Color change when using Sulfur cure vs peroxide cure

[markbr57]

We produce a light colored EPDM / SBR copolymer that is usually cured with peroxide. If we attempt to use a sulfur cure system, there is a serious shift in the materials color, becoming much yellower...and even has signs of scorched places (a golden brown). Is it possible to overcome this? Or should we just stick with the peroxide cure system?

 

[GrahamBennett]

The colour change is due to the sulphur/accelerator package. You could try reducing the sulphur content somewhat but, I would stick with the peroxide system for efficient curing if not for the colour shift. Even the fastest curing grades of EPDM are slower curing compared to SBR with sulphur/accelerators.

 

[markbr57]

is the color shift because of the sulfur reacting with the epdm? or is that somehow due to the type of acceleration that is used?

 

[GrahamBennett]

I'm no expert on discolourisation, being an almost totally carbon black man, however I do know a little about it. It woulod help if you could tell me what and how much of the sulphur and accelerators you are using but, in general, keep the sulphur content as much below 1.3phr as possible.

If you are using Crystex OT20 or other insoluble sulphur keep your final mix temperature below 105°C and cool the batch as quickly as possible after dumping otherwise you will convert the sulphur to the soluble form and that will produce a creamy-white colouration and/or bloom on the surface of the rubber.

Other ingredients that will cause discolouration are PPD-type antioxidants - use BHT or similar hindered phenol - and TMQ, although with EPDM in the compound you do not need these protective agents.

 

[tom1953]

The curing temperature will affect the cured color somewhat, so lowering the cure temp, along with increasing the cure time, may minimize the yellowing.

Tom Jablonowski, TSE Industries, Inc.

http://www.tse-industries.com

 

[markbr57]

Graham and Tom, we started off with 3 phr sulfur, 1.5 phr CBTS and 0.2 phr of TMTD..along with 5 phr ZnO. Reducing the sulfur to 2 phr helps some, but not enough. Also curing at a lower temperature helps some. The reson for the cure change, was there was some problem with spontaneous combustion when using peroxide at a 3 phr level. I'm considering reducing peroxide to 2 phr because of the color. It is just unacceptable with the sulfur cure thus far.

 

[tom1953]

Spontaneous combustion??!! Is that happening during mixing or during down-like processing? What type of peroxide are you using? And how are you mixing the compound (time, temp, mixer type)?

If you're using a neat peroxide, you might consider one that's absorbed on a filler, such as DiCup 40C or Varox DBPH-50.

Tom Jablonowski, TSE Industries, Inc.

http://www.tse-industries.com

 

[markbr57]

Tom, this is Dicup 40c that we use. The combustion is happening after curing...in dust that has been collected after the sheet faces have been sanded. The dust is collected in a bin, and sometimes in a few days time, the dust will begin to smolder. It doesn't slways occur, but occurs often enough to make everyone nervous.

 

[tom1953]

Companies that grind rubber covered rollers oftimes have such problems with grinding dust catching on fire. This is usually due to grinding the roller too quickly (grinding too much off the roll too quickly) and the grinding dust getting very hot, hot enough to catch fire sometimes. Sometimes the hot grinding dust will smolder for a time, not appearing to be hot or on fire, but when given enough time and maybe better exposure to oxygen in the air, can start burning.

Some types of rubber, and different rubber compounds (probably those that are more resistant to abrasion), can build up more heat than others during grinding. I hadn't heard that this phenomenon is related to a type of cure system, seemingly mostly related to grinding technique/speed.

Hope this helps.

Tom Jablonowski, TSE Industries, Inc.

http://www.tse-industries.com

 

[GrahamBennett]

Tom, we've had this happen with peroxide mixes. Slabs of hot, mixed rubber left on a cold floor to cool down over the weekend have been smoldering quite nicely on the Monday morning. We've found it's the peroxide (DiCup 40C in our case too).