ABSTRACT
Polyhydroxybutyrate (PHB) is a promising biopolymer. However, processing PHB in pure form in thermoplastic processes is limited due to its rapid degradation, very low initial crystallization rate, strong post-crystallization, and its low final stretchability. In this article, we screened commercial PHBs for morphological characteristics, rheological properties, and "performance" in the meltblown process in order to reveal process-relevant properties and overcome the shortcoming of PHB in thermoplastic processes for fiber formation. An evaluation of degradation (extruded (meltblown) material vs. granules) was performed via rheological and SEC analysis. The study revealed large differences in the minimum melt temperature (175 up to 200 °C) and the grade-dependent limitation of accessible throughput on a 500 mm plant. The average fiber diameter could be lowered from around 10 µm to 2.4 µm in median, which are the finest reported values in the literature so far. It was found that the determination of the necessary process temperature can be predicted well from the complex shear viscosity. Different to expectations, it became apparent that a broader initial molar mass distribution (>8) is suitable to overcome the state-of-the art limitations of PHAs in order to stabilize fiber formation, increase the productivity, and obtain better resistance towards thermal degradation in process. Accordingly, longer polymer chain fractions could be more affected by degradation than medium and short polymer chains in the distribution. Further, a low initial narrow distributed molar mass resulted in too brittle fabrics.
ABSTRACT
The idea of "Nanoval technology" origins in the metal injection molding for gas atomization of metal powders and the knowledge of spunbond technologies for the creation of thermoplastic nonwovens using the benefits of both techniques. In this study, we evaluated processing limits experimentally for the spinning of different types of polypropylene, further standard polymers, and polyphenylene sulfide, marked by defect-free fiber creation. A numerical simulation study of the turbulent air flow as well as filament motion in the process visualized that the turnover from uniaxial flow (initial stretching caused by the high air velocity directed at the spinning die) to turbulent viscoelastic behavior occurs significantly earlier than in the melt-blown process. Modeling of the whole process showed that additional guide plates below the spinneret reduce the turbulent air flow significantly by regulating the inflow of secondary process air. The corresponding melt flow index of processible polymer grades varied between 35 g·10min-1 up to 1200 g·10min-1 and thus covering the range of extrusion-type, spunbond-type, yarn-type, and meltblown-type polymers. Hence, mean fiber diameters were adjustable for PP between 0.8 and 39.3 µm without changing components of the process setup. This implies that the Nanoval process enables the flexibility to produce fiber diameters in the typical range achievable by the standard meltblown process (~1-7 µm) as well as in the coarseness of spunbond nonwovens (15-30 µm) and, moreover, operates in the gap between them.