Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 8
SOFT PAPER OF HIGH STRENGT~ AND METHOD FOR
PRODUCTION THEREOF
The present invention concerns a soft but also strong
paper which is based on a mixture of a hardwood pulp, a
waste paper pulp or a mechanical or semi-mechanical cellu-
losic pulp, or a mixture thereof, and a sulphate pulp
and/or sulphite pulp based on softwood; as well as a
method for the production thereof.
Usually, it is required that a soft paper, e.g. tis-
sue paper, should be not only soft, but also strong. To
achieve a satisfactory compromise between qualitative
properties, such as softness and strength, on the one
hand, and financial consideratlons, on the other hand, one
has mixed different cellulosic pulps of differin~ origins
and properties when producing soft paper, for example
tissue paper. Generally, one main component is long-fibred
to impart strength to the paper, while the other main
component is short-fibred to give the paper its softness
and the de~ired absorption qualities.
The long-fibred pulp is usually based on softwood,
such as pine wood or spruc~ wood, which has been chemi-
cally delignified by a sulphate or sulphite process. The
short-fibred pulp is generally based on hardwood, such as
birch wood, cucalyptus wood, aspen wood or oak wood, which
has been delignified by a sulphato proceQS. At times, the
celluloslc raw material may to a certain extent be based
al30 on mechanical and semi-mechanical pulp, such as
groundwood pulp, TMP and CTMP pulp and waste paper pulp.
The long-fibred pulps, or example chemical pulp from
spruce wood or pine wood, has a fibre length of about 3-
3c5 mm and a fibre width of about 0.04 mm. A short-fibred
pulp based on birch sulphate has an average fibre length
of 1.3 mm and a fibre thickness which is about half of
that of conifer fibres. The proportion of short fibres,
so-called fines, is high. Mechanical, semi-mechanical and
waste fibre pulp have a fibre length which usually is
'~5~2~
shorter than that of chemical pulp from spruce wood or
pine wood. The proportion of fines may be high. When
producing soft paper, it is desirable that the proportion
of fines be kept as low as possible in order to reduce
dusting.
To impart sultable paper-forming properties to the
pulp, the latter is usually ground, e.g. in a beater or a
refiner, which results in a paper of higher tensile
strength. The degree of grinding i~ generally measured as
the drainage resistance of the pulp according to Schopper-
Riegler (SCAN C 19:65). The SR value increases with in-
creasing grinding of the pulp. Already during the produc-
tlon of cellulosic pulp for paper, the pulp usually is
refined to 10-20SR.
When maklng tissue paper, the different pulps can be
refined separately or in mixture. Grinding not only
results in a higher tensile streng~h, but also in a higher
tensile stiffness of the paper. Tahle 1 below illustrates
this fact in connection with hand-made sheets of a mixture
of 70~ birch sulphate and 30% pine sulphate pulp. In TAPPT
Journal 66 (2), 1983, pp 97-99, H. Hollmark states that
the tensile stiffness of a paper correlates extremely well
with softness determined by means of panel tests. The
lower the tensile stiffness, the softer the paper, accord-
ing to the test panel.
US Patent 2,706,155 discloses a method for producingsoft paper, the starting material being a mixture of
25-70% oak wood pulp, the remainder being softwood pulp.
The oak wood pulp is essentially unground, whereas the
softwood pulp is refined. In an example, the softwood
sulphate pulp was ground to 500 ml CSF, which corresponds
to 25SR, and was then mixed with equal parts of essen-
tlally unrefined oak wood sulphate pulp to achieve the
desired combination of tensile strength, tearing strength,
softness and absorption quali~ies of the paper.
~ ~3 ~
Soviet Patent 779,483 discloses the production of a
paper from 40-60~ bleached softwood sulphate pulp, 30-54%
chemically refined aspen wood pulp and 5-15~ birch wood
sulphate pulp which has been further chemically refined to
increase the strength of the paper.
An article in the Soviet periodial Sb. Tr. TsNIIB
No. 15: 72-77 (1978) deals with hand-made sheets produced
from softwood sulphite pulp, softwood sulphate pulp and
hardwood sulphate pulp ground to 13-30SR, said sheets
being tested as to absorption, compressibility, softness,
ten~ile strength, bulk and strain. According to th~ ar-
ticle, a three-component mixture consisting of 50% soft-
wood sulphate pulp (<25SR), 30~ hardwood sulphate pulp
(20-21S~) and 20~ sof~wood sulphite pulp (20-21SR)
resulted in the tissue paper with the best properties.
Soviet Patent 775,212 states that tissue paper pro-
duced rom a mixture of softwood sulphate pulp, hardwood
sulphate pulp and softwood sulphite pulp ground to 23-
25SR becomes softer if the softwood sulphate pulp has
first been ground to 18-20SR.
SV 1,008,324 discloses the production of typographic
paper of good opa ity and ink absorbency ~rom a paper-
making pulp containlng 30-40~ by weight of bleached soft-
wood sulphate pulp ground to 50-55SR and 60-70% by weight
of hardwood sulphate pulp ground to 30-35SR.
One method of imparting increased softness to the
paper is to treat the paper or the paper stock with a
fibre-fibre-bond-rsducing agent, often called debonding
agent. A fibre-fibre-bond-reducing agent usually comprises
a prlmary, secondary, tertiary or quaternary ammonium com-
pound containing a hydrocarbon group having 8-30 carbon
atoms and, optionally, nonionic hydrophilic chains. It is
common to combine ths cationic ammonium compound with a
nonionlc surface-active compound. Such fibre-fibre-bond-
reducing agen~ are inter alia described in US Patent Spe-
cification~ 3,554,862, 3,554,863 and 4,144,122, as well as
in GB Patent Specification 2,121,449. The fibre-fibre-
~ i 2~ 8
bond-reducing agent markedly reduces the strength of the
bonds between the fibres in the paper, while the softness
increases. This is apparent from Table 1 bearing upon
hand-made sheets from a mixture of 70% birch wood sulphate
pulp and 30~ pine wood sulphate pulp. US Patent Specifica-
tion 4,795,530 tries to solva tha inconvenience of
strength reduction by applying the fibre-fibre-bond-
reducing agent only to part of the thickness of the tissue
paper, thereby to obtain an untreated part of paper
maintaining its origlnal strength. As is apparent from
Table 1 below, the changes in tensile stiffness and
tensile strength of the paper owing to a conventional
increased grinding of a pulp mixture and the addition of a
fibre-flbre-bond-reducing agent to the ground fibre
mixture cancel each other out. When grinding is increased,
the strength and the stiffness increase proportionally.
When the amount of fibre-fibre-bond-reducing agent added
is increased, the tensile stiffness as well as the tensile
strength are proportionally reduced. Thus, the gain in
strength is cancelled out by the loss in softness, and
vice versa. There is, therefore, a generally expressed
desire to improve the softness of a paper while main-
taining a satisfac~ory strength.
It has now suprisingly been found that a paper advan-
tageously combining softness and strength is obtained if
b~sed on a mlxture of
a) a hardwood pulp, a waste paper pulp or a mecha-
nical or semi-mechanical celluloslc pulp, or a mixture
thereof, constituting 55-90% by weight, of the total
amount of cellulose fibres and having a drainage resist-
ance below 25SR, and
b) a sulphate pulp and/or sulphite pulp based on
softwood and constituting 10-45% by weight, of the total
amount of cellulose fibres and having a drainage resist-
ance exceeding 30SR. The difference in drainage resist-
ance between the cellulosic pulps b) and a) is preferably
at least 10SR. The paper can be produced by preparing a
~ ~ r~
stock from the above cellulosic pulps a) and b) in the
given amounts, whereupon the stock mixture is taken up on
a w~re, and is drained and dried in per se known manner.
In a preferred embodiment, the soft paper also con-
tains a fibre-fibre-bond-reducing agent in an amount of
0.05-2.5% by weight, as based on the amount of cellulose
fibres. As mentioned earlier, a soft paper according to
the invention has a surprisingly advantageous rat$o of
softness to strength. To achieve this effect, the cellu-
losic pulp b) should be ground to above 30SR, but prefer-
ably not above 80SR, since pul~s of so high grinding
degrees require comparatively large amount~ of fibre-
fibre-bond-reducing agents to give the paper a satisfac-
tory softness. The celluloslc pulp b) preferably has 35-
60SR. The cellulosic pulp a) should be essentially un-
ground or ground to less than 25SR, preferably less than
20SR.
Whether the long-fibred pulp b) has been obtained by
a sulphate process or by a sulphite process is of no
decisive $mportance. Also, whether it originates from pine
wood, spruce wood or another conifer is of no vital impor-
tance elther. It is, however, dssirable that it has been
ground in such a manner that the fibres have been shorten-
ed a~ little as possible. The grind$ng results in a fibre
of higher flexibility. To benef$t from this increased
flexibility of ~he long-fibred ground cellulosic pulp,
there is preferably an addition of a fibre-fibre-bond-
reduclng a~ent serving to reduce the increase in strength
re~ulting ~rom the grinding, when the pulp forms a sheet
of paper. The agent is added in such a manner as to be
able to act on the bonds between the fibres. Prsferably,
the addltlon takes place at a stage during the preparation
of the stock, but the fibre-fibre-bond-reducing agent may
also be added to the cellulosic pulp a) and/or the cellu-
losic pulp b) or to the wet, formed or dried paper web.
~ 3 ~
Preparatlon of Hand-made Sheets and Measurin~ Techniques
The cellulosic pulps were ground in a beater accord-
ing to SCAN C 25:67 to the desired drainage resistance
determined in a Schopper-Riegler apparatus according to
SCAN Standard C 19:65. In those cases when one did not
want to notlceably change the dra~nag~ resistance of the
cellulosic pulp, the latter wa~ wet-defibrated according
to ~CAN C 18:65.
Before the formation of sheets, the cellulosic pulp,
alternatively the mixture of cellulosic pulp, was stirred,
optionally in the presence of a fibre-fibre-bond-reducing
agent, at a pulp concentration of about 2% by weight for
10 min. In the prsduction of sheets, use was made of tap
water of 30C whose pH had been ad~usted to 6-7. The
sheets were dried and conditloned according to SCAN P
2:75, whereupon the basis weight of the sheets was deter-
mined according to SCAN P 6:63. When measuring tensile
strength and tensile stiffnes~ according to SCAN P 44:81,
but with 15 mm broad straps, ons used a tensile-strength
tester of the mark Alwetron TH1, made by Lorentzen &
Wettre, Stockholm. The indices of tensile strength and
tensile ~tiffnes~, respectively, were determined by
dlvision by the basis weight of the she~t, in order to
eliminate the influence thereof.
Comparison
In the comparative study, plne wood sulphate pulp and
birch wood sulphate pulp were mixed. The pulps, ground as
bslow, were mixed in such a manner that 70% by weight con-
sisted of birch wood sulphate pulp and 30% by weight con-
slsted of pine wood sulphate pulp. Hand-made sheets were
~ormed in accordance wi~h the above-described method. The
following results were obtained.
2 ~ c~ 8
Table 1
,
Birch Plne Ten- Stiff- Strength/
wood wood sile ness Stiffness
pulp pulp index index
(SR) (SR) (Nm/g) (Nm/g) *1000
Without debonder 12 12 19.3 2780 6.9
Without debonder 18.5 18.5 35.6 5250 6.8
With, 2 kg ptp ) 18.5 18.5 32.3 4680 6.9
10 With, 4 kg ptp 18.5 18.5 28.0 3960 7.1
With, 16 kg ptp 18.5 18.5 19.9 2740 7.3
Wlthout debonder 26 26 55.0 7950 6.9
With, 2 kg ptp 26 26 38.3 5600 6.8
15 With, 4 kg ptp 26 26 30.6 4400 7.0
W~th, 16 kg ptp 26 26 22.3 3140 7.1
~ ~ . _ ,
) ptp = per ton pulp
As is apparent from these results, an i~creased
grinding of a pulp mixture combined with an addltion of
fibre-fibre-bond-reducing agent to the ground fibre mix-
ture does not noticeably affect the ratio of strength to
stiffness (se~ the last column of the Table). When
grinding is increased, the strength as well as the tensile
stiffness are proportionally increased. When more of the
flbre-flbre-bond-reducing agent is added, the tensile
stiffness ls :reduced proportionally, as is the strength.
Thu~, the gain in tensile strength is cancelled out by a
reduced softness, and vlce versa.
E~ample 1
A long-fibred pine wood sulphate pulp was ground to 13,
16.5, 20, 27 and 45SR. Then, 30 parts by weight of the
long-fibred pulp was mixed with 70 parts by weight of short-
fibred wet-defibrated birch wood sulphate pulp, whereupon
hand-made shee~s were produced. The following results were
obtalned.
~3r~ 2~8
Table 2
. .
Birch Pine Ten- Stiff- Strength/
wood wood sile ness Stiffness
pulp pulp index index
5 (SR) (SR) (Nm/g) (Nm/g~ *1000
~ . . . . .
Wlthout debonder 14 13 15.5 2240 6.9
Without debonder 14 16.5 18.7 2750 6.8
With, 4 kg ptp 14 16.5 12.2 1770 6.9
Wlthout debonder 14 20 21.5 3110 6.9
With, 4 kg ptp 14 20 14.8 2110 7.0
Without debonder 14 27 26.1 3640 7.2
15 Wlth, 4 kg ptp 14 27 15.3 2120 7.2
Without debonder 14 45 32.8 4050 8.1
With, 4 kg ptp 14 45 16.6 1750 9.5
As is apparent from these results, the ratlo
strength/stiffness of the paper is roughly constant at a
drainage resistance of 13-27SR of the pine wood pulp, but
i5 considerably improved when pine wood pulp of 45SR is
used~
Further, it can be seen that a stock containing pine
wood pulp ground to a drainage resistance of 45SR and
with an addition of a fibre-fibre-bond-reducing agent
results in an even better ratio.
Example 2
A plne wood sulphate pulp according to Exa~ple 1 and
ground as below was mixed with a short-fibred pulp con-
sisting of a wet-defibrated eucalyptus wood sulphate pulp.
For the shee~ formation, use was made of a pulp mixture of
70% eucalyptus wood sulphate pulp and 30~ ground pine wood
sulphate pulp~ The following results were obtained.
2 1 8
Table 3
-
Euca- Pine Ten- Stiff- Strength/
lyp- wood slle ness Stiffness
tus pulp index index
wood
pulp
(SR) (SR) (Nm/g) (Nm/g) *1000
.
Without debonder 16 12 19.0 2740 6.9
10 Wlthout debonder 16 45 29;6 3620 8.2
With, 4 kg ptp 16 45 18.9 2100 9.0
~ , .
From the above Table, it can b~ gathered that the
ratio of tensile strength to tensile stiffness is advan~
tageous for the paper according to the invention.
Example 3
A spruce wood sulphite pulp ~round as below was mixed
with a short-fibred wet-defibrated birch wood sulphate
pulp. For the sheet formation, use was made of a pulp mix-
ture of 70% birch wood sulphate pulp and 30% ground sprucewood sulphite pulp. The following results were obtained.
Table 4
Birch Spruce Ten- Stiff- Strength/
wood wood sion ness Stiffness
pulp pulp index index
(SR) (SR) (Nm/g) (Nm/g) *1000
~ . _
Without debonder 13 12 18.0 2790 6.5
30 Without debonder 13 39 30.9 4510 6.9
With, 4 kg ptp 13 39 15.5 1990 7.8
Without debonder 13 47 33.2 4660 7.1
Wlth, 2 kg ptp 13 47 25.7 3420 7.5
35 With, 4 kg ptp 13 47 19.9 2520 7.9
2~ '1Y
From the above Table, it can be gathered that the
ratlo of tenslle strength to tenslle stlffness also in
this case is advantageous for the paper according to the
invention.
Example 4
A long-fibred pine wood sulphate pulp ground as below
was mixed with a short-flbred wet-defibrated birch wood
sulphate pulp. For the sheet formation, use was made of a
pulp mixture of 80% birch wood sulphate pulp and 20%
ground pine wood sulphate pulp. The following results were
obtained.
Table 5
~ ~ .
Birch Plne Ten- Stiff- Strength/
wood wood sile ness Stiffness
pulp pulp index index
(SR) (SR) (Nm/g) (Nm/g) *1000
,
Without debonder 14 12 18.1 2830 6.4
20 Without debonder 14 28 22.3 3410 6.5
With, 4 kg ptp 14 28 11.4 1630 6.7
Without debonder 14 46 27.4 3530 7.8
With, 4 kg ptp 14 46 14.7 2030 7.9
~
It i8 apparent from these result~ that the ratio of
ten~ile strength to tensile stiffness is advantageous when
the paper has a composition according to the invention.
Example 5
A pine wood sulphate pulp ground as below was mixed
with a short-fibred wet-defibrated birch wood sulphate pulp.
For the sheet formation, use was made of a pulp mixture of
60% birch wood sulphate pulp and 40% ground pine wood sul-
phate pulp. The following results were obtained.
J 'i 2 ~ ~
Table 6
Birch Plne Ten- Stiff- Strength/
wood wood sile ness Stiffness
pulp pulp index index
(SR) (SR) (Nm/g) (Nm/g) *1000
. . . ~ . .
Without debonder 14 12 19.1 2750 6.9
Without debonder 14 28 25.5 3640 7.0
With, 4 kg ptp 14 28 16.7 2350 7.1
Without debonder 14 46 34.5 3970 8.7
With, 4 kg ptp 14 46 24.3 2620 9.3
_
As ls apparent from these results, the ratio of ten-
sile strength to tensile stiffness is advantageous when the
paper has a compositlon according to the inventlon.
Example 6
A pine wood sulphate pulp ground as below w~s mixed
with a deinked waste-paper-based pulp. The pulp had been
produced ln a deinklng plant, the waste paper consis~ing of
computer printouts, boo~s, brochures and the like. For the
sheet formation, use was made of a pulp mixture of 70% waste
paper pulp and 30~ ground pine wood sulphate pulp. The
followlng results were obtained.
Tabls 7
Waste Pine Ten- Stiff- Strength/
paper wood sile ness Stiffness
pulp pulp index index
(SR) (SR) (Nm/g) (Nm/g) *1000
Without debonder 24 12 32.0 4100 7.8
Without debonder 24 45 39.6 4660 8.5
With, 4 kg ptp 24 45 33.8 4020 8.4
35 With, 8 kg ptp 24 45 32.4 3700 8.8
With, 16 kg ptp 24 45 24.6 2660 9.2
2~21~
It is apparent from these results that the ratio of
tenslle strength to tenslle stiffness is advantageous when
the paper has a composition according to the invention.
Example 7
A pine wood sulphate pulp ground as below was mixed
with a wst-defibrated CTMP pulp. For the sheet formation,
use was made of a pulp mixture of 70% CTMP and 30% ground
pine wood sulphate pulp. The following results were
obtained.
Table 8
CTMP Plne Ten- Stlff- Strength/
pulp wood sile ness 5tiffness
pulp index index
_ (SR) (SR) (Nm/g) (Nm/g) *1000
Wlthout debonder 11 12 18.2 2200 8.3
Without debonder 11 45 31.6 2900 10.9
With, 4 k~ ptp 11 45 21.6 2090 10.3
20 Wit~, 6 kg ptp 11 45 17.5 1650 10.6
~ . -- . . .
It is apparent from these results that the ratio of
tenslle strength to tensile stiffnss~ i~ advantageous when
the paper ha~ a composition according to the invention.
