PLA fiber material is a new type of bio-based biodegradable materials, lactic acid as the main raw material polymerization of polymers, raw materials from sufficient and renewable sources, mainly corn, cassava and other raw materials, can be used in melt spinning, electrostatic spinning and other ways of processing.PLA fiber material due to good biocompatibility, easy to degrade renewable and other characteristics, so that it has good prospects for application in the fields of biomedicine, filtration and separation, packaging, and so on. It has good application prospects. This study focuses on the preparation of bio-based PLA fibers.
1,Melt spinning
Melt spinning method is based on polymer melt as raw material, extruded through the spinneret, and solidified into fibers by rapid condensation in the air. Melt spinning process is simple, spinning liquid for the fiber-forming polymer melt itself, do not need to spinning solvent or condensation bath recovery, and fiber forming process is completed in the gas phase, frictional resistance is small, and can be used to higher coil speed, high production efficiency. However, not all fiber-forming polymers can be used to prepare fibers by melt spinning, one of the conditions for the preparation of fibers by melt spinning: the polymer melt temperature must be lower than its thermal decomposition temperature of about 30 ℃, otherwise it is difficult to use the classic melt method for spinning.
The production process of PLA 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 heat sensitivity of PLA degradation reaction and the high viscosity of the melt, resulting in an extremely narrow temperature range for PLA melt spinning processing and the need to control the water content in the masterbatch to prevent hydrolysis and carbonization in the melt extrusion process. At the same time, the low crystallization rate of PLA leads to low heat distortion temperature, brittle material, poor toughness and long forming cycle. In order to improve the performance of PLA melt spinning, Pan Xiaodi et al. found that increasing the shear rate, i.e., increasing the spinning speed has less effect on the apparent viscosity of PLA melt, and the spinning process is easier to control.
Li et al. prepared polypropylene/poly(lactic acid) (PP/PLA) fibers by melt spinning and investigated their properties, and found that there was a small decrease in the thermal stability of PLA with the addition of PP, but the crystallinity was improved, and the orientation and mechanical properties of the PP/PLA blended fibers were improved.
CLARKSON et al. prepared high stiffness cellulose nanofiber/poly(lactic acid) (CNF/PLA) composite fibers by melt spinning under anhydrous and solvent-free conditions using poly(ethylene glycol) (PEG) as a bulking agent, and the mechanical properties of the fibers were increased by 600% after thermal stretching when CNF with a mass fraction of 1.3% was added.
2,Solution Spinning
Solution spinning is divided into two types of solution dry and wet methods. dichloromethane, trichloromethane or toluene are often used as solvents for the preparation of PLA fiber spinning stock solutions, such as YANG S et al. who studied the crystallization of solution cast high molecular weight PLA/CNT composites in the presence of solvents such as dichloromethane (CH2Cl2), trichloromethane (CHCl3), N,N-dimethylformamide (DMF), and 1,4-dioxane (DIOX). -dioxane (DIOX) solvents. It was found that the addition of carbon nanotubes (CNTs) with a mass fraction of 0.1% could promote the formation of neutral conformation crystals (SCs) of iso-PLLA/PDLA blends.
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 in descending order of DMF, DIOX, CHCl3, and CH2Cl2. In particular, distinctive SC microcrystals are formed in DMF. This difference can be explained by solubility parameters and solvent vapor pressure. The results of the study also provide possible solutions to regulate the crystalline composition of PLLA/PDLA/CNT blends.
Solution spinning to prepare PLA fiber research less, with melt-spun fibers, solution spinning has the following advantages: in the spinning process, the polymer entanglement of the network structure is less, so that the primary filament has a high tensile properties; spinning temperature is low, the thermal degradation is lower than the melt-spun fibers; the mechanical properties of the fiber is good, the strength of melt-spun fibers is high, but solution spinning there are slower spinning, spinning process of solvent contamination and recycling problems, but the industrial production applications are more limited. Therefore, it is more limited in industrial production applications.
3,Electrostatic spinning
Electrostatic spinning refers to the spinning process of polymer solutions or melts under the action of an applied electric field, and the prepared fibers can reach the nanoscale (5 nm~1000 nm), but the spinning conditions are prone to have a large impact on the fiber morphology and properties. Yin Xuebing et al. investigated the effects of dichloromethane (DCM), hexafluoroisopropanol (HFIP), and dimethylformamide (DMF) on the filament-forming ability of PLLA solution, the microstructure of the spinning products, and the filtration properties.
It was found that the mixed solvent of DCM/DMF could effectively improve the filament formation and jet stability of PLLA solution, the fiber diameter decreased significantly, and a special structure of coarse and fine crosses was formed between fibers, and the best overall performance of fiber membrane was obtained from the PLLA spinning solution when the volume ratio of DCM/DMF was 0.2.
Wang et al. used melt differential electrostatic spinning to prepare PLA fibers, and the average diameter of the fibers reached a minimum of 400 nm at a spinning temperature of 260 ℃, an airflow flow rate of 20 m3/h, an airflow temperature of 100 ℃, and a spinning distance of 5.5 cm. In addition, Zhong Guo-cheng et al. used hydroxyl-capped D-type polylactic acid (D PLA) as a macromolecular initiator to initiate the ring-opening polymerization of the L-propyl lactide backbone to produce different In addition, Zhong et al. used hydroxyl-capped D-type PLA as macromolecular initiator to initiate the ring-opening polymerization of L-propyl lactide body to prepare linear cubic diblock PLA with different number-average molecular weights, and prepared submicron fibers by means of electrostatic spinning.
The results showed that the melting points of the formed cubic composite crystals were over 215 °C, and the thermal stability was improved and good toughness was exhibited. Electrostatic spinning can realize the refinement of fiber materials compared with the traditional spinning technology, and the formation of PLA cubic composite crystals can help to improve the mechanical properties of fiber materials.
4. Concluding remarks
At present, the forming and application of bio-based PLA fibers and products in China are still in the primary stage. Data show that by the end of 2021, the production capacity of PLA in China is about 452,000 t, and it is expected that it will reach 5 million t in 2025.PLA, as a kind of green and environmentally friendly material, has the potential to replace the traditional petroleum-based fibrous materials.Analyzing and comparing the existing biobased PLA fiber forming methods and their advantages and disadvantages, the PLA degradation reaction needs to be solved in the melt spinning process with industrialization prospects In the process of melt spinning with industrialization prospect, it is necessary to solve the contradiction between heat sensitivity and high viscosity of melt, and to broaden the processing temperature range of PLA melt spinning.
At the same time, with the help of PLA recycling technology to accelerate the stable supply of PLA fiber raw materials in China. In the national "double carbon" strategy and other favorable policies, it can be expected that bio-based PLA fiber materials and products will usher in the development of leapfrog, in the field of biomedicine, filtration and separation, packaging and other areas show good prospects for application.