Plasticization and Toughening Modification of PLA

Polylactic acid (PLA) has excellent properties such as high mechanical strength, ductility and good biocompatibility. It is easy to process and can be processed into products by various methods, among which blow molding and thermoplastic are the most commonly used. Its application involves a wide range of fields such as aerospace, medicine, packaging, etc., and has a very promising application prospect.

However, the shortcomings of PLA, such as excessive brittleness, poor heat resistance, and difficulty in film formation, greatly limit its application development in various fields. Therefore, it is necessary to modify PLA to increase its flexibility and expand its application range. At present, the methods to increase the flexibility of PLA mainly include two methods: plasticizing modification by adding plasticizer and toughening modification by adding tough material blending.

01. Plasticizing modification of PLA

When synthesizing PLA composite materials, a small amount of plasticizer can be added to enhance the flexibility of PLA, reduce the glass transition temperature (Tg), enhance the processing properties and mechanical properties of PLA, and make PLA easier to form.

Plasticization mechanism of PLA

The molecular chain of PLA is easily entangled with each other due to its polar groups, which hinder its movement after entanglement, resulting in poor flexibility. Plasticizing PLA means reducing the mutual attraction between PLA molecular chains, making it more fluid and less likely to entangle each other. At present, the method of plasticizing PLA is to add an appropriate amount of plasticizer during PLA processing, and there are two main methods for plasticizer to play a role:

The first is the direct action method, that is, when PLA and plasticizer are blended, the plasticizer molecules are filled between the PLA molecular chains, the distance between the large molecular chains is enlarged, and the moving space is increased, so that the molecular chains are not easily entangled with each other, and the purpose of plasticizing PLA is achieved.

The second is the indirect action method, that is, add polar plasticizer, the polar group of plasticizer acts on the polar part of PLA, destroys the polar ester group on PLA molecular chain, weakens the attraction between molecular chains, and increases fluidity.

Polyethylene Glycol (PEG) Plasticized PLA

PEG is one of many plasticizers suitable for PLA, and it has remarkable effect in plasticizing and modifying PLA. PEG plasticized PLA can increase the elongation at break and impact strength, but the tensile strength sometimes decreases. The reason is that although PEG has a good effect on plasticizing and toughening PLA, due to the poor compatibility between PEG and PLA, the strength of the plasticized PLA is greatly reduced and the thermal stability is poor. This issue still needs to be resolved.

Citrate plasticized PLA

Citrate has a polar ester group, which directly acts on the polar group on the molecular chain when plasticizing PLA, causing damage, thereby enhancing the motion activity of the PLA segment and improving the flexibility of PLA. Tributyl citrate (TBC) and triethyl citrate (TEC) are common among citrate plasticizers. Acetyl butyl citrate (ATBC) is obtained by processing and optimizing TBC. In terms of performance, ATBC retains the advantages of original TBC, but its effect as a plasticizer is better than TBC.

Vegetable oil-based plasticized PLA

As the name suggests, vegetable oil plasticizers are extracted from plants and belong to natural environmental protection materials. They have a good development trend in plasticizing and modifying PLA. Modification of bio-based vegetable oil plasticizers can improve the plasticization effect and may ensure the original mechanical properties of PLA. The use of vegetable oil plasticizer polymerization can increase its plasticizing effect. Some scholars used polyepoxy palm oil (PEPO) as plasticizer to prepare blend PLA/PEPO by dynamic vulcanization technology. The results show that plastic deformation occurs between the PEPO phase and PLA. When 20% PEPO is added, the elongation at break increases to 100%, which is 10 times that of pure PLA.

02. Toughening modification of PLA blend

At present, the blend materials used for PLA toughening and modification mainly include flexible and degradable resins, elastomers, inorganic nanoparticles, plant fibers, starch, etc.

Flexible degradable resin toughened modified PLA

Flexible and degradable resins have achieved excellent results in toughening PLA due to their excellent flexibility. However, the compatibility of degradable resins when blended with PLA is not good, and compatibilizers need to be added for compatibilization. When the flexible resin is blended and toughened, the strength of PLA is likely to be greatly reduced, which can be improved by adding other materials with excellent strength. For example, PLA/PBS can be enhanced with microcrystalline cellulose (MCC). When 20% PBS was added, the blends had the best compatibility, and the tensile properties decreased. When MCC was added, the thermal stability was enhanced, and the tensile properties did not decrease significantly. When the MCC mass content was higher than 0.5%, the tensile properties increased.

Elastomer toughened modified PLA

Elastomers have achieved good results in toughening PLA due to their excellent flexibility. At present, there are mainly three kinds of natural rubber, synthetic rubber and thermoplastic elastomer. Among them, natural rubber and thermoplastic elastomer are more common in toughening and modified PLA.

Thermoplastic polyurethane (TPU) has high toughness, durability and good biocompatibility, and has great potential in toughening PLA. Elastomers can effectively improve the toughness of PLA, but the composites do not have biodegradability, which is not conducive to the environment and needs to be improved.

Inorganic rigid particle toughening modified PLA

Inorganic rigid particles have rigidity and can increase the strength of PLA materials while toughening PLA. At present, nano-CaCO3, nano-SiC, nano-SiO2 and carbon nanotubes (CNTs) are mainly used for toughening PLA.

Inorganic rigid particles have limited effect on toughening PLA. Blending it with other flexible materials to toughen PLA can make PLA both tough and tough. For example, PLA can be melt-blended by adding epoxidized natural rubber (ENR) and nano-silica (nSiO2) to prepare PLA/ENR-30/nSiO2 composites. After adding nSiO2, the impact strength of the material is significantly improved, which is 1.1-1.8 times higher than that of pure 80/20 PLA/ENR blend and 6.3-10.8 times higher than that of PLA.

Plant fiber toughening modified PLA

Plant fibers are extracted from natural plants and have excellent biocompatibility and flexibility. Blending plant fibers with PLA can improve the mechanical and thermal properties of PLA, reduce its cost, and retain the good biocompatibility and biodegradability of PLA itself. Plant fibers can also synergistically toughen PLA together with plasticizers, and the toughening effect is better.

Starch Toughened Modified PLA

Starch is a pure natural biological material with excellent biocompatibility, ductility and mechanical properties. Because it has similar properties to PLA, it is often used to fill PLA, which can strengthen PLA and improve its toughness while reducing the cost of PLA. Due to the thermodynamic incompatibility between starch and PLA, a compatibilizer needs to be added to improve the interfacial compatibility during blending.

For example, non-oxidized sesame oil (ESO) was added to PLA and thermoplastic yam starch (TPS) to prepare a PLA/TPS/ESO mixture by extrusion. The results showed that when 3% ESO was added, the tensile elastic modulus and tensile strength approximately doubled, and the deformability of the ESO-free mixture increased by more than 70%.

Using starch epoxidation as a core-shell structure and then blending with PLA can not only greatly enhance the toughness, but also ensure that the strength of PLA does not decrease significantly.

03. Conclusion

Modified by adding plasticizer or blending with flexible degradable resin and elastomer, the tensile strength and thermal stability of PLA will be greatly reduced, and the material obtained by blending with elastomer is non-biodegradable. and the blending effect of inorganic rigid particles is not obvious, and the blending compatibility with plant fibers and starch is poor.

The following suggestions are put forward for the future plasticization and toughening modification of PLA:

(1) Develop new plasticizers to make up for the deficiencies of current plasticizers to improve the mechanical properties of PLA;

(2) Add a suitable compatibilizer to improve the compatibility between PLA and other materials;

(3) Find suitable bio-based materials to ensure the mechanical strength of PLA as much as possible when blending with PLA.

Nanjing Haisi Extrusion supply twin screw extruder for PLA plasticization and toughening modification. 

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