Choosing the proper transfer adhesive for your membrane switch is one of the more critical decisions you can make during the design process. Achieving a satisfactory bond between your membrane switch and the surface to which it is mounted is vitally important to reducing field failures and ensuring the long-term reliability of your product.

This article will deal with “permanent” adhesives designed to achieve a long-term and permanent bond between the rear surface of the membrane switch and the mounting surface to which it is applied.

Your choice of adhesive essentially boils down to identifying the appropriate “family” of adhesives, and once identified, then choosing the proper adhesive thickness. There are two families of adhesive used for these applications: “acrylic” adhesives, and “modified acrylic” adhesives.

Acrylic Adhesives

Acrylic adhesives are a popular choice, and generally considered the industry standard, for membrane switch attachment applications. They provide outstanding adhesion to metal and high surface energy plastics. These adhesives provide some initial repositionability for placement accuracy when bonding to plastics. They also perform well after exposure to humidity and hot/cold cycles.

Typical metals to which acrylic adhesives bond well to are aluminum and steel. Compatible high surface energy plastics are usually ABS, acrylic, and polycarbonate, to name a few.

Acrylic adhesives offer outstanding temperature and chemical resistance, as well as excellent shear strength to resist slippage and edge lifting.

A popular acrylic adhesive choice is 3M™ High Performance Acrylic Adhesive 200MP adhesive, which offers the following performance characteristics:

  • Humidity Resistance: High humidity has a minimal effect on adhesive performance. Bond strength shows no significant reduction after exposure for 7 days at 90⁰F (32⁰C) and 90% relative humidity.
  • UV Resistance: When properly applied, membrane switches are not adversely affected by outdoor exposure.
  • Water Resistance: Immersion in water has no appreciable effect on the bond strength. After 100 hours at room temperature, the high bond strength is maintained.
  • Temperature Cycling Resistance:  High bond strength is maintained after cycling four times through: 4 hours at 158⁰F (70⁰C) 4 hours at -20⁰F (-29⁰C) 4 hours at 73⁰F (22⁰C)
  • Chemical Resistance: When properly applied, nameplate and decorative trim parts will hold securely after exposure to numerous chemicals including oil, mild acids and alkalis.
  • Bond Build-up:  The bond strength of 3M™ 200MP increases as a function of time and temperature
  • Temperature/Heat Resistance: 3M™ 200MP is usable for short periods (minutes, hours) at temperatures up to 400⁰F (204⁰C) and for intermittent longer periods (days, weeks) up to 300⁰F (149⁰C).
  • Lower Temperature Service Limit: The glass transition temperature for 3M™ 200MP is -31°F (-35°C). Many applications survive below this temperature (factors affecting successful applications include: materials being bonded, dwell at RT before cold exposure, and stress below the TG[i.e.expansion/contraction stresses, impact]). Optimum conditions are: bonding high surface energy materials, longer time at RT before cold exposure, and little or no stress below the TG. The lowest service temperature is -40°F (-40°C).

Modified Acrylic Adhesives

These adhesives provide high bond strength to most surfaces, including many low surface energy plastics such as polypropylene and powder coated paints. The modified acrylic adhesives also provide excellent adhesion to surfaces contaminated lightly with oil typically used with machine parts.

An often-used modified acrylic adhesive family is 3M™ Adhesive 300LSE. Typical performance characteristics are:

  • Bond Build-up:  The bond strength of 3M™ Adhesive 300LSE increased as a function of time and temperature, and has very high initial adhesion.
  • Humidity Resistance: High humidity has a minimal effect on adhesive performance. No significant reduction in bond strength is observed after exposure for 7 days at 90°F (32°C) and 90% relative humidity.
  • U.V. Resistance:  When properly membrane switches are not adversely affected by exposure.
  • Water Resistance:  Immersion in water has no appreciable effect on the bond strength. After 100 hours at room temperature, the high bond strength is maintained.
  • Temperature Cycling Resistance:  High bond strength is maintained after cycling four times through: 4 hours at 158°F (70°C) 4 hours at -20°F (-29°C) 4 hours at 73°F (22°C)
  • Chemical Resistance:  When properly applied, membrane switches will hold securely after exposure to numerous chemicals including oil, mild acids and alkalis.
  • Temperature Resistance: 3M™ 300LSE adhesive is usable for short periods (minutes, hours) at temperatures up to 300°F (148°C) and for intermittent longer periods of time (days, weeks) up to 200°F (93°C).
  • Lower Service Temperature: -40°F (-40°C).

Choosing the Proper Adhesive Thickness

Once you have determined the appropriate adhesive family for your membrane switch adhesive, it is necessary to specify the thickness of the adhesive required to achieve a successful bond of the membrane switch to the mounting surface. When bonding the membrane switch to a smooth surface, it is generally acceptable to use a 2-mil (.002″) thick adhesive. If a texture is visible on the mounting surface, a 5 mil (.005″) thick adhesive would be suggested., in order to maximize the adhesive’s ability to flow around the textured surface profile, maximizing the surface area to which the adhesive can bond.

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