Tribology of 2-Mercaptobenzothiazole in Lithium Complex Grease

Abstract

Globally, extreme pressure greases are prepared by incorporating sulfurized fats, fatty acids, esters, sulfurised olefins, dialkyl polysulfides, organo-sulphur compounds and antimony dialkyl dithiocarbamate. A mixture of amine phosphates or Zinc dialkyl dithiophosphate types of compounds are used in combination with above EP additives to reduce the wear. The load carrying capacity of these EP additives is limited. Additionally, these additives contribute corrosion to yellow metals. In order to protect yellow metals, certain metal passivators are incorporated in grease, which contribute additional cost. Normally, derivatives of tolutriazole, benzotriazole or Mercaptobenzothiazole are incorporated to protect yellow metal corrosion.  There are various automotive and industrial applications, where very high shock load and high temperature is encountered by bearings. For such severe applications a high performance grease with high load carrying capacity is required. Normally, solid lubricants i.e. Molybdenum di sulphide or graphite is used in combination with antiwear additive for such applications. To meet the requirements of such applications, very high treat level of such solid lubricants required. Higher particle size of such solids may also cause abrasion/wear in bearings. For prolong use at high temperature these solids tend to lead cake formation. Considering the severity of such applications, we have attempted to study the tribological properties of 2-Mercaptobenzothiazole (MBT) in lithium complex grease. The EP properties of MBT were studied in lithium complex grease alone and in combination with amine phosphates type ashless anti-wear compound and Zinc dialkyl dithiophosphate. Load carrying properties were checked by conducting series of Extreme pressure tests by four ball method and Timken method. Frictional property was studied by universal tribometer. The developed Lithium complex grease has shown very high four ball OK load, Timken OK load and lower wear scar diameter.  The grease has shown very low coefficient of friction. The developed complex grease was tested for other regular characteristics in addition to tribological properties.

Introduction

There has been no much difference in usage pattern of EP/antiwear additive chemistry of  additives in greases and other lubricants like cutting fluids and gear oils. These additives perform very well in some greases but not able to perform up to desired level in other applications. Due to high active sulfur content, they need supplementary additive with them for non-ferrous metal corrosion protection. Additive chemistry plays a vital role in performance of greases, none the less base fluid and thickener play an important role. These additives though are in liquid states, seems well mixed in greases but not able to perform up to desired level in actual operating conditions. As the greases is worked and churned in bearing, the grease is required to liberate some oil to lubricate race way surfaces. Some of the additives also released with oil but not able to go back in grease’s jell structure. As a result depletion of additive takes place gradually and performance level of grease decrease. Thus the life of grease and bearings reduced. In view of this, the usage of solid additives with nano particle size is increasing in high performance greases in recent times. The biggest advantage of solid additives with nano particle size in  greases is it provides the structure to grease by working as the thickener/fillers and support the development of lubricating film on friction contact surfaces. Thus reduces the chances of depletion of additives with oil due to break down of grease structure due to high temperature and high load during operation. Analyzing the properties of 2-Mercaptobenzothiazole, we have attempted to study the performance of this additive in lithium complex grease. We have evaluated this additive in wide perspective of tribological behaviour in different combination with antiwear additives. Though the primary use of 2-Mercaptobenzothiazole is vulcanization accelerator for rubber, fungicide, copper corrosion inhibitors in various lubricants and coolants at a very low treat level. 2-Mercaptobenzothiazole has very high sulfur content hence it can also be used as an excellent EP additive.  Due to presence of two nitrogen atom in MBT, it is slightly basic in nature which  helps to neutralize the acidic components formed in application thus increases the life of grease. Additionally, due to solid nature it can work as a filler in greases and subsequently  increases  the yield of greases.

Experimental Details

Lithium complex grease was prepared by conventional method by using 12-hydroxy stearic acid and lithium hydroxide. Dodecanedioic acid was used as complexing agent in Lithium soap grease. An ISO VG 220 mineral oil was selected as a base fluid for better lubricity at high temperature application. The grease consistency was kept in NLGI 2 range.  This grease was fortified with commercially available antioxidant, rust inhibitor to meet other requirements i.e. Thermal and  oxidation stability, rust & corrosion resistance, water washout resistance property. Series of extreme pressure properties of Lithium complex grease were checked in following combinations of  EP and antiwear additives.

1.     Lithium complex grease without any EP and antiwear additive

2.     Lithium complex grease with only 2.5% 2-Mercaptobenzothiazole (MBT)

3.     Lithium complex grease with combination of 2.5% 2-Mercaptobenzothiazole and 1.0% Zinc dialkyl dithiophosphate antiwear additive

4.     Lithium complex grease with combination of 2.5% 2-Mercaptobenzothiazole and 1.0% ashless antiwear additive

5.     Lithium complex grease with combination of 2.5% conventionally available sulfur-phosphorous chemistry based EP additive and 1.0% Zinc dialkyl dithiophosphate antiwear additive.

The coefficient of friction was tested by universal tribometer in following combinations of EP and antiwear additives.

1.     Lithium complex grease with combination of 2-Mercaptobenzothiazole and Zinc dialkyl dithiophosphate antiwear additive

2.     Lithium complex grease with combination of 2-Mercaptobenzothiazole and ashless antiwear additive

The testing conditions were as under.

1. Load               -    300 N

2. Temperature   -   60⁰C

3. Frequency      -   50 HZ

4. Time               -   120 minutes

5. Contact           -   Pin-on-disk mode 

Results and Discussion

The test results of Lithium complex grease with 2- Mercaptobenzothiazole and ZDDP vis-à-vis Lithium complex grease with conventional EP additive and ZDDP are provided in Table 1. The various regular properties of grease with new additive combinations i.e. combination of 2-Mercaptobenzothiazole and  Zinc dialkyl dithiophosphate antiwear additive are similar with Lithium complex grease  with combination of conventional EP additive and Zinc dialkyl dithiophosphate. This reveals that 2-Mercaptobenzothiazole has compatibility and good additive response in lithium complex grease.  The series of extreme pressure properties of greases with different combinations of 2-Mercaptobenzothiazole with Zinc dialkyl dithiophosphate, ashless antiwear additive and grease with commercially available extreme pressure additive in combination with  Zinc dialkyl dithiophosphate are provided in table 2. The four ball weld load of lithium complex grease 2 ( only 2.5% MBT) has 500 kg weld load. The wear scar dia is 0.78 mm. After addition of  1.0%  Zinc dialkyl dithiophosphate and 1.0%  ashless antiwear additive respectively in this grease the wear scar dia has come down to 0.30 mm in grease 3 and 4 respectively. Additionally, the weld load is increased by 50 kg. Whereas, the grease with conventional EP additive & antiwear additive has shown a weld load of 315 kg and wear scar dia 0.42 mm. The Timken OK  load is also slightly higher with MBT compared to conventional EP additive. These higher values of four ball weld load, higher Timken load and lower wear scar diameter indicate that 2- Mercaptobenzothiazole has shown better response with lithium complex grease. The results of coefficient of friction of grease 3 (Lithium complex grease in combination of 2-Mercaptobenzothiazole and Zinc dialkyl dithiophosphate antiwear additive) and grease 4 (Lithium complex grease in combination of 2-Mercaptobenzothiazole and ashless antiwear additive) are provided in Figure 1. The coefficient of friction of 2-Mercaptobenzothiazole in combination with Zinc dialkyl dithiophosphate and ashless antiwear additive is very low.

           

                                     Figure 1. 

  

Conclusions

2-Mercaptobenzothiazole has shown better compatibility with Lithium complex grease and resulting good additive response.

Due to solid nature of 2-Mercaptobenzothiazole the yield of grease has increased marginally.

The series of four ball EP tests and Timken Ok load test reveal that 2- Mercaptobenzothiazole has exceptional load carrying capacity compared to conventional EP additives.

This is supported by low coefficient of Friction of greases compared to conventional EP additives.

The Based on these test results, it can be summarized that 2- Mercaptobenzothiazole has exceptional load carrying capacity compared to conventional EP additives.

References 

[1] Avery ,  et al. Organosulfur adducts as multifunctional additives for lubricating oils and fuels and as multifunctional lubricants, United States Patent, April 9, 1991

[2]Denis Smit & Sam Lane, High Performance Products from Invista  C12 Business, Tribology &  Lubrication Technology, pages 36-39

[3] Wilfried J. Bartz, The Importance of Synthetic Greases for Future High Performance Applications.  ELGI Annual Meeting  2005, Edenburgh

[4]  [4] Kavin  J. Chase & Gaston Aguilar, R.T. Vanderbilt Co., Inc. metal Dithiocarmates:  A New Approach to  Old technology, NLGI Spokesman, Vol. 71 No. 4, July 2007

[5] William c. ward jr.,  Richard A. denis, Morey Najman & Carlos L. Cerda de Groote:  EP Additive Response and Tribochemical film formation in Lithium and lithium complex Grease,    NLGI Spokesman, Vol. 71 No. 7, October  2007

[6] Polish, A. F. , & farmer, H.H. Dithiocarbamate additives in Lubricating greases, NLGI Spokesman, pg 200-205, 1979

Table 1. Test Results of  Lithium Complex Grease with  2 - Mercaptobenzothiazole and ZDDP antiwear additive vis-à-vis Lithium Complex grease with conventional EP additive and ZDDP antiwear additive

Characteristics	Lithium Complex Grease with  2 – Mercaptobenzothiazole and ZDDP	Lithium Complex grease with conventional EP additive and ZDDP	Test Method
NLGI Grade	NLGI 2	NLGI 2	NLGI
Consistency, @ 25oC Worked, 60 X Worked, 100, 000 X	281 305	283 309	ASTM D 217
Drop Point,oC	272	272	ASTM D 566
Copper Corrosion @ 100 oC, 24 hrs	1 a	1a	ASTM D 4048
Heat Stability, @ 100oC, 30 hrs % loss	1.85	1.76	ASTM D 6184
Wheel Bearing test, Leakage by mass, gm Slump test	2.63 Pass	2.66 Pass	ASTM D 1263
Water washout @ 80oC, % loss wt.	4.66	4.98	ASTM D 1264
Roll Stability, % change @ ambient, after 16 hrs @ 82oC, after 48 hrs.	6 16	7 18	ASTM D 1831
Oxidation Stability, @ 100oC Drop in psi, @ 100 hrs.	6	6	ASTM D 942
Emcor Rust Test, rating	0,0	0,0	IP 220

           

 Table 2. Tribological properties of Greases

Characteristics	Grease 1	Grease 2	Grease 3	Grease 4	Grease 5	Test Method
Load wear Index, kg	66	132	166	166	112	ASTM D 2596
Four ball weld Point,kg	200	500	550	550	315	ASTM D 2596
Four ball Wear scar dia, mm	0.80	0.78	0.30	0.30	0.42	ASTM D 2266
Timken OK load, lb	15	40	50	50	45	ASTM D 2509