Influence of the styrene content of thermoplastic styrene–butadiene rubbers in the effectiveness...

* Corresponding author. Tel.: #34-96-5903977; fax: #34-96-5903454.E-mail address: [email protected] (J.M. MartmHn-MartmHnez).

International Journal of Adhesion & Adhesives 21 (2001) 161}172

In#uence of the styrene content of thermoplastic styrene}butadienerubbers in the e!ectiveness of the treatment with sulfuric acid

Carmen M. Cepeda-JimeH nez, M. Mercedes Pastor-Blas, T.P. FerraH ndiz-GoH mez,J.M. MartmHn-MartmHnez*

Adhesion & Adhesives Laboratory, Department of Inorganic Chemistry, University of Alicante, 03080 Alicante, Spain

Accepted 22 November 2000

Abstract

Three thermoplastic block styrene}butadiene}styrene (TR) rubbers were treated with sulfuric acid to improve their adhesion topolyurethane adhesives. T-peel test, scanning electron microscopy (SEM), contact-angle measurements (water, ethane diol), infra redspectroscopy (ATR-IR) and stress}strain experiments were used to analyze the nature of surface modi"cations produced in the rubber.The length of the treatment and the neutralization procedure (with and without ammonium hydroxide) were considered in this study.The treatment produced the sulfonation of the butadiene units of the rubber and the creation of highly conjugated C"C bonds, whichproduced a change in the color of the TR rubbers. The treatment also produced a decrease in the tensile strength and the elongation atbreak of the TR rubbers. This suggests that the treatment with sulfuric acid was not restricted to the rubber surface but also produceda bulk modi"cation. The lower the styrene content in the TR rubber, the more signi"cant modi"cations were produced on the surface.The styrene content (33}55 wt%) in three thermoplastic styrene}butadiene (TR) rubbers a!ected the extent, but not the nature of thesurface modi"cations produced by treatment with sulfuric acid. The H

�SO

�treatment increased the T-peel strength of S1 and S2

rubber/polyurethane adhesive joints and produced a mixed failure mode (adhesion#cohesive failure in the rubber). Sulfonation ofthe TR rubbers is fast and needs only a 30 s immersion in sulfuric acid to produce high adhesion. Furthermore, the neutralization ofthe acidic surface with ammonium hydroxide is critical to assure an adequate durability of the adhesive joints. � 2001 ElsevierScience Ltd. All rights reserved.

Keywords: A. Polyurethane; B. Surface treatment; B. Rubbers; C. Infrared spectra

1. Introduction

Thermoplastic rubbers (block styrene}butadiene}styrene copolymers * TR) are widely used in footwear,adhesives manufacturing, molded or extruded goods andas modi"ers for asphalt and thermoplastic resins. How-ever, due the non-polar nature of these rubbers, pooradhesion is produced with polyurethane adhesives. Inconsequence, a surface treatment is required to chemic-ally modify the rubber surface and produce suitablejoints.

Several surface treatments for TR rubbers have beenproposed. Halogenation with organic chlorine donors(trichloroisocyanuric acid or chloramine) improves thewettability of these rubbers and enhances their adhesion

[1]. Cyclization (i.e. treatment in sulfuric acid) has beenproposed as an alternative to the halogenation [2]. Bothtreatments have provided excellent and durable ther-moplastic rubber/polyurethane adhesive joints. How-ever, previous studies [3] have shown that the nature ofthe rubber may a!ect the extent of the chemical modi"ca-tions produced on the rubber after surface treatment. Forinstance, TR rubbers with a low polybutadiene contentare very di$cult to adhere even after a chlorinationsurface treatment with trichloroisocyanuric acid (TCI)[4]. In this case, the joint strength can be increased usinga chloramine as a chlorine donor [5]. If the TR rubberhas a relatively high polybutadiene content, treatmentwith TCI is e!ective [6]. On the other hand, when evalu-ating adhesion strength by means of a peel test, unex-pected trends in the joint strength or in the locus offailure sometimes occur. This may be the consequence ofthe dependence on both, the styrene content and themechanical properties of the TR rubber [1].

0143-7496/01/$ - see front matter � 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 1 4 3 - 7 4 9 6 ( 0 0 ) 0 0 0 4 8 - 8

Table 1Properties of the TR rubbers�

Rubber Oil content (phr)� Styrene block (wt%) Tensile strength (MPa) Hardness (3Shore A)

S1 45 33 13 49S2 50 44 14 72S3 40 55 21 88

�Data provided by REPSOL QUIMICA.�Data are expressed in parts per hundred of rubber (phr).

Cyclization is often used for bonding rubber to metalwith epoxy or phenol}formaldehyde adhesives and ap-pears to be more successful with unsaturated diene rub-bers. The treatment of natural rubber with sulfuric acidproduces cyclization of the surface polymer chains andyields a brittle surface layer that, when #exed, developsmicrocracks, which are believed to act as centers formechanical interlocking of the adhesive to the rubber[2,7]. Most polymers that contain tertiary C}H bondsare suitable for sulfonation [8]. During sulfonation withsulfuric acid, the hydrogen of a C}H bond is removedand replaced by a SO

�molecule, which is then hydro-

genated to form a sulfonic acid at the site of attachment.The sulfonic acids created during sulfonation can beneutralized to create more stable species. The neutraliza-tion can be accomplished with ammonium hydroxide,which extracts the hydrogen from the sulfonic acid andleave a stabilized ion pair (}SO�

�NH�

�) [8].

As an alternative treatment to halogenation, cycliz-ation of TR rubbers provide promising results [9]. Thetreatment of SBS rubbers in sulfuric acid involves severalstages, which determine the treatment e!ectiveness (e.g.time of immersion in sulfuric acid, neutralization of theresidual acidity on the rubber surface, etc.). On the otherhand, the nature and formulation of the TR rubbersmight a!ect the extent of the surface modi"cationsproduced by the treatment with sulfuric acid. In thisstudy, three oil-extended TR rubbers with di!erentstyrene content have been used and the surface modi"ca-tions produced and their adhesion properties will beconsidered.

2. Experimental

2.1. Materials

Three TR rubbers (S1, S2, S3) provided by REPSOLQUIMICA (Santander, Spain) were used in this study.The rubbers have di!erent styrene content (33}55 wt%)and also contains 40}50 wt% oils in their formulation.Those oils were added during the processing to facilitatethe manufacture of the TR rubbers and will probablya!ect their adhesion properties. According to Table 1, theincrease in the styrene content in the rubber formulationresults in an increase in tensile strength and Shore

A hardness. The TR samples were prepared by injectionof the polymer into a heated mold at 1503C. Samples of30 mm width, 150 mm length and 4 mm thickness wereobtained.

The treatment of the TR rubbers with sulfuric acid wascarried out following di!erent consecutive steps: (i) im-mersion of the rubber in concentrated sulfuric acid; (ii)removal of the TR rubber from the acid and allowed tocontinue to react in air for 1 min; (iii) neutralization usinghot distilled water and/or ammonium hydroxide (15 wt%),followed by exhaustive washing with distilled water; theneutralization was carried out until a pH"7 was ob-tained in the washing water; (iv) dry o! the treated rubberunder infrared radiation at moderate temperature (lowerthan 603C) for 30 min.

To determine the adhesion strength of surface-treatedTR rubbers, two identically treated specimens werejoined using an adhesive based on a one-componentthermoplastic polyester urethane (Desmocoll 510)provided by Bayer AG (Leverkusen, Germany). Theadhesive solution was prepared by mixing 18 wt%polyurethane in 2-butanone; a Brook"eld viscosity of1.7$0.1 Pa s (203C) was obtained. The dried adhesive"lm on the TR surfaces was 0.4 mm thick (about 100 mgpolyurethane adhesive solution). The strength of thejoints was obtained from T-peel tests.

2.2. Experimental techniques

2.2.1. Contact-angle measurementsThe wettability of the treated TR surfaces was ob-

tained using contact-angle measurements. The surface-treated rubber pieces were placed into the thermostatedchamber of a RameH Hart 100 goniometer. The chamberwas previously saturated with the vapor of the test liquidat 253C for at least 10 min before placing the drop of theliquid on the rubber surface. The contact-angle values onthe surface-treated rubbers were measured 15 min afterplacing 4 �l drops of bidistilled deionized water or ethanediol on the sample. The error in the measurementswas$23.

2.2.2. Attenuated total multiple reyection-fouriertransform infrared spectroscopy (ATR-FTIR)

The ATR-IR spectroscopy allows the analysis of about5 �m depth of the rubber surface. The ATR-IR spectra of

162 C.M. Cepeda-Jime&nez et al. / International Journal of Adhesion & Adhesives 21 (2001) 161}172

Fig. 1. Variation in the contact-angle values (253C) of H�SO

�-treated

S1, S2 and S3 rubbers as a function of the immersion time in sulfuricacid.

the treated rubber samples were obtained using a NicoletFTIR 205 spectrometer. To avoid deep penetration of theIR radiation into the sample, the attenuated total mul-tiple re#ection method was employed (ATR-IR spectro-scopy), a KRS-5 (thallium bromo-iodide) crystal wasused. 200 scans were recorded and the resolution was4 cm��.

2.2.3. SEMScanning electron microscopy (SEM) allowed the ex-

ternal modi"cations on the rubber surfaces to be ana-lyzed. A JEOL SEM JSM 840 system was used. Thesamples were secured on copper mounts using silverpaint and coated with gold.

2.2.4. Stress}strain measurementsThe mechanical properties of the treated rubber sam-

ples were measured in an Adamel-L'Homargy DY-32 testinstrument. All experiments were carried out usingdumb-bell test pieces 2$0.5 mm thick and 4$0.5 mmwide (in the center of the test sample) and following thetest procedure given in the ISO 37}77 standard; a pullingrate of 0.1 m/min was used. The tensile strength and theelongation at break were measured. The values obtainedwere the average of at least three experimental deter-minations. The error in the measurements was less than15%.

2.2.5. T-peel strength measurementsT-peel strength measurements were carried out for

adhesive joints produced with similarly treated strip testpieces (150 mm�30 mm�4 mm). The polyurethane ad-hesive solution was applied using a brush. After allowingthe solvent to evaporate for 1 h, a PU "lm was obtained.This PU "lm was melted at 1003C under infrared radi-ation. Afterwards, the melted PU "lms on the two stripswere then placed in contact under a pressure of 0.8 MPafor 10 s to achieve a suitable joint. The T-peel strengthwas measured in an Adamel-L'Homargy DY-32 testinstrument (peeling rate"0.1 m/min). The values ob-tained were the average of three replicates (standarddeviation was less than 1 kN/m). The adhesive joints weremaintained at 253C and 50 wt% relative humidity beforeundergoing the T-peel test. The T-peel strength valueswere monitored for di!erent time (15 min}72 h) after jointformation. In order to more precisely assess the locus offailure of the joints, the failed surfaces were analyzedusing ATR-IR spectroscopy and SEM.

3. Results and discussion

The in#uence of the immersion time (t�) in concen-

trated (95 wt%) sulfuric acid on the properties of thethree TR rubbers was studied. The neutralization wasperformed using ammonium hydroxide solution, fol-

lowed by extensive washing with distilled water (up topH"7), and "nally the surface was dried under IRradiation.

Fig. 1 shows the contact-angle values obtained usingwater and ethane diol as test liquids on the treated S1, S2and S3 rubber surfaces. The as-received TR rubbers showhigh contact-angle values (higher than 703 with ethanediol, and higher than 903 with water) due to the poorwettability of these non-polar rubbers. The high con-tact-angle values can be explained using ATR-IR spec-troscopy. The ATR-IR spectra of the as-received S1, S2and S3 rubbers (Fig. 2) show the presence of "CHstretching at 3300 cm��, C}H stretching at 2920 and2850 cm��, }CH

�scissoring at 1452 cm��, }CH

�twist at

1380 cm��, and trans 1,4}C"C stretching band at968 cm��. The CH

�bands correspond to both the bu-

tadiene and oils. The presence of oils on the surfacejustify the low values of contact angle. Furthermore,some styrene absorption bands appear at 696 and749 cm�� (C}H out-of-plane deformation of vinylgroup), 1601 cm�� (aromatic C}C stretching) and3034 cm�� (aromatic C}H stretching). The relative inten-sity of the butadiene bands at 968, 2850 and 2920 cm��

decreases by increasing the styrene content in the rubber.The treatment of S1 rubber with H

�SO

�produced

a noticeable decrease in contact-angle values (Fig. 1). The

C.M. Cepeda-Jime&nez et al. / International Journal of Adhesion & Adhesives 21 (2001) 161}172 163

Fig. 2. ATR-IR spectra of the as-received S1, S2 and S3 rubbers.

increase in styrene content in the TR rubber results ina lower increase in wettability on the sulfuric acid-treatedrubber surfaces, i.e. S2 and S3 rubbers (whose styrenepercentage is 44 and 55 wt%, respectively) show a lessmarked decrease in contact-angle values than for S1rubber (33 wt%). The increase in the immersion time (t

�)

does not produce further noticeable variations incontact-angle values for any of the TR rubbers. There-fore, a short immersion time is enough to increase thewettability of the TR rubbers.

The noticeable decrease in contact-angle values on TRrubbers after treatment with H

�SO

�can be ascribed to

severe modi"cations in the chemistry and morphology ofthe surfaces. The variations in surface chemistry wereobtained using ATR-IR spectroscopy. According to pre-vious studies [10}12], the treatment with H

�SO

�is ex-

pected to produce severe modi"cations on the TR rubbersurface, mainly the formation of sulfonic acid by sulfona-tion of the rubber surface. Strong absorption in the1200}1000 cm�� region is indicative O"S"O stretchingof sulfonic acid. The ATR-IR spectra (Fig. 3a) show twonew bands at 1040 and 1167 cm�� after treating the TRrubbers with H

�SO

�, which correspond to the formation

of sulfonic acid moieties. Furthermore, there is a newband at 1350 cm�� due to O"S"O linkages which doesnot correspond to a sulfonic acid, and may correspond toa sulfonate (}C}SO

�}OR}). On the other hand, the treat-

ment with sulfuric acid produces a noticeable decrease inthe relative intensity of C}H stretching (2850, 2920 cm��)and C"C stretching (968 cm��) absorption of butadiene.The decrease is more important in the ATR-IR spectra ofS1 rubber (the one with the highest butadiene content).On the other hand, the styrene unit in the TR rubberdoes not seem to be sulfonated, considering that nomodi"cation of the styrene bands (696, 749, 1601,3034 cm��) is produced after treatment with sulfuric acid.

The treatment with sulfuric acid modi"es the color ofthe TR rubber surface (from white in the as-receivedrubbers to yellow-yellowish brown in the acid-treatedrubbers), which is consistent with the creation of highlyconjugated C"C unsaturated bonds [8,13]. The ATR-IRspectra of the TR rubbers treated with sulfuric acid(Fig. 3a) show new broad and relative low intense bandsat 1600}1700 cm��; the relative intensity of these bandsis more important in the rubber with low styrene content(S1).

The increase in the length of the treatment with sulfur-ic acid enhances the chemical modi"cations produced onthe rubber surface (Fig. 3b). A more marked decrease inthe C}H stretching absorption of CH

�, CH

�units in

butadiene is found, which is more important in the rub-ber with the lowest styrene content (S1).

Scanning Electron Microscopy (SEM) is a powerfultechnique to show the morphological modi"cations ofthe rubber surface produced by the treatment with sulfur-ic acid. Fig. 4a shows as a typical example the SEMmicrograph corresponding to the as-received S3 rubberwhich is quite homogeneous; some small white particlesappears on the surface which could be contaminants. Thesurface morphology of the TR rubbers drastically cha-nges after treatment with sulfuric acid, although the ex-tent of the modi"cations depends on the styrene contentin the rubber (Fig. 4b). Therefore, big and deep cracksappear on the S1 surface, showing strong degradation bythe acid. Lower degree of degradation is produced in thetreated S2 rubber, and some holes and surface heterogen-eity are obtained. However, the surface of the S3 rubberseems not to be degraded in any extent, indicating a lackof morphological modi"cation by the treatment withsulfuric acid. Therefore, the styrene content of the TRrubber may a!ect the degree of external degradation bythe sulfuric acid.


Fig. 3. (a) ATR-IR spectra of the H�SO

�-treated S1, S2 and S3 rubbers. Immersion time"0.5 min. (b) ATR-IR spectra of the H

�SO

�treated S1, S2

and S3 rubbers. Immersion time"2 min.

The degree of surface degradation may a!ect the mech-anical properties of the treated TR rubbers. The tensilestrength and elongation-at-break of the as-received andthe treated TR rubbers were obtained from stress-strainmeasurements (Fig. 5). The increase in styrene contentproduces an increase in the tensile strength of the as-received rubber. Fig. 5 shows the decrease in tensilestrength and elongation-at-break of the three TR rubbersafter treatment with H

�SO

�, which is more pronounced

as the butadiene content in the SBS rubber increases.This might be due to a hardening of the rubber surfaceproduced by sulfuric acid, because a noticeable decreasein tensile strength corresponds to a relatively reduceddecrease in elongation-at-break. High tensile strength isobtained in S3 rubber immersed in sulfuric acid for 2 min,as a consequence of the stability of this rubber against the

acid-degradation provided by its high styrene content.These "ndings are in agreement with the lack of mor-phological modi"cations in the sulfuric acid-treated S3rubber surface (SEM micrographs in Fig. 4b). The in-crease in the length of the treatment produces onlya slight decrease in both tensile strength and elongation-at-break of the three SBS rubbers.

Adhesion properties were measured from T-peel testsof sulfuric acid-treated TR rubber/polyurethane adhesivejoints performed 15 min (immediate adhesion) and 72 h("nal adhesion) after joint formation (Fig. 6). T-peel testsof S3 rubber/adhesive joints resulted in the breaking ofthe rubber strip pieces due to the high tensile strength ofthe treated S3 rubber, so it was not possible to obtain theT-peel strength values for S3 rubber joints. Thus, Fig. 6only shows the peel strength values for S1 and S2 rubber


Fig. 4. (a) SEM micrograph of the as-received S3 rubber. (b) SEMmicrophotographs of the H

�SO

�-treated S1, S2 and S3 rubbers. Im-

mersion time"2 min.

Fig. 5. Variation of the tensile strength (MPa) and elongation at break(%) of the as-received and H

�SO

�-treated S1, S2 and S3 rubbers as

a function of the immersion time.

joints. The treatment with sulfuric acid of both S1 and S2rubbers produces an increase in the immediate and "naladhesion. The increase in the length of treatment resultedin a slight decrease of peel strength values likely due tothe degradation of the rubber surface produced by thetreatment. It has been reported that the cohesive strengthof the outermost surface layer becomes the limiting factorin the adhesion of thermoplastic rubbers [14}17]. In allcases, the "nal adhesion is higher than the immediateadhesion, and similar trends are obtained for both rub-bers. On the other hand, higher adhesion is obtained inthe treated S2 rubber/polyurethane adhesive joints thanin the S1 rubber/polyurethane adhesive joints, probablydue to the reduced mechanical properties and the higherdegree of surface degradation in the treated S1 rubber.

The di!erences in peel strength values may also a!ectthe locus of failure of the joints. In general, by visualinspection, the locus of failure of the joints was ad-hesional i.e. one of the failed specimens corresponds tothe rubber (R surface) and the other to the polyurethane(A surface). In order to more precisely assess the locus offailure of the joints, the ATR-IR spectroscopy was usedin this study. Fig. 2 shows the ATR-IR spectra of the


Fig. 7. ATR-IR spectrum of the polyurethane (PU) adhesive "lm.

Fig. 6. T-peel strength values of the H�SO

�-treated S1 and S2 rub-

bers/polyurethane adhesive joints as a function of the immersion time.

as-received S1 and S2 TR rubbers, and the ATR-IRspectrum of the polyurethane (PU) is given in Fig. 7. TheATR-IR spectra of the raw materials show di!erent typi-cal bands, which may allow distinguishing the locus offailure in the joints. The ATR-IR spectrum of the PU(Fig. 7) shows C}H stretching bands at 2939, 2860 cm��,

and other typical bands associated to the urethane group: N}H stretching (3250 cm��), N}H bending and N}C"Osymmetric stretching (1533 cm��), C"O stretching(1717 cm��), C}O stretching (1100}1200 cm��), and C}Nstretching (1250 cm��). The ATR-IR spectra of the twoTR rubbers show typical bands due to styrene (696,749 cm��) and butadiene (968 cm��). Furthermore, thebands in the TR rubbers corresponding to C}H stretch-ing at high wavenumbers are displaced about 15 cm��

to lower wavenumbers values in the rubber compared tothe PU.

The ATR-IR spectra of the R surfaces of the as-re-ceived TR rubber/PU adhesive joints are quite similar tothose corresponding to the as-received TR rubbers(Figs. 8a and b, respectively). The ATR-IR spectra of thefailed A surfaces mainly show the bands correspondingto the PU and a few corresponding to the rubber (749,968 cm��). This is consistent with an adhesional failurewhich also contain some rubber on the A surface (mixedfailure). Considering the relatively low mechanical prop-erties of S1 and S2 rubbers, it seems that thin externalrubber pieces are transferred from the rubber surface tothe PU surface during the peel test.

The treatment with sulfuric acid modi"es the locus offailure in the joints. For the peel test measured 15 minafter joint formation, the ATR-IR spectrum of the failedR surface of the joint of S1 rubber (Fig. 9a) correspondsto the treated S1 rubber before the joint was produced(Fig. 2), but more intense O"S"O band and less intensebutadiene band (968 cm��) appear. The ATR-IR spec-trum of the failed A-surface (Fig. 9a) shows the absorp-tion bands of both the PU and S1 rubber. Therefore,a mixed failure mode is obtained and during the peel testa thin layer of rubber is transferred from the S1 rubber tothe PU surface. When the peel test is performed 72 h afterjoint formation, the ATR-IR spectrum of the failed A sur-face (Fig. 9b) also shows bands due to the S1 rubber


Fig. 8. ATR-IR spectra of the failed surfaces of as-received TR rubber/PU adhesive joints, 72 h after joint formation; (a) S1 rubber; (b) S2 rubber.

indicating a similar kind of failure than in the 15 minjoint formation although more S1 rubber is transferred tothe PU surface.

The S2 rubber has higher tensile strength and showsa lower degree of degradation after treatment with sulfur-ic acid than the S1 rubber. Therefore, the locus of failurein the joints produced with S2 rubber could be di!erent.For the peel values obtained 15 min after joint formation,the ATR-IR spectrum of the failed R surface (Fig. 10a)corresponds to the treated S2 rubber before the joint isproduced, but the ATR-IR spectrum of the A failedsurface (Fig. 10a) shows the bands of the PU and the S2rubber. Therefore, a mixed failure, similar to that for theS1 rubber joint, is produced, although the relative

amount of rubber transferred to the PU surface is lessimportant. A similar situation is obtained in the peeledspecimens 72 h after joint formation (Fig. 10b). Conse-quently, similar locus of failure is obtained in the jointsproduced with treated S1 and S2 rubber, although theamount of rubber transferred to the PU surface duringthe peel test is less important with the S2 rubber, due toits better mechanical properties.

3.1. Neutralization with and without ammonia

It has been established [9] that the use of ammonia toneutralize the sulfuric acid-treated rubber surface is a keyaspect to obtain an adequate adhesion performance in


Fig. 9. ATR-IR spectra of the R and A failed surfaces of H�SO

�-treated S1 rubber/PU adhesive joints. Immersion time"0.5 min (a) Joint peeled

15 min after formation. (b) Joint peeled 72 h after formation.

vulcanized rubber joints. The neutralization usingammonia extracts the hydrogen from the sulfonic acidmoieties produced during the immersion of the rubberin sulfuric acid and leaves a stabilized ion pair(-C}SO�

�NH�

�). Therefore, the neutralization of the three

TR rubbers treated with H�SO

�was carried out using

two di!erent procedures: (i) Immersion in hot waterfollowed by extensive washing with distilled water untilneutral pH in the rubber surface; (ii) Immersion in hotwater followed by neutralization in 15 wt% NH

�OH

solution, and a "nal extensive washing in distilled water.In this study, these two neutralization procedures will bereferenced as `withouta and `withaNH

�OH, respectively.

Fig. 11 shows the variation in the ethane diol andwater contact-angle values of the TR rubbers treatedwith H

�SO

�without NH

�OH as a function of the im-

mersion time in sulfuric acid. Lower contact-angle valuesare obtained compared to those measured when theneutralization is performed with NH

�OH (Fig. 1). The

increase in the styrene content produces a slight decreasein the wettability of the rubber surface. As for the rubberneutralized using ammonia, the increase in immersiontime does not produce noticeable changes in contact-angle values of the three rubbers.

The ATR-IR spectra of the three rubbers treated withsulfuric acid without NH

�OH (Fig. 12) show similar


Fig. 10. ATR-IR spectra of the R and A failed surfaces of H�SO

�-treated S2 rubber/PU adhesive joints. Immersion time"0.5 min (a) Joint peeled

15 minutes after formation. (b) Joint peeled 72 h after formation.

features than those neutralized using ammonia (Fig. 3a),although the extent of modi"cation is higher when theneutralization is produced without ammonia. The di!er-ences are more pronounced with the S1 rubber, i.e. theTR rubber with high butadiene content. Furthermore,the ATR-IR spectrum of the three TR rubbers withoutNH

�OH shows }OH stretching absorption at

3250}3500 cm��. The relative intensity of this band be-comes less relevant as the styrene content in the TRrubber increases.

The T-peel strength values of H�SO

�-treated S1 and

S2/polyurethane adhesive joints are shown in Fig. 6

(neutralization with NH�OH) and Fig. 13 (neutralization

without NH�OH). The immediate adhesion (15 min after

joint formation) is, in general, similar or higher if theammonia is not used for neutralization; lower peelstrength is obtained only for high immersion time. Acohesive failure in the rubber (assessed by visual inspec-tion) was obtained in those joints formed after immersiontimes lower than 2 min. Therefore, the interlocking be-tween the PU and the TR rubber surface is, in general,su$cient for both TR rubbers and di!erent immersiontime in sulfuric acid. However, the "nal adhesion (72 hafter joint formation) dramatically decreases (peel values


Fig. 12. ATR-IR spectra of the H�SO

�-treated S1, S2 and S3 rubbers. Immersion time"0.5 min. Neutralization without NH

�OH.

Fig. 11. Variation of the contact-angle values of the H�SO

�-treated S1,

S2 and S3 rubbers as a function of the immersion time in sulfuric acid.Neutralization without NH

�OH. Fig. 13. T-peel strength values of H

�SO

�-treated TR rubber/PU ad-

hesive joints as a function of the immersion time in sulfuric acid.Neutralization without NH

�OH.

are near zero in most of the joints), mostly for immersiontime in sulfuric acid higher than 1 min. This decrease inpeel strength can be ascribed to a degradation of thepolyurethane by the remaining acidic moieties on the


Fig. 14. SEM micrographs of the failed surfaces of the H�SO

�-treated

S1 rubber/PU adhesive joint (72 h after joint formation). Immersiontime"1 min. Neutralization without NH

�OH.

treated rubber surface which was not neutralized withammonia. The SEM micrographs of failed surfaces corre-sponding to the joint produced with S1 rubber treatedwith sulfuric acid for 1 min followed by neutralizationwithout NH

�OH (Fig. 14), show a highly degraded

A surface. Consequently, the removal of the acidic moie-ties from the rubber surface with ammonia is mandatoryto prevent degradation of the polyurethane after jointformation and to produce adequate adhesive joints.

4. Conclusions

The treatment with sulfuric acid modi"es the surfacechemistry of three TR rubbers having di!erent styrene

content. Sulfonation mainly a!ects the butadiene in therubber and conjugated C"C bonds are created, which areresponsible for the change in color of the treated rubbers.Furthermore, a noticeable improvement in wettability isproduced and some cracks (depending on the styrenecontent in the rubber) are created on the surface.The treatment a!ects not only the surface, but thebulk rubber producing a noticeable decrease in tensilestrength without a signi"cant loss in elongation-at-break. The increase in styrene content in the rubberprovides higher resistance against acid degradation.The treatment with sulfuric acid increases the immediateand the "nal adhesion of the adhesive joints. Theincrease in the styrene content enhances the mechanicalproperties of the rubber, and as a consequence, thestrength of the joints is improved. Neutralization withammonium hydroxide is critical to assure adequateadhesive joints.

Acknowledgements

Financial support from CICYT (Project MAT98-0611)is gratefully acknowledged. The authors thanksREPSOL QUIMICA (Santander, Spain) for providingthe TR pellets and INESCOP (Footwear ResearchInstitute, Elda, Spain) for the mold injection of the TRpellets.

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Influence of the styrene content of thermoplastic styrene–butadiene rubbers in the effectiveness...

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