Polylactic Acid is highly favored because of its biodegradability, excellent performance and wide application. This paper introduces the degradation mechanism of PLA,including the current situation of its application as degradable plastic, as well as the improvement methods and the future development trend.
What is the Polylactic Acid?
A single lactic acid molecule has a hydroxyl group and a carboxyl group. When multiple lactic acid molecules come together, the -OH is dehydrated and condensed with the -COOH of other molecules, and the -COOH is dehydrated and condensed with the -OH of other molecules. And in this way, they work hand in hand to form a polymer called polylactic acid. Polylactic acid is also known as poly(propylene glycol) and belongs to the polyester family. PLA is obtained by polymerizing lactic acid as the main raw material, which is well-sourced and renewable. The production process of PLA is pollution-free . And the product is biodegradable and recyclable, making it an ideal green polymer material.
Degradation mechanism of PLA
PLA is a typical “green plastic” . And it is one of the most valued materials in the field of biodegradable materials due to its good biocompatibility, complete degradability and bio-absorbability. And the degradation mechanism of PLA is described below.
PLA is a synthetic aliphatic polyester. And its degradation can be divided into simple hydrolytic (acid-base catalyzed) degradation and enzyme-catalyzed hydrolytic degradation. From the physical point of view, there is homogeneous and non-homogeneous degradation. Non-homogeneous degradation means that the degradation reaction occurs on the polymer surface, while homogeneous degradation occurs inside the polymer. From the chemical point of view, there are three main ways of degradation.
Firstly, Main-chain degradation to oligomers and monomers.
Secondly, hydrolysis of side chains to produce soluble main chain polymers;,.
Thirdly , cross-chain point cleavage to generate soluble linear macromolecules. The ontogenetic erosion mechanism suggests that the main mode of PLA degradation is ontogenetic erosion, and the underlying cause is the hydrolysis of ester bonds in the PLA molecular chain. The end carboxyl groups of PLA polymers play a catalytic role in their hydrolysis. As degradation proceeds, the amount of end carboxyl groups increases, and the degradation rate accelerates, resulting in autocatalysis.
Because PLA comes from renewable resources, it is polymerized, modified and processed into products. When the products are discarded, they can be completely absorbed by the human body or degraded by environmental organisms into carbon dioxide and water, thus benefiting humans and returning to nature without pollution. The production process of PLA is pollution-free. And the products are biodegradable and recycled in nature, so it is an ideal green polymer material. The diagram below shows the recycling process of PLA in nature.
3、Polylactic acid preparation
Currently, the chemical synthesis of PLA includes two methods: propylene glycol ester ring-opening polymerization (also known as the two-step method) and direct lactic acid condensation (also known as the one-step method). Among them, the propyleneglyceride ring-opening polymerization method has simple equipment and can obtain large molecular weight PLA. The disadvantage is that the cost is higher, the whole process is complicated, and the route is long. The direct lactic acid condensation method has sufficient sources of raw lactic acid, cheap, high monomer conversion rate, simple process . And it does not need to go through the purification of intermediates. Thus the cost is lower; the defect is that it is more difficult to obtain high molecular weight polymer.
4、Application status
Packaging, fiber and medical are the popular areas of PLA market applications. In the packaging market, its consumption accounts for about 70% of the total PLA consumption. In the medium and long term, the proportion of fiber and textile will increase to 50%, becoming the largest consumer market for PLA. The amount of PLA in the medical field is relatively small, but the threshold is higher, and the profit is more profitable. In this report, we focus on the use of PLA in packaging materials.
4.1 Polylactic Acid Film
Polylactic Acid Film refers to raw materials from renewable substances, which have the functions and characteristics of traditional plastics, but can also be degraded and reduced in the natural environment by the action of microorganisms in soil and water, as well as through the action of ultraviolet light in sunlight. And they will eventually become carbon dioxide and water, and re-enter the ecological environment in a non-toxic form, so it is also known as “green plastic. Therefore, it is also called “green plastic”.
Currently, PLA, succinic acid copolymers and modified starch plastics are available in the market. Still, they have not been popularly used because of the high price (3-8 times higher than the general plastic). They are limited to when it is difficult to recycle or the recycling cost is too high, such as bone nails, surgical sutures, artificial organs, and slow-release pharmaceutical materials.
4.2 Polylactic acid packaging film
In April 2004, the U.S. college farm brand candy was packaged in a biodegradable resin polylactic acid (PLA) Natureworks natural material packaging film. This film’s appearance and performance and traditional candy packaging film (cellophane or biaxially oriented polypropylene film) are the same, with crystalline transparency, excellent twist retention, printability and strength, and a higher barrier, and can better retain the flavor of candy.
A German company using polylactic acid as raw material successfully developed a green food cup with a rapid natural decomposition function to solve the previous problem of disposable plastic packaging degradation, opening up a new way of practicality. The degradable material developed by the company belongs to the polyester polymer. The lactic acid can be extracted from sugar beet fermentation liquid and undergo a ring-opening polymerization reaction to produce polylactic acid.
In December 2002, the Japanese government introduced the “Bioengineering Strategy Outline” and other initiatives to replace fossil-based energy or products with biological materials to prevent global warming. Increased consumer awareness of environmental protection has also encouraged companies to use plastics made from plant materials fully.
4.3 Polylactic acid disposable tableware
While PLA production capacity and consumption are expanding, PLA applications are increasingly maturing. 2010 global PLA consumption of PLA bioplastic disposable lunch boxes (PLA) was about 120,000 tons (pure resin), with demand dominated by Western Europe and North America, and consumption in Asia is growing. Currently, the main consumption area of PLA is packaging materials, accounting for about 65% of the total consumption, followed by the biomedical field, accounting for about 26% of the total consumption.
At present , the PLA biodegradable material has been used to make all kinds disposable tableware, such as the biodegradable fork, knife , cold drink cup and so on .
5. Disadvantages of PLA
By the direct polycondensation method of lactic acid, higher molecular weight PLA cannot be synthesized. And there are some defects in PLA materials, such as poor mechanical properties, very low heat resistance temperature. Also it is expensive and difficult to control degradation time. All of these restricts the application range of PLA.
6、Improvement of Polylactic Acid
6.1 Physical modification
6.1.1 Blending modification
Blending modification refers to melt-blending two or more polymers together. Then to achieve the purpose of modification through the compounding of the properties of the polymer components. Polymer blend modification is a physical modification, not changing the polymer macromolecular chain structure, retaining the original polymer’s advantages while adding new substances. And changing the structure of the aggregation state, thus giving the polymer some new properties. Blending modification can improve polymer properties, reduce costs, and make inexpensive and versatile materials. Polymer blending is one way to improve the toughness of polymers.
6.1.2 Plasticizer Modification
By adding biocompatible plasticizers to the PLA matrix, it is clear from studying the changes in glass transition temperature, crystallization temperature, melting point, crystallinity, elastic modulus, and elongation at the break of PLA after plasticization that the addition of plasticizers increases the flexibility of PLA macromolecular chains, decreases the glass transition temperature very significantly, decreases its elastic modulus, and increases the elongation at break, i.e., the toughness increases to some extent. Comparing its deformation and elastic properties shows that these plasticizers can improve PLA’s flexibility and impact resistance.
6.2 Chemical modification
6.2.1 Copolymerization modification
The copolymerization modification of PLA changes the properties of PLA copolymer by adjusting the ratio of lactic acid and other copolymer monomers. Homopolymerized PLA is a hydrophobic material, and the degradation cycle is difficult to control. By copolymerizing with other monomers, the hydrophobicity and crystallinity of the material can be improved. And the degradation rate of the polymer can be controlled according to the molecular weight of the copolymer, the type and ratio of the copolymer monomers, etc.
6.2.2 Cross-linking modification
Cross-linking refers to the chemical reaction process between polymer macromolecule chains, resulting in the formation of chemical bonds. The general process of PLA cross-linking is to improve the properties of PLA by adding other monomers to cross-link with PLA to form a network polymer under the action of cross-linking agents or radiation. The crosslinking agent is usually a polyfunctional substance , such as polyfunctional anhydride or polyisocyanate. The cross-linking method and degree of cross-linking will vary depending on the situation.
6.3 Compound modification
Another way to achieve PLA modification is to compound PLA using compounding technology. After compounding, most PLA composites have excellent performance and special features with excellent biocompatibility, better mechanical strength, elastic modulus and thermoformability.
Conclusion.
Biodegradable plastic PLA is a green material with multifaceted application development and great development prospect. And its important position in today’s biodegradable materials is irreplaceable. As the most important biopolymer product, PLA has a broad development prospect and will be the focus of attention in the chemical field in the coming years. And it is expected to be competitive in industrial plant construction, application market and demand development, price and performance.
Sources:
