Utilized in a wide variety of adhesion applications — from urinary catheters to cardiac pacemakers — one-part condensation-cure adhesives have traditionally been the most commonly used silicone adhesives in the healthcare industry. Despite their acclaimed and proven effectiveness, the downside to using these adhesive systems is their long cure time (many hours or even days at room temperature) and inability to be accelerated with heat. Alternatively, using a two-part, addition-cure silicone adhesive — also known as a fast-cure adhesive — is often helpful or even necessary, depending on the application at hand.
An adhesive is considered a fast-cure adhesive if it fully cures in less than 10 minutes using heat, or if it partially cures for handling in less than five minutes and fully cures at room temperature in eight hours. Fast cures as defined here are induced by bringing the adhesive into contact with heat, usually via an air-circulating oven.
Cure by Infrared
A modern, more specialized thermal technology is utilized by the iCure® AS200 Spot Curing System by IRphotonics (Hamden, CT). This system cures adhesives by emitting infrared radiation (IR) to a targeted area of material to create an environment in which the adhesive generates heat internally. Once absorbed by the adhesive, IR causes the atoms comprising the adhesive to vibrate, resulting in a temperature increase.
Accelerating cure time with IR can be more energy-efficient than the heat-by-oven method: Combustion byproducts are bypassed, and time spent waiting for an oven to warm up and cool down is eliminated, as heat is not transferred to but generated inside the material. This latter circumstance — that the temperature increase occurs from the inside out — decreases the opportunity for product contamination because the adhesive experiences less contact with other materials (e.g., ovens).
Two studies were conducted using silicone materials by NuSil Technology (Carpinteria, CA) and the iCure® Thermal Spot Curing System by IRphotonics. The first study demonstrated the ability of IR to cure healthcare-related silicone adhesives within seconds. In this study, all participating materials were checked for cure progression at the same intervals (10, 30, 60, 120, and 180 seconds), but the individual cure time of each adhesive was not obtained. In an effort to quantify exactly how long these materials take to cure with IR, a second study was initiated in which the cure rate of each sample was more attentively monitored than in the previous study.
Three silicone adhesives by NuSil Technology — MED1-4213, MED2-4213 and MED3-4213 — were heat-cured in an oven and with IR using the iCure machine. Each sample was used to adhere two silicone tubes to each other, one larger than the other. The outer diameter of the smaller tube was inserted inside the inner diameter of the larger tube, and these were bonded together with adhesive covering approximately one centimeter in length (see Figs. 1 and 2). Adhesion strength was tested at various intervals using a tensile tester, which pulled on the assembled tubing specimens at a rate of 20 in./min.
MED1-4213, MED2-4213 and MED3-4213 are two-part, addition-cure adhesives commonly used in the assembly of medical devices. According to their cure schedules, MED1-4213 and MED3-4213 fully cure in 24 hours at room temperature (RT), 25 ºC, and are often cured in shorter time periods when exposed to heat. MED2-4213 has a longer work time and requires heat to initiate curing.
For each set of test specimens, samples were cured according to their respective cure schedules listed in Table 1. Once cured, MED1-4213 and MED3- 4213 samples were tested for adhesion strength at 25 ºC, 100 ºC and 150 ºC, for comparative purposes. MED2-4213 samples were tested at 100 ºC and 150 ºC, as this adhesive is not designed to cure at room temperature.
For the IR cure, samples absorbed infrared at 6.5 watts and were monitored for rate of cure. All had fully cured after 180 seconds, showing that IR provides a significantly faster cure than the more conventional oven cure. The chosen amount of wattage was determined by the size of the area to be targeted by infrared — the one centimeter-long bond line. These samples were tested for adhesion strength after a five-minute exposure to IR.
Figure 3 depicts the results of the break force test for all samples regardless of cure schedule utilized. Comparative analysis shows that silicone adhesives cured via IR in less than 180 seconds can exhibit similar adhesive properties as those cured in an oven, according to their cure schedules.