Cold-bonding aerobic structural adhesives, those that cure “on demand” without heat, allow manufacturers to assemble parts when they want to without being limited by two-component mixing, additional heating and cool downtimes or oxygen presence within the bond area. Cold-bonding aerobic adhesives provide companies with the ability to bond dissimilar substrates within the production line without having to worry about long fixture times. Because the adhesives experience rapid bond strength development, the parts can continue through the production line without increasing work in process (WIP) times. This means companies who switch to cold-bonding adhesives will improve production efficiencies within their process while experiencing decreased energy costs. All of these are extremely important to companies in today’s current economic conditions. Companies need to explore different methods to become more efficient without sacrificing quality. Cold-bonding aerobic adhesives are one method to help companies.

It reminds me of a story of a company who was using an induction heat-curing epoxy to bond magnets into a cold-rolled-steel motor housing. Not only were they experiencing a number of in-line quality issues but the cool down time to handle the parts added significant WIP and expense to their process. By switching to an aerobic structural adhesive they were able to improve their quality and eliminate any non-valued added steps in their process. This allowed the company to produce a quality assembly quickly while improving their profits.

These adhesives cannot, however, be used in all applications and are only recommended when alternative bonding methods such as ultraviolet light-curing adhesives cannot be used. Aerobic adhesives have been successfully used in DC and brushless DC motor, speaker hardware, and opaque metal and glass bonding applications, in some cases exceeding the strength of one of the substrates.

How do they cure? Aerobic structural adhesives are two-component but in a non-traditional manner. Cold bonding technology incorporates a liquid activator and (usually) a gelled adhesive. As shown in the picture to the left, adhesive is applied to parts of a motor, in this case the magnets. Unlike a two-part epoxy where the components are statically mixed together, the activator for the aerobic adhesive is applied to the opposite part, in this case to the motor housing. Once the parts are ready to assemble, the magnets are pressed into the motor housing. The adhesive spreads through the activator, filling any gaps that exist between mating parts. As the adhesive spreads through the bond area and activator, this mixing action starts the curing within seconds and eventually finishes it. Again, there is no need to batch or rack curing parts since the buildup of strength is immediate and ongoing, allowing manufacturers to keep the assembly within the production flow.

Aerobic adhesives do not work in all applications, but should be selected as an alternative to cyanoacrylates, second-generation acrylics, induction heat-curing and two-part epoxies, and anaerobic adhesives. Each application should be reviewed with the technical staff of the manufacturers’ products that you are considering using. Changing to a new adhesive should only be considered after you have thoroughly tested the performance of the assembly to see if it meets your established test criteria.

Structural Adhesives, Cold Bonding, Dissimilar Substrates, Structural

An inquiry that came through:

“We have an application that requires a hermetic seal between dissimilar materials.  The bonds must be able to withstand the conditions of autoclaving and sustain immersion in a fluid for approximately 30 days.  One bond is between ceramic and silicon, and the other is ceramic and SS.

Please advise on materials and other recommendations for surface prep, bond line, etc.”

Response:

There are a few materials that may be candidates for evaluation to bond dissimilar materials. To withstand one autoclaving cycle, followed be immersion in a fluid (I am going to assume a water/aqueous liquid), and give good adhesion to ceramic and silicon, and ceramic and stainless steel, I would recommend either an epoxy or an acrylic-based adhesive.  Two-part epoxies will generally withstand these conditions, come in a wide variety of hardnesses, and give good adhesion.  One-part thermal-cure acrylates will survive the autoclaving (1 cycle) with a protected bond line.  A protected bond line can be best described as large mating surface areas between the two substrates, with only the edge of the adhesive being exposed to the steam or fluid.  I would recommend a bond line thickness of 0.002-0.004 inches for this type of application.  If it’s too thin you might have voids.  If it’s too thick you might have too much surface area of the adhesive being exposed to these conditions.  In this case, smaller bond line thicknesses are better. Products with a viscosity of 200-1000 cP would be ideal for this bond line thickness. Another adhesive option to improve efficiency in your manufacturing is to look at products classified as Multi-Cure^®.  Products of these types cure in different ways, including the ability to cure with heat or light.  The ability to cure with light would allow these parts to be assembled and tacked in place in seconds, and then exposed to heat to cure the remaining shadowed area.

To verify one point:  We always double check if the substarte is silicon or silicone.  While made up of the same elements, silicone is a flexible, rubbery material.  Silicon, as in silicon wafers, are generally metallic, hard, rock-like surfaces.  We work with both materials, but there has been enough confusion over the years that we like to double check.  That little “e” at the end can make a big difference in selecting the proper adhesive.  If you are looking at bonding ceramic to silicone, then I would recommend a silicone one- or two-part adhesive. 

Regarding surface preparation:  A rough surface will (generally)  give better adhesion than an electropolished surface. A rough-surface topography often has microscopic mountains, valleys, and pores that the adhesive can fill, which provides additional surface area, as well as a mechanical interlock.  A smooth-surface topography only gives one value of surface area, and no mechanical interlock.  If the surfaces can be roughened by abrasion, shot preening, scoring, or a chemical primer - these methods will improve the overall bond strength.  Making sure that the bonding surface is free of contaminants, oils, release agents, cutting lubricants, or even finger oils can help yield a repeatable bond strength.

Structural Adhesives, Dissimilar Substrates, Epoxies, Structural