Interactions at the interface between thermoplastic rubber and polychloroprene adhesive

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Research ArticleReceived: 30 April 2007 Revised: 16 November 2007 Accepted: 21 December 2007 Published online in Wiley Interscience: 1 February 2008

(www.interscience.com) DOI 10.1002/sia.2770

Interactions at the interface betweenthermoplastic rubber and polychloropreneadhesiveM. Virtudes Navarro-Banon, M. Mercedes Pastor-Blas∗and Jose Miguel Martín-Martínez

The interface produced between a chlorinated thermoplastic styrene-butadiene-styrene rubber and a polychloroprene (PCP)adhesive has been studied and compared to the interface produced using a polyurethane (PU) adhesive. Chlorination of therubber was produced by spin coating solutions of trichloroisocyanuric acid in methyl ethyl ketone. The adhesive solution wasspin coated on to the chlorinated rubber and the interface between the chlorinated rubber and the adhesive was analyzed byinfrared spectroscopy.

Chlorination of the rubber produces cross-linking of the outermost chlorinated and oxidized rubber surface, which becomesinsoluble in toluene. The chlorinated rubber chains are able to migrate through the chlorinated rubber/adhesive interfaceand produce a cross-linked interface. Similar interfaces are obtained with PU or PCP adhesive. However it is the addition ofa thermoreactive phenolic tackifier resin to the PCP adhesive, which imparts appropriate rheological properties to the PCPadhesive, responsible for the increased adhesion properties. Copyright c© 2008 John Wiley & Sons, Ltd.

Keywords: SBS rubber; halogenation; surface treatment; polychloroprene; polyurethane; adhesive; adhesion; interface; rheologicalproperties

Introduction

Previous studies[1 – 4] showed that chlorination of a thermoplas-tic styrene-butadiene-styrene (SBS) rubber with a solution oftrichloroisocyanuric acid in methyl ethyl ketone (3 wt% TCI/MEK)increases the SBS rubber surface energy from 41.2 to 51.2 mJ/m2,mainly due to an increase of the polar component of the surfaceenergy which increases from 1.2 to 10.7 mJ/m2, (the surface energywas evaluated by the Owens and Wendt[5] approximation fromcontact angle measurements using water and diiodemethane asprobe test liquids). The increase of the polarity of the rubbersurface is a consequence of the creation of chlorinated and oxi-dized moieties on the rubber surface, which were assessed by XPSand IR-ATR spectroscopies.[4] Besides, the chlorinating treatmentintroduces surface roughness (assessed by SEM). As a result, theadhesion properties of the SBS rubber surface are expected to beimproved.

Traditionally, polyurethane (PU) adhesives are used to joinchlorinated rubber to leather in applications such as footwearmanufacturing. However, PU adhesives need reactivation byheating under infrared radiation prior to bonding. This adhesivereactivation is a time consuming process and temperature mustbe carefully controlled. On the other hand, polychloroprene(PCP) adhesives have longer open time than PU adhesives, soreactivation of the adhesive is not necessary. Thus, PCP adhesivescould be proposed as an alternative to PU adhesives.

Although it was believed that an incompatibility between thechlorinated rubber surface and the PCP adhesive was responsiblefor the lack of adhesion between them,[5] recent studies[6,7]

have shown that in some cases suitable adhesion propertiesare obtained using some PCP adhesives, the nature of the tackifierin the adhesive formulation being a key factor.[8 – 13]

Thus, the aim of this study is to analyze the mechanismsresponsible for the improved adhesion properties between thechlorinated rubber surface and PU or PCP adhesive. Consideringthat adhesion is an interfacial process, the interactions producedat the interface between the surface chlorinated SBS rubberand PU and PCP adhesive will be studied. The analysis of theinterface between the rubber and the adhesive is essential tofully understand the improved adhesion of the rubber afterchlorination, and the influence of the tackifier in the PCP adhesiveformulation.

Experimental

Materials

The thermoplastic SBS rubber used in this study was obtainedfrom Kraton D-4270CS pellets, an oil-extended (45 phr nonstainingparaffin oil) radial block SBS copolymer, which contains 10 wt% ofcalcium carbonate as inorganic filler. Table 1 shows some typicalcomposition and properties of Kraton D-4270CS rubber. TheSBS rubber test samples (25-mm width, 150-mm length, 3-mmthick) were prepared by Synthelast, S.A. (Elche, Alicante, Spain) byinjection molding in a heated mold (150 ◦C) and contain 10 wt%calcium carbonate as filler and 0.3 wt% phenolic antioxidant.

∗ Correspondence to: M. Mercedes Pastor-Blas, Department of Inorganic Chem-istry, University of Alicante, 03080 Alicante, Spain.E-mail: [email protected]

Department of Inorganic Chemistry, University of Alicante, 03080 Alicante,Spain

Surf. Interface Anal. 2008; 40: 107–120 Copyright c© 2008 John Wiley & Sons, Ltd.

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M. V. Navarro-Banon, M. M. Pastor-Blas and J. M. Martín-Martínez

Table 1. Some properties of Kraton D-4270 CS rubber

PropertyTEST

Method Value

Styrene content SM 32 wt%

Volatile matter SMS 2404 0.3 wt%

Ash content ISO 247-B 0.3 wt%

Total extractable SMS 2401 31.5 wt%

Density (20 ◦C) ISO 2781 0.94 g/cm3

Tensile strength ISO 37 12 mPa

Modulus 300% ISO 37 1.9 mPa

Elongation at break ISO 37 1100%

Abrasion resistance DIN 53516 100 mm3

ISO, International Organization for Standardization; ASTM, AmericaSociety for Testing and Materials; DIN, German National StandardsOrganization; SMS, Safe Minimum Standard; SM, Standard model.

The surface of SBS rubber was treated with methyl ethyl ketone(MEK) solutions containing 0.5–3 wt% trichloroisocyanuric acid(98 wt% purity, Merck): (TCI/MEK). The solution was brushed orspin coated onto the SBS rubber surfaces and allowed to react atroom temperature in air for 1 h.

Adhesive strengths were evaluated from T-peel tests on jointsproduced between the surface chlorinated SBS rubber and splitsuede leather using five different PCP adhesives. All the PCPadhesives were formulated with the same base polymer (DENKAA-90) but PCP0 adhesive did not contain any resin and theother PCP adhesives were formulated with 30 phr of one ofthe following tackifier resins: a thermoreactive phenolic resin(PCP30P), a terpene phenolic resin (PCP30TP), a glycerol esthercolophony resin (PCP30EC), and an aromatic hydrocarbon resin(PCP30AH). Table 2 shows the properties of the tackifier resinsand Table 3 shows the formulation of nonresin-containing andresin containing polychloroprene adhesives. To prepare the resin-containing PCP adhesive solutions, the resin and the metallicoxides were previously stirred in a laboratory mixer at 1000 rpmfor 30 min. After that, the PCP was added and the solution wasstirred at 2000 rpm for 1 h and later on, at 2500 rpm for three morehours. To prepare the PCP adhesive solution with no resin in itsformulation, all the ingredients were simultaneously mixed andstirred at 2500 rpm for 3 h.

The results obtained with PCP adhesives were compared withthose obtained with a solvent-based PU adhesive. To prepare thesolvent-based PU adhesive solution, 18 wt% Desmocoll 540 (Bayer,Leverkusen, Germany) PU pellets were added to MEK and stirred

Table 3. Formulation of polychloroprene adhesives

Parts perhundred

PCP – noresin

PCP-30wt% resin

PCP 100 100

MgO – 4

ZnO – 5

Antioxidant 2 2

Resin – 30

Solvents 333 431

H2O – 2

PCP, DENKA A-90; ZnO, ZINKOXYD AKTIV (Rubber Chemicals); MgO,ANSCOR Q (Britmag); Antioxidant, IRGANOX 1010 (Ciba); Solvents,45 : 20 : 35 (Hexane : toluene : acetone).

Table 4. Brookfield viscosity (spindle #3, 100 rpm, 20 ◦C) ofpolyurethane and polychloroprene adhesives

AdhesiveBrookfield

viscosity (mPa·s)Solids

content (wt%)

PU 2865 ± 60 17.2 ± 0.1

PCP 11 710 ± 90 23.4 ± 0.2

PCP-P 4180 ± 60 26.3 ± 0.3

PCP-TP 2550 ± 35 23.9 ± 0.1

PCP-EC 2465 ± 75 24.0 ± 0.1

PCP-AH 2185 ± 40 24.4 ± 0.1

at 1500 rpm for 3 h to obtain the adhesive solution. Table 4 showsBrookfield viscosity (spindle #3, 100 rpm, 20 ◦C) and solids contentof the adhesive solutions.

Split suede leather used to prepare the adhesive joints wassupplied by INESCOP (Elda, Alicante, Spain) (tensile strength =13 mPa; elongation-at-break = 60%; ash content at 950 ◦C lessthan 5 wt%).

Experimental techniques

Fourier transform infrared (FTIR) spectroscopy

The chemical modifications produced in about 3 µm depth of theSBS rubber by treatment with TCI/MEK were assessed by ATR-IRspectroscopy. A Bruker Tensor 27 FTIR spectrometer (Bruker Optik,Ettlingen, Germany) was used and a KRS-5 (thallium bromoiodide)crystal was employed. The angle of incidence of the IR beam was

Table 2. Properties of the tackifier resins used in the formulation of the polychloroprene adhesives

Properties of the tackifier resin

Polychloropreneadhesive

Tackifierresin Tackifier resin provider Softening point, (◦C)

Density(25 ◦C)

Brookfieldviscosity (mPa·s)

PCP-P SP154 SAFIC-ALCAN Division ISISA(Barcelona, Spain)

71–88 (Capillary method) 1.08–1.11 –

PCP-TP SP553 SAFIC-ALCAN Division ISISA(Barcelona, Spain)

112–118 (Ball and ring method) 1.08–1.11 –

PCP-EC RESIESTERGUM 35V

Union Resinera Espanola(Barcelona, Spain)

93 (Ball and ring method) – 170 ± 20 (200 ◦C)

PCP-AH ESCOREZ2203

Exxon Mobil Chemical(Barcelona, Spain)

93 (Ball and ring method) – 1250 (150 ◦C)

www.interscience.wiley.com/journal/sia Copyright c© 2008 John Wiley & Sons, Ltd. Surf. Interface Anal. 2008; 40: 107–120

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Interface

SBS rubber / adhesive / leather

0

1

2

3

4

5

6

7

T-p

eel s

tren

gth

(kN

/m)

A: interfacial failure

PC

P0

PC

P30

P

PC

P30

TP

PC

P30

AH

PC

P30

EC

PU

A AA

AA

(a) As-received SBS rubber

0

1

2

3

4

5

6

7

PC

P0

PC

P30

P

PC

P30

TP

PC

P30

AH

PC

P30

EC

PU

A

M

A A A

(b) 3 wt% TCI/MEK-SBS rubberM

M: cohesive failure within the rubber

A

Figure 1. T-peel strength of joints produced between (a) as-received-SBS rubber and leather joint and (b) chlorinated SBS rubber and leather usingpolyurethane (PU) and polychloroprene (PCP) adhesives. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

Abs

orba

nce

0.0

0.2

0.4

0.0

0.5

-0.0

0.1

0.2

20004000

Wavenumber (cm-1)

(a) As-received SBS rubber

(b) 3 wt% TCI/MEK-SBS rubber

(c) Leather

540

700

875

910965

1375

14502855

2925

3000

540760

10651400

14201720

28552925

10301240

145015401650

2845

29203300

1740

3025

1600

Figure 2. ATR-IR spectra of the (a) as-received and (b) chlorinated SBS rubber and (c) leather. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

45◦, and 200 scans were collected at a resolution of 4 cm−1 andaveraged.

Transmission-IR spectra of solid residues and solutions wereperformed using KBr pellets. The KBr pellets containing thesample were prepared using a Spectra Tech. Qwik Handy Press.A KBr background was collected prior to collecting the sample IRspectrum. As many as 32 scans were obtained and were averagedwith a resolution of 4 cm−1.

Dynamical mechanical thermal analysis (DMTA)

The mechanical and viscoelastic properties of the adhesives wereassessed by dynamical-mechanical-thermal analysis (DMTA) usinga Rheometric Scientific DMTA MK III equipment. The measurementswere obtained in two points bending mode, using a frequencyof 1 Hz and a strain amplitude between peaks of 32 µm. Thetemperature range was varied from −80 to 80 ◦C using aheating rate of 5 ◦C/min. Rectangular test sample pieces of(1.0–2.0 mm) × (10.0–20-0 mm) × (60.0–70.0 mm) were usedin the measurements.

T-peel tests

Adhesive joints between test pieces (150 mm × 30 mm × 3 mm)of chlorinated SBS rubber and split suede leather were producedusing a solvent-based PU adhesive and five PCP adhesives. Then,two consecutive adhesive coatings (the second coating wasapplied 10 min after applying the first one) were applied witha brush on the leather surface. On the other hand, 1 h afterchlorination, the adhesive solution was brushed on the treatedSBS rubber surface. The adhesive was allowed to dry for 45 minat room temperature. When using a PU adhesive, the dried solidPU adhesive film was melted at 80 ◦C under infrared radiation(reactivation process) and immediately placed in contact undera pressure of 0.8 mPa for 10 s to achieve a suitable joint. PCPadhesives were not reactivated under infrared radiation beforeputting the two adherents in contact under pressure (0.8 mPa for10 s). To determine the adhesive strength values of the treated SBSrubber/PU adhesive/leather joints, an Instron 4411 (Instron Ltd.,Buckinghamshire, UK) was used at a peel rate of 0.1 m/min. Theadhesive joints were tested 72 h after bond formation.

Surf. Interface Anal. 2008; 40: 107–120 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia

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0


3030

3030

1600

1610

1600

1605

1735

1460

1460

1360

1360

1385

1370

1210

1215

1240

1070

1085

875

730

730

730

1460

Abs

orba

nce

15251460

1255 11651065

PU

PCP0

PCP30P

PCP30TP

PCP30EC

PCP30AH

2915 2885 1720 1600 965 730

30252915

28551660 1445

830775 675

0.2

0.0

0.2

0.00.10.2

0.0

0.2

-0.00.10.2

-0.0

0.1

20004000

Wavenumber (cm-1)

Figure 3. ATR-IR spectra of the polyurethane (PU) and polychloroprene (PCP) adhesives before the joint formation. This figure is available in colour onlineat www.interscience.wiley.com/journal/sia.

Results and Discussion

The nonpolar nature of the as-received SBS rubber is responsiblefor the lack of adhesion toward a polar adhesive such as PU or PCP(Fig. 1(a)). In all cases, an interfacial (adhesion) failure was obtained.Chlorination with a 3-wt% TCI/MEK solution produces an increaseof T-peel strength of this rubber toward a PU adhesive (Fig. 1(b)).However, although five different PCP adhesive formulations wereconsidered, only when a thermoreactive phenolic tackifier resinwas added to the PCP adhesive (PCP30P), adhesion was improved.

The ATR-IR spectrum of the SBS rubber (Fig. 2(a), Table 5)shows typical absorption bands due to polybutadiene ( C–Hstretching at 3000 cm−1, C–H stretching at 2925 and 2855 cm−1,C–H bending at 1450 cm−1, C–H symmetrical bending at1375 cm−1, and C–H out-of-plane bending of 1,4-trans C Cat 965 cm−1) and polystyrene (stretching of aromatic C–H at3025 cm−1, out-of-plane bending of aromatic C–H at 910 and700 cm−1) units. There is also O–C–O bending absorption at875 cm−1 of calcium carbonate in the rubber formulation and abroadening of the band of C–H bending at 1450 cm−1 due tothe contribution of the absorption of COO− symmetric stretchingof calcium carbonate. The treatment of the SBS rubber with a 3-wt% TCI/MEK solution (Fig. 2(b), Table 5)) introduces oxygenated[C O stretching at 1720 cm−1, C–H (–CH2CO) bending at1400 cm−1 and C–O stretching at 1065 cm−1] and chlorinated[C–H (–CH2Cl) bending at 1420 cm−1 and C–Cl stretching at760 and 540 cm−1] moieties on the rubber surface. The relativeintensity of bands from polybutadiene (2925, 2855, and 965 cm−1)becomes lower compared to polystyrene bands, indicating thatchlorination of double bonds of polybutadiene is produced.Besides, C O stretching (1740 cm−1) and N–C O symmetricstretching (1600 cm−1) bands appear, which are probably dueto the TCI and/or isocyanuric acid crystals, reaction by-productbetween TCI and rubber, deposited on the treated rubber surface.The ATR-IR spectrum of the split suede leather used in the adhesivejoints (Fig. 2(c), Table 5) shows several bands due to collagen (themain constituent of animal skin): N–H and C–N bands (N–Hstretching at 3300 cm−1, C–N stretching and N–H bending at

1540 cm−1, C–N bending at 1240 cm−1), methylene bands (C–Hstretching at 2920 and 2845 cm−1, C–H bending at 1450 cm−1),and ester bands (C O stretching at 1650 cm−1, C–O stretching at1030 cm−1).

On the other hand, the ATR-IR spectrum of the PU film (Fig. 3,Table 5) shows typical bands due to N–H stretching at 3370 cm−1,C–H stretching at 2915 and 2885 cm−1, C O stretching dueto urethane at 1720 cm−1, aromatic C–aromatic C stretching at1600 cm−1, N–CO stretching, N–H bending, and C–N stretchingdue to urethane at 1525 cm−1, C–H bending at 1460 cm−1, C–Nstretching and N–H bending at 1255 cm−1, C–O–C stretching at965, 1065, and 1165 cm−1 and N–H out-of-plane deformation at730 cm−1.

ATR-IR spectra of the PCP adhesives (Fig. 3, Table 5) show thefollowing absorption bands: C–H stretching of aromatic rings(3025 cm−1), C–H stretching (2914, 2859 cm−1), C C stretching(1665 cm−1), C–H bending (1440 cm−1), out-of-plane bending

C–H (769, 677 cm−1), C–Cl stretching (825 cm−1). The additionof a tackifier resin to the PCP formulation results in the presence oftypical resin absorptions (Table 6). Thus, ATR-IR spectrum PCP30PPCP adhesive (Fig. 3, Table 5) also shows absorption from thethermoreactive phenolic resin (Table 6): aromatic C–aromatic Cstretching (1600 cm−1), asymmetric C–H bending (1460 cm−1),CO–H bending and symmetric C–H bending (1360 cm−1), out-of-plane C–O stretching (1210 cm−1), asymmetric C–O–C stretching(1070 cm−1), and symmetric C–O–C stretching and aromaticC–H out-of-plane bending (875 cm−1). Absorption from terpenephenolic resin appears in PCP30TP adhesive spectrum at 3030,1610, 1460, 1360, 1215, and 730 cm−1 (Table 6 for bandsassignment); bands from glycerol esther colophony at 1735, 1600,1460, 1385, 1240, and 730 cm−1 are shown on PCP30EC spectrum;and absorption from aromatic hydrocarbon resin is present inPCP30AH spectrum at 3030, 1605, 1370, 1085, and 730 cm−1.

ATR-IR spectroscopy of the failed surfaces was used to determinethe locus of failure of the joints. Thus, spectra of the failed surfacethat visually corresponded to the rubber and the failed surfacethat visually corresponded to the leather after the T-peel testwere compared to the spectra of the as-received (Fig. 2(a)) and


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Interface

Table 5. ATR-IR absorption peaks of as-received and chlorinated SBSrubber, leather, and PU and PCP adhesives

MaterialWavenumber

(cm−1) Absorption

As-received SBSrubber

3025 st arC–H

3000 st C–H

2925, 2855 st C–H

1450 δ as C–H, sy st COO−

1375 δ sy C–H

965 δ oop C C

875 δ O–C–O

910, 700 δ oop arC–H

Chlorinated SBSrubber

1740 st sy (N–C O)

1720 st C O

1600 st N–C O

1420 δ C–H (–CH2Cl)

1400 δ C–H (–CH2CO)

1065 st C–O

760, 540 st C–Cl

Leather 3300 st N–H

2920, 2845 st C–H

1650 st C O

1540 st C–N, δ N–H

1450 δ C–H

1240 δ C–N

1030 st C–O

873 oop δ N–H

PU 3370 st N–H

2915, 2885 st C–H

1720 st C O

1600 st arC–arC

1525 st N–C O, δ N–H, st C–N

1460 δ C–H

1255 st C–N, δ N–H

1165, 1065, 965 st C–O–C

730 oop δ N–H (HNC O)

PCP 3025 st C–H

2914, 2859 st C–H

1665 st C C

1440 δ C–H

825 st C–Cl

769, 677 oop δ = C–H

st, stretching; δ, bending; γ , rocking; oop, out of plane; as, asymmetric;sy, symmetric; ar, aromatic.

chlorinated rubber (Fig. 2(b)), the leather (Fig. 2(c)), and the PUand PCP adhesives (Fig. 3). ATR-IR spectra of the failed surfacesafter the T-peel test of chlorinated SBS rubber/leather joints (Fig. 4)using a PCP adhesive showed that only when a thermoreactivephenolic resin is added to the formulation of the PCP30P (Fig. 4(c)),a cohesive failure (Fig. 5(b)) within the chlorinated SBS rubber wasproduced, similar to the failure produced with the PU adhesive(Fig. 4(a)). Thus, with PU and PCP30P adhesives, both failed surfacesshowed absorption from chlorinated SBS rubber, and ATR-IRspectra were similar to the chlorinated SBS rubber spectrumin Fig. 2(b). However, when the PCP adhesive with no resin in itsformulation or with any other tackifier resin different from the

Table 6. ATR-IR absorption peaks of the resins in the polychloropreneadhesives

ResinWavenumber

(cm−1) Absorption

Thermoreactivephenolicresin

3330 st OH

(P) 3030 st C–H

2960, 2925, 2875 st C–H

1600 st arC–arC

1490 δ as C–H

1360 δ CO–H; δ sy C–H

1210 st oop C–O

1070 st as C–O–C

875 st sy C–O–C, δ oop arC–H

Terpenephenolicresin

3030 st C–H

(TP) 2950, 2925, 2870 st C–H

1610 st arC–arC

1460 δ as C–H


1215 st oop C–O

730, 700 δ oop CH

Glycerol esthercolophonyresin

3465 st OH

(EC) 2930, 2870 st C–H

1735 st C O

1600 st C C

1460 δ as C–H

1385 δ sy C–H

1240 st C O

730 δ oop CH

Aromatichydrocarbonresin

3030 st C–H

(AH) 2950, 2930, 2870 st C–H

1605 st arC–arC

1455 δ as C–H


1085, 1030 δ ip arC–H

970 δ oop CH

730, 695 δ ip arC–H

st, stretching; δ, bending; γ , rocking; oop, out of plane; as, asymmetric;sy, symmetric; ar, aromatic.

thermoreactive phenolic resin was used to perform the bondingjoints, an interfacial (adhesion) failure mode (Fig. 5(a)) was alwaysproduced and a lack of adhesion was obtained (Fig. 1). Thus, theATR-IR spectrum of the surface that visually corresponded to therubber showed bands from chlorinated rubber and the surfacethat visually corresponded to the leather showed mainly PCPadhesive absorption (Fig. 4(b)).

Chlorination of SBS rubber with TCI/MEK produces depositionof nonreacted TCI crystals on the rubber surface.[14] In order toinvestigate a possible chlorination of the adhesive by residualchlorinating TCI crystals on the rubber surface, PU and PCPadhesive films were immersed in the chlorinating solution. Thedry adhesive films were prepared by placing an adhesive solution


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Failure mode of the adhesive joint:3 wt% TCI/MEK SBS rubber / PU / leather

rubber

-0.0

0.1

0.2

0.3

Abs

-0.00

0.05

0.10

0.15

0.20

Abs

20004000

Wavenumber (cm-1)

Leather surface

Rubber surface

chlorinated rubberrubber

rubber

rubber

TCI

chlorinated rubber

TCI

rubber chlorinated rubber

chlorinated rubber

rubber

(a)

Failure mode of the adhesive joint:3 wt% TCI/MEK SBS rubber / PCP0 / leather

Rubber surface

Leather surface PCP0

0.0

0.1

0.2

Abs

0.0

0.1

0.2

Abs

20004000

Wavenumber (cm-1)

chlorinated rubber

(b)

Failure mode of the adhesive joint:3 wt% SBS rubber/ P30P/ leather

Rubber surface

Leather surface0.0

0.2

0.4

Abs

0.0

0.2

0.4

Abs

20004000

Wavenumber (cm-1)

chlorinated rubber

chlorinated rubber

rubber

rubber

rubber

rubber

(c)

Figure 4. ATR-IR spectra of the failed surfaces obtained after T-peel test of (a) chlorinated SBS rubber/PU adhesive/leather joint; (b) chlorinatedSBS rubber/PCP adhesive/leather; (c) chlorinated SBS rubber/PCP-P adhesive/leather joint. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

(18 wt%) on a Teflon mold and allowing the solvent to evaporate.The dry PU or PCP films were then immersed in 0.5 and 3wt% TCI/MEK solution for 5 s. ATR-IR spectra of the treatedadhesive films were obtained 72 h after chlorination. Figure 6shows chlorination of the PU film: C O stretching bands of bothPU (1720 cm−1) and TCI (1740 cm−1) are present, and new bands

from isocyanuric acid (by-product of the reaction of TCI withrubber) (N–H stretching at 3208, 3079 cm−1) and oxidized andchlorinated hydrocarbons (CH2C–O bending at 1400 cm−1, CH2Clbending at 1460 cm−1, and C–Cl stretching at 760 and 540 cm−1)are present. This suggests a chemical modification of the PUhydrocarbon chains.


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Interface

3 wt% TCI/MEK SBS rubber / PU or PCP30P adhesive / leatherLeather

SBS rubber

AdhesiveLeather

Chlorinated SBS rubberPeel test

SBS rubber

AdhesiveChlorinated SBS rubber

Chlorinated SBS rubber

(b)

As-received SBS rubber / adhesive / leather(a)

SBS rubber

Adhesive

Leather

Peel test

SBS rubber

Adhesive

Leather

Figure 5. (a) Interfacial (adhesion) failure mode; (b) cohesive failure mode obtained after T-peel test. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

Interactions PU / chlorinating agent

PU

PU Immersion in 0.5 wt% TCI/MEK

PU Immersion in 3 wt% TCI/MEK

st C-H st C=O

st arC-arC

st C-O-C

st C=O

st C-Cl

δ CH2C-O

st N-CO

st C=O

δ CH2C-O

st N-H

st C=O

-0.0

0.2

Abs

0.0

0.2

Abs

0.0

0.2

0.4

Abs

20004000

Wavenumber (cm-1)

δ C-H

δ N-Hst C-N

oop N-H

δ CH2Cl

Figure 6. ATR-IR spectra of polyurethane adhesive film (a) as-received; (b) immersed in 0.5 wt% and (c) 3 wt% TCI/MEK solution for 5 s. Spectrarecorded 72 h after immersion. st = stretching; δ = bending; oop = out-of-plane; ar = aromatic. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

Interactions PCP / chlorinating agent

PCP0

PCP0 Immersion in 0.5 wt% TCI/MEK

PCP0 Immersion in 3 wt% TCI/MEK

st C=Ost C-Clδ CH2CO

st C-H st C=C δ C-H st C-Clδ CH2Cl

st C=Oδ CH2CO st C-Cl

st ClC-Hst N-H

st C=O

-0.0

0.2

Abs

0.0

0.2

Abs

0.0

0.2

0.4

Abs

20004000

Wavenumber (cm-1)

Figure 7. ATR-IR spectra of polychloroprene adhesive film (a) as-received; (b) immersed in 0.5 wt% and (c) 3 wt% TCI/MEK solutions for 5 s. Spectrarecorded 72 h after immersion. st = stretching; δ = bending. This figure is available in colour online at www.interscience.wiley.com/journal/sia.


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Wavenumber(cm-1)

-0.0

0.1

-0.0

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20004000

st C=Oδ CH2ClC-H wag (O-CH-O)

st C=O

δ CH2CO st C-Cl

C-H wag O-CH-O

st O-H

st C=O

C-H wag O-CH-O

(a) immersion in 0.5 wt% TCI/MEK

PCP30P

PCP30TP

PCP30EC

PCP30AH

Abs

orba

nce

(b) immersion in 3 wt% TCI/MEK

st C=O

δ CH2Clst C-Clst O-H

st N-Hst ClC-H

st C=Ost O-H

δ CH2Cl δ CH2COC-H wag O-CH-O

st C-Cl

st N-Hst ClC-H st C=Oδ CH2CO st C-Cl

-0.00

0.05

-0.0

0.2

0.4

-0.0

0.2

-0.0

0.1

20004000

C-H wag (O-CH-O)st C-Cl

Figure 8. ATR-IR spectra of the resin-containing polychloroprene adhesive films immersed in (a) 0.5 wt%; (b) 3 wt% TCI/MEK solutions for5 s. Spectra recorded 72 h after immersion. st = stretching; δ = bending; wag = wagging. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

(a)

rubber

chlorinating solution or adhesive solution

TCI/MEK

SBS rubber

(b)

(c)

SBS rubber

PU or PCP adhesive

PU or PCPChlorinated

rubber

Figure 9. (a) Scheme of a spin coating process; (b) spin coating of a TCI/MEK solution on the rubber surface; (c) spin coating of a polyurethane (PU) or a poly-chloroprene (PCP) adhesive solution of the chlorinated rubber surface. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

On the other hand, PCP adhesive with no resin in its formulationalso shows chlorination and oxidation (Fig. 7). However, the natureof the resin in the formulation of the PCP adhesive affects thedegree of chemical modification of the PCP adhesives. A moreconsiderable extent of chlorination (more intense CH2Cl bendingabsorption at 1460 cm−1, and C–Cl stretching absorption at760 and 540 cm−1) and oxidation (C–H wagging (O–CH–O) at1380 cm−1) is observed on the PCP-P adhesive film surface after

immersion in a 0.5 wt% TCI/MEK solution (Fig. 8(a)), compared tothe as-received PCP adhesives (Fig. 3). However, the use of a moreconcentrated chlorinating solution (3 wt% TCI/MEK) producesimportant chemical modifications (chlorination and oxidation) inall PCP adhesives (Fig. 8(b)).

A possible modification of the adhesives by nonreacted TCIdeposited on the rubber surface after chlorination treatment hasbeen considered and the interaction between the chlorinated


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Interface

2 h700 rpm

FILTER

TOLUENETOLUENE

DISOLUCIÓ

Transmission-IR

ATR-IR

SOLUTION

Transmission-IR

PU or PCPChlorinated

rubber

Figure 10. Experimental procedure showing the immersion of the spin coated rubber in toluene for 1 h and ulterior filtering of the solids and insolubleadhesive film. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

-0.000

0.001

0.002

0.003

Abs

0.00

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0.04

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0.08

Abs

20004000

Wavenumber (cm-1)

Filter paper

Solids

Solution

29252855

1720

14201400

700965 910

1450

2855

2925

SBS rubber + 0.5 wt% TCI/MEK + PU

3025

3000

760

1375 875

(a)

0.00

0.01

0.02

0.03

Abs

0.00

0.01

0.02

0.03

Abs

20004000

Wavenumber (cm-1)

Surface 1

Surface 2

1720

1420

760

1720

1420

760

SBS rubber + 0.5 wt% TCI/MEK + PU

PU

PU

PU

3025

3000

3025

30001740

1740

1600

1600

2855

2925

2855

2925

1400

1400

965910

875

965 910

875

700

700

(b)

Figure 11. (a) Transmission-IR spectra of solids and solution from PU adhesive spin coated chlorinated rubber immersed in toluene; (b) ATR-IR spectra of both surfaces of insoluble PU adhesive film spin coated onto chlorinated rubber. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.


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0.000

0.005

0.010

Abs

-0.00

0.05

0.10

Abs

20004000

Wavenumber (cm-1)

2925

2855

1720

1420

1400

760

Filter paper

700965910

1450

2855

2925

SBS rubber + 0.5 wt% TCI/MEK + PCP30P

Solids

Solution

30253000

1375

875

(a)

SBS rubber + 0.5 wt% TCI/MEK + PCP30P

0.00

0.01

0.02

0.03

Abs

0.00

0.01

0.02

0.03

Abs

20004000

Surface 1

Surface 2

1720

1420

760

Wavenumber (cm-1)

PCP30P

PCP30P

PCP30P

1720

1420

7603025

30001740

16002855

2925

1400

965

910

875

700

3025

30002855

2925

1740

1400

965910

875

700

(b)

Figure 12. (a) Transmission-IR spectra of solids and solution from PCP-P adhesive spin coated chlorinated rubber immersed in toluene; (b) ATR-IR spectra of both surfaces of insoluble PCP-P adhesive film spin coated onto chlorinated rubber. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

rubber and the adhesive polymeric chains at the interface hasbeen studied. Chlorination of the rubber was performed by spincoating a 0.5 wt% TCI/MEK solution on the rubber surface. Then,thin layers (approximately 230 Å thick, assessed by elipsometry[15]

of PU adhesive, PCP adhesive with thermoreactive phenolicresin (PCP30P), and PCP adhesive with a terpenophenolic resin(PCP30TP) were deposited by spin coating solutions of theadhesive solutions on the chlorinated SBS rubber (Fig. 9). Thechlorinated rubber samples previously spin coated by the adhesivewere immersed in toluene and mechanically shaken at 700 rpmfor 2 h (Fig. 10). Toluene was selected due to the differences ofsolubility of the rubber and the adhesives: as-received rubber issoluble in toluene but PU and PCP are insoluble in this solvent. After2 h, the PU and PCP adhesives remained as films in the solution, asexpected due to their nonsolubility in toluene. The rubber became

partially insoluble in toluene and solid sediment precipitated atthe bottom of the flask. The solid precipitate was filtered off fromthe solution and analyzed by ATR-IR; the solution was analyzed byTransmission-IR; and both surfaces of the insoluble adhesive filmwere analyzed by ATR-IR.

The Transmission-IR spectrum of the solids deposited as aprecipitate and filtered off from the solution (Fig. 11(a), Solids)shows typical absorption bands from chlorinated rubber (Table 5).On the other hand, some drops of the solution were depositedon a KBr pellet. After evaporation of the solvent, a Transmission-IRspectrum was recorded (Fig. 11(a), Solution). This spectrum showsabsorption from nonchlorinated rubber (Table 5). This suggeststhat chlorination by spin coating the 0.5 wt% TCI/MEK solutiononto the rubber surface produces cross-linking of the chlorinatedchains of the outermost rubber surface; as a consequence, this


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Interface

0.000

0.005

0.010

Abs

-0.00

0.05

0.10

Abs

20004000

Wavenumber (cm-1)

2925

2855

1720

1420

1400

760

Filter paper

700965915

1450

2850

2925

SBS rubber + 0.5 wt% TCI/MEK + PCP30TP

Solids

Solution

3025

3000

1375

875

(a)

SBS rubber + 0.5 wt% TCI/MEK + PCP30TP

0.00

0.01

0.02

0.03

Abs

0.00

0.01

0.02

0.03

Abs

20004000

Surface 1

Surface 2

Wavenumber (cm-1)

PCP30TP

PCP30TP

PCP30TP

1720

1420

7603025

3000 2855

2925

1740

1400

965910

875

700

1720

1420

760

3025

3000

2855

2925

1740

1400

965

910 875

700

(b)

Figure 13. (a) Transmission-IR spectra of solids and solution from PCP-TP adhesive spin coated chlorinated rubber immersed in toluene; (b) ATR-IR spectra of both surfaces of insoluble PCP-TP adhesive film spin coated onto chlorinated rubber. This figure is available in colour online atwww.interscience.wiley.com/journal/sia.

Metering rod ~132 µm

Adhesiveapplication

Figure 14. Adhesive deposition onto rubber using a metering rod. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

thin layer of cross-linked chlorinated rubber becomes insolublein toluene and precipitates as a solid sediment (nonchlorinatedrubber is soluble in toluene, but chlorinated rubber is nonsolublein toluene due to cross-linking of rubber by chlorination[14,15]).

On the other hand, ATR-IR spectra were recorded on both sidesof the PU adhesive film that remained insoluble in the toluene.

Both surfaces of the adhesive film, the one that was in contactwith the chlorinated surface as well as the one that did nothave direct contact with the chlorinated rubber surface solutionshowed absorption from chlorinated rubber, (Fig. 11(b), Table 5),which suggests that migration of chlorinated rubber through theadhesive/chlorinated rubber interface is produced and a cross-


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-0.00

0.05

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Abs

0.0

0.1

0.2

Abs

20004000

Wavenumber (cm-1)

RubberPU

PU

Chlorinated Rubber

PU

Rubber

PU

PU

PU

Rubber

Chlorinated Rubber

Rubber

SBS rubber + 3 wt% TCI/MEK + PU

Surface 1

Surface 2

PU

PU

PU

(a)

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0.10

Abs

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20004000

SBS rubber + 3 wt% TCI/MEK + PCP30P

Rubber

PCP30PPCP30P

RubberChlorinatedRubber

Rubber

PCP30PPCP30P

Rubber ChlorinatedRubber

Wavenumber (cm-1)

Surface 1

Surface 2

PCP30P

PCP30P

PCP30P

(b)

-0.00

0.05

0.10

Abs

-0.00

0.05

0.10

Abs

20004000

SBS rubber + 3 wt% TCI/MEK + PCP30TP

Wavenumber (cm-1)

Rubber

PCP30TP

RubberChlorinatedRubber

Rubber

PCP30TP

ChlorinatedRubber Rubber

Surface 1

Surface 2

PCP30TP

PCP30TP

PCP30TP

(c)

Figure 15. (a) ATR-IR spectra of both surfaces of insoluble (a) PU; (b) PCP-P and (c) PCP-TP adhesive films deposited onto chlorinated rubber. This figureis available in colour online at www.interscience.wiley.com/journal/sia.


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linking between the chlorinated rubber and the PU adhesive isproduced.

When performing the same experiment using the PCP adhesiveinstead of the PU adhesive, similar results were obtainedirrespective of the resin in the formulation of the PCP: athermoreactive phenolic resin (PCP30P) (Fig. 12, Table 5) or aterpene phenolic resin (PCP30TP) (Fig. 13, Table 5). Therefore, IRspectra of the solids corresponded to the cross-linked chlorinatedrubber insoluble in toluene and the solution showed absorptionfrom nonchlorinated rubber, which is soluble in toluene (Fig. 12(a),Table 5). Besides, both sides of the PCP adhesive showedabsorption from chlorinated rubber (Fig. 13(b), Table 5).

Considering that in the actual bonding joints the adhesivesolution is deposited not as a thin film (240 Å), but as a thickfilm (approximately 132 µm) using a metering rod (Fig. 14),ATR-IR spectra of the adhesive film (Fig. 15) (PU, PCP30P, orPCP30TP) deposited on the chlorinated rubber were monitoredafter obtaining the insoluble adhesive film in the toluene (Fig. 10).In all cases, both sides of the adhesive showed absorption fromboth, the chlorinated rubber and the adhesive.

From the above evidence, it seems that a similar interface isproduced between the chlorinated rubber, and the PU and thePCP adhesives, irrespective of the addition of a resin to the PCPadhesive. Thus, the increase in the adhesion properties producedby the addition of a thermoreactive phenolic resin to the PCPadhesive (Fig. 1) is not explained only on the basis of the interfaceproduced between the chlorinated rubber and the adhesive.

Therefore, viscoelastic properties of the adhesives were as-sessed. DMTA shows that PU adhesive is more elastic that anyof the studied PCP adhesives as the PU adhesive has the highestlogarithm of the storage modulus (log E′) (Fig. 16(a)). However,the addition of a thermoreactive phenolic resin to the PCP formu-lation produces an increase of elasticity of the PCP, which resultsin a rheological behavior more similar to that of the PU adhe-sive. Similar results are observed in the variation of tan δ versustemperature (Fig. 16(b)): There is a decrease of the tan δ valuewhen a resin is added to the PCP adhesive. The thermoreactivephenolic resin is the one that imparts the most elastic behaviorto the PCP adhesive, making it more similar to the PU adhesive.Consequently, it seems that similar adhesion properties obtainedwith the PU and the PCP adhesive with a thermoreactive phenolicresin (PCP30P) (Fig. 1) are mainly determined by the appropriaterheological properties of both adhesives. Besides, both adhesivesare chlorinated and oxidized by contact with the chlorinated SBSrubber and a cross-linking between the adhesive film and thechlorinated SBS rubber layer is produced. Thus, adhesion whenusing a PCP adhesive is improved as a consequence of the for-mation of a cross-linked interface between the adhesive and thechlorinated rubber surface and the suitable rheological propertiesimparted by a thermoreactive phenolic resin to the PCP adhesiveformulation.

When any other tackifier resin is used in the PCP adhesiveformulation, although a cross-linked interface between thechlorinated rubber and the PCP adhesive is produced, theviscoelastic properties of the PCP adhesives are not suitable,so a lack of adhesion is obtained and an interfacial (adhesion)failure is produced due to poor elasticity of the PCP adhesive.

Conclusions

Chlorination of a thermoplastic SBS rubber by solutions of TCI/MEKproduces chlorination and oxidation of the rubber surface, which

5.5

6

6.5

7

7.5

8

8.5

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-80 -60 -40 -20 0 20 40 60

Temperature (°C)

log

E' (

Pa)

Rheological properties ofthe PU/PCP adhesives

PCP0

PCP30EC

PCP30TP

PCP30P

PCP30AH

PU

0

0.1

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-80 -60 -40 -20 0 20 40

Temperature (°C)

tan

δ

PCP0

PCP30EC

PCP30TP

PCP30PPCP30AH

PU

Figure 16. (a) Logarithm of the storage modulus (log E′) versus tempera-ture and (b) Tan δ versus temperature for polyurethane and polychloro-prene adhesives obtained by DMTA. This figure is available in colour onlineat www.interscience.wiley.com/journal/sia.

results in a cross-linked outermost rubber surface. The depositionof a PU or PCP adhesive on the chlorinated rubber results inthe interlocking between the chlorinated rubber chains and theadhesive chains at the interface, which is partially responsible foran increase of adhesive properties.

However, the viscoelastic properties of the adhesive also playan important role. Thus, it is necessary to modify the viscoelasticproperties of the PCP adhesive by the addition of a thermoreactivephenolic resin to its formulation in order to increase its elasticityand obtain rheological properties similar to those of the PUadhesive.

Therefore, adhesion properties of thermoplastic SBS rubber/PCPjoints are increased as a consequence of the formation of a cross-linked interface between the chlorinated SBS rubber and the PCPadhesive and the appropriate rheological properties imparted bythe addition of a thermoreactive phenolic resin to the PCP adhesiveformulation.

Acknowledgements

Financial support from Science and Technology Ministry – MCYT,Spain (project MAT 2001-0625-C02-01) and Generalitat Valenciana,Spain (project GV04B-567) is gratefully acknowledged. The authors


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would like to thank INSOCO (Elche, Alicante, Spain) for supplyingPCP and PU; SYNTHELAST (Elche, Alicante, Spain) for providing theSBS rubber used in this research; and SAFIC-ALCAN (Barcelona,Spain) for supplying the resins used in this study.

References

[1] Pastor-Blas MM, Torregrosa-Macia R, Martín-Martínez JM, Dil-lard JG. Int. J. Adhesion and Adhesives 1997; 17(2): 133.

[2] Iniesta-Jaen J, Pastor-Blas MM, Mahiques-Bujanda MM, Martín-Martínez JM, Dillard JG. J. Adhesion Sci. Technol. 1999; 13(8): 903.

[3] Navarro-Banon MV, Pastor-Blas MM, Martín-Martínez JM. ExpressPolym. Lett. 2007; 1(4): 236.

[4] Navarro-Banon MV, Pastor-Blas MM, Martín-Martínez JM. RubberChem. Technol. 2007; (in press).

[5] Owens DK, Wendt RC. J. Appl. Polym. Sci. 1969; 13: 1741.[6] Pastor-Blas MM, Martín-Martínez JM, Dillard JG. J. Adhes. 1997; 63:

121.

[7] Pedron-Haba S, Sanchez-Navarro MM, Aran-Aís F, Torro-Palau AM,Orgiles-Barcelo AC, Martín-Martínez JM. Properties of polychloro-prene adhesives containing terpene phenolic resin tackifiers. 28thAnnual Meeting of the Adhesion Society, Mobile, USA, February13–16, 2005; 267.

[8] Verghese LA, Thachil ET. J. Adhes. Sci. Technol. 2004; 18(2): 181.[9] Verghese LA, Thachil ET. J. Adhes. Sci. Technol. 2004; 18(11): 1217.

[10] Dusescu I, Dan IL, Stanescu PO, Hodosan C, Hubca G. Mater. Plast.2006; 43(3): 181.

[11] Lubi MC, Thachil ET. Polym. Plast. Technol. Eng. 2006; 45(1): 43.[12] zukiene K, Jankauskaite V. J. Adhes. Sci. Technol. 2005; 19(8): 627.[13] Font R, Sabater MC, Martínez MA. J. Adhes. Sci. Technol. 2001; 15(14):

1677.[14] Pastor-Blas MM, Martín-Martínez JM. Mechanisms of formation of

weak boundary layers in styrene-butadiene rubber. In AdhesionScience and Technology, Mizumachi Hiroshi (ed). Gordon & BreachScience Publishers: Australia, 1997; 215, ISBN: 90-5699-588-X.

[15] Pastor-Blas MM, Martín-Martínez JM, Boerio FJ. Rubber Chem.Technol. 2002; 75(5): 825.


Interactions at the interface between thermoplastic rubber and polychloroprene adhesive

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