CA2032736A1 - Silicone treatment of porous substrates - Google Patents

Abstract

SILICONE TREATMENT OF POROUS SUBSTRATES
ABSTRACT OF THE DISCLOSURE
In accordance with the present invention there is provided a water repellency system and a method for its application on a porous substrate. The system comprise a low viscosity polydiorganosiloxane which is characterized in having reactive end-groups and a cross-linking agent therefor. Additionally, the system is characterized in that it is free from catalysts and or fillers.
Upon its application to a porous substrate, the water repellency system of the invention penetrates the pores and then cures to form a protective film which resists the formation of efflorescence. The film thus formed is characterized by its good adherence to the protected substrate.

Description

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Mo-3332 SILICONE TREATMENT OF POROUS SUBSTRATES
FIELD OF THE INVENTION
The present invention relates to water repellent porous substrates and in particular to silicone rubber treated substrates.
BACKGROUND OF THE INVENTION
Efflorescence can occur on the surface of cementitious substrates, e.g. concrete roof tiles and concrete pavers. A
typically white residue thus formed is aesthetically objectionable and means to prevent its formation have long been sought. Hydrated cement contains calcium hydroxide as an inevitable product of the reaction between cement and water.
When this slightly insoluble (1 lb. in 72 gallons of water) calcium hydroxide is brought to the surface by water it combines with carbon dioxide in the air to form the insoluble calcium carbonate which then appears as a whitish deposit, called efflorescence.
Water borne acrylic based sealers have been proposed yet are of but limited usefulness against efflorescence. Moreover, acrylic sealers are notoriously poor in terms of weather resistance and they, at best, afford but short lived protection.
Compositions based on silicone rubber, useful as sealants, caulking compounds and the like are known. For the most part these are room temperature vulcanizable (RTV) compounds which 25 cure under the influence of atmospheric moisture. Silicone rubber compounds useful for molding are also known and have been described, for instance in U.S. Pa~ent 3,897,376. Adhesion problems encountered in the application of RTV to substrates have been addressed in U.S. Patent 4,036,813. A method for bonding 30 silicone rubber to a primed surface has been disclosed in U.S.
Patent 3,935,348 and in U.K. Patent Specification 1,571,199.
An RTV silicone rubber having low modulus and good adhesion to concrete has been disclosed in U.S. Patent 4,323,489. Water proofing of masonry walls have been discussed in U.S. Patent 35158Ap0765 .. , .. . , : . . .
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4,273,813. Accordingly, a method was provided for coating the surface of a masonry wall with a silicone emulsion and then drying the emulsion ~o provide an elastomeric film bonded to the wall. A self bonding room temperature vulcanizable sil;cone 5 ` rubber composition suitable in water repellency applications has been disclosed in U.S. Patent 4,273,698.
An asbestos-cement shingle having water repellent properties was disclosed in U.S. Patent 2,833,673. Accordingly, the surface of an article to be protected has been treated by hydrolysis and condensation copolymer of amyltrichloro silane and methyl trichloro silane.
The art is also noted to include U.S. Patent 3,032,528 which disclosed a room temperature vulcanizing silicone rubber which consists of a linear siloxane polymer having reactive end groups 15 and an amino silane. The silicone rubber is said to be suitable as a coating.
U.S. Patent 3,674,738 disclosed an RTV silicone rubber consisting of a linear diorganopolysiloxane having chemically reacti~e end-blocking units and a cross-linking agent. The 20 disclosed silicone rubber is said to be vulcanizable and curable in the form of a thin film on any of a variety of substrates, including concrete, mortar and construction materials. In U.S.
Patent 3,678,003 there is disclosed a silicone rubber which includes a linear diorganopolysiloxane having chemically reactive end groups, a cross-linking silicone compound and a silane. The disclosed elastomer is said to adhere to any of a wide variety of substrates including stone, concrete and mortar and be useful for the preparation of protective coatings. An RTV silicone elastomer consisting of a hydroxylated organopolysiloxane and a cross-linking agent, said to have excellent adhesion and be useful as a coating material for buildings and structures has been disclosed in U.S. Patent 4,1169935.

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SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a water repellency system and a method for its application on a porous substrate. The system comprise a low viscosity polydiorganosiloxane which is characterized in having reactive end-groups and a cross-linking agent therefor. Additionally, the system is characterized in that it is free from catalysts and or fillers.
Upon its application to a porous substrate, the water repellency system of the invention penetrates the pores and then cures to form a protective film which resists the formation of efflorescence. The film thus formed is characterized by its good adherence to the protected substrate.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention there is provided a water repellency system for porous substrates comprising (i) an RTV silicone compound, exclusive of silicone fluids, and (ii) a suitable cross-linking agent therefor. The system is a one component prepared from a polydiorganosiloxane, preferably polydimethylsiloxane, which is characterized in that it contains reactive end groups.
It is critical to the present invention that the system penetrates the pores of the substrate to which it is applied and that no continuous silicone coating is formed on the protected substrate. In order to achieve the degree of penetration which is a requisite of the invention, the siloxane of the invention is characteristically of a low viscosity. The viscosity of the system is one which is sufficient to permit its penetration of the pores of the substrate to be protected. Typically, the viscosity of the siloxane has a viscosity of 50 to 10,000 cps at 25C, preferably 100 to about 5,000 cps. It is also critical that the system of the invention contains neither catalysts nor fillers which normally characterize RTV siloxane compounds because the fillers tend to increase the viscosity and the .

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~ , 3 catalysts tend to complete the reaction before the siloxane had suffic;ently penetrated the pores of the substrate.
The polydiorganosiloxanes of the invention are known. These include any polydiorganosiloxane wh~ch have reactive terminal groups such as hydroxyl, vinyl or amine groups, preferably terminal hydroxyl groups. Generally, the polydiorganosiloxane compounds (siloxane) of the invention conform to:

R' R' RO-Si- - O-Si - - OR
R' R' wherein o R~ denotes a monovalent hydrocarbon radical having 1 to 8 carbon atoms and R denotes a hydrogen atom, a hydroxyl group, an amino group or a monovalent hydrocarbon radical having 1 to 8 carbon atoms, and n is an integer such that the viscosity of the siloxane renders it suitable in accordance with the criterion noted abnve.
Polydiorganosiloxane suitable in the practice of the invention have been disclosed in U.S. Patents 3,294,732, 3,127,393 and 3,108,061, the specifications of all of which are incorporated herein by reference.
~o Optionally, the system may contain an adhesion promoter such as an amino-functional alkoxy silane.
The cross-linking agent of the invention are silanes which conform to R'-Si-(OR)3 wherein R and R' independently of each other are lower alkyl radicals, preferably a C1-C4 alkyl radical or an amino functional group, preferably .

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The amount of cross-linking agent to be used in conjunction with the polydiorganosiloxane of the invention is that amount which would permit the penetration of the rubber into the pores 5 of the substrate before final cure is attained. Preferably the amount of cross-linking agent is in the range of about 0.2~ to about 10% more, preferably 1.0 to about 5% relative to the weight of the system.
A convenient method to quantify the amount of cross-linking lo agent is to prepare a sample of the formulation and determine its tack free time in accordance with ASTM C-679. Essentially, the tack-free time of the formulations of the invention should not be less than two hours nor more than 48 hours.
The system of the invention has good adhesion properties but more importantly, it penetrates the pores of the substrate to be protected and cures within the pores. Importantly, the application of the system of the invention preferably entails a substrate having a pH of about 4.5 to 10.
The substrate to be protected in accordance with the invention includes masonry materials, concrete and cement blocks, bricks and pavers and leather goods, preferably concrete blocks, bricks and pavers.
EXAMPLES
A water repellency system in accordance with the invention ~5 has been prepared and its properties evaluated. The system was based on Baysilone C 0.7 polydiorganosiloxane, a hydroxy terminated polydimethylsiloxane having a viscosity of about 700 cps, from Mobay Corporation and 3 to about 7~ of methyltris(methylethylketoxime)silane as the cross-linking agent.
The systems thus prepared were brushed on the surface of cured concrete and brick pavers and demonstrated sufficient penetration before cure. The water repellency of the thus protected substrates was confirmed. It was noted that the system of the ~;
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invention also provide an aesthetic benefit. The application of the system to paver bricks appears to intensify the color of the brick substrate.
In a parallel set of experiments it was established that a s system based on the above noted polydimethylsiloxane which was cross-linked by ethyltriacetoxysilane, was not suitable in accordance with the invention. Similarly, methylethoxybis-(N-methylbenzamido)silane was found to be unsuitable as a cross-linking agent for the system.

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Claims (11)

1. A porous substrate which is rendered water repellent by the application to its surface, a protective silicone system comprising (i) a polydiorganosiloxane which is characterized in that its viscosity is in the range of about 50 to 10,000 cps at 25 degrees centigrade and in that it contains chemically reactive end-groups and (ii) a cross-linking agent conforming to wherein R and R1 independently denote a lower alkyl radical or a radical containing an amino-functional group, said system being further characterized in the absence of catalysts and fillers therefrom.

2. The porous substrate of Claim 1 wherein said polydi-organosiloxane conforms to wherein R1 denotes a hydrocarbon radical having 1 to 8 carbon atoms, R denotes a hydrocarbon radical having 1 to 8 carbon atoms, a hydrogen atom, a hydroxyl group or an amine radical and n is an integer such that that the viscosity of said siloxane is such that it is sufficiently fluid to penetrate the pores of said substrate without forming a continuous silicone coating.

3. The porous substrate of Claim 1 wherein said substrate is a member selected from the group consisting of concrete blocks, concrete tiles, cement products, bricks and leather.

4. The porous substrate of Claim 1 wherein said chemically reactive end group is a member selected from the group consisting of hydroxyl, vinyl and amine groups.

5. The porous substrate of Claim 1 wherein said polydi-organosiloxane is polydimethylsiloxane.

6. The porous substrate of Claim 1 wherein said viscosity is in the range of about 100 to 5,000 cps.

7. The porous substrate of Claim 1 further containing an adhesion promoter.

8. A method for rendering a porous substrate water repellent comprising applying to the surface of said substrate a polydiorganosiloxane based system and curing said system, said polydiorganosiloxane being characterized in that it contains chemically reactive end groups, said system being characterized in that it contains a cross-linking agent and in that it exhibits a tack-free time in accordance with ASTM C-679 of between about two and 48 hours.

9. The method of Claim 7 wherein said polydiorganosiloxane is polydimethylsiloxane.

10. The method of Claim 7 wherein said reactive end group is a member selected from the group consisting of hydroxyl, vinyl and amino functional radicals.

11. The method of Claim 7 wherein said system further comprise an adhesion promoter.