Progress in the development of bead foams – A review

For a long time, the number of available bead foam variants limited to standard polymers which restricted their functionality mainly to packaging, thermal insulation (e.g. in construction) and shock absorption (e.g. in transportation). In particular, standard polymers such as expanded polystyrene, expanded polyethylene and expanded polypropylene were used for components requiring good insulating properties and high energy absorption at low cost. Mainly since the last two decades, new polymer variants have found their way into the world of bead foams and are currently adding further functionalities, such as sustainability, flame retardancy, increased thermal stability and enhanced mechanical performance (e.g. improvements in energy absorption and impact resistance). Versatile fields of application open up, revolutionizing both industry and design sectors. This review article emphasizes the special development progress of new bead foam variants and their processing technologies. Upcoming opportunities of digital methods for modelling and simulation are highlighted.

Low-Density Polybutylene Terephthalate Foams with Enhanced Compressive Strength via a Reactive-Extrusion Process

Due to their appealing properties such as high-temperature dimensional stability, chemical resistance, compressive strength and recyclability, new-generation foams based on engineering thermoplastics such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) have been gaining significant attention. Achieving low-density foams without sacrificing the mechanical properties is of vital importance for applications in the field of transportation and construction, where sufficient compressive strength is desired. In contrast to numerous research studies on PET foams, only a limited number of studies on PBT foams and in particular, on extruded PBT foams are known. Here we present a novel route to extruded PBT foams with densities as low as 80 kg/m³ and simultaneously with improved compressive properties manufactured by a tandem reactive-extrusion process. Improved rheological properties and therefore process stability were achieved using two selected 1,3,5-benzene-trisamides (BTA1 and BTA2), which are able to form supramolecular nanofibers in the PBT melt upon cooling. With only 0.08 wt % of BTA1 and 0.02 wt % of BTA2 the normalized compressive strength was increased by 28% and 15%, respectively. This improvement is assigned to the intrinsic reinforcing effect of BTA fibers in the cell walls and struts.

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.