3 preparation methods of bio-based PLA fiber, which one is the best?
PLA fiber material is a new type of bio-based degradable material. It is a polymer obtained by polymerization of lactic acid as the main raw material. The raw material source is sufficient and renewable. It mainly uses corn and cassava as raw materials. It can be melt-spun, electrospun, etc. Processing in various ways. PLA fiber material has good application prospects in biomedicine, filtration separation, packaging and other fields due to its good biocompatibility, easy degradation and renewable characteristics. This paper mainly discusses the preparation of bio-based PLA fibers.
1. Melt spinning
The melt spinning method is to use polymer melt as raw material, extrude through the spinneret hole, condense rapidly in the air and solidify into fiber. The melt spinning process is simple, the spinning liquid is the molten liquid of the fiber-forming polymer itself, and there is no need to recover the spinning solvent or coagulation bath, and the fiber forming process is completed in the gas phase, with small frictional resistance, and a higher Winding speed, high production efficiency. However, not all fiber-forming polymers can be melt-spun to prepare fibers. One of the conditions for preparing fibers by melt-spinning is that the melting temperature of the polymer must be about 30°C lower than its thermal decomposition temperature, otherwise it is difficult to use the classic melting method. method for spinning.
The production process of polylactic acid melt spinning is similar to the spinning process of polyethylene terephthalate PET, which is divided into high-speed spinning one-step method and spinning-stretching two-step method. In the melt spinning process, there is a contradiction between the thermal sensitivity of the PLA degradation reaction and the high viscosity of the melt, resulting in an extremely narrow temperature range for PLA melt spinning, and it is necessary to control the water content in the masterbatch to prevent hydrolysis during the melt extrusion process. carbonization. At the same time, the low crystallization rate of PLA leads to low thermal deformation temperature, brittle material, poor toughness and long forming cycle. In order to improve the melt-spinning performance of PLA, Pan Xiaodi et al. found in the research on the rheological properties of four kinds of polylactic acid slices and their influence on the melt-spinning performance that increasing the shear rate means increasing the spinning speed on the surface of PLA melt. The smaller the effect of apparent viscosity, the easier the spinning process is to control.
Li Xiaochuan and others prepared polypropylene/polylactic acid (PP/PLA) fibers by melt spinning and studied their properties. They found that the thermal stability of PLA decreased slightly with the addition of PP, but the crystallinity increased, and PP/ The degree of orientation and mechanical properties of PLA blended fibers were improved.
CLARKSON et al prepared high-stiffness cellulose nanofiber/polylactic acid (CNF/PLA) composite fibers by melt spinning with polyethylene glycol (PEG) as a compatibilizer under anhydrous and solvent-free conditions. When adding a mass fraction of 1.3 % CNF, the mechanical properties of the fiber after thermal stretching increased by 600%.
2. Solution spinning
Solution spinning is divided into solution dry method and wet method. The preparation of PLA fiber spinning stock solution often uses dichloromethane, chloroform or toluene as solvents, such as YANG S et al. who study solution casting of high molecular weight polylactic acid/carbon nanotube (PLA/CNT) composites in dichloromethane (CH2Cl2 ), chloroform (CHCl3), N,N-dimethylformamide (DMF) and 1,4-dioxane (DIOX) and other solvents to crystallize stereocomplexes. It was found that the addition of carbon nanotubes (CNTs) at a mass fraction of 0.1% could promote the formation of stereocrystalline (SC) in iso-PLLA/PDLA blends.
The results of wide-angle X-ray diffraction and differential scanning calculations show that the ability of solvents to increase the SC content in PLLA/PDLA/CNT composites is DMF, DIOX, CHCl3, and CH2Cl2 in descending order. Especially unique SC crystallites were formed in DMF. This difference can be explained by solubility parameters and solvent vapor pressure. The findings also provide a possible solution for adjusting the crystallographic composition of PLLA/PDLA/CNT blends.
The preparation of PLA fibers by solution spinning is less researched. Compared with melt spinning fibers, solution spinning has the following advantages: the network structure of polymer entanglement is less in the spinning process, which makes the as-spun silk have high tensile properties; The spinning temperature is low, and the thermal degradation is lower than that of melt-spun fibers; the mechanical properties of fibers are good, and the strength is higher than that of melt-spun fibers, but solution spinning has problems of slow spinning speed, solvent pollution and recycling during spinning, so it is widely used in industrial production relatively restricted.
3. Electrospinning
Electrospinning refers to the spinning process of polymer solution or melt under the action of an external electric field. The prepared fibers can reach nanometer scale (5 nm~1000 nm), but the spinning conditions tend to have a great influence on the morphology and properties of the fibers. Influence. Yin Xuebing and others studied the effects of dichloromethane (DCM), hexafluoroisopropanol (HFIP), and dimethylformamide (DMF) on the silk-forming ability of PLLA solution, the microstructure of spinning products and the filtration performance.
The study found that the DCM/DMF mixed solvent can effectively improve the filamentation and jet stability of the PLLA solution, the fiber diameter is significantly reduced, and a special structure of thick and thin intersecting is formed between the fibers. When the volume ratio of DCM/DMF is 0.2, the PLLA spinning The fiber membrane obtained from the silk solution has the best overall performance.
Wang Xiaohui and others used melt differential electrospinning to prepare PLA fibers. When the spinning temperature was 260 ℃, the airflow rate was 20 m3/h, the airflow temperature was 100 ℃, and the spinning distance was 5.5 cm, the average fiber diameter reached the minimum value of 400 nm. . In addition, Zhong Guocheng et al. used hydroxyl-terminated D-type polylactic acid as a macroinitiator to initiate bulk ring-opening polymerization of L-lactide, and prepared linear stereo diblock polylactic acid with different number-average molecular weights. Electrospinning to produce submicron fibers.
The research results show that the melting point of the formed stereocomplex crystals exceeds 215°C, the thermal stability is improved and good toughness is exhibited. Compared with traditional spinning technology, electrospinning can realize the refinement of fiber materials. At the same time, the formation of PLA stereocomplex crystals helps to improve the mechanical properties of fiber materials.
