Article pubs.acs.org/IECR
Curing Behavior Study of UV-Curable Coatings Containing Nanosilica and Different Multifunctional Monomers via Depth Profiling Assessment Ehsan Zarshenas,† Saeed Bastani,*,†,§ and Malihe Pishvaei‡ †
Surface Coating and Corrosion Department, Institute for Color Science and Technology (ICST), Tehran, Iran Resin and Additives Department, Institute for Color Science and Technology (ICST), Tehran, Iran § Center of Excellence for Color Science and Technology, Tehran, Iran ‡
ABSTRACT: This study was done to assess the curing behavior of UV-curable coatings containing the multifunctional acrylate monomers, 1,6-hexanediol diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA), and dipentaerythritol pentaacrylate (DiPEPA). Functionalized nanosilica was used for nano-based coating preparations. A microtome was used to cut each coating into three sections in order to study depth profiles of the coatings. FTIR and HPLC analyses were applied to determine degrees of double-bond conversion and quantities of residual monomer. The results showed that the biggest monomer in terms of size, DiPEPA, mainly remained at the surface of the microtomed sections. However, the bottom of microtomed sections had residual HDDA and TMPTA monomers. Similar behavior was observed in the nano-based coatings, and the degree of conversion at the bottom of the microtomed sections had noticeably increased.
1. INTRODUCTION Photoinduced polymerization is a rapidly expanding technology because of advantages such as high-speed curing, low VOC content, and low energy consumption.1 Such polymerization processes are affected by variables of monomer functionality, such as temperature, synthesis environment, irradiation rate, and photoinitiator type. This method is used to produce coatings that are suitable for many applications. The material properties of such products are dependent on the particular polymer structure, which is determined by the method of production. Therefore, a study to determine effective parameters for the polymerization process, such as behavior of the UV-curing reaction and monomer structure, is of significant importance. Considerations of monomer composition and amount of functionality strongly affect the curing behavior of the polymerization process. This behavior can be studied by measuring the degree of double-bond conversion and the quantity of the remaining unreacted monomers. The main components of a UV-curable coating are a UVcurable oligomer, a photoinitiator, and monomers (reactive diluents).2 The photoinitiator plays a key role in governing the initiation rate, and penetration of light into a sample has an effect on the cure depth of the process.3 The structure and composition of monomers may be varied according to a particular application and property requirements of a product.4 There are many reports on the effect of monomer structure on polymerization rate and double-bond conversion.5−8 Linear acrylates are generally used as monomers to reduce the viscosity of the UV-curable formulation (containing UV-curable oligomers with high viscosity) to facilitate processing. Multifunctional acrylates increase the mechanical strength of a polymer product by forming cross-linked networks rather than linear polymer chains.3,9 Increasing the monomer functionality accelerates the curing reaction, but it also serves to reduce the final degree of conversion because of gelation of the irradiated volume, which © 2013 American Chemical Society
restricts the mobility of the reactive species and prevents completion of the polymerization reaction.10 An important feature in terms of network formation is radical entrapping. In cross-linked systems, mobility of the radicals is severely hindered, so they are susceptible to becoming entrapped. This susceptibility appears when radicals are surrounded by polymer chains because there is a severe restriction of space available for chain movement.11 Under these circumstances, entrapped radicals become inaccessible for further reaction to take place. Radical entrapping occurs from the formation of a microgel, which then interrupts the very beginning of the polymerization process.6,12 This causes a substantial quantity of unreacted double bonds in cross-linked polymer systems.10 Bowman’s group has focused on microgel formation and radical entrapping for di(meth)acrylates.12−14 It has been reported that in the case of automotive clearcoating, the UV-curing system involves the kinetics of photopolymerization and synthesis of nanocomposite materials.15 Techniques have also been developed for detailed behavior analysis of the different components.16 This study reports on the curing behavior of three multifunctional monomers, 1,6-hexanediol diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA), and dipentaerythritol pentaacrylate (DiPEPA). The study of the curing behavior of the system involving these three monomers, facilitated by depth profiling following high-performance liquid chromatography, is a unique research investigation. The novelty of this work is to develop an insight to the curing behavior of specific monomers that are located in the film in terms of size and depth. The results of the Received: Revised: Accepted: Published: 16110
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Figure 1. Chemical structures of the monomers.
Figure 2. HPLC chromatograms of standards. The x axis represents retention time, and AU on the y axis represents absorbance units (a signal corresponding to the response created by the detector) at 210 nm. 16111
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Figure 3. HPLC chromatograms of HCC microtomed sections.
2.2. Preparation of Nano-Based Formulations. Functionalized nanosilica particles (4% of the total formulation) were added to the proposed formulations. The process was followed by mixing for 5 min, and then the samples were subjected to a powerful ultrasonic treatment through a five-step process for 15 min (each step lasted 3 min). Ultrasonic waves were applied under atmospheric conditions at room temperature, and the power was set at 280 W. In order to prevent any prepolymerization, all opaque glass bottles containing nano-based formulations were held in an ice bath for the duration of the process.17 2.3. Curing Conditions. All formulations were applied on glass plates using a film applicator (35 μm). The samples were then irradiated with a high-pressure mercury lamp equipped with a conveyer. Irradiation was applied at full power, with the lamp set at 1650 mJ/cm2, and the conveyer speed was 2 m/min. The
study also supply useful information comparing the curing behaviors of nano-based and neat formulations.
2. EXPERIMENTAL SECTION 2.1. Materials. The acrylate prepolymer used for the tests was aromatic urethane acrylate (Cytec Co., Ebecryl 204). HDDA and TMPTA were provided by Eternal Co., and DiPEPA was purchased from Sartomer Co.. The chemical structures of these monomers are shown in Figure 1. Benzophenone (Eternal Co., Eterphoto PI BP) was used as the photoinitiator (PI); it is an Habstraction type of photoinitiator. A tertiary amine acrylate (Eternal Co., Etermer 641) was used as the co-photoinitiator. UV absorber (UVA) and hindered amine light stabilizer (HALS) with the trade names TINUVIN 400 and TINUVIN 292, respectively, were obtained from CIBA Co. 16112
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Figure 4. HPLC chromatograms of HCNC microtomed sections.
2.5. Microtomy Technique. Microtomy was performed using a Leica Microsystems Jung MultiCut 2045 microtome, which was used to cut the coating horizontally into thin sections. At first, specimens had to be kept fixed and adequately firm. In order to do this precisely, they were embedded in a solid paraffin medium with appropriate physical properties to enable thin sections to be cut easily. Subsequently, three sections were cut with various thicknesses. One of the sections had a surface of 5 μm and the other two sections had surfaces of 15 μm. 2.6. HPLC Measurements. The technique of solvent extraction was used to quantify monomers from cured films, and they were analyzed with high-performance liquid chromatography (HPLC). In solvent extraction, all of the microtomed sections were immersed into acetonitrile. A 1 cm × 1 cm portion of each section was immersed in 5 mL of acetonitrile. All of the samples were kept in opaque glass bottles for 48 h. Samples were
light intensity was measured using a UV Runwing (UV mini) radiometer. 2.4. Measurement of CC Conversion. The obtained films were analyzed by infrared spectroscopy (Spectrum One, PerkinElmer). Quantitative measurements of the degree of conversion (X) were calculated according to the absorbance of two particular bonds using the following equation: X=
A 0810 − A t810(A 0base /A tbase) A 0810
810 −1 where A810 0 and At represent the absorbance at 810 cm (CC 18−20 stretching vibration for acrylate double bonds ) before and after base UV exposure, respectively, and Abase represent the baseline 0 and At absorbance before and after UV exposure, respectively. The baseline absorbance point was set at 1730 cm−1 (CO stretching vibration for carbonyl bonds21).
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Figure 5. Main entrapped residual monomers in different microtomed sections.
Table 1. Compositions of the Four Formulationsa resin (wt %) HDDA (wt %) TMPTA (wt %) DiPEPA (wt %) PI and amine (% of total formulation) UVA (% of binder solid weight) HALS (% of binder solid weight) nanoparticle (% of total formulation) degree of conversion
Table 2. Degrees of Conversion Derived from FT-IR Spectra
HCC
LCC
HCNC
LCNC
50 40 5 5 4 1.3 0.7 − 0.93
33 23 22 22 4 1.3 0.7 −0.6
50 40 5 5 4 1.3 0.7 4 0.9
33 23 22 22 4 1.3 0.7 4 0.9
coating HCC
HCNC
LCC
a
LCNC
Abbreviations used for the different formulations: HCC, High Converted Coating; LCC, Low Converted Coating; HCNC, High Converted Nano-Based Coating; LCNC, Low Converted Nano-Based Coating.
section
conversion (%)
surface section second section third section surface section second section third section surface section second section third section surface section second section third section
100 88 84 81 95 95 81 78 58 90 90 92
conversion generally increased, and there was also a dramatic increase in the degree of conversion for the third sections. This provides a uniform cure through the bulk of the coating. One possible factor that could be responsible for this behavior is that aggregates comprising several nanoparticles (other than large particles that cause a break in dispersion) would cause scattering and thus reflect the UV light. But this may effectively enhance the efficiency of photoinitiation and cause the final conversion rate to increase.22 Furthermore, light scattering causes UV radiation to penetrate deeper and thus increases the degree of conversion in the bulk.15 3.2. Determination of Residual Monomers. HPLC chromatograms for pure monomers (termed “standards”) are shown in Figure 2. Samples were analyzed by evaluating the establishment of standards peaks as described in the Experimental Section. Measurement values indicate an amount for each monomer within a sample. The evaluations show amounts of extracted residual independent monomer in cured formulations that had not fused to the network. Moreover, in Figure 3, the retention times of the HPLC measurement show close values for the HDDA and TMPTA standards, even closer than measuring error amount (1%). Considering this, overlap between the peaks in the HPLC chromatograms was deemed to be possible. This means that the amounts of residual HDDA and TMPTA monomers could
then shaken for 12 h at 35 °C. A Waters pump (model 600E), a Waters variable-wavelength detector (model 2487) at 210 nm, and a Nova-Pak silica chromatographic column were used for HPLC analysis. The flow rate was 1.5 mL/min at room temperature. The gradient was 100% H2O to 100% MeCN in 20 min for both standards and examined samples. It was necessary to hold the system for 15 min on 100% MeCN to examine the samples. HPLC analysis was repeated five times. The retention time measurement error was calculated to be 1%.
3. RESULTS AND DISCUSSION The curing behaviors of coatings with different compositions were studied using two selected formulations [High Converted Coating (HCC) and Low Converted Coating (LCC)], and a great difference was determined for the degree of conversion within the acceptable range. Two nano-based formulations [High Converted Nano-Based Coating (HCNC) and Low Converted Nano-Based Coating (LCNC)] were produced by adding nanoparticles to HCC and LCC. The compositions of the four formulations are presented in Table 1. 3.1. Degree of Conversion. Evaluations of the degree of conversion calculated by FTIR analysis are given in Table 2. The results indicate that in the nano-based formulations, the degree of 16114
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Figure 6. HPLC chromatograms of LCC microtomed sections.
not be discriminated from a single peak and therefore had to be considered as an aspect of the whole. The results of HPLC measurements on high converted coating samples without and with nanoparticles (HCC and HCNC) are shown in Figures 3 and 4, respectively, while the results measured on low converted coating samples (LCC and LCNC) are shown in Figures 6 and 7, respectively. Numbers that determine the peak location and HPLC value are presented next to the each peak for ease of evaluation. The peak location numbers were used to recognize the corresponding monomers by matching the numbers to location values for standard peaks and for recognition of the related monomers. Furthermore, the HPLC values show the quantities of the specified residual monomers. Although it was possible to convert the relative numbers (quantitative amounts of each independent monomer) into a weight unit, the main conclusion was drawn from comparing these numbers. According to Figure 3, the amounts of residual monomers in the surface of microtomed sections were much less than other ones.
Clearly, UV irradiation at the top of the coating was more extreme, and it was reduced as the thickness increased. High irradiation leads to high initiation rates. With high initiation rates and the resulting fast propagation, volume shrinkage rates experienced a severe decline. This slowed down the reaction rate much more than expected, which in turn temporarily provided excessive free volume in the system to enhance the mobility of monomers (more notably, the smaller ones). Under these conditions, entrapped monomers became capable of approaching the active sites and taking their part in the network. As a result, there was less residual monomer in the more highly irradiated parts of the coating such as surface thicknesses. Besides, as more monomers joined the network, final conversion evaluation increased (as shown in Table 2).23 Furthermore, the amount of residual DiPEPA monomer in the surface of the microtomed section was more than that in the others. It seems that the mentioned temporary excessive free volume provided by high UV irradiation was not enough to move 16115
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Figure 7. HPLC chromatograms of LCNC microtomed sections.
concentrations in the vicinity of active sites. The cyclization reaction leads to the formation of compact structures that form microgel particles. These compact structures are formed at the beginning of the polymerization process. However, the higher reaction rates provided by higher irradiation in the surface of the microtomed section cause the formation of microgel particles at earlier degrees of conversion, and many monomers get entrapped among the chains.24,25 In view of the fact that the temporary excessive free volume can release only HDDAs and TMPTAs, many residual DiPEPAs will remain entrapped.
the entrapped large DiPEPA monomers. Conversely, the third microtomed section was replete with residual HDDA and TMPTA monomers. As expected on the basis of their better functionality, DiPEPA monomers were more highly reactive than the other monomers. Thus, DiPEPA joined the network more easily, and as per expectations, this monomer disappeared faster.24,25 Figure 5 schematically shows the main entrapped residual monomers in different microtomed sections. Figure 6 shows similar behavior, but the amount of residual monomer decreased entirely and the degree of conversion was increased, as shown in Table 2. The possible reason for these behavior responses, as mentioned before, could be attributed to the effects of light scattering by nanoparticle aggregates. The same behavior responses can be seen in Figures 6 and 7. The only difference is the noticeable amount of residual DiPEPA monomer in the surface of the microtomed section of LCC formulation. Higher concentrations of DiPEPA in LCC formulations caused an increase in the average number of functionality (favg) in this formulation. As the favg increased, cyclization reactions such as intramolecular cross-linking increased. This means that active sites on the chains were more willing to attack the pendant double bonds on their own backbones. This can be attributed to the apparent increased reactivity of the pendant double bonds on the same chain compared with monomeric double bonds as a result of higher
4. CONCLUSION The curing behaviors of three different monomers, namely, HDDA, TMPTA, and DiPEPA, have been studied. Nanosilica particles were also added to the samples. FTIR and HPLC techniques were used to analyze the samples. The results indicate that in neat formulations, higher irradiation at thicknesses close to the surface allowed the system to reach a higher degree of conversion. Furthermore, the free volume provided by high UV irradiation in the surface section was not enough to move the entrapped large DiPEPAs. However, higher reactivity of DiPEPAs compared with the other monomers in the third section caused them to join the network earlier. This response can explain why the surface section and the third section were replete with residual DiPEPAs and residual HDDA and TMPTA, respectively. 16116
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In nano-based formulations, because of the scattering effect caused by nanoparticle aggregation, the degree of conversion in different sections did not get closer together. Moreover, an increase in average functionality number shows a decrease in the final degree of conversion. It also increased the amount of remaining unreacted monomers.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +98 21 22956126. Notes
The authors declare no competing financial interest.
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REFERENCES
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