Process for recycling silicone scrap and products relating thereto

Abstract: The present invention relates generally to the re-utilization of silicone scrap material. More specifically, the process of the present invention is directed to dissolving silicone scrap in an appropriate solvent and chemically converting the scrap into dimethyl silicone cyclic trimer, tetramer, pentamer and/or the like by means of a two step acid/base catalyzed cracking process.

Description: The present invention relates generally to the re-utilization of silicone scrap material. More specifically, the process of the present invention is directed to dissolving silicone scrap in an appropriate solvent and chemically converting the scrap into dimethyl silicone cyclic trimer, tetramer, pentamer and/or the like by means of a two step acid/base catalyzed cracking process followed by distilling out the silicone cyclics. Chemical waste and scrap material is of increasing concern to society, and as a result, the chemical industry is increasingly focusing its attention on the recycling and re-using of chemical materials. Silicones, otherwise known as organosiloxanes, are polymeric materials having alternate silicon and oxygen atoms with various organic radicals attached to the silicon. These compositions can be liquid, semisolid or solid, depending upon the molecular weight and degree of crosslinking. Such polymers may be chain extended and/or crosslinked chemically, with or without catalyst. Silicones are generally stable over temperature ranges from about -50 to about +250 °C and sold in the form of fluids, powers, emulsions, solutions resins, pastes, elastomers and the like. Liquid silicones are used in adhesives, sealants, lubricants, protective coatings, coolants, mold-release agents, dielectric fluids, heat transfer, wetting agents and surfactant, foam stabilizer for polyurethanes, diffusion pumps, antifoaming agent for liquids, textile finishes, water repellents, weatherproofing concrete, brake fluids, cosmetic items, polishes, foam shields for solar energy collectors and rust preventives. Silicone resins are used in coatings, molding compounds, laminates (with glass cloth), filament winding materials, room temperature curing cements, electrical insulation, impregnating electric coils, bonding agents, modifiers for alkyd resins, and vibration damping devices. Elastomer or silicone rubber is used in encapsulation of electronic parts, electrical insulation, gaskets, surgical membranes and implants automobile engine components, flexible windows for face masks, sealants, air locks and the like. Once used, silicones generally cannot be melted down and reused. Once crosslinked, molded or reacted, any scrap must generally be discarded. FR-A-1 176 541 discloses that silicone cracking is accomplished in the presence of a base or an acid. GB-A- 2,015,550 to Heidingfeldova, et al., is directed to a method of reclaiming siloxanes by dissolving the siloxane in an amine. However, amines produce noxious odors and the overall process is rather inefficient. U.S.-A-3,989,733 to Okamotoa, et al., is directed to a process for the preparation of cyclic polydiorganosiloxanes by a complex distillation process having a vertical packed catalysts zone with an inlet stream at about the middle of the catalyst zone. The process is run at a pot temperature in excess of about 240°C and is a fix bed, base catalyzed reaction. U.S.-A-3,714,213 to Ryan is directed to a method for making cyclopolysiloxanes by cracking the poly(siloxanes) over acid-treated clay or synthetic aluminum silicate. A drawback to this method is that it must be conducted at extreme temperatures. In Chemistry of Materials , Vol. 1, 445-451, 1989, "Synthesis of Cyclic Siloxanes by the Thermal Depolymerization of Linear poly(siloxanes)" is directed to a fixed bed, acid catalyzed distillation process run at temperatures of from about 500°C to about 700°C. As exemplified by the above references, the thermal depolymerization of linear poly(siloxanes) has generally been achieved by either an acid catalyzed cracking reaction or a base catalyzed cracking reaction, typically in combination with temperatures in excess of 180°C. Cracking reactions less than about 180°C have been achieved by use of an amine solvent, but such processes have proven to be inefficient and have a tendency to produce noxious odors. It is therefore and object of the present invention to provide a process for recycling silicone materials which is efficient, which can be accomplished at temperatures less than about 180°C, which does not have a tendency to produce noxious odors and which is sufficiently economical to justify full scale commercialization. In accordance with the present invention, high molecular weight silicone elastomers are cracked to useful, low molecular weight, silicone cyclics. Such cyclics can be used as raw materials for synthesizing linear silicone prepolymers. In detail the present invention relates to a process of cracking a high molecular weight silicone polymer obtained from silicone scrap to silicone cyclics, said process comprising the steps of: (1) dissolving a silicone polymer in an organic solution comprising an acid R 3 SO 3 H, in which R 3 is selected from alkyl, aryl or hydroxyl; and a water insoluble solvent, wherein said solvent is not highly hydroxylated, is capable of swelling said silicone polymer, and has a higher boiling point than the silicone cyclics from step (3), and heating the resulting mixture at a temperature range of 150 °C to 180 °C until the silicone polymer is substantially dissolved; (2) adding a base MOH, in which M is selected from lithium, sodium, potassium, rubidium, cesium, tetraalkylammonium or tetraalkylphosphonium, and reducing the temperature to a range of 80 °C to 115 °C; and (3) distilling out the silicone cyclics from the solution. The process of the present invention requires three steps. The first step involves dissolving the silicone scrap in a solution of an appropriate acid in a solvent having the properties as defined before. The preferred solvent of the present invention is Butyl Carbitol™ (diethylene glycol monobutyl ether) or Koppers' Methylnaphthalene Fraction™ (73-78 wt.% alkylnaphthalenes, 3-8 wt.% naphthalene, 5-8 wt.% biphenyl, 5-10 wt.% acenaphthene, 1-5 wt.% dibenzofuran). Alternative solvents are identified in Table 1, and as can be seen by this table, triglyme and Methyl Cellosolve would be possible useful solvents, but are less preferred due to their high cost and hazardous nature, respectively. Water, diethylene glycol and tripropylene glycol were generally ineffective in aiding the dissolution of the scrap, suggesting that all highly hydroxylated materials would be poor media for this step; these solvents do not swell cured silicone, so perhaps, if an additive could be found to cause such swelling, such hydroxylated solvents might be useful. The preferred solvent for the cracking reaction must not only promote the cracking reaction, but also must be easy to separate from the silicone cyclic end product and the solid byproducts. It may also be necessary to remove dissolved impurities from the solvent. The solvent has a substantially higher boiling point than the silicone cyclics, so that substantially pure silicone cyclics can be distilled from the solution. The solvent generally is water insoluble, so that water soluble byproducts, impurities and the like can be extracted from the solvent in a water extraction process or the like. The first step of the process of the present invention also requires the addition of an acid catalyst as defined before. The most preferred acid catalyst is sulfuric acid. In the first step, the silicone scrap is dissolved in the solvent comprising the acid catalyst, and the mixture is heated, to a temperature between 150°C and 180°C. The high temperature and acid catalyst then causes the dissolution of the silicone scrap. For the acid cracking reaction, useful ratios appear to be in the range of 39 weight percent silicone scrap, 57.5 weight percent solvent and 2.5 weight percent concentrated sulfuric acid. The cracking reaction is then driven to completion by a second step of the process of the present invention. In this second step, the temperature of the solution is reduced to 80°C to 115°C and a base catalyst MOH as defined before is added to the solution, most preferably, potassium hydroxide. The base catalyzed reaction at the lower temperature drives the cracking reaction substantially to completion. The potassium hydroxide and additional solvent are added, and the final weight ratio (ignoring reactions) is preferably 31.5 weight percent silicone scrap, 63 weight percent solvent, 2 weight percent concentrated sulfuric acid and 3.5 weight percent potassium hydroxide. At the end of the potassium hydroxide catalyzed cracking reaction, the reaction mixture consists primarily of silicone cyclics, solid potassium compounds, filler (i.e., fume silica) from the original silicone scrap and solvent. Because the solvent boils substantially higher than the expected cyclic tetramer and pentamer, these products can be distilled out in fairly pure state. No solvent need be distilled. Solvent and silicone cyclic boiling points appear in Table 2. Filtration of the distillation residue probably is not much affected by choice of solvent. However, cleaning up the solvent may be easier if it is water insoluble. Dissolved impurities might include potassium salts of sulfated or sulfonated solvent. These could be easily removed by water extraction from a water insoluble solvent, while a water soluble solvent would require a second distillation for separation. In the third step of the process of the present invention, the silicone cyclics are distilled from the reaction mixture. This step allows for the distillation of substantially pure silicone cyclic vapors which in turn can be liquified by means of a water-cooled condensation chamber or the like. Although the process is preferably a batch, three step process, a continuous process is conceivable. In a continuous process, it is theorized that a tube flow reactor could be used and operated at atmospheric pressure, wherein the reactor is vented to the atmosphere, and the silicone cyclic vapors are condensed by a condenser prior to reaching the exhaust outlet. At one stage of the tube reactor, the solvent could be heated to 160°C and sulfuric acid and scrap material could be added. Downstream, the solution could be cooled to 100°C and potassium hydroxide could be added. Further downstream, a water extraction could be conducted to remove the sulfuric acid, potassium hydroxide, salts thereof and any byproducts from the reaction. Thereafter the solvent could be recycled into the initial stage of the continuous reactor. Generally, the preferred batch process of the present invention yields 80-95% by weight based upon the silicone available in the scrap or 45-55% by weight based upon the weight of the scrap used. Preferably, the temperature of the distillation is adjusted so that only cyclics are distilled from the reaction mixture. Generally speaking, the cyclics produced are dimethyl silicone cyclic trimer, tetramer, pentamer or the like. The resulting cyclics can then be used as raw materials to prepare the silanol and vinyl terminated linear silicone prepolymers used in, for example, silicone sealants. In accordance with the present invention, silicone scrap can be defined as liquids or elastomeric solids comprising high molecular weight (i.e., solid) networks of silicone polymer having a siloxane backbone and pendant lower alkyl, typically methyl, groups. A typical two-step catalyzed cracking reaction can be defined as follows: where: q ≥ 3, typically 4, p = is a sufficiently high number to indicate a polymeric material, R 1 and R 2 = can be the same or different and are any conventional silicone end groups such as an alkyl group, R 3 = an alkyl, aryl or hydroxide group, and M = lithium, sodium, potassium, rubidium, cesium, tetraalkylammonium, or tetraalkylphosphonium. The resulting cyclics can be used as raw materials for linear silicone prepolymers according to any one of a number of reactions, such as: wherein: X and X' can be the same or different and are vinyl or hydroxyl groups The product of reaction II, vinyl or silanol terminated linear silicone polymer, is a common starting material for many sealant and mold making materials. A typical curing reaction for such materials is: wherein "n" is a number sufficient to indicate a liquid. EXAMPLE D. Cracking Reaction Conditions Silicone scrap (20.0g), solvent (30 ml), and concentrated sulfuric acid (50 drops = 1.29g) were combined in a 250 ml three neck round bottom flask equipped with Teflon paddle stirrer, thermometer, and 15.24cm (six inch) vacuum jacketed Vigreux condenser equipped for vacuum distillation. The vacuum pump was protected by a Dry ice-isopropyl alcohol condenser. Heating and slow stirring were begun at atmospheric pressure under air. Temperature was held at 150-180°C for about an hour. Traces of water in the mixture would sometimes make the mixture foam; in these cases, vacuum was cautiously applied to remove the water. Eventually, all silicone scrap would dissolve, giving an opaque gray liquid. No silicone cyclic could be distilled from this mixture; it apparently contained silicone as linear oligomers. The acid cracking reaction mixture was then let cool with continued stirring to about 50°C. Potassium hydroxide (2.2g) was then added, and heating resumed at a pressure of 1.6 kPa-2.4 kPa (12-18 torr). This vacuum is used only to prevent dense foam in the reaction vessel as the boiling point of silicone cyclic is approached. Without vacuum, the foamy pot material would tend to move up the distillation column. After the potassium hydroxide dissolves, there is no further visible change in the pot contents. Pot temperature is slowly raised to about 115°C. (No distillate has come over before this temperature.) Schlieren at a distillation temperature of about 85°C show that some solvent is beginning to come over. By 95°C, solvent comprises most of the distillate. Distillate and cold trap condensate were weighted an analyzed via gas chromatography. Trap residues amounted to a few grams, about equal parts water and organic layer. E. Yield of Silicone Cyclics Actual yield of cyclics was taken as the weight of distillate and trap residue times the weight fraction of silicone cyclic in these phases. Yields of dimethylsilicone cyclics were generally 9-11g from 20g of silicone scrap. Both Silastic E (from Dow Corning™) and scrap of unknown origin gave cyclic yields in this range. Thermogravimetric analysis of these silicone elastomers shows them to be about 45 wt. % filler, or about 55% silicone polymer. Thus in each 20g scrap sample, there was about llg of actual polymer. Recovery of 9-llg silicone cyclic thus corresponds to 80-100% recovery of the silicone available in the scrap. SOLVENT PERFORMANCE Solvent H 2 SO 4 Cracking Reaction KOH Cracking Reaction Triglyme ++ ++ 1-Methylnaphthalene + + Water 0 ? Aliphatic Hydrocarbon + ? Diethylene Glycol 0 ? Methyl Cellosolve™ ++ ++ Tripropylene Glycol 0 ? Lauryl Alcohol + 0 Aliphatic Oil + 0 N,N-Dimethylacetamide ? ++ N-Methylpyrrolidinone ? 0 N,N,N',N'-Tetramethylurea ? + Dimethyl Sulfoxide ? + Sulfolane ? 0 3-Methylsulfolane ? 0 Polyoxyethylene 0 0 Diphenyl Ether ++ 0 2-Pyrrolidinone 0 0 Butyl Carbitol™ ++ ++ Tripropylene Glycol Monomethyl 0 + Ether BOILING POINTS, °C Material Atmospheric Pressure Reduced Pressure Dimethylsilicone, Cyclic Tetramer 175-6 ~ 85/2.4kPa (18 torr) Dimethylsilicone Cyclic Pentamer 205 101/2.67kPa (20 torr) Triglyme 216 111-112/2.4kPa (18 torr) 1-Methylnaphthalene ~ 240 171/2.27kPa (17 torr) Diethylene Glycol 244 Methyl Cellosolve™ 124 Lauryl Alcohol 260-2 138/1.73kPa (13 torr) Dimethylacetamide 163-5 N-Methylpurrolidinone 202 N,N,N'N'-Tetramethylurea 176.5 63-64/1.6kPa (12 torr) Dimethyl Sulfoxide 189 Sulfolane 285 3-Methylsulfolane 276 Diphenyl Ether 259 ~ 130/2.0kPa (15 torr) 2-Pyrrolidinone 245 Butyl Carbitol™ 231 Tripropylene Glycol Monomethyl 121-2/1.87kPa (14 torr) Ether Koppers Methylnaphthalene Fraction 230-75 PROPERTIES OF SILANE ELASTOMERS Silicone Scrap A Silastic E™ Appearance Turbid brownish gray Opaque white Chief filler Fume silica Fume silica Filler loading via TGA 43.8 wt. % 45.7 wt. % Wt. loss, THF extract 7.2 wt. % <1 wt. % Wt. loss, heptane extract 1.4 wt. % <1 wt. % Extracted material, via IR dimethylsiloxane polymer dimethylsiloxane polymer

Claims: A process of cracking a high molecular weight silicone polymer obtained from silicone scrap to silicone cyclics, said process comprising the steps of: (1) dissolving a silicone polymer in an organic solution comprising an acid R 3 SO 3 H, in which R 3 is selected from alkyl, aryl or hydroxyl; and a water insoluble solvent, wherein said solvent is not highly hydroxylated, is capable of swelling said silicone polymer, and has a higher boiling point than the silicone cyclics from step (3), and heating the resulting mixture at a temperature range of 150 °C to 180 °C until the silicone polymer is substantially dissolved; (2) adding a base MOH, in which M is selected from lithium, sodium, potassium, rubidium, cesium, tetraalkylammonium or tetraalkylphosphonium, and reducing the temperature to a range of 80 °C to 115 °C; and (3) distilling out the silicone cyclics from the solution. The process of claim 1 wherein the acid is sulfuric acid. The process of claim 1 or 2 wherein the base is potassium hydroxide. The process of claim 3 wherein the final weight ratio of the solution (ignoring reactions) is 31.5 weight percent silicone polymer, 63 weight percent solvent, 2 weight percent concentrated sulfuric acid and 3.5 weight percent potassium hydroxide. The process of anyone of claims 1 to 4 wherein said solvent is selected from Butyl Carbitol (diethylene glycol monobutyl ether), methyl Cellosolve (diethylene glycol monomethyl ether), triglyme (triethylene glycol dimethyl ether), and Koppers' Methylnaphthalene Fraction (73-78 wt.% alkylnaphthalenes, 3-8 wt.% naphthalene, 5-8 wt.% biphenyl, 5-10 wt.% acenaphthalene, 1-5 wt.% dibenzofuran).

Inventors: GREENLEE THOMAS W, GREENLEE, THOMAS W.
Applicants: TREMCO INC, TREMCO INCORPORATED
Application year: 1992
Main classification: C08J11
IPC codes: C08J11, C07F7