Flame retardant organosilicon polymer composition, process for making same, and article produced therefrom

Abstract: Disclosed are flame retardant organosilicon compositions, wherein the organosilicon compositions comprise residues derived from (a) polycyclic polyene compounds and (b) residues derived from cyclic polysiloxanes, tetrahedral siloxysilanes and linear poly(organohydrosiloxane)s. Preferably the compositions contain flame retardants, such as a phosphorus containing or halogen containing flame retardant, which may be microencapsulated. Also disclosed are processes for making such compositions and articles made from such compositions.

Description: The present invention is directed to the field of organosilicon polymeric materials and articles, and to processes for preparing such materials and articles. Leibfried, in U.S. Patent Nos. 4,900,779, 4,902,731, 5,013,809, and 5,077,134, Bard and Burnier, in U.S. Patents 5,008,360 and 5,068,303, and Burnier, in U.S. Patent No. 5,025,048, describe crosslinked organosilicon polymers and crosslinkable organosilicon prepolymers comprised of polycyclic hydrocarbon residues and cyclic polysiloxanes or siloxysilane residues linked through carbon to silicon bonds, and processes useful for preparing the same. Cowan, in U.S. Patent No. 4,877,820, and Burnier, in U.S. Patent No. 5,025,048, disclose crosslinked or crosslinkable linear poly(organohydrosilane) polymers having at least 30% of their ≡ SiH groups reacted with hydrocarbon residues derived from polycyclic polyenes. The crosslinked polymers have the desirable characteristics of high glass transition temperature (Tg), low dielectric constant, and low moisture absorption, in addition to other desirable properties. The polymers and prepolymers are useful for electronic applications, such as preparing printed circuit boards (including substantially tack-free prepreg and laminates useful for preparing such circuit boards) and encapsulated electronic components, and structural applications. U.S. Patent Nos. 4,900,779 and 4,902,731 state that the thermoset polymers are fire retardant and burn very slowly when subjected to flame. In addition, these patents state that the organosilicon polymers described therein are tough thermoset polymers which pyrolyze upon heating to a temperature greater than 1000°C and that this high temperature resistance makes these polymers useful as refractory materials, fire resistant materials, and ablative materials. While some of the above polymers are fire retardant in that they eventually extinguish when removed from a flame, they are not sufficiently fire retardant (do not self extinguish quickly enough) to meet standards required for many electronic applications. Accordingly, it is an object of the invention to provide an organosilicon composition that self extinguishes quickly enough to meet fire retardant standards required for electronic applications. It is also an object of the invention to provide an organosilicon composition that self extinguishes quickly enough to meet fire retardant standards required for electronic applications, and also has properties desirable for electronic applications such as high glass transition temperature (Tg), low dielectric constant, and low moisture absorption. It is another object of the invention to provide such flame retardancy properties to the organosilicon compositions without adversely affecting processing conditions for forming such organosilicon compositions, for instance without substantial degree of inactivation of the catalyst. It is a further object of the present invention to provide an organosilicon prepolymer having improved storage stability. According to this invention, an organosilicon composition is provided which comprises at least one member selected from the group consisting of an organosilicon polymer and an organosilicon prepolymer. The organosilicon polymer or prepolymer comprises residues of at least one silicon-containing compound and at least one polycyclic polyene compound linked through carbon to silicon bonds, wherein the silicon-containing compound comprises at least one member selected from the group consisting of: A. cyclic polysiloxanes compounds containing at least two hydrosilation reactive ≡ SiH groups; B. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡ SiH groups; and C. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡ SiH groups; wherein the polycyclic polyene compound contains at least two hydrosilation reactive carbon-carbon double bonds; wherein at least a substantial portion of at least one member selected from the group consisting of the silicon compound and the polycyclic polyene compound has more than two reactive sites. The organosilicon composition further comprises at least one flame retardant comprising a microencapsulated bromine containing flame retardant. The composition comprises a flame retardant compound or residue thereof. The flame retardant compounds may be reactive (contain functional groups reactive in hydrosilation and react with the silicon-containing compounds or polycyclic polyene compounds) or nonreactive. The flame retardants may be present in the composition in the form of a compound in admixture with the polymer or may be present in the composition in the form of a residue on the prepolymer or polymer. Preferably the flame retardant comprises at least one member selected from the group consisting of phosphorus containing compounds, phosphorus containing residues, halogen containing compounds and halogen containing residues. In a more preferred embodiment the flame retardant composition comprises at least one member selected from the group consisting of phosphorus containing compounds or residues. Also in a more preferred embodiment the flame retardant composition comprises at least one member selected from the group consisting of halogen containing compounds or residues. In an additional more preferred embodiment the flame retardant composition comprises at least one member selected from the group consisting of bromine containing compounds or residues. In another more preferred embodiment the flame retardant composition comprises at least one member selected from the group consisting of chlorine containing compounds or residues. In a more preferred embodiment the flame retardant comprises at least one member selected from the group consisting of ammonium polyphosphates, residues of ammonium polyphosphates, phosphazenes, residues of phosphazenes, phosphine oxides, residues of phosphine oxides, phosphate esters, residues of phosphate esters, elemental red phosphorus, brominated diphenyl oxides, brominated polystyrenes, brominated bisphenol A's, hexachlorocyclopentadiene derivatives, and residues of hexachlorocyclopentadienes. Preferably the microencapsulated bromine containing flame retardant is microencapsulated in at least one member selected from the group consisting of gelatins, waxes, cellulose derivatives, epoxies, acrylates, nylons, urethanes, urea formaldehyde resorcinol resins, and melamine-, urea- and phenol- formaldehyde resins. More preferably the microencapsulated flame retardant comprises at least one member selected from the group consisting of brominated alkyls, brominated diphenyl oxides, brominated polystyrenes, brominated bisphenol A's. Preferably, the composition further comprises a flame retardant synergist comprising a metal oxide. More preferably, the flame retardant synergist comprises at least one member selected from the group consisting of antimony oxide, sodium antimonate, zinc borate, zinc stannate, iron oxide, and titanium dioxide, and wherein the synergist is present in an amount of from 0.5 weight percent to 8 weight percent based on the weight of the flame retardant. Flame retardants are preferably present in an amount of about 2 to about 90 weight percent, more preferably about 2 to about 50 weight percent, and most preferably about 3 to about 35 weight percent, based on the weight of the composition. Preferably, the ratio of hydrosilation reactive carbon-carbon double bonds in the polycyclic polyene compound and the flame retardant to hydrosilation reactive ≡ SiH groups in the silicon containing compound and the flame retardant is in the range of from 0.5:1 up to 1.8:1, and the composition comprises the flame retardant in an amount of from 2 weight percent to 90 weight percent, based on the overall weight of the composition. More preferably, the ratio of hydrosilation reactive carbon-carbon double bonds in the polycyclic polyene compound and the flame retardant to hydrosilation reactive ≡ SiH groups in the silicon containing compound and the flame retardant is in the range of from 0.6:1 to 1.3:1, and the composition comprises the flame retardant in an amount of from 2 weight percent to 50 weight percent, based on the weight of the composition. Most preferably, the ratio of hydrosilation reactive carbon-carbon double bonds in the polycyclic polyene and the flame retardant to hydrosilation reactive ≡ SiH groups in the silicon containing compound and the flame retardant is in the range of from 0.8:1 to 1.1:1, and the composition comprises the flame retardant in an amount of from 3 weight percent to 35 weight percent, based on the weight of the composition. Preferably the polycyclic polyene compound contains two hydrosilation reactive carbon-carbon double bonds in its rings. More preferably the polycyclic polyene is selected from the group consisting of dicyclopentadiene, tricyclopentadiene, norbornadiene dimer, bicycloheptadiene, dimethanohexahydronaphthalene, and methyl dicyclopentadiene, and the silicon containing compound is a cyclic polysiloxane. Preferably, the composition comprises up to 90%, by weight of the composition, of at least one filler. More preferably, the filler is selected from the group consisting of carbon black, vermiculite, mica, wollastonite, calcium carbonate, silica, fused silica, fumed silica, synthetic silica, glass spheres, glass beads, ground glass, waste glass and glass fibers. Preferably, 127 mm by 12.7 mm by 12.7 mm (5 inch by 1/2 inch by 1/2) inch crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. Also according to this invention, an organosilicon composition is provided comprising at least one member selected from the group consisting of an organosilicon polymer and an organosilicon prepolymer as described above, wherein 127 mm by 12.7 mm by 12.7 mm (5 inch by 1/2 inch by 1/2 inch inch) crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. Also according to this invention an organosilicon composition is provided comprising at least one member selected from the group consisting of an organosilicon polymer and an organosilicon prepolymer as described above, wherein 127 mm by 12.7 mm by 3.18 mm (5 inch by 1/2 inch by 1/8 inch inch) crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. Also according to this invention, an organosilicon composition was provided comprising at least one member selected from the group consisting of an organosilicon polymer and an organosilicon prepolymer as described above, wherein 127 mm by 12.7 mm by 50.8 µm (5 inch by 1/2 inch by 2 mils mils) crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. Most preferably, the crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the UL94V-0 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. Also according to this invention, a method for making a flame retardant organosilicon composition, is provided which comprises the steps of: A. providing at least one silicon-containing compound wherein the silicon-containing compound comprises at least one member selected from the group consisting of: (i) cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡ SiH groups; (ii) tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡ SiH groups; and (iii) linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡ SiH groups; B. providing at least one polycyclic polyene monomer having at least two non-aromatic hydrosilation reactive carbon-carbon double bonds; C. providing at least one flame retardant compound comprising a microencapsulated bromine containing flame retardant ; D. reacting the at least one silicon-containing compound and the at least one polycyclic polyene monomer in the presence of a hydrosilation catalyst, so that an organosilicon crosslinkable prepolymer or crosslinked polymer is formed; wherein at least a substantial portion of at least one member selected from the group consisting of the at least one silicon-containing compound and the at least one polycyclic polyene monomer, has more than two reactive sites. Preferably, the polycyclic polyene monomer is reacted with the silicon-containing compound so that a prepolymer is produced, followed by the addition of at least one member selected from the group consisting of a reactive flame retardant compound and a nonreactive flame retardant compound to the prepolymer, followed by polymerization of the prepolymer so that a crosslinked organosilicon polymer is formed. Also according to this invention, a method for making a flame retardant, crosslinked organosilicon composition is provided which comprises the steps of: (1) providing a crosslinkable organosilicon prepolymer comprising residues of at least one silicon-containing compound and at least one polycyclic polyene compound linked through carbon to silicon bonds, when the silicon-containing compound comprises at least one member selected from the group consisting of: A. cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡ SiH groups; B. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡ SiH groups; and C. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡ SiH groups, wherein the polycyclic polyene compound contains at least two hydrosilation reactive carbon-carbon double bonds; wherein at least a substantial portion of at least one member selected from the group consisting of the silicon-containing compound and the polycyclic polyene compound has more than two reactive sites; and wherein 30% to 70% of the hydrosilation reactive ≡ SiH groups are reacted; (2) providing a flame retardant compound comprising a microencapsulated bromine containing flame retardant ; (3) mixing the crosslinkable organosilicon prepolymer and the flame retardant compound; and (4) curing the organosilicon prepolymer in the presence of a hydrosilation catalyst to form a crosslinked organosilicon composition; and wherein 12.7 cm by 1.27 cm by 1.27 cm (5 inch by 1/2 inch by 1/2 inch) polymer specimens of the crosslinked organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94 V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. Also according to this invention an article of manufacture is provided which comprises a cured, crosslinked organosilicon polymer, wherein the article of manufacture is selected from the group consisting of an electronic component which comprises, or an electronic component coated or encapsulated with, a crosslinked organosilicon polymer comprising: A. cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡ SiH groups; B. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡ SiH groups; and C. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡ SiH groups; wherein the polycyclic polyene compound contains at least two hydrosilation reactive carbon-carbon double bonds; wherein at least a substantial portion of at least one member selected from the group consisting of the silicon-containing compound and the polycyclic polyene compound has more than two reactive sites; and wherein the article of manufacture exhibits flammability characteristics which meet the criteria for the UL94 V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test, and wherein the organosilicon composition contains at least one flame retardant, comprising a microencapsulated bromine containing flame retardant. The article is preferably selected from the group consisting of electronic components comprising the organosilicon composition or an electronic component coated or encapsulated (potted or sealed) with the organosilicon composition. Preferably, the component comprises at least one member selected from the group consisting of electronic circuit boards, circuit board laminates, circuit board prepregs, semiconductor devices, capacitors, resistors, diodes, transistors, transformers, coils, wires, hybrid circuits, and multichip modules. In the article of manufacture the organosilicon composition comprises at least one flame retardant. Preferably, in the article the crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the UL94V-0 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. The article may preferably contain at least one filler or a substrate which is coated (either fully or partially coated) with the flame retardant crosslinked organosilicon polymer composition. Herein, "SiH" is used to describe hydrosilation reactable ≡SiH groups. As used herein, the term "flame retardant" refers to any agent which, when combined with an organosilicon polymer, inhibits combustion of the polymer. The flame retardant may be nonreactive or reactive, relative to any monomers which react to form the organosilicon polymer described hereinafter. A current criterion for demonstrating flame retardant properties is to meet the specifications for the Underwriter Laboratory's UL94 tests described in "Standard for Test for Flammability of Plastic Materials for Parts in Devices and Appliances, UL94", June 16, 1988 for UL94-80, Third Edition. Briefly, this test involves burning five samples from the bottom and measuring the time for the flame to extinguish. Two flame applications on each of the five samples are required. The best rating is V-0, which has the requirement that the total flaming time for the samples be less than 50 seconds, with no single burn exceeding 10 seconds. The next rating is V-1, which requires the total burn time to be less than 150 seconds, with no single burn exceeding 30 seconds. The sample sizes required by the test are 1.27 cm wide by 12.7 cm long (1/2 inch wide by 5 inches long). The sample thickness is variable but the test must be passed with the thicknesses which will be used. For circuit board laminate this would range from a single sheet to a rigid laminate having a thickness of from 50.8 mm to 1.52 µm (2 mils (1/500 inch) to 60 mils (1/16 inch)). Encapsulant formulations are typically tested at 1/8 inch thickness. So that the organosilicon composition and process are defined with respect to their uses, they are preferably defined with respect to crosslinked specimens having thicknesses of 1.27 cm, 3.18 mm, 1.59 mm, and 50.8 µm (1/2 inch, 1/8 inch, 1/16 inch and 2 mil). In the case of articles of manufacture, the entire article of manufacture must meet the flame retardancy test for Underwriter Laboratory certification. As used herein, the term "residue" is defined as a component in a composition which is present as that portion of a reactant which resides upon a larger molecule, which larger molecule is a product of a chemical reaction (e.g. reactions resulting in the formation of prepolymers and polymers), and/or substituents on said polymers and prepolymers. Thus, the term "residue" as used herein, generally refers to that portion of a reactant which resides in the resulting prepolymer or polymer. The present invention may advantageously comprise at least one "filler". Typical fillers include but are not limited to carbon black, vermiculite, mica, wollastonite, calcium carbonate, sand, glass spheres, glass beads, ground glass, waste glass, fused silica, fumed silica, synthetic silica, glass fibers, and glass flakes. Other useful fillers include the other fiber reinforcements which are described in U.S. Patent Nos. 4,900,779, 4,902,731, 5,008,360 and 5,068,303. Fillers can be present in amounts up to about 90% by weight, preferably 25% to 85% by weight, based on the weight of the composition. As used herein, the term "substrate" refers to any article which may be coated (either wholly or partially) with the organosilicon composition of the present invention. The article of the present invention preferably comprises a crosslinked organosilicon composition and a flame retardant component (present as a residue and/or in admixture with the polymer). In one preferred embodiment, the article of the present invention comprises an article of manufacture which is coated (wholly or in part) with the crosslinked organosilicon composition having the flame retardant. As used herein, the term "microencapsulated" is used with reference to materials which are coated so that interaction of the core material with the monomer, prepolymer, and/or polymer is prevented, or at least substantially reduced. A more detailed explanation of this term is provided below. As used herein, the term "synergist" is defined as a substance which, when employed in combination with one or more flame retardant species, enhances the flame retardancy of the composition. As described herein, the phrase "at least a substantial portion of a reactive monomer having more than two reactive sites thereon" is met if a substantial portion of at least one of the reactive compounds has more than two reactive sites thereon, so that a desired degree of crosslinking may be produced in a final polymerization (i.e. curing) step. The phrase "reactive monomer" may include the silicon-containing compound and the polycyclic polyene compound. In some embodiments described below additional compounds are considered in determining the necessary number of reactive groups for the desired degree of crosslinking. The flame retardant (i.e. flame retardant compound or residue) comprising a microencapsulated bromine containing flame retardant may, in general, comprise any material which has the effect of increasing the level of flame retardancy of a composition. Preferably the flame retardant comprises at least one phosphorus atom or at least one halogen atom. Phosphorus compounds, such as ammonium polyphosphates and phosphazenes, impart flame retardance to the organosilicon polymer formulations described herein, and are among the preferred flame retardants for use in the present invention. Useful phosphorus flame retardants include microencapsulated ammonium polyphosphate derivatives (such as Exolit® 455 and Exolit® 462 from Hoechst Celanese), intumescent ammonium polyphosphate formulations (such as Exolit® IFR20, also from Hoechst Celanese), phosphine oxide derivatives (such as Cyagard® RF 1204 produced by American Cyanamid), phosphate ester derivatives and elemental red phosphorus (such as Amgard® CPC, produced by Albright and Wilson Americas). The effectiveness of phosphorus additives as flame retardants in organosilicon polymers is based on the absolute loading of the active element, phosphorus, with respect to the total weight of the composition. Compositions with burn times within the UL94V-1 rating have been obtained using phosphorus additives such as ammonium polyphosphate or phosphine oxides at a loading of from about 3.5% by weight phosphorus to about 7% by weight phosphorus, with the most preferred range being from about 3.5% by weight phosphorus to about 5% by weight phosphorus. Intumescent flame retardant formulations and formulations containing flame retardant components microencapsulated in nitrogen-containing polymers, such as melamine-formaldehyde resins, show decreased burn times over the phosphorus flame retardant additives which contain little to no nitrogen other than ammonium ions. A preferred flame retardant additive is Exolit® 462 (available from Hoechst Celanese Corp.). This additive is an ammonium polyphosphate microencapsulated within a melamine formaldehyde wall polymer. Samples with burn times within the UL94V-1 rating have been obtained using these phosphorus additives at loadings of from about 3% by weight phosphorus to about 7% by weight phosphorus. Samples containing phosphorus flame retardant additives with burn times within the UL94V-0 rating have been obtained using these additives at a loading of greater than about 10% by weight phosphorus, preferably greater than about 12% by weight phosphorus. Another class of flame retardants useful in the present invention comprises halogenated compounds. Preferred are those containing chlorine and bromine. Chlorine containing flame retardants, such as hexachlorocyclopentadiene derivatives and residues of hexachlorocyclopentadiene, are useful in these organosilicon polymers. Chlorinated flame retardants such as Dechlorane Plus® available from Occidental Chemical Corporation are especially effective in combination with tin synergists. Samples with burn times in the UL94V-1 range are obtained with combinations of greater than about 17% by weight chlorine together with greater than about 1.0% by weight zinc stannate, ZnSnO 3 . The preferred range is about 20% by weight chlorine together with about 2.0% by weight zinc stannate. The ratio of chlorine to tin can be varied to obtain optimal burn performance. Many brominated flame retardants, even at low levels, interfere with the catalyzed hydrosilation cure reaction to prepare the organosilicon compositions. It has been discovered that minimal interference with the catalyzed cure reaction is observed when these brominated flame retardants are microencapsulated. It has been found that high levels of the additive can be incorporated into the organosilicon prepolymer without adversely affecting the catalyzed hydrosilation cure reaction when the brominated flame retardants are microencapsulated. Examples of these types of additives include but are not limited to the following: brominated alkyls, brominated diphenyl oxides such as decabromodiphenyl oxide, coctabromodiphenyl oxide and higher oligomers of brominated phenyl ethers, for example Saytex® 102, Saytex® 111 and Saytex® 120 available from Ethyl Corporation and DE-83 from Great Lakes Chemical Corporation. Brominated polystyrene can also be used, such as Pyro Chek® LM or Pyro-Chek® 68PB from Ferro Corporation. Substituted and nonsubstituted brominated bisphenol A additives, such as BE-51 available from Great Lakes Chemical Corporation or Saytex® RB-100 from Ethyl Corporation, are also useful. Any other brominated compound suitable for use as a flame retardant additive which has low solubility in the resin solutions or is able to be microencapsulated can be used in this invention. The microencapsulation wall or coating material may be comprised of any material typically used to form a protective wall. Suitable encapsulants include: gelatins, waxes, cellulose derivatives, epoxies, acrylates, nylons, urethanes, urea formaldehyde resorcinol resins, and melamine-, urea- and phenol-formaldehyde resins. Additional wall materials can also be used. Any materials which effectively shield a halogenated flame retardant additive from the hydrosilation catalyst, and which do not interfere with the high temperature cure of the organosilicon polymers, are potentially useful in the present invention. Preferred wall materials include urea formaldehyde resorcinol resins and melamine-formaldehyde resins, combinations of these resins with other additives, and polyacrylates. These resins do not interfere with the catalyst effectiveness in the high temperature cure of organosilicon polymers. The effectiveness of microencapsulated brominated additives as flame retardants in organosilicon polymers is based on the loading of the active element, bromine, with respect to the total weight of the formulation. The preferred flame retardant additive is Pyro-Chek® 68PBG, a finely ground high molecular weight brominated polystyrene available from Ferro Corporation, Hammond, Indiana. The preferred microencapsulation material for this brominated additive is a urea formaldehyde resorcinol or melamine-formaldehyde, where the wall comprises from about 5% to about 19% by weight of the flame retardant additive. Melamine-formaldehyde is a preferred wall material because it acts as both a wall material and a flame retardant. Urea formaldehyde resorcinol is a more preferred resin because it has the least affect on polymer properties, such as low moisture absorption and low dielectric constant. In general, microencapsulated flame retardants are present as microscopic particles which generally are substantially spherical in shape, as opposed to fibrous or flake-shaped. These particles generally have a diameter of from about 2 µm (microns) to about 150 µm (microns). The particles are comprised of a core of the flame retardant, which core is surrounded by a wall of the encapsulating material. The phrase "wall-to-core" ratio is used to express the ratio of (1) the weight of the "wall" component which surrounds and encapsulates the core of flame retardant agent to (2) weight of the "core" of flame retardant agent. In general, the wall-to-core ratio should be high enough that the flame retardant component therewithin is effectively shielded from the polymerization catalyst. The wall-to-core ratio can be varied over a wide range to minimize interaction between the hydrosilation catalysts and the brominated flame retardant additives, or to maximize active flame retardant payload in the capsules. The wall-to-core ratios for this invention typically range from about 50% by weight wall/50 % by weight core, up to 2% by weight wall/95 % by weight core. The preferred range is from 20% by weight wall/80% by weight core up to 5% weight wall/95% by weight core. In the case of nitrogen-containing wall materials, such as a melamine-formaldehyde resin, the wall may also impart a measure of flame retardance to the polymers. It has also been discovered that certain brominated additives having low solubility in the resin or resin solution can be used without microencapsulating them. (Resin soluble halogenated retardants generally must be microencapsulated to prevent inhibition of catalyst activity.) For example, Pyro-Chek® 68PBG can also be used without a coating as a flame retardant for laminates prepared with organosilicon resins. Resin samples containing Pyro-Chek® 68PBG show substantial increases in the gel point of the resin, indicating that the flame retardant does interact with the catalyst. At high temperatures the activity of the resin is increased sufficiently to overcome the catalyst inhibiting effect of the flame retardant additive. Laminate samples can be prepared using various levels of Pyro-Chek® 68PBG as a flame retardant additive if sufficiently high temperatures are used during processing. The low temperature catalyst inhibiting effect provides low temperature storage stability to prepolymers made with this flame retardant. Samples with burn times within the UL94V-1 range were obtained when the bromine level in the sample was from about 6% to about 18% by weight, based on the weight of the entire formulation. A preferred range is from about 7% to about 10% by weight bromine. Samples having burn times within the UL94V-0 range were obtained using a minimum bromine level of from about 18% to about 20% by weight, based on the weight of the organosilicon composition. These ranges can change with the sample thickness and the brominated flame retardant employed. Flame retardant polymer formulations can comprise combinations of flame retardants, such as combinations of both phosphorus and halogen-containing additives (preferably phosphorus containing and microencapsulated bromine containing additives) improve the flame retardant performance of these organosilicon polymers. The phosphorus and bromine flame retardant additives described above can be combined at various loadings and phosphorus-to-halogen ratios to give organosilicon polymer formulations covering a range of flame retardant performance. Loadings of the phosphorus-containing and bromine-containing flame retardant additives which give desirable flammability performance in organosilicon polymers (i.e. as determined by the UL94V burn test) are as follows. Samples with burn times within the UL94V-1 rating were obtained using a combination of additives which ranged from about 0.5% by weight phosphorus to about 4% by weight phosphorus, as the bromine content of the polymer was varied inversely from 6% by weight bromine to 1 % by weight bromine. Samples with bum times within the UL94V-0 rating have been obtained using from about 0.5% by weight phosphorus, or greater, as the bromine content of the polymer was varied inversely from at least about 19% by weight bromine to about 9% by weight bromine. Preferred ranges include from about 2.5% to about 5% by weight phosphorus, as the bromine content of the polymer was varied inversely from about 15% to about 11 % by weight bromine. It has been found that the performance of flame retardant agents can be enhanced through the use of "synergists", i.e. substances which, although not recognized as flame retardants, enhance the performance of flame retardant agents present in combination with organosilicon polymers. Synergists have been found to be especially effective when used in combination with halogen-containing flame retardants. The synergist may be any substance which enhances the performance of a flame retardant when present in combination therewith, including the following: antimony oxide, sodium antimonate, zinc borate, zinc stannate, iron oxide, and titanium dioxide, as well as others. The synergist may be present in any amount which is effective in enhancing the flame retardancy of the composition. Preferably the synergist is present in amount of from 0.5 weight percent to 10 weight percent, based on the weight of the flame retardant present in the composition. Still more preferably, the synergist is present in an amount of from 0.5 weight percent to 8 weight percent, based on the weight of the flame retardant present in the composition. Most preferred for laminate for electronic circuit boards are high molecular weight brominated polystyrenes (such as Pyro-Chek® 68PBG), which is not microencapsulated, because they have minimal effect on electronic and mechanical properties of the organosilicon composition. Most preferred for encapsulants for electronic components is microencapsulated high molecular weight brominated polystyrenes (such as Pyro-Chek® 68PBG encapsulated in urea formaldehyde resorcinol). Any cyclic polysiloxane or tetrahedral siloxysilane with two or more hydrogen atoms bound to silicon can be used to form the crosslinked organosilicon polymer or hydrosilation crosslinkable organosilicon prepolymer. Cyclic polysiloxanes useful in forming the products of this invention have the general formula: wherein R is hydrogen, a saturated, substituted or unsubstituted alkyl or alkoxy radical, a substituted or unsubstituted aromatic or aryloxy radical, n is an integer from 2 to about 20, and R is hydrogen on at least two of the silicon atoms in the molecule. Suitable such silicon compounds include those disclose in U.S. Patent No. 4,900,779, 4,902,731, 5,013,809, 5,077,134, 5,008,360, 5,068,303, and 5,025,048. Examples include trimethylcyclotrisiloxane, tetraoctyl-cyclotetrasiloxane, and hexamethylcyclotetrasiloxane; tetra- and penta-methycyclotetrasiloxanes; tetra-, penta-, hexa- and hepta-methycyclopentasiloxanes; tetra-, penta- and hexamethylcycloexasiloxanes, tetraethylcyclotetrasiloxanes and tetraphenylcyclotetrasiloxanes. Preferred are polysiloxanes comprising 1,3,5,7-tetramethycyclotetrasiloxane, 1,3,5,7,9-pentamethylcyclopentasiloxane and 1,3,5,6,9,11-hexamethylcyclohexa-siloxane, or blends thereof. Preferred cyclic polysiloxanes are the methylhydrocyclosiloxanes. In most cases, what may be used is a mixture of a number of species of these, wherein n can vary widely. Reference to methylhydrocyclosiloxanes herein is intended to refer to such mixtures. The tetrahedral siloxysilanes are represented by the general structural formula: wherein R is as defined above and is hydrogen on at least two of the silicon atoms in the molecule. Examples of reactants of Formula (Ib) include, e.g., tetrakisdimethylsiloxysilane, tetrakisdiphenylsiloxysilane, and tetrakisdiethylsiloxysilane. Tetrakisdimethylsiloxysilane is the best known and preferred species in this group. Polymers and prepolymers made with cyclic polysiloxanes or tetrahedral siloxysilanes may also contain other hydrosilation reactable polysiloxanes bearing two or more SiH groups. For instance, they may contain linear, short chain SiH terminated polysiloxanes having the general formula: wherein n is 0 to 1000 and R is alkyl or aryl, preferably methyl or phenyl, as described by Leibfried in U.S. Patent Nos. 5,013,809 and 5,077,134. These linear, short chain SiH terminated polysiloxanes impart flexibility to the cured polymers and can be used to produce elastomers. The linear poly(organohydrosiloxane) preferably has the general formula: wherein R is a substituted or unsubstituted, saturated alkyl radical or a substituted or unsubstituted phenyl radical, and about 5% to about 50% of the R's are hydrogen and m is an integer from about 3 to 100, and the maximum value of m is preferably 40. Exemplary linear poly(organohydrosiloxanes) include: trimethylsiloxy-terminated dimethylsiloxane-methylhydrosiloxane copolymer, dimethylsiloxy-terminated dimethylsiloxane-methylhydrosiloxane copolymer, dimethylsiloxy-terminated polydimethylsiloxane, trimethylsiloxy-terminated methyloctylsiloxane-methylhydro-siloxane copolymer, dimethylsiloxy-terminated phenylmethylsiloxane- methylhydro-siloxane copolymer, trimethylsiloxy-terminated methylcyanopropyl-siloxane-methylhydrosiloxane copolymer, trimethylsiloxy-terminated 3,3,3-trifluoropropyl-methylsiloxane methylhydrosiloxane copolymer, trimethylsiloxy-terminated 3-aminopropylmethyl siloxane-methylhydrosiloxane copolymer, trimethylsiloxy-terminated 2-phenylethylmethylsiloxanemethylhydrosiloxane copolymer, and trimethylsiloxy-terminated 2-(4-methylphenyl)-ethylmethyl-siloxane-methylhydrosiloxane copolymer. Polycyclic polyene compounds useful in preparing the composition of this invention are polycyclic hydrocarbon compounds having at least two non-aromatic carbon-carbon double bonds that are reactive in hydrosilation. Preferably they are in the rings of the compound. Illustrative are compounds selected from the group consisting of cyclopentadiene oligomers (e.g., dicyclopentadiene, tricyclopentadiene and tetracyclopentadiene), norbornadiene dimer, bicycloheptadiene (i.e., norbornadiene) and its Diels-Alder oligomers with cyclopentadiene (e.g., dimethanohexahydro-naphthalene), and substituted derivatives of any of these, e.g., methyl dicyclopentadienes. Preferred are cyclopentadiene oligomers, such as dicyclopentadiene and tricylopentadiene. Two or more polycyclic polyenes can be used in combination. Other hydrocarbon compounds may also be used. For instance, according to one embodiment described in U.S. Patent No. 5,008,360, the hydrocarbon component comprises (a) at least one low molecular weight (typically having a molecular weight less than 1,000, preferably less than 500) polyene having at least two non-aromatic carbon-carbon double bonds highly reactive in hydrosilation (they may contain other less reactive (including unreactive) double-bonds, provided that those double bonds do not interfere with the reactivity of the highly reactive double bonds; but, compounds having only two highly reactive double bonds are preferred), the carbon-carbon double bonds being either in an alpha, beta or gamma position on a linear carbon moiety, next to two bridgehead positions in a strained polycyclic aliphatic ring structure, or in a cyclobutene ring, and (b) at least one polycyclic polyene having at least two chemically distinguishable non-aromatic carbon-carbon double bonds in its rings that are reactive in hydrosilation. Examples of component (a) include 5-vinyl-2-norbornene, o-, m- or p-diisopropenylbenzene, o-, m- or p-divinylbenzene, diallyl ether, diallyl benzene, dimethanohexahydronaphthalene and the symmetrical isomer of tricyclopentadiene. By "having at least two chemically distinguishable carbon-carbon double bonds" it is meant that at least two carbon-carbon double bonds have widely different rates of reaction in hydrosilation and that one of the double bonds will react prior to substantial reaction of the other double bond(s). This first double bond must be quite reactive in hydrosilation. Reactive double bonds include those that are next to two bridgehead positions in a strained polycyclic aliphatic ring structure or in a cyclobutene ring, as per component (a) of the embodiment described directly above. The other carbon-carbon double bond(s) may be any other non-aromatic, 1,2-disubstituted non-conjugated carbon-carbon double bond that is not next to two bridgehead positions in a strained polycyclic aliphatic ring structure and is not in a cyclobutene ring. Exemplary are dicyclopentadiene and the asymmetrical isomer of tricyclopentadiene. Preferred, for some applications, when using these hydrocarbon compounds are cyclic polysiloxanes containing three or more SiH groups. It is also possible to add other groups to the crosslinked structure. For instance, short chain alkyl groups can be incorporated into the system by attaching them to SiH bearing compounds (e.g., the cyclic polysiloxanes described above) and reacting them with polycyclic polyenes and silicon containing compounds (e.g., the cyclic polysilxoanes and tetrahedral siloxysilanes described above). The reactions for forming the organosilicon prepolymers and polymers of this invention are described in U.S. Patent Nos. 4,900,779, 4,902,731, 5,013,809, 5,077,134, 5,008,360, 5,068,303, 5,025,048, and 4,877,820. The reactions for forming the prepolymers and polymers can be promoted thermally or by the addition of a hydrosilation catalyst or radical generators such as peroxides and azo compounds. Hydrosilation catalysts include metal salts and complexes of Group VIII elements. The preferred hydrosilation catalysts contain platinum (e.g., bis(acetronitrile)platinum dichloride, bis(benzonitrile)platinum dichloride, platinum on carbon, platinum dichloride, cyclooctadieneplatinum dichloride, dicyclopentadieneplatinum dichloride, chloroplatinic acid, etc.). The preferred catalyst, in terms of both reactivity and cost, is chloroplatinic acid (H 2 PtCl 6 .6H 2 O), which is preferably complexed with the olefin as described in U.S. Patent Nos. 4,900,779 and 5,008,360. Catalyst concentrations of 0.0005 to about 0.05% by weight of platinum, based on the weight of the monomers, are preferred. To prepare the thermoset and thermoplastic polymers, several approaches are available. It is possible, by selection of reactants, reactant concentrations and reaction conditions, to prepare polymers exhibiting a broad range of properties and physical forms. Thus, it has been found possible to prepare tacky solids, elastomeric materials, and tough glassy polymers. In one approach, the correct relative ratios of reactants and the hydrosilation catalyst are simply mixed and brought to a temperature at which the reaction is initiated and proper temperature conditions are thereafter maintained to drive the reaction to substantial completion (typically, with a ratio of carbon-carbon double bonds to SiH groups of about 1:1, when 70 to 90% of the SiH groups are consumed). Generally, the ratio of carbon-carbon double bonds in the polycyclic polyene compounds to SiH groups in the silicon containing compounds is in the range of about 0.5:1 to about 0.8:1. Thermoset polymers having a crosslinked structure result when the ratio of carbon-carbon double bonds to SiH groups is in the range of from about 0.6:1 up to about 1.3:1, more preferably from about 0.8:1 up to about 1.1:1. When the flame retardant(s) used contains hydrosilation reactive functional groups, these groups should be considered part of the total reactive functionality of the organosilicon composition. The level of hydrosilation reactive carbon-carbon double bonds of the polycyclic polyene compounds or the level of hydrosilation reactive SiH groups in the silicon-containing compounds should be adjusted in view of these reactive groups to give the desired ratio for the composition. (This is true with respect to other such ratios described in this specification.) B-stage type prepolymers can be prepared as disclosed in U.S. Patent Nos. 4,877,820, 5,008,360 and 4,902,731. Generally, the initial product of the reaction at lower temperatures, e.g., about 25 to about 80°C, is a crosslinkable prepolymer, which may be in the form of a solid or a flowable, heat-curable liquid, even though the ratio of carbon-carbon double bonds to SiH groups is otherwise suitable for cross-linking. The prepolymers generally have 30 to 70% of the SiH groups reacted, and when liquids are desired preferably about 30 to 65% of the SiH groups reacted. Such prepolymers, analogous to the so-called B-stage resins encountered in other thermoset preparations, can be recovered and transferred prior to curing. These prepolymers are prepared using polycyclic polyenes having at least two chemically distinguishable non-aromatic carbon-carbon double bonds in their rings. Illustrative are compounds selected from the group consisting of dicyclopentadiene, asymmetrical tricyclopentadiene, and methyl dicyclopentadiene, and substituted derivatives of any of these. Preferred is dicyclopentadiene. Such prepolymers can also be prepared with the hydrocarbon combinations described in U.S. Patent No. 5,008,360. The prepolymers, including the viscous, flowable liquid prepolymers, are stable at room temperature for varying periods of time, and cure upon reheating to an appropriate temperature, e.g., about 100 to about 250°C. Frequently, additional catalyst is added to the prepolymer prior to cure to further promote the reaction. A second type of prepolymer can be prepared by a process described in U.S. Patent Nos. 5,013,809 and 5,077,134. In this process, an olefin rich prepolymer is prepared by reacting a large excess of polycyclic polyene compounds with cyclic siloxanes or tetrahedral siloxysilanes. The olefin rich organosilicon prepolymer is blended with additional cyclic polysiloxane or tetrahedral siloxysilane before cure. According to this process, organosilicon prepolymers are made with a large excess of carbon-carbon double bonds available for reaction with SiH groups. That is, the ratio of carbon-carbon double bonds in the rings of the polycyclic polyenes used to form the polycyclic polyene residues to SiH groups in the cyclic polysiloxanes and tetrahedral siloxysilanes used to form the cyclic polysiloxane or tetrahedral siloxysilane residues is greater than 1.8:1, preferably greater than 1.8:1 and up to 2.2:1. The prepolymers of this embodiment are generally in the form of flowable liquids, which are stable at room temperature. The most stable prepolymers are formed at a double bond to SiH ratio of about 2:1 since virtually all SiH is reacted and excess polycyclic polyene need not be removed. (Due to their odor, the presence of unreacted polycyclic polyenes may be undesirable. When necessary or desirable, unreacted polycyclic polyenes can be stripped, e.g., using a rotoevaporator, to form odorless compositions.) Later, crosslinked polymers are formed by mixing the prepolymers with the polysiloxanes/siloxysilanes such that the total ratio of non-aromatic carbon-carbon double bonds in the rings of the polycyclic polyenes used to form the polycyclic polyene residues (a) to SiH groups in the polysiloxanes and siloxysilanes used to form the polysiloxane/siloxysilane residues (b) is in the ratio of 0.4:1 to 1.7:1; preferably 0.8:1 to 1.3:1, most preferably about 1:1, and curing the mixture in the presence of a hydrosilation catalyst. Preferably, according to this embodiment, the organosilicon prepolymers are reacted with the polysiloxanes and/or siloxysilanes to form a crosslinked polymer in a mold. The prepolymers and polysiloxanes/ siloxysilanes are stored separately and are blended before entering the mold. The hydrosilation catalyst may be present in either or both stream(s) or injected directly into the mixer. The reaction is exothermic and proceeds rapidly so that the polymer gels and the product can be removed from the mold in minutes. The components of the blends are completely stable until they are mixed. This permits indefinite ambient storage of the materials. Alternately, the blend components can be premixed and stirred in a tank. These blends have low viscosity and are pumpable. Addition of catalyst and/or application of heat can be used to cure the prepolymer composition. The reaction may be carried out in an extruder, mold or oven, or the blend may be applied directly on a substrate or part. With all of the above processes, the reaction speed and its accompanying viscosity increase can be controlled by use of low levels of a cure rate retardant (complexing agent), such as N,N,N',N'-tetramethyl-ethylenediamine, diethylenetriamine or phosphorus compounds, such as those described in "Phosphorus Based Catalyst Retardants for Silicon Carbon Resin Systems", Research Disclosure 326103 (June 1991). Stabilizers (antioxidants) are useful to maintain storage stability of B stage materials and thermal oxidative stability of the final product. Preferred are bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-(3,5-di-tert-butyl-4-hydroxybenzyl)butyl-propanedioate, (available as Tinuvin™ 144 from Ciba-Geigy Corp., Hawthorne, NY) or a combination of octadecyl, 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (also known as octadecyl 3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate), available as Naugard™ 76 from Uniroyal Chemical Co., Middlebury, CT, and bis(1,2,2,6,6-pentamethyl-4-piperidinylsebacate), available as Tinuvin™ 765 from Ciba-Geigy Corp. Stabilizers and their use are described in U.S. Patent No. 5,025,048. An elastomer can be added to improve the toughness of the organosilicon polymer-containing compositions of the present invention. Although any elastomer may be added to impart toughness to the organosilicon polymer compositions of the invention, hydrocarbon elastomers are preferred for use in the present invention. Preferred are ethylene-propylene-ethylidenenorbornene polymers having a molecular weight of from about 5500 to about 7000. Most preferred is Trilene 65 elastomer (obtainable from Uniroyal of Middlebury, CT). Elastomers are generally used in an amount of from about 0.5 to 20 weight percent, preferably from about 3 to about 12 weight percent, based on the weight of the total composition. Elastomers may be added to the monomers or to a prepolymer. Use of elastomers is described in European Patent Application No. 482,404 and U.S. Patent No. 5,171,817, as well as "Organosilicon Compositions Containing Hydrocarbon Elastomers", Research Disclosure 33082 (October 1991). The organosilicon composition of this invention have excellent electrical insulating properties and resistance to moisture. Many also have high glass transition temperatures. Best moisture resistant properties are obtained with brominated flame retardants. The phosphorus flame retardants lower the amount of smoke produced during combustion of the organosilicon compositions. The polymers and prepolymers of this invention are well suited for electronic applications, e.g., composites, adhesives, encapsulants, potting compounds and coatings. They are especially useful for preparing laminates and prepregs, such as those used for printed circuit boards. Such prepregs and laminates are described in U.S. Patent No. 5,008,360. The article of manufacture of this invention is selected from the group consisting of an electronic component comprising the organosilicon composition or an electronic component coated or encapsulated (potted or sealed) with the organosilicon composition. The component typically comprises at least one member selected from the group consisting of electronic circuit boards, circuit board laminates, circuit board prepregs, semiconductor devices, capacitors, resistors, diodes, transistors, transformers, coils, wires, hybrid circuits and multichip modules. The following examples serve to illustrate the invention. They are not intended to be limiting. All percentages and parts are by weight. Flame retardant additives were incorporated into catalyzed prepolymer solutions with stirring to give well dispersed mixtures. These prepolymer formulations were cured or fabricated into laminates using processing conditions similar to non-flame retarded prepolymers. Prepolymer solutions were prepared from the following components. Catalyst solution A was prepared by adding 100 parts dicyclopentadiene and 0.3 parts chloroplatinic acid to a glass container. The mixture was heated at 70°C for 1 hour and cooled to room temperature. Catalyst solution B was prepared by dissolving divinyltetra-methyldisiloxane platinum complex (PC072 Huls America, Bristol, PA) in toluene to give a solution which was 0.4 wt% platinum (Example I - III, V - IX, XI, XII and XXV (except Sample 7), and some of the samples in Example IV) or dissolving cyclooctadienep latinum dichloride (available from Degussa, South Plainfield, NJ) in methyl ethyl ketone to give a solution which was 0.28 wt % platinum (Examples X, some of the samples in Example IV and Sample 7 in Example XXV). Reaction solution A was prepared by combining 252.5 parts methylhydrocyclosiloxane, 11.2 parts octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (antioxidant) available as Naugard™ 76 from Uniroyal Chemical Co., Middlebury, CT, and 2.3 parts bis(1,2,2,6,6-pentamethyl-4-piperidinylsebacate) (antioxidant) available at Tinuvin™ 765 from Ciba-Geigy Corp., Hawthorne, NY, and 13.5 parts toluene. Reaction solution B was prepared by combining 330.3 parts dicyclopentadiene 3.7 parts catalyst solution A and 116.5 parts toluene. A rubber solution was prepared by combining 30 parts Trilene™ 65 rubber available from Uniroyal Chemical Co., Middlebury, CT, with 70 parts toluene. The prepolymer was prepared by heating reaction solution A to 70°C in a suitable glass reaction vessel. Reaction solution B was added dropwise to solution A with stirring to maintain a reaction temperature of 75-85°C. The reaction solution was heated at 70°C for 1 hour after the addition was complete if needed to ensure complete reaction. The prepolymer was activated by adding 3 - 6 parts catalyst solution B to the prepolymer to give a gel point of nearly 5 minutes at 130°C. (The gel point of the prepolymer formulation was the time required for the activated prepolymer formulation to gel or cure on the face of a hot block at a specified temperature. A gel point of nearly five minutes at 130°C indicated the prepolymer had sufficient catalyst activity to cure completely to a high Tg polymer.) The rubber solution (105.3 parts) was then added to the prepolymer solution. Flame retardant additives were incorporated into catalyzed prepolymer solutions with stirring to give well dispersed mixtures. The prepregs were prepared by dipping the appropriate glass (7628 glass for 8 layer, 1.59 mm (0.0625 inch) laminates and 1080 glass for 2 layer, 0.127 mm (0.005 inch) laminates, style CS256 available from Clark Schwebel, White Plains, NY) in the prepolymer varnish solution containing the appropriate flame retardant additives. These prepregs were cured to a nontacky state by placing them in a forced air oven at 130°C - 165°C for 30-80% of the gel time at the prepreg temperature. The appropriate number of prepreg layers were then aligned between the plates of a press and cured at 170°C for 1 hour at 0.689 MPa (100 psi) pressure to form a laminate. The cured laminates were postcured at 250°C for 4 hours. Burn tests were performed using the Underwriters Laboratories UL94V test procedure, as described in Underwriters Laboratories Inc., Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, UL 94-80 Third Edition, June 1988. A Tirrell type burner was used with an inner diameter of 1.27 cm (0.5 inches) and a tube length of 8.89 cm (3.5 inches). Technical grade methane gas was used as recommended in the UL procedure. The gas was passed through a rotameter to get a flow of approximately 0.20 standard liters per minute. The flame height was approximately 19 mm (0.75 inches). A portable ventless fume hood (Captair, model 5008, available from Captair LabX, Inc., N. Andover, MA) was utilized to ensure that the airflow around the sample during burn testing was negligible. The airflow was shut off during burns, but soot and smoke were vented between burns. The sample was clamped at the top so that the bottom of the sample was approximately 12.7 mm (0.5 inches) from the burner. The samples of polyorganosilicon resin did not drip upon testing. The flame was applied to the sample for 10 seconds, following which the flame was removed. The time required for the flaming sample to extinguish was then recorded. When the flaming sample extinguished, the burner flame was reapplied for an additional 10 seconds, and the time required for the sample to extinguish a second time was also recorded. This procedure was repeated five times for the five samples in each set. The sum of all 10 burns (5 samples, 2 burns each) was compared to the rating criteria for burn testing. Example I This example illustrates the effect of brominated additives on the cure kinetics, and the advantage of microencapsulating these brominated additives. The additives were combined with the catalyzed prepolymer solutions and mixed well. The gel point of the resin at a given temperature was taken as a measure of the catalyst activity at this cure temperature. Sam. Br Additive Br GP 130°C GP 150°C GP 170°C 1 none - 5min 45s 1min 50s 50s 2 Pentabromotoluene 1 part >10 min >10 min >10 min 3 Saytex® 102 Decabromodiphenylether 1 part >10 min >10 min >10 min 4 Pyro-Chek® 68PBG Brominated polystyrene 5 parts >10 min 8min 30s 1min 45s 5 Pyro-Chek® 68PBG Brominated polystyrene 15 parts >10 min >10 min 4min 20s 6 Pyro-Chek® 68PBG microencapsulated 15 parts 6min 05s 2min 35s 45s Samples 1 to 5 are comparative Samples 2 and 3, prepared with low levels of non-encapsulated brominated additives, showed gel times of greater than 10 minutes at all temperatures tested. Samples 4 and 5, prepared with Pyro-Chek 68PBG, a high molecular weight brominated polystyrene of low solubility in the resin, retained a gel point at high temperatures. These gel points were substantially longer than the plain resin. Sample 6 was microencapsulated in a urea formaldehyde resorcinol based wall polymer at a loading of 19% by weight wall, 81% by weight core. The samples containing microencapsulated brominated flame retardants showed minimal degradation of the prepolymer gel point even at the high loadings required to meet the UL94V-0 criteria. These results indicate that when the brominated flame retardant additives were microencapsulated there was little or no deleterious interaction between the catalyst and the flame retardants. Example II This example illustrates the effectiveness of microencapsulated brominated additives as flame retardants in glass filled 8 layer, 1.52 mm (0.060 inch) laminates prepared with the organosilicon polymer described above. The flame retardant (Pryo-Chek® 68PBG obtained from Ferro Corporation, Hammond, IN) was microencapsulated in various wall polymers of different core to wall ratios. Two of the samples were prepared using Pyro-Chek 68 PBG, which had not been microencapsulated, using a cure temperature of 160-165°C. Sample BR Level Wall Material Wall Level Burn Time UL Rating 1 0 parts - - 226 sec N.R. 2 20 parts 1 19% 32 sec V-0 3 20 parts 1 10% 48 sec V-0 4 20 parts 1 5% 52 sec V-1 5 7 parts 1 19% 119 sec V-1 6 20 parts 2 19% 24 sec V-0 7 7 parts 2 19% 93 sec V-1 Microencapsulation Wall Polymers: 1 - urea formaldehyde resorcinol 2- melamine formaldehyde These results show that microencapsulated brominated flame retardant additives are effective in producing formulations with UL94V-1 and V-0 formulations, depending on loading. The capsule wall thickness was not critical as long as the core was effectively shielded from the catalyst. Example III This example illustrates the effectiveness of synergists with brominated flame retardant additives. Antimony oxide was added to organosilicon compositions containing brominated flame retardants and 1.52 mm (60 mil) thick glass filled laminated were prepared from the organosilicon composition. Sample Br %Sb 2 O 3 Burn Time UL Rating 1 0 parts 0 parts 226 sec N.R. 2 15 parts 0 parts 83 sec V-1 3 15 parts 3.7 parts 82 sec V-1 4 15 parts 7.5 parts 36 sec V-0 Antimony oxide was found to be a synergist with brominated flame retardants in these organosilicon polymers. Other metal oxides behave similarly. Example IV This example illustrates the effectiveness of combinations of phosphorus and bromine containing flame retardant additives in 8 layer, 1.52 mm (60 mil) thick glass filled laminates. The brominated flame retardant, Pyro-Chek 68PBG (obtained from Ferro Corporation), was microencapsulated with a urea formaldehyde resorcinol based polymer 19% by weight wall, 81% by weight core. The phosphorus flame retardant was Exolit® 422 from Hoechst Celanese Corporation, Charlotte, N.C. Sample P (parts) Br (parts) Burn Time UL Rating 1 0 0 226 sec N.R. 2 1.25 3.75 123 sec V-1 3 5 11 23 sec V-0 4 2.5 15 26 sec V-0 A variety of UL94V-1 and V-0 formulations were obtained by changing the ratio of phosphorus to bromine in the samples as well as the total flame retardant loading. Example V This example illustrates the effectiveness of various flame retardant additives in thin laminates 50.8 µm or 127 µm (2 mil or 5 mil) thick laminates prepared with 1080 or 2313 glass, style CS256 Clark Schwebel, White Plains, N.Y.). The flame retardant additive used was Pyro-Chek® 68 PBG. The encapsulation wall polymer was present as 19% by weight of the brominated flame retardant additive. The symbol N.R. indicates the sample could not be rated by UL94V burn test criteria. Sample Laminate Thickness Bromine Wall Material UL Rating 1 127 µm (5 mils) 20 parts 1 N.R. 2 " 7 parts 1 N.R. 3 " 20 parts 2 V-0 4 " 7 parts 2 N.R. 5 " 20 parts 3 V-0 6 " 7 parts 3 N.R. 7 50.8 µm (2 mils) 20 parts 1 V-0 Microencapsulation Wall Polymer 1 - no wall, cure temperature 160-165°C ; comparative 2 - urea formaldehyde resorcinol (19 parts wall, 81 parts core) 3 - melamine formaldehyde (19 parts wall, 81 parts core) The results from this example showed the additives described impart sufficient flame retardancy to meet UL94V-0 criteria on thin laminate samples prepared with the organosilicon polymers described. Example VI This example illustrates the effect of cyclic phosphazenes as flame retardant additives in 1.52 mm (60 mil) laminate samples prepared from the organosilicon composition as described above. Sample Phosphazene Phosphorus Burn Time UL Rating 1 16.7 parts 1.7 parts 73 sec V-1 2 9.1 parts 1.0 part 110 sec V-1 3 13.5 parts 1.5 parts 70 sec V-1 Samples 1 and 2 are comparative Example VII This example illustrates the burn performance of samples of the organosilicon composition described above. These samples contain no added filler material. The burn time is the time required for a 6.35 cm by 1.27 cm by 0.318 cm (2.5 inch by 0.5 inch by 0.125 inch) bar to extinguish after applying a methane flame to the sample for 10 sec. Sample Flame Retardant Parts Additive Burn Time 1 none 0 parts 61 seconds 2 Pyro-Chek 68 PBG 29.2 parts 2 seconds The sample containing the brominated flame retardant additive would be expected to meet UL94V-0 burn test criteria. Example VIII This example illustrates the burn performance of the organosilicon polymer described above in the presence of a particulate filler material. Samples were prepared as follows. The solvent free prepolymer was formulated to give a mixture with 65-80 parts fused silica, 0.02 parts methacryloxypropyltrimethoxysilane coupling agent (A174, Union Carbide Danbury, CT), 0.4 parts carbon black (Lampblack 101, Degussa, Frankfurt, Germany) and 34.4 to 19.4 parts organosilicon prepolymer. The formulation was mixed and the flame retardant was added at 5 to 10 weight percent based on the total formulation (this decreased the effective percentage of filler). After mixing, the formulation was placed in a flask and degassed in a rotary evaporator for 10-15 min at 70°C. The warm mixture was transferred to a preheated mold, 100°C, and cured in an oven for 3 hr at 180°C. The burn times for filled systems containing various flame retardant additives are given below. Sample Flame Retardant %Filler %Flame Retardant Burn Time UL Rating 1 none 65 0 burned completely - 2 Pyro-Chek® 68 PBG 62 5 111 sec V-1 3 Pyro-Chek® 68 PBG 54 10 19 sec V-0 4 Exolit® 422 / Pyro-Chek 68 PBG 54 10 10 sec V-0 This example illustrates a variety of additives as well as combinations of additives were effective flame retardants for the organosilicon formulations described above in the presence of particulate fillers. Example IX The following example illustrates the effect of increased filler loadings on the burn performance of the organosilicon polymer in the presence of flame retardant. The flame retardant was Pyro-Chek® 68 PBG available from Ferro Corporation, Hammond, IN, which was microencapsulated in a urea formaldehyde resorcinol wall polymer with a wall level shown in the table. Sample %Filler %Flame Retardant Wall Level Burn Time UL Rating 1 60 wt% 8 wt% 5 wt% 72 sec V-1 2 60 wt% 8 wt% 19 wt% 85 sec V-1 3 64 wt% 8 wt% 5 wt% 41 sec V-0 4 69 wt% 8 wt% 5 wt% 33 sec V-0 5 80 wt% 5 wt% 5 wt% 28 sec V-0 The burn time of filled polymer samples containing the same level of flame retardant decreased with increasing percentage of nonflammable filler incorporated into the formulation. Example X The following example serves to illustrate the effect of various flame retardant additives on the moisture absorption and electrical properties of 1.52 mm (60 mil) thick laminate samples prepared with the organosilicon prepolymer as described above. Moisture absorption was measured as follows. A 5.08 cm by 5.08 cm (2 inch by 2 inch) piece of laminate was dried under vacuum at 100°C for nearly one hour and weighed immediately on removing from the oven. The samples were then placed in a constant humidity chamber (85°C, 85 % relative humidity) for 7 days. The weight difference of the samples was recorded immediately on removal from the constant humidity chamber. The moisture absorption is reported as the percentage weight increase. The dielectric constant and dissipation factor were measured using ASTM method D-150. Flame retardant loadings are reported as the percentage of the active flame retardant element in the polymer composition. Sample Flame Retardant Flame Retardant Loading Moisture Absorption Dielectric Constant Dissipation Factor UL Rate 1 None 0% 0.05 wt% 3.60 0.00171 N.R. 2 A 3% 0.25 wt% 3.91 0.01355 V-1 3 B 3% 0.20 wt% - - V-1 4 B 12% 0.92 wt% - - V-0 5 C 5%-P 0.48 wt% 4.09 0.00989 V-0 11%-BR 6 D 20% 0.25 wt% 3.83 0.00324 V-0 7 E 20% 0.20 wt% 3.95 0.00172 V-0 Flame Retardant Additives: A Exolit® 422 from Hoechst Celanese, Charlotte, NC B Exolit® 462 from Hoechst Celanese, Charlotte, NC C Exolit® 422 from Hoechst Celanese, Charlotte, NC with Pyro-Chek® 68PBG from Ferro Corporation, Hammond, IN microencapsulated with 19% urea formaldehyde resorcinol. D Pycro-Chek® 68PBG from Ferro Corporation, Hammond, IN microencapsulated with 19% urea formaldehyde resorcinol. E Pyro-Chek® 68PBG from Ferro Corporation, Hammond, IN (comparative) This example shows the flame retardant compounds which alter the moisture absorption and the electrical properties of the organosilicon polymer the least are microencapsulated brominated compounds having a urea formaldehyde resorcinol wall.

Claims: An organosilicon composition comprising at least one member selected from a crosslinked organosilicon polymer and a crosslinkable organosilicon prepolymer, wherein the organosilicon polymer or prepolymer comprises residues of at least one silicon-containing compound and at least one polycyclic polyene compound linked through carbon to silicon bonds, wherein the silicon-containing compound comprises at least one member selected from : A. cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡SiH groups; B. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡SiH groups; and C. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡SiH groups; wherein the polycyclic polyene compound contains at least two hydrosilation reactive carbon-carbon double bonds; wherein at least a substantial portion of at least one member selected from the silicon compound and the polycyclic polyene compound has more than two reactive sites; and wherein the composition further comprises at least one flame retardant, comprising a microencapsulated bromine containing flame retardant. An organosilicon composition as claimed in claim 1, wherein the flame retardant comprises at least one member selected from phosphorus containing compounds, phosphorus containing residues, halogen containing compounds and halogen containing residues. An organosilicon composition as claimed in claim 2, wherein the flame retardant composition comprises at least one member selected from phosphorus containing compounds or residues. An organosilicon composition as claimed in claim 2, wherein the flame retardant composition comprises at least one member selected from halogen containing compounds or residues. An organosilicon composition as claimed in claim 2, wherein the flame retardant composition comprises at least one member selected from bromine containing compounds or residues. An organosilicon composition as claimed in claim 2, wherein the flame retardant composition comprises at least one member selected from chlorine containing compounds or residues. A composition as claimed in claim 1, wherein the flame retardant comprises at least one member selected from ammonium polyphosphates, residues of ammonium polyphosphates, phosphazenes, residues of phosphazenes, phosphine oxides, residues of phosphine oxides, phosphate esters, residues of phosphate esters, elemental red phosphorus, brominated diphenyl oxides, brominated polystyrenes, brominated bisphenol A's, hexachlorocyclopentadiene derivatives, and residues of hexachlorocyclopentadienes. A composition as claimed in claim 1, wherein the flame retardant is microencapsulated in at least one member selected from gelatins, waxes, cellulose derivatives, epoxies, acrylates, nylons, urethanes, urea formaldehyde resorcinol resins, and melamine-, urea- and phenol- formaldehyde resins. A composition as claimed in claim 8, wherein the flame retardant comprises at least one member selected from brominated alkyls, brominated diphenyl oxides, brominated polystyrenes, brominated bisphenol A's. A composition as claimed in claim 9, wherein the composition further comprises a flame retardant synergist comprising a metal oxide. A composition as claimed in claim 10, wherein the flame retardant synergist comprises at least one member selected from antimony oxide, sodium antimonate, zinc borate, zinc stannate, iron oxide, and titanium dioxide, and wherein the synergist is present in an amount of from 0.5 weight percent to 8 weight percent based on the weight of the flame retardant. A composition of matter as claimed in any of the preceding claims, wherein the ratio of hydrosilation reactive carbon-carbon double bonds in the polycyclic polyene compound and the flame retardant to hydrosilation reactive ≡SiH groups in the silicon containing compound and the flame retardant is in the range of from 0.5:1 up to 1.8:1, and the composition comprises the flame retardant in an amount of from 2 to 90 weight percent, based on the overall weight of the composition. A composition of matter as claimed in claim 12, wherein the ratio of hydrosilation reactive carbon-carbon double bonds in the polycyclic polyene compound and the flame retardant to hydrosilation reactive ≡SiH groups in the silicon containing compound and the flame retardant is in the range of from 0.6:1 to 1.3:1, and the composition comprises the flame retardant in an amount of from 2 to 50 weight percent, based on the weight of the composition. A composition of matter as claimed in claim 13, wherein the ratio of hydrosilation reactive carbon-carbon double bonds in the polycyclic polyene and the flame retardant to hydrosilation reactive ≡SiH groups in the silicon containing compound and the flame retardant is in the range of from 0.8:1 to 1.1:1, and the composition comprises the flame retardant in an amount of from 3 to 35 weight percent, based on the weight of the composition. A composition as claimed in any of the preceding claims, wherein the polycyclic polyene compound contains two hydrosilation reactive carbon-carbon double bonds in its rings. A composition as claimed in claim 12, wherein the polycyclic polyene is selected from dicyclopentadiene, tricyclopentadiene, norbornadiene dimer, bicycloheptadiene, dimethanohexahydronaphthalene, and methyl dicyclopentadiene, and the silicon containing compound is a cyclic polysiloxane. A composition as claimed in any of the preceding claims, wherein the composition comprises up to 90%, by weight of the composition, of at least one filler. A composition as claimed in claim 17, wherein the filler is selected from carbon black, vermiculite, mica, wollastonite, calcium carbonate, silica, fused silica, fumed silica, synthetic silica, glass spheres, glass beads, ground glass, waste glass and glass fibers. An organosilicon composition as claimed in any of the preceding claims wherein 12,7 cm x 1,27 cm x 1,27 cm (5 inch by 1/2 inch by 1/2 inch) crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. A composition as claimed in claim 19, wherein the crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the UL94V-0 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. A method for making a flame retardant crosslinkable organosilicon composition, the method comprising the steps of: A. providing at least one silicon-containing compound wherein the silicon-containing compound comprises at least one member selected from: i. cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡SiH groups; ii. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡SiH groups; and iii. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡SiH groups; B. providing at least one polycyclic polyene monomer having at least two non-aromatic hydrosilation reactive carbon-carbon double bonds; C. providing at least one flame retardant compound comprising a microencapsulated bromine containing flame retardant; D. reacting the at least one silicon-containing compound and the at least one polycyclic polyene monomer in the presence of a hydrosilation catalyst, so that an organosilicon crosslinkable prepolymer or crosslinked polymer is formed; wherein at least a substantial portion of at least one member selected from the at least one silicon-containing compound and the at least one polycyclic polyene monomer, has more than two reactive sites. A method as claimed in claim 21, wherein the polycyclic polyene monomer is reacted with the silicon-containing compound so that a prepolymer is produced. followed by the addition of at least one member selected from a reactive flame retardant compound and a nonreactive flame retardant compound to the prepolymer, followed by polymerization of the prepolymer so that a crosslinked organosilicon polymer is formed. A method for making a flame retardant, crosslinked organosilicon composition, the method comprising the steps of: (1) providing a crosslinkable organosilicon prepolymer comprising residues of at least one silicon-containing compound and at least one polycyclic polyene compound linked through carbon to silicon bonds, when the silicon-containing compound comprises at least one member selected from: A. cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡SiH groups; B. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡SiH groups; and C. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡SiH groups, wherein the polycyclic polyene compound contains at least two hydrosilation reactive carbon-carbon double bonds; wherein at least a substantial portion of at least one member selected from the silicon-containing compound and the polycyclic polyene compound has more than two reactive sites; and wherein 30% to 70% of the hydrosilation reactive ≡SiH groups are reacted; (2) providing a flame retardant compound, comprising a microencapsulated bromine containing flame retardant (3) mixing the crosslinkable organosilicon prepolymer and the flame retardant compound; and (4) curing the organosilicon prepolymer in the presence of a hydrosilation catalyst to form a crosslinked organosilicon composition; and wherein 12,7 cm x 1,27 cm x 1,27 cm (5 inch by 1/2 inch by 1/2 inch) polymer specimens of the crosslinked organosilicon composition exhibit flammability characteristics which meet the criteria for the UL94 V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test. An article of manufacture comprising a cured, crosslinked organosilicon polymer, wherein the article of manufacture is selected from an electronic component which comprises, or an electronic component coated or encapsulated with, a crosslinked organosilicon polymer comprising residues of at least one silicon-containing compound and at least one polycyclic polyene compound linked through carbon to silicon bonds, wherein the silicon-containing compound comprises at least one member selected from: A. cyclic polysiloxane compounds containing at least two hydrosilation reactive ≡SiH groups; B. tetrahedral siloxysilane compounds containing at least two hydrosilation reactive ≡SiH groups; and C. linear poly(organohydrosiloxane) polymers containing at least two hydrosilation reactive ≡SiH groups, wherein the polycyclic polyene compound contains at least two hydrosilation reactive carbon-carbon double bonds; wherein at least a substantial portion of at least one member selected from the silicon-containing compound and the polycyclic polyene compound has more than two reactive sites; and wherein the article of manufacture exhibits flammability characteristics which meet the criteria for the UL94 V-1 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test, and wherein the organosilicon composition contains at least one flame retardant, comprising a microencapsulated bromine containing flame retardant. An article of manufacture as claimed in claim 24, wherein the component comprises at least one member selected from electronic circuit boards, circuit board laminates, circuit board prepregs, semiconductor devices, capacitors, resistors, diodes, transistors, transformers, coils, wires, hybrid circuits, and multichip modules. An article as claimed in claims 24 or 25, wherein the crosslinked polymer specimens of the organosilicon composition exhibit flammability characteristics which meet the UL94 V-0 rating according to Underwriters Laboratories Tests for Flammability of Plastic Materials UL94 Vertical Burn Test.

Inventors: BABCOCK LAURA MAE, BARD JOHN KENNETH, LEIBFRIED RAYMOND THOMAS SR, BABCOCK, LAURA MAE, BARD, JOHN KENNETH, LEIBFRIED, RAYMOND THOMAS, SR.
Applicants: HERCULES INC, HERCULES INCORPORATED
Application year: 1993
Main classification: C08L83
IPC codes: H01L23, C08L83, C08K3, H01C1, H05K1, C08K5