I agree that the suspect is the plasticizer migrating during the sterilization and accelerated aging process. Plasticizers such as DEHP and butyl octyl phthalate (BOP) often migrate with heat and time from areas of high concentration to areas of low concentration. It does not matter if the adhesive is cured or uncured. Plasticizers in effect solvate the adhesive and migrate into it, often causing it to change color and become gummy or tacky. Just as plasticizers keep PVC nice and flexible in the cured state, they migrate away from the PVC under the right conditions.

In this case, the plasticizer migrated into the adhesive, eventually leading to bond failures. This result can be tested by subjecting the PVC tubing alone to the same heating and accelerated aging conditions and wiping the surface periodically throughout the process. Testing the wipe media for contaminants such as DEHP or BOP can indicate the process step that causes this migration and how much contaminant is migrating. Instead of wiping, you can also “chemically wash” the part with a proper solvent, collect the solvent, and analyze it using gas chromatography.

To fix the problem, we would recommend that you try different PVC tubing with a less mobile plasticizer, or switch to comparable polyurethane tubing that has physical properties similar to those of PVC but does not require a plasticizer. Changing the chemistry of the adhesive, while possible, is a last resort in most cases.

There are a number of silicone mold-making materials available from silicone-manufacturing companies. Products from Wacker, Rhodia, NuSil, Momentive, and Dow Corning all have a typical relative density of 1.06 to 1.15 g/cm³. While Dow Corning has a nice Mold Making Solutions Web page, product selection may best be achieved by contacting a distributor in your area that carries these products. They are set up to answer questions quickly and serve as a conduit to the larger companies.

Urethane rubbers can also be used for mold making. During a quick search, I found a urethane rubber product, PMC-121, from Smooth-On Liquid Rubber, that has a relative density of 1.04 g/cm³. There are many urethane materials for mold making, so the choice is yours to pick a vendor that specializes in the mold-making materials that suit your needs.

Adjusting the feed rates, order of addition, and feed composition will help create a block copolymer–type system with blocks of soft and hard segments. The chain lengths of the polymers will dictate the length of the blocks, but in a straight blending-type reaction vessel, how the blocks form together will still be random.

You can get AAABBBAAABABABAABAAABAABBBBBAAABBA—a random sequence that will form if you are not careful. A step-growth block copolymer system can help form a repeatable and controlled AABBAABBAABBAABB structure. Vendors of the various polyols may have ideas on how to assemble a stepwise growth structure.

To bond materials such as Ultem and stainless steel for reusable devices that will see autoclave conditions, the right choice in adhesives is important. As a general category, acrylates, urethanes, and cyanoacrylates will survive only limited autoclaving. Epoxies and silicones will have better success. Ultem is a difficult material, but with proper evaluation, you might find a successful candidate.

Silicone materials are supplied by such companies as Dow Corning, Momentive, NuSil, and Wacker. Epoxy materials can be obtained from Epoxy Technology, Henkel, and numerous other companies. Silicones will be softer and flex with the thermal stresses, while epoxies will be more rigid. Bonding will ultimately depend on the part design.

Bonding any type of cloth requires a little bit of care in the application and process. Low-viscosity adhesives will penetrate into the fibers, and may wet/migrate farther than intended. Higher-viscosity adhesives will have a better chance of success. A hot-melt-type adhesive may be an option for laying down a highly viscous liquid in a bead or pattern, and lightly pressing the cloth to the PMMA plate. A high-viscosity cyanoacrylate adhesive like Dymax 222/GEL may be an option. A high-viscosity light-curing adhesive (light beaming through the PMMA) like Dymax 1187-M-GEL may also work well, as long as it does not penetrate into the cloth too far and create a shadow area where the Dacron blocks light from curing the adhesive. A two-part epoxy such as those available from Epoxy Technology or Loctite/Henkel will work well, and be most moisture resistant as well.

There are a few light-curable adhesives that initially come to mind that will bond to both metals and plastics and have good adhesion to both polyether block amide (PEBA) and PE. Treatment of the PE is a good idea. Methods include corona treatment of the PE or plasma treatment with an oxygen-gas attachment.

Adhesion to PEBA, a block copolymer, is affected by the material’s variable nylon characteristics. Usually, the higher the material’s durometer, the more like nylon it will behave, which is a little more difficult to bond to. A lower-durometer PEBA is typically easier to adhere another material to. If you are bonding PEBA to PE, generally you would only need to treat the PE, but if you are using a high-durometer PEBA, it would be wise to test treat the PEBA and see if better bond results are found. If you are looking for a recommendation from Dymax, I would recommend evaluating either 1161-M or 1168-M. Other companies provide some very good adhesives for these substrates, and I am sure their technical support groups can help make a proper recommendation.

There is a new technology available that lets you see the blue-colored adhesive as it is dispensed. When it cures, it becomes clear. See Cure Technology, as found in a product such as 1203-M-SC, can bond to metal, PEBA, and PE (with treatment). The adhesive’s ability to allow the operator to see where the adhesive is dispensed and when it reaches full cure also functions as a safety measure.

There are a few options available for bonding PET to PET, whether you use injection molding or stretch-blow molding. One technology available is light-curable adhesives. These adhesives and curing systems are available from Partner Pak (Huntington Beach, CA) and a few other players in the market that work closely with the container and packaging industry.

While Dymax has not tested each of its adhesives for recyclability, David Cornell, technical director of the Association of Postconsumer Plastic Recyclers (Washington, DC), offers these guidelines on PET clamshells:

1. Does the presence of the adhesive cause the PET to yellow when it is remelted?

2. Does it cause the intrinsic viscosity of the PET to fall excessively when it is melted?

3. Does the adhesive produce haze in melted PET?

4. Does it cause films to crystallize too quickly?

5. Does it create black specks in melted PET?

Generally, one should find out by testing a controlled lot of unbonded components and a test sample of bonded components. Make new components by grinding up bonded and unbonded structures separately and processing them as regrind after the hot water wash. If none of the above changes in appearance occur at 25 and 50% regrind levels, the adhesive is probably not going to be an issue if the clamshells are recycled.

Generally, clamshells are not pursued as sources of recycled PET, but PET clamshells do occasionally end up in bales—whether requested or not. As such, real-world experience dictates that the adhesive present should be much less than 25% of the recyclate. But if you can show that there is no problem with using a 25% adhesive content, concerns will be allayed. The cured adhesive should not affect solid stating performance, but it should be tested fully.

My thought is that there is no problem with recycling PET sealed with Dymax’s adhesive, because when it is cured, this adhesive is a clear thermoset that constitutes a small percentage of the total assembled package.

I would recommend a soft, flexible A-70-durometer material with a viscosity of 1500 cP so that the printing paper can be easily rolled onto the surface, thereby eliminating the entrapment of bubbles. A suitable material is Dymax light-curable dome coatings, which are designed to provide crystal-clear coating and adhesive properties. At the same time, they stick well to plastic lenses, thin films, and professional photo printing paper without wrinkling or distorting the image. It is important to use a low-medium intensity light to reduce stress during cure. Other coatings are available to provide a glossy scratch-resistant coating over the photograph, usually in applications without a covering PET or acrylic lens.

UV-light-curable epoxies typically only cure with the UV spectrum from 300 to 390 nm and do not make use of visible light to cure. Given the semihidden bond described in your application, switching to a visible-light-curable acrylated urethane adhesive would definitely be worth trying. A visible-light-curable adhesive will allow more of the available light to hit the adhesive, curing deeper within the FEP tubing.

One of the issues is that the borosilicate glass capillary is going to act like a light fiber. It will take the light and carry it like a fiber-optic cable, not allowing it to get to the adhesive as it is bouncing the light internally within the glass and moving it past the bond area. The visible-light-curable adhesive should be exposed with high-intensity light, and the adhesive itself should allow the light to penetrate into the 0.03-mm gap.

There are a few other alternatives. A cyanoacrylate adhesive would cure deep within the FEP tubing without light. Because of the deep overlap area, only the top surface will be exposed to water, possibly providing enough protection to create a watertight seal. Another alternative is to use Dymax 9440, a light-curable silicone adhesive that can be exposed to light as it is being dispensed during assembly. This material may provide you with enough time to assemble the part before the material starts to set up.

Our experience has been that adhesives that bond to other inorganic materials such as glass and metal will bond to inorganic salts. We have seen applications in which inorganic salt plates or crystals are bonded together with very good success. We have also had success depositing saltlike crystals onto a thin film of adhesive that embed themselves into the coating and are then cured in place. It really comes down to selecting an adhesive that gives you the right properties and choosing among different levels of hardness, viscosity, and any other properties of importance to your application. There are several light-curing materials to choose from.