3,393,116 United States Patent Office Patented July 16, 1968 1. 2 ments embedded beneath a plaster surfacing and which 3,393,116 may be disposed over a gypsum board plaster base to GYPSUM COMPOSITION HAVING DEHYDRATION which the elements are secured. Since the temperature RESISTANT CHARACTERSTCS Kenneth R. Larson, Mount Prospect, Ill., assignor to created by the heat radiating from the elements in such United States Gypsum Company, Chicago, Ill., a systems may reach 120-125 F. or higher to appreciably corporation of Illinois warm the atmosphere adjacent the plaster covering, the No Drawing. Filed Aug. 30, 1965, Ser. No. 483,776 plaster adjacent the heating elements has heretofore on 7 Claims. (Cl. 161-43) occasion cracked and the strength thereof deteriorated while the gypsum was dehydrated as its water of crystal O lization was driven off. Similarly, the plaster base had deteriorated in structural integrity at heating system tem ABSTRACT OF THE DESCLOSURE peratures which caused dehydration of the gypsum. A composition comprising calcium sulfate hemihy It is an object of this invention to provide a novel con drate, boric acid and glycerol; said composition being position adapted to form hard gypsum bodies which convertible into a formable mixture that is capable of 5 tenaciously retain water of crystallization at tempera setting to a hard gypsum upon admixture With an amount tures in excess of those normally found in radiant heat of water in excess of that required to hydrate said hemi ing systems. hydrate to the dihydrate; said hard gypsum product being More specifically, it is another object of this invention characterized by substantial resistance to dehydration at to provide the combination of boric acid and glycerol a temperature of 150 F. 20 as additives to calcium sulfate hemihydrate for purposes of significantly retarding the dehydration of the set gypsum product formed therefrom after the hemihydrate This invention relates to a composition of matter has been combined with the necessary water of hydration. adapted to form gypsum products having significantly im It is another object of this invention to provide addi proved resistance to dehydration. More particularly, this 25 tives to retard the dehydration of gypsum, said additives invention relates to the combined use of boric acid and not being noticeably affected by other additives normally glycerol as additives to calcium sulfate hemihydrate to employed in modifying properties of the initial slurry or retard the dehydration of gypsum products formed after the final gypsum product formed therefrom. combining the hemihydrate with necessary water of crys The above and other objects of this invention will be tallizaiton. 30 come more apparent in the light of the following detailed Crystallized calcium sulfate dihydrate or gypsum may discussion and appended claims. be in a myriad of forms such as planar plaster surfacings It has been found that if calcium sulfate hemihydrate and cores for gypsum boards including paper-covered is combined with minute quantities comprising approxi wallboards, sheathing and the like. Gypsum is formed by mately 0.05% to 0.65% of boric acid and 0.05% to combining calcium sulfate hemihydrate with adequate 1.25% of glycerol based on the weight of the hemihy water of hydration. In the course of "setting,” the dihy drate, gypsum products formed from such combination drate crystallizes out. If the hemihydrate or plaster of after mixing the same with a quantity of water necessary Paris is 100% pure, the amount of water needed to form to effect formation of the dihydrate crystalline structure the dihydrate comprises 18.6% by weight of the hemi will tenaciously retain such water of crystallization. Since hydrate. In normal practice, an excess of water is neces 40 substantially all major sources of gypsum rock contain sary for producing a workable and flowable slurry which some impurities, such impurities should be deducted from is formed into a wall coating, wallboard core or the like. the hemihydrate weight in the course of ascertaining the The amount of this excess varies and is readily deter weight of glycerol and boric acid additives based thereon. minable by those skilled in the art. This excess water Thus, although gypsum water of crystallization is normal must, of course, be removed by drying after the dihydrate 45 ly driven off at a temperature of about 105-110 F., gyp has formed so that the full strength of the resulting sum products formed in combination with the above ad gypsum may be realized. ditives will release substantially no water whatsoever, Ordinary gypsum products, when subjected to elevated even after being subjected to a temperature of 150 F. temperatures of about 105-110 F. tend to lose water of for two hours. Such gypsum products thus are most gain crystallization. As a result, the strong calcium sulfate fully employed in radiant heating installations in which dihydrate crystals degenerate and revert to the hemihy temperatures of 125 F. are imparted to gypsum ma drate. The destruction of the dihydrate crystalline struc terials disposed adjacent the radiant heating means. ture results in a loss in strength as the gypsum body such Also, the use of additives normally employed in plaster as a plaster wall or gypsum board core becomes soft and compositions to improve the working properties of the chalky. A plaster surface will reveal increasingly unsight 55 paster nix or the properties of the final gypsum surfacing ly cracks as the dehydration progresses. has been found to be in no way materially affect the de A gypsum board core will become progressively weak, sired water-retaining qualities imparted to the gypsum beginning at the outer surface and working inwardly as surfacing by the boric acid and glycerol additives. Similar the zone of dehydration extends inwardly. Thus, the ly the boric acid and glycerol do not interfere with the initial core dehydration will be evidenced by poor ad 60 functioning of other conventional additives. hesion or lack of bond between the core and paper cover; For purposes of demonstrating the effects of the com as the dehydration continues the inner core portion also bination of boric acid and glycerol on calcium sulfate becomes dehydrated resulting ultimately in a board which hemihydrate adapted to form gypsum products, the fol has insufficient strength for any structural use. lowing table is presented. The various specimens set forth It is appreciated from the above, that unless the water 85 in the following table were in the form of slabs /2 inch of crystallization of gypsum products is retained in high by 2 inches by 6 inches. Each slab was dried at 110 F. temperature environments, deterioration of the gypsum to constant weight and then exposed to a constant tem will result. This problem has been particularly acute in perature of 150 F. in a laboratory oven. The additive plaster coatings and the supporting plaster base therefor percentages are based on the weight of the calcium sul utilized in radiant heating systems. Such systems employ 70 fate hemihydrate employed in forming the various speci heat-radiating means, such as electrical resistance ele S. 3,393,116 4. Calcium sulfate hemihydrate ------lbs. 100 ABLE Calcium sulfate anhydrite, insoluble (adds to plas Percent Moisture Loss ter fluidity during troWelling) ------lbs. 10 Time of No. Gye- Q.5% 0.5% Boric Sand (aggregate) ------lbs.-- 60 Exposure At ero or 0.5% Boric Acid, 0.5% 150°F. (days) Boric Acid Glycerol Acid Glycerol 5 Double hydrated dolomitic lime (assists action of the

retarder and improves workability of plaster dur 3.85 1.07 89 .33 5.87 .54 1, 46 39 ing troWeling) ------lbs.-- 5 7.67 1.91 2.20 46 8,97 2.8 3.2 .56 Polyvinyl acetate, spray dried (retains water during 9.53 2.46 4.0 70 Working of the plaster) ------lbs. 55 O Perlite fines (aggregate, facilitates troweling of finish coat) ------cubic foot. 0.05 It will be seen from the table that the combination of Tartaric acid (retarder) ------lbs., .025 boric acid and glycerol is vastly superior to either addi Boric acid ------lbs.-- .25 tive alone for purposes of retaining the Water of crystal Glycerol ------IbS-- .5 lization in the gypsum specimens. At the end of Seven 5 The cable covered by the above composition was a days or 168 hours, the specimen employing both boric "Ceil-Heat' if C-102 cable manufactured by Ceil-Heat, acid and glycerol had not yet lost 1% by weight of mois Inc., of Knoxville, Tenn. The cable was arranged in loops ture. The reason for the beneficial synergistic effect pro in which all cable portions were at least 1/2 inches from vided by the combination of boric acid and glycerol is the plaster edges and the cable runs were on 1% inch not yet known; however, the above table clearly displays 20 centers. A voltage was applied to the cable to attain a a cooperation between the additives which provides a temperature of 150 F. as determined by a thermocouple desirable result unattainable by either additive alone. attached to the cable sheath at the plaster panel center. It should be appreciated that although the specimen Continuous maintenance of the 150. F. temperature for of the table employing both additives lost some mois two hours resulted in no detectable weight loss in either ture, the applied heat was greatly in excess of that nor 25 the plaster base (having a core containing 0.5% boric mally imparted by heating cables which are actuated in acid and 0.5% glycerol) or the plaster covering. termittently for short periods of time in a radiant heating It will be noted that the foregoing plaster surfacing installation. employed a number of conventional additives, the func The above specimens of the table were free of any addi tions of which are set forth. The plaster base core sim tives normally found in gypsum products. It has been 30 ilarly possessed conventional additives normally found found, however, that the presence of foams, fibers, bond in gypsum cores. It is apparent that the additives in no ing agents, accelerators, retarders and other additives nor way interfered with the ability of the boric acid and mally found in gypsum bodies such as gypsum board glycerol to retain the water of crystallization of the dihy cores in no way materially affects the increase in reten drate surfacing. tion of the water of crystallization imparted to the cores 35 The glycerol and boric acid additives provide no proc by the combination of boric acid and glycerol. essing difficulties in formation of either the plaster or It has in addition been found that when gypsum board core compositions. The plaster composition may be sold cores contained the boric acid and glycerol additives in a bag for use at the job site whereat all that need be within the prescribed ranges of .05%-65% and .05%- added is the necessary water to hydrate the hemihydrate 1.25%, respectively, based on the weight of calcium sul 40 and provide a consistency suitable for troweling. The fate hemihydrate, the bond of the board fibrous wrapper glycerol which is a syrupy liquid at room temperature to the core remained firm and secure despite the presence may be first mixed with the sand normally employed in of elevated temperatures, as demonstrated by the foll the plaster, after which the coated sand and other addi lowing test. tives including boric acid are mixed and packaged with An X was cut in an edge portion of a paper wrapper 45 the hemihydrate. normally employed in plaster base and disposed about a In the course of gypsum core manufacture the boric gypsum core of a plaster base board adapted for use in acid may be added at room temperature to a stucco stream a radiant heating System. The core contained 0.3% of at normal processing temperatures. The glycerol additive boric acid and 0.5% of glycerol based on the original is introduced into the gauging water. Thus, neither addi weight of the hemihydrate and was subjected to an oven 50 tion affects normal manufacturing procedures. Both glyc erol and boric acid of substantially 100% purity are avail temperature of 150 F. for 168 hours. When an attempt able for carrying out this invention, and the additives in was made to pull each of the four paper tabs defining the Such purity were employed in the above specimens set X from the underlying gypsum surface, paper lamina forth in the table and test. All of the hemihydrate in the tions parted. The bond between the gypsum and the paper 55 examples above set forth was formed from gypsum of did not give nor did outer surface portions of the gypsum 94.4% purity. core fracture during this test. The latter test clearly dis In a typical electric cable, radiant heating system as closed that the bond of the paper to the gypsum core was above indicated, the cable is normally stapled in place still excellent and that the outermost gypsum surface over a gypsum board or plaster base, after which a plaster adjacent the paper had not been weakened by dehydration 60 Surfacing is applied to completely cover the cable. Ob despite being subjected to the elevated temperature of viously the thinner the plaster coating the more readily 150 F. for an extended period of time. the cable heat is radiated to the atmosphere adjacent the In addition to core compositions, the additives of glyc Quter plaster Surface comprising the heat-radiating sur erol and boric acid may be incorporated in plaster com face. It is desirable, therefore, that the plaster coating in positions intended to form a surfacing adapted to be sub 65 addition to being non-cracking at elevated temperatures, jected to temperatures as high as 150° F. without dele be as thin as possible while affording complete coverage of terious effects. the underlying heating cable. The desired electric cable, radiant heating system would employ both plaster and The following plaster composition when combined with plaster base able to retain water of crystallization attem Sufficient water to crystallize the hemihydrate and render 70 peratures of 125 F. or greater. the resulting plaster mix workable was applied in a one Although the above-discussed plaster and core compo quarter inch thick layer over a plaster base 18 inches sitions are primarily intended for use in radiant heating square having an electric cable securely stapled thereto. Systems wherein an electrical cable, hot fluid or other heat The plaster was allowed to dry to a constant Weight in Source generates an elevated temperature in the neighbor an Oven maintained at 110° F. 75 hood of 125 F., the gypsum compositions of this inven 3,393,116 5 6 tion have other obvious uses as in boiler rooms, uninsul to .65% and .05% to 1.25% respectively, based on the lated attics, etc., where temperatures of this magnitude weight of the calcium sulfate hemihydrate; said gypsum are encountered. being characterized by substantial resistance to dehydra Without further elaboration, the foregoing will so fully tion when subjected to a temperature of 150 F., and by explain the character of my invention that others may, secure bonding thereof to said fibrous wrapper. by applying current knowledge, readily adapt the same for 5. A plaster composition comprising for every 100 use under varying conditions of service, while retaining pounds by weight calcium sulfate hemihydrate, 10 pounds certain features which may properly be said to constitute insoluble calcium sulfate anhydrite, 60 pounds sand, 5 the essential items of novelty involved, which items are pounds double hydrated dolomitic lime, 0.55 pounds intended to be defined and secured to me by the following O spray-dried polyvinyl acetate, 0.5 pound glycerol, 0.05 claims. cubic foot of perlite fines, 0.25 pound boric acid and I claim: .025 pound tartaric acid; said composition being conver 5. A composition comprising calcium sulfate hemihy tible into a mix that is capable of setting to a hard gypsum drate, boric acid, and glycerol; said composition being con Surfacing characterized by a substantial resistance to de vertible into a formable mixture that is capable of setting 5 hydration at a temperature of 150 F. upon admixture of to a hard gypsum characterized by a substantial resistance said composition with an amount of water adequate to to dehydration at a temperature of 150 F. upon admix hydrate said hemihydrate to calcium sulfate dihydrate and ture with an amount of water in excess of that required facilitate troweling of the mix. to hydrate said hemihydrate to the dihydrate; said boric 6. The plaster composition of claim 5 in which said acid being present in the amount of .05% to .65% by 20 glycerol is coated on said sand. weight of the calcium sulfate hemihydrate; said glycerol 7. A hard gypsum surfacing formed from a mix com being present in the amount of .05% to 1.25% by weight prising the composition of claim 5 and an amount of water of the calcium sulfate hemihydrate. adequate to hydrate the calcium sulfate hemihydrate to 2. A hard gypsum product formed from water and cal calcium sulfate dihydrate and to facilitate troweling of cium sulfate hemihydrate having uniformly admixed there 25 said mix. with boric acid and glycerol present in the amounts of References Cited .05% to .65% and .05% to 1.25% respectively based on UNITED STATES PATENTS the weight of said hemihydrate; said gypsum product hav ing a substantial resistance to dehydration at a tempera 1,708,436 4/1929 Weinstein ------106-110 tre of 50 F. 30 1,732,737 10/1929 Wiggin et al. ------106-315 3. The hard gypsum product of claim 2 disposed in the 2,078,199 4/1937 King ------106-114 form of a gypsum board core enveloped by a paper wrap 2,557,083 6/1951 Eberl ------106-111 per and in which effective quantities of an accelerator, an 3,305,518 2/1967 Jakacki ------106-110 extender, a retarder, fibers, a soap and a bonding agent are contained in said core. FOREIGN PATENTS 4. A gypsum board comprising a fibrous Wrapper dis 491,826 9/1938 Great Britain. posed about a gypsum core formed from Water, calcium Sulfate hemihydrate, boric acid and glycerol; said boric TOBIAS E. LEVOW, Primary Examiner. acid and glycerol being present in the amounts of .05% S. E. MOTT, Examiner.