GB1573871A

GB1573871A - Starch coated paper and gypsum wallboard process therewith

Info

Publication number: GB1573871A
Application number: GB6427/78A
Authority: GB
Original assignee: United States Gypsum Co
Current assignee: United States Gypsum Co
Priority date: 1977-02-18
Filing date: 1978-02-17
Publication date: 1980-08-28
Also published as: BE864079A; SE7801477L; NO780539L; AU3305278A; CA1087080A; SE427833B; NO150767B; JPS53122806A; NO150767C; NZ186253A; AU510925B2; DK69378A; FR2380876A1; FR2380876B1; DE2806923A1; MX149542A; ZA78937B

Description

(54) STARCH COATED PAPER AND GYPSUM WALLBOARD PROCESS THEREWITH (71) We, UNITED STATES GYPSUM COMPANY, a corporation organised and existing under the laws of the State of Delaware, United States of America, of 101 South Wacker Drive, Chicago, Illinois 60606, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement::- The present invention relates to gypsum wallboard and more particularly refers to a method for coating the surface of a bonding layer, known as the bond liner, of a paper cover sheet with 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 surface of the paper cover sheet.

Conventional wallboard manufacture has in the past been based on the theory that the wet and dry bond were one and the same, that the paper-to-core bond was mechanical and crystalline, and that drying calcined the crystalline bond to the point that the bond was essentially destroyed. To prevent this, starch was usually introduced into the core slurry and permitted to migrate to the paper-core interface to protect the gypsum crystals forming the mechanical bond. In contrast to this, it has been discovered that the paper-to-core bond actually proceeds in a two-step fashion, as follows. When the slurry is cast onto the paper cover sheets, and specifically the bond liner thereof, wet or green bonding will occur between the bond liner and the slurry due to hydrogen bonding, unless the paper is treated to prevent such hydrogen bonding.For example, any bond liner sizing tends to reduce such hydrogen bonding, and any film completely coating the liner eliminates the hydrogen bonding altogether. 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 replacement for the destroyed hydrogen bonds must be introduced. Since the dry bond takes place between the bond liner and the gypsum core, the adhesive must be maintained at the paper-core interface until it cures or sets.

It was generally found that the adhesive must be coated upon the bond liner of the cover sheet in such a manner as to permit and maintain wet bonding of the cover sheet during the casting of the wallboard. This required that the adhesive must be non-film forming until it cures, and must be for that reason 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 during 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.As a result of the interplay of wet bonding and subsequent dry bonding, a wallboard is formed which, when dry, retains the adhesive in a discontinuous or spaced-apart or partially covered pattern at the core-to-paper interface, because of the non-migratory nature of the adhesive.

It was previously found that certain uncooked or raw starches and other adhesives are in fact non-migratory and therefore do not spread to completely cover the surface of the cover sheets, but leave substantial areas uncoated and free of the adhesive. Consequently a wallboard paper-to-core bond-is formed which is uniformly free of "peelers" and paper "blows".

It was then found that when a slurry of raw starch was coated on a paper cover sheet in a discontinuous pattern and in such a manner that substantial areas of the cover sheet were left uncoated even when dry, the thus coated cover sheets could be utilized either in the wet or dried stage to form gypsum wallboard by apple ing an aqueous calcined gypsum slurry which itself did not contain starch to the coated paper cover sheets.

In seeking convenient commercial methods for preparing the starch-coated paper cover sheets, it was found that an excellent product could be produced by means of a roller coater having spaced-apart annular ridges. However, because of the relatively low viscosity of the starch coating composition, it was found difficult to apply sufficient coating material without having the slurry creep over on areas which were desired to be left uncovered.

In preparing starch-coated paper having substantial areas free of starch it has been found that an excellent product can be produced by providing the starch coatings in the form of discrete and definite "striped" patterns or designs wherein the stripes are substantially parallel to each other. It was found that such patterns could be applied to paper with better control and reproduceability by means of a grooved, resilient applicator roll using a direct roll coater. However, in attempting to use a direct roll coater to apply the starch coating compositions it was found that the coating material was not completely satisfactory for such application due in part to the properties of the composition, and due particularly to a low Brookfield viscosity of about 1000 cps.It became 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 efforts to improve the physical properties of the coating composition to obtain a high viscosity were generally unsuccessful. When the proportion of starch was increased to obtain the necessary viscosity, this resulted in a coating of relatively dry character and which could not be utilized for forming a suitable coated paper by means of the direct roll coater.Moreover, an increase in the amount of dispersing agent to obtain a suitable viscosity resulted in the formation of a "gummy" coating which caused additional problems in the coating operation, as for example, extensive water holding capacities which affected both the drying rate of 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 areas intended to be free of the adhesive coating.

Moreover, the use of the additional dispersing agent resulted in a prohibitively high material cost.

Accordingly, it is an object of the invention to provide an improved papercovered gypsum wallboard and process for making the same, wherein the bonding of the paper to the gypsum core is predictable and relatively unaffected by variations in the board-making process or components.

According to the invention, there is provided a process for producing a starchcoated 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 the remaining areas free of said composition.

The invention also provides 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 composition at limited areas of said bond liner while leaving the remaining areas free of said composition, said coating composition comprising water, nonmigratory raw starch, and finely particulate calcium sulfate.

According to the invention, there is also provided 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 the remaining areas free of said adhesive composition, said adhesive composition comprising 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.

Finally. the invention also provides 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 the remaining areas free of said starch adhesive composition, said coating composition providing an adhesive bond between said core and said paper cover sheets at the limited areas containing said starch adhesive.

Thus, by means of the invention it is possible to provide an improved wallboard and process which are substantially more economical due to the elimination of unnecessary amounts of the components. The bonding of the paper to the gypsum core is based solely upon an adhesive located only at the paper-core interface, which adhesive permits proper wet bonding of the paper sheet during the process.

It is also possible to provide a process, and an improved cover sheet for gypsum wallboard, wherein the cover sheet is coated with an adhesive and which nevertheless is capable of being roll-wrapped and shipped to a board processing station. The paper bond liner or cover sheet is coated with starch which can be applied and dried at limited areas of the paper and wherein substantial areas of the paper are free of starch.

According to the invention it has been found that the bond liner of paper cover sheets may be provided with a starch coating of limited areas and having substantial areas of the paper uncoated by utilizing as a coating medium a composition comprising starch and calcium sulfate in finely particulate form. The calcium sulfate may be used to increase the viscosity of the coating composition within carefully controlled limits. The controlled viscosity permits these 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 gypsum wallboard having excellent paper adhesion.

An embodiment of the invention is hereinafter described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a photograph, in actual size, of a paper bond liner coated with starch with a composition of the present invention; Figure 2 is a photograph of the coated paper bond liner shown in Figure 1, but enlarged ten times; Figure 3 is a photograph of a paper bond liner coated with raw starch, and Figure 4 is a photograph of the bond liner shown in Figure 3, but enlarged ten times.

In carrying out the present invention a coating composition is first formed comprising calcium sulfate in a finely divided state, preferably calcined calcium sulfate or calcium sulfate hemihydrate. Very finely particulate calcium sulfate dihydrate may also be used. The composition additionally contains raw starch, sufficient water to make a slurry and, optionally, other common additives such as dispersing agents and preservatives. Further water may then be added to form a slurry of a suitable consistency. The composition is coated over paper cover sheets, preferably with a direct roll coater to form a plurality of parallel lines or stripes which are spaced apart. Because of the gypsum composition present in the slurry, a sufficiently high viscosity is obtained to enable lines or stripes having sharp edges or high resolution with respect to the uncoated areas.The coated paper may then be utilized either in the wet or subsequently dried stages to form gypsum wallboard by depositing a calcined gypsum slurry intermediate a pair of coated cover sheets.

In carrying out the procedures described in the following examples, which illustrate preferred embodiments of the invention, 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 of .010 inch depth. The width of the bands between the grooves was .031 inch. The roll provided a pattern of parallel stripes along the length of the paper, with the width of the stripes being substantially equal to the space intermediate each pair of stripes. The coating operation comprised passing 25 inch wide paper sheets between the top grooved resilient roll and a bottom smooth resilient roll. The grooved roll rotated in a reservoir of liquid adhesive composition contained between a steel doctor roll and the grooved applicator roll.Metal endplate dams were used to hold an adhesive reservoir of approximately 800 grams. The steel doctor roll was adiusted free and/or in contact with the grooved roll to affect coating deposition from the grooves only. The amount of coating adhesive transferred from the applicator roll to the sheet was controlled by the pressure between the top applicator roll and the bottom resilient backing roll. The distance between the top and bottom roll is held to about .007 inch and the coating speed was about 100 ft per minute.

The gypsum slurry utilized with the coated paper cover sheets in all the examples below was a conventional gypsum slurry comprising 600 parts of stucco (calcium sulfate hemihydrate), 500 parts water, 1.35 parts of an accelerator comprising calcium sulfate dihydrate finely ground together with 5 n 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 finished board to 1750 pounds per thousand square feet of one-half inch board.

In the tests described below, the effectiveness of 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 of the adhesion of the paper surface to the wet cast gypsum board prior to entering the final drying stage which normally takes place in a drying kiln. Dry bond is a measure of the final attachment of 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.xl5 in. with each test strip made 3 in.x5 in. with the machine direction in the 5 in.

direction. The wet bond test is made ten minutes after the dry stucco is poured into the water in the mixer to form the gypsum 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 1/2 in.x 1/2 in. square rod frame. The wet gypsum slurry is poured onto the paper samples and then a top cover sheet is laid over and pressed down to the 1/2 in. thickness with a flat plate large than the 5 in.x 15 in. paper size. The gypsum is allowed to set until the initial Vicat set of 4-1/2 to 5-1/2 minutes has passed, and then the framing rods and top plate are removed. After a ten minute total elapsed time the board is turned over and the 5 paper samples are then scored parallel to the 15 in. direction, about 3/4 in. in from one edge.The cut is made with a sharp board knife cutting through the paper. 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 damp gypsum core is rated from 0 to 100% wet bond, 0 /,, representing the condition when no visible fibers are left adhering to the gypsum core, and 100% representing the condition of complete fiber coverage over the surface. Wet bond testing in no way reflects the final starch bonded achieved after the board dries, but gives an indication of some of several forces, chemical bond, gypsum-to-fiber attachment, mechanical bond, and other 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 is further dried in a circulating air oven or kiln at a temperature of about 360"F for 50 minutes or until the final weight 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 board sample is then scored through the back paper or opposite to the five test pieces in a direction parallel to the 15 in. length about 1 inch in from the edge. The board is then 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 bond failure, that is, 100% being no paper to gypsum core bond, and 0% being no paper to core failure, or complete and full paper bond ply coverage to the board core surface. Humidified bond is obtained by placing the sample in a 90% relative humidity at 90"F for a period of three hours.

EXAMPLE 1 AND 2 A starch coating composition was prepared according to the invention having the following formulation: Percent Parts by Weight Tapioca starch 20.0 160.0 Calcium sulfate hemihydrate 10.3 80.0 Kelzan M 0.3 2.4 Dow 'G' ("Dow" is a registered 0.1 .8 trade mark) Water 69.6 556.8 100.0 800.0 Total The Brookfield viscosity of the above composition at the time of coating was measured as 2800 cps. The composition was coated on paper as described above by means of a roller coater. In Example I the composition was coated on manilla paper. In Example 2 the composition of Example 1 was coated on Newslined paper.

Test gypsum boards were made as described with each type of paper. 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 the parent application referred to above.

EXAMPLES 3 AND 4 Percent Parts by Weight 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 time of coating had a viscosity of 1600 cps. This composition was also coated on both manila paper and Newslined paper, and coated in the same manner and with the same apparatus as described above with regard to Examples 1 and 2. The results of tests on the finished board made with the coated papers are also shown in Table I below.

TABLE I Humidified* Starch Dry* Bond Bond Composition Wet Bond /a Failure % Failure Manila Example 1 Invention composition Excellent 0 0 starch/stucco/kelzan (800/,--100"/, Fiber attach ment on core) Example 3 Control Good 20 20 starch/kelzan (6080 /a Fiber attach ment on core) Newslined Example 2 Invention composition Excellent 0 0 starch/stucco/kelzan Example 4 Control Good 20 20 starch/kelzan *Ave. 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 formed with the coated paper of Examples 1 and 2 according to the invention give 0% of failures whereas the board of Examples 3 and 4 formed with the prior art coating composition gave about 20% failures in all cases.

Figs. 1 and 2 of the drawing are photographs of coated paper cover sheets made with the composition of Example 1. As can 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 10x enlargement of Fig. I and illustrates the high resolution achieved by the composition having sharp defined outer edges of the coating lines or stripes 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 edges of the stripes or lines more diffused and in part spreading to the spaces intermediate the lines.

One of the important factors in providing a coating composition which will provide a coating of sufficient thickness and excellent resolution is the viscosity of the composition. To determine the effect on viscosity of various compositions containing stucco with a control free of stucco, the following coating compositions were prepared:: EXAMPLE 5 Percent of Total by Weight Parts by Weight 20% Starch 80 g Starch 0.3% Kelzan M 1.2 g Kelzan M 0.1% Dow G 0.4 g Dow G 80% Water 320 ml Water EXAMPLE 6 Percent of Total by Weight Parts by Weight 20% Starch 80 g Starch 10% Shoals stucco gypsum 40 g Gypsum 0.3 Kelzan M 1.2 g Kelzan M 0.1% Dow G 0.4 g Dow G 70% Water 280 ml Water EXAMPLE 7 Percent of Total by Weight Parts by Weight 20% Starch 80 g Starch 15% Shoals stucco gypsum 60 g Gypsum 0.3% Kelzan M 1.2 g Kelzan M 0.1% Dow G 0.4 g Dow G 65% Water 260 ml Water EXAMPLE 8 Percent of Total by Weight Parts by Weight 20% Starch 80 g Starch 20% Shoals stucco gypsum 80 g Gypsum 0.3% Kelzan M 1.2 g Kelzan M 0.1% Dow G 0.4 g Dow G 60% Water 240 ml Water EXAMPLE 9 Percent of Total by Weight Parts by Weight 20% Starch 80 g Starch 30% Shoals stucco gypsum 120 g Gypsum 0.3% Kelzan M 1.2 g Kelzan M 0.1%DowG 0.4 g Dow G 50% Water 200 ml Water The viscosities of the compositions of Examples 5-9 measured at various times from initial mixing are shown in Table II below.

TABLE II Sample Identification 30 minutes 3 hours 6 hours 24 hours Example 5 1525 cps 1625 cps 1625 cps 1700 cps Example 6 2787.5 cps 2850 cps 2900 cps 5800 cps Example 7 4025 cps 4150 cps 4650 cps 11,250 cps Example 8 5650 cps 5950 cps 8800 cps Solid Example 9 18,750 cps 21,750 cps Solid Solid It has generally been found that coating compositions having viscosities in the range from 2500 cps to 8000 cps provide excellent coatings when prepared according to the invention. In the case of compositions containing smaller amounts of calcium sulfate hemihydrate, it may be necessary to let the composition stand for a period sufficient to raise the viscosity.On the other hand, for those compositions, as for example that of Example 7 the composition should be used within 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 preparing the coating compositions described above in Examples 1--9, the dry ingredients were first mixed together and then the mixture added to water under agitation using a "Lightnin Mixer" ("Lightnin" is a registered trade mark) at a medium speed for ten minutes. It was found, that, contrary to expectations, the final coating contained a major proportion of unchanged calcium sulfate hemihydrate, as determined by X-ray diffraction studies. It was subsequently determined that the starch acts as an inhibitor to hydration of the hemihydrate.

It has been found that in addition to common stucco (beta-calcium sulfate hemihydrate), other forms of gypsum are suitable for practicing the invention, as for example, alpha-calcium sulfate hemihydrate, and calcium sulfate dihydrate.

The following examples illustrate the effect on the coating composition of various factors such as the composition and grain size or surface area of the gypsum, the sequence used in adding the ingredients, the intensity and length of time of stirring the mixture and of length of time of standing after mixing but before application of the composition on the paper.

EXAMPLE 10 Formulation: Percent Parts Tapioca starch 20.0 160.0 Stucco 10.0 80.0 Kelzan M 0.3 2.4 Dow G 0.1 0.8 Water 69.6 556.9 Total 100.0 800.0 All the dry ingredients (starch, stucco, Kelzan M, and Dow G) were mixed together and then added to water under medium agitation (800 RPM) for a mixing period of ten minutes'.

EXAMPLE 11 The formulation shown in Example 10 was prepared by combining the dry ingredients and adding them to water under high agitation (1400 RPM) and mixing for thirty minutes.

EXAMPLE 12 The formulation shown in Example 10 was utilized. Only the stucco was added to the total water and mixed for twenty minutes under high agitation (1400 RPM).

The remaining ingredients were combined and added to the stucco slurry, and then mixed for an additional ten minutes under agitation.

EXAMPLE 13 The following formulation was prepared, utilizing only 7% stucco.

Formulation: Percent Parts Tapioca starch 20.0 160.0 Stucco 7.0 56.0 Kelzan M 0.3 2.4 Dow G 0.1 0.8 Water 72.6 580.8 Total 100.0 800.0 All of the dry ingredients were combined together, added to water under high agitation and mixed for thirty minutes.

EXAMPLE 14 The following formulation was prepared.

Formulation: Percent Parts Tapioca starch 20.0 160.0 Finely ground calcium 7.0 56.0 sulfate dihydrate Kelzan M 0.3 2.4 Dow G 0.1 0.8 Water 72.6 580.8 Total 100.0 800.0 In this Example, 7% finely ground calcium sulfate dihydrate normally used as a gypsum setting accelerator was substituted for stucco. All the dry ingredients were combined together and added to water under high agitation and mixed for thirty minutes.

EXAMPLE 15 The following formulation was prepared: Formulation: Percent Parts Tapioca starch 20.0 160.0 Land plaster 7.0 56.0 Kelzan M 0.3 2.4 Dow G 0.1 0.8 Water 72.6 580.8 Total 100.0 800.0 In Example 15, Land plaster, which is a calcium sulfate dihydrate of moderate particle size, was substituted for stucco. All the dry ingredients were combined together, added to water under high shear and mixed for thirty minutes.

EXAMPLE 16 The following formulation was prepared: Formulation: Percent Parts Tapioca starch 20.0 160.0 Kelzan M 0.3 2.4 Dow G 0.1 0.8 Water 79.6 636.8 Total 100.0 800.0 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.

TABLE III Brookfield Viscosity-Cps Gypsum Form Board Bond 1 hr 4 hrs 24 hrs 24 hr Sample Pattern Dry Example Standing Standing Standing X-ray Diffraction Definition Wet % Failure Example 10 2800 2900 5600 90% Hemihydrate Ex. Ex. 0 (10% Stucco-10' Mixing) Example 11 5000 5200 5300 90% Hemihydrate Ex. Ex. 0 (10% Stucco-30' Mixing) Example 12 6600 6900 7600 90% Dihydrate Ex. Ex. 0 (two mixing stages) Example 13 3800 3900 4100 90% Hemihydrate Ex. Ex. 0 (7% Stucco-30' Mixing) Example 14 4100 4200 4500 95% Dihydrate Ex.Ex. 0 (7% Calcium sulfate dihydrate 30' Mixing) Example 15 2000 2050 2100 95% Dihydrate Poor Good 20 (7% Land plaster-30' Mixing) Example 16 1525 1625 1700 - Poor Good 20 (starch only 30' mixing) The results shown above in Table III indicate that the viscosity of the coating composition is closely related to its ability to form starch coated paper from which gypsum board can be made having excellent pattern definition, and excellent board strength, both wet and dry. As shown, all of the compositions of Examples lO--14 form starch-coated paper of excellent pattern definition and the coated paper used to form gypsum board having excellent wet and dry board bond. The compositions used in every case were those which were allowed to stand for only one hour before coating the paper.It is thus seen that where the one hour viscosity was from 2800 to 6600, excellent products were produced. Viscosities of as high as 7600, as for example that of Example 12 which was permitted to stand for 24 hours, may also be used but the coating process becomes more difficult. The particular compositions of Examples 15 and 16 having viscosities of 2000 or under form coated paper of poor pattern definition and, although they exhibited good wet bond, the dry bond failure was less acceptable at 20%.

Even the composition of Example 15 which exhibited a viscosity of 2100 cps after standing for 24 hours was too thin to provide good definition and good dry bond. An explanation for the poor performance of the composition of Example 15 is that the product was common land plaster of the type used for calcining, whereas the dihydrate of Example 14 was very finely ground and qf the type normally used as a gypsum set accelerator.

It was further found that, although the dihydrate form of gypsum may be used to form the coating composition, somewhat better results are obtained by using the hemihydrate form and by premixing the dry ingredients prior to adding water so that the presence of the starch cause the hemihydrate to remain in that form even after standing and even after coating of the paper.

In order to determine the lower limits of calcium sulfate hemihydrate addition which would still provide a good starch-coating composition were prepared utilizing 20 zn starch in each example but varying the calcium sulfate hemihydrate from 0 to 10%. Mixing was carried out for 30 tninutes at high shear and the coating composition then permitted to stand for an additional hour, at which time the viscosities of the compositions were measured.

TABLE IV % Calcium Sulfate Viscosity cps Example Hemihydrate O/, Starch after I hr Example 17 0 20 2200 Example 18 1 20 2625 Example 19 3 20 3025 Example 20 5 20 3400 Example 21 10 20 4650 As can be seen from the results in Table IV, which sets out percent of ingredients by weight a composition containing as low as 1% calcium sulfate hemihydrate provided a composition having a viscosity of 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.

The viscosities of the coating compositions of the invention preferably range from at least 2500 cps to 8000 cps. However, compositions having higher viscosities may be satisfactorily 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 viscosities. 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 effective coating composition than the very finely ground dihydrate which is conventionally used as an accelerator for setting a gypsum slurry.

The data above also shows that when high shear mixing at extended periods is utilized, the viscosity reaches a maximum early and does not change materially after being allowed to stand for a period of time.

In listing the percentage or parts of the various ingredients in the Tables above, the reference in every case is to percentage or parts by weight. In the case of an aqueous composition, the percentage is by weight of the entire composition including the water.

The material listed as Kelzan M is a conventional dispersing agent comprising xanthane gum.

Dow G is a conventional preservative having the chemical name sodium pentachlorophenate.

The raw starch may be present in an amount from 5% to 30% by weight of the coating composition.

The calcium sulfate such as calcium sulfate hemihydrate may be present in an amount from 0.75In to 30% of the weight of the coating composition.

It is to be understood that the invention is not to be limited to the exact details of composition, materials, or operation shown or described, as obvious modifications and equivalents will be apparent to one skilled in the art.

WHAT WE CLAIM IS: 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 the remaining areas free of said composition.

2. A process according to Claim I, 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 2500 to 8000 cps.

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

6. A process according to Claim 1, wherein said starch is present in an amount from 5% to 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.

Il. 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 comprixes xanthate 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 composition at limited areas of said bond liner while leaving the remaining 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 2500 to 8000 cps.

20. A paper cover sheet according to Claim 16, wherein said calcium sulfate is present in an amount of from 0.75 / to 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 5% to 30 n by weight of said composition.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The material listed as Kelzan M is a conventional dispersing agent comprising xanthane gum. Dow G is a conventional preservative having the chemical name sodium pentachlorophenate. The raw starch may be present in an amount from 5% to 30% by weight of the coating composition. The calcium sulfate such as calcium sulfate hemihydrate may be present in an amount from 0.75In to 30% of the weight of the coating composition. It is to be understood that the invention is not to be limited to the exact details of composition, materials, or operation shown or described, as obvious modifications and equivalents will be apparent to one skilled in the art. WHAT WE CLAIM IS:

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 the remaining areas free of said composition.

22. A paper cover sheet according to 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 xanthate 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 the remaining areas free of said adhesive composition, said adhesive composition comprising 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 finely particulate calcium sulfate is calcium sulfate dihydrate.

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

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

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

36. A process according to Claim 31, wherein said starch is present in an amount from 5% to 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 xanthate 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 the remaining areas free of said starch adhesive composition, said coating composition providing an adhesive bond between said core and said paper cover sheets at the limited areas containing said starch adhesive.

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 2500 to 8000 cps.

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

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

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

53. A process 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 xanthate 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.

61. A process, for producing a starch-coated paper cover sheet, substantially as hereinbefore described with reference to Examples 1, 2 and 10 to 15.

62. A starch coated paper cover sheet substantially as hereinbefore described with reference to the accompanying drawings.

63. A process, for producing gypsum wallboard, substantially as hereinbefore described with reference to Examples 1, 2 and 10 to 15.

64. Gypsum wallboard, substantially as hereinbefore described with reference to Examples 1, 2 and 10 to 15.