UV Sensor Based on Fiber Bragg Grating Covered with Graphene Oxide Embedded in Composite Materials

sensors

Letter UV Sensor Based on Fiber Bragg Grating Covered with Graphene Oxide Embedded in Composite Materials

Piotr Lesiak 1,* , Karolina Bednarska 1,2 , Krzysztof Małkowski 1, Łukasz Kozłowski 1, Anna Wróblewska 1, Piotr Sobotka 1, Kamil Dydek 3 , Anna Boczkowska 3 , Tomasz Osuch 4 , Alicja Anuszkiewicz 4 , Wojciech Lewoczko-Adamczyk 5, Henning Schröder 5 and Tomasz Ryszard Woli ´nski 1

1 Faculty of Physics, Warsaw University of Technology, 00-662 Warszawa, Poland; [email protected] (K.B.); [email protected] (K.M.); [email protected] (Ł.K.); [email protected] (A.W.); [email protected] (P.S.); [email protected] (T.R.W.) 2 Centre for Advanced Materials and Technologies CEZAMAT, 02-822 Warszawa, Poland 3 Faculty of Materials Science and Engineering, Warsaw University of Technology, 02-507 Warszawa, Poland; [email protected] (K.D.); [email protected] (A.B.) 4 Faculty of Electronics and Information Technology, Institute of Electronic Systems, Warsaw University of Technology, 00-665 Warszawa, Poland; [email protected] (T.O.); [email protected] (A.A.) 5 Abt. SIIT/Optical Interconnection Technology, Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (IZM), 13355 Berlin, Germany; [email protected] (W.L.-A.); [email protected] (H.S.) * Correspondence: [email protected]

Received: 25 August 2020; Accepted: 21 September 2020; Published: 24 September 2020

Abstract: Polymer–matrix composites degrade under the influence of UV radiation in the range of the 290–400 nm band. The degradation of polymer–matrix composites exposed to UV radiation is characterized by extensive aging of the epoxy matrix, resulting in deterioration of their mechanical properties. Glass fibers/epoxy resin composites were made by an out-of-autoclave method whereas a fiber optic sensor was placed between different layers of laminates. In our work, we used a fiber Bragg grating sensor covered with graphene oxide and embedded in a polymer matrix composite to monitor UV radiation intensity. Measurements of UV radiation may allow monitoring the aging process of individual components of the polymer composite. In order to estimate the number of microcracks of epoxy resin, microstructure observations were carried out using a scanning electron microscope.

Keywords: composites; ﬁber Bragg grating sensor; UV radiation; ﬁber optic sensor

1. Introduction Due to their very good mechanical properties, epoxy resins are very often used as a matrix of composites or coatings in a variety of industrial applications: automotive, energy or construction, electromagnetic, electric and electromechanical, particularly in concrete constructions. Such nanocomposites should possess good adhesion with cement/concrete, as well as high ﬁre resistance and good weatherability against UV, alkaline and saline environments [1–3]. Chemical and physical changes of structures made of epoxy resins occur through the action of atmospheric conditions such as sunlight, oxidation, and other external factors [4–6]. The reason for the absorption of UV radiation by epoxy resin and as a result its degradation is the presence of an aromatic

Sensors 2020, 20, 5468; doi:10.3390/s20195468 www.mdpi.com/journal/sensors Sensors 2020, 20, 5468 2 of 10 moiety in their structure. UV radiation, which has influence on discoloration and chalking of epoxy surface, is the major reason for limiting the use of epoxies for outdoor applications [7,8]. Previous studies have confirmed the negative effect of UV radiation on epoxy matrix composites. However, not all the types of UV radiation can reach the composite material exposed to sunlight because the ozone layer absorbs 100% of UVC radiation (100–280 nm) and majority of UVB radiation (280–315 nm). Only UVA radiation (315–400 nm) and a small part of UVB radiation reach the Earth’s surface and adversely affect epoxy resin [9]. Yan, Chouw and Jayaraman have studied the combined effect of ultraviolet (UV) radiation and water spraying on the mechanical properties of a flax fabric reinforced epoxy composite [10]. The aim of their work was to study the durability of such composites used in civil engineering. The research conducted by the authors confirmed that UV radiation and water spraying had an impact on degradation of epoxy matrix composites resulting in discoloration, matrix erosion, microcracking and voids formation. Other studies confirm the negative effect of UV radiation on epoxy composites, which resulted in darkening of the surface and formation of microcracks in epoxy resin/nanoclay composites [1]. The effect of UV radiation on carbon fiber reinforced polymers (CFRP) was also investigated and, as in the above studies, epoxy matrix degradation was also observed, which resulted in deterioration of the mechanical properties of composites. In that case, microcracks, matrix erosion, and fiber debonding were also observed [11]. Methods to counteract epoxy resin degradation can be found in the literature. By introducing antioxidants and photo stabilizers, which include titanium dioxide (TiO2), and zinc oxide (ZnO), high-energy radiation can be attenuated or suppressed [7]. Carbon black (CB) is another additive used to stabilize polymers against UV radiation. The effectiveness of application depends on the type and size of CB particles as well as the degree of dispersion and concentration of the filler in the polymer matrix [12,13]. Interesting possibilities to monitor the aging processes of the epoxy matrix are provided by the use of fiber optic sensors laminated in a composite material [14]. Fiber optic sensors have many advantages over conventional sensors [15], such as small size, electrical safety or immunity to electromagnetic interference. Solutions are known to monitor the distribution of stress or temperature [16,17], however the measurement of UV radiation using fiber optic sensors has not been reported. In this manuscript we propose the use of a laminated fiber optic sensor with a graphene oxide (GO) coated fiber Bragg grating (FBG) for UV measurement inside the composite material. A general idea of this sensor assumes that a temperature change around the fiber can be induced by UV radiation as shown in our previous publication [18]. The absorbance spectrum of an aqueous solution of GO typically peaks at 230 nm while visible and infrared radiation is not significantly absorbed. In laminated FBG sensors, scattered UV light increases the internal energy of GO and consequently locally raises the surface temperature of optical fiber [19]. Consequently, temperature changes modify stress distribution within the fiber and result in a change in Bragg’s wavelength [20].

2. Materials and Methods Typically, the refractive index variation in an FBG changes sinusoidally along the grating length [21,22]. The wavelength λB which is reﬂected due to these periodic changes is called Bragg wavelength and is expressed by the following equation:

λB = 2ne f f Λ (1) with ne f f being the effective refractive index of propagating core mode and Λ the grating period, i.e., a distance between neighboring maxima in periodically induced refractive index changes in the fiber core. Any temperature change in the vicinity of the FBG results in a Bragg wavelength shift. This shift is due to two effects: thermal expansion (which changes the period of the FBG) and thermo-optic effect (stemming from a change in the effective refractive index in the optical fiber). In the case of Sensors 2020, 20, 5468 3 of 10 silica optical fibers, the latter is predominant. Formula (2) describes the influence of the change of temperature ∆T on the Bragg wavelength [23]: ! ∂ne f f ∂Λ ∆λ = 2 Λ + n ∆T (2) B ∂T e f f ∂T

A typical temperature sensitivity of the FBG within the 1550 nm wavelength range is c.a. 10 pm/K. FBGs were written by a krypton fluoride (KrF) excimer laser manufactured by Coherent using the phase mask technique. This laser generates 15 ns pulses at a wavelength of 248 nm. During the gratings inscription, 3 mJ energy and 500 Hz repetition rate of laser pulses were set. Before the inscription of the grating, an SMF-28 standard single-mode optical fiber manufactured by Corning was high-pressure hydrogen loaded at room temperature to enhance its photosensitivity. During the grating inscription, 3 mJ laser pulse energy and 500 Hz repetition rate were set, while the process of FBG formation was monitored on-line to obtain similar spectral responses for all in-written gratings. Thin graphene oxide films (manufactured by Graphene Supermarket) were prepared by the vacuum filtration method. GO suspension with a concentration of 0.1 mg/mL was used in the filtration process. The suspension was filtered onto a mixed cellulose ester membrane filter (MCE, 0.45 µm pore-size, 25 mm diameter). The formed layer on the filter was left under air flow to complete drying. Next, the dry GO film was cut and immersed in an acetone bath to dissolve the filter. After removing the filter residue, the clean GO layer was transferred on a selected optical fiber and gently dried with a nitrogen stream. The thickness of the GO film was about 100 nm (measured by atomic force microscopy). Unfortunately, when applying the GO layer to the optical fiber, the layer tends to surface irregularly and sticks. Therefore, in microscopic images (Figure1c,d), GO creates layers that appear thickerSensors than 2020 100, 20,µ xm. FOR PEER REVIEW 4 of 11

FigureFigure 1. Images 1. Images of optical of optical fiber fiber without without any any protectiveprotective coating coating (a (),a ),partially partially covered covered by poly(methyl by poly(methyl methacrylate)methacrylate) (PMMA) (PMMA) (b ),(b partially), partially covered covered byby graphenegraphene oxide oxide (GO) (GO) film film (c), ( coptical), optical fiber fiber and andGO GO film coveredfilm covered by PMMA by PMMA (d) ( observedd) observed in in an an optical optical microscopemicroscope (magnification (magnification 20×). 20 ). ×

Figure 2. Raman spectrum of the GO thin film. Intensity ratio and peak positions were obtained from Lorentzian fit.

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Since the GO layer located on the surface of the optical fiber is very fragile, that is why it should be properly protected against lamination in a composite material. Poly(methyl methacrylate) (PMMA) was used as a protective material for the GO layer [24], as it exhibits good adhesion with both the silica glass from which the optical fiber is made (Figure1b) and GO (Figure1d). In addition, it protects the GO layer from reduction. The absorption of PMMA peaks for wavelengths below 300 nm [25]. Since the wavelength of the laser we used in experiments was 404 nm, the PMMA layer did not contribute to the temperature change. Additionally, UV radiation for wavelengths below 280 nm is negligible (absorption in the atmosphere), therefore PMMA does not affect the sensor in any way because the sensor’s range of interest is between 300 and 400 nm. Figure2 presents the Raman spectrum of the thin GO film deposited on the surface of the optical 1 1 fiber. Two main bands were observed in the spectrum: D (1345.9 cm− ) and G (1590.5 cm− ), in which peak positions and intensity of each band were obtained from Lorentzian fitting. Based on a relative intensity ratio of the D and G modes (ID/IG = 1.20) we confirmed the purity and presence of GO on the Figure 1. Images of optical fiber without any protective coating (a), partially covered by poly(methyl optical fiber before covering with a thin layer of PMMA [26]. The Raman spectra were collected by methacrylate) (PMMA) (b), partially covered by graphene oxide (GO) film (c), optical fiber and GO using a 514 nm laser in a Renishaw inVia spectrometer in ambient conditions. Measurements were film covered by PMMA (d) observed in an optical microscope (magnification 20×). performed for low laser power (below 0.3 mW) to avoid the sample heating and reduction processes.

Figure 2. Raman spectrum of the GO thin film. Intensity ratio and peak positions were obtained from Figure 2. Raman spectrum of the GO thin film. Intensity ratio and peak positions were obtained from Lorentzian fit. Lorentzian fit. In order to verify the performance of a UV sensor located at a different distance from the surface of composite, three types of laminates were made using out-of-autoclave method, formed by hand lay-up. In all cases, epoxy resin with a trade name Epidian52, supplied by Sarzyna S.A. mixed with a triethylenetetramine hardener (Z1) in a ratio 100:13 was used as a polymer matrix, while Interglas 92125 glass fabric supplied by Interglas (280g/m2, twill 2/2 weave) was used as reinforcement. Laminates were made using a glass plate as a base, from which they were separated with a release film. No chemical release-agent was used. The depth of introduction of the FBG based UV sensor was different in all laminates—in the first case it was laid on the surface, in the second between the first and second layers, and in the third laminate—between the second and third one (Figure3). In addition, a reference FBG sensor without GO coating was placed in each sample (Figure4). All laminates were cured for 24 h at room temperature. Sensors 2020, 20, x FOR PEER REVIEW 5 of 11

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Figure 3. Schematic of UV sensors arrangement in a composite sample. Figure 3. Schematic of UV sensors arrangement in a composite sample.

Figure 3. Schematic of UV sensors arrangement in a composite sample.

Figure 4. The image of FBG UV sensor arrangement in a composite sample. Figure 4. The image of FBG UV sensor arrangement in a composite sample. The experiment was conducted as follows: light from a superluminescent diode (SLD) source with a centralThe wavelength experiment of was 1550 conducted nm and a spectralas follows: width ligh oft 100from nm a wassuperluminescent launched into thediode fiber. (SLD) The source Bragg wavelengthwith a centralFigure reflected wavelength 4. The from image theof 1550of FBG FBG nm was UV and sensor redirected a spectral arrangement by width the circulatorin ofa composite 100 nm through was sample. launched its third into port the into fiber. the opticalThe Bragg spectrum wavelength analyzer reflected (OSA). from A 404 the nm FBG ‘heating’ was redirected laser beam by was the focusedcirculator on through the GO coveredits third partport ofintoThe the theexperiment fiber optical where spectrum was the FBGconducted analyzer was inscribed as(OSA). follows: (FigureA 404 ligh nm5).t from‘heating’ The size a superluminescent oflaser the beam beam was in focus focused diode was on(SLD) 2 the3 GOsource mm covered and it × withwaspart a central positionedof the wavelengthfiber inwhere a way the of that 1550FBG only wasnm 3 inscribedand mm a long spectral (Figure central width 5). section The of size100 of the ofnm the FBG was beam was launched in illuminated. focus intowas the2 × 3fiber. mm and The Braggit was wavelengthpositioned in reflected a way that from only the 3 FBG mm was long redirected central section by the of circulator the FBG wasthrough illuminated. its third port into the optical spectrum analyzer (OSA). A 404 nm ‘heating’ laser beam was focused on the GO covered part of the fiber where the FBG was inscribed (Figure 5). The size of the beam in focus was 2 × 3 mm and it was positioned in a way that only 3 mm long central section of the FBG was illuminated.

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FigureFigure 5. 5. ExperimentalExperimental setup. setup. Figure 5. Experimental setup. 3. Results3.and Results Discussion and Discussion 3. ResultsInitially and Discussion the FBG sensor was investigated prior to lamination in the composite material. For this Initially the FBG sensor was investigated prior to lamination in the composite material. For this purpose,Initially FBGthe FBGcovered sensor with was GO investigated and a protective prior layer to lamination was placed in in the the composite measurement material. system For this purpose, FBG covered with GO and a protective layer was placed in the measurement system presented purpose,presented FBG in coveredFigure 5. Thewith Bragg GO wavelength and a protective changed bylayer 1.1 nm was when placed the UV in sensor the measurement was illuminated system by the 404 nm laser light with intensity equal to 500 mW/cm2 (Figure 6a). An accurate measurement in Figurepresented5. The in Bragg Figure wavelength 5. The Bragg wavelength changed by changed 1.1 nm by when 1.1 nm the when UV the sensor UV sensor was illuminated was illuminated by the of the dependence of the Bragg wavelength change on 2the intensity of light illuminating the sensor 404 nm laser light with intensity equal to 500 mW/cm (Figure2 6a). An accurate measurement of the by the(inset 404 in nm Figure laser 6) lightyields with sensor intensity sensitivity equal equal to to 500 3 pm/mW/cm mW/cm (Figure2 (Figure 6a).6b). An accurate measurement dependenceof the dependence of the Bragg of wavelengththe Bragg wavelength change on change the intensity on the intensity of light of illuminating light illuminating the sensor the sensor (inset in Figure(inset6) yields in Figure sensor 6) yields sensitivity sensor equal sensitivity to 3 pmequal/mW to 3/ cmpm/mW/cm2 (Figure26 (Figureb). 6b).

Figure 6. Bragg wavelengths of the fiber Bragg grating (FBG) sensor covered by PMMA before (blue) and after (orange) illumination by the 404 nm laser light with intensity 500 mW/cm2 (a) and dependence of the Bragg wavelength change on the intensity of UV light in the same sensor (b).

FigureFigure 6. InBragg addition, 6. Bragg wavelengths wavelengthsit was noticed of theof that the ﬁber thanks fiber Bragg Bragg to the grating grating PMMA (FBG)(F protectiveBG) sensor sensor layer, covered covered the UV by by sensorPMMA PMMA could before before safely (blue) (blue) operate for a greater range of light intensity changes. Without the protective layer, the safe level of and afterand (orange)after (orange) illumination illumination by the by 404 the nm 40 laser4 nm light laser with light intensity with intensity 500 mW /500cm 2mW/cm(a) and2 dependence(a) and light intensity for which the GO layer was not damaged was 160 mW/cm2 (Table 1). After coating GO of thedependence Bragg wavelength of the Bragg change wavelength on the intensitychange on of th UVe intensity light in ofthe UV samelight in sensor the same (b). sensor (b). with the PMMA protective layer, the light intensity range increased to 500 mW/cm2.

In addition,In addition, it was it was noticed noticed that that thanks thanks to to the the PMMA PMMA protective layer, layer, the the UV UV sensor sensor could could safely safely operateoperate for a for greater a greater range range of lightof light intensity intensity changes. changes. Without the the protective protective layer, layer, thethe safe safe level level of of 2 light intensitylight intensity for which for which the the GO GO layer layer was was not not damaged damaged waswas 160 mW/cm mW/cm (Table2 (Table 1).1 ).After After coating coating GO GO with the PMMA protective layer, the light intensity range increased to 500 mW/cm2. with the PMMA protective layer, the light intensity range increased to 500 mW/cm2.

As explained in [27], the Bragg wavelength slightly changes after lamination in the composite material. This is the eﬀect of technological shrinkage that arises in the composite material as a result of the lamination process. The impact of technological shrinkage can be minimized by adding a protective layer [28]. In our case it is PMMA, however, the thickness of the protective layer turned out to be insuﬃcient and a residual Bragg wavelength shift of 0.1 nm was observed (Figure7). In the following measurements, the Bragg wavelength after lamination was taken as the reference value. Sensors 2020, 20, x FOR PEER REVIEW 7 of 11

Table 1. Comparison of safe use of a fiber optic UV sensor before and after coating with a UV protective layer.

Sample Type GO GO + PMMA Sensors 2020, 20, 5468 Light intensity [mW/cm2] 160 500 7 of 10

As explained in [27], the Bragg wavelength slightly changes after lamination in the composite Tablematerial. 1. Comparison This is the effect of safe of technological use of a ﬁber shrinkag optice UV that sensor arises in before the composite and after material coating as with a result a UV protectiveof the lamination layer. process. The impact of technological shrinkage can be minimized by adding a protective layer [28]. In our case it is PMMA, however, the thickness of the protective layer turned out to be insufficient andSample a residual Type Bragg wavelength GO shift of 0,1nm was GO observed+ PMMA (Figure 7). In the following measurements,Light intensity the Bragg [mW wavelength/cm2] after lamination160 was taken as 500 the reference value.

Figure 7. Bragg wavelengths before (yellow) and after (blue) of lamination process FBG sensor in sampleFigure 3. 7. Bragg wavelengths before (yellow) and after (blue) of lamination process FBG sensor in sample 3. The Bragg wavelength of the UV sensors laminated in the sample 3 and the sample 4 illuminated The Bragg wavelength of the UV sensors laminated2 in the sample 3 and the sample 4 illuminated by 404by nm 404 laser nm laser light light with with intensity intensity 500500 mW/cm mW/cm2 is presentedis presented in Figure in Figure 8. The 8difference. The di betweenﬀerence the between the BraggBragg before before the the UV UV illumination illumination for for both both samp samplesles was was small, small, therefore therefore these thesetwo samples two samples were were selectedselected for comparison, for comparison, and and for for greater greater clarity, clarity, oneone spectrum spectrum was was used used before before irradiating irradiating the samples the samples with UV.with A UV. change A change in the in the Bragg Bragg wavelength wavelength is is equal equal to to0.35 0.35 nm nmand and0.23 nm 0.23 for nm sensors for sensorslaminated laminated in in samplessamples 3 and 3 and 4, respectively.4, respectively. TheThe change decreases decreases with with the thedepth depth of FBG of FBGlamination lamination but the but the sensitivity of the laminated sensor is constant. Moreover, within the scope of its action this change sensitivity of the laminated sensor is constant. Moreover, within the scope of its action this change results directly from the light scattering in the composite material. resultsSensors directlyLight 2020, from20,passing x FOR the PEERthrough light REVIEW scatteringthe composite in thematerial composite is strongly material. dispersed due to its large heterogeneity8 of 11 [29]. This heterogeneity results from the fact that the composite material consists of two components: resin and reinforcement. In addition, the laminates were made using the out-of-autoclave method, which is why the composite material used in our experiment is characterized by very high dispersion. In general, the absorption can be expressed by the following equation [30]: 𝐼=𝐼𝑒 (3)

where α is the attenuation coefficient, d is the sample thickness, I and I0 are output and input light intensity, respectively.

Figure 8. The Bragg wavelength of a UV sensor before (blue) and after illumination by the 404 nm laser Figure 8. The Bragg wavelength of a UV sensor before (blue) and after illumination by the 404 nm 2 light withlaser intensity light with 500 intensity mW/cm 500for mW/cm FBG sensors2 for FBG laminated sensors laminated in the sample in the 3sample (orange) 3 (orange) and the and sample the 4 (red). sample 4 (red). Light passing through the composite material is strongly dispersed due to its large heterogeneity [29]. This heterogeneityThe measurement results fromof the the effect fact of that thickness the composite on light transmission material consists was made of twousing components: composite resin and reinforcement.materials with varying In addition, thicknesses, the laminates from 0.4 wereto 2.0 mademm. This using allowed the out-of-autoclave to determine the attenuation method, which is factor α = 1.52 mm−1 (Figure 9).

Figure 9. Dependence of light transmission on the thickness of the composite material.

Both light scattering as well as absorption result in less light reaching the UV-laminated sensor in the composite material in comparison with the non-laminated sensor. The reduction in the Bragg wavelength change for UV sensors laminated at different depths in the composite material confirms this effect. When comparing the percentage change in sensitivity of the laminated UV sensor versus lamination depth to the change in light intensity presented in Figure 10, it can be seen that these two relationships are consistent. It follows that the change in sensitivity to UV radiation results directly

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Sensors 2020, 20, 5468 8 of 10 why the composite material used in our experiment is characterized by very high dispersion. In general, the absorption can be expressed by the following equation [30]:

αd I = I0e− (3)

Figure 8. The Bragg wavelength of a UV sensor before (blue) and after illumination by the 404 nm where α is the attenuation coeﬃcient, d is the sample thickness, I and I0 are output and input light laser light with intensity 500 mW/cm2 for FBG sensors laminated in the sample 3 (orange) and the intensity, respectively.sample 4 (red). The measurement of the eﬀect of thickness on light transmission was made using composite materials withThe varying measurement thicknesses, of the effect from of 0.4thickness to 2.0 on mm. light transmission This allowed was tomade determine using composite the attenuation materials with1 varying thicknesses, from 0.4 to 2.0 mm. This allowed to determine the attenuation factor α = 1.52factor mm α = −1.52(Figure mm−1 (Figure9). 9).

Figure 9. DependenceFigure 9. Dependence of light of light transmission transmission on th thee thickness thickness of the of composite the composite material. material.

Both light scatteringBoth light scattering as well as as well absorption as absorption result result in lessin less light light reachingreaching the the UV-laminated UV-laminated sensor sensor in the composite materialin the composite in comparison material in with comparison the non-laminated with the non-laminated sensor. sensor. The reduction The reduction in thein the Bragg Bragg wavelength wavelength change for UV sensors laminated at different depths in the composite material confirms change for UVthis effect. sensors laminated at different depths in the composite material confirms this effect. When comparingWhen comparing the percentage the percentage change change in sensitivity in sensitivity of the of laminatedthe laminated UV UV sensor sensor versus versus lamination depth to theSensorslamination change 2020, 20, indepth x FOR light toPEER the intensity REVIEW change in presented light intensity in presented Figure 10 in, Figure it can 10, be it seen can be that seen these that these two9 of two relationships11 relationships are consistent. It follows that the change in sensitivity to UV radiation results directly are consistent.from the It followsattenuation that of the the composite change material in sensitivity and the sensors to UV were radiation not damaged results during directly the from the attenuationlamination of the composite process. material and the sensors were not damaged during the lamination process.

Figure 10. ComparisonFigure 10. Comparison between between percentage percentage sensitivity sensitivity of of the the laminated laminated UV sensor UV (red sensor dots)(red and the dots) and the change in lightchange intensity in light intensity at a given at a given plate plate depth depth (blue (blue line). line).

4. Summary The presented work shows that it is possible to laminate a fiber optic UV sensor using FBG in a composite material. However, to protect the GO layer on the surface of the optical fiber, it is necessary to cover the GO layer with a thin layer of PMMA. As demonstrated by the tests, the PMMA layer does not adversely affect the UV sensor. What is more, the protective layer allows to significantly increase the sensor's operating range. The presented UV sensor allows to measure UV radiation both on the surface of the composite material and inside of it. For use in real measurement systems, however, both temperature and deformation must be monitored to distinguish them from changes in the measured UV light intensity. Thanks to this, it may be possible in future to link the intensity of UV radiation falling on the composite material with its degradation under the influence of this radiation. On the other hand, the presented sensor can be used to monitor UV radiation during the lamination process of light-curing materials. However, both solutions require further research.

Author Contributions: Conceptualization, P.L. and K.D.; formal analysis, P.S., K.B.; investigation, P.L., P.S., K.M., A.W. and Ł.K.; resources, K.B., K.D., P.S., A.B., W.L.-A., T.O. and A.A.; writing—original draft preparation, P.L., A.W., T.O. and K.D.; writing—review and editing, K.B. and P.L.; visualization, K.B. and P.S.; supervision, P.L. and T.R.W.; project administration, P.L. and W.L.A.; funding acquisition, H.S., W.L.A. and P.L. All authors have read and agreed to the published version of the manuscript.

Funding: This project is co-financed by the European Regional Development Fund (ERDF).

Conflicts of Interest: The authors declare no conflict of interest.

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4. Summary The presented work shows that it is possible to laminate a fiber optic UV sensor using FBG in a composite material. However, to protect the GO layer on the surface of the optical fiber, it is necessary to cover the GO layer with a thin layer of PMMA. As demonstrated by the tests, the PMMA layer does not adversely affect the UV sensor. What is more, the protective layer allows to significantly increase the sensor’s operating range. The presented UV sensor allows to measure UV radiation both on the surface of the composite material and inside of it. For use in real measurement systems, however, both temperature and deformation must be monitored to distinguish them from changes in the measured UV light intensity. Thanks to this, it may be possible in future to link the intensity of UV radiation falling on the composite material with its degradation under the influence of this radiation. On the other hand, the presented sensor can be used to monitor UV radiation during the lamination process of light-curing materials. However, both solutions require further research.

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