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Determination of zinc and calcium in multigrade crankcase oils
Wear, 140 (1990)
49-61
49
Determination of zinc and calcium in multigrade crankcase oils H. H. Abou El Naga, M. M. Mohamed and M. F. El Meneir Research
Centre,
Misr Petroleum
Co., P.O. Box 228, Cairo
(Egypt)
(Received May 5, 1989; revised February 13, 1990; accepted March 5, 1990)
Abstract Atomic absorption spectroscopy is used for the determination of zinc and calcium in multigrade crankcase oils. It has been found that zinc and calcium concentrationsare subjected to matrix interferences from polymers incorporated in the oil formulations as viscosity index (VI) improvers. Results indicate that the two most important factors are concentration and chemicai structure type for the following VI improvers: styrene-isoprene, styrene-butadiene, poly(aIkyImethacrylate) and ethylene-propylene. Correlations between the zinc or calcium concentrations and either the concentration or the type of each VI improver are established via a regression analysis statistical technique. Acceptable models have been found with confidence higher than 99.5%.
1. Introduction Atomic absorption spectroscopy (AAS) has been rapidly developed and widely applied over the last three decades. The original papers on this technique were first published by Walsh and coworkers [ 1, 21. The main reasons for its wide applications are its simplicity, sensitivity, portability and accuracy. The theoretical principles and description of basic apparatus have been given elsewhere [l-5]. AAS is widely used in the petroleum industry, either for quality control purposes or for research activities. One of its versatile applications is the determination of zinc and calcium in new and used lubricating oils [3-61. The presence of these two metals in lubricating oil formulations is a result of the incorporated detergent and antioxidant additives. The methods applied to the determination of zinc and calcium via AAS basically rely on the dilution of tested samples with kerosene followed by aspiration into the flame. It has been shown in the case of zinc determination that such methods are subjected to matrix interference from other additives, especially copolymers incorporated as viscosity index (VI) improvers in crankcase lubricating oil formulations [ 7, 81. Generally, the following copolymers are used in lubricant formulations as VI improvers: styrene-isoprene copolymer (SICP), styrene-butadiene copolymer (SBCP), ethylene-propylene (oleiln) copolymer (OCP) and 0043-1648/90/$3.50
(DElsevier SequoWPrinted in The Netherlands
50
poly(alkylmethacrylate) (PMA). References 9-14 include more data about the function and mechanism of action of these VI improvers, The effects of VI improvers on measured zinc concentrations in unused crankcase oils have been studied [ 7, S] where only one concentration from each incorporated VI improver was considered and precision limits for these measurements were not established. On the contrary, the dete~ation of calcium concentrations in these oils has not been considered before. Therefore the aims of the present study are as follows: (1) to confirm previous published results for zinc determination, using variable concentrations of different commercial VI improvers in multigrade crankcase oil formulations (these concentrations are selected to be within the applicable commercial doses for VI improvers in multigrade crankcase oils); (2) to establish precision limits for zinc concentrations in these formations; (3) to calculate regression equations for each type of VI improver incorporated, so that those equations can be used in estimating the actual concentrations of zinc in multigrade crankcase oil formulations; (4) similarly, to measure the calcium concentrations in the presence of different VI improvers.
2, Experimental
details
A basic lubricating oil formula (Table l), i.e. without a VI improver, was used in all measurements. Components of this formula are neutral base oil (300 N), bright-stock (Brt 135) and a multipurpose additive (i.e. detergent, dispersant, antiwear and antioxidant). The zinc and calcium contents in this additive are 1.46 wt.% and 1.68 wt.% respectively. VI improvers were ~co~o~~d in this basic blend at different concentrations, which are within the applicable commercial doses for VI improvers in multigrade crankcase oils (Table 2). The base oil content is reduced so TABLE
1
Basic oil: composition
and properties
Composition Neutral base oil (300 N) (wt.%) Bright stock (Brt 135) (wt.%) Multipurpose additive (wt.%)
60 32.1 7.9
Properties Kinematic viscosity (cSt) at 40 “C Kinematic viscosity (cSt) at 100 “C Viscosity index Pour point (“C) Total base number (mg KOH (g sample>-‘> Sulphated ash (wt.%) Zn (wt.%) Ca (wt.%)
90.45 10.56 99 -9 6.45 0.82 0.1150 0.1325
51 TABLE 2 Viscosity index improver characteristics Polymm
type
Symbol
Cf-wo~ymer cOntent
W.%)
Copolymer content in crankcase oil formulations
As diluted f(wt.%)
As solid
f(wt.%)
Styrene-isoprene copolymer
SICP
Solid
lo-20
0.6-1.2
Styrene-butadiene copolymer
SBCP
Solid
11-21
0.88-l .68
Olefin copolymer
OCP
13
Poly(allqImethacrylate)
PMA
26
6-l 1 3.5-5
0.78-1.43 0.78-1.43
as to keep the zinc and calcium concentrations at constant values in all formulations. The following commercial VI improvers were tested: SICP at concentrations from 0.6 to 1.2 wt.%, SBCP at concentrations from 0.88 to 1.68 wt.%, OCP at concentrations from 0.78 to 1.43 wt.% and PMA at concentrations from 0.78 to 1.43 wt.%. Table 3 includes the percentage of ingredients in the formulated multigrade crankcase oils. The kinematic viscosities at 40 and 100 “C, the VI, the total base number and sulphated ash for these blends are also listed in Table 3. Neat blends with the same viscosity as these VI-improver-containing formulations were prepared in order to study the effect of viscosity on the measured zinc and calcium concentrations. Table 4 includes the percentages of ingredients and properties for these neat blends. The zinc and calcium concentrations were measured using a Perkin-Elmer model 460 atomic absorption spectrophotometer and according to operating conditions given in Table 5. A fraction of kerosene with boiling point range 150-250 “C was used as the solvent. To estimate the precision limits of the method, one sample was tested at least ten times under the same working conditions and by the same operator. The results obtained were substituted in the standard deviation equation. Duplicate results by the same operator should be considered suspect if the average of two results differ by more than the percentage repeatability limits. To estimate the correction factors, a computer program was used to explore and derive the anticipated statistical correlations between the percentages of reduction in zinc and calcium concentrations in the presence of each type of incorporated VI improver.
VI i?npraver (~.%I
0.60 0.72 0.84 0.96 1.04 1.20
0.88 1.04 1.20 1.36 1.62 I.68
0.78 0.91 1.04 1.17 1.30 1.43
0.78 0.91 1.04 1.17 1.30 1.43
CopOl~ tme
SICP
SBCP
OCP
PMA
59.36 59.27 59.19 69.10 59.02 58.94
59.36 59.27 59.19 59.10 59.02 58.94
69.29 69.lQ 59.09 58.99 58.89 58.79
59.48 59.40 59.32 59.24 59.19 59.09
Oil (300 N) (~*%I
31.96 31.92 31.87 31.83 31.78 31.73
31.9% 31.92 31.87 31.83 31.78 31.73
31.93 31.87 31.81 31.76 31.69 31.63
32.02 31.98 31.94 31.90 31.87 31.81
Brt 135 (~.%I
7.9 7.9 7.9 7.9 7.9 7.9
7.9 7.9 7.9 7.9 7.9 7.9
7.9 7.9 7.9 7.9 7.9 7,Q
7.9 7.9 7.9 7.9 7.9 7.9
111.17 119.91 132.65 142.38 163.11 168.56
123.21 136.17 143.71 169.11 173.21 189.24
119.72 136.49 148.51 174.98 180.61 221.45
100.05 111.10 128.76 142.60 156.32 182.91
MuLtipurposs Kinematic additive At 40 “C (wt.%)
Viscosity index improver blends: composition and properties
TABLE 3
15.70 16.54 17.72 18.86 19.71 20.60
15.6% 16.68 17.65 18.57 19.80 21.17
14.61 15.87 17.22 18.72 19.53 21.96
13.99 15.26 16.22 17.89 19.29 21.43
At 100 “C
viscositgj (cSt)
150 149 148 150 148 152
134 133 13% 131 132 133
124 122 12% 120 124 120
142 144 130 140 142 140
VI
6.38 6.44 6.4% 6.51 6.43 6.48
6.51 6.50 6.44 6.49 6.42 6.44
6.39 6.42 6.36 6.32 6.41 6.44
6.49 6.42 6.43 6.38 6.37 6.41
Total base number (mg KON (g sample) - ‘)
0.85 0.82 0.84 0.82 0.82 0.81
0.84 0.82 0.81 0.82 0.83 0.82
0.82 0.85 0.81 0.84 0.84 0.81
0.81 0.85 0.80 0.82 0.84 0.81
T&t%)
Sulphuted
53
TABLE 4 Neat blends: composition and properties
Composition Neutral base oil (3OON) (wt.%) Bright-stock (Brt 135) (wt.%) Multipurpose additive (wt.%) Properties Kinematic viscosity (cSt) At 40 “C At 100 “C Viscosity index Total base number (mg KOB (g sample)-‘) Sulphated ash (wt.%) Zn (wt.%) Ca (wt.%)
BO I
BO 2
BO 3
BO 4
BO 5
44.0 56.0 7.90
35.5 64.5 7.90
33.5 66.5 7.90
11.0 69.0 7.90
24.5 75.5 7.90
134.88 13.69 97 6.49
149.32 14.96 100 6.39
158.44 15.5 99 6.51
173.53 16.23 97 6.52
195.95 17.5 96 6.39
0.83 0.1150 0.1325
0.81 0.1150 0.1325
0.84 0.1150 0.1325
0.82 0.1150 0.1325
0.85 0.1150 0.1325
TABLE 5 Operating conditions for atomic absorption spectrophotometer Instrument Wavelength (nm) Zn Ca Lamp current (mA) Air flow (l min-‘) Acetylene flow (I minSlot width (mm) Response time (s) Burner head (in)
Perkin-Elmer model 460 213.9 422.7 15 35 13 0.7 1 4 (one slot)
3. Results and discussion 3.1. Precisicm limits of the method The calculated precision limits for zinc and calcium are found to be I- 2.62 ppm and f 3.00 ppm from the obtained concentrations respectively. Therefore, duplicate results by the same operator should be considered suspect if the average differs by more than 0.23%. These results indicate that the method has a high precision and good sensitivity. 3.2. Eflect of viscosity index improvers on the mtxsured zinc and calcium concentrations Table 6 includes the measured zinc and calcium concentrations in the presence of variable concentrations of tested VI improvers. The percentage reductions in measured concentrations from the actual concentrations, i.e.
10 12 14 16 18 20
11 13 15 17 19 21
6 7 8 9 10 11
3 3.6 4 4.5 5 5.5
SBCP
OCP
PMA
in measured
SICP
Reductions
TABLE 6
0.78 0.91 1.04 1.17 1.30 1.43
0.78 0.91 1.04 1.17 1.30 1.43
0.88 1.04 1.20 1.36 1.52 1.68
0.60 0.72 0.84 0.96 1.04 1.20
zinc and calcium
15.70 16.54 17.72 18.86 19.71 20.60
15.66 16.68 17.65 18.57 19.80 21.17
14.61 15.87 17.22 18.72 19.53 21.96
13.99 15.25 16.52 17.89 19.29 21.43
concentrations
1108 1087 1053 1021 969 942 1126 1111 1090 1067 1040 1020 1136 1129 1117 1113 1087 1076
1150 1150 1150 1150 1150 1150 1160 1150 1150 1150 1150 1150
1111 1086 1052 1014 966 939
1150 1150 1150 1150 1150 1150 1150 1150 1150 1150 1150 1150
Measured
Cured
1.22 1.83 2.87 3.22 5.48 6.43
2.17 3.39 5.22 7.22 9.57 11.3
3.65 5.48 8.43 11.22 15.74 18.09
3.39 5.57 8.52 11.83 16.00 18.35
(%I
Reduction
index improver
(ppm)
viscosity
Zn comentration
with increasing
1325 1325 1325 1325 1325 1325
1325 1325 1325 1325 1325 1325
1325 1325 1325 1325 1325 1325
1325 1325 1325 1325 1325 1325
Calculated
(ppm)
1316 1304 1297 1285 1276 1272
1309 1288 1281 1256 1245 1232
1300 1270 1254 1225 1198 1188
1300 1269 1252 1222 1197 1187
Measured
Ca concentratim
concentration
0.68 1.58 2.11 3.02 3.70 4.15
1.21 2.79 3.32 5.21 6.04 7.17
1.89 4.15 5.36 7.55 9.58 10.42
1.89 4.23 5.51 7.77 9.66 10.49
1%)
Reduction
55
-
SICP
-
SBCP
-
OCP
o----o
PMA
a I
0.6
0.8
1.2
1.0
1.L
1.6
1.8
SOLID COPOLYMER CONCENTRATION (WT %1 Fig.
1. Effect of copolymer concentration
on measured zinc and calcium
concentrations.
as incorporated in the basic formula, were calculated. F’igure 1 illustrates the percentage reduction in AAS analytical data for zinc and calcium against the solids content in weight per cent for each of the four tested VI improvers. According to these results,it is possible to make the following observations. (1) An increase in the VI improver concentrations resulted in a signifhxmt reduction in both the zinc and the calcium contents as analysed. (2) The VI improver chemical type has a sign&ant effect on both the zinc and the calcium measured concen~atio~. Percentage reduction for a solid VI improver of 1 wt.% concentration and for the maximum concenkation of each of the incorporated VI improvers used are listed in Table 7. (3) It is possible to rank the percentage reduction in zinc and calcium concentration in terms of VI improver chemical type according to the following: percentagereduction high
4
SICP > SBCP
> OCP 5 PMA
low
3.3. Eflect of viscosity cm percentage reduction in mmsured cmLcentratim It was very spout to End a clear answer to the following question: can such a reduction in measured zinc and calcium concen~tio~ be a
56 TABLE 7 Percentage reductions in zinc and calcium at 1 wt.% and maximum viscosity index improver concentrations VI improver
SICP SBCP OCP Ph‘IA
Concentration (wt.961
1 1 1 1
M&mum
Reduction (%) for I wt.% concentration Zn
Ca
13.90 4.60 4.30 2.35
8.68 3.06 3.03 1.85
concatrat&m
(wt.%)
1.20 1.68 1.43 1.43
Reduction (%) for -mum ci3ncentratia Zn
Ca
18.35 18.09 11.30 6.43
10.49 10.42 7.17 4.15
TABLE 8 Percentage reductions in zinc concentration for neat and viscosity-index-improver-containing blends CopOl@mer wpe
Kinsnzatic viscosity (cSt) at 100 “C
2% @pm)
Po&?ner
Neat
Po&??m
NeUt
P&$p&-
Neai
0.60 0.66 0.72 0.84 0.96
13.99 14.52 15.25 16.52 17.89
13.69
1111 1094 1086 1052 1014
1139 1138 1136 1133 1130
3.390
14.96 15.50 16.23 17.50
4.870 5.570 8.520 11.83
0.96 1.04 1.22 1.48 1.74
SBCP
1.20
17.22
17.50
1053
1130
8.430
1.74
OCP
1.04
17.65
17.50
1090
1130
6.220
1.74
PMA
1.04
17.72
17.60
1117
1130
2.870
1.74
SICP
S&id POW=-(wt.%)
Reduction (%) in zn cmmmti
result of the increase in the blend viscosity or to the presence of the different types of VI improver at variable ~oncen~~o~? Therefore, neat blends without any VI improver were formulated (Table 4) to give nearly the same viscosities as the VI-improver-containing formulations (Table 3). Zinc and calcium were also measured in these neat blends. Table 8 includes the results obtained for the zinc concentration. As the SICP concentration is increased from 0.6 to 0.96 wt.% (as solid copolymer weight percentage), the reduction in measured zinc concentration is increased from 3.39% to 11.83%. On the contrary, increasing the viscosity for neat blends by the same magnitude increased the percentage reduction by only 0.96%-1.74%. Figure 2 illustrates the change in percentage reduction of measured zinc concentration for neat blends and SICP-containing formulations. These results clearly show that the viscosity increase has a limited effect on reducing the measured zinc ~oncen~~on, and the reduction in concen-
57
L
I 0.60
I
I
-
SICP
BLENDS
-
NEAT
BLENDS
I SOLID
Fig.
2.
Percentage
reduction
I
I
0.80
0.70
SICP
I
CONCENTRATION
in zinc concentration
I
I 0.10
0.90 (WT %)
for neat and SICP blends.
tration is attributed mainly to the presence of the VI improvers. Other measurements in the presence of SBCP, OCP and PMA have also confirmed these SICP results. Measurements of calcium concentrations also con&m that the neat blend viscosity has a limited effect on the percentage reduction in the measured calcium concentration. 3.4. Estimating the correcticm factors To determine the zinc and calcium concentrations in multigrade oil formulations, the regression equations were estimated by considering formulations either with variable VI improver concentrations or with variable viscosities.
3.4.1. Via viscosity index improver concentration Table 9 includes F (the statistical parameter which is taken to assess the whole model significance), R2 (the correlation coefficient), confidence percentages and the proportionalities for correlations between the VI improver solid concentration and the percentage reduction in zinc or calcium concentrations. The derived models for these correlations are as in Table 10.
9
10
SBCP OCP PMA
SICP
Percentage
TABLE
99.5 Direct
Confidence percentage Proportionality
0.9840 99.5 Direct
99.5 Direct
26.5756 X (concentration (wt.%)) 18.9705 x (concentration (wt.%)) 14.4231 X (concentration (wt.%)) 8.2937 X (concentration (wt.%))
in Zn concentration
393.774 0.9777 99.5 Direct
175.930 0.9513 99.5 Direct
-
6.8646+ 15.2669 x (concentration (wt.%)) 7.3502 -t 11.0534 X (concentration (wt.%)) 5.4322 f 8.9690 x (concentration (wt.%)) 3.3392 + 5.4690 x (concentration (wt.%))
Reduction (96)
in Ca
132.777 0.9365 99.5 Direct
PMA
in Ca concentration
OCP
SBCP
concentration
99.5 Direct
0.9769
380.395
SICP
for reduction VI improvers
and viscosity index improver concentrations
Value of regression parameter cimcenWation fur the folknuing
index improver
554.815
1262.573
0.9929
via viscosity
PMA
in
OCP
concentrations
- 12.9412 + - 13.7367 + -9.2920+ - 5.4281+
Reduction (3)
reductions in zinc and cakium
99.5 Direct
805.191
0.9769
426.713
0.9777
SBCP
SICP
Value of regression parawzetar for reduction Zn concentration for the following VI improvers
between the reductions in zinc and calcium concentrations
F R2
parameter
Regreak3n
Correlation results and proportionalities via single linear regression analysis
TABLE
g
OCP 1467.325 0.9939 99.5 Diiect
SBCF 327.134 0.9732 99.5 Direct
SICP
702.406 0.9873 99.5 Direct
1401.06 0.9396 99.5 Direct
PUA
Valueof regression parameter for redw;tion in Zn concentration for the following VI improve?3
SICP SBCP OCP PMA
-27.3514+2.1845 - 28.6886 + 2.1708 -26.8449+ 1.7707 - 15.6308-k 1.0505
[vkosity (viscosi~ (viscosity (viscosity
(Cst) (cSt) (Cst) (cSt)
at at at at
100 100 100 100
“C) “C) “C) “C)
448.885 0.9803 99.5 Direct
239.499 0.9638 99.5 Direct
(c&j (cSt) (cSt) fcSt)
DCP
Saw
-14.4186f1.2119x(viscosity -15.8972-k 1.2477x(viscosi~ - 15.5983 c 1.0909 ~(viscosity -10.1314+0.6972X&iscosity
267.332 0.9674 99.5 Direct
SICP
at at at at
100 100 100 100
“C) “C> “C) “C)
1467.679 a.9939 99.5 Direct
PM2
Value of regression parameter for wduction in Ca concentrati4m for the following VI improvers
between reduction in zinc and calcium concentrations and viscosity via single linear regression analysis
Percentage reductions in zinc and cakiusn concentrations via blend viscosity
TABLE: 12
F RZ Confidence percentage Proportionality
Regression parameters
Correlation results and propo~onaiities
TABLE 11
60
3.4.2. Via blend viscosity
Table 11 includes F and R2 values, confidence percentages and the proportionalities for correlations between blend viscosity and percentage reduction in zinc or calcium concentration. The derived models for these correlations are as in Table 12. Testing of these models with several multigrade crankcase oil samples have proved their suitability for prediction of the actual zinc and calcium concentrations. 3.5. Explanation of results Previous workers [ 8, 151 have explained the phenomena for the reduction in measured metal concentration in the presence of VI improvers as follows: the break-up point of a polymer-thickened fluid exiting from a capillary is affected and proportional to the polymer concentration. Upon break-up, the polymer-containing viscoelastic fluid forms droplets connected to other droplets by thread-like structures, thus producing droplets that are effectively larger. A more dilute solution can result in shorter threads and smaller-sized particles, which affects the capability of the spectrometer to analyse the metals contained in the oil completely. 4. Conclusions This study shows that VI improvers can affect the determination of the zinc and calcium concentrations in unused multigrade crankcase oils formulations using AAS. The differences in the zinc and caIcium concentrations are found to be up to 18.5% and 11% respectively. Furthermore, it has been demonstrated that the VI improver concentration as well as its chemical structure type greatly affect these determinations. Regression equations are calculated to assist in the estimation of the actual content of zinc and calcium in the presence of the following VI improvers: SICP, SBCP, OCP and PMA. It is essential in applying of these equations to know the type of incorporated VI improver. The models developed can be run either via the polymer concentration or via the blend viscosity at 100 “C. References 1 A. Walsh, Spectrochim. Acta, 7 (1955) 108. 2 B. J. Russell, J. P. Shelton and A. Walsh, Spectrochim. Acta, 8 (1956) 317. 3 V. Sychra, I. Lang and G. Sebar, Analysis of petroleum products by atomic absorption spectroscopy and related techniques, Frog. Anal. At. Spectrosc., 4 (1981) 341. 4 H. H. Abou El Naga and M. Nader, The application of atomic absorption spectroscopy for measuring metals in petroleum products, Proc. 8th Arab Petroleum Congr., Al&et-s, May 28-Jww 3, 1972, 1972, Paper 81, Cl, p. 1. 5 W. B. Barnett, H. L. Kahn and G. E. Peterson, Rapid determination of several elements in a single lubricating oil sample by atomic absorption spectroscopy, At. Abscwpt.NewsZ., 10 (1971) 106.
61 6 E. A. Means and D. Ratcliff, Determination of Wear metals in lubricating oils by atomic absorption spectroscopy, At. Absorpt. News.!., 4 (1965) 174. 7 R. J. Lukasiewicz and B. F. Buell, Anal Chem., 47 (1975) 1673. 8 T. L. Oliphant, J. R. Terry and E. E. Klaus, The effect of viscosity index improvers on the determination of zinc using atomic absorption spectroscopy, SAE Reprint 860548, 1986 (Society of Automotive Engineers). 9 W. T. Stewart and F. A. Start, Lubricating oil additives, in K. A. Kobe and J. J. McKetta (eds.), Advances in Petrokum Chemistry and Refining, Vol. 7, Wiley, New York, 1963, p. 2.28-2.43 10 A. Schilhng, Motor Oils and Engine Lubrication, Vol. 1, Scientific Publications, 2nd edn., 1968, Chapter 2. 11 T. W. Selby, The non-Newtonian characteristics of lubricating oils, ASLE Iprans., I (1958) 68. 12 J. P. Arlie, J. Denis and G. Part, Relations between the structure and viscometric properties of polymethacrylate solutions in lube oils, J. Inst. Pet., Landon, IP 75-006 (1975). 13 R. J. A. Eckert and D. F. Covey, Developments in the field of hydrogenated diene copolymers as viscosity index improvers, Pt-oc. 5th Int. CouOq. on Additives for Lubricants and Operational Fluids, EssLingg January 14-16,1986, Technische Akademische, Esshngen, 1986, pp. 8.5.1-8.5.14. 14 G. T. Spiess, J. E. Johnston and G. Verstrate, Ethylene propylene copolymers as lube viscosity modifiers, Proc. 5th Int. CoUoq. on Additives for L&&ants and Operational FhGds, Esslingen, Januaw 14-16, 1986, Technische Akademische, Esslingen, 1986, pp. 8.10.1-8.10.11. 15 M. Goldin, J. Yerushalmi, R. Pfeffer and R. Shinnar, .I Fluid Mech., 38 (4) (1969) 689.
49-61
49
Determination of zinc and calcium in multigrade crankcase oils H. H. Abou El Naga, M. M. Mohamed and M. F. El Meneir Research
Centre,
Misr Petroleum
Co., P.O. Box 228, Cairo
(Egypt)
(Received May 5, 1989; revised February 13, 1990; accepted March 5, 1990)
Abstract Atomic absorption spectroscopy is used for the determination of zinc and calcium in multigrade crankcase oils. It has been found that zinc and calcium concentrationsare subjected to matrix interferences from polymers incorporated in the oil formulations as viscosity index (VI) improvers. Results indicate that the two most important factors are concentration and chemicai structure type for the following VI improvers: styrene-isoprene, styrene-butadiene, poly(aIkyImethacrylate) and ethylene-propylene. Correlations between the zinc or calcium concentrations and either the concentration or the type of each VI improver are established via a regression analysis statistical technique. Acceptable models have been found with confidence higher than 99.5%.
1. Introduction Atomic absorption spectroscopy (AAS) has been rapidly developed and widely applied over the last three decades. The original papers on this technique were first published by Walsh and coworkers [ 1, 21. The main reasons for its wide applications are its simplicity, sensitivity, portability and accuracy. The theoretical principles and description of basic apparatus have been given elsewhere [l-5]. AAS is widely used in the petroleum industry, either for quality control purposes or for research activities. One of its versatile applications is the determination of zinc and calcium in new and used lubricating oils [3-61. The presence of these two metals in lubricating oil formulations is a result of the incorporated detergent and antioxidant additives. The methods applied to the determination of zinc and calcium via AAS basically rely on the dilution of tested samples with kerosene followed by aspiration into the flame. It has been shown in the case of zinc determination that such methods are subjected to matrix interference from other additives, especially copolymers incorporated as viscosity index (VI) improvers in crankcase lubricating oil formulations [ 7, 81. Generally, the following copolymers are used in lubricant formulations as VI improvers: styrene-isoprene copolymer (SICP), styrene-butadiene copolymer (SBCP), ethylene-propylene (oleiln) copolymer (OCP) and 0043-1648/90/$3.50
(DElsevier SequoWPrinted in The Netherlands
50
poly(alkylmethacrylate) (PMA). References 9-14 include more data about the function and mechanism of action of these VI improvers, The effects of VI improvers on measured zinc concentrations in unused crankcase oils have been studied [ 7, S] where only one concentration from each incorporated VI improver was considered and precision limits for these measurements were not established. On the contrary, the dete~ation of calcium concentrations in these oils has not been considered before. Therefore the aims of the present study are as follows: (1) to confirm previous published results for zinc determination, using variable concentrations of different commercial VI improvers in multigrade crankcase oil formulations (these concentrations are selected to be within the applicable commercial doses for VI improvers in multigrade crankcase oils); (2) to establish precision limits for zinc concentrations in these formations; (3) to calculate regression equations for each type of VI improver incorporated, so that those equations can be used in estimating the actual concentrations of zinc in multigrade crankcase oil formulations; (4) similarly, to measure the calcium concentrations in the presence of different VI improvers.
2, Experimental
details
A basic lubricating oil formula (Table l), i.e. without a VI improver, was used in all measurements. Components of this formula are neutral base oil (300 N), bright-stock (Brt 135) and a multipurpose additive (i.e. detergent, dispersant, antiwear and antioxidant). The zinc and calcium contents in this additive are 1.46 wt.% and 1.68 wt.% respectively. VI improvers were ~co~o~~d in this basic blend at different concentrations, which are within the applicable commercial doses for VI improvers in multigrade crankcase oils (Table 2). The base oil content is reduced so TABLE
1
Basic oil: composition
and properties
Composition Neutral base oil (300 N) (wt.%) Bright stock (Brt 135) (wt.%) Multipurpose additive (wt.%)
60 32.1 7.9
Properties Kinematic viscosity (cSt) at 40 “C Kinematic viscosity (cSt) at 100 “C Viscosity index Pour point (“C) Total base number (mg KOH (g sample>-‘> Sulphated ash (wt.%) Zn (wt.%) Ca (wt.%)
90.45 10.56 99 -9 6.45 0.82 0.1150 0.1325
51 TABLE 2 Viscosity index improver characteristics Polymm
type
Symbol
Cf-wo~ymer cOntent
W.%)
Copolymer content in crankcase oil formulations
As diluted f(wt.%)
As solid
f(wt.%)
Styrene-isoprene copolymer
SICP
Solid
lo-20
0.6-1.2
Styrene-butadiene copolymer
SBCP
Solid
11-21
0.88-l .68
Olefin copolymer
OCP
13
Poly(allqImethacrylate)
PMA
26
6-l 1 3.5-5
0.78-1.43 0.78-1.43
as to keep the zinc and calcium concentrations at constant values in all formulations. The following commercial VI improvers were tested: SICP at concentrations from 0.6 to 1.2 wt.%, SBCP at concentrations from 0.88 to 1.68 wt.%, OCP at concentrations from 0.78 to 1.43 wt.% and PMA at concentrations from 0.78 to 1.43 wt.%. Table 3 includes the percentage of ingredients in the formulated multigrade crankcase oils. The kinematic viscosities at 40 and 100 “C, the VI, the total base number and sulphated ash for these blends are also listed in Table 3. Neat blends with the same viscosity as these VI-improver-containing formulations were prepared in order to study the effect of viscosity on the measured zinc and calcium concentrations. Table 4 includes the percentages of ingredients and properties for these neat blends. The zinc and calcium concentrations were measured using a Perkin-Elmer model 460 atomic absorption spectrophotometer and according to operating conditions given in Table 5. A fraction of kerosene with boiling point range 150-250 “C was used as the solvent. To estimate the precision limits of the method, one sample was tested at least ten times under the same working conditions and by the same operator. The results obtained were substituted in the standard deviation equation. Duplicate results by the same operator should be considered suspect if the average of two results differ by more than the percentage repeatability limits. To estimate the correction factors, a computer program was used to explore and derive the anticipated statistical correlations between the percentages of reduction in zinc and calcium concentrations in the presence of each type of incorporated VI improver.
VI i?npraver (~.%I
0.60 0.72 0.84 0.96 1.04 1.20
0.88 1.04 1.20 1.36 1.62 I.68
0.78 0.91 1.04 1.17 1.30 1.43
0.78 0.91 1.04 1.17 1.30 1.43
CopOl~ tme
SICP
SBCP
OCP
PMA
59.36 59.27 59.19 69.10 59.02 58.94
59.36 59.27 59.19 59.10 59.02 58.94
69.29 69.lQ 59.09 58.99 58.89 58.79
59.48 59.40 59.32 59.24 59.19 59.09
Oil (300 N) (~*%I
31.96 31.92 31.87 31.83 31.78 31.73
31.9% 31.92 31.87 31.83 31.78 31.73
31.93 31.87 31.81 31.76 31.69 31.63
32.02 31.98 31.94 31.90 31.87 31.81
Brt 135 (~.%I
7.9 7.9 7.9 7.9 7.9 7.9
7.9 7.9 7.9 7.9 7.9 7.9
7.9 7.9 7.9 7.9 7.9 7,Q
7.9 7.9 7.9 7.9 7.9 7.9
111.17 119.91 132.65 142.38 163.11 168.56
123.21 136.17 143.71 169.11 173.21 189.24
119.72 136.49 148.51 174.98 180.61 221.45
100.05 111.10 128.76 142.60 156.32 182.91
MuLtipurposs Kinematic additive At 40 “C (wt.%)
Viscosity index improver blends: composition and properties
TABLE 3
15.70 16.54 17.72 18.86 19.71 20.60
15.6% 16.68 17.65 18.57 19.80 21.17
14.61 15.87 17.22 18.72 19.53 21.96
13.99 15.26 16.22 17.89 19.29 21.43
At 100 “C
viscositgj (cSt)
150 149 148 150 148 152
134 133 13% 131 132 133
124 122 12% 120 124 120
142 144 130 140 142 140
VI
6.38 6.44 6.4% 6.51 6.43 6.48
6.51 6.50 6.44 6.49 6.42 6.44
6.39 6.42 6.36 6.32 6.41 6.44
6.49 6.42 6.43 6.38 6.37 6.41
Total base number (mg KON (g sample) - ‘)
0.85 0.82 0.84 0.82 0.82 0.81
0.84 0.82 0.81 0.82 0.83 0.82
0.82 0.85 0.81 0.84 0.84 0.81
0.81 0.85 0.80 0.82 0.84 0.81
T&t%)
Sulphuted
53
TABLE 4 Neat blends: composition and properties
Composition Neutral base oil (3OON) (wt.%) Bright-stock (Brt 135) (wt.%) Multipurpose additive (wt.%) Properties Kinematic viscosity (cSt) At 40 “C At 100 “C Viscosity index Total base number (mg KOB (g sample)-‘) Sulphated ash (wt.%) Zn (wt.%) Ca (wt.%)
BO I
BO 2
BO 3
BO 4
BO 5
44.0 56.0 7.90
35.5 64.5 7.90
33.5 66.5 7.90
11.0 69.0 7.90
24.5 75.5 7.90
134.88 13.69 97 6.49
149.32 14.96 100 6.39
158.44 15.5 99 6.51
173.53 16.23 97 6.52
195.95 17.5 96 6.39
0.83 0.1150 0.1325
0.81 0.1150 0.1325
0.84 0.1150 0.1325
0.82 0.1150 0.1325
0.85 0.1150 0.1325
TABLE 5 Operating conditions for atomic absorption spectrophotometer Instrument Wavelength (nm) Zn Ca Lamp current (mA) Air flow (l min-‘) Acetylene flow (I minSlot width (mm) Response time (s) Burner head (in)
Perkin-Elmer model 460 213.9 422.7 15 35 13 0.7 1 4 (one slot)
3. Results and discussion 3.1. Precisicm limits of the method The calculated precision limits for zinc and calcium are found to be I- 2.62 ppm and f 3.00 ppm from the obtained concentrations respectively. Therefore, duplicate results by the same operator should be considered suspect if the average differs by more than 0.23%. These results indicate that the method has a high precision and good sensitivity. 3.2. Eflect of viscosity index improvers on the mtxsured zinc and calcium concentrations Table 6 includes the measured zinc and calcium concentrations in the presence of variable concentrations of tested VI improvers. The percentage reductions in measured concentrations from the actual concentrations, i.e.
10 12 14 16 18 20
11 13 15 17 19 21
6 7 8 9 10 11
3 3.6 4 4.5 5 5.5
SBCP
OCP
PMA
in measured
SICP
Reductions
TABLE 6
0.78 0.91 1.04 1.17 1.30 1.43
0.78 0.91 1.04 1.17 1.30 1.43
0.88 1.04 1.20 1.36 1.52 1.68
0.60 0.72 0.84 0.96 1.04 1.20
zinc and calcium
15.70 16.54 17.72 18.86 19.71 20.60
15.66 16.68 17.65 18.57 19.80 21.17
14.61 15.87 17.22 18.72 19.53 21.96
13.99 15.25 16.52 17.89 19.29 21.43
concentrations
1108 1087 1053 1021 969 942 1126 1111 1090 1067 1040 1020 1136 1129 1117 1113 1087 1076
1150 1150 1150 1150 1150 1150 1160 1150 1150 1150 1150 1150
1111 1086 1052 1014 966 939
1150 1150 1150 1150 1150 1150 1150 1150 1150 1150 1150 1150
Measured
Cured
1.22 1.83 2.87 3.22 5.48 6.43
2.17 3.39 5.22 7.22 9.57 11.3
3.65 5.48 8.43 11.22 15.74 18.09
3.39 5.57 8.52 11.83 16.00 18.35
(%I
Reduction
index improver
(ppm)
viscosity
Zn comentration
with increasing
1325 1325 1325 1325 1325 1325
1325 1325 1325 1325 1325 1325
1325 1325 1325 1325 1325 1325
1325 1325 1325 1325 1325 1325
Calculated
(ppm)
1316 1304 1297 1285 1276 1272
1309 1288 1281 1256 1245 1232
1300 1270 1254 1225 1198 1188
1300 1269 1252 1222 1197 1187
Measured
Ca concentratim
concentration
0.68 1.58 2.11 3.02 3.70 4.15
1.21 2.79 3.32 5.21 6.04 7.17
1.89 4.15 5.36 7.55 9.58 10.42
1.89 4.23 5.51 7.77 9.66 10.49
1%)
Reduction
55
-
SICP
-
SBCP
-
OCP
o----o
PMA
a I
0.6
0.8
1.2
1.0
1.L
1.6
1.8
SOLID COPOLYMER CONCENTRATION (WT %1 Fig.
1. Effect of copolymer concentration
on measured zinc and calcium
concentrations.
as incorporated in the basic formula, were calculated. F’igure 1 illustrates the percentage reduction in AAS analytical data for zinc and calcium against the solids content in weight per cent for each of the four tested VI improvers. According to these results,it is possible to make the following observations. (1) An increase in the VI improver concentrations resulted in a signifhxmt reduction in both the zinc and the calcium contents as analysed. (2) The VI improver chemical type has a sign&ant effect on both the zinc and the calcium measured concen~atio~. Percentage reduction for a solid VI improver of 1 wt.% concentration and for the maximum concenkation of each of the incorporated VI improvers used are listed in Table 7. (3) It is possible to rank the percentage reduction in zinc and calcium concentration in terms of VI improver chemical type according to the following: percentagereduction high
4
SICP > SBCP
> OCP 5 PMA
low
3.3. Eflect of viscosity cm percentage reduction in mmsured cmLcentratim It was very spout to End a clear answer to the following question: can such a reduction in measured zinc and calcium concen~tio~ be a
56 TABLE 7 Percentage reductions in zinc and calcium at 1 wt.% and maximum viscosity index improver concentrations VI improver
SICP SBCP OCP Ph‘IA
Concentration (wt.961
1 1 1 1
M&mum
Reduction (%) for I wt.% concentration Zn
Ca
13.90 4.60 4.30 2.35
8.68 3.06 3.03 1.85
concatrat&m
(wt.%)
1.20 1.68 1.43 1.43
Reduction (%) for -mum ci3ncentratia Zn
Ca
18.35 18.09 11.30 6.43
10.49 10.42 7.17 4.15
TABLE 8 Percentage reductions in zinc concentration for neat and viscosity-index-improver-containing blends CopOl@mer wpe
Kinsnzatic viscosity (cSt) at 100 “C
2% @pm)
Po&?ner
Neat
Po&??m
NeUt
P&$p&-
Neai
0.60 0.66 0.72 0.84 0.96
13.99 14.52 15.25 16.52 17.89
13.69
1111 1094 1086 1052 1014
1139 1138 1136 1133 1130
3.390
14.96 15.50 16.23 17.50
4.870 5.570 8.520 11.83
0.96 1.04 1.22 1.48 1.74
SBCP
1.20
17.22
17.50
1053
1130
8.430
1.74
OCP
1.04
17.65
17.50
1090
1130
6.220
1.74
PMA
1.04
17.72
17.60
1117
1130
2.870
1.74
SICP
S&id POW=-(wt.%)
Reduction (%) in zn cmmmti
result of the increase in the blend viscosity or to the presence of the different types of VI improver at variable ~oncen~~o~? Therefore, neat blends without any VI improver were formulated (Table 4) to give nearly the same viscosities as the VI-improver-containing formulations (Table 3). Zinc and calcium were also measured in these neat blends. Table 8 includes the results obtained for the zinc concentration. As the SICP concentration is increased from 0.6 to 0.96 wt.% (as solid copolymer weight percentage), the reduction in measured zinc concentration is increased from 3.39% to 11.83%. On the contrary, increasing the viscosity for neat blends by the same magnitude increased the percentage reduction by only 0.96%-1.74%. Figure 2 illustrates the change in percentage reduction of measured zinc concentration for neat blends and SICP-containing formulations. These results clearly show that the viscosity increase has a limited effect on reducing the measured zinc ~oncen~~on, and the reduction in concen-
57
L
I 0.60
I
I
-
SICP
BLENDS
-
NEAT
BLENDS
I SOLID
Fig.
2.
Percentage
reduction
I
I
0.80
0.70
SICP
I
CONCENTRATION
in zinc concentration
I
I 0.10
0.90 (WT %)
for neat and SICP blends.
tration is attributed mainly to the presence of the VI improvers. Other measurements in the presence of SBCP, OCP and PMA have also confirmed these SICP results. Measurements of calcium concentrations also con&m that the neat blend viscosity has a limited effect on the percentage reduction in the measured calcium concentration. 3.4. Estimating the correcticm factors To determine the zinc and calcium concentrations in multigrade oil formulations, the regression equations were estimated by considering formulations either with variable VI improver concentrations or with variable viscosities.
3.4.1. Via viscosity index improver concentration Table 9 includes F (the statistical parameter which is taken to assess the whole model significance), R2 (the correlation coefficient), confidence percentages and the proportionalities for correlations between the VI improver solid concentration and the percentage reduction in zinc or calcium concentrations. The derived models for these correlations are as in Table 10.
9
10
SBCP OCP PMA
SICP
Percentage
TABLE
99.5 Direct
Confidence percentage Proportionality
0.9840 99.5 Direct
99.5 Direct
26.5756 X (concentration (wt.%)) 18.9705 x (concentration (wt.%)) 14.4231 X (concentration (wt.%)) 8.2937 X (concentration (wt.%))
in Zn concentration
393.774 0.9777 99.5 Direct
175.930 0.9513 99.5 Direct
-
6.8646+ 15.2669 x (concentration (wt.%)) 7.3502 -t 11.0534 X (concentration (wt.%)) 5.4322 f 8.9690 x (concentration (wt.%)) 3.3392 + 5.4690 x (concentration (wt.%))
Reduction (96)
in Ca
132.777 0.9365 99.5 Direct
PMA
in Ca concentration
OCP
SBCP
concentration
99.5 Direct
0.9769
380.395
SICP
for reduction VI improvers
and viscosity index improver concentrations
Value of regression parameter cimcenWation fur the folknuing
index improver
554.815
1262.573
0.9929
via viscosity
PMA
in
OCP
concentrations
- 12.9412 + - 13.7367 + -9.2920+ - 5.4281+
Reduction (3)
reductions in zinc and cakium
99.5 Direct
805.191
0.9769
426.713
0.9777
SBCP
SICP
Value of regression parawzetar for reduction Zn concentration for the following VI improvers
between the reductions in zinc and calcium concentrations
F R2
parameter
Regreak3n
Correlation results and proportionalities via single linear regression analysis
TABLE
g
OCP 1467.325 0.9939 99.5 Diiect
SBCF 327.134 0.9732 99.5 Direct
SICP
702.406 0.9873 99.5 Direct
1401.06 0.9396 99.5 Direct
PUA
Valueof regression parameter for redw;tion in Zn concentration for the following VI improve?3
SICP SBCP OCP PMA
-27.3514+2.1845 - 28.6886 + 2.1708 -26.8449+ 1.7707 - 15.6308-k 1.0505
[vkosity (viscosi~ (viscosity (viscosity
(Cst) (cSt) (Cst) (cSt)
at at at at
100 100 100 100
“C) “C) “C) “C)
448.885 0.9803 99.5 Direct
239.499 0.9638 99.5 Direct
(c&j (cSt) (cSt) fcSt)
DCP
Saw
-14.4186f1.2119x(viscosity -15.8972-k 1.2477x(viscosi~ - 15.5983 c 1.0909 ~(viscosity -10.1314+0.6972X&iscosity
267.332 0.9674 99.5 Direct
SICP
at at at at
100 100 100 100
“C) “C> “C) “C)
1467.679 a.9939 99.5 Direct
PM2
Value of regression parameter for wduction in Ca concentrati4m for the following VI improvers
between reduction in zinc and calcium concentrations and viscosity via single linear regression analysis
Percentage reductions in zinc and cakiusn concentrations via blend viscosity
TABLE: 12
F RZ Confidence percentage Proportionality
Regression parameters
Correlation results and propo~onaiities
TABLE 11
60
3.4.2. Via blend viscosity
Table 11 includes F and R2 values, confidence percentages and the proportionalities for correlations between blend viscosity and percentage reduction in zinc or calcium concentration. The derived models for these correlations are as in Table 12. Testing of these models with several multigrade crankcase oil samples have proved their suitability for prediction of the actual zinc and calcium concentrations. 3.5. Explanation of results Previous workers [ 8, 151 have explained the phenomena for the reduction in measured metal concentration in the presence of VI improvers as follows: the break-up point of a polymer-thickened fluid exiting from a capillary is affected and proportional to the polymer concentration. Upon break-up, the polymer-containing viscoelastic fluid forms droplets connected to other droplets by thread-like structures, thus producing droplets that are effectively larger. A more dilute solution can result in shorter threads and smaller-sized particles, which affects the capability of the spectrometer to analyse the metals contained in the oil completely. 4. Conclusions This study shows that VI improvers can affect the determination of the zinc and calcium concentrations in unused multigrade crankcase oils formulations using AAS. The differences in the zinc and caIcium concentrations are found to be up to 18.5% and 11% respectively. Furthermore, it has been demonstrated that the VI improver concentration as well as its chemical structure type greatly affect these determinations. Regression equations are calculated to assist in the estimation of the actual content of zinc and calcium in the presence of the following VI improvers: SICP, SBCP, OCP and PMA. It is essential in applying of these equations to know the type of incorporated VI improver. The models developed can be run either via the polymer concentration or via the blend viscosity at 100 “C. References 1 A. Walsh, Spectrochim. Acta, 7 (1955) 108. 2 B. J. Russell, J. P. Shelton and A. Walsh, Spectrochim. Acta, 8 (1956) 317. 3 V. Sychra, I. Lang and G. Sebar, Analysis of petroleum products by atomic absorption spectroscopy and related techniques, Frog. Anal. At. Spectrosc., 4 (1981) 341. 4 H. H. Abou El Naga and M. Nader, The application of atomic absorption spectroscopy for measuring metals in petroleum products, Proc. 8th Arab Petroleum Congr., Al&et-s, May 28-Jww 3, 1972, 1972, Paper 81, Cl, p. 1. 5 W. B. Barnett, H. L. Kahn and G. E. Peterson, Rapid determination of several elements in a single lubricating oil sample by atomic absorption spectroscopy, At. Abscwpt.NewsZ., 10 (1971) 106.
61 6 E. A. Means and D. Ratcliff, Determination of Wear metals in lubricating oils by atomic absorption spectroscopy, At. Absorpt. News.!., 4 (1965) 174. 7 R. J. Lukasiewicz and B. F. Buell, Anal Chem., 47 (1975) 1673. 8 T. L. Oliphant, J. R. Terry and E. E. Klaus, The effect of viscosity index improvers on the determination of zinc using atomic absorption spectroscopy, SAE Reprint 860548, 1986 (Society of Automotive Engineers). 9 W. T. Stewart and F. A. Start, Lubricating oil additives, in K. A. Kobe and J. J. McKetta (eds.), Advances in Petrokum Chemistry and Refining, Vol. 7, Wiley, New York, 1963, p. 2.28-2.43 10 A. Schilhng, Motor Oils and Engine Lubrication, Vol. 1, Scientific Publications, 2nd edn., 1968, Chapter 2. 11 T. W. Selby, The non-Newtonian characteristics of lubricating oils, ASLE Iprans., I (1958) 68. 12 J. P. Arlie, J. Denis and G. Part, Relations between the structure and viscometric properties of polymethacrylate solutions in lube oils, J. Inst. Pet., Landon, IP 75-006 (1975). 13 R. J. A. Eckert and D. F. Covey, Developments in the field of hydrogenated diene copolymers as viscosity index improvers, Pt-oc. 5th Int. CouOq. on Additives for Lubricants and Operational Fluids, EssLingg January 14-16,1986, Technische Akademische, Esshngen, 1986, pp. 8.5.1-8.5.14. 14 G. T. Spiess, J. E. Johnston and G. Verstrate, Ethylene propylene copolymers as lube viscosity modifiers, Proc. 5th Int. CoUoq. on Additives for L&&ants and Operational FhGds, Esslingen, Januaw 14-16, 1986, Technische Akademische, Esslingen, 1986, pp. 8.10.1-8.10.11. 15 M. Goldin, J. Yerushalmi, R. Pfeffer and R. Shinnar, .I Fluid Mech., 38 (4) (1969) 689.