BEATER CURVES OF A PULP
Introduction
Laboratory evaluation of a pulp aims to characterize the pulp's papermaking quality and strength potential. TAPPI standard T 200 gives a detailed procedure that, in the main, will be followed here except for certain simplifications. In essence, this procedure is intended to establish the "beater curves" for the pulp. This is done by beating a pulp sample in a small laboratory beater known as the Valley beater, for different time intervals, with samples withdrawn after each interval for the determination of Canadian Standard Freeness (CSF) and the preparation of laboratory handsheets. In addition to the apparent density, the most important strength properties, like tensile strength, bursting strength, tearing resistance, and folding endurance are measured on these handsheets along with any other properties that may be of interest. Similar measurements are also made on the unbeaten pulp, corresponding to zero beating time. Finally, one plots each property against CSF to obtain the "beater curves" for the different properties of the pulp. Inspection of these curves and comparison with similar data obtained on a variety of pulps in the past can tell the experienced papermaker much about the most appropriate uses of the pulp in question.
The procedure is also fraught with certain difficulties, and one of its most outspoken and persistent critics has been the late James d'A. Clark who summarized his arguments in his well-known book (see references). His chief criticisms are two. The first relates to the Valley beater as being notoriously difficult to keep in a standardized condition (standardization is of course absolutely necessary if consistent and reproducible results are to be obtained); he also points out that the kind of beating action the Valley beater imparts to a pulp is quite different from that of typical refining machines used in the mill. His second criticism concerns the concept of CSF. He presents persuasive evidence to show that CSF is not only useless as a predictor of the drainage rate of a pulp on the paper machine but is equally useless as a predictor of any paper property. He suggests various alternatives that will, however, not be considered here.
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In partial recognition of these criticisms, the most recent versions of TAPPI standard T 200 acknowledge that CSF has meaning only if the same type of beating apparatus is used for all tests. This means, among other things, that CSF-strength relationships of a pulp will in general be quite different if obtained in the laboratory with different beaters, and especially if obtained with refining equipment in the mill. It also implies that a pulp beaten to the same CSF in different beaters or refiners will in general have quite different characteristics. All these statements are amply supported by experimental evidence.
Of what value, then, are the beater curves of a pulp to the practical papermaker? On the negative side, examples are known of naive and overconfident reliance on beater curves leading to decisions that were soon found to be wrong. On the positive side, the beater curves enable an experienced papermaker, who is aware of the pitfalls and exercises proper caution, to draw useful conclusions about the pulp.
Outline
Most of the work is carried out in the "Pulp Testing Laboratory", Room 1118.
Each task group consists of four students and receives either a hardwood, a softwood or will be given both types of pulp to obtain a 50% hdw/50% swd blend. Each group will obtain the consistency of the pulp according to the consistency worksheet provided. This must be done a day or two before the lab session. The task is to determine the beater curves for apparent density, tensile index, tensile energy absorption (TEA), burst index, tear index, and folding endurance of their pulp by the procedures outlined in the following sections. These procedures are similar but not identical in all respects to TAPPI standard T 200.
Economy of time is essential if the entire task is to be accomplished in one afternoon. A particularly critical step is the determination of OD consistency of the pulp, requiring drying of pulp samples overnight in an oven. Each team should designate a person who will place the pulp samples in the oven on the afternoon preceding the actual laboratory period.
The entire task of pulp evaluation can, for convenience, be broken down into the following eight jobs:
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1Determine the OD consistency of the pulp(s). The wet pulp must be placed in the drying oven on the afternoon before the laboratory period. See the section on "Determining the OD Consistency of the Initial Wet Pulps".
2Set the Valley beater up for the beater run. The corresponding procedure is outlined in the section on "Furnishing the Valley Beater".
3Make the actual beater run; this consists of removing the weight periodically and withdrawing pulp samples for CSF measurement and handsheet making; of restarting the beater and estimating the time for the next stop; and of keeping accurate records of the total beating time elapsed. The goal is to collect five beaten pulp samples at intervals of beating time so chosen that the last or fifth sample has CSF slightly less than 300, about 290; and that the CSF range from this low of 290 to the high of pulp sample is divided into roughly equal CSF intervals. Follow the procedure in the section on "Beater Run".
4Measure CSF on these pulp samples according to the section on "Measurement of Canadian Standard Freeness (CSF)".
5Make laboratory handsheets as described in the section on "Forming Handsheets for Physical Tests of Pulp".
6Test the handsheets in the Paper Testing Laboratory according to the section on "Physical Testing of Pulp Handsheets".
7Plot all properties tested against CSF. Specifically, in these graphs CSF is plotted along the horizontal axis (abscissa), decreasing from left to right; and each property is plotted along the vertical axis (ordinate).
8Prepare a report; see report ! instructions for what should be included in the report and which groups data your group will compare with..
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Determining the OD Consistency of the Initial Wet Pulps
•COD is determined in Room 1107. From your assigned pulp(s), fill (3) preweighed (tared and recorded) with a representative sample of your pulp. Determine the wet pulp weight by reweighing on the Mettler PE 600 balance.
•Place the beakers with the pulp samples in the Fischer Isotemp Oven (200 Series, Model 255 G) at 105C. Leave them in the oven overnight.
•The next day, quickly remove one of the beakers from the oven and place it on the balance. The balance soon displays a stable reading. Note that reading. Continue in this manner and weigh all tins. Subtract the tare weight of the tins from the weights just determined to obtain the OD weights of your pulp samples.
•Calculate the oven-dry consistencies according to COD = S/P, where S is the solids weight or OD weight of a pulp sample, and P is the weight of the wet pulp sample.
•Calculate the average COD from your three determinations and report these mean values.
Tin IDTare weight
Wet wt +
Tare
Wet wt – tare
Dry wt. + tare
Dry wt. – tare
Consistency
(o.d./a.d.)
x 100= %
Furnishing the Valley Beater
•Weigh the equivalent of 360 ± 5 g OD of the original wet pulp.
•Before furnishing the beater, remove the weight on the lever-arm, fill the tub with 12 L of water, and start the motor.
•In a graduated bucket, slush the pulp with water to a volume of 11 L and slowly add it to the beater. At this point, the beater stock has COD = 1.57%.
•Allow about 10 min for the pulp to become thoroughly dispersed and disintegrated.
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Beater Run
•When the pulp is well dispersed and disintegrated, do the following:
• Withdraw a 190 mL sample for a CSF measurement and an additional 1275 mL sample for making 10 handsheets. Dilute the 190 mL sample with water to 1000 mL for a consistency of COD = 0.3%. Determine the CSF according to the section on "Measurement of Canadian Standard Freeness (CSF)". Transfer the pulp in the freeness tester to a ¾ filled sheet mould to measure the consistency. Fill to the line. DO NOT MIX! Drain and couch off . Place on the speed dryer to dry. Weigh and adjust the freeness if the consistency of not 0.3% .
Pour the 1275 ml sample into a bucket containing 5392 mls of water. This is also 0.3% consistency. See section “Forming Handsheets for Physical Tests of Pulp”
•The resulting data are those on the unbeaten pulp. For the purpose of further discussion, call the Canadian Standard Freeness of this unbeaten pulp CSF0. On graph paper, make a plot of CSF versus beating time. At beating time zero, mark the point for CSF0.
•To establish your beater curves, your aim is to obtain, in addition to the unbeaten point, 4 (normally 5) beaten strength points at approximately equal freeness intervals during the beater run. The final point should be at a freeness of about CSF = 290. A little thought will show that the equidistant target values of freeness you should aim for in your beaten samples are
as follows: CSF1 = 0.8 CSF0 + 58 (do not do for this class)
CSF2 = 0.6 CSF0 + 116
CSF3 = 0.4 CSF0 + 174
CSF4 = 0.2 CSF0 + 232
CSF5 = 290
Thus, having measured CSF0, you should now calculate these four CSF levels to aim for in your beater run.
•Next, add the standard weight to the lever-arm of the Valley beater and start to time the beater operation.
•The beating characteristics of the unknown pulps are not known, and so the withdrawal schedule must be determined by trial and error as follows. After 3 min of beating, stop the beater and the stopwatch.
• Take a 190 mL sample for CSF measurement. Enter the new point in your plot of CSF versus beating time. Use this plot as a guide for estimating withdrawal times.
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•Restart the beater and the stopwatch. Continue beating for an additional time that you estimate will get you close to your first target freeness CSF2. Stop the beater and stopwatch. Withdraw a 190 mL sample and determine the CSF as above. If you are within ±10 freeness units of target, you are close enough; withdraw an additional 1275 mL for handsheet making.
•Restart the beater and stopwatch. Continue in this way for a total of 4 beaten samples for both CSF measurement and handsheet making. As mentioned above, aim for a CSF value of 290 in your 4th beaten sample.
Also remove another 1275 at the 350~400 freeness level for your “poorly” made handsheets. (see instructions at the end of this chapter)
Measurement of Canadian Standard Freeness (CSF)
•The freeness test requires a pulp suspension in water of volume 1 L and of OD consistency COD = 0.30 ± 0.02 %. Ideally, the temperature of this specimen should be 200 C, but TAPPI standard T 227 provides a table of freeness correction terms in case the temperature is off.
•Make sure the CSF tester is clean. Place the drainage chamber on its supporting bracket. Secure the lower lid. Open the air-cock in the upper lid. Place a graduated cylinder of capacity 1 L and with 10 mL or smaller divisions in position to receive the discharge from the side orifice.
•Fill the thoroughly mixed and homogeneous stock into a clean 1 L cylinder. Take its temperature to the nearest 0.50 C. Mix the sample in the cylinder by covering the opening with your hand and turning the cylinder upside down three times.
•Pour the sample gently but as rapidly as possible into the chamber of the CSF tester.
•As soon as the stock has become motionless, close the top lid; close the air-cock (lever lies horizontally); open the bottom lid; and open the air-cock (vertical) to start the flow.
•Observe the discharge from the side orifice. When the discharge has ceased, record the total volume discharged, in mL. This is the Canadian Standard Freeness (CSF) as measured.
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•Use Table 1 to adjust the observed CSF to the standard temperature of 200 C.
Measure the consistency of the freeness as you did for csf0 and adjust freeness according to Table 2.
Forming Handsheets for Physical Tests of Pulp
•The pulp sample withdrawn from the beater for handsheet making has volume 1275 mL, COD = 1.57%, and therefore 20 g solid fiber, enough for 10 standard handsheets of 1.2 g each.
•Pour the sample into bucket containing 5392 mls of water. Your stock is now 0.3%, the consistency required for handsheet making.
•Important: Before dividing the stock up into 410 ml portions, mix well by stirring vigorously. The intent is to eliminate flocculation as much as possible, thereby obtaining 1.2od handsheets.
•Measure out 410 mL of stock into each of 5 –500ml graduated cylinders for making 5 handsheets. Repeat for a total of 10 handsheets.
•Open the handsheet mold by unscrewing the clamp and tilting the hinged cylindrical deckle backwards until it touches the sheetmold rest behind the handsheet mold. Turn on the water. Gently rub the surface of the wire with the fingers to clear away any adhering fibers.
•Close the handsheet mold and half fill with water. Pour one of the 410 mL stock samples into the mold. Add water up to the mark inscribed around the inside of the cylinder.
•It is extremely important to acceptable handsheet quality and meaningful test data to stir the stock in the cylinder before draining. Proceed as follows. Insert the stirrer, a perforated metal plate, into the stock. Move the stirrer up and down several times at moderate speed, without introducing air bubbles into the stock. During this stirring action, be careful about the following two points: On the downstroke, the stirrer should not touch the wire; and on the upstroke, the perforated disc should always remain beneath the level of the
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•After you have withdrawn the stirrer from the stock, quickly open the drain cock of the handsheet mold. The drain cock is the long handle about 16 inches off the floor. Pulled all the way up the drain is closed. Push it down to open drain.
•Immediately after the water has drained from the sheet, open the cylinder to release the suction.
•To couch the sheet off the wire, proceed as follows. Place three pieces of standard blotting paper squarely on the drained sheet, with the smoother side (felt side) of the lower blotter against the wet sheet. Lay the flat brass couch plate squarely on the blotters. Place the brass couch roll very gently on the middle of the plate. Carefully move the roll away from you, without applying additional downward pressure. Move it close to but not beyond the far edge of the plate. Prevent the plate from tilting by placing your fingers on the front edge. Next, move the roll toward you and to the front edge. Continue this back and forth movement for a total of 4 cycles, ending in the middle. Lift the roll.
•Remove the pulp sheet, blotters, and couch plate together from the wire much like you open the cover of a book. The sheet will be found to stick to the blotter that is in direct contact with it; this is known as the couch blotter.
•Immediately remove the couch blotter together with the attached wet handsheet from the other blotters, the so-called couch filler, and the couch plate. Discard the couch filler.
•Place the couch blotter, handsheet side up, on a fresh blotter called the press filler. Place them together centrally on the press plates using the press template. On the extreme edge of the handsheet, in small area, write group number and elapsed time sample was drawn from beater. Cover the handsheet with a polished metal plate, polished side down. Lay a fresh blotter on top of the metal plate. This stack is now ready to receive the next couch blotter and handsheet.
•Continue placing the remaining 9 sheets on this stack for a total of 10 sheets, and finally lay a single blotter on the uppermost plate. The stack is now ready for pressing.
•For pressing, use the TAPPI handsheet press. Place the cover of the press in position and screw the wing nuts hand-tight. Start the press by turning the switch to "1". The press will shut itself off after 5 ½ min. Now, turn the switch to "2". The press shuts off after about
2 1/2 min. Remove the press cover. The sheets are now ready for drying.
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•To dry the sheets, take them to the Paper Testing Laboratory, Room 1228. Fit a metal plate with the sheet sticking to it into a drying ring, sheet side up. Place the next drying ring, with its rubber against the sheet, on top of the first. Continue until all 10 sheets have been placed into the drying rings. Finally, clamp the whole pile of rings together with a clamping device. This is extremely important! Clamping prevents the handsheets from shrinking as they dry. Any shrinkage would change the sheet properties significantly.
•Leave the handsheets in the drying rings overnight for drying. After that time, the sheets can be removed from the drying rings and tested for the different physical properties.
•Be sure to remove your handsheets from the drying rings early the following morning because the rings are needed by other teams in the afternoon.
Physical Testing of Pulp Handsheets