CA1045866A - Bonding properties of mechanical pulps - Google Patents

Abstract

IMPROVING THE BONDING PROPERTIES OF
MECHANICAL PULPS

ABSTRACT OF DISCLOSURE

Process of producing a mechanical pulp of improved lint-ing properties by fractionating the mechanical pulp by means of a hydrocyclone into at least two fractions one fraction having an average specific surface less than a predetermined value between 1.2 and 4 m2/g and the second fraction having an average specific surface greater than the first fraction and subjecting the first fraction to mechanical processing thereby to form a processed fraction having an average spe-cific surface at least 2 m2/g larger than the initial average specific surface of said first fraction and recombi-ning said process fraction and said second fraction into a combined pulp.
The invention also relates to a method of determining specific surface distribution of fibres of the mechanical pulp by fractionating the pulp in a hydrocyclone system into a plurality of underflow fractions and a plurality of overflow fractions each of said underflow and overflow fractions containing a different portion of fibres of said sample and analysing each of said underflow fractions or of said overflow fractions or both to determine the specific surface of each of said selected underflow or overflow fractions.

Description

FIELD OF T~E INVENTION
The present invention relates to a method of improving the properties of a mechanical pulp and of paper made there-from. It relates particularly to the improvement of the bonding properties of such pulp and the resulting surface properties of such paper.
sackground of the Invention By mechanical pulps are understood pulps produced prima-rily by mechanical processing with or without auxiliary steps of a chemical or physical nature. Such pulps include conven-tional (stone) groundwood and refiner groundwood and pulps produced by an array of chemi-mechanical and thermo-mechanical processes. Such pulps generally have lower bonding properties than chemical pulps and conventionally have been used, e.g.
in the making of newsprint, with substantial admixtures of chemical pulp. The tendency has been to reduce more and more such admixtures of chemical pulp and even to use mecha-nical pulp alone. This trend has been greatly encouràged by the development of the more recent processes of thermomechan-ical and chemi-mechanical pulping and the improvements in the properties of the resulting pulps.
However, the surface properties of paper produced from furnishes of preponderantly mechanical fibers often present problems, particularly accentuated in connection with changes in printing technology, such as the growing acceptance of off-set printing. One such problem is ~linting" i.e. the phenomenon of fibers being picked out of the sheet in the process o~ printing and accumulating on the printing press.
Related phenomena are scuffing and dusting and also "picking~
in both, wet and dry webs; and in most cases where the term -"linting~ is used, it is meant to cover all these related phenomena. A high linting propensity of the paper or . ~', '~ . ,. . ' .

-newsprint will be a nuisance to the printer and in certain cases may cause the paper to be rejected by the customer.
The linting propensity ancl the associated other defi-ciencies of the paper are essentially a surface phenomenon and are due to the relatively weak bonding of and between certain fibers on or close to the surface of the paper. As such, the phenomenon is connected in some way with the type of paper machine, the condition of forming, draining and so on. But on any given machine there will still be differences between paper obtained at different times and these differ-ences are accounted for primarily by differences between the pulps used.
It has been proposed to reduce the linting of paper by applying adhesive materials to the surface of the paper, however, this procedure adds considerably to the expenses and is not always effective. It is of course, known to improve the bonding properties of fibres by beating or re-fining, with the attendant decrease in the freeness of the pulp. However, this method of improving bonding properties does not always solve the problem of linting. Furthermore, excessive refining of the pulp may degrade its properties and may also lengthen the drainage time beyond what is acceptable in the papermaking operation. It is also known that the bonding properties of a mechanical pulp are con-nected in a positive way (and conversely, the freeness value in a negative way) with the specific surface of the pulp, a property which is measurable and which expresses the ratio of the surface of the fibers to their weight (e.g.
in square metres per gram). However, pulps having substan-tially the same average specific surface often have differ- -ent linting properties and some pulps of quite low freeness give paper which still lints badly.

The specific surface may vary with the method by which it is determined. The specific surface as referred to herein is obtained using the Robertson ~ Mason method des-cribed in "Specific Surface of Cellulose Fibres by Liquid Permeability Method" Pulp & Paper magazine of Canada, page 103 December l9, 1949.
It is also known to separate from a pulp a "rejects"
fraction by means of a hydrocyclone, such fraction being gene-rally very small and consisting of the relatively unrefined particles, and to subject such "rejects" to further refining.
BRIEF DESCRIPTION OF THE INVENTION
We have found that the tendency to lint (insofar as it is caused by the characteristics of the pulp and not by those of the paper machine1 is connected not with the overall or ~average value of the specific surface of the pulp but with the fractional distribution of fibers of different specific surface in the pulp. The distinction is important. The same average value may result from a summation of values which are all close to the average but also of values which deviate -~ -.. . .
widely from the average toward both extremes. A pulp mayhave a high average specific surface, yet if it contains a substantial fraction of low specific surface, it will still have a tendency to lint. To overcome the linting propensity it i~ necessary that the pulp contain as small a fraction as possible of lOw specific surface.
- Accordingly, the present invention broadly provides a process for reducing the linting propensity of a mechanical pulp and the linting of paper made therefrom in which said pulp is fractionated into at least two fractions, one of said fractions having a specific surface lower than the other fraction, said fraction of lower specific surface is sepa-rated and subjected to mechanical processing thereby to increase the specific surface thereof, and the processed . . .

--` i045866 fraction is recombined with said other fraction.
The present invention more specifically provides a process wherein a mechanical pulp is fractionated into at least two fractions, one of said fractions having a selected average specific surface value between 1.2 and 4m2/g and the other fraction having a specific surface greater than said one of said fractions, said one of said fractions is subjected to mechanical processing thereby to increase the specific surface to above 4 and generally to not above 10 m2/g and said mechanically processed fraction is recomb-ined with said other fraction.
The present invention also broadly relates to a method of determining specific surface distribution of the fibre in a mechanical pulp by fractionating in a hydrocyclone system to provide a plurality of overflow and underflow fractions and analyzing at least some of said fractions to determine the specific surface of the selected fractions.
Brief Descriptions of the Drawings The invention will be further illustrated by means of the attached drawings in which Figure l-represents a schematic flow sheet of one embodiment of the invention integrated with a refiner ground mill operation, - Figure 2 represents the specific surface distribution of fibers in a pulp, the curve being a plot of specific surfaces versus weight fraction (in % of total pulp).
- Figure 3 represents the relationship between the linting propensity of a pulp and the presence in the pulp of a frac-tion of low specific surface;
Figure 4 shows a comparison between the specific surface distribution in a thermomechanical pulp before and after treatment in accordance with the present invention.

Figures 5 a, b, and c illustrate schematically methods of fractionating on the basis of specific surface by means of systems of hydrocyclones.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, a mechanical pulp, e.g. one produced from wood fragments in a refiner, is divided into at least two fractions, one being characterized by a low specific surface, and the fraction of low specific surface is passed through a second refiner. The fractionation of the pulp on the basis of specific surface i8 conveniently - achieved by means of hydrocyclones. These are familiar devices in the pulp and paper industry where they have been used for many years to remove from pulp a ~rejects" fraction, ~.
usually consisting of shives, fibre bundles and heavy particles. Usually the ~rejectsn from the cyclones are kept as low as possible; in mechanical pulp they rarely exceed about 10% of the pulp. Normally the material reject-ed by the cleaners in a conventional system would have an average specific surface of about .8 to 1, and would not exceed 1.2 m2/g.
Fractionation by means of hydrocyclones takes place on the basis of various physical and geometrical characteristics of the fibers but we have found that for mechanical pulps the predominant measurable characteristic which differentia-tes the "overflow" from the "underflow" fraction issuing from a hydrocyclone is the specific surface.~ (By "underflow~
fraction i8 understood the fraction issuing from the apex and the hydrocyclone). Accordingly, in the present invention, the pulp is passed into a hydroclyclone or a set of hydro-cyclones adapted to produce an underflow fraction having a specific surface not exceeding a certain value; the overflow fraction will then have a value higher than said value. The value of specific surface which is thus chosen as criterion ` ' .

may vary, depending on how exacting are the specification~
for the paper, the latter in turn depending, among othexs, on the method of printing to be used, the type of ink and so on. It has been found by studying lint obtained from a S blanket of a commercial offset printing press from a printing job of medium difficulty (two color, medium tack ink) that the lint fibres have an average specific surface of approxi-mately 2.5 m2/g. This average specific surface for lint fibres would change depending on the difficulty of the prin-ting job but normally is within a range of about 1 to 4 m2/g,.
In all cases individual lint fibres will be present having specific surfaces above and below the average value, however, the lint material will not be expected to contain significant numbers of fibres with specific surfaces more than about 1 m2/g above the average specific surface of the lint mate-rial for a given paper machine and printing operation since material of this increased specific surface would tend to bond into the sheet. Based on these findings it was deter mined that to improve the lint qualities of a paper or a pulp those fibres with the lower specific surface should be further worked by mechanically refining so that the amount of lint candidate material in the pulp for the specific application for which the paper is intended would be reduced to accepta-ble limits.
Hydrocyclones are generally set to fractionate a pulp into fractions according to a ratio of underflow to overflow fractions, such ratios being determined by the geometrical features of the hydrocyclone (relative sizes of input to output openings, cone angle etc.) as well as by conditions of operation (pressure, consistency). With the present in-vention the hydrocyclones will be set to produce an underflow fraction of an average specific surface in the range of 1.2 -4 m2/g preferably 1.5 - 4 m2/g is separated. The size . ' ' :

-. , ~ . :
- . . ~ .

- 8 - 1~866 of this fraction will necessarily vary with the conditions of pulping, type of wood, etc. In the case of refiner ground-wood and in the freeness range within which most refiner plants operate, the underflow fraction will amount to between - 5 15-70% preferably 25-50% of the total pulp. The desirable size of the underflow fraction may be determined in the mill by varying the reject rate and determining the linting of the paper produced or by preliminary tests in the laboratory, - e.g. by multiple fractionation of a sample of the pulp using a hydrocyclone and measuring the average specific surface of each fraction to obtain the specific surface distribution of the fibers in the sample. A method for determining the fractional distribution of specific surface in a mechanical pulp will be described in more detail hereinbelow.
The underflow fraction is passed through a refiner where it is subjected to sufficient refining to increase the aver-age specific surface to between 4-lOm2/g preferably to bring its specific surface to a value at least equal to that of the initial pulp and even more preferably to a value equal to or above that of the overflow fraction. Refined underflow fraction is then recombined with the overflow fraction and the recombined pulp of improvea properties is further processed as desired in a conventional manner and forwarded to the paper machine.
Referring to Figure 1, WOOd chips, or other comminuted cellulosic raw material, are fed through line 11 to refiner 10 in which the chips are mechanically disintegrated to form a pulp. Refiner 10 represents diagramatically a refiner plant, which may be operated in any of the known ways and may thus include features which are not specif1cally shown in the figure but are known ln tne art, SUCh as pre-steam1ng the chips, maintaining tne requisite pressure in an enclosure - 1045866 around the refiner or otherwise controlling the conditions of refini~g, chemical treatment accompanying any of the stages of refining and so on. The resulting mechanical pulp is usually passed to a tank (not shown) for latency development and also to dilute the pulp to a suitable consistency and is conveyed through line 12 to screen 13 which rejects oversize particles or particularly long fibres in a conventional manner. The rejects constituting generally about 5 - 10% of the pulp are removed through line 21, and can further be processed, if desired. The portion of the pulp which passes through the screens is passed through line 14 into hydro-cyclones, e.g. Centricleaners sold by C-E Bauer Company, which fractionate the pulp into an overflow fraction removed from the hydrocyclones via line 17 and an underflow fraction removed via line 18. A two stage hydrocyclone fractionating system is shown, represented respectively by hydrocyclones 15 and 16, but a single stage or a multiple stage system greater than two stage may be used. The overflow fraction in line 17 is the accepted fraction of the pulp having the requisite specific surface diQtribution; as mentioned, it will constitute about 30 - 85% percent of the pulp passed through the screens. The underflow fraction in line 18 is the fraction of low average specific surface (1.2 to 4 m2/g) and will constitute about 15 - 70~ of the pulp (after the screen). The underflow fraction is then cleaned, if desired, in hydrocyclone 19 (Magnacleaner) to remove, in a conven-tional way, dirt and other alien particles (ngrit~) and the thus cleaned underflow fraction is passed through line 22 where, if desired, is combined with the screen rejects from line 21. Obviously, the cleaning by means of a magnacleaner may be omitted, if no cleaning is necessary, and similarly the underflow fraction need not be combined with the screen -, ' ' ' . , : .- '.............. - : ' ~ - , I
- , . . .

-- 10 -- .
~45866 rejects if it is preferred to treat these streams separately.

The underflow fraction from line 22 is then thickened, e.g.
in a press 23, to a consistency suitable for refining in a refiner and passed into refiner 20 where it is refined to an acceptable average specific surface value, preferably one substantially equal to the specific surface of the overflow fraction in line 17. The pulp from refiner 20 is conveyed through line 24 and, either directly mixed with the pulp in line 17 or subjected to further treatment before the paper machine.
The fractional distribution of specific surface in a thermomechanical pulp, i.e. the percentage by weight of fractions having a given specific surface, is shown in Figure 2. ~oth Pulp I and Pulp II are samples collected from line 12 in Figure 1, but they differ as to fractional -~ distribution of specific surface, Pulp I containing close to 60~ of fibers of a specific surface of 2.5 or less, while Pulp II has about 40% of such low specific surface fibers.
In fact, Pulp I has a much higher linting propensity. The linting-propensity is measurable by means of a simple appa-ratus, similar in principle to a printing press, in which the lint picked out from the sheet of paper is collected under controlled conditions and subsequently weighed or otherwise measured to establish the relative proportion of fibers so picked out from the sheet. This procedure permits the com-parison of the degree of linting of various papers by means of a linting index, the latter being expressed in terms in weight or number of fibres (lint) picked out from the sheet per unit area of the paper. The direct relationship existing between the weight of a low specific surface fraction in the pulp, e.g. less than 2.5 m2/g, (which we shall call ~the linting propensity index~) and the linting index of the paper, is shown in Figure 3. This is a plot of the linting propensity index (weight fraction of fibres with specific surface of equal to a less than 2.5 m2/g) against the paper linting index (number of linting fibres per unit area of paper surface). Curve B provides such a plot for one particular paper machine and curve A for a second machine. The values on the vertical axis represent the fraction (as measured by the method described hereinbelow) of the fibers in various thermomechanical pulps having an average specific surface of 2.5 m2/g or less, while the values on the horizontal axis represent the paper linting index. It can be seen that, while for different paper machines the same relative content of low specific surface fibers may result in different values of the paper linting index, for any given paper machine the paper linting index is in a direct relationship to the content of such low specific surface fibers.
Figure 4 shows the specific surface distribution of, respectively a pulp directly from a refiner, a pulp treated according to the present invention and one of the prior art. Curve A indicates the specific surface distribution for pulp as discharged from the refiner (before screening and cleaning, line 12 of figure 1), curve B indicates the pulp to the paper machine when treated according to the present invention, and curve C is the pulp to the paper ma-chine when treated according to a conventional process.
The data for these curves is provide in table 1.

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1~4S866 AVERAGE SPECIFIC SURFACE m /g PULP SAMPLEConventional ProcessPresent Invention 3% hydrocyclone 14% hydrocyclone underflow and underflow and 10% Screen rejects10% Screen rejects Refiner Discharge 4.8 4.8 (before screening and cleaning) (line 12, figure 1) Screen Rejects (line 21, figure 1) 1.6 1.6 Underflow fraction (line 22, figure 1) 0.9 1.2 Total Rejects - Before Refining 1.5 1.3 (Press 23, figure 1) Rejects - After 6.7 6.2 Refining (line 24, figure 1) Centricleaner Accepts 5.2 5.8 (line 17, figure 1) Pulp to Papermachine 5.4 5.9 Linting Propensity ; Index of pulp to the machine (weight . fraction of spec~fic 0.41 0.28 surface of 2.5 m /g or less) Paper Linting Index 62 47 (Curve A figure 3) It will be seen that in this example the original pulp had close to 50g of fibres with a specific surface of 2.5 or less and that by separating only 14% of the pulp in the cyclone and mechanically processing the percentage of fibres with such a low specific surface is reduced about 28%
in the pulp to the machine. This reduced the linting index of the paper significantly (from 62 to 47) Curve C represents -the specific distribution of a pulp obtained from pulp A -by separating from said pulp in a cyclone an underflow ' . , ' . . .

1~)45866 fraction of about 3% and processing this fraction in the refiner. It is apparent that no substantial reduction in the quality of fibre with a specific surface below 2.5 (linting candidate material) is obtained.
- 5 No method for readily obtainin- specific surface dis-tribution of a pulp is available and therefore it was neces-sary to develop a method of fractionating pulp into frac-tions each of which has a relatively narrow range of specific surface. Hydrocyclones have been found to provide an appro-priate means for separating fractions of pulp of relatively narrow specific surface distribution. Conventional methods of measuring specific surface are relatively complicated.
There is however, a correlation between the measurements of specific surface and freeness for a wide range of these two properties (correlation is not 100% accurate and there may be significant deviation if the pulp properties vary widely). Thus while the basic criteria for characterizing the linting propensity is fractional distribution according to specific surfa~e, in practice, the freeness value may also be used instead of specific surface because of the inverse - relationship of specific surface to freeness.
Several arrangements of hydrocyclones may be used to fractionate pulps by specific surface, for example, parallel arrangements as illustrated in figure 5A, underflow cascade arrangement as illustrated in figure 5B, and an overflow cas-cade arrangement as illustrated in figure 5C.
In each of these arrangements the cyclones will be de-signed to reject different fractions, for example in a pa-rallel arrangement as illustrated in figure 5A for a 2 inch diameter cyclone the apex outlets cyclone may be 10/32 in., 7/32 in. and 5/32 in. which will provide underflow fractions of about 60% to 40% and 12%.
:.

104S86~
The cascading arrangements shown in figures SB and SC
wherein the underflow or overflow fraction of the first cyclone becomes the feed to the second, similarly the flow from the second becomes the feed to the third and so on step by step down the line provide a narrow fractionation on the basis of specific surface.
In order to obtain the reading of the specific surface, e.g. in figure 5A, the flow from the apex of each of the cleaners is measured, consistency of the underflow is mea-sured and the specific surface is measure. As indicatedabove, freeness values may measure instead of specific surface, keeping in mind, however, the limitations on the correlation between specific surface and freeness.
In figure 5B, the specific surface distr~bution is determined by measuring the rate, consistency, and specific surface (or, with the above reservations, freeness) of the underflows from each of the cleaners, while the index of the specific surface distribution with the arrangement shown in figure SC is obtained by measuring the flow, consistency and specific surface of the overflow of each of the cleaners.
Obviously, in each of the systems the flow, consistency and specific surface of the whole pulp is measured before it is fed to the cleaners.
It has been found that these arrangements provide adequate indication of the specific surface distribution of the given pulp.
In using the equipment of figure 5, the flow to the cleaners should be at low consistency say about 0.15%.
It will be understood that various modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims (4)

1. A process for producing a mechanical pulp comprising;
generating a mechanical pulp from wood material, screening said mechanical pulp into a through fraction and a retained fraction, fractionating by hydrocyclone means substantially all of said through fraction into an under-flow fraction containing between about 15 and 70% of said through fraction and having an average specific surface less than the pre-determined value between 1.2 and 4 m2/g and an overflow fraction having an average specific surface greater than said underflow fraction, subjecting said under-flow fraction to a mechanical processing thereby to form a processed fraction having an increased average specific surface value between 4 and 10 m2/g, combining said processed fraction and overflow fraction into combined pulp having a significantly lower linting propensity than said generated mechanical pulp.

2. The process of claim 1 wherein said underflow fraction constitutes between 25 and 50% of said through fraction.

3. The process of claim 1 wherein said mechanical pulp is fractionated so that said underflow fraction has an average specific surface less than a predetermined value between 1.5 and 4 m2/g.

4. The process of claim 1 wherein said processed fraction has an average specific surface at least equal in value to that of said overflow fraction.