CA1043556A - External sizing of extensible paper - Google Patents

Abstract

Abstract of the Disclosure There is disclosed a method and apparatus for producing externally sized compacted paper which imparts to such paper radically improved stiffness, tensile, TEA and burst characteristics. The appartus comprises a compactor for compressively compacting a web, means for receiving said compacted web and for impregnating both sides thereof with a solution containing starch, and means for drying said web after the impregnation of said starch solution.

Description

Back~round of the Inventio~
Extansible (compacted) paper produced, for example, in accordance with the apparatus and process disclosed in U.S.
Patent No. 2,624,245 has certain well recognized advantages and commercial uses. Such paper is subjected, while in a partlally moistened condition, to compressive compactior. in the direction of web movament tmachine direction or ~D) between a pressure nip, thus compacting and forcing the fibers together to produce an inherent stretchabilit~ without creping. Compacted paper has improved tensile energy absorption (TEA) burst and tear characteristics which are highly desirable for such end uses as the manufacture of paper sacks. However, extensible paper has reduced tensile strength and stiffness, tne diminution of the latter characteristic increasing the difficulty of convert-ing this paper into sacks.
It is the purpose of the present invention to improve the tensile strength and stiffness of compacted paper without sacrificing tear strength and additionally, to produce a ràdical improvement in TEA and burst strengths. The method by which these objectives are achieved is by the selective im.pregnation of starch as an external sizing, .~.J' .~7";~

lO~S~i parti-~ularly in a manner whic'.l will not destroy the exten-sible na~ure of the paper, External sizing of non-extensible paper is well known and has generally been used to improve the printing qualities of higher grade papers by applying to one or both sides a coating of starch, clay9 polyvinyl alcohol or the like. In a pap~r entitled "On Machine Surface Sizing Trials With Acid Mcdified Wheat Flour" Vol. 53 No. 8 August 1970/Tappi, it was noted that the sizing of Kraft paper by applying thereto an a~id modified wheat flour or a hypochlorite - oxidized starch has the effect of increasing burst, tensile and stretch strengths~ U,S.
Paten~ No~ 3,362,869~to Welch discloses the application o a starch coating to one side of a moist compacted paper prior to drying of the paper in order to provide glazing thereof. It is the teaching of this patent that ordinary glazing of compacted paper by reintroducing sufficient moisture for this purpose into the web after compacting would destroy much o the extensible nature of the web and its attendant advantages. Therefore, in order to avoid undue wetting of the webg a coating of starch is applied to one surface of a still moist web after compacting and immediately prior to a drying roll~ which inhibit3 increas-ing the moisture content of the web while the starch adhes ively bonds the web to the drying roll and when the web is held under pressure against the roll9 results in glazing of the starch coated side of the web.
It is the purpose of the present invention to treat a compacted paper web with a starch solution in a particular way which overcomes or disregards the adverse ef.ect upon the compac~ed web caused by introducing moisture thereto. 1~4~55~
In accordance with the present invention, an apparatus is provided for treating a paper web to improve its tensile strength, TEA, burst strength and stiffness consistin~ essentially of in combination and in series a compactor for compressively compacting said web, means for receiving said compacted web from said compactor and feeding said web to a starch impregnating means for impregnating both sides thereof witn a solution containing starch, said starch impregnating means including at least two counter-rotating rollers defining a pressure nip therebetween, nozzle means for feeding said starch solution to said web as it enters said nip, means for transporting said web into said nip wherein said solution is forced under pressure to impregnate said web witn 1 to 10% by weight of said starch based on weight of said web, and means for drying said web a~ter the impregnation of said starch solution.
The apparatus preferably includes drying meansdisposed ahead of said compactor to adjust the moisture content of saia web to 20 to 50% prior to tne web entering said compactor.
In accordance with another Draf~rred ~rbodiment, the apparatus includes drying means interposed between the compactor and said counter-rotating rollers to adjust the moisture content of said web after compacting to 1 ~o 15% by weight.
In another embodiment of the present invention, the apparatus includes drying means disposed after the starch impregnating means for adjusting the moisture content of said web after impregnation of said starch solution to a moisture content of 5 to 10% by weight. The apparatus may also include means for recovering and reusing Al S~ii6 excess starch solution from the starch impregnating means.
The compactor may consist of an elastomeric belt and rotary cylinder compactor unit.
Further aspects and objectives of the invention will be seen by examination of the following detailed specification, including specific examples, and accompany~
ing drawings in which:
Fig. 1 is a schematic representation of a com pactor for producing extensible paper~
Fig~ 2 is a schematic representation of dryer sections which receive the compacted web after processing by the apparatus of Fig~ 1 and a size press intermediate said dryer sections for applying a solution of starch into the web; and Figo 3 is a schematic illustration of an alternate form of the invention wherein the compacted web is fed .,~ .

~ 5 5~
directly to a ~ize press and ~herefrom to a dryer section, and;
Figs. 4-9 illustrate grap~ically the effects of varying percentages of starch vis a vis different ~trength factors 9 compacted and uncomp~cted paper~
Referrin~ now ~o the drawing3 Fig~ 1 illustrates generalLy ~ c.mpactor 10 for producing an uncreped exter.si ble paper web 11. The apparatu3 10 in~ludes a looped elastomeric traveling belt 1~ and a rotar~ cylinder 13 having a smooth (chrome plated) outer surf~ce for rPceiving the web 11 thereon. A plurality of guide rolls 14, 15, 16 and 17 are po~i~ioned with-n the loop of the belt 129 guide roll 14 operating to squeeze the belt 12 against the cylino der 13 at a press nip N wherein the belt 12 is temporarily reduced in thickness as it travels therethrough~ A section of the guide rolls 15 is spaced from the first guide roll 14 and urges a portion 12a of the belt against the cylinder 13 at the off-running side o~ the nip N9 and web guide means 18, 19 feed the web onto the cylinder 13 for move-ment through the nip N. The apparatus 10 is shown located in a paper machine between the second and third dryer sec-tions. A dryer roll shown fragmentarily at 20 indicates the last roll of the ae~ond dryer section and the roll 33 (also shown fragmentarily) indicates the first roll of the third dryer section 30. The web is conveyed from the dryer roll 20 beneath the paper roll 19 and over an ex-pander (paper guide) roll 183 and then into the nip N. The web 11 is brought into contact with the surfa~-e of the chrome plated roll 13 before it enters the nip N. The web 11 is then held agains~ the surace of the roll 13 for an appreciable angular distance by the belt portion 12a a~d lV4;~55d~
the ~eb is then separa.ed ~rom both the belt 12 and the surface of the roll 13 and trained around a paper guide roll 22 and over another expander guide roll 23 and onto the dryer roll 33~
The web 11 coming off the dryer drum 20 has the optim~m amount of moisture required for the compacting trea~ment. This optimum may vary between 20 to 50 percent moisture as the web 11 enters the ~pparatus 10~
As previously indicated, the roll 13 has a smooth surface, preferably chrome plated~ The roll 13 is mounted on bearings 13a for rotation and is driven by a suitable source of power, such as a motor indicated diagrammatically at M. In fact, thelmotor M is the driving power for the entire device 10. In this way, the roll 13 drives the belt 12 primarily through pre3sure engagement at the nip N so that tension in the belt 12 is reduced at the nip N~ As will be appreciated, some power is required to drive the belt ~2 around the various guide rolls, such as the rolls 14, 15, 16 and 17 and the point at which the belt 12 is driven (which in this case is at the nip N? is a point at which tension in the belt is reduced. Since the belt is elastomeric in character there is a slight reduction in the stretching of the belt resulting from the reduction in tension and this ~auses the belt 12 to compress the web 11 longitudinally of its direction of travel as the web ll passes through the nip, and immediately thereafter~ The smooth surface of the roll 13 is conducive to relative sliding bet~een the web 11 and the roll 13~ The web 11 which is held in close frictionaL contact with the belt is thus compacted in the machine direction (MD) without creping, A lubricant shower 25 is provided just beneath Sf~
the web 11 at the oncoming side of the nip N, Preferably the lubricant shower 25 is a silicone shower but other re-lease agents are suitable which applies a relatively small amount of liquid, ln the fonm of a film~ to the surface of the roll 13.
The roll 13 is heatedg as shown here diagram~a-tically, by a suitable source of steam S fed axially into the roll 13 in the manner in which steam is fed into the usual paper machine dryer dr~m. The steam maintains a rel-atively high temperature about 212 and up to 350 F, at ,the surface of the roll 13 so as to create a very thin steam film or cushion between the web 11 and the drum 130 This steam cushion assists in permitting relative sliding between the web 11 and the drum 13 (which is essential to permit the web to be compacted by the belt) and tends to soften some of the ingredients of certain types of paper~
The apparatus thus far described is well known and forms no part of the present invention beyond illus~
trating the production of compacted paper which is sub~
sequently treated according to the invention~ The remain-der of this specification will be devoted to specific as-pects of the treating of compacted pape~in-accordance:'w~th the teachings and discoveries afforded by the invention~
After leaving the compacting apparatus 10 9 the web 11 will pass through the dryer section 30 which in cludes conventional drying rolls,31 - 36, the web 11 first passing over drying roll 33 thence to roll 34 etc. as shown~
Felt dryer belts 37 and 38 are arranged upon guide rolls 39 and in contact with the respective dryer rolls 31 - 36 to assure the intimate engagement between the web 11 and the respective dryer rolls~

After leaving the compactor 10, the moisture contained in the web is approximately 35 to 45% moisture by weight. After passing through dryer section 309 the moisture content of the web 11 will be approximately 1 to 15 percent of ~hoisture by weight, Reference numeral 4û indicates a size press ar-rangement for treating the dried compacted web 11 in accord-a~ce with the present invention. Web 11 first passes over guide rolls 41, 42 and therefrom proceeds between a nip formed by two counter~rotating size press rolls 43 and 44O
Nozzles 46, 47 control the starch metering valves 48, 49 in conduit 50, 51 which permit a metered quanity of a starch solution 52 to flow into the nip between size press rolls 43, 44, Edge catch pans 53 collect the excess starch solution at each end of the nip~, The starch solution is supplied from a starch solution supply tank 54 having there-in an agitator 56, starch in solution being drawn from the tank 54 through conduit 57 to the pump 58 which supplies metering valves 48 and 49. A starch return pump 60 ser-viced by line 61 from catch pans 53 returns the starch solution via line 62 to the supply tank 54~, During the passage of web 11 between the nip of press roll 43 and 44, a solution of starch is applied under pressure to both sides of the web in accordance wit4 vary-ing percentages indicated below by way of specific examples.
The passage of the web 11 `~hrough the starch solùtion and between the rolls,43, 44 will be sufficient and will be so regulated as to the speed of application and the ~trength of the starch solution to permit and to cause impregnation of starch into both surfaces of the web 11 in accordance with a predetermined amount~ The weight of starch impreg-.. . . ... ...... . .. ..

_ 1~ 3~ ~ ~
nated and deposited in the ~7eb will be from 1 percent tolO percent by weight to the total weight of the web after drying in order to achieve good results 9 wi~h optimums of different strength characteristics being obtained by im-pregnating starch in accordance with the percentage shown in Figs~ 49~ During this process ~he moisture content of the web will increase from the 1 to 15 percent ~-alue at the outlet of the dryer section 30 to a value of 35 to 45 percent.
Following the appli~ation and impregnation of starch into both sections of the web 11 ~he web passes around guide roll 64 and thence into the second dryer section 66. ~ryer section 66 contains a plurality of dryer rollers 67,689 69, 709 etc. about which pass dryer felts 71~ 72 guided by rolls 73, The dryer section 66 is similar in all respects to the dryer section 30 and the web 11 passing th~rethrough will los~ thP moisture imparted by the size press existing from the dryer section 66 with moisture content o~ approximately 5 to 10 percent by weight.
F~gure 3 discloses an alternate embodiment of the invention wherein the same reference numerals have been used for si~ilar parts as shown in Fig. 2~ In accordance with the embodiment of Fig. 39 the web 11 proceeds directly from the compactor 10 without first going through dryer section 30. As a ~onsequence9 the w~b 11 contains ap~
proximately 30 to 40 percent moisture by weight prior to passing between t~e size press rolls 439 44. It will be appreciated that since the web 11 co~tains this amount of mois.ure3 lesser amounts ol the starch solution will be caused to impregnate the web, however9 a sufficient amount of starch in solution will impregnate the web to provide 104;~55~i significant and beneficial results in regard to increasing tensile, burst and TE~ characteristics9 and may be expected to be in the order o~ approximately 50 percent of the im-provement achieved with the configuration of Fig. 2, After passing through the size pre.s 40~ the web 11 feeds into the dryer section 66 which redllce~ th2 moisture content of the web to a moisture level of 5 to 10 percent by weight.
The advantage of the arr3ngement sk.own in section 3 is the use of a single dryer sectlon to receive the web fro~ the size press, although the mcisture content of a web will ordinarilly require a larger dryer section to reduce the moisture content to acceptable levels~
It should be noted that with respect to both ar~
rangements, Figs. 2 and 3, it should be possible to obtain glazing of the paper web 11 by feeding the web down~tream of the size press 40 around a single dryer roll provided that the starch solution when applied in the press 40 provides sufficient bonding of the web to the cylinder.
Inr.,order to achieve glazing9 a pressure contact.with the drying cylinder, as shown for example in United States Patent No~ 3,362,869~ The difference between the presse~
arrangement and that shown in Patent NoO 3~36298699 referred to above in describing the background of the present in~
vention, is that the starch applied in the said United States Patent is a surface coating -used to achieve adhesive bonding between the web and the dryer roLl9 whereas in the present in*ention the web becomes ~mpregnated with starch on both sides to ~chieve the subseq~ent additional improve~
ments of radically increased tensile~ burst and T~A stren~ths~
Such additional effects are made possible by the use of the size press, which includes the size press rolls for forcing the starch solution .into both web surfaces, .

~()4;~55~
Having described the apparatus of the invention we will now set for~h several examples illustrating the particular constituents used in carrying out the invention and the measured results~ Example NoO 1 EXAMPLE_l ~URNISH of unbleached 30% Softwood unbleached 70% Hardwood UN~EFI~ED = (i2 Sec~ W) STARCH - ~acid modified corn starch) .. . . . .
STARCH (% Add~?
o% 2.8%5.7%
Tensile ~lbs,/in.~
: MD 6.0 14~021.0 CD 5O5 80810.6 Elong, (%) MD 9.1 8018,5 CD 1.3 2032~8 TEA (ftolbs~/ft~ ) MD 4.17 7.95 14,12 CD 0.62 1,66 2,60 TEAR (~ms~
MD 54~8 630666,8 CD 61~6 76~588,8 Burs~ si) 5~4 17.723.7 Stif~ness (Tabor Unit~
MD .40 0~98 1~19 CD o48 1~01 1,13 ~.

104;~S~

URNISH of unbleached 30~/~ Softwood unbleached 70% Hardwood REFINED (14 Sec. Williams Freeness) STARCH Unmodified Potato Starch STARCH %
0% 1.3% 3,0% 507% 10,7%
Tens ~ bs./in~ 7.912.8 14.0 15,1 1505 Stretch % 8.5 7.2 7.1 7.2 701 TE~ ~ - 8.2 8.9 809 Stiffness Tabor ~,56 - 1.02 1.15 1.20 1033 Tear (gms) MD 69~783.1 88.8 88.1 84.6 CD 88.4100~4 103,2 101.6 lOloC

Burst ~psi) 8.716.6 17.7 19.3 25.8 URNISH o unbleached 30% Softwood unbleached 70% Hardwood REFINED (40 Sec. Williams Freeness) STARCH (Acid Modified Corn Starch) STARCH ~% Add,) 0% 1.4% 3.8% 5~7%
Tensile (lbs~/in ~
MD 140621.7 26.3 2906 CD 9.010.1 12.4 12.6 ~ 2 l~k'~;~556 EXAMPLE 3 (con't) O% 1,4%3,8% 5.7%

Elon~, (%) MD 10.4 g.7 10.9 8,6 CD 2,4 3.4 3.5 3O9 TEA ~ft.lbs./ft.2) MD 1207 16,621,1 27.2 CD L.97 2,803,87 4~42 Tear (~ms) - MD 78.7 75~2 73.9 68~5 CD 97.6 1~1.1121,4 90D9 Burst (psi) 18~8 28,6 30.8 35~6 ., Stiffness (Tabor Uni~
MD .65 1.16 1.20 1~73 CD .85 .~9 1.01 1.31 ~ . . ..
From the above examples, it is apparent that ~wo factors strongly influenced the results. These were (1) the degree of refining applied and (2) the amount or type of starch added, Corn starch promoted greater improvements in tensile strength and stiffness than potato starch under identical conditions. These were the properties it was desired to improve for better converting characteristics.
Moreover, it was concluded that although tear strength on compacted paper was generally reduced by increased refining and increased starch levels, the effect at freeness levels usually associated with sack production may not be great enough to offset the other advant~ges of si2ing~ Generally ~ 3 5~

at between 3 and ~% added potato starch, properties levelled out, showing no ~ar~ed changes beyond this additive level.
Contrasted to this with added corn starch, tren~s in properties continued beyord this point~ No tackiness or brittleness was apparent in any of the samples tested up to about 8%. Above this, however~ some brittleness was observed.
In keeping with the principles of the present in-vention, it is suggested that sli~htly converted (plas-ticized) starch should be s~perior to highly converted starches, particularly at high percentage levels of starch impreg-nation. Such plasticized starch should be highly compato ible with the extensible qualities of compacted paper.
This is not, however, in any way to suggest that the use of unconverted starch is undesirable, since the examples disclosed herein were made using starch in the highly converted form.
~ eferring to Fig~ 4, a diminution of tear occurs on compacted paper with increasing starch content at 525 C~Fo (23,7 Schopper Riegler)O This work was done with an acid modified corn starch (Eclipse N) manufactured by AoE~ Staley4 With a lower level of refining9 i.e~ 9 at 730 CoFo tl4~3 Schopper Riegler), tear strength is markedly increased with increased starch levels~ At some i~ermediate level of re~
fining then, one would expect to see small effects~ That this is true can be seen in the more extensive work done on potato starch (see Fig, 5), where the effect of increased starch on tear is in fact rather small, The tear appears to approach some fixed value which is probably dependent on pulp ~ariables~
Tear strength of unrefined paper increases with 10~55~;
starch addition bec~use of greater Eiber bonding which re-sults in greater forces required to pull fibers loose dur-ing tearing, On refined stock used in thîs study it is believed that optimum bonding for tear development has already been achieved by refining, so that additional bonding caused by the-~tarch leads to ir.creased shear failo ure of the fibers and reduced tear. The CD ~Cross Direc~
tion, as opposad to Machire Direction9 MD3 tear reuslts which are in all cases considerably better for compacted than for uncompacted papers can be explained by the fact that in CD tear, the failure must propagate across com-pacted fibers9 and that compacted fibers with their lower modulus and higher elongation are more difficult to shear than uncompacted fibers. The initial increase in CD tear as starch is increased results from greater bonding and more difficulty in rupturing these bondsO Further in-creases in starch level however, leads to i~creased fiber modulus or stiffness, as well as further increases in bonding, which favors fiber shear failure and reduced tear.
As is normal for extensible paper9 W tear was some-what lower than the CD vlaues. The refined stoc~ lost tear strength on adding starch as previously discussed ( see Fig. 6).
Now to direct attention to properties improved hy the addition of starch, that is 9 tcnsile~ stiffness and TEA, on Fig~ 7 are shown MD tensile results for potato and corn starch at different levels~ Two different refining levels were explored in each instance~ Note that where~
as tensile gains for potato starch occur during the first 3% addition~ increased tensile with corn starch continues to higher additive levels~ Also it may be observed that 16- 10~ 56 the addition of 3% starc~ to compacted paper brings the tensile up to the level of the untreated (uncompacted) sample~ This may be understood by recalling that the molecular structure of corn starch9 having a more linear orientation than potato starchg creates a fil~ with higher tensile strength~
The use of corn starch ~t high levels ~5%~ doubled stiffness of compacted sheets9 approaching the untreated (uncompacted) control. Or. Figo 8 can be seen the results for potato starch al-so3 where gains are not as dramatic9 particularly at the interm~diate refining levels normal to sack paper production. At 3% potato starch addition9 the maximum benefit appears t~ have been achieved 9 an approxi~
mately 40% increase in stiffness.
Another property increased by starch addition is the work to break in the machine direction ~TEA) largely co~-tributed to by the increased tensile (see Fig. 9). While a 30% increase is evident with corn starch, only a~out half this ~an be obtained with the same level of potato starch~ The usual five to seven fold MD TEA increase over uncompacted paper is evident9 whether starch treated or not~
No significant differences in burst strength occurred between compacted and non-compacted sheets with increased size addition. About 30% to 40% increase at the 3% level were obtained in either instance~ This i3 expected since CD tensile and TEA properties which affect burst, are not markedly changed during the compaction process. The re-sults of the burst test are thus dependent on the starch level~

~ 6 It will be understood that the foregoing description has been of specific embodiments of the invention and i8 therefore representative, In order to understand the scope of the invention9 reference should b~ ~ade to the appended claims.

~7

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for treating a paper web to improve its tensile strength, TEA, burst strength and stiffness consisting essentially of in combination and in series a compactor for compressively compacting said web, means for receiving said compacted web from said compactor and feeding said web to a starch impregnating means for impregnating both sides thereof witn a solution containing starch, said starch impregnating means including at least two counter-rotating rollers defining a pressure nip therebetween, nozzle means for feeding said starch solution to said web as it enters said nip, means for transporting said web into said nip wherein said solution is forced under pressure to impregnate said web with 1 to 10% by weight of said starch based on weight of said web, and means for drying said web after the impregnation of said starch solution.

2. The apparatus of claim 1 including drying means disposed ahead of said compactor to adjust the moisture content of said web to 20 to 50% prior to the web entering said compactor.

3. The apparatus of claim 1 including drying means interposed between said compactor and said counter-rotating rollers to adjust the moisture content of said web after compacting to 1 to 15% by weight.

4. The apparatus of claim 1 including drying means disposed after said starch impregnating means for adjusting the moisture content of said web after impregnation of said starch solution to a moisture content of 5 to 10%
by weight.

5. The apparatus of claim 1 including means for recovering and reusing excess starch solution from said starch impregnating means.

6. The apparatus of claim 1 wherein said compactor consists of an elastomeric belt and rotary cylinder compactor unit.