Polymers and Degradable Resins

In an increasingly ‘Green’ world, some companies seek advantage by claiming that their products are biodegradable. The Federal Government requires that such claims be supported by scientific evidence.

Anaerobic biodegradation as occurs in landfills can be simulated in the laboratory under controlled conditions using compost and landfill leachate incubated at temperatures seen in landfills. Many materials naturally degrade when exposed to landfill conditions while others such as plastics do not.

To aid plastic materials to biodegrade or to speed-up the degradation process, plastic producers often add ingredients to the plastic resin that act as food for microbes commonly found in landfills. The microbes attack the plastic structure, allowing a more rapid breakdown of the molecular chain.

SR5000 was added to Foamex urethane in 1% and 2% concentrations and evaluated for weight loss after six months. The two samples, one incubation jar with foam samples in each concentration recorded a mass loss of 1.7% and 2.9% loss respectively. While the mass loss by weight of solid plastics can be fairly accurate, mass loss of foamed plastic by weight loss is a more conservative method as the cellular structure of the foam traps denser compose particulate preventing a more accurate weight loss calculation.

Scope

To determine the degree or percentage of anaerobic biodegradation of an EDS Group project 100919L, for two different Foamex Polyurethane foam preparations under a controlled composting environment designed to yield reproducible and results that resemble composting conditions.

Background

Biodegradation is defined as the process wherein organic materials are broken down into simple components such as carbon dioxide and methane through the metabolic activity of a wide variety of microorganisms. Organic materials may be degraded aerobically (with oxygen) or anaerobically (without oxygen). Degradation of plastics is observable by changes in the physical properties such as mass loss. Mass loss can be both visually observed and quantified by sample weights before and after exposure to microorganisms. Anaerobic biodegradation occurs in the absence of light deep within a landfill when the organic materials are exposed to moist soil containing microorganisms.

Simulated landfill conditions are used in the laboratory to evaluate the biodegradation of materials. Prepared compost, saturated with landfill leachate containing microorganisms typically found in landfills provides a controlled environment for such evaluations. Several methods exist for the determination of biodegradation of plastics materials. ASTM D-5338 Determining Aerobic Biodegradation of Plastic Materials Under High Solids Anaerobic Digestion Conditions utilizes both gaseous and mass loss measurement techniques for determining the biodegradation rate. This study utilized mass loss only for determining the degree of degradation. Mass loss measurement alone is a more conservative measurement than gaseous measurement insofar as solid particles trapped within the cellular structure of the test specimen add weight to the final results, reducing the measured mass loss values. However, degradation can be seen visually as a discoloration, embrittlement, and/or mass reduction.

Materials Tested

Two Foamex open cell flexible polyurethane foam formulations, each containing Sierra Resin SR 5000 in either 1% or 2% concentrations were molded in large blocks, cut into sheets and die cut into ASTM dog bone shapes for biodegradation testing. One material, identified by the manufacturer as 12-2-10A-3 and referred to as 1% SR5000A contained 1% SR5000 by volume while the second material, identified as 12-2-10A-6 and referred to as 2% SR5000B contained 2% SR5000 by volume. In addition to the test samples, a negative control, identified by the manufacturer as1-12-11A-7 did not contain Sierra resin SR5000. Speaking of Code Complexity.  A positive control, chromatography paper, used for its ability to easily degrade in landfills, was also molded into dog bone shapes and tested.

Test Methods

Accepted methods for simulating landfill conditions in the laboratory are prepared by using compost comprised of leaf scraps, soil, household vegetables, and chipped wood. For each tested Foamex material, six test specimens in a dog bone shape along with both positive and negative controls also in a dog bone shape are photographed and weighed. They are layered in a one-liter jar with compost to separate and prevent contact with other test specimens. Jars are completely filled with compost to minimize air space, hydrated to 50% ± 10% by weight with landfill leachate from the Crapo Hill Landfill in New Bedford, MA and incubated at 58°C in the absence of light. At predetermined time periods, jars are inspected, weighed and rehydrated to specification. After pre-determined time periods, jars are removed from the incubator and the test specimens removed, cleaned, dried, weighed and photographed.

Results

One liter glass sample containers, each used to contain the Foamex urethane material test specimens and controls were removed from the incubation chamber after 45 days. Upon inspection of the contents, it was observed that the positive controls in both jars had decomposed to the point that there were no longer recognizable positive control segments in either jar, indicating that an anaerobic biodegradation environment did exist within each jar.

Weight measurements and change for the 1% & 2% SR resin specimen at 180 days ranged from negative 10.1% to negative 18.3% change for the 1% SR and negative 4.2% to negative 30.5% for the 2% SR, averaging a loss of 14.1% and 16.0% respectively. Positive controls and conditional coverage in both jars degraded completely while the negative controls for the two specimens measured a 12.4% loss for the 1% SR and a 13.1% loss for the 2% SR resulting in an overall loss of 1.7% for the 1% SR and 2.9% loss for the 2% SR. Test specimen #6 from the 2% SR test jar showed a positive 13.4% weight gain most likely as a result of an inability to remove compost particulate from the test specimen.

This specimen was considered an outlier and not included in calculations. Figures 2 illustrates the cellular wall before testing while Figure 3 illustrates the effects of degradation of the cellular structure and walls after 48 days. Figures 4 and 5 illustrate the test specimen dog bones prior to testing. Figure 6 illustrates test specimen degradation after 152 days in a landfill environment while the control specimen 7, exposed to the same environment in jar #9 shows little degradation.

Weight loss variation between test specimens exists both in the test jars as well as in landfills. We believe this occurs as a result of contact between specimens, preventing microbial contact with the plastic. This condition mimics landfills, thus sample weight change is averaged rather than shown as maximum values.