Improved two component polymerizable polyorganosiloxane compositions are described, particularly for use in making dental impressions, having improved tear strength and wettability. Improved tear strength results from inclusion of a quadri-functional polysiloxane having a vinyl content of 0.16 to 0.24 m-mole/g. Working time is maintained by including sufficient amounts of a retarder composition that delays onset of the vinyl polymerization. Wettability is improved by including a surfactant resulting in a surface contact angle with water at three minutes of less than 50.degree.. The surfactant chosen has an HLB of 8-11, such that the wetting contact angle is achieved within less than two minutes and remains wetting throughout the working time of the impression taking, substantially improving impression quality. A low viscosity impression material is provided and includes a base component and a catalyst component. Both components are siloxane-based materials.
In employing polyorganosiloxanes as dental impression materials, a number of difficulties have arisen. First of all, tear strength tends to be low. It is necessary, in effectively taking an impression, to be able to easily remove the impression, from the dentition without tearing, particularly at thin marginal areas, to preserve fine detail. In the past, fillers of various types have been added to improve tear strength. Such additions may result in some improvement, on the order of about 10%, but such improvements have proved inadequate.
Paradiso in WO 93/17654 describes improving tear strength by incorporating multi-functional, including quadri-functional, polysiloxane components into the impression material, to add increased cross-linking to the resulting cured impression material matrix, particularly along the length of the linear vinyl end-stopped polysiloxane principal component. The Paradiso composition comprises SiOH groups capped off with Me.sub.3 Si units that form pendants from the molecule. These pendants provide only mechanical or physical interlinking between the linear polysiloxane chains. This solution is deficient, being non-chemical and low in cross-linking density.
Voigt et al in EP 0 522 341 Al describes very short processing times of 35-45 seconds for forming dentition bite registration devices, utilizing a "QM" resin as a means of speeding and increasing cross-linking. These resins comprise as Q, the quadri-functional SiO.sub.4/2 and as M, building blocks such as monofunctional units R.sub.3 SiO.sub.1/2 wherein R is vinyl, methyl, ethyl or phenyl, or similar tri or bi-functional units. voigt notes that an elastomer with small elastic deformation having a higher tenacity and hardness results. However, such material lacks flexibility, having a low strain value, and is unsuitable for impression taking. The increased cross-linking rate of the QM resin also results in very limited processing times that are unsatisfactory.
The other major, well-known difficulties with polyorganosiloxane impression materials are caused by its inherent hydrophobic character. Such characteristics make reproduction of hard and soft oral tissue difficult since the oral cavity environment is wet and often contaminated with saliva or blood. The hydrophobicity of the impression material can result in loss of surface detail often at critical surfaces of the dentition.
A number of improvements of polyorganosiloxane impression materials focus upon adding a surfactant component to the dental impression material in order to reduce the hydrophobic nature of the polysiloxanes and make the composition more hydrophilic. Thus, Bryan et al in U.S. Pat. No. 4,657,959 describes adding an ethoxylated nonionic surface active agent containing siloxane or perfluoroalkyl solubilizing groups to achieve a three minute water contact angle below about 65.degree.. While surfactants including hydrocarbyl groups, for rendering the surfactant soluble or dispersible in silicone prepolymer, are mentioned, including ethyleneoxy groups, the results achieved appeared to be less than optimal.
As stated above, all silicone material are known to have highly hydrophobic properties. Therefore, these materials are usually not able to wet the surface of the teeth properly, especially under moist conditions. Hydrophilic properties can be achieved in a silicone with the addition of dipoler surfactants. These additives are usually not soluble in the silicone matrix, but rather form an emulsion together with the silicon system. The Theological properties of such materials are characterized by a typical non-Newtonian flow behavior with a high yield stress and a highly sheer stress-dependent viscosity. Non-dipolar surfactants include for example, polyether modified silicones, and do not build proper micelles. The resulting emulsions are therefore not stable and tend to separate. However, the resulting multiple phase systems have a high yield stress which avoids the flow on the tooth surface when no or low stress is applied. Hydrophilic silicones therefore usually have poor flow characteristics under low stress.
Conventional "light bodies" formed from a two component system are usually applied in one of two forms. The first is a handmixed form in the case when the two components have to be mixed by hand. Second is an automixed form when the two components have to be released through a static mixer out of a cartridge. In both cases, the mixablility of the two components are strongly influenced by the rheological properties of the individual components making up the light body. Especially in the most common automixed form, the force to release the material out of the cartridge is influenced by the yield stress of the pastes.
Because of the Theological sub-structures, most conventional hydrophilic silicones have high yield stresses. To take advantage of low forces for releasing the material, large static mixers have to be employed. This leads to a high rate of waste because much of the material often remains in the mixer. Therefore, it is desired to achieve a low yield stress of both the single paste component and the mix in order to minimize the force which is necessary to remove the paste from the cartridge.
With conventional light body formulations a high stress has to be applied to obtain a flow of the impression material into the sulcus and into the other details of the preparation. Low viscosity type materials ("light bodies") are therefore always used in combination with a high viscosity type material in the so called "putty/wash" technique or in the "double mix" technique. To improve the mixablility of the putties, the viscosity of these products must be low. Even in the case where machine mixed heavy bodies are used the stress for releasing is limited by the technical properties of the machine. Higher viscosities lead to longer release times. In cases where these so-called soft putties or heavy bodies are used together with low viscosity silicones, the high yield stress and the highly stressed viscosity of the light body causes problem because it is impossible to generate sufficient pressure by the unset soft putty or heavy body during the taking of the impression. Therefore, a flow into the details of the preparation is not guaranteed. This problem is even more evident when the low viscosity material has a high yield stress.
In addition, the hydrophilic components to improve the wetting properties of the silicone tend to create a new problem. This problem is a stability problem of the cross linking SiH-components against moisture because these functional groups are sensitive against hydrolysis reactions especially under basic conditions. Therefore, it is a preferred embodiment of the present invention to add a water absorbing inorganic filler such as calcium sulfate hemihydrate, anhydrous calcium sulfate, calcium cloride, and the like and adsorbing compounds such as zeoliths, molecular sieves and other similar adsorbing and absorbing compounds.
It is known in the art that a low viscosity can be obtained by the use of a short chain dimethylvinylsiyl terminated polydimethylsiloxane in combination with either a low filler content or no filler at all. These materials usually have a very low mechanical strength such as a low tear strength making them too weak for use as a dental impression material. In cases where the viscosity is too low, the material tends to drop from the teeth and the fillers separate after certain periods of time.
In sum, polyorganosiloxane impression materials still need improvement in viscosity, tear strength and wettability in order to provide improved use of these compositions for taking impressions of oral hard and soft tissues such that adequate working time, tear strength and wettability are provided.
SUMMARY OF THE INVENTION
The new polyvinylsiloxane impression materials are useful in low and high viscosity impression compositions to record hard and soft tissues in the mouth. The new impression material is a platinum-catalyzed, vinylpoly-siloxane material, preferably a two component polymerizable organosiloxane composition, one component including a catalyst for polymerization, comprising:
(a) a QM resin, containing vinyl groups;
(b) a linear vinyl terminated polydimethylsiloxane fluid, forming with said QM resin a dispersion having a vinyl content of about 0.16 to 0.24 m-mole/g;
(c) an organohydrogen polysiloxane for cross-linking said vinyl groups;
(d) an organoplatinum catalyst complex for accelerating polymerization of said components;
(e) a retarder component in sufficient amount for temporarily delaying the onset of said polymerization;
(f) a filler; and
(g) a surfactant that imparts wettability to said composition, wherein said composition surface contact angle with water is less than 50.degree. after three minutes.
Preferably, the dispersion of (a) and (b) has a viscosity of about 5,000-60,000 cps. The dispersion of (a) and (b) may comprise a plurality of dispersion components having desired viscosities and QM resin contents. Preferably, the QM resin-containing dispersions comprise a first dispersion component having a viscosity of about 5,000-7,000 cps and a second dispersion component having a viscosity of about 45,000-60,000 cps, said QM resin comprising about 20-25 weight % of each dispersion.
A preferred QM resin comprises a polyorganosiloxane comprising units of SiO.sub.4/2 and units of R.sup.1 R.sup.2.sub.2 SiO.sub.1/2 wherein R.sup.1 is unsaturated, preferably vinyl and R.sub.2 is alkyl, aryl, etc., such as methyl, ethyl, phenyl, etc. More preferably, the QM resin comprises the formula: ##STR1##
The retarder component of the composition is a low molecular weight, vinyl functional fluid that is a linear or cyclic polysiloxane in an amount of at least about 0.030 weight percent of said composition. Preferably, the retarder component comprises: a fluid 1,3-divinyl, tetramethyldisiloxane, in an amount of about 0.030 to 0.12 weight percent of said composition.
The filler component of the invention comprises about 15 to about 45 weight percent of said composition and preferably includes a filler mixture of about 20 to about 40 weight percent.
A key component of the composition of the invention is the surfactant for imparting wettability, preferably comprising an HLB of about 8-11 and a pH of about 6-8. A most preferred surfactant is a nonionic surfactant, nonylphenoxy poly(ethyleneoxy) ethanol having an HLB of about 10.8.
For compositions of the invention of relatively high viscosity, the composition includes an emulsifying plasticizer that imparts desired handling and flow properties to the catalyst complex, to match those of the second component, wherein a suitable composition for taking a dental impression may conveniently be formed. Preferably, the plasticizer comprises an alkylphthalate at about 0.5 to 2.0% by weight of said catalyst component and is, most preferably, octyl benzyl phthalate.
After polymerization, the compositions of the invention include a tear strength of 270-300 PSI (1.86-2.06 MPa) and a contact angle with water of less than about 50.degree. at three minutes. For the lower viscosity impression material of the invention, tear strength will be somewhat lower at about 200 PSI (1.38 MPa) which is still substantially improved over the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing Wetting Contact Angle, in degrees, as a function of Time, in minutes.
FIG. 2 is a graph showing Impression Material Viscosity as a function of Time, in minutes.
FIG. 3 is a graph showing the difference in the charcteristics of the base and catalyst components of the present invention.
FIG. 4 is a graph showing the thixotropy of a base paste according to the invention by a time sweep measured with an oscillation rheometer.
FIG. 5 is a graph representing the development of viscosity during the setting reaction of the material accorindg to the invention by a time sweep with an oscillation rheometer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polymerizable polysiloxane compositions of the instant invention comprise, in general: an organopolysiloxane having at least about two vinyl groups per molecule, further including, dispersed therein, a quadri-functional vinyl polysiloxane resin; an organohydrogen-polysiloxane having at least about two hydrogen atoms bonded to at least two silicone atoms per molecule; a catalyst for accelerating the addition of the silicone atoms bonded to the hydrogen atoms to the polysiloxane vinyl groups; a filler; a low molecular weight retarder composition for delaying onset of polymerization; and an emulsifying surfactant that imparts wettability to said impression material.
The composition of the invention is preferably divided into two components. A first component, which is conveniently referred to as a "Base Paste", contains the vinylorganopolysiloxanes dispersion, the organo-hydrogen-polysiloxane, a portion of the filler and the surfactant. The second component of this two-part composition is referred to as a "Catalyst Paste" and comprises a second portion of the vinyl polysiloxanes, together with the catalyst for accelerating the addition reaction, a scavenging agent for hydrogen released during polymerization and usually, additional quantities of fillers and pigments. Where high viscosity impression materials are desired, an emulsifying plasticizer may be added to the catalyst paste component such that the working viscosities of its two components are compatible and have desired flow characteristics.
A wide variety of organopolysiloxanes having at least about two vinyl groups per molecule are known for inclusion in the dental polysiloxane compositions of the invention to form the dispersion including a quadri-functional vinyl polysiloxane. Each of these materials may be included in greater or lesser degree in accordance with the practice of the instant invention. Preferred for use herein are linear vinyl terminated polydivinylsiloxanes preferably a divinyl polydimethylsiloxane. Such polymers are sold having varying average molecular weights with concomitant variations in viscosity. It is preferred that these materials be selected to have a viscosity appropriate for the conditions to be experienced by the resulting silicone material.
The dispersions of interest have a viscosity range of 5,000-60,000 cps. In practice, it is convenient to employ a blend of the dispersing polymers having differing viscosities and physical properties to provide compositions having a desired thixotropicity and viscosity.
The dispersions of interest are preferably formed in two viscosity ranges: (1) a first dispersion having a viscosity of about 5000-7000 cps; and (2) a second dispersion having a viscosity of about 45,000-65,000 cps. While it is convenient to provide polysiloxane oligomers for this purpose having methyl substituents, other substituents may also be included in the compositions in accordance with this invention. Thus, alkyl, aryl, halogen, and other substituents may be included in greater or lesser degree as part of the vinyl polysiloxanes which are useful. Those of ordinary skill in the art will be able to determine which polysiloxane materials are preferred for any particular utility from the foregoing considerations.
The quadri-functional polysiloxanes, designated and known in the art as QM resins, provide improved tear strength to the polymerized impression composition, by increasing its resulting polymerized crosslink density. As is known, the QM resin is made up of: Q units of quadri-functional SiO.sub.4/2 ; and M units, such as R.sup.1 R.sup.2.sub.2 SiO.sub.1/2 wherein R.sup.1 is unsaturated, preferably vinyl and R.sup.2 is alkyl, aryl or the like, such as methyl, ethyl or phenyl. In a preferred composition R.sup.1 is vinyl and both R.sup.2 are methyl. A most preferred composition is represented by the formula: ##STR2##
The QM resin provides a vinyl concentration in the dispersions with the vinyl-terminated polydivinylsiloxanes of at least about 0.16 m-mole/g. Preferably the vinyl concentration is 0.16-0.24 m-mole/g. The amount of QM resin is preferably about 20-25% by weight of the dispersion. Such dispersions are sold by Miles, Inc. of Pittsburg, Pennsylvania. Other QM resin formulations may be used, including those that are "neat" or dispersed in carriers other than the preferred fluid polydivinylsiloxane.
A key element of the invention is a retarder component that delays onset of polymerization of the QM resin/dispersion such that sufficient working times to employ the composition are provided. It functions, as it is consumed, to offset what would otherwise be a too rapid polymerization. The preferred retarder fluid in the preferred impression material of interest is 1,3 divinyltetramethyldisiloxane at a sufficient concentration level to perform its retarding functions, which is in at least about 0.03 weight percent of the composition, preferably within a range of about 0.03 to 0.12 weight percent. This preferred amount is in contrast with the lower amounts of 0.0015-0.020 weight percent typically employed in PVS systems to stabilize compositions. Other suitable retarders are any low molecular weight, vinyl functional material that would be initially consumed in the polymerization, to delay hardening suitably and as desired, including linear and cyclic polysiloxanes.
The organohydrogen-polysiloxanes useful in the practice of the present inventions are well-known to those of ordinary skill in the art. It is required only that polysiloxanes having hydrogen atoms directly bonded to silicone atoms be employed, and that they have suitable viscosities and other physical properties. Substituents in the molecules such as alkyl (especially methyl), aryl, halogen, and others may be employed as well. It is necessary only that such substituents not interfere with the platinum-catalyzed addition reaction. It is preferred that molecules be employed having at least two silicone-bonded hydrogen atoms per molecule. Polymethylhydrogensiloxane is preferred, having a viscosity range of about 35-45 cps.
The catalysts which are useful for catalyzing the reaction of the silicone atoms (bonded to hydrogen atoms) to the vinyl groups of the vinyl polysiloxane molecules are preferably based upon platinum. In this regard, it is preferred to employ a platinum compound such as chloroplatinic acid, preferably in admixture or complex with one or more vinyl materials, especially vinyl polysiloxanes. While such materials have been found to be preferred, other catalysts are also useful. Thus, platinum metal together with other noble metals including palladium, rhodium, and the like and their respective complexes and salts are also useful. In view of the toxicological acceptability of platinum, however, it is greatly to be preferred for dental use.
The compositions of the present invention also include a filler, preferably a mixture of hydrophobic fillers. A wide variety of inorganic, hydrophobic fillers may be employed such as silicas, aluminas, magnesias, titanias, inorganic salts, metallic oxides and glasses. It is preferred, however, that forms of silicone dioxides be employed. In accordance with the present invention, it has been found to be preferable to employ mixtures of silicone dioxides, including those derived form: crystalline silicone dioxide, such as pulverized quartz (4-6.mu.); amorphous silicone dioxides, such as a diatomaceous earth (4-7.mu.); and silanated fumed silica, such as Cab-o-Sil TS-530 (160-240 m.sup.2 /g), manufactured by Cabot Corporation. The sizes and surface areas of the foregoing materials are controlled to control the viscosity and thixotropicity of the resulting compositions. Some or all of the foregoing hydrophobic fillers may be superficially treated with one or more silanating or "keying" agents, as known to those of ordinary skill in the art. Such silanating may be accomplished through use of known halogenated silanes or silazides. The fillers are present, preferably, in amounts of from about 15 to about 45 weight percent of the composition, forming an impression composition that is polymer rich and, thus, having improved flow properties. The fillers, more preferably, are about 35-40 weight percent of the composition. A preferred filler mixture for a higher viscosity formulation includes 14-24 weight percent crystalline silicone dioxide, 3-6 weight percent amorphous silicone dioxide and 4-8 weight percent silanated fumed silicone dioxide. A most preferred filler is about 19% cristobalite at about 4-6.mu.particle diameter, about 4% diatomaceous earth at about 4-7.mu.particle diameter and about 6% silanated fumed silica at about 160-240 m.sup.2 /g.
A chemical system may be employed to diminish the presence or degree of hydrogen outgassing which may be typically generated as a result of the vinyl polymerization. The composition thus may comprise a finely divided platinum metal that scavenges for and takes up such hydrogen. The Pt metal may be deposited upon a substantially insoluble salt having a surface area of between about 0.1 and 40m.sup.2 /g. Suitable salts are barium sulphate, barium carbonate and calcium carbonate of suitable particle sizes. Other substrates include diatomaceous earth, activated alumina, activated carbon and others. The inorganic salts are especially preferred to lend improved stability to the resulting materials incorporating them. Dispersed upon the salts is about 0.2 to 2 parts per million of platinum metal, based upon the weight of the catalyst component. It has been found that employment of the platinum metal dispersed upon inorganic salt particles substantially eliminates or diminishes hydrogen outgassing during curing of dental silicones.
An important improvement of the invention is inclusion in the composition of a surfactant that imparts wettability to said composition, as indicated by a surface contact angle with water at three minutes of less than 50.degree.. An unexpected result of the selection of surfactant provides a major clinical advantage in that the wetting contact angle of less than 50.degree. is achieved in less than about two minutes, decreasing and remaining below 50.degree. throughout the working time of the composition, in contrast with prior art polyvinylsiloxanes and surfactant formulations that require more time to wet out. This higher wetting rate of the composition of the invention is particularly advantageous during the impression taking process and is shown in the Drawings.
Referring to FIG. 1, the Wetting Contact Angle, in degrees, as a function of Time, in minutes, is shown for the polyvinyl siloxane composition of the invention, in comparison with prior art compositions. Curve A is the composition of the invention showing a wetting contact angle of about 50.degree. at two minutes after mixing of the base and catalyst components. FIG. 1 demonstrates that good wettability is achieved early and improves at a fast rate over the about 3.5 minutes of useful working life of the impression taking material. Curves B and C are, respectively, polyether and conventional polyvinyl siloxane impression materials of the prior art. FIG. 2 shows Impression Material Viscosity as a function of Time for composition of the invention, Curve A, and the two prior art compositions B and C noted above. It shows the progression of the polymerization process from mixing and, in combination with FIG. 1, demonstrates that the improved wettability of the composition of the invention occurs during the critical working time for the impression material, an important advantages over other known systems.
The surfactant of the invention may be of cationic, anionic, amphoteric or nonionic type. A key criteria for selection is that the Hydrophobic Liphophilic Balance (HLB) value (described by Gower, "Handbook of Industrial Surfactants", 1993) must be in the range of 8-11. As is well-known, the higher the HLB the more hydrophobic is the substance. In addition, the pH of the surfactant must be in the 6-8 range to prevent side reactions that may be detrimental the polymerization of the impression. A preferred surfactant is nonionic, having an HLB value of 10.8 comprising nonylphenoxypoly(ethyleneoxy) ethanol, sold by Rhone-Poulenc of Cranbury, NJ as Igepal CO-530. In comparison it is noted above with respect to Bryan et al, in U.S. '959 that Igepal CO-630, having an HLB of 13.0, differing in structure from CO-530 wherein the number of repeating units in CO-630 is 9 and those of CO-530 is 6, is not effective, demonstrating the criticality of the HLB limitation. The amount of surfactant used to render the composition hydrophilic is based on the rate of wetting. The desired contact angle at three (3) minutes is less than about 50.degree..
The composition of the invention may include plasticizers for the higher viscosity material that beneficially alter the handling and flow properties of the impression material, particularly the catalyst component. A preferred emulsifying plasticizer is octyl benzyl phthalate. Other phthalates are useful. The plasticizer composite is not necessary for lower viscosity wash type impression materials.
The selection of QM resin dispersion viscosity depends upon the overall impression material characteristics desired. To give higher viscosity handling characteristics more of the 60,000 cps component is employed. For a low viscosity type, more of the 6,000 cps component is employed to increase flow. In addition to the QM resin dispersion characteristics filler selection affects overall viscosity characteristics. Using a higher loading of low surface area fillers, such as cristobalite, gives more flow to a low viscosity type impression material. Using a higher loading of high surface area fillers, such as diatomaceous earth, reduces flow and gives more body to a high viscosity type impression material. The use of an emulsifying plasticizer makes the composition more thixotropic, which is useful for a high viscosity material. However, a plasticizer is generally not used in the low viscosity material since high flow is desired.
The composition of the invention may include various pigments to achieve a preferred color. Such pigments are well known and include titanium dioxide as well as many others.
The two component compositions prepared in accordance with the instant invention are employed in the same way that conventional impression materials have been employed. Thus, appropriately equal portions of base paste and catalyst paste are mixed together thoroughly and applied to the oral dentition or other region for a period of time sufficient for the polymerizations or hardening of the composition. Once the composition has been substantially hardened, it is removed from the mouth or other surface and used for the elaboration of casts and the like from which representations of the casting surface are subsequently prepared.
As will be appreciated by those of ordinary skill in the art, it is important that dental silicone materials be capable of being stored for reasonably long periods of time and at reasonable storage temperature in order to maximize their commercial utility. Accordingly, it is necessary that such materials not suffer from decreased physical properties or substantial changes in working time or hardening time upon such storage. In this regard, accelerated storage tests employing high ambient temperatures are now capable of determining the shelf stability of such materials.
Certain embodiments of the present invention are described below. Numerous other compositions and formulations may be prepared within the spirit of the invention. The following examples are not to be construed as limiting and are offered by way of illustration.
EXAMPLE 1
A two component composition of the invention is formulated in a Base Paste and Catalyst Paste components. Mixing of each component's ingredients is done in a double planetary mixer having a mixing pot heated with circulating water at 45.degree. C.-50.degree. C. and under 65 mm mercury vacuum.
BASE PASTE COMPONENT
In making the Base Paste, the mixing pot is first charged with all organohydrogen polysiloxane and incrementally thereafter, with QM dispersion and filler component, with mixing continuing until a uniform mixture is achieved. The finished Base Paste is discharged into a storage container.
CATALYST PASTE COMPONENT
The Catalyst Paste component is formulated and mixed under conditions and in equipment as described above. The platinum catalyst, 1,3 divinyldimethyldisiloxane, QM resin dispersions, fillers and pigments are added incrementally to the mixing pot and mixing carried out until a uniformly mixed mass is achieved. The compounded Catalyst Paste is then discharged into a storage container.
The composition of each component is indicated in the table below, wherein amounts are in weight percent of the component.
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BASE CATALYST
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Organohydrogen Polysiloxane
9.00 0.00
(5000-7000 cps) QM resin dispersion
19.62 23.95
(45000-60000 cps) QM resin dispersion
34.59 42.89
Cristobalite 19.01 19.06
Diatomaceious earth 6.53 6.41
Cab-O-Sil TS-530 6.53 6.00
Pigments Predispersed in
0.65 0.25
Divinyl Polysiloxane
Titanium Oxide Pigment
0.07 0.07
Surfactant (Igepal CO-530)
4.00 0.00
Plasticizer 0.00 0.50
Platinum Catalyst 0.00 0.64
1,3-Divinyldimethyidisiloxane
0.00 0.07
Finely divided Platinum metal
0.00 0.16
on Calcium Carbonate
100.00 100.00
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EXAMPLE 2
A two component composition of the invention is made by first
