Altaf Basta*a,
Mauro Missori*b,
Adel S. Girgisc,
Marco De Spiritod,
Massimiliano Papie and
Houssni El-Saieda
aCellulose and Paper Dept., National Research Centre, Dokki-12622, Cairo, Egypt. E-mail: altaf_basta@yahoo.com; Fax: +20-23-3371719; Tel: +20-12-22174239
bIstituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, Via Salaria Km 29.300, 00016 Monterotondo Scalo (Rome), Italy. E-mail: mauro.missori@isc.cnr.it; Fax: +39-06-49934168; Tel: +39-06-49934166
cChem. Pesticide Dept., National Research Centre, Dokki-12622, Cairo, Egypt
dIstituto di Fisica, Università Cattolica del Sacro Cuore, Largo F. Vito 1, 00168, Rome, Italy
eFacoltà di Medicina e Chirurgia, Università Cattolica del Sacro Cuore, Largo F. Vito 1, 00168, Rome, Italy
First published on 27th October 2014
Application of newly investigated fluorescent nanoparticles of 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles as security markers for enhancing the safety property of bagasse-based paper sheets (as valuable documents) was studied. The role of sonication conditions and the type of molecule (morpholinyl or piperidinyl) contained in the heterocyclic compound on production of nanoparticles and their fluorescent behavior were evaluated. The quality of the obtained treated paper was evaluated from examining the fluorescence behavior of the particles-treated paper sheets, in comparison with their fluorescence behavior in water suspension. The success of these investigated fluorescent particles as a security marker was recommended from examining the strength properties of the treated paper and the interaction between the fluorescent particles and the cellulose fibers, via FT-IR-spectra and thermal stability tests, and moreover, from the unfalsifiable safety of the treated documents by erasure technique (chemical and mechanical).
The majority of safety papers currently available on the market react quickly to such attempts of falsification with ink-erasing pencils. This often causes the appearance of a fluorescent yellow color which is not easily visible to the naked eye and which furthermore proves troublesome for certain uses.11 Several methods and systems to mark and protect from falsification valuable documents made of, or supported on paper, are based on the use of secret fluorescent dyes.12 In this context, 2-alkoxy-3-pyridinecarbonitrile derivatives have been synthesized.12,13 These derivatives exhibit remarkable fluorescence properties enabling their employment on paper sheets prepared from non-wood fibrous material (bagasse pulp) to produce security paper. However, while these secret dyes show intense fluorescence and leave the paper substrate on which they are deposited unaffected, known problems include low resistance to chemical degradation and photodegradation.12 In addition, the optical properties are specific to each compound and cannot be modified. The same authors14 succeeded in synthesizing a variety of pyridine derivatives possessing both the aforementioned functional groups, i.e. amino and alkoxy groups oriented at the o- and o′-positions of the pyridine nucleus and neighboring to nitrile functions. In other words, gathering the whole functional moieties responsible for fluorescence properties in a sole structure system in an attempt to optimize a fluorescent active agent with high quantum yield.
On the other hand, fluorescent nanoparticles of organic molecules are attracting increasing interest for their good fluorescence quantum yield and long stability over time.15–19 The electronic and the optical properties of organic nanoparticles, due to weaker intermolecular forces such as hydrogen bonds, van der Waals and hydrophilic/hydrophobic interactions, can easily be modulated to obtain many unusual optical properties and potentially useful emission.20,21
In particular, when the relative orientation of the transition dipole moments of molecules in nanoparticles results in a head-to-tail arrangement, a so-called J-aggregate is formed.20 The optical properties of this structural packing are characterized by the shift of the excitonic band towards lower energy with respect to the absorption of the monomers (bathochromic shift), and high fluorescence quantum yield.22 The opposite behaviour is found for the parallel arrangement of dipole moments (H-aggregates), with excitonic absorption at higher energy than that of the isolated molecule (hypsochromic shift) and quenching of fluorescence.21
When the organic nanoparticles tend to be unstable in solution, often aggregating or precipitating over the course of few days,22 their use is therefore severely hampered. Therefore advances have been gained by adopting the reprecipitation method. This method consists of injecting a dilute solution of monomers into a poor solvent. Being insoluble, monomers aggregate and form a colloidal suspension of particles, whose size ranges from nanometers to micrometers.23,24 To control particle size, sonication was used during the reprecipitation process.25–27 Sonochemical preparation of nanophase materials relies on acoustic cavitation, which is the formation, the growth and implosive collapse of bubbles in a liquid irradiated with high-intensity ultrasound.28 The extremely high temperatures (∼4000 K in water), pressures (>20 MPa) and cooling rates (>107 K s−1) attained during acoustic cavitation inside of the collapsing bubbles are exploited to initiate chemical reactions such as oxidation, reduction, decomposition and promotion of polymerization.29,30 Although details on the formation of organic nanoparticles under sonication still need further investigation, acoustic cavitations seems to be the most probable driving force providing the energy needed in the formation of small nuclei.25
Converting the previously investigated 2-amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile (AEMP) fluorescent active prepared heterocycles to nanoparticles of sizes ranging from tens to hundreds of nanometers was provided by the combination of reprecipitation and sonication.27 This method provides an effective way to produce fluorescent organic nanoparticles for several applications that rely on strong and stable fluorescence, for example for marking important or valuable documents. The surprising results were that the suspension of nanoparticles was found to be stable for more than two years when stored at 4 °C in darkness. It was also found that a remarkable increase in the fluorescence yield was observed as the nanoparticle size decreases. The above features, together with the striking stability of optical and mechanical properties over the course of months, allow for straightforward applications that rely on strong and stable fluorescence such as marking important documents.
These results persuaded us to pursue the surface application of AEMP on paper sheet for production valuable documents (unfalsifiable safety paper). To the best of our knowledge, this work is the first work about fluorescent organic nanoparticles that can be used as secret markers for paper documents. In parallel experiments, 2-amino-6-ethoxy-4-[4-(1-piperidinyl)phenyl]-3,5-pyridinedicarbonitrile particles (AEPP) were synthesized and surface treated the paper sheets.
The overarching objective of this present work is to assess the possibility of applying newly synthesized fluorescent nanoparticles of 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles as security markers for enhancing the safety properties of bagasse-based paper sheets (as valuable documents). The role of sonication conditions and type of molecule (morpholinyl or piperidinyl) contained in the heterocyclic compounds on the production of nanoparticles of fluorescent compounds, and which fluorescent particles are more efficient for document quality, especially against strength and falsification, were also optimized.
According to the fluorescence behavior data (quantum yield), the synthesized 2-amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile (3c), possesses higher quantum yield (ϕs relative to the fluorescence quantum yield of quinine sulfate, reaching ∼0.8314).
Sample code | Fluorescent cpd | Solution volume | Water volume | Sonication power (W) | Process time (min) | Temperature (°C) | 〈Rh〉 ± σ (nm) | σ/〈Rh〉 | Zeta potential |
---|---|---|---|---|---|---|---|---|---|
a Process conditions used to obtain the AEMP and AEPP particles with an indication of the size and size distribution observed (〈Rh〉is the mean hydrodynamic radius, σ/〈Rh〉 is the polydispersity index of the size distribution; zeta potential is the electric potential between nanoparticles which is correlated to the stability of the nanoparticles suspension). | |||||||||
S1 AEMP | 3c | 100 μL | 10 mL | 12.5 | 1 | 0 | 479 ± 378 | 0.79 | |
S2 AEMP | 3c | 100 μL | 10 mL | 12.5 | 5 | 0 | 264 ± 137 | 0.52 | Approx −20 mV |
S3 AEMP | 3c | 100 μL | 10 mL | 12.5 | 30 | 0 | 97 ± 39 | 0.40 | |
S4 AEPP | 3d | 100 μL | 10 mL | 12.5 | 1 | 0 | 499 ± 265 | 0.53 | |
S5 AEPP | 3d | 100 μL | 10 mL | 12.5 | 5 | 0 | 224 ± 110 | 0.49 | Approx −10 mV |
S6 AEPP | 3d | 100 μL | 10 mL | 12.5 | 30 | 0 | 240 ± 139 | 0.58 |
Sonication was induced by a Vibra-Cell ultrasonic processor (Sonics & Materials, Inc., USA), at 12.5 W power (see Table 1), connected to a titanium horn to radiate the ultrasonic energy to the liquid contained in a beaker. The beaker was kept in a thermostatic bath to maintain the water at 0 °C temperature during the sonication process. The sonication conditions were selected based on the previous study,27 in order to obtain smaller nanoparticles avoiding their degradation. Therefore we used lower power and low solution temperature and sonication times for compounds 3c and 3d.
Excitation spectra and photoluminescence emission of AEMP and AEPP heterocyclic fluorescent particles dispersed in water and on paper substrate were carried out by using a Perkin-Elmer LS 45 fluorescence spectrometer. Quartz cuvette with 1 cm optical path length was used to house the samples. The spectral resolution was 10 nm and data were pitched every 0.5 nm.
Front-surface excitation and collection geometries were used for the investigation of photoluminescence spectra of particles on paper substrate.
Strength properties, e.g., tensile, tear, and burst indices of paper samples were measured before and after treatment, according to standard procedure.38 Surface ultra-violet examination was carried out using a Documenter® 4500, Spectral Analysis System (Projectina, Swiss).
The paper samples were subjected to non-isothermal thermogravimetric analysis. Thermogravimetric analyses were performed in a Perkin-Elmer Thermogravimetric Analyzer TGA7. The samples were heated in pure nitrogen (flow rate 50 mL min−1) at 10 °C minute−1, and within the typical temperature range: 35–600 °C, i.e., until no additional weight loss was observed. Measurements were made using calcined alumina as the reference material. Differential thermogravimetric (DTG) peaks were examined for evidencing different behaviours between the samples, and also for understanding how the surface treatments affected the thermal stability of the fibers. The kinetic parameters based on the weight loss data from TG curve analysis were determined according to the equations described elsewhere.40,41
(1) |
(2) |
For linear heating rate, a, (deg min−1):
(3) |
The activation energy, Ea, of thermal decomposition when n = 1, was calculated by using eqn (4).
(4) |
When n ≠ 1, eqn (5) was used;
(5) |
Plotting the left-hand-side value of the equation {i.e., log[1 − (1 − α)1 − n/T2(1 − n)]} against 1/T using various values of “n” should give a straight line with the most appropriate value of “n”.41 The least squares method was applied for the equation, using values of “n” ranging from 0.0 to 3.0 in increments of 0.5.41 The correlation coefficient (r) and the standard error (SE) were calculated for each value of “n”. The “n” value, which corresponds to the maximum r and minimum SE, is the order of the degradation process. The activation energies and frequency factors were calculated from the slope and intercept, respectively, of the Coats–Redfern equation with the most appropriate value of “n”.
In all eradicators, the ballpoint ink (Reynolds)® type, the erasing of ink by chemical agents (e.g., eradicators, commercial alkaline Clorox® (NaOCl), or benzylalcohol), from different examined hand sheet papers done using a piece of cotton moistened with the erasure solution, and the wiping was done several times carefully with ethanol to avoid destruction of paper or removal of fiber from the surface of the paper as much as possible. While, the mechanical eradication was done using erasers. Papers were examined visually by naked eye and by ultraviolet device by Docucenter® 4500 instrument at a wavelength of 365 nm.
The mean hydrodynamic radii, 〈Rh〉, together with the polydispersity index of the size distribution (σ/〈Rh〉) and the zeta potential are reported in Table 1. Zeta potential is the electric potential between nanoparticles which is correlated to the stability of the nanoparticles suspension.
The mean hydrodynamic radii 〈Rh〉 and the polydispersity index of the AEMP and AEPP particles dispersed in water are shown in Fig. 1.
Fig. 1 The mean hydrodynamic radii 〈Rh〉 together with the polydispersity index of the size distribution (σ/〈Rh〉) of 3c (AEMP) and 3d (AEPP) particles dispersed in water. |
DLS measurements showed that the sonication time has a profound effect besides the included groups of novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles. AEMP and AEPP samples show two distinct behaviors of 〈Rh〉 and σ/〈Rh〉 with sonication time. For the sample containing the morpholinyl group (AEMP; 3c), a maximum value of 〈Rh〉 = 479 nm was observed for 1 minute of sonication. A smaller particle size was obtained with 5 minutes of sonication while extending the sonication time to 30 min resulted in the smallest size of 97 nm. The polydispersity index, instead, decreased from 0.79 to 0.33 and rose slightly to 0.40 with increasing the time from 1 to 5 and finally to 30 min (Table 1 and Fig. 1).
As can be noted, all the average sizes of piperidinyl-containing fluorescent compounds (AEPP; 3d) lie in the hundred nanometres region. Particle sizes are less affected by the sonication time, especially when extending the time from 5 min to 30 min, where the particle sizes were 224 nm, and 240 nm and polydispersity indices were about 0.49 and 0.58, respectively. It could be concluded that substituting the morpholinyl groups with piperidenyl groups in the investigated fluorescent heterocyclic compounds does not provide nanoscale particles, but rather in the hundreds of nanometers scale.
The zeta potential, which indicates the electrical potential between nanoparticles, of the AEPP is two times that of AEMP, indicating the superior stability of the sonicated particles of AEPP fluorescent compound over AEMP.
UV-vis absorption spectra of sonicated AEMP and AEPP particles, in water suspension, are shown in Fig. 2. For AEMP particles in water suspension two maxima in the absorption spectrum (Fig. 2) at about 340 nm (3.65 eV) and 370 nm (3.35 eV), and a tail towards longer wavelength are evident, in agreement with previously published results.27 The spectroscopic features have been assigned to the n–π* and π–π* transitions of the pyridine nuclei of AEMP molecules in nanoparticles, and a charge-transfer (CT) exciting band of the extended crystal structure, as previously observed in the absorption spectra of other organic nanoparticles.15,22 For AEPP particles in water suspension only one shoulder at about 325 nm in the absorption spectrum is evident (Fig. 2).
Fig. 2 Ultraviolet-visible absorption spectra of fluorescent heterocyclic sonicated AEMP and AEPP particles in water suspension. |
The fluorescence excitation spectra of AEMP and AEPP nanoparticles with 5 minutes of sonication time, displayed in the lower panels of Fig. 3 and 4, show similar features to those of the absorption spectra in Fig. 2.
It is obvious that the synthesized morpholine-containing heterocyclic compounds (AEMP) exhibits two excitation bands with peaks at wavelengths of about 340 nm and 375 nm, while the piperidinyl-containing compound (AEPP) has only one excitation band with a peak at a wavelength of about 335 nm, corresponding to UV absorption maxima (Fig. 2). However, one intense emission band was observed for both compounds at a wavelength of 4002540 nm when exciting at 360 nm. The emission band in the morpholinyl-containing compounds is broader with maximum at higher wavelength (458 nm), and shoulder at 550 nm, than the piperidinyl-containing compounds (only at 432.5 nm). The best fluorescence is obtained when the morpholine (AEMP) ring was introduced comparable with the piperidinyl ring (AEPP).
Fig. 3 and 4 show that the positions of the emission bands of paper treated with morpholinyl-containing fluorescent compounds (S2, AEMP) were shifted to shorter wavelengths (red shift 18.5 nm) compared to that treated with the piperidinyl-containing compound (S5, AEMP); (blue shift 7 nm), both with respect to the emission of fluorescent samples in water. This is ascribed to the probable interaction of the functional groups of the fluorescent compounds with the hydroxyl and carbonyl groups of the paper sheets. This may affects on functional moieties responsible for fluorescence properties (6-alkoxy-2-amino-4-aryl-pyridine functional groups). The presence of a lone-pair electron of oxygen in the morpholinyl ring may promote this interaction effect. This view is in agreement with the assumption reported before that the strong interaction between AEMP and cellulose occurred due to a resonant energy transfer from cellulose to nanoparticles.12
As can be seen, changing the substituted groups is accompanied by changing the position of the emission band. The intensities of the emission bands were nearly the same for paper sheets treated with the two sonicated compounds, S2 and S5, despite the weight gain on the paper of 1.3 g m−2, and 1.7 g m−2, respectively. The relatively lower weight gain of the paper with morpholinyl-containing compound did not affect either the position or the intensity of the emission band, compared with the relatively higher weight gain of the piperidinyl compound (AEPP).
For the mechanical properties, the histograms in Fig. 5 show that treating the paper sheets made from bagasse with the investigated fluorescent compounds, in addition to providing the paper samples with fluorescent character, also improves the quality number, Qz. Quality number (Qz) refers to the trend of all strength properties, i.e., breaking length, burst and tear factors, and number of double folds.41 This observation is reverse to that observed in the case of bagasse pulp-containing paper sheets treated with these unsonicated fluorescent compounds.14 In other words, subjecting the investigated fluorescent compounds to sonication reduced the bad effect of surface treating the local-bagasse paper surface with fluorescent compounds. The extent of improving is not only dependent on the fluorescent compound but also on average particle size. As can be seen, there is a good relation between the decrease in particle size, under the effect of sonication, and the improvement in quality number (Qz). Moreover, treating with the morpholinyl group-containing fluorescent compound (AEMP; 3c) achieves relatively high improvement in the double fold property of paper sheets compared with AEPP (3d). These data confirmed that the presence of the morpholinyl group instead of the piperidinyl group may enhance the interaction of fluorescent compounds with paper fibers and weaken the fiber–fiber hydrogen bonding.
Fig. 5 Strength properties of surface treated paper sheets with sonicated morpholinyl containing fluorescence (AEMP) and piperidinyl containing fluorescence (AEPP) compounds. |
It is interesting to note that all treated paper sheets are characterized by relatively high strength properties (Qz), compared to untreated local bagasse-based paper sheets.
Fig. 6 FT-IR spectra of surface treated paper sheets with sonicated morpholinyl-containing (AEMP) and piperidinyl-containing (AEPP) fluorescent compounds. |
Paper sample | MHBS | νOH or νNH (stretching) | νCH (stretching) | νCO (stretching) amide I and II (1558–1705 cm1) | νOH or νNH (bending) | νC–O 1000–1280 cm−1 | νCH (rocking) | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
cm−1 | E/A1050 | cm−1 | E | cm−1 | E | cm−1 | E | cm−1 | E | cm−1 | E | ||
a MHBS: AOH(Str.)/ACH(Str.). | |||||||||||||
Untreated | 0.917 | 3856.35 | 0.294 | 2933 | 0.522 | 1730.2 | 0.231 | 1466.9 | 0.293 | 1279 | 0.184 | 927.0 | 0.275 |
3752.22 | 0.295 | 2857 | 0.465 | 1679.1 | 0.259 | 1375.3 | 0.203 | 1158.4 | 0.263 | 879.8 | 0.992 | ||
3335.67 | 0.479 | 1533.5 | 0.2461 | 1322.4 | 0.171 | 1033.1 | 1.00 | 741.9 | 0.157 | ||||
S1 AEMP | 0.887 | 3745.5 | 0.259 | 2930 | 0.448 | 1725.4 | 0.235 | 1468.9 | 0.326 | 1270.3 | 0.189 | 887.5 | 0.348 |
3659.7 | 0.283 | 2860 | 0.395 | 1646.3 | 0.319 | 1329.1 | 0.209 | 1109.3 | 1.122 | 841.2 | 0.159 | ||
3322.2 | 0.398 | 1560.5 | 0.307 | 1111.2 | 0.543 | 645.5 | 0.474 | ||||||
3177.5 | 0.352 | 1031.2 | 1.00 | 594.4 | 1.183 | ||||||||
S2 AEMP | 0.934 | 3679.9 (sh) | 0.252 | 2925 | 0.433 | 1720.6 (s) | 0.200 | 1464.1 | 0.302 | 1279.9 | 0.245 | 899.1 | 0.367 |
3328.9 | 0.404 | 2865 | 0.364 | 1655.011 | 0.286 | 1431.3 | 0.294 | 1109.3 (sh) | 0.568 | 779.5 | 0.276 | ||
1563.4 (sh) | 0.239 | 1370.6 | 0.265 | 1036 | 1.00 | 659.9 | 0.630 | ||||||
S3 AEMP | 0.9529 | 3755.1 | 0.153 | 2931 | 0.394 | 1719.6 | 0.149 | 1464.1 | 0.229 | 1276.1 | 0.127 | 887.5 | 0.189 |
3289.4 | 0.376 | 2860 | 0.326 | 1653.1 | 0.222 | 1435.1 | 0.210 | 1150.7 | 0.204 | 781.4 | 0.142 | ||
1552.8 | 0.173 | 1371.5 | 0.155 | 1108.3 | 0.511 | 649.3 | 0.526 | ||||||
1322.4 | 0.134 | 1032.1 | 1.00 | ||||||||||
S4 AEPP | 0.9674 | 3743.5 (sh) | 0.394 | 2931 | 0.573 | 1711.9 (s) | 0.423 | 1466.9 | 0.475 | 1263.5 | 0.419 | 893.3 | 0.502 |
3613.4 | 0.481 | 2860 | 0.526 | 1658.9 | 0.46 | 1342.6 | 0.440 | 1161.3 | 0.474 | 841.2 | 0.382 | ||
3299.9 | 0.554 | 1540.3 (sh) | 0.449 | 1111.2 | 0.685 | 755.4 | 0.497 | ||||||
1032.1 | 1.00 | 650.3 | 0.754 | ||||||||||
S5 AEPP | 0.989 | 3753.2 (sh) | 0.1200 | 2929 | 0.440 | 1718.7 | 0.129 | 1461.2 | 0.221 | 1276.1 | 0.158 | 900.0 | 0.223 |
3335.7 | 0.435 | 2867 | 0.345 | 1653.1 | 0.207 | 1435.2 | 0.222 | 1159.41105 | 0.243 | 792.9 | 0.083 | ||
1368.6 | 0.200 | 1032.1 | 0.543 | 642.5 | 0.178 | ||||||||
1322.3 | 0.188 | 1.0 | 0.189 | ||||||||||
S6 AEPP | 1.225 | 3751.3 | 0.140 | 2860 | 0.368 | 1718.7 (s) | 0.133 | 1461.2 | 0.223 | 1289.6 | 0.124 | 887.5 | 0.219 |
3335.7 (sh) | 0.429 | 1663.7 | 0.189 | 1368.6 | 0.182 | 1163.3 | 0.216 | 841.2 | 0.068 | ||||
3229.7 | 0.451 | 1104.4 | 0.550 | 645.5 | 0.450 | ||||||||
1034.0 | 1.00 | 594.4 |
For all paper samples surface treated with AEPP, the relative absorbances of bands related to CO or CN decreased, and the band corresponding to OH is shifted to lower wavelength with increasing MHBS. The foregoing results concluded that subjecting the fluorescent compounds, especially those containing morpholinyl, to sonication leads to motivating their application as a security marker for high strength quality paper sheets.
Analysis of the TGA curve also can be helpful in studying the thermal stability property which is provided to local bagasse-based sheets by surface treatment with the investigated fluorescent compounds; especially with morpholinyl-containing compounds (Table 3 and Fig. 7 and 8). The TGA curves exhibit similar decomposition stages to the untreated paper samples, with differences only in onset temperatures and maximum peak temperatures. Thermogravimetric (TGA and DTGA) degradation curves for untreated paper sheets are represented by Fig. 7, and three stages can be observed. The first stage, from 50–102 °C represents the evolution of residually absorbed water that corresponds to 5.5% of the total weight. The second starts at 209.2 °C and ends at 343.1 °C, with peak maximum at 289.49 °C and occurs due to the depolymerization of paper components (cellulose and hemicellulose). This stage represents a prominent thermal degradation of weight loss of the cellulose to levoglucosan, with weight loss ∼76.5%, and is known as the volatilization stage. The third stage occurs in the region of 380.6–470 °C and has a peak maximum at 438.8 °C, and occurs due to rapid volatilization accompanied by the formation of carbonaceous residue, with weight loss of 96.8% (Table 3). For surface-treated paper samples, the onset temperatures are 210–241 °C and peak temperatures are 296–307 °C. Also, their activation energies (Ea) for the main depolymerization stages are increased to 148–162 kJ mol−1, while Ea for untreated paper is 131 kJ mol−1. It is surprising to notice that the surfaces treated with these investigated compounds retard the degradation, leading to a reduction in the weight loss observed in the volatilization stage to about 75–70.5%, and the final degradation stages to about 95.2–90%. This means that the fluorescent heterocyclic compounds in addition to providing thermal stability to paper sheets also exhibit to some extent fire resistance in the produced treated paper. This view based on the main theory of flame retardance, is to minimize the formation of levoglucosan by lowering the decomposition temperature of cellulose and enhancing char formation by catalyzing the dehydration and decomposition reactions.43–45
Paper sample | Stage | Temp. range °C | DTG peak temp. °C | “n” | −r | Se | Ea kJ mol−1 | Wt (final) |
---|---|---|---|---|---|---|---|---|
Untreated | 1st | 50–102.41 | — | — | — | — | — | 94.563 |
2nd | 209.19–343.11 | 289.49 | 1.5 | 0.989 | 0.17 | 131 | 23.485 | |
3rd | 380.6–469.96 | 438.8 | 3.16 | |||||
S1 AEMP | 1st | 50–125.09 | — | — | — | — | — | 93.555 |
2nd | 223.39–345.96 | 307.68 | 1.5 | 0.986 | 0.14 | 143.32 | 29.498 | |
3rd | 381.87–477.01 | 424.46 | 9.797 | |||||
S2 AEMP | 1st | 50–99.44 | — | — | — | — | 93.374 | |
2nd | 226.33–344.4 | 302.2 | 1.5 | 0.991 | 0.15 | 138.38 | 25.403 | |
3rd | 392.03–487.14 | 452.51 | 5.833 | |||||
S3 AEMP | 1st | 50–109.37 | — | — | — | — | 93.355 | |
2nd | 233.46–356.07 | 305.68 | 1.5 | 0.984 | 0.19 | 162.07 | 23.94 | |
3rd | 392.1–491.41 | 455.6 | 6.351 | |||||
S4 AEPP | 1st | 50–137.24 | — | — | — | — | 91.686 | |
2nd | 210.1–346.22 | 303.81 | 1.5 | 0.946 | 0.22 | 131.15 | 24.984 | |
3rd | 389.1–478.5 | 455.26 | 6.781 | |||||
S5 AEPP | 1st | 50–102.29 | — | — | — | — | 92.866 | |
2nd | 232.04–344.8 | 296.52 | 1.5 | 0.983 | 0.19 | 148.08 | 27.945 | |
3rd | 392.87–516.04 | 459.85 | 7.846 | |||||
S6 AEPP | 1st | 50–115.3 | — | — | — | — | 91.846 | |
2nd | 240.78–360.49 | 304.84 | 1.5 | 0.989 | 0.16 | 152.13 | 22.02 | |
3rd | 386.3–498.7 | 452.6 | 4.751 |
Fig. 7 TGA and DTGA curves of surface-treated paper sheets with sonicated morpholinyl-containing fluorescent compound (AEMP). |
Fig. 8 TGA and DTGA curves of surface-treated paper sheets with sonicated piperidinyl-containing fluorescent compound (AEPP). |
From Fig. 9 and 10, it was noticed that sodium hypochlorite erasure had a great effect on erasing ink from both the morpholinyl-containing fluorescent compound (AEMP) and the piperidinyl-containing fluorescent compound (AEPP) treated paper sheets. This agent acts as a reducing agent for the ink color and the erased area appears as a dark stain by ultraviolet light as it destroys the fluorescence behavior of the fluorescent compounds in this area.
Fig. 9 Chemical and mechanical erasure of surface-treated paper sheets with morpholinyl-containing fluorescent compound (AEMP). |
Fig. 10 Chemical and mechanical erasure of surface-treated paper sheets with piperidinyl-containing fluorescent compound (AEPP). |
For the case of benzyl alcohol followed by ethanol, when applied on ballpoint ink strokes that were written on treated paper sheets, it was difficult to erase the ink color completely, so the erased area appears under ultraviolet light.
On comparing with untreated paper sheets, it was noticed that sodium hypochlorite or benzyl alcohol when applied for erasing ink color cannot be detected easily and gave good erased sites without changing the color of the untreated paper itself. Time requires for erasing the ink color from the paper increased in the case of paper treated with AEMP than AEPP. This view emphasized the highest surface bonding capacity of sonicated AEMP with relatively lower particle size (S3, 〈Rh〉 = 97 nm). As can be noticed, erasure by benzyl chloride was less effective for removing the ink stroke letters from treated paper produced from different sonicated fluorescent particles, and left a zone that appeared under ultraviolet light. For AEPP-treated paper sheets this zone appeared only for fluorescent particles resulting from sonication for 5 and 30 min.
For mechanical erasure (Fig. 9 and 10), it was noticed that trials required for erasing the ink stroke from the treated paper sheets increased with the treated fluorescent particles resulted from sonication times of 5 min and 30 min. Moreover, the mechanical erasure was less effective for removing the written letters than benzyl chloride.
It was found that the shaved area exhibited more intense fluorescence than the neighboring non-shaved area. So we can conclude that fluorescent compounds, especially with lower particle size, are penetrated inside paper sheets to a greater depth than the ink stroke itself.
The application of the resulted particle dispersions of AEMP and AEPP in water to bagasse-based paper sheets provided excellent results. For strength properties, the results showed that the extent of improving is not only dependent on the fluorescent compound but also on the average particle size. There was a good relation between the decrease in particle size, under the effect of sonication, and the improvement in quality number (Qz). Moreover treating with the morpholinyl group-containing fluorescent compound (AEMP) achieved relatively high improvement in the double fold property of paper sheets, compared with AEPP.
It is surprising to notice that these investigated compounds, besides providing thermal stability to the treated paper sheets, also retarded the degradation, and decreased the formation of levoglucosan, and consequently the weight loss. The weight loss of the volatilization stage is about 75–70.5%, and the final degradation stages are about 95.2–90%. This means that the fluorescent heterocyclic compounds, in addition to providing thermal stability to the paper sheets, also provide, to some extent, fire resistant properties to the produced treated paper.
The unfalsifiable safety property of the treated documents by erasure techniques showed that fluorescent compounds, especially with lower particle size, penetrated inside the paper sheets which makes it difficult for them to be falsified.
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