GB2285340A - Elastomeric keypads - Google Patents

Abstract

An elastomeric keypad is coated with a fluoroaliphatic polymer 17 to improve its resistance to skin oil. Specifically, the use of a pentadecafluoroctyl methacrylate polymer coating increases by a factor of 5 the resistance of silicone rubber keypads to skin oil. Each button 11 of the keypad has a conductive region 13 in registration with a conductive or resistive region 16 on a substrate 14. <IMAGE>

Description

IMPROVED RELIABILITY ELASTOMERIC KEYPADS AND METHOD FOR MAKING SAME Field of the Invention This invention relates to keypads such as elastomeric keypads used in telephones. The invention is particularly useful for providing a silicone rubber keypad highly resistant to contamination by human skin oil.

Background of the Invention Elastomeric keypads provide easily made highly useful interfaces between human users and a variety of electronic instruments including telephones. A typical elastomeric keypad comprises a molded array of projecting elastomeric buttons each of which includes beneath the button, a concave region containing a conductive region. When the button is pressed, the concave region is flattened, pushing the conductive region into contact with conductors or resistors placed below.

One shortcoming of such devices, however, is the tendency of elastomeric materials to absorb human skin oil. After repeated use the skin oil diffuses through the keypad and into the conductive region, greatly increasing its contact resistance and leading to failure of the keypad.

A variety of steps have been taken to deal with this problem including the development of "oil resistant" silicones and the use of hard plastic caps over the elastomeric buttons. The new materials, however, still suffer the same problem.

Accordingly, there is a need for a way of improving the reliability of elastomeric keypads.

Summary of the Invention Applicants have discovered that coating elastomeric keypads with fluoroaliphatic polymers improves their resistance to skin oil. Specifically, the use of a pentadecafluoroctyl methacrylate polymer coating increases by a factor of 5 the resistance of silicone rubber keypads to skin oil.

Brief Description of the Drawings In the drawings: FIG. 1 is a schematic cross section of an elastomeric keypad in accordance with the invention; and FIG. 2 illustrates the repeat unit of the preferred polymeric coating for the keypad of FIG. 1.

Detailed Description Referring to the drawings, FIG. 1 is a schematic cross section of an elastomeric keypad comprising a molded array 10 of projecting elastomeric buttons 11, which includes beneath each button a concave region 12 containing a conductive region 13. The array 10 is advantageously disposed over a substrate 14, such as a circuit board, having a pattern of conductors 15 and resistors 16 in registration with conductive regions 13. For example, depression of a button 11 in the structure illustrated would force a conductive region 13 into contact across a resistor 16, reducing the resistance between the pair of adjacent conductors 15.

The outer surface of array 10 is provided with a coating 17 of a fluoroaliphatic polymer to enhance resistance of the structure to human skin oiL The polymer is advantageously a pentadecafluoroctyl methacrylate polymer comprised of the repeating unit shown in FIG. 2. A suitable solution of such a polymer is available from 3 M Corporation as Fluorad fluorochemical coating FC-722.

Tests demonstrated that an AT & T 5500 telephone keypad coated with FC-722 exhibited more than 5 times the resistance of an uncoated 5500 keypad to simulated skin oil. (The formulation for synthetic skin oil is that described in D. Downing, "The Role of Sebum and Epidermal Lipids in Cosmetic Properties of Skin", 8 International Journal of Cosmetic Science, pp. 115-123 (1986)).

Specifically, the keypads were initially washed with laboratory detergent and tap water then blotted dry with a paper towel. The non-control pads were then given 1-3 coats of FC-722 by brushing, and were air-dried 30 minutes between coats. 30 pI drops of simulated skin oil were applied to all the keypads. The keypads were then placed in a 500C oven for 5 hours, were again washed and dried, and then were weighed with the results shown in Table 1: Table 1 Keypad Coating Application Weight Change AT & T 5500 None 1.0 AT & T 5500 FC-722 1 coat-brush 0.31 AT & T 5500 FC-722 2 coats-brush 0.19 AT & T 5500 FC-722 3 coats-brush -0.045 Thus the application FC-722 reduces the amount of skin oil absorbed, and its effectiveness is proportional to the number of coatings applied. Two coatings reduce the absorption by a factor of 5.

While brushing was the preferred method of application for testing, it is believed that in commercial production an adequate thickness of polymer can be achieved by spraying an increased concentration of polymer in solution.

Claims (4)

Claims:

1. Tn an elastomeric keypad comprising a plurality of elastomeric buttons, each having a conductive region disposed within an associated concave region, and a substrate having conductive or resistive regions in registration with said buttons for sensing the depression of one of said buttons, the improvement wherein: said keypad is coated with a fluoroaliphatic polymer for reducing absorption of skin oil.

2. The improved keypad of claim 1 wherein said elastomeric keypad comprises an array of silicone rubber buttons.

3. The improved keypad of claim 2 wherein said fluoroaliphatic polymer comprises pentadecafluoroctyl methacrylate.

4. The improved keypad of claim 2 wherein said fluoroaliphatic polymer comprises repeated units of the structure of FIG. 2.