Processing and experimental micromechanics of Elium® thermoplastic composites

Developing new structural materials with high specific strength and stiffness is no longer sufficient: good environmental performance has become a prerequisite for reaching the market. In this context, the composite community has been faced with the pressing challenge of transitioning from thermosetting to recyclable thermoplastic matrices. The recent development of Elium, a range of methyl methacrylate-based resins compatible with liquid composite moulding (LCM) methods, has marked a turning point in this transition. Mitigating porosity in thick Elium laminates manufactured by LCM was the primary goal of this work, given the suspected tendency of the monomer formula to boil during the in-situ polymerisation step. Thermochemical modelling was used for assessing the influence of various parameters on the evolution of preform temperature during polymerisation, thus preventing the occurrence of thermal porosity. In parallel, a miniature infusion setup was designed for monitoring void growth in 2 cm-thick Elium laminates via dynamic X-ray computed tomography, which helped identify and decouple various mechanisms responsible for void formation in a proof-of-concept study. The second part of the work explored the deformation and failure mechanisms occurring at the constituent level in Elium composites. Using nanoscale digital image correlation, the strain fields in between two adjacent fibres of a composite specimen subjected to transverse compression could be successfully captured. Small-scale indentation techniques were also used to quantify the influence of fibre proximity and physical ageing on the elastic properties of the Elium polymer, and assess the thickness and mechanical response of the fibre-matrix interphase region.(FSA - Sciences de l'ingénieur) -- UCL, 202

Nanoscale digital image correlation at elementary fibre/matrix level in polymer–based composites

Multiscale mechanical modelling aims at predicting the failure of composites from the fibre/matrix level up to the component scale. Existing frameworks are limited by the lack of reliable experimental data and by an incomplete understanding of the submicron deformation and failure mechanisms. A novel digital image correlation (DIC) method has been developed for the characterisation of the nanoscale mechanical response in composites, based on latest advances in surface patterning. Indium has been deposited on unidirectional composites leading to a dense, homogeneous speckle with particle diameter around 15 nm. The specimens were subjected to transverse compression in a scanning electron microscope, while minimising distortion effects. Strain concentration areas, like submicron shear bands and fibre–matrix interphases, were successfully captured for two systems: a carbon fibre-reinforced thermoset and a glass fibre-reinforced thermoplastic. DIC results were compared with alternative experimental data, obtained by atomic force microscopy, and with finite element simulations based on a conventional elastoplastic model

Relationships between processing parameters, mechanical and chemical properties of thick glass fibre reinforced thermoplastic methacrylic composites

As the energy transition unfolds, the recycling targets applied to composite materials are on the rise, thus getting more and more challenging to meet. In this context, continuous fibre reinforced thermoplastic composites and their recyclable matrix have gained increasing interest over the past twenty years – especially for the design of lightweight and high-performance structural parts. In order to produce such materials, specific monomers are usually vacuum-infused through glass or carbon fabric before undergoing in-situ polymerization. While most parts obtained this way are only a few millimetres thick, some industrially important applications require the manufacturing of much thicker components – up to several centimetres. The present work focuses on the links between the infusion parameters, physico-chemical state of the matrix and mechanical properties of 6-cm-thick glass fibre reinforced thermoplastic methacrylic composite plates, at both the micro- and macroscopic scales. More precisely, different composite plates were infused with Elium® resin and left to polymerize at different temperatures, while recording the temperature profiles at several locations. The evolution of the microstructure was then studied along the thickness of each sample, and emphasis was placed on the characterization of the porosity distribution and morphology by optical microscopy and computed microtomography. Heating up the bottom part of the plates after infusion - in order to trigger the polymerization reaction - can in turn lead to monomer boiling and thus favour the formation of porosity inside the matrix, with more and larger pores observed near the surface. While it seems obvious that such cavities affect the overall mechanical response of composite parts, the decrease of modulus and strength as a function of the volume fraction of pores proved much more significant than first expected, at least under uniaxial compression. In order to get a better understanding of the phenomena, the local mechanical response of the matrix was then measured further to various infusion conditions by carrying out nano-indentation tests inside the matrix pockets, while chromatographic analyses gave access to the molecular weight distributions and monomer conversions achieved in the Elium® matrix. The results revealed small but mutually consistent differences in matrix properties across the thickness of the plates. Though the latter were slight enough to have a limited influence on the chemical and micromechanical properties of the matrix, they give important insights into the complexity of the thermal and chemical phenomena occurring in the system during polymerization and cooling. In a nutshell, this work suggests that changing the infusion temperature strongly impacts the amount and distribution of porosity in Elium® composites. This governs in turn the macroscopic mechanical properties of the final part, and implies significant variations in the course of the polymerization reaction along the thickness of the plate

Vacuum infusion of thick glass-fibre reinforced methacrylic composites: computationally efficient modelling of temperature profiles and kinetics during in-situ polymerization

Recycling composite materials is a key challenge in the context of energy efficiency and climate change mitigation. Among other strategies, the use of thermoplastic matrices has emerged as an effective way to produce partially recyclable composite parts. Ensuring the compatibility of such new resins with pre-existing processes – in particular liquid moulding techniques - is of prime importance for current manufacturers of polymer-based composite structures. In this perspective, dedicated monomers have been specifically formulated to enable the vacuum-assisted resin infusion through a fibrous preform, before undergoing in-situ radical polymerization. However, in some cases, the inherent exothermic nature of polymerization reactions can lead to thermal runaway - often compromising the quality of the final part. This is especially true for thick composite plates infused with methyl methacrylate (MMA)-based formulas, which are prone to an acute auto-acceleration phenomenon during polymerization known as the “gel (or Trommsdorff [1]) effect”. While ensuring optimal conversion values, this also gives rise to a sudden and non-homogenous temperature rise in the reaction medium, leading to unpredictable properties and/or the presence of voids in the polymerized matrix. In order to design a relevant manufacturing method for thick MMA-based composite plates, it is thus crucial to understand and model the influence of the (numerous) process parameters on the heat release pattern induced by polymerization across the thickness of the part. To this end, a one-dimensional finite difference numerical model was developed in MATLAB, combining heat transfer and free-radical polymerization kinetics. Using either an empirical approach [2] or a semi-empirical approach [3]–[5] based on the free-volume theory [6] for the reaction kinetics, the framework is used to predict the temperature and monomer conversion profiles achieved during the in-situ polymerization of a MMA-based resin infused into a 7-cm thick glass fibre preform. Experimental validation was conducted by comparing the calculated temperature and conversion profiles with those measured during the corresponding real-scale infusion experiments. The predictive capacity, robustness and computation time of both empirical and semi-empirical approaches were assessed and compared for various sets of boundary and initial conditions involving isothermal vs. non-isothermal external temperatures (thermal triggering of the reaction by heating up the bottom part of the plate), and different resin formulations. Both models successfully capture the non-linear behaviour characteristic of the gel effect across the thickness of composite plates. [1] V. E. Trommsdorff, H. Köhle, and P. Lagally, « Zur polymerisation des methacrylsäuremethylesters », Die Makromolekulare Chemie, vol. 1, no 3, p. 169-198, Jan. 1948. [2] P. Hayden and H. Melville, « The kinetics of the polymerization of methyl methacrylate. I. The bulk reaction », Journal of Polymer Science, vol. 43, no 141, p. 201-214, Mar. 1960. [3] D. S. Achilias and C. Kiparissides, « Development of a general mathematical framework for modeling diffusioncontrolled free-radical polymerization reactions », Macromolecules, vol. 25, no 14, p. 3739-3750, Jul. 1992. [4] D. S. Achilias and I. D. Sideridou, « Kinetics of the Benzoyl Peroxide/Amine Initiated Free-Radical Polymerization of Dental Dimethacrylate Monomers: Experimental Studies and Mathematical Modeling for TEGDMA and Bis-EMA », Macromolecules, vol. 37, no 11, p. 4254-4265, Jun. 2004. [5] A. Zoller, D. Gigmes, and Y. Guillaneuf, « Simulation of radical polymerization of methyl methacrylate at room temperature using a tertiary amine/BPO initiating system », Polymer Chemistry, vol. 6, no 31, p. 5719-5727, 2015. [6] J. S. Vrentas and J. L. Duda, « Diffusion in polymer—solvent systems. I. Reexamination of the free-volume theory », Journal of Polymer Science: Polymer Physics Edition, vol. 15, no 3, p. 403-416, Mar. 1977

Processing maps based on polymerization modelling of thick methacrylic laminates

Control of the in-situ polymerization of thick methacrylic laminates is essential to minimize cavitation due to monomer boiling. To this end, a computationally efficient thermochemical model is developed to predict the temperature and degree of monomer conversion during polymerization of a methyl methacrylate (MMA)- based resin in a fibrous preform. The model couples the self-accelerating exothermic free-radical bulk polymerization ofMMA(gel effect)with the heat transferwithin the preformand to the environment. The predicted temperature profiles are in excellent agreement with experimental data measured during a series of in-situ polymerization tests with different heating conditions and preform thicknesses. Processing diagrams are constructed to reveal the regimes of interest for the production of thick fiber-reinforced methacrylic composites without voids induced by monomer boiling

A computationally efficient thermomechanical model for the in-situ polymerization of a methyl methacrylate-based resin in a thick glass fiber laminate

A computationally efficient thermochemical model is used to predict thermal runaway and the occurrence of monomer boiling during the in-situ polymerization of a methyl methacrylate (MMA)-based resin within a thick glass fiber layup. The framework couples the auto-accelerating exothermic free-radical bulk polymerization of MMA with the transfer of heat within the system and to the environment. Three approaches are considered for the kinetic treatment of the auto-acceleration: (1) an empirical model, (2) a semi-empirical model and (3) an analytical model. Predictions made via the three kinetic models are compared with experimental data measured during infusion tests with different processing conditions. Processing diagrams are constructed to reveal manufacturing regimes of interest for the production of thick fiber-reinforced methacrylic composites free of voids due to monomer boiling

Relationship between processing parameters and mechanical properties of thick glass fibre reinforced pmma composites

micro- and macroscopic scales. More precisely, composite plates are infused at different temperatures, and the evolution of the microstructure along the thickness of each sample is studied. Emphasis is placed on the characterization of porosity distribution and morphology by optical microscopy and X-ray microtomography. Additionally, chromatographic chemical analysis is carried out for the assessment of molecular weight distribution and residual monomer content. The local mechanical response is evaluated by carrying out nano-indentation tests inside the matrix pockets, while in situ transverse compression experiments are performed within a SEM. Thermal analysis as well as a simple thermomechanical method are used to quantify the amplitude of the residual stresses. In parallel, the viscoelastic-viscoplastic response of the bare matrix is determined by combining various macroscopic tests in order to allow micromechanical modelling of representative volume elements. Current results suggest that, while the in situ micromechanical properties of the methacrylic matrix barely vary with the infusion temperature, it strongly affects the amount and distribution of porosity in the composite part – which, in turn, governs the macroscopic mechanical properties of the final composite part and their variability

Nanomechanics serving polymer-based composite research

Tremendous progress in nanomechanical testing and modelling has been made during the last two decades. This progress emerged from different areas of materials science dealing with the mechanical behaviour of thin films and coatings, polymer blends, nanomaterials or microstructure constituents as well as from the rapidly growing field of MEMS. Nanomechanical test methods include, among others, nanoindentation, in-situ testing in a scanning or transmission electron microscope coupled with digital image correlation, atomic force microscopy with new advanced dynamic modes, micropillar compression or splitting, on-chip testing, or notched microbeam bending. These methods, when combined, reveal the elastic, plastic, creep, and fracture properties at the micro- and even the nanoscale. Modelling techniques including atomistic simulations and several coarse graining methods have been enriched to a level that allows treating complex size, interface or surface effects in a realistic way. Interestingly, the transfer of this paradigm to advanced long fibre-reinforced polymer composites has not been as intense compared to other fields. Here, we show that these methods put together can offer new perspectives for an improved characterisation of the response at the elementary fibre-matrix level, involving the interfaces and interphases. Yet, there are still many open issues left to resolve. In addition, this is the length scale, typically below 10 micrometres, at which the current multiscale modelling paradigm still requires enhancements to increase its predictive potential, in particular with respect to non-linear plasticity and fracture phenomena