Previously, the globally renowned consulting firm IHS Markit analyzed that actions initiated by Western European countries such as "restricting or stopping the use of plastic bags and other disposable plastic products" will further drive the market demand for biodegradable plastics. In 2018, the global market value of biodegradable plastics exceeded 1.1 billion US dollars, with a demand of 360000 tons and an average annual growth rate of 9%. It is expected to reach 1.7 billion US dollars by 2023, with a demand of 550000 tons. PLA will play an important role, and the food packaging and other fields will develop the fastest. In 2018, PLA film accounted for over 25% of the biopolymer film market share.
Food packaging, disposable tableware (including cups, plates, knives, forks, etc.), and plastic bags are the largest user markets for biodegradable plastics. Coupled with the current popularity of e-commerce express delivery, these are the main sources of "white pollution" in China. In a previous article, the editor pointed out that biodegradable plastics may not be able to bear the responsibility of solving "white pollution", but they are currently a good choice.
The application of PLA in the market is mainly concentrated in three major fields: packaging, medical, and textile (fiber), among which its application in the packaging industry is particularly prominent. Let's take a look at the application of modified PLA in the packaging industry.
Introduction to the Application of PLA in the Packaging Industry
The purpose of packaging is to protect the product, facilitate storage and transportation, and promote sales. The specific performance and basic requirements are as follows.
Compared with traditional PE and PP materials, PLA has a higher melting point (about 180 ℃) and better processability. The hardness and mechanical properties of PLA are similar to traditional resins such as PS, with good transparency and excellent glossiness, making it suitable for use as a packaging material. But its disadvantages are also quite prominent. Compared with commonly used high barrier nylon, polyvinylidene chloride (PVDC), etc., PLA has insufficient oxygen and water vapor barrier properties, and insufficient protection for oily foods. In addition, PLA has poor toughness, low Tg, and poor impact resistance, which means that PLA is temporarily not suitable for packaging fields with high durability. In addition, the degradation cycle of PLA is difficult to control, and it is prone to varying degrees of degradation or hydrolysis during use, resulting in a decrease in its performance. The material cost is more than 2.5 times that of traditional PE and PP, which cannot be ignored.
Main performance parameters of PLA
The application of PLA in the packaging industry mainly benchmarks modified PP, PE, and PET, while cushioning materials benchmark EPS, EPE, and EPU. Given the advantages of PLA, it can be used as a packaging material to make films, foam materials, and containers, such as flexible packaging bags, plastic bottles, cushioning materials, disposable tableware, etc. From a morphological perspective, the main material is film, followed by containers and sheets. The degradability, barrier properties, mechanical properties, optical properties, antibacterial properties, conductivity, and piezoelectricity of PLA are mostly related to crystallinity. If the crystallinity of PLA is increased, its barrier properties and mechanical properties will generally improve, but its processability, degradability, and transparency will usually decrease. So when using modified PLA as a packaging material, the key is to control its crystallization behavior and achieve optimal performance in all aspects.
(1) PLA film
PLA cast film (left) and biaxially oriented film (right)
PLA film mainly includes high transparency film, heat shrink film, pellet blend blown film bag, and PLA based composite material film. PLA film can replace traditional film materials as new packaging materials. For example, Wal Mart Supermarket requires fully biodegradable PLA film bags to replace traditional non degradable PE bags, and Japanese electronic products also begin to use PLA nanocomposite film packaging.
(2) PLA container
PLA bottles (disposable) and water cups
Due to the degradability of PLA, it is used in the production of water and juice bottles, but the market is limited. Injection stretch blow molding is a typical production method for PLA bottles, which are mostly used in food packaging on short-term shelves. The transparency and oxygen barrier properties of PLA bottles are better than those of PE bottles. Due to the insufficient creep behavior of PLA and its low barrier to CO2, the application of PLA bottles is limited to non carbonated beverages. NatureWorks uses PLA bottles to package organic fruit juice beverages, ensuring a shelf life of 60 days. Danish company Faerch Plastics uses PLA packaging for low-temperature fresh foods such as pasta, meat, salads, etc., which can achieve a long shelf life. The European market has launched yogurt cups and beverage packaging bottles made of PLA material, which can meet the standards of EU related food containers. A high transparency and heat-resistant PLA water bottle that has been developed by a company can meet the application conditions of being both transparent and heat-resistant (heat-resistant temperature ≥ 100 ℃). In the domestic market, PLA is coated on paper to make coated paper cups, which are used for various waterproof and oil resistant food packaging.
Application of PLA by NatureWorks in the field of food and beverage packaging (official website)
(3) PLA foam material
PLA foam tableware
PLA foam material has both biodegradability and can be used to produce disposable tableware (such as fast food boxes, instant noodle bowls, etc.) and cushioning packaging liners, without producing white pollution, especially PLA heat-resistant tableware will become more practical. The "Gift" series disposable tableware is designed with PLA foam material to create a realistic fruit and vegetable appearance, combining environmental protection and aesthetics. It is worth noting that foaming is an effective method for low-cost use of PLA. Pure PLA melt has low strength and its viscosity is not significantly affected by shear, making it unsuitable for processing methods such as extrusion coating and foaming. However, its melt strength can be improved by changing the molecular chain topology, such as long-chain branching.
Degradation
PLA is relatively stable at room temperature, but it rapidly degrades in high-temperature, acid-base, or microbial environments (involving microbial degradation, hydrolysis, photolysis, radiation degradation, and thermal degradation, etc.), leading to a rapid decline in performance and ultimately becoming CO2 and water. The factors that affect the degradation of PLA include resin molecular weight, crystalline state, microstructure, environmental temperature and humidity, pH value, oxygen, light exposure time, and environmental microorganisms. The degradation kinetics of PLA in natural environments is very important for its use and disposal, but there is currently limited research on this topic. The degradation of PLA is very complex and can be roughly divided into two stages: the early stage is physical degradation, which relies on the temperature and humidity (water) in the environment to break the ester chain and break it down into oligomers or monomers, and the later stage is converted into CO2 and water through microbial and enzyme action. Generally, it starts from the amorphous region and extends to the crystalline region.
The biological composting degradation of PLA generally utilizes the biodegradability of PLA itself, which can be achieved through PLA film, and the weight loss rate before and after film degradation is used as the evaluation criterion for degradation performance. Due to the long degradation time and low degradation rate of PLA in biological composting, effective microorganisms in compost and sludge are often used for its degradation. The following table summarizes the PLA degrading strains discovered in recent years and examines their degradation rates.
Partially reported PLA degrading microorganisms
If other materials are mixed or blended into PLA to control its degradation rate based on factors such as the product's circulation environment and shelf life, ensuring that the product can be safely protected within its shelf life and can be degraded in a timely manner after disposal. It should be noted that blending or doping non degradable fillers into modified PLA will affect the degradability of the terminal material.
barrier property
Barrier property is the ability to block the passage of gases and water vapor, which is particularly important for food packaging. For example, vacuum packaging, inflatable packaging, and modified atmosphere packaging require materials with good barrier properties as much as possible, while spontaneous modified atmosphere preservation of fresh fruits and vegetables requires materials with different breathability to gases such as oxygen and carbon dioxide; Moisture resistant packaging requires materials with good moisture resistance; Rust proof packaging requires materials that can block air and moisture. The factors that affect the barrier properties of PLA mainly include intrinsic factors (molecular structure and crystalline state) and external factors (temperature, humidity, external forces).
Heating or stretching treatment of PLA film can affect its barrier properties. For example, studying the effect of different stretching ratios on the barrier properties of PLLA film, it was found that when the stretching ratio increased from 1 to 6.5, the crystallinity of PLLA increased from 0.2% to 41.8%, and the crystallization rate also increased, resulting in a certain improvement in barrier properties. Adding some barriers (usually nano filled particles such as clay and fibers) to the PLA matrix can also improve the barrier properties of PLA. The principle is to prolong the curved path of water or gas permeation of small molecules. Three types of nanocomposites, namely PLA/CNF (cellulose nanofibers), PLA/CNC (nanocrystalline cellulose), and PLA/OMMT (organic modified montmorillonite), were prepared separately. The results showed that these composites had better barrier properties than pure PLA and significantly reduced water vapor transmission rate. If a barrier coating is composed of graphene oxide (FGO) treated with silane coupling agent and waterborne polyurethane acrylic acid (WPUPA) photopolymerization material, the surface coating barrier coating method can also significantly improve the oxygen barrier performance of PLA film.
mechanical property
PLA lacks flexibility and elasticity, and is prone to bending and deformation. When applied in packaging, it needs to be toughened and modified. The specific modification methods are described in the previous text. Many studies have shown that PLA/non degradable resin blends can significantly improve their impact resistance, but the modified products do not degrade completely and can also reduce the transparency of PLA. Considering safety, such products are not suitable for direct use in food packaging. PLA used for food packaging is usually blended with biodegradable resins for toughening. Commonly used flexible polymers include polyethylene glycol (PEG), polyglycolic acid (PGA), polycaprolactone (PCL), polybutylene succinate (PBS), and polybutylene adipate/terephthalate (PBAT). In addition, organic small or large molecules such as triacetin, tributyl citrate, gluconate, glycerol, various vegetable oils, etc. can increase the toughness of PLA, but some may reduce the mechanical properties such as Tg and elastic modulus.
PLA plastic bag
optical performance
PLA has rare transparency and glossiness compared to other biodegradable plastics, comparable to glass paper (regenerated cellulose film) and PET, and is suitable for use as visual packaging with good decorative effects. The transparency and glossiness of PLA generally do not need to be improved. It should be noted that when modifying in other aspects, the good transparency should not be reduced as much as possible. In 2004, College Farm brand candies in the United States used transparent PLA film, which precisely utilizes its low toughness to achieve stability at the candy twist end. The gloss effect is similar to traditional candy glass paper packaging film, higher than BOPP, but the barrier properties are better than traditional packaging. In the case of similar effects, environmental protection materials are more favored by customers.
Thermal performance
The thermal stability of PLA material is comparable to PVC, but not as good as PP, PE, and PS. The processing temperature is generally controlled between 170~230 ℃, and it is suitable for ordinary processes such as injection, stretching, extrusion, blow molding, and 3D printing. In the actual processing, the crystallization rate of PLA is slow, resulting in a lower thermal deformation temperature of PLA, which limits its application in hot filling or hot sterilization product packaging. Zhejiang Haizheng Biomaterials Co., Ltd. and Changchun Institute of Applied Chemistry, Chinese Academy of Sciences have been committed to the development of heat-resistant PLA and have achieved certain results. The heat-resistant modification method of PLA has been detailed in the previous text and will not be shared here.
antibacterial activity
Antibacterial packaging is a comprehensive method that combines packaging materials and preservatives to improve food shelf life and safety, and cleverly combines various factors that restrict food preservation. The effectiveness of antibacterial packaging is influenced by factors such as antibacterial agents, microorganisms, and the environment. In addition, the antibacterial properties of the material itself are also crucial. PLA can create a weakly acidic environment on the surface of the product, with antibacterial and anti mold effects. If other antibacterial agents are used in combination, the antibacterial rate can reach over 90%. Commonly used inorganic antibacterial agents mainly include silver, copper, zinc and other metal ions or oxides. Commonly used organic antibacterial agents for packaging include vanillin, clove essential oil, cinnamon essential oil, fennel essential oil, lemongrass essential oil and other natural plant antibacterial essential oils or ethyl vanillin compounds. Food safety of other antibacterial agents needs to be studied. Generally, organic antibacterial agents have poor heat resistance and short validity period. Smooth and dense PLA/nano ZnO composite films were prepared using solution casting method. The composite films have UV blocking properties without reducing transparency. After adding 0.5% mass fraction of ZnO, the tensile strength of ZnO/PLA films increased by 27.5% compared to PLA films, and the water vapor transmission rate decreased by 30.5%. It has inhibitory effect on foodborne pathogenic bacteria such as Escherichia coli and Listeria monocytogenes.
Electrical performance
Conductive polymer composite materials (CPCS) are widely used in anti-static plastics, electromagnetic shielding materials, temperature controlled heating materials, environmental sensitive devices, and other fields. In the field of functional packaging, CPCS based on PLA has both degradability and biocompatibility, and can be used for anti-static packaging, electromagnetic shielding packaging, and intelligent packaging. At present, the conductivity of PLA is mostly obtained by adding conductive particles such as carbon black (CB), carbon nanotubes (CNTs), carbon fibers (CFs), or graphene. The synergistic modification of PLA using carbon black and carbon nanotubes as fillers, and the foaming of the PLA/CB/CNTs three-phase system with supercritical CO2, resulted in better conductivity than CB and CNTs particles. In addition, some studies have combined PLA with some highly conductive polymers to obtain PLA based conductive materials. Japanese scholars have developed flexible piezoelectric films with high sensitivity and large-area response using PLLA and PDLA laminates, and successfully rolled PLA piezoelectric laminates into roller shaped piezoelectric rollers, which can be used to make sensors or transducers.
Overall, due to cost and some performance limitations, PLA has not yet been widely applied in the packaging industry. The next step is to further improve the degradation, toughness, heat resistance, barrier properties, conductivity, etc. of PLA, optimize the production process of PLA, and reduce material costs. It is worth noting that general modification methods do not individually improve a certain performance, and there is a certain correlation between various performances. The demand is based on the usage requirements, pursuing a balance of various performances. At present, the application of PLA in the packaging industry is mainly in short-term use areas such as disposable plastic bags, tableware, and cushioning materials. With more balanced indicators such as heat resistance, barrier properties, and transparency, it will gradually expand to high heat resistance tableware, visible packaging films for food and beverage, and containers in recent years. It is believed that related products will soon be available in the market.
