By S.P. Van Remortel, R.E. Ratcliff and D.D. Kragten, Unimin Corporation
Although it is deficient in crystalline or free-silica, nepheline syenite otherwise provides physical performance properties that match ground crystalline silica fillers. Since nepheline syenite functional fillers are naturally derived, deficient in free silica and heavy or transition metals, they typically are less burdened by regulatory requirements, such as REACH, RoHS and TSCA.
Figure 1 shows the typical particle size and shapes of ultrafine nepheline syenite. Table 1 lists the typical properties of nepheline syenite. Nepheline syenite is considered a moderate gloss reducer, based on its low oil absorption and combination of angular, rectangular and nodular shapes. Mohs hardness on the 1-to-10 scale is about 6. The particles themselves are moderately hard or rigid and possess high compressive strength, providing scratch and abrasion resistance in the polymer matrix. The low oil absorption and alkali aluminosilicate surface chemistry contribute to the ease of wetting and rapid dispersion with low viscosity build.
The essential physical property of nepheline syenite that provides high clarity in clear UV-cured resins is the ideal match in Refractive Index (R.I.) with that of the resins. The R.I. of nepheline syenite is compared with the R.I. for several resin systems and other common mineral fillers in Figure 2. The R.I. for nepheline syenite is in the range of 1.50 to 1.53, matching several types of resin systems and oligomers used for radiation curing. Nepheline syenites R.I. matches particularly well with acrylic, urea and urethane monomers and oligomers for exceptional clarity when properly wet out and dispersed in the host binder system.
Previous investigations showed that ultrafine nepheline syenite has exceptionally high light transmittance in organic binder systems commonly used for radiation curing in the critical UVA and UVB wavelengths1. Thus, unlike other mineral fillers and pigments, ultrafine nepheline syenite is not expected to interfere with the UV curing process.
Additionally, it was discovered that nepheline syenite, with sufficiently controlled particle top-size (i.e. D99 <5µm), not only maintains but enhances key properties in radiation-curable clear coatings – such as optical clarity, image clarity, % haze, scratch resistance and gloss – when compared to conventional sizes (i.e. D99 <30µm) of nepheline and other mineral fillers1. An illustration of the improved optics and relationship to particle top-size is found in Figures 3a and 3b. The quality, clarity and warmth of a UV-cured high-solids acrylic topcoat, applied over stained and sealed maple veneer, filled at 10% with nepheline syenite with a top-size of 5 microns (5µm N.S.), is shown in Figure 4.
Figure 3b. Optical clarity as a function of N.S. top-size and concentration in an aqueous UV-cure PUD with ~2 mil DFTFurthermore, initial testing by UV coating producers and contract labs with nepheline syenite fillers in UV-cured formulations indicated that ultrafine prototypes could have a positive impact on hardness and cure rate. Further experiments measuring pendulum hardness and percent double-bond conversion of filled films while regulating the cure energy showed that ultrafine nepheline improved cure rates1. This is illustrated in Figures 5a and 5b, respectively. Improved cure rates by filler addition opens the possibility for greater line speeds or reduced energy requirements simply by adding ultrafine nepheline syenite to the system.
Experimental
Previous experiments noted above confirmed ultrafine nepheline syenite has a positive effect on cure rate and film hardness in UV-cure PUD systems, opposed to unmodified systems using indirect measures. To better investigate the real-time curing behavior of ultrafine nepheline syenite, the effects of loading level and varying film thickness, and the potential to reduce the required amount of costly photoinitiators, an improved analytical technique was required.
UV-Cure PUD Test Formulations
The particle size statistics for ultrafine nepheline syenite (5µm N.S.) used in the experiments are found in Table 2. The 5µm N.S. was first incorporated into a stable resin-free pre-dispersion found in Table 3 so it could be added directly to test formulations. The UV-cure PUD test formulations used in this investigation are found in Table 4. The photoinitiator selected for this study functions optimally at higher UV wavelengths of 360 to 440 nm to match the energy of the UV-LED source used for curing.
Cure rate determination using modified FTIR and spot cure apparatus
A Dymax DX-1000 Visicure Spot was utilized to cure the samples during testing in the FTIR. This UV spot curer has a peak wavelength of 405 nm. The spot curer power can be digitally set from 0 to 100%. At peak power the output energy is 15 kW/cm2. The power was set at 40% and held constant for all curing tests and is fed via fiber-optic cable to the area to be cured. This high-intensity source provides virtually instantaneous and controlled power output, making it particularly suitable for measuring real-time cure rates.
The test formulations were applied to Leneta release charts form RC-5C with a wire wound bar at the specified wet thickness. The coatings were allowed to air dry for 10 minutes, followed by 10 minutes at 50ºC. A small section of the coating was scored with a razor blade, then peeled off the release chart using transparent tape. The coating was then attached to a metal holder with tape for measurement in the FTIR instrument.
The curing reaction was measured with a Bruker Vertex 70 FTIR in rapid scan mode. A typical 3D plot of the spectrum for the area of interest can be found in Figure 6. The resolution for testing was set at 8 cm-1 and measured from 1800 to 600 cm2 with the liquid N2MCT detector and 0.5mm aperture. Approximate scan time was about one minute, in which a total of 750 measurements were performed.
Curing was followed by taking the FTIR band area of the reacting resin carbon-carbon double bond (C=C, 800-820 cm-1) and normalizing with a band of the resin that does not change during curing (825-845 cm-1). The cure rate was determined by measuring the time required to react half of the available C=C (t1/2), where complete curing is taken as the intensity of the C=C band after exposure to UV radiation for the full one minute.
The 5µm N.S. loading and photoinitiator interaction via DOE
A design of experiments (DOE) was employed to determine the effects of loading levels of 5µm N.S. and photoinitiator on the cure rate of the UV-cure PUD test formulation. The design of experiments was set up using statistical software as a multifactorial design with two factors and one response. The two factors studied in the DOE were loading levels of photoinitiator (g) and of 5µm N.S. (% solids in cured film). The response was t1/2, which is the time required to react half of the available C=C in seconds, and is expressed as the desirability level. A desirability of 1 is ideal, as it represents the lowest cure time for t1/2. The fastest t1/2 was typically around 0.5 seconds. A desirability of 0 represented the slowest t1/2 and was typically around 1.5 seconds. A diagram illustrating this DOE can be found in Figure 7. This DOE was run in triplicate, producing 27 separate measurements.
The cure rate was determined by measuring the time required to react half of the available C=C (t1/2), where completed curing is taken as the intensity of the C=C band after exposure to UV radiation for one minute. The total degree to which the coatings were cured could be determined by analyzing the amount of this band remaining at the end of curing. The degree of curing, or crosslinking, is expected to have a substantial effect on coating properties such as hardness and flexibility.
Assignment of the C=C bands in the FTIR spectra of the urethane acrylate resin were based on comparison with other spectra. Good agreement was found with spectra of various methacrylate resins published on the NIST Chemistry Webbook2. The vibrational modes of the vinyl moiety can be assigned by comparison with the well-known assignments of the vibrational spectrum of ethylene. FTIR bands at 800-820 and 1400-1410 cm-1 are both due to C-H bending modes of the vinyl moiety of the acrylate3. Two bands in the FTIR spectra at approximately 800 and 1400 cm-1 were used to determine the cure rates of the testing coatings. The 1400 cm-1 band was relatively stronger than the band at 800 cm-1 and could potentially be used to consider other filler types that might otherwise interfere with the 800 cm-1 band.
Results and discussion
The first series of experiments studied the kinetics of the UV curing reaction as affected by factors of applied film thickness, 5µm N.S. filler loading and photoinitiator levels (100% and reduction by 50%). Of most interest was the effect of ultrafine nepheline syenite curing behavior in filled UV-cure PUD clear versus an unmodified formulation.
As noted, the time required to react half of the available C=C is presented as a measure of cure rate, which occurs in the first couple of seconds after the UV source is turned on. Maximum cure for the system was determined to be about 80%, based on the available double bonds when cured for a full minute. The UV-curable binder was a polyurethane dispersion functionalized with acrylate for double-bond reactivity. UV-curable polyurethane dispersions are higher molecular weight, and not all C=C are available for conversion due to mobility issues. Sufficiently cured film properties at 50% double bond conversion are obtained and selected as the benchmark for cure rate testing.
Loading level of filler and cure rate
The cure rate results for 5µm N.S. filled systems at 6 and 12% by weight vs. the unmodified control are presented in Figures 8a and 8b at both 1.5 and 3.0 wet film thickness, respectively. The unmodified control at 3.0 mils cured slower than at 1.5 mils. However, both addition of 5µm N.S. at 6% and 12% loadings based on resin solids had a positive effect on cure rate. Both filler loadings exhibited faster cure rates than the unmodified control. The 5µm N.S. loading at 6% had equivalent cure rates at both film thickness. Moreover, there appears to be an optimal loading level of 5µm N.S. to accelerate the curing time in this system. Optical factors, such as haze, may be coming into play with increased loadings. The data also suggest that 5µm N.S. also is more beneficial for curing at greater film thickness by potential mechanisms that make better use of the limited UV energy. Potential mechanisms that could explain this behavior are enhanced light transmission, diffusion or even control of oxygen inhibition at the surface when transparent 5µm N.S. particles are present vs. when they are not.
Level of photoinitiator and cure rate
The second series of FTIR experiments studied the effect of 5µm N.S. on cure rate of formulations with standard and 50% reduced concentration of the photoinitiator. The results shown above suggested that nepheline syenite has a UV cure synergy with the photoinitiator, and photoinitiator level can be adjusted with no loss of cure. The results for testing at standard and at 50% reduced photoinitiator levels were
measured as percentage cured vs. time. The plotted functions are found in Figure 9. Time for initial cure (50% C=C conversion) is found in Figure 10. Reducing the photoinitiator level by 50%, or half, in the unmodified control has a significant negative effect on the initial cure rate and cure profile, which is not surprising. However, when using 12% 5µm N.S. in the coating with half the normal amount of photoinitiator, the cure rate decreased only slightly and is comparable. Again, the initial cure rate with 5µm N.S. is much faster and superior as compared with the unmodified control with standard photoinitiator level, and with more thorough cure after one minute.
DOE: 5µm N.S. and photoinitiator impact on cure
To further study the effect of 5µm N.S. on radiation curing and interactions with the photoinitiator, a design of experiments (DOE) was conducted. The design parameters and results of these measurements are listed in Table 5 and Table 6. The standard deviation in the curing rate, as expressed by the half-time, is approximately 10%. The error analysis for each FTIR measurement band showed the R2 or correlation values both exceeded 90%, which is excellent for this model and shows a high level of correlation and confidence in the predictions.
With the aid of statistical graphing software, the contributions of 5µm N.S. and photoinitiator to the curing rate were compared with response surface plots, contour plots and Pareto charts. The response surface plots are useful to show the correlation of the two factors (5µm N.S. and photoinitiator) with the response (t1/2). The contour plot is useful to make predictions of the cure time based on 5µm N.S. and photoinitiator loading levels. The Pareto chart determines the importance of each factor in the response.
The response surface plots for the DOE for each band (800-810 cm-1 and 1400-1410 cm--1) can be found in Figure 11a and Figure 11b. A desirability of 1 is ideal, as it represents the fastest, or lowest, cure time for t1/2. The response plots show a large correlation between the level of photoinitiator and the cure rate, which is expected. To a lesser degree, the amount of 5µm N.S. influences cure rate. For each FTIR band, both surfaces are almost identical, confirming that both represent the same chemical conversion.
Contour plots for the response surfaces are shown in Figure 12a and Figure 12b. These plots predict the possibility of reduction in photoinitiator in combination with the use of 5µm N.S. For example, at desirability of 0.5, or approximately a t1/2 of 0.8 seconds, a loading level of 6% solids of 5µm N.S. could help reduce the photoinitiator level by almost 20%. For large-scale production lines, this reduction could be substantial. To achieve the fastest cure, the photoinitiator can be reduced by the same amount at a loading level of 12% 5µm N.S.
The Pareto charts for each band can be found in Figure 13. Pareto charts illustrate the importance of each factor being studied. The standardized effect is the absolute value for that factor and includes a reference line. Effects beyond this line can be considered potentially important. The level of photoinitiator is the most important factor; however, a loading level of 5µm N.S. also has a significant effect on the curing time.
Economic considerations with synergistic ultrafine nepheline syenite
An important factor in the formulation of commercial UV-cure coatings is the raw material costs. One of the main benefits of a natural mineral filler suitable for use in clear UV-cure formulations is the much lower cost compared with that of the resin. Fillers typically cost on the order of 10 to 20 times less. This suggests that adding 5µm N.S. at 5% by weight on resin solids will reduce the formulation cost by approximately 5%. Adding 10% by weight will reduce applied film cost by 10%; 15% addition equals 15% reduction in cost, and so forth.
Additionally, it was shown that 5µm N.S. has a synergistic effect with the photoinitiator, allowing the system to maintain cure with a lower percentage of photoinitiator in the system on the order of 10% to 20%. The ability to lower the amount of photoinitiator by 10% to 20% at any given loading level can lower the formulation cost by an additional 1% to 2%. In thin film UV-cured applications like OPV and inks, the potential to load 5µm N.S. as high as 20% to 25% is realistic. Of course, the maximum allowable filler loading depends on the applied film thickness, haze and clarity tolerances in each application.
Conclusion
Ultrafine nepheline syenite possesses unique transparent optical and physical properties that are compatible and particularly well suited for enhancing performance in radiation-curable applications. Using real-time FTIR analysis methods to measure double conversion rates, it was found that ultrafine nepheline syenite significantly accelerates cure rate in UV-cure PUD at 6% and 12% loading levels vs. the unmodified or conventional system. The mechanism by which 5µm N.S. accelerates cure is still unknown, but possibly explained by either increased oxygen inhibition or more efficient transmittance and diffusion of the low energy UV light with these fine optically transparent particles present in the film matrix.
A DOE also was employed to consider the interaction of photoinitiator with 5µm N.S. The DOE, with high correlation, confirmed that the peroxide photoinitiator level is the most important factor in determining cure rate, which was expected, and the level of 5µm N.S. also is an important factor. The DOE also predicts at that 5µm N.S. can potentially replace up to 20% of the photoinitiator with no loss in cure. Additionally, 5µm N.S. is an excellent tool for lowering formulating cost while enhancing mechanical performance and ensuring enhanced and noninterfering UV-curing behaviors in clear systems and composites.
References
- Van Remortel, S., Ratcliff, R. “Ultrafine Nepheline as a Durable and Transparent Additive to Accelerate Radiation Cure.” RadTech, May 2010.
- NIST Chemistry Webbook, NIST standard reference database number 69, webbook.nist.gov/chemistry.
- Ethylene has vibrational modes at 835 cm-1 and 1413 cm-1 due to H-C-H in-plane rocking and H-C-C in-plane scissoring, respectively.
Unimin Corporation is a producer of low-iron nepheline syenite used in glass, ceramic, paint and plastics; quartz proppants for oil and natural gas stimulation and recovery; and high purity quartz, used in the fabrication of integrated circuits, solar photovoltaic cells and high-intensity lighting. For more information, call 203.442.2500 or visit [email protected].
