PLA is one of the most researched biodegradable materials at home and abroad, and medical, packaging and fiber are its three popular application fields. PLA uses natural lactic acid as the main raw material. It has good biodegradability and biocompatibility. Its life cycle load on the environment is significantly lower than that of petroleum-based materials. It is considered to be the most promising green packaging material.
Biodegradable plastic refers to a material that can be degraded into low molecular weight substances by microorganisms themselves or their secretions under certain conditions. The U.S. Food and Drug Administration stipulates that except for biodegradable plastics and very few water-degradable plastics that can be used in the field of food packaging, others such as photodegradable plastics or photo-biodegradable plastics fail to meet the regulations as food packaging materials.
Polylactic acid (PLA) can be completely decomposed into carbon dioxide and water under natural conditions after being discarded. It has good water resistance, mechanical properties, biocompatibility, can be absorbed by organisms, and has no pollution to the environment. At the same time, PLA also has good mechanical properties. It has high impact strength, good flexibility and thermal stability, plasticity, processability, no discoloration, good permeability to oxygen and water vapor, good transparency, mildew resistance and antibacterial , service life of 2 to 3 years.
Film food packaging
The more important performance index of packaging materials is air permeability, and the application field of this material in packaging can be determined according to the air permeability of the material. Some packaging materials are required to be breathable to oxygen to supply the product with enough oxygen; some packaging materials are required to be barrier to oxygen in terms of material, such as packaging for beverages, which require materials that can prevent oxygen from entering the packaging to inhibit mold The role of growth. PLA has gas barrier, water barrier, transparency and good printability.
Transparency
PLA has good transparency and gloss, and its excellent performance is equivalent to that of cellophane and PET, which is a property that other degradable plastics do not have. The transparency and gloss of PLA are 2~3 times that of ordinary PP film, and 10 times that of LDPE. Its high transparency makes the use of PLA as a packaging material beautiful in appearance. For example, for candy packaging, at present, many candy packaging on the market use PLA packaging film.
The appearance and performance of this packaging film are similar to those of traditional candy packaging films, with high transparency, excellent kink retention, printability and strength, and excellent barrier properties, which can better retain the flavor of candies . A Japanese company uses the "racea" brand PLA of Cargill Dow Polymers of the United States as the packaging material for new products. The appearance of the packaging is very transparent. Toray Industries has developed PLA functional films and slices using its proprietary nanoalloy technology. The film has the same heat and impact resistance as petroleum-based films, but also has excellent elasticity and transparency.
Barrier
PLA can be made into film products with high transparency, good barrier properties, excellent processability and mechanical properties, and can be used for flexible packaging of fruits and vegetables. It can create a suitable storage environment for fruits and vegetables, maintain the life activities of fruits and vegetables, delay aging, and maintain the color, aroma, taste and appearance of fruits and vegetables. However, when applied to actual food packaging materials, some modifications are needed to adapt to the characteristics of the food itself, so as to achieve better packaging effects. For example, in practice, experiments have found that hybrid films perform better than pure films
Bacteriostatic
PLA can form a weakly acidic environment on the surface of the product, and has the basis of antibacterial and anti-mildew. If other antibacterial agents are used in combination, the antibacterial rate can reach more than 90%, and it can be used for antibacterial packaging of products. Taking Agaricus bisporus and golden ear as examples, Yin Min studied the fresh-keeping effect of the new PLA nano-antibacterial composite film on edible fungi, so as to prolong the shelf life of edible fungi and keep their quality in good condition. The results showed that PLA/rosemary essential oil (REO)/AgO composite film could effectively delay the decrease of vitamin C content in Auricularia auricula.
Compared with LDPE film, PLA film and PLA/REO/TiO2 film, the water permeability of PLA/REO/Ag composite film is significantly higher than other films. It can be concluded that it can effectively prevent the formation of condensed water and achieve the effect of inhibiting the growth of microorganisms; at the same time, it also has excellent antibacterial effect, can effectively inhibit the reproduction of microorganisms during the storage of golden ears, and can significantly extend the shelf life to 16 sky.
At the same time, compared with ordinary PE plastic wrap, PLA has better effect
PLA film packaging can extend the shelf life of broccoli at room temperature by 1 to 2 days, and the preservation effect is obvious.
Straw Disposable Tableware
The degradation products of PLA can assist plants to become the raw material of PLA through photosynthesis, so PLA is a "cradle to cradle" green material. At present, PLA is the raw material of mainstream disposable tableware on the market. PLA is relatively stable at room temperature, but it is easy to degrade rapidly in a slightly high temperature environment, acid-base environment or microbial environment, and has poor impact resistance, which often cannot meet the requirements of circulation in harsh environments.
The thermal stability of PLA material is comparable to that of PVC, but lower than that of PP, PE and PS. The processing temperature is generally controlled between 170 and 230 °C, and it is suitable for common processes such as injection, stretching, extrusion, and blow molding. However, in the actual processing process, the crystallization rate of PLA is slow, and generally needs to be modified. At the same time, due to the slow crystallization rate and low crystallinity, the heat distortion temperature of PLA is low, which limits its application in hot filling or heat sterilization product packaging.
In order to improve PLA crystallization rate and crystallinity, the optical purity of PLA can be increased as much as possible during production. The crystallinity of PLLA containing a small amount of PDLA will be greatly reduced. Annealing treatment is also a method to improve the crystallinity of PLA. In addition, a nucleating agent can be added to improve the crystallization behavior and increase its crystallinity, thereby increasing the heat distortion temperature and improving its heat resistance. Some studies have also obtained products with special properties such as high temperature resistance through the stereocomposition of PLLA and PDLA.
At present, blending with inorganic fillers or natural fibers is the main modification method to improve the performance of PLA materials in all aspects. For example, LiWH et al. used bamboo fiber to modify PLA, and the bending strength and elongation at break of the obtained composite material were increased by 19.3% and 30.1%, respectively. Shyif et al. prepared PLA banana fiber composites by melt blending method. By using coupling agent and chemical modification to combine banana fibers to PLA chains, the thermal stability and mechanical properties of the composites were significantly improved.
Medical material
PLA is widely used in sutures, bone fixation, repair materials, carriers for sustained and controlled release of drugs, tissue engineering cell growth scaffold materials and other fields, and has become one of the important biomaterials in the medical field. Sutures made of PLA can meet the requirements of sutures in terms of strength. As the wound heals, it is slowly decomposed and disappears in the body. Compared with conventional sutures, it does not need to be removed, and is especially suitable for suturing those deep tissue wounds.
PLA can be used as a bone fixation material. As the bone grows, the bone fixation material made of PLA slowly decomposes, and there is no need for secondary surgery to remove it, avoiding the physical pain and surgical expenses caused by the secondary surgery. PLA can be used as a carrier for slow- and controlled-release drugs. It has the advantages of low toxicity, high efficiency, sustained release, and long action time. It has good compatibility with organisms and a long residence time. It can also be copolymerized with other monomers. Adjust the release rate of the drug, so as to improve the activity of the drug and reduce the toxicity and side effects to the body.
PLA can be used as a scaffold material for tissue engineering. By culturing cells on the PLA scaffold, the cells slowly grow and differentiate into tissues and organs. At the same time, the PLA scaffold material slowly degrades. This technology has made progress in the research of skin and blood vessels. .
Summarizing the main applications of PLA in the packaging field mentioned above, we will find that PLA, as a new type of pure bio-based material, has great market application prospects. Its good physical properties and the environmental protection of the material itself will inevitably make PLA be used more widely in the future.
However, when PLA materials are used in packaging materials, there are still many properties that need to be further improved, such as degradability, toughness, heat resistance, barrier properties, and electrical conductivity. In addition, the current high raw material prices are also an important factor limiting the wide application of PLA materials. It is necessary to further optimize the production process of PLA to reduce material costs and improve its competitiveness as packaging materials.
