JP2517770B2 - Silicone rubber composition, its curing method and cured product - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicone rubber composition, and more particularly to a silicone rubber composition that can be cured by an ultra-high frequency vulcanization method, which is also the silicone rubber composition. The present invention relates to a method of subjecting a composition to ultra-high frequency vulcanization and a silicone rubber cured product which is thus obtained and can be colored in various color tones.

[Prior Art] Conventionally known methods for continuous vulcanization of silicone rubber include atmospheric pressure hot air vulcanization (HAV), steam continuous image vulcanization (CV), and molten salt vulcanization (LCM). However, these methods are not suitable for obtaining a long body of silicone rubber.

On the other hand, recently, in order to obtain gaskets for construction, tube materials, rolls, etc., the demand for stable and inexpensive production of continuously extruded rubber moldings is increasing. For this reason, ethylene propylene diene rubber (EPDM), Regarding chloroprene rubber (CR), etc., a method of vulcanizing and curing this continuously extruded product by an ultra high frequency vulcanization method (Ultra High Frequency vulcanization, hereinafter abbreviated as UHF vulcanization) has been put into practical use, but silicone rubber is Loss Index
Has a disadvantage that it cannot be vulcanized by this UHF vulcanization.

This UHF vulcanization method is usually 2,450 ± 50MHz and 915 ± 25MHz.
An electromagnetic wave of MHz is used to irradiate the object to be processed, and the object to be processed absorbs the microwave to generate heat and is crosslinked. That is, when the energy (P) oscillated by the microwave is applied to the microwave heating oven, this energy causes a dielectric loss coefficient (ε · ta).
It is absorbed by the heated part (dielectric) having nδ) and this heats up. This energy (P) is expressed by the following equation: P = (5/9) f · E ² · ε · tan δ × 10 ¹⁰ This UHF vulcanization method is loss index ε ・ tanδ
Is more than 0.08, preferably 0.2 or more, so this loss index is about 0.03 (at3GH
It is believed that z) and small silicone rubbers cannot apply this UHF vulcanization.

[Problems to be Solved by the Invention] Therefore, the organic group of the organosiloxane that constitutes the organopolysiloxane composition is aryl radicals, chlorinated aliphatic hydrocarbon radicals, fluorinated aliphatic hydrocarbon radicals, at least 1 5% or more of silicon-bonded organic radicals selected from hydrocarbon radicals having 1 carbon-bonded mercapto group, hydrocarbon radicals having at least 1 carbinol group, and aliphatic hydrocarbon ether radicals A method has also been proposed in which UHF vulcanization is carried out according to the content (Japanese Unexamined Patent Application Publication No. Sho 5).
However, there is a practical problem because it deteriorates the heat resistance, weather resistance, electric characteristics and surface characteristics which are the characteristics of silicone rubber.

[Means for Solving the Problems] The present invention relates to a silicone rubber composition capable of UHF vulcanization, in which such disadvantages are solved, and it relates to 1) an average composition formula RaSiO _{4-a /} (here R has 1 to 10 carbon atoms
An unsubstituted or substituted identical or different monovalent hydrocarbon group, a = 1.95 to 2.05 positive number), the average degree of polymerization is
100 parts by weight of 3,000 to 30,000 diorganopolysiloxane, 2) 10 to 300 parts by weight of finely divided silica filler, 3) a cyanoalkyl group, an acryloxyalkyl group, a methacryloxyalkyl group directly bonded to a silicon atom, 10 to 200 parts by weight of polyorganosilsesquioxane containing at least one group selected from γ-trifluoroalkyl groups, and 4) a curing agent, which is also characterized in that The present invention relates to a method of vulcanizing this foamable silicone rubber composition containing a foaming agent by UHF and a cured silicone rubber composition obtained by the method.

That is, as a result of various studies to develop a silicone rubber composition capable of UHF vulcanization without impairing the characteristics of the silicone rubber, the present inventors have directly linked a silicon atom to a conventionally known silicone rubber composition. If a polyorganosilsesquioxane containing at least one group selected from the group consisting of a cyanoalkyl group, an acryloxyalkyl group, a methacryloalkyl group, and a γ-trifluoroalkyl group is added, the composition It has been found that UHF vulcanization is possible because of the improved microwave absorptivity of, thus enabling the production of foamed molded products by continuous extrusion of silicone rubber compositions, and furthermore, this composition provides silicone rubber. It can be colored in various color tones, which is the original feature of the product, and its fashionability can be maintained. Ensure the above-mentioned polyorganosilsesquioxane type, amount, and further completed the study present invention complete the on such UHF vulcanizing method of the composition.

 This will be described in detail below.

[Function] The diorganopolysiloxane as the first component constituting the silicone rubber composition of the present invention is represented by the formula RaSiO _{4-a /} , and R is an alkyl group such as a methyl group, an ethyl group, a propyl group or a butyl group. Group, vinyl group, alkenyl group such as allyl group and butenyl group, aryl group such as phenyl group and tolyl group, or some or all of hydrogen atoms bonded to carbon atoms of these groups are substituted with halogen atom, cyano group, etc. Chloromethyl group, chloropropyl group, 3,3,3-trifluoropropyl group, 2-cyanoethyl group, etc., having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, which are the same or different and are unsubstituted or Substituted monovalent hydrocarbon group, a is 1.95 to 2.
It is assumed to be a positive number of 05. However, in this case, 98% or more of R is an alkyl group having 1 to 4 carbon atoms, particularly a methyl group, and may further include a vinyl group, a phenyl group, a trifluoropropyl group, It is preferable that this compound has a linear molecular structure, but there is no problem even if it contains a partially branched molecular structure. Further, this may have a degree of polymerization of 3,000 to 30,000, and from the viewpoint of workability, it is preferably 4,000 to 10,000.

The fine powder silica filler as the second component in the composition of the present invention is added for the purpose of reinforcing silicone rubber, increasing viscosity, improving processability, increasing amount, and the like. Examples include wet silica, fumed silica having a hydrophobic surface, wet silica, quartz fine powder, diatomaceous earth, and the like. Other types of fillers such as calcium silicate, calcium carbonate, carbon black,
It may contain some glass fibers and the like, but it is preferable that these have a specific surface area of 1 m ² / g or more, preferably 50 m ² / g or more. If the amount of this compound is 10 parts by weight or less with respect to 100 parts by weight of the above-mentioned organopolysiloxane, the desired reinforcing property cannot be obtained, and the processability becomes insufficient. If it is 300 parts by weight or more, mold flowability, Since processing characteristics such as dischargeability are extremely deteriorated, it is necessary to set the range of 10 to 300 parts by weight, and the preferable range is 25 to 20.
It is 0 parts by weight.

Next, the third component constituting this composition is for improving the microwave absorptivity of this composition to make it exothermic, and this is directly linked to the silicon atom. The polyorganosilsesquioxane contains at least one group selected from a cyanoalkyl group, an acryloxyalkyl group, a methacryloxyalkyl group, and a γ-trifluoroalkyl group. This polyorganosilsesquioxane is, for example, cyanoalkyltrialkoxysilane, γ-methacryloxypropyltrimethoxysilane or γ-trifluoroalkyltrialkoxysilane and methylalkoxysilane or its partial partial hydrolysis by alcal earth metal hydroxide. Or hydrolysis in an aqueous solution of alkali metal carbonate or amine, ammonia, etc.,
After condensation, it is dried and pulverized to obtain a spherical fine particle powder. This is also the residual silanol group in the polymethylsilsesquioxane obtained by hydrolysis of methylalkoxysilane. It can also be obtained by treatment with alkyltrialkoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-trifluoroalkyltrialkoxysilane, or the like. Further, the polyorganosilsesquioxane may be one obtained by cohydrolyzing vinyltrialkoxysilane as a vinyl source or introducing a vinyl group as a surface modifier, and the vinyl group is finally When the resulting silicone rubber composition is cured, it helps improve the physical properties such as mechanical properties and compression recovery of the molded product, but the introduction of this vinyl group is determined depending on the physical properties required for the final cured product. Good.

If the average particle size of the polyorganosilsesquioxane is smaller than 0.1 μm, it will be difficult to fill this sufficient amount, and the hardness of the rubber cured product obtained by curing the silicone rubber composition will increase significantly. 50 disadvantages
If it is larger than μm, the mechanical properties of the rubber cured product obtained by curing the silicone rubber composition of the present invention filled with a sufficient amount thereof will be significantly impaired and the required functions will not be obtained. Is preferably 0.1 to 50 μm.

If the amount of this polyorganosilsesquioxane is less than 10 parts by weight with respect to 100 parts by weight of diorganopolysiloxane as the first component, the microwave absorptivity will be low, and if it is more than 200 parts by weight, physical absorption will be low. Since it causes deterioration of characteristics, it is necessary to use 10 to 200 parts by weight, but a preferable range is 25 to 100 parts by weight.

Next, the curing agent as the fourth component constituting this composition is generally used when curing the silicone rubber composition, and specifically, organic peroxide, or organohydrogen siloxane and platinum. Examples of the organic peroxide include benzoyl peroxide, monochlorobenzoyl peroxide, p-methylbenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate, and dichromyl peroxide. , 2,5-bis- (t-butylperoxy) -2,5-dimethylhexane, 2,5-bis- (t-butylperoxy) -2,5-dimethylhexyne and dimyristyl peroxycarbonate, Dicarbonates such as dicyclododecyl peroxy dicarbonate, t-butyl monoperoxy carbonates, Serial type (However, R in the formula is a monovalent hydrocarbon group having 3 to 10 carbon atoms.) And the like are exemplified, and these may be used alone or in combination of two or more thereof. This amount may be 0.5 to 5 parts by weight with respect to 100 parts by weight of the diorganopolysiloxane as the first component. Further, when the diorganopolysiloxane as the first component contains an alkenyl group as the curing agent, the hydrogen atom bonded to the alkenyl group is 1
Organohydrogensiloxane containing two or more molecules is added, and platinum-based additions such as chloroplatinic acid, acetic acid of chloroplatinic acid and olefin or vinylsiloxane, and alcoholic solution of chloroplatinic acid are used as a curing catalyst. A reaction catalyst may be added, and the amount of this catalyst is the platinum amount based on the diorganopolysiloxane as the first component.
It may be 0.5 to 500 ppm, preferably 2 to 200 ppm. As the organohydrogen siloxane, for example, the general formula (In the formula, R ² is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, b and c are positive numbers, and b + c is 1.0
To 3.0) having at least two silicon-hydrogen bonds in the molecule (R ² is an alkyl group such as a methyl group, an ethyl group, a propyl group or a butyl group, a phenyl group or a tolyl group). It is preferably an aryl group such as a group), and the organohydrogensiloxane as the curing agent for the addition reaction is a silicon-hydrogen bond (≡Si-H) that reacts with the vinyl group in the first component. 2) is contained, the degree of polymerization is preferably 300 or less. In addition, this is a conventionally known linear structure, annular structure,
The branched chain structure may be any, and the amount added is such that the ≡Si-H bond is in the range of 50 to 300% in terms of moles with respect to the vinyl group amount (moles) of the first component. And it is sufficient.

The silicone rubber composition of the present invention is the above-mentioned first to fourth
It can be obtained by mixing predetermined amounts of the components and uniformly kneading, and as a dispersant, low molecular weight siloxanes having a polymerization degree of 100 or less, silanol group-containing silanes, alkoxy group-containing silanes, iron oxides, cerium oxides. , Iron octylate, heat resistance improvers such as titanium oxide, pigments for coloring, flame retardant aids such as platinum compounds and palladium compounds, and other additives usually added to this type of silicone rubber composition It is optional to add agents and the like.

The silicone rubber composition of the present invention may be added with a foaming agent to give a foamable silicone rubber composition.
The foaming agent added here is stable at room temperature but may be one that releases a foaming gas when exposed to high temperatures, and this foaming gas is generally considered to be nitrogen gas. , Which may be carbon dioxide or other gas. This product may be commercially available, and includes azobisisobutyronitrile, dinitrosopentamethylenetetramine, benzenesulfone hydrazide, N, N'-dinitroso-N, N'-dimethylterephthalamide, azodicarbonamide, etc. However, the blending amount may be 1 to 10 parts by weight, and usually 3 to 7 parts by weight, based on 100 parts by weight of the diorganopolysilylsiloxane as the first component.
The foamable silicone rubber composition to which the foaming agent is added has a high expansion ratio and gives a foamed molded product having fine and uniform foam cells.

The thus-produced silicone rubber composition of the present invention is used in a conventional method for vulcanizing silicone rubber, for example, press mold, atmospheric pressure hot air vulcanization, steam vulcanization, transfer molding, injection molding, etc. It can be vulcanized and molded by the method of
A cyanoalkyl group directly connected to a silicon atom as a component, an acryloxyalkyl group, a methacryloxyalkyl group and / or a polyorganosilsesquioxane containing a γ-trifluoroalkyl group is added,
When UHF is vulcanized, it absorbs microwaves well, generates heat, and is easily vulcanized. This makes it possible to obtain a silicone rubber molded product from a silicone rubber composition by continuous extrusion in a stable and inexpensive manner. Gives you an advantage,
The composition is also given the advantage that it can be colored in different shades.

As described above, the silicone rubber composition of the present invention has UHF
A silicone rubber molding can be obtained by vulcanization, but this UHF vulcanization is performed using a vent type extruder → UHF heating section →
It may be carried out by using a continuous extrusion vulcanizer comprising a secondary vulcanization section and a take-up machine. Therefore, in this molding, first, the silicone rubber composition is supplied to a vent type extruder to continuously extrude a silicone rubber molded body in a desired shape, and then this is assigned as a heating device of this type 2,450. It can be sent to a UHF heating section equipped with a high frequency oscillator of ± 50 MHz or 915 ± 25 MHz and irradiated with this high frequency, according to which the extruded silicone rubber molded body has a cyanoalkyl group, Since polyorganosilsesquioxane containing a roxyalkyl group, a methacryloxyalkyl group and / or a γ-fluoroalkyl group is mixed, it absorbs microwaves to generate heat and is heated to 160 ° C. It is sulphated by the accompanying decomposition of organic peroxide, but in order to speed up this vulcanization, a heater is added to this UHF heating section. It may be be built to promote the temperature rise. Also,
The vulcanization of the silicone rubber molding is almost completely vulcanized and cured by this UHF vulcanization. This is followed by secondary vulcanization such as conventionally known hot air vulcanization (HAV) and fluidized bed (PCM) vulcanization. It is preferable to perform vulcanization, which allows complete vulcanization and removal of decomposition products generated by UHF vulcanization, so that the desired silicone rubber molding can be easily and stably obtained. The advantage is given.

The silicone rubber molding obtained by such a method is not special in that the first component, the second component, the organosiloxane, and the silica-based filler constituting the silicone rubber composition of the present invention used here are special. A cyanoalkyl group, an acryloxyalkyl group, a methacryloxyalkyl group and / or γ-trifluoro, which is of the same kind as an ordinary silicone rubber and is directly bonded to a silicon atom as a third component added to the same. Polyorganosilsesquioxane containing an alkyl group does not impair the physical properties of silicone rubber, so it has excellent heat resistance, cold resistance, weather resistance, and electrical properties, and has a low compression set. However, it is also said that this UHF vulcanization can perform high-speed vulcanization not only in the surface layer but also in the internal heat almost at the same time. Since the one with the benefit of, this compound can be industrially advantageously used for construction gaskets, tubing, a thickness molded body such as a roll.

(Example) Next, a synthesis example of polyorganosilsesquioxane constituting the composition of the present invention, an example of the present invention, and a comparative example will be described. In the examples, "part" means "part by weight".

Synthesis Example 1 21.7 g of cyanoethyltriethoxysilane and 122.4 g of methyltrimethoxysilane were mixed, and these were dropped into a basic aqueous solution for hydrolysis and condensation, followed by drying and pulverization, and an average particle size of 1.2 was obtained. As a result, 67 g of polymethylsilsesquioxane (hereinafter abbreviated as polyorganosilsesquioxane A) having a particle size of μm and containing a cyanoethyl group bonded to a silicon atom in the form of spherical fine powder was obtained.

Synthesis Example 2 γ-trifluoropropyltriethoxysilane 26.0 g
After mixing with 122.4 g of methyltrimethoxysilane,
This was added dropwise to a basic aqueous solution to be hydrolyzed and condensed, then dried and pulverized to contain spherical fine powder having an average particle diameter of 1.8 μm and containing a γ-trifluoropropyl group bonded to a silicon atom. Polymethylsilsesquioxane (hereinafter abbreviated as polyorganosilsesquioxane B)
72 g were obtained.

Synthesis Example 3 After mixing 173.6 g of cyanoethyltriethoxysilane and 27.2 g of methyltrimethoxysilane, these were dropped into a basic aqueous solution to hydrolyze and condense, followed by drying and crushing. The average particle diameter was 3.5 μm. 91 g of polymethylsilsesquioxane (hereinafter abbreviated as polyorganosilsesquioxane C) containing a cyanoethyl group bonded to a silicon atom in the form of spherical fine powder was obtained.

Examples 1-3, Comparative Examples 1-2 Dimethylsiloxy unit 99.825 mol%, methylvinylsiloxy unit 0.15 mol%, dimethylvinylsiloxy unit 0.
100 parts of a gum-like organopolysiloxane having an average degree of polymerization of 8,000 consisting of 025 mol%, 3 parts of diphenylsilanediol as a dispersant, 4 parts of terminal silanol group dimethylpolysiloxane (polymerization degree n = 10) and fumed silica. 40 parts of Aerosil 200 [trade name of Nippon Aerosil Co., Ltd.] was added, kneaded with a two-roll mill, and then heat-treated at 150 ° C. for 4 hours to prepare a base compound.

Then, 100 parts of this base compound was added to 50 parts of the polyorganosilsesquioxane A, B or C prepared in Synthesis Examples 1 to 3 shown in Table 1 and 2,4-dichlorobenzoyl peroxide (50 parts) as a vulcanizing agent. % Silicone oil paste) was added and kneaded using a two-roll to prepare a silicone rubber composition.

Next, apply this silicone rubber composition to a frequency of 2,450 ± 50M.
Heated and vulcanized by a microwave oscillator with a frequency of Hz and an output of 0.6 KW,
At that time, the temperature rising rate, the vulcanization state, and the appearance and colorability of the obtained cured product were examined, and the results shown in Table 1 were obtained. For comparison, the above-mentioned polyorganosilsesquioxane is not added (Comparative Example 1), and Bengala KN-320 [trade name of Toda Kogyo Co., Ltd.] is used instead of polyorganosilsesquioxane. The physical properties of the cured product obtained by treating in the same manner as above for the product with addition of parts (Comparative Example 2) were also obtained, and the results as shown in Table 1 were obtained.

Examples 4 to 6 and Comparative Examples 3 to 4 Polyorganosilsesquioxane A prepared in Synthesis Example 1 was added to 100 parts of the base compound prepared in Examples 1 to 3.
Was added in an amount shown in Table 2, 1.5 parts of 2,4-dichlorobenzoyl peroxide (50% dimethyl silicone oil paste) as a vulcanizing agent was added to these,
A two-roll kneading was performed to prepare a silicone rubber composition.

Next, this silicone rubber composition has a cylinder diameter of
At 40 mm / φ, the ratio of cylinder length L and diameter D is L / D = 12
It is supplied to an extruder equipped with a 15 mm / φ die, and from this, a round bar-shaped silicone rubber molded body is continuously molded at room temperature (15 to 30 ° C), and this is radiated with a microwave with a frequency of 2,450 ± 50 MHz. A hot air of 150 ℃ was passed through the UHF heating part of 1.5 m in length at a conveying speed of 1.5 m / min for vulcanization and curing, and the vulcanized state of this cured product and the extrudability of this composition were examined. The results as shown in Table 2 were obtained together.

However, for comparison, those without addition of the polyorganosilsesquioxane (Comparative Example 3) and those with addition of 310 parts of polyorganosilsesquioxane (Comparative Example 4) were treated in the same manner as above. When the vulcanized state of the cured silicone rubber obtained by vulcanization and the extrudability of this composition were examined, the results as shown in Table 2 were obtained.

Examples 6 to 8 and Comparative Example 5 100 parts of the base compound prepared in Examples 1 to 3 was added to 50 parts of the polyorganosilsesquioxane A, B or C obtained in Synthesis Examples 1 to 3 and azobis as a foaming agent. 2.5 parts isobutyronitrile, 0.5 parts 2,4-dichlorobenzoyl peroxide as a vulcanizing agent and dicumyl peroxide
1.5 parts was added and kneaded with a two-roll mill to prepare a foamable silicone rubber composition.

Next, this foamable silicone rubber composition was used with a cylinder diameter of 40 mm / φ and the ratio of the cylinder length L to the diameter D was L / D =
It is supplied to an extruder equipped with a 20mmφ / 10mmφ die at 12, and then at room temperature (15-30 ° C), the outer diameter is 20mmφ and the inner diameter is 10mm.
φ tubular silicone rubber molded product was continuously extruded and molded, and hot air at 130 ° C was circulated in a 1.5 m long UHF vulcanization part that transmits a microwave of frequency 2,450 ± 50 MHz with an output of 2 kw. While passing it at a conveying speed of 1.5 m / min, foam and vulcanize and cure to make a silicone rubber foam molded body,
When the physical properties of this molded product were examined, the results as shown in Table 3 were obtained.

However, for comparison, the foamable silicone rubber composition to which the above polyorganosilsesquioxane was not added was treated in the same manner as above to prepare a silicone rubber foamed molded product, and its physical properties were examined. The results as shown in the table were obtained.

Examples 9-14, Comparative Example 6 The polyorganosilsesquioxane B obtained in Synthesis Example 2 was added to 100 parts of the base compounds prepared in Examples 1-3 in the amounts shown in Table 4, and 50 parts of polyorganosilsesquioxanes B-1, B-2 and B-2 made by changing the mixing molar ratio of cyanoethyltriethoxysilane and methyltrimethoxysilane in 2 were added, and then these were added as a foaming agent. Azobisisobutyronitrile
2.0 parts and 0.5 parts of 2,4-dichlorobenzoyl peroxide as a vulcanizing agent and 1.5 parts of dicumyl peroxide were added and kneaded with a two-roll to prepare a foamable silicone rubber composition.

Then, this foamable silicone rubber composition was used in Example 6.
The properties as shown in Table 4 were obtained when the physical properties of the obtained silicone rubber foamed molded product were vulcanized by UHF vulcanization in the same manner as in ~ 8, and the results shown in Table 4 were obtained. A foamable silicone rubber composition (Comparative Example 6) obtained by treating in the same manner as above except that the addition amount of Oxane B was 5 parts was also subjected to UHF vulcanization in the same manner as above,
When the physical properties were examined, the results as shown in Table 4 were obtained.

(Effects of the Invention) The present invention relates to a silicone rubber composition that can be cured by an ultra-high frequency vulcanization method. As described above, this is a polyorganosiloxane, a silica filler, and a cyanoalkyl directly bonded to a silicon atom. Group, acryloxyalkyl group, methacryloxyalkyl group and / or γ
-Organopolysilsesquioxane containing a trifluoroalkyl group, a curing agent, and optionally a foaming agent are added, but this organosilsesquioxane enhances microwave absorption. Since it generates heat when irradiated with microwaves, it can be subjected to high-frequency vulcanization, and the silicone rubber molded product obtained by this method is in a good vulcanized state and can be colored in various hues and foamed. Addition of the agent gives a foamed molded product with a high expansion ratio and fine and uniform foam cells, so this product is a material for continuous extrusion molding such as construction gaskets, tube materials, rolls, and thick foams. Is said to be useful.

Claims (3)

(57) [Claims] 1. An average composition formula RaSiO _{4-a /} (wherein R is an unsubstituted or substituted monovalent hydrocarbon group having 1 to 10 carbon atoms, which is the same or different, and a is a positive number of 1.95 to 2.05). , 100 parts by weight of diorganopolysiloxane having an average degree of polymerization of 3,000 to 30,000, 2) 10 to 300 parts by weight of finely divided silica filler, 3) cyanoalkyl group or acryloxyalkyl directly bonded to silicon Group, a methacryloxyalkyl group, and a polyorganosilsesquioxane containing at least one group selected from γ-trifluoroalkyl groups 10 to 200 parts by weight, and 4) a curing agent. Rubber composition. 2. The silicone rubber composition according to claim 1.
A method of curing a silicone rubber composition, which comprises curing with microwave irradiation of 900 to 5,000 megahertz. 3. The silicone rubber composition according to claim 1
A cured silicone rubber, which is obtained by curing with microwave irradiation of 900 to 5,000 megahertz.