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Anti-fingerprint strategies for post-deposition of fingerprints on non-porous surfaces.

Touch screens are ubiquitous in modern electronic world and are quickly being adapted to more and more devices which previously lacked touch interface. The rapid adaptation of touch-based technology has created a market need for human smudge and residue minimization. The residual fingerprint from direct contact with these devices has resulted in a common aggravation. To counter this persistent and pervasive problem, manufacturers are rapidly developing different technologies to reduce the appearance of fingerprints and contact residue. One technique to improve image clarity has been to reduce the gloss and refractive index difference between the contact surface and the residue as a means to diminish the visual impact. Another surface modification method is based in decreasing the coating surface energy to reject wetting of fingerprint oils most often via an oleophobic material. The poor wetting of fingerprints by oleophobic materials often results in an increase in difference of refractive index between the coated surface and the finger oil, which is easier to clean, yet more visually noticeable. Each of these methods leave the residue chemically unchanged.

In an attempt to improve residue control technologies, after market temporary foil appliques have become commonplace as a means to reduce device damage and control human and other residue without replacing the entire device. Reactive Surfaces has successfully engineered an additive specifically to impart self-cleaning functionality to almost any surface, e.g., original equipment manufacturers (OEM) and aftermarket foil appliques. The Reactive Surfaces additive has been successfully reduced to practice in solvent and waterborne coatings that can be applied to virtually any material substrate. The unique difference between Reactive Surfaces' technology occurs when the residue is chemically and catalytically converted, resulting in a surface that is more self-cleaning with use. The chemically converted residue shifts from hydrophobic to more hydrophilic, and thus increases the compatibility between the residue and the coated surface and decreases the refractive index difference between the coating and the residual fingerprint. The fingerprint appearance is noticeably reduced and is easier to wipe from the surface.

When human fingers contact an object, many substances transfer to the surface of the object and leave behind fingerprints. The substances on human fingerprints can arise endogenously from the sweat glands within the skin surface of a finger or exogenously from foreign materials like cosmetics, perfumes, food residues, and substances from other glands. The combination of these endogenous and exogenous materials, along with the environmental conditions such as humidity and temperature, cause a fingerprint on a surface to be increasingly visible. As a strategy to counter the annoyance of accumulated residual fingerprints, we developed coatings for non-porous surfaces, i.e., screen protectors appliques for any touchscreen device.

The palms of human hands and the soles of human feet are covered by skin with patterns that can enhance frictional forces between skin and contacted surfaces. The skin topography greatly assists human activities, like walking, holding, and picking-up articles. Skin without hair or sebaceous glands and with ridges is collectively termed frictional ridge skin (Figure 1). Sweat glands are found on the groves of frictional ridge skins, and these skin types are the most frequently used contact areas for electronic devices (Figure 2).

A description of common compositions for latent print residue has been reported in several studies (see Ramotowski, Advances in Fingerprint Technology, 2001 CRC Press LLC, source of Table 1). A substantial weight percent of the latent print residue is attributed to lipid components and lipids are susceptible to ester hydrolysis. Knowing the composition and spatial distribution of the contaminating fingerprint residue on touch screen surfaces, Reactive Surfaces engineered these additives and developed these coatings as a useful counter strategy.

Quantifying functional coatings efficacy in a laboratory setting was performed using the indicator, 4-nitrophenyl acetate (pNp-OAc) possessing an ester linkage. The catalytic reaction rate using pNP-OAc was monitored using UV absorption at 401 nm and these values correspond directly with the degree of chemical conversion versus time while monitoring for the absorbance of the hydrolyzed product p-Nitrophenyl. Increase in absorbance per unit time can then be converted to a concentration using the extinction coefficient of 4-nitrophenol (e = 3.2 x 10-5 M cm-1 at pH 7.1) as a conversion factor. The routine analysis sample size was standardized to 1 cm2 and all assays results contain triplicate data values within the same assay. Control coating materials lacked the functional additive as an internal standard of the assay conditions. The functional coating activity levels shown in Figure 3, indicated by the 4-nitrophenyl acetate hydrolysis rates for a series of solvent based polyurethane overcoated commercially available screen protectors. The results support robust coating functionality over multiple substrate types. The urethane coating was applied directly to the unprotected screen protectors as received. The samples were cured for 30 minutes at 60 [degrees]C, cooled to ambient, and cut into lcm2 test coupons. Coupons were assayed over a one hour period to obtain the relative functionality of each screen protector. The activity ranged between 9.6 to 19.67 pmole/ min cm2 with at the same additive loading level of 1.5% based upon total resin solids. The hydrophilic or hydrophobic character of the varying film materials exhibited minimal effect on the coating functionality.

Table 2 shows the fingerprint removing capabilities of eRACE[TM] in an acrylic copolymer waterborne coating applied to a PET substrate. Note the difference in the eRACE[TM] sample after 24 hours as compared to the control where the fingerprint became more visible over the observation period.

Reactive Surfaces have launched a line of touch screen protective and self-cleaning functional coatings and these appliques are being marketed under the trademark eRACE[TM]. It is now possible to convert the lipid components of the fingerprint and other related contaminants to soap-like and glycerin hydrolysis products using functionalized screen protectors on touch screen devices. The new technology facilitates the removal of subsequent latent residues, and maintains clearer and cleaning touch screen surfaces. Reactive Surfaces is currently producing screen protectors for leading cell phone brands, other handheld devices, and automotive surfaces. CW

Dwaine A. Braasch, Eric B. Williams, James W. Rawlins (1) and Steve McDaniel (2)

(1) The University of Southern Mississippi, (2) Reactive Surfaces

* Fig 1-3 Photos Courtesy of Dr. Jorn Chi-Chung Yu, Forensic Science Program, College of Criminal Justice, Sam Houston State University

Table 1. Organic Composition of
Sebaceous Secretions

A. Organic (major)

Triglycerides        30-40%
Free fatty acids     15-25%
  Saturated          50%
  Monounsaturated    48%
  Polyunsaturated    2%
Wax esters           20-25%
Squalene             10-12%
Cholesterol          1-3%
Cholesterol esters   2-3%

eRACE Activity on Commercial Phone Screen Protectors

Buffer                   0.00
Control w/o eRace       -0.90
Supplier E sample B     15.10
Supplier E sample E     17.95
Supplier E sample F     18.33
Supplier E sample I     19.63
Supplier E sample 2B     9.60
Supplier E sample 2F    19.67
Supplier Z              15.20

Note: Table made from bar graph.
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Author:Braasch, Dwaine A.; Williams, Eric B.; Rawlins, James W.; McDaniel, Steve
Publication:Coatings World
Geographic Code:1USA
Date:Jun 1, 2014
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