PLA is the acronym of Poly Lactic Acid, and its scientific name is Poly Lactide. First found in yogurt (or sour milk) by Sdude in the U.S.A. in the year 1850, lactic acid, also named α-hydroxypropionic acid, with molecular formula CH3CHOHCOOH, exists in yogurt and blood, or be produced by muscle exercise. Lactic acid has three types of structure: two optical isomers–labeled D and L; one structure without optical activity–labeled DL, namely racemic structure.
Due to the progress of human civilizations and enormous use of petrochemical plastics, quantities of plastics that hardly degrade without incurring pollution are discarded in the nature, resulting in one of the most severe environmental pollution after industrialization in human history. Through the past 50 years, there has been a stunning increasing in the packaging application of petroleum plastic and various types of high polymer. At present, the plastics consumption in the whole world has exceeded 160 million tons, covering all kinds of plastic-related materials valued at 150 billion USD, 35% among which was used in packaging materials, and the treatment of these packaging wastes has caused a huge impact on our environment. Therefore, recycling mechanism for plastic materials and the study on dissolvable plastic have become more and more.
PLA is not a new plastic material. As early as the year 1932, Wallace Caruthers in DuPont Company in the United States was already able to polymerize lactic acid in vacuum to produce polymer with low molecular weight. However, because production cost was too high, no progress was made until Cargill Company invested on the research of new PLA production process in 1987, and then in 2001, Cargill and Dow Chemical Company established by joint venture Nature-Works LLC Company that commercially and massively produce PLA products named “ECODEAR”.
Raw material to produce PLA is organic acid lactic acid. Because some features of this material are superior to present petrochemical plastics: polyethylene, polypropylene and polystyrene, so PLA is taken as the most promising new packaging material in the new century and it is a superstar of packaging materials related to environmental protection. In the future, it is expectable that PLA can replace polyethylene, polypropylene and polystyrene to be widely used in plastics. Besides, PLA has a vast development feature since its raw material is taken mainly from plants, instead of limited petroleum resources. Another fact worth pointing out is that PLA can be degraded by microorganisms in the soil into carbon dioxide (CO2) and water without making any pollution at all. Within ten years in the future, PLA will gradually replace plastic with similar natures like PET and PS, and it will become the most promising polymer raw material.
PLA is a biodegradable and compostable material. Glucose molecules in plant fibers or starch ferment and become lactic acid which further polymerizes to PLA. Corn and wheat are the main resource of plant fibers. PLA is completely produced by starch isolated from plant, and then go through fermentation, dewatering and polymerization processes. PLA in the early stage was researched for medical applications, e.g.: surgical stitches and bone nails. PLA is a new kind of degradable high polymer with multiple usages; it is produced by 100% regeneratable resources: plant starch ingredient from corn, beets or rice.
Under natural environment, PLA can be completely degraded into CO2 and water in several months. In preliminary tests, tray made by PLA is inferior to Styrofoam in thermal insulating and buffer effects; its strength is not enough and can be scraped easily. PLA polymer has been included in the index of food contact materials announced by FDA. Under 65℃, this material can be used to contain food. But Taiwan MOH did not set a food safety standard for PLA.
PLA’s merits are indicated in the following aspects:
- Excellent biodegradability. After using, PLA can be completely degraded by microorganisms in natural environment and is finally degraded into CO2 and water. It will not pollute the environment and is ideal for environmental protection.
- Excellent processability and physical properties. PLA is suitable for various processing methods, such as blowing molding and thermoplastic technique. It can be easily processed and widely used. PLA processing ranges from plastics, food packaging, fast food container, nonwoven fabric and cloth for industrial and civil purposes to further processed fabrics for agricultural and health care purposes, wipers, sanitary products, outdoor anti-UV fabrics, tent cloth, and floor mat side, etc. It is a promising product in future market.
- Excellent products compatibility. PLA is also widely used in medical and pharmaceutical field. For example, it can be used to produce one-off transfusion instrument, nontakenout surgical stitches, etc., and low molecular weight PLA can be used to produce sustained-release drug delivery packs.
Lactic acid is the raw material to produce PLA. Traditional lactic acid fermentation used starch as raw material. Presently, United States, France and Japan are developing methods to utilize agricultural and sideline products as fermentation raw materials to produce lactic acid and then further produce PLA. The PLA-producing technique of LLC Company in United States is: corn starch becomes glucose through hydrolysis, and then introduce lactic acid bacillus to conduct anaerobic fermentation; during fermentation process, liquid alkaline is used for neutralization to produce lactic acid. After being purified, the fermentation liquid is used to produce L-lactic acid with 99.5％ purity by electrodialysis.
There have been two processes in the synthesis of PLA out of lactic acid:
- direct polycondensation—a process of using solvent in vacuum to directly dehydrate polycondense the lactide.
- nonsolvent method— a process of firstly synthesizing become cyclic dimer PLA from lactic acid and then ring-opening polymerizing to make PLA. PLA molecular polymerization reaction is showed as follows:
One research institute in the United States successfully converse the waste whey liquid left by cheese-producing process to glucose syrup, and then use germs to ferment the glucose syrup to produce lactic acid contained fermentation broth. Through isolation using electrodialysis and vaporizing waters by heating, PLA for making film and coating is produced and it can be used to produce fresh-keeping bags and packaging materials free of polyethylene and waterproofing wax.
Erstein’s Sugar Refinery in France cooperated with one university to invent another process to prepare PLA. They use beets as raw materials, firstly degrade into monosaccharide and then ferment into lactic acid, then use chemical method to polymerize lactic acid into PLA. PLA can also be produced by waste sugar liquid of industrial sugar-refining process and thus greatly curtail producing cost.
Kinebo Company in Japan uses corn as raw materials to ferment into PLA, which is further used to produce biodegradable foaming material. This is a process of mixing a special additive into PLA so as to control its molecular structure and make easily foaming particles. And then they add organic compound foam agent made by carbonhydrate; the materials are molded in molder and then heated by high-pressure water vapor to make foaming materials. Such materials are the same as polystyrene plastics in terms of strength compressive stress, cushioning, and drug tolerance. They do not pollution the environment when incinerated, and they can be used as fertilizer.
PLA produced by a commercial mode uses crop containing enormous amount of starch, e.g.: corn, as raw materials. Reaped corn is sent to corn factory and starch are isolated from corn endosperm and hydrolyzed by liquid enzyme into glucose. Glucose ferments to lactic acid in a neutral environment. Under mild conditions and nonsolvent, lactic acid is dewatered to produce low molecular weight prepolymer. Through catalytic depolymerization, this prepolymer forms cyclic intermediate dimer, called dewatered lactic acid. The purity of dewatered lactic acid can be controlled to produce molecular weight with a wider range. By distillation purification, dewatered lactic acid can be purified to polymer level. After nonsolvent ring-opening polymerization, purified dewatered lactic acid can be further processed into plastic granules to be used as PLA raw materi