Experimental and computational study of the polymerization kinetics of a reactive acrylic thermoplastic resin
Topic(s) : Material science
Co-authors :
Damien GACON (FRANCE), Olivier DE ALMEIDA (FRANCE), Raffaele D'ELIA (FRANCE), Fabrice SCHMIDTAbstract :
The high viscosity of thermoplastics, which implies significant liquid processing temperatures and pressures, has long held back market growth for these composites. However, they are now gaining in popularity thanks to the development of liquid-reactive processes suitable for thermoplastic composites. In these processes, polymerization of the reactive mixture of low-viscosity monomers takes place in-situ, enabling fibrous reinforcements to be impregnated at lower temperatures. Due to the low evaporation temperature of the monomers and the highly exothermic nature of the reaction, monomer boiling-induced voids can form during processing of thick composite parts, leading to a sharp decrease of their mechanical properties.
The nucleation and growth of voids by monomer evaporation is a complex multiphysical phenomenon, requiring a good understanding of the polymerization kinetics of these reactive mixtures. In this study, the reaction kinetics of an Elium® acrylic resin grade (C195E) initiated by three different peroxides is investigated by differential scanning calorimetry (DSC). Dynamic and isothermal scans were performed in the ranges [1 – 20]°C/min and [50 – 120]°C respectively.
A total enthalpy of polymerization of more than 450 J/g was measured from dynamic tests carried out at 10°C/min. Figure 1 shows several thermograms obtained from DSC tests at different isothermal temperatures. A sudden auto-acceleration phenomenon, known as Trommsdorff effect, is observed during the free-radical polymerization of methyl methacrylate. Its occurrence and intensity depend strongly on the curing temperature, the viscosity and the composition of the reactive mixture. The obtained curves highlight various polymerization regimes and the influence of peroxide dissociation kinetics on the overall reaction rate. A first step is initiated by the exothermic dissociation reaction of peroxides, followed by an acceleration of chain propagation due to the Trommsdorff effect. Depending on the polymerization temperature and initiator nature and concentration, a third polymerization phase may occur. After vitrification, the reaction becomes diffusion-controlled and reaction kinetics drop drastically.
The measured data are fitted to a temperature, conversion and composition dependent kinetic model. Based on a hybridization of mechanistic and semi-empirical approaches, the model defines polymerization in two coupled parts: polymerization initiated by the formation of free radicals and an autocatalytic stage simulated by a modified Kamal and Sourour equation. This model transcribes the conversion and the different regimes observed during this polymerization over a wide range of reaction parameters. Preliminary results are encouraging, enabling the Elium® composites manufacturing process to be optimized. Coupled with thermal and evaporation models, this kinetic model will help prevent overheating of the reactive mixture and thus predict void growth within the composite part.
The nucleation and growth of voids by monomer evaporation is a complex multiphysical phenomenon, requiring a good understanding of the polymerization kinetics of these reactive mixtures. In this study, the reaction kinetics of an Elium® acrylic resin grade (C195E) initiated by three different peroxides is investigated by differential scanning calorimetry (DSC). Dynamic and isothermal scans were performed in the ranges [1 – 20]°C/min and [50 – 120]°C respectively.
A total enthalpy of polymerization of more than 450 J/g was measured from dynamic tests carried out at 10°C/min. Figure 1 shows several thermograms obtained from DSC tests at different isothermal temperatures. A sudden auto-acceleration phenomenon, known as Trommsdorff effect, is observed during the free-radical polymerization of methyl methacrylate. Its occurrence and intensity depend strongly on the curing temperature, the viscosity and the composition of the reactive mixture. The obtained curves highlight various polymerization regimes and the influence of peroxide dissociation kinetics on the overall reaction rate. A first step is initiated by the exothermic dissociation reaction of peroxides, followed by an acceleration of chain propagation due to the Trommsdorff effect. Depending on the polymerization temperature and initiator nature and concentration, a third polymerization phase may occur. After vitrification, the reaction becomes diffusion-controlled and reaction kinetics drop drastically.
The measured data are fitted to a temperature, conversion and composition dependent kinetic model. Based on a hybridization of mechanistic and semi-empirical approaches, the model defines polymerization in two coupled parts: polymerization initiated by the formation of free radicals and an autocatalytic stage simulated by a modified Kamal and Sourour equation. This model transcribes the conversion and the different regimes observed during this polymerization over a wide range of reaction parameters. Preliminary results are encouraging, enabling the Elium® composites manufacturing process to be optimized. Coupled with thermal and evaporation models, this kinetic model will help prevent overheating of the reactive mixture and thus predict void growth within the composite part.