Self-healing epoxies using two-part epoxy-amine chemistry

• Main Author: Zhang He
• Other Authors: School of Mechanical and Aerospace Engineering
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/60756
• Institution: Nanyang Technological University

Epoxy adhesives and epoxy based composites have been extensively applied in engineering fields due to their excellent properties. However, due to its relatively low fracture toughness, this thermosetting polymer is highly sensitive to the defects or cracks. How to toughen it and prolong its service life is one of the most practical and attractive topics. Among all the techniques developed, self-healing approach using micro-carriers has been demonstrating its promising potential since it can not only toughen the matrix by the included carriers, but also extend its lifespan by healing the induced micro-cracks timely using the reagents encapsulated in the micro-carriers. Although various techniques were explored to autonomously heal epoxy materials, amine-based curing chemistry as a major process of producing industrial epoxies indicates that a similar self-healing chemistry is needed in order to achieve a homogeneous and fully compatible healed structure. Therefore, it is demanded to explore two-part epoxy-amine self-healing systems with low cost, wide adaptability, and ease of processing, and most importantly, to develop robust micro-containers for amines with chemical and thermal stabilities in consideration of nature of amines and material processing requirements. In this study, glass bubbles (GBs) with micro-through-holes were fabricated and adopted as the healing agent carriers for self-healing epoxies. This kind of GB was successfully achieved by etching the commercial products using 1 wt% hydrofluoric acid (HF) solution in an innovatively designed reaction system, which can realize the controllable etching, concurrent separation and continuous collection of GBs simultaneously. The function of each element in the etching system was carefully studied to guarantee the repeatable and controllable etching reaction. In order to achieve the best quality and the highest yield of the etching process, various factors were investigated to optimize the etching condition. It is found that the bigger GBs (67.4 ± 7.4 μm) give better etching quality and yield, compared with the smaller ones (45.1 ± 7.5 μm). The optimal conditions were concluded, under which the highest gross yield of about 86% and the highest effective yield of about 48% were obtained. The versatility of the etched GBs as robust micro-carriers for self-healing purpose was demonstrated by loading the GBs with healing agents to develop two self-healing systems. First, amine solution loaded GBs (AM-GBs) together with epoxy solution filled microcapsules (EP-Capsules) were incorporated into epoxy matrix to fabricate self-healing samples. The measured core percentages for EP-Capsules and AM-GBs are about 80 wt% and 33 wt%, respectively. The amine in the GBs after incorporation into epoxy matrix show high stability at ambient temperature, but diffuses out gradually during heat treatment at 80 °C. Various factors including the ratio, the total concentration, and the size of the two carriers were studied as well as the healing temperature and the post heat treatment process. The best healing performance was obtained at ratio of 1 : 3 for EP-Capsules to AM-GBs. It is observed that the higher concentration of bigger carriers, together with higher healing temperature, enable better healing behavior. Healing efficiency up to 93% was obtained in this system. In addition, post heat treatment decreases the healing efficiency due to the loss of amine in GBs at elevated temperature. Second, another self-healing epoxy was developed using epoxy-amine chemistry both carried by the etched GBs. The purpose using epoxy solution loaded GBs (EP-GBs) is to diminish the compromised mechanical properties caused by EP-Capsules. In this investigation, the mechanical response of the GBs was obtained using a micro-compressive tester, which reveals the brittle feature and relatively high normalized strength of the GBs compared with other polymer-based microcapsules. The self-healing performance was systematically studied, and the highest healing efficiency of about 62% was achieved at 50 °C for 24 h when 12.5 - 15.0 wt% GBs was incorporated at the optimized ratio of 4 : 1 for EP-GBs to AM-GBs. It is also found that the healing efficiency increases with increase of the healing duration at 50 °C. For a better understanding of the healing behavior of two-part self-healing systems, modelling of dual-carrier systems was conducted to qualitatively predict and analyse the healing performance. The amount and the mass ratio of the two released healants at the crack plane were correlated with the size, concentration, and core percentage of the healing agent carriers. A simplified cubic array model for randomly distributed healing agent carriers was adopted to depict the longest diffusion distance of the released healants, which is inversely proportional to the cubic root of the carrier concentration. In summary, the innovatively designed etching device is proven to be an efficient approach to produce robust GBs as healing agents carriers. The etched GBs demonstrated their versatility in developing novel self-healing material systems. It is believed that etched GBs that can load a large variety of chemicals and substances will inspire self-healing in other host polymer systems and even other applications.