Friction of polyethylene-terpolymer mixtures on polymer substrates: adhesion dependence

181

Wear, I57 (1992) 181-187

Friction of polyethylene-terpolymer substrates: adhesion dependence

mixtures on polymer

L. Lavielle Centre de Recherches SW la Physico-Chin& des Surfaces Solides- CNRS, 24 Avenue du Prbsident Kennedy, F-68200 Mulhouse (France) (Received December 2, 1991; revised and accepted January 30, 1992)

Abstract A wide variety of friction coefficients can be obtained with mixtures of polyethylene and (polyethylene-butylacrylate-maleic anhydride) terpolymers as films rubbing on smooth polymer substrates (poly(viny1 chloride), polymethylmethacrylate). Owing to the acid-base properties of the coupled polymers, adhesion is favoured. A linear relation is observed between the steady state friction coefficient and the potential adhesion, as determined by peel testing of assemblies on an aluminium foil. This generalizes previous results obtained with grafted polyethylenes and resolves the duality between the influence of surface properties and bulk mechanical properties observed in polymer-polymer friction. 1. Introfluction The widespread use of polymers in diverse applications requires the study of new polymers and their tribological properties. A lot of polymers are used as solid lubricants: polytetrafluoroethylene (PTPE), polyethylene (PE), polyamides or polyimides [l, 21. Knowledge of the mechanisms of polymers friction and wear is necessary in order to improve their characteristics. More recently, polymers displaying a high coefficient of friction have also been in demand for certain applications. The friction of polymers generally depends to a large extent upon interfacial adhesion [3,4]. With modified polyethylenes it has been shown that a linear relationship exists between the friction coefficient EL and the potential adhesion on a model aluminium substrate [4]. Poly(ethylene-butylacrylate) copolymers grafted with maleic anhydride and their mixtures with polyethylene have interesting mechanical and adhesion properties [5]. They show elasto-plastic behaviour and, applied as a hot melt, have adhesion ability on substrates such as aluminum, as determined by peel testing. The highest adhesion is observed not for the pure terpolymer but for the 75% terpolymer mixture. The energy expended in the plastic stretching of the peeled film represents 40% of the input energy of separation [5]. These polymer mixtures also exhibit tackiness or instantaneous adhesion. The influence of coupling of surface interactions and bulk mechanical properties [6] as well as acid-base properties [7j is well known. Thus either acidic poly(viny1 chloride) (PVC) or basic polymethylmethacrylate (PMMA) discs have been used for our friction experiments. The aim of this work is to see if any relationship exists in general between adhesion properties and friction coefficient for polymer-polymer sliding, as suggested in ref. 4.

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0 1992 - Elsevier Sequoia. All rights reserved

182

2. Experimental details This study is based on the same experimental procedure published results on modified ~lyethylenes [4].

as described

in previously

2.1. Friction tests A classical pin-on disc apparatus is used. The PMMA pin is worn in situ against an abrasive paper (800 grit) to ensure a Bat contact between pin and disc. The rubbing film is stretched over a PMMA pin with a contact area of 6x6 mm2 and is mechanically fixed on both sides of the pin holder by fixing bars as described in ref. 4. The normal load, FN is 2.2 N, giving an average contact pressure of 0.06 MPa. The disc is rotated at 1 rev min-I, giving a linear speed of 1.6 mm s-r. The pin is then brought into contact. Each experiment has a duration of 1 h or 60 cycles, but steady state is sometimes observed at shorter sliding times. The friction coefficient is recorded as a function of time. Experiments are performed at room temperature and about 45% relative humidity. 2.2. Materials

Polymers are in the form of films 40 pm thick, industrially prepared by Orkem (France). The polymers studied are polyethylene and its mixtures with a terpolymer (polyethylene-maleic anhydride-butylacrylate) for the four compositions 25%, 50%, 75% and 100%. The pure terpoiymer contains mainly polyethylene, less butylacrylate and 3% maleic anhydride. Surface energy components are determined by contact angle measurements and there is a linear increase in the polar component jps with terpolymer content. The specific morphology of these polymers shows that polar domains containing the polar groups are present in the polyethylene matrix. The adhesion properties on an aluminium substrate have been extensively examined as a function of temperature and speed of peeling [S]. Typical results are given in Fig. 1 for a peel rate of 0.83 which is significantly higher than the polymer film’s mm s-r at room temperature, glass transition temperature. 1000 -

800 -

600 -

200 -

0 %

25%

50%

75%

loo%

%

terpolymer

Fig. 1. Peeling work as a function of polymer mixture composition at a 50 mm min-’ (0.83 mm s-l) peel rate 1.51.

183

2.2.2. Substrates The discs are 45 mm in diameter and 8 mm thick. They are machined in PMMA (Orkem) or in PVC (Simona). The surfaces are smooth and covered by a weakly adherent protective polymer film which is wrapped off before each sliding experiment. The disc is then rinsed with ethanol to clean the surface of residual impurities and dried using a hair-dryer [4]. 2.2.3. Microscopic examinations The disc surfaces are examined by optical magnification scale is given on the figures.

microscopy

in reflected

light. The

3. Results 3.1. Friction tests Usually the steady state value is obtained after rubbing for 1 h. The typical evolution of the friction coefficient p over 60 disc rotations is shown in Fig. 2. A mean value of the friction coefficient can be given for each polymer mixture, calculated as the average of at least 10 identical experiments. The steady state value of friction coefficient is the parameter used in this study. Table 1 summarizes the results. The friction coefficients are nearly identical on the PMMA and PVC substrates, taking into account the experimentally deduced accuracy (&-0.X). Figures 3(a) and 3(b) show typical results for the five polymers on a PVC and a PMMA substrate respectively. A linear relationship is observed between the friction coefficient at steady state and the adhesion energy of the same polymers on an aluminium foil. 3.2. Microscopic

examination By examination of the PVC surface by optical microscopy, we can see shallow scratching and arrow-shaped deposits on the sliding area, as shown in Fig. 4 for the

Fig. 2. Typical friction curve for a 75% terpolymer-polyethylene mixture. TABLE 1 Friction coefficients at steady state for the different mixtures

F PVC (*o.ls) p PMMA (f0.15)

0% PE 100% terpolymer

25% 75%

50% 50%

75% 25%

100% 0%

0.90 0.80

1.10 1.20

0.65 0.50

0.30 0.35

0.20 0.25

184

1,4 1,2 1.0 0.8

P

0.6 OS4 0,2 0.0

0

200

400

600

800

1000

WA (J.m”)

@I

-I

0

200

400

600

@I

Fig. 3. Variation in friction coefficient p as a function on (a) a PVC substrate and (b) a PMMA substrate.

600

1000

WA (J.mm2) of adhesion

energy of polymer mixtures

Fig. 4. Micrograph of a PVC substrate after 1 h of sliding of a 75% terpolymer-25% mixture film (+, direction of disc rotation).

polyethylene

185

Fig. 5. Micrograph of a PMMA substrate after 1 h of sliding of a 75% terpolymer-25% mixture film (+, direction of disc rotation).

polyethylene

75% terpolymer content mixture. The extremity of these arrow deposits points in the direction opposite to that of disc rotation. With 25% terpolymer content, deep scratches are observed and only a few deposits. With the polyethylene film alone the arrow-shaped deposits are never observed, but only rare platelets of polyethylene and scratches are present on the substrate. The same experiments with the PMMA substrate sometimes show the presence of a small amount of debris next to the scratched lines, but arrow-shaped deposits, as in the case of the PVC substrate, are also present (Fig. 5). 4. Discussion 4.1. Intelfacial adhesion in friction

It is known that adhesion is an important parameter in friction [3, 8, 91. A linear relationship between the frictional work and the critical surface energy of some polymers (PE, PTFE, PMMA, PVC, polystyrene (PS)) rubbing on a common glass counterface is observed 191. Critical surface energy is a parameter of wettability rather than of adhesion. We have recently shown that a linear relationship exists between friction coefficient and adhesion ability [4]. By adhesion ability we mean W,, the adhesion energy as determined by a peel test, which is the work necessary to rupture the adhesive bonding and not the reversible adhesion energy W,, calculated by the classical relation wo- 2(yDsOns2YR + 2(yPs1QsY (1) where yn and 9 are the dispersive and non-dispersive components of the free energy of the two solid surfaces respectively. WA is the product of the reversible adhesion energy W. and a dissipation factorf(v, 7) depending upon the speed and temperature of peeling [6]: W*=WQf(v,

7)

(2) In adhesion studies it is well known that interfacial properties and bulk mechanical properties have an influence. If the interfacial adhesion is high, the bulk mechanical properties often play a greater role. Adhesion is responsible for the transfer of polymer on to the substrate. The micrographs in Figs. 4 and 5 show that transfer occurs on both the PVC and PMMA substrates and adhesion is high enough for polymer deformation to occur. In the present study the terpolymer is acidic owing to the maleic anhydride group and basic because of the butylacrylate group, so adhesion ability is possible on either

186 a basic or an acidic substrate and is perhaps favoured by an acidic substrate, the basic groups being more numerous in the terpolymer. The typical linear relation between the steady state friction coefficient and the adhesion ability of the same films on an aluminium foil substrate is very clear on the PVC substrate (Fig. 3(a)) and works also for the PMMA substrate (Fig. 3(b)). This result is thus a generalization of the observations originally made with modified polyethylene films on PMMA substrates [4]. No relation can be found with reversible adhesion energy W,, calculated from the surface energy components of the two solids in contact, which increases with terpolymer content. This shows that in such a case, as in adhesion, the mechanical properties of the polymer also play a role. Clearly, when adhesion is possible, the friction coefficient is linearly related to the adhesion energy as measured by a peel test for the same polymer adhering on an aluminium substrate, and this energy takes into account the deformation of the polymer. This brings a confirmation of the linear relation between adhesion ability and friction coefficient of smooth polymers.

5. Conclusions

For smooth polymer mixture films containing polyethylene and elasto-plastic terpolymers (polyethylene-butylacrylate-maleic anhydride) rubbing against smooth PVC or PMMA discs, we have found a linear relationship between friction coefficient p at steady state and adhesion energy as determined for the same polymer mixtures by peel tests on an aluminium substrate. These acido-basic polymers have the ability to adhere to the two types of acidic or basic substrates studied, PVC or PMMA, and this allows us to give an extension to previous observations made with modified polyethylenes [4]. As is the case in adhesion studies, it clearly appears that in some cases the interfacial interactions due to the physical van der Waals forces are sufficient to explain the tribological behaviour of polymers [9, 10, 111. In most cases it is necessary to take into account, as for adhesion, the influence of mechanical properties and resulting energy dissipation, and usually with the polymers studied here, the higher the interfacial adhesion, the higher is the energy dissipation [5]. A great number of publications on polymer friction [l-3] conclude that the relation is between the friction coefficient and reversible adhesion energy or surface properties [ll], whilst others attribute the behaviour to the mechanical properties [12,13]. In fact, the two factors are simultaneously involved as accepted in adhesion [6], and so the term WA is a more convenient representative parameter for the friction coefficient evolution in polymer-polymer sliding. This brings to an end the duality of the relation of polymer friction to either the surface properties or the bulk mechanical properties.

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