AU2014100053A4 - Low Dust Gypsum Wallboard - Google Patents

Abstract

This invention provides low ust low density gypsum wallboard products havig high total core void volumes corresponding to tow densities in the range of about 10 to 30 pot, The wailboards have a set gypsum core formed between two substantially parallel cover sheets, the set gypsum core preferably having a total void volume from about 80% to about 92%, and made from a slurry including stucco. pregelatinized starch, and a:naphthalenesuffonate dispersant The combination of the pregelatinized starch and the naphthiaenesulfonate dispersant also provides a glue-4ike effect in biriding the set gypsum crystals together, The wallboard formulation, along with srall air bubble voids (and water voids) provides dust control during cutting, sawing, rbuting snapping, nailing or screwing down, or drilling of the gypsum-containing products. This invention also provides a method of making the low dust low density gypsum products including the introduction of soap foam in an amount sufficient to form a total void volume, including air voids, preferably from about 80% to about $2% in the set gypsum core, corresponding to a set gypsum core density from aboot 10 pcf to about 30 poft The waliboards produced by the method generate significantly less dust during working.

Description

LOW DUST GYPSUM WALLBOARD FIELD OF THE INVENTION 10001 t This invention relates to a method of making gypsum waoard in which the generation of dust during wing of the waiboard is significarnty reduced. More particularly, the method includes the introduction of soap foam in an amount sufficient to form a total void volume from about 0% to about 92% in a set gypsum core, corresponding to a set gypsum core density fror about 10 pot to tbout 30 pot which provides significantly reduced dust formation dunng working. It also pertains to a low dusting gypsum wallboard made using the method. BA CKGROUND OF THE iNVENION [00021 Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products, such as gypsum wallboard. Gyposu is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydrador, can be cast, molded or otherwise formed into useful shapes. The base material from which gypsum wallboard and other gypsum products are manufactured is the Namihydrate form of calcium sulfate (CaSOI/2H20), commonly tented "stuco." which is produced by heat conversion of the dehydrate fom of calcium sulfate (CaSO2H2O), from which 1-1/2 water molecules been removed (0003J Conventional gypsum-containing products such as gypsum wlbard have many advantages, such as low cost and easy workability, although substantial amounts of gypsum dust can be generated when the products are cut or drilled Vanous improvements have been achieved in making gypsum-containin products using starches as ingredients in the slurres used to make such products. Pregelatinized starch, like glue, can increase flexural strength and compressive strength of gypsum-containing products including gypsum wallboard. Known gypsum wallboard contains starch at levels of less than about 10 lbs/MSF [0004) It is also necessary to use substantial amounts of water in gypsum slurries containing pregelatinized starch in order to ensur proper owabity of the slurry. Unfortunately, most ; this water eventually must be driven off by drying, which is expensive due to the igh cost of the fuels used in the drying process. This drying step is also timeconsuming. It has been found that the use of naphthalenesuifonate dispersants can increase the fluidity of the slurres, thus overcoming the water demand problem, In addition, it has also been found that the naphinalenesulfonate dispersants, if the usage level is high enough, can cross--ink to the pregelatinized starch to bind the gypsum crystals together after drying. thus increasing dry strength of the gypsurn composite. Thus, the combination of the pregelatinized starch and the naphthalenesulfonata dispersant provide a glue-like effect in bmding the set gypsum crystals together. Trimetaphosphate salts have not in the past been recognized to affect gypsum s hurry water requirements. However, the present inventors have discovered that increasing the level of the inmetaphosphate salt to hitherto unknown levels in the presence of a specific dispersant makes it possible to achieve proper slurry flowability with unexpectedly reduced amounts of water, even in the presence of high starch levels. This, of course, is highly desire because it in trn reduces fuel usage for drying as well as the process time associated with subsequent water removal process steps. Thus the present inventors have also discovered that the dry strength of gypsum board can be increased by using a naphthalenesltonate dispersant in combination with pregelatinized starch in the slurry used to make the wallboard {0005I The gypsum wallboards of the instant invention should be distingushed from acoustical boards or tles that do not have face sheets. Also, the wallboards of the instant invention should be distinguished from acoustical boards or tiles that includee polystyrene as a lightweight aggregate, Importanty, the aforementioned acoustical boards and bles do not meet many ASTM standards that apply to gypsum wallboards. For example, known acoustical boards do not have the flexuras strength required of gypsum wallboards including those of the present invention. Conversely, in order for aooustcal boards or tiles to meet AS'TM standards, iA is required that an exposed surface of the acoustcal boards or tiles have holow voids or depressions that would be undesirable in a gypsum wallboard and would adversely effect hail pull resIstance propertIes. 1(MX6I Dust generation is a potential problem during the installation of all walboard. VWhen gypsum wallboard is worked, for example, by cutting, sawing, 2 routing, snapping, nailing or screwing down, or drnling, substanbal amounts of gypsum dust can be generated. For the purposes of the instant disclosure, d usting* and dust generation" means the release of dust into the surrounding workspace during working of a gypsum-containing product, by, for example. cuUngi sawing, routing. scorelsnapping, nailing or screwing down, or drilling the waflboard. Working can also generally include normal board handling, including dust produced on accidentaly scraping and gouging the boards during transport, carrying. and installation If a way could be found to produce a low density wallboard in which such dust generation is sgnifianty reduced, this would represent a particularly useful contribution to the art. BRIEF SUMMARY OF THE INVENTION [0007) The invention generally comprises a low dusting gypsum walboard including a set gypsum core formed between two substantially parallel cover sheets, the set gypsum core having a total void volume from about 75% to about 95%, the set gypsum core made from a gypsum-containing slurry comprising water stucco, pregelatinized starch, and a naphthalenesultonate dispersant, wherein the pregeatmnized starch is present in an amount from about 0,5% by weight to about 10% by weight based on the weight of stucco. Preferably, the naphthalenesulfonate dispersant is present in an amount of about 01% 3.0% by weight based on the weight of dry stcco, Optionally, sodium trimetaphosphateis present in an amount of at least about 0.12% by weight based on the weight of stuc-o In a preferred embodimentthe trimetaphosphate salt is present in an amount of about 012 -a4% by weight based on the weight of dry stucco 100081 In a preferred embodiment, the invention comprises low dusting gypsum wallboard comprising a set gypsum core including pregelatinized starch and a naphthalenesulfonate dispersant formed between two substantially paralel cover sheets, the set gypsum core having a total void volume from about 80% to about 92% wherein at least 60% of the total void volume comprises air voids having an average diameter less than about 100 microns, and the set gypsum core having a density from about 10 pof to about 30 pof. The tem "pci is defined as pounds per cubic foot (lb/ft. The set gypsum core is made from a gypsum-containing slurry composing stucco. pregelatinized starch, and a naphthalenesulfonate dispersant, 3 wherein the pregelatniaed starch is present in an amount from about 05% by weight to about 10% by weIght based on the weight of stucco. Preferably. the naphthaienesurfonate rspersant is present m an amount of about 0.1% - 3.0% by weightibased on the w of dry stucco. 0009 Gypsum wallboard made in accordancetwithe :nvention has high strength, yet much lower weightthan conventions V yards in adlion, it has been found that by is total core void volumes in the set gypsum core fom about 75% to about 95%, and preferatly from about 80% to about 92%, much less dust is generated on cutting, sawing, routing, snapping, naling or screwing down, or drilling the waiboards made according to this embodiment, Wj010U In yet another embodiment the invention constitutes a method of making high strength, low dusting gypsum walboard by moxng a gypsum-containing slurry comprising water, stucco, pregeatintized starch, and a naphthalenesu fonate dispersant, wherein the naphthalenesulfonate dispersant is present in an amount of about 0.1% - 3,0% by weigIt based on the weight of dry stucco, where the pregelatinized starch Is present in an amount of at least about 0.5% by weight up to about 10% by weight based on the weight of stucco, and adding sufficient soap oam to the gypsum-containing slurry to form a total void volume. including air voids, of from about 7% to about 95% in a finished waoard. The resulting gypsum-containing slurry is deposited on a first paper or other approtpnate cover sheet and a second paper or other appropriate cover sheet is placed over the deposited slurry to form a gypsum waibcard. The gypsum wallboard is cut alter the gypsum-containing slurry has hardened sufficiently for cutting, and the resuming gypsum watboard is dried, to provide a set gyprn core in the Wnished valboard with a total void volume, including air voids, from about 75% to about 95% The gypsum-containing slurry can optionaly contain a tuimetaphosphate salt, for example, sodium timetaphosphate. Other converdional ingredients will also be used in the slur-ry including, as appropriate, acceirators, binders, waterproofing agents, paper fiber, glass fiber, clay. biocde, and other known ingredients I I} In yet another embodiment the invention constitutes a method of using low dusting gypsum lpsumboalboard with a 4 set gypsum cOre having a total void volume from about 75% to about 95% wherein at least 60% of the tolvaid volume comprises air voids having an average diameter less than about 100 microns and including water voids having an average diameter less than about 5 microns, wooing the wallboard in a manner that produces gypsum dust (C g cutting sawing, routing, score/snapping, nailing or screwing down, or diingq; and camping a substlan portion of the gypsum dust in the voids. BRIEiF DESCRPTRON OF THE DRAWINGS 100124 FIG. 1 is a scanning eectron photomicrograph of a cast gypsum cube sample (1 1:08 at 15X magnification illustrating one embodiment of the present invention. 10013 FiG. 2 is a scanning electron photomicrgraph of a cast gypsum cube sample (11:30) at 1 5X magnMcation illustating one embodiment of the present Invention. O141 FIG.3 is a scaning electron photomicrograph of a castgypsum cube sample (11:50)at 15X magnfication illustrating one embodiment of the present invention. [015 FIG. 4 is a scanning electron photonicrograph of a cast gypsum cube sample (IT:08) at 50X magnifcation illustrating one embodiment of the present inventions L0016] FIG, 5 is a scanning electron photomicrograph of a cast gypsum cube sample (11:30) at 50X magnification lustrating one embodiment of the present invention. 100171 FIG S i a scanning electron photomicrograph Of a cast gypsum cube sample (50) at 50X magifiatn stratingonerembdmentofTeprsent nvention. 5 j0OA8~ RG. 7 is a scanNeg electron photomlcrogrph of a cast gypsum cube sample (11:50) at 5OX magnification miustrating one embodiment of the present mention 100191 FG. 8 is a scanning electronphotomicrgmph of a cast gypsum cube sample (11.50) at 2,5OX magnif:ation llustratng one embodiment of the present invention, 10020} FIGS, 910 are scanning electron photomiemgiphs of a cast gypsum cube sample (11:50) at 10,000X magnification illustrating one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [002! it has unexpectedly been found that gypsum wafiboard made using a gypsum-conaining siury including stucco. pregelatiized starch, and a naphthalenesulfonate d spersant, and an appropriate amount of soap foam. provides not only very low board core densities of about 10 to 30 pcf (and thus low board weight), but also low dusting upon normal board handling and upon working. such as, for example, ctiing, sawing, routing, score/snapping, nailing or screwing dow, or dnling. when the total void volume of the set gypsum core is from about 80% to about 92%. This wallboard is consequently easier to cut than other known products. The introduction of the soap foam produces Small air (bubble) voids, twch on average can be less than about 100 microns in diameter, but are generally greater than about 10 nucrons in diameter, and preferably greater than about 20 microns m diameter. The invention requires that these small air bubbles along with evaporative water voids (genera!!y about 5 microns in diameter, or less, normal less than about 2 microns in diameter), are generally evenly distributed throughout the set gypsum core in the finished wallboard products. For example, the set gypsum core can have a total vcd volume from about 80% to about 92% wherein at least 60% of the total void volume comprises air voids having an average diameter grater than about 10 microns and at least 10% of the total void volume comprises water voids having an average diameter less than about 5 microns it is believed that the low density board core prepared in this manner with a total void volume of the set gypsum core forom about 80% to about 92% as air and 6 water voids (total core voidvoume) captures a substaI amount of the sma dust and other debris in the voids exposed on cutting. sawing, routing, snapping, nailng or screwing down, or drilling the boards so that dust generation is significantly reduced and does not become air-bome (002 The rehiydration of calcium sulfate hemihydrate (stucco) and consequent hardening requKes a spel, theoretical amount water n1o2 meoes water? mole of stucco) to fom calcimn sulfate 4hydrae crystals However, the commercial process generally ca Is far excess water his excess process water produces evaporative waler vodsi thegypsum rystamatrix which are general substantially iregular in shape, and alsoae linked to other water voids, forming irregular channels in a general contnuous netwo*between set gypsum crystals. In contrast air (bubble) vods are introduced into the gypsum slurry using soap foam The air voids are general spheica round in shape, and also are general separated from other air voids and tus generallydiscontinuous The water vods cai be distributed withith as sb of the ar'vidt see, for example, Figs. 0 J0023] The effectiveness of dust Capture depends upon the composition of the set gypsum core. it has been found that the naphihaenesulfonate dispersants the usage level is high enough, can romss4ink to the pregelatinized starch to bind the gypsum crystals together afterdryig thus increasig dry strength of the gypsum composite, Further, it has now unexpectedly been found that the combination of the pregelatinized starch and the naahthaenesulfenate dispersant organicc phase provides a gluedli e effect in binding the set gypsum crystals together, and when this formulation is combined wih a particular void volume and void distribution larger sized fragments are generated on score/snapping of the finished wallboard, Larger gypsum fragments generally produce less air-borne dust. In contrast, if a conventional wallboard formulation is used, smaller fragments are generated and thus more dust, For example, convention wallboards can generate dust fragments on saw cutting hang g an average diameter of about 20-30 microns. and a minimum diameter of about 1 micron. in contrast, the gypsum walboards a the present invention generate dust fragments on saw cutting having an average 7 diameter of about 30-50 microns, and a minimum diameter of about 2 microns; score/snapping can produce even larger fragments. 10024] In softer wallboards, dust can be captured in both the water voids and air voids ( g. capture of small gypsum needles as single crystal dust). Harder wallboards favor dust capture in the air voids, since larger chunks or fragments of the set gypsum core are generated on working of these boards. In this case the dust fragments are too lange for the water voids, but'are tramped in the air voids. It is possible;according to one embodiment of the poserit invention, to achieve increased dust capture by introducing a preferred void/pore size distribution within the set gypsum core, !t is prefen-ed Ao have a distribution of small and large voi sizes. as a distribution of ait and water voids, In one embodiment, a preferred air void distribution can be prepared using soap foam, See Examples 6 and 7 below. !0025] The ratio of air voids (greater than about 10microns) to water voids (less than about 5 microns) withn the set gypsum care can range from about 1.8:1 to about 9:1. A preferred ratio of air voids (greater than about 10 microns) to water voids (less than about 5&arons) within the set gypsum core can range from about 2:1 to about 31. in one embodiment, the void/pore size distribution within the set gypsum core should range from about 10 - 30% of voids less about S microns and from about 70 - 90% o voids greater than about 10 microns, as a percentage of total voids measured, Stated in another way, the ratio of air voids (greater than 10 microns) to water voids (less ton 5 microns) within the set gypsum core ranges from about 2.3:1 to about S: n a preferred embodiment, the void/pore size distribution within the set gypsum core should range from about 30-35% of voids less about S microns and from about 65 - 70% of voids greater than about 10 microns, as a percentage of total voids measured. Stated in another way, the ratio of air voids (greater than 10 microns) to water voids iess than microns) within the set gypsum core ranges frm about 1.8:1 to about 2.3:1, 100261 It is preferred that the average air (bubble) void size be less than about 100 microns in diameter. in a preferred embodiment the void/pore size distribution within the set gypsum core is. grater than about 100 microns (20%), from about 50 microns to about, 100 microns (0%), arnd Jess than about 50 microns (50%). That is, a preferred median void/pore sie is about 50 microns (00271 Soap foam is preferred to introduce and to control the air (bubble) void sizes and distribution in the set gypsum core, and to control the density of the set gypsum core, A prfemed range of soap is from about 0.2 ib/MSF to about Ct Ib/MSF: a more preferred level of soap is about 0.45 lb/MSF. [0028) Soap foam must be added in an amount effective to produce the desired densities, and in a controlled manner. In order to control the process, an operator must monitor the head of the board forming line, and keep the envelope filled. If the envelope is not kept filled, wallboards with hollow edges resut since the slurry cannot fill the necessary volume. The envelope volume is kept filed by increasing the Soap usage to prevent rupture of air bubbles during manufacturing of the board (to better retaining the air bubbles), or by increasing the air blast rate. Thus, general, the envelope volume is controlled and adjusted either by increasing or decreasing the soap usage, or by increasing or decreasing the air blast rate. The art of controlling the head includes adjustments to The 'dynamic slurry' on the table by adding soap foam to increase slurry volume, or by decreasing soap foam usage to decrease slurry volume 100291 According to one embodiment of the present invention, there are provided finished gypsum-containing products made from gypsumcontaining surries containing stucco, pregelatinized starch, and a naphthalenesulionate dispersant. The naphthalenesulfonate dispersant is present in an amount of about 0 1% - 3.0% by weight based on The weight of dry stucco. The pregelatinized starch is present in an amount of at least about 0.5% by weight up to about 10% by weight based on the weight of dry stucco in the formulatio. Other ingredients tha may be used in the slurry include binders. waterproofing, agents paper fiber, glass fiber. clay, biocide, ard accelerators, The present{ invention requires the addition of a scap foam to the newly formulated gypsum-ontaining slurries to reduce the density of the finished gypsum-containing product for example, gypsum wallboard and to control dusting by introduction of a total void volume of from about 75% to about 95% and preferably from about 50% to about 92% in the form of small a (bubble) voids and water voids in the set gypsum core. Preferably, the average pore size distribution will be from about 1 micron (water voids) o about 40 - 50 microns (air voids). 100301 Opbonaly, the combination of from about 0.5% by weight up to about 10% by weight pregeiatnizeci starch, from about 0. % by weight up to about 3 0% by weight naphthalenesufonate dispersant and a minimum of at least about 0,12% by weight up to about 04% by weight of rimetaphosphate salt (all based on the weight of dry stucco used in the gypsum slurry) unexpectedly and significantly increases the 4uidity of the gypsum slurry. This substantially reduces the amount of water required to produce a gypsum slurry wrth suffcient ilowability to be used in makig gypsumicontaining products such as gypsum wallboard. The level of rimetaphosphate salt, which is at least about twice that of standard formulations (as sodium trimetaphosphate) is believed to boost the dispersant achvity of the naphthalenasulfonate dispersanit. [00311 A naphthalenesulfonatedispersant must be used in gypsum-containing slun 4 rs prepared in accordance with tle present invention. The naphthaenesulfonate dispersants used in the present invention include polynaphthalenesulfonic acid and its sats (polynaphthalenesulfonates) and derivatives, which are condensation products of naphthalenesulfonic acids and fomaldehyde. Particularly desirable polynaphthalenesulfonates include sodium and calcium naphthialenesulfonate. The average molecular weight of the naphthalenesulfonates can range from about 3,000 to 27 000, although it is preferred that the molecular weight be about 8000 to 22,000, and more preferred that the molecular weight be about 12,000 to 17,000. As a commercial product, a higher molecular weight dispersant has higher viscosity, and lower solids content than' a lower molecular weight dispersant. Useful naphthaenesulfonates indude DiLOFLO, available from GEO Speciatty Chemicals, Cleveland, Ohio; DAXAD. available from Hampshire Chemical Corp, Lexington, Massachusetts; and LOMAR D. available from GEO Specialty Chemicals, Lafayette, Indiana. The naphthaenesulfonates are preferably used as aqueous solutions in the range 35 55% by weght sold content, for example. It is most preferred to use the naphthalenesulfonates in the fom> of an acueous solution, for example, in the range of about 40-45% by weight solids content, AltemativeLy, where appropriate the naphthalenesulfonates can be used in dry soiid orpowder form, such as LOMAR D, for example. j00321 The polynaphthalenesultoates usefuLrT the present invention have the general structure (I)

CH

2 wherein nis 2 ard wherein Mis sodium, potassum, caliumand the like 19033] The naphthalenesulfonate dispersant, preferably as an about 45% by weigh solution in water, may be used in a range of from about 0.5% to about 3.0% by weight based on the weight of dry stucco used in the gypsum composite formulation. A more prefened range of naphthalenesulfonate dispersant is from about .5% to about 2,0% by weight based on the weight of dry stucco, and a most preferred range from about 0.7% to about 2.0% by weight based on the weight of dry stucco. in contrast, known gypsum wallboard contains this dispersant at levels of about 04% by weight or less, based on the weight of dry stucco. 10,34j Stated in an another way, the naphthaleneufonate dispersant. on a dry weight basis, may be used in a range from about 0 1% to about 15% by weight based of the weight of dry stucco used in the gypsum composite formulation A more preferred nange of naphthaienesulfonate dispersant, on a dry solids basis, is from about 0.25% to about 07% by weight based on the weight of dry stucco, and a most preferred range (on a dry solids basis) from about 0.3% to about 0.% by weight bai oni the weight of dry stuo.o I0 35] The gypsum-containing slurry can optionally contain a trimetaphosphate salt, for example, sodium lrmetaphosphata. Any suitable water-soluble metaphosphate or polyphosphate can be used in accordance with the present invention. It is preferred that a trimetaphosphate sait be used, including double saits, that is trimetaphosphate salts having two nations. Particularly useful trimetaphosphate saits include sodium trimetaphosphate, potassium trimetaphosphate, calcium trimetaphosphale, sodium calcium trlmetaphosphate, lithium trimetaphosphate, ammonium trimetaphosphate, and the like, 'or combinations thereof, A preferred trimetaphosphate sat is sodium tnimetaphosphate, It is preferred to use the trimetaphosphate sait as an aqueous solution, for example, in the range of about 10-15% by weight solids content. Other cyclic or acyclic polyphosphates can also be used, as described in U.S. Patent No. 6,409625 to Yu et al., herein incorporated by reference. j0036} Sodium trimetaphosphate is a known additive in gypsum-containing compositions, although it is generally used in a range of from about 0.05% to about 0.08% by weight based on the weight of dry stucco used in the gypsum slurry, in the embodiments of the present invention, sodium ttmetaphosphate (or other water-soluble metaphosphate or. polyphosphate) can be present in the range of from about 0.12% to about 0.4% by weight based on the weight of dry stucco used in the gypsum composite formulatioW A preferred range of sodium tamrnetaphosphate (or other water-soluble metaphosphate or polyphosphate) is from about 0.12% to about 0.3% by weight based on the weight of dry stucco used in the gypsum composite formation 10037 There are two forms of stucco, alpha and beta These two types of stucco are produced by ditferer means of calination. In the present inventions either the beta or the alpha form of stucco may be used. O38] Starches, including pregelatnized starch in particular, must be used in gyps'umcontaining slunTies prepared in accordance with the present invention. A preferred pregelatinized starch is pregelatinized corn starch, for example pregelatnized corn flour avaIable from Bunge Miling, SL Louis. Missouri, having the following typical analysis: mosture 7.5%, protein 850%, oil 0.5%, crude fiber 0.5%, ash 0.3%; having a green length of GAS psi; and having a loose bulk density of 359 Ifb. Pregeatinized eommr starch sheadd be utsed in an amount of at least about 0,5% by-weight up to about 10% by weight based on the weight of dry stucco used in hegypsum-cntaining slurry rou39j The present inventors have further discovered that an unexpected increase in dry strength particulary in walboar) can be ebined by using at least about 0.5% by weight up toout ut 10% by weight pregelatinized starch (preferably pregelatized corn starch) in the presence of about 0.1% by weight to 3.0% by weight naphthalenesulfonate dispersant (starch and naphthalenesulfonaze levels based on the weight of dy stucco present in the formulation). This unexpected result can be obtained whether or-not water-sokible frimetaphosphate or polyphosphate is present 004o in addition i has unexpectedy been found that pregeanized starch can be used at leves of atleast about10 bMSF, ormore, ir he dried gypsum wallboard made in accordance with the present invenion, yet high strength and lov weight can be achieved Levels ashgh as 35~45 i/MSF pregeatinized starch in the gypsum walboard have been shwnto be effective. As en example FormulationB, as shown in Tables i and2 below inciudts 45 lbIMSF yet produced a board weight cf 1042lbMSF having exOAlle strength In this example ornulation B), naphthlenesulfonate dispersant as a 45% by weight soution inwater was used at lev of .28% by weight. {0041] A further unexpected result may be achieved with the present invention when the naphthaienesulfonate dispersant tnmetaphosphate salt combination is cornbined with pregelatinized corn starch, and optionally, paper fiber or glass fiber, Gypsum wallboard made from formulations containing these three ingredients have Increased strength and reduced weight, and are more economical desirable due to the reduced water requirements in their manufacture. Useful ieveis of paper liber can range up to about 2% by weight based on the weight of dry stucco. Useful lees of glass fiber can rnge up to about 2% by weiht based on the weight of dry stucco [042] Accelerators can be used in the gypsum-containing compositions of the present inventon, as described in U.S. Patent No. 6,409,825 to Yu et at, herein incorporated by reference. One desirable heat resistant accelerator (HRA I can be made from the dry grindnig of landpiaster (calcium sulfate dihydrate). Small amounts of additives (normally about 5% by weight) such as sugar, dextrose, boric acid, and starch can be used to make tits HRA. Sugar, or dextrose, is currently preferred, Another useful accelerator is '%lirnate stabilized accelerator or climatee stable accelerator (CSA) as described in US. Patent No. 3.573,947, hereIn incorporated by reference. (00431 Water/stacco (w/s) ratio is an important parameter, since excess water must eventually be driven off by heating, In the embodiments of the present invention, a preferred ie ratio is from about 0.7 to about 1 3 j0044j Other gypsum slurry additives can include accelerators, binders, waterproofing agents, paper or glass fibers, clay, biocide, and other known ccndttuents. 10045) Cover sheets may be made of paper as in conventional gypsum wallboard although other useful cover sheet materials known in the art (e. fibrous glass mats) may be used. Paper coversheets provide strength characteristics in the gypsum wallboard. Useful cover sheet paper includes Manila 7-ply and News-Une 5ply available ftrm United States Gypsum Corporation, Chicago, lilinois; and Grey-Back 3 ply and Manila tvory 3-ply. available from Caraustar, Newport Indiana, The paper cover sheets comprise top cover sheets, or face paper, and bottom cover sheets, or back paper. A preferred back cover sheet paper is 5-ply News-Line. A preferred face cover sheet paper is Mana 7y. 00m461 Fibrous mats may also be used as one or both of the cover sheets. One usefuibrous mat is a glass fiber mat in which iamentsof glass fiber are bonded together by an adhesive. Preferably the fbrous mats will be nonwoven glass fiber 14 mats in which filaments of glass ftibet are bonded together by an adesive, Most preferably, the nonwoven glass fiber mats will have a heavy resin coang, For e xarnple, Duragiass nornvoven glass fiber mats available from Johns4Aanville. having a weight of about 1.2-2.0 /100 f wim about 40~50% of the mat weight coming from the resin coating, could be used, Other useful breus mats incude, but are not ;mited to, woven glass maIs ant orceluoseIabi (00471 The folowing examplesfurther iuste the invention, They should not be construed as In any way limiting the scope of The invent EXAMPLE I (0048J Sample Gypsum Slurry Formu ations [01491 Gypsum slumy formuations are shown in Table 1 below. Al values in Table I are expressed as weight percent based on the weight of dry stuco Values in parentheses are dry weight in pounds (ib/MSF). Component Formulation A Formulation B Stu lb/M(72) (704) sdum 0otae20( 50} 0.30 (2.14' Dispent t (n2al~aenesulfoate)I 0.j! 35 I05'(4.0 Pregelatinized starch Board starch ,041 ( Heat resistant accelerator (H-RA)(1)15 G iber 027 (2.0) 0.28 (2,0) 0 852 *Used to regenerate foam 1.28% by weight a- 45% aqueous solution 5s EXAMPLE 2 [0050] Preparaton of Wailboards [9951] Sample gypsum wallboards were prepared in accordance with U.S Patent Nos. 6,342.284 to Yu et at and 6,632,550 to Yu et al, herein incorporated by reference. This includes the separate generation of foam and introduction of the foam into the slurry of all of the other ingredients as described in Example 5 of these patents. [0521 Test results for gypsum wailboards madei using the Formulations A and B of Example 1 and a normal control! board are shown in Table 2 below. As in this example and other examples below, nall pull resta , corehardness, and flexural strength tests were performed according to ASTM C-473. Addiionally, it is noted that typical gypsum wallboard is approximately M inch thick and has a weight of between about 1600 to 1800 pounds per 1,000 square feet of material, or IbIMSF. (''MSF' is a standard abbreviation in the art for a thousand square feet;t ia an area measurement for boxes, corrugated media and wallboard.) 16 TABLE 2 Lab test result Control Board Formulation A Fornulation B Board Board Board weIght i157 1066 1042 Na1 pull resistance 13 1502 28 Core hardness Ib) 16. 5.2 Humidified bond load 17. 20.3 15,1 (db) __ Humidfedbond 0 6 5 11,1 Flexural strength, 47 47,2 52.6 face-up (MD) (fo) ____ __ __ Fexurl strength, 51.5 56.7 78.8 ae-own (MO) (Ib) Flexural strength, 150 135 .9 173,1 face-up (XMD)-_____b)__ exural strength 144.41 4 iface-down (XJ)(b)_______~ MD: machine direction XMD: across machine direction M0053} As illustrated in Table 2, gypsum wallboards prepared using the Fornulation A and B sluiies have significant reductions in weight compared to the control! board With reference again to Table 1; the comparisons of the Formulation A board to the Formulation B board are most striking. The waterisbucco (wis) ratios are similar in Formulaion A and Formulation B, A signifcantly higher level of naphthaienesu.fonate dispersant is also used in Formulation B. Also, in Formulation B substantally mere pregelatinized starch was used about 6% by weight, a greater than 100% incease over Formulation A accompanied by marked strength increases. Even so, the water demand to produce the required flowability remained low in the Formulation B sury the difference being about 10% in comparison to Formulation A. The low water demand in both Formulations is attributed to the synergistic effect of the combination of naphthalenesulfonate dispersant and sodium trimetaphosphate in the gypsum surr which increases the 7 fluidity of the gypsum slurry, even in the presence of a substantialy higher level of pregelatinizedl starch. !O054i As illustrated in Table 2, the wallboard prepared using the Formulation B slurry has substantially increased strength compared with the wallboard prepared using the Fornulation A slurry. By incorporating increased amounts of pregelatinized starch in combination with increased amounts o naphthalenesulfonate dispersant and sodium trimetaphosphate, nail pul resisance in the Formulation 8 board improved by 45% over the Formulation A board Substantial increases in filexural strength were also observed in the Formulation B board as compared to the Formulailon A board. EXAMPLE 3 10055) 112 nch Gypsum Wallboard Weight Reduction Trias (00561 Further gypsum walboard examples (Boards C; 0 and E) including slurry formuations and test results are shown in Table 3 below. The slurry formulations of Table 3 include the major components of the slurries Values in parentheses are expressed as weight percent based on the weight of dry stucco.

TAOLE 3 Ti formulation Conto onuin F r.o. -or-l com n arameter Board c Board D Board E Boa Dry tuo 1/S) 1300 1281 1196 1Q0 Accelerator /IMSF ~ 9229 DRLOC (IJSF 41 (232%) 8(.3%) . (0 %) 5(7% Regular starch 5. (43%) (b/MSF) TPatie ctrn 1 0 F8 10 403) ASstandrd (IbM D oF u 0.7 ( 0%) 1. (nUtio (n wa(r tnetaphosphatenomnsnwtthcotobar(auttwfldires Tota water i stucco 0.8 SO 082.8 r....o............ ----- Cal Moad wIght 1611 1501451 1320 --- .-----.

.........-----

tASTAI standard 77 lb j00574 A llsrae in TMbe 3, Boards , D, ad E were made from s un-y having substantialy increased aonsof sah 1 UILOFLO dispersant, and sodium trimetaphosphate- in3 copaisnwih ecntol board (aboute a two'-old ices 19 on a percentage basis for the starch and dispersant and a two- to trefold increase for the trtmetaphsphate) while maintaining the w/a ratio constant Neverthelessboard weight was significantly educed and strength as measured by nail pul resistance was rot damatically afected. Therefore, in this example of an embodiment of the inventionthe new formulation "such as, for example, Board 0) can provide increased starch formulated in a usable, flowable surry, while maintaining the same w/s rato and adequate strength EXAMPLE 4 IOassl Wet Gypsum Cube Strength Test [10591 The wet cube strength tests were carried out by using Southard CK1 board stucco, avaiable from United States Gypsum Corp., Chicago, linois and tap water in the laboratory to determine their wet compressive strength. The following lab test procedure was used. (l060C Stucco (1000 g), CSA (2 g), and tap water (1200 cc) at about 7L*F were used for each wet gypsum cube cast Pregelatinized corn starch (20 g, 2.0% based on stucco wt) and CSA (2 g, 0.2% based on stucco wt) were thoroughly dry mixed first in a plastic bag with the stucco prior to mixing with a tap water solution containing both niaphthalenesulfonlate dispersant and sodium trimetaphosphate. The dispersant used was DILOFLO dispersant (1.0 - 2 0%. as indicated in Table 4). VaryIng amounts of sodium trimetaphosphate were used also as indicated in Table 4. 1]1 The dry ingredients and aqueous solution were initially combined in a laboratory Vaming blender, the mixture produced allowed to soak for 10 seo. aid then the mixture was rmxed at low speed for 10 sec in order to make the slury. The slurnes thus formed were cast into three 2'X2"X2' cube molds The cast cubes were then removed from the molds, weighed, and sealed inside plastic bags to event moisture loss before the compressive strength test was performed. The compressive strength of the wet cubes was measured using an ATS machine and recorded as an average in pounds per square inch (psi). The results obtained were as oows 20 TABLE 4 SodiulDhOPL trimtaphosphate, (wt% tasod IWet cube Wet cube Test grams (wt% basd o dry weight compressive Sample on ry tsfuto) (2"X2X2"), strength, psi No. 1 0 15 183.57 321 2 0.5(005) j 15 183.11 357 31(1)1 5 183.19 380 4 2 (0 2) 3 5 183.51 361 5 4(0t4) 1,5 3.65 381 10 (10) 15 183,47 369 -4 ' ....- '...... 7 0 10 1 84 02 8 0.5 (0.05) 1.0 183.66 349 .... .... 91(0,1) 1t0 183.93 356 10 2 (0.2) 1.0 182.67 36 .......... . . . - - * . . . ............. 1~ 4 (04) 10 183753 355 1210(1.0) 10 1834 341 13 0 2 0 j183 33 345 _I b3 _ 14 0.5(005) 20 I 1840 356 5 1(0.1) 2143 363 Sodium DILOFLO trimetaphosphate~ wt% basd Wet eube Wet cube TOs grams (wt% bed on dry weight compressive Sa y stucco) stucco) (2X2"X2) snre gth, psi 6 2(0.2 20 48402 363 17 4(CA 2.0 1368 18 0(0 2.0 18 68 339 DILOFLO is a 45% Na n nate so in wte [0962] As illustrated in Table 4, Samples 4-5, 10-11, and 17, hav ing levels of sodium trnmetaphosphate in the about 0 12- 0.4 % range of the present invention generaly provided superior wet cube compressive strength as compared to samples with sodium trimetaphosphate outside This range EXAMPLE S [0963) 1/2 Inch Light Weight Gypsum Wallboard Plant Production Trials [0064] Further trials were performed (Trial Boards I and 2, including slurry formulations and test results are shown in Table 5 below. Te surry formulations of Table 5 include the major components of the scurries Values in parentheses are expressed as weIght percentbased on The weight of dry stucco 22 TABLE 5 Tralfomuaton Control 7PlanU Cntrot ln component/paramleter Board 1 Formnutation Board 2 $ormulaton Tnal Board Tri Board 20 Dr tco(Ib/IMSF) 1381160 1212 1 12~ DiLOFLO 1 ' (lb/MSF) 5.98 7,98 7.8.99 (0.457%A) (0.88% ,0.592%) (0.803% Regular starch 5 0.38%) 0 4 6 (0.38%) 0 (Ib/MSF) Pregelatirdzed cor 2.0 (0.15%) 10 (08$%) 2.5 (0.21%) 9 0(0 80%) starch (lb/MSF) ------j Th Y~----- ......' timetaphosphate tio (WAS) Trlmrlation test results Dry b.ard weight 1458 1553 44 (1b/MSF) uI Naiipu reisistanceJb) 81 5 82.4 8O.7 8& Flexural strength 47 43 44. 49

------

vrge(MD) (b5 ....... _ 4 average (XMD) (3b ~Hum idified bond 90a 2 2I 9 9 average (b Humidified bond 1 01 2 failure (%) 23 ASTM standard 77 lb tAD machine direction XMD: across machine direction DILOFLO isa 45% Naphthajnsulfonate solution in water 2 9FI 90% Relative Humidity it is we understood that under these test conditons, percentage faure rates 50% are acceptable. 10065] As illustrated in Table 5, TraI Boards 1 and 2 were made from a slurry having substantially iread amounts of starch; DILOFLO dispersant, and sodium rimetaphoisphate, while slightly decreasing the wfs ratio, in comparison with the control boards. Nevertheless, strength as measured by nail pull resistance and flexural testing was maintained or improved, and board weight was significantly reduced. Therefore. in this example of an embodiment of the invention, the new formulaton (such as, for example, Tri Boards I and 2) can provide increased trimetaphosphate and starch formulated in a usable flowable slurry, wine maintaining substantially the same ws rat and adequate strength EXAMPLE 6 f0i6 1/U2 Inch UltradUght Weight Gypsum Wallboard Plant Productin Trials {po067] Further trials were performed (Thai Boards 3 and 4) using Formulation B (Example 1) as in Example 2, except that the pregelatinized com starch was prepared with water a10% concentration (wet starch prepararonand a blend of HYONiC 25 AS and PFM 33 soaps (available fm GE Specialty Chemicals, Lafbyett, Indiana) was used For exampledaI Board 3 was prepared with a blend of HYONIC 25 AS and PFM 33 ranging from 05-70% by weight of 25A3, and the balance PFM 33. For example, Tia Board 4 was prepared with a 70/30 wiwt. blnd of HMONIC 25ASYONIC PFM 33. The triai retiults are shown in Table 6 below.

TABLE 6 Trial Board 3 Trial Board 4 (Formulation B plus (Eormulatnn YONIC Soap plus HYONIC soap Lab test result blend 65435) blend 7W1) (n=12) n=3) Board weight 1106 1013 Nai pull resistance a Flexural strength, 55.66. averag---(MD) (..i..). Flexurai strength, 140. 142.3 averaged (XMptb) Except as marked. n = 4 MD: machine direction XM 13 aross machine direction SASTM standard: 77 lb MASTM standard: 11 lb CASTM standard:36 lb d ASTM standard: 107 lb 100681 As Illustrated irn Table 5 strength characteristics measured by nal pull and core hardness were above the ASTM standard, Flex ral strength was also measured to be above the ASTM standard. Again, in this example of an embodiment of the inventionthe new formulation (such as; for example Trial Boards 3 and 4) can provide Increased trimetaphosphate and starch formated in a usable, fiowabie slurry while maintaining adequate strength EXAMPLE 7 [0069J Percentage Void Volume Calculation in 1/2 Inch Thick Gypsum Wallboard Core As A Function pf Board Weight and Saw Cuting Resuits 25 [0070J Further trials were performeed in order to determine void volumes and densities (^ilal Boards No. 5 to 13) using Formulation B (Example 1) as in Example 2, except that the pregelatinized corn starch was prepared with water at 10% concentration (wet starchpreparation), 0,5% glass fiber was used, and naphthalenesulfonate (DILOFLO) was used at a level of 1.2% by weight a 45% aqueous solution, Soap foam was made using a a foam generator and introduced into the gypsum slurry in an amount effective to provide the desired densities. In the present example, soap was used at a level from 0.25 bWMSF to T.4 IbIMSF. That is, the soap foam usage was increased or decreased as appropriate, in each sarple, the wallboard Thickness was 12 inch, and the core volume was assumed to be uniform at 39.1 ffIMSF. Void volumes were measured across 4 ft Wde wallboard samples from which the fm0nt and back paper was removed 'The front and back papers can have a thickness n the range 11-18 mU (each side). Void volumes? pore sizes and pore size distribution were determined by scanning electron microscopy (see Example 8 below) and X-ray CTscanning 26 TABLE 7 T oard Foam Foam Evaporad Evap. Tot..o Board Weight Vod Pore Void Pore Core Core No, OIb!MSF Voltunel Size Void Density I f'?SF) 0I0tri (ft $SF) Dfistrk- Volume' 4pt) ton tion (%) (%f (%) is80- 1 54 1 12,7 463 70 5 3-4 ----------- - k -- --- ........ ..... - - ........ (Control)I 6 140 19.6 S- 10.3 34 7 1300 211 69 9,4 31 78.0 --- --- --- -- - 8 1200 20.9 68 10.0 32 79 0 28 92 , 9 1100 21 1 671 10.4 j33 80.6. 26 10 1000 20.9 65 11.1 35 81.6 23 -- - -- 23.4 95 29 841 21 7 . ... .

...- L----- ... ... ....... 13 800 255 76 .24 85.9 13 500 31.5 1 86 45 12 921 10 10 micron air (bubble) voids 5 micron water voids * Based on uniform core vo. 391 f9/MSF: ., Total core void volume = foam void volt + evaporative oid vo. 39,1 X 100 4 Based on uniform Core vol 39.1 ft/MSF; ie., Board core density (pef) Board weight (lb/MSF) - weight of paper cover sheets (b/MSF)/ 39.1 f'/MSF Board weight (Ib/MSF) -90 Ib/MSF/ 39,1 ff/MSF i Percent of total voids measured [0071) As illustrated in Table 7, tial board samples having total core void volumes ranging from 79.0% to 92.1% were made, which con-espond to board core densities ranging from 28 pcf down to 10 pot. respectively, As an example. saw 27 cutting of Trial board 10, having a total core void volume of 818% and a board core density of 23 pf, generated about 30% less dust than control board, As an additional example 1 if wailboards with a conventional formulation having less binder (as starch with or without dispersant) were made that had significant y less tat about 75 --- 80% total core void volume, significantly greater dust generation would be expected on cutting, sawing, routng, snapping, nailing or screwing down, or drilling. For example, conventional walboards can generate dust fragment on saw cutting having an average diameter of about 20-30 microns, and a minimum diameter of about 1 micron. in contrast the gypsum wailboards of the present invention will generate dust fragments on saw cutting having arn average diameter of about 30-50 microns, and a minimum diameter of about 2 microns; score/snapping will produce even larger fragments. 100721 It has been shown that the combinaUsn o several key components used to make the gypsumm-onttaining slurry, namely: stucco, naphthalenesullonate dispersant, pregelatnized corn starch, sodium tretaphosphate. and glass and/or paper fibers, in combination with a sufficient and effective amount of soap foam, can have a synergistic effect in producing a useful low density gypsum wallboard that also dramatically reduces gypsum dust formation during knife cutting saw cutting, score/snapping, drilling, and normal board handling. EXAMPEE 100731 Determination of Air Bubble VodSizes and Water Void Sies in Trial Bord No. 10, and Gypsum CrystalMorphology '00741 Cast gypsum cubes (2 inch X 2 inch X 2 inch) from the plant thai to prepare Tai Board No. 10 were analyzed by scanning electron microscopy (SEM). Air bubble voids and evaporative water voids were observed and measured as well as gypsum crystallize and shape. [00751 Three sample cubes were made and labeled 11:08. 1:30, and 11:50, respectively. Figures 1 to 3 illustrate the air bubble void sizes and distribution for each sample at 1SX magnrficaton. Figures 4 to 6 ilstrate the air bubble void sizes and distabution for each sample at S0X magnification.

(0076) At higher magnications, water voids were bs for example in the generally substantially larger air bubble void walls, as shown in Figures 7 to 10 for sample cube 1:50. up to 10,OX magnication. Almost all of the gypsum crystals were needles; few platelets were observed. The density and packing of the needles varied onh surfaces of the air bubble vols. needles were also observed in the water voids in the air bubble void' walls [0077 The SEM results deronstate that in the gypsum-containing products made according tohe present invention, the air and water voids are generally evenly disribied throughout the set gypsum core. The observed void sizes and void distributions also demonstrate that suldent free space is formed as air and water voids (total core void volume) such dhat a substantial amount of The gypsum dust produced wil be captured in the surrounding voids exposed upon normal board handling and during he cutUng, sawing, routing, snapping, naiing or screwing down, or drriing and does not become air-borne EXAMPLE 9 [0078] Dust Capture in Low Dust Gypsum Wallboard [O{V79] if a wallboard were prepared according to the teachings of the present invention as in Example ' it is expected Thatthe gypsum dust producedon worldng the walboard would comprise atleast 50% by weight gypsum fragments larger than about 10 microns in diameter. At least about 30% or more of theota dust generated by working the wahboard by cutting, sawing, routing, scoreisnappng, naing or screwing down, and dnlng, would be captured. [Q80) The use of the terms 'a and 'n an the' and simrniar referents in the context of describing the inventon (especialy In the context of the following clams are to be construed to cover bot the singular and the plural) unless otherwise indicated herein or clearly contradicted by conte. Recitatiorn of ranges of vaues herein are merely intended to serve as a shorthand method of referrng individual, to each separate value facing within the range uness otherwise indicated here and each separate values ncorporated into the specification as ift'were individually recited herein- All methods descbed herin can be performed in any 30 suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and al examples, or exerplary language (eg, "such A) provided herein, is intended merely to better illuminate the invention and does not pose a linitaton on the scope of the inventin unless otherwise claimed. No language in The 5 specification should be construed as indicating any non-claimed element as essential to The practice of the invention, [0081] Preferred embodiments of this invention are described herein, including thre best moce known to the inventors for carrying out the invention, It should be understood that 10 the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention [0082 Comprises/comprising and grammatical variations thereof when used in this specification are to be taken to specify the presence of stated features, integers, steps or 15 components or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims (5)

1. A light weight gypsum board comprising: a set gypsum core disposed between two cover sheets; the set gypsum core comprising a gypsum crystal matrix having a pore size distribution comprising (i) voids having a pore size less than about 50 microns in diameter, (ii) voids having a pore size from about 50 microns to about 100 microns in diameter, and (iii) voids having a pore size greater than about 100 microns in diameter such that the voids having a pore size greater than about 100 microns in diameter comprise at least about 20% of the total void volume of the set gypsum core, wherein the voids are measured using scanning electron photomicrograph imaging; and the board having a density of about 35 pcf (about 560 kg/M 3 ) or less.

2. A light weight gypsum board comprising a set gypsum core disposed between two cover sheets, wherein the set gypsum core comprises air voids and water voids in a volume ratio of air voids to water voids from about 1.8 to 1 to about 9 to 1, the set gypsum core comprises voids having a pore size greater than about 100 microns in diameter such that the voids having a pore size greater than about 100 microns in diameter comprise at least about 20% of the total void volume of the set gypsum core, wherein the voids are measured using scanning electron photomicrograph imaging, and the board has a density from about 24 pcf (about 380 kg/m 3 ) to about 35 pcf (about 560 kg/m 3 ).

3. A light weight gypsum board comprising: a set gypsum core disposed between two cover sheets; the set gypsum core comprising a gypsum crystal matrix having a pore size distribution comprising (i) voids having a pore size less than about 50 microns in diameter, (ii) voids having a pore size from about 50 microns to about 100 microns in diameter, and (iii) voids having a pore size greater than about 100 microns in diameter such that the voids having a pore size greater than about 100 microns in diameter comprise at least about 20% of the total void volume of the set gypsum core, wherein the voids are measured using three-dimensional imaging acquired by X-ray CT-scanning analysis (XMT); and 31 the board having a density of about 35 pcf (about 560 kg/M 3 ) or less.

4. The light weight board of claim 1 or 3, wherein the board has a density from about 24 pcf (about 380 kg/m 3 ) to about 35 pcf (about 560 kg/m 3 )

5. The light weight board of any one of claims 1-4, wherein at a board thickness of about inch (about 1.3 cm), the board has a dry weight from about 1000 lb/MSF (about 5 kg/m2 ) to about 1400 lb/MSF (about 6.8 kg/m2). 32