EFFECT OF CARBON BLACK ON THE TEARING OF DICUMYL PEROXIDE (DCP)-CURED NATURAL RUBBER VULCANIZATES

Transcription

1 EFFECT OF CARBON BLACK ON THE TEARING OF DICUMYL PEROXIDE (DCP)-CURED NATURAL RUBBER VULCANIZATES A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Zhenpeng Li May, 2014

2 EFFECT OF CARBON BLACK ON THE TEARING OF DICUMYL PEROXIDE (DCP)-CURED NATURAL RUBBER VULCANIZATES Zhenpeng Li Thesis Approved: Accepted: Advisor Dr. Gary R. Hamed Dean of the College Dr. Stephen Z. D. Cheng Faculty Reader Dr. Ali Dhinojwala Dean of the Graduate School Dr. George R. Newkome Department Chair Dr. Coleen Pugh Date ii

3 ABSTRACT Dumbbell specimens have been prepared from dicumyl peroxide (DCP)-cured natural rubber (NR) vulcanizates containing 0-60 phr of carbon black (N115). For normal specimens with no pre-cut, tensile strength increases as the carbon black level increases up to 45 phr, and then slightly decreases. When a pre-cut is introduced, specimens with 6-25 phr carbon black are weaker than gum (unfilled) specimens, and rupture occurs by lateral growth of a single crack. Specimens with 38 phr carbon black have similar strength as the gum, and multiple cracks and crack deflection occur before rupture. Specimens with phr carbon black are stronger than gum, and rupture occurs via multiple cracks and crack deflection. The difference in cut growth resistance with low and high levels of black is attributed to competitive effects. A bound rubber/continuous network first forms at 15 phr of black. Tearing of DCP-cured vulcanizates has been compared with sulfur-cured ones. Sulfur-cured vulcanizates have normal tensile strengths that are similar for all levels of black (0-60 phr). Cut growth resistance of sulfur-cured specimens is much greater than DCP-cured specimens at all levels of black. The onset of reinforcement for sulfur-cured vulcanizates is 15 phr of black, while for DCP-cured vulcanizates, it is 38 phr of black. iii

4 ACKNOWLEDGEMENTS Thanks to Dr. Hamed and Dr. Dhinojwala (reader). Moreover, thanks to group members: Yu Sun, Minghang Yang, Yanxiao Li, Qinwei Wang, Hanki Park and Xin Tan. iv

5 TABLE OF CONTENTS v Page LIST OF TABLES... vii LIST OF FIGURES... viii CHAPTER I. INTRODUCTION... 1 II. BACKGROUND STATEMENT Natural Rubber Vulcanization by Organic Peroxides Definition of Vulcanization Organic Peroxides Vulcanization Mechanism Comparison of Peroxide and Sulfur Vulcanization Effects of Vulcanization on Vulcanizate Properties Characterization of Vulcanization Determination of Crosslink Density Carbon Black Reinforcement Carbon Black Effects of Carbon Black on Reinforcement Effect of Carbon Black Concentration on Cut Growth in Sulfur Cured NR Vulcanizates Fracture of Rubber Fracture Mechanism Rubber Fracture Mechanics Effect of Edge-cut on the Fracture of Natural Rubber III. EXPERIMENTAL Materials and Formulation Internal Mixing... 24

6 3.3 Milling Measurement of Vulcanization Parameters Molding Cut Size Measurement Tensile Test Swelling Test Bound Rubber Crack Patterns IV. RESULTS AND DISCUSSION Empirical Equation to Determine DCP Levels Vulcanization Characterization Swelling Results Normal Tensile Tests (Cut Depth c=0) Pre-cut Tensile tests Effects of Carbon Black on Cut Growth Content of Bound Rubber Comparison of DCP-cured Vulcanizates and Sulfur-cured Vulcanizates Tearing Behavior Percolation Threshold V. CONCLUSION Tearing Behavior of DCP-cured NR Vulcanizates Comparison of DCP-cured and sulfur-cured NR Vulcanizates REFERENCES APPENDIX vi

7 LIST OF TABLES Table Page 3.1 Formulations Mixing procedure in the internal mixer Mixing procedure on the two-roll mill Crosslink densities of DCP-cured NR specimens with different levels of DCP Crosslink densities of NR specimens with 1.55 phr DCP and 1.75 phr sulfur at different levels of carbon black Comparison of crosslink densities between DCP-cured and Sulfur-cured vulcanizates Cure characteristics of DCP cured NR compounds (150 ) Results of swelling test Normal tensile properties of gum and filled vulcanizates Content of bound rubber in specimens with different carbon black levels Percolation threshold of carbon black in DCP-cured vulcanizates and sulfur-cured vulcanizates vii

8 LIST OF FIGURES Figure viii Page 2.1 The structure of natural rubber macromolecule (cis-1,4-polyisoprene) Network formation by sulfur The basic process of peroxide vulcanization Typical cure curves Typical carbon black morphology Bound rubber/continuous network Results of the specimens with low carbon black levels Results of the specimens with high carbon black levels Sample with an edge-cut c The results of tensile tests on natural rubber vulcanizates Fitting curve of crosslink densities of specimens with 1.75 phr sulfur versus crosslink densities of specimens with 1.55 phr DCP Fitting curve of crosslink densities with 1.55 phr DCP versus various carbon black levels Cure curves of NR compounds Cure curves of NR compounds (log-log) Normal (uncut) stress-strain curves Variation of tensile strength with carbon black concentration for normal tensile tests Strength versus cut depth for gum (CD0) σ bc /σ b0 versus cut depth for gum (CD0) Strength versus cut depth for CD Strength versus cut depth for CD Strength versus cut depth for CD Strength versus cut depth for CD0, CD6, CD12 and CD25. The line is fitted to CD σ bc /σ b0 versus cut depth for CD

9 4.14 σ bc /σ b0 versus cut depth for CD σ bc /σ b0 versus cut depth for CD σ bc /σ b0 versus cut depth for CD0, CD6, CD12 and CD25. The line is fitted to CD Crack pattern of CD Strength versus cut depth for CD σ bc /σ b0 versus cut depth for CD Crack pattern of CD Strength versus cut depth for CD Strength versus cut depth for CD Strength versus cut depth for CD Strength versus cut depth for CD0, CD45, CD50 and CD60. The line is fitted to CD σ bc /σ b0 versus cut depth for CD σ bc /σ b0 versus cut depth for CD σ bc /σ b0 versus cut depth for CD σ bc /σ b0 versus cut depth for CD0, CD45, CD50 and CD60. The line is fitted to CD Strain amplification Comparison (strength) of CD0 versus CS Comparison (strength) of CD25 versus CS Comparison (strength) of CD50 versus CS Comparison (σ bc /σ b0 ) of CD0 versus CS Comparison (σ bc /σ b0 ) of CD25 versus CS Comparison (σ bc /σ b0 ) of CD50 versus CS ix

10 CHAPTER I INTRODUCTION In the previous investigations, the effect of carbon black level on cut growth in sulfur-cured natural rubber (NR) vulcanizates was studied. Vulcanizates with a certain crosslink density, but containing various N115 levels were tested. Normal tensile strengths (no pre-cut) of the gum (unfilled) and all black-filled specimens were similar. However, when a pre-cut is present, specimens with low levels of black, are weaker than the gum and there is only a single crack that propagates laterally from the original cut tip. Specimens with high levels of carbon black are stronger than the gum and crack splitting occurs near the original cut tip. At least two longitudinal cracks are formed prior to the catastrophic rupture. This reduces the stress concentration by releasing stored strain energy. Rupture is delayed. 1 In this research, the effect of carbon black (N115) level on cut growth in NR vulcanizates cured with dicumyl peroxide (DCP) is studied. The goals of the research are to systematically investigate the tearing of DCP-cured black-filled NR vulcanizates and compare these results with those of sulfur-cured systems at similar crosslink density. 1

11 CHAPTER II BACKGROUND STATEMENT 2.1 Natural Rubber There are nearly 2000 species of trees, shrubs, or vines found in tropical and temperate regions that produce NR latex. The latex from the Hevea brasiliensis tree is the most important commercial source of natural rubber. The latex is mostly produced in Asia. 2 Latex is obtained from vessels of trees by tapping. Natural rubber latex consists primarily of rubber hydrocarbon, proteins, carbohydrates, neutral lipids, glycolipids, phospholipids, inorganic constituents and water. Commonly, the dry-rubber content of the latex is 30%-45%. The dry rubber can be obtained by coagulation of the latex. 2 Natural rubber macromolecules consist almost entirely of cis-1,4-polyisoprene (Figure 2.1). The molecular weight of natural rubber is high. Nair showed that the number-average molecular weight of natural rubber ranges from (g/mol) to (g/mol) and the weight-average molecular weight ranges from (g/mol) to (g/mol). The dispersion index ranges from 3.6 to

12 Figure 2.1 The structure of natural rubber macromolecule (cis-1,4-polyisoprene) 2 3

13 As seen in Figure 2.1, the natural rubber molecule is an unsaturated aliphatic macromolecule with a regular structure. Double bonds are reactive sites. Natural rubber polymer has a T g Natural rubber can crystallize at low temperature, even without stretching. Crystallization can also occur when the rubber is stretched. This imparts a high tensile strength. Natural rubber is widely used in tyres, beltings, hoses, foot wear, engineering products, etc. because of its high tensile strength, high resilience, low heat build-up, excellent dynamic properties and outstanding processability Vulcanization by Organic Peroxides Definition of Vulcanization Unvulcanized rubber is weak. Crosslinking (vulcanization) increases strength. The first commercial method of vulcanization was found by Charles Goodyear. Natural rubber with sulfur was vulcanized by heating. 5 Vulcanizates retract to their approximately original shape after large deformation. According to the theory of rubber elasticity, the retractile force is proportional to the concentration of network-supporting chains (molecular chains between network junctions). 6 Figure 2.2 shows network formation by sulfur-cure. The most common vulcanization agent is sulfur. Other curing agents include organic peroxides, phenolic curatives and quinoid curatives. 4

14 Figure 2.2 Network formation by sulfur. 5 5

15 2.2.2 Organic Peroxides Vulcanization Mechanism Another curative sometimes used is organic peroxide. This was first reported by Ostromislensky in Organic peroxides curatives can be divided into three classes: diacyl peroxides, dialkyl peroxides and peresters. 7 The first step in peroxide vulcanization is the production of free radicals. Three steps are involved: thermal decomposition of the peroxide, hydrogen abstraction from the polymer, and crosslink formation (Figure 2.3). The actual process can be complex due to many side reactions, including acid-catalyzed decomposition of the peroxide, beta cleavage of the oxy radical, addition, radical transfer, polymer scission, dehydrohalogenation, and oxygenation. 8 6

16 Figure 2.3 The basic process of peroxide vulcanization. 8 7

17 2.2.3 Comparison of Peroxide and Sulfur Vulcanization Peroxide vulcanization introduces carbon-carbon crosslinks. These vulcanizates have poorer mechanical properties than those obtained by sulfur vulcanization. However, peroxide vulcanizates age better and have lower compression set. Also, peroxide curatives can be used to cure fully saturated rubbers, which sulfur cannot Effects of Vulcanization on Vulcanizate Properties As crosslink density increases, plasticity, hysteresis, permanent set, and friction decrease, while elasticity and stiffness increase. Tear strength, fatigue life, toughness and tensile strength maximize at a certain crosslink density. 5, Characterization of Vulcanization During vulcanization, it is important to know scorch time, rate of crosslink formation and extent of crosslinking. The scorch time must be long enough to permit mixing, shaping, forming, and flowing in the mold before vulcanization. Crosslink formation must be relatively rapid, and the extent of crosslinking must be controlled. 5 The state of crosslinking can be obtained from cure meter. One type of cure meter is called oscillating disk rheometer (ODR). Here, a sample is enclosed in a heated cavity. A metal disk, surrounding by rubber, oscillates sinusoidally. Vulcanization is measured by the increase in the torque. Another type of cure meter is the moving-die rheometer (MDR). It uses a smaller sample and heat transfer is faster. 5 Typical cure curves are shown in Figure 2.4. At the beginning of vulcanization, the torque may decrease due to decreased viscosity. After crosslinking begins, torque increases with time. There are three types of curves: creep, plateau and reversion. These result from competition of crosslink formation and chain scission or crosslink breakage. If crosslink formation dominates, the torque keeps increasing (creep). If the two effects are equal, there is a plateau. If chain scission dominates, torque drops (reversion). 5 8

18 Figure 2.4 Typical cure curves. 5 9

19 2.2.6 Determination of Crosslink Density Crosslink density can be measured by swelling. According to the Flory-Rehner equation, 10, 11 crosslink density can be determined: ρ c = 1 ln(1 ν r )+ν r +χν 2 r 1 2ν s ν r 3 ν r 2 where ρ c is the crosslink density (number of moles of crosslinks per unit volume); 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 solvent and rubber. Another method to obtain crosslink density is taking stress-strain measurements and making a Mooney-Rivlin plot. 12 (2.1) σ = C 2(λ λ 2 ) 1 + C 2 λ (2.2) where σ is the stress obtained from stress-strain measurements; λ is the extension ratio obtained from stress-strain measurements; C 1 = ρ c RT, where ρ c is the crosslink density (moles of crosslinks per unit volume); R is the gas constant; T is temperature. 2.3 Carbon Black Reinforcement Most rubber materials contain filler. The most common are carbon black and silica. Other fillers include silicates, clays, whiting (calcium carbonate) and other mineral fillers Carbon Black There are two common types of carbon black: furnace black and thermal black. Furnace blacks are prepared by incomplete combustion of gas or oil in excess of air. Thermal blacks are prepared by thermal cracking of natural gas or oil in absence of air. 13 Figure 2.5 shows a typical carbon black morphology with three levels of structure. The smallest are primary particles, also called nodules. The primary particles are fused together to form aggregates. The aggregates are not destroyed by normal processing. The aggregates can come together to form agglomerates. These 14, 15 are broken down during processing. 10

20 2.3.2 Effects of Carbon Black on Reinforcement Reinforcement by carbon black depends on four factors: primary particle size, specific surface area, structure and surface activity. 16 Small particle size gives larger specific surface area, which gives more interaction between rubber and carbon black. Particle size is the most important parameter in reinforcement. When particles are small enough, the space between the particles is similar to crosslink spacing. Fracture is hindered due to restriction of the chain motion. 17 Another factor is carbon black structure. Some rubber can be within the internal void of the aggregates: this is called occluded rubber. 15 Occluded rubber is shielded from deformation. This increases the effective volume fraction of carbon black. Furthermore, rubber can absorb onto carbon black physically or chemically. Bound rubber is formed. As the content of carbon black increases, a continuous network can be formed as shown in Figure 2.6, which cannot be dissolved in good solvent. 15 There is a critical black level for bound rubber/continuous network formation. This critical level is called the percolation threshold. When the carbon black level is higher than the critical level, some properties of the material would vary dramatically, such as tearing resistance, moduli or electric properties. The relation of the modulus of filled and unfilled rubber is given by the following equation. 13 E = E 0 ( φ φ 2 ) (2.3) 11

21 Figure 2.5 Typical carbon black morphology

22 Figure 2.6 Bound rubber/continuous network

23 2.3.3 Effect of Carbon Black Concentration on Cut Growth in Sulfur Cured NR Vulcanizates Hamed has investigated the effect of carbon black (N115) concentration on cut growth in sulfur-cured natural rubber (NR) vulcanizates. Normal tensile strengths (no pre-cut) of the gum (unfilled) and all black-filled specimens are similar. If a pre-cut is introduced into the samples, the results can be very different. With low levels of carbon black, filled specimens are weaker than the gum, while with high levels of carbon black, filled specimens are stronger than the gum. Figure 2.7 shows the results of the specimens with low carbon black levels and Figure 2.8 shows the results of the specimens with high carbon black levels. 1 14

24 Figure 2.7 Results of the specimens with low carbon black levels. 1 15

25 Figure 2.8 Results of the specimens with high carbon black levels. 1 16

26 2.4 Fracture of Rubber Fracture is a process in which new free surface area is created in a solid. 18 It is caused by the growth of a crack which is introduced to the solid intentionally or it is an inherent flaw Fracture Mechanism The criterion for fracture occurring can be described by two methods: stress concentration and energy conservation. When a solid is subjected to a stress, the local stress at the tip of a crack or flaw is magnified as many times compared to the average applied stress. If the local stress reaches a critical level larger than that which the solid can withstand, fracture occurs. Inglis was the first person to give an expression relating the local stress at the tip of a crack or flaw to the applied average stress. 19 σ t = σ[1 + 2(l/r) 1/2 ] (2.4) where σ t is the local stress at the flaw; σ is the average stress; l is the depth of the edge flaw; r is the radius of the flaw tip; If the flaw length is much larger than the radius of the tip, the equation can be simplified to: σ t = 2σ(l/r) 1/2 (2.5) According to this equation, long, sharp cracks produce the greatest stress concentration. An energy criterion for fracture was proposed by Griffith. 20 The energy balance for fracture can be expressed by the equation below. W S + W P = G( A) + W e (2.6) where W S is the stored strain energy released from the bulk when a crack grows; W P is the change of energy from the loading machine when a crack grows; G is the fracture energy per unit area; A is the newly created surface due to the growth of the crack; W e is the stored strain energy gained as a result of crack growth. The left side of the equation stands for the energy supplied in the process of fracture and the right side of the equation stands for the energy expended in the 17

27 process of fracture. Conservation of energy requires the energy supplied equals the energy expended Rubber Fracture Mechanics Thomas and Rivlin applied the energy criterion for fracture proposed by Griffith to elastomers. 21 For a rubber sheet, the energy criterion for fracture can be expressed as: ( W ) = Gt (2.7) c l where l means that the equation is for constant overall deformation of the sample; G is the characteristic fracture energy for the sample and is independent of the test geometry: it includes energy dissipation during crack growth; t is sample thickness. For samples with an edge-cut (Figure 2.9), the results are: σ b = ( GE kc )1/2 (2.8) where σ b is the stress at break; G is the fracture energy; E is Young s modulus; k is a function which varies slowly with strain; c is the cut depth. Equation 2.8 is only valid for the samples, where there is only one crack. It cannot be used for the samples where multiple cracks occur. 18

28 Figure 2.9 Sample with an edge-cut c

29 2.4.3 Effect of Edge-cut on the Fracture of Natural Rubber According to Equation (2.8), the stress at breaking of an edge-cut specimen under simple tensile stress is proportional to c -1/2. However, natural rubber which can crystallize when stretched does not obey this relationship. Thomas and Whittle 23 studied σ b versus c using tensile tests of a natural rubber vulcanizate. Figure 2.10 shows results. The tensile strength decreased as c increasing. An abrupt drop of σ b occurred at a critical size (c cr ). This was attributed to the bulk crystallization prior to crack propagation when c<c cr, but the absence of bulk crystallization when c>c cr. 20

30 Figure 2.10 The results of tensile tests on natural rubber vulcanizates

31 CHAPTER III EXPERIMENTAL 3.1 Materials and Formulation Ingredients include natural rubber SMR CV-60, curative dicumyl peroxide (DCP), phenyl-alpha-naphthylamine (PANA) as antioxidant and carbon black N115. Formulations are shown in Table 3.1. (phr stands for parts per hundred of rubber. The designation of CD means carbon black filled composition cured by DCP, the suffix indicates the phr of carbon black. NR indicates natural rubber.) 22

32 Table 3.1 Formulations. Ingredient (phr) CD0 CD6 CD12 CD25 CD38 CD45 CD50 CD60 NR (SMR CV-60) DCP PANA Carbon Black

33 3.2 Internal Mixing Natural rubber SMR CV-60 was cut into small pieces (about 20g each). A water-cooled internal mixer (Farrel, Midget Size Banbury Mixer) was employed. Small pieces of rubber were added into the mixer and mixed for 1 minute. Then, half of the carbon black was added and mixing continued for 0.5 minute. The other half of carbon black together with PANA was added and mixed for 3.5 minutes. The gate of the mixer was opened and the mixture discharged. The mixing procedure is given as Table

34 Table 3.2 Mixing procedure in the internal mixer. Time (min) Procedure 0-1 Natural Rubber /2 carbon black the other 1/2 carbon black and PANA 5 Dump and weigh 25

35 3.3 Milling A two-roll mill (Reliable, 15 cm diameter and 30 cm roll length) was used after internal mixing. The batch from the mixer was milled at a nip of 1.9 mm for 2 minutes. Then, a rolling bank was formed (0.6 mm nip for 1 minute). Curing agent (DCP) was added to the rolling bank with alternating cut. After 3 minutes, 10 end-rolls were made at a 1.2 mm nip. The compound was sheeted off at a 1.9 mm nip, and the direction of milling was marked on the sheet. The sheet was stored at room temperature for at least 16 hours before vulcanization. The milling procedure is shown in Table

36 Table 3.3 Mixing procedure on the two-roll mill. Time (min) Procedure 0-2 Mix with the nip at 1.9 mm 2-3 Form rolling bank at 0.6 mm nip 3-6 Add curing agents (DCP) at 0.6 mm nip - 10 end-roll with the nip at 1.2 mm and then sheet off at 1.9 mm 27

37 3.4 Measurement of Vulcanization Parameters Vulcanization parameters were determined with an Alpha Moving Die Rheometer (MDR), equipped with the Advanced Polymer Analyzer (APA) About 5g of rubber was cut and placed between two plastic sheets. This was placed in the MDR at 150 for 100 minutes. The t min value was chosen as the molding time. 3.5 Molding A Dake Press (1800 watts, 120 volts, 30 tons load) was used to mold a rubber sheet at 150 (These had been preheated to 150 ). An unvulcanizated rubber sheet was cut into pieces (about 20g of each). One of the pieces was placed at the center of a window mold (180 x 180 x 0.5 mm), sandwiched between two Mylar films and two steel plates. Then the sheet was vulcanized in the press at 150 for t min. The vulcanized sheet was cut into dumbbells with a Relco compression machine and a dumbbell die (6.35mm width). The length of the dumbbell was along the milling direction. 3.6 Cut Size Measurement An edge-cut was introduced into a dumbbell, using a razor blade which had been dipped into a soap solution. Cut depths were measured with a travelling microscope, three times on both sides. The final value of cut depth was taken as the average of the six values. The cut depths ranged from 0.1 mm to 3 mm. 3.7 Tensile Test Tensile tests were performed at room temperature with an Instron 5567 tensile tester. Data were analyzed with the Instron software. To begin a test, calibration of the clamps and the extensometer was done. Two ends of a dumbbell sample were clamped in the extensometer. One end moved with the extensometer and the other one was fixed. The test stopped automatically at break. 3.8 Swelling Test A rubber sheet was cut into 10 small rectangular pieces, which then were dipped into toluene in the dark for about one week. Swollen pieces were taken out, 28

38 weigh, and then put into a vacuum oven to deswell. Deswelled pieces were then reweighed. Crosslink density was calculated using Equation (2.1). 3.9 Bound Rubber Uncured specimens were mixed as given Table 3.2 and Table 3.3, but with no curing agent. Mixing time was 5 minutes. Then, about 1 g (W 0 ) of rubber was swelled in 80 ml of toluene with no agitation. After four days, the supernatant was pipetted out. If the sample remained as a chunk of gel, a bound rubber network had formed. The gel was dried in vacuum for two days and weighed (W t ). On the other hand, if the sample disintegrated completely, no bound rubber network was formed. The bound rubber fraction was determined using Equation 3.1. Bound Rubber w.t.%= 3.10 Crack Patterns W W CB phr / (102 CB phr) t 0 W 100 / (102 CB phr) 0 (3.1) After tensile tests, crack patterns of ruptured samples were obtained using an optical microscope (Olympus SZX16). 29

39 CHAPTER IV RESULTS AND DISCUSSION 4.1 Empirical Equation to Determine DCP Levels To compare DCP-cured vulcanizates and sulfur-cured ones, crosslink densities must be similar. DCP levels to match the crosslink densities of sulfur-cured specimens at all levels of carbon black were calculated. For the sulfur-cured gum, crosslink density mol/m. From Table c 4.1, a DCP level of 2 phr gives mol/m for the gum. c For black-filled specimens, determination of the appropriate DCP levels is complicated because of the interaction between carbon black and DCP. Thus, more than 2 phr of DCP must be used for black-filled specimens to match the crosslink density of sulfur-cured specimens. Firstly, a series of specimens cured with 1.55 phr of DCP with different levels of carbon black were made. The crosslink densities were measured (shown in Table 4.2). Then, these crosslink densities were plotted versus the corresponding crosslink densities of sulfur-cured specimens (Figure 4.1). This gives: (S1.75phr) = 1.39 c c (DCP1.55phr) (4.1) The ratio between and c( S 1.75 phr ) c( DCP 1.55 phr ) is shown as: (S1.75phr) / (DCP1.55phr) = / (DCP1.55phr) (4.2) c c c It can be assumed that the ratio of crosslink densities equals to the ratio of curing agent. So, the DCP level can be determined multiplying the ratio / by c( S 1.75 phr ) c( DCP 1.55 phr ) 30

40 Then crosslink densities of DCP-cured versus corresponding phr of carbon black are plotted in Figure 4.2: c (DCP1.55phr) = CB phr (4.3) Thus, DCP levels can be expressed as: DCP phr = 1.55 ratio = 1.55 ( / c (DCP1.55phr) ) (4.4) Combining Equation 4.3 and Equation 4.4, DCP phr CB phr 0.25 (4.5) Using Equation 4.5, the DCP level for a certain carbon black level can be predicted. Crosslink densities are shown in Table 4.3. The discrepancies between DCP-cured and sulfur-cured crosslink densities are within 10%. 31

41 Table 4.1 Crosslink densities of DCP-cured NR specimens with different levels of DCP. DCP levels (phr) Crosslink densities *10 4 (mol/ml)

42 Table 4.2 Crosslink densities of NR specimens with 1.55 phr DCP and 1.75 phr sulfur at different levels of carbon black. Black levels (phr) Crosslink desities *10 4, DCP (1.55phr) (mol/ml) Crosslink desities *10 4, Sulfur (1.75phr) 24 (mol/ml)

43 Table 4.3 Comparison of crosslink densities between DCP-cured and Sulfur-cured vulcanizates. Composition Carbon DCP levels Crosslink Crosslink Error black levels (Equation 4.5) desities desities percent (phr) (phr) *10 4, DCP *10 4, Sulfur (DCP-cured to (mol/ml) (1.75 phr) 24 Sulfur-cured) (mol/ml) CD0 0 / % CD % CD % CD % CD % CD % CD % CD % 34

44 c, S1.75 *10 4 (mol/ml) y=1.39x c, DCP1.55 *10 4 (mol/ml) Figure 4.1 Fitting curve of crosslink densities of specimens with 1.75 phr sulfur versus crosslink densities of specimens with 1.55 phr DCP. 35

45 c, DCP1.55 *10 4 (mol/ml) y=0.0044x Carbon Black Level (phr) Figure 4.2 Fitting curve of crosslink densities with 1.55 phr DCP versus various carbon black levels. 36

46 4.2 Vulcanization Characterization Cure characteristics at 150 for all the eight natural rubber (NR) compounds are shown in Table 4.4. Cure curves are shown in Figure 4.3 and Figure 4.4. The scorch time decreases as carbon black concentration increases except CD50 and CD60. The optimum cure time decreases as carbon black concentration increases except CD50. The maximum cure time of each compound is around 100 min (the test setting time is 100 min). The minimum torque increases as carbon black concentration increases. The maximum torque increases as carbon black concentration increases. 37

47 Table 4.4 Cure characteristics of DCP cured NR compounds (150 ). Compound t s2 (min) a t c (90) b t c (100) c Minimum torque Maximum torque (min) (min) M L (dnm) M HF (dnm) CD CD CD CD CD CD CD CD a t s2 : scorch time (time for torque to raise 2 dnm above minimum value). b t c (90): optimum cure time (time for torque to reach torque M 90, M 90 =M L + 0.9(M HF -M L )) c t c (100): maximum cure time (time for torque to reach maximum torque M HF ). 38

48 Torque (dnm) phr 45 phr 50 phr 38 phr 25 phr 12 phr 6 phr 0 phr Time (min) Figure 4.3 Cure curves of NR compounds. 39

49 10 60 phr Torque (dnm) 50 phr 45 phr 12 phr 38 phr 25 phr 6 phr 1 0 phr Time (min) Figure 4.4 Cure curves of NR compounds (log-log). 40

50 4.3 Swelling Results The weight of swollen rubber sheets w r together with the weight of the original rubber sheets w 0 was used to calculate the volume fraction of rubber in the swollen gel v r. Crosslink densities were calculated using Equation (2.1). The molar volume of toluene v s is m 3 /mol at 25, and the interaction parameter of toluene and gum natural rubber χ is Results are shown in Table 4.5. The corresponding crosslink densities of sulfur cured samples are also shown. 24 The two crosslink densities are similar. 41

51 Table 4.5 Results of swelling test. Compound DCP Fraction Crosslink density (10 4 mol/ml) Corresponding Crosslink Density of Sulfur Cured Sample (10 4 mol/ml) CD0 2 phr CD phr CD phr CD phr CD phr CD phr CD phr CD phr

52 4.4 Normal Tensile Tests (Cut Depth c=0) Table 4.6 and Figure 4.5 show the results of normal tensile tests (no cut) of gum (CD0) NR vulcanizates and compounds with all carbon black levels. 100% modulus is the tensile stress at 100% strain. This increases monotonically as carbon black concentration increases. The strain at break decreases as carbon black concentration increases. Tensile strength behavior is complex: this is shown in Figure 4.6. The gum and CD6 have similar tensile strength. Therefore, tensile strength increases as the carbon black concentration increases. CD45 has the highest tensile strength. For CD50 and CD60, tensile strength decreases slightly. 43

53 Table 4.6 Normal tensile properties of gum and filled vulcanizates. Property Compound 100% modulus Tensile strength at Strain at break (MPa) break b0 (MPa) b0 (%) Gum (CD0) 0.51± ± ±39 CD6 0.79± ± ±69 CD ± ± ±15 CD ± ± ±21 CD ± ± ±31 CD ± ± ±10 CD ± ± ±8 CD ± ± ±39 44

54 CD45 CD38 CD CD50 CD12 20 Tensile Stress (MPa) CD60 CD6 gum Strain (%) Figure 4.5 Normal (uncut) stress-strain curves. 45

55 Tensile Strength (MPa) Carbon Black Concentration (phr) Figure 4.6 Variation of tensile strength with carbon black concentration for normal tensile tests. 46

56 4.5 Pre-cut Tensile tests Figure 4.7 shows the effect of cut depth on the tensile strength of gum vulcanizates (CD0). The horizontal line depicts the normal strength (σ b0 ). The tensile strength (σ bc ) decreases as cut depth, c, increases. The data are discontinuous, divided into two populations (strong and weak). The vertical dotted lines indicate two critical cut depths, c s =0.68 mm and c w =1.11 mm. The strong population is the region of c<c s and the weak population is the region of c>c w. Bulk crystallization occurs in the strong population, but is absence in the weak population. In the region of c s <c<c w, the data points are quite scattered (unstable region). The ratio σ bc /σ b0 is plotted versus cut depth (c) in Figure

57 b0 =13.7MPa b0 =706% 10 CD0 Strong Population bc (MPa) 1 Unstable Weak Population c s =0.68mm c w =1.11mm c (mm) Figure 4.7 Strength versus cut depth for gum (CD0). 48

58 1 b0 =13.7MPa b0 =706% CD0 Strong Population bc / b0 0.1 Unstable Weak Population c s =0.68mm c w =1.11mm c (mm) Figure 4.8 σ bc /σ b0 versus cut depth for gum (CD0). 49

59 Figures show the effect of cut depth on the tensile strength of CD6, CD12 and CD25. The horizontal lines show the normal tensile strengths. Like CD0, tensile strengths decrease as cut depths increase. For CD 6, similar to CD0, there are strong and weak populations (c s =0.23 mm, c w =0.43 mm). The two critical cut depths are smaller than the two of CD0. For CD12 and CD 25, no critical cut depths are apparent: tensile strengths decrease continuously as cut depths increase. Tensile strengths of CD6, CD12 and CD25 are lower than the gum (CD0), especially in the strong population of gum (CD0). CD6 and CD25 are stronger than CD12. CD12 has a lowest tensile strength. CD0, CD6, CD12 and CD25 are plotted together in Figure σ bc /σ b0 is plotted versus cut depths (c) for CD6, CD12 and CD25 in Figures In CD0, CD6, CD12 and CD25, crack growth occurs without multiple cracks or crack deflection. Figure 4.17 shows the crack pattern of CD12. 50

60 b0 =12.8MPa b0 =495% 10 CD6 bc (MPa) 1 c s =0.23mm c w =0.43mm c (mm) Figure 4.9 Strength versus cut depth for CD6. 51

61 b0 =19.6MPa b0 =472% 10 CD12 bc (MPa) c (mm) Figure 4.10 Strength versus cut depth for CD12. 52

62 b0 =24.4MPa b CD25 10 bc (MPa) c (mm) Figure 4.11 Strength versus cut depth for CD25. 53

63 10 CD0 CD6 CD12 CD25 bc (MPa) c (mm) Figure 4.12 Strength versus cut depth for CD0, CD6, CD12 and CD25. The line is fitted to CD0. 54

64 1 b0 =12.8MPa b0 =495% CD6 bc / b0 0.1 c s =0.23mm c w =0.43mm c (mm) Figure 4.13 σ bc /σ b0 versus cut depth for CD6. 55

65 1 b0 =19.6MPa b0 =472% CD12 bc / b c (mm) Figure 4.14 σ bc /σ b0 versus cut depth for CD12. 56

66 1 b0 =24.4MPa b CD25 bc / b c (mm) Figure 4.15 σ bc /σ b0 versus cut depth for CD25. 57

67 1 CD0 CD6 CD12 CD25 Fit of CD0 bc / b c (mm) Figure 4.16 σ bc /σ b0 versus cut depth for CD0, CD6, CD12 and CD25. The line is fitted to CD0. 58

68 Original Cut Tip Figure 4.17 Crack pattern of CD12. 59

69 Figure 4.18 shows the effect of cut depth on the tensile strength of CD38. The horizontal line gives the normal tensile strength and the tensile strength decreases as cut depth increases. Like CD0 and CD6, there are strong and weak populations (c s =0.33 mm, c w =0.88 mm) in CD 38. The tensile strength of CD38 is a little higher than CD25 and is similar to CD0. σ bc /σ b0 is plotted versus cut depths (c) for CD38 in Figure Figure 4.20 shows the crack pattern of CD38. Multiple crack and crack deflection occur near the cut tip. This reduces stress concentration and delays rupture. CD38 is considered the onset of reinforcement. 60

70 b0 =25.6MPa b0 =403% CD38 10 bc (MPa) 1 c s =0.33mm c w =0.88mm c (mm) Figure 4.18 Strength versus cut depth for CD38. 61

71 1 b0 =25.6MPa b0 =403% CD38 bc / b0 0.1 c s =0.33mm c w =0.88mm c (mm) Figure 4.19 σ bc /σ b0 versus cut depth for CD38. 62

72 Original Cut Tip Figure 4.20 Crack pattern of CD38. 63

73 Figures show the effect of cut depth on the tensile strength of CD45, CD50 and CD60. The horizontal lines show the normal tensile strengths. There are strong and weak populations (c s =0.55 mm, c w =0.84 mm) in CD 45 and there are no critical cut depths in CD50 and CD60. The tensile strengths of these three compounds are all higher than gum (CD0). CD45 has the lowest strength. CD 50 and CD 60 have similar strengths, which are nearly two times that of CD0. CD0, CD45, CD50 and CD60 are plotted together in Figure σ bc /σ b0 is plotted versus cut depths (c) for CD45, CD50 and CD60 in Figure Figure 4.16 and Figure 4.28 show that strengths of specimens with carbon black decrease more than the gum when a pre-cut is introduced. This indicates black-filled specimens are more sensitive to cut than the gum. Multiple cracks and crack deflection occur in CD45, CD50 and CD

74 b0 =27.6MPa b0 =382% CD45 10 bc (MPa) 1 c s =0.55mm c w =0.84mm c (mm) Figure 4.21 Strength versus cut depth for CD45. 65

75 b0 =22.0MPa b0 =339% CD50 10 bc (MPa) c (mm) Figure 4.22 Strength versus cut depth for CD50. 66

76 b0 =19.3MPa b0 =275% CD60 10 bc (MPa) c (mm) Figure 4.23 Strength versus cut depth for CD60. 67

77 CD0 CD45 CD50 CD60 10 bc (MPa) c (mm) Figure 4.24 Strength versus cut depth for CD0, CD45, CD50 and CD60. The line is fitted to CD0. 68

78 1 b0 =27.6MPa b0 =382% CD45 bc / b0 0.1 c s =0.55mm c w =0.84mm c (mm) Figure 4.25 σ bc /σ b0 versus cut depth for CD45. 69

79 1 b0 =22.0MPa b0 =339% CD50 bc / b c (mm) Figure 4.26 σ bc /σ b0 versus cut depth for CD50. 70

80 1 b0 =19.3MPa b0 =275% CD60 bc / b c (mm) Figure 4.27 σ bc /σ b0 versus cut depth for CD60. 71

81 1 CD0 CD45 CD50 CD60 Fit of CD0 bc / b c (mm) Figure 4.28 σ bc /σ b0 versus cut depth for CD0, CD45, CD50 and CD60. The line is fitted to CD0. 72

82 4.6 Effects of Carbon Black on Cut Growth From the results, it is interesting that low levels carbon black-filled vulcanizates are weaker than gum but high levels carbon black-filled vulcanizates are stronger than gum. This behavior is like that for sulfur-cured vulcanizates studied previously. 1 There must be competitive effects of carbon black on cut growth. Natural rubber vulcanizates can crystallize when stretched. This imparts high strength. Crystallization is determined by two factors: nucleation and crystal growth. The addition of carbon black influences both. Nucleation requires sufficient molecular chain orientation, which is determined by the extent of strain. For high levels of carbon black-filled vulcanizates, strain amplification dominates. This is shown in Figure A NR gum vulcanizate sample, shown in Figure 4.29 (a), is stretched. The same sample but filled with sufficient carbon black is shown in Figure 4.29 (b). It can be imagined that the carbon black is all concentrated at the two ends of the sample. Because carbon black cannot deform, the total strain is given by the rubber part. This favors chain orientation and increases the rate and degree of crystallization. As a result, vulcanizates with high levels of carbon black are stronger than the gum. On the other hand, for low levels of black, decreased chain mobility dominates and inhibits crystal growth. As a result, vulcanizates with low levels of black are weaker than the gum. 73

83 (a) (b) Figure 4.29 Strain amplification. 74

84 4.7 Content of Bound Rubber Table 4.7 shows the result of the bound rubber content of specimens with different carbon black levels. The result indicates the percolation threshold of carbon black is 15phr, where a bound rubber network first forms. 75

85 Table 4.7 Content of bound rubber in specimens with different carbon black levels. Level of carbon black Weight fraction of bound rubber (phr)

86 4.8 Comparison of DCP-cured Vulcanizates and Sulfur-cured Vulcanizates Tearing Behavior Figures compare the cut growth resistance of DCP-cured and sulfur-cured vulcanizates. 24 Figure show the relationship between strength and cut depth and Figure show the relationship between σ bc /σ b0 and cut depth. For normal tensile strength, sulfur-cured specimens are higher than DCP-cured ones for not more than 50%. CD0 and CS0 both have critical cut depths. But, the critical cut depths of CS0 are larger than the ones of CD0. The strength of CS0 is nearly two times of CD0 at small cut depths. The σ bc /σ b0 of CD0 and CS0 are similar. This shows CD0 and CS0 are sensitive to cut equivalently. CD25 and CS25 have no apparent critical cut depths. The strength of CS25 is nearly ten times that of CD25 and the strength of CS50 is nearly twice that of CD50. For 25 phr of carbon black, CS25 has been reinforced by carbon black, but CD25 has not. For 25 and 50 phr of carbon black, the ratio σ bc /σ b0 of sulfur-cured specimens is obviously higher than DCP-cured ones. This shows that for black-filled specimens, DCP-cured specimens are much more sensitive to cut than sulfur-cured ones. 77

87 b0 (CS0)=23.6MPa b0 (CS0)=739% b0 (CD0)=13.7MPa b0 (CD0)=706% 10 CS0 CD0 bc (MPa) 1 c s = 0.68mm c s = 1.96mm c w = 2.65mm c (mm) c w = 1.11mm Figure 4.30 Comparison (strength) of CD0 versus CS0. 78

88 b0 (CS25)=28.6MPa b0 (CS25)=596% b0 (CD25)=24.4MPa b0 (CD25)=470% 10 CS25 bc (MPa) CD c (mm) Figure 4.31 Comparison (strength) of CD25 versus CS25. 79

89 b0 (CS50)=27.8MPa b0 (CD50)=22.0MPa b0 (CS50)=517% b0 (CD50)=339% CS50 10 bc (MPa) CD c (mm) Figure 4.32 Comparison (strength) of CD50 versus CS50. 80

90 1 b0 (CS0)=23.6MPa b0 (CS0)=739% b0 (CD0)=13.7MPa b0 (CD0)=706% CD0 CS0 bc / b0 0.1 c s = 0.68mm c s = 1.96mm c (mm) c w = 1.11mm c w = 2.65mm Figure 4.33 Comparison (σ bc /σ b0 ) of CD0 versus CS0. 81

91 1 b0 (CS25)=28.6MPa b0 (CD25)=24.4MPa b0 (CS25)=596% b0 (CD25)=470% CD25 CS25 bc / b c (mm) Figure 4.34 Comparison (σ bc /σ b0 ) of CD25 versus CS25. 82

92 1 b0 (CS50)=27.8MPa b0 (CD50)=22.0MPa b0 (CS50)=517% b0 (CD50)=339% CD50 CS50 bc / b c (mm) Figure 4.35 Comparison (σ bc /σ b0 ) of CD50 versus CS50. 83

93 4.8.2 Percolation Threshold Table 4.8 compares the percolation threshold of carbon black in DCP-cured and sulfur-cured vulcanizates. With sulfur-cured vulcanizates, bound rubber/continuous network begins to form from 15 phr of carbon black. This indicates the percolation threshold of sulfur-cured vulcanizates is 15 phr of black. And also, for sulfure-cured vulcanizates, the onsets of reinforcement and occurrence of multiple cracks are both 15 phr of black. With DCP-cured vulcanizates, same as sulfur-cured ones, bound rubber/continuous network begins to form from 15 phr of black. However, the onset of reinforcement and the occurrence of multiple cracks are both 38 phr of black. From this result, it could be inferred that reinforcement of rubber is not only dependent on the formation of bound rubber/continuous network, but also on crosslink type. 84

94 Table 4.8 Percolation threshold of carbon black in DCP-cured vulcanizates and sulfur-cured vulcanizates. Properties Carbon black levels (DCP-cured) Carbon black levels (Sulfur-cured) 24 Tearing 38 phr 15 phr Bound rubber 15 phr 15 phr Crack pattern 38 phr 15 phr 85

95 CHAPTER V CONCLUSION 5.1 Tearing Behavior of DCP-cured NR Vulcanizates Normal tensile strengths are different for specimens with different carbon black levels. Specimens with low levels of carbon black are weaker than the gum. There are no longitudinal cracks or crack deflection. Specimens with high levels of carbon black are stronger than the gum. Longitudinal cracks and crack deflection occur before rupture. CD38 is the onset of reinforcement. 5.2 Comparison of DCP-cured and sulfur-cured NR Vulcanizates Sulfur-cured vulcanizates are stronger than DCP-cured ones. A percolation threshold of carbon black for sulfur-cured vulcanizates is 15 phr. Tearing and cracking pattern give a percolation threshold for DCP-cured vulcanizates of 38 phr, but bound rubber commences at 15 phr of black. 86

96 REFERENCES 1. Hamed, G. R.; Al-Sheneper, A. A. Effect of Carbon Black Concentration on Cut Growth in NR Vulcanizates. Rubber Chem. Technol. 2003, Subramaniam, A. Natural Rubber. Rubber Technology, 3rd ed.; Morton, M., Eds., Van Nostrand Reinhold: New York, Brydson, J. A. Natural Rubber. Rubbery Materials and Their Compounds. Elaevier Science Publishers LTD: New York, Hofmann, W. Natural Rubber. Rubber Technology Handbook; Hanser Publisher: New York, Coran, A. Y. Vulcanization. Science and Technology of Rubber; Mark, J. E., Erman, B., Eirich, F. R., Eds.; Elsevier Academic Press: San Diego, Flory, P. J. Ch. 11. Principles of Polymer Chemistry. Cornell Univ. Press: New York, Loan, L. D. Mechanism of Peroxide Vulcanization of Elastomers. Rubber Chem. Technol. 1967, Dluzneski, P. R. Peroxide Vulcanization of Elastomers. Rubber Chem. Technol. 2001, Hamed, G. R.; Rattanasom, N. Effect of Crosslink Density on Cut Growth in Gum Natural Rubber Vulcanizates. Rubber Chem. Technol. 2002, Flory, P. J.; Rehner, J. Statistical Mechanics of Cross-Linked Polymer Networks I. Rubberlike Elasticity. J. Chem. Phys. 1943, 11, Flory, P. J.; Rehner, J. Statistical Mechanics of Cross-Linked Polymer Networks II. Swelling. J. Chem. Phys. 1943, 11, Rivlin, R. S. Saunders, D. W. Large Elastic Deformation of Isotropic Materials VII. Experiments on the Deformation of Rubber. Phil. Trans. Roy. Soc. 1951, A243,

97 13. Medalia, A. I.; Kraus, G. Reinforcement of Elastomers by Particulate Fillers. Science and Technology of Rubber; Mark, J. E., Erman, B., Eirich, F. R., Eds.; Elsevier Academic Press: San Diego, J. T. Byers. Filler Part I: Carbon black. Rubber Technology, 3rd ed. Morton, M., Eds., Van Nostrand Reinhold: New York, Donnet, J. B.; Custodero, E. Reinforcement of Elastomers by Particulate Fillers. Science and Technology of Rubber, Mark, J. E.; Erman, B.; Eirich, F. R., Eds., Elsevier Academic Press: San Diego, Norman, D. T. Rubber Grade Carbon Blacks. The Vanderbilt Rubber Handbook, 13th ed., Ohm, R. F. Eds., R. T. Vanderbilt Company, Inc.: Connecticut, Hamed, G. R.; Hatfield, S. On the Role of Bound Rubber in Carbon-Black Reinforcement. Rubber Chem. Technol., 1989, 62, Hamed, G. R. Energy Dissipation and the Fracture of Rubber Vulcanizates. Rubber Chem. Technol., 1991, Inglis, C. E. Trans. Inst. Nav. Archit., 1913, 55, Griffith, A. A. Philos. Trans. R. Soc., 1921, Ser. A, 221, Rivlin, R. S., Thomas, A. G. Rupture of Rubber. I. Characteristic Energy for Tearing, J. Polym. Sci., 1953, 10, Gent, A. N. Strength of Elastomers. Science and Technology of Rubber, Mark, J. E.; Erman, B.; Eirich, F. R., Eds., Elsevier Academic Press: San Diego, Thomas, A. G.; Whittle, J. M. Tensile Rupture of Rubber. Rubber Chem. Technol. 1970, 43, Rong, G. Doctor Dissertation, 2011, The University of Akron, Akron, OH. 25. Ma, J. H., Zhang, L. Q., Wu, Y. P. Characterization of Filler-rubber Interaction, Filler Network Structure, and Their Effects on Viscoelasticity for Styrene-butadiene Rubber Filled with Different Fillers. J. Macromol. Sci., Phy. 2013, 52,

98 APPENDIX 1. Tensile Properties of Uncut NR Compounds Table 1 Tensile properties of uncut gum (CD0). (Table 4.6, Figure 4.5 and Figure 4.6) Specimens Cut Size Thickness Tensile Stress Ultimate Strain 100% Modulus (mm) (mm) (MPa) (%) (MPa)

99 Table 2 Tensile properties of uncut CD6. (Table 4.6, Figure 4.5 and Figure 4.6) Specimens Cut Size Thickness Tensile Stress Ultimate Strain 100% Modulus (mm) (mm) (MPa) (%) (MPa)