CA1127374A - Fibrous calcium sulfate - Google Patents
Fibrous calcium sulfateInfo
- Publication number
- CA1127374A CA1127374A CA387,331A CA387331A CA1127374A CA 1127374 A CA1127374 A CA 1127374A CA 387331 A CA387331 A CA 387331A CA 1127374 A CA1127374 A CA 1127374A
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- Canada
- Prior art keywords
- fiber
- temperature
- calcium sulfate
- gypsum
- fibers
- Prior art date
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Abstract
ABSTRACT
Fibrous calcium sulfate is produced by autoclaving a dilute aqueous suspension of gypsum, at a temperature between the minimum fiber forming temperature and about 20°C above, in the presence of a dispersing agent to provide for discrete whisker crystal development and protecting the whisker crystals from destruction or conversion to a different physical or chemical form until they have been stabilized by calcining at a temperature in excess of 500°C.
The process of the present invention provides a fibrous calcium sulfate product of markedly improved dispersibility, longer fibers and with more complete conversion of the calcium sulfate fibers than the prior art processes. The fibrous calcium product is useful as a reinforcing agent for either organic or inorganic matrix materials.
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Fibrous calcium sulfate is produced by autoclaving a dilute aqueous suspension of gypsum, at a temperature between the minimum fiber forming temperature and about 20°C above, in the presence of a dispersing agent to provide for discrete whisker crystal development and protecting the whisker crystals from destruction or conversion to a different physical or chemical form until they have been stabilized by calcining at a temperature in excess of 500°C.
The process of the present invention provides a fibrous calcium sulfate product of markedly improved dispersibility, longer fibers and with more complete conversion of the calcium sulfate fibers than the prior art processes. The fibrous calcium product is useful as a reinforcing agent for either organic or inorganic matrix materials.
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Description
~1273~4 BACK~RO~ND
....
Calcium sulfate is known to exist in several different chemical forms generally differing in gross chemical composition only by the amounts of hydrogen and o~ygen expressed as water (i.e in the same 2 to 1 ratio as in water) relative to the amount of calcium sulfate expressed as the simple salt, CaSO4. In this way, the art recognizes at least;
a hydrate (CaSO~.2H2O); a hemihydrate (CaSO~1/2 H2O); and, .an anhydrite (CaSO4~. Additionally, the anhydrite and the hemi-hydrate exist in at least two different forms, based on different degrees of water solubility In addition to the several different chemical forms that have been proposed for . calcium sulfate, it is known to be polymorphous as well, having at least two distinct crystalline forms: rhombic and orthorneubic or mono-clinic, Other crystalline variations including acicular, needle-like and whisker crystals have also been recognized Reference to such variations in physical form, as ribbon or tape-like crystals, columnar and rod-like, . twinned and swallow-tailed, curved and prismatic, are frequently encountered in di.scussions of calcium sulfate or gypsum as the hydrated form is commonly known 7 Thus, it appears that authors disagree as to the exact chemical -composition and the physical form of the various transformation products of gypsum The numerous attempts to define the various forms of the product and its transformation mechanism have not remoyed the uncertainty as to an understanding of the composition, structure, properties, and behavior of calci~ sulfate, nor provided a generally accepted correlation between them, There, nevertheless, have been numerous attempts to explain the behavior of calcium sulfate either by transformation of physical forms or by the addition or loss hrn:~3-.~
~Z7374 of water of hyd.ration; see for example, The Chemi.stry of Cement and Concrete by Lea and Desch (Longmans; Green &
_ .
Co., New York, 1935), pages 17 to 20; Journal of The So _ tX
of The Chemical Industry No. 13 Vol, XXVI (July 15, 1907), . _, . _~....
pages 727-738; U, S, Patent No, 3,59~,123; U. S, Patent No, 1,901,051, Elsewhere, such as J. Appl. Chem. 1968, pages 307 to 312, differences in various forms of calcium sulfate formed in autoclaving are attributed to differences in such ` things as purity, particle size and thermal history of the : 10 gypsum startlng materials~
The commonly used crystalline form of calcium . sulfate known as plaster has long been an impor-tant product of the rehydration of dehydrated gypsum stored as a powdered heml-hydrate or anhydrite which has the ability to set when rehydrated by simply mixing with water. Plaster products, which generally are stated to be the hemi-hydrate or anhydrite form, are generally manufactured by dehydrating gypsum, One commonly used process involves autoclaving ground, natural gypsum in an atmosphere of steam, The commercial methods generally seek to obtain a powdered produc-t of uniform particle size with indiYidual particles of minimum surface area; though there are frequent references ~o the formation of undesirable acicular or needle-like crystals, Early references to needle~like crystals formed in producing Plaster of Paris can be found for-example in U.S, Patent Nos, 757,649 and 782,.321~ Manufacturers sought to eliminate acicular crystals either by avoiding the conditions of their formation as in U,S, Patent No~ 3,~10,655, or in breaking them up by pulveriæing, Only within about the last ten years has it been recognized that calcium sulfate whisker crystals could be ad~antageously employed as reinforcement fiber and that bm~
~lZ737~
conditions under which acicular crystals form might be controlled to give high yield conversion of gypsum to whisker ~ibers having length to diameter ratio of at least 6 to 1 and as high as 100 to 1 or higher, Whisker fibers of this type have been produced for example as described by Eberl, e-t al in U.S, Patent Nos, 3,822,340 and 3,961,105, While these patents recognize a variety of uses for . such fibrous calcium sulfate product, particularly as a substitute for asbestos fibers, none of the products made in accordance with these methods has actually achieved commercial use, The lack of commerical acceptance i5 believed to be largely due to the difficulty in dispersing and effectively employing the fibrous product obtained as a tangled mass of fibers that cannot be readily separated, When reinforcement is attempted with such fibers, they remain in the form of tangled lumps or balls and offer little practical benefit, . Accordingly, a high strength calcium sulfate fiber that is readily dispersible in liquid media is highly desirable and it is an object of this invention to proYide an improved 20 method for the production of calcium sulfate whisker crystals which can be readily dispersed for use as reinforcing fiber.
A further object of this invention is to provide a method by w~ich ~ypsum is converted into stable whisker crystals with average fiber length of 100 to 300 microns having strength and aspect ratio conducive to greater usefulness as reinforcement, It is a further object of this invention to provide a method wherein substantially all of the gypsum is converted to whisker crystals 50 microns long or longer and aspect ratio of 10 to 1 or greater which crystals are obtained as substantially discrete fibers easily separated and capable of being uniformly dispersed in liquid media using conventional techniques, These and other objects of the invention will be bm ~
~2~374 more fully understood from the description and examples which follow `DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has been found that a calcium sulfate whisker fiber product of markedly improved dispersibility can be made from gypsum by the autoc].ave method if the temperature and pressure are maintained substantially constant at or just above the minimum fiber forming condi.tions during substantially the entire perioa of fiber foxmation The improved process also gives longer fibers and more'complete conversion of the calcium sulfate to fibers Further', the presence of small amounts of dispersing agent such as tannins or tannic acid during the fiber formation step eliminates the clumping or balling generally encountered during the formation of calcium sulfate whisker fibers especially when the average length exceeds about 100 microns r . As used herein, the term "calcium sulfate whisker fiber" is intended to mean any filamentary form of calcium sulfate including single crystals that haYe grown in filamentary form and which can be composed of any of the hydrated, hemi-hydrated, anhydrous or other calcium sulfate compositions or comhinations thereof Whereas, the whisker fibers made in accordance-with this invention may be individual fibers of h.eterogenous compositions or different fibers may have different compositions, it is belieyed that generally the whisker fibers are formed as single crystals of - calcium sulfate hemi-hydrate which upon calcining are converted to crystals of the anhydrite As alrèady discussed~ calcium sulfate is known to exist in several differént chemical combinations with water and one or more of these chemical forms can exis-t in several . _4_ bm:Y~
~I~LZ7374 physical forms. The combinations and permutations of chemical and physical forms is substantial and except when otherwise specified herein, the term calcium sulfate is intended to cover all of the various forms Where a particular chemical or physical form is specified, it will be understood that the calcium sulfate composition intended is predominantly in that form though part of the composition may be present as one or more of the transformation products.
As used herein, the term calcine is intended to mean heating calcium sulfate to a temperature in excess of about 500C and covers elimination of water and volati]es ordinarily removed at those temperatures as well as any transformations in the product which may occur during such heat treatment The term, fiber~ depending on the context, is used - alternatively to mean an individual filament of one or more calcium sulfate whisker crystals or collectively, the fibrous mass of calcium sulfate filaments The fibrous calcium sulfate produced by the method of this invention is composed of discrete fibers having an average length of about 100 to 500 microns and a diameter or average cross section of about 2 to 10 microns~ with an overall average aspect ratio ~length to diameter3 in the range of about 50:1 to 100:1, and is substantially free of non-fibrous calcium sulfate In referring to diameter of the fiber and to aspect ratios, it will be understood that the fibers may have varied cross-sectional configurations. As used herein the terms diameter or cross-sectional dimensions are intended to mean the average of the characteristic dimensions of the cross section Any commercially available gypsum, either naturally occuring mined gypsum or gypsum produced as a by-product of ~n iS~
~Z73 industrial waste treatments, such as limed spent sulfuric ; acid wastes, can be used as starting material, Preferably, the gypsum used will have a calcium sulfate content of 95%
; or greater and will be substantially free of organic impurities, As a general matter, the presence of unknown impurit.ies should be avoided either as an ingredient in the gypsum or in the water used in making up the gypsum slurry, since impurities, particularly some organic impurities, can materially affect the formation and quality of whisker crystals, In most instances, however, particularly in the case of dissolved inorganic salts, the effect will be to shift the minimum fiber forming temperature, as described elsewhere herein, which can be determined emplrically and the process conditions adjusted accordingly, Gypsum with impurities in almost any amount can be used, if the impurity does not adversely affect the fiber formation and provided it can be tolerated as a diluent, The gypsum used in the preparation of the fiber described herein f was mined gypsum containing 96~ plus of calcium sulfate dihydrate~ Distilled water was used to make up the slurry though generally a~ailable water can be used without requiring any significant change in ~he method, As dispersing agent, there was used a reagent grade tannic acid supplied by Fisher Scientific Company, Fisher Building, Pittsburgh~ Penna3 Such tannic acids are used in an amount between about 1 and about 100 ppm, desirably between about 5 and about 50 ppm and preferably between about 10 and about 30 ppm of the total reaction mixture on a w~igh-t basis, There can also be utilized as a dispersing agent any of the available tannic acid or tannic products including synthetic tannins, e,g, naphthalenic syntans, and ~egetable extracts, such as, tea extract, A
~m /~
737~
variety of tannin products are available commerically. Any of the generally available materials such as are disclosea, for example in M Nierenstein, The Natural_Organic Tannins ~London, 1934) can be used. Other materials which function as dispersing agents without inhibiting fiber formation can also be employed Certain of the known crystal habit modifiers have been found to assist in dispersibllity as well, though in using such materials,-it will be appreciated that usually the form of the product will be substantially altered as well, and the choice may be determined by the characteristics required for the fiber product. Ohter materials suitably employed to improve dispersibility, are for example, the polycarboxylic acids such as succinic acid or benzoic acid derivatives such as anthranilic acid. When using a dispersing agent which also modifies , the crystal structure, the concentration of the dispersing agent can be chosen so as to achieve the dispersing effect without significantly altering the crystal structure~ The amount required to produce ~ dispersion will be considerably less than the amount used for crystal habit modification.
Use of organic additives, particularly the poly-carbo~ylic acids~ such as, succinic acid, have long been kno~n in the preparation of Plaster of Paris where these materials have been added as crystal modifiers or as agents to retard setting or curing after the plaster has been gauged Such use as crystal habit modifiers, crystallization inhibitors, or retarding agents, are disclosed for example by U. S~ Patent Nos. 2,460,267 and 2,448,218, British Patent No 563,019, U. S. Patent Nos 3,520,708 and 2,044,942 These materials especially the preferred tannic acids or tannins, have not, however, been previously used to prevent c]umping or balling of whisker crystal fibers during formation ~Z~37~
The product fibers are obtained in readily separable form arranged in generally parallel planes and their dispersibility may be maintained during subsequent filtering, drying, and calcining The fiber product can then be readily dispersed in an organic or aqueous liquid media using ordinary mixing : and dispersing techniques to provide a uniform dispersion of reinforcing fiber throughout the product In carrying out the process, a gypsum slurry is prepared by mixing finely powdered gypsum with water in a mixing tank heated either directly or by steam injection The concentration of the slurry-should be between about 5%
and about 10% by weight of gypsum and preferably about 7% by weight. Concentrations below 5% could be used but require handling increased volumes Concentrations above 10% could be used but there is a gradual decrease of average fiber length as concentration is increased The dispersing agent is added in an amoùnt between 0 0005% and about 0,5% by weight of slurry/ The exact amount of tannic acid, tannin, synthetic tannin, or other dispersing agent used will depend upon the specific choice of dispersing agent In all cases, the amount of dispersing.agent to be used is that amount which;is sufficient to prevent clumping and balling without inhibiting the nucleation and development of whisker crystal fibers The entire mixture is maintained in suspension by stirring while the temperature is raised to about 80~ to 90C
The hot slurry is then pumped into a vented autoclave capable of being rotated, to provide mild agitation during the fiber formation, and equipped with a heating mantel and temperature control to enable the temperature of the a~ltoclave to be elevated at a controlled rate With the autoclave vented, the temperature is raised to abou-t 100C and the autoclave is sealed. Thereafterj the pressure in the autoclave will blll f~ J.15~
correspond to the autogenous pressure developed by heating and, depending upon the exact composition of the slurry, it will approximate the pressure of saturated steam at the corresponding temperature The temperature is then raised rapidly to about 112C, after which the temperature is raised at a rate of about 0.1C to about 2 5C per minute, desirably at about 0.5 to 1.5C per minute and preferably at about 1C
per minute, up to about 5C to 20C above the minimum fiber forming temperature Generally, the temperature is raised to an optimum reaction temperature between about 115C and about 130C and held constant at that temperature for a period of up to two hours, and preferably for about 5 minutes to one hour, during which period all or substantially all of the calcium sulfate is converted to the fiber form -The autoclavin~ is preferably done at a temperature no higher than about 10.C~ and preferably no more than about 6C~ above the minimum fiber forming tempera-ture, This is generally sufficient to achie~e complete conversion to the whisker form in about 1 hour. It has been found that the rate of whisker formation increases as the temperature in the autoclave is increased beyond a certain minimum or threshold temperature below which whisker crystals do not form within any reasonable period of time Such te~perature is referred to herein as the minimum fiber forming temperature, As the temperature is raised beyond the minimum fiber forming temperature,`fiber is formed more rapidly though it has also been found tha~ the longest fibers and the most complete conversion of calcium sulfate to fibrous product is obtained by allowing the fiber formation to proceed at the slowest rate In prac-tice the temperature will be selected so as to allow for substantially total conversion to fibers within a reasonable time~ say about 1 hour. The m:inimum fiber forming bm~
~L~Z/~374 temperature can be determined by trial and error for a particular gypsum slurry dependiny on the quality of the gypsum and water used for make-up. When a dispersing agent is added, it generally causes the minimum fiber forming temperature to rise by about 4 to 8C and the temperature for the autoclave step should be adjusted accordingly The calcium sulfate slurry is not stirred during the fiber forming period, although, gentle agitation is desired. A
sufficient degree of agitation to prevent sedimentation can be easily accomplished by periodically inverting the autoclave during the fiber forming stage. Other methods for achieving gentle agitation such as use of a vertical gravity pipe reactor or other continuous autoclaving methods such as disclosed, for example, in U S Patent No 3,579,300 can be employed.
Upon completion of the fiber formation, the loose unfelted mass of fiber is separated from the excess water~
The separation can be readily accomplished by pressure filtration without allowing the temperature to drop below 100C and preferably~ not below 105C.
The filter cake i5 immediately dried at a temperature in excess of 200C and preferably, at about 400C The dewatered product consists of easily separable free flowing whisker crystal fibers that have a silky feel~ The reaction product is consistently recovered as fiber in quantative yield based on the amount of calcium sulfate in the starting mixture The pressure filtration can be conveniently carried out in a rotar~ pressure filter such as a BHS-FEST-Filter which is a rotating drum type filter manufactured by BHS-Werk Sonthofen, D-~g72, Sonthofen, West Germany Ordinarily, the fibrous product can be fed directly from -the autoclave to the filter using the autoclave pressure, with additional steam bm~
~Z7374 under about the 30 psig pressure, to maintain the product under pressure during the entire removal and filtering operation while the water content is reduced to less than about 50% by weight of water in the filter cake As the product is removed from the f ilter, the individual f ibers appear layered in the f ilter cake or mat indicating little three dimensional entangl.ement and it is desired to maintain the f iber mass free of such entanglement during the subsequent drying and calcining. In order to do so, the drying and calcining are preferably carried out by transferring the fiber mat, as removed from the filter, onto an endless conveyor belt and passing it through an oven for drying and calcining.
Alternat.ively, the fiber mass can be dried and calcined by other conventional means, It is, however~ preferred to minimize the amount of -tumbling in order to maintain the individual fibers relatively free of entanglement to permit easy fiber dispersion when subsequently ~ixed with a resin or cement.
Calcining is carried out at a temperature between about 500C and about 650C/ preferably at about 600C, for a period of less than 15 minutes and mostly for no longer than.about 1 minute in order to avoid degradation of the product~ The pH obtained with the final product immersed in water should not exceed pH lO and preferably should be less than pH 8 ~ower pH is obtained by lower calcining temperatures and shorter calcining times After removal from the calciner, the product is allowed to cool in air. This product, which is believed to consist of the insoluble anhydrite of calcium sulfate, will remain stable for periods up to a year and longer; it will not readily reconvert to hydrated forms in the presence of moisture so that it can be used to reinforce aqueous matrix ~)rn: ft SJ. ~., Z737~ .
materials such as -the hydraulic cements~ Alternatively, if the product is to be used in an organic resin matrix, without a preliminary prolonged exposure to moist air, it can be used without calcinlng simply removing it from the drying stage and cooling The bulk density of the produc-t after calcining is between about 10 and 20 lbs. per cubic foot, usually about 15 to 18 lbs ; the product can be readily compressed up to about 30 lbs. per cubic foot without damaging the individual fibers The calcium sulfate fibers are sufficiently stable after drying for use as reinforcement in non-aqueous systems and after calcining for use in aqueous media However~ if desired, the fiber can be coated as described by Eberl, et al U S Patent No~ 3,961~105. If a coating is applied~ it is necessary only to dry the product to a water content of less than 1~ which is generally accomplished by drying at 300C to 400~C for about 5 to 15 minutes As described herein the process is essentially a batch process, though it can be made continuous by use of a continuous pressure reactor for the autoclaving step In either batch or continuous~ it is important that the handling, from the fiber forming stage to complétion, be done with a minimum of mechanical agitation, tumbling, shearing and the like~ which would cause the fiber to clump or ball making it difficult to disperse uniformly in the subsequent product If desired, the water removed in filtering can be recycled to the slurry stage with additional make-up water as re~uired. In recycling a certain amount of the dispersing agent is retained in the water and the amount added in make up of additional slurry must be adjusted accordingly~
If desired~ coatings can also be applied to the fibers to improve the wetability or the adhesion of the fibers blt~
....
Calcium sulfate is known to exist in several different chemical forms generally differing in gross chemical composition only by the amounts of hydrogen and o~ygen expressed as water (i.e in the same 2 to 1 ratio as in water) relative to the amount of calcium sulfate expressed as the simple salt, CaSO4. In this way, the art recognizes at least;
a hydrate (CaSO~.2H2O); a hemihydrate (CaSO~1/2 H2O); and, .an anhydrite (CaSO4~. Additionally, the anhydrite and the hemi-hydrate exist in at least two different forms, based on different degrees of water solubility In addition to the several different chemical forms that have been proposed for . calcium sulfate, it is known to be polymorphous as well, having at least two distinct crystalline forms: rhombic and orthorneubic or mono-clinic, Other crystalline variations including acicular, needle-like and whisker crystals have also been recognized Reference to such variations in physical form, as ribbon or tape-like crystals, columnar and rod-like, . twinned and swallow-tailed, curved and prismatic, are frequently encountered in di.scussions of calcium sulfate or gypsum as the hydrated form is commonly known 7 Thus, it appears that authors disagree as to the exact chemical -composition and the physical form of the various transformation products of gypsum The numerous attempts to define the various forms of the product and its transformation mechanism have not remoyed the uncertainty as to an understanding of the composition, structure, properties, and behavior of calci~ sulfate, nor provided a generally accepted correlation between them, There, nevertheless, have been numerous attempts to explain the behavior of calcium sulfate either by transformation of physical forms or by the addition or loss hrn:~3-.~
~Z7374 of water of hyd.ration; see for example, The Chemi.stry of Cement and Concrete by Lea and Desch (Longmans; Green &
_ .
Co., New York, 1935), pages 17 to 20; Journal of The So _ tX
of The Chemical Industry No. 13 Vol, XXVI (July 15, 1907), . _, . _~....
pages 727-738; U, S, Patent No, 3,59~,123; U. S, Patent No, 1,901,051, Elsewhere, such as J. Appl. Chem. 1968, pages 307 to 312, differences in various forms of calcium sulfate formed in autoclaving are attributed to differences in such ` things as purity, particle size and thermal history of the : 10 gypsum startlng materials~
The commonly used crystalline form of calcium . sulfate known as plaster has long been an impor-tant product of the rehydration of dehydrated gypsum stored as a powdered heml-hydrate or anhydrite which has the ability to set when rehydrated by simply mixing with water. Plaster products, which generally are stated to be the hemi-hydrate or anhydrite form, are generally manufactured by dehydrating gypsum, One commonly used process involves autoclaving ground, natural gypsum in an atmosphere of steam, The commercial methods generally seek to obtain a powdered produc-t of uniform particle size with indiYidual particles of minimum surface area; though there are frequent references ~o the formation of undesirable acicular or needle-like crystals, Early references to needle~like crystals formed in producing Plaster of Paris can be found for-example in U.S, Patent Nos, 757,649 and 782,.321~ Manufacturers sought to eliminate acicular crystals either by avoiding the conditions of their formation as in U,S, Patent No~ 3,~10,655, or in breaking them up by pulveriæing, Only within about the last ten years has it been recognized that calcium sulfate whisker crystals could be ad~antageously employed as reinforcement fiber and that bm~
~lZ737~
conditions under which acicular crystals form might be controlled to give high yield conversion of gypsum to whisker ~ibers having length to diameter ratio of at least 6 to 1 and as high as 100 to 1 or higher, Whisker fibers of this type have been produced for example as described by Eberl, e-t al in U.S, Patent Nos, 3,822,340 and 3,961,105, While these patents recognize a variety of uses for . such fibrous calcium sulfate product, particularly as a substitute for asbestos fibers, none of the products made in accordance with these methods has actually achieved commercial use, The lack of commerical acceptance i5 believed to be largely due to the difficulty in dispersing and effectively employing the fibrous product obtained as a tangled mass of fibers that cannot be readily separated, When reinforcement is attempted with such fibers, they remain in the form of tangled lumps or balls and offer little practical benefit, . Accordingly, a high strength calcium sulfate fiber that is readily dispersible in liquid media is highly desirable and it is an object of this invention to proYide an improved 20 method for the production of calcium sulfate whisker crystals which can be readily dispersed for use as reinforcing fiber.
A further object of this invention is to provide a method by w~ich ~ypsum is converted into stable whisker crystals with average fiber length of 100 to 300 microns having strength and aspect ratio conducive to greater usefulness as reinforcement, It is a further object of this invention to provide a method wherein substantially all of the gypsum is converted to whisker crystals 50 microns long or longer and aspect ratio of 10 to 1 or greater which crystals are obtained as substantially discrete fibers easily separated and capable of being uniformly dispersed in liquid media using conventional techniques, These and other objects of the invention will be bm ~
~2~374 more fully understood from the description and examples which follow `DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has been found that a calcium sulfate whisker fiber product of markedly improved dispersibility can be made from gypsum by the autoc].ave method if the temperature and pressure are maintained substantially constant at or just above the minimum fiber forming condi.tions during substantially the entire perioa of fiber foxmation The improved process also gives longer fibers and more'complete conversion of the calcium sulfate to fibers Further', the presence of small amounts of dispersing agent such as tannins or tannic acid during the fiber formation step eliminates the clumping or balling generally encountered during the formation of calcium sulfate whisker fibers especially when the average length exceeds about 100 microns r . As used herein, the term "calcium sulfate whisker fiber" is intended to mean any filamentary form of calcium sulfate including single crystals that haYe grown in filamentary form and which can be composed of any of the hydrated, hemi-hydrated, anhydrous or other calcium sulfate compositions or comhinations thereof Whereas, the whisker fibers made in accordance-with this invention may be individual fibers of h.eterogenous compositions or different fibers may have different compositions, it is belieyed that generally the whisker fibers are formed as single crystals of - calcium sulfate hemi-hydrate which upon calcining are converted to crystals of the anhydrite As alrèady discussed~ calcium sulfate is known to exist in several differént chemical combinations with water and one or more of these chemical forms can exis-t in several . _4_ bm:Y~
~I~LZ7374 physical forms. The combinations and permutations of chemical and physical forms is substantial and except when otherwise specified herein, the term calcium sulfate is intended to cover all of the various forms Where a particular chemical or physical form is specified, it will be understood that the calcium sulfate composition intended is predominantly in that form though part of the composition may be present as one or more of the transformation products.
As used herein, the term calcine is intended to mean heating calcium sulfate to a temperature in excess of about 500C and covers elimination of water and volati]es ordinarily removed at those temperatures as well as any transformations in the product which may occur during such heat treatment The term, fiber~ depending on the context, is used - alternatively to mean an individual filament of one or more calcium sulfate whisker crystals or collectively, the fibrous mass of calcium sulfate filaments The fibrous calcium sulfate produced by the method of this invention is composed of discrete fibers having an average length of about 100 to 500 microns and a diameter or average cross section of about 2 to 10 microns~ with an overall average aspect ratio ~length to diameter3 in the range of about 50:1 to 100:1, and is substantially free of non-fibrous calcium sulfate In referring to diameter of the fiber and to aspect ratios, it will be understood that the fibers may have varied cross-sectional configurations. As used herein the terms diameter or cross-sectional dimensions are intended to mean the average of the characteristic dimensions of the cross section Any commercially available gypsum, either naturally occuring mined gypsum or gypsum produced as a by-product of ~n iS~
~Z73 industrial waste treatments, such as limed spent sulfuric ; acid wastes, can be used as starting material, Preferably, the gypsum used will have a calcium sulfate content of 95%
; or greater and will be substantially free of organic impurities, As a general matter, the presence of unknown impurit.ies should be avoided either as an ingredient in the gypsum or in the water used in making up the gypsum slurry, since impurities, particularly some organic impurities, can materially affect the formation and quality of whisker crystals, In most instances, however, particularly in the case of dissolved inorganic salts, the effect will be to shift the minimum fiber forming temperature, as described elsewhere herein, which can be determined emplrically and the process conditions adjusted accordingly, Gypsum with impurities in almost any amount can be used, if the impurity does not adversely affect the fiber formation and provided it can be tolerated as a diluent, The gypsum used in the preparation of the fiber described herein f was mined gypsum containing 96~ plus of calcium sulfate dihydrate~ Distilled water was used to make up the slurry though generally a~ailable water can be used without requiring any significant change in ~he method, As dispersing agent, there was used a reagent grade tannic acid supplied by Fisher Scientific Company, Fisher Building, Pittsburgh~ Penna3 Such tannic acids are used in an amount between about 1 and about 100 ppm, desirably between about 5 and about 50 ppm and preferably between about 10 and about 30 ppm of the total reaction mixture on a w~igh-t basis, There can also be utilized as a dispersing agent any of the available tannic acid or tannic products including synthetic tannins, e,g, naphthalenic syntans, and ~egetable extracts, such as, tea extract, A
~m /~
737~
variety of tannin products are available commerically. Any of the generally available materials such as are disclosea, for example in M Nierenstein, The Natural_Organic Tannins ~London, 1934) can be used. Other materials which function as dispersing agents without inhibiting fiber formation can also be employed Certain of the known crystal habit modifiers have been found to assist in dispersibllity as well, though in using such materials,-it will be appreciated that usually the form of the product will be substantially altered as well, and the choice may be determined by the characteristics required for the fiber product. Ohter materials suitably employed to improve dispersibility, are for example, the polycarboxylic acids such as succinic acid or benzoic acid derivatives such as anthranilic acid. When using a dispersing agent which also modifies , the crystal structure, the concentration of the dispersing agent can be chosen so as to achieve the dispersing effect without significantly altering the crystal structure~ The amount required to produce ~ dispersion will be considerably less than the amount used for crystal habit modification.
Use of organic additives, particularly the poly-carbo~ylic acids~ such as, succinic acid, have long been kno~n in the preparation of Plaster of Paris where these materials have been added as crystal modifiers or as agents to retard setting or curing after the plaster has been gauged Such use as crystal habit modifiers, crystallization inhibitors, or retarding agents, are disclosed for example by U. S~ Patent Nos. 2,460,267 and 2,448,218, British Patent No 563,019, U. S. Patent Nos 3,520,708 and 2,044,942 These materials especially the preferred tannic acids or tannins, have not, however, been previously used to prevent c]umping or balling of whisker crystal fibers during formation ~Z~37~
The product fibers are obtained in readily separable form arranged in generally parallel planes and their dispersibility may be maintained during subsequent filtering, drying, and calcining The fiber product can then be readily dispersed in an organic or aqueous liquid media using ordinary mixing : and dispersing techniques to provide a uniform dispersion of reinforcing fiber throughout the product In carrying out the process, a gypsum slurry is prepared by mixing finely powdered gypsum with water in a mixing tank heated either directly or by steam injection The concentration of the slurry-should be between about 5%
and about 10% by weight of gypsum and preferably about 7% by weight. Concentrations below 5% could be used but require handling increased volumes Concentrations above 10% could be used but there is a gradual decrease of average fiber length as concentration is increased The dispersing agent is added in an amoùnt between 0 0005% and about 0,5% by weight of slurry/ The exact amount of tannic acid, tannin, synthetic tannin, or other dispersing agent used will depend upon the specific choice of dispersing agent In all cases, the amount of dispersing.agent to be used is that amount which;is sufficient to prevent clumping and balling without inhibiting the nucleation and development of whisker crystal fibers The entire mixture is maintained in suspension by stirring while the temperature is raised to about 80~ to 90C
The hot slurry is then pumped into a vented autoclave capable of being rotated, to provide mild agitation during the fiber formation, and equipped with a heating mantel and temperature control to enable the temperature of the a~ltoclave to be elevated at a controlled rate With the autoclave vented, the temperature is raised to abou-t 100C and the autoclave is sealed. Thereafterj the pressure in the autoclave will blll f~ J.15~
correspond to the autogenous pressure developed by heating and, depending upon the exact composition of the slurry, it will approximate the pressure of saturated steam at the corresponding temperature The temperature is then raised rapidly to about 112C, after which the temperature is raised at a rate of about 0.1C to about 2 5C per minute, desirably at about 0.5 to 1.5C per minute and preferably at about 1C
per minute, up to about 5C to 20C above the minimum fiber forming temperature Generally, the temperature is raised to an optimum reaction temperature between about 115C and about 130C and held constant at that temperature for a period of up to two hours, and preferably for about 5 minutes to one hour, during which period all or substantially all of the calcium sulfate is converted to the fiber form -The autoclavin~ is preferably done at a temperature no higher than about 10.C~ and preferably no more than about 6C~ above the minimum fiber forming tempera-ture, This is generally sufficient to achie~e complete conversion to the whisker form in about 1 hour. It has been found that the rate of whisker formation increases as the temperature in the autoclave is increased beyond a certain minimum or threshold temperature below which whisker crystals do not form within any reasonable period of time Such te~perature is referred to herein as the minimum fiber forming temperature, As the temperature is raised beyond the minimum fiber forming temperature,`fiber is formed more rapidly though it has also been found tha~ the longest fibers and the most complete conversion of calcium sulfate to fibrous product is obtained by allowing the fiber formation to proceed at the slowest rate In prac-tice the temperature will be selected so as to allow for substantially total conversion to fibers within a reasonable time~ say about 1 hour. The m:inimum fiber forming bm~
~L~Z/~374 temperature can be determined by trial and error for a particular gypsum slurry dependiny on the quality of the gypsum and water used for make-up. When a dispersing agent is added, it generally causes the minimum fiber forming temperature to rise by about 4 to 8C and the temperature for the autoclave step should be adjusted accordingly The calcium sulfate slurry is not stirred during the fiber forming period, although, gentle agitation is desired. A
sufficient degree of agitation to prevent sedimentation can be easily accomplished by periodically inverting the autoclave during the fiber forming stage. Other methods for achieving gentle agitation such as use of a vertical gravity pipe reactor or other continuous autoclaving methods such as disclosed, for example, in U S Patent No 3,579,300 can be employed.
Upon completion of the fiber formation, the loose unfelted mass of fiber is separated from the excess water~
The separation can be readily accomplished by pressure filtration without allowing the temperature to drop below 100C and preferably~ not below 105C.
The filter cake i5 immediately dried at a temperature in excess of 200C and preferably, at about 400C The dewatered product consists of easily separable free flowing whisker crystal fibers that have a silky feel~ The reaction product is consistently recovered as fiber in quantative yield based on the amount of calcium sulfate in the starting mixture The pressure filtration can be conveniently carried out in a rotar~ pressure filter such as a BHS-FEST-Filter which is a rotating drum type filter manufactured by BHS-Werk Sonthofen, D-~g72, Sonthofen, West Germany Ordinarily, the fibrous product can be fed directly from -the autoclave to the filter using the autoclave pressure, with additional steam bm~
~Z7374 under about the 30 psig pressure, to maintain the product under pressure during the entire removal and filtering operation while the water content is reduced to less than about 50% by weight of water in the filter cake As the product is removed from the f ilter, the individual f ibers appear layered in the f ilter cake or mat indicating little three dimensional entangl.ement and it is desired to maintain the f iber mass free of such entanglement during the subsequent drying and calcining. In order to do so, the drying and calcining are preferably carried out by transferring the fiber mat, as removed from the filter, onto an endless conveyor belt and passing it through an oven for drying and calcining.
Alternat.ively, the fiber mass can be dried and calcined by other conventional means, It is, however~ preferred to minimize the amount of -tumbling in order to maintain the individual fibers relatively free of entanglement to permit easy fiber dispersion when subsequently ~ixed with a resin or cement.
Calcining is carried out at a temperature between about 500C and about 650C/ preferably at about 600C, for a period of less than 15 minutes and mostly for no longer than.about 1 minute in order to avoid degradation of the product~ The pH obtained with the final product immersed in water should not exceed pH lO and preferably should be less than pH 8 ~ower pH is obtained by lower calcining temperatures and shorter calcining times After removal from the calciner, the product is allowed to cool in air. This product, which is believed to consist of the insoluble anhydrite of calcium sulfate, will remain stable for periods up to a year and longer; it will not readily reconvert to hydrated forms in the presence of moisture so that it can be used to reinforce aqueous matrix ~)rn: ft SJ. ~., Z737~ .
materials such as -the hydraulic cements~ Alternatively, if the product is to be used in an organic resin matrix, without a preliminary prolonged exposure to moist air, it can be used without calcinlng simply removing it from the drying stage and cooling The bulk density of the produc-t after calcining is between about 10 and 20 lbs. per cubic foot, usually about 15 to 18 lbs ; the product can be readily compressed up to about 30 lbs. per cubic foot without damaging the individual fibers The calcium sulfate fibers are sufficiently stable after drying for use as reinforcement in non-aqueous systems and after calcining for use in aqueous media However~ if desired, the fiber can be coated as described by Eberl, et al U S Patent No~ 3,961~105. If a coating is applied~ it is necessary only to dry the product to a water content of less than 1~ which is generally accomplished by drying at 300C to 400~C for about 5 to 15 minutes As described herein the process is essentially a batch process, though it can be made continuous by use of a continuous pressure reactor for the autoclaving step In either batch or continuous~ it is important that the handling, from the fiber forming stage to complétion, be done with a minimum of mechanical agitation, tumbling, shearing and the like~ which would cause the fiber to clump or ball making it difficult to disperse uniformly in the subsequent product If desired, the water removed in filtering can be recycled to the slurry stage with additional make-up water as re~uired. In recycling a certain amount of the dispersing agent is retained in the water and the amount added in make up of additional slurry must be adjusted accordingly~
If desired~ coatings can also be applied to the fibers to improve the wetability or the adhesion of the fibers blt~
2~3~4 to the plastic component when used as a reinforcing fiber with~ for example, phenol formaldehyde, urea-formaldehyde~
melamine formaldehyde, polyurethane or other polymers The use of such coatings is disclosed in Eberl, et al U. S.
Patent No. 3,9~1,105.
The calcium sulfate whisker fibers prepared by the method of this invention as already stated are useful as reinforcing agents for either organic or inorganic matrix materials, particularly, the synthetic polymers. These fibers ; are especially suited for blending with organic or inorganic matrix materials in liquid or in dry solid form.
Various methods of dispersion and blending of the fiber in the plastic or other matrix materials can be employed, although properties r especially mechanical properties~ of the resultant composite materials may be affected by the choice of dispersing means ~any varieties of commonly used processing equipment, such as, single and twin screw extruders, two-roll mills and Banbury mixers can be used to produce excellent dispersions The-equipment must be specifically run to produce good fiber dispersion in the plastic matrix with a minimum amount of damage to the fiber A two-roll mill can be advantageously used to blend into small batches of thermoplastic materials. Usually~
two-roll mills are operated under atmospheric conditions As with other types of processing equipment; the plastic material should be fluxed on the mill before the addition of fiber. It is recommended that the roll stock be removed by a doctor blade or other means and then returned to the mill. In order to attain recommended shear levels, it is suggested that nip clearances greater than 25 mils be used and that the roll speeds be in the range 100 to 125 r p m bm~
~L~27374 with a speed differential of 25 r,p.m, It is apparent that the temperature at which the rolls are maintained is dictated by the properties of the plastic material being worked, As a guiding principle, roll temperatures should be maintained at such a level that addition of the fiber to the plastic roll stock causes the resultant roll stock to lose tackiness and thus become easily workable.
A Banbury mixer can also be employed, ~s in the case of the two-roll mill discussed above, it is very important that the plastic material be in a condition of flux before addition of the fiber, This is best accomplished by feeding the plastic into a hea~ed hopper or holding bin and then holding it under force in the mixing area until fluxing is observed. Upon completion of fluxing the fiber may be added, The blending should then be continued for a period of time sufficient to achieve the desired distribution of fibers, It is necessary to control the viscosity of the fluxed resin~
fiber composite. This can be done by controlling the melt temperature by means of varying speed of rotation, cycle time and the circulation through the rotors of either steam or water as the situation requires. Generally speaking, it is possible to increase the blending time so that improved dispersion may be attained without degrading the fiber. For example, a typical compounding-cycle for a thermoplastic material would total three minutes, Twenty seconds would allow fluxing to occur, and forty seconds would suffice to add the fiber, Thus, two minutes are available for the blending process. A temperature rise during the final -two minutes of the cycle is commonly observed, For extrusion, single or twin screw extruders can be used to compound the fibers into resin compositions, The temperature variation along the length of a twin screw bm:~u~
~2~737~
extruder should be controlled so that fiber degradation is kept to a minimum, Generally speaking, with higher melting polymers, such as nylon, the temperature in the feed section should be maintained about 25 to 50C above the required melt temperature for the polymer involved, In the case of low melting polymers such as polyolefins, it is suggested that the feed section be maintained 10 to 30C above melt temperature, The temperature profile in -the barrel will depend on the speed of rotation of the screws since this dictates the time the solid resin remains in the feed section, Thus, larger temperature differences between feed and completion sections are required at higher screw ro-tations, It is essential to recognize that degradation of fiber during the extrusion process may occur before the fluxing of the resin, ~ccordingly~ configuration of the screws should be such that the fiber and resin mix are gently conveyed until softening of the resin occurs by the transfer of sufficient heat~ Once fluxing has taken place, much compressive and dispersive working can be accomplished with little or no degradation of the fiber, Reduction of compression in the feed section can be accomplished by using deeper channels here than in the remainder of ~he screw, -Good engineering practice would suggest a gradual reduction in screw pitch following the feed section to avoid a situation of immediate compression of the material, It is desirable that the screw design incorporate kneading blocks since these are known to aid in dispersion of the fiber. In accordance with the infor~ation given above, these kneading blocks should be placed in a section where the polymer has already melted so that unnecessary mechanical working is avoided.
bm~
~2~37~
In cJeneral~ standard desi~3ns and configurations for dies and adaptcrs are suitable for use in fiber extrusion and those configurations commonly used for PVC, work well, Combinations of calcium sulfate fiber and other reinforcing fiber, such as chopped fiberglass, can be used for example in polyester, or the calcium sulfate fiber can be used to replace fiberglass, Cast composite specimens of ~ polyester resin and calcium sulfate fiber containing 5,5% by volume or approximately 13% by weight of fiber are prepared by dispersing the fiber in a polyester resin stirring and cuxing at room temperature using 0,75% by weight methyl-ethyl-ketone peroxide as a curing agent, ~esidual air is removed from the mix under a vacuum of about two millimeters mercury for a time period of about 5 minutes.
Irhe fiber is dispersed easily and gives a pourable mixture of fairly low viscosity, The composite mix is then cast into a block with dimensions approximately one quarter inch thick by three inches wide by six inches long, The test blocks are allowed to set at room temperature and are then post cured at 110C for 30 minutes, The mechanical properties of these polyester/fiber composites compare well with corresponding asbestos and iberglass reinforced composites, Calcium sulfate fiber can be bonded with magnesium ~xysulfate (Sorél Cementl, By way of example, a mixture composed of: calcium sulfate fiber, 38,7 wt, %; magnesium oxide, 13.6 wt. ~; magnesium sulfate, 4.8 wt, ~; Natrosel*
250, 1.0 wt. %; Duponol*ME (dry)~ 0,3 wt. %; and water~
~1.6 wt, ~, is prepared by adding the water all at once and using a high speed mixer to foam the mix. The mix, which appears to be dry at first, becomes a heavy foamy mass with * T~de ~arks -lG~
L~m~
~Z~374 continued mixing and is suitable for use as a foam-in-place insulation.
The whisker fibers can also be used in place of or in combination with asbestos in other applications such as for reinforcement in cas-t magnesia cement compositions and the like.
Particularly useful products prepared by using the calcium sulfate fiber prepared in accordance with this invention, are the formulations for friction elements such as brakes, clutches, transmission bands, etc. wherein the calcium sulfate fiber is used to replace part or all of the asbestos fiber presently employed in such formulations.
Formulations for friction elements generally utilize phenolic resins of the pulverized noyolak type. These resins are utilized primarily in dry mix processes for the production of disc pads, linings and truck blocks. A suitable formulation is prepared by blending about 35 to 75 parts preferably about 60 parts by weight of calcium sulfate fiber prepared in accordance with the process of this invention, with about 10 to 15 parts preferably about 13 parts by weight of barytes, about 5 to 10 parts preferably about 7 parts by weight of Cardolite brand epoxy resin flexibilizer made by the 3M Company of Minnesota, and about 15 to 25 parts preferably about 20 parts by weight of a phenolic resin of the novolak type. If desired the fiber ingredient can be used as a blend, suitably at a ratio between about 1 to 1 and about ~ to 1 of calcium sulfate fiber to asbestos.
The blended formulation is compression molded at about 200C
and post cured for about ~ hours at about 230~C. The cured friction element when evaluated by various fric-tion -testing procedures, which are standardized in the industry show the calcium sulfate Eiber to be an acceptable reinEorcing material, particu-larly, for light duty applications and as a diluent for asbestos. The drn~ 17-~,7374 calcium sulfate fiber is especially beneficial for improved wear performance.
The following examples are given by way of illustrating the improved gypsum transformatiorl process for producing an improved dispersible calcium sulfate iber product.
Example 1.
Twenty-two grams of a commercial gypsum (Terra Alba) is slurried in 200 cc of water (a concentration of 10% by weight of total slurry). The slurry is heated to a temperature of about 80C and pumped into an autoclave equipped with an electric heating mantel and Variac control, With the autoclave vented, the temperature is raised to about 100C and maintained ; until substantially all air is removed. The vent is then sealed and the temperature raised to about 116C for about 15 minutes after which, the temperature is raised to about 126C and held for an additional 40 minutes at which time the transformation to whisker crystals is complete. The slurry is maintained in suspension throughout by mild stirring or rotating the autoclave periodically. Excess water is removed and the remaining fibrous mass is dewatered. The excess of supernatent water is first blown out of the autoclave without cooling using the pressure from the autoclave. A sample of the fibrous product is r~moved and put into ethylene gl~col to prevent rehydration Other organic water immiscible solvents such as acetone, ethanol, etc could be used The solvent is removed under vacuum and the sample fibers are examined under the microscope. The product is composed of whisker crystals having an overall average length of about 200 microns and cross sectional dimensions of about 2 to 3 microns. The remaining product while maintained at a temperature over 100~C is filtered under pressure, the filter cake is transferred immediately to an oven where it is dried bm~
- ~Z7;3'~4 to about 1% moisture content at about 400C for about 15 minutesO The dried fiber is then fed to a calcining oven and rapidly heated to about 600C and held at that temperature for no longer than about 0,5 to 1~5 minutes, After cooling the product remains unchanged indefinitely.
Exampl The experiment is carried out as in Example 1 except that before autoclaving, tannic acid (Fisher Chemical) in an amount equal to 0~0025% by weight of the total, is added to the gypsum slurry. After venting and sealing the autoclave~
the slurry is heated to 116C and held for 15 minutes The mixture is then heated to 122C and held for an additional 20 minutes after which, the partially reacted product is heated to 128C for an additional 20 minutes. The sample fibers examined under the microscope are between 200 and 300 microns long with some even longer and an average diameter of 4 to 5 microns~ The reaction mixture from the autoclave has a low ~iscosity and is easily pourable, The filter cake is similar in appearance to that of Example 1 but with little ~r no th~ee dimensional int~ungling of fibers and a layered appearance.
Example 3, This experiment is carried out as in Example 2 except that the reaction is stopped at 124C after about 20 minutes and partial fiber formation, The product is cooled and reheated to 128C for about 20 minutes The sample fibers are flat ribbon-like crystals, 200 to 300 microns long and longer with cross sectional dimensions of about 5 to 10 microns The filtered product is similar in appearance to that obtained in the preceding example, Example 4.
., A solution of succinic acid is prepared by dissolving 0 0712 grams of succinic acid in 300 cc of water.
bm~
~2'7374 Experimental runs are carried out as in Example 1 using a 7% by weight gypsum slurry. The autoclaving is done at 130C
for 45 minutes. Three separate autoclave runs with slurries containing, respectively 1 cc, 2 cc, and ~ cc of th succinic acid solution are carried out~ The sample fibers removed from each product and examined under the microscope were rod-like crystals having the following average dimensions:
with 1 cc succinic acid ---- length 150 microns diameter 2 microns with 2 cc succinic acid ---- length 150 microns diameter 2 microns with 4 cc succinic acid ---- length 80-100 microns diameter 5-7 microns The filtered and dried products are similar in appearance to those of preceding examples with little three dimensional mixing, Example 5.
.
This experiment is carried out as in Example 4 by adding 0.15% by weight anthranilic acid to the slurry instead of succinic acid, The sample fibers had an average length of 150 microns and an average diameter of 2 to 3 microns and the filtered and dried fiber product is easily separated into discrete fibers similar to that obtained in the previous experiments~
Example 6, This experiment is carried out similar to the procedure of Example 5 using a 7% by weight gypsum slurry to which is added 0.007% by weight succinic acid and 0,15%
by weight anthranilic acid, The autoclaving is carried out for 35 minutes at temperatures between 122 and 13~C, The : sample fiber had an average whisker crystal length of 150 microns and an average diarneter of 5 to 8 microns, bm~ t~
Example 7 ~ ~
A group of experiments is run using a 250 milliliter Parr Bomb reactor and a stirrer running at high speed until the temperature is nearly at the min.imum fiber forming temperature tca 110C) and thereafter~ the gypsum is maintained in suspension by rotating the reactor a full 360 about every two minutes while the temperature is raised at a controlled rate to 140C, The controlled rates of temperature increase are 2,5C/min~ 1C/min " and 0,5C/min " respectively, In each instance, the conversion goes substantially to completion.
Examples of the fibers produced in each case have the following composition based on total number of fibers in the sample, @ 2,5C/min, Length: 0 to 50 microns 5 - .7%
50 to 100 microns 28 - 30%
100 to 200 microns 50 - 52%
. 200 to 300 microns 14 - 16~
! @ 1. 0C/min.
Length: 0 to 50 microns 4 - 5%
.50 to 100 microns 8 - 10%
100 to 200.microns 27 - 30%
. 200 to 300 microns 28 - 30%
300 to 400 microns 14 - 16%
400 to:500 microns 10 - 12%
@ 0,5C/min, ..
Length: 0 to 50 microns 4 - 5%
50 to 100 microns 4 - 5%
100 to 200 microns 16 - 18%
200 to 300 microns 20 - 30%
300 to 400 microns 22 - 24%
400 to 500 microns 24 - 26%
A similar experiment is carried out with 0~0125%
by weight tannic acid added to the slurry, I'he product formed has lower viscosity, is easily poured, has about the same distribution on length o~ fibers, a somewhat larger average diameter and is considerably more easily dispersed, brn ~
~L2737~
Example 8 In this experiment a fiber sample prepared according to Example 2 followed by drying at 400C and calcining at 600~C Eor 1 minute is tested to determine its reinforcing characteristics in commonly used engineering thermoplastics. Separate samples for injection molding were prepared usi.ng nylon and polyester. Pelletized resins of Nylon 6,6 (Zytel 101 available from DuPont Corporation, Wilmington, Delaware) and polyester (Valox 310, available from General Electric Corporation, Pittsfield, Massachusetts) are used as the resin~ The nylon and polyester pellets are yround separately to about 50 mesh~ and each is blended with 50% by weight of calcium sulfate fiber prepared as above to provide injection molding samples of reinEorced nylon and reinforced polyester, Substantially no clumping and balling occurs : during blending. In each case, the blend is dried for 2 hours j at 100C, The resin/fiber pellet is in~ection molded in a : five-ounce Van Dorn hydraulic press capable of 75-ton clamp pressure according to the conditions below:
. 20- Temperatures (~C? Mylon Polyester :- rear cylinder 300 238 ; front cylinder 313 241 . nozzle 316 243 : . mold . 135 66 melt 316 243 back pressure (psi)50 50 injection pressure (psi) stage I 1500 1500 : stage II 1300 1300 injection speed (sec) stage I
stage II 1.5 2.5 mold case tsec) 30 30 injection forward (sec) 4~0 4.0 screw speed (rpm) 70 70 total cycle time (sec) 34 34 The resulting reinforced molded plastic is subjected to the following tests:
* Trademark bm: t~J,i./~
~273~4 Tests .....
Flexural: Strengths and moduli are measured on a Baldwin-Tate-Emery Testing.
machine according to ~STM D-790.
Tensile: Strengths are obtained according to ASTM D-638 on an Instron test-ing machine, Wet Tensile: Tests are conducted on specimens exposed to 50C water for 24 hours, Hea-t Distortion: Tests are performed with a Tinius Olsen HDT~ bath using phenyl sili-cone oil elevated at a rate of 2C
per.minute (ASTM D-648), Izod Impact: Notched and unnotched tests are performed according to ASTM D-256, Modified Gardner A 1/2 lbo weight is dropped until Impact Test: the bottom of a 4" x 2 1/2" x 1/8"
plaque exhibited a fracture, Results 20 ~ Polyester Flexural Strength~ psi containing 50 wt,% containing 50 wt,%
calcium sulfate calcium sulfate fiber fiber . Dry lg,000 14,000 Wet 9~000 11,000 Tensile Strength, psi. 11~000 8,000 Izod Impact Notched 0~5 0 5 ft,lb,/in~
.30 Unnotched 4,0 4,0 Gardner Impact, in,lb, 3,5 3,5 Heat Deflection Temp.! F 450 350 Scanning electron microscope studies o~ fracture surfaces of the calcium sulfate whisker fiber reinforced nylon show good fiber distribution and a good degree of bonding between matrix and reinforcement, brn~
Z~374 Example 9 . . ~
Calcium sulfate fibers prepared by using a dispersing agent in accordance with the methods of Examples 2 and 9 are prepared for evaluation as reinforcement for PVC pipe using powdered compound with the fiber added in a latter stage of the blending cycle. Pipe extruded from PVC
compound blended with 20 and 30 parts of calcium sulfate fiber per hundred parts of resin show good impact and a high modulus of elasticity~ The physical properties of the fiber reinforced PVC pipe are shown below with comparison to extruded P~C pipe using asbestos. PVC powder compound is used for all pipe extrusionî the reinforcement being introduced with the compound at the blender.
Physical Properties of Reinforced PVC
Càlcium Sulfate Fiber Asbestos ` Physical Property 20 pts. 30 pts, 20 pts, " .
Modulus (Long)676~000786,000 550,000 Modulus (Cir.)585~000691~000 556,000 Tensile (Long)6,600 6,200 5,810 Tensile (Cir,)6,400 4,830 5,450 Impact 23C
Ave. ft,-lb./in. wall 1~050 - 185 195 This application is a division of copending Canadian application Serial No. 314,239, filed October 25, 1978.
hm~
melamine formaldehyde, polyurethane or other polymers The use of such coatings is disclosed in Eberl, et al U. S.
Patent No. 3,9~1,105.
The calcium sulfate whisker fibers prepared by the method of this invention as already stated are useful as reinforcing agents for either organic or inorganic matrix materials, particularly, the synthetic polymers. These fibers ; are especially suited for blending with organic or inorganic matrix materials in liquid or in dry solid form.
Various methods of dispersion and blending of the fiber in the plastic or other matrix materials can be employed, although properties r especially mechanical properties~ of the resultant composite materials may be affected by the choice of dispersing means ~any varieties of commonly used processing equipment, such as, single and twin screw extruders, two-roll mills and Banbury mixers can be used to produce excellent dispersions The-equipment must be specifically run to produce good fiber dispersion in the plastic matrix with a minimum amount of damage to the fiber A two-roll mill can be advantageously used to blend into small batches of thermoplastic materials. Usually~
two-roll mills are operated under atmospheric conditions As with other types of processing equipment; the plastic material should be fluxed on the mill before the addition of fiber. It is recommended that the roll stock be removed by a doctor blade or other means and then returned to the mill. In order to attain recommended shear levels, it is suggested that nip clearances greater than 25 mils be used and that the roll speeds be in the range 100 to 125 r p m bm~
~L~27374 with a speed differential of 25 r,p.m, It is apparent that the temperature at which the rolls are maintained is dictated by the properties of the plastic material being worked, As a guiding principle, roll temperatures should be maintained at such a level that addition of the fiber to the plastic roll stock causes the resultant roll stock to lose tackiness and thus become easily workable.
A Banbury mixer can also be employed, ~s in the case of the two-roll mill discussed above, it is very important that the plastic material be in a condition of flux before addition of the fiber, This is best accomplished by feeding the plastic into a hea~ed hopper or holding bin and then holding it under force in the mixing area until fluxing is observed. Upon completion of fluxing the fiber may be added, The blending should then be continued for a period of time sufficient to achieve the desired distribution of fibers, It is necessary to control the viscosity of the fluxed resin~
fiber composite. This can be done by controlling the melt temperature by means of varying speed of rotation, cycle time and the circulation through the rotors of either steam or water as the situation requires. Generally speaking, it is possible to increase the blending time so that improved dispersion may be attained without degrading the fiber. For example, a typical compounding-cycle for a thermoplastic material would total three minutes, Twenty seconds would allow fluxing to occur, and forty seconds would suffice to add the fiber, Thus, two minutes are available for the blending process. A temperature rise during the final -two minutes of the cycle is commonly observed, For extrusion, single or twin screw extruders can be used to compound the fibers into resin compositions, The temperature variation along the length of a twin screw bm:~u~
~2~737~
extruder should be controlled so that fiber degradation is kept to a minimum, Generally speaking, with higher melting polymers, such as nylon, the temperature in the feed section should be maintained about 25 to 50C above the required melt temperature for the polymer involved, In the case of low melting polymers such as polyolefins, it is suggested that the feed section be maintained 10 to 30C above melt temperature, The temperature profile in -the barrel will depend on the speed of rotation of the screws since this dictates the time the solid resin remains in the feed section, Thus, larger temperature differences between feed and completion sections are required at higher screw ro-tations, It is essential to recognize that degradation of fiber during the extrusion process may occur before the fluxing of the resin, ~ccordingly~ configuration of the screws should be such that the fiber and resin mix are gently conveyed until softening of the resin occurs by the transfer of sufficient heat~ Once fluxing has taken place, much compressive and dispersive working can be accomplished with little or no degradation of the fiber, Reduction of compression in the feed section can be accomplished by using deeper channels here than in the remainder of ~he screw, -Good engineering practice would suggest a gradual reduction in screw pitch following the feed section to avoid a situation of immediate compression of the material, It is desirable that the screw design incorporate kneading blocks since these are known to aid in dispersion of the fiber. In accordance with the infor~ation given above, these kneading blocks should be placed in a section where the polymer has already melted so that unnecessary mechanical working is avoided.
bm~
~2~37~
In cJeneral~ standard desi~3ns and configurations for dies and adaptcrs are suitable for use in fiber extrusion and those configurations commonly used for PVC, work well, Combinations of calcium sulfate fiber and other reinforcing fiber, such as chopped fiberglass, can be used for example in polyester, or the calcium sulfate fiber can be used to replace fiberglass, Cast composite specimens of ~ polyester resin and calcium sulfate fiber containing 5,5% by volume or approximately 13% by weight of fiber are prepared by dispersing the fiber in a polyester resin stirring and cuxing at room temperature using 0,75% by weight methyl-ethyl-ketone peroxide as a curing agent, ~esidual air is removed from the mix under a vacuum of about two millimeters mercury for a time period of about 5 minutes.
Irhe fiber is dispersed easily and gives a pourable mixture of fairly low viscosity, The composite mix is then cast into a block with dimensions approximately one quarter inch thick by three inches wide by six inches long, The test blocks are allowed to set at room temperature and are then post cured at 110C for 30 minutes, The mechanical properties of these polyester/fiber composites compare well with corresponding asbestos and iberglass reinforced composites, Calcium sulfate fiber can be bonded with magnesium ~xysulfate (Sorél Cementl, By way of example, a mixture composed of: calcium sulfate fiber, 38,7 wt, %; magnesium oxide, 13.6 wt. ~; magnesium sulfate, 4.8 wt, ~; Natrosel*
250, 1.0 wt. %; Duponol*ME (dry)~ 0,3 wt. %; and water~
~1.6 wt, ~, is prepared by adding the water all at once and using a high speed mixer to foam the mix. The mix, which appears to be dry at first, becomes a heavy foamy mass with * T~de ~arks -lG~
L~m~
~Z~374 continued mixing and is suitable for use as a foam-in-place insulation.
The whisker fibers can also be used in place of or in combination with asbestos in other applications such as for reinforcement in cas-t magnesia cement compositions and the like.
Particularly useful products prepared by using the calcium sulfate fiber prepared in accordance with this invention, are the formulations for friction elements such as brakes, clutches, transmission bands, etc. wherein the calcium sulfate fiber is used to replace part or all of the asbestos fiber presently employed in such formulations.
Formulations for friction elements generally utilize phenolic resins of the pulverized noyolak type. These resins are utilized primarily in dry mix processes for the production of disc pads, linings and truck blocks. A suitable formulation is prepared by blending about 35 to 75 parts preferably about 60 parts by weight of calcium sulfate fiber prepared in accordance with the process of this invention, with about 10 to 15 parts preferably about 13 parts by weight of barytes, about 5 to 10 parts preferably about 7 parts by weight of Cardolite brand epoxy resin flexibilizer made by the 3M Company of Minnesota, and about 15 to 25 parts preferably about 20 parts by weight of a phenolic resin of the novolak type. If desired the fiber ingredient can be used as a blend, suitably at a ratio between about 1 to 1 and about ~ to 1 of calcium sulfate fiber to asbestos.
The blended formulation is compression molded at about 200C
and post cured for about ~ hours at about 230~C. The cured friction element when evaluated by various fric-tion -testing procedures, which are standardized in the industry show the calcium sulfate Eiber to be an acceptable reinEorcing material, particu-larly, for light duty applications and as a diluent for asbestos. The drn~ 17-~,7374 calcium sulfate fiber is especially beneficial for improved wear performance.
The following examples are given by way of illustrating the improved gypsum transformatiorl process for producing an improved dispersible calcium sulfate iber product.
Example 1.
Twenty-two grams of a commercial gypsum (Terra Alba) is slurried in 200 cc of water (a concentration of 10% by weight of total slurry). The slurry is heated to a temperature of about 80C and pumped into an autoclave equipped with an electric heating mantel and Variac control, With the autoclave vented, the temperature is raised to about 100C and maintained ; until substantially all air is removed. The vent is then sealed and the temperature raised to about 116C for about 15 minutes after which, the temperature is raised to about 126C and held for an additional 40 minutes at which time the transformation to whisker crystals is complete. The slurry is maintained in suspension throughout by mild stirring or rotating the autoclave periodically. Excess water is removed and the remaining fibrous mass is dewatered. The excess of supernatent water is first blown out of the autoclave without cooling using the pressure from the autoclave. A sample of the fibrous product is r~moved and put into ethylene gl~col to prevent rehydration Other organic water immiscible solvents such as acetone, ethanol, etc could be used The solvent is removed under vacuum and the sample fibers are examined under the microscope. The product is composed of whisker crystals having an overall average length of about 200 microns and cross sectional dimensions of about 2 to 3 microns. The remaining product while maintained at a temperature over 100~C is filtered under pressure, the filter cake is transferred immediately to an oven where it is dried bm~
- ~Z7;3'~4 to about 1% moisture content at about 400C for about 15 minutesO The dried fiber is then fed to a calcining oven and rapidly heated to about 600C and held at that temperature for no longer than about 0,5 to 1~5 minutes, After cooling the product remains unchanged indefinitely.
Exampl The experiment is carried out as in Example 1 except that before autoclaving, tannic acid (Fisher Chemical) in an amount equal to 0~0025% by weight of the total, is added to the gypsum slurry. After venting and sealing the autoclave~
the slurry is heated to 116C and held for 15 minutes The mixture is then heated to 122C and held for an additional 20 minutes after which, the partially reacted product is heated to 128C for an additional 20 minutes. The sample fibers examined under the microscope are between 200 and 300 microns long with some even longer and an average diameter of 4 to 5 microns~ The reaction mixture from the autoclave has a low ~iscosity and is easily pourable, The filter cake is similar in appearance to that of Example 1 but with little ~r no th~ee dimensional int~ungling of fibers and a layered appearance.
Example 3, This experiment is carried out as in Example 2 except that the reaction is stopped at 124C after about 20 minutes and partial fiber formation, The product is cooled and reheated to 128C for about 20 minutes The sample fibers are flat ribbon-like crystals, 200 to 300 microns long and longer with cross sectional dimensions of about 5 to 10 microns The filtered product is similar in appearance to that obtained in the preceding example, Example 4.
., A solution of succinic acid is prepared by dissolving 0 0712 grams of succinic acid in 300 cc of water.
bm~
~2'7374 Experimental runs are carried out as in Example 1 using a 7% by weight gypsum slurry. The autoclaving is done at 130C
for 45 minutes. Three separate autoclave runs with slurries containing, respectively 1 cc, 2 cc, and ~ cc of th succinic acid solution are carried out~ The sample fibers removed from each product and examined under the microscope were rod-like crystals having the following average dimensions:
with 1 cc succinic acid ---- length 150 microns diameter 2 microns with 2 cc succinic acid ---- length 150 microns diameter 2 microns with 4 cc succinic acid ---- length 80-100 microns diameter 5-7 microns The filtered and dried products are similar in appearance to those of preceding examples with little three dimensional mixing, Example 5.
.
This experiment is carried out as in Example 4 by adding 0.15% by weight anthranilic acid to the slurry instead of succinic acid, The sample fibers had an average length of 150 microns and an average diameter of 2 to 3 microns and the filtered and dried fiber product is easily separated into discrete fibers similar to that obtained in the previous experiments~
Example 6, This experiment is carried out similar to the procedure of Example 5 using a 7% by weight gypsum slurry to which is added 0.007% by weight succinic acid and 0,15%
by weight anthranilic acid, The autoclaving is carried out for 35 minutes at temperatures between 122 and 13~C, The : sample fiber had an average whisker crystal length of 150 microns and an average diarneter of 5 to 8 microns, bm~ t~
Example 7 ~ ~
A group of experiments is run using a 250 milliliter Parr Bomb reactor and a stirrer running at high speed until the temperature is nearly at the min.imum fiber forming temperature tca 110C) and thereafter~ the gypsum is maintained in suspension by rotating the reactor a full 360 about every two minutes while the temperature is raised at a controlled rate to 140C, The controlled rates of temperature increase are 2,5C/min~ 1C/min " and 0,5C/min " respectively, In each instance, the conversion goes substantially to completion.
Examples of the fibers produced in each case have the following composition based on total number of fibers in the sample, @ 2,5C/min, Length: 0 to 50 microns 5 - .7%
50 to 100 microns 28 - 30%
100 to 200 microns 50 - 52%
. 200 to 300 microns 14 - 16~
! @ 1. 0C/min.
Length: 0 to 50 microns 4 - 5%
.50 to 100 microns 8 - 10%
100 to 200.microns 27 - 30%
. 200 to 300 microns 28 - 30%
300 to 400 microns 14 - 16%
400 to:500 microns 10 - 12%
@ 0,5C/min, ..
Length: 0 to 50 microns 4 - 5%
50 to 100 microns 4 - 5%
100 to 200 microns 16 - 18%
200 to 300 microns 20 - 30%
300 to 400 microns 22 - 24%
400 to 500 microns 24 - 26%
A similar experiment is carried out with 0~0125%
by weight tannic acid added to the slurry, I'he product formed has lower viscosity, is easily poured, has about the same distribution on length o~ fibers, a somewhat larger average diameter and is considerably more easily dispersed, brn ~
~L2737~
Example 8 In this experiment a fiber sample prepared according to Example 2 followed by drying at 400C and calcining at 600~C Eor 1 minute is tested to determine its reinforcing characteristics in commonly used engineering thermoplastics. Separate samples for injection molding were prepared usi.ng nylon and polyester. Pelletized resins of Nylon 6,6 (Zytel 101 available from DuPont Corporation, Wilmington, Delaware) and polyester (Valox 310, available from General Electric Corporation, Pittsfield, Massachusetts) are used as the resin~ The nylon and polyester pellets are yround separately to about 50 mesh~ and each is blended with 50% by weight of calcium sulfate fiber prepared as above to provide injection molding samples of reinEorced nylon and reinforced polyester, Substantially no clumping and balling occurs : during blending. In each case, the blend is dried for 2 hours j at 100C, The resin/fiber pellet is in~ection molded in a : five-ounce Van Dorn hydraulic press capable of 75-ton clamp pressure according to the conditions below:
. 20- Temperatures (~C? Mylon Polyester :- rear cylinder 300 238 ; front cylinder 313 241 . nozzle 316 243 : . mold . 135 66 melt 316 243 back pressure (psi)50 50 injection pressure (psi) stage I 1500 1500 : stage II 1300 1300 injection speed (sec) stage I
stage II 1.5 2.5 mold case tsec) 30 30 injection forward (sec) 4~0 4.0 screw speed (rpm) 70 70 total cycle time (sec) 34 34 The resulting reinforced molded plastic is subjected to the following tests:
* Trademark bm: t~J,i./~
~273~4 Tests .....
Flexural: Strengths and moduli are measured on a Baldwin-Tate-Emery Testing.
machine according to ~STM D-790.
Tensile: Strengths are obtained according to ASTM D-638 on an Instron test-ing machine, Wet Tensile: Tests are conducted on specimens exposed to 50C water for 24 hours, Hea-t Distortion: Tests are performed with a Tinius Olsen HDT~ bath using phenyl sili-cone oil elevated at a rate of 2C
per.minute (ASTM D-648), Izod Impact: Notched and unnotched tests are performed according to ASTM D-256, Modified Gardner A 1/2 lbo weight is dropped until Impact Test: the bottom of a 4" x 2 1/2" x 1/8"
plaque exhibited a fracture, Results 20 ~ Polyester Flexural Strength~ psi containing 50 wt,% containing 50 wt,%
calcium sulfate calcium sulfate fiber fiber . Dry lg,000 14,000 Wet 9~000 11,000 Tensile Strength, psi. 11~000 8,000 Izod Impact Notched 0~5 0 5 ft,lb,/in~
.30 Unnotched 4,0 4,0 Gardner Impact, in,lb, 3,5 3,5 Heat Deflection Temp.! F 450 350 Scanning electron microscope studies o~ fracture surfaces of the calcium sulfate whisker fiber reinforced nylon show good fiber distribution and a good degree of bonding between matrix and reinforcement, brn~
Z~374 Example 9 . . ~
Calcium sulfate fibers prepared by using a dispersing agent in accordance with the methods of Examples 2 and 9 are prepared for evaluation as reinforcement for PVC pipe using powdered compound with the fiber added in a latter stage of the blending cycle. Pipe extruded from PVC
compound blended with 20 and 30 parts of calcium sulfate fiber per hundred parts of resin show good impact and a high modulus of elasticity~ The physical properties of the fiber reinforced PVC pipe are shown below with comparison to extruded P~C pipe using asbestos. PVC powder compound is used for all pipe extrusionî the reinforcement being introduced with the compound at the blender.
Physical Properties of Reinforced PVC
Càlcium Sulfate Fiber Asbestos ` Physical Property 20 pts. 30 pts, 20 pts, " .
Modulus (Long)676~000786,000 550,000 Modulus (Cir.)585~000691~000 556,000 Tensile (Long)6,600 6,200 5,810 Tensile (Cir,)6,400 4,830 5,450 Impact 23C
Ave. ft,-lb./in. wall 1~050 - 185 195 This application is a division of copending Canadian application Serial No. 314,239, filed October 25, 1978.
hm~
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a process for preparing a calcium sulfate whisker fiber, which comprises preparing an aqueous slurry of fine particle gypsum with the gypsum comprising about 5% to 10% by weight of the slurry; agitating said slurry to maintain a uniform dispersion while heating to a temperature of about 100°C, the improvement which comprises heating said slurry under autogenous pressure until the minimum fiber-forming temperature is reached and thereafter continuing heating to raise the temperature above the minimum fiber-forming temperature at a rate of no more than 2.5°C per minute and maintaining the temperature above the minimum fiber-forming temperature until substantially all of the gypsum is transformed to fiber.
2. A process according to claim 1 wherein the minimum fiber-forming temperature is between about 115°C
and about 120°C and the temperature of the reaction mixture is raised above the minimum fiber-forming temperature at a rate of no more than about 1°C per minute.
and about 120°C and the temperature of the reaction mixture is raised above the minimum fiber-forming temperature at a rate of no more than about 1°C per minute.
3. A process according to claim 2 wherein the product obtained on completion of heating under autogenous pressure is filtered under pressure at a temperature in excess of 100°C to recover the fiber product which is transferred to a drying oven without cooling below about 100°C and dried to a moisture content of about 1% or less and thereafter calcined at a temperature above about 500°C
for a period of about 0.5 minute to about one hour.
for a period of about 0.5 minute to about one hour.
4. A process according to claim 3 wherein the calcining is carried out at about 600°C for about 0.5 to 1.5 minutes.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA387,331A CA1127374A (en) | 1977-11-25 | 1981-10-05 | Fibrous calcium sulfate |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US854,774 | 1977-11-25 | ||
| US05/854,774 US4152408A (en) | 1977-11-25 | 1977-11-25 | Fibrous calcium sulfate |
| CA314,239A CA1115487A (en) | 1977-11-25 | 1978-10-25 | Fibrous calcium sulfate |
| CA387,331A CA1127374A (en) | 1977-11-25 | 1981-10-05 | Fibrous calcium sulfate |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1127374A true CA1127374A (en) | 1982-07-13 |
Family
ID=27165936
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA387,331A Expired CA1127374A (en) | 1977-11-25 | 1981-10-05 | Fibrous calcium sulfate |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1127374A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115432986A (en) * | 2022-09-28 | 2022-12-06 | 宜都兴发化工有限公司 | Preparation method of high-strength water-permeable composite material |
-
1981
- 1981-10-05 CA CA387,331A patent/CA1127374A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115432986A (en) * | 2022-09-28 | 2022-12-06 | 宜都兴发化工有限公司 | Preparation method of high-strength water-permeable composite material |
| CN115432986B (en) * | 2022-09-28 | 2023-10-24 | 宜都兴发化工有限公司 | Preparation method of high-strength water-permeable composite material |
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