SI23002A - Procedure for sol-gel preparation of corrosion protection coatings for solar collectors - Google Patents

Abstract

The subject of this invention is a procedure for a sol-gel preparation of corrosion protection coatings for solar collectors. It is a sol-gel procedure for the preparation of corrosion protection of solar collector surfaces made of thin layers of cermet materials applied onto metal substrates, which includes mixing of at least one mercapto silane with, but not necessarily with, another silane in an organic solvent with water and an acid catalyst, immersion of the substrate into the solution for a certain time required to create a self-ordered layer on the immersed substrate, retrieval, drying and thermal treatment of the said substrate. The method comprises soaking of substrate in the mixture of at least one mercapto silane, a solvent, an acid catalyst, water and, but not necessarily, another hydrolysable silane for a certain time during which a quasi self-composable monolayer of sol molecules is formed on the surface of the substrate, removal of the substrate from the solution, drying of the coated substrate and finally thermal treatment of the coated substrate.

Description

Sol-gel process for preparing corrosion protection coatings for solar panels

It is an object of the present invention to provide a process for sol-gel preparation of corrosion protection coatings for solar panels. It is a sol-gel process for the preparation of corrosion protection of the surfaces of solar panels made of thin layers of cermet materials applied to metal substrates, which includes: mixing at least one mercapto silane with, but not necessarily, another silane in an organic solvent with water and an acid catalyst , immersing the substrate in a solution for a limited time, which is necessary to create a self-assembled layer on the immersed substrate, extracting, drying and heat treating said substrate.

BACKGROUND OF THE INVENTION

The high costs of environmental disruptive forces on materials, especially metals and metal-coated materials, can be reduced by the use of suitable protective coatings applied by various technologies.

The increasing need to replace fossil fuels used to heat buildings, which accounts for 50% of total energy consumption, has led to the development of solar façade collection systems. Although many commercially available kermet coatings are dark blue in color and represent a good trade-off between color and spectral selectivity, their corrosion resistance must be improved before being used in uncovered facade systems. Corrosion stability studies are essentially due to the long-term stability of solar absorbers in relation to the efficiency of the conversion of solar radiation into thermal energy (S. Brunold; U. Frei; B. Carlsson; K. Moller; M. Kohl, Solar Energy Materials and Solar Celiš 2000, 61, (3), 239253). The problem is not easy, since it requires, in addition to a detailed knowledge and application of corrosion prevention, knowledge of the manufacture of barrier coatings that can prevent the absorber from collapsing without altering their optical properties.

One option to avoid insufficient corrosion stability may be the production of absorber surfaces composed of multiple layers, as envisaged by EP 1867934. A multilayer structure can only be made with multiple sequential PVD or CVD processes, which significantly increases the cost of such absorbers . Also, satisfactory corrosion protection can be achieved by applying a thin coating (up to 1000 nm) by chemical vapor loading (CVD), achieved by applying a thin coating (up to 1000 nm) by chemical loading vapor (CVD), physical vapor loading (PVD), Dust and Plasma Reinforced Chemical Pair Loading (PECVD), but their preparation also requires expensive vacuum techniques as described in EC 2 061 399.

By contrast, sol-gel technology can be used with relatively simple equipment, a simple process and room conditions.

Simple silanes (RS- (OR ') 3, R = aminoalkyl, acryloxypropyl, glycidoxypropyl, isocyanatopropyl, alkyl, aryl ...; OR ¹ = ethoxy, methoxy, acetoxy ... are mostly used for the preparation of thin films for corrosion protection of metals. ) and bilaterally functionalized (OR'-polymer-OR ') silanes. These coatings act most often as barrier coatings, as they isolate the corrosion medium from the metal, and the corrosion reactions (cathode and anode) are diffusion controlled. Unfortunately, the effectiveness of such a corrosion protection coating depends on its thickness and thick coatings as described in US 3935349, US 4754012, US 5814137, US 6261638, US 7141306, WO 2007/085339 must be applied for excellent protection; M. Fir; B. Orel; A. Surca Wolf; A. Vilčnik; R. Jese; V. Francetic, Langmuir 2007, 23, (10), 5505-5514.

Mercapto silanes have also been used for the preparation of protective films, mostly on aluminum alloys as described in WO 02/072283 and US 5750197, but have not been recognized as effective corrosion inhibitors of spectrally selective surfaces.

In the past, extremely thin sol-gel films were also produced that were invisible to human eyes. But their protection declined sharply with the aging of the salt, this process being burdened with a very short open time from preparation to application as described in US 5175027. They used rather thick sunscreens used to concentrate sunlight in high-temperature reservoirs. films as described in WO 2007/059883, but there is no requirement for low thermal emission. However, such an approach is not sufficient for use on solar panels, since the thickness of the coating and the insufficient corrosion resistance greatly affect thermal emissivity and solar absorption.

It is an object of the present invention to provide processes for the preparation of ultra-thin coatings, i.e. from 1 to 500 nm, coatings that will not impair the optical properties of sun-selective surfaces,

-3 that the solar absorptance as does not decrease and the thermal emission ε _τ remains below 0.20; on the other hand, these coatings must provide sufficient corrosion stability.

SUMMARY OF THE INVENTION

The present invention relates to a method for the preparation of mercaptosilane-based sol-gel corrosion-protection nano-thin films with a thickness between 0.5 and 500 nm deposited on various metal substrates including metal sheets coated with highly selective kermet absorber layers used in solar thermal collectors. Most important is the use of coatings on the surfaces of solar absorbers, since these coatings provide very good corrosion protection and, on the other hand, do not spoil the low thermal emissivity of these surfaces, which increases by only a few%; solar absorption even increases, which is very advantageous.

DETAILED DESCRIPTION OF THE INVENTION

Short description of the pictures:

Figure 1 presents photographs of samples exposed in a salt chamber.

Figure 2 presents the potentiodynamic curves for protected and unprotected kermet.

Figure 3 represents the hemispherical reflectance spectra of uncoated and MPTMS coated kermet.

Figure 4 presents the results of the salt chamber, i.e. photographs of exposed aluminum samples.

Process for the preparation of nano-thin anti-corrosion films, 0.5 to 500 nm thick, on solar spectral selective surfaces - solar absorbers made of aluminum, copper, steel and alloy sheets coated with kermet absorption coatings such as commercially available Sunselect, TiNOX, Eta plus made from chromium and titanium oxynitride. The good corrosion properties of these coatings on commercial Sunselect are presented in Figure 1, where we can see 3 differently protected specimens that were exposed in the salt chamber after protection. The left sample is unprotected cermet after 24 hours of exposure, the middle is identical to cermet protected by inappropriate coating after 120 hours of saline chamber, and the right sample is protected by mercaptopropyltrimethoxysilane (MPTMS) salt after 480 hours of saline

-4 chamber. Protection with MPTMS salt significantly improves corrosion resistance in the salt chamber. Figure 2 shows the potentiodynamic curves of MPTMS-protected Sunselect. The drop in corrosion current and the shift of corrosion potential towards more negative values are clearly seen, indicating that MPTMS acts as a mixed-type inhibitor with more pronounced cathodic inhibition. Figure 3 shows the hemispherical reflectance spectra of protected and unprotected Sunselect with calculated values of solar absorption as and thermal emissivity ετ · From the data it can be seen that the coating has no significant effect on selectivity. The increase in thermal emissivity of the protected samples was below 0.35, typically below 0.10. An increase of less than 0.05 can be achieved by paying special attention to film making. The most important parts of the invention are simultaneous corrosion stability and maintaining selectivity.

The corrosion protection process is also suitable for metal substrates such as copper, aluminum, iron and their alloys. This is shown by the results in Figure 4, where photographs of Al 2204-T3 specimens exposed for certain time intervals in the salt chamber are presented. The top row of photos represents poor protection without MPTMS, and the bottom row contains specimens protected with a coating containing MPTMS and exhibiting excellent corrosion stability in the salt chamber - after 13 days only corrosion onset - compared to samples without MPTMS that completely corrode after one day.

Substrate is protected by immersion in a mixture consisting of:

i) 30% to 98% w / w, preferably more than 60% w / w of an organic solvent, preferably an aliphatic alcohol, preferably ethanol;

ii) 0.01% to 70% w / w, preferably 0.02% to 60% w / w of at least one mercaptosilane, which can be described by the following formula:

(R ¹ ) m

HS- (CH ₂ ) _n -Si- (OR ² ) _r wherein R and R are independently selected from alkyl (1-14C), cycloalkyl and aryl groups and n is an integer from 0 to 20, m is an integer from 0 to 2 and r are an integer from 1 to 3 such that: m + r = 3, iii) an acid catalyst (that is, any compound whose aqueous solution has a pH between 0-6.9) selected from the acid group containing at least: HF, HCI, HBr, HI, HNO3, H2SO4, H3PO4, ant, • ·

-5 acetic, propanoic, butanoic, salicylic, trifluoroacetic, trichloroacetic, triflic, methanesulfonic acid; thus giving a final concentration of ΙμΜ to lmM;

iv) 0.001% to 50% w / w, preferably between 0.01% and 45% w / w of water;

(v) 0% to 50% w / w, preferably between 0% and 40% w / w, of another single end-capped hydrolysable silane represented by the following general formula:

(R ¹ ) m (R ³ ) n-Si- (OR ² ) r

2 where R and R are independently selected from alkyl (1-14C atoms), cycloalkyl (1-10C atoms) and aryl groups and n is an integer from 0 to 3, m is an integer from 0 to 2 and r is an integer from 1 to 4, so that: m + n + r = 4 and R is independently selected from the group consisting of at least: alkyl, cycloalkyl, substituted alkyl, substituted cycloalkyl, aryl, substituted aryl, fluorinated alkyl, fluorinated aryl and fluorinated cycloalkyl groups:

or / and bis end-capped silane represented by the following formula:

(R ¹ ) m (OR ² ) _r -Si-R (R ¹ ) m

-Si- (OR ² ) _r

2 wherein R and R are independently selected from alkyl (1-14C atoms), cycloalkyl (1-10C atoms) and aryl groups and m is an integer from 0 to 2 and r is an integer from 1 to 3, such that: m + r = 3 and R ⁴ is independently selected from the group consisting of at least different terminated alkyl, substituted alkyl, fluoroalkylated, polydialkylsiloxane and polyether chains.

The mixture is prepared in such a way that only monomeric and oligomeric species are formed in the sol-gel process; it is best if the salt consists only of fully hydrolyzed monomers and dimers with a very small amount of fully hydrolyzed trimers. The hydrolysis / condensation ratio can be adjusted with an appropriate amount of water. In particular, more than one water equivalent is used for each hydrolysable alkoxy group.

• ·

-6The sample is soaked in this solution long enough to form a quasi-self-assembling monoplast salt molecule on the surface of the assisted substrate. Commercially available solar absorbers are porous, so this step is especially important as salt molecules can penetrate deep into the porous structure and thus provide better corrosion protection. The self-organization / breakthrough of the genre is completed within 1 s to 5 hours, preferably with a soaking time of more than 5 s and less than 4 hours.

The sample can be removed from the solution in various ways, but it should be noted that the rate of extraction from the salt has a significant effect on the thickness and corrosion properties of the film, but the thick films impair the spectral selectivity of the sample. The extraction rate should be between 0.1 and 100 cm / s or the sample can be easily removed from the solution by hand in batch operations.

After removal from the sample, the sample is removed by air-drying solvent for a period of between 1 s and 2 days, preferably between 1 s and 36 hours.

After drying, the sample is heat treated to complete condensation and complete crosslinking. In this step, the air-dry gel collapses into a thick, even, watertight film. Samples can be heat treated at different temperatures, which means reducing the duration of heat treatment with increasing temperature. Protected samples are heat treated at 70 - 300 ° C for 1 s to 5 hours, preferably at 80 - 250 ° C for 1 s to 4 hours.

Samples can be protected in at least two different ways with respect to coating. If the objects are in the form of panels, they can be protected by the batch diving technique described above; of course, it is desirable that specimens having the form of a long strip rolled up in a roll may also be protected. A continuous process for these types of samples can be done by the following non-limiting procedure. From the roll of the unprotected substrate, a strip of sample is passed through the cylinders to a container containing a salt - mixture for preparation of the coating, where the strip is immersed in the mixture and therein is retained for a certain time, which is very similar to the residence time in said batch process. The strip is then released from the solution, dried by blowing air and then brought to a furnace where it is heated to complete the condensation. It is then cooled to room temperature and rolled into a roll of protected substrate. In order to determine the speed of movement of the strip, the temperatures of the individual sections, in particular the furnace, and the overall construction of the machinery, the conditions laid down for the batch processes must be observed; preferably, these process parameters should be as similar as possible.

-ΊTo illustrate the preparation of coatings, we give the following examples:

Example I:

A thin MPTMS coating of g 3-mercaptopropyltrimethoxysilane was dissolved in 25 g ethanol. The solution was acidified with 0.1 ml of 0.1 M HCl, then stirred for 4 hours. During this time, only hydrolyzed monomers and dimers are formed. During hydrolysis, the salt is further diluted with 75 ml of ethanol. This salt was applied to commercial Sunselect as follows: the sample was immersed in salt, 240 s the salt was then penetrated into the porous structure of Sunselect then the sample was extracted at a rate of 10 cm / s. The samples were then air-dried for 60 min and then heat-treated at 140 ° C for one hour. Sunselect thus protected exhibits excellent anti-corrosion properties (stable over 20 days in the salt chamber) and shows no significant deterioration in optical properties. The increase in solar absorptivity was in the interval 0.00-0.05 and the increase in thermal emissivity 0.00 - 0.10, mostly below 0.05. Based on these properties, it can be argued that this coating is superior to others. Last but not least, Sunselect is also fingerprint proof.

Example II:

A thin mixed MPTMS coating of g 3-mercaptopropyltrimethoxysilane was mixed with 1 g of 1H, 1H, 2H, 277-perfluorooctyltriethoxysilane and dissolved in 25 g of ethanol. The solution was acidified with 0.1 ml of 0.1 M HCl, then stirred for 4 hours. During this time, only hydrolyzed monomers and dimers are formed. During hydrolysis, the salt is further diluted with 75 ml of ethanol. This salt was applied to commercial Sunselect as follows: the sample was immersed in the salt, 240 s then the salt penetrated into the porous structure of Sunselect, then the sample was extracted at a rate of 10 cm / s. The samples were then air-dried for 60 min and then heat-treated at 140 ° C for one hour. Sunselect thus protected exhibits excellent anti-corrosion properties (stable over 20 days in the salt chamber) and shows no significant deterioration in optical properties. The increase in solar absorptivity was in the range of 0.00 - 0.05 and the increase in thermal emissivity was 0.00 - 0.10, mostly below 0.05. Based on these properties, it can be argued that this coating is superior to others. Last but not least, Sunselect is also fingerprint proof. The contact angles for water for this coating are significantly higher (100 °) than for the example I coating (75 °).

-8Example III:

g of 3-mercaptopropyltrimethoxysilane is mixed with 1 g of trimethoxysilylpropylureido terminated polydimethylsiloxane 1000 (PDMSU 1000) and dissolved in 25 g of ethanol. The solution was acidified with 0.1 ml of 0.1 M HCl then stirred for 4 hours. During this time, only hydrolyzed monomers and dimers are formed. During hydrolysis, the salt is further diluted with 75 ml of ethanol. This salt was applied to AA 2024 -T3 alloy samples as follows: the sample was immersed in salt, 240 s was soaked in salt then the sample was extracted at a rate of 10 cm / s. The samples were then air-dried for 60 min and then heat-treated at 140 ° C for one hour. The aluminum alloy thus protected exhibits excellent anti-corrosion properties (stable over 13 days in the salt chamber). The contact angles for water for this coating are significantly higher (100 °) than for the example I coating (75 °).

Claims (13)

A method for the preparation of sol-gel corrosion protection coatings for solar absorbers based on nano-thin corrosion films, which involves soaking the substrate in a mixture of at least one mercaptosilane, solvent, acid catalyst, water, and, but not necessarily, another hydrolysable silane, for a particular the time during which a quasi-self-assembling monoplast is formed from salt molecules on the substrate surface, removal of the substrate from the solution, drying of the coated substrate, and final heat treatment of the coated substrate. 2. The method of claim 1, wherein the mercaptosilane is a compound having the following chemical structure: (R ¹ ) m HS- (CH ₂ ) _n -Si- (OR ² ) _r wherein R and R are independently selected from alkyl (1-14C), cycloalkyl and aryl groups and n is an integer from 0 to 20, m is an integer from 0 to 2 and r are an integer from 1 to 3 such that m + r = 3 and the final concentration of mercaptosilane in the coating solution is 0.01% to 50% w / w. The method according to claims 1-2, wherein the amount of the second hydrolysable silane is from 0% to 50% w / w in the final mixture and wherein the second hydrolysable silane is represented by the following general formulas: <R ¹ ) m (r \ -Si- (OR ² ) _r wherein R and R are independently selected from alkyl (1-14 C atoms), cycloalkyl (1-10 C atoms) and aryl groups and n is an integer of 0 to 3, m is an integer from 0 to 2 and r is an integer from 1 to 4 such that m + n + r = 4 and R ³ is independently selected from the group consisting of at least: alkyl, cycloalkyl, substituted alkyl , substituted cycloalkyl, aryl, substituted aryl, fluorinated alkyl, fluorinated aryl and fluorinated cycloalkyl groups, and: -10 < ^R (R ¹ ) _m (OR ² ) r-Si-R - Si- (OR ² ) r 1 2 wherein R and R are independently selected from alkyl (1-14C atoms), cycloalkyl (1-10C atoms) and aryl groups and m is an integer from 0 to 2 and r is an integer from 1 to 3, such that : m + r = 3 and R ⁴ is independently selected from the group consisting of at least different terminated alkyl, substituted alkyl, fluoroalkylated, polydialkylsiloxane and polyether chains. The method according to claims 1-3, wherein the solvent is, but is not limited to, aliphatic alcohol. The method according to claims 1-4, wherein the acid catalyst is selected from the group consisting of at least: HF, HC1, HBr, HI, HNO3, H2SO4, H3PO4, formic, acetic, propanoic, butanoic, salicylic, trifluoroacetic, trichloroacetic , triflic, methanesulfonic acid and the amount of catalyst added is from ΙμΜ to lmM throughout the mixture. The method according to claims 1-5, wherein the amount of water used is between 0.001% to 50% w / w of the total mixture. The method according to claims 1-6, wherein the immersion substrate is a septerally selective solar absorber made from cermet, preferably chromium oxynitride or titanium oxynitride, applied to copper, aluminum, iron and their alloys. The method according to claims 1-7, wherein the submerged substrate is a bare metal surface of, but not limited to, copper, aluminum, iron and their alloys. The method according to claims 1-8, wherein the removal of the sample is carried out at a specified speed between 0.1 and 100 cm / s or the sample is removed manually. The method according to claims 1-9, wherein the drying of the aforementioned coated samples at room temperature takes 1 s to 2 days. The method according to claims 1-10, wherein the heat treatment of the aforementioned samples is carried out at a temperature of from 70 to 300 ° C for 1 s to 5 hours. The method of claims 1-11, wherein the increase in thermal emissivity is up to a maximum of 0.35. The method according to claims 1-12, wherein the substrate coverage is made by a continuous or batch process.