Structure Effect of Refrigeration Polyolester Oils
for use in HFC-134a Compressors
J.-C. Remigy, E. Nakache, ISMRA, CNRS URA 480, Caen, France, and P.D.
Brechot, Mobil Oil Francaise, Research Centre, Notre Dame de
Gravenchon, France
A study of polyolester oils (POE)for compressors using an ozone-friendly
refrigerant is presented here. The effect of the chemical structure of POE was
studied in relation to three lubricant properties: miscibility with HFC-134a
(CF,-CH,F), lubricity (steel-on-steel and aluminium-on-steel contacts), and
viscosity. The paper shows the strong structure effect of POE on miscibility
and lubricity. Based on the data obtained, some model lubricants were
blended, and the pe~ormanceof these lubricants was assessed on refrigera-
tion test rigs.
KEYWORDS: polyolester oils, compressor, refrigeration, HFC-I 34a, lubricity,
viscosity, miscibility, steel, aluminium
INTRODUCTION
Since late 1995, to comply with the Montreal protocol and its amendments,
CFC has no longer been manufactured.’ This protocol has brought about im-
portant modifications in refrigeration technology, and has made it necessary
to find suitable materials for refrigeration systems that are compatible with the
new refrigerants. In particular, the choice of lubricant is crucial, since the re-
liability of the compressor cannot be assured without a suitable lubricant. The
new refrigerants proposed as replacements for chlorofluorocarbons (CFCs)
are hydrofluorocarbons (HFCs).
The leading substitute for CFC-12 (CCl,F,) is HFC-134a (CF,-CH,F).
This refrigerant is not compatible (i.e. is immiscible) with conventional refrig-
eration oils (e.g. mineral oil, polyalphaolefins, alkylbenzenes). Two oils that
have been found to be miscible with HFC-134a are polyalkylene glycols
(PAG) and polyolesters (POE). PAGs are so hygroscopic that the water ab-
sorbed can produce damage to the refrigeration system. POEs are less
~ - ~ PAGs, and several studies show their good thermal and
h y g r o s ~ o p i cthan
chemical ~tability.~.’
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238 Remigy, Nakache, and Brechot
Table 1 Typical properties of some POE oils used
PE-A PE-B TMP-A NPG-A DPE-A
Viscosity at 40°C, mm2/s 21.3 15.6 13.7 5.61 52.3
Viscosity at 1OO'C, mm2/s 4.61 3.61 3.39 1.89 8.76
Viscosity index 'f 35 115 122 122 146
Total acid number, mg 10.05 <0.05 <0.05 <0.05 <0.05
KOH/g
Water, ppm <I 00 400 <I 00 1100 <I 00
Pour point, "C <54 <-54 <-54 <-54 -
PE: pentaerythritol ester
TMP: trirnethylolpropaneester
NPG: neopentylglycol ester
DPE: dipentaerythritol ester
The present study focuses on the influence of the chemical structure of
POE on various properties involved in the lubrication of an HFC-134a com-
pressor: miscibility, lubricity and viscosity. The influence of chemical struc-
ture was studied in terms of acid chain length, branching and number of ester
groups for the same basic structure (i.e. neopentyl). From this study, we pro-
pose a general method to select pure POE and formulate a refrigeration oil.
Two oils were blended and tested on refrigeration test rigs.
MATERIAL AND METHOD
The oils used in this study are available commercially. The POE oils have the
same basic structure (similar to a pentaerythritol structure) and different num-
bers of ester groups: neopentylglycol esters (NPG)have two ester groups, tri-
methylolpropane esters (TMP) have three ester groups, pentaerythritol esters
(PE) have four ester groups, and dipentaerythritol esters (DPE) have six ester
groups. Typical properties of these esters are presented in Table 1.
Miscibility
Lubricant miscibility with HFC-134a was tested in sealed glass tubes with a
mixture of 10% vol. oil in 90% vol. refrigerant. The tubes were immersed in
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Structure Effect of Refrigeration Polyolester Oils in HFC-134a Compressors 239
Figure 1 Miscibility temperature range for typical oils with HFC-l34a,
according to reference 6
a constant temperature alcohol bath and controlled to k 0.5”Cwithin temper-
ature range tested -75°C to +65”C. We measured the temperature at which
two phases became visible; this temperature is called the ‘miscibility’ temper-
ature.
Tribological properties
The tribological properties were characterised with a Falex (pin on v-block)
load tester (ASTM D-3233). These tests were carried out for steel-on-steel and
aluminium-on-steel (aluminium pin) contacts at ambient temperature in an air
atmosphere.
RESULTS AND DISCUSSION
The ester’s structure is decomposed in alcohol (alcohol used to synthesise the
ester) and in acid (acid used to synthesise the ester). In this paper, the terms
‘acid’ and ‘alcohol’ mean the acid, or the alcohol, used to synthesise the ester.
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240 Remigy, Nakache, and Brechot
Figure 2 Influence of acid structure on miscibilityof polyolesterswith HFC-134a
The arrows mean either below or above the temperature studied (-75 to 65°C)
60
50
40
2 30
a 20
!! 10
ln
a 0
-5
-
0
.-
-10
-20
-30
P -40
2
._ -50
r -60 I I I I I I
-70
3
rTi
5 7 9 11 13 15 17 19
Principal acid chain carbon number
MISCIBILITY
Lubricant miscibility with a liquid refrigerant is one of the most important re-
quirements for trouble-free operation of any refrigeration system. Lubricant
miscibility is critical in flooded evaporators, because a phase separation will
lead to poor oil return and poor lubrication. Miscibility is a key factor in any
non-flooded evaporator, because the separated lubricant forms a layer which
creates additional heat transfer resistance.
Miscibility varies with the chemical structure of the base oil. Figure 1
shows that aliphatic or aromatic oils, such as mineral oil, polyalphaolefin and
alkylbenzene, are immiscible, whereas the others show either partial or total
miscibility.
The miscibility temperature of an ester varies with its type and chemical
structure, so selected polyolesters are miscible with HFC- 134a, unlike diesters
or aromatic esters. The miscibility of POE depends on the structure of the al-
cohol and acid used to manufacture the ester.
Figure 2 shows that the miscibility temperature depends strongly on the
acid chain length: for the same alcohol, an ester with a 5-carbon acid is very
miscible, whereas a 9-carbon acid is not. A branched chain acid is more mis-
cible than a linear chain acid (with the same carbon number); for example, an
ester of pentaerythritol and 3,5,5-trimethylhexanoic acid (PE-iC9) has a
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Structure Effect of Refrigeration Polyolester Oils in HFC-134a Compressors 241
Figure 3 Influence of alcohol structure on miscibility of polyolesters with
HFC-134a
70
60
50
40
g 30
2 20
!! 10
al
--&
a 0
-10
& -20
.-
._ -30
a -40
.4 -50
-60
-70
-80
1 2 3 4 5 6 7
Hydroxyl number
miscibility temperature of -32"C, whereas an ester of pentaerythritol n-
nonanoic acid (PE-nC9) has an upper miscibility temperature (+60°C).
The influence of alcohol on miscibility is shown in Figure 3, which plots
miscibility data for esters based on a single straight-chain acid (n-heptanoic
acid) and different alcohol hydroxyl numbers (i.e. different alcohols, NPG,
TMP, PE, DPE).
This study shows that miscibility decreases when the number of ester
groups increases. This can be explained by the steric hindrance of the ester
groups: the ester groups of a polyolester with 4 or 6 ester functions are hin-
dered because of the neopentylic structure of these esters. The HFC-134a mol-
ecule cannot reach the ester group to interact, which induces the high
miscibility temperature of this ester, as shown in Figure 4 (overleaf). The mis-
cibility behaviour is very dependent on the accessibility of HFC- 134a to the
ester groups.
Tribological properties
The general regime of refrigeration compressor lubrication is hydrodynamic,
but compressor start-up, shutdown and overload conditions may also require
good boundary lubrication properties. Hydrodynamic lubrication depends on
lubricant viscosity (for a fixed compressor) and good boundary lubrication
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242 Remigy, Nakache, and Brechot
Figure 4 Steric hindrance of polyolester
,---/--
PE-nC7: highly hindered ester groups NPG-nC7: less hindered ester groups
Figure 5 Influence of acid chain length on tribological properties of polyolesters
6,000
-z 5,000
g
7J
4,000
-
$ 3,000
.-N
$ 2,000
-Steel-on-steel
- - - - - Aluminium-on-steel
1,000
1.000
4 5 6 7 8 9 10
Acid chain length in terms of carbon number
properties depend on lubricant tribological properties. Steel-on-steel and alu-
minium-on-steel contacts are usually found in refrigeration compressors, in
particular, where the connecting rods are aluminium.
Figure 5 shows that for steel-on-steel contact, the seizure load is fairly in-
dependent of the acid chain length, whereas the influence of acid chain length
on seizure load for aluminium-on-steel contact is important. The antiwear pro-
tection of aluminium-on-steel will be very positively influenced by the
increase in the acid chain length of the ester.
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Structure Effect of Refrigeration Polyolester Oils in HFC-134a Compressors 243
Figure 6 Influence of acid branching on seizure load
Steel-on-steel
6,000
5,000
z
h
'0 4,000
8
- 3,000
2?
3
.-a, 2,000
N
(0
1,000
-
n ~~
PE-nC9 PE-iC9 DPE-nC9 DPE-iC9
iC9 = 3,5,5-trimethylhexanoic acid
Figure 7 Influence of alcohol structure on tribological properties of polyolesters
6,000
-z 5,000
u 4,000
8
-
2?
.-a, 3,000
(0
- - - - - - - Aluminium-on-steel
2,000
1,000
1 2 3 4 5 6 7
Hydroxyl number
Figure 6 shows variation of seizure load versus acid chain length. What-
ever the type of metallurgy considered, linear acids (PE-nC9, DPE-nC9) con-
fer higher seizure load and better anti-wear properties on the polyolester than
do the branched acids (PE-iC9, DPE-iD9).
The influence of the ester group number on tribological properties is
shown in Figure 7.Whatever the type of metallurgy considered, an optimum
is reached in seizure load versus the number of hydroxyl groups (i.e. the num-
ber of ester groups of the ester) for pentaerythritol ester (four ester groups).
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244 Remigy, Nakache, and Brechot
Figure 8 Structure of TMP and DPE
oh /
Trimethylolpropane ester Dipentaerythritol ester
Figure 9 Influence of acid structure on viscosity of polyolesters
120
* 100
iii
PE: branched chain acid
.-V
iii
$ 20 ._._._._.....-.-
C
3
t 7 ! 1 , l , l ,
1
I , l
0
4 5 6 7 8 9 10
Total carbon number
The increase in seizure load with the increasing number of ester groups is like-
ly to be due to various phenomena, including the increase in the molecule size
of the ester increasing the film thickness, and the increase in the fixing points
of the molecule on the surface (i.e. polar ester groups).
The lower seizure load of dipentaerythritol ester (six ester groups) may be
explained by the fact that its basic structure is not strictly neopentyl. Its struc-
ture is like two trimethylolpropanes linked with an epoxide bond (see Figure
S), and thus the behaviour of dipentaerythritol may be the same as that of tri-
methylolpropane.
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Structure Effectof Refrigeration Polyolester Oils in HFC-134a Compressors 245
Table 2 Properties of two model oils
Oil 1 Oil 2
Viscosity at 40°C cSt 20.84 21.21
Viscosity at 100°C cst 4.37 4.48
Water content PPm 60 90
Miscibility temperature (90% wt HFC-134a) "C -45 -45
Seizure load (steel on steel) N 4,900 4,800
Seizure load (aluminiumon steel) N 4,750 5,000
Number of short chain carbon (<8C)/
number of long chain carbon (>9C) 3.72 3.82
Viscosity
Figure 9 shows the influence of the carbon number of the acid on viscosity:
the higher the carbon number, the higher the viscosity. The branched acid es-
ters have higher viscosities than the linear acid esters (with the same carbon
number), as shown in Figure 9.
Model lubricant
Because of the higher seizure load observed for pentaerythritol ester, and from
these results, good tribological properties will be obtained with an ester of
pentaerythritol and a long and linear chain acid, but good miscibility with
HFC-134a will be obtained with short and branched acid esters. Thus, these
properties require two opposing types of ester, and a POE lubricant should be
a mixture of different types of ester. Therefore, to obtain both good miscibility
and good tribological properties for a pentaerythritol, the concentration of
long- and linear-chain acid esters should be limited, and, preferably, the car-
bon number of the linear acid should be limited to 7 carbons; the viscosity will
be affected by long and branched acids, which is a good balance between mis-
cibility and viscosity.
Based on these considerations, two IS0 VG 32 model oils (see Table 2
for oil properties) were tested on a compressor rig. Table 3 (overleaf) gives
the operating conditions.
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246 Remigy, Nakache, and Brechot
Table 3 Operating conditions of compressor rig test
Type Hermetic compressor unit
Single-stagepiston
Refrigerant HFC-134a
Lubrication mode Splash
Outlet pressure, bar 31f l
Inlet pressure, bar lM.l
Evaporation temperature High temperature loop only
Duration, h 500
Table 4 Rig test result
Oil 1 Oil 2
Wear demerit scale 17 20
Ap (cSt) at 40°C +0.02 -0.15
ATAN (mg KOH/g) <0.01 <0.01
Water (ppm) 26 64
Metal (ppm) Al =O =O
Fe 1 3
Si 1 1
Others -0 -0
To assess the product performance, a demerit scale going from 0 (best) to
5 (worst) was established, which rates the level of wear on each of the 11 com-
pressor parts. If the sum of this rating is lower than or equal to 20, the oil
passes the test.
As shown in Table 4, the two lubricants give good results on this test. The
wear was low and the oil properties did not change after 500 h.
CONCLUSION
This study shows the strong influence of chemical structure of polyolesters on
their miscibility with HFC-l34a, and on tribological properties. Good misci-
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Structure Effect of Refrigeration Polyolester Oils in HFC-134a Compressors 247
bility will be obtained with short- and branched-chain acid esters, and good
tribological properties will be obtained with an ester of pentaerythritol and a
long- and linear-chain acid. Based on the data obtained regarding the behav-
iour of each of the POEs studied, model lubricants were blended. These oils
passed the compressor rig test with good results. This study shows how chem-
ical structure influences lubricant properties, and can thus be a powerful tool
in formulating lubricants.
Acknowledgement
The authors acknowledge A.D.E.M.E. (the French Environmental Agency),
Paris, France, for their financial support.
References
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stances that deplete the Ozone Layer’, 3rd ed., Ozone Secretariat, UNEP, August 1993.
2. Sanvordenker, K.S., ‘Material compatibility of R- I34a in refrigeration systems, CFCs:
time of transition’,ASHRAE Trans., 95 (1989), 211.
3. Sundaresan, S.G., and Finkenstadt, W.R., ‘Polyalkylene glycol and polyol ester lubricant
candidates for use with HFC-I 34a in refrigeration compressors’, ASHRAE Trans., 98,
(1992), 1.
4. Johnston, G.J., Shim, J., and Brechot, P.D., ‘Refrigeration oils for use with non-CFC, non-
ozone-depleting refrigerants - key performance and retrofitting issues’, J. Synth. Lubr.,
11,4 (1995), 243.
5. Bakunin, V.N., and Parenago, O.P., ‘A mechanism of thermo-oxidative degradation of
polyol ester lubricants’, J. Synth. Lubr., 9, 2 (1992), 127.
6. Komatsuzaki, S., and Kishi, A,, ‘Lubricants for HFC-134a automobile air conditioners’,
in Proc. Int. CFC Halon Alternatives ConL, Baltimore, USA, 1991, p. 553.
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