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Biodegradable plastics allow the integration of plastics into the natural cycle of life. Many processes in nature require biodegradation to occur. For example, plants require microorganisms in the soil to break down organic biomass such as fallen leaves and dead animal remains. These microorganisms biodegrade the organic matter producing simpler compounds such as nitrates and amino acids that plants can absorb through their roots and use as nutrients for their growth and metabolism.
For humans, biodegradation is important for us to obtain nutrients from food. The enzymes, acids, microorganisms, and conditions (such as temperature, moisture, and pH) in our digestive system facilitate the biodegradation of foods into fuel and food for our growth and sustenance. For example, carbohydrates are broken down into sugars, and proteins are broken down into amino acids.
How biodegradation occurs is an important factor in the overall sustainability of the material. For all biodegradable bioplastics and other biodegradable materials, biodegradation can be aerobic (occur in the presence of oxygen) or anaerobic (occur in the absence of oxygen).
Anaerobic biodegradation results in the production of methane, a greenhouse gas that is up to 25 times more potent than CO2. Therefore aerobic biodegradation may be more desirable, in open-air composting. Anaerobic may be desirable in a closed system where the methane needs to be captured and used as biofuel.
Biodegradation occurs in 3 stages: Bio-deterioration, bio-fragmentation, and bio-assimilation (figure 1).Biodeterioration is the change in the material’s chemical, mechanical, and physical properties upon exposure to abiotic factors. This results in the weakening of the material.
Bio-fragmentation of polymers is when breaks occur within the polymer chain. This is when the polymer breaks down into oligomers and monomers, and eventually into smaller units like water, methane, hydrogen, CO2, and other compounds. This can be an aerobic or anaerobic process depending on the availability of oxygen.
Biodegradation can also be categorized based on whether the biodegradation is occurring under normal household or field composting conditions or occurring under highly controlled conditions in industrial composts. This is where it is important for consumers to be able to differentiate between bioplastics that can be tossed in the garden compost heap and those that need to be separated and sent out for industrial composting.
Bioassimilation is when the products of biofragmentation get acted on by microorganisms, and are converted into other products. These microorganisms do this in order to obtain energy and carbon. In the process, they produce byproducts that they may retain within their cells (for example PHA PHA-producing bacteria) or they may excrete them (for example alcohol-producing yeast).
Figure 1. The Stages of Biodegradation according to the Industrial Biotechnology Innovation Centre.
One of the approaches to addressing the global plastic challenge is to make plastics biodegradable. This offers benefits that include:
It is important to differentiate between plastic pieces breaking up into smaller fragments and into microplastics via biodeterioration. This is not complete biodegradation as they still exist as plastics, just smaller. Complete biodegradation requires biofragmentation and bioassimilation.
In the next sections, we compare the biodegradation of three different classes of bioplastics of commercial significance; PHA, PLA, and starch-based bioplastics. These are based on available data of specific formulations or grades of each bioplastic.
Microorganisms produce PHAs and therefore have the enzymes that can biodegrade PHAs. These are the PHA depolymerases that break down the polymer chain through hydrolysis of the ester linkages. This results in oligomers and monomers that dissolve in water. In one study, at 58oC and pH of 8.2, all samples of PHA showed 15 -25% degradation after 15 days. After 70 days, all the PHA samples had degraded by 80-90%. Actual degradation rate varied with type of PHA as shown in Table 1.
|PHA||Degradation after 70 days|
Table 1. Degradation rate of different PHAs at 58oC after 70 days
The use of additives and the thickness of the material has also been shown to affect the biodegradation rate of PHAs. For example, 0.25mm and 1.2mm thick PHA samples were completely biodegraded after 16 weeks. Meanwhile, thicker samples of 5mm thickness were not completely biodegraded after 16 weeks under the same conditions. The use of tributyl citrate plasticizer significantly reduced the biodegradation rate of PHB.
PLA biodegradation mechanism occurs in two stages; hydrolysis of the ester bonds to form lactic acid oligomers and then digestion of the oligomers by microorganisms. CO2 and water are produced in the process. Because microorganisms are not known to produce PLA, the first stage of biodegradation is not easily mediated by microorganisms. Therefore special conditions like pH and temperature need to be created to aid biodegradation of PLA.
Other factors like the type of PLA also affect biodegradation. For example, when composted at 63oC and a pH of 8.5 under aerobic conditions, PLA trays and deli containers made up 94% L-lactide showed decreased molar mass after 15 days. This indicates that biodegradation has occurred. After 30 days the trays had completely decomposed.
On the other hand, PLA bottles with 96% L-Lactide took longer to biodegrade under the same conditions. This is attributed to the higher crystallinity of the particular PLA. The PLA showed a molar mass reduction of 5kDa after 57 days.
Temperature significantly affects PLA biodegradation. Increasing the temperature of the degradation environment significantly increases the biodegradation rate as shown in Table 2.
|37oC||20% after 12 months|
|45oC||57% after 9 weeks|
Table 2. Effect of temperature on biodegradation of PLA
Starch is used in the form of derivatized starch or starch derivatives in bioplastic formulations. Starch derivatization can be through acetylation, hydroxypropylation, oxidation, or other reactions that lead to improved barrier properties, solubility, and mechanical properties.
The biodegradability of starch-based bioplastic blends is affected significantly by the composition. For example, acetylated starch blends reached 25.3% biodegradation after 2 months when a composition of 80/20 PCL/acetylated starch was used. However, when the blend was 60/40 29.8% biodegradation was reached after 2 months.
Plasticizers like glycerol and sorbitol tend to increase the hydrophilicity of thermoplastic starch. This may contribute to an increased biodegradation rate as the glucose linkages become more prone to hydrolytic cleavage. On the other hand, derivatization results in reduced hydrophilicity. Therefore the overall biodegradation depends on the type of derivatization, additives, and the composition of the bioplastic formulation.
Biotechnology makes it possible to formulate bioplastics to meet a wide range of specifications. This can be done in various ways. VEnvirotech for example produces bioplastic formulations with varying compositions of bioplastics and bio-based additives to meet different specifications.
From the discussion so far, we see that it is not simply a matter of which material degrades faster or better. Rather, there are several factors to be considered when comparing the biodegradation of different bioplastics. VEnvirotech team of experts consults with clients to determine their specific needs and create customized bioplastic formulations to meet specific demands.
Contact VEnvirotech’s team of experts to find out the best bioplastic formulation that suits your specific needs.