PLA (polylactic acid) filament is a popular material for 3D printing, owing to its low cost, ease in handling, and safety. As 3D printing becomes more popular and accessible to the average consumer, more offcuts such as supports, purged filament, and failed prints are entering the waste stream each year, adding to existing landfill and oceanic pollution issues. Due to immaturity of the 3D printing industry, there are no current studies that specifically track the amount of PLA polymer entering the waste stream from individuals, workshops, maker spaces, and offices. According to a news article by UC Berkeley, their 100+ printers generate at least 600 lbs (approximately 272 kg) of 3D printing-related waste each year. [1]
Current plastic recycling efforts mainly focus on the following polymer types: Polyethylene terephthalate (PET), High density polyethylene (HDPE), Low density polyethylene (LDPE), Polyvinyl chloride (PVC), Polypropylene (PP), and Polystyrene (PS). Other forms of plastic are commonly discarded or incinerated, as the effort and energy needed to sort, clean and recycle may not be feasible. PLA polymer currently falls in the category where it mainly is disposed of.
Despite this, many PLA manufacturers claim that their filament is fully biodegradable in normal waste streams. [2][3] These claims mislead consumers into further consumption, despite the risks of microplastic shedding, pollution, and further damage to ecosystems. PLA can only decompose under specific conditions, which the ocean nor most industrial waste processing plants do not have. The abiotic decomposition of PLA is mainly achieved via hydrolysis and thermal decomposition. UV exposure (photodegradation) also assists in the degradation of the polymer via breakage of polymeric chains and free radical production.
The hydrolytic reaction of PLA is shown below:
$$ -COO+H_2O->-COOH+-OH^- $$
Decomposition of PLA is best achieved within water at elevated temperatures of 60°C and above. Seawater maintains an average temperature of 25°C, which is far below the ideal temperatures for decomposition of PLA. A study conducted by A. R. Bagheri et al from Macromolecular Chemistry II and Bayreuth Centre for Colloid and Interfaces, University of Bayreuth, investigated the effects of PLA in oceanic environments for a year, and showed no loss of polymer mass. [4]
The aim of this experiment is to design and construct a setup where:
It is known that PLA can decompose in sufficiently hot water over a period of time, but the difficulty is designing and constructing a setup that can fully decompose PLA in a safe, cost efficient and accessible manner. To make the best use of the environment and resources available, the setup utilised solar thermal energy to heat the water in which the PLA will be submerged to 55°C and above. PLA pieces would ideally be fully submerged during the entire duration of this experiment to allow hydrolysis to occur efficiently.
To investigate PLA manufacturer claims of their product being “eco-friendly” and “biodegradable”, 2 identical setups will be constructed, the only difference being the brand of PLA being decomposed. Setup A will have 1kg of standard PLA, whereas Setup B will have 1kg of PLA marketed to be “biodegradable”.
The experimental steps are shown below:
Figure 1: Experiment flowchart
Due to the nature of the experiment, where slow changes occur every day and may not be visible to the casual observer, the experiment was to be continued despite no visible loss of mass observed. However, to ensure that the setup is not left outside for no good reason, daily maximum temperatures reached were noted with scrutiny to ensure that the setup reached minimum temperatures of 55°C.
Two SCD-30 multifunction sensors were placed inside the glass containers to continuously measure the temperature, humidity, and total CO2 concentration within the experimental setup. CO2 concentration is to be monitored to ensure that the decomposition remains abiotic.
Keeping in mind the total setup cost and accessibility to laymen, items should be easily acquired from common consumer brands, such as IKEA, or neighbourhood stationery shops. 1kg of PLA scraps were placed inside a thick glass container purchased from IKEA, exposing the scraps to direct sunlight exposure and creating a greenhouse effect within. To increase experimental temperatures, a piece of Fresnel lens was placed on top of the glass lid to concentrate sunlight, further heating up the water.
Figure 2: Experimental setup
Figure 3: Experimental setup in action
During data collection, 2 NDIR CO2 sensors (model: Sensiron SCD-30) were used to sense the CO2 levels within the glass container. Both sensor modules are also equipped with temperature and humidity sensors that allows them to quantify exactly how hot and humid the inside of the container is.
The data collected from this has several key observed patterns: