Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
2002-09-09
2004-10-05
Chapman, Mark A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S108700, C423S335000, C423S336000
Reexamination Certificate
active
06800413
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a silylated silica and to processes for preparing this silica.
2. Background Art
Silylated silicas having a residual silanol content of more than 25% of the silanol group content of the parent silica, corresponding to an SiOH density per nm
2
of more than 0.6 based on the BET-method surface area (DIN 66131 and 66132), have numerous disadvantages. The high residual silanol content leads to strong interactions between the silica particles through hydrogen bonds between the silanol groups and the silica surface, and hence to a severe thickening effect which is disrupted in processing, particularly when attempting to attain high filler levels in polymer and resin systems.
In powder systems, a residual silanol content above 25% leads to particle interactions and hence reagglomeration and separation. This has adverse effects on the flow-behavior stability and triboelectric stability properties of powder systems such as powder resins and toners on storage.
Existing processes for coating silica with organosilicon compounds, in accordance with the prior art, are based on the addition of silylating agents. A precondition for this treatment is the use of silylating agents which possess sufficient reactivity, for example silylating agents which bear reactive halide radicals (Cl, Br), alkoxy radicals (OR), amino radicals (—N—R), or silazane radicals (—N—Si—). Under industrial conditions, to accelerate the reaction and hence improve economy, it is common to add protic solvents, such as water. However, this process may lead to side reactions culminating in the formation of stable siloxane bonds (Si—O—Si), which are of little or no reactivity under the reaction conditions of the prior art, and which are therefore no longer available under industrial silylating conditions. Ultimately, this leads to a lower degree of silylation than intended, and to a lower reaction yield than expected.
Existing processes for preparing pyrogenic silica from organosilicon compounds are based on the use of a pyrogenic silica raw material, for example, in accordance with EP 0 855 368 A1 and the literature cited therein, wherein silica is freed of adhering hydrogen chloride by further addition of water or steam. According to the prior art, the further treatment with water results in improved silica purification but also, at the same time, in an increased surface density of hydroxyl groups attached to surface silicon atoms (silanol groups). Normally, this increased surface silanol group density is correlated with an increased reactivity of the pyrogenic silica thus treated.
It has now been surprisingly discovered that increased silanol group density on hydrophilic parent silica is disruptive to the achievement of a high degree of silylation, i.e., a high degree of coverage with silylating agent and a high yield in the reaction of the surface silanol groups with silylating agent radicals. The yield is desirably more than 75%, corresponding to a residual silanol content of less than 25%.
To attempt to overcome the inadequate reaction of the silanol groups and thus a deficient reaction yield by using a large excess of silylating agent to attain the target degree of silylation is economically unattractive. The high levels of hydrophobicizing agent that are needed for high degrees of hydrophobicization produce a high organosilicon waste load, which must be disposed of at great expense.
EP 579 049 discloses the addition of alcohols as a catalyst for the purpose of improving the reaction yield. This has the disadvantage, however, that the alcohol must subsequently be disposed of as an organic load on waste treatment facilities, and additionally, may create emissions to air and water.
From DE 02211377 and DE 2344388 it is known to silylate silica under mechanical load. In the processes described therein, however, residual silanol contents of less than 25% are not obtained.
Another disadvantage of silicas with high residual silanol content arises when they are used as an active filler in liquid systems, polymer systems, and resin systems of moderate and high polarity. In such systems, problems of miscibility and compatibility arise. This is particularly disadvantageous when close integration between the silica and the surrounding matrix is required in order to obtain good mechanical properties such as hardness, strength, toughness, and abrasion resistance, etc.
Another disadvantage displayed by such silica arises when used as additives, e.g., as antiblocking agents, as free-flow aids, or for controlling or regulating triboelectric properties of solids, especially with finely divided solids such as crosslinked polymer systems and resin systems or finely divided inorganic solids. Problems of miscibility and compatibility result, leading to separation of the silica when used as a free-flow aid and/or charge regulator in pulverulent products such as powder resin systems, powder coatings, and in toners and developers. This separation is associated with poor silica-powder particle interaction. The consequences of this separation include the destabilization and poor service life of the system, i.e., in the case of toner-silica mixtures, for example, poor stability on exposure to repeated copying and activating processes.
It would be desirable to overcome the disadvantages of the prior art, and to provide silica with a low concentration of surface silanol groups.
SUMMARY OF THE INVENTION
The present invention provides a process for the economical preparation of low surface silanol-content silicas by a multistage procedure, wherein in separate steps, a hydrophilic silica is first provided; the hydrophilic silica is loaded with silylating agent; the silica is then reacted with the silylating agent; and the silylated silica is then purified.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention thus firstly provides a silylated silica having an SiOH density per nm
2
of less than 0.6, based on the BET-method surface area (DIN 66131 and 66132). These are silicas having a residual silanol group content of less than 25%, preferably less than 15%, and with particular preference less than 10%, of that of the parent silica. Particular preference is given to silylated silica of this kind with a residual silanol group content of less than 8%, and especially with a residual silanol group content of less than 5% that of the parent silica. The residual silanol group content is determined by the Sears method of acid-based titration, in a solvent mixture of methanol (50% by volume) and saturated aqueous sodium chloride solution (50% by volume) in deionized water (G. W. Sears, Anal. Chem., 28(12), (1956), 1981).
The invention further provides a process for preparing a silylated silica in separate, successive steps which each preferably take place in a separate vessel: (A) first, preparation of the hydrophilic parent silica, then (B) silylation of the silica by (1) loading of the hydrophilic silica with silylating agent, (2) reaction of the hydrophilic silica with the silylating agent, and (3) purification of the hydrophilic silica to remove silylating agent and products of side reaction.
Parent Silica
The base (parent) silica for the silylation is a hydrophilic pyrogenic silica which is prepared under substantially anhydrous conditions. By anhydrous in this context is meant that neither in the hydrothermal preparation process nor in the further steps of the process, such as cooling, purifying, and storage, continuing to the ready-prepared and purified, packaged and ready-to-dispatch product, is substantial additional water supplied to the process, in either liquid or vapor form. In any case, no more than 10% by weight of water, based upon the total weight of the silica, is added; preferably, not more than 5% by weight, more preferably not more than 2.5% by weight, and with particular preference, no water at all is added.
It is preferred to use a silica of increased surface activity, which can be described in terms of increased surface homogeneity, characterize
Barthel Herbert
Grünwald Franz
Heinemann Mario
Maginot Helmut
Völkel Ute
Brooks & Kushman P.C.
Chapman Mark A.
Wacker-Chemie GmbH
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