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Characterization of a water-based paint for corrosion protection.

Abstract Corrosion of steel rebars in reinforced concrete is one of the major problems in the construction industry. Carbonation reactions of concrete with carbon dioxide and, mainly, the chloride salts action are the main causes responsible for concrete degradation. Protective coatings help to improve the durability of concrete structures by acting as a physical barrier against the corrosion agents. Waterborne paints are usually used for concrete protection rather than solvent-based paints since they are less pollutant. The aim of this work is to investigate the influence of the pore size and porosity on the permeability of the paints films toward sodium chloride. Three characterization methods from membrane science were implemented to characterize paint coatings. The time-lag method was used to determine the permeability toward the sodium chloride and toward helium and argon, these for approximately 100% relative humidity. From the seven waterborne paints formulated, only one was found to be suitable for surface protection of reinforced concrete, since its permeability toward NaCl was smaller than [10.sup.-14] [m.sup .2] [s.sup.-1], the threshold value required by National Laboratory of Civil Engineering (LNEC) in Portugal. For the formulated paints, it was observed that the average pore size correlates well with the permeability toward sodium chloride. This is an important result since obtaining the permeability toward sodium chloride of corrosion protective paints is very time consuming, while the average pore size can be obtained in a much shorter time.

Keywords Concrete protection, Water-based paints, Chloride ion permeability, Transport mechanism, Time-lag method

Introduction

Despite being thought as a modern material, concrete is being used for thousands of years. It is a construction material made up of aggregates of various sizes (fine aggregates such as sand and course aggregates such as crushed stone or gravel), combined with a cement paste (a mixture of cement and water), which acts as binder, and chemical admixtures to give special final charactcristics. (1), (2) Concrete is very strong in compression but relatively weak in tension. Therefore, in the areas where tension may occur, reinforcement has to be included by adding steel reinforcing rebars. (1) Reinforced concrete has a high alkalinity ([pH.sub.initial] [approximately equal to] 12.5) and steel rebars become covered by a protective iron oxide layer highly stable and insoluble, named passive layer; in these conditions, embedded steel is well protected. (2) However, the corrosion of the concrete reinforcement is the main deterioration process of these structures. The main phenomena that can bring on corrosion are: the chloride salts action (as NaCl and Ca[Cl.sub.2]) that destroys the passive layer and the reaction of carbon dioxide from the air with calcium hydroxide present in concrete. This latter process is known as carbonation, which causes the decrease of pH in the interstitial pores of the concrete, exposing the surface of the rebars to corrosion. (3), (4)

The surface protection of concrete by coating systems is considered as a very feasible and cost-effective option to retard the degradation process, not only from a technical point of view but also due to economical reasons. (5) The use of coatings can help to reduce concrete degradation by acting as a physical barrier against the external corrosion agents. Solvent-based paints usually have an excellent protective behavior but their usage is now limited due to their high content of volatile organic compounds (VOC). Thus, it is now encouraged the developing of water-based paints for concrete protection, less pollutants and able to fulfill all the environmental protection requirements.

All surface protection systems for concrete should comply to the requirements of standard EN 1540-2. (6) Protection systems for concrete should be an effective barrier to carbon dioxide and to liquid water permeation (water transports salts that degrade the concrete surface and block the pores originating erosion and cracking) and should allow the passage of water vapor from inside the concrete structures. Nevertheless, the actual maximum permeability of protective coatings toward chloride salts is not defined by this standard but only the limit for the liquid water uptake--the capillary adsorption of water should be lower than 0.01 kg [m.sub.-2] [h.sup.-0.56] In principal, the diffusion of chloride salts does not need to be regulated for coalings aiming to meet this requirement since it is not expected to happen at least in a significant extension. The above-mentioned standard directs then the responsibility to the national entities for defining a permeability threshold value. In Portugal, for coatings claiming to be corrosion protective the National Laboratory of Civil Engineering (LNEC) sets this permeability value to [10.sup.-14] [m.sup.2] [s.sup.-1].

The durability of the reinforced concrete protected by coatings is strongly affected by the mass transport properties of the aggressive species through this protective layer. This way, the mass transport through the coating layer is related to its pore size distribution. Four types of mass transport mechanisms: molecular diffusion, Knudsen diffusion, surface diffusion, and viscous flow. (7), (8) The predominance of one of these mechanisms depends on the pore size, on the driving force (e.g., concentration) and on the interaction between the permeant and the surface that contact with them.

Nowadays, there are many characterization techniques of porous systems such as gas adsorption-desorption, permporometry, thermoporometry, liquid displacement, rejection measurements, (9) of which applicability depends on the average pore size and on the material properties. Also, most of these techniques need the previous knowledge of the pore geometry and since this is not generally known the experimental results are often difficult to interpret. (9- 13) Paint coats, which can be viewed as a polymer composite membrane, have pores in the range of 0.1-2 [micro]m. (14) This way, the main goal of the present work is to relate the pore size and porosity with the permeability toward sodium chloride. Three characterization methods were used to optimize a paint formulation for corrosion protection. The so-called time-lag method was used to assess the coatings permeability toward chloride salts (15-17) and the permeability toward helium and argon that allow to estimate the average pore size. (8), (15), (18) As a complementary study, helium picnometry and mercury porosimetry were used to determine the internal porosity of these coatings. (19)

Experimental section

Coatings preparation

In the present study, seven different water-based paints have been prepared--Table 1. These acrylic paints will be hereafter coded as paints A, B, C, D, E, F, and G. The paints were formulated based on paint A by changing the amount of fillers and maintaining the total solids weight percentage. Paint A is a standard aqueous-based product of low brightness used for concrete protection, but it does not comply with the established permeation limit of chloride salts. Three different types of fillers with different geometries were used, two of them showing a lamellar geometry (CI and C2) and the third one with a spherical geometry (C3). Modifying the volume fraction of the three fillers also changes the pigment volume concentration (PVC) and the critical pigment volume concentration (CPVC) and, consequently, the ratio between them (denoted by A). The PVC of a paint is the volumetric percentage of pigments and fillers in the total solids of a paint system, excluding all volatiles from the calculation. However, the CPVC is the transition point where the binder present in the paint formulation is responsible for covering all the particles of pigments and fillers; above or below this point, substantial differences in the appearance and behavior of the paint film can occur, as described elsewhere. (20) The PVC and CPVC were calculated using equation (1).
Table 1: Characteristic parameters of the different
formulated paints

Paint  Amount of fillers  (wt%)        A (%)
                      C1     C2    C3

A                   14.6   14.6  70.7   33.3
B                   20.5   48.5  31.0   43.4
C                   29.3   63.4   7.3   49.4
D                   63.4   29.3   7.3   51.1
E                   22.0   22.0  56.1   37.4
F                    7.3   36.6  56.1   36.6
G                   36.6    7.3  56.1   38.1


PVC (%) = (volume of pigments + fillers)/total volume of dry paint x 100 (1)

Application of coatings

To use the time-lag characterization method, either for liquid or gas systems, paints have to be supported. The paint films were applied on a sheet of Kraft paper (Europac[R], Portugal), according to the recommendations of the paint manufacturer, namely in what concerns the drying time and paint film thickness. The minimum drying time for water-based paints is 7 days at (296.15 [+ or -] 1) K and (50 [+ or -] 5) % of relative humidity (EN 23 270). These conditions allow the formation of a homogeneous paint film and also ensure the complete solvent evaporation. The thicknesses of the different paint films were measured using a digital micrometer (Mitutoyo[R], 0-25 mm [+ or -] 1 [micro]m). The thickness of the support (Kraft paper) is around 400 jam and the typical thicknesses of paint films are between 140 and 180 [micro]m. The applied films of paints A-G presented thicknesses in the range of 170-180 [micro]m.

For the helium picnometry and mercury porosimetry methods, paint films do not need support and so paints were applied on a glass support coated with Teflon[R] and allowed to dry for 7 days. After drying, the paints were peeled off from the glass substrate, obtaining "free paint films.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Time-lag method: determination of paint films permeability toward chloride salts

The permeability of paint films toward chloride salts was obtained using the time-lag method. (15-17) This method considers a permeation cell with two chambers divided by the supported paint film as sketched in Fig. 1. Both chambers should be completely filled, one with an aqueous sodium chloride solution (2.5 M) and the other with distillated water. A conductivity electrode should be placed in the water chamber. Both chambers should contact to the atmosphere for preventing the osmotic pressure to drive the mass transport across the paint film. Moreover, the two chambers should also be homogenized using a magnetic stirrer. (15) The permeability is obtained when a steady conductivity increase is recorded, corresponding to a steady permeation of sodium chloride. For low permeating concentrations, the permeation driving force is mostly constant and the permeation rate can be easily obtained from the conductivity history slope--Fig. 4.

Time-lag method-determination of paint films permeability toward gases

The time-lag method was applied to determine the permeability of paint films to different gases saturated with humidity at 25[degrees]C for obtaining the pore size of the experimental coatings in these conditions. (15), (18) The selected gases, helium and argon, were fed to the permeation cell at 1 and 2 bar. A simple sketch of the experimental setup is shown in Fig. 2.

An experimental run starts by evacuating the permeating cell followed by filling the storage tank with the desired gas (helium or argon) at the desired pressure. This gas permeates the paint film driven by the partial pressure difference between the retentate and permeate sides. The permeate flux is obtained from the pressure history derivative in the permeate chamber. The passage of gas through the water bubbling tank (Fig. 2) and the presence of liquid water in the permeate chamber insure 100% of relative humidity in both retentate and permeate chambers and thus there should be no net water vapor mass transport. The paint film was applied on a Kraft paper placed on top of a sintered metal disc in the retentate chamber. None of the supports add significant resistance to the permeating gas since they are very porous.

Helium picnometry

Helium picnometry was used to determine the structural density. A helium pyenometer consists of two chambers with calibrated volumes--Fig. 3. Chamber 1 is filled with helium, while the sample under analysis is placed in chamber 2. Then, helium from chamber 1 is fed to chamber 2, occupying the free volume inside the paint flakes. (19), (21) The actual volume of the sample can be determined since its initial mass and volume of chamber are known. Degassing processes involving a series of purges with helium should be performed to remove impurities, moisture and volatiles.

Mercury porosimetry

Mercury porosimetry was performed using the equipment PoreMaster from Quanta Chrome[R]. This equipment is used to characterize porous media with pores between ca. 30 nm and 1 x [l0.sup.6] nm, corresponding to a maximum and minimum pressures of [P.sub.max] = 427.21 bar and [P.sub.min] = 3.56 bar of mercury, respectively. (22)

Results and discussion

Time-lag method was used to evaluate chloride salts permeation. However, these experiments can take several weeks. Since the permeability to chloride salts should be related to the average pore size of the paint samples, it is proposed to use the time-lag method to evaluate the permeability of the paint films toward different gases (much faster experiments) and thus calculate their pores sizes.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Time-lag method

The time-lag method is a powerful and yet simple method to characterize permeation processes and it is used to determine the permeability of membranes toward gases and solutes. Because of the simple nature of the permeation experiment, transport parameters can be directly obtained from experimental data, thus avoiding the complex mathematical treatment required by other techniques. (15), (23) The experiment consists on monitoring continuously the amount of penetrant permeating through the solid membrane into a closed vessel. At a given instant, a step perturbation is performed in the feed chamber and the concentration (or pressure) history in the permeate chamber is recorded. (15) The permeation process can be divided into its transient and steady-stale components--Fig. 4.

This method is known as "time-lag method" because there is a delay between the perturbation and the onset of the concentration (or pressure) rise on the other side of the membrane. Extrapolating the linear branch of the response curve until the abscissa defines an interception point known as "time-lag"--shown in Fig. 4--that is related to the mass transport coefficient of the species across the membrane. On the other hand, the slope of the linear branch is related to the steady-state permeating flux and then to the permeability of the paint film.

Determination of paint films permeability toward chloride salts

During a time-lag experiment mass transfer occurs between the two chambers. When the solid curve in Fig. 4 becomes straight, it means that a steady-state mass transfer is occurring and the membrane permeability can be obtained from the slope of that line. The driving force derives from the concentration gradient. i.e., the concentrations difference between the upper chamber and the lower chamber ([C.sub.U] and [C.sub.L], respectively). At the steady-state condition, the amount of species leaving the retentate chamber (solution with higher concentration) equals the amount of species that reaches the permeate one and the following equation can be written (15):

ln ([C.sub.U] - [C.sub.L]) = -[L.sub.C]A/[l.sub.C](1/[V.sub.U] + 1/[V.sub.L])t + ln([C.sub.U.sup.0] - [C.sub.L.sup.0])(2)

where [L.sub.C] and [l.sub.C] are the permeability and the thickness of the paint film plus support, respectively; A is the permeation area; [V.sub.U] and [V.sub.L] are the volume of the upper and the lower chambers; t is time; [C.sub.U.sup.0] and [C.sub.L.sup.0] are the initial concentrations at the upper chamber (this concentration is constant and equal to 2.5 M) and at the lower chamber, respectively. According to equation (2), the permeability of the paint film plus support is obtained from the linear part of the representation of ln([C.sub.U] - [C.sub.L]) as a function of time t--Fig. 5.

Analogously to electrical resistances combined in series, the overall permeability can be written as a combination of the permeabilities of the support and of the paint film:

[l.sub.C]/[L.sub.C] = [l.sub.f]/[L.sub.f] + [l.sub.s]/[L.sub.S] (3)

where [L.sub.f] and [l.sub.f] are the permeability and the thickness of the paint film, respectively; [L.sub.s] and [l.sub.s] are the permeability and the thickness of the support, respectively. The paint film permeability, [L.sub.f], can be easily obtained since the support permeability, [L.sub.s], is experimentally known ([L.sub.s] = 1.5 x [10.sup.-11] [m.sup.2] [s.sup.-1]). The results obtained for all paints are given in Table 2 and clearly show that only paint B complies to the requirement defined by LNEC concerning the permeability toward NaCl.
Table 2: Permeability of the paint films ([L.sub.f])
toward NaCI

Paint          A     B     C     D     E     F     G

[L.sub.f] x  1.48  0.95  3.27  4.52  8.26  4.08  2.58
[10.sup.14]
([m.sup.2]
[S.sup.-1])


Determination of paint films permeability toward gases

The mass transport of pure gases in porous media can be described by Knudsen diffusion and viscous flow mechanisms since for meso- and macropores, such as the pores in a paint film, the surface diffusion is normally negligible. (8), (13), (24) The permeability of the permanent gases (L) is given by the ratio between the steady-state gas flow that crosses the membrane and the correspondent driving force normalized by the membrane thickness, l. (9) In gas permeation, the driving force of the mass transport is the difference between retentate and permeate partial pressures. Thus, the gas permeability is written as (9), (18):

L = 1/A F/([P.sub.h] - [P.sub.1])/l (4)

where F is the volumetric flow of the permanent gas, A is the membrane area, [P.sub.h] and [P.sub.1] are feed and permeate partial pressures of the permanent gas, respectively. On the other hand, the gas volumetric flow (F) is calculated by (9), (18):

[FIGURE 5 OMITTED]

F = V[V.sub.m]/RT d[P.sub.1]/dt (5)

V is the volume of the permeate tank, [V.sub.m] is the molar volume, R is the gas constant, and T is the absolute temperature of the cell. The paint film permeability is obtained from the slope, d[P.sub.1]/dt, of the permeate pressure history--Fig. 6. Important to holdback that the record of the experiment starts (t = 0) when the relation between permeate pressure and time becomes linear, what happens for pressures higher than 3200 Pa--Fig. 6. This pressure corresponds to the water vapor pressure value at the working temperature of 25[degrees]C Using the mentioned procedure, Fand L were calculated and presented in Table 3 for all formulated paints. The permeability results for each gas correspond to the average value obtained for the two pressures.
Table 3: Volumetric flow (F) and permeability (L) for the
formulated paints and experimental ratio between
permeabilities toward helium and argon

Paint   F x [10.sup.11]([m.sup.3]    L x [10.sup.17]    [L.sub.He]/
                [s.sup.-1])         ([m.sub.PTN.sup.3]   [L.sub.Ar]
                                        m [m.sup.-2]          (exp)
                                         [s.sup.-1]
                                        [Pa.sup.-1])

            He            Ar            He       Ar

       1 bar  2 bar  1 bar  2 bar

A      3.85   7.64   1.71   3.53       6.73    3.06            2.20

B      2.58   5.46   1.00   1.74       4.60    1.62            2.84

C      1.88   4.01   1.14   2.55       3.07    1.89            1.62

D      2.51   6.30   1.79   3.87       4.34    2.83            1.53

E      2.86   5.79   2.67   4.66       4.50    3.92            1.15

F      3.20   5.51   2.19   3.98       4.73    3.25            1.46

G      2.74   5.38   1.80   2.91       3.87    2.38            1.63


[FIGURE 6 OMITTED]

The ideal selectivity of a medium to gases helium and argon is given by the ratio of their permeability Coefficients (9), (25):

[alpha]/Ar = [L.sub.He]/[L.sub.Ar] (6)

This selectivity factor points out about the permeation mechanisms and elucidates about the porous structure. Since the partial pressure of water is very low (3.2 kPa), it was neglected the molecular diffusion mass transport. Assuming just Knudsen diffusion, the ideal selectivity is given by the ratio of the square root of the atomic masses of helium and argon--[L.sub.He]/[L.sub.Ar] (Knudsen) = 3.16. Table 3 shows the ratios of the experimental permeabilities toward both gases for the formulated paints. It is expected that the mass transport mechanism combines Knudsen diffusion and viscous flow since the experimental ratios are smaller than the Knudsen selectivity for all cases. Moreover, paint B shows the smaller difference between experimental and Knudsen selectivities, indicating that it may have the lowest average pore size.

Knudsen diffusion occurs when the pore diameter is of the same order of magnitude of the free pathway for gas molecules. On the other hand, when the pores are large, intermolecular collisions leads and a viscous flow is noticed. These two phenomena may occur together. Combining in parallel both mechanisms one obtain (8), (24):

N = [epsilon]/l [tau] 1/RT {[D.sub.k]([P.sub.h] - [P.sub.1]) + [B.sub.0]/2[micro]([P.sub.h.sup.2] - [P.sub.1.sup.2])} (7)

where N is the molar flux of the pure gas, [epilon] is the

porosity, [tau] is the tortuosity, [D.sub.k] = 2/3[r.sub.P] [square root of (8RT/[Pi]M)], [B.sub.0] = [r.sub.P.sup.2]/8 and [mu] is the viscosity of the pure gas.

Linearizing equation (7), it is possible to calculate the average pore size, [r.sub.P]:

NRT/([P.sub.h] - [P.sub.1])[v.sub.k] = [epsilon]/l[tau] [r.sub.P] + [epsilon]/l[tau] [r.sub.P.sup.2]/16 ([P.sub.h] + [P.sub.1])/[mu][v.sub.k] (8)

where [v.sub.k] = 2/3 [square root of (8RT/[Pi]M)]. Plotting Y = NRT/([P.sub.h] - [P.sub.1])[v.sub.k] vs X = ([P.sub.h] + [P.sub.1])/[mu][v.sub.k] and fitting a straight line to the experimental points, the correspondent slope and the intercept can be evaluated--Fig. 7. These values depend only on parameters [epsilon]/[tau] and [r.sub.p] and thus the average pore radius can be calculated--Table 4.
Table 4: Average pore radius ([r.sub.p]) for the formulated paints

Paint            A   B    C    D     E    F    G

[r.sub.p] (nm)  220  24  752  918  1929  868  471


At this point, important conclusions can be drawn from Table 4. Paint B is the paint with the lowest permeability toward NaCl ([L.sub.f] = 0.95 x [10.sup.-14] [m.sup.-2] [s.sup.-1], cf. Table 2) and the only one complying with the permeability threshold to be considered corrosion protective. Moreover, this paint film has the smallest pore size, [r.sub.p] = 24 nm, which corresponds to a mesoporous medium (1 nm--[r.sub.p]--25 nm). (9) On the other hand, paint E has the largest [r.sub.p] (corresponding to a macroporous medium), responsible for high permeability toward sodium chloride ([L.sub.f] = 8.26 x [10.sup.-14] [m.sup.2][s.sup.-1], cf. Table 2).

Figure 8 plots the permeability toward NaCl as a function of the average pore size, showing that there is a direct relation between them. However, this relationship was only observed within the same paint family (data not shown). This indicates that it is possible to characterize the corrosion protection performance based on the average pore size of the paint films. Obtaining this parameter, as mentioned before, is cheaper than the long lasting experiments for obtaining the permeability toward NaCl. This approach is particularly advantageous for the development of very low permeable paint films for corrosion protection.

Helium picnometry and mercury porosimetry methods: internal porosity determination

Helium picnometry and mercury porosimetry methods were used to determine the structural and apparent densities of the paint films, respectively. The density of a material is defined as its mass per unit volume. The so-called structural density is the ratio between the mass of the paint film sample and its solid volume, this means the volume of the paint film sample not taking into account the volume occupied by the pores. The structural density ([[rho].sub.str]) is calculated by:

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[[rho].sub.str] = [m.sub.s]/[V.sub.s] (9)

where ms is the mass of the paint sample and [V.sub.s] is the solid volume of the sample. On the other hand, the apparent density ([[rho].sub.ap]) is defined as the ratio between the mass of the paint sample ([m.sub.s]) and its apparent volume ([V.sub.ps]). The apparent volume of the paint sample comprehends the solid volume and the intra-particle voidage. (26)

[[rho].sub.ap] = [m.sub.s]/[V.sub.ps] (10)

Combining the results of helium picnomctry and mercury porosimetry methods, it is possible to provide information about the intra-porosity of the paint samples prepared. The intra-porosity, or just the paint film porosity, is related to the number of pores and together with the average pore size are key parameters to characterize the mass transport. The paint film porosity ([epsilon]) is given by:

[epsilon] = [V.sub.p]/[V.sub.ps] = 1 - [V.sub.s]/[V.sub.ps] = 1 - [[rho].sub.ap]/[[rho].sub.str] (11)

where [V.sub.p] is the intra-particle voidage of the sample.

In helium picnometry, the volume displacement is measured indirectly from the equilibrium pressure after controlled expansion of a given volume of gas, cf. "Helium picnometry" section. Helium is usually used because it does not sorb and it is an inert gas that can penetrate into the finest pores. (19), (21) [V.sub.s] can be determined since the volumes of the picnometer cells are known:

[V.sub.s] = ([p.sub.1][V.sub.1] + [p.sub.0][V.sub.2] - [p.sub.1][V.sub.1] - [p.sub.1][V.sub.2])/([p.sub.0] - [V.sub.f]) (12)

[V.sub.1] and [V.sub.2] are the volume of chambers 1 and 2, respectively; [P.sub.0] is the pressure read when chamber 2 is at vacuum; [P.sub.i] is the initial pressure when chamber 2 is filled with helium; and [P.sub.f] is the final pressure in chamber 2.

In what concerns mercury porosimetry method, this technique involves the intrusion of mercury into the porous network of a material. As the pressure increases, the mercury progressively enters into narrower pores. The apparent volume is obtained when the applied pressure is just enough for mercury to contact with the whole paint film surface. (19), (22) In the present case, the apparent volume was obtained for a mercury pressure of 2 bar.

Table 5 summarizes the structural and apparent densities and the porosity for each paint analyzed. Il can be concluded that all paints have a very low internal porosity, about 2-7%. Paint B exhibits the lowest porosity making it less permeable.
Table 5: Structural density (pstr), apparent density (pap) and
porosity (s) for the formulated paints

Paint  Pest (g [cm.sup.-3])  Pap (g [cm.sup.-3])  [epsilon] (%)

A                    1.5804               1.5495           1.96
B                    1.4927               1.4660           1.79
C                    1.5258               1.4748           3.34
D                    1.5586               1.4952           4.07
E                    1.5644               1.4580           6.80
F                    1.5547               1.4945           3.87
G                    1.5001               1.4615           2.58


Figure 9 plots the average pore size of the paint films according to their internal porosity. The internal porosity of the media increases almost linearly with the average pore radius. Assuming that the pores shape is the same, it is possible to conclude that the number of pores decreases with the internal porosity, meaning that the pore size increase is made cannibalizing the neighboring pores. Moreover, as the pore size varies linearly with the permeability of the paint films toward sodium chloride, it can also be concluded that there is a linear relation between the porosity and the permeability toward NaCl. Thus, higher porosity and average pore radius of paint films implies higher permeation of chloride salts.

[FIGURE 9 OMITTED]

Analyzing all the results obtained it is possible to conclude that the PVC/CPVC ratio also affects the proprieties of the paints, but it is not devised a direct relationship between this ratio and the paint film permeability. Composite polymer membrane films containing impermeable particles present enhanced barrier properties, depending on the particle shape, size and size distribution, orientation, and concentration. (27), (28) It should be concluded that there is no direct relation between the particle concentration alone (here quantified by the PVC parameter) and the barrier properties of the paint film since many other variables play simultaneously a critical role in the process. This fact highlights the importance of the present work, which describes an expedite strategy to experimentally assess the barrier properties of paint films.

Conclusions

The present work describes a simple method for obtaining the average pore size of a paint film that can be easily implemented and used by the paint industry. This method is based on the permeation of two gases, helium and argon, obtained by the so-called time-lag method.

Helium pycnometry and mercury porosimetry techniques were used to obtain the internal porosity of the coatings that, together with the average pore size, are key parameters for characterizing the mass transport through paint films.

It was also concluded that for the family of paint formulations studied, the average pore size correlates well with the permeability toward sodium chloride, which was obtained by the time-lag method. Since, for corrosion protective paint film obtaining the permeability toward sodium chloride is very time consuming (each determination takes more than 3 weeks), determining the average pore size can shorter significantly the time needed for optimizing a protective paint formulation.

Finally, the water-based paint B showed the lowest permeability toward sodium chloride, 9.45 x [10.sup.-15] [m.sup.2] [s.sup.-1] the lowest average pore size, [r.sub.p] = 24 nm, and the lowest porosity, [epsilon] = 1.79%. Paint B is the only one that meets the corrosion protective threshold permeability toward sodium chloride, 1.0 x [10.sup.-14] [m.sup.2] [s.sup.-1]. The average pore size of this paint is one order of magnitude smaller than the other paint formulations produced changing only the proportion of three different fillers.

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P. Dias, L. Andrade, J. Sousa, A. Mendes (*)

LEPAE, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

e-mail: mendes@fe.up.pt

C. Carneiro, J. Machado

CIN, Corporacao Industrial do Norte, S. A., Avenida Dom

Mendo 831, Apartado 1008, 4471-909 Maia, Portugal

J. Sousa

Departamento de Quimica, Escola de Ciencias da Vida e do

Ambiente, Universidade de Tras-os-Montes e Alto Douro, Apartado 1013, 5001-911 Vila-Real Codex, Portugal

DOI 10.1007/s1198-011-9388-0
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Title Annotation:BRIEF COMMUNICATION
Author:Dias, Paula; Carneiro, Catarina; Andrade, Luisa; Sousa, Jose; Machado, Joao; Mendes, Adelio
Publication:JCT Research
Date:May 1, 2012
Words:5781
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