Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/12—Powdering or granulating
  • C08J3/122—Pulverisation by spraying
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/26—Nozzle-type reactors, i.e. the distribution of the initial reactants within the reactor is effected by their introduction or injection through nozzles
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/0008—Organic ingredients according to more than one of the "one dot" groups of C08K5/01 - C08K5/59
  • C08K5/0016—Plasticisers
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L79/00—Compositions of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing nitrogen with or without oxygen or carbon only, not provided for in groups C08L61/00 - C08L77/00
  • C08L79/04—Polycondensates having nitrogen-containing heterocyclic rings in the main chain; Polyhydrazides; Polyamide acids or similar polyimide precursors
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/00049—Controlling or regulating processes
  • B01J2219/00184—Controlling or regulating processes controlling the weight of reactants in the reactor vessel
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2379/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing nitrogen with or without oxygen, or carbon only, not provided for in groups C08J2361/00 - C08J2377/00
  • C08J2379/04—Polycondensates having nitrogen-containing heterocyclic rings in the main chain; Polyhydrazides; Polyamide acids or similar polyimide precursors

Definitions

• the present invention relates to processes for forming polymeric compactible compositions suitable for sintering.
• the powders employed in accordance with the present invention are preferably those containing a polybenzimidazole (PBI) resin prepared by atomization/quenching as hereinafter described.
• PBI polybenzimidazole
• a desirable group of polymers are those that retain excellent mechanical properties at high temperatures.
• polymers in this group often melt at very high temperatures or decompose without melting.
• their viscosities in the melt phase are extremely high. Therefore, these polymers are considered to be intractable,
• nylons of hexamethylene diamine and terephthalic acid exhibit excellent temperature resistance but cannot be melt-spun or molded because they decompose before their crystalline melting temperatures are reached.
• many other wholly aromatic polymers such as polyimides of pyromellitic anyhydride and aromatic diamines cannot be melt-processed in polyamic acid or fully imidized form. Powder processing and sintering techniques have been used to process such intractible polymers into useable articles.
• polymeric particulate matter can be compacted into green bodies and sintered thereafter.
• Halldin studied the density of green bodies as a function of compaction pressure, and among other things, concluded that the strength of an unsintered body is determined by its density. Generally speaking, there is an upper limitation of compaction pressure where increasing the pressure will not increase the density of the object, i.e., plateau density. Halldin concluded that the compressibility of polymer powders can be best modeled by the equation:
• p* p 0 /p t
• p g green density, gm/cm 3
• p t theoretical density, gm/cm 3
• p* p relative plateau density
• p* a relative apparent density
• k compaction constant
• P compaction pressure.
• the compaction constant, k has units which are the reciprocal of the pressure units, for example, if P is expressed as Pa, k is expressed as Pa "1 or if P is expressed as psi, k is expressed as psi "1 .
• UHMWPE ultra high molecular weight polyethylene
• ASTM No. D-1895-89 The apparent density as measured by ASTM No. D-1895-89 is the density that the powder exhibits in its free flowing state.
• a composition including a polybenzimidazole containing (PBI) powder and a plasticizer exhibits superior compaction behavior.
• Preferred embodiments utilize a PBI powder having a plasticizer content of about 1 to about 25 weight percent such that the composition exhibits a cold compressibility characterized by a compaction constant, k, ranging from about 0.100 to about 0.500 MPa "1 , and an apparent density ranging from about 0.100 to about 0.300 gm/cm 3 .
• the preferred plasticizer is water
• the preferred PBI resin is poly- 2,2'-(m-phenlyene)-5,5'-bibenzimidazole.
• the PBI resin may be the sole resin employed in the powder or used in combination with another resin, for example, solution blended with a polyester or polyimide resin prior to powder preparation.
• FIG. 1 is a schematic view in elevation of an atomization reactor system utilized to form the highly porous particles of the invention
• FIG. 2 is a compressibility curve of atomized porous PBI disks containing moisture levels of 8.2, 10.3 and 14.8 weight percents;
• FIG. 3 is a plot illustrating the incremental pore volume versus pore radius of the particles useful in connection with this invention.
• FIG. 4 is a compressibility curve of atomized, porous PBI disks containing moisture levels of about 13.1 weight percent.
• Polymers useful in the highly porous powders of this invention may comprise any polybenzimidazole resins known to those skilled in the art. Typical polymers of this class and their preparation are more fully described in U.S. Patent No. 2,895,946; U.S. Patent No. Re. 26,065; and the Journal of Polymer Science, Vol. 50, pages 511-539 (1961), which are herein incorporated by reference. These polybenzimidazoles consist essentially of recurring units of the following Formulae I and II.
• R is a tetravalent aromatic nucleus, preferably symmetrically substituted, with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms, i.e., ortho carbon atoms of aromatic nucleus
• R' is selected from the group consisting of (1) an aromatic ring, (2) an alkylene group, and (3) a heterocyclic ring selected from the group consisting of (a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e) thiophene, and (f) pyran.
• Formula II is :
• Z is an aromatic nucleus having the nitrogen atoms forming the benzimidazole ring paired upon adjacent carbon atoms of the aromatic nucleus.
• aromatic polybenzimidazoles are selected from polymers consisting essentially of the recurring units of Formulae I and II, wherein R' is at least one aromatic ring of a heterocyclic ring.
• a preferred polybenzimidazole for use in the present invention is poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole, the recurring unit of which is:
• PBI-containing powders useful in connection with the present invention may be prepared by a variety of techniques, the atomization/quenching technique described below is particularly preferred.
• the first step of the process involves forming a solution or dope of polymeric resin dissolved in a suitable solvent.
• the polymeric solution utilized herein generally contains from about 1 to about 25 percent by weight of polymer solids. Typically, however, the solution will contain from about 5 to about 20 percent, by weight of polymer solids and preferably, from about 12 to about 15 percent by weight of polymer solids is utilized.
• the resins are generally selected from the group consisting of polybenzimidazoles, polybenzimidazolones, polybenzoxazoles, polyesters, polyimides, polyamides, polyamideimides, partial and wholly aromatic aramides, nylons of hexamethylene diamine and terephthalic acid, polyarylketone, polyarylsulfide resins and mixtures thereof.
• the resin solution will contain polybenzimidazole in combination with at least one of the aforementioned resins.
• Polymer component of the solution can be PBI alone or a blend of PBI with another polymer. Generally the PBI will be about 10 to about 100 weight % of the blend. Preferably, PBI will be about 20 to about 80 wt.
• the other polymer can be polybenzimidazolones, polybenzoxazoles, polyesters, polyimides, polyamides, polyamideimides, partial and wholly aromatic aramides, nylons of hexamethylene diamine and terephthalic acid, polyarylketones, polyarylsulfides.
• the solvents utilized to form the solution or dope include those solvents which are commonly recognized as being capable of dissolving the particular polymer resin being used.
• suitable solvents include N, N' -dimethylacetamide, N,N'-dimethyl formamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone.
• Additional representative solvents include formic acid, acetic acid, sulfuric acid, polyphosphonic acid and methanesulfonic acid.
• the preferred solvent is N,N'- dimethylacetamide (DMAc) having a concentration of from about 90 to 100 percent and preferably about 99 percent by weight.
• One suitable method for dissolving polybenzimidazoles is by mixing and heating the materials at a temperature above the normal boiling point of the solvent, for example, about 25° to about 120° C. above such boiling point, and at a pressure from about 2 to about 15 atmospheres for a period from about 1 to about 5 hours.
• Preferred conditions will usually comprise heating the mixture in a stainless steel reactor at a pressure of about 7 atmospheres (for high molecular weight polymers, low molecular weight polymers require less pressure) for about 2 hours at a temperature of about 235° C.
• the resulting solution is then filtered to remove any undissolved resin.
• a minor amount of lithium chloride e.g., about 2 percent by weight, can be added in the solution to prevent the polymeric resin from phasing out of the solution upon standing for extended periods of time.
• the non-solvent component of the present invention is any substance which can be atomized or formed into an aerosol in which the polymer is insoluble. Any component in which the polymer is insoluble is sufficient.
• the non-solvent is a C x to C 4 alcohol or water.
• the non-solvent component is water.
• the polybenzimidazole powder produced by the process of the present invention is capable of being compressed into shaped articles having an ASTM No. D-1895-89 apparent density ranging from about 0.100 to about 0.300 gm/cm 3 , a compaction constant ranging from about 0.100 to about 0.500 MPa "1 , and a plasticizer content ranging from about 1 to about 25 weight percent. Furthermore, the powder exhibits a porosity, as determined by mercury porosimetry (see Lowell & Shields, p. 217) ranging from about 0.5 to about 1.5 cmVgm, a surface area, as measured by N 2 adsorption in the standard BET technique (see Lowell & Shields, p.
• the reactor system (10) has a cylindrical vessel (12) with upstanding side walls (14) and a cone- shaped bottom (16) with an outlet (18) equipped with a particle separation system (20) .
• Typical reactor dimensions are about 36 inches in diameter by about 37 inches in height.
• a plurality of nozzles are contained within the reactor.
• the first set of nozzles (22) slidably attached to the top-center of the reactor, are utilized to atomize the resin solution downwardly into the reactor.
• a second set of nozzles (24) equaling about four times the quanity of the first nozzle set, separated into an equal number of rows (i.e., four), equally spaced and attached to the reactor side walls at positions well below the first nozzle set, are utilized to atomize the non- solvent component in a direction perpendicular to that of the polymer solution and horizontally into the reactor to create an aggregation zone.
• a recirculation pump (26) is positioned in the bottom of the reactor to maintain suspension of the rounded particles in the non-solvent solution prior to discharging through outlet (18) .
• the atomization nozzles manufactured by Spraying Systems of Wheaton, Illinois, produce a wide-angle rounded spray pattern using a No.
• nozzles 1 spray set-up containing Fluid Cap 2050 and Air Cap 64. Under normal operation at 60 psi the nozzles are capable of delivering 0.79 gallons of liquid per hour at a spray angle of about 18 degrees.
• the polymer solution is pumped from a reservoir through a filtering system (28) to remove undissolved resin before feeding it through the first inlet of the first set of nozzles (22) .
• a gas such as air or nitrogen is fed into a second inlet of each nozzle (22) as is typical of gas-operated atomization nozzles.
• Nozzles (22) are suspended along the upper-vertical axis of the reactor in such a way as to spray fine droplets of the resin solution down into the reactor.
• the non-solvent component e.g., water or alcohol
• a compressed gas such as air or nitrogen
• a polymer solution aerosol jet is discharged from the first set of nozzles (22) downwardly into the atomized atmosphere of the non- solvent component, which is discharging horizontally into the aggregate (i.e., reaction) zone (30) in the form of an aerosol jet discharged from the second set of nozzles (24) to form highly porous particles.
• An initial quantity of non-solvent component e.g., water, is accumulated in the reactor bottom to maintain suspension of the highly porous particles as they are formed, as well as to complete any residual precipitation from solution that may be necessary.
• the reactor is operated at atmospheric pressure, but other pressures are possible.
• the contents in the bottom of the reactor i.e., polymeric particles suspended in the non-solvent component
• the outlet of the reactor is typically equipped with a separation system
• the separation system can comprise any of several methods known in the art including vacuum, gravity and pressure filtrations or centrifugation.
• the particles can be collected on the filter and washed to remove residual solvent and non-solvent components, or the slurry can be centrifuged to produce a wet cake which can be washed and dried.
• the separated powder may be redispersed in water to remove additional residual solvent and separated again by any of the aforementioned means.
• the wet cake that results after filtration or centrifugation must be dried to a controlled or defined moisture content that assures adequate cold compactibility.
• wet cake powder can be dried to low non-solvent content ( ⁇ 7 wt%) and humidified to the desired moisture content (7-12 wt%) ; or 2) the wet cake powder can be dried directly to the desired moisture content (i.e., the non-solvent is water).
• size can be controlled by adjusting the viscosity of the polymer solution through control of nozzle temperature and/or polymer concentration in the solution and adjusting the amount of non-solvent component droplets in the atmosphere.
• the particle size can be changed by adjusting pressure on the nozzle, especially the polymer solution nozzles (22). Thereafter, the dried, free-flowing highly porous particles can be milled and screened if so desired to produce submicron particles which produce an even higher apparent density powder which compress into high bulk density disks.
• the polybenzimidazole powders produced by the method of this invention will generally exhibit a compaction constant, k, in accordance with Halldin's compressibility equation in the range of about 0.100 to about 0.500 MPa "1 .
• k a compaction constant
• powders characterized by higher values of k i.e., k approaching 1.0 MPa "1
• the compaction constant approaches 1.0 MPa "1
• less force and time are required to compress the powder into a dense shape.
• the powder when water is the plasticizer and polybenzimidazole is at least one of the resins the powder is humidified to a moisture content ranging from about 1 to about 25 weight percent water, typically, about 5 to about 20 weight percent water, and preferably, about 12 to about 15 weight percent water based on the total weight in preparation for the cold compaction process.
• water is the preferred plasticizer
• other components containing small molecules which are souble in PBI may be useful, e.g., formic acid, carbon dioxide, phosphoric acid, hydrogen cyanide, methylamine, hydroxylamine, phosphine, ammonia, sulfuric acid, dimethlyacetamide, hydrochloric acid, hydrogen sulfate and hydrogen selenide.
• plasticizers While many of these plasticizers are effective in improving cold compressibility, many of them are difficult to remove from the shaped polymeric article prior to sintering.
• a shaped article containing a volatile plasticizer can develop fractures when heated to quickly to sintering temperatures. If the plasticizer cannot diffuse out of the compressed part, it may volitilize as a gas pocket with sufficient pressure to create a fracture in the part.
• an amount of the polymeric powder can be shaped by pressing in a mold at pressures from about 3,000 to about 60,000 psi for dwell times ranging from about 1 second to about 10 minutes. After the predetermined dwell time has elapsed, the compaction pressure is released and the shaped article is removed from the mold. If the shaped article is to be sintered, any volatile plasticizer must be removed slowly. Moisture can be removed from shaped article by raising the temperature from about 80° to about 300° C. The time- temperature conditions of drying the shaped article must be done in a way to avoid generating defects (e.g., cracks, blisters, etc.) due to vaporization of volatiles in the article.
• defects e.g., cracks, blisters, etc.
• the article can be dried in a nitrogen purged oven to remove any unwanted plasticizer and finally sintered to a near net shape.
• the following examples are general illustrations of preparing highly porous PBI particles and cold compaction of shaped articles therefrom in accordance with the present invention; other polymeric particles can be produced using similar procedures. They are provided for purposes of exemplification only as should be appreciated from the foregoing discussion.
• FIG. 2 is a plot of bulk densities of the disk versus the pressure of powders produced according to this process having 8.2, 10.3 and 14.8 weight percent moisture. Disks having the higher moisture content will generally exhibit the higher densities at an applied pressures.
• FIG. 3 represents a plot of the incremental pore volume versus the pore radius of melt-derived and atomized PBI powder.
• the total porosity, measured by mercury intrusion analysis, of the atomized powders range from 0.5 to 2.0 cm 3 /gm of polybenzimidazole, however, the total porosity of melt-derived powder is typically only 0.02 cmVgm of polybenzimidazole.
• This internal porosity is believed to promote cold compaction by allowing the particles to crush and pack more efficiently.
• the cold compacted article has strength which is due in part to the enhanced mechanical interlocking that the crushed particles present compared to the uncrushed particles.
• the total pore volume of the atomized polybenzimidazole powder is 0.885 cm 3 /gm, and the total pore volume of the melt-derived powder is 0,0213 cm 3 /gm.
• FIG. 4 represents a compressibility curve for atomized polybenzimidazole powder produced according to this process which was compressed into disks; the water content of the powder was about 13.1 weight percent.
• the data points on the curve represent the relative densities of disks made at various applied pressures based upon a theoretical density.
• the compaction constant, k calculated according to a best fit curve of the Halldin equation was about 0.11 ksi "1 for PBI powder containing about 13.1 wt.% moisture made by the atomization process described herein.
• poly-2,2'-(m-phenylene)-5,5'- benzimidazole (PBI) resin was dissolved into DMAc to produce a 12 weight percent PBI solids solution.
• the PBI solution was fed through a pumping system to nozzles in the reactor at about 12.4 cmVmin and about 30 psi. Nitrogen gas at a pressure of about 15 psi was fed to the same nozzles to atomize the PBI solution. Quenching water
• BET surface area was measured to be approximately 37 m 2 /g.
• PBI-DMAc feed 24.8 cm 3 /min at 65 psi; Nitrogen feed (PBI-DMAc): 40 psi; Water feed: 600 cmVmin at 50 psi; and Nitrogen feed (water) : 10 psi.
• the powder was recovered by centrifuging without prescreening and washed in hot water followed by centrifuging three times. Finally, the powder was dried in a planetary mixer at 1 atmosphere of N 2 for 22 hours at 200°C.
• a representative characterization of the powder is as follows:
• Moisture content 11.5 - 12.7 wt.%
• DMAC content 0.5 wt.%
• LiCl content ⁇ 20 ppm
• BET Surface Area 59 m /gm Mean Particle Size: ⁇ 150 ⁇ m for 67.7% fraction
• Example II a 50/50 w/w of PB /Isaryl 25H (an aromatic polyester manufactured by Isonova Technische Innovationen G.m.b.H. of Austria) powders were dissolved in DMAc to form a solution containing 12 weight percent polymer solids.
• a 4.4 kg of the solution was atomized at a rate of 12.4 cm 3 /minute and a pressure of about 30 psi with the assistance of N 2 gas at a pressure of about 15 psi.
• the non-solvent, water was fed to the nozzle at a rate of about 1080 cmVmin and a pressure of about 25 psi.
• the atomized polymers in solution contacted the atomized water to form polymer particles which were filtered, washed and dried. This powder was dried, moisturized and pressed into disks. Bulk density results are reported below in Table II. The particles exhibited a porosity ranging from about 0.69 to about 1.41 cmVgm, and a surface area of about 81 m 2 /gm.
• Solutions containing dissolved PBI and Isaryl 25H in 1/1 weight ratio and containing total polymer solids contacts amounting to 12 and 15 weight percent has viscosities of 3 and 10 poise, respectively.
• Powders are most useful in compression molding-type operations if they exhibit, among other desirable characteristics, high apparent density. Powders with high apparent densities can be compressed to maximum density with smaller compression ratios, i.e., the ratio of powder height in a mold before compression to the height after compression.
• apparent densities were measured in accordance with ASTM D-1895-89; commonly referred to as the loose bulk density of a powder.
• Polymer powders produced by gas-assisted atomization have different apparent densities depending on 1) the dope nozzle orientation with respect to the quench fluid, and 2) whether or not the quench fluid is atomized.
• High apparent densities result from the process of this invention if both the polymer solution and the quench fluid are atomized and caused to mix while both are suspended as droplets in air (or other gas) so that polymer precipitation occurs in this state of suspension. If the atomized polymer spray is quenched by directing it into a static quench fluid layer, a lower apparent density will result.

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• Organic Chemistry (AREA)
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• Processes Of Treating Macromolecular Substances (AREA)
• Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)

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• 1991
  • 1991-12-19 US US07/810,663 patent/US5247010A/en not_active Expired - Lifetime
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