A brittle adhesive that failed after a year in service forced the maker of a radioisotope imaging scanner to find a replacement. The AR-2000 scanner from Bioscan, Washington D.C., is equipped with a self-adjusting detector arm that houses the electronics circuit board and a high-sensitivity collimator plate which optimizes isotope counting resolution and detection speeds.

Different collimators are required for different applications, so they must be easy to remove and replace. Four rare earth magnets hold the collimator securely in a specific orientation to ensure accuracy and ease plate switch-out. American Products Inc., York, Pa., who manufactures the scanner, tested a variety of adhesives to bond magnets to the aluminum detector arm. The company’s engineers found that the ideal product for the magnet-bonding project is rapid-setting, high-strength Araldite 2021 methacrylate adhesive from Huntsman Advanced Materials, The Woodlands, Tex.

The original detector arm design specified cyanoacrylate adhesive to install the magnets. However, after 12 months in service the adhesive often became brittle resulting in small fissures in the bondline. When technicians attempted to switch plates, the magnets came loose.

Bioscan and American Products engineers then tried a longer lasting Araldite 2043 epoxy adhesive that sets in 90 sec. The epoxy provided the desired mechanical strength but did not allow adequate time to install all four magnets using a single batch of mixed adhesive.

Huntsman then suggested Araldite 2021 methacrylate adhesive that has the strength of the rapid-setting epoxy but a slightly longer work life. Testing on the methacrylate demonstrated that it satisfied both the handling and performance criteria for the project. The adhesive has a two to three-minute work life and an easy-to-mix viscosity. The material attains handling strength after eight minutes at room temperature. Cured Araldite 2021 methacrylate adhesive has a lap shear strength of 3,625 psi at temperatures of up to 104ºF (40ºC). The adhesive maintains bondlines even when exposed to fluids, oils, and water, has an elongation of 50 to 75% and a roller peel of 62 pli. A further advantage is the product’s packaging in 50 ml dual barrel cartridges that provide for precise control of the resin-to-hardener mix ratio and eliminates material waste.

American Products technicians begin the bonding process by cleaning predrilled cavities into which the magnets will be placed. The surfaces surrounding each of the four holes are then covered with masking tape to prevent excess adhesive from flowing onto the Iridite chromate-coated aluminum detector arm.

Next a small amount of resin and hardener is dispensed from the cartridge and mixed until thoroughly blended. Technicians use a small stick to add a drop of adhesive to the inside of each of two side-by-side cavities. Then magnets are installed on a small placement jig machined from tool steel that holds each magnet in the correct position for insertion in the hole. The process is repeated for the second set of two magnets.

Magnet orientation is critical. The scanner design calls for a 0.010 to 0.030-in. air gap between the top of the magnets and the surface of the collimator plate to eliminate any interference that can affect the accuracy of radioisotope readings.

Earl Snyder, design engineer at American Products, says, “Araldite 2021 methacrylate adhesive provides us with the handling time we need to effectively insert magnets in the detector arm according to design specifications and then cures quickly at room temperature.” He adds, “In service, the Huntsman adhesive maintains its resilience producing long-lasting bond strength.”

After the magnets cure for 30 minutes, air-gap spacing is measured before detector arm assembly continues. Then each unit is calibrated and tested.