Preparation of Cellulose-based uorescent materials as coating pigment by use of reversible DMSO/DBU/CO2 system

A series of cellulose-based uorescent materials are prepared under relative mild condition by use of the particular reversible DMSO/DBU/CO 2 system to utilize as coating pigments. Through the observation under 365nm UV light, the cellulose-based uorescent materials exhibit good uorescence response and bright color. Furthermore, due to the limitation of the molecular skeleton of cellulose, the intrinsic aggregation caused quenching phenomenon commonly existed in conventional organic uorescent pigments can be effectively inhibited, which is very helpful to retain good uorescence response in epoxy-based coating material and its coating lms. Moreover, the addition of cellulose-based uorescent materials also increases the mechanical properties of the coating lm. The increase of tensile strength and tensile modulus respectively reaches ~ 39% and ~ 66%. Solvent resistance and thermal property of the coating lms generally remain unchanged. The fabrication of cellulose-based uorescent materials in DMSO/DBU/CO 2 system provides a feasible way to develop the functional application of cellulose.


Introduction
With the constant exhaustion of petrochemical energy and the aggravation of environmental pollution, the development and utilization of clean and renewable bioresources have been widely attended by people (Llevot et al., 2016;Teng et al., 2010). As one of the most important green bioresources with good performance, cellulose is strongly expected to be used as renewable material (Druel et al., 2018). But, in general, cellulose has very strong intermolecular and intramolecular hydrogen bonds. It makes cellulose di cult to processing and forming (H. Zhang et al., 2005;L. Zhang et al., 2019). This defect weakens the applicability of cellulose.
With the help of these dissolving systems, the more e ciently functional modi cation of cellulose can be achieved. In recent years, a reversible DMSO/organic base/CO 2 system has been further developed to dissolve cellulose (Q. Zhang et al., 2013). In this particular system, the hydroxyl of cellulose rstly reacts with amine base and CO 2 to produce a kind of temporary carbonate structure, which forms similar effect of ionic liquid.
This process activates the hydroxyl of cellulose and promotes the solubility of microcrystalline cellulose up to 10wt% (Xie et al., 2014). Such promising method not only produces a homogeneous dissolution system, but also improves the modi cation ability of cellulose through its particular "self-activation" even under mild reaction condition, such as the acylation with anhydride , grafting with polylactide  and polyester (Xu et al., 2017), and substitution with halides (Onwukamike et al., 2017). Though many researches have involved the modi cation of cellulose, few reports mentioned the functional cellulose derivatives. Furthermore, the uorescent cellulose derivative obtained by reversible DMSO/DBU/CO 2 system has not been reported. In fact, the uorescent derivative modi cation of cellulose is an important way to create cellulose-based uorescent materials which promote the application of this green bioresource material. Furthermore, common uorescent pigments usually have advantages as diverse structure, bright color and strong tinting strength. But as relatively small molecular chemicals, they tend to migrate and to reunite (Rahman et al., 2009). This phenomenon leads to the well-known aggregation caused quenching (ACQ) (Birks, 1971). As a result, the actual color of the common uorescent pigments will be much dim. In contrast, the cellulose-based uorescent materials can be easily controlled with stable chemical structure to achieve good solvent resistance and chemical stability. More notably, it can inhibit the unnecessary migration of uorescent groups and possibly avoid the ACQ to retain bright color.
In this work, corncob cellulose was utilized to dissolve in the reversible DMSO/DBU/CO 2 system to prepare cellulose-based uorescent materials. And four kinds of uorescent groups were respectively introduced into cellulose derivatives. The uorescent response of the cellulose-based uorescent materials was researched. After that, these uorescent cellulose derivatives were further tried to add into epoxy resin as pigments to fabricate the thermosetting uorescent coatings. Fluorescent response and mechanical properties of the uorescent coatings were also discussed in the experiment.

Dissolving cellulose in DMSO/DBU/CO 2 system and synthesis of cellulose-based uorescent materials
Corncob cellulose (0.235g), DMSO (10g, 128mmol) and DBU (0.75g, 4.93mmol) were added into a highpressure reactor, and then kept stirring while continuously pumping CO 2 (5bar) for 2h at 50 o C to obtain clear and transparent cellulose solution (2wt%). The cellulose solution was kept at 50 o C in the reactor for 1h and then cooled to room temperature. Then 2-BMN (0.32g, 1.45mmol) was carefully added to the obtained cellulose solution and then followed with a vigorous mechanical stirring for 4h at 30 o C. After nishing the derivative reaction, the mixture was poured into methanol (100mL) and washed twice continuously with distilled water (50mL) and methanol (50mL), respectively. The precipitate was dried at 80 o C for 24h to obtain powder as nal product A. The synthesis processes of the other cellulose-based uorescent materials were exactly the same. The cellulose-based uorescent materials produced with Coumarin 151 (0.332g, 1.45mmol), 1-BMP (0.428g, 1.45mmol), and 9-CMA (0.328g, 1.45mmol) are respectively marked with B, C and D. The schematic process to synthesize cellulose-based uorescent materials is shown in Figure 1.

Preparation of epoxy coating lms
The cellulose and cellulose-based uorescent materials were immersed in 2wt% NaOH aqueous solution (500mL), respectively. The container was placed in a magnetic stirrer for 4h at 60 o C until the reaction was completed. Alkali swelling treatment, the powders were ltrated and washed several times to neutral before dried at 80 o C. The KH560 solution was prepared by 2.5mL KH560, 77.5mL distilled water and 120mL anhydrous ethanol with stirring continuously for 2h. Then cellulose and cellulose-based uorescent materials were added into KH560 solution for 4h to fully coupling and then dried at 80 o C for further use. 0.5g of cellulose and cellulose-based uorescent materials were dispersed in 12mL of water and treated with ultrasound for 4h.
Epoxy resins (10g) and curing agent (DETA, 1.3g) in the stoichiometric ratio (epoxy group/N−H=1:1) were wellmixed in a beaker. After that, cellulose-based uorescent materials (0.5g) were added with stirring continuously for 10min. Then the mixtures were quickly transferred to a stainless mold. The thermal curing reaction was performed on a heater at 80°C for 3h. Finally, the mold was slowly cooled down to room temperature. The cured epoxy resin lms were removed from the mold and then left at room temperature for 24h. The epoxy coating lm with cellulose (0.5g) was also prepared as blank sample.

Characterization
The FT-IR analysis of the synthesized cellulose-based uorescent materials was performed on a NICOLET 6700 FT-IR using the KBr pellet method with transmittance mode. The spectra recorded from 400 to 4000 cm -1 at room temperature and 16 scans were collected for each sample. Fluorescence spectra of the cellulosebased uorescent materials and epoxy coating lms were measured by Hitachi F-4600 Spectro uorometer equipped with a xenon (Xe) lamp (150W). The differential scanning calorimetry (DSC) measurements were performed on a Mettler-Toledo MET DSC under a high-purity nitrogen atmosphere with a owing rate of 60mL/min. Each sample (~7mg)

Fluorescence properties of cellulose-based uorescent materials
Corncob cellulose was rstly dissolved in DMSO/DBU/CO 2 system and then reacted with four aromatic organics contained the luminescent groups of 2-BMN, Coumarin 151, 1-BMP and 9-CMA to form cellulosebased uorescent materials (namely the obtained uorescent cellulose carbonates), respectively. FT-IR spectra of the corncob cellulose and the synthesized cellulose-based uorescent materials are shown in Figure 2.
From the gure, 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 at 1523cm -1 , 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. The results clearly exhibit uorescent groups of 2-BMN, Coumarin 151, 1-BMP and 9-CMA are all successfully grafted onto the molecular chains of the corncob cellulose.
The normalized uorescent emission spectra of the cellulose-based uorescent materials are shown in Figure  3(a). Under the irradiation of 365nm ultraviolet (UV) light, the four kinds of cellulose-based uorescent materials show different uorescence response. From Figure 3(a), it is clear that the emission peaks (λ max ) of these cellulose-based uorescent materials are separated at 424nm (A), 470nm (B), 505nm (C), and 579nm (D). Figure 3(b) gives the UV-vis absorption of the cellulose-based uorescent materials. Good absorption in the range from 250nm to 365nm can be clearly exhibited for all the cellulose-based uorescent materials. Under UV light, four different bright colors of purple, blue, green and orange can be shown by these cellulosebased uorescent materials, respectively (as shown from Figure 3(c)), which are different with the colors shown under visible light.

Fluorescence properties of epoxy coating lms
As shown in Figure 4(a-d), Coumarin 151 were simply mixed with cellulose and utilized to graft onto the molecular skeleton of cellulose, respectively. 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   4(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 uorescent material) is much brighter under 365nm UV irradiation (Figure 4(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 4(e). There was no obvious uorescence response inside the coating material under UV irradiation (365nm). Only weak uorescence response appeared on the surface of the coating material. It probably owes to the serious ACQ of Coumarin 151 inside the coating material. The ACQ phenomenon can be also found in the coating lm (see Figure 4(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 uorescence under UV irradiation (see Figure 4(f)). Furthermore, the coating lm with the addition of cellulose-based uorescent material as pigment also shows rather bright blue color under UV-light (Figure 4(i-j)). That is to say, ACQ phenomenon in the coating material has been greatly suppressed. Zhang and coworker (Tian et al., 2016(Tian et al., , 2018 had ever noticed the similar inhibition of ACQ by grafting uorescent groups on the molecular skeleton of cellulose with the use of common ionic liquid system at 150 o C. The coating lms obtained from cellulose-based uorescent material showed rather good uorescence response and bright colors. By using of the reversible DMSO/DBU/CO 2 system in our research, due to the particular "self-activation" of the solvent system to cellulose, cellulose-based uorescent materials (i.e the cellulose carbonates) are successfully synthesized under relative mild reaction condition at 30-50 o C.
Then they can be prepared of epoxy-based uorescent coating material for e cient inhibition of ACQ, which has not been clearly reported previously.
The prepared epoxy-based uorescent coatings materials were continuously cured into coating lms. The uorescent emission spectra of the lms are shown in Figure 5. Meanwhile, the epoxy resin lms with pure corncob cellulose and without any additives were used as comparable samples. From Figure 5, the emission peaks of uorescent lms including the cellulose-based uorescent materials had obvious uorescence response with the emission peaks (λ max ) at 435nm(A), 476nm(B), 512nm(C) and 594nm(D), respectively. The λ max is basically the same and very slightly higher than the value from the cellulose-based uorescent materials. However, the pure epoxy resin lm and the lm only with corncob cellulose don't show obvious uorescence response. These results show that the uorescence response and the color under 365nm UV light of the epoxy-based coating lms are actually decided by the added cellulose-based uorescent materials.
Therefore, it is feasible to use the obtained cellulose-based uorescent materials to prepare epoxy-based uorescent coating materials and lms.

Standard, mechanical and thermal properties of epoxy coating lms
The standard properties of epoxy coating lms are shown in Table 1. In comparison with pure epoxy lm, the addition with cellulose and its uorescent derivatives induces the increase of the hardness level from H to 2H. Meanwhile, the increase of hardness doesn't impact the exibility of the coating lms. The only decrease exists on the adhesion level between coating lms and metal substrate, which is from 0 to 1. However, the result of "level 1" is still satis ed with the performance standard of national standard (GB/T9286-1988) as a high-quality coating. Furthermore, all the coating lms show good water and acetone resistances. The addition of cellulose and its uorescent derivatives doesn't change the stability of epoxy-based coating lms. Mechanical properties of epoxy coating lms are investigated by the tensile test. The tensile curves of the coating lms are shown in Figure 6 and the mechanical properties are listed in Table 2. After the addition of cellulose and its uorescent derivatives into coating lms, 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 lms decreases from 4.30±0.02% to 2.46-2.78%, it doesn't re ect obvious change in the essential stiffness and toughness. That is because cellulose and its uorescent derivatives are rigid additives as to epoxy resin. The addition of cellulose and its uorescent derivatives increase hardness and rigidity of epoxy coating lms. It increases the mechanical properties to some extent. That is consistent with previous research ndings 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).

Conclusion
Cellulose-based uorescent materials were successfully achieved by taking advantage of reversible DMSO/DBU/CO 2 system under relative mild condition. The obtained cellulose-based uorescent materials show good uorescence response and bright colors under 365nm UV light. Due to the limitation of the molecular structure, the intrinsic ACQ phenomenon of conventional organic uorescent pigments can be inhabited. Then the cellulose-based uorescent materials are introduced into epoxy resin to prepare uorescent coating. It also shows rather good uorescence response and the bright color. Moreover, the addition of cellulose derivatives increases the mechanical properties of the coating lm. Solvent resistance and thermal property of the coating lms generally remain unchanged.

Competing interests
There is no competing interest to declare.

Data Availability
All data generated or analyzed during this study are included in this published article.

Author contribution
Qinghua Cao carried out the majority parts of experiments such as dissolving and modi cation of corncob cellulose. He also carried out the majority parts of analysis of experimental results and writing of manuscript.
Jinyue Dai guided the preparation and characterization of epoxy coating lm.
Xin Bao helped performing the analysis and gave constructive discussion during experiment.
Zhenyu Zhang helped to carry out the analysis of experiment and some part of manuscript preparation.
Fei Liu helped performing the analysis of partially experimental data.
Yuhong Feng guided the research methodology of this manuscript.
Haining Na guided the analysis of partially experimental data and the writing of this manuscript.
Jin Zhu guided the general conception of this study.