Patent Publication Number: US-5837047-A

Title: Heat curable binder systems and their use

Description:
FIELD OF THE INVENTION 
     This invention relates to heat curable binder systems comprising as separate parts: a Part I comprising (a) a soluble source of silica, (b) an alcohol, and preferably (c) a surfactant; and a Part II comprising an alkali aluminate. The binder components are mixed with an aggregate to form a mix. The resulting mix is shaped and heated at an elevated temperature to form a cured shape, particularly ceramic preforms. Heat is applied by warm air, baking in an oven, microwave, or preferably by hot-box equipment. 
     BACKGROUND OF THE INVENTION 
     Several patents describe methods for making metal matrix composite bodies by spontaneously infiltrating porous ceramic filler materials. U.S. Pat. Nos. 5,500,244, 5,298,339, 4,923,832 and 5,433,261 focus on infiltration techniques, promoters, control of atmosphere during infiltration, etc. Very little development is devoted to making ceramic preforms and binders used for this process. U.S. Pat. Nos. 5,433,261 and WO 9,414,725 describe liquid organometallic ceramic precursor for making rigidized ceramic preforms. This binder has many disadvantages, such as slow cure rate, low hot and cold strength and high cost. U.S. Pat. No. 5,500,244 discloses the use of colloidal oxides, e.g. colloidal alumina or silica, to rigidize ceramic fillers. The process involves freezing of colloidal oxide/filler mix and then heating to about 900° C. before contacting with molten matrix metal. This is not a high production process. There is a possibility of complete closure of interconnected porosity during sintering which would inhibit infiltration. 
     SUMMARY OF THE INVENTION 
     The subject invention relates to heat curable binder systems comprising as separate parts: 
     A. a Part I comprising: 
     (a) soluble source of silica, 
     (b) an alcohol, and preferably 
     (c) a surfactant; and 
     B. a Part II comprising: 
     an alkali aluminate. 
     The binder system is mixed with an aggregate to form a mix. The resulting mix is shaped and cured by heating in one continuous stage with warm air, baking in oven, microwave, and preferably in hot-box equipment. 
     Curing of the binder system does not occur spontaneously when the Parts are mixed with an aggregate and shaped, but instead occurs when the shaped mix is heated to temperatures of at least 150° C. to 300° C., preferably 200° C. to 275° C., most preferably 230° C. to 260° C. 
     C 13  NMR studies suggest that alcohol is not only a solvent, but is reacted with the other binder components. Shaping mixes prepared with the subject binder systems do not substantially react until heated and have a benchlife of at least five hours. 
     Shapes prepared with the binder systems show improved immediate strength, strength development over twenty-four hours, humidity resistance, and scratch hardness when compared to binder systems which do not contain an alcohol. The polyhydric alcohols are especially useful for making cores and molds with various sands as aggregates. Some polyhydric alcohols, such as sorbitol control the rate of dehydration under hot box conditions and prevent excessive drying of shapes during storage at ambient conditions. 
     The binder system with aliphatic alcohols is particularly useful for making porous ceramic preforms suitable for the production of metal matrix composites. These preforms can be made economically with high productivity in an environmentally acceptable way and have good hot and cold strength. Ceramic preforms rigidized with the binder can be spontaneously infiltrated with molten aluminum to form ceramic/metal composites. Infiltration promoters, such as magnesium metal powder and magnesium oxide, can be added to preforms. Evidence related to the microstructure of the composite preforms suggests that alumina particles are embedded into aluminum matrix and there is no porosity at aluminum/alumina interface. 
     ENABLING DISCLOSURE AND BEST MODE 
     For purposes of describing the invention herein the following definitions will be used: 
     Aggregate--any material which can be used to make foundry shapes or refractory shapes. 
     Mix--a mixture of aggregate and binder system used for making shapes. 
     Mold or Core--any disposable shape which is use to create empty space in the object to be made. 
     Pattern--a disposable shape which has the shape of the object made. 
     Preform--a pattern which is infiltrated with a metal and is the object made for use. 
     Shape--any shape used in the molding or extrusion industry including foundry shapes or refractory shapes, particularly molds, cores, patterns, and preforms. 
     The binder system consists of two parts: a Part I comprising (a) a soluble source of silica, (b) an alcohol, and preferably (c) a surfactant; and a Part II comprising an alkali aluminate. Either Part may be liquid or solid, but at least one Part must be liquid. If either Part is a solid Part, the solid Part will dissolve in the liquid Part when the two Parts are mixed. 
     The amount of solids in the Part I typically ranges from 40-50 weight percent. The amount of solids in the Part II typically ranges from 35-45 weight percent. The weight percent is based upon the total weight of the Part I and Part II respectively. The alkali aluminate is a powder alkali aluminate, or an alkali aluminate in solution, preferably where sodium aluminate is the alkali aluminate. The powder form provides greater flexibility to the formulator when adding other Parts to the binder, but liquid Parts are easier to use. 
     Although various combinations of soluble silica source and alkali aluminate are possible, the best results are obtained when a colloidal silica solution is used with solid or liquid sodium aluminate. 
     For purposes of describing this invention, term alcohol means monofunctional alcohols, glycols, and polyhydric alcohols. The alcohols used in the binder systems are added to the source of silica, typically a colloidal silica solution. Although a variety of alcohols can be used, for example xylitol, erythritol, glycerin, ethylene glycol, aliphatic alcohols, the best results are obtained with sorbitol, ethanol, isopropanol and butanol. The optimum alcohol concentration, particularly sorbitol, is from about 0.05% to about 0.40% bos, preferably 0.15% to about 0.25% bos. 
     The binder system will typically contain 10-35 weight percent colloidal silica, 1-20 weight percent alcohol, 60-80 weight percent sodium aluminate solution having solids concentration at 35-45 weight percent, 0-1 weight percent surfactant where said weight percent is based upon the total weight of the binder system. The preferred ranges are 15-30 weight percent colloidal silica, 2-10 weight percent alcohol, 65 -75 weight percent sodium aluminate solution and 0-0.5 weight percent surfactant. This would correspond to a molar ratio of Na 2  O:Al 2  O 3  from 1:1 to 1.4:1 and SiO 2  :Al 2  O 3  from 1:1 to 1.7:1, preferably Na 2  O:Al 2  O 3  from 1:1 to 1.3:1 and SiO 2  :Al 2  O 3  from 1:1 to 1.5:1. 
     Other additives, such as surfactants, can be added to silica solution to improve flowability of the sand mix. Preferably used for this purpose is sodium dodecylbenzene sulfonate, WITCONATE 45 of Witco. 
     The binder system composition is mixed with an aggregate at a binder level of 1-15 percent based on filler (BOF), preferably 2-8 percent, to form a shaping mixture, and cured in conventional hot box equipment. Preferably the Part I is first applied to the aggregate when making a mix. Then the Part II is added. 
     Hot box, warm air/warm box, baking in conventional oven and microwave can be used to cure the mixture of binder system and aggregate. The alcohols incorporate into the structure of the binder at elevated temperature, resulting in improved film properties. The binder exhibits high immediate strength and good strength development over 24 hrs. The shapes have good scratch hardness and good humidity resistance with silica, lake sands and various alumina grades. The polyhydric alcohols control the rate of dehydration under hot box conditions and prevent excessive drying on the surface of cores, resulting in improved scratch hardness. 
     The aggregate can be any conventional aggregate used in the foundry industry or any refractory used in making shapes from refractory materials. Sand, typically sand which contains at least 70 percent by weight silica, is used as the aggregate. Other suitable aggregate materials include zircon, olivine, alumino-silicate sand, chromite sand, and the like. Sand used for die casting preferably has a fineness grade of 70 to 105. For making preforms, the preferred aggregate is a ceramic refractory, particularly alumina, silicon carbide, silicon nitride, and used at a binder level of 4-8 weight percent. 
     The curing of the shapes is accomplished by the crystallization and dehydration of the reaction product of the silica and sodium aluminate at elevated temperature. The curing time depends on the temperature and the equipment used. With conventional hot box equipment, the temperature range typically used is 175°-300° C. which results in cure times of 30-90 seconds. 
     The following abbreviations are used in the Examples and Tables which follow: 
     bos=based on sand 
     bof=based on filler 
     BP=blow pressure (psi) 
     CH=constant humidity 
     CT=constant temperature 
     DT=dwell time (seconds) 
     Imm.=immediate (strength out of box) 
     PI=Part I (source of silica) 
     PII=Part II (source of alumina) 
     RH=room humidity 
     RT=room temperature 
     Example A is a control which does not employ an alcohol. Examples 1-2 show formulations containing various amounts of sorbitol which are used in making preforms with alumina refractory. 
    
    
     EXAMPLE A AND 1 
     A mixture containing tabular alumina refractory and binder were formulated by first mixing the Part I with the alumina and then mixing the Part II using a paddle mixer Hobart N-50. Both the Part I and Part II were liquid. The mix was made in the shape of dog bones and cured in a conventional hot box system. The curing conditions were as follows: 
     blow time/pressure 0.5 sec at 65 psi 
     box temperature 230° C. 
     dwell time 90 sec 
     The tensile strength of preforms was measured with a Thwing-Albert TA 500. The humidity resistance was tested by placing samples which have cured for 24 hrs into a humidity chamber at 25 C. and 99% RH. The tensile strength was measured after 1 hr exposure and is designated as 24+1 hrs. Tabular alumina 60 mesh was used in Examples A, 1, 2 and 100 mesh tabular alumina with fines removed was used in the remaining examples. 
     In Example A, no sorbitol was added while Examples 1-2 contained varying amounts of sorbitol. The test conditions and the amount of binder components are described below: 
     TEST CONDITIONS 
     Refractory: Alumina (4000 parts) 
     CT 1  Room: 50% Relative Humidity, 25° C. 
    
     
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BINDER FORMULATION                                                        
Example      A           1       2                                        
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Part I                                                                    
Ludox TM-50  2.0%        2.0%    2.0%                                     
Sorbitol     none        0.15%   0.25%                                    
Witconate 45 0.01%       0.01%   0.01%                                    
Part II                                                                   
Sodium Aluminate                                                          
             4.5%        4.5%    4.5%                                     
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                       TABLE I                                                     
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TENSILE PROPERTIES AND SCRATCH HARDNESS                                   
         TENSILE STRENGTH                                                 
         (psi)                                                            
         zero benchlife Scratch                                           
Example  24 hrs.      24hrs. + 1                                          
                                Hardness                                  
______________________________________                                    
A        103           54       60                                        
1        112          106       87                                        
2        163          152       86                                        
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     The results in Table I indicate that tensile strengths of the shapes measured 24 hours after curing and 24 hours after curing and aging under high humidity for one hour are improved by the addition of the sorbitol. Scratch hardness is also improved. Observation of the shapes also shows that friability is improved. 
     EXAMPLES 3-5 
     Example 1 was followed except the Part I/Part II ratio was varied. The amount of sorbitol was at 0.25% bof and the amount of surfactant Witconate 45 was 0.01% bof. The results are shown in Table II. 
     
                       TABLE II                                                    
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TENSILE PROPERTIES AND SCRATCH HARDNESS                                   
Part I/Part Tensiles (psi)      Scratch                                   
Example                                                                   
       II ratio Imm.   1 hr.                                              
                            24 hrs.                                       
                                  24 hrs. + 1                             
                                          Hardness                        
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3      0.0/6.5  0      0    0     0       0                               
4      1.0/5.5  22     242  126   70      79                              
5      1.5/5.0  43     299  186   121     80                              
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     EXAMPLE 6-9 
     The procedure in Example 1 was used to prepare alumina preforms but aliphatic alcohols were added to Part I of the binder. 
     The results are shown in Table III. They indicate that the addition of alcohols, such as ethanol, isopropanol, n-butanol, increase strength, density and scratch hardness of alumina preforms. The flowability is also significantly improved, resulting in increased density and better surface finish of preforms. 
     
                       TABLE III                                                   
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TENSILE PROPERTIES AND SCRATCH HARDNESS                                   
Exam-           Tensiles (psi)      Scratch                               
ple   Alcohol   Imm.   1 hr.                                              
                            24 hrs.                                       
                                  24 hrs. + 1                             
                                          Hardness                        
______________________________________                                    
6     none      35     225  203   149     60                              
7     ethanol   64     306  272   168     72                              
8     isopropanol                                                         
                75     313  301   226     70                              
9     n-butanol 67     332  302   201     75                              
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     EXAMPLE 10-13 
     The procedure in Example 1 was used, but some water based colloidal silica solution was replaced with colloidal silica solution in isopropanol. This is a commercial product of Nissan Chemicals with trade name IPA-ST. It contains 30% of colloidal silica. These formulations show the best mixing properties with alumina and the best flowability. The strength results are summarized in Table IV. 
     
                       TABLE IV                                                    
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TENSILE PROPERTIES AND SCRATCH HARDNESS                                   
Ex-             TENSILES (PSI)                                            
am-  Ludox/IPA-ST                                                         
                (zero benchlife)    Scratch                               
ple  % bof      Imm.   1 hr.                                              
                            24 hrs.                                       
                                  24 hrs + 1                              
                                          Hardness                        
______________________________________                                    
10   2.0/0      35     225  203   149     60                              
11   1.5/0.83   65     278  267   207     68                              
12   1.0/1.65   61     276  252   196     66                              
13    0/3.3     65     188  204   146     67                              
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     EXAMPLE 13 
     The procedure of Example 1 was used (formulation 2) to prepare preforms except 2 weight percent of magnesium powder was added to alumina to facilitate infiltration with molten aluminum. The dog bone shaped preforms were successfully infiltrated with aluminum at 880° C. The composite materials were completely infiltrated, with no residual porosity, as shown by Scanning Electron Microscopy data. They had excellent mechanical and thermal properties and very good surface finish. 
     EXAMPLES 14-15 
     Example 1 was followed except New Jersey silica sand (grain finess 80 ) was used instead of alumina. The binder composition was as follows: 
     
         ______________________________________                                    
Part I: 1.5% colloidal silica Ludox TM 50 + sorbitol, see below           
Part      II:     4.0%     sodium   aluminate.                            
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     Solid additives such as aluminum silicate, were added to those formulations to accelerate strength development. Core making conditions were: 
     hot box temperature 230° C. 
     dwell time 50 sec. 
     The results are summarized in Table V. 
     
                       TABLE V                                                     
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TENSILE PROPERTIES AND SCRATCH HARDNESS                                   
          Tensiles (psi)    Scratch                                       
Example                                                                   
       % Sorbitol                                                         
                Imm.   1 hr.                                              
                            24 hrs.                                       
                                  24 hrs. + 1                             
                                          Hardness                        
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14     0        20     140  112   53      50                              
15     0.25     32     212  135   129     65                              
16     0.35     29     162  147   93      66                              
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     Table V shows that the addition of sorbitol improves sand performance of the cores made with the binder, especially the strength after 24 and 24+1 hrs. Also the scratch hardness of cores is significantly improved. The optimum sorbitol concentration is about 0.25% bos.