Rapid advances are occurring in various electronic devices especially display devices that are used for various communicational, financial, and archival purposes. For such uses as touch screen panels, electrochromic devices, light-emitting diodes, field-effect transistors, and liquid-crystal displays, electrically-conductive films are essential and considerable efforts are being made in the industry to improve the properties of those electrically-conductive films and particularly to improve metal grid or line conductivity and to provide as much correspondence between mask design with resulting user metal patterns.
Electrically-conductive articles used in various electronic devices including touch screens in electronic, optical, sensory, and diagnostic devices including but not limited to telephones, computing devices, and other display devices have been designed to respond to touch by a human fingertip or mechanical stylus. Typically, touch screen technology incorporates the use of resistive or capacitive sensor layers that make up part of the display.
Typically, touch screen technology incorporates the use of resistive or capacitive sensor layers that make up part of the display. There is a need to provide touch screen sensors and displays that contain improved electrically-conductive film elements. Currently, such resistive and capacitive touch screen displays use Indium Tin Oxide (ITO) coatings to create arrays used to distinguish multiple points of contacts. Efforts are underway in the industry to find useful replacements for ITO coatings including the use of various other electrically-conductive metallic compositions. ITO is in limited supply and exhibits undesirable fragility, lack of flexibility, and low conductivity compared to other materials.
As noted, touch screens are often prone to damage due to the increased level of direct contact (touching) by the user of the display or from moisture or water in the environment. Both resistive and capacitive touch sensors can include translucent (or nearly transparent) electrically insulating covering materials disposed on the display structure in order to protect and isolate the touch screen sensors from environmental conditions (such as moisture), abrasion, oxygen, and any harmful chemical agents.
There is also a need to protect the electrically-conductive portions of the sensor from environmental damage (such as from moisture) and both environmental and physical damage during manufacturing and integration operations.
Such electrically insulating covering materials include glass or polyester layers as protective covers. Each of these materials has advantages and disadvantages. WO 2013/062630 (Petcavich) and WO 2013/063051 (Petcavich et al.) both describe the formation of crosslinked polymeric protective layers over touch sensors (and display screens) using photocurable compositions containing various photoinitiators and photocuring materials.
U.S. Pat. No. 7,569,250 (Nelson) describes a process for applying a protective coating to a flex circuit having conductive traces on one surface and by applying a protective coating in substantially a liquid state to the one surface from a roller including a protective coating in a patternwise fashion. Portions of the flex circuit are left exposed (uncoated) for connection to an electronic device such as a print head assembly. A protective coating can be applied to a surface of the flex circuit and then further treated for example by crosslinking or thermal curing.
While such materials can provide protective surfaces in touch sensors, it would be desirable to avoid crosslinkable materials because of the additional processing procedure that is needed as well as the potential problems associated with photoinitiators or other crosslinking agents that may remain chemically reactive in the final protective surface and that can cause yellowing if left as residue in the protective coatings.
In addition, residual photoinitiators used in photocuring operations used to prepare protective coatings can pose adhesion and shrinkage problems, at least partially due to their low molecular weight, high mobility, and likely high initial concentrations so that residual concentrations after photocuring can be as much as 15% of the final protective covering weight. These problems can be particularly apparent in photocurable compositions that are applied to electrically-conductive patterns using printing methods such as flexography at high curing and printing speeds.
Thus, there is a need for improved protection of electrically-conductive patterns especially when high concentrations of photocuring photoinitiators are present so that yellowing and other problems are minimized.