E.C. Lupton, Jr., D.F.C. Simmonds, and R. Longo
Coatings: application and 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. Dying of coated parts is usually equally unsuccessful.
Two grades were examined: a water-clear coating a one that produces a “wet-look” finish, and an antiglare coating b for surface-glare reduction.
a VueGuard 901 WC, made by Performance Coatings International
b VueGuard 901 AG, made by Performance Coatings International
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.
Relative 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.
The development of a large number of brand-new resins to meet major engineering requirements appears to be remote. R&D and product-development costs are frequently prohibitive. A natural consequence has been the introduction of composite structures, by which resins are elevated to engineering status and by which parts can thus be made to meet the requirement of specialized markets. One such development 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 and are applied to part surfaces.
| Table 1 – Taber abrasion-resistance test: percent change in haze measured as a function of cycles.a | ||||||
|---|---|---|---|---|---|---|
| Change in haze, percent Cycles | ||||||
| Material | Coating | 0 | 10 | 100 | 500 | 1000 |
| Acrylic | None | 0.4 | 7.6 | 21.0 | — | — |
| Acrylic | Water-Clearb | 0.6 | — | 1.6 | 10.8 | 18.2 |
| Acrylic | Anti-glarec | 9.8 | — | 9.1 | 12.1 | 14.6 |
| Polycarbonate | None | 0.8 | 17.4 | 26.8 | — | — |
| Polycarbonate | Water-Clearb | 0.6 | — | 2.6 | 6.1 | 10.8 |
| Polycarbonate | Anti-Glarec | 9.8 | — | 9.4 | 8.8 | 11.1 |
| A ASTM D-1044 b VueGuard 901 WC UV-cured coating, made by PCI Labs c VueGuard 901 AG UV-cured coating, made by PCI Labs |
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Abrasion Resistance
Three different tests were performed to determine the
abrasion resistance of uncoated and coated acrylic and polycarbonate samples (See the section Abrasion Test Methods for test details).
Taber abrasion-resistance tests (See Table 1) showed that haze development for each abrasion cycle is substantially greater for uncoated acrylic and polycarbonate than for uncoated samples. The Princeton abrasion-resistance tests (See Table 2) compares uncoated and coated polycarbonate for visible scratches. After only one cycle, the uncoated sample exhibited visible scratches, while, after as many as 1000 cycles, the sample with a water clear coating showed no visible scratches and the sample with an antiglare coating showed only a slight polish. The PCI abrasion-resistance test (the rotary steel-wool method) was performed on both acrylic and polycarbonate samples at loadings of 12 psi (See Table 3) and 24 psi (See Table 4). Haze was found to increase at a greatly accelerated rate for both uncoated samples. Antiglare coatings exhibited substantial haze with no abrasion, but subsequent abrasion actually reduced haze by polishing the surface.
| Table 2 Princeton abrasion-resistance test: visual evaluation of coated and uncoated polycarbonate subjected to the linear steel wool test. |
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|---|---|---|---|
| Material | Coating | Cycles | Evaluation |
| Polycarbonate | None | 1 | Visible Scratches |
| Polycarbonate | Water-Clear a | 1000 | No Visible Scratches |
| Polycarbonate | Anti-Glare b | 1000 | Slight Polish |
| a VueGuard 901 WC UV-cured coating, made by PCI Labs b VueGuard 901 AG UV-cured coating, made by PCI Labs |
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Glare measurement
Current techniques for measuring surface glare reduction were originally developed for metals with nonshiny finishes. The techniques assumed that light is reflected from or absorbed by a surface (none is transmitted) and that the surface is flat. Through the use of standard measurement techniques (such as with a Gardener 60-degree glossmeter), however, serious errors were found to occur when transparent plastics (especially thin film) were being evaluated. Gloss readings as much as 20 units too high resulted unless precautions were taken to guard 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 there 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.
A clear substrate printed with several different colors can show different gloss readings for each color even though coatings are identical and uniform. Curved surfaces can also generate erroneous readings.
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. Given enough time, therefore, there may be some chemical attack on the base substrate even though the coating itself remains unaffected. Practical guidelines based on the method described (See the section Chemical Resistance Test Methods) were developed governing contact of certain substances with coated and uncoated acrylic (See Table 5) and polycarbonate (See Table 6).
Chemical resistance test methods
The chemical resistance test used in this study are more vigorous than those described in ASTM D 1308, where the low-viscosity, volatile liquids used tend to evaporate rather quickly, thus shortening the true exposure time. In our test, samples were in continuous contact with reagents via a wick or a pad set in the mouth of an inverted bottle placed on the samples.
| Table 3 The PCI test: abrasion resistance determined by the rotary steel wool test (12 psi loading). Percent haze measured as a function of revolutions. |
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|---|---|---|---|---|---|
| Haze, percent Revolutions |
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| Material | Coating | 0 | 1 | 50 | 100 |
| Acrylic | None | 0.4 | 4.6 | 22.1 | 20.8 |
| Acrylic | Water-Clear a | 0.4 | 0.4 | 0.9 | 0.9 |
| Acrylic | Anti-Glare b | 1.9 | 1.9 | 1.8 | 1.7 |
| Polycarbonate | None | 0.4 | 8.1 | 17.6 | 23.3 |
| Polycarbonate | Water-Clear a | 0.4 | 0.4 | 0.8 | 0.9 |
| Polycarbonate | Anti-Glare b | 7.6 | 7.6 | 3.8 | 3.1 |
| a VueGuard 901 WC UV-cured coating, made by PCI Labs b VueGuard 901 AG UV-cured coating, made by PCI Labs |
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| Table 4 The PCI test: abrasion resistance determined by the rotary steel wool test (24 psi loading). Percent haze measured as a function of revolutions. |
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|---|---|---|---|---|---|
| Haze, percent Revolutions |
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| Material | Coating | 0 | 1 | 50 | 100 |
| Acrylic | None | 0.4 | 8.5 | 25.8 | 23.9 |
| Acrylic | Water-Clear a | 0.4 | 0.4 | 1.0 | 1.0 |
| Acrylic | Anti-Glare b | 1.9 | 1.9 | 1.5 | 1.4 |
| Polycarbonate | None | 0.4 | 11.3 | 20.8 | 24.0 |
| Polycarbonate | Water-Clear a | 0.4 | 0.4 | 1.3 | 1.5 |
| Polycarbonate | Anti-Glare b | 7.6 | 7.6 | 3.7 | 3.6 |
| a VueGuard 901 WC UV-cured coating, made by PCI Labs b VueGuard 901 AG UV-cured coating, made by PCI Labs |
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| Table 5 Effect of various reagents on coated and uncoated acrylic. |
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|---|---|---|---|
| Reagent | Uncoated | Water-Clear a | Antiglare b |
| Toluene | N | M | S |
| Acetone | N | S | S |
| Trichloroethylene | N | M | S |
| a VueGuard 901 WC UV-cured coating, made by PCI Labs b VueGuard 901 AG UV-cured coating, made by PCI Labs N = Do not use M = Medium term resistance (up to 8 hours) liquid exposure S = Short term resistance (up to 1 hour) from drops, spills, etc. |
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| Table 6 Effect of various reagents on coated and uncoated polycarbonate. |
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|---|---|---|---|
| Reagent | Uncoated | Water-Clear a | Antiglare b |
| Toluene | N | S | S |
| Acetone | N | S | S |
| Trichloroethylene | N | S | S |
| Gasoline | N | M | M |
| Caustic Soda, 50 percent |
N | S | S |
| a VueGuard 901 WC UV-cured coating, made by PCI Labs b VueGuard 901 AG UV-cured coating, made by PCI Labs N = Do not use M = Medium term resistance (up to 8 hours) liquid exposure S = Short term resistance (up to 1 hour) from drops, spills, etc. |
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Abrasion test methods
The Taber test (ASTM D 1044: “Resistance of Transparent Plastic Materials to Surface Abrasion”) uses a Taber abrader (or its equivalent) loaded at 500 grams with CS-10F wheels and rotated for a specified number of cycles. The visual appearance of the sample is then measured as percent changes in haze according to ASTM D 1003: “Measurements of Haze and Luminance Transmittance of Transparent Plastics.”
The Princeton abrasion test uses a 0.5-inch-square pad of 000 grade steel wool loaded to 1000 grams. The pad is drawn over the test sample by a Princeton scratch tester.. Two 2-inch strokes constitute a cycle.
The PCI abrasion test, which is a severe test, uses a 1.25-inch-square pad of 0000 grade steel wool loaded either to 12 or 24 psi. The pad is revolved for a specified number of cycles. Results are reported in percent haze for 0, 1, 50, and 100 revolutions.
| Table 7 The resolvable separation between a series of parallel lines viewed through uncoated and coated polycarbonate film. |
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|---|---|---|---|
| Resolvable separation, microns Image-to-coating distance, inches |
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| Coating | 0.015 | 0.75 | 1.5 |
| None | <4.3 | <4.3 | <4.3 |
| Water-Clear a Antiglare b | <4.3 | <4.3 | <4.3 |
| 75 degree gloss Antiglare b |
<4.3 | <4.3 | 5.5 |
| 50 degree gloss Antiglare b |
<4.3 | 5.5 | 7.0 |
| 32 degree gloss Antiglare b |
6.2 | 7.0 | 8.8 |
| 10 degree gloss Antiglare b |
31 | 360 | 400 |
| a VueGuard 901 WC UV-cured coating, made by PCI Labs b VueGuard 901 AG UV-cured coating, made by PCI Labs |
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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 theapplication of melamine or silicone based coatings. UV-curable coatings can be applied to one or both sides of substrates, therefore being capable of covering exposed surfaces. Water-clear coatings lower the surface coefficient of friction or parts, providing slip characteristics. Although dyeing of UV-cured-coated parts is usually unsuccessful, coatings can sometimes be made that contain the dye.
Resolution of antiglare
Coatings
High performance organic coatings reduce or eliminate glare from flat surfaces by
counteracting the flatness. But, unfortunately, non-flat surfaces tend to distort the light passing through them. Light distortion, of course, reduces image resolution. How much distortion is to be expected for given conditions can be determined by a resolution test, in which a series of parallel lines – arranged in patterns of
increasingly fine separation – is viewed through an antiglare surface. When the separations can no longer be completely discerned, as determined by the values given in (See Table 7), the image is no longer resolvable.
Article first appeared in Plastics Engineering Magazine.