The antiwear mechanism of zddp's. Part I

The antiwear mechanism of zddp's Part I H. Speddingand R.C. Watkins*

Zinc dialkyldithiophosphates (zddp's) have been used in internal combustion engine oils for over 30 years as antiwear and antioxidant additives. In this paper, their mode of action is investigated. It was concluded, using various analytical techniques, that zddp's decompose in oil solution predominantly by a hydrolytic mechanism, ultimately to zinc polyphosphate and a mixture of alkyl sulphides, which are the precursors of the antiwear action of zddp Keywords: wear resistance, zinc dialkyldithiophosphates (zddp's), cams, cam followers

Zinc dialkyl-phosphorodithioates or, as they are more commonly known, zinc dialkyldithiophosphates (zddp's) have been used in internal combustion engine oils for over 30 years as antiwear and antioxidant additives. Their mode of action is still a subject of controversy in both fields. One of the most highly stressed components of the modern internal combustion engines is the cam and lifter, or finger follower. In the absence of zddp type antiwear additives, these can fail prematurely due to catastrophic wear. The cam and lifter mechanisms commonly work in the 1 m s -~ speed and 1 GPa pressure region where simpler additives, such as fatty acids etc, cannot function due to the severity of surface conditions. Over the years, engine designers, striving for greater efficiency, have increased the stresses on engine components. Additive packages in lubricants have also become more complex to cope with other engine problems (rust, detergency, sludge dispersancy, etc). This has, inevitably, increased the requirement for more antiwear performance from the zddp ~'2 . Despite a great deal of research into alternatives, no more efficient substitute has yet been found. An obvious conclusion from this, therefore, has been to study the wear mechanism o f zddp to try to increase its effectiveness. Again, over the last 30 years, a large amount of research has been undertaken in this field, but no unequivocal mechanism has emerged. This can be attributed to two factors. Firstly, the multidisciplinary nature of the antiwear process and, secondly, the complexity of zddp's reaction mechanisms. The work o f Czichos 3 has highlighted the multidisciplinary nature of all tribological problems. For the cam and lifter wear case. the important areas are: • • • • • •

Organic solution chemistry Surface adsorption Inorganic chemistry Metallurgy Metrology Mechanical design

Since each of the above headings can be further subdivided tinder a number o f sub-headings, the complexity becomes *Esso Chemical Research Centre, PO Box No 1, Abingdon, Oxfordshire. UK. OX13 6BB

0301 - 6 7 9 X / 8 2 / 0 1

apparent. So, in order to make the problem manageable, we have chosen to leave the mechanical/metallurgical aspects till last and to concentrate on the solution chemistry and surface aspects. This is on the assumption that there will be some common factors among the mechanical - metallurgical inputs.

Solution chemistry Chemically, zddp may be written as:

\RO

\

S

t/\

OR

it decomposes in oil, due to high temperature reactions, to give a wide variety o f products. Our own work, and that of others (see Part II) has shown that zddp itself is not substantially chemisorbed on iron, but that its decomposition products are adsorbed and these supply the antiwear function. There is a considerable amount of work, therefore, in the literature in the area of bulk and solution decomposition. The reactions involved have been variously interpreted as free radical 4'6 or ionic 4,s . Evidence for a third interpretation based on a hydrolytic mechanism 2'7'8 has also been mentioned. Such is the number of side reactions and possible structures, that any of these can be made plausible. The primary reaction is loss o f the hydrocarbon alkyl groups and loss of sulphur in various forms. Since we are dealing with four alkyl groups and four sulphur atoms per molecule, many structures are possible. Sulphur-oxygen interchange is also very facile (all sulphur atoms are equivalent) and polymerisation to higher phosphorus acids is possible after the initial decomposition. The zincphosphorus bond can be represented as covalent or ionic. A wide variety of structures have been postulated in the literature 4,s,9-11 . Commercial zddp's also contain a variety of impurities from the synthesis, the most important of which is the basic salt. This may be present in varying quantities and acts as a basic stabiliser to the product. Quite apart from antiwear, one of zddp's primary purposes is to act as an antioxidant. This may remove part of the zddp from the antiwear decom-

0009--04503.00 © 1982 Butterworth & Co (Publishers) Ltd

9

SpeddL~g and VVatkins -- A n t i w e a r mechanisms o f zddp's Psrt ?

position path. Thus, the complexity of the decomposition mechanism is apparent and it is, therefore, necessary to find some way through this maze of possible reactions. Since there appears to be so much confusion as to the exact decomposition mechanism, we have preferred, in the overail picture of zddp antiwear action, to treat solution decomposition as a black box into which goes zddp and out of which come the ultimate decomposition products.

Experiment~

Ail reactions were carried out on a pure zinc di-n-pen[yb dithiopbosphate. This was prepared by ccnventionai r~ac~ tion of the pure alcohol with phosphorus pentasuiphide, neutralisation and separation of tb~e arnmon~um s i t , and finally double decomposition with zinc chlori4e. ~itially a wide variety of analytical techniques were applied to the butk, and sotution decomposition ia o,il:

zdds +-

Transmission ir Eieme~ta[ analysis Mass spectrometry Proton nmr 31 P ,~mr

Decomposit Zinc po~yphosphate

Nixed "alkyl sul~hide~"

What goes on in the box may be the cause of academic argument, but in terms of antiwear potency the two ulti° mate products are a zinc polyphosphate and a mixture of alkyt suiphides (H2S, RSH, RSR, RSSR, RSSSR). In this first paper we propose to demonstrate this, including some ideas on the preferred decomposition mechanism, which we believe to be predominantly hydrolytic. The reaction of these two ctasses of decomposition product with the iron wear surfaces to give antiwear effects will be discussed in the second paper.

Each of these co~tributed to the overall picture but 3 ~ P :'~r;~ was by far the most useful since it is capable of distinguishing all the phosphorus acids (ortho, poly~ pyre and the thio analogues). ~nitial]y bulk decompositions were carried out on 5g quaa. titles of zddp in a boiling tube heated to 160 and 200°C under N2- The resultant solid was then stripped under vacuum using a liquid nitrogen trap to collect the distiilate. At the end of the experiment the distitiate was ailowed to warm up to room temperature and any gaseous products were vented. The residues were then subjected to detailed .analysis (Table !). [n other experiments a 5% solution cf

Table I nCs zddp decomposition products analysis 160°C under N2 Test Elemental analysis

infrared

Zn P S

Mass

spectroscopy

10

5% in Hexadecane

Residue

Distillate

Residue

Distillate

Precipitate

l 5.8 ~3.0 19.4

Nil -

21.5 18.3 7o7

Nii 3,1 25.8

2~.4 ~5,9 13,8

Strong phosphate band 900--1300cm -1

-POC -P=S

Strong phosphate band 900-1300cm -~

-P=S no - P O e

Phosphate band 900-1300cm-

Proton nmr

31 P nmr

200°C under N z

-POCH2 -PSCH2 RSR RSSR no zddp {POs)(P= 06S) 4 (POS3)3 (PS4)3 -

{RS)3 PS (RS)3P (RS)3PO

{ RO)z PSSH (RO)2 {RS)PS IRS)3 PS (RS)aP {RS)3 PO {RS)2 PSOH RSR RSSR

TR~BOLOGY international February 1£82

Filtrate

-P--SCH2 RSR RSSR ~qo - P - O C H 2 o~~ zddp (PO3t{P2 06S} 4"(P03 S)3 { PS2$2 )3 {POS3 )3 {PS4i 3 -

(RS)3 PS (RS)3P (RS)3PO

Confirms 31 P nm: RSR RSSR

(POs ~-

{ RS) 3 PS stron~ (RS)3 P weak
Conf{rm~ 31 F~nmr

Spedding and Watkins - A n t i w e a r mechanisms o f zddp's Part 1

zddp in hexadecane was decomposed for 16 h at 200°(? under nitrogen and filtered. The filtrate and precipitate were also subject to detailed aualysis (Table 1). 31 P lmlr was carried out on the solids by dissolution in NaOH + D2 O, or sodium edta and conducting the test within 30 rain to prevent hydrolysis of the phosphorus acids.

I00

80

In all cases the distillate/filtrate showed a complex mixture of sulphides as found by other workers in the field 4'7 , dialkyl-dithio-(dioxy)-phosphoric acids, and a range of compounds of the forms (RX)3-PX; (RX)3-P, where X = S or O. These latter compounds confirm the work of Grishina and Kuzmina 6'~2-~4 and the more recent papers of Coy and Jones 15,16 , The analysis of the residues and precipitate showed a complex mixture of zinc phosphates/thiophosphates from the o r t h o acids (PX4 3-) to pyro (P2X7 4-) to polyacids (PX31-). The less rigorous the condition the more complex the mixture found. The solution precipitate was unique in its simplicity, being exclusively polyphosphates, with little residual sulphur. All this demonstrates the reduction in complexity of the solid products formed in solution, probably due to the lesser opportunity for intermolecular reactions. It also shows the greater migration of sulphur into oil soluble compounds. It is evident that all this is consistent with our theoretical mechanism with the possible exception of the (RX)3 PS and ( RX)3 P compounds. However, it has been shown ~7 that these types of compound can decompose fairly readily to phosphorus acids and alkyl sulphides. They can, therefore, share in the ultimate fate of the rest of the phosphorus and sulphur. In order to study the fate of the hexadecane precipitate a large sample was prepared and analysed after being subjected to further heat treatments for 1 h at 300--800°C (Table 2). The initial precipitate corresponded approximately to one R O - group and one sulphur per original zddp molecule and a polyphosphate chain length of approximately seven phosphorus atoms. Heat treatment at 300°C removed virtually all of the sulphur and the hydrocarbon group leaving a polyphosphate of the same chain length. Healing at higher temperatures gave little further cha~lge. The chai~l le,lgths were collfirmed by 31 P llmr. Since in compounded oils this precipitate was not formed, we hypothesized that it was micro-dispersed in oil by the detergent or dispersanl components in the oil. This was found to be true since decomposition in a solution of a polyisobutenyl succinimide dispersant showed the presence of a solid phase of approximately 5.0 nm particle size (ultracentrifuge method).

6O

40

20 I

Precipitate (1) @ 300 ° C (2) @ 400°C @ 500°C @ 800°C

Zn%

P%

S%

C%

23.6 30.2 31.1 32.9 33.0

17.6 20.3 23.0 24.2 23.9

3.2 0.4 0.2 < 0.1 < 0.1

9.2 5.4 1.2 0.4 < 0.1

(1) Di-n-pentyl dithiophosphate 5% in base oil heated at 200°C for 16 h, washed and dried precipitate (2) Precipitate heated in a furnace for 1 h in air at stated temperature

3

4

Fig 1 zddp decompositio,t at 170°C. l%nCs zddp in hexadecan e/dodecan e

This work gave us a relatively clear idea of the product chemistry from our black box but it gave little information on the mechanism of zddp breakdown inside the box. While this is largely irrelevant to the subsequent antiwear process, it does control the rate and to some extent the direction of breakdown to the antiwear precursors, and for this reason we have spent some effort on examining it. As stated above, three possible mechanisms have been postulated in the literature: • • •

Free radical Ionic (thermal) Hydrolytic

Our results tend to confirm the hydrolytic mechanism and we will produce supporting evidence. Our initial choice of this route was based on the assumption that it was the simplest of the three and thereafter we wielded Occams razor (invent all possible solutions and then cut off all but the simplest). Our reaction scheme In our reaction scheme we have left out the zinc and shown the reaction of only one of the four possible alkyl or sulphur groups. We have also omitted the rest of the pentavalent phosphorus bonds in order to simplify the writing of equations. We can write the hydrolysis reactions in simplified form: RO

Table 2 Analyses of heated zddp oil precipitate

2 Time, h

P - + H20 . . . . H+ ....

ROH

RO-

> ROH + H ÷ O - P

(1)

> R' = CH 2 + H 2 0

(2)

_H2_O_>R, = C H z + H * O

P -

P-

(3)

We postulate the formation of an alcohol followed immediately by dehydration by the phosphate acid group to olefin. Thus if we sum reaction (1) + (2), water acts as a catalyst for the overall reaction (3). A number of side reactions are then involved, polyphosphate formation generating water: -P-OH+HO-P

---->

-P-O

P

+H20(4)

TRIBOLOGY international February 1982

11

Spudding and Watkins - A n t i w e a r mechanisms o f zddp % P~rt :~

sulphur release, consuming water:

- P=S+H20

....

>-P=O+H2S

(5)

The surface reactions by which these precursors ieai ~o :>.e antiwear process will be discussed in Part H,

sulphur-oxygen interchange (probably intramo!ecu!ar):

S :P-

OR ....

>O:P-

SR

(6)

Subsequent reactions of H2 S plus olefin can then generate the k n o w n sulphides, the precise mixture depending upon the specific R group present: R' = CH2 + }t2 S - - - - > RSH . . . .

> RSR . . . .

> RSSR

Acknowledgements We wouM like to acknowledge the contributio,qs G£ ou:;anaiytical co~leagues at the Esso Chemical Research C e e t e to this work programme, and a!so to thank A~ Hubbard {br contributions to the experimentm wcrk.

(7)

Thus the problem of proof of the hydrolysis mechanism lies in a study of the effect of water on the reaction. However, since water: e is produced in reaction (4) e catalyses the reaction (3) is consumed in the reaction (5) it is impossible to study the effect of water on reaction ~ . We therefore decided to study the effect of water removal from the reaction on the assumption that if water removal stopped the reaction, then water must take part in it. We studied the loss of the R O - group in pure n-pentyl zddp by quantitative infrared using the 990 cm -~ peak at 1% concentration in hexadecane at 170 ° C. The results are given in Fig I. Using H2 O-saturated nitrogen a rapid autocatalytic decomposition was found as expected. Drying the nitrogen through a liquid nitrogen trap slowed the reaction b u t failed to stop it. Finally, we added a Dean and Stark trap to our test system and used a mixture of dodecane and hexadecane boiling at t70°C, to remove any water oveffnead as fast as it was formed° (it was necessary to dehydrate the system i i t i a l l y with boiling 4 0 - 6 0 petroleum ether and to remove this by distillation before beginning the test.) This approach gave almost complete suppression of the decomposition for a five hour period. This result confirms our original hypothesis that zddp breakdown is water-catalysed. Further tests, this time at 200°C, failed to stow the reaction as much. We were unable to distinguish whether this was due to a change to an ionic mechanism or to failure of the system to remove water fast enough to prevent reaction at 200°C. However, since the water content of automotive sumps is quite high enough to postulate a hydrolytic reaction and since sump temperatures can range from 100 ° 170 ° C, it almost certai~tly predominates in the automotive eng~neo

Conclusions Zinc dialkyldithiophosphates decompose in oi! soiution, predominantly by a hydrolytic mechanism, ultimately tc zinc polyphosphate and a mixture of alkyl sulphides. These two products are the precursors of the antiwear process of zddp.

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TR~BOLOGY [nternationai February 1982

References 1. Rounds F.G° Additive ~nteractions and Their Effect cn the Performance of a Zinc Dialkyl Dithiophosphate. ASLE Tra~s. Aprii 7978. 2i (2), 9 t - 1 0 t 2. Rounds F.G° Some Functions Affecting the Decompos~,tiono~ Three Commercial Zinc Organo-Dithiophosphates ASLE" Tra~s, April 1975, t8 (2), 79-89 3. Czichos H. Tribology: A Systems Approach. E&eviee. i 978 4. Brazie~A.D. and Elliott J.S. The Thermal Stabi!ity of Zim: Dithiophosphates. J. Inst. Per.., Februao, 196 7, 53 (5I S) 5. Ming.Fe:ag. [. Pyrolysis of Zinc Diaikyt Phosphorodithioate and Boundary Lubrication. Wear, i960, 3,209 3,i I 6. Grishina O.N° et al. Concerning Thermai Transformations of ZDDPs and O Aiky!, Alkyl Dithiophosphates. Nej?ekhgmdya, t974, 14 (l), 147-151 7, Lu~he~H~, ~iaumgarten E. and Staeck D. The Decomposition cf Dialkyl DithiophospF.ate in Hydrocarbons. Erdoei und KoMe, September J969, 22 {9), 530 8. Corinson Ao The Antiweaz Action of Zinc P_,i-n-Buty~Phos~]~a~,'= ASLE Tram, ApriI i979, 22 (2), 1 9 0 - i 9 2 9. Stanton G.M. and But'rail E.M. Decomposition Characteristics o£ Some 1DiarylCadmium, Ammonium, and Nicket Dithiophosphates. A CS PIeprint, (Div° Pet. Chera) , September ]96£ 14 (4/, A 5 9 - 6 7 10. Luther V.H. and Sinha S.K. Zur Reactionskiaetik der thermischee Zersetzung yon Zinkdialkyl Dithiophosphaten. E'rdoei u~d Koh!e February i964, !7 (2), 9t ! I. Ashfor8 etal. The Thermal Decompositio~ of Zinc Di(4-Me~i~,y?~ pentyl-2) Dithiophosphate. d. AppL Chem., April t 965, 15, 7 7C 12.Gfishina O.N. et ai. Comparative Assessment o.f the Therma~ Stability of Various Organic Compounds Containing Phosphorus and Sulphur. Ne]tekhimiya, 197!, ?,~ (2), 298-302 13.Kuzmiaa GoN. et aL Therma~Transformations of ZD~2:P Antioxidant. Neflekhim@a, 1972, ~2(I), i 12-.~ ] 7 14.Kuzmina G.N. et aL Investigation of Thermal Transi'ormatioas oi Zinc Salts of Certain Organophosphorus Acids. Ncf?ekhim~ya; t971, ~ (3), 465-469 !5.Coy RoCoand Jones R.B. The Thermal Degradatior and E? Performance of Zinc Dialkytdithiophosphate Additives in Wh).te OiL ASL£" Trans, January 1981, 24 (t), 77-90 16.Jones R.Bo and Coy R.Co The Chemistry of the Therma! Degradation of Zinc Dialkyldithiophosphate. A SLE T,'a,'~s~ January !981, 24(I), 9 t - 9 7 !7.Enge~ R.Ro and Liotta D. Thermal Deeompositio~ o£ P.hospho:o. thioates. Z C.S.( @, 1970, 522