Biologically degradable plastic is being used to an increasing extent in the manufacturing of packaging and disposable products. Polylactic acid is a major component of this type of sustainable material. In order to meet the growing demand, scientists have developed a process with the aim of obtaining lactic acid from a waste/by-product of biodiesel manufacturing.
Poly(lactic acid) or polylactide (PLA) is a biodegradable thermoplastic aliphatic polyester derived from renewable resources, such as corn starch (in the United States), tapioca roots, chips or starch (mostly in Asia), or sugarcane (in the rest of the world).
Strong demand for polylactic acid
Plastic waste is one of the biggest environment problems of our age. A look at the environment reveals that most plastics are not degradable at all or only degrade to a limited extent. They merely disintegrate into smaller and smaller pieces, that not only pollute the environment but also have an impact on human beings. In addition to this, most plastics are made from oil, a raw material with reserves that are being depleted, even if the current slump in the oil price suggests otherwise. Alternatives, i.e. sustainable polymers and/or raw materials for biodegradable plastics, have already been introduced or are being discussed, however. They include such agricultural products as sunflower seeds, but also meadow grass or crab shells.
Polylactic acid (PLA) is one of the materials that are currently being used to an increasing extent to manufacture bioplastics. This polymer, which is based on lactic acid, is biologically degradable and the raw material for it is renewable. Many disposable cups, plastic sacks and packaging materials are already being made from PLA nowadays.
Strong demand for bioplastic
Demand for bioplastic is rising steadily. Experts expect it to increase by one megatonne per year up to 2020. How and with the help of which sources the growing demand is to be covered has not evidently been settled adequately yet. Or perhaps it has. Scientists from the Institute for Chemical and Bioengineering Sciences at the Swiss Federal Institute of Technology (ETH) in Zurich recently presented a process for the production of lactic acid which is said to satisfy the exacting sustainability requirements that will be made in future .
The main features of their process: ETH reports that it is more productive, cost-efficient and climate-friendly than the fermentation process by which lactic acid is normally obtained. Another particularly interesting aspect is that the new process is based on what has classically been a waste product – glycerol – large quantities of which are obtained as a by-product of biodiesel manufacturing.
Crude biodiesel contains substantial amounts of glycerol (dark phase). (Photo: Bo Cheng / ETH Zurich)
Glycerol has potential
It is estimated that the amount of glycerol produced in biofuel manufacturing will increase from three megatonnes at the present time to more than four megatonnes in 2020. The glycerol obtained in biodiesel production is not really clean, however; it contains traces of ash and methanol, which explains why it is not suitable for the production of food, cosmetics or pharmaceuticals.
But what can the glycerol from biodiesel production be used for, if it is unsuitable for food or pharmaceutical applications and is not a viable alternative fuel in energy generation either due to its poor combustion properties?
“Glycerol from biodiesel manufacturing should normally be treated and processed like (industrial) waste water. To save money and because it is not very toxic, some companies discharge it into rivers, however”, says Merten Morales, a doctoral student at the ETH and lead author of the study on which this article is based. A closer look at the issue shows, however, that simple disposal in the environment like this is a waste of a valuable resource.
Focus on catalysis technology
The fact that the method chosen by the ETH research scientists is based on what has up to now been a waste product is one of its outstanding advantages. Process engineering skill is, however, needed in the final analysis, in order to create a valuable material from it. Catalysis plays a central role in the transformation of glycerol into polylactic acid. Let us take a look at a few details: there are two basic stages in the process. First of all, enzymes, i.e. biocatalysts, turn glycerol into an intermediate product (dihydroxyacetone). After this, secondly, a heterogeneous catalyst drives the further reaction which leads to the production of lactic acid.
Catalysis (from the Ancient Greek word κατάλυσις katálysis "dissolution") is the word used to describe the change in speed (kinetics) of a chemical reaction by means of a catalyst, generally with the aim of initiating it, accelerating it or directing selectivity in a desired direction. In a living cell, enzymes – which catalyse biochemical processes – play a fundamental role in metabolism, from digestion to reproduction and transcription of genetic information. Both natural catalytic processes like smog formation and the catalytic reduction of pollutants in automotive and power plant applications are of major importance in the environmental field. New systems for the generation and storage of energy, such as fuel cells, are based on catalytic processes.
Substances that increase the speed of a chemical reaction by reducing activation energy without being consumed themselves in the process are known as catalysts. Catalysts accelerate the forward and backward reaction of a chemical process, i.e. the change in a substance, equally effectively. A catalyst participates in a chemical reaction by forming an intermediate stage with the reaction partners (reactants), with the catalyst remaining unchanged after the product has been created. A catalyst can complete this catalysis cycle repeatedly. Depending on which phases the catalysts and reactants are in, the catalysts are called homogeneous or heterogeneous. By way of explanation: in homogeneous catalysis, the catalyst is in a phase (solution or gas phase) with the reacting substances, while the catalyst is a phase of its own – generally solid – in heterogeneous catalysis.
Incidentally: adding value via catalysis is of considerable economic significance in the chemical industry, because more than 80% of all chemical products are manufactured with the help of catalytic processes. The amount of energy and resources required can be reduced decisively by optimising these processes. Global catalyst sales amounted to about USD 16 billion in 2007, with more than 90% of this figure being generated with catalysts for heterogeneously catalysed processes.
Catalysed polylactic acid production
In the case of biopolymers based on PLA, the catalyst consists of a microporous mineral, a zeolite , the structure of which encourages chemical reactions in the microspaces of the pores. The catalysis operation, which represented a key element in successful implementation of the project, was optimised step by step in the course of the study. The research scientists were able to outperform the fermentation process at both the environmental and economic levels by improving various aspects of catalyst design.
Cecilia Mondelli, a member of the scientific staff involved in the study, says that industrial processes were often made “sustainable” simply by switching to a renewable raw material. “However, if the entire process – from the source of the original material to the finished product, including waste disposal channels – is taken into consideration, what are allegedly sustainable processes are not necessarily more sustainable than the conventional ones.”
The use of biodegradable plastic packaging made of polylactic acid (PLA) is spreading. Since this year, PLA cups are available also in the ETH canteens. (Photo: Bo Cheng / ETH Zurich)
A reduction of one third in carbon dioxide emissions
If the increase in productivity and the energy that the new process saves by providing a new recycling option for a waste material are taken into account as well, carbon dioxide emissions are said to be reduced by 30 per cent over the fermentation process. It is reported that the new process produces 6 kilograms of carbon dioxide per kilogram of lactic acid obtained compared with 7.5 kilograms in the conventional method. The research scientists have also calculated that profits seventeen times higher can be generated, because the overall costs of the process are lower.
According to Morales, “the assumptions made in this context were on the conservative side and were based on a relatively high glycerol quality”. The process did, however, work with highly contaminated glycerol too, which would be even more cost-effective, even if this seems difficult to believe. The scientist is convinced that manufacturers could improve their profits even more. And not only major bioplastic manufacturers like those based primarily in the USA today would benefit from this. Because the research scientists are convinced: “The process is relatively simple and can be established everywhere where biodiesel and/or glycerol is made as a by-product”.
 Morales, M., Dapsens, P. Y., Giovinazzo, I., Witte, J., Mondelli, C., Papadokonstantakis, S., Hungerbühler, K., Pérez-Ramírez, J.: Environmental and economic assessment of lactic acid production from glycerol using cascade bio- and chemocatalysis. Energy Environ. Sci. 5 November 2014, doi: 10.1039/C4EE03352C
There are limits to the use of olfactorily contaminated plastics for the production of fresh materials. Odorous contamination of them needs to be minimised so that they are suitable for manufacturing new plastic products. A combination of analytical and sensor technology can help to identify olfactory contaminants and to check the efficiency and effectiveness of cleaning processes.