Dinesh Fernando * , Dmitri Gorski , Marc Sabou in r and Geoffrey...

DOI 10.1515/hf-2012-0135 Holzforschung 2013; aop

Dinesh Fernando * , Dmitri Gorski , Marc Sabourin and Geoffrey Daniel

Characterization of fiber development in high- and low-consistency refining of primary mechanical pulp Abstract: Primary refined softwood was subjected to high-

consistency (HC) or low-consistency (LC) secondary refin-

ing, and the nature of the development of the internal

and external fiber microstructure and ultrastructure has

been compared. The primary refining of mixed softwood

as a raw material was performed in pilot scale by the

advanced thermomechanical pulp process. The study was

aiming at the comparative characterization of LC and HC

pulps at the fiber level when produced with similar and

well-characterized handsheet properties. The formerly

described Simons ’ staining method was applied. A signifi-

cant degree of fiber wall delamination/internal fibrilla-

tion (D/IF) was observed during both LC and HC refining.

Both the energy input and the refining consistency had a

significant impact on elevating the degree of fiber wall D/IF.

The statistical evaluation of internal fiber development

indicated that the fiber populations in LC- and HC-refined

pulps had a similar degree of fiber wall D/IF despite hav-

ing a large difference in refining energy input (420 kW h

odt -1 ), confirming that D/IF was promoted more energy-

efficiently in LC than in HC refining. The characteristic of

the external fiber development from HC and LC refining

was very different. Secondary LC refining promoted fiber

surfaces with ribbons of thin hairlike threads arising from

the inner secondary S2 layer that occasionally developed

along the whole fiber length. Broad sheet- and lamellae-

type external fibrillation from the S2 was typical for HC

refining, and these characteristics were rarely observed in

the LC pulps. The mechanisms for LC and HC fiber devel-

opment are proposed. The cell wall characteristics (inter-

nal and external) of the pulp fibers appear to govern most

of the physical and optical properties in handsheets.

Keywords: ATMP, delamination/internal fibrillation

(D/IF), fiber characterization, fiber development, surface

ultrastructure of fibers, HC refining, LC refining, SEM,

Simons ’ staining

*Corresponding author: Dinesh Fernando , Department of Forest

Products/Wood Science, Wood Ultrastructure Research Centre

(WURC), Swedish University of Agricultural Sciences, SE-75007

Uppsala, Sweden, e-mail: [email protected]

Dmitri Gorski: Norske Skog Saugbrugs , N-1756 Halden, Norway; and

Department of Mechanical Engineering , Pulp and Paper Centre, The

University of British Columbia, 2385 East Mall, Vancouver, British

Columbia, Canada V6T 1Z4

Marc Sabourin: Andritz , Inc., 3200 Upper Valley Pike, Springfield,

OH 45504, USA

Geoffrey Daniel: Department of Forest Products/Wood Science ,

Wood Ultrastructure Research Centre (WURC), Swedish University of

Agricultural Sciences, SE-75007 Uppsala, Sweden

Introduction Mechanical pulping is expected to produce pulp of suit-

able quality that meets the customer requirements at the

lowest possible energy expenditure. The advanced thermo-

mechanical pulp (ATMP) process gives rise to pulps with

similar properties to those produced by a traditional TMP

process, but the energy input is significantly lower (Gorski

et al. 2011a, 2012c ; Johansson et al. 2011 ). The low-consist-

ency (LC) refining of primary ATMP further reduces the total

energy requirement: ~ 300 kW h odt -1 less energy is needed to

achieve the target pulp and paper properties by secondary

LC refining compared with the secondary high-consistency

(HC) refining of primary ATMP (Gorski et al., 2012a).

The cross-sectional dimensions of fibers decrease with

HC refining (Kure 1999 ) but remain unchanged or even

increase with LC refining perhaps due to internal delami-

nation and swelling (Gorski et al., 2012a). The distribution

of fiber fractions from the two processes also differs con-

siderably. During LC refining, the long fiber fraction (R30)

is lowered and the middle fraction (P30/R100) is elevated

(Gorski et al. 2012b ). LC-refined fibers are more flexible

but have a smaller external surface area compared with

HC-refined fibers (Gorski et al., 2012a). The interpretation

is that LC refining creates less external fibrillation (EF)

and more internal delamination compared with HC refin-

ing, although the characteristic of the fibrillation has not

been studied. The mechanisms leading to these observa-

tions are unknown.

The quality of the final paper product correlates

strongly with the development of fiber properties such as

the amount of split fibers and decreased cross-sectional

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2 D. Fernando et al.: Fiber development in refining

dimensions (Reme et al. 1998 ; Kure 1999 ; Gorski and Hill

2012 ), fiber flexibility (Corson 1989 ; Fernando et al. 2011 ,

2012; Gorski et al. 2011b,c , 2012a ), fiber wall thickness

(Braaten 2000 ; Ferluc et al. 2010 ), fiber bonding (Skowron-

ski 1990 ; Reme et al. 1998 ), collapsibility of fibers (Reme et

al. 1998 ; Kure 1999 ), development of the long fiber fraction

(Corson et al. 2003 ), and EF/internal fibrillation (EF/IF)

(Fernando et al. 2011 , 2012; Gorski et al., 2012a ). EF/IF can

presumably be tailored by the parameters of the refining

process (Miles and Karnis 1991 ; Kang and Paulapuro 2006 ;

Fernando et al. 2011 ). It is known that IF (Claudio -da-Silva

1983 ; Abitz and Luner 1989 ; Mohlin 1989 ; Paavilainen

1993 ; Fernando et al. 2011 , 2012) and EF (Mohlin 1989 ;

Kang and Paulapuro 2006 ; Fernando 2007 ) determine, to

a large extent, both the strength and the optical properties

of paper.

The focus of this study was to investigate the mecha-

nisms of fiber development in secondary HC and LC refin-

ing of primary ATMP. Therefore, the characterization of

fiber development was performed, aiming to improve the

understanding of the interrelation between HC- and LC-

refined fiber profiles and the optical and strength properties

of paper sheets. The intention was to contribute to a better

understanding of the fundamental mechanisms gover-

ning this fiber property development in LC- and HC-refined

pulps at the microstructural and ultrastructural levels of

cell walls. Most of all, the influence of refining processes

on the structural changes to the fiber walls and thereby the

final paper quality should be better understood.

Materials and methods

Raw materials and refi ning The mixed soft wood chips were obtained from a Canadian TMP

mill. Composition: ~ 80 % lodgepole pine and 20 % Sitka spruce

and western balsam fi r from the interior of British Columbia. The

ATMP process (Hill et al. 2010 ; Johansson et al. 2011 ) for HC refi n-

ing was conducted at the Andritz pilot plant (Springfi eld, OH, USA).

The chips were preheated and defi brated by means of a mechanical

pretreatment consisting of a modular screw device (Impressafi ner;

Andritz, 40 kW h odt -1 energy input) and a Fiberizer (Andritz; 220

kW h odt -1 energy input). The fi berized material was fi rst stage re-

fi ned under conditions of RTS (low retention time, high tempera-

ture, and high refi ner speed) according to Sabourin et al. (1997) ,

with an energy input of 530 kW h odt -1 ; 3.1 % bisulfi te was added

through the dilution water.

A part of the fi rst-stage pulp was further refi ned by an atmos-

pheric HC double-disc refi ner with four diff erent energy inputs

(610 – 1000 kW h odt -1 ). LC refi ning of the fi rst-stage ATMP was

conducted in two optimized stages at the pilot plant in the Pulp

and Paper Centre, The University of British Columbia (Vancouver,

British Columbia, Canada). The pulp was disintegrated in 60 ° C

water for 4 h before refi ning at LC (3 – 3.4 % ) using a 14-inch LC pilot

refi ner (Aikawa Fiber Technologies, Sherbrooke, Quebec, Canada)

equipped with a 16-inch overhung segments and driven by a 110

kW variable frequency motor. The refi ning conditions were varied

by changing the gap of the refi ner while the throughput was kept

constant. This gave specifi c energy inputs of 70 – 180 kW h odt -1 in

the fi rst LC stage and 70 – 240 kW h odt -1 in the second LC stage.

The FineBar (Aikawa Fiber Technologies) segments were used (bar

edge length 5.59 km, groove depth 4.8 mm, groove width 2.4 mm,

bar width 1 mm, and bar angle 15 ° ). The pulps and handsheets

formed from the pulps (Table 1 ) were subjected to the Technical

Association of the Pulp and Paper Industry standards testing as

described previously (Gorski et al. 2012b ).

Pulps for fiber characterization Five pulps were chosen for the characterization of fi ber properties

(Table 1) based on specifi c energy consumption (SEC) and the properties

of handsheets made from the refi ned pulps for representing extreme

levels and/or similar properties (Figure 1 ; cf. Table 1). This is due to the

fact that paper with similar physical properties can be produced from

LC-refi ned pulp with signifi cantly lower energy input compared with

HC-refi ned pulp. Therefore, in this study, an emphasis was given to

the comparisons of pulps with a similar level of handsheet properties.

Simons ’ staining of pulp fibers The development of fi ber wall delamination/IF (D/IF) was evaluated

according to Fernando and Daniel ’ s (2010) method of Simons ’ stain-

ing (SS). Staining was performed on ~ 1 g of never-dried pulp from

each of the fi ve samples and then immediately examined and ana-

lyzed by light microscopy as described in the quoted literature.

Table 1 Abbreviations and essential data of the pulps observed.

Code Position (stage) SEC (kW h odt -1 ) Basic properties

TI (N m g -1 ) Density (kg m -3 ) TEA (J m -2 ) CSF (ml) LS (m 2 kg -1 )

HC1 First HC stage 800 21.6 298 16.4 487 50.1

HC2a Second HC stage 1410 37.8 371 37.8 170 57.0

HC2b Second HC stage 1570 43.6 401 50.0 117 60.3

LC2 Second LC stage 980 29.7 351 29.2 238 55.3

LC3 Third LC stage 1150 39.4 398 32.2 110 59.2

TI, tensile index; CSF, Canadian standard freeness; LS, light scattering coefficient.



D. Fernando et al.: Fiber development in refining 3

20

25

30

35

40

45

50

55

600 800 1000 1200 1400 1600 1800 2000

Tens

ile in

dex

(Nm

g-1

)

SEC (kW h odt-1)

HC Primary

HC Secondary

LC Secondary

HC2a

HC2b

HC1

LC2

LC3

Figure 1 Tensile index of handsheets formed from pulps character-

ized in this study (labeled inside squares; see Table 1).

Data collection and statistical analysis Five subfi ber populations (SFPs; see Fernando and Daniel 2010 )

that represent the diff erent categories of fi bers possessing varying

degrees of D/IF in a given pulp were identifi ed following SS. Each

category refl ects varying levels of internal fi ber development in a

given pulp. The SFPs from stained samples were obtained manually

by light microscopy (200 fi bers from each pulp were evaluated) and

analyzed as described (Fernando and Daniel 2010 ). Raw data were

fi rst summarized graphically for the ease and direct observation of D/

IF development in the cell walls. Thereaft er, the data were evaluated

and statistically analysed with SAS soft ware (SAS/STAT version 9.3

for Windows XP-Pro platform; SAS Institute, Cary, NC, USA). Ordinal

logistic regression tests were performed for the assessment of the sig-

nifi cance of the diff erences concerning the degree of D/IF of the pulp

fi ber cell walls.

Scanning electron microscopy The characterization of surface ultrastructure and EF of pulp fi bers

was performed by scanning electron microscopy (SEM). Approxi-

mately 2 g of wet samples were dehydrated separately in a series of

increasing ethanol and then acetone concentrations (Fernando and

Daniel 2008 ). The samples were then dried by means of an Agar

E3000 critical point dryer (Agar Scientifi c, Stansted, UK) with CO 2 as

a drying agent and coated with gold with an Emitech K550X sputter

device (Quorum Technologies Ltd, Ashford, Kent, UK). SEM instru-

ment: Philips XL 30 ESEM (FEI Company, Eindhoven, Netherlands)

operated at 10 kV.

Results and discussion The handsheets with a tensile index of 40 Nm g -1 and a

light scattering of 59 m 2 kg -1 were produced from both HC-

and LC-refined ATMP. The HC refining required ~ 1450 kW h

odt -1 SEC, whereas ~ 300 kW h odt -1 less SEC was consumed

in LC refining (HC2a vs. LC3; Figure 1). The apparent

density of the handsheets developed similarly for both

HC and LC refining compared at equal tensile index level,

whereas the tear index, length weighted average fiber

length, stretch, and tensile energy absorption (TEA) were

lower for the LC-refined pulp (Table 1). LC refining signifi-

cantly reduced the size of R30 fiber fraction and consider-

ably increased the middle fraction (Gorski et al. 2012a,b ).

Internal fiber development

The response of individual fibers to SS differed; fibers

stained in varying color intensities from blue to yellow/

orange with increasing severity of the fiber wall D/IF.

The observation is in line with that of previous works

(Blanchette et al. 1992 ; Fernando and Daniel 2010 ). This

indicates a diverse distribution of the individual fiber

development within a given pulp. This can be expected

because the heterogeneous wood fibers are treated dif-

ferently depending on SEC, temperature, and intensity,

and these parameters change both the chemical and the

morphologic structures of the native fiber wall as illus-

trated in the literature (Claudio -da-Silva 1983 ; Fernando

2007 ; Daniel et al. 2009 ). For example, an increase in SEC

or refining intensity enhances the proportion of treated

fibers with high degree of D/IF, thereby improving the

overall internal fiber development of a pulp (Fernando

et al. 2012 ).

An overview of the results from the SS study is shown

in Figure 2 and outlines an initial assessment of fiber

development. The five SFPs were simplified into three

major groups: “ non-D/IF ” , “ low D/IF ” , and “ high D/IF ” ,

as described in the method for ease of understanding

(Figure 2a). As expected, increasing the SEC significantly

reduced the percentage of untreated fiber population

resulting in enhanced fiber development (Figure 2a, red

line). The difference was especially pronounced when the

initial primary-stage pulp (HC1) was compared with any

of the secondary pulps irrespective of refining consist-

ency (e.g., HC1 vs. LC2). The primary pulp with the lowest

refining energy input (HC1; 800 kW h odt -1 ) consisted of

~ 63 % untreated stiff fibers and only 16 % fibers with a high

degree of D/IF. In contrast, pulp HC2b with the highest

SEC (1570 kW h odt -1 ) was the most developed at the fiber

wall level. It was dominated by a treated fiber population

( ~ 65 % ), with the majority of fibers from the high D/IF

group ( ~ 38 % ) that represents the most flexible fiber frac-

tion. The preliminary analysis also indicated that pulps

LC3 and HC2b appeared to have more or less similar fiber

populations concerning the degree of D/IF (61 % and 65 %




treated fiber population, respectively). However, LC3 con-

sumed 420 kW h odt -1 less SEC than HC2b (Figure 1). This

finding confirms that LC refining induces wall D/IF more

energy-efficiently, thereby generating more flexible fibers

than during HC refining. These fiber development values

are in good agreement with mill-scale pulp data, where

primary double-disc refined pulp had ~ 40 % untreated

and 35 % well-treated fibers at 1700 – 1900 kW h odt -1 and

tensile index of 45 Nm g -1 (Fernando et al. 2011 ).

The percentage of almost untreated fibers (i.e.,

SFP light blue) was lower in the secondary HC- and LC-

refined pulps compared with the primary pulp (Figure 2b).

However, the secondary LC pulps also contained more of

the almost untreated fibers ( ~ 25 % light blue compared

with ~ 15 % in the HC-refined pulps). If it is assumed that

a fiber always develops some degree of D/IF on receiving

an impact (mechanical action) inside a refiner, a con-

clusion can then be drawn from this finding; less fibers

receive any impact during LC compared with HC refining

at a given pulp quality level (LC3 vs. HC2a; Figure 2b). The

proportion of well-treated fibers increased with energy

input for both HC and LC. However, the energy increment

required was significantly smaller in LC refining ( ~ 350

vs. 750 kW h odt -1 SEC for LC3 and HC2b, respectively;

Figure 2a) to achieve a similar level of internal fiber devel-

opment. Accordingly, each impact on fibers in a LC refiner

HC1 LC2 LC3 HC2a HC2b

(800) (980) (1150) (1410) (1570)SEC

Pulp name

Per

cent

age

of fi

bres

(%)

0

10

20

30

40

LB DB Green Or/Yl < 1/2 Or/Yl > 1/2

100

50

0

HighD/IF

NonD/IF

LowD/IF

a

b

Figure 2 Percentage of fibers stained in different color intensities

following SS.

(a) Fractions of the three major groups of fibers representing differ-

ent levels in the degree of wall D/IF. (b) Raw data for each of the five

SFPs in the pulps investigated.

promotes D/IF of the cell wall more efficiently than HC

refining. This implies that, within the LC system, more

energy was transferred effectively onto fibers (i.e., interac-

tion effect of SEC and LC system) as discussed below.

Statistical analysis for the degree of D/IF

Detailed statistical analyses by means of the ordinal logis-

tic regression test (Table 2 ) shows (a) the overall signifi-

cance in the degree of D/IF among all pulps, (b) the effect

of the two refining conditions (i.e., the refining energy

input and the refining consistency), and (c) the impact of

the different refining conditions on morphologic changes

(i.e., D/IF) in the different pulps by analyzing two at a

time.

The greatest difference was found in the degree of

wall D/IF among the five pulps at 0.01 % significance

level (P < 0.0001; Table 2). This resulted from both increas-

ing SEC, which is in line with previous results (Fernando

et al. 2011 , 2012), and changing the refining conditions

(HC/LC). The statistical evidence for this was provided

when analyzing the two variables separately. The results

indicated that both the energy input (P < 0.0001) and the

LC/HC refining conditions (P < 0.0001) have highly signifi-

cant influence on enhancing fiber wall D/IF (i.e., making

stiff wood fibers flexible).

The statistical analysis performed pairwise is also

informative and provides further evidence on the novel

information regarding the internal fiber development

mechanisms during the two processes. There is a sig-

nificant difference in the development of fiber wall D/

IF between primary HC and secondary LC pulps (HC1

vs. LC2; P = 0.0472). The difference in SEC between the

Table 2 Logistic regression statistics for type 3 analysis of ordinal

logistic regression test for significant differences between HC- and

LC-refined pulps on the degree of fiber wall D/IF.

Source DF χ 2 Pr > χ 2

Pulps a 4 53.18 < 0.0001

Energy b 1 41.39 < 0.0001

HC/LC system b 1 15.66 < 0.0001

HC1 vs. LC2 c 1 3.94 0.0472

LC2 vs. LC3 c 1 8.43 0.0037

LC3 vs. HC2a c 1 0.04 0.8461

HC2a vs. HC2b c 1 2.36 0.1241

LC3 vs. HC2b c 1 1.50 0.2213

a Overall significance.

b Two refining conditions (energy and HC/LC system effect).

c Comparing two pulps for differences in overall degree of D/IF of

their fibers.




two pulps was 180 kW h odt -1 . The internal fiber develop-

ment between the two LC pulps is also significantly dif-

ferent with an increase in SEC of 170 kW h odt -1 (LC2 vs.

LC3; P = 0.0037). However, the degree of D/IF developed

between the two HC pulps was not significantly different,

although the SEC difference of the two pulps was similar

to that of the two LC pulps (HC2a vs. HC2b; P = 0.1241). The

results thus indicate an interaction effect of energy and LC

system that accelerates internal fiber development during

LC refining. This is reflected in LC3 pulps that contained

a greater percentage of treated fibers compared with LC2

(Figure 2a). An evidence for the interaction effect is also

given by the raw data itself (Figure 2b) as discussed above.

However, it should be noted that the interaction effect

could not be statistically estimated because all the pos-

sible combinations of energy and consistency were not

available in the data. For example, the combination high

energy input and LC was missing due to the practical limi-

tations of the refining system.

Furthermore, statistical analysis indicated that LC

refining with low SEC produced pulps that was similar

to high-energy HC pulps regarding the degree of internal

fiber development (LC3 vs. HC2b; P = 0.2213). This provides

a statistical evidence for the preliminary analysis and con-

firmed the improved energy efficiency of LC refining.

S1

S1

S1

S1

S1

S1

S2

CML

CM

LS

2

a b

dc

S2

S2

S1

S2

S2

S2

S2

S1S2 S1

S2

S2

S2 S2 S2

Figure 3 SEM micrographs of HC- and LC-refined TMP fibers with their surface morphologic characteristics (see Table 1).

(a) HC1 fibers with the secondary S1 layer of the cell wall as the outer layer. The retention of the native geometrical shape of the fibers indi-

cates inherent fiber stiffness. (b) Almost all the HC2b fibers possess the S2 as the outer layer with ribbon-type fibrillation or clear surfaces

presumably due to complete peeling of fibrils. Collapsed never-dried fibers lost their square-like form. (c) Most fibers of LC2 show S1 as the

outer layer. (d) The majority of LC3 fibers have the S2 layer exposed most often with hairlike fibrils protruding from the surface. Bars, 40 μ m

(a – d).

Characterization of the fiber surface ultra-structure by SEM

There were differences in the fiber surfaces especially

between the primary HC1 pulp (Figure 3 a) and the second-

ary HC2b pulp produced with high energy input (Figure 3b).

The majority of fibers in HC1 (SEC of 800 kW h odt -1 )

had the S1 secondary wall as their outer surface layer

(Figures 3a and 4a). It was most often fibrillated into

typical flake-like fibrils (Figure 4 a, arrows), sometimes

with remaining parts of the compound middle lamella

(CML; Figures 3a and 4a). In addition, most of the HC1

fibers retained their native geometrical shape of wood

fibers, which is indicative of intrinsic stiffness. On the

contrary, almost all the fibers in the HC2b pulp (SEC of

1560 kW h odt -1 ) displayed an exposed cellulose-rich S2

layer with a typical ribbon-type fibrillation and the fibers

exhibited greater collapsibility/conformability (Figure 3b).

The collapse of the fibers was most likely attributable to

their thinner walls, which resulted from the removal of the

major part of fiber wall materials as shown in Figure 3b.

Most of the fibers from the LC2 and LC3 pulps also

exhibited an exposed S2 layer, often with typical ribbon-

like fibrils projecting from the fiber surface (Figure 3c

and d). Both LC-refined pulps appeared morphologically




similar, although the LC2 with lower energy input had

more fibers covered with outer S1 wall material, indicat-

ing an inferior peeling effect compared with the LC3 pulp

from higher SEC.

A detailed SEM observation revealed important infor-

mation related to external fiber development mecha-

nisms during HC and LC refining. Severe fiber splitting

was already initiated during the ATMP refining process

in the first-stage pulp (Figure 4b, arrows). This is consist-

ent with earlier observations where a split fiber index of

a

f g h

i j k

l

m

d

cb

e

S1

S2

CML

S2

S2

S2 S2

S2

S2

Figure 4 SEM micrographs detailing surface ultrastructure and EF of HC- and LC-refined fibers.

(a) HC1 fiber has a rugged appearance due to bricks-like structures (arrows) from the fibrillated S1 surface layer. Parts of the remaining CML

and retention of the square-like native fiber form are visible. (b) Opening of some HC1 fibers exposing the cell lumen due to fiber splitting

(arrows). (c) HC2b fibers with typical ribbon-type S2 fibrillation reflecting the native hierarchical structure of the fiber wall. (d) Thin thread/

string-like ribbons most likely representing the smallest macrofibrils (arrows). (e) Wide band-like ribbons formed by aggregates of smaller

numbers of macrofibrils (arrowheads in d and e). (f) Broad sheet-like ribbons (arrows). (g and h) Broad carpet forming lamellae sheet

peeling from the S2 (curved arrow). (i) Hairlike structures of ribbons from the S2 surface layer developed along the fibers (pulp LC3) during

LC refining. (j and k) Hairlike ribbons were either thin thread and string-like fibrils presumably representing single or small macrofibrils

(arrowheads) or slightly wide band-like ribbon fibrils (arrows) from the S2. (l and m) LC3 fibers illustrating distinct S2 EF along the fiber so

that they appear as “ hairs ” projecting from the surface. Bars, 20 μ m (a, d – f, and j), 40 μ m (b and c), 10 μ m (g, h, and k – m), and 100 μ m (i).

~ 0.30 was determined for first-stage pulp (Gorski et al.,

2012a ). Although this implied a strong refining action on

the fibers already in the first stage, an extensive peeling of

the fiber wall material into the inner secondary wall layers

was observed only at high SEC.

Strong dissimilarities in fiber surface morphology were

found between HC and LC pulps. HC refining with high

SEC generated various types of characteristic S2 fibrilla-

tion (Figure 4c; e.g., ribbon-type fibrils) ranging from thin

thread-like fibrils (Figure 4d, arrows) to broad sheet-like




fibrils (Figure 4e and f) and lamellar sheets (Figure 4g

and h) peeling from the fiber surfaces. Particularly, the

broad sheet-like fibrils and lamellar sheets were common

on fibers in HC2b, although they were also observed occa-

sionally in the HC2a pulp with lower energy input.

LC refining promoted a mechanical interaction that

led to a distinct fiber development mechanism, where fiber

surfaces most often showed thin hairlike threads, some-

times along the whole fiber length (Figure 4i, l, and m).

Broad sheet-like and lamellae types of EF were rarely

observed in the LC pulps and the major hairlike surface

wall structures were thread-like fibrils (i.e., similar to

single macrofibrils; Figure 4j and k, arrowheads) or very

thin ribbon-like fibrils originating from the S2 layer (Figure

4j and k, arrows). In mechanical pulping refining, micro-

cracks are known to develop along naturally occurring

weak zones in the lamellae of already exposed S2 layer

(i.e., between single macrofibrils or aggregates of various

sizes of macrofibrils within lamellae; Fernando and Daniel

2004 ). In this trial, LC refining enhanced the development

of microcracks predominantly between smaller aggregates

of macrofibrils (Figure 5 a). The resulting ribbons of thin

threads were thereby easily peeled off from the surface by

the action of shearing forces inside the refiner (Figure 5b

and c). The actual events taking place in the fiber outer

wall during LC refining are illustrated in Figure 5.

A possible cause for the differences in the fiber surface

ultrastructure may be the different refining environments

in HC and LC refiners, which could influence the soften-

ing and surface properties of refined fibers. For example,

Kerekes (2011) describes the difference in the coefficient of

friction measured in HC (0.5 – 0.8) and LC (0.1 – 0.15) refin-

ers. The motor effect and typical energy inputs in HC and

LC refiners are also reported to follow the same relation-

ship and are approximately 5 – 10 times less in the case of

LC. The lower friction inside the LC refiners suggests a dif-

ferent type of mechanical interaction, which may explain

the distinct fibrillation type observed.

Hypothesis on the mechanisms of HC and LC refining

Based on the results, it can be hypothesized that shear

forces may dominate in HC refining and influence the

observed fibrillation types (e.g., broader sheets/lamellae

fibrils), which have a greater relative bonded area (RBA).

In the case of LC refining, on the contrary, compressive

forces are dominant. They are generated from fiber-to-

fiber interaction by the hitting of less concentrated fibers/

fiber bundles against each other, which is less probable in

HC refining. The refiner bars play also a more prominent

role (plate-to-fiber action) for cyclic compression in LC

refiners. Therefore, it is most probable that these mechan-

ical interactions (impact on fibers) in LC refiners impose

high internal stresses/strains on fibers that should trigger

a c d

b

e

Figure 5 SEM micrographs of LC3 fibers at high magnifications highlighting the phenomena occurring in the fiber wall during LC refining.

(a) Development of many microcracks (arrows) along the inherent weak zones within lamellae. These zones lie between individual mac-

rofibrils (red circle; popping up of a single macrofibril) or small aggregates of macrofibrils, which form band-like ribbons (arrowheads). (b)

Microcrack on the wall (arrowhead) that developed into a large opening stimulating peeling of the respective band-like narrow fibril (curved

arrow) and strings of small macrofibrils peeling and protruding from the surface contributing to the hairlike appearance. (c) Remnants on

the wall of a fiber providing evidence for stripping of narrow band-like ribbons (yellow lines indicate the width of the individual ribbon

peeled out). (d and e) Latter stage of hairlike narrow S2 fibrillation by peeling (curved arrows). Bars, 1.0 μ m (a and b), 2.0 μ m (c), and 8.0 μ m

(d and e).




fiber wall D/IF efficiently and generate more microcracks

in the wall leading to hairlike fibrillation. The previous

studies on the action of shear and compressive forces con-

clude that the former contributes predominantly to EF via

peeling mechanisms, whereas IF is the primary effect of

cyclical repetitive compression (Hartman 1984 ; Kang and

Paulapuro 2006 ; Kerekes and Senger 2006 ). Accordingly,

the hypothesis proposed on refining mechanisms with dif-

ferent and selective refining actions between HC and LC

refiners is the principle reason for the different fibrillation

mechanisms and fiber responses observed.

Several studies report on the importance of plate

clearance in terms of fiber properties ( H ä rk ö nen et al.

2003 ; Murton and Duffy 2005 ; Kerekes and Senger 2006 ;

Mohlin 2006 ; Eriksen and Hammar 2007 ). Compres-

sive forces are known to dominate when the plate gap

is small, and shearing forces are prominent when the

gap is large. H ä rk ö nen et al. (2003) showed how consist-

ency controls fiber residence time and plate clearance.

The smallest values for the latter two parameters are

attained with the lowest consistency. The plate clear-

ance in LC refiners is very small compared with HC

refiners, where it is several times larger (Eriksen and

Hammar 2007 ). In addition, a very intensive turbulent

motion of the pulps and the extension of the turbulence

toward the outer radius of the refiner plate are obtained

when the pulp consistency is lowest ( H ä rk ö nen et al.

2003 ). These conditions most likely prevailed in the LC

refiners of the present trial (at ~ 3 % consistency), which

promoted strong fiber-to-fiber as well as fiber-to-bar

impacts (i.e., compressive forces). In contrast, at higher

consistencies, repetitive compression was less influen-

tial during HC refining. However, more fundamental

work is needed to clarify the mechanisms proposed for

HC and LC refining.

Characteristic of fiber development and final product quality

The relationship of the three major D/IF groups to the

tensile strength is established in earlier work (Fernando et

al. 2011 ). The present study also showed very similar rela-

tionships (Figure 6 a). A very strong positive correlation

(r 2 = 0.96) to tensile index of handsheets was exhibited by

pulp fibers with high D/IF. Untreated non-D/IF fibers had

an inverse correlation with the same strength (r 2 = 0.99).

Fibers with low D/IF showed a positive trend with lower

strength gain. Furthermore, the three groups of fibers

were correlated to the density of the handsheets with

exactly the same strength and direction as they did with

tensile strength (Figure 6b). The results thus emphasize

the direct influence of the relative proportions of the two

groups of high and low D/IF, which contain desirable flex-

ible fibers, in a given pulp for the development of strength

and density.

Accordingly, the significant differences in the degree

of D/IF among the five pulps are the fundamental bases for

varying tensile strengths, freenesses, and densities of the

pulps developed (Table 1). The elevated D/IF in LC3 and

HC2b pulps made them highly flexible and thereby improved

the collapsibility and conformability of fibers. This accounts

for the enhanced fiber-to-fiber bonding and better densifica-

tion of handsheets (Stone and Scallan 1965 ; Abitz and Luner

1989 ; Heikkurinen et al. 1991 ; Paavilainen 1993 ).

In addition, the surface ultrastructure of each pulp

also contributed substantially to most of the physical and

optical properties of their handsheets. For example, the

exposed S2 layer with enhanced EF (long ribbon-like cel-

lulose fibrils), particularly the broad sheet-like and lamel-

lae fibrils, exhibited by pulp HC2b were key characters

for good bonding ability of the pulp (Kang and Paulapuro

R2=0.9907

R2=0.7277

R2=0.9599

0

10

20

30

40

50

60

70

Fibr

es s

tain

ed w

ith d

iffer

ent c

olou

rs (%

)

Tensile index (Nm g-1)

Non D/IF

Low D/IF

High D/IF

R2=0.9358

R2=0.6618

R2=0.9171

0

10

20

30

40

50

60

70

20 25 30 35 40 45 280 300 320 340 360 380 400 420Density (kg m-3)

Non D/IF

Low D/IF

High D/IF

a b

Figure 6 Linear correlation of the three major groups of SFPs in a pulp to the tensile index (a) and apparent density (b) of their handsheets.

The non D/IF and high D/IF groups had very strong but opposite correlation. The figure illustrates the influence of D/IF on the properties of

the sheets produces from the fibers.




2006 ; Li et al. 2010 ; Fernando et al. 2012 ). Ribbon-type

fibrils acquire higher bonding potential, particularly the

broad and long fibrils possessing greater RBA (Braaten

2000 ) and promote sheet density, tensile index, and Scott

bond strength (Kang and Paulapuro 2006 ). The higher

tensile index and density of HC2b and LC3 pulps were

thus explained by enhanced fiber development (i.e., inter-

nal and external cell wall characteristics) of their fibers.

In contrast, the inferior EF reflected by the lignin-rich S1

surface layer with flake-like short S1 fibrils of poor bonding

potential is partly responsible for the lowest density and

strength properties in HC1 and LC2 (Table 1). A consider-

ably larger proportion of their untreated stiff fibers (63 %

for HC1 and 54 % for LC2; Figure 2a) also collectively con-

tributed to the observed lower quality of the pulp.

Light scattering was on the same level for LC- and HC-

refined pulps compared at equal tensile indices (HC2b vs.

LC3; Table 1). Gorski et al. (2012a) observed lower EF for

LC pulps than for HC pulps. However, this study demon-

strates that LC refining has a different mechanism of EF,

which promoted hairlike long ribbons. Braaten (2000)

describes RBA of external fibrils as the determining factor

for both strength and optical properties of a paper sheet

and the important role of thread-like and narrow ribbon-

shaped fibrils for higher light scattering properties. Con-

sequently, the morphologic characteristic of EF found in

LC3 pulps was presumably the principle reason for its

equally high light scattering ability. According to Braaten

(1997, 2000) , the RBA of the hairlike narrow long ribbons

of LC3 fibers is very low and thus significantly influences

the light scattering coefficient of the pulp.

A significantly lower stretch and TEA of handsheets

was shown by the LC compared with the HC-refined pulp

(Table 1). This means that the fiber network of sheets

from the LC pulps was not able to stretch as much as

the network of HC-refined fibers. This may be caused

by several factors. LC-refined fibers are straighter and

shorter compared with HC-refined fibers (Gorski et al.,

2012a), which would lead to less stretch in the individual

LC fibers, when a handsheet is subjected to tension. The

morphologic characteristic of EF, which was deficient

in carpet forming broad sheets and lamellae (which are

excellent binders between fibers; Braaten 1997, 2000 )

compared with that of HC fibers, may have also collec-

tively contributed to the inferior stretchability of the LC

fiber network. A higher degree of D/IF normally causes

higher flexibility and stretchability of individual fibers

(Fernando et al. 2012 ), which did not seem to compensate

here for the higher straightness of the LC-refined fibers. A

possible explanation is that the highly flexible LC fibers

formed bonds with a larger bonded area, which could

stretch out the fiber segments between the bonds leading

to less network stretchability.

Conclusions HC and LC refining of primary ATMP along with the increas-

ing SEC significantly elevated the degree of fiber wall D/

IF. The internal fiber development during secondary HC

and LC refining was similar at a similar level of handsheet

tensile index. However, the refining energy input for the LC

refining was 420 kW h odt -1 less. LC promoted the fiber wall

D/IF more energy-efficiently than HC refining. The external

fiber development during HC and LC refining was very dif-

ferent as revealed by the ultrastructural characterization of

the pulp fiber surfaces. LC refining is suggested to induce

a distinct action that promotes fiber surfaces rich in thin

hairlike threads of S2 ribbons, sometimes along the whole

fiber length in addition to efficient D/IF development. In

the course of HC refining, broad sheet and lamellar types

of EF of the S2 layer occur, leading to high bonding poten-

tial of the fibers. These characteristics were rarely observed

in LC-refined pulps. The different mechanisms of fiber

development at the cell wall level were explored in HC and

LC refining that governs most of the physical and optical

properties of the pulps investigated.

Acknowledgements: Our work was carried out within the

framework of the “ Branschforskningsprogram f ö r skogs-

och tr ä industrin ” and “ Process and product developments

through unique knowledge of wood fiber ultrastructure ”

(2007 – 03230). The program is financed by VINNOVA, six

forest-based industries (Eka Chemicals, Holmen, SCA,

Smurfit Kappa Kraftliner, StoraEnso and S ö dra Cell), and

involves collaborative work among SLU, Innventia, KTH,

and MSU. The authors would like to express their grati-

tude to the staff of The University of British Columbia Pulp

and Paper Centre and the Andritz pilot plant in Spring-

field, OH, especially Dr. Antti Luukkonen. Dr. Jens Heymer

(Aikawa Fiber Technologies) and Prof. James Olson (The

University of British Columbia Pulp and Paper Centre) are

acknowledged for their advice and support during the LC

refining trials. Jan Hill (QualTech AB) is acknowledged for

valuable comments on the article. Dmitri Gorski would

also like to acknowledge the financial support of G å l ö stif-

telsen, Hans Werth é n Fonden, and Jansons Legat who all

contributed to his postdoctoral at The University of British

Columbia.

Received August 13, 2012; accepted February 11, 2013; previously

published online xx




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