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No.6（2012）

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

Strength Properties of Wet-combination Paper
Containing Dry Strength Resin*
Takushi SAKAEMURA *1, and Tatsuo YAMAUCHI *2*This paper was presented in part at the 21st Annual Meeting of the Packaging Science andTechnology, Japan.
The effect of wet-combination on some basic strength properties was examined by using a series of
wet-combined papers made from paper plies containing cationic polyacrylamide (C-PAM) dry strength
resin. As the basic in-plane and out-of plane strength properties, tensile, compressive, and internal bondstrength were examined. Microfailures, which are detectable as acoustic emissions (AEs) and occurred
during the tensile deforming process, were further analyzed to investigate the effect of the paper ply
combination on in-plane tensile strength. The results showed that wet-combination with paper plies
containing C-PAM brought about an increase in the plastic deformation region with a gradual increase in
microfailures and delayed the acute occurrence of microfailures just before the maximum load. The
higher strain at breakage rather than the increase in fiber bonding led to an increase in tensile strength.
Heterogeneous location of strong paper ply containing C-PAM, especially on one side, decreased theincrement of the tensile strength. The compressive strength in the X-Y direction increased with increasing
total C-PAM content, regardless of wet-combination, although the increment was small for papers inwhich the strong paper ply containing C-PAM was located at the center or on one side. Tensile strength in
the Z-direction measured as internal bond strength was governed by that of the weakest paper ply, andwet-combination brought about a further decrease in the Z-directional tensile strength.
Keywords ： Acoustic emission, Compressive strength, Dry strength resin, Internal bond strength,

Microfailures, Strain at breakage, Tensile strength, Wet combination

1. Introduction
In the manufacturing of paperboard, wet-combination has been exclusively employed for a long time,and application of a dry strength agent has also become popular to make up for the strength decrease due
to an increased use of recycled fibers. However, the effect of wet-combination on the strength propertiesof paper has not been thoroughly examined. In the previous report 1), the effects of wet-combination of
paper plies from lightly and heavily beaten pulps on some basic strength properties of paper was
examined, and microfailure occurrence during the tensile strength testing process, detectable as acoustic
emissions (AEs), were investigated. They found that the layered paper structure made by
wet-combination brought about an increase in the period of the plastic deformation region, leading to a
*1

Arakawa Chemical Industries Ltd., Paper Chemicals Division
Kyoto University, Graduate School of Agriculture
Corresponding author, Kitashirakawa Oiwake-cho, Sakyo-ku,Kyoto 606-8502, Japanyamauchi@kais.kyoto-u.ac.jp*2－ 461 －Strength Properties of Wet-combination Paper Containing Dry Strength Resinhigher strain at breakage with an increase in tensile strength. Compared with homogenous paper madeby pulp mixing, the presence of a strong paper ply in wet- combination, irrespective of its Z-directional
position, may increase the period of the plastic deformation region with gradual increase in microfailure
and thus delay the acute occurrence of microfailures just before the maximum load. Further, including
strong paper plies on both outer sides could prevent the development of microfailures in paper sheet
failure, leading to an increase in strain at breakage and ultimately to higher tensile strength. In contrast to
tensile strength, compressive strength in the X-Y direction was simply determined by the content ofhighly beaten pulp, regardless of the manner of combination, i.e. wet-combination or pulp mixing. Tensile
strength in the Z-direction measured as internal bond strength was governed by the strength of theweakest paper ply, and wet-combination brought about a further decrease in the Z-directional tensile
strength. As a serial study on the effect of wet-combination with consideration of effective application ofa dry strength agent, wet-combination was conducted with paper plies containing dry strength resin,slightly containing it and not including it as strong, less strong and weak paper layers in this study. Firstly,
the basic strength properties of the wet-combination papers varying in total content of dry strength resin
and in location of strong or less strong paper plies were examined. Secondly, microfailures occurred
during the tensile testing process were investigated with a help of AE analysis as a fundamental study into
the effect of wet-combination on the tensile strength properties.

2.Exerimental
2.1Materials
The cationic polyacrylamide (C-PAM) used in this study as a dry strength resin was a commerciallyavailable standard C-PAM (relative molecular mass: 2 x 106, charge density: 0.6 meq/g) 2) from Arakawa
Chemical Industries, Ltd. (Osaka, Japan) at 7% concentration solution. No further purification of C-PAM
was conducted.
The pulp used in this study was a commercially available fully bleached hardwood (mixed species)
kraft pulp and was in advance screened using a single compartment classifier with a 100-mesh screen 3).
The fines-free pulp was lightly beaten with a PFI mill to 510 ml CSF.
Aliquots of the lightly beaten pulp were diluted with tap water to 0.3% concentration (at pH 7) andthen an aqueous solution of the C-PAM, which was adjusted to 1.0% concentration with deionised water,
was added to the furnish to produce the papers containing C-PAM in the range of its concentrations 0 to
2%( mass of C-PAM on o.d. paper). No other additives (such as alum) were added to the furnish tosimplify the experiment. After the addition of C-PAM, the furnish was continuously stirred at room
temperature for 30 min before sheet molding using the internal application method 2) to mold handsheets
as follows. Couched wet handsheets of 30 or 60 g/m2 basis weight were made from the above mentioned
furnish, according to TAPPI Test Method T-205. In the case of 30 g/m2 sheet, the water level in themolding machine was kept at half the normal level to keep the fiber consistency on molding constant, andthus the sheet formation was nearly constant between handsheets of 30 or 60 g/m2 basis weight. The
couched wet handsheet was once or twice overlaid on the next wet handsheet just after drainage to－ 462 －

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prepare the sample combination papers (final basis weight: ca 120 g/m2). Thus, various combinations ofpaper plies were prepared. In more detail, the first series of papers were made of the same furnish but
varying in number of paper plies (number of combination), and the second series of papers were made of
two furnishes with and without C-PAM, keeping their total C-PAM content constant and varying thelocation of the paper ply with C-PAM. A third series of papers (basis weight: ca 120 g/m2) varying in
C-PAM content were directly made without the combination operation. All wet papers were dried withdrying rings under restraint by air blowing at room temperature. The total C-PAM content retained in thepapers was determined by nitrogen analysis (Total Nitrogen Analyzer TN-110, Mitsubishi Chemical
Analytec Ltd.) 4).

2.2 Basic and strength properties of handsheets
The thickness for determination of sheet density and Young’s modulus was measured by the rubber
platen method according to TAPPI test method T-551. For determination of the Young’s modulus andtensile strength, test specimens were strained using an Instron type machine (Shimadzu Autograph
AGS-100) with a pair of line type PAPRICAN clamps 5) to secure a measurement under in-plane stressloading. Measurements of edgewise in-plane compressive strength (short span compressive strength) andinternal bond strength were conducted according to TAPPI test method T-826 and -541, respectively.
Internal bond strength is the physical breaking energy with constant strain at breakage, and thus could be
proportional to Z-directional tensile strength. As a result it is conventionally considered to be a substitutefor Z-directional tensile strength 6). The basic and strength properties of the paper samples tested are
given in Table 1.
Table 1 Basic properties of the tested papers－ 463 －Strength Properties of Wet-combination Paper Containing Dry Strength Resin

2.3 AE measurement
The experimental arrangement for AE measurement during tensile straining and for analyticalprocessing of the detected AE was essentially the same as in previous papers 1,4). A commerciallyavailable piezoelectric AE sensor (Model AE 901U; NF Corporation, Yokohama, Japan) with afundamental resonant frequency of 150 kHz was attached to the lower part of the specimen. The sensor
and the specimen were pinched with a lightweight clip-type clamp across the thickness of the specimen.
The sensor surface, as a result, contacted well with the surface of the specimen. The detected AE signal
was then processed in an AE data processor (Model AE 9640; NF Corporation, Yokohama, Japan), andseveral parameters of each AE event including the time when the AE signal first crossed the thresholdlevel set at 50 dB, the maximum amplitude (expressed in dB), and the tensile load were successively
recorded on magnetic tape. The recorded AE data were further processed with a computer (AE analysis
software 0963; NF Corporation, Yokohama, Japan) after the test. The relationships between cumulative
AE event counts and tensile load were examined as the selected AE parameters in this study. A slightvariation in tensile and cumulative AE event count behaviour among samples was found due to thefundamental characteristics of micro- and macro-failures, but the general trend of these results was clear 7).
AE study of the compressive strength and internal bond strength was not applicable, since setting of the
AE sensor on the paper sample was difficult for both tests and both measurements were conducted at
high-speed and thus the periods of their deforming processes were too short to examine the AE behavior.

3. Results and discussion
3.1 Effects of total PAM content, wet-combination and position of strong paper ply
containing C-PAM on basic and strength properties of papers
As reported in a previous paper 3), the effect of C-PAM addition on sheet density was small. In
contrast, wet-combination brought about an evident decrease in the density for papers having
homogenous Z-directional distribution of C-PAM regardless of the C-PAM content, although the total
PAM content differed to some extent between one and three ply papers. This was probably because an
incomplete overlaying accompanied with wet-combination brought about a slight increase in thickness
and thus a decrease in density, as described in the previous report 1).
The effects of wet-combination and location of the C-PAM contained paper ply on tensile strength,edgewise compressive strength and internal bond strength are shown in Fig.1a, b, c, respectively in
relation to total C-PAM content. The two former show strength in the X-Y direction and the latter shows
strength in the Z-direction of paper sheets. Furthermore, tensile strength and internal bond strength arestrength in tension mode, while edgewise compressive strength is strength in compression mode.
As previously reported 2), the increase in tensile strength with increasing C-PAM content of the paper
was generally large for C-PAM content lower than 1% and quite small for higher content. That is to say,an increase in total C-PAM content does not yield a proportional increase in tensile strength, in contrast to
the proportional increase in tensile strength with content of highly beaten pulp in the previous paper 1).
Homogeneous paper made by wet-combination of three plies had a higher tensile strength regardless of－ 464 －

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Fig. 1aVol.21

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Relationship between tensile index and total C-PAM content in the papers.

Fig.1b Relationship between compressive index and total C-PAM content in the papers.

Fig.1c Relationship between internal bond strength and total C-PAM content in the papers.

－ 465 －Strength Properties of Wet-combination Paper Containing Dry Strength Resinthe total C-PAM content, and the increment of tensile strength for lower C-PAM content wascomparatively bigger that that for higher C-PAM content. A heterogeneous C-PAM location in theZ-direction (located at both outer sides or at one side) gave a lower tensile strength for the same total
C-PAM content, suggesting a poor cost-performance of C-PAM addition.
Tensile strength had linear relationships with Young’ modulus and strain at breakage as shown in
Fig.2 a, b. respectively for all tested papers. Young’s modulus, showing little variation between
measurements, is a basic mechanical property and is essentially a physical constant of a solid material. Its
increase means an increase of fiber-fiber bonding, leading to an increase in tensile strength. In contrast,
strain at breakage is not a constant and shows some variation arising from the character of large
deformation. Both generally showed linear relationships; the correlation coefficient for Young’s modulus
was 0.780 and that for strain at breakage was 0.913, and thus the latter was more decisive factor on thetensile strength, as mentioned in the previous paper 1). That is to say, the contribution of the elongation on
tensile strength increase was bigger than that based on an increase in fiber bonding. The increase intensile strength with increasing strain at breakage by wet-combination is further examined in the AE study
in 3.2.
Edgewise compressive strength increased by C-PAM addition, and the increment was large for
C-PAM content lower than ca 0.5 % and small for higher C-PAM content, regardless of wet-combination.
Heterogeneous Z-directional C-PAM content by wet-combination gave a lower compressive strength thanhomogeneous Z-directional C-PAM content by wet combination for the same total C-PAM content.
Notably in the case of one side location of strong paper ply containing C-PAM, the compressive strength
was lower than that of paper containing no C-PAM. This result differed from the previous study 1) inwhich compressive strength increased proportionally with the ratio of strong paper ply (from heavily
beaten pulp) irrespective of the location of the strong paper ply.

Young's modulus (GPa)

5

4

3
50

60

70

80

90

Tensile index (Nm/g)

Fig. 2a Relationship between tensile index andYoung’s modulus for papers containing C-PAM.

Fig. 2b Relationship between tensile index andstrain at breakage for papers containing C-PAM.

－ 466 －

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The internal bond strength of papers not made by wet-combination steeply increased with the
C-PAM content for C-PAM content lower than 1 % and leveled off at higher C-PAM content, while thatof papers made of paper plies having the same C-PAM content by wet combination moderately increased
with total C-PAM content. At any total C-PAM content, the former was higher than the latter. In contrast,
the internal bond strength of papers made of paper plies having different C-PAM content by
wet-combination was equal to or lower than that of paper containing no C-PAM. Thus, tensile strength in
the Z-direction was determined by that of the weakest paper ply. Furthermore, incomplete overlaying
accompanied by wet-combination could bring about some mechanical defects within the combinedinterface, easily leading to Z-directional sheet failure, i.e., low internal bond strength.

3.2 AE examination of papers made by wet combination (Effect of number of
paper plies)
The occurrence of microfailures detected as AE events was examined from the relationships between
tensile load and the cumulative AE event count up to the maximum tensile load, in order to investigate theeffect of the number of paper plies in wet-combination. A representative result of papers made by
wet-combination is shown in Fig.3 for papers made of paper plies containing C-PAM at total C-PAM
content of ca. 1 %. Little difference was found in the load at which AE events began to occur regardlessof wet-combination. Further, the shapes of the load-AE curves of the papers were generally similar toeach other, i.e., the slopes of the curves were quite low at first, and then gradually and finally steeply
increased just before the maximum load. However, the shape of the curve differed somewhat withincreasing number of paper plies, i.e., the period of the gradual increase in AE events before the acuteoccurrence was prolonged for papers made by wet-combination. The longer period indicates an increase

Fig, 3 Representative relationship between the cumulative AE event count and tensile load for thepapers containing C-PAM at the total content of 1 %, showing the effect of the number of paper plies.

－ 467 －Strength Properties of Wet-combination Paper Containing Dry Strength Resinin the plastic deformation region, leading to higher strain at breakage and an increase in tensile strength.
The longer period of gradual increase in AE events (microfailures) could be interpreted as follows: a kindof discontinuity of paper structure caused by wet-combination might reduce a possible development of
microfailures leading to sheet failure. Better sheet formation caused by wet-combination could be anotherreason for the strength increase. However, paper samples in this study were made as the sheet formation
kept constant as possible.

4. Conclusions
Wet-combination with paper plies containing C-PAM brought about an increase in the plastic
deformation region, leading to higher strain at breakage. The increase in tensile strength of the
wet-combination papers was strongly related to the enlarged strain at breakage rather than the increase infiber bonding. The existence of a layered structure formed by wet-combination may lengthen the periodof gradual increase in microfailures at the plastic deformation region and delay the acute occurrence of
microfailures just before the maximum load. Heterogeneous location of a strong paper ply containing
C-PAM, especially one side location, decreased the increment of the tensile strength. In contrast, thecompressive strength increased with increasing total C-PAM content, regardless of wet-combination.
However, the increment was small for papers in which the strong paper ply containing C-PAM located at
the center or on one side. Tensile strength in the Z-direction, shown as internal bond strength, wasgoverned by that of the weakest paper ply, and wet-combination brought about a further decrease in theZ-directional tensile strength.

Acknowledgments
The authors express their huge thanks to Arakawa Chemical, Rengo, and Showa Products Cos. Ltd,for their financial support and to the Laboratory of Wood Processing in the Graduate School of
Agriculture, Kyoto Univ for permission to use the AE measurement system.

References
1)
2)
3)
4)
5)
6)
7)

T. Sakaemura and T. Yamauchi, the79th Pulp and Paper Research Conference Japan p.24 (2012)
T. Sakaemura and T. Yamauchi, JAPAN. TAPPI J. 64(11)1303(2010)
T. Yamauchi and T. Hatanaka, Appita J. 55(3) 240(2002)
T. Sakaemura and T. Yamauchi, Appita J. 64(4) 331(2011)
R.S.Seth, A.G.Robertson, Y.W.Mai, and J.D.Hoffman TAPPI J. 76(2)109 (1993)
Tappi standard T-541, Internal bond strength of paperboard (z-direction tensile)
1
T.Yamauchi, S. Okumura, and M. Noguchi, J. Pulp Paper Sci. 16(2):J44 (1990).
（原稿受付 2012 年 6 月 27 日）（審査受理 2012 年 8 月 27 日）－ 468 －

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紙力増強剤を含むすき合わせ紙の強度的性質
栄 村 拓 史*1、

山 内 龍 男*2

カチオン性ポリアクリロアミド系紙力増強剤(C-PAM)を加えて強度を増加させた紙をモデル
的にすき合わせ、それらの強度的性質に及ぼすすき合わせ、すなわちすき合わせ層数およびC-PAM を加えた高強度の紙層内位置の影響を検討した。その結果すき合わせは、微小破壊が徐々に増大する塑性引張変形域の増加をもたらし、微小破壊が急激に増大して破壊に至るまでの引張
破断伸びを増加させ、結果として引張強度を増大させる。ただし C-PAM 含有紙層が偏在するとその増分は比較的小さい。他方紙面方向の圧縮強度は、すき合わせの影響をあまり受けずに、単に紙力増強剤量の増加と共に増大する。ただし、C-PAM 含有紙層が全紙層の中央あるいは片側に偏在するとその増分は比較的小さい。厚さ方向の引張強度でもある内部結合強度は、強度の低
い C-PAM 非含有紙層のそれで決まり、さらにすき合わせ面に生じる欠陥によりさらに強度を減少させる。
キーワード: 紙力増強剤、引張強度、圧縮強度、内部結合強度、すき合わせ、微小破壊、引張破
壊伸び，アコースティックエミッション－ 469 －