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Abstract
This paper proposes the recycling of poly (methyl methacrylate) plates, formerly used in LCD monitors to produce polymer optical fibers without cladding for sensor systems and a discussion about the fabrication process of the fiber cladding is briefly presented. After disassembling LCD monitors the acrylic plate is cleaned and submitted to an extrusion process. Extrusion temperatures of 220°C, 230°C and 240°C were applied, and the produced polymer fibers were characterized by infrared and visible spectrometry, as well as evaluated for thermal analysis through differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Furthermore, a refractive index sensor was developed with the recycled fibers. Results show that the recycled fiber refractive index sensor is linear (R2 = 0.99) and presents a sensitivity of more than 4 times higher when compared to a sensor using a commercial POF.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The government, industries, society and educational institutions, in various segments, should make a commitment regarding the recycling and reuse of electronic equipment [1,2]. According to the United Nations Environment Program (UNEP) report, 40 million tons of electronic waste are generated per year worldwide, mainly from developed countries [3]. The European Union annually generates 8.3 to 9.1 million tons [1-2, 4]. Moreover, they estimate that the sale of electronic equipment is expected to grow substantially in developing countries over the next 10 years [1]. In Latin America, Brazil ranks first as waste producer and the second is Mexico [1].
The environmental impact of electronic equipment waste has attracted attention in recent years. This problem is caused by the reduction of prices due to high productivity of the current industries. Thereby, new technologies intensify the problem, because it increases the replacement of electronic equipment [5]. It is estimated that 1 billion computers will be discarded, which causes to an estimated discard of elements like lead, iron, aluminum and plastics of about million of tons [5]. To minimize this problem, various solutions are being proposed, such as new legislation, development of products from recycled materials, and implementation of appropriate methodologies for discard of electronics waste [6–8].
Recycling is an important option that has been implemented and encouraged by governments, institutions and non-governmental organizations. Recycling electronic waste contributes to the preservation of the environment and reduce the extraction of non-renewable natural resources. Mainly in developing countries, the possibility of marketability of reprocessed materials is encouraging new industries to recycle some materials [9–12]. One of the materials to be recycled include polymers of different types, such as polycarbonate, polyethylene terephthalate (PET), polyurethane, polylactic acid (PLA) [13]. There are, however, different methods of recycling them. The reuse is the most common, which comprises of employing the component for the same application as before without any chemical or mechanical processing [14]. In the chemical recycling method, the polymer is returned to its hydrocarbon component for the processing of new polymers [14]. Whereas, the physical or mechanical processing comprises of reprocess the polymer to produce different component that can be employed on different applications [9]. Since the component to be recycled is already made of poly (methyl methacrylate) (PMMA), the mechanical recycling is the most suitable method for the recycled fiber proposed in this work.
Optical fibers are compact, lightweight, allow multiplexing systems, electromagnetic fields immunity and present chemical stability [15]. These advantages enable the application of optical fibers as sensors for different parameters like strain [16], force [17], curvature [18], refractive index [19], acceleration [20], among others. Although optical fibers are generally made of silica, polymer optical fibers (POF) present the additional advantages of higher fracture toughness, higher strain limits and biocompatibility when compared to silica optical fibers [15].
This paper proposes a new application for recycling acrylic sheets that are used as light dispersion agent in LCD monitors, where PMMA is a polymer used for this purpose due to its transparency and low optical losses, as well as high durability and no toxicity. PMMA has characteristic of being thermoplastic that enables its recycling process, thus, being the main motivation of this work. The contribution of this work is the process of recycling PMMA contained in monitors for the construction of POFs for optical sensors applications.
This paper is organized as follows. Section 2 presents production of recycled PMMA optical fibers at different temperatures and the methods for their optical and thermal characterization. Section 3 is presented the results and discussion of all obtained data and the application of the recycled fibers as a refractive index sensor. Conclusions and future works are discussed in Section IV.
2. Methods
2.1 Preparation of the PMMA samples
The first stage of this work consisted in dismantling of LCD monitors to remove the PMMA. The removed sheets of PMMA were weighed (100 grams), washed with water and left to air dry. The sheets were cut into pieces and crushed with a model 300 Sm (Retsch, Germany) mill. Then, the crushed material was dried in a vacuum oven during 24 hours. After the drying period, the material was processed in a mini-extruder Haake MiniLab II (Thermo Scientific, USA), with twin screw at temperatures of 220 °C, 230 °C and 240 °C, under 60 rpm. Although the PMMA glass transition temperature is about 110°C [21-22], operating limitations of the employed extruder led us to work with temperatures above 200°C. The temperature adjustment was carried out empirically, since in temperatures lower than 200°C the produced fiber did not present suitable morphology for applications in optical devices. Since the heater element is physically positioned at point “a” and the extruder output is on point “b” (see Fig. 1), there is possibly a temperature gradient due to heat exchange with the environment until the fused PMMA reaches point “b”. Thus, it was needed to work with higher temperatures to keep whole region of the axis of rotation with the ability to drain the PMMA above its glass transition temperature.
Fig. 1 Mini-extruder at temperatures of 220 °C, 230 °C and 240 °C, under 60 rpm. Points a and b indicate the position of the extruder heater and output, respectively.
The selected extrusion orifice diameter was similar to the diameter of commercial POF, i.e. 1 mm with an average diameter fluctuation below 10%. Nevertheless, it is possible to produce different diameters by changing the extruder output orifice. Figure 1 shows the cross-sectional view of the mini-extruder employed.
2.2 Thermal characterization
In order to evaluate the structural characteristics and the presence of impurities of the obtained material, samples taken at different temperatures were studied via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). DSC is a thermo analytic technique that compares the enthalpy variation of a material under different temperatures and a thermally inert reference. Whereas, TGA comprises of measuring the mass variation of a sample with different temperatures. A variation of TGA is the differential thermal analysis (DTA) where there is a derivation of the mass variation with respect to temperature. In DTA, the peaks are related to the loss of mass at a certain temperature. TGA was performed using the Q600 SDT (TA Instruments, USA). Whereas, DSC Q200 (TA Instruments, USA) was employed on the DSC analysis. The results obtained for the recycled fibers at 220°C, 230°C and 240°C were compared with the material removed from the monitors without heat treatment. DSC analyzes the heat flow variation of each sample with the temperature. Whereas, the variation of the sample’s weight with the increase of temperature is analyzed by TGA. Finally, the weight derivative with respect to the temperature is evaluated through DTA.
2.3 Optical characterization
Optical characterization was made with two different techniques: infrared spectroscopy and absorbance in the visible region. The degradation of the PMMA fibers produced in different temperatures was evaluated by infrared spectroscopy. In this test, the PMMA plates were thinned with solvent. Thus, two grams of the plates (before the extrusion process) were dissolved in acetone at room temperature and left to air dry on a surface of glass, which leads to the formation of plates of 1 mm thickness that were applied in the process of checking the absorbance in the region from about 700 nm to about 4000 nm. For the analysis of the fibers after the extrusion in different temperatures, the attenuated total reflection (ATR) probe was employed. It comprises of a crystal plate where each fiber is positioned on the top of it and the evanescent wave resulting from the total internal reflection on the sample is analyzed [23].
The analysis of the absorption in the visible range was made with the equipment 800XI (FEMTO, Brazil). In order to characterize the produced PMMA fibers processed in different temperatures, the samples were displayed on plates with thickness of 1 mm and submitted to the absorption process. In this characterization, there is also a comparison between the fibers extruded in different temperatures and the material without the extrusion process.
2.4 Experimental setup of the recycled POF refractive index sensor
In order to present potential applications for the polymer fibers developed in this work, a recycled fiber of about 3 cm, produced at a temperature of 220°C was used to build a refractometer due to its better optical transmission characteristics.
The fiber produced at 220°C is bended in a 25° angle to increase the sensor sensitivity. Its ends are connected to a photodetector and a laser operating at 630 nm. The system is placed in a medium containing water and glycerin that is gradually added in order to verify the changes in refractive index of the medium. The refractive index variation is corresponding to light intensity variation observed in the photodetector. Figure 2(a) shows the block diagram of the experimental setup. Whereas, Fig. 2(b) presents the sensor on different solutions of glycerin and distillated water that leads to different refractive indices. The distillated water volume is 8.0 mL and 1.0 mL of glycerin was added at every minute until a total volume of 10.0 mL of glycerin is added. Since the temperature is constant, the refractive index of each water-glycerin mixture can be obtained from well-known relations between the glycerin and water volumes such as the one presented in [24].
Fig. 2 (a) Block diagram of the experimental setup employed for the refractive index sensor. (b) Recycled-POF sensor positioned on a water-glycerin solution.
3. Results and discussion
The results of thermal and optical characterization lead to the assessment of the main features of produced PMMA fibers and design conditions for specific applications. Moreover, these results allow defining the best extrusion temperature, among the ones tested, for the application of the recycled-POF as a refractive index sensor.
3.1 Thermal characterization results
The thermal analysis enabled the evaluation of possible structural variations of the processed fiber. This work considered DTA and TGA techniques. Comparing with the result of the same material without heat treatment, the polymer fiber obtained in the extrusion process has small incidence of impurities. As can be seen in Fig. 3, there are only minor deviations on the curves with respect to the thermal data obtained on both TGA and DTA, which evidences the lower amount of impurities in the composition of PMMA.
Fig. 3 TGA and DTA analysis of PMMA before and after the extrusion process at different temperatures.
Physical changes in the PMMA due to the presence of impurities are relatively common. The sample degradation temperature processed at 220 °C showed the same value of temperature as the material without heat treatment. However, the temperature difference compared to the other samples indicates that there is a small presence of impurities due to the small variation presented [22], [25-26]. This behavior can be explained by the conformational changes between chains of a polymer that can generate changes in the thermal properties of the fiber.
It is interesting to note that the process to produce the fibers with a mini-extruder presents a significant variation temperature between the heating structure and the external environment (at approximately 23 °C). Therefore, the process carried out in PMMA at lower temperatures takes less time for the chains in the polymer organize themselves in the process cooling to the room temperature due to the smaller temperature difference than the one obtained on the polymer processing at higher temperatures.
However, the process carried out at higher temperature provides a better organization of the polymer chain, generating more similar chains of the material before the extrusion. Thus, it was interesting to evaluate the samples produced via DSC analysis [25-26] to obtain another approach of the temperature effect that can provide a better understanding of the presence of impurities.
Regarding to DSC data, two different processes can be seen in Fig. 4. The first one occurs at about 110°C and is related to the glass transition. Whereas, the second process refers to the merger process [27] and occurs at about 200°C. As it can be observed, the curve of the sample processed at 220°C showed the greatest heat flow between all samples tested. This effect is related to the poor organization of polymer chains due to the lower cooling process temperature gradient. However, smaller cooling temperature gradient can be advantageous, since the maximum reached temperature is far from the polymer degradation temperature, generating a polymeric structure less susceptive to the thermal degradation [22], [25-26]. Whereas, samples processed at higher temperatures tends to exhibit a higher absorbance, which can be related to the amount of impurity present in the samples due to process at high temperature, that promotes thermal degradation of polymers. Although the TGA analysis shows a small variation of the mass with the temperature increase that can be related to the low presence of impurities, the DSC analysis presents higher variation of the heat flow, which is related to the thermal degradation and presence of impurities in the processed polymer. For this reason, it is necessary to apply both TGA and DSC tests to obtain a broader understanding of the extrusion process on POFs. Nevertheless, the optical characterization provides a quantitative analysis of the absorbance that also can be related to the presence of impurities.
3.2 Optical characterization results
As optical fibers present a cylindrical geometry, there was a partial adherence on the spectrometer for infrared characterization tests. This means that the ATR probe was partially measured. Thereby, the absorption intensity values were lower than the observed for the plate, i.e. the material removed from the LCD monitor. Nevertheless, the measured wavelength values were very close in all cases. Figure 5(a) shows the PMMA absorption measured before performing the extrusion process. Whereas Fig. 5(b) shows the PMMA absorbance after the extrusion process on each of the three temperatures tested. It was possible to verify that there was no degradation of the PMMA functional groups, since the absorbance of the PMMA samples with respect to the wavelength before and after the extrusion are very similar. Furthermore, the same wavelengths caused a sharp decrease on the absorbance curve when comparing the samples before and after extrusion with different temperatures. Table 1 presents the attribution of each sharp decrease of the indicated wavelengths on the absorbance curve presented in Fig. 5. Therefore, the extrusion process keeps the chemical characteristics of the polymer, which enable its application in the fabrication of optical fibers.
Fig. 5 (a) Infrared PMMA before the extrusion process. (b) Infrared PMMA after extrusion at different temperatures.
Table 1. Attribution of the decrease absorbance at each wavelength
Figure 6 shows the spectral data in the visible spectral region of fibers processed at different temperatures with raw and recycling material. Raising the process temperature may promote thermal degradation of the PMMA increasing its characteristic attenuation due to the emergence of impurities. This characteristic can be observed in the obtained data for the samples produced at higher temperatures (230°C and 240°C). Whereas, the sample produced at 220°C presents a behavior similar to the PMMA before the extrusion process. Therefore, the optimal temperature for the extrusion process, among the ones tested, is 220°C.
Fig. 6 Analysis absorption in the visible PMMA before and after the extrusion process at different temperatures.
3.3 Recycled POF refractive index sensor results
To demonstrate the viability of using the recycled PMMA material as a sensing element, a refractive index sensor based on POF is proposed. The results obtained are shown in Fig. 7, where it is possible to observe the good detection capability of the system. The sensor was tested in different solutions of glycerin in water, inferring a refractive index between 1.3342 and 1.3680. The correlation coefficient of the sensor response with a linear regression is 0.99, which means that the sensor is linear. In addition, the root mean squared error (RMSE) between the measured and reference refractive index is 0.02. For the sensor variation on the measurement range, it is possible to observe that the sensor proposed presents high sensitivity (about 31 u.a./RIU). The sensitivity of the sensor with the recycled-POF is higher than the one presented in literature [19] for a commercial POF (about 7.15 u.a./RIU). Commercial POFs presents a cladding of fluorinated polymer that presents a lower refractive index than the core and is employed to reduce optical power transfer from fiber core to the POF surroundings, since conventional POFs are designed for transmission and not for sensing applications. However, for sensors applications based on optical power variation, it is desirable that the POF presents higher attenuation with the variation of the measurand. In order to achieve its higher variation, a side-polish is generally made on commercial POFs that removes the fiber cladding and part of its core [19]. This leads to a reduction of the guiding properties of POFs [19] and the increase of the power leakage along the polished region that can increase the sensor sensitivity due to better interaction with the measurand [28]. Therefore, cladless fibers will interact closely with the measurand than any mechanically modified conventional POFs. Since the recycled-POF does not have cladding, it presents higher attenuation with the refractive index variation than commercial POFs. For this reason, the POF proposed in this work may be more suitable for sensors applications than commercial POFs.
4. Conclusion
As electronic waste is growing across the world, our research seeks for new solutions to reuse and recycle these materials in order to preserve the environment. For this reason, this paper proposed recycling PMMA plates, formerly used in LCD monitors of notebooks, to produce optical fibers for sensor systems. The processing of polymers obtained after recycling PMMA plates from notebooks resulted in polymeric fiber with good transmission characteristics.
The fibers obtained from recycling material using a commercial regular extruder present only small degradation of the PMMA for the tested temperatures. This result indicates the possibility of producing polymer optical fibers with good light transmission characteristics for applications in fiber sensors or even in telecommunications, without requiring high temperatures. The recycled PMMA POF was employed in refractive index sensing, which presents high linearity and sensitivity higher than the one obtained with commercial POFs [19].
Other sensor systems can be set and detect different material and physical properties of a medium, such as onset measures for characterizing petroleum, turbidity levels, and chemical composition of mixtures, temperature measurement or evaluation of tilt changes as inertial sensors in rehabilitation engineering, which are topics for further investigations. Another future work is the development of a cladding layer on the recycled POF presented here to employ it in different photonics applications, which can be done by another extrusion or chemical deposition of a material that presents lower refractive index than PMMA material.
Acknowledgments
This research is supported by CAPES (88887.095626/2015 -01), FAPES (72982608), CNPq (304192/2016 -3), Petrobras (23068.013529/2012 -56) and FCT (SFRH/BPD/109458/2015). This work was funded by FCT/MEC through national funds and when applicable co- funded by FEDER- PT2020 partnership agreement under the project UID/EEA/50008/2013.
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