Premature Ozone Cracking

[davidtire]

Making bias tires with a 60 natural 40 sbr N299 tread compound. 4.5 phr 6PPD and Wingstay combined. Two phr blended wax. 1 TMQ 2 sulfur 2 TBBS. Some tires are cracking horribly within 3 months of being mounted and only mounted on a rim, never used, loaded or flexed. Any ideas about what to look at? black dispersion? Too much cure?

 

[GrahamBennett]

Are these tyres passenger or truck? Rib or block tread patterns?

 

[davidtire]

Tires are R1 irrigation/rear tractor traction pattern. Cracks are showing up throughout tread and sidewall.

 

[GrahamBennett]

Are you storing them outside in full sunlight, or near to an ozone generator (electric motor)? What is the hardness of the tread rubber (measured on the top of the blocks)?

 

[davidtire]

Tires are stored outside. There is limited distortion. Tires are four deep on a skid. Shore A spec hardness range is 57 to 61.

 

[GrahamBennett]

Thank you. Your first mistake was to store the tyres outside and not in the shade. Your formulation looks OK and is well protected. Are you sure the problem is not just a surface phenomenon caused by the wax and/or sulphur bloom cracking in response to temperature changes?

 

[Compositepro]

It is unlikely to be ozone cracking in such a short time. More likely is poor mixing resulting in poor distribution of your ingredients or perhaps using poor quality or recycled rubber.

 

[stancom]

I too am starting to think ozone is not involved, especially as the compound is well protected, assuming the 6PPD level is at least 2phr. You can check whether the antiozonant is drawn to the surface by scratching an uncracked area. You should see some 'tyre browning'. 6PPD reduces the rate of crack growth and needs a wax bloom to confer static protection. If the tyre is unloaded I can't see how ozone cracking can occur unless there are severe inbuilt stresses arising from cure. Are any tyres kept under different conditions and indeed in an unmounted state for comparision?

 

[davidtire]

Recycled Rubber - The compound has powdered tread peel.
Tires are stored outside next to competition. Competition tires are performing well.
Ozone - I have had several chemists and consultants view tire pieces. Consistently get, "Yes, that is ozone," when looking at the tires and "No way should that formula ever have an issue with ozone," when looking at the formula. Compounds are mixed 2-pass with AO protection, PPD, wax etc., in the first of two mixes so there should not be a problem with AO dispersion. My black dispersion was rated as OK to good from consultants. I have made mix cycle changes to improve it. Thanks for all your suggestions and comments.