PRINTED ELECTRONICS: A LANDFILL SIMULATION STUDY TO ASSESS ENVIRONMENTAL IMPACTS
JAMES ATKINSON
Dr. Thomas Joyce - Dept. of Chemical and Paper Engineering, Western Michigan University
Dr. Margaret Joyce - Dept. of Chemical and Paper Engineering, Western Michigan University
First Analytical Labs - Raleigh, NC
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ABSTRACT
A landfill simulation study was conducted using EPA Method 1311 and SW 846 in combination to assess the potential environmental impacts that could occur by the landfilling of printed electronics containing silver flake, silver nanoparticle, and nickel conductive inks. It appears that the amounts of inks used to print electronics might pose a threat to the environment even at low application rates. It was found that the nickel ink used could have a potential environmental impact if printed electronics were landfilled in large quantities. The silver flake and nanoparticle inks did not exceed the concentration threshold named in TCLP procedure. However, due to the low ratio of ink to substrate in this experiment, it is recommended that more research should be conducted to fully evaluate these inks.
Keywords: TCLP, Printed Electronics, Environmental toxicity, Silver, Nickel, Nanoparticles, EPA method 1311, EPA method SW 846
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1. INTRODUCTION
Printed electronics (PE) are cost-effective ways to create an electrically functional device by printing with a conductive ink, typically containing silver, copper, or nickel, on various substrates, including paper and film. Examples of PE are RFID (radio-frequency ID tags), glucose sensors for diabetics, advertising displays, and LED lighting. PE are expected to be very useful in the near future for applications that require low-performance, low-cost electronics. (Subramanian et al.) Currently, the most common substrates being used are polymer films, glass, silicon, and ceramics. Paper is also a possible functional substrate, but further research is needed to ensure the functionality of the electronic circuit when printed on paper. Paper is an attractive substrate due to its low cost, recyclability, and sustainability. (Tobjörk and Österbacka)(Berggren, et al.) A key consideration is that the metals used in the inks are heavy metals, which are known to be toxic and will be scrutinized for regulation. Research suggests that nanoparticle silver does have a damaging effect on photosynthesis in in certain types of algae. (Navarro et al.) Even if the heavy metals did not reach the environment, a recycled paper mill would still be faced with the ultimate disposal of a sludge containing a high concentration of these toxic metals. It appears that production technology for PE has been the focus of most of the research and very little attention has been paid to the life cycle or end fate of the PE material or the heavy metals contained in PE. The exact disposal route will strongly affect the amount of Ag nanoparticles that might be released into the environment. (Fabrega et al.) A recent study also suggests that there needs to be a plan for recovering these metals, due to their high cost and also that it is a limited availability. (Oguchi et al.)
The way the inks are applied on the substrate depends on whether the inks are solvent-based, water-based, or UV curable. (Tobjörk and Österbacka) The functionality of the PE circuit is minimal on a paper substrate because of paper’s high roughness and absorbency, when compared with film and other substrates. Currently the biggest challenge in the printed electronics industry is finding suitable materials, besides film and other non-sustainable materials, to use for substrates. Paper is a leading candidate for inclusion as a substrate, but only a few specific paper grades are available and known to be functional. (Tobjörk and Österbacka)(Bollström et al.)(Kim, et al.) There are multiple nanoparticle inks and other materials available, but researchers are working on formulating functional inks with such materials (carbon nanotubes and other organic and inorganic conductive polymers) for successful PE. (Subramanian et al.)(Tobjörk and Österbacka)(Berggren, et al.)(Bollström et al.)(Ko et al.) Much research is being conducted to find a way to better improve the use of PE with paper as the substrate because its low cost and multiple applications make it an attractive substrate. (Subramanian et al.)(Tobjörk and Österbacka) Eventually, the fate of the toxic metal containing inks printed on paper versus film will become a significant factor for consideration. This would particularly be the case if there were significant differences in the ability of film and paper to bind or release the ink over the substrate’s life cycle. IT is desirable in the production of PE have a responsibility to society as a whole to protect the environment for the future generations by reclaiming heavy metals, i.e. gold, silver, copper, nickel, etc. This not only prevents environmental contamination, it promotes sustainability based on the use of natural resources.
According to the IDtechEx 2014-2024 forecast, the utilization of PE for consumer and military uses is expected to expand rapidly in the next ten years. (Ghaffarzadeh and Zervos) Research is needed to understand the effects that disposal or recycling these printed materials might have on the environment. Virtually no research has been done on the potential environmental problems that could be caused by recycling or landfilling PE materials and the toxic heavy metal-containing inks. Separation of PE items from the household good waste stream is difficult and uncertain, and therefore should a source of concern. (Niu and Li)(Köhler, et al.) Research is being conducted on nanoparticles and there is significant concern about the silver nanoparticle. For example, a study done by Dr. Samuel N Luoma in 2008 calls for in depth study and regulation on production and use of nanosilver in consumer products due to its, toxic, bioaccumulative, and persistence properties. There is debate about the mechanism by which the silver nanoparticle creates toxicity, but in general it is agreed that releasing silver into the water or waste stream has a great potential for ecological damage. (Navarro et al.)(Tolaymat et al.)(Xiu et al.)(Levard et al.) (Luoma)
The purpose of this research is to understand the potential environmental concerns that might arise from recycling or disposal of PE material. It is noted that most of PE material presently uses film as the substrate. Paper as a substrate for PE should be investigated to assess any advantages/disadvantages it might have on the environmental effect of disposal or recycling of these materials. For example, film might release the environmentally toxic conductive inks quite easily, while paper, through different binding mechanisms, may retain these toxic metals with it.
2. EXPERIMENTAL PROCEDURES
2.1 Sample Preparation
Paper and polyethylene terephthalate (PET) substrate samples were printed with conductive inks using laboratory scale proofing techniques. The silver flake and silver nanoparticle inks were printed using the flexographic process, while the nickel ink was screen-printed. The printed samples were cured using the Novacentrix (Austin, TX) Pulseforge 1200 photonic sintering unit. All of the print samples displayed some resistance somewhere on the sheet, and could therefore be considered representative of PE. Substrate samples were weighed prior to and after printing in order to quantify the weight of ink transferred onto the substrate. Table 1 below shows the average amount of ink transferred to the sheet for each substrate/ink combination. The sheet used to print the samples was delivered as an 8.5x11 inch sheet, but was cut in half lengthwise and joined to form a sheet 4.25x22 inches for printing.
Table 1: Printed ink weights
Sample / Average ink weight deposited – g/m2Nickel Paper / 0.1015
Nickel PET / 0.0473
Silver Flake -Paper / 0.0221
Silver Flake - PET / 0.0034
Silver Nano - Paper / 0.0351
Silver Nano - PET / 0.0109
2.2 TCLP Testing Procedures
EPA method 1311 is a landfill simulation procedure. SW 846 is a solid waste analysis procedure. These two standards were used in combination to create the testing methodology. The TCLP test is designed to determine the mobility of both organic and inorganic materials present in liquid, solid, and multi-phase waste. For this procedure to be carried out, the solid waste needed to be reduced in particle size. A crosscut office shredder was used in order to ensure that no particle was larger than 1 square centimeter in area. The extraction fluid was made by first measuring 5.7 mL of acetic acid and adding to 500 mL of reagent water. The next step is to add 64.3 mL of sodium hydroxide to this mixture and then dilute to 1000mL. The pH of the fluid after preparation was 4.93 ± 0.05. This was measured on a calibrated electronic pH meter from Fisher Scientific. The samples were separated according to deposited ink weights and placed into the extraction vessels, with a volume of extraction fluid equal to 20 times the weight of the waste sample. The extraction vessel, made of borosilicate glass by Wheaton Industries Inc. (Millville, NJ), and the rotary agitator by Associated Design and Manufacturing Company, model no. 3740-6-BRE (Alexandria, VA) can be seen below in Figure 1. The samples were agitated for 18±2 hours and then filtered through a 0.7 micron, acid-washed, glass fiber filter. The filtered liquid is analyzed as the TCLP extract.
Figure 1: TCLP Rotary Agitator and Extraction Vessel
The printed samples were arranged in three sets for TCLP extraction. The paper extraction samples used seven sheets. The PET samples used four printed sheets to achieve the weight indicated as minimally acceptable by SW 846, which is 30 grams of solid waste for each sample to be tested. The ink weights are significantly different between paper and PET, although are representative of commercial practice. The weights are also different between inks as well. PE are concerned with small features, so the low ink weights are an artifact of creating representative PE samples to analyze for toxicity. The average ink weight deposited for TCLP extraction is listed in Table 2.
Table 2: Ink weights deposited into extraction vessels
Sample / Ink weight into extraction -gNickel - Paper / 1.6821
Nickel -PET / 0.7843
Silver Nano - Paper / 0.3656
Silver Nano - PET / 0.0572
Silver flake - Paper / 0.582
Silver Flake - PET / 0.1802
2.4 AA Spectrometer analysis
The TCLP samples were collected and stored at 4oC until analysis. Each sample was acidified with 70% nitric acid below a pH 2. In this case, a 2% v/v of the nitric acid was sufficient to acidify the samples for analysis. Concurrently, standard samples of known concentration were made from a 1000-ppm solution purchased from Fisher Scientific (Waltham, MA). The standard solutions were made at 1, 5, 10, 20, 50, 100, and 500 ppm concentrations, respectively. The Varian AA240FS Flame AA spectrometer (Mulgrave, Victoria Australia) was used to analyze the acidified TCLP samples. Flame Atomic Absorption Spectrometer can be used to determine the concentration of the silver by plotting the Atomic Absorption (AA) values against a calibration curve and interpolating the values. Atomic Absorption Spectroscopy is a spectroanalytical procedure for the quantitative determination of chemical elements using the absorption of optical radiation (light) by free atoms in the gaseous state. Figure 2 below is a diagram showing how the AA spectrometer gathers its data.
Figure 2: www.orgspectroscopyint.blogspot.com
3. RESULTS AND DISCUSSION
3.1 Nickel Ink Results
3.1.1 Visual Observations
The previously described substrate extractions were performed and the extraction fluid was filtered through a 0.7 micron glass fiber filter. The PET samples appeared to readily release the metal into the extraction fluid, while the paper samples did not show a ready release of the metal into the extraction fluid. Figure 3 shows the glass filter fiber pads used. The filter for the PET sample is on the right and the paper sample filter is on the left. The initial observation was that the PET sample would definitely have a significantly higher amount of metal in the extraction fluid. This was based on the observation that the PET releases the ink quite readily. This could correlate to the release into the environment if PE printed with nickel ink was placed into a landfill.
Figure 3: Nickel Ink Extraction Filters
3.1.2 AA Spectrometer Analysis
The TCLP extracts were all collected and analyzed. Nickel ink appears to have surpassed the regulatory limit for hazardous material. The TCLP extract contained a concentration of 6.46 parts per million (ppm) on paper, and 6.82 ppm on PET. The wastewater regulation for nickel is 5 ppm. (“Wastewater Discharge Regulations -- Introduction”) This means the nickel ink used in this experiment presents a potential hazard if placed into a landfill. It is important to note that only about 5% of the total weight placed into the extraction vessel was nickel ink. Nickel inks are still largely in the developmental stages for use in PE, so it will be important to formulate these inks in the future to prevent the release into the environment, or should not be used for PE due the potential for environmental damage.
3.2 Silver Flake Ink Results
3.2.1 Visual Observations
The silver flake ink samples were collected, extracted and filtered. The filter papers for the PET samples appeared to release a more significant amount of metal into the extraction fluid than did the paper samples. This observation led to the belief that the PET sample’s potential for environmental damage was more significant than that of the paper PE samples. The filter papers are shown in Figure 5 below.
Figure 5: Silver Flake Extraction Filters
3.1.2 AA Spectrometer Analysis
The AA spectrometer in the lab at WMU has detection limits in the hundreds of parts per billion (ppb). The absorption numbers appeared inconsistent, so First Analytical Labs (Raleigh, NC) was contacted and agreed to carry out the analysis by Inductive Coupled Plasma Mass Spectometry (ICP-MS) for verification. This spectrometer has a much lower detection limit, as in fractions of ppb. The silver flake samples were analyzed by FAL and the concentration levels were quite low. The paper sample had a concentration in the extraction fluid of 0.054 mg/L. The PET sample had a concentration of 0.117 mg/L. This translates to less than 1 ppm for both of the samples. The maximum concentration limit for TCLP method 1311for silver is 5 ppm.(“The EPA TCLP: Toxicity Characteristic Leaching Procedure and Characteristic Wastes (D-Codes)”) Both of these samples are below the limit, but it is important to note that PET sample had more than double the amount of silver in the extraction fluid. It is also important to note that the weight of ink placed into the extraction vessel was quite low. For the PET samples, the ink only accounted for approximately 0.6%, and around 2% of the total weight for paper. This artifact is a result of the thin ink films printed and the nature of PE having small features.