3.1. Fluorescence properties of cellulose-based fluorescent materials
Corncob cellulose and MCC were firstly dissolved in DMSO/DBU/CO2 system and then reacted with four aromatic organics contained the luminescent groups of 2-BMN, Coumarin 151, 1-BMP and 9-CMA to form cellulose-based fluorescent materials (namely the obtained fluorescent cellulose carbonates), respectively. FT-IR spectra of the corncob cellulose, MCC and the synthesized cellulose-based fluorescent materials are shown in Figure 2(a,b). From the figure, it appears the absorbance peak of C=O at ~1743 cm-1 in the curves of cellulose derivatives (A, B, C, and D), which indicate the formation of ester groups. Furthermore, the absorption peaks of benzene ring from 1523cm-1 to 1445cm-1 and 725cm-1 are also found on the FT-IR curves of these cellulose derivatives, which is different from the original corncob cellulose and MCC. The results clearly exhibit fluorescent groups of 2-BMN, Coumarin 151, 1-BMP and 9-CMA are all successfully grafted onto the molecular chains of the corncob cellulose and MCC.
1H-NMR spectra of synthesized cellulose-based fluorescent materials using MCC are shown in Figure 3. The obvious presence of the introduced aromatic group and heterocyclic groups are seen in graphs. The degree of substitution (DS) of the cellulose-based fluorescent materials were calculated (DSA=0.40,DSB=0.32,DSC =0.28,DSD=0.23).
Under the irradiation of 365nm ultraviolet (UV) light, the four kinds of cellulose-based fluorescent materials prepared by MCC show different fluorescence responses. From Figure 4, four different bright colors of purple, blue, green and orange can be shown by these cellulose-based fluorescent materials, respectively, which are different from the colors shown under visible light.
The normalized fluorescent emission spectra of the cellulose-based fluorescent materials are shown in Figure 5(a). Under the irradiation of 365nm ultraviolet (UV) light, the four kinds of cellulose-based fluorescent materials show different fluorescence response. From Figure 5(a), it is clear that the emission peaks (λmax) of these cellulose-based fluorescent materials are separated at 424nm (A), 470nm (B), 505nm (C), and 579nm (D). Figure 5(b) gives the UV-vis absorption of the cellulose-based fluorescent materials. Good absorption in the range from 250nm to 365nm can be clearly exhibited for all the cellulose-based fluorescent materials. These cellulose-based fluorescent materials prepared by corncob cellulose show the same fluorescence response with MCC derivatives (as shown in Figure 5(c)).
These various color and fluorescence response of cellulose-based fluorescent materials further reflect that 2-BMN, Coumarin 151, 1-BMP and 9-CMA are all successfully grafted onto the molecular chain of cellulose. Generally, simple unmodified fluorophores (i.e. naphthalene, anthracene and pyrene without any push-pull substituents) cannot produce relative large stokes shifts. The emission peaks of them (λmax) in fluorescent emission spectra usually should be at relative low value. However, in the cellulose-based fluorescent materials, 2-BMN, Coumarin 151, 1-BMP and 9-CMA are grafting onto the molecular chain of cellulose through carbonate bond. The carbonate bonding group and molecular chain of cellulose produce the effect of push-pull substituents. It induces the fluorescent groups of naphthalene, anthracene and pyrene (namely 2-BMN, Coumarin 151, 1-BMP and 9-CMA) to produce relative large stokes shifts. As a result, the emission peaks (λmax) are improved at 424nm (A), 470nm (B), 505nm (C), and 579nm (D), respectively. This phenomenon is accord with the references (K. A. Fletcher et al., 2001; W. Z. Jiang et al., 2020; Y. F. Sun et al., 2013; M. Sugino et al. 2014).
3.2. Fluorescence properties of epoxy coating films
As shown in Figure 6(a-d), Coumarin 151 were simply mixed with cellulose and utilized to graft onto the molecular skeleton of cellulose, respectively. Both the above ways added the same mass of Coumarin 151. And then both of them were exposed under visible and 365nm UV light. It can be clearly seen that the color of the mixture of cellulose and Coumarin 151 is rather dim (Figure 6(c)). This result is attributed to the ACQ of Coumarin 151. After grafting Coumarin 151 on the molecular skeleton of cellulose, because of the limitation of the molecular structure, it shows that the ACQ phenomenon is obviously inhibited. As a result, the color of the Coumarin 151 grafted onto cellulose (i.e. the obtained cellulose-based fluorescent material) is much brighter under 365nm UV irradiation (Figure 6(d)).
After that, the mixture of cellulose and Coumarin 151 is introduced into epoxy resin to prepare coating material with stirring continuously for 10min, as exhibited in Figure 6(e). There was no obvious fluorescence response inside the coating material under UV irradiation (365nm). Only weak fluorescence response appeared on the surface of the coating material. It probably owes to the serious ACQ of Coumarin 151 inside the coating material. Even small amounts without good dispersion will cause fluorescence quenching. The ACQ phenomenon can be also found in the coating film (see Figure 6(g-h). However, when the similar process was used to mix Coumarin 151 grafted cellulose derivative with epoxy resin, it presents a rather bright blue fluorescence under UV irradiation (see Figure 6(f)). Furthermore, the coating film with the addition of cellulose-based fluorescent material as pigment also shows rather bright blue color under UV-light (Figure 6(i-j)). That is to say, ACQ phenomenon in the coating material has been greatly suppressed. Zhang and coworker (Tian et al., 2016, 2018) had ever noticed the similar inhibition of ACQ by grafting fluorescent groups on the molecular skeleton of cellulose with the use of common ionic liquid system. The coating films obtained from cellulose-based fluorescent material showed rather good fluorescence response and bright colors. By using of the DMSO/DBU/CO2 system in our research, cellulose-based fluorescent materials (i.e the cellulose carbonates) are successfully synthesized under relative mild reaction condition at 30-50oC, which is lower than the reaction temperature reported by using common ionic liquid at ~150oC (Tian et al., 2016, 2018). Then they can be prepared of epoxy-based fluorescent coating material for efficient inhibition of ACQ, which has not been clearly reported previously.
The prepared epoxy-based fluorescent coatings materials were continuously cured into coating films. The fluorescent emission spectra of the films are shown in Figure 7. Meanwhile, the epoxy resin films with pure corncob cellulose and without any additives were used as comparable samples. From Figure 7, the emission peaks of fluorescent films including the cellulose-based fluorescent materials had obvious fluorescence response with the emission peaks (λmax) at 435nm(A), 476nm(B), 512nm(C) and 594nm(D), respectively. The λmax is basically the same and very slightly higher than the value from the cellulose-based fluorescent materials. However, the pure epoxy resin film and the film only with corncob cellulose do not show obvious fluorescence response. These results show that the fluorescence response and the color under 365nm UV light of the epoxy-based coating films are actually decided by the added cellulose-based fluorescent materials. Therefore, it is feasible to use the obtained cellulose-based fluorescent materials to prepare epoxy-based fluorescent coating materials and films.
Table 1. Standard properties of coating films prepared by epoxy with or without corncob cellulose and its fluorescent derivatives.
Sample
|
Hardness
|
Adhesion/level
|
Flexibility/mm
|
Water resistance
(25℃,240h)
|
Acetone resistance
(wiping method)/time
|
Epoxy/A
|
2H
|
1
|
1
|
Normal
|
>500
|
Epoxy/B
|
2H
|
1
|
1
|
Normal
|
>500
|
Epoxy/C
|
2H
|
1
|
1
|
Normal
|
>500
|
Epoxy/D
|
2H
|
1
|
1
|
Normal
|
>500
|
Epoxy/Cellulose
|
2H
|
1
|
1
|
Normal
|
>500
|
Epoxy
|
H
|
0
|
1
|
Normal
|
>500
|
Table 2. Mechanical properties of epoxy coating films with or without corncob cellulose and its fluorescent derivatives.
Sample
|
Tensile strength
(MPa)
|
Tensile modulus
(MPa)
|
Elongation at break
(%)
|
Epoxy/A
|
78.17±1.53
|
35.44±1.60
|
2.48±0.01
|
Epoxy/B
|
75.03±2.28
|
31.97±0.54
|
2.78±0.05
|
Epoxy/C
|
75.13±2.29
|
36.11±0.88
|
2.56±0.03
|
Epoxy/D
|
83.21±0.67
|
36.69±1.20
|
2.60±0.02
|
Epoxy/Cellulose
|
75.73±1.66
|
35.85±1.52
|
2.46±0.05
|
Epoxy
|
55.63±0.85
|
19.21±0.78
|
4.30±0.02
|
3.3. Standard, mechanical and thermal properties of epoxy coating films
The standard properties of epoxy coating films are shown in Table 1. In comparison with pure epoxy film, the addition with cellulose and its fluorescent derivatives induces the increase of the hardness level from H to 2H. Meanwhile, the increase of hardness does not impact the flexibility of the coating films. The only decrease exists on the adhesion level between coating films and metal substrate, which is from 0 to 1. However, the result of “level 1” is still satisfied with the performance standard of national standard (GB/T9286-1988) as a high-quality coating. Furthermore, all the coating films show good water and acetone resistances. The addition of cellulose and its fluorescent derivatives does not change the stability of epoxy-based coating films.
Mechanical properties of epoxy coating films are investigated by the tensile test. The tensile curves of the coating films are shown in Figure 8 and the mechanical properties are listed in Table 2. After the addition of cellulose and its fluorescent derivatives into coating films, the tensile curves show greatly change (see Figure 6). Tensile strength and tensile modulus are respectively increase from 55.63±0.85MPa to 75-83MPa and from 19.21±0.78MPa to 31-35MPa (see Table 2). The increase of tensile strength and tensile modulus reaches ~39% and ~66%. Although, at the same time, the elongation of the coating films decreases from 4.30±0.02% to 2.46-2.78%, it does not reflect obvious change in the essential stiffness and toughness. That is because cellulose and its fluorescent derivatives are rigid additives as to epoxy resin. The addition of cellulose and its fluorescent derivatives increase hardness and rigidity of epoxy coating films. It increases the mechanical properties to some extent. That is consistent with previous research findings that cellulose has ability to improve the mechanical property of epoxy matrix (Alamri & Low, 2013; Canche-Escamilla et al., 2002; HerreraFranco & AguilarVega, 1997; Jang et al., 2012; Masoodi et al., 2012; Sdrobis et al., 2012).
DSC measurement is selected to determine the glass transition temperature (Tg) of all the epoxy coating films. The DSC heating curves are shown in Figure 9. The Tg of pure epoxy coating film was 90.0oC. After added cellulose and its fluorescent derivatives, the Tg of coating films slightly decrease to range from 88.4℃ to 89.6oC. The use of cellulose-based fluorescent materials does not change the thermal property of the coating films.