Atomic-level understanding of the rubber-brass adhesion and the effect of additives

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DISSERTATIONS | CHIAN YE LING | ATOMIC-LEVEL UNDERSTANDING OF THE RUBBER–BRASS ADHESION... | No 376

CHIAN YE LING

ATOMIC-LEVEL UNDERSTANDING OF THE RUBBER–BRASS ADHESION AND

THE EFFECT OF ADDITIVES

THE UNIVERSITY OF EASTERN FINLAND

Rubber–brass adhesion has high importance in practical applications like tire production.

A detailed understanding of the adhesion is essential to enhance the rubber–brass interaction for better performance. The thesis focuses on the adsorption of rubber adsorbate models on ZnS, Cu_xS and ZnO surfaces to study the role of rubber functional groups and effect of different dopants at atomic level. The results can be used in practical applications for optimum

enhancement of the rubber–brass adhesion.

CHIAN YE LING

Chian Ye Ling

ATOMIC-LEVEL UNDERSTANDING OF THE RUBBER – BRASS ADHESION AND THE EFFECT

OF ADDITIVES

Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences

No 376

University of Eastern Finland Joensuu

2020

Academic dissertation

To be presented by the permission of the Faculty of Science and Forestry for public examination in the Auditorium F101 in the Futura Building at the University of Eastern

Finland, Joensuu, on April 24, 2020, at 12 o’clock noon

Grano Oy Jyväskylä, 2020 Editor: Nina Hakulinen

Distribution: University of Eastern Finland / Sales of publications www.uef.fi/kirjasto

ISBN: 978-952-61-3366-9 (Print) ISSNL: 1798-5668

ISSN: 1798-5668 ISBN: 978-952-61-3367-6 (PDF)

ISSNL: 1798-5668 ISSN: 1798-5676

Author’s address: Chian Ye Ling

University of Eastern Finland Department of Chemistry P.O. Box 111

80101 JOENSUU, FINLAND Email: chian-ye.ling@uef.fi

Supervisors: Professor Emeritus Tapani Pakkanen, Ph.D.

University of Eastern Finland Department of Chemistry P.O. Box 111

80101 JOENSUU, FINLAND Email: tapani.pakkanen@uef.fi

Docent Janne Hirvi, Ph.D.

University of Eastern Finland Department of Chemistry P.O. Box 111

80101 JOENSUU, FINLAND Email: janne.hirvi@uef.fi

Reviewers: Professor Tapio Rantala, Ph.D.

Tampere University Department of Physics P.O. Box 692

33101 TAMPERE, FINLAND Email: tapio.rantala@tuni.fi

Professor Emeritus Markku Räsänen, Ph.D.

University of Helsinki Department of Chemistry P.O. Box 55

00014 HELSINKI, FINLAND Email: markku.rasanen@helsinki.fi

Opponent: Professor Jouni Pursiainen, Ph.D.

University of Oulu Sustainable Chemistry P.O. Box 8000

90014 OULU, FINLAND Email: jouni.pursiainen@oulu.fi

5 Ling, Chian Ye

Atomic-level understanding of the rubber–brass adhesion and the effect of additives Joensuu: University of Eastern Finland, 2020

Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences 2020; 376

Abstract

Rubber–brass adhesion is highly important in practical applications such as tire manufacturing, conveyor belts, and hydraulic hoses. The adhesion is influenced by several factors, which include the compositional change of components in adhesive interlayer, the composition of the rubber structure, and the inclusion of additives. Thus, to achieve a stronger interaction to enhance the adhesion between rubber and brass, it is essential to have a thorough understanding of rubber–brass adhesion. In this study, an atomic level investigation is carried out via a computational approach at the Density Functional Theory (DFT) level to study the role of rubber functional groups and different components present at the adhesive interlayer, as well as the effect of additives on the rubber–brass adhesion.

The adhesion interface is very complex, because copper and zinc hydroxides and oxides are also formed simultaneously along with copper and zinc sulfides. Thus, throughout our study, the adsorption of the rubber adsorbate models is studied on copper and zinc sulfide surfaces, which are responsible for adhesion. In addition, zinc oxide is also examined. Among all the functional groups available on the rubber structure, carbon-carbon double bonds and sulfur containing functional groups in rubber are found to be responsible for the interaction with the surfaces. Between sulfide and oxide surfaces, a stronger interaction is exhibited in the interaction with the sulfide surfaces. Adsorption on zinc and copper sulfide surfaces exhibit a different order of the adsorption strength. Sulfur containing adsorbates interacted stronger with zinc sulfide while carbon-carbon double bonds adsorbed stronger on copper sulfide. Despite weaker interaction, the adsorption strength order on zinc oxide is similar to that on zinc sulfide.

A doping approach has been utilized to investigate the effect of additives on rubber–

brass adhesion. The adsorption study is performed only on the promising surfaces, which are zinc and copper sulfides. Cobalt has been known to enhance the adhesion between rubber and brass. From our study, an improvement in adsorption strength is observed with cobalt doping.

Cobalt doping promotes the interaction of all the functional groups of rubber with the zinc sulfide surface, which include the carbon-carbon double bonds and sulfur containing functional groups and activates the carbon-hydrogen bonds of saturated hydrocarbon adsorbates. On the other hand, when used with the copper sulfide surface, the promotional effect is only shown through the carbon-carbon double bonds for the rubber adsorbate models.

However, due to the demand of finding a substitute for cobalt additives, several transition metals were studied as alternative dopant atoms, such as manganese, iron, and nickel.

Among these alternatives, manganese showed promise in improving the adhesion on both zinc and copper sulfide. Comparing manganese to cobalt results in a weaker promotional effect on the zinc sulfide but a greater promotional effect on copper sulfide. Iron doping, which only improved the adhesion on copper sulfide, showed a weaker effect than manganese; nickel

doping showed the weakest promotional effect. With different promotional effects shown through different dopant atoms, the best way to enhance the interaction between rubber and brass is by using combinations of dopant atoms.

Universal Decimal Classification: 544.72

Library of Congress Subject Headings: Adhesion; Sulfides; Rubber; Adsorption; Zinc sulfide;

Copper sulfide; Zinc oxide

Other keywords: Rubber–brass adhesion; Adhesive interlayer; Rubber adsorbate models, Dopant atom

7

Acknowledgements

This research was carried out at the Department of Chemistry, University of Eastern Finland during the years 2014-2018. The work was funded by the Finnish Funding Agency for Technology and Innovation TEKES and the European Union/ European Regional Development Fund (ERDF) for the ‘‘Smart Active Materials’’ project (70058/11) and “Vauhtia Renkaisiin” project (3246/31/2015). We acknowledge grants of computer capacity from the Finnish Grid and Cloud Infrastructure (persistent identifier urn:nbn:fi:research-infras- 2016072533).

I would like to express my utmost gratitude and appreciation to my supervisor, Prof.

Tapani Pakkanen, and Doc. Janne Hirvi for their guidance, advice, patience, and encouragement. Their continuous support helped me throughout the duration of my study.

It was an honor to work in close collaboration with teams from Nokian Renkaat Oyj. I am particularly grateful for their professional guidance with respect to our research. I would also like to thank the entire VR research group for numerous fruitful discussions and support.

I would like to express my sincere appreciation to the staff of the chemistry department, friends, and everyone that has assisted, guided, encouraged, and supported me throughout my undertaking of this study. You were always there to assist and help me, whenever I faced problems and needed valuable advice and support. I also had a pleasant time working together with Balogun Fatimoh Olayinka for her Master thesis. To Linlin and Wenjuan, my office mates, you have provided me valuable feedback, and I thank you for sharing the ups and downs with me and for the support.

My warmest thanks go to my family for your endless supports and encouragements.

Your support means a lot to me.

Joensuu, 2020 Chian Ye Ling

List of Abbreviations

AES Auger electron spectroscopy BSSE Basis set superposition error CuS Copper(II) sulfide

Cu2S Copper(I) sulfide DFT Density functional theory

PBE0 Hybrid exchange-correlation functional of Perdew, Burke, and Ernzerhof Def-TZVP Triple zeta valence basis set with polarization functions

XPS X-ray photoelectron spectroscopy

ZnO Zinc oxide

ZnS Zinc sulfide

9

List of original publications

This dissertation is a summary of publications I-III.

I. Ling, C.Y.; Hirvi, J.T.; Suvanto, M.; Bazhenov, A.S.; Ajoviita, T.; Markkula, K.;

Pakkanen. T.A. A computational study of adhesion between rubber and metal sulfides at rubber–brass interface. Chem. Phys. 2015, 453-454, 7.

II. Ling, C.Y.; Hirvi, J.T.; Suvanto, M.; Bazhenov, A.S.; Markkula, K.; Hillman, L.;

Pakkanen, T.A. Effect of cobalt additives and mixed metal sulfides at rubber–brass interface on rubber adhesion: a computational study. Theor. Chem. Acc. 2017, 136, 24.

III. Ling, C.Y.; Hirvi, J.T.; Markkula, K.; Hillman, L.; Pakkanen, T.A. Computational approach to study the influence of Mn, Fe, and Ni as additives towards rubber–brass adhesion. Theor. Chem. Acc. 2018, 137, 64.

Author’s Contribution

The key ideas for the topic in publications I, II, and III are based on conversations and discussions between the author and co-authors. The author has the main responsibility in the writing of the manuscripts and has carried out all computational works.

11

Contents

1 Introduction ……… 13

1.1 Rubber ……… 13

1.2 Brass-plated steel cord ………14

1.3 Rubber–brass adhesion ……… 15

1.4 Formation of ZnO ...……… 17

1.5 Additives ……….………... 17

1.6 Summary ……… 18

1.7 Aims of the study ...………. 18

2 Models and methods ………... 19

2.1 Models ……… 19

2.1.1 Surfaces and dopants ………. 19

2.1.2 Rubber adsorbate models ………... 21

2.2 Computational details ……… 22

3 Adsorption of rubber adsorbate models on sulfide surfaces ………. 23

3.1 Adsorption on zinc sulfide surfaces ………23

3.2 Adsorption on copper sulfide surfaces ………... 24

4 The effect of dopant atoms on adsorption of rubber adsorbate models on doped sulfide surfaces ………...……… 26

4.1 Adsorption on doped ZnS(110) surfaces ……… 26

4.2 Adsorption on doped Cu2S(111) surfaces ……….. 27

4.3 Promotional impact of dopant atoms on sulfide surfaces ………... 29

5 Adsorption of rubber adsorbate models on zinc oxide surfaces ……… 31

6 Discussion ………... 33

7 Conclusions ……… 34

Bibliography ………. 35

13

1 Introduction

Technologies that promote the adhesion between rubber and metal are important in various industries. A typical case is in tire manufacturing, and a schematic of an automobile’s radial tire is illustrated in Figure 1. Other than rubber tires, conveyor belts and hydraulic hoses are also among the products of steel cord reinforced rubber products. The role of the steel cord is to give structural strength to the rubber while maintaining its flexibility. Therefore, it is important to have strong rubber to steel cord adhesion for good performance and prolong the lifespan of the products, but the direct adhesion of rubber to most metals, including steel cord, is weak. To cause such stiff adhesion, a few principal methods have been employed, including the use of an ebonite interlayer, brass plating the metal, or the use of a polymeric adhesive interlayer.¹ The most widely used of these methods, especially in tire manufacturing, is plating brass onto the steel cord. During the curing process, strong bonds between the rubber and brass are established, and a rubber–brass adhesion interface is formed. In this chapter, the general knowledge regarding the rubber–brass adhesion will be presented, including the components of rubber and brass as well as the formation of the rubber–brass adhesive interlayer and the importance of additives.

Figure 1. Typical schematic of the cross section of an automobile radial tire. Figure obtained from ref 2.

1.1 Rubber

In tires, the most widely used elastomer is natural rubber. More than 90% of natural rubber consists of cis-1,4-polyisoprene and the rest of the components are made of naturally occurring non-rubber constituents, such as proteins and carbohydrates.³ However, when used alone, natural rubber does not have sufficient strength to endure the forces that it encounters in its applications. Therefore, the incorporation of either textile or steel reinforcement is essential in

order to boost the strength of the rubber. One of the ways to improve the strength of natural rubber is by vulcanizing the rubber with sulfur in order to create a network of chemical crosslinks.⁴ The generalized structure of vulcanized rubber is shown in Figure 2. Vulcanized rubber exhibits better properties when compared to natural rubber, which include higher elasticity, a lack of stickiness, and a resistance to abrasion. Furthermore, vulcanized rubber does not harden at cold temperatures or soften much except at high temperatures. Properly formulated sulfur-vulcanized rubber is known to have better adhesion with brass and forms a rubber–brass bonding that exceeds the cohesive strength of the rubber.^4,5

Figure 2. Generalized structures in sulfur-vulcanized rubber. Figure obtained from ref 6.

1.2 Brass-plated steel cord

Because of its low cost and good mechanical properties, steel cords are often chosen as the reinforcement material in rubber tires.⁷ However, bare steel cord adheres rather poorly with rubber, and hence, the steel cord is coated with a layer of brass in order to increase the adhesion strength between the rubber and the steel cord. The schematic diagram of the steel cord plated with brass, which is an alloy of copper and zinc, is presented in Figure 3. The thickness of the brass plating can vary by up to 0.3 μm.² At room temperature, an oxide layer is normally formed at the surface of the brass. This layer is formed when the zinc ions at the surface are being oxidized during the forming process.⁸ The zinc oxide layer is then overlaid with a very thin layer of copper oxide.^8,9

15 Figure 3. Schematic of the brass-plated steel cord. Figure obtained from ref 8.

1.3 Rubber–brass adhesion

A strong rubber–brass bonding is created during the vulcanization of rubber and brass due to the existence of non-stoichiometric copper sulfide (CuxS) (1.8 < x < 2.0)^10,11 and CuS alongside ZnS. Copper sulfides have been speculated to be responsible for the rubber–brass interface bonding since the early 1970s.¹² The sulfides are formed when copper ions, zinc ions, and the free electrons, which migrate to the brass surface via cationic diffusion, undergo the sulfidation process.¹³ The amount of copper sulfide formed is directly related to the degree of sulfidation, and it is essential to control the thickness of the copper sulfide layer because exceeding the optimum thickness will prevent the formation of strong rubber–brass bonding. The formation of copper and zinc sulfides have been confirmed by X-ray photoelectron spectroscopy (XPS)^12,14,15 and Auger electron spectroscopy (AES)^15-17 measurements. A schematic illustration of a typical rubber–brass interface is shown in Figure 4.

Figure 4. Interface structure of the rubber–brass-plated steel cord. Figure obtained from ref 8.

Many studies have examined the mechanism behind rubber–brass adhesion, but this is still the subject of much interest, since a detailed understanding of the adhesion mechanism of rubber on brass remains elusive. Moreover, the knowledge of how the alteration of the composition of the rubber and the surface structure at the rubber–brass interface can affect the rubber–brass adhesion is also important in enhancing the bonding between rubber and brass. Even so, mechanisms such as mechanical interlocking¹⁸ and the chemical bonding model¹⁹ are two possible mechanisms that have been proposed. Thus far, out of the two, the mechanical interlocking mechanism is more widely accepted for rubber–brass adhesion, as the rubber–

brass bonding strength is seen to be related to the thickness of the copper sulfide layer.

The mechanical interlocking model was proposed by MacBain and Hopkins in 1925.²⁰ This model proposes that mechanical interlocking of adhesives into the irregularities of adhered surface is the main source of the intrinsic adhesion. However, this theory is not universally applicable, because good adhesion occurs between smooth surfaced substrates and it is not a mechanism at the molecular level. Therefore, many researchers have noted that mechanical interlocking has significance in explaining the adhesion phenomena but must be used in conjunction with other forces. For instance, the effects of both mechanical interlocking and adsorption theory could be taken into account to describe the adhesion process in polymer–

metal systems, such as rubber–brass adhesion.²¹ The adsorption theory was proposed by Sharpe and Schonhorn,²² and it is a more generally accepted model in adhesion science. Adhesion via this mechanism is based on the surface chemical forces and the chemisorption or physisorption of atomic and molecular species. Thus, in our adsorption study of rubber–brass adhesion at the atomic level, we will discuss the interaction between the rubber adsorbate and the brass surface based on the adsorption theory.

17

1.4 Formation of ZnO

ZnO is readily found on the brass surface, since zinc is oxidized more easily than copper.

Moreover, additional ZnO is also present in the rubber–brass interlayer as ZnO is formed because of the oxygen in rubber.^23,24 The existence of ZnO can affect the rubber–brass adhesion.

Thus, the thickness of the ZnO layer is very crucial. An overly thick layer of ZnO leads to weak adhesion as it causes insufficient formation of copper sulfide, while a proper thickness of ZnO can help in preventing the overgrowth of copper sulfide and hence contribute to the stability of the adhesion interlayer. Therefore, the optimum growth of the ZnO layer is essential in the rubber–brass adhesion.

However, a rupture in the adhesion interface due to humidity aging is possible because of the excessive growth of ZnO and due to the loss of the metallic zinc.^25-27 The removal of zinc from the brass surface under aqueous conditions during aging, which is known as dezincification,^28,29 results in an increase of the copper content in the outermost layers. This process leads to an overgrowth of copper sulfide, where the thicker layer becomes brittle and nonbonding, leading to the failure of the rubber–brass adhesion. It has been suggested that the dezincification of brass leads to the degradation of rubber–brass adhesion.²⁸ Therefore, it is essential to inhibit the dezincification of brass when exposed to moisture. One way to inhibit this dezincification is by using additives, which have a function in inhibiting the dezincification process. Moreover, the inclusion of ZnO in the rubber compound also contributes in inhibiting the dezincification by reducing the diffusion of zinc to the surface of the steel cord.

1.5 Additives

The adhesion interlayer is very complex, because other than copper and zinc sulfides, copper and zinc hydroxides and oxides are also formed through the reaction between copper and zinc with water and oxygen in the rubber compound.11,14-17,23,24 Therefore, a suitable thickness and a stable structure at the adhesion interface is essential to achieve a good adhesion. When insufficient copper sulfide is formed, the adhesion becomes weak, while excessive growth of any component leads to their own cohesive failure.^23,30 Therefore, the role of additives is important for achieving good adhesion. Additives are necessary in forming the required copper sulfide needed in the adhesion interlayer by enhancing the migration of copper.^31,32

Cobalt salts^31,33,34 as additives have been widely used, because they affect both the initial adhesion and the longevity of the rubber to metal bonding.³⁵ Cobalt is known to promote the sulfur activation in the interlayer and affect sulfidation, as it inhibits the formation of zinc sulfide, thus allowing a suitable amount of copper sulfide to form to improve the adhesion.³⁵ For the longevity of the rubber–metal bonding, cobalt is able to suppress the excessive growth of the copper sulfide layer by slowing down the dezincification via the formation of cobalt oxide, which delays the debonding process.

Although cobalt acts as a promising additive, efforts to find other suitable alternatives, which are cost-effective and better performing than cobalt adhesion promoters, have been made.

Additionally, high content of cobalt salts and its existence in the rubber can lead to an adverse effect on the rubber compounds by degrading the rubber properties.³⁶ Therefore, the use of other transition metal salts has been proposed.

1.6 Summary

In tire manufacturing, a layer of brass is employed to the steel cord to increase the adhesion of rubber to the steel cord. Even though the formation of a rubber–brass adhesive interlayer and the adhesion mechanisms have been extensively studied experimentally, these are still the subject of much research and discussion due to the higher demand for better quality rubber tires. Furthermore, the complexity of the adhesion interlayer in terms of its structure and composition, as well as all the possible factors that influence the interfacial adhesion, further complicate the detailed understanding of rubber–brass adhesion. Additionally, little research has examined the computational studies of rubber–brass adhesion. Therefore, in our study, a computational approach is applied since it helps to understand the system behavior at the atomic level. The theoretical investigation on the mechanisms of interaction in rubber–brass adsorption is done at the level of Density Functional Theory (DFT) using a periodic supercell approach.

1.7 Aims of the study

The study described in this dissertation aims to achieve a detailed atomic-level understanding of rubber–brass adhesion through a computational approach. In particular, the following questions should be answered:

• What is the role of the functional groups of the rubber structure in interfacial interaction?

• How do copper sulfide, zinc sulfide and zinc oxide affect the interaction between rubber and brass at the adhesive interlayer?

• What is the promotional effect of cobalt as an additive in rubber–brass adhesion?

• What are the possible, more economical, and effective transition metals that can be used as alternatives to substitute for the currently used cobalt additives?

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2 Models and methods 2.1 Models

2.1.1 Surfaces and dopants

Because both non-stoichiometric copper sulfide (CuxS) and stoichiometric mono copper sulfide (CuS) are formed at the adhesive interlayer of rubber and brass, two bulk models that consist of hexagonal copper(II) sulfide and ideal cubic antifluorite copper(I) sulfide are constructed³⁷ for the adsorption studies. The usage of two idealized stoichiometric copper sulfides is a simplification that does not cover all the specific details of the complicated adhesive interlayer system. On the other hand, only one bulk model is created for zinc sulfide, where cubic zinc blende polymorph³⁸ is used. Other than sulfide, adsorption on the oxide surface has also been studied as the presence of zinc oxide at the adhesive interlayer has been reported. The bulk model of zinc oxide is constructed from hexagonal wurtzite.³⁹ The details of the optimized cell dimensions alongside the experimental values are tabulated in Table 1.

Table 1. Optimized cell dimensions (in Å) within the experimental space group symmetries for ZnS, Cu2S, CuS, and ZnO bulk structures. The values shown in parentheses refer to experimental counterpart.

ZnS (F-43m)⁴⁰ Cu2S (Fm-3m)⁴¹ CuS (P63/mmc)⁴² ZnO (P63mc)³⁹ a = 5.474 (5.400) a = 5.652 (5.629) a = 3.873 (3.788) a = 3.278 (3.249) b = 5.474 (5.400) b = 5.652 (5.629) b = 3.873 (3.788) b = 3.278 (3.249) c = 5.474 (5.400) c = 5.652 (5.629) c = 16.673 (16.333) c = 5.244 (5.204)

A (2 x 2) supercell of ZnS(110),^43,44 Cu2S(111),⁴⁵ and CuS(001)⁴⁶ surfaces (Figure 5), which is the most stable surfaces for zinc and copper sulfide, are cleaved from the respective optimized bulk structures. The thickness for the ZnS(110) and CuS(001) slabs, and the Cu2S(111) slab, are four and six atomic layers, respectively. Two slab models for zinc oxide are cleaved from the optimized bulk structure, since both ZnO(001)⁴⁷ and ZnO(110)⁴⁸ have been reported to be stable surfaces. A (2 x 2) supercell is created for the ZnO(001) surface, while a (1 x 1) supercell is created for the ZnO(110) surface. Both surfaces consist of four atomic layers. The corresponding surface models are visualized in Figure 6. Throughout the study, the lower halves of all sulfide and oxide surfaces are fixed at the equilibrium bulk environment, and the upper halves are relaxed.

Figure 5. Top and side view of (a) ZnS(110), (b) Cu2S(111), and (c) CuS(001) surfaces.

Figure 6. Top and side view of (a) ZnO(001) and (b) ZnO(110) surfaces.

To study the influence of different transition metals as surface dopant atoms on the adsorption of the rubber adsorbate models, the doping method is utilized, where a metal atom on the topmost of the unconstrained side is substituted with the desired dopant atom. We studied the effect of doping only on the ZnS(110) surface, since it exhibits a stronger interaction with the rubber adsorbate models than the ZnO surfaces. With similar reasoning, out of the two copper sulfide surfaces, we only focused on the Cu2S(111) surface. Doped sulfide surfaces are shown in Figure 7. Cobalt was chosen as our primary dopant atom since it has been widely used as an additive in rubber–brass adhesion. Cobalt also acted as our reference for the investigation of the effect of other transition metals as dopant atoms, such as manganese, iron, and nickel, in order to investigate their potential as cobalt substitutes in rubber–brass adhesion. For the influence of the mixed zinc-copper sulfide surface, the simplest case, which is the copper doped ZnS(110) surface, was chosen.

21 Figure 7. Top and side view of the doped sulfide surface models of (a) ZnS(110) and (b) Cu2S(111).

2.1.2 Rubber adsorbate models

The rubber adsorbate models used throughout the study were constructed based on the rubber structure (cis-1,4-polyisoprene) shown in Figure 8. During the adsorption calculations, the rubber adsorbate models were allowed to relax freely after positioned on the unconstrained side of the surfaces. Due to the complication of multiple functional groups on the rubber structure in the adsorption process, simpler prototypes of the rubber adsorbate models were created based on the functional groups present in the rubber structure. These prototypes consist of elementary functional group models that only contain one functional group, such as saturated hydrocarbons, ethene, and hydrogen sulfide. Larger models with two functional groups were constructed by replacing one of the hydrogen atoms in ethene with either a methyl or a thiol group.

We divided the chosen rubber adsorbate models into three different groups based on the functional group present in rubber structure, which include saturated hydrocarbons, unsaturated hydrocarbons, and sulfur-containing models. Each group consisted of smaller-to-larger adsorbate models, which are listed in Table 2. The use of models such as methane and hydrogen sulfide is a simplification, which does not directly represent the rubber structure.

Even so, by studying the adsorption of these models, we can predict the relationship between the smaller and larger models and the similarity between the adsorption trends with the larger models in each respective group of the rubber adsorbate models. This finding could suggest that the smaller adsorbate models are equally efficient at representing adsorption on sulfide and oxide surfaces.

Figure 8. Rubber adsorbate models constructed from cis-1,4-polyisoprene rubber structure.

Table 2. List of rubber adsorbate models

Saturated hydrocarbons Unsaturated hydrocarbons Sulfur-containing models

Methane Ethene Hydrogen sulfide

Ethane Methyl-substituted ethene Thiol-substituted ethene

Propane

2.2 Computational details

All calculations in this study are carried out using the Crystal09 program⁴⁹ at the DFT level alongside the PBE0 hybrid exchange-correlation functional.^50-52 DFT calculations were employed since they are suitable for modeling molecular and crystal systems as well as adsorption processes. DFT has been widely used because it provides both qualitative and quantitative insights into the structures of active surfaces and the surface reactions.⁵³ The standard def-TZVP basis sets are used for iron, nickel, manganese, sulfur, carbon, hydrogen, and oxygen atoms,⁵⁴ while for zinc, copper, and cobalt atoms, the optimized def-TZVP basis sets are applied [supplementary data in ref 55 and 56]. Spin-paired calculations are used for the adsorption study on pure sulfide and oxide surfaces while spin-unpaired calculations are employed for the doped sulfide surfaces due to the magnetic properties of the dopant atoms.

The density of the k-point is set high enough to ensure convergence. In this work, the adsorption energy (ΔE^ads) is calculated by ΔE^ads = EA/S – EA – ES, where EA/S, EA, and ES are the calculated energies of adsorption system, adsorbate, and surface, respectively. The counterpoise method has been utilized to correct the basis set superposition error (BSSE) in the calculated adsorption energies.⁵⁷

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3 Adsorption of rubber adsorbate models on sulfide surfaces 3.1 Adsorption on zinc sulfide surfaces

The calculated adsorption energies of various substances onto a zinc atom of a ZnS(110) surface are compiled in Table 3. The adsorption strength of elementary functional group models from the weakest to the strongest are as follows: saturated hydrocarbons < ethene <

hydrogen sulfide. This adsorption pattern is also reflected in larger models, where methyl- substituted ethene exhibits lower adsorption energy than thiol-substituted ethene. As seen from Table 3, there are only minor quantitative differences between the results obtained using smaller and larger adsorbate models in the respective adsorbate models grouping. In addition, similar adsorption trends are predicted for the different functional groups of rubber adsorbate models. Therefore, throughout our study, the same rubber adsorbate models are used, since the smaller adsorbate models are found to be as effective as the larger models in terms of the adsorption on sulfide and oxide surfaces.

Table 3. The calculated adsorption energies on the zinc and copper sulfide surfaces (in kJ mol^‒

1).⁵⁵

Rubber adsorbate models ZnS(110) Cu2S(111) CuS(001)

Methane –3.0 –0.5 ‒0.4

Ethane ‒4.3 ‒0.8 ‒1.1

Propane ‒3.0 ‒1.0 ‒1.8

Ethene ‒29.8 ‒48.2 ‒36.6

Hydrogen sulfide ‒61.4 ‒39.3 ‒17.4

Methyl-substituted ethene ‒39.8 ‒53.7 ‒37.4

Thiol-substituted ethene ‒50.5 ‒41.5 ‒29.4

The outward relaxation of interacting zinc atom in ethene and hydrogen sulfide cases shown in Figure 9(d) and 9(e) is not visible in the case of saturated hydrocarbons (Figure 9(a)-(c)), indicating a stronger interaction is achieved with ethene and hydrogen sulfide. Saturated hydrocarbons only undergo weak physisorption. On the other hand, ethene interacts through the carbon-carbon double bond and the hydrogen sulfide interaction occurs via the lone pair electrons in sulfur. Notable outward relaxation is also visible in the case of larger models (Figure 9(f) and 9(g)). The way that the models interact relate closely to ethene and hydrogen sulfide, since methyl-substituted ethene adsorbs via a carbon-carbon double bond while the thiol-substituted ethene interaction occurs through the lone pair sulfur electrons instead of a double bond, as hydrogen sulfide interacts stronger than ethene.

Figure 9. Side view of the optimized configurations on the ZnS(110) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.

3.2 Adsorption on copper sulfide surfaces

The obtained adsorption energies are presented in Table 3. Both copper sulfide surfaces show the same adsorption trend, with the rubber adsorbate models having a stronger interaction with the Cu2S(111) than with the CuS(001) surface. Saturated hydrocarbons still show the weakest adsorption, as seen with the ZnS(110) surface, but the adsorption strength for ethene and hydrogen sulfide on both copper sulfide surfaces are reversed when compared to ZnS(110) surface. This phenomenon is further confirmed by the interaction with larger models because of the similar adsorption strengths seen in ethene and hydrogen sulfide is reflected on both copper sulfide surfaces.

Despite the fact that the order of the adsorption strength of ethene and hydrogen sulfide is opposite of the order than on ZnS(110) surfaces, they behave similarly as on ZnS(110) surface in that they interact through a carbon-carbon double bond and the lone pair electron of sulfur, respectively. The only difference occurs in thiol-substituted ethene, where the carbon-carbon double bond dominates the interaction as opposed to the thiol group, since the adsorption strength of ethene is higher than that of hydrogen sulfide. The optimized structures of the rubber adsorbate models on both Cu2S(111) and CuS(001) surfaces are illustrated in Figure 10 and Figure 11, respectively. For both copper sulfide surfaces, stronger interactions caused by ethene, hydrogen sulfide, and substituted ethenes lead to outward relaxation of the interacting copper atom.

25 Figure 10. Side view of the optimized configurations on the Cu2S(111) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.

Figure 11. Side view of the optimized configurations on the CuS(001) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.

4 The effect of dopant atoms on adsorption of rubber adsorbate models on doped sulfide surfaces

4.1 Adsorption on doped ZnS(110) surfaces

The calculated adsorption energies for the rubber adsorbate models positioned above the dopant atom on doped ZnS(110) surfaces are tabulated in Table 4. The energies are then compared to those on the undoped ZnS(110) surface. Despite having different dopant atoms, the adsorption trend seen on the undoped ZnS(110) surface is followed, but different adsorption strengths are observed. Among the elementary functional group models, saturated hydrocarbons still show the weakest interactions, but there is a notable enhancement in adsorption due to cobalt and manganese doping. For the ethene and hydrogen sulfide cases, only copper dopant atom decreases the adsorption strength, while cobalt doping has the highest positive effect showing notable enhancement in the calculated adsorption energies. The next most effective dopant atom is manganese, while both iron and nickel doping atoms display similar adsorption strengths as the undoped ZnS(110) surface. A similar qualitative observation on the changes in the adsorption strength can also be deduced for methyl- and thiol-substituted ethenes, depending on the inclusion of the corresponding dopant atom.

Table 4. The calculated adsorption energies on the undoped and doped ZnS(110) surfaces (in kJ mol^–1)

Rubber adsorbate models Undoped⁵⁵ Mn doped Fe doped Co doped⁵⁶ Ni doped Cu doped⁵⁶

Methane –3.0 –10.2 –2.4 –27.0 –3.1 –2.0

Ethane –4.3 –13.0 –1.9 –30.1 –3.5 –2.4

Propane –3.0 –12.8 –4.0 –29.6 –3.7 –2.7

Ethene –29.8 –46.7 –39.4 –71.5 –28.6 –15.5

Hydrogen sulfide –61.4 –72.9 –56.2 –98.8 –60.5 –36.8

Methyl-substituted ethene –39.8 –56.4 –42.0 –78.5 –39.6 –24.2

Thiol-substituted ethene –50.5 –61.5 –53.8 –89.1 –52.3 –33.9

Despite the differences in the adsorption strength, the functional group models interact similarly on all the doped surfaces, which are comparable to those on the undoped surface, with the exception of saturated hydrocarbons on cobalt and manganese doped zinc sulfide.

Optimized adsorption geometries on cobalt doped zinc sulfide are shown as an example in Figure 12. The promotional effect caused by cobalt doping in all rubber adsorbate models, including saturated hydrocarbons, is visible in the optimized adsorption geometries, where the interacting cobalt atom relaxed outward, indicating a stronger interaction. Moreover, the stronger interaction between methane and the dopant atom leads to a larger distortion of bound methane from the free geometry, as listed in Table 5. The dominant functional group that is responsible for the interaction with the ZnS(110) surface is further proved through larger model cases that have two functional groups. For methyl-substituted ethene, the interaction is dominated through the carbon-carbon double bond, while the thiol group is responsible for and dominates the adsorption with thiol-substituted ethene, rather than the carbon-carbon double bond, since the preferential binding is via sulfur containing groups.

27 Figure 12. Side view of the optimized configurations on the cobalt doped ZnS(110) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.

Table 5. Adsorption energies (ΔEads) and geometrical parameters of methane interacting with metal on the undoped and doped ZnS(110) surfaces. Included as a reference are the geometrical parameters of free methane.

Free methane Undoped Mn doped Fe doped Co doped Ni doped Cu doped

ΔEads (kJ mol^‒1) - ‒3.0 –10.2 –2.4 ‒27.0 –3.1 ‒2.0

Longest C-H bond length (Å) 1.091 1.092 1.098 1.092 1.099 1.094 1.091 Largest H-C-H angle (^o) 109.5 110.8 114.4 110.4 114.9 111.9 110.2 Smallest H-C-H angle (^o) 109.4 108.3 106.8 108.8 106.6 107.4 108.9

4.2 Adsorption on doped Cu

S(111) surfaces

The calculated adsorption energies of the rubber adsorbate models interacting with a dopant atom on doped Cu2S(111) surfaces are presented in Table 6 and compared to those of the undoped Cu2S(111) surface. Generally, the adsorption trend on doped Cu2S(111) surfaces also follows the trend seen on the undoped Cu2S(111) surface, with the exception of methyl- and thiol-substituted ethenes on manganese and iron dopants, where the thiol-substituted ethene exhibits a slightly higher adsorption energy than methyl-substituted ethene when compared to the undoped Cu2S(111) surface. Moreover, a remarkable increase in the adsorption strength is achieved through the olefinic group interacting with the dopant atoms. The strongest effect is from the manganese and iron dopant atoms, followed by cobalt and nickel dopant atoms. This enhanced effect also results in obtaining higher adsorption energies for the methyl- and thiol- substituted ethene models.

Table 6. The calculated adsorption energies on the undoped and doped Cu2S(111) surfaces (in kJ mol^–1).

Adsorbate Undoped⁵⁵ Mn doped Fe doped Co doped⁵⁶ Ni doped

Methane –0.5 0.0 –5.6 –0.9 0.0

Ethane –0.8 –0.1 –5.8 –1.0 –0.3

Propane –1.0 –0.2 –6.0 –1.2 –1.3

Ethene –48.2 –111.3 –104.4 –79.1 –69.0

Hydrogen sulfide –39.3 –34.3 –47.8 –43.8 –40.8

Methyl-substituted ethene –53.7 –106.5 –97.9 –79.7 –71.3

Thiol-substituted ethene –41.5 –110.1 –101.4 –69.7 –59.6

The optimized structures on cobalt doped copper sulfide are illustrated in Figure 13. The results show that rubber adsorbate models on all doped Cu2S(111) surfaces including cobalt dopant have similar interactions to those that occurred on the undoped Cu2S(111) surface.

Stronger interactions lead to the outward relaxation of the dopant atom interacting with the rubber adsorbate models. This phenomenon is clearly visible in all adsorbate models containing a carbon-carbon double bond, which includes ethene and both of the substituted-ethene models.

In addition to the relative evidence shown through the optimized structures that connect this effect to stronger interaction, the larger distortion of the ethene compared to the free ethene also indicates a stronger interaction and higher adsorption energies, as shown in Table 7.

Figure 13. Side view of the optimized configurations on the cobalt doped Cu2S(111) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene

Table 7. Adsorption energies (ΔEads) and geometrical parameters of ethene interacting with metal on undoped and doped Cu2S(111) surfaces. The geometrical parameters of free ethene are included as a reference.

Free ethene Undoped Mn doped Fe doped Co doped Ni doped

ΔEads (kJ mol^‒1) - ‒48.2 –111.3 –104.4 ‒79.1 –69.0

C=C bond length (Å) 1.32 1.35 1.42 1.39 1.38 1.37

Bending of CH2 groups (^o) 0.0 7.3 26.9 19.5 17.0 14.9

29

4.3 Promotional impact of dopant atoms on sulfide surfaces

The adsorption of rubber adsorbate models on the sulfide surfaces generally show weak interactions via the saturated hydrocarbons while carbon-carbon double bonds and thiol groups of rubbers lead to stronger interactions. However, with the inclusion of different dopant atoms on both sulfide surfaces, the adsorption strength of the rubber adsorbate models can either be enhanced or reduced. Therefore, the use of different transition metals as dopant atoms on ZnS(110) and Cu2S(111) surfaces has been studied to determine their interaction with rubber adsorbate models, and the promotional impacts are presented in Figure 14 and Figure 15, respectively.

Figure 14. The effect of doping on interaction of the rubber adsorbate models and the doped ZnS(110) surfaces. Saturated and unsaturated hydrocarbons present an average effect on the adsorption of methane, ethane, and propane, and ethene and methyl-substituted ethene, respectively.

Figure 15. The effect of doping on the interaction of the rubber adsorbate models and the doped Cu2S(111) surfaces. Saturated and unsaturated hydrocarbons present an average effect on the adsorption of methane, ethane, and propane, and ethene and methyl-substituted ethene, respectively.

In the case of the ZnS(110) surface (Figure 14), the copper dopant notably weakens the adsorption strength of all the functional groups of rubber. On the other hand, the iron and nickel dopants on the ZnS(110) surface display little or no effect on the adhesion. Manganese and cobalt doping not only enhance the adsorption strength of the carbon-carbon double bonds and thiol groups of the rubber adsorbate but also trigger the saturated hydrocarbon groups, leading to higher adsorption energies. These positive effects are more significant in the case of cobalt doping than when manganese is used for doping.

The enhancement of the adsorption strength of all the functional groups of rubber by cobalt and manganese doping on the ZnS(110) surface is not shown on the doped Cu2S(111) surfaces.

Only the adsorption involving the carbon-carbon double bonds of rubber is initiated on all the doped Cu2S(111) surfaces. Even so, the effect is significantly stronger, as shown in the case of manganese and iron doping when compared to the doped ZnS(110) surfaces (Figure 15). While the highest promotional effect is shown through cobalt doping on the ZnS(110) surface, this is not in the case on the Cu2S(111) surface, since the manganese and iron doping on the Cu2S(111) surfaces have a higher promotional effect than the cobalt doping. On the other hand, while the nickel dopant does not show a promotional effect on the ZnS(110) surface, the adsorption strength of rubber to Cu2S(111) surface is also increased through carbon-carbon double bond when the nickel dopant is used.

31

5 Adsorption of rubber adsorbate models on zinc oxide surfaces

The calculated adsorption energies on both the ZnO(110) and ZnO(001) surfaces are tabulated in Table 8. Even though the most stable polymorph of ZnO and ZnS are different, the results obtained from the adsorption of rubber adsorbate models on the ZnS(110) surface is being used as a reference. The adsorption trend between functional group models and the ZnO(110) surface follow the same trend seen on the ZnS(110) surface, in which saturated hydrocarbons exhibit the weakest interaction, followed by ethene while hydrogen sulfide had a slightly stronger interaction than ethene. Even so, the adsorption energies are slightly enhanced, by approximately 4-7 kJ mol^-1 through the saturated hydrocarbons and ethene but are reduced by 20 kJ mol^-1 when interacting with hydrogen sulfide. The adsorption strength for larger adsorbate models is also shown to be similar to that on the ZnS(110) surface, where thiol- substituted ethene shows a stronger interaction than methyl-substituted ethene, but the adsorption energies are lower compare to ZnS(110) surface.

On the other hand, the adsorption is noticeably stronger on the ZnO(110) surface than on the ZnO(001) surface. The adsorption strength for ethene and hydrogen sulfide on the ZnO(110) surface is approximately 30 kJ mol^-1 higher than on the ZnO(001) surface. A similar observation can also applied to larger models, where the ZnO(110) surface exhibits higher adsorption strength than the ZnO(001) surface.

Table 8 The calculated adsorption energies on the ZnO surfaces. Calculated adsorption energies on the ZnS(110) surface are included as a reference (in kJ mol^–1).

Adsorbate ZnS(110)⁵⁵ ZnO(110) ZnO(001)

Methane –3.0 –7.6 –1.1

Ethane –4.3 –8.6 –0.4

Propane –3.0 –5.2 –1.4

Ethene –29.8 –37.2 –2.9

Hydrogen sulfide –61.4 –41.7 –9.8

Methyl-substituted ethene –39.8 –26.3 –5.0

Thiol-substituted ethene –50.5 –39.3 –13.1

As on the ZnS(110) surface, saturated hydrocarbons only exhibit weak physisorption on the ZnO(110) surface, while ethene interacts via the carbon-carbon double bond and hydrogen sulfide through the donation of the lone pair electrons of sulfur. On the other hand, for the adsorption on the ZnO(001) surface, all the functional group models exhibit weak physisorption as they display low adsorption strength. Even so, as predicted via adsorption on the ZnS(110) surface, the preferred binding mechanism is through the sulfur containing models, since hydrogen sulfide exhibits a stronger interaction than ethene. The optimized structures of the rubber adsorbate models on the ZnO(110) and ZnO(001) surfaces are presented in Figure 16 and Figure 17, respectively. The outward relaxation of the interacting zinc atom on the zinc oxide surfaces caused by rubber adsorbate models having carbon-carbon double bond and sulfur containing models is smaller than in zinc and copper sulfide surfaces cases. Similar

relative observations can be seen in the adsorption of larger adsorbate models on both ZnO surfaces.

Figure 16. Side view of the optimized configurations on the ZnO(110) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.

Figure 17. Side view of the optimized configurations on the ZnO(001) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.

33

6 Discussion

The role of the functional groups of the rubber structure in rubber–brass adhesion was explored on copper and zinc sulfide and on zinc oxide surfaces at the atomic level, finding distinct differences in the preferential binding between the rubber adsorbate models and the surfaces involved. In general, carbon-carbon double bonds and the thiol groups of rubber are responsible for the interaction with the surface metal atoms. However, zinc and copper sulfide surfaces display contradicting binding preferences. Zinc sulfide surface prefer to bond with sulfur containing functional groups, while copper sulfide preferentially bonds with the carbon-carbon double bond. The preferential binding of the sulfur containing functional groups also shown on the zinc oxide surfaces, but the adsorption strengths are lower compared to zinc sulfide surface.

The bonding via the carbon-carbon double bond occurs in the π-adsorption mode, where a π- donation from the adsorbate containing the carbon-carbon double bond into the empty orbitals of the metal occurs together with a back-donation from the metal into the empty π*-orbital of the adsorbate. This is the Dewar-Chatt-Duncanson adsorption model.⁵⁸ On the other hand, the bonding that occurs through the sulfur-containing adsorbate is due to lone pair electron donation from the sulfur in the sulfur-containing adsorbate to the metal atom of the surface and occurs together with the back-donation of the surface electrons to the sulfur-containing adsorbates.⁵⁹

Additives play an important role in rubber–brass adhesion, as they enhance the interaction.

Hence, the promotional effect of additives towards the rubber–brass adhesion has also been studied. Cobalt is widely used as additive especially in tire manufacturing since it assists in the formation of an essential amount of the copper sulfide layer and affects the durability of rubber–metal bonds.³⁵ From our findings, only the adsorption involving carbon-carbon double bonds is enhanced by the cobalt doping on the copper sulfide surface. However, a notable promotional effect is achieved on all the functional groups of rubber, including saturated hydrocarbons, on the cobalt doped zinc sulfide surface. The dissociation of methane is known to occur on transition metal surfaces, and for a cobalt surface the activation barrier is 120 kJ mol^–1.^60,61 This could be the reason why a stronger interaction is observed, as the cobalt dopant on the zinc sulfide moderately activates the carbon-hydrogen bond of the saturated hydrocarbons.

Despite its advantages, the limitations of cobalt usage as an additive, which include the cost, have increased the demand for finding an alternative to substitute for cobalt as an additive.

Therefore, we substituted cobalt with manganese, iron, and nickel. The results were compared with those of cobalt doped substances. In the case of zinc sulfide, the promotional effect from cobalt is still the strongest. On the other hand, manganese and iron doping on the coper sulfide surface show a greater promotional effect than cobalt due to the enhancement in the interaction via the carbon-carbon double bond. This finding is important for the interaction between rubber and brass at the adhesive interlayer, since copper sulfide is speculated to be responsible for the interfacial adhesion.¹²

7 Conclusions

The rubber–brass adhesion process has been investigated using DFT involving the adsorption of rubber adsorbate models on copper and zinc sulfide and on zinc oxide surfaces at the adhesive interlayer. It was found that the functional groups of the rubber structure that are responsible for the interfacial interaction with metal are olefinic and sulfidic. The obtained adsorption energies reveal that preferential binding on copper sulfide is via the double bond, while sulfur containing functional groups are preferable for zinc sulfide and zinc oxide. Both copper and zinc sulfide exhibit a stronger interaction with the rubber structure than zinc oxide does. The different behavior of the functional groups of the rubber structure and the different preferential binding on the copper and zinc sulfide at the adhesive interlayer can play an important role in rubber–brass adhesion.

The doping of zinc and copper sulfides is a useful tool in the optimization of the interfacial interaction. Cobalt as an additive is found to be important in rubber–brass adhesion, as it enhances the rubber adhesion on both sulfide surfaces, with a greater promotional effect shown through the interaction with zinc sulfide. Cobalt even activates carbon-hydrogen bonds, which can be useful in enhancing rubber–brass adhesion. However, the search for more economical and effective additives has been carried out. Considering the promotional effect on both copper and zinc sulfide, out of manganese, iron, and nickel additives, manganese seems to be a good substitute. Compared to cobalt, manganese displays a lower effect on the zinc sulfide but a notable effect on copper sulfide. Therefore, from our findings, the best method for achieving optimum enhancement of the rubber–brass adhesion in practical applications is by using a combination of dopants.

35

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