Dow Corning offers Silastic MDX4-4210, a two-component silicone adhesive that has a tensile strength of 5 MPa. NuSil offers MED-1000 one-part silicone adhesive and MED2-4013 two-part silicone adhesive, which have similar strength characteristics. Momentive Performance Materials, Rhodia, and Wacker offer alternatives as well. These adhesives are listed as tensile-strength materials. Their operating data are acquired by molding a dog-bone tensile bar and pulling it 180° apart until it breaks. The breaking point determines the tensile strength. Bonding two pieces of a substrate is tensile shear strength, but adhesion depends on the substrate itself as well as the surface area. Most vendors cannot test all combinations and substrates, so evaluation of these candidate adhesives determines if they will reach 4–5 MPa, as you require.

I recommended these materials based on their tensile strength. If a material has a lower tensile strength, it might tear cohesively in the application, leaving silicone adhesive on both substrates. By selecting a high-tensile-strength material, the cohesive strength of the adhesive will be above your minimum value, so that you can focus on the actual adhesion of the silicone adhesive to the substrate.

I would initially start looking at either Epoxy Technology’s Epo-Tek 730 or Henkel’s Hysol M-21 HP epoxy adhesives for medical device applications. This is a good place to start, but there are other products on the market as well. Pebax is a copolymer of nylon and urethane-type polymers. The 55D is a mid-range polymer, while the 72D is more nylonlike and 32D is more urethanelike.

There are a number of clear adhesives that are suitable for bonding glass to plastic and that remain clear and colorless. Dymax 429 or 4-20418 are candidates that cure with UV light in seconds to form a strong bond between multiple substrates. Epotek 353ND is a clear two-part epoxy that would also work well. These few options might get you started looking in the right direction.

A key criterion for selecting the right material for your application is to identify how many parts you will make per day, per month, and per year and what type of process you can envision. Is high-speed UV curing the best option, or should you perhaps consider using the benefits of light-curing materials, which cure “on-demand”? Such adhesives allow you to align the parts, and once they are aligned, you shine light on them for a few seconds, locking them in place. Or are you more comfortable using a two-part material that needs to be mixed and degassed prior to use to avoid air bubbles? Some epoxies are available in 1:1 mix-ratio cartridges, allowing you to dispense through a static mixer system and easing the formation of air bubbles. Dymax ER1196/CT1196 is an 8000 cP two-component epoxy available in a 1:1 mix.

Another factor in selecting the right material is to determine what viscosity will work best for your part or application. To put this in perspective, water = 1 cP, honey = 10,000 cP, Dymax 429 = 2,500 cP, and Dymax 4-20418 = 450 cP.

There are a couple of choices that come to mind based on your application. Several epoxy options can survive these temperatures and exhibit good adhesion. Dymax ER1196/CT1196 has shown good adhesion up to 285°F. Epotek 353ND is very popular for these types of medical applications. And Epotek 375, which is classified as a high-temperature epoxy, can also be used. Basically, any epoxy that is designed for rugged or ruggedizing applications will be more flexible (i.e., less brittle) so that it can absorb the torsional strain.

Another option is high-temperature epoxies that move the glass transition temperature (Tg) above the sterilization temperature. A typical epoxy can have Tg values in the 80° to 120°C range. When it undergoes sterilization at 260°F/126°C, it crosses this glass transition temperature. An epoxy with a process temperature higher than the Tg value will exhibit more rubberlike behavior, while an epoxy with a process temperature below the Tg value will exhibit more glasslike behavior. Moving the part through this transition (change in temperature) causes stress in the bond line. A high-temperature epoxy should have a Tg above the sterilization temperature so that the bonded part does not have to experience this transition.

To increase adhesion to the Vectra LCP or similar materials, you have a few options:

1. Use an alcohol wipe with a 70:30 solution of IPA:water to clean the surface and put a uniform layer of -hydroxy groups onto the glass beads. As the glass ages, the humidity in the air deposits a buildup of hydroxyl units onto the surface. They are held in place by weak van der Waals forces. When an adhesive is placed on top, it is easy to remove these weak boundary layers. By removing them and starting fresh, you will achieve better adhesion.

2. Surface abrasion will increase the surface area and create a mechanical interlock, both yielding better adhesion to the substrate.

3. Dow Corning offers a few different primer systems, such as Dow Corning 1200. The primer can be applied by brush or spray. Allow the solvent to evaporate and then apply the adhesive to the bond line.

4. An air plasma treatment can also increase adhesion. Enercon and other companies offer gas or flame plasma treatment systems that can deposit bonding groups onto a substrate surface, enabling better chemical bonds to the surface.

5. Sometimes, a combination of some of the above options will have good results—in other words, surface abrasion with an alcohol wipe.

Where there are a number of different options available, I would initially start with Dymax 846-GEL/501-E acrylated urethane, with other options being 3M ScotchWeld 2216, Hysol 9462 toughened epoxy, or Hysol U-05FL urethane, to name just a few. One thing to keep in mind is the failure mode of the current material and other eventual materials. Is the adhesive sticking to one side versus the other, as in the case of PVC or stainless steel? This would indicate adhesive failure. Or is it sticking to both, indicating cohesive failure? Or are you getting a mixed result—adhesion and cohesion failure? Are you testing in a shear mode (pulling the materials along the bond line in opposite directions), or in more of a peel mode (a 90° peel)? Evaluating the bond line for these issues will help you select adhesives for your application.

ASTM 2475-05 describes the current requirements for demonstrating biocompatibility in product contact packaging. The selection of which tests to choose depends on the interaction of the packaging and the device. Typically, the packaging material contacts only the medical device and is considered indirect contact to the patient. This condition therefore reduces the level of testing required. If the device involves a fluid or drug that will come in contact with the packaging, more-stringent testing is be required. Consultation with a biocompatibility testing firm is recommended.

The following table provided by SGS (Rutherford, NJ) summarizes the suggested testing for different medical device packaging materials.

RTV silicone adhesives rely on moisture and humidity in the air to cure properly. Generally the conditions have to be 40 to 60% RH but can extend down to 20% and up to 70% in certain cases. The moisture in the air reacts with the stabilizer in the RTV, and once the stabilizer is removed, the adhesive can cure fully. In a very-high-humidity environment, the humidity in the air can saturate the surface of the RTV and effectively seal it off, limiting the penetration of the humidity to deeper levels. If you have a thick bond line or cross-section of material, it may take longer than one week to cure fully. The silicone manufacturers generally set a five- to seven-day cure schedule for RTVs before they can test the physical properties in a thick slab of material, and that’s with the condition of 40 to 60%. If you have a 70% RH condition during the summer time, it may take longer or disrupt the cure enough for the adhesive to appear gummy or semicured. Acidic surfaces may also cause problems with the cure mechanism.

Another avenue to explore: Was this failure linked to just one lot of material? And was it a 100% failure for this lot, or a 1% failure of one tube within the lot? These answers can lead the manufacturer of the RTV to help determine the root cause of the failure.

Cyclohexanone is a popular solvent that solvates or melts the plastic of the bond line and fuses it together. The solvent then has to evaporate from the bond line. With the recent spells of hot and humid weather, the density of the air changes, changing the vapor pressure of the cyclohexanone. It is possible that the solvent is not evaporating as fast as it does during other times of the year.

As the solvent sits in contact with the plastic for longer periods of time, the impact of the solvent on the plastic is often seen in weakening or stress cracking the ABS. If you can increase the number of turns your air conditioning provides in a room, or if you can use additional fans to circulate and exchange the air in the manufacturing environment, this will help the solvent to evaporate faster and stop those pesky cracks from appearing.

There are a few different options to explore. 206-CTH-T is a thickened version of 206-CTH, with a viscosity of approximately 5000 cP. 206-CTH is closer to the viscosity of water or thin oil, with a viscosity of approximately 135 cP. This should help limit the wicking of the adhesive. Note the very scientific naming convention: T stands for “thick.”

Other options could include high-viscosity permanent or temporary removable damming materials. These materials would put a dam in the way of the adhesive and prohibit wicking. Permanent materials could include a number of products that are available in a VT (“very thick”) or a GEL grade. Temporary “masking” materials include peelable masks that can be applied to protect the desired area. Apply the 206-CTH and then remove the peelable mask just prior to performing light curing.