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Prof. Petr Kalenda, CSc.

University of Pardubice, Institute of Chemistry and Technology of Macromolecular Compounds,

Faculty of Chemical Technology, Studentská 573, 53210 Pardubice, Czech Republic,

e-mail: petr.kalenda@upce.cz

Ing. Pavel Menc University of Pardubice, Institute of Chemistry and

Technology of Macromolecular Compounds, Faculty of Chemical Technology,

Studentská 573, 53210 Pardubice, Czech Republic Abstract

Driers are used to catalyse auto-oxidation reactions that take place between the oxidation-drying paint and air oxygen. Co(II) salts of organic acids like, e.g., Co(II) octoate, represent the most efficient and, accordingly, most often used drier. Some selected inorganic compounds have been studied, in most instances oxide pigments used to produce paints - ZnO, nano-ZnO, TiO2, and V2O5. Chalcogenide compounds (ZnS) have been also studied. The objective was to reveal whether the inorganic compounds investigated have any positive effect on the time of drying and the surface hardness of oxypolymerising drying paint films. The examined compounds were mostly added together with the primary drier. The drying time and the surface hardness of the paint film as a function of time were studied.

Keywords Alkyd paint, Paint driers, Cobalt, ZnO, TiO2, V2O5, ZnS, Oxypolymerising drying, Organic coatings

INTRODUCTION

Alkyd resins are polyesters of polyfunctional alcohols and polyfunctional carboxylic acids, where at least one component is at least trifunctional. If some other, third component is present during the manufacture of a given alkyd resin, such resin is called modified alkyd [1]. The modifying components are most often fatty acids of drying, semi-drying or non-drying oils. Alkyd resins modified by drying and semi-drying oils rank among oxidation-drying paints (oxypolymerising drying paints). Their fundamental feature is a self-reaction with oxygen from air, resulting in the formation of a three-dimensional polymer structure [2]. Such paint materials thus form a solid film without addition of other components - hardeners. The reaction between the oxidation-drying paint and oxygen is known as auto-oxidation [3]. The fundamental condition underlying auto-oxidation is

the presence of oxygen, which diffuses through the paint film, as well as the existence of a sufficient number of reactive sites inside the paint system. In alkyd resins such reactive sites are represented by double bonds introduced into the structure by the side chains of the unsaturated fatty acids. The autooxidation reaction proper affects an activated methylene group either situated in position α with regard to the double bond or localised between two double bonds. The polymer network is formed via hydroperoxides, relatively stable intermediate products of oxidation [4]. Their decomposition to free radicals, necessary for the subsequent reaction, is very slow at room temperature. Consequently, drying of the paint film is slow and the resulting hardness is inadequate. Hydroperoxide decomposition can be however accelerated by means of suitable catalysts, known as driers [5]. As a result the drying time is shortened substantially and the drying process is completed within several hours.

Organic salts of transition metals [6] are used as compounds catalysing decomposition of hydroperoxides. The metal ion takes part in the catalytic reactions proper while the anionic part of the salt (in most instances 2-ethylhexanoic, neodecanoic or naphthenic acid) provides for good solubility of the metallic soap in the paint system. Depending on the activity of the metal ion in the autooxidative reaction metals are divided into primary and secondary. Primary driers actively support hydroperoxide decomposition, potentially accelerate the subsequent stages of the autooxidative reaction and can also play the role of oxygen carrier, thus affecting its solubility and diffusion in the resin, etc. Cations present in primary driers include primarily cobalt, but also manganese, vanadium and, at elevated temperatures, cerium and iron [7]. The most popular secondary driers include zirconium, calcium, barium, zinc, potassium, formerly also lead. Secondary driers are in themselves ineffective but exhibit a synergetic effect if combined with primary driers. In such mixed systems they improve drying of the paint film across its thickness, enhance the overall hardness and mechanical stability of the paint, modify its rheological characteristics, etc. Demand for low-toxic products has been rising recently and has handicapped some very efficient but environmentally dubious driers. At present these include cobalt compounds [8, 9]. Driers based on manganese(2+) combined with certain nitrogen-containing substances capable of forming complexes with the metal ion offer a certain solution in searching for suitable

CONTRIBUTION OF INORGANIC PIGMENTS TO THE FORMATION OF

PAINT FILMS

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alternatives [10, 11]. The central manganese ion in such complexes exhibits increased efficiency with regard to catalysis of autooxidative reactions. One can also make use of the synergetic effect of different metals to create a highly efficient drier system. The present study follows the effects of some inorganic pigments combined with a cobalt(2+) salt on the formation of a paint film from an alkyd resin during drying. Should a positive effect of inorganic pigments on the rate of drying and the surface hardness of the paint film be demonstrated, this approach would enable one to reduce the amount of the cobalt siccative added to oxypolymerising paint materials. EXPERIMENTAL Specification of the raw materials used

CHS alkyd S621 W 60 is a 60-% solution in lacquer petroleum of a drying alkyd resin based on pentaerythritol and phthalic anhydride, modified by soy soya bean oil (Spolchemie, a.s., Czech Republic)/ Octa-Soligen Kobalt 10 in D 60 is a primary drier, containing a cobalt(2+) salt of 2-ethylhexanoic acid, dissolved in lacquer petroleum (Borchers GmbH, Germany).

Preparation of model paint materials and test films

To test the drying properties of the investigated pigments, model paint materials based on an alkyd resin were formulated. To do that one has to know the critical pigment volume concentration CPVC and the density of individual pigments. The pigment volume concentration (PVC) in the alkyd resin paint materials was selected in the series 0, 1, 2, 3, 5, 10, 15, 20, and 30 vol. per cent. Cobalt(2+) salt of 2-ethyl-hexanoic acid was used as the primary drier. Accordingly, Co(2+) (0.02 % of the alkyd resin dry residue) was added by means of a micropipette to the prepared mixture of the resin and the pigment. Model alkyd-resin-based paint materials were prepared by dispersing the pigment, using the pearl mill Dispermat CV (WMA Getzmann GmbH, Verfahrenstechnik, Germany) filled with glass pearls of 2-mm diameter as the grinding medium.

The model paint materials so prepared were applied by means of an applicator with slot width of 0.09 mm onto cleaned glass panels 100 x 200 x 5 mm. The paint samples applied onto the panels were kept under standard conditions in a climatised laboratory (temperature 23 °C, relative air humidity 50 %).

Determination of degree of dryness the paint films

The time necessary for reaching a specified degree of non-tackiness of the paint film (the drying time) was determined using a paint applied onto a glass panel; each measurement was carried out on a different part of the film surface.

Drying to stage 1: About 0.5 g of Ballotini glass beads was poured onto the surface of the film placed horizontally to form a band 20 mm wide, in a manner ensuring that individual beads form a single layer. After 60 seconds the tested film is tilted by about 20o and several times wiped over with a brush. Drying to stage 1 is attained when all beads are removed without leaving any trace on the film surface. The determination is repeated each 10 minutes after paint application until the above degree of non-tackiness is reached.

Drying to stage 2: A square of special paper is carefully put onto the film surface; a rubber ring is then placed on the paper and a 20-g rider is then placed in the centre of the rubber ring; the weight and the ring are removed after 60 seconds. The tested panel is then allowed to fall freely by the longer edge from a height of 30 mm. Stage 2 of drying is reached when the paper square falls off the film after the impact. The determination is repeated each 5 minutes. Determination of surface hardness of the paint film

The progress of the drying process was followed by measuring the surface hardness of the paint films for 60 days, i.e., as long as the film hardness continued to rise. The surface hardness thus reached its maximum value and did not appreciably rise any longer. Hardness of the paint films applied onto the glass panels was measured by means of Persos-type pendulum (Elcometer Pendulum Hardness Tester, United Kingdom). The measured hardness was related to the hardness of a glass standard and expressed as relative hardness in per cent.

The measurements were carried out in conformity with ISO 1522 between 1 day and 60 days after application; during that period the films were kept in a climatised laboratory at 23 °C and 50 % relative humidity. Figure 2 illustrates the progressing formation of the paint film, revealed by individual types of measurement, from the stage of liquid film until a paint with the final, constant parameters has been reached.

The SEM pictures in Figure 1 show the morphology of the investigated pigments.

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(a)

(b)

(c)

(d)

(e)

fig. 1 SEM pictures showing the morphology of particles of individual pigments: (a) ZnO, magnification 1,000x;

(b) nano-Zn0, magnification 2,000x; (c) V2O5, magnification 1,000x; (d) ZnS, magnification 3,000x; (e) TiO2, magnification 13,000x.

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Fig. 2 Progressing paint formation from the liquid stage until a solid paint having the final constant parameters

has been reached. RESULTS AND DISCUSSION Drying of paint films containing the tested pigments

The progress of drying of alkyd paint films containing inorganic pigments (ZnO, TiO2, V2O5, and ZnS) together with a Co(2+) drier. Drying to stage 1 and stage 2 was determined for all films. Table 1 summarises the results demonstrating the catalytic contribution of the investigated inorganic compounds to drying of alkyd based paints to stage 1, where the paint becomes non-tacky. Both the type of the pigment and its content in the paint affect film formation. The catalytic effect during the first stage of film formation was established for the two pigments ZnO and V2O5. No appreciable catalytic effect of ZnS was found; TiO2 inhibited paint drying inside the entire concentration range PVC = 1 to 30 volume per cent. Table 1 shows the effect of microcrystalline nano-ZnO on the oxypolymerisation reactions. The results

demonstrate that at low concentrations ZnO does not accelerate oxopolymerisation and more extensive acceleration of the crosslinking reaction is only apparent for PVC = 5 volume per cent ZnO in the paint. ZnO nanoparticles lead to a much more apparent acceleration of the crosslinking process than is the case when microcrystalline ZnO is used. One can conclude that in the region of low concentrations the specific surface area of the particles employed substantially influences the catalytic effect.

The ZnS pigment has no significant effect on drying of the paint film to stage 1. Nevertheless, the rate of drying to the higher stage 2 of paint films containing ZnS is higher by a factor of 1.5 compared with films without ZnS and, moreover, the time of drying does not depend very much on the volume concentration of the ZnS pigment in the paint film.

Tab. 1 Effect of inorganic pigments on oxypolymerising crosslinking of alkyd paints (paint drying to stage 1)

Time (in hours) of film drying to stage 1

PVC (volume per cent) Pigment

0 1 2 3 5 10 15 20 30ZnO 5.0 5.0 4.8 4.3 3.1 2.8 2.5 1.9 1.5nano-ZnO 5.0 6.3 4.2 3.4 2.5 1.7 - - -V2O5 5.0 5.7 4.8 3.5 3.5 3.2 2.5 2.1 1.6ZnS 5.0 5.3 4.5 4.5 4.2 - - - -

day 1 day 30

Stage-1 drying

Stage-2 drying

Increasing filmhardness

Pain

t film

har

dnes

s

10 h max.

Final paint hardness

Time

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TiO2 5.0 11.3 11.7 10.3 10.0 10.3 10.5 10.5 10.2

0

5

10

15

20

0 5 10 15 20 25

PVC (%)

Tim

e of

film

dry

ing

to s

tage

2 (i

n h)

nano-ZnO ZnO TiO2ZnS V2O5

Fig. 3 The time of film drying to stage 2 as a function of volume concentration of the pigments at a constant

amount of the Co(2+) drier.

Considering the good oxidative properties

of V2O5 one would expect it to act as a catalyst of oxopolymerisation reactions. Table 1 however shows that this is not the case. The system containing high concentrations of the V2O5 pigment dried at a rate almost comparable to that containing

ZnO. On the other hand, at low PVC values an inhibiting effect was observed in films containing V2O5. In paint films containing higher PVC concentrations the oxidative properties probably won through and resulted in noticeable acceleration of film formation.

Measurement of relative hardness of films containing the investigated compounds

The relative hardness of the prepared paint films was measured using the Persoz pendulum tester. The results were used to determine the relative hardness of the film as a function of the film drying time. It is apparent that the presence of the ZnO pigment in the paint film markedly influences its hardness. Relative hardness of 27 % was found in films without the pigment, while the film containing ZnO at PVC = 15 % exhibited relative hardness of almost 52 %, almost twice as high. All films containing ZnO exhibited higher hardness than films without the pigment. The relative hardness increases with increasing content of the pigment in the paint film. Hardness at first increases quite rapidly, but after some 15 days the rate of increase slows down and subsequently remains almost constant. In all paint films the relative hardness was more than twice that found for the film without the pigment. A significant effect, not found for microcrystalline ZnO, consisted in that pigment concentration of 1 volume per cent was sufficient to double the film hardness. The increasing film hardness is probably attributable to substantially higher specific surface area of the pigment particles compared with microcrystalline ZnO.

Similarly to the measurement of the drying time of paint films, we also compared the relative hardness of paints containing the investigated pigments together with the Co(2+) salt of 2-ethylhexanoic acid. The most conclusive results concerning the relative paint hardness were obtained with ZnO pigments. Zinc oxides generally enhance the hardness of paint films. According to the effect of the added pigment on the relative hardness of the paint films the pigments can be ordered as follows:

nano-ZnO > ZnO > ZnS > TiO2 > V2O5

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0

10

20

30

40

50

60

0 10 20 30 40 50 60Time [days]

Film

har

dnes

s [%

]PVC = 0 % PVC = 1 % PVC = 2 %PVC = 3 % PVC = 5 %

Fig.4 Relative hardness of paint films containing the nano-ZnO pigment together with the Co(2+) drier as a

function of time.

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60Time [days]

Film

har

dnes

s [%

]

PVC = 0 % PVC = 1 % PVC = 2 %PVC = 3 % PVC = 5 %

Fig. 5Relative hardness of paint films containing ZnS pigment together with the Co(2+) drier as a function of

time.

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10

15

20

25

30

35

40

45

50

55

60

0 5 10 15 20 25 30 35

PVC (%)

Film

har

dnes

s [%

]nano-ZnOZnSV2O5TiO2ZnO

Fig. 6 Final hardness of paint films containing the investigated pigment together with the Co(2+) drier after 60

days of drying as a function of their volume concentrations.

Taking into account the obtained results one can say that the most favourable values of relative hardness were observed in paint films containing nano-ZnO and primary driers (Co(2+)). Paint films containing the microcrystalline ZnO and ZnS also exhibited increased hardness. Only V2O5 impaired the relative hardness of paint films across the entire concentration range examined. Owing to its oxidative properties V2O5 crosslinks the surface of the paint film and, consequently, impedes diffusion of oxygen into the film (Fig. 6). CONCLUSIONS

Some shortening of the drying time was observed with several pigments (ZnO, nano-ZnO, V2O5 and ZnS); the pigment TiO2 inhibited film formation over the entire range of its concentration. With regard to the drying time of the paint films zinc-containing system achieved excellent results; among them nano-ZnO was the most efficient thanks to the high specific surface area of its particles.

The study of relative hardness of the films led to conclusions similar to those deduced from the measured drying time. The paint containing the ZnO pigment exhibited the best results, where the relative hardness of the paint films was increased by 100 %. Films containing ZnS also exhibited high

values of hardness. Pigmentation generally improves the physical characteristics of paint films and has a certain effect on autooxidation reactions that take place in paints modified by unsaturated higher fatty acids. Excellent results were observed for paint films containing nano-ZnO; this pigment markedly and demonstrably improved the relative hardness of the paint film. References [1] Bieleman, J. H., (2002), “Progress in the

Development of Cobalt-free Drier Systems”, Macromol. Symp., Vol. 187, pp 811-21.

[2] Bieleman, J. H., (2002), „Driers”, Chimia, Vol. 56, pp 184-90.

[3] Erich , S.J.F., Laven, J., Pel, L., Huinink, H.P. and Kopinga, K., (2006) “NMR depth profiling of drying alkyd coatings with different catalysts”, Progress in Organic Coatings, 55, pp 105-11.

[4] Erich, S.J.F., Laven, J., Pel, L., Huinink, H.P., Kopinga, K., (2006) „Influence of catalyst type on the curing process and network structure of alkyd coatings”, Polymer, Vol. 47, pp 1141-9.

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[5] Mallégol, J., Gonon, L., Commereuc, S. and Verney, V., (2001), “Thermal (DSC) and chemical (iodometric titration) methods for peroxides measurements in order to monitor drying extent of alkyd resins”, Progress in Organic Coatings, Vol. 41, pp 171-6.

[6] Mallégol, J., Lemaire, J. and Gardette, J-L., (2000), “Drier influence on the curing of linseed oil”, Progress in Organic Coatings, Vol. 39, pp 107-13.

[7] Micciché, F., Oostveen, E., van Haveren, J. and van der Linde, R., (2005), “The combination of reducting agents/iron as environmentally friendlier alternatives for Co-based driers in the drying of alkyd paints”, Progress in Organic Coatings, Vol. 53, pp 99-105.

[8] Oyman, Z.O., Ming, W., van der Linde, R., van Gorkum, R., Bouwman, E., (2005), “Effect of [Mn(acac)3] and its combination with 2,2‘-bipyridine on the autoxidation and oligomerisation of ethyl linoleate”, Polymer, Vol. 46, pp 1731-8.

[9] Stava, V., Vesely D. and Kalenda, P., (2008), “Catalytic effects of transition metals in the form of the salts of organic acids in the cross linking of alkyds” , Pigment and Resin Technology, Vol. 37, pp.67-72.

[10] van Gorkum, R. and Bouwman, E., (2005), “The oxidative drying of alkyd paint catalysed by metal complexes”, Coordination Chemistry Reviews, Vol. 249, pp 1709-28.

[11] Wu, J-Z., Bouwman, E. and Reedijk, J., (2004), “Chelating ligands as powerful additives to manganese driers for solvent-borne and water-borne alkyd paints”, Prog. in Org. Coat., Vol. 49, pp 103-8.

Acknowledgement

This work was supported by the Ministry of Education of the Czech Republic under project MSM 0021627501 and by the grant of the Ministry of Industry and Trade of the CR MPO 2A-1TP1/014.