Development of multifunctional polymer composites with self-healing capability

Self-healing is an inherent property of living organisms, which poses a significant challenge for materials science. In recent years, self-repair mechanisms observed in plants have been recognized as promising models for the development of bio-inspired self-healing materials. The potential of biomimetic approaches to develop self-healing materials has been widely studied in the literature. In the field of composite materials, the concept of self-healing composites refers to the design of materials capable of autonomously restoring lost mechanical properties. The advantages of self-healing composites are numerous, including reduced maintenance and repair costs, and improved service life, leading to enhanced sustainability. Two types of self-healing composites have been extensively studied: extrinsic and intrinsic. This PhD Thesis focuses on investigating the intrinsic self-healing mechanism within polymer composites, which involves the ability of polymer matrices to heal micro-damage, such as cracks, under external stimuli. This Thesis aims to develop a thermoplastic matrix possessing self-healing properties using polyamide 6 (PA6), which is the most commonly utilized thermoplastic polymer in the production of thermoplastic composites. As there is a lack of systematic investigation on this particular research topic in the scientific literature, various combinations of PA6 and thermoplastic healing agents, along with different types of compatibilizers, were employed. The optimized matrix has been used for the manufacture of both short and long-carbon fiber composites. This PhD Thesis not only focuses on the production of thermoplastic self-healing composites but also investigates thermosetting intrinsic self-healing composites. Two distinct systems are examined, and in both cases, thermoplastic healing agents has been applied by depositing them on top of the fiber fabrics. A crucial aspect of this PhD Thesis is the fractography analysis, which enables an understanding of the reasons behind the failure of several healing mechanisms and the factors contributing to the success of other healing mechanisms. The PhD Thesis is divided into eight Chapters. Chapter I highlights the aim of this work together with the outline of the Thesis. Chapter II provides a brief introduction and the theoretical background of self-healing composites. Chapter III details all the experimental techniques utilized for the characterization of the polymer blends and for the characterization of the prepared composites. All the obtained results are thoroughly reported in Chapters IV-VIII. Chapter IV presents the results of PA6 with the combination of two different healing agents, i.e., Polycaprolactone (PCL) and Cyclic olefinic copolymer (COC) and it is subdivided into four different parts. The first investigated system was PA6/PCL and the latter was melt compounded with PA6 in different amounts. PCL caused a decrease in the mechanical properties of PA6, due to its immiscibility and low mechanical properties. Nevertheless, acceptable fracture toughness values in quasi-static mode were obtained. Samples were thermally mended at 80 and 100 °C, and the healing efficiency (HE) was assessed by comparing the fracture toughness of virgin and repaired samples both in quasi-static and in impact mode. The blend with a PCL content of 30 wt% showed limited HE values (up to 6%) in quasi-static mode, while interesting HE values (53%) were detected under impact conditions. This discrepancy was explained through microstructural analysis and correlated to a different fracture morphology. In fact, under quasi-static mode, the PA6 matrix was severely plasticized, while under impact a brittle fracture surface was obtained favoring thus the flow of PCL during the thermal healing process. The second investigated system was PA6/COC and the latter was melt compounded with PA6 in different amounts. From scanning electron microscope micrographs, it was possible to highlight the immiscibility and the lack of interfacial adhesion between the constituents. The HE of the system was evaluated by comparing the fracture toughness of the produced blends, both in quasi-static and impact mode, before and after the healing process performed at 140°C by applying a pressure of 0.5 MPa. Through the addition of 30 wt% of COC, the fracture toughness of the virgin samples slightly decreased, passing from 2.3 MPa·m1/2 of neat PA6 to 2.1 MPa·m1/2. However, the presence of the 30 wt% of COC homogeneously distributed within the PA6 matrix led to a HE of 11% in quasi-static mode and 35% in impact mode. From the analysis of these preliminary systems, it was decided that the best matrix/healing agent combination with the highest potential was the one reported by PA6/COC system. At this aim, since the lack of interfacial adhesion between the two different constituents severely decreased the healing performances of the system, different types of compatibilizers were selected in order to enhance the interphase between PA6 and COC. Three different types of compatibilizers were selected, i.e., poly(ethylene)-graft-maleic anhydride (PE-g-MAH), polyolefin elastomer-graft-maleic anhydride (POE-g-MAH), and ethylene glycidyl methacrylate (E-GMA), and thoroughly investigated in the third subchapter. The dynamic rheological analysis revealed that E-GMA played a crucial role in reducing interfacial tension and promoting PA6 chain entanglement with COC domains. Mechanical tests showed that PE-g-MAH and POE-g-MAH compatibilizers enhanced elongation at break, while E-GMA had a milder effect. A thermal healing process at 140 °C for 1 h was carried out on specimens broken in fracture toughness tests, performed under quasi-static and impact conditions, and HE was evaluated as the ratio of critical stress intensity factors of healed and virgin samples. All the compatibilizers increased HE, especially E-GMA, achieving 29% and 68% in quasi-static and impact conditions, respectively. SEM images of specimens tested in quasi-static conditions showed that all the compatibilizers induced PA6 plasticization and crack corrugation, thus hindering COC flow in the crack zone. Conversely, under impact conditions, E-GMA led to the formation of brittle fractures with planar surfaces, promoting COC flow and thus higher HE values. This study demonstrated that compatibilizers, loading mode, and fracture surface morphologies strongly influenced self-healing performance. From this study, it was evident that the best compatibilizer, in terms of HE performance, was E-GMA. For these reasons, it was decided to perform a fine-tuning of both the E-GMA content in the PA6/COC matrix and also a tuning of the temperature of the healing process. The experimental results of this investigation are reported in the fourth subchapter. From the capillary rheometer analysis, it was possible to assess that the addition of E-GMA improved both the melt strength (MS) and the breaking stretching ratio (BSR). The enhancement of these parameters reflected better processability and an improved capability of forming film by the optimized blend. From the performed fracture toughness tests, both in quasi-static mode and impact mode, it was possible to obtain, utilizing analysis of variance (ANOVA) statistics, the optimum E-GMA content, and healing temperature. The HE values in quasi-static mode at a healing temperature of 160 °C passed from 12 % for the non-compatibilized blend up to 38 % for the blend containing 5 wt% E-GMA. Passing to the performance in impact mode, the HE values at a healing temperature of 160 °C pass from 57 % for the non-compatibilized blend up to 82 % for the blend containing 5 wt% E-GMA. The differences in these two HE values for quasi-static conditions and impact mode were investigated through field emission scanning electron microscopy and it was noticed that the specimens tested in quasi-static mode showed severe plasticized fractured surfaces. On the other hand, the specimens tested in impact mode reported brittle fractured surfaces. The differences between the severely plasticized surfaces and the brittle surfaces explained the difference between the HE values of the two different tests. Severely plasticized surfaces hindered the flow of the healing agent during the thermal mending process, while the brittle surfaces allowed a better distribution of the healing agent during the thermal mending process. In conclusion, from the performed analysis, it was possible to obtain an optimized thermoplastic self-healing matrix to be used in structural composite applications. Chapter V presents the results of both short and long-carbon fiber composites produced by using the optimized self-healing thermoplastic blend detected in Chapter IV. The first investigated system was composed of short carbon fiber composite with self-healing properties. All the prepared compositions were produced in collaboration with the University of Pisa by means of a semi-industrial extruder, followed by an injection molding machine. Thanks to the remarkably high quality of the prepared specimens, the thermal mending capability was assessed through Charpy impact testing and plane-strain fracture toughness tests. The HE values of the self-healing composites were remarkable, and the system was successfully proven with HE values of approximately 10 % in quasi-static mode and approximately 50 % in Charpy impact tests. From the fractography analysis, it was possible to assess that the healing agent was capable of flowing in the crack plane but since, in both tests, a catastrophic rupture took place, the fiber integrity was thus lost. Thus, it was decided to perform fatigue testing and implement a statistical method found in the literature. In particular, a damage criterion was adopted to predict the fatigue life of these materials. Through the presented statistical approach, the Wöhler curves for both reinforced systems, i.e., the neat containing only PA6 and short carbon fibers and the self-healing short carbon fiber composites, were produced. Through the damaging/healing process, it was possible to highlight that the mending process was able to improve the fatigue life of the self-healing composites by approximately 77 %. The obtained results highlighted the potential of the self-healing composites in prolonging the fatigue life and therefore enhancing the working life of structural components. From the presented results it was highlighted that the prepared self-healing thermoplastic blend was capable of effectively repairing micro-damages and not catastrophic damages. The second investigated system was composed of long carbon fiber composites with self-healing properties prepared starting from the thermoplastic blend developed in Chapter IV. Long carbon fiber composites are prepared through film stacking and hot pressing process, the thermoplastic thin films were produced in collaboration with Professor Pietro Russo from the University of Naples by using an extruder equipped with a calender. A thorough analysis of the thermal and mechanical properties of these laminates highlighted the repair capabilities of PA6 and self-healing blend long carbon fiber laminates. The optical microscope revealed matrix-rich and fiber-rich regions, which could potentially undermine the mechanical integrity of the laminates due to incomplete impregnation of the carbon fiber by the matrices. However, pycnometer analysis confirmed that the void percentage within the composites remained acceptable for structural applications. The evaluation of the interlaminar shear strength (ILSS) through short beam shear (SBS) tests highlighted that there was no difference between the two different laminates. Through the thermal mending process, it was possible to demonstrate that the neat laminates were not able, as expected, to recover their mechanical properties. On the other hand, the self-healing laminates were capable of restoring the mechanical properties with a healing efficiency value of 104 %. From the analysis of the fracture surfaces, before and after the thermal mending process, it was possible to understand the reason behind the high value of healing efficiency. SBS tests induced mainly micro damages in the matrix and delamination. The damages were totally recovered upon the thermal mending process since there were no cracks or evident delamination on the observed specimens. In conclusion, this Chapter substantiated the efficacy of the developed thermoplastic self-healing blend in producing intrinsic self-healing composites. The self-healing laminates, with their superior tensile properties and robust self-healing performance, highlighted their potential for advanced applications in structural components with enhanced working life. Chapter VI reports the two different studies conducted on intrinsic self-healing thermosetting composites. The first investigated system was focused on the self-healing behavior of carbon fiber (CF) reinforced composites by depositing jet-spun COC meshes on dry carbon fiber plies before lamination with epoxy resin (EP). Three different laminates were prepared, including neat EP/CF and two composites with 4 wt.% and 8 wt.% in the form of a jet-spun COC network. The introduction of COC mesh reduced flexural stress by 26% and interlaminar shear strength by 50%. Mode I interlaminar fracture toughness was evaluated and specimens were mended at 110 °C by resistive heating generated by an electrical current flowing within the samples. The laminates containing 8 wt% COC reported a healing efficiency, evaluated as the ratio between the GIC and the maximum load of virgin and healed samples, of 9.4% and 33.7%, respectively. Fractography analysis highlighted the poor adhesion between the COC mesh and EP matrix, and several COC microfibers were trapped inside the epoxy matrix, hindering their diffusion inside the crack zone, which limited the healing capability of the prepared laminates. The second investigated system was based on the intrinsic-extrinsic self-healing laminates in which different healing agents were directly 3D printed on top of the fiber fabrics. Different amounts of thermoplastic healing agents were deposited through a specifically designed 3D printed process on top of fiber fabrics and with different percentages of covered area. Through vacuum assisted resin transfer molding (VARTM) process it was possible to produce, two reference laminates containing only carbon fibers and glass fibers, and laminates containing polyamide 11 (PA11), thermoplastic polyurethane (TPU) and PA11 with carbon nanotubes (PA11CNT). All the samples were labeled according to the following code “XX_YY_ZZ”, where “XX” stands for the selected reinforcements (CF or GF), “YY” stands for the thermoplastic polymer utilized, and “ZZ” stands for the percentage of the covered area by the thermoplastic polymers. A complete characterization of the thermal and mechanical properties was performed to assess the effect of the thermoplastic insertion on the physical properties of the composites. From the measurement of mode I fracture toughness, it was possible to assess the extremely positive effect of the healing agent on the GIC values. CF_PA11 laminates were demonstrated to be the best systems thanks to the toughening effect generated in the thermoplastic enriched plane. The fracture toughness was 674% higher with respect to the neat reference laminates in the case of the CF_PA11_36 system (GIC = 1641 J/m2). This exceptional result was attributable to the enhanced adhesion of the deposited thermoplastic pattern within the midplane laminae, while the large data scattering is related to the concomitant delamination processes induced in the adjacent planes. The same trend was recorded also for the CF_PA11_24 and the CF_PA11_12 laminates with a fracture toughness increase of 516 % and 359 %, respectively. On the other hand, for the TPU and PA11+CNTs laminates, the fracture toughness was marginally affected due to the possible degradation of TPU and the lack of interfacial adhesion of the PA11+CNTs thermoplastic healing agent with the GF. The specimens used for the determination of the mode I fracture toughness were healed at a temperature of 210 °C allowing the flow of the introduced healing agent in the crack plane thus restoring the loss of mechanical properties. The healing efficiency was successfully determined by calculating the variation of the fracture toughness upon the thermally activated healing cycles. In the considered analysis, the best systems were proved to be the CF_PA11_36 and the GF_PA11_CNTs laminates with a healing efficiency of 74%. Nevertheless, the best system was the one presenting the PA11 thermoplastic healing agent due to the much higher virgin fracture toughness value. Since the best system was the one composed of CF_PA11 laminates, several healing cycles were performed in order to assess the healing efficiency also for subsequent damage/healing processes. By evaluating the healing efficiency through the fracture toughness, it was possible to assess recovery of almost 50% after the three subsequent healing cycles for the CF_PA11_36 system. In conclusion, the results reported in this Chapter demonstrated that CF/epoxy laminates enriched with the 36% covered area pattern of PA11_20C were the best system in terms of both healing efficiency and fracture toughness. Chapter VII reported the final conclusion of the PhD Thesis and the general evaluation of the performances of the produced systems. Chapter VIII reported a summary of all the side activities performed during the PhD program.