Electrode discharge for plasma surface treatment of polymeric materials

Jo~eml o[

Materials rocess|ng nology ELSEVIER

Journal of Materials Processing Technology 58 (1996) 96-99

Electrode discharge for plasma surface treatment of polymeric materials F. Caiazzo a, P. Canonico b, R. Nigro% V. Tagliaferri d "lstituto di btgegneria Meccanica, Universitt) di Sah, rmt. Via pnnte thin MefillooS4084 Fi.whmo (SA ), Italy bSAAT! SpA, Appumo Gentih, (MIh htdv ~Dip. blgegneria (Yfinlica e Alimel.ttare, Unwersitt'a di Sah, rno-Via mmte don Melillo-84084 Fascitmo (SA 1. Italy alstituto di Tecnoh~gie e hnpianti Meccanici, Universitd di Genova°Via dell'Ooera Pitt 15, 161(~ Genoa, Italy

Received 23 June 1994: accepted 23 November 1994

Industrial summary

A cold plasma prototype electrode discharge apparatus to perform polymer surface treatment is described. The plasma generator, operating with oxygen and/or nitrogen, works in glow regimeat low pressure (10 mbar) and at relatively high gas flow rate, in the !!0=340 l/h range. The effects of treatments on PET monopolyester and glass fibre reinforced plastics (GFRP) surfaces are analysed. The treatment points out, in all operating conditions, an increase of both wettability (PET) and adhesivity (GFRP). The treatment times obtained are one order of magnitude lower than those of the traditional treatmen,~ apparatus. Keyu'ords: Electrode dist:harg¢; Plasma surfi~ce treatment

I, Introduction

In recent years, plasma technology hus been diffused because it is possible Io improve the superficial characteristics either of metallic or of polymeric material [1,2]. Plasma generators permit good control of the treatment process, low-cost treatments and low environmental pollution [2]. Plasmas used for surface treatments are identified and subdivided according to the thermal state of the gas, i.e, thermal and cold plasmas. The thermal plasmas, characterized by an average temperature between 1500 and 3500 °C, are used for the surface treatment of metallic materials to increase the hardness of metallic alloys with or without a d d ~ material, The cold plasmas, with a temperature lower than 100 °C, can activate selective treatment processes due to the contact between the material surface and the reactive gases [3]. This reactivity is dependent on the ¢r~ergetic state of the species and on the high energy difference between the ions and the electrons, respectively, characterized by temperatures of 300-500 K and 10 000-30 000 K (Te=(2/3)ek, where ¢'k is the kinetic energy of the

species}.

0~24-0136i96:$15,00 © 1996 Elsevier Science S.A, All rights reserved SSD!0924.0136(95)02112-Y

Owing to lheir characleristics, cold plasmas are used in industrial trcalmen| processes of low melting material, In polymeric material, cold plasma is employed to increase the wettabi!ity, adhesion and all surface characteristics dependent on the crossqinking process [3,4]. In most polymers the wettability and the adhesion are dependent on the increase of surface energy. This energy is increased by the activated gas, i.e. oxygen that diffuses into the first layer of the material. Moreover, the linking of the oxygen molecules and/or atoms into the polymeric chain increases the microporosity of the surface, thus increasing its characteristics [3,5]. The apparatus for industrial treatments employs cold plasmas at low pressure (0.01-10 mbar) produced by means of generators with a resistive coupling (DC or AC supply), or with a capacitive/inductive RF (i.5-50 MHz) or MW (150-10000 MHz) supply [2]. The treatments are performed in autoclave reactors causing the treatment cycles to be long. In this work an electrode cold plasma generator, in lab scale, has been designed and tested to perform treatments on polymeric materials. The lab prototype is able to operate with gas flow and continuously from low up to atmospheric pressure. Only 10 mbar treatments have been performed using oxygen and nitrogen.

2. Experimental zpp~wz~us

F<, 1',~ iV~

In Fig. 1 a diagram of the experimenlal apparatus is shown. The plasma generalor consisls of a glass tube equipped with two stainless steel electrodes 10 mm in diameter. The particular angle (90 '>) between the electrodes improves the interaction between gas and electromagnetic fields produced by the current flow. The electrodes are hollow to enhance the emission of UV radiation 13]. The distance between the electrodes can be varied simply by adjusting the screw clamps S E t SE_,. The discharge gas is supplied to the reactor through inputs 1-3 in order to obtain gas flow parallel to the current (inputs 1 and 3) and/or perpendicular to it (input 2). The gas pressure is measured by means of inputs 4 and 5. Oxygen and nitrogen are used as discharge gases, owing to their high reactivity with respect to many polymeric materials [2]. During the tests pure gases have been used and the apparatus is also able to supply mixed gases. Valves V~ and V~, placed upstream to the vacuum pump, permit control of pressure levels from 1 torr up to atmospheric pressure and gas flow rate in the 0 - 1 0 m3/h range. The AC power supply is obtained by means of a high voltage one-phase transformer of 900 VA (30 mA and

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Fig. 2. Electrical characteristics of cold plasma generator. |/,l discharge voltage, #d current discharge, ]'~ suppb ~ohage, B'~ poaer discharge. 30 kV). The primary voltage is controlled by a voltage regulator. The electric discharge is stabilized by means of a I MIq! resistance. Tile V-A characteristic of the discharge and the power dis:..ipated in the gas is obtained measuring the discharge current and the voltage. Both current and voltage are time-resolved. The electrical characteristic has been measured at three different voltage levels and at differell| gas flow rates: 110, 190 and 340 l'h. Fig. 2 shows lJ:~ voltage al~.d Ll current, supply voltage 1I~ and power Wd= V, lt) ld{t), tbr the case of oxygen gas~ I / = 7,5 kV and flow rate 190 l/h, The operation of the disclmrge is a typical quasi:~finu: soidal regime. At the beginning of each ha!f-cyc!e, the discharge is inactive until the Vd voltage reaches break° down voltage ( ~ !500 V). When the discharge is active, the discharge voltage decreases to an almost constant value much lower than the supply voltage, while the current increases as shown in Fig. 2. The discharge is Table 1 Values of maximum discharge current, l,ll,~,,~,,discharge voltage, l/~l, and average power. I'I/,i,~ve, with varying supply voltage I,".. and

oxygen flow rate, O Fig. I. Experimental get~erator of cold plasma. The distance between the electrodes can be varied adjusting the screw clamf,s SEI- SEa. The discharge gas is supplied through inputs i-3 in order to obtain the gas flow parallel to th~:current (inputs i and 3} and/or perpendicular to it (input 2). The gas pressure is measured by means of inputs 4 and 5. Valves V~ and V~ permit control of pressure levels fi'om I torr up to atmospheric pressure and gas flow rate in the 0-10 m-~th range. The AC power supply is obtained by means of a high voltage oae-phase transforxner of 900 VA (30 mA and 30 kV). The primary voltage is controlled by a voltage iegulator. The electric discharge is stabilized by means of a ! M~ resistance.

V.II~ ...... (%)

Q (l/h)

4~p,,,,k (mA}

li~ (V)

lt~i .... ¢W)

25

I!0 190 340 II0 190 340 II0 190 340

8 8 8 15 15 15 22 22 22

80O !070 1150 850 1070 1180 900 il00 1120

5 5.5 6 8,5 !1 12 13 16 18

50 75

b: Cttta::o et ,t. : Jo,mtal of ~htterials Processi,g Tcchmdo,O, 58 (I 996) 96- 99

98

Table 2 Values of maximum di~harge current, I d ~ , , discharge voltage, I.~. and average power, |I-~,,,~.with varying supply voltage V~, and nitrogen ttow rate. Q I",/V,.....(%)

Q (I/h)

Idoe,,~ (mA)

I~ (V)

:.I~,,,~(W)

25

110 190 340 ! 10 190 340 1 I0 190 340

7 7 7 14.5 14.5 14.5 22 22

900 I100 1200 870 1070 1190 920 1150 i 230

4.2 5 5.5 8.5 10.5 ! 1.5 13,5 17 18

50

75

22

Table 3 Values of maximum force, F,.....; separation of glue fi'om the surta=e, SRa; break energy, E,, and standard deviations, a, at ditferent treatment conditions: no treatment ---; treatment time 5 min, PI. 5; treatment time 15 rain, PI 15; surlace abrasion treatment, Abr. Sure Treatment

F~,,~ (N)

O"

SRd (%0

E~ (Nm)

a

--PI.5 Pi.15 Abr. Surf.

18.6 19.2 19.3 17.3

15.9 10 6.2 9.2

13.9 10.2 !.9 22.6

58.7 61.7 76 60.7

4.9 3.7 3.3 7,5

3. Results

active until the supply voltage is greater than the discharge sustaining voltage ( ~ 1100 V). In the following half-cycles the same behaviour is repeated. The discharge voltage has a stable behaviour in the active half-cycles and is equal to 1070 V. The power of the discharge has an alternating behaviour with a maximum value of 16 W. The average power dissipated into the discharge is 11 W. In Tables I and 2 the peak current, the voltage and the average dissipated power are indicated in respect to oxygen and nitrogen. These ele,ctrical parameters are measured at the mentioned gas flow rates and at the three level~ of supply voltage.

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100

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o

~o

40

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Wvm,, (%) Fig, 3, Minimum treatment time, r, versus supply voltage at different exxon flow rates,

The treatment tests have been perlbrmed by changing the supply voltage, gas flow rate, gas type and treatment time setting constant gas pressure (10 mbar) as well as the gap between the electrodes ( ~ 50 mill),

.~. 1. lYetthlg test

PET specimens of 5 x 4 cm a have been treated either in oxygen or in nitrogen, with treatment time ranging between 5 and 1020 s. The increase in wettability is evaluated by measuring the contact angle between the liquid phase and the treated surface, as recommended ill Ref, [6]. The liquids used for the above analysis are water, 95% ethi!ic alcohol and ink. Tile analysis has been performed on whole specimen area, by sampling the wetlabi!ity on a square grid of 5 mm side. The material has been considered as treated if the contact angle was lower than at least of 10%. The treatments have been carried out at the three voltage levels mentioned above (25%, 50%, 75% of the maximum supply voltage 30 kV) and at the gas flow rate of 110, 190 and 340 l/h, The minimum time to obtain the surface treatment has been measured, In Fig, 3 this minimum time versus the supply voltage is shown at different gas flow rates with respect to oxygen. The experimental data are line-connected in order to individuate the locus of the points below which the surface is considered treated. The decrease in the treatment time is evident as the supply voltage increases, The minimum treatment time, 5 s, is observed working with oxygen gas, gas flow rate of 340 1/h and 50% of the maximum supply voltage, in the operating regimes, the range of treatment time in the case of oxygen is wide (5-1020 s), whereas for nitrogen, it varies between 40 and 200 s. The repeatability of the treatments with both gases is good for all operative conditions. The decay time of the treatments has also been tested. The contact angle has been measured on differ-

F, Caiaz=o et ~d. / ,hmrmd of .'lhm*rhaL~"Process#gg TecgmMogy 58 (~996) 96 99

ent specimens every 10 rain afler the end of the treatment. The increased weltabilily has been observed ibr periods u D t,O 3 h. 3.2. Adhesion tests

Surface treatments of GFRP composites have been performed. Specimens 4 x 18 x 60 mm were obtained by manual deposition of glass cloth and polyester resins. Alter plasma treatment, two samples were joined according to the standard procedure proposed in Ref. [7]. The glue used was a two-component epoxy type. The joined sample was tested by a peeling test using a tensile test INSTRON machine. The operative treatment conditions were: oxygen pressure 10 tabor; flow rate 340 l/h; supply voltage 75% of maximum supply voltage; treatment time between 5 and 15 rain. Untreated joined samples, previously abraded by rubbing paper according to industrial practice, were adopted as reference. Table 3 reports the maximum force, total energy dissipated in separating the joint and relative standard deviation. The results show that the cold plasma treatment significantly influences both the total energy required to separate samples and the standard deviation, a, of the maximum force measured. On increasing treatment time, an increase in the break energy and a lower value of a were observed. Morphological analysis of fracture surfaces evidences two characteristic areas: one shows separation of the glue from the surface; the olher shows a thin layer of material peeled away from the surface. In Table 3 the values of lirst separa|hm area (SR) are indicated, it is evident that there is close correlation between the break energy and the SR vab ues; higher break energy values are seen at lower SR values.

99

4. Conc|usions The resuRs obtained lead to the following conclusions: - the experimental apparatus can produce high flow rate of gas energetically activated for the surface treatment of polymeric materials; PET wettability increases sensibly in short treatment times, much lower than those used in traditional technology; GFRP adhesion increases with an increase in treatment time improving the gluing quality. In the last case, composite gluing problems can easily be overcome by the ldcafized treatment technology of cold plasma. This aspect will be studied in a suitable experimental analysis aimed at understanding the adhesion mechanism and at evaluating the technological limits of the proposed technique. -

-

References [!] D,S. Rickerby and A. Matthews, A&'am'ed SmJace Coat~:gs: a Handbook of Smface Enghwering. Chapman and Hall. New York (1991) pp. 1-14. [2] J.R. Hollallan and AT. Bell, Techniques attd Applications o.l Plasma Chemisto,. John Wiley & Sons, New York (1987) pp. 379-391. [3] S.L. Perrorte, Low Temperature Plasma for surface treatment of polymers, in Ann. Pac. Tech. Col!f. Tech. Disp. (Society Plast. Eng.) (1979~ pp. 191-194. [4] C.A. Wcsterdahl, Gas plasma effects on polymer surface, ,Im~r~m/ o/(~dhJid am/Duet?/ace Science, 3 ( !984} 610 620. [5] R.R, Sowcll, Effects of RF ~ctiv~llcd inert gas plasmas on critical ~iurface tension and bondability of PET. Report No SC- R R°71° (1483, Sandi~ kaboratorics, Alberqucrque, New Mexico (1971)r [O] ASTM, Wetting Ten.sion of Polyethylene and Polypropylcnc lilms, Standard Methods ol I'ests D2578o07, [7] ASTM, Adhesion Ratio of Polyethylene lilm, Slandard Methods of Tests D2141-68.