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High performance UV-cured coatings for wood protection
Progress in Organic Coatings 45 (2002) 359–363
High performance UV-cured coatings for wood protection R. Bongiovanni a,∗ , F. Montefusco a , A. Priola a , N. Macchioni b , S. Lazzeri b , L. Sozzi b , B. Ameduri c a
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, c.so Duca degli Abruzzi, 24 10129 Torino, Italy b Istituto per la Ricerca sul Legno—CNR, Via Barazzuoli, 23 50136 Firenze, Italy c UMR 5076 CNRS 8, rue Ecole Normale, F-34296 Montpellier Cedex, France Received 25 February 2002; accepted 15 June 2002
Abstract UV-curable systems based on the copolymerisation of a typical acrylic resin with a low amount of a fluorinated monomer (<1%, w/w) were used for the protection of wood panels. In the presence of the additives, the bulk properties and the adhesion of the acrylic films were unchanged, while a strong modification of the surface was obtained. The quality aspects and the chemical resistance of the coatings applied to the wood panels were also enhanced. © 2002 Elsevier Science B.V. All rights reserved. Keywords: UV-curing; Fluorinated monomers; Surface properties; Wood coating
1. Introduction The UV-curing technique is based on the polymerisation of a multifunctional system induced by an incident UV radiation to obtain a three-dimensional network. The reaction allows to transform at room temperature in a fraction of a second a liquid system into a solid having rubbery or glassy properties. It has been widely applied in many industrial fields for the manufacturing, the decoration and the protection of different materials, including wood [1]. The main advantages of the technique are the following [2]: • • • •
the energy needed for the process is low; the polymerisation is very fast and highly efficient; the cure is selectively limited to the irradiated area; no solvent is required (100% solid formulation), therefore the environmental pollution by VOC is avoided.
In addition to these important features, this technology can satisfy new requirements for traditional or advanced applications, as it can offer a broad range of final properties, changing the formulation and the curing conditions. The general chemical formulation contains a photoinitiator, reactive monomers and functionalised oligomers, namely un∗ Corresponding author. E-mail address: [email protected] (R. Bongiovanni).
saturated polyesters, tiol-ene compounds, acrylic and epoxy resins. In order to obtain high performance advanced coatings, particular structures have to be introduced in the polymeric network, e.g. fluorinated groups. In fact peculiar characteristics are given by the presence of fluorine [3,4]: hydrophobicity, chemical stability and weathering resistance, good release properties and low coefficient of friction, water permeability, refractive index. The cost of these products is usually very high and depends on the fluorinated structures content. In previous investigations interesting results were obtained by copolymerising low amounts of fluorinated monomers or oligomers with a typical UV-curable resin [5,6]. In this work we applied these fluorinated coatings on a wood substrate and studied their properties, such as chemical resistance, wettability, hardness and adhesion. Sweet Chestnut (Castanea sativa Mill.) solid wood panels were chosen: they find wide application in furniture and for interior applications [7]. Largely used for its aspect, Sweet Chestnut has other attractive characteristics, such as an average wood shrinkage–swelling behaviour and it is easy to glue [8]. However it is very sensitive to hygrothermal variations and on the panels the wood surface deformations are more dramatic than on other kind of panels such as plywood. Therefore hygrothermal tests were performed on the coated material.
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R. Bongiovanni et al. / Progress in Organic Coatings 45 (2002) 359–363
2. Experimental 2.1. Materials An acrylic UV-curable resin was used, BisphenolAdihydroxyethyldiacrylate (BHEDA), kindly supplied by UCB, Belgium. A series of blends were prepared by adding the following acrylates: C6 F13 (CH2 )2 OCOCH = CH2
(additive F1)
C6 F13 CH2 CF2 (CH2 )3 OCOCH = CH2
(additive F2)
C6 F13 (CH2 )3 S(CH2 )2 OCOCH = CH2
(additive F3)
Additive F1 was purchased from Daikin (Japan) and used as received. Additives F2 and F3 were synthesised according to a procedure reported elsewhere [9]. The concentration of the fluorinated acrylates was varied from 0.01 up to 1% (w/w). 2,2-Dimethoxy-2-phenylacetophenone (Ciba-Geigy) was added as photoinitiator (4%, w/w). The substrates employed were glass slides and wood panels. 12 cm×12 cm solid wood panels of Sweet Chestnut were chosen, glued with polyvinylacetate, obtained from the same piece, in order to reduce the variability of the behaviour in different humidity conditions. 2.2. Films formation The films were obtained by applying the resin on the solid substrates (wood panels and glass slides) with a calibrated wire-wound applicator to obtain a thickness of about 100 m. The resins were cured under N2 atmosphere (O2 content < 20 ppm) by UV light with a 500 W medium pressure Hg lamp (light intensity = 12 mW cm−2 on the film surface), equipped with a water jacket for IR radiation screening and a camera shutter to control the UV exposure time. The irradiation was stopped when a constant double bond conversion was found, as determined by FTIR measurements (absorption band at 1640 cm−1 ). The experimental conditions assured the complete crosslinking of the polymeric films, their high insolubility in water and in organic solvents. The films prepared on glass slides could be peeled away from the substrate for the characterisation; the surface in contact with the glass was labelled as the glass side, the other one as the air side. 2.3. Films characterisation Differential scanning calorimetry analyses were performed by means of a Mettler DSC 30 Instrument, calibrated with indium. First the sample was cooled down to −150 ◦ C and was immediately heated at a heating rate of
10–150 ◦ C/min. The Tg was determined as the middle point in the transition. The gel content was determined by measuring the weight loss of the sample after extraction with CHCl3 for 16 h at room temperature. Contact angle measurements were made by means of a Kruss G1 goniometer, in air at room temperature (20 ◦ C) with doubly distilled water. The sessile drop technique was applied (10). The adhesion test were performed according to the cross-cut method (ASTM D 3359). The film hardness was evaluated according to the standard test ASTM D 3363. The chemical resistance to reagents and stains was assessed according to the standard practice described in ASTM D 3023. In order to assess the behaviour of the samples under hygrothermal stresses, the coated panels were put for 1 month at 20 ◦ C and 100% UR in order to reach the so called “fiber saturation point” (FSP), corresponding to the maximum swelling of wood and, for chestnut, to a mean moisture content of 29% (w/w). The panels then stayed for 1 month in an oven at 55 ◦ C, reaching a mean moisture content of 3.5% (w/w). The image analysis of the scanned surfaces of the panels was performed before and after the treatments. The painted faces of the samples were scanned on a normal HP scanner and on the images the detached surfaces were measured through the Philip’s Analysis software. At a microscopic level, the coatings and the wood surfaces, as well as their interface were observed by means of a scanning electron microscope (Philips XL 20).
3. Results and discussion 3.1. Properties of the wood coatings Different coatings were obtained by introducing into the BHEDA resin the fluorinated monomers under investigation at a concentration ranging from 0.01 up to around 1% (w/w). In these conditions clear transparent films were always obtained. For the copolymeric coatings the gel percentage was always higher than the pure BHEDA for which the insoluble fraction was about 96%. The glass transition temperature Tg was 70◦ for all the cured films, equal to that of the pure resin: therefore the fluorinated comonomers, due to their low concentration, do not affect the glass transition of the coatings. On the contrary, these products could dramatically chang the surface of the films as shown in Figs. 1–3. The data concern the surface analysis of coatings applied on glass slides, from which they could be easily detached. The advancing contact angle of the pure acrylic resin with water was around 70◦ on the air side and 50◦ on the substrate side. The hysteresis, i.e. the difference between the advancing and the receding contact angle, evaluated on the air side was around 34◦ . As shown in the figures, the fluorinated monomers reduced the wettability of the UV-cured films as far as the air
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361
Fig. 4. Wettability with hexadecane: effect of additive F2. Fig. 1. Effect of additive F1 on the wettability of cured films.
Fig. 2. Effect of additive F2 on the wettability of the cured films.
Fig. 3. Effect of additive F3 on the wettability of the cured films.
side is concerned. On the glass side of the films the contact angle data were always found corresponding to the value obtained by using the pure hydrogenated resin. It is evident that the wettability of the films (air side) depends on the fluorinated monomer concentration: the advancing contact angle increases by increasing the amount of the fluorinated acrylate. The films become highly hydrophobic while the dependence on the concentration is asymptotic. The maximum values obtained by the different monomers are very high and approaches those of fully fluorinated surface [10]. From a practical point of view it is interesting that the concentration of the fluorinated monomer at which the hydrophobicity is reached (critical concentration) is always extremely low, e.g. below 0.2% (w/w) for the F2 monomer.
Similar trends were found for the films coated on the wood panels. In this case it was not possible to peel them off and examine the substrate side. However it is feasible to assume that the selective modification of the air side takes place also with the wood substrate as suggested in previous works [11,12] examining the substrate effect on the surface modification of films. Comparing the data of the different additives, which contain the same fluorinated chain C6 F13 , it is clear that the surface properties also depend on the complex monomer structure as already shown and discussed in [13]. The surface characteristics achieved indicate that there is a change in the surface composition of the external layers of the films and that the surface segregation of the low surface energy components (the fluorinated monomers) takes place at the air interface. Therefore the surface concentration of the fluorinated monomers is very high: this was assessed by XPS analyses performed on similar systems [13,14]. The change in surface composition, i.e. the presence of fluorine in the very surface of the films affects the interaction of the coatings with organic media. Hexadecane was used as an example: the films showed a high oleophobicity, as reported in Fig. 4 for the additive F2. Similar results were obtained using the other additives. Because of the surface enrichment of the fluorinated additives, the chemical resistance of the films was expected to improve. This was confirmed by the results of the ASTM D 3023 test, collected in Table 1. The data show that the coatings having a fluorinated surface have a better gloss than the unmodified film. Moreover the gloss is better maintained when the fluorinated monomers are present, showing that the chemical resistance is improved. Notwithstanding their low concentration, the additives can protect the coatings from aggressive solvents. Only in the case of chloroform there is a reduction of the gloss: however it is less relevant for the fluorinated systems than for the acrylic resin.
Table 1 Chemical resistance of the coatings: 45◦ gloss before and after treatment Solvent
Reference resin (BHEDA)
Film containing F1 (1%, w/w)
Film containing F2 (1%, w/w)
Film containing F3 (1%, w/w)
– Water Ammonia HCl (conc.) Chloroform
100 99 96 95 77
105 104 104 104 99
105 104 104 104 97
104 104 103 102 97
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Table 2 Films hardness by pencil test
Table 3 Cross-cut adhesion of the films on wood panels
Coating
Hardness
Coating
Adhesion (%)
Reference resin (BHEDA) Film containing F1 Film containing F2 Film containing F2
7B HB B HB
Reference resin Film containing F1 Film containing F2 Film containing F3
93 93 94 93
As far as the hardness of the coatings is concerned, it was evaluated by means of the ASTM D 3363 test (pencil test). The results are reported in Table 2. According to the scale of hardness of the calibrated wood pencils, the hydrogenated film is the easiest to damage. The coatings with the fluoroadditives show similar resistance. 3.2. Adhesion of the coatings on wood and effect of the hygrothermal treatments It is well known that fluorinated coatings show poor adhesion due to the very low surface tension developed by those compounds. In our case the fluorinated monomers migrate selectively to the air side, while the side in contact with the substrate is practically free of fluorine. Therefore we expected to have a reasonable adhesion on the wood panels. The adhesion was assessed immediately after the curing of the resin, according to the ASTM D 3359 method (cross-cut adhesion). The results are reported in Table 3. The data show a good adhesion of the acrylic resin, for which the percentage of detachment is very low; moreover the modified systems show the same behaviour. The adhesion behaviour of the coating was assessed also after a severe hygrothermal treatment of the samples. The
Table 4 Cross-cut adhesion of the films on wood panels Detached surface (%) Reference resin Film containing F1 Film containing F2
7.2 6.9 4.1
goal of this treatment was to subject the coated panels to the highest movements of the wood structure (swelling and shrinkage) in order to evaluate the mutual behaviour of the substrate and the coating. After the treatment, the degree of detachment of the coatings was measured by means of image analysis. The results are collected in Table 4 and show that the adhesion is retained through the severe ageing treatment to which the panels were subjected. Therefore the coatings performance is satisfying. On the panels a microscopic analysis was done. The SEM micrographs show that the surface of the commercial coating is more fissured than that of the modified coatings. In all the cases there is a good interface between wood and coating (Fig. 5) and the surface of wood is perfectly “copied” by the inner layer of the resin causing at least a mechanical interlocking of the two compounds (Fig. 6). This result is
Fig. 5. SEM micrograph of the wood panel coated by the film monomer F2.
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363
Fig. 6. SEM micrograph of the wood panel coated by the film containing monomer F3.
interesting, especially considering that it is rather unusual among acrylic resins coated on wood. 4. Conclusions The addition of fluorinated monomers in the formulation of UV-curable wood coatings permitted the modification of the surface properties of the films, while the bulk remained unchanged. This fact can be attributed to the migration of the fluorinated monomers to the air side surface. As a consequence the coatings applied to wood panels showed high hydrophobicity and oleophobicity, an improved resistance to chemicals and a better scratch resistance. The surface segregation was found selectively limited to the air side of the coatings, thus the adhesion on the wood panel was satisfactory and maintained even after a severe hygrothermal treatment. References [1] S.P. Pappas (Ed.), Radiation Curing, Science and Technology, Plenum Press, New York, 1992, Chapter 1. [2] J.P. Fouassier, J.F. Rabek, Radiation Curing in Polymer Science and Technology, vols. 1–4, Elsevier, London, 1993.
[3] B.G. Willoughby, R.E. Banks, in: D. Bloor, R.J. Brook, M.C. Flemings, S. Mahajan (Eds.), Encyclopedia of Advanced Materials, Pergamon Press, Oxford, 1994. [4] R. Thomas, in: G. Hougham, K. Johns, P.E. Cassidy, T. Davidson (Eds.), Fluoropolymers 2: Properties, Plenum Press, New York, 1999, Chapter 4. [5] B. Ameduri, R. Bongiovanni, G. Malucelli, A. Pollicino, A. Priola, J. Polym. Sci. A 37 (1999) 77. [6] R. Bongiovanni, A. Pollicino, G. Gozzelino, G. Malucelli, A. Priola, B. Ameduri, Polym. Adv. Technol. 7 (1996) 403. [7] S. Berti, Researches on the Use of Secondary Quality Wood in Italy: Production of Solid Wood Panels, China Agricultural Scientech Press, 1998, pp. 583–588. [8] S. Berti, Utilizing secondary quality of wood: manufacturing and testing chestnut solid wood panels, Actes: Technologies de transformation et de valorisation des bois de qualité secondaire, Cluny, Décembre 7–9, 1994. [9] B. Ameduri, R. Bongiovanni, F. Montefusco, A. Priola, in preparation. [10] S. Wu, Polymer Interface and Adhesion, Marcel Dekker, New York, 1982. [11] R. Bongiovanni, G. Malucelli, A. Priola, C. Tonelli, G. Simeone, A. Pollicino, Macromol. Chem. Phys. 199 (1998) 1099. [12] R. Bongiovanni, A. Priola, M. van der Grinten, A.S. Clough, T.E. Shearmur, J. Colloid Interf. Sci. 182 (1996) 511. [13] B. Ameduri, R. Bongiovanni, V. Lombardi, A. Pollicino, A. Priola, A. Recca, J. Polym. Sci. A 39 (2001) 4227. [14] R. Bongiovanni, G. Beamson, A. Mamo, A. Priola, A. Recca, C. Tonelli, Polymer 41 (2000) 409.
High performance UV-cured coatings for wood protection R. Bongiovanni a,∗ , F. Montefusco a , A. Priola a , N. Macchioni b , S. Lazzeri b , L. Sozzi b , B. Ameduri c a
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, c.so Duca degli Abruzzi, 24 10129 Torino, Italy b Istituto per la Ricerca sul Legno—CNR, Via Barazzuoli, 23 50136 Firenze, Italy c UMR 5076 CNRS 8, rue Ecole Normale, F-34296 Montpellier Cedex, France Received 25 February 2002; accepted 15 June 2002
Abstract UV-curable systems based on the copolymerisation of a typical acrylic resin with a low amount of a fluorinated monomer (<1%, w/w) were used for the protection of wood panels. In the presence of the additives, the bulk properties and the adhesion of the acrylic films were unchanged, while a strong modification of the surface was obtained. The quality aspects and the chemical resistance of the coatings applied to the wood panels were also enhanced. © 2002 Elsevier Science B.V. All rights reserved. Keywords: UV-curing; Fluorinated monomers; Surface properties; Wood coating
1. Introduction The UV-curing technique is based on the polymerisation of a multifunctional system induced by an incident UV radiation to obtain a three-dimensional network. The reaction allows to transform at room temperature in a fraction of a second a liquid system into a solid having rubbery or glassy properties. It has been widely applied in many industrial fields for the manufacturing, the decoration and the protection of different materials, including wood [1]. The main advantages of the technique are the following [2]: • • • •
the energy needed for the process is low; the polymerisation is very fast and highly efficient; the cure is selectively limited to the irradiated area; no solvent is required (100% solid formulation), therefore the environmental pollution by VOC is avoided.
In addition to these important features, this technology can satisfy new requirements for traditional or advanced applications, as it can offer a broad range of final properties, changing the formulation and the curing conditions. The general chemical formulation contains a photoinitiator, reactive monomers and functionalised oligomers, namely un∗ Corresponding author. E-mail address: [email protected] (R. Bongiovanni).
saturated polyesters, tiol-ene compounds, acrylic and epoxy resins. In order to obtain high performance advanced coatings, particular structures have to be introduced in the polymeric network, e.g. fluorinated groups. In fact peculiar characteristics are given by the presence of fluorine [3,4]: hydrophobicity, chemical stability and weathering resistance, good release properties and low coefficient of friction, water permeability, refractive index. The cost of these products is usually very high and depends on the fluorinated structures content. In previous investigations interesting results were obtained by copolymerising low amounts of fluorinated monomers or oligomers with a typical UV-curable resin [5,6]. In this work we applied these fluorinated coatings on a wood substrate and studied their properties, such as chemical resistance, wettability, hardness and adhesion. Sweet Chestnut (Castanea sativa Mill.) solid wood panels were chosen: they find wide application in furniture and for interior applications [7]. Largely used for its aspect, Sweet Chestnut has other attractive characteristics, such as an average wood shrinkage–swelling behaviour and it is easy to glue [8]. However it is very sensitive to hygrothermal variations and on the panels the wood surface deformations are more dramatic than on other kind of panels such as plywood. Therefore hygrothermal tests were performed on the coated material.
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 1 1 9 - 4
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R. Bongiovanni et al. / Progress in Organic Coatings 45 (2002) 359–363
2. Experimental 2.1. Materials An acrylic UV-curable resin was used, BisphenolAdihydroxyethyldiacrylate (BHEDA), kindly supplied by UCB, Belgium. A series of blends were prepared by adding the following acrylates: C6 F13 (CH2 )2 OCOCH = CH2
(additive F1)
C6 F13 CH2 CF2 (CH2 )3 OCOCH = CH2
(additive F2)
C6 F13 (CH2 )3 S(CH2 )2 OCOCH = CH2
(additive F3)
Additive F1 was purchased from Daikin (Japan) and used as received. Additives F2 and F3 were synthesised according to a procedure reported elsewhere [9]. The concentration of the fluorinated acrylates was varied from 0.01 up to 1% (w/w). 2,2-Dimethoxy-2-phenylacetophenone (Ciba-Geigy) was added as photoinitiator (4%, w/w). The substrates employed were glass slides and wood panels. 12 cm×12 cm solid wood panels of Sweet Chestnut were chosen, glued with polyvinylacetate, obtained from the same piece, in order to reduce the variability of the behaviour in different humidity conditions. 2.2. Films formation The films were obtained by applying the resin on the solid substrates (wood panels and glass slides) with a calibrated wire-wound applicator to obtain a thickness of about 100 m. The resins were cured under N2 atmosphere (O2 content < 20 ppm) by UV light with a 500 W medium pressure Hg lamp (light intensity = 12 mW cm−2 on the film surface), equipped with a water jacket for IR radiation screening and a camera shutter to control the UV exposure time. The irradiation was stopped when a constant double bond conversion was found, as determined by FTIR measurements (absorption band at 1640 cm−1 ). The experimental conditions assured the complete crosslinking of the polymeric films, their high insolubility in water and in organic solvents. The films prepared on glass slides could be peeled away from the substrate for the characterisation; the surface in contact with the glass was labelled as the glass side, the other one as the air side. 2.3. Films characterisation Differential scanning calorimetry analyses were performed by means of a Mettler DSC 30 Instrument, calibrated with indium. First the sample was cooled down to −150 ◦ C and was immediately heated at a heating rate of
10–150 ◦ C/min. The Tg was determined as the middle point in the transition. The gel content was determined by measuring the weight loss of the sample after extraction with CHCl3 for 16 h at room temperature. Contact angle measurements were made by means of a Kruss G1 goniometer, in air at room temperature (20 ◦ C) with doubly distilled water. The sessile drop technique was applied (10). The adhesion test were performed according to the cross-cut method (ASTM D 3359). The film hardness was evaluated according to the standard test ASTM D 3363. The chemical resistance to reagents and stains was assessed according to the standard practice described in ASTM D 3023. In order to assess the behaviour of the samples under hygrothermal stresses, the coated panels were put for 1 month at 20 ◦ C and 100% UR in order to reach the so called “fiber saturation point” (FSP), corresponding to the maximum swelling of wood and, for chestnut, to a mean moisture content of 29% (w/w). The panels then stayed for 1 month in an oven at 55 ◦ C, reaching a mean moisture content of 3.5% (w/w). The image analysis of the scanned surfaces of the panels was performed before and after the treatments. The painted faces of the samples were scanned on a normal HP scanner and on the images the detached surfaces were measured through the Philip’s Analysis software. At a microscopic level, the coatings and the wood surfaces, as well as their interface were observed by means of a scanning electron microscope (Philips XL 20).
3. Results and discussion 3.1. Properties of the wood coatings Different coatings were obtained by introducing into the BHEDA resin the fluorinated monomers under investigation at a concentration ranging from 0.01 up to around 1% (w/w). In these conditions clear transparent films were always obtained. For the copolymeric coatings the gel percentage was always higher than the pure BHEDA for which the insoluble fraction was about 96%. The glass transition temperature Tg was 70◦ for all the cured films, equal to that of the pure resin: therefore the fluorinated comonomers, due to their low concentration, do not affect the glass transition of the coatings. On the contrary, these products could dramatically chang the surface of the films as shown in Figs. 1–3. The data concern the surface analysis of coatings applied on glass slides, from which they could be easily detached. The advancing contact angle of the pure acrylic resin with water was around 70◦ on the air side and 50◦ on the substrate side. The hysteresis, i.e. the difference between the advancing and the receding contact angle, evaluated on the air side was around 34◦ . As shown in the figures, the fluorinated monomers reduced the wettability of the UV-cured films as far as the air
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Fig. 4. Wettability with hexadecane: effect of additive F2. Fig. 1. Effect of additive F1 on the wettability of cured films.
Fig. 2. Effect of additive F2 on the wettability of the cured films.
Fig. 3. Effect of additive F3 on the wettability of the cured films.
side is concerned. On the glass side of the films the contact angle data were always found corresponding to the value obtained by using the pure hydrogenated resin. It is evident that the wettability of the films (air side) depends on the fluorinated monomer concentration: the advancing contact angle increases by increasing the amount of the fluorinated acrylate. The films become highly hydrophobic while the dependence on the concentration is asymptotic. The maximum values obtained by the different monomers are very high and approaches those of fully fluorinated surface [10]. From a practical point of view it is interesting that the concentration of the fluorinated monomer at which the hydrophobicity is reached (critical concentration) is always extremely low, e.g. below 0.2% (w/w) for the F2 monomer.
Similar trends were found for the films coated on the wood panels. In this case it was not possible to peel them off and examine the substrate side. However it is feasible to assume that the selective modification of the air side takes place also with the wood substrate as suggested in previous works [11,12] examining the substrate effect on the surface modification of films. Comparing the data of the different additives, which contain the same fluorinated chain C6 F13 , it is clear that the surface properties also depend on the complex monomer structure as already shown and discussed in [13]. The surface characteristics achieved indicate that there is a change in the surface composition of the external layers of the films and that the surface segregation of the low surface energy components (the fluorinated monomers) takes place at the air interface. Therefore the surface concentration of the fluorinated monomers is very high: this was assessed by XPS analyses performed on similar systems [13,14]. The change in surface composition, i.e. the presence of fluorine in the very surface of the films affects the interaction of the coatings with organic media. Hexadecane was used as an example: the films showed a high oleophobicity, as reported in Fig. 4 for the additive F2. Similar results were obtained using the other additives. Because of the surface enrichment of the fluorinated additives, the chemical resistance of the films was expected to improve. This was confirmed by the results of the ASTM D 3023 test, collected in Table 1. The data show that the coatings having a fluorinated surface have a better gloss than the unmodified film. Moreover the gloss is better maintained when the fluorinated monomers are present, showing that the chemical resistance is improved. Notwithstanding their low concentration, the additives can protect the coatings from aggressive solvents. Only in the case of chloroform there is a reduction of the gloss: however it is less relevant for the fluorinated systems than for the acrylic resin.
Table 1 Chemical resistance of the coatings: 45◦ gloss before and after treatment Solvent
Reference resin (BHEDA)
Film containing F1 (1%, w/w)
Film containing F2 (1%, w/w)
Film containing F3 (1%, w/w)
– Water Ammonia HCl (conc.) Chloroform
100 99 96 95 77
105 104 104 104 99
105 104 104 104 97
104 104 103 102 97
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Table 2 Films hardness by pencil test
Table 3 Cross-cut adhesion of the films on wood panels
Coating
Hardness
Coating
Adhesion (%)
Reference resin (BHEDA) Film containing F1 Film containing F2 Film containing F2
7B HB B HB
Reference resin Film containing F1 Film containing F2 Film containing F3
93 93 94 93
As far as the hardness of the coatings is concerned, it was evaluated by means of the ASTM D 3363 test (pencil test). The results are reported in Table 2. According to the scale of hardness of the calibrated wood pencils, the hydrogenated film is the easiest to damage. The coatings with the fluoroadditives show similar resistance. 3.2. Adhesion of the coatings on wood and effect of the hygrothermal treatments It is well known that fluorinated coatings show poor adhesion due to the very low surface tension developed by those compounds. In our case the fluorinated monomers migrate selectively to the air side, while the side in contact with the substrate is practically free of fluorine. Therefore we expected to have a reasonable adhesion on the wood panels. The adhesion was assessed immediately after the curing of the resin, according to the ASTM D 3359 method (cross-cut adhesion). The results are reported in Table 3. The data show a good adhesion of the acrylic resin, for which the percentage of detachment is very low; moreover the modified systems show the same behaviour. The adhesion behaviour of the coating was assessed also after a severe hygrothermal treatment of the samples. The
Table 4 Cross-cut adhesion of the films on wood panels Detached surface (%) Reference resin Film containing F1 Film containing F2
7.2 6.9 4.1
goal of this treatment was to subject the coated panels to the highest movements of the wood structure (swelling and shrinkage) in order to evaluate the mutual behaviour of the substrate and the coating. After the treatment, the degree of detachment of the coatings was measured by means of image analysis. The results are collected in Table 4 and show that the adhesion is retained through the severe ageing treatment to which the panels were subjected. Therefore the coatings performance is satisfying. On the panels a microscopic analysis was done. The SEM micrographs show that the surface of the commercial coating is more fissured than that of the modified coatings. In all the cases there is a good interface between wood and coating (Fig. 5) and the surface of wood is perfectly “copied” by the inner layer of the resin causing at least a mechanical interlocking of the two compounds (Fig. 6). This result is
Fig. 5. SEM micrograph of the wood panel coated by the film monomer F2.
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363
Fig. 6. SEM micrograph of the wood panel coated by the film containing monomer F3.
interesting, especially considering that it is rather unusual among acrylic resins coated on wood. 4. Conclusions The addition of fluorinated monomers in the formulation of UV-curable wood coatings permitted the modification of the surface properties of the films, while the bulk remained unchanged. This fact can be attributed to the migration of the fluorinated monomers to the air side surface. As a consequence the coatings applied to wood panels showed high hydrophobicity and oleophobicity, an improved resistance to chemicals and a better scratch resistance. The surface segregation was found selectively limited to the air side of the coatings, thus the adhesion on the wood panel was satisfactory and maintained even after a severe hygrothermal treatment. References [1] S.P. Pappas (Ed.), Radiation Curing, Science and Technology, Plenum Press, New York, 1992, Chapter 1. [2] J.P. Fouassier, J.F. Rabek, Radiation Curing in Polymer Science and Technology, vols. 1–4, Elsevier, London, 1993.
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