Abstract:
A low viscosity filler boron nitride agglomerate particles having a generally spherical shape bound together by an organic binder and to a process for producing a BN powder composition of spherically shaped boron nitride agglomerated particles having a treated surface layer which controls its viscosity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/754,154, filed Jan. 3, 2001, now U.S. Pat. No. 6,713,088, issued Mar. 30, 2004. 
    
    
     FIELD OF INVENTION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/386,883 filed Aug. 31, 1999 and relates to a low viscosity filler composed of boron nitride agglomerated particles of spherical geometry, a process for forming a low viscosity filler of boron nitride agglomerated particles of spherical geometry and to a low viscosity boron nitride filled composition composed of a polymer selected from the group consisting of a polyester, epoxy or polyamide loaded with a low viscosity filler composition of BN particles in a concentration of between 30-50 wt. % BN with the composition having a viscosity below about 300 cp and preferably below about 250 cp. 
     BACKGROUND OF THE INVENTION 
     Boron nitride (EN) is a chemically inert non-oxide ceramic material which has a multiplicity of uses based upon its electrical insulating property, corrosion resistance, high thermal conductivity and lubricity, A preferred use is as a filler material additive to a polymeric compound for use in semiconductor manufacture as an encapsulating material or to form a low viscosity thermosetting adhesive or in formulating a cosmetic material. As presently manufactured boron nitride is formed by a high temperature reaction between inorganic raw materials into a white powder composition of BN particles having an hexagonal structure similar to graphite in a platelet morphology. The platelet morphology is for many applications undesirable and of limited utility. A conventional powder composition of BN particles has the physical attributes of flour in terms of its inability to flow. Accordingly, when added as a filler to a polymer a blended material is formed having poor rheologial properties and at loaded concentrations above 30 wt. % BN the blended material is so viscous that it is difficult to dispense from a mechanical dispenser such as a syringe. The physical characteristics of the filled polymer are enhanced at loading concentrations above 30 wt. % BN. Accordingly, a powder composition of BN particles with an improved rheology for use as a filler at high loading concentrations would be desirable. 
     The surface morphology and shape of conventional platelet BN particles are modified in accordance with the present invention to form boron nitride agglomerated particles bound by an organic binder such that when filled into a polymeric compound at loading levels between 30 to 50 wt. % BN the viscosity of the filled composition remains below 300 cp and preferably below a viscosity of 250 cp. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a low viscosity composition of spherically shaped agglomerated particles of boron nitride can be formed by spray drying an aqueous slurry composed of boron nitride particles of random irregular shape in combination with an organic binder and a base adapted to maintain the pH of the slurry above about 7.3 and optimally above a pit of 7.5, at a sustained elevated temperature into a dry powder composition of spherically shaped RN agglomerated particles with the concentration of the organic binder in the slurry adjusted to at least above about 1.8 wt. % of the slurry to form a residue of organic binder and/or a decomposition layer from said organic binder on said particles for modifying the surface viscosity of the composition without degrading the physical properties attributable to boron nitrate such as high thermal conductivity. 
     Each BN particle in the composition of the present invention represents a composite agglomerate of non-spherical BN particles bound together by an organic binder in a generally spherical geometry. The diameter of each spherically shaped BN particle formed by the spray drying method of the present invention may vary in size over a relatively wide size distribution of sizes but may be controlled so that the majority of particles and up to about 98% of the BN particles have a minimum diameter above one micron and preferably a minimum diameter above about 5 microns. The size distribution of the BN particles may extend to a maximum diameter of about 275 microns. Although the size distribution is relatively wide the BN particles have an average size which falls into a much narrower size range between about 10 microns and 150 microns in diameter and can be adjusted to form an even narrower size range by adjustment of the physical parameters of the spray drying operation and/or the initial size of the non-spherical particles of BN in the slurry. Accordingly, the size of the spherical BN agglomerated particles formed in the spray drying process of the present invention can be controllably varied over of a preferred range of from as low as 1 micron in diameter to a preferred maximum diameter of about 75 microns so as to accommodate a variety of end uses. 
     The spherical shape of the BN particles formed in accordance with the present invention and the weight concentration of organic binder in the slurry controls the degree to which the particles flow and, in turn, the viscosity of the polymeric compound into which the particles are loaded. The ability to “flow” is an essential characteristic of the spray dried BN material when used as a low viscosity filler. The degree to which a material can “flow” is readily measurable as is well known to those skilled in the art. In contrast, a powder composition of conventional non-spherical BN is unable to flow and inhibits the flow characteristic of the filled polymer. The standard used in the present invention to establish the existence or non-existence of a flowable material is the ASTM B213-77 hall flow standard as is well known to those skilled in the art. In the present invention, it is essential to be able to load the BN spray dried particles into a polymeric compound at loading levels of above at least 30 wt. % BN and preferably between about 35 to 50 wt. % BN without increasing the viscosity of the blend above about 250 cp. The BN particles can be loaded into any polymer selected from the group consisting of a polyester, a polymide or an epoxy. 
     A low viscosity BN filled composition is formed in accordance with the method of the present invention comprising the steps of, forming an aqueous slurry composed of irregular non-spherically shaped BN particles, water, an organic binder and a base for maintaining the pH of the slurry at a pH above 7.3, adjusting the concentration of organic binder to a minimum level above about 1.8 wt. % of the slurry and preferably above about 2 wt. %; spray drying the aqueous slurry into a powder consisting of agglomerated BN particles of generally spherical shape and adding the powder as a filer into a polymeric compound at a loading level of between 30 to 50 wt. % BN. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiment when read in conjunction with the accompanying drawings: 
         FIG. 1  is a block diagram of a conventional spray drying apparatus for producing the agglomerated spherically shaped BN particles in accordance with the present invention; 
         FIG. 2  is a photomicrograph of the spherically shaped BN particles formed by the spray drying operation of the present invention at a magnification of 50×; 
         FIG. 3  is a typical graph of the particle size distribution of the collected BN particles from the spray drying operation of the present invention; and 
         FIG. 4  is a graph showing the relationship of viscosity at a given loading of spray dried BN filler particles in an organic binder relative to the weight percent of binder in the slurry forming the spray dried BN particles. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic block diagram of the spray drying apparatus used in the method of the present invention to form a powder composition of BN composite particles each of generally spherical shape. The spray drying apparatus  10  may consist of conventional equipment including an atomizer  2  and a source of air or an inert gas  3 , such as nitrogen, which forms an atomized spray of particles from an aqueous feed slurry  6  of water, a polymeric binder in the liquid phase and a base selected to maintain the pH of the slurry above a pH of 7.3 and preferably above a pH of 7.5. The atomized particle spray is preheated to a temperature in a range of 25° C.-360° C. preferably by preheating the nitrogen or air before injection at a desired feed rate into a spray drying chamber  1  with the outlet temperature between 110° C.-250° C. The BN particles in the feed slurry  6  preferably have a hexagonal crystalline structure although they may have a turbostratic structure. A dispersant, cross-linking agent and defoamer may also be included in the aqueous feed slurry  6  but are not essential. A polymerization initiator such as ammonium, sodium or potassium persulfate or other peroxide compound or other known polymerization initiator can be included to complete polymerization of the binder. 
     The particles formed in the spray drying chamber  1  are dried at an elevated temperature to a moisture level typically below 1% and collected. A cyclone  8  may be incorporated to remove superfine size particles before collection. The collected particles are solid particles having the same structure as the initial BN particles in the slurry  1  and will vary in diameter over a distribution range as shown in  FIG. 3  from a minimum diameter size of one about micron up to about 275 microns with a mean particle size which varies based upon the size of the non-spherical BN particles, the concentration of binder, and the selected spray drying parameters of operation such as slurry ratio, feed rate, gas pressure etc. The mean particle size for the distribution of particles in  FIG. 3  is about 55 microns but can be controllably adjusted. 
     In accordance with the present invention the powder BN product collected from the spray drying operation possesses particles which are essentially all of generally spherical geometry as evident from the photomicrograph of  FIGS. 2 and 3 . Each of the collected particles is a solid agglomerated particle formed of irregular non-spherical BN particles bound together by the organic binder in a spherical geometry. The high concentration of the organic binder in the slurry forms a coating over each of the recovered particles which at a concentration of over about 1.8 wt. % of the slurry varies the surface characteristic of the spray dried BN particles such that when added as a filler to a polymer selected from a polyester, epoxy or polyimide even under high loading levels at concentrations of between 30-50 wt. % BN, the flow characteristic of the filled polymer is not inhibited. In fact the viscosity of the filled polymer can be tailored to below about 250 cp. provided the concentration of organic binder is above about 2 wt. % of the slurry and optimally at about 2.5 wt. %. At a viscosity below about 250 cp. the filler polymer is easily dispensed through any conventional mechanical dispenser. 
     The organic binder is needed to bond the BN particles during spray drying and to modify its viscosity characteristic. The latter requirement limits the choice of organic binder to a water soluble acrylic or acetate which at high concentration has been found to function as a viscosity modifier. A preferably acrylic binder is formed from monoethylenically unsaturated acid free monomers comprising C 1 -C 4  alkyl esters of acrylic or methacrylic acids such as methly acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate and isobutyl methacrylate; hydroxylalkyl esters of acrylic methacrylic acids such as hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate; acrylamides and alkyl-substituted acrylamides including acrylamide, methacrylamide, N-tertiarybutylacrylamide, N-methacrylamide and N,N-dimethacrylamide, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate; acrylonitrile and methacrylonitrile. The monoethylenically unsaturated acid free monomer may include the acrylic monomer styrene so as to form a copolymer or may be formed solely from styrene. Preferred examples of acid free monomers include butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, acrylamide, methacrylamide, N-tertiarybutylacrylamide and styrene as a copolymerization agent. Acid containing monomers are less desirable but may equally be used. Such acid containing monomer may be selected from any carboxylic acid monomer preferably acrylic acid and methacrylic acid. 
     Although any acetate may be used for the organic binder a metal acetate is preferred over a non-metal acetate. The preferred metal acetates include nickel acetate, aluminum acetate, titanium acetate and any transition metal oxide acetate such as zinc acetate. Ammonium acetate is less desirable but is an acceptable non-metal acetate. The elevated drying temperatures used in the spray drying operation may cause the acetate to partially or entirely decompose to an hydroxide film on the surface of the BN agglomerated particles. The concentration of binder and any hydroxide decomposition layer formed on the agglomerated BN particles following spray drying should remain essentially at the same molar ratio as the corresponding weight ratio of binder to boron nitride in the slurry. Accordingly, for a concentration of at least 1.8 wt. % of binder in the slurry, the molar ratio of binder to boron nitride should be in a range of 0.00170-0.008 particularly for metal acetate binders. 
     The base can be selected from any suitable alkaline which will enable the pH of the slurry to be controllably maintained above a pH of 7 and preferably above 7.3. A preferred base is an hydroxide such as ammonium hydroxide or an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide or a methyl or ethyl ammonium hydroxide. 
     The following are examples of four ceramic slurries spray dried in accordance with the present invention to substantiate the production of spherical BN particles from a feed slurry of non-spherical irregular shaped BN particles. The four slurries consisted of conventional non-spherical BN powder in water with feed solids ranging from 32% to 49%. The pH of each slurry sample varied between 7.75 and 8.5. The pH was controlled by the addition of an ammonium hydroxide buffer at a concentration of less than 0.5 wt %. The binder selected for each example was “Efla” 4550 in a concentration of between about 1 to 5 wt %. “Efka” is a registered trademark of the Lubrizol Corporation and is a polyacrylate. A resin dispersant Efka 1501 which is a polyester fatty acid from Lubrizol at a concentration of about 0.25-2.00% was also added. Alternate binders which were found to be just as effective include “DURAMAX” 1022, “DURAMAX” 1020 and “DURAMAX”3007. “DURAMAX” is a registered trademark of the Rohm and Haas Company in Philadelphia Pa. “DURAMAX” 1020 and “DURAMAX” 1022 are styrene/acrylic copolymers formed from acrylic monomers. It was not necessary to add any resin dispersant. However, a buffer such as ammonium hydroxide used to adjust the pH of the non-spherical BN particle aqueous slurry to above 7.3 was essential. 
     The following four tables contain all of the process conditions of the spray drying operation: 
     
       
         
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 BN SLURRY FEED PROPERTIES 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Feed Number 
                 1 
                 2 
                 3 
                 4 
               
             
          
           
               
                 Feed Material 
                 Boron nitride (BN) shiny in Water 
               
               
                 Percent EFKA binder 
                 0.25-4.0  
               
             
          
           
               
                 Run Number 
                 1 
                 2 
                 3 
                 4 
               
             
          
           
               
                 Solids, %* 
                 32.7-49.0 
               
             
          
           
               
                 Temperature, ° C. 
                 19 
                 23 
                 19 
                 19 
               
               
                 Density, g/cm 3   
                 1.292 
                 1.195 
                 1.194 
                 0.963 
               
               
                 pH 
                 7.75 
                 8.44 
                 8.38 
                 7.77 
               
               
                 Viscosity, Avg. CP 
                 104 
                 706 
                 806 
                 3006 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 BN TEST DATA PROPERTIES 
               
             
          
           
               
                 PRODUCT: 
                   
                   
               
               
                 Run no 
                 1 
                 2 
               
             
          
           
               
                 Sample location 
                 Chamber 
                 Chamber 
                 Cyclone 
               
               
                   
               
               
                 Sample time 
                 10:40 
                 11:10 
                 12:15 
               
               
                 Sample weight, g 
                 195.8 
                 210.3 
                 199.5 
               
               
                 Total weight, kg 
                 0.59 
                 7.17 
                 9.48 
               
               
                 Total residual moisture, % 2   
                 0.30 
                 0.50 
                 0.35 
               
               
                 Bulk density, g/cc 
                 0.551 
                 0.562 
                 0.521 
               
               
                 Tapped density, g/cc 
                 0.621 
                 0.631 
                 0.611 
               
               
                 Particle size, microns, 
                   
                   
                   
               
               
                 10% less than 
                 27.02 
                 30.09 
                 11.00 
               
               
                 50% less than (median size) 
                 94.07 
                 112.13 
                 23.31 
               
               
                 90% less than 
                 189.84 
                 180.28 
                 87.87 
               
             
          
           
               
                 Chamber-to-cyclone ratio (kg/kg) 
                 0.82 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 BN TEST DATE PROPERTIES 
               
             
          
           
               
                 PRODUCT: 
                   
               
               
                 Run no. 
                 2B 
               
             
          
           
               
                 Sample location 
                 Chamber 
                 Cyclone 
               
               
                   
               
             
          
           
               
                 Sample weight, g 
                   
                 132.6 
                 74.9 
               
               
                 Total weight, kg 
                   
                 5.26 
                 5.33 
               
               
                 Total residual moisture, % 
                   
                 0.45 
                 0.79 
               
               
                 Bulk density, g/cc 
                   
                   
                   
               
               
                 Particle size, microns 
                   
                   
                   
               
               
                 10% less than 
                   
                 23.49 
                 15.09 
               
               
                 50% less than (median size) 
                   
                 55.03 
                 25.42 
               
               
                 90% less than 
                   
                 142.60 
                 43.90 
               
             
          
           
               
                 Chamber-to-cyclone ration (Kg/Kg) 
                 2.15 
                 0.99 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 BN TEST DATA PROPERTIES 
               
             
          
           
               
                 Run no. 
                 3 
                 4 
               
             
          
           
               
                 Sample location 
                 Chamber 
                 Cyclone 
                 Chamber 
                 Cyclone 
               
               
                   
               
             
          
           
               
                 Sample weight, g 
                 141.6 
                 85.2 
                 110.5 
                 NA 
               
               
                 Total weight, kg 
                 3.04 
                 6.24 
                 1.13 
                 1.50 
               
               
                 Total residual moisture, % 
                 0.59 
                 0.43 
                 0.41 
                 0.39 
               
               
                 Bulk density, g/cc 
                 0.331 
                 0.221 
                 0.305 
                 0.273 
               
               
                 Tapped density, g/cc 
                 0.409 
                 0.287 
                 0.382 
                 0.342 
               
               
                 Particle size, microns 
                   
                   
                   
                   
               
               
                 10% less than 
                 10.83 
                 8.46 
                 7.22 
                 6.91 
               
               
                 50% less than (median size) 
                 25.85 
                 14.59 
                 14.69 
                 12.87 
               
               
                 90% less than 
                 102.38 
                 21.66 
                 25.89 
                 20.37 
               
             
          
           
               
                 Chamber-to-cyclone ration 
                 0.49 
                 0.75 
               
               
                 (kg/kg) 
               
               
                   
               
             
          
         
       
     
     The following is another example for forming spray dried boron nitride particles in accordance with the present invention. In this example aluminum acetate is used as the organic binder and bismaliamide is used as the polymer. 
     A boron nitride powder PT120 (Lot 5151) was used to demonstrate the effect of surface modification on the viscosity of the BN filled resin. A conventional thermosetting resin, bismaliamide (BMI), from Quantum Chemical was used as the polymer into which non-spherical BN particles were loaded. PT 120 is a conventional boron nitride powder with a platelet morphology. The physical and chemical characteristics are shown in the following Tables 5A-5C. 
     The PT120 filled resin was spray dried using a laboratory scale spray dryer, Mobile Minor Hi Tec, made by Niro Atomizer. 
     A slurry was prepared by mixing boron nitrate in de-ionized water using a high intensity mixer. Aluminum acetate-dibasic was added to the slurry and the slurry was mixed. After stabilizing the slurry, spray dying was initiated. The slurry composition for three separate runs is described in Table 5A. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5A 
               
               
                   
               
               
                   
                 Run No. 
                 5D7 
                 SD8 
                 SD9 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Water (gm 
                 3000 
                 3500 
                 3000 
               
               
                   
                 Boron Nitride (gm) 
                 1050 
                 1225 
                 1050 
               
               
                   
                 Aluminum Acetate- 
                 52 
                 91.88 
                 26.25 
               
               
                   
                 Dibasic (gm) 
                   
                   
                   
               
               
                   
                 BN/Water (wt. %) 
                 35 
                 35 
                 35 
               
               
                   
                 Al. Acet./BN (wt. %) 
                 5 
                 7.5 
                 2.5 
               
               
                   
               
             
          
         
       
     
     After the slurry was prepared, it was kept agitated by the mixer. The slurry was then pumped into the feed section of the spray dryer by using a peristalic pump. The spray dryer was operated with its fan on, inlet temperature in the range of 250° C.-270° C. The outlet temperature was in the range of 110° C. to 150° C. Air flow was set in the range of 17 to 22 on the gauge. Boron nitride feed rate (powder basis) was 1016, 1050 and 841 gm/hr for SD7, SD8 and SD9 respectively. Powders were collected from chamber and cyclone and then tested for their rheological properties. 
     Rheological Testing: 
     Powders were mixed with the BMI resin alone at 37.4 wt. % loading level to form a baseline. About 30 gm. of resin was used in each case. After careful mixing in a cup, it was placed in a vacuum chamber for removal of trapped air. After evacuating for a few hours, it was carefully mixed and then placed into evacuation chamber again. Once air bubbles stopped rising to the surface, the cup was removed. The resultant paste was gently stirred and placed in a water-cooled bath for equilibrating to 25° C. After it reached a constant temperature of 25° C., viscosity was measured by Brookfield rheometer DVII using spindle no. 96. Viscosity was measured at various speeds but the measurements taken at 5 rpm ware used for comparison. Measurements were taken after at least 5 minutes from the start of the rotation to obtain steady state value. 
     The results of viscosity tests and analytical data are given in Table 5B and 5C for powders collected from chamber and cyclone respectively. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5B 
               
               
                   
               
               
                   
                 PT120 - 
                   
                   
                   
               
               
                   
                 Baseline 
                 SD7 - Chamber 
                 SD8 - Chamber 
                 SD9 - Chamber 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 % Oxygen 
                 0.371 
                 5.17 
                 5.71 
                   
               
               
                 % Carbon 
                 0.021 
                 0.58 
                 0.84 
                   
               
               
                 Surface Area 
                 2.97 
                 4.5 
                 7.91 
                 8.68 
               
               
                 MicroTrac Size 
                   
                   
                   
                   
               
               
                 D-10 (Microns) 
                 6.15 
                   
                   
                   
               
               
                 D-50 (Microns 
                 12.32 
                   
                   
                   
               
               
                 D-90 (Microns 
                 21.71 
                   
                   
                   
               
               
                 Shape 
                 Plate 
                 Spheroidal 
                 Spheroidal 
                 Spheroidal 
               
               
                 Agglomerage Size 
                   
                 70-150 
                 70-150 
                 70-150 
               
               
                 microns 
                   
                   
                   
                   
               
               
                 Viscosity @5 RPM 
                 400,000 
                 141,000 
                 74,000 
                 242,000 
               
               
                 (cps) 
                   
                   
                   
                   
               
               
                 Comments 
                 No aluminum 
                 Increased surface 
                 Increased surface 
                 Increased surface 
               
               
                   
                 acetate - no 
                 area due to 
                 area due to 
                 area due to 
               
               
                   
                 sphericalization 
                 coating 
                 coating 
                 coating 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 5C 
               
               
                   
               
               
                   
                 PT120 - 
                 SD7 - 
                 SD8 - 
                 SD9 - 
               
               
                   
                 Baseline 
                 Chamber 
                 Chamber 
                 Chamber 
               
               
                   
               
             
             
               
                 Agglomerage Size 
                   
                 10-50 
                 10-50 
                 10-50 
               
               
                 (microns) 
                   
                   
                   
                   
               
               
                 Viscosity @5 RPM 
                 400,000 
                 258,000 
                 216,000 
                 402,000 
               
               
                 (cps)