Multifunctional UV Curable Oligomers for Hard Optical Coatings
Abstract
Hard, optically clear, protective UV curable oligomers for hard optical coatings are necessary for a variety of demanding applications in the automotive, aviation, electronics and scientific instrumentation markets. Coatings with special properties, such as anti-fog, conductivity, anti-Newton ring, and anti-glare performance, are highly sought-after products. However, due to limited choices of oligomers, current coatings cannot easily meet customers’ varied demands.
Oligomers with low functionality may hardly meet scratch (mar) resistance or hardness specifications. Some coatings can only be applied as very thin films due to brittleness. Chemical resistance, anti-fog, anti-static and anti-glare properties all require artful formulation choices between monomers, oligomers, additives, photo-initiators and diluents.
New oligomers have been developed to solve problems like these. This paper discusses new polyurethane acrylate oligomers developed using grafting and block copolymerization that, under adequate formulation, provide excellent optical properties while presenting great toughness after UV curing. Raw materials with different characteristics of hydrophilicity, hydrophobicity, conductivity, elasticity, mechanical properties, and formulation compatibility are selected to obtain oligomers of desired properties.
Oligomers targeting anti-fog applications, glass adhesion, and soft touch applications are discussed. Specifically, anti-fog coatings for Polycarbonate (PC) substrate have been evaluated extensively for performance against many real-life use conditions, with some formulations compared against other commercial products in the marketplace.
Introduction
In modern society, advanced products often require outstanding materials not only for their bulk properties, but also improved surface properties. Polymeric materials found widespread applications due to easy processing, cost-effectiveness and versatile properties. However, surface characteristics at times cannot meet all the requirements of designated applications — hydrophilicity, hydrophobicity, conductivity, transparency, thermal, mechanical, and chemical resistance not found in bulk materials.
UV/EB curable coatings provide the versatility for fast processing that can complete the curing process in a few seconds, for environmentally friendly applications that essentially eliminate solvent usage. UV as a “cold” energy source allows curing to occur at low temperature. Polyurethane acrylates have been one of the key materials for optical coatings — their mechanical properties, wide range of hardness, high bearing capacity, flexibility, abrasion and weather resistance, adhesion, and low cost can be exploited through creative chemistry for performance coatings hardly matched by any other material.
However, due to limited choices of oligomers, current coatings cannot easily meet customers’ varied demands. Lack of functionalized oligomers often restricts creative venues for multipurpose applications. Through precision functionalization of polyurethane acrylates, many new oligomers were explored. Star shaped oligomers, and/or dendrimers, can be desirable candidates — they usually possess significantly lower viscosity than their linear chain counterparts, and multiple functions can be more easily derived from a dendrimer than linear counterparts.
One straightforward way to modify dendrimeric raw materials is by converting unifunctional groups to hetero functional groups (see Figure 1 below). Branched or dendrimeric raw materials with hydroxyl functional groups were converted to multifunctional UV curable polyurethane acrylate oligomers by reacting the hydroxyl groups with heterofunctional isocyanates.
Function Conversion
R1-OH + OCN-R2-NCO + HO-R3-Acrylates = R1-urethane-R2-urethane-R3-Acrylates
Figure 1. Conversion of hydroxyl dendrimer to multifunctional dendrimers
UV Curable Oligomer Synthesis & Development
Polyether polyols, polyester polyols, branched polyols, and dendrimeric polyols were tested from Perstorp Group, Panolam, Dow Chemical, Momentive, and Cray Valley. Heterofunctional monomers were sourced from Kowa, Bayer, and Sartomer. Multifunctional isocyanates were tested from Momentive, Kowa, Duranate (Asahi Kasei), Vencorex, and Vestanat (Evonik).
FTIR was measured using Perkin Elmer Spectrum 100
Viscosity was measured by a Brookfield viscometer RVF 100
Molecular Weight was evaluated by gel permeation chromatography, PL-GPC-50
A typical synthesis for anti-fog applications: isophorone diisocyanate was reacted with branched polyether polyol under constant stirring at 70°C for two hours with tin octoate catalyst. Stoichiometry was controlled to allow half of the isocyanate to react. Hydroxyl-functional acrylates were then added to react the remaining isocyanate, also at 70°C. Reaction progress and final product were monitored by FTIR.
For glass adhesion anti-fog applications, two oligomers were combined: Oligomer 1 reacted isophorone diisocyanate with HEMA, then polyether polyol, then dendrimeric polyol at 80°C for four hours. Oligomer 2 reacted triisocyanate TPA-100 with polyester polyol, then multifunctional hydroxyl acrylate and a glass adhesion monomer at 80°C.
For soft-touch applications, branched polybutadiene polyol and polyG polyol were reacted with isophorone diisocyanate at 70°C, followed by HEMA addition, with stoichiometry controlled to achieve appropriate viscosity.
Figure 2. FTIR of a reaction mixture including branched polyester polyol, diisocyanates, and hydroxyfunctional acrylates at different reaction stages. The peak at 2200 cm⁻¹ indicated unreacted isocyanates at an early stage; its absence indicated completion of the urethane reaction.
Figure 3. GPC of a reaction product including polyester polyol, diisocyanates, and hydroxyfunctional acrylates. Multiple peaks reflect the presence of molecules of different molecular weight and chain connection.
Formulation, Coating & Curing
Multifunctional UV curable oligomers were mixed with appropriate amounts of complementary oligomers, monomers, adhesion promoters, flow and leveling adhesives, and UV initiators. The mixture was diluted with suitable diluents under stirring to adjust viscosity for better flow and leveling. Formulations were coated onto substrates such as PC, PMMA, PET, or glass under spray or dip coating processes.
Spray coating used an EJN 208 spray nozzle. Dip coating was held at room temperature at a dipping speed of 2.54 cm/min. Upon drying at 40°C for approximately 3 minutes, coatings were further cured under UV irradiation.
Coating Evaluation Methods
Adhesion
ASTM D-3359 cross-cut test. A lattice pattern with 1mm spacing, brushed 5 times, then tape-pulled. Score 0–5 based on pattern removal.
Boiling Water Adhesion
Samples placed in 100°C boiling water for 30 minutes, air dried 2 hours, then cross-hatch adhesion tested.
Scratch Resistance
Steel-wool rotary test, 1.25 sq. in. #0000 pad at 2 psi for 5 rotations.
Taber Abrasion
ASTM D-1044. CS10F wheels with 500g load.
Bayer Abrasion
ISO CD 15258 / ASTM F735. Oscillating hard particles (Kryptonite) simulate everyday wear scratches.
Pencil Hardness
ASTM 3363, 750g weight, Mitsubishi Hi Uni pencils.
Optical Properties
Optical properties were measured using Color Quest II. Coated samples are generally stored after being masked by plastic films in a cold room before delivery. In masking tests, samples were masked and stored for predetermined times, then evaluated for adhesion, anti-fog, and optical performance changes as an indicator of storage stability.
Anti-Fog Properties Testing
Anti-fog properties were evaluated using a fogmaster/breath test and a beaker test. In the breath test, moisturized air was exhaled toward the sample surface, recording three phases: loss of transmission to an opaque film, spotted film from water drops, then a clear uniform water film that doesn’t reduce transmission. In the beaker test, samples were placed over a beaker filled with 60°C water and evaluated per ASTM D618-00.
Anti-fog properties against freezing cycles were also tested — samples cooled to -15°C for 45 minutes, checked at room temperature, then subjected to a second freeze/beaker cycle.
Chemical resistance was tested with chemical-soaked cotton balls placed on the coated substrate, observing for loss of coating, anti-fog properties, or integrity. Contact angle tests measured surface tension by placing a water droplet on the sample and recording the spreading diameter over time — a direct indicator that correlates closely with anti-fog performance.
Stability was characterized by performing the same set of tests described above, at predetermined time intervals after stability treatment. The tested values were compared against those derived before stability treatment.
Results & Discussion
Synthesis
Highly branched oligomers, especially dendrimers, generally possess lower viscosity than their linear counterparts — an important characteristic for coating process flow requirements. Curability, and therefore mechanical properties like hardness, highly correlate with functionality. By increasing functionality per chain, coatings more readily reach hard, scratch resistant, chemical resistant, and abrasion resistant properties. However, highly functionalized monomers can cause shrinkage stress during polymerization that yields brittle, cracking coatings. Because multifunctional oligomers can include both pro-adhesion segments and performance-enforcing segments in the same chain, coating performance using the developed oligomers was significantly better than commercial counterparts.
The derivation of quality multifunctional dendrimer depends on purity of reactants, reaction condition, stoichiometry, and sequence of feedstocks. High moisture level in raw materials, high reaction temperature, or inappropriate reactant ratio may lead to premature gelling or products with an inappropriate amount of functional groups for targeted applications.
Viscosity of samples was monitored within a tight range as a spec for the final product. The oligomers synthesized were applied across a number of applications — anti-fog coatings on PC and glass substrates, as well as soft touch applications, are presented below.
Coating Formulation & Property Evaluation
Anti-Fog Coatings
Anti-fog oligomers were mixed with a combination of acrylate monomers, anti-fog promoters, adhesion promoters, photo initiators, and solvent mixtures. Viscosity of formulations was 10 cP for spray coating and 50 cP for dip coating, using polycarbonate as the substrate. Coating samples were obtained by dipping at 2.5 cm/min, drying 90 seconds at 30°C, and UV curing at 700 mJ/cm².
One formulation, 931-PC, gave satisfactory properties across the spectrum. Results for a subset of samples — 931-PC-1, 931-PC-2, and 931-PC-3 — are reported below, with noncoated PC substrate used as the control. In the fogmaster/breath test, 931-PC showed no fogging while the control showed obvious fogging immediately.
Table 1 — Anti-Fog Beaker Test at 60°C
| Coating ID | Rainbow | Fogging |
|---|---|---|
| 931-PC-3 | 30 sec | No fog ≥ 30 min |
| Control | 30 sec | Fog immediately |
Table 2 — Duration Test
| Coating ID | Fog Start (s) | Fog End (s) | Non-Uniform Start | Non-Uniform End | Uniform Start |
|---|---|---|---|---|---|
| 931-PC-2 | 2-4 | 10-13 | 10-13 | 113-115 | 115 |
| Control | Immediate | 60 | 60 | 120 | 120 |
Sample 931-PC significantly outperformed the uncoated control in both fog resistance duration and uniform-film formation time.
Stability & Durability Testing
Stability of samples against storage — masking, cleaning, and freezer placement — was evaluated by examining anti-fog three-phase performance after predefined time intervals. For masking, samples were checked for fogging after 3 and 6 months.
In the cleaning test, samples were sprayed with Windex and wiped clean with a dry, scratch-free rag, then repeated until the sample began to fog longer than three seconds. The anti-fog property proved sustainable against cleaning agents.
Table 3 — Cleaning Cycle Durability (931-PC-1)
| Coating ID | Before Test | Cycles 1–11 | Cycle 12 |
|---|---|---|---|
| 931-PC-1 | No fog 1-30 sec | No fog 1-30 sec | No fog 1-6 sec (fail) |
| PC Substrate | Fail | ||
931-PC-1 sustained 12 cycles of cleaning treatment before performance degraded.
The freezer test indicates stability of anti-fog properties after cold temperature exposure — samples placed at -15°C for 45 minutes, checked at room temperature, then treated a second time before a beaker test at 60°C.
Table 4 — Freezer Test (In Air / Over Beaker)
| Coating ID | Fog Start | Fog End | Non-Uniform Start | Non-Uniform End | Uniform Start |
|---|---|---|---|---|---|
| 931-PC-1, in air | 3-6 | 41-43 | 41-43 | 96-98 | 96-98 |
| 931-PC-1, over beaker | 2-5 | 8-18 | 8-18 | 30-33 | 30-33 |
| Control | Immediate | 140 | 140 | 180 | 180 |
High Temperature Stability
Haze, light transmittance, and yellow index were measured on samples before and after high temperature treatment — a preheated oven at 100°C for five hours, followed by cooldown to room temperature for at least 2 hours. Tested values were expected to stay within specified ranges: transmittance within 95% of original, Yellow Index within 3%, and Haze within 5%.
Table 5 — Change After High Temperature Treatment (931-PC-1)
| Before | After | Gain | |
|---|---|---|---|
| YI E (%) | 0.74 | 0.83 | 12% |
| YI D (%) | 0.88 | 0.98 | 10% |
| Haze (%) | 0.22 | 0.24 | 8% |
| Luminous Transmission (%) | 89.63 | 90.68 | 1% |
| Adhesion (%) | Pass | Pass | No Change |
| Anti-fog | No Fog | No Fog | No Change |
Surface Tension & Contact Angle
Three perfect scores are required for the cross-cut adhesion test to indicate good adhesion (ASTM 3359). Anti-fog surfaces can be hydrophilic or hydrophobic, and this study observed a dynamic change of surface tension over time. The advancing contact angle of anti-fog samples changed from 40° to 10° within 4 seconds, while untreated PC showed a constant angle of 72°.
The contact angle after a month’s placement was 7°, indicating long-term coating stability. Within a few seconds, a water droplet spread quickly on the anti-fog surface, reaching a plateau in three minutes — indicating an extremely hydrophilic surface.
Figure 4. Water contact area change with time as a measure of surface tension for 931-PC.
Table 6 — Water Contact Angle (°) Over Time
| 1 sec | 2 sec | 3 sec | 4 sec | 30 days | |
|---|---|---|---|---|---|
| 931-PC | 40 | 25 | 15 | 10 | 7 |
| PC (control) | 72 | — | — | — | — |
Figure 5. Surface tension is caused by the unbalanced forces of liquid molecules at the surface.
The contact angle is governed by the interaction of liquid-vapor, solid-vapor, and solid-liquid interfacial tensions, according to Young’s equation. Because solid-vapor tension does not change over time, the liquid-vapor and solid-liquid tensions are responsible for the observed changes — likely due to surface ligands rearranging over time from exposure to water, leading to a more hydrophilic surface.
Mechanical Properties & Chemical Resistance
Adhesion test was performed using the cross-cut method. A score of 5, corresponding to 100% adhesion, indicates good adhesion and is denoted as “pass.” Three passes have to be satisfied for a good coating. Adhesion tests of anti-fog samples prior to and after 100°C oven treatment were performed on samples, and adhesion after boiling was also tested for robustness of coating. All coated samples reported in this study showed good adhesion for PC.
In general, addition of anti-fog enabling components to the formula impacts adhesion, most of the time inversely — therefore, formulation is challenging in achieving desired performance. Adding heterofunctional oligomer should provide advantages in terms of performance, storage stability against phase separation, and easy processing.
Scratch resistance steel wool tests, pencil hardness, and Taber abrasion tests evaluated coating quality against wear forces. In scratch resistance tests, both the depth and the width of the scratches by steel wool were recorded and compared for scratch resistance. Pencil hardness had to be B or higher for qualification.
Table 7 — Haze Increase After Abrasion (931-PC-2)
| After 100 Cycles | After 500 Cycles | |
|---|---|---|
| Haze gain (%) | 9.7% | 31.5% |
Bayer test carried out using 500g of Kryptonite with 600 cycles. Bayer ratio for 10–11 micron film thickness is 1.43. Pencil hardness of B to H was obtained on PC samples showing quality anti-fog performance.
Chemical resistance of coated and noncoated samples was compared for the length of time necessary to produce noticeable changes. Coated samples did not show significant changes against most organic chemicals tested, though they were more susceptible under basic solutions such as sodium hydroxide — likely due to the hydrophilicity of the coating allowing easier penetration.
Table 8 — Chemical Resistance (931-PC vs. Uncoated PC)
| Chemical | Uncoated | Coated |
|---|---|---|
| Distilled White Vinegar | 2 min | 10 min |
| 28% Ammonium Hydroxide | 2 min | 8 min |
| Butyl Acetate | 2 min | 4 min |
| MIBK | 4 min | 8 min |
Glass Adhesion Anti-Fog Coating
Standard anti-fog formulations do not have sufficient adhesion onto glass without primers. Replacing key components, especially highly ethoxylated oligomers, with glass adhesion components results in a loss of anti-fog properties. An effort was made to develop two multifunctional oligomers: one with a glass adhesion promoter, one with an anti-fog promoter.
Oligomer 1 (glass adhesion) carries acrylate and hydroxyl groups, while Oligomer 2 (anti-fog) carries acrylate and isocyanate groups. Mixed together, they provide dual cure reaction mechanisms — UV cure and thermocure. With the proper mixture of Oligomer 1, Oligomer 2, and anti-fog enablers, formulations achieved better glass adhesion while maintaining anti-fog performance.
Soft Touch
Soft touch coatings are another area where UV-curable oligomers play a significant role. To achieve soft touch performance, the final product must have high elasticity and low Young’s modulus at usage temperature — properties found in natural rubber, PBD rubber, and silicones. UV curable soft touch formulations demand conversion of these existing oligomers to UV curable functional oligomers.
Polybutadiene polyol was used as a key raw material to derive the final oligomer, with PBD polyol of different molecular weight and functionality combined with other polyols to develop proper formulations.
Table 9 — 931-PC vs. Commercial Anti-Fog Product
| 931-PC | Commercial Sample | |
|---|---|---|
| Haze | 0.15 | 1.23 ± 0.63 |
| Yellow Index | 1.03 | 1.1 ± 0.01 |
| Transmittance % | 92.37 | 92.40 ± 0.55 |
| Adhesion % | 100 | 95 ± 5 |
| Thickness, mil | 0.3 | 0.5 |
| Scratch Resistance, psi | 2 to 3 | 1 to 2 |
| Pencil Hardness | HB | B |
| Breathing Test | Anti-fog | Anti-fog |
| Water Immersion, % adh. | 100 | <50 |
| Boiling Test, % adh. | 100 | Failed |
| Fogging Test, min w/o fog | 120 | Flash, 57 |
| Freezer Test, % adhesion | 100 | 68 ± 17 |
931-PC showed significant improvement over the commercial sample in haziness, adhesion, and anti-fog performance against harsh conditions — water immersion, boiling, freezing, and fogging duration. For applications that require better hardness, toughness, and minimum thickness, other product formulations are feasible.
Conclusions
Heterofunctional polyurethane acrylates have been developed based on highly branched polyol raw materials, including dendrimeric polyols. Raw materials with different characteristics — hydrophilicity, hydrophobicity, conductivity, elasticity, mechanical property, and formulation compatibility — were selected to obtain oligomers of desired properties.
Thin film UV-curable oligomers targeting anti-fog, glass adhesion, and soft touch applications have been developed. Anti-fog coatings for PC substrates were evaluated extensively for performance against many application scenarios, from masking and freezing to abrasion resistance, time degradation, high temperature stability, and longevity against water immersion. Some formulations were compared against commercial products and found to outperform their commercially available counterparts in many performance qualities.
Acknowledgements
The authors want to thank the following companies for their free samples that allowed the work to continue and finish: the Perstorp Group, Panolam, Dow Chemical, Momentive, Cray Valley, Kowa, Evonik, Sartomer, and Asahi Kasei, among others.
Many colleagues gave their precious time to help this project. Especially, Ms. Colleen Heiser and Mr. George Drazinakis have been constantly helping out with matters big and small.
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