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Behaviour of polymer-thickened oils in lubricated concentrated contacts
Wear, 98 (1984)
63
63
- 78
BEHAVIOUR OF POLYMER-THICKENED CONCENTRATED CONTACTS KAZUO
YOSHIDA
Nippon Minani,
HOSONUMA
Mining Co. Ltd., Lubricants and Petroleum Toda-shi, Saitama 335 (Japan)
TOSHIO Tokyo
and KUNIHIKO
OILS IN LUBRICATED
Products
Laboratory,
3-17-35
Niizo-
SAKURAI Institute
of
Technology,
12-1
Ookayama
2-Chome,
Meguro-ku,
Tokyo
152
(Japan) (Received
April 18,1984;
accepted
September
14,1984)
Summary Measurements of elastohydrodynamic (EHD) oil film thicknesses have been carried out to assess the lubricating performances of polymer-thickened oils using an optical interferometric technique. Five commercially available polymethacrylates (PMAs) with weight average molecular weights of 40 000 900 000 were used as polymer additives. The film thicknesses of all PMAcontaining solutions became less than those of corresponding straight oils. Furthermore, provided that the PMA solutions had the same viscosity at a given experimental temperature, the EHD film thicknesses decreased to that of the base oil with increasing average molecular weight of the PMA. In order to elucidate the results and to clarify the influences of permanent viscosity loss, further experiments have been conducted for PMA-thickened oils previously degraded by means of a sonic shear tester of frequency 10 kHz. It is very interesting that an increase in the EHD film thickness with a decrease in the ambient viscosity resulted from a molecular weight decrease. In order to evaluate the above results with respect to antiwear properties, four-ball wear experiments were also conducted with PMA-thickened oils having the highest and the lowest average molecular weights. These results showed that the wear scar diameters for oils containing a PMA of higher molecular weight were larger than those for lower molecular weight oils. It is suggested that it is difficult for the polymer molecules of larger size to pass through the contact region; therefore these molecules may accumulate at the inlet region and/or sweep around the contact. The behaviour of PMA-thickened oils has been investigated by taking photomicrographs in the presence of graphite particles under pure sliding conditions.
1. Introduction With the increasing practical use of engine lubricants polymers to improve their viscosity-temperature relationship, 0043-1648/84/$3.00
@ Elsevier
Sequoia/Printed
containing it becomes
in The Netherlands
64
exceedingly important to clarify their lubricating properties. Because of the non-newtonian behaviour of such oils, lubricating performances have been studied under high shear rates, e.g. using a high speed journal bearing test rig. Therefore many research investigations have been carried out under elastohydrodynamic (EHD) conditions, and these studies have revealed that oils containing polymers are subject to both reversible and irreversible viscosity decreases. The reversible viscosity decrease is well known as the temporary viscosity loss resulting from the orientation of polymer molecules parallel to the flow directions and the loosening of entanglements between polymers. The irreversible viscosity decrease is known as the permanent viscosity loss which is due to the breakdown of polymer molecules that is caused by severe shear stresses. The reduction in fluid film thickness resulting from these viscosity decreases may accelerate wear. Studies of the lubricating performance of polymer-thickened oils in concentrated contacts have also been carried out by some investigators [ 1 - 61. Dyson et al. [l] revealed by using a two-disc machine that an oil thickened with polymethacrylate @‘MA)formed a thinner EHD lubricating film than the corresponding straight oil. Hamilton and Robertson [Z] also observed the same trends for several polymer-thickened oils, and they ascribed the observations to the temporary and the permanent viscosity losses. Cameron and coworkers [ 7 - 91 have developed the optical interference method which allows direct observations of the film shape and film thickness measurements. Using this technique, Foord et al. [3], Sanborn and Winer [ 51 and Hironaka et al. [ 4 ] have evaluated the EHD film-forming ability in polymerthickened oils under rolling and sliding conditions. All these results demonstrate that oils containing polymers are generally inferior to straight oils with respect to EHD film-forming ability. Hirata and Cameron recently demonstrated that if the entry viscosity of polymer-thickened oils, in which the applied polymers had a number average molecular weight of 11500 - 95 000, differed hardly at all from that of the straight oil, then the resulting film thicknesses of the polymer-thickened oil and the straight oil were almost the same [6]. The purpose of the present paper is to clarify the effects of the average molecular weight of the PMA on EHD film thickness and to demonstrate the influences of the permanent viscosity loss on the film thickness.
2. Experimental
details
EHD oil film thickness measurements have been carried out using the friction machine shown schematically in Fig. 1 [lo], in which nominally rolling concentrated contacts were formed. This machine consisted of a glass disk driven by a d.c. motor and a freely rotating steel ball of 25.4 mm diameter. Optical interferometry, of which the fundamentals are fully explained in detail elsewhere [ 7 - 91, allowed direct measurements to be made of the EHD film thickness. A thin chromium layer was deposited on the
65
MICROSCOPE
Thermally
DRIVEN BY DC MOTOR b
insulated LOAD
jacket
Fig. 1. Schematic diagram of the test apparatus [lo]. disk, which was made up of a Pyrex glass, to enhance the interference fringe patterns, and its r.m.s. surface roughness was 0.018 I.cm.The steel ball was made of JIS SUJ-2 (corresponding to AISI 52100) and had an r.m.s. surface finish of 0.039 pm. Therefore the composite r.m.s. roughness was 0.043 pm. The speed of rotation of the glass disk was accurately determined by an electrical pick-up and a counter. This apparatus was enveloped by a thermally insulated jacket, the water being circulated through an isothermal water bath. All experiments were conducted at approximately 25 “C (to within 1 “C!). To enable photomicrographs of the contact region to be taken another apparatus was used under pure sliding conditions; in this apparatus a highly polished steel ball (SUJ-2) of 38 mm diameter was loaded against a stationary glass plate [ll]. The steel ball could be rotated at various speeds. The fundamentals of this apparatus were the same as those for the rolling contact machine. Wear experiments have been performed using a Roxana four-ball wear tester. The ball was rotated at a low speed of 0.038 m s-l to eliminate an excessive temperature rise, which would have lead to a substantial viscosity decrease, and the top ball load was 0.29 - 1.03 kN. All the experiments were performed at a temperature of 25 “C which was the same as that for the film thickness measurements. lower
2.1. Materials The base oils investigated are tabulated in Table 1; oils A, B and C are common neutral oils, oil D being a highly refined naphthenic oil. The five commercially available PMAs listed in Table 2 were used as the polymer additives. They have widely different weight average molecular weights, ranging from 40 000 to 900 000, and were dissolved into base oils A and D, which have the same viscosity at the experimental temperature. The physical properties of PMA-thickened oils added to oils A and D are tabulated in Tables 3 and 4. The refractive indices of the lubricants listed in Tables 1,3 and 4 were calculated from a general pressure-density relation and the Lorenz-Lorentz equation.
66 TABLE
1
Physical
properties
of the hydrocarbon
Viscosities (cSt) At 25 “C At 40 “C! At 100 “C Viscosity index Refractive index ng Density di5 aHighly refined b Hydrogenated TABLE
oils
Oil Aa
Oil Ba
Oil Ca
Oil Db
153.5 67.5 8.83 104 1.4835 0.8766
191.6 82.0 10.0 101 1.4828 0.8795
222.3 93.0 10.8 99 1.4851 0.8809
155.0 65.1 7.93 84 1.4792 0.8789
neutral oil. naphthenic oil.
2
Weight average molecular
weights of the polymethacrylates
PMA
Average
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
3. Experimental
molecular
weight Mw
results
3.1. Base oils With respect to the several base oils listed in Table 1, the relationships between viscosity at ambient conditions and central film thickness are shown in Fig. 2. As to the three neutral oils (oils A - C), the central film thickness gradually increased with increasing viscosity, while base oil D formed thicker EHD films than oil A. This discrepancy could be explained by the difference in viscosity-pressure coefficients inferred from the viscosity-temperature coefficients of these two oils. For a given fluid, the EHD film thickness increased with increasing entrainment velocity, whilst it decreased slightly with an increase in load. 3.2. Polymer-thickened oils The variation in the central EHD film thickness with viscosity for several PMA-thickened oils are shown in Figs. 3 - 6. For the five PMA-base oil A systems listed in Table 3, the film thickness is plotted against the ambient viscosity at an entrainment velocity of 0.3 m s-’ and a load of 9.8 N in Fig. 3. With the given series of PMA solutions, no matter how the viscosity
67 TABLE
3
Physical
properties
of polymethacrylate-thickened
oil A
(wt.%)
Density df
Refractive index ng
Viscosity at 25 “C (cSt)
Viscosity index
PMA-1
2.0 4.0 5.8 7.5
0.8781 0.8862 0.8818 0.8835
1.4836 1.4836 1.4830 1.4829
164.25 178.30 194.82 214.33
113 121 128 133
PMA-2
2.0 4.0 5.8 7.5
0.8780 0.8799 0.8810 0.8819
1.4829 1.4830 1.4825 1.4826
171.47 195.03 220.62 249.27
124 139 152 163
PMA-3
1.5 2.1 2.8 4.1
0.8778 0.8775 0.8781 0.8796
1.4815 1.4811 1.4814 1.4814
180.66 195.13 209.78 243.65
129 139 149 165
PMA-4
1.2 1.5 2.3 3.3
0.8776 0.8777 0.8781 0.8786
1.4811 1.4815 1.4810 1.4811
185.30 194.07 220.11 259.12
132 140 155 172
PMA-5
0.9 1.7 2.5
0.8768 0.8766 0.8778
1.4808 1.4809 1.4808
171.08 192.34 220.57
139 173 187
using the ASTM viscosity-temperature
equation
PMA
Average
Concentration
viscosity
at 25 “C calculated
[ 121.
increased with changes in PMA concentration, the rate of increase in film thickness decreased with an increase in the average molecular weight of the PMA. That is, provided that solutions of several PMAs have the same ambient viscosity at an experimental temperature of 25 “C, the EHD film thickness decreased and approached that of the base oil as the average molecular weight increased. The film thicknesses of the same fluids at 0.3 m s-l and 29.4 N are shown in Fig. 4, in which each plot was at a lower film thickness than the corresponding plot recorded at 9.8 N. However, an increase in average molecular weight of the PMA led to a decrease in the film thickness with the same gradient as that in Fig. 3. For the PMA-base oil D systems shown in Figs. 5 and 6, the relationship between film thickness and viscosity for various series of PMA solutions showed a similar tendency to those obtained in PMA-thickened oil A. When PMA-4 was added to A and D, these solutions showed different film-forming abilities. To elucidate the effects of the average molecular weight of the PMA and of differences in base oil on the EHD film thickness, relationships be-
68 TABLE 4 Physical properties of polymethacrylate-thickened
oil D
(wt.%)
Density d;’
Refractive index n&’
Viscosity at 25 OC (cSt)
2.0 4.0 5.8 7.5
0.8807 0.8825 0.8837 0.8853
1.4780 1.4781 1.4781 1.4781
164.86 177.37 193.14 211.11
106 113 120
PMA-2
2.0 4.0 4.5 5.8 7.5
0.8802 0.8810 0.8818 0.8825 0.8835
1.4791 1.4791 1.4791 1.4790 1.4790
170.95 190.97 193.02 215.44 241.71
110 127 127 141 152
PMA-3
1.5 2.4 2.8 4.1
0.8790 0.8798 0.8799 0.8807
1.4779 1.4774 1.4780 1.4780
178.43 194.75 204.67 235.70
114 129 138 155
PMA-4
1.2 1.5 2.3 3.3
0.8791 0.8786 0.8798 0.8804
1.4781 1.4775 1.4781 1.4781
181.97 192.32 213.96 250.95
118 128 144 163
PMA-5
0.9 1.7 2.5
0.8789 0.8792 0.8794
1.4779 1.4776 1.4778
168.84 191.12 210.60
124 155 178
Concentration
PMA
PMA-1
Average viscosity at 25 “C calculated using the ASTM viscosity-temperature
Viscosity index
97
equation [ 121.
tween the film thickness and the average molecular weight of PMA are shown in Fig. 7. 3.3. Effective viscosity The following parameter, being the ratio of the effective viscosity of the polymer-thickened oil to that of the base oil, has been used in evaluating the EHD film-forming ability of polymer solutions [ 131: r)eff -= qB
hp ( h,
1’a 1
where qeff is the effective viscosity of the polymer-thickened oil, qa is the viscosity of the base oil, hp and h, are the central film thicknesses of the polymer-thickened oil and the base oil respectively and (1is the speed parameter index in the EHD film thickness formula (a = 0.67). This parameter is derived from the ratio of the calculated EHD film thickness for the polymerthickened oil to that of the base oil. All data were obtained at the same load,
69
41
, 160
4
180
200 Viscosity
240x10
220 at
25%
-6
(I+s~)
Fig. 2. Central film thickness us. ambient viscosity for various base oils and loads (6, entrainment velocity): O,*, ox Q, 9.8 N;A, A,A, A, 19.6 N;n, n,m, m, 29.4 N.
Viscosity
at
25%
(m*
a’)
Fig. 3. Relation between central film thickness and ambient viscosity of PMA-thickened oil A (entrainment velocity ii = 0.3 m s-r ; load P = 9.8 N; maximum hertzian pressure prnax = 0.35 GPa). Symbol
Molecular weight Mw of PMA
Lubricant Base oil Base oil Base oil Base oil Base oil Base oil
A A A A A A
with with with with with
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
70
/ 150
1
200
250X10-” Viscosity
at
25%
(m’
5’)
Fig. 4. Relation between central film thickness and ambient oil A (ii = 0.3 m s-‘;P = 29.4 N; pmax = 0.50 GPa).
Symbol
Lubricant
l
Base Base Base Base Base Base
0
n V :
oil oil oil oil oil oil
viscosity
Molecular A A A A A A
of PMA-thickened
weight Mw of PMA
with with with with with
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
speed and temperature, and it was assumed that the pressure-viscosity coefficient of the polymer-thickened oil would be independent of the addition of a few per cent of polymer additive [ 141. The parameter is displayed for several results in Table 5 on the basis of Figs. 3 and 5. The relationship between the weight average molecular weight of PMA and the effective viscosity enhances the previous results. 3.4. Shear degradation of polymer-thickened oils Further experiments have been undertaken, both to confirm the effects of average molecular weight on EHD film thickness and to assess the influences of the permanent viscosity loss on EHD film-forming ability in concentrated contacts. Polymer-thickened oils were prepared by the addition of PMA-5 to base oils A and D, which were then degraded using a sonic shear tester of frequency 10 kHz. The changes in physical properties of the test oils with irradiation time are listed in Table 6, in which the average molecular weights determined by means of gel permeation chromatography are given. The parameter Mw/MN , being the ratio of the weight average molecular weight to the number average molecular weight, represents the molecular weight distribution of the polymer. The value of M,/MN became smaller as the irradiation time increased and the ambient viscosity of PMA solutions
71
400 t
.L
I
I
1
25OXlO-6
200
150 Viscosity
at
25
“C (m’
<‘)
Fig. 5. Relation between central film thickness and ambient oil D (ii = 0.3 m s-l ;P = 9.8 N;p,, = 0.35 GPa).
Symbol
Lubricant
0
Base Base Base Base Base Base
0
a D :
oil oil oil oil oil oil
viscosity
of PMA-thickened
Molecular weight MW of PMA
D D D D D D
with with with with with
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
decreased almost exponentially with decreasing average molecular weight. This change means that the higher molecular weight polymers were selectively broken down by an oscillating shear, resulting in a narrower molecular weight distribution. The appearance of these changes, which is shown more clearly in Fig. 8, may be the same as that which occurs in machine elements [ 151. The influences of a decrease in the molecular weight of the polymer on the EHD film thickness are shown in Fig. 9. It is obvious that the EHD film thicknesses of oils A and D thickened with PMA-5 increase with a decrease in ambient viscosity; thus this tendency is in complete disagreement with general knowledge. The increase in EHD film thickness for PMA-5 in oil D with a viscosity decrease is higher than that for oil A, and the difference is strongly consistent with the trend shown in Fig. 7. Comparison of Figs. 8 and 9 shows that there is an interesting relationship between the changes in molecular weight distribution of the polymer and the EHD film thickness that accompany the average molecular weight decrease. That is, the molecular weight distribution becomes narrower with the irradiation time whereas the increase in EHD film thickness decreases gradually. 3.5. Wear tests In the lubrication grade oil was observed
of cam followers, the wear associated with a multito be profoundly larger than that with a single-grade
72
150
200 Viscosity
250x10-” at
25°C
(d
s-‘)
Fig. 6. Relation between central film thickness and ambient oil D (ii = 0.3 m s-‘;P = 29.4 N;nmax = 0.50 GPa).
Symbol
Lubricant
0
Base Base Base Base Base Base
0
a D
oil oil oil oil oil oil
viscosity
of PMA-thickened
Molecular weight MW of PMA
D D D D D D
with with with with with
40000 100000 350000 480000 900000
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
o
PMA-Thickened
Oil
A
0
PMA
Oil
D
Thickened
350 5 Weight
Average
10 Molecular
Fig. 7. Variations in central film (ii=0.3ms-‘;P=9.8N;p,,= Figs. 3 and 5, arising from oils mental temperature. The broken oil A.
50 Weight
100~10’
of PMA
thickness with weight average molecular weight of PMA 0.35 GPa). All plots are derived from results obtained in having approximately the same viscosity at the experiline for base oil B is a reference to the polymer-thickened
oil [16]. To evaluate the results mentioned above, wear experiments have been conducted to verify the rate of wear for the polymer-thickened oils having the highest and the lowest average molecular weight of PMA. Changes
73 TABLE 5 Effective viscosity of polymethacrylate-thickened PMA
%ffhB
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
1.16 1.11 1.07 1.01 1.00
oils
%fflrlB for oil D
for oil A
1.16 1.08 1.03 0.99 0.99
TABLE 6 Changes in physical properties of polymethacrylate-thickened Irradiation (min)
time
Viscosity (cSt)
0
Molecular
at 25 “C
220.57 192.41 184.03 179.07 171.51
5 10 15 30
oil A with irradiation time weighta
900000 461000 334000 276000 189000
aWeight average molecular weight measured by gel permeation chromatography by using the molecular weight of polystyrene.
3
4
5 LOG
6
MwIMN
4.38 2.92 2.38 2.16 1.68 corrected
7
CM)
Fig. 8. Changes in the molecular weight distribution of PMA-5 after treatment with the sonic shear tester.
in wear scar diameters associated with incremental changes in total load are shown in Fig. 10. Wear scar diameters obtained with the lower molecular weight PMA-1 solution were smaller than those for the higher molecular weight PMA-5 solution within, at least, these frictional conditions.
74
I
0
PM-5
(25rt%)~OiI
A
0
M-5
(2.5wt%)bOiI
t
M-5
450 -
MW - 900.000
-y*
\ 400 -
-c--n-
y__=_______
35ok r,
’
I
160
180
I 200
Viscosity
at
220 ‘( 10
25%
(m’
Fig. 9. Effect of shear degradation pmax = 0.35 GPa).
s-‘)
on the central
film thickness
(U = 0.3 m s-l ; P = 9.8 N;
500
0
PMA
0
I’M&-Bare
B-Base
Oil
A (220.57
Oil
A (214.33
V..,
I.”
Total
Fig. 10. duration
Load
(kN)
Variation in wear scar diameter with total time t = 1 h; temperature, 25 “C).
load (sliding speed
u = 0.077
m s-‘;
4. Discussion It has been shown that the EHD film thicknesses of polymer-thickened oils are less than those of the corresponding base oils. There are two main explanations why polymer-thickened oils are inferior to base oils in their ability to form EHD films: (i) temporary viscosity loss and (ii) permanent
15
viscosity loss. Dyson and Wilson have drawn attention to the fact that high shear rates occur at the inlet region [1’7], and this seems to lead to the temporary viscosity loss. Values of the maximum shear rates in the inlet region can be of the order of 10’ - lo6 s-’ even under pure rolling conditions. This may lead polymer solutions to the second newtonian region [ 181. The permanent viscosity loss is ascribed to a decrease in the molecular weight of the polymer due to severe shear stresses in the contact region. Walker et al. performed excellent work in which small samples of polymer solutions were extracted from both the inlet and the exit regions of pure sliding EHD contacts and then analysed to determine any changes in the molecular weight and its distribution [ 191. They revealed that decreases in the molecular weight of the polymer occurred in the EHD contacts and thus that the film thickness of the polymer-thickened oils decreased. 4.1. Film thickness of polymer solutions The viscosity of polymer solutions subjected to both high pressures and high shear rates is higher than that of the base oil [ 181; thus the EHD film thickness should be increased by the addition of polymers. The EHD film thickness for PMA-5 solutions was, however, found to be approximately equal to that of the base oil even though the ambient viscosity was considerably raised. The complex behaviour of polymer-thickened oil could be affected by the following factors: (1) the molecular size, molecular structure and molecular weight distribution of PMA; (2) the solubility and/or the dispersibility of PMA in the base oil, particularly under higher pressures; (3) the mechanical shear degradation of PMA at or near the EHD contact. It appears to be difficult for the larger size PMA molecules to pass through the contact; thus some molecules may accumulate in the inlet and others sweep around the EHD contact. It is considered that the accumulation of PMA in the inlet may primarily be caused by the adsorption of PMA onto the steel surface. However, some of the larger molecules may pass through the contact if alignment of the molecules occurs in the flow and if the molecules can be shear degraded in the inlet region. This behaviour should be related to the molecular weight and the distributions of molecular weight of the PMA, i.e. whether it has a narrow or a wide distribution. The polymer additives exert a thickening action which increases the viscosity by means of their solubility in the base oil. However, as the molecular weight increases considerably, the solubility decreases markedly and the molecules are likely to separate from the base oil. This is particularly true under high pressures because the solubility of the polymers in mineral oils becomes poorer, such that the larger size molecules are likely to remain in the inlet region. Furthermore, this tendency may be facilitated by the reverse flow which occurs in the convergent inlet region. 4.2. Shear degradation of polymethacrylate-thickened oil With respect to the PMA solutions degraded by means of the sonic shear tester, the EHD film thickness gradually increased with decreasing
76
average molecular weight of the PMA. This suggests that the higher molecular weight polymers may sweep around the contact and/or may accumulate in the inlet, obstructing some of the oil supply to the contact. It is observed in Fig. 8 that the average molecular weight of the PMA decreases as the larger PMA molecules are selectively degraded. This may also be concerned with the film thickness. 4.3. Photomicrographs of elastohydrodynamic contacts under pure sliding conditions Cusano and Sliney have recently presented many interesting photomicrographs which show the distribution of graphite in and around EHD contacts by using graphitedispersed oils [20]. In the present paper, photomicrographs of the contact under pure sliding conditions are shown in Figs. 11 and 12. The graphite particle size was less than 0.3 E.trn and the concentrations of graphite in oil A and oil A thickened with PMA-5 were 1.0 wt.%.
Fig. 11. Graphite-dispersed perature, 8 - 10 “C).
base oil A (LJ = 0.01
Fig. 12. Graphite-dispersed oil A thickened pmax = 0.27 GPa; temperature, 8 - 10 “C).
m s-l;
with
P = 9.8
PMA-5
N;p,,
(u = 0.01
= 0.27
m s-l;
GPa; tem-
P = 9.8
N;
The results presented in these figures are representative of many photomicrographs which were taken. The load was 9.8 N and the sliding velocity was 0.01 m s-i at a temperature of 8 - 10 “C. It was observed that graphite particles go through the contact as shown in Fig. 11. In the base oil alone, graphite did not accumulate in the inlet region. In contrast, in the PMA-thickened oil, the amount of graphite passing through the contact seemed to be very small in the same condition as shown in Fig. 12. In pure sliding under boundary conditions, it was observed that the graphite accumulated, in varying degrees, in the inlet and reverse flow took place in the inlet region. In these experiments, graphite particles passed through the contact when the base oil only was used. It seems that the contact takes place under partial EHD conditions. These pure sliding conditions are quite different from the nominal rolling conditions, but the same phenomena probably occur under thin film lubrication conditions. This suggests that the higher molecular weight PMA accumulates with graphite particles in the inlet region. From these results, it is considered that it is generally difficult for PMA molecules to pass through the contact as the molecular size increases, even under rolling conditions. The blocking action of polymers at the inlet region decreases the oil supply to the contact to cause a form of lubricant starvation. Therefore there may be a critical range of molecular weights for polymers in base oils for successful operation which depends on the structure and the distribution of molecular weight of the polymers. 5. Conclusions The behaviour of PMA additives under the partial EHD or EHD conditions has been studied under nominally rolling and pure sliding conditions. (1) Under rolling conditions, the oil film thickness of PMA-thickened oils gradually increases with increasing viscosity, but its rate of increase slowly falls as the average molecular weight increases until the thickness remains the same as that of the base oil. (2) In PMA-thickened oils, the larger size molecules in the inlet may find it difficult to pass through the contact and they may accumulate at and/or sweep around the contact. (3) With the shear-degraded PMA-thickened oils, the oil film thickness increases as the molecular weight of the PMA decreases under rolling conditions. In the practical use of polymer additives, it appears that film formation at the contact becomes effective in the moderate average molecular weight range, even if a permanent viscosity loss occurs under the operating conditions. (4) Under sliding conditions, graphite particles dispersed in the base oil and PMA-thickened oil have been studied by taking photomicrographs. It was observed that the larger size PMA molecules accumulated with the graphite particles in the inlet region. It seems that the same behaviour may occur under rolling conditions.
78
Acknowledgments
The authors would like to thank Mr. T. Uchikawa, Director, Lubricants and Petroleum Products, for his valuable advice, and they also thank sincerely the Nippon Mining Co. Ltd. We appreciate the kind support of Mr. H. Kageyama, Vice-president, Kyodo Yushi Co. Ltd. References 1 A. Dyson, H. Naylor and A. R. Wilson, The measurement of oil-fifm thickness in elastohydrodynamic contacts, Proc., Inst. Mech. Eng., London, 180 (3B) (1965 1966) 119 _ 134. 2 G. M. Hamilton and W. G. Robertson, Lubrication of rollers with oils containing polymers, Proc., Inst. Mech. Eng., London, 181 (3B) (1966 - 1967) 192 - 199. 3 C. A. Foord, W. C. Hamman and A. Cameron, Evaluation of lubricants using optical elastohydrodynamics, ASLE Trans., 11 (1) (1968) 31 - 43. 4 S. Hironaka, M. Nagai and T. Sakurai, Behavior of polymer additives in elastohydrodynamic contact, J. Jpn. Pet. Inst., 22 (1) (1979) 52 - 58. 5 D. M. Sanborn and W. 0. Winer, Fluid rheological effects in sliding elastohydrodynamic contacts with transient loading, I, Film thickness, J. Lubr. Technol., 93 (April 1971) 262 - 271. 6 M. Hirata and A. Cameron, The use of optical elastohydrodynamics to investigate viscosity loss in polymer-thickened oils,ASLE Trans., 27 (2) (1984) 114 - 121. 7 A. Cameron and R. Gohar, Theoretical and experimental studies of the oil film in lubricated point contact, Proc. R. Sot. London, Ser. A, 291 (1966) 520 - 536. 8 R. Gohar and A. Cameron, The mapping of elastohydrodynamie contacts, ASLE Trans., 10 (1967) 215 - 225. 9 C. A. Foord, L. D. Wedeven, F. S. Westlake and A. Cameron, Optical elastohydrodynamics, Proc., Inst. Mech. Eng., London, 184 (Part 1, 28) (1969 - 1970) 487 - 505. 10 H. Hamaguchi, H. A. Spikes and A. Cameron, Elastohydrodynamic properties of water in oil emulsions, Wear, 43 (1) (1977) 17 - 24. 11 W. Machidori, H. Kageyama and T. Moriuchi, Grease lubrication in elastohydrodynamic contacts, 50th National Lubricating Grease Institute Annu. Meet., Kansas City, MO, October 23 - 26, 1983. 12 A. Cameron, Principles of Lubrication, 1966, p. 23. 13 B. J. Hamrock and D. Dowson, Isothermal elastohydrodynamie lubrication of point contacts, Part III, Fully flooded results, J. Lubr. Technot., 93 (April 1971) 264 - 276. 14 W. 0. Winer and J. 13. Novak, Some measurements of high pressure lubricant rheology, J. Lubr. Technol., 90 (1968) 580 - 591. 1.5 N. V. Messina, V. I. improvers for multigrade oils. In T. Sakurai (ed.), Proc. JSLEASLE Int. Lubrication Con& Tokyo, 1975, Elsevier, Amsterdam, 1976, pp. 880 889. Engineers Wear 16 J. Chida, Wear on engine parts, 7th Jpn. Society of Lubrication Symp., March 1975. 17 A. Dyson and A. R. Wilson, Film thickness in elastohydrodynamic lubrication by silicone fluids, Proc., Inst. Mech. Eng., London, 180 (3K) (1965 - 1966) 97 - 112. 18 J. D. Novak and W. 0. Winer, The effects of pressure on the non-newtonian behavior of polymer blended petroleum oils, J. Lubr. Technol., 91 (July 1969) 459 - 463. 19 D. L. Walker, D. M. Sanborn and W. 0. Winer, Molecular degradation of lubricants in sliding elastohydrodynamic contacts, J. Lubr. Technol., 97 (July 1975) 390 - 397. 20 C!. Cusano and H. E. Sliney, Dynamics of solid dispersions in oil during the lubrication of point contacts, Part I, Graphite, ASLE Trans., 25 (2) (1982) 183 - 189.
63
63
- 78
BEHAVIOUR OF POLYMER-THICKENED CONCENTRATED CONTACTS KAZUO
YOSHIDA
Nippon Minani,
HOSONUMA
Mining Co. Ltd., Lubricants and Petroleum Toda-shi, Saitama 335 (Japan)
TOSHIO Tokyo
and KUNIHIKO
OILS IN LUBRICATED
Products
Laboratory,
3-17-35
Niizo-
SAKURAI Institute
of
Technology,
12-1
Ookayama
2-Chome,
Meguro-ku,
Tokyo
152
(Japan) (Received
April 18,1984;
accepted
September
14,1984)
Summary Measurements of elastohydrodynamic (EHD) oil film thicknesses have been carried out to assess the lubricating performances of polymer-thickened oils using an optical interferometric technique. Five commercially available polymethacrylates (PMAs) with weight average molecular weights of 40 000 900 000 were used as polymer additives. The film thicknesses of all PMAcontaining solutions became less than those of corresponding straight oils. Furthermore, provided that the PMA solutions had the same viscosity at a given experimental temperature, the EHD film thicknesses decreased to that of the base oil with increasing average molecular weight of the PMA. In order to elucidate the results and to clarify the influences of permanent viscosity loss, further experiments have been conducted for PMA-thickened oils previously degraded by means of a sonic shear tester of frequency 10 kHz. It is very interesting that an increase in the EHD film thickness with a decrease in the ambient viscosity resulted from a molecular weight decrease. In order to evaluate the above results with respect to antiwear properties, four-ball wear experiments were also conducted with PMA-thickened oils having the highest and the lowest average molecular weights. These results showed that the wear scar diameters for oils containing a PMA of higher molecular weight were larger than those for lower molecular weight oils. It is suggested that it is difficult for the polymer molecules of larger size to pass through the contact region; therefore these molecules may accumulate at the inlet region and/or sweep around the contact. The behaviour of PMA-thickened oils has been investigated by taking photomicrographs in the presence of graphite particles under pure sliding conditions.
1. Introduction With the increasing practical use of engine lubricants polymers to improve their viscosity-temperature relationship, 0043-1648/84/$3.00
@ Elsevier
Sequoia/Printed
containing it becomes
in The Netherlands
64
exceedingly important to clarify their lubricating properties. Because of the non-newtonian behaviour of such oils, lubricating performances have been studied under high shear rates, e.g. using a high speed journal bearing test rig. Therefore many research investigations have been carried out under elastohydrodynamic (EHD) conditions, and these studies have revealed that oils containing polymers are subject to both reversible and irreversible viscosity decreases. The reversible viscosity decrease is well known as the temporary viscosity loss resulting from the orientation of polymer molecules parallel to the flow directions and the loosening of entanglements between polymers. The irreversible viscosity decrease is known as the permanent viscosity loss which is due to the breakdown of polymer molecules that is caused by severe shear stresses. The reduction in fluid film thickness resulting from these viscosity decreases may accelerate wear. Studies of the lubricating performance of polymer-thickened oils in concentrated contacts have also been carried out by some investigators [ 1 - 61. Dyson et al. [l] revealed by using a two-disc machine that an oil thickened with polymethacrylate @‘MA)formed a thinner EHD lubricating film than the corresponding straight oil. Hamilton and Robertson [Z] also observed the same trends for several polymer-thickened oils, and they ascribed the observations to the temporary and the permanent viscosity losses. Cameron and coworkers [ 7 - 91 have developed the optical interference method which allows direct observations of the film shape and film thickness measurements. Using this technique, Foord et al. [3], Sanborn and Winer [ 51 and Hironaka et al. [ 4 ] have evaluated the EHD film-forming ability in polymerthickened oils under rolling and sliding conditions. All these results demonstrate that oils containing polymers are generally inferior to straight oils with respect to EHD film-forming ability. Hirata and Cameron recently demonstrated that if the entry viscosity of polymer-thickened oils, in which the applied polymers had a number average molecular weight of 11500 - 95 000, differed hardly at all from that of the straight oil, then the resulting film thicknesses of the polymer-thickened oil and the straight oil were almost the same [6]. The purpose of the present paper is to clarify the effects of the average molecular weight of the PMA on EHD film thickness and to demonstrate the influences of the permanent viscosity loss on the film thickness.
2. Experimental
details
EHD oil film thickness measurements have been carried out using the friction machine shown schematically in Fig. 1 [lo], in which nominally rolling concentrated contacts were formed. This machine consisted of a glass disk driven by a d.c. motor and a freely rotating steel ball of 25.4 mm diameter. Optical interferometry, of which the fundamentals are fully explained in detail elsewhere [ 7 - 91, allowed direct measurements to be made of the EHD film thickness. A thin chromium layer was deposited on the
65
MICROSCOPE
Thermally
DRIVEN BY DC MOTOR b
insulated LOAD
jacket
Fig. 1. Schematic diagram of the test apparatus [lo]. disk, which was made up of a Pyrex glass, to enhance the interference fringe patterns, and its r.m.s. surface roughness was 0.018 I.cm.The steel ball was made of JIS SUJ-2 (corresponding to AISI 52100) and had an r.m.s. surface finish of 0.039 pm. Therefore the composite r.m.s. roughness was 0.043 pm. The speed of rotation of the glass disk was accurately determined by an electrical pick-up and a counter. This apparatus was enveloped by a thermally insulated jacket, the water being circulated through an isothermal water bath. All experiments were conducted at approximately 25 “C (to within 1 “C!). To enable photomicrographs of the contact region to be taken another apparatus was used under pure sliding conditions; in this apparatus a highly polished steel ball (SUJ-2) of 38 mm diameter was loaded against a stationary glass plate [ll]. The steel ball could be rotated at various speeds. The fundamentals of this apparatus were the same as those for the rolling contact machine. Wear experiments have been performed using a Roxana four-ball wear tester. The ball was rotated at a low speed of 0.038 m s-l to eliminate an excessive temperature rise, which would have lead to a substantial viscosity decrease, and the top ball load was 0.29 - 1.03 kN. All the experiments were performed at a temperature of 25 “C which was the same as that for the film thickness measurements. lower
2.1. Materials The base oils investigated are tabulated in Table 1; oils A, B and C are common neutral oils, oil D being a highly refined naphthenic oil. The five commercially available PMAs listed in Table 2 were used as the polymer additives. They have widely different weight average molecular weights, ranging from 40 000 to 900 000, and were dissolved into base oils A and D, which have the same viscosity at the experimental temperature. The physical properties of PMA-thickened oils added to oils A and D are tabulated in Tables 3 and 4. The refractive indices of the lubricants listed in Tables 1,3 and 4 were calculated from a general pressure-density relation and the Lorenz-Lorentz equation.
66 TABLE
1
Physical
properties
of the hydrocarbon
Viscosities (cSt) At 25 “C At 40 “C! At 100 “C Viscosity index Refractive index ng Density di5 aHighly refined b Hydrogenated TABLE
oils
Oil Aa
Oil Ba
Oil Ca
Oil Db
153.5 67.5 8.83 104 1.4835 0.8766
191.6 82.0 10.0 101 1.4828 0.8795
222.3 93.0 10.8 99 1.4851 0.8809
155.0 65.1 7.93 84 1.4792 0.8789
neutral oil. naphthenic oil.
2
Weight average molecular
weights of the polymethacrylates
PMA
Average
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
3. Experimental
molecular
weight Mw
results
3.1. Base oils With respect to the several base oils listed in Table 1, the relationships between viscosity at ambient conditions and central film thickness are shown in Fig. 2. As to the three neutral oils (oils A - C), the central film thickness gradually increased with increasing viscosity, while base oil D formed thicker EHD films than oil A. This discrepancy could be explained by the difference in viscosity-pressure coefficients inferred from the viscosity-temperature coefficients of these two oils. For a given fluid, the EHD film thickness increased with increasing entrainment velocity, whilst it decreased slightly with an increase in load. 3.2. Polymer-thickened oils The variation in the central EHD film thickness with viscosity for several PMA-thickened oils are shown in Figs. 3 - 6. For the five PMA-base oil A systems listed in Table 3, the film thickness is plotted against the ambient viscosity at an entrainment velocity of 0.3 m s-’ and a load of 9.8 N in Fig. 3. With the given series of PMA solutions, no matter how the viscosity
67 TABLE
3
Physical
properties
of polymethacrylate-thickened
oil A
(wt.%)
Density df
Refractive index ng
Viscosity at 25 “C (cSt)
Viscosity index
PMA-1
2.0 4.0 5.8 7.5
0.8781 0.8862 0.8818 0.8835
1.4836 1.4836 1.4830 1.4829
164.25 178.30 194.82 214.33
113 121 128 133
PMA-2
2.0 4.0 5.8 7.5
0.8780 0.8799 0.8810 0.8819
1.4829 1.4830 1.4825 1.4826
171.47 195.03 220.62 249.27
124 139 152 163
PMA-3
1.5 2.1 2.8 4.1
0.8778 0.8775 0.8781 0.8796
1.4815 1.4811 1.4814 1.4814
180.66 195.13 209.78 243.65
129 139 149 165
PMA-4
1.2 1.5 2.3 3.3
0.8776 0.8777 0.8781 0.8786
1.4811 1.4815 1.4810 1.4811
185.30 194.07 220.11 259.12
132 140 155 172
PMA-5
0.9 1.7 2.5
0.8768 0.8766 0.8778
1.4808 1.4809 1.4808
171.08 192.34 220.57
139 173 187
using the ASTM viscosity-temperature
equation
PMA
Average
Concentration
viscosity
at 25 “C calculated
[ 121.
increased with changes in PMA concentration, the rate of increase in film thickness decreased with an increase in the average molecular weight of the PMA. That is, provided that solutions of several PMAs have the same ambient viscosity at an experimental temperature of 25 “C, the EHD film thickness decreased and approached that of the base oil as the average molecular weight increased. The film thicknesses of the same fluids at 0.3 m s-l and 29.4 N are shown in Fig. 4, in which each plot was at a lower film thickness than the corresponding plot recorded at 9.8 N. However, an increase in average molecular weight of the PMA led to a decrease in the film thickness with the same gradient as that in Fig. 3. For the PMA-base oil D systems shown in Figs. 5 and 6, the relationship between film thickness and viscosity for various series of PMA solutions showed a similar tendency to those obtained in PMA-thickened oil A. When PMA-4 was added to A and D, these solutions showed different film-forming abilities. To elucidate the effects of the average molecular weight of the PMA and of differences in base oil on the EHD film thickness, relationships be-
68 TABLE 4 Physical properties of polymethacrylate-thickened
oil D
(wt.%)
Density d;’
Refractive index n&’
Viscosity at 25 OC (cSt)
2.0 4.0 5.8 7.5
0.8807 0.8825 0.8837 0.8853
1.4780 1.4781 1.4781 1.4781
164.86 177.37 193.14 211.11
106 113 120
PMA-2
2.0 4.0 4.5 5.8 7.5
0.8802 0.8810 0.8818 0.8825 0.8835
1.4791 1.4791 1.4791 1.4790 1.4790
170.95 190.97 193.02 215.44 241.71
110 127 127 141 152
PMA-3
1.5 2.4 2.8 4.1
0.8790 0.8798 0.8799 0.8807
1.4779 1.4774 1.4780 1.4780
178.43 194.75 204.67 235.70
114 129 138 155
PMA-4
1.2 1.5 2.3 3.3
0.8791 0.8786 0.8798 0.8804
1.4781 1.4775 1.4781 1.4781
181.97 192.32 213.96 250.95
118 128 144 163
PMA-5
0.9 1.7 2.5
0.8789 0.8792 0.8794
1.4779 1.4776 1.4778
168.84 191.12 210.60
124 155 178
Concentration
PMA
PMA-1
Average viscosity at 25 “C calculated using the ASTM viscosity-temperature
Viscosity index
97
equation [ 121.
tween the film thickness and the average molecular weight of PMA are shown in Fig. 7. 3.3. Effective viscosity The following parameter, being the ratio of the effective viscosity of the polymer-thickened oil to that of the base oil, has been used in evaluating the EHD film-forming ability of polymer solutions [ 131: r)eff -= qB
hp ( h,
1’a 1
where qeff is the effective viscosity of the polymer-thickened oil, qa is the viscosity of the base oil, hp and h, are the central film thicknesses of the polymer-thickened oil and the base oil respectively and (1is the speed parameter index in the EHD film thickness formula (a = 0.67). This parameter is derived from the ratio of the calculated EHD film thickness for the polymerthickened oil to that of the base oil. All data were obtained at the same load,
69
41
, 160
4
180
200 Viscosity
240x10
220 at
25%
-6
(I+s~)
Fig. 2. Central film thickness us. ambient viscosity for various base oils and loads (6, entrainment velocity): O,*, ox Q, 9.8 N;A, A,A, A, 19.6 N;n, n,m, m, 29.4 N.
Viscosity
at
25%
(m*
a’)
Fig. 3. Relation between central film thickness and ambient viscosity of PMA-thickened oil A (entrainment velocity ii = 0.3 m s-r ; load P = 9.8 N; maximum hertzian pressure prnax = 0.35 GPa). Symbol
Molecular weight Mw of PMA
Lubricant Base oil Base oil Base oil Base oil Base oil Base oil
A A A A A A
with with with with with
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
70
/ 150
1
200
250X10-” Viscosity
at
25%
(m’
5’)
Fig. 4. Relation between central film thickness and ambient oil A (ii = 0.3 m s-‘;P = 29.4 N; pmax = 0.50 GPa).
Symbol
Lubricant
l
Base Base Base Base Base Base
0
n V :
oil oil oil oil oil oil
viscosity
Molecular A A A A A A
of PMA-thickened
weight Mw of PMA
with with with with with
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
speed and temperature, and it was assumed that the pressure-viscosity coefficient of the polymer-thickened oil would be independent of the addition of a few per cent of polymer additive [ 141. The parameter is displayed for several results in Table 5 on the basis of Figs. 3 and 5. The relationship between the weight average molecular weight of PMA and the effective viscosity enhances the previous results. 3.4. Shear degradation of polymer-thickened oils Further experiments have been undertaken, both to confirm the effects of average molecular weight on EHD film thickness and to assess the influences of the permanent viscosity loss on EHD film-forming ability in concentrated contacts. Polymer-thickened oils were prepared by the addition of PMA-5 to base oils A and D, which were then degraded using a sonic shear tester of frequency 10 kHz. The changes in physical properties of the test oils with irradiation time are listed in Table 6, in which the average molecular weights determined by means of gel permeation chromatography are given. The parameter Mw/MN , being the ratio of the weight average molecular weight to the number average molecular weight, represents the molecular weight distribution of the polymer. The value of M,/MN became smaller as the irradiation time increased and the ambient viscosity of PMA solutions
71
400 t
.L
I
I
1
25OXlO-6
200
150 Viscosity
at
25
“C (m’
<‘)
Fig. 5. Relation between central film thickness and ambient oil D (ii = 0.3 m s-l ;P = 9.8 N;p,, = 0.35 GPa).
Symbol
Lubricant
0
Base Base Base Base Base Base
0
a D :
oil oil oil oil oil oil
viscosity
of PMA-thickened
Molecular weight MW of PMA
D D D D D D
with with with with with
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
40000 100000 350000 480000 900000
decreased almost exponentially with decreasing average molecular weight. This change means that the higher molecular weight polymers were selectively broken down by an oscillating shear, resulting in a narrower molecular weight distribution. The appearance of these changes, which is shown more clearly in Fig. 8, may be the same as that which occurs in machine elements [ 151. The influences of a decrease in the molecular weight of the polymer on the EHD film thickness are shown in Fig. 9. It is obvious that the EHD film thicknesses of oils A and D thickened with PMA-5 increase with a decrease in ambient viscosity; thus this tendency is in complete disagreement with general knowledge. The increase in EHD film thickness for PMA-5 in oil D with a viscosity decrease is higher than that for oil A, and the difference is strongly consistent with the trend shown in Fig. 7. Comparison of Figs. 8 and 9 shows that there is an interesting relationship between the changes in molecular weight distribution of the polymer and the EHD film thickness that accompany the average molecular weight decrease. That is, the molecular weight distribution becomes narrower with the irradiation time whereas the increase in EHD film thickness decreases gradually. 3.5. Wear tests In the lubrication grade oil was observed
of cam followers, the wear associated with a multito be profoundly larger than that with a single-grade
72
150
200 Viscosity
250x10-” at
25°C
(d
s-‘)
Fig. 6. Relation between central film thickness and ambient oil D (ii = 0.3 m s-‘;P = 29.4 N;nmax = 0.50 GPa).
Symbol
Lubricant
0
Base Base Base Base Base Base
0
a D
oil oil oil oil oil oil
viscosity
of PMA-thickened
Molecular weight MW of PMA
D D D D D D
with with with with with
40000 100000 350000 480000 900000
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
o
PMA-Thickened
Oil
A
0
PMA
Oil
D
Thickened
350 5 Weight
Average
10 Molecular
Fig. 7. Variations in central film (ii=0.3ms-‘;P=9.8N;p,,= Figs. 3 and 5, arising from oils mental temperature. The broken oil A.
50 Weight
100~10’
of PMA
thickness with weight average molecular weight of PMA 0.35 GPa). All plots are derived from results obtained in having approximately the same viscosity at the experiline for base oil B is a reference to the polymer-thickened
oil [16]. To evaluate the results mentioned above, wear experiments have been conducted to verify the rate of wear for the polymer-thickened oils having the highest and the lowest average molecular weight of PMA. Changes
73 TABLE 5 Effective viscosity of polymethacrylate-thickened PMA
%ffhB
PMA-1 PMA-2 PMA-3 PMA-4 PMA-5
1.16 1.11 1.07 1.01 1.00
oils
%fflrlB for oil D
for oil A
1.16 1.08 1.03 0.99 0.99
TABLE 6 Changes in physical properties of polymethacrylate-thickened Irradiation (min)
time
Viscosity (cSt)
0
Molecular
at 25 “C
220.57 192.41 184.03 179.07 171.51
5 10 15 30
oil A with irradiation time weighta
900000 461000 334000 276000 189000
aWeight average molecular weight measured by gel permeation chromatography by using the molecular weight of polystyrene.
3
4
5 LOG
6
MwIMN
4.38 2.92 2.38 2.16 1.68 corrected
7
CM)
Fig. 8. Changes in the molecular weight distribution of PMA-5 after treatment with the sonic shear tester.
in wear scar diameters associated with incremental changes in total load are shown in Fig. 10. Wear scar diameters obtained with the lower molecular weight PMA-1 solution were smaller than those for the higher molecular weight PMA-5 solution within, at least, these frictional conditions.
74
I
0
PM-5
(25rt%)~OiI
A
0
M-5
(2.5wt%)bOiI
t
M-5
450 -
MW - 900.000
-y*
\ 400 -
-c--n-
y__=_______
35ok r,
’
I
160
180
I 200
Viscosity
at
220 ‘( 10
25%
(m’
Fig. 9. Effect of shear degradation pmax = 0.35 GPa).
s-‘)
on the central
film thickness
(U = 0.3 m s-l ; P = 9.8 N;
500
0
PMA
0
I’M&-Bare
B-Base
Oil
A (220.57
Oil
A (214.33
V..,
I.”
Total
Fig. 10. duration
Load
(kN)
Variation in wear scar diameter with total time t = 1 h; temperature, 25 “C).
load (sliding speed
u = 0.077
m s-‘;
4. Discussion It has been shown that the EHD film thicknesses of polymer-thickened oils are less than those of the corresponding base oils. There are two main explanations why polymer-thickened oils are inferior to base oils in their ability to form EHD films: (i) temporary viscosity loss and (ii) permanent
15
viscosity loss. Dyson and Wilson have drawn attention to the fact that high shear rates occur at the inlet region [1’7], and this seems to lead to the temporary viscosity loss. Values of the maximum shear rates in the inlet region can be of the order of 10’ - lo6 s-’ even under pure rolling conditions. This may lead polymer solutions to the second newtonian region [ 181. The permanent viscosity loss is ascribed to a decrease in the molecular weight of the polymer due to severe shear stresses in the contact region. Walker et al. performed excellent work in which small samples of polymer solutions were extracted from both the inlet and the exit regions of pure sliding EHD contacts and then analysed to determine any changes in the molecular weight and its distribution [ 191. They revealed that decreases in the molecular weight of the polymer occurred in the EHD contacts and thus that the film thickness of the polymer-thickened oils decreased. 4.1. Film thickness of polymer solutions The viscosity of polymer solutions subjected to both high pressures and high shear rates is higher than that of the base oil [ 181; thus the EHD film thickness should be increased by the addition of polymers. The EHD film thickness for PMA-5 solutions was, however, found to be approximately equal to that of the base oil even though the ambient viscosity was considerably raised. The complex behaviour of polymer-thickened oil could be affected by the following factors: (1) the molecular size, molecular structure and molecular weight distribution of PMA; (2) the solubility and/or the dispersibility of PMA in the base oil, particularly under higher pressures; (3) the mechanical shear degradation of PMA at or near the EHD contact. It appears to be difficult for the larger size PMA molecules to pass through the contact; thus some molecules may accumulate in the inlet and others sweep around the EHD contact. It is considered that the accumulation of PMA in the inlet may primarily be caused by the adsorption of PMA onto the steel surface. However, some of the larger molecules may pass through the contact if alignment of the molecules occurs in the flow and if the molecules can be shear degraded in the inlet region. This behaviour should be related to the molecular weight and the distributions of molecular weight of the PMA, i.e. whether it has a narrow or a wide distribution. The polymer additives exert a thickening action which increases the viscosity by means of their solubility in the base oil. However, as the molecular weight increases considerably, the solubility decreases markedly and the molecules are likely to separate from the base oil. This is particularly true under high pressures because the solubility of the polymers in mineral oils becomes poorer, such that the larger size molecules are likely to remain in the inlet region. Furthermore, this tendency may be facilitated by the reverse flow which occurs in the convergent inlet region. 4.2. Shear degradation of polymethacrylate-thickened oil With respect to the PMA solutions degraded by means of the sonic shear tester, the EHD film thickness gradually increased with decreasing
76
average molecular weight of the PMA. This suggests that the higher molecular weight polymers may sweep around the contact and/or may accumulate in the inlet, obstructing some of the oil supply to the contact. It is observed in Fig. 8 that the average molecular weight of the PMA decreases as the larger PMA molecules are selectively degraded. This may also be concerned with the film thickness. 4.3. Photomicrographs of elastohydrodynamic contacts under pure sliding conditions Cusano and Sliney have recently presented many interesting photomicrographs which show the distribution of graphite in and around EHD contacts by using graphitedispersed oils [20]. In the present paper, photomicrographs of the contact under pure sliding conditions are shown in Figs. 11 and 12. The graphite particle size was less than 0.3 E.trn and the concentrations of graphite in oil A and oil A thickened with PMA-5 were 1.0 wt.%.
Fig. 11. Graphite-dispersed perature, 8 - 10 “C).
base oil A (LJ = 0.01
Fig. 12. Graphite-dispersed oil A thickened pmax = 0.27 GPa; temperature, 8 - 10 “C).
m s-l;
with
P = 9.8
PMA-5
N;p,,
(u = 0.01
= 0.27
m s-l;
GPa; tem-
P = 9.8
N;
The results presented in these figures are representative of many photomicrographs which were taken. The load was 9.8 N and the sliding velocity was 0.01 m s-i at a temperature of 8 - 10 “C. It was observed that graphite particles go through the contact as shown in Fig. 11. In the base oil alone, graphite did not accumulate in the inlet region. In contrast, in the PMA-thickened oil, the amount of graphite passing through the contact seemed to be very small in the same condition as shown in Fig. 12. In pure sliding under boundary conditions, it was observed that the graphite accumulated, in varying degrees, in the inlet and reverse flow took place in the inlet region. In these experiments, graphite particles passed through the contact when the base oil only was used. It seems that the contact takes place under partial EHD conditions. These pure sliding conditions are quite different from the nominal rolling conditions, but the same phenomena probably occur under thin film lubrication conditions. This suggests that the higher molecular weight PMA accumulates with graphite particles in the inlet region. From these results, it is considered that it is generally difficult for PMA molecules to pass through the contact as the molecular size increases, even under rolling conditions. The blocking action of polymers at the inlet region decreases the oil supply to the contact to cause a form of lubricant starvation. Therefore there may be a critical range of molecular weights for polymers in base oils for successful operation which depends on the structure and the distribution of molecular weight of the polymers. 5. Conclusions The behaviour of PMA additives under the partial EHD or EHD conditions has been studied under nominally rolling and pure sliding conditions. (1) Under rolling conditions, the oil film thickness of PMA-thickened oils gradually increases with increasing viscosity, but its rate of increase slowly falls as the average molecular weight increases until the thickness remains the same as that of the base oil. (2) In PMA-thickened oils, the larger size molecules in the inlet may find it difficult to pass through the contact and they may accumulate at and/or sweep around the contact. (3) With the shear-degraded PMA-thickened oils, the oil film thickness increases as the molecular weight of the PMA decreases under rolling conditions. In the practical use of polymer additives, it appears that film formation at the contact becomes effective in the moderate average molecular weight range, even if a permanent viscosity loss occurs under the operating conditions. (4) Under sliding conditions, graphite particles dispersed in the base oil and PMA-thickened oil have been studied by taking photomicrographs. It was observed that the larger size PMA molecules accumulated with the graphite particles in the inlet region. It seems that the same behaviour may occur under rolling conditions.
78
Acknowledgments
The authors would like to thank Mr. T. Uchikawa, Director, Lubricants and Petroleum Products, for his valuable advice, and they also thank sincerely the Nippon Mining Co. Ltd. We appreciate the kind support of Mr. H. Kageyama, Vice-president, Kyodo Yushi Co. Ltd. References 1 A. Dyson, H. Naylor and A. R. Wilson, The measurement of oil-fifm thickness in elastohydrodynamic contacts, Proc., Inst. Mech. Eng., London, 180 (3B) (1965 1966) 119 _ 134. 2 G. M. Hamilton and W. G. Robertson, Lubrication of rollers with oils containing polymers, Proc., Inst. Mech. Eng., London, 181 (3B) (1966 - 1967) 192 - 199. 3 C. A. Foord, W. C. Hamman and A. Cameron, Evaluation of lubricants using optical elastohydrodynamics, ASLE Trans., 11 (1) (1968) 31 - 43. 4 S. Hironaka, M. Nagai and T. Sakurai, Behavior of polymer additives in elastohydrodynamic contact, J. Jpn. Pet. Inst., 22 (1) (1979) 52 - 58. 5 D. M. Sanborn and W. 0. Winer, Fluid rheological effects in sliding elastohydrodynamic contacts with transient loading, I, Film thickness, J. Lubr. Technol., 93 (April 1971) 262 - 271. 6 M. Hirata and A. Cameron, The use of optical elastohydrodynamics to investigate viscosity loss in polymer-thickened oils,ASLE Trans., 27 (2) (1984) 114 - 121. 7 A. Cameron and R. Gohar, Theoretical and experimental studies of the oil film in lubricated point contact, Proc. R. Sot. London, Ser. A, 291 (1966) 520 - 536. 8 R. Gohar and A. Cameron, The mapping of elastohydrodynamie contacts, ASLE Trans., 10 (1967) 215 - 225. 9 C. A. Foord, L. D. Wedeven, F. S. Westlake and A. Cameron, Optical elastohydrodynamics, Proc., Inst. Mech. Eng., London, 184 (Part 1, 28) (1969 - 1970) 487 - 505. 10 H. Hamaguchi, H. A. Spikes and A. Cameron, Elastohydrodynamic properties of water in oil emulsions, Wear, 43 (1) (1977) 17 - 24. 11 W. Machidori, H. Kageyama and T. Moriuchi, Grease lubrication in elastohydrodynamic contacts, 50th National Lubricating Grease Institute Annu. Meet., Kansas City, MO, October 23 - 26, 1983. 12 A. Cameron, Principles of Lubrication, 1966, p. 23. 13 B. J. Hamrock and D. Dowson, Isothermal elastohydrodynamie lubrication of point contacts, Part III, Fully flooded results, J. Lubr. Technot., 93 (April 1971) 264 - 276. 14 W. 0. Winer and J. 13. Novak, Some measurements of high pressure lubricant rheology, J. Lubr. Technol., 90 (1968) 580 - 591. 1.5 N. V. Messina, V. I. improvers for multigrade oils. In T. Sakurai (ed.), Proc. JSLEASLE Int. Lubrication Con& Tokyo, 1975, Elsevier, Amsterdam, 1976, pp. 880 889. Engineers Wear 16 J. Chida, Wear on engine parts, 7th Jpn. Society of Lubrication Symp., March 1975. 17 A. Dyson and A. R. Wilson, Film thickness in elastohydrodynamic lubrication by silicone fluids, Proc., Inst. Mech. Eng., London, 180 (3K) (1965 - 1966) 97 - 112. 18 J. D. Novak and W. 0. Winer, The effects of pressure on the non-newtonian behavior of polymer blended petroleum oils, J. Lubr. Technol., 91 (July 1969) 459 - 463. 19 D. L. Walker, D. M. Sanborn and W. 0. Winer, Molecular degradation of lubricants in sliding elastohydrodynamic contacts, J. Lubr. Technol., 97 (July 1975) 390 - 397. 20 C!. Cusano and H. E. Sliney, Dynamics of solid dispersions in oil during the lubrication of point contacts, Part I, Graphite, ASLE Trans., 25 (2) (1982) 183 - 189.