Iron as an alternative drier for curing of high-solid alkyd coatings.
Keywords HS alkyd coatings, Driers, FTIR, Rheology, EIS
Solvent-based alkyd resins are polyesters modified with oils or fatty acids. They have been the subject of considerable research interest over the past decade because of their low price, diversity, and composition of bio-based raw materials. (1) Due to new EU legislation that requires reduced volatile organic content (VOC) in solventborne decorative paints, high-solid (HS) alkyd resins can be used as an appropriate alternative (1); their curing process and application properties, however, are specific.
Film formation from the application point of view is one of the most important factors in ensuring the proper chemical and physical characteristics of cured films. (2) Issues with HS alkyd coatings are typically associated with the film formation process and can be expressed as long Set-to-Touch Times, poor through drying, and consequent low film hardness. (1)
Chemical curing, as part of the film formation process, is the result of autooxidative crosslinking and is accelerated by the addition of a metal catalyst. (3) The most widely used catalysts are metal soaps composed of carboxylic acids also known as driers. (5) They can be classified as surface, through, and auxiliary driers. Cobalt (Co), manganese (Mn), cerium (Ce), iron (Fe), and vanadium (V) belong to the group of surface driers. They exhibit catalytic effects and participate in different reactions in the autooxidation process. (4,5) One of the most active surface driers and therefore often used is Co. (6,7) Due to the likely classification as a carcinogen within the framework of REACH legislation the use of Co driers will be limited. (8) However, changing of the catalyst in alkyd coatings has an impact not only on the chemical reactions during the curing process but also on the physical properties of the cured film. This is even more evident in HS alkyd coatings. (9)
In order to find a replacement for Co driers, numerous studies were performed on different metal salts. According to studies thus far, Fe salts can be used as alternative driers. The first--iron carboxylate driers--exhibited poor activity at ambient temperature and introduce colouration problems in bright colors. (5,8) Further development was focused on different Fe driers. Ferrocene-based Fe (II.) driers, (10-12) acyl-substituted ferrocenes, (13) and a combination of reduced agents/iron were tested. (14) Significant work was done on the Fe complex containing ligands (Fe-bispidon). Reactivity studies of the Fe-bispidon were tested on model substances like ethyl linoleate (EL), where a similar reactivity relative to Co carboxylate was observed. Paint drying capabilities were also tested in medium-oil alkyd paints. Drying times were, in most cases, comparable or even shorter with Fe-bispidon than with Co driers. Nevertheless, no significant work was done on the testing of Fe driers in HS long-oil alkyds, which are the most widely used alkyd systems in decorative and industrial applications owing to applicable VOC legislation.
The aim of our work was to perform a comparative study of commercially available Fe-bispidon driers and Co-octoate driers in air-dried HS long-oil alkyd coatings. We assume that driers perform differently in HS alkyd coatings than in model substances due to significant chemical differences in polymers and structural changes during film formation. The study was focused on drying performance and curing kinetics. Our work concluded by evaluating the physical properties of formed HS alkyd coatings in terms of film hardness, rheological parameters, and EIS.
Materials and sample preparation
We used the following components in the preparation of coatings: resin, driers, antiskinning agent, and a solvent. Synolac 4047 WD 90, a long-oil (72%) alkyd resin with low VOC, was used as a resin, supplied by Cray Valley (90 wt% of solid content based on phthalic acid anhydride, polyol-pentaerythritol, and linoleic fatty acids).
As driers, we used a commercially available Fe complex (ligand bispidon), supplied by OMG (Borchi Oxy Coat with 1 wt% of Fe complex in 1,2-propylene glycol, with 0.09 wt% of Fe metal content; clear yellow liquid; viscosity maximum 200 mPa s; density approx. 1.04 g/[cm.sup.3]). Commercially available Co drier was also obtained from OMG (Cobalt bis (2-ethylhexanoate) with 10 wt% of Co metal content).
An antiskinning agent (Methyl Ethyl Ketoxime--MEKO) was added to inhibit the drier effect in closed containers for up to 28 days.
Samples were prepared by adding individual components (drier, antiskinning agent, and solvent) to an alkyd resin and mixed with a propeller stirrer for 10 min at room temperature. The amount of resin used was 83 wt%. Applied concentrations of driers are presented in Table 1. The addition of MEKO was 0.7 wt%. The rest of the mixture consisted of aromatic-free Shellsol D-40 gasoline that was added to adjust the viscosity of the coating. Tests were performed after 24 h of sample storage.
The influence of Co and Fe driers on HS alkyd coating characteristics was determined using different analytical methods.
Drying performance of samples as part of the curing process was determined according to ASTM D5895-03 using a BYK drying recorder. The samples were applied to plain glass of approx. 60 [micro]m wet film thickness. The instrument included needles that travel on the test strip within a chosen range of time. In our case the time was 6 h. The trace left on the film during drying was used to define different stages of the film formation process.
The weight loss experiment for the coating sample was performed on a balance. Coatings of different drier concentrations were applied on a glass slide with an applicator (60 [micro]m) at 23[degrees]C and 50% humidity. The average value of the weighted coating samples was 0.265 [+ or -] 0.018. The weights of the coating films were recorded during the process of solvent evaporation.
Time-resolved FTIR spectra were obtained using a Thermo Nicolet 6700 spectrometer using the transmission technique (KBr crystal, range 400-4000 [cm.sup.-1], resolution 4 [cm.sup.-1]) vs time. Samples were applied on a KBr crystal plate with 7 [micro]m spiral film applicator. FTIR spectrum readings were taken every 30 min. In the meantime, the samples were placed in the conditioner at 23[degrees]C and 50% humidity. We monitored the peaks at 3007 [cm.sup.-1] (ds-C=C-H stretching) and 3450 [cm.sup.-1] (O-H stretch) normalized against the ester peak (COOC stretching) at 1272 [cm.sup.-1] vs time. Relative reaction rates of radical-initiated crosslinking were calculated after the induction period with a linear regression of data as In ([A.sub.t]/[A.sub.0] x 100) vs time.
Phase angles ([delta]) of cured coating film as a [tan.sup.-1] ratio of loss module (G") and storage module (G') were determined using an Anton Paar Rheometer MRC301. Sensor amplitude was selected in the elastic deformation range. First, linear viscoelastic behavior (LVE) was determined using an amplitude sweep test where G' and G" displayed a constant value at low deformation. The amplitude test is an oscillatory test with variable amplitude deformation [[gamma].sub.A] or stress [[tau].sub.a] and constant frequency value. The frequency (f) was kept constant (6.28 rad [s.sup.-1]), with amplitude varying from 0.01% to 100%. Phase angle ([delta]) was determined by a frequency sweep test and was measured in the LVE range between region 0.1 and 100 [s.sup.-1]. Constant amplitude value was 0.05%. Coating samples were applied on metal pots by spraying in three layers over three consecutive days. Tests were carried out after 20 days in the conditioner at 23[degrees]C and 50% humidity, and generated information on material structure.
To determine hardness, we used the standard Konig method according to ISO 1522. Samples were applied on glass with an applicator. Wet film thickness was 60 [micro]m. Measurements were taken successively after 1, 3, 7, 14, 21, and 28 days. The results were related to glass standard hardness and expressed as relative hardness.
Electrochemical impedance spectroscopy (EIS) is an established quantitative method by which to examine anticorrosion properties. EIS was used to evaluate the protective properties of the coatings, particularly barrier properties that are indicators for crosslinking of the coating film. Coating samples were applied on metal plates by spraying in three layers over three consecutive days. The final dry film thickness was measured with an Elcometer 456 (F1 magnetic induction probe from Elcometer Ltd.). Film thickness was 72-88 [micro]m. For each sample, 15 measurements were taken in compliance with the ISO 2178 standard.
Prepared samples for the EIS test remained in the conditioner for 5 days at 23[degrees]C and 50% humidity. After 5 days, the samples were measured and then placed in a humidity chamber for 4 h. Samples were placed in the humidity chamber to observe the difference after said exposure. Temperature in the Ericssen HIGROTHERUS 519 thermostatic humidity chamber was 40 [+ or -] 0.5[degrees]C, and relative humidity was maintained at 95 [+ or -] 0.5%. All EIS measurements were carried out in duplicate.
EIS measurements were performed using a Parstat 2273 potentiostat. The measuring cell was a standard three-electrode Tait cell, consisting of a working electrode of 32 [cm.sup.2] immersed in 0.1 M NaCl solution, a Flastelloy counter electrode, and a standard calomel reference electrode (SCE). The potential difference between the working and counter electrodes was -0.6 V vs SCE, and the amplitude of the signal was 30 mV. Data were collected and analyzed using Electrochemistry Power Suite and PowerSINE software (Princeton Applied Research, USA). Impedance spectra analysis was performed after numerical fitting using an equivalent circuit and presented in Fig. 1.
Results and discussion
Comparing the results from different analytical methods, we evaluated the effect of Co and Fe surface driers on the curing kinetics and physical properties of HS alkyd coatings. Due to the influence of MEKO on inhibition activity, (15) some tests were also performed on coatings without MEKO.
Film formation characteristics
Drying time is a film formation characteristic of coatings. The influence of Fe and Co driers was initially compared using the drying time recorder (DTR) method, which evaluates a maximum of four stages during film formation. (16) The first stage--Set-to-Touch Time--represents times for solvent evaporation and can also be related to physical drying. (2,4,16) The remaining stages present chemical curing. Final Drying Time is ascertained when no visible mark is left on the film. Our research was focused on Set-to-Touch Time and Final Drying Time of HS alkyd coatings, with results presented in Table 2.
Set-to-Touch Times were differentiated using Fe and Co driers and lasted from 3.8 to 1.7 h. Similar values have also been observed by other researchers. (7,12) HS alkyd coatings with Fe driers have longer Set-to-Touch Times compared to Co driers. Set-to-Touch Time values of HS alkyd coatings with Co driers were significantly reduced from 3.0 to 1.7 h by increasing drier concentrations; times for HS alkyd coatings with Fe driers were reduced from 3.8 to 2.1 h. The influence of MEKO on Set-to-Touch Time was observed only for Co driers where the values were reduced by [approximately equal to] 50% for the samples without MEKO. Final Drying Times were comparable for Co and Fe driers and lasted from 4.4 h to more than 6 h.
According to the literature data, Set-to-Touch Times are mostly attributed to the evaporation of a solvent from a coating film. (16) During the process, a rapid bulk loss of approximately 90% of the solvent by mass can occur. After bulk evaporation, any remaining solvent is lost through diffusive evaporation. (17) In order to confirm those findings also for HS alkyds, we used the weight calculation method on applied paint on a glass substrate and compared the results with Set-to-Touch Times. Solvent evaporation time for all HS alkyd samples was 30 min. Equally long times are observed due to equal solvent/polymer systems. Differences between Set-to-Touch Times and solvent evaporation times can be observed for all samples. From these data, we can anticipate that Set-to-Touch Times in the case of HS alkyd coatings with Fe and Co driers reveal not only evaporation of solvent from coating films but also oxygen penetration, the inhibition of MEKO, and probably part of the curing process. The difference in the Set-to-Touch Times and the influence of MEKO also indicate that Co and Fe driers have different catalytic characteristics. (5,11) We assumed that the curing process of HS alkyds with Co drier starts on the surface film soon after solvent evaporation, (11) which results in lower Set-to-Touch Time values compared to HS alkyd coatings with Fe driers.
Chemical curing during film formation process
The influence of Co and Fe driers on chemical curing was evaluated by FTIR analysis. From the FTIR spectra of HS alkyd coating (Fig. 2), we can observe different band assignments that belong to chemical groups or bonds according to autooxidation mechanism. (4) The band at 3540 [cm.sup.-1] represents the -O-H stretching of the alkyd resin and does not change. (13,18) The broad intense band at ~3450 [cm.sup.-1] was assigned to the appearance of -O-H stretching that exhibits ROH and ROOH. (8) The band at 3007 [cm.sup.-1] corresponds to unsaturated c/s-C=C-H symmetric stretching. Intense aliphatic -C-H stretching is observed in FTIR spectra between 3000 and 2800 [cm.sup.-1] and indicates oil modification. (18) The bands at 1735 and 1275 [cm.sup.-1] correspond, respectively, to -C=0 and -C-O bonds of a COOC- ester group. (18) The appearance of a shoulder at ~1710 [cm.sup.-1] corresponds to a carbonyl band due to the formation of various aldehydes, ketones, and esters as the result of [beta]-scission reaction. (5,8) From the FTIR spectra, we can also observe that new bands appear at ~987 as 948 [cm.sup.-1] (conjugated trans-C=C-H) and at ~970 [cm.sup.-1] (isolated trans-C=C-H). (8)
The first changeable peak from the FTIR spectra of HS alkyd coatings during chemical curing occurs at a band of frequency 3540 [cm.sup.-1]. It represents the -OH group as a result of the formation of hydroperoxides. These two propagation reactions lead to hydroperoxide formation (R + O2 [right arrow] ROO; ROO + RH [right arrow] ROOH + R) (4) After oxygen absorption, we observed the increase in the concentration of -OH groups that is related to the disappearance of cis-C=C-H double bonds (Fig. 3). With Co driers, hydroperoxides start to form earlier in coatings with Co driers than with Fe driers.
The sharp band at 3007 [cm.sup.-1] is characteristic of unsaturated cis-C=C-H symmetric stretching double bonds and decreases during curing. (14) Changes for this band area are a consequence of the double bond activation due to autooxidation mechanism (RCH=CH-R + [Co.sup.3+] [right arrow] R-CH-C[H.sup.+]-R+[Co.sup.2+]). Then hydroperoxide decomposes into alkoxy and hydroxyl radicals. Radicals are involved in the direct or indirect crosslinking of cis-C=C-H by further formation of hydroperoxides as part of the curing process. (4)
Formation of crosslinks takes place by recombination of radicals and by addition of radicals to carbon-carbon double bonds. With FTIR spectroscopy, the addition of radicals to carbon-carbon double bonds was monitored (R + C=C [right arrow] R-C-C; RO + C=C [right arrow] R-O-C-C; ROO + C=C [right arrow] R-O-O-C-C). (4,8) Differences in FTIR spectra values for HS alkyd coatings were also observed at 987, 948, and 970 [cm.sup.-1]. Conjugated trans-C=C-H at 987 and 948 [cm.sup.-1] rises for some hours. Although they begin to decrease, isolated trans-C=C-H, however, continues to increase, which indicates that a crosslinking process is taking place. In HS alkyds with Co driers, this change occurs after 2.5 h, and with Fe driers after 5 h (Figs. 4 and 5). After 24 h, we noticed that all conjugated trans-C=C-H has not been transformed completely. It can be assumed that film viscosity in alkyd coatings increases intensively, with the structure becoming more rigid. (4) Consequently, chains with conjugated trans-C=C-H cannot move and instead react with other groups.
For the kinetic evaluation of Co and Fe driers on HS alkyd autooxidation curing processes, we selected band at 3007 [cm.sup.-1], which was also observed by other authors in previous publications. (9,19-22) FTIR spectra of HS alkyd coatings were recorded after different times and quantitatively evaluated. Relative reaction rates have been calculated with linear regression of the data ln ([A.sub.t]/[A.sub.0] x 100) vs time after the induction period (Fig. 6). The plateau at the beginning of each curve corresponds to induction time, which can vary-according to the literature-from a few minutes to a few hours for similar systems. (21) It is the period during which the peak at 3007 [cm.sup.-1] remains unchanged which indicates that hydroperoxide formation has not yet begun. It is evident that induction times lasted [approximately equal to] 1 h for samples with Co driers (Fig. 6). On the other hand, induction time with Fe driers lasted [approximately equal to] 3 h. We also observed that MEKO increased the induction time by [approximately equal to] 1 h in the case of Fe drier, (8,15) but there was no change in samples with Co drier.
The logarithmic graph in Fig. 6 gives a straight line after the induction time and can be correlated to the rate of reaction. (15,21) Comparing the coatings with Fe and Co driers, we can conclude that the highest reaction rates have HS alkyd coatings with Co driers (Fig. 7). Different drier concentrations do not exert significant influence on the relative rates of reactions. In the case of samples with different concentrations of Co drier, 95% cis-C=C-H groups transform approximately after 4.5 h. On the other hand, samples with Fe driers only transform up to 85% after 24 h.
From these findings, we can confirm that Co and Fe driers have different catalytic activities because the same changes on FTIR spectra occur with a time lag in HS alkyd coatings. Lower induction times and the influence of MEKO on lower Set-to-Touch Time for coatings with Co drier indicates a faster reaction of Co drier with oxygen and further on higher rate of crosslinking. On the other hand, HS alkyds with Fe drier have longer induction times and lower crosslinking reaction rates. MEKO has an influence on kinetic parameters of HS alkyd coatings with Fe drier; however, this fact does not have significant influence on established conclusions.
Physical properties of coating film
Regarding the findings that Co and Fe driers have an influence on very different induction times and reactions rates, we also evaluated viscoelastic properties on cured coating samples.
Using the amplitude test, we determined the linear viscoelastic region (LVR) for cured coating films. As long as we remain within the LVR, Hooke's law is valid. (23) In our case, the samples fall within LVR in the 0.1-1% strain range. Determined LVR values provide suitable conditions for frequency sweep tests where changes in the ratios of storage module (G') and loss module (G") expressed as phase angle [delta] [equation (1)] were observed.
[delta] = [tan.sup.-1] (G"/G') (1)
The results are presented in Fig. 8. Phase angle at low loads in coatings with Co is constant, which then starts to decrease at higher loads. This behavior corresponds to a stronger structure of the cured solid top layer because Co has a high catalytic activity effect on surfaces where oxygen concentrations are high. With high loads loss factors diminish, which can be attributed to softer layers below the surface. In the literature, it was confirmed that Co drier does not crosslink homogeneously along the depth of an alkyd film. (4) Oyman et al. have also found that double bond conversion with a Co drier was higher on surfaces due to the very high catalytic activity. Diffusion of oxygen to the deeper layer was therefore limited and further crosslinking was reduced. On the other hand, the phase angle of HS alkyds with Fe drier increases linearly for small to large loads, which indicates that Fe drier crosslinks more uniformly throughout the entire thickness of the HS alkyd coating in comparison with Co drier.
Evaluation of the viscoelastic behavior of HS alkyd films during film formation with Co and Fe driers was also performed by Konig hardness measurements, which indicates the hardness of the upper film layer (Table 3). Values for samples with Co drier were significantly higher than those for the samples with Fe drier, which is consistent with viscoelastic measurements. A similar trend of reduced film hardness with Fe-bispidon drier in HS alkyds was also observed. (8)
Electrochemical characteristics of cured coating film
In order to determine the influence of different Co and Fe drier concentrations on HS alkyd film characteristics, we performed a series of experiments measuring EIS. Figure 1 shows the used equivalent circuit after 4-h exposure in a humidity chamber. Differences in impedance spectra between coatings with Co and Fe driers were observed (Fig. 9) and quantified with equivalent circuits.
Capacitor [C.sub.c] represents the charge separation across the coating film and can provide information related to water absorption characteristics of the coating. Lower coating capacitance correlates to lower water absorption and consequently improved protective properties. (24) Coatings with different Co and Fe drier concentrations exhibit equal capacitance values due to similar film thicknesses (between 72 and 88 [micro]m) and the same dielectric constant of the polymer. (25)
The Warburg element [[sigma].sub.c] represents the diffusion of ions through the coating film. (24) Lower diffusion values through the coating film can be attributed to increased crosslinking of the polymer structure. The relationship between the Warburg coefficient and film thickness is taken to compensate the influence of coating thickness on equivalent circuit parameters (Fig. 10), where we observed differences between coatings with Co and Fe driers. Higher diffusion values were recorded for coatings with Co drier, and lower for coatings with Fe driers. The results indicate that the film with Fe drier crosslinks through the entire thickness of the film, which was also assumed previously according to rheological parameters of the frequency sweep test.
By measuring the pore resistance [R.sub.po] of HS alkyd films of different Co and Fe drier concentrations (Fig. 11), we observed that the Co drier (PCo3) sample did not develop pores. Fe drier concentration has no impact on pore resistance; however, they did exhibit higher coating film pore resistance than the PCo2 and PCo3 samples.
Comparing the overall equivalent circuit parameters, we can conclude that HS alkyd samples with Fe driers exhibit different pore resistances and lower ion diffusion levels than Co drier samples. It is also evident that Fe drier concentrations do not significantly affect equivalent circuit parameters, whereas Co drier concentrations, however, do. This is important information for the coating quality optimization considering film formation parameters.
From the literature, it is known that Co and Fe are both classified as surface driers. In tested model substances, they had comparable catalytic activity; however, FTIR study confirms different catalytic activity of Co and Fe driers in HS alkyd coatings. First, changes can be observed from film formation properties where Set-to-Touch Time is longer for Fe drier. The samples had similar solvent/polymer composition, so the evaporation process and penetration of oxygen should be comparable. Anyway, the activation of autooxidation process with oxygen starts faster with Co drier than with Fe drier. From kinetic evaluation, it was also determined that the induction times were longer for the samples with Fe drier. The reason could be slower activation by Fe drier or different autooxidation mechanism. Further kinetic evaluation according to double bond changes (cis-C=C-H to isolated trans-C=C-H) confirms that autooxidation process takes place via the same mechanism. However, the autooxidation process is slower for the HS alkyds with Fe drier than with Co drier. Further, elastic and viscous properties of the HS alkyd coatings during film formation process at room temperature were studied. Significant differences were observed from which it can be concluded that autooxidation process of HS alkyds with Co drier crosslinks more intensively on the surface of the coating, while in the case of HS alkyds with Fe drier crosslinks uniformly throughout the entire thickness of the coating film. It can be assumed that lower hardness measurement values for the samples with Fe driers are cause of through drying process. Equally, it can be assumed that through drying process of Fe drier also has an influence on Final Drying Time which was, despite lower catalytic activity of Fe drier, comparable with Co drier. Consequently, the autooxidation process of HS alkyd coatings with Fe drier compared to Co drier is not only slower but also catalyzes the process as a through drier. The diffusion of ions through cured film that was made by EIS method confirms this final assumption.
New legislative requirements have significant impact on the development of alkyd coatings. The most important changes limit the use of Co driers and lower the VOC levels. As a result, numerous studies have been undertaken on different surface driers in modeling substances or medium-solid alkyds as testing polymers to find an alternative to Co driers. We used high-solid alkyds to evaluate Fe-bispidon drier as a Co alternative.
This study has shown that Fe drier and Co drier have different catalytic activities in HS alkyd coatings. HS alkyd coatings with Fe driers exhibit longer induction times and lower relative reaction rates compared to coatings with Co driers. The reaction of double bond activation of cts-C=C-H is complete after 4.5 h for the coatings with Co driers and more than 24 h for the coatings with Fe driers. The effect of MEKO on curing kinetics was only observed on HS alkyd coatings with Fe drier; however, this fact does not have significant influence on established conclusions. The study of viscoelastic properties (phase angle) demonstrates that Fe drier displays significantly different viscoelastic properties for HS alkyd coating film. Hardness values for HS alkyd coatings with Fe drier were significantly lower as compared with those with Co drier. This result lead us to a conclusion that Fe drier in HS alkyd coatings works more uniformly throughout the coating film compared to Co drier where higher intensity of crosslinking can be observed on the coating surface. The EIS study proved that HS alkyds with Fe drier have increased crosslinking in coating films in comparison with Co drier. The concentrations of Fe drier have no significant impact on film pore resistance, which offers engineers favorable conditions to find the balance between requirements for certain film formation characteristics for HS alkyd coatings.
B. Pirs ([mail]), B. Znoj, J. Zabret
Helios TBLUS, d.o.o., Kolicevo 65, 1230 Domzale, Slovenia
S. Skale, P. Venturini
Helios Domzale d.d., Kolicevo 2, 1230 Domzale, Slovenia
Center of Excellence for Polymer Materials and Technologies, Tehnoloskipark 24, 1000 Ljubljana, Slovenia
Acknowledgments Operation part was financed by the European Union, European Social Fund. Operation was implemented in the framework of the Operational Program for Human Resources Development for the Period 2007-2013, Priority axis 1: Promoting entrepreneurship and adaptability, Main type of activity 1.1: Experts and researchers for competitive enterprises.
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Table 1: Weight percent of added drying agent and metal content on resin solids in HS alkyd coating samples Coating Drying agents Metal content on samples (wt%) resin solids (wt%) PCo1 0.8 0.08 PCo2 1.0 0.10 PCo3 1.2 0.12 PCo4 1.4 0.14 PFe1 0.62 0.56 x [10.sup.-3] PFe2 0.78 0.70 x [10.sup.-3] PFe3 0.93 0.84 x [10.sup.-3] PFe4 1.09 0.98 x [10.sup.-3] Table 2: Set-to-Touch Time and Final Drying Time values for HS alkyd coatings with different Fe and Co drier concentrations Coating Set-to-Touch Final Drying samples Time (h) Time (h) PCo1 3.0 5.9 PCo2 2.5 4.7 PCo3 1.4 5.6 PCo4 1.7 5.9 PFe1 3.8 >6 PFe2 2.1 5.0 PFe3 2.9 4.4 PFe4 2.5 5.3 Table 3: Relative film hardness of coatings with different Co and Fe drier concentrations after 28 days Coating samples Film hardness after 28 days (%) PCo1 15.0 PCo2 16.3 PCo3 16.7 PCo4 18.0 PFe1 7.8 PFe2 7.4 PFe3 7.8 PFe4 8.3
Please note: Some tables or figures were omitted from this article.
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|Author:||Pirs, Barbara; Znoj, Bogdan; Skale, Sasa; Zabret, Jozefa; Godnjavec, Jerneja; Venturini, Peter|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Nov 1, 2015|
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