NBR tearing - material identification

[TedinCalif]

Hi,
I have some failures of a loudspeaker surround - actually most of this batch - it is a half roll (semi-circle). When the material is folded it breaks like a metal. Also, I can tear it like paper - does not seem to have many rubber qualities. I am trying to determine if the cause is the high durometer rating (shore 90), the wrong kind of NBR or a processing issue in the heat forming of the shape. We use NBR surorunds all the time and this is the first time I have ever seen this.
I have attacehd a photo of the tearing.
Also, any advice on specifying the a NBR wouldbe helpful.
Thanks
Ted

 

[GrahamBennett]

Did your company mix the compound or did you buy it in? Either way you need to talk to the technical guy responsible. Such a high hardness would cause the part to become brittle and have a fairly low tear strength. Causes include massively long curing times or too high a temperature or even the wrong grade of NBR. The photo shows flow cracking in my opinion, which could be due to a very short scorch time.

 

[TedinCalif]

Hi Graham,
No the surrounds are made in China and then attached to the cones (in this case aluminum) in China and then made into tranducers (also in China) so I am not involved with the surround process, though I did ask for the material composition (see below). They have supplied samples in a shore 80 which does not tear when folded but I am concerned they might have a process issue. "Short scroch time" means it was not in the heat or cpmression press long enough?


material constituent content
NBR 52.0%
Carbon black 13.0%
Light Calcium Carbonate 15.6%
vulcanizing agent 1.1%
vulcanization accelerator 11.8%
zinc stearate 1.6%
Antioxidant 1.1%
zinc oxide 3.8%

I am not sure if we should be specifying the type of NBR or not?

Thanks
Ted

 

[TedinCalif]

Buy the way, I can duplicate that crack on a new unit by just pushing down on the roll ,and then the crack will appear. Also, Once I have a piece in my hand, I can tear it like paper - has none of the rubber characteristics I am use to.

 

[Metten]

vulcanization accelerator 11.8%
This seems excessive unless they mean sulphur and then I can imagine it may become brittle and scorchy. High amounts of sulphur will lead to products which aren't very elastic, but they should still have better properties then what you are seeing.

10 points harder then a supplied sample would be out of specification and a reason for rejection in my eyes.

Have the properties tested on a test slab, if the properties are still bad, then it is inherent to the compound and not the processing. Check the rubber part for microscopic air or other inclusions.

 

[GrahamBennett]

Metten is right regarding the amount of sulphur - it's too high.

Scorch safety is the time available between the uncured compound going into the heated mould and the startt of curing. This time allows the compound to flow to all parts of the mould before it becomes too stiff (cured) to move any further.

I'm not suprised about the varying quality from Chinese-made parts. I personally know of several companies that have pulled their orders from China and given them to Malaysian companies.

 

[Ltormento1]

I think they are using a regular NBR and due the high level of vulcanization accelerator they heve the breaking problem.

My suggestion is to use a NBR with low viscosity (30 ML1+100) increase the level of carbon black and add a litle quantity of polyesther plasticizer as processing aids. This eventually will change the caracteristic of the compound and it will be more flexible even with high hardness.

Luis