WO2007049169A2

WO2007049169A2 - Carbon dioxide permeability of coatings determining unit

Info

Publication number: WO2007049169A2
Application number: PCT/IB2006/053674
Authority: WO
Inventors: Adélio Miguel MAGALHÃES MENDES; Paulo Jorge Ribeiro Da Cruz; João Carlos GODINHO DE FARIA DOS SANTOS; Catarina De Carvalho Carneiro; José Luís PINHEIRO DE SOUSA NOGUEIRA
Original assignee: Universidade Do Porto; Cin - Corporação Industrial Do Norte, S.A.
Priority date: 2005-10-27
Filing date: 2006-10-06
Publication date: 2007-05-03
Also published as: WO2007049169A3; PT103373B; PT103373A

Abstract

This invention refers to a unit for determining the carbon dioxide permeability of coatings, comprising a thermostatic cabinet (10) containing inside a permeation cell (11), a CO2 specific analyser (12), a mixing container (13), three pressure transducers (14) and two mass flow controllers (15) to regulate the feeding stream in the permeation cell. This unit is connected to a data processor (16) and applies the Wicke-Kallenback method. The project of the carbon dioxide permeability of coatings determining unit was performed in order to promote its functionality and effectiveness. With this unit it is possible to get results in a short period of time, which is a major benefit in comparison with other existing methods. The coatings are applied on Kraft paper, as a result of its effectiveness in these tests and its low cost.

Description

CARBON DIOXIDE PERMEABILITY OF COATINGS DETERMINING UNIT

Technical issue

[1] The present invention refers to a unit designed for determining the CO permeability of coatings. This determination is of extreme importance concerning facade coatings, because it allows the coatings reference as anticarbonation.

[2] The unit for the determination of carbon dioxide permeability of coatings uses the

Wicke-Kallenback method and is specifically applied to the coatings branch (such as facade coatings).

Invention antecedents

[3] Nowadays, it is acknowledged that concrete surfaces need protection against the environment's corrosive agents. As a result, architectural facade coatings, keeping its decorative feature, acquire a new functionality - anticorrosive protection.

[4] Carbon dioxide (CO ) is one of the corrosive agents that has a major impact on concrete degradation and has been increasing its concentration in the atmosphere due to the rise on the consumption of fossil fuels.

[5] Carbonation process is a result of CO penetration through concrete pores. CO will react with calcium hydroxide present in the concrete, originating calcium carbonate and water. Carbonation promotes the concrete pH decrease, which results in the exposure of the reinforced steel (armature), opening the way to corrosion.

[6] In the last few years, different studies have been developed in order to better understand the mechanism of concrete degradation due to CO (Jerga, J., 2004). Therefore, it is essential to coat concrete surfaces in order to protect them against corrosive agents, such as CO .

[7] The present invention refers to a unit designed for determining the CO permeability of coatings. This determination is of extreme importance concerning facade coatings, because it allows the coatings reference as anticarbonation.

[8] Throughout the years, the concern about permeability to some gases has suffered an increase, especially in repect of packages used in food industry. In July 1971, an instrument to determine membrane permeability was patented ( US 3590634). In April 1987 the patent about the determination of permeability to gases or vapour of films or membranes ( US 4656865) was registered. In July 2002 was published another patent (US 6422063) concerning to a unit that determines oxygen permeability of micro- perforated films, used to pack fruits and vegetables. In July 2004 was patented an equipment to determine the permeability to isotopic gases, i. e., in plastic films, US Invention description

[9] This invention refers to a unit for determining the carbon dioxide permeability of coatings, comprising a thermostatic cabinet (10) containing inside a permeation cell (11), a CO specific analyser (12), a mixing container (13), three pressure transducers (14) and two mass flow controllers (15) to regulate the feeding stream in the permeation cell. This unit is connected to a data processor (16) and applies the Wicke- Kallenback method.

[10] The permeation cell (11) was carefully design in order to assure that its configuration can guarantee that the concentration, inside each chamber, is homogeneous. To the upper chamber (32) it is fed a 15% carbon dioxide in pure nitrogen mixture stream and to the lower chamber (33) it is fed a pure nitrogen carrier stream, such as there is no total pressure difference between the two chambers. Outlet stream (24) from lower chamber is composed by nitrogen and the carbon dioxide that permeates through the coat film. This stream is fed to the CO analyser (12) in order to determine the permeate concentration. All unit is controlled by a software, developed for this unit, installed in a data processor (16). This software controls all pressures and flowrates and permits to read CO concentration present in a gas stream as well as its temperature. All these data could be record in a file to posterior CO permeability calculation.

[11] For this kind of determination, temperature is a very important factor, thus the unit has to be inside the thermostatic cabinet (10). This cabinet (10) allows us to maintain the temperature constant during the determination.

[12] The project of the carbon dioxide permeability of coatings determining unit was performed in order to promote its functionality and effectiveness. With this unit it is possible to obtain results in a short period of time (between 5 and 8 hours), which could be very advantageous when compared with other methods (more laborious).

[13] The coatings are tested on Kraft paper, as a result of its effectiveness in these tests and its low cost.

Brief figures description

[14] The present invention implements Wicke-Kallenback method (described in EN

1062-6) and it could be more easily understood with the attached figures: • Figure 1 represents a sketch of carbon dioxide permeation unit where (10) is the thermostatic cabinet, (11) is the permeation cell, (12) is the CO analyser, (13) is the mixture container, (14) is the pressure transducers, (15) is the mass flow controllers, (16) is the data processor, (17) is the globe valves, (18) is the needle valves, (19) is the three way valve, (20) is the five way valve, (21) is the upper chamber inlet stream, (22) is the upper chamber outlet stream, (23) is the lower chamber inlet stream, (24) is the lower chamber outlet stream, (25) is the mixture container evacuation, (26) is the vacuum pump and (27) is the unit ventilation;

• Figure 2 presents permeation cell 3D sketch (11), which is composed by four parts (28 to 31), where 28 is equal to 31 and 29 is equal to 30. Upper chamber (32) is composed by 28 and 29 and the lower chamber (33) by the 30 and 31;

• Figure 2a presents one part of the permeation cell (28) where one can visualize the upper chamber (32) feed lateral hole (34);

• Figure 2b presents one part of the permeation cell (29) where one can visualize the distribution channel (35) of the upper chamber inlet stream (21) and the collected hole (36) of the upper chamber outlet stream (22);

• Figure 2c presents one part of the permeation cell (30) where one can visualize the distribution channel (35) of the lower chamber inlet stream (23) and the collected hole of the lower chamber outlet stream (24);

• Figure 2d presents one part of the permeation cell (31) where one can visualize the lower chamber (33) feed lateral hole (34);

• Figure 3 represents a sketch of the upper chamber (32) of the permeation cell

(H);

• Figure 4 corresponds to a scheme of the software FrontPage which controls, acquire and registers all CO pressure, flow and concentration variations in during the test.

Detailed invention description

[15] The unit designed for determining the carbon dioxide permeability of coatings, comprising a thermostatic cabinet (10) containing a permeation cell (11), a CO analyser (12), a mixing container (13), three pressure transducers (14) and two mass flow controllers (15). In thermostatic cabinet left lateral wall are supported all valves that regulate the entire inlet streams of this unit (17 and 18) and, in the other wall is sustained a five way valve (20) which direct any stream to the CO analyser (12) to determinate this gas concentration. The three way valve, inside the thermostatic cabinet, is necessary to direct the upper chamber inlet stream, N stream for cleaning this chamber or 15% CO in a N mix stream to determine some coating CO permeability. This unit is connected to a data processor (16) which controls, acquires and records all pressures, flows and CO concentration variations - see Figure 1.

[16] The permeation cell (11) design was performed in order to maximize its functionality and effectiveness. This cell is composed by four parts (28 to 31), where 28 is equal to 31 and 29 is equal to 30 - see Figures 2, 2a, 2b, 2c and 2d. Upper chamber (32) is composed by 28 and 29 and the lower chamber (33) by the 30 and 31. These chambers are separated by the test coating, in this coating is imposed partial pressure difference but the total pressure difference is null. In the upper chamber (32) flows a 15% CO in nitrogen (N ) mixture and the lower chamber (33) is fed with a N carrier

2 2 2 stream, which works as a carrier gas. Permeation cell inlet streams (21 and 23) are done by a lateral hole (34) which communicates with a distribution channel (35), near the external perimeter of the chambers. This channel has 45 little holes with 1 mm diameter. The permeation cell outlet streams (22 and 24) are collected by a central chambers hole (36). The surface between the inlet holes (34) and the outlet holes (36) is conic - Figure 3 - in order to promote greater radial velocity homogeneity.

[17] This cell allows performing experiences with coatings applied in different supports like Kraft paper, fibber glass paper and other materials with very low thickness and high permeability or in porous ceramic tiles, sinterized glass or metal plates and other materials with high thickness and high permeability. As a result, the test panels have 9 cm of diameter but thickness could vary between 0.4 mm to 6 mm.

[18] This cell is versatile concerning the paint film support. The upper and lower chambers are structurally identical. The internal volume of the upper chamber (32) is variable (depending on the used support) and the lower chamber (33) volume is constant. The lower chamber (33) has a reduced volume (approximately 5 cm³) with the aim of to increase response sensibility to CO permeation, reduce the system response time, minimize dead volumes inside this chamber and guarantee the homogeneity of the concentration in each chamber point. The stream concentration, fed to the upper chamber, does not suffer any variation between inlet and outlet because its flow is raised enough (approximately 10 times superior then the flow fed to the lower chamber). The reduced flow fed to the lower chamber, allied to its configuration, assurance that the lower chamber outlet stream concentration contain exactly the CO concentration which has permeate through the test coating panel.

[19] The big advantages of using Kraft paper as support are its large porosity, its easily handling, application and accessibility, facilitating permeabilities determinations. Other advantage, and a very important one, is its low cost (it could be considered negligible when compared with other supports like sinterized glass).

[20] Permeation cell is made in stainless steel, has an external diameter around 13 cm and 4.5 cm high, comprising o-rings that are placed to assure the permeation cell gas staunchness.

[21] These characteristics allow to distinguish this cell from all the existing ones mainly in the way how the gas distribution is done inside the upper and lower chambers as well as its conic configuration and its reduced volume.

[22] Analysing the related patents, one may easily prove these differences. In the US

4656865 patent the retentate and permeate chambers of the permeation cell are cylindrical and the flow rate is equal in both chambers. In the US 2004/0123646 patent, the permeation cell is composed by two chambers, one hyperbaric and the other hypobaric.

[23] The outlet stream of the lowerchamber (24) is composed by N_^ and CO which has peπneated through the tested coating. To determine this stream CO concentration is used a specific CO analyser, CA-2A Carbon Dioxide Analyser®, Sable Systems® (12). This unit also holds the possibility to analyse other streams like upper chamber inlet (21) and outlet (22). Thus it is possible to guarantee that it is no concentration difference between these streams. This equipment (12) could be used in many detection ranges between 0-500 ppm and 0-100000 ppm, it has a resolution of about 0.0001% (1 ppm) and has an accuracy of about 1% of the selected detection range. Besides de CO concentration determination in the fed stream, this device is also able to measure the temperature and pressure of the fed stream.

[24] Analysing US 6422063 and US 4656865 it is possible to verify that the detection method used to analyse the permeated is gases chromatography. In US 2004/0123646 is used a mass spectrophotometer.

[25] This unit has a tubing system which compose the ventilation (27). Ventilation evacuates part of 22 and 24 streams and the stream which is fed to the CO specific analyser (12) to the unit's exterior.

[26] The CO / N mixture used in this unit is made in mixture container (13), with 5 1 of capacity. This container is in the inferior part of this unit and in the horizontal line. The container inlet and outlet for gases is made through a tube in the center of one of its sidewalls to assure the better mixture homogeneity and to minimize the gases stratification. Mixture is prepared according to partial pressures of the evolved gases, following the next equations (assuming perfect eases behaviour):

PT = PcO₁ + PN₂ (I) PCO₂ = X Co₁ ' ^Pr (II) PN₁ = X _Nl ^• P₇ (III)

where P is total pressure inside the mixture container, p _^ is CO_^ partial pressure, X

CO2 is CO 2 molar fraction in mixture, p ΛG is N2 partial pressure and X N2 is N 2 molar fraction in mixture.

[27] The gas to be first fed to the container is the one which exists in lower concentration in the mixture, this is CO . By this way it is possible to assure that with the N₁ inlet the mixture will be perfectly homogeneous.

[28] Temperature control is very important for this system because permeability vary significantly with temperature. To make it possible, one thermostatic cabinet (10) was made which consists in a domestic refrigerator with a fan heater inside, connected to a control system that allows the temperature to be kept constant. Concerning the fan heater connection to the controller, the fan element is separated from the heating element. The fan runs continuously while the heating element only works when the controller switch it on. This kind of operation prevents the resistance's damage by superheating. When the temperature is inferior to the set-point, the resistance element from the fan heater is switched on bringing heat to the system; the resistance element is switch off when the temperature reaches the desired value.

[29] It was acquired Eurotherm® 2216L to control the temperature as well as a solid state relay Eurotherm® SSR 50^A which is responsible to send the control signal to the fan heater. The temperature is read using a type K thermocouple placed inside the thermostatic chamber. The use of a controller in the system allows read temperature variations within 0.1 ⁰C from the set-point inside the controlled region. The controller was inserted in a specially designed box where it was also installed the fan heater power supply.

[30] This thermostatic chamber is characterized by its simplicity and precision at low cost (between 70 and 80% cheaper than a conventional thermostatic chamber with the same effective volume).

[31] This unit has three pressure transducers (14) with high precision. These devices are fundamental to monitor pressure inside the upper chamber, the permeation chamber and in the mixture container. The upper and lower chambers must be at atmospheric pressure. The pressure sensor connected to the mixture container is used to produce the mixture (used in permeability measurements) because this mixture is prepared based on gases partial pressures.

[32] The transducers connected with the cell chambers works in a range of 0 to 2 absolute bar and the one which is connected to the container works in a range of 0 a 7 absolute bar. These devices are from Druck® and have an accuracy of about 0.1% F.S..

[33] This unit has two mass flow controllers (15). These devices control upper (32) and lower (33) chambers inlet streams of the permeation cell (11). These devices work range are 0 to 100 ml mi rf¹ for the mass flow controller connected to the upper chamber and O a IO ml mi rf¹ for the mass flow controller connected to the lower chamber. The upper chamber flow rate should be the highest possible. Thus, simultaneously with permeation cell configuration, it is possible to assure that there is no concentration difference between upper chamber inlet (21) and outlet (22). The lower chamber flow rate must be low enough to guarantee the higher CO concentration in this chamber outlet (24) - this contributes to the unit accuracy increase. These devices are from Bronkhorst Hi-Tec® and have an accuracy of about 1% F. S..

[34] As related, this unit is connected to a data processor (16) and is totally controlled by a software. This software allows to see, each instant, the flow rates fed to the permeation cell, CO concentration present in the stream which is fed to the detector and all unit operation pressures. All these values have to be recorded in order to turn possible the CO permeability determination. Thus, data acquisition has a very important role and is done through this software - see Figure 4. The acquired data are: Time, in seconds; CO concentration (in lower chamber outlet stream or in upper chamber inlet or outlet streams), in ppm; Detector inlet stream pressure, in kPa; Detector inlet stream temperature, in ⁰C; Upper and lower chambers pressures and mixture container pressure, in bar; Upper and lower chambers flow rates, in ml mi rf¹

[35] CO analyser data is acquired via RS 232. Mass flow controllers and pressure transducers are connected to an acquisition board.

[36] Later, this data is treated in order to obtain CO permeability of the tested coating.

This software could be developed using some kind of programming language or using programming macro-languages.

[37] Mixture container evacuation (25) is very important to assure that tests are correctly performed, since it is in the mixture container that the CO / N mixture is prepared. This evacuation is done using a rotating vacuum pump (26) from Edwards® model V4. It is considered absolutely evacuated when pressure inside this container is lower than 5 mbar.

[38] In this unit all connections are done using Swagelok® material, which is very accurate for gases.

[39] This unit global project is extremely user friendly because its operation can be done by non-specialized workers. Using this unit it is possible to obtain results in a few hours (between 5 and 8 hours). This is a major benefit of this unit when compared with other methods (that are slower).

[40] This unit has a precision equilibrium between each transducer/analyser which contributes for CO coatings permeability determination precision. Thus the global precision is maximized and allows CO permeability of industrial coatings, even the most impermeable ones. The major error obtained for this permeability determination is 10% when nominal permeability is 10

(EN 1062-6 tolerates errors up to 30%).

Exanrole [41] The product to be tested has to be applied in Kraft paper, accordingly to the producer specifications. It should be dried at a temperature between 21 and 25°C but better at 23°C , and at a relative humidity between 45 and 55% but better at 50%, for a minimum period of 7 days (the time and drying conditions are similar for any CO permeability determination method). The paper samples with the coating applied should be cut at the permeation cell's dimension with the help of a special cutting tool. The thickness of the dried coating film should be determined using a measuring tool, such as a micrometer. The dry thickness value is necessary for obtaining the CO permeability.

[42] After, the sample should be inserted into the permeation cell (11) and this placed inside the unit for determining the permeability towards CO .

[43] The unit is controlled through software, which should be open before anything else.

[44] Previously to start the procedures of the permeability determination it should be made to flow nitrogen through both chambers of the permeation cell in order to remove tracers of CO . The cleanness of the permeation cell can be checked by reading the CO concentration coming out especially from the lower chamber of the permeation cell (24).

[45] The next step comprises the evacuation of the mixture tank and then the performance of the gas mixture of CO2 (15%) in N (85%) according to the procedure described before.

[46] The mass flowrates of the gas streams fed to the upper and lower chambers should be set using the controlling software, which is also used to assign a name to the file where the operating conditions and experimental results data are stored and frequency of the acquisition is defined- figure 4.

[47] At the end of the test, all valves should be shut down and the unit for determining the carbon dioxide permeability of coatings switched off.

[48] The experimental data are recorded in a text file. To obtain a graphical representation of them and perform the algebraic transformations in order to obtain the permeability it is necessary to load the file using a speed sheet application such as the Microsoft Excel®.

[49] The permeability is normally given in

or mo l m in^'* s^"1 Pa^"1

. Meanwhile, in the standards concerning the characterization of coatings, e.g. EN 1504-2, the permeability is given as the equivalent thickness, S (m), of a stagnant air film with the same permeation properties as the coating. This thickness relates with the permeation resistance factor, μ, through:

Deo, /Z = -^ ( IV )

where D is the diffusivity of CO in air,

C02 m^< -s-¹

Di is the effective diffusivity of CO^ in the coating, m* -s^"1 , and

S_D = μ - l (V ) where, is the thickness of the coating film, m. , [50] It is the necessary to obtain D and

CO2

Dl accordingly with:

^D _co2 W ^■ s^~] ) = 1,395 x 1 (T^{5 •} (VI ]

where T is the absolute temperature of the test, K, and

L

Dl = R - T (vii :

Vco,

where L is the permeability towards CO ,

, V is the molar volume of CO^, m³, and R is the perfect gases constant ( R = 8.314

J - mo I^"1 . K^"1

). [51] The permeability towards CO can be obtained from: TV^"

L = — / (VII i :

Ap

where N is the flux of CO , j -j -i

In PTM • IU ' S

is the partial pressure difference between the upper and lower chambers, Pa, and/ is the thickness of the dried film, m. [52] The CO flux can be obtained from:

* = --% ( IX)

/I x lO⁶

where y is the molar fraction of CO 2 at the lower chamber, ppm, Q lτtf is the flowrate leaving the lower chamber,

, and A is the permeation effective area, m^". [53] The partial pressure difference between the two chambers can be obtained from:

where P is the total pressure, Pa.

Bibliography

[54] i Jerga, J., 'Physico-mechanical properties of carbonated concrete', Construction and Building Materials, Vol. 18, 645-652, 2004. ii US 3590634, 'Instrument for determining permeation rates through a membrane', July, 1971. iii US 4656865, 'System for analyzing permeation of a gas or vapour through a film or membrane', April, 1987. iv US 6422063, 'Rapid method to experimentally measure the gas permeability of micro-perforated films', July, 2002. v US 2004/0123646, 'Gas permeability measurement method and gas permeability measurement device', July, 2004. EN 1062-6 - 'Paints and varnishes - Coating materials and coating systems for exterior masonry and concrete - Part 6: Determination of carbon dioxide permeability', 2004. EN 1504-2 - 'Products and systems for the protection and repair of concrete structures - Definitions, requirements, quality control and evaluation of conformity - Part 2: Surface protection systems for concrete', April, 2004.

Claims

[1] Unit for determining the permeability of coatings towards carbon dioxide, comprising: a) a thermostatic chamber (10); b) a permeation cell (11); c) a CO specific analyzer (12); d) a mixture tank (13); e) a data processor (16) where a software to control the unit is installed; f) three pressure transducers (14), which read the pressure at the upper and lower chambers of the permeation cell (11) and at the mixture tank (13); g) two mass flow controllers (15), which control the feed gas flowrate in the permeation cell (11).

[2] Unit for determining the permeability of coatings towards carbon dioxide, according to claim 1, characterized by a thermostatic chamber (10) made of a domestic freezer and a fan heater, this one placed inside the freezer and connected to a temperature controller through a solid state relay, where the temperature probe is placed inside the chamber, close to the permeation cell (11); the chamber is characterized for its simplicity, precision of the temperature control, uniformity of temperature and low cost (between 70 and 80% cheaper than a conventional thermostatic cabinet of an equivalent volume).

[3] Unit for determining the permeability of coatings towards carbon dioxide, according to claim 1, characterized by the permeation cell (11) being of cylindrical shape and made of two chambers, the upper (32) and the lower (33).

[4] Unit for determining the permeability of coatings towards carbon dioxide, according to the previous claim, characterized by the upper (32) and lower (33) chambers allow an uniform radial flow along the coating and with the support of the coating contacting surfaces; this configuration, together with the use of a high enough feed flowrate (21) to the upper chamber (32), let the stream composition being approximately constant over the complete contacting surface with the coating.

[5] Unit for determining the permeability of coatings towards carbon dioxide, according to claim 3, characterized by the upper (32) and lower (33) chambers having a small volume and a surface slightly conic that together with the coating surface to test, or the support surface, allows the flow with an approximately constant radial velocity.

[6] Unit for determining the permeability of coatings towards carbon dioxide, according to claim 3, characterized by the input of the feed streams (21 and 23) to the permeation cell (11) being through a side orifice (34), which communicate with a distribution channel (35), close to the external perimeter of the cell, in which there are many orifices of reduced diameter; this stream is then collected by a single central orifice (36); the flow direction can also be made in the reverse way.

[7] Unit for determining the permeability of coatings towards carbon dioxide, according to the previous claim, characterized by the distribution channel being closed on its upper part with the help of a lid (28 and 31) and o-rings.

[8] Unit for determining the permeability of coatings towards carbon dioxide, according to claims 1 and 2, characterized by the modification of the fan heater in such a way that the fan works continuously and the resistances are switched on and off through a solid state relay controlled by a temperature controller.

[9] Unit for determining the permeability of coatings towards carbon dioxide, according to claim 1, characterized by being controlled by a software that reads/controls the pressures, flowrates, the CO concentration of the permeate stream (24) and the temperature of the feed stream of the CO analyzer.

[10] Unit for determining the permeability of coatings towards carbon dioxide, according to claim 1, characterized by having a cylindrical tank to make and contain the gas mixture (13) placed horizontally and with the output placed at the center of one of the tank lids.

[11] Utilization of the unit for determining the permeability of coatings towards carbon dioxide, according to the previous claims, characterized by being used to determine the permeability of coatings towards pure gases when the CO specific analyzer is replaced by a specific analyzer of the target pure gas.

[12] Utilization of the unit for determining the permeability of coatings towards carbon dioxide, according to the previous claim, characterized by being used to determine the permeability of coatings towards oxygen (O ) when the CO analyzer is replaced by a O analyzer.

[13] Utilization of the unit for determining the permeability of coatings towards carbon dioxide, according to the previous claims, characterized by the use of Kraft paper as support of the coatings.