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Sliding friction behaviour of polymer-polymer material combinations
Wear, 84 (1983)
167
167 - 181
SLIDING FRICTION BEHAVIOUR OF POLYMER-POLYMER MATERIAL COMBINATIONS
GUNTER ERHARD BASF
AG.,
D-6700
Ludwigshafen
(F.R.G.)
(Received August 3,1982)
Summary The intermolecular bonding energies of polymeric materials can be employed in the evaluation of sliding under conditions determined predominantly by adhesion. The effect at the interface of these energies can be evaluated by reference to the surface energy and to its polar and dispersion-related components. Wetting angle measurements were used to determine the work of adhesion for a group of polymer-polymer material combinations. An exponential relationship exists between the work of adhesion and the coefficients of friction of these combinations, these coefficients of friction being determined primarily by adhesion.
1. Introduction The sliding friction and we= behaviour of polymer-steel combinations have been reviewed [ 11. Corresponding basic literature on the sliding friction behaviour of polymer material combinations is largely lacking, although combinations which are suitable for many industrial applications have already been found, either empirically or, for certain individual cases, by experimental methods. The present paper is a report of the results of experiments on the frictional behaviour of polymer materials in contact, the object of the experiments being to determine whether, under sliding conditions which were predominantly adhesive, relationships could be discovered between the sliding friction and other properties of the polymer materials. Such relationships provide the basis for optimum materials selection in cases involving industrially relevant sliding combinations [ 21. 2. Basic principles According to present knowledge [3 - 81, the frictional force FF which must, macroscopically speaking, be exerted during the process of sliding 0043-1648/83/0000-0000/$03.00
@ Elsevier Sequoia/Printed in The Netherlands
168
comprises an adhesion the formula
term FFadh and a deformation
FF = F~aa +FFdef Two factors thus emerge as principal solids: (1) adhesion of the interfacial contact points.
term FFdef according
to (1)
influences on sliding friction between layers; (2) deformation of the micro-
2.1. Discussion of the adhesion term 2.1.1. Intermolecular bonding energies In the regions corresponding to the actual contact surface, the surfaces of two rubbing partners approach each other sufficiently closely for atomic and/or molecular interactions to occur. With frictional contact between polymer-polymer combinations, an interaction of this type is the development of physical adhesion forces between the polymer material surfaces brought into contact. These interactions are electrostatic forces of the type which act, as intermolecular bonding forces, in the polymers themselves [7,81. It is well known that these interactions result from dispersion forces, dipole interactions (permanent and induced), hydrogen bridge bonds and ionic forces. Intermolecular bonding forces between macromolecules in electrostatic equilibrium are termed dispersion forces. These forces are attributable to electron movements, which momentarily generate dipoles. Physical bonds of this type occur in all polymer materials, but in materials such as poly(tetrafluoroethylene) (PTFE) and poly(ethylene) (PE) they are the only type of intermolecular bond present. Their strength is approximately two to three orders of magnitude lower than that of the covalent bonds [9, lo]. In contrast, interactions which result from the presence of permanent dipoles are markedly more powerful than these dispersion forces. Permanent dipoles occur when positive and negative poles are formed as a result of charge displacements in the electron shell (electronegativity). Permanent dipoles occur for example in the molecular structure of poly(viny1 chloride) (PVC), poly(methy1 methacrylate) (PMMA), poly(butylene) terephthalate (PBTP) and poly(oxymethylene) (POM). Their strength is approximately two orders of magnitude less than that of a covalent bond. A permanent dipole can even produce a dipole in an adjacent molecule which is initially non-polar or can increase the strength of a dipole which is already present. Bonding forces that are due to induced dipoles of this type are termed induction forces. Hydrogen bridge bonds such as those which occur for example in the polyamides (PAS) produce even stronger intermolecular forces. In a PA a hydrogen atom which is covalently bonded to a nitrogen atom acts as a “bridge” between this nitrogen atom and an oxygen atom belonging to an adjacent chain. As a result of the strong electronegativities of nitrogen and oxygen this mechanism generates attractive forces of the order of magnitude of 20 kJ mol-‘.
169
Ionic bonds result from the electrostatic interaction between ions carrying different charges. They occur only in a particular group of polymer materials, the ionomers, which were not investigated in the present work. 2.1.2. Surface energy of solid materials The intermolecular forces discussed above also determine the surface energy y of a polymer material. This parameter can be broken down into a disperse component yd and a polar component yp in accordance with the respective effects of the dispersion forces and dipole interactions (polymeric materials with surface energy components which result exclusively from dispersion forces are also called non-polar, while those with polar components are designated as weakly polar or strongly polar, depending on the magnitudes of these components) according to the formula y = yd + yp
(2) is valid when two bodies with surface energies ‘ya and +& are
Dupre’s equation in contact:
(3) Ya + ?b = Yab + wab In this equation w&, is the work of adhesion which has been performed, referred to unit surface area, and which is transformed into “heat of contact”, while Tab is the interfacial energy of the surfaces which are in contact. In order to separate the two bodies a and b, work per unit of surface area equivalent to W& has to be expended. If the polar and disperse surface energy components of the combination partners a and b are known, the work w,b of adhesion can be calculated, either according to Owens and Wendt [ 111 who showed that Wab
= 2hadybd
or according
)1’2
)1’2
(4)
to Wu [ 121 who showed that
4%dybd wab
+ 2haPrbP
+
4YapYbp
=
(5) Yad
+ybd
Yap
+?bp
Alternatively, the polar and disperse surface energy components of a polymeric material can be determined analytically from wetting angle measurements with at least two test liquids having known surface energy components [ 11, 121 or graphically [ 131 from eqns. (2) - (5). The inter-facial energy ysL is obtained by means of Young’s equation: YSL
=rs
-YL
cost
(6)
In this equation, 19 is the measured wetting angle, while the indices S and L respectively denote solid and liquid in cases where solids come into contact with liquids. 2.1.3. The part played by surface energy in the sliding process When relative movement takes place the adhesive contacts formed when solids touch are repeatedly separated and re-formed in accordance with the
170
jump distance between the points at which contact occurs. A loss of energy is associated with this process, and this energy loss can be described by means of the surface energies of the materials involved [ 14 - 171. The work W,, of adhesion provides a direct means for measuring the effect of adhesive contacts [ 181, 2.2. Discussion of the deformation term Deformations occur as soon as the actual contact surface starts to form. Deformations of segments of molecules, and within parts of molecules, are also necessarily involved in the separation of contact bridges, these deformations being associated, as complex dynamic processes, with hysteresis losses. When contact occurs between materials possessing markedly different moduli of elasticity, the surface asperities of the harder partner will penetrate into the surface of the softer partner. When relative movement occurs, a pile-up (resembling a bow wave) ,will be generated in front of each asperity. As the sliding process proceeds, the surface asperity displaces this frontal bow wave in the direction of movement and continuously rolls it flat. As a result of this process the material, or rather its surface layers, is subjected to continuous alternating stressing, and lost work is performed [7, 81. These deformation processes were not investigated in the present work and were substantially excluded from the friction experiments through suitable choice of the stressing parameters.
3. Material combinations investigated
The combinations Table 1.
of polymer materials investigated are summarized in
4. Experimental apparatus and procedure
4.1. Ring-rung uncrates for determining coeffic~~t$ of friction The test arrangement represented di~matic~ly in Fig. 1 was utilized for the experiments involving the determination of coefficients of friction. The two test rings 1 are fitted and centred in the receiving plates by means of three close-tolerance pins in each case. The lower plate is driven by a variable-speed drive 2 via a hollow shaft. The upper plate is connected to a torsion spring 3; the twist of this spring provides a measurement of the frictional torque. The angle of twist is indicated by a measuring pointer 4 and can be read off on a scale 5 directly as the coefficient of friction. For the torsion spring used, one scale division represents a value of p of 0.02. The normal force is applied by attaching weights 6 to a lever arm. The weight of the lever arm itself, and of the upper test specimen complete with holding devices, is balanced by means of a ~o~~~eight 7.
171 TABLE 1 Combinations
of polymer materials investigated*
HDPE PA 6 PBTP PMMA (52612) (84) (4550) HDPE (5261 Z) PA6 (B4) PBTP (4550) PMMA POM (N 2200) ;:68 N) PTFE PVC (516) SAN (368 R)
PTFE PVC SAN POM PS (N2200) (168N) (516) (368R)
x
X
X
X
X
X
X
x
X
X
X
X
X
X
X
x
X
X
X
X
X
X
X
x x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
x x
X
X
x
X
X
X
X
X
X
X
X X
X
X
X
X
X
*HDPE, high density PE; PS, poly(styrene); SAN, styrolacrylnitrile; the designations in parentheses identify the materials according to the commercial listings of BASF AG., Ludwigshafen.
Fig. 1. Diagram of the ring-ring
apparatus showing the test piece dimensions.
Changes in the temperature in the vicinity of the surface on which sliding is occurring are measured by means of a thermocouple which is implanted 1 mm beneath this surface; the changes are recorded during the test with the aid of a pen recorder 8.
172 4.1.1~
Conduct of the friction experiments
Before each friction experiment, the test rings were ultrasonically cleaned in 2% detergent solution (concentrated SU-40; Lever Industries) for 3 min at room temperature, after which they were rinsed in distilled water and dried. In the friction tests, the load was chosen to give an average surface pressure jj of 0.09 N mm-‘. The wear measurements were carried out at mean surface pressures p ranging from 0.09 to 0.6 N mme2. In both cases the sliding velocity t; was kept constant at 0.12 m s-i. Each test of a sliding combination ran for a minimum of 24 h corresponding, at the sliding velocity of 0.12 m s-‘, to a sliding distance in excess of 10 km. The deflection of the measuring pointer indicating the coefficient of friction was read off at regular intervals. After the running-in period, the average value was taken as representative of the mean coefficient pcm of friction in continuous operation. Before the test was started, and after completion, the height of the specimen was measured at three points, and the amount S of wear of each ring was determined by taking the difference of the mean specimen heights before and after test. The wear intensity S* is given by dividing the amount of wear by the sliding distance. The wear incurred during the running-in phase, which cannot be determined separately, is also contained in S*, but this runn~g-in wear is negligible when the me~~ements are made over a sliding distance exceeding 10 km. 4.2, Measurement of the surface energy The surface energy of the polymer materials investigated was determined, as was its breakdown into the polar and disperse components, according to the methods of Owens and Wendt [ll] and of Wu [12]. For this purpose, it is first necessary to measure the wetting angles whi_ch various test liquids, having different surface energies, make with the polymers. These measurements were carried out by the capillary rise method usingan apparatus described by Neumann [ 191 and Giickel and Synnatschke [ 201. A sketch of the experimental apparatus is shown in Fig. 2. A vessel 2 is located in a frame 1; the vessel contains a cell 3 filled with the liquid used for the measurement. Both the vessel and the cell can be moved vertically by means of a micrometer screw 4. By operating this screw so that they move upward, the polymer sample 5, which is attached to a holding fixture 6, can be dipped vertically into the measuring liquid. The dimensions of the polymer specimen are 4 mm X 6 mm X 60 mm. A meniscus forms on the polymer specimen thus immersed, and the height to which this meniscus rises is measured with the aid of a cathetometer 7. A glass capillary tube 8, which can likewise be shifted in the vertical direction, is used for accurately sighting on the liquid surface. The space in which the measurements are carried out is enclosed within a t~sp~ent d~u~e-~~ed cover 9 ~0~ which thermostatically controlled liquid‘ flows. The vessel 2 is likewise double walled and
I
1
/////////////////////
Fig. 2. Diagrammatic representation by the capillary rise method.
of the test apparatus for measuring wetting angles
the same liquid flows through it, keeping it at a constant temperature. The cover 9 also ensures that the gas phase in the measuring system reaches an equilibrium corresponding to the vapour pressure of the measuring liquid. 4.2.1. Conduct of the tests and evaluation of the results Before each measurement the polymer specimens were carefully cleaned. The technique whereby a fresh surface is prepared by cutting with a microtome proved to be the most reliable method. Before cutting, the pieces of specimen material were rinsed in alcohol to prevent traces of grease from contaminating the microtome blade. The specimens thus cleaned were inserted into the holding fixture of the test apparatus without being touched by fingers. Before each test, the measuring fluid cell was cleaned with a chromic acid-sulphuric acid mixture and rinsed with distilled water. The measuring liquids used were a-bromonaphthalene and water or, for PE and PS, formamide. The surface energies of these liquids are summarized in Table 2. The measurement temperature was 20 + 0.1 “C. On each occasion the measurement was not made until 3 - 4 h had elapsed to allow the temperature and vapour pressure to reach equilibrium within the measuring space. TABLE 2 Surface energies and densities of the measuring liquids used Measuring liquid
cu-bromonaphthalene Water Formamide
Surface energies (mN m-l)
Density p (g cm+)
Y
Yd
YP
43.9 72.8 58.3
42.3 21.8 37.9
1.57 51.0 20.4
1.482 1.0 1.132
174
On immersing the polymer specimen, the measuring liquid rises on its surface in a meniscus-like manner. The height to which it rises, referred to the undisturbed liquid, was measured with the aid of the cathetometer. (The height to which the liquid rises varies depending on whether the specimen is being dipped into the fluid or is being pulled out of the fluid. Because of this, a distinction is made between the advancing angle and the retiring angle. In the present tests, only the advancing angle was determined.) For each polymer material, four measurements were carried out on each of four test pieces, and a mean value was consequently obtained from the 16 measurements. The height h to which the liquid rises is related to the wetting angle 0 according to the following function, which involves the density p, the surface energy yL of the measuring liquid and the gravitational acceleration g: sin 0 = 1 -
.E!C_ (7) 2% The wetting angles for the various combinations of polymer materialmeasuring liquid were determined from this relationship. The measured wetting angles were then used to determine the surface energies of the polymer materials, and the dispersion-related and polar components of these energies, from eqns, (2) - (6) according to the methods of Owens and Wendt [ll] and of wu [IZ] .
5. Results of the experiments 5.1. Surface energies of the polymer materiab investigated 5.1.1. Determination of wetting angles The wetting angles, determined from the results of the capillary measurements, are summarized in Table 3,
TABLE3 jetting angles for various polymer material-me~uring combinations Polymer
HDPE PA6 PBTP PMMA POM PS PVC SAN
liquid
Contact angle 8 with the following liquids Water
a-bromonaphthalene
Formamide
65.20 77.99 72.79 75.31 70.98 82.00
43.70 9.87 10.59 7.46 23.00 21.67 12.22 10.95
79.06 74.3 -
rise
175
Since, for the combinations PS-water and HDPE-water the wetting angles exceed 90”, they could not be determined by means of the capillary rise method. For this reason the wetting angles of the combinations PSformamide and HDPE-formamide are given in place of the above values in Table 3, fourth column. 5.1.2. Surface energy and breakdown into polar and dispersion-related components The results of calculations of the surface energies y of the polymer materials, based on the wetting angle measurements, are reproduced in Tables 4 and 5, second to fourth columns, together with the corresponding polar components yp and dispersion-related components rd. A compilation of the results evaluated in accordance with the method of Owens and Wendt is given in Table 4, while Table 5 presents the results evaluated in accordance with the method of Wu. In addition, the results of measurements performed by other researchers are included with the present results for comparative purposes. The results relating to PS are unreliable, and they are accordingly shown in parentheses. This unreliability reflects the fact that PS underwent incipient solution by cu-bromonaphthalene during the test, which led to surface energy values which appear to be comparatively high. TABLE 4 Surface energiesa of various polymer materials measured by the method of Owens and Wendt [ll] Polymerb
PTFE HDPE LDPE PP POM PA6 PBTP PETP SAN PMMA PVC PS PC
Surface energies (mN m-‘)
obtained by the following researchers
Measurements present paper
in the
Measurements by Owens and Wendt [II]
Measurements by Rabel [13]
Measurements by Koerner et al. [21]
Yd
Y
Yd
Yd
YP Y
Yd
YP Y
18.5 31.6
0 0.2
18.5 31.8
32.1 35.1
0 0
30.5
0.7
31.2
34.5
YP 0.1
YP Y
34.6 33.2
36.0 36.8 39.6
6.1 10.7 4.2
42.1 47.5 43.8
40.5 38.8 37.7 (44.6)
2.7 6.4 7.5 (0.8)
43.2 45.2 45.2 (45.4)
0
33.2
32.1 35.1
43.2
4.1
47.3
32.9
4.5
37.4
37.8
3.1 40.9
35.9 40.0 41.4
4.3 40.2 1.5 41.5 0.6 42.0
36.6
1.6 38.1
36.0
3.9
37.0
1.8 38.8
39.9
‘The significance of the values in parentheses is discussed in Section 5.1.2. bLDPE, low density PE; PC, polycarbonate; PETP, poly(ethylene) terephthalate; poly(propylene).
PP,
176 TABLE
5
Surface
energies
Polvmer
HDPE PP POM PA6 PBTP SAN PMMA PVC PS PC
With not differ the polar non-polar
of various polymer
materials
Surface energies (mN m-‘l
measured
by the method
of Wu [ 121
obtained by the following researchers
Measurements in the present paper
Measurements wu [12]
by
Measurements by Potente and Kriiger [22]
Yd
Yd
Y
Yd
YP
Y
35.0 36.8 39.2 39.4 39.5 39.6 39.0
11.1 15.4 9.4 7.7 11.7 12.7
47.9 54.6 48.8 47..2 51.3 51.7
YP 0.7
YP
Y
35.7
29.8
11.6
41.4
33.6
6.9
40.7
25.8
1.3
27.1
25.6
12.7
38.3
27.1 25.7 26.0 23.3 27.3
4.0 14.6 11.3 5.7 6.0
31.1 40.3 37.3 29.0 33.3
the exception of HDPE, the polymeric materials investigated do significantly with respect to their surface energies y, In contrast, component y* can be distin~ished as being markedly different in materials (y” < 1 mN m-l) and in polar materials (rP > 1 mN m-l).
5.2. Work of adhesion in polymer-polymer materiui combinations The values for the work W,, of adhesion calculated according to the method of Owens and Wendt [ll] and according to the method of Wu [12] are summarized in Table 6 for the various possible combinations of the polymer materials investigated. The measured values cited by Rabel [13] and by Owens and Wendt [ll] (cf. Table 4) were used as the basis of the calculations of the work of adhesion in PTFE combinations and PS combinations. 5.3. ~ornp~r~on of the results of friction experiments on polyme~polymer rn~teri~~combinations with surface energy data The results of the friction experiments on the combinations of polymer materials investigated are summarized in Figs. 3 - 9. As the graph of the coefficient of friction plotted against the work of adhesion (evaluated according to the method of Owens and Wendt) shows, a similar exponential relationship occurs for all the material combinations. The coefficient of sliding friction rises sharply as the work of adhesion increases. In the individual cases, the measured values can be represented, to a good approximation, by the curve-fitting functions given in the figures. Regarded as a whole, the results of the friction experiments on polymer material combinations, under sliding conditions which were pr~omin~tly adhesive, are suggestive of the rela~onship, justified at the beginning of this
0
0.2
Work 0‘ adhesion, W&
0.L
: :I 8 0
O-1
0.2
% 0.3
0.6 0.5
g
0.6 0.7
4
0.9
g f
1.0
.$
5 1.2 - 1.1
1.3
1.6
Work of adhesion, Wab
6
!
55
I
w
I
65
I
I I
sliding friction equation, i(h sliding friction equation, @m
a0 70 75 Work of edherion. W*
Fig. 5. The coefficient @cm of conditiona as for Fig. 3): fitted Fig. 6. The coefficient &rr, of conditions as for Fig. 3): fitted
t I 90
_
-.
as a function of the work WA of adhesion for PBTP slid against various polymers (test = 0.104 * 6.1 x 10m6 exp(O.lSW& as a function of the work Wh of adhesion for WDPE slid against various polymers (test = 0.132 + 2.6 x low6 exp(0.13 Wd)"
85mNlm
I
Fig. 3. The coefficient p* of sliding friction aa a function of the work IV* of adhesion for POM slid against various poIymers (test conditions: ring-ring apparatus; 5 = 0.09 N mme2; d = 0.12 m s- 1; dry atmosphere (normal industrial standard); room temperature): fitted equation, pw = 0.108 + 4.9 X lo+ exp(O.l2W&. Fig. 4. The coefficient &b of sliding friction as a function of the work Wan of adhesion for PA 6 slid against various polymers (test conditions as for Fig. 3): fitted equation, &m = 0.094 + 5.1 x lop6 exp(O.l3W&.
x 8
F 0.4 15 d % ; 0.3 ‘y p
g- 0.5 ‘Z $
5
0.6
Work
Of adheri‘m,
W&
60
1..
.
! 60 f!INllIl
90
0 -i-i-0
35
LO
15
50
55
60
T---7t
65 70 75 80 Work of adfles,on. W,b
65
I
,
90 mNlm
l&l
Fig. 9. The coefficient pcm of sliding friction as a function of the work Wh of adhesion for SAN slid against various polymers (test conditions as for Fig. 3): fitted equation, pcrn = 0.108 + 4.7 x 10M6 exp(O.l3W&. Fig. 10. The coefficient pcm of sliding friction as a function of the work Wb of adhesion for all polymer- polymer combinations: fitted equation, pELcm = 0.12 + 4.8 x loo6 exp(0.13Wah).
Work of adheslan, W&
0.1
0.2
u
0.1
E 0.5
0.6.
0.7.
6*
0.8.
Tj
i6
;
1.1 0.9 l.O-
4
s OL ; ’ jj 0.3.
. . . * 70!
1.3 1.2-
0.2
0.3
0.4
OS
O-6
Fig. 7. The coefficient pcIcmof sliding friction as a function of the work Wg, of adhesion for PS slid iagainst various polymers (test conditions as for Fig. 3): fitted equation, /..&, = 0.098 + 5.6 X lo@ exp(O.l3W&. Fig. 8. The coefficient pcrn of sliding friction as a function of the work Wd of adhesion for PMMA slid against various polymers (test conditions as for Fig. 3): fitted equation, /.&, = 0.179 + 4.5 x 10s6 exp(O.l3W&.
PMMA~---
179 TABLE 6 Work W, of adhesion for various polymer-polymer
combinations
Work W, of adhesion for the followingpolymer-polymer combinations evaluated by the methods of Owens and Wendt and of Wu HDPE
PA 6
PBTP
PMMA
69.2; 71.4 73.4; 76.6 75.3; 76.8 74.8; 77.0 72.1; 74.4 76.1; -
73.4; 76.6 95.0; 109.2
75.3; 76.8 89.7; 102.0
74.8; 77.0 92.1; 105.4
89.7; 102.0 92.1; 105.4 88.9; 101.7 83.1; -
87.6; 97.6
88.8; 99.9 90.4; 102.6 87.3; 99.1 84.0; -
PTFE
50.5; -
-
PVC
73.9; 76.4 75.9; 76.8
92.4; 106.0 88.0; 99.2
HDPE PA6 PBTP PMMA POM PS
SAN
52.2;
-
88.8; 99.9 85.6; 96.5 84.1; 54.1;
88.5; 100.0 86.8; 95.8
53.6; 90.4 ; 103.0 87.6; 97.7
POM
SAN
PS
PTFE
72.1; 74.4
76.1; -
50.5; -
73.9; 76.4
75.9; 76.8
88.9; 101.7
83.1; -
52.2; -
92.4; 106.0
88.0; 99.2
85.6; 96.5
84.1; -
54.1; -
88.5; 100.0
86.8; 95.8
87.3; 99.1
84.0; -
53.6; -
90.4; 103.0
87.6; 97.7
84.2; 95.8 81.0;
81.0; 84.0; -
51.6; 55.4; -
87.2; 99.4 83.3;
84.5; 94.4 84.4; -
51.6;
55.4; -
37.0; -
-
52.8;
54.7; -
87.2; 99.4 84.5; 94.4
83.3; 84.4; -
52.8; 54.7; -
90.4; 103.4 87.1; 97.7
87.1; 97.7 86.4; 94.4
-
PVC
paper, between the frictional work and/or the coefficient of friction and the work of adhesion which is required in order to separate the areas over which contact is occurring. Since the coefficient of friction of a combination rises exponentially with the work of adhesion, it is thus possible to explain the fact that when a large amount of work IV,,, is required in order to separate the adhesive contacts the deformation of those regions of the molecules which are associated with the separation also rises in a corresponding manner. A further compilation of ail 44 results from the friction experiments and measurements of the work of adhesion is given in Fig. 10. Comparatively large deviations usually occur when wear was observed on both of the materials employed in the friction experiment. It is understandable that the adhesive mechanism of sliding, assumed in this paper, will be disturbed by the presence, in the sliding surface, of particles resulting from wear effects. 5.4. Wear measurements On plotting the wear intensity of the sliding partners against the mean surface pressure a qualitative relationship is clearly recognizable between the intermolecular bonding energies of the polymer materials tested or between the polar surface energy component of these bonding energies and the
180
0
0.1 0.2
0.3
0.l 0.5 %/mm2
Meansurface
pressure,
b
0.7
0
Ll,l 0.2
0.3
0.L
05 Nlmmz
Mean surface pressure,
0.7
P
Fig. 11. The coefficient of sliding friction and the wear intensity as functions of the mean surface pressure for the combination POWHDPE. Fig. 12. The coefficient of sliding friction and the wear intensity as functions of the mean surface pressure for the combination POM-PA 6.
amount of wear which is observed. By taking the sliding combination POMHDPE as an example (Fig. ll), it can be shown that both a higher wear intensity and a more rapid increase in wear intensity with rising surface pressure can be expected in the sliding partner having the lower intermolecular bonding energies (HDPE). In combination with HDPE, POM proves to be significantly more wear resistant. If, by contrast, POM is combined with PA 6, in which the high strength of the ~~rmolecul~ hydrogen bridge bonds results in yp being markedly higher than in POM (Table 4), the higher wear intensity is found to affect the POM as shown in Fig. 12.
References G. Erhard and E. Strickle, Maschinenelemente aus Thermoplastischen Kunststoffen, Vol. 1, Verein Deutscher Ingenieure, Dusseldorf, 1974. G. Erhard, Zum Reibungs- und Verschleissverhalten von Polymerwerkstoffen, Dissertation, University of Karlaruhe, Karlsruhe, 1980. F. P. Bowden and D. Tabor, R&bung und Schmierung Fester Grper, Springer, Berlin, 1969. I. UT.Kragelski, Reibu~ und Verschleiss, Hanser, Munich, 1971.
181 5 E. Rabinowicz, Friction and Wear of material, Wiley, New York, 1965. 6 K. Tanaka, A review of recent studies on polymer friction and wear in Japan, Froc. Int. Solid Lubrication Symp., Tokyo, 1975, pp. 57 - 66. 7 D. T. Clark and W. J. Feast (eds.), Polymer Surfaces, Wiley, New York, 1977. 8 L.-H, Lee (ed.), Polymer Science and Technology, Vol. 5B, Advances in Polymer Friction and Wear, Plenum, New York, 1974. 9 H. A. Stuart, Molekiiistruktur, Springer, Berlin 1967. 10 G. W. Ehrenstein, Polymer-Werkstoffe, Struktur und Mechanisches Verhalten, Hanser, Munich, 1978. 11 D. K. Owens and R. C. Wendt, Estimation of the surface free energy of polymers, J, Appl. Poiym. Sci., 13 (1969) 1741- 1747. 12 S. Wu, Polar and non-polar interactions in adhesion, J. Adhes., 5 (1973) 39 - 55. 13 W. Rabel, Einige Aspekte der ~enetzungstheorie und ihre Anwendung auf die Untersuchung und Verinderung der Oberfl~cheneigenschaften von Polymeren, Farbe Lack, 77(10)(1971)997-1006. 14 W. A. Zisman, Friction, durability and wettability, properties of monomolecular films on solids. In R. Davies (ed.), Friction and Wear, Elsevier, Amsterdam, 1959, pp. 110 148. 15 A. W. Neumann and P.-J. Sell, Bestimmung der Oberflichenspannung von Kunststoffen aus Benetzungsdaten unter Berfcksichtigung des Gleichgewichts-Spreitungsdrucks, Kunststoffe, 57 (10) (1967) 829 - 834. 16 M. Massin, Synthese des traveaux relatifs 1 l’utilisation des fluides silicones comme lubrifiants en micromecanique, Eurotrib ‘77, Proc. European Conf on Tribology, October 3 - 5, 1977, Diisseldorf, Vols. II, III, Gesellschaft fur Tribologie, Dusseldorf, pp. 48-1 - 43-4. 17 A. J. G. Allan, Wettability and friction of polytetrafluoroethylene film: effect of prebonding treatments, J. Polym. Sci., 24 (1957) 461 - 466. 18 E, Rabinowicz, Surface energy approach to friction and wear, Prod. l&g,, (March 15, 1965) 95 - 99. 19 A, W. Neumann, Gber die Messmethodik zur Bestimmung grenzfllchen-energetischer Grossen, parts I and II, 2. Phys. Chem., 41 (1964) 339 - 352; 43 (1964) 71- 83. 20 W. Giickel and G. Synnatschke, Eine Methode zur vergleichenden Priifung des Netzvermogens von Netzmitteln, Tenside, 7 (2) (1970) 75 - 80. 21 G. Koerner, G. Rossmy and G. Sanger, Oberfllchen und Grenzfllchen, GoldschmidtHauszeitschrift 2, Vol. 29 (1974) pp. 2 - 41. 22 H. Potente and R. Kruger, Bedeutung polarer und disperser Oberfliichenspannungsanteile von Plastomeren und Beschichtungsstoffen fir die Haftfestigkeit von Verbundsystemen, Farbe Lack, 84 (2) (19’78) 72 - 75.
167
167 - 181
SLIDING FRICTION BEHAVIOUR OF POLYMER-POLYMER MATERIAL COMBINATIONS
GUNTER ERHARD BASF
AG.,
D-6700
Ludwigshafen
(F.R.G.)
(Received August 3,1982)
Summary The intermolecular bonding energies of polymeric materials can be employed in the evaluation of sliding under conditions determined predominantly by adhesion. The effect at the interface of these energies can be evaluated by reference to the surface energy and to its polar and dispersion-related components. Wetting angle measurements were used to determine the work of adhesion for a group of polymer-polymer material combinations. An exponential relationship exists between the work of adhesion and the coefficients of friction of these combinations, these coefficients of friction being determined primarily by adhesion.
1. Introduction The sliding friction and we= behaviour of polymer-steel combinations have been reviewed [ 11. Corresponding basic literature on the sliding friction behaviour of polymer material combinations is largely lacking, although combinations which are suitable for many industrial applications have already been found, either empirically or, for certain individual cases, by experimental methods. The present paper is a report of the results of experiments on the frictional behaviour of polymer materials in contact, the object of the experiments being to determine whether, under sliding conditions which were predominantly adhesive, relationships could be discovered between the sliding friction and other properties of the polymer materials. Such relationships provide the basis for optimum materials selection in cases involving industrially relevant sliding combinations [ 21. 2. Basic principles According to present knowledge [3 - 81, the frictional force FF which must, macroscopically speaking, be exerted during the process of sliding 0043-1648/83/0000-0000/$03.00
@ Elsevier Sequoia/Printed in The Netherlands
168
comprises an adhesion the formula
term FFadh and a deformation
FF = F~aa +FFdef Two factors thus emerge as principal solids: (1) adhesion of the interfacial contact points.
term FFdef according
to (1)
influences on sliding friction between layers; (2) deformation of the micro-
2.1. Discussion of the adhesion term 2.1.1. Intermolecular bonding energies In the regions corresponding to the actual contact surface, the surfaces of two rubbing partners approach each other sufficiently closely for atomic and/or molecular interactions to occur. With frictional contact between polymer-polymer combinations, an interaction of this type is the development of physical adhesion forces between the polymer material surfaces brought into contact. These interactions are electrostatic forces of the type which act, as intermolecular bonding forces, in the polymers themselves [7,81. It is well known that these interactions result from dispersion forces, dipole interactions (permanent and induced), hydrogen bridge bonds and ionic forces. Intermolecular bonding forces between macromolecules in electrostatic equilibrium are termed dispersion forces. These forces are attributable to electron movements, which momentarily generate dipoles. Physical bonds of this type occur in all polymer materials, but in materials such as poly(tetrafluoroethylene) (PTFE) and poly(ethylene) (PE) they are the only type of intermolecular bond present. Their strength is approximately two to three orders of magnitude lower than that of the covalent bonds [9, lo]. In contrast, interactions which result from the presence of permanent dipoles are markedly more powerful than these dispersion forces. Permanent dipoles occur when positive and negative poles are formed as a result of charge displacements in the electron shell (electronegativity). Permanent dipoles occur for example in the molecular structure of poly(viny1 chloride) (PVC), poly(methy1 methacrylate) (PMMA), poly(butylene) terephthalate (PBTP) and poly(oxymethylene) (POM). Their strength is approximately two orders of magnitude less than that of a covalent bond. A permanent dipole can even produce a dipole in an adjacent molecule which is initially non-polar or can increase the strength of a dipole which is already present. Bonding forces that are due to induced dipoles of this type are termed induction forces. Hydrogen bridge bonds such as those which occur for example in the polyamides (PAS) produce even stronger intermolecular forces. In a PA a hydrogen atom which is covalently bonded to a nitrogen atom acts as a “bridge” between this nitrogen atom and an oxygen atom belonging to an adjacent chain. As a result of the strong electronegativities of nitrogen and oxygen this mechanism generates attractive forces of the order of magnitude of 20 kJ mol-‘.
169
Ionic bonds result from the electrostatic interaction between ions carrying different charges. They occur only in a particular group of polymer materials, the ionomers, which were not investigated in the present work. 2.1.2. Surface energy of solid materials The intermolecular forces discussed above also determine the surface energy y of a polymer material. This parameter can be broken down into a disperse component yd and a polar component yp in accordance with the respective effects of the dispersion forces and dipole interactions (polymeric materials with surface energy components which result exclusively from dispersion forces are also called non-polar, while those with polar components are designated as weakly polar or strongly polar, depending on the magnitudes of these components) according to the formula y = yd + yp
(2) is valid when two bodies with surface energies ‘ya and +& are
Dupre’s equation in contact:
(3) Ya + ?b = Yab + wab In this equation w&, is the work of adhesion which has been performed, referred to unit surface area, and which is transformed into “heat of contact”, while Tab is the interfacial energy of the surfaces which are in contact. In order to separate the two bodies a and b, work per unit of surface area equivalent to W& has to be expended. If the polar and disperse surface energy components of the combination partners a and b are known, the work w,b of adhesion can be calculated, either according to Owens and Wendt [ 111 who showed that Wab
= 2hadybd
or according
)1’2
)1’2
(4)
to Wu [ 121 who showed that
4%dybd wab
+ 2haPrbP
+
4YapYbp
=
(5) Yad
+ybd
Yap
+?bp
Alternatively, the polar and disperse surface energy components of a polymeric material can be determined analytically from wetting angle measurements with at least two test liquids having known surface energy components [ 11, 121 or graphically [ 131 from eqns. (2) - (5). The inter-facial energy ysL is obtained by means of Young’s equation: YSL
=rs
-YL
cost
(6)
In this equation, 19 is the measured wetting angle, while the indices S and L respectively denote solid and liquid in cases where solids come into contact with liquids. 2.1.3. The part played by surface energy in the sliding process When relative movement takes place the adhesive contacts formed when solids touch are repeatedly separated and re-formed in accordance with the
170
jump distance between the points at which contact occurs. A loss of energy is associated with this process, and this energy loss can be described by means of the surface energies of the materials involved [ 14 - 171. The work W,, of adhesion provides a direct means for measuring the effect of adhesive contacts [ 181, 2.2. Discussion of the deformation term Deformations occur as soon as the actual contact surface starts to form. Deformations of segments of molecules, and within parts of molecules, are also necessarily involved in the separation of contact bridges, these deformations being associated, as complex dynamic processes, with hysteresis losses. When contact occurs between materials possessing markedly different moduli of elasticity, the surface asperities of the harder partner will penetrate into the surface of the softer partner. When relative movement occurs, a pile-up (resembling a bow wave) ,will be generated in front of each asperity. As the sliding process proceeds, the surface asperity displaces this frontal bow wave in the direction of movement and continuously rolls it flat. As a result of this process the material, or rather its surface layers, is subjected to continuous alternating stressing, and lost work is performed [7, 81. These deformation processes were not investigated in the present work and were substantially excluded from the friction experiments through suitable choice of the stressing parameters.
3. Material combinations investigated
The combinations Table 1.
of polymer materials investigated are summarized in
4. Experimental apparatus and procedure
4.1. Ring-rung uncrates for determining coeffic~~t$ of friction The test arrangement represented di~matic~ly in Fig. 1 was utilized for the experiments involving the determination of coefficients of friction. The two test rings 1 are fitted and centred in the receiving plates by means of three close-tolerance pins in each case. The lower plate is driven by a variable-speed drive 2 via a hollow shaft. The upper plate is connected to a torsion spring 3; the twist of this spring provides a measurement of the frictional torque. The angle of twist is indicated by a measuring pointer 4 and can be read off on a scale 5 directly as the coefficient of friction. For the torsion spring used, one scale division represents a value of p of 0.02. The normal force is applied by attaching weights 6 to a lever arm. The weight of the lever arm itself, and of the upper test specimen complete with holding devices, is balanced by means of a ~o~~~eight 7.
171 TABLE 1 Combinations
of polymer materials investigated*
HDPE PA 6 PBTP PMMA (52612) (84) (4550) HDPE (5261 Z) PA6 (B4) PBTP (4550) PMMA POM (N 2200) ;:68 N) PTFE PVC (516) SAN (368 R)
PTFE PVC SAN POM PS (N2200) (168N) (516) (368R)
x
X
X
X
X
X
X
x
X
X
X
X
X
X
X
x
X
X
X
X
X
X
X
x x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
x x
X
X
x
X
X
X
X
X
X
X
X X
X
X
X
X
X
*HDPE, high density PE; PS, poly(styrene); SAN, styrolacrylnitrile; the designations in parentheses identify the materials according to the commercial listings of BASF AG., Ludwigshafen.
Fig. 1. Diagram of the ring-ring
apparatus showing the test piece dimensions.
Changes in the temperature in the vicinity of the surface on which sliding is occurring are measured by means of a thermocouple which is implanted 1 mm beneath this surface; the changes are recorded during the test with the aid of a pen recorder 8.
172 4.1.1~
Conduct of the friction experiments
Before each friction experiment, the test rings were ultrasonically cleaned in 2% detergent solution (concentrated SU-40; Lever Industries) for 3 min at room temperature, after which they were rinsed in distilled water and dried. In the friction tests, the load was chosen to give an average surface pressure jj of 0.09 N mm-‘. The wear measurements were carried out at mean surface pressures p ranging from 0.09 to 0.6 N mme2. In both cases the sliding velocity t; was kept constant at 0.12 m s-i. Each test of a sliding combination ran for a minimum of 24 h corresponding, at the sliding velocity of 0.12 m s-‘, to a sliding distance in excess of 10 km. The deflection of the measuring pointer indicating the coefficient of friction was read off at regular intervals. After the running-in period, the average value was taken as representative of the mean coefficient pcm of friction in continuous operation. Before the test was started, and after completion, the height of the specimen was measured at three points, and the amount S of wear of each ring was determined by taking the difference of the mean specimen heights before and after test. The wear intensity S* is given by dividing the amount of wear by the sliding distance. The wear incurred during the running-in phase, which cannot be determined separately, is also contained in S*, but this runn~g-in wear is negligible when the me~~ements are made over a sliding distance exceeding 10 km. 4.2, Measurement of the surface energy The surface energy of the polymer materials investigated was determined, as was its breakdown into the polar and disperse components, according to the methods of Owens and Wendt [ll] and of Wu [12]. For this purpose, it is first necessary to measure the wetting angles whi_ch various test liquids, having different surface energies, make with the polymers. These measurements were carried out by the capillary rise method usingan apparatus described by Neumann [ 191 and Giickel and Synnatschke [ 201. A sketch of the experimental apparatus is shown in Fig. 2. A vessel 2 is located in a frame 1; the vessel contains a cell 3 filled with the liquid used for the measurement. Both the vessel and the cell can be moved vertically by means of a micrometer screw 4. By operating this screw so that they move upward, the polymer sample 5, which is attached to a holding fixture 6, can be dipped vertically into the measuring liquid. The dimensions of the polymer specimen are 4 mm X 6 mm X 60 mm. A meniscus forms on the polymer specimen thus immersed, and the height to which this meniscus rises is measured with the aid of a cathetometer 7. A glass capillary tube 8, which can likewise be shifted in the vertical direction, is used for accurately sighting on the liquid surface. The space in which the measurements are carried out is enclosed within a t~sp~ent d~u~e-~~ed cover 9 ~0~ which thermostatically controlled liquid‘ flows. The vessel 2 is likewise double walled and
I
1
/////////////////////
Fig. 2. Diagrammatic representation by the capillary rise method.
of the test apparatus for measuring wetting angles
the same liquid flows through it, keeping it at a constant temperature. The cover 9 also ensures that the gas phase in the measuring system reaches an equilibrium corresponding to the vapour pressure of the measuring liquid. 4.2.1. Conduct of the tests and evaluation of the results Before each measurement the polymer specimens were carefully cleaned. The technique whereby a fresh surface is prepared by cutting with a microtome proved to be the most reliable method. Before cutting, the pieces of specimen material were rinsed in alcohol to prevent traces of grease from contaminating the microtome blade. The specimens thus cleaned were inserted into the holding fixture of the test apparatus without being touched by fingers. Before each test, the measuring fluid cell was cleaned with a chromic acid-sulphuric acid mixture and rinsed with distilled water. The measuring liquids used were a-bromonaphthalene and water or, for PE and PS, formamide. The surface energies of these liquids are summarized in Table 2. The measurement temperature was 20 + 0.1 “C. On each occasion the measurement was not made until 3 - 4 h had elapsed to allow the temperature and vapour pressure to reach equilibrium within the measuring space. TABLE 2 Surface energies and densities of the measuring liquids used Measuring liquid
cu-bromonaphthalene Water Formamide
Surface energies (mN m-l)
Density p (g cm+)
Y
Yd
YP
43.9 72.8 58.3
42.3 21.8 37.9
1.57 51.0 20.4
1.482 1.0 1.132
174
On immersing the polymer specimen, the measuring liquid rises on its surface in a meniscus-like manner. The height to which it rises, referred to the undisturbed liquid, was measured with the aid of the cathetometer. (The height to which the liquid rises varies depending on whether the specimen is being dipped into the fluid or is being pulled out of the fluid. Because of this, a distinction is made between the advancing angle and the retiring angle. In the present tests, only the advancing angle was determined.) For each polymer material, four measurements were carried out on each of four test pieces, and a mean value was consequently obtained from the 16 measurements. The height h to which the liquid rises is related to the wetting angle 0 according to the following function, which involves the density p, the surface energy yL of the measuring liquid and the gravitational acceleration g: sin 0 = 1 -
.E!C_ (7) 2% The wetting angles for the various combinations of polymer materialmeasuring liquid were determined from this relationship. The measured wetting angles were then used to determine the surface energies of the polymer materials, and the dispersion-related and polar components of these energies, from eqns, (2) - (6) according to the methods of Owens and Wendt [ll] and of wu [IZ] .
5. Results of the experiments 5.1. Surface energies of the polymer materiab investigated 5.1.1. Determination of wetting angles The wetting angles, determined from the results of the capillary measurements, are summarized in Table 3,
TABLE3 jetting angles for various polymer material-me~uring combinations Polymer
HDPE PA6 PBTP PMMA POM PS PVC SAN
liquid
Contact angle 8 with the following liquids Water
a-bromonaphthalene
Formamide
65.20 77.99 72.79 75.31 70.98 82.00
43.70 9.87 10.59 7.46 23.00 21.67 12.22 10.95
79.06 74.3 -
rise
175
Since, for the combinations PS-water and HDPE-water the wetting angles exceed 90”, they could not be determined by means of the capillary rise method. For this reason the wetting angles of the combinations PSformamide and HDPE-formamide are given in place of the above values in Table 3, fourth column. 5.1.2. Surface energy and breakdown into polar and dispersion-related components The results of calculations of the surface energies y of the polymer materials, based on the wetting angle measurements, are reproduced in Tables 4 and 5, second to fourth columns, together with the corresponding polar components yp and dispersion-related components rd. A compilation of the results evaluated in accordance with the method of Owens and Wendt is given in Table 4, while Table 5 presents the results evaluated in accordance with the method of Wu. In addition, the results of measurements performed by other researchers are included with the present results for comparative purposes. The results relating to PS are unreliable, and they are accordingly shown in parentheses. This unreliability reflects the fact that PS underwent incipient solution by cu-bromonaphthalene during the test, which led to surface energy values which appear to be comparatively high. TABLE 4 Surface energiesa of various polymer materials measured by the method of Owens and Wendt [ll] Polymerb
PTFE HDPE LDPE PP POM PA6 PBTP PETP SAN PMMA PVC PS PC
Surface energies (mN m-‘)
obtained by the following researchers
Measurements present paper
in the
Measurements by Owens and Wendt [II]
Measurements by Rabel [13]
Measurements by Koerner et al. [21]
Yd
Y
Yd
Yd
YP Y
Yd
YP Y
18.5 31.6
0 0.2
18.5 31.8
32.1 35.1
0 0
30.5
0.7
31.2
34.5
YP 0.1
YP Y
34.6 33.2
36.0 36.8 39.6
6.1 10.7 4.2
42.1 47.5 43.8
40.5 38.8 37.7 (44.6)
2.7 6.4 7.5 (0.8)
43.2 45.2 45.2 (45.4)
0
33.2
32.1 35.1
43.2
4.1
47.3
32.9
4.5
37.4
37.8
3.1 40.9
35.9 40.0 41.4
4.3 40.2 1.5 41.5 0.6 42.0
36.6
1.6 38.1
36.0
3.9
37.0
1.8 38.8
39.9
‘The significance of the values in parentheses is discussed in Section 5.1.2. bLDPE, low density PE; PC, polycarbonate; PETP, poly(ethylene) terephthalate; poly(propylene).
PP,
176 TABLE
5
Surface
energies
Polvmer
HDPE PP POM PA6 PBTP SAN PMMA PVC PS PC
With not differ the polar non-polar
of various polymer
materials
Surface energies (mN m-‘l
measured
by the method
of Wu [ 121
obtained by the following researchers
Measurements in the present paper
Measurements wu [12]
by
Measurements by Potente and Kriiger [22]
Yd
Yd
Y
Yd
YP
Y
35.0 36.8 39.2 39.4 39.5 39.6 39.0
11.1 15.4 9.4 7.7 11.7 12.7
47.9 54.6 48.8 47..2 51.3 51.7
YP 0.7
YP
Y
35.7
29.8
11.6
41.4
33.6
6.9
40.7
25.8
1.3
27.1
25.6
12.7
38.3
27.1 25.7 26.0 23.3 27.3
4.0 14.6 11.3 5.7 6.0
31.1 40.3 37.3 29.0 33.3
the exception of HDPE, the polymeric materials investigated do significantly with respect to their surface energies y, In contrast, component y* can be distin~ished as being markedly different in materials (y” < 1 mN m-l) and in polar materials (rP > 1 mN m-l).
5.2. Work of adhesion in polymer-polymer materiui combinations The values for the work W,, of adhesion calculated according to the method of Owens and Wendt [ll] and according to the method of Wu [12] are summarized in Table 6 for the various possible combinations of the polymer materials investigated. The measured values cited by Rabel [13] and by Owens and Wendt [ll] (cf. Table 4) were used as the basis of the calculations of the work of adhesion in PTFE combinations and PS combinations. 5.3. ~ornp~r~on of the results of friction experiments on polyme~polymer rn~teri~~combinations with surface energy data The results of the friction experiments on the combinations of polymer materials investigated are summarized in Figs. 3 - 9. As the graph of the coefficient of friction plotted against the work of adhesion (evaluated according to the method of Owens and Wendt) shows, a similar exponential relationship occurs for all the material combinations. The coefficient of sliding friction rises sharply as the work of adhesion increases. In the individual cases, the measured values can be represented, to a good approximation, by the curve-fitting functions given in the figures. Regarded as a whole, the results of the friction experiments on polymer material combinations, under sliding conditions which were pr~omin~tly adhesive, are suggestive of the rela~onship, justified at the beginning of this
0
0.2
Work 0‘ adhesion, W&
0.L
: :I 8 0
O-1
0.2
% 0.3
0.6 0.5
g
0.6 0.7
4
0.9
g f
1.0
.$
5 1.2 - 1.1
1.3
1.6
Work of adhesion, Wab
6
!
55
I
w
I
65
I
I I
sliding friction equation, i(h sliding friction equation, @m
a0 70 75 Work of edherion. W*
Fig. 5. The coefficient @cm of conditiona as for Fig. 3): fitted Fig. 6. The coefficient &rr, of conditions as for Fig. 3): fitted
t I 90
_
-.
as a function of the work WA of adhesion for PBTP slid against various polymers (test = 0.104 * 6.1 x 10m6 exp(O.lSW& as a function of the work Wh of adhesion for WDPE slid against various polymers (test = 0.132 + 2.6 x low6 exp(0.13 Wd)"
85mNlm
I
Fig. 3. The coefficient p* of sliding friction aa a function of the work IV* of adhesion for POM slid against various poIymers (test conditions: ring-ring apparatus; 5 = 0.09 N mme2; d = 0.12 m s- 1; dry atmosphere (normal industrial standard); room temperature): fitted equation, pw = 0.108 + 4.9 X lo+ exp(O.l2W&. Fig. 4. The coefficient &b of sliding friction as a function of the work Wan of adhesion for PA 6 slid against various polymers (test conditions as for Fig. 3): fitted equation, &m = 0.094 + 5.1 x lop6 exp(O.l3W&.
x 8
F 0.4 15 d % ; 0.3 ‘y p
g- 0.5 ‘Z $
5
0.6
Work
Of adheri‘m,
W&
60
1..
.
! 60 f!INllIl
90
0 -i-i-0
35
LO
15
50
55
60
T---7t
65 70 75 80 Work of adfles,on. W,b
65
I
,
90 mNlm
l&l
Fig. 9. The coefficient pcm of sliding friction as a function of the work Wh of adhesion for SAN slid against various polymers (test conditions as for Fig. 3): fitted equation, pcrn = 0.108 + 4.7 x 10M6 exp(O.l3W&. Fig. 10. The coefficient pcm of sliding friction as a function of the work Wb of adhesion for all polymer- polymer combinations: fitted equation, pELcm = 0.12 + 4.8 x loo6 exp(0.13Wah).
Work of adheslan, W&
0.1
0.2
u
0.1
E 0.5
0.6.
0.7.
6*
0.8.
Tj
i6
;
1.1 0.9 l.O-
4
s OL ; ’ jj 0.3.
. . . * 70!
1.3 1.2-
0.2
0.3
0.4
OS
O-6
Fig. 7. The coefficient pcIcmof sliding friction as a function of the work Wg, of adhesion for PS slid iagainst various polymers (test conditions as for Fig. 3): fitted equation, /..&, = 0.098 + 5.6 X lo@ exp(O.l3W&. Fig. 8. The coefficient pcrn of sliding friction as a function of the work Wd of adhesion for PMMA slid against various polymers (test conditions as for Fig. 3): fitted equation, /.&, = 0.179 + 4.5 x 10s6 exp(O.l3W&.
PMMA~---
179 TABLE 6 Work W, of adhesion for various polymer-polymer
combinations
Work W, of adhesion for the followingpolymer-polymer combinations evaluated by the methods of Owens and Wendt and of Wu HDPE
PA 6
PBTP
PMMA
69.2; 71.4 73.4; 76.6 75.3; 76.8 74.8; 77.0 72.1; 74.4 76.1; -
73.4; 76.6 95.0; 109.2
75.3; 76.8 89.7; 102.0
74.8; 77.0 92.1; 105.4
89.7; 102.0 92.1; 105.4 88.9; 101.7 83.1; -
87.6; 97.6
88.8; 99.9 90.4; 102.6 87.3; 99.1 84.0; -
PTFE
50.5; -
-
PVC
73.9; 76.4 75.9; 76.8
92.4; 106.0 88.0; 99.2
HDPE PA6 PBTP PMMA POM PS
SAN
52.2;
-
88.8; 99.9 85.6; 96.5 84.1; 54.1;
88.5; 100.0 86.8; 95.8
53.6; 90.4 ; 103.0 87.6; 97.7
POM
SAN
PS
PTFE
72.1; 74.4
76.1; -
50.5; -
73.9; 76.4
75.9; 76.8
88.9; 101.7
83.1; -
52.2; -
92.4; 106.0
88.0; 99.2
85.6; 96.5
84.1; -
54.1; -
88.5; 100.0
86.8; 95.8
87.3; 99.1
84.0; -
53.6; -
90.4; 103.0
87.6; 97.7
84.2; 95.8 81.0;
81.0; 84.0; -
51.6; 55.4; -
87.2; 99.4 83.3;
84.5; 94.4 84.4; -
51.6;
55.4; -
37.0; -
-
52.8;
54.7; -
87.2; 99.4 84.5; 94.4
83.3; 84.4; -
52.8; 54.7; -
90.4; 103.4 87.1; 97.7
87.1; 97.7 86.4; 94.4
-
PVC
paper, between the frictional work and/or the coefficient of friction and the work of adhesion which is required in order to separate the areas over which contact is occurring. Since the coefficient of friction of a combination rises exponentially with the work of adhesion, it is thus possible to explain the fact that when a large amount of work IV,,, is required in order to separate the adhesive contacts the deformation of those regions of the molecules which are associated with the separation also rises in a corresponding manner. A further compilation of ail 44 results from the friction experiments and measurements of the work of adhesion is given in Fig. 10. Comparatively large deviations usually occur when wear was observed on both of the materials employed in the friction experiment. It is understandable that the adhesive mechanism of sliding, assumed in this paper, will be disturbed by the presence, in the sliding surface, of particles resulting from wear effects. 5.4. Wear measurements On plotting the wear intensity of the sliding partners against the mean surface pressure a qualitative relationship is clearly recognizable between the intermolecular bonding energies of the polymer materials tested or between the polar surface energy component of these bonding energies and the
180
0
0.1 0.2
0.3
0.l 0.5 %/mm2
Meansurface
pressure,
b
0.7
0
Ll,l 0.2
0.3
0.L
05 Nlmmz
Mean surface pressure,
0.7
P
Fig. 11. The coefficient of sliding friction and the wear intensity as functions of the mean surface pressure for the combination POWHDPE. Fig. 12. The coefficient of sliding friction and the wear intensity as functions of the mean surface pressure for the combination POM-PA 6.
amount of wear which is observed. By taking the sliding combination POMHDPE as an example (Fig. ll), it can be shown that both a higher wear intensity and a more rapid increase in wear intensity with rising surface pressure can be expected in the sliding partner having the lower intermolecular bonding energies (HDPE). In combination with HDPE, POM proves to be significantly more wear resistant. If, by contrast, POM is combined with PA 6, in which the high strength of the ~~rmolecul~ hydrogen bridge bonds results in yp being markedly higher than in POM (Table 4), the higher wear intensity is found to affect the POM as shown in Fig. 12.
References G. Erhard and E. Strickle, Maschinenelemente aus Thermoplastischen Kunststoffen, Vol. 1, Verein Deutscher Ingenieure, Dusseldorf, 1974. G. Erhard, Zum Reibungs- und Verschleissverhalten von Polymerwerkstoffen, Dissertation, University of Karlaruhe, Karlsruhe, 1980. F. P. Bowden and D. Tabor, R&bung und Schmierung Fester Grper, Springer, Berlin, 1969. I. UT.Kragelski, Reibu~ und Verschleiss, Hanser, Munich, 1971.
181 5 E. Rabinowicz, Friction and Wear of material, Wiley, New York, 1965. 6 K. Tanaka, A review of recent studies on polymer friction and wear in Japan, Froc. Int. Solid Lubrication Symp., Tokyo, 1975, pp. 57 - 66. 7 D. T. Clark and W. J. Feast (eds.), Polymer Surfaces, Wiley, New York, 1977. 8 L.-H, Lee (ed.), Polymer Science and Technology, Vol. 5B, Advances in Polymer Friction and Wear, Plenum, New York, 1974. 9 H. A. Stuart, Molekiiistruktur, Springer, Berlin 1967. 10 G. W. Ehrenstein, Polymer-Werkstoffe, Struktur und Mechanisches Verhalten, Hanser, Munich, 1978. 11 D. K. Owens and R. C. Wendt, Estimation of the surface free energy of polymers, J, Appl. Poiym. Sci., 13 (1969) 1741- 1747. 12 S. Wu, Polar and non-polar interactions in adhesion, J. Adhes., 5 (1973) 39 - 55. 13 W. Rabel, Einige Aspekte der ~enetzungstheorie und ihre Anwendung auf die Untersuchung und Verinderung der Oberfl~cheneigenschaften von Polymeren, Farbe Lack, 77(10)(1971)997-1006. 14 W. A. Zisman, Friction, durability and wettability, properties of monomolecular films on solids. In R. Davies (ed.), Friction and Wear, Elsevier, Amsterdam, 1959, pp. 110 148. 15 A. W. Neumann and P.-J. Sell, Bestimmung der Oberflichenspannung von Kunststoffen aus Benetzungsdaten unter Berfcksichtigung des Gleichgewichts-Spreitungsdrucks, Kunststoffe, 57 (10) (1967) 829 - 834. 16 M. Massin, Synthese des traveaux relatifs 1 l’utilisation des fluides silicones comme lubrifiants en micromecanique, Eurotrib ‘77, Proc. European Conf on Tribology, October 3 - 5, 1977, Diisseldorf, Vols. II, III, Gesellschaft fur Tribologie, Dusseldorf, pp. 48-1 - 43-4. 17 A. J. G. Allan, Wettability and friction of polytetrafluoroethylene film: effect of prebonding treatments, J. Polym. Sci., 24 (1957) 461 - 466. 18 E, Rabinowicz, Surface energy approach to friction and wear, Prod. l&g,, (March 15, 1965) 95 - 99. 19 A, W. Neumann, Gber die Messmethodik zur Bestimmung grenzfllchen-energetischer Grossen, parts I and II, 2. Phys. Chem., 41 (1964) 339 - 352; 43 (1964) 71- 83. 20 W. Giickel and G. Synnatschke, Eine Methode zur vergleichenden Priifung des Netzvermogens von Netzmitteln, Tenside, 7 (2) (1970) 75 - 80. 21 G. Koerner, G. Rossmy and G. Sanger, Oberfllchen und Grenzfllchen, GoldschmidtHauszeitschrift 2, Vol. 29 (1974) pp. 2 - 41. 22 H. Potente and R. Kruger, Bedeutung polarer und disperser Oberfliichenspannungsanteile von Plastomeren und Beschichtungsstoffen fir die Haftfestigkeit von Verbundsystemen, Farbe Lack, 84 (2) (19’78) 72 - 75.