Strontium Aluminate-Based Long Afterglow PP Composites: Phosphorescence, Thermal, and Mechanical Characteristics

A tremendous potential has been observed in the designing of long afterglow materials for sensing, bioimaging, and encryption applications. In this study, two different strontium aluminate-based luminescent materials; SrAl2O4: Eu, Dy (S1), and Sr4Al14O25: Eu, Dy (S2) were melt-mixed with polypropylene (PP) matrix, and the phosphorescence properties were evaluated. After excitation at 320 nm, the PP/S1 composite exhibited a green emission and the PP/S2 generated a blue emission at 520 nm and 495 nm, respectively. The emission spectra intensity increased by increasing the content of these luminescent fillers. The attenuated total reflection-Fourier transform infrared (ATR-FTIR) experiments show that no chemical reaction occurred during the melt-mixing process. The differential scanning calorimetry (DSC) results revealed that the total crystallinity of the composites reduced by increasing the amount of the fillers; however, no changes in the temperature of melting (Tm) and crystallization (Tc) of PP were observed. Both fillers improved the impact strength of the composites, but the tensile strength (TS) and modulus (TM) decreased. Poly (ethylene glycol) dimethyl ether (P) plasticizer was used to improve the filler-matrix interaction and its dispersion; nevertheless, it adversely affected the intensity of the luminescence emissions.


Introduction
Luminescent materials emit light, especially in the visible region. When a material continuously emits visible light for longer time (hours) after stopping the radiating source (visible, UV, X-ray, or gamma-ray radiation), a persistence of luminescence or phosphorescence is observed [1]. The first-generation phosphors were Cu or Mn-doped ZnS-based materials (green emission at 530 nm) [2,3]. These materials have been exploited in catalysts and optoelectronic devices. However, their functions are limited because of their low brightness and short afterglow time, and chemical instability in the presence of moisture and CO 2 .
A new luminescence era started with the detection of rare-earth-doped phosphors. The first batch of modern luminescent materials was the rare-earth (R 3+ ) and Eu 2+ doped alkaline-earth aluminates (MAl 2 O 4 :Eu 2+ , R 3+ ; M = Ca, Sr, or Ba) [4][5][6][7][8][9]. The Eu 2+ doped phosphor exhibits a bluish-green luminescence, and the glowing time can be enhanced by adding rare-earth ions, such as neodymium (Nd) or dysprosium (Dy) (e.g., SrAl x O y : Eu 2+ , Dy 3+ ), or by adding Al 2 O 3 [4,10]. These phosphors have attracted much attention because of their long phosphorescence, greater stability (i.e., moisture and photo-stability), and high quantum efficiency compared with sulfide-based phosphors [9,11,12]. These materials can obtain radiation energy from solar light, remaining photo-luminescent for long periods of time (12-20 h) [13,14]. The luminescent properties of these phosphors allowed their commercial acceptance because of the suitable usage in fluorescent lamps, glowing paints for highways buildings and airports, cathode ray tubes, plasma displays, textile, ceramic area, nighttime clocks, safety displays, among others. Significant growth in the use of these phosphors was observed in optoelectronics, telecommunications, optically active commercial products, and biomedical and way-finding systems [15][16][17].
Recently, various phases of strontium aluminates with rare-earth-doped were developed, such as SrAl 2 [18][19][20][21]. The wavelength of the emitted visible light is decided by the crystalline phase structure of the resultant strontium aluminate [22,23]. Among the different SrAl x O y : Eu 2+ , Dy 3+ phosphors reported, Sr 4 Al 14 O 25 : Eu, Dy, and SrAl 2 O 4 : Eu, Dy have exhibited the strongest potential for long phosphorescence and are commercially available [8,21,24]. Different synthesis techniques such as solid-state reactions [14,20,25], sol-gel method [11,24,26], combustion method [27], solvothermal method [28], chemical precipitation [29], microwave processing, and hydrothermal reaction [30] have been developed. These synthesis methods are often complex and require high temperatures for long-duration phosphorescence materials [31]. A mechanism for luminescence persistence was proposed for SrAl 2 O 4 :Eu 2+ , Dy 3+ and has been implied to explain the luminescence in several Dy 3+ and Eu 2+ co-doped silicates and aluminates. The mechanism is related to the thermally activated release of a hole from Eu 2+ in its excited 5d state to the valence band, in which it is then trapped by Dy 3+ . Luminescence is generated when the excited electron relaxes back to the ground state of Eu 2+ [10,32]. The detailed mechanism of the process has been described [15]. However, SrAl x O y : Eu 2+ , Dy 3+ phosphors are not exempt from gradual luminescence decay due to their affinity towards moisture. Various encapsulation procedures have been described for gaining stable phosphor, such as Al 2 O 3 [33], SrF 2 coating [34], phosphoric acid [35], organic ligands [36], amino-functionalized [37], among others, have been implied and reported. These processes are complicated, require elaborate equipment, and inversely affect the luminescence output. The easy and inexpensive method reported in the literature to prolong the afterglow properties of SrAl x O y : Eu, Dy is the encapsulation within a polymer matrix, which acts as an insulator for moisture. This process enables the composites to exhibit better chemical stability, good physical properties and can be processed very easily [38][39][40][41][42][43][44][45]. In this study, two different strontium aluminate materials doped with Eu, Dy were incorporated in the selected poly(propylene) matrix, and the phosphorescent characteristics of the resultant composites were studied in detail. Moreover, a known plasticizer poly (ethylene glycol) dimethyl ether (P) was used to enhance the phosphor dispersion in the PP matrix, and also to evaluate its effect on the phosphorescence emission. The characterization studies on these composites provide valuable information on the fabrication of polymer-based luminescent films.

Phosphorescence measurements
The phosphorescence tests were carried out in a Fluorescence Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) using a Xe ultraviolet (UV) lamp. The emission spectra were collected at the wavelength of excitation 320 nm.
Scanning Electron Microscope (SEM) The morphological and elemental analyses were performed in a JEOL JSM-6360A, Japan SEM model, with energy-dispersive X-ray spectroscopy (EDS) facility. A thin cut surface of the composite was prepared for the analyses. The gold coating for these samples is performed in an auto fine coater (JFC/1600) for 30 s. The coating with gold was carried out to prevent the effect of charging and to improve the quality of the image.
Differential Scanning Calorimetry (DSC) DSC tests were done in Shimadzu DSC-60A model. Approximately 6-10 mg of sample were taken in an aluminum pan and is heated from 30 • C to 220 • C at a ramp of 10 • C/min with 4 min holding time.
The percent crystallinity was evaluated as follows where (Φ) is the filler weight fraction in the composites, (∆H m ) is the melting enthalpy, and (∆H 0 m ) is the melting enthalpy of 100% crystalline PP, and was reported as 207 J/g [46]. X-Ray Diffraction (XRD) The crystalline studies were performed in a wide-angle XRD (Bruker D8 advance). The diffractometer was endowed with a wide-angle goniometer attached to a sealed-tube Cu-Kα radiation source (λ = 1.54056 Å). The scanning was done in the 2θ range of 5 • to 50 • at 5 • /min in the reflection mode.
Thermo-gravimetric analysis (TGA) TGA was done in a Shimadzu DTG-60H model. For the analysis, 10 ± 1.5 mg of the samples were maintained in an aluminum pan and is heated to a temperature of 600 • C (inert atmosphere) with a heating rate of 20 • C/min; and the loss in weight was monitored.
Mechanical properties The standard tensile testing specimens (ASTM Type1, Dumb-bell shaped) were prepared using a DSM Xplore, Netherlands (12 cm 3 , microinjection molding). The mold was maintained at room temperature and at a pressure of 6 bar. Tensile testing was performed in Hounsfield H100 KS model UTM (ASTM D638), and the mean of five test results was reported.

DSC and ATR-FTIR Data
The DSC data for the PP and its composites are displayed in Tables 1 and 2. The presence of both S 1 and S 2 did not considerably affect the temperature of melting (T m ) and crystallization (T c ) of the composites, demonstrating that the PP was not interacting chemically with S 1 and S 2 and the mixing process was purely physical. Additionally, the FTIR spectra also support this observation as PP, PP/10S 1 , PP/10S 2 , and PP/10S 1 /5P have similar FTIR spectra, in which no new peaks were observed nor either peaks diminished ( Figure 1). This observation confirms the absence of chemical reactions between both S 1 and S 2 and PP. The sharp peaks at 2900 cm −1 were due to the asymmetrical CH 2 bending and the peaks at 1450 cm −1 and 1380 cm −1 were assigned to the symmetrical CH 3 bending and asymmetrical CH 3 bending, respectively, of PP [47]. Table 1. DSC data of PP, PP/S 1 , and plasticized composites.  crystallization (Tc) of the composites, demonstrating that the PP was not interacting chemically with S1 and S2 and the mixing process was purely physical. Additionally, the FTIR spectra also support this observation as PP, PP/10S1, PP/10S2, and PP/10S1/5P have similar FTIR spectra, in which no new peaks were observed nor either peaks diminished ( Figure  1). This observation confirms the absence of chemical reactions between both S1 and S2 and PP. The sharp peaks at 2900 cm −1 were due to the asymmetrical CH2 bending and the peaks at 1450 cm −1 and 1380 cm −1 were assigned to the symmetrical CH3 bending and asymmetrical CH3 bending, respectively, of PP [47].  Conversely, the crystallinity of the composites decreased by increasing the S1 and S2 contents. The crystallinity decrease was more pronounced in S2 because of the bulky chemical structure of S2 compared with S1, and in turn, restricts the PP chains mobility; thus, decreasing the crystallinity values [48]. The incorporation of the plasticizer in the composites led to a further reduction in the crystallinity percentage of the composites, as shown in Table 1. Conversely, the crystallinity of the composites decreased by increasing the S 1 and S 2 contents. The crystallinity decrease was more pronounced in S 2 because of the bulky chemical structure of S 2 compared with S 1 , and in turn, restricts the PP chains mobility; thus, decreasing the crystallinity values [48]. The incorporation of the plasticizer in the composites led to a further reduction in the crystallinity percentage of the composites, as shown in Table 1.   [49]. Hence it was clear the absence of chemical reaction between PP and S1, S2 fillers, or the plasticizer. The (020) peak at 20°; (-211), (220), (211) peaks at 30°, and (031) peaks at 35° are the characteristics diffraction peaks of S1 [39], and S2 shows characteristics orthorhombic crystal structure with diffraction peaks at 25°, 27°, and 32° [50].

Thermal Gravimetric Analysis (TGA)
The TGA results collected for PP, PP/S1, PP/S2, S1, and S2 under an inert atmosphere are shown in Figure 3A-C, respectively. For both S1 and S2 composites, the degradation process occurred in a single step and the composite with the highest filler loading have better thermal stability than that of neat PP. The better thermal stability of the composites is because of the fact that the inorganic filler particles (S1 and S2) can act as a barrier, slowing down the decomposition process of PP [43]. However, in low filler loading concentrations, the thermal stability decreased. Moreover, the residual weight left at the end of the TGA curve was proportional to the loading percentage of the S1 and S2 fillers. Both S1 and S2 are inorganic materials and are very much stable as seen from the TGA graph ( Figure 3C). The weight loss is very much negligible for both the S1 and S2, and on comparing S1 and S2; S2 is found to be slightly more stable than S1. The S1 and S2 are found to be stable until 300 °C and the minor weight loss starts from that temperature, as shown in Figure 3C.

Thermal Gravimetric Analysis (TGA)
The TGA results collected for PP, PP/S 1 , PP/S 2 , S 1 , and S 2 under an inert atmosphere are shown in Figure 3A-C, respectively. For both S 1 and S 2 composites, the degradation process occurred in a single step and the composite with the highest filler loading have better thermal stability than that of neat PP. The better thermal stability of the composites is because of the fact that the inorganic filler particles (S 1 and S 2 ) can act as a barrier, slowing down the decomposition process of PP [43]. However, in low filler loading concentrations, the thermal stability decreased. Moreover, the residual weight left at the end of the TGA curve was proportional to the loading percentage of the S 1 and S 2 fillers. Both S 1 and S 2 are inorganic materials and are very much stable as seen from the TGA graph ( Figure 3C). The weight loss is very much negligible for both the S 1 and S 2, and on comparing S 1 and S 2 ; S 2 is found to be slightly more stable than S 1 . The S 1 and S 2 are found to be stable until 300 • C and the minor weight loss starts from that temperature, as shown in Figure 3C.

Phosphorescence Emission
The phosphorescence emission spectra of PP/S1 and PP/S2 composites are presented in Figures 4 and 5, respectively. The emission spectra were collected at an excitation of 320 nm. As expected, the intensity of emission in the spectra of S1 and S2 were higher than that of PP composites because of the opacity and UV resistance of PP. In the PP/S1 and PP/S2

Phosphorescence Emission
The phosphorescence emission spectra of PP/S 1 and PP/S 2 composites are presented in Figures 4 and 5, respectively. The emission spectra were collected at an excitation of 320 nm. As expected, the intensity of emission in the spectra of S 1 and S 2 were higher than that of PP composites because of the opacity and UV resistance of PP. In the PP/S 1 and PP/S 2 composites spectra, the emission intensity increased by increasing the percent loading of both S 1 and S 2 . The PP/S 1 composite generated a green emission at 520 nm attributed to the electronic transition of europium divalent ion (Eu 2+ ) in the S 1 phosphors (4f 6 5d 1 to 4f 7 ) [10,51]; the detailed mechanism of phosphorescence has already been described [52]. The PP/S 2 composites produced a blue emission at 495 nm and the emission intensity increased by increasing the S 2 content ( Figure 5). The green (PP/S 1 ) and blue (PP/S 2 ) emissions in the dark are shown in Figure 6A,B, respectively.

Phosphorescence Emission
The phosphorescence emission spectra of PP/S1 and PP/S2 composites are presented in Figures 4 and 5, respectively. The emission spectra were collected at an excitation of 320 nm. As expected, the intensity of emission in the spectra of S1 and S2 were higher than that of PP composites because of the opacity and UV resistance of PP. In the PP/S1 and PP/S2 composites spectra, the emission intensity increased by increasing the percent loading of both S1 and S2. The PP/S1 composite generated a green emission at 520 nm attributed to the electronic transition of europium divalent ion (Eu 2+ ) in the S1 phosphors (4f 6 5d 1 to 4f 7 ) [10,51]; the detailed mechanism of phosphorescence has already been described [52]. The PP/S2 composites produced a blue emission at 495 nm and the emission intensity increased by increasing the S2 content ( Figure 5). The green (PP/S1) and blue (PP/S2) emissions in the dark are shown in Figure 6A,B, respectively. . Phosphorescence emissions of S1 and PP/S1 composites (excitation wavelength: 320 nm; green emission at 520 nm) Figure 4. Phosphorescence emissions of S 1 and PP/S 1 composites (excitation wavelength: 320 nm; green emission at 520 nm).    To investigate the effect of incorporation of plasticizer on the phosphorescence emission, 2.5 and 5 wt.% of plasticizers were added to composite with 10 wt.% of S 1 ; however, the plasticizer exhibited an adverse effect on the intensity of emissions. The incorporation of the plasticizer in the 10 wt.% S 1 composite decreases the overall phosphorescence emission intensity, as shown in Figure 7. The excitation process in S 1 and S 2 by absorbing UV light may get hindered in the presence of the plasticizer, which is more prone to degradation in UV light [53]. Because of the negative outcome of the plasticizer incorporation on the phosphorescence intensity of the PP/S 1 composites, they were not studied for the PP/S 2 composites.   To investigate the effect of incorporation of plasticizer on the phosphorescence emission, 2.5 and 5 wt.% of plasticizers were added to composite with 10 wt.% of S1; however, the plasticizer exhibited an adverse effect on the intensity of emissions. The incorporation of the plasticizer in the 10 wt.% S1 composite decreases the overall phosphorescence emission intensity, as shown in Figure 7. The excitation process in S1 and S2 by absorbing UV light may get hindered in the presence of the plasticizer, which is more prone to degradation in UV light [53]. Because of the negative outcome of the plasticizer incorporation on the phosphorescence intensity of the PP/S1 composites, they were not studied for the PP/S2 composites.   To investigate the effect of incorporation of plasticizer on the phosphorescence emission, 2.5 and 5 wt.% of plasticizers were added to composite with 10 wt.% of S1; however, the plasticizer exhibited an adverse effect on the intensity of emissions. The incorporation of the plasticizer in the 10 wt.% S1 composite decreases the overall phosphorescence emission intensity, as shown in Figure 7. The excitation process in S1 and S2 by absorbing UV light may get hindered in the presence of the plasticizer, which is more prone to degradation in UV light [53]. Because of the negative outcome of the plasticizer incorporation on the phosphorescence intensity of the PP/S1 composites, they were not studied for the PP/S2 composites.

Mechanical Characteristics of PP/S 1 and PP/S 2 Composites
For practical application purposes, the composites must exhibit suitable physical properties. Therefore, the Izod impact strength and tensile data of the PP with PP/S 1 and PP/S 2 composites were evaluated and are illustrated in Figures 8 and 10, respectively.

Mechanical Characteristics of PP/S1 and PP/S2 Composites
For practical application purposes, the composites must exhibit suitable physical properties. Therefore, the Izod impact strength and tensile data of the PP with PP/S1 and PP/S2 composites were evaluated and are illustrated in Figure 8 and Figure 10, respectively. The impact strength of PP/S1 and PP/S2 composites increased gradually by increasing the S1 and S2 contents as shown in Figure 8. The PP composites with 10 wt.% filler (S1 and S2) showed the highest notch impact strength, which is ~ 32% greater than that for PP. The increase in the impact strength of the composites is because of the better interfacial adhesion among the PP and fillers (S1 and S2), which allows more efficient stress transfer. The distribution of fillers and the increased adhesion between the fillers and PP are visible in the SEM images of the composites with the highest S1 and S2 loading of 10 wt.% ( Figure 9A,B). The impact strength of PP/S 1 and PP/S 2 composites increased gradually by increasing the S 1 and S 2 contents as shown in Figure 8. The PP composites with 10 wt.% filler (S 1 and S 2 ) showed the highest notch impact strength, which is~32% greater than that for PP. The increase in the impact strength of the composites is because of the better interfacial adhesion among the PP and fillers (S 1 and S 2 ), which allows more efficient stress transfer. The distribution of fillers and the increased adhesion between the fillers and PP are visible in the SEM images of the composites with the highest S 1 and S 2 loading of 10 wt.% ( Figure 9A,B). The impact strength of PP/S1 and PP/S2 composites increased gradually by increasing the S1 and S2 contents as shown in Figure 8. The PP composites with 10 wt.% filler (S1 and S2) showed the highest notch impact strength, which is ~ 32% greater than that for PP. The increase in the impact strength of the composites is because of the better interfacial adhesion among the PP and fillers (S1 and S2), which allows more efficient stress transfer. The distribution of fillers and the increased adhesion between the fillers and PP are visible in the SEM images of the composites with the highest S1 and S2 loading of 10 wt.% ( Figure 9A,B).  The tensile properties of PP and PP/S 1 and PP/S 2 composites are presented in Figure 10A,B, respectively. A gradual decrease in TS and tensile TM of the composites can be seen with the increase in the weight content of S 1 and S 2 . In the highest filler loading (10 wt.% S 1 and S 2 ), the TS and TM decreased from 34.5 to 30.5 MPa and 1.1 to 0.94 GPa, respectively. The decrease in the tensile properties of PP/S 1 and PP/S 2 composites by increasing the filler loading can be due to the fact that the presence of inorganic fillers (S 1 and S 2 ) generally influences the elastic properties of PP because of their intrinsic stiffness and incapability to transfer the applied stress [37]. These observations are in agreement with the decrease in elongation at the yield values for these composites, as shown in Figure 10C. This is because of the decrease in PP ductility in the presence of S 1 and S 2 particles, which decreases the PP chain mobility. The inorganic-polymer composites often cause phase separation due to their incompatibility leading to a reduction in elongation at yield and break [54]. Additionally, the agglomeration of S 1 and S 2 filler, as shown in the SEM images ( Figure 9A-C), adversely impacts the tensile modulus values. and incapability to transfer the applied stress [37]. These observations are in agreement with the decrease in elongation at the yield values for these composites, as shown in Figure 10C. This is because of the decrease in PP ductility in the presence of S1 and S2 particles, which decreases the PP chain mobility. The inorganic-polymer composites often cause phase separation due to their incompatibility leading to a reduction in elongation at yield and break [54]. Additionally, the agglomeration of S1 and S2 filler, as shown in the SEM images ( Figure 9A-C), adversely impacts the tensile modulus values.

Conclusions
In this study, strontium aluminate-based phosphors (S1 and S2) were melt-mixed with PP matrix to achieve the long afterglow properties. A long-lasting, PP encapsulated S1 and S2 composites with long afterglow properties, which lasts for hours, were obtained. The ATR-FTIR spectra confirmed that the melt-blending process was physical. Moreover, the luminescence spectra of the composite have a major excitation peak at 320 nm and an emission peak at 520 nm (S1; green) and 495 nm (S2; blue), respectively. The thermal studies show that the Tc and Tm of PP were not affected by the S1 and S2 fillers. However, there was a significant decrease in the crystallinity of the composites with S2 fillers, owing to the comparatively bulky chemical structure of these fillers. The impact strength of the resultant composites increased with the filler amount, but an adverse effect was witnessed

Conclusions
In this study, strontium aluminate-based phosphors (S 1 and S 2 ) were melt-mixed with PP matrix to achieve the long afterglow properties. A long-lasting, PP encapsulated S 1 and S 2 composites with long afterglow properties, which lasts for hours, were obtained. The ATR-FTIR spectra confirmed that the melt-blending process was physical. Moreover, the luminescence spectra of the composite have a major excitation peak at 320 nm and an emission peak at 520 nm (S 1 ; green) and 495 nm (S 2 ; blue), respectively. The thermal studies show that the Tc and Tm of PP were not affected by the S 1 and S 2 fillers. However, there was a significant decrease in the crystallinity of the composites with S 2 fillers, owing to the comparatively bulky chemical structure of these fillers. The impact strength of the resultant composites increased with the filler amount, but an adverse effect was witnessed on the TS and TM. These results demonstrated the satisfactory prospects for the formulation of phosphorescence films based on low-cost PP, which has great potential for applications in a new generation of light sources such as traffic signage and emergency signals.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.