If you’re looking for a tape that can bond silicone rubber or silicone foam, 3M 9731 is an interesting product because it uses two different adhesives on the same tape.
One side has a high-performance silicone adhesive, while the opposite side uses 3M’s 350-series acrylic adhesive.
That’s what makes 9731 a differential adhesive tape—each side is designed to bond to a different type of surface.
The silicone side is designed for bonding to silicone rubber and silicone foam, while the acrylic side provides strong bonding to a wide range of other materials.
What about the liners?
There is a reason the two sides use different liners.
The silicone adhesive side uses a clear PET liner, because silicone adhesive can have difficulty releasing from a silicone-coated liner.
The acrylic side uses a silicone-coated kraft paper liner, which is typical for this type of adhesive construction.
So, essentially, you have a silicone adhesive on one side and a high-performance acrylic adhesive on the other.
What does the acrylic side do well?
The 350-series acrylic adhesive provides high shear strength, good edge-ooze resistance, and strong adhesion to many common industrial materials.
It can bond to both high-energy surfaces and a number of more difficult substrates, including materials such as polypropylene, polyethylene, some ABS, powder-coated surfaces, metals, and glass.
3M also indicates that it can provide good adhesion to some lightly oily surfaces.
However, as a general practice, we recommend applying the tape to a clean, dry surface whenever possible.
Where 9731 gets particularly interesting
The real specialty of 9731 is its ability to bond to silicone sponge, silicone foam, and other silicone rubber materials.
And there’s an important distinction here.
Just because silicone rubber is considered a low-surface-energy material does not mean you should automatically think of 9731 as simply an “LSE tape.”
In fact, if you’re trying to bond silicone to another material, a tape specifically designed with a silicone adhesive is generally the type of product you want to investigate first.
But don’t expect every silicone-to-silicone bond to be permanent
One thing to keep in mind is that bonding to silicone foam or sponge rubber can be challenging.
You may get a very useful and reliable bond for a particular application, but you should not assume that the bond will be permanent under every condition.
And this brings up an important point about silicone adhesive tapes that is sometimes overlooked.
It’s not just how strong the adhesive is.
It’s also how the force is being applied to the bond.
Think about the direction of the force
In a real-world application, the tape may be exposed to different types of loading.
For example, you can have peel forces, where the adhesive is being lifted away from the substrate.
You can also have shear forces, where the load is trying to slide one bonded surface across the other.
And these different forces can produce very different results.
For example, imagine a strip of tape supporting a load.
If the load is trying to pull the tape away from the surface, you’re putting the adhesive into a peel-type loading condition.
But if the load is trying to slide the bonded materials past each other, you’re applying shear.
This distinction can be extremely important with silicone adhesive.
Where silicone adhesive can perform well
For silicone-to-silicone applications, silicone adhesive can provide very useful performance under shear loading.
Peel loading—particularly forces that try to lift the edge of the tape away from the silicone surface—can be much more challenging.
So when you’re evaluating a silicone adhesive tape, don’t just ask:
“What’s the adhesion number?”
Ask:
“How is this tape actually going to be loaded in my application?”
That’s often a much more useful question.
Why data sheets aren’t the whole story
This is also why adhesive data-sheet numbers should be viewed as a starting point for comparison, rather than a guarantee of how a tape will perform in your particular application.
For the best evaluation, you want to test the tape using:
- The actual substrate
- The actual surface condition
- The actual tape construction
- The actual direction of force
- The actual loading conditions
- And, when important, the actual temperature and environmental conditions
In other words, there really is no substitute for application-specific testing.
That’s particularly true when you’re bonding silicone foam, sponge, or rubber.
If you’re considering 3M 9731 for an application, we have sample-size sheets available for initial evaluation.
Testing a sample on your actual materials is usually the best way to determine whether 9731 is the right tape for the job.