surface characterization of synthetic vulcanized rubber treated with oxygen plasma

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 385È399 (1998)

Surface Characterization of Synthetic VulcanizedRubber Treated with Oxygen Plasma

M. Mercedes Pastor-Blas,1 Jose� Miguel Mart•�n-Mart•�nez1,* and John G. Dillard21 Adhesion & Adhesives Laboratory, Department of Inorganic Chemistry, University of Alicante, 03080 Alicante, Spain2 Center for Adhesive and Sealant Science, Department of Chemistry, Virginia Polytechnic Institute and State University,Blacksburg, VA 24061-0201, USA

The surface of a synthetic vulcanized styrene–butadiene rubber (R2) was treated in an oxygen plasma to improveadhesion in joints prepared with a one-component solvent-based polyester–urethane adhesive. The modiÐcationsproduced on the rubber surface by plasma treatment were assessed using advancing and receding contact anglemeasurements, x-ray photoelectron spectroscopy, (XPS), infrared-attenuated total reÑection spectroscopy andscanning electron microscopy. Adhesion was obtained from T-peel tests of treated R2 rubber/polyurethane adhesivejoints. Several experimental variables were considered, such as the radio-frequency power and the length and life-time of the plasma treatment. The treatment in the oxygen plasma produced a noticeable decrease in contact angle,which can be mainly ascribed to the creation of CwO and CxO moieties on the rubber surface. Advancing andreceding contact angles only di†ered by ¿10Ä. Depending on the experimental conditions used, some ablation wasproduced on the surface, which was more noticeable as the length and power of the treatment increased. Anadequate performance of adhesive joints was obtained using a power of 50 W and a time for oxygen plasmatreatment of Æ10 min. The changes in the rubber surface remained for 2 h after plasma treatment, as indicated bythe variation in peel strength and XPS data. Although improved adhesion was obtained by treating the rubber in anoxygen plasma, the T-peel strength values are not sufficient to assure technical use, probably due to the migrationof waxes and zinc stearate to the surface once the treatment was carried out. Finally, sulfur oxidation was produc-ed by the plasma treatment, and for severe conditions solid crystals of a sodium salt of an oxidized sulfur com-pound (sodium sulphate or an organic sulphate) appeared on the treated rubber surface. 1998 John Wiley &(

Sons, Ltd.

Surf. Interface Anal. 26, 385È399 (1998)

KEYWORDS: oxygen plasma; surface treatment ; styreneÈbutadiene rubber ; adhesion ; T-peel strength ; XPS; IR-ATRspectroscopy ; contact angle measurements ; SEM

INTRODUCTION

A number of polymers, especially rubbers, requiresurface treatment to achieve a satisfactory level of adhe-sion for bonding. Most of the adhesion problems inrubbers are usually related to additives in the rubberformulation (antioxidants, mold-release agents), whichmigrate to the surface once the adhesive joint is madeand contribute to a lack of adhesion. Actually, thesecompounds create a weak boundary layer (WBL) on therubber surface and prevent some degree of interactionwith the adhesive. To avoid the migration of these com-pounds to the surface, a surface treatment must be used.

The most commonly proposed surface treatments forrubber materials include mechanical/physical andchemical modiÐcation. Solvent wiping produces a swell-ing of the rubber surface, giving rise to improved adhe-sion in some rubber formulations.1 Roughening hasbeen proposed as an e†ective physical surface treatment

* Correspondence to : J. M. Mart•� n-Mart•� nez, Adhesion & Adhe-sives Laboratory, Department of Inorganic Chemistry, University ofAlicante, 03080 Alicante, Spain.Contract grant sponsor : Spanish Research Foundation : Contractgrant number : MAT95/0729.

to remove zinc stearate and waxes from the rubbersurface, but a progressive migration of these compoundsfrom the bulk to the surface with time may occur.2,3 Onthe other hand, some of the abrasive particles can belodged into the roughened rubber surface, constituting ahighly undesirable source of contamination.

Chemical treatments (organic peroxides,4 cycliza-tion,5 reactive polyisocyanates,6 halogenation7h9)involve the use of chemicals that have potentialenvironmental threats and may involve expensive wastedisposal. In contrast, atomic oxygen treatment usuallyinvolves no hazardous chemicals and does not depositsolid contaminants onto the treated surfaces, and theexhaust products of this treatment are gaseous oxides ofhydrocarbons, which exert minimal adverse environ-mental e†ects.

In this study atomic oxygen treatment is performedon rubber surfaces exposed to an oxygen plasma gener-ated by a radio-frequency discharge. Monatomicoxygen formed in the plasma arrives approximately iso-tropically on all exposed surfaces, oxidizing the rubberin a process that is spatially non-uniform on a micro-scopic scale and results in a simultaneous erosion andoxidation process.10 The improved adhesion obtainedby low-pressure plasma treatment is attributable to theformation of chemical double bonds, cross-linkages andredox reactions.11 In addition, this treatment results in

CCC 0142È2421/98/050385È15 $17.50 Received 31 October 1997( 1998 John Wiley & Sons, Ltd. Accepted 8 February 1998

386 M. M. PASTOR-BLAS, J. M. MARTIŠ N-MARTIŠ NEZ AND J. G. DILLARD

surface ablation of polymer components with lowmolecular weight (which constitute a WBL). On theother hand, the e†ects of the plasma treatment arerestricted to the uppermost layers, thus not a†ecting theoverall bulk properties of the substrate. The energeticenvironment of the plasma creates surface free radicalsand is able to produce di†erent e†ects, such as cleaning,ablation, cross-linking or chemical modiÐcation on thesubstrate surface.12

There has been much activity in the scientiÐc liter-ature over the past ten years in plasma polymerizationand, to a lesser extent, in the plasma modiÐcation ofsurfaces.13 The treatment of plastics for adhesivebonding with plasmas of argon, air, ammonia, nitrogenor oxygen has been reported.14 Treatment of polymericsurfaces with low-temperature gaseous plasmas hasfound widespread utility in the area of adhesion. Plasmapretreatment is used in several di†erent applications,including cleaning and activation of organic polymersurfaces,15 microelectronics,16 biocompatiblematerials17 and carbon Ðbers.18

Studies concerning the surface modiÐcation of rubbermaterials by plasma are scarcely found. Treatment ofnatural rubber in a plasma has been reported.19C2F6Both EPDM and natural rubber surfaces can be modi-Ðed by plasma polymerization and photografting oflow-molecular-weight acrylic compounds to create anoxygen permeation barrier.20 Details have been given ofimprovements in peel strength of silicone rubber jointsby oxygen plasma pretreatment followed by graftingwith acrylamide and acrylic acid.21,22 Furthermore,plasma can be used as a means of providing hydropho-bicity to silicone rubber.23

The increase in adhesive strength by means of plasmatreatment has been well documented and a number ofstudies have investigated several polymers other thanrubbers. In many cases, an increase in adhesive strengthcan be produced with plasma treatment,24 but thechemical nature of the particular polymer used, as wellas the speciÐc plasma treatment conditions, seem toplay a vital role in the resultant adhesive strength. Onthe other hand, oxygen plasma treatment has also beenused to prevent sticking of some materials that tend tostick to each other (e.g. rubber food-dispensing pro-ducts, rubber nipples for baby bottles, rubber gloves,packaging materials, solar-array blankets and thermal-control polymers).10

Rubber/metal joints are generally used in the widevariety of industrial applications. Thorough cleaningand pretreatment of the metal surfaces and the rubberare important to produce a strong rubber-to-metalbond.25 Plasma-sprayed NiÈCr and NiÈCrÈZn coatingsprotect steel adherends from corrosion in ambient andforced ageing conditions (up to 8 days in alternatecycles of immersion and removal from salt water).26The coatings exhibited high bondability to rubber andepoxy adhesives. Peel and tensile button-pull tests failedwithin the polymer for bonds formed on both “as-sprayedÏ and aged surfaces for specimens maintained inaggressive environments. The coatings provided longhold times prior to bonding and were tolerant to pro-cessing and handling damage or defects.

Although a number of studies in the literature discussplasma treatment of polymers in general, the surfacemodiÐcation by plasma in vulcanized styreneÈbutadiene

rubbers (SBR) has not been well described and there areonly a few papers related to the e†ect of discharge treat-ment on vulcanized rubber surfaces.27 Several dischargetreatments were applied to vulcanized natural rubber(NR), SBR and nitrile rubber (NBR) compounds withdi†erent degrees of e†ectiveness ; after plasma treat-ments, NR did not adhere to urethane adhesives,whereas the adhesion of SBR is slightly improved. Thecorona discharge treatment of NBR exhibited poorbonding.

The plasma polymerization of SBR onto the surfaceof natural bauxite has also been studied.28 Styrene orbutadiene was plasma polymerized onto the surface ofnatural bauxite. The properties of coated and uncoatedbauxite as a Ðller were compared in SBR vulcanizateformulations, using dynamic thermal mechanicalanalysis, tensile testing, ¡Shore A hardness and scanningelectron microscopy (SEM). Hardness was greatly inÑu-enced by the nature of the coating on the bauxite :plasma applied to polystyrene made the vulcanizatessofter, whereas plasma applied to polybutadiene madethem harder. Systems had low tensile strength regard-less of whether the bauxite was treated.

StyreneÈbutadiene rubber has also been employed tomodify the surface of Nylon 6 and polyimide Ðlms byplasma-induced polymerization to improve their adhe-sion to elastomers.29 A layer of rubber chain containingunsaturated moieties was implanted onto the Ðlm sur-faces using butadiene as the monomer gas. The struc-ture of the plasma polymer was characterized bysolubility measurements and NMR, Fourier transforminfrared spectroscopy (FTIR) and thermal analyses, andthe adhesion on laminates with rubber materials (blendsof NR, butyl rubber and SBR) was evaluated by peeltesting.

Several experimental techniques were used to studythe e†ect of ozone on the properties of styrene-butadiene block and random copolymers.30 Morphol-ogy di†erences in the copolymers signiÐcantly a†ectedthe rate of oxygen attack and subsequent static fatiguelifetime. A radio-frequency (rf) plasma produced a poly-styrene coating on the copolymers that resulted inlonger static fatigue lifetimes in an ozone environment.

Considering the few studies dealing with the plasmatreatment of rubber materials to improve their adhesionproperties, in this paper a study that considers thee†ects of oxygen plasma on the adhesion of a syntheticvulcanized rubber has been carried out. Several experi-mental parameters were considered, mainly the rfpower, the treatment time (length) and the lifetime ofthe treatment. Di†erent surface analysis techniques wereused to establish the nature of the surface modiÐcationsproduced on the plasma-treated rubber.

EXPERIMENTAL

Materials

A sulfur-vulcanized synthetic styreneÈbutadiene rubber(R2) manufactured by Caster S.A. (Elche, Spain) wasused in this study. An SBR 1502-type rubber was used.This is a “cold-SBRÏ rubber obtained using emulsionpolymerization at low temperature. The formulation of

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 385È399 (1998) ( 1998 John Wiley & Sons, Ltd.

SYNTHETIC VULCANIZED RUBBER TREATMENT IN O2 PLASMA 387

Table 1. Formulation of R2 synthetic vulcanized styrene–butadiene rubber

Percentage

(parts per hundred

Component rubber, phr)

SBR1502 100

Precipitated silica 42

Hydrocarbon resin 5.0

Sulfur 2.0

N-Cyclohexyl-2-benzothiazole

sulphenamide 2.0

Stearic acid 2.4

Zinc oxide 1.5

Phenolic antioxidant 0.5

2-Mercaptobenzothiazole

disulphide 2.5

Microcrystalline paraffin wax 0.8

Hexamethylene tetramine 1.0

Fatty acid zinc salts 5.4

the rubber is given in Table 1. This rubber containssilica as a Ðller and relatively large amounts of zincstearate, stearic acid and microcrystalline wax ; some ofthese components are responsible for the poor adhesionof this rubber.2 Some properties of the rubber wereobtained using standardized procedures : Hardness \72 ¡Shore A; density (20 ¡C) \ 1.1 g cm~3 ; Tensilestrength \ 11.4 MPa; maximum elongation atbreak \ 612%; tear resistance\ 14.7 kN m~1.

Oxygen plasma treatment was carried out in a MarchI Instruments solid-state unit (Concord, CA,Plasmod}USA) at 13.56 MHz and variable rf power. The pressureinside the chamber was maintained at 1 Torr using aMarch GCM-200 gas control module. Care was takento pump down and purge the treatment chamber for atleast 10 min prior to activating the rf Ðeld. The sampleswere placed on an aluminium plate inside a quartz/glasschamber. Oxygen (99.95% minimum purity) used forthe plasma treatment was supplied by Airco, Inc. Theoxygen plasma treatment was carried out between 30 sand 40 min. Additionally, the power of the oxygenplasma was varied between 25 and 125 W. Before treat-ing the samples in the oxygen plasma, the rubbersamples were wiped with 2-butanone to remove dustand surface contaminants and the solvent was allowedto evaporate for 30 min. It has been tested that the wipewith 2-butanone prior to plasma treatment does nota†ect the nature of the surface modiÐcation producedby plasma treatment on the rubber samples. Allbonding operations were performed within 1È2 minafter plasma treatment was carried out, in order tominimize the possibility of surface molecular rearrange-ment and/or contamination from the atmosphere.

To determine the T-peel strength, a one-componentthermoplastic polyesterÈurethane adhesive (Pearlstick45/40-15) manufactured by Merquinsa S.A. (Barcelona,Spain) was used. This polyesterÈurethane has a highcrystallization rate and a short open time. The adhesivewas prepared by dissolving 15 wt.% polyurethane in 2-butanone in a laboratory mixer (500 rpm for 2.5 h). Theadhesive solution obtained had a BrookÐeld viscosity of1690 mPa É s (23 ¡C).

Adhesive joints were made using two rubber strip testpieces (150 mm] 30 mm) that had been similarly

treated. The polyurethane adhesive was applied with abrush (150 mg of dried adhesive was placed on theadherend). After allowing the solvent to evaporate for1 h, the dry adhesive was melted at 80 ¡C under IR radi-ation. The coated species were placed into contactimmediately under a pressure of 0.8 MPa. The thicknessof the adhesive layer was D0.5 mm. The adhesive jointswere conditioned at 23 ¡C and 50% relative humidityfor 72 h before undergoing the T-peel test.

Experimental techniques

X-ray Photoelectron Spectroscopy (XPS). X-ray photoelec-tron spectroscopy was used to determine the modiÐ-cations produced on the outermost (50È100 rubberÓ)surface. The plasma-treated rubber materials wereanalyzed using a Perkin Elmer PHI 5400 spectrometerwith a Mg Ka achromatic x-ray source (1253.6 eV)operating at 14 KeV and 300 W with an emissioncurrent of 30 mA. The pressure inside the analysischamber was held below 5] 10~7 Torr (6.6 ] 10~5 Pa)during the course of the analysis. Samples weremounted onto the spectrometer probe with double-sided tape. Rectangular sample pieces (10 mm] 20mm) were used, although the dimension of the analyzedareas on the samples was 1 mm] 3 mm. The measure-ments were taken using a take-o† angle of 45¡. Thespectrometer was calibrated to the Au photopeak4f7@2at 83.8 eV and the Cu photopeak at 932.4 eV.2p3@2Survey scans were taken in the range 0È1100 eV andnarrow scans were obtained on all signiÐcant peaks inthe survey spectra. Binding energies of all photopeakswere referenced to the C 1s photopeak position forCwC and CwH species at 285.0 eV. Atomic concentra-tion calculations and curve Ðtting were carried out onan Apollo 3500 computer using PHI software, version4.0. In general, two spots of the same sample were mea-sured. Multicomponent C 1s photopeaks were curveÐtted using photopeaks of Gaussian peak shape with afull width at half-maximum (FWHM) of 1.6^ 0.1 eV.The C 1s Ðt was adjusted to the high binding energyside of the photoelectron peak. At least three analysesand curve Ðts were performed independently for eachplasma-treated sample to assure reproducibility.

Contact angle measurements. The surface-treated rubberpieces were introduced into the 25 ¡C thermostatedchamber of a Rame� Hart 100-00 115 NRL goniometerequipped with a video monitor. The chamber was pre-viously saturated with the vapour of the test liquid forat least 10 min before placing a liquid drop on thesurface of the rubber. The contact angles on the surface-treated rubbers were measured immediately afterplacing 4 ll drops of bidistilledÈdeionized water on thesample. Both the advancing and receding contact anglesof at least Ðve drops were measured and averaged foreach experiment. The experimental error was ^ 2¡. Theadvancing and receding contact angles were obtainedusing the tilting plate procedure.31

Fourier transform infrared spectroscopy (FTIR). The IRspectra of plasma-treated rubber were obtained using aNicolet FTIR 5DXB spectrophotometer. To avoid deeppenetration of the IR radiation into the sample, attenu-

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 385È399 (1998)


ated total multiple reÑection was employed (IR-ATR)and a zinc selenide crystal was used ; the incident anglewas 45¡. The spectrometer bench was purged with drynitrogen gas prior to analysis. Two hundred scans wereobtained and averaged at a resolution of 4 cm~1.

Scanning electron microscopy. A Phillips EM-420T scan-ning transmission electron microscope, operated at anacceleration voltage of 20 kV, was used to take photo-micrographs of the as-received and plasma-treatedrubber samples. The samples were secured on coppermounts using silver paint and coated with gold in aPolaron high-resolution sputter coater.

T-Peel strength measurements. The strength of the adhe-sive joints was determined using a T-peel test (EuropeanStandard PREN 1391) in an Instron 1123 test instru-ment (peel rate 0.1 m min~1). Five experimental deter-minations for each analyzed experimental variable wereobtained ; the standard deviation was \0.7 kN m~1.

RESULTS AND DISCUSSION

Three experimental parameters were considered in thisstudy : the rf power, the length of treatment and thee†ective lifetime of the surface treatment. The surfacetreatment lifetime was determined by considering the

maximum time for which the e†ects of the treatmentremained on the rubber surface. The treated sampleswere stored under open air in the laboratory bench toanalyze the lifetime of the surface treatment. Eachparameter will be considered separately in this study.

Power of oxygen plasma treatment

Figure 1 shows the variation of the water advancingand receding contact angle of oxygen plasma-treatedrubber using di†erent rf power ; the length of the treat-ment was 2 min. In this Ðgure and others the questionmarks indicate the experimental parameters that werespeciÐcally varied : e.g. in Fig. 1 the R2 rubber has beentreated in oxygen plasma for 2 min and the rf powerwas varied between 25 and 125 W. In general, the di†er-ence between advancing and receding contact angles issmall and remains almost constant (D10¡), indicatingthat the surface roughness is not very important in theuntreated and treated rubber samples. The treatmentwith oxygen plasma at only 25 W produces a noticeabledecrease in contact angle (from 113¡ to 61¡) due to theincrease in surface energy produced by the treatment.Increasing the power up to 100 W produces a lessmarked and gradual decrease in contact angle. The useof 125 W produces a slight increase in contact anglevalues, which is the reverse of that for rubber treated atlower power.

Figure 1. Variation of advancing and receding contact angle (water, 25 ¡C) on oxygen plasma-treated rubber as a function of power.Length of treatment ¼2 min.



Figure 2. Influence of the oxygen plasma rf power on oxygenplasma-treated rubber : (a) atomic composition (%) ; (b) carbonspecies (at.%). Length of treatment ¼2 min.

Analysis of treated samples by XPS permits an expla-nation of the variations in contact angle values. Thesurface chemical composition of untreated rubber ismainly composed of carbon [Fig. 2(a)] and a smallamount of oxygen. Carbon corresponds mainly toCwC and CwH species. The rubber formulation (Table1) contains relatively large amounts of sulfur andsilicon, but the XPS results show the absence of theseelements, indicating the presence of hydrocarbon com-pounds on the rubber surface. These hydrocarbon moi-eties may correspond to the presence of an SBR-richsurface layer and/or a microcrystalline wax-rich layerproduced by migration of the antioxidant (i.e. the wax)for the bulk to the surface. Treatment with the oxygenplasma at 25 W produced an important decrease incarbon content and a noticeable increase in oxygen percent, but no important changes in other elements [Fig.2(a)]. Therefore, the plasma treatment oxidizes CwCbonds to create CwO species and, in a smaller percent-age, CxO functionalities. The partial removal of hydro-carbon and the creation of the oxygen species areprobably responsible for the decrease in contact angle

values (Fig. 1). An increase in rf power to 100 W pro-duces a gradual decrease in carbon content and anincrease in oxygen, without signiÐcant variation in thesilicon, nitrogen, sulfur or zinc contents [Fig. 2(a)].According to Fig. 2(b), the oxygen species are mainlycomposed by CwO compounds, although somewCOO~ (carboxylic) species are created for rf powerhigher than 75 W. The plasma treatment at 125 Wfollows a di†erent trend : there is a smaller amount ofoxygen species than in the rubber treated with a powerof 100 W [Fig. 2(a)] and these species mainly corre-spond to CwO compounds because there are no car-boxylic groups [Fig. 2(b)] on the rubber surface. Thisdi†erent trend corresponds to that found in contactangle values (Fig. 1). Although the oxygen plasma pro-duces a noticeable removal of carbon moieties, there isno evidence of important amounts of sulfur and siliconand the rubber surface, indicating that this treatment isnot e†ective enough to remove the wax-rich surfacelayer.

Treatment in the oxygen plasma may modify thesurface morphology of rubber. Figure 3 shows the SEMmicrographs of untreated and plasma oxygen-treatedrubber samples for 3.5 min. The untreated rubber showsthe existence of a relatively heterogeneous surface, witha few cracks and di†erent morphologies. The treatmentwith oxygen plasma at 25 W produces surface cleaningby removal of some surface heterogeneities and createsa new kind of morphology similar to ribbons. Theincrease in plasma power up to 100 W enhances thesee†ects, increasing the number of ribbons and the degreeof surface cleaning. This corresponds to the increase insulfur and nitrogen shown by XPS [Fig. 2(a)], indicat-ing an ablation of the rubber surface as a consequenceof the increased aggressiveness of the plasma treatmentwhen the rf power is raised. The uppermost layers of therubber are removed and thus sulfur and nitrogen fromthe bulk rubber are detected by XPS.

The treatment with oxygen plasma at 125 W creates adi†erent kind of surface morphology (Fig. 3) due to animportant degree of degradation because the treatmentis too aggressive. Thus, in this sample there is an impor-tant degree of surface ablation, a higher contact anglethan expected and a lower oxygen content, indicatingthat the treatment should be carried out for an rf powerof \100 W.

Summarizing, the oxygen plasma treatment of rubberproduced an important decrease in contact angle (or anincrease in surface energy) due to the creation of CwOand CxO moieties on the surface. Furthermore, thistreatment produces some degree of surface ablation.The power during plasma treatment should be main-tained below 100 W to ensure that no excessive degra-dation is produced on the rubber surface. However,under the experimental conditions used in this study, itwas not possible to completely remove the wax-richlayer on the rubber produced by migration from thebulk to the surface.

Length of oxygen plasma treatment

Because the variation of the rf power during oxygenplasma was not able to remove the weak boundarylayer, the length of the treatment was extended up to 40



Figure 3. Scanning electron micrographs of untreated (R2-MEK) and oxygen plasma-treated rubber using different rf power. Length oftreatment ¼3.5 min.

min. Figure 4 shows the variation in contact angle ofoxygen plasma-treated rubber at 50 and 100 W for dif-ferent lengths of time. The inÑuence of the length of thetreatment is similar when a di†erent power is used : theplasma treatment for 1 min produced a noticeabledecrease in contact angle (from 113¡ to 66¡). Theincrease in length of treatment produces a gradualdecrease in contact angle, but there is no variation incontact angle for a treatment time higher than 10 min.Therefore, a treatment time lower than 10 min producesthe highest increase in surface energy.

The IR-ATR spectra of oxygen plasma-treated rubberare given in Fig. 5. The IR-ATR spectrum of untreatedrubber (R2-MEK) shows the presence of wax and zincstearate on the surface (the depth analyzed by IR-ATRis D5 lm thick). The IR-ATR spectrum of untreatedrubber shows some bands corresponding to hydrocar-bon moieties : CwH stretching (2872 and 2930 cm~1),

asymmetric bending and bending (1460wCH3 CH2cm~1), symmetric bending (1370 cm~1) andwCH3rocking (722 and 872 cm~1). The bands at 872,wCH31370, 1460, 2872 and 2930 cm~1 can be ascribed to

rubber, zinc stearate and wax. However, the band at722 cm~1, which is usually weak in rubber and zincstearate,8 is relatively intense in the IR-ATR spectrumof Fig. 5 and is typical of paraffin waxes. Furthermore,the intensity of the SiwO bands is relatively less impor-tant than expected due to the existence of the wax-richlayer on the rubber surface. Treatment in the oxygenplasma produced a noticeable removal of hydrocarbonmoieties on the surface (there is an increase of theSiwO bands at D1100 cm~1 and the intensity of CH2peaks at 2872 and 2930 cm~1 is reduced). Furthermore,the treatment creates carbonÈoxygen functionalities(bands at 1600È1730 cm~1), and the extent of the treat-ment up to 10 min increases the relative intensity ofthese bands ; a treatment of [10 min does not producea noticeable change in the relative intensity of carbonÈoxygen bands. At the same time, the band at 1436 cm~1(stretching band of OxSxO) increases in intensity byincreasing the length of plasma treatment due to oxida-tion of sulfur.24

Figure 6 shows the chemical composition of treatedrubber obtained from XPS analysis. The oxygen plasma



Figure 4. Variation of advancing and receding contact angle(water, 25 ¡C) on oxygen plasma-treated rubber as a function ofthe length of treatment. The rf power of the oxygen plasma was 50and 100 W.

treatment for a short time (i.e. 1 min) produces adecrease in carbon and an increase in oxygen content,which is mainly due to the oxidation of CwC bonds tocreate CwO functionalities. The curve Ðtting of C 1sphotopeaks (Fig. 7) shows an increase in the intensity ofCwO moieties and the creation of CxO functionalities.The increase in the length of plasma treatment up to 10min does not produce noticeable modiÐcations inchemical composition of the treated rubber, althoughthe amount of CxO groups is increased (Figs 6 and 7).Between 10 and 20 min of plasma treatment, a notice-able modiÐcation in surface composition is producedthat mainly consists of a sudden decrease in carbon anda signiÐcant increase in oxygen [Fig. 6(a)]. Further-more, the surface concentrations of sulfur and sodiumbecome signiÐcant. The presence of sodium was unex-pected because none of the rubber components containsodium; its presence in the treated samples may becaused by deposition of sodium impurities from thewalls of the plasma chamber. In any case, the presenceof sodium in the rubber has been conÐrmed in severalsamples treated in the oxygen plasma for [10 min.

Figure 5. Influence of the length of oxygen plasma treatment onthe IR-ATR spectra of untreated and oxygen plasma-treatedrubber. The rf power of the oxygen plasma was 50 W.

The carbon species [Figs 6(b) and 7] for rubbertreated in the oxygen plasma for [10 min are mainlyCwC and CwH, with a relatively small percentage ofsingle CwO and double CxO functionalities. Con-sidering that the oxygen content for the treated rubberis D40 at.%, most of this oxygen does not correspondto carbon oxidation. The curve Ðtting for S 2p photo-peaks (Fig. 8) and the increase in sulfur and oxygen[Fig. 6(a)] in the oxygen plasma-treated samples indi-cated the formation of an inorganic or organic sulphateproduced by oxidation of sulfur during the plasmatreatment. According to Fig. 8, the sulfur in theuntreated rubber is mainly SwS specimens (BE\ 164.0eV, corresponding to but the treatment with oxygenS

n),

plasma for 1 min produces a noticeable degree of oxida-tion (BE\ 168.5 eV), which may correspond to a sul-phate compound containing oxidized sulfur. This is ingood agreement with the results obtained usingIR-ATR spectroscopy (Fig. 5). The rubber sampletreated for 3.5 min shows a poor signal-to-noise ratio,which may be caused by surface roughness (created byoxygen plasma treatment). Except for this sample, the



Figure 6. Influence of the length of oxygen plasma treatment onoxygen plasma-treated rubber : (a) atomic composition (%) ; (b)carbon species (at.%). The rf power of the oxygen plasma was50 W.

increase in the length of the treatment produces ahigher degree of sulfur oxidation. Oxidized sulfur asinorganic or organic sulphate is the exclusive specimenwhen the treatment is extended for [10 min.

A further conÐrmation of the existence of oxidizedsulfur moieties on the oxygen plasma-treated rubbercan be obtained from SEM. According to Fig. 9, theoxygen plasma treatment removes surface heter-ogeneities on the rubber surface, producing surfacecleaning and ablation. The increase in the length of thetreatment favours the formation of rounded small whiteparticles that, according to energy-dispersive x-rayanalysis, contain sulfur, oxygen and sodium; therefore,the chemical composition of these particles is sodiumsulphate or an organic sulphate. The concentration ofthese particles is more signiÐcant for an oxygen plasmatreatment of [10 min.

T-Peel strength values of adhesive joints are given inFig. 10. By visual inspection, an adhesional failure wasalways obtained. The joints produced between

untreated rubber materials present a lack of adhesionbecause of the existence of a wax-rich layer on therubber surface (constituted by hydrocarbon species).Atomic composition of the failed surfaces after peel testwas assessed using XPS. The atomic percentages corre-sponding to the elements on the adhesive failed surface(surface A) (Table 2) show a higher amount of carbonand a lower amount of oxygen than in the polyurethaneadhesive (before bonding). Furthermore, silicon andsulfur are present on that failed surface A. On the otherhand, the analysis of the failed rubber surface (surfaceR) shows a relatively similar composition to theR2-MEK surface (before bonding) (Table 2). Therefore,the failure of the untreated rubber/polyurethane adhe-sive joint is not interfacial but due to a failure in theweak thin rubber layer. During the peel test, the failurein the joint will be produced in the weakest part of thejoint, which in that joint corresponds to a thin rubberlayer on the surface. Considering the reduced amountsof elements other than carbon in the failed R surface, itseems that wax still remains on the failed surface R afterthe T-peel test is carried out.

The oxygen plasma treatment produces an increase injoint strength that is maximum for 10 min of treatment ;the increase in the length of the treatment produces adecrease in adhesion strength that can be ascribed tothe existence of oxidized sulfur particles on the treatedrubber surface and to the excessive surface ablation pro-duced by an extended length of the treatment. The trendin joint strength agrees well with the trends shown bycontact angle measurements, IR-ATR spectroscopy andXPS analysis.

Summarizing this section, the oxygen plasma treat-ment of rubber carried out for \10 min is e†ective inincreasing the adhesive strength of adhesive joints. Anextended treatment favoured surface ablation and theformation of oxidized sulfur particles that act as con-taminants on the surface, decreasing the joint strength.

Lifetime of oxygen plasma treatment

The plasma treatment of polymers creates unstableshort-life species on the surface that act as reactivecenters against the adhesive to produce strong inter-actions.12 For practical purposes it is important todetermine the stability of the chemical species createdon the rubber surface after oxygen plasma treatment,and therefore the lifetime of the treatment was con-sidered in this study. The treated rubber samples were

Table 2. Chemical composition (at.% ) by XPS of untreated R2rubber (0 min) and polyurethane adhesive, and failedsurfaces after T-peel test

Before bonding Failed surfaces

Element Polyurethane adhesive R2-MEK Surface A Surface R

C 1s 77.4 96.5 91.7 94.0

O 1s 21.5 2.1 6.1 2.8

Si 2p — 0.3 1.5 1.4

N 1s 1.1 1.0 0.5 1.7

S 2p — 0.1 0.2 0.1



Figure 7. Curve fitting of C 1s photopeaks of oxygen plasma-treated rubbers : influence of the length of treatment.

kept under open air on the laboratory bench to test thelifetime of the plasma treatment. Figure 11 shows thewater advancing and receding contact angles measuredfor the rubber treated at 25 W for 2 min, as a functionof the time after plasma treatment. There is a rapidincrease in contact angle during the Ðrst hour aftertreatment, and for longer times there are no signiÐcant

changes in contact angle. This indicated that the e†ectsof the plasma treatment remain on the rubber surfacefor a short time, suggesting that the adhesive must beapplied to the rubber surface immediately after thetreatment.

The same tendencies are obtained from XPS analysisof treated rubber materials. According to Table 3, the



Figure 7. (Continued)

increase of time after oxygen plasma treatment produc-ed a gradual increase in carbon content and a decreasein oxygen, silicon, nitrogen and sulfur content. Thesechanges suggest the migration of hydrocarbon speciesto the rubber surface after plasma treatment. In fact,

curve Ðtting of the C 1s photopeak (Table 4) shows theincrease in CwC and CwH moieties and the decreaseof CwO and CxO percentages, conÐrming the lack ofe†ectiveness when the time to bond increases afterplasma treatment. In fact, the strength measured in



Figure 8. Curve fitting of S 2p photopeaks of oxygen plasma-treated rubbers : influence of the length of treatment.

treated rubber/polyurethane adhesive joints produced 1h after oxygen plasma treatment shows a negligiblevalue, due to the re-formation of a weak boundary layercomposed by hydrocarbon species.

CONCLUSIONS

The oxygen plasma treatment of a synthetic vulcanizedstyreneÈbutadiene rubber (R2) is a promising alternative

to chemical surface treatments (halogenation,cyclization). The surface of this rubber is mainly consti-tuted by a wax-rich layer produced by migration ofmicrocrystalline wax from the rubber bulk to thesurface. The oxygen plasma treatment is not completelye†ective in the removal of that wax-rich layer, but arelatively good adhesion strength can be obtained. Theoxygen plasma treatment produced a noticeabledecrease in contact angle even under soft conditions (25W for 30 s) ; however, the decrease in contact angle does



Figure 8. (Continued)

not correspond to an increase in joint strength due tothe gradual migration of hydrocarbon moieties from thebulk to the rubber surface once the adhesive joint isproduced. The e†ectiveness of the oxygen plasma treat-ment is due to the creation of CwO and CxO func-tionalities, the removal of surface contaminants and tosurface ablation. The best performance of the treatmentwas obtained for a time of \10 min, an rf powerbetween 50 and 100 W and by applying the adhesive

just after the treatment is carried out. The use ofextreme treatment conditions (high power or too long atreatment time) to produce the removal of the wax-richlayer is not completely successful and creates oxidizedsulfur moieties on the treated rubber surface. Moreexperimental work is necessary to produce a best per-formance in the adhesive joints produced with oxygenplasma-treated rubber materials.



Figure 9. Scanning electron micrographs of untreated (R2-MEK) and oxygen plasma-treated rubber : influence of the length of oxygenplasma. The rf power of the oxygen plasma was: 50 W.

Acknowledgements

Financial support from the Spanish Research Foundation CICYT(project MAT95/0729) is acknowledged. The authors would like tothank Mr Frank Cromer (Center for Adhesive and Sealant Science,Virginia Tech, Blacksburg, VA, USA) for his assistance with XPSmeasurements.

Table 3. Chemical composition (at.% ) of oxygen plasma-treated rubber analysed by XPS (rf power = 100 W,length of treatment = 3.5 min)

Time after oxygen plasma treatment

Element As-received 0 min 1 h 2 h 24 h

C 96.5 86.1 83.4 88.2 91.0

O 2.1 10.5 12.3 8.8 6.5

Si 0.3 0.7 0.9 0.7 0.5

N 1.0 1.5 1.9 1.4 1.2

S 0.1 1.2 1.2 0.9 0.8

Zn — — 0.3 — —

Table 4. Carbon species (curve Ðtting of C 1s photopeak) ofoxygen plasma-treated rubber (rf power = 100 W,length of treatment = 3.5 min)

Time after oxygen plasma treatment

Species As-received 0 min 1 h 2 h 24 h

CwC, CwH 96.7 91.5 91.2 92.1 95.3

CwO 3.3 6.3 6.3 6.1 2.9

†Œ

CxO — 2.2 2.5 1.8 1.8



Figure 10. T-Peel strength of oxygen plasma-treated rubber/polyurethane adhesive joints as a function of the length of treatment. The rfpower of the oxygen plasma was: 50 W and 100% adhesional failure (visually inspected) was always obtained.

Figure 11. Variation of advancing and receding contact angle (water, 25 ¡C) on oxygen plasma-treated rubber as a function of the timeafter plasma treatment (rf power of oxygen plasma ¼50 W, length of plasma treatment ¼2 min).



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surface characterization of synthetic vulcanized rubber treated with oxygen plasma

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