Nicolas Boyard

Chemical shrinkage characterization techniques for thermoset resins and associated composites

Control and optimization of curing process is very important for the production of high quality composite parts. Crosslinking of molecules of thermoset resin occurs in this phase, which involves exothermy of reaction, chemical shrinkage (Sh) and development of thermo-physical and thermo-mechanical properties. Exact knowledge of the evolution of all these parameters is required for the better understanding and improvement of the fabrication process. Sh is one such property of thermoset matrix, which is difficult to characterize due to its coupling with thermal expansion/contraction. A number of techniques have been used to determine volume Sh of thermoset matrix, which later on has been used to find tensor of Sh for the simulation of residual stresses and shape distortion of composite part, etc. Direct characterization of volume Sh of composites has also been made by some authors. Though not much, but some work has also been reported to determine the Sh of composite part in a specific direction. In this article, all the techniques used in the literature for the characterization of Sh of resin and composite are reported briefly with their respective advantages, disadvantage and important results.

Evolution of chemical and thermal curvatures in thermoset-laminated composite plates during the fabrication process

Residual deformations and stresses formation in the thermoset-laminated composite is a frequently studied subject in the recent years. During fabrication, the laminated composites undergo chemical deformation during cross-linking and thermal deformation while cooling. In thin laminates, due to large displacements and complex evolution of shape, these deformations can only be explained by using nonlinear strain–displacement relationship. In the present article, we calculated together for the first time, the thermal and chemical deformations occurring in carbon/epoxy laminates by considering a nonlinear geometrical approach to understand the evolution of shape and hence residual stresses induced during fabrication process. The effect of fibre fraction on the chemical and thermal deformations is studied as well.

Characterization of the cure shrinkage, reaction kinetics, bulk modulus and thermal conductivity of thermoset resin from a single experiment

The use of thermoset composites has increased remarkably during the recent past in naval, automobile and aeronautical applications. Despite superior mechanical behaviour, certain problems, e.g. shape distortion, fibre buckling and matrix cracking, are induced in composite part, especially during fabrication due to the heterogeneous nature of such materials. Excellent control of the curing process is required for production of a composite part with required shape and properties. For an accurate simulation of the curing process, exact knowledge of cure-dependent polymer properties and heat transfer is needed. Several instruments are required to identify these parameters, which is time consuming, and costly. In the present study, results on the simultaneous characterization of bulk modulus, chemical shrinkage and degree of cure of vinylester resin using PVT-α device are presented. Determination of cure and temperature-dependent thermal conductivity of the matrix using the same device is also discussed. The obtained results are compared with the available literature results.

Atomic force microscopy, a powerful tool to study blend morphologies based on polyester resins

Atomic Force Microscopy (AFM) was an unusual but effective tool used to investigate the morphology of cured blends based on UP (unsaturated polyester). The pertinence of AFM was evaluated by studying four miscible UP/LPA (low profile additive)/ST (styrene) blend systems. The morphology of these cured blends before and after LPA solubilization was analogous in SEM (Scanning Electron Microscopy) and AFM. However, in AFM the particles boundaries were more defined compared to SEM. Before treatment, nanoparticles (less than 60 nm) and aggregates (140 to 250 nm) were discernible. After treatment, nanogels (less than 50 nm) and microgels (80 to 220 nm) were observed. The aggregates composed of linked nanoparticles, were connected together to form a whole network. The microgels were composed of linked nanogels and were connected to form a polyester network. The LPA solubilization reduced the nanoparticles to nanogels in extracting the LPA phase out of the nanoparticles. The particles size depended on the miscibility of the system UP/LPA/ST and was related to the void volume. Shrinkage and light opacity were macroscopic properties which characterized the void volume and therefore the particle sizes.

A Device to Measure the Shrinkage and Heat Transfers during the Curing Cycle of Thermoset Composites

Residual stresses development during manufacturing of composites depends mostly on the shrinkage behaviour of the polymer matrix from the point where stresses cannot be relaxed anymore. The matrix shrinkage may have a thermal and/or chemical origin and can leads to dimensional instability, ply cracking, delamination and fibre buckling. The approaches for measuring cure shrinkage can be classified as volume and non-volume dilatometry. Each technique has corresponding advantages and drawbacks but volume dilatometry is the one that is mostly used. In the present article, we report a home-built apparatus, named PVT-a mould, on which temperature, volume change and reaction conversion degree are measured simultaneously for an applied pressure. It can also be used to study the composite during curing and for the bulk samples having several millimetre thicknesses. The instrument is preferred over other techniques as it works in conditions close to the industrial ones. This device was used to measure cure shrinkage of resin and thermoset composite material with different fibre fractions as a function of temperature and reaction conversion degree. The heat of cure of the resin measured by PVT-a mould was compared to the results obtained by DSC.

Heat Transfer and Crystallization Modeling during Compression Molding of Thermoplastic Composite Parts

We present in this paper, the coupling of heat transfer to the crystallization of composite in a closed mold. The composite is based on thermoplastic resin (low viscosity PA 66) with glass fiber (50% volume fraction). In order to realize this coupling, an accurate characterizationof thermo physical properties in process conditions, especially in the molten and solid state is needed. In addition, theidentification of the parameters of crystallization kinetics is required. Therefore, we present the methods that were used to study the thermo physical properties as the thermal conductivity, heat capacity and the specific volume. Moreover, the kinetic of crystallization was estimated over a large temperature range by using Flash DSC and classical DSC. In order to validate the measurements, the whole process was modeled by finite elements. The model includes the resolution of the strong coupling between the heat transfer and crystallization. Finally, the experimental and numerical results were compared.

In situ characterization of in-plane chemical shrinkage of thermoset laminated composites using a simple setup

Cure shrinkage in the thermoset matrix is the major source of cure-induced defects in composite parts for industrial applications. Thus, its correct determination is very important to optimize the composite fabrication process. In general, volume chemical shrinkage of resin is tested and assuming it is isotropic, rule of mixture or a homogenization technique is used to model the linear chemical shrinkage of composite. Some studies are also found in the literature on the measurement of linear chemical shrinkage of very small composite samples under atmospheric pressure. In the present article, a new setup is presented for the measurement of evolution of in-plane chemical shrinkage of thermoset laminated composite during curing. Using this setup, characterization of mass scale samples was done under pressure and heating ramp conditions. Degree of cure of composite during the test was determined using differential scanning calorimeter. Results show that chemical shrinkage in the composite appears from gel point and its evolution with the degree of cure is nonlinear. Experimental results also led to conclusion that most of the chemical shrinkage occur along the thickness direction.

Estimation of effective thermal conductivity tensor from composite microstructure images

The determination of the effective thermal properties of inhomogeneous materials is a long-standing problem of continuously interest. The impressive number of methods developed to measure or estimate the thermal properties of composite materials clearly exhibits the importance given to their knowledge. Homogenization models are a cheap way to determine or predict them. Many different approaches of homogenization were developed, but the last advances are credited to numerical methods. In this study, a new computational model is developed to estimate the 2D thermal conductivity tensor and the thermal main directions of a pure carbon/epoxy unidirectional composite. This tool is based on real composite microstructure.

Effect of pressure and reinforcement type on the volume chemical shrinkage in thermoset resin and composite

The diverse use of thermoset composite materials is increasing day by day in industrial applications. This has led to the development of several fabrication techniques, use of various reinforcement types, and different fabrication conditions to achieve a composite part with required properties. Despite all these technological advancements, there is a shear need to investigate and understand the effect of all these factors on the curing process. Volume chemical shrinkage of resin is one such property, which has been studied by several authors for a given value of applied pressure. A few studies have reported results on volume chemical shrinkage of composites for one type of reinforcement and for a single applied pressure. In the present work, experiments on vinylester resin and associated glass fibres composites were conducted under two different pressures. The tested composites were containing unidirectional fibres ([0] and [0/90]) and plain woven fabric with two different fibre volume fractions. The results of these experiments, carried out in a plunger type dilatometer, led us to show the effect of fibre fraction, type of reinforcement, and applied pressure on the volume chemical shrinkage of vinylester resin.

Experimental Determination and Modeling of Thermal Conductivity Tensor of Carbon/Epoxy Composite

In this study, the effective thermal conductivity tensor of carbon/epoxy laminates was investigated experimentally in the three states of a typical LCM-process: dry-reinforcement, raw and cured composite. Samples were made of twill-weave carbon fabric impregnated with epoxy resin. The transverse thermal conductivity was determined using a classical estimation algorithm, whereas a special testing apparatus was designed to estimate in-plane conductivity for different temperatures and different states of the composite. Experimental results were then compared to modified Charles & Wilson and Maxwell models. The comparison showed clearly that these models can be used to accurately and efficiently predict the effective thermal conductivities of woven-reinforced composites.