J. Pac .Sci. Tech.

Vol.23No.5（2014）

Original Paper～～～～～～～～～～～～～～～～～～～～～～～～～～～～

Effects of Repeated Drying-and-rewetting and Disintegration
Cycles on Fundamental Properties of Dissolving Pulp Fibers
and Paper Made from Them
Tatsuo YAMAUCHI* and Yousuke AMANO **In order to study paper strength reduction on recycling and the effect of hemicellulose on thedecrease of specific bond strength during recycling, handsheets made from prehydrolyzed kraft pulp for
dissolving were repeatedly recycled up to 30 times with and without disintegration. The results werecompared with those from kraft pulp for papermaking. The effects of recycling with and without
disintegration on the fundamental properties of dissolving pulp fibers and paper made from them werequite similar to those of papermaking pulp. The effect of hemicellulose on the specific bond strength withrecycling may be negligible. The decrease of external fibrillation due to disintegration during paper
recycling affected CSF and WRV and could cause a decrease of specific bond strength, resulting infurther strength reduction of paper after several times recycling.
Keywords: Paper recycling, Strength reduction, Disintegration, External fibrillation, Hornification,
Dissolving pulp, Papermaking pulp

Introduction
One of the most characteristic properties of paper material is its high ability to be recycled; thedisintegration, molding, drying and rewetting of paper are easily repeated. Thus, almost all paper material
for packaging use is made from recycled pulp. However, the strength of paper made from kraft pulp
remarkably decreases through recycling because the recycling causes fiber hornification, loss of external
fibrillation and so on 1, 2). In the first report of a serial study on paper recycling3), handsheets made fromsoftwood and hardwood bleached kraft pulps for papermaking were repeatedly subjected to only
drying-and-rewetting cycles up to 30 times and properties of pulp fiber and the paper made from themwere compared with those recycled through the drying-and-rewetting and disintegration processes inorder to study relation between the strength reduction and hornification, since the “net hornification” iscaused only by repeated drying-and-rewetting without disintegration. The results suggested that decreasein external fibrillation with increasing recycle number might have a large effect on the changes in freeness(CSF) and water retention value (WRV) and further could partly explain the large decrease in tensile
strength of the paper made from recycled pulp with disintegration. In the second report of the serial study4), a further reduction of the strength at many times recycling with disintegration could be partly attributed
to decrease in specific bond strength. In this study, taking into account the role of hemicellulose infiber-fiber*

Graduate School of Agriculture Kyoto Univ.

**Faculty of Engineering Kyoto Univ.
Kitashirakawa Oiwake-cho,Sakyo-ku,Kyoto 606-8502,Japan
E-mail: yamauchi@kais.kyoto-u.ac.jp－ 361 －

Effects of Repeated Drying-and-rewetting and Disintegration Cycles on Fundamental Properties
of Dissolving Pulp Fibers and Paper Made from Them

fiber-fiber bonding and specific bond strength, handsheets from prehydrolyzed kraft pulp for dissolving,
the hemicellulose content of which is low, were recycled in the same way as in the previous study, 3) andthe fundamental properties of pulp fibers and papers made from them were examined and compared withthose from papermaking pulp.

Experimental
Materials 3) As the base pulp a commercially available prehydrolyzed kraft pulp for dissolving use,in the form of dry pulp sheet, was used without classification to remove the fines fractions in this study.
The pulp (α-cellulose content:91%) was beaten with a PFI mill to 115 ml in CSF. Handsheets of 60 g/m2
were first made from the pulp according to TAPPI Test Method T 205. After taking some sheets toprovide a sample before recycling, the remainder were soaked in water overnight, and then re-dispersed
for 25 min with a standard laboratory disintegrater. After measurement of CSF and WRV, handsheets weremolded, wet-pressed and dried overnight with drying rings. The drying condition was an air blowing at
60℃ to avoid heat deterioration. Recycling without disintegration was also conducted. These recycling
procedures were repeated 30 times with sampling for some sheets at 1, 2, 3, 5, 10, and 30 cycles.

Measurement
The water retention value (WRV) of wet handsheets was determined according to Japan TAPPI Test
Method 26 (3000g, 15min). Other fiber properties except CSF were measured optically using a fiber
quality analyzer (OpTest Equipment Inc.). All dry handsheets before and after recycling were thoroughly
conditioned at 23℃ and 50% relative humidity, and the following physical measurements were made.
The thickness value needed to determine density and Youngs modulus was measured by the rubber platen
method. The tensile strength and Youngs modulus were measured using an InstronType machine(Shimadzu Autograph GS-100) with a pair of line type PAPRICAN clamps 5) to secure a measurement
under in-plane stress loading. Tear strength was measured using the same tensile tester 6). The
measurement of zero-span tensile strength as a measure of fiber strength was performed using the IPC
Table 1 Fundamental properties of the pulp fibers before and after thirty times recycling with andwithout disintegration
Percent fines*1%
Before recycling

Coarsenessmg/100m

Mean fiber length*2 Mean curl

Mean kink Zero-span strength

weight-weighted length-weighted kinks per mm

Nm/g

17

16.0

1.2mm

0.07

0.37

68

4

15.7

1.2mm

0.06

0.38

70

17

16.7

1.2mm

0.07

0.38

64

After recyclingWithdisintegration
Without
disintegration*1:less than 200μm,*2::70μm―10mm－ 362 －

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Vol.23No.5（2014）

type attachment. Optical reflectance of the paper with black velvet backing (R0), and that with a backing
consisting of a pad of the same papers (R∞) were measured at several points on the specimen using aphotovolt type reflectrometer (Tokyo Denshoku TC-6D). The scattering coefficient was calculated fromthe values of the measured R0, R∞, and basis weight using the Kubelka-Munk equation.

Results and discussion
Change in fundamental properties of pulp fibers before and after recycling
The fundamental properties of the pulp fibers before and after 30times recycling are shown in Table 1.
On recycling almost no change in the properties including fiber length and fiber strength (indexed aszero-span strength) was found, regardless of whether disintegration was included or not. The exception
was a marked reduction of fines fraction by recycling with disintegration, which is natural as a result of
not having carried out screening with a classifier in advance.
Freeness (CSF) unexceptionally increased with increasing recycle number, although the rate ofincrease and its detail differed with variations in sample and recycling conditions. The increase of CSF in
this experiment was sharp in the early recycling stages and gradually decreased with increasing recycle
number as shown in Fig.1. The change in internal (fiber wall) fibrillation by recycling is commonlyreferred to as hornification and WRV is used as a fiber hornification index. The decrease in WRV withincreasing recycle number is shown also in Fig.1 as a percentage decrease (100x(WRV before recycling—
WRV at nth recycling)/WRV before recycling) with and without disintegration. WRV decreased rapidly atthe early recycling stage and gradually leveled off with further increase in recycle number. The WRV
decrease of pulp without disintegration was larger than that of pulp with disintegration at the early
recycling. Since the change in WRV by recycling without disintegration is thought to be a net change ofhornification during the drying-rewetting cycle, the disintegration might have an additional reverse effecton the decrease of WRV. One explanation of the reverse effect could be that a loss of external fibrillation
by disintegration contributes to the difference of WRV between with and without disintegration, as
suggested in the previous study for papermaking pulp3).

Fig.1 CSF and decrease in WRV as the percent decrease with increasing recycle number.

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Effects of Repeated Drying-and-rewetting and Disintegration Cycles on Fundamental Properties
of Dissolving Pulp Fibers and Paper Made from Them

Effect of recycling on the fundamental properties of the papers
The percentage changes in the density and other properties, defined as 100x(property value before
recycling—the value at nth recycling) divided by the value before recycling, are shown against recycle
number in Fig.2 (with disintegration) and Fig.3 (without disintegration). As to the papers from recycled
pulp with disintegration, decreasing behaviors of density, tensile index and Youngs modulus with recycle
number were similar each other, i.e., the decrease of these on the first and second recycling was eminent.
In contrast, the decrease of density with recycling was quite small for papers from recycled pulp without
disintegration, while the hornification or reduction of wet fiber flexibility due to repeateddrying-and-rewetting should be the same as that with disintegration, as suggested from the small
difference in WRV between with and without disintegration. Further, the decrease of tensile index andYoungs modulus was smaller than that for paper from recycled pulp with disintegration. The difference
between with and without disintegration was eminent in early recycling.

Fig.2 Decrease in density, Youngs modulus, Tensile and Tear indexes and increase in scattering
coefficient as percent changes with increasing recycle number for the papers made fromrecycled pulp with disintegration.

Relationship between Youngs modulus and tensile strength
Fiber bonding is a key factor in paper strength. Thus, tensile strength correlates very well with Youngs
modulus. Further, Youngs modulus, which represents mechanical fiber bond strength, could have a linear
relation with the mechanical fiber bonding area, assuming that the specific bond strength (bond strength
per bonded area) is independent of beating degree and constant. A slightly curved linear relation was
found for papers from papermaking pulp, regardless of disintegration 4). The relationship between Youngs
modulus and the tensile index of the papers from recycled dissolving pulp generally gave a similarslightly curved linear relationship, as shown in Fig.4. However, the strength of papers from recycled pulp
without disintegration, excluding paper recycled 30 times, was lower than that with disintegration, as longas the Youngs modulus level was the same. The result suggested that a factor other than mechanicalfiber bonding, e.g. fracture toughness, was additionally related to tensile strength. Tear strength, as a
substitute for fracture toughness, of the papers tested is discussed below.

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Fig.3 Decrease in density, Youngs modulus, Tensile and Tear indexes and increase in scattering
coefficient as percent changes with increasing recycle number for the papers made fromrecycled pulp without disintegration.

Fig.4 Relationship between Youngs modulus and tensile index for papers made from recycled
pulp for dissolving with (solid symbol) and without (open symbol) disintegration: Figures indicate
recycle number.

Relationship between tear strength and tensile strength
The relationship between tear index and density of the papers from recycled dissolving pulp is shownin Fig.5. Incidentally, the tear index of papers from virgin and recycled LBKP for papermaking increases
with density or tensile strength and levels off at ca. 9 Nm2/kg 6). Although the tear index of paper from
dissolving pulp was generally smaller than that of paper from papermaking pulp, the tear index gradually
increased with increasing density for paper from recycled pulp with disintegration and rapidly increased
with increasing density for papers made from recycled pulp without disintegration. Thus, the tear index of
paper made from recycled pulp without disintegration is lower than that with disintegration, at the same

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Effects of Repeated Drying-and-rewetting and Disintegration Cycles on Fundamental Properties
of Dissolving Pulp Fibers and Paper Made from Themlevel of density. Taking into consideration of no difference in fiber length between with and without
disintegration, the lower tear index or fracture toughness could be a possible additional cause of strength
reduction for paper made from recycled dissolving pulp without disintegration.

Fig.5 Relationship between tear index and density for papers made from recycled dissolving pulp
with (solid symbol) and without (open symbol) disintegration: Figures indicate recycle number.

Fig.6 Relationship between scattering coefficient and tensile index for papers made fromrecycled dissolving pulp with (solid symbol) and without (open symbol) disintegration: Figuresindicate recycle number.

Relationship between scattering coefficient and tensile strength
The mechanical fiber bonding area is surrounded by the optical fiber bonding area (optical contact).
Further, non-bonded area spreads outside of the optical fiber bonding area. The non-bonded area can be
optically measured and is expressed as the scattering coefficient of the Kubelka- Munk equation. Thus, adecrease of the coefficient means an increase in optical fiber bonding area and further an increase inmechanical fiber bonding area. In fact, an increase in tensile strength accompanied a decrease in thecoefficient for a series of papers made from papermaking pulp beaten to various degrees and papers madefrom recycled pulp without disintegration 4). Fig.6 shows the relationship between tensile index and the－ 366 －

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Vol.23No.5（2014）

coefficient for papers made from recycled dissolving pulp. As to the papers made from recycled pulp
without disintegration, a decrease of tensile index caused by recycling accompanied an increase in
scattering coefficient, regardless of recycle number. In contrast, the scattering coefficient of papers madefrom recycled pulp with disintegration increased with a decrease of tensile index caused by recycling up
to 5 times and then decreased with further recycling from 5 to 30 cycles. The same behavior occurred for
papers made from recycled papermaking pulp with and without disintegration and thus, the strength
reduction at many times recycling could be partly explained by a decrease in specific bond strength 4).
The similarity between papers with and without disintegration from both pulps for papermaking anddissolving suggested that the effect of hemicellulose on the decrease of specific bond strength was quite
small. That is to say, cause of the decrease in specific bond strength due to multiple recycling could be
mainly attributed to the decrease of external fibrillation caused by disintegration during recycling 3,4).

Conclusions
The effects of recycling with and without disintegration on the fundamental properties of dissolving
pulp fibers and paper made from them are similar to those of papermaking pulp. The decrease of external
fibrillation due to disintegration during paper recycling caused a decrease of specific bond strength, which
resulted in further strength reduction of paper after several times recycling.

Acknowledgements
The authors wish to express their gratitude to Nippon Paper Mills Limited for the gift ofprehydrolyzed kraft pulp for dissolving. They also thank Arakawa Chemical Industries for use of the fiber
quality analyzer and the Laboratory of Biomaterial Chemistry, Grad. School of Agriculture Kyoto Univ.
for measurement of α-cellulose content.

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