RESEARCH ARTICLE CREEP AND RECOVERY OF BITUMEN-ACACIA SAP COMPOSITES

Transcription

1 International Research in Arts and Sciences Vol. 1, No, 11, pp , November, 213 RESEARCH ARTICLE CREEP AND RECOVERY OF BITUMEN-ACACIA SAP COMPOSITES *Mbithi, N. M., Merenga, A. S. and Migwi, C. M. School of pure and applied sciences, Department of Physics, Kenyatta University, P. O. Box 43844, Nairobi- Kenya Accepted 1 th October, 213; Published Online 21 st November, 213 Bitumen (B) binder creep and recovery properties have been unsatisfactory. Different synthetic binder modifies that have been used to improve its properties have led to environmental problems such as incineration and landfill. Acacia Sap (AS) a natural polymer and environmental friendly has been used successfully as a modifier. Composites of bitumen-acacia sap with different composition of sap percentage ranging from % to 62.% cell sap were prepared by injection drawing process. The composites were analyzed by Dynamic Mechanical Analysis (DMA) for creep and recovery. It was found that the creep strain for all composites decreased with increase of acacia sap loading in the binder and also increased with time and temperature. This implied increased resistance to deformation of the binder. The incorporation of acacia sap improved the recovery property. In addition, modification of bitumen binder with 2% acacia sap loading gave optimum creep behavior. The Arrhenius and William Landel Ferry models were employed in the analysis. Key words: Dynamic mechanical analysis, creep and recovery analysis, Bitumen-acacia. INTRODUCTION Bitumen is a low molecular weight hydrocarbon obtained by fractional distillation of crude oil and catalytic cracking of hydrocarbons []. It s a black, sticky thermoplastic hydrocarbon made of four main components - saturates, aromatics, resins and asphaltenes. Because of its good adhesive, impermeability and viscoelastic properties, It has wide range of applications such as used as a binder of aggregate in road construction, waterproofing agent, coating, insulation, to more specialized purposes such as when blended with proportion of polymers used in built up membranes for the roofing industry [2]. However, its performance when subjected to high traffic levels and rigorous climatic changes has been unsatisfactory. Road performances depend mainly on the rheological properties of the binder [4]. Different types of binder modifiers used to improve performance have led to environmental problems such as incineration and landfill. The modifiers used should have good mechanical properties such as high strength, good elasticity and be able to resist deformation and maintain desired properties of the binder during its applications. Viscoelastic properties of the modifiers are important in predicting road characteristics over wide range of temperatures and traffic loadings. Creep is an important characteristic of polymeric binders. Knowledge of the long term durability of composites structures is essential for assessing conditions for survival time under load. Creep in polymers is a complex phenomenon, which depends on both material properties and external parameters. It occurs because of a combination of the viscous flow components of viscoelastic deformation. The resulting creep strain increases non-linearly with time. Creep strain in polymers and polymeric composites depend on stress level and temperature [11].Creep is a time dependent deformation resulting from the application *Corresponding author: Mbithi, N. M., School of pure and applied sciences, Department of Physics, Kenyatta University, P. O. Box 43844, Nairobi- Kenya of a constant stress to a material at a constant temperature, as time progress, creep may exceed the structural limitation and rupture occurs. Creep analysis is essential if bitumen-acacia sap composites are to be shown to have long-term loading applications. This study was carried out with the objective of investigating the creep and recovery behavior of bitumen binders modified using different proportions of acacia sap. MATERIALS AND METHODS Materials The materials used in the study were bitumen and acacia sap. Bitumen, 8/1 penetration grade was obtained from Kenya oil Refineries companies in Mombasa, Kenya, with an asphaltene content of 2 wt.% and softening point of 2 o C. The acacia sap was obtained from Acacia trees in Taita-Taveta District. Sample Preparation Bitumen (solid) was put in a melting chamber with the opening closed by a stopper screw. The melting chamber was placed on a hot plate at 16 o C and maintained at that temperature for minutes to ensure that all the bitumen melted. Still molten, dry sap was added and the mixture mechanically stirred continuously with a screw for minutes to obtain a homogeneous composite. The mixing process was achieved by using the mixing screw that almost fitted to the inside walls of the chamber as shown Figure 2.1. During injection, the mixing screw and the stopper screw were carefully removed. A piston was used to quickly inject the mixture into the Petri dish as illustrated in Figure 2.2. The mixture was allowed to cool at room temperature. This procedure was repeated in preparing all the composites. However, bitumen-acacia sap composites samples which were highly viscous in such a way that they could not be injected were prepared by mechanical stirring. The corresponding polymer acacia sap weight percentage was added to bitumen and the mixing was maintained at 16 o C. After mixing, the resulting dispersion was poured into the Petri dish and

2 International Research in Arts and Sciences 3 then stored at room temperature to retain the morphology. The paste was scooped and stuck in the jaws of the shear sandwich clamp. It was then compressed by application of force on the screw producing circular sheets. The sheets were then cut into rectangular shapes of dimensions 1.mm x 1.mm x 2.mm. Figure 2.1: Heating process blend that is phase inversion occurs due to weakening of bitumen-acacia sap interface thus allowing deformation. This is attributed to chains untangling and slipping relative to one another which increase with temperature and time. From the Figure 3.1a, it is also evident that at 3 and 3 o C creep values for all composites are very small and almost overlapping. This is due to the fact that at higher temperature polymers become too viscous and flow like liquids. This shows that creep increases with increase with temperature until it reaches a threshold value where it no longer creeps. The creep behaviour of a material indicates its susceptibility to deformation under stress. Recovery is the ability of a material to return to its original state once the stress or stretching force acting on it has been removed. From the recovery data, the recovery percentage versus time was plotted as shown in Figure 3.1b. Creep Analysis Figure 2.2: Injection drawing process The creep strain behaviour, recovery and creep compliance as a function of time and temperature were measured by Dynamic mechanical analyzer DMA 298, using Shear sandwich clamp in the creep mode system. The DMA equipment was set in a temperature range of 2-37 o C and the samples were allowed to creep for 2 min, displace 12 min, and equilibrate 12 min and then released to unstressed state and allowed to recover for 2 min. All the creep tests were carried out by applying a constant shear stress level at the selected temperature of 2, 2, 3 and 3 o C. Stress of.87 MPa, static force of. and 18 N was applied and the corresponding strain recorded at an interval of 1 min for 3 hours. Measured data was analyzed using microcal card origin 8 software. RESULTS AND DISCUSSION Creep and recovery The viscoelastic creep and recovery data were obtained under constant stress. On the base of creep data the isothermal curves of the creep strain percentage versus time for each composite was obtained as shown in Figure 3.1. The creep data for bitumen at 2 o C are great, while at 2, 3 and 3 o C creep values are very small due to viscous flow as shown in Figure 3.1a. This is in agreement with results reported in literature for bitumen primary viscoelastic functions [1]. The creep values of the bitumen-acacia sap composites at 2 o C are small in comparison with pure bitumen. As the content of acacia sap increased in the composites the creep at 2 o C also decreased due to stiffening. This shows good creep response. It is also evident from the curves that creep values of the composites at 2 o C are greater than at 2 o C and increased with increase of acacia sap loading in the % creep strain B1AS BAS a) %creep strain % creep strain o C 3 o C C 2 C 3 C 3 C o C Figure 3.1a: Creep master curve at 2, 2, 3 & 3 o C at constant stress as a function of temperature for bitumenacacia sap composites; B1AS,, BAS &. Inset: magnified creep curves at 2, 3 & 3 o C % strain recovery B1AS BAS Figure 3.1b: Recovery master curves vs time for bitumenacacia sap composites at 2, 2, 3 and 3 o C b)

3 International Research in Arts and Sciences 31 creep modulud [MPa] creep modulus [MPa] 8 7 (a) B1AS (b) o C 2 o C 3 o C 3 o C 3 3 (c) BAS (d) Figure 3.2: Creep modulus versus time for bitumen-acacia sap for a) B1AS, b), c) BAS& d) respective fully a) B1AS BAS 2 O C c) 3 O C B1AS BAS b) 2 O C B1AS BAS d) 3 O C B1AS BAS Figure 3.3: Creep modulus master curves versus time for bitumen-acacia sap composites at a) 2 o C, b) 2 o C, c) 3 o C & d) 3 o C

4 International Research in Arts and Sciences 32 The recovery data for bitumen at 2 o C is small, while at 2, 3 and 3 o C the creep recovery is very high due to flow. The recovery values at 2 and 2 o C for bitumenacacia sap composites are negligible due to molecular reorientation and slippage of chains. The composites recovered fully at 3 and 3 o C due to less chain entanglement. The recovery increased with acacia sap loading in the composites. Acacia sap stiffens the bitumen matrix thus decreasing chain entanglement this results to increase in recovery. The hydroxyl groups in the bitumenacacia sap composites together with bitumen-acacia sap composites chains untangling and slipping relative to one another results in a continuous polymer network formation in the bitumen and imparts elastic behaviour to the composites. This elastic behaviour is reflected through the higher creep and recovery values of composites at all examined temperature in comparison with bitumen. This suggests that the network formation strongly depends on the bitumen-acacia sap ratio and their interactions. Creep modulus, E It s the ratio of stress to strain. The creep modulus is dependent on time and temperature. It decreases with increase of time and temperature. From the creep data the isothermal curves of the creep modulus, E, versus time was plotted as shown in Figures 3.2 (a) to Figure 3.2 (d). From these Figures, it can be seen that the creep modulus for all composites decreased with increase in temperature and time due to softening as a result of long periods of heating. From the master curves in Figures 3.3 (a) to (d), it can be seen that the creep modulus, E, for all composites increased with increase in acacia sap loading at the same temperature. This is because acacia sap stiffened the bitumen matrix. It can also be inferred from the Figures that the creep moduli obtained for all composites at 2 o C are greater than at 2, 3 and 3 o C. This shows that creep modulus decreases with increase in time and temperature. At low temperature (short times) polymers are stiff thus strain less, as temperature increases (longer times) polymers become soft and strains more. By selecting as the reference the curve for 2 o C, and then shifting all other isothermal curves of the creep modulus vs. time obtained at 2, 3 and 3 o C with respect to time 3 3 horizontally, the master curves of creep modulus vs. time at references temperature for all composites are regenerated as shown in Figure 3.4. The shift factor is a measure of how a material s frequency response changes with the variation of temperatures. From the curves it was deduced that at short time intervals the composites exhibited a relatively high creep modulus while at longer times viscous flow occurred and the composites exhibited a relatively low modulus. Creep modulus is very high for bitumen-acacia sap composites at short times because the acacia sap resists the slippage, re-orientation and motion of the polymer chain in the bitumen. As time increases the creep modulus decreases this is due to the fact that stress decreases the activation barrier for bond dissociation, thus allowing the molecular chains to move more easily. Hence under a constant stress polymers undergo molecular relaxation or rearrangements which become more pronounced with time and are faster at higher temperatures. From the shift master curves in Figure 3.4, it can be inferred that creep modulus for pure bitumen is lower than that of composites and is highest for blend with highest acacia sap content. This shows that creep modulus increases with increase in acacia sap loading which indicates the reinforcing effectiveness of acacia sap. The experimental data for shift factors, a T, were tested with Arrhenius and WLF [8] models. E a o exp kt, log a T C C 1 ( T To ) 2 T T o C 1 & C 2 are empirically determined constants, T is the selected temperature, in C or K, and T o is the reference temperature, in C or K. is relaxation time, o is the preexponential factor, Ea is the activation energy and k is the Boltzmann constant B1AS BAS creep modulus [MPa] BAS B1AS log(a T ) Temperature ( o C) Figure 3.4: Master curves of creep modulus vs. time for all bitumen-acacia sap blends Figure 3.: Temperature dependence of the shift factor for bitumen-acacia sap blends at The experimental data showed good agreement with the William Landel Ferry as shown in Figure 3.. It was

5 International Research in Arts and Sciences 33 inferred that the shift factor increased with increase in acacia sap being greatest in composites with 62.% acacia sap loading. The values of C 1 and C 2 were determined from WLF equation. Conclusions On the mechanical properties of the injected drawn composites of bitumen and acacia sap has been presented. From the analysis of the results, the following conclusion was made: The creep modulus of the bitumen-acacia sap composites was found to increase with increase of acacia sap loading resulting to decreased creep strain. This showed good creep response. The optimum acacia sap loading was found to be 2% based on the rheological properties. Acknowledgement We thank the laboratory technicians from the physics Department, Kenyatta University for providing materials and apparatus during experimental work. REFERENCES Table 3.1: Values of C 1 and C 2 Sample C 1 C 2 T o ( o C) B1AS BAS B37.A Carbognani, L., Hassan, A. and Pereiralmao, P. (21). Oxidation of oils and bitumen at various oxygen concentrations, Journal of energy fuel; 24: Fawcett A. H. and Lort, S. K. (23). Structure Formation in Polymeric Fibers, Journal of Polymer; 29: Fawcett, A. H., McNally, T., McNally, G.M., Andrew, S.F. and Clarke, J. (1999). Blends of Bitumen with various Polyolefins; Journal of Polymer; 41: Frederika, P. P., Donatella, D., Clara, S., Sossio, C. and Aneta V. (21). Viscoelastic properties and morphological characteristics of polymer-modified bitumen blends, Journal of Applied Polymer Science; 118: Saleh, M. F. (24). New Zealand Experience with Foam Bitumen Stabilization, Journal of Transport Research Board; 1868: Vasiljevic, A., Frederika, P. and Cimmino, S. (21). Viscoelastic properties and morphological characteristics of polymer modified bitumen blends. Journal of applied polymer science; 118: Vogel, A. (1921). Non-linear Dielectric responses of a model glass former under oscillating temperature. Journal of Physical society Japan; 22: Ward, I.M. and Hadley D.W. (1993.). An introduction to the mechanical properties of solid polymers; John Wiley and Sons Ltd, New York. 9. William, G. and Perkins N. (1999). Polymer toughness and impact resistance, Journal of polymer engineering and science; 39: Xiaohu, L. and Isacsson, U. (21). Modification of road bitumen with thermoplastic polymers, Journal of Polymer testing; 2: Houshyar, S., Shanks, R.A. and Hodzic, A. (2). Tensile creep behaviour of polypropylene fibre reinforced polypropylene composites, journal of polymer; 24: *******