Effects of Surface Modification by Oxygen Plasma on PeelAdhesion of Pressure-Sensitive Adhesive Tapes

M. KAWABE,1 S. TASAKA,2 N. INAGAKI2

1 Reliability Evaluation Center, Nitto Denko Co., Toyohashi, Aichi 441-3194, Japan

2 Laboratory of Polymer Chemistry, Shizuoka University, Hamamatsu, Shizuoka 432-8561, Japan

Received 7 November 1999; accepted 22 January 2000

ABSTRACT: Surfaces of poly(isobutylene) (PIB) and poly(butylacrylate) (PBA) pressure-sensitive adhesive tapes were treated by oxygen plasma, and effects of surface modifi-cation on their adhesive behavior were investigated from the viewpoint of peel adhe-sion. The peel adhesion between PIB and PBA pressure-sensitive adhesive tapes andstainless steel has been improved by the oxygen plasma treatment. The surface-modification layer was formed on PIB and PBA pressure-sensitive adhesive surfaces bythe oxygen plasma treatment. The oxygen plasma treatment led to the formation offunctional groups such as various carbonyl groups. The treated layer was restricted tothe topmost layer (50–300 nm) from the surface. The GPC curves of the oxygenplasma-treated PBA adhesive were less changed. Although a degradation product of1–3% was formed in the process of the oxygen plasma treatment of the PIB adhesive.There are differences in the oxygen plasma treatment between the PIB and PBAadhesives. A close relationship was recognized between the amount of carbonyl groupsand peel adhesion. Therefore, the carbonyl groups formed on the PIB and PBA adhesivesurfaces may be a main factor to improve the peel adhesion between the PIB and PBAadhesive and stainless steel. The peel adhesion could be controlled by changing thecarbonyl concentration on the PIB and PBA adhesive surfaces. We speculate that thecarbonyl groups on the PIB and PBA adhesive surface might provide an interactionwith a stainless steel surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1392–1401,2000

Key words: poly(butyl acrylate); poly(isobutylene); oxygen plasma treatment; car-bonyl group; surface modification; peel adhesion

INTRODUCTION

Adhesion using pressure-sensitive adhesive(PSA) tape easily joints two substrates, but themechanism of the adhesion is complicated. Themechanism is related to surface science and rhe-ology of the PSA tapes. The fundamental adhe-sion mechanism of the adhesion with PSA tapecan be considered as due to three factors. For the

first factor, when the PSA adhesive touches thesurface of the substrate, the contact area of thePSA adhesive and the substrate is increased. Forthe second factor, a bonding force between thePSA adhesive and the substrate surface isformed. For the final factor, when the adhesivejoins are peeled off by external force, the PSAadhesive layers are deformed by the externalforce. The first two factors are related to the sur-face science of the PSA tape, and the last factor isrelated to the rheology of the PSA tape. Therefore,the adhesion behavior1–8 of PSA tapes may beinterpreted in terms of a combination of surface

Correspondence to: M. Kawabe.Journal of Applied Polymer Science, Vol. 78, 1392–1401 (2000)© 2000 John Wiley & Sons, Inc.

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science including wettability, interfacial bondingforce, and rheology including viscoelastic proper-ties, peel speed, and peel temperature. Many in-vestigators have pointed out the importance ofthe viscoelastic properties of the PSA tape inmanufacture of PSA tapes. Recently, in the fieldof the adhesive, many theories and experimentresults focusing on the importance of the acid-base interaction have been reported.9–17 How-ever, in the field of PSA tape, data on the casewith the acid-base interaction is not found. How-ever, some investigations have suggested that aninterfacial bonding force induced from acid-baseinteraction contributed greatly to the peel adhe-sion of PSA tape with a stainless steel surface. Itis thought that functional groups such as the car-bonyl group on the PSA surface play an importantrole in adhesion with stainless steel. To confirmthe effects of the functional groups in the PSAtape experimentally a tape sample, whichchanges only the surface without changing thebulk properties is needed.

There are many ways for the surface modifica-tion technique of polymers: flame treatment, ozo-nization, plasma treatment, UV treatment, lasertreatment, graft copolymerization, etc. Theplasma treatment18 is a surface modificationmethod that is able to introduce functional groupsinto the surface layer without changing the bulkproperties of polymers. The formation of func-tional groups on a polymer surface by plasma isinterpreted from actions of the radicals in theplasma. When the radicals touch a polymer sur-face, hydrogen atoms in polymer chains are re-moved to form carbon radicals on the polymersurface. Some of the carbon radicals combine withother radicals in the plasma to form functionalgroups on the polymer surface. Other carbon rad-icals, when exposed to air after the plasma treat-ment, are oxidized to form oxygen functionalgroups such as carbonyl and carboxyl groups. Theformation of these functional groups is restrictedto few hundred nanometers in depth from thepolymer surface. Therefore, oxygen plasma canmake oxygen functional groups such as carbonylgroups on the PSA surfaces. How the functionalgroups contribute to adhesion with stainless steelwill be investigated.

This study investigated what functional groupswere formed on PSA surfaces by oxygen plasma,how deep the functional groups existed from thePSA surface, and how the oxygen functionalgroups contribute to the adhesion with stainlesssteel.

EXPERIMENTAL

Materials

Two adhesives,19 Poly(isobutylene) (PIB) and Po-ly(butylacrylate) (PBA) were used as raw materi-als of PSA tapes. PBA was polymerized in anethyl acetate solution containing AIBN (0.2%) asan initiator at 60°C for 10 h. The molecularweight of the product was Mw 5 510,00, Mw/Mn5 7.2. PIB was purchased from Exxon ChemicalCo. (the trade name Bistanex MML-80). The mo-lecular weight of PIB was Mw 5 690,000 andMw/Mn 5 1.67. Poly(ethylene terephthalate)(PET) (trade name Lumira S-10, 38 mm thick-ness) and polypropylene (PP) films (trade nameTorefan BO 40-T2745, 40 mm thickness) pur-chased from Toray Co., were used as backings forPSA tapes.

PSA tapes of PBA and PIB adhesives (PBA/PET and PIB/PP PSA tapes) were prepared bycoating of PBA and PIB onto the PET and PPfilms, respectively. An ethyl acetate solution ofPBA and a heptane solution of PIB were used ascoating solution, and the coating procedure wasdone using a wire-bar apparatus. The PBA andPIB adhesives coated on the PET and PP filmswere maintained at 80°C for 3 min to evaporatethe solvents from the adhesive layers. After evap-oration of the solvents, to become 50% of the frac-tion of the gel the PBA adhesive layer wascrosslinked by exposure to an electron beam (12Mrad). The PBA and PIB adhesive layers of PBA/PET and PIB/PP PSA tapes were 30 mm thick-ness.

Oxygen Plasma Treatment of PSA Surfaces

A special reactor20 was used for surface modifica-tion of the PSA surfaces by oxygen plasma. De-tails of the reactor are shown in Figure 1. Thereactor consisted of a cylindrical Pyrex glass tube(45 mm in diameter, 1000 mm in length) and acolumnar stainless steel chamber (300 mm in di-ameter, 300 mm in height). The Pyrex glass tubehad a gas inlet for the injection of oxygen gas anda copper coil for the energy input of rf power (at afrequency of 13.56 MHz). The stainless steelchamber contained a Barocel pressure sensor(type 622, Edwards) and a vacuum system of acombination of a rotary pump (320 dm3/min) anda diffusion pump (550 dm3/s) (type YH-350A, Ul-vac Co., Japan). The Pyrex glass tube was jointedto the columnar chamber using a Viton O ringflange.

SURFACE MODIFICATION BY OXYGEN PLASMA 1393

Specimens (20 mm width and 50 mm length) ofthe PSA tapes were positioned at a constant dis-tance of 0 to 800 mm from the center of the coppercoil. Afterwards, the reaction chamber was evac-uated to approximately 1.3 3 1022 Pa, and thenoxygen at a flow rate of 10 cm3 (STP)/min wasintroduced into the Pyrex glass tube. The oxygenplasma was operated at rf powers of 50 to 100 Wat a frequency of 13.56 MHz at a system pressureof 13.3 Pa. The specimens were exposed to theoxygen plasma for 30 to 120 s.

Peel Adhesion(180Peel Test) with Stainless Steel

The peel test was carried out according to the JIS(Japanese Industrial Standard) method NumberC2107-1991. Specimens for the peel adhesionmeasurement were 20 mm width and 50 mmlength. The specimens were touched with stain-less steel substrates (SUS430BA), and pressedusing a 2-kg roller that passed slowly over thespecimen at a rate of 300 mm/min two times eachin a lengthwise direction. The PBA and PIB PSAtapes/stainless steel joints were stored at roomtemperature for 48 h, and the peel adhesion in the180° direction was measured at a peel rate of 300mm/min at room temperature using an Instrontype adhesion tester (Minebia Co., Japan; modelTCM-1kNB). The peel adhesion was evaluated ina unit of N/20 mm.

X-Ray Photoelectron Spectroscopic (XPS) Analysisof the Oxygen Plasma-Treated PSA

PBA and PIB PSA surfaces were treated with theoxygen plasma, and analyzed on a Perkin-Elmer-Phi ESCA 5400 spectrometer with a nonmono-chromatic MgK X-ray source at an anode voltage

of 15 kV, at an anode current of 20 mA. Thetake-off angle of photoelectrons against the PSAsurface was 45°.

Even if the C1s spectrum is divided by decon-volution, the carboxyl group and ester group can-not be distinguished by the usual XPS technique.A gas chemical modification technique combinedwith XPS is usually applied to cope with theseproblems.21–23 The concept of gas chemical modi-fication is to label a functional group of interestselectively with an element that is not present inthe original material, and has a large cross-sec-tion for XPS analysis. Reagents containing fluo-rine are often used for this purpose. To analyzefor the carboxyl groups, the gas chemical modifi-cation technique combined with XPS21–23 wasused. Trifluoroethanol was used to detect the car-boxyl group. The PSA tapes treated with oxygenplasma and treated with trifluoroethanol wereanalyzed for XPS analysis on a SSI Co. modelSSX-100 spectrometer with a monochromatic AlK X-ray source at an anode voltage of 10 kV, at ananode current of 20 mA. The take-off angle ofphotoelectrons was 35°. The concentrations of thecarboxyl groups were estimated from fluorine con-centration.

FTIR Attenuated Total Reflection (ATR) Analysis ofthe Oxygen Plasma-Treated PSA

FTIR-ATR spectra of the PBA and PIB PSA sur-faces treated with the oxygen plasma were ob-tained on a Perkin-Elmer Spectrum 2000 FT-IRusing a Multiple Internal Reflectance Apparatus,for the surface analysis of the PSA. A germaniumprism (45°) was used as an internal reflectionelement (IRE).

TEM Observation of the Oxygen Plasma-TreatedPSA

Usually for the preparation of the TEM the spec-imens were imbedded in epoxy resin. However,the PSA might swell and the thickness of the PSAlayer change when the PSA samples were imbed-ded in epoxy resin. A special preparation tech-nique was applied to cope with this problem.

The oxygen plasma-treated PBA and PIB PSAtapes were laminated to other PSA tapes. Afterthese specimens were bonded to the supportstand, these PSA samples were trimmed and sec-tions made using a cryosystem ultramicrotome(Leica Co., model Reichert-Nissei fcs and EM-Ultracut-S). The sample sections were dyed with

Figure 1 Schematic representation of plasma reac-tor.

1394 KAWABE, TASAKA, AND INAGAKI

ruthenium oxide, and sliced using a cryosystemultramicrotome. TEMs of the cross-sections of thesliced PSA layers were obtained on an HitachiH-7100FA TEM.

GPC Analysis of the Oxygen Plasma-Treated PSA

GPC curves of the PBA and PIB PSA treated withthe oxygen plasma were obtained on a TOSOHHLC-8120 GPC using a differential refractive in-dex (RI) detector. The PSA samples were dis-solved in tetrahydrofuran (THF), and adjusted toa 0.3% concentration. The PSA solutions werefiltered with a membrane filter of 0.45 mm. TheGPC curves of the filtered solutions, which ex-cluded the gel, were measured.

RESULTS AND DISCUSSION

Peel Adhesion of the Oxygen Plasma-TreatedPBA/PET and PIB/PP PSA Tapes

The PBA and PIB PSA surfaces were treated withthe oxygen plasma at 50–100 W for 30–120 s, andthen peel adhesions between the PBA PSA tape orPIB PSA tape and the stainless steel substratewere measured. Typical results of the peel adhe-sion are shown in Figures 2 and 3 as functions ofthe oxygen plasma treatment time and the sam-ple position in the plasma reactor. The oxygenplasma treatment gave a large effect on the peeladhesion for the PBA and PIB PSA tapes. Thepeel adhesion for the PIB PSA tapes/stainlesssteel joint is shown in Figure 2. The peel adhesionof the PIB PSA tapes treated with oxygen plasmaat an rf power of 100 W, 600 mm sample position

was increased from 3.3 N/20 mm to 8.7 N/20 mmby increasing the oxygen plasma treatment, lev-eled off after a treatment time of 90 s, andreached a maximum of 8.7 N/20 mm. The sampleposition also influenced the peel adhesion. Thepeel adhesion first increased, and then decreasedwith increasing the sample distance from theplasma. For example, when plasma treated, max-imum peel adhesions were 6.1 N/20 mm at asample position of 300 mm, 8.7 N/20 mm at 600mm, and 6.5 N/20 mm at 800 mm. These effects ofthe oxygen plasma treatment on the peel adhe-sion suggest that oxygen plasma treatment is ef-fective in improvement of the peel adhesion. Thisimprovement may be due to the formation of ox-ygen functional groups.

The PBA PSA tapes also, when treated withoxygen plasma, showed large changes in the peeladhesion (Fig. 3). The peel adhesion of PBA PSAtapes treated with oxygen plasma at an rf powerof 100 W, 800-mm sample position, as shown inFigure 3, was increased from 3.9 N/20 mm to 13.7N/20 mm with increasing plasma treatment timereached a maximum, and then decreased. Thesample position also influenced the peel adhesion.Maximum peel adhesions were 6.1 N/20 mm at300 mm, 6.9 N/20 mm at 600 mm, and 13.7 N/20mm at 800 mm.

From these results we conclude that the oxy-gen plasma treatment causes improvement of PIBand PBA PSA tapes peel adhesion. The improve-ments are related to plasma treatment time andalso the sample position. We believe that the im-provement results from the formation of oxygenfunctional groups on the PSA surfaces. The oxy-gen plasma-treated PSA surfaces were analyzedwith FTIR-ATR and XPS.

Figure 3 Peel adhesion of PBA PSA tapes treatedwith oxygen plasma at rf power 100 W as functions ofplasma treatment time and sample position.

Figure 2 Peel adhesion of PIB PSA tapes treatedwith oxygen plasma at rf power 100 W as functions ofplasma treatment time and sample position.

SURFACE MODIFICATION BY OXYGEN PLASMA 1395

Chemical Composition of the PIB, PBA PSASurfaces Treated by Oxygen Plasma

To clarify functional groups formed by the oxygenplasma treatment, the treated PIB and PBA ad-hesives were analyzed by FTIR-ATR. Specimensfor FTIR-ATR spectroscopy were PIB and PBAadhesives treated with the oxygen plasma at 100W for 120 s. Typical IR spectra are shown inFigures 4 and 5. Theoretically, IR radiation pen-etration depth in adhesive layers depends onmany factors, as shown by the equation24 for thedepth calculation.

dp 5 l/2 p nc@~sin u!2 2 ~ns/nc!2#1/2

In the equation dp is the penetration depth, l isthe wavelength, u is the incidence angle, ns is therefractive index of the sample, and nc that of theIRE. The calculated penetration depth is about0.4 mm in this FTIR-ATR method. Comparisonbetween spectra for the oxygen plasma-treatedPIB and untreated PIB (Fig. 4) shows a new ab-sorption appearing at 3400 and 1700 cm21, whichhave been assigned25 to a hydroxyl group and acarbonyl group. Comparison between spectra forthe oxygen plasma-treated PIB and untreatedPBA (Fig. 5) shows new absorption appearing at3500, 3200, and 1700 cm21, also assigned25 to ahydroxyl group and a carbonyl group. These newabsorptions suggest that carbonyl groups and hy-droxyl groups were formed on the PIB and thePBA adhesive surfaces.

To investigate the concentration of carbonylgroups, formed on the PIB and PBA adhesive

surface, the relative ratio (A1703 cm21/A1470cm21) of the absorbance peak area at A1703 cm21

and at A1470 cm21, which was due to carbonyland CH2, CH3 groups, calculated from the FTIR-ATR spectra (Fig. 4), respectively, was used todeduce the amount of carbonyl group introducedin the PIB adhesive. The ratio increased from0.03 to 1.0 with increasing oxygen plasma treat-ment time, and then leveled off after a treatmenttime of 60 s. On the other hand, the relative ratio(A1700 cm21/A1730 cm21) of the absorbances atA1700 cm21 and at A1730 cm21, which was due tocarbonyl and ester groups and calculated from theFTIR-ATR spectra (Fig. 5) respectively, was usedto determine the level of the carbonly group in-troduced in the PBA adhesive. The ratio in-creased from 0.05 to 0.15 with increasing oxygenplasma treatment time, reached a maximum, andthen decreased. Typical results of the increase ofcarbonyl groups are shown in Figures 6 and 7 asfunctions of the oxygen plasma treatment timesand the sample position. The carbonyl groupswere introduced into the PIB and PBA adhesivesby the oxygen plasma treatment.

The concentration of carbonyl groups formedon the PIB and PBA adhesive surfaces was afunction of the oxygen plasma treatment time.The variation of the carbonyl concentration withincreasing oxygen plasma treatment is similar tothat of the peel adhesion (Fig. 2 and 3). Influenceof the carbonyl groups on the peel adhesion willbe discussed in a later section.

The PBA and PIB adhesive surfaces treatedwith the oxygen plasma at 100 W for 120 s wereanalyzed by XPS. The O/C atomic ratio was esti-

Figure 5 FTIR-ATR spectra of PBA PSA untreatedand treated with oxygen plasma at rf power 100 W for120 s.

Figure 4 FTIR-ATR spectra of PIB PSA untreatedand treated with oxygen plasma at rf power 100 W for120 s.

1396 KAWABE, TASAKA, AND INAGAKI

mated from the relative peak intensity of the O1sand C1s spectra (Table I). The O/C atom ratio forthe PIB adhesive was increased from 0.01 (un-treated) to 0.13–0.15 by the oxygen plasma treat-ment. The PBA adhesive also increased the O/Catom ratio from 0.26 to 0.29–0.44.

It is known that carboxyl groups in PSA arevery effective in peel adhesion improvement,3,26

But, even if the C1s spectrum is divided by thedeconvolution, the carboxyl group and ester groupcannot be distinguished by the usual XPS tech-nique.

Possible carbonyl groups formed by the oxygenplasma treatment are ketone, aldehyde, ester,and carboxyl groups. To clarify which carbonylgroups were formed by the oxygen plasma treat-ment, the treated PIB and PBA adhesives were

analyzed by a gas chemical modification tech-nique combined with XPS21–23 and transcribedspectrum method of FTIR-ATR.

To detected the concentration of carboxylgroups, a gas chemical modification techniquecombined with XPS21–23 was used. The PBA andPIB adhesive surfaces treated with the oxygenplasma at 100 W for 120 s were analyzed by thegas chemical modification technique combinedwith XPS. The concentration of carboxyl groupswas calculated as carbon contents of carboxylgroups in all carbons (the rightmost column ofTable I). The concentration of carboxyl groups forthe PIB was increased from 0.000 (untreated) to0.002 by the oxygen plasma treatment. The PBAadhesive also increased the concentration of car-boxyl groups from 0.000 to 0.004. However, theamount of carboxyl groups are less than theamounts of the increase of the oxygen functionalgroups.

The oxygen plasma-treated PSA tapes showedcohesive failure. Then, to judge the kind of car-bonyl groups, the stainless steel surface residues,after the peel tests of oxygen plasma-treated PIBPSA tapes, were analyzed by Micro-FTIR-ATR,and the germanium IRE surface residues, afterFTIR-ATR analysis of oxygen plasma-treatedPBA PSA tapes, were analyzed by FTIR-ATR.This method is described as the FTIR-ATR tran-scribed spectrum method. A typical Micro-FTIR-ATR spectrum of stainless steel surface residue ofoxygen plasma-treated PIB PSA tape is shown inFigure 8. Comparison between spectra for residue

Figure 6 Concentration of carbonyl groups of PIBPSA treated with oxygen plasma at rf power 100 W asfunctions of plasma treatment time and sample posi-tion.

Figure 7 Concentration of carbonyl groups of PBAPSA treated with oxygen plasma at rf power 100 W, asfunctions of plasma treatment time and sample posi-tion.

Table I Chemical Composition of PIB and PBAAdhesives Treated with Oxygen-Plasmaat 100 W for 120 s

Adhesives

SamplePosition

(mm)

O/CAtomRatio

Concentration ofCarboxyl Groups

(Carbon Contents ofCarboxyl Groups/

All Carbons)

PIB (untreated) 0.01 0.0000 – –

300 0.15 –600 0.14 –800 0.13 0.002

PBA (untreated) 0.26 0.0000 0.34 –

300 0.44 –600 0.37 –800 0.29 0.004

SURFACE MODIFICATION BY OXYGEN PLASMA 1397

of PIB (Fig. 8) and oxygen plasma-treated PIB(Fig. 4) shows almost the same spectra. The 1710cm21 peaks were composed from several carbonylpeaks due to ketone, aldehyde, ester, and car-boxyl groups. A typical FTIR-ATR transcribedspectrum of oxygen plasma-treated PBA PSAtape is shown in Figure 9. On the other hand,comparison between spectra for residue of PBA(Fig. 9) and oxygen plasma-treated PBA (Fig. 5)shows quite a difference. In this FTIR-ATR spec-tra (Fig. 9), several absorption appear at 3220,3060, 2960, 1730, 1600, 1430, and 1170 cm21,which have been assigned25 to hydrogen-bondedcarboxyl groups, a carbonyl group, water adsorp-tion, and a CH2 band. These new absorptionssuggest that a special carbonyl group like a per-oxide structure was formed on the PBA adhesivesurfaces by the oxygen plasma treatment.

Thickness of Treated PSA Layer and BulkProperties of Adhesives

How deeply the PBA and PIB adhesives wereaffected by oxygen plasma was estimated from

TEMs. The PBA PSA tapes were treated with theoxygen plasma at 100 W for 30 s, the PIB PSAtapes were treated with the oxygen plasma at 75W for 30 s, and then dyed with ruthenium oxide.The dyed PBA and PIB PSA tapes were sliced offusing a cryosystem ultramicrotome, and thesliced PSA layer was observed with TEMa. Figure10 shows the typical TEMs of the PBA and PIBPSA layers. The oxygen plasma-treated PBA ad-hesives showed a hyperchromic line on the TEMs.The line was estimated to be 120–270 nm widefrom the magnitude of the photograph. The sam-ple position influenced the thickness of dyedlayer. The thicknesses of the dyed layers were 270nm at 0 mm, 180 nm at 300 mm, 140 nm at 600mm, and 120 nm at 800 mm. Similarly, the oxy-gen plasma-treated PIB adhesives also showed ablack line that was estimated to be 50–280 nmwide. The sample position influenced the thick-ness of the dyed layer. The thicknesses of thedyed layers were 280 nm at 0 mm, 125 nm at 300mm, 80 nm at 600 mm, and 50 nm at 800 mm. Onthe other hand, the untreated PBA and PIB ad-hesives never showed such a black line on theTEMs, even when the adhesives were dyed withruthenium oxide. We believe that the dyed layeron the TEMs may be due to ruthenium atomsabsorbed on the carbonyl groups, unsaturatedgroups, or long-lived free radicals in the adhe-sives. The thickness of the dyed layer was propor-

Figure 10 TEM photographs of PBA and PIB PSAtapes treated with oxygen plasma. (A) PBA PSA tapetreated with oxygen plasma at rf power 100 W for 30 s.(B) PIB PSA tape treated with oxygen plasma at rfpower 75 W for 30 s.

Figure 8 Micro-FTIR-ATR spectrum for stainlesssteel surface peeled off from stainless steel/PIB PSAtape treated with oxygen plasma at rf power 100 W for120 s.

Figure 9 FTIR-ATR spectrum of germanium IREsurface after FTIR-ATR analysis of PBA PSA tapetreated with oxygen plasma at rf power 100 W for 90 s.

1398 KAWABE, TASAKA, AND INAGAKI

tional to the absorbance of increased carbonylgroups by FTIR-ATR spectrum (PIB data areshown in Fig. 11). This results suggested that thedyed thin layer on the PBA and PIB adhesivesurfaces was a modified layer where the carbonylgroups were introduced by the oxygen plasmatreatment.

From TEM observation, the layer affected bythe oxygen plasma is extremely thin (50–300 nm)compared with the adhesive layer (30 mm). Evenif viscoelastic properties of the treated layer werechanged drastically, the whole of the adhesivelayer shows little change in viscoelastic proper-ties. We investigated how the whole adhesivelayer was affected from the view point of themolecular distribution.

Figures 12 and 13 show GPC curves of the solingredient of PBA and PIB adhesives. There wasless difference in the GPC curve between the ox-ygen plasma-treated PBA adhesive and the un-

treated. The weight-average molecular weight(Mw) and the ratio of the weight-average andnumber-average molecular weight (Mw/Mn) were1.4–1.5 3 105 and 4.0–4.6 for the oxygen plasma-treated PBA adhesives, and 1.4 3 105 and 4.3 forthe untreated PBA adhesive. The oxygen plasma-treated PIB adhesive showed a new and smallpeak in addition to a main peak on the GPCcurve. The main peak corresponded to a molecu-lar weight of 5.4–6.3 3 105 and the new peakcorresponded to a molecular weight of 4.3–8.13 103. The relative peak area of the two peakswas 99 : 1–97 : 3. The untreated PIB adhesiveshowed a weight-average molecular weight of 6.93 105. These GPC curves indicates that the oxy-gen plasma treatment gave less influence on mo-lecular distribution of the adhesives, although adegradation product of 1–3% was formed in theprocess of the oxygen plasma treatment of thePIB adhesive. We believe that there is no changein viscoelastic properties for the whole adhesivelayers.

What Contributes to Improvement of the PeelAdhesion?

The oxygen plasma treatment, as shown in Fig-ure 2 and Figure 3, improved the peel adhesion.The peel adhesion, as described in the introduc-tory section, is considered in terms of a combina-tion of two factors—interfacial bonding force be-tween the PSA and stainless steel surfaces, andviscoelastic properties of the whole PSA layer. Webelieve that the improvement of the peel adhesionby the oxygen plasma may be due to an interfacialbonding force rather than viscoelastic properties,because the treated layer was restricted to the

Figure 13 GPC curves of PBA PSA untreated andtreated with oxygen plasma at rf power 100 W for 120 s.

Figure 11 Relationship between the thickness ofdyed layer and absorbance area ratio (A1703 cm21/A1470cm21) of the carbonyl groups on PIB PSA sur-faces treated with oxygen plasma at rf power 75 W for30 s.

Figure 12 GPC curves of PIB PSA untreated andtreated with oxygen plasma at rf power 100 W for 120 s.

SURFACE MODIFICATION BY OXYGEN PLASMA 1399

topmost layer (50–300 nm) from the surface, andthe oxygen functional groups were formed in thetreated layer.

To confirm the effect on peel adhesion of car-bonyl groups formed on the PSA surface by theoxygen plasma, we performed correlation analy-sis of the peel adhesion and the concentration ofcarbonyl groups on the PSA surfaces. Typical re-sults of peel adhesion for the oxygen plasma-treated PSA/stainless steel joints are shown inFigures 14 and 15 as function of the concentrationof carbonyl groups on the PSA surfaces. The car-bonyl concentration was estimated from FTIR-ATR spectra. The estimation of the peel adhesionfor both joints, as shown in Figures 14 and 15,showed a linear relationship with the carbonylconcentration.

Figures 14 and 15 indicate that the carbonylgroups formed on the PSA surfaces may be a mainfactor in improving the peel adhesion between thePSA and stainless steel. The peel adhesion couldbe controlled by changing the carbonyl concentra-tion on the PSA surfaces. We speculate that thecarbonyl groups on the PSA surface might inter-act with a stainless steel surface.

CONCLUSION

The influence of the surface modification of PIBand PBA PSA tapes on their adhesion behaviorhas been investigated. Oxygen plasma treatmentwas used for the surface modification of the adhe-sives. Peel adhesions of PIB and PBA PSA tapeswere evaluated as a measurement of their adhe-sion behaviors. Functional groups formed on the

adhesive surfaces by oxygen plasma were ana-lyzed, and thickness of treated PSA layer wasmeasured. The contribution of introduced func-tional groups to the adhesion with stainless steelwas discussed. The effects of surface modificationby the oxygen plasma treatment are summarizedas follows.

1. The peel adhesion between the PSA andstainless steel was improved by the oxygenplasma treatment. For the PIB PSA tapes,the peel adhesion improved with increas-ing the plasma treatment time, but leveledoff after a treatment time of 90 s. For thePBA PSA tapes, the peel adhesion im-proved with increasing the plasma treat-ment time, reached a maximum, and thendecreased. The sample position also influ-enced the peel adhesion.

2. A surface modification layer was formed onthe PSA surfaces by the oxygen plasmatreatment. The oxygen plasma treatmentled to the formation of functional groupssuch as various carbonyl groups. The af-fected layer was restricted to the top-mostlayer (50–300 nm) from the surface.

3. The GPC curves of the oxygen plasma-treated PBA adhesive were less changed,although a degradation product of 1–3%was formed in the process of the oxygenplasma treatment of the PIB adhesive.There are differences in the oxygen plasma

Figure 14 Relationship between the peel adhesionand absorbance area ratio (A1703 cm21/A1470cm21) ofthe carbonyl groups on PIB PSA surfaces treated withoxygen plasma at rf power 50 W for 120 s.

Figure 15 Relationship between the peel adhesionand absorbance ratio (A1700 cm21/A1730cm21) of thecarbonyl groups on PBA PSA surfaces treated withoxygen plasma at rf power 100 W for 30 to 90 s. (1)untreated, (2) 30 s at 0 mm position, (3) 30 s at 300 mmposition, (4) 30 s at 600 mm position, (5) 60 s at 0 mmposition, (6) 60 s at 300 mm position, (7) 60 s at 600 mmposition, (8) 90 s at 0 mm position, (9) 90 s at 300 mmposition, (10) 90 s at 600 mm position.

1400 KAWABE, TASAKA, AND INAGAKI

treatment between the PIB and PBA adhe-sives.

4. A close relationship was recognized be-tween the amount of carbonyl groups andpeel adhesion. Therefore, the carbonylgroups formed on the PSA surfaces may bea main factor in improving the peel adhe-sion between the PSA and stainless steel.The peel adhesion could be controlled bychanging the carbonyl concentration on thePSA surfaces. We speculate that the car-bonyl groups on the PSA surface mighthave an interaction with the stainless steelsurface.

This article could not be published without generoussupport of Mr. H. Onishi, Mr. K. Sano, Mr. N. Okochi,Mr. M. Horada, and Miss M. Nishikawa, to whom I amindebted and deeply grateful.

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