PEEK composites

Adhesion properties of plasma-treated carbon/PEEK composites E. OCCHIELLO, 114.MORRA, G.L. GUERRINI and F. GARBASSI (Istituto Guido Donegani SpA, Italy) Received 17 September 1991; accepted in revised form 20 November 1991 Carbon/PEEK composites have been treated by plasma to improve adhesion with conventional epoxy adhesives. Oxidizing (oxygen, air) and inert (nitrogen, argon) mixtures have been used, and the effects of treatment time and radio frequency power investigated. Plasma treatments resulted in the introduction of oxygen- and, in some cases, nitrogen-containing functionalities at the surface of the composite, as observed by X-ray photoelectron spectroscopy, and in increased wettability. Hydrophobic recovery occurred during ageing, but to a very limited extent that was reduced with increasing treatment time. While untreated samples showed no pull strength and adhesive failure, plasma-treated samples reached high pull strengths and cohesive failure even at very short treatment times (<30 s). Pull strength values were relatively insensitive to the nature of the treatment gas and plasma parameters; furthermore, ageing did not affect adhesion but in some cases improved it. These effects suggest the occurrence of plasma-induced cross-linking of the surface layer.

Key words: composite materials; adhesive bonding; plasma treatment; wettabilitw pull strength; epoxy adhesive; carbon fibres; PEEK matrix Joining thermoplastic composites is an important technological issue. The topic has recently been reviewed for carbon fibre-reinforced polyetheretherketone (carbon/PEEK) composites by Silverman and Griese ~, who mention adhesive bonding, mechanical fastening and welding as viable alternatives. In the case of adhesive bonding, surface preparation is particularly critical 1"2. A comparison between different surface preparation methods showed that plasma treatment, albeit with particularly 'hard' parameters (long treatment time and use of a CF4/O2 mixture), scored remarkably well I . Plasma treatment is particularly interesting since it is reasonably uniform over the whole part, compared with flame or corona treatments, and can be made environment-friendly by an appropriate choice of treatment gases 3--5. These evidences, along with our former experience on adhesion testing of plasma-treated polymers 6"7, prompted us to assess the efficiency of plasma treatments in improving the adhesion of carbon/PEEK composites with epoxy adhesives. Effort was concen-

trated first of all on finding the minimum requirements for adhesion, in order to minimize treatment time and the toxicity and pollution due to the plasma gas and exhausts. In a recent paper, Brennan e t al. 8 studied the ageing behaviour of oxygen plasma-treated unreinforced PEEK, showing that some hydrophobic recovery occurs upon ageing; however, no adhesion tests were performed. We further elaborated on this theme by studying the effect of ageing on the adhesion of carbon/ PEEK composites. This is particularly important from the application point of view, since a material's response to ageing affects the possibility of storing treated parts and therefore the site of treatment. In fact, for materials that are unaffected by ageing the treatment can be performed where the part is moulded, as in the case of plastic films, which are routinely corona-treated and then shipped to converters for further operations such as printing and laminating. On the other hand, materials affected by ageing must be treated close to the assembly site, to minimize adhesion loss.

0010-4361192/030193-08 (~) 1992 Butterworth-Heinemann Ltd COMPOSITES . VOLUME 23 . NUMBER 3 . MAY 1992 193

EXPERIMENTAL DETAILS

Table 1. XPS compositions (atomic %) of untreated and plasma-treated* carbon/PEEK samples

Materials

Carbon]PEEK plaques, 0.5 mm thick, were prepared from textile products obtained from filaments produced by the FIT technology9. The carbon to PEEK weight ratio was 60:40, pressing was performed at 380°C, under 1.6 MPa pressure. In Fig. 1 an optical micrograph of the as-moulded surface is presented.

Sample Theoretical Untreated Oxygen plasma-treated Oxygen plasma-treated, aged 15 days Air plasma-treated Nitrogen plasma-treated Argon plasma-treated

Plasma treatments

Plasma treatments were performed using a parallel plate reactor, with the samples located on the watercooled ground electrode 1°. The plasma parameters were as follows: excitation frequency 13.56 MHz; power 100 W; pressure 2 Pa; gas flow 8 c m 3 min -~ (STP); treatment time in the range 5-60 s. Oxygen, nitrogen and argon were used from lecture bottles supplied by Carlo Erba, air was taken from the environment without further purification. Treated samples were aged in atmosphere either at room temperature (20°C) or at 120°C.

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13.6 13.9 24.9

86.4 86.1 75.1

24.5 23.3 24.5 20.3

75.5 72.5 70.8 76.7

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4.2 4.7 3.0

* Treatment conditions: 50 W, 30 s XPS compositions determined immediately after treatment, except for the sample aged for 15 days

Adhesion tests

Pull tests were performed using a Sebastian II instrument, produced by Quad Group. Epoxy-coated pull studs, 12 mm long and with a cross-section in the bond area of 0.55 cm 2, were provided by the manufacturer.

Characterization

Water contact angles were measured by both the sessile drop and the Wilhelmy plate techniques6'~]. In the former case advancing and receding angles were obtained using a Ram6-Hart contact angle goniometer by increasing or decreasing the drop volume until moving the three-phase boundary over the surface. In the second case a Cahn DCA 322 dynamic contact angle analyser was used. The stage speed was held at 40 Ixm s-t.

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X-ray photoelectron spectroscopy (xPs) was carried out using a PHI model 548 XPS spectrometer, using experimental procedures extensively described elsewhere 6. Scanning electron microscopy (SEM) micrographs were obtained using a Cambridge Stereoscan 604 microscope; optical micrographs were obtained using a Leitz Orthoplan microscope.

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At least 10 specimens per treatment condition were glued to the studs following procedures suggested by the manufacturer (cure cycle: 1 h at 373 K) and then tested 6. RESULTS Treatments

The effect of the treatment parameters was assessed by performing plasma experiments at variable gas mixture, radio frequency (RF) power and treatment

time. The choice of treatment gases was restricted to environmentally friendly systems: two inert and two oxidizing gases. The former (argon and nitrogen) are expected to act following the CAS1NGmechanism (cross-linking activated by chain scission induced by the plasma) and by oxidation promoted by long-living radicals when the treated sample is brought into contact with the atmosphere 5. The latter (oxygen and air) can induce both cross-linking and oxidation by CASING and hydroperoxide mechanisms5"6. Other important parameters are RF power and treat-

Table 2. Advancing and receding contact angles of plasma-treated carbon/PEEK composites as a function of treatment gas, RF-power, treatment time and ageing time at room temperature Treatment gas

. 02 N2 Ar 02 N2 Ar 02 N2 Ar 02 N2 Ar 02 N2 Ar 02 N2 Ar 02 Air N2 Ar 02 Air N2 Ar 02 Air N2 Ar 02 N2 Ar 02

N2 Ar O2 N2 Ar 02 N2 Ar

RF power (W)

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. 20 20 20 20 20 20 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 100 100 100 100 100 100

Treatment time (s)

Ageing time (days)

. 30 30 30 30 30 30 5 5 5 5 5 5 10 10 10 10 10 10 30 30 30 30 30 30 30 30 30 30 30 30 60 60 60 60 60 60 30 30 30 30 30 30

6 6 6 6

0 0 0 14 14 14 0 0 0 14 14 14 0 0 0 14 14 14 0 0 0 0 14 14 14 14 h, 120°C h, 120°C h, 120°C h, 120°C 0 0 0 14 14 14 0 0 0 14 14 14

Advancing angle

Receding angle

74 33 43 46 42 49 50 28 40 42 39 44 45 23 35 34 39 41 40 23 24 28 27 32 34 32 35 35 34 33 36 23 25 26 31 32 31 23 26 25 30 31 30

41 10 10 10 8 10 11 6 10 10 8 10 11 6 10 10 9 11 10 7 8 9 10 8 10 9 9 10 9 8 11 7 7 6 8 10 11 7 9 6 10 11 10

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ment time. Increasing the former results in a higher energy density in the medium, hence favouring the reaction of plasma species with the polymer surface 12. Treatment time is important since, as shown previously 13, the chemistry of the discharge (depending on plasma parameters and the sample geometric area) needs some time (typically tens of seconds in our system) to reach a steady state, due to competing etching and modification reactions.

affect the carbon/PEEK surface composition as determined by xPs, thus involving segmental motions within the surface oxidized layers. Nitrogen and argon treatments caused the introduction of oxygen- and nitrogencontaining groups at the composite surface, due to the above-mentioned reaction of radicals formed by plasma treatment with molecules ~oxygen and nitrogen) present in the atmosphereL The lesser amount of nitrogen with respect to oxygen is due to the much lower reactivity of the nitrogen molecule.

Physico-chemical characterization

Water wettability hysteresis cycles obtained by the Wilhelmy technique are presented in Fig, 2. In the case of untreated carbon/PEEK (Fig. 2(a)), the first cycle is relative to wetting of the dry surface. The second advancing path does not repeat the former track. The uneven surface obtained by pressing a textile surface (Fig. 1) presents crevices filled by water, so that in the second cycle water advances on water, as shown by the much more 'hydrophilic' values which have been measured. Interestingly, when the water front gets to the position reached in the first cycle, the measured

In Table I surface compositions from xPs spectra of untreated and plasma-treated carbon/PEEK samples are reported. The composition of the untreated surface is in reasonably good agreement with the theoretical composition of PEEK. Samples treated with the oxygen and air plasmas both show an increase in oxygen concentration due to the introduction of oxygen functionalities; in the case of air-treated samples this is accompanied by a few nitrogen-containing groups. As already observed for other polymers6"7, ageing does not

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force goes back to 'hydrophobic' values. For the oxygen plasma-treated sample (Fig. 2(b)) much more hydrophilic values are obtained and the difference between first and second cycle is quite less evident, due to the much higher hydrophilicity of the treated surface.

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Contact angle data, obtained from sessile drop measurements for untreated and plasma-treated carbon/ PEEK samples, are presented in Table 2. A first observation is that while advancing angles do show some trends, receding angles are rather constant. Actually this behaviour is not unexpected, since advancing angles, relative to the least wettable portion of the surface 12, are most sensitive to both treatment and ageing effects. Receding angles, relative to the most wettable fraction of the surface~2, are least sensitive, since they are related to strongly oxidized islets, always formed on plasma-treated polymer surfaces and quite insensitive to reorientation effects6"7A°. More information can be gathered by looking at Figs 3(a) to 3(d) where the data of Table 2 have been elaborated by plotting the advancing angle of oxygen and argon plasma-treated surfaces as a function of RF power and treatment time, before and after ageing. In general higher powers and treatment times mean lower advancing angles, both immediately after treatment and after prolonged ageing (15 days). Furthermore, the difference between oxygen and argon plasmas tends to diminish at higher powers and treatment times. A better view of the ageing time dependence of contact angles of treated samples is given in Figs 4(a) and 4(b), which present the behaviour of samples treated with 50 W oxygen and argon plasmas as a function of ageing time, at treatment times of 5 and 30 s respectively. In both cases ageing is mostly over in one day, yet it is interesting to observe that ageing is very limited, similar to that observed for polyethylene6, and therefore suggesting plasma treatment-induced crosslinking, more evident in the case of argon plasmas. Ageing the samples at high temperature (120°C, which is below the glass transition temperature (Tg) of the PEEK), the limiting contact angles are close to those observed after ageing at room temperature (Table 2) but they are reached much faster, being stable after 6 h. As a general statement, given the differences in the plasma reactor and plasma conditions (lower pressure in our case), our data are in reasonably good agreement with those reported by Brennan et al. ~.

Adhesion testing Pull strength data as a function of plasma treatment conditions are reported in Table 3. With the exception of those treatments lasting 5 s, most of the data appear crowded in the 9-12 MPa area. Further evidence is presented in Fig. 5, where pull strength data after oxygen and argon plasmas are plotted vs. treatment time, both before and after ageing. In the case of oxygen plasma-treated samples there is little dependence upon both treatment time and RF power, in agreement with the fact that even a few adhesion sites are enough to provide good pull strengths 6"7. In the

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Fig. 4 Water advancing angles for oxygen (E3) and a r g o n (11) plasma-treated samples as a f unct i on of ageing time: (a) 50 W, 5 s t r e a t m e n t time; (b) 50 W, 30 s t r e a t m e n t t i m e

case of argon plasmas, a clear increase was observed with increasing treatment time, in agreement with contact angle data. The similarity of pull strengths observed using oxidizing and inert gases for treatment points to cross-linking effects, as reported in Reference 5, pp 212-213, specifically for not readily oxidizable polymers such as polyether sulphone and polyaryl suiphone. Another important influence on adhesion is probably the mechanical interlocking due to much easier penetration of the liquid adhesive during curing into crevices on the composite surface, the hydrophilicity of which has been much improved by plasma treatment. More evidence is presented in Fig. 6, which shows the behaviour of pull strengths of oxygen- and plasmatreated samples vs. ageing time, for 5 s and 30 s treatment times. Interestingly in the case of 5 s treated samples some decay is observed, in agreement with the contact angle behaviour (Fig. 4(a)) and as found for other plasma-treated polymers6. The decay is stronger for oxygen than for argon plasma, again in good agreement with contact angle evidence and pointing to

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Table 3. Pull strengths of plasma-treated carbon/PEEK composites as a function of treatment gas, RF power, treatment time and ageing time at room temperature Treatment gas

. 02 N2 Ar 02 N2 Ar Oz N2 Ar 02 N2 Ar 02 N2 Ar 02 N2

Ar 02 Air N2 Ar 02 Air N2 Ar 02 Air N2 Ar 02 N2

Ar 02 N2 Ar 02 N2 Ar 02 N2 Ar

RF power (W)

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Treatment time (s)

. 30 30 30 30 30 30 5 5 5 5 5 5 10 10 10 10 10 10 30 30 30 30 30 30 30 30 30 30 30 30 60 60 60 60 60 60 30 30 30 30 30 30

a 'wettability' control on the process. For 30 s treatments no decay is observed for oxygen plasma treatment, while for argon plasma treatment some increase in adhesion with ageing is suggested. The error margin is too large compared with the observed differences to draw conclusions, yet this behaviour could be related to further cross-linking induced by long-living radicals as a result of plasma treatment. As observed for contact angles (Table 2), ageing at 120°C rather than at room temperature does not affect pull strengths (Table 3).

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Ageing time (days)

6 6 6 6

0 0 0 14 14 14 0 0 0 14 14 14 0 0 0 14 14 14 0 0 0 0 14 14 14 14 h, 120°C h, 120°C h, 120°C h, 120°C 0 0 0 14 14 14 0 0 0 14 14 14

Pull strength (MPa) Average

Standard deviation

0 9.1 11.3 9.9 9.6 11.6 10.8 8.2 9.7 6.3 5.5 5.6 5.4 9.6 7.8 5.3 10.9 12.1 6.4 8.9 8.8 11.8 9.9 9.3 9.2 12.1 12.9 9.3 9.2 12.1 12.9 9.7 11.8 11.5 9.8 12.0 12.4 9.8 11.0 9.6 9.9 11.5 11.7

0 2.3 2.3 1.5 2.1 1.2 2.4 2.8 2.4 1.8 2.2 2.1 1.5 0.5 2.5 1.3 2.3 3.2 1.2 0.6 2.5 1.9 2.4 3.0 2.2 2.4 0.6 3.0 2.2 2.4 0.6 1.0 2.3 2.1 1.3 2.1 2.1 2.5 2.1 2.0 1.2 0.9 2.1

In Fig. 7 SEM micrographs of the fracture surfaces of untreated (Fig. 7(a)) and oxygen plasma-treated (Fig. 7(b)) carbon/PEEK composites are presented. In the case of the untreated sample an adhesive failure clearly occurred, while for the plasma-treated specimen cohesive failure is evident, with a layer of composite being ripped off. All plasma-treated specimens showed cohesive failure, even at 5 s treatment time, suggesting that very short plasma treatments are enough to dramatically improve adhesion.

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CONCLUSIONS This study has shown that plasma treatments are indeed very efficient in improving adhesion of carbon/ PEEK composites with epoxies, obtaining cohesive failure of the adhesive bonds. There is no need to pursue exotic (and potentially polluting) mixtures such as those suggested elsewhere ~'3. Friendly gases such as oxygen, nitrogen, argon or even air are quite sufficient. The latter solution is particularly interesting, since it lifts requirements for gas handling and lowers those concerned with pumping. In fact, before plasma treatment, pumping is typically performed to reach a pressure at least one order of magnitude, better two, lower that that used for treatment; then the treatment gas is introduced and the plasma is ignited. In the case of air it is sufficient just to reach the treatment pressure and then perform the treatment, with evident savings in time and equipment. Another interesting finding is that, for plasma treatments lasting more than 5-10 s, much lower than the 5 min quoted in Reference 1 or the 2 min of Reference

3, there is no real increase in pull strength with treatment time. Also RF power seems a rather mild requirement, since 20 W treatments provide excellent adhesion, even if some caution should always be suggested on the immediate reactor-to-reactor transferability of treatment effects5. Oxygen provides consistent improvements in adhesion even at very low treatment times. Argon is most efficient for treatments lasting at least 30 s, nitrogen is somewhat in between. Moreover, the ultimate pull strength is close for all gases, a behaviour reminiscent of that observed on other oxidation-resistant polymers, i.e., polyarylsulphones, formerly attributed to surface cross-linking induced by plasma treatment 5. Another important factor could be mechanical interlocking, favoured by easier penetration of the adhesives in surface imperfections made hydrophilic by the treatment. For 5 s treatments some decrease of pull strength with ageing time was observed, along with increases in water contact angles. Yet at higher treatment times ageing

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Fig. 6 Pull strengths for oxygen ([7) and argon (11) plasmatreated samples as a function of ageing time: (a) 50 W, 5 s treatment time; (b) 50 W, 30 s treatment time

did not occur, a rather unique characteristic paralleled only by polymers where surface cross-linking occurs induced by plasma treatment, namely polyethylene6. Actually some increases in pull strength with ageing time were observed, albeit close to error margins, which could have originated by the reaction during ageing of long-living radicals formed by the plasma treatment.

ACKNOWLEDGEMENTS The authors wish to thank Mr G.B. Morelli, Mr L. Pozzi and Mr L. Torelli for experimental assistance.

REFERENCES 1 Silverman, E.M. and GHese, R.A. 'Joining methods for graphite/ PEEK thermoplastic composites' SAMPEJ 25 (1989) p 34 2 Goeders, D.C. and Perry, J.L. 'Adhesive bonding PEEK/IM-6 composite for cryogenic applications' in Proc 36th lnt SAMPE Symp and Exhibition edited by J. Stinson, R. Adsit and F. Gordaninejad (SAMPE, Covina, CA, USA, 1991) p 348 3 Yoon, T.-H. and McGrath, J.E. 'Adhesion study of PEEK/ graphite composites' ibid. p 428

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Fig. 7 SEM micrographs from the composite side of fracture surfaces with epoxy-coated pull studs: (a) untreated carbon/ PEEK; (b) oxygen plasma-treated carbon/PEEK, 50 W RF power, 30 s treatment time, 14 days ageing

4 Occhiello, E. and Garbassi, F. "Surface modifications of polymers using high energy density treatments' Polymer News 12 (1988) p 365 5 Boenig, H.V. Plasma Science and Technology (Cornell University Press, Ithaca, NY, USA, 1982) 6 Morra, M., Ocehiello, E., Gila, L. and Garbassi, F. 'Surface dynamics vs. adhesion in oxygen plasma treated polyolefins' J Adhesion 33 (1990) p 77 7 Ocehiello, E., Morra, M., Morini, G., Garbassi, F. and Johnson, D. 'On oxygen plasma treated polypropylene interfaces with air, water and epoxy resins Part II: Epoxy resins"J Appl Polym Sci 42 (1991) p 2045 8 Brennan, W.J., Feast, W.J., Munro, H.S. and Walker, S.A. 'Investigation of the ageing of plasma oxidized PEEK' Polymer 32 (1991) p 1527 9 Ganga, R.L. US Pat 4 614 678 (1986) 10 Morra, M., Ocehiello, E. and Garbassi, F. 'Contact angle hysteresis on oxygen plasma treated polypropylene surfaces" J ColllnterfSci 132 (1989) p 504 11 Wu, S. Polymer lnterface and Adhesion (Dekker, New York. NY, USA, 1982) 12 Melliar-Smith, C.M. and Mogab, C.J. 'Plasma-assisted etching techniques for pattern delineation' in Thin Film Processes edited by J.L. Vossen and W. Kern (Academic Press, New York, NY. USA, 1978) p 497 13 Morra, M., Occhiello, E. and Garbassi, F. 'Surface characterization of plasma treated PTFE' SurflnterfAna116 (1990) p 412

A U THORS

The authors are with the Istituto Guido Donegani SpA, via G. Fauser 4, 28100 Novara, Italy. Enquiries should be directed to Dr Garbassi.