Poly Lactic Acid (PLA) is a new biodegradable material called polylactide. Polylactic acid (PLA) is produced by fermentation and polymerized into Biodegradable plastic with good biodegradability and Biocompatibility. The raw materials of PLA are mainly derived from sugar from renewable plant resources such as corn, cassava and sugarcane
PLA has good application prospects in the field of packaging, but it is not perfect in toughness, heat resistance, antibacterial and barrier properties, the Institute of biodegradable materials reported. When applied to transport packaging, antibacterial packaging and intelligent packaging with high requirements for these properties, further improvement is needed. How about the application of PLA in the field of packaging? What are the advantages and limitations?
The modification of PLA, such as copolymerization, blending and plasticizing, can correct these shortcomings of PLA, and keep the advantages of PLA, such as transparency and degradability, it can further improve the properties of PLA, such as controllable degradation, toughness, heat resistance, barrier and conductivity, and reduce the production cost, so that PLA can be widely used in packaging collar.
Degradation performance
PLA itself is stable at room temperature, but it is easy to degrade rapidly in slightly high temperature, acid-base environment or microbial environment. The factors affecting the degradation of PLA are the molecular weight, crystalline state, microstructure, environmental temperature and humidity, PH value, illumination time and environmental microorganism.
When applied to packaging, the degradation cycle of PLA is not easy to control. For example, because of its degradability, PLA containers are mostly used in food packaging on short-term shelf. Therefore, it is necessary to control the degradation rate of PLA by doping or blending other materials according to the circulation environment and shelf life of the products, so as to ensure that the packaged products can be safely protected during the shelf life, the waste can be degraded in time.
It should be noted that in order to improve the properties of PLA and composites, blending or doping non-degradable fillers may affect the degradation of the final packaging materials.
Wan et al mixed PLA with wood flour (WF) and used poly(methyl methacrylate)(PMMA) to improve the compatibility, and found that the degradation rate of PLA WF PMMA composites was much higher than that of pure PLA.
Barrier performance
Barrier, that is, the ability to block gas and water vapor through, also known as moisture and gas resistance performance, barrier to food packaging is particularly important. For example, vacuum packaging, inflatable packaging, modified atmosphere packaging materials are required to be as good as possible barrier; Moisture-proof packaging requires materials with good moisture resistance; rust-proof packaging requires materials with air and moisture resistance.
Compared with high barrier nylon and acrylate, PLA has poor oxygen and water vapor barrier property, and is not protective enough for fatty foods when used in packaging.
The factors that affect the barrier property of PLA mainly include self factors (molecular structure and crystalline state) and external factors (temperature, humidity, external force) .
The barrier properties of PLA films are affected by heating or stretching treatment. Zhou gang studied the effect of cooking sterilization (3 minutes) and microwave treatment (3,6 minutes) on the barrier properties of PLA film, it may even cause small surface cracks, so pure PLA is not suitable as a food packaging material to be heated. It was found that the crystallinity of Plla increased from 0.2% to 41.8% and the crystallization rate increased when the tensile ratio increased from 1 to 6.5, the barrier property was improved to some extent.
The barrier properties of PLA can also be improved by adding some barrier materials (usually nano filling particles, such as clay and fiber) to the PLA matrix, this is because the barrier extends the curved path of small molecules through water or gas. The PLA/CNF (cellulose nanofiber) , PLA/CNC (nanocrystalline cellulose) and PLA/OMMT (organic modified montmorillonite) nanocomposites were prepared by Trifol et al. , moreover, the water vapor permeability is greatly reduced, so adding nano-fillers into PLA matrix is an effective method to improve the barrier property of PLA.
Heat resistance
With the promotion of public awareness of environmental protection and the guidance of national policy, the traditional polypropylene straw has been gradually eliminated and replaced by the biodegradable straw represented by PLA.
The traditional straw in the high temperature performance, has withstood the test. Will it stand the test at high temperatures as a substitute?
The poor heat resistance of PLA is due to its slow crystallization rate and low crystallinity. The thermal deformation temperature of amorphous PLA is only about 55 °C. The heat resistance of the unmodified polylactic acid pipette is poor. Therefore, PLA straw is suitable for warm and cold drinks with tolerance temperature ranging from -10 ° C to 50 ° C.
However, in practical use, milk tea drink straw, coffee stirrer, etc. , need to meet the heat resistance of more than 80 °C. This needs to be modified on the basis of the original, from the physical and chemical modification of two aspects to change the performance of PLA. The technology of multi-component composite, chain expansion and inorganic filling can be used to change the problem of poor heat resistance of PLA and break down the technical barrier of PLA pipette.
The results show that the branched chain length of PLA can be controlled by changing the feed ratio of PLA and nucleating agent. The longer the branched chain, the larger the molecular weight, the larger the TG, the stronger the rigidity and the higher the thermal stability of PLA, therefore, the heat resistance of PLA is improved and the thermal degradation of PLA is inhibited.
Post time: Jul-28-2023