CA1087080A - Starch-coated paper and gypsum wallboard prepared therewith - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A composition comprising water, finely particulate gypsum and starch is coated on a paper cover sheet in a selective pattern such that the starch composition is disposed only at limited areas of the bond liner surface, and wherein substantial areas of the bond liner surface are free of the coating; the coated paper, either in the wet stage or after drying being used as paper cover sheets to form gypsum wallboard by applying an aqueous slurry of calcined gypsum, which may be free of starch, to the coated surface and setting the gypsum, resulting in a gypsum wallboard having excellent adhesion between the paper cover sheets and the gypsum core.

Description

o~

:

`
Bl\CRGl~OUND OF T}IE: INVE~TION
;
. (1) F ld of Invention The present invention relates to gypsum wallboard and .
m~re particularly refers to a meth~d ~or coating tlle bond liner surface o a paper cov~r sheet wi~h a starch adhesive composition in a selected pattern, to the product formed thereby, and to gypsum wallboard formed by applying a calcined gypsum slurry to the coated bond liner surf~ce o the paper cover sheet.

(2) Description Or ~he Prier Art ConventioIlal wallboard manufacture has in the past been based on the ~heory that the wet and dry ~ond were one and ~.
the same, tha~ the paper-to-core bond was mechanical and crystalline, a~d that dr~in~ calcined the crystalline ~ond :
to the point th~t the bond was ~s~entially des~roy~d. To prevent this, starch was usually introduced into the core slurry and permitted to migrate to the paper-core interface !
. .
j ~ to protect the gypsum crystals orming the mechanical bond.
In contrast to this, it was disclosed in U~S. Patent 4,051,291 issued September 27, 1971 . . ~
that the paper-to-core bond actually proceeds in a two-step fashionO as follows~ When th~ slurry is cast onto the paper ~o~er shcets, an~ specific~lly ~he bond liner thereof, wet or gree~ bonding will oocur between the ~ond liner and the ~lurry due ~co hydrogen bondin~O unless ~.ho p~par ~s tr~ted ol 1!.
~8708~

to prevent such hyd~ogen b~nding. For example, any bond liner ~izing tcnds to reduce such hydrogen bonding, and any film completely coatin~ ~he liner eliminates the hydrogen bonding alto~ether. Upon drying the board in conventional high temperature kilns, no matter to what degree wet bonding existed prior to drying, it is all destroyed during drying.
Thus, for dry bond to take place, a replacemcnt for the destroyed hydrogen bo11ds must 1~e introducea~ Since the dry bond takes place between the bond liner and the gypsum core, the adhesive must be maintained at the paper-core interface un~il it cures or sets.
It was gener~lly found that ti1e adhesive must be coated upon the bond liner of the cover sheet in such a manner as to permit and maintain we~ bonding oE the covcr sheet durincJ
the casting o~ ~he wallboard. This required that the ad-hesive must be nol1-Eilm forming until it cures, and must be for that re~son applied in a discontinuous manner in order that a substantial area of the bond liner surface remains uncoated. The adhesive must further be one which will not migrate from the paper-core interface durin~ the casting and drying of the wallboard, and yet one which will set or cure during the drying before the wet bond has been completely destroyed. ~s a result o the interplay of wet bonding and ~ubsequel1t dry bondin~, a wallboard i5 formed whiah, when dry, ratain~ thc adh~ivc in a di~continuou~ or spaced-apart or partially covered pattern at the core-to-paper interface, because of the non-migratory nature of the adhesive.
~ t was previously found that certain uncooked or raw starches and other adhesives are in fact non-migratory and therefore do not spraad to completely cover the surface of .'' .
' ' , '''~, ;~ ` ::

~L~87~80 th~ cover shcets, but leave substantial arca~ uncoated and freo of the adhesive. Consequently a wallboard paper-to-core bond is ~orlllcd which is uni~ormly ~ree of "peelers" a~ld paper "blows".
~ s disclosed irt the prior ~ppli~; ~ reLerred to above, it was found that wllen a slurry of r~w starch was coated on a paL-er cover sheet in a discontinuous pattern and in such a manner that substantial areas of the cover sheet were left uncoated e~ven when dry, it was ~ound that the thus coated cover she~ts could be utilized either in the wet or dried stage to form gypsum wallboard by applying an aqueous calcined gypsuln slurry which itsel~ did not contain starch to the coated paper cover sheets.
In seeking convenient commercial me~hods ~or preparing the starch-coated paper cover sheets, it was found ~nat an excell~llt product could be producod by means of a roller coater having spaced-apart annular ridges. However, because of the relatively low viscosity o~ the starch coating composition, it was found difficult to apply su~ficient ~oating material without having the slurry creep over on areas which were desired to be left uncovered.
IJ1 preparing starch-coated papor having substantial area~ frea of starch it has been foulld that an excellent produc~ can ba produced by providincJ ~ho F~tarch coatings in thc form o~ di~creto and definite "strlpod" pattarn~ or designs wherein the stripes are substantially parallel to each other. It w~s found that such patterns could be applied to papcr with better control and reproduc~ability by means of a grooved, resilient applicator roll using a direct roll coater. However, in attempting to usa a direct roll coater ;~

-3-'' ' ' ` ' ' -:

iL08708~

to apply ~he starch coating composition~ disclosed i~ ~he a~ove-mentioned . ~ , U.S. ~ No~ 49l,0~7l, it was found that the coating matcrial was not completely satisfactory for such application due in part to the properties of the composition, and due particularly to a low srookfield viscosity of about ~000 c~s. It ~ecam~ clear that compositions having higher viscosities and providing "stiffer" coatings were needed to maintain extended, continuous and trouble-free operations with the direct roll coater process. Original eforts to in\prove t11e physical properties of the coating composition to obtain a high ~iscosity w~re generally unsuccessful.
When the proportion of starch was increased to obtain the necessary viscosity, this resulted in a coating of relatively dry charact~r and which could not be utilized ~or forming a suitable coated paper by means of the direct roll aoa~er.
Moreover, an increase in the amount of disper~ing agent to obtain a suil:able viscosity resulted in ~he o~mation oE a ~gummy" coating which caused additional problems in the coating operation, as for example, extensive water holding capacities which affected both the drying rate o the coating on the paper as well as the imprinting operation, as for example, flowing of the coating on the total sheet which resulted in poor pattern definition and some overcoating of the area~ intended to be Eree of the adhesive coatlng.
Moreovor, the use the ndditional disporsing a~ant r~ulted in a prohibitively high matcrlal cost.
SU~RY OF THE INVENTION
Accordingly, it is an objeat of the invention to provide an improved paper-covercd gypsum wallboard and process for making the same, wherein the bonding o~ the paper to the _4_ , . ' '`I ' 1~87~8(~

gypsum core is predictable and relatively una~fected by varlances in the hoard-making process or components.
A further object of the invention is to provide such an improved walll,oard and process which are substantially more e~onomical due to the elimination of unnecessary amounts of the components.
~ related object of the invention is to provide such an improved wallboard and m~nufacturing process wherein the bonding of the paper to the gypswn core is based solely upon an adhcsive located only at the paper-core interface, which adhesive permi~s proper wet bonding of the pap~r sheet during the process.
Still another object is to provide such a process, and an improved cover sheet for gypsum wallboard, wherein the cover sheet is coated wi~h an adhesive and which never~heles~
i~ capable of being roll-wrapped and shipped to a board processing station.
It is still further an object to provide a paper bond liner or cover sheet coated with starch wherein the coating is applied and dried at limited areas of the paper and wherein substantial areas of the paper are ree of starch.
It is still further an object to provide a coating composition for coating paper cover sheets with a starch adhesive having,a viscosity within well defined limits, and which aoating matoriAl can b~ u~ed to provlda coatinqs O.e certain limited areas adjacent areas free o~ coating, with well de~ined boundarie~ between them.
Still other objects and advantages will readily present ,thomselves to one ~killed in the art upon reerence to the following speci~ication and claimq.

, ~
,. ' . ' ' ' . ''~

:

i.

~ID871~813 ~ ccording to the invention it has been found that the bond lincr of paper cover sheets may be provided with a superior starch coating of limited areas and having substantial areas of the paper uncoated by utilizing as a coating medi~m a composition comprising starch and calcium sulfate in finely particulate form. The calcium sulfate increases the viscosity of the coating composition within carefully controlled limits. The controlled viscosity permits coatings of fine boundary definition. This process results in a starch-coated paper bond liner which can be used with a calcined gypsum slurry to produce excellent gy~sum wallboard having excPllent paper adhesion.
BRIEF DESCRIPTE~N OF TIIE DRAWING
FIG. 1 is a photograph, in actual size, of a paper bond liner coated wlth starch wi~h a composition of the present invention.
FIG. 2 is a photograpll of the coated paper bond liner shown in FIG. 1, but enlarged ten times.
FIG. 3 is a photograph of a paper bond liner coated with starch utilizing a composition as disclosed in U.S.
, referred to above, and FIG. 4 is a photograph of the bond liner shown in FIG.
3, but enlargcd ten tlmes.

In aarrying out the present invention a coating com-position is first formed comprising calcium sulfate in a finely divided state, preferably calcined calcium sulfate or calcLum sulf~te hemihydratc. Very finely particulate calcium ; sulfate dihydrate may also be used. Thc composition additionally contains starch, preferably raw starch, sufficient water to .
..

~L~87~8~
make a slurry and other common additivos such as dispersing agents and preservativcs. Sufficient water is then addcd ~o form a slurry o~ the proper consistency. The composition is coated over paper cover shee~s, preferably with a direct roll coater to form a plurality of parallel lines or stripes which are spaced apart. secause of the gypsum composition present in the slurry, a sufficiently high viscosity is obtainod to enable lines or stripes having sharp edges or high resolution with respect to the uncoated areas. The coated p~per may then be utiliæed either in the wet or subsequently dried stages to form gyp-.um wallboard by depositing a calcined gypsum slurry intermediate a pair of coated cover sheets.
The following examplcs are provided to illustrate preferred embodimcnts o~ thc invention, but ara not intcnded to bc limiting in any ma~ner.
In carrying out the procedures d~scribed in the following examples, the coating composition was applied to the paper by means of a 26 inch wide roll covered by a resilient material and grooved to provide sixteen annular V-grooves per inch o .010 inch depth. The width of the land of the grooves was .031 inch. The roll provided a pattcrn of parallel strlpes along the longth o~ thc paper, with the Width oE the stripe9 bein~ substantially cqual to the space intcrmadiato o~ch pair oC stripes. ~`ho coatin~ oparation aomprised passing 25 inch wide pa~er sheets between the top grooved resilicnt roll and a bottom smooth resilient roll.
The grooved roll rotated in a resorvoir o liquid adhesive composition contained between a steel doctor roll and the ;

O ` . ~ ~

7~

grooved applicator roll. Metal endplate dams were used to hold an adhesive resexvoir oE a~proximately 800 grams. The steel doctor roll was adjust~d free and/or in contact with the grooved roll to affect coating deposition from the grooves only. The amount of coating adhasive transferred from the applicator roll to the sheet was controlled by the pressure between the top applicator roll and the bottom resilient backing roll. lhe distance beween the top and bottom roll is held to about .007 inch and the coating spe~d was about 100 ft. per minute.
The gypsum slurry utili~ed with the coated paper cover sheets in all the examples below was a conventional gypsum slurry comprising 600 parts o~ stucco ~calcium sulfate hemi-hydrate), 500 parts water, 1.35 parts of an accelerator com-prising calcium sulfate dihydra~e finely ground together with 5~ sugar based on the total accelerator by weight, 0.66 parts potassium sulfate, and sufficient conventional wood rosin foam to bring the density of the ~inished board to 1750 pounds per thousand square feet of one-half inch board.
In the tests described below, the effectivaness o' the adhesion of the paper cover sheets to the gypsum core are given in terms of wet bond and dry bond. Wet bond is the measure o~ the adhe~lion of the paper sur~ace to the wet cast gypsum board prior to entering the final drying ~tage which normally take~ placo ln a dryinq kiln. Dry bond i8 a measu~a of the final attachment o~ the paper to the gypsum core after the board has progressed through the drying kiln.
In carrying out the testing in the laboratory, the laboratory boards are made 5 in. X 15 in. with each test strip made 3 in. X 5 in. with the machine direction in the -~

' , .. .

,: : . : ,, . , : : : :i . . ~.: : :

:

~7Cli8a~
:' 5 in. direction. ~he wet bond test i5 made ten minutes ater the dry stucco is poured into the water in the mixer to form the gy~sum slurry. The test paper specimens are laid down with the bond side uppermost on a flat surface adjacent to each other and confined by a l/2 in. X l/2 in.
square rod frame. The wet g~psum slurry is poured onto the paper samples and then a top cover sheet is laid over and pressed down to the l/2 in. thickness with a flat plate larger than the 5 in. X 15 in. paper si~e. The gypsum is allowed to set until the initial Vicat set of 4-l/2 to 5-l/2 minutes has passed, and then the framing rads and top plate are removod. ~fter a ten minute total elapsed time the board is turned over and the 5 pa~er samples nre then scored parallel to the lS in. direction, about 3/4 in. in from one edge. The cut is made with a sharp board knife cutting through the ~aper. Each piece of paper is then pulled off in a direction parallel to the 5 in. board direction. The degree of paper fibers left on the da~lp gypsum core is rated from 0 to lO0~ wet bond, 0~ representing the condition when no visible fibers are left adhering to the gypsum core, and 100% representing the condition of comulete fibor covernge over the surfaca. Wet bond testing in no way re1ects tho final starch bonded achiaved after the board dries, but ~ivcs ~n indication o~ somc o several eorcos, chemical bond, gypsum-to-eiber attachment, mechanical bond, and o~her possible forces as well.
In carrying out the tests on laboratory boards to determine the measure of dry bond, the same board that has ; been previously tested for wet bond i5 further dried in a circulating air oven or kiln at a tempera~ure of about 360F

_g_ - , .. ' . . ~,. .

- \

~L~870813 , for 50 minutes or until th~ final weigllt has reached 70~ of the original wet weight. Final drying is accomplished at 100 F for 16 additional hours or until a constant weight has been attained. The bo~d sample is then scored through the back paper or opposite to th~ five test pieces in a direction parallel to the 15 in. length about 1 inch in from the edge. ~he board is th~n broken through the core parallel to this cut score and bent back upon itself and one side or both sides peeled back parallel to the 5 in. direction to expose the paper to core interface and to show the attachment of the paper to the gypsum core surface. The test results are given in terms of percentage of boncl failure, tha~ is, 100~ being no paper to gypsum core bond, and 00 boincJ no paper to core failure, or complete and ~ull paper bond ply coverage to the board core sureace. ~lumidified bond is obtained by plncing the sample in a ~0~ r~lative humity at 90 F for a period of three hours.

EXAMP~E 1 AND 2 A starch coating composition was prepared according to the invention having the following formulation:
Percent Part~ by W~i~ht Tapioca Starch 20.0 160.0 Calcium Sulfate ~lemihydrate 10.3 80.0 Kelz~n M 0.3 2.4 Dow 'G' Q.l .8 Water 69.6 556.8 100.0 800.0 Tota~

, ~ e ~ a. r \~
.~, ' ' ' ~

~ll087C~8~ ~

The ~rookfield viscosity of the above ~omposition at the time of coating was moeasured as 2800 cps. The composition was coatcd on paper as dcscribed above by means of a roller coater. In ~xample 1 the composition was coated on manilla paper. In Example 2 the composition of Example 1 was coated on Newslirled paper. Test gypsum boards were made as described with each type of papor. The results of tests on the board as described above are shown in Table I below.
The following composition which does not contain stucco (calcium sulfate hemihydrate) was prepared to be used as a control. The composition is disclosed in thc parent application referred to above.
EX~MPL~S 3 ~N~
Percent P
Tapioca Starch 20.0 160.0 Kelzan M 0.3 2.4 Dow 'G' 0.1 .8 Water 79.6 636.8 100.0 800.0 The composition used for Examples 3 and 4 at the timc of coating had a viscosity o 1600 Cp5. This composition was al~o coatcd on both manila papcr and Ncwsllned papcr, and coated in tho sam~ manner and with the samo apparatus as described above with regard to Examples 1 and 2. The resul's of tests on the finished board made with the coated papers arc al~o shown in Tablo I below.

... .

Tr ~Je ~4 ' ., ' ~ ~ ' I

' . L

~lCl 8708(3 rr~BLE I
Starch Wet Dry* Humidified*
Composition Bond Bond Bond ~ Failure ~ ~ailure Manila ~xample 1 -Inve~ Comp~.sition Exccllent O O
Starch/Stucco/Kel~an~ (80~-1no%
Fiber Attach-ment on Core) Example 3 - :
Control Good 20 20 .
Starch/Kelzan~ (60-80~
Fiber Attach-ment on Core) Newslined Exam~le 2 -Invent.~on Composi.ti.onExcellent 0 . 0 Starch/Stucco/Kelzan Example 4 -Control Good 20 20 Starch~Kelæan~
~*~ve. of 6 Breaks on Same Board) As can be seen from the results shown in Table I, although good wet bond results were obtained from all the samples tested, the dry bond and humidified bond failure tests show that the board ormed with tho coated paper of Examples 1 and 2 according to thc invention gave 0~ oE
Eailuras wher~as the board of ~xamples 3 and 4 Eormed with the prior art coating aomposition gave about 20~ Eailur~s in all cases.
FIGS. 1 and 2 of the drawing are photographs of coated paper cover sheets made with tlle composition of Example 1.
~s aan be seen, the lines are very sharp and the areas intermediate the lines are substantially free of coating . material. This can be seen especially in FIG. 2 which is a lOX enlargement of FIG. 1 and illustrates the high resolution ~r~e~ ~ , . . ..
~- . . ' 1 :. : . . , .,: ., .. . ::: .:, .. , :. : .. ~

371:118~

achievcd by th~ compositiorl having sharp defined outer ed~es of the coating lin~s or stripcs and substantially no coating in the space intermediate the stripes or lines. As opposed to this, the photographs of the coated paper formed with the composition of Example 3 are shown in actual size in FIG. 3 and in 10X enlargement in FIG. 4. As can be seen, the resolution is much lower, and the edyes of the stripes or lines more diffused and in part spreading to the spaces inter~ediate the lines.
One of the ilnportant factors in prov~ding a coating composition which will provide a coating of sufficient thickness an~ excellent resolution is the viscosity of the composition. To determine the ~fe~c~ on viscosity o~ varlous compositions containing s~ucco wlth a control ~rc~ o stucco, the following coating compo~itions were prepared:

PERCENT OF TOTAL BY WEIGHT PARTS BY WEIGHT
20~ Starch 80g Starch 0.3g Kelzan~M 1.2q Kelzan M
0.1~ Dow~G 0.4g Dow G
80~ Water 320 ml Water LXP~lPLE 6 PERCENT OF T~TAL BY WEICIIT
20 ~ Starch 80g S~arch 10~ Shoals Stu~co Gypsum 40g Gypsum 0 3~ Relzan~M 1.2g ~olzan M
0 1~ Dow~G 0.~ Dow G
70~ Wat~r 2U0 nll Water PÉRCEWT O~ TOTAL BY WEIC;IIT PARTS ~Y WEIGIIT
20~ Starch 80g Starch 15~ Shoals ~Stucco Gypsum 60g Gypsum 0.3~ ~el~an M 1.2g Kelzan M
0 l~ Dow~G 0.4g Dow G
6;~ Water 260 ml Water , , -13-, I ~r~le ~
..
r .~ ~ . .

~70~3~

EX~MPLE 8 PERCENT OF TOTAL BY WEIGI~T PM TS BY WEICHT

20~ Starch 80g Starch 20~ Shoals Stucco Gypsum 80g Gypsum 0.3~ Kel~an~M 1.2g Kelzan M
0.1% Dow~G 0.4g Dow G
60~ Water 240 ml Water PERCENT OF TOTIIL BY WEIc~HT PAR~S BY WÆIGHT

20~ Starch 80g Starcll 30~ Shoals Stueco Gypsum 120g (;y~sum 0.3~ Kelzan~M 1.2g Kelzan M
0.1~ Dow~G 0.4g Dow G
50% Water 200 ml Water The viscosities of the compositions of Examples 5 -. 9 measured at various times from initial mi.xing are shown in Table II below.

~ ~8LE II

Sample Identifieation 30 minutes 3 hours 6 hours 24 hours _ Example 5 1525 eps 1625 eps 1625 eps 1700 eps Example 6 2787.5 eps 2850 eps 2900 eps 5800 Cp9 Example 7 4025 eps 4150 eps 4650 eps 11,250 eps Example 8 5650 cps 5950 eps 8800 eps Solid Example 9 18750 eps 21750 eps Solid Solid It has gonerally been found that eoating eompositions having vlscosikies in thQ range from about 2500 eps. to about 8000 eps. ~rovid~ exeellent Coating when prepared aeeording to the invention. In the ease of eompositions eontaining smaller amoun~s of caleium sulfate hemihydrate, it may be neeessary to let the eomposition stand for a ~14-. . .

~ \ r~e -~

:
.

~8708~

per~od su~icient to rai.se the viscosity. On the otherhand, for those compositions, as for exampl~ that o Example 7 the composition should be us~d wit~in a short time of mixing, since when left to stand more than about 6 hours, it may attain a viscosity too high for adequate coating.
In preparin~ the coating compositions described above in Examples 1 - 9, the dry ingredients were irst mixed together and then the mixture added to water under aggitation using a "~igAtnin Mixer" at a medium speed for ten minutes.
It was found, that, contrary to expectations, the final coating contained a major proportion of un~hanged calcium sul~ate hemihyclrate, as determined by X-ray defraction studies. It was subseciuently determined ~hat the starch acts as an inhibitor to hydration of the hemihydrate.
It has been found that in addition to common stucco ~beta-caloium sulfate hemihydrate), other forms of gypsum are suitabl0 Eor practiciny the invention, as for example, alpha-calcium sulfate hemihydrate, and calcium sulfate dihydrate.
The following examples illustrate the effect on the coAting composition of varioua f~ctors such a3 the composition and ~rain ~ize or surface area oE the gypsum, the sequence used in adding ~he incJrcdicnts, thc intcnsity and length oE
tim~ o stirrin~ th~ mixturo and o len~th of time of standing after mixing but before application of the composition on the paper.

.i .
.. . .

~ r 4 ~

' ''~ .

1~)87~0 EX~MI'I,E 10 ~ P~rcent Parts Tapioca Starch 20.0 160.0 Stu~co 10.0 80.0 Kelz~n~M0.3 2.4 Dow~G 0.1 0.8 Water 69.6 556.9 Total 100.0 800.0 /~11 the dry ingredien~s (starch, stucco, Kelzan~M, and Do ~ G) wer~ mixed toc~eth~r and then added to water under medium agitation (800 RPM) for a mixing period of ten minutes.
EX~MPLE 11 The Eormula~ion shown in ~xample 10 was preparod by combining the dry ingredients and addlng them to water under high agitation ~1400 RPM) and mixing Eor thirty minutes.
~XAMPLE 12 The ~ormulation shown in Example 10 ~as utilized. Only the stucco was added to the total wat~r and mixed for twenty minutes under high agitation (1400 RPM~. The remaining ingredients were combined and added to tile stucco slurry, and th~n mixed ~or an additional ten minut~ under agitation.
F,X~MP r.~ 13 q'he ~ollowing Eormulation was pr~pared, utilixing only 7r~ ~tucao.
Formulation: Percent Parts Tapioca Starch 20.0 160.0 Stu~¢o 7.0 56.0 Kelxan ~ 0.3 2.4 Dow~G 0.1 0.8 ~' Water 72.6 580.8 Total 100.0 800.0 ` i ~ T r~r~r~r~r ~
. r ,- ' ' . :' '! , i :

1~87~8~

~ ll of the dry ingredients were combined together, added to water under hlyh ayitation and m.ixecl ~or thirty minutes.

The followincJ formulation was prepared.
Formulation:Percent Parts Tapioca StarchZ0.0 160.0 Finely Groun~l Calcium sulfate dihydrate 7.0 56.0 Kelzan*M 0.3 2.4 Dow G 0.1 0.8 Water 72.6 580.8 Total100.0 800.0 In this Example, 7% finely yround calcium sulfate di-hydrate normally used as a g~psum setting accelera~or was substituted for stucco. All the dry ingredients were combined together and added to water under high agitation and mixed for thirty minutes.
EXAMPI.E l5 Thc eollowirlcJ ~ormulation was prepared:
Formulation:PerceTIt Parts Tapioca Starch20.0 160.0 Land Plaster 7.0 56.0 Xelza~M 0.3 2.~
Dow~G 0.1 0.8 Water 72.6 580.8 TotallO0.0 800.0 ln Examplo 15, Land Plaster, whlch i8 a calcium sulEatt~
dihydrate o~ moderate particle siza, was substltuted for ~tucco. All the cl~y ingrcdictlt~ w~ra combint~d togath~r, add~d to water und~r hicJh shear and mixed ~or thirty minutes.

' -17-, ., . .
.. , ~.

8 7 ~ ~

F,X~M~Ll~ 16 The follo~ing ormulation was pr~pared:
Formulation:Percent Parts Tapioca Starch 20.0 160.0 Kelzan*M 0.3 2.q Dow*G 0.1 0.8 Water 79.6 636.8 Total100.00 800.0 The formulation of Example 16 was used as a control and does not contain any calcium sulfate additive. The dry ingredients were combined, added to water under high shear and mixed for thirty minutes.

* Trademark ; -18-: , .

~L~8708(~ ~

.
. .~
h .
~ ~ ~ o o o o o o o h ~ ~4 X X X X O O

h rl O ho JJ rl ~ X X X X X O O
P. a, ~ ~ ~
h ~ ~ a ~ ~ , ~, h O ~U O 51 .C rd ~C~ rJ ~a I
V~ rl r~ rl rl :~ rl Q ~ E3 e ~ e ~, ~rl ~ ~ ~ ~ :r a ~ a n. æ dP
~ ~r I o o o o ~ u7 o~ a~
n .
H ~ & ~ O O O O O r-l -H ~) 111 ~ O O O O O O O
~1 ~ ~ C ~D 1~ 0 _1 ~n O 1 ~d O 1~ U~ ~1 n U~ ~D ~ ~ r~
I
~-1 U) E~ ~ C C o 00 0 00 o O ~"
~ ~ c c~ ~ o ~o ~
rt ~ u~ r~ ~I It~ u~ trl ~ ~ ~1 ~u l O ~ a ~ O 0O 0 0 o o ~ a ~ O
r o o tJ r ~ u t~ ~ o t~
r l u c ~1 ~ x ~ tr~ ~) u x a) Ir~ x ~ J C r I O C
o 1~ u~ r ~ Ei r ~ ~ r ~ Il~ r~ r I ~

~;! x r I o x r I o x ~) ~ x t~ o x r~ :J rl o x r r-l o x u~ o t-l ~ 1 ~ 1--U~ ~1--~ W--Ul ~ r~ ~ _ ~ 1 . ~

' ' - ~ .

~ I .. .. , ., . . , j, ; . . . .

i.

0~

The results shown above in Table III indicate that the viscosity Oe the coating compo~ition is closely related to its ability to Eorm starch coated paper Erom which gypsum board can belmade having excellent pattern definition, and excellent board strength, both wet and dry. As shown, all of the composition~ of Examples 10-14 form starch-coated paper of excellent pattern definition and the coated paper used to form gypsum board having excellent we~ and dry board bond. Tlle compositions used in every case were those which were allowed to stand for onl~ one hour before coating the paper. It is thus seen that where the one hour ~iscosity was from 2800 to 6600, excellent products were produced.
Viscosities of as higll a~ 7600, as for example that of Example 12 WhiCIl was permitt~d to stand for 24 hours, may also be used but the coating process becomes more diEEicult.
Tho compositions of Examples 15 and 16 having viscosities of 2000 or under Eorm coated paper of poor pattern deÇinition and, although they e~hibited good wet bond, the dry hond failure was unacceptable at 20~.
Even the composition of Example lS which exhibited a viscosity of 2100 cps. after standing for 24 hours was too thin to provide good definition and good dry bond. ~n oxplanation eor tha poor p~rformance o~ tho composition oE
Exampl.e 15 i~ tllat tho product w~s common lAndplas~or Oe thQ
typQ u~od Eor calcining, wharQn~ tho d.;hydr~to o~ Exampl~ 14 WA5 very finely ground and o the type normally used as a gypsum set accelerator.
It WA9 urther ound that, although the dihydrate form . . .
of gypsum mny ~e used to form the coating aompo~ition, ':
: , .
., .

' ' ' ' ' ' ' ' ~ ' -' : ' ' ' " ' ' "
,' . '' ,' : . ', ' ,: ', ' , ~. ': :'''": ''~ "' ,:,,.. ,,: .'' '"" '' '' '',`.' ' ' ,,';', , , ~ ~ 0b~ ~

~omewhat better re~ul ts are obtained by using the hemihydrate form and by premixing th~ dry ingredien~ prior to adding water so that the presence of the s~arch cause the hemihydr~te to remain in tha~ form even after stal~diny and even after . coating of the paper.
: - In order to determine tlle lower limi~s of calcium sulfate hemihydrate addition which would still provide a good starch-coatin~ composition were prepared utilizing 20%
starch in each example but varying the calcium sulfate ,hemihydrate ~rom 0 to 10%. Mixing was carried out or 30 minutes at high s~lear and the coating composition then permitted to stancl for an additional hour, at which time th~
viscosities of the compositions were measured.
TA~LE IV

; ~ Calcium Sulfate Viscosity cps Example lleln.ihydrale~ Starch after l Hr.
Example 17 0 20 2200 -Example 18 1 20 2625`
Example 19 3 20 302S
Example ~0 5 20 3~00 Example 21 10 20 4650 Ag can be s~en ~rom the results in Table IV, which sets out percent of inyredients by weight a composition containing as low as 1~ calcium sulfate hemihydrate provided a composition havlng a viscosity o~ 2625 cps. A composition having as little as 0.75~ and providing a viscosity of about 2500 cps.
also can be used to form starch coated paper which may be used to ; form gypsum wallboard having excellent wet and dry bond properties.

'': ' : '' '' : ' '' ~370~

The viscosities oE ~he coatincJ compositions of the invention may range from at le~st about 2500 cps to about 8000 cps. However, compositions having higher viscosities may be applied by means other than roller coating.
It has also been found that although dihydrate gypsum may be utilized, hemihydrate gypsum provides superior coating compositions at comparable or greater viscositles.
It can also be seen that the particle surface magnitude plays an important part in attaining the proper viscosity, since the medium ground dihydrate gypsum or land plaster provides a considerably less e~fective coating composition that the very finely ground dihydrate which is conventlonally used as an accelerator for setting a gypsum slurry.
The data abovc also shows that when high shear mixing at extended periods is utilized, the viscosity reaches a maxlmum early and does not chan~e materially after being allowed to stand or a period of time.
In listing the percentage or parts of the various in-- 20 gredients in the Tables above, the reference in every case is to percentage or parts by weight. In the ~ase o an aqueous compoæition, the percentage is by wolght o~ the entlre composltlon including the water.
The mntcrlal li~ted a~ Kelzan~M is a conventional dlspcr0iny n~ent ~omprlsing xan~han gum.
Dow~G is a conventional preservative having the chemical name sodlum pentachlorophenate.
; Thq starch is preferably a raw starch and may be present in an amount from about 5~ to about 30~ by weight of , -22-~, Tr~e~`k '-I

L

ilD87~

th~ coating compositlon.
l'he calcium ~sulfate SUC~I a5 calcium sulfate hemihydrate may be present in.an amount from about 0.75~ to about 30% of ;:
the weight of the coating composition.
It i~ to be wlderstood that the invention is not to be limited to the ~xact details o~ composition, materials, or operation shown or described, as obvious modificiations and equivalents will be apparent to one skilled in the art.

~-23-.
. ~ . . . . , . , , . , . , . ., . . . . . . . . . . . . . ~ .

r .. . . . . . .
,. . . ..
, . . . . . . .. . . .
. .... . . . . . .. ... .... . .. . .

Claims (60)

Invention is claimed as follows:

1. A process for producing a starch-coated paper cover sheet having a bond liner and being suitable for use in manufacturing gypsum wallboard, which comprises:
(a) preparing a coating composition comprising water, non-migratory raw starch, and finely particulate calcium sulfate, and (b) applying said coating composition on the surface of said bond liner in a pattern having said coating composition at limited areas of said bond liner while leaving substantial areas free of said composition.

2. A process according to Claim 1, wherein said calcium sulfate is calcium sulfate dihydrate.

3. A process according to Claim 1, wherein said calcium sulfate is calcium sulfate hemihydrate.

4. A process according to Claim 1, wherein the viscosity of said coating composition is from about 2500 to about 8000 cps.

5. A process according to Claim 1, wherein said calcium sulfate is present in an amount of from about 0.75% to about 30% by weight of said coating composition.

6. A process according to Claim 1, wherein said starch is present in an amount from about 5% to about 30%
by weight of said composition.

7. A process according to Claim 1, wherein said coating is applied by a roller coater.

8. A process according to Claim 7, wherein said coating is applied in the form of parallel spaced-apart stripes or lines.

9. A process according to Claim 8, wherein said parallel stripes or lines and the space intermediate said stripes or lines are of substantially equal widths.

10. A process according to Claim 7, wherein said parallel stripes or lines are applied at about 16 lines per inch.

11. A process according to Claim 1, wherein said coating composition is dried after it is applied to said paper cover sheet.

12. A process according to Claim 1, wherein said coating composition additionally includes a dispersing agent.

13. A process according to Claim 12, wherein said dispersing agent comprises xathane gum.

14. A process according to Claim 1, wherein said coating composition additionally contains a preservative.

15. A process according to Claim 14, wherein said preservative is sodium pentachlorophenate.

16. A starch-coated paper cover sheet having a bond liner and being suitable for use in manufacturing gypsum wallboard, said bond liner having a coating composition on the surface thereof in a pattern having said coating com-position at limited areas of said bond liner while leaving substantial areas free of said composition, said coating composition comprising water, non-migratory raw starch, and finely particulate calcium sulfate.

17. A paper cover sheet according to Claim 16, wherein said calcium sulfate is calcium sulfate dihydrate.

18. A paper cover sheet according to Claim 16, wherein said calcium sulfate is calcium sulfate hemihydrate.

19. A paper cover sheet according to Claim 16, wherein the viscosity of said coating composition is from about 2500 to about 8000 cps.

20. A paper cover sheet according to Claim 16, wherein said calcium sulfate is present in an amount of from about 0.75% to about 30% by weight of said coating composition.

21. A paper cover sheet according to Claim 16, wherein said starch is present in an amount from about 5% to about 30% by weight of said composition.

22. A paper cover sheet according Lo Claim 16, wherein said coating has been applied by a roller coater.

23. A paper cover sheet according to Claim 16, wherein said coating is in the form of parallel spaced-apart stripes or lines.

24. A paper cover sheet according to Claim 23, wherein said parallel stripes and the space intermediate said stripes or lines are of substantially equal widths.

25. A paper cover sheet according to Claim 22, wherein said parallel stripes or lines are present at about 16 lines per inch.

26. A paper cover sheet according to Claim 16, wherein said coating composition is dry.

27. A paper cover sheet according to Claim 16, wherein said coating composition additionally includes a dispersing agent.

28. A paper cover sheet according to Claim 27, wherein said dispersing agent comprises xathane gum.

29. A paper cover sheet according to Claim 16, wherein said coating composition additionally contains a preservative.

30. A paper cover sheet according to Claim 29, wherein said preservative is sodium pentachlorophenate.

31. A process for producing gypsum wallboard comprising the following steps:
(a) providing at least one paper cover sheet having a bond liner and applying a coating of a non-migratory raw starch adhesive composition to provide a pattern having said starch adhesive composition at limited areas of said bond liner while leaving sub-stantial areas free of said adhesive composition, said adhesive composition com-prising water, starch and finely particulate calcium sulfate, and (b) casting an aqueous slurry of calcium sulfate hemihydrate on said paper cover sheet over said bond liner with the water from said aqueous slurry establishing wet bonding between said slurry and the uncoated portions of said bond liner, and (c) setting said slurry to form a core and setting said starch adhesive composition to establish an adhesive bond between said core and said paper cover sheet at the limited areas containing said starch adhesive.

32. A process according to Claim 31, wherein said calcium sulfate is calcium sulfate dihydrate.

33. A process according to Claim 31, wherein said calcium sulfate is calcium sulfate hemihydrate.

34. A process according to Claim 31, wherein the viscosity of said coating composition is from about 2500 to about 8000 cps.

35. A method according to Claim 31, wherein said calcium sulfate is present in an amount of from about 0.75%
to about 30% by weight of said coating composition.

36. A process according to Claim 31, wherein said starch is present in an amount from about 5% to about 30% by weight of said composition.

37. A process according to Claim 31, wherein said coating is applied by a roller coater.

38. A process according to Claim 37, wherein said coating is applied in the form of parallel spaced-apart stripes or lines.

39. A process according to Claim 38, wherein said parallel stripes or lines and the space intermediate said stripes or lines are of substantially equal widths.

40. A process according to Claim 37, wherein said parallel stripes or lines are applied at about 16 lines per inch.

41. A process according to Claim 31, wherein said coating composition is dried after it is applied to said paper cover sheet.

42. A process according to Claim 31, wherein said coating composition additionally includes a dispersing agent.

43. A process according to Claim 42, wherein said dispersing agent comprises xathane gum.

44. A process according to Claim 31, wherein said coating composition additionally contains a preservative.

45. A process according to Claim 44, wherein said preservative is sodium pentachlorophenate.

46. Gypsum wallboard comprising:
(a) a core comprising rehydrated calcium sulfate hemihydrate, and (b), a cover sheet affixed to each surface of said core comprising:
(1) a bond liner having a coating composition thereon comprising non-migratory raw starch and finely particulate calcium sulfate, said coating composition being present in a pattern having said coating composition at limited areas of said bond liner while leaving substantial areas free of said coating composition, said coating composition providing an adhesive bond between said core and said paper cover sheets at the limited areas containing said coating composition.

47. Gypsum wallboard according to Claim 46, wherein said calcium sulfate is calcium sulfate dihydrate.

48. Gypsum wallboard according to Claim 46, wherein said calcium sulfate is calcium sulfate hemihydrate.

49. Gypsum wallboard according to Claim 46, wherein the viscosity of said coating composition is from about 2500 to about 8000 Cps.

50. Gypsum wallboard according to Claim 46, wherein said calcium sulfate is present in an amount of from about 0.75% to about 30% by weight of said coating composition.

51. Gypsum wallboard according to Claim 46, wherein said starch is present in an amount from about 5% to about 30% by weight of said composition.

52. Gypsum wallboard according to Claim 46, wherein said coating is applied by a roller coater.

53 Gypsum wallboard according to Claim 52, wherein said coating is in the form of parallel spaced-apart stripes or lines.

54. Gypsum wallboard according to Claim 53, wherein said parallel stripes or lines and the space intermediate said stripes or lines are of substantially equal widths.

55. Gypsum wallboard according to Claim 52, wherein said parallel stripes or lines are applied at about 16 lines per inch.

56. Gypsum wallboard according to Claim 52, wherein said coating composition is dried after it is applied to said paper cover sheet.

57. Gypsum wallboard according to Claim 46, wherein said coating composition additionally includes a dispersing agent.

58. Gypsum wallboard according to Claim 57, wherein said dispersing agent comprises xathane gum.

59. Gypsum wallboard according to Claim 46, wherein said coating composition additionally contains a preservative.

60. Gypsum wallboard according to Claim 59, wherein said preservative is sodium pentachlorophenate.