JP2008031309A - Method for producing surface-treated inorganic filler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing inorganic filler surface-treated with a silane coupling agent and suitable for producing a composite material for restorative dental materials giving cured product with high strength and causing almost no color tone change. <P>SOLUTION: The inorganic filler surface-treated with a silane coupling agent is produced by performing vacuum distillation of almost all organic solvent from an inorganic powder dispersion liquid obtained by dispersing the inorganic powder in an organic solvent and containing a silane coupling agent and amine, then performing blast drying of suitable drying conditions comprising 0.01-3.0 m/sec wind velocity and 20-100°C temperature, and performing vacuum drying under heating. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a surface-treated inorganic filler, and more specifically, a surface treatment with a silane coupling agent, which is suitable for producing a composite material for a dental restorative agent having high strength of a cured product and almost no color tone change. The present invention relates to a method for producing an inorganic filler.

  In order to improve properties such as strength and durability of plastics, it is generally known to add a filler to make a composite material. Such polymer composite materials are electrically conductive materials, recording materials, sliding materials. It is applied in various fields such as materials, sealing materials, damping materials, dental / medical materials, and heat insulating materials. Particularly in recent dental treatments, polymer composite materials for dental restoration materials such as composite resins and resin cements are frequently used. Inorganic fillers are often used as fillers blended into the resin matrix, which is the base material, but these inorganic fillers improve the wettability with the resin matrix and, moreover, are firmly bonded to the resin matrix. In addition, it is common to use after surface treatment with a surface treatment agent such as a silane coupling agent.

  The surface treatment method of the inorganic filler using the silane coupling agent includes spraying the silane coupling agent to the fluidized inorganic filler, adding the inorganic filler in an organic solvent such as alcohol and toluene, and further adding the silane coupling agent. A method of heating after adding water and water, evaporating and drying water and an organic solvent with an evaporator, a method of hydrolyzing a silane coupling agent using a hydrolysis aid, and then mixing and spray drying with a dispersion of inorganic particles (Patent Document 1), a method of treating an inorganic powder with an amine and then reacting it with a silane coupling agent (Patent Document 2), and the like have been reported. Among these, a surface treatment method using an amine is suitable as a surface treatment method for an inorganic filler for dental materials because it is excellent in water resistance of the material.

JP-A-10-36705 JP-A-63-159214

  However, as described above, when the manufactured composite material was used as a product using an inorganic filler obtained by a surface treatment method involving amine treatment, there was a problem that the cured product gradually discolored after curing. This problem is that, when the product made of the composite material is a dental material such as a composite resin, these products are products that particularly require aesthetics, and even if they are slightly colored, their commercial value is greatly impaired. It was a big problem.

  Although the cause of this coloring is not clear, the amine used in the surface treatment process remains on the surface of the inorganic filler, and when the inorganic filler and the resin matrix are mixed, the amine is eluted, and the components in the resin matrix Is expected to cause discoloration by forming some interaction or derivative.

  It was thought that this problem could be solved by increasing the drying time and increasing the drying temperature to accelerate the volatilization of the amine, but in reality, these methods did not solve the problem and the amine was volatilized. It was expected to remain on the surface of the inorganic filler in a difficult state.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it has been found that an inorganic filler that hardly causes discoloration even when an amine is used can be produced through a specific drying step, and the present invention has been completed.

  That is, in the present invention, an inorganic powder is dispersed in an organic solvent, the crude organic solvent is distilled off under reduced pressure from an inorganic powder dispersion containing a silane coupling agent and an amine, and then blown and dried. After the above, a method for producing a surface-treated inorganic filler is characterized in that drying under reduced pressure is performed under heating.

  Moreover, it is more preferable that the present invention is performed under the conditions that the wind speed of the blast drying is 0.01 to 5.0 m / sec and the temperature is 20 to 100 ° C. Moreover, it is more preferable to carry out air drying within 5 hours after distilling off the crude organic solvent from the inorganic powder dispersion under reduced pressure. Furthermore, it is an effective method in surface treatment using a silane coupling agent represented by the following general formula.

^{_{CH 2 = C (R 1)}} -COO- (CH 2) n -Si-R 2 m R 3 (3-m) [I]
{Wherein R ¹ is a hydrogen atom or a methyl group, R ² is an alkoxy group having 1 to 6 carbon atoms, an isocyanate group, or a halogen atom, and R ³ is a hydrocarbon group having 1 to 6 carbon atoms. Yes, m is 2 or 3, and n is an integer of 5-20. }

  The composite material using the inorganic filler produced by the production method of the present invention provides a cured product having high mechanical strength and excellent color tone stability. Therefore, when it is used as an inorganic filler to be blended in a composite material for a dental restorative material represented by a composite resin or resin cement, it is particularly preferable because the same product having excellent aesthetics can be produced.

  The inorganic powder used in the production method of the present invention is not particularly limited, and amorphous silica, silica zirconia, silica titania, aluminosilicate glass, barium glass, quartz used in conventional dental restorative materials compositions, etc. Inorganic oxides such as alumina and powders of other inorganic compounds can be used. As a material of the inorganic powder, a composite oxide having silica and zirconia as main components is particularly preferably used because a cured body having X-ray contrast properties and more excellent wear resistance can be obtained. . The inorganic powder is preferably an inorganic powder having a surface acid point from the viewpoint of improving water resistance. The surface acid point is preferably an inorganic powder having an acid strength (pKa) in the range of −3.0 to 3.3, and a silica composite oxide is suitable. The shape and particle size of the inorganic powder are not particularly limited.

  The method for producing these inorganic powders is not particularly limited. For example, the inorganic powder is selected from a solution containing a hydrolyzable organosilicon compound, or a group consisting of metals in Groups I, II, III, and IV of the periodic table. A mixed solution containing a hydrolyzable organic compound containing at least one metal is added to an alkaline solvent that dissolves these raw materials but does not substantially dissolve the reaction product, and performs hydrolysis to produce a reaction. A so-called sol-gel method for precipitating a product is preferably employed.

As the organic solvent used in the production method of the present invention, known ones can be used without any particular limitation. Hydrocarbons (aliphatic hydrocarbons such as hexane, naphthenes, linear or branched paraffins, cyclohexane, etc.) Alicyclic hydrocarbons, aromatic hydrocarbons such as toluene and xylene), alcohols (C _1-4 alcohols such as methanol, ethanol, isopropanol), esters (ethyl acetate, ethylene glycol monobutyl ether acetate, etc.) ), Ketones (aliphatic ketones such as acetone, ethyl methyl ketone, isobutyl methyl ketone, etc.), ethers (dioxane, tetrahydrofuran, etc.), cellosolves (ethylene glycol ethyl ether, etc.), carbitols (ethylene glycol monobutyl ether) Acetate) And halogenated hydrocarbons (chlorinated solvents such as tetrachloroethylene and trichloroethane, etc., or a mixed solvent thereof). An organic solvent capable of dissolving a silane coupling agent in order to perform a uniform surface treatment. In this respect, it is particularly preferable to use methanol, ethanol, acetone, toluene, chloroform, or methylene chloride.

  The mixing ratio of the inorganic powder and the organic solvent may be in a range that makes a slurry. Generally, the organic solvent is used in an amount of 100 to 1000 parts by weight, preferably 150 to 400 parts by weight, based on 100 parts by weight of the inorganic powder.

  A well-known thing is used without a restriction | limiting as a silane coupling agent used by this invention. Examples of suitable silane coupling agents include vinyltriethoxysilane, vinyl-tris (β-methoxyethoxy) silane, γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, γ-methacryloyloxypropyl. Methyldimethoxysilane, β- (3,4-epoxycyclohexyl) -ethyltrimethoxysilane, γ-glycidoxypropyl-trimethoxysilane, N-β- (aminoethyl) -γ-aminopropyl-trimethoxysilane, γ -Ureidopropyl-triethoxysilane, γ-chloropropyltrimethoxysilane, vinyltrimethoxysilane, etc., and particularly γ-methacryloyloxypropyltrimethoxysilane having a polymerizable functional group, γ-methacryloyloxypropylene. Triethoxysilane and the like are suitable.

The production method of the present invention is particularly suitable for surface treatment using a silane coupling agent represented by the following general formula [I].
^{_{CH 2 = C (R 1)}} -COO- (CH 2) n -Si-R 2 m R 3 (3-m) [I]
{Wherein R ¹ is a hydrogen atom or a methyl group, R ² is an alkoxy group having 1 to 6 carbon atoms, an isocyanate group, or a halogen atom, and R ³ is a hydrocarbon group having 1 to 6 carbon atoms. Yes, m is 2 or 3, and n is an integer of 5-20. }
When surface treatment is performed using a silane coupling agent represented by the general formula [I], it becomes easy to fill the resin matrix with an inorganic filler at a high density. Since such a silane coupling material is generally poorly water-soluble due to its structure, it is particularly suitable for application to a surface treatment method using an organic solvent as shown in this patent. Examples of suitable silane coupling agents include 10-methacryloyloxydecyltrimethoxysilane, 11-methacryloyloxyundecyltrimethoxysilane, 11-methacryloyloxyundecylmethyldimethoxysilane, and the like.

  These silane coupling agents may be used alone or in combination of two or more. Moreover, the said silane coupling agent can also be used in combination with well-known surface treatment agents other than a silane coupling agent in the range which does not impair the effect of this invention.

  The content of these silane coupling agents in the organic solvent may be an amount necessary for imparting desired surface properties to the inorganic particles. In general, the silane coupling agent is used in an amount of 0.1 to 30 parts by weight, preferably 0.5 to 10 parts by weight with respect to 100 parts by weight of the inorganic particles. It may be determined appropriately in consideration of the minimum covering area, the results of preliminary experiments, and the like.

  As the amine used in the present invention, known amines can be used without any particular limitation. From the viewpoint of silane coupling reactivity and amine removal, primary or secondary amine compounds, and aliphatic An amine is preferred. Specific examples thereof include n-propylamine, isopropylamine, n-butylamine, isobutylamine, tert-butylamine, diethylamine, di-n-propylamine and the like.

  These amines are used in an amount of 0.1 to 5 moles, preferably 0.5 to 2.0 moles of the silane coupling agent.

  In the production method of the present invention, first, an inorganic powder dispersion liquid in which an inorganic powder is dispersed in an organic solvent containing a silane coupling agent and an amine is prepared. The mixing order of these raw materials is not particularly limited, and after dispersing inorganic powder in an organic solvent, a method of adding a silane coupling agent and an amine to the dispersion, a silane coupling agent and an amine Any method, such as a method of adding and dispersing inorganic powder in a dissolved organic solvent, may be employed, but in order to prevent unintended reaction between the amine and the silane coupling agent, addition of an amine It is preferable to add a silane coupling agent to the organic solvent dispersion of the inorganic powder. Addition of an amine also has an effect of improving the dispersibility of the inorganic powder in the organic solvent. Therefore, it is preferable to add the amine before dispersing the inorganic powder in the organic solvent.

  In order to perform surface treatment uniformly, this dispersion liquid preferably has good dispersibility of the inorganic powder, and is preferably in a state where the inorganic powder is crushed as much as possible to the primary particles. A method for producing the dispersion is not particularly limited, and a suitable method includes a method of mixing and dispersing with a ball mill, a homogenizer, an ultra-high pressure impact type emulsifying dispersion device or the like.

  In the inorganic powder dispersion obtained by the above method, the reaction of the silane coupling agent to the inorganic powder usually does not proceed to a high degree in this state. The amine is adsorbed at the reaction point on the surface of the inorganic powder, and it is considered that the amine does not volatilize completely when the solvent is distilled off under reduced pressure and blown dry. Usually, the added amine is added in a slight excess in consideration of the effects of volatilization and loss, but the amine that is not adsorbed on the reaction point on the surface of the inorganic powder remains due to such excessive addition. It is estimated that discoloration is promoted by this. Blow drying is performed to volatilize excess amine, and in the subsequent heating and drying process, this amine is used to promote the condensation reaction between the silane coupling agent and the reactive sites on the surface of the inorganic powder. It is thought that there is.

  The vacuum distillation step in the production method of the present invention is performed for the purpose of roughly removing the organic solvent. Although there is no restriction | limiting in particular as a method of depressurizingly distilling an organic solvent, Generally, an evaporator is used suitably. There is no limitation on the degree of solvent removal by distillation under reduced pressure, but the process may proceed to the next step when the liquid is not visible and solidifies. Generally, round or eggplant-shaped flasks are used for distillation under reduced pressure. However, when moving to the next step, it is preferable to collect in a separate container such as a vat from the viewpoint of work efficiency and drying efficiency. It is.

  In this distillation under reduced pressure, it is difficult to completely remove the organic solvent, and the rough side is removed to obtain a wet body. Usually, the residual weight of the organic solvent is removed to 30% or less, preferably 1 to 10%.

The blow drying in the production method of the present invention refers to a drying step performed using a blower having a discharge pressure of 1 kg / cm ² or less. By incorporating this blow drying process, it is considered that the excess amine used for the surface treatment can be volatilized appropriately and the residue in the inorganic filler can be suppressed. Without this process, when vacuum drying is performed directly after distillation under reduced pressure, excess amine is not sufficiently volatilized due to poor air exchange efficiency and heat transfer efficiency, and a cured dental composite material using an inorganic filler. Will lead to discoloration.

  A fan is generally used as the air blowing mechanism. Moreover, although it can use regardless of a circulation type and a discharge type as a ventilation system, the discharge type is more preferable from a viewpoint of the exchange efficiency of air.

  After the solvent is distilled off under reduced pressure, it is better that the time required for air drying is shorter. In order to increase the volatilizing effect of excess amine, the solvent is distilled off under reduced pressure within 5 hours, preferably 3 hours. Preferably within. The reason for this is not clear, but it is thought that the bond between the amine and the inorganic powder becomes stronger and does not volatilize easily by leaving it for a long time with the amine remaining concentrated in the inorganic powder. It is done.

  The wind speed of the blast drying is preferably 0.01 to 3.0 m / sec, more preferably 0.5 to 1.5 m / sec. Moreover, it is preferable that the temperature of ventilation drying is performed on the conditions of 20-100 degreeC, More preferably, it is 50-80 degreeC. Although there is no restriction | limiting in particular also in drying time, It is suitable to carry out in less than 48 hours and also less than 24 hours.

  In the production method of the present invention, after the blast drying, drying under reduced pressure further removes the condensate such as water and alcohol, the remaining organic solvent, and the amine generated by the silane coupling reaction almost completely. Here, drying under reduced pressure refers to drying performed by promoting volatilization of a solvent or the like by lowering the partial pressure of air around the inorganic powder. Generally, a vacuum dryer with a heating mechanism connected to a vacuum pump is used. This step is performed for the purpose of removing condensates such as water and alcohol produced by the silane coupling reaction, and removing the remaining organic solvent and amine to completely dry the inorganic filler. The degree of vacuum is preferably 20 mmHg or less, more preferably 0.01 to 10 mmHg, and particularly preferably 0.03 to 1 mmHg.

  The reduced-pressure drying step is performed while heating. By heating, the silane coupling reaction can be advanced to promote the formation of a covalent bond by the condensation reaction between the filler and the silane coupling agent, and the drying of the inorganic powder can be accelerated. The heating temperature is preferably 50 to 150 ° C, more preferably 70 to 110 ° C. Although there is no restriction | limiting in particular also in drying time, It is suitable to carry out in less than 48 hours and also less than 24 hours.

  The surface-treated inorganic filler produced according to the present invention can be used as a polymer composite material by dispersing in a suitable resin matrix. Surface treatment of the inorganic filler can improve compatibility with the resin matrix and introduce functional groups that bind to the resin matrix, improving various physical properties such as strength and durability of the composite material To do.

  Preferred examples of the use of the present invention include a sealing material, a recording material, a sliding material, a vibration damping material, a heat insulating material, and the like. Particularly, it can be suitably used for dental materials such as a composite resin and a resin cement. Taking a dental composite resin as an example, the surface-treated inorganic filler by the production method of the present invention and a (meth) acrylate monomer containing a photopolymerization initiator as a resin matrix are sufficiently kneaded and further subjected to reduced pressure. A composite material obtained by defoaming is provided as a product. Such a dental composite resin is prepared by directly filling the cavity of the tooth to be repaired with an appropriate pretreatment material or adhesive, and forming it into a tooth shape. It can be used by irradiation and polymerization curing.

EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not restrict | limited to these Examples. In addition, the surface treating agent, the polymerizable monomer, and the polymerization initiator used in Examples and Comparative Examples are as follows.
Surface treatment agent:
MPS: γ-methacryloyloxypropyltrimethoxysilane MUDS: 11-methacryloyloxyundecyltrimethoxysilane polymerizable monomer:
GMA: 2,2-bis (4- (3-methacryloyloxy) -2-hydroxypropoxyphenyl) propane
TEGDMA: Triethylene glycol dimethacrylate polymerization initiator CQ: camphorquinone DMBE: dimethyl benzoic acid ethyl ester The test method is as follows.
(1) Adjustment of curable composition:
To 60 parts by weight of GMA and 40 parts by weight of TEGDMA, 0.5% by weight of CQ and 0.5% by weight of DMBE are added and mixed to prepare a uniform polymerizable monomer composition. A matrix was used.
400 parts by weight of the inorganic filler prepared according to the example is put in an agate mortar, and 100 parts by weight of the polymerizable monomer composition is gradually added thereto, and sufficiently kneaded in a dark place to obtain a uniform curable paste. . Furthermore, this paste was degassed under reduced pressure.
(2) Discoloration degree of the cured product:
The paste of the curable composition was put into a mold having a hole of 7 mmφ × 1 mm, and both surfaces were pressed with a polyester film. Both surfaces were irradiated with a visible light irradiator (Tokuyama, Powerlight) for 30 seconds each and cured, then removed from the mold, and a color difference meter (Tokyo Denshoku, TC-1800MKII) was used. * ₁ , a * ₁ , and b * ₁ values were measured. After that, it is stored in an incubator at 50 ° C. for 7 days, and after collection, the L * ₂ , a * ₂ , and b * ₂ values are measured again with the background whiteness, and the discoloration degree of the cured product is obtained based on the following formula (II). It was.
_{ΔE * = √ {(L *} 2 -L * 1) 2 + (a * 2 -a * 1) 2 + (b * 2 -b * 1) 2} (II)
Example 1
In a rotating ball mill in which φ5 mm zirconia balls are put, 0.5 kg of pulverized silica zirconia having an average particle diameter of 1.2 microns, 1 kg of methylene chloride, and 12.7 g of isopropylamine are charged, and the pulverized silica zirconia is run for 30 minutes. After dispersing, MPS 15.9 was further added and the operation was continued for 30 minutes. The obtained dispersion was transferred to an eggplant flask, and methylene chloride was distilled off under reduced pressure using an evaporator. After carrying out the vacuum distillation until the liquid content can no longer be confirmed visually, the resulting wet inorganic powder was collected in a vat and charged into a blower dryer after 1 hour, with a wind speed of 1.0 m / sec and hot air temperature. Was dried at 50 ° C. for 15 hours. Then, inorganic filler F-1 was obtained by performing vacuum drying for 15 hours at 0.3 mmHg and heating temperature 80 degreeC with a vacuum dryer.

  As a result of adjusting the curable composition using this inorganic filler F-1 and curing it to measure the color change degree, ΔE * = 0.3.

Example 2
Into a rotating ball mill in which zirconia balls with a diameter of 5 mm are charged, 0.5 kg of pulverized silica zirconia having an average particle diameter of 1.2 microns, 1 kg of methylene chloride and 12.7 g of n-propylamine are charged and operated for 30 minutes. After zirconia was dispersed, 22.0 g of MUDS was further added and the operation was continued for 30 minutes. The obtained dispersion was transferred to an eggplant flask, and methylene chloride was distilled off under reduced pressure using an evaporator. After carrying out the vacuum distillation until the liquid content could not be confirmed visually, the resulting inorganic powder wet body was collected in a vat and charged into a blower dryer after 1 hour, with a wind speed of 1.0 m / sec and hot air temperature. Was dried at 50 ° C. for 15 hours. Then, inorganic filler F-2 was obtained by performing vacuum drying for 15 hours at 0.3 mmHg and heating temperature 80 degreeC with a vacuum dryer. As a result of adjusting the curable composition using this inorganic filler F-2 and curing it to measure the degree of discoloration, ΔE * = 0.9.

Example 3
In a rotating ball mill in which φ5 mm zirconia balls are charged, 0.5 kg of pulverized silica zirconia having an average particle diameter of 1.2 microns, 1 kg of methanol, and 15.0 g of n-butylamine are charged, and the pulverized silica zirconia is run for 30 minutes. After the dispersion, 22.0 g of MUDS was further added and the operation was continued for 30 minutes. The obtained dispersion was transferred to an eggplant flask, and methanol was distilled off under reduced pressure at room temperature using an evaporator. After performing distillation under reduced pressure until the liquid content could not be confirmed visually, the resulting wet inorganic powder was collected in a vat and charged into a blast dryer after 3 hours, with a wind speed of 0.8 m / sec, hot air temperature Was dried at 80 ° C. for 15 hours. Then, inorganic filler F-3 was obtained by performing vacuum drying for 15 hours at 0.05 mmHg and heating temperature 100 degreeC with a vacuum dryer.

  As a result of adjusting the curable composition using this inorganic filler F-3 and curing it to measure the degree of discoloration, ΔE * = 0.8.

Example 4
Into a rotating ball mill in which zirconia balls with a diameter of 5 mm are charged, 0.5 kg of pulverized silica zirconia having an average particle diameter of 1.2 microns, 1 kg of methylene chloride and 12.7 g of n-propylamine are charged and operated for 30 minutes. After zirconia was dispersed, 22.0 g of MUDS was further added and the operation was continued for 30 minutes. The obtained dispersion was transferred to an eggplant flask, and methylene chloride was distilled off under reduced pressure using an evaporator. After carrying out the vacuum distillation until the liquid content could not be confirmed visually, the resulting wet inorganic powder was collected in a vat and after 7 hours charged into a blast dryer, the wind speed was 1.0 m / sec, the hot air temperature Was dried at 50 ° C. for 15 hours. Then, inorganic filler F-4 was obtained by performing vacuum drying for 15 hours at 0.3 mmHg and heating temperature 80 degreeC with a vacuum dryer.

  As a result of adjusting the curable composition using this inorganic filler F-4 and curing it, the discoloration degree was measured. As a result, ΔE * = 2.6.

Comparative Example 1
Into a rotating ball mill in which zirconia balls with a diameter of 5 mm are charged, 0.5 kg of pulverized silica zirconia having an average particle diameter of 1.2 microns, 1 kg of methylene chloride and 12.7 g of n-propylamine are charged and operated for 30 minutes. After zirconia was dispersed, 22.0 g of MUDS was further added and the operation was continued for 30 minutes. The obtained dispersion was transferred to an eggplant flask, and methylene chloride was distilled off under reduced pressure using an evaporator. After carrying out the vacuum distillation until the liquid content could not be confirmed visually, the wet body of the obtained inorganic powder was collected in a vat and dried under reduced pressure at 0.3 mmHg and a heating temperature of 80 ° C. for 15 hours with a vacuum dryer. To obtain an inorganic filler F-5.

The curable composition was adjusted using this inorganic filler F-5, cured, and the color change was measured. As a result, ΔE * = 7.9.

Claims (4)

The inorganic powder is dispersed in an organic solvent, and the crude organic solvent is distilled off under reduced pressure from the inorganic powder dispersion containing the silane coupling agent and amine, and then blown and dried before heating. A method for producing a surface-treated inorganic filler, characterized by performing vacuum drying under pressure. The method for producing a surface-treated inorganic filler according to claim 1, wherein the blast drying is performed under conditions of a wind speed of 0.01 to 3.0 m / sec and a temperature of 20 to 100 ° C. The method for producing a surface-treated inorganic filler according to claim 1 or 2, wherein the organic solvent is roughly distilled off from the inorganic powder dispersion under reduced pressure and then blown and dried within 5 hours. The method for producing a surface-treated inorganic filler according to any one of claims 1 to 3, wherein a silane coupling agent represented by the following general formula [I] is used.
^{_{CH 2 = C (R 1)}} -COO- (CH 2) n -Si-R 2 m R 3 (3-m) [I]
{Wherein R ¹ is a hydrogen atom or a methyl group, R ² is an alkoxy group having 1 to 6 carbon atoms, an isocyanate group, or a halogen atom, and R ³ is a hydrocarbon group having 1 to 6 carbon atoms. Yes, m is 2 or 3, and n is an integer of 5-20. }