Magnetic polyamide 6 nanocomposites for increasing damage tolerance through self-healing of composite structures.

Self-healing materials have the ability to repeatedly repair damages that occur before complete failure of materials. The development of stimuli-responsive self-healing materials has been in demand recently for composite structures, since their failure is relatively uncertain and can result in major expenses. Using such materials can enhance damage tolerance, leading to greater asset reliability - it also limits expenditure and keeps the need for human interventions to a minimum. This work investigates Fe3O4 magnetic polymer nanocomposites that can be used to intrinsically heal composites through thermal stimuli, followed by self-healing of glass fibre reinforced polymer (GFRP) composites, which are fabricated by embedding the healable polymer nanocomposite as one of the sacrificial layered matrices. However, performance of nanocomposites depends on various parameters, including nanoscale dispersion of nanoparticles. Specifically, a lack of hierarchical dispersion of nanoparticles in three-dimensional polymer matrices prevents electron tunnelling and deteriorates the nanocomposites' ability to conduct heat stimuli or otherwise lead to pyrolysis. To address this issue, two functionalisation techniques - viz. silica (Stöber method for lower silica loading and tri-phasic reverse emulsion method for higher silica loading), and oleic acid (22%, 33%, 44%, and 55% w/w of nanoparticles) variations - were experimentally investigated as capable of changing hydrophobic characteristics for facilitating uniform dispersion of the Fe3O4 magnetic nanoparticles (MNPs). The main focus of the presented work is to understand the role of functionalisation routes in the particle-polymer interface in forming a uniformly dispersed and hierarchical network of MNPs in a polymer matrix. Emphasis was on understanding and optimising the role of activator and initiator proportions in controlling the in-situ polymerisation of PA6, capturing the MNPs dispersion state. The resulting dispersion state due to functionalised Fe3O4 MNPs determined the properties of magnetic PA6 nanocomposites (PMC) to help achieve a generic set of principles for designing the desired materials for stimuli-induced self-healing of GFRP composites. The method used to achieve this involved firstly undertaking anionic ring-opening in-situ polymerisation of PMC by experimental synthesis within the laboratory, with optimised EtMgBr (activator) and NACL (initiator) proportions for improved degree of crystallinity, and capturing the MNPs dispersion state attained by probe ultrasonication of the melt monomer & MNPs solution mixture. Based on this, 50% EtMgBr (activator) and 30% NACL (initiator) were assessed as the optimised proportions for giving the highest possible crystallinity amongst all the prepared PMC variations. Secondly, as per the functionalisation type of the MNPs, the prepared PMC samples were tested based on chemical, thermal, structural and magnetic characterisations, for the purpose of assessing their self-healing capability by microwave stimuli. The physical characterisation results were also used to train a simulation model to create the 3D dispersion state, for better studying the dispersion state and interaction region defined by the interaction radius (IR) of each MNP/agglomerate of the MNPs. Based on this overall comparison, the most suitable PMC of 22 w/w % OA loading was selected and formed into thin films. Sandwiched tensile testing samples were then prepared using this PMC film as a sacrificial layer between GFRP tapes. Both bare and modified Fe3O4 MNPs PMC exhibited paramagnetic behaviour, with average particle sizes ranging from 30-60 nm. The saturation magnetisation (Ms) of the unmodified MNPs PMC was around 65% and that of the selected PMC with 22 wt/wt % OA loading was 47%. The self-healing concept was demonstrated with the prepared composite samples' microwave induction heating, and the efficiencies based on strength recovery were calculated as 84%, 58% and 34% after first, second and third healing, respectively. This can essentially increase the life-cycle viability of the composite structure by over 175% (with 60% certainty) compared with that of an otherwise damaged structure, hence promising cost saving by extending the structural life

Rapid multifunctional composite part manufacturing using controlled in-situ polymerization of PA6 nanocomposite.

Currently, mass composite manufacturing is dominated by the fiber-reinforced thermosets due to the ease of its processing. However, the addition of multifunctionality to the thermoset composite requires an additional step of manufacturing. This work presents the single-step rapid manufacturing of bespoke designed multifunctional nanocomposite PA6 components. In this method, firstly, the liquid PA6 nanocomposite containing inorganic nano-inclusions was infused into a glass or carbon fibre matrix placed within the mould. Subsequently, the multifunctional composite component was formed via the anionic in-situ synthesis route. The flexibility of this method presented is akin to that of a polymer moulding process, which ensures a reduction in the manufacturing cycle times and increases the production efficiency of the bespoke component. The in-situ reaction is optimised by setting the specific volume of the activator, initiator and including polymer chain terminator, PEG. To prepare the polymer with at least 50% and above crystallinity, the most suitable activator-initiator combination was found to be 0.143 ml and 0.096 ml respectively, with 0.1 g of PEG-6000 for 5 g sample preparation. Such thermoplastic-based components can be easily recycled, present better impact resistance and can be easily formed or fused with heat; eliminating the cost and time associated with the assembly of composite part, unlike thermoset components

Optimising crystallisation during rapid prototyping of Fe3O4-PA6 polymer nanocomposite component.

Polymer components capable of self-healing can rapidly be manufactured by injecting the monomer (ε-caprolactam), activator and catalyst mixed with a small amount of magnetic nanoparticles into a steel mould. The anionic polymerisation of the monomer produces a polymer component capturing magnetic nanoparticles in a dispersed state. Any microcracks developed in this nanocomposite component can be healed by exposing it to an external alternating magnetic field. Due to the magnetocaloric effect, the nanoparticles locally melt the polymer in response to the magnetic field and fill the cracks, but the nanoparticles require establishing a network within the matrix of the polymer through effective dispersion for functional and uniform melting. The dispersed nanoparticles, however, affect the degree of crystallinity of the polymer depending on the radius of gyration of the polymer chain and the diameter of the magnetic nanoparticle agglomerates. The variation in the degree of crystallinity and crystallite size induced by nanoparticles can affect the melting temperature as well as its mechanical strength after testing for applications, such as stimuli-based self-healing. In the case of in situ synthesis of the polyamide-6 (PA6) magnetic nanocomposite (PMC), there is an opportunity to alter the degree of crystallinity and crystallite size by optimising the catalyst and activator concentration in the monomer. This optimisation method offers an opportunity to tune the crystallinity and, thus, the properties of PMC, which otherwise can be affected by the addition of nanoparticles. To study the effect of the concentration of the catalyst and activator on thermal properties, the degree of crystallinity and the crystallite size of the component (PMC), the ratio of activator and catalyst is varied during the anionic polymerisation of ε-caprolactam, but the concentration of Fe3O4 nanoparticles is kept constant at 1 wt%. Differential Scanning Calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), XRD (X-ray diffraction) and Thermogravimetric analysis (TGA) were used to find the required concentration of the activator and catalyst for optimum properties. It was observed that the sample with 30% N-acetyl caprolactam (NACL) (with 50% EtMgBr) among all of the samples was most suitable to Rapid Prototype the PMC dog-bone sample with the desired degree of crystallinity and required formability

Insulating MgO–Al2O3–LDPE nanocomposites for offshore medium-voltage DC cables.

A polymer–metal oxide nanocomposite is a key in developing a high-temperature insulation material for power electronics and high-voltage direct current (HVDC) and medium-voltage direct current (MVDC) subsea cables having the capability of transmitting offshore renewable energy with lower losses and higher reliability. To achieve a higher operation voltage level and larger power capacity at a reduced cable size, weight, and volume, the lighter material offering improved electrical insulation at a high operating temperature is required. Addition of metal oxide ceramics in the polymer is shown to improve the insulating properties of the polymer used in the cable and power electronic applications; however, their performance deteriorates at elevated temperatures as thermal energy facilitates the electron injection to the bulk material by following conduction according to the Schottky emission. In this work, the heat insulating Al2O3 nanoparticles are added to the MgO–polyethylene nanocomposite to observe the effect of the interface between mix oxide nanoparticles on current density and breakdown strength of the nanocomposite compared to the MgO–polyethylene nanocomposite at room and elevated temperatures (90 °C). The concentrations of the MgO and MgO + Al2O3 mixture were varied from 1 to 12 wt % to find out that the nanocomposite containing MgO showed the best response than MgO + Al2O3 at elevated and room temperatures. There was no unified trend observed in the leakage current density and breakdown strength results for the MgO + Al2O3 nanocomposite, indicating the absence of the interface formation between MgO and Al2O3. The decrease in the interaction radius, calculated using numerical simulation of the nanoparticle dispersion state, resulted in the high breakdown strength. Addition of 12 wt % MgO helped achieving the highest breakdown strength, but overall breakdown strength for the MgO + Al2O3 nanocomposite improved at elevated temperatures. All nanocomposites showed improved electrical insulating properties compared to virgin low-density polyethylene (Pure LDPE)

Flexible low-density polyethylene–BaTiO3 nanoparticle composites for monitoring leakage current in high-tension equipment.

Polymer–nanoparticle composites prepared using a low-density polyethylene (LDPE) matrix with BaTiO3 nanoparticle compositions of 6, 9, 12, and 15 wt % have shown insulating behavior and are evaluated for their applicability as flexible strain sensors. With increasing percentage of the nanoparticles, the LDPE crystallinity decreased from 38.11 to 33.79% and the maximum electrical displacement response was seen to increase from 2.727 × 10–4 to 4.802 × 10–4 C/cm2. The maximum current, remnant current, and coercive field, all increased with the increasing nanoinclusion loading. Furthermore, the interaction radius values derived from the three-dimensional (3D) model of the nanoparticle dispersion state in polymer–nanoparticle composites were found to be correlated with its key properties. The interaction radius values from the simulated 3D model gave a clear basis for comparing the electrical properties of the samples with the effect of the nanoparticles’ functionalization on the dispersion state in the context of the increased NP loading and giving the values of 275, 290, 310, and 300 nm, respectively. The 12 wt % nanoparticulate-loaded sample demonstrates the best overall trade-off of key parameters studied herein. Overall, the results demonstrate that these flexible polymer–nanoparticle composites could be used for strain-based sensors in the high-tension applications

Tuneable magnetic nanocomposites for remote self-healing.

When polymer composites containing magnetic nanoparticles (MNPs) are exposed to an alternating magnetic field, heat is generated to melt the surrounding polymer locally, partially filling voids across any cracks or deformities. Such materials are of interest for structural applications; however, structural polymers with high melting temperatures pose the challenge of generating high localised temperatures enabling self-healing. A method to prepare a multiferroic-Polyamide 6 (PA6) nanocomposite with tuneable magnetocaloric properties is reported. Tunability arises from varying the MNP material (and any coating, its dispersion, and agglomerate sizes in the nanocomposite). The superparamagnetic MNPs (SMNPs) and iron oxide MNPs with and without surface functionalization were dispersed into PA6 through in situ polymerization, and their magnetic properties were compared. Furthermore, computer simulations were used to quantify the dispersion state of MNPs and assess the influence of the interaction radius on the magnetic response of the self-healable magnetic nanoparticle polymer (SHMNP) composite. It was shown that maintaining the low interaction radius through the dispersion of the low coercivity MNPs could allow tuning of the bulk magnetocaloric properties of the resulting mesostructures. An in-situ polymerization method improved the dispersion and reduced the maximum interaction radius value from ca. 806 to 371 nm and increased the magnetic response for the silica-coated SMNP composite. This sample displayed ca. three orders of magnitude enhancement for magnetic saturation compared to the unfunctionalized Fe3O4 MNP composite

Novel method of healing the fibre reinforced thermoplastic composite: a potential model for offshore applications.

Continuous fibre reinforced thermoplastic composites are increasingly finding their use as engineering materials in many industries due to the excellent fire, smoke and toxicity performance. However, the composite component produced using automated continuous fibre reinforced thermoplastic tapes laying machine are susceptible to sudden failure emanating from microscale cracks. This study demonstrates the healing potential of a layered Glass Fibre Reinforced Polymer (GFRP) composite consisting of alternative layers of GFRP and a magnetic polyamide-6 (PA-6) nanocomposite (PNC) film. The self-healing process is presented in three steps, viz. (i) polymer nanocomposite synthesis, (ii) preparation of the layered GFRP layered composite sample and (iii) self-healing and testing of GFRP layered composite sample. Firstly, the multilayer dog bone sample consisting of a magnetic polymer nanocomposite (PNC) film sandwiched between thermoplastic unidirectional GFRP tapes are prepared. Healing is triggered by exposing the damaged multilayer sample to microwave causing selective heating of nanocomposite film and its subsequent melting. The healing process completes when liquid polymer fills the micro-crack in the multilayer tape through capillary action and solidifies upon cooling. The healing yields 84% of the undamaged tensile strength recovery. Results demonstrate the potential application of an autonomous self-healing method for thermoplastic composite used in the offshore environment

Role of interface in optimisation of polyamide-6/Fe3O4 nanocomposite properties suitable for induction heating.

Induction heating of magnetic nanoparticles (MNPs) and localised melting of the surrounding high temperature engineering polymer matrix by generating microscopic or macroscopic eddy currents during magnetisation of a polymer nanocomposite (PMC) is crucial for realising induction heating-aided structural bonding. However, the polymer heating should be homogeneous and efficient to avoid local pyrolysis of the polymer matrix, which results in degraded mechanical properties, or requiring a large coil for generating a high frequency magnetic field. Increasing the interfacial area by homogeneously dispersing the MNPs in the polymer matrix provides many microscopic eddy currents to dissipate the power through magnetisation and polarisation, leading to micro eddy current induced uniform heating of the PMC. However, the application of a hydrophobic coating on MNPs to aid dispersion can perturb the generation of eddy currents and affect the crystallinity and size of the crystallites responsible for the mechanical properties. In this work, the dielectric and magnetic properties, as well as the degree/size of crystallinity of a PMC containing oleic acid (OA) (22 and 55 w/w%) and silica coated (Stöber and reverse emulsion method) Fe3O4 MNPs were measured to evaluate the effect of the interfacial coating and its chemistry. The correlation between the measured properties and dispersion state of the MNPs was established to demonstrate the comprehensive effects of interfacial coating on the PMC and this is a unique method to select a suitable PMC for induction aided structural bonding applications. The results showed that the lower amount of OA (22 w/w%) helped achieve the best dispersion to reduce the crystallinity size and increase degree of crystallinity, and to give the best candidate for achieving mechanical properties of the bonded carbon fibre reinforced polymer (CFRP). Moreover, the low concentration of OA helped achieve high polarisation for dielectric heating as well as eddy current formation due to the relatively high magnetic saturation. The silica coating proportionally reduced the magnetic response and electric polarisation of the PMC, which could affect its eddy current generation that is responsible for induction heating

Insulating polymer nanocomposites for high thermal conduction and fire retarding applications.

The possibility of combining the flexibility and light - weight of polymers with the highest insulation of ceramics, drives the field of nanocomposites for potential commercial application. The inclusion of nano-sized insulating particles in the polymer matrix, and orienting the fillers along the direction of heat flow results in modifying the induced interfaces for effective phonon propagation. Such flexible polymer nanocomposites (PNC) offer easy workability and refined insulating effect with high thermal conductivity and fire-retardancy. Hence, opening a wider arena of applications with the advantage of their light-weight. The engineering of the interfaces, is the key for dictating the desired properties at the macro-scale. Consequently, silane functionalisation of nanoparticles with designed dispersion technique was tried for achieving this purpose. Transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, Differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), and Dynamic mechanical analysis (DMA) were done to characterize the properties and structure of the synthesised nanocomposite. This paper reports that surface modification of the nanoparticles can effectively solve the dispersion problem and reduces the electric field charge concentration at the interface. Synthesising PNC with selective nanoparticle loading percentage can yield a lmost 6-12% increase in the thermal capacity and fire retardability of the base polymer. Presenting an effective way of resulting in a commercially promising PNC suitable for various defence applications of radome technology, energy storage (e.g. batteries), structural bodies and cables in general