Summary

Polylactic acid (PLA) is a biodegradable plastic that is advertised to degrade in a composting environment while maintaining its integrity under normal use. The objective of this report is to summarize the results of composting PLA in laboratory composting system.

PLA film and sheet samples were placed in composting vessels with composted yard waste. PLA was put in the compost at 0%, 10% and 30% concentration on a weight basis. Samples of PLA and compost were taken once per week for four weeks to test for mechanical strength, pH and to monitor moisture. Exhaust gases were analyzed for carbon dioxide concentrations two times per week using a gas chromatograph. Statistical analyses were implemented to determine if PLA concentrations and composting time had an affect on the mechanical strength of the PLA, carbon dioxide concentration of the exhaust gases and the pH of the compost. Gel permeation chromatography and electron microscopy slides were used to determine if the molecular weight of composted PLA changed and if signs of microbial feeding could be seen.

Results from the experiment show that PLA degraded in the compost because of the reduction in mechanical strength, the carbon dioxide given off and the loss of molecular weight during composting. It was also shown that as the concentration of PLA is increased in a compost pile then the pH decreased during degradation. It was concluded that the safe amount of PLA to be placed in a compost pile is at or below 10 percent. If pH modification of the compost is needed then PLA concentrations exceeding 10 percent could be used to lower the pH in a four week period.

Introduction

Polylactic acid (PLA) is a polymer that can replace petroleum based polymers used to make single use items. The advantage of using PLA Is in its ability to degrade in a composting system while maintaining its integrity under normal use. This would allow a reduction in the waste stream to the landfill.

Another benefit of using PLA is that it comes from a renewable resource. Lactic acid can be derived from starchy products such as corn. The polymer PLA is made by a condensation reaction process which binds the lactic acid chains together. It degrades by first going through a hydrolysis reaction and then through microbial decomposition during which carbon dioxide and water are generated.

The objective of this report is to summarize the results of a study on controlled composting of PLA strips and films. The research objectives were to study PLA degradation, to recommend an efficient and economic composting environment and to determine the extent of degradation using physical property characterization.

Materials and Methods

PLA was extruded into sheets averaging 0.060" in thickness. They were cut into 1" x 3" strips for testing. Films were extruded using a film blowing die. The thickness of the films averaged 0.004" and were cut into 1" x 6" strips. The samples were placed into laboratory composting vessels (Figure 1) with previously composted (burned out) yard waste. The yard waste was composed of grass, wood mulch and tree leaves. It was composted for 7 months with grass being added periodically. The vessels had a 5" inside diameter and were filled 12.5" deep. This gave a height to diameter ration of 2.5.
The laboratory composting vessels were then placed in the composting system (Figure 2). This system supplied humidified air, passed through flow meters, into the composting vessels. External heat maintained constant composting temperature which was monitored by a data acquisition system. The exhaust air was directed through a two-way valve attached to a gas chromatograph (Perkin Elmer Corporation, Norwalk, CT) to measure carbon dioxide concentration.

The compost in the vessels were stirred once per week. During this time, compost and PLA samples were taken. The PLA films and strips were tested to determine their strength. The compost samples was used to monitor and measure the moisture content and pH. Carbon dioxide content of the exhaust gases was measured two times per week.

PLA was added to the compost in three different concentrations. They are 0%, 10% and 30% on weight basis. Each concentration was replicated three times. Four composting vessels were available to run the experiment. The concentrations were randomly assigned to the chambers over three time periods. Table 1 shows the assignments made to the chambers.

A statistical analysis was conducted to determine whether the pH and carbon dioxide (dependent variables) were affected by the concentration of PLA and/or by the time period in the compost pile. The models evaluated are:

Carbon Dioxide = Concentration + Concentration*Time = Time

pH = Concentration + Concentration*Time + Time

If the p value was found to be less than 0.05, then the model was considered significant. If the models were found significant then the concentrations were contrasted to determine if there were any differences, while using the same p value.

Electron microscopy pictures and gel permeation chromatography (GPC) tests were performed on the "before" and "after" samples of PLA sheets to determine microbial feeding and molecular weight.

Results and Discussion

PLA, through hydrolysis and microbial action, will break down into carbon dioxide and water. This occurs in a compost situation because of the available microbes, moisture and heat. The heat helps the moisture with hydrolysis and keeps the microbes active. Figure 3 shows a graph of the carbon dioxide measurements (replications for each concentration were averaged together for that day) and pH measurement over the four week time period. The carbon dioxide graph shows that during the first four days the carbon dioxide emission levels were approximately the same. After four days the carbon dioxide levels separate and the 10 and 30% concentrations had more carbon dioxide emissions than the 0%.

Statistical analyses show that the model was significant and the carbon dioxide level did depend on the time period and the concentration level. When the concentrations were contrasted, there was a difference between the 0% and the other concentrations but not between the 10 and 30% concentrations levels.

Figure 3 shows that at the end of the four week period the pH drops for the 30% concentration while the 0 and 10% concentrations remained similar. At this low pH level, the carbon dioxide emission dropped while the 10% concentration remained above the 0% concentration. It is speculated that the hydrolysis rate was faster than the microbial action, because the lactic acid would be more prominent in the compost, causing the pH to decrease.

Mechanical analysis shows that after one week the PLA strips and films break apart and have no strength left after being composted. This was not different for any of the concentrations.

The composting conditions were an average moisture content range from 49 to 55 % on wet basis, 52 degrees C average temperature and an average air flow rate of 25 ml min-1. There was adequate moisture and temperature for the decomposition of PLA. If the compost became too moist, water leached to the bottom of the composting vessel.

Figure 4 shows a graph of the GPC results. The results show that degradation of PLA occurred with composting. It also shows that there was degradation during extrusion of the PLA. This is attributed to a moisture content higher than recommended for processing PLA.

Electron microscopy slides of composted PLA (left) show definite cracking, which would contribute to the loss of mechanical strength. But, it was uncertain whether there was microbial feeding when compared to the electron microscopy slide of the extruded PLA (right).

Information about PLA has concentrated on its ability to degrade. It has been proven that PLA can degrade in a composting system. The next question is how much PLA should be placed in a compost pile for safe and effective composting. There was no difference in the amount of carbon dioxide given off between the 10 and 30 percent load levels. The only difference was the pH of the compost. If a low pH is desired for the compost, then above 10 percent should be placed in the compost pile. If a low pH is a concern, then the 10 percent level should not be exceeded.

Conclusions

Laboratory biodegradation chambers were used to degrade PLA films and sheets in yard compost. The chambers were used because of their ability to control the environment of the compost and to monitor gases coming off of the compost. Results from the test show that PLA is degrading in the compost because of the carbon dioxide given off, the reduction in mechanical strength and the loss of molecular weight during composting. It also was shown that as the concentration of PLA was increased in a compost pile then the pH will decrease during degradation. From the information above, it was concluded that the safe amount of PLA to be placed in a compost pile is at or below 10 percent. If pH modification of the compost is desired then PLA concentrations exceeding 10 percent can be used to lower the pH.

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