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Solubility study of curatives in various rubbers
European Polymer Journal 44 (2008) 3890–3893
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Short communication
Solubility study of curatives in various rubbers R. Guo, A.G. Talma, R.N. Datta, W.K. Dierkes, J.W.M. Noordermeer * Elastomer Technology and Engineering Department, Faculty of Engineering Technology, University of Twente, P.O. Box 217, 7500AE, Enschede, The Netherlands
a r t i c l e
i n f o
Article history: Received 4 May 2008 Received in revised form 24 July 2008 Accepted 27 July 2008 Available online 13 August 2008
Keywords: Solubility Curatives Rubber Vulcanisation Solubility parameters
a b s t r a c t The previous works on solubility of curatives in rubbers were mainly carried out in natural rubber. Not too much information available on dissimilar rubbers and this is important because most of the compounds today are blends of dissimilar rubbers. Although solubility can be expected to certain level by the previous studies, the current work provides a much precise view in the solubility behavior of curatives. Solubility of sulphur and several accelerators N-cyclohexylbenzothiazole-2-sulphenamide (CBS), N-dicyclohexylbenzothiazole-2-sulphenamide (DCBS), and 2-mercaptobenzothiazole (MBT) is measured in dicumyl peroxide vulcanised Styrene-Butadiene rubber (SBR), Acrylonitrile-Butadiene rubber (NBR) and Ethylene-Propylene-Diene rubber (EPDM) rubber at room temperature and at 60 °C. The experimental results can be correlated with the calculated solubility parameters d, as determined using the method of Hoftijzer and Van Krevelen. Results of Dd are used to judge the solubility of curatives in a specific rubber in blends. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Attempts to determine the solubility of the curatives in rubbers have already been made for quite some time. Initial values of sulphur solubility in natural rubber (NR) were measured by Venable and Greene [1] in 1922. Since then, other experimental methods, e.g. equivalent solvents [2], weight take-up, microscopy [3–6], radioactively labeled sulphur [7–10], ToF-SIMS [11] etc. were used to determine the solubility of curatives, in most cases the solubility of sulphur in rubbers. Amongst all these methods, the method of Morris and Thomas [12] is most effective to give reliable solubility results. The internal crystallisation of sulphur is excluded due to the isothermal experimental procedure. What is more, the peroxide cure applied in the method can reduce the modification of chain structures of rubber, which consequently reduce the influence on solubility compared to sulphur curing. * Corresponding author. Tel.: +31 53 4892529; fax: +31 53 4892151. E-mail address: [email protected] (J.W.M. Noordermeer). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.07.054
Although quite some experimental data have been obtained, an apparent lack of consistence exists due to the drawbacks of each method and the experimental conditions. On the other hand, the wide-spread use of rubber blends makes people become much more interested in the solubility of sulphur in rubbers other than NR. Especially EPDM is interesting due to its excellent ozone- and oxygen-resistance. As the distribution and dispersion of curatives other than sulphur in blends of dissimilar rubbers is also vital for the properties of such vulcanised blends, it is of great interest to determine the solubility of accelerators in various rubbers as well, in an attempt to obtain some insight into the mechanistic aspects involved in curing the blends. The solubility parameters of the rubbers and the curatives involved in this study can be calculated by the method of Hoftijzer and Van Krevelen*** [13], by adding the contributions from all functional groups. The solubility parameter d (J1/2/cm3/2) can then be used to predict the mutual solubility. The details of measurement and calculation are described earlier.
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R. Guo et al. / European Polymer Journal 44 (2008) 3890–3893
2.1. Solubility calculations of the elastomers and curatives The calculated solubility parameters of SBR is dd 16.6, dp 0.5, dh 0; NBR is dd 17.2, dp 8.6, dh 4.3; EPDM is dd 16.4, dp 0, dh 0. It should be mentioned that both the content of more polar groups (e.g. the styrene group in SBR, the acrylonitrile contained in NBR) and the molar ratio of the copolymers influence the polarity and amount of hydrogen bonding in these polymers. The solubility parameters of the curatives are summarized in Table 1. From the values of dp it can already be deduced that the polarities of the accelerators are much higher than that of sulphur. The mutual solubilities between rubbers and sulphur or curatives as reflected in the values of Dd, are given in Table 2. It illustrates the preference of each curative towards the three rubbers used in this study at room temperature.
Table 2 Calculated Dd between rubbers and curatives at room temperature [J1/2/cm3/2]
SBR NBR EPDM
a
S8
OT20
CBS
DCBS
MBT
16.5 18.6 16.8
5.4 10.7 5.6
8.1 5.9 8.5
8.3 8.1 8.5
11.4 6.0 11.8
8.0 7.0 6.0
% wt increase
2. Results and discussion
2.2. Solubility of sulphur and polymeric sulphur in different elastomers
5.0 4.0 3.0 2.0 1.0
2.3. Solubility of accelerators in different elastomers The highest solubility of CBS is in NBR at both temperatures. This correlates with the fact that the polarity of CBS is much higher than that of sulphur, which is also Table 1 Calculated solubility parameters of curatives [J1/2/cm3/2]
dd dp dh
S8
OT20
CBS
DCBS
MBT
33.1 0 0
22.0 0 0
20.4 4.0 6.3
21.8 2.2 6.2
22.5 7.5 6.8
0.0 S8
OT20
CBS
DCBS
MBT
S8
OT20
CBS
DCBS
MBT
b 15.0 12.0
% wt increase
A comparative overview of the solubilities, the highest values taken from the measurements, of all the curatives for a certain temperature is given in Fig. 1a and b according to the three rubbers involved in this study. The weight up-take of sulphur (S8) and polymeric sulphur (OT20) increases with time and temperature. The preference of sulphur is SBR>EPDM>>NBR. As the solubility difference of sulphur between SBR and EPDM is small, therefore, it can be expected that in case of blends of dissimilar rubbers, a homogeneous dispersion of sulphur is achieved for the blend SBR/EPDM. On the contrary the solubility difference of sulphur in NBR and EPDM is large, therefore, sulphur is expected to accumulate in the EPDM phase. The solubility of polymeric sulphur (OT20) is 10-fold higher than that of elemental sulphur for SBR and EPDM. However, it is still insoluble in NBR. The preference of polymeric sulphur is EPDM>SBR>>NBR. The higher solubility of polymeric vs. elemental sulphur observed in EPDM can be related to the fact that there is some 20% oil mixed in OT20, which has a high solubility in EPDM. However, the solubilities of OT20 in EPDM and SBR are still very close, so that it will still result in a good dispersion of OT20 in a blend of SBR/EPDM. In a NBR/EPDM blend the OT20 will participate more to the EPDM phase than the NBR phase, similar to the elemental sulphur.
9.0
6.0
3.0
0.0
Fig. 1. (a) Comparison of solubility of curatives in SBR, NBR and EPDM gum rubber at room temperature. (b) Comparison of solubility of curatives in SBR, NBR and EPDM gum rubber at 60 °C. (j) SBR; NBR; hPDM.
in accordance with the higher value of dp of CBS. DCBS shows the highest solubility in EPDM at 60 °C, different to CBS. The thiazole-type curative, MBT is a decomposition product of all sulphenamide accelerators. It has the highest solubility in NBR at room temperature as well as at 60 °C, which matches with the high polarity of MBT shown in the dp in Table 1. It has a great tendency to partition into the NBR phase in rubber blends like NBR/SBR and NBR/EPDM. 2.4. Correlation between the calculated solubility parameters with the experimental results As stated before, mutual solubility can happen only when the value of Dd is smaller than 5 J1/2/cm3/2. This
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R. Guo et al. / European Polymer Journal 44 (2008) 3890–3893
a
8.0 OT20 OT20
Solubility (%wt increase at R.T.)
7.0 6.0 5.0 4.0 3.0 2.0
CBS MBT
DCBS DCBS CBS CBS DCBS
1.0 0.0 0.0
2.5
5.0
7.5
10.0
Δδ
Solubility (%wt increase at 60 οC)
b
20.0
OT20 MBT MBT 12.5
S8 S8 15.0
S8
17.5
20.0
(J1/2/cm3/2)
OT20 OT20
15.0
10.0 MBT
5.0
0.0
0.0
DCBS
DCBS CBS CBS CBS DCBS 2.5
5.0
7.5
OT20
10.0
Δδ
S8 S8
MBT MBT 12.5
15.0
17.5
S8 20.0
(J1/2/cm3/2)
Fig. 2. (a) Correlation between the calculated solubility parameters and the experimental data at R.T. (b) Correlation between the calculated solubility SBR; NBR; EPDM. parameters and the experimental data at 60 °C.
rule can now be used to check if the calculated Dd is sufficiently predictive for the experimental values. In Fig. 2, the solubility data are plotted against the value of Dd for both R.T. and 60 °C. Due to the complexity involved in calculating the solubility parameters at 60 °C, especially the refined solubility parameters, dd, dp and dh, the solubility measured at 60 °C is also plotted against the Dd at room temperature. For this situation, it is still possible to observe the same trend in Fig. 2b as in Fig. 2a. It is clear from Fig. 2 that a higher solubility is found with smaller Dd value, regardless what rubbers or curatives are involved. An extremely high solubility is observed for OT20, but this must most probably be attributed to the 20% oil contained.
3. Conclusions From the solubility data of sulphur, it is clear that elemental sulphur does not dissolve well at room temperature. The difference in solubility of sulphur in the different rubbers is more pronounced at higher temperatures (SBR > EPDM >> NBR). This is the main reason for cure incompatibility in rubber blends. Polymeric sulphur shows a 10-fold higher solubility than elemental sulphur. The high solubility of OT20 can partially explain its reduced blooming tendency. The other reason for reduced blooming is that polymeric sulphur is not migrating, which is not accounted for in this study. The solubility of accelerators is much higher than that of elemental sulphur in NBR,
R. Guo et al. / European Polymer Journal 44 (2008) 3890–3893 Table 3 General formulation for gum rubber compounds Component SBR NBR EPDM ZnO Stearic acid Dicumyl peroxide (40%)* *
Amount (phr) 100 0 0 5 2 1
0 100 0 5 2 4.8
0 0 100 5 2 6.25
PERKADOX BC-40B.
SBR and EPDM rubber. CBS and MBT are very polar, which gives them a preference towards NBR rubber (NBR >> SBR > EPDM). However, in the case of DCBS, the sequence of solubility is SBR > EPDM > NBR, explained by the molecular structure of DCBS, where the two benzene rings cause symmetry. The experimental data of solubility of curatives at room temperature can be correlated to the Dd values calculated by the method of Hoftijzer and Van Krevelen, as shown in Fig. 2, where a lower value of Dd correlates with a higher value in solubility. 4. Experimental 4.1. Materials Three different kinds of rubbers: S-SBR (BunaÒ VSL 5025-0HM from LANXESS), NBR (PerbunanÒ 3446F from LANXESS), and EPDM (KeltanÒ 4703 from DSM Elastomers) were employed. Zinc-oxide and elemental sulphur were purchased from Sigma–Aldrich and polymeric sulphur (CrystexÒ HS OT 20) was obtained from Flexsys; accelerators (SantocureÒ CBS, SantocureÒ DCBS and PerkacitÒ MBT) were also provided by Flexsys and stearic acid used as commercial type. Dicumyl peroxide was provided by Akzo Nobel, with active content of 40%. 4.2. Solubility measurements and calculations All the rubber samples were slightly crosslinked by peroxide before the solubility measurements were carried out. The general formulations are shown in Table 3. The vulcanisation was carried out at 160 °C for 15 min. The gel content was measured after extraction with toluene in a Sohxlet extractor for 2 days: 96.6% for SBR, 99.6% for NBR, and 98.2% for EPDM. The levels of crosslinking were kept low and in the similar range for the three different rubbers to exclude the influence of crosslink density on solubility. The vulcanised samples were then cut into sheets with a size of 10 10 2 mm. These sheets were extracted with acetone in a Sohxlet extractor for two days to remove the non-rubber parts. Finally, all the samples were dried in a vacuum oven for 24 h at room temperature. Samples
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were stored in a plastic bottle and protected from light before use. The solubility measurements were carried out in triplicate by placing the accurately weighed sample of vacanisate in a glass bottle such that they were packed on all sides with either sulphur or the accelerators of interest in the form of fine powders. The glass bottle was placed in a thermo-stated oven at room temperature or 60 °C. The sample weights were measured every day. The sample surfaces were cleaned with a sharp blade, followed by treatment with an adhesive tape before weighing in order to remove remaining adhering material. Blank experiments were carried out by inserting rubber samples into the curatives for a few seconds and weighing the sample after the cleaning procedure. The aim of a blank experiment was to check if the cleaning procedure was good enough to remove all the adhered powders from the surface of the samples. This proved to be the case: no weight increase was observed. The solubilities of both elemental sulphur and polymeric sulphur (OT20) were measured as the materials as received. The solubilities of the accelerators CBS, DCBS, and MBT were determined using the same procedure. CBS and DCBS were first ground into fine powders before use. MBT was used as received in powder form. Calculations of the solubility parameters were carried out according to the method of Van Krevelen [13]. The solubility parameter is calculated into three parts, dd: the solubility parameter component from the dispersion forces; dp: the component from polar forces; dh: the component from hydrogen bonding. Acknowledgements This project is financially supported by the Dutch Technology Foundation (STW), the Applied Science Division of NWO, the technology program of the Ministry of Economic Affairs of the Netherlands. TIMCAL Graphite and Carbon and Hexagon compounding in Belgium are also gratefully acknowledged for their support. References [1] Venable CS, Greene CD. Solubility of sulfur in rubber. Ind Eng Chem 1922;14:319. [2] Guillaumond F. The influence of the solubility of accelerators on the vulcanization of elastomer blends. Rubber Chem Technol 1976;49:105. [3] Gardiner JB. Rubber Chem Technol 1968;41:1312. [4] Graf HJ, Issel HM. New evaluation of dithiophosphates with reference to accelerators incapable of forming N-Nitrosamines. Kautsch Gummi Kunstst 1995;48:600. [5] Morris TC. Ind Eng Chem 1932;24:584. [6] Kemp AR, Malm FS, Stiratelli B. Ind Eng Chem 1944;36:109. [7] Baldi L, Zannetti R. Rubber Chem Technol 1962;44:1350. [8] Frederick Ignatz-Hoover, Byron To, Datta RN, Arie De Hoog, Huntink N, Talma A. Rubber Chem Technol 2003;76:747. [9] Mozisek M. Pol Eng Sci 1970;10:383. [10] Berry BS, Susko JR. IBM J Res Develop 1977. [11] Dias AJ, Galuska AA. Rubber Chem Technol 1996;69:615. [12] Morris MD, Thomas AG. Rubber Chem Technol 1995;68:794. [13] Krevelen DWV, Hoftijzer PJ. Properties of polymers. 3rd ed. Amsterdam: Elsevier Science B.V.; 1990.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Short communication
Solubility study of curatives in various rubbers R. Guo, A.G. Talma, R.N. Datta, W.K. Dierkes, J.W.M. Noordermeer * Elastomer Technology and Engineering Department, Faculty of Engineering Technology, University of Twente, P.O. Box 217, 7500AE, Enschede, The Netherlands
a r t i c l e
i n f o
Article history: Received 4 May 2008 Received in revised form 24 July 2008 Accepted 27 July 2008 Available online 13 August 2008
Keywords: Solubility Curatives Rubber Vulcanisation Solubility parameters
a b s t r a c t The previous works on solubility of curatives in rubbers were mainly carried out in natural rubber. Not too much information available on dissimilar rubbers and this is important because most of the compounds today are blends of dissimilar rubbers. Although solubility can be expected to certain level by the previous studies, the current work provides a much precise view in the solubility behavior of curatives. Solubility of sulphur and several accelerators N-cyclohexylbenzothiazole-2-sulphenamide (CBS), N-dicyclohexylbenzothiazole-2-sulphenamide (DCBS), and 2-mercaptobenzothiazole (MBT) is measured in dicumyl peroxide vulcanised Styrene-Butadiene rubber (SBR), Acrylonitrile-Butadiene rubber (NBR) and Ethylene-Propylene-Diene rubber (EPDM) rubber at room temperature and at 60 °C. The experimental results can be correlated with the calculated solubility parameters d, as determined using the method of Hoftijzer and Van Krevelen. Results of Dd are used to judge the solubility of curatives in a specific rubber in blends. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Attempts to determine the solubility of the curatives in rubbers have already been made for quite some time. Initial values of sulphur solubility in natural rubber (NR) were measured by Venable and Greene [1] in 1922. Since then, other experimental methods, e.g. equivalent solvents [2], weight take-up, microscopy [3–6], radioactively labeled sulphur [7–10], ToF-SIMS [11] etc. were used to determine the solubility of curatives, in most cases the solubility of sulphur in rubbers. Amongst all these methods, the method of Morris and Thomas [12] is most effective to give reliable solubility results. The internal crystallisation of sulphur is excluded due to the isothermal experimental procedure. What is more, the peroxide cure applied in the method can reduce the modification of chain structures of rubber, which consequently reduce the influence on solubility compared to sulphur curing. * Corresponding author. Tel.: +31 53 4892529; fax: +31 53 4892151. E-mail address: [email protected] (J.W.M. Noordermeer). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.07.054
Although quite some experimental data have been obtained, an apparent lack of consistence exists due to the drawbacks of each method and the experimental conditions. On the other hand, the wide-spread use of rubber blends makes people become much more interested in the solubility of sulphur in rubbers other than NR. Especially EPDM is interesting due to its excellent ozone- and oxygen-resistance. As the distribution and dispersion of curatives other than sulphur in blends of dissimilar rubbers is also vital for the properties of such vulcanised blends, it is of great interest to determine the solubility of accelerators in various rubbers as well, in an attempt to obtain some insight into the mechanistic aspects involved in curing the blends. The solubility parameters of the rubbers and the curatives involved in this study can be calculated by the method of Hoftijzer and Van Krevelen*** [13], by adding the contributions from all functional groups. The solubility parameter d (J1/2/cm3/2) can then be used to predict the mutual solubility. The details of measurement and calculation are described earlier.
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R. Guo et al. / European Polymer Journal 44 (2008) 3890–3893
2.1. Solubility calculations of the elastomers and curatives The calculated solubility parameters of SBR is dd 16.6, dp 0.5, dh 0; NBR is dd 17.2, dp 8.6, dh 4.3; EPDM is dd 16.4, dp 0, dh 0. It should be mentioned that both the content of more polar groups (e.g. the styrene group in SBR, the acrylonitrile contained in NBR) and the molar ratio of the copolymers influence the polarity and amount of hydrogen bonding in these polymers. The solubility parameters of the curatives are summarized in Table 1. From the values of dp it can already be deduced that the polarities of the accelerators are much higher than that of sulphur. The mutual solubilities between rubbers and sulphur or curatives as reflected in the values of Dd, are given in Table 2. It illustrates the preference of each curative towards the three rubbers used in this study at room temperature.
Table 2 Calculated Dd between rubbers and curatives at room temperature [J1/2/cm3/2]
SBR NBR EPDM
a
S8
OT20
CBS
DCBS
MBT
16.5 18.6 16.8
5.4 10.7 5.6
8.1 5.9 8.5
8.3 8.1 8.5
11.4 6.0 11.8
8.0 7.0 6.0
% wt increase
2. Results and discussion
2.2. Solubility of sulphur and polymeric sulphur in different elastomers
5.0 4.0 3.0 2.0 1.0
2.3. Solubility of accelerators in different elastomers The highest solubility of CBS is in NBR at both temperatures. This correlates with the fact that the polarity of CBS is much higher than that of sulphur, which is also Table 1 Calculated solubility parameters of curatives [J1/2/cm3/2]
dd dp dh
S8
OT20
CBS
DCBS
MBT
33.1 0 0
22.0 0 0
20.4 4.0 6.3
21.8 2.2 6.2
22.5 7.5 6.8
0.0 S8
OT20
CBS
DCBS
MBT
S8
OT20
CBS
DCBS
MBT
b 15.0 12.0
% wt increase
A comparative overview of the solubilities, the highest values taken from the measurements, of all the curatives for a certain temperature is given in Fig. 1a and b according to the three rubbers involved in this study. The weight up-take of sulphur (S8) and polymeric sulphur (OT20) increases with time and temperature. The preference of sulphur is SBR>EPDM>>NBR. As the solubility difference of sulphur between SBR and EPDM is small, therefore, it can be expected that in case of blends of dissimilar rubbers, a homogeneous dispersion of sulphur is achieved for the blend SBR/EPDM. On the contrary the solubility difference of sulphur in NBR and EPDM is large, therefore, sulphur is expected to accumulate in the EPDM phase. The solubility of polymeric sulphur (OT20) is 10-fold higher than that of elemental sulphur for SBR and EPDM. However, it is still insoluble in NBR. The preference of polymeric sulphur is EPDM>SBR>>NBR. The higher solubility of polymeric vs. elemental sulphur observed in EPDM can be related to the fact that there is some 20% oil mixed in OT20, which has a high solubility in EPDM. However, the solubilities of OT20 in EPDM and SBR are still very close, so that it will still result in a good dispersion of OT20 in a blend of SBR/EPDM. In a NBR/EPDM blend the OT20 will participate more to the EPDM phase than the NBR phase, similar to the elemental sulphur.
9.0
6.0
3.0
0.0
Fig. 1. (a) Comparison of solubility of curatives in SBR, NBR and EPDM gum rubber at room temperature. (b) Comparison of solubility of curatives in SBR, NBR and EPDM gum rubber at 60 °C. (j) SBR; NBR; hPDM.
in accordance with the higher value of dp of CBS. DCBS shows the highest solubility in EPDM at 60 °C, different to CBS. The thiazole-type curative, MBT is a decomposition product of all sulphenamide accelerators. It has the highest solubility in NBR at room temperature as well as at 60 °C, which matches with the high polarity of MBT shown in the dp in Table 1. It has a great tendency to partition into the NBR phase in rubber blends like NBR/SBR and NBR/EPDM. 2.4. Correlation between the calculated solubility parameters with the experimental results As stated before, mutual solubility can happen only when the value of Dd is smaller than 5 J1/2/cm3/2. This
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R. Guo et al. / European Polymer Journal 44 (2008) 3890–3893
a
8.0 OT20 OT20
Solubility (%wt increase at R.T.)
7.0 6.0 5.0 4.0 3.0 2.0
CBS MBT
DCBS DCBS CBS CBS DCBS
1.0 0.0 0.0
2.5
5.0
7.5
10.0
Δδ
Solubility (%wt increase at 60 οC)
b
20.0
OT20 MBT MBT 12.5
S8 S8 15.0
S8
17.5
20.0
(J1/2/cm3/2)
OT20 OT20
15.0
10.0 MBT
5.0
0.0
0.0
DCBS
DCBS CBS CBS CBS DCBS 2.5
5.0
7.5
OT20
10.0
Δδ
S8 S8
MBT MBT 12.5
15.0
17.5
S8 20.0
(J1/2/cm3/2)
Fig. 2. (a) Correlation between the calculated solubility parameters and the experimental data at R.T. (b) Correlation between the calculated solubility SBR; NBR; EPDM. parameters and the experimental data at 60 °C.
rule can now be used to check if the calculated Dd is sufficiently predictive for the experimental values. In Fig. 2, the solubility data are plotted against the value of Dd for both R.T. and 60 °C. Due to the complexity involved in calculating the solubility parameters at 60 °C, especially the refined solubility parameters, dd, dp and dh, the solubility measured at 60 °C is also plotted against the Dd at room temperature. For this situation, it is still possible to observe the same trend in Fig. 2b as in Fig. 2a. It is clear from Fig. 2 that a higher solubility is found with smaller Dd value, regardless what rubbers or curatives are involved. An extremely high solubility is observed for OT20, but this must most probably be attributed to the 20% oil contained.
3. Conclusions From the solubility data of sulphur, it is clear that elemental sulphur does not dissolve well at room temperature. The difference in solubility of sulphur in the different rubbers is more pronounced at higher temperatures (SBR > EPDM >> NBR). This is the main reason for cure incompatibility in rubber blends. Polymeric sulphur shows a 10-fold higher solubility than elemental sulphur. The high solubility of OT20 can partially explain its reduced blooming tendency. The other reason for reduced blooming is that polymeric sulphur is not migrating, which is not accounted for in this study. The solubility of accelerators is much higher than that of elemental sulphur in NBR,
R. Guo et al. / European Polymer Journal 44 (2008) 3890–3893 Table 3 General formulation for gum rubber compounds Component SBR NBR EPDM ZnO Stearic acid Dicumyl peroxide (40%)* *
Amount (phr) 100 0 0 5 2 1
0 100 0 5 2 4.8
0 0 100 5 2 6.25
PERKADOX BC-40B.
SBR and EPDM rubber. CBS and MBT are very polar, which gives them a preference towards NBR rubber (NBR >> SBR > EPDM). However, in the case of DCBS, the sequence of solubility is SBR > EPDM > NBR, explained by the molecular structure of DCBS, where the two benzene rings cause symmetry. The experimental data of solubility of curatives at room temperature can be correlated to the Dd values calculated by the method of Hoftijzer and Van Krevelen, as shown in Fig. 2, where a lower value of Dd correlates with a higher value in solubility. 4. Experimental 4.1. Materials Three different kinds of rubbers: S-SBR (BunaÒ VSL 5025-0HM from LANXESS), NBR (PerbunanÒ 3446F from LANXESS), and EPDM (KeltanÒ 4703 from DSM Elastomers) were employed. Zinc-oxide and elemental sulphur were purchased from Sigma–Aldrich and polymeric sulphur (CrystexÒ HS OT 20) was obtained from Flexsys; accelerators (SantocureÒ CBS, SantocureÒ DCBS and PerkacitÒ MBT) were also provided by Flexsys and stearic acid used as commercial type. Dicumyl peroxide was provided by Akzo Nobel, with active content of 40%. 4.2. Solubility measurements and calculations All the rubber samples were slightly crosslinked by peroxide before the solubility measurements were carried out. The general formulations are shown in Table 3. The vulcanisation was carried out at 160 °C for 15 min. The gel content was measured after extraction with toluene in a Sohxlet extractor for 2 days: 96.6% for SBR, 99.6% for NBR, and 98.2% for EPDM. The levels of crosslinking were kept low and in the similar range for the three different rubbers to exclude the influence of crosslink density on solubility. The vulcanised samples were then cut into sheets with a size of 10 10 2 mm. These sheets were extracted with acetone in a Sohxlet extractor for two days to remove the non-rubber parts. Finally, all the samples were dried in a vacuum oven for 24 h at room temperature. Samples
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were stored in a plastic bottle and protected from light before use. The solubility measurements were carried out in triplicate by placing the accurately weighed sample of vacanisate in a glass bottle such that they were packed on all sides with either sulphur or the accelerators of interest in the form of fine powders. The glass bottle was placed in a thermo-stated oven at room temperature or 60 °C. The sample weights were measured every day. The sample surfaces were cleaned with a sharp blade, followed by treatment with an adhesive tape before weighing in order to remove remaining adhering material. Blank experiments were carried out by inserting rubber samples into the curatives for a few seconds and weighing the sample after the cleaning procedure. The aim of a blank experiment was to check if the cleaning procedure was good enough to remove all the adhered powders from the surface of the samples. This proved to be the case: no weight increase was observed. The solubilities of both elemental sulphur and polymeric sulphur (OT20) were measured as the materials as received. The solubilities of the accelerators CBS, DCBS, and MBT were determined using the same procedure. CBS and DCBS were first ground into fine powders before use. MBT was used as received in powder form. Calculations of the solubility parameters were carried out according to the method of Van Krevelen [13]. The solubility parameter is calculated into three parts, dd: the solubility parameter component from the dispersion forces; dp: the component from polar forces; dh: the component from hydrogen bonding. Acknowledgements This project is financially supported by the Dutch Technology Foundation (STW), the Applied Science Division of NWO, the technology program of the Ministry of Economic Affairs of the Netherlands. TIMCAL Graphite and Carbon and Hexagon compounding in Belgium are also gratefully acknowledged for their support. References [1] Venable CS, Greene CD. Solubility of sulfur in rubber. Ind Eng Chem 1922;14:319. [2] Guillaumond F. The influence of the solubility of accelerators on the vulcanization of elastomer blends. Rubber Chem Technol 1976;49:105. [3] Gardiner JB. Rubber Chem Technol 1968;41:1312. [4] Graf HJ, Issel HM. New evaluation of dithiophosphates with reference to accelerators incapable of forming N-Nitrosamines. Kautsch Gummi Kunstst 1995;48:600. [5] Morris TC. Ind Eng Chem 1932;24:584. [6] Kemp AR, Malm FS, Stiratelli B. Ind Eng Chem 1944;36:109. [7] Baldi L, Zannetti R. Rubber Chem Technol 1962;44:1350. [8] Frederick Ignatz-Hoover, Byron To, Datta RN, Arie De Hoog, Huntink N, Talma A. Rubber Chem Technol 2003;76:747. [9] Mozisek M. Pol Eng Sci 1970;10:383. [10] Berry BS, Susko JR. IBM J Res Develop 1977. [11] Dias AJ, Galuska AA. Rubber Chem Technol 1996;69:615. [12] Morris MD, Thomas AG. Rubber Chem Technol 1995;68:794. [13] Krevelen DWV, Hoftijzer PJ. Properties of polymers. 3rd ed. Amsterdam: Elsevier Science B.V.; 1990.