Thomas Köppl

Competition in aluminium phosphinate-based halogen-free flame retardancy of poly(butylene terephthalate) and its glass-fibre composites

Abstract Aluminium diethylphosphinate (AlPi-Et) and inorganic aluminium phosphinate with resorcinol-bis(di-2,6-xylyl phosphate) (AlPi-H+RXP) were compared with each other as commercially available halogen-free flame retardants in poly(butylene terephthalate) (PBT) as well as in glass-fibre-reinforced PBT (PBT/GF). Pyrolysis behaviour and flame retardancy performance are reported in detail. AlPi-H+RXP released phosphine at very low temperatures, which can become a problem during processing. AlPi-Et provided better limiting oxygen index (LOI) values and UL 94 ratings for bulk and PBT/GF than AlPi-H+RXP. Both flame retardants acted via three different flame-retardancy mechanisms in bulk as well as in PBT/GF, namely, flame inhibition, increased amount of char, and a protection effect of the char. AlPi-Et was more efficient in decreasing the total heat evolved of PBT in the cone calorimeter test. AlPi-H+RXP reduced the peak heat release rate of PBT more efficiently than AlPi-Et. An optimum loading of AlPi-Et in PBT/GF was found, which was below the supplier’s recommendation. This loading provides a maximum increase in LOI and a maximum decrease in total heat evolved.

Influence of graphene on the cell morphology and mechanical properties of extruded polystyrene foam

The incorporation of nanoparticles to polymer foams not only reinforces the cell walls and struts but can also lead to a decrease of cell size and enhanced cell morphology which in turn, yield foams with superior mechanical properties. For this purpose, several studies have focused on identifying close-to-ideal nucleating agents as well as understanding the influence of processing parameters on foam cell morphology. This research provides a systemic approach to low-density polystyrene foams produced with graphene (thermally reduced graphite oxide), talc and carbon nanotubes (MWCNTs) via foam extrusion. Remarkably, the cell morphologies of polystyrene/thermally reduced graphite oxide foams show enhanced cell homogeneity with a tremendous increase of the cell densities by more than one order of magnitude compared to neat polystyrene and its counterparts.

The miscibility of poly(butylene terephthalate) (PBT) with phosphorus polyester flame retardants

Phosphorus polyesters provide sustainable flame retardancy to conventional polyesters like PET (poly(ethylene terephthalate)) and PBT (poly(butylene terephthalate)). Their use as a flame retardant additive results in the formation of a classical polymer blend. The morphology of such blends should determine the resulting fire behavior and has therefore to be examined in detail. This study investigates the state of miscibility in blends of standard polyesters (mainly PBT) with phosphorus (co)polyesters mainly using the behavior of the glass transition (T g). The T g values were determined by modulated DSC measurements and compared to the ones calculated assuming complete miscibility according to the Fox–Flory concept. The homopolyesters are completely immiscible with each other. Miscibility can be improved by transesterification reactions in the melt as well as by copolyester consisting of segments of PBT or PET and the phosphorus polymer.

E-PBT—Bead foaming of poly(butylene terephthalate) by underwater pelletizing

Foams from engineering thermoplastics like poly(butylene terephthalate) (PBT) are a new generation of polymer foams and, probably, the future for lightweight, insulation and damping materials. By means of foam extrusion or bead foaming, it is possible to achieve low-to-medium density foams (< 500 kg/m3). However, foam extrusion of PBT is quite challenging due to its low melt strength and drawability combined with a small temperature-processing window, which is a characteristic of semi-crystalline thermoplastics. This work proves that the problem of cell coalescence and insufficient cell stabilisation can be reduced by choosing the right material and processing parameters in foam extrusion with underwater pelletizing. Therefore, expanded PBT beads could be realised for the first time using CO2 as supercritical blowing agent. To obtain spherical low-density PBT beads with a homogenous foam structure, different process parameters were systematically studied with two different commercial extrusion grades and different blowing agent concentrations. The influence of water pressure and cutting speed of the underwater pelletizer on foam morphology of E-PBT and bead structure was studied. It was shown that using a polymer grade with a sufficiently high-melt viscosity helps to reduce cell coalescence. The lowest achieved density was 230 kg/m3. An increase of the blowing agent concentration did not help in reducing the density. The melting range was investigated by differential scanning calorimetry and yielded reasonable moulding temperatures of 205–215 ℃. This corresponds to steam pressures of 17–21 bar in a steam-moulding machine.