The Effects of Recycling on the
Chemical Properties of Pulps
J. BOUCHARD and M. DOUEK
Changes in the physical properties of
pulp during recycling are sometimes reported to be caused by changes in chemical
properties offibres. In order to determine the
relationship, if any, between physical and
chemical properties, a chemical characterization was carried out of several recycled
pulps. Measurements of carbohydrates, lignin, crystallinity index, and DP of cellulose
were carried out as well as FTIR spectra.
The results showed no direct relationship
between the removal of hemicelluloses or
lignin and the loss or gain of strength properties during recycling. Slight cellulose depolymerization occurred during recycling of
kraf pulps. Howevel; it was largely inhibited
in the presence of deinking chemicals. Also,
there were no increases in cellulose crystallinity index afer recycling.
INTRODUCTION
The papermaking potential of a recycled pulp is often claimed to be lower than
that of a virgin pulp. As pointed out by
Howard in a recent literature survey [I],
there is general agreement that recycling
causes a significant reduction in breaking
length, burst, and fold, with a lesser reduction in apparent density, and stretch. Other
properties, such as tear, stiffness, and porosity, are usually increased. These changes
have been largely ascribed to decreased
swelling capacity and flexibility of the fibres
which, in turn, result in a loss of bonding
potential.
JP
ps
J. Bouchard and M. Douek
Paprican
570 St. John’s Blvd.
Pointe Claire, Que.
H9R 3x9
It has also been speculated that, in
addition, loss of bonding potential could be
due to changes in surface properties during
recycling. According to Eastwood and
Clarke [2], a surface which is “gummy” with
hemicelluloses might be particularly good
for bonding. They speculated that, during
recycling, hydrogen bonding is inhibited resulting in a loss in “surface condition”. They
also reported a small solubilization of pentosans from a beaten semi-bleached kraft
pulp after recycling three times. Changes in
surface properties during recycling have also
been reported by Chemaya and Bryantseva
131. Handsheets of bleached softwood sulphite pulp were aged for six months and then
repulped. They found from light microscopy
and electron microscopy that the surface of
the fibres was covered by a coating which
was attributed to low molecular weight products from hemicelluloses and lignin degradation. There was also a significant drop in
the physical properties of the handsheets,
which the authors attributed to the observed
changes to the fibre surface without mentioning if these changes were due to the
aging or to the recycling of pulp.
In spite of such speculations about the
possible effect of hemicelluloses and lignin,
there are very limited data in the literature on
the chemical changes which pulp constituents undergo during recycling. Yamagishi
and Oye [4] found a drop in EWNN solubility of bleached kraft pulp after recycling and
drying at 80°C. They also showed that the
viscosity of pulp decreased with recycling
which, in turn, corresponded to a decrease in
the degree of polymerization of cellulose.
However, there was no indication
from these studies whether there is a relationship between such chemical changes and
changes in physical properties which take
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994
place during recycling. In addition, other
types of pulps which were not examined
previously, such as mechanical or CTMP
pulps, may respond differently to recycling.
In a recent study on laboratory broketype recycled pulps, it was found that the
effect of recycling on physical properties
varied greatly depending on pulp type and
history, whether beaten or unbeaten, and
whether chemical or mechanical 151. This
provided us with the opportunity to characterize some of these pulps for their chemical
properties.
It is also reported in the literature [6]
that, when deinking chemicals are used during recycling, there is no loss of strength
properties. In particular, the beneficial effect
of sodium hydroxide on strength properties
is recognized 171. However, the effect of
deinking chemicals on the chemical properties of pulp has not been clearly established.
Accordingly, another objective of this work
was to assess the effect of repulping and
flotation, with and without the presence of
deinking chemicals, on the chemical properties of two pulp samples: a TMP, and an
unbleached kraft pulp.
Several authors have also claimed
that a small increase in cellulose crystallinity
index occurs during recycling. For example,
Yamagishi and Oye [4] have shown, using
the procedure proposed by Segal et al. [8],
that the increase in crystallinity index of
recycled pulps ranged between 1.4 and 2.7%
after five recycles. It is not clear, however,
whether this was a real increase in cellulose
order, or simply due to an increase of cellulose concentration in pulp caused by solubilization of hemicelluloses and/or lignin.
Similarly, Bugajer [9] concluded from his
work that there was an increase in cellulose
crystallinity of beaten and unbeaten kraft
J131
pulp as a function of the number of recycles.
His conclusion was based on the decrease of
the width of the peak at a 2 0 angle of 22.6"
of the X-ray diffractogram after the fifth
recycle. This is an indication of an increase
in crystal width but does not necessarily
mean an increase of crystallinity. The number of defects in the crystal must also be
considered. In an attempt to settle this apparent controversy, the crystallinity index, corrected for yield, was determined on some of
the samples examined in this study.
The overall objectives of this work
were to determine the effect of different recycling and flotation procedures on the
chemical properties of various types of pulp;
and secondly, to assess if there is a correlation between chemical changes and changes
in physical properties which take place during recycling.
EXPERIMENTAL
Description of Samples
Broke-Type Recycling
Broke-type recycled pulps used in
this work were selected from those prepared
by Howard and Bichard [5] for their study
on the effect of recycling on physical properties of the pulps. The procedure, described
in detail in their paper, consisted in the
preparation of pulp test handsheets followed
by reslushing and handsheet making at room
temperature for a total of five recycles. All
handsheets were prepared according to
Technical Section, CPPA Standard C.4 using
a handsheet-making apparatus equipped for
the recirculation of pulp fines. From the
eleven commercial pulps used in their study,
six were selected for chemical characterization. Details of the pulps are presented in
Table I.
Repulping and Flotation
Two samples were used in this work.
One of the samples was a thermomechanical
pulp (TMP) obtained from an eastern Canadian mill, and the other was an unbleached,
unbeaten, black spruce kraft pulp prepared
in the Paprican pilot plant. Unprinted handsheets from these two samples were repulped in a Hobart mixer either with or
without deinking chemicals. For the TMP
sample, the following chemicals were added
during repulping based on the 0.d. weight of
fibres: NaOH (1%), DTPA (0.4%), NazSiO3
(2S%),Hz02(1%), andsodiumoleate(l%).
For the kraft pulp sample, 2% NaOH and 1 %
sodium oleate were added to the pulper. After repulping, portions of each pulp were
subjected to flotation in a flotation cell built
at Paprican. Handsheets were prepared both
after repulping and after flotation.
Analytical Procedures
Monosaccharides were determined
on each pulp by gas-liquid chromatography
of their alditol acetate derivatives, according
to the method of Theander and Westerlund
[IO]. Acid-insoluble lignin was determined
using the Technical Section, CPPA Standard
Method (2.9. Acid-soluble lignin was meaJ132
t
sured by UV spectrophotometry following
TAPPI's Useful Method UM-250.
Molecular Weight Distribution
of Cellulose
The molecular weight distribution
(MWD) of the cellulose in pulps was determined by size exclusion chromatography
(SEC) of their tricarbanilate derivatives
(CTC). Carbanilation was conducted following the procedure proposed by Wood et
al. [1I] and scaled down for 50 mg samples.
In the case of pulps having a lignin content
greater than 3%, holocelluloses were prepared following the acid chlorite delignification procedure of Schroeder and Haig [12].
The SEC was performed using tetrahydrofuran (THF) as eluent (Curtin Matheson
Scient. Inc.) and three columns of pore size
of 105, IO4 and 500 A connected in series
(Ultra Styragel, 300 x 7.5 mm, Waters
Chrom.). One hundred microlitres of 3-6
mg/mL sample in THF (filtered on 0.45 pm
filter) were injected and eluted at a flow rate
of 1 .O mL/min. Detection was made using a
differential refractometer (Waters 410)
thermostated at 35°C. Determination of the
MWD was made from universal calibration
[I31 using narrow molecular weight distrbution polystyrenes as standards (Supelco
Inc.) and the Mark-Houwink coefficients ( K
and a)proposed in the literature for polystyrene [ 141 and CTC [ 151 in THE
Infrared Spectroscopy by
Diffuse Reflectance (DRIFT)
Air-dried pulps were milled in a steel
ball vibratory mill (Wig-L-Bug, Crescent
Co.) for 30 s, mixed with potassium bromide
(FUR grade, Aldrich Chem.) in order to
achieve a fibre concentration of
2.00 f 0.05% in KBr and milled again for
30 s. This procedure eliminates variations
due to spectral distortion [16]. Each milled
sample was transferred to the diffuse reflectance sample cup and levelled off gently
using a blade. DRIFT spectra were obtained
with a 1725 Perkin Elmer lTIR spectrophotometer and 400 scans were collected
and averaged at a resolution of 4 cm-1 for
the 4600400 cm-1 range. Spectra were expressed as ratios to KBr background and
converted to values of the Kubelka-Munk
function. Baselines were linearly corrected
in the 1900-800 cm-1 region and spectra
were normalized to 1 K-M unit for the
1512 cm-1 band.
Crystallinity Index of Cellulose
X-ray diffractograms were recorded
using a Philips diffractometer equipped with
a copper source ( h= 1.542A). Spectra were
recorded between 10" and 30" of 2 0 angle
at a speed of 2"/min. Crystallinity indexes
were determined using the procedure of
Segal et al. [8] and subsequently corrected
for pulp yield.
RESULTS AND DISCUSSION
Effect of Recycling on
Hemicelluloses and Lignin
Content of Pulps
It has been known for some time that
the disintegration and beating of pulps cause
a partial dissolution of certain wood components in water. Sjostrom and Haglund [17]
showed that between 0.3 and 0.6% by
weight of bleached sulphate and sulphite
hardwoods and softwoods was dissolved
during disintegration and beating. Roberts
and Awad El-Karim [18] showed that between 1.2 and 1.6% of a softwood bleached
kraft pulp can be solubilized by beating.
Both groups demonstrated that carbohydrates were the main component of the dissolved materials and that xylose was the
principal sugar making up between 70 and
100% of the hydrolyzable sugars. In most
cases, arabinose was the second most abundant hydrolyzable sugar, accounting for up
to 10% of the total.
Based on these results, the pentose
content of pulp seems to be a good indicator
of hemicellulose solubilization during recycling. From carbohydrates analyses, the ratio
of [xylose + arabinose] to the total content
of hemicellulosic carbohydrates in our samples has been calculated. The glucose contri-
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994
bution to the hemicellulosic carbohydrates
has been estimated on the basis of the ratio
of 3:l for mannose:glucose proposed by
Mills and Time11 [191 for galactoglucomannans. These pentosans to total hemicelluloses ratios as well as the loss (%) of pentose
sugars based on hemicellulose and on pulp
are shown in Table II. Lignin contents in
pulp are also given. For the repulping flotation experiments, values for the initial pulp,
as well as for pulp after repulping and flotation without and with chemicals, are presented.
As indicated in Table 11,the most pronounced decrease in pentosans level after the
fifth recycle was observed with the unbeaten
kraft and TMP samples (1.2 and 1.3%, based
on pulp, respectively); however, the breaking lengths (Table 11, from Ref. [5]), as well
as burst, Scott bond and fold 151 were significantly increased. Despite a significant
decrease in strength properties after recycling of the beaten kraft pulps, the level of
pentosans dropped by only 0 . 2 4 3 % (for
pulps with and without fines, respectively).
These latter values agree with the results
reported by Sjostrom and Haglund [171 for
the dissolution of carbohydratesduring beating of chemical pulps (0.3-0.6%, based on
Pulp).
There was no measurable loss of pentosans in the CTMP sample, although the
breaking length was increased by about 15%
after the fifth recycle. With the 50/50 blend
of TMPand bleached kraft, there was a small
decrease in pentosans with no significant
change in strength properties.The lignin
content of all the samples examined, with the
possible exception of the TMP which
showed a 6% decrease after the fifth recycle,
remained essentially unchanged during recycling.
Preliminary data on physical properties of pulps from the repulping/flotation
experiments suggest that there is a loss of
tensile after repulping and flotation, which
is partly recovered after addition of chemicals. As indicated in Table II, none of these
effects is reflected in the changes in hemicelluloses or lignin content.
On the basis of these results, there
seems to be no direct correlationbetween the
dissolution of pentosans and lignin during
recycling and loss of strength properties. In
fact, an inverse relationship was observed
with the TMP and the unbeaten bleached
kraft. For these types of samples, the factors
proposed by Howard and Bichard [ 5 ] , viz
fibre flattening and flexibilizing for mechanical pulps, and curl removal for un-
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994
beaten bleached chemical pulp, appear to be
largely responsible for the recycling effects
that they observed.
With beaten unbleached kraft pulps,
there is also no evidence that the small decrease in pentosans after the fifth recycle has
much influence on the breaking length reduction (17 and 25% for pulps with and
without fines, respectively).This substantial
loss of strength properties in beaten chemical pulps has been attributed mainly to loss
of fibre swelling [11 which, in turn,results in
a reduction of bonding potential.
Effect of Rec cling on Molecular
Weight Distri ution
of Cellulose
L
The molecular weight distribution
(MWD) was determined only on the chemical pulps. For the unbleached kraft pulps, the
carbanilation reaction did not go to completion because of interference from residual
lignin and a colloidal suspension remained
present. Consequently, the delignification
procedure referred to in the experimental
section was used to achieve a complete carbanilation of these samples. The fact that the
delignified CTC samples gave higher average molecular weights than the lignified
5133
samples is an indication of successful carbanilation. With high-yield mechanical
pulps, TMP and CTMP, the carbanilation
was ineffective even after four delignification treatments.
The different average degrees of polymerization calculated from the MWD of
chemical pulps are presented in Table 111.
Number-average DPs have been omitted because the low DP part of thedistribution
- is
__
not of particular relevance. DP,, DP,, and
DP,+1 better reflect the presence of relatively
longer cellulose chains.
For the broke-type recycled pulps,
there was no straightforward relationship between the changes in DP and the changes in
physical properties observed by Howard and
Bichard. However, some rationalization of
the results can still be made.
The unbeaten bleached kraft pulp
showed some depolymerization during recycling. T h e m , decreased by about 200 units
after five recycles, corresponding to the drop
measured by viscometry by Yamagishi and
Oye [4]. Moreover, in our case, the reduction
in DP, in going from the initial pulp to the
pulp after the fifth recycle, was constant at
19% for the three averages. In fact, the logarithmic DP of these samples, shown in
Fig. 1, indicates a homogeneous displacement of the distribution toward lower molecular weight with increasing levels of
recycling. In this case, depolymerization appears to be a random process with equal
probability for the breakdown of glycosidic
bonds for all the cellulose chains with a DP
As noted preequal to or greater than
viously, strength properties for this pulp
were increased with recycling.
In the case of the beaten, unbleached
kraft pulp, the DP results are somewhat different. Depolymerization of the high MW
cellulosic chains seems to be more pronounced than for the unbeaten kraft. The DP,
and m,+1decreased by 20 and 25%, respectively. On the other hand, the shorter cellulose chains were considerably less affected
m,.
4.0
3.5
3.0
2.5
2.0
log DP
Fig. 1. DP distribution curves for the unbeaten bleached kraft sample at various
stages of recycling. 0: Initial pulp; 3: pulp
after 3 stages of recycling; 5: pulp after 5
stages of recycling.
m,).
(10% drop in
However, we should
note that, if the higher DP cellulosic chains
are more frequently broken than others, the
resulting generation of lower DP cellulose
chains can thwart the depolymerization efmeasurement. This possibilfect in the
ity is reinforced by the cross-over of the
cumulative DP curves shown in Fig. 2. Significant decreases in strength properties with
recycling observed for this pulp could be
partially attributed to the breaking of longer
cellulosic chains.
The results for the bleached sulphite
pulp were surprising. In spite of a considerable drop in strength properties during recycling, there was no change in any of the DPs.
A possible explanation is that sulphite pulp
is less sensitive to mechanical action because the cellulose chains are of much lower
MW compared with kraft pulp. In addition,
the arithmetic average fibre length of sulphite pulp is about half that of kraft pulp [5].
The reduction in strength properties may
thus be only due to physical factors.
Evidence presented suggests that
m,
there is no direct relationship between the
decrease in strength properties during recycling and the breaking of cellulosic ch'ains.
The partial depolymerization of cellulose
observed for kraft pulps may be caused by
mechanical action or stress imposed on the
fibres during the sequence repulping-drying-repulping ... It is difficult, however, to
assess the contribution, if any, from cellulose
depolymerization to the decrease in strength
properties.
In the pulps from the repulping-flotation experiments without chemicals, there
was a loss of 8-13% in DP. In the presence
of deinking chemicals, there is clearly inhibition of depolymerization for the high DP
l
by only 1%
celluloses. The mz+drops
compared with about 12% without chemicals. It is believed, however, that the improvement in tensile properties in the
presence of deinking chemicals is due to a
partial recovery of fibre saturation point
[20]. This, in turn, may result in inhibition of
depolymerization by reducing the extent of
mechanical stress imposed on the fibres.
Effect of Recycling on the
DRIFT Spectra of Pulps
In an attempt to detect chemical
changes from broke-type recycling and repulping/flotation experiments, the DRIFT
spectra of all the pulp samples were recorded. In the case of the broke-type recycling, the absolute amount of organic
materials (lignin and hemicellulose) which
has been solubilized was so small that no
significant difference could be detected in
the corresponding infrared spectra.
However, minor differences were observed in the DRIFT spectra of the repulping-flotation pulps before and after
addition of chemicals. This is illustrated in
Fig. 3 which shows superimposed DRIFT
spectra of the TMP pulps between 1540 and
1800 cm-1. The three major bands in this
area are related to the following functional
n
8
100-
C
80
.-0
5
.-P
z
.n
L
60
40
Q,
.->
c,
4.0
3.5
3.0
2.5
2.0
20
cp
3
o
E
E
6
log DP
Fig. 2. DP distribution curves for the beaten unbleached kraft
sample (with fines). Cumulative curves are also shown. Solid
line: initial pulp; dotted line: pulp after 5 stages of recycling.
J134
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994
,
group vibrations:
1738 cm-l:
unconjugated ketone and
*
carbonyl group; uronic acid
carbonyl stretching
carbonyl stretching of aryl1660 cm-l:
aryl ketone and quinone
structures
1593 cm-l:
aromatic ring (in-plane) vibration and carbonyl stretching in carboxylate ion
As indicated in Fig. 3, a decrease of
intensity of the 1738 cm-l band and an increase of intensity of the 1593 cm-1 band
was observed after flotation with chemicals.
Table IV compares the ratio of these band
intensities for several pulps over that of the
1512 cm-l band associated with aromatic
ring vibration. There were practically no differences for TMP or CTMP before and after
recycling or for TMP before and after repulping-flotation without chemicals. However, a significant decrease of the 173811512
ratio and an increase of the 1593/1512 ratio
were detected when flotation was performed
in the presence of chemicals. This is likely
due to a vibration frequency shift from
1738 cm-1 to 1593 cm-l due to the transformation of a portion of the aliphatic carboxylic acids to their corresponding carboxylate
ions in the presence of NaOH.
e
.-0
1660
Y
1800
1760
1720
1680
1640
1600
1560
Wavenumber (cm-1)
Fig. 3. Superimposed DRlFTspectra of the carbonyl group absorptions (1800-1540 cm-')
of the repulping-flotationTMP pulp. 1: without deinking chemicals; 2: with deinking
chemicals.
Effect of Recycling on the
Crystallinity Index of Cellulose
Crystallinity indexes were determined for three recycled pulps and also for
pulps which had been repulped and floated
in the presence of deinking chemicals. The
results, presented in Table V, give the crystallinity as measured by X-ray diffraction
and after correction for pulp yield. This correction is based on the assumption that the
small amount of material which is solubilized during recycling and flotation is
amorphous. Consequently, the concentration of crystalline cellulose is slightly increased in the remaining pulp.
From these results, we can conclude
that, once yield corrections have been applied, there is no significant increase in cellulose crystallinity index either during
recycling or flotation. This does not agree
with previous claims [4,9] which suggest
that there is a small increase (up to 2%) of
this index during recycling. However, these
claims are based on crystallinity indexes uncorrected for yield changes.
CONCLUSIONS
There appears to be no direct relationship between the removal of hemicelluloses
from pulp and the loss or gain of strength
properties during recycling. There is also no
evidence for the contribution by lignin degradation products to the loss of strength
properties, since the concentration of lignin
remains essentially unchanged at various
stages of recycling. Changes in bonding potential are therefore likely to be largely govemed by physical factors, such as fibre
flattening and flexibility for mechanical
pulps, curl removal for unbeaten chemical
pulp, and loss of fibre swelling for beaten
chemical pulps.
There is a partial depolymerization of
cellulose during recycling of kraft pulp. The
longer cellulosic chains appear to be more
sensitive towards depolymerization possibly because of mechanical action and stress
imposed on the fibres during the sequence
repulping-drying-repulping
... It is difficult,
however, to assess the contribution, if any,
from cellulose depolymerization to the decrease in strength properties. In the repulping/flotation experiments, cellulose
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994
depolymerization is largely inhibited in the
presence of oleate and NaOH. This may be
due to the fact that there is less mechanical
stress on the fibres because of a partial recovery in fibre saturation point.
No significant changes in infrared
spectra were evident in the pulps from
broke-type recycling. However, for repulping/flotation with deinking chemicals,
there was a frequency shift from 1738 cm-l
to 1593 cm-1, which is probably due to the
conversion of carboxylic acid to carboxylate
5135
ions in the presence of sodium hydroxide.
Also, contrary to previous claims, there is no
increase in cellulose crystallinity after recycling.
In conclusion, this work has shown
that relatively minor changes in chemical
properties occur during recycling. This suggests that the chemical analyses of the fibres
explored in this paper cannot be used as a
tool for differentiating between virgin and
recycled fibres in a paper fumish.
ACKNOWLEDGEMENTS
The authors wish to express their appreciation to the National Sciences and Engineering Research Council of Canada for
an Industrial Post-Doctorate Fellowship to J.
Bouchard. We also wish to thank JeanFranGois Revol for the X-ray diffraction
analysis, Wayne Bichard, Roger Howard
and Norayr Gurnagul for the pulp samples
and the latter for reviewing the manuscript.
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