EFFECT OF CROSSLINK DENSITY AND N660 CARBON BLACK ON TEARING BEHAVIORS OF NATURAL RUBBER VULCANIZATES A Thesis Presented to The Graduate Faulty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Tingling Rao December, 2012 EFFECT OF CROSSLINK DENSITY AND N660 CARBON BLACK ON TEARING BEHAVIORS OF NATURAL RUBBER VULCANIZATES Tingling Rao Thesis Approved Accepted Advisor Dean of the College Dr. Gary R. Hamed Dr. Stephen Z. D. Cheng Co-Advisor or Faculty Reader Dean of the Graduate School Dr. Darrell H. Reneker Dr. George R. Newkome Department Chair or School Director Date Dr. Ali Dhinojwala ii ABSTRACT All tires contain black-filled natural rubber compounds in their belts, the edges of which experience high stress concentration. Black filled natural rubber vulcanizates have tearing resistance unsurpassed by any other elastomer. Previous studies have examined the effect of filler and cut size on tearing resistance and cracking behavior at a single crosslink density. Studies have now been extended to include NR vulcanizates with various crosslink densities. Conventional sulfur-cured, gum natural rubber vulcanizates were prepared with various crosslink densities. Tensile and tearing strength are quite sensitive to changes in crosslink density. For lightly and moderately crosslinked vulcanizates, the tear strength of edge-cut specimens shows two populations as a function of cut size. The drop-off in strength at the critical cut size is attributed to the lack of bulk crystallization. When crosslink density is increased enough the two populations disappear. Also, the cracking behavior changes as crosslink density in varied. When crosslinked lightly, gum NR vulcanizates exhibit simple lateral cracking with relatively smooth fracture surfaces. When crosslink density increases, gum NR vulcaniztes show increasingly complicated cracking behavior. The complexity of the fracture surfaces is incrased. Over a limited range of cut size, moderately crosslink vulcanizates exhibit multiple cracks prior to rupture. iii In other experiments, low concentrations of a coarse carbon black (N660) were added to the specific gum vulcanizate that showed crack deviation. Instead of promoting cracking deviation, carbon black (N660) of low concentration suppressed anisotropy. Multiple cracks only occur when the carbon black loading reaches a critical level. iv TABLE OF CONTENTS Page LIST OF TABLES ............................................................................................................. vii LIST OF FIGURES ............................................................................................................ ix CHAPTER I. INTRODUCTION ............................................................................................................ 1 II. HISTORICAL REVIEW ................................................................................................. 3 2.1 Natural rubber ............................................................................................................ 3 2.2 Fracture mechanics .................................................................................................... 4 2.2.1 Stress and energy approaches .............................................................................. 4 2.2.2 Fracture energy .................................................................................................... 7 2.3 Effect of strain-induced crystallization, carbon black and vulcanization .................. 7 2.3.1 Strain-induced crystallization ............................................................................. 9 2.3.2 Effect of carbon Black ...................................................................................... 12 2.3.3 Effect of vulcanization ...................................................................................... 16 2.4 Crack deviation ........................................................................................................ 21 III. EXPERIMENTAL ....................................................................................................... 28 3.1 Materials ................................................................................................................... 28 v 3.2 Compounding ........................................................................................................... 28 3.3 Specimen preparation ............................................................................................... 33 3.4 Tensile Testing ......................................................................................................... 33 3.5 Swelling testing ........................................................................................................ 34 3.6 Cracking pattern photographs .................................................................................. 36 IV. EFFECT OF CROSSLINK DENSITY ON PROPERTIES OF GUM NR VULCANIZATES ............................................................................................... 37 4.1 Crosslink density of gum vulcanizates .................................................................... 37 4.2 Normal tensile testing .............................................................................................. 42 4.3 Precut tensile testing ................................................................................................ 45 4.4 Comparison of properties of gun vulcanizates ......................................................... 90 V. THE EFFECT OF LOW CONCENTRATIONS (0-20 PHR) OF CARBON BLACK N660 ON THE REINFORCEMENT OF NR ............................................... 95 5.1 Swelling and curing testing ...................................................................................... 95 5.2 Normal tensile testing .............................................................................................. 97 5.3 Precut tensile testing .............................................................................................. 102 5.4 Comparison between gum and black-filled NR vulcanizates ................................ 141 VI. CONCLUSION .......................................................................................................... 147 REFERENCES ................................................................................................................ 149 APPENDIX ...................................................................................................................... 155 vi LIST OF TABLES Table Page 3.1 Compound formulations for varying crosslink density ............................................... 29 3.2 Compound formulations for vary carbon black loading .............................................. 30 3.3 Internal mixing procedure (UA0.6x-0 – UA2.0x-0) .................................................. 32 3.4 Internal mixing procedure (UA1.7x-6—UA1.7x-20) ................................................ 32 3.5 Mixing procedure on the two-roll mill ......................................................................... 33 4.1 Crosslink density of gum NR vulcanizates .................................................................. 37 4.2 APA cure characteristics (T = 140 °C , 3 ° arc) ............................................................... 39 4.3 Normal (uncut specimens) tensile properties ............................................................... 42 4.4 Critical cut size from tensile testing of pre-cut specimens .......................................... 94 5.1 Values of vvrr0 / obtained from swelling measurement ................................................. 95 5.2 APA cure characteristics (T = 140 °C , 3 ° arc) ............................................................... 97 5.3 Normal (uncut specimens) tensile properties ............................................................. 99 5.4 Crack pattern distribution ........................................................................................ 103 APPENDIX ...................................................................................................................... 155 A. 1 Tensile properties of UA0.6x-0 ................................................................................ 155 A. 2 Tensile properties of UA0.8x-0 ................................................................................ 157 vii A. 3 Tensile properties of UA1.0x-0 ................................................................................ 159 A. 4 Tensile properties of UA1.2x-0 ................................................................................ 160 A. 5 Tensile properties of UA1.4x-0 ................................................................................ 162 A. 6 Tensile properties of UA1.6x-0 ................................................................................ 164 A. 7 Tensile properties of UA1.8x-0 ................................................................................ 165 A. 8 Tensile properties of UA1.9x-0 ................................................................................ 167 A. 9 Tensile properties of UA2.0x-0 ................................................................................ 169 A. 10 Tensile properties of UA1.7x-0 .............................................................................. 171 A. 11 Tensile properties of UA1.7x-6 .............................................................................. 173 A. 12 Tensile properties of UA1.7x-10 ...........................................................................