Resting of rubber bases before acceleration finals (two cycle mixing)

[JDLSHF]

Greettings!

Even being a possible simple question to anwser, I always retain the question as to why, in some cases of course, should a rubber base or master rest before adding the acceleration. The main answser usually is for stabilization. But, for a carbon back, RAE oil, antioxidant and antiozonant, C9 resin and a dispertion agent filled rubber base of lets say NR, SBR and BR mix, after the 1st mixing cycle, where is the need for stabilization and what does it actually affects?

Thanks.

 

[stancom]

Stabilization is one of those catch-all terms we're never quite sure of what it means. In this case I don't think of chemical change but primarily of control and sometimes bringing the mix down to room temperature. For test mixes ISO 2393 specifies a minimum of 30 minutes for this purpose and an upper storage time of 24 hours. Under factory conditions the period can be anything between a few minutes when equipment is close by and several weeks when the mix is despatched to another plant for completion. Other factors include availability of staff, intermediate control testing and cross blending of batches. If the period is prolonged there could be an increase in viscosity with some mixes and blooms may form. I mention the latter because I remember a keen recipient of a master-batch wiping the surface to remove talc and bloom he mistook for contamination. Control is key.

 

[JDLSHF]

stancom: I would agree with you, and I truly believe that in some cases, it's just a, lets say, precaution measure. But one of the cases that I found, was a thread mix started to have some problems in the final application (rethreading of a tyre carcace), and the main issue was cracking of the rubber (I think after vulcanization). There was an instruction to have a waiting period between the 1st mixing cyle and the 2nd (final rubber mix) of no less then 24h. According to the client, the problem disappeared.

Nevertheless, I'm placing a disclamer, because sometimes clients have that "placebo effect", and complain heaven and earth about rubber problems just because the compound is a little different in colour, but can receive worst rubber with the colour that they are used to, and the product is alright again! In short, the client is not always the best way to find out true problems, if they truly exist.

Indeed that several testings ask for specific conditions to test the rubber, and the usual ones always state that the rubber should be at room temperature (or specified in the norm, but usually around 20ºC to 25ºC with a +/-2ºC). But that makes sence, because if you place, for instances, a not "rested" rubber to test rheometry, well the curve will be strange, different. You might not detect ts2, torques will be off, and so on. But the problem that I state (or situation if you will), supposably there were differences in the final rubber compound, that I cannot figure out as to why.

Thanks for the help. Neveetheless, if there are other explanations, go for it.