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Effect of binding free sulfur in vulcanized soybean oil.

Vulcanized Vegetable Oil (VVO) is used in robber compounds to improve ozone resistance, increase dimensional uniformity, reduce hardness and improve extrusion and calendering. VVO is an effective means to boost ozone resistance in NR applications that require non-staining and low bloom capabilities. VVO is also an effective plasticizing agent in compounds that require high tensile strength and modulus, which limits the amount of process oil which can be used.

VVO is made by blending vegetable oil, sulfur and other raw materials above the required 160[degrees]C reaction temperature. Residual sulfur within the VVO itself can cause adverse effects on the long term aging properties of a rubber compound. This effect on aging properties limits the use of VVO in certain NR applications. Research on using a thiuram accelerator (TMTD) to bind the free sulfur in VVO to improve heat aging properties has been done in the past (ref. 1).

In this study, five different accelerators are evaluated at different loadings to characterize their effect on binding the free sulfur in VVO. The accelerators evaluated were TMTD (tetramethylthiuram disulfide), ZDMC (zinc dimethyldithiocarbamate), MBTS (2,2'-dibenzothiazole disulfide), TBBS (N-tertbutyl-2- benzothiazolesulfenamide) and DPG (N,N'-diphenylguanidine). Each accelerator was added in a VVO compound at 1.0 and 2.0 phr to evaluate the accelerator and loading effects on heat aging properties. The formulas in table 1 were used to evaluate the VVO compounds.

VVO preparation

VVO is made by blending vegetable oil and other raw materials together above the 160[degrees]C reaction temperature. The degummed soybean oil was heated in a beaker to 165[degrees]C on a hot plate. Once the desired temperature was reached, a powder blend of the sulfur, stearic acid, zinc oxide and accelerator was added to the soybean oil and continuously stirred. The pre-blended powder additives were blended in plastic bags and worked mechanically to remove any clumps within the powders to ensure uniformity of the VVO before they were added to the soybean oil. The time needed for the reaction to take place depended on the type and loading of the accelerator used, as shown in table 2.

Once the VVO was synthesized, it was removed from the beaker and put on an aluminum tray to cool. All VVOs rested for at least 72 hours before being mixed in a rubber compound or tested. All accelerated VVOs reacted faster than the control VVO without accelerators. VVO compounds with 1.0 phr of DPG and 2.0 phr DPG reacted at roughly the same time, regardless of accelerator level. The VVOs with TMTD, ZDMC, MBTS and TBBS had shorter reaction times with increased level of accelerator.

Chemical testing

The WO was tested for percent free sulfur and percent acetone extract to see what effects the accelerator additions made to the WO. The percent free sulfur is the unreacted or lightly bound sulfur during the vulcanization process (ref. 5). The percent free sulfur was analyzed using ASTM procedure D-297. The extraction time was 16 hours and the method of detection was titration. The results are shown in figure 1.

The percent free sulfur was lower in the VVO compounds with 1.0 phr ZDMC, 2.0 phr TMTD, 2.0 phr ZDMC and 2.0 phr DPG than the control compound with no acceleration. Acetone extract is the amount of unreacted oil and partially sulfurized glyceride oil extractable from the VVO (ref. 5). The percent acetone extract was tested per ASTM procedure D-297, and the extraction time was 16 hours at the reflux temperature (figure 2).

All accelerated VVO compounds had equivalent to lower acetone extract than the control compound with no acceleration. Acetone extracts of 35% or higher are considered standard grades of VVO (ref. 5).

Rubber formulas

The VVO compounds were mixed into a NR compound to evaluate aging properties and ozone resistance. Three control compounds were mixed and are as follows:

* Control 1: No VVO

* Control 2: Commercially available 2L Brown VVO

* Control 3: Compounded VVO with no accelerators.

The VVO was evaluated at 20 phr or 10% total weight of the mixed rubber compound. The mixed compounds were two pass mixed in a BR laboratory internal mixer and sheeted out and cooled on a two-roll mill between passes. The formulas are shown in table 3.

Rubber testing

The mixed rubber compounds were tested for Mooney viscosity/scorch, cure properties (MDR), unaged physical properties, aged physical properties, ozone resistance and compression set. The Mooney viscosity was tested on a large rotor at 100[degrees]C, and the Mooney scorch was tested at 121[degrees]C per ASTM D1646. The results are shown in table 4.

The scorch time was much shorter on the VVO synthesized in the lab (control 3), and decreased when the loading level of accelerator was raised.

The cure properties were tested on a moving die rheometer (MDR) per ASTM procedure D-5289 at 160[degrees]C (table 5).

Once again, the scorch time and Tc90 are much faster on the VVO synthesized in the lab (table 5, figure 3). This is more than likely the result of missing stabilizers used in the commercial manufacturing of VVO.

Slabs were cured for unaged physical properties, aged physical properties and ozone resistance using the MDR Tc90 value at 143[degrees]C. The unaged physical properties were tested on an Alpha Technologies T2000 tensiometer per ASTM procedure D-412.

Overall, the addition of VVO increases elongation (figure 5) and lowers 100% modulus (figure 4). However, VVOs with the higher loading of accelerators have higher 100% modulus and lower elongation than the lower level of accelerators.

There is a noticeable drop in tensile from the Control 1 compound with no VVO and the Control 2 compound with commercially manufactured 2L Brown VVO (figure 6). Overall tensile properties drop with VVOs with the higher loading of accelerators. The VVO synthesized with TBBS retained the same physical properties, regardless of loading. All synthesized VVOs with lower loadings had higher tensile properties than all three control compounds.

Durometer A was tested per ASTM D2240. There was not a significant difference in durometer between any of the compounds (figure 7). In fact, what variation exists seems to be inherent to the test itself.

Aged physical properties were tested per ASTM procedure D-573 for 96 hours at 70[degrees]C. The change in physical properties is shown in figure 8.

The commercially manufactured 2L Brown VVO has a higher percent change on 100% modulus than the Control 1 compound with no VVO. Compounds with 1.0 phr ZDMC, 2.0 phr MBTS and 2.0 phr DPG have better aging properties than the Control 1 compound with no VVO. The compound with 2.0 phr TMTD had lower modulus change than the Control 1 compound (figure 8).

The compounds with 1.0 phr ZDMC, 1.0 phr TBBS and 2.0 phr DPG had the same change in durometer that the Control 1 compound has with no VVO. The compounds with 1.0 phr DPG and 2.0 phr TMTD had a lower change in durometer units than the Control 1 compound. Control 2 (2L Brown VVO) and Control 3 (no accelerator VVO) had a higher change in durometer units than the Control 1 compound with no VVO (figure 9).

Compression set properties were tested per ASTM procedure D-395, Method B for 22 hours at 70[degrees]C (figure 10).

The compound with 2.0 phr TBBS has lower percent set than the Control 1 compound. The compounds with 1.0 phr MBTS and 2.0 phr MBTS have a compression set equivalent to the Control 2 (2L Brown VVO) compound.

The test compound was natural rubber (NR), which has unsaturation in the polymer backbone with no wax and no antiozonants, so as expected, all samples failed ozone resistance. However, we were looking for differences in ozone failure (figures 11, 12 and 13).

All of the accelerated VVO compounds performed better in ozone testing than the Control 1 and Control 2 compounds, which experienced complete shear.

Theory

It is known that using VVOs in a NR compound with lower percent free sulfur and acetone extract will provide superior heat aging properties (ref. 2). Factice or WO is made from fatty oils (soybean oil) that are mixtures of triglycerides of mono and polyunsaturated fatty acids. It is this unsaturation that allows for crosslinking (ref. 3). It has been found that in oil and sulfur mixtures, the sulfur combines at a ratio of greater than one, but less than two atoms of sulfur per double bond when vulcanized. When the oil sulfur mixture is activated or accelerated, the ratio increases to two sulfur atoms for each double bond lost (ref. 4). Accelerating the mixture ensures that more sulfur will be used up faster in the early part of vulcanization as diatomic sulfur leaving less free sulfur in the compound (ref. 4).

Conclusion

All accelerated VVOs had faster reaction times and lower percent acetone extract than unaccelerated VVO. The dithiocarbamate (1.0 and 2.0 phr ZDMC), thiuram (2.0 phr TMTD) and guanidine (2.0 phr DPG) had lower percent free sulfur than the control VVO. In a mixed robber compound, all accelerated VVOs had faster scorch time, TC (90), and Ts2 than the compounds mixed with unaccelerated VVO. All compounds with VVO had decreased modulus and increased percent elongation. There was a significant loss in tensile properties from the control compound with no VVO and the Control 2 compound with the commercially available VVO. VVO with 1.0 phr of acceleration had higher tensile properties than the Control 1 compound with no VVO. The commercially available VVO also had a higher change in durometer and 100% modulus when heat aged than the Control 1 compound with no VVO. VVO accelerated with a dithiocarbamate (2.0 phr ZDMC), thiazole (2.0 phr MBTS) or guanidine (2.0 phr DPG) had better heat aged properties overall than all of the control compounds. The VVO accelerated with a sulfenamide (2.0 phr TBBS) had lower compression set than the control compound with no VVO. VVO accelerated with high and low levels of a thiazole curative (MBTS) had equivalent compression set to the control compound with no VVO. All accelerated VVO compounds showed better ozone retention than the Control 1 compound with no VVO and the Control 2 compound with commercially available VVO.

Overall, VVO derived from degummed soybean oil and accelerated with a low loading dithiocarbamate (1.0 phr ZDMC), a high loading thiazole (2.0 phr MBTS) and a high loading guanidine (2.0 phr DPG) had superior reaction times to un-accelerated VVO. In a mixed rubber compound, these accelerated VVOs showed improved heat aging and ozone retention properties compared to the control compounds with no VVO and commercially available VVO. Therefore, a VVO accelerated with one of these accelerators could be used at higher loading levels in a mixed rubber compound without adversely affecting aging properties, allowing for growth in the soybean oil derived VVO market.

This article is based on a paper presented at the 184th Technical Meeting of the Rubber Division, ACS, October 2013.

References

(1.) Samir H. Botrost, Fawzia E Ada El-Moshen and Eberhard A. Meinecke, "Effect of brown vulcanized vegetable oil on ozone resistance, aging and flow properties' of rubber compounds, " Rubber Chemistry and Technology." March 1987, Vol. 60, No. 1, pp. 159-175.

(2.) John S. Dick, How to Improve Rubber Compounds: 1,500 Experimental Ideas for Problem Solving, Hanser Gardner Publications Inc., Cincinnati, OH, 2004.

(3.) Robert Brentin and Phil Sarnacke, "Rubber compounds." A market opportunity study," September 2011, United Soybean Board, Omni Tech International Ltd.

(4.) E.A. Hauser and M.C. Sze, "Chemical reactions during vulcanization III," Journal of Physical Chemistry." January 1942, Vol. 46, No. 1, pp. 118-131.

(5.) R.O. Ebewele, A.E Iyayi, F.K. Hymore, S.O. Ohikhena, P.O. Akpaka and U. Ukpeoyibo, "Polymer processing aid from rubber seed oil, a renewable resource." Preparation and characterization, "African Journal of Agriculture: May 2013, Vol. 8 (18), pp. 1,925-1,928.

by Nicole Hershberger, Akron Rubber Development Laboratory

Table 1--VVO formulas

                Control      TMTD      ZDMC       MBTS
                      1   1.0 phr   1.0 phr    1.0 phr
Soybean oil         100       100       100        100
Sulfur               25        25        25         25
Zinc oxide            5         5         5          5
Stearic acid          1         1         1          1
TMTD                            1
ZDMC                                      1
MBTS                                                 1
TBBS
DPG
Total               131       132       132        132

                   TBBS       DPG      TMTD      ZDMC
                1.0 phr   1.0 phr   2.0 phr   2.0 phr
Soybean oil         100       100       100       100
Sulfur               25        25        25        25
Zinc oxide            5         5         5         5
Stearic acid          1         1         1         1
TMTD                                      2
ZDMC                                                2
MBTS
TBBS                  1
DPG                             1
Total               132       132       133       133

                  MBTS      TBBS       DPG
               2.0 phr   2.0 phr   2.0 phr
Soybean oil        100       100       100
Sulfur              25        25        25
Zinc oxide           5         5         5
Stearic acid         1         1         1
TMTD
ZDMC
MBTS                 2
TBBS                           2
DPG                                      2
Total              133       133       133

Table 2--reaction time

                  Control         2        3        4
            No accelerator    1 phr    1 phr    1 phr
                               TMTD     ZDMC     MBTS
Reaction                50       24       21       27
time,
minutes

                 5        6        7        8
             1 phr    1 phr    2 phr    2 phr
              TBBS      DPG     TMTD     ZDMC
Reaction        25       33       16       15
time,
minutes

                 9       10       11
             2 phr    2 phr    2 phr
              MBTS     TBBS      DPG
Reaction        17       23       35
time,
minutes

Table 3--rubber compounds

                        Phr       Phr
Raw material        control       VVO

SMR CV60                 100       100
N550                      40        40
N990                      40        40
HV naphthenic oil         10        10
Zinc oxide                 5         5
Stearic acid               1         1
TMQ                      1.5       1.5
Sulfur                   0.8       0.8
TBBS                       1       1.0
Bismate                  0.3      0.05
Various VVOs              -       20.0
Total                  199.6    219.35

Table 4--compound Mooney viscosity and scorch

                        Control 1                                  2
                               no                              1 phr
                      accelerator    Control 2    Control 3     TMTD
ML 1+4 @
100[degrees]C, MU            57.5         51.7         52.0     49.8

Ts5 @
121[degrees]C, min.           25+          22.4         9.58    13.0

                          3        4        5        6        7
                      1 phr    1 phr    1 phr    1 phr    2 phr
                       ZDMC    MBTS      TBBS      DPG     TMTD

ML 1+4 @
100[degrees]C, MU       50.3     49.1     50.5     54.2     50.9

Ts5 @
121[degrees]C, min.     11.7      8.8     10.8     17.2     11.2

                          8        9       10       11
                      2 phr    2 phr    2 phr    2 phr
                       ZDMC     MBTS     TBBS      DPG

ML 1+4 @
100[degrees]C, MU       48.3     48.7     49.9     54.8

Ts5 @
121[degrees]C, min.      9.0      7.7     10.5     15.7

Table 5--MDR properties at 160[degrees]C

                                   Tc50,    Tc90,    Ts2,
Test I.D     MH, N-m    ML, N-m     min.     min.    min.

Control 1        1.13       0.18     2.90     3.71    2.45
Control 2        0.95       0.18     2.20     3.54    1.85
Control 3        1.09       0.21     1.19     2.12     0.9
1 phr TMTD       1.05       0.20     1.44     2.41    1.12
1 phr ZDMC       1.05       0.20     1.37     2.33    1.07
1 phr MBTS       1.15       0.20     1.16     2.07    0.86
1 phr TBBS       1.10       0.20     1.27     2.27    0.95
1 phr DPG        1.10       0.20     1.75     2.82    1.39
2 phr TMTD       1.07       0.20     1.38     2.32    1.09
2 phr ZDMC        1.0       0.19     1.27     2.27     1.0
2 phr MBTS       1.14       0.20     1.10     1.98    0.82
2 phr TBBS       1.18       0.20     1.33     2.29    0.98
2 phr DPG        1.05       0.21     1.77     2.82    1.43

Figure 1--percent free sulfur

          Free
          sulfur,%

Control    0.8
1 TMTD     0.9
1 ZDMC     0.45
1 MBTS     1.26
1 TBBS     0.97
1 DPG      1.17
2 TMTD     0.64
2 ZDMC     0.39
2 MBTS     1.2
2 TBBS     1.59
2 DPG      0.68

Note: Table made from bar graph.

Figure 2--percent acetone extract

          Acetone
          Extract, %

Control    51.05
1 TMTD     43.83
1 ZDMC     47.58
1 MBTS     51.33
1 TBBS     45.24
1 DPG      37.31
2 TMTD     47.6
2 ZDMC     49.27
2 MBTS     47.32
2 TBBS     50.43
2 DPG      40.65

Note: Table made from bar graph.

Figure 10--% compression set

            Percent
            set

Control 1   26.1
Control 2   30
Control 3   35.2
1 TMTD      34.7
1 ZDMC      32.7
1 MBTS      30
1 TBBS      34.8
1 DPG       34.8
2 TMTD      33.2
2 ZDMC      35.1
2 MBTS      28
2 TBBS      24.5
2 DPG       30.2

Note: Table made from bar graph.
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Publication:Rubber World
Date:Nov 1, 2013
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