Micromorphology Influence on the Color Performance of Lignin and Its

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Micromorphology Influence on the Color Performance of Lignin and Its Application in Guiding the Preparation of Light-colored Lignin Sunscreen Hui Zhang, Fangeng Chen, Xinxin Liu, and Shiyu Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03464 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Sustainable Chemistry & Engineering

Micromorphology Influence on the Color Performance of Lignin and Its Application in Guiding the Preparation of Light-colored Lignin Sunscreen Hui Zhang, Fangeng Chen,* Xinxin Liu and Shiyu Fu State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road No. 381, Guangzhou 510640, Guangdong, China *Corresponding author: Fangeng Chen. E-mail: [email protected]; Tel.: +8613430364029 ABSTRACT:

The

lignin

with

different color

degree

was

obtained

by

micromorphology regulation and was characterized by SEM, brightness test, bulk density, particle diameter, zeta potential, specific surface area and visible light diffusion to investigate the relationship between micromorphology and color performance. The results show that the lignin that consists of fine particles and contains dense interval space between particles exhibits obvious advantages on color reduction which is highly lightened by more than 3 times compared with the untreated lignin. The loose structure of lignin leads to a low bulk density which by the way decreases the concentration of chromophores in the macroscopic scale. Rely on the new fundamental basis for the color reduction of lignin, a kind of light-colored lignin was obtained and was used as additive in the preparation of sunscreen. The lignin-based sunscreen reached SPF 50.69 under an addition amount of 5 wt%, and barely showed staining on skin. KEYWORDS: Kraft lignin; Color reduction; Micromorphology; Sunscreen; Sun protection factor

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INTRODUCTION As one of the most abundant renewable resources, lignin has attracted worldwide attention because of the growing crisis in oil resources.1 Large amounts of lignin are produced yearly as the by-product of paper and pulping industry,2 but less than 2% of technical lignin is utilized for value added products.3 Recently, lignin from different resources has been proved safe and has no cell cytotoxicity.4-8 Besides, lignin, the most productive aromatic biopolymer in nature,9,10 contains abundant benzene ring and ketone structure, which make it possible to be used in UV defense.11,12 The evaluation of lignin as a sunscreen additive pushes lignin into high-end utilization.13,14 However, the dark color of industrial lignin hinders the market promotion of lignin-based sunscreens. It is imperative to whiten lignin to an acceptable color. While mentioning the color reduction of lignin, people will think of bleaching15 or effluent treatment.16 But these traditional methods aim to destroy or eliminate lignin molecule as much as possible. Therefore, the product was hard to be further used. In the recent years, some new color reduction methods17-19 were carried out and achieved favorable results, but most of them were focus on the removing or restriction of one or several chromophores, which inevitably changed the main structure of lignin or have undergone tedious steps and consumed too much time in the color reduction process. As we all know, the color of lignin is influenced by many factors. Even the chromophores have been classified into many different categories,17,20 let alone plenty of auxochromic groups.21-23 Besides, the contribution proportions of different chromophores and auxochromes to the color of lignin are still uncertain. Therefore,


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the color reduction mechanism that only based on the elimination of one or several chromophores are not very convincing. Is there any other factor that can influence the color of lignin? Or in other words, is there any other way that we can rely on to get the color of lignin lightened except the removing or restriction of chromophores? Fortunately, we found that the micromorphology also has relationship with the color of lignin except the chromophores influence. In this work, lignin was grinded and sieved by different mesh screens without changing the chromophores and main structure, and the color of lignin was obviously reduced. The micromorphology difference was observed by a Scanning Electron Microscopy (SEM). Besides, the color degree, bulk density, particle diameter, zeta potential and specific surface area of lignin were also characterized. According to the evidences, we provided a new fundamental basis for the color reduction of lignin, which mainly depended on the micromorphology regulation. Furthermore, this basis was successfully applied in the preparation of light-colored sunscreens and reached agreeable results. Since the solvent and reagent used in the research can be recycled, this whitening method is green and practicable. EXPERIMENTAL SECTION Materials Sulfuric acid and diethyl ether anhydrous were produced by Guangzhou Chemical Reagent Factory. Acetic anhydride was produced by Hengyang Kaixin Chemical Reagent Factory. Methanol, ethanol absolute and pyridine were purchased from Guangzhou Jinhuada Chemical Reagent Co., Ltd. All the chemical reagents were



of analytical grade and were used without further purification. The eucalyptus kraft lignin was acid-precipitated from black liquor supplied by a pulp mill in south China. The constitutional unit of the lignin is C9H6.21O3.03S0.26(OCH3)1.32. The Sunscreen lotion was Haiermian SPF 15 bought from Jingdong Mall. The main components of the lotion were listed in the Supporting Information. Drying of lignin Acid-precipitated lignin was separated and washed using a centrifuge for ten times. Then the lignin slurry was divided into four parts and was dried by different methods, i.e. oven drying, vacuum drying, freeze drying and spray drying. Due to a lower concentration was requested in freeze and spray drying, the lignin slurry was further dispersed in water and the initial solid content in freeze and spray drying was 56 g/L. Preparation of lignin particles with different diameters Oven dried lignin was grinded using a ceramic mortar, and then the lignin powder was sieved through different mesh screens (mesh size: 60, 80, 120, 160, 200, 300, 400 and 500). Therefore, lignins with different particle size distribution were obtained. In order to obtain smaller lignin particles, the sieved part between 60 and 80 mesh was selected and further dissolve in pyridine and finally reprecipitated in diethyl ether to get lignin microspheres. The redundant pyridine was extracted by diethyl ether and the lignin microspheres was vacuum rotary evaporated to solid powder at 40 °C. Acetylation of lignin


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Kraft lignin was dissolved in acetic anhydride/pyridine (1:1, V/V) solution, and was stored in a dark room at 50 °C for 48 h. Then it was precipitated in diethyl ether. The redundant acetic anhydride/pyridine solution was extracted by diethyl ether and the acetylated lignin was vacuum rotary evaporated to solid powder at 40 °C. Preparation of lignin-based sunscreen All the lignin-based sunscreens were prepared by simple mechanical agitation. For example, the lignin sunscreen with an addition amount of 5 wt% was prepared by blending 0.2 g lignin and 4.0 g pure lotion at 1000 rpm for 8 h under room temperature. After blending, these lignin sunscreens were transferred onto a transpore tape (3M Company, U.S.A.) surface for the measurement of UV transmittance. Measurements and Characterizations Bulk density was determined by transferring a known quantity of lignin particles into a 5 ml measuring cylinder and tapping it five times. The bulk density of the lignin was determined by dividing the weight of the sample by the volume of the lignin. The micromorphology of lignin was observed by a scanning electron microscopy (Merlin, Zeiss, Germany). N2 adsorption-desorption isotherms of lignin samples were measured at the temperature of liquid N2 (77 K) using an ASAP 2460 Surface Area and Porosity Analyzer (Micromeritics, U.S.A.). The specific surface area was calculated with the Brunauer-Emmett-Teller (BET) equation. Particle size distribution of lignin was determined by Laser Scattering Particle Size Distribution Analyzer (LA-960S, Horiba, Japan). 50 mg of sample was added in



the measuring cell and it was treated by ultrasonic for 1 min before testing. Zeta potential of lignin particle was obtained by Particle Analyzer (SZ-100Z, Horiba, Japan). 0.025 g of lignin was well dispersed in 5 mL deionized water. Then the suspension was transferred into a quartz tube and the Zeta potential was tested using electrophoresis method. The quantitative comparison of color performance of lignin was carried out in the solid state by using an L&W brightness and color tester (Elrepho 070, Sweden). The tested items include brightness and CIE L*a*b* values. A higher brightness and L*a*b* value is related to a lighter color. The visible light diffusion reflectance spectra of lignin samples were recorded in solid state by Shimadzu UV-3600 with an integrating sphere (Shimadzu, Japan). BaSO4 standard sample was tableted and scanned to build a base line. The test area was set between 400-760 nm with an interval of 1 nm. The UV transmittance of the lignin sunscreen was also measured by the same instrument. Transmittance measurement per 1 nm was collected in the wavelength range from UVB (290–320 nm) to UVA (320–400 nm). The Sun Protection Factor (SPF) value was calculated by the method mentioned in reference (13). RESULTS AND DISCUSSION Color difference from drying of lignin During the drying process of lignin, we find that different drying method makes different color performance. As seen in Figure 1, the same lignin was dried using oven, vacuum, spray and freeze methods. The oven and vacuum drying makes the lignin


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form large solid particles which is with surface gloss but presents really dark color among all the drying methods. On the contrary, the spray and freeze drying makes the lignin fine powders, but there is obvious color difference between these two drying methods. The spray dried lignin is brown, which can be easily distinguished from the freeze-dried lignin. Fourier transform infrared spectroscopy and 31P nuclear magnetic resonance spectra analysis (figure not shown here) were carried out to investigate the structure difference, but the data were nearly all the same and no difference was find out. Since the chemical structure has not been changed, what exactly causes the difference in color performance?

Figure 1. Color comparison of lignin dried by different method. After dried in different ways, the biggest difference of lignin samples should be in the micromorphology aspect. Therefore, the micromorphology of lignin was observed by SEM and the pictures were showed in Figure 2. The oven dried lignin looks like irregular clods (Figure 2a). After further magnified, we can see the surface of lignin clods is flat (Figure 2b). Similarly, the vacuum dried sample also shows massive clods (Figure 2c) with a smooth surface which is as same as the observation in Figure 2b. As seen in Figure 2d, the spray dried lignin formed rounded particles with diameter ranging from hundreds of nanometers to tens of microns. Under a low



magnification, the freeze-dried lignin presents no obvious difference with the oven and vacuum dried lignin, but the further magnified picture (Figure 2f) indicate that the surface of freeze dried sample is quite different, where large amounts of lignin particles can be obviously observed. It is reasonable that the small particles of acid precipitated lignin should be maintained after the freeze drying, but it is unexpected that the particles aggregated closely and formed large lumps.

Figure 2. Micromorphology of lignin dried in different ways. a Oven dried lignin (×500). b Surface of oven dried lignin (×50,000). c Vacuum dried lignin (×500). d Spray dried lignin (×2,000). e Freeze dried lignin (×100). f Surface of freeze dried lignin (×30,000). In consideration of the differences of micromorphology of lignins combined with the color distinction, it seems like a small particle size make contributions to the light color of lignin. As we know, small particles can create high specific surface area and by the way give rise to large interval spaces which finally lead to a low bulk density of samples.24 In another word, the chromophores in lignin are constant, but a low bulk density decreases the chromophore concentration in the macroscopic scale, thus the


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color of lignin is lightened macroscopically. As expected, the spray dried lignin, which is with the lightest color, shows the lowest bulk density of the four samples (figure not shown here). The freeze-dried sample also consist of numerous small particles with diameter around one hundred nanometers, but they are tightly aggregate. Therefore, the advantages on high specific surface and large interval spaces disappear, which finally result in a dark color of freeze dried lignin. Relationship between color and micromorphology In order to prove our assumption that the apparent color of lignin has relationship with the specific surface area and interval spaces, i.e. the bulk density performance in macroscopic scale, the oven died lignin was selected to be grinded using a ceramic mortar and then it was fractionated by mesh screens as lignin particles of different size can be separated by the mesh screens with different mesh numbers. A higher mesh number represent a smaller particle size, therefore the lignin that get through a higher mesh number screen should present a lighter color. As shown in Figure 3, the bulk density of grinded lignin was decreased from 0.69 to 0.34 g/cm3 with the increase of mesh number (the specific surface area was gradually increased, it was explained in detail in the rear section of this paper). As was expected, the color of lignin was indeed reduced only by grind and mesh screens fractionation. Photo comparison of several lignin samples was showed in Figure 4. According to a previous literature,18 the color degree of lignin samples was quantified using a brightness and L*a*b* value method and the result was illustrated in Figure 5. The brightness value in Figure 5a was gradually increased with the increase of mesh



number. Similarly, the lignin shows a same trend on L*(Figure 5b), a*(Figure 5c), b*(Figure 5d) values. Compared with the lignin between 60 and 80 mesh, the color performance of the part that has got through the screen of 500 mesh (sample named 500+) are improved by 30%, 47%, 390% and 220% respectively regarding the brightness, L*, a* and b* values.

Figure 3. Bulk density of grinded lignin.

Figure 4. Color comparison of grinded lignin.


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Figure 5. Color degrees of grinded lignin samples. (a) Brightness value; (b) L* value; (c) a* value; (d) b* value. The SEM observation of the grinded lignin was showed in Figure 6. The samples look similar (Figure 6a-c), except that the size of the particles is getting smaller with the increase of mesh number. It can be verified by a laser scattering particle size


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distribution analyzer. As seen in Figure 7a, the particle sizes show narrow distributions and the average diameter decrease from 143.7 to 13.3 µm. Although the particle size may differ, the surface of the grinded lignin is still flat (Figure 6d), which indicate the lignin molecules in the irregular bricks is still tightly bonded. The zeta potential of grinded lignin was also detected in Figure 7b. With the decrease of particle size, the zeta potential gradually increases. As we know, higher zeta potential causes a stronger repulsive interaction which is helpful to creates a larger interval space between lignin particles. It may make contributions to the decrease of bulk density of lignin samples in Fig. 3. But beyond the mesh number of 500, the zeta potential shows a slight decrease. At the same time, an abnormal, i.e. a larger size distribution around 110 µm, also can be observed in the size distribution regarding sample 500+ in Figure 7a. Due to the equipment limitation, the lignin that has got through a screen with a mesh number of 500 was not further sieved, in another word, the sample of 500+ contains more fine particles even with a diameter around hundreds of nanometers. Besides, the tests were carried out in water which is easy to cause the aggregation of fine lignin particles. Therefore, in view of these factors, the sample of 500+ exhibits an abnormal in the size distribution and zeta potential determination.



Figure 6. Micromorphology of lignin prepared in different ways. a Grinded lignin between mesh 160-200 (×200). b Grinded lignin between mesh 300-400 (×200). c Grinded lignin got through mesh 500 (×200). d Surface of grinded lignin (×50,000). e Solvent precipitated lignin (×5000). f Surface of solvent precipitated lignin (×50,000). g Acetylated and precipitated lignin (×5000). h Surface of acetylated and precipitated lignin (×50,000).


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Figure 7. Particle size distribution (a) and Zeta potential of lignin (b). Design and preparation of light-colored lignin Based on the above surveys, it seems reasonable that the apparent color of lignin dose has relationship with the specific surface area and interval spaces, namely the bulk density performance in macroscopic scale. On account of this conclusion, we start to design a kind of light-colored lignin which may have an opportunity to be used as additives in sunscreens. The design comes from two ideas. Firstly, we can infer that if the obtained lignin has smaller particle size and lower bulk density, it should exhibit a light color. According to previous works, solvent precipitated lignin can form rounded particles with diameter from tens of nanometers to several microns. Therefore, the dark colored oven dried lignin was dissolved in pyridine, and then it was precipitate in diethyl ether and vacuum rotary evaporated to solid powder. As expected, the color of the solvent precipitated lignin is even lighter than that of the grinded lignin 500+ (Figure 8). We can see in Figure 6e and f, the precipitated lignin



formed loose structure with a large interval space, although some of the particles aggregated. Secondly, our previous works19,25 confirmed that the phenolic hydroxyl blocking makes contribution to the color reduction of lignin and acetylation is a simple and efficient way to block the hydroxyls in lignin molecules. Thus, inspired by the two points the lignin was acetylated first and then it was precipitate in diethyl ether to prepare a light-colored lignin. As seen in Figure 8, the color performance of the acetylated and precipitated lignin was further improved and quantified color degrees are showed in Table 1. Compared with the oven dried lignin between 60-80 mesh in Figure 5, the brightness, L*, a* and b* value of lignin was improved by 318%, 145%, 318% and 328%, respectively.

Figure 8. Color comparison of lignin prepared by different method.


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Table 1. Color degrees of lignin samples. Samples

Brightness ISO (%)

L*

a*

b*

Diethyl ether precipitated

8.33±0.01

46.45±0.01

7.75±0.01

17.43±0.02

Acetylated and precipitated

10.40±0.01

51.01±0.02

8.03±0.02

22.68±0.01

The acetylated and precipitated lignin looks similar with the diethyl ether precipitated lignin regarding the micromorphology (Figure 6g and h) which also consists of fine particles and is with dense interval space. It also can be confirmed by N2 adsorption-desorption isotherms. As shown in Figure 9a, different from the grinded lignin 500+ which showed a rather weak adsorption-desorption signal, a hysteresis loop was found in the adsorption-desorption isotherm of acetylated and precipitated lignin although it is not obvious, which resembled type IV of Brunauer’s classification.26 At the lower relative pressure, the increasing speed of quantity adsorbed was slower, indicating monolayer adsorption. However, when it came to higher relative pressure, the increasing speed became higher, owing to the capillary condensation.27 The pore width curves are shown in Figure 9b. Obviously, the acetylated and precipitated lignin contains more pores compared with the grinded lignin. Strictly speaking, the calling of pore is not proper, since there is no pore in the lignin structure but some dense interval space according to the SEM observation. Therefore, the large pore volume represents the dense interval space, which well agreed with the loose structure of the acetylated and precipitated lignin. The specific surface areas of samples were calculated with the Brunauer-Emmett-Teller (BET)



equation; meanwhile the Langmuir surface areas were also listed in Table 2. We can tell that the two test items show a same trend although specific data may differ. As for the grinded lignin, the specific surface area was gradually increased with the increase of mesh number, which is in accordance with the increase of bulk density and well agreed with the color changes. It is noteworthy that the specific surface area of lignin was highly improved by the acetylation and precipitation method, at the same time, the color of this kind of lignin behaved the best of all.


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Figure 9. (a) N2 adsorption–desorption isotherms of lignin samples. (b) Pore width curves of lignin samples. Table 2. Specific surface areas of lignin samples. Samples

BET Surface Area (m2/g)

Langmuir Surface Area (m2/g)

Grinded 160-200

0.1480

0.1228

Grinded 300-400

0.7076

0.9916

Grinded 500+

1.5849

4.0089

Acetylated and precipitated

19.8598

113.7818

The reason that the acetylated and precipitated lignin presents the lightest color can also be explained by visible light diffusion reflectance. As seen in Figure 10, the analyzed lignin samples show an obvious distinction regarding the diffusion reflectance. The grinded lignin between mesh 160 and 200 absorbed most of the incident light between 400 and 760 nm thus few visible light reflected into human eyes which as a consequence led to a dark vision of this lignin sample. With the



decrease of particle size and the increase of specific surface areas, the surface reflection of lignin in the visible areas became stronger which result in a lighter and brighter color of grinded 500+ and diethyl ether precipitated lignin. Besides the fine particle size and the high specific surface area, some of the chromophores in acetylated and precipitated lignin were weakened due to a phenolic hydroxyl blocking effect which has already been confirmed in our previous works.18,19 Considering of these advantages, the diffusion reflectance of acetylated and precipitated lignin showed a further improvement especially in the long wavelength area, which naturally created a light-colored sense when the reflected lights of different wavelength got into the human eyes.

Figure 10. Visible light diffusion spectra of lignin samples. From the above illustrations we can summarize that small particle size and large interval spaces make contributions to the light color of lignin together. This kind of micromorphology presents a high specific surface area and leads to a low bulk density


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in the macroscopic scale. In the micromorphology regulation process, the chromophores in lignin are constant, but a low bulk density decreases the chromophore concentration in the macroscopic scale, thus the color of lignin is lightened macroscopically. With the help of phenolic hydroxyl blocking effect, i.e. acetylation of lignin, the transformation from lignin to quinones and quinone radicals is limited, which further improves the color reduction of lignin. Preparation of light-colored lignin sunscreen The lightest-colored that is acetylated and precipitated lignin was blended with a SPF 15 lotion to prepare a kind of light-colored sunscreen. In this part, the interactions between lignin and the ingredients, like avobenzone, octinoxate and TiO2, contained in the commercial sunscreen are not further demonstrated as the relative researches have been investigated in the previous works.4,14,28 Our works are based on these researches and our main purpose is to address the color issues. The color performances of lignin-based sunscreen with different additive amount were compared. As shown in Figure 11a. The sunscreen without any addition of lignin was labeled as Blank which showed a bright white. The color of sunscreen was gradually darkened with the increase amount of lignin. Correspondingly, in Figure 11b, the UV transmittance was also decreased gradually. It is worth noting that the transmittance of the lignin sunscreens with a higher addition amount was much lower than that of the blank both in UVB (290–320 nm) and UVA (320–400 nm) area. This suggested that lignin is a very good natural candidate for broad spectrum sunscreens as most of the sun blockers only work in a single area.29-32 The SPF of the



lignin-based sunscreens were further calculated according to the methods mentioned in previous works,13,33-35 and the values were listed in Table 3. The blank, i.e., the pure SPF 15 lotion was calculated to be 16.49 which corresponded closely to the value of 15. Under an addition amount of 8 wt%, the SPF was highly boosted to 80.03, but a staining on skin still cannot be ignored although efforts have been made to reduce the color of lignin. Fortunately, with an addition of 5 wt%, the staining barely could be seen when the sunscreen was evenly spread on skin (Figure 11c). At the same time, the SFP reached 50.69, which is totally enough to meet our daily needs. Considering the acceptable staining performance under 5 wt%, there is no doubt that a lower addition amount presents a better staining performance. A lighter colored sunscreen with a lignin amount of 2 wt% reached 34.54 regarding SPF value. It also can be widely used due to the SPF value around 30 well meet the common demand in some less requested sun protection field.


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Figure 11. (a) Appearance of SFP 15 sunscreen blended with different amount of lignin. (b) Typical UV transmittance of SFP 15 sunscreen blended with different amount of lignin. (c) Appearance performance of lignin sunscreen evenly spread on skin.



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Table 3. SPF values of the sunscreens blended with different amounts of lignin. Lignin (wt%)

0

0.5

1

2

5

8

SPF

16.49±0.88

21.28±1.02

27.66±0.79

34.54±1.21

50.69±1.05

80.03±1.98

CONCLUSIONS Lignin that consists of fine particles and at the same time is with dense interval space exhibits a light color performance. This kind of lignin shows a low bulk density and presents a high specific area surface. Considering this character and in combination with the hydroxyl blocking effect, a kind of light-colored lignin was successfully obtained by an acetylation and precipitation method and the color degree was highly improved by multi times. The light-colored lignin was used as an additive in the preparation of lignin-based sunscreen. Under an additive amount of 5 wt%, the SPF of lignin-based sunscreen reached 50.69 which is fair enough to meet the daily needs, meanwhile the sunscreen did not present obvious staining on skin. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Active and inactive ingredients of purchased sunscreen lotion. AUTHOR INFORMATION Corresponding Author Fangeng Chen* E-mail: [email protected] ORCID Fangeng Chen: 0000-0001-9161-8196


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Notes The authors declare no competing financial interest.

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The figure below is for Table of Contents use only.

Synopsis A light-colored lignin sunscreen was obtained with the help of micromorphology regulation, which performed well regarding sun-blocking and anti-staining.


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Micromorphology Influence on the Color Performance of Lignin and Its

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