 Þ was established as the necessary time to reach 90% of the maximum torque.) Then, dumbbell shaped tensile test specimens (ASTM 638- testing number 5) were punched out of the compression molded sheets. Tensile testing was performed on an Instron 4204 Universal Testing Machine, at a crosshead speed of 100 mm min1 . The experiments were performed before and after ageing the specimens in an air-circulating oven at 70 °C for 3 days. Crosslink densities of the blends were obtained from swelling experiments in toluene. Cured test pieces of 20 20 2 mm dimension were accurately weighted ðwÞ and immersed in toluene in closed bottles for 7 days. Then, the surfaces were dried with filter paper and the samples were quickly weighed ðw1 Þ. The swollen samples were then dried completely at 60 °C for 48 h under reduced pressure and weighted again ðw2 Þ. The volume
The vulcanization parameters of compatibilized and noncompatibilized blends, cured with different amounts of MBTS single accelerator are listed in Table 2. The effect of MBTS content on the rheograph profiles obtained at 160 °C is illustrated in Fig. 2. The presence of higher amount of MBTS (0.8 phr) in noncompatibilized blend resulted in an increase of scorch safety, which is expected since MBTS is classified as a delayed-action semi-ultra accelerator [13]. This behavior is observed in all temperatures studied but the difference on scorch time in the two MBTS concentration becomes smaller as the cure temperature increases. Blends containing 2.5 phr of EPDMTA also presented similar behavior in experiments performed at 150 and 160 °C, but the difference on scorch safety was smaller as compared to noncompatibilized blend. At 170 °C, a better scorch safety was achieved with lower amount of MBTS. In blends compatibilized with EPDMSH, a higher scorch time with higher amount of MBTS was only observed in experiments performed at lower temperature (150 °C). As indicated in Table 2, the scorch time of blends containing lower amount of MBTS did not change significantly with the presence of EPDMTA or EPDMSH. However, at higher amount of MBTS, the effect of both functionalized EPDMs was significant, indicating an accelerated action of the
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Table 2 Vulcanization parametersa of NR/EPDM blends as a function of compatibilization and MBTS content Temperature (°C) 150 b
160
170
c
d
b
c
d
b
c
d
S/MBTS (0.4 phr) 15.7 MH (lb in1 ) ML (lb in1 ) 1.4 ts1 (min) 4.8 t90 (min) 17.8
18.8 1.5 5.0 17.9
17.9 1.7 4.8 16.0
16.2 1.3 3.0 12.8
18.8 2.3 3.0 11.4
19.6 1.3 2.8 10.1
15.2 1.3 2.2 7.3
17.7 2.0 2.0 6.5
16.4 1.5 2.0 5.7
S/MBTS (0.8 phr) 17.5 MH (lb in1 ) ML (lb in1 ) 2.1 ts1 (min) 7.0 t90 (min) 15.3
17.1 2.6 5.7 17.9
20.0 2.2 5.1 15.1
17.1 2.0 4.0 8.4
19.5 1.5 3.2 8.6
20.1 2.3 2.3 7.8
16.5 1.9 2.5 5.1
18.7 1.7 1.1 4.4
18.3 2.4 1.8 4.7
a
t90 ––optimum cure time; ts1 ––scorch time; ML ––minimum torque; MH ––maximum torque. Blend without compatibilizer. c Blends with 2.5 phr of EPDMTA. d Blends with 2.5 phr of EPDMSH. b
mercapto or thioacetate groups when combined with higher amount of MBTS. This behavior is better observed in Fig. 3, where the rheographs of compatibilized and noncompatibilized blends are compared. As stated in several papers [13–15], during the scorch delay period or induction period (first region of a curometer curve), the majority accelerator chemistry takes place. Therefore, one can assume the participation of mercapto and thioacetate groups in EPDMSH and EPDMTA, respectively, on the chemical reactions that occur in this stage. The more effective participation is observed for the mercapto groups. The presence of EPDMTA or EPDMSH also resulted in a substantial increase of maximum torque ðMH Þ, which is normally related to the crosslink density. It is interesting to observe that, for blends vulcanized at 160 °C, the MH values achieved in compatibilized blends containing 0.4 phr of MBTS were even higher than that found in noncompatibilized blend containing higher amount of MBTS, indicating a strong influence of these functionalized copolymers on the crosslink density. Concerning the rheometric experiments carried out at 160 °C, one can observe that, in addition to the accelerating action, the increase of MH values was more pronounced in blends containing EPDMSH, indicating a more effective participation of mercapto groups than thioacetate groups on the vulcanization process. The accelerating action of the functionalized EPDMs was also evaluated in blends compounded with unaccelerated-sulfur formulations, that is, without the presence of MBTS. Table 3 presents the curing parameters of these blends, measured at different temperatures. The maximum torque values in all blends were low, indicating poor crosslink density. EPDMSH resulted in a slight increase of MH . Both EPDMTA and EPDMSH
resulted in a decrease of scorch time and optimum cure time in experiments performed at higher temperatures, confirming the accelerating action of both functionalized EPDMs, even in absence of MBTS. The cure rate index of these blends vulcanized in unaccelerated-sulfur system was calculated, according to Eq. (2) [16]: CRI ¼
100 t90 ts1
ð2Þ
where t90 is the optimum cure time and ts1 is the scorch time. The effect of the functionalized compounds on the cure rate index is more pronounce at 180 °C. At this temperature, a substantial increase of the CRI values has been observed, mainly in blends containing EPDMSH, confirming the higher reactivity of this group on the vulcanization process. The effect of the functionalized EPDMs on the reversion phenomenon, which occurs at the end of the curing process in the rheometer, is summarized in Table 4, for accelerated-sulfur vulcanizing systems. The reversion tax was calculated, according to Eq. (3) [17]: Reversion tax ¼
MH MHþ30 min MH
100
ð3Þ
where MH is the maximum torque and MHþ30 min is the torque 30 min after maximum torque occurs. The reversion tendency was only observed at higher temperature (160 and 170 °C). At 160 °C, only blends formulated with higher amount of MBTS presented reversion tendency, whereas at 170 °C, one can distinguish the reversion process in the two MBTS concentration
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Fig. 3. Effect of the compatibilization on the rheograph profiles of vulcanized NR/EPDM blends: (b) noncompatibilized; compatibilized with (c) EPDMTA and (d) EPDMSH.
3.2. Kinetic parameters of vulcanization
Fig. 2. Effect of the MBTS concentration on the curing parameters of 70:30 blends, obtained at 160 °C; MBTS content ¼ (a) 0.4 phr and (b) 0.8 phr.
employed. The addition of EPDMTA or EPDMSH resulted in a considerable decrease of the reversion tax, indicating an improvement on thermal stability and a lower dissociation of sulfur crosslink. Normally, natural rubber shows rapid curing and high reversion effect because of its high content of allylic hydrogens. Because all allylic hydrogens are not used for crosslinking, the remaining allyls may provide site to be radicals, which cause dissociation of sulfur crosslinks. The good protection effect against reversion achieved with EPDMTA or EPDMSH may be attributed to the ability of the corresponding thioacetate or mercapto groups in reacting with these radicals before they have time to promote the sulfur crosslinks dissociation.
The kinetic equation that describes the vulcanization process can be obtained from the torque–time values of the rheographs [15,18–21]. If the process is considered as a first-order reaction, it would be expressed by Eq. (4), as follows: MH ML ln ¼ kt ð4Þ MH Mt where MH , ML and Mt are the maximum torque, the minimum torque and the torque at a cure time t and k is the vulcanization kinetic constant. Because the rate of the first stage, associated to the rate of conversion of the cure complex, reflects the character of the main forward reaction kinetically, M values between 25 and 45% of torque changes were chosen to estimate the rate constants in this work [21]. By plotting lnðMH Mt Þ against t, a straight line graph is obtained, indicating that the cure reaction follows first-order kinetic. Theory and practice show that the vulcanization rate depends on the reciprocal temperature. So, it is possible to use the Arrhenius equation to express this dependence, according to Eq. (5):
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Table 3 Vulcanization parametersa of NR/EPDM blends vulcanized with unaccelerated-sulfur system Temperature (°C) 150 1
MH (lb in ) ML (lb in1 ) ts1 (min) t90 (min) CRI (min1 )
160
170
180
b
c
d
b
c
d
b
c
d
b
c
d
4.8 3.0 4.8 23 5.5
4.1 2.1 4.3 21 5.8
5.3 2.9 3.6 21 5.6
6.7 2.8 2.6 21 5.4
6.3 2.0 3.0 21 5.5
7.8 2.8 2.0 21 5.3
9.2 2.8 2.6 19 6.2
9.3 1.8 2.0 17 6.7
9.6 2.5 1.8 17 6.6
9.2 2.8 2.0 13 9.1
9.8 1.8 1.8 10 12.2
9.9 2.5 1.6 9 14.3
a
t90 ––optimum cure time; ts1 ––scorch time; ML ––minimum torque; MH –maximum torque. Blend without compatibilizer. c Blends with 2.5 phr of EPDMTA. d Blends with 2.5 phr of EPDMSH. b
Table 4 Effect of the compatibilization and cure temperature on the reversion tax of NR/EPDM blends obtained from ODR experiments Compatibilizer
Temperature (°C) 150
None EPDMTA (2.5 phr) EPDMSH (2.5 phr) a b
160
170
a
b
a
b
a
b
– –
– –
– –
6.0 4.0
28.0 10.0
15.0 12.0
–
–
–
3.0
2.0
7.0
Sulfur curing system containing 0.4 phr of MBTS. Sulfur curing system containing 0.8 phr of MBTS.
k ¼ A exp
Ea RT
ð5Þ
where A is the pre-exponential factor, R represents the gas constant, T is the absolute temperature and Ea is the apparent activation energy for the curing process. The values of the rate constants and activation energy of the cure reaction of all blend formulations are given in Table 5. In all curing conditions and in both S/MBTS curing systems, the rate constant values increase in this order: Noncompatibilized blends < blends with EPDMTA < blends with EPDMSH Therefore, the Ea decreases in the same order. Also the cure rate index (CRI), obtained from Eq. (1), increases considerably with the presence of EPDMTA and EPDMSH, in this order, for all curing conditions and in both curing systems. All these results confirm the accelerating action of both functionalized EPDMs. 3.3. Mechanical properties and crosslink density
or ln k ¼ ln A
Ea 1 R T
ð6Þ
These properties were determined from samples vulcanized at 160 °C. Table 6 presents the values of
Table 5 Kinetic parameters of vulcanization of NR/EPDM blends Compatibilizer
CRIa (min1 ) 150 °C
Rate constant,b k (min1 )
Ea c (kJ/mol)
160 °C
170 °C
150 °C
160 °C
170 °C
S/MBTS (0.4 phr) None EPDMTA EPDMSH
7.73 7.73 9.00
10.24 11.90 13.53
19.72 22.00 26.95
0.204 0.261 0.299
0.289 0.365 0.502
0.526 0.536 0.658
73.63 61.52 58.74
S/MBTS (0.8 phr) None EPDMTA EPDMSH
12.05 8.19 10.0
22.73 18.52 18.45
38.91 29.76 34.50
0.263 0.266 0.329
0.534 0.517 0.659
0.643 0.752 0.887
82.10 77.52 69.06
a
Calculated from Eq. (1). Calculated from Eq. (4). c Calculated from Eq. (6). b
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Table 6 Mechanical properties and crosslink density of NR/EPDM blends as functions of compatibilization and amount of accelerator Compatibilizer
rB a (MPa)
eB a (%)
Vr a
Unaccelerated-sulfur system None EPDMTA (2.5 phr) EPDMSH (2.5 phr)
9.3 0.5 12.4 0.2 10.8 1.0
1020 24 1100 6 800 40
0.100 0.100 0.103
S/MBTS (0.4 phr) system None EPDMTA (2.5 phr) EPDMSH (2.5 phr)
10.4 0.6 12.6 0.5 15.7 0.8
990 10 980 10 780 9
0.121 0.115 0.159
S/MBTS (0.8 phr) system None EPDMTA (2.5 phr) EPDMSH (2.5 phr)
10.3 0.9 12.2 1.0 13.5 0.5
790 10 960 32 740 30
0.134 0.140 0.196
a
rB ––ultimate tensile strength; eB ––elongation at break; Vr––volume fraction of the polymer in a swollen mass.
ultimate tensile strength, elongation at break and Vr, as functions of compatibilization and curing system. The noncompatibilized blends present similar values of tensile strength whatever be the curing system employed, whereas the elongation at break decreases as the MBTS content in the formulation increases, as a consequence of an increase on crosslink density, confirmed by the increasing on Vr values. It is important to observe the increase on tensile strength with the addition of EPDMTA or EPDMSH in all unaccelerated and accelerated sulfur-curing systems employed. The presence of 2.5 phr of EPDMTA resulted in an increase on tensile strength for blend containing no accelerator, but the addition of MBTS did not change the corresponding values, in spite of the increase on crosslink density, mainly in blends vulcanized with 0.8 phr of MBTS. The effect of the EPDMSH addition on the accelerated-sulfur curing blends is significant. An important increase on tensile strength with a decrease on elongation at break was observed in blends containing 0.4 phr of MBTS. Also, a significant increase on Vr values indicates an increase on the crosslink density. At higher MBTS concentration, the tensile properties of EPDMSH-compatibilized blend decrease, in spite of an increase on crosslink density. Probably, an ageing process in these blends is favored by the presence of EPDMSH with high amount of MBTS. The better tensile strength of EPDMSH-modified blends may be attributed to the increase of the crosslink density but also to the interfacial action of the functionalized copolymer, as a consequence of the reactions between the double bonds of the rubber phase and the mercapto groups of the compatibilizing agent, which occur during blending process. Blends compatibilized with EPDMTA did not present any substantial increase on the crosslink density, when compared to noncom-
patibilized blends. Therefore, the increase on the tensile strength in these EPDMTA-modified blends may be better related to the increase on the reactive compatibilization. In order to evaluate the extent of chemical reaction between the thioacetate- or mercapto-modified EPDM and the rubber matrix, a small amount of each blend has been withdrawn from the roll-mill before the addition of curatives and submitted to extraction with hot toluene for 24 h. Noncompatibilized blend did not present any insoluble material, as expected since there is no vulcanizing system in the formulation and no reaction is expected to occur between NR and EPDM during the blend processing. The presence of EPDMTA or EPDMSH resulted in an amount of insoluble material corresponding to around 7% and 23%, respectively. These proportions are higher than the amount of the functionalized copolymer used in the blend, suggesting the formation of a network during the process. This phenomenon is more effective with EPDMSH, as expected since the reaction between mercapto groups and carbon–carbon double bond is known to occur promptly [22,23]. In the case of EPDMTA-modified blends, the formation of insoluble material may be attributed to the generation of mechano-radicals formed by the dissociation of the weak S–C bond in thioacetate pendant groups, which could react with the double bond of the rubber matrix. The tensile properties were also determined from aged specimens. Fig. 4 presents the retention of the tensile properties of the blends after treatment in an air-circulating oven at 70 °C for 72 h. The best ageing resistance has been achieved with the addition of EPDMTA, except in terms of elongation at break in blends vulcanized with higher amount of MBTS. Concerning blends compatibilized with EPDMSH, the ageing resistance decreases as the concentration of the accelerator in
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the vulcanization of some fraction of the EPDM phase, thus promoting the covulcanization in some extent and (c) the reactive compatibilization, as a consequence of chemical reactions between the mercapto groups and the rubber matrix. EPDMTA was not so effective in improving the mechanical performance or crosslink density of the blends because of the lower reactivity of the thioacetate groups. However, it was efficient in increasing the ageing resistance of the corresponding blends.
Acknowledgements We would like to acknowledge the Conselho Nacional de Desenvolvimento Cientıfico e Tecnol ogico ao de Aperfeicßoamento de Pessoal (CNPq), Coordenacß~ ao de Amparo a de Nivel Superior (CAPES), Fundacß~ Pesquisa do Estado do Rio de Janeiro (FAPERJ), and PADCT/CNPq for the financial support for this project.
References Fig. 4. Retention of the tensile properties of NR/EPDM blends with thermal aging: (a) noncompatibilized; compatibilized with (b) EPDMTA and (c) EPDMSH.
the blend increases. The ageing resistance of EPDMSHcompatibilized blends is lower than noncompatibilized blends, for accelerated-sulfur curing system.
4. Conclusion The above results show that modification of NR/ EPDM blends with 2.5 phr of EPDMTA or EPDMSH results in an increase in the cure rate index and rate constant of vulcanization in both accelerated-sulfur curing systems containing 0.4 or 0.8 phr of the MBTS accelerator. The accelerating effect of these functionalized copolymers was also confirmed by the comparatively lower values of activation energy for the crosslink process. In unaccelerated-sulfur system, this behavior has been observed at higher cure temperature. The best results have been achieved with EPDMSH. This functionalized copolymer has also increased the crosslink density, as indicated by the higher maximum torque values observed from ODR experiments and by the higher Vr values observed from swelling experiments. Both EPDMTA and EPDMSH resulted in important increase of the ultimate tensile strength of the vulcanized blends. Nevertheless, the efficiency of EPDMSH was superior, which was attributed to the combination of several factors: (a) increase of the crosslink density; (b)
[1] Andrews EH. Rubber Chem Technol 1967;40:435. [2] Hess WM, Herd CR, Vegvari PC. Rubber Chem Technol 1993;66:329. [3] Woods ME, Davidson JA. Rubber Chem Technol 1976;49:112. [4] Chang Y-W, Shin YS, Chun H, Nah C. J Appl Polym Sci 1999;73:749. [5] Baranwal KC, Son PN. Rubber Chem Technol 1974;47:88. [6] Coran AY. Rubber Chem Technol 1988;61:281. [7] Coran AY. Rubber Chem Technol 1991;64:801. [8] Suma N, Joseph R, Francis DJ. Kautsch Gummi Kunstst 1990;43:1095. [9] Sirqueira AS, Soares BG. J Appl Polym Sci 2002;83:2892. [10] Poh BT, Ng CC. Eur Polym J 1998;34:975. [11] Oliveira MG, Soares BG. J Appl Polym Sci 2001;82:38. [12] Oliveira MG, Soares BG, Santos CMF, Diniz MF, Dutra RCL. Macromol Rapid Commun 1999;20:526. [13] Akiba M, Hashim AS. Prog Polym Sci 1997;22:475. [14] Krejsa MR, Koenig JL. Rubber Chem Technol 1993; 66:376. [15] Ding R, Leonov AI, Coran AY. Rubber Chem Technol 1996;69:81. [16] Menon ARR, Pillai CKS, Nando GB. Polymer 1998; 39:4033. [17] Poh BT, Kwok CP, Lim GH. Eur Polym J 1995;31:223. [18] Konar BB. J Appl Polym Sci 1997;63:233. [19] Mathew G, Singh RP, Lakshminarayanan R, Thomas S. J Appl Polym Sci 1996;61:2035. [20] Elvira MR, Macios A, Oteo JL, Royo J, Rubio JD. Angew Makromol Chem 1995;227:43. [21] Chough S-H, Chang D-H. J Appl Polym Sci 1996;61:449. [22] Boutevin B, Fleury E, Parisi JP, Pietrasnta Y. Makromol Chem 1989;190:2363. [23] Romani F, Passaglia E, Aglietto M, Ruggeri G. Macromol Chem Phys 1999;200:524.