Shootout: strain gage adhesives

mtngun
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Shootout: strain gage adhesives

Postby mtngun » Sun Feb 17, 2008 6:55 pm

from the old forum:

After fighting adhesive failures all year with my RSI Pressure Trace system, it was time to get serious about testing adhesives. Strain gages are encapsulated in a rubbery plastic that most glues don't stick to. The gages are too expensive to use as guinea pigs, so I used strips of a bicylcle innertube glued to pieces of scrap steel. The innertube seems like a similar surface so hopefully it will give similar results.

The pieces were washed in hot soapy water and dried prior to gluing.

Loctite 401 was recommended by RSI at one time.

JB weld is the favorite of Denton Bramwell.

Devcon 30340 "Flex Superglue" elastrosized ahehsion creeates a flexible bond, won't crack when bending, resists impact, great for rubber, vinyl and leather. It sounded good.

Devcon 30350 "Plastic Superglue" works on all kinds of plastics.

After drying overnight, I tried peeling the rubber strips off the metal.




  glue     peel test  
  Loctite 401    peeled off, glue stuck to rubber but not steel  
  JB    peeled off, glue stuck to steel but not rubber  
  Devcon 30340    stuck, couldn't remove with fingers  
  Devcon 30350    peeled off, glue stuck to rubber but not steel  


Image

I'll be the first to say that this is not a highly scientific test, and I can't be sure that the innertube is a good substitute for the strain gage material. Also, this test does not consider the high temperatures that will be encountered on a rifle barrel, or the effect of cleaning solvents and oils. Still, even a crude test is better than nothing.

Only the Devcon 30340 could not be peeled off. The others came off with very little effort -- just like most of the strain gages that I have removed from barrels.

RSI has recently dropped Loctite 401 in favor of another product. At one point they told me they were experimenting with a Devcon glue that was very flexible but when I asked for a product number they would only say
We found another cyano with the same properties as the Locktite but at half the price.

mtngun
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Re: Shootout: strain gage adhesives

Postby mtngun » Sun Feb 17, 2008 7:18 pm

The strain gage substrate is a polyimide. I don't which flavor of polyimide, though. There are several varieties like Vespel and Kapton. It is very expensive so it is only used for high tech applications. I will try to locate some polyimide material that I can use for glue tests.

Plastic-to-steel test
Lacking any polyimide, I ran some tests on a hard plastic. To be specific, a plastic bag closer. As before, I glued a strip of the plastic to a small piece of scrap steel. Both the plastic and the steel were cleaned with acetone before gluing.

Loctite 401 popped off easily. Stuck to plastic, not metal.

Devcon 30340 popped off easily. Stuck to plastic, not metal.

JB weld popped off easily. Stuck to metal, not plastic.

Plastic-to-steel test, surface roughened

I repeated the plastic bag closer test except emory cloth was used to roughen both surfaces before gluing.

Loctite 401 popped off easily. Stuck to plastic, not metal.

Devcon 30340 moderate effort required, but eventually it popped off. Stuck to plastic, not metal.

I also tested a used strain gage with Devcon 30340. It held pretty firmly, but eventually I was able to loosen one edge and after that the whole thing popped off pretty easily. The Devcon had stuck to the gage but not to the steel.

What I have learned so far
1) Adhesion is very dependent on the material, so I really need to use polyimide for the tests.

2) JB works great on metal but not so well on plastic or rubber.

3) Failure seems to occur through crack propagation. Effort is required to initiate a crack along an edge, but once the crack has started, the rest of the bond cracks or can be peeled up quite easily. This is consistent with what I have observed on gage autopsies. The ideal glue should be flexible and resist peeling (but if it is too soft and flexible then it may distort the strain signal).

4) The super glues failed by not sticking to the metal in my tests, however, failures on real installations have always been a failure to stick to the polyimide.

5) It may help to roughen the surfaces with emory cloth.

6) The gage manufacturer recommends either a superglue similar to Loctite 401 or else their proprietary 2-part epoxy.

What's next?
1) Try an installation with Devcon 30340.

2) Purchase a 2-part epoxy that is flexible, resists peeling, and sticks to polyimide. This is not the kind of epoxy that is sold at the local hardware store but it can be ordered. Long term, I would rather use an epoxy because the quick set-up time of the super glues doesn't give you much time to position the gage correctly.

mtngun
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Re: Shootout: strain gage adhesives

Postby mtngun » Thu Feb 21, 2008 7:22 pm

I did my standard test on 3M Scotch-Weld 2216 two-part epoxy. It is advertised as a "highly flexible epoxy provides a high-strength bond with excellent resistance to peeling and high shear strength. Bonds rubber, metal, .... ABS, PVC, acrylic, nylon, and polycarbonate. Rated for -67° to +350°F." It is not rated for polyimide, however. The McMaster-Carr part number is 75045A65 and it's about $25 for 3.3 oz.

The plastic bread wrapper-to-metal test passed with flying colors. When I tried to pull it off, the unglued portion simply broke off. I tried prying off the rest with a sharp knife, but it wouldn't budge.

The rubber intertube did peel off with some resistance. It felt like peeling off a piece of duct tape.

I haven't tried 3M 2216 on a strain gage installation yet. Update: I ended up using the 3M 2216 for all kinds of plastic repairs around the house. It worked great.

Update on Devcon 30340 So far I have installed four gages with 30340 "flex" superglue and none of them have failed due to the adhesive (one did fail but it was stuck on good, and in retrospect it was probably a bad electrical connection). Devcon 30340 only costs $1.99 at my local farm and ranch store.

I have located an epoxy that is rated for gluing polyimide to metal, however, so far I haven't accumulated enough money in my piggy bank to purchase it. It is Bondit B45TH, McMaster-Carr #7513A1, costing $47.85. That's a lot of money, but the cost of the glue has to balanced against the cost and aggravation of having a strain gage fail on a gun. If Bondit performs as advertised, it would be worth $45 to me.

Update McMaster-Carr now offers a thinner version of the Bondit epoxy for a mere $58. Due to the cost, it may be several months before I am able to give it a test drive. The thinner Bondit may work better for our purposes.

mtngun
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Re: Shootout: strain gage adhesives

Postby mtngun » Fri Feb 22, 2008 7:35 pm

The bonding agent that pressure trace is sending out now is Turbo Fuse Super Glue. Just for your information! I received some and hope to use it in 2 or 3 weeks.

Glenn

eagle1
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Re: Shootout: strain gage adhesives

Postby eagle1 » Fri Jul 31, 2009 3:46 pm

:D I just ran accross your discussion of last year for bonding to strain gauges with rubber substrate for testing using the RELTEK B-45TH product. I can help if you's like. I'd be happy to send you a cartridge of the B-45TH and give you engineering support on how to get your desired results. If you'd like to follow up you are welcome to send an email to my attention at info@reltekllc.com and reference this conversation.
Bob
Cheif Technology Officer

mtngun
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Re: Shootout: strain gage adhesives

Postby mtngun » Sat Aug 01, 2009 5:23 am

Email sent.

I bought a small cartridge from McMaster a while back, but haven't had time to follow up on the formal tests.

I did not buy the gun that you're supposed to use to dispense the mix from the cartridge. To save money, I was planning to try to squeeze it out by hand.

I did use B-45TH in an attempt to repair a broken plastic oil separator on a backhoe, which of course, is not what B-45TH was designed to do. Nonetheless, I could see that squeezing the cartridges manually was a problem. It was difficult to control and as a result, I apparently didn't get the mix ratio correct, and the epoxy never fully hardened. I could also see that long term storage could be a problem once the cartridge has been opened.

If B-45TH works as advertised, it should be great for gluing strain gages to barrels. However, in order to be feasible for the hobbyist, it would have to be available in small, not-so-expensive packages that could be dispensed by hand, similar to JB Weld.

The firearms application for this product is tiny, so I don't expect you to jump through a lot of hoops for this application alone. Your best bet would be to approach the strain gage vendors and try to get your product approved and recommended (and probably distributed) by one of those vendors.

As it is, I get the impression that the strain gage vendors have not been doing their homework on adhesives.


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