Fabrication of Melt Spun Polypropylene Nanofibers by Forcespinning

Melt spinning of polypropylene nonwoven nanofiber mats is demonstrated using Forcespinning The effects of rotational speed and melt flow rate of polymer on fiber diameter were studied. It was observed that melt flow rate of the polymer plays a major role in nanofiber formation and uniformity. Differential scanning calorimetry and X-Ray Diffraction analysis indicate that the crystallinity of nanofibers was decreased when compared to bulk polypropylene and the appearance of an oriented meso-phase (β phase) became evident with nanofiber development. Thermogravimetric analysis demonstrated that melt spinning of polypropylene using the Forcespinning method did not cause molecular degradation of the polymer. INTRODUCTION Nonwoven fabrics used in filtration applications are an important part of the global nonwoven industry making up the fourth largest end-use market in North America [1]. The global market for nonwoven filter media was estimated to be $2.5 billion in 2010 and is expected to increase to more than $3.5 billion in 2015 [2]. Filter manufacturers have used micron sized nonwoven fiber mats in filter media since about the 1950s. Melt blowing and melt spinning are the most common processes used for the fabrication of nonwoven micron sized fiber mats. Polymer melt is extruded through the orifice die and fibers are collected on the take up wheel during the melt spinning process. The fiber diameters of melt spun fibers are higher than 10 μm and are produced in high volumes. The melt blowing is performed by extruding the polymer melt through the orifice die and molten filaments are attenuated by hot air (usually the same temperature as molten polymer) to form microfibers. Filter efficiency is known to improve as the size of the fibers decreases since surface area increases [3]. Therefore, since the mid 1990s research and development in the nanofiber area has heavily intensified. Polymeric nanofibers have gained significant attention given their potential applications in the filtration industry and in general in the nonwoven industry. Some of the methods that have been used to produce nanofibers are drawing [4], template synthesis [5-6], phase separation [7], self-assembly [8-9], electrospinning [10-12] and melt-blown spinning [13]. Many of these methods have advantages for specific fiber formations, but are limited from an industrial processing standpoint. Mass production of nanofibers is of great importance to meet the current demands in the market [14]. Process control and design are some of the key parameters in mass production of nanofibers, and desirable properties are small fiber diameter and tight homogeneity (narrow fiber size distribution). The drawing process is one where the viscoelastic materials can forgo large deformation and are mechanically pulled into nanofibers though usually encapsulated into a matrix and known as “island in a sea”. The drawing process has the advantage of being a low cost process since there is minimum equipment requirement, though its disadvantage is that materials must possess enough ductility to undergo large deformations without failure and it is either a discontinuous, batch process where fiber dimensions and morphology are difficult to control or if prepared as island-in-a-sea, there is a need to remove the matrix. In the case of template synthesis, fibers of different diameters and shapes can easily be obtained. Nanoporous membranes have been used as a template to produce nanofibers of fibril or hollow shape. Its disadvantage is that the process cannot be scaled to obtain high yields and continuous nanofibers. The phase separation process is limited to certain polymeric systems and, as with the other processes, it cannot be scaled up. However, its advantages are that the consistency of the nanofibers from batch to batch is quite homogeneous and the process can be tailored to directly fabricate nanofiber matrices