Polylactic acid (PLA), also known as polylactide, is an aliphatic thermoplastic polyester. It is an earlier commercial bio-based, biodegradable and biocompatible polymer. PLA can be prepared by direct condensation polymerization of lactic acid monomers or ring-opening polymerization of cyclic lactide dimers. It is derived from corn starch, sugar cane and other renewable biomass products. After use, it can be completely degraded by microorganisms in nature to generate carbon dioxide and water. PLA are used in plastic products, packaged food, fast food lunch boxes, non-woven fabrics and other fields. In addition, PLA also has good gloss, transparency and tensile strength. However, PLA also has shortcomings such as low melt strength, slow crystallization rate, lower use temperature and heat distortion temperature (HDT), which hinder its application. In addition, PLA is very brittle, and its elongation at break is less than 10%; its glass transition temperature (Tg) is only 55-60°C. These shortcomings greatly limit its application areas. Research strategies and technologies include modification of PLA with plasticizers, copolymerization or blending with other polymers, and preparation of composite materials.
Compared with chemical copolymerization, polymer blending can combine the performance advantages of many existing polymers, changing the components and composition of the blend to adjust its properties. For example, blending with bio-based biodegradable polymers, such as starch, polycaprolactone (PCL), polyadipate/butylene terephthalate (PBAT), etc.; blending with other non-degradable thermoplastics, Such as acrylonitrile, butadiene, styrene terpolymer (ABS) resin, polyoxymethylene, etc.; blended with elastomeric polymers, such as natural rubber (NR), polyurethane (PU), styrene-based thermoplastic elastomer ( SBS), etc.; blending with inorganic materials, such as nano-silica, graphene, etc.; or forming a ternary polymer blend, such as PLA/thermoplastic elastomer (TPS)/PCL, PLA/TPS/polybutadiene Butylene Diester (PBS) and so on.
Blending of PLA with biodegradable polymers
The polyhydroxybutyrate-based polymer (PHB-di-rub) has a synergistic effect on the nucleation and toughening of PLA. Adding 10% PHB-di-rub/PLA blend can increase its storage modulus by 32%, elongation by 128 times, toughness by 84 times, and slight changes in strength and stiffness. PHB acts as a rigid core and poly(lactide-co-caprolactone) random copolymer acts as a soft shell. Differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized light microscopy (POM) analysis showed the excellent nucleation ability of PHB-di-rub. Small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and WAXD clarify that the toughening mechanism is the rubber-induced crazing effect. This high-strength, high-toughness degradable blend is expected to replace the existing petroleum-based polymers, and has broad application prospects in the fields of biomedicine, automobiles, and structures.
A reinforced and toughened polylactic acid/polyadipate-butylene terephthalate (PLA/PBAT) blend through a die-drawing process, introduced PBAT into the PLA matrix, and introduced it through a die-drawing process. Oriented structure, can prepare reinforced and toughened materials.
Polylactic acid-caprolactone (PLA-r-PCL) random copolymer films with different component ratios. Compared with pure PLA, when the PCL content is 30%, the elongation at break reaches 611%, which is 70 times that of PLA. At the same time, at 40℃, the CO2 and O2 permeability coefficients of PLA-r-PCL increased by 801% and 523%, respectively; at 5℃, the CO2/O2 permeability ratio of PLA-r-PCL reached 7.0, which increased Up 115%. In addition, the water vapor transmission rate of PLA-r-PCL increased by 355% at 20°C. Can be used for food packaging materials.
Blending of PLA with non-degradable thermoplastics
ABS is a thermoplastic polymer structural material with high strength, good toughness, and easy processing and forming, and it has been widely used in 3D printing materials. Abt et al. melt blended polylactic acid (PLAREx) modified by reaction extrusion with ABS, and used maleic anhydride (MA) grafted ABS-g-MA as a compatibilizer to improve the compatibility of PLA and ABS. Studies have found that the presence of the ABS phase makes the PLA/ABS-g-MA blend microfibrillate, hinders the propagation of free cracks, and increases the fracture energy.
By the research of the blend of polylactic acid and polybutylene terephthalate (PLA/PBT), ethylene-glycidyl methacrylate (E-GMA) copolymer is added as a compatibilizer. The epoxy group on the compatibilizer undergoes a ring-opening reaction with the hydroxyl group or carboxylic acid of the blending component in the molten state. The tensile test shows that when 3% (wt, mass fraction, the same below) PBT is contained, the elongation at break increases from 3% of pure PLA to 49%, while the tensile modulus only drops from 3.59GPa to 3.35GPa. PLA changes from brittleness to toughness. Therefore, the nano-scale dispersed phase can effectively change the deformation behavior of the matrix without significantly reducing its modulus.
Blend of PLA and elastomer
Poly(ethylene-butyl acrylate-glycidyl methacrylate copolymer) (PTW) is a very active elastomer containing epoxy groups (glycidyl groups) and can be used as a toughening agent for PLA.
Through the research of the miscibility, thermal properties, degradation behavior and toughening mechanism of PLA/PTW mixtures, the PLA/PTW blend was prepared by melt blending. During the mixing process, a chemical reaction occurred between the PTW epoxy group and the PLA end group, which improved the compatibility of the blend. As the PTW content increased from 0 to 20%, the impact strength of PLA/PTW blends increased from 4.6kJ/m2 to 54.1kJ/m2, and the elongation at break increased from 5.6% to 270%. The large number of holes and plastic deformation on the impact fracture surface are caused by the interaction between the elastomer and the terminal functional groups, thereby increasing the interfacial adhesion.
The compatibility of PLA with ethylene-vinyl acetate-glycidyl methacrylate elastomer (EVM-GMA) have been adjusted by changing the blending temperature. The PLA/EVM-GMA (80/20) blend prepared at 230°C showed the best mechanical properties, with a tensile strength of 37.2MPa, an elongation at break of 176%, and an Izod notched impact strength of 61.2kJ/ m2; The annealed blend also has good heat resistance (HDT>90°C).
By studying hydrogen bonding to improve the toughness of PLA blends with a small amount of bio-based polyamide elastomer, a series of sustainable functional polyamide copolymers (PUDA-co-BUDA) have been successfully prepared using castor oil derivatives. The sustainable functional polyamide uses N,N'-(2-hydroxypropane-1, 3-Diyl) 0 bis(undecyl-10-enamide) (UDA) and 1,3-bis(undecyl-10-enamide) prop-2-yl butyrate (BUDA) A copolymer synthesized from monomers, PUDA-co-BUDA copolymer contains a large number of —NH and —OH groups, which will form a strong enough hydrogen bond network after blending with PLA, only adding 1% PUDA-co-BUDA , The elongation at break of the PLA/PUDA-co-BUDA blend is increased by about 23 times.
Using core-shell nanoparticles, PLA is toughened, which is a methyl methacrylate-butyl acrylate (ACR) core-shell structured copolymer. The rubber core can resist impact, and the glass shell provides rigidity and compatibility. . When the ACR content reaches 20%, the elongation at break is increased by 65 times compared with pure PLA, the Izod impact strength of the blend is 8 times higher, and the transmittance is as high as 75%. The shear yield of the matrix and the cavitation of the rubber particles during the impact result in the excellent toughness of the PLA/ACR blend.
The dispersed spherical silica nanoparticles with rubber elastomer and biodegradable poly(ε-caprolactone-co-L-lactic acid) are designed as the rigid core and rubber shell of core-shell nanoparticles, respectively, by increasing the thickness of the rubber shell, rigid core-soft shell particles can greatly improve the toughness of PLA. The main mechanism of toughening is the cavitation of rubber, which causes strong plastic deformation of the matrix.
Blending of PLA with inorganic materials
The functionalized multi-walled carbon nanotubes (FMWCNTs) are introduced into ethylene-vinyl acetate (EVA) to toughen PLA. FMWCNTs has a good nucleation effect on the cold crystallization of the PLA matrix during the annealing process, which increases the crystallinity (Xc); after annealing, it can compensate for the decrease in the stiffness and strength of the PLA caused by the addition of EVA.
Based on PLA/PCL blends and high surface area graphite (HSAG), a simple and low environmental impact new composite preparation method is developed . Through the ultrasonic treatment of HSAG in PCL, the dispersion and peeling of HSAG in PLA/PCL blends are realized. Adding 0.3% HSAG filler to the blend can obtain large elongation at break and strength.
By the use of shell powder (SP) and stearic acid surface modified shell powder (SSP) as fillers and PLA as a matrix, PLA/SP and PLA/SSP composites are prepared by melt blending. The results show that with the increase of filler content, the tensile strength and bending strength of PLA/SP and PLA/SSP composite materials decrease, while the impact strength first increases and then decreases. When the SP content is 10%, PLA/SP composite materials Its impact strength is 0.58kJ/m2, which is equivalent to pure PLA; when the amount of SSP is 30%, the impact strength of PLA/SSP composite is 1.16kJ/m2, which is 114.81% higher than pure PLA. The dispersion of SSP in the PLA matrix is more uniform than that of SP, and the mechanical properties of PLA/SSP composites are relatively better. SP and SSP can be used as nucleating agents to promote the crystallization of PLA, and the crystallization promotion effect of SSP is more obvious.
Blending of PLA with ESO and APP
Through reasonable design, controlled the reaction of PLA, epoxidized soybean oil (ESO, an inexpensive biological toughening agent) and ammonium polyphosphate (APP, an effective environmentally friendly flame retardant) during the blending process. , PLA composite material with high mechanical strength and good flame retardancy is made. The PLA/ESO/APP ternary blend not only increased the tensile strength to 42.0Mpa, but also increased the ductility and fracture toughness of PLA by 21 times and 14 times, respectively.
At the same time,through the use of the reinforcement and interface compatibility of microfibrillated cellulose, it’s succeful to realize a strong and tough PLA-based composite material. Blend PBS and PLA, and then use epoxidized microfibrillated cellulose (MFC-EPI) as interfacial compatibilizer and reinforcing filler. The effect of the amount of PBS and MFC-EPI on the crystallization behavior and thermal stability of PLA-based materials was studied. The impact of sex and mechanical properties. The "bridging" effect of the filler helps the energy transfer and dissipation during the deformation process. This "two-in-one" modification strategy ensures high strength and high toughness, and can be used to develop more materials with high mechanical properties.
Conclusion
PLA is a bio-based biodegradable and biocompatible plastic. Due to its excellent mechanical properties (such as high modulus and high strength), it has shown great potential in various fields. However, due to its inherent brittleness, low elongation at break (<10%) and low impact strength (2.6kJ/m2), the application areas of PLA are limited. Various biodegradable and non-biodegradable polymers have been blended with PLA to obtain new materials with unique properties. Among them, biodegradable polymers are widely used in the toughening of PLA due to their biocompatibility, rapid and complete biodegradability, and high toughness. In addition, non-biodegradable polymers are also widely used in the toughening research of PLA due to their low cost, excellent mechanical properties, high thermal stability and processability.
In addition, PLA is mixed with nanocomposites, nano core-shell particles, cellulose nanocrystals, etc., which can improve the interface compatibility of the mixture, especially with selective dispersion effect, and control the compatibility and phase structure of the blend. So as to achieve the effect of modification and toughening.
The main purpose of most PLA blends is to improve the impact toughness of PLA, but PLA toughening is usually related to the loss of tensile strength or modulus. Therefore, in the future, more polymer blending technology should be used to achieve long-lasting toughening of PLA without affecting the tensile strength and modulus. Especially in the context of global attention to environmental issues, especially strengthening the evaluation of the degradability of PLA-based biomaterials can effectively promote the development and application of biodegradable materials, which is very important to promote the development of the industry chain in the field of biodegradable materials.
