US2346999A - Manufacture of composite wallboard - Google Patents

Description

Patented Apr. 18, 1944 muurac'rona F comrosrrn I wmnom John mime mam, Saltburn, and Victor Lefebnre, London, England, minors to 11mperlal Chemical Industries Limited, a corporation of Great Britain No Drawing. Application October 15, 1941, Serial No. 415,148. In Great BrltalnJune 30, 1937 32 Claims.

This invention relates to the manufacture of composite wall board consisting of a .layer of water-setting cementltlous material such as calcium sulphate plaster covered on one or both sides with relatively thin flbre liner, e. g. pulp board, thick paper, cardboard or the like, which is caused to adhere firmly to the cementitious material by allowing the latter to set in contact therewith, if necessary under pressure.

In plasterboard of this type the cementitious material is most generally plaster of Paris, hereinafter called stucco, but catalysed anhydrite plaster, and mixtures of stucco and catalysed anhydrlte can also be used. As is well known the board is usually made by a continuous sheeting method. The dry plaster, with certain modifying agents to be described later, ismixed with water to give a semi-liquid plastic mix and this is delivered on to the lower of two moving webs of vertically superimposed rolls, known as master rolls, which mould the board. The bottom liner is arranged to be wider than the top. one and is scored and folded over by means of side edge formers which ar placed in front of the master rolls and extend into the nip. Boards having edges totally enclosed in liner are produced in this way. The-wet formed board travels down a long conveyor placed beyond the rolls and of such a length that the core has set and become suflicient hard to permit the board to be cut by the time it reaches the knife situated at the end of the conveyor. The. knife generally consists of a double rotary cutter with two blades having saw-toothed edges. Theblades are super;- imposed, move in the same direction at the moment of cutting, and operate by severing the liner and cutting into the set core. An accelerat ing device forces the board forward and facilitates cleancutting. The knife can be set by suitable means to cut boards of different thicknesses and lengths. It will be clear that the distance between the master rolls and the knife, or in other words the length of the conveyor, will be transfer to the kiln, and lower drying temperatures are generally necessary owing to the smaller amount of water to be evaporated.

The following is a brief explanation of 'what. happens during the process oi-manufacture. The

wet mixed plaster as it is applied to the paper has not hydrated and does not indeed hydrate appreciably until some time after the board has passed the master rolls. If any degree of hydration were allowed to take place prior to the rolls pre-setting of the core would occur and this would cause poor core adhesion and weakness in the core. Retarder is added to the mix to prevent the occurrence of such conditions. As the board passes down the conveyor the plaster gradually sets and some of the supersaturated gypsum solution from. the .core mix passes into the paper. The plaster sets before the knife is reached, and just before the completion of set the gypsum in the solution which has penetrated into the liners is precipitated, and the crystals so formed provide the bond between the core and the paper. this initial bond, as it is called, betweenpaper and core should develop well before the knife to prevent detachment of the liners from the core during the cutting operation. The core is completely hydrated before the boards enter the kiln and the drying process simply consists in evaporating off the free moisture. Sharp rapid drying without dehydration produces the-best boards.

The wet plastic mix invariably contains additional ingredients which are added to confer special properties. Thus wood fibre and other cel-, lulosic materials are added to reducedensity, to confer improved strength, and to improve the nailing and sawing qualities of the boards; foam produced by the aeration of aqueous solutions of organic, colloidal matter isfrequently added to the wet plastic mix to reduce density, or is formed in situ by incorporating in the-mixchemicals which interact to give gas bubbles; retarders of plaster set, suchas scutch prepared for example from keratinous refuse, are added with the object of facilitating the passage of the core mix through the mixers without setting; accelerators, such as ground set gypsum blocks or masses derivedfrom plaster' of Paris for example the ground cores of old plaster boards, are added with the object of causing the core mix to set sufiiciently hard to be capable of, being cut within the time interval permitted by plant de sign; and materials known as core adhesives are In the manufacture of good board added to promote bond between the setting coreand the paper.

This quality of bond between core and liner is of the utmost importance in the manufacture of good plasterboard because upon it depends the strength and integrity and performance in use of the board. Indeed it is true to say that without it commercially sound plasterboard does not exist. Poor bonding yields a weak board in which the full strength of the reinforcing liners is not utilised and one which will disintegrate when subjected to the effect of damp or when a coat of plaster is applied to it. Even if the core bond is patchy it frequently happens that when a plaster face coat is applied, the-water from it soaks into the liner, which then commences to bulge over the patches. Later, as the plaster dries, the paper is pulled away from the core where the adhesion is poor and cracking and detachment of the plaster take place. It will thus be seen that gOOd bond between core and liner is a sine qua non in plasterboard of commercial standards and quality.

The mechanism of the formation of bond between core and liners has already been described. It is well known, however, in the art of plasterboard manufacture that the bond so produced by any type of plaster whether stucco, or anhydrite, or indeed any other type is not sumciently good to give a commercially sound plasterboard and it is'invariably necessary to incorporate in the core mix an adhesive, known as a core adhesive, to promote the necessary good bond. It is not known why such adhesives are necessary but it is nevertheless a fact that they are necessary. Suitable adhesives are starches, chemically treated and so-called soluble starches, dextrines, gums, urea formaldehyde and thiourea formaldehyde resin syrups, and other water soluble or dispersible substances. The amount of the core adhesive'necessary varies greatly and is determined partly by its own properties and partly by thecharacteristics of the plaster: gen erally, however the extreme proportions vary between 0.25% and 3% by weight of the dry plaster. (In the following description the proportions of all ingredients in any mix or composition are expressed as a percentage by weight of the dry plaster used in the said mix or composition.) Thus from 0.3-1.5% starches, gums and dextrine and 1.0-2.0% urea formaldehyde syrup containing 75% resin substance or equivalent amount of different syrups, may be used for stuccos manufactured from rock gypsum and 0.75%-3% of the former adhesives and 1.5-2.0% of the latter may be used for stucco prepared from by-product gypsum, and for other types of plaster. In commercial practice it is never possible, even with the best plasters, to employ less than 0.25% of core adhesive if adequate bond is consistently to be obtained in continuous operation, and in order to provide a safety factor at least about 0.5% is generally employed. All these adhesives are many times as expensive as the plaster and the cheapest will cost twenty times as much as the plaster per unit weight.

core mix and very many attempts have been made to do so but none of these so far as we are aware has been successful.

It is an object of this invention to make commercially sound plasterboard'from ordinary plas-- ters without the necessity of having to add adhesive to the core mix, and particularly to do this It would therefore be extremely desirable to eliminate them from the on continuous plasterboard machines. It is also an object to provide an improved method of securing good adhesion between the cementitious material and the liner or other fibrous surface, especially where the cementitious material has poor bonding powers or where the liners do not possess suitable properties for bonding. A further object is to provide a method of securing a much improved adhesion between the cementitious material and the liners or other fibrou surface. A further object is to provide an adhesive composition specially adapted to secure a good bond between the cementitious material and the liners or other'fibrous surface. A further object is to provide this improved bond by a method which is suitable for use in connection with fast continuous plasterboard machines and in which the initial wet bond between plaster and liner develops sufflciently quickly to enable the machine to be run at the highest speeds. A

tween the core and the liner. This method consists in applying to the liners before they come into contact with the wet core mix a coating consisting of a water setting plaster or cement, in

admixture with water and with a water-soluble or water-dispersible adhesive. It would appear that the water setting cement and water soluble adhesive'in the coating bond to the paper and that the water setting cement of the core bonds to the water setting cement in the coating, since a coating of water setting cement alone or of water soluble adhesive alone does not give the same effect. In the production of plasterboard from plaster core mixes on fast continuous machines the water setting cement will generally be plaster of Paris, i. e. stucco. The proportion of water soluble or dispersible adhesive should not be less than 5% by weight of the stucco in the coating. The amount of water varies betwen 45% and 600% by weight of the stucco and is determined by the properties of the water setting cement of which the core is composed, the properties of the to the liners.

- troughs, spray or the like.

paper, and the method of applying the coating The coating may'be applied by means of doctors, applicator rolls, brushes, The coating may be used in either of two ways, viz. the wet way which consists in applying it to the liners on the plasterboard machine a short distance in front of the rolls and allowing it to become tacky before it comes in contact with the plastic core mix, or the dry way, according to which it is applied to the liner in a prior operation and dried and the dry coated liner is reeled for subsequent use on the plasterboard machine. For various reasons, which will be given later, the first method is preferred.

In using the wet way it is important that the coating should be in the correct tacky condition before it is brought into contact with the core mix. Several factors contribute towards this condition, among which the speed of running, the water absorptive properties of the liner and the atmospheric temperature appear to be the more important. The tackiness appears to be brought about by the absorption of water from the coating by the liner and by surface evaporation into the air, and so far as we are aware, practically no hydration has occurred in the coating at this stage. Indeed marked hydration in the coating at the tacky stages results in poor final bond. If the tacky condition is not sufflciently attained before the core mix is applied, the coating will be too liquid and will tend to be rubbed off by the friction of the latter, particularly in the neighbourhood of the master rolls: on the other hand, if the coating has gone beyond the tacky stage when the core mix is applied it will be too dry and the final bond will be poor. The correct conditions are achieved by adjusting the position of the application means and by varying the proportion of water in the coating mix. The weight of the coating applied will depend partly upon its composition, partly upon that of the. core mix, and partly upon the nature of the paper, but it will not in general exceed 0.4 lb. per sq. yard of liner and will preferably amount to about 0.2 lb.

We have found that the invention may be successfully carried out on continuous machines byemploying the following range of variations in the composition and weight of the coating.

Stucco parts by weight 100 Adhesive do 5-50 Water do -600 Weight of coating lb. per sq. yd. of liner (depending upon the water proportion) .15-06 The preferred ranges are Plaster parts by weight 100 Adhesive do 540 Water l -do -100 Weight of coating lb. per sq. yd. of liner- 0.2

It i generally unnecessary to employ more than 20 parts of adhesive because no further benefit is obtained. However with catalysed urea formaldehyde or thiourea formaldehyde syrups up to 50% (based on the weight of plaster) of syrups containing 75% resin substance may be used to give a bond having improved water resistance resulting from the resinification of the syrups during the drying operation. Beyond thisproportion the bond is not so good and its development initially is slow.

0f the adhesives gum arable, karaya gum, and dextrine are preferred for ordinary plasterboard, and urea formaldehyde syrup for boards to be used in outside situations where more long delay in the attainment of initial adhesion with coating mixes containing a large proportion or an adhesive, such as gum arable, which is itself a retarder.

The invention may be used in any method of making plasterboards, but as already mentioned the primary object is to produce on rapid continuous machines plasterboard possessing adequate core adhesion from plaster mixes containing no core adhesive. It is, however, a requirement of production on rapid continuous plasterboard machines that by the time the board hasreached the knife the core should be suiliciently hard, and the bond between core and liner should be sufficiently strong, to-enable the bqardto be out. If the second condition is not fulfilled, the

knife as it penetrates into and is again withdrawn from the board during the cutting operation will detach the liners from the core at the ends. The liners so detached do not bond again to the core in the subsequent process and consequently the boards have weak ends which are liable to be easily damaged by knocking or by water ingress. It is a secondary advantage of our invention that initial adhesion develops rapidly. This is due to the presence in the coating of the setting cement, viz. plaster of Paris, because initial adhesion only develops slowly if the attempt is made to coat the liner with solutions of gum, dextrine, and urea formaldehyde syrup or starch paste, containing no added water setting cement. (It should be mentioned that the final dry bond produced in these last I ways is inferior.) This is of particular advantage where the water setting cement or plaster employed in the core mix develops initial bond slowly. This advantage of early initial adhesion is best achieved by using coatings containing plaster of Paris. The development of initial bond can be hastened by introducing an accelerator such as ground set gypsum in the coating.

It is important that the liquid coating should not contain gypsum (other than accelerator) or at any rate very little gypsum, otherwise the final bond is not good. This can be achieved by arranging that it i used before any noticeable stiffening occurs or better by determining the time for an increase in hydration of 1.0% and ensuring that the mix is used before this is attained. Where the mix is made and used continuously this dimculty does not occur.

In carrying out the invention by the dry way referred to herein, the composition of the mix will generally-be as'before, but as the coated liner is rolled and there is therefore some danger of the dry coating cracking and flaking ofirit is desirable to use theminimum weight of coatmg and also to use more water in the Very good results are obtained by using 0.2 lb. per sq. yard of a coating having the composition:

} Parts by weight Stucco a .1 Adhesive $920 Water recs The method of use is to apply the coating to the liner by means of a doctor, roller, brush. trough, or other known means of application and to dry rapidly by passing it over a hot cylinder at -180 C. with the coating towards the hot surface or to direct a current of air at a temperature of lam- C. onto the coating.

The time of application of the heating medium should be about-2M0 seconds. Apparently very little gypsum is formed and consequently good I core bond is obtained when the dry coated liner is brought-into contact with the wet plaster core mix. -Ifthe coating is allowed to'dry slowly in air hydration of the plaster of Paris occurs and essential to plasterboard manufacture.

poor core bond is obtained in the plasterboard process. As we have said it is desirable that the amount of gypsum, other than accelerator, present in the coating at the time of application of the core mix should be kept low and should correspond to an increase in hydration of not more than 1.0%. Liner coated in this way may be stored for many months and at the end of that time can be used to make plasterboard possessing adequate core adhesion. The disadvantages of employing the dry way are that the extra drying and handling operations add to the expense and reduce the economic advantages of the process, and that it is difficult unless expensive reeling equipment is provided to reel the dry coated liner so that the reels have the square edges that are If the edges are not square the liners weave in the plasterboard machine and in consequence efficiency of production falls and a large number of low quality board having poor edges is produced. It will thus be seen that the dry way has a more limited sphere of usefulness than the wet way, I

It will be understood that the present invention has valuable application to any core mix which is, of itself, incapable of giving good bond to plasterboard liners, and in particular to any core mix which is incapable of being used on a continuous plasterboard machine to yield a commercia'ly acceptable product. It will also be understood that in the preferred form of the invention the core mix is one free from adhesive or is one containing a proportion of adhesive relatively small compared with that hitherto employed in commercial practice, since it is thereby possible to achieve the maximum economy as a result of the discovery on which the present invention is based, namely, the discovery that it is possible to provide a positive bonding interlayer between the core and the liner instead of relying upon modification of the core mix as has hitherto been believed to be necessary.

The present invention therefore does not exclude the uses of core mixes modified for purposes other than that of giving good bond to the plasterboard liners. Thus it is common practice in the art of plasterboard manufacture to add prepared foams to the core mix to reduce the density of the boards and to improve the insulating properties. Liners coated by the wet and dry methods of our invention can be used on the plasterboard machines with core mixes containing foam to give the aforesaid advantages of good bond and early development of initial bond, and this without the addition of special core adhesives. Frequently the added foam contains gum, dextrine, starch, glue or casein as stabiliser. The amount of this stabiliseris generally about 0.3% of the weight of stucco and it does not give adequate core adhesion in the made plasterboard. Our invention covers the use of the positive bond coating of the invention with such foamed core mixes in which the total adhesive substance in the mix, exclusive of glue or casein, does not exceed 0.3% by weight of the stucco. As foamed core mixes do not bond so well to liner as ordinary mixes the application of our invention to them is of great advantage. Foams having glue as stabiliser, although possessing excellent stability, could be used in core mixes heretofore because the presence of the glue prevents bonding to the paper even although excessive proportions of the ordinary core adhesives are used. We have found that mixes containing foams of this type can be made to bond to liner by using our new positive bond coating. It is therefore to be understood that the invention covers the use of these coatings in combination with foamed plaster core mixes which do not of themselves bond to the liners.

The strength of the core bond is assessed in the following way. A sample of the board to be tested, about 6" square, is taken and the attempt is made to strip the liner from the core. If after repeated attempts it is foundimpossible to expose the core and the liner continues to tear in itself, the bond is regarded as excellent, and an arbitrary adhesion value, of 10 is assigned. If after repeated attempts it is found impossible to expose an area of core exceeding 222 cated. If after similar treatment the core can be exposed over an area of n l/ H," in depth, an adhesion value of 7 is allocated. If after similar treatment the core can be exposed over an area T M- A in depth an adhesion value of 6 is allocated. If the liner can be completely stripped from the'core and leaves no fibre attached to the latter an adhesion va1ue'0 is allocated. Intermediate values are allocated to all degrees of bond between 10 and 0. In commercially sound plasterboard the bond or adhesion value as determined in this way should preferably be 8 or more, and must in no circumstances be less than 7.

The advantages of the invention are illustrated by the following series of tests, and the method of execution by Examples 1-10.

Plasterboards made from wallboard liner and core mixes having the following compositions were dried in the usual way and gave the adhesion values indicated.

Stucco (made from rock gypsum) 100 100 100 Stucco (made from by-product gypsum 100 100 100 100 Sawdust 4 4 4 4 4 4 4 4 Dextrine (09% 501- uble in cold water) 0.25 0.4 0.5 0.25 0.4 0. 6 Organic scutch reta er 0.05 0.05 0.05 0.05005 0.05 0.05 0.05 Water 65 65 65 65 62 62 62 62 Adhesion values 0 6 8 8 0 5 7 8 These results indicate that with the stucco made from rock gypsum adequate bond could not be obtained with less than 0.3% dextrine, and that with stucco made from by-product gypsum adequate bond could not be obtained with less than 0.4% dextrine. Similar results are obtained with other adhesives.

Further boards were made from the same liner using the core mixes (1), (2), (5) and (6) respectively and applying coatings having the following composition by the wet method. The coatings were applied by brush and the core mix was brought into contact with the liners 20 seconds after aplication. The weight of coating applied was 0.2 lb. per sq. yard of liner.

0 Parts by weight These results demonstrate the improved core bond obtained by using the method of our invention. g

A further lot of boards was prepared from in depth, an adhesion value of 8 is allobond.

aseaeoe core mixes (1),(2), and (6) using the se liners and applying by the wet method 0.20 lb.

- per square yard of a coating having the following composition. Parts by weight Stucco 109 Dextrine. 2.5 Retarder 0.14 Water 100 Example 1 finished board to water was also perfect, which result would not have been obtained otherwise.

Example 2 100 parts of plaster of Paris were mixed with 7.6 parts of water soluble gum, 12.5 parts of urea formaldehyde resin syrup and 5'? parts of water. A team plasterboard core containing no added core adhesive was employed contacting with the coated liners within three minutes of application of the coating. Perfect adhesion was obtained in the final board, far better than any adhesion previously obtained with foamed cores without the use of the positive bond coating.

Example 3 Parallel tests were made with the mixes of Examples 1 and 2, with board liners of very low absorption and very smooth surface which did not give any adhesion when normal plaster of Paris board cores were employed. These liners, however, gave perfect adhesion with such normal plaster of Paris cores when the liners were treated with the mixes and in the manner indicated in Examples 1 and 2.

Example 4 100 gms. of the kind of plaster of Paris known as coarse boiled plaster was mixed with 25 to cos. of catalysed urea formaldehyde resin syrup (containing 45 per cent of solids) in admixture with 45-55 cos. of water. The resulting coating composition was applied to the liners as in. Example l with similar excellent results.

Example 5 100 gms. of the kind of plaster or Paris known as coarse boiled plaster was ed with 70-l5,ccs.

Bit

til)

Enample 6 Plasterboard was made from a core mix ha a w 1;- the following composition.

Parts by weight By-product plaster of Paris 1W Sawdust Organic retarder..; 9.10 Water $2 The water was heated to 40 C. to promote rapid hardening. I

The plaster of Paris used in this exple was obtained as .a by-product in the manufacture of phosphoric acid from phosphatic rock and is referred to in subsequent examples as phosphate stucco.

A liquid coating having the formula:

- Parts by weight By-product plaster of Faris ltd Gum arable 5 Organic retarder aces Water was applied by means of two V shaped troughs bearing upon the paper and set so that the clearance between the bottom face of thetrough and the paper surface was of an inch. These troughs were each placed about 6 feet in front of the master rolls. The. weight or coating applied was approximately 0.25 lb. per square yard or liner. The board was fabricated in the usual way. Initial adhesion developed at is minutes. The boards were cut and transferred to the kiln where they were dried for l. hours using an inlet temperature of 200 C... an outlet temperature of 75 C., and an air velocity of 10 feet per second. The dried boards had a core adhesion value of 9.,

Example 1? thick boards were made from a core mix consisting of a mixture of plaster of Paris and- Parts by weight Ground anhydrite -i... a. we.

3 Plaster or Paris from rock. gypsum 30 Potassium sulphate in Zinc sulpha ill Water 32 thus enabling a plant spec of 20 per cent gum arabic solutiomtogether with up to 59 cos. of water if necessary to strain a suitable consistency.' This again gave a perfect Parts by weight Phosphate stucco Dextrine 5 Water v 30 Calgon 'to give an mun set by Vicat of 20 minutes Initial. adhesion developed at 5% minutes and it was possible to cut at 7 minutes after mixing, (1 of 15 feet per minute to be obtained. It had not been possible previousiy to make board from this coremix owing to the very long delay-several hours--in the development of initial adhesion. Theimprovementwas due to the quick initial bonding of the coating to the liner, followed by the bond-1 ing of the core mix to the coating. The boards were allowed tosnature in air for z i hours and were then dried for 1% hours with an inlet temperature of 150 C. and an outlet temperature 01' 70 C. The final core adhesion value was 9.

Example 8 Boards were made from a core having the composition:

Part by weight Ground anhydrite 70 Plaster of Paris from rock gypsum 30 Potassium sulphate 1.0 Zinc sulphate 1.0 Water 32 Calgon 0.04

A liquid coating having the formula:

- Parts by weight Phosphate stucco 100 Urea formaldehyde syrup (75% solids)--- 50 Water 145 H2804 (catalyst) 0.05

was applied by brush to strawboard liners at the rate of 0.2 lb. per square yard of liner. 20 seconds after application the liners were brought to contact with the cor mix. Initial adhesion developed at 4 minutes. The board was allowed to mature in air for 24 hours and was then dried.

The final core adhesion was 10.

Example 9 Boards were made from a core having the composition:

, Parts by weight Phosphate stucco -s 100 Sawdust -L 4 Calgon 0.03 Water 60 To the liners, 'which were of strawboard, 2.0 seconds before the application of the core mix there was applied by means of a brush 0.2 lb. per square yard of liner a positive :bond coating having the following composition:

Parts by weight Stucco Dextrine 5 Scutch retarder; 0.03 Water 100 Initial adhesion developed at 6 minutes and after drying the core adhesion value was 10.

Example 10 0.2 lb. liquid coating was applied per square yard of liner. They were dried at 160 C. for 40 seconds and were then laid aside.

Boards were made from these dry pre-coated liners using a core mix as follows:

Parts by weight Ground anhydrite 70 Plaster of Paris from rock gypsum 30 Potassium sulphate. 1.0 Zinc sulphate 1.0 Water 32 Calgcn 0.04

A thin coating or water was applied to the liners seconds before applying the plaster nnx. Imtial adhesion developed in 5 minutes. The boards were allowed to mature in air for 24 hours and were then dried. The final adhesion was 10-9. In Examples 1, 2, 4 and 5, retarded hemihydrate plasters could be used to modify the behaviour of the wet coating, with'a view to prolonging the life of the wet mix in the mixing vessel and in the coating appliance. Accelerators and retarders could also be added as required. A larger proportion of urea resin syrup could also be used, e. g. 2550 ccs. per 100 gms. of plaster. This would delay the development of adhesion, but would be feasible in a plant oping andboard-making operations.

crating at a low speed or with a very long conveyor.

Instead of the gum arabic used in Example 5, other gums, such as karaya gum, water-soluble derivatives of cellulose, gum tragacanth, starch, or mixtures thereof, could be used. If dextrine is used as water-soluble adhesive, it should contain the minimum of insoluble material.

By varying the proportions of the constituents the setting time of the coating compositions can be adjusted to suit the requirements of the coat- Fi-bre liners treated with these compositions adhere firmly to the water-setting cementitious material forming the body of the composite board. If the plaster is omitted from the composition, or is replaced by an inert filler, adhesion to the cementitious materials falls far short of that required for commercial purposes.

Good results have been obtained with aqueous urea-formaldehyde or thio-urea-iormaldehyde resin syrups such as are obtained by reactin urea or thio-urea with formaldehyde in aqueous medium and removing a portion of the water or not as required. A typical resin syrup may contain 50 %-'75% of resin substance, the balance being water. It will be understood that the resin is in the reversible or uncured state and that the resin may be applied in any solid form capable of being dissolved or dispersed in water to give a solution, colloidal dispersion or emulsion. Resin accelerators may be employed in the mix. The plaster may be mixed with the syrup .or solid form of resin and water if necessary to a consistency suitable for the chosen method of application, e. g. a thin paint for brushing or spraying, or a thicker paint for application by rolling, or by means of a spreading knife.

The plaster used in preparing the coating composition may be any known kind of calcium sulphate plaster other than dead burnt plasters, for example, stucco, plaster of Paris, coarse boiled plaster, anhydrite plaster and the like. However, in order to obtain the early development of initial adhesion necessary on a fast continuous machine, plaster of Paris or its modifications i preferred. The plaster forming the core may be any of the above mentioned plasters, or a dead burnt plaster, e. g. Keenes cement, or any other known kind of calcium sulphate plaster.

This invention is a valuable advance in the art as the bond between the liners and the core of the plaster board is one of the most important properties of a good plasterboard as well as one of the largest factors in providing the strength of the final plaster board. The invention provides a method whereby this bond can be both improved and made more lasting.

An important advantage introduced by the coatings which contain the urea or thio-urea synthetic resins is in regard to waterproofing efiect. Thus for example a typical normal plasterboard with edges treated to prevent the entry assaeeo of water so that water could only enter through the liner laces was immersed in water the board absorbed about 50 per cent. oi its weight oif-water, with considerable weakening of the core and thus oi the board itself. A board was made identical in all respects with the above except that the liner was treated with the resin composition coating in accordance with this invention In the same period of time this board had only absorbed 6 per cent of its own weight of water.

Further, this new method of producing plasterboard whereby the liners are coated with the resins instead of modification of the core with resins is more rapid, more foolproof, and cheaper.

The economic advantages are well illustrated by the following example:

It was found that 0.75% dextrine or gum arable was necessary as core adhesive to give a core adhesion value of 8-9 with a certain stucco. On eliminating the core adhesive and applying a positive bond coating by the wet method it was found possible to obtain a core adhesion value of 9. The positive bond coating was as follows, 0.2

lb. of the liquid composition being applied per square yard.

Parts by weight Stucco 100 Dextrine 5 Emtarder... 0.10 Water we The amounts of adhesive used per 1000 square feet or board in the two cases were:

As the adhesive costs about 6 cents per lb. the benefits are obvious. With favourable conditions even e eater economies are achieved than this.

As many apparently widely diiierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specified embodiments thereof, except as defined in the appended claims.

In the claims, the term water-difiusible to define the adhesive material is used as including both water-soluble and water-dispersible adhesives.

W e claim:

l. A method of manufacturing composite plasterboard which comprises applying to a liner a coating comprising an aqueous mixture of a water-setting cementitious material with a water-difiusible adhesive, and applying said coated liner to an unset plastic core mix containing a water-setting cementitious materiel, said core mix being incapable, in the absence of the said coating, of bonding to the liner.

2. A method as claimed in claim 1, in which.

the core mix is one free from adhesive.

3. A method' as claimed in claim 1, in which the core mix contains less than 0.5 per cent. of adhesive.

4. A method as claimed in claim 1, in which the water-setting cementitious material in the core mix is a calcium sulphate plaster. V

5. A method of manufacturing composite plasterboard, which comprises applying to a continuously advancing liner sheet an aqueous mixture of a water-setting cementitious. material and a drying the coating so 75 water-difiusible adhesive,

formed to the tee stage, app I? .1. the said coated liner while it is still advancing to unset plastic core a watch-setting cementitious material, said core mix being incapable, in the absence of the said coating, or bonding to the liner, allowing the core mix to set, cutting the advancing set board into lengths and drying the cut lengths.

6. A method as claimed in claim 5, in which the coating comprises a mixture of plaster of Paris together with between 5 per cent. and 20 per cent. by weight of adhesive calculated on the weight of the plaster.

Z. A method as claimed in claim 5, which comprises applying to the liner a coating composition comprising a mixture oi a calcium sulphate plaster with a water-difluslble adhesive together with an amount oi water between 45 per cent. and 600 per cent. by weight of the plaster.

8. A method as claimed in claim 5, in which the weight of coating applied to the liner is 0.2 lb. per square yard.

9. A method as claimed in claim 1, which com-= prises treating a continuously advancing liner sheet with an aqueous mixture at a water=setting cementitious material and a water-difiusi- 'ble adhesive, rapidly drying the coating so 12. A plasterboard as claimed in claim 10, in.

which the core mix contains less than $.25 per cent. of adhesive.

13. A plasterboard as claimed in claim 1c, and comprising a learned core mix containing not more than 0.3 per cent. of adhesive calculated on the weight of the cementitious material.

is. A plasterboard as claimed in clain i 10, in which the water-setting cementitious material in the core mix is a calcium sulphate plaster.

15. A plasterboard comprising a core oi a set composition containingplaster oi lai'is and oi the kind incapable by itself of bonding to plas terboard liner, and a liner bonded to said core by means of a coating comprising a mixture or set plaster of Paris together with between 5 and 50 per cent, calculated on the weight oi the plaster, of a resin syrup selected from the class con sisting of urea formaldehyde resin syrup and thiourea formaldehyde resin syrup.

16. A plasterboard as claimed in claim 10, in which the coating comprises a mixture of plaster of Paris to ether with between 5 per cent. and 20 per cent. by weight of adhesive calculated on the weight of the plaster.

17. A plasterboard as claimed in claim 10, in which the adhesive is selected from the class of acid-catalysed urea formaldehyde resin syrups and thiourea formaldehyde resin syrups.

18. A plasterboard as claimed in claim 10, in which the adhesive is selected from the class consisting of gunarabic, karaya gum and dextrine.

19. A continuous method of. manufacturing composite board having a core and a liner which comprises rue-coating a web of fibrous liner with sheets to dry them.

an adhesive composition comprising a water-settable cementitious material intimatel com'min- Bled with a water-soluble adhesive to form a binder coating, continuously forming a plasterboard by continuously applying a core layer of water-settable unset cementitious material to the treated side of a continuously advancing web of said liner; before the binder has set traveling the composite web and core until the initial set an adhesive composition comprising a mixture of plaster'of Paris intimately commingled with between 5 and 50 per cent, calculated on the weight of the plaster of Paris, of a' water-soluble adhesiveselected from the class consisting of urea formaldehyde resin syrups and thiourea formaldehyde resin syrups, continuously forming a plasterboard by continuously applying a core layer of plaster of Paris to the treated side of a continuously advancing web of said liner;

before the adhesive composition has set traveling the composite web and core until the initial set has occurred, then cutting the composite web into sheets, allowing the core and binder to set' completely and thereafter heating the 21. A method of obtaining a bond between a surface-of fibrous material and a surface of water-setting cementitious material in the plastic or'unset -state which comprises causing the two surfaces to adhere together by means of an adhesive comprising an aqueous mixture of a watersetting cementitious material with a watersoluble adhesive, said adhesive being applied as a coating to said fibrous surface.

22. A method of manufacturing composite boards which comprises treating a liner with an adhesive which comprises an aqueous mixture of a water-setting cementitious material with a water-soluble adhesive and applying it to a core of water-setting cementitious material while the latter is in the plastic or unset state.

23. A method as set out in claim 22 in which the core comprises a plaster of Paris mix.

24. A method as claimed in claim 21 in which the adhesive comprises a water-setting cementltious material in admixture with an aqueous solution of a resin syrup.

25. A method as claimed in claim'21 in which the adhesive comprises a water-soluble adhesive in admixture with plaster of Paris.

26. A continuous method of manufacturing composite board which comprises treating a liner with an adhesive which comprises an aqueous mixture of a water-setting cementitious maon the weight of the plaster of Paris, of a water-' soluble adhesive selected from the class consisting of gum arabic, dextrine and karaya gum, and applying a core layer of plaster of Paris in the plastic or unset state to said coated liner sheet while it is being advanced.

28. The method of manufacturing composite boards which comprises treating a liner with an adhesive which comprises an aqueous mixture of plaster of Paris with a water-soluble adhesive selected from the group consisting of gum arabic, dextrine, karaya gum, starch, gum tragacanth, water-soluble cellulose derivatives, urea formaldehyde syrups, and thiourea formaldehyde syrups, and applying said liner to a core of plaster'of Paris while the latter is in a plastic state. I

29. A plasterboard comprising a core of water-setting cementitious material firmly bonded to a fibrous liner by means of an adhesive comprising an aqueous mixture of a watersetting cementitious material with a watersoluble adhesive, said adhesive forming a coating over said fibrous liner.

30. A plasterboard comprising a core of watersetting cementitious material firmly bonded to a fibrous liner by means of an adhesive comprising an aqueous mixture of a water-setting cementitious material with a resin syrup, said adhesive forming a coating over said fibrous liner.

31. A plasterboard comprising a core of Watersetting oementitiousmaterial firmly bonded to a fibrous liner by means of an adhesive comprising an aqueous mixture of a water-setting cementitious material with a resin syrup of the class consisting of urea formaldehyde resin syrups and thiourea formaldehyde resin syrups, field adhesive forming a coating over said fibrous ner.

32. A plasterboard comprising a core of set