Note: Descriptions are shown in the official language in which they were submitted.
2095~
PATENT
BLEACHED CHEMITHERMOMECHANICAL HARDWOOD FIBERS FOR SOFT TISSUE
Backqround of the Invention
In the paper industry, chemithermomechanical pulps such as
bleached chemithermomechanical pulp (BCTMP) have been used as a
source of relatively inexpensive fiber for produc:ts such as multi-
ply board and the like. These pulps are generally produced by a mild
chemical treatment of wood chips and bleached with peroxide. Such
processes are well known in the industry. Because BCTMP is a high
yield pulp (about 90 percent or greater) containing a substantial
amount of lignin, paper sheets made with such pulps are generally
characterized as having high strength and stiffness. For this reason
BCTMP pulps have not been previously used for making high-quality,
soft t;ssue products such as facial tissue and bath tissue, where
softness is a primary attribute. Instead, pulps used for making soft
tissue products have been made using chemical pulps, such as kraft
pulps, which have a much lower yield of about 50 percent or less and
are therefore significantly more expensive.
Therefore there is a need for fibers or pulps which provide the
necessary levels of softness required for today's tissue products,
yet which are less costly than the chemical pulps currently in use.
SummarY of the Invention
It has now been discovered that hardwood BCTMP pulp fibers,
particularly maple BCTMP fibers, have unexpectedly good softness and
strength properties which enable one to make a relatively high-
quality tissue with a relatively low cost fiber source. As a result,
it is not necessary to bury the BCTMP fibers in the middle of the
tissue sheet by layering. Instead, the tissue sheets can be blended
using a mixture of hardwood BCTMP fibers (for softness) and longer
softwood fibers (for strength). If a layered tissue is preferred, the
hardwood BCTMP fibers can be utilized in the outer layer(s).
Hence, in general, the invention resides in a soft tissue,
useful as a facial or bath tissue, comprising a mixture of softwood
fibers and hardwood fibers wherein about 5 dry weight percent or more
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of the tissue fibers are hardwood BCTMP fibers. The softness of a
tissue can be measured by trained softness panels, consumer testing,
or by objective test methods which quantify factors known to
contribute to softness, such as stiffness, surface depth, smoothness,
protruding surface fibers, bulk, etc.
The amount of hardwood BCTMP fibers in the tissue will depend on
the tissue properties desired. Suitable amounts include from about
10 to about 60 weight percent, or alternatively from about 20 to
about 40 weight percent.
The softwood fibers can be any softwood fibers useful for
tissue, such as southern or northern softwood kraft fibers or fibers
produced by other pulping means. The presence of softwood fibers,
which are longer than hardwood fibers, provides strength to the
tissue. Preferred amounts of softwood fibers in the tissue are from -
about 10 to about 60 dry weight percent. Lower amounts will be
insufficient to provide adequate strength and stretch and higher
amounts will detract from the softness of the tissue.
The hardwood fibers can comprise blends of hardwood species,
including chemical pulp fibers or fibers produced by other pulping
methods, as well as hardwood BCTMP fibers. Established softness
fibers, such as are produced from eucalyptus chemical pulps, can also
be blended with the hardwood BCTMP fibers to further enhance the
softness. Preferred amounts of hardwood fibers in the tissue are
from about 40 to about 90 dry weight percent. Lesser amounts will
have little impact on softness, and higher amounts will lack adequate
strength or stretch.
Suitable BCTMP fiber species include, without limitation, maple,
birch, aspen and eucalyptus. Particularly preferred BCTMP fibers are
blends of maple and birch. It is believed that, during the pulping
operation, the longer birch fibers improve or enhance the break-up of
the shorter maple fibers by some mechanical interaction, thereby
resulting in pulp fibers more suitable for making soft tissue.
Examples
In order to illustrate the effectiveness of using hardwoGd BCTMP
fibers for making tissue, two-ply, blended, wet-pressed facial
tissues were made in a conventional manner using a number of
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differen-t papermaking furnishes. Specifically, the tissues were made
by depositing an aqueous suspension of papermaking fibers on~o a
forming fabric to form a web, dewatering and transferring the web to
a papermaking felt, pressing the web onto the surface of a Yankee
dryer, drying the web, creping the web, and plying the web together
with another like web to form a two-ply tissue sheet. The two-ply
tissues were calendared in some cases to improve the surface
smoothness and hence softness. All of the produc:ts were evaluated by
a trained sensory panel to ascertain their softness. Higher Panel
Softness numbers correlate with greater softness. The various
hardwood BCTMP fibers used herein were purchased from Tembec Company
in Canada.
The results are set forth in the Table below:
TABLE
Sample Furnish Calendared? Basis Weiqht Tensile Softness
Control 50% SW No 19.2 569 7.54
50% Euc Yes 19.3 540 7.93
1 35% SW No 19.4 610 6.73
35% Euc Yes 19.2 590 7.61
30% Aspen BCTMP
2 35% SW No 19.5 668 6.70
35% Euc Yes 18.5 627 7.40
30/O Birch BCTMP
3 20% SW No 19.3 694 6.30
20% Euc Yes 18.7 694 6.71
60% Birch BCTMP
4 35% SW No 19.0 408 7.55
35% Euc Yes 18.5 432 7.88
30% Maple BCTMP
35% SW No 19.1 473 7.60
35% Euc Yes 18.7 476 8.05
30% Maple BCTMP
6 35% SW No 19.5 593 7.51
35% Euc Yes 18.3 535 7.87
30% Maple BCTMP
7 40% SW No 19.2 488 7.17
60% Maple BCTMPYes 18.7 486 7.80
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As shown by these results, tissues containing substantial
amounts of hardwood BCTMP pulp fibers exhibit levels of softness
comparable to that of the control tissue made from hardwood kraft and
softwood kraft fibers.
It will be appreciated that the foregoing examples, given for
purposes of illustration, are not to be construed as limiting the
scope of this invention, which is defined by the following claims and
all equivalents thereto.
