Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-08-11
2002-05-07
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C523S213000
Reexamination Certificate
active
06384125
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to chemically modified silica fillers, methods for making chemically modified silica fillers, and the use of such silica fillers in polymeric compositions, especially silicone compositions. Most particularly, this invention relates to a process for producing a functionalized and hydrophobed silica filler, hereinafter referred to as modified silica filler.
The modified silica fillers prepared by the present method are useful in many applications where fumed silicas, precipitated silicas, colloidal silicas, and silica gels are useful. However, the modified silica fillers prepared by the present method are particularly useful as reinforcing and extending fillers in organic rubbers and silicone rubber compositions including silicone adhesives, rubbers, coatings, foams, and sealants. It is well known that cured silicone compositions, such as silicone rubbers, formed from the curing of polydiorganosiloxane fluids and gums alone generally have poor elongation and tensile strength properties. One means for improving the physical properties of such silicone compositions involves the incorporation of silica filler into the fluid or gum prior to curing. However, such silicas typically contain hydroxyl functionalities that cause the silicas to be hydrophilic. Such hydrophilic silicas are difficult to incorporate into silicone compositions comprising hydrophobic polydiorganosiloxane polymers and further cause the silica to react with the polymers causing a phenomenon typically referred to as “crepe hardening”. A great deal of effort has been made in the past to treat the surface of aggregated reinforcing silica fillers with organosilanes or organosiloxanes to make the surface of the silica hydrophobic. This surface treatment reduces or diminishes the tendency of the silicone compositions to crepe harden and improves the physical properties of the cured silicone compositions.
Various coupling agents, e.g., titanates, zirconates and silanes, have been suggested for use with silica fillers when such fillers are incorporated into polymeric compositions, e.g., rubber, in order to improve the performance of the rubber vulcanizate.
U.S. Pat. No. 5,116,886 describes a two-step process in which the surface of natural or synthetic, oxide or silicate fillers is modified by using certain organosilicon compounds. In the first step, the organosilicon compound is mixed intensely with the filler at a temperature below 60° C. In the second step, the homogenous mixture is subjected to a temperature of from 60 to 160° C. to complete the surface modification of the filler.
U.S. Pat. No. 5,908,660 also describes a two-step method for the preparation of hydrophobic silica. In the first step, an aqueous suspension of precipitated silica is contacted with an organosilicon compound in the presence of a catalytic amount of an acid to effect hydrophobing of the precipitated silica. In the second step, the aqueous suspension of the hydrophobic precipitated silica is contacted with a water-immiscible organic solvent at a solvent to silica weight ratio greater than 5:1 to effect separation of the hydrophobic precipitated silica from the aqueous phase.
U.S. Pat. No. 6,051,672 describes a method for making hydrophobic non-aggregated colloidal silica. The method comprises reacting an aqueous suspension of hydrophilic non-aggregated colloidal silica having an average particle diameter greater than about 4 nm with a silicon compound at a pH less than about pH 4 in the presence of a sufficient quantity of a water-miscible organic solvent to facilitate contact of the hydrophilic non-aggregated colloidal silica with the silicon compound. The method is conducted at a temperature within a range of about 20° C. to 250° C. for a period of time sufficient to form a hydrophobic non-aggregated colloidal silica. The hydrophobic non-aggregated colloidal silicas prepared by the present method are particularly useful as fillers in silicone compositions such as adhesives, rubbers, and sealants.
SUMMARY OF THE INVENTION
It has now been discovered that the processes described in U.S. Pat. No. 5,908,660 and U.S. Pat. No. 6,051,672 may be improved and used to produce the modified silica filler of the present invention, having a pH of from about 5.0 to 10, by utilizing a certain combination of functionalizing and hydrophobing agents in an aqueous suspension of silica having a pH of 2.0 or less and treating the acidic aqueous suspension of modified silica fillers with acid neutralizing agents to increase the pH of the suspension to a range of from greater than 2.0 to 10.
As used herein, a functionalizing agent is a reactive chemical that can cause silica to be covalently bonded to the polymeric composition in which it is used. A hydrophobing agent is a chemical that can bind to or be associated with silica to the extent that it causes a reduction in the affinity for water of the silica while increasing the silica's affinity for the polymeric composition in which it is used.
It has also been found that polyorganosiloxanes in silicone compositions containing the modified silica fillers of the present invention, which have a pH of 5.0 to 10.0, show less decomposition than those in compositions containing lower pH silica fillers.
DETAILED DESCRIPTION OF THE INVENTION
This invention is a process for making modified silica filler comprising the steps of:
(A) contacting an acidic aqueous suspension of precipitated or colloidal silica, optionally in the presence of a surfactant and/or a water-miscible solvent, with a combination of:
(a) a functionalizing coupling agent, and
(b) an organometallic hydrophobing compound in a weight ratio of (a) to (b) of at least 0.001 to 1 to form an acidic aqueous suspension of chemically treated silica filler having a pH of 2.0 or less
(B) treating the acidic aqueous suspension of the chemically treated filler with an acid-neutralizing agent to increase the pH of the suspension to a range of from greater than 2.0 to 10, and
(C) recovering the chemically treated filler.
This invention is also modified silica filler that has a pH of from about 5.0 to 10.0, where the silica filler is made by the method of this invention. This invention is also a curable silicone elastomer composition comprising a polyorganosiloxane, the modified silica filler produced by the method of this invention, and a means of curing the polyorganosiloxane into an elastomer.
The silica used to make the modified silica filler of the present invention is a colloidal or precipitated silica of the type used to formulate polymeric compositions such as rubber.
The colloidal silicas useful in the present invention are hydrophilic and are non-aggregated, essentially spherical-shaped particles having an average particle diameter greater than about 4 nm (nanometer). By “non-aggregated”, it is meant that the colloidal silica exists in aqueous suspension in large part as discrete spherical particles. Preferred is when the colloidal silica has an average particle diameter within a range of greater than about 4 nm to about 150 nm. Even more preferred is when the colloidal silica has an average particle diameter within a range of about 5 nm to 100 nm. The method of making the hydrophilic non-aggregated colloidal silica is not critical and can generally be any of those known in the art. For example, a silica hydrosol may be prepared by acidifying a sodium silicate mixture with subsequent stabilization of the silica particles by a method such as described in U.S. Pat. No. 6,051,672 to provide an essentially spherical-shaped, hydrophilic non-aggregated colloidal silica in an aqueous suspension.
The hydrophilic non-aggregated colloidal silica is added to the present method as an aqueous suspension. The concentration of colloidal silica in the aqueous suspension is not critical and can be varied within wide limits. To facilitate mixing and dispersion of the silicon compound in the method of the present invention it is generally preferred that the colloidal silica comprise about 1 weight percent to 50 weight
Bergstrom Debora Frances
Boswell Lisa Marie
Fisher Mark David
Hahn James Richard
Okel Timothy A.
Brown Catherine
Dow Corning Corporation
Michl Paul R.
Warren Jennifer
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