Note: Descriptions are shown in the official language in which they were submitted.
~ 2~4
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to paper-making, and more
particularly refers to the production of a calcium sulfate-
filled and neutral sized paper particularly well adapted for
use as cover sheets in the production of gypsum wallboard.
Description of the Prior Art
. . _ _ . _ .
Paper for gypsum board is conventionally made by pulping
up waste paper constituents o~ old corrugated paper, or kraft
cuttings and waste news. In cleaning, screening and refining
the suspended materials in water suspension, the process paper
stock is diluted still further with water and then formed by
draining tlle plies of paper on several continuously moving wire
cylinders, where the separate plies are joined together by a
carrying felt. The weak paper web is then dewatered in a press
--3--
~;~2090~L
section where water is pressed out o~ the web. The pre~ssed
paper is dried in a multi-cylinder dryiny section wi-th steam
added to each cylinder. I'he dried paper is subjected to a
squeezin~ or calendaring operation for uniformity in thickness
and is then finally wound into rolls. The paper is subsequently
utilized as paper cover sheets to form gypsum wallboard by de-
positing a calcined gypsum slurry between -two shee-ts, and per-
mitting the gypsum to set and dry.
Conventional paper used in gypsum wallboard has definite
limitations with regard to the utilization of heat energy.
First, it has definite drainage limitations in forming and
pressing, and additional limitations in the drying rate. The
drainage rate limitations impose a large paper drying energy
load on the mill. It would be highly desirable to have a more
porous paper ~or utilization as paper cover sheets in the for-
mation of gypsum wallboard to permit the achievement of a sub-
stantial reduction in drying energy load, while still having a
paper which has the requisite physical properties with regard
to physical strength even though less pulp is utili~ed.
2 0 SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide
paper for use as paper cover sheets in the production of
gypsum wallboard.
It is another object of the invention to provide paper
for use in making gypsum wallboard which is highly porous
and requires less energy for drying than conventional paper
previously utili~ed for this purpose.
~o~
It :Ls sti:l.J. another object to provi.de a paper o.E
the type descrlbed whi.ch has sufEic.lently hiyh tensile
strength :Eor use in gypsum wallboarcl.
It is stlll. a further object to provide a porous
paper :Eor makiny gypsum wal:Lboarcl wh:Lch :Ls so treated
that excellent adheaion is ob-tained between the paper
cover sheet and the gypsum core even though the paper
has a greater porosity than that f~und in conven-tional
paper.
Other objec-ts and advantages of the invention will
become apparent upon reference to the description below.
According to an aspect of the invention, a paper
eminently suitable for use in fabricating gypsum wallboard
is produced using substantially conventional paper processes,
and having the following composition (dry weight basis):
(A) a major proportion of cellulose fibers;
(B) from about 10 to about 35 percent calcium sulfate,
as a filler;
(C) from about one -to abou-t 3.5 percent of a binder;
(D) a neu-tral sizing agent in an effec-tive amount to
prevent water penetration;
(E) from about .25 to about 10 percent of a buffering
agent comprising a salt of a cation of a strong base
and an anion of à weak acid; and
(F) an anionic retention aid in an amount suitable for
retaining the filler in the paper.
In a preferred embodiment, after the paper is treated
wi-th a neutral internal sizing agent during its formation,
it is subsequently treated with a surface sizing agent after
formation of t.he paper, in order to provide certain
ye/
~oso~
properties includLng be-tter adhesLon to the gypsurn core
when used to make gypsum wallboard.
In a Eurther preEerred embodiment, the paper comprises
from about .1 to about .2 dry weigh-t percen-t of a cationic
flocculant.
Durlng the paper-making process, rapid drying is obtained
with less than the normal amount of heat energy required. The
finished paper has excellent porosity, tensile streng-th and
fire resistant properties. Further, when the paper is utilized
as paper cover sheets in the manufacture of gypsum wallboard,
the porosity of the paper facilitates the drying and se-tting
of finished wallboard.
The paper may be utilized as paper cover sheets for the
production of gypsum wallboard. In the setting and drying
of the wallboard, because of the excellent porosity of the
paper, less energy need be utilized and more rapid drying
is obtained, to produce a wallboard wherein the paper has
excellent tensile strength and Eire resistant properties.
In a preferred embodiment the paper is treated with an
internal sizing agent during its formation, and subsequently
treated with a surface sizing agent after formation, in order
to provide better adhesion to the gypsum core.
6 -
12~ 4
DESCRIPl`ION OY THE PR~[F,RRED EMsoDrMENTs
In the examples which follow the paper was pr~apared by
the method oE Procedure A which followso
PROCEDURE A
An aqueous slurry was prepared comprising 20 oven dry
grams of fiber and 3500 ml of water. The slurry was sub-
jected to stirring with a three bladed propeller a-t ~00 RPM.
During the agitation, the designated ,mount of CaSO4 land-
plaster filler in amounts of from 10-30% were added dry to
the slurry. After three minutes of agitation, 4 lb/ton of
the designated flocculant were added in a solution containing
0.1% solids. After agitation was carried out for an additional
three minutes, the designated amount of ~inder in amounts from
about 0.5% to 3% were added at a total solids content from about
3% to 50%. Stirring or agitation was continued at 1250 RPM for
an additional three minutes after which time the slurry was
diluted to a consistency of 0.3% total solids content. A
sufficient amount of the slurry was then added to a standard
6-1/4" (159mm) diameter sheet mold. Size emulsion in desig-
nated amounts was added to the sheet mold contents whichwere subsequently agitated. After agitation, the anionic
polymer retention aid was added to the sheet mold in de-
signated amounts followed by agitation. A 1.50 gram handsheet
was subsequently formed in the sheet mold. The drainage time
was recorded and the wet sheet was couched off the 150 mesh
sheet mold screen.
~22~90~
Handsheets were stacked while s-till wet on blotters and
then covered with a mlrror polished disc. The handsheets
were then pressed at 50 pourlds/square inch ~or five and one
half minutes. At this point the wet blotters were removed
and the handsheets were inverted so that the metal plate was
on the bottom. Dry blotters were utilized to replace the wet
ones and the stack was pressed at the same pressure for two
and one-half minutes. The partially dry handsheets were peeled
`off the metal plates and dried on a rotating drum dryer for one
pass which took approximately 40 seconds. At the end of this
period the handsheets were dry.
The dried handsheets were then coated with 0.35 lb/ton of
a silicone surface sizing agent and then redried for 20 seconds
in the handsheet dryer. Afterwards the handsheets were oven-
cured at 140F for 24 Hrs. and then allowed to come to equili-
brium at room conditions for 1 hour before testing.
Additionally, in the examples which follow, where gypsum
board was prepared from the papers which were fabricated and
described in the tables, the gypsum wallboard was prepared
from the papers utilizing the method of Procedure B which
follows:
PROCEDURE B
PRODUCTION OF GYPSUM WALLBOARD
Gypsum wallboard was produced by discharging a stucco
slurry from a mixer onto prepared paper with the topliner
face downward while the paper was moving continuously. A
top sheet, which is newslined, was brought into contact with
the upper surface of the slurry, and subse~uently the combina-
tion of facing papers and slurry was passed under a forming
~20~
roll to distribute the slurry uniforrllly and to form the board
into a uniform cross-section. The edyes of the paper were
Eolded up and over the edges of the top paper, and the edges
of the board were formed in the same operation.
The wet gypsum board was carried through the forming
section of the board machine on a continuously moving belt
unti:L the board core was fully hydrated to calcium sulfate
dihydrate. Subsequently, the board was conveyed onto con-
tinuously moving strip belt conveyors to the knife section
where the board was cut into conventionally desired lengths.
The board was then inverted with the manila face up and
fed into a drying kiln on continuously turning rollers,
where it was dried to a uniform 5-6% moisture content. The
board was inspected and then stacked into packages.
Testing of Gypsum Wallboard
Before gypsum wallboard is marketed it is first sub-
ected to specific quality control tests to ascertain that
the board meets quality standards~ A~ong the various tests
which are generally conducted are ASTM nail pull and trans-
verse strengths~ Also tested are humidified bond for bothface and backsides of the board, face Cobbs and total immersion
absorption water resistance tests on board to be used for
high humidity application and/or sheathing board, and face
absorption water absorptiveness tests on board for plaster
application.
The nail pull test consists of applying an ever-increasing
amount of weight on a specially designed nail until the head is
pulled through the board sample. Weight at ~ailure is recorded.
C~9~
Trallsvers~ strell-Jth te~ts are carried out by ar~plyirlg a
~orce downwar(lly in the center of the s~)ecilllell Wtlic~l is sup-
ported at two opposing outer edges. The face which is ~0'3i-
tioned downwardly is the face wh;c~l is tested. E'or(:e applied
at Eailure ls the measurement of transverse strength.
The humidified bond test consists of humidifying the board
~or three hours at 90~ relative humidity and 90F tem~erat:ure,
and then applying a force on the board sufficient to break the
- bond between the paper and the board core. The applied force
or weight at failure is the measure of bond strength.
Face Cobb and absorption tests are carried out by con-
ventiorlal methods.
The total i~nersion water absorption tests are conducted
- by immersing a 12 inch by 12 inch sample of board for two hours
in 70F temperature water. The weight of water absorbed is
determined by aifference and converted to percent absorption
based on dry weight.
In U.S. patent 4,372,8l4, issued
February 8, 1983 results are
given of tests ~ade utilizing calcium sulfate as a mineral
filler for paper to be used in making gypsum walLboard. The
results of those experiments were not entirely satisfactory
since insufficient calcium sulfa-te was retained in the paper
when used with the retention aids disclosed therein~ It has
been subsequently found that excellent retention of calcium
sulfate as a mineral filler may be accomplished by -the use
of an anionic polyacrylamide retention aid when added to the
--10--
~22~
dilute furnish used when making the paper. When this anionic
polyacrylamide polymer was added, the filler retention was
vastly improved and the handsheets streng-ths were also
improved.
In Examples 1-17 below are results for various handsheets
containing various proportions of calcium sulfate landplaster
(CaS04 . ~H20) as a filler together with varying amounts
of an anionic polymer utilized as the retention aid. The
handsheets were all prepared according to Procedure A.
The compositions and the results of conventional paper tests
are shown in Table I below. Since the experiments were de-
signed primarily to test the effect of the anionic polymer as
a retention aid, convention ingredients normally used in making
paper suitable for use in making gypsum wallboard were not in-
corporated.
Further, the white water from a given handsheet was re-
circulated to make the subsequently formed handsheets.
39(~L
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--12-
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From the results shown above, it is eviden-t that as the
number of recirculations increases Ca ion concentration
builds up, reten-tion of the landplaster deteriorates wit~out
the admixture of the anionic polymer retention aid. This is
true for every level of landplaster evaluated. This constituted
27~ in examples 1-7, 20% in examples 8-12, and 10% in examples
13-17. The results of examples 6 and 7, where a cationic poly-
mer instead oE an anionic polymer was used clearly illustrate
the deterioration in both filler retention by percent and breaking
length with the admixture of the cationic polymer. These results
clearly show the need for the use of an anionic polymer for proper
landplaster retention in a system that is highly anionic as in
dicated by the negative Zeta potential.
In examples 18-27 below paper handsheets utilizing calcium
suifate dihydrate as a filler, an anionic polymer as a retention
aid and a ketene dimer as an internal si~ing agent were prepared
according to the method of Procedure A above. The compositions
utili2ed in examples 18-27 and the conventional tests results
are shown below below in Table II:
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r~
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r tD ~:
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~ ~ w r N N N r.~ N N N t`l r. N O
O ,a r~ rD rl r~,
r~ rn .~c rr. ~ ù u
~ r 1 ~:: Ei r
t~ r~ ~ o ~ u~ r~ ~ N r,~ r,~
r~ . ~ (n rn ~t~ w un w m w -r Ir~ In In U rl
æ r~ a a v ~n rE~ O o o o c; o o o c; c; ~ r
r.~ O ~n ~ ~ o
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H ~C :~ D ~r ~ r~ ~ rl 1~1 r~ ~ r~ t~ r,~ ro .~
m r`æ~; n r~ ~n ~ ~ r r j r c~ c~ t~ t~a; W t O ~r
t~, r~ r~ ~n ta ,1 r .
E~ æ a t~P r~ ~ r~ rD u
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rn m C r~ r r_ t~ rJ~ r_ ~ . n al
r4 _ al ,1~ ~r r~ w o o o ~ n r~
W ra D a u~ w'n ~r n In~n ~r ~r ~r o ~r W
Q N . t'l
rL~ 'D ~ ~C t~ ~ t~
ræ~ p~~0 "~O ,~N t'l ~r mw .~ g ,~
X ''3 Z ~ ~I N N N N N N N N ~ D
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~1~221~90~
The results o the e~perLrlle~lts of Examples 18-27 ai~ove
and sho~wn in Table II indicate the suitability oE a ketellc
dimer as arl interncll size for calcluln sulEate dih~drate
(landplaster) filled gypsum board paper. The results stl0wn
in the table are based on tests conducted on handsheets
prepared by the method of Procedure A except that satllr~lted
calci~n sulfate water was used in ~laking up the Eurnish arld
in dilutinq the furnish in a lar~e 12" x 12" shee-t mold
where heavier basis weight handsheets were produced than
those produced by the basic method of Procedure A. The
data shown above in Table II indicate that where a proprietary
ketene dimer size such as HERCON 32 marketed by the Eiercul;es
Company, is used either with an anionic or a cationic polymer
retention aid, it provides improved landplaster retention in
the presence of calcium ions. The paper sizin~ tests indicate
that the HERCON 32 ketene dimer size provided excellent
sizing results regardless of the charge of the re-tention aid
and with or without silicone surface size application. The
board test data also shown above in Table II demonstrate
that with few exceptions the ketene dimer internally sized
paper, when additionally surface sized with a silicone
polymer, provides good bond to gypsum board. The desirability
of the use of the siIicone surface size is indicated by the
generally lower bond failures and higher bond strength which
were obtained when the silicone size was applied, compared
to the halldsheets where it was not applied.
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The ketene dimer has the following structural formula:
R - C~I = C - CM - R
O - C = O
~'
`- wherein the two radicals, R, each may have 8-18 carbon
atoms, and the indicated ring is a lactone ring.
Conventionally, the ketene dimers are foI~ed from a 50/50
mixture of palmitic and stearic fatty acids, although they may
be formed from any fatty acids with 10 to 20 carbon atoms. The
ketene dimers are usually emulsified in a cationic starch solu-
tion in the ratio of 3 or 4 parts of dimer to l part of cationicstarch. Proprietary ketene dimers usually contain a cationic
polymer which ac~ts as a size retention aid in the paper machine
furnish.
Succinic Acid Anhydride as an Internal Sizing Agent for CaSO4-
Filled Gypsum ~oard Paper
In Examples 28-30 succinic acid anhydride internal size
was utilized in preparin~ a calcium sulfate-filled gypsum board
paper, utilizing an anionic polymer as a retention aidO Addition-
ally a silicone polymer size acidified with 1~ alum solids was
applied as a surface size to the dried paper. The formulations
of Examples 28-30 and tests results of the prepared paper, as
well as gypsum board samples prepared from the papers as shown
below in Table III. Additionally tests results of the paper and
of the prepared gypsum board are shown~
-16-
~2~09~
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0 rn H ~¢ ~n ~ u R~ ~ U ~C
r4 ~ ~ .5 .~
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rUn~ n ~ ~ ~ r~ u r~ r.~ c~
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--17--
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The data obtained from tes~in~ the samples oE Examples
28-30 show that at all levels of anionic polymer application,
paper sizing by the combination of internal size and silicone
resin surface sizes produced excellent paper. The tests made
on the finished gypsum wallboard show that the anionic polymer
is particularly use~ul in providing a sheet which demonstrates
superior bond to the gypsum hoard to which it is applied.
This is shown by the decreasing bond failure and the increasing
bond strength with increasing anionic polymer addition. The
handsheets prepared and illustrated in Table III were prepared
in a method similar to that of Procedure A but with a large
12" x 12" sheet mold to produce 12" x 12" 55 lb/1000 ft2 basis
weight handsheets.
The internal size utilized was ACCOSIZE 18, a trademarked
product marketed by American Cyanamid which contains 1% anionic
surfactant as an emulsifying agent, and which was emulsified in
a turbine emulsifier with 3% cationic potato starch as the
emulsifying medium. Two pounds o~ starch were used with each
pound o sizing material.
In Examples 31 and 32 a ketene dimer was compared to succinic
acid anhydride as an internal size for calcium carbonate-
filled gypsum board paper. The handsheets of these examples
were prepared in a manner similar to that of Procedure A, except
that the large 12" x 12" sheet mold was used to make 12" x 12"
handsheets of 55 lb/1000 ft2 basis weight and a cationic in
place of an anionic retention aid was used. The formulations
and xesults are shown below Table IV.
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b ul~al) a~r~ rurJ~ coco a.
O > u~ ~1 ol r~ U~ o er ,1 ~r
I rlr~l r,l C o u; ~r r~l r; ,i u; .1 rJ
D ~1~ s ~o ~DU3 ~D U~ ~O
D CU~ ro ~ ~ rD u~ ~ o o ul Ir~ U
r ~~ rQ, ~ r~ a~ O r; ~ C U
: r ~ r
~: V C o _I .1 r,l u o o o c~ o o rJ ~
- : ~n ~rl.rl ~t:l rJ D r~ r~ ~1 ~I h r~ 3
~ C~ ~ r . a~ ~ ~ r~
r~ rar~ U u
O O ~ O ~ ~ ~ _~ r; ~ U ~
O rl Ul r~ 1 Q 0~ ~1
o ~t D O Q, r r~ er u~ ~o 1- ru
~ _ ~ CU W Z ~
J~ X ~ :
-20-
,
f
~2~
~ s indicated in the above table tllt! keterle dim-~r ~)rovides
excellenL sizinq ancl paper borld perEornlallce, comE)are~l to the
succinic acld anllytlride.
Exanlples 33-38, the data for which are shown in ~'ahle IV
above, demonstrate the advantages to be obtained by the use
of STA-LOK 500 cationic potato starch as a binder when used
together with an anionic polymer as a retention aid. The
handsheets prepared in examples 33-38 used ln this st:uciy
were prepared in a manner similar to that of Procedure ~,
except that saturated calcium carbonate (CaCO3) wa-ter was
used to make the handsheets within the sheet mold as dilution.
The results show that cationic starch binder provide-; excellent
retention of the filler and that improvement in filler
- retention and porosity is provided by the anionic polymer.
The examples above resulted in the preparation of
handsheets by laboratory methods. Consequently a buffer
such as calciurn carbonate was not added. However, in a full
scale paper-making operation, a buffer sucll as calcium
carbonate must be added to the calcium sulfate-fillecl furnish
because the calcium sulfate tends to buffer the system to a
lower pH. This is not a problem in the laboratory where
there is no build up in acidity from the lab furnish, and
consequently no buffers were used in the laboratory experi-
ments. HoweVer, on a paper-making machine which engages in
a large amount of recirculation of water which is drained
from the furnish in making the sheet~ continual input of
acidic paper stock causes a build up of acidity in the
-21-
*trade mark
. . ~
,
12;~09(~
sys-tem which must be buffered to maintain neutral to slightly
alkaline conditions in order to insure that the strenyth o~
the sheet will be optimum.
Example 3_
A commercial run is carried out in the plant to produce
a calcium sulfate dihydrate paper for conversion to market-
able gypsum board. The paper line is firs-t set up to make
conventional paper utiliæing 100~ conventional paper stock.
After the line is running, the process is converted to making
calcium sulfate paper by adding a cationic flocculant, finely
ground calcium sulfate dihydrate filler and calcium carbonate
buffer to the filler refiner dump chest. Latex binder is
added to the filler machine chest followed by addition of
anionic polyacrylamide retention aid to the dilute machine
furnish after the fan pumps.
The initial paper is comprised of succinic acid anhydride
internally sized regular furnish manila paper which is the cover
sheet which faces outward when the gypsum board is attached to
the wall ~rame. The change over to calcium sulfate furnish is
accomplished ~y adding calcium sulfate landplaster and latex to
the filler portion of the sheet at twice the steady state rate
and the cationic flocculant, and anionic retention aid at the
steady state rate. Water is added to the topliner and dilute
aqueous silicone emulsion is added to the bondliner in the
wet calender stack after the dryers. The silicone emulsion
contains 1% alum solids. Internal sizing levels are adjusted
to provide sufficient moisture pickup, 2.5%, in the calender
stack~ Internal sizing levels applied to the various plies
-22-
12~
are 3, 8, 5, and 9 lb/ply ton of succinic aci.d anhydride
cationi~ed with 2.0 lb cationic starch/lb of size utilized
respectively in the two bondliner plies, the filler ply
beneath the topliner and the two topliner plies. The bond-
liner of the filler portion of the sheet is the part in
contact with the gypsum core of the board. The topliner is
the portion of the sheet facing outwarclly. The bonAliner
internal and surface sizing levels are set to provide uniform
resistance to excessive wetting of the sheet in board manu-
facture. The topliner internal sizing is set to obtain adequatedecorating properties of the dried board.
Steady state proportions in the filler stock portion of
the sheet are achieved as given below following conversion to
calcium sulfate dihydrate filled paper:
Fiber : Kraft Cuttings 56%
Waste News 14
Fillers : Calcium Sulfate 22-25
(Dihydrate)
Calcium Carbonate 2-5
(Buffer)
Chemicals : Styrene-Butadiene
Latex 3%
Cationic Polyacrylamide
Flocculant 2-4 lb/ton
Anionic Polyacrylamide
Reten-tion Aid 0.5-1.50 lb/ton
Silicone Surface Size
Solids 0.35-0.50 lb/ton
The manila topliner comprising.25% of the total manila
sheet consists of flyleaf or magazine trimmings.
~l2~)90~l
Following manuf~cture of fill~d manila, nçwslined, -the
covering paper which Eaces toward the house frarrle is made
using above filled-paper stock proportions throughout all of
the sheet. Sizing levels of succinic acid anhydride employed
are 4, 8, 8 and 9 lb/ply ton in the bondliner plies and the
two top plies respectively, where the bondliner is the
portion of the sheet against the gypsum core.
The papers so formed as above are more porous and give up
moisture by drainage and drying more readily than conventional
gypsum board cover sheets. These properties provide substantial
drying steam energy savings of 27%. The papers formed above are
then used to produce gypsum wallboard in the conventional manner,
as described in Procedure B above. The more open porosity of
the filled-paper compared to conventional paper provides a 5%
board drying energy savings due to easier drying. The con-
verted board demonstrates excellent paper-to-core bond, trans-
verse strengths and decorating characteristics.
The following are the desired ranges for the various con-
stituents utilized:
Fiber Freeness
Range : 300-550 ml. CSF
Optimum : 350 ml~ CSF
Filler, as Cal_ um Sulfate or Calcium Carbonate
Range : 10-35 dry weight %
Binder, as Latex or Cationic Starch
Range : 1-3%
Ratio : 1~ Binder/10% Filler
-24-
~0~30~
Cat onic Floccu~L-lnt, wlth L :ex Onl~
Range : 2-4 lb/torl or 0.1-0.2~
Buffer for Calcium Sulfate - Filled Furnish
As either CaCO3 or Na2CO33
Range : 0.25-10
5izing Agent
As either Ke-tene Dimer or Succinic Acid Anhydride
Compound
Range : 3-7 lbfton or 0.15-0.35%
Retention Aids
Cationic Starch: 10-14 lb/ton or 0.5-0.7%
Anionic or Cationic Polymers: 0~5-1.5 lb/ton
or 0.025-0.075%
The composite paper of the present invention utilizing
calcium sulfate as a filler has several advantages when utilized
as paper cover sheets for making gypsum wallboard over other
papers conventionally used which do not have a mineral filler.
First, it is more porous than conventional papers. Conse-
quently, in the fabrication of the paper, the water utilized
drains off more rapidly so that the amount of heat energy
required for drying the paper is about 27~ less than that
required for drying conventional paper. Furthermoxe, the
porcus structure of the sheet provides faster drying, higher
machine speeds and greater production with existing papermill
equipment. Further, when the paper is utilized in the fabri-
cation of gypsum wallboardr because it is porous, about 5%
less heat energy is required in drying and setting the
wallboard than is required for use with conventional paper
-25-
~L220~0~
cover sheets. Additionally, because of the selected ratios
of filler to paper fibers, and because o~ the binders and
binder ratios utilized, the paper has excellent physical
properties. Further, in the improved embodimerlt, utilizing
an additional surface size on the side of the paper which
engages the gypsum core results in considerably improved
bond between the paper and the gypsum core even when sub-
jected to elevated temperature and humidity. Additionally,
from an economic standpoint, the use of plentiful and in-
expensive gypsum as a filler leads to substantial materialeconomies. Further, the presence of gypsum in the paper
leads to excellent adhesion between the paper and the gypsum
core of -the final gypsum board.
Additional advantages accrue from the use of an internal
neutral or slightly alkaline size which results in a paper
sheet which is stronger than that made with an acid size such
as rosin and alum. Consequently, a sheet of comparable strength
to that of the conventional rosin-alum sized sheet may be
obtained while using less cellulose fibers. This results in
a thinner sheet which drains more readily and more rapidly,
and requires less heat for drying, resulting in substantial
fuel savings. Alternatively, weaker and less expensive
fiber may be utilized, since neutral size does not weaken
the fibers. When an acid size such as rosin and alum is
used the fibers are materially weakened. An alum and rosin
sized sheet is acid by nature due to the addition of the
al~m. Being acid, the fibers which make up the sheet are
stiff and generally tubular and non~conformable. As a
-26-
~7~20904
result, the bonding provided by these fibers is poorer than
that which may he obtained with ~ more conformable flbe~.
In contrast, paper which is made with neutral size con~ists
of fibers which are conformable. They assume a flatter
position more readily than fibers which are subjected to
acid. ~s a result they provide better bonding and better
strength. Consequently, as stated, the improved strength
properties of the sheet imparted by the neutral sized fibers
can be utili~ed to reduce the basis weight of the sheet,
that is, the amount of materials utilized, and/or to reduce
the amount of hard stock used to maintain the strength of
the sheet. Other advantages obtained through the use of
neutral size are reduced corrosion on the paper machine and
a generally cleaner system than an alum and rosin system.
Additionally through the use of a surface size, im-
proved uniformity of internal sizing is obtained. Because
of this, the amount of the internal size application may
be reduced, while still obtaining good results. Moreover,
when manila paper is used, a significant increase in the
soft stock content may be utilized. This is made possible
by the improved strength of the sheet under like conditions
when neutral size is used. The same advantages are obtained
when using other papers.
A further advantage has been observed. When paper
machines formed of non-corrosion-resistant metal parts are
used, such as those made of steel and iron, corrosion is
greatly reduced. This result is obtained because the system
utilizing neutral size is maintained at a pH of about 7.0-7.8.
-27-
~2~)90~
Consequently the ~errous metal par~s ar~ not attacked. On
the other hand, the pH con~itions of ~.5~5.0, as e~perienced
in the use of an alum and rosin size, cause corrosion of
unprotected non-corrosion-resistant metals.
The large reduction or elimination of both alum and
rosin size results in a stock system which is a lot cleaner
ionically and chemically. This means that fewer problems
are encountered with chemical buildup which causes varia-
tions in paper quality and excessive filling of the paper
machine cylinder wiresO Additionally fouling of carrying
felts results in a high frequency of shutdowns for cleaning.
The use of neutral size also greatly reduces the conditions
of high chemical buildup in the system, which may contribute
to the above difficulties.
The cationic starch of the invention has several func-
tions. First, it acts as an emulsifying medium in which the
size particles are dispersed. Second, it serves to coat the
individual particles of size to protect them from hydrolysis.
Third, the cationic starch imparts a positive charge to the
individual size particles causing them to remain separated
from each other. Fourth, the cationic starch serves to
attach the size particles electrostatically to individual
cellulose fibers. Fifth, the cationic starch acts as a
retention aid or binder for the size particles and maintains
them affixed to the cellulose fibers. Sixth9 the cationic
starch enhances the tensile strength of the final paper by
improving the fiber-to~fiber bond. Finally, the cationic
starch acts as a retention aid to retain the buffer particles,
such as calcium carbonate, to the paper fibers.
-28
2~
The buffering ayent is utilized to maintain the in-
ternal neutral size at a pH of at least 7 and preferably
7 to 7.8. This prevents acid conditions from occurring
which would be detrimental to fiber s1:rength. If the
acidity oE the furnish in the system is not neutralized by
the presence of the buffer, the system becomes acid from
the acidity in the waste paper furnish and the beneits o
the neutral size such as high sheet strength and reduced
furnish cost can not be achieved.
The surface size utilized on the surface of the bond
liner prevents migration of starch out of ~he gypsum core
and contributes towards better bond between the paper and
the core. Suitable surface size materials are silicone
resins. Their efficiency may be enhanced by the addition of
an acid material to the silicone resin prior to application
which assists in the polymerization of the silicone resin.
Suitable acidic materials are alum and boric acid.
The neutral or slightly alkaline sizing agents of the
present invention may be of two kinds. The first type are
the suhstituted cyclic dicarboxylic acid anhydrides corres-
ponding to the following structural formula:
O R - R'
C
O
-29-
~220~0~
wherein R represents a climethylene or krimethylene radical
and wherein R' is a hydrophobic group containing more than
5 carbon atoms which may be selected from the group consist-
ing of alk~l, alkenyl, aralkyl or aralkenyl groups. Sub-
stituted cyclic dicarbo~ylic acid anhydrides falling within
the structural formula above are the substituted succinic
and glutaric acid anhydrides.
Specific examples of the above described sizing agents
include iso-octadecenyl succinic acid anhydride, n-hexadecenyl
succinic acid anhydride, dodecenyl succinic acid anhydride,
dodecyl succinic acid anhydride, decenyl succinic acid anhydride,
octenyl succinic acid anhydride, nonenyl succinic acid anhydride,
triisobutenyl succinic acid anhydride, capryloxy succinic acid
anhydride, heptyl glutaric acid anhydride, and benzyloxy succinic
acid anhydride. It has been found that optimum results are
obtained ~ith acid anhydrides in which R' contains more than
twelve carbon atoms. In addition to the above individual
compounds, mixtures of these compounds may also be employed.
Among the preferred neutral sizing compositions are
Accosize 18 and Fibran 68. Accosize 18 is a trademarked
product of American Cyanamid Company and is a substituted
succinic acid anhydride having a total of from 15 to 20
carbon atoms, and contains about 1% of an anionic surfactant.
Fibran 68 is a trademarked product o~ National Starch and
Chemical Corporation and is a substituted succinic acid
anhydride having a total o~ 15-20 carbon atoms. Fibran 68
-30-
~L2~(~90~
nor111ally does not cont~lln any em~11siEyir1g acJerll:. Howeve~,
it is adva11t~1geous to ac1d sucll all a~3f!11t to pronlo1f? the
emulsiEicatio11 of t1le product~ ~L~hc~ a1~1o~u1t oE sizi!1cJ agc~r1t
em~loyed 1n~1y range from ahout 0.]5~ to a~1out 0.35'~ of the
dry weight of the finished paper. Larger amounts may bc used
without adverse effects, but the excess adds l;ttle to the
sizing properties.
Other useful neutral or alkaline sizing agents for use
in the present inventior1 are ketene dimers, the structuraL
formula of which has been set out above. ~mong the useEu]
materials are Hercon 32 and Hercon 40 marketed by the
Hercules Company.
In those examples where it is used, the cationic
retention agent is useful in promoting or aiding the re-
tention of the siæing agents and for bringi1lg the agen-ts
into close proximity to the pulp fibers. Althou~h any oE a
large number of cationic agents may be utilized in the
invention, such as alum, alurninum chloride, long chain fa-tty
amines, sodlum aluminate, thermosetting resins and polyamide
polymers, the preEerred cationic agents are the various
cationic starch derivatives including primary, secondary,
tertiary or quarternary amine starch derivatlves. Such
derivatives are prepared from all types of starches including
corn, tapioca, pota-to, waxy mai~e, wheat and rice. The
cationic starch agent may be used in an amoun-t by weight o-f
from about 0.5% to about 0.7% based on the dry weight of the
paper. A preEerred cationic starch is STA-LOK 500 manufac-
tured by the A. E. Staley Manufacturing Company.
*trade mark
122~91Q~
The bufEer material may be any of a number of compounds
which are salts of a cation Oe a strong base and an anion
of a weak acid. Although a number of materials may be
utilized such as sodium carbonate and sodium bicarbonate,
the preferred buffering agent is calcium carbonate. This
material is instrumental in maintaining the pH of the sizing
agent and paper in a range of from about 7 to about 7.3.
Additionally, the CaCO3 buffer as filler improves sheet
porosity and improves drainage rate, thereby facilitating
the drying of the paper and reducing the amount of energy
necessary to manufacture the paper and the resultant gypsum
wallboard. An amount of at least 2% should be utilized. An
amount greater than about 6~ is no longer functional as a
buffer, but larger amounts up to 10% and greater may be used
where the calcium carbonate serves as both a buffer and a
filler.
It has been found advantageous to provide a surface
coating on the bond liner of the paper, that is, the suxface
of the paper`which becomes affixed to the gypsum core of the
wallboard. A preferred material is an epoxy resin such as
a silicone emulsion RE-30 a -trademarked material marketed by
Union Carbide Corporation. Additionally, a silicone emulsion,
Tego 5342A, a trademarked material manufactured and marketed
by the Goldschmidt Chemical Corporation is suitable. Further,
it has been found that even though the use of an acid material
to facilitate setting or curing of a sizing agent is detri-
mental when used as an internal sizing agent, the use of an
-32-
~;~2()9~
acid materlal such as alum or boric acid with the epo~y
sizing agent as a surEace size facilitates the cure of the
epoxy resin, and, because it does not enter internally into
the paper, does not adversely affect the strength of the
paper fibers.
As stated, in order to achieve the required quality
performance of neutral-size paper utilized to fabricate
gypsum wallboard, the addition of a weak acid material such
as alum to the dilute silicone emulsion in the concentration
of 1~ alum solids is critical for achieving optimum per-
formance.
Prior to the use of the present novel application of
alum to the external silicone size itself, it was found that
neutral-sized paper which was contaminated at discreet
points in the surface of the paper with dirt, shives and
bark, and which was surface sized with untreated silicone
emulsion had a tendency to form mini-cockles (dimples) in
the gypsum wallboard. Subsequent field tests showed that
the paper in the area of the dimpling was poorly sized
internally and had substantial amounts of dirt in it.
When alum-treated silicone was applied to the surface
of the paper in manufacture, the dimpling of the board was
eliminated. It is believed that the alum-acidified silicone
did not strike into the paper in the areas of poor internal
sizing, whereas the untreated silicone did strike in. This
strike-in defeated the purpose of the silicone which was to
give uniform paper sizing to provide a cockle-free board.
~2090~
It is believed that where a surEace size strikes into the
sheet oE paper it is unavailable at the paper surface to
provide sur-face sizing.
Alum-trea-ted silicone size is most effective when
applied to the surface of a sheet having a filler of a
material such as calci~ carbonate which acts as a buf~er.
When the alum-treated silicone comes in contact with the
calcium carbona-te,`the pH changes from 3.5 - 4.0 to neu-
trality~ It is believed that this causes the silicone to
cure out on the paper surface, thereby providing the desired
sizing uniformity. The alum addition appears to have no
appreciable adverse effect on the tensile strength of the
resulting paper, nor any visible adverse effect on the
stability of the silicone em~lsion nor on its tendency to
polymerize. Whatever curing effect takes place occurs as
the silicone is applied to the surface of the unsized, neutral
and 5% calcium carbonate filled paper.
It is to be understood that the invention is not to be
limited to the exact details of operation or materials
described, as obvious modifications and equivalence will be
apparent to one skilled in the art.-
-34-
