Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to an improved process for
making paper. More particularly, it relates to a process
in which the improvement concerns the mitigation or complete
avoidance of "wet press pi~king" (or buildup on the wet
presses) as commonly occurs on industrial paper machines.
When a web is first formed on the wire or otherwise
in a papermaking process, it thereafter is threaded over and
under or between more than a dozen rolls including press rolls,
dryin~ and calendering rolls. As the web approaches the press
rolls it contains large amounts of water which previously
served to carry the paper fiber for web formation, and
considerable amounts of this water are removed by the action
of press rolls operating in pairs, a top roll and a cooperative
bottom roll. In usual practice~ the wet web is carried ~n a
felt through the nip of two or more pairs of press roll8, as
well as a pair of rolls commonly referred to as the "smooth-
ing press" which, together with the press rolls, constitutes
the wet press section of the paper machine. The web then
enters the dryer section of the paper machine immediately
following the smoothing rolls.
The web makes direct contact with the upper pres~
, rolls and it is at this point that a problem termed "wet press
`, p~cking" (or buildup on the wet presses) is often developed.
The press rolls nearest the headbox are often referred to as
"wet" press rolls in the industry. For convenience, the
term "press rolls" as used herein will generally designate
"wet" press rolls, as well as the smoo~hing press rolls.
The bottom press rolls may be slotted or vacuum equipped
for improved dewatering and, as previousl~ indicated,
covered b~ an endless moving felt to absorb water from the web.
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The problem of wet press plcking is manifested
wherein small agglomerates of fibers from the web, with or
without pigment or other particles (at times, just barPly
visible to the naked eye) are picked up from the web and
attach themselves to the surface of the press rolls which come
in direct contact with the travelling web. The deposited
particles in turn create an obstruction on the press roll
surface sufficient to detach a small portion of w~b from the
moving web surface, constituting a singular point of wet
press picking. The press roll turns at high speeds, and it
is understandable that the picking may be repeated at localiz-
ed areas of the press roll and in many cases the progressive
buildup may be serious enough to cover the entire contacting
surace of the roll. In extreme cases, the tacky roll surface
may cause the moving web to ollow and wrap itself around the
roll resulting in a web breakage and considerable downtime.
Even in less e~treme cases, the surface of the paper will be
generall~ gouged or badly disrupted causing serious quality
problems
2~ The main cause or causes of wet press picking have
not been identi~ied with any certaint~. It is known, however,
that a number of factors tend to initiate, contribute or
aggravate the problem. Among these factors are included:
(a) origin and type of pulp with hardwood pulps generally
being more susceptible to wet press picking, (b) operating
with little stock refining or web moisture outside of a
prescribed moisture range, ~c) impurities such as residual
pulping impurities, pitch, slime or oam in the papermaking
furnish (or feed stock), and (d) the inclusion of various
additives in the stock slurry prior to sheet formation,
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` particularly high concentrations of rosin or other commerciallysupplied sizing agents, as well as relatively high concentra-
tions of polymeric binders and other chemical additives.
Various measures have been contemplated by the
industry in recent years for dealing with wet pres~ picking.
To correct wet press picking according to recent prior art,
most often the press roll would be equipped with a doctor
blade and/or water shower. Such measures are not fully effec-
tive, h*wever, and the US2 of a water shower also introduces
undesired moisture to the web. Use of press rolls of various
surface com~ositions, for exam~le, specially compounded hard
or soft rubber, granite or stone, polymer coated or filled
plastic surfaces, have also been tried in an attempt to over-
come the problem, but none have resolved the problem and no
definite conclusions as to the superiority of one composition
over another have been reached. Machine grinding of the rolls
to effect special surace characteristics have also proven to
i be unsuccess~ul. The addi~ion to the feed stock of extra
amounts of alum over tha~ ordinarily used has been found to
reduce picking at times, but the procedure is not generally
reliable and may actually create more picking under certain
I conditions. Careful control o~ operating conditions, wlthin
limits imposed by specific manu~acturing obiectives, can
alleviate the problem, but not fully eliminate it when it
arises. These measures incLude maximizing fiber refining and
retention characteristics in the sheet-forming process, opti-
mization of vacuums, draws and other machine variables, etc.
Certain materials including, for example, natural gums, various
pitch dispersants, talc, sequestering agents, etc. added to
the furnish have provided only modest improvements in some
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cases. On an industry basis, wet press picking is still a
common occurrence and none of the attempts to correct the
; picking can be said to be fully successful in eliminating the
problem.
DESCRIPTION OF THE INVENTION
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I have now discovered that wet press picking is
substantially mitigated or essentially eliminated when a
specified polymeric siloxane is added to the feed stock in a
specified concentration range in accordance with the process
of this invention. Moreover, the prevention of wet press
picking in this manner is independent of the composition o
the press roll or its surface characteristics. The process
~ has been found successful in papermaking conditions employing
-~ high or, alternatively, low concentrations of alum or sizing
agents. The process is not dependent on the source or type
of pulp used in the feed stock, the presence of other additives,
etc.
, The principal object of this invention is to pro-
vide an improved process of making paper wherein wet press
picking is substantially or completely eliminated 90 that it
no longer presents a problem to the paper maker under normal
, papermaking conditions.
The principal ob~ect of the present invention may
be accomplished by reference to the following detailed de-
scription.
The polymeric siloxane additives useful in the
process of this invention are of two classes: .(~) a 1uid,
.~ater-soluble copolymer of dimethylpolysiloxane-polyoxyalkylene
ether wherein the alkylene moiety may be ethylene, propylene
or mixtures thereof. Examples of this copolymer available
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; commercially are the products designated "SF-1066" sold by
General Electric Company and "L-7001" sold by Union Carbide
Corporation, and ~B) an aqueous emulsion of dimethylpoly-
siloxane or se1f-emulsiying mixture of dimethylpolysiloxane
and surfactant. The emulsified dimethylpolysiloxane must
remain water dispersible in all proportions. Examples of
useful dimethylpolysiloxane emulsions commerc~ally available
are the products designated "SM-2061" sold by General Electric
Com~any; "LE-466" sold by Union ~arbide Corporation; and
"HV-490" sold by Dow Corning Corporation.
A~ to their actual use, the polymers are preferably ~-
diluted with water and thereafter added to the headbox or
stock preparation system containing cellulose fibers and
other papermaking ingredients to provide an amount ranging
from about 0.005 to 0.15~/o~ preferably 0.01 to ~.05% of siloxane
polymer based on dry fiber weight. When sizing agents,
strength additives, or retention aids are used in the s~ock,
the siloxane polymer may be added in a~propriate amounts
directly to aqueous dispersions of ~hese materials which, in 20 turn, are subsequently added to the headbox or stock prepara-
tion system. The aqueous siloxane dispersions may also be
sprayed on the web as it travels over the forming wire. The
manner of addition is of no serious conQequence, it being
necessary only to see that the siloxane polymer is uniformly
present in the stock in the required concentration pr~or to
the we~ entering ~he press section of the paper machine.
It is to be noted that since these aqueous siIoxane
dispersions are chemically and electrochemically inert in the
stock system and are also added to the paper stock during the
"wet end" portion of the paper making operation, most of the
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siloxane is released in the plant effluen~ and little, if
any, of the siloxane polymer i9 actually present in or on the
final drled paper. Although measurement of such small quan-
tities of polymer i9 virtually impossible, it is estimated
that, at most, approximately one-quarter of the initially
charged polymer is pres~nt in the paper after the drying is
completed.
It is also noted that the addition of the poly-
siloxane polymer within the specified con~entrations does not
affect other variables in the papermaking process. Thus, the
polymer has no deleterious effects on the properties of the
resultant paper, for example, strength, porosity, smoothness,
printability, optical properties, and the like, since measure-
ments of these proper~ies show them to be within established
statistically variable limits for untreated paper products.
The invention will be more fully illustrated by
the examples which follow representing specific embodiments
of the invention and is not to be construed as a limitation
thereon.
EXAMPLE I
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A series of tests were conducted on a Fourdriner
paper machine wherein the press section onsisted of two
main presses followed by a smoothing press, each press con-
sisting of a top and bottom roll. The first press consisted
of a straight-through plain press ~ith a standard rubber
covered top roll typically used in the industry and the
second press was a plain reversing press with a comyosition
("Microrok") covered top roll, also typically used in the
industry. The smoothing press consisted of a straight-through
set o~ rolls with a "Press-Tex" metal surfaced top roll and
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composition covered ("Micromate") bottom roll. The basic
papermaking furnlsh consisted of a very lightly refined mix-
ture of approximately 80% bleached hardwood kraft pulp and
20% bleached softwood kraft pulp.
Additives noted below, commonly used in paper
making operations to impart sizing and ~trength, were added
continuously to the stock preparation system and a sheet
of paperboard was formed at apprGximately 123 lbs. per 3,000
sq. ft. basis weight. When equilibrium conditions were
reached, two polymeric siloxane additives of this invention
(as further identified below) were diluted to 1~ solids and
added continuously to the ~tock system in concentrations
specified in the following table. Build-up due to picking
was ascertained under the indicated conditions on each of
the press rolls and noted in descriptive terms: none,
slight, moderate, heavy, etc. The flnished paper was tested
in terms of its physical properties.
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Table I
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~ Stock Conditions Buildup Noted ~Eter Running 15 Min.
; 1st Press 2nd Press SMoothing
Press
-- . -- . .
(1) Base sheet - No additives Moderate Slight Slight
(2) 0.6% Alum added Slight Slight Sli~ht
(3) 1% Rosin ~ 2% Alum added Moderate Slight Moderate
(4) 0.5% Starch added Moderate None Sligllt
(5) 0.25% Synthetic Size* Heavy Moderate ~leavy
- 10 ~ 0.25% Alum added
(6) 0.25% Synthetic Size* Very heavy Heavy Very heavy
.05% caustic added
~ (7) 0.025% Dimethylpoly- None None None
r siloxane/Polyethyleneoxide
polypropyleneoxide
copolymer (SF-1066) aclded
to stock condition (5)
(8) 0.01% Dimethylpoly- None None Slight
siloxane/ Polyethyleneoxide -
polypropyleneoxide
I copolymer (SF-1066) added
to stock condition (5)
(9)0.025% Dimethylpoly- Slight Slight Slight
siloxane/Polyethyleneoxide
polypropyleneoxide
copolymer (SF-1066) added
to stock condition (6)
(10) 0.025% polyclimethyl- None None None
siloxane em-llsion
(SM-2061) adcled to stock
condition (5)
(Ll) 0.01~/~ polyclimethyl- None None Slight
siloxane emulsion
(SM-2061) added to stock
;~ condition (5)
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*alkenyl succinic anhydride
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The concentrations of the various ingredients listed
in the above table are expressed in term~ of per cent active
ingredient by weight of dry pulp.
The aboue results clearly illustrate the improved
results with respect to wet press picking obtained with the
use of two polysiloxane polymers typical of this invention
under various conditions contrasted to a number of stock
~ conditions in which no polysiloxane polymer was added.
i Subsequent physical testing of a~l paper produced
in terms of strength factors, sizing, porosity, ~urface
characterlstics, etc., showed no statistically significant
differences due to the addition o the siloxane polymers. In
order to illustrate the fact that there were no statistically
significant changes in physical property, the water resistance
; (sizing properties) and MuIlen burst strength of the samples
~; produced in Sample 5, 7 a~d 8 were tested.
~ Water resistance was measured using the TAPPI
-~ stand~rd method T4410s-69 wherein the amount of water absorbed
by the sheet over a period of two minutes was measured. The
values obtained, des~gnated Cobb size values, are shown in
grams/sq. meter. Measurements are taken on both the top (fel~3
an~ bottom (wlre) sides of the paper. In this testing proce-
dure, lower Cobb values indicate higher water resistance.
The Mullen Burst Strength wa~ measured using ASTM
te~ting method D774-67. According to this method a sheet of
the paper is clamped between two ring shaped platens, thus
leaving an exposed circular surface of paper under which there
is an inflatable rubber diaphragm. As air is pumped into this
diaphragm, it expands and comes into contact with the exposed
surface of the paper. Note is made of the pressure in p.s.i.,
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at which the diaphragm caused the paper to burst.
Higher values indicate stronger paper. The values
are shown in p.s.i.g.
;~ Mullen Burst
Sample Stock Conditions Cobb SizeStren~th
~ elt/wire) ~~
0.25% Synthetic size 31/32 35.0
(alkenyl succinic anhydride)
0~25~/o alum added
7 0~ 025~/o Dimethylpolysiloxane/ 34/36 34.0
polyethylene oxide poly-
propyleneoxide copolymer
(SF-1066~ added to stock
condition (5)
8 0.01% Dimethylpolysiloxane/ 31/34 35.0
. polyethyleneoxide poly-
~; propyleneoxide copolymer
(SF-1066) added to stock
-, condition (5)
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Although there were minor variati~ns pre~ented above,
they are within the range of experimental error and the limits
` of the tests and there were no statistically significant
; var~ations in water resistance or strength between the samples
prepared with the polysiloxane in the stock system and those
prepared without the additive. Thus, the fact that in the
Cobb size test, higher amounts of siloxane yield apparently
poorer water resistance (and is therefore in fact contrary to
what would be expected) i9 considered to be due to the
experimental error and imprecise nature of the test.
3~ EXAMPLE ~I
In crder to shcw that addition of the particular
polysiloxanes oE the pr~sent invention in amounts as high as
0..15% based on the dry film weight have no siz~ng effec~s on
the final paper, handsheets were prepared and te~t~d a~ follows.
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Sh?eets were prepared according to TAP~I ~tandards
using bleached kraft of 50 lb./3000 sq. ft. basis weight,
adjusted to pH 6 with alum. All the sheets contained 0~2V/o
of the alkenyl succinic anhydride synthetic size and 0.03~/O
of a cationic retention aid. Sheets were tested immediately
after drying and again after one hour cure at 105C.
In comparing the water resistance of the~e sheets,
use was made of a dye test employing crystals of potassium
permanganate and an acid ink penetration test. In the dye
test several crystals of potassium permanganate are placed on
the upper surface of a swatch of test paper which is then set
afloat in distilled water at room temperature. As the water
is absorbed into the paper the crystals are moistened and
impart a characteristic deep violet color to the paper. The
time measured in seconds required for an end-point where
three colored spots first appear on the paper surface is
noted and is in direct relation to the water resistance since
a more water resistant paper will retard the moistening of
the permanganate crystals which had been placed upon its upper
surface.
The acid ink penetration test is a c~pari.son test
wher~in a swatch of test paper is floated in a dish of acid
ink (pH 1.5) at 100F. and the time measured in seconds
required for the ink to penetrate through the paper to reach
an end-point where about 50% of the paper is colored is noted.
The following table presents data on the various
paper sheets which were compared in the described testing
procedures.
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,~ Acid Ink Penetration --
(Time in Seconds) KMnO4
Uncured Cured (Time in Seconds)
Sheets containing no 65 60 62
silicone (control)
Sheets containing 0.15%62 62 66
dimethy~polysiloxane/
~- polyethyleneoxide poly-
--~ propyleneoxide copolymer
Since the above tests are con~idered to be valid
within a tolerance of + 10~/o ~ it is seen that there are no
statistically significant sizing effects apparent from the use
of up to 0.15% of the polysiloxanes of the present invention.
Summarizing it is seen the invention provides an
improved process for making paper essentially eliminating the
problem of wet press picking b~ use of speci~ied polymeric
siloxane additives. Variations may be made in materials,
proportions and procedures without departing from the scope
of this invention.
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