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Stabilizing the adhesioninterphase between rubbercompounds and brass filmby the addition of resorcinolformaldehyde resin to the rubberGon Seo aa Department of Chemical Technology, Chonnam NationalUniversity, Kwangju 500-757, KoreaPublished online: 02 Apr 2012.

To cite this article: Gon Seo (1997) Stabilizing the adhesion interphase betweenrubber compounds and brass film by the addition of resorcinol formaldehyde resinto the rubber, Journal of Adhesion Science and Technology, 11:11, 1433-1445, DOI:10.1163/156856197X00228

To link to this article: http://dx.doi.org/10.1163/156856197X00228

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Stabilizing the adhesion interphase between rubber

compounds and brass film by the addition of resorcinol

formaldehyde resin to the rubber

GON SEO*

Department of Chemical Technology, Chonnam National University, Kwangju 500-757, Korea

Received in final form 14 July 1997

Abstract-The effect of resorcinol formaldehyde (RF) resin on the stability of the adhesion interphase between rubber compounds containing different amounts of RF resin and the thin brass film deposited on a glass plate was investigated under the conditions of humidity aging. The depth profiles of the adhesion interphase were examined by AES (Auger electron spectroscopy). In the unaged state, there was no significant difference in the adhesion interphase with varying content of RF resin, but the migration of copper and zinc to the bulk of the rubber during humidity aging was considerably suppressed with high levels of RF resin in the rubber compound. The pull-out force and rubber coverage of TCAT (tire cord adhesion test) samples also showed an improvement in the adhesion durability to humidity aging, caused by the addition of RF resin. The improvement in the adhesion stability to humidity aging obtained by adding RF resin to the rubber compound can be explained by a barrier formation effect, which prevents further migration of reactive materials inducing the degradation of the adhesion interphase.

Keywords: Adhesion interphase; rubber-to-brass bonding; RF resin; AES; depth profile.

1. INTRODUCTION

Brass-plated steel cords are inserted in the belts and carcasses of tires to improve mechanical stability. Brass reacts with the rubber compound during the curing stage of tire manufacturing, forming an adhesion interphase between the rubber compound and the steel cord. Copper and zinc sulfides form in the adhesion interphase as a result of the reaction between the brass and sulfur in the rubber compounds. Oxides and hydroxides of copper and zinc also form in the adhesion interphase, due to the reaction of brass with oxygen and residual water in the rubber compound [1]. This demonstrates the chemical complexity of the adhesion interphase, which varies with the composition of the rubber compound and brass as well as the curing conditions [2].

The structure and composition of the adhesion interphase are important for the

strength and stability of the adhesion between rubber compounds and the brass-plated

*E-mail: gseo@orion.chonnam.ac.kr

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steel cords. Adhesion becomes weak when the copper sulfide layer is not sufficiently thick, but the excessive formation of copper sulfide and zinc oxide brings about their

own cohesive failures. Excessive migration of copper sulfide and zinc oxide into the

rubber compound also leads to poor adhesion.

Adhesion stability is important for tire performance and so the degree of adhesion

must withstand the conditions of use. The degradation of adhesion must be slowed

down as much as possible to prolong the stable performance and long service of

the tire. Many reports have thus already been written regarding the optimal rubber

composition and curing conditions, as well as the copper content and plating thickness

of brass, required to achieve strong and stable adhesion [3-5]. Resorcinol fomaldehyde resin (Penacolite B-18-S, Indspec Chem. Co., Pittsburgh,

PA, USA; hereafter designated as RF resin) is used as an adhesion stabilizer for

suppressing adhesion degradation. Hamed and Huang explained that the migration of RF resin to the brass surface led to the formation of a barrier against attack by moisture, thus preserving adhesion against humidity aging [4]. Even though the

migration of RF resin to the brass surface has been observed using IR spectroscopy in a simple mixture of natural rubber and RF resin, the contribution of RF resin to

stabilizing adhesion at microscopic levels is not yet clear.

Since the strength and stability of the adhesion are dependent on the structure of the

adhesion interphase, the structure of this interphase has been studied using various

surface analysis techniques such as XPS (X-ray photoelectron spectroscopy) [6] and

AES [5, 7]. The schematic structure of the adhesion interphase and its change when

subjected to aging treatment are well described in van Ooij's review papers [1-3]. Surface analysis is helpful for studying the effect of rubber additives on the formation

and degradation of the adhesion interphase, but in samples obtained by mechanical

breaking, it is difficult to ensure enough cross-sample homogeneity and reproducibili-

ty. However, adhesion samples prepared by curing rubber compounds with thin brass

films deposited on glass plates do not require any mechanical treatment [8]. Surface

analysis with sputtering through the brass film towards the bulk of the rubber provides a complete depth profile of the brass layer, the rubber, and the interphase between

them, showing the structure of the adhesion interphase. The aim of this study was to show the enhancing role of RF resin in the stability of

the adhesion interphase. AES was used to investigate the role of RF resin in suppress-

ing the degradation of the adhesion interphase between the rubber compound/brass- film samples against humidity aging. Stabilization of the adhesion upon addition of

RF resin was also confirmed by the adhesion test of the rubber compound/brass-plated steel cord system.

2. EXPERIMENTAL

A thin brass film with a Cu/Zn ratio of 70: 30 was sputtered onto glass plate (Men-

zenglasser, Germany, 76 x 26 mm) using an RF Magnetron sputterer for 70 s at

2 x 10-6 Torr. RF power was controlled to 400 W. The thickness of the thin brass

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film was confirmed to be 180 nm using a SEM (scanning electron microscope, JEOL JSM 7400).

Three rubber compounds with different amounts of RF resin were prepared. Master- batch components were as follows: natural rubber (Lee Rubber Co., Malaysia, SMR

100), 100 phr; carbon black N326 (Lucky Co., Korea), 67 phr; aromatic processing oil (Michang Co., Korea, A#2), 8.0 phr; zinc oxide (Hanil Co., Korea), 8.0 phr; an- tioxidant (Monsanto Co., USA, Kumanox-RD, 2,2,4-trimethyl-1,2-dihydroquinone), 4.0 phr; cobalt boroacylate (Rhone-Poulenc Co., France, Manobond 680C), 0.75 phr. Final rubber compound components were as follows: masticated rubber masterbatch, 100 phr; stearic acid (Pyungwha Co., Korea), 1.2 phr; hexamethoxymethylmelamine (Cytec Co., USA, Cyrez-964, 35 wt% Si02), 3.7 phr; accelerator (Monsanto Co., USA, Santocure MOR, 2-(morpholinothio)-thio-benzothiazole), 0.8 phr; insoluble sul- fur (Akzo Chemicals Co., The Netherlands, Crystex HS OT 20), 5.0 phr.

The amount of RF resin added to the compound was 1, 2, and 3 phr to make the rubber compounds, which were designated as R-l, R-2, and R-3 rubber, respectively. The rubber compounds were mixed following the procedures described in ASTM

D3184-80, using an internal mixer (Farrel Co., USA, Banbury Mixer model 82). All the masterbatch components were mixed for 10 min at a rotor speed of 40 rpm and

dumped at 150 °C. After the masterbatch compound was cooled to room temperature, the final mixing components were mixed for 5 min at a rotor speed of 30 rpm and

dumped at 90°C. After dumping, the batches were sheeted out using a two-roll mixing mill (Farrel Co., USA, model MKIII).

A brass-on-glass plate was sandwiched between two uncured pads of each rubber

compound, which were then placed in a pad mold. In order to determine the curing time, the T90 time at 160°C was measured based on the rheocurve obtained from a Monsanto Rheometer (model Rheo-100). The T90 times of R-1 and R-3 rubbers were 12.0 and 9.0 min, respectively. Since the variance of the T90 time with the level of RF resin was small, all samples were cured at 160°C for 15 min using a Cure Press (Osaka Jack Co. Ltd., Japan), regardless of the RF resin content. The adhesion

samples were placed in a humidity chamber for 7, 14, and 21 days under conditions of 85°C and 85% relative humidity.

The depth profiles from the outer brass surface to the bulk of rubber were recorded on a Perkin-Elmer Auger spectrometer (Phi 670). A surface area of 10 x 10 f.Lm2 was examined at a potential of 5.0 keV, a current of 0.2 ,u,A, and an incident angle of 60° to the specimen. Surface concentrations were determined from the Auger peaks of detected elements with compensation for their sensitivities at every 0.5 min. A sputter gun with an argon ion beam rastered on a 2 x 2 mm2 area was used for the depth profiling. The sputtering rate for the brass film was determined to be 25 nm/min. It is difficult to determine the sputtering rate for the adhesion interphase precisely, because it includes various chemical components with variable concentrations. Therefore, the

sputtering time of the absolute depth instead is used to indicate the depth of the adhesion interphase in this paper.

In order to examine the stabilizing effect of RF resin on the adhesion against hu-

midity aging, the pull-out force and rubber coverage of a brass-plated steel cord to the rubber compound were measured using the TCAT method. A six-cavity mold

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was used to prepare the TCAT sample of the brass-plated steel cord and the rubber

compound following the procedure described in [4]. Brass-plated steel cord (4 x 0.28) manufactured by the Tongyang Nylon Co. (Korea) was used. The plating weight of

brass on the steel cord was 3.6 g/kg and the copper content of the brass was 63.6%.

The curing and aging treatments were the same as those for the preparation of the rub-

ber compound/brass-film adhesion samples described above. The pull-out force was

determined as the maximum force exerted by the tensile tester (Shimadzu Co., Japan,

Autograph AGS-1000) on the TCAT sample during the pull-out test, with 10 mm/min

of crosshead speed. Rubber coverages were also noted. Each value reported was the

average derived from six specimens.

3. RESULTS AND DISCUSSION

3.1. Adhesion interphase

Figure I shows the AES depth profiles obtained from the adhesion samples prepared with brass films and three rubber compounds after curing at 160°C for 15 min. The concentrations of carbon and oxygen are high on the outer surfaces of the brass films.

In the R-1 /brass-film sample, the copper content increases after sputtering for 0.5 min

and then the intensity is maintained constant to the depth of sputtering for 5 min. A

peak of zinc is observed after sputtering for 0.5 min, which then flattens until another

peak of zinc is observed. The region with constant contents of copper and zinc is

due to a brass film which does not react. Small peaks of zinc, sulfur, and oxygen are

observed after sputtering for 5 min, showing the formation of an adhesion interphase composed of sulfides and oxides of copper and zinc. After sputtering for 7 min, the

signal intensities of carbon and sulfur are constant, indicating the bulk of rubber. So, the adhesion interphase between the rubber compound and brass can be deduced from

the depth profiles of the rubber compound/brass-film adhesion sample. The concentrations of zinc and oxygen on the outer surface were almost the same,

indicating the formation of zinc oxide due to surface oxidation. Zinc hydroxide

may also be formed on such surfaces [8], but it is not possibile to distinguish zinc oxide and zinc hydroxide precisely from AES spectra. Therefore, zinc oxide and zinc hydroxide are considered together as oxidized zinc. The fact that the detected

depth for the oxidized zinc is different for each sample is due to the variation in the

thickness of the surface contamination layer. At the depth of sputtering for 6 min from the peak of surface oxidized zinc, the

adhesion interphase between the rubber compound and the thin brass film was ob- served. As the sum of copper and zinc does not coincide with the sum of sulfur and

oxygen, part of the brass remains in a metallic state. These profiles are almost the same as those reported by Coppens et al. [8].

In the unaged state (Fig. 1), the depth profiles of the three adhesion samples of

rubber compound/brass film are similar, regardless of the RF resin content. This means that the effect of RF resin addition does not significantly alter the structure of the adhesion interphase in the unaged state.

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Figure 1. Depth profiles of the unaged rubber compound/brass-film adhesion samples: (a) R-1 rubber; (b) R-2 rubber; and (c) R-3 rubber.

Figure 2 shows the depth profiles of copper and sulfur in the R-1 and R-3 adhesion

samples after humidity aging. The copper peak in the R-1 rubber compound sample spreads over a wide area with humidity aging. After humidity aging for 21 days, copper signals are observed even at the depth of 30 min of sputtering. In contrast, there is no significant migration of copper for the R-3 rubber compound. The migration of copper during humidity aging is significantly suppressed by the addition of high levels of RF resin. The changes in the sulfur peaks are similar to those of the copper peaks.

The shapes and distributions of the zinc and oxygen signals in the adhesion in-

terphase also change during humidity aging. These signals are widely spread in the

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Figure 2. Depth profiles of copper and sulfur for the rubber compound/brass-film adhesion samples with humidity aging: (a) unaged; humidity aging for (b) 7 days, (c) 14 days, and (d) 21 days.

R-1 adhesion sample after humidity aging, as shown in Fig. 3, but spreading is not

observed in the R-3 adhesion sample. Although the signals of zinc and oxygen on the outer surfaces increase after humidity aging, the migration of these elements towards

the rubber bulk is negligible in the R-3 sample.

Figure 4 shows the depth profiles of the adhesion samples after 21 days of humidity

aging. The distributions of copper, zinc, sulfur, and oxygen differ considerably,

according to the RF resin content. Carbon signals are omitted in order to show

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Figure 3. Depth profiles of zinc and oxygen for the rubber compound/brass-film adhesion samples with humidity aging: (a) unaged; humidity aging for (b) 7 days, (c) 14 days, and (d) 21 days.

clearer profiles. Copper and zinc are distributed very widely across the R-I adhesion

sample, while the spread of these signals is negligible in the sample made with R-3 rubber.

Very complicated reactions between the rubber compound and the brass film occur

during the curing process and humidity aging. But these results show clearly that the migration of copper and zinc towards the rubber bulk during humidity aging is

considerably suppressed with the increases in the amount of RF resin added to the

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Figure 4. Depth profiles of the rubber compound/brass-film adhesion samples after humidity aging for 21 days: (a) R-1 rubber; (b) R-2 rubber; and (c) R-3 rubber.

rubber compounds. RF resin in the rubber compound migrates to the brass surface and reacts with hexamethoxymethylmelamine acting as a methylene donor, forming a high molecular weight layer during the curing process, as suggested by Hamed and

Huang [4]. The barrier suppresses further migration of sulfur and moisture to the adhesion interphase. Reduction in the migration of reactive materials contributes to

the preservation of the adhesion interphase against humidity aging.

3.2. Adhesion between the rubber compound and brass-plated steel cord

The pull-out force and rubber coverage of TCAT samples differ with the amount of

RF resin added to the rubber compound as shown in Table 1. The unaged pull-out

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Table 1. Adhesion properties of the rubber compounds (R-1 to R-3) with different contents of RF resin to brass-plated steel cord

"Force per cord.

force decreases with increasing RF resin content. The pull-out force of R-3 rubber is

low, about 80% of that of R-1 rubber, indicating that high levels of RF resin lead to low adhesion in the unaged state. But after humidity aging for 21 days, the pull-out force of R-3 rubber is considerably higher than that of R-1 rubber. The pull-out force decreases for all the samples during humidity aging, but the decrease rate is

considerably slower for the rubber compound containing large amounts of RF resin.

Rubber coverages for unaged samples are high, near 80-90% for all three rubber

compounds, regardless of the RF resin content. But rubber coverage after humidity

aging for 21 days varies with the RF resin content. The rubber coverage of R-3 rubber after humidity aging for 21 days is as high as 80%, while that of R-1 rubber is as low

as 20%. It is worth noting that in the unaged state, RF resin does not produce good adhesion, but the adhesion durability against humidity aging is considerably improved with the addition of high levels of RF resin.

Figure 5 shows the scanning electron micrographs of cords pulled out from unaged TCAT samples. The fraction of exposed metal on the cord pulled out from the R-3 rubber sample is smaller compared with the other samples. Only a thin layer of rubber remains adhered to the cord pulled out from the R-1 sample, compared with the thicker

layer on the R-3 sample. Although the difference in rubber coverage among the RF

resin variant samples is not large in the unaged state, the rubber coverage of R-3

rubber is better than those of the other rubbers. The shape and coverage of rubber on pulled-out cords change with humidity aging,

as shown in Fig. 6. After humidity aging for 21 days, the exposed metallic area increases for R-1 and R-2 rubbers. On the other hand, the fraction of exposed metal does not increase for the cord pulled out from R-3 rubber, indicating a negligible change in the adhesion interphase with humidity aging for 21 days.

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Figure 5. Scanning electron micrographs of cords pulled out from unaged TCAT samples: (a) R-1 rubber; (b) R-2 rubber; and (c) R-3 rubber.

In the unaged state, there is no large improvement in adhesion between the rubber

compound and the brass-plated steel cord with the addition of RF resin, but the

durability of the adhesion against humidity aging is improved with the addition of

high levels of RF resin.

3.3. The role of RF resin in enhancing adhesion stability

The adhesion properties between the rubber compound and the brass-plated steel cord

are considerably improved with the addition of high levels of RF resin, especially after humidity aging, although the difference in the amount added is relatively small, about 2 phr.

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Figure 5. (Continued).

Figure 6. Scanning electron micrographs of cords pulled out from TCAT samples aged for 21 days under humid conditions: (a) R-1 rubber; (b) R-2 rubber; and (c) R-3 rubber.

The curing characteristics and physical properties of these rubber compounds were also examined. The tensile strength decreased slightly for the rubber compound with

high levels of RF resin after humidity aging, while the modulus was slightly higher, as previously reported [4]. However, the effects of RF resin on these properties are not large enough to induce a significant change in the adhesion properties in the range of 1-3 phr.

The adhesion interphases between the rubber compound and the thin brass film are almost similar in the unaged state, regardless of the RF resin content in the range from I to 3 phr. But the change of the adhesion interphase with humidity aging is greatly

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Figure 6. (Continued).

affected by the RF resin content. The adhesion interphase of the rubber compound containing I phr of RF resin is widely spread after humidity aging, but that of the rubber compound containing 3 phr of RF resin is still retained after humidity aging for 21 days. This shows that RF resin forms a protective barrier in the adhesion interphase to suppress the degradation, resulting in a high resistance to humidity aging.

Since moisture is one of the reactive materials which induce adhesion degradation, the barrier produced from the reaction of RF resin and hexamethoxymethylmelamine may play a role in preventing moisture attack, as suggested by Hamed and Huang [4]. In addition to this suggestion, the barrier prevents the migration of reactive materials such as copper and zinc towards the bulk rubber, and the migration of sulfur and

oxygen to the adhesion interphase, resulting in suppression of the degradation of the adhesion interphase under the conditions of humidity aging.

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4. CONCLUSION

The depth profiles of the adhesion interphase in the rubber compound/brass-film samples are almost the same in the unaged state, regardless of the RF resin content in the rubber. But the degradation in the adhesion interphase due to the migration of copper and zinc towards the bulk of rubber during humidity aging is considerably suppressed in the rubber compound containing 3 phr of RF resin. The durability of the adhesion to humidity aging, measured using the TCAT method, also improves with high levels of RF resin, even though the adhesion is poorer in such samples in the unaged state. The improvement in adhesion stability due to the addition of RF resin under conditions of humidity aging can be explained as an effect of barrier

formation, which prevents degradation of the adhesion interphase.

Acknowledgements

I wish to acknowledge Chonnam National University for its award of a Research Grant

(1995) and the invaluable assistance of Dr. G. S. Jeon, Dr. B. Y. Sohn, Mr. D. M. Lim, and Mr. S. J. Choi.

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