Evaluation of Fracture Toughness of Paper in Wet State (II) - Effect of Fiber Curl Treatment -

Evaluation of Fracture Toughness of Paper in Wet State (II) - Effect of Fiber Curl Treatment -

You Sun Roh

and Yung Bum Seo

Received September 8, 2016; Received in revised form October 1, 2016; Accepted October 4, 2016

ABSTRACT

Paper breaks can cause a large economic loss in the paper mill and papermakers should take all possible precaution measures to prevent them. Fracture mechanics was employed for the evaluation of the paper break tendency on the paper machine and, until now, has been considered to be one of the most logical approaches to the problem. Fiber curl treat- ment usually increases paper strain to failure in tensile loading and, in the present study, we tried to relate fiber curl treatment to fracture energy. Two different furnishes (soft- wood and hardwood bleached kraft pulp) were used with three different refining levels;

furthermore, their fracture energy values were measured at different dryness levels (40%, 60%, 80%, and 95%). While fiber curl treatment greatly increased fracture energy for the hardwood pulp, the effect was marginal for the softwood pulp. The highest stretch was mostly at 80% solid content, where the highest fracture energy occurred.

Keywords: Fracture mechanics, fiber curl, dryness, stretch, load-widening, paper break

• Dept. of Bio-based Materials, Chungnam National Univ., Daejun, Yousung-Gu, Gung-Dong 220, Republic of Korea 1 Hansol Paper Co. Republic of Korea

† Corresponding Author: E-mail: [email protected], Fax: 82-42-821-6159

http://dx.doi.org/10.7584/JKTAPPI.2016.10.48.5.22 ISSN (Print): 0253-3200

Printed in Korea

1. Introduction

Although paper breaks can lead to a large eco- nomic loss in the paper mill, papermakers cannot totally avoid them from happening.

What they can do is to reduce the frequency of breaks. Page et al. showed that wet web stretch is proportion- al to the curl index irrespective of fiber sources.

The authors also suggested that fiber curl treat-

ment causes paper stretch (strain to failure in per- cent in a tensile test).

Furthermore, Roh et al.

demonstrated that the highest fracture energy was observed where the highest wet web stretch was measured.

To induce fiber curl for the wood chemical pulp,

commercial mechanical devices, such as Frauta

pulper, disperser, and screw press, can be used at

a high pulp consistency.

In a lab experiment, a

Hobart mixer caused fiber curl very conveniently.

Fiber curl treatment usually increases the paper stretch, but decreases tensile and burst strength.

Further refining treatment on curled fibers at a low consistency increases strength properties while keeping paper stretch high.

Roh et al. showed a low fracture toughness for paper at a low dryness level on a paper machine.

They also demonstrated that the papers made with softwood pulp fibers, among other fibers, give the highest fracture toughness and the maximum fracture toughness was shown at ca. 80% dryness.

The influencing factors for fracture energy include the dryness of the web, fiber types (mechanical or chemical pulp, softwood or hardwood), refining degree, paper machine speed, wire type, wet press type, size press type, and web tension.

In the present study, we focused on the first three fac- tors (web dryness, fiber type, and refining degree).

Other factors are mainly due to the paper machine itself, which is difficult and expensive to change.

The measurement of fracture energy is not easy, even nowadays. Paper is a nonlinear viscoelas- tic material and, therefore, linear elastic fracture mechanics is not applicable to it; however, elas- tic-plastic fracture mechanics can be applied. In

this study, essential work of fracture

and the stress-widening curve method

, both of which are applicable to nonlinear elastic materials, were used to estimate fracture energy.

2. Materials and methods

2.1 Materials

We used SwBKP (softwood bleached kraft pulp: a mixture of hemlock, Douglas fir, and cedar, Can- ada) and HwBKP (hardwood bleached kraft pulp: a mixture of aspen and poplar, Canada). The curled fibers were prepared by the Hobart mixer treat- ment

for 3 hours in 10% consistency at room temperature for each furnish; then, they were re- fined to three levels in a valley beater for making handsheets. The physical properties of the sample are shown in Table 1, where breaking length and stretch were averages of at least 10 measurements.

The solid contents of the wet paper webs were con- trolled by changing the drying times (40%, 60%, 80%, and 95%). We made square handsheets with a Williams type square handsheet former (Daelim Paper Machinery Co. in Korea). After wet pressing, we applied the predetermined drying time for the

Sample Refining

(min)

Freeness (mL CSF)

Density (g/cc)

Breaking length (km)

Stretch (%)

No curl

Softwood

30 590 0.52 7.97 3.93

50 540 0.65 8.50 4.30

70 470 0.70 8.56 4.95

Hardwood

15 570 0.54 2.28 0.92

30 520 0.57 4.41 2.23

45 430 0.65 5.52 2.27

Curl

Softwood

30 560 0.52 7.24 4.22

50 530 0.57 7.69 4.71

70 430 0.62 7.59 5.84

Hardwood

20 521 0.57 5.40 4.09

40 361 0.58 5.80 4.82

60 258 0.65 6.96 5.36

Table 1. Physical properties of the fiber furnishes refined to different freeness levels

(Dried and conditioned handsheets at standard condition, ISO 187)

wet webs, and sampled small pieces from the wet webs to measure their dryness levels. After the wet webs were dried to the desired solid content, they were put into a vinyl folder and sealed. This vi- nyl-bagged wet web was bagged again with a larg- er vinyl folder and then sealed. The double-time bagged wet web was preserved in a refrigerator until testing. We checked the solid contents of the sample after 3 weeks in the refrigerator and no significant property changes were observed.

2.2 Physical testing

When testing the samples, we brought the sample in the vinyl folder and cut the sample to the de- sired shape with the vinyl still attached. After the sample was securely connected between the grips in the tensile tester (Micro 350 tensile tester; Tes- tometrics, UK), we removed the vinyl and ran the test. After the test, we measured the weight of the sample and checked its solid contents. No signifi- cant differences in the solid contents of the samples before and after the test were observed.

2.3 Fracture energy measurement

Deep double-edge notched tension specimens (DENT) containing varying ligament lengths were used in the measurement method of essential work of fracture. As the ligament length of the specimen changes, the total strain energy of the specimen changes as well. The intercept of the line, which is drawn from the linear relationship between lig- ament length and total strain energy, is called the essential work of the fracture. It is already well established that the essential work of the fracture is an appropriate estimate of the fracture behavior of a material, independently of the specimen’s size and shape. The listed references

describe the method in detail.

Another way of measuring fracture toughness used in the present study was the stress-widening curve method at the stable fracture region sug-

gested by Tryding.

In this method, the specimen dimension should be controlled in such a way that the specimen fails at the stable fracture region in the tensile test. To ensure that the fracture occurs at the stable fracture region, one should find an appropriate length-to-width ratio of the speci- men before the fracture test. In our experiment, we used the specimen dimension of 50 mm in width and decided on the span length depending upon the dryness level before the experiment by pretesting.

For 40% and 60% dryness cases, the span length of 50 mm; for 80% dryness, the span length of 25 mm; and for 95% dryness, the span length of 15 mm were chosen, respectively. At those length- to-width ratios, no unstable fractures or abrupt breaks of the samples were found.

Fracture energy is defined as the square of frac- ture toughness value divided by Young’s modulus.

Fracture energy can be measured directly from the essential work of fracture and Tryding’s stable fracture widening method, and we used the frac- ture energy for the analysis of the fracture tenden- cy evaluation of the paper web.

3. Results and discussions

3.1 Curl treatment effects

The effect of fiber curl treatment is shown in Fig.

1 for softwood fiber furnish and Fig. 2 for hard- wood fiber furnish, respectively. There was a sig- nificant loss of breaking length in softwood by the curl treatment; however, a large gain in hard- wood in the figures was observed. Stretch was in- creased for both fiber furnishes by the curl treat- ment. Stretch increase was expected, as Page et al.

showed in their study. If there was no refining af-

ter curl treatment, there would be breaking length

loss.

As demonstrated by Seo et al., refining after

curl treatment usually straitens curled fibers and

increases both tensile strength and stretch. In this

case, softwood showed exceptional behavior. It was concluded that there were too many curls to be re- covered by the curl treatment of the long fibers, such as softwood fiber furnish in this case.

Fig. 3 shows the effects of curl treatment on stretch increase for both furnishes. Hardwood fiber furnish remarkably increased stretch, but softwood did much less than hardwood by the curl treat-

ment. While large differences in stretch between two furnishes initially without the curl treatment were found, little differences were seen after the curl treatment.

3.2 Fracture energy

Fracture energy was measured in two different methods - namely, the essential work of fracture and the Tryding stress-widening method. Figs. 4 and 5 show the results of two fracture energy mea- surement methods at different dryness levels. We used the specific fracture energy, which is the frac- ture energy divided by its density, instead of frac- ture energy to mitigate the effects of paper sample density variation. There were no major differences between two fracture energy measurement meth- ods with respect to curve shapes; however, minor differences were observed in the magnitude of the measured values. Two conspicuous conclusions can Fig. 1. Breaking length and stretch differences for the softwood by curl treatment (95% dryness).

Fig. 2. Breaking length and stretch differences for the hardwood by curl treatment (95% dryness).

Fig. 3. Effects of fiber curl treatment on stretch

increase for softwood and hardwood

(95% dryness).

be made. One is that the curl-treated fiber fur- nishes yield higher fracture energy values than the untreated ones. The other is that the fracture en- ergy is highest at 80% dryness level. This behavior has already been reported in a previous study.

The case of the softwood fiber furnish refined for 30 minutes was exceptional, where the curl treat- ment caused less fracture energy than the untreat- ed one and the stretch at that refining level was not improved considerably either (see Table 1 and Fig. 3). Therefore, it appears that, for long-length fibers such as softwood fibers, curl treatment may not be so effective as for short fibers (hardwood fibers, in this case) in terms of improvement of fracture energy.

3.3 Fracture energy and stretch

Fracture energy has been reported to have a pos- itive relationship with stretch.

Fig. 6 and Fig. 7 show a closely positive relationship between the two factors again for the furnishes without curl treatment. Their regression coefficients (R) were over 0.9.

However, for the curl-treated fiber furnishes, their regression coefficients were below 0.5. The stretch of the curl treated furnish was increased to a narrow range of 4.0-5.5% while that of control furnish was 1.0-5.0%. We believe the variation of fracture energy measurement was high enough to give high regression coefficient for the curl treated furnish.

Fig. 4. Specific fracture energy measured at different dryness levels by essential work of fracture.

(a) Softwood (b) Hardwood

Fig. 5. Specific fracture energy measured at different dryness levels by Tryding stress-widening method.

(a) Softwood (b) Hardwood

4. Conclusions

The curl treatment using a Hobart mixer was ap- plied to the bleached kraft chemical pulp (softwood and hardwood) to find out its effect on fracture energy. The fracture energy of two different fiber furnishes with three refining levels were measured at four different dryness levels (40%, 60%, 80%, and 95%). Two different fracture toughness mea- surement methods-namely, the essential work of fracture and the Tryding stress-widening meth- od -were used in the measurements. Based on the results of the present study, the following conclu- sions can be made:

* The essential work of fracture and the Tryding stress-widening method gave identical fracture energy values for different fiber furnishes and at various wet web dryness levels.

* The fiber curl treatment followed by refining treatment resulted in stretch increase.

* The fiber curl treatment also increased fracture energy. A positive relationship between stretch and fracture energy was observed.

* The fracture energy and stretch curves gave maximum values at ca. 80% dryness levels at different refining degrees.

Literature Cited

1. Park, J. M. and Thorp, J. L., Yield and fracture of paper, Journal of Korea TAPPI 31(5):57-72 (1999).

2. Page, D. E., Seth, R. S., Jordon, B. D., and Barbe, M. C., Curl, crimps, kinks and micro- compressions in pulp fibres - Their origin, measurement, and significance, In Paper- making Raw Materials, Transactions of the 8

Fundamental Research Symposium, Vol.

1, Mechanical Engineering Publications Ltd., London, pp. 183-227 (1985).

3. Seth, R. S., Optimizing reinforcement pulps by fracture toughness, Tappi J. 79(1):170-178 (1996).

4. Roh, Y. S. and Seo, Y. B., Evaluation of frac- ture toughness of paper in wet state (I) - Ef- fect of fiber furnish and refining, Journal of Korea TAPPI 48(4):62-70 (2016).

5. Seo, Y. B., Choi, C. H., Seo, S. W., Lee, H. L., and Shin, J. H., Fiber property modification by mechanical pretreatment, Tappi J. 1(1):8-13 (2002).

6. Seth, R., Plane stress fracture toughness and its measurement for paper, In Products of Papermaking, Transactions of the 10th Fun- damental Research Symposium, Vol. III, Bak- Fig. 6. Fracture energy vs. stretch relationship

for softwood and hardwood at 80%

dryness.

Fig. 7. Specific fracture energy vs. stretch

relationship for softwood and hardwood

at 95% dryness.

er, C. F. (ed.), Pira, UK, pp. 1529-1560 (1993).

7. Mai, Y. W. and Cotterell, B., On the essential work of ductile fracture in polymers, Int. J.

Fract. 32:105-125 (1986).

8. Mai, Y. W., Cotterell, B., Horlyck, R., and Vigna, G., The essential work of plane stress ductile fracture of linear polyethylenes, Polym.

Eng. Sci. 27(11):804-809 (1987).

9. Seth, R., Robertson, A. G., Mai, Y. W., and Hoffmann, J. D., Plane stress fracture tough- ness of paper, Tappi J. 76(2):109-115 (1993).

10. Paton, C. A. and Hashemi, S., Plane-stress essential work of ductile fracture for polycar- bonate, J. Mater. Sci. 27:2279-2290 (1992).

11. Wu, J., Mai, Y. W., and Cotterell, B., Fracture toughness and fracture mechanics of PBT/PC/

IM blend: Part 1. Fracture properties, J. Mater.

Sci. 28:3373-3384 (1993).

12. Tryding, J. and Gustafsson, P. J., Character-

ization of tensile fracture properties of paper,

Tappi J. 83(2):84-89 (2000).