Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

リサイクルにおける繊維の角質化が繊維交点の潜在結合力に及ぼす影響：
共焦点型レーザー走査顕微鏡を用いて東京大学大学院農学生命科学研究科
カンタヤーヌウオン ソムワン、江前敏晴、尾鍋史彦
広葉樹クラフトパルプから調製した手すき紙に、リサイクル処理をモデル化した熱処理を施し、リサイクルを行った。4 回のリサイクルによって密度は 0.67 から 0.51 g/cm3 に、比引張り強さは46.3 から 12.9 Nm/g にそれぞれ低下した。リサイクル手すき紙の全体的な結合強度の低下と関係のある微細繊維量の減少と繊維の角質化は、密度と引張強さの低下に影響を及ぼす要素になっていると考えられるため、それらの挙動を調べた。共焦点型レーザー走査顕微鏡（CLSM）を用いた観察によって、微細繊維が繊維表面を覆ったり、繊維間結合部の周囲を埋めたりすることによって紙力を向上させる役割を果たしている可能性が示された。広葉樹クラフトパルプ繊維の断面の CLSM 写真は、角質化が起これば湿潤状態での繊維の再膨潤が妨げられることを明らかに示していた。その結果、手すき紙のリサイクルのどの段階においても角質化は紙層形成過程の繊維の柔軟性低下に大きな影響を及ぼしていた。リサイクル繊維は膨潤性と柔軟性に劣るため、リサイ
クル手すき紙では十分な繊維間の接触がないということも CLSM 写真ははっきりと示していた。
手すき紙のリサイクルでは、十分な微細繊維量の減少は検出されなかったことから、非結合面積の増加は、再湿潤リサイクル繊維の再膨潤能力又は柔軟性の低下に因ることが示唆された。CLSM
観察でも、リサイクル手すき紙の強度低下は明らかに繊維交点の非結合面積の増加に起因するものであることがわかった。

1

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

Effect of fiber hornification in recycling on bonding potential at interfiber crossings:
Confocal laser-scanning microscopy

Khantayanuwong Somwang, Toshiharu Enomae, and Fumihiko Onabe
Paper Science Laboratory, Graduate School of Agricultural and Life Sciences
The University of Tokyo

Abstract

Hardwood bleached kraft pulp (HBKP) handsheets were recycled with heat treatment as a modelof paper recycling.

The apparent density and the tensile index decreased from 0.67 to 0.51 g/cm3 and 46.3

to 12.9 Nm/g, respectively, after they were recycled for four times.

Decrease in fines content andhornification of fibers with recycling, which were related to the reduction in total bonding strength ofrecycled handsheets, were investigated as possibly influential factors on this behavior. A possibility ofstrengthening role of fines by covering fiber surfaces including filling peripheral regions of interfibercrossings was presented by confocal laser-scanning microscopy (CLSM).

The CLSM micrographs ofcross-sections of HBKP fibers evidently showed hornification effect on re-swelling in the wet state, whichstrongly affected their conformability during wet web forming, in each cycle of handsheets recycling.
Recycled fibers showed inferior re-swelling ability and conformability, i.e. they could not provide sufficientinterfiber contacts in recycled handsheets, as in CLSM micrographs clearly demonstrated.

Because no

decrease in fines content was noticeable by handsheets recycling, the increase in un-bonded areas ofrecycled handsheets is suggested to rely on the reduction in re-swelling ability or conformability ofrewetted recycled fibers. The decrease in strength of recycled handsheets was also certainly attributed tothe increase in un-bonded areas of interfiber crossings by CLSM.

Keywords: Confocal laser-scanning microscopy, Hardwood bleached kraft pulp, Hornification, Recycling

1.

Introduction

Recycled paper has been increasingly produced in various grades in paper industry.

However,

there are still technical problems including reduction in mechanical strength for recycled paper.
Especially, chemical pulp-origin paper, that is, fine paper requires a certain level of strength. Howard andBichard1 reported that beaten bleached kraft pulp produced handsheets which were bulky and weak intensile and burst strengths, except in tear strength that was increased, by handsheet recycling.

Thisbehavior could be explained by the reduction in re-swelling ability or the reduction in flexibility ofrewetted pulp fibers due to fiber hornification and, possibly, by fines loss during recycling processes, which

2Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

decrease both total bonding area and the strength of paper1-4).

For thermomechanical pulp (TMP), thereduction in flexibility of wet fibers due to recycling treatment had a strong relation to the reduction in
conformability of the wet fibers during paper forming, i.e. paper strength was also reduced due to reduction
in interfiber bonding areas, as evidently substantiated by Cao et al5).

Furthermore, they also demonstrated

that the specific strength of interfiber bonding was not affected by the recycling treatment.

Jang et al6)

studied the transverse dimensions of chemical and mechanical softwood pulp fibers in the dry state, which
was subjected to the recycling treatment, with a confocal laser-scanning microscope (CLSM).

They

reported that the decrease in lumen area as well as the significant decrease in fiber wall thickness of the
TMP fibers occurred due to the recycling treatment; however, this behavior did not clearly occur inchemical pulp fibers.

The credibility of CLSM application for determining the collapse behavior andfibril angle of various fibers has also been demonstrated in many publications of Jang and his colleagues6-9).
Weise and Paulapuro10) found a greater reduction in the magnitude of re-swelling of softwood bleachedkraft pulp fibers in the wet state using CLSM images of fiber cross-sections. They considered it was a
result of repeated wet-and-dry cycles.

This result could possibly imply that re-swelling ability of recycled

softwood chemical fibers decreased due to the repeated wet-and-dry cycles of the recycling treatment.
Furthermore, they also confirmed that the dry state cross-sections of fibers did not significantly changewith the wet-and-dry cycles, which was similar to the result of Jang et al6).

Moss and Retulainen11)

applied CLSM technique to clarify the effect of chemical pulp fines on the interfiber bonding in TMP
handsheets as a strengthening material. They demonstrated that the chemical pulp fines could possibly
strengthen the TMP handsheets predominantly by covering fibers surfaces and filling peripheral regions ofinterfiber crossings.

Though in all these researches the CLSM technique was applied to study themorphological aspects of various types of fibers as well as in the related research fields, they wereseemingly quite limited to softwood fibers.

Changes in morphological aspects of hardwood bleached kraft

pulp (HBKP) fibers directly induced by recycling treatment have not been understood well even by meansof the CLSM technique.

Understanding of them would lead to important fundamentals of strength

recovery of HBKP recycled paper that is sourced partially from HBKP contained in waste office paper andrequires strength to be improved.

Therefore, the application of CLSM to study the effects of fines andhornification of HBKP fibers, due to the recycling treatment, on the bonding potential at interfibercrossings affecting mechanical properties of the recycled paper is a main objective in this research.

2.

Materials and Methods

Handsheets preparation and measurement of mechanical and physical properties

According to TAPPI test methods, all of handsheets R0 were first made from a virgin HBKP
beaten for 5,000 revolutions in a PFI mill to obtain a freeness of 480 ml CSF (Canadian Standard Freeness).

3

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

In order to simulate a part of recycling process, the following treatment was conducted as a model
treatment in paper recycling.

That is, a laboratory-level treatment was applied to some of the handsheetsby keeping them in a well-ventilated electric oven at 105 ℃ and then soaked in de-ionized water, both for
24 hours, before disintegration for making handsheets R1 (recycled once).
“recycling treatment” hereafter.

This procedure is named as

The recycling treatment was repeated for one, two and three more cycles

to produce handsheets R2, R3 and R4 from some of handsheets R1, respectively.

The handsheets R0-R4

were each randomly determined six replications for subsequent mechanical and physical properties tests,
also according to TAPPI test methods.

Fiber and fines analysis

Some sheets of handsheets R0 to R4 were soaked in de-ionized water for 24 hours and thendisintegrated to fiber slurries; post-R0 slurry to post-R4 slurry.

Parts of them were each subjected toanalysis for fines content and mean fiber length with a fiber quality analyzer (FQA, OpTest Equipment Inc.,
Canada).

Other parts of the rest of them were each stained with acridine orange, a fluorescent dye, andfiltered with filter paper for retaining fines.

A small piece of the wet dyed fibers and fines retained on thefilter paper were cut out and placed on a slide glass, and a piece of cover glass was mounted on the top
before subjecting to CLSM (LSM 510, Carl Zeiss Jena GmbH, Germany) observation.

The micrographsof XY plane and XZ or YZ sectional plane of the dyed fibers and fines, both in the wet and dry states, weretaken using a 65x water immersion lens.

Micrographs of only XZ or YZ sectional plane of the dyed fibers,

which were perpendicular to their fiber axes, were used for the interpretation of changes in the
cross-section and phenomena induced by the recycling treatment.

3.

Results and Discussion

Fig. 1 shows that apparent density and tensile index of the handsheets decreased from 0.67 for R0
to 0.51 g/cm3 for R4 and 46.3 for R0 to 12.9 Nm/g for R4, respectively.

These results suggest that thedecrease in fines content of handsheets and the hornification of recycled fibers reduced the total bonding
strength in those recycled handsheets.
observed by CLSM.

Fig. 2 shows typical wet HBKP fines in the post-R0 slurry

These fines could play a role of strengthening material, as suggested by Moss and11)

Retulainen . As can be seen in Fig. 3, a thin layer of fines covered a surface of a dry fiber from post-R0
slurry and filled the peripheral region of the interfiber crossing potential to make a bond in the dry state,
which is similar to the CLSM micrograph demonstrated by Moss and Retulainen11).

However, there was

not any difference in fines content between any pairs of post-R0 to post-R4 slurries as shown in Fig. 4.
Because all the slurries used for the fiber and fines analysis were prepared by the same method, theconstant fines content of post-R0 to post-R4 slurry was possibly caused by the similarity in fines retention

4

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

during handsheet making, i.e. similar amounts of fines were retained in wet webs consisting of fibers ofequivalent lengths.

The mean fiber length that was also constant with cycles of the recycling treatment asalso shown in Fig. 4 which strongly supported this interpretation.

Fig. 5 shows the hornification effect on

the re-swelling ability of HBKP fibers when soaked in water.

It can be noticed that the recycling

treatment had great influence on re-swelling of fiber walls of recycled HBKP fibers and the collapse of
their lumens.

In the dry state, however, the change in transverse dimension of fibers through the recycling

treatment was hardly noticeable because the lumens completely collapsed as shown in Fig. 6.

Thisbehavior in the wet and dry states was similar to that of the softwood chemical fibers studied by Jang et al6)and Wiese and Paulapuro10).

Less swollen fibers wall and more collapsed lumens of wet recycled fibers

tend to loose conformability for promoting good interfiber contact during a handsheet making process
because the plasticity of fiber walls directly depends on their swelling ability12).
representative example to prove this mechanism.

Fig. 7 gives a

It can also be seen that wet-collapsed fibers of post-R4

slurry lost their conformability and produced loose contact at the interfiber crossing in the dry state,
whereas wet-swollen ones of post-R0 slurry were fairly collapsed but maintained superior conformability to
produce good interfiber contact in the dry state.

These results also strongly suggest that rewetted fibers inpost-R0 slurry, which were not subjected to the recycling treatment, still had good re-swelling ability andconformability for improving good interfiber contact.

Therefore, it undoubtedly implies that virgin fibers

had much superior swelling ability and conformability to produce good interfiber contact in the firsthandsheets (R0).

Furthermore, the fiber behavior of post-R4 slurry could possibly be applied forpredicting those of the other fibers of the recycled slurries because of the similarity in re-swelling ability
and dried cross-sections of them all as can be seen in Figs. 5 and 6. As a result, it is no exaggeration to
say that the recycled fibers had the inferior re-swelling ability and conformability, i.e. they could not
produce the good interfiber contact in the recycled handsheets.

The top micrograph of Fig. 8 shows a gap

representing a loosely bonded or un-bonded area, which possibly often occurred in the recycled handsheets,at the interfiber crossing of dried fibers of post-R4 slurry.

Because none of the handsheets used in thisresearch was calendered, un-bonded areas including interfiber gaps in the handsheets were measured interms of light scattering coefficient13-14). Fig. 9 shows that measured light scattering coefficients of all therecycled handsheets were significantly higher than those of handsheets R0. This finding strongly suggeststhat there were larger interfiber un-bonded areas in the recycled handsheets than that in handsheets R0.
Though there was a possibility that fiber wall breakage created light scattering surfaces, but it was notnoticeable in any of the samples observed with the CLSM.

Fig. 10 shows the strong relationship between

the light scattering coefficient and the tensile index of the handsheets.

This relationship could possibly

explain that the decrease in strength of the recycled handsheets was mainly related to their increased
un-bonded areas if the specific strength of interfiber bondings in the handhseets was not affected by therecycling treatment at all, as clearly recognized by some experts 5). As can be seen, the handsheets withhigh light scattering coefficients had low strengths. According to all the results mentioned so far, the5Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

increase in un-bonded areas of the recycled HBKP handsheets is also suggested to rely on the reduction inre-swelling ability or conformability of rewetted recycled HBKP fibers during the handsheet making
process.

This hypothesis was proved by the significant increase in light scattering coefficient anddecrease in tensile index of the recycled handsheets while there was no significant difference in fines
content among post-R0 to R4 slurries.

4.

Conclusions

The strengthening role of fines layer, the adverse effect of hornification of recycled HBKP fiberson the re-swelling ability, and gaps at interfiber crossings of dry recycled fibers were proved to affectstrongly the potential of interfiber bondings in the handsheets by the powerful function of CLSM.

The

increase in un-bonded areas including interfiber gaps in the recycled handsheets is suggested to rely on the
reduction in re-swelling ability or conformability of recycled fibers during the handsheet making process.
The unique CLSM micrographs in this research also showed that the fibers treated with the recycling
treatment had the inferior re-swelling ability and conformability, i.e. they could not produce the goodinterfiber contact in recycled handsheets.

The decrease in strength of recycled HBKP handsheets was

confirmed as being due to the increase in un-bonded areas of interfiber crossings.

Acknowledgements

The authors wish to acknowledge the useful advice on the CLSM operation given by Associate
Professor Hiroyuki Nakayama and Ms Kyoko Miyawaki, Department of Veterinary Pathology, Graduate
School of Agricultural and Life Sciences, The University of Tokyo.

We are also grateful to Associate

Professor Akira Isogai, Department of Biomaterial Sciences, Graduate School of Agricultural and Life
Sciences, The University of Tokyo, for valuable discussion and suggestions.

Sincere appreciation must be

hereby extended to Nippon Paper Industries Co., Ltd. for allowing us to use the fiber quality analyzer in thepart of fiber and fines analysis of this research. Ms Hitomi Hamada, a Ph.D. candidate at our laboratory,
is thanked for her assistance in the light scattering coefficient measurement.

References

1. Howard R. C. and Bichard W.: J. Pulp Pap. Sci. 18 (4) J151-J159 (1992)
2. Howard R. C.: Technology of paper recycling, ed. by R. W. J. Mc Kinney, Chapman & Hall, London,
1995, p 180-203
3. Nazhad M. M. and Paszner L.: Tappi J. 77 (9) 171-179 (1994)
4. Phipps J.: Pap. Technol. Jul.-Aug. 34-40 (1994)

6

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

5. Cao B., Tschirner U. and Ramaswamy S.: Pap. Ja Puu-Pap. Timber 81 (2) 117-122 (1999)
6. Jang H. F., Howard R. C. and Seth R. S.: Tappi J. 78 (12) 131-137 (1995)
7. Jang H. F.: J. Pulp Pap. Sci. 24 (7) 224-230 (1998)
8. Jang H. F. and Seth R. S.: Tappi J. 81 (5) 167-173 (1998)
9. Jang H. F., Amiri R., Seth R. S. and Karnis A.: Tappi J. 79 (4) 203-210 (1996)
10. Weise U. and Paulapuro H.: J. Pulp Pap. Sci. 25 (5) 163-166 (1999)
11. Moss P. A. and Retulainen E.: J. Pulp Pap. Sci. 23 (8) J382-J388 (1997)
12. Young R. A.: Cellulose-Structure, modification and hydrolysis, ed. by R. A. Young and R. M. Rowell,
John Wiley and Sons, New York, 1986, p 91-128
13. Kurdin J. A.: Pulp and paper-Chemistry and chemical technology (1), ed. by J. P. Casey, John Wileyand Sons, New York, 1980, p 197-252
14. Robinson J. V.: Pulp and paper-Chemistry and chemical technology (2), ed. by J. P. Casey, John Wileyand Sons, New York, 1980, p 915-963

7

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

60

0.65

50

0.60

40

0.55

30

0.50

20

0.45

10

0.40

Tensile index(Nm/g)

Apparent density (g/cm

3

)

0.70

0
R0

R1

R2

R3

R4

N umber of ha nds heets recy cl i ng

Fig. 1.

Decrease in apparent density and tensile index of HBKP handsheets due to the recycling
treatment.

A pair of bars denotes a range of 95% confidence level.

8

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

20μm

Fig. 2.

CLSM micrograph of typical fines in post-R0 slurry.

9

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

X
Z

20μm

Y
X

Fig. 3.

CLSM XZ-sectioned micrograph (top) along the dotted white line in XY-micrograph (bottom)
shows a thin layer of fines (arrowed) covering a fiber surface of post-R0 slurry surface in thedry state at the potentially interfiber crossing as a kind of strengthening material.

10Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

0.86

15

0.82
10

0.80
0.78

Mean fiber length(length weighted, mm)

Fines content (%)

0.84

0.76
5

0.74
R0

R1

R2

R3

R4

N umber of ha nds heets recy cl i ng

Fig. 4.

Fines content and mean fiber length of post-R0 to R4 slurries.

A pair of bars denotes a rangeof 95% confidence level.

11

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

Post-R0
Post-R1

Post-R2

Post-R3

Post-R420μm

Fig. 5.

CLSM micrographs of HBKP fiber (wet) cross-sections show the stepwise effect of recycling
treatment on the re-swelling ability.

12

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

Post-R0

Post-R1

Post-R2

Post-R3

Post-R420μm

Fig. 6.

CLSM micrographs of HBKP fiber (dry) cross-sections at the same location as in Fig. 5.

13

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

Wet post-R0 fibers

Dry post-R0 fibers

20μm

Wet post-R4 fibers

Dry post-R4 fibers

20μm

Fig. 7.

CLSM micrographs of HBKP fiber cross-sections at the interfiber crossings show theconformability of wet-swollen fibers of post-R0 slurry is superior, for promoting the goodinterfiber contact in the dry state, to that of wet-collapsed fibers of post-R4 slurry.

14

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

X
Z

20μm

Y
X

Fig. 8.

CLSM XZ-sectioned micrograph (top) along the dotted white line in XY-micrograph (bottom)
shows an un-bonded area at the interfiber crossing of dried fiber of post-R4 slurry.

15

Somwang, K., Enomae, T. and Onabe, F., Effect of fiber hornification in recycling on bonding potential at interfiber crossings: Confocal laser-scanning microscopy
Japan TAPPI Journal 56(2), 79-85(2002)

42

2

Light scattering coefficient (m /kg)

44

40
38
36
34
32
30
R0

R1

R2

R3

R4

Number of handsheets recycling

Fig. 9.

The increase in light scattering coefficient of recycled HBKP handsheets due to the recycling

treatment.

A pair of bars denotes a range of 95% confidence level.

50
45Tensile index (Nm/g)

40
35
30
25
20
15
y = -3.9257x + 177.71
R2 = 0.9551

10
5
0
30

32

34

36

38

40

42

44

Light scattering coefficient (m2/kg)

Fig. 10. Relationship between light scattering coefficient and tensile index of HBKP handsheets.

16