Thermal behaviour of benzoxazine blends based on epoxy and cyanate ester

In the present work, an attempt has been made to develop high-performance polymeric hybrid binary blends of epoxy/benzoxazine and benzoxazine/cyanate ester with varying weight percentages (25/75, 50/50 and 75/25 wt%) of resins, namely, bisphenol-F epoxy resin (DGEBF), benzoxazines [bisphenol–F/aniline (BF-a) and imidazole core-based bisphenol/aniline (IBP-a)] and cyanate ester [bisphenol-F bifunctional cyanate ester (BF-CE)]. The molecular structure, polymerisation temperature/cure behaviour, glass transition temperature (Tg) and thermal stability of the neat polymeric matrices and binary hybrid blends of polymeric matrices were characterised using different analytical techniques, viz. Fourier Transform infra-red spectroscopy (FTIR), Nuclear Magnetic Resonance spectroscopy (NMR), Differential Scanning Calorimetry (DSC) and thermogravimetric analysis (TGA). Among the binary hybrid blends, the lowest polymerisation temperatures (Tp) were noticed in the case of blends of epoxy/benzoxazine were 219°C for DGEBF/BF-a (25/75 wt%) and 170°C for DGEBF/IBP-a (25/75 wt%). Similarly, in the case of blends of benzoxazine/cyanate ester, the lowest values of Tp observed were 155°C and 153°C for BF-a/BF-CE (75/25 wt%) and IBP-a/BF-CE (75/25 wt%), respectively. The highest values of Tg observed for the blends of epoxy/benzoxazine were 175°C and 254°C for DGEBF/BF-a (25/75 wt%) and DGEBF/IBP-a (25/75 wt%), respectively. Whereas, the highest values of Tg observed in the case of blends of benzoxazine/cyanate ester were 234°C and 278°C for BF-a/BF-CE (25/75 wt%) and IBP-a/BF-CE (75/25 wt%), respectively. From the TGA results of blends, the maximum degradation temperature (Tmax) and limiting oxygen index (LOI) value calculated from the char yield, which ascertain that almost all the binary hybrid blends of epoxy/benzoxazine and benzoxazine/cyanate ester possess good flame retardant behaviour.

Thermal dimensioning of manufacturing moulds with multiple resistively heated zones for composite processing

Multi-zonal, electrically heated moulds for composite processing offer the potential of a direct heat introduction with low thermal lag and high energy efficiency. However, appropriate thermal dimensioning of these tools requires the consideration of the thermal response of the tool itself as well as the thermal and cure behaviour of the part, which is to date mostly estimated based on experience. To realize the full potential of this tool class, a numerical method is presented to determine a sound partitioning of the designated heating area utilizing 3D finite element cure simulation. Further, a cure simulation model of an application case is set up and validated. The capability of the numerical method to significantly increase the temperature accuracy and the degree of cure homogeneity are demonstrated in an evaluation of the numerically improved application case. Finally, the effect of the tool material on the zone allocations and temperature accuracy is studied.

Preparation and Processing Characterization of Glass/Phenolic Prepregs

Glass/phenolic prepreg is one of the most applicable prepregs used for making composites for structural parts. To investigate the effective parameters on the processing and properties of these prepregs, thirty samples of glass/phenolic prepregs containing about 50 wt.% resins were prepared by using a resole type phenolic resin and satin glass fibre fabric. They were B-staged or pre-cured at 110 °C and 120 °C for different times from 15 to 50 min, to control the flow behaviour and tack properties. Tack and resin flow were characterized, in order to determine the conditions in which flow and tack properties are optimum. Results show that the levels of pre-cure or conversion were between 2% and 55%. The maximum tack was achieved at 5.3% conversion. Cure behaviour and rheological properties of these prepregs were studied by using differential scanning calorimetry and rheometry. An appropriate cure cycle is presented with the aid of these results. After curing the prepregs on the basis of this cure cycle, a flexural strength of 172.6 MPa and flexural modulus of 17 GPa were obtained.