Strength in Wood Pulp Papers Effect of Beating H. AINSWORTHHARRISON, Horwich, Lancashire, England
T
Factors hffecting ihe strength of paper are conas being readily d i s c e r n i bl e deeply involved in under the microscope to about sidered. It is believed thatfibrillation in itself is numerous factors that to half the wave length of light not of such importance in producing strength as differentiate and assess the imand by stating that the magnifiis cOmrnOr1ly thought* Evidence in favor of this portance of each is a difficult cation used by the author problem. Eight of these factors view is adcanced. Experiments are giuen to show (x 150) was therefore not sufthe effect of wetting-out agents on the rate of hyficient to identify them. The which m a y be c o n s i d e r e d of reply to that argument was four&ation and strength development. Primary importance in their influence on h y d r a t i o n , paper fold (3): I n the first place, fibrilformation, and strength are as lae are shown in the later stages follows: felting, fibrillation, surface cohesion, plasticity, plia- of beating under the same magnification. Secondly, “incibility, mucilage production, individual fiber strength, and dental” fibrillation of kraft pulp produced by Hollander fiber length-diameter ratio. Of course, between some of beaters is clearly shown in Figure 2 ( X 150). The fibers in them there are no definite lines of demarcation. this series, like those in Figures 1 and 2, were mounted in Felting capacity, for instance, is intimately bound up with glycerol jelly, and the lighting conditions (lens aperture, fibrillation, cohesion, and plasticity, with fiber length- condenser aperture, etc.) were identical throughout. Thirdly, diameter ratio, and to some extent with the presence or ab- microscopical examination of mechanical pulp, at the same sence of mucilage. Felting, however, is a means to an end, magnification, when beaten for 15 minutes in a Lampen not the end itself. Adequate felting, as indicated by satis- mill, shows marked fibrillation although there is an allfactory sheet formation, simply enables the other factors to round decrease in strength with increasing wetness from 66” become operative. Nakano (6) has shown that, although to 75.5” Schopper-Riegler. And in the fourth place, photonatural silk and asbestos split into fibrils when beaten, the micrographs (Figure 3) taken on the original unbleached resulting sheets have little strength owing to slippage of the sulfite pulp, but at 800 diameters and in one case (pulp 16) fibers. even a t 1000 diameters magnification, still show no signs of fibrillation in the early stages of beating. (The fibers in this FIBRILLATION case were stained with Safranine and mounted in Hyrax; Strachan (7) has shown that imbibition is enormously for pulp 16 a Special 8-mm. objective, numerical aperture facilitated by increase in specific surface as brought about by 0-54,and flattening ocular m‘ereused.) fibrillation, and there has arisen a general impression that From the foregoing evidence, there can be little doubt that fibrillation produces strength, probably because they usually the initial rapid rise in strength Occurs in complete absence Occur together in Hollander beating. Although in this article Of fibrillation, and there is an indication that what fibrillation by Strachan no statement is made explicitly that strength is does Occur in the early stages of Hollander beating of wood dependent on fibrillation, there is an assumption to that pulp as a result of direct blows from the roll is entirely incidental, perhaps almost accidental; this is characteristic effect, and this relationship is actually maintained in a recent article (8) in answer to a challenge which the author of such beating, no doubt, but is without any significance made three years ago ( 2 ) that the assumption is an entirely as far as strength development by facilitating felting is erroneous one. concerned. The absence of fibrillation in strong Swedish The evidence on which the author first questioned the kraft papers is considered to be important evidence in favor fibrillation-strength theory is set out a t some length (1). of this view. It is chiefly concerned with the rapid development in strength At this stage disagreement should be expressed with the of unbleached sulfite pulps &,hen beaten in a Lampen mill, “crystallization theory of strength” originated by 51;. B. and the appearance of the fibers at each stage of beating Campbell, a t least SO far as it has been represented in techwhen photographed at a magnification of 150 diameters. nical literature available in England. It is an accepted fact Figure 1 gives these photomicrographs, and Table I the essen- among crystallographers that pressure cannot effect appretial strength figures of each pulp. Unbeaten test 1 is that ciable union between two crystalline surfaces unless they are obtained on dry pulp after 4 hours of soaking in water and SO oriented that their space lattices are Congruent. Obstandard disintegration in the British evaluation apparatus; viously this can occur only fortuitously and comparatively the other tests have had the same preliminary treatment rarely in a sheet of paper, and the strength so produced can only be a small part of the total strength of the sheet resulting before the specified beating period. It is quite certain that fibrillation does occur during beating. from other Causes. Figure 1 clearly shows, however, that such fibrillation by no EFFECTOF BEATINGO N MICELLAR STRUCTURE means coincides with the period of greatest development of Regarding the effect of beating on the micellar structure of strength but commences a t a later stage, when, in fact, the strength is remaining constant (the tearing strength is the cellulose fiber and on the length of the unit chains of actually decreasing owing to fiber shortening). The air glucose residues, the evidence is overwhelmingly in favor of permeability curves (and possibly the drainage time curves the view that no detectable change in the x-ray photographs is brought about even by excessive beating; this has been also) illustrate the same phenomenon. Strachan countered this evidence by seeking to reduce the confirmed recently by H. Mark of Vienna. So far as the size of the individual fibrillae from that originally described micellar structure is concerned, the bulk of the evidence, HE strength of paper is so
458
459
1
4
2
3
0
J
Iiiostly Eroiii Aiiierican liiboraturiea, iiidicetes tliiit no ~ i i i d l n r breakdown, as iwlicaterl by reduction of viscosity in cupramnniorrium iiylroxiile, occur,^. Kresv arid Bialkowsky (.$) have noted slight cimnges in viscosity on beating wood pulp but attribute this to the fillers being rendered more rea.ctive by the ineclianical disintegratiori. Apjiarently only in one case has an apprei;iable decrease in vismsiby been recorfIecl---namely, by Sakano (G), wlio observed a 38 per cotit decrease when surgical cotton was severely beaten. It appears tlmt with ii fiber of this sort, having initially a ii~nclilnrgrr average micelle size tiinn has i ~ ~ opulp, i l exacssive bethiig n1a.y prodiicc some rniccllar detritus ani1 so reduce via:osily, espeaially if iiicipient destruction has already occurred during the hleacliing process. Severtheless, the case is esceptionai, and, so far as the consideration of wood pulp is concerned, unimportant. The significaiice of this absence iif micellar destruction by !)eating is tiiat discussion can l i e liinited to
coiisiileratioirs of surface change without being concerned in tiie kast with anything more deep-seated. SURFACE COHESION Consiileriition will iiow be given to tlre most important reimining factor of the eight laid down a t the beginning of this article-the cohesion brought about by changes in the colIoirld nalure of the fiber surface. The physical phenomena which give rise to a surface of this kind are Iiighly complex and are only just beginning to be understood. Nter the oinaiitiuln amount of swelling has taken place up to saturation point by imnrersion in water, further ciinnges brought about by beating include additional iinbibition of wat.er by tiie wlloidd eiiveiope of the fiber and a diange in the nature of the surface, both ilronroted by tiie pressure of the beater roll. Time two changes affect the pliability of the fillers, tlios enabling the pressires to which the moist web is after-
10. neaten 5 s*i""iea:
IO' Sehopper-ltiiegler
ward subjected to colriprehb tlieni Lo a greater extent, a i d its described by the iiutlror ( 1 ) . To oiic Jut, however, was thus forming a denser sheet having a greater area of fiber-to- added a t the start of beating 5 per cent of Penninal W (a fiber contact. By platting the wetness (degree8 Scliopper- wetting-mt agent of the alkylnaphthalene suifonic a d type), Riegler) against tlie apparent specific gravity of the laboratory calculated on the weight of ovendry fiber. The wetness of test sleets as given in Table I, all of which were twice pressed the pulp so obtained mas only 1 9 O Schopper-Riegler as comaccording to the British standard conditions of 50 pounds per pared with 31" for that beaten in the absence of a wetting square inch (3.5 kg. per sq. cm.), a typical cubic curve is agent. A third lot of pulp was then beaten alone for 25 obtained, showing in a marked degree how the apparent minutes so as to give 19", thus enabling strength comspecific gravity increases rapidly with aliglit anionnts of parisons to be made after equal beating periods and at the beating, then remains almost coilstant oyer tlie middle same degree of wetness. The results are given in the following range of wetness (from 30' to GO" Scliopper-Riegler), and table [relative humidity 65 per cent; teinperat,rrre 6R'? F. finally increams again up to and probably lxyond 90" (corn- (19" C.) diirine tcitinej: pare citation I , page 332). It sliould perhaps be stated dliat tlie last portion of the curve is probably more dependent on the insensitivity of the Scliopper-Itiegler apparatus at high degrees of wetness than on any renewed rapirlity of change in the apparent specific gravity. It may also be significant that on plotting cold copper number (whicli is a measure of hydration) against air space for a wide variety of papers ranging irom blottings to Cellopliane and made Cuniparison of the first, two ooluinna sliows that botli from different kinds of fibers, an almost identical cubic curve bursting and tensile strengths are appreciably lower, while the is obtained. tearing strength is considerably higher when R wetting-out
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Pdp Beating t i m e Burat (setor Breskiog length Tssr factor Wstneas. * Sehopyei-
1
Uubeaieo
28.7
4.460 203
Ri'Xki 14 Air permeshiiity (Dur~
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ADpweat
3.1 xcu.
0.585
BIJ. gr.
Basis weight
=
1.
3 5 mi". 57.0 s.110 213
71.8
so ,"in 75.0
180
168
11,250 169
15
23
28
13.1 iec. 0.577
62 B e e
2.25 ,mi*,.
4 15 min.
in.410
0.726
io,wn
0.740
6
en mijj.
7 2 hour.
s
Y
80.6
42/z hours 83.7
17 Ihu18 52.4
158
136
10,3?0 114
45
53
72.5
89
8.25 mi". 0.761
14 mi".
1 . 5 houin U.7Y1
0 . 2 5 hours 0.886
81.5
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11,500
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GY. burst, d a i i . m i . bsais weiilht. c./w. m. SV. tensile strriigth, kg, X 13renkinp leii*tii (",eterd besia weight, a./aq. m. x 3 ,~Bsr(sotoi By. ~ ~ iuiee n LO tear s ( x ion) weight. &./SR. ,a, basis weight, g./sq. 10 Apl'Brent sp. vi-. jly. tilickness of aingiesh8et. i/inn ram.
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It might be expected from current theories regarding the nature of wetness and liydri~lionthat the presence during heating of a substance whicli facilitates watcr penetration would increase the rate of absorption and so facilitate hot11 strength and wetness developinent. The following experiinentsindicate that the opposite effect is obtained: Two lots of a dry kraft pulp (24 grains ovendry fiber) were each beaten for precisely GO minutes in a Lamp6n mill (IO-kg. hall model) at 300 r. p. in.. consistency 3.0 per cent,
agent is present. These three features, as well as the lower wetness degree, siniply show that beatiiig has not proceeded to tlie same extent. This is confirnied by a comparison of the second and third columns wliicli slrow that the strength r:aliics o1)tained in presence of tlie wetting agent used are, witiin t,iie. liiiiits of experiniental error, equal to those obhined in the abscuce of wetting agent at the same wetness. This iiieiins that tlic presence of the wetting-out agent has inhibited wctncss development but has not impaired strength at, tho wetness attained. A somewhat more extensive investigation, this time with different percentages of Nekal BX (also an alkylnaphthalene sulfonic acid) on kraft and unbleached sulfite pulps, gave tlic results shown in Table 11. While the strength and wetness development of the kraft pulp (pH 7.8) were considerably repressed (thus oonfirming
tire prcviz,u> result aitli 1)criiiiiiaI W), the siiltite puljr (pH 5.8) was practically unaffected. The chief difference during tlie beatiug OS t,he two pulps was the formation of froth witli
the kraft piilp, and the comparative freeilom from it with tile sulfite, traces of rrsidual acid prthl,ly heing sufficient to account for this difference. (This froth was of course dispersed before siicct,-uiakiug.) It was therefore (kcided to beat another batch of the same krtift pulp for 60 minutes in the presence of 5 per cent Xekal BX hut with the addition also of 2 per cent kerosene to prevent frothing. Tlie results are given below and should be compared with those obtained on the kraft pulp (Table It) alter GO minutes of heating alone and with 5 per cent Xekal RX.
11 No 1 s t 800diamdein
14
No 3 st 800 diamstara
'I'lius, irotliiog done is sufficient to account al-
iiiost completely for the inability of kraft pulps to develop in strength and hydration during beating wlieii a w e t t i n g agent, is p r e s e n t . Frothing may, by flotation, remove tho fiiiors from the sphere of action of the ball mill, or there may be B tuore otmcure explanation depending on clmnges in t h e d e g r e e of .gelatinization of the ~~elliilo~c surface in presence of ari air filrri
15.
No. 6 nt 800 diameter8
16. No. 3 nt 1000 dinmelers
'I'nnm 11. STRENO
Ham$ *eight Burst 1soLor Hreaking lengtla Stretch. %
'rearinem WeLnesa.
* Sohuggor-itiedei
l ~ i n i i i n g etime. aeo. \ir i,eimesbility. ~ e c .
Ussir weip1,t Burst factor I(renkin8 l e w t h St7oti.h. 9% 1'CW iaetor
\vetoes*.
SelloyPer-Riegler
i>r,,i,,r.ge time, sac. \ii wrineability, aec.
60.2 102.6 12.700 3.4
?no 28
5.3 ( i n , 4 * c.1 52
60.2 104.3 13.400 3.2
184 42 7 . 8 (12.0 185
c.!
fiJ.9 94.8 11,420 3.5 21 I 35 5.4 (11.730
61.8 90.0 11.350 3.5
216
c.i
62.4 96.3 12.100 3.5 206 36 6 . 6 (10.5- (:.I 96
21 4 . 2 ( 8 1.4' c.1
ti2.9 66.0
60.5
a .4 272 17.5
220
3.8 (in.5O
15
i
61.9
61.4
95.2
12.100
3.5
204 30 3.7 5R
(10.4- c.)
91.0
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11,400 3.5
c.1
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80.7
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3.5 260
17.5 3.9 c . 4 " 8
21 4.2 ( 1 i . 0 " (: i I4
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