Simplifying Failure Analysis for Select Medical Device Adhesives

Troubleshooting a failure in an adhesive bonded joint can be a daunting task due to the wide variety of processing and performance variables that can affect joint performance. The adhesive families most used for bonding plastics on medical devices are cyanoacrylates and light cure acrylics.

When examining a failed adhesive joint, one of three failure modes will usually present itself:

  1. The adhesive is still liquid or partially cured
  2. The joint is incompletely filled with adhesive
  3. The adhesive has failed to one surface

Cyanoacrylates

Cyanoacrylates are one-part, room temperature-curing adhesives available in viscosities ranging from water-thin liquids to thixotropic gels. When a cyanoacrylate comes in contact with a weak base, such as moisture present on most surfaces, acidic stabilizers in the adhesive are neutralized and the cyanoacrylate hardens rapidly to form a rigid thermoplastic that adheres to most substrates. 

Light Curing Acrylics

Light Curing Acrylics are one-part adhesives that cure via a free radical cure mechanism upon exposure to light of proper wavelength and intensity to form thermoset plastics with good bond strength to a wide variety of substrates. Products may respond to ultraviolet light, visible light or both. The sensitivity to visible light is particularly helpful when working with substrates that transmit UV light poorly, such as plastics that contain UV stabilizers.

Bond Failures: Causes & Solutions

Wrong Adhesive for the Application

When all other options have been exhausted, it may be that the adhesive selected for an application is simply a poor choice relative to the performance characteristics required. This problem may seem unlikely to occur in a production environment; however, devices are often used in more strenuous conditions than those for which they were originally qualified. Single lot testing of an adhesive may have failed to identify lot to lot variance in key properties that can affect device performance. In addition, factors that are critical to actual use performance may have been overlooked during the qualification process.

Summary

Determining which category an adhesive failure falls into and reviewing the potential causes and solutions for that type of failure should assist in resolving bond failure problems when bonding plastics with cyanoacrylates and light curing acrylic adhesives. Considering the potential sources of adhesive bonding problems up front will aid in developing more robust adhesive bonding processes. In cases that cannot be solved using this methodology, it is suggested to involve the adhesive supplier.  The supplier’s experience and detailed knowledge of their specific formulations can often allow them to identify the key parameters that will spell the difference between success or failure.

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