Final Report Summary - NANCORE (Microcellular nanocomposite for substitution of Balsa wood and PVC core material)
The major objective of this project is to developed novel nanocomposite foams that can replace polyvinyl chloride (PVC) and Balsa wood in sandwich structure applied in wind turbine, yachting and railway industries. Through the combination of a strong product oriented focus and the requirement of a range of scientific and development activities to meet associated scientific and technological objectives in the project, a multi-scale approach is implemented for synthesis, characterisation, modelling and processing of materials in order to relate nano and microscale controlling mechanisms and parameters to the macro performance. After the formulation of major material specification requirements, processing and manufacturing issues related to new nanocomposite foams are first addressed comprising activities related to the selection of raw materials and components, development of formulations and processing parameters to produce the first, and after optimisation, the second generation of nanocomposites and nanocomposite foams. This is followed by the activities focusing on up-scaling possibilities for different manufacturing methods. Then characterisation and modelling is considered comprising standard and developed methods for the assessment of nanocomposite foams' mechanical properties, in-situ investigations of different material mechanisms operative either under processing, such as evolution of foam cells during foaming, or under mechanical loading where the deformation pattern of cells may be quantified and related to the underlying microstructure. Modelling approaches are based on the multi-scale bridging concept, where the lower scale model is experimentally verified and embedded into a larger length scale model with a subsequent experimental confirmation of its predictive capabilities.
Manufacturing and characterisation of sandwich elements including determination of fundamental mechanical and hydrothermal properties, in-situ characteristics related to strength and fracture of sandwich components, as well as considerations towards the design and manufacturing of possible demonstrators constitute guidelines for eventual full-scale testing if it is feasible within project's incurred restrictions.
The technologies developed in the project are assessed in a life cycle perspective in order to disclose whether a real environmental improvement from the developments is achieved or not. Furthermore, activities on safety issues of nanoparticles from nanocomposites and nanocomposite foams are broadly addressed in areas of exposure, hazard ad risk assessment.
Project context and objectives:
Development of new materials is one of the most important drivers for progress in a wide range of technological areas of high strategic interest for the future progress of our societies. In this sense basic materials research is the most cross disciplinary field in modern science, with direct impact on all areas of everyday life. The fast development of both synthesis and characterisation capabilities at the nanometre scale over the past decade enable new approaches to design of materials with properties which are not available from conventional materials. This approach, with a high degree of control from the atomic level over nanostructures to the macroscopic scale, opens a wealth of opportunities for tailoring combinations of materials properties to specific applications. Building a number of functionalities into the final products by proper design of the constituent materials holds a large number of advantages with respect to cost effective and environmentally friendly design. This has led to a continuously increasing interaction between engineering design of structures and components and basic materials science.
The success of new materials developments depends on development of new processing techniques that can provide cost-effective fabrication of structures and components with improved performance. The success factor is the ability to make these material-related developments timely at relatively low-costs. This demands not only rapid development of new / improved processing techniques but also better understanding and control of material chemistry, processing, structure, physics, performance, durability, and, more importantly, relationships between these fields. This scenario usually involves multiple length (space) and time scales and multiple processing and performance stages, which are sometimes only accessible via multi-scale / multi-stage modelling or simulation. It is this integrated approach to materials, processes, and applications together with the span from physics and chemistry at the basic level to synthesis of macroscopic materials that has been the specific hallmark of the activities in the project.
The principal objective of the NANCORE project is to design novel microcellular polymer nanocomposite foams, with mechanical properties and cost characteristics allowing for a substitution of Balsa wood and PVC foam as core material for lightweight composite sandwich structures. Advanced composite sandwich structures represent a special type of composite laminate where two thin, stiff, strong and relatively dense skins or faces, usually made from polymer based fibre-reinforced materials, are separated by a thick, lightweight and compliant core material. Sandwich structures find a wide spread applications in different industrial sectors including transport, building, yachting and aviation.
This project concentrates on developing new core materials which can be applied mainly in wind turbine industry including also yachting and rail transportation applications. The core materials of sandwich structures typically fall into one of three groups: polymer foams, Balsa wood and honeycomb materials. Traditionally, sandwich elements of wind turbine blades contain either PVC foam, which is not environmentally friendly, or Balsa wood exhibiting recurrent shortages and supply problems. From the outset of the project, thermosetting polyurethane (PU) and thermoplastic polypropylene (PP) systems were selected as potential replacement candidates. These two polymer systems occupy the large market share in terms of the amount consumed and are relatively cheap. In order to fulfil specification requirements for the sandwich core material, PU and PP polymer system need to be reinforced with nanoparticles and subject to foaming processes. Thus, manufacturing of sandwich core materials combines two different processing routes, one for the synthesis of nanocomposites and another for foaming. Therefore the choice of material components constituting a final core material must simultaneously compromise adequateness of the material composition to both processes.
Polymer nanocomposites represent a radical alternative to conventional filled polymers, since nanocomposite materials have large surface area-to-volume ratios for the reinforcing phase with nano-sized fillers. Therefore, an improvement in property arises because the length scale of the nanocomposites morphology and fundamental physics associated with a property coincide, i.e. surface effects that govern chemical and physical interactions are operative on the scale comparable to the size of nanofillers. However, there is still a lack of understanding of the interfacial bonding between the reinforcements and the matrix material from both analytical and experimental viewpoints and considerable uncertainty in theoretical modelling and experimental characterisation of the nano-scale reinforcement materials. The main challenges have mainly to do with issues related to composites processing: assessing a good dispersion quality of nano-inclusions in polymer matrix, achieving high volume and high rate fabrication which is fundamental to manufacturing of nanocomposites as a commercially viable product, and acceptable cost effectiveness. A consensus is yet to emerge on those issues.
Nanocomposite polymer foam is defined as a two phase material in which air bubbles are entrapped in a continuous nanocomposite phase. Two steps are involved in the foaming process: bubble nucleation and growth. Both processes are affected by many physical properties which are inter-related and many are complex functions of foaming conditions. Enhanced thermo-mechanical properties of nanocomposite foams result from improved cell morphology that is mainly attributable to the role of nanoparticles as nucleation agents for bubble generation. Although the nucleation mechanism is still not fully understood, it is generally recognised that the size, shape and distribution of the nanoparticles, as well as the surface treatment, can affect the nucleation efficiency. Due to the high nucleation efficiency, nanoparticles provide a powerful way to increase cell density and reduce cell size. The nanometre dimension of nanoparticles is especially beneficial for reinforcing effect of foam materials, considering the thickness of cell walls is usually in the range of few micrometres. Nanocomposite foams can be obtained by several processing protocols depending on the type of component materials used. For the selected PP and PU polymer systems, logistics for manufacturing of nanocomposite foams is illustrated. The PP nanocomposite foams need to be processed in two major steps, first preparing an adequate nanocomposite material and then using appropriate foaming technique, whereas manufacturing PU nanocomposite foams follows a different path is also illustrated. Each box represents several alternatives and possible choices, which leads to large number of constituent materials and components, processing protocols, manufacturing conditions and hence all meaningful combinations needed to be investigated.
Synthesised nanocomposite foams are used as sandwich core materials to assess load bearing capability and compatibility with face sheets of sandwich structures. Nanocomposite foam cell structure and physical properties of nanocomposites determine the quality of the interface between the core and face sheets of the sandwich elements. During flexural loading, the core basically controls the failure of the sandwich structures and therefore, if characteristics of the core material can be appropriately designed, the enhancement will be reflected in the performance of the entire sandwich structure.
There is a general concern that nanoscale particles may have negative health and environmental impacts. This is because the size of the nanomaterials is comparable to human cells and large proteins thus the regular human immune system may not work against them. While nanoparticles may enter the body by inhalation, due to skin contact, or by ingestion, the major concern is by inhalation during processing. The safety issues must address exposure, hazard and risk assessment with nanomaterials and processing technologies worked with at different laboratories of involved in the project partners. Toxicology can provide data on hazard of nanoparticles based on in vitro assays and input from the production, materials processing and material characterisation activities. Furthermore, physicochemical and other characterisations of nanocomposites and any particle emissions can provide insight into the determinants of the toxicity and the potential for human exposure. This is a key to ensuring integration of the information from the exposure and hazard assessments into the risk assessment where dose-response models allow estimating the health risk and developing control measures for workers in the exposure scenarios.
Project's major objectives need to be reached through the combination of a strong, product oriented focus and implementation of scientific and technological development within each of the central problem areas mentioned above. The group of scientific objectives comprises objectives related to:
- Synthesis and processing of nanocomposites and nanocomposite foams including:
a) selection of raw materials and additives;
b) impurity removal from raw material precursors;
c) clarification of role of nanofillers in promoting polymer foamability and foam morphologies;
d) functionalisation of nanofillers focussing on physical/chemical compatibility with foaming processes;
e) development of new paradigms for creating nano-scale building blocks and processing protocols;
f) development of novel processing routes for nanocomposite foams.
- Characterisation and modelling including:
a) characterisation and modelling tools for mechanical behaviour of novel materials;
b) real-time characterisation and materials' structure evolution during synthesis, where possible;
c) spatially resolved microstructure and in situ monitoring;
d) testing and characterisation of structural sandwich elements with nanocomposite foams as core material and conclusive guidelines for scale-up processing of nanocomposite foams;
e) fundamental models to accurately predict nanostructure formation;
f) nano / micro scale predictive models of nanocomposite foams.
- Safety issues of nanoparticles from nanocomposites including:
a) determinants of environmental impacts and potential health and safety risks of nanoparticles used in nanocomposite materials;
b) risk management and techniques to minimise and control health risks to manufacturers, downstream users and impacts to the environment.
The group of technological objectives includes:
- development of a novel polymer foams reinforced with nano-sized fillers which display mechanical properties in the require range;
- cost-effective production of the new materials and the manufacturing of structural sandwich demonstrators with nanocomposite foams;
- risk assessment and estimation of safe occupational exposure levels to particles associated with the production and machining of nanocomposites and development of eco-efficiency indicators and documentation of life cycle impacts.
Project results:
Significant results were obtained in different areas by tuning scientific and technological achievements towards project's objectives:
Synthesis and processing of nanocomposites and nanocomposite foams
PP-based systems
Several different grades of PP materials were tested in order to find the most suitable matrix for nanocomposites. Three selected PP materials were successfully used to produce first generation of nanocomposites with different nanofillers, such as multiwall carbon nanotubes (MWCNTs), carbon nanofibres (CNFs), polytetrafluoroethylene fibres (PTFE) and montmorillonite (MMT) platelets. Further trials showed that high melt strength of PP materials is a crucial factor for subsequent foaming ability of PP nanocomposites. Therefore, two other grades of PP matrix were thoroughly tested and accepted as basic PP systems for second generation of nanocomposites.
Nanocomposites containing PTFE fibres differ from other nanocomposites containing reinforcing nanoparticles as PTFE fibres are created in-situ during blending with PP matrix. Pilot experiments using PP of high melt viscosity and reaction grade PTFE clearly showed that it is relatively easy to generate all-polymer nanocomposites with fibril reinforcement just by compounding the two polymers. All-polymer nanocomposites are a new type of nanocomposite characterised by many unusual properties. One of the most interesting features of PP / PTFE nanocomposite is its high melt strength and high elongation viscosity, the property that is required for efficient foaming. PTFE nanofibrils built up a continuous network, which proved to be necessary for melt strengthening even for PP matrix that originally does not show any melt strength. This fibril structure was the object of different observations made by scanning electron microscopy (SEM), transmission electron microscopy, melt and solid rheology, microcalorimetry and X-ray methods.
The idea for simplification of producing nanocomposites with fibril reinforcement was to find a way to strongly deform PTFE inclusions during shearing or compounding and preserve their shape after deformation, i.e. to act against capillary instabilities. The use of crystalline PTFE inclusions instead easy to deform nanoparticles seemed to be a promising option to generate polymer nanofibres through shearing of a polymer matrix in which PTFE crystalline inclusions are dispersed. Crystals are not subjected to capillary instabilities and they preserve their shape. Shearing of the matrix led to a mechanical instability of a crystal which results in sliding layers as the shear deformation increases. The shear modulus and shear yield stress of the PTFE assumes values characteristic for crystals of many polymers at temperatures few tens of degrees below their melting. The above estimation made the idea to transform PTFE polymer crystals into nanofibres by shear deformation quite realistic. Another problem was to obtain very large deformation of polymer crystals and hence thin fibrils. That can be solved by using crystalline powder formed by disentangled macromolecules. The reaction grade PTFE in the powder form fulfils this condition and as a result, PTFE macromolecules are not entangled and PTFE powder grains can be easily deformed to very large deformations.
In several experiments, molten PP premixed with 3 wt % of PTFE particles were subjected to shearing with suitable rate and temperature range until the requested deformation of the matrix was achieved. It appeared that deformation of PTFE particles follow nearly strictly the deformation of PP matrix. Those experiments in uniform shearing proved that it is possible to produce all-polymer nanocomposites by shearing starting with raw polymers: chain disentangled polymer powder grains and thermoplastic polymer matrix. Twin screw extruders and also twin blades batch mixers were used for fabrication of nanocomposites from PP pellets and PTFE chain disentangled powder. The method of manufacturing of all-polymer nanocomposites is the application's subject for the European patent.
A series of protocols, compositions and processing conditions were employed to optimise manufacturing PP / MMT with nearly fully exfoliated MMT platelets. Foaming involves an extensive deformation of polymer matrix significantly influencing the crystallisation of PP-based nanocomposites with organo-modified MMT and compatibilised PP matrix. Neat PP was also studied for comparison. At lower shear rates the effect of shear deformation on the crystallisation kinetics and orientation of crystals was stronger in the nanocomposites than in neat PP. The differences between the nanocomposites with different MMT contents were minor, as well as between those differing in the compatibiliser contents. Pre-orientation of the clay achieved by pre-shearing did not have any significant effect on shear induced crystallisation and crystal orientation of PP / MMT nanocomposites. A new staining method for the purpose of transmission electron microscopy investigations was developed to show compatibiliser's localisation in PP / MMT nanocomposites and confirmed that no MMT platelets exfoliation could occur without compatibiliser surrounding. A novel method to exfoliate MMT platelets by using azodicarbonamide as additive has been discovered. This is a simple procedure allowing improving the exfoliation of nanoclays in thermoplastic non-polar matrices without the use of compatibilisers. The key advantage of the method is its very low cost in comparison to other alternatives. Yet another novel method of examining the exfoliation of MMT platelets from rheological data was developed showing that platelet exfoliation could be sensed as a function of certain rheological properties. It has been concluded that complete exfoliation of MMT platelets does not guarantee melt strengthening necessary for the foaming process. While it is a drawback in terms of technological issues, it remains as an important scientific observation. In the molten state PP is separated from MMT platelets through compatiliser and can freely slip along platelets not contributing to melt strain hardening. The lack of melt strengthening is prevailing at relatively slow rates of material flow characteristic for foaming processes. Hence, the use of high melt strength PP for foamable nanocomposites is strongly advised and recommended. For such systems transmission electron microscopy images show homogeneous distribution of MMT platelets in foam walls oriented along the wall expansion direction.
Transmission electron microscopy studies revealed that the presence of agglomerates in CNTs and CNFs nanocomposites is related to agglomerates that are present in CNF and CNT masterbatches. Even very high shearing is not sufficient to eliminate nascent agglomerates because of strong interactions between single CNTs and CNFs. Neither usage of pure fillers, their filtration, ultrasound applied during compounding in a batch mixer nor the change of compounding machine from dedicated twin-screw co-rotating extruder to twin-screw self-cleaning co-rotating extruder was helpful. Many aggregates of CNTs and CNFs were still visible in ultrathin sections of nanocomposites. It is very probable that a significant improvement of CNT and nanofibres CNF dispersion avoiding agglomerates can be achieved using powdered PP that will be dusted with plain CNT or CNF and no use of masterbatches will be then needed.
The compression moulding technique has been improved to accommodate control of cellular structure in PP foams. The precursor material or the foamable pellets are introduced in a mould and the piston is introduced in the mould to apply a certain pressure. Temperature is increased to values higher than the decomposition temperature of the blowing agent. When all the gas from the chemical blowing agent is generated, pressure is released and foaming takes place. Expansion ratio (ER), i.e. density, is controlled by the piston's displacement of the piston. This has a significant advantage over the conventional method; the density can be easily controlled and foams expand only in one direction which allows producing anisotropic foams.
Amongst different nanocomposite PP foams manufactured with this technique, the foams based on blends of a high melt strength PP and a linear PP have been produced varying the amount of linear polymer in the blends. For these particular materials, the cellular structure (open cell content, cell size and anisotropy ratio) and mechanical properties have been measured.
The new developed procedure based on extensional rheology tests, which has the capability of predicting whether a given formulation would have sufficient foaming behaviour, limited the number of investigated PP nanocomposites to ten systems, allowing for a significant time saving in the development of nanocomposite formulations for foaming.
PU-based systems
Reactive foaming at atmospheric pressure and room temperature has been used to produce the foamed PU nanocomposites. In this technique, the preformulated polyol is mixed with isocyanate, the foaming starts by the generation of carbon dioxide (CO2) due to the chemical reaction of isocyanate and water contained in the polyol. At the same time the polymerisation reaction of PU is produced, i.e. foaming and polymerisation occurs simultaneously. An adequate chemistry allows obtaining low density rigid foams. This is the technique which evolved during the course of the project by systematically changing components' chemistry and assuring that the final PU system fulfils specification requirements with respect to density, closed cell content and mechanical properties. The main processing parameters identified as essential to control the final foam characteristics comprise pre-mixing of the polyol compound, mixing method of the nanoparticles in the polyol and / or the isocyanate, mixing method between the polyol and the isocyanate and foaming temperature. Different types of commercial nanoparticles, as received or after a surface treatment, have been tested as reinforcing additives for the nanocomposite foams, and then eleven different PU nanocomposite foams were thoroughly investigated. Due to processing limitations for up-scaling of nanocomposites only pristine PU foams were successfully manufactured in a pilot semi-industrial production line.
A novel approach has been developed for the synthesis of hybrid nanofillers by growing CNTs between galleries of MMT platelets. The process involves a cation exchange process of the clay, calcinations step and finally growth of CNTs in between the silicate layers in the clay using chemical vapour deposition. The nanohybrids have been used as reinforcing filler in PU foam and have been shown to improve significantly the mechanical properties of the resulting PU nanocomposite foams.
Characterisation and modelling
Special equipment has been designed for the analysis of the foaming process of cellular polymers allowing quantifying phenomena such as nucleation, growth, coalescence and diffusion coarsening of foam cells. Specific hardware and software have been developed to implement this technique in investigations clarifying foaming mechanisms operative during processing.
The system includes X-ray source and a detector. The fundamentals of the technique are based on the interaction between radiation and matter, i.e. the transmitted radiation carries the information captured by a flat panel as a radiographic image. Standard acquisition time for X-ray radioscopy experiments is between one and two seconds, limiting the time resolution to these values. However, PU foaming process takes place very fast. Once the two reactants are mixed the process is already in progress. For this reason, it was necessary to increase the time resolution by special procedures in order to obtain motionless and not blurred images.
A comparison between pure PU foam reaction sequences and those corresponding to PU / MMT nanocomposite system is shown at different foaming stages. The frame-by-frame visual inspection permitted to determine absence of bubble ruptures in both materials. It indicates that the addition of nanoclays has more influence on the initial nucleation and does not substantially affect the coarsening or coalescence phenomena. Zoomed images of both cured PU foams, with and without nanoclays, clearly indicate how different the cellular structure in both materials is. The neat PU cellular structure shows larger and fewer cells in comparison with nanocomposite PU foam.
The microstructure and deformation of foams was characterised using in-situ compression tests inside the SEM chamber with an aim to record deformation pattern of cell walls within the elastic mode of deformation and investigate typical failure modes. A similar procedure was developed for X-ray microtomography investigations; in both cases a dedicated in-situ loading device was designed for an accurate control of deformation applied to the foam samples. The evolution of deformation pattern in PP foams under compressive loading from the SEM is illustrated. Wrinkling of cell walls, initiation and propagation of micro-cracks in cell walls and buckling of cell walls are major mechanisms of cells deformation leading to final failure. Buckling of cell walls, is more apparent on images from X-ray microtomography tests, which provide visualisation of deformation pattern in three-dimensional space.
During measurements of mechanical properties of polymeric foams it is often a challenge to ensure an appropriate load transfer from the experimental equipment to material samples. This discrepancy may result in large variations / uncertainties in the measured values or in worst case erroneous values. This is particularly true for the measurement of shear stiffness and strength. The dedicated method from project's background has been extended for present purposes avoiding these difficulties and unifying calibration of measurements among participating laboratories. The method is a combination of traditional mechanical testing, digital image correlation and numerical modelling. Using this approach, the tensile, compressive and shear properties may be determined from just one type of test samples which is much easier to produce than traditional samples for the same test. Furthermore, the problems related to the load transfer from the test equipment to the test samples has been largely reduced, if not solved. Finally, through newly developed test method different bi-directional loading conditions can be applied to foam materials. The method allows unambiguous determination of foam shear properties, which are of primary interest in foam applications in sandwich structures.
Theoretical modelling and simulations are the key enabling fields for reducing the time needed to design new materials, increasing the predictability of properties and proper interpretation of experimental measurements due to the complexity of the measurement at atomic and nano-scales. Optimisation of nanomaterials requires exploration of many alternatives prior to synthesis. Therefore modelling and simulation efforts that may provide the most quickly realisable benefit include the development of computational models to predict the properties of materials at different scales and bridging length scales in order to predict overall, macroscopic properties of materials. Appropriate validation experiments are also crucial to verify that the models predict the correct behaviour.
Reliable determination of elastic constants for clays is still unresolved problem. Direct experimental measurements are not available by any technique. To overcome this obstacle the assignment of elastic constant to clays is usually based on indirect measurements. Quantum mechanical simulations revealed that the clay platelet possesses overall orthotropic symmetry and is characterised by five elasticity constants whose numerical values were calculated for the first time. Molecular model of the intercalated region of clay gallery was built and molecular dynamic simulations were performed in order to determine the interlayer distance from the model. Single layer clay was simulated with a purpose to calculate vibrational spectra and compare them to experimentally available data.
Furthermore, exfoliated model of clay was simulated to disclose local interactions between the clay surface, surfactant and polymer matrix. The interlayer distance resulting from the simulation very closely resembles the experimental value. Molecular dynamic simulation provides also valuable information on interfacial properties and possibility to optimise organo-modification of clays. With growing length of surfactant molecules (or increased volume of the surfactant) the binding energy clay / surfactant and clay / polymer decreases whereas binding energy surfactant / polymer increases. Thus optimisation of the organic modification is possible maximising interfacial strength with binding energies constraints. This information was used for optimisation of PP nanocomposites. Molecular model of clays reproduces also very well vibrational infrared spectra obtained from experimental measurements. These results suggest that created molecular models are reliable and correctly reproduce selected experimental data.
Similarly, successful models at micro and mesoscale were developed and tested against experimental data. A model predicting elastic constants of the cell wall material reinforced with nanoclays was verified taking into account such effects as clustering of nano-clays, proximity of nanoclays to surfaces (e.g. the cell walls in the microcellular foam), the presence of the interphase and re-orientation of nanoclay platelets during the foaming process. This model has been consecutively embedded in macro-scale model to predict stiffness of pure and nanocomposite foams. Results showed good correlation with experimentally measured values of Young's moduli for PU foams whereas properties of PP foams were significantly overestimated, which could be due to different structure and properties of the polymer in the cell walls and in the bulk.
Safety issues of nanoparticles from nanocomposites
The risk assessment provides information based on the analysis of scientific data which describe the form, magnitude, and characteristics of a risk, i.e. the likelihood of harm to humans or the environment, following the identification (characterisation) of exposure and hazards. Although risk assessment is mainly a scientific task, political decisions are required on matters such as: 'What are we trying to protect and to what extent should it be protected'?
The purpose of exposure characterisation was to identify and describe the potential exposure scenarios relevant to the tasks which were to be carried out within the project. Information was collected from the peer-reviewed literature in the public domain that was relevant to the health and environmental safety of nanocomposites using PubMed and Web of Knowledge. Furthermore, an online workplace exposure survey was developed to help identify generic and substance-specific activities which may present an exposure potential for subsequent investigation. The exposure scenarios identified for use in the occupational exposure assessment and the complete set of questions and anonymised responses from the Online Workplace Survey serve as a primer for the subsequent exposure measurement and risk assessment. The findings related to quantitative exposure and contextual data for key exposure scenarios' span all NANCORE project activities encompassing the manufacture and processing of PU and PP nanocomposites.
Measurements and observations of the manufacturing and processing activities were conducted at four partner sites, using either all or a selection of nanomaterials adopted by the project consortium (CNTs, CNFs and nanoclays).
The hazard assessment comprises the toxicity profiles of the NANCORE compounds to address the fibrogenic potential of the materials, their bio durability in the lung environment and further material characterisation. This information enables the contextualisation of dose used within in vitro assessments to human exposure and provides a benchmarked profile of toxicity compared to control particles of known toxicity.
The risk assessment task contains the evaluation of the exposure risk and the estimation of the overall risk using exposure scenarios within the project through a comparison of the output given in the control banding tools with the results from the exposure measuring campaign to evaluate whether the tools can be used by the beneficiaries to plan exposure control strategies.
The research-based approach to safety issues and very comprehensive documentation collected during project's duration may be considered as an important step towards lacking standardisation within this area.
Technological issues
The project was successful in providing well documented compositions, processing protocols and manufacturing techniques for variety of nanocomposite foams that fulfil specification requirements in terms of fundamental mechanical properties. However, up-scaling problems from the laboratory scale to industrial or semi-industrial scale still persist and present challenges with regards to feasible and cost-effective processing techniques. The major hurdle relates to bulk production of nanocomposites where all composition and processing protocols from small laboratory scale manufacturing need to be projected to large industrial scale. Furthermore, processing technique itself may not be easily accommodated in the large scale production. For instance, up-scaling the improved compression moulding technique, which is successful at the laboratory scale, would be limited by a realistic size of mould and press. Also physical aspects of foaming in large quantities may pose difficulties not observed at small scales for example heat transfer from interior of the foam and its dissipation. It has been observed that during laboratory scale continuous extrusion, when foamed nanocomposite reaches the thickness approximately 3 mm, the foaming dye heat transfer from the inside of the foam occurs slower than the bubble wall crystallisation. This results in bubble collapse and worsening the foam properties. Following the problems with up-scaling the extrusion foaming process of PP nanocomposites, some activities were initiated to use another foaming technique focusing on micro-bead technology foaming. Micro-bead structure analysis revealed that foams have fully closed cell structure and have a density within the desired range. This technology is actually under further development in collaboration with industrial partners.
Nano technological processes and systems are not yet well investigated from a life cycle perspective. Performing lifecycle assessment (LCA) in this project is an important contribution not only to the project itself but also to the availability of data for the processes included. The available literature on environmental impacts of the suggested polymers and nanoparticles in a life cycle perspective was scrutinised and a first simple assessment performed. Further analysis was performed using SimaPro LCA software and connected data bases, as well as using data received for operations related to foaming processes and recycling from other partners in the consortium, and recycling opportunities and eco-indicators were established. The recommendations are focused on low energy requirements, limitations in the use of acid for CNT purification, the selection of catalysts according to their associated environmental burden when produced and disposed (most catalysts are either highly toxic heavy metals or organic substances). A special attention also concerns human toxicity via air, which might be relevant in case of releases and exposures to nanoparticles during the life cycle of the nanocomposite, and ecotoxicity in aquatic ecosystems due to potential long-term emissions from landfills.
In order to serve as a benchmark / reference for the new technology, data for current practice and technology was collected from partners and their suppliers as well as from the literature. The general goal has been to construct an inventory of the current state of practice and technology regarding the manufacture and application of structural composites in the wind turbines, maritime, and rail industries in Europe, in order to define a common analytical framework for the further LCA. The scope included the inventory of technologies, materials and energy involved in the manufacturing, use and disposal of products in the three studied industries. Each product defined different boundary and object for analysis and a particular technological setting.
For the NANCORE technologies, data were collected from industrial partners to the extent possible, in order to clarify their coming use of the new technology in sandwich structures. Product system models were built focusing the modelling on wind turbine blades. In addition, a review of the LCAs on wind turbines was performed in order to put the results of the blades into the perspective of the whole turbine. A number of hypothetical cases / scenarios for the use of nanocomposite foams in sandwich structures were considered. Four different nanoparticle types with three different concentrations embedded in PU foam as well the pure PU foam were compared to the use of an equivalent volume of Balsa wood under the assumption that a direct substitution would not have any impact on the use stage of the wind turbine. The models show that using nanocomposites in sandwich structures as replacement for Balsa wood or PVC foam is generally not environmentally beneficial. This is mainly due to high environmental impact associated with the production of nanoparticles but also due to the fact that production of PU foams has a higher environmental impact as compared with Balsa wood.
Potential impact:
The potential impact and added value from obtained results follows the multidisciplinary character of the project and may be categorised according to major research areas involved. The scientific and technological impact on processing comprises:
- establishing processing protocol to produce fully exfoliated PP/MMT nanocomposites, PP / PTFE nanocomposites with high melt strength properties and gaining an extensive know-how on nanofillers treatment in PP matrix;
- development of various foaming protocols for PP / MMT and PP / PTFE materials;
- testing up-scaling capability for industrial application for various foaming methods;
- development of the novel method to exfoliate nanoclays without using compatibilisers;
- establishing processing protocol for novel CNT/MMT nanoparticles for reinforcing as well as other functional purposes such as electrical conductivity and gas barrier properties.
The impact related to characterisation of materials covers:
- examining several PPs and nanofillers in terms of melt strength, foaming capability, final material durability and possibility to form sandwich structures;
- establishing extensive database of connections between processing parameters and final characteristics of the foamed materials;
- development of methodology based on extensional rheology tests allowing predicting appropriateness of given polymer formulation for foaming purposes;
- development of X-ray radioscopy technique for the analysis of foaming processes allowing quantifying phenomena such as nucleation, growth, coalescence and diffusion coarsening;
- new rheological methods to assess the quality of nanoparticles dispersion in the polymer matrix;
- new methodology to validate the wetting behaviour of clays in nanocomposites and foamed nanocomposites;
- development of characterisation procedures for in-situ monitoring of the deformation pattern in foams and stress transfer in CNT nanocomposites.
The impact related to measurement techniques contains:
- development of the transmission electron microscopy staining approach to determine interface behaviour between clay and PP matrix;
- development of the nuclear magnetic resonance procedure to examine polymer chains dynamics and structure in foams;
- parallel plate rheometer has been modified to validate the measurement of rheological properties of flexible PU foams during foaming;
- development of the experimental setup and measurement procedure for non-standardised fracture toughness properties of foams;
- utilisation of the specialised setup to unify and unambiguously determine shear properties of materials.
The scientific impact on modelling methodologies includes:
- development of the atomic simulation procedure for the model prediction of properties not accessible for direct measurements but validated by parent measurements;
- development of predictive multiscale modelling technique for foams and nanocomposite foams including sequential incorporation of material parameters measured at different length scales.
The impact on safety issues incorporates:
- Advances in the assessment of the potential risks posed by nanomaterials in a variety of lab and pilot-scale processing activities has been gained through the experience in NANCORE, specifically allowing reflection on the strengths and limitations of the techniques and instruments employed to gather, analyse and interpret multi-component data from complex workplaces and laboratory environments. The pragmatic outputs from the 'Safety' work package, in the form of the case examples of control measures to mitigate exposure cutting across the site-specific activities, have been drawn together with the intention of being as relevant as possible to a variety of manufacturers and researchers both within the sectors specific to NANCORE and beyond. These include general nanomaterial handling tasks in the extrusion laboratory (i.e. weighing and mixing, batch mixing precursor powders, and extrusion), lab-scale master batch extrusion, compression moulding, lab-scale PU foam synthesis, pilot-plant PU foam synthesis, sawing, milling, and materials testing.
- Formalisation of the institutional policy on nanofiller processing and waste disposition.
These impact elements are joint for project's beneficiaries as their accessibility and dissemination was freely available for all partners. The impact on some other areas specific for an individual partner may include issues related to education, learning and/or training as well as issues related to company / institution / department strategy. The impact gained by the manufacturer (Recticel) of PU foams, comprises strengthening the concept 'strong & light' as a strategic research and development (R&D) platform for Recticel and the possibility of further tailor-made design / development regarding the end users applications. The Azimut-Benetti (AB) recognises the impact on applied research and design of structures with the new foam where the project allowed accomplishing the first, decisive steps in the development of new structural design and its parametric analyses including the investigation of some details which are often only partially analysed in current solutions. LM WindPower sees the impact through a more realistic picture of how matured are nanocomposite foams for the company's product strategy. A larger time frame is needed for the research before the results could be incorporated in the production, both in terms of processing, pricing and technology readiness in general. It has been also recognised that the extension of know-how provided by the project is beneficial for the future strategy; the element of impact appreciated also by the Focal partner.
All participating academic partners strongly appreciate the impact on advancement of knowledge in several interconnected scientific disciplines, fostering involvement in new strategic scientific and technological programmes both on national and international level. A significant added value from the project's results has been created through the education at undergraduate and graduate level. Furthermore, the education of candidates with a sharpened profile within advanced materials and processes makes them attractive to industry, which leads to the transfer of technological expertise from research to industry and substantially contributes to future society. A high level of the scientific research exercised in the project has been one of the prerequisite requirements for the new Master programme in materials technology approved by the Danish Ministry of Science, Innovation and Higher Education at the Aalborg University commencing September 2013.
Dissemination activities were mainly carried out via publications in recognised scientific journals and presentations at national and international conference, symposia, workshops and other meetings. During the time interval allocated to the project, 10 research papers were published in peer reviewed journals, 8 have been submitted, 9 are in the preparation, and attendance and presentations at 50 conferences have been recorded. Furthermore, seven doctorate (PhD) theses are expected to be completed no later than medio 2014.
The exploitation of result fall into two foregrounds categories: exploitation of R&D results via standards and a general advancement of knowledge. The first category comprises the exploitation of physical testing, material characterisation, lightweight and durable materials, new products and new production and characterisation technique whereas materials characterisation tools, new products, functional materials and predictive simulation model at nanoscale belong to the second foreground category. For one exploitable product / measure the national patent was granted and one European patent application is on its way. Patent applications are under consideration for another two exploitable products / measures.
Manufacturing and characterisation of sandwich elements including determination of fundamental mechanical and hydrothermal properties, in-situ characteristics related to strength and fracture of sandwich components, as well as considerations towards the design and manufacturing of possible demonstrators constitute guidelines for eventual full-scale testing if it is feasible within project's incurred restrictions.
The technologies developed in the project are assessed in a life cycle perspective in order to disclose whether a real environmental improvement from the developments is achieved or not. Furthermore, activities on safety issues of nanoparticles from nanocomposites and nanocomposite foams are broadly addressed in areas of exposure, hazard ad risk assessment.
Project context and objectives:
Development of new materials is one of the most important drivers for progress in a wide range of technological areas of high strategic interest for the future progress of our societies. In this sense basic materials research is the most cross disciplinary field in modern science, with direct impact on all areas of everyday life. The fast development of both synthesis and characterisation capabilities at the nanometre scale over the past decade enable new approaches to design of materials with properties which are not available from conventional materials. This approach, with a high degree of control from the atomic level over nanostructures to the macroscopic scale, opens a wealth of opportunities for tailoring combinations of materials properties to specific applications. Building a number of functionalities into the final products by proper design of the constituent materials holds a large number of advantages with respect to cost effective and environmentally friendly design. This has led to a continuously increasing interaction between engineering design of structures and components and basic materials science.
The success of new materials developments depends on development of new processing techniques that can provide cost-effective fabrication of structures and components with improved performance. The success factor is the ability to make these material-related developments timely at relatively low-costs. This demands not only rapid development of new / improved processing techniques but also better understanding and control of material chemistry, processing, structure, physics, performance, durability, and, more importantly, relationships between these fields. This scenario usually involves multiple length (space) and time scales and multiple processing and performance stages, which are sometimes only accessible via multi-scale / multi-stage modelling or simulation. It is this integrated approach to materials, processes, and applications together with the span from physics and chemistry at the basic level to synthesis of macroscopic materials that has been the specific hallmark of the activities in the project.
The principal objective of the NANCORE project is to design novel microcellular polymer nanocomposite foams, with mechanical properties and cost characteristics allowing for a substitution of Balsa wood and PVC foam as core material for lightweight composite sandwich structures. Advanced composite sandwich structures represent a special type of composite laminate where two thin, stiff, strong and relatively dense skins or faces, usually made from polymer based fibre-reinforced materials, are separated by a thick, lightweight and compliant core material. Sandwich structures find a wide spread applications in different industrial sectors including transport, building, yachting and aviation.
This project concentrates on developing new core materials which can be applied mainly in wind turbine industry including also yachting and rail transportation applications. The core materials of sandwich structures typically fall into one of three groups: polymer foams, Balsa wood and honeycomb materials. Traditionally, sandwich elements of wind turbine blades contain either PVC foam, which is not environmentally friendly, or Balsa wood exhibiting recurrent shortages and supply problems. From the outset of the project, thermosetting polyurethane (PU) and thermoplastic polypropylene (PP) systems were selected as potential replacement candidates. These two polymer systems occupy the large market share in terms of the amount consumed and are relatively cheap. In order to fulfil specification requirements for the sandwich core material, PU and PP polymer system need to be reinforced with nanoparticles and subject to foaming processes. Thus, manufacturing of sandwich core materials combines two different processing routes, one for the synthesis of nanocomposites and another for foaming. Therefore the choice of material components constituting a final core material must simultaneously compromise adequateness of the material composition to both processes.
Polymer nanocomposites represent a radical alternative to conventional filled polymers, since nanocomposite materials have large surface area-to-volume ratios for the reinforcing phase with nano-sized fillers. Therefore, an improvement in property arises because the length scale of the nanocomposites morphology and fundamental physics associated with a property coincide, i.e. surface effects that govern chemical and physical interactions are operative on the scale comparable to the size of nanofillers. However, there is still a lack of understanding of the interfacial bonding between the reinforcements and the matrix material from both analytical and experimental viewpoints and considerable uncertainty in theoretical modelling and experimental characterisation of the nano-scale reinforcement materials. The main challenges have mainly to do with issues related to composites processing: assessing a good dispersion quality of nano-inclusions in polymer matrix, achieving high volume and high rate fabrication which is fundamental to manufacturing of nanocomposites as a commercially viable product, and acceptable cost effectiveness. A consensus is yet to emerge on those issues.
Nanocomposite polymer foam is defined as a two phase material in which air bubbles are entrapped in a continuous nanocomposite phase. Two steps are involved in the foaming process: bubble nucleation and growth. Both processes are affected by many physical properties which are inter-related and many are complex functions of foaming conditions. Enhanced thermo-mechanical properties of nanocomposite foams result from improved cell morphology that is mainly attributable to the role of nanoparticles as nucleation agents for bubble generation. Although the nucleation mechanism is still not fully understood, it is generally recognised that the size, shape and distribution of the nanoparticles, as well as the surface treatment, can affect the nucleation efficiency. Due to the high nucleation efficiency, nanoparticles provide a powerful way to increase cell density and reduce cell size. The nanometre dimension of nanoparticles is especially beneficial for reinforcing effect of foam materials, considering the thickness of cell walls is usually in the range of few micrometres. Nanocomposite foams can be obtained by several processing protocols depending on the type of component materials used. For the selected PP and PU polymer systems, logistics for manufacturing of nanocomposite foams is illustrated. The PP nanocomposite foams need to be processed in two major steps, first preparing an adequate nanocomposite material and then using appropriate foaming technique, whereas manufacturing PU nanocomposite foams follows a different path is also illustrated. Each box represents several alternatives and possible choices, which leads to large number of constituent materials and components, processing protocols, manufacturing conditions and hence all meaningful combinations needed to be investigated.
Synthesised nanocomposite foams are used as sandwich core materials to assess load bearing capability and compatibility with face sheets of sandwich structures. Nanocomposite foam cell structure and physical properties of nanocomposites determine the quality of the interface between the core and face sheets of the sandwich elements. During flexural loading, the core basically controls the failure of the sandwich structures and therefore, if characteristics of the core material can be appropriately designed, the enhancement will be reflected in the performance of the entire sandwich structure.
There is a general concern that nanoscale particles may have negative health and environmental impacts. This is because the size of the nanomaterials is comparable to human cells and large proteins thus the regular human immune system may not work against them. While nanoparticles may enter the body by inhalation, due to skin contact, or by ingestion, the major concern is by inhalation during processing. The safety issues must address exposure, hazard and risk assessment with nanomaterials and processing technologies worked with at different laboratories of involved in the project partners. Toxicology can provide data on hazard of nanoparticles based on in vitro assays and input from the production, materials processing and material characterisation activities. Furthermore, physicochemical and other characterisations of nanocomposites and any particle emissions can provide insight into the determinants of the toxicity and the potential for human exposure. This is a key to ensuring integration of the information from the exposure and hazard assessments into the risk assessment where dose-response models allow estimating the health risk and developing control measures for workers in the exposure scenarios.
Project's major objectives need to be reached through the combination of a strong, product oriented focus and implementation of scientific and technological development within each of the central problem areas mentioned above. The group of scientific objectives comprises objectives related to:
- Synthesis and processing of nanocomposites and nanocomposite foams including:
a) selection of raw materials and additives;
b) impurity removal from raw material precursors;
c) clarification of role of nanofillers in promoting polymer foamability and foam morphologies;
d) functionalisation of nanofillers focussing on physical/chemical compatibility with foaming processes;
e) development of new paradigms for creating nano-scale building blocks and processing protocols;
f) development of novel processing routes for nanocomposite foams.
- Characterisation and modelling including:
a) characterisation and modelling tools for mechanical behaviour of novel materials;
b) real-time characterisation and materials' structure evolution during synthesis, where possible;
c) spatially resolved microstructure and in situ monitoring;
d) testing and characterisation of structural sandwich elements with nanocomposite foams as core material and conclusive guidelines for scale-up processing of nanocomposite foams;
e) fundamental models to accurately predict nanostructure formation;
f) nano / micro scale predictive models of nanocomposite foams.
- Safety issues of nanoparticles from nanocomposites including:
a) determinants of environmental impacts and potential health and safety risks of nanoparticles used in nanocomposite materials;
b) risk management and techniques to minimise and control health risks to manufacturers, downstream users and impacts to the environment.
The group of technological objectives includes:
- development of a novel polymer foams reinforced with nano-sized fillers which display mechanical properties in the require range;
- cost-effective production of the new materials and the manufacturing of structural sandwich demonstrators with nanocomposite foams;
- risk assessment and estimation of safe occupational exposure levels to particles associated with the production and machining of nanocomposites and development of eco-efficiency indicators and documentation of life cycle impacts.
Project results:
Significant results were obtained in different areas by tuning scientific and technological achievements towards project's objectives:
Synthesis and processing of nanocomposites and nanocomposite foams
PP-based systems
Several different grades of PP materials were tested in order to find the most suitable matrix for nanocomposites. Three selected PP materials were successfully used to produce first generation of nanocomposites with different nanofillers, such as multiwall carbon nanotubes (MWCNTs), carbon nanofibres (CNFs), polytetrafluoroethylene fibres (PTFE) and montmorillonite (MMT) platelets. Further trials showed that high melt strength of PP materials is a crucial factor for subsequent foaming ability of PP nanocomposites. Therefore, two other grades of PP matrix were thoroughly tested and accepted as basic PP systems for second generation of nanocomposites.
Nanocomposites containing PTFE fibres differ from other nanocomposites containing reinforcing nanoparticles as PTFE fibres are created in-situ during blending with PP matrix. Pilot experiments using PP of high melt viscosity and reaction grade PTFE clearly showed that it is relatively easy to generate all-polymer nanocomposites with fibril reinforcement just by compounding the two polymers. All-polymer nanocomposites are a new type of nanocomposite characterised by many unusual properties. One of the most interesting features of PP / PTFE nanocomposite is its high melt strength and high elongation viscosity, the property that is required for efficient foaming. PTFE nanofibrils built up a continuous network, which proved to be necessary for melt strengthening even for PP matrix that originally does not show any melt strength. This fibril structure was the object of different observations made by scanning electron microscopy (SEM), transmission electron microscopy, melt and solid rheology, microcalorimetry and X-ray methods.
The idea for simplification of producing nanocomposites with fibril reinforcement was to find a way to strongly deform PTFE inclusions during shearing or compounding and preserve their shape after deformation, i.e. to act against capillary instabilities. The use of crystalline PTFE inclusions instead easy to deform nanoparticles seemed to be a promising option to generate polymer nanofibres through shearing of a polymer matrix in which PTFE crystalline inclusions are dispersed. Crystals are not subjected to capillary instabilities and they preserve their shape. Shearing of the matrix led to a mechanical instability of a crystal which results in sliding layers as the shear deformation increases. The shear modulus and shear yield stress of the PTFE assumes values characteristic for crystals of many polymers at temperatures few tens of degrees below their melting. The above estimation made the idea to transform PTFE polymer crystals into nanofibres by shear deformation quite realistic. Another problem was to obtain very large deformation of polymer crystals and hence thin fibrils. That can be solved by using crystalline powder formed by disentangled macromolecules. The reaction grade PTFE in the powder form fulfils this condition and as a result, PTFE macromolecules are not entangled and PTFE powder grains can be easily deformed to very large deformations.
In several experiments, molten PP premixed with 3 wt % of PTFE particles were subjected to shearing with suitable rate and temperature range until the requested deformation of the matrix was achieved. It appeared that deformation of PTFE particles follow nearly strictly the deformation of PP matrix. Those experiments in uniform shearing proved that it is possible to produce all-polymer nanocomposites by shearing starting with raw polymers: chain disentangled polymer powder grains and thermoplastic polymer matrix. Twin screw extruders and also twin blades batch mixers were used for fabrication of nanocomposites from PP pellets and PTFE chain disentangled powder. The method of manufacturing of all-polymer nanocomposites is the application's subject for the European patent.
A series of protocols, compositions and processing conditions were employed to optimise manufacturing PP / MMT with nearly fully exfoliated MMT platelets. Foaming involves an extensive deformation of polymer matrix significantly influencing the crystallisation of PP-based nanocomposites with organo-modified MMT and compatibilised PP matrix. Neat PP was also studied for comparison. At lower shear rates the effect of shear deformation on the crystallisation kinetics and orientation of crystals was stronger in the nanocomposites than in neat PP. The differences between the nanocomposites with different MMT contents were minor, as well as between those differing in the compatibiliser contents. Pre-orientation of the clay achieved by pre-shearing did not have any significant effect on shear induced crystallisation and crystal orientation of PP / MMT nanocomposites. A new staining method for the purpose of transmission electron microscopy investigations was developed to show compatibiliser's localisation in PP / MMT nanocomposites and confirmed that no MMT platelets exfoliation could occur without compatibiliser surrounding. A novel method to exfoliate MMT platelets by using azodicarbonamide as additive has been discovered. This is a simple procedure allowing improving the exfoliation of nanoclays in thermoplastic non-polar matrices without the use of compatibilisers. The key advantage of the method is its very low cost in comparison to other alternatives. Yet another novel method of examining the exfoliation of MMT platelets from rheological data was developed showing that platelet exfoliation could be sensed as a function of certain rheological properties. It has been concluded that complete exfoliation of MMT platelets does not guarantee melt strengthening necessary for the foaming process. While it is a drawback in terms of technological issues, it remains as an important scientific observation. In the molten state PP is separated from MMT platelets through compatiliser and can freely slip along platelets not contributing to melt strain hardening. The lack of melt strengthening is prevailing at relatively slow rates of material flow characteristic for foaming processes. Hence, the use of high melt strength PP for foamable nanocomposites is strongly advised and recommended. For such systems transmission electron microscopy images show homogeneous distribution of MMT platelets in foam walls oriented along the wall expansion direction.
Transmission electron microscopy studies revealed that the presence of agglomerates in CNTs and CNFs nanocomposites is related to agglomerates that are present in CNF and CNT masterbatches. Even very high shearing is not sufficient to eliminate nascent agglomerates because of strong interactions between single CNTs and CNFs. Neither usage of pure fillers, their filtration, ultrasound applied during compounding in a batch mixer nor the change of compounding machine from dedicated twin-screw co-rotating extruder to twin-screw self-cleaning co-rotating extruder was helpful. Many aggregates of CNTs and CNFs were still visible in ultrathin sections of nanocomposites. It is very probable that a significant improvement of CNT and nanofibres CNF dispersion avoiding agglomerates can be achieved using powdered PP that will be dusted with plain CNT or CNF and no use of masterbatches will be then needed.
The compression moulding technique has been improved to accommodate control of cellular structure in PP foams. The precursor material or the foamable pellets are introduced in a mould and the piston is introduced in the mould to apply a certain pressure. Temperature is increased to values higher than the decomposition temperature of the blowing agent. When all the gas from the chemical blowing agent is generated, pressure is released and foaming takes place. Expansion ratio (ER), i.e. density, is controlled by the piston's displacement of the piston. This has a significant advantage over the conventional method; the density can be easily controlled and foams expand only in one direction which allows producing anisotropic foams.
Amongst different nanocomposite PP foams manufactured with this technique, the foams based on blends of a high melt strength PP and a linear PP have been produced varying the amount of linear polymer in the blends. For these particular materials, the cellular structure (open cell content, cell size and anisotropy ratio) and mechanical properties have been measured.
The new developed procedure based on extensional rheology tests, which has the capability of predicting whether a given formulation would have sufficient foaming behaviour, limited the number of investigated PP nanocomposites to ten systems, allowing for a significant time saving in the development of nanocomposite formulations for foaming.
PU-based systems
Reactive foaming at atmospheric pressure and room temperature has been used to produce the foamed PU nanocomposites. In this technique, the preformulated polyol is mixed with isocyanate, the foaming starts by the generation of carbon dioxide (CO2) due to the chemical reaction of isocyanate and water contained in the polyol. At the same time the polymerisation reaction of PU is produced, i.e. foaming and polymerisation occurs simultaneously. An adequate chemistry allows obtaining low density rigid foams. This is the technique which evolved during the course of the project by systematically changing components' chemistry and assuring that the final PU system fulfils specification requirements with respect to density, closed cell content and mechanical properties. The main processing parameters identified as essential to control the final foam characteristics comprise pre-mixing of the polyol compound, mixing method of the nanoparticles in the polyol and / or the isocyanate, mixing method between the polyol and the isocyanate and foaming temperature. Different types of commercial nanoparticles, as received or after a surface treatment, have been tested as reinforcing additives for the nanocomposite foams, and then eleven different PU nanocomposite foams were thoroughly investigated. Due to processing limitations for up-scaling of nanocomposites only pristine PU foams were successfully manufactured in a pilot semi-industrial production line.
A novel approach has been developed for the synthesis of hybrid nanofillers by growing CNTs between galleries of MMT platelets. The process involves a cation exchange process of the clay, calcinations step and finally growth of CNTs in between the silicate layers in the clay using chemical vapour deposition. The nanohybrids have been used as reinforcing filler in PU foam and have been shown to improve significantly the mechanical properties of the resulting PU nanocomposite foams.
Characterisation and modelling
Special equipment has been designed for the analysis of the foaming process of cellular polymers allowing quantifying phenomena such as nucleation, growth, coalescence and diffusion coarsening of foam cells. Specific hardware and software have been developed to implement this technique in investigations clarifying foaming mechanisms operative during processing.
The system includes X-ray source and a detector. The fundamentals of the technique are based on the interaction between radiation and matter, i.e. the transmitted radiation carries the information captured by a flat panel as a radiographic image. Standard acquisition time for X-ray radioscopy experiments is between one and two seconds, limiting the time resolution to these values. However, PU foaming process takes place very fast. Once the two reactants are mixed the process is already in progress. For this reason, it was necessary to increase the time resolution by special procedures in order to obtain motionless and not blurred images.
A comparison between pure PU foam reaction sequences and those corresponding to PU / MMT nanocomposite system is shown at different foaming stages. The frame-by-frame visual inspection permitted to determine absence of bubble ruptures in both materials. It indicates that the addition of nanoclays has more influence on the initial nucleation and does not substantially affect the coarsening or coalescence phenomena. Zoomed images of both cured PU foams, with and without nanoclays, clearly indicate how different the cellular structure in both materials is. The neat PU cellular structure shows larger and fewer cells in comparison with nanocomposite PU foam.
The microstructure and deformation of foams was characterised using in-situ compression tests inside the SEM chamber with an aim to record deformation pattern of cell walls within the elastic mode of deformation and investigate typical failure modes. A similar procedure was developed for X-ray microtomography investigations; in both cases a dedicated in-situ loading device was designed for an accurate control of deformation applied to the foam samples. The evolution of deformation pattern in PP foams under compressive loading from the SEM is illustrated. Wrinkling of cell walls, initiation and propagation of micro-cracks in cell walls and buckling of cell walls are major mechanisms of cells deformation leading to final failure. Buckling of cell walls, is more apparent on images from X-ray microtomography tests, which provide visualisation of deformation pattern in three-dimensional space.
During measurements of mechanical properties of polymeric foams it is often a challenge to ensure an appropriate load transfer from the experimental equipment to material samples. This discrepancy may result in large variations / uncertainties in the measured values or in worst case erroneous values. This is particularly true for the measurement of shear stiffness and strength. The dedicated method from project's background has been extended for present purposes avoiding these difficulties and unifying calibration of measurements among participating laboratories. The method is a combination of traditional mechanical testing, digital image correlation and numerical modelling. Using this approach, the tensile, compressive and shear properties may be determined from just one type of test samples which is much easier to produce than traditional samples for the same test. Furthermore, the problems related to the load transfer from the test equipment to the test samples has been largely reduced, if not solved. Finally, through newly developed test method different bi-directional loading conditions can be applied to foam materials. The method allows unambiguous determination of foam shear properties, which are of primary interest in foam applications in sandwich structures.
Theoretical modelling and simulations are the key enabling fields for reducing the time needed to design new materials, increasing the predictability of properties and proper interpretation of experimental measurements due to the complexity of the measurement at atomic and nano-scales. Optimisation of nanomaterials requires exploration of many alternatives prior to synthesis. Therefore modelling and simulation efforts that may provide the most quickly realisable benefit include the development of computational models to predict the properties of materials at different scales and bridging length scales in order to predict overall, macroscopic properties of materials. Appropriate validation experiments are also crucial to verify that the models predict the correct behaviour.
Reliable determination of elastic constants for clays is still unresolved problem. Direct experimental measurements are not available by any technique. To overcome this obstacle the assignment of elastic constant to clays is usually based on indirect measurements. Quantum mechanical simulations revealed that the clay platelet possesses overall orthotropic symmetry and is characterised by five elasticity constants whose numerical values were calculated for the first time. Molecular model of the intercalated region of clay gallery was built and molecular dynamic simulations were performed in order to determine the interlayer distance from the model. Single layer clay was simulated with a purpose to calculate vibrational spectra and compare them to experimentally available data.
Furthermore, exfoliated model of clay was simulated to disclose local interactions between the clay surface, surfactant and polymer matrix. The interlayer distance resulting from the simulation very closely resembles the experimental value. Molecular dynamic simulation provides also valuable information on interfacial properties and possibility to optimise organo-modification of clays. With growing length of surfactant molecules (or increased volume of the surfactant) the binding energy clay / surfactant and clay / polymer decreases whereas binding energy surfactant / polymer increases. Thus optimisation of the organic modification is possible maximising interfacial strength with binding energies constraints. This information was used for optimisation of PP nanocomposites. Molecular model of clays reproduces also very well vibrational infrared spectra obtained from experimental measurements. These results suggest that created molecular models are reliable and correctly reproduce selected experimental data.
Similarly, successful models at micro and mesoscale were developed and tested against experimental data. A model predicting elastic constants of the cell wall material reinforced with nanoclays was verified taking into account such effects as clustering of nano-clays, proximity of nanoclays to surfaces (e.g. the cell walls in the microcellular foam), the presence of the interphase and re-orientation of nanoclay platelets during the foaming process. This model has been consecutively embedded in macro-scale model to predict stiffness of pure and nanocomposite foams. Results showed good correlation with experimentally measured values of Young's moduli for PU foams whereas properties of PP foams were significantly overestimated, which could be due to different structure and properties of the polymer in the cell walls and in the bulk.
Safety issues of nanoparticles from nanocomposites
The risk assessment provides information based on the analysis of scientific data which describe the form, magnitude, and characteristics of a risk, i.e. the likelihood of harm to humans or the environment, following the identification (characterisation) of exposure and hazards. Although risk assessment is mainly a scientific task, political decisions are required on matters such as: 'What are we trying to protect and to what extent should it be protected'?
The purpose of exposure characterisation was to identify and describe the potential exposure scenarios relevant to the tasks which were to be carried out within the project. Information was collected from the peer-reviewed literature in the public domain that was relevant to the health and environmental safety of nanocomposites using PubMed and Web of Knowledge. Furthermore, an online workplace exposure survey was developed to help identify generic and substance-specific activities which may present an exposure potential for subsequent investigation. The exposure scenarios identified for use in the occupational exposure assessment and the complete set of questions and anonymised responses from the Online Workplace Survey serve as a primer for the subsequent exposure measurement and risk assessment. The findings related to quantitative exposure and contextual data for key exposure scenarios' span all NANCORE project activities encompassing the manufacture and processing of PU and PP nanocomposites.
Measurements and observations of the manufacturing and processing activities were conducted at four partner sites, using either all or a selection of nanomaterials adopted by the project consortium (CNTs, CNFs and nanoclays).
The hazard assessment comprises the toxicity profiles of the NANCORE compounds to address the fibrogenic potential of the materials, their bio durability in the lung environment and further material characterisation. This information enables the contextualisation of dose used within in vitro assessments to human exposure and provides a benchmarked profile of toxicity compared to control particles of known toxicity.
The risk assessment task contains the evaluation of the exposure risk and the estimation of the overall risk using exposure scenarios within the project through a comparison of the output given in the control banding tools with the results from the exposure measuring campaign to evaluate whether the tools can be used by the beneficiaries to plan exposure control strategies.
The research-based approach to safety issues and very comprehensive documentation collected during project's duration may be considered as an important step towards lacking standardisation within this area.
Technological issues
The project was successful in providing well documented compositions, processing protocols and manufacturing techniques for variety of nanocomposite foams that fulfil specification requirements in terms of fundamental mechanical properties. However, up-scaling problems from the laboratory scale to industrial or semi-industrial scale still persist and present challenges with regards to feasible and cost-effective processing techniques. The major hurdle relates to bulk production of nanocomposites where all composition and processing protocols from small laboratory scale manufacturing need to be projected to large industrial scale. Furthermore, processing technique itself may not be easily accommodated in the large scale production. For instance, up-scaling the improved compression moulding technique, which is successful at the laboratory scale, would be limited by a realistic size of mould and press. Also physical aspects of foaming in large quantities may pose difficulties not observed at small scales for example heat transfer from interior of the foam and its dissipation. It has been observed that during laboratory scale continuous extrusion, when foamed nanocomposite reaches the thickness approximately 3 mm, the foaming dye heat transfer from the inside of the foam occurs slower than the bubble wall crystallisation. This results in bubble collapse and worsening the foam properties. Following the problems with up-scaling the extrusion foaming process of PP nanocomposites, some activities were initiated to use another foaming technique focusing on micro-bead technology foaming. Micro-bead structure analysis revealed that foams have fully closed cell structure and have a density within the desired range. This technology is actually under further development in collaboration with industrial partners.
Nano technological processes and systems are not yet well investigated from a life cycle perspective. Performing lifecycle assessment (LCA) in this project is an important contribution not only to the project itself but also to the availability of data for the processes included. The available literature on environmental impacts of the suggested polymers and nanoparticles in a life cycle perspective was scrutinised and a first simple assessment performed. Further analysis was performed using SimaPro LCA software and connected data bases, as well as using data received for operations related to foaming processes and recycling from other partners in the consortium, and recycling opportunities and eco-indicators were established. The recommendations are focused on low energy requirements, limitations in the use of acid for CNT purification, the selection of catalysts according to their associated environmental burden when produced and disposed (most catalysts are either highly toxic heavy metals or organic substances). A special attention also concerns human toxicity via air, which might be relevant in case of releases and exposures to nanoparticles during the life cycle of the nanocomposite, and ecotoxicity in aquatic ecosystems due to potential long-term emissions from landfills.
In order to serve as a benchmark / reference for the new technology, data for current practice and technology was collected from partners and their suppliers as well as from the literature. The general goal has been to construct an inventory of the current state of practice and technology regarding the manufacture and application of structural composites in the wind turbines, maritime, and rail industries in Europe, in order to define a common analytical framework for the further LCA. The scope included the inventory of technologies, materials and energy involved in the manufacturing, use and disposal of products in the three studied industries. Each product defined different boundary and object for analysis and a particular technological setting.
For the NANCORE technologies, data were collected from industrial partners to the extent possible, in order to clarify their coming use of the new technology in sandwich structures. Product system models were built focusing the modelling on wind turbine blades. In addition, a review of the LCAs on wind turbines was performed in order to put the results of the blades into the perspective of the whole turbine. A number of hypothetical cases / scenarios for the use of nanocomposite foams in sandwich structures were considered. Four different nanoparticle types with three different concentrations embedded in PU foam as well the pure PU foam were compared to the use of an equivalent volume of Balsa wood under the assumption that a direct substitution would not have any impact on the use stage of the wind turbine. The models show that using nanocomposites in sandwich structures as replacement for Balsa wood or PVC foam is generally not environmentally beneficial. This is mainly due to high environmental impact associated with the production of nanoparticles but also due to the fact that production of PU foams has a higher environmental impact as compared with Balsa wood.
Potential impact:
The potential impact and added value from obtained results follows the multidisciplinary character of the project and may be categorised according to major research areas involved. The scientific and technological impact on processing comprises:
- establishing processing protocol to produce fully exfoliated PP/MMT nanocomposites, PP / PTFE nanocomposites with high melt strength properties and gaining an extensive know-how on nanofillers treatment in PP matrix;
- development of various foaming protocols for PP / MMT and PP / PTFE materials;
- testing up-scaling capability for industrial application for various foaming methods;
- development of the novel method to exfoliate nanoclays without using compatibilisers;
- establishing processing protocol for novel CNT/MMT nanoparticles for reinforcing as well as other functional purposes such as electrical conductivity and gas barrier properties.
The impact related to characterisation of materials covers:
- examining several PPs and nanofillers in terms of melt strength, foaming capability, final material durability and possibility to form sandwich structures;
- establishing extensive database of connections between processing parameters and final characteristics of the foamed materials;
- development of methodology based on extensional rheology tests allowing predicting appropriateness of given polymer formulation for foaming purposes;
- development of X-ray radioscopy technique for the analysis of foaming processes allowing quantifying phenomena such as nucleation, growth, coalescence and diffusion coarsening;
- new rheological methods to assess the quality of nanoparticles dispersion in the polymer matrix;
- new methodology to validate the wetting behaviour of clays in nanocomposites and foamed nanocomposites;
- development of characterisation procedures for in-situ monitoring of the deformation pattern in foams and stress transfer in CNT nanocomposites.
The impact related to measurement techniques contains:
- development of the transmission electron microscopy staining approach to determine interface behaviour between clay and PP matrix;
- development of the nuclear magnetic resonance procedure to examine polymer chains dynamics and structure in foams;
- parallel plate rheometer has been modified to validate the measurement of rheological properties of flexible PU foams during foaming;
- development of the experimental setup and measurement procedure for non-standardised fracture toughness properties of foams;
- utilisation of the specialised setup to unify and unambiguously determine shear properties of materials.
The scientific impact on modelling methodologies includes:
- development of the atomic simulation procedure for the model prediction of properties not accessible for direct measurements but validated by parent measurements;
- development of predictive multiscale modelling technique for foams and nanocomposite foams including sequential incorporation of material parameters measured at different length scales.
The impact on safety issues incorporates:
- Advances in the assessment of the potential risks posed by nanomaterials in a variety of lab and pilot-scale processing activities has been gained through the experience in NANCORE, specifically allowing reflection on the strengths and limitations of the techniques and instruments employed to gather, analyse and interpret multi-component data from complex workplaces and laboratory environments. The pragmatic outputs from the 'Safety' work package, in the form of the case examples of control measures to mitigate exposure cutting across the site-specific activities, have been drawn together with the intention of being as relevant as possible to a variety of manufacturers and researchers both within the sectors specific to NANCORE and beyond. These include general nanomaterial handling tasks in the extrusion laboratory (i.e. weighing and mixing, batch mixing precursor powders, and extrusion), lab-scale master batch extrusion, compression moulding, lab-scale PU foam synthesis, pilot-plant PU foam synthesis, sawing, milling, and materials testing.
- Formalisation of the institutional policy on nanofiller processing and waste disposition.
These impact elements are joint for project's beneficiaries as their accessibility and dissemination was freely available for all partners. The impact on some other areas specific for an individual partner may include issues related to education, learning and/or training as well as issues related to company / institution / department strategy. The impact gained by the manufacturer (Recticel) of PU foams, comprises strengthening the concept 'strong & light' as a strategic research and development (R&D) platform for Recticel and the possibility of further tailor-made design / development regarding the end users applications. The Azimut-Benetti (AB) recognises the impact on applied research and design of structures with the new foam where the project allowed accomplishing the first, decisive steps in the development of new structural design and its parametric analyses including the investigation of some details which are often only partially analysed in current solutions. LM WindPower sees the impact through a more realistic picture of how matured are nanocomposite foams for the company's product strategy. A larger time frame is needed for the research before the results could be incorporated in the production, both in terms of processing, pricing and technology readiness in general. It has been also recognised that the extension of know-how provided by the project is beneficial for the future strategy; the element of impact appreciated also by the Focal partner.
All participating academic partners strongly appreciate the impact on advancement of knowledge in several interconnected scientific disciplines, fostering involvement in new strategic scientific and technological programmes both on national and international level. A significant added value from the project's results has been created through the education at undergraduate and graduate level. Furthermore, the education of candidates with a sharpened profile within advanced materials and processes makes them attractive to industry, which leads to the transfer of technological expertise from research to industry and substantially contributes to future society. A high level of the scientific research exercised in the project has been one of the prerequisite requirements for the new Master programme in materials technology approved by the Danish Ministry of Science, Innovation and Higher Education at the Aalborg University commencing September 2013.
Dissemination activities were mainly carried out via publications in recognised scientific journals and presentations at national and international conference, symposia, workshops and other meetings. During the time interval allocated to the project, 10 research papers were published in peer reviewed journals, 8 have been submitted, 9 are in the preparation, and attendance and presentations at 50 conferences have been recorded. Furthermore, seven doctorate (PhD) theses are expected to be completed no later than medio 2014.
The exploitation of result fall into two foregrounds categories: exploitation of R&D results via standards and a general advancement of knowledge. The first category comprises the exploitation of physical testing, material characterisation, lightweight and durable materials, new products and new production and characterisation technique whereas materials characterisation tools, new products, functional materials and predictive simulation model at nanoscale belong to the second foreground category. For one exploitable product / measure the national patent was granted and one European patent application is on its way. Patent applications are under consideration for another two exploitable products / measures.
