Research on toughening modification of polylactic acid material (PLA)

Research on toughening modification of polylactic acid material (PLA)

Polylactic acid (PLA) is a thermoplastic aliphatic polyester that can be extracted from corn, potato or starch materials. And can be completely degraded into carbon dioxide and water under certain soil and compost conditions without causing environmental pollution. PLA has excellent biocompatibility, biodegradability, thermal properties and mechanical properties. And is widely used in disposable tableware, food packaging and biomedical devices. However, PLA has poor toughness, which limits its wide application.
Currently, there are two common methods to improve the toughness of PLA:

(1) Graft flexible molecular chains into PLA as block or graft copolymers to improve the ductility of the PLA matrix.
(2) Physical blending method. Blend PLA with materials such as elastomers, inorganic fillers or plasticizers to improve the toughness of PLA. The physical modification method is simple and convenient, and has become the most commonly used modification method in industrial production.

  1. Blended with elastomer
    Blending PLA with elastomers is an effective way to toughen PLA. When impacted by an external force. The elastomer will produce streaks or shear bands, which can absorb the energy from the outside and improve the impact strength of the material. But this method will reduce the strength and modulus of PLA. Therefore, researchers focus on developing high-performance elastomers. Blending elastomers with PLA can improve toughness without reducing strength and modulus.

Xi Lifeng and others used discrete metallocene catalysis technology to develop propylene-based elastomers (PBE) and toughened and modified PLA to obtain high-toughness PLA/PBE melt-blown nonwoven materials. The results show that as the PBE content increases. the stress of PLA/PBE melt-blown nonwoven materials increases. And the elongation at break increases accordingly. When the PBE addition amount is 20%. The elongation at break and tensile strength of PLA/PBE melt-blown nonwoven materials increase by 455% and 25% respectively. The filtration efficiency increases by about 1.1 times, and the filtration performance of the material is enhanced.

Researchers use the superior thermoplasticity and high elasticity of elastomers to toughen PLA so that the blends exhibit excellent toughness. However, as the toughener content increases, the tensile strength and modulus of PLA blends are affected. In addition, currently commonly used elastomers are all petroleum-based. and their extensive use causes serious pollution to the environment and affects the degradation performance of PLA. Therefore, the development of bio-based/degradable elastomers is a research hotspot for elastomer-toughened PLA.

  1. Blended with flexible polymers

2.1 Flexible biodegradable polymers
In order to maintain the biodegradability and compostability of PLA, polycaprolactone (PCL), starch, polybutylene adipate-terephthalate (PBAT). And polybutylene succinate (PBS) are usually used Toughened PLA with biodegradable polymers such as polyhydroxyalkanoate (PHA) to prepare a variety of biodegradable polymers.
The addition of flexible biopolyester causes the tensile strength of PLA to drop from 57MPa to about 20MPa.
Some biodegradable polymers have poor compatibility with PLA, and compatibilizers usually need to be added for adjustment. Wu Haojie et al. melt-blended PLA, PBAT and a compatibilizer containing glycidyl methacrylate (GMA) functional groups to study the toughening effect of the PBAT content on the blended system. Research shows that when 20% PBAT and 6% compatibilizer are added. The elongation at break of the PLA/PBAT blend increases from 8.9% of pure PLA to 80.7%. It shows that under the action of compatibilizer. The blend of PLA and PBAT has good toughness and biodegradability, which broadens the application scope of PLA.

2.2 Non-biodegradable polymers
In addition to blending with flexible bio-based polymers, non-degradable polymers can also be added to achieve a toughening effect. Such as polyethylene, polypropylene, aromatic polyester, polysiloxane, polyformaldehyde, etc.
The addition of flexible polymer materials can improve the toughness of PLA materials. Especially flexible biodegradable polymers, and also solve the problem of non-degradable blend systems. However, most flexible polymers and PLA are incompatible or partially compatible during the blending process. Resulting in low mechanical strength of the system and insignificant toughening effect. Therefore, it is necessary to add compatibilizers. Nucleating agents or inorganic nanoparticles to improve the interfacial compatibility between the two and improve the performance of the material.

  1. Blended with inorganic materials
    PLA can be toughened by blending with elastomers and flexible polymers. But this will reduce the material’s rigidity and heat resistance. Inorganic nanoparticles have a special structure and excellent properties. Which can significantly improve the toughness, rigidity and heat resistance of polymers, and can also reduce costs. However, the higher surface area of nanoparticles can easily lead to aggregation, thereby reducing some other properties of the material.

Studies have shown that the elongation at break of composite materials first decreases and then increases with the addition of fGO. Indicating that the flexible molecular chain TDI-BD has a greater contribution to improving the toughness of composite materials. Therefore, fGO has excellent toughening and strengthening composite effects and can be used to toughen and modify PLA.

Some scholars used aluminate coupling agent (ACA) to modify calcium carbonate (CaCO3) and filled the modified PLA with aluminate calcium carbonate (AlCaCO3). The results showed that the addition of Al-CaCO3 improved the toughness of the material. The elongation at break and impact strength of the blend are increased by up to 350% and 150%, maintaining good comprehensive mechanical properties. The addition of AlCaCO3 also improves the degradation performance of the system. When the Al-CaCO3 content is greater than 30%, the blend can be completely degraded within 3 days.

The inorganic filler forms a certain physical cross-link with PLA, which can reinforce, toughen and improve the rigidity of the system. It also has certain thermal stability. However, the poor adhesion and dispersion between the filler and the matrix is a difficulty that needs to be solved.

  1. Add plasticizer

Plasticization modification refers to mixing a certain amount of high boiling point, low volatility and non-toxic plasticizer into PLA to weaken the force between PLA molecular chains, reduce the rigidity of PLA molecular chains, and enhance the mobility of PLA chain segments. However, plasticizers may leak out during use, affecting the performance of PLA. Commonly used plasticizers include low molecular polymers such as citrate esters, polyethylene glycol, glyceryl butyrate and glycerin. Citrate ester plasticizer is an important environmentally friendly plasticizer, and the commonly used ones are acetyl tributyl citrate (ATBC) and tributyl citrate (TBC).

Plasticizers can weaken the forces between PLA molecules, thereby improving the material’s flexibility and processability. However, there are still problems with plasticizers: when a certain amount of plasticizer is added, the plasticizing effect will be more obvious, but the compatibility with PLA is poor. As the plasticizer content increases, the tensile strength of PLA decreases and the modulus decreases; the boiling point of the plasticizer is low, and volatilization occurs during processing; the cost of the plasticizer is high, which limits its practical use. Therefore, bio-based plasticizers with low cost, stable performance and good compatibility are the future development direction.

5 Conclusion

Some results have been achieved in the physical blending and toughening modification of PLA, which has broadened the market prospects of PLA in high-toughness applications. However, most of the traditional modification methods can only improve the single property of PLA toughness, and rarely can take into account both the mechanical properties and degradation properties of PLA. Therefore, the focus of future research is to develop low-cost toughening modifiers with excellent comprehensive properties, green and environmental protection, so that PLA can meet more application fields.

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