Thin SiO.sub.2 films, a process for producing them and their use

Details Thin SiO.sub.2 films, a process for producing them and their use Thin SiO.sub.2 films, a process for producing them and their use

1998-03-19

2000-01-25

Brunsman, David

Compositions: coating or plastic

Coating or plastic compositions

Silicon containing other than solely as silicon dioxide or...

10628711, 10628712, 10628713, 10628714, 10628715, 501 12, 2105001, 428447, C09D18304

Patent

active

060173890

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to thin, tearfree, preferably transparent and colorless SiO.sub.2 films, a process for producing them according to the sol-gel process and their use as, e.g., membranes, filters, components of laminates or support materials having functional additives incorporated therein.
Films made of glass are usually produced by drawing or extrusion processes from the melt. Said processes depend on the thermal properties of the glass (softening point, crystallization rate etc.) and thus limited to specific glass compositions. There is a further limitation regarding the minimum thickness of the glass films which can be produced, this being the reason that so far it has not been possible to produce films of silica glass having a thickness below about 250 .mu.m by melt and subsequent molding processes.
It is known that by sol-gel techniques the densification temperature of vitreous and/or ceramic materials can be substantially reduced. However, the production of SiO.sub.2 films according to the sol-gel process has so far been prevented by the problems in the conversion of the mostly aqueous-alcoholic precursors (sols) to xerogel bodies by solvent removal or addition of a gelling agent. In the drying process of sols on substrates the formation of tears can easily take place due to capillary forces and according to different drying rates at the upper and lower side, respectively. Furthermore, due to the preferential solvent evaporation at the surface in the course of the drying operation said gel films separate from the substrate and roll up. A process according to which transparent, tearfree SiO.sub.2 films having larger dimensions can be produced by casting and gelling of SiO.sub.2 sols on a base has not been known.
Surprisingly it has now been found that thin SiO.sub.2 films of substantially any dimensions (length and width) can very easily be produced in a sol-gel process.
Object of the present invention is a process for the production of thin SiO.sub.2 films which is characterized in that
a) 40 to 100% by weight of one or more silanes of general formula (I) groups or hydrolytically removable groups, the radicals R are the same or different and are hydrolytically non-removable groups and x represents 0, 1, 2 or 3, provided that x.gtoreq.1 in at least 70% by moles of said silanes; optionally in the presence of
b) 0 to 50, preferably 0 to 25% by weight of colloidal SiO.sub.2 and/or
c) 0 to 10% by weight of an organic binder gel film and said gel film is heat-treated.
Objects of the present invention also are SiO.sub.2 films which can be produced in said manner and SiO.sub.2 films having a thickness of 2.5 to 250 .mu.m and a surface area of at least 25 cm.sup.2, preferably at least 50 cm.sup.2, particularly preferred at least 100 cm.sup.2.
The SiO.sub.2 films according to the present invention are thin, tearfree, preferably transparent and colorless, and are characterized by a high flexibility and minimum shrinkage.
Details regarding the sol-gel process are disclosed in C. J. Brinker, G. W. Scherer: "Sol-Gel Science--The Physics and Chemistry of Sol-Gel-Processing", Academic Press, Boston, San Diego, New York, Sidney (1990), and in DE 1941191, DE 3719339, DE 4020316 and DE 4217432.
Said references also describe specific examples of the silanes which can be employed in the process according to the present invention as well as of hydrolytically removable radicals A and hydrolytically non-removable radicals R.
Preferred examples of hydrolytically removable groups A are hydrogen, halogen (F, Cl, Br and I, particularly Cl and Br), alkoxy (particuarly C.sub.1-4 alkoxy such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy), aryloxy (particularly C.sub.6-10 aryloxy, e.g. phenoxy), alkaryloxy (e.g. benzyloxy), acyloxy (particularly C.sub.1-4 acyloxy such as acetoxy and propionyloxy) and alkylcarbonyl (e.g. acetyl). Radicals A which are also useful include amine groups (e.g. mono- or dialkyl, aryl and aralkyl amino groups featuring the above mentioned alkyl, aryl and aralkyl radicals), amide groups (

REFERENCES:
patent: 5496402 (1996-03-01), Sakamoto et al.
patent: 5858280 (1999-12-01), Zhang et al.

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