Foamable silicone composition, silicone sponge, and silicone sponge roll

Abstract: A foamable silicone composition comprising (A) a vinyl-­ free organopolysiloxane R a SiO (4-a)/2 wherein R is a monovalent hydrocarbon or hydroxyl group and a is 1.95 to 2.05, (B) a finely divided silica filler, (C) an azodicarbonamide foaming agent, and (D) a diacyl peroxide foams and cures into a silicone sponge having a uniform and dense cellular structure and toner releasability. The silicon sponge is suitable to form a fixing roll for use in electrographic copying machines.

Description: This invention relates to a foamable silicone composition. It also relates to silicone sponge obtained by foaming and curing the composition and articles formed of such sponge. In prior art electrostatic copying machines, a toner image is most often fixed to a sheet of paper using a heating roller. That is, a sheet of paper having a toner image formed thereon is passed through a pair of rollers in pressure contact, one or both of which are internally heated: Most advanced electrostatic copying machines adopt this fixing method using a heating roller because it has the advantages of heat efficiency and high speed operation over other fixing methods using an oven, for example. Most of the currently used rolls for fixing toner images are silicone sponge rolls because they are heat resistant and can provide clear and soft images. However, the conventional silicone sponge rolls are insufficient in toner release so that the resulting images are somewhat vague. For promoting toner release, it was a common practice to coat the silicone sponge roll on the surface with a liquid silicone rubber layer having toner releasability or to integrate a fluoride release layer to the roll surface. These methods undesirably complicate the overall manufacture process and add to the cost of fixing rolls. The silicone sponge for fixing rolls is required to be a foam body having a dense and uniform cellular structure Azobisisobutyronitrile is used as a foaming agent in the prior art. Decomposition residues of this foaming agent are detrimental to a safe and hygienic environment and require long term heating for substantial decay. JP-A-55 029 565 discloses a foamable silicone rubber composition comprising a silicone raw rubber, a decomposition type foaming agent, an organic peroxide, 0.5 to 10 parts by weight of a decomposition accelerator (foaming aid) for the foaming agent, and 3 to 15 parts by weight, per 100 parts by weight of the silicone rubber, of an organic liquid having a boiling point of 90 to 150°C and being compatible with the silicone raw rubber. It is the object of this reference to provide a silicone sponge having a high degree of expansion. Foamable silicone compositions are further generally known from CH-A-487 969 and DE-A-25 46 698. Therefore, a primary object of the present invention is to provide a silicone sponge roll having a dense and uniform foam structure and improved toner releasability, and suitable for use as a fixing roll. Another object of the invention is to provide a foamable silicone composition from which the roll can be produced without a problem in safety and hygienic aspects. A further object of the invention is to provide a silicone sponge. According to the present invention there is provided a foamable silicone composition comprising (A) 100 parts by weight of an organopolysiloxane free of an aliphatic unsaturated group having the general formula: R a SiO (4-a)/2 (1) wherein R is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms or a hydroxyl group and letter a is a positive number in the range of from 1.95 to 2.05, (B) 3 to 100 parts by weight of a finely divided silica filler, (C) 0.1 to 10 parts by weight of an azodicarbonamide foaming agent having a decomposition temperature of up to 160°C, and (D) 0.1 to 10 parts by weight of a diacyl peroxide having a decomposition temperature of up to 100°C to provide a half-life of 10 hours, said composition does not contain an organic liquid having a boiling point of 90 to 150°C and being compatible with component (A) in an amount of 3 to 15 parts by weight per 100 parts by weight of component (A). A silicone sponge is obtained by causing the foamable silicone composition to foam and cure. A silicone sponge roll can be obtained from the silicone sponge. Upon foaming, the foamable silicone composition leaves decomposition residues which invite little problem in safety and hygienic aspects, thus eliminating a need for long-term heating to remove such decomposition residues. The composition foams and cures into a silicone sponge at a high blowing magnification (ratio of foam volume to before-foaming volume). The silicone sponge which is fully cured from the interior to the surface is free of surface tack uniform, and dense, and has improved toner releasability. Formation of an additional composite layer is needed no longer. Thus silicone sponge rolls having improved performance as fixing rolls are obtained at low cost. FIGS. 1 to 3 are sectional views of rolls according to different embodiments of the present invention. The silicone sponge roll of the invention is mainly formed from a silicone sponge which is obtained by foaming and curing a foamable silicone composition comprising (A) an organopolysiloxane, (B) a silica filler, (C) a foaming agent, and (D) a peroxide. Component (A) is an organopolysiloxane having the general formula: R a SiO (4-a)/2 (1) and substantially free of an aliphatic unsaturated group In formula (1), R is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms or a hydroxyl group. Examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, propyl, and butyl, aryl groups such as phenyl, and substituted ones of these groups in which some or all of the hydrogen atoms are replaced by halogen atoms, such as γ-trifluoropropyl group. For ease of synthesis, heat resistance, and toner release, the R moiety preferably contains at least 50 mol%, especially at least 80 mol% of methyl. Letter a is a positive number in the range of from 1.95 to 2.05, The organopolysiloxane of formula (1) is free of an aliphatic unsaturated group. If an alkenyl group such as vinyl is present in the organopolysiloxane even in a small amount, for example, as small as 0.2 mol%, then there would result a hard foam or sponge which has a short cured surface layer and is discolored. These factors reduce the commercial value as the sponge for fixing rolls. In addition, the toner releasability is substantially lost. The organopolysiloxane of formula (1) desirably has a molecular structure which is as linear as possible, although a partially branched structure is acceptable. Its molecular chain preferably terminates with a triorganosilyl group such as trimethylsilyl or a hydroxyl group. The organopolysiloxane preferably has a viscosity of at least 0.1 m²/s (100,000 centistokes), preferably 0.1 to 10 m²/s (100,000 to 10,000,000 centistokes) at 25°C although its degree of polymerization is not critical. Component (B) is a finely divided silica filler which is added for the purpose of reinforcement, viscosity build-up, processability improvement, and extension of the silicone rubber. To this end, fumed silica, wet silica, quartz powder, and diatomaceous earth are useful. These fillers preferably have a specific surface area of at least 1 m²/g, preferably 1 to 800 m²/g as measured by the BET method. The silica filler preferably has an average particle size of 0.005 to 100 µm especially 0.005 to 5 µm. Filler (B) is blended in an amount of 3 to 100 parts, preferably 5 to 30 parts by weight per 100 parts by weight of organopolysiloxane (A). The composition with less than 3 parts by weight of the filler would be less reinforced and lose processability, whereas the flow in a mold would become extremely low with more than 100 parts by weight of the filler. Component (C) is an azodicarbonamide foaming agent having a decomposition temperature of up to 160°C, preferably 100 to 160°C, which is higher than the decomposition temperature for allowing component (D) to have a half-life of 10 hours. This foaming agent is generally prepared by adding an additive to azodicarbonamide. Examples of the additive include urea, urea compounds (e.g., lower alkyl ureas such as methylurea, ethylurea, propylurea, and butylurea and higher fatty acid salts of urea such as urea laurate, urea stearate, and urea oleate), dibasic lead phthalate, zinc oxide, stearic acid, zinc stearate, and polyethylene glycol. Mixtures of azodicarbonamide with at least one of urea, urea compounds, zinc oxide, stearic acid, and zinc stearate are preferred from safety and hygiene aspects. Preferred mixtures contain 100 parts by weight of azodicarbonamide and 1 to 300 parts, especially 30 to 150 parts by weight of urea or urea compound and 0 to 100 parts by weight of zinc oxide or any other additive, with the total of the additives being, limited to 1 to 500 parts, especially 50 to 200 parts by weight. Such mixtures are commercially available as CELMIC® CAP and CAP-250 from Sankyo Chemicals K.K. Foaming agent (C) is blended in an amount of 0.1 to 10 parts, preferably 0.5 to 7 parts by weight per 100 parts by weight of organopolysiloxane (A). Less than 0.1 parts by weight of the foaming agent would be ineffective for foaming whereas more than 10 parts by weight of the foaming agent would produce unnecessary decomposition residues and cause discoloration or cracks due to excessive foaming. Component (D) is a diacyl peroxide having a decomposition temperature of up to 100°C, preferably 30 to 100°C, to provide a half-life of 10 hours. This peroxide is added as a vulcanizing agent for promoting heat curing of silicone rubber. Exemplary of the peroxide are benzoyl peroxide, di(2,4-dichlorobenzoyl) peroxide, di(p-chlorobenzoyl) peroxide, di(o-chlorobenzoyl) peroxide, di(p-methylbenzoyl) peroxide, di(o-methylbenzoyl) peroxide, di(p-bromobenzoyl) peroxide, di(o-bromobenzoyl) peroxide, di(2,4-dimethylbenzoyl) peroxide, di(p-trimethylsilylbenzoyl) peroxide, and mixtures thereof. Peroxide (D) is blended in an amount of 0.1 to 10 parts, preferably 0.1 to 3 parts by weight per 100 parts by weight of organopolysiloxane (A) although its amount may be suitably determined depending on the type and amount of other components. Less than 0.1 parts by weight of the peroxide would result in a foamed body which is less elastic because of short vulcanization. With more than 10 parts by weight of the peroxide, vulcanization would occur in advance of foaming, failing to form a soft foamed body. In addition to components (A) to (D), the foamable silicone composition of the present invention may contain any desired additives. Such additives include a dispersant such as a low-molecular weight siloxane having a degree of polymerization of up to 100, a silanol group-containing silane, and an alkoxy group-containing silane; a heat resistance modifier such as iron oxide, cerium oxide, iron octylate, and titanium oxide; a pigment for coloring; a flame retardant such as platinum compounds and palladium compounds; and other additives commonly used in conventional silicone rubber compositions. The silicone sponge may be obtained by uniformly mixing the above-mentioned components in a conventional manner and causing the mixture to foam and cure. The blowing magnification preferably ranges from 200 to 500%. The foaming and curing method is not particularly limited and any desired method commonly used in the manufacture of silicone rubber foams may be used. For example, hot air vulcanization may be carried out at a temperature of 150 to 300°C and an atmospheric pressure for 10 seconds to 30 minutes. Another useful method is by charging a mold with the silicone composition and vulcanizing the composition at a temperature of 100 to 200°C for 2 to 10 minutes. A fixing roll may be produced by these methods. Alternatively, a fixing roll can be produced by extruding the silicone composition through an extruder, and causing the extrudate to foam and cure in an atmospheric steam vulcanization tower, infrared vulcanization tower, UHF vulcanization tower or similar vulcanization apparatus. A fixing roll can also be manufactured by extruding the silicone composition into a roll form, cutting the unvulcanized extrudate to a suitable length, heating it for foaming in an oven, and inserting a metal mandrel through the resulting roll. It is also possible to cause the silicone composition to foam and cure in a mold with a metal mandrel inserted therein. After the foaming and curing step, if desired, the silicone sponge or roll is post cured at 150 to 220°C for 0 to 8 hours, completing vulcanization and decomposition of the foaming agent. The resulting silicone sponge has a high foaming factor, a uniform and dense cellular structure, and improved toner releasability so that it may perform well as a fixing roll. It can be advantageously used as fixing rolls for electrostatic copying machines, oil coating rolls, and cleaning rolls. The silicone sponge can also find applications as gaskets for building utility and heat insulating tubes. A silicone sponge roll, particularly a fixing roll may include a roll body and a rubber layer formed on the outer surface of the roll body. The rubber layer is made of the silicone sponge of the invention. The roll body may be made of a metal such as aluminum, aluminum alloy, and stainless steel, or a metal having a rubber applied thereon. The roll body may have a heater received therein. FIGS. 1 to 3 show some embodiments of the fixing roll. In FIG. 1, a roll body 10 consisting of a metal core 12 is covered with a layer 20 formed from the silicone composition. In FIG. 2, a roll body 10 consisting of a metal core 12 and a rubber sleeve 14 which is a conventional one for a fixing roll is covered with a layer 20 of the silicone composition. In FIG. 3, a roll body 10 consisting of a metal core 12 having a heater 16 inserted therein is covered with a layer 20 of the silicone composition. If desired, the silicone sponge roll of the invention may be coated on the surface with a polyfluoroalkylene (PFA) resin, polytetrafluoroethylene (PTFE) resin, room temperature vulcanizable silicone fluid, or low temperature vulcanizable silicone fluid which is eventually integrated with the substrate. The foamable silicone composition according to the present invention can foam and cure into a silicone sponge at a high foaming factor without a safety and hygiene problem. The silicone sponge has a uniform and dense cellular structure and improved toner releasability. Rolls comprising the silicone sponge are very suitable as fixing rolls for electrostatic copying machines. EXAMPLE Examples of the invention are given below by way of illustration and not by way of limitation. All parts and percents are by weight unless otherwise stated. Viscosity is measured at 25°C. Example 1 A base compound was prepared by adding 30 parts of fumed silica (trade name Aerogel A-200, Nihon Aerogel K.K.) and 5 parts of dimethylpolysiloxane [viscosity 0.15x10⁻⁴ m²/s (∼15 centistokes)] blocked with a hydroxyl group at each end as a dispersant to 100 parts of dimethylpolysiloxane [viscosity ∼10 m²/s (∼10,000,000 centistokes)] having structural units shown in Table 1, blocked with a trimethylsilyl group at each end, and free of a vinyl group. The ingredients were mixed in an internal mixer to form a dispersion, which was heat treated for about 2 hours at 160°C. A composition was prepared by adding 5 parts of azodicarbonamide foaming agent (trade name CELMIC CAP-250, Sankyo Chemicals K.K.) and 2 parts of a 50% silicone oil paste of di(p-methylbenzoyl) peroxide vulcanizing agent to 100 parts of the base compound and thoroughly milling the mixture. The composition was rolled into a sheet of 50 mm x 50 mm x 4 mm (thick). It was foamed and cured into a silicone sponge sample by heating for 20 minutes in an oven at 200°C. The sample was visually observed for outer appearance, measured for specific gravity and hardness (Ascar C scale), and examined for toner stain by the following method. Toner stain A pair of silicone sponge sheets having toner sandwiched therebetween were placed in an oven and heated to 250°C at which the toner was melted, and then cooled down. The sheets were separated and visually observed how the sheet surface was stained with the toner. The results are shown in Table 1. Example 2 For comparison purposes, a silicone sponge sample was prepared by the same procedures as in Example 1 except that vinyl-containing methylpolysiloxane having structural units shown in Table 1 and blocked with a trimethylsilyl group at each end. The sample was examined for outer appearance, specific gravity, hardness, and toner stain. The results are shown in Table 1. Structural unit Example 1 Example 2 (CH₃)₂SiO 99.975 mol% 99.775 mol% (CH₂=CH)CH₃SiO - 0.200 mol% (CH₃)₃SiO 1/2 0.025 mol% 0.025 mol% Total 100.0 mol% 100.0 mol% Specific gravity 0.31 0.78 Hardness, Ascar C 20 42 Appearance aesthetic, uniform foam brown, short-foamed/cured Toner stain no stain stained Example 3 A composition was prepared by adding 5 parts of azodicarbonamide foaming agent (trade name CELMIC CAP-250, Sankyo Chemicals K.K.) and 1.2 parts of a 49% silicone oil paste of di(2,4-dichlorobenzoyl) peroxide vulcanizing agent to 100 parts of the base compound used in Example 1 and thoroughly milling the mixture. The composition was extruded into a cylinder having an outer diameter of 12 mm and an inner diameter of 6 mm through an extruder having a barrel diameter of 70 mm. The unvulcanized cylinder was cut to a length of 30 cm. The molded cylinder was heated for 20 minutes in an oven at 200°C for foaming, and then post cured at the temperature for 4 hours, obtaining a silicone sponge roll. A metal core was inserted into the sponge roll, obtaining a fixing roll as shown in FIG. 1. The fixing roll was mounted in a commercial electrostatic copying machine where copying operation was conducted for a test. The roll was set in pressure contact conditions including a total pressure of 1.96 MPa (20 kgf) and a circumferential speed of 70 mm/sec. Plain sheets of A-4 size were copied at a rate of 10 sheets per minute to a total of 10,000 sheets. In the copying test, the fixing roll using the silicone sponge of the invention was free of toner stain and allowed for continuous reproduction of copies of high image quality.

Claims: A foamable silicone composition comprising (A) 100 parts by weight of an organopolysiloxane free of an aliphatic unsaturated group having the general formula: R a SiO (4-a)/2 (1) wherein R is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms or a hydroxyl group and letter a is a positive number in the range of from 1.95 to 2.05, (B) 3 to 100 parts by weight of a finely divided silica filler, (C) 0.1 to 10 parts by weight of an azodicarbonamide foaming agent having a decomposition temperature of up to 160°C, and (D) 0.1 to 10 parts by weight of a diacyl peroxide having a decomposition temperature of up to 100°C to provide a half-life of 10 hours, said composition does not contain an organic liquid having a boiling point of 90 to 150°C and being compatible with component (A) in an amount of 3 to 15 parts by weight per 100 parts by weight of component (A). The composition as set forth in claim 1 wherein the majority of R is a monovalent hydrocarbon. The composition as set forth in claim 2 wherein the majority of R is methyl. The composition as set forth in claim 1 wherein component (B) is selected from fumed silica, wet silica, quartz powder, and diatomaceous earth. The composition as set forth in claim 1 wherein component (C) is azodicarbonamide in admixture with at least one member selected from urea, a urea compound, zinc oxide, stearic acid, and zinc stearate. The composition as set forth in claim 1 wherein component (D) is selected from benzoyl peroxide, di(2,4-dichlorobenzoyl) peroxide, di(p-chlorobenzoyl) peroxide, di(o-chlorobenzoyl) peroxide, di(p-methylbenzoyl) peroxide, di(o-methylbenzoyl) peroxide, di(p-bromobenzoyl) peroxide, di(o-bromobenzoyl) peroxide, di(2,4.dimethylbenzoyl) peroxide, di(p-trimethylsilylbenzoyl) peroxide, and mixtures thereof. A silicone sponge which is obtained by causing a foamable silicone composition as set forth in any one of claims 1 to 6 to foam and cure. A silicone sponge roll comprising a silicone sponge as set forth in claim 7.

Inventors: HATAKEYAMA JUN, TAKAHASHI MASAHARU, TAKITA KEN-ICHI, HATAKEYAMA, JUN, TAKAHASHI, MASAHARU, TAKITA, KEN-ICHI
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1990
Main classification: C08L83
IPC codes: C08K3, G03G15, C08J9, F16C13, C08K5, B29D99