Making plastic biodegradable can be accomplished either by making resin from plants to create bioplastics or adding a catalyst to conventional polyethylenes to hasten the degradation time to a few years instead of hundreds of years.
We are neutral about which approach to biodegradable bags or biodegradable food packaging is the most sustainable packaging. We only point out the basic pros and cons of biodegradable plastic to help you decide what is right for you and your budget.
Which feedstock (plants or natural gas) is most sustainable has been complicated by the advent of "green polyethylene" produced by Braskem. Braskem makes polyethylene from sugar cane. This polyethylene plastic made from plants is a bioplastic, yet it is not biodegradable. Bioplastics are not by definition biodegradable. Here is a recent success story about Braskem:
The following is a condensation of definitions from The American Society of Testing Materials test method D 6400 which is specific to plastic:
Degradable Plastic: a plastic designed to undergo a significant change in its chemical structure under specific environmental conditions, resulting in a loss of some properties that may be measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification.
Biodegradable Plastic: a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae.
Compostable Plastic: a plastic that undergoes degradation by biological processes during composting to yield CO2, water inorganic compounds, and biomass at a rate consistent with other known compostable materials and leave no visible, distinguishable or toxic residue.
The array of biodegradability standards can be confusing:
ASTM D 6400, ASTM D 6954-04, ASTM D 5526-94, ASTM D 5511, ASTM D6868,, ASTM D 6002, ASTM 1991, ASTM D 5209-91, ASTM D 5338.98, ISO 14855, EU 13432
The subsets of degradable plastic – bioerodable, compostable, hydrobiodegradable, photodegradable or biodegradable, can be derived from natural or synthetic polymers.
All biodegradable plastics have one thing in common – they have "triggers" of U V light, heat or humidity which initiate the breakdown reaction. For this reason, biodegradable plastics require more TLC in shipment and storage than conventional polymers to stave off the initiation of the breakdown reaction. They must be stored in cool, dark conditions and turned within a few months.
Final degradation time is not an exact science. There are too many interdependent factors such as exposure to triggers and ambient heat and humidity.
Destination must be taken into account. It is illegal to claim that a plastic film or plastic bag is biodegradable in California unless it passes ASTM D 6400. SB 1972 has softened this somewhat. In any case, thick film made from the same resin will not pass D 6400. A leaf usually will not pass ASTM D 6400.
Disposal environment is a big factor in deciding on which biodegradable plastic is appropriate, especially if the disposal environment is unknown. For example, if a biodegradable bag or biodegradable food package is disposed of as litter, either a compostable bag (D 6400) or oxobiodegradable (D 6954-04) plastic bag would be appropriate. Both will degrade outdoors. A compostable plastic will degrade in a landfill. Whether an oxobiodegradable plastic will degrade in a landfill is a matter of debate.
If biodegradable bags are destined for the landfill and evaluated in context of ASTM D 5511, Eco Logic's additive would make sense as they claim their additive will render conventional plastics completely degradable and digestible by microbes in an anaerobic environment.
Price Oxobiodegradables and other technologies are the lowest cost because they simply incorporate an additive to polyethylene.
Bioplastics have the highest green premium.
What are Bioplastics?
According to Cereplast, bioplastics are a sustainable alternative to traditional plastics, bioplastics are plastics that are fully or partially biobased, and/or biodegradable or compostable. In other words, they are plastics that are made from renewable resources (plants like corn, tapioca, potatoes, sugar and algae), and they will break down faster than traditional plastics, which are typically made from petroleum, and other fossil resources such as natural gas.
The non-profit Biodegradable Products Institute lists resins approved as being biodegradable.
The USDA has recently begun to issue specific approvals of bioplastics.
The Benefits of Bioplastics
• Reduced carbon footprint
• Reduced use of fossil resources
• Improved end-of-life options
The Carbon Cycle
When a plant grows, it takes in carbon dioxide, and when it biodegrades, it releases the carbon dioxide back into the earth - it's a closed loop cycle. When we extract fossil fuels from the earth, we disrupt the natural cycle, and release carbon dioxide into the atmosphere faster than natural processes can take it away. As a result, the atmosphere is getting overloaded with carbon dioxide. Additionally, fossil fuels take millions of years to form, and are therefore non-renewable resources. In other words, we are using our fossil resources faster than they can be replaced.
When we make products like plastics from fossil fuels, we are contributing to the imbalance in the environment while depleting valuable fossil resources, thereby increasing the carbon footprint of the product. Bioplastics can replace nearly 100% of the fossil fuel content found in conventional plastics, and require considerably less energy to produce.
Many bioplastics are 100% compostable and will biodegrade in 180 days or less when disposed of in a municipal composting facility, whereas traditional plastics can take decades to break down. Initially, when conventional plastics begin to break down, they fragment into smaller and smaller particles that often end up in our water stream, and in our food stream when animals eat the plastic particles. Conversely, compostable plastics are absorbed back into the earth and become nutrients for the soil – closing the loop.
We have evaluated many bioplastic film resins. Cereplast makes the most sense for us. Cereplast bioplastic resins are manufactured in Indiana, resulting in the lowest carbon footprint for transport to our factory in St. Louis.
Oxobiodegradable plastics work by adding what amounts to a catalyst to ordinary polyethylene or polypropylene to make the polymer degrade in months after the service life rather than hundreds of years.
Oxobiodegradables are the lowest cost biodegradables because they utilize conventional monomer feedstocks and an inexpensive additive. They have zero effect on extrusion conditions, printing or bagmaking or physical properties.
The yield, or square inches per pound for film resins is greater than bioplastic film resins. This means about 30% more area per pound at any given gauge.
The price per pound of oxobios is one third the price of compostable plastic resins. They have the same throughput rates as conventional LDPE and do not require special handling such as drying.
Oxobiodegradables cannot be composted.
The most common criticisms are breakdown rate in an anaerobic environment, whether they are totally biodegradable, and that oxobiodegradables contain heavy metals. While it is true that EU 13142 classifies cobalt as a heavy metal, cobalt is not a toxic heavy metal such as lead, cadmium, molybdenum. Cobalt is an essential component of the B12 molecule.
The most popular oxobiodegradable additive manufacturers are EPI, Symphony and E2S green club.
Check out these links for more information:
|The differences between compostable and biodegradable plastics.|
|"Biodegration of thermally-oxidized, fragmented low-density polyethylenes." (University of Pisa)|
|Epi-Environmental Products Responds to SPI Bioplastics Panel Questioning Oxo-Degradability Claims|
|Oxo-biodegradable Additive Suppliers Rebut Critics|
|Frequently Asked Questions|
|Definitions and Terms|
|Details of Test Method D 6954 - 04|
|International Standards and Compliance Status|