Balanced Crosslink Network Created in Natural Rubber by using

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					                                               ELASTOMERE UND KUNSTSTOFFE
                                                  ELASTOMERS AND PLASTICS

                                                Balanced Crosslink Network
Durability Á Balanced network Á Tetra-
methyl thiuram disulphide (TMTD) Á Tet-
rabenzyl thiuram disulphide (TBzTD) Á

                                                Created in Natural Rubber by
Cure efficiency

The durability of rubber goods often re-
quires heat resistance in combination
with good dynamic properties and ade-
quate oxygen/ozone resistance. A prop-
                                                using Sulphenamide/Tetrabenzyl
er balance of the network is important
for flex and heat aging properties. The
use of a binary system of accelerators
                                                Thiuram Disulphide Cure System
such as sulfenamide and a commonly
used thiuram such as Tetramethyl thiur-
am disulphide (TMTD) improves cure ef-          R. Datta, B. Oude Egbrink, F. Ingham and T. Mori (NL)
ficiency but suffers from an adverse ef-
fect on scorch and flex related proper-
ties. In this article, the possibility of an
N-nitrosamine safe thiuram, Tetrabenzyl
thiuram disulphide (TBzTD), is explored
to obtain an improvement in cure effi-
ciency together with a balanced network
in typical natural rubber compounds.            A binary combination of accelerators, pri-         In earlier studies tetrabenzyl thiuramdi-
A network study has been carried out in         mary plus booster, is widely used [1 ± 3] in    sulphide (TBzTD) [7, 8] has been used in
order to correlate cured properties to the      the rubber industry in order to achieve in-     higher concentration, affecting both pro-
fine structure of the vulcanizates.
                                                creased cure rates. The choice of accel-        cessing and flex related properties. In the
                                                erators is mainly dictated by the design        present study the use of lower amount of
Herstellung von stabilen                        properties of the end product and the           TBzTD has been explored to achieve a
Kautschuknetzwerken durch                       processing capabilities. Another restric-       faster cure without negatively affecting
                                                tion for the choice of accelerators may         scorch resistance and flex related proper-
Verwendung von Sulfenamid/                      be imposed due to the N-Nitrosamine is-         ties. The use of TMTD as a booster is re-
Tetramethylthiuramdisulfid als                  sue [4]. Accelerators derived from sec-         ported also in this paper as a compari-
Beschleunigerkombination                        ondary amines, for example MBS,                 son.
                                                TMTD and TMTM, would fall into this ca-
Verschleiûfestigkeit Á stabiles Netzwerk Á
Tetramethylthiuramdisulfid (TMTD) Á Te-         tegory. The combination of a sulfen-            Experimental
trabenzylthiuramdisulfid (TBzTD) Á Ver-         amide, such as CBS or TBBS, and a
netzungseffizienz                               thiuram, such as TMTD or TETD, shows            Compound formulations are shown in
                                                high cure rates but suffers from adverse        Tables 1 and 2.
Die Verschleiûfestigkeit von Reifen er-         effects on scorch resistance and vulcani-          Compound mixing was carried out in
              È                 È
fordert Bestandigkeit gegen Warme,
Sauerstoff, Ozon und Walzarbeit. HierfurÈ
                                                zate dynamic properties [5]. Additionally,      two stages. In the first phase rubber
ist ein stabiles Netzwerk wichtig. Der          as previously mentioned, the use of             was mixed with carbon black, Zinc oxide
Einsatz binarer Beschleunigerkombina-           TMTD or TETD is undesirable [6] due to          and all other ingredients except CBS and
tionen bestehend aus einem Sulfenamid           concerns over the carcinogenic nature           sulfur for five minutes in an internal mixer
und ublicherweise Tetramethylthiuram-           of the N-Nitrosamines formed from the           of capacity 5 kg at a dump temperature
disulfid (TMTD) verbessert zwar die Ver-
netzungseffizienz, fuhrt aber gleichzeitig      parent secondary amines. A demand of            of 130 ± 135 8C. The masterbatch was
zu einer Reduzierung der Scorch-Zeit            the rubber industry today is to formulate       kept for 24 hrs before finalising on a
und Verschlechterung der Eigenschaften          a cure package which can increase cure          two-roll mill. Compound cure charac-
die von der Verformung bestimmt wer-            rate without a significant, negative effect     teristics were determined using an
den. In der vorliegenden Arbeit wird der
Einsatz von Tetrabenzylthiuramdisulfid
                                                on scorch resistance and physical and           MDR 2000E. The mixes were vulcanised
(TBzTD) vorgestellt, das ohne N-Nitrosa-        mechanical related properties.                  for t90 and 60 minutes (overcure) at
mine zu bilden, zu einer verbesserten                                                           150 8C.
Vernetzungseffizienz und ausgewogenen
Netzwerkeigenschaften fuhrt. Der Effekt
geringer TBzTD Dosierungen in Sulfen-
                                                 Table 1. Formulations
amid beschleunigten Vernetzungssyste-
men wird mit dem Ziel untersucht, bes-           Ingredients/Mixes           1            2             3            4            5
sere Leistungen zu erhalten. Eine Netz-
                                                 NR SMR CV                 100          100           100          100          100
werkuntersuchung wurde durchgefuhrt È
                                                 N-330                      50           50            50           50           50
um die Eigenschaften mit der Mikro-              Zinc oxide                  5            5             5            5            5
struktur der Vulkanisate in Beziehung            Stearic acid                2            2             2            2            2
setzen zu konnen.                                Santoflex 6PPD              2            2             2            2            2
                                                 Santocure CBS               0.6          0.6           0.6          0.6          0.6
                                                 Perkacit TBzTD              ±            0.1           0.2          ±            ±
                                                 Perkacit TMTD               ±            ±             ±            0.1          0.2
                                                 Sulfur                      2.3          2.3           2.3          2.3          2.3

612                                                                                 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 11/2001
                                                  Balanced Crosslink Network Created F F F

KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 11/2001                                    613
                                                Balanced Crosslink Network Created F F F

 Table 2. Formulations                          Table 3. Processing and cure characteristics
 Ingredients/mixes       1          1A          Properties                1             2               3            4             5
                                                                          Control       0.1 phr         0.2 phr      0.1 phr       0.2 phr
 NR SMR CV               100        100                                                 TBzTD           TBzTD        TMTD          TMTD
 N-330                    50         50
 Zinc oxide                5          5         Processing data
 Stearic acid              2          2         Mooney viscosity          51            52              52           53            51
 Santoflex 6PPD            2          2         ML(1 ‡ 4) 100 8C, MU
 Santocure CBS             0.6        0.4       Mooney scorch, 121 8C     30            30              27           22            18
 Perkacit TBzTD            ±          0.2       t5, min
 Sulfur                    2.3        2.3       Rheometer at 150 8C
                                                ML, Nm                     0.20            0.20          0.20          0.21         0.20
                                                MH-ML, Nm                  1.66            1.82          1.90          1.85         2.03
                                                Ts2, min                   3.8             3.7           3.4           3.0          2.5
                                                T90, min                  10.0             7.3           5.8           6.2          4.6
   Stress strain and tear properties of the     Cure rate, t90-ts2         6.2             3.6           2.4           3.2          2.1
vulcanizates were measured using a
Zwick universal testing machine (model
1445) in accordance with ISO 37 and
ISO 34 respectively. Heat build up was         Results and discussion                             shorter optimum cure time compared to
measured on a Goodrich Flexometer ac-                                                             TMTD. Thus, the addition of small
cording to the method described in             The effect of TBzTD and TMTD is clear              amounts of TBzTD provides the potential
ASTM D623. Fatigue to Failure properties       from the data presented in Table 3. The            to increase productivity during vulcaniza-
were measured on a Monsanto Fatigue to         Mooney scorch values show significant              tion without hampering compound pro-
Failure tester at 23 8C and 1.67 Hz with       differences; TBzTD has far less effect             cessing characteristics.
0 ± 100 % extension. Abrasion resistance       on scorch safety than TMTD. Even at                   Compounds were cured to t90 and
(DIN) was measured following ISO 4649.         the reduced level of 0.1 phr TMTD the              60 minutes, the latter to simulate the ef-
Viscoelastic properties were determined        scorch safety is far inferior to the com-          fect of anaerobic aging. The cured sam-
using a Rheometric Dynamic Analyzer            pound containing 0.2 phr of TBzTD.                 ples were tested for stress/strain proper-
(RDA 700). The tests were carried out          TBzTD affects scorch time to a lesser ex-          ties, flex to fatigue failure, abrasion resis-
at 15 Hz and 60 8C with a dynamic strain       tent at 0.2 phr with hardly any effect at          tance, heat build up and visco-elastic
of 0.75 % and a prestrain of 1 %. For          0.1 phr (Fig. 1).                                  properties. Test data are listed in Tables 4
structural elucidation, the volume fraction       Reduction of the optimum cure time              and 5.
(Vr) in the swollen vulcanizate was deter-     (t90) can be achieved both with TBzTD                 The addition of both TBzTD and TMTD
mined by equilibrium swelling in toluene       and TMTD. The reduction is more pro-               provides better retention of hardness,
using the method reported by Ellis and         nounced with TMTD compared on an                   tensile strength and abrasion resistance
Welding. The Vr value so obtained was          equal phr basis. Nevertheless even at              upon overcure compared to the control
converted to the Mooney-Rivlin elastic         the low loading of TBzTD (0.1 phr) a de-           compound 1. Compression set is re-
constant (C1 ) and finally to the concentra-   crease of optimum cure time of the order           duced on overcure for the compounds
tion of chemical crosslinks by using equa-     of 30 % can be achieved. As indicated              containing TBzTD or TMTD. However,
tions described in the literature. The pro-    above, TMTD reduces scorch safety,                 flex resistance is negatively affected on
portions of mono-, di- and polysulphidic       ts2, significantly.                                incorporation of TMTD, particularly in
were determined using the thiol-amine             Compared at near equimolar levels               the overcured condition. This points to
chemical probe method. The details of          (0.2 phr TBzTD vs 0.1 phr TMTD) TBzTD              a crosslink network containing a higher
the method have been reported earlier          exhibits increased scorch safety with a            proportion of mono/disulfidic crosslinks
[9 ± 14].                                                                                         in the compound boosted with TMTD.
                                                                                                     The network structure of the vulcani-
                                                                                                  zates, the distribution of crosslink types,
                                                                                                  is described in Table 6. The total concen-
                                                                                                  tration of crosslinks is reflected by differ-
                                                                                                  ences in torque values observed in the
                                                                                                  rheometer data (see Table 3.) It is clear
                                                                                                  that the CBS/TMTD combination gener-
                                                                                                  ates a higher level of monosulfidic cross-
                                                                                                  links whereas the CBS/TBzTD cure sys-
                                                                                                  tem generates a more balanced network
                                                                                                  with regard to the relative levels of poly-
                                                                                                  sulfidic and monosulfidic crosslinks, al-
                                                                                                  most equal levels.
                                                                                                     This balance in mono- and polysulfidic
                                                                        Fig. 1. Mooney            crosslinks provides a good balance of
                                                                        scorch at 121 8C          properties.

614                                                                                 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 11/2001
                                                    Balanced Crosslink Network Created F F F

 Table 4. Vulcanizate properties (Cure: 150 8C/t90)                                               Table 7. Solubility of complexes in natural
                                                                                                  rubber (temp. 23 8C)
 Properties                 1 Control     2             3             4               5
                                          0.1 phr       0.2 phr       0.1 phr         0.2 phr     Complexes         v-value      Solubility
                                          TBzTD         TBzTD         TMTD            TMTD
                                                                                                  Complex (I)       1.72         LOW
 Hardness, IRHD              72            74            75            74              75         Complex (II)      1.32         HIGH
 M100 %, Mpa                  2.9           3.6           3.8           3.8             4.2
 M300 %, Mpa                 14.0          15.7          16.4          16.0            17.5
 Tensile Strength, Mpa       27.7          28.1          28.7          29.0            28.3         The improved cure efficiency of a sul-
 Elongation at break, %     530           500           505           525             480
                                                                                                 fenamide/thiuram cure system is believed
 Compression set, %
 (100 8C/72 h)               55            53            50            49              45
                                                                                                 to be due to synergistic effects, a result of
 (70 8C/24 h)                37            32            29            33              29        the formation of zinc/accelerator com-
 Fatigue to Failure, kc     143           142           142           122             122        plexes, I and II respectively for TBzTD
 (100 % ext.)                                                                                    and TMTD. The solubility in rubber com-
 Abrasion loss, mm3         104           101            94            93               91       pounds of these complexes (v) deter-
                                                                                                 mines their reactivity during vulcanization:
                                                                                                 the higher the solubility, the more rapid
                                                                                                 will be the desulfurization process of initi-
 Table 5. Vulcanizate properties (Cure: 150 8C/60 minutes)
                                                                                                 ally formed polysulfidic crosslinks. This
 Properties                             1 Control           3                   5                will result in a network containing a higher
                                                            0.2 phr TBzTD       0.2 phr TMTD
                                                                                                 concentration of monosulfidic crosslinks.
 Hardness, IRHD                          66                  71                  74              Table 7 gives solubility data of these com-
 M100 %, Mpa                              1.8                 2.4                 3.0            plexes in natural rubber [15].
 M300 %, Mpa                              8.8                11.1                13.2
 Tensile strength, Mpa                   21.8                25.3                25.7               The visco-elastic properties of the vul-
 Elongation at break, %                 560                 540                 410              canizates are shown in Table 8. The com-
 Compression set, %                      40                  33                  30              pounds containing TBzTD show inter-
 (100 8C/72 h)                                                                                   mediate values compared to the TMTD
 Fatigue to Failure, kc                  95                   70                 50              and control compound. This tendency
 (100% ext.)
                                                                                                 of a reduction in tangent delta suggests
 Abrasion loss, mm3                     120                   90                 80
                                                                                                 that the rolling resistance of tire tread
                                                                                                 compounds can be reduced through
                                                                                                 the incorporation of TBzTD, with the
                                                                                                 added advantages of no loss in scorch
 Table 6. Distribution of crosslink types (Cure 150 8C/t90)                                      safety during processing and increased
 Crosslink                  1 Control     2             3             4               5          cure rate.
 Concentration*                           0.1 phr       0.2 phr       0.1 phr         0.2 phr       However, compounds containing
                                          TBzTD         TBzTD         TMTD            TMTD
                                                                                                 TMTD, with even lower tan delta values,
 Total                      5.20          5.41          5.62          5.86            6.25       suffer from a reduction in scorch safety
 Poly-S                     4.15          2.34          2.41          2.30            2.33
 Di-S                       0.88          0.78          0.60          0.58            0.55       and dynamic properties.
 Mono-S                     0.17          2.29          2.61          2.98            3.37
* Expressed in gram mole/gram rubber hydrocarbon X 105                                           Comparison at equal moduls
                                                                                                 The data shown so far describe the con-
                                                                                                 cept of the addition of a small amount of
                                                                                                 TBzTD resulting in an increased cure rate
                                                                                                 at almost equal scorch safety, in compar-
                                                                                                 ison with the control. Physical and dy-
                                                                                                 namic properties might, however, be
                                                                                                 somewhat affected due to a difference
                                                                                                 in modulus. To compare compounds
                                                                                                 boosted with TBzTD on an equal modu-
                                                                                                 lus basis, the amount of primary accel-
                                                                                                 erator was decreased. The compound
                                                                                                 composition is shown in Table 2.
                                                                                                    As shown in Table 9 both compounds
                                                                                                 have the same MH-ML value (delta tor-
                                                                                                 que). From this table and Figure 2 it can
                                                                                                 be concluded that the addition of TBzTD
                                                                             Fig. 2. Rheometer   increases the cure rate significantly. There
                                                                             at 150 8C           is a minimal drop in scorch safety on the

KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 11/2001                                                                                 615
                                                      Balanced Crosslink Network Created F F F

                                                                                                   addition of 0.2 phr of TBzTD as observed
                                                                                                   also in the previous study.
                                                                                                       The vulcanizate properties are tabu-
                                                                                                   lated in Table 10. It is clear that the mod-
                                                                                                   ified system (compound 1A) possesses
                                                                                                   equal stress-strain properties but im-
                                                                                                   proved retention of tensile properties fol-
                                                                                                   lowing aging.
                                                                                                       Worthwhile mentioning is the signifi-
                                                                                                   cant reduction in heat build up (HBU) of
                                                                                                   the TBzTD boosted compound due to
                                                                                                   the more balanced crosslink network ob-
                                                                                                   tained (Table 11).
                                                                                                       Another advantage of using the sulfen-
                                                                                                   amide/TBzTD system is reflected in an
                                                                                                   improvement in the hysteresis character-
                                                                                                   istics. The data summarized in Table 12
                                                                                                   clearly indicate a reduction in tangent
                                                                                                   delta (tan d), indicating that improved roll-
                                                                                                   ing resistance can be achieved by using
                                                                                                   this accelerator system.

 Table 8. Vulcanizate visco-elastic properties (Cure: 150 8C/t90) Temp: 60 8C, Frequency:
 15 Hz and Strain: 2 %.                                                                            Conclusions
 Properties                   1 Control   2                 3          4            5
                                                                                                   A sulfenamide/TBzTD accelerator sys-
                                          0.1 phr           0.2 phr    0.1 phr      0.2 phr
                                          TBzTD             TBzTD      TMTD         TMTD           tem offers an attractive combination of
                                                                                                   compound characteristics: efficient vul-
 Storage mod. (E H ) Mpa      7.9         8.6               8.9        8.7          9.0
 Loss mod. (E HH ) Mpa        1.13        1.19              1.21       1.17         1.12           canization with little or no loss in scorch
 Tangent delta                0.143       0.139             0.137      0.135        0.125          safety. In addition, TBzTD in combination
                                                                                                   with a sulfenamide generates a crosslink
                                                                                                   network containing a more balanced dis-
                                                                                                   tribution of polysulfidic and monosulfidic
 Table 9. Processing and cure characteristics
                                                                                                   crosslinks compared to a compound
 Properties                                         1                     1A                       containing the sulfenamide alone or in
                                                    Control               TBzTD
                                                                                                   combination with TMTD.
 Mooney scorch, 121 8C                              30                    27
 t5, min
 Rheometer at 150 8C
 ML, Nm                                              0.20                  0.16
 MH-ML, Nm                                           1.66                  1.65
 Ts2, min                                            3.8                   3.3                      Table 11. Distribution of crosslink types
 T90, min                                           10.0                   6.2                      (Cure 150 8C/t90)
 Cure rate, t90-ts2                                  6.2                   2.9                      Crosslinks*             1            1A
                                                                                                                            Control      TBzTD
                                                                                                    Total                   5.20         5.22
                                                                                                    Poly-S                  4.15         2.33
 Table 10. Vulcanizate properties (Cure: 150 8C/t90; hot air aging: 100 8C/3 days))                 Di-S                    0.88         0.70
 Properties                                           1                    1A                       Mono-S                  0.17         2.19
                                                      Control              TBzTD                   *Expressed in gm mole/gm rubber hydrocarbon
 M100 %, MPa                                            2.9                  3.0                   Â105
 Aged                                                   4.2                  4.4
 M300 %, MPa                                           14.0                 13.7
 Aged                                                  16.5                 16.6
 Tensile Strength, MPa                                 27.7                 27.7
                                                                                                    Table 12. Vulcanizate visco-elastic prop-
 Aged                                                  18.2                 23.1
                                                                                                    erties (Cure: 150 8C/t90) Temp: 60 8C,
 Elongation at break, %                               540                  530
                                                                                                    Frequency: 15 Hz and Strain: 2 %.
 Aged                                                 350                  400
 Tear strength, kN/m                                  140                  130                      Properties                 1          1A
 Fatigue to Failure, kc (100 % ext.)                  143                  140                                                 Control    TBzTD
 HBU @ 100 8C in 2 hrs                                                                              Storage mod. (E H )        7.9        7.9
 Holder, delta, T, 8C                                  79                      45                   MPa
 Needle, delta T, 8C                                  BO1^*                    78                   Loss mod. (E HH ) Mpa      1.13       0.99
                                                                                                    Tangent delta              0.143      0.125
*Blow out after 115 min

616                                                                                     KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 11/2001
                                                  Balanced Crosslink Network Created F F F

   The above suggests that the sulfen-          References                                                 [9] B. Ellis and G.N. Welding, Rubber Chem. Tech-
                                                                                                               nol., 37 (1964) 571.
amide/TBzTD cure system can lead to               [1] J.F. Krymowski and R.D. Taylor, Rubber Chem.       [10] P.J. Flory and J. Rehner, J. Chem. Phys., 11
gains in productivity during the curing               Technol., 50 (1977) 671.                                 (1943) 521.
process with no loss in processing safety         [2] B. Adhikari, D. Pal, D.K. Basu and A.K. Chaud-     [11] L. Mullins, J. Appl. Polym.Sci., 2 (1959) 1.
                                                      huri, Rubber Chem. Technol., 56 (1983) 327.        [12] B. Saville and A.A. Watson, Rubber Chem.
together with improved vulcanizate per-           [3] M.P. Ferrandino, J.A. Sanders and S.W. Hong,             Technol., 40 (1967) 100.
formance characteristics.                             Paper presented at 147th meeting of Rubber di-     [13] A.H.M. Schotman, P.J.C. Van Haeren, A.J.M.
                                                      vision, ACS, Philadelphia, Pennsylvania, May 2 ±         Weber, F.G.H. Van Wijk, J.W. Hofstraat. A.G. Tal-
   With the use of TBzTD the curing sys-                                                                       ma, A. Steenbergen and R.N. Datta, Rubber
                                                      5, 1995.
tem is safe regarding N-nitrosamine for-          [4] H. Lohwasser, Kautschuk Gummi Kunststoffe,               Chem. Technol., 69 (1996) 727.
mation.                                               42 (1989) 22.                                      [14] R.N. Datta and J.C. Wagenmakers, J. Polym.
                                                  [5] R.M. Russell, T.D. Skinner and A.A. Watson,              Materials, 15 (1998) 379.
   Potential applications may be found in             Rubber Chem. Technol., 42 (1969)418.               [15] M.D. Morris and A.G. Thomas, Rubber Chem.
the tire industry and in compression and          [6] R.N. Datta and F.A.A. Ingham, Kautschuk Gum-             Technol., 68 (1995) 794.
injection molded general rubber goods.                mi Kunststoffe, 52 (11/1999) 758.
                                                  [7] D. Seeberger, Flexsys Techical Bulletin            Corresponding author
                                                      2AC.0.12.1 A/09-92.                                Dr. R.N. Datta, Flexsys BV, Technology Center
                                                  [8] D. Seeberger and G. Raabe, Kautschuk Gummi         Zutphenseweg 10
                                                      Kunststoffe, 42 (1989) 27.                         7418 AJ Deventer, NL
                                                                                                         Tel. ‡ 3 15 70 67 92 93


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KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 11/2001                                                                                                 617

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