W. Lamar Miller, PhD
University of Florida
State University System of Florida
FLORIDA CENTER FOR SOLID AND HAZARDOUS WASTE
MANAGEMENT
2207 NW 13 Street, Suite D
Gainesville, FL 32609
Report #9l-2, May 1991
EXECUTIVE SUMMARY
The primary objective of this study was to evaluate the degradation of commercial polyethylene and polystyrene as a function of their exposure to various environmental conditions selected to enhance photodegradation and biodegradation. The plastics selected for these studies were consumer products and were not classified as degradable by the manufacturers. The consumer products used were polyethylene freezer bags of 0.7 mils thickness and bags of 1.75 mils thickness. The foamed polystyrene products tested were hot drink cups and picnic plates. Samples from these products were exposed to environmental conditions at the following sites: IFAS roadside, IFAS wetlands, IFAS submerged, beach roadside, beachfront, beach intertidal, beach submerged, 35 degree anaerobic digester, 55 degree anaerobic digester, yard waste compost and municipal compost.
The amount of degradation which occurred in polystyrene samples was determined based on the loss of molecular weight of the sample, the loss of flexural strength of the material and the loss of tensile strength. Polyethylene degradation was measured by determining the loss of tensile strength of the material.
Degradation of the polyethylene bags varied with material thickness, orientation induced during manufacture and exposure site. The samples which showed the most significant loss in elongation to break were those exposed at the beachfront, beach roadside and IFAS roadside locations. At these sites, over 75% of the original elongation was lost after eight weeks of exposure for the 1.75 mil thickness, and over 98% for the 0.7 mil. Samples with the least amount of degradation had less than 40% loss of elongation at 120 days for the 1.75 mil material, and generally less than 55% loss for the 0.7 mil. These sites were the IFAS submerged and floating locations, beach submerged and digester exposures. The loss of elongation for the samples placed in anaerobic digesters averaged less than 10%. The 0.7 mil material showed a greater tendency to lose elongation ability than did the 1.75 mil. An edge effect appeared to be observed during the testing, but could not be correlated from available data. However, this effect could indicate that some portion of the observed tensile strength loss is a result of accelerated decomposition of the exposed edges of the polyethylene films.
Degradation of foamed polystyrene varied by type of product and exposure location. Gel permeation chromatographic analysis indicated that more than 20% of the average molecular weight value was lost after 120 days for picnic plates and cups at the IFAS roadside, beach intertidal, beachfront and IFAS floating locations. This was also true for cups at the beach roadside site.
The flexural strength of the foamed polystyrene materials was determined by utilizing four point bending analysis. The foamed polystyrene materials would rarely actually fracture or rupture prior to the ASTM specified strain value for test termination. For the plates, failure occurred by creases forming at or near the loading points. A significant loss in flexural strength was observed for the IFAS roadside, beach roadside and beachfront locations.
An 80% loss in flexural strength occurred after only 720 hours (30 days) of exposure for samples in each of these locations. For the cups, neither creasing or actual rupture of the material occurred frequently. None of the exposed samples showed a significant loss in flexural strength after 120 days.
The tensile strength of the polystyrene materials showed the most significant change at the beachfront, beach roadside and IFAS roadside locations. The plates and cups at these locations lost approximately 40% of their original tensile strength after 120 days. Other exposure sites did not show a significant loss in tensile strength.
This report concludes that the loss in tensile properties and molecular weight of foamed polystyrene consumer products of the nature tested did not occur at a rate that would result in degradation of the materials under common exposure conditions. Loss of tensile properties may occur sufficiently rapidly in very thin films of unstabilized polyethylene, but this rate of degradation diminishes rapidly with thicker materials.
No significant degradation was observed to take place in either polyethylene or foamed polystyrene by a biological pathway in this study. Essentially all of the tensile loss observed is attributed to either mechanical loss or photolytic degradation, but in no case is it concluded that the samples tested would qualify to be determined as environmentally degradable within the context of the Florida statutes.
Thin film polyethylenes and materials such as a sensitized foamed polystyrene are available in commercial quantities and may be sufficiently degraded by exposure to sunlight to meet degradability standards. The more important question, however remains, what is degradable.
The regulatory agencies will have to define the term in terms of physical and chemical parameters which can be reproducibility measured, and in terms which can result in reproducible exposures. Requiring outdoor exposures gives rise to many uncontrolled parameters. Relying on tensile properties results in too much variation as a function of manufacturers methods. These and other variables need to be addressed before the term "degradable plastic" will have real meaning.
At the time of the initiation of this study there were no promulgated rules governing exposure and testing of the materials. The Florida Department of Environmental Regulation has since that time issued rules on testing of polyethylene (FAC 17-707).
The limited term of this study did not allow development of appropriate test procedures or definitions of degradation. This study was expected to be a preliminary study in that effect.
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