Intact and broken cellulose nanocrystals as model ......1 Intact and broken cellulose nanocrystals...

Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9

Intact and broken cellulose nanocrystals as model nanoparticles to promote 1

dewatering and fine-particle retention during papermaking 2

3 Connor J. Lenze,1 Caryn A. Peksa,1 Weimin Sun,1,2 Ingrid C. Hoeger,1 Carlos Salas,1 and 4

Martin A. Hubbe 1,* 5

6

North Carolina State University,1 Department of Forest Biomaterials, Campus Box 8005, 7

Raleigh, NC 27695; Northwestern Polytechnical University,2 Department of Applied 8

Chemistry, School of Science, 127 Youyi Xilu, Xi'an, Shaanxi, China, 710072; * Author 9

for correspondence (e-mail: [email protected]) 10

11 Key words: Paper machine chemical programs; drainage aids; retention aids; 12

microparticles; nanocellulose; colloidal silica 13

14

Abstract 15 16

Cellulose nanocrystals (CNCs), either in intact form or after mechanical shortening, were 17

used as a model nanoparticle for enhancement of dewatering and fine-particle retention 18

during lab-scale papermaking process evaluations. Cryo-crushing, using dry or wet 19

CNCs, was performed to shorten the particles from an initial mean value of 103.1 nm to 20

either 80.4 nm (wet crushed) or 63.4 nm (dry crushed). Papermaking-related tests were 21

performed with the solids from 100% recycled copy paper, which were prepared as a 22

0.5% solids suspension in dilute Na2SO4 solution and then treated successively with 23

0.05% of poly-diallyldimethylammonium chloride, 0.05% of very-high-mass cationic 24

acrylamide copolymer, and then various types and dosages of negatively charged 25

nanoparticles. The performance of the CNCs, relative to papermaking goals, was 26

compared to that of two colloidal silica products that are widely used in industry for this 27

purpose. All of the nanoparticles were observed to promote both dewatering and fine-28

particle retention. The intact CNCs were more effective than the broken CNCs with 29

respect to fine-particle retention. Effects on flocculation of the fiber suspension were 30

detectable, but not large relative to the sensitivity of the test employed. Results are 31

discussed in the light of concepts of polyelectrolyte bridges and the participation of 32

elongated nanoparticles in completing those bridges in such a way as to form shear-33

sensitive attachments among solids surfaces in the suspension. 34

35

Introduction 36 37

Nanoparticle-based and microparticle-based additive systems have been widely 38

used within the paper industry since the 1980s as a means of achieving favorable 39

combinations of the rate of water release, fine-particle retention, and uniformity of 40

formation during the preparation of a paper sheet (Andersson et al. 1986; Langley and 41

Litchfield 1986; Breese 1994; Anderson and Lindgren 1996; Hubbe 2005). In particular, 42

it has been found that higher rates of drainage, including water release assisted by 43

hydrofoil action and vacuum, can be achieved by such programs (Andersson et al. 1986; 44

Langley and Litchfield 1986; Swerin et al. 1995; Wågberg et al. 1996; Harms 1998). 45

The most widely used particulate additives to promote dewatering and fine-particle 46


retention are colloidal silica (Swerin et al. 1995; Anderson and Lindgren 1996; Sang et 47

al. 2012) and sodium montmorillonite, which is often called “bentonite” (Langley and 48

Litchfield 1986; Asselman and Garnier 2001). Certain highly cross-linked anionic 49

polyelectrolyte products also have been used, and similar effects have been reported 50

(Honig et al. 2000). One of the distinguishing features of these widely used systems is 51

that they depend on a strong interaction between the negative charge at the micro- or 52

nano-particle surface and the positive charge of a very-high-mass cationic polyelectrolyte 53

(Hubbe 2005). It is very common for the micro- or nano-particle to be added as late as 54

possible to the system, just before the fiber suspension is formed into paper (Langley and 55

Litchfield 1986). Alternatively, it is also common to invert the order of addition, mixing 56

the micro- or nanoparticle additive with the fiber suspension just before addition of a 57

cationic flocculant, after which the suspension is almost immediately formed into paper. 58

In the 1990s some published articles announced the development of a class of 59

new “structured” colloidal silica products for papermaking applications (Moffett 1994; 60

Andersson and Lindgren 1996; Harms 1998; Main and Simonson 1999). Unlike the 61

colloidal silica products used up to that time as papermaking additives, the new products 62

were comprised of irregular chains of fused primary SiO2 nanoparticles, i.e. the “gel” 63

form of colloidal silica particles. It was stated that such products would be inherently 64

more effective for promotion of fine-particle retention, while still having a strong ability 65

to promote dewatering. The enhanced capability was attributed to the high aspect ratio of 66

the gel type of colloidal silica. 67

Recently, cellulose nanocrystals (CNCs) have become widely available in 68

quantities suitable for laboratory research and pilot-scale development (Reiner and Rudie 69

2013; Zhang et al. 2013). The availability of these materials provides an opportunity to 70

use them as a model nanoparticle, as a means to probe some aspects of the mechanism by 71

which certain dewatering and fine-particle retention programs function (Xu et al. 2014). 72

CNCs offer the advantages of having a simpler geometry in comparison to structured 73

colloidal silica (gel-type) products, as well as having a relatively narrow length 74

distribution. CNCs, when produced using concentrated sulfuric acid, also resemble 75

colloidal silica (Sears 1956) in having a strongly negative surface charge when immersed 76

in water (Araki et al. 1998, 1999; Habibi et al. 2010; Abitbol et al. 2013). In this work, 77

CNCs having different mean lengths were used for the first time in a study of the 78

papermaking process. 79

80

Experimental 81 82

Nanoparticles 83

84

The cellulose nanocrystals used in this work were all based on the spray-dried CNC 85

powder (Item number 6) supplied by the University of Maine 86

(http://umaine.edu/pdc/process-and-product-development/selected-87

projects/nanocellulose-facility/nanocellulose-requests/). As is well known, this material 88

is prepared at the Forest Product Laboratory, Madison, Wisconsin, by digestion of 89

cellulose in concentrated sulfuric acid, thus giving rise to particles that have sulfate 90

groups at their surfaces (Habibi et al. 2010; Abitbol et al. 2013). The product is known 91

to be prepared with Na+ as the associated cation. Upon placement in water, the ionizable 92


groups at the cellulosic surfaces contribute to rapid wetting, as well as good dispersion in 93

water on account of the repulsive electrostatic forces between adjacent particles. 94

95

Crushing of the CNC was carried out by chilling the nanoparticles to liquid nitrogen 96

temperature. Two batches of crushed CNCs were prepared. The first batch (dry crushed) 97

was prepared by adding 1.5 to 2 mm3 of dry CNC powder (no packing pressure applied) 98

into the cell of a Model 6750 Freezer/Mill cryo-crushing device supplied by SPEX 99

Certiprep, Metuchen, NJ, USA. The device was run for two standard cycles of 5 minutes 100

each. The full procedure was done again the same way, but with a 10% slurry of CNC in 101

water (wet crushed), such that a frozen combination of water-ice and CNC particles was 102

crushed together. 103

104

Micrographs of representative particles from the intact and crushed CNC batches were 105

obtained by atomic force microscopy (AFM). In preparation, silicon wafers (1 cm X 1 106

cm) were cleaned with water, then ethanol, followed by exposure to UV and ozone. The 107

wafers were then immersed in a 500 ppm solution of poly-ethyleneimine (PEI), followed 108

by rinsing with water. A dilute suspension of 0.01wt% CNCs (of each type) in water was 109

prepared by sonication at 10% to 30% amplitude of a Branson Digital Sonifier, Model 110

No. 102C (CE) device for about 30 s, and then placed dropwise onto a PEI-treated wafer 111

and allowed to dry. AFM tests were carried out with a NanoScope III D3000 device 112

from Digital Instruments Inc. 113

114

For comparison purposes, two widely used industrial products were included in the 115

experiments, representing the “gel” (structured) and “sol” (singlet particles) types of 116

colloidal silica (Andersson and Lindgren 1996; Harms 1998). Product NP 320, a 117

structured colloidal silica product obtained from Kemira, has been described as consisting 118

of irregular chains of fused nano-sized primary silica particles. The primary particles are 119

understood to be spheres having diameters near to 2 to 4 nm. Those primary particles are 120

fused together into irregular chains or extended clusters having lengths in the range of 121

about 10 nm to 60 nm. Product NP 440 was selected as a representative for non-122

structured silica. The particles of NP 440 are understood to be spheres having diameters 123

of about 5 nm. Such products have been discussed and characterized in other studies 124

(Carlson 1990; Andersson and Lindgren 1996; Harms 1998; Hangen and Tokarz 2002; 125

Carr 2005). 126

127

The surface charge of each type of nanoparticle was evaluated by streaming current tests, 128

using a PCD-pH device from BTG Corp. After rinsing the equipment, 10 mL of 129

deionized water was combined with 1 ml of 1% nanoparticle suspension and sufficient 130

sodium carbonate to make a 10-4 M buffer solution (pH ~ 9.5) in order to convert any 131

weak acidic groups to their anionic form. Four replicate titrations were carried out in 132

each case using poly-DADMAC (see next). 133

134

Preparation of cellulosic fiber and fine-particle suspensions 135 136

The pulp suspensions used throughout the work were prepared with fiber from 100% 137

recycled Envirocopy standard white copy paper (Office Depot) at a 0.5% solid 138


suspension level using a TAPPI disintegrator (TAPPI Method T 205). After resuspension 139

of the paper solids into deionized water, giving a consistency of 0.5%, sufficient sodium 140

sulfate was added to achieve an electrical conductivity of 1000 S/cm at a laboratory 141

temperature of approximately 22 C. A confirmatory set of experiments (evaluating the 142

retention effects) was carried out at 2000 S/cm; since the main effects were generally 143

the same at the two conductivity levels, only the results corresponding to 1000 S/cm are 144

reported in this work. The adjustment of electrical conductivity is important so that the 145

system can be representative of salt ion levels that are likely to be encountered in paper 146

manufacturing facilities. Due to the presence of calcium carbonate filler in the recycled 147

paper, the pH of the suspensions was in the range of 8 to 8.5. 148

149

Before being mixed with the pulp suspension, all nanoparticle materials (i.e. either 150

cellulose nanocrystals or colloidal silica) were pre-diluted to 1.0% solids to facilitate 151

volumetric measuring and efficient mixing with the pulp suspensions. The poly-152

DADMAC (Aldrich Cat. No. 40,901-4, low mass, supplied as 20% solids solution) was 153

prepared as a 1.0% solution in water. The cationic acrylamide copolymer (Percol 175, 154

BASF, dry bead form) was prepared as a 0.1% solution. 155

156

Rate of water release during papermaking 157 158

Rates of water release from differently treated fiber suspensions were evaluated using a 159

modified Schopper-Riegler test device (Sampson and Kropholler 1995; Sampson 1997). 160

This device differs from a standard Schopper-Riegler tester with respect to its base. 161

Rather than having the filtrate pass through a capillary opening at the base of the 162

collection funnel, there was a large opening, allowing immediate flow of the filtrate into a 163

tared beaker resting on an electro-balance. Also, the device had no “side arm” for 164

collection of filtrate. Such a modified system offers two advantages when used for 165

research work that involves the addition of polyelectrolytes such as cationic acrylamide 166

copolymer (cPAM). First, the results are easier to interpret. Secondly, any high-mass 167

polyelectrolyte present in the aqueous phase would be expected to markedly slow down 168

the flow through a capillary (Sridhar et al. 1991; Larson 2005), and such issues are 169

avoided by the modified system. 170

171

To run an individual test, 1000 ml of optionally treated fiber suspension, having a 172

consistency of 0.5% and a conductivity of 1000 S/cm, was prepared. In a large beaker, 173

with continued stirring at 500 rpm, the suspension was treated first with 0.05% poly-174

DADMAC solids (1% solution concentration), followed by 30 s of continued agitation, 175

then 0.05% cPAM solids (0.1% solution concentration), followed by 30 s of continued 176

agitation, then by nanoparticle suspension addition at an amount to be specified. The 177

treated suspension then was placed at the top of the modified Schopper-Riegler device, 178

while holding down a sealing cone. At time equal to zero, the sealing cone was allowed 179

to spring upwards, by spring action, and simultaneously a timer was started. The 180

amounts of filtrate accumulated in the tared beaker were recorded at elapsed times of 5, 181

10, 20, 30, 40, 50, and 60 seconds. These data were captured by videography, showing 182

both the times and the output of a tared electronic balance. The reported mean values are 183

based on four replicate tests of each condition. The 90% confidence intervals for the 184


observations were generally within the range of plus or minus 15 g from the mean values 185

reported. 186

187

Retention of fine particles during papermaking 188 189

To evaluate the effects of different nanoparticle treatments (following 0.05% poly-190

DADMAC and 0.05% cPAM) on the fine-particle retention, tests were carried out using a 191

Dynamic Drainage/Retention Jar (Britt 1973; Britt and Unbehend 1976). The suspension 192

was prepared in 500 ml batches having a consistency of 0.5% in each case. An agitation 193

speed of 500 rpm was used throughout all of the testing. In each case the first additive 194

was poly-DADMC at the 0.05% level based on solids. This was followed, after 30 s, by 195

cPAM at the 0.05% level. After an additional 30 s of agitation, the first portion of 196

nanoparticles was added. 197

198

To allow for rapid collection and replication of data, turbidity was used as a means of 199

quantifying the relative amounts of fine particulate matter (presumably mainly CaCO3 200

particles and cellulosic fines) present in the filtrate. Each turbidity measurement 201

involved 10-fold replication, using a DRT turbidimeter, Model 15CE. Filtrate was 202

collected directly into the turbidimeter vial from the base of the Britt Jar device. The first 203

portion was immediately placed back into the top of the jar, and tests were based on a 204

second portion. The first replication of each test was obtained by gently inverting the 205

capped vial, and then, after about 2 seconds, obtaining a new reading. Five such pairs of 206

measurements were taken, with each pair of tests completed in about 60 seconds. In 207

addition, each experimental run, as described above, was independently repeated, 208

providing either three or four replicate tests for evaluation of mean and standard 209

deviation values. 210

211

Two series of tests were performed. In an initial full set of tests, after the poly-212

DADMAC and cPAM were in the system, the nanoparticles were added in successive 213

amounts of 0.05%, 0.05% (for a net amount of 0.1%), 0.1% (for a net amount of 0.2%), 214

and 0.2% (for a net amount of 0.4%) with continuous stirring. A second series of tests 215

was very similar except that a separate preparation involving poly-DADMAC and cPAM 216

addition was carried out for each separate level of nanoparticle addition. In addition, a 217

parallel series of tests in the continuous mode was carried out at an electrical conductivity 218

of 2000 S/cm; because the results showed basically the same trends, only the results of 219

tests at 1000 S/cm are reported in this article. 220

221

222

Fiber flocculation extent during papermaking 223 224

The extent of fiber flocculation, as a function of nanoparticle addition, was evaluated by 225

means of a Photometric Dispersion Analyser (PDA) from Rank Bros, Manchester, UK. 226

A specialized “low gain” PDA version (Burgess and Phipps 2000; Hubbe 2000; Burgess 227

et al. 2002) was employed in this work, allowing usage of a relatively large Tygon tube 228

(internal diameter ca. 6.35 mm) and the sensing of the state of flocculation in a 0.5% 229

consistency stock suspension. Each batch had a volume of 500 mL, and it was stirred 230


continuously, using a magnetic stirrer at a rate just sufficient to prevent settling. A 231

peristaltic pump was used to circulate the fibrous suspension from the beaker, through the 232

PDA sensing zone, and back to the beaker, at a flow rate of 50 L/min. The gain setting 233

was at 1.0 throughout the testing, and the output was selected as “RMS”, meaning the 234

root-mean-squared of fluctuations in intensity of transmitted light through the tubing. In 235

addition, the “filter” setting was selected in order to reduce the amount of noise in the 236

output signal. 237

238

Results and discussion 239 240

Characterization of intact and broken CNC 241

242

Figure 1 shows atomic force micrographs of as-received CNC particles, comparing two 243

modes of imaging. The depth-based image (Fig. 1A) provides a sense of the bulky nature 244

of the layers of cellulosic material, with particles present in different planes. Because the 245

phase-based images (as in Fig. 1B) were better resolved, those images were used as a 246

means of estimating particle lengths. Because at least some of the observed particles can 247

be an angle to the plane of view, one can expect that there will be a systematic 248

underestimation of CNC particle length based on measurements from the two-249

dimensional images. However, for the sake of making relative comparisons, it is 250

assumed here that the degree of out-of plane orientation was similar for each of the 251

samples evaluated. Based on visual measurements of 100 each representative cellulose 252

nanocrystals appearing in the phase-based images (Figs. 1B, 2A, and 2B) the mean, 253

standard deviation, and 90% confidence interval for the intact and the broken CNC 254

material were as shown in Table 1. 255

256

Table 1. Length estimates (nm) of cellulose crystals 257

Description Mean St. Dev. 90% Confidence interval

Lower Upper

Intact CNC 103.1 39.1 96.6 109.5

Wet crushed CNC 63.4 33.2 57.9 68.8

Dry crushed CNC 80.4 35.7 74.5 86.2

258

It is clear from these results that cryo-crushing can be regarded as an effective way to 259

reduce the average length of cellulose nanocrystals. Secondly, the crushing achieved a 260

greater reduction in size when the procedure was carried out in the absence of frozen 261

water. 262

263

Optimization of treatment level for poly-DADMAC 264

265

Figure 3 shows results of a set of preliminary tests aimed at determining the optimum 266

addition level of the highly cationic agent poly-DADMAC. In each case the poly-267

DADMAC addition was followed by 0.05% of cPAM (based on solids vs. furnish solids) 268

and then by sequential addition of four cumulative dosage levels of silica nanoparticles. 269

Criteria for selection of an optimum set of treatments included achievement of a low 270

turbidity value, consistent with very high retention of fine particles onto fibers, such that 271


the filtrate passing through 76-m openings, which are too small to admit fibers, would 272

be relatively free of particles. In addition, out of a concern to minimize costs, it is 273

desirable to minimize the levels of both poly-DADMAC and the colloidal silica or other 274

nanoparticle additive. Based on these criteria, the most favorable combination of results 275

was achieved with a 0.05% addition level of poly-DADMAC. This level was utilized in 276

the subsequent work. 277

278

Based on previous work, there are two primary reasons to anticipate that a well-selected 279

pretreatment with high-charge cationic polymer may lead to optimized performance of a 280

nanoparticulate-based drainage-retention system. As has been shown (Wågberg et al. 281

1996; Swerin et al. 1997), pretreatment with the high-charge polymer can favorably 282

influence the adsorbed conformation of the high-mass cPAM added subsequently. Rather 283

than lying down flat on the cellulosic surfaces, the cPAM is forced to adopt a more 284

extended conformation having loops and tails extending from the surface into the solution 285

phase. Secondly, a high-charge cationic polymer can be optimized to neutralize most of 286

the excess of negatively charged polyelectrolytes, such as hemicellulose byproducts in 287

solution, such that the more expensive cPAM is not wasted in complexing with such 288

species. 289

290

It is notable in Fig. 3 that relatively high levels of poly-DADMAC pretreatment initially 291

increased the filtrate turbidity, which is consistent with charge-reversal of the system. 292

When cationic polyelectrolytes are in a sufficient excess, all of the wetted surfaces repel 293

each other. Even in such an overdose situation, it was still possible to achieve relatively 294

low values of turbidity by adding relatively large levels of negatively charged 295

microparticles; however this would not be cost-effective. 296

297

Dewatering rates affected by nanoparticle addition 298

299

Figure 4 shows results of dewatering rate tests, using the modified Schopper-Riegler test 300

device. As shown, the mass of filtrate collected as a function of time after initiating the 301

drainage test depended strongly on whether or not cPAM (at the 0.05% level, following 302

0.05% poly-DADMAC) had been added. It is notable that all of the conditions involving 303

nanoparticles were clustered together, and all of them achieved an additional increment 304

of drainage rate enhancement over what was achieved with just the cationic polymers. 305

306

The 90% confidence intervals for this set of data typically extended about 16 g units from 307

the plotted mean values. Therefore, one cannot conclude that any of the nanoparticles 308

was more effective than another in promoting dewatering. The highest values were 309

obtained with the default CNC, and this was closely followed by the sol-type (singlet) 310

colloidal silica nanoparticle product. These results support the working hypothesis that 311

cellulose nanoparticles can be utilized as a model to study the factors affecting retention 312

and drainage programs. 313

314

As shown in Fig. 5, the mass of filtrate collected after 20 seconds of gravity drainage 315

increased as a function of nanoparticle addition level (with the zero level corresponding 316

to systems treated with poly-DADMAC and cPAM, at the same levels mentioned before). 317


As already noted, based on statistical analysis one cannot conclude from this data that the 318

nanoparticles had different effects from each other. 319

320

Fine-particle retention during papermaking 321

322

As noted in the introduction, it has long been hypothesized that nanoparticles having a 323

higher aspect ratio should be more effective in promoting fine-particle retention (Hubbe 324

2005). The approach used to examine that hypothesis, for the first time in this work, was 325

to systematically break the cellulose nanocrystals into shorter pieces. The tests 326

represented in Fig. 6 were carried out in continuous mode, meaning that there was not 327

any interruption in agitation of the suspension (500 rpm), portions of nanoparticle 328

suspension were added successively to the same mixture, and “zero time” corresponds to 329

addition of just 0.05% poly-DADMAC followed by 0.05% cPAM. 330

331

As shown in Fig. 6, the default (unbroken) CNCs were consistently the most effective in 332

reducing turbidity of the filtrate. Thus, following each addition of fresh CNC (see the 333

upward-facing arrows), the lowest turbidity values were those associated with the 334

unbroken CNC particles. However, no consistent difference was detected with respect to 335

the turbidity-reducing ability of the two “crushed” CNCs, even though a difference in 336

average length had been observed (see Table 1). One possible interpretation is that a 337

certain threshold of length of CNC must be reached before greater flocculating ability can 338

be achieved. Further study is recommended in order to provide a better test for such a 339

hypothesis. Another issue that may be considered in future studies is whether or not the 340

nanoparticles become aligned in flow and whether such alignment affects results such as 341

retention or flocculation. 342

343

It is also clear from Fig. 6 that turbidity of the filtrate increased rapidly with the passage 344

of time following the most recent addition of either cPAM (after time equal to zero) or 345

CNC (at the indicated times after each of the upward-pointing arrows). This shows that 346

the bridging effect of the cPAM and of the cPAM/nanocellulose combinations was 347

progressively broken down by hydrodynamic shear. However, the results in Fig. 6 show 348

for the first time that freshly-added CNC repeatedly was able to restore the flocculating 349

effect, holding fine particles onto cellulosic fibers. In terms of mechanisms presented 350

earlier (Hubbe 2005), such behavior is consistent with “completion” of bridges by the 351

freshly-added nanoparticles, which interact with adsorbed cPAM on the respective 352

surfaces. 353

354

In theory, one end of a high-aspect-ratio nanoparticle will associate with the positively 355

charged cPAM adsorbed on one of the surfaces, and the other end will be associated with 356

cPAM adsorbed on another surface. Results in Fig. 6 suggest that it would be 357

advantageous to carry out such treatment just seconds before formation of a sheet of 358

paper, thus achieving efficient retention of the fine particles in the paper sheet as it is 359

being formed. Indeed, such timing is consistent with modern papermaking practices. A 360

shear-sensitive system also could allow the fiber-to-fiber attachments to become 361

redispersed, which can be favorable to achieving uniform formation in the resulting paper 362

(Hubbe and Wang 2002; Hubbe 2007). When comparing the three batches of CNC, the 363


results followed the order that would be expected based on the hypothesis given earlier. 364

In other words, the longest CNCs were the most effective, and so on. 365

366

Figure 7 compares the fine-particle retention results obtained with the default CNCs vs. 367

two types of commercially available silica nanoparticle products. As shown, both of the 368

colloidal silica products achieved lower turbidity values in this test series in comparison 369

to the CNCs. Moreover, both SiO2 nanoparticle products achieved low turbidity values at 370

a low addition level of the nanoparticle (each time following 0.05% poly-DADMAC and 371

0.05% cPAM additions). By contrast, an addition level of 0.5% CNC was needed in 372

order to achieve a turbidity value as low as about 20 NTU. Results of repeat tests with 373

other pulp batches (not shown) indicated basically the same trends as shown in Fig. 7. 374

375

To explain these results, it was hypothesized that the surface charge density of the SiO2 376

nanoparticles was substantially higher than that of the CNCs. Results from streaming 377

current titrations, comparing the surface charge amounts, are shown in Table 2. Notably, 378

the result obtained for the CNC was 0.288 meq/g, which is within the range of values 379

obtained for CNC from the same source by Beck et al. (2015). The cited authors 380

pretreated the material with a strong cationic exchange resin and determined charge 381

density by a conductometric titration. On the other hand, the value obtained in the 382

present work was greater than the value 0.084 meq/g that was obtained by Araki et al. 383

(1998, 1999) for CNC prepared in a generally similar manner. The difference might be 384

due to differences in the source material and preparation detail of the CNC batches 385

considered in the two studies. In addition, the methods used for evaluation of surface 386

charge were different. 387

388

The higher relative standard deviation of replicate determinations, in Fig. 7 compared to 389

Fig. 6, can be attributed to a shifting of the baseline from batch to batch. In each set of 390

tests it was observed that the effects of the nanoparticles within each replication were 391

well reproduced. However, the cationic polyacrylamide retention aid treatment by itself 392

already produced a strong flocculating effect, which provided a slightly different starting 393

point from which to evaluate the combined effect of the retention aid and the 394

nanoparticle. The relative standard deviation was greater for the experiments represented 395

in Fig. 7 because the cationic polyacrylamide treatment by itself happened to be more 396

effective in comparison to the runs represented in Fig. 6. 397

398

As shown in Table 2, the differences in charge density of the three types of nanoparticle 399

were in the correct order in order to account for the effects of nanoparticle dosage in Fig. 400

7. 401

402

Table 2. Comparison of the (negative) surface charge of nanoparticles (meq/g) 403

Description Charge density (meq/g) 90% Confidence interval

Mean St. Dev.

Lower Upper

Intact CNC 0.288 0.028 0.255 0.320

Singlet SiO2 nanoparticles 0.550 0.018 0.528 0.571

Chained SiO2 nanoparticles 0.686 0.043 0.635 0.737


404

Based on these results, one can understand that a greater quantity of the CNCs would be 405

needed to balance the combined positive charge of the poly-DADMAC and cPAM 406

already present in the system. Past work has shown that nanoparticle-based retention and 407

dewatering systems are often optimized under charge-balanced conditions (Andersson 408

and Lindgren 1996; Hubbe 2001). 409

410

Because the chains of SiO2 (indicated by yellow triangles in Fig. 7) had the highest 411

negative charge density among the nanoparticle types considered, it makes sense that a 412

high dosage of such particles would overwhelm the system with excess negative surface 413

charge. This can explain the relatively high turbidity values for this series of tests, as can 414

be seen in the upper right of Fig. 7. In other words, the negatively charged particles, 415

when they were present in excess, had the effect of a dispersant. The fact that the curve 416

rose continuously with time, when the SiO2 dosage was above 0.2%, indicates a 417

progressive detachment of particles with the passage of time during the continued 418

stirring. 419

420

Confirmation of the effects just mentioned can be seen in the results obtained with the 421

singlet SiO2 additive (green triangles in Fig. 7). Notably, that curve did not exhibit a 422

continuous rise until a higher level of SiO2 had been reached. Such a difference is 423

consistent with the lower charge density, compared to the SiO2 chains. 424

425

In a real paper mill it would be most common to add cPAM at just a single addition point, 426

and this often would be shortly followed by nanoparticle addition to the mixed 427

suspension, also at a single addition point. Because of this, some of the fine-particle 428

retention tests were repeated in a “discontinuous” mode of addition of the poly-429

DADMAC, cPAM, and then each different level of nanoparticles. In other words, a new 430

experiment was conducted for each level of nanoparticle addition. Results are shown in 431

Fig. 8. As indicated, at the lowest dosage of nanoparticles (0.05% on a solids basis), the 432

nano-SiO2 chains achieved the lowest turbidity (achieved in the second sets of aliquots of 433

the replicated tests). This is consistent with the higher negative charge density of the 434

silica nanoparticles. At the next higher level of nanoparticle addition (0.1%), all the 435

types of nanoparticles performed well, though the singlet SiO2 particles gave the most 436

persistent effect in reducing the turbidity. At the highest dosage of nanoparticles, the 437

CNCs outperformed the silica particles – showing the lowest value of turbidity achieved 438

in the whole series of tests. The fact that a greater addition of CNCs was needed to 439

maximize its performance, relative to the SiO2 products, in consistent once again with the 440

lower negative charge density of the CNCs (Table 2). 441

442

The shear-sensitivity of the systems shown in Fig. 8 was generally in agreement with Fig. 443

7, indicating progressive detachment of the particles from fibers with the passage of time 444

during continued agitation. In addition, the range of turbidity values shown in Fig. 8 was 445

generally less than in Fig. 7. This difference is tentatively attributed to the lesser total 446

time that any treated system was exposed to stirring (about 5 min in Fig. 8 vs. up to about 447

25 minutes in Fig. 7). 448

449


Fiber flocculation extent 450

451

Figure 9 reports the results for fiber flocculation tests, utilizing the Potentiometric 452

Dispersion Analyser (PDA) device. To summarize, addition of any of the nanoparticles 453

to fiber suspensions that already had been treated by 0.5% poly-DADMAC, followed by 454

0.05% cPAM, generally increased the PDA output value (settings: RMS, filter), 455

indicating an increased extent of flocculation among the fibers. In the case of SiO2 456

nanoparticles, the largest increase in fiber flocculation was detected when the first 457

incremental amount of 0.05% colloidal SiO2 was added to the suspension. Further 458

incremental additions of SiO2, with continued stirring and recirculation of the suspension 459

through the PDA device, yielded lower levels of flocculation. In fact, at a net addition of 460

0.4% colloidal silica, the PDA output values for flocculation intensity were not 461

significantly different from the default level, representing treatment only with poly-462

DADMAC and cPAM. In these determinations the standard deviation of output, for 463

specified conditions, were in the range of 0.01 to 0.04 PDA units. In other words, the 464

apparent changes in flocculation extent were minor, or in a few cases insignificant, 465

relative to the repeatability of the test. The fact that the flocculation effects were near or 466

below the detection limit of the method can be regarded as a favorable result relative to 467

the papermaker’s usual goal of achieving a relatively uniform, unflocculated distribution 468

of fibers within the paper product. 469

470

Results for the CNC particles, as shown in Fig. 9, generally exhibited a different trend 471

with respect to the amount of nanoparticle added. Again, the addition of nanoparticles 472

generally increased the value of the PDA output. However, there was no clear maximum 473

in the results corresponding to a low nanoparticle addition level of 0.05%. Rather, some 474

of the highest values were obtained at an addition level of 0.2% CNC. At the highest 475

level of CNC addition, as in the case of SiO2 nanoparticles, the PDA output was at or 476

below the default level representing just poly-DADMAC and cPAM treatment. In other 477

words, it took as much as four times as much CNC in some cases to achieve maximum 478

flocculation, in comparison to SiO2 nanoparticles. To explain the fact that maximum 479

flocculation was shifted to higher levels of nanoparticles, when using CNC, it was 480

hypothesized that this was due to a lower surface charge density, in comparison to the 481

same mass of SiO2 nanoparticles. This hypothesis was confirmed by the test results 482

shown in Table 2. 483

484

Conclusions 485 486

The principal finding of this work is that longer cellulose nanocrystals, serving as model 487

nanoparticles in a retention and drainage aid program, were more effective in enhancing 488

the fine particle retention, compared to shorter CNCs. This is the first set of experiments 489

in which it was possible to change just the length of the nanoparticles, while leaving other 490

attributes of the system the same. The CNCs were also found to be effective in 491

promoting dewatering, showing that indeed they can be regarded as a successful model or 492

even a possible substitute for colloidal silica products in additive programs that involve 493

cationic acrylamide-type retention aid addition. 494

495


Surprisingly, however, the present work did not reveal any strong advantage of structured 496

chain-like (gel-type) colloidal silica over unstructured singlet (sol-type) colloidal silica 497

with respect to fine-particle retention effects. A possible explanation for this is that the 498

primary particles making up the singlets are understood to be larger than those making up 499

the chain-like silica product. So one cannot say that “all other things are kept equal” 500

when comparing these two colloidal silica products having different lengths. 501

502

It was also interesting to find that the drainage-boosting effects of CNCs of various 503

length and SiO2 nanoparticles of either type all were statistically indistinguishable from 504

each other. This was despite the finding that the surface charge per unit mass was 505

distinctly lower in the case of the CNCs, compared to the colloidal silica products. It 506

follows that the negative charge degree of the CNCs was sufficient to achieve a 507

sufficiently strong interaction with the cationic polyelectrolytes present in order to 508

accomplish the strong drainage promotion, for which micro- and nanoparticle retention 509

and drainage systems are well known. 510

511

One of the most striking and potentially advantageous attributes of the CNCs, acting as 512

retention-promoting agents, was their sensitivity to hydrodynamic shear. Thus, with 513

continued stirring of the Britt Jar apparatus at 500 rpm, repeat measurements of turbidity, 514

representing the matter in the filtrate that was not adhering to fibers, ramped steeply 515

upwards with time during continued stirring. Subsequent incremental additions of CNCs 516

were able to re-set the turbidity values to low values, and the pattern was repeated. Such 517

transient, relatively weak bridging effects demonstrated by the CNCs used in this work 518

suggest a potential to achieve favorable combinations of paper uniformity and retention 519

of fine particles. This possibility is also supported by the low levels of fiber floc 520

formation that were observed with the PDA test results. Various questions related to 521

charge interactions will be pursued in ongoing research. 522

523

524

Acknowledgements 525 526

The authors are grateful for support from North Carolina State University for an award of 527

an Undergraduate Research Grant to Caryn Peksa in 2014, for the Buckman Foundation, 528

for support of undergraduate research of Connor Lenze in 2015, and for funding from 529

Northwestern Polytechnical University in Xian, China to support the work of visiting 530

scholar, Dr. Weimin Sun. 531

532

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668


FIGURES AND CAPTIONS 669

670

671

A.

B.

672

Figure 1. Atomic Force Micrographs (AFM) of intact cellulose nanocrystals. A: 673

Imaging based on height, with a sensing range of 40 nm. B: Imaging based on 674

phase, using a 20 degree angle. 675

676

677

A.

B.

678

Figure 2. Atomic Force Micrographs (AFM) of two types of broken cellulose 679

nanocrystals, with phase-based imaging. A: Batch 1, prepared under dry 680

conditions (dry crushed). B: Batch 2, prepared with the freezing of a water 681

suspension (wet crushed). 682

683

684

685

100 nm100 nm

100 nm100 nm


686 687

Figure 3. Optimization of poly-DADMAC level (on furnish solids) to achieve the 688

lowest turbidity after subsequent treatment with 0.05% cationic acrylamide 689

copolymer, with final addition of different amounts of colloidal silica 690

691

692 693

Figure 4. Filtrate mass as a function of drainage time in modified Schopper-694

Riegler test with 0.05% poly-DADMAC, 0.05% cPAM, and 0.4% nanoparticles. 695

Error bars indicate 90% confidence intervals of the mean. 696

1200

1000

800

600

400

200

0

Tu

rbid

ity

(NT

U)

0.0

0.0

50

.10

.20

.40

.00

.05

0.1

0.2

0.4

0.0

0.0

50

.10

.20

.40

.00

.05

0.1

0.2

0.4

0.0

0.0

50

.10

.20

.40

.00

.05

0.1

0.2

0.4

Colloidal Silica Addition Level vs. Furnish Solids (%)

No poly-

DADMAC

0.05% poly-

DADMAC

0.1% poly-

DADMAC

0.2% poly-

DADMAC

0.4% poly-

DADMAC0.8%

poly-

DAD-

MAC

All:

1. Optional poly-DADMAC

2. 0.05% cationic PAM

3. Optional colloidal silica

0 5 10 20 30 40 50 60

Time (s)

Filtr

ate

Ma

ss

(g

)

900

800

700

600

500

400

300

200

CNC system (cat. PAM)

CNC (dry crushed) sys.

SiO2 chains system

SiO2 singlets system

Fibers + cationic PAM

Fiber suspension alone


697 Figure 5. Filtrate mass after 20 seconds of gravity drainage, following treatment 698

with 0.05% poly-DADMAC, 0.05% cPAM, and the shown levels of nanoparticles. 699

700 Figure 6. Results of continuous tests with Britt’s Dynamic Drainage/Retention 701

Jar. Systems were treated at time zero with 0.05% poly-DADMAC and 0.05% 702

cPAM. Cellulose nanocrystals (of three types) were added at the levels shown, 703

always just before the strong reductions in filtrate turbidity. Nanoparticle 704

amounts shown represent the total of NPs present in the system at that point. 705

Average values of the relative standard deviation for four independent 706

determinations were 0.025, 0.020, and 0.018 for intact, dry crushed, and wet 707

crushed CNCs, respectively. 708

0 0.05 0.1 0.2 0.3 0.4

Nanoparticle Addition Level (%, solids basis)

820

800

780

760

740

720

Filtr

ate

Ma

ss

(g

)

CNC system (cat. PAM)

CNC (dry crushed)

CNC (wet crushed)

SiO2 chains system

SiO2 singlets system

200

300

400

500

600

700

800

900

1000

Standard CNC Cryocrushed CNC 10% Water Cryocrushed CNC

Tu

rbid

ity

(NT

U)

0.05%CNC

0.1%CNC

0.2%CNC

0.4%CNC

Time (~ 60 s intervals)

CNC (unbroken)

CNC (dry crushed)

CNC (wet crushed)


709 Figure 7. Results of continuous tests with Britt’s Dynamic Drainage/Retention 710


cPAM. Unbroken cellulose nanocrystals and two types of colloidal silica were 712

added at the levels shown. Nanoparticle amounts shown represent the total of 713

NPs present in the system at that point. Average values of the relative standard 714

deviation for three independent determinations were in the range of 0.1 to 0.5. 715

716

717 Figure 8. Results of discontinuous tests with Britt’s Dynamic Drainage/Retention 718


cPAM. Unbroken cellulose nanocrystals and two types of colloidal silica were 720

added at the levels shown. Average values of the relative standard deviation for 721

four independent determinations were in the range 0.25 to 0.32. 722

160

140

120

100

80

60

40

20

0

Tu

rbid

ity

(NT

U)

Cellulose nanocrystals

Nano SiO2 chains

Nano SiO2 singlets

0.05%NP

0.1%NP

0.2%NP

0.4%NP

Time (~ 60 s intervals, 2 replicates each point)

0.4%NP

35

30

25

20

15

10

5

0

Tu

rbid

ity

(NT

U)

Cellulose nanocrystals

Nano SiO2 chains

Nano SiO2 singlets

0.05%NP

0.1%NP 0.2%

NP

Time (~ 60 s intervals, 2 replicates each point)


723

724

A.

B.

725

Figure 9. Flocculation intensity, based on Photometric Dispersion Analyser (PDA) 726

measurements with furnish treatment with 0.05% poly-DADMAC, 0.05% cPAM, 727

and the nanoparticles of the types and amounts indicated. A: Comparison 728

between effects of intact cellulose nanocrystals and two types of colloidal silica. 729

B: Comparison between effects of intact nanoparticles and two types of broken 730

cellulose nanocrystals. Average values of the relative standard deviation for four 731

independent determinations were in the range 0.008 to 0.03. 732

733

1.15

1.2

1.25

1.3

1.35

1.4

1.45

0.05 0.1 0.2 0.4

Nanoparticle Addition Level (%)

Flo

cc

ula

tio

n In

ten

sit

y

(PD

A o

utp

ut)

SiO2 chains

SiO2 singlets

CNC (unbroken)

Control

0

1.15

1.2

1.25

1.3

1.35

1.4

1.45

0.05 0.1 0.2 0.4

Nanoparticle Addition Level (%)

Flo

cc

ula

tio

n In

ten

sit

y

(PD

A o

utp

ut)

CNC (unbroken)

CNC (dry crushed)

CNC (wet crushed)

Control

0

Intact and broken cellulose nanocrystals as model ......1 Intact and broken cellulose nanocrystals...

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