SBR Banbury Mixing

[donaloriain]

I'm mixing a very basic SBR compound on our 270 liter Banbury mixer. The compound has carbon black, the standard small powders, some synthetic resin for tack and no oil. It really is a basic formulation. But I'm having an issue with the compound discharging from the mixer in pieces rather than as one single mass, which is always the ideal scenario for reduced milling time. I tried single stage initially but ended up switching to two-stage to allow higher temperatures. I have tried various fill factors, different mix steps, different order of loading and different rotor speeds on discharge - all to no avail. Has anyone any tips on improving this?

 

[donaloriain]

I forgot to add - I have mixed the exact same compound on an intermix without any problem.

 

[btrueblood]

You have a top-load, bottom-discharge mixer? Don't know for sure, but would expect a low-plasticity mix to come out in pieces, given that most the the mass has to come through the rotors to discharge after opening the door.

 

[GrahamBennett]

What water temperatures do you have set on the chamber sides, rotors? What rotor speeds do you have available? How much C Black (in phr) are you adding to the SBR?

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[donaloriain]

It's a top loader, variable speed mixer. Sides, chamber and rotors at 35 degrees C but i also tried increasing to 45C. Carbon black loading is 55phr.

 

[GrahamBennett]

Have you tried a mix cycle like the following?

1. Water temperature 40°C
2. Rotor speed 50 rpm
3. Add SBR, ram down. Mix for 60secs.
4. Add ½ carbon black, ram down. Mix for 45 secs.
5. Add rest of black, ram down. Mix for 45 secs.
6. Add rest (but NOT accelerators or sulfur), ram down. Mix to indicated 165-170°C.
7. Dump masterbatch.

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[donaloriain]

Thanks Graham. I have tried a similar cycle previously but not with this higher dump temperature. The highest temperature I went to previously was 150 degrees C. I was concerned about thermal degradation of the SBR. But I will certainly try going to 165 and assess the effects on the mechanical properties.

 

[GrahamBennett]

Don't worry about thermal degradation of SBR - you can take the dump temperature up to about 175°C quite safely if you wish.

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