Fiber Quality Analysis: OpTest Fiber Quality Analyzer versus L&W

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Fiber Quality Analysis: OpTest Fiber Quality Analyzer versus L&W Fiber Tester Bin Li,* Rohan Bandekar, Quanqing Zha, Ahmed Alsaggaf, and Yonghao Ni

Ind. Eng. Chem. Res. 2011.50:12572-12578. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/21/19. For personal use only.

Department of Chemical Engineering and Limerick Pulp and Paper Centre, University of New Brunswick, P. O. Box 4400, Fredericton, NB, Canada E3B 5A3 ABSTRACT: In this paper, the measurements from OpTest FQA (fiber quality analyzer) and L&W Fiber Tester were compared on 95 TMP (thermomechanical pulp) samples from a TMP-based newsprint mill in Eastern Canada that uses mixed black spruce and balsam fir as raw material. It was found that both FQA and L&W Fiber Tester provide repeatable measurements for fiber length, fines, coarseness, curl, and kink index. However, there were significant differences in the results obtained from the two instruments. Compared to FQA, the L&W Fiber Tester measurements were on average 59% higher for the arithmetic mean fiber length and arithmetic fines content, about 6% higher for the length weighted mean fiber length, approximately 44% lower for the coarseness, and about 37% lower for the kink index.

1. INTRODUCTION In pulping and papermaking processes, the fiber properties and fiber morphology, such as fiber length, width, coarseness, curl, and kink, change during mechanical or chemical treatments. These changes can greatly affect the quality and end-use performance of the products.1
5 Therefore, reliable measurements of fiber properties are of critical importance for quality control.6 There are a number of techniques to measure fiber properties: for example, a set of TAPPI Test methods describe detailed procedures of using the Bauer-McNett fiber classifier for fractionating fibers and determining the weight percentage of each fraction;7 subsequently, the fiber length can be further measured by projection8 or using an automated optical analyzer.9 The measurement of fiber length and other morphological properties based on optical analyzers, particularly from the Kajaani units (FS-100, FS-200, and FS-300), have been well-reported in the literature.10
13 The primary principle of these systems is to allow a dilute pulp suspension to go through a flow cell. A light source illuminates the flow for detection of fibers using a camera. The concept of the combination of a flow cell (one-dimensional capillary tube or two-dimensional plate channel) and the image analysis in an automated manner is used for fiber characterization. On the basis of this concept and the rapid development of digital image analysis technology, several new fiber analyzers have been developed and brought to market by different companies. These new instruments, such as PQM (pulp quality monitor), Galai CIS-100, Fiberlab, MorFi, FiberMaster, FQA (fiber quality analyzer), and L&W Fiber Tester (FT), provide fast measurements with the capability of both laboratory and online analysis. However, the measurement differences among these instruments are expected due to the different designs of hardware and software. Although comparisons of some of this equipment have been reported,10,14
17 results from FT are scarcely published, and there are no systematic studies available in the literature comparing the results between FQA and FT. In this paper, we aimed to provide some direct comparison of the results obtained from a large body of pulp samples measured r 2011 American Chemical Society

by FQA and FT. FQA is a common instrument in North America, and the latest development leads to a new highresolution (HiRes) FQA. Over 150 units of FQA have been sold around the world so far. This includes 84 HiRes FQA units, 18 of which were upgraded from the “Classic FQA (code LDA96)”, while FT (over 130 units have been sold) is a new advanced instrument for fiber analysis and its predecessor is FiberMaster, which has similar technologies with FT. Both FQA and FT, like other optical fiber analyzers, are developed on the basis of the combination of a flow cell and the image analysis, but they do have some unique features. The FQA comprises hydraulic, optical, and image-processing systems, with a special sheath flow cell design aiming to resist fouling and the formation of deposits.18,19 Three layers of current go through the cell. A dilute suspension of pulp fibers travels up the cell in the thin middle layer of the current. The two sheath currents, which are clean and mineral free water, are situated on both sides and maintain the fibers in the middle current which is 0.5 mm thick. This keeps the fibers at the correct focal length from the camera. In the tapered region of the cell, preceding the imaging region, fibers are gradually oriented and positioned by the flow fields imposed by the cell taper.18,20 The sheath currents also keep the inner walls of the cell clean. The classic FQA can measure fiber length, coarseness, curl, kink, and fines content, while the new HiRes FQA with a higher pixel resolution (14  7 μm2) can also simultaneously measure the fiber width, shive content, and vessel elements. FT uses two-dimensional imaging technology. The measuring cell is comprised of two glass plates with a very small distance (0.5 mm, according to ISO standard 16065-2), thus limiting the movement of fibers in one direction. This can create a true projection of the fiber length and fiber deformations. The cleaning is Received: July 27, 2011 Accepted: September 27, 2011 Revised: September 23, 2011 Published: September 27, 2011 12572

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Table 1. Major Features of FQA and FT

a

instrument

camera optical resolution, μm2

light source

FQA

35  35

circular polarized

0.75
2.0

g5000

0.07
10

FT

10  10

nonpolarized

100

g20000

0.05 a
7.5

sample size, mg

no. of fibers analyzed

length range limits of tested fibers, mm

The minimum length detection used in this study is 0.05 mm, and it could be lowered to 0.01 mm.

Figure 1. Fiber images from FQA (a) and from FT (b).

automatic so that it is done after each analysis, which is effective in preventing residual fibers and fiber particles from clogging or fouling. During the cleaning process, the gap between the cell plates is increased to 3 mm, having the maximum effect.21 The water used for FT is clean water (free from particles greater than 5 μm). FT (code 912) measures the fiber length, width, fines, shape, and coarseness. Other parameters, including kink, vessel cells, minishives, fiber species content, fiber distribution, and classification can also be added. Table 1 summarizes the major features of the FQA (code LDA96) and FT (code 912). It is seen that FT has higher optical resolution (10  10 μm2) than FQA (35  35 μm2). Furthermore, FQA uses circular polarized light, whereas nonpolarized light is used by FT. Air bubbles may be detected as fines in nonpolarized light. Thus, FT uses vacuum to remove air before the measurement. Circular polarized light can measure twodimensional objects, such as curled fibers, while linear polarized light can only measure a one-dimensional object. Circular polarized light can measure fiber ends more accurately than linear polarized light, but polarized light may detect less fines and fibers than nonpolarized light. In addition, the minimum length detection of FT is lower (0.05 mm) than that of FQA (0.07 mm), and the sample size of FT is clearly larger compared to FQA (for FQA analysis, the sample consistency is about 2 mg/L for softwood samples and 0.75 mg/L for hardwood samples, while the sample consistency is approximately 1 g/L or more for FT). Examples of one fiber image from FQA and FT are illuminated in Figure 1a,b, respectively. Please note that both the length ranges and fiber counts can be changed at the discretion of the operator. In Eastern Canada, black spruce (Picea mariana (Mill.) B.S.P.) and balsam fir (Abies balsamea (L.) (Mill.)) are the most popular species for producing TMP-based (thermomechanical-pulpingbased) newsprint.22,23 Certainly, monitoring the development of TMP quality is an integral part of the mill’s strategy to improve the overall mill performance. There is a strong interest in comparing the results on fiber quality obtained from FQA and L&W Fiber Tester.

2. EXPERIMENTAL SECTION Pulp samples were collected from a TMP-based newsprint mill in Eastern Canada. The mill uses a mixture of black spruce and

Figure 2. Trends of the spruce content and CSF (95 samples).

balsam fir. The samples were collected at the primary cleaner accepts of a two-stage TMP line. Thus, samples were already screened and cleaned with latency removal. The wood species content was determined on an online chip sensor (FPInnovations). All samples were collected for a period of 7 days. A classic FQA (code LDA96, OpTest Equipment Inc.) and an L&W Fiber Tester (FT, code 912, Lorentzen & Wettre Co.) were used in this study. Before testing, samples were disintegrated. The whole pulp for each sample was measured on FQA and FT. Both FQA and FT were calibrated and used according to the manufacturers’ specifications.21,24 In addition, CSF (Canadian Standard Freeness) for each pulp sample was measured following the TAPPI Test method: T 227 om-99. The results on the spruce content and CSF for all pulp samples have been shown in Figure 2. It can be seen that there are fluctuations of both the spruce content and freeness, although for the first 32 samples (collected during the first 2 days) they were rather constant. In total, 95 pulp samples were analyzed for the fiber length, fines, curl, and kink index, by the two instruments. The fiber coarseness was not measured for the first 32 pulp samples, and only done on the last 63 samples. A total of 10 replicates of sample no. 86 were analyzed for the determination of repeatability. For the analysis of other samples, two or three replicates were tested and the average was reported for each sample.

3. RESULTS AND DISCUSSION 3.1. Fiber Length. During a typical mechanical pulping, fibers are separated from the ordered wood configurations and then further developed with the generation of fibrils and fines, leading to the reduction of the fiber length. The average fiber length is an important fiber property that influences the paper sheet strength but can also have a large effect on the sheet formation.1,2,25 Thus, 12573

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Figure 3. Comparison of arithmetic mean fiber length between FQA and FT (95 samples).

Figure 4. Comparison of length weighted mean fiber length between FQA and FT (95 samples).

the original function of optical fiber analyzers was to measure fiber length. The common fiber length measurements of FQA and FT are arithmetic mean, length weighted mean, and weight weighted mean as defined by eqs 1, 2, and 3, respectively. 20,26

∑i ni li ln ¼ ∑i ni

ð1Þ

∑i ni li 2 llw ¼ ∑i ni li

ð2Þ

∑i ni li 3 lww ¼ ∑i ni l2i

ð3Þ

Fibers are grouped into various length classes, and ni is the number of fibers in the specified length class li. The presence of fines will significantly affect arithmetic mean, and it tends to have smaller impact on the length weighted mean, whereas its influence on the weight weighted mean would be the least among the three parameters.24 The comparison of arithmetic, length weighted, and weight weighted mean fiber length between FQA and FT is given in Figures 3
5, respectively. It can be seen from Figure 3 that FT measurements provided systematically higher arithmetic mean than FQA (by an average of 59%). Such a difference may be due to the difference in the design of the optical measurement systems: FT, which has higher camera resolution and a nonpolarized LED (light-emitting diode) light source, can detect the fines and fiber ends (tails) more accurately than FQA. As shown in Table 1, the minimum of fiber length detection for FT is 0.05 mm which is lower than that for FQA. In particular, the measurement difference could be larger for short fibers. Although FT detects more fines, the length of fines tested by FT could also be longer than FQA measurements.

Figure 5. Comparison of weight weighted mean fiber length between FQA and FT (95 samples).

As mentioned above, in the calculation of length weighted mean and weight weighted mean, the influence of fines is decreased. As shown in Figure 4, the differences of length weighted mean between the two instruments were smaller compared to the results given in Figure 3, but the results from FT were still on average about 6% higher than those from FQA. However, the FT and FQA measurements for the weight weighted mean fiber length are in good agreement for all samples, as presented in Figure 5. On the other hand, as illustrated in Figures 3
5, the correlation of arithmetic mean is not as good as the length weighted and weight weighted mean, which is also due to the impact of fines. It is noted that some samples in Figures 3
5 exhibited higher fiber length, which can be explained by the higher spruce (lower fir) content in the wood furnish (shown in Figure 2), while the lower 12574

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Figure 6. Comparison of arithmetic fines content between FQA and FT (95 samples).

fiber length can be explained by the lower spruce (higher fir) content in the wood furnish. For example, the length weighted mean fiber length of sample no. 43 with the spruce content of 70% is 1.70 and 1.59 mm for FT and FQA, respectively, while the corresponding results on sample no. 68 with the spruce content of 40% is 1.46 and 1.37 mm for FT and FQA, respectively. This is because balsam fir could generate much more fines at a given refining energy than black spruce,27,28 although the average virgin fibers in balsam fir wood are only about 10% shorter than those of black spruce.29 Therefore, a higher spruce content in the furnish tends to give a longer mean fiber length, and a higher CSF as well. As shown in Figure 2, the trend of CSF basically follows the trend of spruce content, although other parameters may also affect the relationship. In the pulp and paper industry, for optical methods, the length weighted mean fiber length is most widely used.24 Lanouette and Law27 reported the length weighted mean fiber length of FQA results is in the range of 1.7
2.1 mm for pure black spruce TMP and 1.4
1.6 mm for pure balsam fir TMP (at the CSF level of 100 mL). Omholt et al.30 found that the length weighted mean fiber length from FQA is 1.78 ( 0.07 and 1.32 ( 0.09 mm at 100 mL of CSF for pure black spruce TMP and pure balsam fir TMP, respectively. 3.2. Fines. In mechanical pulps, fines are not only essential for the improvement of strength properties31,32 but also critical for the optical properties, surface properties,33,34 and other end-use properties, such as interactions with OBA (optical brightening agents) and dyes.35,36 Fines are usually defined as the fraction of the pulp passing through a 200 mesh screen in a Bauer-McNett classifier,37 while optical fiber analyzers typically define fines as objects that are less than 0.20 mm in length and the results are reported as a percentage based on an arithmetic basis or length weighted basis. The arithmetic fines content is the number of fines divided by the total number of fibers (fines included) multiplied by 100, while the length weighted fines content is the sum of the fines length divided by the total length of fibers and fines in the samples.17,24 Figures 6 and 7 present the comparison of the arithmetic fines and the length weighted fines contents obtained from FQA and FT. As can be seen from Figure 6, the arithmetic fines content measured by FT was higher than FQA measurements (by an average of 59%), which is in agreement with the difference of the

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Figure 7. Comparison of length weighted fines content between FQA and FT (95 samples).

Figure 8. Comparison of coarseness between FQA and FT (63 samples).

results of arithmetic mean fiber length in Figure 3. Again, the difference can be explained by the fact that the arithmetic results are sensitive to the number of fines.20,38 As shown in Figure 7, the difference of length weighted fines value between FT and FQA is smaller than the arithmetic fines results. Furthermore, it can also be seen from Figures 6 and 7 that the samples with the sample ID between 60 and 70 have significantly higher fines content, because these samples have a higher fir content and fir can lead to the generation of more fines. 3.3. Coarseness. Coarseness is a very important fiber characteristic related to tear strength39 and also to some extent the optical properties.34 The decrease in fiber coarseness may be small in the commercial pulp refining since the actions of the commercial refiners are more perpendicular than tangential.40 The fiber coarseness is defined as the mass (m) of the oven-dried weight of pulp by the total measured length of the fibers.12,41 In practice, the total measured length of the fibers is calculated by arithmetic mean length (l) of the fibers multiplied the total number (n) of fibers in the mass of fibers.20,24,41 The analyzer 12575

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Figure 9. Difference between the true contour fiber length (L) and the projected fiber length (l).

provides the l and n. The accuracy of coarseness depends on the accuracy of the measurement of the three factors. The comparison of coarseness between FQA and FT is given in Figure 8. It is seen that the fiber coarseness results obtained from FT were smaller than FQA by an average of approximately 44%. This is due to the fact that (1) the arithmetic mean fiber length measured by FT is systematically higher than that measured by FQA (Figure 3) and (2) the total number of arithmetic fines, thus, fibers, measured by FT in the same pulp mass may be greater than the FQA measurements. In addition, the correlation of coarseness between the results obtained from the two instruments is not good, to a large extent depending on the effect of fines. 3.4. Curl Index. Typically, fiber shape is reported as a mean curl or kink index.3,42
44 Fiber curl describes the deviation from straightness of the fiber axis. Fibers, which are straight in wood, become curled during pulping, mixing, and refining through exposure to bending and axial compressive stresses, particularly at the medium and high consistency conditions.20 The fiber curl index (CI)18,20,24 and the shape factor (SF)21 are defined in eqs 4 and 5, respectively: CI ¼

L
1 l

ð4Þ

SF ¼

l  100 L

ð5Þ

where L is the true contour fiber length and l is the projected fiber length. The difference between L and l is illustrated in Figure 9. One can find that, for each individual fiber, CI ¼ ð100=SFÞ
1

ð6Þ

However, the length weighted mean CI is the sum of the individual CI of each fiber multiplied by its contour length divided by the summation of the contour lengths.24 Hence, in practice, the approximate length weighted mean CI measurement for FT was converted from the corresponding SF by eq 6. Figure 10 shows the comparison of length weighted mean CI between FQA and FT (calculated from the SF). As can be seen, the CI from the FQA was much lower than that from FT (on average 59% lower). This may be due to the more accurate measurement of fiber ends by FT; longer fibers have a higher probability of having higher values of CI than shorter fibers. Another source of the difference between the two types of equipment may be the different designs of flow cell: the measurement gap of the flow cell for FT is 0.5 mm,45 smaller than that of FQA (total measuring gap of the flow cell is 3 mm). In addition, the CI is very low and the absolute difference in CI is

Figure 10. Comparison of length weighted mean curl index between FQA and FT (95 samples).

Figure 11. Comparison of mean kink index between FQA and FT (95 samples).

small for the two instruments, because the latency of collected samples was already removed in the mill process. 3.5. Kink Index. Kink is the abrupt change in fiber curvature. The most widely used definition of kink is Kibblewhite’s equation,24,43 which gives more weight on higher angles (severity kink), because it was found that higher angled kinks in fibers had a more negative impact on paper properties, i.e., tensile and tear, than lower angled kinks.46,47 Both FQA and FT use a modified form of Kibblewhite’s equation. The calculation of kink index (KI) for FQA and FT is given in eq 724,43 and eq 8,21 respectively. KI ¼

2Nð21
45Þ þ 3Nð46
90Þ þ 4Nð91
180Þ Ltotal

ð7Þ

KI ¼

2Nð20
50Þ þ 3Nð50
90Þ þ 4Nð90
180Þ Ltotal

ð8Þ

where N is the number of kinks with the specified range of kink angle. For example, N(21
45) is the number of kinks with the kink angle between 21 and 45°; L is the total fiber length (arithmetic length) of all of the fibers. 12576

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Table 2. Repeatability of FQA and FT a

a

length

weight

statistic items

arithmetic length

weighted length

weighted length

arithmetic fines content

weighted fines content

coarseness

curl index

kink index

RFQA

0.030 mm

0.034 mm

0.029 mm

1.813%

0.693%

0.007 mg/m

0.001

0.032 mm
1

MFQA

0.601 mm

1.541 mm

2.202 mm

48.0%

9.2%

0.287 mg/m

0.040

0.660 mm
1

length

%RFQA

5.0%

2.2%

1.3%

3.8%

7.5%

2.4%

2.5%

4.8%

RFT

0.020 mm

0.011 mm

0.033 mm

1.921%

0.562%

0.012 mg/m

0.002

0.003 mm
1

MFT

0.935 mm

1.610 mm

2.178 mm

74.5%

10.1%

0.154 mg/m

0.088

0.419 mm
1

%RFT

2.1%

0.7%

1.5%

2.6%

5.6%

7.8%

2.3%

0.7%

R, repeatability; M, mean value; %R, repeatability ratio.

The comparison of the mean KI results between FQA and FT is presented in Figure 11. It is seen that the KI from FT measurements were on average about 37% lower than that from FQA. This is partially due to the fact that the arithmetic fiber length given by FT is longer than that from FQA, as shown in Figure 3. It can be found from eqs 7 and 8 that the range of kink angle with the weight of 2 for FT (20
50°) is larger than that for FQA (21
45°), while the one with the weight of 3 for FT (50
90°) is smaller than that for FQA (46
90°). This means that FQA assigns the weight of 3 for the kinks with the kink angle range of 46
50°, while it is 2 for FT. Thus, it may also lead to a lower KI value from FT than that from FQA. In addition, as shown in Figure 11, FQA and FT give similar trends of KI, indicating a strong correlation. 3.6. Repeatability. Repeatability is an estimate of the maximum difference that is expected 95% of the time between two test results obtained under the same testing conditions and from the same homogeneous source of material. The results √
on the repeatability were calculated by multiplying 2.77 (1.96 2) with the sample standard deviation and then divided by the square root of the number of replicates.20,48 A lower value indicates a higher repeatability for each parameter. The repeatability of FQA and FT is given in Table 2, which shows that both FQA and FT can give good repeatability. In addition, the repeatability ratio can reflect measurement variability of each parameter for a given instrument, and it is presented in the percent of repeatability by mean value.48 As shown in Table 2, the length weighted fines content has the highest measurement variability for FQA, while coarseness has the highest repeatability ratio for FT.

4. CONCLUSIONS A total of 95 samples from a TMP-based paper mill in Eastern Canada was analyzed by a Classic OpTest FQA (fiber quality analyzer, code LDA96) and an L&W Fiber Tester (FT, code 912). The two instruments do have some unique features, in particular, with respect to optical resolution and light source. When compared with FQA, FT measurements were on average 59% higher for the arithmetic mean fiber length and arithmetic fines content, about 6% higher for the length weighted mean fiber length, approximately 44% lower for the coarseness, and about 37% lower for the kink index, while the curl index of FQA measurements was 59% lower than that from FT, which was calculated on the basis of the shape factor. In addition, there are good correlations of the results of fiber length, fines content, and kink index, between the measurements from FQA and FT. Both FQA and FT provide repeatable measurements, while it is noted

that FT deals with larger sample size and does not require further dilution during the analysis.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: (506) 453-3535. Fax: (506) 453-4767.

’ ACKNOWLEDGMENT The financial support for this project was from an NSERC CRD grant of the Government of Canada. The authors also thank (1) Ms. Cathy Joss, who is the technical representative of OpTest Equipment Inc., for providing the details of FQA and HiRes FQA, and (2) Mr. Fadi Nohra, who is the regional sales manager of L&W Co., for providing the detailed information of L&W Fiber Tester. ’ REFERENCES (1) Clark, J. d’A. Effects of fiber coarseness and length. Tappi J. 1962, 45, 628–634. (2) Kerekes, R. J.; Schell, C. J. Effects of fiber length and coarseness on pulp flocculation. Tappi J. 1995, 78, 133–139. (3) Mohlin, U.-B.; Alfredsson, C. Fiber deformation and its implications on pulp characterization. Nord. Pulp Pap. Res. J. 1990, 5, 172–179. (4) Mohlin, U.-B.; Dahlbom, J. Fiber deformation and sheet strength. Tappi J. 1996, 79, 105–111. (5) Eeth, R. S. The importance of fibre straightness for pulp strength. Pulp Pap. Can. 2006, 107, 34–42. (6) Heikkurinen, A.; Levlin, J.-E.; Paulapuro, H. Principles and methods in pulp characterization—Basic fibre properties. Pap. Puu (Pap. Timber) 1991, 73, 411–417. (7) Fiber length of pulp by classification; TAPPI Test Method T233 cm-95; TAPPI Press: Atlanta, GA, USA; 1995. (8) Fiber length of pulp by projection; TAPPI Test Method T232 cm01;TAPPI Press; Atlanta, GA, USA; 2001. (9) Fiber length of pulp and paper by automated optical analyzer using polarized light; TAPPI Test Method T271 om-02; TAPPI Press: Atlanta, GA, USA; 2002. (10) Bichard, W.; Scudamore, P. An evaluation of the comparative performance of the Kajaani FS-100 and FS-200 fiber length analyzers. Tappi J. 1988, 71, 149–155. (11) Jackson, F. Fibre length measurement and its application to paper machine operation. Appita J. 1988, 41, 212–216. (12) Dentley, R. G.; Scudamore, P.; Jack, J. S. A comparison between fibre length measurement methods. Pulp Pap. Can. 1994, 95, 41–45. (13) Gerard, J.; Ring, F.; Bacon, A. J. Multiple component analysis of fiber length distributions. Tappi J. 1997, 80, 224–231. (14) Hirn, U.; Bauer, W. A review of image analysis based on methods to evaluate fiber properties. Lenzinger Ber. 2006, 86, 96–105. 12577

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