UV-Cured Coatings Protect Optical-Grade Resins
Abrasion and chemical attack, enemies of optical surfaces, can be tamed by coatings. The coatings can reduce glare but in some cases at the expense of image resolution. Designers can choose a reasonable compromise.
Coatings: Application & Finishing Methods
UV-curable coatings are usually applied to substrates by spray coating, dip coating, curtain coating, direct or offset gravure coating, or reverse roll coating. Of particular interest is a development in which a mold is coated and the mold subsequently filled — the coating becomes an integral part of the part.
Coated parts can be machined, drilled, routed, and blanked with no more difficulty than uncoated parts. Thermoforming is usually unsuccessful because of the highly crosslinked structure of the coatings. Dyeing of coated parts is usually equally unsuccessful.
Two grades were examined: a water-clear coating that produces a “wet-look” finish, and an antiglare coating for surface-glare reduction.
a VueGuard® 901 WC, made by Performance Coatings International
b VueGuard® 901 AG, made by Performance Coatings International
Optical Resin Challenges
Relatively few engineering resins have outstanding optical properties. Two that do are polycarbonate and acrylic. PVC and cellulosics could make the grade in applications less demanding of superior thermal and mechanical properties. Polyester is used for thin films requiring optical properties.
The surfaces of parts made from all of these resins are easily scratched. Scratches are ruinous to their optics. Furthermore, polycarbonate, acrylic, cellulosics, and PVC are vulnerable to attack by many chemicals that at best destroy surface clarity and in many cases induce crazing and cracking.
One solution has been the formulation of high-clarity, high-performance ultraviolet cured coatings to protect optical-quality acrylic and polycarbonate substrates from abrasion and chemical attack while balancing image resolution against glare from ambient lighting. The coatings described here are based on acrylic chemistry and range from 2 to 4 microns in thickness.
Abrasion Resistance
Three different tests were performed to determine the abrasion resistance of uncoated and coated acrylic and polycarbonate samples. Taber abrasion tests showed haze development for each abrasion cycle is substantially greater for uncoated samples than for coated samples.
Table 1 — Taber Abrasion Test (% Change in Haze by Cycles)
| Material | Coating | 0 | 10 | 100 | 500 | 1000 |
|---|---|---|---|---|---|---|
| Acrylic | None | 0.4 | 7.6 | 21.0 | — | — |
| Acrylic | Water-Clear | 0.6 | — | 1.6 | 10.8 | 18.2 |
| Acrylic | Anti-Glare | 9.8 | — | 9.1 | 12.1 | 14.6 |
| Polycarbonate | None | 0.8 | 17.4 | 26.8 | — | — |
| Polycarbonate | Water-Clear | 0.6 | — | 2.6 | 6.1 | 10.8 |
| Polycarbonate | Anti-Glare | 9.8 | — | 9.4 | 8.8 | 11.1 |
ASTM D-1044 • Water-Clear = VueGuard® 901 WC • Anti-Glare = VueGuard® 901 AG
Table 2 — Princeton Abrasion Test (Visual Evaluation)
| Material | Coating | Cycles | Evaluation |
|---|---|---|---|
| Polycarbonate | None | 1 | Visible Scratches |
| Polycarbonate | Water-Clear | 1000 | No Visible Scratches |
| Polycarbonate | Anti-Glare | 1000 | Slight Polish |
Linear steel wool test • Water-Clear = VueGuard® 901 WC • Anti-Glare = VueGuard® 901 AG
Table 3 — PCI Test, Rotary Steel Wool (12 psi Loading)
| Material | Coating | 0 rev | 1 rev | 50 rev | 100 rev |
|---|---|---|---|---|---|
| Acrylic | None | 0.4 | 4.6 | 22.1 | 20.8 |
| Acrylic | Water-Clear | 0.4 | 0.4 | 0.9 | 0.9 |
| Acrylic | Anti-Glare | 1.9 | 1.9 | 1.8 | 1.7 |
| Polycarbonate | None | 0.4 | 8.1 | 17.6 | 23.3 |
| Polycarbonate | Water-Clear | 0.4 | 0.4 | 0.8 | 0.9 |
| Polycarbonate | Anti-Glare | 7.6 | 7.6 | 3.8 | 3.1 |
Table 4 — PCI Test, Rotary Steel Wool (24 psi Loading)
| Material | Coating | 0 rev | 1 rev | 50 rev | 100 rev |
|---|---|---|---|---|---|
| Acrylic | None | 0.4 | 8.5 | 25.8 | 23.9 |
| Acrylic | Water-Clear | 0.4 | 0.4 | 1.0 | 1.0 |
| Acrylic | Anti-Glare | 1.9 | 1.9 | 1.5 | 1.4 |
| Polycarbonate | None | 0.4 | 11.3 | 20.8 | 24.0 |
| Polycarbonate | Water-Clear | 0.4 | 0.4 | 1.3 | 1.5 |
| Polycarbonate | Anti-Glare | 7.6 | 7.6 | 3.7 | 3.6 |
Rotary steel wool test, 1.25 in² #0000 pad • Water-Clear = VueGuard® 901 WC • Anti-Glare = VueGuard® 901 AG
Glare Measurement
Current techniques for measuring surface glare reduction were originally developed for metals with nonshiny finishes, assuming light is reflected or absorbed (none transmitted) and the surface is flat. Serious measurement errors occur when transparent plastics, especially thin film, are evaluated with standard techniques like a Gardener 60-degree glossmeter — gloss readings as much as 20 units too high can result unless precautions are taken against second surface reflections.
A preferred technique involves either printing a black color onto the back of a clear substrate, or temporarily laminating a black substrate using water or glycerine as an adhesive. Simply pressing the clear material against a black background is not sufficient unless the contact surfaces are wet.
Chemical Resistance
High performance, UV-cured coatings not only protect substrates from chemical attack for a period of time, but greatly increase the time in which they can be in contact with a chemical before damage occurs. Coatings of 2 to 4 microns do not provide chemical resistance indefinitely, since solvents eventually permeate coatings on the molecular level.
Table 5 — Reagent Effects on Coated & Uncoated Acrylic
| Reagent | Uncoated | Water-Clear | Antiglare |
|---|---|---|---|
| Toluene | N | M | S |
| Acetone | N | S | S |
| Trichloroethylene | N | M | S |
Table 6 — Reagent Effects on Coated & Uncoated Polycarbonate
| Reagent | Uncoated | Water-Clear | Antiglare |
|---|---|---|---|
| Toluene | N | S | S |
| Acetone | N | S | S |
| Trichloroethylene | N | S | S |
| Gasoline | N | M | M |
| Caustic Soda, 50% | N | S | S |
Test Methods
Chemical Resistance: More vigorous than ASTM D-1308. Samples were in continuous contact with reagents via a wick or pad set in the mouth of an inverted bottle placed on the samples.
Taber Test (ASTM D-1044): Uses a Taber abrader loaded at 500 grams with CS-10F wheels, rotated for a specified number of cycles. Visual appearance measured as percent change in haze per ASTM D-1003.
Princeton Test: Uses a 0.5-inch-square pad of 000 grade steel wool loaded to 1000 grams, drawn over the sample by a Princeton scratch tester. Two 2-inch strokes constitute a cycle.
PCI Test: A severe test using a 1.25-inch-square pad of 0000 grade steel wool loaded to 12 or 24 psi, revolved for a specified number of cycles.
UV-Curing Benefits
Because they are radiation-cured, these coatings can also be used on plastics with low-heat distortion temperatures (PVC, polystyrene, ABS) that cannot withstand the long thermal bakes required for melamine or silicone based coatings. UV-curable coatings can be applied to one or both sides of substrates, covering exposed surfaces. Water-clear coatings lower the surface coefficient of friction of parts, providing slip characteristics.
Resolution of Antiglare Coatings
High performance organic coatings reduce or eliminate glare from flat surfaces by counteracting the flatness. But non-flat surfaces tend to distort the light passing through them, reducing image resolution. How much distortion is to be expected can be determined by a resolution test, viewing a series of parallel lines through an antiglare surface. When the separations can no longer be discerned, the image is no longer resolvable.
Table 7 — Resolvable Separation Through Coated Polycarbonate Film
| Coating | 0.015 in | 0.75 in | 1.5 in |
|---|---|---|---|
| None | <4.3 | <4.3 | <4.3 |
| Water-Clear | <4.3 | <4.3 | <4.3 |
| Antiglare, 75° gloss | <4.3 | <4.3 | 5.5 |
| Antiglare, 50° gloss | <4.3 | 5.5 | 7.0 |
| Antiglare, 32° gloss | 6.2 | 7.0 | 8.8 |
| Antiglare, 10° gloss | 31 | 360 | 400 |
Values shown in microns of resolvable separation vs. image-to-coating distance
Article first appeared in Plastics Engineering Magazine.