Material carbon footprintReference Narayan^1, Reference Narayan, Graziani and Fornasiero²

Carbon is the major basic element that is the building block of all plastics, fuels, and even life itself. Therefore, discussions on sustainability and environmental responsibility center on the issue of managing carbon (carbon-based materials) in a sustainable and environmentally responsible manner. Indeed, the burning issue of today is concern over increasing human-made CO₂ emissions with no offsetting sequestration and removal of the released CO₂. Reducing our carbon footprint is a major challenge. Reduced CO₂ emissions translate to minimizing global warming-climate change problems.

Switching the manufacturing base (the origins of the carbon) from petro-fossil carbon feedstock to bio-renewable carbon feedstock offers an intrinsic zero material carbon footprint value proposition. This can be seen by reviewing biological carbon cycle. Nature cycles carbon through various environmental compartments with specific rates and time scales, as shown in Figure 1. Carbon is present in the atmosphere as inorganic carbon in the form of CO₂. The current level of CO₂ in the atmosphere is around 380 ppm (parts per million) and is increasing. CO₂ and other greenhouse gases in the atmosphere trap the sun’s heat from radiating back to space, thereby providing a life-sustaining average planet temperature of 7.2^°C (45^°F). Increasing levels of CO₂ and other greenhouse gas emissions to the atmosphere would trap more of the sun’s heat, thereby raising the average temperature of the planet. While one may debate the severity of the effects associated with this or any other target level of CO₂, there can be no disagreement that an uncontrolled, continued increase in levels of CO₂ in the atmosphere will result in a slow perceptible rise of the earth’s temperature, global warming, and with it an associated severity of effects that will affect life on this planet as we know it. It is therefore prudent and necessary to try and maintain current CO₂ levels—the “zero carbon” approach. This can best be done by using renewable biomass crops to manufacture carbon-based products so that the CO₂ released at the end-of-life of the product is captured by planting new crops in the next season. Specifically, the rate of CO₂ release to the environment at end-of-life equals the rate of photosynthetic CO₂ fixation by the next generation of crops planted—a zero material carbon footprint. In the case of fossil feedstock, the rate of carbon fixation is measured in millions of years, while the end-of-life release rate into the environment is in 1–10 years. The math is simple, and this is not sustainable. It causes more CO₂ release than fixation, resulting in an increased carbon footprint, and with it attendant global warming and climate change problems.

Figure 1.

Biological carbon cycle—value proposition for using biorenewable carbon instead of petro-fossil carbon feedstock.

Based on the previous carbon cycle discussion and using basic stoichiometry, we calculate that for every 100 kg of polyolefin (polyethylene, PE; polypropylene, PP) manufactured, a net 314 kg CO₂ is released into the environment at its end-of-life (100 kg of PE contains 85.7% kg carbon and upon combustion will yield 314 kg of CO₂ (44/12) × 85.7). Similarly, PET (polyethylene terephthalate) contains 62.5% carbon and would result in 229 kg of CO₂ released into the environment at end-of-life. However, if the carbon in the polyester or polyolefin comes from a biological feedstock, the net release of CO₂ into the environment is zero, because the CO₂ released is fixed in a short time period by the next crop or biomass plantation (Figure 2). This is the intrinsic zero material carbon footprint value proposition for using a bio/renewable feedstock.

Figure 2.

Material carbon footprint value proposition for bio-based plastics.

Thus, the fundamental driver or switch to bio-based products is the material carbon footprint reduction arising from the “short-term” biogenic carbon cycle—the rate and time scales of CO₂ sequestration is in balance with the use and release, resulting in a carbon neutral footprint. This is in contrast to the “long-term” carbon cycle for petro/fossil feedstock; the rate and time scale of CO₂ sequestration is in millions of years, and the rate and time scales of use and release is in a 1–10 year time frame.

Process carbon and environmental footprint

An equally important consideration is the impact on the environment in terms of CO₂ and other emissions arising from the process of converting the carbon feedstock to product, the impact during product use, and ultimate disposal. LCA methodology and standards³ (ISO 14040 standards) are the accepted tools to compute the process environmental footprint. Unfortunately, LCA focuses almost exclusively on the process (carbon and environmental) footprint. The impact of the carbon present in the product, the material carbon footprint, is treated as feedstock energy or embodied carbon energy for potential use in the next product cycle. For example, compare the case of poly(lactic acid) (PLA), a bio-based product obtained from corn, with fossil-based polyolefin products (PE, PP) and PET. Figure 3 shows the process and material carbon footprint of the products. As can be seen from the figure, the process carbon footprint for PE/PP is 205 kg CO₂ emitted per 100 kg plastic manufactured,⁴ whereas that for PLA is around 385 kg CO₂ emitted per 100 kg plastic manufactured,Reference Vink⁵ a much higher carbon footprint for the bio-based product. However, if one includes the material carbon footprint, then PLA has a lower total carbon footprint—385 kg CO₂ emission per 100 kg plastic manufactured versus 519 kg CO₂ emission per 100 kg plastic manufactured. This is because the material carbon footprint for the PLA plastic is zero, whereas PE/PP has a material carbon footprint of 314 kg CO₂ emissions per 100 kg plastic manufactured. In the case of PLA, the total (net) CO₂ released to the environment, taking into account the intrinsic carbon footprint as discussed earlier, is lower and will continue to get even lower as process efficiencies are incorporated and renewable energy is substituted for fossil energy, as shown in Figure 4.Reference Vink, Glassner, Kolstad, Wooley and O’Connor⁶

Figure 3.

Process carbon footprint and material carbon footprint.

Figure 4.

Continuing improvements in the carbon footprint of poly(lactic acid) (PLA). Ingeo is the trademark name for NatureWorks LLC’s synthetic fiber made from corn. NatureWorks LLC PLA carbon footprint data.Reference Vink, Glassner, Kolstad, Wooley and O’Connor⁶

What is biodegradability?

Biodegradability is an end-of-life option that allows one to harness the power of microorganisms present in the selected disposal environment to completely remove biodegradable plastic products from the environmental compartment in a timely, safe, and efficacious manner.Reference Narayan^1, Reference Narayan, Graziani and Fornasiero^2, Reference Narayan, Hoitink and Keener¹¹

The “disposal environment” must be clearly identified when discussing or reporting the biodegradability of a product. Some examples include biodegradability in a composting environment (compostable plastic), biodegradability in a soil environment, biodegradability under anaerobic conditions (in an anaerobic digestor environment or landfill environment), or biodegradability in a marine environment.

Reporting the time to complete biodegradation or, more specifically, the time required for the complete microbial assimilation of the plastic in the selected disposal environment is an essential requirement. Stating that it eventually will biodegrade based on data showing an initial 10–20% biodegradability is not acceptable and misleading, especially since the percent of biodegradation levels off and reaches a plateau after some initial rate and level of biodegradation. Drawing a dotted line extrapolation from the initial rate and value to 100% biodegradation is scientifically untenable.

Measuring biodegradability—The scientific basis

Microorganisms utilize carbon substrates to extract chemical energy for their life processes. The carbon substrates become “food” that microorganisms use to sustain themselves. For this to occur, the carbon substrate needs to be transported inside the cell. Molecular weight is important but is not the only criterion for transport across cell membrane. Factors such as hydrophobic-hydrophilic balance and molecular and structural features also govern transport. Under aerobic conditions, the carbon is biologically oxidized to CO₂ inside the cell, releasing energy that is harnessed by the microorganism for its life processes. Under anaerobic conditions, CO₂+CH₄ (landfill gas) are produced. Thus, a measure of the rate and amount of CO₂ or CO₂+CH₄ evolved as a function of total carbon input to the process as a direct measure of the amount of carbon substrate being utilized by the microorganism (percent biodegradation). This is fundamental, basic biology and biochemistry that can be found in any biochemistry textbook. This forms the basis for various national (ASTM, EN, OECD) and international (ISO) standards for measuring biodegradability or microbial utilization of chemicals and biodegradable plastics.

One can measure the rate and extent of biodegradation or microbial utilization of the test materials by using it as the sole carbon source in a test system containing a microbial-rich matrix such as compost in the presence of air and under optimal temperature conditions (preferably at 580°C—representing the thermophilic phase). Figure 6 shows a typical graphical output that would be obtained if one were to plot the percent of carbon in a test material converted to CO₂ as a function of time in days. First, there is a lag phase during which the microbial population adapts to the available test C-substrate. Then, the biodegradation/utilization phase occurs, during which the adapted microbial population begins to utilize the carbon substrate for its cellular life processes, as measured by the conversion of the carbon in the test material to CO₂. Finally, the output reaches a plateau when the substrate is completely utilized.

Figure 6.

Graphical output of a typical composting experiment showing lag, biodegradation, and plateau phase.

Linear or any other form of data exploration for these complex biological systems is not acceptable and is very misleading, because credible scientific substantiation for the extrapolation model does not exist to date; in fact, current biodegradability data for polyolefin + additives (oxo- or other additives) formulations show the percent of biodegradation leveling off and reaching a plateau after some initial level of biodegradation.

Compostable plastics—Standard specifications

To meet the requirements of compostability, a material must satisfy the primary requirement of complete biodegradability under composting conditions, as discussed in an earlier section. In addition, it has to meet the disintegration and safety criteria to make a claim of compostability. ASTM D6400, D6868, ISO 17055, and EN 13432 are specification standards for compostable plastics and require:

1. • 90%+ of the test material’s carbon conversion to CO₂ via microbial assimilation of the test polymer material in powder, film, or granule form in 180 days or less—a laboratory scale test method, as described in the previous section.

2. • Disintegration of the test material in both shape and thickness of the products intended for use, such that 90% of the test material must pass through a 2 mm sieve using ISO 16929 or ISO 20200 standard test methods.

3. • Safety considerations where the resultant compost should have no impact on plants, using OECD Guide 208, Terrestrial Plants Growth Test. Furthermore, regulated (heavy) metals content in the polymer material should be 50% or lower than prescribed thresholds in the country of use (e.g., 50% of the U.S, and Canadian Environmental Protection Agency’s prescribed threshold).

Misleading biodegradability claims within the context of the environment

Some studies use the term “biodegradable” to indicate that the plastic samples were subjected to a biotic environment (soil, compost) as part of their experimental procedure. The study goes on to measure weight loss, molecular weight reductions, carbonyl index, and mechanical property loss (films becoming brittle). Additive manufacturers reference these studies and extrapolate, stating that their product is “completely (100%) biodegradable” in the environment based on weight loss and physical, chemical, or mechanical property loss. However, the fundamental biology/biochemistry data showing carbon utilization by the microorganisms, as measured by the evolved CO₂ (aerobic) or CO₂+CH₄ (anaerobic), are missing.

A peer-reviewed paperReference Galgali, Varma, Puntambekar and Gokhale¹² reported increasing the rates of biodegradation of polyolefins by anchoring minute quantities of glucose, sucrose, or lactose onto functionalized polystyrene. A mere 2–12% weight loss and formation of carbonyl groups was evidence for biodegradation. There were no data showing microbial utilization evidenced by measuring evolved CO₂, as discussed in previous sections.

In another report,Reference Scott¹³ polyethylene and polypropylene were placed in a composting environment after solvent extraction to remove the antioxidants, and it was reported that polypropylene lost 60% of its mass over six months, whereas low density polyethylene (LDPE) lost only 10%. It is well known that unstabilized polypropylene will degrade in the environment. The report concludes as follows: polypropylene biodegrades more rapidly than LDPE by mass loss in compost, and ethylene-propylene copolymers biodegrade at rates intermediate between polypropylene and ethylene. This implies that 60% of the polypropylene carbon has been utilized by microorganisms present in compost, as measured by evolved CO₂; however, no CO₂ data are provided to substantiate the claim.

There are many more examples where physical, chemical, and mechanical property losses are used to claim “biodegradability.” In some papers, microbial colonization or biofilm formation is used to make claims of biodegradability. Weight loss, molecular weight reductions, carbonyl index, mechanical property loss, biofilm formation, and microbial colonization do not confirm the microbial utilization of the polymeric carbon substrate, nor do they provide the amount of carbon utilized or the time to complete microbial utilization.

In a recent paper,Reference Ojeda, Dalmolin, Forte, Jacques, Bento and Camargo¹⁴ oxo-biodegradable polyethylene samples were abiotically degraded in natural and saturated humidity for one year. The degraded fragments then were biodegraded in a mixture of soil:compost:perlite (1:1:2) at 58°C for three months. The percent biodegradability, as measured by evolved CO₂, was 3.61% (abiotic natural humidity) and 5.70% (abiotic saturated humidity). The percent of biodegradability for samples weathered for one year in polypropylene enveloped in compost at 58°C was 12.4%, and after three months, 25°C was 5.4%. Given this kind of almost negligible biodegradability data after one year of weathering and subsequent exposure to an aggressive, biologically active compost environment for three months, it is surprising to note that companies claim that such oxo-biodegradable products will biodegrade 100% in the environment. The previously mentioned study shows that a significantly large amount of the degraded plastics, some of which could be microscopic, would be released into the environment.

Environmental and health consequences

Making hydrophobic polyolefin plastics such as polyethylene unstable and degradable and releasing them into the environment without ensuring that the degraded fragments are completely assimilated by the microbial populations in a short time period has the potential to harm the environment and create human health risks. The fragments, some of which could be microscopic, can be transported through the ecosystem and could have serious environmental and health consequences. In fact, stringent “European community REACH (registration, evaluation, authorization, and restriction of chemical substances) laws” governing the release of almost all chemicals (small molecules) is becoming the norm in Europe and other countries. It requires the chemical to be completely assimilated by microorganisms in the ecosystem if it is to be released into the environment.

ThompsonReference Thompson, Olsen, Mitchell, Davis, Rowland, John, McGonigle and Russell¹⁵ reported that plastic debris around the globe can erode (degrade) away and end up as microscopic granular- or fiber-like fragments, and these fragments have been steadily accumulating in the oceans. The experiments show that marine animals consume microscopic bits of plastic, as seen in the digestive tract of an amphipod.Reference Thompson, Olsen, Mitchell, Davis, Rowland, John, McGonigle and Russell¹⁵

The U.S.-based Algalita Marine Research Foundation¹⁶ reported that degraded plastic residues can attract and hold hydrophobic elements such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT) up to one million times the background levels. PCBs and DDTs are at background levels in soil and diluted out, so as to not pose significant risk. However, degradable plastic residues with high surface areas concentrate these chemicals, resulting in a toxic legacy in a form that may pose risks in the environment.

Japanese researchersReference Mato, Isobe, Takada, Kahnehiro, Ohtake and Kaminuma¹⁷ have similarly reported that PCBs, DDT, and nonylphenols can be detected in high concentrations in degraded polypropylene resin pellets collected from four Japanese coastal areas. This work indicates that plastic residues may act as a transport medium for toxic chemicals in the marine environment. More recently, issues were reported surrounding microscopic plastics being released into the environment and causing environmental and human health problems.Reference Teuten, Saquing, Knappe, Barlaz, Jonsson, Björn, Rowland, Thompson, Galloway and Yamashita^{18, 19}