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Ind. Eng. Chem. Res. 2010, 49, 9830–9833
Optical Properties of Red Algae Fibers Yung Bum Seo,*,† Youn Woo Lee,‡ Chun Han Lee,‡ and Min Woo Lee† Department of Bio-based Materials, College of Life and Agriculture, Chungnam National UniVersity, Yousung-Gu, Daejun, Republic of Korea, and Pegasus Research Inc., 216 Uheun-Dong, Yousung-Gu, Daejun, Republic of Korea
Red algae fibers extracted from red algae (seaweed) showed very high opacity when used in the manufacture of white paper. For a basis weight of 60 g/m2, over 90% opacity was observed in paper made from red algae fibers without the aid of mineral fillers, whereas papers made from wood fibers had 70-80% opacity. Mercury porosimetry and light reflectance measurements at various wavelengths were used to investigate the origin of such high opacities in red algae fibers. The results revealed that red algae fibers had an extraordinarily high specific surface area on account of their very narrow widths (2-4 µm) and short lengths (500-800 µm). This unique property of red algae fibers can be best utilized for manufacturing highly opaque, premiumgrade, lightweight printing papers without lowering the paper strength properties. 1. Introduction 1
Red algae fibers were introduced by Seo et al. as raw materials for the manufacture of paper. They were produced from red algae species by bleaching the solid remnants obtained after mucilaginous material had been extracted from the red algae in an aqueous medium. Remarkable differences were observed between the physical properties of handsheets made from red algae pulp fibers and those made from wood pulp. Paper made from wood pulp, which consists of a mixture of bleached softwood pulp (50%) and hardwood pulp (50%), was found to be considerably less smooth and opaque than that made from red algae pulp fibers. The smoothness and the opacity are the most important and value-adding properties of all in printing and writing papers. The smoother is the paper, the higher is the printing resolution that can possibly be obtained. One of the limitations in using premium-grade thin paper for printing is the paper’s opacity. Therefore, papermakers usually add fillers such as opaque minerals to thin paper to increase the opacity; however, most fillers greatly decrease the paper strength. In contrast, paper made from red algae pulp fibers does not need additional fillers to increase the opacity or additional strength additives to compensate for the decreased paper strength that can result from filler addition.2 Paper opacity can be measured by using the international standard method, ISO 2471-1977(E). It is defined as R0/R∞ × 100%, where R0 is the reflectance factor that corresponds to the attribute of visual sensation by which a single sheet of paper with a black backing is judged to reflect incident light, while R∞ is the luminous reflectance factor of a pad of material thick enough to be opaque. The Kubelka-Munk (K-M) theory is conventionally used for estimating the optical properties of paper.3 In this theory, light flux is treated as two individual yet dependent parameters, and their intensities of light are determined on the basis of the light scattering and light absorption coefficients. The light scattering coefficient in K-M theory plays a major role in enabling us to understand the paper structure due to its physical ability to efficiently reflect the specific surface area of the material.4-6 Thus, the light scattering coefficient is a very useful parameter in estimating the surface area (or * To whom correspondence should be addressed. Tel.: 82-42-8215759. E-mail:
[email protected]. † Chungnam National University. ‡ Pegasus Research Inc.
unbounded area) of paper and has been widely used in white paper.7 The light scattering coefficient is dependent upon R0, R∞, and the basis weight of paper, as shown in eq 1.5 sW ) (1/2b) ln(1 + η)/(1 - η)
(1)
where s is the light scattering coefficient, and W is the basis weight of paper. b ) ((1/R∞) + R∞)/2 η ) opacity ) R0/R∞ In eq 1, η is greater than 0.0 and less than 1.0. Therefore, as η increases, the term sW increases. This implies that when the opacity of paper increases, the scattering coefficient increases for a constant basis weight. For bleached chemical pulp, the light absorption coefficient has little effect on the opacity of paper, and b in eq 1 is stable. Thus, as the scattering coefficient increases, the opacity of paper also increases for a constant basis weight. Mercury porosimetry can be used to measure the void surface area of paper on the basis of an increase in the pressure level, and each pressure level can be converted to a particular pore diameter. In mercury porosimetry, the pore diameters are calculated by assuming that the voids in paper have a cylindrical shape. As a result, the void surface area can be calculated from the volume of the cylindrical void. Alince et al. reported a correlation between the void surface area of paper as measured by mercury porosimetry and the light scattering coefficient of paper.8 An improved correlation was obtained when the pore diameters of voids with surface areas smaller than 100-200 nm were excluded from the data. This indicates that the minimum pore diameter at which paper is optically effective is in the range of 200 nm. One concern in mercury porosimetry is whether or not compression of the specimen and collapse of the voids takes place during measurement because of high pressure. Murakami et al. indicated that the mechanism of compression in the mercury method would be quite different from that in compression testing of paper.9 They said that the intruded mercury would not act to collapse the specimen. Rather, it would tend to support the porous structure. However, Levlin et al. recommended using an applied pressure of below about 2.5 MPa (above 300 nm in equivalent
10.1021/ie101194g 2010 American Chemical Society Published on Web 09/28/2010
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Table 1. Samples Prepared sample
abbreviation
red algae fibers
Rap
wood pulp mixture
WP
cotton fibers
cotton
WP 95% + Rap 5%
WP95
WP 90% + Rap 10%
WP90
WP 80% + Rap 20%
WP80
WP + filler 6.6%
WPF6.6
WP + filler 18.1%
WPF18.1
description fibers prepared from red algae (Gelidium corneum) SwBKP, 50% + HwBKP, 50%, refined to 450CSF donated from KOMSCO, refined to 450CSF wood pulp mixture, 95% + red algae fibers, 5% wood pulp mixture, 90% + red algae fibers, 10% wood pulp mixture, 80% + red algae fibers, 20% wood pulp mixture, 93.4% + CaCO3 6.6% wood pulp mixture, 81.9% + CaCO3 18.1%
pore radius) to negate the effects of high pressure on the fiber network.10 Lehtonen et al. showed that the mercury porosimetry specific surface area excluding surface area from pores smaller than 284 nm gave the highest linear relationship with BET specific surface area.4 They said mercury intrusion may destroy the paper sheet beyond the intrusion pressure of 4.4 MPa (equivalent to the pore radius of 284 nm). So, it seems that there is a range of an optimum pore size (around 200-300 nm), where effects of mercury intrusion pressure on sheet structure are insignificant, and where the high correlation coefficient between scattering coefficients of sheets and specific surface areas measured by mercury method holds. We will briefly discuss this issue using our data. We measured the specific surface areas of the paper samples by mercury porosimetry and used them to explain the specific scattering coefficients and opacities of different types of white paper, irrespective of the characteristics of the paper, such as the type of fiber, presence or absence of fillers, and degree of refining. 2. Experiment Red algae fibers were prepared from Gelidium corneum imported from Morocco. First, agar was extracted from the raw material in hot water; then, the solid remnant was bleached with chlorine dioxide and peroxide. The details of the preparation of red algae fibers are provided elsewhere.1 A wood pulp mixture (50:50) of commercial SwBKP (a mixture of hemlock, douglas fir, and cedar) and HwBKP (a mixture of aspen and poplar) was prepared for comparison. For improving the opacity of the wood pulp mixture, we prepared samples by adding small amounts of calcium carbonates or red algae fibers to the wood pulp mixture. Cotton fibers, which are the major component of the raw materials used for making banknotes and which are considered to give rise to high opacity in paper, were obtained from Korea Minting & Security Printing Corp., and sample
Figure 1. Breaking lengths of the samples.
Figure 2. Opacities of the samples.
handsheets were made from them. The prepared samples are described in Table 1. The table shows that calcium carbonates were added to the wood pulp mixtures, and their ash values were found to be 6.6% for WPF6.6 and 18.1% for WPF18.1. The light reflectances (R0 and R∞) at wavelength intervals of 20 nm from 360 to 740 nm were measured for each sample. The scattering coefficient at each wavelength was calculated using R0, R∞, and the handsheet basis weight.4 The brightness (ISO 2470) and opacity (ISO 2471) were measured using the Color Touch model manufactured by Technidyne Co. The void volumes and void surface areas of sample handsheet papers were measured by using a mercury porosimeter (Autopore IV 9500, Micromeritics, U.S.) for pore diameters of g3 nm (equivalent to 60 000 psi). The density (T410 om-98, T411 om-97), breaking length (a measure of tensile strength; T494 om-96), and drainage (T221 cm-99) of the handsheets were measured according to the TAPPI test method. The zero span (Z-span) strengths (T231 cm-96) were measured to compare the fiber strengths of different furnishes using the Z-span 1200 model (Pulmac International, U.S.). 3. Results Table 2 lists the physical properties of the sample handsheets. Rap (100% red algae fibers) has much lower double folds and
Table 2. Physical Properties of the Samples
sample
basis weight (g/m2)
density (g/cm3)
bulk (cm3/g)
breaking length (km)
stretch (%)
double folds (count)
ISO brightness (%)
ISO opacity (%)
scattering coeff.a (%)
tear index (mNm2/g)
red algae fibers wood pulp mixture cotton fibers WP95 (WP9 5% + RAP 5%) WP90 (WP9 0% + RAP 10%) WP80 (WP8 0% + RAP 20%) WPF6.6 (WP + filler 6.6%) WPF18.1 (WP + filler 18.1%)
63 62 57 60 65 63 63 60
0.77 0.64 0.51 0.63 0.65 0.69 0.65 0.65
1.30 1.56 1.98 1.58 1.54 1.45 1.54 1.54
6.34 5.64 3.46 5.61 5.95 6.71 4.14 2.91
3.21 2.23 2.32 2.49 3.05 3.28 1.82 1.41
3 40 52 63 61 161 14 6
82.16 81.48 84.18 82.25 82.25 80.35 84.48 86.89
93.8 77.1 77.8 78.5 81.4 81.8 84.6 87.3
94.48 36.73 42.96 40.39 43.48 43.49 56.20 72.97
1.65 6.29
a
Scattering coefficients at the same wavelength used for ISO opacity measurement (557 nm).
4.87 5.04 7.21
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Table 3. Cumulative Specific Surface Areas (m2/g) Measured by Mercury Porosimetry pore diameter (nm) sample
3
50
100
150
200
250
300
500
1000
2000
5000
10 000
red algae fibers wood pulp mixture cotton fibers WP 95% + RAP 5% WP 90% + RAP 10% WP 80% + RAP 20% WP + filler 6.6% WP + filler 18.1%
5.494 1.163 1.063 1.384 2.332 1.183 4.991 4.653
2.196 1.003 1.063 1.079 1.212 1.182 2.513 3.101
2.128 0.956 1.063 1.038 1.134 1.160 2.075 2.560
2.0962 0.9306 1.0381 0.9939 1.0923 1.1271 1.7292 2.0703
2.060 0.899 1.000 0.967 1.065 1.098 1.469 1.692
2.028 0.874 0.960 0.958 1.044 1.070 1.285 1.419
1.982 0.836 0.936 0.939 1.039 1.049 1.175 1.272
1.832 0.753 0.816 0.845 0.941 0.958 0.874 0.886
1.165 0.586 0.649 0.643 0.714 0.683 0.574 0.538
0.085 0.402 0.412 0.421 0.422 0.268 0.348 0.322
0.059 0.102 0.142 0.092 0.066 0.065 0.096 0.112
0.028 0.064 0.076 0.060 0.052 0.058 0.056 0.060
tear index than does WP (wood pulp mixture). We think that is because of short fiber lengths of Rap. However, in WP80 (WP 80% + Rap 20%), its double folds and tear index are higher than are WP’s. This cannot be explained by the simple arithmetic averaging process of the properties of two components, Rap and WP. It suggests there may be positive interactions between extremely narrow fibers such as Rap and wood fibers, which could be studied further in the future. Figures 1 and 2 show that WPF6.6 and WPF18.1 (wood pulp mixture + CaCO3) have much lower breaking lengths and lower opacities than does Rap, even though WPF6.6 and WPF18.1 contain 6.6% and 18.1% CaCO3, respectively. Table 3 lists the cumulative specific surface areas at various pore diameters as measured by mercury porosimetry. Figure 3 shows the regression analysis of cumulative specific surface areas against the ISO opacities of the sample handsheets. The figure revealed that the regression coefficients vary according to the pore diameters. For the scattering of visible light, the reflectance of light at wavelengths of less than 200 nm may not be sufficiently effective to influence the ISO opacity. Figure 4 shows the plot of regression coefficients obtained from Figure 3 versus the pore diameter. The results revealed that pore diameters of around 250 nm produced the highest regression coefficient, while Lehtonen’s study revealed that a pore diameter of 284 nm yielded the highest regression coefficient. Lehtonen used mechanical pulps and changed the refining degree and the wet pressing level. In this paper, three different fiber furnishes were used and calcium carbonate was added to the wood pulp mixture on two levels. Red algae fibers were also added to the wood mixture furnish in an attempt to improve the opacities of the wood pulp mixture. Irrespective of the presence of calcium carbonate or the type of fiber furnish, the cumulative specific surface areas for pore diameters of 250 nm yielded a high regression coefficient with ISO opacities.
The cumulative specific surface areas obtained by mercury porosimetry at a pore diameter of 250 nm are plotted in Figure 5. The red algae fibers (Rap) provided the largest surface area, followed by the wood pulp mixture with 18.1% calcium carbonate (WPF18.1), while the 100% wood pulp mixture (WP) provided the smallest surface area. The handsheet made from red algae fibers had the highest cumulative specific surface area at 250 nm; therefore, it provided the highest opacity. Red algae fibers developed the largest optically significant specific surface areas while maintaining the highest tensile strength. Thus far, no study has reported on the production of wood pulp with high opacity as well as high tensile strength without the aid of special paper chemicals; in any case, such wood pulp would be highly costly. Handsheet strength can be explained in terms of two important factors: fiber strength and interfiber bonding. In Figure 6, the Z-span strengths of sample handsheets were compared, and Rap provided a relatively lower value than WP. The Z-span strength is generally used as a means to compare fiber strengths.2 It is clear that red algae fibers do not have fiber strength higher than that of wood pulp. Therefore, the high breaking length of Rap cannot be explained by the Rap fiber strength. Cotton fibers provided the highest Z-span strength, but they were also characterized by a low breaking length and low opacity. The basis weight of the cotton fiber
Figure 4. Pore diameters versus regression coefficients between surface areas and ISO opacities.
Figure 3. Specific surface areas of sample handsheets measured by mercury porosimetry according to each pore diameter VS (ISO opacities).
Figure 5. Cumulative specific surface areas (m2/g) measured by mercury porosimetry.
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4. Conclusions
Figure 6. Zero span strengths of sample handsheets.
Handsheets made from red algae fibers showed higher opacity as well as higher tensile strength than those made from wood pulp. It was found that handsheets made from red algae fibers had a higher specific surface area at 250 nm, as measured by mercury porosimetry, than wood pulp mixture alone (SwBKP: HwBKP ) 50:50) or the wood pulp mixture with 18.1% CaCO3. The highest specific surface area led to the highest opacity. The small width of red algae fibers increased the number of interfiber bonds and the total bonding area in the handsheets; consequently, high breaking lengths were obtained even though the fiber strength of red algae fibers was less than that of wood pulp. In practical use of the red algae fibers, one may have to be concerned about their relatively slow drainage and low tear strength due to their exceptionally large specific surface areas and short fiber lengths.1 Proper mixing with wood fibers will make better use of these unique fibers. Acknowledgment This research was supported by a grant (20088081) from the Future Marine Technology Development Program funded by the Ministry of Maritime Affairs and Fisheries of the Korean government. Literature Cited
Figure 7. Diagrams of handsheet structures.
handsheet was slightly lower than those of the other fibers, and this may have contributed to its low opacity. Figure 7 can be used to explain why red algae fibers provide such unique properties. As per this figure, a model was developed to show that the total area of bonding in the red algae fiber handsheet was much greater than that in the wood pulp handsheet. Even though wood pulp fiber can develop a larger bonding area in combination with other fibers, the number of wood pulp fibers would be much less than that of red algae fibers in a handsheet. For the red algae fiber handsheet, the bonding area of a red algae fiber is much smaller than that of a wood pulp fiber, but the number of bonds for red algae fibers should be much more because the fiber width is much smaller (one-tenth of wood pulp fiber). The total bonding area of the red algae fiber handsheet (which is the product of the average bonding area by the number of bonds) must be much larger than that of the wood pulp mixture handsheet. As a result, the red algae fiber handsheet has a higher tensile strength. Red algae fibers also allow a larger unbonded area to provide a large specific surface area.
(1) Seo, Y. B.; Lee, Y. W.; Lee, C. H.; You, H. C. Red Algae and Their Use in Papermaking. Bioresour. Technol. 2010, 101, 2549–2553. (2) Scott, W. E.; Trosset, S. Properties of Paper: An Introduction; TAPPI Press: Norcross, GA, 1989. (3) Kubelka, P. New Contributions to the Optical Properties of Intensely Light Scattering Materials, Part I. J. Opt. Soc. Am. 1948, 44, 488. (4) Lehtonen, L. K.; Dyer, T. J. Light-Scattering Coefficient as a Measure of Specific Surface Area in Mechanical Pulp Laboratory Sheets. Pap. Puu 2005, 87, 517–524. (5) Borch, J. Optical and Appearance Properties. In Handbook of Physical Testing of Paper; Borch, J., Mark, R. E., Habeger, C. C., Eds.; Marcel Dekker Inc.: New York and Basel, 2001. (6) Arnold, E. A. Light Scattering in Fibrous Sheets. Tappi J. 1963, 46, 250–256. (7) Swanson, J. W.; Steber, A. J. Fiber Surface Area and Bonded Area. Tappi J. 1959, 42, 986–994. (8) Alince, B.; Pobuska, J.; Van De Ven, T. G. M. Light Scattering and Microporosity in Paper. JPPS 2002, 28, 93–98. (9) Murakami, K.; Imamura, R. Porosity and Gas Permeability. In Handbook of Physical and Mechanical Testing of Paper and Paperboard; Mark, R. E., Murakami, K., Eds.; Marcel Dekker Inc.: New York and Basel, 1984. (10) Levlin, J. E.; Nordman, L. On the Penetration of Ink into Paper. In Paper in the Printing Process, AdVances in Printing Science and Technology; Banks, W. H., Ed.; Pergamon: Oxford, 1967; Vol. 4.
ReceiVed for reView May 31, 2010 ReVised manuscript receiVed September 14, 2010 Accepted September 15, 2010 IE101194G
