40A Neoprene degradation 2

[Dardak32]

In a stepper motor driven linear motion system with guide rails I have used 1/32" thickness neoprene as a spacer between a couple screw heads and steel surfaces. This was one to allow some extra play in the mating surfaces to reduces the rigidity.

The neoprene spacer is a rectangle about .5"X1" and has two holes to allow the screws to pass through. One side of the neoprene has an adhesive that sticks to the steel surface. After repeated use, the neoprene begins to bubble in between the two screw heads pressing against it and eventually cracks and breaks up into small pieces. My first instinct says to try using washers below the screw heads to distribute the load on the neoprene over a wider area. However, I wanted to see if anyone could shed some light on neoprene degradation in the form of cracking and breaking up. Does this mean that the environment is typically too dry for the neoprene?

 

[EdStainless]

So there is no oil and no ozone and no UV reaching the neoprene?
Sounds like environmental degradation to me.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[3DDave]

One feature of elastomers is they are considered incompressible**. I'm guessing this is solid and not foam, right?

This means that any displacement in one direction means there must be displacement in some other direction which maintains a constant volume. The adhesive is trying to resist that displacement and producing a shear force. I presume the adhesive creeps a bit when the compression load is applied, either by external force or by thermal load. Since the top surface of the bubble is under some tension to counter the compression on the bottom, it cracks there.

**An interesting discussion of compressibility:

http://imechanica.org/node/10589

 

[WKTaylor]

Dardak32... Hmmmm... whatever happens, appears to happen fast.

This 'sounds' a bit like high temp rubber deterioration driven by reversion or loss of plasticizers.

Are the screws plated? IE: cadmium or zinc or nickel or phosphate or organic coated? Platings/coatings can drive chemical reversion of rubbers/plastics.

Not all Neoprene [Polychloroprene] is the same. Have You confirmed the origin and general quality of the sheet material You are using?

Does the deteriorated PCR damage or discolor the screws or mating surfaces? Do You clean the PCR before installation to eliminate contaminates before install?

Is the service temperature of the PCR pad exceeding 180F [way-too-hot-to-touch?]; and/or is the operating environment 'wet/moist'?

Perhaps other rubber/plastic compounds should be tested for improved durability. For grins check-out NAS1515 WASHER, PLASTIC AND SYNTHETIC RUBBER, which specifies various silicone rubber compounds, synthetic rubbers PTFE, PCTEF etc would be a better option.

There are several good texts/documents published for designing with rubber... they might help.

MIL-HDBK-149B RUBBER [helped a little, but didn't provide a lot of environmental exposure data]
SAE AIR786 Elastomer Compatibility Considerations Relative to Elastomeric Sealant Selection [I'll check this out later].



Regards, Wil Taylor

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[Compositepro]

A single example means very little. The rubber in this part may have been mixed incorrectly or the part may have been made from one bad spot in the rubber batch, or any of hundreds of other reasons.