Method of protecting a surface

Details Method of protecting a surface Method of protecting a surface

1994-10-27

2001-05-01

Beck, Shrive (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Chemical agent applied to treat coating

C427S397700, C427S419200, C427S419300, C427S136000, C427S299000

Reexamination Certificate

active

06224944

ABSTRACT:

The invention concerns a method of protecting a surface of a carbonatic material from corrosion, the method comprising treatment of the material surface with an acidic cationic silica sol, the pH of the sol preferably being from about 4 to about 7. The invention also concerns a new silica sol useful for performing the method and a method of preparing the new silica sol. Further, the invention concerns carbonatic material comprising a surface layer of silica, the surface of the silica particles comprising a polyvalent metal oxide/hydroxide.
Many buildings, ordinary houses as well as historical monuments and statues, are made of porous carbonate rich material such as limestone, dolomite, marble or calcareous sandstone which materials are sensitive to pollutants present in the air, particularly the oxides SO
X
, NO
X
and CO
2
giving acids when dissolved in rain water. It has been established that a major cause of limestone deterioration is acidic rain and dry deposition of SO
2
into the stone pore system, i.e. corrosive reaction progressing between the rainfalls, for which reason also material non-exposed to rain becomes deteriorated.
It has been found that the dominating deterioration factor of carbonatic stone is the formation of a hard black crust on the surface of the stone, the crust mainly consisting of CaSO
4
.2H
2
O (gypsum) and depositions of dust and dirt. The crust detaches very easily from the underlying stone, and when exposed to rain water, the gypsum dissolves and is transported through the pore system deep into the bulk of the stone where it crystallizes during the drying period, disrupting the stone fabric.
A common method of preserving stone involves treatment with water repellents, for example silicon organic products such as alkylsilanes or silicon resins, preventing the penetration of water into the stone and hindering the damage due to crystallization of gypsum in the pores. However, the treatment does not hinder the corrosive reaction at the stone surface and therefore does not counteract the formation of the gypsum crust which can easily be detached from the underlying stone. Further, water solutions of salts appearing behind the silicone treated layers lead to accelerated decay of the stone. Moreover, the silico-organic materials are relatively expensive and must also be applied as solutions in organic solvents.
Another method involves coating of the stone with lime sacrificial layers, introducing fine reactive calcite at the stone surface and thus creating particularly favorable conditions for the corrosive reaction to occur. Therefore, the lime coating treatment must be repeated after a certain time interval, and after some time also the surface of the coated stone can be affected by the corrosive reaction.
U.S. Pat. No. 4,423,096 discloses treatment of ceramic construction materials with a composition comprising a silica sol having finely divided granular ceramic powder suspended therethrough. The patent does not mention cationic sol and treatment of carbonatic material is not mentioned.
U.S. Pat. No. 3,252,917 relates to the production of “salt free” cationic silica sol. The sol is said to be useful for waterproofing building material constructed from hydraulic binding agents such as concrete or mortar, the sol being incorporated together with the other components during preparation of the material. The patent also discloses treatment of asbestos plates, cork plates or the like, but does not mention surface treatment of solid carbonatic material.
Thus, there is a current demand for a method of protecting solid carbonatic building material against pollutants present in the air. It is therefore an objective of the invention to provide a method of inhibiting the corrosion caused by acid rain and by dry deposition of SO
2
at the surface of carbonatic materials. It is also an objective of the invention to prevent water from reaching the bulk of a porous material without stopping it from breathing. It is another objective of the invention to provide an effective, non-toxic and comparatively inexpensive agent for treating the surface of a carbonatic material.
According to the invention, the above objectives have been achieved by providing a method of protecting a surface of a carbonatic material, particularly a material having a high content of calcium carbonate and comprising a pore system, the method comprising treatment of the surface with an acidic cationic silica sol, the pH of the sol suitably being from about 4 to about 7, preferably from about 5 to about 7, most preferably from about 5 to about 6. Cationic silica sol refer to an aqueous sol comprising dense, non-agglomerated positively charged particles, the surface of the particles comprising a polyvalent metal oxide/hydroxide, preferably aluminum oxide/hydroxide.
Without being bound to any theory, the following mechanism behind the stone protection is assumed. At slightly acidic pH, i.e. below the iso-electric point of the metal oxide/hydroxide on the SiO
2
particles in the sol, which point in the case of alumina is between 6 and 8, the metal oxide/hydroxide is in its protonated form, the particles thus comprising a great number of positively charged groups. When the surface of a carbonatic material, such as carbonatic stone, is contacted with the weakly acidic sol, the carbonate dissolves slightly and reacts with H
+
to HCO
3
−
. The resulting deprotonation of the sol particles brings about a fast increase of the pH in the sol and gelling of the metal oxide/hydroxide near its zero-point of charge. Since the supply of proton-binding species proceeds from the surface of the carbonate grains, the gelling proceeds at the stone surface, coating it with a thin dense protective layer mainly consisting of silica and metal oxide, suitably from about 0.05 to about 2 mm thick, preferably from about 0.1 to about 0.2 mm thick, partly above the stone surface and partly in the outer pores. The portion above the stone surface should preferably be thinner than 1 mm, most preferably thinner than 0.2 mm. It has been found that the protective layer is effectively prevented from being washed out of the stone surface. The silica protects the carbonatic material against acids and also restrains rain water from penetrating into the stone pore system. On the other hand, the protective layer has been found to be permeable to water vapour, enabling the stone to breath and preventing moisture from being permanently entrapped in the pores.
In order to avoid corrosion caused by easily soluble salts, the silica sol used should contain as small amounts as possible of both dissolved anions and cations. The conductivity at pH 4 should preferably be less than about 3000·10
−6
S/cm, most preferably less than about 500·10
−6
S/cm. Chloride and other halide ions are particularly harmful, and the content of those ions should preferably be less than 0.05 mol/liter, most preferably less than 0.01 mol/liter.
Cationic silica sols comprising alumina or other polyvalent metal oxides/hydroxides are well known and commercially available, for example under the trademark Bindzil® CAT 80 (Eka Nobel AB, Bohus Sweden). The preparation of cationic sols involve surface modification of an anionic sol and is described in numerous patents, for example U.S. Pat. Nos. 3,007,878, 3,252,917, 3,139,406, 3,620,978, 3,719,607 3,745,126 and 3,956,171, GB patent 1265550 and CA patent 953605. The surface modification does not significally change the size of the silica particles. The average particle size and the particle size distribution can therefor be controlled when preparing the starting anionic silica sol. In order to obtain the required average particle size and particle size distribution, the reaction time has to be carefully controlled at chosen pH and temperature, see for example U.S. Pat. Nos. 2,244,325 and 2,574,902. However, most of the commercially available cationic sols are too acidic and contain too much of chloride or other dissolved anions, and must therefore be modified prior to use in the present method. The mo

Affiliated with

Also associated with

Rating

Method of protecting a surface does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method of protecting a surface, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of protecting a surface will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2452805