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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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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Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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)
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
709 Figure 7. Results of continuous tests with Britt’s Dynamic Drainage/Retention 710
Jar. Systems were treated at time zero with 0.05% poly-DADMAC and 0.05% 711
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
Jar. Systems were treated at time zero with 0.05% poly-DADMAC and 0.05% 719
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)
Author version. Citation: Cellulose 23(6), 3951-3962; DOI: 10.1007/s10570-016-1077-9
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