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J. Adhesion Sci. Technol., Vol. 17, No. 6, pp. 797–814 (2003)Ó VSP 2003.Also available online - www.vsppub.com

Boric acid ester as an adhesion promoter for rubber

compounds to brass-plated steel cord

GYUNG SOO JEON ¤

Department of Environmental Information, Provincial College of Damyang, Damyang,

Chonnam 517-802, South Korea

Received in nal form 21 January 2003

Abstract—The adhesion between a boric acid ester (BAE) containing rubber compound and a brass-

plated steel cord was investigated to understand the role of BAE as an adhesion promoter. An

improvement in adhesion was shown with the low loading of BAE in the range 0.5–1 phr, while

signicant decline of adhesion was not observed with high loading at 2 phr and a long aging time of

15 days. The interphase between the brass plated steel cord and the rubber compound studied using

Auger electron spectroscopy (AES) showed a stabilized adhesion interphase by BAE incorporation,resulting in the enhancement of the adhesion retention.

Keywords: Boric acid ester; adhesion promoter; cobalt salt; rubber-to-brass bonding; adhesion

interphase; AES; depth prole.

1. INTRODUCTION

Adhesion of rubber to brass-plated steel cord is of paramount importance in steel

belted tires. Brass-plated steel cords inserted in the belt and carcass of tires has long

been used as a reinforcing material to provide a sufcient mechanical strength and

stability to endure cars themselves and their loads. Brass plating on the surface of

steel cords reacts with sulfur in the rubber compound during the curing process of

tire manufacturing, forming an adhesion interphase between the rubber compound

and the steel cord. Copper and zinc also react with oxygen and water in the

rubber, forming oxides and hydroxides of copper and zinc. Therefore, the adhesioninterphase is very complex in terms of components and content, so good adhesion

can be achieved when the adhesion interphase is formed with a sufcient thickness

and a stable structure.

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798 G. S. Jeon

The major components of the adhesion interphase are suldes, oxides and

hydroxides of copper and zinc [1–3]. Adhesion becomes weak when copper sulde

is not sufciently grown in the interphase, but the excessive growth of copper sulde

or zinc oxide brings about their own cohesive failures [4]. Thus the optimum growth

of copper sulde is essential to form a large contact interface between the rubberand the brass, resulting in good adhesion [5– 7]. Several reagents such as cobalt

salt, resorcinol formaldehyde resin and methylene donor are commercially used

as adhesion promoters to enhance the migration of copper, forming the necessary

amount of copper sulde in the adhesion interphase [8, 9]. A large content of zinc

oxide at the outer surface of the brass induces a cohesive failure, since its mechanical

strength is very weak. On the other hand, the coexistence of zinc oxide with zinc

at the interface is helpful to control the mass transfer rate of reacting species in the

formation of the adhesion interphase [10]. This contributes to the stability of theadhesion interphase by preventing the excessive growth of its components.

Cobalt salt has been used as an adhesion promoter to accelerate the activation of

sulfur in the interphase by inducing the formation of an adequate copper sulde

layer, thus making better adhesion possible [11, 12]. However, an adverse effect is

observed for rubber compounds with high levels of cobalt salt, or after humidity

aging, due to the formation of an excessive copper sulde layer which induces

cohesive failure [7]. Boron is well known as a corrosion inhibitor in the adhesion

interphase [13].

Some tire chemicals manufacturers have incorporated boric acid ester (BAE)

into rubber compounds as an organic adhesion promoter together with cobalt

salt [14, 15]. The BAE has been incorporated in the rubber compound together with

Co naphthenate [16]. The enhanced effect of loading of BAE on the adhesion was

reported, but there is no systematic report on the role and optimal loading amount

of BAE.

The interaction between boric acid ester and copper is known. The formation of

the copper complex between the boric acid ester and copper has been studied by

means of Nuclear Magnetic Resonance Spectroscopy [17]. The adsorption of boric

acid ester on silica has attracted considerable attention from surface chemists using

FT-IR spectroscopy [18]. Li et al. reported a strong surface interaction of boric acid

ester with metal oxides [19].

A moderate diffusion of copper into rubber, due to the interaction of boric acid

ester with copper, may be benecial in the formation of copper sulde with a high

surface area in the adhesion interphase and the control of zinc oxide formation in

the adhesion interphase due to the increased freedom of zinc arising from a lack of

the copper on the cord surface. The afuence of copper sulde with a high surfacearea in the adhesion interphase is necessary for the mechanical interlocking of the

rubber compound to the brass-plated steel cord.

Since adhesion between two phases implies that some kind of chemical or

physical interaction has taken place, much attention has been paid to characterize

the adhesion interphase formed between two phases (rubber and brass-plated steel



Improving the adhesion by boric acid ester 799

cord). The samples of adhesion interphases were prepared by inserting lter paper

between rubber compound and brass plated steel cord and they were analyzed by

AES in order to interpret the adhesion properties [20]. The effect of BAE on the

adhesion property between the rubber compound and brass-plated steel cord was

examined based on the formation and degradation of the adhesion interphase fromthe depth proles of the rubber compound/ brass-plated steel cord samples.

The pull-out force and coverage of the rubber compound containing BAE were

compared to those of a BAE-free rubber compound. The inuences of humidity

aging as well as BAE concentration on the adhesion property of BAE-containing

rubber compounds were also investigated.

2. EXPERIMENTAL

2.1. Preparation of rubber compounds

Five rubber compounds with different BAE loadings were prepared. Compositions

of the masterbatch and nal mixed compounds are given in Table 1. Bonding

systems composed of resorcinol formaldehyde resin and a methylene donor such

as hexamethoxymethylmelamine or nitromethylpropanol was excluded in order

to observe the effect of BAE on the structure and composition of the adhesion

interphase more clearly.The loading amounts of BAE were varied as 0, 0.5, 1.0, 2.0, and 4.0 phr. All

the rubber compounds were mixed as described in ASTM D-3184 using an internal

Table 1.

Composition of rubber compounds used

Component Chemical or Manufacturer Content

Trade name (phr)

MasterbatchNR SMR-CV60 Lee Rubber Co., Malaysia 100

carbon black N351 Lucky Co., Korea 50

processing oil A#2 Michang Co., Korea 5

activator ZnO Hanil Co., Korea 10

antioxidant Kumanox-RDa Monsanto Co., USA 1

adhesion promoter BAEb Bayer Co., Germany Varied

adhesion promoter Co stearate Manchem Co., UK 0.8

Final Mixing

activator Stearic acid Pyungwha Co., Korea 1.5

accelerator Santocure MORc Monsanto Co., USA 0.7

sulfur Crystex HS OT 20 Akzo Co., The Netherlands 5

a 2,2,4-trimethyl-1,2-dihydroquinone.b boric acid ester.c 2-(morpholinothio)-benzothiazole sulfenamide.



800 G. S. Jeon

mixer (Banbury Mixer model 82, Farrel Co., USA). Ingredients for the masterbatch

were mixed for 10 min at a rotor speed of 40 rpm and discharged at 150 ±C. After

the masterbatch had cooled to room temperature, the nal mixing components were

mixed for 5 min at 30 rpm and discharged at 90±C. After mixing, the compounds

were carefully remilled into at sheets on a two-roll mill (model MKIII, Farrel Co.USA).

Rheocurves were recorded using a Monsanto Rheometer 100 at 160±C. The

t 90 time, i.e. the time required to achieve 90% cure, and maximum torque were

determined from the rheograms. Mooney viscosity was also obtained using a

Monsanto MV-200 instrument according to ASTM D-1646.

The hardness of vulcanizates was measured using a Shore A durometer according

to ASTM D-2240, and tensile properties were measured with a tensile tester (model

6021, Instron, USA) according to ASTM D-412.

2.2. Adhesion test

Based on the procedure described in ASTM D-2229, T-test specimens were cured at

160±C on a cure press. Curing was continued for 5 min more than the t 90 time. The

brass-plated steel cords with 4£0:28 construction in which 4 steel wires having the

same diameter of 0.28 mm were twisted together, manufactured by Hyosung T&C

Co., Korea, were used. The plating weight of the brass was 3.6 g/kg and the copper

content 63.6%. For humidity aging, specimens were placed in a humidity chamber

at 85±C under 85% relative humidity for 5, 10, and 15 days.

Pull-out force was determined as the maximum force exerted by the tensile tester

on the T-test adhesion sample during the pull-out test, at a crosshead speed of 10

mm/ min. Rubber coverage was also noted. Each value reported is an average of

six specimens tested. The morphology of the pulled-out steel cord surface after

measuring pull-out force was studied using a scanning electron microscope (JEOL

JSM 7400, Japan).

2.3. Analysis of the adhesion interphase

A brass plated steel cord was covered with a lter paper (pore size: 5 ¹m; catalog

no 142 50, Millipore Co., USA), sandwiched between two uncured pads of rubber

compound, and then placed in a pad mold [20]. Curing and aging conditions for the

rubber compound/ brass plate samples were the same as in the preparation of the

T-test specimens. After the various treatments, samples for the surface analysis of

the adhesion interphase were obtained by peeling away the lter paper. Sulfur fromthe rubber compound migrated through the pores of the lter paper and reacted with

the copper and zinc of the brass-plated steel cord, forming an adhesion interphase.

After removing the rubber and lter paper from the brass-plated steel cord, the

adhesion interphase, including copper sulde and zinc oxide, remained on the brass-

plated steel cord.




The depth proles from the interphase in contact with the rubber compound to

the bulk of the brass were recorded on a Perkin-Elmer Auger spectrometer (model

Phi 670, Perkin-Elmer Co., USA). An area of 10£ 10 ¹m2 was examined using an

ion beam with a potential of 5.0 kV, a current of 0.03 ¹A, and an incident angle to

the specimen of 60

±

, i.e. the same conditions as described in previously publishedpapers [6, 9]. Surface concentrations were determined every 0.5 min from the Auger

peaks of detected elements with compensation for their sensitivities. A sputter

gun with an argon ion beam rastered a 2 £ 2 mm2 area for depth proling. The

sputtering rate for the brass lm was determined to be 13 nm min¡1. It was difcult,

however, to determine the sputter rate precisely for the adhesion interphase because

it included various chemical components with variable concentrations. Therefore,

the sputter time instead of the absolute depth was used to indicate the depth of the

adhesion interphase in this paper.

3. RESULTS AND DISCUSSION

3.1. Physical properties of BAE-containing rubber compound

The cure rates of the rubber compounds varied with the loading amount of BAE as

listed in Table 2. The t 90 time did not change signicantly with an increase in the

loading amount of BAE. Therefore, the cure rate index (CRI) calculated using the

equation 100=.t 90 ¡ t 2/ was nearly constant.

The mechanical crosslink density, which was calculated by the difference in the

maximum torque and the minimum torque, decreased with increasing BAE loading

in rubber compounds. The mechanical crosslink density changed by incorporating

BAE into rubber compounds. The lower cure rate and the higher crosslink density

of rubber compound are essential to good adhesion. The slow cure rate improved

the mobility of the crosslink agent resulting in its afuence in the region of adhesion

interphases.

Table 2.

Cure propertiesa of green rubber compounds with respect to BAE loading

BAE loading Time (min) CRIe Torque (J)

(phr) (min¡1)t 2

b t 40c t 90

d min max

0 2.2 3.9 6.6 22.7 1.85 6.72

0.5 2.3 3.9 6.7 22.7 1.84 6.64

1.0 2.2 3.8 6.5 23.3 1.76 6.53

2.0 2.2 3.8 6.5 23.3 1.79 6.43

a The rubber compounds were sheared to §1± and 1.67 Hz at 160±C.b Time required to attain 2% cure.c Time required to attain 40% cure.d Time required to attain 90% cure.e CRI D 100=.t 90 ¡ t 2).



802 G. S. Jeon

Table 3.

Mooney viscoelastic propertiesa of green rubber compounds with respect to BAE loading

BAE loading Time (min) Torque (J)

(phr)t 5

b t 35c initial minimum at 4 mind

0 11.8 17.8 8.37 7.07 7.10

0.5 11.9 17.8 7.93 6.77 6.77

1.0 12.4 18.5 7.34 6.08 6.11

2.0 11.7 17.5 7.57 6.18 6.21

a The rubber compounds were sheared to 2 rpm at 125 ±C.b Time required to attain minimum torque plus 5 point torque.c Time required to attain minimum torque plus 35 point torque.d Viscosity, ML1C4;125 ±C.

Table 4.

Mechanical properties of unaged vulcanizates with respect to BAE loading

BAE loading Hardness Modulus (MPa) T.S.a E.B.b

(phr) (Shore A) (MPa) (%)50% 100% 200% 300%

0 77 2.62 5.08 11.4 17.2 23.5 431

0.5 76 2.47 4.80 11.1 17.0 24.5 454

1.0 76 2.49 4.87 11.2 17.2 23.8 433

2.0 77 2.51 4.90 11.1 17.0 23.9 441a Tensile strength.b Elongation at break.

The viscosity of green rubber compound also varied with BAE loading as shown

in Table 3. The t 5 time, indicating the onset of scorch time, was nearly constant

regardless of BAE loading except for 1 phr BAE loading. Viscosity, represented as

torque at 4 min, decreased steadily with BAE loading.

The changes in mechanical properties of vulcanizates with BAE loading were notremarkable (Table 4). Consistent trends in modulus, tensile strength and elongation

at break with BAE loading were not observed. Although cure characteristics,

viscosity and mechanical properties of the rubber compound slightly varied with

BAE loading, the degrees of variances in these properties were not signicant. Since

the change in the rubber property with BAE loading is small, the contribution of the

rubber property on the adhesion between the BAE-containing rubber compound and

brass-plated steel cord can be neglected in the discussion of BAE as an adhesion

promoter.

3.2. Adhesion properties of the BAE-containing rubber compound

The change in the adhesion properties of the rubber compound with brass-plated

steel cord was comparable with the incorporation of BAE into the rubber compared

to that of the mechanical properties. Figure 1 shows the pull-out force and rubber




Figure 1. Adhesion properties of BAE-containing rubber compounds after cure; (A) pull-out force

and (B) rubber coverage.

Table 5.

Adhesion properties of humidity ageda adhesion samples for various rubber compounds with respect

to BAE loading

BAE loading Pull-out force (N) Rubber coverage (%)(phr)

0b 5 10 15 0 5 10 15

0 610 319 285 258 75 35 30 15

0.5 628 467 353 353 80 65 45 45

1.0 621 434 386 344 80 50 45 40

2.0 610 400 324 300 75 50 45 40

a The adhesion samples were humidly aged at 85 ±C and 85% relative humidity.b Aging period (days).

coverage of the BAE-containing rubber compounds after curing without any aging.

There were no signicant increases with BAE loading in the pull-out force and

rubber coverage in the unaged state. No improvement in adhesion property was

shown with the addition of BAE. Rubber coverage showed the same trend as the

pull-out force. Figure 2 shows that the rubber attached to pulled-out cord surface of

unaged adhesion samples is signicant regardless of BAE loading amount.

Humidity aging deteriorates adhesion properties, so long aging causes poor

adhesion (Fig. 3). The data after humidity aging for 5 days and 10 days are shownin Table 5. After humidity aging for 15 days, the pull-out force decreased, but the

decrease was small for the 0.5 – 1.0 phr BAE-containing rubber compounds. Further

increase of BAE loading decreased pull-out forces of the rubber compounds. Also

rubber coverage showed similar behavior to that of the pull-out force. The rubber

coverage of the BAE-containing rubber compounds up to 2 phr was higher than that



804 G. S. Jeon

(A) (B)

(C) (D)

Figure 2. SEM micrographs of pulled-out steel cord from unaged adhesion samples with respect to

BAE loading: (A) 0 phr; (B) 0.5 phr; (C) 1 phr; (D) 2 phr.

Figure 3. Adhesion properties of BAE-containing rubber compounds with humidity aging; (A) pull-

out force and (B) rubber coverage. Humidity aging: 15 day, 85 ±C and 85% relative humidity.




of the BAE-free rubber compound after humidity aging of 15 days. As shown in

Fig. 3, a small incorporation of BAE into rubber compound signicantly improves

adhesion stability against humidity aging. Figure 4 shows that BAE addition into

the rubber compound is very benecial to increase the rubber coverage for pulled-

out cord surface of humidity aged adhesion samples. The BAE-free and humidityaged adhesion samples mainly showed interfacial failure but BAE-containing

and humidity aged adhesion samples dominantly showed cohesive failure in the

adhesive.

Figure 5 and Table 6 show the pull-out force and rubber coverage of the BAE-

containing rubber compounds after thermal aging. The trend of adhesion property

with BAE loading after thermal aging is similar to that after humidity aging.

Figure 6 shows SEM micrographs of pulled-out steel cord from adhesion samples

thermally aged for 15 days with respect to BAE loading.The effect of BAE loading into the rubber compound on its adhesion properties

can be summarized as: (1) The enhancement in the unaged adhesion properties was

not signicant, (2) a low loading, below 1.0 phr, was sufcient for the enhancement

(A) (B)

(C) (D)

Figure 4. SEM micrographs of pulled-out steel cord from adhesion samples humidity aged for 15

days with respect to BAE loading: (A) 0 phr; (B) 0.5 phr; (C) 1 phr; (D) 2 phr.



806 G. S. Jeon

Figure 5. Adhesion propertiesof BAE-containing rubber compounds with thermal aging; (A) pull-out

force and (B) rubber coverage. Thermal aging: 15 day, 95 ±C.

Table 6.

Adhesion properties of thermally-ageda adhesion samples for various rubber compounds with respect

to BAE loading

BAE loading Pull-out force (N) Rubber coverage (%)(phr)

0b 5 10 15 0 5 10 15

0 610 453 396 287 75 80 80 70

0.5 628 550 432 368 80 90 80 80

1.0 621 520 442 399 80 80 65 85

2.0 610 492 424 359 75 95 80 75

a The adhesion samples were thermally aged at 95±C.b Aging period (days).

of the adhesion retention even after humidity aging treatment, and (3) severe

degradation of adhesion properties was observed with humidity aging for the BAE-

free rubber compound. This summary suggests that BAE loading is considerably

effective in maintaining adhesion stability against hostile environment.

3.3. Examination of the adhesion interphase

The formation, growth and deformation of an adhesion interphase between the

BAE-containing rubber and the brass-plated steel cord can be monitored from theadhesion sample with lter paper placed at the interphase. Since the lter paper

adheres neither to rubber nor to brass, the brass-plate steel cord is easily detached

from the rubber compound after cure. The analysis of the surface layer on the brass-

plated steel cord provides information on the formation and disappearance of chem-

ical components in the adhesion interphase during curing process. Figure 7 shows




(A) (B)

(C) (D)

Figure 6. SEM micrographs of pulled-out steel cord from adhesion samples thermally aged for 15

days with respect to BAE loading: (A) 0 phr; (B) 0.5 phr; (C) 1 phr; (D) 2 phr.

AES depth proles of the unaged adhesion interphase formed between the BAE-

free rubber compound and brass plated steel cord. At the outer surface of the brass

plated steel cord adhered to the BAE-free rubber compound, carbon, copper and

sulfur were detected. Beneath these elements, zinc, oxygen and iron were detected.

The afuence of carbon at outermost interphase is due to surface contamination.

With increasing sputter time, carbon concentration decreased exponentially. Iron

detected signicantly from 6 min of sputtering and increased linearly up to 20 min

of sputtering. After 12 min of sputtering, copper and zinc contents were constant

with depth, indicating non-reacted brass. This depth prole shows that copper sul-

de is formed on the outer surface of the brass plated steel cord and zinc oxide on the

inner side, although the oxidation states of these elements are not clearly identied.The contents of copper and sulfur on the outer surface decreased on the BAE-

containing rubber compounds (Fig. 8). A copper peak shoulder was observed in

the adhesion interphase adhered to the rubber compound regardless of BAE loading

but the ratio of sulfur content to copper shoulder content increased on the outer

surface with increasing BAE loading. Also copper peak shoulder decreased with



808 G. S. Jeon

Figure 7. AES depth proles of C, O, S, Fe, Cu and Zn for the adhesioninterphaseof unaged adhesion

samples between the BAE-free rubber compound and brass plated steel cord.

increasing BAE loading. Zinc and oxygen peaks were observed on the inner surface

of the brass plated steel cord rather than copper and sulfur peaks after cure. Both

zinc and oxygen peaks decreased with increasing BAE loading and the zinc peak

shape was not the same as that of oxygen peak. This indicates that the incorporation

of BAE induces formation of a new compound in the adhesion interphase and the

suppression of dezincication which is important for good adhesion stability. As

shown in Fig. 8D, the S peak is larger than that of shoulder copper peak in the

adhesion interphase for BAE-containing adhesion samples. This result suggests the

formation of ZnS layer in the adhesion interphase which suppresses the growth of

adhesion interphase, especially copper sulde layer. Since it is known that BAE

interacts strongly with copper [17], the migration of copper to the rubber side will

be accelerated with BAE loading. Though the BAE has a high afnity to copper,

this afnity does not exclusively play a role in the formation of adhesion interphase

(copper sulde). As soon as an adhesion sample cures, parallel reactions of ZnS and

copper sulde layers formation occur. For the BAE-containing adhesion samples,

the formation of ZnS is faster than that of the copper sulde layer compared to

BAE-free adhesion samples. This was conrmed in Fig. 8 where a signicantdifference in the shapes of copper and sulfur peaks was present for the BAE-

containing adhesion samples.

The zinc prole was similar to the oxygen prole on the outer surface, regardless

of BAE incorporation, suggesting the formation of zinc oxide. However, the

detected depth and the amount of zinc oxide formed varied with BAE incorporation.




F i g u r e 8 .

A E S d e p t

h p r o

l e s o

f C u ,

S ( t o p

) a n

d Z

n ,

O

( b o t t o m

) f o r t h e a

d h e s i o n

i n t e r p

h a s e s o

f u n a g e

d a

d h e s

i o n s a m p

l e s

b e t w e e n t h e r u

b b e r c o m p o u n

d a n

d

b r a s s p

l a t e d s t e e

l c o r d w

i t h r e s p e c t t o B A E l o a

d i n

g :

( A ) 0 p

h r ;

( B ) 0

. 5 p

h r ;

( C )

1 p

h r ;

( D ) 2 p

h r .



810 G. S. Jeon

With an increase in the loading amount of BAE, the detected depth of zinc shifted to

the outer surface and the content of zinc oxide in the adhesion interphase decreased.

The fast migration of copper to the rubber side results in voids at the brass side,

inducing the shift of zinc to the outer surface and activating the zinc due to the loss

of bonded copper. Therefore, the formation of ZnS instead of ZnO in the adhesioninterphase is accelerated by virtue of BAE as corrosion inhibitor and prevents

excessive growth of copper sulde, resulting in the lowering of dezincication in

the adhesion interphase.

Humidity aging is a very effective way to investigate the degradation of adhesion

properties due to the change in the adhesion interphase, compared to thermal aging

which induces mainly the decline of mechanical properties of the rubber compound.

Figure 9 shows the depth proles of the humidity-aged adhesion interphases formed

on the brass side adhered to the rubber compound. The width and shape of copperand sulfur peaks at the outer surface were unchanged with an increase in the loading

amount of BAE. For the BAE-free adhesion samples, adhesion interphase grows

excessively and deforms as shown in Fig. 9A. Intense sulfur peaks were observed

on the outer surface of the brass plated steel cord adhered to the rubber compound

with low BAE loading.

As in unaged adhesion samples, the ratio of sulfur content to copper shoulder

content increased signicantly with BAE loading after humidity aging. The high

concentration of S compared to Cu suggests the afuent formation of ZnS. For the

low BAE loading, the high concentration of ZnS plays a major role in high adhesion

stability. The contents of zinc and oxygen in the adhesion interphase decreased with

an increase in the BAE loading, indicating that BAE suppressed the dezincication

during humidity aging.

The enhancement of zinc activity due to the loss of bonded copper brings

about large formation of ZnS instead of severe dezincication in humidity aging

treatments because of BAE as corrosion inhibitor. The role of zinc sulde is

suggested to retard migration of reacting species across the interface between rubber

and brass [7, 21]. The suppression of the migration improves the stability of the

adhesion interphase. The depth proles of the humidity-aged adhesion interphase

for the BAE-containing rubber compounds with the brass plated steel cord were

considerably different from the unaged ones in contents of copper sulde and zinc

oxide on the outer surface of the brass plated steel cord.

In the unaged sample, copper was depleted from the outer surface by BAE loading

into the rubber, and loss of copper was signicant after humidity aging. Depletion

and accumulation of surface components with BAE loading was different before

and after humidity aging, especially the sulfur concentration.This can be explained by a strong interaction between BAE and copper. The

acceleration of copper migration to the rubber will be helpful in the formation of

copper sulde, which is the major component of the adhesion interphase between

the rubber compound and brass. In the unaged sample, copper was depleted by

BAE, but high reactivity of zinc with water induced a severe dezincication in




F i g u r e

9 .

A E S d e p

t h p r o

l e s o

f C u ,

S ( t o p

) a n

d Z n ,

O

( b o t t o m

) f o r t h e a d

h e s

i o n

i n t e r p

h a s e s o

f h u m

i d i t y a g e

d a

d h e s i o n s a m p

l e s


b b e r

c o m p o u n

d a n

d b r a s s p

l a t e d s t e e

l c o r d w

i t h r e s p e c

t t o B A E l o a

d i n g :

( A ) 0 p

h r ;

( B ) 0

. 5 p

h r ;

( C ) 1 p

h r ;

( D ) 2

p h r .

H u m

i d i t y a g

i n g :

1 5 d a y s ,

8 5 ±

C a n

d 8 5 %

r e l a t i v e

h u m

i d i t y

.



812 G. S. Jeon

F i g u r e

1 0 .

A E S d e

p t h p r o

l e s o

f C u ,

S ( t o p

) a n

d Z n ,

O

( b o t t o m

) f o r t h e a d

h e s i o n

i n t e r p

h a s e s o

f t h e r m a l

l y a g e

d a

d h e s

i o n s a m p

l e s


b b e r

c o m p o u n

d a n

d b r a s s

p l a t e d s t e e

l c o r d w

i t h r e s p e c t t o B A E l o a

d i n g :

( A ) 0 p

h r ; (

B ) 0

. 5 p

h r ;

( C ) 1 p

h r ;

( D ) 2 p

h r .

T h e r m a

l a g

i n g :

1 5 d a y s a n

d 9 5 ±

C .




humidity aging, causing a high accumulation of copper sulde in the adhesion

interphase.

This means that the BAE enhances the migration of copper and the formation of

a large amount of zinc sulde, and not of copper sulde. The thick layer of zinc

sulde suppresses the rapid further growth of copper sulde and zinc oxide duringhumidity aging. Slow growth of copper sulde during humidity aging proceeded at

the BAE-containing rubber compound. Moderate amounts of copper sulde and

zinc sulde formation in the adhesion interphase of the BAE-containing rubber

compound are also conrmed from the observation of the high depletion of copper

in the AES depth prole of the unaged brass plated steel cord adhered to the rubber

compound containing high loading of BAE.

Also, the effect of adhesion interphase on thermal aging is shown in Fig. 10. For

the BAE-free adhesion sample, the adhesion interphase is severely oxidized. The

shape difference between copper shoulder peak and sulfur peak is prominent with

increasing BAE loading, supporting the formation of zinc sulde. With increasing

BAE loading, dezincication in the adhesion interphase was signicantly less.

3.4. Role of BAE as an adhesion promoter

BAE has a strong interaction with copper and forms a metal complex [17]. Though

the migration of copper to the rubber side is expected to be enhanced by the

incorporation of BAE, the BAE across the interface between the rubber compoundand brass leads to moderate amounts of copper sulde and zinc sulde formation in

the adhesion interphase. The signicant formation of zinc sulde in the adhesion

interphase of BAE-containing adhesion samples is very benecial to improve the

adhesion stability against hostile environments. Incorporation of BAE into the

rubber suppresses the growth of copper sulde resulted from afuence of zinc

sulde, which is regarded as a diffusion barrier for copper in brass plated steel,

in the adhesion interphase during hostile aging treatment.

The incorporation of BAE into the rubber compound does not increase unagedadhesion properties. The acceleration of copper migration by BAE resulted

in activated zinc which was easily suldized, not oxidized, in the humidity

aging treatment. Therefore, moderate level of zinc sulde plays a major role in

maintaining adhesion stability in various aging treatments.

4. CONCLUSIONS

Adhesion stability between rubber compounds and brass-plated steel cords isconsiderably enhanced by incorporation of BAE. Copper migration to the rubber

bulk was accelerated by BAE incorporation, resulting in moderate copper sulde

and zinc sulde formation and the enhancement of adhesion stability. The loading

of BAE suppressed dezincication signicantly after humidity aging. BAE acts as

an adhesion promoter in hostile environments.



814 G. S. Jeon

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3 boric acid brass plated steel cord

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