Polylactic acid, a biodegradable material in the new era, is the "key to the future" to green environmental protection

Produced by: Science Popularization China

Author: Ding Jianxun, Li Hongjie , Yang Jiazhen (Changchun Institute of Applied Chemistry, Chinese Academy of Sciences)

Producer: China Science Expo

Plastics have brought many conveniences to our lives. We use many plastic products in our daily life. However, many people have a common impression of plastics - they are not environmentally friendly. This is because commonly used petroleum-based plastics are difficult to degrade in the natural environment, and their pollution control is also a global problem.

In recent years, issues such as "white pollution", "microplastics" and "garbage continent" have received widespread attention. We are paying more and more attention to the burden that non-degradable plastics bring to nature. The restriction of non-degradable plastics by human society has become a mainstream trend. However, plastic products are everywhere in our daily lives. Therefore, degradable plastics have gradually become a new trend.

To this end, scientists have proposed a new concept of "from nature, back to nature" and developed a synthetic route to convert corn and other biomass into biodegradable polylactic acid (PLA) plastics, providing a feasible solution to plastic pollution control through the magic of chemistry. This plastic, which is converted from plant starch, has abandoned environmentally unfriendly petrochemical raw materials in its preparation process, has excellent biodegradability, and is an environmentally friendly plastic.

The popularization of PLA can effectively reduce the use of non-degradable plastics and has great significance for the treatment of plastic pollution. So, how is PLA transformed step by step from biomass such as corn, and how does it replace traditional petroleum-based plastics that have prospered for a century in daily life, biomedical research and development, agricultural production, textiles, engineering manufacturing, etc.? Let's find out!

PLA recycling process and its application

(Image source: self-made, some illustrations cited from references 26-29)

The magic of turning crops into plastic

PLA is an aliphatic polyester made from lactic acid (LA) through oligomerization, cyclization, polymerization and other processes. The conversion process of PLA is as follows: chemists can efficiently convert starch extracted from crops such as corn into LA through hydrolysis and microbial fermentation, and further convert it into PLA through condensation polymerization or ring-opening polymerization, thus achieving the "magic" of turning crops into plastics.

Since LA monomers contain both hydroxyl (-OH) and carboxyl (-COOH), they can react with the carboxyl and hydroxyl groups of another LA monomer. In this way, LA monomers react alternately to obtain high molecular weight PLA materials.

So why didn’t such an environmentally friendly material get widespread use in the beginning? In fact, the process is not as simple as it sounds. There are many chemical and engineering problems and bottlenecks in the preparation process. For example, the water molecules (H2O) generated during the condensation reaction cannot be discharged in time, which will greatly inhibit the reaction process; in addition, the harsh reaction conditions also reduce the potential of this solution for industrial production.

However, the method for preparing PLA has been improved, making the whole process more controllable. The current mainstream route for industrial production of PLA, the lactide ring-opening polymerization method, can achieve the controllable synthesis of PLA molecules.

However, in order to obtain PLA plastics that meet daily needs, a specific molecular weight alone is far from enough. Advances in processing technology and modification methods are also important. Next, we will introduce the characteristics of PLA and how to transform PLA into various common items in life.

PLA synthesis route and chemical structures of LA, lactide and PLA. A. Synthesis of PLA by direct polycondensation; B. Synthesis of PLA by lactide ring-opening polymerization; C. Stereoisomers of LA, lactide and PLA

(Photo source: Homemade)

Properties and Modification of PLA

When it comes to the properties of PLA, one thing needs to be pointed out: LA contains a chiral carbon atom and can be divided into L-lactic acid (L-LA) and D-lactic acid (D-LA). The former has the same structure as the LA metabolized by the human body, but large intake of D-LA will have toxic side effects.

Therefore, commercial PLA is generally L-LA synthesized left-handed PLA (PLLA). In addition, there is also right-handed PLA (PDLA) synthesized from D-LA, and poly (D, L-LA) (PDLLA) synthesized from the copolymerization of L-LA and D-LA.

From the chemical structure of PLA, it can be inferred that the large number of ester bonds in the PLA main chain is the key to its good degradation performance. Under composting conditions (high temperature, high humidity and microbial action), PLA can be fully degraded into H2O and carbon dioxide (CO2) in just a few months, and then recycled through plant photosynthesis.

In addition, PLA can be degraded in the human body, and its hydrolysis product LA can be absorbed and utilized by the human body. This excellent biocompatibility makes it show extremely high application value in the field of biomedicine.

Schematic diagram of the main degradation pathways of PLA: PLA photooxidation and hydrolysis (left), biodegradation of PLA by enzymes produced by microorganisms (right)

(Photo source: Homemade)

In addition to its excellent eco-friendliness, biodegradability and biocompatibility, PLA also has physical properties comparable to those of traditional petroleum-based plastics.

Note: PET is polyethylene terephthalate, PS is polystyrene, HIPS is high impact styrene, PP is styrene

(Data source: Reference 10)

Due to its excellent thermoforming properties, PLA can be processed into various plastic products through extrusion molding, vacuum forming, laminating, blow molding, injection molding, fiber spinning, foaming, etc., and has a wide range of applicability.

(Data source: Bengbu Bio-based Materials Industry Development Plan)

However, PLA also exposes some shortcomings in its applications, such as brittle fracture, uncontrollable degradation cycle, insufficient biocompatibility, etc.

Scientists are also actively looking for solutions to these problems of PLA. For example, through blending, copolymerization, nanocomposite, stereocomposite and other means, the toughness, controllable degradation cycle, hydrophilicity, antibacterial properties of PLA have been further improved, greatly broadening the application range of PLA.

Note: PBS is polybutylene succinate, PBAT is polybutylene adipate, PHDA is poly (hexamethylene-decamethylene adipate)

Application and Development of PLA

PLA is actually not unfamiliar and you may encounter it in every aspect of your life.

From the perspective of eating, the commercialization of PLA products has made great strides forward in recent years, driven by the rapid development of the takeaway, express delivery and catering industries and the "plastic ban" and other policies issued by the state. For example, when we drink milk tea every day, in addition to paper straws, PLA straws are also more commonly used.

From the perspective of wearing, PLA clothing made by blending PLA and antibacterial agents is favored by consumers.

From the perspective of daily use, PLA that has been plasticized and toughened is used in daily necessities shells and children's toys, which can effectively prevent children from ingesting harmful plastic particles.

From the perspective of medical use, by improving the hydrophilicity and biocompatibility of PLA and strengthening the control of PLA degradation time, biomedical polymers such as PLA vascular stents, absorbable materials, and surgical sutures are gradually being put into use.

In addition, materials such as PLA mulch and sand barriers have a broad market, are of great significance to environmental governance, and have a very large application market.

Statistics of China’s polylactic acid consumption from 2017 to 2021

(Data source: Guanyan Report Network)

At present, the domestic PLA production capacity is about 250,000 tons/year, but the demand for PLA has been on the rise in recent years. At present, the total capacity of PLA production lines under construction or planned by various companies has exceeded 1.5 million tons/year. It is expected that PLA will be more widely popularized in the next 3-5 years.

Although the domestic PLA industry has developed rapidly, it still faces two major challenges. First, the key technologies of the industry need to be improved urgently, and second, the production cost of PLA needs to be further reduced. At present, the production cost of PLA is 3 times or even more than that of commonly used petroleum-based plastics, making it difficult to replace traditional plastics. In addition to reducing costs in production technology, it is also necessary to strengthen the layout of the PLA industry and accelerate the construction of PLA production lines to reduce its cost to a "cheap price" comparable to that of traditional plastics.

Conclusion

Currently, PLA has gained a place in daily life, biomedicine research and development, agricultural production, textiles, engineering manufacturing, etc. due to its excellent biodegradability, biocompatibility and wide applicability. However, compared with petroleum-based plastics that have been developed for a hundred years, PLA's higher production cost and complex production process are still the main reasons hindering its widespread popularity.

Today, driven by the implementation of the "dual carbon" strategy and the ban on plastics, PLA has ushered in new market development opportunities. This plastic produced from grains realizes the environmental protection concept of "originating from nature and returning to nature", and points the way to solving the problem of plastic pollution. Perhaps one day, we will be able to fully popularize environmentally friendly and degradable plastics, so that we can enjoy the convenience brought by plastic products and achieve harmonious coexistence with nature.

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Editor: Guo Yaxin