Keyboard Coatings
The ever growing and constantly changing electronic industry has established very stringent requirements for the durability and aesthetic appearance of computers and mobile input devices. Computer keyboards, the most commonly used input devices, tend to wear under everyday use. Acrylonitrile butadiene styrene (ABS) keyboard keys become glossy and often legends are erased with constant use. This wear can be prevented by the use of high performance UV curable anti-glare protective keyboard coatings applied by the spray process.
Background
The UV curable protective coatings for computer keyboards have to meet a demanding set of conflicting specifications. Some of them are low gloss, extended abrasion and wear resistance, transparency, temperature and humidity tolerance, high pencil hardness and acceptable aesthetic appearance. In addition, these coatings have to be cost effective and meet stringent international health, safety and environmental requirements.
The majority of keyboard manufacturers demand coatings with low gloss levels. It has been proposed that gloss levels at 3 to 5 gloss units are best suited for computer keyboards, where the unpleasant glare is reduced significantly to provide minimum distraction to computer users. In addition to reducing light reflection, low gloss coatings usually hide fingerprints and provide better visual appearance. Typically, low gloss coatings are obtained by incorporating matting particles with various sizes into the UV curable formulation. Low gloss coatings require a substantial amount of matting agent, and usually, the higher the amount of particles the lower the abrasion resistance is.
The industry has adopted numerous tests for abrasion and wear resistance, the most challenging of which are key wear durability and legend abrasion tests. Both tests are performed on an abrasion tester as shown in Figure 1. Wear durability involves linear abrasion with CS-5 Taber Industries® Jumbo Wearaser under 200g load. The keycap coating should withstand 250,000 cycles at 38mm stroke, 36 cycles per minute without visible change to the surface. The legend abrasion test is performed with Taber Industries® Wearaser CS-10F under 500g weight load at 9.5mm stroke at 26 cycles per minute. After 1,250 cycles the legend should not show any visible wear.
Figure 1. Abrasion tester for Keyboard Coatings
Other key technical aspects for keyboard coatings that have to be taken into consideration are high chemical resistance to household cleaners and chemicals, as well as resistance to varied temperature and humidity conditions computers might be exposed to. Matting particles and coating additives which are not chemically bound to the resins tend to migrate to the surface when the coating is exposed to harsh environmental conditions, leading to whitening of the surface and/or lowering of the wear resistance of the coating. The UV curable anti-glare keyboard coatings transparency is essential, as they are applied over screen printed legends.
Gloss Reduction Considerations
Most often, the anti-glare effect in coatings is achieved by use of matting agents. The degree of resulting matte finish is a function of the degree of surface roughening and is represented as gloss. Gloss is an optical property of a surface to reflect light in a specular direction. Micro rough surfaces diffusely scatter the incident light and reduce the amount of reflected light. The light-diffraction phenomenon is a rather complex process that depends on the particle size, chemical, physical and morphological properties of the matting agents.
One of the most powerful gloss reducing particles are silica particles. Most of them are fairly easy to incorporate and are cost effective. We probed several grades of silica particles into a model UV-acrylate system (ca. 30% resins), achieving coatings with desired gloss level. Figure 2 demonstrates the efficiency of gloss reduction for different grades of silica. The percentage represents the overall amount of silica in the UV curable coating formulation. Gloss is measured with a BYK® micro-gloss meter at 60 degrees.
While the desired gloss can be easily achieved, the coatings are not the best performers in long-term wear resistance testing or lack the required smooth appearance. Greater particle size silica requires a smaller amount of matting agent but produces quite rough, dry and unpleasant to touch surfaces at low coating thickness. Smaller particles produce smoother coating finish but require higher load. High loads of silica lower the abrasion and wear resistance of the resulting coating — particles are easily dislodged and embedded into the wool pad during the wear test, acting as additional abrasive media that leads to fast destruction of the coating finish.
Figure 2. Gloss reduction power of different grades silica in UV-acrylate system
Wax treated silica particles produce smoother and more slippery coating finish which tends to polish upon wear test. Combination of different size and types of silica contributes to better packing of the particles at the surface and produces smooth finish. We have found that silica S5 produces considerably smooth coating finish at 12–14 µm coating thickness, withstanding the long term abrasion test without polishing or scratching.
Thickness & Matting Load
The gloss reduction efficiency is a function of not only particle size but also of coating thickness. Figure 3 shows the gloss dependency of a model UV-acrylate system as a function of coating thickness and load of matting agent with an average particle size of 3 µm. Higher percentage of matting agent at low coating thickness produces low gloss coatings, but the surface smoothness is compromised. As thickness increases the surface becomes smoother but the gloss gets higher.
Figure 3. Gloss dependence as a function of coating thickness and matting agent load
Numerous mineral fillers such as kaolin, cristobalite, calcium carbonate, etc. have been screened as potential matting additives in UV curable coatings for keyboards. The tested compounds either influence the opacity of the coating or diminish the abrasion resistance. However, we noticed an interesting effect with ceramic microspheres. Although not efficient gloss reducers, ceramic microspheres contributed to increased pencil hardness. Average pencil hardness of the model UV acrylate system applied on polycarbonate at dry film thickness of 8–10 µm is 1H. The same coating with 6% load of ceramic spheres increases the pencil hardness to 2H, though gloss is reduced to only 23 units at 60 degrees. Additional ceramic microspheres don’t result in substantial gloss reduction — further matting effect is achieved by the addition of silica, though this deteriorates abrasion resistance and pencil hardness. As expected, when greater particle sizes of ceramic spheres are used the coating surface becomes coarser.
Finely grained organic matting materials gained a lot of popularity in recent years. Contemporary grinding techniques allow for producing particles with round shape and very narrow particle size distribution, which promotes a soft feel effect. Typically, organic particles are easy to disperse and have very low effect on formulation viscosity. Other advantages of these particles are their chemical resistance and temperature stability. Micronized PMMA beads, polymethyl urea resins, polystyrene and polyamide resins have been screened as matting agents. The effect of particle size, coating thickness and amount of load on gloss reduction follows the same pattern as observed for silica — the major difference is the gloss reduction efficacy and behavior upon wear test.
Figure 4 demonstrates the gloss efficiency of several organic matting agents with average particle size of 5 µm at 5% load at 15 µm coating thickness. The coating surfaces are considerably smoother in comparison to silica containing coatings but exhibit polishing upon wear test.
Figure 4. Gloss reduction with organic matting particles
Wax Additives
Micronized waxes, such as polyethylene, polypropylene, amide and Carnauba, are common additives to coatings known to significantly improve surface properties. Waxes tend to migrate to the coated surface, thus improving slip, mar, rub, scratch and abrasion resistance. Some types of waxes are suggested for matting and gloss control — their gloss reduction efficacy is considerably lower than that of silica and organic matting agents, but in combination with other matting agents, waxes contribute drastically to improving surface properties of anti-glare coatings. We tested several waxes in combination with silica, achieving high performance UV curable protective coatings for keyboards. The best combination of properties and visual appearance is achieved at dry coating thickness of approximately 8 µm.
The type of acrylic resin incorporated into the anti-glare formulation contributes to the overall coating gloss. Table 1 illustrates the overall gloss achieved with solvent borne anti-glare formulations using different resins and the same amount and type of matting agents. While common multifunctional acrylate monomers and oligomers produce coatings with similar gloss, silica nanocomposite in acrylate leads to lower gloss with the same amount of matting particles. Abrasion resistant studies show that silica nanocomposites in acrylate enhance long term abrasion resistance and do not influence pencil hardness.
Table 1 — Gloss Reduction & Abrasion Resistance by Resin Type
| Resin Type | 60° Gloss (8µm on PC) | Taber¹ 100 cyc. | Taber¹ 500 cyc. | Pencil² |
|---|---|---|---|---|
| Multifunctional Acrylate | 15 | 2.3 | 9.4 | H |
| Polyether Acrylate | 13 | 3.2 | 7.0 | H |
| Polyurethane Acrylate | 15 | 2.8 | 8.5 | H |
| Silica Nanocomposite / Multifunctional (1:1) | 5 | 1.8 | 2.3 | H |
1. ASTM D-1044, CS-10 wheels, 500g load @ 100 & 500 cycles, %Δ Haze — PC data
2. ASTM D-3363, 750 grams @ Mitsubishi Hi Uni pencils — PC data
Self-Matting Resins
Keyboards are produced by different manufacturers and keycaps differ in type of material, thickness, color, gloss and surface texture. Some materials require substantial coating thickness in order to cover the surface and hide keycap imperfections and surface roughness. Thus, coatings with dry film thickness above 15 µm become necessary. Such coatings require more load of matting particles, create issues with UV cure at low irradiance, and typically compromise wear resistance and transparency.
To overcome problems associated with thick coatings we turned our attention to UV curable self-matting resins. Such agents produce smooth particle-free finish. Several acrylate resins, polyurethane acrylates and polyester acrylates possessing self-matting properties have been screened. Figure 5 shows values of minimum gloss achieved utilizing UV curable self-matting resins at 15 µm thickness. The self-matting resins are less sensitive to solvents and flash off temperatures.
Figure 5. Gloss reduction with self-matting resins
High Performance Keyboard Coatings
Coating 1
Numerous anti-glare coating formulations have been applied and tested; based on their overall performance, three UV curable coatings were selected. Major properties of these low gloss UV curable coatings are outlined in Table 2. Coating 1 is a premier coating which possesses exceptional wear and chemical resistance at a low dry film thickness. The chemistry is based on a combination of nanotechnology and conventional acrylate chemistry, with the matting effect resulting from a proper combination of waxes and micronized silica. Coating 1 has the best appearance and properties when applied at 8 µm coating thickness. The major disadvantage is that gloss must be controlled within a quite limited thickness range — a coating thickness of 11 µm produces gloss of 25 gloss units, and bringing gloss down to 5 or lower requires substantial matting agent that worsens overall coating properties. This gloss dependency trend limits the coating’s application to smooth, non-textured keyboard keys. Coating 1 is a commercialized product used in the keyboard industry.
Coating 2
Coating 2 is a cost effective coating developed for a broad spectrum of keyboard materials and textures. Gloss is relatively easy to adjust without altering overall coating performance, making it applicable to different substrate finishes at various coating thicknesses. Overall coating properties are achieved by incorporating treated silica with 6 µm average particle size into a multifunctional acrylate-based formulation. Due to the size of matting particles, the coated surface is somewhat compromised, though at 12 µm dry film coating thickness an acceptable smooth finish can be produced.
Coating 3
Coating 3 is a product based on self-matting resins and is quite universal with regard to coated material. The tested self-matting resins do not produce finish with gloss of 3–5 units when used by themselves — further gloss reduction is attained with the help of silica particles, though at a much lower amount than used in Coating 2. This formulation uses only 1% silica particles, with desired gloss achieved at 15 µm. The gloss is easy to control, making the coating suitable for different materials and colors.
Table 2 — Coating Properties
| Coating 1 | Coating 2 | Coating 3 | |
|---|---|---|---|
| Adhesion¹ | 100% | 100% | 100% |
| Gloss² | 5 | 3 | 3 |
| Coating Thickness³, µm | 8 | 12 | 15 |
| Legend Test⁴ | Pass | Pass | Pass |
| Wear Resistance⁴ | >250,000 | >250,000 | >250,000 |
| RCA⁵ | >150 | 150 | 150 |
| Pencil Hardness⁶ | 1H | 1H | 1H |
| Steel Wool Scratch⁷, psi | 24 | 32 | 5 |
1. ASTM D-3359 2. ASTM D-523 3. Measured with micrometer 4. Described in Background section
5. ASTM F-2357 6. ASTM D-3363, 750g load, Mitsubishi Hi Uni pencils, ABS
7. Rotary test using #0000 steel wool pad at load @ 5 rotations. No scratches at load — PC data
Anti-Microbial Performance
Antimicrobial surfaces gain more and more popularity in various industries and applications. Computer keyboards harbor harmful bacteria for extended periods — taking into consideration the magnitude of computer use, bacterial contamination can become a health threatening issue. Antimicrobial action is achieved by various agents, such as silver and copper containing compounds and quaternary ammonium compounds. We incorporated into the UV curable anti-glare coating formulation described above several agents known to suppress microbial growth. Satisfactory results are achieved with a silver containing compound in concentration of only 0.2%.
Table 3 demonstrates the antimicrobial effect of Coatings AB1, AB2 and AB3 against Staphylococcus aureus and Escherichia coli. The test is performed in accordance with JIS 2801.
Table 3 — Anti-Microbial Effect Against S. aureus & E. coli
| Sample | E. coli Start | E. coli After 24h | S. aureus Start | S. aureus After 24h | Efficacy Reduction % |
|---|---|---|---|---|---|
| Coating AB1 (blank) | 1.4×10⁵ | 3.0×10⁷ | 1.8×10⁵ | 9.5×10⁵ | — |
| Coating AB1 (0.2%) | 1.4×10⁵ | <1×10² | 1.8×10⁵ | <1×10² | >99.9 |
| Coating AB2 (blank) | 1.4×10⁵ | 3.2×10⁷ | 1.8×10⁵ | 2.8×10⁵ | — |
| Coating AB2 (0.2%) | 1.4×10⁵ | <1×10² | 1.8×10⁵ | <1×10² | >99.9 |
| Coating AB3 (blank) | 1.4×10⁵ | 3.2×10⁷ | 1.8×10⁵ | 2.8×10⁵ | — |
| Coating AB3 (0.2%) | 1.4×10⁵ | <1×10² | 1.8×10⁵ | <1×10² | >99.9 |
| Control (uncoated ABS) | 1.4×10⁵ | 3.7×10⁷ | 1.8×10⁵ | 3.7×10⁷ | — |
Tested per JIS Z 2801 protocol.
Chemical Resistance
The UV-curable Coatings 1, 2 and 3 have excellent chemical resistance to household cleaning solutions, hand lotions, and sunscreens. Common foods and drinks such as coffee, Pepsi cola, ketchup and mustard do not stain the coating surface upon contact for over 8 hours. Chemical resistance tests show that the coatings significantly improve the chemical resistance of the uncoated material.
Table 4 — Chemical Resistance Test
| Chemical | Uncoated ABS | Coating 1 | Coating 2 | Coating 3 |
|---|---|---|---|---|
| Gasoline | C | A | A | A |
| Sodium Hydroxide (10%) | C | B | A | B |
| Acetone | C | B | B | B |
| Methyl Ethyl Ketone | C | B | B | B |
| Propyl Alcohol | A | A | A | A |
| Toluene | C | A | A | A |
| Sulfuric Acid (10%) | A | A | A | A |
| Ethyl Alcohol | A | A | A | A |
Other Considerations
Solvents and diluents are important ingredients of UV curable coatings applied by spray technique for better flow and leveling. Proper selection of solvent blends is quite a complex process, with solvency, evaporation rate, and compliance with health, safety and regulatory requirements as key factors. Oxygenated solvents are known to dissolve acrylic resins well. Typically spray application requires at least three types of solvents — fast, medium and slow evaporating. The fast evaporating solvent lowers initial viscosity and allows for good atomization of the spray. The medium evaporating solvent accounts for controlled release of formulation and prevents dripping and sagging. The slow evaporating solvent is crucial for final flow and leveling of the coating.
The solvent blend optimized for anti-glare coatings contains n-butyl acetate, n-propyl alcohol, diacetone alcohol and methyl isoamyl ketone. Typical processing parameters for UV curable anti-glare coatings by spray application include solvent evaporation in a convection recirculating oven for 3 minutes at 35–40°C and UV cure at 460mJ/cm² (EIT UVA, Fusion H lamp, 7.6 m/min). These parameters allow for a smooth coating finish with targeted mechanical properties. However, when IR ovens are used to flash off solvents at 35°C for 3 minutes or longer, severe chemical attack on the ABS key surface has been observed — due to low chemical and temperature resistance of the ABS substrate. For applications requiring IR assisted flash off, a solvent blend of isobutyl isobutyrate, n-propyl alcohol and n-butyl alcohol has been proposed.
Both insufficient evaporation of solvents and less UV energy used to cure coatings can lead to “whitening” phenomena, when the coated material is exposed to harsh environmental conditions. The effect is due to leaching of matting agents, surface modifiers and other coating additives that are not chemically bound in the polymer network. Figure 6 shows the whitening issue after the coated material has been exposed to 70°C temperature at 90% humidity for five days. Insufficient flash-off time leads to solvent entrapment within the polymer network, and inadequate UV cure energy results in lower crosslink density and low degree of polymerization. When recommended processing parameters are followed the whitening issue is eliminated and the coating demonstrates good surface and mechanical properties.
Coating 1 processed within thickness, flash off and UV energy recommended ranges
Coating 1 coated with insufficient UV energy
Figure 6. Coating 1 after temperature and humidity test
Coating Thickness Measurement
The overall gloss of a particular coating is influenced by many factors. Material surface finish, spray application conditions, solvent blend evaporation rate, flash off temperatures and UV energy are key factors, though one of the most critical remains coating thickness. Dry film thickness of optically clear coatings can be measured easily with instruments analyzing reflected light, but fast thickness measurement of thin anti-glare coatings remains a challenge. Measurement with micrometer is a common lab and production quality control routine — the reported coating thickness is the difference between the thicknesses of a coated and uncoated substrate, though this type of measurement can give misleading values depending on surface smoothness, degree of particle wetting, and variations in surface profile.
Anti-glare coating thickness can also be measured with a universal scope Tooke gage observation of a precision cut. The V-shaped cut, produced by a cutting tip with specific geometry, is observed vertically through the Tooke gage microscope with reticle allowing measurement in different units. Both methods give an approximation of coating thickness that might differ significantly from measurements with contemporary microscope techniques.
Summary
New abrasion resistant UV-curable coatings for keyboards have been developed. These low-gloss, thin dry film thickness UV curable coatings exhibit exceptional chemical resistance, soft feel, and high performance antimicrobial properties. Coated keycaps possess long term wear resistance and maintain keycap legend integrity and durability under harsh environmental conditions, while ensuring compliance with international health, safety and environmental requirements.
References
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