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Preparation and characterization of corn starch–calcium carbonate hybrid pigments
Industrial Crops and Products 83 (2016) 294–300
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Preparation and characterization of corn starch–calcium carbonate hybrid pigments Jonna Kuusisto ∗ , Thad C. Maloney Department of Forest Products Technology, School of Chemical Technology, Aalto University, PO Box 16300, FI-00076 AALTO, Finland
a r t i c l e
i n f o
Article history: Received 6 October 2015 Received in revised form 17 December 2015 Accepted 12 January 2016 Available online 25 January 2016 Keywords: Corn starch Calcium carbonate Composite Thermal treatment Crystallization Paper
a b s t r a c t This article presents a novel method for integrating native corn starch and calcium carbonate (CaCO3 ) into a hybrid pigment, which can be applied in papermaking to increase the amount of starch in paper and improve sheet bonding without impairing dewatering. Starch was first thermally treated to partially gelatinize and swell the granules. The aim of the treatment was to change the surface properties of starch and thus to improve the interaction with CaCO3 particles. CaCO3 shell was precipitated around the swollen granules and the encapsulated starch was then cooked to achieve complete gelatinization. When the granules were cooked, the starch became active for bonding, but its solubility was very limited due to the encapsulation. Particle size measurement and scanning electron microscopy were used to analyze the sample properties during the preparation. The hybrid pigment was added to paper sheets and its effect on the paper properties was examined. Starch was shown to be effectively bound to the pigment structure. The hybrid pigment allowed a large amount of native corn starch to be added to paper without interfering with the papermaking process. Furthermore, it gave excellent mechanical paper properties. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Starch is one of the most abundant organic polymers on earth after cellulose. Despite being both composed of the glucose monomer (C6 H10 O5 ), starch and cellulose have different molecular structures and properties. Cellulose consists of linear glucose units linked by beta linkages, whereas the glucose units in starch form both linear (amylose) and branched (amylopectin) molecules connected by alpha linkages (Zobel, 1988). Their roles in plants are completely different; starch is in the form of granules and acts as an energy storage, whereas strong cellulose fibers provide structural support for plants. The crystallinity of cellulose is high and it is insoluble in water, while the semicrystalline starch is watersoluble when heated. In paper and paperboard products, cellulosic fibers are the main raw material and provide the strength of the network by bonding to each other via a combination of hydrogen bonds, and van der Waals and electrostatic interactions (Hubbe, 2006; Lindström et al., 2005). Starch is also capable forming hydrogen bonds and it is used in papermaking as an additive mainly to improve both the dry tensile strength and the surface strength. Besides its availability, starch is also inexpensive, biodegradable
∗ Corresponding author. E-mail addresses: jonna.kuusisto@aalto.fi, [email protected] (J. Kuusisto), thaddeus.maloney@aalto.fi (T.C. Maloney). http://dx.doi.org/10.1016/j.indcrop.2016.01.026 0926-6690/© 2016 Elsevier B.V. All rights reserved.
(Dipa and Sonakshi, 2012) and is widely used by many industries such as food, paper, textile as well as in adhesives. Starch is stored in plants in the form of granules and has no binding properties in this form (Bruun, 2009). Prior to its use in papermaking, starch needs to be cooked into a dissolved form before it is added to the wet end of a paper machine. However, because the retention of native starch is very low, the starch is generally cationized to improve its adsorption onto the anionic pulp fibers. The cationic starch is significantly more expensive than native starch. The amount of cationic starch that can be added to the wet end is limited by the adsorption capacity of the other furnish components. Typically the retention of cationic starch reaches a plateau at 1.5–2% and the further addition of wet end starch will lead to a number of production problems such as high biological oxygen demand (BOD) levels, foaming, slime and sticky problems as well as interferences in runnability and drainage (Krogerus, 2007; Yoon and Deng, 2006). If it is desired to increase the starch content of paper or board beyond this amount, the starch must be added to the paper surface with a size press. This approach has a detrimental effect on paper machine speed and energy consumption since the surface starch must be dried with thermal energy. Gelatinization means the irreversible process of swelling and finally solubilizing the starch granules when they are heated in an aqueous medium, typically in water (Hari et al., 1989; Lund and Lorenz, 1984). It can be ascribed as a reorganization of the
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molecular structure of starch, which is related to the hydration and swelling of the granules, leaching of amylose and changes in viscosity. During gelatinization, the intermolecular hydrogen bonds between the chains break and hydroxyl groups form bonds with water molecules. First the amorphous parts of the granules, which mainly consist of amylose, start to soften and swell. The water molecules and swollen amorphous parts push amylopectin chains apart breaking the crystalline structure of the granules. The swelling and gelatinization are greatly affect by the origin of starch as well as the amylose/amylopectin content (Fredriksson et al., 1998; Liu et al., 2006). Inorganic minerals, like calcium carbonate (CaCO3 ), clay and talc, are the second largest constituent in paper and board used either for filling or coating the product. Mineral pigments give good optical and surface properties to paper whilst improving also the dewatering and economic performance. However, when minerals are added to stock, they prevent fiber–fiber bonding leading to the decreased strength and stiffness of paper as well as increased dusting and size demand (Cao et al., 2011; Zhao et al., 2005). Recently, improved cost-efficiency via increased mineral content has been the general trend in the paper industry. To minimize the strength reduction effects, paper manufacturers have developed methods to modify pigments with different strength adding polymers, such as starch. To avoid the difficulties which may result when adding large amounts of dissolved starch into the papermaking process, different techniques, such as precipitation of starch on top of the pigment (Yoon and Deng, 2006) or spray-drying of starch-coated pigments (Deng et al., 2011; Zhao et al., 2008) have been examined. Another possibility to prevent the accumulation of molecular starch has been the treatment of starch in a high solids content into a highly swollen (not dissolved) form followed by a rapid mixing with a pigment to give the pigment a shell structure of starch gel before adding it to the papermaking furnish (Hirvikoski and Laakso, 2014; Laleg, 2013a,b; Zhao et al., 2005) However, most of the reported techniques have been rather complex or have faced challenges in scaling up from a laboratory to an industrial level. Composites from starch and filler for other applications than paper filling have also been widely developed and examined. Mixture of starch, fibers and CaCO3 have been used to produce food containers from baked foams, where Glenn et al. (2001) have reported CaCO3 to have negative impact on the mechanical properties of the foams. The replacement of synthetic materials by biodegradable starch-clay nanocomposites for packaging and food industry has also gained a lot of interest (Aouada et al., 2013; Avella et al., 2005; Chiou et al., 2006). The target of this research is to form an encapsulated native starch hybrid pigment which imparts useful properties to paper and board products. The starch should be activated for bonding, but restricted from full dissolution. This gives increased sheet strength while maintaining good dewatering and wet end characteristics. Because the hybrid pigment is formed from low cost raw materials, i.e. native corn starch, lime (i.e. calcium oxide, CaO), and carbon dioxide gas (CO2 , typically available from flue gas as a waste product), the potential cost structure of the new type of hybrid pigment is excellent and can be used to improve the overall cost structure of the paper or board where it is used. In addition, the process developed for the pigment preparation has been kept simple in order to generate an industrially feasible technique and a product, which could found use in many applications like as a fiber replacement in paper, additive in coating or packaging and in different composite applications.
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2. Materials and methods 2.1. Materials Lime was provided by Lhoist, Ltd. (La Mède, France) and native corn starch containing 25% amylose (560P) by Roquette, Ltd. (Lestrem, France). Bleached chemical pulps (pine and birch) were obtained from Stora Enso, Varkaus mill (Finland). Kemira Ltd. (Finland) provided cationic polyacrylamide (C-PAM, grade Fennopol K3400R), which was used as a retention aid in hand sheet preparation. As a reference pigment, commercial scalenohedral precipitated calcium carbonate (PCC, grade Syncarb FS-240 from Omya AG, Switzerland) was utilized in hand sheets. Deionized water was used in the experiments. 2.2. Thermal treatment of starch Starch was thermally treated before using it in the hybrid pigment preparation. Granular starch was suspended in water and heated to initiate the gelatinization. The target of the treatment was to swell the granules, loosen their tightly packed semi-crystalline structure and make their surface stickier and more accessible for calcium ions to adsorb. To determine the temperature of the thermal treatment, the effect of various temperatures on the swelling, solubility and gelatinization of starch were investigated. To identify the gelatinization parameters, the corn starch was analyzed with the differential scanning calorimetry (DSC, Mettler Toledo DSC821). The thermal transitions, i.e. the onset temperature (To ), peak temperature (Tp ) and the conclusion temperature (Tc ), and the heat of gelatinization i.e. the enthalpic change (H) during the endothermic process were determined. Calorimetric measurement was conducted by heating the starch suspension in excess water from 25 ◦ C to 120 ◦ C at a scanning rate of 10 ◦ C/min in a sample pan. H was calculated from the peak area of the endotherm. The degree of starch gelatinization (DG) after the thermal treatment was determined from the DSC data with the following procedure. The starch suspension was heated from 25 ◦ C to the target temperature at a scanning rate of 10 ◦ C/min, held at the temperature for a designated time, cooled to 25 ◦ C and heated again to 120 ◦ C. Various treatment temperatures and times were examined. DG was calculated by comparing H for untreated starch with that for treated starch (Holm et al., 1988; Ratnayake et al., 2009): DG (%) =
Huntreated − Htreated × 100 Huntreated
(1)
To measure the swelling power and solubility of starch, the thermal treatment of starch was conducted by heating the 8 wt.% starch suspension in a water bath for 10 min at various temperatures around To and cooling the starch suspension to room temperature. Swelling power and solubility of starch after thermal treatment were measured with the procedure similar to the method developed by Holm et al. (1988). Treated starch suspensions were decanted into a centrifuge tubes and diluted to 3 wt.%. Untreated starch was also weighed into a centrifuge tube and diluted to the same concentration. The tubes were mixed with an overhead shaker (Heidolph Reax 2) for 1 h and centrifuged for 15 min at 3000G. The supernatant was removed from the centrifuged samples and dried in the oven. Swelling power and solubility of untreated and treated starches were calculated according to Leach et al. (1959): Swelling power =
massgel massstarch,total − masssolute
(2)
and Solubility (%) =
masssolute × 100 massstarch,total
(3),
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Fig. 1. Procedure for the hybrid pigment preparation.
Degree of gelatinization (%)
100 80 60 40 20 0 15
20
55 60 65 70 Treatment temperature (°C)
75
Fig. 2. The effect of the treatment temperature on the degree of gelatinization of starch granules. The treatment time after reaching the target temperature was 10 min.
Hand sheet
Fiber (%)
PCC (%)
Starch (%)
Unfilled Reference PCC 1 Reference PCC 2 Reference PCC 3 Starch-PCC 1 Starch-PCC 2 Starch-PCC 3
100 95 91 87 90 82 74
0 5 9 13 5 9 13
0 0 0 0 5 9 13
temperature 50 ◦ C. After slaking, the Ca(OH)2 slurry was filtered through 100 mesh (equal to 149 microns) screen to remove impurities. Precipitation of CaCO3 was performed in the presence of treated starch in a laboratory-scale reactor equipped with a highshear mixer, a CO2 gas feed system with rotameter, and a pH, conductivity, and temperature sensors. The set-up for the batch reactor is presented in Kuusisto and Maloney (2015). A quantity of Ca(OH)2 was added into the reactor sufficient to form PCC equal to the amount of starch. Starch and Ca(OH)2 were mixed together before starting to feed the gas. CO2 gas was fed through Ca(OH)2 slurry at the pH of 11.5–12.0, while monitoring temperature, conductivity and pH. The completion of the reaction was seen as an abrupt, consecutive decrease of conductivity and pH, and the reaction was finished when pH had dropped to 7–8. The initial precipitation temperature was 22–23 ◦ C, CO2 flow rate 2 L min−1 and final solids content 10%. After precipitation, the hybrid pigment was cooked at 95 ◦ C for 20 min to get starch into a molecular form and exploit its bonding potential. The cooked pigment was mixed with a high-shear mixer and screened through 100 mesh screen. 2.4. Preparation of handsheets
Degree of gelatinization (%)
95
90
85
80
75 0
5
10
15
20
25
Bleached softwood and hardwood pulps were refined with a Valley beater to a Schopper Riegler value (◦ SR) of 27 and 21, respectively. A 70/30 mixture of hardwood and softwood pulps was used to prepare handsheets with the basis weight of 80 g m−2 . The composition of the sheets is shown in Table 1. 0.02% of C-PAM was used as a retention aid in the sheets, which contained either reference PCC or hybrid pigment. Handsheets were done with a Moving Belt Former (MBF), which has been designed to simulate the pulsation drainage and turbulence on the wire section of paper machine (Strengell et al., 2003). Retention aid was added 15 s prior to drainage. The applied vacuum during drainage was 25 kPa and suction time 500 ms. Handsheets were then wet-pressed at 100 kPa for 2 times 2 min, dried between the drying-plates at ambient temperature and conditioned according to the standard ISO 187 before measuring paper properties.
Treatment time (min) 2.5. Characterization Fig. 3. The effect of the treatment time on the degree of gelatinization of starch granules. The treatment temperature was 66 ◦ C.
where massgel is the wet mass of the centrifuged sample after removing the supernatant, massstarch,total is the dry mass of starch and masssolute is the dry mass of supernatant. 2.3. Preparation of hybrid pigments The procedure for the preparation of the composite pigments is presented in Fig. 1. The starch treatment temperature was chosen on the grounds of the results from the swelling power, solubility and DG measurements. Slaking was performed by mixing lime and water with a highshear mixer for 10 min to form calcium hydroxide (Ca(OH)2 ) i.e. slaked lime. Lime-to-water ratio was 1:8 and initial water
To analyze the size and morphology during the hybrid pigment preparation, samples were taken after the starch treatment, coprecipitation and cooking. Particle size was measured with the particle size analyzer (PSA, Malvern Mastersizer 2000) by using the general purpose model and the refractive index of 1.53 and the absorption index of 0.01 for starch and 1.572 and 0.1 for composites, respectively. Morphologies of the freeze-dried, Au-coated specimens were investigated by a field-emission scanning electron microscope (FE-SEM, Zeiss Sigma VP) operated at 2.5 kV. The retention of starch in a composite structure was evaluated by measuring the amount of starch in the composite before cooking and after washing the cooked composite. The washing procedure was repeated three times. After diluting the composite with deionized water, the sample was centrifuged for 15 min at 3000G. The proportions of PCC and starch in the composite before cooking
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297
Fig. 4. SEM micrographs of raw starch (a), thermally treated starch (b), hybrid pigment after precipitation (c) and after cooking (d and e, higher magnification in e) and reference PCC (f). Scale bars in images correspond to 10 m.
and after washing were determined from the ash content of dried samples. Handsheet properties were analyzed according to the following standards: T569 pm-00 (internal bond strength), ISO 1924-2 (tensile strength and elastic modulus), ISO 1974 (tear strength), ISO 534 (density), ISO 5631-1 (opacity and light scattering) and ISO 5636-3 (air permeability). The effect of composites on the dewatering of hand sheets was analyzed by measuring the solids content of hand sheets after couching.
Table 2 The effect of the thermal treatment (10 min) on the swelling power and solubility of starch.
3. Results and discussion
level of swelling, as well as the solubility, was affected by the temperature during the treatment. After 66 ◦ C, no further increase in the swelling power of starch was detected, but the solubility was still increased. Due to the high swelling power, relatively low solubility, and high DG, 66 ◦ C was chosen for the treatment temperature in hybrid pigment preparation. The effect of treatment time at 66 ◦ C on the gelatinization was also examined. As shown in Fig. 3, longer treatment time induced higher DG of the granules. However, after 7.5 min, the duration of the treatment had only a very small effect on the DG. Thus, 10 min was chosen for the treatment time in the composite preparation.
3.1. Thermal treatment of starch To of the corn starch was identified to be at 66 ◦ C, Tp at 73 ◦ C and Tc at 82 ◦ C. Fig. 2 illustrates the impact of treatment temperature on the DG of starch. When the treatment time was set to 10 min, it was observed that the partial gelatinization of the granules occurred already at the temperatures below the To . However, when moving closer to the To , there was a sharp increase in the gelatinization degree and at 66 ◦ C more than 90% of the starch was gelatinized. At 68 ◦ C, DSC results indicated a complete gelatinization of starch. Table 2 summarizes the effect of treatment temperature on the swelling power and solubility of the starch granules. Untreated starch granules had very low solubility and the swelling power similar to the values reported by previous authors (Holm et al., 1988). After the thermal treatment the granules were swollen, and the
Starch treatment (◦ C) Untreated (20) 58 62 66 70
Swelling power 2.23 ± 0.11 4.47 ± 0.06 5.35 ± 0.03 7.92 ± 0.33 7.78 ± 0.06
Solubility (%) 0.38 ± 0.02 1.67 ± 0.07 2.44 ± 0.11 3.56 ± 0.10 4.53 ± 0.06
3.2. Hybrid pigment properties From the SEM micrographs taken during composite preparation (Fig. 4), it was observed that the shape of the originally angular starch granules was changed after the treatment, but the starch
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80
12 Sample
8
Raw starch
13.8
Thermally treated
23.5
Precipitated comp.
28.5
Cooked comp.
32.8
Air permeability (µm Pa-1s-1)
Volume (%)
10
d(0.5) µm
6 4 2 0 0.1
Unfilled Reference PCC Starch-PCC
60
40
20
0 1
10
100
1000
Size (µm)
was still mainly in a granular form. Holes in the nuclei indicate that gelatinization started from the hilum of the granules, which has also been observed by other authors (Liu and Zhao, 1990; Peng et al., 2007; Ratnayake and Jackson, 2006, 2007). Due to the swelling, the particle size of the thermally treated granules (Fig. 5) shifted toward bigger values. After precipitation the surfaces of the swollen starch granules were found to be coated with PCC and the median particle size was slightly increased. It is possible that the CaCO3 shell is formed when Ca(OH)2 particles are adsorbed on the sticky outer surface of partially gelatinized granules, followed by carbonization. After cooking, the particle size was further increased, but the starch was mostly retained in the composite structure. This was seen both with the microscopy and from the retention measurement. Before cooking the starch content of the composite was 56% and after vigorous washing of the cooked composite it had decreased to 49%. This gives starch the retention of 87% in the composite structure. According to PSA, the fraction of smaller particles (<10 m) was minor after precipitation, which shows that PCC particles were mostly bound to the starch. In addition, the particle size distribution remained narrow throughout the composite preparation procedure. The morphology of the formed PCC was colloidal (Fig. 4e), which results from the fast precipitation rate (i.e. high CO2 flow rate) and low precipitation temperature. As commonly known (Garcı´ıa-Carmona et al., 2003; Jung et al., 2000; Ukrainczyk et al., 2007), the ratio of dissolved calcium to carbonate species ([Ca]tot /[CO3 ]tot ) as well as the precipitation temperature are the main parameters, which determine the morphology of the precipitated particles. 3.3. Paper properties The properties of PCC and hybrid pigment (i.e. starch-PCC) filled papers are reported at the same PCC contents of paper. This means that in the starch-PCC filled papers, there is also an equal amount of starch and the fiber content is smaller than in the PCC filled papers. Even though large amounts of starch were incorporated into the paper within the composite, the solids content of sheets after couching were slightly higher than when PCC was used (Fig. 6), which means that the dewatering was maintained at a desirable level. Good dewatering can result from the bonding of starch into the composite structure and large size of the composites. At the same time, the use of hybrid pigment led to extremely small permeability values, whereas the air permeability of PCC filled sheets was increased as the content of pigment in a sheet was increased. It is unusual that a papermaking raw material can yield a combination of high dewatering and very low permeability,
15 16 17 Solids content after couching (%)
18
Fig. 6. Air permeability of the paper sheets vs. their solids content after couching. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
Scott internal bonding strength (J m-2)
Fig. 5. Particle size distribution and volume median particle size d (0.5) during the hybrid pigment preparation.
14
1800
Unfilled Reference PCC Starch-PCC
1600 1400 1200 600 400 200 0 0
5
10
15
PCC content (%) Fig. 7. Scott internal bonding strength at different PCC contents of the paper sheets. Starch-PCC filled paper with the highest loading level was above the measurement range of the device.
because small particles that plug interfiber pores also inhibit dewatering. Unlike traditional pigments, the composite had a positive effect on the strength properties of paper. As shown in Fig. 7 , the internal bonding strength was significantly improved, and increasing the amount of composite in paper further increased the bonding strength. Also the tensile strength (Fig. 8) and elastic modulus (Fig. 9 ) of the sheets containing composite were in a much higher level than in unfilled and PCC filled papers and remained in a constant level regardless of the content of composite in paper. Tear strength was the only mechanical property, which was decreased when using high loading level of composite. At lower addition levels, the composite gave similar tear strength to paper as traditional PCC, but the highest addition level resulted in impaired tear strength when compared to PCC filled paper (Fig. 8). This was likely due to the much smaller fiber content of starch-PCC filled papers and the high density of the sheets with the hybrid pigment. For example at the highest composite addition, the content of starch in the paper was 13% which means that the content of fibers in the sheet was decreased by an equal amount. As shown in Fig. 9, the density of paper was increased when the composite was used instead of PCC, which means that the
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55 Unfilled Reference PCC Starch-PCC
60
Light scattering (m2 kg-1)
Tensile index (Nm g-1)
70
50
40
30
Unfilled Reference PCC Starch-PCC
50 45 40 35 30
20 6
8
10
12
25 75
Tear index (mN m2 g-1)
5.0 4.5
3.5 3.0 2.5
0.6
85
90
Fig. 10. Light scattering vs. opacity of the paper sheets. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
unmodified starch was used instead of more expensive, cationic starch. Due to the extremely high bonding strength, the use of hybrid pigment could eliminate the need for the surface sizing of paper in the future. Since the size press limits the production speed of paper machine and is an expensive equipment, its elimination would also cut down the production costs. Besides fiber replacement, the hybrid pigment could also find use in paper coating or as an additive in board production. The content of the hybrid pigment depends on the end product and desired properties. In full-scale paper production, adding small amount (a few percent) of hybrid pigment along with the traditional filler would already give considerable cost savings and strength benefits, whereas in some other products the content of the hybrid pigment could be much higher.
Unfilled Reference PCC Starch-PCC
4.0
2.0 0.5
80 Opacity (%)
Fig. 8. Tensile index vs. tear index of the paper sheets. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
Elastic modulus (GPa)
299
0.7
0.8
0.9
Density (g cm-3) Fig. 9. Elastic modulus vs. density of the paper sheets. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
composite pigment decreased the bulk of paper. Due to the scalenohedral morphology, which make it a high-bulking pigment (Hubbe, 2004; Laufmann et al., 2000), reference PCC showed the opposite behavior. The optical properties of composite-filled papers, such as light scattering and opacity (Fig. 10) were in the same level with the unfilled paper and much lower than those of PCC filled paper sheets, which can again be attributed to the morphological and size-related differences between the hybrid and traditional pigment. The technique used for the preparation of the hybrid pigment was fairly simple consisting only of two heating steps and precipitation. Thus, it could easily be scaled up into the industrial process and is feasible in the traditional, existing precipitation reactors. In addition, the hybrid pigment was prepared from the raw materials, which are commonly used in papermaking and have low cost thus giving it a very attractive cost structure when compared to kraft pulp. Thus, if used as a fiber replacement in paper, significant cost savings could be achieved already at low hybrid pigment contents. When the composite was added into a paper to partially replace fiber raw material, excellent strength properties as well as some exceptional property combinations, such as low air permeability and good dewatering, were achieved. Within the composite, the amount of starch incorporated into the paper sheet could be increased far above the typical limitations, even though
4. Conclusions A new type of hybrid pigment was produced by encapsulating a thermally activated native corn starch granule with a layer of precipitated calcium carbonate. It was found that the new hybrid pigment had desirable process and product properties. By heating the encapsulated granules at 90 ◦ C the starch could be fully swollen, and thus in a suitable form for bonding, but the full dissolution into the papermaking furnish was restricted. Therefore, higher amounts of starch could be introduced into the paper via wet end addition than is normally done and good dewatering could still be achieved, even though low cost, native corn starch was used instead of more expensive, cationic starch. Regardless of the good dewatering, the addition of hybrid pigment resulted in low air permeability, which is a very unique combination. In addition, the composite gave extremely good internal bonding strength to paper, and the other strength properties except the tear strength were also improved. Besides the decreased paper bulk, the disadvantage of composite was that the optical properties of paper remained in the same level with the unfilled paper. The hybrid pigment was prepared with a simple technique from the raw materials, which have much lower cost than chemical pulp. This means that besides its unique paper properties, the hybrid pigment could be used to improve the cost-efficiency of papermaking. In addition, the composite has a potential to eliminate the need of the size press, since it gives a very high bonding (or surface) strength to the paper. In the future experiments, the authors are interested in the further development of hybrid pigment and examining its effects on
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paper properties. For example, the improvement of bulk and optical properties of paper could be done by altering the ratio of starch and PCC, changing the morphology of PCC on top of the starch granules or decreasing the size of the hybrid pigments. The size of the hybrid pigments could be changed by either changing the origin of starch or grinding the material. In addition, the applicability of the pigment as an additive in board production or as a coating material for paper and board will be explored. Acknowledgements The financial support from the Academy of Finland is gratefully acknowledged. Microscopy work made use of the Aalto University Nanomicroscopy Center (Aalto-NMC) premises. Tuyen Nguyen, Leena Nolvi and Victoria Lindqvist are acknowledged for their assistance with the experimental work. References Aouada, F.A., Mattoso, L.H.C., Longo, E., 2013. Enhanced bulk and superficial hydrophobicities of starch-based bionanocomposites by addition of clay. Ind. Crop. Prod. 50 (10), 449–455, http://dx.doi.org/10.1016/j.indcrop.2013.07.058. Avella, M., De Vlieger, J.J., Errico, M.E., Fischer, S., Vacca, P., Volpe, M.G., 2005. Biodegradable starch/clay nanocomposite films for food packaging applications. Food Chem. 93 (3), 467–474, http://dx.doi.org/10.1016/j. foodchem.2004.10.024. Bruun, S., 2009. Binders. In: Paltakari, J. (Ed.), Papermaking Science and Technology. Pigment Coating and Surface Sizing of Paper, vol. 11. Paper Engineers’ Association/Paperi ja Puu Oy, Helsinki, Finland, pp. 192–225. Cao, S., Song, D., Deng, Y., Ragauskas, A., 2011. Preparation of starch—fatty acid modified clay and its application in packaging papers. Ind. Eng. Chem. Res. 50 (9), 5628–5633, http://dx.doi.org/10.1021/ie102588p. Chiou, B.-S., Yee, E., Wood, D., Shey, J., Glenn, G., Orts, W., 2006. Effects of processing conditions on nanoclay dispersion in starch–clay nanocomposites. Cereal Chem. 83 (3), 300–305, http://dx.doi.org/10.1094/CC-83-0300. Deng, Y., Yoon, S., Ragauskas, A., White, D., 2011. Making modified fillers for papermaking; applying a modified filler to a composition comprising fiber to form a mixture, processing the mixture whereby producing a paper. US 7964063 B2. Dipa, R., Sonakshi, M., 2012. Starch nanocomposites. In: Maya, J.J., Sabu, T. (Eds.), Natural Polymers, Nanocomposites, vol. 2. Royal Society of Chemistry, UK, pp. 185––233. Fredriksson, H., Silverio, J., Andersson, R., Eliasson, A.-C., Åman, P., 1998. The influence of amylose and amylopectin characteristics on gelatinization and retrogradation properties of different starches. Carbohydr. Polym. 35 (3), 119–134, http://dx.doi.org/10.1016/S0144-8617(97)00247-6. Garcı´ıa-Carmona, J., Morales, J.G., Clemente, R.R., 2003. Morphological control of precipitated calcite obtained by adjusting the electrical conductivity in the Ca(OH) 2-H2O-CO2 system. J. Cryst. Growth. 249 (3–4), 561–571, http://dx.doi. org/10.1016/S0022-0248(02)02173-5. Glenn, G.M., Orts, W.J., Nobes, G.A.R., 2001. Starch, fiber and CaCO3 effects on the physical properties of foams made by a baking process. Ind. Crops. Prod. 14 (3), 201–212, http://dx.doi.org/10.1016/S0926-6690(01)00085-1. Hari, P.K., Garg, S., Garg, S.K., 1989. Gelatinization of starch and modified starch. Starch 41 (3), 88–91, http://dx.doi.org/10.1002/star.19890410304. Hirvikoski, L.K., Laakso, A.J., 2014. Filler suspension and its use in the manufacture of paper. WO 2014055787 A1. Holm, J., Lundquist, I., Bjorck, I., Eliasson, A.C., Asp, N.G., 1988. Degree of starch gelatinization, digestion rate of starch in vitro, and metabolic response in rats. Am. J. Clin. Nutr. 47 (6), 1010–1016.
Hubbe, M.A., 2004. Filler particle shape vs. paper properties—a review. In: TAPPI Spring Technical Conference Proceedings, Atlanta, pp. 1–10. Hubbe, M.A., 2006. Bonding between cellulosic fibers in the absence and presence of dry-strength agents—a review. BioResources 1 (2), 281–318. Jung, W.M., Kang, S.H., Kim, W.S., Choi, C.K., 2000. Particle morphology of calcium carbonate precipitated by gas–liquid reaction in a Couette–Taylor reactor. Chem. Eng. Sci. 55 (4), 733–747, http://dx.doi.org/10.1016/S00092509(99)00395-4. Krogerus, B., 2007. Papermaking additives. In: Alén, R. (Ed.), Papermaking Science and Technology, Papermaking Chemistry, vol. 4. Finnish Paper Engineers’ Association/Paperi ja Puu Oy, Helsinki, Finland, pp. 56––121. Kuusisto, J.E., Maloney, T.C., 2015. The effect of carbonation conditions on the properties of carbohydrate-calcium carbonate hybrid pigments. BioResources 10 (2), 3277–3292, http://dx.doi.org/10.1016/S0009-2509(99) 00395-4. Laleg, M., 2013a. Unruptured, ionic, swollen starch for use in papermaking. US 8354004 2013 B2. Laleg, M., 2013b. Swollen starch compositions for use in papermaking. CA 2671973C. Laufmann, M., Forsblom, M., Strutz, M.D., Yeakey, S.R., 2000. GCC vs. PCC as the primary filler for uncoated and coated wood free paper. Tappi J. 83 (5), 76. Leach, H.W., McCowen, L., Schoch, T.J., 1959. Structure of the starch granule I. Swelling and solubility patterns of various starches. Cereal Chem. 36 (6), 534––544. Lindström, T., Wågberg, L., Larsson, T., 2005. On the nature of joint strength in paper—a review of dry and wet strength resins used in paper manufacturing. In: 13th Fundamental Research Symposium Proceedings, Cambridge, pp. 457–562. Liu, J., Zhao, S., 1990. Scanning electron microscope study on gelatinization of starch granules in excess water. Starch-Stärke 42 (3), 96–98, http://dx.doi.org/ 10.1002/star.19900420305. Liu, H., Yu, L., Xie, F., Chen, L., 2006. Gelatinization of cornstarch with different amylose/amylopectin content. Carbohydr. Polym. 65 (3), 357–363, http://dx. doi.org/10.1016/j.carbpol.2006.01.026. Lund, D., Lorenz, K.J., 1984. Influence of time, temperature, moisture, ingredients, and processing conditions on starch gelatinization. Crit. Rev. Food Sci. Nutr. 20 (4), 249–273, http://dx.doi.org/10.1080/10408398409527391. Peng, L., Zhongdong, L., Kennedy, J.F., 2007. The study of starch nano-unit chains in the gelatinization process. Carbohydr. Polym. 68 (2), 360–366, http://dx.doi. org/10.1016/j.carbpol.2006.11.021. Ratnayake, W.S., Jackson, D.S., 2006. Gelatinization and solubility of corn starch during heating in excess water: new insights. J. Agric. Food Chem. 54 (10), 3712–3716, http://dx.doi.org/10.1021/jf0529114. Ratnayake, W.S., Jackson, D.S., 2007. A new insight into the gelatinization process of native starches. Carbohydr. Polym. 67 (4), 511–529, http://dx.doi.org/10. 1016/j.carbpol.2006.06.025. Ratnayake, W.S., Otani, C., Jackson, D.S., 2009. DSC enthalpic transitions during starch gelatinisation in excess water, dilute sodium chloride and dilute sucrose solutions. J. Sci. Food Agric. 89 (12), 2156–2164, http://dx.doi.org/10.1002/jsfa. 3709. Strengell, K., Stenbacka, U., Ala-Nikkola, J., 2003. Advanced wire part simulation with a moving belt former and its applicability in scale up on rotogravure printing paper. In: 89th Annual Meeting, Pulp and Paper Technical Association of Canada, Montreal. ´ ´ V., Breˇcevic, ´ L., Kralj, D., 2007. Ukrainczyk, M., Kontrec, J., Babic-Ivanˇ cic, Experimental design approach to calcium carbonate precipitation in a semicontinuous process. Powder Technol. 171 (3), 192–199, http://dx.doi.org/ 10.1016/j.powtec.2006.10.046. Yoon, S., Deng, Y., 2006. Clay-starch composites and their application in papermaking. J. Appl. Polym. Sci. 100 (2), 1032–1038, http://dx.doi.org/10. 1002/app.23007. Zhao, Y., Hu, Z., Ragauskas, A., Deng, Y., 2005. Improvement of paper properties using starch-modified precipitated calcium carbonate filler. Tappi J. 4 (2), 3–7. Zhao, Y., Kim, D., White, D., Deng, Y., Patterson, T., Jones, P., Turner, E., Ragauskas, A.J., 2008. Developing a new paradigm for linerboard fillers. Tappi J. 7 (3), 3–7. Zobel, H., 1988. Molecules to granules: a comprehensive starch review. Starch-Stärke 40 (2), 44–50, http://dx.doi.org/10.1002/star.19880400203.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Preparation and characterization of corn starch–calcium carbonate hybrid pigments Jonna Kuusisto ∗ , Thad C. Maloney Department of Forest Products Technology, School of Chemical Technology, Aalto University, PO Box 16300, FI-00076 AALTO, Finland
a r t i c l e
i n f o
Article history: Received 6 October 2015 Received in revised form 17 December 2015 Accepted 12 January 2016 Available online 25 January 2016 Keywords: Corn starch Calcium carbonate Composite Thermal treatment Crystallization Paper
a b s t r a c t This article presents a novel method for integrating native corn starch and calcium carbonate (CaCO3 ) into a hybrid pigment, which can be applied in papermaking to increase the amount of starch in paper and improve sheet bonding without impairing dewatering. Starch was first thermally treated to partially gelatinize and swell the granules. The aim of the treatment was to change the surface properties of starch and thus to improve the interaction with CaCO3 particles. CaCO3 shell was precipitated around the swollen granules and the encapsulated starch was then cooked to achieve complete gelatinization. When the granules were cooked, the starch became active for bonding, but its solubility was very limited due to the encapsulation. Particle size measurement and scanning electron microscopy were used to analyze the sample properties during the preparation. The hybrid pigment was added to paper sheets and its effect on the paper properties was examined. Starch was shown to be effectively bound to the pigment structure. The hybrid pigment allowed a large amount of native corn starch to be added to paper without interfering with the papermaking process. Furthermore, it gave excellent mechanical paper properties. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Starch is one of the most abundant organic polymers on earth after cellulose. Despite being both composed of the glucose monomer (C6 H10 O5 ), starch and cellulose have different molecular structures and properties. Cellulose consists of linear glucose units linked by beta linkages, whereas the glucose units in starch form both linear (amylose) and branched (amylopectin) molecules connected by alpha linkages (Zobel, 1988). Their roles in plants are completely different; starch is in the form of granules and acts as an energy storage, whereas strong cellulose fibers provide structural support for plants. The crystallinity of cellulose is high and it is insoluble in water, while the semicrystalline starch is watersoluble when heated. In paper and paperboard products, cellulosic fibers are the main raw material and provide the strength of the network by bonding to each other via a combination of hydrogen bonds, and van der Waals and electrostatic interactions (Hubbe, 2006; Lindström et al., 2005). Starch is also capable forming hydrogen bonds and it is used in papermaking as an additive mainly to improve both the dry tensile strength and the surface strength. Besides its availability, starch is also inexpensive, biodegradable
∗ Corresponding author. E-mail addresses: jonna.kuusisto@aalto.fi, [email protected] (J. Kuusisto), thaddeus.maloney@aalto.fi (T.C. Maloney). http://dx.doi.org/10.1016/j.indcrop.2016.01.026 0926-6690/© 2016 Elsevier B.V. All rights reserved.
(Dipa and Sonakshi, 2012) and is widely used by many industries such as food, paper, textile as well as in adhesives. Starch is stored in plants in the form of granules and has no binding properties in this form (Bruun, 2009). Prior to its use in papermaking, starch needs to be cooked into a dissolved form before it is added to the wet end of a paper machine. However, because the retention of native starch is very low, the starch is generally cationized to improve its adsorption onto the anionic pulp fibers. The cationic starch is significantly more expensive than native starch. The amount of cationic starch that can be added to the wet end is limited by the adsorption capacity of the other furnish components. Typically the retention of cationic starch reaches a plateau at 1.5–2% and the further addition of wet end starch will lead to a number of production problems such as high biological oxygen demand (BOD) levels, foaming, slime and sticky problems as well as interferences in runnability and drainage (Krogerus, 2007; Yoon and Deng, 2006). If it is desired to increase the starch content of paper or board beyond this amount, the starch must be added to the paper surface with a size press. This approach has a detrimental effect on paper machine speed and energy consumption since the surface starch must be dried with thermal energy. Gelatinization means the irreversible process of swelling and finally solubilizing the starch granules when they are heated in an aqueous medium, typically in water (Hari et al., 1989; Lund and Lorenz, 1984). It can be ascribed as a reorganization of the
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molecular structure of starch, which is related to the hydration and swelling of the granules, leaching of amylose and changes in viscosity. During gelatinization, the intermolecular hydrogen bonds between the chains break and hydroxyl groups form bonds with water molecules. First the amorphous parts of the granules, which mainly consist of amylose, start to soften and swell. The water molecules and swollen amorphous parts push amylopectin chains apart breaking the crystalline structure of the granules. The swelling and gelatinization are greatly affect by the origin of starch as well as the amylose/amylopectin content (Fredriksson et al., 1998; Liu et al., 2006). Inorganic minerals, like calcium carbonate (CaCO3 ), clay and talc, are the second largest constituent in paper and board used either for filling or coating the product. Mineral pigments give good optical and surface properties to paper whilst improving also the dewatering and economic performance. However, when minerals are added to stock, they prevent fiber–fiber bonding leading to the decreased strength and stiffness of paper as well as increased dusting and size demand (Cao et al., 2011; Zhao et al., 2005). Recently, improved cost-efficiency via increased mineral content has been the general trend in the paper industry. To minimize the strength reduction effects, paper manufacturers have developed methods to modify pigments with different strength adding polymers, such as starch. To avoid the difficulties which may result when adding large amounts of dissolved starch into the papermaking process, different techniques, such as precipitation of starch on top of the pigment (Yoon and Deng, 2006) or spray-drying of starch-coated pigments (Deng et al., 2011; Zhao et al., 2008) have been examined. Another possibility to prevent the accumulation of molecular starch has been the treatment of starch in a high solids content into a highly swollen (not dissolved) form followed by a rapid mixing with a pigment to give the pigment a shell structure of starch gel before adding it to the papermaking furnish (Hirvikoski and Laakso, 2014; Laleg, 2013a,b; Zhao et al., 2005) However, most of the reported techniques have been rather complex or have faced challenges in scaling up from a laboratory to an industrial level. Composites from starch and filler for other applications than paper filling have also been widely developed and examined. Mixture of starch, fibers and CaCO3 have been used to produce food containers from baked foams, where Glenn et al. (2001) have reported CaCO3 to have negative impact on the mechanical properties of the foams. The replacement of synthetic materials by biodegradable starch-clay nanocomposites for packaging and food industry has also gained a lot of interest (Aouada et al., 2013; Avella et al., 2005; Chiou et al., 2006). The target of this research is to form an encapsulated native starch hybrid pigment which imparts useful properties to paper and board products. The starch should be activated for bonding, but restricted from full dissolution. This gives increased sheet strength while maintaining good dewatering and wet end characteristics. Because the hybrid pigment is formed from low cost raw materials, i.e. native corn starch, lime (i.e. calcium oxide, CaO), and carbon dioxide gas (CO2 , typically available from flue gas as a waste product), the potential cost structure of the new type of hybrid pigment is excellent and can be used to improve the overall cost structure of the paper or board where it is used. In addition, the process developed for the pigment preparation has been kept simple in order to generate an industrially feasible technique and a product, which could found use in many applications like as a fiber replacement in paper, additive in coating or packaging and in different composite applications.
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2. Materials and methods 2.1. Materials Lime was provided by Lhoist, Ltd. (La Mède, France) and native corn starch containing 25% amylose (560P) by Roquette, Ltd. (Lestrem, France). Bleached chemical pulps (pine and birch) were obtained from Stora Enso, Varkaus mill (Finland). Kemira Ltd. (Finland) provided cationic polyacrylamide (C-PAM, grade Fennopol K3400R), which was used as a retention aid in hand sheet preparation. As a reference pigment, commercial scalenohedral precipitated calcium carbonate (PCC, grade Syncarb FS-240 from Omya AG, Switzerland) was utilized in hand sheets. Deionized water was used in the experiments. 2.2. Thermal treatment of starch Starch was thermally treated before using it in the hybrid pigment preparation. Granular starch was suspended in water and heated to initiate the gelatinization. The target of the treatment was to swell the granules, loosen their tightly packed semi-crystalline structure and make their surface stickier and more accessible for calcium ions to adsorb. To determine the temperature of the thermal treatment, the effect of various temperatures on the swelling, solubility and gelatinization of starch were investigated. To identify the gelatinization parameters, the corn starch was analyzed with the differential scanning calorimetry (DSC, Mettler Toledo DSC821). The thermal transitions, i.e. the onset temperature (To ), peak temperature (Tp ) and the conclusion temperature (Tc ), and the heat of gelatinization i.e. the enthalpic change (H) during the endothermic process were determined. Calorimetric measurement was conducted by heating the starch suspension in excess water from 25 ◦ C to 120 ◦ C at a scanning rate of 10 ◦ C/min in a sample pan. H was calculated from the peak area of the endotherm. The degree of starch gelatinization (DG) after the thermal treatment was determined from the DSC data with the following procedure. The starch suspension was heated from 25 ◦ C to the target temperature at a scanning rate of 10 ◦ C/min, held at the temperature for a designated time, cooled to 25 ◦ C and heated again to 120 ◦ C. Various treatment temperatures and times were examined. DG was calculated by comparing H for untreated starch with that for treated starch (Holm et al., 1988; Ratnayake et al., 2009): DG (%) =
Huntreated − Htreated × 100 Huntreated
(1)
To measure the swelling power and solubility of starch, the thermal treatment of starch was conducted by heating the 8 wt.% starch suspension in a water bath for 10 min at various temperatures around To and cooling the starch suspension to room temperature. Swelling power and solubility of starch after thermal treatment were measured with the procedure similar to the method developed by Holm et al. (1988). Treated starch suspensions were decanted into a centrifuge tubes and diluted to 3 wt.%. Untreated starch was also weighed into a centrifuge tube and diluted to the same concentration. The tubes were mixed with an overhead shaker (Heidolph Reax 2) for 1 h and centrifuged for 15 min at 3000G. The supernatant was removed from the centrifuged samples and dried in the oven. Swelling power and solubility of untreated and treated starches were calculated according to Leach et al. (1959): Swelling power =
massgel massstarch,total − masssolute
(2)
and Solubility (%) =
masssolute × 100 massstarch,total
(3),
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Fig. 1. Procedure for the hybrid pigment preparation.
Degree of gelatinization (%)
100 80 60 40 20 0 15
20
55 60 65 70 Treatment temperature (°C)
75
Fig. 2. The effect of the treatment temperature on the degree of gelatinization of starch granules. The treatment time after reaching the target temperature was 10 min.
Hand sheet
Fiber (%)
PCC (%)
Starch (%)
Unfilled Reference PCC 1 Reference PCC 2 Reference PCC 3 Starch-PCC 1 Starch-PCC 2 Starch-PCC 3
100 95 91 87 90 82 74
0 5 9 13 5 9 13
0 0 0 0 5 9 13
temperature 50 ◦ C. After slaking, the Ca(OH)2 slurry was filtered through 100 mesh (equal to 149 microns) screen to remove impurities. Precipitation of CaCO3 was performed in the presence of treated starch in a laboratory-scale reactor equipped with a highshear mixer, a CO2 gas feed system with rotameter, and a pH, conductivity, and temperature sensors. The set-up for the batch reactor is presented in Kuusisto and Maloney (2015). A quantity of Ca(OH)2 was added into the reactor sufficient to form PCC equal to the amount of starch. Starch and Ca(OH)2 were mixed together before starting to feed the gas. CO2 gas was fed through Ca(OH)2 slurry at the pH of 11.5–12.0, while monitoring temperature, conductivity and pH. The completion of the reaction was seen as an abrupt, consecutive decrease of conductivity and pH, and the reaction was finished when pH had dropped to 7–8. The initial precipitation temperature was 22–23 ◦ C, CO2 flow rate 2 L min−1 and final solids content 10%. After precipitation, the hybrid pigment was cooked at 95 ◦ C for 20 min to get starch into a molecular form and exploit its bonding potential. The cooked pigment was mixed with a high-shear mixer and screened through 100 mesh screen. 2.4. Preparation of handsheets
Degree of gelatinization (%)
95
90
85
80
75 0
5
10
15
20
25
Bleached softwood and hardwood pulps were refined with a Valley beater to a Schopper Riegler value (◦ SR) of 27 and 21, respectively. A 70/30 mixture of hardwood and softwood pulps was used to prepare handsheets with the basis weight of 80 g m−2 . The composition of the sheets is shown in Table 1. 0.02% of C-PAM was used as a retention aid in the sheets, which contained either reference PCC or hybrid pigment. Handsheets were done with a Moving Belt Former (MBF), which has been designed to simulate the pulsation drainage and turbulence on the wire section of paper machine (Strengell et al., 2003). Retention aid was added 15 s prior to drainage. The applied vacuum during drainage was 25 kPa and suction time 500 ms. Handsheets were then wet-pressed at 100 kPa for 2 times 2 min, dried between the drying-plates at ambient temperature and conditioned according to the standard ISO 187 before measuring paper properties.
Treatment time (min) 2.5. Characterization Fig. 3. The effect of the treatment time on the degree of gelatinization of starch granules. The treatment temperature was 66 ◦ C.
where massgel is the wet mass of the centrifuged sample after removing the supernatant, massstarch,total is the dry mass of starch and masssolute is the dry mass of supernatant. 2.3. Preparation of hybrid pigments The procedure for the preparation of the composite pigments is presented in Fig. 1. The starch treatment temperature was chosen on the grounds of the results from the swelling power, solubility and DG measurements. Slaking was performed by mixing lime and water with a highshear mixer for 10 min to form calcium hydroxide (Ca(OH)2 ) i.e. slaked lime. Lime-to-water ratio was 1:8 and initial water
To analyze the size and morphology during the hybrid pigment preparation, samples were taken after the starch treatment, coprecipitation and cooking. Particle size was measured with the particle size analyzer (PSA, Malvern Mastersizer 2000) by using the general purpose model and the refractive index of 1.53 and the absorption index of 0.01 for starch and 1.572 and 0.1 for composites, respectively. Morphologies of the freeze-dried, Au-coated specimens were investigated by a field-emission scanning electron microscope (FE-SEM, Zeiss Sigma VP) operated at 2.5 kV. The retention of starch in a composite structure was evaluated by measuring the amount of starch in the composite before cooking and after washing the cooked composite. The washing procedure was repeated three times. After diluting the composite with deionized water, the sample was centrifuged for 15 min at 3000G. The proportions of PCC and starch in the composite before cooking
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297
Fig. 4. SEM micrographs of raw starch (a), thermally treated starch (b), hybrid pigment after precipitation (c) and after cooking (d and e, higher magnification in e) and reference PCC (f). Scale bars in images correspond to 10 m.
and after washing were determined from the ash content of dried samples. Handsheet properties were analyzed according to the following standards: T569 pm-00 (internal bond strength), ISO 1924-2 (tensile strength and elastic modulus), ISO 1974 (tear strength), ISO 534 (density), ISO 5631-1 (opacity and light scattering) and ISO 5636-3 (air permeability). The effect of composites on the dewatering of hand sheets was analyzed by measuring the solids content of hand sheets after couching.
Table 2 The effect of the thermal treatment (10 min) on the swelling power and solubility of starch.
3. Results and discussion
level of swelling, as well as the solubility, was affected by the temperature during the treatment. After 66 ◦ C, no further increase in the swelling power of starch was detected, but the solubility was still increased. Due to the high swelling power, relatively low solubility, and high DG, 66 ◦ C was chosen for the treatment temperature in hybrid pigment preparation. The effect of treatment time at 66 ◦ C on the gelatinization was also examined. As shown in Fig. 3, longer treatment time induced higher DG of the granules. However, after 7.5 min, the duration of the treatment had only a very small effect on the DG. Thus, 10 min was chosen for the treatment time in the composite preparation.
3.1. Thermal treatment of starch To of the corn starch was identified to be at 66 ◦ C, Tp at 73 ◦ C and Tc at 82 ◦ C. Fig. 2 illustrates the impact of treatment temperature on the DG of starch. When the treatment time was set to 10 min, it was observed that the partial gelatinization of the granules occurred already at the temperatures below the To . However, when moving closer to the To , there was a sharp increase in the gelatinization degree and at 66 ◦ C more than 90% of the starch was gelatinized. At 68 ◦ C, DSC results indicated a complete gelatinization of starch. Table 2 summarizes the effect of treatment temperature on the swelling power and solubility of the starch granules. Untreated starch granules had very low solubility and the swelling power similar to the values reported by previous authors (Holm et al., 1988). After the thermal treatment the granules were swollen, and the
Starch treatment (◦ C) Untreated (20) 58 62 66 70
Swelling power 2.23 ± 0.11 4.47 ± 0.06 5.35 ± 0.03 7.92 ± 0.33 7.78 ± 0.06
Solubility (%) 0.38 ± 0.02 1.67 ± 0.07 2.44 ± 0.11 3.56 ± 0.10 4.53 ± 0.06
3.2. Hybrid pigment properties From the SEM micrographs taken during composite preparation (Fig. 4), it was observed that the shape of the originally angular starch granules was changed after the treatment, but the starch
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80
12 Sample
8
Raw starch
13.8
Thermally treated
23.5
Precipitated comp.
28.5
Cooked comp.
32.8
Air permeability (µm Pa-1s-1)
Volume (%)
10
d(0.5) µm
6 4 2 0 0.1
Unfilled Reference PCC Starch-PCC
60
40
20
0 1
10
100
1000
Size (µm)
was still mainly in a granular form. Holes in the nuclei indicate that gelatinization started from the hilum of the granules, which has also been observed by other authors (Liu and Zhao, 1990; Peng et al., 2007; Ratnayake and Jackson, 2006, 2007). Due to the swelling, the particle size of the thermally treated granules (Fig. 5) shifted toward bigger values. After precipitation the surfaces of the swollen starch granules were found to be coated with PCC and the median particle size was slightly increased. It is possible that the CaCO3 shell is formed when Ca(OH)2 particles are adsorbed on the sticky outer surface of partially gelatinized granules, followed by carbonization. After cooking, the particle size was further increased, but the starch was mostly retained in the composite structure. This was seen both with the microscopy and from the retention measurement. Before cooking the starch content of the composite was 56% and after vigorous washing of the cooked composite it had decreased to 49%. This gives starch the retention of 87% in the composite structure. According to PSA, the fraction of smaller particles (<10 m) was minor after precipitation, which shows that PCC particles were mostly bound to the starch. In addition, the particle size distribution remained narrow throughout the composite preparation procedure. The morphology of the formed PCC was colloidal (Fig. 4e), which results from the fast precipitation rate (i.e. high CO2 flow rate) and low precipitation temperature. As commonly known (Garcı´ıa-Carmona et al., 2003; Jung et al., 2000; Ukrainczyk et al., 2007), the ratio of dissolved calcium to carbonate species ([Ca]tot /[CO3 ]tot ) as well as the precipitation temperature are the main parameters, which determine the morphology of the precipitated particles. 3.3. Paper properties The properties of PCC and hybrid pigment (i.e. starch-PCC) filled papers are reported at the same PCC contents of paper. This means that in the starch-PCC filled papers, there is also an equal amount of starch and the fiber content is smaller than in the PCC filled papers. Even though large amounts of starch were incorporated into the paper within the composite, the solids content of sheets after couching were slightly higher than when PCC was used (Fig. 6), which means that the dewatering was maintained at a desirable level. Good dewatering can result from the bonding of starch into the composite structure and large size of the composites. At the same time, the use of hybrid pigment led to extremely small permeability values, whereas the air permeability of PCC filled sheets was increased as the content of pigment in a sheet was increased. It is unusual that a papermaking raw material can yield a combination of high dewatering and very low permeability,
15 16 17 Solids content after couching (%)
18
Fig. 6. Air permeability of the paper sheets vs. their solids content after couching. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
Scott internal bonding strength (J m-2)
Fig. 5. Particle size distribution and volume median particle size d (0.5) during the hybrid pigment preparation.
14
1800
Unfilled Reference PCC Starch-PCC
1600 1400 1200 600 400 200 0 0
5
10
15
PCC content (%) Fig. 7. Scott internal bonding strength at different PCC contents of the paper sheets. Starch-PCC filled paper with the highest loading level was above the measurement range of the device.
because small particles that plug interfiber pores also inhibit dewatering. Unlike traditional pigments, the composite had a positive effect on the strength properties of paper. As shown in Fig. 7 , the internal bonding strength was significantly improved, and increasing the amount of composite in paper further increased the bonding strength. Also the tensile strength (Fig. 8) and elastic modulus (Fig. 9 ) of the sheets containing composite were in a much higher level than in unfilled and PCC filled papers and remained in a constant level regardless of the content of composite in paper. Tear strength was the only mechanical property, which was decreased when using high loading level of composite. At lower addition levels, the composite gave similar tear strength to paper as traditional PCC, but the highest addition level resulted in impaired tear strength when compared to PCC filled paper (Fig. 8). This was likely due to the much smaller fiber content of starch-PCC filled papers and the high density of the sheets with the hybrid pigment. For example at the highest composite addition, the content of starch in the paper was 13% which means that the content of fibers in the sheet was decreased by an equal amount. As shown in Fig. 9, the density of paper was increased when the composite was used instead of PCC, which means that the
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55 Unfilled Reference PCC Starch-PCC
60
Light scattering (m2 kg-1)
Tensile index (Nm g-1)
70
50
40
30
Unfilled Reference PCC Starch-PCC
50 45 40 35 30
20 6
8
10
12
25 75
Tear index (mN m2 g-1)
5.0 4.5
3.5 3.0 2.5
0.6
85
90
Fig. 10. Light scattering vs. opacity of the paper sheets. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
unmodified starch was used instead of more expensive, cationic starch. Due to the extremely high bonding strength, the use of hybrid pigment could eliminate the need for the surface sizing of paper in the future. Since the size press limits the production speed of paper machine and is an expensive equipment, its elimination would also cut down the production costs. Besides fiber replacement, the hybrid pigment could also find use in paper coating or as an additive in board production. The content of the hybrid pigment depends on the end product and desired properties. In full-scale paper production, adding small amount (a few percent) of hybrid pigment along with the traditional filler would already give considerable cost savings and strength benefits, whereas in some other products the content of the hybrid pigment could be much higher.
Unfilled Reference PCC Starch-PCC
4.0
2.0 0.5
80 Opacity (%)
Fig. 8. Tensile index vs. tear index of the paper sheets. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
Elastic modulus (GPa)
299
0.7
0.8
0.9
Density (g cm-3) Fig. 9. Elastic modulus vs. density of the paper sheets. Symbol size illustrates the PCC content of the paper sheet (small = 5%, medium = 9%, large = 13%).
composite pigment decreased the bulk of paper. Due to the scalenohedral morphology, which make it a high-bulking pigment (Hubbe, 2004; Laufmann et al., 2000), reference PCC showed the opposite behavior. The optical properties of composite-filled papers, such as light scattering and opacity (Fig. 10) were in the same level with the unfilled paper and much lower than those of PCC filled paper sheets, which can again be attributed to the morphological and size-related differences between the hybrid and traditional pigment. The technique used for the preparation of the hybrid pigment was fairly simple consisting only of two heating steps and precipitation. Thus, it could easily be scaled up into the industrial process and is feasible in the traditional, existing precipitation reactors. In addition, the hybrid pigment was prepared from the raw materials, which are commonly used in papermaking and have low cost thus giving it a very attractive cost structure when compared to kraft pulp. Thus, if used as a fiber replacement in paper, significant cost savings could be achieved already at low hybrid pigment contents. When the composite was added into a paper to partially replace fiber raw material, excellent strength properties as well as some exceptional property combinations, such as low air permeability and good dewatering, were achieved. Within the composite, the amount of starch incorporated into the paper sheet could be increased far above the typical limitations, even though
4. Conclusions A new type of hybrid pigment was produced by encapsulating a thermally activated native corn starch granule with a layer of precipitated calcium carbonate. It was found that the new hybrid pigment had desirable process and product properties. By heating the encapsulated granules at 90 ◦ C the starch could be fully swollen, and thus in a suitable form for bonding, but the full dissolution into the papermaking furnish was restricted. Therefore, higher amounts of starch could be introduced into the paper via wet end addition than is normally done and good dewatering could still be achieved, even though low cost, native corn starch was used instead of more expensive, cationic starch. Regardless of the good dewatering, the addition of hybrid pigment resulted in low air permeability, which is a very unique combination. In addition, the composite gave extremely good internal bonding strength to paper, and the other strength properties except the tear strength were also improved. Besides the decreased paper bulk, the disadvantage of composite was that the optical properties of paper remained in the same level with the unfilled paper. The hybrid pigment was prepared with a simple technique from the raw materials, which have much lower cost than chemical pulp. This means that besides its unique paper properties, the hybrid pigment could be used to improve the cost-efficiency of papermaking. In addition, the composite has a potential to eliminate the need of the size press, since it gives a very high bonding (or surface) strength to the paper. In the future experiments, the authors are interested in the further development of hybrid pigment and examining its effects on
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paper properties. For example, the improvement of bulk and optical properties of paper could be done by altering the ratio of starch and PCC, changing the morphology of PCC on top of the starch granules or decreasing the size of the hybrid pigments. The size of the hybrid pigments could be changed by either changing the origin of starch or grinding the material. In addition, the applicability of the pigment as an additive in board production or as a coating material for paper and board will be explored. Acknowledgements The financial support from the Academy of Finland is gratefully acknowledged. Microscopy work made use of the Aalto University Nanomicroscopy Center (Aalto-NMC) premises. Tuyen Nguyen, Leena Nolvi and Victoria Lindqvist are acknowledged for their assistance with the experimental work. References Aouada, F.A., Mattoso, L.H.C., Longo, E., 2013. Enhanced bulk and superficial hydrophobicities of starch-based bionanocomposites by addition of clay. Ind. Crop. Prod. 50 (10), 449–455, http://dx.doi.org/10.1016/j.indcrop.2013.07.058. Avella, M., De Vlieger, J.J., Errico, M.E., Fischer, S., Vacca, P., Volpe, M.G., 2005. Biodegradable starch/clay nanocomposite films for food packaging applications. Food Chem. 93 (3), 467–474, http://dx.doi.org/10.1016/j. foodchem.2004.10.024. Bruun, S., 2009. Binders. In: Paltakari, J. (Ed.), Papermaking Science and Technology. Pigment Coating and Surface Sizing of Paper, vol. 11. Paper Engineers’ Association/Paperi ja Puu Oy, Helsinki, Finland, pp. 192–225. Cao, S., Song, D., Deng, Y., Ragauskas, A., 2011. Preparation of starch—fatty acid modified clay and its application in packaging papers. Ind. Eng. Chem. Res. 50 (9), 5628–5633, http://dx.doi.org/10.1021/ie102588p. Chiou, B.-S., Yee, E., Wood, D., Shey, J., Glenn, G., Orts, W., 2006. Effects of processing conditions on nanoclay dispersion in starch–clay nanocomposites. Cereal Chem. 83 (3), 300–305, http://dx.doi.org/10.1094/CC-83-0300. Deng, Y., Yoon, S., Ragauskas, A., White, D., 2011. Making modified fillers for papermaking; applying a modified filler to a composition comprising fiber to form a mixture, processing the mixture whereby producing a paper. US 7964063 B2. Dipa, R., Sonakshi, M., 2012. Starch nanocomposites. In: Maya, J.J., Sabu, T. (Eds.), Natural Polymers, Nanocomposites, vol. 2. Royal Society of Chemistry, UK, pp. 185––233. Fredriksson, H., Silverio, J., Andersson, R., Eliasson, A.-C., Åman, P., 1998. The influence of amylose and amylopectin characteristics on gelatinization and retrogradation properties of different starches. Carbohydr. Polym. 35 (3), 119–134, http://dx.doi.org/10.1016/S0144-8617(97)00247-6. Garcı´ıa-Carmona, J., Morales, J.G., Clemente, R.R., 2003. Morphological control of precipitated calcite obtained by adjusting the electrical conductivity in the Ca(OH) 2-H2O-CO2 system. J. Cryst. Growth. 249 (3–4), 561–571, http://dx.doi. org/10.1016/S0022-0248(02)02173-5. Glenn, G.M., Orts, W.J., Nobes, G.A.R., 2001. Starch, fiber and CaCO3 effects on the physical properties of foams made by a baking process. Ind. Crops. Prod. 14 (3), 201–212, http://dx.doi.org/10.1016/S0926-6690(01)00085-1. Hari, P.K., Garg, S., Garg, S.K., 1989. Gelatinization of starch and modified starch. Starch 41 (3), 88–91, http://dx.doi.org/10.1002/star.19890410304. Hirvikoski, L.K., Laakso, A.J., 2014. Filler suspension and its use in the manufacture of paper. WO 2014055787 A1. Holm, J., Lundquist, I., Bjorck, I., Eliasson, A.C., Asp, N.G., 1988. Degree of starch gelatinization, digestion rate of starch in vitro, and metabolic response in rats. Am. J. Clin. Nutr. 47 (6), 1010–1016.
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