Meaning
Measurement unit describing the rate of radioactive decay within a polymer sample to determine the proportion of modern biological material present. Carbon 14 activity is expressed in disintegrations per minute per gram of carbon and shows the amount of the unstable isotope remaining in the material. This value differentiates between resins sourced from petroleum and those derived from recent plant matter.
The method works because fossil fuels have been isolated from the atmosphere for millions of years, causing their carbon 14 to decay completely. It ceases to provide useful data once the biogenic content falls below the detection threshold of the liquid scintillation counter.
Decay Quantification
Living plants constantly absorb carbon 14 from the atmosphere, maintaining a specific level of activity that remains constant until the plant is harvested. When these plants are processed into polymers, the radioactive clock begins to tick as the isotope decays without replenishment. The carbon 14 activity of a bio based plastic shows how much time has passed since the feedstock was part of a living organism.
Because polymer production usually occurs shortly after harvest, the activity level remains close to the modern atmospheric standard. Testing labs compare the measured activity of a sample against a known reference standard for oxalic acid. This comparison allows for the calculation of the percentage of modern carbon in the resin blend.
A sample with half the activity of the standard is considered to have a fifty percent biogenic content.
Biogenic Fraction
Manufacturers use this measurement to substantiate environmental claims and satisfy regulatory requirements for sustainable product labelling. High carbon 14 activity confirms that the carbon atoms in the polymer chain are renewable rather than fossil based. This data is critical for companies participating in carbon credit schemes or marketing compostable plastics.
The presence of the isotope does not change the physical or chemical properties of the resin, so a bio based polyethylene behaves identically to its fossil counterpart. It provides a definitive way to audit the supply chain without relying on mass balance accounting. Moulders can use these results to prove to their customers that the material they use matches the specification on the purchase order.
The activity level acts as a permanent chemical signature of the resin origin.
Accuracy Threshold
Variations in atmospheric carbon 14 levels over recent decades require the use of correction factors to maintain the precision of the activity measurement. Isotopic fractionation can also occur during the chemical processing of the feedstock, slightly altering the expected decay rate. To ensure accuracy, laboratories must remove all inorganic carbon and surface contaminants from the polymer before combustion.
The resulting CO2 gas is then measured to determine the specific activity of the sample. Modern accelerator mass spectrometry has largely replaced traditional decay counting because it requires a much smaller sample size. Small errors in the measurement can lead to a miscalculation of the bio based percentage by several points.
Every report includes a margin of error that reflects the statistical uncertainty of the decay count.
