Steam-Generating Composite Sheets Prepared Using Techniques in

Oct 6, 2009 - Effective Retention of Iron Powder in a Sheet Using ... Flexible heat- and steam-generating sheets are promising medical devices for hum...
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MATERIALS AND INTERFACES Steam-Generating Composite Sheets Prepared Using Techniques in the Papermaking Process. 1. Effective Retention of Iron Powder in a Sheet Using Fibrillated Cellulose Fibers Yoshiaki Kumamoto,†,‡ Masataka Ishikawa,† Hironobu Kawajiri,† Takeshi Nakajima,† and Akira Isogai*,‡ Processing DeVelopment Research Lab., Kao Corporation, 2606 Akabane, Ichikai-machi, Haga-Gun, Tochigi 321-3497, Japan, and Department of Biomaterial Sciences, School of Agricultural and Life Sciences, The UniVersity of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

Flexible heat- and steam-generating sheets are promising medical devices for human healthcare, and oxidation of iron powder in sheets has potential applications for this purpose. Hence, the preparation conditions of composite sheets containing iron powder, cellulose fiber and activated carbon powder using techniques in the papermaking process were investigated to increase the iron powder content as much as possible and improve retention ratios of iron powder in the sheets. When unfibrillated cellulose fibers were used without any retention aids, the total retention ratios of the components were less than 40%. By contrast, when highly fibrillated cellulose fibers were used in combination with a cationic/anionic dual polymer system, the total retention ratios at a basis weight of 217 g m-2 reached about 90%. Moreover, iron powder aggregates in the sheet surfaces as well as throughout the thickness of the sheet, and higher tensile and internal strengths were obtained by using highly fibrillated cellulose fibers. Introduction

Fe3O4 f 3Fe2+ + 2O2 + 6e-

(2)

Heat- and steam-generating mats, when attached to human bodies, have a significant influence on enhancement of blood circulation and are effective in alleviation of chronic lumbago, stiff shoulders, eyestrain, etc.1-7 Application of these mats to the lumbar or abdominal region as body warmers also affects some autonomic nerve activities by physiological functions.8 Oxidation of iron powder to iron oxide in the mats is generally used to generate heat and steam in disposable body warmers for human healthcare, according to the following eq 1.

Fe3O4 f 3Fe3+ + 2O2 + 9e-

(3)

Fe + (3/4)O2 + (3/2)H2O f (1/2)Fe2O3 · (3/2)H2O + 402 kJ

(1)

These disposable mats for body warmers generally consist of iron powder, activated carbon powder, vermiculite, waterabsorbing polymer like sodium polyacrylate, and a NaCl solution. Activated carbon powder and the salt solution enhance the oxidation of iron when the mat is exposed to air or oxygen, and vermiculite and water-absorbing polymer play a role in adsorption (or retention) of the salt solution in the mat. Each iron powder particle is covered with a thin layer of iron oxide (Fe3O4) to protect the iron from oxidation under conditions without Cl- ions. When Cl- ions are in contact with the iron powder, the iron oxide layer is dissolved in water as ferrous ions according to eq 2 or 3, and the iron in the interior of the iron powder particles can be oxidized to iron oxide, producing heat and steam, by contact with activated carbon powder and oxygen in air. * To whom correspondence should be addressed. Phone: +81 3 5841 5538. Fax: +81 3 5841 5269. E-mail: [email protected]. † Kao Corporation. ‡ The University of Tokyo.

However, current steam-generating body warmers are not sufficiently flexible to properly fit various parts of the human body. Moreover, because not all of the components and oxygen are in close contact in the mats, oxidation of iron does not proceed smoothly in all parts, resulting in insufficient steam generation. Thus, some unoxidized iron powder remains in the mats. To overcome these problems, we investigated the conditions for preparation of flexible and thin composite sheets consisting of iron powder at high weight ratios, using techniques in the papermaking process. As shown in the above eqs 2 or 3, oxidation of iron powder will not occur even in the presence of water in the papermaking process, so long as the concentration of salt ions can be controlled to be as low as possible. However, because iron powder has small surface area and high density, it is generally difficult to retain the particles efficiently or evenly in cellulose fiber sheets, when cellulose slurries containing iron powder at high ratios are subjected to the basic filtration step of the papermaking process. In this study, two well-known techniques used in the papermaking process were adopted to improve retention when making composite sheets consisting of iron powder, cellulose fiber, and activated carbon powder. One technique involved fibrillation of cellulose fibers using standard “beating” of the pulp, prior to the addition of the iron and carbon and the sheet forming step. Native celluloses have hierarchical structures from cellulose chains to fibers via highly crystalline fibrils 3-4 nm in width and bundles of fibrils.9,10 Fibrillation of native cellulose fibers can be achieved by mechanical treatment in water (beating or refining of paper pulps),11,12 or by chemical modification such

10.1021/ie900875u CCC: $40.75  2009 American Chemical Society Published on Web 10/06/2009

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Figure 1. Typical thermogravimetric (TG) curve of iron powder/cellulose/activated carbon powder composite sheet and relationships between actual weights of iron powder, cellulose, or activated carbon powder and their weights determined from TG curves.

as catalytic oxidation using nitroxyl radical in water.13,14 Mechanical fibrillation of cellulose fibers is expected to bring about efficient entrapment of iron powder particles in the fibril matrices. The other technique used involved the addition of a dual polymer retention system, where cationic and anionic polymers were added in that order to the other sheet components suspended in water prior to the sheet forming step. The dual polymer system is known to be effective in retention of inorganic powderlike particles such as papermaking fillers, zeolite, and titanium oxide (photocatalyst) in sheets, when the weight ratios of the inorganic powders are relatively high in the sheet components.15-20 Poly(diallyldimethyl)ammonium chloride and anionic polyacrylamide have been predominantly used as the dual polymer system in previous applications. In the present study, polyamideamine-epichlorohydrin and carboxymethyl cellulose were used as cationic and anionic polymers, respectively, after preliminary experiments suggested that this combination would provide the best retention. Experimental Procedures Materials. Commercial softwood and hardwood bleached kraft pulps were used as the original cellulose fibers, both of

them having approximately 90% R-cellulose content, the remainder being hemicelluloses. The cellulose fibers were beaten to various degrees for fibrillation using a Niagara beater (Kumagai Riki Kogyo, Japan). The commercial iron powder (RKH, Dowa Iron Powder, Co., Japan), whose surface was covered with a thin layer of iron oxide (Fe3O4) to protect the interior iron from oxidation, had average particle size, specific surface area, and zeta potential of 45 µm, 3 m2 g-1, and -12 mV, respectively. The activated carbon powder with an average particle size of 43 µm (Carboraffin, Japan Enviro Chemicals, Co., Japan) had a specific surface area and zeta potential of 1000 m2 g-1 and -24 mV, respectively. A commercial polyamideamine-epichlorohydrin (PAE) resin solution (WS4020, Seiko PMC Co., Japan) and carboxymethyl cellulose (CMC) powder (HE1500F, Dai-Ichi Kogyo Seiyaku Co., Japan) were used as cationic and anionic polymers, respectively. The cationic charge of PAE determined by colloid titration was 3.2 mequiv g-1, and the degree of substitution of CMC was 1.30-1.45. The viscosity of the CMC solution at 25 °C was 2500-3500 mPa s. Solutions of PAE and CMC at 4 wt % and 1 wt %, respectively, were used as retention aids. Preparation of Composite Sheets. Iron powder, cellulose fiber, and activated carbon powder in proportions by weight 75, 15, and 10 wt %, respectively (total weight 75 g), were

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Figure 3. Optical microphotographs of slurries of iron powder and softwood or hardwood cellulose fiber with or without fibrillation. The weight ratio iron powder:cellulose was 9:1.

Figure 2. Optical microphotographs of slurries of unfibrillated softwood cellulose fiber and either activated carbon powder or iron powder without additives.

suspended in tap water (1 L); the total solids content in the slurry was adjusted to 7.5% w/v. The slurry was agitated at 120 rpm, and 0.5% PAE and 0.18% CMC (both percentages based on the total weight of the components) were added in that order. The interval between PAE and CMC additions was 30 s. After stirring the slurry for an additional 100 s, it was diluted to 0.1% total solids content with tap water and subjected to sheet-making using a 250 mm × 250 mm static sheet former. The total dry weight of the sheet components at 100% retention was adjusted to be 240 g m-2; the actual basis weights of the sheets prepared varied, depending on the retention ratios of the sheet components. The wet webs were dried at 110 °C for 3 min using a rotary drum dryer to decrease their moisture contents to below 1%. Retention ratios of total sheet components were determined from the total weights of the dried sheets and solid contents in the original slurry fed to the sheet-making device. Determination of Each Component in Sheets. Because not only iron content but also cellulose fiber and activated carbon powder contents in the sheets should be determined, the combustion method for determination of ash could not be used in this study. For that reason, the TG method was established. A thermogravimetric (TG) method to determine the three components in the sheets was established in preliminary experiments. An accurately known amount of an air-dried sheet sample (ca. 15 mg) was set in a platinum pan and subjected to the TG measurement using a TG/DTA-6300 apparatus (Seiko Instrument Co., Japan). The TG test program was as follows: heating at 10 °C min-1 in N2 atmosphere from 25 to 105 °C, keeping the temperature at 105 °C in N2 atmosphere for 30 min,

Figure 4. Retention ratio of total components (iron powder + cellulose fiber + activated carbon powder) prepared from unfibrillated or fibrillated cellulose fiber with or without a dual polymer system (0.5% PAE + 0.18% CMC, based on the weight of total components). The original slurry contained iron powder/cellulose fiber/activated carbon powder in the ratios 75:15:10 by weight, and the basis weight of the sheets at 100% retention was set to be 240 g m-2.

heating at 6 °C min-1 in N2 atmosphere from 105 to 800 °C, heating at 10 °C min-1 in air from 800 to 1000 °C, and keeping the temperature at 1000 °C in air for 40 min. From the TG curves obtained, iron powder, cellulose fiber, and activated carbon powder contents in the sheet were calculated (see the Results and Discussion section). Three samples were measured for each sheet to obtain an average value. Calibration lines were obtained beforehand using known amounts of iron powder, cellulose, and activated carbon powder. Other Analyses. Degrees of fibrillation of cellulose fibers were evaluated by measuring Canadian Standard Freeness (TAPPI Test Methods T 227 om-99, 2001), a measure of drainage rate which is inversely related to fibrillation. Optical microphotographs of the cellulose fiber/activated carbon powder and cellulose fiber/iron powder slurries without retention aids were taken using a microscope (Olympus BX50) equipped with a phase-contrast lens (Olympus UPlanFLN-PH) and a digital camera (Olympus PD20). Scanning electron microphotographs

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(SEM) and reflection electron microscopic images of surfaces of the sheets were obtained at 15 kV by means of a field emission-type SEM (Hitachi S-4300SE) after platinum-palladium coating for 40 s using a sputtering apparatus (Hitachi E-1030). Tensile strengths of the sheets were measured based on the TAPPI Test Method T 494 om-96 (2001). Internal fiber bond strengths of the sheets or sheet strengths in the thickness direction were evaluated visually by the so-called adhesive tape method. A commercial pressure sensitive adhesive tape 24 mm in width was applied to the sheet sample surface, and the tape was pressed on the sheet by back-and-forth rolling movement of a 1 kg roller. After removing the tape by hand, the sheet components on the adhesive tape were visually observed. Fiber lengths of softwood and hardwood celluloses were measured by means of a Kajaani FS-200 fiber-length analyzer. Results and Discussion Determination of Each Component in the Composite Sheets. The upper graph in Figure 1 presents a typical TG curve of the composite sheet sample. The weight decrease occurring from 25 to 105 °C (A) was due to loss of moisture present in the sample. The weight decrease from 105 to 500 °C (C) is ascribed to thermal degradation of cellulose to volatile compounds. The weight decrease from 800 to 900 °C is explained in terms of oxidation or combustion of carbon components, originating from both thermally degraded cellulose and activated carbon powder, to carbon dioxide. Simultaneously, iron was oxidized to iron oxide in air at temperatures above 800 °C, the weight increases rapidly, and iron oxide remained as a residue in the platinum pan. When iron, cellulose, and moisture contents are obtained from the TG curve, the amount of activated carbon powder in the sheet sample can be calculated from these values and the original sample weight. On the basis of these assumptions, correlation lines were obtained using known amounts of iron, cellulose, and activated carbon; the results are shown in the lower three graphs in Figure 1. Good linear relationships between the actual and measured weights were obtained for all three components. Approximately 15 wt % of the original cellulose was present as char after thermal degradation (see the lower and center graph in Figure 1). Thus, the contents of three components and moisture in the sheet were calculated from eqs 4-7. moisture content (%) ) iron content (%) )

A × 100 W

(4)

1 W-B+A × 0.70 × × 100 W 0.94 (5)

cellulose content (%) )

C × 100 0.85W

(6)

activated carbon content (%) ) 100 - moisture content iron content - cellulose content (7) W is the air-dried weight of the sheet sample, and A-C are the corresponding weights in Figure 1. The values 0.94 and 0.85 were derived from the slopes of the correlation lines for iron and cellulose, respectively, in Figure 1. The iron powder used contained 18.3% iron oxide (Fe3O4) as the protecting surface layer, when calculated from the slope coefficient 0.94. The slope coefficient 0.85 shows that 85% of the cellulose weight was lost as volatile gases by thermal degradation under the conditions used. The value 0.70 was calculated from the molecular weights of iron (M ) 55.85) and iron oxide (M ) 159.7). Thus,

Figure 5. Weight of each component in sheets prepared from unfibrillated or fibrillated cellulose fiber with dual polymer system (0.5% PAE + 0.18% CMC, based on the weight of total components), tensile strength of the sheets, and retention ratio of iron powder in the sheets. The original slurry contained iron powder/cellulose fiber/activated carbon powder in the ratio 75:15:10 by weight, and the basis weight of the sheets at 100% retention was 240 g m-2.

quantitative analysis of three components, i.e. iron powder, cellulose fiber, and activated carbon powder, of the composite sheets was accessible by the TG method. Retention of Iron Powder in Sheet by Fibrillation of Cellulose and Dual Polymer System. Figure 2 shows optical microphotographs of cellulose fiber/activated carbon powder and cellulose fiber/iron powder mixtures in water. The activated carbon powder particles were well adsorbed on cellulose fibers, forming some aggregates. Because the activated carbon had density similar to that of cellulose fiber and a remarkably large active surface area of 1000 m2 g-1, van der Waals forces between them were sufficiently strong to overcome electrostatic repulsions (both activated carbon and cellulose have negative surface charges). On the other hand, iron powder particles were hardly adsorbed on cellulose fibers, because iron power has a much higher density (7.8 g cm-3) than that of cellulose and much lower surface area than that of activated carbon. Softwood and hardwood cellulose fibers were then fibrillated, and interactions with iron powder particles in water were observed by optical microscopy. Cellulose fibrillation was effective in physical entrapment of iron powder particles in the cellulose fibril matrices in both cases (Figure 3). However, the retention ratios of the total components were still lower than 70% (as described later, the iron powder retention ratios were especially low). Cellulose fibrillation was then combined with the technique using the dual polymer system, which has proven effective in improving retention of inorganic fillers or particles in papermaking. In many preliminary experiments, the combina-

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Figure 6. SEM and reflection electron microscopic images of the surface of a composite sheet of iron powder/softwood cellulose fiber/activated carbon powder.

tion of PAE and CMC was found to be the best system. Detailed experimental data for various dual polymer systems and related discussion will be presented in the following paper. Figure 4 shows retention ratios of the total components in the sheets when fibrillated cellulose fibers and the PAE/CMC dual polymer system were adopted. Detailed studies concerning the dual polymer systems, i.e. the reason that the PAE/CMC system gave the highest retention ratios of iron powder, and steam-generating behavior of the composite sheets will be reported in other papers. When unfibrillated celluloses were used without retention aids, the retention ratios were below 40% for both softwood and hardwood celluloses. Fibrillation of the celluloses to the extent of producing about 100 mL Canadian standard freeness increased the retention ratios to about 70%. When the dual polymer system was used, the retention ratios of total components increased to approximately 65% and 90% for unfibrillated softwood and hardwood celluloses, respectively. Thus, especially for unfibrillated hardwood cellulose fiber, the dual polymer system per se was quite effective in increasing retention ratios of the sheet components. In the case of softwood cellulose fiber, the combination of cellulose fibrillation and use of the dual polymer system resulted in the increase in the retention ratios up to approximately 90%. Figure 5 presents the sheet weights, weights of each component in the sheets, retention ratios of iron powder, and tensile strength of the sheets, when unfibrillated and fibrillated cellulose fibers were used in combination with the PAE/CMC dual polymer system as the retention aid. As the degree of cellulose fibrillation was increased, the iron powder content in the sheets clearly increased, while the contents of cellulose fiber and activated carbon powder in the sheets were almost constant.

Figure 7. Reflection electron microscopic images of top and bottom sides of iron powder/cellulose fiber/activated carbon powder composite sheets prepared from fibrillated or unfibrillated softwood or hardwood cellulose fiber. Sheet-making conditions are the same as those for Figure 5.

Thus, the use of fibrillated cellulose fibers in combination with the PAE/CMC dual polymer system was required to prepare acceptable composite sheets. As described in the following paper, high iron contents are needed for flexible steamgenerating sheets. As expected, the tensile strength of the composite sheets was also increased by increasing the degree of cellulose fibrillation for both softwood and hardwood celluloses. The absolute tensile strengths of the sheets prepared from softwood cellulose were always higher than for hardwood cellulose at the same freeness because the fiber lengths of the former (1.7-2.1 mm) were always greater than those of the latter (0.6-0.8 mm), when measured using a fiber length analyzer. Since sheet strength is one of the significant factors for flexible steam-generating composite sheets as disposable body warmers, it was concluded that the use of fibrillated softwood cellulose in combination with the dual polymer system should be preferred for preparing suitable composite sheets. Distribution of Iron Powder Particles in the Composite Sheets. As shown in the upper graph in Figure 5, when sufficiently fibrillated softwood cellulose beaten to 65 mL Canadian standard freeness was used in sheet-making with the slurry containing iron powder/cellulose fiber/activated carbon powder in the ratio of 75:15:10 by weight using the PAE/CMC dual polymer system, the total retention ratio reached approximately 90%. Moreover, the sheets contained a high iron powder content of approximately 75 wt %. The SEM and

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Figure 8. Appearance of adhesive tapes pressed on the top and bottom surfaces of the iron powder/cellulose fiber/activated carbon powder composite sheets. The sheets were prepared from fibrillated or unfibrillated softwood or hardwood cellulose fiber.

reflection electron microscopic images of a surface of the above composite sheet are depicted in Figure 6. Iron and activated carbon powder particles were aggregated and entrapped in the cellulose fiber matrix. Although the aggregates of iron and activated carbon powder particles were similar in shape, the iron particles were distinguishable from activated carbon particles by reflection electron microscopy. It seems from Figure 6 that the iron particle aggregates and activated carbon powder aggregates were formed separately in the slurry, and also retained separately in the sheet without any strong interaction. Both surfaces of the composite sheets prepared from softwood and hardwood cellulose fibers with and without fibrillation treatment were observed by reflection electron microscopy (Figure 7). When fibrillated softwood cellulose fibers were used, the iron powder aggregates were evenly distributed on each surface as well as through the thickness of the sheet, indicating that the iron powder aggregates were adsorbed on fibrillated softwood cellulose fibers and/or entrapped in fibrillated softwood cellulose matrices at the slurry stage. These strong interactions between fibrillated softwood cellulose fibers and iron powder aggregates might have brought about higher retention ratios of iron powder in the composite sheets. By contrast, the iron powder aggregates were smaller in number for the sheets prepared from unfibrillated softwood cellulose. Moreover, when unfibrillated softwood cellulose was used, the heavy iron powder aggregates were predominantly present on the wire side (or bottom side of the sheet as formed in the sheet former), showing that interactions between cellulose fibers and iron powder

aggregates were insufficient to achieve a good distribution of the iron powder aggregates. These results are related to the structures of the fibrous networks. When the fibers have low coarseness like hardwood cellulose fibers, they may collapse and get close to each other irrespective of fibrillation treatment, resulting in a more closed structure. On the other hand, stiff fibers with high coarseness such as the unfibrillated softwood cellulose fibers form an open network structure, which facilitates the flow of iron particles to the bottom side of the sheet former. For highly fibrillated fibers, this difference was not observed for both the hardwood and softwood cellulose fibers, because fibrils played a major role in the iron retention process (Figure 7). Internal bond strengths of the composite sheets or sheet strengths in the thickness direction were evaluated by the adhesive tape method (Figure 8). When fibrillated celluloses were used, only small amounts of powders and fibers were peeled off from the sheet surfaces and transferred to the tapes, indicating sufficiently strong adhesive forces between the three components. By contrast, large amounts of fibers and powders were transferred to the tapes when unfibrillated cellulose fibers were used. These results reveal that use of fibrillated cellulose fibers is required for achieving not only high retention ratios of iron powder, uniform distribution of iron powder aggregates, and high tensile strengths of the composite sheets but also for high internal bond strengths. It appears that contact points between iron powder particles and cellulose fibers by electrostatic interactions needed to be increased as much as possible at the slurry stage. When

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Figure 9. Schematic representation of interactions between iron powder and either unfibrillated or fibrillated cellulose fiber with the dual polymer system (cationic PAE and anionic CMC) at the slurry stage.

unfibrillated cellulose fibers were used, the contact points were very few in number, resulting in weak resistance to shear stress in the slurries. However, the number of contact points could be increased by the use of fibrillated cellulose fibers, thus providing more stable adhesion of iron powder aggregates to cellulose fibers by both electrostatic and physical interactions. Schematic representations of the hypothesized interactions between iron powder particles and cellulose fibers are illustrated in Figure 9. On the basis of the laboratory results obtained, we succeeded in manufacturing flexible composite sheets with high content of iron powder without encountering iron corrosion problems (Figure 10). The as-produced iron-containing sheet after drying resisted corrosion for at least three years, when the sheet was exposed to air at 23 °C and 50% relative humidity. However, corrosion of the iron component occurred immediately in air at 40 °C and 90% relative humidity. When the sheet was placed in a polyethylene bag, the iron component was stable without corrosion at 40 °C and 90% relative humidity for at least 3 months. Conclusions Iron powder, cellulose fiber, and activated carbon powder were suspended in water, and composite sheets for steamgeneration were prepared from the slurry both with and without the use of a PAE/CMC dual polymer system. Two techniques used in the papermaking process were used to increase iron powder retention ratios, namely fibrillation of cellulose fiber and application of PAE/CMC dual polymer system. High degrees of fibrillation of cellulose fiber were effective in improving iron powder retention, and about 90% of the added components were retained in the sheets when

Figure 10. Flexible composite sheets with iron powder:softwood fibrillated cellulose fiber:activated carbon powder weight ratio of 182.2:22.3:18.5. The basis weight is 223 g m-2.

fibrillated softwood cellulose fiber was used in combination with the PAE/CMC dual polymer system, giving composite sheets with a basis weight about 217 g m-2 and iron powder, cellulose fiber, and activated carbon powder contents of 75, 16, and 9%, respectively. The iron powder aggregates were evenly distributed on each sheet surface as well as between the two surfaces, i.e. throughout the thickness of the sheet. Softwood fibrillated cellulose was superior to eucalypt fibrillated fibers in terms of higher tensile strength of the composite sheets.

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ReceiVed for reView May 28, 2009 ReVised manuscript receiVed September 12, 2009 Accepted September 25, 2009 IE900875U