Vitrimer carbon fibre composites based on a novel polybenzoxazine matrix
Topic(s) : Special Sessions
Co-authors :
Festus ANAGWU (UNITED KINGDOM), Alex SKORDOS (UNITED KINGDOM)Abstract :
A high-temperature polybenzoxazine vitrimer has been synthesised based on a solventless method [1] involving the Mannich condensation of a phenolic disulphide, paraformaldehyde, and aniline. The material has thermosetting behaviour below its glass transition temperature of 155°C. Dynamic bond exchange by disulphide metathesis with a topological freezing temperature of 78°C enables self-healing over the glass transition temperature (Fig. 1). The benzoxazine vitrimer system exhibits reprocessability within realistic time scales at 190°C, a glassy modulus of 3.6 GPa, a high limiting oxygen index of 40.5% making it a fire-retardant material, and excellent environmental resistance. This combination of attributes positions the polybenzoxazine vitrimer system as a promising matrix for recyclable and self-repairable composites.
A study of cure kinetics, glass transition evolution, and chemorheology of the new polybenzoxazine was carried out to optimise the processing of its composites. The cure kinetics of the system which is autocatalytic was modelled using the Karkanas model [2] with a diffusion term, resulting in a relative error of 4.4% in degree of cure. The rheological model, which is based on the kinetics of the evolution of a reference viscosity acting as a state variable, represents accurately the advancement of viscosity during cure with an average relative error of 9.4%. The rheological behaviour of the system is appropriate for processing through a film or prepreg route.
Resin film infusion was utilised to manufacture composites using the polybenzoxazine matrix and a pseudo-UD carbon fabric (Tenax HTA 5131). Composite specimens were subjected to multicycle mode I delamination and healing. The healing conditions were within the range of conventional composite manufacturing processing, using a temperature of 190°C and a pressure of 4 bar. The strain energy release rate of the pristine polybenzoxazine-carbon composite is 285 ± 45 J/m². Self-healing of the material is controlled by the level of previous crack opening and is gradual from a level of about 15% when two surfaces of a crack have been completely separated to 100% at the point of fresh interface. The variable controlling the level of healing is maximum historical crack opening, with the dependence of restored toughness following an exponential decay as a function of it (Fig.2). This behaviour is universal across different cycles of healing and for the case of the polybenzoxazine-carbon material investigated here has a characteristic length of about 0.3 mm – corresponding to the level of mode I crack opening at which healing efficiency reaches 50%. Consequently, delamination cracks in the interior of composite laminates can be healed under realistic manufacturing temperatures and pressures. The dependence of healing efficiency on historical crack opening shown for the first time in this study is an important consideration for the future design of self-repairing composite structures.
A study of cure kinetics, glass transition evolution, and chemorheology of the new polybenzoxazine was carried out to optimise the processing of its composites. The cure kinetics of the system which is autocatalytic was modelled using the Karkanas model [2] with a diffusion term, resulting in a relative error of 4.4% in degree of cure. The rheological model, which is based on the kinetics of the evolution of a reference viscosity acting as a state variable, represents accurately the advancement of viscosity during cure with an average relative error of 9.4%. The rheological behaviour of the system is appropriate for processing through a film or prepreg route.
Resin film infusion was utilised to manufacture composites using the polybenzoxazine matrix and a pseudo-UD carbon fabric (Tenax HTA 5131). Composite specimens were subjected to multicycle mode I delamination and healing. The healing conditions were within the range of conventional composite manufacturing processing, using a temperature of 190°C and a pressure of 4 bar. The strain energy release rate of the pristine polybenzoxazine-carbon composite is 285 ± 45 J/m². Self-healing of the material is controlled by the level of previous crack opening and is gradual from a level of about 15% when two surfaces of a crack have been completely separated to 100% at the point of fresh interface. The variable controlling the level of healing is maximum historical crack opening, with the dependence of restored toughness following an exponential decay as a function of it (Fig.2). This behaviour is universal across different cycles of healing and for the case of the polybenzoxazine-carbon material investigated here has a characteristic length of about 0.3 mm – corresponding to the level of mode I crack opening at which healing efficiency reaches 50%. Consequently, delamination cracks in the interior of composite laminates can be healed under realistic manufacturing temperatures and pressures. The dependence of healing efficiency on historical crack opening shown for the first time in this study is an important consideration for the future design of self-repairing composite structures.