Bioplastic — the good, the bad and the ugly. What is this group of materials and why are they so promising for sustainable design?

Lignin is the glue that binds cellulose fibres in trees. It is an abundant natural polymer, second only to cellulose, but is not readily available in its isolate form. Naturally occurring in plants, it is chemically bound with the cellulose and hemicellulose. It is the rigid woody part that is removed in the paper making process, and often incinerated for energy recovery. When heated to around 170 degC it behaves thermoplastically and can be moulded, extruded and 3D printed for example. As a reinforcement material in bio-based composites, lignin can significantly improve the mechanical properties and offers additional enhancements such as antimicrobial, antioxidant, flame retardant and UV blocker.

Starch is a natural polymer, a polysaccharide which functions as a carbohydrate (sugar) store, formed by photosynthesis when CO2 reacts with water. There are many possible sources, but the majority used in bio-based plastics comes from corn, wheat, cassava and potatoes. It is mixed with plasticiser to make thermoplastic starch (TPS), which can be formed using conventional moulding, extrusion and film-making processes. It is available in many formats, from edible take away containers to industrial polylactic acid (PLA). The difference comes from the way it is processed. As a raw material, it can be converted into simple items by cooking it with water. Alternatively, it is fermented with the help of bacteria to produce lactic acid, which forms the basis of PLA, the most popular bioplastic.

Cellulose is an abundant renewable material, found in plants, algae and some bacteria. The raw materials used for industrial cellulose acetate production, such as wood and cotton, are converted into useable monomer through a process of esterification. The pulp is processed with acetic acid and acetic anhydride in the presence of sulphuric acid to produce acetate flake from which products are made. While the chemical processes used to make these semi-synthetic plastics can be pretty nasty, there are some developments in closed-loop manufacturing, such as lyocell, that are yielding much more sustainable plastics and textiles. Some types of cellulose bioplastic are biodegradable and home compostable at the end of their useful life, but not all.

All manner of cellulose containing materials may be used in the production of these bio-based plastics, including bamboo, eucalyptus and bacteria. While the choice can affect the efficiency of the process and overall sustainability, it will not affect the properties – these are determined more by the manufacturing process.

Seaweed algae is an exciting area of development and there is huge investment pouring into the area with several start-ups working hard to offer a cost-effective alternative to conventional plastic packaging. Seaweed-based biopolymers include alginate, agar and carrageenan. These are polysaccharides made up of sugar molecules bonded together in long chains. There are many types of seaweed and several of them are suitable for bioplastic production. Each has its own qualities and benefits, such as providing an effective barrier, water solubility, printability, or thermoplastic properties. Read more about seaweed developments here What the kelp! Exploring this pioneering new breed of materials made from seaweed, including paper, packaging, foams and textiles.

Natural rubber is an important industrial material produced from the Hevea rubber tree. The raw latex is converted into a resilient and high strength material through chemical treatment and vulcanisation (reacted with sulphur at high temperature to harden it). While it has some impressive qualities and there is potential for it to be renewable and certified, unfortunately the majority of production is not particularly sustainable in terms of land use and chemical processing.

In addition to all these bio-derived plastics, there have been major developments in the production of drop-in replacements of conventional plastic using fossil-free ingredients, such as bio-based polypropylene (bio-PP), polyethylene (PE), polyurethane (bio-PUR) and epoxy (bio-EP) among others. Not without their limitations, these plastics offer a way to reduce our consumption of oil and connect into the circular economy. For example, using bio-PP can more than half the environmental impact compared to regular oil-based PP.

Plastics and landfill 

Estimates vary, but somewhere between half and three-quarters of plastic waste ends up in landfill. The problem with this is plastic materials persist and pollute long after their intended use is finished. Landfilling plastics loses all the energy put into its production, and it starts a longterm degradation process that results in microplastics and, in some cases, the release of some toxic chemistry such as heavy metals, flame retardants and plasticisers. Plastic waste is dominated by polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyester, and polystyrene (PS), including expanded polystyrene (EPS).

Landfill is an ecological system. The inputs are solid waste and water and the outputs are leached liquids and gases. Once landfilled, materials first undergo aerobic degradation, but soon make the transition to anaerobic conditions. Most of the polymers in plastics in landfill remain unchanged by the processes as the material is broken down into smaller and smaller fragments. Oxo- or oxy-degradable plastics contain photo-active or thermo-active ingredients that cause the material to fragment into tiny particles — microplastics that biodegrade very slowly like regular plastic.

By contrast, biodegradable materials like food waste (and bioplastics) breakdown relatively quickly. Microbes including bacteria, mould and yeast work to breakdown biological materials, releasing chemicals and gases. To be certified compostable, bioplastics must break down at a rate consistent with other biomaterials and there must be no harmful chemicals released. Under anaerobic conditions, biomaterials release methane (CH4), a powerful greenhouse gas (GHG) >20 times more powerful than CO2. Therefore, these types of materials must be disposed in facilities that have controlled conditions. Either, they can be composted in aerobic conditions (oxygen present) to reduce the release of methane, or anaerobically and the methane captured and burnt to produce oxygen and CO2, such as in a biogas facility. Anaerobic digestion is feasible for several types of polymers, including thermoplastic starch (TPS), polycaprolactone (PCL) and polyhydroxyalkanoates (PHA), as well as for polylactic acid (PLA) at elevated temperatures.

Composting and the circular economy

One solution for circularity in plastics is to produce them from renewable ingredients, using renewable energy, and to either recycle them or compost them, returning their nutrients back to the earth — in a kind of ‘bio-cycle’. This is possible with today’s technology, but depends on alignment between material, context of used and end of life. For example, compostable food packaging at large public events, whereby the packaging is collected along with the food residue and composted like Notpla; reusable packaging in a closed loops system, such as returnable transit packaging (RTP); and construction products used in conjunction with bio-based building materials like hempcrete.

Compostable plastics are classified as either industrially compostable, or home compostable. The composting period is governed by a number of factors including temperature, moisture, oxygen levels, particle size, carbon-to-nitrogen ratio and the degree of turning involved. Home compostable, or backyard compostable, materials breakdown under normal conditions as would be found naturally. Industrial composting takes place in specialised units that create an artificially hotter environment (50—60 degC) which speeds up the degradation process and ensures the material completely disappears. The outcomes of industrial composting process are CO2, water and compost. The compost includes nutrients that can be used in agriculture to enhance the quality of soil, for example.

Not all biodegradable waste goes to composting, and not all compostable waste goes to these specialised facilities. This causes some problems with so-called compostable plastics. If they are industrially compostable, but end up in landfill, then they can produce methane. Likewise, if they end up being recycled with similar looking non-compostable plastics, they cause contamination.

To be labelled as commercially ‘compostable’, a plastic must meet the specifications set out by ASTM standards D6400 and D6868 in the US and EN 13432 for the European market. These include the test requirements and evaluation criteria for the compostability and anaerobic processing of packaging materials in controlled waste treatment plants. They cannot automatically be considered to be suitable for home composting, because the conditions are different. While third-party certification may not be required to meet the standards, the most well known in the US is the Biodegradable Products Institute (BPI) and in Europe is TUV Austria and their OK Compost mark.

While it may look like a gimmick (and the speckle effect is sometimes), including wood waste in bioplastic is a very effective method for ensuring compostability. The rate of biodegradation depends on the type of polymer and its thickness. Thicker wall sections take considerably longer to breakdown, because they have relatively less surface area. Films breakdown quickly and are certified home compostable in many cases for this reason. Anything thicker than 1 mm, depending on the polymer, can take more than a month and so fall outside the acceptable time limit for certification. On top of this, biodegradable plastics like PLA breakdown very slowly in sea water and soil — they need high temperature and humidity, such as found in industrial composting facilities. Including wood or other water absorbing additives draws moisture into the plastic and so initiates biodegradation from within, accelerating the composting process. This opens up a wider range of applications, in particular in packaging, where petrochemical plastics pose a significant problem. Adding wood also helps to reduce the cost, reduce weight and improve the sustainability of bioplastic — wood waste is cheaper, lighter and has lower carbon footprint and embodied energy than biodegradable plastic.