The Effect of Binding Free Sulfur in Vulcanized Soybean Oil

ABSTRACT

The effect that different accelerators have on binding the free sulfur in vulcanized vegetable oil (VVO) to improve long term heat aging properties in a natural rubber (NR) compound were evaluated. Research on using 1.0 PHR of a thiuram accelerator (TMTD) to bind the free sulfur in VVO to improve heat aging properties has been undertaken in the past1. The purpose of this study is to see if other accelerators or higher loading levels of accelerators can bind the free sulfur within the VVO as well as, or better than 1.0 PHR of TMTD. Five different types of accelerators were reviewed at different levels and 1.0 PHR of a dithiocarbamate (ZDMC), 2.0 PHR of a thiazole (MBTS), and 2.0 PHR of a guanidine (DPG) had the most dramatic effects on aging. Most accelerators evaluated, when used at a higher loading level, improved their ability to bind free sulfur. Some accelerators improved both aging and ozone properties.

INTRODUCTION

Vulcanized Vegetable Oil (VVO) is used in rubber compounds to improve ozone resistance, increase dimensional uniformity, reduce hardness, and improve extrusion and calendaring. 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 and modulus strength 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°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.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:

1). TMTD (tetramethylthiuram disulfide)
2). ZDMC (zinc dimethyldithiocarbamate)
3). MBTS (2,2′-dibenzothiazole disulfide)
4). TBBS (N-tert-butyl-2-benzothiazolesulfenamide)
5). 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 affects on heat aging properties. The below formulas were used to make the VVO compounds evaluated.

VVO FORMULAS: TABLE 1 

VVO PREPERATION

VVO is made by blending vegetable oil and other raw materials together above the 160°C reaction temperature. The degummed soybean oil was heated in a beaker to 165°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.

REACTION TIME: TABLE 2 

Once the VVO was synthesized it was removed from the beaker and put on an aluminum tray to cool. All VVO’s rested for at least 72 hours before being mixed in a rubber compound or tested. All accelerated VVO’s 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 VVO’s with TMTD, ZDMC, MBTS, TBBS had shorter reaction times with increased level of accelerator.

CHEMICAL TESTING

The VVO was tested for % free sulfur and % acetone extract to see what effects the accelerator additions made to the VVO. The % free sulfur is the un-reacted or lightly bound sulfur during the vulcanization process5. The % free sulfur was analyzed using ASTM procedure D-297. The extraction time was 16 hours and the method of detection was titration. The results were as follows:

CHART1

The % 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 then the control compound with no acceleration. Acetone extract is the amount of un-reacted oil and partially sulferized glyceride oil extractable from the VVO5. The % acetone extract was tested per ASTM procedure D-297 and the extraction time was 16 hours at the reflux temperature.

CHART 2

All accelerated VVO compounds had equivalent to lower acetone extract then the control compound with no acceleration. Acetone extracts of 35% or higher are considered standard grades of VVO5.

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 lab banbury and sheeted out and cooled on a two roll mill between passes. The formulas are as follows:

TABLE 3 

RUBBER TESTING

The mixed rubber compounds were tested for Mooney Viscosity / Scorch, Cure Properties(MDR), Un-aged Physical Properties, Aged Physical Properties, Ozone Resistance and Compression Set. The Mooney Viscosity was tested on a large rotor at 100°C and the Mooney Scorch was tested at 121°C per ASTM D1646. The results are as follows:

TABLE 4

The scorch time was much shorter on the VVO synthesized in the lab 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°C.

TABLE 5 

CHART 3 

Once again the scorch time and Tc90 are much faster on the VVO synthesized in the lab. 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°C. The Un-aged Physical Properties were tested on an Alpha Technologies T2000 Tensiometer per ASTM procedure D-412.

Overall the addition of VVO increases elongation and lowers 100% Modulus. However, VVOs with the higher loading of accelerators have higher 100%Modulus and lower elongation then the lower level of accelerators.

CHART6

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. 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 then all three control compounds.

CHART 7  

Shore A Durometer was tested per ASTM D2240. There was not a significant difference in Durometer between any of the compounds. 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°C. The change in physical properties is as follows:

CHART 8

 

The commercially Manufactured 2L Brown VVO has a higher % 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.

CHART 9 

The compounds with 1.0 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 then the Control-1 compound. Control-2 (2L Brown VVO) and Control-3 (No accelerator VVO) had a higher change in Durometer units then the Control-1 compound with no VVO.

 

Compression set properties were tested per ASTM procedure D-395, Method B for 22 hours at 70°C.

The compound with 2.0 PHR TBBS has lower % set then the Control-1 compound. The compounds with 1.0 PHR MBTS and 2.0 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.

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

THEORY

It is known that using VVO’s in a NR compound with lower % free sulfur and acetone extract will provide superior heat aging properties.2 Factice or VVO 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.3 It has been found that in oil and sulfur mixtures that 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.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.

CONCLUSION

All accelerated VVO’s had faster reaction times and lower % acetone extract than un-accelerated VVO. The dithicarbamate (1.0 and 2.0 PHR ZDMC), thiuram (2.0 PHR TMTD), and guanidine (2.0 PHR DPG) had lower % free sulfur than the control VVO. In a mixed rubber compound all accelerated VVO’s had faster scorch time, TC (90), and Ts2 than the compounds mixed with un-accelerated VVO. All compounds with VVO had decreased modulus and increased % 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 then 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 age 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 VVO’s showed improved heat ageing 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.

REFERENCES
1 Samir H. Botrost, Fawzia F. ADA El-Moshen, 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: 1500 Experimental Ideas for Problem Solving, Hanser Gardner Publications Inc., Cincinnati, OH 2004.
3 Robert Brentin, Phil Sarnacke, “Rubber Compounds: A Market Opportunity Study”, September 2011, United Soybean Board, OMNI Tech International LTD.
4 E.A . Hauser, M.C. Sze, “Chemical Reactions During Vulcanization III”, Journal of Physical Chemistry: January 1942, Vol. 46, No. 1, pgs 118-131.
5 R.O Ebewele, A.F. Iyayi, F.K. Hymore, S.O. Ohikhena, P.O. Akpaka, and U. Ukpeoyibo, “Polymer processing aid from rubber seed oil, a renewable resource: Preperation and Characterization.” African Journal of Agriculture: May 2013, Vol 8 (18), pgs. 1925-1928.

 

 

(Visited 40 times, 1 visits today)