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Anti Carbonation Coatings

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“Anti-carbonation coatings” is a buzz word used in concrete protection. In this article, test methods adopted by BASF in determining carbon dioxide diffusion rate and consequently the requirements for a coating to be called “Anti Carbonation Coating” will be introduced. Besides key factors that need to be considered when formulating Anti carbonation coatings and the minimum thickness required to achieve desired CO2 permeability will be mentioned. The test method was developed by referring EN ISO 12572 and this paper also gives guidelines to make water based anti-carbonation coatings based on dispersions.

What is carbonation?

Carbonation means chemical conversion to a carbonate; Carbonation is a term which is widely used in beverage industry. It refers to the impregnation of carbon dioxide into a fluid. However in construction industry, carbonation is referred to “the process of chemical weathering by which minerals containing soda, lime, potash and other basic oxides are changed to carbonates by the action of carbon dioxide and water”.

The process is represented by the reaction below,
CO2 + H2O ——> H2CO3
H2CO3 + CaO ——> CaCO3 + H2O

As represented by the above chemical reaction, carbon dioxide with water forms carbonic acid. This carbonic acid subsequently reacts with alkaline material such as lime to form calcium carbonate.

Effects of Carbonation on concrete structure

The action of atmospheric carbon dioxide on lime mortars and cement concretes has been known for centuries. Roman plasters used air-hardened lime, which gained strength due to the reaction of carbon dioxide with calcium hydroxide formed an interlocked mass of calcium carbonate. But, traditionally concrete is reinforced with steel to increase tensile strength.

Concrete protects steel from corrosion by forming a “passive layer” because of very high alkalinity induced by cement and other minerals. But many concrete structures are exposed to atmospheric carbon dioxide emitted from various sources. In presence of moisture / water and as described in the above reaction, atmospheric carbon dioxide diffuses slowly through the concrete and the process of carbonation is initiated. Due to this process, the pH of concrete slowly turns acidic and destroys the passive layer protecting the reinforcing steel bars. Once the passive layer is destroyed, rusting of steel bars begins.

Carbonation is a slow process but is detrimental to concrete structures as life expectancy of concrete structures is between 50 –100 years.

Preventing Carbonation -Anti Carbonation coatings

It is possible to formulate coatings which can prevent the diffusion of Carbon dioxide and thereby protect concrete structures from the detrimental effects, carbonation. This article discusses the functionality of coatings which can be called “Anti Carbonation coatings”.

Overview

Test method

Formulation of Anti Carbonation coating
Test results of Anti Carbonation coating

References

Overview

Generally, water based coatings is more permeable to gases and water vapour. These coatings are formulated by using polymer dispersions. Water vapour permeability is a desirable property to allow water vapour to escape from the substrate. But, if the permeability is too high, carbon dioxide can diffuse from atmosphere into the substrate easily. Hence there is a need for coatings with moderate gas permeability which can prevent Carbon dioxide from diffusing into the substrate while allowing moisture to escape.

Test Method for determining Carbon dioxide transmission rate Principle :
This test method describes how to determine the Carbon dioxide transmission rate gravimetrically. The test sample is used to seal off a space within a test cell is kept in a dry atmosphere comprising air and 10% ( v/v) carbon dioxide and weighed at regular intervals.

The carbon dioxide transmission rate or carbon dioxide diffusion flux is measure of the amount of carbon dioxide diffusing in unit time through unit area of the coating.

By Fick’s first law, the carbon dioxide permeability  is calculated using the carbon dioxide transmission rate. It is the rate of carbon dioxide transmission through unit area of coating of unit thickness induced by unit partial pressure difference between two specific surfaces.

(1)S = thickness of the sample
?P = P1 – P2 , the partial pressure difference between the two surfaces

The reciprocal of permeability is called diffusion resistance.

The diffusion resistance number µ is the quotient of the diffusion resistance of the sample and of air. This number indicates how many times a coating is impermeable to carbon dioxide than a static air layer under the same conditions.

(2)Where  is permeability of carbon dioxide in the coating
And L is permeability of carbon dioxide in air

The diffusion –equivalent air layer thickness Sd (m) is the thickness of a static air layer that possess, under the same carbon dioxide permeability as the coating.

(3)p1 = carbon dioxide partial pressure above the sample (Pa)
p2 = carbon dioxide partial pressure below the sample (Pa)
K = L.(p1-p2) = 250 ( for p1-p2 = 10% ( v/v CO2, T = 23 °C)

Experiments

In order to estimate the desired thickness of coatings required to achieve satisfactory diffuse of CO2, few experiments were carried out. Water based coatings films were applied at different film thickness dried at standard condition and subjected to CO2 diffusion test. Fillers were also added in the coatings to understand the effect of addition. It is also compared with solvent (resin) based coatings.

Samples Preparation

Drying and conditioning: all samples are dried at 23°C, 50% humidity to constant mass before test.

Sample A and B are water based coatings in blue color without fillers. Their membranes were made by casting the solution in the square silicone mold. 3 pieces of sample B membrane were selected out of several pieces of membrane we made, no obvious defect such as hole and air bubble can be observed.
Sample C and D, white color, contain fillers. Sample E and F are solvent based and without fillers. Membranes were made by casting the solution on the PTFE panel sealed with foam tape to form a square mold. They are also difficult to make continuous film and those without visual defects are selected for test.

Testing Method

Apparatus: HTS-12X concrete carbonation machine, Donghua Apparatus Co. Ltd

Carbon dioxide Transmission –

Procedure:

The test sample is used to seal off a space within a test cell containing NaOH as a carbon-dioxide-absorbing substance.
The test cell is kept in CO2 chamber under dry atmosphere comprising air and 10 % (v/v) carbon dioxide. CaCl2 was used as the desiccant in the chamber.

Weigh the test cell at time intervals of 24hours, until the rate of weight change of test cell remains constant over several subsequent intervals.

Results & Discussion

The following table shows CO2 permeability results obtained from sample films. It is clear from the result that as the thickness of the coatings increases, CO2 permeability decreases. It is also observed that there is no significant effect on CO2 permeability after addition of filler. This could be due to the fact that the ratio of Binder and Filler is too low. In case of solvent based coatings even 200 micron thickness is sufficient to get required CO2 permeability (Sd value above 50)

CO2 Permeability Results

where
I : Carbon dioxide diffusion flux
Sd : Diffusion-equivalent air layer thickness
µ : Diffusion resistance number

Appendix: Figures and Charts

Formulation of Anti Carbonation coating

Although solvent based coatings give desired CO2 permeability at lower film thickness, it is not preferred for reasons such as very low water vapor permeability and very high Volatile organic compound (VOC). Low water vapor permeability leads to deboning even if moisture content of the substrate is on slightly higher side. All over world VOC restriction is regulated for environmental reason. Hence we decided to formulate water based coatings with lowest possible VOC. Based on the results obtained on earlier experiments, an ideal water based formulation was developed to achieve desired properties

It is well known that PVC of the coatings plays a major role in deciding coatings quality in terms of durability. Since higher filler coatings impart porosity to the film it adversely affects the carbon dioxide diffusion Hence the filler loading is restricted to get PVC level of Approx. 30%.

The viscosity especially that of high shear is very important as it determines the thickness of the coatings. The ICI viscosity is maintained above 2.0 Poise by using high shear thickeners such as polyurethane. If it is lower, problem such as sagging is faced during application. Volume solids should be more than 50% unlike normal exterior coatings.

Typical formulation
Tego fomex 810 Evonik,
Rheolate 278 Elementis,
Natrosol 250 HBR Aqualon
Ultradisperse W 30 BASF
Acronal PS 715 ap BASF
Collacral 8990 BASF
Texanol Eastman
Acticide SPX Thor
Acticide EPW Thor

Factors affecting Anti-carbonation coatings :

There are two major factors that are affecting the anti-carbonation property of coatings.
1. Thickness of coatings
2. Durability of coatings

Thickness Of coatings

The rate of carbon dioxide diffusion into the substrate mainly depends upon the thickness of the film and partly depends upon how compact is the film. A compact film can be obtained by using cross-linked polymer such as Acronal PS 715 ap, a special styrene/acrylic water based dispersion developed by BASF.

The study shows that an ideal formulation can give anti-carbonation properties at around thickness of 300 micron DFT. (Sd value obtained anything above 50 m is sufficient to give necessary anti-carbonation properties.2)

Carbon dioxide diffusion results wrt thickness

A graph of Sd value v/s film thickness is plotted, it clearly shows that below 200 micron film thickness, the coatings cannot give sufficient anti carbonation properties.

Durability of coatings

The durability of the coatings depends upon its stability against the harsh environment. Apart from carbon dioxide diffusion, other factors like industrial pollution, rain, wind etc are playing major role in deciding durability of the coatings. It also depends upon the properties of the coatings such as its stability against UV light, water alkali etc. Most importantly, the coatings with crack bridging property have proven that they give maximum durability. They are also called as elastomeric coatings in some part of the world. The crack bridging capacity of the coatings is determined by mechanical strength such as,
1. Elongation
2. Tensile strength
3. Modulus of elasticity (E)
4. Toughness

Physical properties

The formulation obtained using Acronal PS 715 ap exhibit following properties:
The high tensile strength indicates the toughness of coatings, which is much higher than paint formulations. The elongation of 270% is sufficient to give crack bridging capacity to the coatings.

Conclusions

The study shows that film thickness plays a major role in carbon dioxide permeability. An ideal formulation can be formulated by using dispersions such as Acronal PS 715 ap with low VOC. It is quite clear from the discussion that quality of anti-carbonation coatings depends not only on carbon diffusion, but also on overall durability of the coatings to give long lasting performance.

References

–    prEN 1062-6 Paints and varnishes – Coating materials and coating systems for exterior masonry – Part 6: Determination of carbon dioxide permeability
–    Carbonisation von Beton, ihre Bedeutung und Beeinflussung durch Beschichtungen (Carbonation of concrete, its significance and how it is affected by surface coatings); Ing. (grad) Robert Engelfried, Esslingen: defazet, 31 (1977), 9, 353-359
–    Die carbonisation von Sichtbeton und ihre Bekampfung (carbonation of exposed concrete and how to combat it); Prof. Dr.-Ing. Heinz Klopfer, Dortmund; Bautenschutz und Bausanierung, 3 (1978), 3, 86-97.
–    Diffusionswiderstandszahlen fur Kohlendioxid und Wasser und deren praktische Anwendung (Diffusion resistance numbers for carbon dioxide and water and their practical significance; Dipl. -lng. Robert Engelfried, Dortmund University; Farbe und Lack, 89 (1983), 7, 513 – 518
–    TP-OS German technical standards for concrete protection systems used in civil engineering;  ZTV-SIB 87 (1990 issue).

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