Ultrafine Nepheline Syenite as a Durable and Transparent Additive to Accelerate Radiation Cure S.P. Van Remortel and R.E. Ratcliff-Unimin Corp. Nepheline syenite is a naturally occurring feldspathic ore deficient in crystalline silica. Certain geological formations can be processed into very pure, bright and translucent pigments and performance additives for radiation cured coatings, adhesives, and overprint ink varnishes. Pure and beneficiated nepheline syenite exhibits high light transmission behavior in filled polymer and resin systems across the UV and visible wavelengths. This paper reviews the key properties of nepheline syenite and performance benefits obtained in clear UV cured systems with standard and new ultrafine sizes. The unique light transmission properties of nepheline syenite in the UV and visible wavelengths are investigated further along with the accelerated cure rate potential in radiation curable applications. INTRODUCTION Nepheline syenite is a silica-deficient functional filler and additive used globally in a variety of polymer filled coating, adhesive, and ink applications. Micronized sizes are valued for purity of color, gloss control, ease of dispersion with minimal viscosity build, surface property modification, and outstanding durability. It is composed of three minerals: soda and potash feldspar, and the mineral nepheline. Although it is deficient in crystalline silica, nepheline syenite otherwise provides physical performance properties which duplicate ground silica fillers. Since nepheline syenite functional fillers are naturally derived, and deficient in free-silica and heavy or transition metals, they are typically less burdened by regulatory requirements such as REACH, RoHS and TSCA. Figure 1 shows the typical particle shapes while 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 contributes to the ease of dispersion and low viscosity build. Chemically, commercial nepheline syenite is anhydrous and consists of sodium potassium alumino silicates. The surface chemistry of nepheline syenite is also ideal for 100% solids and low VOC systems. Owning a net negative surface charge or a natural “detergency” in aqueous systems, nepheline syenite accelerates dispersion times with little or no polymeric dispersant requirements, although dispersants aid in suspension and shelve life. Micronized nepheline syenite is utilized in air dry, baked and radiation curable clear coating application for its purity of color, light transmission and refractive index features. The Refractive Index (R.I.) of nepheline syenite is compared with the R.I. for several resin systems and other more 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 monomers used for radiation curing. Nepheline syenite R.I. is a particularly close match with acrylic, urea and urethane monomers and oligomers for exceptional clarity when properly wet out and dispersed in the host binder system.

Figure 2 - R.I. Comparison of Binder Systems with Mineral Fillers and Extenders

1.25 1.35 1.45 1.55 1.65 1.75

Aluminum Oxide

Calcium Carbonate

Barium Sulfate

Talc

Kaolin

Fumed Silica

Nepheline Syenite

Polyurethane

Epoxy

Alkyd

Nitrocellulose

Acrylic

Urea

Refractive Index (R.I.)

Clear and opaque protective and packaging coatings, adhesives and inks are increasingly radiation cured to improve process efficiency, lower energy costs, and satisfy sustainability objectives1. Conventional wisdom suggests filled or pigmented systems do not cure as well as unfilled systems since the pigment or filler can compete for the UV curing energy required to activate the UV cure initiator, especially in opaque systems as film thickness increases. The light transmittance behavior of nepheline syenite is unique, especially in the critical UV curing range. Figure 3 represents an early look into the light absorbance behavior of nepheline syenite. A micronized nepheline syenite was formulated into a radiation curable solvent borne urethane-acrylate at loadings of zero, 8, 12, and 16 % of the dry film weight. The coatings were applied over a thin UV transparent fused quartz disk and then scanned for absorbance versus wavelength. The preliminary findings suggested a radiation curable urethane filled with micronized nepheline syenite has similar and high light transmission in the visible range (400-700 nm), and even higher transmittance in the ultraviolet range (280-400 nm) compared to unfilled systems.

Figure 1 - SEM Photomicrograph of Nepheline Syenite at 5,000X

Table 1 - Typical Properties of Nepheline Syenite

Particle Shape Rectangular, angular, nodular Specific Gravity, g/ml 2.56-2.61 Mohs Hardness 6.0 Brightness (tappi) 85-94 Oil Absorption, % (ASTM 281) 22-35 Refractive Index 1.51-1.53 Moisture, % (ASTM C-566) .05 -.15 pH (20 % Slurry) 9.5 –10.5

Figure 3 - Absorbance of Micronized Nepheline Syenite vs. Wavelength in a Solventborne UV Urethane-Acrylate.

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8% Neph Sye

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Unfilled (solvent UV urethane-acrylate)

Figure 4 - Haze, Optical Clarity and Gloss as a Function of N.S. Top-size and Concentration in an Aqueous UV cure PUD a) % Haze (ASTM D1003-61)

R2 = 0.9949

R2 = 0.9962

R2 = 0.9959

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New Ultrafine Nepheline Syenite for Optical and Physical Performance Recently, new ultrafine sizes of nepheline syenite have been developed that offer superior performance in clear wood and industrial coatings compared to more expensive and lower clarity alternatives. These engineered particle size distributions of nepheline syenite provide desirable gloss modification, optical, surface hardness, scratch, abrasion and suspension characteristics in clear UV coating systems2. Extensive testing was completed with the ultrafine nepheline syenite in conventional clear solvent, waterborne and commercial UV cure resin systems. Figure 4 demonstrates the importance that nepheline syenite particle top-size and concentration play with respect to film haze, optical clarity, and gloss. The new ultrafine five micron top-size nepheline syenite (5 µm N.S.) has lower haze, higher clarity and gloss as loadings are increased compared to standard fifteen (15 µm N.S.) and thirty (30 µm N.S.) top-sizes when formulated neat into a commercial aqueous UV cure PUD system commonly used in wood floor and cabinetry applications.

Most often the purpose of adding a hard mineral additive to a clear coating system is to increase

physical performance. Figure 5 demonstrates that loading nepheline syenite into a UV cure PUD provides improved steel wool scratch resistance. Figure 6 compares detailed micrographs of the test surfaces before and after 50 cycles with steel wool. The systems modified with micronized nepheline syenite have little visible surface change, while the surface of the unfilled coating system has been visibly scuffed and eroded. This feature is especially useful in flexible coatings where the hardness cannot simply be increased by elevated cross-linking, since this would cause a loss of flexibility. Ultrafine nepheline syenite is particularly suitable to increase abrasion while retaining flexibility.

Figure 5 - Gloss Change for 50 cycles #1 Grade Steel Wool as a Function of N.S. Particle Size

and Concentration (1500 g load)

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Figure 6 – Micrographs (200x) of Coating Surfaces before and after Scratch Testing a) No filler i) before

b) 18% 30 um N.S. i) before

c) 24% 5 um N.S. i) before

ii) after 25 double rubs

ii) after 50 double rubs

ii) after 50 double rubs

Ultrafine nepheline syenite fillers also compare favorably with other pigment fillers such as synthetic silica (fumed, precipitated, or colloidal suspension), micron alumina and nano-size alumina pigment types. Figure 7 demonstrates that ultrafine nepheline has high image clarity relative to micron-size alumina (10 µm particle top-size), nano-size alumina, and colloidal silica pigments. Five µm N.S. has the best image clarity and approaches the image clarity of the unmodified UV cure PUD systems. Figure 7 - Relative Image Clarity of Pigment Additives at 6 % by Wt. in Aqueous UV cure PUD

30 µm N.S. 15 µm N.S. 5 µm N.S.

0%

unmodified 10 µm

Alumina Synthetic

silica Nano Alumina

Figure 8 compares the gloss of the filled and unmodified systems when subjected to Scotchbrite A testing after 10, 25 and 50 cycles. Comparative Scotchbrite scratch resistance confirms hard pigment additives provide improved Scotchbrite A resistance versus the unmodified system which drops more significantly in gloss. Ultrafine nepheline syenite provides comparable performance with micron and nano-size alumina, or could be blended with alumina to provide systems with even better clarity, higher pigment loading and abrasion resistance.

Ultrafine nepheline syenite is also an effective choice to lower the Coefficient of Friction (COF).

COF is the amount of force required to pull or slide like coatings or films apart and is a good indicator of handling properties of finished goods. Figure 9 compares the coefficient of friction for a modified and unmodified UV cure PUD system. Ultrafine grades of nepheline syenite are the most effective at reducing the COF. Ultrafine nepheline syenite can also reduce the tackiness and improve the handling features prior to cure in UV curable applications. Improved handling features lowers the risk of film damage prior to cure, saving time and money otherwise spent on fixing the defect.

Figure 8 - Scotchbrite Resistance (Fine A) Comparison of Abrasion Resistant Pigments at 6% by Wt. in Aqueous UV cure PUD

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Figure 9 - Coefficient of Friction Comparison of Pigments at 6 % by Wt in Aqueous UV cure PUD

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EXPERIMENTAL During development, laboratory testing and in preliminary customer trials, finer particle size distributions of nepheline syenite were believed to have a favorable impact on the cure rate in radiation cure applications. Systems where ultrafine nepheline syenite was tested with suspected improvements in hardness or cure time were: UV aqueous cure PUD, 100 % solids acrylic, and UV cure polyurethane powder coatings. The UV light transmission properties for nepheline syenite were known to be good. However, the relationship of UV transmission with nepheline syenite particle size had not been investigated in detail in a controlled test matrix. Furthermore, no standard test method existed to study the interaction with light and the varying wavelengths when dispersed in a polymer matrix. Also of particular interest was how finer nepheline syenite behaves in the UVA and UVB regions. Higher transmission in this specific region might possibly provide a positive effect in the acceleration of radiation cure times. This would be surprising since it is commonly believed that most hard durable filler pigments hinder the curing process, acting as either radiation absorbers or reflectors instead of transmitters.

Light Transmission To investigate the light transmission properties of nepheline syenite based on particle size distribution, three distinct particle sizes were chosen. Particle size statistics are found in Table 2. One standard size nepheline grade with a thirty micron particle top-size, an existing and ultrafine grade with a fifteen micron top-size and the new ultrafine five micron top-size were formulated into an aqueous UV cure polyurethane PUD (acrylate functionality) formulation as provided in Table 3. Test formulations were prepared for the three nepheline syenite sizes at 8% by Wt. based on clear resin solids. Ultrafine grades were sifted into the mixing vessel pre-charged with resin, UV initiator and the rheology modifier. The filler pigment additive was sifted in slowly and Cowles dispersed using medium speed with good agitation for twenty minutes. Modified and unmodified formulations were applied at three mils over thin UV transparent fused quartz disks (25 mm x 500µm), air dried ten minutes, forced oven dried for ten minutes at 49º C, and then UV cured in an Edmun ELC-500 UV oven ( peak UV, Mercury lamp 365 nm) for nine minutes. Only test specimens that had a film thickness within 3.0 mil ± 0.10 were used. The light transmission was then measured with a Shimadzu Mini 1240 UV/VIS Spectrophotometer. Additional fillers were tested using the same procedure.

Table 3—Clear Aqueous UV cure PUD formulation % Wt. of filler on clear solids 0 % 8 % Ingredient UV curable PUD 100 100 UV initiator 1.5 1.5 Urethane rheology modifier 1.0 1.0 Test filler 0 2.34 DI water 23.33 25.31 Siloxane wetting additive 0.62 0.62

Cure Rate by Pendulum Hardness Development and Double Bond Conversion vs. Cure Energy Two methods were used to study the cure rate effect of ultrafine nepheline syenite. The aqueous polyurethane formulation from Table 3 was employed for both methods. The first cure rate method involved measuring the surface hardness development as a function of applied cure energy. The second method considered the amount of double bond conversion as a function of applied cure energy measured by FTIR analysis3. For pendulum hardness the coatings were applied to glass at a 6-mil wet thickness and allowed to air dry for 10 minutes. The coatings were placed in a forced air oven at 45°C for 10 minutes. The test panels were then cured with regulated curing energy by turning down the power on an American Ultraviolet Company mini-conveyer, where one pass equals 100mW/cm2. After each pass, the pendulum hardness was tested and recorded with a Sheen Instruments Persoz pendulum instrument. FTIR double bound analysis was accomplished by applying the coating to a standard polyester transparency (3M CG3300) at a 0.5-mil wet thickness then allowing it air-dry for 15-minutes. Transparencies are fairly thin and have minimal contribution to the FTIR spectra in the range of 802-817cm-1. Small samples were then prepared and cured at increasing time intervals in an Edmund Optics ELC-500 oven. Samples were analyzed on a Nicolet Magna-IR 560. Both ovens used mercury lamps for a peak wavelength of 365 nm.

Table 2— Nepheline syenite Particle Size

Test Filler

Median, µm (D50) 99 % < µm (D99)

30 µm N.S. 5.0 30

15 µm N.S. 3.0 15

5 µm N.S. 1.5 5

RESULTS AND DISCUSSION In the radiation curable industry, there has been much speculation about the impact that mineral fillers have on radiation cure applications. It is suggested filled systems do not cure as well as unfilled systems. In some instances, whole cure lines are modified to accommodate certain pigment additions, or lamps with longer wavelengths added to provide adequate cure. Transmittance of Mineral fillers in the UV Curing Range Figure 10 shows the % transmittance results for the thirty, fifteen and five micron N.S. fillers versus the unmodified aqueous UV cure PUD formulation. Both the ultrafine fifteen and five micron N.S, grades have similar or greater transmission and with no interference in the UVB (280-320 nm) range. Thirty micron N.S., with its large particle top-size, does in fact lower the transmission in the UVB range, but starts to recover in the top end of UVA (320-400 nm) range. The ultrafine fifteen and five micron N.S fillers provide very little absorption or reflectance over both the UVA and UVB ranges suggesting they would cure efficiently in typical UV light configurations and photoinitiator types. The higher UV transmittance of ultrafine nepheline syenite also suggests enhanced cure efficiency in radiation curable coatings is possible. Figure 10 - % Transmission as function of N.S. Top-size and wavelength in Aqueous UV cure PUD at 8% Wt. Solids.

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Figure 11 compares the % Transmittance of unmodified and modified aqueous UV cure PUD systems when filled with ultrafine N.S. and several other common mineral filler types. Conventional fillers like barium sulfate, calcium carbonate, crystalline silica and calcined silica reduce the transmission of UV light through the film in both the UVB and UVA curing range. Ultrafine nepheline syenite sizes offer superior transmission properties when compared to these standard filler types. The results suggest that ultrafine N.S. formulations may have enhanced cure efficiency when compared to the unfilled formulation and when filled with other minerals. Using ultrafine N.S. could also eliminate the need to alter UV cure equipment and processes when selected as a performance additive in UV cure applications.

Figure 11 - % Transmission of Fillers as a Function of Wavelength in Aqueous UV PUD at 8 % by Wt.

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blank15 um N.S.15 um SiO25 um N.S.10 um barium sulfate10 um CaCO320 um calcined SiO2

UV Curing Performance To verify that the superior light transmission features of ultrafine nepheline syenite lead to enhanced radiation curing, test coatings were prepared with the UV curable PUD with 15 and 5 um N.S. sizes at 0 and 6% weight on total resin solids. The coatings were then cured one pass on the mini-conveyer and then measured for pendulum hardness. One pass on the mini-conveyor was equal to 100mW/cm2. Pendulum hardness measures the number of oscillations. Softer coated surfaces deform more easily and thereby absorb more energy which results in fewer oscillations. Greater pendulum hardness as a function of applied curing energy provides evidence of an accelerated cure rate. The results are shown in Figure 12 and demonstrate that systems filled with ultrafine N.S. develop film hardness at a faster rate than the unmodified system. The five µm N.S. size is particularly effective for hardness development with less cure energy suggesting line speeds or production rates could be increased. Energy consumption could also be reduced by as much as fifty percent in systems filled with ultrafine nepheline syenite while providing equal or superior cure. Figure 12 – Pendulum Hardness Development with Increasing Energy at 6% N.S by Wt. in a UV PUD

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The same test formulation using five µm N.S. was also used to measure the double bond conversions by FTIR after half minute cure intervals in the Edmund Optics ELC-500 oven. Normally, nine minutes are required to ensure a complete cure. FTIR analysis can measure the depletion of carbon-carbon double bonds in the acrylate group. The rate at which double bonds decrease is a measure of the cure rate of the formulation. The conversion of the double bonds was determined by the formula:

0

0 )(

A

AAConversion t−

=

Where A0 is the area of the peak at 802-817cm-1 before cure. At is the area of the peak at some time t cure. This formula gives a percentage of the cure of the coating. Coatings with little or no haze are required for this method. Figure 13 compares the results for percent double bond conversion versus cure time. The initial results show some scatter in the data, but the general trend indicates that this method is viable and agrees fairly well with the with pendulum hardness finding that ultrafine nepheline can accelerate the conversion via the acrylate carbon double bond conversion step and not just by the addition of a hard filler alone. Figure 13 – % Acrylate Double Bond Conversion as a Function of Cure Time at 12% N.S. by Wt in UV PUD (FTIR Method)

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CONCLUSION Nepheline syenite is a versatile and unique functional filler, possessing properties that are useful in a wide variety of clear and opaque coating applications. Newer ultrafine nepheline syenite sizes have refractive index and physical properties that are particularly well suited for use as a performance additive in radiation curable coatings, inks and adhesives.

It was demonstrated that ultrafine nepheline syenite has exceptionally high light transmittance in organic binder systems commonly used for radiation curing in the critical UVA and UVB wavelengths. Thus, unlike other mineral fillers and pigments, ultrafine nepheline syenite is not expected to “interfere” with the UV curing process. Furthermore, experiments with pendulum hardness development and double bond conversion rate, with regulated curing energy, suggest that it is possible to actually improve cure rates for greater line speeds or reduced energy consumption when ultrafine nepheline syenite is added to the system.

Future work will be aimed at developing more precise methods to study the rate of double bond conversion with ultrafine grades, testing finer nepheline syenite sizes, and evaluating performance in other radiation curable applications such as 100% solids, overprint ink varnish, and adhesives for additional performance benefits and cost saving opportunities. REFERENCES (1) Wright T., “Introduction UV/EB Coatings Market Analysis Moving Forward.” Coatings World-April 2008. (2) Van Remortel. S., Ratcliff. “Ultra-fine Nepheline Syenite as a Hard, Transparent Filler for Increased Performance in Clear Wood, Industrial and UV Coatings.” Advancement in Coatings Series, September 2009. (3) Yang, B “Investigation of UV Curable Coatings and Adhesives by Real Time FTIR,” Sartomer Technical

Publication, 2005.