Ferrari Mailing List: tire cracks

[Doug & Terri (dnt]

Cracks in tire rubber?  Just your everyday chemistry in action.

 

Compounding tire rubber is the original compounding witch’s brew.  But what cause’s the cracks?  Why it when the sulfur atom is robbed from the tire by ozone.  Sulfur?  What?  Let’s take a look at what goes into tire rubber. 

 

Incidentally the Brits coined the term “rubber” when latex was first discovered and one of its only uses was for that thing was ‘rubbing out pencil mistakes.”  We, in the US of A call that thing an “eraser.”  You know, the thing at the end of a #2 pencil or both ends of Johnny Carson’s joke pencil he flipped around.  Any how I digress.

 

Back to what goes into tire rubber.  The witches’ brew has, (and for this I have to go back to my basic books because I really don’t carry this stuff around in my head any more) in addition to latex polymers are “Fillers,” Antioxidants,” “Antiozonants,” “Oils,” and “Cures.”  Sulfur resides in the cure batch. 

 

One of the most common fillers is carbon black that gives the tire is common black color.  This strengthens the rubber compound.  But early tires were commonly just latex white.  Where do you think the “Michelin Man” Bibendum (Latin for the act of drinking) or just Bib comes from?  Why it’s a stack of white Michelin tires in the shape of a person drinking champagne with the notice “I drink up obstacles.”  Onward.

 

While the early use of the latex product was water proofing, the rubber had a tendency to decompose and stick together.  You’ll notice that on old rain coats – I mean really old rain coats and water proof shoe covers.  Not until Charles Goodyear in the mid 1800’s discovered heating latex batch and adding sulfur which significantly changed the molecular structure did the rubber we know today become a viable latex tire product.  He called this “Vulcanizing.”  Today the whole batch of stuff and the named products above compounded in a powerful machine using two opposing rollers, like giant rolling pins, working at two separate speeds and creating tremendous heat creates today’s current tire rubber.  Large latex batts are thrown in the mixer along with varying amounts of the other stuff and ground together under tremendous heat and pressure.  It’s finally rolled flat and out comes sheets of black rubber.

 

By varying the ingredients the final rubber product has varying properties of stickiness and longevity. By the way, tires today are still hand made.  A fascinating and hot experience.

 

So why do tires crack?  The ambient O2 (accelerated by heating = sunshine) combines with the weakly linked carbon-sulfur-carbon molecule leaving an ever weaker carbon-carbon-O2 link and since O2 is a lousy binder – cracks form.  Thus protecting tires from sunshine, heat, and ozone goes a long way in tire crack prevention.

 

Cheers

Doug