High resiliency fluorosilicone dispersion

Abstract: A moisture curable fluorosilicone dispersion that cures to an elastomer having high resiliency and low hysteresis loss is made by mixing a trifluoropropylmethyl-containing, hydroxy-terminated polydiorganosiloxane with untreated or partially treated silica, hexamethyldisilazane as a non-fluorine-containing in situ filler treating agent, a solvent and a crosslinking agent. The elastomer is used as coating on a transfer roll for liquid toner photocopy machines.

Description: This invention relates to a fluorosilicone dispersion having an in situ treated filler, said dispersion curing upon exposure to moisture to an elastomer with improved resiliency and low hysteresis loss. This invention also relates to a photocopy machine transfer roll that is coated with said high resiliency elastomer. This invention also relates to a photocopy machine using said transfer roll. Fluorosilicone dispersions which cure upon exposure to moisture are currently available but they lack sufficient resiliency and do not have sufficiently low values for hysteresis loss. Japanese Patent Application (Kokai) 5-311078 describes a fluorosilicone rubber composition consisting of trifluoropropyl group-containing diorganopolysiloxane, silica treated with trifluoropropyl group-containing silane or siloxane, silanol-capped diorganosiloxane and curing agent. U.S. Patent 4,529,774 claims a treatment for silica reinforcing fillers using a fluoroalkyl-functional diorganopolysiloxane treating agent. Such treating agents are especially compatible for use with fluorosilicone gums and fluorosilicone rubbers. U.S. Patent 4,882,369 discloses a high strength fluorosilicone sealant containing an in situ treated reinforcing filler. This sealant is produced in a process which treats the reinforcing filler with di(trifluoropropyl) tetramethyldisilazane. U.S. Patent 4,152,315 teaches the use of bis(polydiorganosiloxanyl)amine in conjunction with a silazane to treat finely divided silica and the subsequent use of such treated silica in curable silicone elastomer compositions. The bis(polydiorganosiloxanyl)amine is used to reduce the amount of silazane treating agent to produce a final product at a lower cost. European Patent Publication 0565100 provides an intermediate recording medium for thermal transfer printing. This recording medium comprises a metallic drum and a thin silicone rubber which is overlaid on the metallic drum. A high resiliency, low hysteresis loss value fluorosilicone elastomer is produced by a process in which a fluorosilicone dispersion, an untreated or partially pre-treated filler and a non-fluorosilicone-containing filler treating agent are blended. A solvent and a crosslinking agent are then added to yield a composition which is capable of curing to an elastomer having improved resiliency and low hysteresis loss. This invention also relates to a photocopy machine transfer roll that is coated with said high resiliency, low hysteresis loss elastomer. This invention also relates to a photocopy machine using said transfer roll. It is an object of this invention to provide a moisture curable fluorosilicone dispersion which is capable of being cured to a fluorosilicone elastomer which has improved resiliency and lower hysteresis loss values. This invention relates to a moisture curable fluorosilicone dispersion produced by the method comprising the steps of (A) mixing: (i) from 30 to 100 parts by weight of a hydroxy-terminated polydiorganosiloxane selected from siloxanes having the unit formula or siloxanes having the unit formula where R 1 and R 2 are individually selected from the group consisting of methyl, ethyl, propyl and butyl, a is an integer with a value of at least one, b is an integer greater than zero; (ii) from 3 to 18 parts by weight of a reinforcing silica filler having a surface area of 50 to 400 m 2 /g, selected from untreated silica or partially pretreated silica; (iii) from 0.1 to 2 parts by weight of hexamethyldisilazane, (B) heating the mixture, (C) applying vacuum to the heated mixture for a sufficient time to remove volatiles, (D) cooling the heated mixture, (E) admixing from 10 to 100 parts by weight of a solvent, and (F) admixing from 2 to 15 parts by weight of a crosslinking agent selected from ketoximo silanes of the general formula (R 3 ) c Si(R 4 ) 4-c where R 3 is a ketoximo group, each R 4 is independently selected from the group consisting of methyl, ethyl, vinyl or Osi(R 5 ) 3 , wherein R 5 is independently selected from methyl, ethyl, propyl, butyl, vinyl and phenyl, and c is an integer with a value of 2 to 4, inclusive, and mixtures thereof, to give a moisture curable fluorosilicone dispersion containing an in situ treated filler, which dispersion cures upon exposure to moisture and which yields, upon evaporation of the solvent, an elastomer having improved resiliency and lower hysteresis loss. In a preferred embodiment, the crosslinking agent admixing step (F) is followed by (G) purging the resulting polymer and solvent containing mixture with dry nitrogen for a time sufficient to remove atmospheric moisture. By fluorosilicone dispersion, it is meant a two phase system in which minute particles of filler are distributed more or less evenly throughout a fluorosilicone polymer. The fluorosilicone polymer molecules are in solution in a solvent. The dispersion resulting from the method of this invention is based upon a hydroxy-terminated polydiorganosiloxane selected from siloxanes having the unit formula or siloxanes having the unit formula where R 1 and R 2 are individually selected from the group consisting of methyl, ethyl, propyl and butyl, a is an integer with a value of at least one, and b is an integer. The preferred R 1 and R 2 are methyl. The preferred polydiorganosiloxane is one in which b is selected such that the resulting polydiorganosiloxane contains no more than 5 mole percent of R 1 R 2 (SiO) b H. The more preferred polydiorganosiloxane is one in which b is equal to zero. The most preferred polydiorganosiloxane is trifluoropropylmethylsiloxane with an average degree of polymerization (DP) of 100 to 200. Such preferred, more preferred and most preferred polydiorganosiloxanes are known to those skilled in the art, as is their method of manufacture. The reinforcing silica filler used in our method can be any one of the commonly used untreated or partially pretreated fumed silicas employed as reinforcement in silicone rubbers. These silicas have a surface area of 50 to 400 m 2 /g. The preferred silica has an average surface area of 200 to 400 m 2 /g and the most preferred silica has an average surface area of 300 to 400 m 2 /g. The amount of filler is from 3 to 18 parts by weight, preferred is 4 to 10 and the most preferred is 6 to 8. If more filler is added, the resulting elastomers will have a higher tensile strength and less resiliency. If less silica is used, the resulting elastomer will have increased resiliency but poor mechanical properties. The silica is added and mixed with the polydiorganosiloxane in small increments to insure that the silica particles are evenly dispersed throughout the polydiorganosiloxane. Typically the silica will be divided into 4 equal segments, each of which will be added and mixed to the polydiorganosiloxane sequentially. The silica must be untreated or only partially pretreated. An untreated silica is one in which the reactive hydroxy sites on the surface of the silica have not been modified by exposure to treating agents. A partially pretreated silica is one in which some, but not all, of the reactive hydroxy sites have been modified by exposure to a treating agent. The silica treatment of this invention is an in situ treatment with hexamethyldisilazane in the presence of the polydiorganosiloxane. The hexamethyldisilazane reacts readily with hydroxy groups at the surface of the silica to treat the silica. If the surface of the silica is not modified by either pretreatment or in situ treatment, then the silica will not be readily dispersible within the polydiorganosiloxane and the resultant silica containing polydiorganosiloxane will not be readily dispersible within the solvent. If fully pretreated silica is used rather than partially pretreated or untreated silica, the resulting elastomer will have a lower durometer and less resiliency and a higher hysteresis loss. The amount of hexamethyldisilazane needed to effectively treat the surface of the silica will range from 0.1 to 2 parts by weight depending on the amount of hydroxy groups present on the surface of the silica. The exact amount for a specific formulation will depend upon the amount and type of silica filler used and can be determined by one skilled in the art without undue experimentation. Hexamethyldisilazane is commercially available. A small amount of water, not to exceed 2 parts by weight, may be added to the polydiorganosiloxane as a processing aid for the filler treating reaction. This water must be removed before the solvent is admixed. The solvent used in this invention can be any highly polar solvent. Suitable solvents include acetone, tetrahydrofuran, methylisobutylketone and methylethylketone. Compositions that do not contain such a solvent will be sticky and difficult to work with on a mill. The solvent selected should be essentially free of free water. If a solvent containing more than 0.2% water is used in this invention, the resulting moisture curable fluorosilicone dispersion may exhibit shelf-stability problems. The amount of solvent will usually range from 10 to 100 parts by weight. The preferred amount of solvent is 25 to 75 parts by weight and the most preferred amount is 30 to 50 parts by weight. The preferred solvent is methylethylketone. Methylethylketone and other highly polar solvents are commercially available. The crosslinking agent used in our method is selected from ketoximo silanes of the general formula (R 3 ) c Si(R 4 ) 4-c where R 3 is a ketoximo group, each R 4 is independently selected from the group consisting of methyl, ethyl, vinyl or OSi(R 5 ) 3 , wherein R 5 is independently selected from methyl, ethyl, propyl, butyl, vinyl and phenyl, and c is an integer with a value of 2 to 4, inclusive, and mixtures thereof. The preferred crosslinking agent is vinyltris(methylethylketoxime)silane. The composition of this invention is moisture curable. The term moisture curable is used to mean a composition that is stable in the absence of moisture and, in the presence of moisture, cures to a crosslinked elastomer. The moisture curable fluorosilicone dispersion of this invention is unique in that, upon exposure to moisture, the composition cures to an elastomer that has high resiliency and low values of hysteresis loss. The resiliency of a cured elastomer is a measure of the tendency of the material, when subjected to strain or compression, to rapidly return to its original shape after the stress has been removed. The hysteresis loss of a cured elastomer is a measure of an elastomer's tendency to retain its resiliency after it has been repeatedly stressed. Elastomers with high hysteresis loss will lose their resistance to strain or deformation after being repeatedly stressed. Elastomers with low hysteresis loss retain their resistance to strain or deformation after repeatedly being stressed. The hysteresis loss value is determined using the formula % Hysteresis Loss =(( A 1 -A 2 )/A 1 ) X 100% where A 1 is the area under the stress-strain curve generated on the first pull of an ASTM die C tensile bar and A 2 is the area under the stress-strain curve on the third pull of the same ASTM tensile bar. Methods of making and using transfer rolls and photocopy machines are well known in the art. Information on the xerography process, the use of transfer rolls and the use of polymeric release layers are found in U.S. Patent 4,185,140. A method of making and using a transfer roll is similarly disclosed in European Patent Publication 0565100. In the process of xerography, more commonly known as photocopying, a light image of an original to be copied is recorded in the form of a latent electrostatic image on a photosensitive member. The latent image is then rendered visible by the application of electroscopic marking particles. These particles are commonly referred to as toners. The visual toner image is either fixed directly upon the photosensitive member, transferred from the member to another support, such as a piece of paper, or transferred to an intermediate transfer roll with subsequent fixing onto another member. The introduction of liquid toner systems in which the electrostatic sensitive particles are dispersed in a solvent have created problems in the conventional xerographic process. The solvents of these liquid systems can degrade the transfer roll coating and thereby decrease the copy quality produced and the operating life of the transfer roll. Although fluorosilicone elastomers are known to be resistant to solvents, conventional fluorosilicones lack the required combination of resiliency and mechanical properties to be effective as transfer roll coatings in photocopy machines employing liquid toners. Photocopy machine transfer rolls coated with the high resiliency, low hysteresis loss elastomer claimed herein are resistant to the solvents used in liquid toner systems and result in improved copy quality and transfer roll life. Liquid toner photocopy machines employing transfer rolls coated with this elastomer produce higher quality copies and have decreased transfer roll replacement. Example A base composition was prepared. First 46.7 parts of a hydroxy-terminated trifluoropropylmethylsiloxane was placed in a "BAKER PERKINS" mixer. "BAKER PERKINS" is a trademark of Baker Perkins, Inc of Philadelphia, PA. Next 0.46 part of water was added and the combination was mixed for 1 minute to distribute the water through the polymer. Then 0.71 part of hexamethyldisilazane was admixed for 3 minutes. Then 1.35 parts of amorphous silica were mixed into the polymer. The silica admixing step was repeated 3 more times to give a total filler content of 6.4 parts. The composition was then mixed for 15 minutes Next, the composition was mixed for another 15 minutes while being heated to 160°C. The composition was mixed and heated to 160°C. for an additional 45 minutes while a vacuum was applied to the composition. The composition was then allowed to cool for 30 minutes to yield a reinforced fluorosilicone base. Next, 54.3 parts of this base were combined with 39.5 parts of dry methylethylketone and mixed on a paint shaker for 15 minutes. The composition was then rolled for 8 hours. Twelve parts of vinyltris(methylethylketoxime)silane were then added. The resulting composition was purged with dry nitrogen and mixed on a paint shaker for 15 minutes. Two comparative compositions were prepared in which the untreated silica and in situ filler treating agent were replaced with pre-treated silica. In the first comparative composition, 6.4 parts of amorphous silica were dispersed in toluene. Then, 0.46 part of water and 0.71 part of hexamethyldisilazane were added to the silica dispersion. The resulting composition was allowed to age for 3 hours at 23°C. The toluene and residual ammonia were then evaporated from the composition by air drying for 16 hours followed by heating for 3 hours at 150°C. The resulting composition was identified as Comparative Composition #1. In the second comparative composition, 0.46 part of water and 0.71 part of hexamethyldisilazane, and 6.4 parts of silica were combined and tumbled for 3 hours. Residual ammonia was removed from the composition by heating for 16 hours at 150°C. The resulting composition was identified as Comparative Composition #2. The three compositions were cured and tested with the results shown in Table 1. IN SITU TREATED Composition PRE-TREATED Comparative Composition #1 PRE-TREATED Comparative Composition #2 VISCOSITY (cs) [mm 2 /s] 500 200 1300 TENSILE (psi) [MPa] 355 [2.45] 95 [0.66] 289 [2.00] ELONGATION (%) 183 85 136 MODULUS 50 % psi 69 71 136 [MPa] [0.48] [0.49] [0.94] 100 % psi 162 --- 222 [MPa] [1.12] [---] [1.53] DURO-METER (Shore A) 33 34 49 BASHORE (%) 43 36 45 SPECIFIC GRAVITY (g/cm 3 ) 1.327 1.323 1.307 HYSTERSIS LOSS (%) 50 % Extension 9.1 12.8 42.7

Claims: A method of producing a moisture curable fluorosilicone dispersion, comprising the steps of: (A) mixing (i) from 30 to 100 parts by weight of hydroxy-terminated polydiorganosiloxane selected from siloxanes having the unit formula or siloxanes having the unit formula where R 1 and R 2 are individually selected from the group consisting of methyl, ethyl, propyl and butyl, a is an integer with a value of at least one, and b is an integer greater than zero; (ii) from 3 to 18 parts by weight of a reinforcing filler having a surface area of 50 to 400 m 2 /g, selected from untreated silica or partially pre-treated silica; (iii) from 0.1 to 2 parts by weight of hexamethyldisilazane; (B) heating the mixture; (C) applying a vacuum to the heated mixture for a sufficient time to remove any volatiles; (D) cooling the heated mixture; (E) admixing from 10 to 100 parts by weight of a solvent; and (F) admixing from 2 to 15 parts by weight of a crosslinking agent selected from ketoximo silanes of the general formula (R 3 ) c Si(R 4 ) 4-c where R 3 is a ketoximo group, each R 4 is independently selected from the group consisting of methyl, ethyl, vinyl or OSi(R 5 ) 3 , wherein R 5 is independently selected from methyl, ethyl, propyl, butyl, vinyl and phenyl, and c is an integer with a value of 2 to 4, inclusive and mixtures thereof. The method of claim 1 further comprising the step of (G) purging the resulting polymer and solvent containing mixture with dry nitrogen for a time sufficient to remove atmospheric moisture. The method of claim 2 wherein up to 2 parts by weight of water is added during the mixing step (A). The method of claim 3 wherein said polydiorganosiloxane is trifluoropropylmethylsiloxane, said filler is untreated silica, said crosslinking agent is vinyltris(methylethylketoxime)silane and said solvent is methylethylketone. The dispersion obtainable by the method of claims 1 - 4. The elastomer obtainable by curing the dispersion of claim 5. A transfer roll for a photocopy machine coated with the elastomer of claim 6. A method of transferring an image using a transfer roll as claimed in claim 7. A method of photocopying in a photocopy machine using a transfer roll as claimed in claim 7. Use of the dispersion of claim 5 for the preparation of the transfer roll for the photocopy machine of claim 7 coated with the elastomer of claim 6.

Inventors: IRISH PAUL THOMAS, MAXSON MYRON TIMOTHY, IRISH, PAUL THOMAS, MAXSON, MYRON TIMOTHY
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1996
Main classification: C08L83
IPC codes: C08L83, C08K9, G03G15