HeatSeal® Reliability – Temperature Test
Test Purpose: To determine the effect of temperature on the bond between the layers of a membrane switch. In this test, we will evaluate switch bodies constructed using a GGI HeatSeal® spacer and a pressure sensitive acrylic spacer.
GGI uses test method ASTM D1876-72 for measuring peel resistance between the switch layers. Samples are prepared using the following spacer materials and cut into one-inch-wide specimens for peel testing:
A) .005″ Mylar™ polyester film (top) – .009″ pressure sensitive acrylic spacer – .005″ Mylar polyester film (bottom).
B) .005″ Mylar polyester film (top) – .008″ HeatSeal® spacer – .005″ Mylar polyester film (bottom).
Three (3) samples of each construction are prepared for testing at eight temperatures: -25, 0, 25, 50, 60, 70, 80 and 100°C.
Each test specimen is placed in the grips of an Instron Test Machine and conditioned at the specified temperature for five (5) minutes. The top and bottom layers are then separated at a rate of ten (10) inches per minute. A chart recorder is used to record load versus displacement. The average peel strength of each specimen is determined by measuring load at each half inch of separation until ten (10) readings are taken. Peel strengths are reported in units of pounds per inch width.
The data demonstrates that the peel adhesion of PSA adhesive varies greatly with temperature, while HeatSeal® peel adhesion remains relatively stable. At temperatures exceeding 50°C, PSA is susceptible to delamination through both internal and external stresses inherent in membrane switch constructions. Examples of these forces are interference fits, differences in coefficients of thermal expansion, and the upward force due to connector/tail placement. Any delamination of the switch layers will leave the circuit vulnerable to fluid ingression (i.e. electrical failure)