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MATERIALS AND INTERFACES The Reinforcement of Calcium Carbonate Filled Papers with Phosphorus-Containing Polymers Xiaonong Chen, Ruixiang Huang, and Robert Pelton* McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
Anionic polyelectrolytes bearing phosphate or phosphonate groups were compared to poly(acrylic acid-co-acrylamide) as reinforcing polymers for paper filled with calcium carbonate. Cationic polyvinylamine, PVAm, was used to promote adsorption of the anionic polymers onto fiber and filler surfaces in the aqueous papermaking suspension. Both poly(acrylamide-co-vinyl phosphonate), PAMVP, and phosphate esters of poly(vinyl alcohol), PVAP, gave stronger paper than did the acrylate copolymers. PAMVP bound soluble calcium ions, resulting in the formation of multichain colloidal-sized clusters. Subsequent addition of PVAm produced dispersed, cationic polyelectrolyte complexes. The polyacrylates bound less soluble calcium and formed weakly scattering clusters. It is proposed that the phosphate and phosphonate groups promote polymer adhesion to the calcium carbonate surfaces, resulting in stronger paper. Introduction Papers for magazines, journals, and books are evolving toward ever increasing contents of calcium carbonate fillers, a trend which is driven by the need for improved print quality with lower cost. However, fillers by definition are materials which decrease the mechanical properties of paper. It has been long known that filler addition decreases the number of strong fiberfiber bonds, weakening the structure. Recently we investigated the mechanism by which precipitated calcium carbonate decreased the mechanical strength of paper.1 In this work, a sparse layer of PCC was printed on the surface of a wet sheet of filler-free paper, and then a second wet sheet of filler-free paper was placed on top of the PCC printed surface. The laminates were pressed and dried to form two-ply composites. Peel tests were used to determine the influence of PCC on the delamination strength which we assumed reflected fiber-fiber bonding in conventionally filled paper. The main conclusions were the delamination force decreased exponentially with the filler coverage; fully covered sheets had essentially no peel strength, suggesting little intrinsic adhesion between PCC and cellulose; and smaller (∼1.3 µm) PCC particles caused a substantially greater decrease in strength than did larger particles (∼3 µm) at the same mass concentration. One approach to overcoming the negative impacts of fillers on paper mechanical properties is to add paper strength enhancing polymers.2 Normally, polymers such as cationic starch are added to the dilute aqueous papermaking suspension, and electrostatic attraction drives starch monolayer adsorption onto fiber and filler surfaces.3 In the papermaking process, the suspension is filtered to give wet paper which is pressed and dried. * To whom correspondence should be addressed. E-mail:
[email protected].
The presence of cationic starch or other added polymers in the fiber-fiber contact zones increased fiber-fiber bond strength and thus paper strength. However, monolayer adsorption limits the quantity of polymer which can be incorporated into the paper structure. For example, surface saturation usually occurs around 1 mg of adsorbed polymer for every square meter of surface. Assuming that wood fibers have a specific surface area of 1 m2/g and the filler 10 m2/g, a paper based on 10% filler could contain a maximum of 0.19 wt % strengthenhancing polymers, which may not be enough. The standard approach to surpassing the monolayer limitation is to add a second oppositely (negatively) charged polymer which will form complexes with the first cationic polymer. By forming insoluble, strongly adsorbing polyelectrolyte complexes, very high polymer contents can be achieved. For example, it is common practice in the paper industry when making wetstrength paper to add a cationic polyamine amide resin with anionic carboxymethyl cellulose to give high polymer loadings.4 Indeed, mixtures of three polymers have been proposed for highly filled papers.5 Recently Wågberg and co-workers showed that repeated sequential addition of cationic and anionic polymer give adsorbed multilayers which enhance paper strength.6 Calcium carbonate fillers partially dissolve in water and thus can influence the properties of water-soluble polymers during the papermaking process. At neutral pH conditions, the calcium ion concentration from filler dissolution is usually about 0.8 mM. Therefore, when considering the mechanisms by which polymers influence both the papermaking process and the mechanical properties of the final product, it is important to understand how polymer solution properties are influenced by soluble calcium ions. For example, calcium ions have been shown to induce the precipitation of wood pitch onto pulp fibers,7 whereas calcium decreases the
10.1021/ie048877k CCC: $30.25 © 2005 American Chemical Society Published on Web 02/22/2005
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efficiency of some flocculants8 and, at high concentrations, can induce cationic polymer desorption from wood pulp fibers.9 In this work, we compare conventional poly(acrylic acid) polymers and copolymers to phosphate- and phosphonate-containing anionic polyelectrolytes as strengthenhancing polymers for high PCC content papers. Polyvinylamine, a highly cationic water-soluble polymer which readily adsorbs onto cellulose, was used as the cationic co-strength-enhancing agent. Our hypothesis was that the phosphate and phosphonate groups, known to have a high affinity for calcium ions, may increase the adhesion between cellulose and calcium carbonate. Although there are no scientific papers on this subject, there are some indications from the patent literature. Nagan described the modification of polyacrylamide with phosphorous acid and claimed the product was an effective filler flocculant for papermaking.10 Iwai et al. prepared copolymers of cationic monomer, acrylamide, and substituted vinyl phosphoric acid and claimed improved filler retention and paper strength.11 Perhaps the closest patent to the present work was assigned to Drummond et al. and describes using copolymers of vinyl phosphonate, acrylamide, and a vinyl sulfonic acid for increased filled paper strength when used with cationic starch.12 Although the use of phosphorus modified polymers has been rarely mentioned in the paper technology literature, the use of such polymers has been discussed for applications including membranes for calcium ionselective electrodes,13 detergent and water-treatment additives,14-16 scale inhibition chemicals,17-20 filler dispersing agents,21-24 and binders for biocompatible organic-calcium salt composites.25 All these applications reflect the tendency for polymeric phosphate and phosphonate groups to bind to calcium and other metal ions. The goals of our work were to compare phosphoruscontaining polymers with carboxylic analogues to see if the phosphoric acid groups gave improvement in paper strength. A secondary objective was to explore the link between polymer properties in the aqueous papermaking suspensions and the ultimate paper properties. Experimental Section Materials. Acrylamide (AM, Aldrich, 99+%) was recrystallized from acetone/pentane (1/5). Benzoyl peroxide (BPO, 97%, Aldrich) was recrystallized from dichloromethane/methanol (1/4) and used as the initiator of polymerization. Dioxane was purified by distillation under a nitrogen atmosphere and used as the polymerization solvent. Tributylamine (Aldrich) was dried over KOH and distilled under reduced pressure before use. The following chemicals were used as received: vinylphosphonic acid (VP, 97%, Aldrich), polyphosphoric acid (115% phosphoric acid equivalent, Aldrich), polyvinylamine (PVAm, BASF, degree of hydrolysis 89.5%, molecular weight 950,000, 7% in aqueous solution, pH 7), poly(vinyl alcohol) (PVA, Fluka, degree of polymerization 1600, degree of hydrolysis 97.5-99.5%), poly(acrylic acid-co-acrylamide) (Aldrich, PAMAA90: 10 wt % AM, average Mw ca. 200,000; PAMAA20: 80 wt % AM, average Mw ca. 200,000), poly(acrylic acid) (Modchrom Inc., PAA1K: Mw 1150, Mn 700; PAA6K: Mw 6600, Mn 3450), N,N-dimethylformamide (DMF, 99.8%), Na2HPO4 (> 99.0%), precipitated calcium carbonate
(PCC: Specialty Minerals, Inc., ALBACAR HO 391105, average particle size 1.3 µm, surface area 12 m2/g). Millipore-Q deionized water was used in the preparation of aqueous solutions or suspensions. Dried softwood bleached Kraft pulp was provided by Bowaters Corp., Thunderbay, Ontario. Preparation of Poly(acrylamide-co-vinyl phosphonic acid) (PAMVP). PAMVP was prepared by nonaqueous dispersion copolymerization of AM and VP using dioxane (9 g of monomer/100 mL of dioxane) as solvent and BPO (0.7 wt %/wt based on monomers) as initiator. Compared with solution polymerization in water this method gave higher molecular weights and easier product isolation. The polymerization was carried out at 80 °C for 3 h. The resulting fine polymer powder was washed with acetone and filtered five times, after which the product was freeze-dried to obtain a white powder (yield > 95%). The composition of the copolymer was controlled by the VP/AM monomer feeding ratio, and the polymer charge was determined by potentiometric titration. The average molecular weight (Mv) of the copolymer was estimated from the intrinsic viscosity ([η]) of the polymer solution (0.08, 0.04, and 0.02 g/dL) in 0.05 M Na2SO4 at 30 ( 0.1°C. Mv was calculated by the Mark-Houwink-Sakurada equation ([η] ) KMvR) using K ) 0.000373 and R ) 0.66 for polyacrylamide in 0.05 M sodium sulfate at 30 °C.26 Preparation of Poly(vinyl alcohol)phosphate, PVAP. Poly(vinyl alcohol) was reacted with polyphosphoric acid in DMF/ tributylamine solution and purified to give the sodium salt of PVAP following the procedure of Whistler and Towle.27 The H NMR spectrum of PVAP was recorded in D2O solution at room temperature using a Brucker Avance 200 NMR instrument, and the peak assignments were 1.6 ppm (CH2), 3.9 ppm (-CH(OH)), and 4.5 ppm (-CH(O-P(dO)(OH)2)-). The degree of substitution (DS) was calculated to be 0.50 from the areas of the peaks at 3.9 and 4.5 ppm. Papermaking and Testing. Laboratory-made paper sheets, called handsheets in the paper industry, were prepared according to TAPPI standard procedures.28 In this procedure, 12.5 g of dry pulp was dispersed in 1000 mL of deionized water by mixing in a British Disintegrator which is a vessel fitted with a single paddle stirrer. After dispersion the pulp was diluted with deionized water to give a 0.35 wt % pulp suspension. Dry PCC (0-40 wt % based on dry pulp) was dispersed in 75 mL of deionized water and added to 400 mL of stirred pulp suspension in a 600-mL plastic beaker at room temperature. The pH of the pulp suspension was adjusted to 7.5 with 0.1 N HCl or 0.1 N NaOH, 0.35 mL of 1 wt % anionic polymer was added to the mixing pulp suspension, and the suspension was allowed to mix for 5 min. PVAm solution (0.35 mL of 1 wt %), was then added, and the suspension was stirred for another 5 min. Wet handsheets were formed by filtration of the pulp suspension in a British Sheet Machine. The wet sheets were removed and pressed by TAPPI standard methods and then dried at 120 °C for 10 min using a drier (Labtech Instruments, Inc.). The resulting dried paper sheets had a fiber-only basis weight of 69 g/m2; the total basis weight depended upon the filler content. Paper samples were conditioned for at least 12 h at 23 °C, 50% humidity. The PCC content in the filled paper sheets was gravimetrically measured by comparison with a PCC-free sheet (as a control). Paper tensile strength was measured with an Instron (model 4411)
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Figure 1. Structures of phosphorus functionalized polymers.
using a strain rate of 25 mm/min and a specimen width of 15 mm. The reported tensile strengths were based on at least six measurements. The internal bond strength (Scott Bond) was measured with a Huygen Internal Bond Tester following standard procedures. Variations in the results were calculated as standard errors and displayed as error bars in the plots. Some paper samples were made using a variation of the above procedure. Instead of sequentially adding anionic polymer and PVAm, 0.35 mL of 1 wt % anionic polymer and 0.35 mL of 1 wt % PVAm were premixed in 50 mL water and then added to the pulp suspension. Solution and Colloidal Characterization. Electrophoretic mobilities were measured at 25 °C using a ZetaPlus zeta potential analyzer (Brookhaven Instruments Corporation) operating in PALS (phase analysis light-scattering) mode (PALS software version 2.5). Potentiometric titrations were used for measuring both polymer composition and the concentration of stock polymer solutions. These measurements were made with a PC-Titrate Module (Man-Tech Associates Inc.) at 25 ( 0.1 °C under N2 atmosphere. Simultaneous conductivity measurements were recorded. Calcium chloride titrations were used to determine calcium ion binding. In a typical experiment 75 mL of 10 mequiv/L CaCl2 solution was placed in the titrator. The pH of the solution was adjusted to 7.0, and the ionic strength was adjusted by the addition of 0.75 mL of 0.1 M KCl. Anionic polymer solution (0.5 wt %, pH 7.0) was added at a rate of 0.2 mL per 20 min, and the calcium ion concentration was measured with a model 93-20, Thermo Orion calcium specific electrode and a model 90-01, Thermo Orion reference electrode. The titrations were conducted under N2 atmosphere at 25 ( 0.1 °C. The electrode was calibrated with CaCl2 solution (in 0.001 M KCl, pH 7.0). Dynamic light scattering was used to characterize dispersed species. Measurements were made with a Lexel 95 ion laser operating at a wavelength of 514 nm and a power of 300 mW. Correlation data were analyzed using a BI-9000AT digital autocorrelator, version 6.1 (Brookhaven Instruments Co.), and the CONTIN statistical method was used to calculate the particle size. All measurements were at 25 ( 0.1 °C. Results Poly(acrylamide-co-vinyl phosphonate) and phosphatemodified poly(vinyl alcohol) were prepared and characterizedsthe structures are shown in Figure 1, and the properties are summarized in Table 1. Also shown in Table 1 are the properties of acrylic acid homopolymers and copolymers with acrylamide which were chosen as representatives of carboxylate polymers. The final column in Table 1 gives the polymer charge densities expressed as the number of anionic charged groups per amine group in an equivalent mass of PVAm. This method of expressing charge densities was chosen because equal quantities of anionic and PVAm were used in the papermaking experiments.
designationa
functional monomer, mol %
molecular weight
PAMAA20 PAMAA90 PAA1K PAA6K PAMVP-1 PAMVP-3 PAMVP-4 PVAP
acrylic acid, 19.8%a acrylic acid, 89.9%a acrylic acid, homopolymer acrylic acid, homopolymer vinylphosphonic acid, 4.5%b vinylphosphonic acid, 8.7%b vinylphosphonic acid, 6.1%b vinyhosphoric acid, 50%c
Mw 200,000 Mw 200,000 Mw 1,150 Mw 6,600 Mv 410,000 Mv 79,000 Mv 84,000 Mn ∼170,000
relative charge density 0.14 0.64 0.063 0.12 0.086 0.48
a PAMAA, polyacrylamide-co-acrylic acid; PAA, poly(acrylic acid); PAMVP, polyacrylamide-co-vinyl phosphonate; PVAP, phosphate ester of poly(vinyl alcohol). The relative charge densities were calculated as the number of anionic charges per amine group in an equal mass of PVAm. a Calculated from the composition in wt %. b Measured by potentiometric titration. c Measured by NMR.
Figure 2. Comparison of PCC retention among different types of anionic polymers. Total polymer dosage 0.5% based on dry fiber weight, anionic polymer/PVAm ) 1:1 (weight ratio) when two polymers were applied. Error bars were calculated as standard error.
A series of papers was prepared by first adding one of the anionic polyelectrolytes, described in Table 1, followed by an equal mass of cationic polyvinylamine to a dilute (∼0.35 wt %) suspension of bleached wood pulp fibers containing various amounts of precipitated calcium carbonate (PCC). Wet paper sheets were then formed by filtering the suspension in a British sheet machine, a vertical cylinder with a diameter of 160 cm and a filtration screen on the bottom. Filler retention, the mass fraction of added filler which is incorporated in the paper, is an important issue in the manufacture of filled papers. Indeed, one of the reasons for adding water-soluble polymers is to induce colloidal filler deposition onto the wood pulp fibers before the filtration process. PCC retention was measured, and the results are summarized in Figure 2 which shows retention as a function of the PCC dose. The main observation was that retention was higher for the two phosphorus-containing polymers than for the polyacrylates. Within experimental uncertainty, there was no retention difference between the poly(vinyl alcohol) derivative, PVAP, and the acrylamide vinyl phosphonate copolymer, PAMVP-3. The following sections will show that the polymers were present as ∼200
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Figure 3. Comparison of internal bond strength of PCC filled hand-sheets using different anionic polymers. Polymer ratio and dosage are the same as that in Figure 2. Error bars were calculated as standard error.
nm colloidally dispersed complexes. Thus, retention involved the heteroflocculation of calcium carbonate and polymer complex particles. In all cases the retention decreases with increasing PCC dose, reflecting the fact that the absolute polymer concentrations were constant; thus, the amount of polymer per surface area decreased with increasing filler content. The maximum possible density of adsorbed polymer on the fiber and filler surface was estimated by assuming the specific surface area of the pulp was 1 m2/g and of PCC was 10 m2/g. Without PCC the maximum theoretical polymer coverage was 5 mg/ m2, whereas at a PCC dose of 0.6 g per g of fiber, the maximum possible polymer coverage was 0.7 mg/m2. Finally, it must be appreciated that handsheet retentions are always much higher than those obtained on a commercial papermachine because of hydrodynamic effects associated with the high dewatering rates typical of modern papermachines.29 The tensile strength and the internal bond strength of the dried papers were measured by standard techniques, and the results are summarized in Figures 3 and 4 which show mechanical properties as functions of PCC content of the paper. To a first approximation, tensile strength is a function of the intrinsic fiber strength, which is constant in this work, as well as the strength and number of fiber-fiber bonds.30 Internal bond strength correlates with the z-direction strength of paper and is most sensitive to the strength of fiberfiber bonds.31 Internal bond strengths obtained with four polymers are shown in Figure 3 as functions of PCC content of the paper. The polymers are conveniently compared in two pairs based upon the charge content. Table 1 shows that, PAMVP-3 and PAMAA20 have approximately equal charge and low charge content. By contrast, PVAP and PAMAA90 are much more anionic with the acrylate being the most charged. For both comparisons the phosphorus-containing polyelectrolytes gave stronger paper than the polyacrylates. The highly substituted poly(vinyl alcohol), PVAP, gave slightly stronger paper than did the much less anionic PAMVP-3. The corresponding tensile results are shown in Figure 4. Although there is a lot of scatter in the PAMAA20
Figure 4. Comparison of tensile strength of PCC filled handsheets using different anionic polymers. Polymer ratio and dosage are the same as that in Figure 2. Error bars were calculated as standard error.
results, it is clear that the two phosphorus-containing polymers gave about the same tensile strengths, which were greater than those in the PAMAA20 results. In the papermaking experiments, 70 ppm of one of the anionic polymers (see Table 1) was mixed with a suspension of fibers and PCC and stirred for 5 min before the cationic PVAm was added. The following paragraphs describe experiments that show that the anionic polymer binds soluble calcium ions. This induces the polymer to form clusters which adsorb onto the PCC/ water interface. Subsequent addition of cationic polyvinylamine (PVAm) results in the formation of polyelectrolyte complexes. Upon addition to a fiber-PCC suspension, the anionic polyelectrolytes will first interact with the soluble calcium ions. This was simulated by titrating 5 mM CaCl2 solutions with polymer and monitoring the soluble calcium ion concentration with an electrode. Figure 5 shows the results for phosphorus-containing polymers and Figure 6 the polycarboxylates. Note that the concentrations are expressed as equivalents per liter which is twice the molarity for the calcium- and phosphorus-containing polymers. In all cases the calcium ion concentration decreased linearly with polymer addition, reflecting the fact that the calcium was in excess. The slopes of the lines are given in Table 2. PAMVP-1 and PVAP, the two high molecular weight phosphorus-containing polymers, had slopes of approximately -2, indicating that every phosphate or phosphonate group bound two calcium ions. Greish and Brown proposed a “two calcium per phosphonate” structure which had two associated hydroxyl groups.25 By contrast, the lower molecular weight polyphosphonates had slopes of approximately 1.2, indicating fewer bound calcium ions. Finally, simple phosphate had a slope of -0.33. There is some evidence in the literature that ion binding increases with polyelectrolyte molecular weight.32 The slopes for the polycarboxylates (Table 2 and Figure 6) were approximately -1.2 for the high molecular weight pair, and much lower for the oligomers.
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Figure 7. Equivalent hydrodynamic diameter and light-scattering intensities of PAMVP-1 and PAMAA20 as functions of calcium ion concentration (0.0025 M KCl, pH 7.0, 25 °C). The PAMAA20 diameters (open squares) have very large error bars which are not shown. Figure 5. Titration of calcium chloride solution with phosphoruscontaining polymers and sodium phosphate. Solutions were prepared in 0.001 M KCl, and the pH and temperature were maintained at 7.0 and 25 ( 0.1 °C, respectively.
Figure 8. Electrophoretic mobility of 70 ppm anionic polymer solutions at various CaCl2 concentrations (25 °C, pH 7.0, 0.0025 M KCl). Error bars were calculated as standard error.
Figure 6. Titration of calcium chloride solution with poly(acrylic acid) polymers and copolymers (pH 7.0, 0.001 M KCl, 25 ( 0.1 °C). Table 2. Slopes of the Polymer Titration of Calcium Solutions Shown in Figures 5 and 6 polymer
MW
slope
PAMVP-1 PAMVP-4 PAMVP-3 PVAP Na2HPO4 PAMAA20 PAMAA90 PAA6K PAA1K
Mv 410,000 Mv 84,000 Mv 79,000 Mn ∼170,000 142 Mw 200,000 Mw 200,000 Mw 6,600 Mw 1,150
-2.05 -1.21 -1.23 -2.08 -0.329 -1.25 -1.16 -0.637 -0.428
a R2 of the fits was greater than 0.99. Calcium ion bonding ability of anionic compounds.
Thus, the polycarboxylates bound less calcium than the PVAP or PAMVP. Dynamic light scattering was used to probe the properties of the anionic polymers with bound calcium. Figure 7 shows the diffusion coefficient, expressed as a diameter, and the light-scattering intensity for
PAMPV-1 as functions of calcium ion concentration. At low calcium ion concentration (∼10-5 M), the polymer was present as large clusters whose size decreased with increasing calcium ion concentration. However the lightscattering intensity increased, suggesting an increase of cluster density. Also shown in Figure 7 are the results for PAMAA20. The light-scattering intensities were much lower than for PAMVP-1, and apparent diameters were outside the accessible range of dynamic light scattering. Thus, it appears that the phosphonate copolymer formed smaller and much denser clusters than did the polyacrylate. The presence of associated anionic polymer clusters in the presence of soluble calcium permitted electrophoretic measurements, and the results for PAMVP-1 and PAMAA20 are summarized in Figure 8. The maximum negative mobility of both polymers occurred at a calcium ion concentration of 10-4 M, and the mobility PAMVP-1 was about twice that of PAMAA20. With increasing calcium concentration the mobilities of both copolymers approached zero and then became slightly positive as the concentration approached 0.1 M Ca2+. Below 10-4 M the mobilities approached zero. The results corresponding to the lowest calcium ion concentration were not reliable because the polymeric species were virtually undetectable by the electrophoresis instrument.
Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2083 Table 3. Electrophoretic Mobility Values for 0.2 wt % PCC Dispersed in 0.001 M KCl at pH 7.5a test suspensiona
electrophoretic mobility, m2/V s × 10-8
PCC PCC + PVAP PCC + PAMVP-3 PCC + PAMVP-4 PCC + PAMVP-1 PCC + PAMAA90 PCC + PAMAA20
-0.50 ( 0.02 -2.07 ( 0.06 -2.02 ( 0.05 -1.87 ( 0.03 -1.51 ( 0.02 -1.64 ( 0.05 -1.46 ( 0.04
a The polymer concentrations were 70 ppm which gives a maximum possible surface coverage on PCC of about 3.5 mg/m2.
Table 4. Properties of Polyanion/PVAm Aqueous Complexes Prepared from a 1:1 wt/wt Mixture of Polymers at pH 7 Giving a Total Concentration of 140 ppma polyanion + PVAm
polyanion + PVAm + CaCl2
polyanion
EM ( STE
size ( STE (nm)
EM ( STE
size ( STE (nm)
PAMAA20 PAMVP-3 PVAP
0.48 ( 0.07 2.75 ( 0.07 3.05 ( 0.18
508 ( 75 211 ( 10 276 ( 28
1.30 ( 0.07 2.01 ( 0.04 2.92 ( 0.07
190 ( 25 175 ( 13 179 ( 9
a The calcium chloride concentration was 0.001 mol/L. Diameters were measured by dynamic light scattering, and EM refers to electrophoretic mobility (m2/V s × 10-8).
In summary, adding either PAMVP-1 or PAMAA20 to aqueous calcium solutions results in the binding of calcium ions and the formation of colloidal-sized, multichain polymer structures bearing a negative charge. Furthermore, the phosphonate polymer bound more calcium and had a greater electrophoretic mobility than did the carboxylate polymer. The interaction of these species with dispersed PCC is now considered. Electrophoresis was used to determine whether the anionic polymer clusters adsorb onto PCC under these conditions. The results are summarized in Table 3. At pH 7.5, 1 mM KCl, the PCC suspension was slightly negative (-0.5 × 10 - 8 m2/V s). However, each of the anionic polyelectrolytes adsorbed onto to the PCC as evidenced by the large increases in the negative mobilities. The phosphorus-containing polymers gave slightly greater mobilities than the carboxylates; however, the results were not very sensitive to polymer charge density. If it is assumed that the PCC had a saturation surface coverage of 1 mg/m2 and that the specific surface area of the PCC was 10 m2/g, then 70% of the added anionic polyelectrolyte remains in solution after the PCC is saturated with an adsorbed monolayer. The excess anionic polyelectrolyte in solution will interact with the cationic polyvinylamine, when it is subsequently added, to form an aqueous complex. The final step in the preparation of papermaking suspensions was simulated by preparing polyanion/ PVAm complexes in the absence of fibers and PCC particles. Table 4 summarizes the complex hydrodynamic diameters and electrophoretic mobilities. PCC releases about 0.8 mM soluble calcium ion so that measurements were made with and without added calcium. In all cases the complexes were positive, reflecting the fact that polyvinylamine has a higher charge density than anionic polymers (see Table 1). For all three polymer combinations in Table 4, the presence of calcium gave smaller hydrodynamic diameters of about 180 nm which is about 1/10 the size of the PCC particles.
Figure 9. Mechanical properties of papers made by premixing the polymers.
Finally, experiments were conducted in which PVAm and the anionic polymers were premixed in water to form complexes which were then added to the papermaking suspension. The mechanical properties of the resulting papers are summarized in Figure 9. The phosphorus-containing polymers gave consistently stronger paper as judged by tensile and internal bond strength. Comparison with Figure 4 shows that the sequential addition of polymers gave somewhat higher tensile strength; however, the internal bond strengths (Figures 3 and 9) were about the same for both methods. Therefore, on the basis of these comparisons, we conclude that the preadsorption of anionic polymer onto the PCC, before PVAm addition, was not an important process. Discussion The material properties of paper have been described in hundreds of scientific publications and a recent text.33 Discussions of the mechanisms by which polymers strengthen paper is a small subset of the paper physics literature, and this subject was recently reviewed.2 The goal of the current work was to determine if and why polyelectrolyte complexes based on polyvinylamine and phosphorus-containing polyanions offer any advantages for paper strength over the corresponding complexes based on polycarboxylates. The inspiration for this project was the expectation that phosphates and phosphonate groups will give strong adhesion to calcium carbonate surfaces which in turn might lead to stronger paper.
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content, the sheet containing larger filler particles will be stronger because larger but fewer particles disrupt fewer fiber-fiber bonds.1 This possibility highlights one of the weaknesses generic to studies involving laboratory-made filled papers. That is, it is impossible to make two sets of papers in which only the polymer content varies but all other structural features are the same. Thus, the measured physical properties reflect a range of microscopic events. Figure 10. Comparison of PCC floc size in paper sheets formed with PAMAA90 (left) and PVAP (right). The PCC content was 25% and the total polymer dosage 0.5% based on dry fiber weight, anionic polymer/PVAm ) 1:1 (weight ratio, sequential addition).
The paper testing results summarized in Figures 2, 3, and 9 showed that the phosphorus-containing copolymers of acrylamide and phosphate esters of poly(vinyl alcohol) do indeed show promise relative to the polyacrylate-based comparison group. Before speculating on the mechanism for the strength enhancement, it is relevant to consider events at colloidal scale, during the paper-formation process. The first step was the addition of dry wood pulp fibers to water. The cellulose fibers used in this work were about 1.5 mm long and 20 µm wide. When added to water, the fibers swelled and dispersed although there was a tendency for them to mechanically entangle into small clumps.34 Colloidal calcium carbonate (PCC) was added next from a concentrated slurry. PCC has little tendency to deposit onto wood pulp fibers in the absence of polymers. Thus, at this point the suspension consisted of fibers and completely dispersed PCC particles (diameter ∼1.3 µm). One of the anionic polymers was added next. If it was a PAMVP or PVAP, calcium ions, released from the PCC, complexed with the polymer, causing it to associate into negatively charged compact clusters which partially adsorbed onto the PCC surfaces. By contrast, PAMAA20 bound less calcium, and formed larger but much less dense clusters which had a smaller electrophoretic mobility. Both types of anionic polymers adsorbed onto PCC; however, they did not induce PCC flocculation or adsorption. Next, the cationic PVAm was added, resulting in the formation of colloidal polyelectrolyte complex with a net positive charge. We presume that the complexes adsorbed onto all surfaces and induced aggregation of the colloidal PCC. Microscopic examination of the final paper revealed filler flocs which must have formed before the filtration process which formed the paper sheets. In view of all the processes leading to the formation of a sheet of paper, there are a number of possible reasons why the phosphorus-containing polymers were superior to the polyacrylates. The obvious explanation is that the polyelectrolyte complexes act as adhesives increasing PCC/cellulose adhesion and that the phosphorus species give enhanced adhesion to the calcium carbonate surface compared with polyelectrolyte complexes based on polyacrylates. However, there are other possible mechanisms. The fact that the phosphorus-containing polymers give higher retention (Figure 2) suggests a greater degree of PCC flocculation before the filtration step. This is turn means that PCC was present as fewer larger flocs in the papers made with PAMVP or PVAP compared with polyacrylates (as shown in Figure 10). Furthermore, we know that when comparing the mechanical properties of two paper samples with the same filler
Conclusions While acknowledging the limitations of a study involving comparisons of a few polymers in laboratorymade paper, the following conclusions are made: 1. Phosphate- or phosphonate-containing anionic polyelectrolytes, used in conjunction with polyvinylamine, gave higher filler retention and stronger paper than did polyacrylamide-co-acrylic acid. This was explained by the stronger interactions of phosphate groups with calcium carbonate surfaces compared with carboxylatecalcium carbonate interactions. 2. Two calcium ions bind to every phosphate group when polymer is added to an excess of calcium ions. By contrast, calcium ion uptake by the polycarboxylates is equivalent to one calcium ion for every two carboxyls. 3. Both the phosphorus- and carboxylate-containing polyelectrolytes show evidence of association when exposed to soluble calcium at concentrations typically present in papermaking. Turbidities and electrophoretic mobilities were higher for the polyphosphonates than for the polycarboxylates, indicating more compact structures for the phosphorus-containing polymers. 4. Colloidal sized (∼200 nm diameter) polyelectrolyte complexes were formed from mixtures of either phosphorus-containing polymers or polycarboxylates and PVAm. We propose that the complexes adsorbed onto both filler and fiber surfaces which improved paper strength. The size and electrophoretic mobilities of both types of complexes were similar, suggesting that these properties do not explain why the phosphorus-containing polymers gave stronger paper. Acknowledgment This work was supported by the Canadian Natural Science and Engineering Research Council CRD award with Mintech Canada. The authors acknowledge Specialty Minerals Inc. for providing samples and many useful suggestions. Literature Cited (1) Li, L.; Collis, A.; Pelton, R. A New analysis of filler effects on paper strength. J. Pulp Paper Sci. 2002, 28, 267. (2) Pelton, R. On the design of polymers for increased paper dry strength - A Review Appita 2004, 57, 181. (3) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Polyelectrolyte adsorption: A subtle balance of forces. Langmuir 1992, 8, 2538. (4) Espy, H. H. The mechanism of wet-strength development in paper: A review. Tappi 1995, 78, 90. (5) Wågberg, L.; Lindstro¨m, T. Method of making paper with high filler content. U.S. Patent 4,824,523, 1989. (6) Wågberg, L.; Forsberg, S.; Johansson, A.; Juntti, P. Engineering of fibre surface properties by application of the polyelectrolyte multilayer concept. Part I: Modification of paper strength. J. Pulp Paper Sci. 2002, 28, 222. (7) Sundberg, K.; Thornton, J.; Pettersson, P.; Holbom, B.; Ekman, R. Calcium induced aggregation of dissolved and colloidal
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Received for review November 21, 2004 Revised manuscript received January 14, 2005 Accepted January 21, 2005 IE048877K
