• Bio-based plastics: Feedstocks, Production and the UK Market

    Introduction

    With the UK government poised to implement a greenhouse gas (GHG) reduction target of 80% from 1995 levels by 2050, it is becoming important to source materials and fuels from renewable resources to reduce our environmental impact.

    Bio-based plastics have been used in medicine for years and were also considered for automotive parts in the days of Henry Ford. However, with climate change on the minds of manufacturers, retailers and consumers alike, there is renewed interest in bio-based plastics.

    Bio-based plastics can be used in a variety of applications to replace petroleum-based plastics (petroplastics). Most bio-based plastic materials used in Europe today are starch-based. In the UK these materials are currently used to manufacture refuse and carrier bags as well as food and consumer goods packaging. Other niche markets for bio-based plastics are under development including electronics and automotive parts.

    1. Feedstocks

    Bio-based plastics can be manufactured from renewable plant materials such as starch, cellulose, oils (e.g. rapeseed oil), lignin (wood), proteins (e.g. maize zein) and polysaccharides (e.g. xylans). Recent technological developments have also proven that it is now possible to utilise organic waste materials and petroplastics (e.g. PET) to produce synthetic bio-based plastics (such as polyhydroxyalkanoates or PHAs).

    2. Starch

    The majority of bio-based plastics are currently manufactured using starch as a feedstock (c.a. 80% of current bio-based plastics). The current major sources of this starch are maize, potatoes and cassava. Other potential sources include arrowroot, barley, some varieties of liana, millet, oats, rice, sago, sorghum, sweet potato, taro and wheat.

    There are several methods for extracting starches depending on the source crop. For wheat, starch is normally extracted using a dry grain milling processes(see figure 1.01a). For potatoes and cassava 'rasping' is used to pulverise cells and release the starch (see figure 1.01b). The extracted starch is then refined and purified further to remove proteins and fibre.

    Figure 1.01.

    Methods of Extracting Starch
    Source; Deahan Group Company Limited, International Starch Institute

    Four fundamental factors determine the choice of starch for bio-based plastic manufacture;

    • Price
    • Location of Source
    • Quality / Purity
    • Starch Characteristics
     

    2.1 Starch price

    Key to developing the production of starch-based plastics in the UK is the sourcing of reliable and relatively inexpensive starch. Starch-plastic companies can either choose to extract starch themselves, or buy starch from milling companies.

    Starch prices vary considerably depending on the type of starch, the availability of crop feedstocks for starch production in the country of origin, the ease by which starch can be extracted and the scale of starch production amongst other factors. For example, table 1. shows that cassava starch imported into the UK had the highest value (£986/tonne) whereas wheat starch exhibited the lowest value (£263/tonne) in 2007. In addition, figure 1.02. indicates the highly variable export value of different starches from different countries. It is clear that there is a complex relationship between the export value of starch and the abundance of the corresponding starch crop. For example; Ghana, the sixth largest producer of cassava starch in 2007 (9.65 million tonnes) had the highest value cassava starch exports ($1192/tonne). However, Thailand, the third largest producer of cassava starch in 2007 (26.41 million tonnes) exhibited relatively low export value cassava starch ($275/tonne).

    Table 1. UK starch import 2007

     

    Starch Type

    Trade Volume [tonnes]

    Import Value [£/tonne]

    Cassava

    3598

    986

    Maize

    77422

    371

    Potato

    63709

    354

    Wheat

    32831

    263

    Source: UN Comrade

    2.2 Location of starch source

    Starch crops are grown in different geographic locations depending on the climate conditions required to grow each crop. For example, wheat is grown in Western Europe including the UK, France and Germany whereas cassava is grown in more equatorial climes such as Ghana, Nigeria and Thailand (see figure 1.02).

    Figure 1.02. Starch production 2007

    starch crop growth around the globe
    Source - UN ComtradeCLICK HERE to see image in full size

    When choosing a starch for bio-based plastic manufacture, transport costs and the greenhouse gas (GHG, e.g. carbon dioxide) emissions generated by production and transportation must be considered.

    Life cycle analysis (LCA) is a means of auditing the GHG emissions (amongst other environmental aspects) throughout production and use of bio-based plastics. Figure 1.03 shows that the most energy intensive and, therefore, GHG emitting steps of the bio-based plastic bag life cycle are the manufacture of starch and bio-based plastic resin. Note that disposal options for bio-based plastic materials greatly influence the GHG output. In addition, for starch-based BioBags, landfill is the least favourable disposal option in terms of methane GHG (23 times more damaging than carbon dioxide) emissions. This, along with European legislative and monetary disincentives, indicates that landfill is not an option for bio-based plastic disposal.

    Figure 1.03. Life Cycle Analysis (LCA) of starch-based BioBags

    Life Cycle Analysis (LCA) of starch-based BioBags
    * Note: raw materials production includes crop production, starch extraction and transportation*
    Source – ITF Rapport: Life cycle assessment of BioBags used for collection of household waste.

    2.3 Starch quality/purity
     

    Starches can vary in quality depending on the source crop and the method of extraction and purification used. In Western Europe starches are extracted from maize, wheat and potatoes on a large scale to high purity. However, the quality of cassava starch can vary due to the fact that small ‘cottage’ industries in countries such as Nigeria, Vietnam and Thailand produce starch using different methods.

    Due to the nature of tuber and root crops, purity can be an issue if dirt is not properly removed prior to ‘rasping’. Storage is another factor to consider since tuber and root crops do not store well, are easily damaged and respire reducing the amount of useable starch available and producing heat. This is in contrast to grain (e.g. wheat) which, when sufficiently dry (13-14% moisture content), can be stored for months prior to use without too much deterioration in starch content.

    2.4 Starch characteristics

    Starches are constructed of unbranched and branched polymers of glucose called amylose (see figure 1.04) and amylopectin, respectively.

    Figure 1.04. Starch amylose structure

    Starch amylose structure
    CLICK HERE to see image in full size.

    Starches from different sources possess different properties that may be useful for bio-based plastic manufacture. For instance, tuber and potato starches tend to have lower protein levels, but higher phosphate levels whereas cereal starches present the opposite characteristics. Bio-based plastics made from high-phosphate starches have been shown to biodegrade faster than plastics manufactured with low-phosphate starches.

    For the production of synthetic bio-based plastics such as polylactic acid (PLA, see figure 1.05a a, b and c) and polyhydroxyalkanoates (PHAs such as PHB, see figure 1.06) it is important to choose an inexpensive starch that can be easily broken down (acid or enzyme hydrolysed) to its sugars for subsequent fermentation. The hydrolysis hierarchy for starches is as follows: wheat followed by maize, cassava and potatoes where wheat is easily broken down to glucose and potato starch is most resistant.

     

    production of synthetic bio-based plastics polylactic acid and polyhydroxyalkanoates

    3. Oils

    Oils from castor beans, soya beans and oilseed rape are used to manufacture bio-based plastics such as polyamides (nylon) and polyurethane for tubing and insulation products, respectively. However, these plastics are typically bio-based and non-biodegradable with very similar (if not identical) properties to their petroplastic counterparts.

    Oils can be extracted from oilseeds by pressing, organic solvent (hexane) or enzyme treatment. The latter two processes produce higher yields. The resultant oil is then purified using various filtering and chemical treatments. Plant-oil polyols (1,3 propanediol and 1,4 butanediol) can be used in the manufacture of plastics such as polyurethane (PU) and polybutylene terephthalate (PBT).

    4. Cellulose and Lignin

    Cellulose, a polymer of glucose and an integral plant cell structural component, has been used to make plastic for nearly 140 years. Common cellulose sources include wood, cotton and hemp. Cellulose-based plastics are usually made from chemically modified cellulose, the most common of which is cellulose acetate used in packaging film.

    Lignin, a complex compound found in woody plants, is also used to manufacture bio-based plastics. Lignin is typically sourced from paper pulping waste.

    5. Proteins 
     

    In the 1920s Henry Ford experimented with the use of soy proteins in plastics. There is currently renewed interest in using both soy and maize proteins in the production of bio-based plastics for horticultural and medical uses. Waste animal products such as bloodmeal proteins can also be used as feedstock for bio-based plastics.

    6. Xylans 
     

    Xylans are complex compounds containing the sugar xylose. They can be isolated from the hulls and husks of cereal grains and are used to make polymer coatings for food packaging. Importantly, removal of xylans from wheat prior to fermentation for alcohol production may be beneficial in reducing the water and energy required during wheat processing for bioethanol manufacture.