# Production and Processing of a Spherical Polybutylene Terephthalate Powder for Laser Sintering
## 1 Introduction
The mechanical properties and crystalline structure of laser-sintered parts made from a cryogenically milled PBT copolymer powder were reported by [PERSON] et al. [13]. The copolymer, which contained 10 mol % isophthal acid as a comonomer, was processed on a laser sintering machine at 190 \({}^{\circ}\)C. The produced parts exhibited only a slightly reduced strength compared to their injection-molded counterparts, but a drastically reduced elongation at break. In a subsequent study [14], the same authors investigated the behavior of PBT powder dry blended with short glass fibers. Most of the work up to this point was carried out on PBT produced by grinding granules into powder, typically resulting in irregularly shaped particles that give a powder suboptimal flowability and packing density. In this article, melt emulsification is used as an alternative method to directly create PBT powder consisting of spherical particles. Various researchers showed the advantages of using powders with spherical particles in laser sintering. [PERSON] et al. [15; 16] characterized different powders with respect to size distribution, and particularly particle shape. The flowability as expressed by measurement of the angle of repose was found to correlate with the circularity and roundness of powder particles. [PERSON] [17] provided screening methodologies for laser sintering powders, which particularly included assessment of the powder flowability. [PERSON] [18], following this methodology, extensively characterized the relations between size, shape, flowability, and performance of a wide range of materials. It was found that in general, a smooth and spherical particle shape contributes to the laser sintering material performance. In a comparison of two laser-sinterable PA12 powders, [PERSON] et al. [19] found increased tensile strength and elongation at break for the powder with the higher sphericity. This was attributed at least partially to the improved packing density achieved with spherical particles. Quantitative results relating a more spherical particle shape to improved flowability and laser sintering performance were presented by [PERSON] [20] and [PERSON] et al. [21]. The term melt emulsification refers to the process of melt blending two incompatible materials to create a structure consisting of droplets of one component embedded in the matrix of the other component. The melt emulsification method is well-established and reviewed [22] in the pharmaceutical domain to produce dispersions of active components with spherical morphology in a polymer matrix. For the production of laser sintering powders, the method is less well documented. [PERSON] et al. [23] reported the production of spherical PA12 particles by melt blending with polyvinyl alcohol (PVA). The authors also showed some preliminary results on blends of PBT and polyethylene glycol (PEG). The production of spherical polypropylene particles via melt blending with hexadecane was described by [PERSON] et al. [24; 25], however no parts were produced with the obtained powder. The current work describes the production of a PBT powder consisting of spherical particles by melt emulsification with PEG as the matrix phase. The advantage of this process route is the absence of organic solvents and the requirement of only elementary polymer processing equipment; as will be demonstrated, it is already possible to produce reasonable amounts of powder that allow for laser sintering trials using a conventional laboratory-scale single-screw extruder. In an emulsion, the equilibrium between break-up and coalescence governs the mean droplet size. For a system in shear, the droplet break-up depends on the shear rate and relative viscosity of the two phases. The coalescence is mainly governed by interfacial tension, which determines the probability of recombination of two colliding droplets. A system consisting of non-Newtonian materials flowing in a non-uniform shear field across a wide temperature range, as is the case in a typical extruder, cannot be described by simple models.