plasma oxigenio

8/7/2019 plasma oxigenio

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Oxygen Plasma Treatment of Sisal Fibersand Polypropylene: Effects onMechanical Properties of Composites

ELISETE COUTO', ING HWIETAN'?,NICOLE DEMARQUEITE",JOSE CLAUD10 CARASCH12*,and ALCIDES L m @lEscola Polit&cnicada Unwersidade de SfioPauloMaterials Engineering Departm entA v . Prof.M e l o Moraes 2463,W aulo, Brazil

2UnwersidadeEstadualPd i s t a - Campus d e BotucatuFaculdade d e Ci~31~iasgronomicasDepto. de Ci&cias AmbientaisF a z e n d a Experimental Lagead oBobcatu, S. P. BrazilPolypropylene powder and sisal fibers were oxygen plasma treated, and the me-chanical properties of their composites were tested. Two main effects were investi-gated: the incorporation of oxygen polar groups in the polypropylene surface andthe surface degradation and chain scission of both polypropylene and sisal fibers.Prior to these treatments, three reactor configurations were tested to invesbgate thebest condition for both effects to occur in PP films.Results showed that polypropy-lene-cellulose adhesion forces are about an order of magnitude higher for PP filmtreatments at 13.56 MH Z than at 40 kHz owing to much hgher chain scission at

lower frequencies, although it probably also occurs at high frequency and highpower. Polypropylene powder treated with oxygen plasmas in optimum conditionsfor polar group incorporation did not result in improvement in any composite me-chanical property, probably owing to the polymer meltmg. Sisal fibers and PP pow-der treated in conditions of surface degradation did not improve flexural or tensileproperties but resulted in higher impact resistance, comparable to the improve-ment obtainedwith the addition of compatibilizer.INTRODUCTION

he growing need for environmentally friendlierT aterials has stimulated research in compositematerials of thermoplastics reinforced with lignocellu-losic fibers. Besides improvement in mechanical prop-erties, the partial replacement of oil-based polymersby renewable vegetable fibers reduces CO, emissionand creates rural obs, among many other advantages(1). However, the lack of compatibility between mostcommonly used nonpolar polymers and polar cellu-lose-based fibers has limited the fiber reinforcement

T o whom comspondenceshould be addnssed.'Pnsent address: nstituto Nadonal de Pesquisas Espaclals. LaboratoxioAdado de Plasmas, Av. dos Astronautas 1758. CEP 12227-010,Sao J m o6Campos. S.P., BI-azll.*Pmsentad- Unfvusidade Es ta dd de Maringa. Departamentode Quimi-ca.Marhga, PR,Brazil.

capability because of the low interfacial adhesion. Agreat deal of work ha s been done to improve the adhe-sion of these materials through: chemical m&cationof either one of the components, addition of compati-bilizers (2). r plasma treatment. This last approachhas the advantage of being an environmentally friend-lier process, owing to the reduced number of chemi-cals involved and low treatment times.When polymer surfaces are exposed to a glow dis-charge plasma, different effects are often observed:surface cleaning by removal of organic contamina-tion, ablation or etching, which M e r s from cleaningby the amount of material removed, and modificationof the chemical composition of the polymer surface(3, ). Oxygen plasma treatment of nonpolar poly-mers such as polypropylene and polyethylene is knownto increase the surface polarity of PP and PE Almsand adhesion to cellulose-based materials (5, ). ow-ever, prolonged plasma exposure has been shown to

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OxygenPlasma 'keatmenfdecrease adhesion, which can be caused by chainscission resulting in low-molecular-weght, low-adher-ing hgments (5, 71, or by differences in the chemicalgroups incorporated during different plasma condi-tions (6).Oxygen plasma treatment of cellulose hasalso been shown to increase adhesion to polyethylene(8, 91, the improvement being attributed by the au-thors to the incorporation of hydroperoxides on thefiber surface, resulting in covalent bonds at the inter-face at high processing temperatures. The effect ofoxygen plasma treatment on the adhesion propertiesbetween PPfilm and wood was evaluated by means ofa peel test. The highest adhesion to wood resultedfi-om the shortest treatment times used: the effects ofplasma on the adhesion properties were more pro-nounced when both the PP film and the wood surfacewere treated (10).Several works on composites made with plasma-treated components have led to different conclusions.Treatment of cellulose fibers with M erent gas plasmasand different polymer matrices showed that acid-baseinteractions are important for several polymers butwere irrelevant to others like polypropylene (11).Di-chlorosilane plasma treatment of sisal fibers and poly-ethylene resulted in poor mechanical properties of theircomposites, which was attributed to the degradationof the fibers and the melting of the polymer, whichwould incorporate the funct iodized groups in the sur-face into the bulk of the material (12). Oxygen coronatreatment of cellulose fibers and polypropylene hassomewhat surprisingly shown that the treatment ofthe cellulose fibers was more efficient in improving thecomposites' mechanical properties than he treatmentof the polypropylene matrix. This improvement corre-lated well with an increase in the dispersive energy ofthe treated fibers, which is directly proportional to th ework of adhesion, and was attributed to the low-molec-ular-weight fragments formed (13, 14).Similar resultsare observed for corona-modified cellulose/polyethylenecomposites (15).Corona treatment of woodfiber andpolyethylene resulted in composites with decreasedmelt viscosity, probably due to low- molecular-weightmoieties acting as ubricants a t the interfaces (16).There is therefore evidence that although oxygenplasma treatment does improve cellulose-plastics ad-hesion by the incorporation of polar groups in non-polar polymers, the effects of low-molecular-weightfragments formed by degradation and chain scissioncaused by intense plasma exposure are still not clear.I t can be deleterious in some cases and beneficial inothers.The main objectiveof this work was to verify the ef-fects of oxygen plasma treatment of polypropylene andsisal fibers on the mechanical properties of their com-posites. Two effects were investigated: 1)polar groupincorporation on polypropylene surfaces for adhesionimprovement, and 2) oxygen plasma-induced degra-dation and chain scission in both polypropylene andsisal fibers.

In order to find the best conditions for these two ef-fects to occur, three different plasma reactor configura-tions were tested. Since chain scission is like@ o be fa-vored by high ionic bombardment, a low frequency (40kHz) apacitive reactor was first tested and comparedto the casewhere the same reactor vessel is powered bya high-frequency (13.56 M H z ) power source. Low-fi-e-quency systems are characterized by high ion bom-bardment, since during one-half cycle, electric fieldsstay in the same direction during a time long enoughfor the ions to respond and acceleratetowards he elec-trodes. At higher frequencies, ions are too massive torespond to the rapidly changing electric fields in thebulk of the plasma, and respond only to the averagepotential Merence between plasma and substrate. Athird reactor configuration involves a second reactorvessel powered by a 13.56 M H z source, but in this caseelectrodesare outside the vacuum chamber. Ion bom-bardment in this case is wen lower compared to the in-ternal elecimde case, since there is no current conduc-tion through the electrodes, current continuity beingachieved by displacement currents only.Polypropylene films were oxygen plasma-treated inthese three reactor configurations with varying power,pressure and treatment times. The purpose for vary-ing these three parameters was to try to find the bestcondition for each reactor in which each of the two ex-pected effects (polar group incorporation and chainscission) would occur. The main effect of increasingpower is mainly to increase electron energies, whichaffects the cross section and reaction rates responsi-ble for the creation of the various reactive oxygenspecies. Ion bombardment energies are also increasedwith power, since voltages between electrodes in-crease. These two effects will affect the number andtype of polar groups incorporated, as well as the de-gree of chain scission. Increasing treatment times willincrease the intensity with which incorporation and/orchain scissionwill occur.Inprinciple, longer treatmenttimes should increase oxygen incorporation, but couldalso induce chain scission by oxygen degradation. Theeffects of pressure are more uncertain, but one wouldthink hat it will affect electron mean free paths, crosssections and reaction rates.Adhesion to cellulose was measured by peel tests oflaminates made with the treated samples and cello-phane paper. XPS analysis of selected samples wasmade in order to determine the oxygen polar groupsincorporated in the surface. Incorporation of polar oxy-gen groupswas alsoestimatedby water contact angles.The external electrode reactor was chosen for treat-ing the sisal fibers and polypropylene powder since itprovided the best condition for adhesion improvementof the PP filmswith cellophane, aswill be shown below,besides the fact thatt this reactor was spec&@built totreat powders and fibers. Chain scission or surfacedegradation was likely to occur in this same reactor athigher power levels and long treatment limes. Sisal/PPcomposites were olbtained in a twin-screw extruder,

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Elisete Couto et dand then injected to form samples for tensile, fleMlraland impact tests. For comparison, pure PP samplesand samples of sisal/PP composites with the additionof maleated polypropylene (MA#P, 1% in weight) wereobtained and tested for the same mechanical proper-ties.

EXPERIMENTALMat e r i a l s

The materials used in this work are listed in Table1. Polypropylene films were cut into 3 cm X 10 cmsamples and were cleaned with acetone and iso-propanol prior to plasma treatment. Traces from theorganic fluids may be present during treatment, butbecause of th e small surface area s involved, theyshould not influence the results significantly, sincebase pressures are essentially the same (less than 20mTorr, while treatments occur at 200 mTorr). Clean-ing the samples by inert gas plasmas was avoidedsince electron and ion impacts could not only removeresidues but also induce chain scission and creationof free radicals. Cellophane paper was chosen as cel-lulose substrate instead of filter paper because of itssmoother surface, which prevents mechanical adhe-sion of the polymer by entanglement with the cellu-lose fibers, so that adhesion is preferentially due tophysical-chemical interactions. Polypropylene pelletswere ground and screened to 325 mesh. Sisal fiberswere cut into lengths of less than 10 mm.

Table 1. Materials.Material Supplier Characteristics

PP film Viopel PT25E homopolymerCellophane paper Nexon, UCB FilmsPolypropylene pellets Solvay MFI 12 g / l O rninSisalfibersMAgPP EastmanG-3002 m < 10 mm in length60,000

PlasmaReactorsOne of the plasma reactors used has been describedelsewhere (17);it consists of a capacitively coupled re-actor with two 20-cm-diameter electrodes, 3 cm apart,with one of them grounded and used as a substrateholder. It can be powered at 40 kHz or at 13.56 MEizusing two different sources. A mechanical pump pro-

vides base pressures of about 20 mTorr. Prior to sam-ple treatments, the reactor vessel was plasma cleanedwith argon at 200 mTorr, 200 W for 5 minutes.The second reactor was specially built for beatingpowders or other irregularly shaped substrates likefibers and pellets. The vacuum vessel is a Pyrex cylin-der with a 14-cm internal diameter, 80 cm long, withstainless steel flanges at both ends, gas being injectedat one end and pumped away at the other end. Inter-nally, a rotating substrate holder consisting of a Pyrexcylinder (12 cm in diameter, 20 cm long connected toa Pyrex rod is supported at both ends by the two &in-ges, one end being connected to an external motor,with controllable rotating speed (see Fig. 1). Fibers,pellets or powder samples are therefore revolved,while the holder rotates. For the PP film treatments,samples were placed inside the substrate holder with-out rotation. Tw o silicone-greased dynamic O-ringsprovided vacuum sealing during rotation. Two semi-cylinder electrodes are placed externally, surroundingthe vessel, and are connected to a 13.56 M H z powersource. The same vacuum and gas injection system asthe first reactor provides base pressures of less than20 mTorr.Argon is preferentially used for cleaning the reactorswalls because of its inertness. However, when oxygenplasmas are used for treatment, it can also be used ascleaning gas. Any polar fragments eventually createdon the reactor walls during oxygen plasma cleaningshould be of the same nature as the ones created dur-ing oxygen plasma treatment itself. This was the casefor the reactor with external electrodes. The lower ionbombardment compared to the low frequency reactor1

g. 1 . Schematicsof the vacuum vessel of the reactor builtfor treatingrns and powder.792 POLYMER ENGINEERING AND SCIENCE, APRIL 2002, Vol. 42, No. 4


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Oxygen P l a s m a neatrnentled us to choose a more aggressive cleaning method,using a reactive gas like oxygen. Plasma cleaning ofthe vacuum vessel is made with oxygen at 400W, 200mTorr, during 15min.OxygenPlasmaTreaments of PolypropyleneFilms and Adhesion Peed Tests

Power, pressure and treatment times were varied forthe three reactor configurations and are reported inTables2.3 and 4.Laminates of PP films and cellophane paper werehot pressed at 130C (below the PP melting point toprevent mechanical entanglement), 19 MPa for 10minutes.We conducted 180peel tests using an EMIC test-ing machine, with a 5 kgf cell, with peeling velocity of20 mm/min.Contact angles of distilled water sessile drops weremeasured using a Rame-Hart goniometer.

Table 2. Peel Strength and Contact Angles for VaryingPower and Pressure (Treatment Time is 10Seconds)of PP Films in the 40 kHzReactor.Pressure Power Peel Strength Contact Angle(mTorr) (w) (N.25 mm) (1

25105000 ..10020040500 50100200

0.0340.0620.0520.0520.0490.0270.010.0140.0120.0160.008

73684433307372736957

Table3. Peel Strength and Contact Angles forVarying Treatment Conditionsof PP Filmsin the 13.56Hz Reactor With Internal Electrodes.Peel Contact

Pressure Time Power Strength Angle(mTorr) ($1 (W) (N.25mm) (1

200 10

5200 4080200300400 10500600

5 0.11410 0.18320 0.20850 0.158100 0.120200 0.0850.11930 0.1960.4020.1100.06030 0.1070.0720.189

75.176.173.376.376.279.578.57371.875.77776.177.277.3

Table 4. Peel Strength and Contact Angles forVarying Treatment Conditions of PP Filmsin the 13.56HI! Reactor With External Electrodes.Peel ContactPressure Time Power Strength Angle

(mTorr) ( 5 ) (W) (N.25mm) (1

200 30

10204080120210200

400700 301000

40 0.405100 0.178200 0.145300 0.043400 0

0.1230.1 520.1940.2050 0.2950.2350.305200 0.1640.066

6770n6661747172736668---

X-ray Photoelectron Spectroscopy (KratosAnalyticalmodel XSAM HS with a 1253,6 eV, 180W Mg Ka radi-ation) were made on PP samples treated at variousconditions for elementalanalysis.oxygen Plasma IkeatmentofPP Porrdu md Sisal F i b

For each composite, 500 g of sisal fiber were usedwith 1500 g of PP powder. The reactor chosen fortreating the PP powder and sisal fibers was the exter-nal electrode, 13.616M H z reactor, which was speciallybuilt with a rotatig sample holder, and, as will beshown in the next section, provided the best condi-tions for oxygen polar group incorporation. Based onthe results obtained for the PP film treatments, twoplasma conditions were chosen. In order to incorporateoxygen polar groups on the PP surface, 200 mTorr,40 W gave the best condition for adhesion with cellu-lose. Considering that the optimum treatment time forPP fXmswas around 2 minutes, and that the powderhad to be revolved in order to expose all of its sur-face, PP powder was exposed to the plasma for 10minutes. For degradation and chain scission condi-tions, 200 mTorr,400W, 30 s resulted in no adhesionof the PP film to cellophane, so that thiswas the con-dition chosen with treatment time of 15 minutes. PPpowder was treated in the rotating sample holder inbatch treatments of about 70 g. Care was taken tomaintain the same plasma conditions and treatmenttime.

Sisal fibers wene also treated for degradation andchain scission at the same conditions as the PP fiber(200mTorr, 400 MI, 15 minutes). Because of the largeamount of fibers necessary (about 500 g) and its lowmass density, treating it inside the substrate holderwould have been too slow (only 15 g would be treatedat each time). Therefore, 125 g of fiber was spreadPOLYMER ENGINEERING AND SCIENCE, APRIL2002, Vol. 42, No. 4 793


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Elisete Couto et aLaround the base of the reactor at each time, withoutsubstrate holder, treated on one side, turned on theother side, and treated again.Preparation of the Composites

Sisal/PP composites were extruded with 25% inweight of fiber. The composites were prepared in atwin-screw extruder (B&P Process Equipment andSystem, model-H.P. 20191, with five zones of tempera-tures ranging from 170C o 200C along the barrel ofthe extruder. The screw speed ranged from 170 to 180rpm. This 19-mm diameter, laboratory-size extruderis capable of processing up to 10 kg of material perhour.Composites were cut into 7-mm-long pellets andoven dried at 50C for 2 hours and then at 105C for2 hours prior to injection molding.Five different composites were manufactured. Theircompositions are given in Table5.Mechan ica l Analysis

For mechanical property analysis. samples for ten-sile and impact tests were obtained by injection mold-ing (in a Mannesmann Demag, model Ergo Tech pro350-1151, with four zones of temperatures rangingfrom 170C to 190C along the barrel of the injectionunit, at a n injection speed of 200 rpm. Mold tempera-ture was kept at 40C. esides the five different com-posites described above, samples of PP were also ob-tained for the sake of comparison. Ten samples weremanufactured for each composite and each mechani-cal test.prior to mechanical testing, the samples were condi-tioned at (40 51% relative humidity, (23 ? 21C for40 hours. Notched Izod Impact tests were made usinga Zwick + Co.Kg - Einsingen Uber Ulm, type 5102 ac-cording to ASTM -D256, tensile and flexural testswere made in an Emic testing machine according toASTM - D790/98 and ASTM - D638, type 1, respec-tively.

RESULTS AND DISCUSSIONPeel Tests, Contact Angles aud XPSAnalpsis ofOxygen Treated PPFilms

Tables 2, 3 an d 4 show the results of peel testsmade on hot pressed laminates of cellophane paperand PP films and contact angles of the PP filmstreated on the three Werent reactor configurations.

Table 5. Types of Composites.Composite Label Sisal Polyprolyplene MagPP

~

SisallPP Untreated Untreated 0SisaVPP/MagPP Untreated Untreated 1%SisaKrPP func Untreated Treated for best adhesion 0SisaUTPP degr. Untreated Treated for degradation 0TsisaWP Treated Untreated 0

Table 2 shows the results of PP samples treated inthe 40 kHz reactor. It can be seen that while contactangles decrease with increasing power, indicating moreincorporation of oxygen polar groups, peel strengthsdecrease with power. Very low power (5 W) with veryshort treatment time (10 econds) gave the best peelstrength, showing that this low-frequency reactor withhigh ion bombardment is probably inducing too muchchain scission, resulting in low-molecular-weght, low-adhering fi-agments. This effect was also observed byCarlsson etd 5), n another low-frequency reactor (127KHz). Increase in pressure decreased peel strengthsand increased contact angles, probably because athlgher pressures, electron mean free. aths are smaller,resulting in lower electron energies, which in turn canresult in less reactive oxygen species, as also shownby XPS analysis described below.Table 3 gives the peel strengths and contact anglesfor treatments of PP filmsmade with the same previ-ous reactor but with a 13.56 M H z power source. Peelstrengths are much higher compared to the low-fi-e-quency case, and have an optimum condition some-where around 30 W with more thana minute of treat-ment time. This corroborates the hypothesis thathigher ion bombardment in low-frequency systemscauses chain scission. At this higher frequency, poly-mer degradation seems to occur at hgher powers, asshown by the lower peel strengths. Dependence onpressure seems to be more complex, and we could notfind a reasonable explanation for the variations meas-ured. Optimum treatment times are over a minute forlower powers. Contact angle measurements showedvery little variation, in alltreatment conditions. As willbe shown by the XPS analysis, the amount of oxygenincorporated at higher frequencies does not seem tovary with treatment conditions, so that longer or moreintense plasma exposure probably results in polymerdegradation.Results of PP films treated in the 13.56M H z reactorwith external electrodes are showed in Table 4.Peelstrengths are of the same order as in the case of theinternal electrodes, with optimum conditions for ad-hesion at about 30W to 60W, 200 mTorr and 2 min-utes of treatment time. There is a strong dependenceof peel strength upon power, with 400W resulting inno adhesion at all with cellophane paper. There is amoderate dependence of peel strength upon treatmenttime, a maximum for this case being around 2 min-utes. Contact angles are similar to the internal elec-trode case, and do not vary with treatment conditions,being around 70".Figure 2 shows a typical XPS survey spectrum ofoxygen plasma-treated PP film, nd Flg. 3 shows thehigh-resolution Cls spectrum of the same film. Table6shows the XPS analysis results for PP films treated atsome plasma conditions at low and hgh frequencies.The C l s spectra were fitted in four peaks, relative toatoms with zero, one, two, and three carbon-oxygenbonds. I t can be seen that at low frequency, oxygen

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OxygenPlasma Tkeatrnent

4

0 3.1r(* 30

dJY

0,

tncu0

4+

- . . . -"1 , . . , , . . , . , . , , . . . . , , . . . , . . . . , . . . . , . . . .T'l!i1100 1000 9 0, 800 700 6 0 0 5 0 0 400 3010 200 10 0Binding Energy (ev)

FQ. 2. XPS survey specbum ofpolypropylene_film reated withoxygenplasma at 40 kkk, 00 rnTorr, 5W , 10s .

incorporation is higher at a higher power, while at highii-equency, oxygen content is much lower, and longertreatment times do not increase oxygen content. Theseresults are consistent with contact angle measure-ments, which seem to depend mainly on the amount ofoxygen incorporated in the surface. The relative con-centrations of the four different carbon-oxygen bondsare roughly the same for all treatment conditionstested. These results corroborate the hypothesis thatchain scission is the most probable cause of low peelstrengths a t lower frequencies and high power, al-though incorporation of different oxygen groups (withthe same number of oxygen-carbon bonds) a t differentconditions could not be ruled out (for example andether type less polar group being incorporated at onecondition, while hydroxyl is incorporated at anothercondition).MechanicalRoperties of SisallPPComposites

Table 7 gives the results of the mechanical testsperformed on the five different composites and onpure PP samples.It can be seen that sisal/PP composites have muchhigher flexural moduli of elasticitythan pure PP sam-ples, as expected, bu t no effect of plasma treatment

or addition of MA&1PPis observed. Addition of MAgPPresults in a slightly higher elastic modulus. Flexuralyield strengths of sisal/PP composites are also higherthanpure PP samples, and only the addition of MAgPPresults in strength improvement, plasma treatmentsbeing also ineffective in this case.For tensile tests, sisal/PP composites have bettermodulus of elasticily compared with pure PP samples,but tensile strengths are about the same. In bothtests, plasma treatments have no effect on mechanicalproperties. However, addition of MAgPP results inabout 25% improvementin yield strength.Sisal/PP composites have much larger impact re-sistance than pure PP samples. This impact resis-tance is improved by both the addition of the compat-ibilizer or by plasma treatment. The plasma treatmentthat is most effective is the treatment of the sisalfibers, which presented similar effects as he additionof MAgPP. Treatment of PP for degradation also re-sulted in impact resistance improvement, while PPplasma treatment for the functionalhation of oxygenpolar groups was even detrimental to this mechanicalproperty.The results shown above indicate that althoughsome improvement in impact resistance was observed

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Elisete Corrtoet al.

00zfPiY

Rg.3. High resolutionC1 XPS spectrum of thepolypropylenej?h hown in Rg. .

for plasma-treated sisal fibers, comparable to addition CONCLUSIONSof compatibilizer, tensile and flexural strengths ofsisal/PP composites are not improved by oxygen plas-ma treatments. In particular, functionalization of polaroxygen groups on the surface of the PP powder failedto cause any beneficial effect, most probably becauseof the melting of the polymer. Surface degmdation ofthe sisal fibers (and to a lesser extent of the PP pow-der) did have a modest effect on impact tests, but thiscould be due to the fact that the fibers were not re-volved and thus not sufficiently exposed to the plas-ma. The installation of a n agitator could help solvethis problem.

Polypropylene powder and sisal fibers were oxy-gen plasma-treated in selected plasma conditions, inorder to functionalize oxygen polar groups in thepolypropylene, and to induce chain scission and low-molecular-weight fragments in both polypropyleneand sisal fibers. These treatments were tested for theireffectiveness in improving mechanical properties ofsisal/PP composites.Oxygen plasma treatment of polypropylene is an ef-ficient method for functionalizing surface polar groupsand improving adhesion to cellulose-based materials

Table 6. RelativeConcentrations(in%) of the Four Cls andTwo 01s Peaks,and the Atomic Ratio OICMeasured by XPS, for PP FilmsTreatedA t DifferentPlasma Conditions.C1 c2 c3CO 286.2 287.5 289.1

Treatment Condition 284.8 eV 20.3 eV k0.4 eV 20.2 eV C 0 OIC40kHz, 0.2 Tom, 70W 77 13 6 3 81.55 18.45 0.2340kHz, 0.2 Torr, 5W 79 12 6 3 86.02 13.98 0.1640kHz,0.5Torr, 5W 80 11 5 4 89.32 10.68 0.1213 MHz, 0.2 Torr, 20W, 10s 83 12 3 2 91.17 8.82 0.09713 MHz, 0.2 Torr, 20W, 80s 78 15 5 2 91.26 8.74 0.096

C,: C-C , C-HC,: C-OH, C - 0 6C,: COOH , COORc,: 0-c-o, c=o

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Oxygen P l a s m a neatmentTable 7. Flexural and Tensile Modulus of Elasticity (MOE) and Yield Strength and Notched lzodImpact Resultsof Composites, With Corresponding Standard Deviations.

Flexural Tensile ImpactMOE Strength MOE Strengh Resistance

PP 1.21 t 0.06 38.52 0.9 0.76 - 0.05 29.5 2 0.2 17.6t 3.7106 t 17P/Sisal 2.44? 0.1 49.2 - 1. 1.092 0.06PP/SisaVMagPP 2.7? 0.13 61 21.2 1.23 - 0.12 39 -t 1.4 127t 24PPrrSisal 2.42t 0.05 48.22 1.1 1.02 - 0.06 31 -t 0.5 130t 24TPP/Sisal (deg.) 2.56t 0.09 50.72 0.75 1.1 +- 0.04 32.52 1.2 118 t 27

Composite (MPa) (GPa) (MPa) J/M

302 .1

TPP/Sisal (func.) 2.51 % 0.12 48.22 0.9 1.05 t 0.07 30.4 - 1.0 93t 22

provided that care is taken in preventing degradationand chain scission.This is best achieved with plasmareactors with higher frequencies and low ion bombard-ment. Oxygen functionalities with one carbon-oxygenbond (hydroxylsand/or ethertypegroups) are the maingroups incorporated, and seems to be independent offrequency, power or time. At low frequencies, higherpower increases oxygen content, causing a decreasein water contact angles, but decreases adhesion tocellulose, indicating that chain scission has occurred,producing low-molecular-weight, low-adhering frag-ments. At higher frequencies, hgher power also de-creases adhesion to cellulose without altering the con-tact angles, so that polymer degradation is probablyoccurring without increasingoxygen incorporation.Functionakation of PP powder surfaces, however,does not improve mechanical properties of compositeswhen the treated powder is used as a matrix, probablybecause of the melting of the polymer, which buriesthe incorporated surfaces groups. Degradation andchain scission on the fiber or PP surfaces do not im-prove tensile and flexural properties but increase thecomposite impact resistance. In the case of fibers, thisimprovement is comparable to addition of compatibi-lizer, and could be improved by a better revolving sys-tem for a more uniform fiber surface exposure to theplasma.

ACKNOWLEDGMENTSThis work was supported by FAPESP (process num-bers 94/03351-4-2.5/09287-0, 6/06947-2. 7/06071 and 01/05382-11,nd CNPq (process no.104670/99-1).e would also like to thank Dr. EliasHage for the use of th e twin-screw extruder, EricaWeruth Ambrozin for contact angle measurements,the Plastics and Rubber Laboratory of the Technologi-cal Research Institute for mechanical and peel-strength analysis, and our materials suppliers, Vitopel,Nexon, Solvay, and Eastman.

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