Polymerization Method for Preparing Conductive Polymer
20170283604 · 2017-10-05
Assignee
Inventors
- Antony P. Chacko (Simpsonville, SC, US)
- Yaru Shi (Simpsonville, SC, US)
- JOHN OLS (SIMPSONVILLE, SC, US)
Cpc classification
C08L65/00
CHEMISTRY; METALLURGY
H01B1/127
ELECTRICITY
C08G2261/1424
CHEMISTRY; METALLURGY
International classification
Abstract
An improved process for preparing a conductive polymer dispersion is provided as is an improved method for making capacitors using the conductive polymer. The process includes providing a monomer solution and shearing the monomer solution with a rotor-stator mixing system comprising a perforated stator screen having perforations thereby forming droplets of said monomer. The droplets of monomer are then polymerized during shearing to form the conductive polymer dispersion.
Claims
1-37. (canceled)
38. A method for preparing a dispersion of conducting polymer comprising: providing a solution comprising a monomer and a polyanion; shearing said solution at a shear rate of at least 10,000 to no more than 800,000 sec.sup.−1 to form droplets; mixing said droplets with at least one oxidizing agent; polymerizing said monomer in said droplet to form said dispersion of polymer.
39. The method for preparing a dispersion of conducting polymer of claim 38 wherein said solution further comprises a steric stabilizer.
40. The method for preparing a dispersion of conducting polymer of claim 38 wherein said shear rate is at least 40,000 to no more than 75,000 sec.sup.−1,
41. The method for preparing a dispersion of conducting polymer of claim 38 where said shearing is by a rotor stator.
42. The method for preparing a dispersion of conducting polymer of claim 38 where rotor stator is selected from the group consisting of single stage batch mixer, single stage inline mixer, multiple stage and multi rowed rotor stator mixers.
43. The method for preparing a dispersion of conducting polymer of claim 38 wherein said solution has a solids content of at least 1.0 wt % solid to no more than 5 wt % solid.
44. A capacitor made by the process of: providing an anode with a dielectric thereon; forming a dispersion of conducting polymer by the method of claim 29; applying said dispersion of conducting polymer to said anode thereby forming a layer over said dielectric; and drying said layer.
45. The capacitor of claim 44 wherein said process comprises providing said monomer with an anion.
46. The capacitor of claim 44 wherein said process comprises providing said monomer with a steric stabilizer.
47. A method of forming a capacitor comprising: providing a solution comprising a monomer and a polyanion; shearing said solution at a shear rate of 10,000 to 800,000 sec.sup.−1 to form droplets; mixing said droplets with at least one oxidizing agent polymerizing said droplets of said monomer to form a polymer; providing an anode with a dielectric over said anode; and forming a cathode of said polymer on said dielectric.
48. The method of forming a capacitor of claim 47 wherein said forming said cathode is after said forming said polymer.
49. The method of forming a capacitor of claim 47 wherein said solution further comprises a steric stabilizer.
Description
BRIEF DESCRIPTION OF FIGURES
[0027]
[0028]
[0029]
[0030]
[0031]
[0032]
[0033]
[0034]
[0035]
[0036]
DESCRIPTION
[0037] The present invention is related to an improved polymerization method for conductive polymer dispersions. More particularly, the present invention is related to the use of perforated stator screens preferably with equivalent hole diameters of at least 0.25 mm no more than about 6 mm, more preferably no more than about 3 mm an even more preferably no more than about 2 mm and even more preferably no more than about 1 mm to control the monomer droplet size during polymerization. The perforations preferably have minimal dimensions of at least 1.0 mm to no more than 3.0 mm and more preferably at least 1.2 mm to no more than 2.5 mm.
[0038] The improved polymerization method eliminates the need for post polymerization homogenization steps. The improved polymerization method using the preferred stator screens provides a uniform droplet size resulting in average polymer particle sizes below about 200 nm, more preferably 150 nm and even more preferably below about 100 nm.
[0039] It has now been found that, surprisingly, conductive polymer dispersions having a lower, and controllable, particle size can be prepared during polymerization, without additional process steps, when the polymerization is carried out using a rotor stator mixing system with perforated screen stators preferably with hole diameters below about 6 mm. The dispersion may further comprise at least one polyanion.
[0040] The invention will be described with reference to the various figures which are an integral, non-limiting, component of the disclosure. Throughout the various figures similar elements will be numbered accordingly.
[0041]
[0042]
Tip speed=π×D×N
wherein:
π is a known constant which is the ratio of a circle's circumference to its diameter;
D is the equivalent diameter of the rotor.
N is the rotation rate of the mixer. As illustrated in
[0043] Shear rate is defined herein as the tip speed/rotor stator gap. By way of example, for a rotor diameter of 1.25 inches and rotational speed of 6000 RPM, tip speed is 42 ft/meter. With rotor gap of 0.009 inches, shear rate is calculated as 51,000 sec.sup.−1. The shear rate is preferably at least about 10,000 to 800,000 sec.sup.−1 and more preferably at least 40,000 to 75,000 sec.sup.−1.
[0044] Rotor/stator mixers are comprised of a rotor turning at high speeds within a stationary stator. As the blades rotate, materials are continuously drawn into one end of the mixing head and expelled at high velocity through the openings of the stator. The resulting hydraulic shear reduces the size of suspended droplets. Inline high shear mixers are used in an inline configuration wherein they behave like a centrifugal pump. The basic single-stage inline high shear mixer consists of a four-blade rotor that turns at high speeds within a stationary stator. Rotor tip speeds between 3,000 to 4,000 ft/min are typical. Rotor/stator mixers are offered with a variety of interchangeable stator designs.
[0045] The “multi-stage” rotor/stator consists of 2-4 rotor/stator pairs nested concentrically, mix material moves outward from the center of the multi-stage unit, and it is subjected to a quick succession of shearing events. Example of the multistage rotor/stator mixer are the ultrahigh shear rate mixers. The X-Series head from Charles Ross and Sons and exemplified in U.S. Pat. No. 5,632,596, consists of concentric rows of intermeshing teeth. The droplets enter at the center of the stator and move outward through radial channels in the rotor/stator teeth. The combination of extremely close tolerances and very high tip speeds (11,300 fpm or higher) subjects the droplets to intense shear in every pass through the rotor/stator. The gap between adjacent surfaces of the rotor and stator are adjustable from 0.010″ to 0.180″ for very high shear rates such as 750,000 sec.sup.−1.
[0046] The MegaShear head, exemplified in U.S. Pat. No. 6,241,472, is capable of the highest peak shear and throughput levels. It consists of parallel semi-cylindrical grooves in the rotor and stator towards which product is forced by high velocity pumping vanes. Different streams are induced within the grooves which collide at high frequency before exiting the mix chamber.
[0047] Such high shear batch, inline, single stage, and multistage rotor-stator mixers are available from various vendors including Charles Ross & sons, Silverson, etc.
[0048] The creation of a small particle sizes during polymerization involves generating small droplets of monomer through a combination of mechanical energy using a rotor-stator mixing system to manipulate the droplet size with an appropriate choice of perforated stator screens, with specific holes having specific equivalent diameters. It is preferable to stabilize the resulting droplets with surfactant. In conventional polymerization the monomer droplets are large which limits the particle size of the polymer. The instant invention includes a mixing system in which the mixer produces intense hydraulic shear wherein the monomer droplets are forced through perforations in the stator screen thereby reducing the monomer droplets into very small droplet sizes. The very small monomer droplets are stabilized by polyanions and the polymerization is believed to be initiated around the monomer droplet wherein the droplet size during polymerization is correlated to the polymer particle size.
[0049] Another advantage of this process is that conductive polymer dispersions with higher viscosity can be prepared effectively with the method of this invention. This is advantageous due to elimination of the step of evaporation typically required to achieve a viscosity suitable for forming a conductive coating as the cathode of a solid electrolytic capacitor.
[0050] When the polymer dispersion is to be used to form the cathode of a capacitor the viscosity of a polymer dispersion must be sufficiently high to adequately cover the corners and edges of the underlying dielectric. Unfortunately, conventional polymerization techniques are more effective in producing low viscosity dispersions. It has therefore been standard practice to form the polymer at a lower viscosity followed by evaporation to achieve a suitable viscosity. The present invention avoids this by allowing for polymer dispersion formation at higher viscosities.
[0051] The viscosities of the polymer dispersion is preferably at least 200 cP @20 RPM to no more than 4000 cP @20 RPM at ambient temperature and preferably at least 2000 to 4000 cP @20 RPM at ambient temperature.
[0052] The polymer dispersion preferably has a percent solids of at least 1.0 wt % to insure adequate viscosity. More preferably, the polymer dispersion has a percent solids of at least 1.3 wt % to no more than 5 wt %. Above about 5 wt % the dispersion does not flow adequately for forming a conductive layer. More preferably, the polymer dispersion has a percent solids of at least 1.7 wt % to no more than 3.5 wt %.
[0053] The dispersion, and polymerization preferably occurs at a temperature of at least about 15° C. to no more than about 35° C. Below a temperature of about 15° C. the polymerization rate is very slow and above about 35° C. conductivity and viscosity can be adversely affected.
[0054] Though not limited thereto, the present invention is particularly suitable for use in forming conductive polymers of polyanilines, polypyrroles and polythiophenes each of which may be substituted. A particularly suitable monomer for demonstration of the teachings herein is 3,4-ethylenedioxythiophene wherein beta-site polymerization is blocked.
[0055] A criteria for polymeric steric stabilizers for ICP dispersion polymerization is that they must be stable during low pH polymerization conditions, stable to oxidizing agents, and that they do not interfere with polymerization of the monomer. An exemplary steric stabilizer is a high molecular weight polyethylene oxide and their copolymers which are preferred as the steric stabilizer due to their stability in low pH reaction conditions. Another exemplary steric stabilizer is polydimethyl siloxane-polyethylene oxide (PDMS-PEO) block copolymer. An advantage of the PDMS-PEO copolymer is that the PDMS block could provide moisture resistance in addition to steric stabilization.
[0056] PEDT:PSSA dispersions can be further stabilized by polymeric steric stabilizers during the polymerization. Coagulation or gel formation is significantly reduced due to the insensitivity of the sterically stabilized system to the fluctuations and increases in electrolyte concentration. In addition, high solids dispersions can be produced by this method due to the higher stabilizing effect of steric stabilizers.
[0057] The polystyrene sulfonic acid has a preferred molecular weight of at least about 500 to no more than about 500,000. Below a molecular weight of about 500 film integrity can be affected and above a molecular weight of about 500,000 conductivity and viscosity can be adversely affected.
[0058] Particularly preferred polymeric steric stabilizers comprise linking groups which, upon formation of a coated layer, crosslink thereby providing an interlinked matrix which functions as a binder thereby providing a coated layer with a suitable structural integrity. Steric stabilzers with a reactive functionality can be employed for post polymerization crosslinking with polyanion. Any reactive steric stabilizer with a reactive functionality which is stable during the polymerization reaction can be used. Examples of such reactive stabilizers are hydroxyl and dihydroxy end capped polybutadiene. Precursors of reactive steric stabilizer can also be employed for post polymerization activation of the steric stabilizer reactive group.
[0059] As used herein, the terminology “steric stabilizer” refers to compounds which are adsorbed to the polymer particles of the dispersion and protective layers around the respective particles to prevent agglomeration of the particles.
[0060] Suitable steric stabilizers include, for example, protective colloids and nonionic surfactants having a hydrophilic/lipophilic balance (HLB) greater than about 10. Hydrophilic/lipophilic balance is a measure of the degree to which a material is hydrophilic or lipophilic.
[0061] For the purposes of the present invention the Griffin's method is used for determining the hydrophilic/lipophilic balance wherein HLB is defined as:
HLB=20*Mh/M
wherein:
Mh is the molecular mass of the hydrophilic portion of the molecule and M is the molecular mass of the molecule. An HLB value of greater than about 10 is a water soluble, lipid insoluble, molecule.
[0062] Suitable protective colloids include polyethylene oxide, fully hydrolyzed polyvinyl alcohol, partially hydrolyzed poly(vinyl alcohol), poly(vinyl pyrollidone), hydroxyethyl cellulose, polyethylene oxide copolymers and their derivatives,and mixtures thereof. Polyethylene oxide is preferred. Suitable nonionic surfactants include ethoxylated alkyl phenols, ethoxylated acetylenic diols, polyethylene oxide-propylene oxide block copolymers as well as mixtures thereof. Steric stabilizers are preferably added to the polymerization reaction as solutions in water or other polar solvents such as dimethyl sulfoxide, ethylene glycol, N-methyl pyrrolidone, etc.
[0063] Another advantage of the use of stator rotors during polymerization is that no post polymerization binder addition is necessary since the polymeric steric stabilizers provide adequate coating performance. Coatings prepared from conventional ICP dispersions prepared by conventional methods without binders tend to crack primarily due to the presence of agglomerated particles. Agglomeration of the particles is significantly reduced by the use of steric stabilizers in the polymerization reaction. The absence of post polymerization binder also eliminates the presence of unwanted electrolytes or ionics, as part of the binders, in the final formulation. Post polymerization binder addition in prior art dispersions may not produce an intimately mixed formulation which could affect percolation and thus equivalent series resistance (ESR) if used as the cathode in a capacitor. In contrast, polymerization with polymeric steric stabilizers results in a molecularly mixed miscible formulation.
[0064] Purification of the polymer by ion exchange is preferred resulting in a final formulation with insignificant amounts of mobile ionic species. Another advantage is that the final formulation is produced in one step.
[0065] The mixing is preferably done in an inert atmosphere with nitrogen being preferred due to cost considerations. A higher flow rate is preferred. While not limited to any theory, it is hypothesized that the inert atmosphere minimizes oxidation.
[0066] The stator rotor will be described with reference to
[0067] The process for forming a capacitor will be described with reference to
[0068] The anode material is not limited herein. A particularly preferred anode material is a metal and a particularly preferred metal is a valve metal or a conductive oxide of a valve metal. Particularly preferred anodes include niobium, aluminum, tantalum and NbO without limit thereto.
[0069] The dielectric is not particularly limited herein. A particularly preferred dielectric is an oxide of the anode due to manufacturing considerations.
[0070] Throughout the description the term “equivalent hole diameter” or “equivalent diameter” refers to a hole wherein the cross-sectional area is the same as that of a circle with the stated diameter.
EXAMPLES
Comparative Example 1
[0071] A 4 L plastic jar, provided with a cooling jacket, was initially charged with 125 g of polystyrene sulfonic acid (PSSA), 3375 g of DI water, 28.5 g of 1% iron(III) sulphate, and 21.5 g of sodium peroxodisulphate. The contents were mixed using a stirrer as shown in
[0072] The viscosity of the dispersion was measured using a Brookfield viscometer. The particle size was measured using a disk centrifuge particle size analyzer from CPS instruments. The conductivity was measured by coating the dispersion with 8% DMSO on a glass slide followed by drying at 150° C. and measuring the resistance by a four point probe.
Comparative Example 2
[0073] The experiment was repeated similar to Comparative Example 1 except that the stirrer used was a high shear impeller shown in
Inventive Example 1
[0074] A 4 L plastic jar, provided with a cooling jacket, was initially charged with 125 g of PSSA, 3375 g of DI water, 28.5 g of 1% iron(III) sulphate, and 21.5 g of sodium peroxodisulphate. The input percent solid was 1.37%. The contents were mixed using a rotor—stator mixing system with perforated stator screen with square holes with a size of 2.54 mm. Subsequently, 11.25 g of 3,4-ethylenedioxythiophene (PEDOT) was added dropwise. The reaction mixture was sheared continuously with a varying shear speed from 3000 RPM to 6000 RPM with the rotor-stator mixing system for an additional 23 hours. The dispersion was treated with cationic and anionic exchanger and filtered. The viscosity of the dispersion was measured using a Brookfield RVII viscometer using spindle number 3. The particle size was measured using a disk centrifuge particle size analyzer from CPS instruments with particle size reported as weight median herein. The conductivity was measured by coating the dispersion with 8% DMSO on a glass slide followed by drying at 150° C. and measuring the resistance by a four point probe. Inductively coupled plasma was used to determine the residual iron and sodium in the dispersion.
Inventive Example 2
[0075] The experiment was repeated similar to Inventive Example 1 except that the shear speed was fixed at 6000 RPM throughout the reaction.
Inventive Example 3
[0076] The experiment was repeated similar to Inventive Example 1 except that a perforated stator screen with circular holes with diameter of 1.6 mm was used.
Inventive Example 4
[0077] The experiment was repeated similar to Inventive Example 1 except that the percent input solid was changed from 1.3 to 1.79%
Inventive Example 5
[0078] The experiment was repeated similar to Inventive Example 1 except that the percent input solid was increased to 1.79%
Inventive Example 6
[0079] The experiment was repeated similar to Inventive Example 1 except that the percent input solid was changed from increased to 2.6%.
Inventive Example 7
[0080] The experiment was repeated similar to Inventive Example 1 except that the percent input solid was changed from increased to 3.2%
Inventive Example 8
[0081] The experiment was repeated similar to Inventive Example 1 except that the percent solids was increased to 2.3% and a steric stabilizer polyethylene oxide (40 g, 1%) was added to the reaction mixture.
[0082] The results for Comparative Examples 1 and 2 and Inventive Examples 1-5 are in Tables 1-4.
TABLE-US-00001 TABLE 1 Comparative examples Mean particle Viscosity, cP Process Shear (RPM) size (nm) @20 RPM Comparative Example 1 2100 488 85 Comparative Example 2 900 478 345
TABLE-US-00002 TABLE 2 Inventive examples - Control of particle size Rotational speed Mean (RPM) and Shear particle Viscosity, cP Process Rate (sec.sup.−1) size (nm) @20 RPM Inventive Example 1 3000-6000 ramped 237 320 (21,800-43,600) Inventive Example 2 6000 fixed 110 145 (43,600) Inventive Example 3 6000-10000 ramped 85 100 (43,600--72,600)
TABLE-US-00003 TABLE 3 Inventive examples - Higher solids and higher viscosity processability % input Shear rate during Viscosity, cP Conductivity Process solids polymerization @20 RPM (s/cm) Inventive 1.3 43600 sec.sup.−1 145 150 Example 2 Inventive 1.79 43600 sec.sup.−1 1560 180 Example 4 Inventive 2.6 43600 sec.sup.−1 3065 220 Example 5 Inventive 3.6 43600 sec−1 5890 270 Example 6
TABLE-US-00004 TABLE 4 Inventive examples - steric stabilizer processability Rotational speed (RPM) and Shear Percent Viscosity, cP Process Rate (sec.sup.−1) solid @20 RPM Inventive Example 2 6000 fixed 1.3 145 (43,600) Inventive Example 5 3000-6000 ramped 2.3 3700 (21,800-43,600)
[0083] Inventive Experiment 1 was repeated with stator screens having differing hole sizes to demonstrate the particle size achieved. The results are presented in Table 5.
TABLE-US-00005 TABLE 5 Effect of shear rate and stator screen diameter on the PEDT:PSSA particle size 6000 RPM + Stator screen 10000 RPM hole equivalent 3000 RPM 6000 RPM (1 hr) (43600- diameter (21800 sec.sup.−1) (43600 sec.sup.−1) 72600 sec.sup.−1) 9.5 mm 472 nm 270 nm 2.54 mm 318 nm 151 nm 72 nm 1.6 mm 166 nm 108 nm 80 nm
[0084] The inventive examples demonstrate that the particle size and the viscosity can be controlled by the intelligent choice of stator screens, shear speed, and reaction conditions. These examples also demonstrate that a particle size below about 150 nm, and more preferably below about 100 nm, can be prepared by proper choice of the stator screen diameter. Furthermore, these examples demonstrate that a higher solids dispersions with high viscosity can be prepared using this mixer system. It was also surprisingly found that the conductivity can be increased by increasing the input % solid. Prior art conductive polymerization conditions lack the capability of processing high solid dispersions due to high viscosity and thus the artisan has not been not able to discover the conductive enhancement obtained by increasing percent input solid
Inventive Example 9
[0085] A 4 L plastic jar, provided with a cooling jacket, was initially charged with 125 g of PSSA, 3375 g of DI water, 28.5 g of 1% iron(III) sulphate, and 21.5 g of sodium peroxodisulphate. Polyethylene oxide (40 gm, 10%) was used as a steric stabilizer in this example. The contents were mixed using a rotor—stator mixing system with a perforated stator screen with square holes with a size of 2.54 mm. Subsequently, 11.25 g of 3,4-ethylenedioxythiophene was added dropwise. The reaction mixture was continued to be sheared with a varying shear speed from 3000 RPM to 6000 RPM with the rotor-stator mixing system for additional 23 hours. Conductivity enhancing additives such as DMSO and other additives such as silane coupling agents can be added to the reaction mixture in the final hour of the reaction or optionally added after the ion exchange treatment. The dispersion was treated with a cationic and anionic exchanger and filtered. The conductive polymer dispersion thus prepared was directly coated on a solid electrolytic capacitor. For the prior art comparative examples, a commercial conductive polymer dispersion and a PEDT:PSSA dispersion prepared by prior art method followed by adding a binder, was compared with the inventive example. Equivalent Series Resistance (ESR) of the comparative and inventive parts are shown in Table 6.
TABLE-US-00006 TABLE 6 Comparative and inventive example from Example 9 Prior Art Present Clevios KV2 PEDOT:PSSA Invention Post With Post PEDOT:PSSA polymerization polymerization with steric Process binder binder addition stabilizer Pre RBI ESR (mohm) 42.8 38.78 NA (Batch 1) Pre RBI ESR (mohm) NA 45.8 42 (Batch 2) Pre RBI Leakage 43.8 5.44 NA (microamp) (Batch 1) Pre RBI Leakage NA 4.1 1.69 (microamp) (Batch 2)
Inventive Example 10
[0086] A 4 L plastic jar, provided with a cooling jacket, was initially charged with 125 g of PSSA, 3375 g of DI water, 28.5 g of 1% iron(III) sulphate, and 21.5 g of sodium peroxodisulphate. The contents were mixed using a rotor—stator mixing system with a perforated stator screen with square holes with a size of 2.54 mm or an equivalent circular area of 6.54 mm.sup.2. Subsequently, 11.25 g of 3,4-ethylenedioxythiophene was added dropwise. The reaction mixture was sheared continuously with a varying shear speed from 3000 RPM to 6000 RPM with the rotor-stator mixing system for an additional 23 hours. The dispersion was treated with a cationic and anionic exchanger and filtered. The conductive polymer dispersion thus prepared was divided into four groups. Group A was used as filtered for the final dispersion formulation. Group B was homogenized using a Silverson rotor-stator mixer at a shear rate of 51,000 sec.sup.−1 for 30 min. Group C was homogenized using a Branson ultrasound sonifier at 50% amplitude for 5 min. Group D was homogenized using the same sonifier at the same amplitude for 30 min. The conductive dispersions thus prepared using each method were mixed with conductive enhancers and additives separately and coated on a solid electrolytic 33 microfarad, 25V capacitor. Equivalent Series Resistance (ESR) of the parts from group A, B, C, D are shown in Table 7.
TABLE-US-00007 TABLE 7 Comparative and inventive example from Example 10 Example 10 ESR (miliohms) (group A-D) Details after encapsulation Group A No post polymerization 38.14 homogenization Group B Homogenized using 39.83 silverson @ 51,000 sec−1 Group C Homogenized using 51.81 Branson sonifier 50%, 5 min Group D Branson sonifier 50%, 59.56 30 min
[0087] It can be seen that the postpolymerization homogenization methods have higher ESR. It can be further seen that the homogenization by ultrasound sonication gave significantly higher ESR than the parts without homogenization or with rotor-stator high shear rate homogenization. These results also suggest that polymerization using a rotor-stator method gives lower ESR than polymerization in a ultrasound irradiation condition.
[0088] The invention has been described with reference to preferred embodiments without limit thereto. One of skill in the art would realize additional embodiments and alterations which are not specifically stated but which are within the scope of the invention as more specifically set forth in the claims appended hereto.