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Stability investigation of self-healing microcapsules containing rejuvenator for bitumen
Polymer Degradation and Stability 98 (2013) 1205e1215
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Stability investigation of self-healing microcapsules containing rejuvenator for bitumen Jun-Feng Su a, b, *, Jian Qiu c, Erik Schlangen b a
Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China Department of Materials and Environment, Faculty of Civil Engineering & Geosciences, Delft University of Technology, Stevinweg 1, 2628CN Delft, The Netherlands c Department of Road and Railway Engineering, Faculty of Civil Engineering & Geosciences, Delft University of Technology, Stevinweg 1, 2628CN Delft, The Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2012 Received in revised form 8 March 2013 Accepted 12 March 2013 Available online 23 March 2013
Preservation and renovation bitumen of pavement is a big problem for the whole world. Traditionally, application rejuvenator is the only one method that can restore the original properties of the pavements. However, some puzzles still restrict its successful usage. Microencapsulation is a promising method to apply rejuvenator in bitumen. These microcapsules can break and leak the oily-liquid rejuvenator into microcracks and self-healing the aged bitumen. Based on our previous work, the objective of this study was to investigate the thermal stability, mechanical stability and interface stability of microcapsules in bitumen. The results showed that these microcapsules containing rejuvenator survived in melting bitumen and in a violent repeated temperature changes. Microcapsules had the elasticeplastic deformation ability resisting the temperature changes and mixing stress. Moreover, the chemical bonds improved the interface stability between shells and bitumen. Microcapsules containing rejuvenator will be a promising product to realize the smart pavements. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Microcapsules Stability Self-healing Bitumen Rejuvenator
1. Introduction The decline in the use of natural bitumen for road construction can be traced to the 1910s when the advent of vacuum distillation made it possible to obtain artificial bitumen from crude oil. Currently, 95% of the almost 100 million tons of bitumen produced worldwide each year is applied in the paving industry, where the bitumen essentially acts as a binder for mineral aggregates to form asphalt mixes [1]. Other uses of bitumen are as emulsions, waterproof materials, or formed materials, but these account for less than 5% of the total bitumen produced. As a widely applied material in pavements, bitumen must be sufficiently fluid at high temperature (around 160 C) to be workable and allow for homogenous coating of the aggregates upon mixing. Another important issue that has to be considered is the extent to which it ages from climate and traffic. After years of use the stiffness of asphalt concrete increases while its relaxation capacity decreases. This causes the binder to become more brittle, causing the development of microcracks and ultimately to cracking of the interface between the aggregates and
* Corresponding author. Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China. Tel./fax: þ86 22 26210595. E-mail addresses: [email protected], [email protected] (J.-F. Su). 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.03.008
binder [2]. This occurs mainly as a result of oxidation of the hydrocarbon compounds contained within the bitumen [3]. Bitumen binders are usually categorized into two subdivisions: solids called asphaltenes and liquids called maltenes. Maltenes can be further divided into polar aromatics, naphthalene aromatics, and saturates (paraffins) [4]. The main aging mechanism of bitumen is the loss of volatiles and oxidation, which leads to bitumen with higher viscosity (stiffer) [5]. In other words, the amount of solid component increases and that of the liquid component decreases, thus resulting in an increase in the rigidity of the pavement. The aging problem of bitumen leads to pavement failure, including surface raveling and reflective cracking. It therefore increases the cost of renovating and preserving bituminous pavements [6]. Several physical and chemical methods are currently employed for bitumen preservation, including the use of rejuvenator emulsions or fog seals, and through additive modification or thin overlay technologies [1,7]. However, of these methods only the first, i.e., the application of rejuvenators, can restore the original properties of the pavement [6]. The most important goal of using rejuvenator product is to restore the asphaltene/maltene ratio to its original balance [8]. Rejuvenating agents have the ability to reconstitute the binder’s chemical composition and they consist of lubricating and extender oils that contain a high proportion of maltene constituents [9]. Rejuvenator can soften the aged binder and provide comprehensive rejuvenation that replenishes the
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volatiles and dispersing oils while simultaneously promoting adhesion. However, for a rejuvenator to be successfully applied the difficulty in penetrating the pavement surface still remains a significant problem. Shen et al. [10] reported on the use of three rejuvenators and found that none could penetrate more than 2 cm into the asphalt concrete. Further issues encountered when applying these materials include the fact that road closures are necessary for some period of time after their application. The rejuvenator may also cause a high reduction in the surface friction of the pavement for vehicles. Moreover, these rejuvenators may also be harmful to the environment. To overcome these issues inspiration is provided by the concept of self-healing based on microcapsules. This particular approach involves incorporation of a microencapsulated healing agent and a dispersed catalyst within a polymer matrix [11e13]. Upon damageinduced cracking, the microcapsules are ruptured by the propagating crack fronts resulting in release of the healing agent into the cracks by capillary action [14]. The method of encapsulating rejuvenators inside the bitumen may be an alternative approach worthy of consideration. The application of microcapsules containing rejuvenator to bitumen derives from the success observed for some polymer self-healing materials [15e17]. García et al. [18] reported a method to prepare rejuvenator capsules by using an epoxy resin as a coating and porous sand as a skeleton. The advantages of these capsules include the fact that they are strong enough to resist the mixing process, the high temperature, and all the years in the road until they are required. However, these capsules have some limitations that restrict their application. The primary limitation is that it is hard for the rejuvenator to flow out from the porous sand when the shell is broken because the
rejuvenator has a high viscidity resulting from it consisting of lubricating and extender oils. The capillary action of the porous structure also limits the rejuvenator release. Another limitation is that the capsule size does not fit with the thickness of bitumen between aggregates. To realize the application of these promising chemical products, we have developed a novel method to fabricate microcapsules containing rejuvenator by in situ polymerization using methanol-melamine-formaldehyde (MMF) prepolymer as shells [19]. A two-step coacervation process, with the aid of styrene maleic anhydride (SMA) as a surfactant, was successfully applied to enhance the thermal stability and compactibility of the shells. It has been shown that this product is an environmentally friendly powder that encapsulates a suitably sized rejuvenator for chemical and construction engineering. To produce microcapsules containing rejuvenator by chemical means, the factors of cost, complexity, and capacity must be considered for the construction industry. These coreeshell microcapsule structures need to meet specific requirements in terms of size distribution, encapsulation ratio, and non-biodegradability because these factors will influence their service performance [20]. As bitumen acts as thin layers between aggregates that are usually less than 50 mm, the size of the microcapsules containing rejuvenators should be smaller than 50 mm to avoid being squeezed or pulverized during asphalt forming. Besides the complex fabrication process of microcapsules containing rejuvenator, their survival ability is another important issue that must be addressed. Fig. 1 illustrates the possible states of the microcapsules in bitumen material. First, for practical application the microcapsules must have high thermal stability and high mechanical strength to resist the melting temperature and mixing stress of the asphalt (Fig. 1aec). The microcapsules must maintain their shape and compatibility at
Fig. 1. Illustration of the possible states of microcapsules containing rejuvenator in bitumen.
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temperatures of 160e200 C during asphalt application. Further, breakage of the shell may occur at high temperatures owing to a mismatch in thermal expansion of the core and shell materials [21]. It is expected that the rejuvenator is protected, with fewer cracks and with lower permeability, during asphalt paving. However, microcapsule shells, such as inorganic or flexible organic shells, will not break when the mechanical strength applied is very high (Fig. 1d). The result is that microcracks may be triggered, leading to fracture without leakage of the rejuvenator. Consequently, the shells employed are more commonly polymeric materials that possess good thermal stability under high temperature and which exhibit an appropriate strength and toughness [22]. Second, interface stability and interfacial interactions are keys for all multicomponent materials, irrespective of the number or type of their components, or of their actual structure. Thus, understanding the relationship between the interfacial behaviors and mechanical properties of bitumen/ microcapsule composites is important for them to be useful under realistic engineering environments. In our previous report [23], it was found that microcracks and interface separation may occur for microcapsule/matrix composites when a repeated, vigorous thermal absorbing-releasing process is undertaken. During a process with repeated temperature changes via heat transmission, expansion and shrinkage of the microcapsules and polymer matrix will occur owing to the different expansion coefficients exhibited. The microcapsule volume can also be affected by the encapsulated rejuvenator upon environmental temperature changes. These phenomena will cause microcracks or fractures in the matrix during heat absorption or resealing, spoiling the thin microcapsule shells such that the encapsulated rejuvenator will lose the protection provided by the shells. Moreover, the mechanical integrity of these composites may decrease because of internal cracking or microcapsule rupture [24]. This is not expected to appear because it is well known that microcapsule rupture and release due to wear and tear can be avoided by proper design of their mechanical properties [25]. Theoretically, the mechanical strength of a microcapsule is determined by its size, and by the shell thickness and structure [26]. The mechanical properties of the shells play an important role in many processes, and therefore, understanding of these properties is desirable for optimizing their application. The theoretical tensile yield strength and ultimate tensile strength of the composites differ for the cases of adhesion and no adhesion between the filler particles and matrix. In the case of no adhesion between the microcapsules and bitumen, the interfacial layers are unable to transfer stress. We are in a position to commence exploring chemical methods capable of providing simple, cheap, robust, and environmentally friendly microcapsules containing rejuvenator for bitumen. In view of the above, the objective of this work was to fabricate microcapsules containing rejuvenator by in situ polymerization using MMFresin shells and to then investigate their stability in bitumen. Thermal stability and mechanical properties were measured to ensure that the microcapsules survived the bitumen during melting and mixing. In addition, interface stability was monitored to evaluate the interface bonding behaviors during the service life of the bitumen. 2. Experimental
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off-white, free flowing power with a faint, aromatic odor. The bitumen used in this study was 70/100 pen obtained from Kuwait Petroleum in a 4.5% by weight [20]. The material used as rejuvenator is dense, aromatic oil obtained from Petroplus Refining Antwerp (800DLA, Belgium). 2.2. Microcapsules synthesis The method of fabrication microcapsules containing rejuvenator by coacervation proceed can be divided into three steps [19]: (1) SMA (10.0 g) and NP-10 (0.2 g) were added to 100 ml water at 50 C and allowed mix for 2 h. Then a solution of NaOH (10%) was added dropwise adjusting its pH value to 10. The above surfactant solution and rejuvenator were emulsified mechanically under a vigorous stirring rate for 10 min using a high-speed disperse machine. (2) The encapsulation was carried out in a 500 ml three-neck roundbottomed flask equipped with a condensator and a tetrafluoroethylene mechanical stirrer. The above emulsion was transferred in the bottle, which was dipped in steady temperature flume (room temperature). Half of MMF prepolymer (16 g) was added dropwise with a stirring speed of 500 r min1. After 1 h, the temperature was increased to 60 C with a rate of 2 C min1. Then another half of prepolymer (16 g) was dropped in a bottle at the same dropping rate. (3) The temperature was increased to 75 C. After polymerization for 1 h, the temperature was decreased slowly at a rate of 2 C min1 to ambient temperature. At last, the resultant microcapsules were filtered and washed with pure water and dried in a vacuum oven. 2.3. Morphologies observations An optical microscope (BX41-12P02, OLYMPUS) was used to check the fabrication process of microcapsules in emulsion. About 1 ml of the colloidal solution was extracted and spread on a clean glass slide (1 3 cm). The dried microcapsules were adhered on a double-side adhesive tape without cracking the shells. The surface morphologies were observed by using an Environmental Scan Electron Microscopy (ESEM, Philips XL30) at an accelerated voltage of 20 kV. 2.4. Mean size and shell thickness of microcapsules For each microcapsule sample, the mean size is the average size value of fifty microcapsules measured from the SEM morphology image. About 2 g MMF-shell microcapsules was mixed in 5 g epoxy resin. After the composite was dried in room temperature, it was carefully cut to obtain the cross-section. The thickness of shells can be measured from the SEM images of cross-section of microcapsules [27]. At least 20 shells of each microcapsule sample were measured and the average data were calculated. 2.5. Thermogravimetric analysis (TGA) The thermal stability characterization of microcapsules was performed on a Dupont SDT-2960 Thermogravimetric analysis (TGA) at a scanning rate of 5 C min1 in a flow of 40 ml min1 nitrogen (N2).
2.1. Materials 2.6. States of microcapsules in bitumen The shell material was commercial prepolymer of melamineformaldehyde modified by methanol (solid content was 78.0%) purchased from Aonisite Chemical Trade Co., Ltd. (Tianjin, China). The rejuvenator was a commercial product. Styrene maleic anhydride (SMA) copolymer (ScripsetÒ 520, Hercules, USA) was applied as dispersant. A small percentage of the anhydride groups have been established with a low molecular weight alcohol and it is fine,
Various bitumen/microcapsule samples were prepared to investigate the thermal stability. A fluorescence microscope (CKX41F32FL, OLYMPUS) was applied to investigate the interface of materials using the light characters such as reflection, diffraction and refraction. A thermal absorbing-releasing process was performed to investigate the thermal stability of bitumen/microcapsule
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composites using a temperature-controlled chest. A similar method has been successfully applied to investigate the thermal stability of microcapsules containing phase change materials in epoxy [28]. The samples (2 1 0.5 cm) were heated to 50 C with the rate of 2 C min1 and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1. This thermal treatment process for each sample was repeated for 20, 40, and 60 times. The interface morphologies of the composites were observed by a fluorescence microscope. The bitumen/microcapsule samples were adhered on a glass sheet. It was carefully poured with a drop of liquid nitrogen at one end. Microcracks were quickly generated in this sample because of the low temperature brittleness. The state of microcapsules in bitumen could be observed by the fluorescence microscope. 2.7. Mechanical properties measurement The mechanical properties of the MMF-shell microcapsules were tested according to our reported two-plate micromanipulation method [25]. Single microcapsule was adhered on a glass sheet (500 500 mm) and pressed by another glass sheet. A pressure sensor under the bottom glass measured the intensity and data were directly recorded and yield stress was calculated automatically by a computer from the forceedisplacement curves. 2.8. Fourier transform infrared spectroscopy (FT-IR) The chemical structures were analyzed by a Perkin ElmerÒ Spectrum 100. FT-IR spectra in absorption modes were recorded among the range of 400e4000 cm1. 3. Results and discussion 3.1. Morphologies and physical structure of microcapsules The fabrication process of microcapsules using melamineformaldehyde (MF) and urea-formaldehyde (UF) resins as shell
materials is usually defined as an in situ polymerization method. Emulsifying agent is used to absorb the shell materials on core droplets. The direct polymerization of a single monomer or prepolymer is carried out on the core-particle surface. It has been proved that the MMF can be successfully applied to fabricate microcapsules using the in situ polymerization method [25]. The shell thickness, surface morphology and average size of microcapsules can be controlled by regulating the core/shell ratio, prepolymer adding speed and emulsion stirring rate. The core/shell ratio means the weight ratio of core material and shell material in original fabrication [19]. Fig. 2(a) shows the optical morphologies of microcapsules in emulsion fabricated by emulsifying rejuvenator with a stirring rate of 4000 r min1. Being encapsulated by shell material, the rejuvenator droplets are ultimately separated through the regulation of hydrolyzed SMA molecules. These microcapsules have regular globe shape with smooth surface. Fig. 2(b,c) and (d) shows the SEM surface morphologies of dried microcapsules with core/ shell ratio of 1/1 fabricated by 4000 and 2000 r min1 emulsion stirring rates. Their mean sizes are about 10 and 20 mm. The dried microcapsules still keep the regular global shape. There is no adhesion and impurity substance between microcapsules. The shells are compact without holes and cracks. To determine the shell thickness, microcapsules were embedded in epoxy resin as shown in Fig. 2(e,f). Microcapsules were uniformly dispersed in epoxy. The cross-section SEM morphologies of a typical single microcapsule are presented in Fig. 2(g). The shell thickness was measured directly. It must be noted that the shell may not be cut across its equator, so the thickness is an average data of at least 10 shells for each microcapsule sample. Fig. 3(a) shows the mean sizes of microcapsules (core/shell ratios of 1/1, 1/2 and 1/3) under various emulsion stirring rates in range of 1000e8000 r min1. With the increasing of stirring rates, the size of these microcapsule samples decreased sharply from 100.5 to 2.0 mm. The reason is that higher stirring rates will disperse the core material into smaller droplets. A similar conclusion has been reported by other researches [29,30]. The spread of the microsphere size distribution was found to decrease with stirring
Fig. 2. Morphologies of microcapsules containing rejuvenator, (a) optical morphology of microcapsules in emulsion, (b) SEM morphology of dried microcapsules, (c) smooth surface of single microcapsule, (d) SEM morphology of dried microcapsules with larger size, (e, f) optical morphology of microcapsules embedded in epoxy resin, and (g) cross-section SEM morphology of microcapsules.
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Fig. 3. Physical properties of microcapsules controlled by core material stirring rates and core/shell ratios, (a) mean size and (b) shell thickness.
speed. With the validation of the mathematical correlation, it is possible to have a good estimate of the average microsphere size prior to microsphere preparation. Moreover, less core/shell ratio leads to a higher shell thickness value. This result accords with reported results and indicates that the size is mainly determined by emulsion stirring rates [29]. In addition, it can be concluded that the MMF prepolymer had cross-linked with a compact structure forming thin shells. Fig. 3(b) shows the data of shell thickness of microcapsules (core/shell ratios of 1/1, 1/2 and 1/3) under core material stirring rates of 2000, 4000, 6000 and 8000 r min1, respectively. Microcapsule samples have the shell thickness from 4.51 0.50 to 0.55 0.12 mm. More shell material leads the shell to a thicker structure for each sample fabricated under the same core material stirring rate. 3.2. Thermal stability of microcapsules Microcapsules containing rejuvenator are expected to keep their shells intact, resisting the high temperatures of the melting bitumen. In the general encapsulation process, the core material is emulsified to form droplets, which are then covered by shell material to obtain the microcapsules. The surface morphology is controlled by the operating conditions. Fan and Zhou [31] reported that the initial pH value, the concentrations of the shell material and surfactant, and the stirring rate used during the stage of microencapsulation are all factors that can influence the surface morphology of the microcapsules obtained by in situ polymerization. In our previous study we also found that the surface morphology greatly affects the thermal stability of microcapsules containing rejuvenator [19]. A lower prepolymer concentration can help to improve the degree of surface smoothness and compactibility of the shells. It is worth noting that the smooth and nonporous shells are believed to result from the deposition of low molecular weight prepolymer at the oilewater interface, while the prepolymer remains soluble. The core/shell ratio is considered one of the main factors to affect the surface morphology. TGA has been widely applied to investigate the encapsulation effect and shell compactness of microcapsules [32]. We have previously reported that the decomposition temperature of microcapsules is higher than the bitumen melting temperature of 180 C. This indicates that the cured MMF resin will not thermally decompose upon mixing with the melting bitumen. On the other hand, there remains a lack of understanding on the thermal stability of microcapsules under extreme temperature changes. The reason for this is that the polymeric shells exhibit a tendency to craze and fracture under forceful thermal transmission [24]. Fig. 4 shows the TGA curves for microcapsules containing rejuvenator with a core/shell ratio of 1/3 under various
temperatures (260 C, 240 C, 220 C, and 200 C) for 1 h. It was found that after heating for 1 h to a temperature of 260 C the microcapsules showed a decomposition temperature from about 150 C. The pure rejuvenator applied in this study showed a marked weight loss between the temperature range 337e469 C owing to the evaporation of the oil substance at high temperature [19]. Therefore, we can confirm that the weight loss of the microcapsules over the temperature range of 150e337 C is a result of the weight loss of the shells. Comparatively, after heating for 1 h to temperatures of 240 C, 220 C, or 200 C, the microcapsules showed decomposition temperatures of 200 C, 240 C, and 290 C, respectively. Heating temperatures have been shown to greatly influence the original decomposition temperatures of microcapsules. Higher temperatures lower the temperature at which the commencement of degradation occurs for microcapsules. This phenomenon can be attributed to two factors. The first is that the shells had broken under the heating temperatures over an hour. Second, the reason may be that the shells did not retain their compact shapes. With an increase in temperature more defects on the shells may occur. These defects will lead the microcapsules to more rapidly decompose. In addition to the above, the initial decomposition temperatures of the microcapsules remain higher than the bitumen melting temperature. This indicates that the microcapsules can survive when in melting bitumen. Fig. 5(aef) shows the SEM morphologies of microcapsules after they have been heated for 1 h to 180 C, 200 C, 220 C, 240 C,
Fig. 4. TGA curves of microcapsules containing rejuvenator with core/shell ratio of 1/3 under various temperatures for 1 h, (a) 260 C, (b) 240 C, (c) 220 C, and (d) 200 C.
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260 C, and 280 C, respectively. It can be seen that the microcapsules retain their compact shell structures for temperatures of 180 C, 200 C, and 220 C for 1 h, as shown in Fig. 5(aec). Under 180 C and 200 C, the microcapsules closely maintain the shape and smooth surfaces of the original states. Although the thermal effect did not break the shells under 220 C, the shells stuck close together because of the softening of the shells. From Fig. 5(d) and (e) it can be seen that the shells contain holes, cracks, or even breaks when the temperature is increased to 240 C and 260 C. The shells are almost entirely broken under 280 C after the hour of heating, as shown in Fig. 5(f). Under these circumstances, the encapsulated rejuvenator leaked out without the protection of the shells. 3.3. Thermal stability of microcapsules in bitumen Microcapsules containing rejuvenator are homogeneously distributed in bitumen after hot-mixing. In addition to the thermal stability of the microcapsules, we also investigated the thermal stability of the microcapsules in bitumen. The mechanical properties of the shells can be enhanced by modifying their chemical structure and by controlling the synthesis conditions [33]. However, until now little has been known about shell stability in bitumen during a repeated thermal transmission process. To investigate the shell stability of microcapsules in bitumen we designed a thermal transmission process to simulate a practical application environment. To verify the thermal stability of microcapsules in bitumen a microcapsule sample was mixed (2.0 wt.%) with melting bitumen under temperatures of 180 C and 200 C for 10 min. The microcapsule sample had the same core/shell ratio (1/1) with a mean size about 15 mm. The frozen bitumen samples were broken apart and the interface morphologies were observed. It can be seen in Fig. 6(a) and (b) that the microcapsules survived in the bitumen under a temperature of 200 C. The microcapsules retained their global shape with no cracks or thermal decomposition. These results indicate that these microcapsules can resist the thermal effects of asphalt for common applications. An alternative thermal process was designed to test the thermal stability of the microcapsules containing rejuvenator in bitumen under an extreme condition. The bitumen/microcapsule samples were repeatedly treated 20, 40, and 60 times with a thermal absorbing-releasing process. For each condition the sample was heated to 50 C at a rate of 2 C min1, with this temperature maintained for 10 min before being reduced to 10 C at a rate of 2 C min1. Fig. 7(a) shows the original state morphologies and fluorescence properties of the microcapsules (2.0 wt.%) dispersed
in bitumen. The amount of rejuvenator retained within the microcapsule compared to the initial amount is indicative of the encapsulation efficiency (E) of the microcapsule fabrication process. We found that the highest E (70%) was obtained under the formation conditions of a core/shell ratio of 1/3, a stirring speed of 3000 r min1, and 2.0e2.5% of SMA [19]. All residues on the shells cannot be removed by water because the rejuvenator cannot be totally encapsulated by MMF shells. Bitumen is stained blue, while the rejuvenator is green. It is clear from our results that the microcapsules have been homogeneously dispersed in the bitumen. Fig. 7(b,c) shows the microcapsules’ states in bitumen after repeated thermal treatment of 20 and 40 times. It is found that the area of blue points had grown larger in comparison to the original state, which indicates that the rejuvenator had permeated out of the shells. It is well known that the release behavior of microcapsules largely depends on the polymer structure of the shell and on the temperature, which in turn is influenced by the preparation conditions employed. As the microcapsules possess the same mean size and core/shell ratio in all bitumen samples studied herein, the temperature change is the only factor influencing the permeability of the microcapsules. The lower the temperature the more time required for the core material to penetrate the shell. Rejuvenator is an oily mixture with high viscosity; therefore, heating effects can accelerate its molecular velocity through the shells. However, most of the rejuvenator is retained in the shells under such a thermal treatment process. This means that the rejuvenator can be protected by the shells for a longer life in practical applications. In Fig. 7(d) it can be seen that some microcapsules had broken after the treatment was repeated 60 times, as evidenced by the rejuvenator having flowed out of the shells. In this instance the rejuvenator permeated the bitumen as small molecules. 3.4. Deformation behaviors of microcapsules in bitumen Another important issue to be considered for microcapsules containing rejuvenator when applied in bitumen is their mechanical properties, including their deformation behavior. In some cases breakage of the microcapsules must be triggered by interior microcracks or by exterior pressure. In other cases, the release or leakage of rejuvenator in bitumen is undesirable. Microcapsules must be sufficiently strong to remain intact during manufacture and further processing, such as during drying, pumping, and mixing [23]. Mechanical properties are, therefore, fundamentally important because they determine the microcapsule’s stability and integrity. It is well known that microcapsule rupture and release due to wear and tear can be avoided by proper design of their
Fig. 5. SEM morphologies of microcapsules after heated to (a) 180 C, (b) 200 C, (c) 220 C, (d) 240 C, (e) 260 C and (f) 280 C.
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Fig. 6. SEM morphologies of microcapsules in bitumen (2.0 wt.%).
mechanical properties [34]. It has been reported that some microcapsules with polymeric shells may act as viscoelastic particles (depending on their size) [35]. Several methods have been reported to measure the mechanical properties of a single microcapsule or cell [29,30]. Because of their small structure, there has been little focus on the relationship between their deformation and shell structure (size and thickness). As a soft and microscale particle, an individual microcapsule’s mechanical properties are difficult to be quantitatively characterized. There is therefore a requirement for a system capable of accurately measuring low-magnitude forces and microscopic material deformation [36]. In this study, a similar method was also applied to investigate the mechanical properties of microcapsules using two-plate micromanipulation [20]. Fig. 8(a) shows a typical loadedisplacement curve for a single microcapsule, where it increases linearly upon loading until remaining constant, then decreases sharply upon unloading. The microcapsule sample was fabricated under a core material dispersal rate of 3000 r min1 with a core/shell ratio of 1/1. The shape of the curve illustrates both elastic (recovery) and plastic (inelastic) deformation behaviors. During unloading, the curve represents elastic behavior with elastic displacement being recovered. This means that a single microcapsule has an ability to recover its original shape. The loadingeunloading curves exhibit significant hysteresis under a large deformation. During the unloading process, the force decreases to zero before the displacement recovers to zero. This means that the microcapsules had a plastic deformation. The ‘Y’ point represents the ‘yield point’ on the concavee convex curve. In theory, creep in polymer materials under a constant load is controlled by the free volume available for the polymer chains and molecular units to move. Thinner shells may provide the cross-linked MMF polymer with more volume to perform molecular chain adjustments. Moreover, a higher proportion of shell material leads the shells to a greater hardness, which means that the thicker shell has an increased ability to resist deformation [27]. However, in this study we pay more interest to the plastic deformation behavior because it determines the rupture of microcapsules. Interestingly, it has also previously been reported that there
is a dependence of the failure strength on microcapsule diameter [37]. The ‘yield point’ was selected as a parameter by which to evaluate the plastic deformation. Fig. 8(b,c) shows the optical morphologies of a microcapsule with a permanent deformation with respect to its yield point. Prior to the yield point, a single microcapsule will deform elastically before returning to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation may become permanent and nonreversible. It should be mentioned that the core materials and environmental temperature will, at the same time, affect the yield points, but these have not been considered in the present study. Because the core and shell materials have different coefficients of expansion, changes in temperature will result in an extrusion force. Fig. 8(d) shows the ‘yield stress’ of MMF-shell microcapsules (under 23 C) with various mean sizes and shell thicknesses, illustrating their plastic deformation behavior. The yield stress of a microcapsule with a core/shell ratio of 1/3 is in the range of 1.20e0.70 MPa, whereas the yield stress of a microcapsule with a core/shell ratio of 1/1 is in the range of 0.75e0.52 MPa. More shell material can provide a shell with greater thickness, thus leading to a higher stiffness of elastic microcapsule. This trend is similar to the results provided by nanoindentation [27]. Fig. 9(a,b) shows the SEM morphologies of microcapsules in bitumen with plastic deformation. Some microcapsules have a large plastic deformation without evidence of any breaks. This deformation may result from a combined effect of the applied force and heat. It can therefore be concluded that the micromechanical properties of microcapsules depend on the thickness and microstructure of the shells. During compression, the volume and Poisson’s ratio of polymer microcapsules may change continuously with the deformation because of their spherical geometry. 3.5. Interface stability of bitumen/microcapsule samples The above results have proved that the MMF-shell microcapsules containing rejuvenator have satisfactory thermal stability and elasticeplastic deformation properties. It can be imaged that these
Fig. 7. Fluorescence microscope morphology of microcapsules (2.0 wt.%) dispersed in bitumen, (a) original state, (bed) states after a thermal treatment process for each sample repeated for 20, 40, and 60 times, each time the sample was heated to 50 C with the rate of 2 C min1and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1.
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Fig. 8. Plastic deformation behaviors of microcapsules containing rejuvenator, (a) a typical loadedisplacement curve of single microcapsule, (b,c) optical morphologies of a microcapsule with a permanent deformation over the its yield point, and (c) yield stress of microcapsules with various mean sizes and shell thicknesses.
microcapsules will survive during mixing with bitumen and aggregates. However, it is reported that microcracks and interface separation may appear in a repeated vigorous thermal absorbingreleasing process for microcapsule/matrix composites [24,38]. During a repeated temperature changes process with heat transmission, expansion and shrinkage of the microcapsules and bitumen will occur due to the different expansion coefficients. Microcapsules volume can be affected by the encapsulated rejuvenator when the environmental temperature changes. These phenomena may cause microcracks or fractures in the bitumen during heat absorbing or resealing, spoiling the thin microcapsule shell, thus the encapsulated rejuvenator will lose the shells protection. Theoretical tensile yield strength and ultimate tensile strength of the composites are different for the cases of adhesion and no adhesion between the filler particles and matrix. In the case of no adhesion, the interfacial layer cannot transfer stress. The main causes of internal thermal stress failure of composites are considered to be the residual stresses due to expansion and shrinkage in the thermal transmission process and mismatch of molecule movement among the components. Therefore, it is vital to investigate the bonding stability between bitumen and microcapsules. A thermal absorbing-releasing process was applied in this study to enlarge the interface change degree and interphase morphologies were observed to give a better explanation of the interface behaviors. A thermal absorbing-releasing process was applied in this study to enlarge the interface change degrees and interphase morphologies were analyzed to give a better explanation of the interface behaviors. The thermal treatment process bitumen sample repeated for 30 times, each time the sample was heated to 50 C
with the rate of 2 C min1 and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1. Fig. 10(a,b) shows the SEM interface morphologies of a bitumen/ microcapsule sample before and after the thermal treatment process. To observe the interface easily, the bitumen were frozen and broken. It can be seen that the microcapsules keep compact structure and global shape in both states. There is no interface debonding emerged between microcapsules and bitumen after a repeated temperature change. Both composites have a compact interphase. It means that the temperature changes may not affect the original adhesive state between shells and bitumen. It is well known that strength of composites strongly depends on the stress transfer between the particles and the matrix. For these wellbonded microcapsules, the applied stress can be effectively transferred to the particles from the bitumen, which will clearly improve the strength. An interphase region is comprised of polymer molecules that are bound at the filled particles surface, and they exhibit unique physical and chemical properties. Polymer molecules within the interphase region are restricted from chain management, molecular cross-linking and dipole polarization, thus they lose one or more degrees of molecular rotational and/or vibrational freedom. The molecule at the interface had modified the agglutination structure, and had moved to fit the violent thermal transmission. From the SEM morphologies in Fig. 10, it can be deduced that the original close interface could resist this molecule modification and the mismatch in expansion coefficients between the shell and bitumen. More heat-transmission cycles may generate destruction in the interface structure. If the interface is only molecular entanglements without chemical bonds, the molecule connecting the shell and
Fig. 9. Deformation morphologies of microcapsules containing rejuvenator in bitumen.
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Fig. 10. SEM interface morphology of bitumen/microcapsules sample, (a) original state and (b) states after a thermal treatment process for each sample repeated for 30 times, each time the sample was heated to 50 C with the rate of 2 C min1and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1.
matrix will undergo structure modification losing its adhesive function under long-time repeated thermal-stress effects. Therefore, it is necessary to investigate the chemical bonding within the interphase region through molecular structure analysis. Fig. 11 illustrates the potential chemical reactions between MMF resin and bitumen. In Fig. 11(a,b), further chemical cross-linking reaction will occur between the methanol-modified MF prepolymer molecular chains. Because this step is complex, the synthetic products are difficult to be controlled. At the end of the chains, hydroxy (eOH) will not be completely consumed. The complexity of bitumen chemistry lies in the fact that many different chemicals are present. It has been reported [2] that bitumen has three typical functional groups (phenolic, anhydride and carboxylic acid) as shown in Fig. 11(cee). They all may have the dehydration reactions or
esterification reactions with MMF resin. In Fig. 11(f), these reactions can form chemical bonds between shells and bitumen, which will enhance the interface stability under heat and stress effects. In addition, these bonds help microcracks to propagate avoiding to be stopped by interface separation or interphase gaps. Microcracks will fairly penetrate the interphase and shells and rejuvenator will outflow the shells. FT-IR was applied to prove the above interface structures. We do not investigate its in-situ molecular structure as the interphase is too very thin. Fig. 12 shows the FT-IR curves of cross-linked MMF, bitumen and their mixture, respectively. Curve (a) is the FT-IR spectrum of the MMF prepolymer. Curve (b) is the FT-IR spectrum of the cross-linked MMF resin. It can determined that the bands at 1598, 1012 and 819 cm1 are C]C stretch, CeO stretch and
Fig. 11. Chemical sketches of interface bonding structures between MMF-resin shell and bitumen, (a, b) MMF crosslinking structure, (cee) there typical functional groups (phenolic, anhydride and carboxylic acid) in bitumen, and (f) the possible dehydration reactions or esterification reactions between the MMF-shell and bitumen.
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and have reasonable absorbance. The chemical structure of cured MMF prepolymer has a wide absorption peak at approximately 3329 cm1attributing to the superposition of NeH stretching vibration. The peaks at 1551 and 815 cm1 are assigned to the vibrations of triazine ring; and the corresponding peaks of cured MMF lie at 1559 and 815 cm1. Curve (c) shows the FT-IR spectrum of the sample of cross-linked MMF/bitumen (98/2, w/w) composite. The peaks at 1071e990 cm1 are assigned to the CeN stretch (aryl). The peaks at 1680e1630 are assigned to the C]O stretch (amides). All these evidences indicate that the MMF resin and bitumen may have chemical reaction forming bonds in interphase. 3.6. Microcapsule states in bitumen with cracks
Fig. 12. FT-IR curves of (a) crosslinked MMF-resin, (b) bitumen, and (c) crosslinked MMF-resin and bitumen mixtures.
CeH bend, respectively. The strong peaks at 2948 and 2850 cm1 are alkyl CeH stretch and aliphatic CeH stretch. All these bands can be used to characterize the existing states of inside or outside of microcapsules, since they do not overlap with other spectral bands
Besides the stability of microcapsules in bitumen, we also need to prove that these microcapsules can break by microcracks. Fig. 13 shows the microcapsule states in bitumen with cracks. As shown in Fig. 13(a), microcapsules are keeping compact structure in bitumen without defects and microcapsule has the mean size about 10 mm. In Fig. 13(b,c), microcracks were generated by liquid nitrogen quickly with a width of 10e20 mm. With the cracks propagation, the shell had been split by the tip stress of cracks (Fig. 13(d)). Interestingly, it was found that the rejuvenators had rapidly filled the cracks under the capillary action (Fig. 13(e)). Then the rejuvenator had permeated through both sides of cracks and the cracks were healed (Fig. 13(f)). Future work will be carried out about the selfhealing behaviors with more details.
Fig. 13. Morphology of bitumen/microcapsules sample treated by liquid nitrogen, (a) cracks appeared in bitumen sample by the low temperature brittleness treatment of liquid nitrogen, (b) original fluorescence microscope morphology of microcapsules in bitumen with cracks, (c, d) fluorescence microscope morphology of microcapsules in bitumen with cracks after 2 h.
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4. Conclusions In this paper, MMF resin was successfully applied to encapsulated rejuvenator. These microcapsules were the first product applied for bitumen self-healing. These microcapsules can be designed with various mean sizes and shell thicknesses. It is necessary to instigate the thermal stability, mechanical stability and interface stability of microcapsules in bitumen. Under 180 C and 200 C, the microcapsules still maintained the globe shape and smooth surface as original state. Microcapsules had survived in bitumen under temperature of 200 C, which indicates that these microcapsules can resist the thermal effect of bitumen in application. Microcapsules had the elasticeplastic deformation ability resisting the temperature changes and mixing stress. Moreover, microcapsules had a large plastic deformation without breaks under a combined effect of the force and heat. The chemical bonds improved the interface stability between shells and bitumen. With the microcracks propagation, the shell had been split by the tip stress of cracks and the rejuvenators had rapidly filled the cracks under the capillary action. Later, the rejuvenator had permeated through both sides of cracks and the cracks were healed. Microcapsules containing rejuvenator will be a promising product to realize the smart pavements. In the second phase of this work, further investigations will be carried out concerning the rejuvenator permeation rate, self-healing effective and mechanical properties recovery. Acknowledgments The authors acknowledge the financial support from the Delft Centre for Materials (DCMat) in the form of Project IOP Self-Healing Materials SHM1036, “Encapsulated rejuvenator for asphalt”. Dr. Jun-Feng Su also thanks the previous financial support of the National Natural Science Foundation of China (No. 50803045). References [1] Lesueur D. The colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen modification. Adv Colloid Interf Sci 2009;145:42e82. [2] Branthaver JF, Petersen JC, Robertson RE, Duvall JJ, Kim SS, Harnsberger PM, et al. Binder characterization and evaluation. In Chemistry, vol. 2. Washington, DC: National Research Council; 1993. SHRP-A-368. [3] Hunter RN. Bituminous mixtures in road construction. London: Thomas Telford; 1997. [4] Corbett LW. Composition of asphalt based on generic fractionation using solvent deasphaltening, elutioneadsorption chromatography and densimetric characterization. Anal Chem 1969;41(4):576e9. [5] Lu X, Isacsson U. Chemical and rheological evaluation of ageing properties of SBS polymer modified bitumens. Fuel 1998;77(9e10):961e72. [6] Zargar M, Ahmadinia E, Asli H, Karim MR. Investigation of the possibility of using waste cooking oil as a rejuvenating agent for aged bitumen. J Hazard Mater 2012;233e234:254e8. [7] Boyer RE. Asphalt rejuvenators: “fact, or fable”. In: Transportation systems 2000 (TS2K) workshop, San Antonio, Texas. [8] Karlsson R, Isacsson U. Material-related aspects of asphalt recycling-state of the art. J Mater Civil Eng 2006;18(1):81e92. [9] Chiu CT, Lee MG. Effectiveness of seal rejuvenators for bituminous pavement surfaces. J Test Eval 2006;34(5):390e4. [10] Shen J, Amirkhanian S, Miller JA. Effects of rejuvenating agents on superpave mixtures containing reclaimed asphalt pavement. J Mater Civil Eng 2007;19(5):376e84. [11] Brown EN, Kessler MR, Sottos NR, White SR. In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene. J Microencapsul 2003;20:719e30.
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[12] Cho SH, White SR, Braun PV. Room-temperature polydimethylsiloxane-based self-healing polymers. Chem Mater 2012;24:4209e14. [13] Li H, Wang R, Liu W. Preparation and self-healing performance of epoxy composites with microcapsules and tungsten (VI) chloride catalyst. J Reinf Plast Comp 2012;31:924e32. [14] Toohey KS, Sottos NR, White SR. Characterization of microvascular-based selfhealing coatings. Exp Mech 2009;49:707e17. [15] Tong XM, Zhang M, Wang MS, Fu Y. Effects of surface modification of selfhealing poly(melamine-urea-formaldehyde) microcapsules on the properties of unsaturated polyester composites. J Appl Polym Sci 2013;127:3954e61. [16] Hansen CJ, White SR, Sottos NR, Lewis JA. Accelerated self-healing via ternary interpenetrating microvascular networks. Adv Funct Mater 2011;21(22): 4320e6. [17] Lee J, Zhang M, Bhattacharyya D, Yuan YC, Jayaraman K, Mai YW. Micromechanical behavior of self-healing epoxy and hardener-loaded microcapsules by nanoindentation. Mater Lett 2012;76:62e5. [18] García A, Schlangen E, van de Ven M, Sierra-Beltrán G. Preparation of capsules containing rejuvenators for their use in asphalt concrete. J Hazard Mater 2010;184(1e3):603e11. [19] Su JF, Schlangen E. Synthesis and physicochemical properties of novel high compact microcapsules containing rejuvenator applied in asphalt. Chem Eng J 2012;198e199:289e300. [20] Su JF, Wang LX, Ren L. Fabrication and thermal properties of microPCMs: used melamine formaldehyde resin as shell material. J Appl Polym Sci 2006;101: 1522e8. [21] Wang XY, Su JF, Wang SB, Zhao YH. The effect of interface behaviors on the mechanical properties of microPCMs/epoxy composites. Polym Compos 2011;32(9):1439e50. [22] Pan X, York D, Preece JA, Zhang Z. Size and strength distributions of melamine-formaldehyde microcapsules prepared by membrane emulsification. Powder Technol 2012;227:43e50. [23] Su JF, Wang XY, Dong H. Interface stability of microencapsulated-paraffin filled epoxy composites: effect of methylation reaction on melamineformaldehyde shell material. Compos Interf 2011;18(8):645e59. [24] Su JF, Zhao YH, Wang XY, Dong H, Wang SB. Effect of interface debonding on the thermal conductivity of microencapsulated-paraffin filled epoxyematrix composites. Compos Pt A-Appl Sci Manuf 2012;43(3):325e32. [25] Su JF, Wang SB, Zhang YY, Huang Z. Physicochemical properties and mechanical characters of methanol-modified melamine-formaldehyde (MMF) shell microPCMs containing paraffin. Colloid Polym Sci 2011;289(2):111e9. [26] Hu JF, Chen HQ, Zhang ZB. Mechanical properties of melamine formaldehyde microcapsules for self-healing materials. Mater Chem Phys 2009;118(1):63e70. [27] Su JF, Wang XY, Dong H. Micromechanical properties of melamineformaldehyde microcapsules by nanoindentation: effect of size and shell thickness. Mater Lett 2012;89:1e4. [28] Su JF, Wang SB, Zhou JW, Huang Z, Zhao YH, Yuan XY, et al. Fabrication and interfacial morphologies of microPCMs/epoxy composites. Colloid Polym Sci 2011;289(2):169e77. [29] Su JF, Schlangen E, Qiu J. Design and construction of microcapsules containing rejuvenator for asphalt. Powder Technol 2013;235:563e71. [30] Zhu K, Qi H, Wang S, Zhou J, Zhao Y, Su J, et al. Preparation and characterization of melamine-formaldehyde resin micro- and nanocapsules filled with n-dodecane. J Macromol Sci Phys 2012;51:1976e90. [31] Fan CJ, Zhou XD. Influence of operating conditions on the surface morphology of microcapsules prepared by in situ polymerization. Colloid Surf A e Physicochem Eng Asp 2010;363(1e3):49e55. [32] Zhang XX, Tao XM, Yick KL, Fan YF. Expansion space and thermal stability of microencapsulated n-octadecane. J Appl Polym Sci 2005;97(1):390e6. [33] Mercadé-Prieto R, Nguyen B, Allen R, York D, Preece JA, Goodwin TE, et al. Determination of the elastic properties of single microcapsules using micromanipulation and finite element modeling. Chem Eng Sci 2011;66(10): 2042e9. [34] Koleva I, Rehage H. Deformation and orientation dynamics of polysiloxane microcapsules in linear shear flow. Soft Matter 2012;8(13):3681e93. [35] Yan Y, Zhang Z, Stokes Jason R, Zhou QZ, Ma GH, Adams MJ. Mechanical characterization of agarose micro-particles with a narrow size distribution. Powder Technol 2009;192(1):122e30. [36] Meier M, John E, Wieckhusen D, Wirth W, Peukert W. Influence of mechanical properties on impact fracture: prediction of the milling behaviour of pharmaceutical powders by nanoindentation. Powder Technol 2009;188(3): 301e13. [37] Liu KK, Williams DR, Briscoe BJ. Compressive deformation of a single microcapsule. Phys Rev E 1996;54(6):6673e80. [38] Wu Y, Huang L, Liu Z, Yang Z. Facile preparation of curved polymer composite nanosheets. Polymer 2012;51:3075e82.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Stability investigation of self-healing microcapsules containing rejuvenator for bitumen Jun-Feng Su a, b, *, Jian Qiu c, Erik Schlangen b a
Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China Department of Materials and Environment, Faculty of Civil Engineering & Geosciences, Delft University of Technology, Stevinweg 1, 2628CN Delft, The Netherlands c Department of Road and Railway Engineering, Faculty of Civil Engineering & Geosciences, Delft University of Technology, Stevinweg 1, 2628CN Delft, The Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2012 Received in revised form 8 March 2013 Accepted 12 March 2013 Available online 23 March 2013
Preservation and renovation bitumen of pavement is a big problem for the whole world. Traditionally, application rejuvenator is the only one method that can restore the original properties of the pavements. However, some puzzles still restrict its successful usage. Microencapsulation is a promising method to apply rejuvenator in bitumen. These microcapsules can break and leak the oily-liquid rejuvenator into microcracks and self-healing the aged bitumen. Based on our previous work, the objective of this study was to investigate the thermal stability, mechanical stability and interface stability of microcapsules in bitumen. The results showed that these microcapsules containing rejuvenator survived in melting bitumen and in a violent repeated temperature changes. Microcapsules had the elasticeplastic deformation ability resisting the temperature changes and mixing stress. Moreover, the chemical bonds improved the interface stability between shells and bitumen. Microcapsules containing rejuvenator will be a promising product to realize the smart pavements. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Microcapsules Stability Self-healing Bitumen Rejuvenator
1. Introduction The decline in the use of natural bitumen for road construction can be traced to the 1910s when the advent of vacuum distillation made it possible to obtain artificial bitumen from crude oil. Currently, 95% of the almost 100 million tons of bitumen produced worldwide each year is applied in the paving industry, where the bitumen essentially acts as a binder for mineral aggregates to form asphalt mixes [1]. Other uses of bitumen are as emulsions, waterproof materials, or formed materials, but these account for less than 5% of the total bitumen produced. As a widely applied material in pavements, bitumen must be sufficiently fluid at high temperature (around 160 C) to be workable and allow for homogenous coating of the aggregates upon mixing. Another important issue that has to be considered is the extent to which it ages from climate and traffic. After years of use the stiffness of asphalt concrete increases while its relaxation capacity decreases. This causes the binder to become more brittle, causing the development of microcracks and ultimately to cracking of the interface between the aggregates and
* Corresponding author. Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China. Tel./fax: þ86 22 26210595. E-mail addresses: [email protected], [email protected] (J.-F. Su). 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.03.008
binder [2]. This occurs mainly as a result of oxidation of the hydrocarbon compounds contained within the bitumen [3]. Bitumen binders are usually categorized into two subdivisions: solids called asphaltenes and liquids called maltenes. Maltenes can be further divided into polar aromatics, naphthalene aromatics, and saturates (paraffins) [4]. The main aging mechanism of bitumen is the loss of volatiles and oxidation, which leads to bitumen with higher viscosity (stiffer) [5]. In other words, the amount of solid component increases and that of the liquid component decreases, thus resulting in an increase in the rigidity of the pavement. The aging problem of bitumen leads to pavement failure, including surface raveling and reflective cracking. It therefore increases the cost of renovating and preserving bituminous pavements [6]. Several physical and chemical methods are currently employed for bitumen preservation, including the use of rejuvenator emulsions or fog seals, and through additive modification or thin overlay technologies [1,7]. However, of these methods only the first, i.e., the application of rejuvenators, can restore the original properties of the pavement [6]. The most important goal of using rejuvenator product is to restore the asphaltene/maltene ratio to its original balance [8]. Rejuvenating agents have the ability to reconstitute the binder’s chemical composition and they consist of lubricating and extender oils that contain a high proportion of maltene constituents [9]. Rejuvenator can soften the aged binder and provide comprehensive rejuvenation that replenishes the
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volatiles and dispersing oils while simultaneously promoting adhesion. However, for a rejuvenator to be successfully applied the difficulty in penetrating the pavement surface still remains a significant problem. Shen et al. [10] reported on the use of three rejuvenators and found that none could penetrate more than 2 cm into the asphalt concrete. Further issues encountered when applying these materials include the fact that road closures are necessary for some period of time after their application. The rejuvenator may also cause a high reduction in the surface friction of the pavement for vehicles. Moreover, these rejuvenators may also be harmful to the environment. To overcome these issues inspiration is provided by the concept of self-healing based on microcapsules. This particular approach involves incorporation of a microencapsulated healing agent and a dispersed catalyst within a polymer matrix [11e13]. Upon damageinduced cracking, the microcapsules are ruptured by the propagating crack fronts resulting in release of the healing agent into the cracks by capillary action [14]. The method of encapsulating rejuvenators inside the bitumen may be an alternative approach worthy of consideration. The application of microcapsules containing rejuvenator to bitumen derives from the success observed for some polymer self-healing materials [15e17]. García et al. [18] reported a method to prepare rejuvenator capsules by using an epoxy resin as a coating and porous sand as a skeleton. The advantages of these capsules include the fact that they are strong enough to resist the mixing process, the high temperature, and all the years in the road until they are required. However, these capsules have some limitations that restrict their application. The primary limitation is that it is hard for the rejuvenator to flow out from the porous sand when the shell is broken because the
rejuvenator has a high viscidity resulting from it consisting of lubricating and extender oils. The capillary action of the porous structure also limits the rejuvenator release. Another limitation is that the capsule size does not fit with the thickness of bitumen between aggregates. To realize the application of these promising chemical products, we have developed a novel method to fabricate microcapsules containing rejuvenator by in situ polymerization using methanol-melamine-formaldehyde (MMF) prepolymer as shells [19]. A two-step coacervation process, with the aid of styrene maleic anhydride (SMA) as a surfactant, was successfully applied to enhance the thermal stability and compactibility of the shells. It has been shown that this product is an environmentally friendly powder that encapsulates a suitably sized rejuvenator for chemical and construction engineering. To produce microcapsules containing rejuvenator by chemical means, the factors of cost, complexity, and capacity must be considered for the construction industry. These coreeshell microcapsule structures need to meet specific requirements in terms of size distribution, encapsulation ratio, and non-biodegradability because these factors will influence their service performance [20]. As bitumen acts as thin layers between aggregates that are usually less than 50 mm, the size of the microcapsules containing rejuvenators should be smaller than 50 mm to avoid being squeezed or pulverized during asphalt forming. Besides the complex fabrication process of microcapsules containing rejuvenator, their survival ability is another important issue that must be addressed. Fig. 1 illustrates the possible states of the microcapsules in bitumen material. First, for practical application the microcapsules must have high thermal stability and high mechanical strength to resist the melting temperature and mixing stress of the asphalt (Fig. 1aec). The microcapsules must maintain their shape and compatibility at
Fig. 1. Illustration of the possible states of microcapsules containing rejuvenator in bitumen.
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temperatures of 160e200 C during asphalt application. Further, breakage of the shell may occur at high temperatures owing to a mismatch in thermal expansion of the core and shell materials [21]. It is expected that the rejuvenator is protected, with fewer cracks and with lower permeability, during asphalt paving. However, microcapsule shells, such as inorganic or flexible organic shells, will not break when the mechanical strength applied is very high (Fig. 1d). The result is that microcracks may be triggered, leading to fracture without leakage of the rejuvenator. Consequently, the shells employed are more commonly polymeric materials that possess good thermal stability under high temperature and which exhibit an appropriate strength and toughness [22]. Second, interface stability and interfacial interactions are keys for all multicomponent materials, irrespective of the number or type of their components, or of their actual structure. Thus, understanding the relationship between the interfacial behaviors and mechanical properties of bitumen/ microcapsule composites is important for them to be useful under realistic engineering environments. In our previous report [23], it was found that microcracks and interface separation may occur for microcapsule/matrix composites when a repeated, vigorous thermal absorbing-releasing process is undertaken. During a process with repeated temperature changes via heat transmission, expansion and shrinkage of the microcapsules and polymer matrix will occur owing to the different expansion coefficients exhibited. The microcapsule volume can also be affected by the encapsulated rejuvenator upon environmental temperature changes. These phenomena will cause microcracks or fractures in the matrix during heat absorption or resealing, spoiling the thin microcapsule shells such that the encapsulated rejuvenator will lose the protection provided by the shells. Moreover, the mechanical integrity of these composites may decrease because of internal cracking or microcapsule rupture [24]. This is not expected to appear because it is well known that microcapsule rupture and release due to wear and tear can be avoided by proper design of their mechanical properties [25]. Theoretically, the mechanical strength of a microcapsule is determined by its size, and by the shell thickness and structure [26]. The mechanical properties of the shells play an important role in many processes, and therefore, understanding of these properties is desirable for optimizing their application. The theoretical tensile yield strength and ultimate tensile strength of the composites differ for the cases of adhesion and no adhesion between the filler particles and matrix. In the case of no adhesion between the microcapsules and bitumen, the interfacial layers are unable to transfer stress. We are in a position to commence exploring chemical methods capable of providing simple, cheap, robust, and environmentally friendly microcapsules containing rejuvenator for bitumen. In view of the above, the objective of this work was to fabricate microcapsules containing rejuvenator by in situ polymerization using MMFresin shells and to then investigate their stability in bitumen. Thermal stability and mechanical properties were measured to ensure that the microcapsules survived the bitumen during melting and mixing. In addition, interface stability was monitored to evaluate the interface bonding behaviors during the service life of the bitumen. 2. Experimental
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off-white, free flowing power with a faint, aromatic odor. The bitumen used in this study was 70/100 pen obtained from Kuwait Petroleum in a 4.5% by weight [20]. The material used as rejuvenator is dense, aromatic oil obtained from Petroplus Refining Antwerp (800DLA, Belgium). 2.2. Microcapsules synthesis The method of fabrication microcapsules containing rejuvenator by coacervation proceed can be divided into three steps [19]: (1) SMA (10.0 g) and NP-10 (0.2 g) were added to 100 ml water at 50 C and allowed mix for 2 h. Then a solution of NaOH (10%) was added dropwise adjusting its pH value to 10. The above surfactant solution and rejuvenator were emulsified mechanically under a vigorous stirring rate for 10 min using a high-speed disperse machine. (2) The encapsulation was carried out in a 500 ml three-neck roundbottomed flask equipped with a condensator and a tetrafluoroethylene mechanical stirrer. The above emulsion was transferred in the bottle, which was dipped in steady temperature flume (room temperature). Half of MMF prepolymer (16 g) was added dropwise with a stirring speed of 500 r min1. After 1 h, the temperature was increased to 60 C with a rate of 2 C min1. Then another half of prepolymer (16 g) was dropped in a bottle at the same dropping rate. (3) The temperature was increased to 75 C. After polymerization for 1 h, the temperature was decreased slowly at a rate of 2 C min1 to ambient temperature. At last, the resultant microcapsules were filtered and washed with pure water and dried in a vacuum oven. 2.3. Morphologies observations An optical microscope (BX41-12P02, OLYMPUS) was used to check the fabrication process of microcapsules in emulsion. About 1 ml of the colloidal solution was extracted and spread on a clean glass slide (1 3 cm). The dried microcapsules were adhered on a double-side adhesive tape without cracking the shells. The surface morphologies were observed by using an Environmental Scan Electron Microscopy (ESEM, Philips XL30) at an accelerated voltage of 20 kV. 2.4. Mean size and shell thickness of microcapsules For each microcapsule sample, the mean size is the average size value of fifty microcapsules measured from the SEM morphology image. About 2 g MMF-shell microcapsules was mixed in 5 g epoxy resin. After the composite was dried in room temperature, it was carefully cut to obtain the cross-section. The thickness of shells can be measured from the SEM images of cross-section of microcapsules [27]. At least 20 shells of each microcapsule sample were measured and the average data were calculated. 2.5. Thermogravimetric analysis (TGA) The thermal stability characterization of microcapsules was performed on a Dupont SDT-2960 Thermogravimetric analysis (TGA) at a scanning rate of 5 C min1 in a flow of 40 ml min1 nitrogen (N2).
2.1. Materials 2.6. States of microcapsules in bitumen The shell material was commercial prepolymer of melamineformaldehyde modified by methanol (solid content was 78.0%) purchased from Aonisite Chemical Trade Co., Ltd. (Tianjin, China). The rejuvenator was a commercial product. Styrene maleic anhydride (SMA) copolymer (ScripsetÒ 520, Hercules, USA) was applied as dispersant. A small percentage of the anhydride groups have been established with a low molecular weight alcohol and it is fine,
Various bitumen/microcapsule samples were prepared to investigate the thermal stability. A fluorescence microscope (CKX41F32FL, OLYMPUS) was applied to investigate the interface of materials using the light characters such as reflection, diffraction and refraction. A thermal absorbing-releasing process was performed to investigate the thermal stability of bitumen/microcapsule
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composites using a temperature-controlled chest. A similar method has been successfully applied to investigate the thermal stability of microcapsules containing phase change materials in epoxy [28]. The samples (2 1 0.5 cm) were heated to 50 C with the rate of 2 C min1 and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1. This thermal treatment process for each sample was repeated for 20, 40, and 60 times. The interface morphologies of the composites were observed by a fluorescence microscope. The bitumen/microcapsule samples were adhered on a glass sheet. It was carefully poured with a drop of liquid nitrogen at one end. Microcracks were quickly generated in this sample because of the low temperature brittleness. The state of microcapsules in bitumen could be observed by the fluorescence microscope. 2.7. Mechanical properties measurement The mechanical properties of the MMF-shell microcapsules were tested according to our reported two-plate micromanipulation method [25]. Single microcapsule was adhered on a glass sheet (500 500 mm) and pressed by another glass sheet. A pressure sensor under the bottom glass measured the intensity and data were directly recorded and yield stress was calculated automatically by a computer from the forceedisplacement curves. 2.8. Fourier transform infrared spectroscopy (FT-IR) The chemical structures were analyzed by a Perkin ElmerÒ Spectrum 100. FT-IR spectra in absorption modes were recorded among the range of 400e4000 cm1. 3. Results and discussion 3.1. Morphologies and physical structure of microcapsules The fabrication process of microcapsules using melamineformaldehyde (MF) and urea-formaldehyde (UF) resins as shell
materials is usually defined as an in situ polymerization method. Emulsifying agent is used to absorb the shell materials on core droplets. The direct polymerization of a single monomer or prepolymer is carried out on the core-particle surface. It has been proved that the MMF can be successfully applied to fabricate microcapsules using the in situ polymerization method [25]. The shell thickness, surface morphology and average size of microcapsules can be controlled by regulating the core/shell ratio, prepolymer adding speed and emulsion stirring rate. The core/shell ratio means the weight ratio of core material and shell material in original fabrication [19]. Fig. 2(a) shows the optical morphologies of microcapsules in emulsion fabricated by emulsifying rejuvenator with a stirring rate of 4000 r min1. Being encapsulated by shell material, the rejuvenator droplets are ultimately separated through the regulation of hydrolyzed SMA molecules. These microcapsules have regular globe shape with smooth surface. Fig. 2(b,c) and (d) shows the SEM surface morphologies of dried microcapsules with core/ shell ratio of 1/1 fabricated by 4000 and 2000 r min1 emulsion stirring rates. Their mean sizes are about 10 and 20 mm. The dried microcapsules still keep the regular global shape. There is no adhesion and impurity substance between microcapsules. The shells are compact without holes and cracks. To determine the shell thickness, microcapsules were embedded in epoxy resin as shown in Fig. 2(e,f). Microcapsules were uniformly dispersed in epoxy. The cross-section SEM morphologies of a typical single microcapsule are presented in Fig. 2(g). The shell thickness was measured directly. It must be noted that the shell may not be cut across its equator, so the thickness is an average data of at least 10 shells for each microcapsule sample. Fig. 3(a) shows the mean sizes of microcapsules (core/shell ratios of 1/1, 1/2 and 1/3) under various emulsion stirring rates in range of 1000e8000 r min1. With the increasing of stirring rates, the size of these microcapsule samples decreased sharply from 100.5 to 2.0 mm. The reason is that higher stirring rates will disperse the core material into smaller droplets. A similar conclusion has been reported by other researches [29,30]. The spread of the microsphere size distribution was found to decrease with stirring
Fig. 2. Morphologies of microcapsules containing rejuvenator, (a) optical morphology of microcapsules in emulsion, (b) SEM morphology of dried microcapsules, (c) smooth surface of single microcapsule, (d) SEM morphology of dried microcapsules with larger size, (e, f) optical morphology of microcapsules embedded in epoxy resin, and (g) cross-section SEM morphology of microcapsules.
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Fig. 3. Physical properties of microcapsules controlled by core material stirring rates and core/shell ratios, (a) mean size and (b) shell thickness.
speed. With the validation of the mathematical correlation, it is possible to have a good estimate of the average microsphere size prior to microsphere preparation. Moreover, less core/shell ratio leads to a higher shell thickness value. This result accords with reported results and indicates that the size is mainly determined by emulsion stirring rates [29]. In addition, it can be concluded that the MMF prepolymer had cross-linked with a compact structure forming thin shells. Fig. 3(b) shows the data of shell thickness of microcapsules (core/shell ratios of 1/1, 1/2 and 1/3) under core material stirring rates of 2000, 4000, 6000 and 8000 r min1, respectively. Microcapsule samples have the shell thickness from 4.51 0.50 to 0.55 0.12 mm. More shell material leads the shell to a thicker structure for each sample fabricated under the same core material stirring rate. 3.2. Thermal stability of microcapsules Microcapsules containing rejuvenator are expected to keep their shells intact, resisting the high temperatures of the melting bitumen. In the general encapsulation process, the core material is emulsified to form droplets, which are then covered by shell material to obtain the microcapsules. The surface morphology is controlled by the operating conditions. Fan and Zhou [31] reported that the initial pH value, the concentrations of the shell material and surfactant, and the stirring rate used during the stage of microencapsulation are all factors that can influence the surface morphology of the microcapsules obtained by in situ polymerization. In our previous study we also found that the surface morphology greatly affects the thermal stability of microcapsules containing rejuvenator [19]. A lower prepolymer concentration can help to improve the degree of surface smoothness and compactibility of the shells. It is worth noting that the smooth and nonporous shells are believed to result from the deposition of low molecular weight prepolymer at the oilewater interface, while the prepolymer remains soluble. The core/shell ratio is considered one of the main factors to affect the surface morphology. TGA has been widely applied to investigate the encapsulation effect and shell compactness of microcapsules [32]. We have previously reported that the decomposition temperature of microcapsules is higher than the bitumen melting temperature of 180 C. This indicates that the cured MMF resin will not thermally decompose upon mixing with the melting bitumen. On the other hand, there remains a lack of understanding on the thermal stability of microcapsules under extreme temperature changes. The reason for this is that the polymeric shells exhibit a tendency to craze and fracture under forceful thermal transmission [24]. Fig. 4 shows the TGA curves for microcapsules containing rejuvenator with a core/shell ratio of 1/3 under various
temperatures (260 C, 240 C, 220 C, and 200 C) for 1 h. It was found that after heating for 1 h to a temperature of 260 C the microcapsules showed a decomposition temperature from about 150 C. The pure rejuvenator applied in this study showed a marked weight loss between the temperature range 337e469 C owing to the evaporation of the oil substance at high temperature [19]. Therefore, we can confirm that the weight loss of the microcapsules over the temperature range of 150e337 C is a result of the weight loss of the shells. Comparatively, after heating for 1 h to temperatures of 240 C, 220 C, or 200 C, the microcapsules showed decomposition temperatures of 200 C, 240 C, and 290 C, respectively. Heating temperatures have been shown to greatly influence the original decomposition temperatures of microcapsules. Higher temperatures lower the temperature at which the commencement of degradation occurs for microcapsules. This phenomenon can be attributed to two factors. The first is that the shells had broken under the heating temperatures over an hour. Second, the reason may be that the shells did not retain their compact shapes. With an increase in temperature more defects on the shells may occur. These defects will lead the microcapsules to more rapidly decompose. In addition to the above, the initial decomposition temperatures of the microcapsules remain higher than the bitumen melting temperature. This indicates that the microcapsules can survive when in melting bitumen. Fig. 5(aef) shows the SEM morphologies of microcapsules after they have been heated for 1 h to 180 C, 200 C, 220 C, 240 C,
Fig. 4. TGA curves of microcapsules containing rejuvenator with core/shell ratio of 1/3 under various temperatures for 1 h, (a) 260 C, (b) 240 C, (c) 220 C, and (d) 200 C.
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260 C, and 280 C, respectively. It can be seen that the microcapsules retain their compact shell structures for temperatures of 180 C, 200 C, and 220 C for 1 h, as shown in Fig. 5(aec). Under 180 C and 200 C, the microcapsules closely maintain the shape and smooth surfaces of the original states. Although the thermal effect did not break the shells under 220 C, the shells stuck close together because of the softening of the shells. From Fig. 5(d) and (e) it can be seen that the shells contain holes, cracks, or even breaks when the temperature is increased to 240 C and 260 C. The shells are almost entirely broken under 280 C after the hour of heating, as shown in Fig. 5(f). Under these circumstances, the encapsulated rejuvenator leaked out without the protection of the shells. 3.3. Thermal stability of microcapsules in bitumen Microcapsules containing rejuvenator are homogeneously distributed in bitumen after hot-mixing. In addition to the thermal stability of the microcapsules, we also investigated the thermal stability of the microcapsules in bitumen. The mechanical properties of the shells can be enhanced by modifying their chemical structure and by controlling the synthesis conditions [33]. However, until now little has been known about shell stability in bitumen during a repeated thermal transmission process. To investigate the shell stability of microcapsules in bitumen we designed a thermal transmission process to simulate a practical application environment. To verify the thermal stability of microcapsules in bitumen a microcapsule sample was mixed (2.0 wt.%) with melting bitumen under temperatures of 180 C and 200 C for 10 min. The microcapsule sample had the same core/shell ratio (1/1) with a mean size about 15 mm. The frozen bitumen samples were broken apart and the interface morphologies were observed. It can be seen in Fig. 6(a) and (b) that the microcapsules survived in the bitumen under a temperature of 200 C. The microcapsules retained their global shape with no cracks or thermal decomposition. These results indicate that these microcapsules can resist the thermal effects of asphalt for common applications. An alternative thermal process was designed to test the thermal stability of the microcapsules containing rejuvenator in bitumen under an extreme condition. The bitumen/microcapsule samples were repeatedly treated 20, 40, and 60 times with a thermal absorbing-releasing process. For each condition the sample was heated to 50 C at a rate of 2 C min1, with this temperature maintained for 10 min before being reduced to 10 C at a rate of 2 C min1. Fig. 7(a) shows the original state morphologies and fluorescence properties of the microcapsules (2.0 wt.%) dispersed
in bitumen. The amount of rejuvenator retained within the microcapsule compared to the initial amount is indicative of the encapsulation efficiency (E) of the microcapsule fabrication process. We found that the highest E (70%) was obtained under the formation conditions of a core/shell ratio of 1/3, a stirring speed of 3000 r min1, and 2.0e2.5% of SMA [19]. All residues on the shells cannot be removed by water because the rejuvenator cannot be totally encapsulated by MMF shells. Bitumen is stained blue, while the rejuvenator is green. It is clear from our results that the microcapsules have been homogeneously dispersed in the bitumen. Fig. 7(b,c) shows the microcapsules’ states in bitumen after repeated thermal treatment of 20 and 40 times. It is found that the area of blue points had grown larger in comparison to the original state, which indicates that the rejuvenator had permeated out of the shells. It is well known that the release behavior of microcapsules largely depends on the polymer structure of the shell and on the temperature, which in turn is influenced by the preparation conditions employed. As the microcapsules possess the same mean size and core/shell ratio in all bitumen samples studied herein, the temperature change is the only factor influencing the permeability of the microcapsules. The lower the temperature the more time required for the core material to penetrate the shell. Rejuvenator is an oily mixture with high viscosity; therefore, heating effects can accelerate its molecular velocity through the shells. However, most of the rejuvenator is retained in the shells under such a thermal treatment process. This means that the rejuvenator can be protected by the shells for a longer life in practical applications. In Fig. 7(d) it can be seen that some microcapsules had broken after the treatment was repeated 60 times, as evidenced by the rejuvenator having flowed out of the shells. In this instance the rejuvenator permeated the bitumen as small molecules. 3.4. Deformation behaviors of microcapsules in bitumen Another important issue to be considered for microcapsules containing rejuvenator when applied in bitumen is their mechanical properties, including their deformation behavior. In some cases breakage of the microcapsules must be triggered by interior microcracks or by exterior pressure. In other cases, the release or leakage of rejuvenator in bitumen is undesirable. Microcapsules must be sufficiently strong to remain intact during manufacture and further processing, such as during drying, pumping, and mixing [23]. Mechanical properties are, therefore, fundamentally important because they determine the microcapsule’s stability and integrity. It is well known that microcapsule rupture and release due to wear and tear can be avoided by proper design of their
Fig. 5. SEM morphologies of microcapsules after heated to (a) 180 C, (b) 200 C, (c) 220 C, (d) 240 C, (e) 260 C and (f) 280 C.
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Fig. 6. SEM morphologies of microcapsules in bitumen (2.0 wt.%).
mechanical properties [34]. It has been reported that some microcapsules with polymeric shells may act as viscoelastic particles (depending on their size) [35]. Several methods have been reported to measure the mechanical properties of a single microcapsule or cell [29,30]. Because of their small structure, there has been little focus on the relationship between their deformation and shell structure (size and thickness). As a soft and microscale particle, an individual microcapsule’s mechanical properties are difficult to be quantitatively characterized. There is therefore a requirement for a system capable of accurately measuring low-magnitude forces and microscopic material deformation [36]. In this study, a similar method was also applied to investigate the mechanical properties of microcapsules using two-plate micromanipulation [20]. Fig. 8(a) shows a typical loadedisplacement curve for a single microcapsule, where it increases linearly upon loading until remaining constant, then decreases sharply upon unloading. The microcapsule sample was fabricated under a core material dispersal rate of 3000 r min1 with a core/shell ratio of 1/1. The shape of the curve illustrates both elastic (recovery) and plastic (inelastic) deformation behaviors. During unloading, the curve represents elastic behavior with elastic displacement being recovered. This means that a single microcapsule has an ability to recover its original shape. The loadingeunloading curves exhibit significant hysteresis under a large deformation. During the unloading process, the force decreases to zero before the displacement recovers to zero. This means that the microcapsules had a plastic deformation. The ‘Y’ point represents the ‘yield point’ on the concavee convex curve. In theory, creep in polymer materials under a constant load is controlled by the free volume available for the polymer chains and molecular units to move. Thinner shells may provide the cross-linked MMF polymer with more volume to perform molecular chain adjustments. Moreover, a higher proportion of shell material leads the shells to a greater hardness, which means that the thicker shell has an increased ability to resist deformation [27]. However, in this study we pay more interest to the plastic deformation behavior because it determines the rupture of microcapsules. Interestingly, it has also previously been reported that there
is a dependence of the failure strength on microcapsule diameter [37]. The ‘yield point’ was selected as a parameter by which to evaluate the plastic deformation. Fig. 8(b,c) shows the optical morphologies of a microcapsule with a permanent deformation with respect to its yield point. Prior to the yield point, a single microcapsule will deform elastically before returning to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation may become permanent and nonreversible. It should be mentioned that the core materials and environmental temperature will, at the same time, affect the yield points, but these have not been considered in the present study. Because the core and shell materials have different coefficients of expansion, changes in temperature will result in an extrusion force. Fig. 8(d) shows the ‘yield stress’ of MMF-shell microcapsules (under 23 C) with various mean sizes and shell thicknesses, illustrating their plastic deformation behavior. The yield stress of a microcapsule with a core/shell ratio of 1/3 is in the range of 1.20e0.70 MPa, whereas the yield stress of a microcapsule with a core/shell ratio of 1/1 is in the range of 0.75e0.52 MPa. More shell material can provide a shell with greater thickness, thus leading to a higher stiffness of elastic microcapsule. This trend is similar to the results provided by nanoindentation [27]. Fig. 9(a,b) shows the SEM morphologies of microcapsules in bitumen with plastic deformation. Some microcapsules have a large plastic deformation without evidence of any breaks. This deformation may result from a combined effect of the applied force and heat. It can therefore be concluded that the micromechanical properties of microcapsules depend on the thickness and microstructure of the shells. During compression, the volume and Poisson’s ratio of polymer microcapsules may change continuously with the deformation because of their spherical geometry. 3.5. Interface stability of bitumen/microcapsule samples The above results have proved that the MMF-shell microcapsules containing rejuvenator have satisfactory thermal stability and elasticeplastic deformation properties. It can be imaged that these
Fig. 7. Fluorescence microscope morphology of microcapsules (2.0 wt.%) dispersed in bitumen, (a) original state, (bed) states after a thermal treatment process for each sample repeated for 20, 40, and 60 times, each time the sample was heated to 50 C with the rate of 2 C min1and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1.
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Fig. 8. Plastic deformation behaviors of microcapsules containing rejuvenator, (a) a typical loadedisplacement curve of single microcapsule, (b,c) optical morphologies of a microcapsule with a permanent deformation over the its yield point, and (c) yield stress of microcapsules with various mean sizes and shell thicknesses.
microcapsules will survive during mixing with bitumen and aggregates. However, it is reported that microcracks and interface separation may appear in a repeated vigorous thermal absorbingreleasing process for microcapsule/matrix composites [24,38]. During a repeated temperature changes process with heat transmission, expansion and shrinkage of the microcapsules and bitumen will occur due to the different expansion coefficients. Microcapsules volume can be affected by the encapsulated rejuvenator when the environmental temperature changes. These phenomena may cause microcracks or fractures in the bitumen during heat absorbing or resealing, spoiling the thin microcapsule shell, thus the encapsulated rejuvenator will lose the shells protection. Theoretical tensile yield strength and ultimate tensile strength of the composites are different for the cases of adhesion and no adhesion between the filler particles and matrix. In the case of no adhesion, the interfacial layer cannot transfer stress. The main causes of internal thermal stress failure of composites are considered to be the residual stresses due to expansion and shrinkage in the thermal transmission process and mismatch of molecule movement among the components. Therefore, it is vital to investigate the bonding stability between bitumen and microcapsules. A thermal absorbing-releasing process was applied in this study to enlarge the interface change degree and interphase morphologies were observed to give a better explanation of the interface behaviors. A thermal absorbing-releasing process was applied in this study to enlarge the interface change degrees and interphase morphologies were analyzed to give a better explanation of the interface behaviors. The thermal treatment process bitumen sample repeated for 30 times, each time the sample was heated to 50 C
with the rate of 2 C min1 and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1. Fig. 10(a,b) shows the SEM interface morphologies of a bitumen/ microcapsule sample before and after the thermal treatment process. To observe the interface easily, the bitumen were frozen and broken. It can be seen that the microcapsules keep compact structure and global shape in both states. There is no interface debonding emerged between microcapsules and bitumen after a repeated temperature change. Both composites have a compact interphase. It means that the temperature changes may not affect the original adhesive state between shells and bitumen. It is well known that strength of composites strongly depends on the stress transfer between the particles and the matrix. For these wellbonded microcapsules, the applied stress can be effectively transferred to the particles from the bitumen, which will clearly improve the strength. An interphase region is comprised of polymer molecules that are bound at the filled particles surface, and they exhibit unique physical and chemical properties. Polymer molecules within the interphase region are restricted from chain management, molecular cross-linking and dipole polarization, thus they lose one or more degrees of molecular rotational and/or vibrational freedom. The molecule at the interface had modified the agglutination structure, and had moved to fit the violent thermal transmission. From the SEM morphologies in Fig. 10, it can be deduced that the original close interface could resist this molecule modification and the mismatch in expansion coefficients between the shell and bitumen. More heat-transmission cycles may generate destruction in the interface structure. If the interface is only molecular entanglements without chemical bonds, the molecule connecting the shell and
Fig. 9. Deformation morphologies of microcapsules containing rejuvenator in bitumen.
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Fig. 10. SEM interface morphology of bitumen/microcapsules sample, (a) original state and (b) states after a thermal treatment process for each sample repeated for 30 times, each time the sample was heated to 50 C with the rate of 2 C min1and keeping for 10 min, and then decreased the temperature to 10 C with the rate of 2 C min1.
matrix will undergo structure modification losing its adhesive function under long-time repeated thermal-stress effects. Therefore, it is necessary to investigate the chemical bonding within the interphase region through molecular structure analysis. Fig. 11 illustrates the potential chemical reactions between MMF resin and bitumen. In Fig. 11(a,b), further chemical cross-linking reaction will occur between the methanol-modified MF prepolymer molecular chains. Because this step is complex, the synthetic products are difficult to be controlled. At the end of the chains, hydroxy (eOH) will not be completely consumed. The complexity of bitumen chemistry lies in the fact that many different chemicals are present. It has been reported [2] that bitumen has three typical functional groups (phenolic, anhydride and carboxylic acid) as shown in Fig. 11(cee). They all may have the dehydration reactions or
esterification reactions with MMF resin. In Fig. 11(f), these reactions can form chemical bonds between shells and bitumen, which will enhance the interface stability under heat and stress effects. In addition, these bonds help microcracks to propagate avoiding to be stopped by interface separation or interphase gaps. Microcracks will fairly penetrate the interphase and shells and rejuvenator will outflow the shells. FT-IR was applied to prove the above interface structures. We do not investigate its in-situ molecular structure as the interphase is too very thin. Fig. 12 shows the FT-IR curves of cross-linked MMF, bitumen and their mixture, respectively. Curve (a) is the FT-IR spectrum of the MMF prepolymer. Curve (b) is the FT-IR spectrum of the cross-linked MMF resin. It can determined that the bands at 1598, 1012 and 819 cm1 are C]C stretch, CeO stretch and
Fig. 11. Chemical sketches of interface bonding structures between MMF-resin shell and bitumen, (a, b) MMF crosslinking structure, (cee) there typical functional groups (phenolic, anhydride and carboxylic acid) in bitumen, and (f) the possible dehydration reactions or esterification reactions between the MMF-shell and bitumen.
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and have reasonable absorbance. The chemical structure of cured MMF prepolymer has a wide absorption peak at approximately 3329 cm1attributing to the superposition of NeH stretching vibration. The peaks at 1551 and 815 cm1 are assigned to the vibrations of triazine ring; and the corresponding peaks of cured MMF lie at 1559 and 815 cm1. Curve (c) shows the FT-IR spectrum of the sample of cross-linked MMF/bitumen (98/2, w/w) composite. The peaks at 1071e990 cm1 are assigned to the CeN stretch (aryl). The peaks at 1680e1630 are assigned to the C]O stretch (amides). All these evidences indicate that the MMF resin and bitumen may have chemical reaction forming bonds in interphase. 3.6. Microcapsule states in bitumen with cracks
Fig. 12. FT-IR curves of (a) crosslinked MMF-resin, (b) bitumen, and (c) crosslinked MMF-resin and bitumen mixtures.
CeH bend, respectively. The strong peaks at 2948 and 2850 cm1 are alkyl CeH stretch and aliphatic CeH stretch. All these bands can be used to characterize the existing states of inside or outside of microcapsules, since they do not overlap with other spectral bands
Besides the stability of microcapsules in bitumen, we also need to prove that these microcapsules can break by microcracks. Fig. 13 shows the microcapsule states in bitumen with cracks. As shown in Fig. 13(a), microcapsules are keeping compact structure in bitumen without defects and microcapsule has the mean size about 10 mm. In Fig. 13(b,c), microcracks were generated by liquid nitrogen quickly with a width of 10e20 mm. With the cracks propagation, the shell had been split by the tip stress of cracks (Fig. 13(d)). Interestingly, it was found that the rejuvenators had rapidly filled the cracks under the capillary action (Fig. 13(e)). Then the rejuvenator had permeated through both sides of cracks and the cracks were healed (Fig. 13(f)). Future work will be carried out about the selfhealing behaviors with more details.
Fig. 13. Morphology of bitumen/microcapsules sample treated by liquid nitrogen, (a) cracks appeared in bitumen sample by the low temperature brittleness treatment of liquid nitrogen, (b) original fluorescence microscope morphology of microcapsules in bitumen with cracks, (c, d) fluorescence microscope morphology of microcapsules in bitumen with cracks after 2 h.
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4. Conclusions In this paper, MMF resin was successfully applied to encapsulated rejuvenator. These microcapsules were the first product applied for bitumen self-healing. These microcapsules can be designed with various mean sizes and shell thicknesses. It is necessary to instigate the thermal stability, mechanical stability and interface stability of microcapsules in bitumen. Under 180 C and 200 C, the microcapsules still maintained the globe shape and smooth surface as original state. Microcapsules had survived in bitumen under temperature of 200 C, which indicates that these microcapsules can resist the thermal effect of bitumen in application. Microcapsules had the elasticeplastic deformation ability resisting the temperature changes and mixing stress. Moreover, microcapsules had a large plastic deformation without breaks under a combined effect of the force and heat. The chemical bonds improved the interface stability between shells and bitumen. With the microcracks propagation, the shell had been split by the tip stress of cracks and the rejuvenators had rapidly filled the cracks under the capillary action. Later, the rejuvenator had permeated through both sides of cracks and the cracks were healed. Microcapsules containing rejuvenator will be a promising product to realize the smart pavements. In the second phase of this work, further investigations will be carried out concerning the rejuvenator permeation rate, self-healing effective and mechanical properties recovery. Acknowledgments The authors acknowledge the financial support from the Delft Centre for Materials (DCMat) in the form of Project IOP Self-Healing Materials SHM1036, “Encapsulated rejuvenator for asphalt”. Dr. Jun-Feng Su also thanks the previous financial support of the National Natural Science Foundation of China (No. 50803045). References [1] Lesueur D. The colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen modification. Adv Colloid Interf Sci 2009;145:42e82. [2] Branthaver JF, Petersen JC, Robertson RE, Duvall JJ, Kim SS, Harnsberger PM, et al. Binder characterization and evaluation. In Chemistry, vol. 2. Washington, DC: National Research Council; 1993. SHRP-A-368. [3] Hunter RN. Bituminous mixtures in road construction. London: Thomas Telford; 1997. [4] Corbett LW. Composition of asphalt based on generic fractionation using solvent deasphaltening, elutioneadsorption chromatography and densimetric characterization. Anal Chem 1969;41(4):576e9. [5] Lu X, Isacsson U. Chemical and rheological evaluation of ageing properties of SBS polymer modified bitumens. Fuel 1998;77(9e10):961e72. [6] Zargar M, Ahmadinia E, Asli H, Karim MR. Investigation of the possibility of using waste cooking oil as a rejuvenating agent for aged bitumen. J Hazard Mater 2012;233e234:254e8. [7] Boyer RE. Asphalt rejuvenators: “fact, or fable”. In: Transportation systems 2000 (TS2K) workshop, San Antonio, Texas. [8] Karlsson R, Isacsson U. Material-related aspects of asphalt recycling-state of the art. J Mater Civil Eng 2006;18(1):81e92. [9] Chiu CT, Lee MG. Effectiveness of seal rejuvenators for bituminous pavement surfaces. J Test Eval 2006;34(5):390e4. [10] Shen J, Amirkhanian S, Miller JA. Effects of rejuvenating agents on superpave mixtures containing reclaimed asphalt pavement. J Mater Civil Eng 2007;19(5):376e84. [11] Brown EN, Kessler MR, Sottos NR, White SR. In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene. J Microencapsul 2003;20:719e30.
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