Heat Aging of Rubber Compounds

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Rulis Heidari

HEAT AGING OF RUBBER COM- POUNDS

Faculty of Engineering and Natural Sciences Master of Science Thesis

August 2021

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ABSTRACT

Rulis Heidari: Heat Aging of Rubber Compounds Master of Science Thesis

Tampere University

Master’s Degree Program in Material Science August 2021

Thermal aging of rubber compounds can be experienced in changes of mechanical as well as chemical properties following softening, hardening, crazing, cracking, and other degradation of rubber compounds. These changes are caused by a single factor or a combination of factors, including light, ozone, oxygen, oil, moisture, heat, water, or any other solvents.

The master’s thesis aimed to study the effect of heat aging at different temperatures and aging times that affect the raw materials of rubber compounds. The master’s thesis includes both theoretical as well as experimental parts. The theoretical background, chemical structures, and reaction mechanisms of elastomers, fillers, antidegradants, and resins are studied in the theoretical part. In addition to this, mixing of rubber compounds, aging of rubber, and analysis of rubber aging are discussed as well. The experimental part of the master’s thesis dealt with the effect of raw materials on rubber aging as well as how the heating temperature affects both the physical and chemical aging of the rubber compounds. The study was done at a temperature of 90 °C, 80 °C, and 70 °C, and at different aging times. The study consisted of mechanical and chemical analyses, such as ozone test, the mass reduction of antidegradants analysis, elastomer, and filler amount analysis.

Thermo-oxidative aging of rubber compounds resulted in a growth in hardness, and modulus as well as a depletion in elongation at the break with a rise in the aging time, and the aging temperature. In this study, polymers and fillers did not show a notable impact on the heat aging of rubber compounds according to the thermogravimetric analysis (TG) curve results. According to the results of gas chromatography-mass spectrometry (GC-MS) analyses, the amount of polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), and N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD) antidegradants reduced logically in the rubber compounds as the aging period and temperature increased.

Based on the test results, the mechanical property test is a suitable method for analyzing the heat-aging of rubber compounds. Nevertheless, depletion of antidegradants analysis testing increased the understanding of the behavior of antidegradants in the rubber aging due to the diffu- sion-controlled process of rubber aging.

Keywords: Elastomer, fillers, rubber aging, rubber compound, thermal aging, silica, antidegradants, thermo-oxidative aging.

The originality of this thesis has been checked using the Turnitin Originality Check service.

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TIIVISTELMÄ

Rulis Heidari: Raaka-aineiden vaikutus kumisekoituksen lämpövanhenemiseen Diplomityö

Tampereen yliopisto

Materiaalitekniikan diplomi-insinöörin tutkinto-ohjelma Elokuu 2021

Kumin lämpövanheneminen on seurausta kumisekoituksen mekaanisten ja kemiallisten ominaisuuksien muutoksista. Kumisekoitukset pehmenevät, kovettuvat, halkeilevat tai hajoavat kemiallisesti vanhetessaan. Nämä muutokset johtuvat joko yhdestä tekijästä tai muiden tekijöiden yhdistelmästä, mukaan lukien valosta, otsonista, hapesta, öljystä, kosteudesta, lämmöstä, vedestä tai muista kemiallisista liuottimista.

Tämän työn tarkoituksena oli tutkia raaka-aineiden vaikutusta kumisekoituksen lämpövanhenemiseen eri vanhenemislämpötiloissa ja -ajoissa. Työ sisältää sekä teoreettisen että kokeellisen osion. Teoreettisessa osiossa käsitellään kumisekoituksen lämpövanhenemiseen vaikuttavia tekijöitä ja vanhenemisen analysointia. Työn kokeellisessa osiossa tutkittiin, miten raaka-aineiden muutokset vaikuttavat kumisekoituksen fysikaalisiin ja kemiallisiin ominaisuuksiin.

Lisäksi tutkittiin suoja-aineiden määrä lämpövanhennetussa kumisekoituksessa kaasukromatografia-massaspektrometrialla (GC-MS). Kumin lämpövanhenemista tutkittiin 70 °C, 80 °C ja 90 °C lämpötiloissa käyttäen neljää eri vanhennusaikaa. Vanhenemista seurattiin analysoimalla kovuutta, venymää ja moduuli 100 %, sekä suoja-aineiden määriä.

Kumisekoitusten termo-oksidatiivinen vanheneminen johti kovuuden ja moduulin kasvuun, murtovenymän ja vetolujuuden pienenemiseen vanhentumisajan funktiona. Mekaanisten ominaisuuksien ja termogravimetrisen analyysin perusteella voitiin päätellä, että raaka-aineilla ei ollut suurta vaikutusta kumisekoitusten lämpövanhenemiseen tässä tutkimuksessa. GC-MS- analyysin perusteella voitiin päättää, että 6PPD ja TMQ suoja-aineiden määrä pieneni loogisesti kumisekoituksissa lämpötilan ja vanhentumisajan kasvaessa.

Tämän työn mekaanisten ominaisuuksien testitulosten avulla voitiin analysoida kumisekoituksen lämpövanhenemista. Toisaalta suoja-aineiden analyysi lisäsi ymmärrystä suoja-aineiden käyttäytymisestä vanhennuksessa, sillä kumin vanheneminen on seurausta diffuusiokontrolloidusta prosessista.

Avainsanat: Kumisekoitus, elastomeeri, suoja-aineet, täyteaineet, kumin lämpövanheneminen, termo-oksidatiivinen vanheneminen.

Tämän julkaisun alkuperäisyys on tarkastettu Turnitin OriginalityCheck –ohjelmalla.

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PREFACE

This Master of Science Thesis was accomplished at the Material Development Department of Nokian Tyres plc, in Nokia between December 2020 and July 2021.

I am very thankful to my supervisors Associate Professor Essi Sarlin and Professor Alexander Efimov from Tampere University for helping me and sharing their knowledge for the research. I wish to thank Senior Laboratory Engineer Mika Junnila, Laboratory Analyst Maarit Myllymaa and the R & D department staff of Nokian Tyres plc.

Furthermore, I wish to thank my master’s thesis supervisor Development Chemist Noora Kemppainen for giving me this fascinating study topic and kindly supporting, encouraging, and guiding me during this project.

My fellow students, friends, and family deserve the most praise for their support and love.

Thank you so much for being there for me throughout my studies.

Tampere, 1.8.2021

Rulis Heidari

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LIST OF FIGURES

Figure 1. The repeating units of polybutadiene rubber. [7] ... 4

Figure 2. The repeating unit of polyisoprene. [7] ... 5

Figure 3. The repeat unit of styrene-butadiene rubber. [7] ... 6

Figure 4. Characterization of carbon black particle sizes. [Modified from [12]] ... 7

Figure 5. Functional groups of carbon black with planar structure. [15] ... 8

Figure 6. A polymer network structure filled with carbon black. [Modified from [13]] ... 8

Figure 7. The structure of different types of silanes. [18] ... 9

Figure 8. Polymer crosslinking mechanism with silane. [Modified from [11, p. 232]] ... 10

Figure 9. Structure of 6PPD, TMQ, and bisfunctional phenolic antioxidants. [11, p. 540] ... 11

Figure 10. A general structure of para-phenylenediamine antiozonant, where R and R’ can be alkyl, an aryl group, or cycloalkyl. [18] ... 11

Figure 11. The mechanism reaction of PPDs with N-alkyl-substituted as well as ozone. [18] ... 11

Figure 12. The structure of paraffin (a) and microcrystalline (ceresin) (b) waxes. [18] ... 13

Figure 13. The structure of hydrocarbon C9 resins consisting of aromatic monomers of nine-carbon polymers. Dashes represent the aromatic monomers.[18] ... 14

Figure 14. The mechanism of thermal aging of polymers in the existence of oxygen, where H˙and P˙ are free radical and P is a polymer. [Modified from [27]] ... 17

Figure 15. Thermal oxidation mechanism of (A) chain scission and (B) crosslinking of rubber, where k is the constant rate of reaction, T temperature, BR butadiene rubber, and NR natural rubber. [18] ... 18

Figure 16. Degradation mechanism of BR, crosslinking of rubber, where k is the constant rate of reaction, T temperature, and BR butadiene rubber. [7]... 21

Figure 17. Thermal degradation mechanism of NR chain scission, where k is the constant rate of reaction, T temperature, and BR butadiene rubber. [7]... 22

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Figure 18. Aging mechanism of SBR. Free radical initiation of SBR

(radicalization). [27] ... 23 Figure 19. The structural formula of cis and trans isomers of NR. [18] ... 25 Figure 20. The migration rates of HPPD (triangles), IPPD (circles) as well as BHT

(squares) antidegradants at 80 °C. [47] ... 27 Figure 21. The TG curves of test specimens for 60, 120, and 180 aging days,

where (a) represents the mass loss (%) and (b) the conversion fraction of the aged specimens against the aging temperature

(°C).[53] ... 31 Figure 22. The mass reduction of TMDQ (a) and 6PPD (b) antidegradants in

heat-aged vulcanized rubber at 60 °C, 80 °C, and 100 °C. [54] ... 32 Figure 23. As a result of the rapid sulfur curing, the structures and network

structures were created. The crosslinks of intermolecular are represented as follow: (a) a crosslink of carbon-carbon, (b) the crosslink of monosulfide (C–S–C), (c) the crosslinks of disulfide (C–S2 –C), and (d) the crosslinks of polysulfide (C–Sy–C or C-Sz- C, y,z ≥ 3). On the other hand, (e) the structure of cyclic sulfide, and (f) the polymer chain intramolecular transformations are presented with the group of pendant sulfide, which is terminated

with the accelerator part. [56] ... 33 Figure 24. The thermomechanical crosslinking decompose mechanism of rubber

compounds, where s is sulfide and x ≥ 3. [58] ... 34 Figure 25. The hardness versus aging time (week) of SBR at 100 °C, where

samples A-D contained different amounts of SBR and E contained no processed oils. [42] ... 35 Figure 26. The tensile strength (MPa) versus aging time (hours) of the vulcanized

NR at a temperature of 100 °C, and 70 °C. [58] ... 36 Figure 27. The modulus 100 % (MPa) against the square root of the aging time

(s^1/2) of (a) the laboratory-tested rubber compound, and (b) tire rubber at a temperature of 70 °C (white circle), 80 °C (white circle with a black diameter), 90 °C (circle with half colored), and 100 °C (black colored circle). [59] ... 37 Figure 28. Ahagon plot for steel belt aging of NR, which shows the ratio of strain

at the break as well as modulus at 100 % strain in logarithm

scales. [59] ... 38 Figure 29. The average values of elongation at break (%) as a function of storage

time (d) and the standard deviation of all rubber compounds at a

temperature of 90 °C. ... 47 Figure 30. The average values of elongation at break (%) against the aging time

(d) at 90 °C. ... 48

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Figure 31. (a) The hardness (Shore A), (b) elongation at break (%), (c) modulus 100 % (MPa), and (d) the tensile strength (MPa) against the aging time (d) of rubber compound 1 at a temperature of 90 °C (circle),

80 °C (square), and 70 °C (triangle). ... 49 Figure 32. The elongation at break (%) against the aging time (d) of rubber

compound 1 at 90 °C, 80 °C, and 70 °C. ... 50 Figure 33. The modulus 100 % (MPa) against the aging time (d) of rubber

compound 1 at 90 °C, 80 °C, and 70 °C. ... 50 Figure 34. Ahagon plot, the logarithm of elongation at break (%) against the

logarithm modulus 100 % (MPa) of rubber compounds 1 (a) and 2 (b) at 90 °C, 80 °C, and 70 °C. ... 51 Figure 35. The elongation at break (%) against the square root of (a) the aging

time (d) at a temperature of 90 °C (circle), 80 °C (square), 70 °C

(triangle) as well as (b) storage time (year) for rubber compound 1. ... 53 Figure 36. The modulus 100 % (MPa) against the square root of (a) the aging

time (d) at a temperature of 90 °C (circle), 80 °C (square), and 70

°C (triangle) as well as (b) storage time (year) for rubber

compound 1. ... 53 Figure 37. The elongation at break (%) versus the aging time (d) of rubber

compounds 1-4 (polymers) at a temperature of 90 °C (circle), 80

°C (square), and 70 °C (triangle). ... 54 Figure 38. The elongation at break (%) and the modulus 100 % [MPa] against

the square root of the aging time (d) for rubber compounds 1–4

(polymers) at a temperature of 90 °C. ... 55 Figure 39. Ahagon plot, the logarithm of elongation at break (%) against the

logarithm modulus 100 % (MPa) of rubber compounds 1-4 at a

temperature of 90 °C. ... 56 Figure 40. (a) The elongation at the break (%), and (b) modulus 100 (MPa)

against the square root of the aging time (d) of rubber compounds 5 and 6 at a temperature of 90 °C (circle), 80 °C (square), and 70

°C (triangle). ... 57 Figure 41. Ahagon plot, the logarithm of elongation at break (%) against the

logarithm of modulus 100 %(MPa) for compounds 5 and 6 at 90 °C (circle), 80 °C (square), and 70 °C (triangle) temperatures. ... 57 Figure 42. (a) The elongation at the break (%) and (b) the modulus 100 % (MPa)

against the square root of the aging time (d) for rubber compounds 7, and 8 at a temperature of 90 °C (circle), 80 °C (square), and 70

°C (triangle). ... 58

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Figure 43. The hardness (Shore A) against the square root of (a) the aging period (d) at 90 °C (circle), 80 °C (square), and 70 °C (triangle) as well as (b) the storage time (year) for rubber compound 1. ... 59 Figure 44. The average hardness (Shore A) as a function of the heat aging

period (year) at 90 °C (blue circle), 80 °C (square), and 70 °C (triangle) as well as storage time (year) for rubber compound 1,

and tires (gray circle). ... 60 Figure 45. The TG curves of rubber compounds (a) 5 and (b) 6 as a function of

time/min with aging times of 0, 3, and 7 days (d) at 90 °C. ... 61 Figure 46. The intensity (mV) versus time (min) of gas chromatograms for 6PPD

antidegradant at 90 °C and the aging time of 7 days. ... 64 Figure 47. The intensity (mV) versus time (min) of gas chromatograms for TMQ

antidegradant at 90 °C and the aging time of 7 days. ... 64 Figure 48. The relative abundance (%) against mass/charge ratio (m/z) of 6PPD

antidegradant at 90 °C and the aging time of 7 days. ... 65 Figure 49. The relative abundance (%) against mass/charge ratio (m/z) of TMQ

antidegradant at 90 °C and the aging time of 7 days. ... 65 Figure 50. Mass (mg) reduction against (a and c) the aging time (d) at a

temperature of 90 °C (circle), 80 °C (square), and 70 °C (triangle) as well as (b and d) the storage time (d) of 6PPD antidegradant for rubber compounds 5 and 6. ... 66 Figure 51. Mass (mg) reduction against (a and c) the aging time (d) at a

temperature of 90 °C (circle), 80 °C (square), and 70 °C (triangle) as well as (b and d) storage time (d) of TMQ antidegradant for

rubber compounds 5 and 6. ... 67 Figure 52. The elongation at the break (%) against the square root of the aging

time (d) of rubber compounds 1 (a), 2 (b), 3 (c), and 4 (d) at 90 °C (circle), 80 °C (square), and 70 °C (triangle). ... 76 Figure 53. The elongation at the break (%) against the square root of the aging

time (d) of rubber compounds 5 (e), and 6 (f) at 90 °C (circle), 80

°C (square), and 70 °C (triangle). ... 77 Figure 54. The elongation at the break (%) against the square root of the aging

time (d) of rubber compounds 7 (g), and 8 (h) at 90 °C (circle), 80

°C (square), and 70 °C (triangle). ... 77 Figure 55. The modulus 100 % (MPa) against the square root of the aging time

(d) of rubber compounds 1 (i), 2 (j), 3 (k), and 4 (l) at 90 °C (circle), 80 °C (square), and 70 °C (triangle). ... 78

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Figure 56. The modulus 100 % (MPa) against the square root of the aging time (d) of rubber compounds 5 (m), and 6 (n) at 90 °C (circle), 80 °C

(square), and 70 °C (triangle). ... 79 Figure 57. The modulus 100 % (MPa) against the square root of the aging time

(d) of rubber compounds 7 (o), and 8 (p) at 90 °C (circle), 80 °C

(square), and 70 °C (triangle). ... 79 Figure 58. Ahagon plot, the logarithm of elongation at break (%) against the

logarithm of modulus 100 % (MPa) of rubber compounds 1 (a), 2 (b), 3 (c), and 4 (d) at 90 °C (circle), 80 °C (square), and 70 °C

(triangle). ... 80 Figure 59. Ahagon plot, the logarithm of elongation at break (%) against the

logarithm of modulus 100 % (MPa) of rubber compounds 5 (e),

and 6 (f) at 90 °C (circle), 80 °C (square), and 70 °C (triangle). ... 81 Figure 60. Ahagon plot, the logarithm of elongation at break (%) against the

logarithm of modulus 100 % (MPa) of rubber compounds 7 (g),

and 8 (h) at 90 °C (circle), 80 °C (square), and 70 °C (triangle)... 81

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LIST OF SYMBOLS AND ABBREVIATIONS

6PPD N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine ACM Alkyl acrylate copolymer

ASTM American Society for Testing and Materials BHT 2,6-ditertrabutyl-4-methyl phenol

BR Butadiene rubber

CR Chloroprene rubber

CPE Chlorinated polyethylene

EPDM Ethylene propylene diene monomer GC-MS Gas chromatography-mass spectrometry

GT Glass transition

HPPD N-phenyl-N’-(1,3-dimethylbutyl)-para-phenylenediamine IPPD N-isopropyl-N’-phenyl-1,4-phenylenediamine

IR Isoprene rubber

IRHD International rubber hardness degree IIR Isobutylene-isoprene rubber

ISO International organization for standardization NBR Nitrile-butadiene rubber

NMR Nuclear magnetic resonance

NR Natural rubber

phr Parts per hundred rubbers

PPDs Para-phenylenediamines

pphm Parts per hundred million

rpm Rounds per minute

SBR Styrene-butadiene rubber

ShA Shore A, measurement of hardness scale S-SBR Solution polymerized styrene-butadiene rubber STDEV Standard deviation

tan δ the loss tangent

TD-NMR Time-domain nuclear magnetic resonance TESPT Bis(3-triethoxysilylpropyl) tetrasulfide Tg Glass transition temperature

TGA Thermogravimetric analysis

TMA Thermomechanical analyzer

TMQ Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline

UV Ultraviolet

ZnO Zinc oxide

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1. INTRODUCTION

The most significant rubber applications are tires, accounting for around 70 percent of the worldwide consumption of natural and synthetic rubber. A tire is a very complex composite item, which consists of many synthetic rubbers such as butadiene, a copolymer of styrene-butadiene, and natural rubbers along with various typical ingredients, such as antioxidants, fillers, crosslinking, and processing aids. Compounded rubber has many special qualities, such as damping properties, high elasticity, and abrasion resistance. For the great efficiency of the resulting rubber compounds, the selection of the correct additives and fillers is important. Without the addition of fillers, mainly silica and carbon black, elastomers give vulcanizates that can barely be used. [1, pp. 50–53]

Rubber compounds experience chemical and physical property changes over time.

These modifications cause unwanted changes to the performance of rubber materials like immoderate cracking, softening, hardening, and some other damage to the surface of materials. These property changes are a result due to the changes to the polymer structures, which may be caused by the deterioration and oxidation processes [2, pp.

67–68]. Rubber compounds can be aged by various types of internal and external factors, such as chemical substances, heat, light and ultraviolet radiation, oxygen, ozone, and water vapor. These factors may accelerate the deterioration process and cause the polymer’s chain scission as well as crosslinking [3].

The thermo-oxidative aging of rubber compounds can be tested following the standard rubber aging testing methods in the laboratory. The storage time of rubber compounds can be predicted with the help of the Arrhenius equation [3]. The heat-aging types of rubber compounds can be characterized by the Ahagon plot, which is the mean of the logarithm of the elongation at the break (%) versus the logarithm of modulus 100 % (MPa) of a rubber compound [4].

This study aimed to define the heat aging of tire compounds. Furthermore, the influence of the raw materials of rubber compounds, such as polymers, fillers as well as the depletion of antidegradants was studied. The thermal and oxidation deterioration can be in- hibited by the usage of the right antidegradants. The effect of raw materials on rubber compound aging was studied at three distinct temperatures 90 °C, 80 °C, and 70 °C, and

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aging periods from 1 day to 42 days in heat ovens. In this study, polymers and the amounts of silica and carbon black of rubber compounds were changed to observe the heat aging of rubber compounds at various temperatures and aging times. Furthermore, deterioration profiles and heat aging characteristics of rubber compounds helped extend- ing their quality during operation, storage, and usage.

This work consists of seven sections. The introduction is presented in section 1. Section 2 deals with the raw materials of tire rubbers. Section 3 describes the aging processes of rubber compounds. Section 4 offers an analysis of the rubber compound aging based on the published literature. Section 5 describes the experimental part of the present work including the compounding of rubber compounds, sample preparation, mechanical and chemical property testing methods, such as ozone test, and depletion of antidegradant investigation. The results and discussion are presented in section 6. The final section includes the conclusions of this work.

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2. TIRE RUBBERS

Rubber, also known as an elastomer, is a durable and elastic material that may be manufactured chemically from natural gas and petroleum or extracted from secretions of some tropical plants. The tree species from which rubber is yielded originates in Africa, America, and the Far East. [5, p. 67] Rubber has been used for thousands of years in the various material industries. It is the fundamental component of tires due to its elasticity, durability, and toughness [2, pp. 67–68].

Compounded rubber consists of a wide range of raw material ingredients, including elastomers, fillers, and antidegradants. The rubber compound’s properties are determined by elastomers, which are the primary components in the rubber compound.

Compounding of a rubber is the process of modifying an elastomer by adding chemicals and additives in the rubber compound to reach the wanted mechanical and chemical properties. In addition to elastomers, ingredients like fillers, antidegradants, activators, accelerators, vulcanizing agents, and processing aids are all included in a simple recipe for the rubber compound. [2, p. 281] Parts per hundred rubber (phr) are used to represent the amount of weight of ingredients in the rubber compound formula. In the rubber compound recipe, the quantity of elastomers is 100 phr [5, pp. 121–155].

This chapter discusses each category of tire rubber ingredients, especially elastomers, fillers, and antidegradants for the sake of the experimental part.

2.1 Elastomers of Tire Rubber

Elastomers are macromolecules of polymers, which are possible to extend to a grand length and yet recover to their primary form. These types of amorphous polymers consist of monomers and they are the major chemical components of the rubber compound [1, pp. 1–2].

The elastomer can be classified into heterochain and carbon chain elastomers.

Heterochain elastomer includes other elements such as silicon and nitrogen besides carbon in the backbone of the polymer. [2, p. 67] The elastomer consists of saturated (single bond), unsaturated (double bonds or triple bonds), polar, or non-polar polymers.

Saturated elastomer contains a single bond in the backbone of the polymer. Then again, unsaturated elastomer including butadiene rubber (BR) styrene-butadiene rubber (SBR) and natural rubber (NR), contains double bonds in the polymer chain’s backbone. [1, pp.

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50–53] Elastomers give the rubber compound the basic properties. Various elastomers are used based on the rubber field’s application. For the purpose of this work, the focus is on polybutadiene rubber, natural rubber, and styrene-butadiene rubber.

2.1.1 Butadiene Rubber

One of the first types of synthetic rubber developed was polybutadiene rubber (BR).

Polybutadiene rubber can be usually manufactured by 1,3-butadiene coordination polymerization. The monomer of butadiene consists of double bonds of carbon-carbon.

The monomer can be grown in three structural isomers in the chain of the polymer including cis-1,4, trans-1,4, and vinyl-1,2 forms (see Figure 1). Butadiene polymerization leads to a polymer containing molecular weight with narrow distribution. [2, p. 54, 4] The glass transition temperature (Tg) of trans-1,4, as well as cis-1,4 polybutadiene, is low around -107 °C. On the other hand, the Tg of 1,2-vinyl is significantly high around 0 °C.

The mechanical properties of trans-1,4 and cis-1,4 polybutadiene are different despite having alike Tg. [4] For instance, polybutadiene with a rich amount of trans-1,4 has a low melting point and crystallinity while BR with a high amount of cis-1,4 is considerably less elastic, much harder, and has higher crystallinity nature [3].

BR has good abrasion resistance, low heat build-up, water resistance, and cracking resistance, which are important for rubber applications. Polybutadiene rubber stands in cold temperatures better than other elastomers. Due to a phenomenon called glass transition, many polymers can become brittle at low temperatures. Polybutadiene glass transition temperature is about -106 °C. Polybutadiene is used as a tread on the side walls in the tire rubber to improve its performance like the resistance of abrasion, rolling, and traction. [2, pp. 53–57]

Figure 1. The repeating units of polybutadiene rubber. [7]

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2.1.2 Natural Rubber

NR, which is a renewable resource, can be isolated from more than 200 different plant species. Natural rubber resources originate primarily in Brasilia, Indonesia, India, Malesia, and the Philippines. The most important source of the tree is Hevea Brasiliensis.

NR is usually gotten from a milky latex of the tree via tapping into the inner bark and extracting the latex in cups from the tree. [2, pp. 1–2] NR mainly consists of cis-trans isomers of 2-methyl-1,3-butadiene (isoprene), as illustrated in Figure 2. Isoprene can undertake additional polymerization in several ways. For instance, synthetic rubber (Natsyn) and natural rubber (Hevea tree) contain cis-1,4-polyisoprene. These rubbers have high stereoregularity isomeric of polyisoprenes [8].

NR has high tensile and tear strength, low rolling resistance, good flexibility, abrasion resistance, excellent tack, and green strength, and is resistant to tearing, cutting, and corrosive substances. [7, 9]

2.1.3 Styrene Butadiene Rubber

SBR is produced from butadiene and styrene copolymers. SBR (see Figure 3) is widely used in the rubber functions with other elastomers like natural rubber. It is normally a mixture of roughly 75 % butadiene and 25 % styrene. Nowadays, there are quite large variations in the styrene and vinyl concentration of SBRs. [2, p. 87] Different types of SBRs can be produced either by emulsion, solution or free radical polymerization. For instance, Bayer Aktiengesellschaft (AG) has made solution styrene-butadiene rubber (SSBR, Buna VSL5025-0) containing vinyl 50 %, trans 15 %, cis 10 % amount of butadiene and styrene 25 % [10].

SBR has various improved physical properties compared to BR in the rubber compound, such as low resistance of rolling, low cost, abrasion, and resistance of crack as well as high properties of mechanical. [5, p. 89] On the other hand, SBR has its limitations such as poor strength, low tear strength, and resilience, poor tack, especially at higher temperatures [9].

Figure 2. The repeating unit of polyisoprene. [7]

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2.2 Fillers

To modify and strengthen the mechanical as well as physical properties of tire rubber compounds, additives like fillers and reinforcements are used. Small hard granules made primarily of carbon or inorganic substances are used as fillers. These are utilized to lower the material cost and improve its processability, electrical, and heat properties. Carbon black, kaolin clay, silica, and natural carbonate are the primary fillers in the rubber compound. The mechanical properties of the rubber compound like abrasion resistance, modulus, fatigue, tear, and tensile strength can be enhanced by fillers. Fillers can be classified according to color or effect. Carbon black is black, and silica, kaolin calcium silicate, and talc are white fillers. On the other hand, precipitated silica and carbon black are reinforcing, kaolin and calcium silicate are semi-reinforcing and calcium carbonate is a non-reinforcing fillers. [11, pp. 281–228]

2.2.1 Carbon Black

Finely separated carbon black manufactured by incomplete combustion of oil or gas is a vital filler material in the rubber compound. Carbon black is composed of small spherical aggregated particles with diameters of just 10–100 nanometers and concentrated carbon graphite layers. Carbon blacks consist mainly of 90–99 % of the elementary carbon with a blended small amount of nitrogen, hydrogen, and oxygen. Carbon black particles come with different shapes, particle size, surface area, surface activities, and aggregate size in colloidal dimensions (see Figure 4). These complexes are linked together through the power of Van der Waals forces. The carbon black particle structure tends to be an intermediate between crystalline and amorphous materials. Carbon black structure differs from a pure crystalline form of carbon, such as diamond and graphite between the atom arrangements in the three-dimensional space. Consequently, carbon blacks have a structure of semi-graphitic. [11, pp. 2 – 4]

Figure 3. The repeat unit of styrene-butadiene rubber. [7]

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The interaction between carbon black and its surroundings is determined by its surface activity. Functional groups including phenolic, lactones, carboxylic, and ketones exist on the surface of carbon black (see Figure 5). These functional groups can respond to interacting with the rubber compound. [13] The interaction of filler and the polymer relies on the internal, geometry, and external factors. The chemical properties and surface activities are accountable for internal factors. Filler porosity and the structure of filler are dependable geometry factors in filler-polymer reinforcing. The chemical properties and surface activities of a filler rely on the existence of a functional group, which has some oxygen. Affinity to nonpolar filler is greater with nonpolar polymer. [14] Figure 6 shows the crosslinked aggregate network of carbon black with a polymer chain of rubber.

In the industrial rubber, carbon black is a broadly utilized filler. It is utilized to give the rubber compound material color, opacity, electrical characteristics, ultraviolet light safety, and thermal conductivity. Carbon blacks have been characteristically designed for the application of elastomers. [11, pp. 60–68] They are usually called according to producing methods: acetylene blacks, furnace blacks, channel blacks, and thermal blacks. Over 90 percentages of carbon blacks come from the oil furnace process. It is an extremely effective system, which allows for strict regulation of physical and chemical properties.

Carbon black gives great processing reliability, dispersibility, and quality consistency for rubber products [15].

Figure 4. Characterization of carbon black particle sizes. [Modified from [12]]

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2.2.2 Silica and Silanes

Silica fillers are used along with silanes to boost the dispersion of filler-rubber interaction in the rubber compound. Silanes inhibit the network of filler-filler formation and reduce the specific surface energy of the rubber compound. This involvement results in a betterment in the physical properties including modulus, the resistance of abrasion, and the resilience of the rubber compound. [11, p. 65]

Silica consists of silicon and oxygen organized in a tetrahedral structure (SiO2). It is an amorphous compound with a particle size between 10–40 nm. Silica is a white crystalline

Figure 5. Functional groups of carbon black with planar structure. [15]

Figure 6. A polymer network structure filled with carbon black. [Modified from [13]]

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compound and is broadly known as quartz [16]. Three unique types of silica fillers, such as fumed or pyrogenic silica, a ground mineral of silica, and precipitated silica are exploited in the industry of rubber compounds. The usage of silica fillers has many advantages in the rubber industry. The chemical and physical properties of silica like chemical composition, pH, oil absorption, and ultimate particle size are the essential properties of silica in reinforcement. [11, pp. 85–88] Then again, silica fillers grow the viscosity of rubber compounds during compounding and deactivates the system of an accelerator, which extends the cure time. Silica fillers provide a variety of benefits like enhancing the tear strength, reducing the abrasive resistance, rolling resistance, and heat [16].

Silanes are organometallic compounds containing silicon-carbon bonds in the molecular structure. In the rubber industry, silanes are utilized as coupling agents. The adhesion promoter of silanes contains a hydrolyzable group like a triethoxysilyl group or trimethoxy. These groups allow the silane to be linked to the organic substrate. There are different functional silane types including di- and polysulfide silanes, mercaptosilanes, and blocked mercaptosilanes (see Figure 7). Di- and polysulfide silanes are commonly used in the rubber compound. [17, pp. 251–261]

The bonding mechanism of silica and silane includes two main phases, namely the process of hydrophobization reaction of silane with silica and crosslinks creation in between elastomer and silica (see Figure 8). The widely utilized silanes are 3-thio- cyanatopropyl triethoxy and bis(3-triethoxysilylpropyl) tetrasulfide (TESPT) silanes. The TESPT coupling agent may act as a sulfur donor or can react with the matrix of the polymer of the rubber compound. As the coupling agents, silanes enhance the affinity between the rubber compound and silica (see Figure 8). They are bifunctional compounds, which are involved in the reaction with the silica surface and the sulfur group in the vulcanized rubber compound. [11, pp. 230 – 231]

Figure 7. The structure of different types of silanes. [18]

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10

2.3 Antidegradants

Ozone, oxygen, and elevated temperatures accelerate the chemical degradation of elastomer materials. Moreover, oxidation of elastomer can be speeded up by different factors covering heavy metal contamination, moisture, solvents, sulfur, swelling in oil, and dynamic fatigue. The degradation of elastomer may be prevented by the cure system, polymer type, and antidegradants. [19] They are important factors to prevent the rubber compound from oxidation and to extend its service life. Different types of antidegradants, such as antioxidants, antiozonants, and waxes are utilized for different protection purposes in the rubber industry. Using the right type of antidegradants depends on various variables like expense, chemical solubility, discoloration, compatibility, performance, staining, toxicity, and conditions of service [20].

Antidegradants can be classified according to staining (discoloring) and non-staining (non-discoloration) antidegradants. Staining antidegradants (amines) are polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), alkyl-aryl para-phenylenediamines (PPDs) and diaryl PPDs. Non-staining antidegradants (phenolics) can be classified into four groups including hindered bis-phenol, hindered phenol, phosphites, and hydroquinones.

[11, pp. 282–284] Antidegradants like aromatic amines (TMQ and PPDs), as well as phenols (see Figure 9), act as chain termination in the thermo-oxidation aging of rubber compounds. Chain termination is the final stage in the chain reaction process where one or more chain-carrying species react without generating another chain-carrying molecule. Whereas, thioesters and phosphites act as decomposers of peroxides. Cyclic amines bounding to the group of alkyl acts as functional initiators [19, 17].

Figure 8. Polymer crosslinking mechanism with silane. [Modified from [11, p. 232]]

CH₂

Polymer chain

+

OH OH OH

R R R

O O O

Si

Rubber Silica surface

Silica coupling

Triethoxysilyl- group

X

Organo functional group Propyl

spacer

Rubber coupling

Coupling during mixing Coupling during vulcanization b

C

H₂ CH Si

O R

R O R O

Radical initiator

Grafting reaction

Water catalyst

Hydrolysis

R

RO O

CH₂ CH₂ Si

CH OR

+

ROH

CH₂ CH₂ Si

CH

OH OH

OH

R

RO O

CH₂ CH₂ Si

CH OR

R

RO O

CH₂ CH₂ Si

CH OR

Catalyst Crosslinking

CH₂ CH₂ Si O

CH

OH

CH₂ CH₂

Si HC

OH

Silane

+ H₂O

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Para-phenylenediamines are the most used antidegradants in the rubber compound [18].

PPDs can be classified into three general groups: Dialkyl PPDs, alkyl-aryl PPDs, and diaryl PPDs (see Figure 10). In dialkyl PPDs, both side substituents (R and R’) are alkyls from 3 carbons to nine carbons. Whereas, in alkyl-aryl PPDs on the group of R’s is an aromatic ring. [17] There are two aromatic pedants’ groups in diaryl PPDs. Alkyl-aryl PDDs have great properties of dynamic resistance of ozone, static, antioxidant, and cracking [20]. PPDs serve as strong anti-fatigue agents, antioxidants as well as antiozonants [11, p. 285]. The mechanism reaction of alkyl-aryl PDDs is shown in Figure 11.

Figure 9. Structure of 6PPD, TMQ, and bisfunctional phenolic antioxidants. [11, p.

540]

Phenolic

Figure 10. A general structure of para-phenylenediamine antiozonant, where R and R’ can be alkyl, an aryl group, or cycloalkyl. [18]

Figure 11. The mechanism reaction of PPDs with N-alkyl-substituted as well as ozone. [18]

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Overall, antidegradants are used as additives and stabilizers in rubber compound materials. They give the material long-term durability properties, high-temperature stability, and fatigue resistance [18].

2.3.1 TMQ

Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) is a polymer with low-molecular- weight and a molecular formula of (C12H15N)n, where n is the number of the monomer in the polymer. The granular form of the pure TMQ’s is an amber to brown color. TMQ softens above 74 °C and it is soluble in several organic solvents including acetone and benzene but insoluble in water. TMQ can be prepared from acetone and aniline with a hydrochloric acid catalyst via condensation at 130–140 °C. [19] TMQ is a low-cost amine utilized as an antioxidant in the rubber industry. It acts as free-radical traps and has high diffusivity in the polymer matrix. TMQ protects the rubber compound’s elastomers from the thermo-oxidative aging [11, pp. 280–282].

2.3.2 6PPD

N-1,3-dimethylbutyl)-N’phenyl-1,4-benzenediamine (6PPD) is known as antioxidant 4020 with the molecular formula of C18H24N2. 6PPD comes with particles ranging in color from grey purple to brown-purple. It melts at a temperature above 45 °C and is soluble in acetone, benzene, and ethyl acetate but insoluble in water. 6PPD is produced from methyl isobutyl ketone and 4-aminodiphenylamine via catalytic hydrogen reactions.

6PPD is usually used as the antioxidant or antiozonant in the rubber industry. 6PPD protects the rubber compound and prevents it from aging. In addition, it reacts with radical sites by creating crosslinks and restoring the network of the rubber compound.

[19]

2.3.3 Waxes

Waxes simply consist of a hydrocarbon (CnH2n+2, n= number of carbon/hydrogen atoms) mixture having an average of 18–50 carbon atoms (see Figure 12) in the backbone of the polymer chains. Waxes originate from plants, mineral oils, animals, or synthetic substances. They are mainly high molecular weight compounds and solids. The rubber compound can be protected from ozone by waxes alongside the formulation of paraffin and microcrystalline. Waxes preserve the rubber compound from ozonation. Microcrystalline wax is obtained from the residuals of refined petroleum. It is made up of saturated aliphatic hydrocarbons (isoparaffinic) and cycloparaffin with a high branched and molecular

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weight. Microcrystalline wax has a melting point between 55–100 ⁰C. On the other hand, paraffin wax comes from crude oil having a melting point between 35–75 ⁰C. Paraffin wax consists mainly of unbranched hydrocarbons. Waxes uphold the rubber compound material from ozonation. They create a layer on the surface of the rubber compound goods. The bloom or creation of a wax film on the surface of the rubber compound may be controlled by the mobility and solubility of waxes. [21, p. 428] The creation of bloom is due to the immoderate solubility of antidegradants in the rubber compound. These layers protect the rubber compound against ozonation. In other words, microcrystalline waxes protect the rubber compound for the long term and paraffin for short-term periods.

Waxes prevent the attack of ozone by making a barrier on the surface of the rubber compound. Using waxes reduces the dynamic resistance of ozone and the life of the fatigue in the rubber compound [19].

2.4 Resins in Rubber Compounds

Resins are natural and synthetic viscous compounds that transform into stiff polymers via the process of curing. Naturally occurring resins are produced from plants. Resins are amorphous compounds, which can be divided into thermoplastic and thermoset resins. They consist of hydrocarbons, terpene (phenolic), and petroleum resins. [2, p. 81]

Resins consist of three functional groups including hydrocarbons, rosin esters, and polyterpenes. They are used to enhance the peel, grip, and tack adhesion. The tack is needed to link various rubbers together. There are different types of resins. These resins include polymer groups like polyester, phenolic, polyamide, polyurethane, silicone, acrylic, polystyrene et cetera [6, pp. 464–467, 18].

The glass transition temperatures (Tg) of resins are normally greater than the polymer of the base. However, they have a smaller molecular weight than polymer bases. Resins regulate the functionality of polymers, which boosts bridging and adhesion. Because of

Figure 12. The structure of paraffin (a) and microcrystalline (ceresin) (b) waxes.

[18]

a) b)

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this molecular cohesion, the system of polymers softens, and its viscosity decreases. [6, pp. 464–467]

Hydrocarbon resins are utilized in the manufacturing of the rubber compound.

Hydrocarbon resins harden at room temperature, thereby preserving the hardness and modulus of the rubber compound. [1, pp. 71–75] DCPD (Dicyclopentadiene), C9 (hydrocarbon of aromatic resins), and C5 (hydrocarbon of aliphatic resins) resins are utilized as homogenizing agents, performance modifiers, and tackifiers in the rubber industry. Figure 13 shows the structure of hydrocarbon C9 resins [18].

2.5 Mixing of Rubber Compounds

The rubber compound components need to be mixed to get the final products.

Awareness of physical features and forms of materials contributes to a specific mixer type. The components of the rubber compound consist of different types of ingredients.

Processing into finish products includes compounding, mixing, shaping, and vulcanizing.

[22, pp. 102–106]

In most cases, the rubber components are mixed in at least two steps. The first step includes polymers, carbon black, antidegradants, stearic acid, and zinc oxide. The second step includes the addition of the rest of the ingredients, which are carried out at a lower temperature than the first step. [23, p. 188]

Figure 13. The structure of hydrocarbon C9 resins consisting of aromatic monomers of nine-carbon polymers. Dashes represent the aromatic monomers.[18]

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The rubber compound can be mixed using an internal mixer or rolling mill. The rubber components are blended within rolls with a selectable space in a roll mill. In the rubber industry, the internal mixer is commonly utilized to mix the rubber compound’s ingredients. The ingredients of the rubber compound are put into the mixer, where they are pushed into the mixing chamber by a ram. The intermeshing rotors mix the material in the mixing chamber. The drop-door is opened and the batch is dropped after the mixing is completed. Power, temperature, and ram position are the most often measured characteristics throughout the mixing. Controlling these parameters allows tracking the development of the mixing. [23, pp. 188 – 193]

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3. AGING OF RUBBER COMPOUNDS

There are various types of degradation that can be caused by oxidation, crack formation, surface changes as well as many other processes [24]. Oxidation can occur at lower and higher temperatures. Heavy metal compounds accelerate the oxidation process in the rubber compound. Heat, moisture, and static ozone action may aid the degradation process. Dynamic stress, high oxygen, and light cause crack formation in the rubber compound [25]. This section will go through the changes in raw materials and the effect of fillers on the rubber compound. In addition, the effect of temperatures on tire rubber aging will be discussed as well.

3.1 Thermal and Thermo-oxidative Aging of Rubber Compound

There are two types of aging of the rubber compound including thermo-oxidative as well as thermal aging. The predominant aging mechanism of the rubber compound products is the thermo-oxidation. Heating results in the thermal aging either by degrading a polymer into a monomer or via random polymer chain scissions. Thermal aging follows thermo-oxidative aging, where the free radicals produced by heat or light react with oxygen. Thermo-oxidative aging causes crosslinking and chain scission. [26, pp. 37–38]

3.1.1 Thermal Aging

Thermal aging happens in the absence of oxygen. Rubber compound materials usually soften at high temperatures and degrade thermally. The thermal aging of the rubber compound is a molecular degradation. The elevated temperature generates the splitting of long chains in the polymer backbone because of the existence of tertiary hydrogen atoms in the polymer’s chain. The weight of molecules usually decreases due to the shortening length of the polymer chain. [24, 25] Thermal degradation, which is influenced by heat, may follow a chain-end or random degradation. Chain-end results in a pure monomer without the formation of a random deterioration. Thermal degradation results in property changes in the polymer, such as color changes, chalking, decreased embrittlement as well as a decrease in the further physical properties. [26, pp. 53–56]

Thermal deterioration begins with chain scission following the generation of free radicals.

Thermal deterioration contains initiation, propagation, and termination steps, as shown in Figure 14. The initiation step of the deterioration process, which starts with the rising temperature including the formation of free radicals by the cleavage of the bond. The

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propagation step contains the creation of peroxy radicals, which attack the polymer chain to create free radicals and hydroperoxide. The termination step or the last step includes hydroperoxide fragmentation as well as the formation of crosslinks between radical polymers. [27]

3.1.2 Thermo-oxidative aging

Thermo-oxidative degradation can either involve softening or hardening, based on the diene elastomer’s microstructure. [28] On the contrary, polymers with bulky side groups may experience softening of strain due to steric hindrance. However, hydrogen abstraction and disproportionation may cause scission of chains in polymers [29].

Exposure to certain reactive gases, such as oxygen significantly speeds up the aging of the rubber compound. In addition to this, stress and heat may cause the aging of the rubber compound. Due to the thermal-oxidative degradation, changes in properties, such as color fading, charring, softening, hardening, and cracking can be seen in the rubber compound. [27] Crosslinking and chain scission (see Figure 15) depend on the composition of the rubber compound. This process is complex and includes various side and intermediates reactions. Chain scission or softening reaction includes the loss of molecular weight in the polymer chain. This happens, when a radical of carbon reacts with an

Initiation

Propagation P.

+ O₂ _Δ POO.

PH P.

+ H. Δ

P.

+ PH P.

+ POOH Δ

POOH POO.

+ OH. Δ

P. Δ

+ POO.

P - P

POO.

+ POO.

P - P Δ

Termination P. Δ

+ P.

P - P

Figure 14. The mechanism of thermal aging of polymers in the existence of oxygen, where H˙and P˙ are free radical and P is a polymer. [Modified from [27]]

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oxygen molecule, which follows softening of the rubber compound. [28] On the other hand, the generated radicals of polymers chain link together to make a crosslink in the polymer network because of hydrogen grab from the polymer chain by a free radical.

This results in the embrittlement and hardening of the rubber compound [29].

The forms of degradation such as chain softening, and hardening rely usually on the chemical composition of the polymer. Crosslinking or hardening occurs in BR and its copolymers like NRB and SBR. It also happens in the certain rubber compound, which has less active double bounds, because of electron-withdrawing groups like halogen. On the other hand, chain scission or softening appears in polymers with electron-donating side groups like methyl (-CH3), linking in the double bonds of the carbon atom. This process happens in elastomers including polyisoprene rubber (IR) and natural rubber. It also occurs in some electron-donating groups of unsaturated polymers. [28, 29]

Crosslinking and chain scission can be experienced, for instance, in styrene-butadiene rubber. However, usage of antioxidants, antiozonants, and ultraviolet stabilizers can merely retard or prevent them [18].

Figure 15. Thermal oxidation mechanism of (A) chain scission and (B) crosslinking of rubber, where k is the constant rate of reaction, T temperature, BR butadiene rubber, and NR natural rubber. [18]

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3.2 Effect of Raw Material on Rubber Compound Aging

Different polymers can be blended together to meet the special application or requirement in processing. The details of the chain structure impact significantly on the polymer properties [24]. Because of the double bonds in the primary chain of BR, SBR, and NR, they are particularly vulnerable to oxidative heat aging. Fillers have also an impact on the rubber compound’s aging resistance. For example, an addition in the content of carbon black filler (phr) reduces the elongation at break induced by thermo- oxidative aging [30]. This results in the increase of oxygen absorption in the vulcanized rubber compound that produces crosslinks of mono- and disulfur from the cleavage of polysulfur linkages during the aging of the vulcanized rubber compound [18]. Different raw materials have different impacts on the thermo-oxidative aging of the rubber compound. In general, changing the raw materials of the rubber compound changes the dynamics of the whole mixtures. Therefore, the comparison between the raw materials is difficult.

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The properties of the rubber compound are influenced by a variety of raw materials. The chemical structure and the effect of raw materials are shown in Table 1. The part that affects the aging of the raw materials is circled with a red circle.

Table 1. Raw material effects on the rubber compound’s properties. The symbol represents the increasing value and the decreasing value of rubber compound properties.

Raw material

Tensile strength [MPa]

Elongation [%]

Hardness [Shore A]

Modulus [MPa]

Effect of raw materials on the rubber compound aging

Notes References

NR

Increasing NR content increases thermal aging of the rubber compound owing to the double bonds in the polymer chain.

[2, p. 87; 3]

BR

Increasing amount of BR increases thermal aging of the rubber compound owing to the double bonds in the polymer chain that are prone to deterioration.

[3; 11, pp. 316 - 310]

SBR

The repeating unit of styrene is chemically stable in SBR, so decreasing of styrene repeating units accelerates thermal and oxidation aging. Increasing of vinyl group increases aging of the rubber compound.

[31]

Silica

Increasing silica amount weakens filler-polymer interactions and strengthens filler-filler interactions. Increasing silica content lowers the tensile strength and increases modulus.

[32,33]

Carbon black

Increasing of carbon black causes thermal stress and softens the rubber compound, where is more filler-polymer, and less filler- filler interactions.

Properties of rubber such as the abrasion resistance and the tensile strength increase with the carbon black content's increase to a maximum and then drops.

[13, 33, 34]

Carbon black Polymer

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3.2.1 Heat Aging of Butadiene Rubber

BR normally experiences hardening when it is exposed to the light or oxygen. The repeating units of 1,2- as well as 1,4-butadiene in the copolymer are unstable. Butadiene rubber may be involved in every reaction within the heat aging of the rubber compound.

This is owing to the double bonds in the butadiene elastomer chain, which are willing to oxidation. [8, 18] Hardening of the rubber compound is far more common in the rubber compound because of the development of new crosslinks in this process. Hardening includes the formation of crosslinks in the BR, where a radical grabs hydrogen from the chain of polymers (see Figure 16). This follows the embrittlement and hardness of BR.

This dramatically decreases the rubber compound flexibility and induces brittleness. [11, pp. 316–319] Figure 16 shows the thermo-oxidative aging of BR.

Chiu et al. [35] studied the changes in blending ratios of butadiene rubber and natural rubber. The blends of NR/BR were aged at 70 °C for 1, 3, 5, 7, and 30 days in a heated oven. The mechanical characteristics including the tensile strength and the elongation at the break were studied at different ratios of blending before and after the thermal aging.

The study found that butadiene rubber had higher stiffness compared to natural rubber.

Under the same load conditions, natural rubber had higher deformation than butadiene rubber. In addition, BR had a lower stress loss than NR indicating better aging resistance.

To sum up, BR had a lower tensile strain, tensile stress, and tear strength than NR. When the ratio of NR was grown, the tensile strength and the elongation decreased and stress loss increased in the NR/BR blends after heat aging.

Figure 16. Degradation mechanism of BR, crosslinking of rubber, where k is the constant rate of reaction, T temperature, and BR butadiene rubber. [7]

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3.2.2 Heat Aging of Natural Rubber

The aging process of natural rubber can be caused by crosslinking, chain scission, or other types of chemical transformations of the polymer chains. This indicates the loss of physical properties in the rubber compound. The main cause of elastomer degradation is oxygen in the form of O3 as well as O2. The light-initiated, and thermal-oxidative indicate progress by the same free-radical chain mechanism. This involves hydroperoxides formation in the rubber compound. [2, 18] The deterioration process affects the cleavage of the polymer chain (see Figure 17). The electron-donating side group, for instance, methyl joined to the double bonds of a carbon atom, is prone to the splitting of the polymer chain, which follows the initiation of crack on the surface of the rubber compound as a result of stress. The length and depth of cracks expand resulting in severe material degradation and the rubber compound materials soften [36].

Chang et al. [37] investigated the impact of the thermal aging on the vulcanized NR in the air circulating ovens at a temperature varying between 50–100 °C for 1–90 days. The mechanical properties were tested according to the ISO–37 standard. The study showed that the modulus 100 % and stiffness increased with an increase in the hardness. On the other hand, the tensile strength as well as the elongation at the break reduced with the aging period and the aging temperature. The failure in the polymer’s molecular chain occurs due to the impact of the heat. The increased aging temperature causes the polymer-filler and filler-filler movement inside the matrix of the polymer following the decrease of the tensile strength and the elongation at the break [38]. As a result, the chain scission increases in the unsaturated polymer and restricts the mobility of the polymer’s chains.

Figure 17. Thermal degradation mechanism of NR chain scission, where k is the constant rate of reaction, T temperature, and BR butadiene rubber. [7]

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3.2.3 Heat Aging of Styrene Butadiene Rubber

SBR can be aged via thermo-oxidative deterioration. This is an oxidation process, which increases the interlinking of the polymer chains by hardening with age instead of softening. SBR is prone to oxidative and thermal deterioration because of the existence of double bonds in the backbone of polybutadiene. [20]. For instance, crosslinking increases the stiffness, embrittlement, and hardness in case the rubber compound’s crosslink density increases. [18, 27] Whereas the chain scission reduces the rubber compound’s crosslink density resulting in softening and loss of elastic characteristics of the rubber compound. There are several complicated interacting reactions in the aging process of SBR. [39] Figure 18 shows free radical initiation in the polymer of SBR affected by heat.

Hao et al. [40] studied the ratio of rubber blends with two types of styrene-butadiene rubber (Krynol 1721, Buna VSL, and Krynol 1712 containing high contents of styrene and vinyl) with natural rubber (SMR 5) in the tire tread compound. The mechanical properties including abrasion resistance, the tensile, and tear strength were measured after heat aging (24 hours at 70 °C). The 50:50 ratio of SBR 1712:NR SMR 5 was used as a reference in the study. In addition, the Buna VSL and SBR 1721 ratio was changed from 30 to 70 phr. The tensile strength increased when the ratio of styrene-butadiene rubbers increased because of the lower heat resistance of NR. The ratio of double bonds in the polymer chain of natural rubber is higher than SBR. The double bonds are seen as weak links in the chain structure where free radicals might migrate to double bonds by making crosslinks, which follows the decreasing of strain at break and rising of modulus. Hence, increasing SBR content in the blend followed the reduction of tear strength and rebound resilience.

Figure 18. Aging mechanism of SBR. Free radical initiation of SBR (radicalization). [27]

Heat Aging of Rubber Compounds

Rulis Heidari