Blooming in NR

[rubberglove]

Hi,
Would somebody like to share his views or experience on this issue i.e. what causes it? how to prevent it? thanks.
In my recent study, the powders that bloomed out seem to comprise of sulphur and zinc. Any idea?

 

[berti]

Hi rubberglove,

blooming of chemicals from NR - but even more from EPDM - is a difficult matter also for your name-giving article.

Basically it means that one of the rubber chemicals is soluble in the polymer at mixing and curing temperature but tends to separate after cooling and "blooms" to the surface in hours or days.

The blooming depends from kind of chemicals used, from the amount in the recipe and much less from other recipe factors.

As you already found out most dangerous are the zinc dithiocarbamates with the short alkyl chains (ZDMC,ZDEC)because of the low solubility in non-polar NR. Unfortunately the corresponding thiurames are not much better because their first reaction step is most probably the reaction with the zinc oxide to form the a.m. dithiocarbamates ... By the way all these chemicals are also the dog's dinner concerning the nitrosamin formation, so blooming problems are nowadays not seen so often aas in the past.

So what to do:
Replace the short chain thiurams and DTC's if you can. You can use zinc dithiophosphates and/or longer chain DTC's and thiurams like Zinc dibutyldithiocarbamate (ZDBC), Z.dibenzylDTC or ZincdinonylDTC (ARBESTAB Z of Robinson in UK)or the respective thiurams. They will not (or much less) bloom, but the main disadvantages are: They are much slower and more expensive.Maybe try also a soluble zinc soap instead of zinc oxide.

If you are fixed to the short chain DTC or thiuram, try to combine, e.g. instead of 3 phr ZDMC use 1 phr ZDMC+1phr ZDEC+1phr ZDBC.

Let me know if it helps.

Berti

 

[1399]

Hi,
In addition to Mr berti and pronate comments I would add that when blooming come from sulfur the process is as follows:
there two kind of sulfur, ordinary orthorombic an insoluble Ordinary sulfur is cristalline and soluble in rubber. It dissolve during the hot mixing process. On cooling the sulfur which has migrated to the surface will cristalyze, changing the appereance of the surface. Relief from sulfur bloom can often be had by using insoluble sulfur. This is an amorphous polymeric sulfur with greatly reduced solubility in rubbers so it does not cristalyze on cooling.

 

[BobFran]

Hello there,

Yes bloom is caused primarily by sulfur &/or chemicals used in the curing process. I see from your query that this is probably related to gloves. The usual chemicals are dithiocarbamates and sulfur, with some thiazoles. If there is an excess of any of these above that which can be used to form the crosslinks, then there is likely to be some bloom or precipitation of the low solubility component to the rubber surface.

Bob