Preparation of cellulose-based fluorescent materials as coating pigment by use of DMSO/DBU/CO2 system

A series of cellulose-based fluorescent materials are prepared under relative mild conditions by use of the DMSO/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/CO2 system to utilize as coating pigments. Through the observation under 365 nm UV light, the cellulose-based fluorescent materials exhibit good fluorescence 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 fluorescent pigments can be effectively inhibited, which is very helpful to retain good fluorescence response in epoxy-based coating material and its coating films. Moreover, the addition of cellulose-based fluorescent materials also increases the mechanical properties of the coating film. The increase of tensile strength and tensile modulus respectively reaches ~39% and ~66%. Solvent resistance and thermal property of the coating films generally remain unchanged. The fabrication of cellulose-based fluorescent materials in DMSO/DBU/CO2 system provides a feasible way to develop the functional application of cellulose.

Abstract A series of cellulose-based fluorescent materials are prepared under relative mild conditions by use of the DMSO/1,.0]undec-7ene (DBU)/CO 2 system to utilize as coating pigments. Through the observation under 365 nm UV light, the cellulose-based fluorescent materials exhibit good fluorescence 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 fluorescent pigments can be effectively inhibited, which is very helpful to retain good fluorescence response in epoxy-based coating material and its coating films. Moreover, the addition of cellulosebased fluorescent materials also increases the mechanical properties of the coating film. The increase of tensile strength and tensile modulus respectively reaches *39% and *66%. Solvent resistance and thermal property of the coating films generally remain unchanged. The fabrication of cellulose-based fluorescent 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 difficult to processing and forming (H. Zhang et al. 2005; L. Zhang et al. 2019). This defect weakens the applicability of cellulose.
By use of functional modification (Heinze and Liebert 2001;Klemm et al. 2005;Wu et al. 2004;Zheng et al. 2005), to convert cellulose into ether or ester derivatives, the processing ability and potential application are definitely improved. Usually, the functionalization of cellulose could be carried out via heterogeneous and homogeneous approaches (Klemm et al. 2005). The preparation of cellulose derivative in heterogeneous non-dissolving system used to be the mainstream approach and mainly for industrial products (Heinze and Liebert 2001). However, by using heterogeneous system, the effect and efficiency of functional modification is commonly at low level (Wu et al. 2004). In order to establish a homogeneous system to increase ability and quality of cellulose functionalization, during past decades, many dissolving systems have been developed such as N,Ndimethylacetamide/LiCl (DMAc/LiCl) (Mccormick and Dawsey 1990), N-methylmorpholine-N-oxidemonohydrate (NMMO) (Fink et al. 2001 (Fischer et al. 2003), NaOH/water/urea (or thiourea) solutions (Zhou and Zhang 2000) and ionic liquids (ILs) (Andanson et al. 2014;Canche-Escamilla et al. 2002). With the help of these dissolving systems, the more efficiently functional modification of cellulose can be achieved. In recent years, a DMSO/organic base/CO 2 system has been further developed to dissolve cellulose (Q. Zhang et al. 2013). In this system, the hydroxyl of cellulose firstly reacts with amine base and CO 2 to produce a kind of temporary carbonate structure, which forms similar effect of ionic liquid. This process can activates the hydroxyl of cellulose and promotes the solubility of microcrystalline cellulose (Xie et al. 2014). This method not only produces a homogeneous dissolution system, but also improves the modification ability of cellulose 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 modification of cellulose, few reports mentioned the functional cellulose derivatives. Furthermore, the fluorescent cellulose derivative obtained by DMSO/1,.0]undec-7-ene (DBU)/CO 2 system has not been reported. In fact, the fluorescent derivative modification of cellulose is an important way to create cellulose-based fluorescent materials which promote the application of this green bioresource material. Furthermore, common fluorescent 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 fluorescent pigments will be much dim. In contrast, the cellulose-based fluorescent 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 fluorescent groups and possibly avoid the ACQ to retain bright color.
In this work, cellulose was utilized to dissolve in the DMSO/DBU/CO 2 system to prepare cellulose-based fluorescent materials. And four kinds of fluorescent groups were respectively introduced into cellulose derivatives. The fluorescent response of the cellulosebased fluorescent materials was researched. After that, these fluorescent cellulose derivatives were further tried to add into epoxy resin as pigments to fabricate the thermosetting fluorescent coatings. Fluorescent response and mechanical properties of the fluorescent coatings were also discussed in the experiments.
Dissolving cellulose in DMSO/DBU/CO 2 system and synthesis of cellulose-based fluorescent materials Cellulose (0.235g), DMSO (10g, 128mmol) and DBU (0.75g, 4.93mmol) were added into a high-pressure reactor, and then kept stirring while continuously pumping CO 2 (5bar) for 2h at 50°C to obtain clear and transparent cellulose solution (2wt%). The cellulose solution was kept at 50°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°C. After finishing 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°C for 24h to obtain powder as final product A. The synthesis processes of the other cellulose-based fluorescent materials were exactly the same. The cellulose-based fluorescent 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 fluorescent materials is shown in Figure 1.

Preparation of epoxy coating films
The cellulose and cellulose-based fluorescent materials (with particle diameter *20lm) were activated to enhance the interaction with by following the coupling agent by immersing in 2 wt% NaOH aqueous solution (500mL) according to the reported method (T. J. Lu et al.; A. K. Bledzki et al.; Y. J. Xie et at.). The container was placed in a magnetic stirrer for 4h at 60°C until the reaction was completed. After alkali treatment, the powders were filtrated and washed several times to neutral before and then dried at 80°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 fluorescent materials were added into KH560 solution for 4h to fully coupling and then dried at 80°C for further use. 0.5g of cellulose and cellulosebased fluorescent 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 well-mixed in a beaker. After that, cellulosebased fluorescent 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 films (thickness *200lm) were removed from the mold and then left at room temperature for 24h. The epoxy coating film with cellulose (0.5g) was also prepared as blank sample.

Characterization
The FT-IR analysis of the synthesized cellulose-based fluorescent 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. The 1 H-NMR spectra were recorded with a Bruker AVANCE III 400MHz NMR spectrometer using TMS (Tetramethyl silane) as the  internal standard. The measurement was performed at 80°C and DMSO-d 6 was used as a solvent. Fluorescence spectra of the cellulose-based fluorescent materials and epoxy coating films were measured by Hitachi F-4600 Spectrofluorometer 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 flowing rate of 60mL/ min. Each sample (*7mg) was sealed in an aluminum crucible and then heated from -30 to 250°C with heating rate of 10°C/min. Mechanical properties of epoxy coating films were carried out using a Universal Mechanical Testing Machine (Instron 5569A) with a crosshead speed of 5mm/min. Before testing, all the samples were cut into 4095mm splines. The tensile properties of each sample were reported as the average of five measurements. The standard properties of epoxy coating films included hardness (GB/T6739-2006), adhesion (GB/T9286-1988), and flexibility (GB/T6742-2007).

Results and discussion
Fluorescence properties of cellulose-based fluorescent materials Corncob cellulose and MCC were firstly 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 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 2a,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. 1 H-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 by following equations (Xu et al. 2017).
where The normalized fluorescent emission spectra of the cellulose-based fluorescent materials are shown in Figure 5a. Under the irradiation of 365nm ultraviolet (UV) light, the four kinds of cellulose-based fluorescent materials show different fluorescence response. From Figure 5a, it is clear that the emission peaks (k max ) of these cellulose-based fluorescent materials are separated at 424nm (A), 470nm (B), 505nm (C), and 579nm (D). Figure 5b 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 5c). 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 (k max ) 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 (k max ) are improved at 424nm (A), 470nm (B), 505nm (C), and 579nm (D), respectively. This phenomenon is accord with the references (Fletcher et al. 2001;Jiang et al. 2020;Sun et al. 2013;Sugino et al. 2014).

Fluorescence properties of epoxy coating films
As shown in Figure 6a-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 6d). 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 6e. 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 6g-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 6f). Furthermore, the coating film with the addition of cellulose-based fluorescent material as pigment also shows rather bright blue color under UV-light (Figure 6i-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 fluorescent groups on the molecular skeleton of cellulose with the use of common Epoxy added with mixture e and f cellulose-based fluorescent material. Epoxy coating film added with the mixture of cellulose and Coumarin 151 powder under g visible light and h 365nm UV-light. Epoxy coating film added with cellulose-based fluorescent material under i visible light and j 365nm UV-light 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/CO 2 system in our research, cellulose-based fluorescent materials (i.e the cellulose carbonates) are successfully synthesized under relative mild reaction condition at 30-50°C, which is lower than the reaction temperature reported by using common ionic liquid at *150°C (Tian et al. 2016(Tian et al. , 2018. Then they can be prepared of epoxybased 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 (k max ) at 435nm(A), 476nm(B), 512nm(C) and 594nm(D), respectively. The k max 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 cellulosebased fluorescent materials. Therefore, it is feasible to use the obtained cellulose-based fluorescent materials   to prepare epoxy-based fluorescent coating materials and films.
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 and Low 2013;Canche-Escamilla et al. 2002;HerreraFranco and AguilarVega 1997;Jang et al. 2012;Masoodi et al. 2012;Sdrobis et al. 2012).
DSC measurement is selected to determine the glass transition temperature (T g ) of all the epoxy coating films. The DSC heating curves are shown in Figure 9. The T g of pure epoxy coating film was 90.0°C. After added cellulose and its fluorescent derivatives, the T g of coating films slightly decrease to range from 88.4 to 89.6°C. The use of cellulose-based fluorescent materials does not change the thermal property of the coating films.

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