Properties of organic coatings depending on chemical composition and structure of pigment particles

Properties of organic coatings depending on chemical composition and structure of pigment particles

Surface & Coatings Technology 204 (2010) 2032–2037 Contents lists available at ScienceDirect Surface & Coatings Technology j o u r n a l h o m e p a...

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Surface & Coatings Technology 204 (2010) 2032–2037

Contents lists available at ScienceDirect

Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t

Properties of organic coatings depending on chemical composition and structure of pigment particles David Veselý a, Andrea Kalendová a, Petr Němec b,⁎ a b

Department of Paints and Organic Coatings, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210 Pardubice, Czech Republic Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210 Pardubice, Czech Republic

a r t i c l e

i n f o

Available online 10 November 2009 Keywords: Organic coating Pigment Anticorrosion efficiency Zincite Periclase Core

a b s t r a c t ZnO, ZnO/core, MgO and MgO/core oxides were synthesized; their morphology was evaluated and their impact on the physical properties of the paint film was assessed. The binder used in the studied coatings was epoxy-ester resin. The physico-mechanical properties and the anticorrosion efficiencies of the pigmented paint films were determined. All the synthesized pigments have good anticorrosion efficiency in epoxy-ester coatings. The synthesized pigments can be conveniently used in coatings protecting metal bases against corrosion. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The organic coatings currently provide the most widespread method of material protection [1]. The anticorrosion efficiency of protective coatings depends on the pre-treatment of the metal surface [2], the type and concentration of the anticorrosion pigment [3,4], the method of film creation [5], the adhesion of the coating to the metal base [6], and on the mechanical properties of the whole coating system. Although inorganic pigments in an organic binder improve its mechanical properties, they simultaneously work as a non-homogenous element in the system. At the organic phase–inorganic phase interface, water and gases quickly penetrate the film, thus disturbing the protective function of the coating. In case of applications on metal materials, this same process is accompanied with metal corrosion and with the disturbance of the organic film by oxidation products [7]. To prevent these undesirable reactions, anticorrosive pigments are applied into organic coatings [8]. Metal oxides (MeO) can be used as special-purpose pigments, as anticorrosive pigments, or as functional additives in paints. The shape of their primary particles significantly affects the mechanical and anticorrosion properties of the resulting paint film [9]. Zinc oxide displays inhibition properties ensuing mainly from its ability to enter into chemical reactions with acidic substances diffusing through the

coating and to create protecting layers by means of electrochemical reactions [10,11]. If ZnO displays various particles shapes, its varying properties in a coating film can be anticipated as well [12]. MgO ranks among alkaline oxides whose ability to neutralize acidic substances can be exploited in order to reduce the speed of corrosion reactions [13]. The aim of this work is to prepare MeO (Me = Zn, Mg) pigments with different shapes of particles as well as core-shell pigments (ZnO/CaSiO3, and MgO/CaSiO3) and to identify and compare their anticorrosive properties in model paints based on epoxy-ester. 2. Experimental 2.1. Synthesis of zincite (ZnO) by the thermal oxidation of metal zinc The ZnO particles were synthesized by the thermal oxidation of metal zinc at 930 °C in an oxygen atmosphere in an electric furnace. 2.2. Synthesis of zincite from a precursor Zn4(SO4)(OH)6 precursor was prepared by hydrolysis of zinc sulfate and urea at 100 °C. The obtained precursor was further gradually annealed to a temperature of 450 °C to obtain lamellar zinc oxide. 2.3. Synthesis of periclase (MgO) based pigment

⁎ Corresponding author. Tel.: + 420 466037247; fax: + 420 466037068. E-mail address: [email protected] (P. Němec). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.11.005

Periclase (MgO) was fabricated by thermal decomposition of MgCO3 in an electric furnace.

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2.4. Synthesis of a zincite-based core-shell pigment (ZnO/CaSiO3) Zincite (ZnO) on a non-isometric core [14,15] with a higher aspect ratio of primary particles; i.e. of calcium metasilicate CaSiO3 (wollastonite) was synthesized. In solid phase, the heterogeneous mixture of CaSiO3(s) + Zn(s) was annealed in an electric furnace to a temperature close to the boiling point of zinc (930 °C). Zinc vapors were oxidized in the oxygen atmosphere to zinc oxide [13]. The resulting product was a ZnO layer precipitated on CaSiO3. 2.5. Synthesis of a core-shell pigment based on periclase MgO/CaSiO3 Periclase (MgO) layer was synthesized on a non-isometric carrier with the higher aspect ratio of primary needle-shaped particles (wollastonite). White periclase was produced by thermal decomposition of MgCO3(s) at 1110 °C. 2.6. Determination of the properties of the synthesized and tested pigments in pulverized state The determination of specific mass was carried out with a gas density bottle Micromeritics AutoPycnometer 1320. “Oil absorption” is the amount of linseed oil (in grams) that yields a paste of defined properties from 100 g of the pigment. The procedure of determining the pH value of the aqueous extract was derived from ISO 789-9 standard. 10%-pigment suspensions in redistilled water were prepared; the pH values were taken regularly for 21 days. The determination of weight loss due to corrosion used steel panels of 20 × 50 × 0.5 mm immersed in the aqueous suspension of the tested pigments. The time of exposure at constant conditions (21 °C) was 21 day. The determination of specific electrical conductivity were taken regularly for 21 days in the same of aqueous suspensions, too (χ1 and χ21). pH and conductivity of distilled water used for tests was 6.9 and 3 μS cm− 1, respectively. Calculations to determine corrosion loss of steel weight per unit of surface (Cw) and weight loss related to the corrosion loss of steel in pure water (Xcorr) were applied [6]. The determination of particles sizes and distribution was carried out by means of Mastersizer 2000 instrument (Malvern, Instruments Ltd., UK). The surface and shape of the pigment particles was examined by means of an electron microscope (JEOL-JSM 5600 LV). Scanning electron microscopy was used for study of morphology of prepared paint films focusing on incorporation of pigment particles into the films. 2.7. Formulation of model paints with the tested anticorrosion pigments To identify anticorrosion and mechanical efficiency, the synthesized pigments were applied in the solution of epoxy-ester resin of solvent type used for the production of anticorrosion paints (ChSEpoxy 101X50, Spolchemie a.s. Czech Republic). The pigment volume concentration (PVC) of the synthesized pigments in coatings was approx. 5 vol.% (the lowest concentration manifesting the effective anticorrosive properties of the pigment). 2.8. Preparation of model paints with tested pigments Epoxy-ester resin modified with the fatty acids of tung oil was used to formulate model anticorrosion paints. To characterize the binder, we show the composition of epoxy-ester resin (60% epoxide, and 40% conjugated fatty acid of tung oil), the density of 1.07 g cm− 3, dry matter of 60% and acidity no. equivalent to 4.1 mg KOH. Model paints were prepared by the dispersion of the binder and the pigment in a pearl mill filled with glass balls of 3 mm in

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diameter. Model paints for the testing of an epoxy-ester resin based binder containing zincite-based pigments (tetrapod-like ZnO, lamellar ZnO and needle-like ZnO) and periclase-based pigment (MgO) were prepared. Model paints containing zincite/wollastonite and periclase/wollastonite core-shell pigments, respectively, were formulated. A non-pigmented paint was prepared for a comparison. 2.9. Preparation of tested samples The formulated paints were applied by a box applicator on steel panels (10 × 10 cm) for mechanical and corrosion tests, respectively; drying of the paint film was performed at room temperature. Dry film thickness, in other words the thickness of the film in dry state was 80 ± 5 μm. The conditioning of the samples on test panels in the standard conditions (50% relative humidity, and temperature of 21 °C) proceeded for 6 weeks in an air-conditioned room. 2.10. Determination of the corrosion-inhibition efficiency of the synthesized pigments in organic coatings During cyclic corrosion test in a chamber with salt mist, the samples of paint films were exposed to the effects of the 5% solution of NaCl at 35 °C following ISO 7253 standard. The testing took place in 12-hour cycles: 10 h of salting with 5% NaCl, 1 h of distilled water condensation at 40 °C, and 1 h of drying at 23 °C. The samples were evaluated after 1100 h' exposure. 2.11. Corrosion tests evaluation methods Corrosion processes were evaluated by means of methods according to standards ASTM D 714-87, ASTM D 610-85 and ASTM D 1654-92 [16]. ASTM D 714-87 method classifies the osmotic blisters formed to groups defined by the sizes designated by 2, 4, 6 and 8 values (2 denoting the highest size, 8 the lowest size). To the blister size, information is attached giving the respective frequency of appearance. The highest frequency of appearance is designated as D (denoting dense), a lower one as MD (denoting medium density) and as F (denoting few). This approach can give a series starting with a surface showing the lowest corrosion attack by few osmotic blisters of small size up to dense large-size blisters. ASTM D 1654-92 method describes a procedure evaluating the extent of corrosion appearing along a cut line and the “subsurface“ corrosion appearing under the coating in the cut-line vicinity. The results of ASTM D 61085 method can be compared to appropriate standards, which are related to the corrosion degree at the surface under the protective coating. The result is defined as a certain degree of the substratesurface corrosion given in %. 2.12. Determination of the effects of the synthesized pigments on the physico-mechanical properties of organic coatings The result of determination of paint resistance against impact test is given by the height of the free fall of a 1000 g weight onto the paint at which the paint film has not yet been damaged. The evaluation was performed on the reverse of the paint according to ISO 6272. The result of determination of paint resistance against cupping in an Erichsen apparatus gives the cupping of a test panel with a coating in mm at which the first impairment of the paint occurred (ISO 1520). The result of determination of paint resistance against bending on a cylindrical spindle indicates the diameter of the spindle on which the paint has not yet been impaired (ISO 1519).

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The evaluation of the degree of coating adhesion by a lattice method was made in accordance with standard ISO 2409 using a special cutting blade with cutting edges positioned 2 mm apart. The hardness of the paint film was determined by means of a pendulum according to Persoz (ISO 1522). The hardness of the paint is indicated in percents related to the hardness of the glass standard.

3. Results 3.1. The properties of the synthesized and tested pigments in a pulverized state The quality of pigments depends on their physical as well as chemical properties that play an important role in applications in

Fig. 1. SEM images of the synthesized and tested pigments: (a) ZnO (tetrapod) particles; (b) ZnO (lamellar) particles; (c) ZnO (needle) particles; (d) ZnO/CaSiO3 particles; (e) MgO/CaSiO3 particles.

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Table 1 Physico-chemical properties of the synthesized and tested pigments. Pigment

Density (g/cm3)

ZnO (tetrapod) ZnO (lamellar) ZnO (needle) MgO ZnO/CaSiO3 MgO/CaSiO3 Blank test (distilled water)

5.49 4.58 5.56 3.20 3.93 3.25 –

Oil absorption (g/100 g)

CPVC (%)

18.60 36.08 19.35 43.21 23.51 26.43 –

49.45 36.02 46.36 27.19 50.24 49.76 –

Particle size d50 (μm)

0.09 48.5 0.55 0.04 11.50 10.76 –

pH21

7.2 6.6 6.7 10.2 9.8 10.2 7.1

Loss due to corrosion

Extract conductivity (μS cm− 1) χ1

χ21

Cw (g m− 2)

Xcorr (%)

60 8310 66 372 1527 1433 3.0

134 965 256 400 140 120 1850

31.20 38.21 37.23 24.45 27.24 14.21 48.12

64.8 79.4 77.4 50.8 56.6 29.5 100

CPVC stands for critical pigment volume concentration. All the parameters are given as arithmetic averages within 10 measured values. d50 stands for medium distribution of particles.

paint binders. Morphology of particles of studied pigments (Fig. 1) is important parameter as well. Table 1 presents the physico-chemical properties of the pigments in a powder state, pH values of aqueous pigments extracts after the 21st day (pH21), specific conductivity values taken after 24 h (χ1) and after the 21st day of extraction (χ21), and the corrosion losses of steel in an aqueous pigment extract (CW — weight corrosion loss, and Xcorr — relative corrosion loss related to steel weight loss in pure water). 3.2. The corrosion-inhibition efficiency of the synthesized pigments in organic coatings The results of the corrosion tests taken in a salt spray chamber are given in Table 2 and Fig. 2. The anticorrosion pigments use their properties to affect the efficiency of a paint film while protecting a metal base. The mechanism and chemical behavior of their action most often cause the creation of protecting passivating layers on a metal surface and affect pH during anodic and cathodic corrosion reactions. The influence of the structure, composition, and shape of the particles of the synthesized pigments on the efficiency of organic coatings in corrosion environments was also examined. 3.3. The effects of the synthesized pigments on the physico-mechanical properties of organic coatings The pigments affect not only the anticorrosion properties but also the physico-mechanical properties of the paint films. Thanks to the Table 2 Results of the corrosion tests of the epoxy-ester coatings after 1100 h' exposure in a salt spray cabinet (dry film thickness — DFT = 80 ± 5 μm). Pigment

Steel base corrosion

Corrosion processes on paint surface

Corrosion in cut Base corrosion Blistering on paint Blistering in (mm) (%) surface (dg.) cut (dg.) Zincite and periclase-based pigments ZnO (tetrapod) 1.4 16 ZnO (lamellar) 2.4 16 ZnO (needle) 2.1 16 MgO 1.3 –

8F – – 8F

– 8F 6F –

Core-shell pigment ZnO/CaSiO3 1.2 MgO/CaSiO3 1.2

10 10

6F 8F

2F 2F

Non-pigmented film (blank test) – 1.2

50

6M

8D

All the parameters are given as arithmetic averages within 5 measured values.

shape of their particles and the optimum distribution of the size of those particles, the pigments can enhance the barrier-base qualities of the paint film binder. The excellent physical properties of paints are the primary preconditions for fulfilling the anticorrosion protection role of the paint system. The results of mechanical testing show good incorporation of pigment particles into polymeric matrix of paint film (Fig. 3). Table 3 shows the results of the mechanical tests of the epoxy-ester coatings. 4. Discussion 4.1. Morphology and structure of the synthesized pigments From a morphological standpoint, the most interesting starshaped particles were obtained during synthesis of metal zinc at 930 °C in an oxygen atmosphere (ZnO tetrapod); the sizes of tetrapod particles range between 100 nm and 2 μm. These particles do not form agglomerates, are white, and do not contain any by-product (Fig. 1a). Zincite formulated by hydrolysis and subsequent calcination shows lamellar particles ZnO (Fig. 1b). The calcination of a precursor was employed to prepare a pigment containing Zn4(SO4)(OH)6 as a by-product. EDX analysis determined the content of Zn and SO2 to be 53.2 and 18.7 wt.%, respectively. The properties of needle-shaped particles ZnO (needle), produced at higher temperatures – between 950 and 1000 °C – with longer periods of annealing were studied as well. In case of this pigment, SEM analysis detected needle-shaped particles prone to form agglomerates (Fig. 1c). The synthesized periclase pigment contains nanoparticles of an isometric shape with the very narrow distribution of particle sizes (Table 1); it is a white product without an admixture of foreign substances products. The core-shell pigment with a ZnO layer (ZnO/CaSiO3) or coreshell pigment with a MgO layer (MgO/CaSiO3) on the surface contain morphologically interesting non-isometric particles with a higher aspect ratio (Fig. 1d and e). 4.2. Physico-chemical properties of the synthesized pigments The zincite-based pigments displayed neutral pH values of the aqueous extract. The identified pH values ranged in a neutral area and showed practically no changes in dependence on the time of extraction. The shell-core pigment MgO/CaSiO3 had a strongly alkaline nature. Thanks to its core (wollastonite), this shell-core pigment provided a more alkaline extract than zincite without a core. Of all the pigments included in the study, periclase exhibited the most alkaline character. The pH value of the aqueous extract of core-shell MgO/CaSiO3 pigment measured after 21 days of extraction was 10.2 (Table 1).

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Fig. 2. Photo images of test panels after removing the paints following 1100 h of exposure in a salt spray cabinet (DFT — dry film thickness = 80 ± 5 μm).

Fig. 3. SEM image of coated surface.

The aqueous pigment extracts that display increasing conductivity in time can be assumed to undergo gradual dissolution and thus to release ions into the solution. The pigment ZnO prepared from a precursor ZnO (lamellar) and the core-shell ZnO/CaSiO3 show conductivity values that drop steeply as the time of extraction progresses. This process can be explained by the fact that initially dissociated ions precipitate into the form of insoluble complexes, causing a significant reduction in aqueous extract conductivity (Table 1). The corrosion of the test steel panels in an aqueous pigment extract is closely connected with the pH value as well as with specific conductivity. When pH values drop into a neutral region, higher corrosion losses of steel are observed. The extracts of the pigments with pH greater than 9 suppress corrosion, thus facilitating lower corrosion steel losses. On the other hand, extreme specific

D. Veselý et al. / Surface & Coatings Technology 204 (2010) 2032–2037 Table 3 Physico-chemical properties of the pigmented epoxy-ester coatings and the values of the surface hardness of epoxy-ester coatings taken after 3 and 65 days from formulation. Pigment

Adhesion (dg.)

Bend test (mm)

Impact (cm)

Cupping (mm)

Hardness (%) After 3 d

After 65 d

Zincite and periclase-based pigments ZnO (tetrapod) 0 4 ZnO (lamellar) 1 b4 ZnO (needle) 0 b4 MgO 1 b4

100 100 100 88

10.1 10.1 11.7 11.9

25.0 21.2 23.0 25.1

65.4 54.5 64.9 62.5

Core-shell pigments ZnO/CaSiO3 1 MgO/CaSiO3 1

b4 b4

100 100

12.2 12.2

23.7 23.7

59.0 59.0

Non-pigmented film (blank test) – 0 b4

100

11.9

22.0

59.0

All the parameters are given as arithmetic averages within 5 measured values.

conductivity enhances corrosion in aqueous extracts, which is most obvious in case of the ZnO (lamellar) pigment made from a precursor. The results suggest that zincite-based pigments prevent corrosion losses in an aqueous extract with less efficiency than periclase-based pigments (Table 1). Test panels' steel is in passive state when pH N 9.5. That is why, for effective protection, it is appropriate to have aqueous extract with pH in alkaline region.

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standpoint of pigments particles morphology, the tetrapod-like particles of zincite enhanced the hardness of the coatings more efficiently than the needle-shaped particles of zincite. The lamellar particles of zincite caused the retarded drying and reduced hardness of the coating. The paint films containing tetrapod-shaped zincite and needleshaped zincite exhibit excellent adhesion to base steel, the same as a non-pigmented binder. The needle-shaped, star-shaped and isometric zincite particles with a small diameter have positive effects on the adhesion of coatings to their base (Table 3). The evaluation of the impact resistance of paints clearly demonstrated that substantially higher values are achieved by the paints pigmented with non-isometric particles of a needle, star and/or lamellar shape. On the other hand, the presence of isometric particles in paints does not contribute to impact resistance to the same extent. When exposing the pigmented paints during to a flexure test and to a cupping test, all the samples showed outstanding physico-chemical properties. Epoxy-ester paints with the content of tetrapod-shaped and needle-shaped pigment particles displayed generally excellent mechanical properties. Paint films containing studied pigments have similar mechanical properties as non-pigmented films have. One can conclude that the pigments particles are well distributed and incorporated in organic coatings. The incorporation of particles in paint films is documented with their surface morphology (Fig. 3); the surface of the films is smooth, without defects.

4.3. Anticorrosion properties of pigments in coatings Osmotic blisters were formed in a paint containing a core-shell pigment based on ZnO/CaSiO3 and in a coating that contained periclase. The ZnO-based pigments showed a strong ability to suppress the formation of blisters on the coating surface, yet they did not prevent the bottom corrosion of the metal base on its surface as well as near a cut (Table 2). On the contrary, the coating with the periclase content showed outstanding resistance against bottom resistance thanks to its alkaline nature. Fig. 2 presents photo images of test panels after removing the paints following 1100 h of exposure in a salt spray cabinet (DFT — dry film thickness = 80 ± 5 μm). In general, the periclase-based pigment exhibited greater anticorrosion efficiency than the zincite-based pigments. The test of resistance against salt mist effects proved the pigments with starshaped and lamellar particles to be more efficient than those with needle-shaped particles. The core-shell pigment ZnO/CaSiO3 is as efficient as the ZnO-based pigments with needle-shaped particles without a core. The evaluation of the anticorrosion efficiency of the pigments is as follows: • Depending on the chemical composition of the pigments particles, anticorrosion efficiency is higher for the periclase (MgO) than for the zincite (ZnO). • Depending on the shape of the zincite particles, anticorrosion efficiency decreases in the order: tetrapod ZnO N lamellar-shaped ZnO N needle-shaped ZnO. • Anticorrosion efficiency of the needle-shaped particles: core-shell pigment ZnO/CaSiO3 is as efficient as the needle-shaped pigment without a core (ZnO needle). • Anticorrosion efficiency of core-shell pigments is higher for the periclase MgO/CaSiO3 than for the zincite ZnO/CaSiO3. 4.4. Physico-mechanical properties of organic coatings In case of an epoxy-ester binder containing the synthesized pigments, the hardness of the film increased and the pigments had positive influence on the speed of film creation (Table 3). From the

5. Conclusions The physico-mechanical and anticorrosion properties of the formulated paint films containing ZnO, ZnO/core, MgO and MgO/ core oxides as anticorrosion pigments displayed better performance compared to the non-pigmented paint film. Pronounced quality of the pigmented coatings was observed when evaluating the anticorrosion tests. Excellent anticorrosion results were achieved especially by the periclase-based pigments and by the star-shaped particles of zincite prepared from zinc metal. Acknowledgment This work was supported by the Ministry of Education, Youth, and Sports of the Czech Republic under the project MSM 0021627501. References [1] L. Veleva, J. Chin, B. del Amo, Prog. Org. Coat. 36 (1999) 211. [2] G. Grundmeier, B. Rossenbeck, K.J. Roschmann, P. Ebbinghaus, M. Stratmann, Corr. Sci. 48 (2006) 3716. [3] L.H. Yang, F.C. Liu, E.H. Han, Prog. Org. Coat. 53 (2005) 91. [4] M.V. Popa, P. Drob, E. Vasilescu, J.C. Mirza-Rosca, A. Santana-Lopez, C. Vasilescu, S.I. Drob, Mater. Chem. Phys. 100 (2006) 296. [5] A. Kalendova, D. Vesely, P. Kalenda, Pigm. Resin Technol. 35 (2006) 83. [6] A. Kalendova, D. Vesely, P. Kalenda, Pigm. Resin Technol. 36 (2007) 3. [7] D. Battocchi, A.M. Simões, D.E. Tallman, G.P. Bierwagen, Corr. Sci. 48 (2006) 2226. [8] G. Blustein, R. Romagnoli, J.A. Jaén, A.R. Di Sarli, B. del Amo, Colloids Surf., A Physicochem. Eng. Asp. 290 (2006) 7. [9] G. Bierwagen, D. Battocchi, A. Simões, A. Stamness, D. Galoman, Prog. Org. Coat. 59 (2007) 172. [10] M. Hernández, J. Genescá, J. Uruchurtu, F. Galliano, D. Landolt, Prog. Org. Coat. 56 (2006) 199. [11] A. Kalendova, D. Vesely, Anti-Corros. Methods Mater. 54 (2007) 3. [12] V. Houšková, V. Štengl, S. Bakardjieva, N. Murafa, J. Phys. Chem. Solids 69 (2008) 1623. [13] A. Kalendova, D. Vesely, Prog. Org. Coat. 64 (2009) 5. [14] H.S. Emira, Pigm. Resin Technol. 34 (2005) 132. [15] M.R. Tohidifar, E. Taheri-Nassaj, P. Alizadeh, Mater. Chem. Phys. 109 (2008) 137. [16] A. Kalendova, D. Vesely, P. Kalenda, Prog. Org. Coat. 57 (2006) 1.