Accelerating Cure Times of Medical Silicone Adhesives With Infrared Radiation

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.

Fig. 1 – Two silicone tubes adhered by silicone adhesive with a 1 cm bond line.

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.

Fig. 2 – The iCure system emits infrared radiation to cure the targeted area of material.

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.

First Study

Table 1 – Cure times and work times of fast-cure silicone adhesives.

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.

Table 2 – Cure Time comparison of silicone adhesives based on backing.

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.

Second Study

Fig. 3 – Break force comparison (lb/ft).

The objective of this study was to quantify how long it takes to cure samples of MED1-4213, MED2-4213, and MED3-4213 approximately one square centimeter in size and with a bond line thickness of one millimeter. A silicone elastomer was used to control the area and cross-sectional thickness of each silicone adhesive. All samples absorbed IR at 5 watts instead of the first study’s 6.5 watts in order to better represent practical use and preserve the life of the IR lamp. Consistent with the previous testing, the space between each sample and the 3.5-millimeter light guide of the iCure system measured 9 mm, the working distance specified by IRphotonics.

The silicone adhesives were cured on glass slides placed on one of two slightly different backings. Samples of each adhesive were divided between a thermally conductive alumina-filled silicone elastomer backing, and a darker gray version containing black pigment. Because the iCure system emanates light energy, or heat, to cure materials, the different colored backings presumably allowed for more extensive inter and intra adhesive cure time comparison. The samples were considered fully cured when they were completely solid and dry to the touch.

Per adhesive, Table 2 lists the average cure times of the samples based on the backing used. The wide discrepancy in cure times of samples of the same adhesives indicates that the substrate used will have significant influence over how much light is reflected back through the silicone versus how much is retained within the specimen. The increased absorbance of light and the subsequent radiation of heat can be seen in the decreased cure times of samples using the darker backing.

Conclusion

Fig. 4 – Pictorial representation of the test samples cured with IR.

In the first study, the tubing samples of MED1-4213 and MED2-4213 reached full cure in a shorter amount of time than even the samples that used the dark gray backing in the second study, MED3- 4213 excluded. Two factors plausibly contributing to this are that the wattage was higher for the first study, and that the description of full cure was more subjective for the second study. The difference is also likely due to the dissimilar geometries of the samples used. Nevertheless, trends from both studies on the iCure system suggest the same conclusion: Infrared radiation can produce faster cure times for medical silicone adhesives than the more traditional fast-cure method of oven heating. Although parameters like substrate and wattage play a role in the rate of cure, the “ultimate” fast-cure method of the iCure system can shave minutes, even hours, off the cure times of silicone adhesives for healthcare applications.

This article was written by Brian Reilly, Product Director, Healthcare Materials for NuSil Technology LLC, Carpinteria, CA, and Élizabeth Brunet, Application Specialist for IRphotonics, Hamden, CT. For more information about NuSil, visit http://info.hotims.com/40434-161 or visit http://info.hotims.com/40434-162 to learn more about IRphotonics.