EFFECT OF TENSILE RATE AND CARBON BLACK ON THE FRACTURE OF NATURAL RUBBER AND STYRENE-BUTADIENE RUBBER. A Thesis. Presented to

Transcription

1 EFFECT OF TENSILE RATE AND CARBON BLACK ON THE FRACTURE OF NATURAL RUBBER AND STYRENE-BUTADIENE RUBBER A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Qinwei Wang May, 2013

2 EFFECT OF TENSILE RATE AND CARBON BLACK ON THE FRACTURE OF NATURAL RUBBER AND STYRENE-BUTADIENE RUBBER Qinwei Wang Thesis Approved: Advisor Dr. Gary R. Hamed Faculty Reader Dr. Li Jia Department Chair Dr. Coleen Pugh Accepted: Dean of the College Dr. Stephen Z. D. Cheng Dean of the Graduate School Dr. George R. Newkome Date ii

3 ABSTRACT Tensile strengths and ultimate elongations of both crystallizing NR and amorphous SBR increase with increasing tensile rate and increasing carbon black N115 loading, although the dependence of N115 loading is at least ten-fold stronger. Previous research has shown that edge-cut specimens of gum NR show an abrupt drop in tensile strength, when tested at 50 mm/min at a critical cut size, because of lack of time to crystallize in bulk. In this thesis, the behavior of critical cut size with respect to tensile rate is completed. Critical cut size increases monotonously, but not linearly, with increasing tensile rate. Changes in precut tensile strength of NR at various tensile rates are measured and compared with those of SBR. Previous research has shown a critical loading of N115 black tested at 50 mm/min, before which precut NR will only be weakened. In this thesis, precut tensile results of NR with various N115 black contents were measured and compared to those of SBR. Only NR shows an critical black loading to reinforce precut specimens and this concentration doesn t change with tensile rate. Reinforcement of precut NR is due to the formation of a rubber/ black network. But, reduce tensile rate can decrease the N115 content for reinforcing NR with all possible cut sizes. This is noted as a second turning point of black concentration. iii

4 DEDICATION To my beloved parents Who support me all the time And to whom I owe everything I have today To my cherished love Who accompanies me in distance And to whom I have a word of promise iv

5 ACKNOWLEDGEMENTS After the accomplishment of my master thesis, I would like to express my sincere gratitude to my advisor Dr. Gary R. Hamed for his guidance, patience, encouragement and supports during my research. I also express my sincere gratitude to Dr. Li Jia for his trust and invitation to his research group for a part of my research. I appreciate Mr. Robert H. Seiple and Mr. Bojie Wang for their guidance in the operation of relevant instruments. I appreciate my roommate Weizheng Fan for his assistance in photographing my samples. I also thank my group members: Tamon Itahashi, Guangzhuo Rong, Adeyemi Adepetun, Tianxiang Xue, Jiali Jiang, Yu Sun, Minghang Yang, Yanxiao Li, Xin Tan, Zhenpeng Li, Chao Wang, Nishant Kumar, Joseph Scavuzzo, Kai Li, for their friendliness, helps and suggestions. Lastly and most importantly, my deepest gratitude goes to my entire family for their unselfish and everlasting love. I wish to thank my parents, Xia Wang and Jianbin Zhang, they have continuously supported me both morally and economically after my arrival of United States without which I would not have a chance to insist to the end. I wish to thank my aunt Qi Wang, uncle Xugang Mei who serve as my health instructor and help me out when I was ill. I wish to thank my grandfather Haishou Wang for his stay in health and save me a lot of time worrying about him. v

6 TABLE OF CONTENTS Page LIST OF TABLES... ix LIST OF FIGURES... x CHAPTER I. INTRODUCTION Gum Natural Rubber Styrene Butadiene Rubber (SBR) System and Role of Vulcanization Fracture of Rubber Mechanism of Rubber Fracture Effect of Chemistry and Fatigue Effect of Crystallization Effect of Energy Dissipation Effect of Crack Blunting and Crack Deviation Effect of Temperature and Strain Rate Pre-introduced Edge Cut Effect of Edge Cut Previous Research on Gum Rubber with Precut Fillers Filler and Carbon Black vi

7 1.6.2 Effect of Carbon Black on Mechanical Properties Fracture of Carbon Black Filled Natural Rubber II. EXPERIMENTAL Materials Formulations Mixing Milling Vulcanization Characterization Molding Swelling Test Bound Rubber Tensile Strength Crack Pattern Photographs III. RESULTS AND DISCUSSION Characterization of Vulcanization Swelling Test and Crosslink Density Bound Rubber Test Normal Tensile Results (c = 0) of SBR and NR at Various Tensile Rates Tensile Tests of SBR and NR at 50 mm/min Rate Dependence of Tensile Strength of gum SBR and NR Rate Dependence of Tensile Results of Black-Filled NR and SBR Precut Tensile Results of SBR and NR at 50 mm/min Precut Tensile Results of Gum Rubbers at 50 mm/min Precut Tensile Results of Black-Filled SBR at 50 mm/min vii

8 3.5.3 Reduced Strength of Slightly Carbon Black Filled NR at 50 mm/min Critical Carbon Black Loading for Partial Reinforcement at 50 mm/min Critical Carbon Black Loading for Completed Reinforcement at 50 mm/min Precut Tensile Results of SBR at Various Tensile Rates Rate Dependence of Precut Tensile Results of Unfilled SBR Rate Dependence of Precut Tensile Results of Black Filled SBR Precut Tensile Results of NR at Various Tensile Rates Completed Tensile Rate- Critical Cut Size Profile of Gum NR Effect of Tensile Rate on the Carbon Black Loading for Reinforcement of NR Slopes of Fitted Line of Strength-Cut Size Plot IV. CONCLUSIONS REFERENCES APPENDIX viii

9 LIST OF TABLES Table Page 2.1 Parts of carbon black and volume fraction Cure characteristics of gum and black filled SBR and NR Swelling tests of unfilled and black-filled SBR and NR Bound rubber test of carbon black N115 filled NR Tensile results of uncut SBR and NR at 50 mm/min Tensile results of uncut B0S and B0N at various tensile rates Tensile strength of SBR with various amount of N115 at various tensile rates Tensile strength of NR with various amounts of N115 at various tensile rates Strength loss of precut SBR at c = 0.2 mm Characteristic values of tensile results of B0N, B5N and B14N at 50 mm/min Collection of SL values of all NR compounds at 50 mm/min Strength loss of precut B0S at various tensile rates Strength loss of B0S, B8S and B20S at various tensile rates Characteristic values of tensile results of B0N at various tensile rates Characteristic values of precut tensile results of black-filled NR Slopes in strength-cut plot of gum SBR/ NR Slopes in strength-cut plot of black-filled SBR Slopes in strength-cut plot of black-filled NR ix

10 LIST OF FIGURES Figure Page 1.1 Chemical structure of natural rubber (cis-1,4-polyisoprene) Chemical structure of SBR Network in rubber cured by sulfur Free radical mechanism of sulfur vulcanization TBBS activation mechanism in curing process Mechanism for peroxide vulcanization Crosslink structures Simple tensile testpiece with edge cut of length c Fraction of crystallinity, Φ c, as a function of time for Hevea as a function of time at different extension ratios. Λ: ( ) 2.0; ( ) 3.0; ( ) 4.0; ( ) 5.0; ( ) Failure envelop of an amorphous gum SBR Effect of edge cut on fracture of gum NR (SMR CV60) Effect of temperature on tensile strength of gum NR (SMR CV60) Morphology of carbon black Particle spacing cubic lattice model Stress-strain for gum and black filled SBR, NR (S1, 2 and N1, 2) Payne Effect: strain on dynamic storage modulus G Effect of precut on the strength of gum NR (N1) and black-filled NR (N2) Tensile strength of precut specimens with 0, 3, 6, 12 phr of N Vulcanization characterization of NR..33 x

11 3.2 Vulcanization characterization of SBR Maximum torque versus true volume fraction of carbon black N115, v Crosslink density ρ c of NR and SBR versus true volume fraction of N115, v Strength and elongation of NR versus N115 concentration, C, at 50 mm/min Strength and elongation of SBR versus N115 concentration, C, at 50 mm/min Comparison of stress-strain curve of B0S and B0N at 50 mm/min Comparison of stress-strain curve of NR with N115-black at 50 mm/min Comparison of stress-strain curve of SBR with N115-black at 50 mm/min Rate dependence of tensile results of B0N and B0S Rate dependence of stress-strain curve of B0N Rate dependence of stress-strain curve of B0S Normal tensile strength versus tensile rate for N115 filled NR Normal tensile strength of NR versus N115 loading at various tensile rates Normal tensile strength versus tensile rate for N115 filled NR Normal tensile strength of NR versus N115 loading at various tensile rates Tensile strength of B0N with precut c at 50 mm/min Strength of B0S with precut c at 50 mm/min Stress-strain curve of B0N at 50 mm/min, with precut specimens labeled Comparison of stress-strain curve of precut B0N in SP and WP at 50 mm/min Comparison of strength of gum SBR and NR with precut c at 50 mm/min Tensile strength of SBR filled with various amount of carbon black with precut c at 50 mm/min Tensile strength of precut B0N, B5N and B14N at 50 mm/min Tensile results of B15N with precut c at 50 mm/min Comparison of precut tensile results of B0N, B14N and B15N at 50 mm/min xi

12 3.26 Crack pattern of specimen of B15N at 50 mm/min (c = 1.09 mm, 8.11 MPa) Crack pattern of specimen of B15N at 50 mm/min (c = 1.40 mm, 7.44 MPa) Comparison of precut tensile results of B0N, B18N and B19N at 50 mm/min Crack pattern of B19N specimen at 50 mm/min (c = 1.09 mm, MPa) Comparison of precut tensile results of N115 filled NR with crack deviation at 50 mm/min Rate dependence of precut tensile strength of B0S Rate dependence of precut strength of B8S Rate dependence of precut strength of B20S Comparison of precut tensile strength of B0S, B8S, B20S at various rates Comparison of precut strength of B0S (r=250, 50) and B8S (r=25, 0.1) SL versus tensile rate to decide slope A Comparison of precut tensile strength of B0N at 250, 50 and 25 mm/min Comparison of precut tensile strength of B0N at 500, 250, 10, 0.05 mm/min Completed tensile rate- c s / c w profile Precut strength of B0N, B5N, B14N and B15N at 250 mm/min Precut tensile strength of B0N, B5N, B14N, B15N at 0.1 mm/min Precut tensile strengths of B0N, B18N, B19N and B20N at 250 mm/min Comparison of precut tensile strength of B0N and B18N at 0.1 mm/min Effect of tensile rates on critical N115 loadings C T1 and C T Comparison of precut strengths of B25N at 250, 50, 10, 0.1 mm/min xii

13 CHAPTER I INTRODUCTION 1.1 Gum Natural Rubber Natural rubber is widely used both in households and in industry. Although synthetic rubbers, such as styrene butadiene rubber (SBR) and synthetic polyisoprene (IR), can replace natural rubber in some situations, the combination of good properties such as high green strength (the strength before cross-linking), good mechanical strength under severe deformations, high tensile strength, high resilience and good low temperature performance make natural rubber irreplaceable for many applications. 1 But, natural rubber has shortcomings. Its resistance to ozone and oxygen is relatively poor. 2 Ozone can react with double bond in natural rubber and thus degrade it. Antioxidants and antiozonants are necessary additives for natural rubber. 2 The chemical structure of natural rubber is believed to be nearly 100% cis-1, 4-polyisoprene (Figure 1.1) 3 with weight average molecular weight ranging from 10 6 to 10 7 g/mol and number average molecular weight ranging from 10 5 to 10 6 g/mol. 4 H 3 C CH 2 C C H 2 C H n Figure 1.1 Chemical structure of natural rubber (cis-1,4-polyisoprene) 1

14 1.2 Styrene Butadiene Rubber (SBR) Styrene butadiene rubber (SBR) is a synthetic rubber and there are two major ways to co-polymerize styrene and butadiene. The first one is emulsion copolymerization (E-SBR) via a free radical mechanism. 5 Two examples of this kind of SBR are high temperature SBR (1000 series) and low temperature SBR (1500 series). 6 The second one is copolymerization in solution (S-SBR) which occurs by an anionic mechanism. 5 The microstructure of SBR is a combination of cis, trans and vinyl butadiene with styrene, as shown in Figure 1.2. C C C C C C C C C C C C C C w x y z n trans cis vinyl styrene Figure 1.2 Chemical structure of SBR Increasing styrene content increases glass transition temperature and modulus, but reduce elasticity. A common styrene content is 23.5% by weight. Increasing vinyl content group increases glass transition temperature, but resistance to abrasion is decreased. Increased cis-butadiene content increases elasticity and reduce tensile strength. Increased trans-butadiene content increases structural symmetry and improves modulus. 7 SBR has better wear resistance than NR (especially at high temperature) and lower elasticity. 8 Also, SBR has better oxidant resistance than NR. 8 The curing rate of SBR is slower than NR due to SBR s lower content of carbon-carbon double bond and bulky styrene substituent. 8 2

15 A very important difference between natural rubber and SBR is in the ability to crystallize. 9 Natural rubber can undergo crystallization upon straining, while SBR remains amorphous. This behavior is discussed extensively later. 1.3 System and Role of Vulcanization Natural rubber and SBR before vulcanization is entangled and viscous liquid with high molecular weight. It is weak and unable to maintain its shape due to molecular slippage under deformation. Once vulcanized, rubber becomes an elastic solid with a three dimensional network, as shown in Figure Various curing agents are used to vulcanize rubber. Sulfur is the most common and extensively used curing agent for diene rubbers, 10 such as natural rubber, styrene-butadiene rubber. Peroxide curing agents are also popular. Figure 1.3 Network in rubber cured by sulfur Free radical mechanism was proposed for sulfur vulcanization, as illustrated in Figure Additives in vulcanization step includes activators (zinc oxide and fatty 3

16 acid) and accelerators. 10 Sulfur alone without accelerator is rarely used because it s inefficient: as many as sulfur atoms are involved in per crosslink; vulcanizes aging properties are poor; curing time is long and crosslinks are unstable. 12 S S S S S S S S S C C C C C n C C C C C n + S x H C C C C C n S S C S S + C C C S S S S C + S 8-x S x n C C C C C S x n + C C C C C m C C C C C C S x C n C C C m C C C C C C S x C n C C C m + C C C C C x C C C C C C S x C n C C C m + C C C C C x Figure 1.4 Free radical mechanism of sulfur vulcanization There are three common types of accelerator that are widely applied. 10 The first one is thiazole type and sulenamides type. 10 Increase the size of substituent R will delay the onset of curing, i.e., increase the scorch time. Therefore, more processing operation are allowed before crosslinking reaction. A famous example is 2-mercaptobenzothiazole (MBT). The second one is dithiocarbamate type. 10 Its cure rate is extremely fast with very short scorch time. It can be applied to the polymer needn t much processing. Example is tetra methyl thiuramdisulfide (TMTD). The last common type of accelerator is amine type (e.g. diphenylguanidine). 10 It s cure rate and scorch time are moderate. 4

17 Scorch time doesn t very sensitive to accelerator concentration or the ratio of sulfur and accelerator, but the accelerator concentration does have important effect on vulcanizate which will be discussed later. Because all processing procedure must be accomplished before scorch time, the first type of accelerator, i.e., thiazole type accelerator, is the most welcomed. Usually, thiazole accelerators perform at the presence of zinc salt, zinc oxide, or fatty acid. Their mechanism and functions are shown in Figure 1.5. The rate of vulcanization is accelerated by addition of zinc-oxide because it activates the breakage of Ac-S bond. N S SH Ac-H Ac-H Ac-Ac S 8 S8-x + Ac-Sx -Ac Ac-H + C C C C C C C C C S x C n m C C C C C fatty acid such as ZnO activates Ac-S bond m C C C C C + n S x Ac C C C C Ac-H C n Figure 1.5 TBBS activation mechanism in curing process Besides sulfur, peroxide type curing agents are also developed and examined. It s mechanism is shown in Figure ,14 Dialkyl peroxides give efficient crosslinking. Di-t-butyl peroxide yield good cured rubbers but di-t-butyl peroxide is too volatile which limits its use. 5

18 O O heat O C C C C C n C C C C C OR n C C C C C OR n C C C C C C OR C C C C n n C C C C ORC C C C C ORC C C C C C ORC C C C C Figure 1.6 Mechanism for peroxide vulcanization Acidic compounding ingredients such as stearic acids can cause decomposition of peroxides can reduce the cross-linking efficiency. Many properties of rubbers cured by sulfur are poorer than that cured by sulfur, but rubbers cured by peroxides have better thermal resistance, excellent aging properties and set resistance. 13 Vulcanization transforms an rubber from a visco-elastic liquid with high molecular weight into an elastic solid. Generally speaking, elastic and recovery stiffness increase as crosslink density increases while hysteresis and friction decrease with increased cross-linking. Some properties show a maximum value with the increasing crosslink density, such as tear strength, fatigue life, and tensile strength, because these properties depend on hysteresis as well as on the number of network chains. The number of network chains increases monotonously with increased cross-linking, but hysteresis, also known as energy dissipation decreases with increased cross-linking. 10 Increase in sulfur and accelerators concentration yields higher crosslink density. Hamed discussed the dependence of strength on crosslink density and accordingly offer 6

19 higher modulus, stiffness. 15 With increasing crosslink density, the fracture changes from viscous flow without breaking chemical bonds to elastic fracture of chemical bonds above the gel point, and finally to brittle fracture at high crosslink levels. Crosslink levels must be high enough to prevent failure by viscous flow, but low enough to avoid brittle failure. The type of crosslinks has effects on properties. 15,16 With increasing ratio of sulfur concentration to accelerator concentration, polysulfide crosslink are increased and more sulfur will be attached to rubber to form sulfur ring structure, as shown in Figure Figure 1.7 Crosslink structures Elastomers with monosulfidic cross-links have better heat stability than elastomers with polysulfidic cross-links, because C-S bonds have better stability than S-S bonds. On the other hand, elastomers with polysulfidic cross-links have better tensile strength and fatigue resistance than elastomers with monosulfic cross-links. This is due to the ability of S-S bonds to break reversibly. 16 7

20 1.4 Fracture of Rubber All solids are heterogeneous and contain flaws. When a solid is exposed to global stress, the local stress at flaws is magnified. 17 When the local stress at a critical flaw is sufficiently high, fracture occurs. Thus, the measured tensile strength is smaller than that predicted theoretically for a perfect solid. 18 Inglis 12 derived an equation relating local stress at crack tip, σ t, to global stress σ: l σ t = σ (1.1) r where l is the depth of an edge flaw; r is the radius of the flaw tip. When the flaw size is much larger than the radius of the flaw tip, Equation 1.1 can be approximated by: 19 2σ l σ t = (1.2) r Sharper and longer flaws increase stress concentration. So introduced sharp precut will give lower strength. Griffith 20,21 proposed another way to analyze fracture. A crack won t grow in a stressed material unless the decrease in elastically stored energy overwhelms the increase in surface energy to form a new crack surface. This theory can be expressed mathematically: W A > γ (1.3) c s c 8

21 where W is the decrease of stored energy caused by crack growth, c; A is surface area created by crack growth; γ s is the surface energy. The Griffith equation is valid for an ideal elastic solid that fractures without any bulk energy dissipation Mechanism of Rubber Fracture For a rubber sheet of thickness t, Thomas and Rivlin 22 proposed that: at fixed deformation l, W = Gt (1.4) c where G is fracture energy, which includes surface energy and bulk energy dissipation during crack growth. Generally, uniaxial tensile testpieces are used (Figure 1.8). An edge-cut of depth c is introduced and a specimen is pulled at constant rate until fracture. The shadowed triangular region in Figure 1.8 is not deformed during stretching; and the remaining parts of the specimen contain elastically stored energy, W. l Figure 1.8 Simple tensile testpiece with edge cut of length c 9

22 Thus, Equation 1.4 yields: G = 2kWc (1.5) where k is weakly dependant on strain. It can be taken as a constant value in many cases. W is equal to σ b 2 /2E, where σ b is the ultimate stress and E is the Young s modulus for a linearly elastic material. Thus, for simple fracture involving the lateral growth of a single crack, Equation 1.5 becomes: 23 σ = GE b kc (1.6) Effect of Chemistry and Fatigue Chain rupture may be mainly due to mechanical loading, but in addition, rubber networks are altered by the environment. Oxygen and ozone are two degradants. Oxidation results in chain scission and chain cross-linking, while chain scission dominates for ozone attack. 2,24 If a test is rapidly carried out, time is insufficient for chemical attack of rubber and fracture is purely mechanical. But, fatigue fracture involves long-time cycling thus chemical attack may occur prior to and during fracture. A description of oxidative attack follows: 25 First, allylic or tertiary hydrogen abstraction initiates oxidation. This creates a macro-radical, which then rapidly reacts with oxygen to form a peroxy radical. Second, this peroxy radical abstracts an active hydrogen to give a hydroperoxide and a new macro-radical, which can add more oxygen. This chain reaction may affect many polymer chains before the propagating radical becomes dormant. The peroxide then goes 10 eptanes 10 cleavage and yields a macro-oxy radical and a hydroxyl radical. These give two different successive reactions: 10

23 one is cross-linking by coupling of macro-radicals or addition of macro-radicals to another unsaturated bond; one is chain scission which produces a dead chain. If the former process dominates, the rubber becomes stiffer, if the latter dominates, the rubber becomes softer. Next, the mechano-chemistry of rubber under fatigue is considered. 25 Oxygen affects both the force necessary to break a network chain and the micro-structure at a crack tip. A smaller stress is needed to break a chain undergoing oxidation. Oxidation is accelerated when a rubber is deformed. 25 It is likely that during oxidation, a stress network will suffer a higher ratio of chain scission to chain cross-linking compared to that of an unstressed network Effect of Crystallization Natural rubber can crystallize when stretched due to its high stereo regularity but SBR remains amorphous. 9 Thermal crystallization of natural rubber occurs slowly at room temperature and is inhibited at elevated temperature. Thermal crystallization reaches a maximum rate at -24 C. 26 This causes hardening of uncured rubber during storage. Strain-induced crystallization imparts high tensile strength and tearing resistance to natural rubber. Synthetic cis-1, 4-polyisoprene contains a very small amount of non-cis units. These irregularities in microstructure hinders crystallization upon straining. Gent 27 demonstrated that, at high strain rate, synthetic cis-1, 4-polyisoprene has lower tensile strength than NR due to a reduced rate of strain-induced crystallization within synthetic cis-1, 4-polyisoprene. 11

24 Shimomura 28 demonstrated that the degree and rate of crystallization increase with increased extension ratio (λ) (Figure 1.9). Gehman and Field 29 found that the crystallization upon straining of gum natural rubber starts at about 250% elongation. With increasing strain, crystalline morphology transitions from spherulitic to fibrillar. 30,31 Figure 1.9 Fraction of crystallinity, Φ c, as a function of time for Hevea as a function of time at different extension ratios. Λ: ( ) 2.0; ( ) 3.0; ( ) 4.0; ( ) 5.0; ( ) Effect of Energy Dissipation Ideally, if stress concentration upon straining in a network were totally absent, all chains that bear load would have the same stress. In this limit, stress would be very high. But, unfortunately, this ideal situation is never true, because, at least in part, of inevitable flaws. Increasing the uniformity of loading among polymer chains in a network is desirable

25 If highly stressed polymer chains can reduce the load they carry without breaking irreversibly, uniformity in chain load is ameliorated and macroscopic strength is enhanced. This mechanism can be involved to explain why sulfur cured rubbers are stronger than peroxide cured ones. 25 Polysulfidic linkages between polymer chains are able to reversibly break and reform when they are overloaded. This results in a more uniformly loaded network. This process is non-catastrophic energy dissipation. On the other hand, peroxide linkages are unable to reform after rupture. Energy dissipation results from nonreversible and catastrophic chain rupture Effect of Crack Blunting and Crack Deviation Microscopic effects of crystallization and energy dissipation have been discussed. These factors manifest macroscopically. A crack tip for a weak rubber remains sharp during deformation and perpendicular to the direction of external stress. But strong rubbers, especially those reinforced with fine fillers, behave differently due to their capability of energy dissipation. 25 When deformed, a crack tip blunts and develops two or more secondary cracks. These grow in two opposite directions. Secondary cracks reduce stress concentration. Therefore, a new crack must initiate before fracture occurs, which enhances strength. Chain alignment at crack tip also occurs in strong rubbers. 25 This leads to the anisotropy at a crack tip. If chain alignment is sufficient, a fibrous structure develops, and cracks propagate between laminae which causes crack deviation. Non-catastrophic energy dissipation at a crack tip allows such a process. If chain rupture before enough anisotropy develops, a crack simply grows forward. 13

26 Kendall proposed a crack deflection mechanism. 32 He proved that a small, initially introduced crack perpendicular to loading direction will deviate along the loading direction if the following condition is met: G G // < (1.7) 4π ν 2 ( 1 ) where ν is Poisson s ratio, G // is the fracture energy for splitting along the stress direction, G is the fracture energy for tearing across the stress direction Effect of Temperature and Strain Rate Smith 33 investigated tensile strength and ultimate strain as a function of temperature and strain rate. For an amorphous elastomer, such as SBR, these properties at different temperatures could be shifted to form a master curve at a reference temperature. The shift factor was defined by the ratio of internal viscosity at various temperatures to the viscosity at a certain reference temperature (WLF equation). 34 When tensile rate is low, polymer chains in a rubber network have enough time to relax, bulk energy dissipation will be hindered and tensile strength is thus relatively low (This only holds for amorphous rubber.). Lake and Lindley 35 found a minimum fracture energy for cured rubber tested at sufficiently high temperature and sufficiently low rate. The threshold mechanic tearing energy below which catastrophic fracture didn t occur was about 50 J/m 2, which is larger than the dissociation energy of chemical bonds per area at the fracture surface. Lake and Thomas 36 proposed a theory to account for this finding. To break a single bond in a network chain, the entire chain must be extended to the break point. As a result, fracture energy and average molecular weight between cross-links was is related by: 36 14

27 where M c is the average molecular weight between cross-links. G M c (1.8) Smith 36 also plotted breaking stress versus the ultimate strain at different temperatures and strain rates. The parabolic shaped curve is called a failure envelope. Figure shows the failure envelope of an amorphous, cured SBR. The anticlockwise direction along the parabolic curve corresponds to increasing strain rate or decreasing temperature. The failure envelope is independent of time and temperature. Figure 1.10 Failure envelop of an amorphous gum SBR 1.5 Pre-introduced Edge Cut An easy way to investigate the effect of flaws on strength is to introduce a precut into specimens before testing. 15

28 1.5.1 Effect of Edge Cut. Hamed 38 studied how edge-cuts of different depth affect the fracture of ethylene propylene diene terpolymer, EPDM, with various cross-link densities. Equation 1.6 is applicable for lightly cross-linked specimens when crack pattern is simple. Natural rubber doesn t follow Equation 1.6 due to strain crystallization. An abrupt drop in strength at a critical cut size, c cr, for gum natural rubber was observed by Thomas and Whittle 38 (Figure 1.11). This sudden drop was attributed to the failure of bulk crystallization when cut size, c, is large. When c is small, bulk crystallization occurs before catastrophic crack growth take place. But when cut size reaches a critical point, upturn occurs prior to the bulk crystallization. Figure 1.11 Effect of edge cut on fracture of gum NR (SMR CV60) 16

29 Hamed and Park 39 compared the strengths of gum natural rubber with SBR filled with carbon black. These two materials have the similar normal tensile strengths about 30 MPa. When c was small, NR crystallized in bulk and the gum natural rubber was stronger than the black filled SBR. However, when c was sufficiently large so that bulk crystallization of NR was absent, the gum NR was weaker than the black-filled SBR Previous Research on Gum Rubber with Precut Hamed and Rattanasom 40 studied the effect of crosslink density on the fracture of sulfur-cured gum natural rubber. An abrupt drop in strength was observed in lightly cured natural rubber in strength at a critical cut size c cr. At the critical cut size, specimens crack patterns after fracture were photographed by scanning electron microscopy, SEM, and only a simple smooth crack was found. The value of c cr deceased quickly with increased crosslink density, ρ c, indicating a hindrance of crystallization at higher ρ c. Sufficiently cross-linked natural rubber did not show a critical cut size. These specimens exhibited simple lateral fracture from the tip of the precut. Crack surfaces were rough. A cured natural rubber specimen with intermediate crosslink density showed slight crack deviation prior to rupture. This indicates a slightly crystalline region, which deviates a crack and strengthens a rubber. Thomas and Whittle 41 examined the effect of temperature on the tensile strength of cured natural rubber cross-linked with DCP (dicumyl peroxide). There was an abrupt drop in tensile strength at a critical temperature, T c, as shown in Figure This was attributed to the lack of strain crystallization at high temperatures, although there was some indirect evidence showing that the slight local stain crystallization at the cut tip might exist. The critical temperature depends on the type and density of cross-linking. 17

30 Polysulfidic crosslinks (cured by sulfur) give high T c and carbon-carbon cross-links (cured by dicumyl peroxide) give low T c. The slippage of polysulfidic cross-links at high stress enables better chain orientation and promotes strain-crystallization and reversibility of polysulfidic linkage facilitates non-catastrophic energy dissipation. High cross-linking restricts chain movement and decreases strain-crystallization. Bell, Stinson and Thomas 42 studied the effect of temperature on the tearing of cured gum natural rubber. In the temperature ranging from 20 to 80 C, the strength decreased abruptly at a critical cut size. The c cr decreased with increasing temperature. At a critical temperature at 80 C, there was no critical cut size, or the c cr reduced to zero. Figure 1.12 Effect of temperature on tensile strength of gum NR (SMR CV60) 1.6 Fillers Fillers are widely used for almost all commercial rubber products because they can reinforce rubbers by increasing their modulus, strength, etc. 18

31 1.6.1 Filler and Carbon Black The most widely used filler is carbon black. 2 Two common types of carbon black are furnace black and thermal black 43. Furnace blacks can be obtained by combustion of oil or natural gas with insufficient oxygen. Thermal blacks are obtained by using natural gas in heated chambers without air. The morphological structure of one carbon black is shown in Figure ,45 The smallest single particles are called primary particles or nodules, consisting overlapping graphitic layers. The primary particles group together to form aggregates whose structure is largely maintained during rubber processing, e.g., mixing, milling, molding. Further combination of aggregates gives agglomerates. These are decomposed during processing. Figure 1.13 Morphology of carbon black Primary particle size and specific area of carbon black are two important factors influencing the reinforcement of rubber. 46 These two factors are inter-related: smaller 19

32 primary particle s have larger specific surface area. Specific surface areas are measured by BET nitrogen adsorption 47 or CTAB adsorption. 48 Particle sizes of carbon black range from 20 to 300 nm. 49 and carbon black. Hamed 50 Increased specific area enhances interaction between rubber showed the crucial importance of particle size in carbon black reinforcement with a particle spacing cubic lattice model (Figure 1.14). It assumes spherical filler with radius r is dispersed on a square lattice at volume fraction, x. For each particle, there is assumed to be a layer with thickness t around each particle. Here the movement of chains is restricted by rubber-filler interactions. The particle spacing s and volume fraction x of this additional layer were calculated for different particle sizes at the same concentration. When particles are fine, s is similar to the distance between cross-links. Fracture is hindered because of the reduced mobility of network chains. On the contrary, with a sufficient increase in particle size, particle spacing is sufficient that the rubber motions are little influenced by the filler. Figure 1.14 Particle spacing cubic lattice model 20

33 Another important parameter of carbon black is its structure. 44 Effective reinforcing volume of aggregates depends on structure. Occluded rubber is defined as the part of a the rubber occupies the internal void space of aggregates. 45 Occluded rubber is shielded from deformation and increases the effective volume fraction of carbon black Effect of Carbon Black on Mechanical Properties The addition of carbon black increases the modulus of rubber due to the strain amplification. 51 Also, the interaction of carbon black with rubber reduces the volume of deformable phase, V d, when compared to unfilled rubber. The V d is further reduced by occluded rubber in the void space of carbon black aggregates. The decrease of V d increases the local strain and strain rate. Carbon black dramatically enhances the tensile strength of amorphous rubbers, but its effect on strain-crystallizing NR is limited. 39 As shown in Figure 1.15, the stress-strain curve of gum NR (without carbon black) is similar to that of SBR up to its breaking point. Carbon black does not increase the tensile strength of strain-crystallizing natural rubber, but the ultimate elongation is nearly reduced by 50%. On the contrary, the addition of carbon black sharply increases the tensile strength of amorphous SBR while maintaining elongation at break. At very low tensile strain (Figure 1.16) 45, carbon black-filled rubber shows a constant dynamic storage modulus G which decreases with increasing strain and reaches a plateau. This phenomenon is called the Payne effect. 52 This effect is explained by the breakage and recovery of a filler network. 21

34 Figure 1.15 Stress-strain for gum and black filled SBR, NR (S1, 2 and N1, 2) Fracture of Carbon Black Filled Natural Rubber Hamed and Park 39 investigated the tensile strengths of gum and carbon black-filled NR, as shown in Figure Gum and black-filled NR have similar normal tensile strength (tensile strength of NR without precut), but their tensile strength with a precut is quite different. At small c, the tensile strength of black-filled NR is two times larger than that of the gum NR. At these cut sizes, both rubbers undergo bulk crystallization. When c is larger, the tensile strength of black-filled NR is nearly 10 times that of the gum NR. The highly improved tensile strength of black-filled NR at large c has been contributed to the enhanced strain-crystallization in the presence of carbon black. Gehman and Field found that crystallization is initiated at elongation of 250% for gum NR and at about 100% for black-filled NR (calculated by X-Ray Diffraction, XRD)

35 Lee and Donovan 54 studied strain-crystallization at the cut tips with XRD and found that the lower limit of strain required to develop crystallization at cut tip is lower for black-filled NR than for gum NR. Figure 1.16 Payne Effect: strain on dynamic storage modulus G Figure 1.17 Effect of precut on the strength of gum NR (N1) and black-filled NR (N2) 23

36 Hamed and Al-Sheneper 55 investigated the effect of a very fine carbon black (N115) on the tensile strength in NR, as shown in Figure Remarkably, the low concentrations of carbon black decreased tensile strength: NR specimens filled with less than 12 phr of N115 were weaker than the gum and c cr decreased with increased black concentration. At about 18 phr of black, the strength of filled specimens increased. Thus, there is a minimum concentration of N115 required to enhance the tensile strength of precut NR. The lowest concentration increases as black filler becomes coarser. 56 Figure 1.18 Tensile strength of precut specimens with 0, 3, 6, 12 phr of N115 24

37 CHAPTER II EXPERIMENTAL 2.1 Materials 1) Nature rubber: SMR CV60 (Akrochem Corporation, received in February 2010). 2) SBR: Plioflex 1502 (Akrochem Corporation, received in April 2012) 3) N115 carbon black: (Cabot Corporation. Average primary particle size if 27nm. DBP absorption value is 112 cm 3 /100g black, received in June 2007) 4) Stearic acid: softening agents (Harwick Chemical Company, received in June 2007). 5) Zinc oxide: reinforcing agents and pigments (Akrochem, received in June 2007) 6) Sulfur: crosslinking agent (Harwick Chemical Company, received in June 2007). 7) TBBS: N-tert-butyl-1,2-benzothiazolesulfnamide, accelerator (Flexsys America, received in June 2007). 8) PD-2: N- (1,3- dimethyl butyl)-n - phenyl- p- phenylene diamine, antiozonant (Akrochem, received in June 2007). 9) Microcrystalline wax: antiozonant (Akrochem received in June 2007). 10) DQ: 2, 2, 4-trimethyl-1-2-hydroquinoline, antioxidant (Akrochem, received in June 2007) 11) Santogard PVI: N-(cyclohexylthio) phthalimide, antiscorching agent (Flexsys America, received in June 2007). 25

38 2.2 Formulations The nomenclature of natural rubber and SBR compounds follows: B-n-R where R indicates the type of rubber (N: natural rubber; S: SBR); B-n means carbon black content. For example, B5N means natural rubber with 5 phr of carbon black. Each compound contains the same composition: 100 phr rubber, 1.8 phr stearic acid, 3.5 phr zinc oxide, 1.5 phr antiozonant PD-2, 1 phr antioxidant DQ, 1 phr microcrystalline wax, 0,1 phr santogard PVI, 1.75 phr sulfur and 0.75 phr accelerator TBBS, where phr means parts of additive per hundred weight of rubber. Densities of carbon black and natural rubber are 1.8 g/cm 3 and 0.9 g/cm 3 respectively. Carbon blacks true volume fraction, v, and effective volume fraction, v eff, are calculated as Equation 2.1 and 2.2, and results are listed in Table 2.1. v eff = v = 26 V B V R +V B (2.1) V B V B +V R DBP W B (2.2) where V B is the volume of carbon black, V R is the volume of rubber, W B is the weight of carbon black, DBP is the indication of occluded rubber which decreases the volume of rubber and increases the effective volume fraction of carbon black. DBP is 113 cm 3 /100g black for N Mixing Rubber masterbatches were initially prepared in a 250 ml Banbury internal mixer with a fill factor of 0.7. After the internal mixer was cleaned with pure natural rubber three times and the machine was allowed to cool down to 90 o C, natural rubber or SBR

39 was added into the mixer in the first 1.5 minutes. Then zinc oxide, stearic acid, PVI, DQ and PD-2 were fed into the mixer and mixed with rubber for the next 1.5 minutes. Then, wax was added and mixing for 2 minutes. Finally the masterbatch was dumped and weighed. 2.4 Milling A masterbatch after mixing was separated into four groups. In the first two minutes, two parts of the masterbatch were mixed at one time, then two new bigger masterbatches were mixed again. This process was done at a nip of 1.9 mm. In the next minute, the nip was adjusted to 0.6 mm and a rolling bank was formed. Then, sulfur and TBBS were fed to the rolling bank and thoroughly mixed with natural rubber and other additives for three minutes. The nip was set to 1.2 mm and the rubber was given ten end-roll passes. Finally, the nip was adjusted to 1.9 mm and the rubber sheeted off. The whole process temperature was about 50 o C. Milling direction was masked with a marker pen. Stocks were stored at room temperature at least 24 hours before vulcanization. 2.5 Vulcanization Characterization Characterization of vulcanization was accomplished using an Alpha Moving Die Rheometer (MDR) Curing temperature was 140 o C for natural rubber and 160 o C for SBR. The cure time t c (100) was the time to reach maximum torque, while t c (90) was the time to reach 90 % of highest torque. 2.6 Molding Milled sheets (about 15.5 g) were put in the center of a window mold ( mm). Both sides covered with Mylar films (for natural rubber) or with Kevlar films (for SBR because SBR was cured at higher temperature) and two aluminum plates. 27

40 Curing took place in a Dake hydraulic press. Press load was 30 tons. After cure, the pressure was relieved and sheets were cooled by quenching with flowing tap water. Cured sheets were dried on paper. The average thickness of the cured sheets ranged from 0.55 to 0.65 mm. Table 2.1 Parts of carbon black and volume fraction Formulation N115 Loading (phr) v v eff B0S B8S B20S B0N B5N B14N B15N B18N B19N B20N B25N Swelling Test About 0.4g cured natural rubber were immersed in toluene and about 0.4 g cured SBR were immersed in heptanes in small bottles and put in the dark for 1 week at room temperature. The swollen rubbers were dried with paper and weighed again and noted as W gel. Then these rubbers were put into a vacuum oven and dried at room temperature for 28

41 24 h until the weight was constant at dried weight W dry. Crosslink densities are calculated by the following equations: V R Wdry = (2.3) ρ dry V S Wgel Wdry = (2.4) ρ solvent v R V V + V R = (2.5) R S where v R is the volume fraction of rubber in swollen gel; V R is the volume of rubber matrix; V S is the volume of solvent; ρ dry is the density of dried rubber which is calculated by dividing the total weight of rubber all additives by total volume; ρ solvent is the density of solvent (toluene: g/ml; heptanes: 0.69 g/ml). The Flory-Rehner equation 32,33 was used to calculate crosslink density, ρ c. ( ν ) 2 1 ln 1 r + ν r + χν r ρ c = (2.4) 1 2ν s v 3 r ν r 2 where ρ c is the crosslink density, ν s is the molar volume of the solvent, ν r is the volume fraction of rubber in the swollen gel, χ is the interaction parameter of the solvent and rubber. The interaction parameter, χ, of the solvent and rubber is calculated by: χ = v r for natural rubber in toluene and χ = v r for SBR in heptanes Bound Rubber About 0.4 g of uncured rubber is weighed (W initial ) and immersed in toluene for one week at room temperature and solvent is changed twice. After one weak, solvent is removed and gel is weighed (W gel ). 29

42 The bound rubber content is calculated by Equation 2.5: 58 Bound rubber % = W gel phr of iller 100+phr of iller W initial phr of iller W initial (2.5) 2.9 Tensile Strength ASTM D412 Type C dumbbells were cut from cured sheets in the milling direction. The width of the narrow section was 6.35 mm and the thickness ranges from 0.55 to 0.65 mm. In tensile strength measurements, crosshead speed varies from 5 to 250 mm/min, with an initial clamp separation of 65 mm. Strain in the narrow section of a specimen was measured by a mechanical clip-on type extensometer. The initial separation of the extensometer was 25 mm. Edge-cuts were introduced with a razor blade that had been wetted in a soap solution to reduce friction with the rubber. Cut depth was measured using a traveling optical microscopy. Cut depth was measured at least three times on each side and the difference between the average of each side was less than 0.08 mm (Otherwise a sample was discarded). The difference of cut depth on each side was minimized by cutting vertically. Calculation of stress was based on the original specimen width, 6.35 mm, regardless of the cut depth Crack Pattern Photographs High magnification photographs of crack patterns were taken with a scanning electron microscopy (SEM) JEOL JSM7401F. Two parts of a fractured specimen were placed together on an aluminum mount using carbon tape and coated with silver. An arrow in an SEM picture indicates the tip of an initial razor cut. 30

43 CHAPTER III RESULTS AND DISCUSSION 3.1 Characterization of Vulcanization The cure characteristics of the natural rubber and SBR are listed in Table 3.1. Rheometer curves are shown in Figure 3.1 (NR) and Figure 3.2 (SBR) Curing of the SBR was carried out at 160 C while that of the natural rubber was done at 140 C. SBR was cured at higher temperature than natural rubber because it contains less unsaturated bonds. Vulcanization time doesn t rely on the loading of carbon black. Curing time (t c (100)) of SBR is nearly 30 min and curing time of NR is around 50 min. Torques of B0N, B5N, B0S, B8S and B20S are stable after t c (100), but torques of B14N, B15N, B18N, B20N and B25N decreases after t c (100). Time for this decrease (t d ) reduces with increasing black concentration: t d (25phr)=50min, t d (20-15phr)=60min, t d (14phr)=75min. Perhaps, rubber/ black network breaks at high temperature when time is enough. Maximum torque, T max, increases monotonously and linearly with increasing true volume fraction of carbon black N115, v, as plotted in Figure 3.3. As v increases, interaction between rubber and carbon black is more intense, thus torque increases. 31

44 Table 3.1 Cure characteristics of gum and black filled SBR and NR t c (90) a t c (100) a t s2 a T min b T max b B0S B8S B20S B0N B5N B14N B15N B18N B19N B20N B25N a. Unit of time is min b. T min is minimum torque and T max is maximum torque, unit is dnm. 3.2 Swelling Test and Crosslink Density Swelling tests were done as described in Chapter 2.7 and crosslink densities of all formulations are calculated by Equation 2.3 to 2.4. Results are listed in Table 3.2 and plotted in Figure 3.4. Crosslink density of both SBR and NR increases with increasing loading of carbon black because of strong rubber/ black interaction. Crosslink density of SBR is smaller than that of NR. Slope of NR in Figure 3.4 is 5 times larger than that of SBR which indicates that NR/ black interaction increases faster with increasing black loading than SBR/ black interaction does. 32

45 Torque 6 (dnm) 5 B25N B20N B18N B15N B14N B0N B5N time (min) Figure 3.1 Vulcanization characterization of NR 33

46 torque (dnm) B20S B8S T0S time (min) Figure 3.2 Vulcanization characterization of SBR 34

47 12 11 SBR: T max =51.7v T max (dnm) NR: T max =35.3v Figure 3.3 Maximum torque versus true volume fraction of carbon black N115, v v 35

48 Table 3.2 Swelling tests of unfilled and black-filled SBR and NR Compounds W dry (g) W gel (g) ρ c (*10-5 mol/cm 3 ) B0S B8S B25S B0N B5N B14N B15N B18N B19N B20N B25N Bound Rubber Test The addition of fine carbon black to gum rubber yields a filler/rubber network when the loading of carbon black is high enough. Black s aggregates are bound together by the adsorption of rubber chains. At this critical loading, swelling ratio of cured rubber decreases and modulus increases because of the rubber/ black interaction. Bound rubber tests were done as described in Chapter 2.8 and results are shown in Table

49 NR: ρ c =1.49v ρ c SBR: ρ c =0.31v Figure 3.4 Crosslink density ρ c of NR and SBR versus true volume fraction of N115, v v 37

50 The critical loading of N115 for the formation of black/ rubber network is 15 phr. No gel can be observed after immersing NR in toluene with carbon black loading less than or equal to 14 phr. After the critical loading, the bound rubber of NR doesn t change a lot with increasing loading of black. SBR starts to have interaction with carbon black N115 and form bound rubber at a relatively low black loading (8 phr), but this interaction is weaker than NR (55.8 ± 4.8% vs ± 2.3%). This explains why the crosslink density of NR depends more on black loading than that of SBR does. Table 3.3 Bound rubber test of carbon black N115 filled NR Compounds Bound Rubber (%) B8S 57.3 B25S 54.2 B5N B14N No coherent gel No coherent gel B15N 61% B18N 64% B19N 62% B20N 62% B25N 63% 3.4 Normal Tensile Results (c = 0) of SBR and NR at Various Tensile Rates Normal tensile test means to stretch a rubber band without a precut. All flaws are microscopic intrinsic flaw. Normal tensile strength is the highest strength that a rubber with fixed compounds can expect. 38

51 3.4.1 Tensile Tests of SBR and NR at 50 mm/min Tensile results of normal specimens (without precut) (tensile strength, σ b0, and elongation at break, ε b0 ) for gum SBR, gum NR and black N115-filled NR tested at 50 mm/min are listed in Table 3.4. The results for all compounds are the average of at least three specimens. Tensile strengths and ultimate elongations versus carbon black N115 concentration, C, are plotted in Figure 3.5 (NR) and Figure 3.6 (SBR), respectively. Table 3.4 Tensile results of uncut SBR and NR at 50 mm/min Rubber N115 Loading σ b0 (MPa) ε b SBR NR Gum SBR has lower extensibility and 8-times lower tensile strength than gum NR. Tensile strength of NR increases very slightly with increasing loading of carbon black N115 while ultimate elongation decreases rapidly. But both tensile strength and ultimate elongation of SBR increase with increasing carbon black N115 concentration. Stress-strain curves of gum SBR (B0S) and gum NR (B0N) at 50 mm/min are compared in Figure 3.7. Stress-strain curves of NR and SBR with various loading of carbon black are compared in Figure 3.8 and Figure

52 σ b0 (MPa) σ=0.09c+23.6 ε=-0.06c ε N115 loading, C (phr) 6.0 Figure 3.5 Strength and elongation of NR versus N115 concentration, C, at 50 mm/min 40

53 σ=0.72c σ b0 (MPa) ε=0.1c+5.56 ε N115 loading, C (phr) 5 Figure 3.6 Strength and elongation of SBR versus N115 concentration, C, at 50 mm/min 41

54 25 rate=50 mm/min σ (MPa) Crystallization period B0N Amorphous period B0S Figure 3.7 Comparison of stress-strain curve of B0S and B0N at 50 mm/min ε 42

55 30 NR rate=50 mm/min 25 σ (MPa) B25N B15N B14N B5N B0N Figure 3.8 Comparison of stress-strain curve of NR with N115-black at 50 mm/min ε 43

56 SBR rate=50 mm/min B20S σ (MPa) 10 8 B8S B0S Figure 3.9 Comparison of stress-strain curve of SBR with N115-black at 50 mm/min σ 44

57 The stress strain curve of the NR (Figure 3.7) at 50 mm/min can be divided into two separate stages. In the first stage (ε < 4.5), stress increases slowly with increasing strain and rubber is soft. In the second stage, stress increases quite rapidly and rubber is stiffer. There is an inflection point between the two stages. No separated stages can be observed in stress-strain curve of SBR. In first stage, the stress-strain curve of natural rubber and SBR are similar and NR and SBR have the same 100% modulus. This indicates similar crosslinking. The upturn in Figure 3.7 is due to the crystallization when the strain is large enough. Crystallization upon straining gives high strength to natural rubber. The crystallized domain must first be destroyed before fracture can occur. Crystallization inhibits mechanical chain rupture. Amorphous gum rubbers such as SBR have lower fracture energy than NR. Tearing in amorphous rubbers often propagates smoothly, with the crack perpendicular to the direction of loading, showing no indication of hindrance to growth. When NR is filled with carbon black, tensile strength increases and the slope in the first stage increases with increasing carbon black loading according to Figure 3.8. Besides, elongation at which upturn takes place decreases with increasing carbon black concentration. Thus, 100% modulus is increased by carbon black. When carbon black concentration reaches 25 phr, the separation of two stages almost disappears. Perhaps, addition of carbon black hinders crystallization and increases stiffness and consequently, the difference in two stages is reduced. When carbon black concentration increases, more polymer chain can be absorbed by carbon black and thus, the rubber/ black interaction is stronger. As a result, the rubber is stiffer. So strength increases with increasing carbon black concentration. And, increase of 45

58 normal tensile strength does not depend on the formation of rubber/ network because strength of NR shows an increase before 15 phr where bound rubber starts to form. Increase of strength of SBR is more obvious than that of NR: slope of σ-c plot of SBR is 8-times larger than that of NR (0.72 vs. 0.09, Figure 3.5, Figure 3.6). When N115 loading is 20 phr, strength of SBR increases 4 times but increases of strength of NR with 20 phr of N115 is negligible (1.1 times). Decrease in chain relaxation and crystallization ability in NR cancels out the effect of rubber/ black interaction Rate Dependence of Tensile Strength of gum SBR and NR Tensile rates for SBR and NR were changed from 0.1 mm/min to 250 mm/min. Tensile results (σ b0 and ε b0 ) of normal specimens (c = 0) of unfilled SBR and NR are listed in Table 3.5 and plotted in Figure Stress-strain curves of B0N and B0S at various rates are compared in Figure 3.11 and Figure 3.12, respectively. According to Figure 3.10, ultimate elongations and tensile strengths of gum SBR and gum NR increase with increasing tensile rate. Changes in ultimate elongations of gum SBR is more obvious than that of gum NR. Stress-strain curves of both gum NR and gum SBR (Figure 3.11, Figure 3.12) are not very distinguishable. Monotonous increase of both tensile rate and ultimate elongation is because of the extensive chain relaxation at sufficient low tensile rate: when specimen is pulled at low tensile rate, polymer chains have more time to relax and beads have more chances to move around, and thus polymer chains will carry on less load. Consequently, polymer is softened and weakened at lower tensile rate. But effect of tensile rate on tensile strength of NR is less significant than that of SBR: when tensile rate decreases from 250 mm/min to 0.1 mm/min, strength of NR reduces 55% 46

59 (27.5 MPa to 12.3 MPa) but strength of SBR reduces 72% (5.0 MPa to 1.4 MPa). NR can undergo strain-induced crystallization and extensive chain relation at low tensile rate favors crystallization: lower tensile rate allows polymer chain to move more freely, which enables polymer chains rearrange to form nuclei and further to crystallize. So, the effect of crystallization partially offset the effect of chain-softening and the slope of fitted lines of gum NR (0.1 for strength-rate and 0.03 for elongation-rate) is smaller than those of gum SBR (0.16 for strength-rate and 0.14 for elongation-rate). Table 3.5 Tensile results of uncut B0S and B0N at various tensile rates Rubber Rate (mm/min) σ b0 (MPa) ε b B0N B0S

60 B0N: σ = R 8 σ b0 10 (MPa) 1 B0N: ε = R B0S: σ = R B0S: ε = R 6 4 ε rate, R (mm/min) Figure 3.10 Rate dependence of tensile results of B0N and B0S 48

61 σ (MPa) r=250 mm/min r=50 mm/min r=25 mm/min r=10 mm/min r=1 mm/min r=0.1 mm/min B0N ε Figure 3.11 Rate dependence of stress-strain curve of B0N 49

62 σ (MPa) r=250 mm/min r=50 mm/min r=25 mm/min r=10 mm/min r=1 mm/min r=0.1 mm/min B0S Figure 3.12 Rate dependence of stress-strain curve of B0S ε 50

63 3.4.3 Rate Dependence of Tensile Results of Black-Filled NR and SBR Tensile results of carbon black N115-filled rubber at various tensile rates are listed in Table 3.6 (SBR) Table 3.7 (NR). Data of NR is plotted in Figure 3.13 (strength vs. rate) and Figure 3.14 (strength vs. rate/ N115 loading). Data of SBR is plotted in Figure 3.15 (strength vs. rate) and Figure 3.16 (strength vs. N115 loading). Tensile strength of both NR and SBR monotonously increases with increasing tensile rate and increasing loading of carbon black when the other variant remains. But the rate dependence is less significant than the carbon black s loading-dependence. Range of slope D (rate-dependence of NR) is while range of slope C (N115 loading-dependence of NR) is ; there is a sharp hundred-folds difference between them. The contrast for SBR is a little smaller: range of slope B (rate-dependence of SBR) is while range of slope A (N115 loading-dependence of SBR) is ; there is only ten-folds difference. Chain relaxation is less decisive for the determination of strength of rubber because the major part of strength comes from other issues such as crosslinking, chain entanglement, crystallization and rubber/ filler interaction. Rate dependence for NR is even weaker because the effect of bulk crystallization of NR upon strain conflicts the effect of chain relaxation and chain softening. SBR doesn t crystallize when stretched, so the difference of rate-dependence and N115 concentration-dependence is smaller. 51

64 Table 3.6 Tensile strength of SBR with various amount of N115 at various tensile rates Rate B0S B8S B20S Slope A a 0.1 b 1.4 d Slope B c a. Slope A is the slope of fitted line of black loading-strength of SBR plot at each rates b. Tensile rates are in unit of mm/min c. Slope B is the slope of fitted line of the rate- strength of SBR plot at each black loading d. Normal tensile strength of a certain compound tested at certain tensile rate. 52

65 Table 3.7 Tensile strength of NR with various amounts of N115 at various tensile rates Rate B0N B5N B14N B15N B25N Slope C a 0.1 b 12.3 d Slope D c a. Slope C is the slope of fitted line of the black loading- strength of NR plot at each rates b. Tensile rates are in unit of mm/min c. Slope D is the slope of fitted line of the rate- strength of NR plot at each black loading d. Normal tensile strength of a certain compound tested at certain tensile rate. 53

66 30 25 B0N B5N B14N B15N B25N 20 σ b0 (MPa) rate (mm/min) Figure 3.13 Normal tensile strength versus tensile rate for N115 filled NR 54

67 32 σ b0 (MPa) R=250 R=50 R=25 R=10 R=1 R= N115 loading (phr) Figure 3.14 Normal tensile strength of NR versus N115 loading at various tensile rates 55

68 B20S 10 σ b0 (MPa) B8S B0S rate (mm/min) Figure 3.15 Normal tensile strength versus tensile rate for N115 filled NR 56

69 25 20 σ b0 (MPa) R=250 R=50 R=25 R= N115 loading (phr) Figure 3.16 Normal tensile strength of NR versus N115 loading at various tensile rates 57

70 Slope B and slope D (rate-dependence) drops as the loading of carbon black increases (Table 3.7). From B0N to B25N, slope D (NR) decreases about 40%; from B0S to B20S, slope B (SBR) decreases about 60%. This indicates that increasing amount of carbon black make the tensile strength less dependent on tensile rate because the rubber/ filler interaction reduces the chain flexibility and consequently, changes in chain relaxation is less prominent when tensile rate changes if rubber is sufficiently filled by carbon black. Slope A (N115 concentration-dependence of SBR) decreases with decreasing tensile rate. Effect of rubber/ black interaction is partially offset by the effect of chain-relaxation at low tensile rate. But changes in slope C (N115 concentration-dependence of NR) with tensile rate doesn t follow a simple rule. Rubber/ black interaction, chain relaxation and chain crystallization affect each other and make this trend hard to predict. 3.5 Precut Tensile Results of SBR and NR at 50 mm/min When SBR and NR, gum or black-filled, are introduced a precut and tested at 50 mm/min, results show a significant difference Precut Tensile Results of Gum Rubbers at 50 mm/min A precut on one edge was introduced to some specimens with a razor blade. Tensile strength σ bc of specimen with a precut c was measured. Results of precut gum NR and SBR tested at 50 mm/min are plotted in Figure 3.17 and Figure 3.18 separately. For natural rubber, when the cut size c is extremely small, as small as only 0.12 mm, the tensile strength drops significantly from 23.6 MPa to 16.5 MPa (30%). The tensile strength is exceedingly sensitive to cut size. Then, until c 0.42 mm, strength doesn t drop much, only from 16.5 MPa to 12.4 MPa (2.5%). At larger c, strength decreases 58

71 faster with increasing c. From c = 0.42 mm to c = 1.61 mm, tensile strength drops from 12.4 MPa to 4.9 MPa (60%). The drop of tensile strength of specimens with 1.61 mm < c < 1.71 mm is sudden and discontinuous: 60% of the tensile strength (from 4.88 MPa to 1.97 MPa) is lost in this 0.10 mm while the total strength loss from 0.12 mm to 1.61 mm is only 70%. For simplification, specimens before this sudden drop are labeled as the strong population, SP, and those after sudden drop are labeled as the weak population, WP. In some cases, there are overlap area in SP and WP, i.e., both SP and WP may be observed in a certain cut size region. This area is unstable. Cut size of strong population, c s, is defined as largest cut size before unstable region; this sample is noted as SS. Cut size of weak population, c w, is defined as the smallest cut size after the unstable region; this sample is noted as WS. Between the SP and the WP is a region with no specimens. Specimens of SBR at cut size of c give one straight and descending line. No separate strong population and weak population can be observed. When c is as small as 0.25mm, strength drops from 3.65 MPa to 1.02 MPa (72%). This drop is larger than NR (30%). The stress-strain curve is shown in Figure Each black solid square represents a precut specimen. All precut specimens, regardless of cut size, have ε bc > 5.19 (ε s ) or ε bc < 2.42 (ε w ) and σ bc > 4.61 MPa (σ s ) or σ bc > 1.99 MPa (σ w ). SS and WS doesn t necessarily has ε s, σ s, ε w, σ w but these four values should be at somewhere near SS and WS. This gives an exclusion zone: Δε = 2.77 or Δσ= 2.62 MPa, where gum NR at 50 mm/min never fails. If the uncut condition is such that the natural flaw can be regarded as a micro circle with l and r nearly the same, σ t can be approximated by 2σ (Eq. 1.2). But for precut specimens, l is far larger than r and therefore, σ t (precut) >> 2σ = σ t (uncut). So a very small precut can cause a large decrease in tensile strength. 59

72 σ b0 = 23.6 MPa, ε b0 = 757% B0N rate = 50 mm/min 10 σ bc (MPa) 1 strong population (SP) weak population (WP) cs=1.61, 526%, 4.89MPa c w =1.71, 282%, 1.97 MPa SS WS c (mm) Figure 3.17 Tensile strength of B0N with precut c at 50 mm/min 60

73 5 4 3 σ b0 = 3.65 MPa ε b0 = 552 B0S rate = 50 mm/min 2 σ bc (MPa) 1 c (mm) Figure 3.18 Strength of B0S with precut c at 50 mm/min 1 61

74 When c is small in NR, the time for bulk crystallization upon straining is sufficiently long, so that the effect of c on σ bc is not much when compared to the changes in cut size in first stage. Although bulk crystallization still plays a role, its extent is somewhat limited by larger c and may be incomplete before catastrophic fracture. Another factor is that the area available for crystallization is decreased due to a precut. Secondly, when the decrease in σ bc becomes discontinuous, this indicates that the rubber lacks the time required to undergo bulk crystallization upon straining. Figure 3.20 compares the stress-strain curve of precut gum NR specimen in SP and WP. WP specimen does not show an upturn stage in stress-strain curve (Figure 3.7, Chapter 3.4.1) while SP specimen does. This plot indicates bulk crystallization in WP is hindered. On the whole range of c, the SBR is several times weaker than the natural rubber (Figure 3.21). Perhaps, crystallization at cut tip makes gum NR in WP still stronger than gum SBR Precut Tensile Results of Black-Filled SBR at 50 mm/min Precut tensile results of carbon black N115-filled SBR tested at 50 mm/min are shown and compared with unfilled SBR in Figure 3.22 and normal tensile strengths are labeled. Tensile strength of all these three compound decreases continuously with increasing cut size. Strength of B20S at c = 2.5 mm is about 1.7 MPa (according to linear fit), almost 2 times than that of B8S and B0S. High concentration of carbon black N115 improves the performance of SBR, especially when SBR contains a large flaw. 62

75 25 B0N rate = 50 mm/min 20 σ (MPa) Exclusion zone ε=1.81, σ=2.62mpa Figure 3.19 Stress-strain curve of B0N at 50 mm/min, with precut specimens labeled ε 63

76 10 B0N rate = 50 mm/min 8 c=0.67 mm, SP σ (MPa) c=2.02 mm, WP ε Figure 3.20 Comparison of stress-strain curve of precut B0N in SP and WP at 50 mm/min 64

77 B0N, Strong population 10 σ bc (MPa) B0N, Weak population 1 B0S c (mm) Figure 3.21 Comparison of strength of gum SBR and NR with precut c at 50 mm/min 65

78 When cut size c is small as 0.2 mm, the percents of strength loss, SL, compared to uncut specimen is defined in Eq.3.1. Difference in normal tensile strength is excluded in SL by definition and only the change in tensile strength is considered. SL of B0S, B8S and B20S are listed in Table 3.8. SL = σ b0 σ bc (c=0.2) σ b0 (3.1) The effect of small edge cut on tensile strength can be expressed by strength loss at a small cut. Strength loss describes how significantly a precut will decrease tensile strength and thus the larger the value is, the more sensitive to precut the rubber is. Table 3.8 Strength loss of precut SBR at c = 0.2 mm Strength Loss at c = 0.2 mm B0S 72% B8S 64% B20S 51% According to Table 3.8, SL decreases with increasing concentration of carbon black, i.e., carbon black reduces sensitivity of SBR to precut. Because additional carbon black introduces interaction with rubber which is only thermodynamically determined and doesn t depend on the edge cut. Therefore, Sensitivity of tensile strength on precut is reduced by the carbon black. 66

79 rate = 50mm/min 10 B20S σ b0 =18.3 MPa σ bc (MPa) B8S σ b0 =11.2MPa 1 B0S σ b0 =3.63MPa c (mm) Figure 3.22 Tensile strength of SBR filled with various amount of carbon black with precut c at 50 mm/min 67

80 3.5.3 Reduced Strength of Slightly Carbon Black Filled NR at 50 mm/min Contrast to the clear increase in tensile strength of precut SBR after addition of carbon black, slight addition of carbon black to precut NR not only doesn t reinforce the rubber, but also reduces the tensile strength as well as c s, c w. (Figure 3.23). Characteristic values are compared in Table 3.9. Table 3.9 Characteristic values of tensile results of B0N, B5N and B14N at 50 mm/min c s c w ε s ε w σ s σ w Δε Δσ SL B0N % B5N % B14N % According to Table 3.9, critical cut size decreases with increasing carbon black loading when N115 loading is below 14 phr. Difference between strong population and weak population diminishes (Δε and Δσ for B15N are only 0.83 and 1.81 MPa, respectively). Strength loss is calculated by Eq SL increases as the carbon black loading increases (less than 14 phr). This indicates that slightly filled rubber is more sensitive to precut when the concentration of carbon black increases, which is contrast to SBR. Slight loading of carbon black can only weaken the NR instead of reinforcing it. Addition of little amount of carbon black interferes strain-induced crystallization negatively. Crystallization requires enough chain orientation to give stable nuclei; crystal growth requires long range chain mobility. Carbon black addition decreases the proportion of deformable phase. The actual deformation and speed of deformation of the rubber phase in filled rubber is higher than that in the gum. But, adsorption of chains on a black surface decreases its mobility. The net rate of crystallization upon strain 68

81 crystallization depends on the relative strength of these effects which depends on the black content. Gent 59 has shown that thermal-crystallization of NR is inhibited by carbon black. X-ray analyses 29,54 have shown that strain-crystallization of black-filled NR initiates at lower strain than the gum. Tensile results will be discussed in three regions: 0.10 mm c 0.85 mm (c s of B14N); 0.85 mm c 1.71 mm (c w of B0N) and c > 1.71 mm. When 0.10 mm c 0.85 mm (c s of B14N), c is smaller than the critical cut sizes of B0N, B5N and B14N. Tensile strength decreases as black concentration increases. B14N has only about 75% of the strength of the B0N. All specimens go bulk crystallization before the rupture occurs. Rate and extent of crystallization decreases as the carbon black concentration increases (less than 14 phr), thus the strength decreases with increasing carbon black concentration. When 0.85 mm < c 1.71 mm (c w of B0N), each compound has a very narrow cut size region where the tensile strength drops significantly. Tensile strength of B0N, B5N and B14N drops about two folds when cut size increases from 0.85 mm to 1.71 mm. All critical cut sizes (c w and c s ) of all three compounds locate in this region. But c w and c s shift to lower size as the carbon black loading increases. This indicates that the crystallization is inhibited when a little amount of carbon black is added: B14N can crystallize upon strain only when c < 0.85 mm while B0N can crystallize until c > 1.61 mm. Nevertheless, although ε s and σ s decreases when N115 loading increases due to the limited crystallization, ε w and σ w increases with increasing N115. In weak population, bulk crystallization is almost absent. Consequently, inhibited crystallization doesn t affect the tensile results in this region. When carbon black concentration increases, rubber/ 69

82 black interaction also increases and thus, σ w of B14N is larger than that of B0N. Difference between strong and weak population is narrowed. When c > 1.71 mm, all specimens are in weak population. Data of three compounds in this region almost overlaps Critical Carbon Black Loading for Partial Reinforcement at 50 mm/min NR with low concentration of carbon black (less than 14 phr) shows a decrease in c s, c w and tensile strength as carbon black concentration increases. But addition of high content of carbon black starts to reinforce NR. Tensile results of B15N are given in Figure 3.24 and compared with those of B0N, B14N in Figure Bold solid line represents for the results of gum NR (B0N) (only linear fit is shown). Solid triangles represent for precut specimens with simple lateral crack when they are pulled apart; hollow triangles represent for precut specimens with multiple cracks or with strong deviation from initial cut when they are pulled apart. The precut tensile tests of B15N also give two separated populations. In the cut region of 1.04 mm c 2.26 mm, strength of B15N is intrinsically unstable: some specimens are two-fold stronger than the others. This separation is based on the different crack patterns instead of the lack in bulk crystallization as in the separation of strong and weak population. Crack patterns of some representative specimens are shown Figure 3.26 and Figure Figure 3.26 shows a specimen with crack deviation at point A. Figure 3.27 shows a specimen with multiple crack which starts at point A and stop at point B and crack deviation at point C, D. 70

83 rate=50 mm/min 10 σ bc (MPa) 1 B14N B5N gum (only line) c (mm) Figure 3.23 Tensile strength of precut B0N, B5N and B14N at 50 mm/min 71

84 The smallest cut size where multiple crack or crack deviation appears is noted as c m,s ; the largest cut size after which multiple crack or crack deviation disappear is noted as c m,l. For B15N at 50 mm/min, multiple crack or crack deviation cannot be observed below c = 1.04 mm and after c = 2.26 mm. Thus, c m,s = 1.04 mm and c m,l = 2.26 mm for B15N at 50 mm/min Crystallization is enhanced with the help of high loading of carbon black. 39 Fiber-like strain-crystallites cause sufficient anisotropy in strength to block lateral crack growth and allow propagation between fibrils. 25 Approved crystallization and rubber/ black network are necessary for crack deviation, i.e., heavily loaded SBR doesn t show multiple cracks. Specimens of B15N with multiple cracks have only about the same strength as the gum NR at comparable cut size. Slow and stable multiple cracks help to blunt the cut tip, and reduce stress concentration. The stable pre-cracks allow more time for the formation crystalline domain and provide additional resistance to fracture. 15 phr of N115 black is the lowest concentration where multiple cracks form and reinforcement begins. This amount of black is also the concentration for incipient bound rubber formation. A continuous filler/rubber network combined with crystallized domain is necessary for reinforcement. 15 phr of N115 black is also the lowest concentration where the separated strong and weak population disappears. But reinforcement at 15 phr is still incomplete: specimens before c m,s and after c m,l still show single lateral crack and have low tensile strength. 72

85 B15N rate=50 mm/min 10 σ bc (MPa) simple crack multiple crack 1 c m,s =1.04mm c m,l =2.26mm Figure 3.24 Tensile results of B15N with precut c at 50 mm/min 1 c (mm) 73

86 rate=50 mm/min 10 σ bc (MPa) B15N B15N 1 B14N gum (only line) c (mm) Figure 3.25 Comparison of precut tensile results of B0N, B14N and B15N at 50 mm/min 74

87 Figure 3.26 Crack pattern of specimen of B15N at 50 mm/min (c = 1.09 mm, 8.11 MPa) Figure 3.27 Crack pattern of specimen of B15N at 50 mm/min (c = 1.40 mm, 7.44 MPa) 75

88 3.5.5 Critical Carbon Black Loading for Completed Reinforcement at 50 mm/min Precut tensile results of NR with higher content of carbon black (B18N and B19N) are plotted and compared with that of B0S in Figure Solid line represents for the linear fit of the results of B0N. Hollow squares or triangles represent for specimens with multiple cracks or crack deviation. Solid triangles represent for specimens with simple lateral crack. Simple lateral crack pattern is still observable in results of B18N: c m,s = 0.36 mm, c m,l = 3.15 mm. But all specimens of B19N show strong crack deviation or multiple cracks. 19 phr of N115 black is the lowest concentration where the simple lateral crack pattern disappears and all specimens show a deviated crack pattern. Reinforcement of precut NR filled with 19 phr of N115 black starts to be completed: all specimens are reinforced regardless of the cut size. First turning point of concentration of black, C T,1, is defined as the concentration of N115 where multiple crack starts to occur. Second turning point of concentration of black, C T,2, is defined as the concentration of N115 where the reinforcement is completed. In the case of NR at 50 mm/min, C T,1 = 15 phr and C T,2 = 19 phr. When N115 black concentration is larger than C T,2 = 19 phr (25 phr), changes in tensile results at given tensile rate are not obvious (Figure 3.30). Tensile strength show some increases in the specimens with multiple cracks from B15N to B19N, but strength-cut profile of B19N and B25N at whole range of cut size is almost overlapped. Reinforcement performance is stabilized after C T,2. 76

89 B19N rate=50 mm/min 10 B18N σ bc (MPa) B18N c (mm) gum (only line) Figure 3.28 Comparison of precut tensile results of B0N, B18N and B19N at 50 mm/min 77

90 When black loading is increased to C T2, crack pattern becomes more complicated. Figure 3.29 shows one representative specimen (B19N at 50 mm/min, c = 1.09 mm) Figure 3.29 Crack pattern of B19N specimen at 50 mm/min (c = 1.09 mm, MPa) When the specimen with initial cut tip (shown by the arrow) was stretched, the initial crack first grew quickly to point A. At point A the crack separated into two direction. One of them stopped in point B as secondary crack and dispersed a lot of energy. The other one formed another secondary crack at point C. Then, the crack grew a little back to cut edge at C (at about 45 o angle). After the short returning crack, the crack deviated and propagated rapidly to point D, where the crack formed into the last secondary crack and propagated until the specimen was pulled apart. 78