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Determination of the effective diffusion coefficient during the drying of paint and varnish films applied on fir wood
Progress in Organic Coatings 137 (2019) 105344
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Determination of the effective diffusion coefficient during the drying of paint and varnish films applied on fir wood ⁎
⁎
A. Mihailaa, C. Lisaa, , A-M. Ipateb, M.F. Zaltariovb, D. Rusub, I. Mămăligăa, , G. Lisaa,
T
⁎
Gheorghe Asachi Technical University, Faculty of Chemical Engineering and Environmental Protection “Cristofor Simionescu”, 73 Prof.dr.doc. D. Mangeron Street, 700050, Iaşi, Romania b Petru Poni Institute of Macromolecular Chemistry, 41A Aleea Gr. Ghica Voda, 700487, Iasi, Romania a
A R T I C LE I N FO
A B S T R A C T
Keywords: Paint Drying process Effective diffusion coefficient SEM EDX ATR-FTIR
The development of the civil and industrial building sector lead to an increase in the demand of lacquers and paints. Wood is one of the long-lasting building materials, for which the demand for exterior landscaping and internal finishes is very high and ever-growing. The drying process is an essential step in obtaining a paint film which shall protect the wood. In this paper, we shall use for the first time the derivatograph to analyse the drying process, respectively, the forming process of the actual film on a series of commercial lacquers and paints deposited directly on fir wood support. Many kinetic drying models were tested. The effective diffusion coefficients and the duration of the drying process were calculated. The use of classical models from literature emphasized the difficulty of modelling the drying of certain paint films, due to their complex composition. The films obtained after the drying process were characterized.
1. Introduction The development of the civil and industrial engineering field, as well as the growing involvement of the population in the refurbishment and decoration of living quarters and furniture has led to higher demand for paints and varnishes. Wood is one of the most durable, aesthetic and non-polluting building materials and the demand for this material for exterior construction and interior finishes is very high and has been continuously growing. It is therefore no wonder that researchers have been concerned to design new surface film formulations (varnishes and paints) to protect the wood that have increased resistance to environmental factors. An important step in obtaining a paint film able to protect the wood is the drying process. As a result of this process a pellicle which strongly adheres to the surface of the wood is achieved, which is called film. Film formation occurs only as a result of the evaporation of the volatile components: solvents or thinners. Once the evaporation process completed, the binder molecules approach one another, a gel is formed and finally the actual film is formed. Since environmental legislation has as a major requirement the reduction of volatile organic compounds (VOCs) released in the environment, specialists have attempted to switch from solvent-based paints to water-based paints [1]. However, since solvent-based paints have a number of advantages, such as: easy application features and better drying tolerance in low temperature and high moisture ⁎
conditions, they still are much more frequently used than water-based ones [1]. The equipment used in thermogravimetric analysis is based on a high degree of precision in mass change and temperature measurement and they therefore have been extensively used in the analysis and modeling of the drying process in the food industry [2–5]. A device such as A Q50 (TA Instruments) and an innovative optical method based on the principle of multi-speckle diffusive wave spectroscopy (MSDWS) have been used by Giraud et al. [6] to analyze the time required to form polydimethylsiloxane (PDMS) films and commercial paint based on an aqueous dispersion of acrylic copolymers. In this paper we will use the Mettler Toledo TGA-SDTA851e derivatograph for the first time to analyze the drying and actual filmforming process for a range of varnishes and paints available on the market, which contain water- and/or solvent-based fatty and medium alkyd resins and acrylic resins. For this purpose, a 0.13-0.31 mm thick coating of varnish or paint has been applied on dry fir wood disks 5 mm in diameter, which have been subjected to a drying process in constant 25 °C temperature conditions. Synthetic air at a flow rate of 20 ml/min was used as drying agent. The films achieved after the drying process, and also after their preservation in laboratory conditions for one year, were characterized by the following techniques: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), water contact angle measurements, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry
Corresponding authors. E-mail addresses: [email protected] (C. Lisa), [email protected] (I. Mămăligă), [email protected] (G. Lisa).
https://doi.org/10.1016/j.porgcoat.2019.105344 Received 17 April 2019; Received in revised form 17 August 2019; Accepted 17 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Quanta 200 microscope is equipped with an Energy Dispersive X-Ray system (EDX) for qualitative analysis which allows us to identify the elements in the samples. The EDX detector used is the Si detector - EDX silicon-drift detector enables rapid determination of elemental compositions and acquisition of compositional maps. Samples are imaged at 10 mm WD (working distance), which is the stage eucentric position and the collection point of the EDX detector. It is used in conjunction with the LFD detector - (Large Field Detector). SEM is a relatively simple technique which requiers minimum preparation of the samples and offers solid, realistic informations concerning the surface structure. Through this analyse method were investigated the apearance, integrity, quality and paint films adhesion degree at the surface of fir wood samples.
(DSC). Thus, information was gathered regarding the influence of the type of resin and solvent used on the drying process and on the quality of the resulting paint films. 2. Materials and characterization methods 2.1. Materials In this study were analyzed three types of varnishing products which are available in market. Paint was applied in one thick layer on dry fir tree wood disks of 5 mm. The three types of paints considered are: a water-based ecolasure, an acrylic impregnant, and a protective lake. Water-based ecolasure (ECM) is a thick coating product based on water containing pigments resistant at UV radiations, precipitations. This product can be successfully used inside and also outside to protect any type of wood structures. It offers resistance to biological factors such fungi, blast, mold, insects; and also against atmospheric factors. The product is characterized by a fast drying, waterproofing of the wood, contains a UV filter that protects against solar radiation. Acrylic impregnant (IAC) is a water-based impregnation primer with addition of fungicides and biocides. This type of product is recommended for any type of wood protection, wood of any essence which is exploated both inside and outside. In the case of resinous wood, use of this product is obligatory. The product penetrates deep in the wood fiber protecting it against blue stains and fungi, ensures microbiological protection against fungi, insects and molds, provides resistance at atmospheric factors and UV radiations. Also the product contributes to reducing of drying and swelling of wood. Wood damping is also allowed by using the impregnant in addition to optimize the adhesion between the substrate layer and the later applied finish. When the product is applied must be respected some conditions for ensuring the desired characteristics of the product like environment relative humidity of 70%, temperature between 10 and 30 °C and also is very important to mention that the product can not be applied of wood which is not dry. Protective lake (LPM) is a glaze type product which can be applied on any type of wood. It can be used for surface protection inside and also outside under high temperature and humidity conditions that can favor microbial attack on wood. The product penetrates deeper in wood fiber providing long-lasting resistance, provides increased wood protection against weathering and UV radiation.
2.2.3. Water contact angle measurements The surface properties of the paint layer were investigated by water contact angle measurements. Water contact angle measurement was done using a KRUSS goniometer. This system allows the recording and assessment of video images and the calculation of surface energies and of surface tension. Two measurements per sample were performed initial for dry varnish and paint films and after one year in the lab conditions (temperature of 23 ± 2 °C and humidity of 42 ± 15%) with distilled water drops with a dosing volume of 5 μL. We performed measurements every other two second for 30 s. The main equipment characteristics are: DSA1 software, EASYDROP, measuring range (contact angle 1 to 180°, surface tension 0.01–1000 mN/m), measuring resolution (contact angle 0.1°, surface tension: 0.01 mN / m, camera system 79 fps (656 × 492 px)). 2.2.4. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) IR spectra were registered using a Bruker Vertex 70 FTIR spectrometer (Bruker Optics, Ettlingen, Germany) equipped with a ZnSe crystal. The measurements were performed in ATR (Attenuated Total Reflectance) mode in the 4000 - 600 cm−1 spectral range at room temperature with a resolution of 4 cm-1 and accumulation of 32 scans. The 1800-600 cm-1 spectral region was deconvoluted by a curve-fitting method, and the areas were calculated with a 50% Lorentzian and 50% Gaussian function. The curve-fitting analysis was performed with the OPUS 6.5 software. The procedure led to a best fit of the original curve with an error of less than 0.002.
2.2. Characterization methods
2.2.5. Differential scanning calorimetry (DSC) The DSC curves for dry varnish and paint films were recorded with a Mettler Toledo DSC1 device in inert atmosphere with a heating rate of 10 °C/min. Scanning was performed in the -80 - 200 °C temperature range. The mass of samples encapsulated in aluminum pans with pierced lids to allow evaporation of the volatile components ranged between 3 and 5 mg. The evaluation of the thermogravimetric and DSC curves was performed using STARe software version 9 of MettlerToledo.
2.2.1. Drying process analysis The analysis of the drying and film forming process for a series of commercial varnishes and paints was performed with the Mettler Toledo TGA-SDTA851e derivatograph. This equipment allows a weighing accuracy of less than 1 μg and a temperature control accuracy of 0.01 °C. Dry fir wood disks 5 mm in diameter were used, on which a 0.13-0.31 mm thick varnish or paint coating was applied, which was left to dry in constant 25 °C temperature conditions, using synthetic air at a flow rate of 20 ml/min as drying agent. The obtained curves were processed using the Mettler Toledo STARe software.
3. Results and discussions
2.2.2. Scanning electron microscopy (SEM) and energy dispersive X-Ray analysis (EDX) SEM measurements were used in order to investigate the appearance and the structural integrity of the treated fir wood samples and also the adhesion between the exterior coating and the wood surface. The surface morphology of the samples was examined by using a Scanning Electron Microscope (SEM) type Quanta 200 (FEI), operating at 20 kV with secondary electrons in low vacuum mode, the magnification being indicated on each micrograph. The SEM studies were performed on uncoated samples fixed on aluminium supports. The
3.1. Drying process analysis Thermogravimetric analysis in isothermal conditions, i.e. 25 °C, allowed using thermogravimetric curves to draw the drying curves (material moisture – drying time) in Fig. 1. Material moisture (MR) [7] was calculated with Eq. (1) in which M0 is the initial content of solvent in the applied paint coating, Me is the solvent content in equilibrium conditions determined from the drying curves at the time the sample mass remains unchanged with time, and M (t) is the solvent content of the sample at any time t. 2
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Fig. 1. Experimental and calculated values for model 4 for MR.
MR =
M(t) − Me M0 − Me
et al. [11] found that when a 25% mass ratio of tackifying resin (TR) was introduced into water-based acrylic latex to improve the adhesion of the resulting film, the drying mechanism was found to change, i.e. the “skin effect”, which lengthened the drying time. The shape of the curves shown in Fig. 2 shows that in the case of IAC water-based acrylic stain and ECM water-based fatty ecolasure containing modified alkyd/ acrylic resins, the moisture migration rate to the surface is higher than the surface evaporation rate. In the case of LPM protective varnish, the evaporation process and external solvent vapor diffusion through the outer boundary coating are rate-dependent. The total drying time, i.e. the time of removal of the volatile components: solvents or thinners, was determined using Eqs. 3–5, based on the drying rate curves shown in Fig. 2:
(1)
MR = f (time) experimental data were processed using the Sigma Plot 11.2 software to check some kinetic drying models in literature [8,9]. These are semi-theoretical models that derive from Fick's second law of diffusion and have been used so far mainly in thin layer drying of foods. The Henderson and Pabis model contains a single exponential term (MR = a ∗ exp(−kt ) ). The logarithmic model includes the logarithmic form of Henderson and Pabis model and an empirical constant (MR = c + a ∗ exp(−kt ) ). The two term model contains two exponential terms (MR = a ∗ exp(−k1 t ) + b ∗ exp(−k2 t ) ), and the modified Midilli and Kucuk model proposed in this paper contains an exponential term, a linear term and an empirical constant (MR = c + a ∗ exp(−kt ) + b ∗ t ). The results obtained are shown in table S1 Supporting Information along with the performance of the models, namely the correlation coefficient (r2) and standard deviation (σ):
∑ i=1
tI =
mi ⋅(ui − ucr ) A⋅wmax
tII =
mi ⋅ A u
(4)
uf
k
σ=
(3)
t = tI + tII
[(MRE )
exp
−
(MRE )calc ]2
/(n − p) (2)
∫ du w cr
(5)
where tI is the constant rate drying time, tII is the decreasing rate drying time, mi initial mass of the paint coating, A is the fir wood disk area, wmax is the maximum drying rate, ui is the initial moisture content, ucr is the moisture content at the beginning of the decreasing rate drying stage and uf is the moisture content at the end of the second drying stage. The results achieved are shown in Table 1. Please note that for the ECM sample, both linear decreasing rate drying and non-linear decreasing rate drying were taken into account when calculating the second stage. When analyzing the results obtained, one may notice that they are comparable to those obtained by Giraud et al. [6] for commercial water-based acrylic resin paints. Considering the analytical solution of Fick's equation, for different geometric shapes [12], i.e. for Cranck's "thin plate" [13], and assuming that moisture transport takes place by diffusion and that temperature and diffusion coefficients are constant, the effective diffusion coefficients (Deff) may be determined based on the following dependence for long solvent removal times:
where n is the number of experimental data, and p is the number of parameters. The kinetic drying models tested and shown in table S1 Supporting Information and the drying curves (material moisture – drying time) in Fig. 1 prove that the best results for ECM, IAC and LPM coatings are obtained with model 4 described in this paper (Modified Midilli and Kucuk). Kucuk et al. [10] states in an extensive study in literature that this type of model yields the best results for thin coating drying processes. Fig. 2 shows the variation of the drying rate - w (kg moisture/m2⋅s) depending on moisture - u (kg moisture/kg initial substance) for ECM, IAC and LPM. The two drying stages are highlighted: I - with constant evaporation rate and II - with decreasing evaporation rate. Higher constant drying rates are found for water-based acrylic stain - IAC and water-based fatty ecolasure containing modified alkyd/acrylic resins ECM. As concerns the LPM protecting coating containing alkyd resins, both the moisture content u and the drying rate are lower. As for ECM, one may notice an acceleration of the drying process with a non-linear descending rate compared to the other samples. One possible explanation would be that after a drying stage at linear descending rate, a “skin effect” film is formed which prevents solvent diffusion. This effect has been noted by other researchers as well. For instance, Mallégol
ln ⎛ ⎝ ⎜
π2MR ⎞ D = −π2 eff t 8 ⎠ δ2 ⎟
(6)
where δ is the thickness of the varnish or paint coating on the wooden disk. According to the graphical representation, 3
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Fig. 2. Moisture-dependant drying rate.
- the second stage, comprising zones II and III in Figure S1 Supporting Information, where the process is influenced by both film diffusion coefficient and film thickness, respectively. In the case of drying films with a low solvent content, the first stage may be missing. As, according to Figs. 1 and 2, the drying time at constant rate (first stage) is insignificant, reference will be made to the second drying stage. When dissolving a solvent in a polymer film during the second drying stage, solvent transfer into the film is described by Fick's second law. Considering that film thickness, diffusion coefficient and temperature remain constant the solution of the equation describing the variation of the solvent content over time is suggested by Crank [13]:
Table 1 Drying time. Sample
tI(s)
tII(s)
t(s)
ECM IAC LPM
119 292 242
7793 1660 4471
7912 1952 4713
Table 2 Effective diffusion coefficients. Sample
r2
δ⋅103(m)
Deff⋅1012 (m2/s)
ECM IAC LPM
0.9969 0.9962 0.9973
0.233 0.130 0.314
2.120 0.712 4.359
ln
( ) = f (t )straight π 2MR 8
MR =
2 Deff ×t 1/2 ⎛ ⎞ π ⎝ δ2 ⎠
∞
∑ n= 0
1 (2 n+ 1)2π2D t ⎤ exp ⎡− ⎥ ⎢ (2 n+ 1)2 δ2 ⎦ ⎣
(8)
MR – standardized residual solvent mass, M0 – initial solution mass, Meq Me – solution mass in equilibrium conditions, M(t)- solution mass at time t, δ - film thickness [m], t – time [s] and D – solvent-film diffusion coefficient [m2/s]. When the first three terms of the sequence are considered, it results from Eq. (8):
lines with negative slope and correlation
coefficients higher than 0.996 were obtained. Table 2 shows: the correlation coefficients (r2), the varnish or paint coating thickness on the fir wood disk (δ) and the effective diffusion coefficients (Deff) values. Other researchers have described in their studies the values of the effective diffusion coefficients determined for different paint systems [14–20]. They generally used the analytical solution of Fick's equation to determine the effective diffusion coefficients for Crank’s "thin plate" [13] for low solvent removal times:
MR =
M(t) − Me 8 = 2 M0 − Me π
MR =
2 2 M(t) − Me 8 1 1 −25D t(π/δ)2 ⎤ = 2 ⎡e−D t(π/δ) + e−9D t(π/δ) + e M0 − Me π ⎣ 9 25 ⎦
(9)
Fig. 3 (a–c) shows the non-dimensional residual moisture variation over time of the three systems analyzed. The experimental data are described compared to the simulated values based on the diffusion model (Eq. 9). The model uses diffusion coefficients and initial film thicknesses values which are identical to those shown in Table 2. According to Fig. 3 (a–c), there is a good correlation between the model and the experimental data in the case of IAC drying (maximum deviation ± 18%) and a satisfactory correlation on LPM drying (maximum deviation ± 40%), and on ECM drying (maximum deviation ± 68%). The cause of these deviations may be the complex composition of the paint film. It also contains a number of fillers with porous structure and the solvent used is not a pure component. Hence the difficulty of modeling the drying of paint films by using classical models in literature. Having analyzed the results achieved, we found that semi-
(7)
The use of Eq. (7) for the experimental data obtained in this paper, i.e. the determination of the MR = f(t1/2) dependencies, led to lower correlation coefficients than for Eq. (6). The physical model of drying a film laid out on a flat surface comprises two stages: - the first stage is represented by zone I in Figure S1 Supporting Information. This may be pointed out especially when films with high solvent content are drying. Solvent evaporation during this stage has the same behavior as pure solvent evaporation on a flat surface. The film diffusion coefficient and film thickness have no influence whatsoever. 4
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Fig. 3. Experimental and calculated values for model 9 for MR.
highlighted during the drying process.
theoretical models, which only take into account the diffusion through the film, generally offer a better match with the experimental data compared to the theoretical ones. However, the advantage of theoretical models is that they provide better understanding of the transport processes and take into account all the conditions of the process (dryer geometry, drying agent speed, moisture level/solvent content in the drying agent, etc.), not only diffusion through the film. Paint film drying is a complex process, so we can say that some of the values of the diffusion coefficient actually obtained are mean values and are valid only for the types of paints used in this study.
3.2.2. Energy dispersive X-ray (EDX) EDX is an X-ray technique that allows identification and characterization of the elemental composition of materials. EDX analysis was carried out on the surfaces of the samples to determine the elements in the paint films. Data were collected from three randomly chosen points by taking arithmetic mean of these values and average weight percent of the elements was calculated. Table S2 Supporting Information reports the data on different elements present in paint film which cover the fir wood samples. According to the EDX analysis, the common elements found in each film are carbon, oxygen, cobalt, aluminium, silicon, phosphorus, chlorine and iron (the percentage of C is between 51%–73 %; oxygen has the second highest quantity within range 20%–32%). In addition, ECM water-based fatty ecolasure contains sodium and calcium. This paint also contains S and I which may be due to the presence of biocides declared by the producer: 3-iodo-2-propynyl Butylcarbamate and 1,2 benzisothiazol-3(2 H)-one. The other IAC water-based paint also contains sodium and calcium. However, the presence of magnesium, titanium and potassium was only identified for the IAC acrylic stain. Topcuoglu et al. [21] identified carbon, oxygen, titanium, calcium, silica, aluminum, magnesium and sodium by using EDX analysis on films of water-based paints containing acrylic resins.
3.2. Characterization studies of paint films 3.2.1. Scanning electron microscope (SEM) In Figs. 4 are presented SEM micrographs of fir wood samples untreated/treated with film paints. We can observe that the cellulose micro-fibrils presented at the wood surface (blind sample) are fully/ completely covered with paint films. The morphological analysis of the surface indicated in the case of ECM and IAC the fact that pigments and fillers exhibit a relatively homogeneous distribution, an uniform aspect, the surface being covered by paint film – however, pigment distribution becomes nonhomogeneous in the case of LPM –some agglomerations zones of the pigment and fillers are present on the surface of wood. Figure S2 Supporting Information shows SEM micrographs for fir wood surfaces treated with the three types of paints containing acrylic/alkyd (ECM), acrylic (IAC) and alkyd (LPM) resins, after they have been preserved in laboratory conditions for one year. The results achieved do not indicate significant changes in the morphology of the analyzed surfaces except in the case of the ECM sample. In this case, we noted the occurrence of wrinkles that may have as a cause the "skin effect"
3.2.3. Water contact angle measurements The mean values of the contact angle measured for the paint and varnish films obtained after the drying process, but also after being kept in laboratory conditions for one year are shown in Table 3. We found that hydrophilic surfaces are obtained in the case of fat water-based 5
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Fig. 4. SEM micrographs for ECM (a), IAC (b), LPM (c) and Blank sample-uncovered fir wood (d) 2000x.
3.2.4. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) The comparative ATR-FTIR spectra of three commercial lacquers and paints films (ECM, IAC and LPM) are shown in Figure S3 Supporting Information. In the IR spectra of the ECM, IAC and LPM samples the characteristic bands assigned to the acrylic/alkyd resins can be observed: 1730 cm−1 specific for carbonyl ester group (C]O), 1708, 1702 and 1676 cm−1 specific for carboxylic groups of the acrylic monomers, 1650-1645 cm−1 specific for OH bond deformation vibrations region (Fig. 5) overlapped with C]C stretches vibrations of the aromatic ring [22]. The spectral region 1500-1300 cm-1 revealed the presence of different vibrations of the C–H in methyl/methylene groups: 1464 and 1458 cm-1 methyl asymmetrical C–H bending vibrations, 1444-1416 cm-1 methylene scissoring vibrations, 1386 – 1372 cm1 methyl symmetrical C–H bending vibrations, 1336 cm-1methylene wagging vibrations (Fig. 5) [23], while asymmetric and symmetric stretches of the methylene groups could be observed in the 30002800 cm−1 region. The overlapped and hidden peak positions of νC–H region of the samples ECM, IAC and LPM were determined with the second derivative of the spectra (Figure S4 Supporting Information). The C–O stretching region (1300 -1000 cm−1) shows a number of strong bands assigned to the C–O-C asymmetric stretching vibration (1192-1000 cm−1) and to the CeOeC symmetric stretching vibration (1268-1234 cm-1) (Fig. 6) [24–26]. The bands assigned to the C-Cl stretching (correlated with the biocide activity of the films) are observed at 728/742/754 cm−1, while those attributed to the pigment can be observed at 990-890 cm−1 (stretching vibrations of calcium phosphate), 844/784 cm−1 (stretching vibrations of calcium carbonate) and 1372/1382/1386 cm−1 (specific for kaolin) [27]. The bands at 702706 cm-1 are assigned to the C–H bending in aromatic ring and the broad spectral region at 600-700 cm−1 in the IR spectrum of IAC water-
Table 3 Contact angle values of paint and varnish films applied on fir wood. Sample
Water contact angle (°) The initial dry film
After one year in the lab
ECM
78
73
IAC
131
111
LPM
91
103
ecolasure containing modified alkyd/acrylic resins – ECM. The mean value of the contact angle is 78° and does not change significantly after the samples have been preserved in laboratory conditions for one year. For the water-based acrylic impregnant – IAC, the mean values obtained for the contact angle show the hydrophobic character of the paint film obtained on fir wood surface, which persists after the samples have been kept in laboratory conditions for one year. The varnish marked LPM containing alkyd resins has contact angle values higher than 90°. After the fir wood samples treated with this varnish have been kept in laboratory conditions for one year, we found that the mean contact angle value increases by 12°, improving the hydrophobicity of the surface. 6
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Fig. 5. Deconvoluted ATR-FTIR spectra of the ECM, IAC and LPM samples in the 1800-1300 cm−1 spectral regions.
based paint can be associated with the presence of the TiO2 pigment [28] which was also confirmed by EDX analysis. The fir wood samples treated with three types of paints: a waterbased ecolasure, an acrylic impregnant, and a protective varnish were kept in laboratory conditions for one year and the ATR-IR spectra shown in figure S5 Supporting Information were also recorded. There were no significant changes compared to the ATR-FTIR spectra shown in figure S3 Supporting Information. There was only a slight increase in the vibration of the specific OeH stretching bands in the 37003100 cm−1 region.
Table 4 DSC characteristics. Sample
Tg1(°C)
Tg2(°C)
ECM IAC LPM
−12.8 7.2 14.6
13.5 24.8 –
drying process were removed from the wooden disks by means of a stainless steel blade and subjected to DSC analysis. The glass transition temperatures (Tg) shown in Table 4 were obtained on the second heating curve. These indicate the fact that in the case of water-based fatty ecolasure containing modified alkyd/acrylic resins – ECM and water-based acrylic stain – IAC, two Tg values were highlighted. According to literature, the negative value of the first Tg in the case of ECM indicates the presence of the alkyd resin and the second one may correspond to the alkyd-acrylic copolymer [29,30]. As concerns the
3.2.5. Differential scanning calorimetry (DSC) The coating properties of the varnish or paint layer laid out on the fir wood disk depend on the integrity of the film obtained. Along with the surface characteristics of the substrate, the glass transition temperature (Tg) of the dry film also has a special importance in the evaluation of the properties. The paint and varnish films obtained after the
Fig. 6. Deconvoluted ATR-IR spectra of the ECM, IAC and LPM samples in the 1300 – 600 cm−1 spectral regions. 7
Progress in Organic Coatings 137 (2019) 105344
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References
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Fan, Nucleation of bubbles during drying of sprayed paint films, Prog. Org. Coat. 994 (2016) 452–462, https://doi.org/10. 1016/j.porgcoat.2016.07.002. [15] N. Saranjam, S. Chandra, Bubble growth and movement in drying paint films, Chem. Eng. Sci. 145 (2016) 149–161, https://doi.org/10.1016/j.ces.2016.02.022. [16] C. Perez, A. Collazo, M. Izquierdo, P. Merino, X.R. Novoa, Characterisation of the barrier properties of different paint systems Part I. Experimental set-up and ideal Fickian diffusion, Prog. Org. Coat. 36 (1999) 102–108, https://doi.org/10.1016/ S0300-9440(99)00030-2. [17] L.A. Ramirez, H.M. Liljestrand, R.L. Corsi, Fate of a coalescing aid after latex paint application, J. Coat. Technol. Res. 7 (2010) 291–300, https://doi.org/10.1007/ s11998-009-9194-0. [18] M.P.M.C. Rodrigues, M.R.N. Costa, A.M. Mendes, M.I.E. Marques, Effectiveness of surface coatings to protect reinforced concrete in marine environments, Mater. Struct. 33 (2000) 618–626, https://doi.org/10.1007/BF02480601. [19] S.A. Altinkaya, O. Topcuoglu, Y. Yurekli, D. Balkose, The influence of binder content on the water transport properties of waterborne acrylic paints, Prog. Org. Coat. 69 (2010) 417–425, https://doi.org/10.1016/j.porgcoat.2010.08.005. [20] C.C. Negoescu, N. Bunduc, E.T.I. Tudose, I. Mamaliga, Aspects on polymer-solvent equilibrium and diffusion in polymeric membranes, Environ. Eng. Manag. J. 12 (2013) 1583–1591, https://doi.org/10.30638/eemj.2013.193. [21] O. Topcuoglu, S.A. Altinkaya, D. Balkose, Characterization of waterborne acrylic based paint films and measurement of their water vapor permeabilities, Prog. Org. Coat. 56 (2006) 269–278, https://doi.org/10.1016/j.porgcoat.2006.02.003. [22] M.S. Metsik, T.I. Shishelova, G.F. Golovko, Z.N. Epifantseva, Region of OH deformation vibrations in micas, J. Appl. Spectrosc. 15 (1971) 1405–1407, https:// doi.org/10.1007/BF00606791. [23] B. Stuart, Infrared Spectroscopy: Fundamentals and Applications, John Wiley & Sons, 2004, pp. 7–8. [24] C. Duce, V.D. Porta, M.R. Tiné, A. Spepi, L. Ghezzi, M.P. Colombini, E. Bramanti, FTIR study of ageing of fast drying oil colour (FDOC) alkyd paint replicas, Spectrochim. Acta A. Mol. Biomol. Spectrosc. 130 (2014) 214–221, https://doi.org/ 10.1016/j.saa.2014.03.123. [25] P.A. Hayes, S. Vahur, I. Leito, ATR-FTIR spectroscopy and quantitative multivariate analysis of paints and coating materials, Spectrochim. Acta A. Mol. Biomol. Spectrosc. 133 (2014) 207–213, https://doi.org/10.1016/j.saa.2014.05.058. [26] R.C. Asensio, M. San Andrés Moya, J.M. de la Roja, M. Gómez, Analytical characterization of polymers used in conservation and restoration by ATR-FTIR spectroscopy, Anal. Bioanal. Chem. 395 (2009) 2081–2096, https://doi.org/10.1007/ s00216-009-3201-2. [27] F. Rosi, C. Miliani, C. Clementi, K. Kahrim, F. Presciutti, M. Vagnini, V. Manuali, A. Daveri, L. Cartechini, B.G. Brunetti, A. Sgamellotti, An integrated spectroscopic
4. Conclusions The thermogravimetrical analysis under isothermal conditions, namely 25 °C, allowed the plotting of the drying curves by using thermogravimetrical curves (material moisture – drying time). Experimental data MR = f(time) was processed with the help of the Sigma Plot 11.2 program, in order to verify certain drying kinetic models indicated by the specialty literature. The best results were obtained with a Midilli and Kucuk modified model, also indicated by the specialty literature as the model with the best results for thick film drying processes. The drying rate curves – w (kg moisture/m2⋅s) according to moisture - u (kg moisture/kg initial substance) stressed out the two stages of drying: I – with a constant evaporation rate and II – with a decreasing evaporation rate. In the case of IAC acrylic-impregnated and water-based ecolasure with alkyd/acrylic resins - ECM, it was concluded that the moisture migration rate from the interior towards the surface is greater than the evaporation rate at the surface. In the case of LPM protective lacquer, the determining factor is the rate of the evaporation process and external diffusion of the solvent vapours through the exterior limit layer. The values of the effective diffusion coefficients (Deff) were calculated. The use of classical models from literature emphasized the difficulty of modelling the drying of certain paint films, due to their complex composition. The characterisation of films obtained after the drying process was carried out by applying: SEM, EDX, water contact angle measurements, ATR-FTIR and DSC. Information about coverage and composition properties as well as paint film integrity was achieved. For ECM and IAC, the morphological analysis of the surface indicates a relatively homogeneous distribution and an even appearance. In the case of LPM, some agglomerations areas of pigment and fillings can be noticed on the wood surface. According to the EDX analysis, the common elements found in each film are carbon, oxygen, cobalt, aluminium, silicon, phosphorus, chlorine and iron (the percentage of C is between 51% and 73%; oxygen has the second highest quantity within the range of 20% and 32%). The ATRFTIR spectra and the DSC curves confirmed the presence of alkyd and/ or acrylic resins in the paint films subjected to analysis. A comparative analysis of the results achieved for the paint and varnish films obtained after the drying process and after being kept in laboratory conditions for one year shows that of all the coating formulations available on the market, which we analyzed, the water-based acrylic impregnant – IAC has the best performance. The mean value of the measured contact angle demonstrates the hydrophobic character of the paint film obtained on a fir wood surface, which is also preserved after keeping the sample in laboratory conditions for one year. It also needs the shortest time to dry and a higher glass transition temperature value compared to ECM alkyd/acrylic water-based ecolasure and LPM protective varnish containing only alkyd resins. Acknowledgments Author Cătălin Lisa is grateful for financial support from the “Program 4, Fundamental and Border Research, Exploratory Research Projects” financed by UEFISCDI, Project No. 51/2017. Authors are grateful to Ph.D. student Roxana Constantinel of the Gheorghe Asachi Technical University of Iasi, Romania, for the water contact angle measuremets. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.porgcoat.2019.105344. 8
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2008.04.011. [30] P. Rämänen, P. Pitkänen, S. Jämsä, S.L. Maunu, Natural oil based alkyd-acrylate copolymers: new candidates for barrier materials, J. Polym. Environ. 20 (2012) 950–958, https://doi.org/10.1007/s10924-012-0492-8. [31] P.M. Spasojevic, V.V. Panic, J.V. Dzunuzovic, A.D. Marinkovic, A.J.J. Woortman, K. Loos, I.G. Popovic, High performance alkyd resins synthesized from postconsumer PET bottles, RSC Adv. 5 (62273-) (2015) 62283, https://doi.org/10. 1039/C5RA11777A.
approach for the non-invasive study of modern art materials and techniques, Appl. Phys. A 100 (2010) 613–624, https://doi.org/10.1007/s00339-010-5744-7. [28] J. Ziebas-Palus, R. Borusiewicz, Examination of multilayer paint coats by the use of infrared, Raman and XRF spectroscopy for forensic purposes, J. Mol. Struct. 792 (2006) 286–292, https://doi.org/10.1016/j.molstruc.2006.03.072. [29] P. Uschanov, N. Heiskanen, P. Mononen, S.L. Maunu, S. Koskimies, Synthesis and characterization of tall oil fatty acids-based alkyd resins and alkyd-acrylate copolymers, Prog. Org. Coat. 63 (2008) 92–99, https://doi.org/10.1016/j.porgcoat.
9
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Determination of the effective diffusion coefficient during the drying of paint and varnish films applied on fir wood ⁎
⁎
A. Mihailaa, C. Lisaa, , A-M. Ipateb, M.F. Zaltariovb, D. Rusub, I. Mămăligăa, , G. Lisaa,
T
⁎
Gheorghe Asachi Technical University, Faculty of Chemical Engineering and Environmental Protection “Cristofor Simionescu”, 73 Prof.dr.doc. D. Mangeron Street, 700050, Iaşi, Romania b Petru Poni Institute of Macromolecular Chemistry, 41A Aleea Gr. Ghica Voda, 700487, Iasi, Romania a
A R T I C LE I N FO
A B S T R A C T
Keywords: Paint Drying process Effective diffusion coefficient SEM EDX ATR-FTIR
The development of the civil and industrial building sector lead to an increase in the demand of lacquers and paints. Wood is one of the long-lasting building materials, for which the demand for exterior landscaping and internal finishes is very high and ever-growing. The drying process is an essential step in obtaining a paint film which shall protect the wood. In this paper, we shall use for the first time the derivatograph to analyse the drying process, respectively, the forming process of the actual film on a series of commercial lacquers and paints deposited directly on fir wood support. Many kinetic drying models were tested. The effective diffusion coefficients and the duration of the drying process were calculated. The use of classical models from literature emphasized the difficulty of modelling the drying of certain paint films, due to their complex composition. The films obtained after the drying process were characterized.
1. Introduction The development of the civil and industrial engineering field, as well as the growing involvement of the population in the refurbishment and decoration of living quarters and furniture has led to higher demand for paints and varnishes. Wood is one of the most durable, aesthetic and non-polluting building materials and the demand for this material for exterior construction and interior finishes is very high and has been continuously growing. It is therefore no wonder that researchers have been concerned to design new surface film formulations (varnishes and paints) to protect the wood that have increased resistance to environmental factors. An important step in obtaining a paint film able to protect the wood is the drying process. As a result of this process a pellicle which strongly adheres to the surface of the wood is achieved, which is called film. Film formation occurs only as a result of the evaporation of the volatile components: solvents or thinners. Once the evaporation process completed, the binder molecules approach one another, a gel is formed and finally the actual film is formed. Since environmental legislation has as a major requirement the reduction of volatile organic compounds (VOCs) released in the environment, specialists have attempted to switch from solvent-based paints to water-based paints [1]. However, since solvent-based paints have a number of advantages, such as: easy application features and better drying tolerance in low temperature and high moisture ⁎
conditions, they still are much more frequently used than water-based ones [1]. The equipment used in thermogravimetric analysis is based on a high degree of precision in mass change and temperature measurement and they therefore have been extensively used in the analysis and modeling of the drying process in the food industry [2–5]. A device such as A Q50 (TA Instruments) and an innovative optical method based on the principle of multi-speckle diffusive wave spectroscopy (MSDWS) have been used by Giraud et al. [6] to analyze the time required to form polydimethylsiloxane (PDMS) films and commercial paint based on an aqueous dispersion of acrylic copolymers. In this paper we will use the Mettler Toledo TGA-SDTA851e derivatograph for the first time to analyze the drying and actual filmforming process for a range of varnishes and paints available on the market, which contain water- and/or solvent-based fatty and medium alkyd resins and acrylic resins. For this purpose, a 0.13-0.31 mm thick coating of varnish or paint has been applied on dry fir wood disks 5 mm in diameter, which have been subjected to a drying process in constant 25 °C temperature conditions. Synthetic air at a flow rate of 20 ml/min was used as drying agent. The films achieved after the drying process, and also after their preservation in laboratory conditions for one year, were characterized by the following techniques: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), water contact angle measurements, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry
Corresponding authors. E-mail addresses: [email protected] (C. Lisa), [email protected] (I. Mămăligă), [email protected] (G. Lisa).
https://doi.org/10.1016/j.porgcoat.2019.105344 Received 17 April 2019; Received in revised form 17 August 2019; Accepted 17 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Quanta 200 microscope is equipped with an Energy Dispersive X-Ray system (EDX) for qualitative analysis which allows us to identify the elements in the samples. The EDX detector used is the Si detector - EDX silicon-drift detector enables rapid determination of elemental compositions and acquisition of compositional maps. Samples are imaged at 10 mm WD (working distance), which is the stage eucentric position and the collection point of the EDX detector. It is used in conjunction with the LFD detector - (Large Field Detector). SEM is a relatively simple technique which requiers minimum preparation of the samples and offers solid, realistic informations concerning the surface structure. Through this analyse method were investigated the apearance, integrity, quality and paint films adhesion degree at the surface of fir wood samples.
(DSC). Thus, information was gathered regarding the influence of the type of resin and solvent used on the drying process and on the quality of the resulting paint films. 2. Materials and characterization methods 2.1. Materials In this study were analyzed three types of varnishing products which are available in market. Paint was applied in one thick layer on dry fir tree wood disks of 5 mm. The three types of paints considered are: a water-based ecolasure, an acrylic impregnant, and a protective lake. Water-based ecolasure (ECM) is a thick coating product based on water containing pigments resistant at UV radiations, precipitations. This product can be successfully used inside and also outside to protect any type of wood structures. It offers resistance to biological factors such fungi, blast, mold, insects; and also against atmospheric factors. The product is characterized by a fast drying, waterproofing of the wood, contains a UV filter that protects against solar radiation. Acrylic impregnant (IAC) is a water-based impregnation primer with addition of fungicides and biocides. This type of product is recommended for any type of wood protection, wood of any essence which is exploated both inside and outside. In the case of resinous wood, use of this product is obligatory. The product penetrates deep in the wood fiber protecting it against blue stains and fungi, ensures microbiological protection against fungi, insects and molds, provides resistance at atmospheric factors and UV radiations. Also the product contributes to reducing of drying and swelling of wood. Wood damping is also allowed by using the impregnant in addition to optimize the adhesion between the substrate layer and the later applied finish. When the product is applied must be respected some conditions for ensuring the desired characteristics of the product like environment relative humidity of 70%, temperature between 10 and 30 °C and also is very important to mention that the product can not be applied of wood which is not dry. Protective lake (LPM) is a glaze type product which can be applied on any type of wood. It can be used for surface protection inside and also outside under high temperature and humidity conditions that can favor microbial attack on wood. The product penetrates deeper in wood fiber providing long-lasting resistance, provides increased wood protection against weathering and UV radiation.
2.2.3. Water contact angle measurements The surface properties of the paint layer were investigated by water contact angle measurements. Water contact angle measurement was done using a KRUSS goniometer. This system allows the recording and assessment of video images and the calculation of surface energies and of surface tension. Two measurements per sample were performed initial for dry varnish and paint films and after one year in the lab conditions (temperature of 23 ± 2 °C and humidity of 42 ± 15%) with distilled water drops with a dosing volume of 5 μL. We performed measurements every other two second for 30 s. The main equipment characteristics are: DSA1 software, EASYDROP, measuring range (contact angle 1 to 180°, surface tension 0.01–1000 mN/m), measuring resolution (contact angle 0.1°, surface tension: 0.01 mN / m, camera system 79 fps (656 × 492 px)). 2.2.4. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) IR spectra were registered using a Bruker Vertex 70 FTIR spectrometer (Bruker Optics, Ettlingen, Germany) equipped with a ZnSe crystal. The measurements were performed in ATR (Attenuated Total Reflectance) mode in the 4000 - 600 cm−1 spectral range at room temperature with a resolution of 4 cm-1 and accumulation of 32 scans. The 1800-600 cm-1 spectral region was deconvoluted by a curve-fitting method, and the areas were calculated with a 50% Lorentzian and 50% Gaussian function. The curve-fitting analysis was performed with the OPUS 6.5 software. The procedure led to a best fit of the original curve with an error of less than 0.002.
2.2. Characterization methods
2.2.5. Differential scanning calorimetry (DSC) The DSC curves for dry varnish and paint films were recorded with a Mettler Toledo DSC1 device in inert atmosphere with a heating rate of 10 °C/min. Scanning was performed in the -80 - 200 °C temperature range. The mass of samples encapsulated in aluminum pans with pierced lids to allow evaporation of the volatile components ranged between 3 and 5 mg. The evaluation of the thermogravimetric and DSC curves was performed using STARe software version 9 of MettlerToledo.
2.2.1. Drying process analysis The analysis of the drying and film forming process for a series of commercial varnishes and paints was performed with the Mettler Toledo TGA-SDTA851e derivatograph. This equipment allows a weighing accuracy of less than 1 μg and a temperature control accuracy of 0.01 °C. Dry fir wood disks 5 mm in diameter were used, on which a 0.13-0.31 mm thick varnish or paint coating was applied, which was left to dry in constant 25 °C temperature conditions, using synthetic air at a flow rate of 20 ml/min as drying agent. The obtained curves were processed using the Mettler Toledo STARe software.
3. Results and discussions
2.2.2. Scanning electron microscopy (SEM) and energy dispersive X-Ray analysis (EDX) SEM measurements were used in order to investigate the appearance and the structural integrity of the treated fir wood samples and also the adhesion between the exterior coating and the wood surface. The surface morphology of the samples was examined by using a Scanning Electron Microscope (SEM) type Quanta 200 (FEI), operating at 20 kV with secondary electrons in low vacuum mode, the magnification being indicated on each micrograph. The SEM studies were performed on uncoated samples fixed on aluminium supports. The
3.1. Drying process analysis Thermogravimetric analysis in isothermal conditions, i.e. 25 °C, allowed using thermogravimetric curves to draw the drying curves (material moisture – drying time) in Fig. 1. Material moisture (MR) [7] was calculated with Eq. (1) in which M0 is the initial content of solvent in the applied paint coating, Me is the solvent content in equilibrium conditions determined from the drying curves at the time the sample mass remains unchanged with time, and M (t) is the solvent content of the sample at any time t. 2
Progress in Organic Coatings 137 (2019) 105344
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Fig. 1. Experimental and calculated values for model 4 for MR.
MR =
M(t) − Me M0 − Me
et al. [11] found that when a 25% mass ratio of tackifying resin (TR) was introduced into water-based acrylic latex to improve the adhesion of the resulting film, the drying mechanism was found to change, i.e. the “skin effect”, which lengthened the drying time. The shape of the curves shown in Fig. 2 shows that in the case of IAC water-based acrylic stain and ECM water-based fatty ecolasure containing modified alkyd/ acrylic resins, the moisture migration rate to the surface is higher than the surface evaporation rate. In the case of LPM protective varnish, the evaporation process and external solvent vapor diffusion through the outer boundary coating are rate-dependent. The total drying time, i.e. the time of removal of the volatile components: solvents or thinners, was determined using Eqs. 3–5, based on the drying rate curves shown in Fig. 2:
(1)
MR = f (time) experimental data were processed using the Sigma Plot 11.2 software to check some kinetic drying models in literature [8,9]. These are semi-theoretical models that derive from Fick's second law of diffusion and have been used so far mainly in thin layer drying of foods. The Henderson and Pabis model contains a single exponential term (MR = a ∗ exp(−kt ) ). The logarithmic model includes the logarithmic form of Henderson and Pabis model and an empirical constant (MR = c + a ∗ exp(−kt ) ). The two term model contains two exponential terms (MR = a ∗ exp(−k1 t ) + b ∗ exp(−k2 t ) ), and the modified Midilli and Kucuk model proposed in this paper contains an exponential term, a linear term and an empirical constant (MR = c + a ∗ exp(−kt ) + b ∗ t ). The results obtained are shown in table S1 Supporting Information along with the performance of the models, namely the correlation coefficient (r2) and standard deviation (σ):
∑ i=1
tI =
mi ⋅(ui − ucr ) A⋅wmax
tII =
mi ⋅ A u
(4)
uf
k
σ=
(3)
t = tI + tII
[(MRE )
exp
−
(MRE )calc ]2
/(n − p) (2)
∫ du w cr
(5)
where tI is the constant rate drying time, tII is the decreasing rate drying time, mi initial mass of the paint coating, A is the fir wood disk area, wmax is the maximum drying rate, ui is the initial moisture content, ucr is the moisture content at the beginning of the decreasing rate drying stage and uf is the moisture content at the end of the second drying stage. The results achieved are shown in Table 1. Please note that for the ECM sample, both linear decreasing rate drying and non-linear decreasing rate drying were taken into account when calculating the second stage. When analyzing the results obtained, one may notice that they are comparable to those obtained by Giraud et al. [6] for commercial water-based acrylic resin paints. Considering the analytical solution of Fick's equation, for different geometric shapes [12], i.e. for Cranck's "thin plate" [13], and assuming that moisture transport takes place by diffusion and that temperature and diffusion coefficients are constant, the effective diffusion coefficients (Deff) may be determined based on the following dependence for long solvent removal times:
where n is the number of experimental data, and p is the number of parameters. The kinetic drying models tested and shown in table S1 Supporting Information and the drying curves (material moisture – drying time) in Fig. 1 prove that the best results for ECM, IAC and LPM coatings are obtained with model 4 described in this paper (Modified Midilli and Kucuk). Kucuk et al. [10] states in an extensive study in literature that this type of model yields the best results for thin coating drying processes. Fig. 2 shows the variation of the drying rate - w (kg moisture/m2⋅s) depending on moisture - u (kg moisture/kg initial substance) for ECM, IAC and LPM. The two drying stages are highlighted: I - with constant evaporation rate and II - with decreasing evaporation rate. Higher constant drying rates are found for water-based acrylic stain - IAC and water-based fatty ecolasure containing modified alkyd/acrylic resins ECM. As concerns the LPM protecting coating containing alkyd resins, both the moisture content u and the drying rate are lower. As for ECM, one may notice an acceleration of the drying process with a non-linear descending rate compared to the other samples. One possible explanation would be that after a drying stage at linear descending rate, a “skin effect” film is formed which prevents solvent diffusion. This effect has been noted by other researchers as well. For instance, Mallégol
ln ⎛ ⎝ ⎜
π2MR ⎞ D = −π2 eff t 8 ⎠ δ2 ⎟
(6)
where δ is the thickness of the varnish or paint coating on the wooden disk. According to the graphical representation, 3
Progress in Organic Coatings 137 (2019) 105344
A. Mihaila, et al.
Fig. 2. Moisture-dependant drying rate.
- the second stage, comprising zones II and III in Figure S1 Supporting Information, where the process is influenced by both film diffusion coefficient and film thickness, respectively. In the case of drying films with a low solvent content, the first stage may be missing. As, according to Figs. 1 and 2, the drying time at constant rate (first stage) is insignificant, reference will be made to the second drying stage. When dissolving a solvent in a polymer film during the second drying stage, solvent transfer into the film is described by Fick's second law. Considering that film thickness, diffusion coefficient and temperature remain constant the solution of the equation describing the variation of the solvent content over time is suggested by Crank [13]:
Table 1 Drying time. Sample
tI(s)
tII(s)
t(s)
ECM IAC LPM
119 292 242
7793 1660 4471
7912 1952 4713
Table 2 Effective diffusion coefficients. Sample
r2
δ⋅103(m)
Deff⋅1012 (m2/s)
ECM IAC LPM
0.9969 0.9962 0.9973
0.233 0.130 0.314
2.120 0.712 4.359
ln
( ) = f (t )straight π 2MR 8
MR =
2 Deff ×t 1/2 ⎛ ⎞ π ⎝ δ2 ⎠
∞
∑ n= 0
1 (2 n+ 1)2π2D t ⎤ exp ⎡− ⎥ ⎢ (2 n+ 1)2 δ2 ⎦ ⎣
(8)
MR – standardized residual solvent mass, M0 – initial solution mass, Meq Me – solution mass in equilibrium conditions, M(t)- solution mass at time t, δ - film thickness [m], t – time [s] and D – solvent-film diffusion coefficient [m2/s]. When the first three terms of the sequence are considered, it results from Eq. (8):
lines with negative slope and correlation
coefficients higher than 0.996 were obtained. Table 2 shows: the correlation coefficients (r2), the varnish or paint coating thickness on the fir wood disk (δ) and the effective diffusion coefficients (Deff) values. Other researchers have described in their studies the values of the effective diffusion coefficients determined for different paint systems [14–20]. They generally used the analytical solution of Fick's equation to determine the effective diffusion coefficients for Crank’s "thin plate" [13] for low solvent removal times:
MR =
M(t) − Me 8 = 2 M0 − Me π
MR =
2 2 M(t) − Me 8 1 1 −25D t(π/δ)2 ⎤ = 2 ⎡e−D t(π/δ) + e−9D t(π/δ) + e M0 − Me π ⎣ 9 25 ⎦
(9)
Fig. 3 (a–c) shows the non-dimensional residual moisture variation over time of the three systems analyzed. The experimental data are described compared to the simulated values based on the diffusion model (Eq. 9). The model uses diffusion coefficients and initial film thicknesses values which are identical to those shown in Table 2. According to Fig. 3 (a–c), there is a good correlation between the model and the experimental data in the case of IAC drying (maximum deviation ± 18%) and a satisfactory correlation on LPM drying (maximum deviation ± 40%), and on ECM drying (maximum deviation ± 68%). The cause of these deviations may be the complex composition of the paint film. It also contains a number of fillers with porous structure and the solvent used is not a pure component. Hence the difficulty of modeling the drying of paint films by using classical models in literature. Having analyzed the results achieved, we found that semi-
(7)
The use of Eq. (7) for the experimental data obtained in this paper, i.e. the determination of the MR = f(t1/2) dependencies, led to lower correlation coefficients than for Eq. (6). The physical model of drying a film laid out on a flat surface comprises two stages: - the first stage is represented by zone I in Figure S1 Supporting Information. This may be pointed out especially when films with high solvent content are drying. Solvent evaporation during this stage has the same behavior as pure solvent evaporation on a flat surface. The film diffusion coefficient and film thickness have no influence whatsoever. 4
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Fig. 3. Experimental and calculated values for model 9 for MR.
highlighted during the drying process.
theoretical models, which only take into account the diffusion through the film, generally offer a better match with the experimental data compared to the theoretical ones. However, the advantage of theoretical models is that they provide better understanding of the transport processes and take into account all the conditions of the process (dryer geometry, drying agent speed, moisture level/solvent content in the drying agent, etc.), not only diffusion through the film. Paint film drying is a complex process, so we can say that some of the values of the diffusion coefficient actually obtained are mean values and are valid only for the types of paints used in this study.
3.2.2. Energy dispersive X-ray (EDX) EDX is an X-ray technique that allows identification and characterization of the elemental composition of materials. EDX analysis was carried out on the surfaces of the samples to determine the elements in the paint films. Data were collected from three randomly chosen points by taking arithmetic mean of these values and average weight percent of the elements was calculated. Table S2 Supporting Information reports the data on different elements present in paint film which cover the fir wood samples. According to the EDX analysis, the common elements found in each film are carbon, oxygen, cobalt, aluminium, silicon, phosphorus, chlorine and iron (the percentage of C is between 51%–73 %; oxygen has the second highest quantity within range 20%–32%). In addition, ECM water-based fatty ecolasure contains sodium and calcium. This paint also contains S and I which may be due to the presence of biocides declared by the producer: 3-iodo-2-propynyl Butylcarbamate and 1,2 benzisothiazol-3(2 H)-one. The other IAC water-based paint also contains sodium and calcium. However, the presence of magnesium, titanium and potassium was only identified for the IAC acrylic stain. Topcuoglu et al. [21] identified carbon, oxygen, titanium, calcium, silica, aluminum, magnesium and sodium by using EDX analysis on films of water-based paints containing acrylic resins.
3.2. Characterization studies of paint films 3.2.1. Scanning electron microscope (SEM) In Figs. 4 are presented SEM micrographs of fir wood samples untreated/treated with film paints. We can observe that the cellulose micro-fibrils presented at the wood surface (blind sample) are fully/ completely covered with paint films. The morphological analysis of the surface indicated in the case of ECM and IAC the fact that pigments and fillers exhibit a relatively homogeneous distribution, an uniform aspect, the surface being covered by paint film – however, pigment distribution becomes nonhomogeneous in the case of LPM –some agglomerations zones of the pigment and fillers are present on the surface of wood. Figure S2 Supporting Information shows SEM micrographs for fir wood surfaces treated with the three types of paints containing acrylic/alkyd (ECM), acrylic (IAC) and alkyd (LPM) resins, after they have been preserved in laboratory conditions for one year. The results achieved do not indicate significant changes in the morphology of the analyzed surfaces except in the case of the ECM sample. In this case, we noted the occurrence of wrinkles that may have as a cause the "skin effect"
3.2.3. Water contact angle measurements The mean values of the contact angle measured for the paint and varnish films obtained after the drying process, but also after being kept in laboratory conditions for one year are shown in Table 3. We found that hydrophilic surfaces are obtained in the case of fat water-based 5
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Fig. 4. SEM micrographs for ECM (a), IAC (b), LPM (c) and Blank sample-uncovered fir wood (d) 2000x.
3.2.4. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) The comparative ATR-FTIR spectra of three commercial lacquers and paints films (ECM, IAC and LPM) are shown in Figure S3 Supporting Information. In the IR spectra of the ECM, IAC and LPM samples the characteristic bands assigned to the acrylic/alkyd resins can be observed: 1730 cm−1 specific for carbonyl ester group (C]O), 1708, 1702 and 1676 cm−1 specific for carboxylic groups of the acrylic monomers, 1650-1645 cm−1 specific for OH bond deformation vibrations region (Fig. 5) overlapped with C]C stretches vibrations of the aromatic ring [22]. The spectral region 1500-1300 cm-1 revealed the presence of different vibrations of the C–H in methyl/methylene groups: 1464 and 1458 cm-1 methyl asymmetrical C–H bending vibrations, 1444-1416 cm-1 methylene scissoring vibrations, 1386 – 1372 cm1 methyl symmetrical C–H bending vibrations, 1336 cm-1methylene wagging vibrations (Fig. 5) [23], while asymmetric and symmetric stretches of the methylene groups could be observed in the 30002800 cm−1 region. The overlapped and hidden peak positions of νC–H region of the samples ECM, IAC and LPM were determined with the second derivative of the spectra (Figure S4 Supporting Information). The C–O stretching region (1300 -1000 cm−1) shows a number of strong bands assigned to the C–O-C asymmetric stretching vibration (1192-1000 cm−1) and to the CeOeC symmetric stretching vibration (1268-1234 cm-1) (Fig. 6) [24–26]. The bands assigned to the C-Cl stretching (correlated with the biocide activity of the films) are observed at 728/742/754 cm−1, while those attributed to the pigment can be observed at 990-890 cm−1 (stretching vibrations of calcium phosphate), 844/784 cm−1 (stretching vibrations of calcium carbonate) and 1372/1382/1386 cm−1 (specific for kaolin) [27]. The bands at 702706 cm-1 are assigned to the C–H bending in aromatic ring and the broad spectral region at 600-700 cm−1 in the IR spectrum of IAC water-
Table 3 Contact angle values of paint and varnish films applied on fir wood. Sample
Water contact angle (°) The initial dry film
After one year in the lab
ECM
78
73
IAC
131
111
LPM
91
103
ecolasure containing modified alkyd/acrylic resins – ECM. The mean value of the contact angle is 78° and does not change significantly after the samples have been preserved in laboratory conditions for one year. For the water-based acrylic impregnant – IAC, the mean values obtained for the contact angle show the hydrophobic character of the paint film obtained on fir wood surface, which persists after the samples have been kept in laboratory conditions for one year. The varnish marked LPM containing alkyd resins has contact angle values higher than 90°. After the fir wood samples treated with this varnish have been kept in laboratory conditions for one year, we found that the mean contact angle value increases by 12°, improving the hydrophobicity of the surface. 6
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Fig. 5. Deconvoluted ATR-FTIR spectra of the ECM, IAC and LPM samples in the 1800-1300 cm−1 spectral regions.
based paint can be associated with the presence of the TiO2 pigment [28] which was also confirmed by EDX analysis. The fir wood samples treated with three types of paints: a waterbased ecolasure, an acrylic impregnant, and a protective varnish were kept in laboratory conditions for one year and the ATR-IR spectra shown in figure S5 Supporting Information were also recorded. There were no significant changes compared to the ATR-FTIR spectra shown in figure S3 Supporting Information. There was only a slight increase in the vibration of the specific OeH stretching bands in the 37003100 cm−1 region.
Table 4 DSC characteristics. Sample
Tg1(°C)
Tg2(°C)
ECM IAC LPM
−12.8 7.2 14.6
13.5 24.8 –
drying process were removed from the wooden disks by means of a stainless steel blade and subjected to DSC analysis. The glass transition temperatures (Tg) shown in Table 4 were obtained on the second heating curve. These indicate the fact that in the case of water-based fatty ecolasure containing modified alkyd/acrylic resins – ECM and water-based acrylic stain – IAC, two Tg values were highlighted. According to literature, the negative value of the first Tg in the case of ECM indicates the presence of the alkyd resin and the second one may correspond to the alkyd-acrylic copolymer [29,30]. As concerns the
3.2.5. Differential scanning calorimetry (DSC) The coating properties of the varnish or paint layer laid out on the fir wood disk depend on the integrity of the film obtained. Along with the surface characteristics of the substrate, the glass transition temperature (Tg) of the dry film also has a special importance in the evaluation of the properties. The paint and varnish films obtained after the
Fig. 6. Deconvoluted ATR-IR spectra of the ECM, IAC and LPM samples in the 1300 – 600 cm−1 spectral regions. 7
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References
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4. Conclusions The thermogravimetrical analysis under isothermal conditions, namely 25 °C, allowed the plotting of the drying curves by using thermogravimetrical curves (material moisture – drying time). Experimental data MR = f(time) was processed with the help of the Sigma Plot 11.2 program, in order to verify certain drying kinetic models indicated by the specialty literature. The best results were obtained with a Midilli and Kucuk modified model, also indicated by the specialty literature as the model with the best results for thick film drying processes. The drying rate curves – w (kg moisture/m2⋅s) according to moisture - u (kg moisture/kg initial substance) stressed out the two stages of drying: I – with a constant evaporation rate and II – with a decreasing evaporation rate. In the case of IAC acrylic-impregnated and water-based ecolasure with alkyd/acrylic resins - ECM, it was concluded that the moisture migration rate from the interior towards the surface is greater than the evaporation rate at the surface. In the case of LPM protective lacquer, the determining factor is the rate of the evaporation process and external diffusion of the solvent vapours through the exterior limit layer. The values of the effective diffusion coefficients (Deff) were calculated. The use of classical models from literature emphasized the difficulty of modelling the drying of certain paint films, due to their complex composition. The characterisation of films obtained after the drying process was carried out by applying: SEM, EDX, water contact angle measurements, ATR-FTIR and DSC. Information about coverage and composition properties as well as paint film integrity was achieved. For ECM and IAC, the morphological analysis of the surface indicates a relatively homogeneous distribution and an even appearance. In the case of LPM, some agglomerations areas of pigment and fillings can be noticed on the wood surface. According to the EDX analysis, the common elements found in each film are carbon, oxygen, cobalt, aluminium, silicon, phosphorus, chlorine and iron (the percentage of C is between 51% and 73%; oxygen has the second highest quantity within the range of 20% and 32%). The ATRFTIR spectra and the DSC curves confirmed the presence of alkyd and/ or acrylic resins in the paint films subjected to analysis. A comparative analysis of the results achieved for the paint and varnish films obtained after the drying process and after being kept in laboratory conditions for one year shows that of all the coating formulations available on the market, which we analyzed, the water-based acrylic impregnant – IAC has the best performance. The mean value of the measured contact angle demonstrates the hydrophobic character of the paint film obtained on a fir wood surface, which is also preserved after keeping the sample in laboratory conditions for one year. It also needs the shortest time to dry and a higher glass transition temperature value compared to ECM alkyd/acrylic water-based ecolasure and LPM protective varnish containing only alkyd resins. Acknowledgments Author Cătălin Lisa is grateful for financial support from the “Program 4, Fundamental and Border Research, Exploratory Research Projects” financed by UEFISCDI, Project No. 51/2017. Authors are grateful to Ph.D. student Roxana Constantinel of the Gheorghe Asachi Technical University of Iasi, Romania, for the water contact angle measuremets. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.porgcoat.2019.105344. 8
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