Interactions of Lignin with Optical Brightening Agents and Their Effect

Article pubs.acs.org/IECR

Interactions of Lignin with Optical Brightening Agents and Their Effect on Paper Optical Properties Hongbin Liu,*,† He Shi,† Yating Wang,† Wei Wu,† and Yonghao Ni‡,† †

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin, China 300457 Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3

‡

S Supporting Information *

ABSTRACT: Optical brightening agents (OBAs) are widely used in the production of uncoated and coated paper grades to improve their optical properties. The presence of lignin in the pulp furnishes is well-known to have a significant effect on the OBA brightening efficiency, but how OBA interacts with lignin is still not well understood. In this study we used wood lignin to investigate the lignin/OBA interactions and its effect on OBA brightening. Three lignin samples isolated from spruce, pine, and aspen were used. Both di- and tetra-sulfonated OBAs were studied. It was found that the OBA addition can effectively improve the optical properties of paper, such as ISO brightness, CIE whiteness, and b*, but disulfonated OBA was found to be more effective at a lower dosage (less than 0.6%) than the tetra-sulfonated OBA. The addition of a small amount of lignin (0.4%) onto filter paper had negative effects on the optical properties, but the impact depends strongly on lignin structures (lignin samples from spruce, pine, and aspen), which explain the early results that mechanical pulps from different wood species respond very differently to OBA brightening. A modified Kubelka−Munk equation was used to predict and model the brightness and whiteness response of different lignin types and OBA, which can be used to provide guidance in determining the amount of OBA needed to reach specified optical property target. significantly.12 Another factor is the filler effect. Fillers such as PCC and GCC have much higher specific surface area and light reflectance than cellulose fibers, they can improve the paper optical properties. However, the presence of fillers decreased OBA efficiency when using OBA in the pulp furnish. Zhang et. al found that the fluorescent composition of filled sheets was significantly lower than that of unfilled sheets at the same OBA dosage.2 Migration of OBA at the size press can also affect the OBA efficiency. Forsstrom found that the sheet structure can affect the OBA migration at size press, and porous base sheet led more OBA migration than dense base sheet.13 Lignin is the obstacle of the improving OBA efficiency in the wood-containing paper grades because of the quenching effect and yellowish hue of itself.8 However, the research of lignin effect on OBA efficiency mainly focused on the interaction of HYP fibers and OBA. Different amounts of HYP addition or different HYP brightness grades were used to compare the different lignin content in the pulps.2,10 Some researchers used different raw materials (HYPs) to examine the effect of lignin content on OBA efficiency, such as aspen, birch and maple HYPs.1,14,15 An earlier study showed that spruce HYP can have similar OBA effectiveness to aspen HYP, although spruce HYP had higher lignin content (28%) than that of aspen HYP (18%).1 Literature results on the interaction of OBA and lignin and its effect on paper optical properties are rather limited. In this study we investigated the lignin/OBA interactions and its effect on the paper optical properties, using milled wood

1. INTRODUCTION In the paper industry, optical brightening agents (OBAs), also known as fluorescent whitening agents (FWAs), are widely used in the production of uncoated and coated paper grades to promote the brightness and whiteness of the paper products.1−4 Optical properties are important performance attributes for printing and writing paper grades, such as brightness and whiteness, which are the key factors to determine the value of the final paper products.3,5 OBAs are dyes that transform energy by absorbing radiation in the UV region and converting it into visible light in the blue region, therefore increasing the paper brightness and whiteness.6,7 OBA efficiency is always a concern in the uncoated paper grades production, particularly, when high-yield pulp (HYP) is the part of the furnish composition. OBA efficiency can be affected by several factors. The first factor is the competition for UV energy from UV absorbers (lignin1,2 and titanium dioxide8,9). Zhang et. al found that OBA efficiency decreased with the increasing of HYP (lignin-containing pulp).2 When OBA is used in the paper coating formula, rutile pigments should not be used due to their high UV absorption characteristics.8 The second factor is OBA retention in paper. Zhang et. al found that OBA retention can be affected by DCS and fines content in the pulp furnish. HYPcontaining furnish had lower OBA retention than HYP-free furnish, which resulted in the decreased OBA efficiency in HYP-containing furnish.10 The third factor is the quenching effect on OBA. It refers to any process or agent which decreases the fluorescence intensity of OBA. Bobu found that a cationic starch decreased OBA efficiency, and the fluorescence component decreased from 7.2% to 4.0% when the cationic starch dosage increased from 0 to 8 kg/t pulp.6,11 He et. al found that PEI can quench OBA and lower OBA efficiency © 2014 American Chemical Society

Received: Revised: Accepted: Published: 3091

September 27, 2013 January 22, 2014 February 2, 2014 February 3, 2014 dx.doi.org/10.1021/ie4032082 | Ind. Eng. Chem. Res. 2014, 53, 3091−3096

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Figure 1. Effect of OBA on the optical properties of paper.

Figure 2. Effect of different lignin on the optical properties of paper.

m2. A 7 × 7 cm2 piece of filter paper was cut and mounted on a module. The lignin and OBA solutions were added to the filter paper by the syringe method to isolate the effect of lignin and OBA retention issue. The MWL, OBA, and their mixtures were applied to the filter surface by the “drop by drop” syringe method. The drops were distributed uniformly on the filter paper and partly overlapped with each other to give a uniform distribution of OBA and/or lignin in the paper sheet. Then the filter paper was air-dried and conditioned at 23 ± 1 °C and 50% ± 2% relative humidity for 24 h. The optical properties of the paper were measured on an Elrepho spectrophotometer. The brightness measurement followed ISO 2470-1:2009 standard, while the CIE whiteness and b* measurements followed ISO 11476-2010 standard. The light source used in this study was D65 and C light.

lignin from spruce, pine and aspen as the lignin model. The lignin effect on the OBA-related brightness gain was modeled by using the earlier modified Kubelka−Munk equation.

2. EXPERIMENTAL SECTION Materials. The disulfonated and tetra-sulfonaned OBA were obtained from BASF, which are widely used in the papermaking industry for the wet end application. Filter paper was purchased from Hangzhou special paper Co., Ltd. The fresh pine, aspen, and spruce wood chips without barks were collected from two Canadian mechanical pulp mills. The samples were air-dried and then stored in plastic bags in a cold room prior to use. Methods. Preparation of milled wood lignin (MWL): MWL of pine, aspen, and spruce was prepared based on the method of Björkman16. The obtained MWL was dissolved in diethyl ether solution. A filter paper was used to study the lignin and OBA interactions. The basis weight of the filter paper was 70 g/ 3092

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Figure 3. Effect of OBA on the optical properties of paper (lignin content was 0.4% when lignin was added).

brightness of the final paper decreased 1.7 units for spruce lignin, 3.5 units for aspen lignin, and 5.4 units for pine lignin. This difference may be attributed to the difference of the pine, spruce, and aspen lignin in chemical and optical properties. Lignin not only absorbs ultraviolet light, but also presents different colors.6,21 Visually, the aspen lignin is darker than the spruce lignin but lighter than the pine lignin. In general, softwood lignin has more intense UV absorption at 350−360 nm than hardwood lignin and more ortho-quinoes that absorb visible light.23 Effect of the Interaction between Lignin and OBA. It is known that OBA efficiency is higher in wood-free paper than in wood-containing paper due to the negative effect of lignin in mechanical pulp.2,10 As shown in Figure 3, lignin addition decreased the OBA performance significantly. Lignin free filter paper always had highest brightness and CIE whiteness and lowest b*, at the same OBA dosage. The milled wood lignin from spruce, pine, and aspen had various degree of negative effects on the OBA efficiency, with the largest effect found from the pine lignin and the smallest effect from the spruce lignin. For example, at 0.6% OBA dosage, addition of 0.4% milled wood lignin decreased the brightness of the paper by 3.8, 5.9, and 10.3 units, respectively, for the spruce, aspen, and pine lignin, when comparing with the control (without lignin addition). The different effect of the lignin from different wood species on the OBA efficiency may be explained by the following reasons. (1) The pine, spruce, and aspen lignin had different chemical structures and had different interactions with the OBA. Forsskahl and Tylli found that lignin model compounds displayed different reflectance spectra when applied to different pulps and showed different photoyellowing behavior.24 (2) These milled wood lignin samples showed different color shades and had various degrees of impact on the brightness of the paper substrate, which in turn affect the OBA efficiency to various extents. Figure 3 further shows that the negatives effects of lignin on the brightness of the paper substrate can be recovered by adding OBA.

3. RESULTS AND DISCUSSION Effect of OBA on the Optical Properties of the Filter Paper. As shown in Figure 1, OBA can effectively improve the ISO brightness, R457 D65, and CIE whiteness and decrease b* of the filter paper. The R457 C and R457 D65 and CIE whiteness sharply increased when the OBA dosage went from 0 to 0.2%. For example, the original brightness of the filter paper was 84.4% ISO. The brightness increased to 91.3% ISO at 0.2% for tetra-OBA and 92.1% ISO for di-OBA. The optical properties can be further improved by increasing the OBA dosage. This is consistent with the results obtained from handsheet study.2,12 It can also be seen that different OBA types had different brightening responses. It is well-known that di-OBA has a higher efficiency than tetra-OBA on brightening at the same dosage.1,2 At the lower dosage (less than 0.8%), diOBA had a higher brightening efficiency than the tetra-OBA. The di-OBA reached the brightening ceiling when the OBA dosage was above 0.8%, as shown in Figure 1. Tetra-OBA can improve the optical properties further when the OBA dosage was above 0.8%, indicating that tetra-OBA has a wider operation window than di-OBA in the paper manufacturing process. OBA addition also decreased the yellowish hue of the paper, as indicated by the b* in Figure 1. Effect of Lignin on the Optical Properties of Filter Paper. As shown in Figure 2, the lignin addition had negative effects on the ISO brightness, R457 D65, CIE whiteness, and b* of the paper. The brightness and whiteness decreased sharply with the addition of milled wood lignin. It has been usually assumed that lignin is the major component responsible for the light absorption from the studies of wood, chemical and mechanical pulp.17−20 Different lignin has different effects on the optical properties of the filter paper. The negative effects of the pine milled wood lignin on the optical properties were the most pronounced. Spruce milled wood lignin had the fewest effects while the aspen milled wood lignin was in the middle of the pine milled wood lignin and spruce milled wood lignin. For example, at 0.6% lignin addition on the filter paper, the 3093

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Figure 4. Calculated optical properties and the experimentally determined optical properties.

Modified Kubelka−Munk Model. Chen et al. developed a modified Kubelka−Munk model to predict the OBA effectiveness on different HYP fibers.14 The modified Kubelka−Munk model was adopted in the present study for the interpretation of OBA effectiveness on different lignin structures. The brightness gain from OBA R∞,F is dependent on (1) OBA dosage and (2) the original optical properties of fiber/ paper substrate (without OBA). Similar to the model by Chen et al., R∞,F can be related to the OBA dosage (X), and the original optical properties of the fiber/paper substrate (a summary function, f1, as defined below), as given in eq 1. R ∞ ,F = kf1 e−0.31/(X + 0.105)

(X = 0.1−0.2 %)

predict the effect of OBA dosage on the individual optical property. As shown in Figure 4, the model predicted results are in good agreement with the experimental results. For the ISO brightness R∞(C) R ∞(C) = R ∞ ,s(C) +

(1)

2 + 2K457S457 K457

k 2 2 + 2k 2s2 )

(3)

R ∞(D65) = R ∞ , S(D65) +

′ )(1 + R ∞ ″ ) exp(−0.31/(0.105 + X )) k1.89(1 + R ∞ 2( k12 + 2k1s1 +

k 2 2 + 2k 2s2 )

(4)

For the CIE whiteness W(D65/10)

[1 + R ∞(360)][1 + R ∞(457)] 2 + 2K360S360 + K360

2( k12 + 2k1s1 +

The D65 light source contains more fluorescence. The fluorescence of D65 is about 1.89 times higher than that of C fluorescence for HYP sheets.14 For the D65 brightness R∞(D65)

f1 describes the effect of paper substrate properties on the fluorescent radiant factor.14,25 It can be calculated by eq 2. The reflectance at 360 or 457 nm, R∞(360), R∞(457), light scattering and absorption coefficients at 360 and 457 nm, namely S360, K360, S457, and K457, were experimentally determined. f1 =

′ )(1 + R ∞ ″ ) exp(−0.31/(0.105 + X )) k(1 + R ∞

W (D65/10) = Ws(D65/10) ′ )(1 + R ∞ ″ ) exp( −0.31/(0.105 + X )) k(1 + R ∞ + 5.106 2 2( k1 + 2k1s1 + k 2 2 + 2k 2s2 )

(2)

R∞ (360): Reflectance factor of the original HYP at 360 nm; R∞(457): Reflectance factor of the original HYP at 457 nm; S360: Scattering coefficient of the original HYP at 360 nm; K360: Absorption coefficient of the original HYP at 360 nm; S457: Scattering coefficient of the original HYP at 457 nm; K457: Absorption coefficient of the original HYP at 457 nm. Based on eq 2, f1 can be calculated from the experimentally determined data (R∞(360), R∞(457), S360, K360, S457, and K457), as listed in Table 1. The k constant can be determined by regression of the R457 fluorescence at different OBA dosages, and the results were shown in Table 1. By following eq 1 (with k and f1 known), one can then predict the brightness gain from OBA (fluorescence). Furthermore, by taking consideration of relationships between different optical properties,14 eqs 3−6 can be obtained that can

(5)

+ 3.18

For the yellowness, b*(D65/10) b*(D65/10) = b ′ )(1 + R ∞ ″ ) exp( −0.31/(0.105 + X )) k(1 + R ∞ *s + 1.078 2 2( k1 + 2k1s1 + k 2 2 + 2k 2s2 ) − 0.694

(6)

R∞ ′ : Reflectance factor of the original HYP at 360 nm; R∞ ″: Reflectance factor of the original HYP at 457 nm; R∞(C): ISO brightness of the HYP pulp sheet at an OBA dosage of X%; R∞(D65): D65 brightness of the HYP pulp sheet at an OBA dosage of X%; R∞,s(C): ISO brightness of the HYP pulp sheet 3094

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ACKNOWLEDGMENTS We would like to acknowledge the financial support from National Natural Science Foundation of China (Grant 31100435), the Foundation for the Development of Science and Technology in Tianjin Universities (Grant 20100509), and Tianjin Municipal Science and Technology Commission (Grant No. 12ZCZDGX01100).

without OBA; R∞,s(D65): D65 brightness of the HYP pulp sheet without OBA; W(D65/10): CIE whiteness of the HYP pulp sheet at an OBA dosage of X%; WS (D65): CIE whiteness of the HYP pulp sheet without OBA (D65/10); b*(D65/10): Yellowness (b*) of the HYP at X% OBA dosage with the D65 illuminant; b*s : Yellowness (b*) of the original HYP (D65/10); R∞: Reflectance factor of the original HYP at 360 nm; R∞: Reflectance factor of the original HYP at 457 nm; K1: Absorption coefficient of the original HYP at 360 nm; K2: Absorption coefficient of the original HYP at 457 nm; S1: Light scattering coefficient of the original HYP at 360 nm; S2: Light scattering coefficient of the original HYP at 457 nm; X: OBA dosage (%, based on pulp), X = 0.1−1.2. As shown in Table 1 in the Supporting Information, the scattering coefficient at 360 nm, S360, decreased with increasing the OBA dosage while the scattering coefficient at 457 nm, S457, increased with increasing the OBA dosage. The absorption coefficient at 360 nm, K360, first increased with the increase of OBA dosage and then decreased when the OBA dosage was higher than 0.2%. The adsorption coefficient at 457 nm, K457, decreased with the addition of OBA. A higher absorption in the UV region means that there would be more lignin available to compete for the limited UV radiation energy with OBA, and a higher scattering coefficient in the UV region will have less UV radiation to travel through the paper thickness to activate OBA that is present in the paper sheet.6

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CONCLUSIONS Although it is well-known that the optical properties of paper, including brightness, CIE whiteness, and b*, can be improved significantly by the addition of OBA, there are a number of key parameters that affect the OBA brightening/whitening performance, including the presence of lignin, the different lignin structures, and the type of OBA and its dosage. The experimental results showed that the presence of lignin, even at a very small amount (e.g., 0.4% on paper) significantly decreases the OBA efficiency. In addition, the negative impact of lignin can also be dependent on the lignin structure: pine lignin has the strongest negative effect, followed by aspen lignin, and spruce lignin has the least effect. This explains the results in the literature that OBA performance varied differently on the high-yield pulps (HYP) from different wood species. The negative effect of lignin is not just related to its amount and the initial brightness of HYP but more to lignin structure. A proposed semiempirical modified Kubelka−Munk equation gave good predictions of the lignin effect on the ISO brightness, D65 brightness, CIE whiteness, and b* of the pulp at a given OBA dosage. This may be practically used to predict the amount of OBA required for reaching the targeted optical properties for a specific HYP. ASSOCIATED CONTENT

S Supporting Information *

Table 1, Optical properties and the calculated f1 value of the lignin-containing paper. This information is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3095

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