Research On Phenol Formaldehyde Resin Microcapsule's Mechanical Properties And Its Use In Self-healing Concrete

Microcapsules are widely used in many different fields of application,such as pharmacy, food industry, agriculture, cosmetics, textile industry, printing, biosensor engineering, active coatings and construction. The mechanical properties of a microcapsule play a very important role in determining its application. Capsule-based self-healing cements are good examples to illustrate the need for tailoring the mechanical properties of the microcapsules, in these materials, the microcapsules should have appropriate mechanical properties to achieve the self-healing effects; in other words, the microcapsules should keep intact during the fabrication of the composite materials and be ruptured when the cracks occur. Due to small size and wide size-distribution of the microcapsules, there are few instruments and methods available for the investigation of microcapsule's mechanical performance. Although much pioneering work has been done on modeling and simulation, to the best of our knowledge, there are few studies reported in literatures on determining the microcapsules' mechannical property parameters to verify these simulation conclusions. In this work, the PF microcapsules were used as an example to determine the microcapsules' mechannical property parameters by nanoindentation; and the influential factors of microcapsules' mechannical property parameters were discussed, systematically. Finally, the feasibility of the PF microcapsules used in self-healing cements was studied. The specific content and conclusions of this paper are as follows:First of all, for mechanical characterization, several series of microcapsules(encapsulated dodecane, dicyclopentadiene and epoxy resin E-51, separately) with various outer diameters and shell thicknesses were successfully prepared by in situ polymerization.Then, two batches of indentation tests were performed to determine the microcapsule's cracking load, shell's modulus and hardness. It is very easy to get the cracking load from the displacement-load curve acquired by the nanoindentation tests under a revised depth-controlled method equipped with a plat indenter. It is not appropriate to use standard Oliver-Pharr method to interpreting the typical displacement-load curve for the shell's modulus and hardness, the reasons were explained and a new way based on energy was figured out. Using the new method, the shell's typical displacement-load curves were interpreted. The result shows the value of the PF shell's modulus is some 2.46 GPa, in corresponding with the estimate result of Oliver-Pharr model.Lastly, the influential factors of microcapsules' mechannical property parameters and the feasibility of the PF microcapsules used in self-healing cements were studied. The experimental results show the cracking load of a microcapsule is mainly depend on its size, shell thickness and core material; it increases with the shell thickness but decreases with the size and core material viscosity. The PF shell's modulus and hardness are independent of microcapsule's size and shell thickness. The synthetic condition has decisive effect on microcapsules' mechannical property parameters. The PF microcapsule has a good durability under alkaline conditions,and appropriate mechanical properties for self-healing cements.