1. Background

Polylactic acid (PLA) is a biodegradable and renewable polymer derived from lactic acid, which is typically produced from fermented plant sugars. It has gained significant attention in recent years due to its environmental friendliness and potential to replace traditional petroleum - based plastics. However, PLA has some inherent limitations, such as brittleness, low impact resistance, and poor toughness, which restrict its wide - spread application in many fields. To overcome these drawbacks, researchers have been exploring various methods to toughen PLA. One promising approach is to use bio - based polyurethane (PU) as a toughening agent. Bio - based PU, derived from renewable resources, not only offers the potential to improve the toughness of PLA but also aligns with the concept of sustainable development.

2. Recent Advances

2.1 New Types of Bio - based Polyurethane for PLA Toughening

• ​​Bio - based Polyols - based PU​​: Researchers have developed new types of bio - based polyols from various renewable resources, such as soybean oil, castor oil, and lignocellulosic biomass, to synthesize bio - based PU for PLA toughening. For example, soybean oil - based polyols can be used to prepare bio - based PU with unique structures and properties. The unsaturated bonds in soybean oil can be modified to introduce different functional groups, which can enhance the compatibility and interaction between PU and PLA. Castor oil, rich in hydroxyl groups, is also a popular raw material for synthesizing bio - based PU. The resulting PU can have good flexibility and toughness, effectively improving the impact resistance of PLA.
• ​​Bio - based Isocyanates - based PU​​: Although the research on bio - based isocyanates is still in its early stages, some progress has been made in using bio - based isocyanates to synthesize PU for PLA toughening. Bio - based isocyanates can be derived from natural products such as amino acids and sugars. These bio - based isocyanates can offer a more sustainable alternative to traditional petroleum - based isocyanates, and the resulting PU may have different reactivity and properties, which can be tailored to achieve better toughening effects on PLA.

2.2 Improved Toughening Mechanisms

• ​​Compatibilization and Interfacial Bonding​​: One of the key factors in toughening PLA with bio - based PU is to improve the compatibility between the two phases. Researchers have developed various methods to enhance the compatibilization and interfacial bonding between PU and PLA. For example, by adding compatibilizers such as block copolymers or reactive agents, the interface between PU and PLA can be strengthened, promoting the transfer of stress and improving the toughness of the composite. The compatibilizers can help to reduce the phase separation between PU and PLA, resulting in a more homogeneous and stable microstructure.
• ​​Multiple Toughening Mechanisms​​: In addition to the traditional mechanism of introducing flexible PU phases to absorb energy, new multiple toughening mechanisms have been explored. For example, the formation of a co - continuous phase structure between PU and PLA can provide better stress distribution and energy dissipation. Some researchers have also found that the interaction between PU and PLA at the molecular level, such as hydrogen bonding or van der Waals forces, can contribute to the toughening effect.

2.3 Application - Oriented Research

• ​​Biomedical Field​​: In the biomedical field, the combination of PLA and bio - based PU has great potential. The improved toughness of PLA through bio - based PU toughening can make it more suitable for applications such as tissue engineering scaffolds, drug - delivery systems, and medical implants. For example, a toughened PLA - bio - based PU composite can provide better mechanical support for cell growth and tissue regeneration in tissue engineering. At the same time, the biodegradability and biocompatibility of both PLA and bio - based PU ensure their safety in biomedical applications.
• ​​Packaging Industry​​: The packaging industry is another area where the toughened PLA - bio - based PU composites can be applied. The improved toughness of PLA can make it more resistant to impact and rupture during transportation and handling, extending the shelf life of packaged products. The biodegradability of the composite also meets the increasing demand for environmentally friendly packaging materials.

3. Challenges and Future Prospects

3.1 Challenges

• ​​Cost​​: The production cost of bio - based PU is still relatively high compared to traditional petroleum - based PU, which limits its large - scale application in toughening PLA. The cost of renewable raw materials, synthesis processes, and purification steps all contribute to the high cost.
• ​​Performance Balance​​: Achieving a good balance between toughness and other properties such as strength, stiffness, and transparency of the PLA - bio - based PU composite is still a challenge. Sometimes, the improvement in toughness may lead to a decrease in other properties, which needs to be carefully optimized.
• ​​Large - scale Production​​: Scaling up the production of toughened PLA - bio - based PU composites from the laboratory to the industrial level is also a significant challenge. Issues such as process stability, quality control, and equipment adaptation need to be addressed.

3.2 Future Prospects

• ​​Technological Innovation​​: Continued research and development in the synthesis of bio - based PU, toughening mechanisms, and composite preparation techniques are expected to lead to further improvements in the performance of PLA - bio - based PU composites. New technologies such as nanotechnology and green chemistry may offer more opportunities for enhancing the toughness and other properties of the composites.
• ​​Market Expansion​​: As the demand for sustainable and high - performance materials increases, the market for toughened PLA - bio - based PU composites is expected to expand. The development of new applications in various industries, such as automotive, construction, and electronics, will drive the growth of this market.
• ​​Sustainable Development​​: The use of bio - based PU to toughen PLA is in line with the concept of sustainable development. In the future, more efforts will be made to develop more environmentally friendly and cost - effective bio - based PU and composite materials, promoting the transition of the plastics industry towards a more sustainable direction.