Spinning of Poly(Lactic Acid) Fibers

Abstract

A fiber or a filament (a continuous form of fiber) is the fundamental unit of textile materials. It has a unique combination of high strength (tensile, bending, torsional, or compression), high flexibility (i.e., low modulus), extensibility, and shows recoverability on deformation. Most of these properties are observed in one principal direction, which is known as the axis of the fiber. Since all textile structures from one to three dimensional (yarn, fabric, or braids, etc.) are built using this basic structural unit, these structures also possess the above unique properties.These unique properties of a fiber or a filament are the result of a microstructure (morphology) in which the majority of the polymer chains are oriented in the direction of the axis of the fiber. The more oriented the polymer molecules are in this direction, the better properties the resulting fiber/ filament is likely to have. For producing man-made fibers, the polymers (either natural or synthetic) must be unfolded and extended unidirectionally to an extremely large extent resulting in a very high aspect (length to diameter) ratio and high orientation. This is known as spinning.In spinning, a small amount of polymer ($1 g) may be elongated to over 9000 m in length while the other dimension (diameter) is only in microns. This requires precise control over the spinning process to enable such unidirectional extension in the melt form. In fiber formation, all efforts are directed at controlling the microstructure of the polymer so that the properties mentioned above are obtained with respect to the principal axis of the fiber. This is unlike other polymer processing methods such as injection molding, compression molding, extrusion, and blowing, and so on, where mostly isotropic properties are desired and only a little attention is directed toward making a controlled microstructure in terms of molecular orientation and spatial arrangement. The engineering complexity of the spinning operation is evident from the fact that, in modern spinning plants, a large amount of polymer is expected to be continuously converted (spun) into filaments with nearly no breaks.In spinning, polymer melt or solution is extruded from a fine hole and is elongated by applying a external tensile force on the extruded portion. As the polymer melt or solution is pulled, it is cooled or precipitated, respectively, to form a solid filament. This filament is then usually subjected to post-spinning operations such as drawing, which applies unidirectional stretching to the material in a semisolid form, and heat setting, which is crystallization to equilibrium. Other post-spinning processes such as texturing are simply variations of the drawing and heat setting processes to impart curvilinear shape to an otherwise straight filament. This gives physical bulk to the filaments. The process of fiber formation is complete only when both spinning and post-spinning operations are carried out. MELT SPINNING LINEMelt spinning is a process of producing filaments from a polymer melt. Ho...

Toward High-Performance Poly(l-lactide) Fibers via Tailoring Crystallization with the Aid of Fibrillar Nucleating Agent

“…Thermal annealing provides a straightforward pathway to improve the heat resistance of PLLA fibers via enhancing crystallinity, but it is time-consuming and only limited improvement has been obtained. 17 Actually, for semicrystalline polymers consisting of alternating crystalline and amorphous phases, their performance is also strongly dependent on crystal structure, crystal morphology, and lamellae orientation. 7,18−25 Formation of an oriented crystal structure is favorable for improving the performance of polymers.…”

“…Recently, PLLA textile fibers with tremendous application value and market potential in various fields have garnered considerable enthusiasm as an eco-friendly alternative to the most widely used nylon and poly­(ethylene terephthalate) (PET) fibers in modern industry and life. − In comparison with these petrochemical-based fibers, PLLA fibers possess not only exceptional sustainability but also some advantageous physical properties, such as better elastic recovery, higher moisture regain, better weathering stability, as well as lower flammability and smoke generation. ,, Various types of commercial PLLA fibers including monofilaments, multifilaments, staple fibers, short-cut fibers, and spunbond fabrics have been fabricated using conventional melt spinning technology. , Unfortunately, the application potential of PLLA fibers has not been fully realized until now, mostly because the very slow crystallization rate of PLLA makes it hard to obtain fibers with a high crystallinity (e.g., 55–60%) even under intensive elongational flow conditions (which can notably accelerate the crystallization kinetics of semicrystalline polymers by orders of magnitude relative to the quiescent condition − ) involved in melt spinning. ,, In this case, the low crystallinity (i.e., 20–30%) cannot provide the melt-spun PLLA fibers with sufficient heat resistance for ironing and dyeing (both of them typically take place above 120 °C) due to the limitation of relatively low glass transition temperature ( T g ) of around 55–60 °C. ,, On the other hand, the insufficient heat resistance also significantly hampers their use in numerous environments where low thermal shrinkage and high dimensional stability are required. Thermal annealing provides a straightforward pathway to improve the heat resistance of PLLA fibers via enhancing crystallinity, but it is time-consuming and only limited improvement has been obtained . Actually, for semicrystalline polymers consisting of alternating crystalline and amorphous phases, their performance is also strongly dependent on crystal structure, crystal morphology, and lamellae orientation. ,− Formation of an oriented crystal structure is favorable for improving the performance of polymers. ,,,, Imposing an intensive flow field seems to be a facile method to trigger dramatically enhanced crystallization kinetics as well as highly oriented crystallization of polyolefin. ,, Nevertheless, for semirigid PLLA chains, flow conditions involved in conventional melt spinning at low speeds (usually 200–1000 m/min) cannot meet the essential requirement for h...…”

Assessment of Polylactide as Optical Material

“…The popularity of polylactide is based on its properties, bio-based producibility 98 and biodegradability [99][100][101] in combination with its processability in most of the established techniques with minimal to no adaptations 102 . Viable processing methods include extrusion [102][103][104] , injection molding [102][103][104] film casting or blow extrusion 102,103 and melt electro spinning 102,104,105 . A number of popular PLA applications according to Auras et al 106 are discussed in this Section.…”

Polylactic acid/wood-based in situ polymerized densified composite material

“…40 Moreover, using biobased resources reduces dependency on fossil fuels and also provides a solution to eco-friendly concerns. 22,41 From a chemistry point of view, PLA is produced from the polycondensation reaction between hydroxyl groups and carboxylic acid groups of lactic acid monomers. 42 And via the removal of water produced as a by-product during condensation reaction, the overall reaction moves in the forward direction to produce PLA.…”