David Southwell, an independent tire consultant and failure analyst from Australia, considers how to maximize component adhesion while minimizing cost and scrap, and enhancing productivity.
Ever since SS Voyutskii’s pioneering work that identified the molecular behavior at the interface of compatible polymer components, we as an industry have sought to better understand, and improve upon the application of these principles in our products. In my view much progress has been made in the intervening 50 or more years – and yet too often we see clear evidence in finished products, of the factors that interfere with interdiffusion and autohesion.
Many people have tested and built upon this important foundation work. There’s a plethora of information in academia and industry that helps us all to understand how to achieve maximum component adhesion, and – importantly – avoid the things that impede it. But still, those of us who are fortunate enough to be able to regularly delve into the deep dark depths of failed truck and bus tires are often amazed by what we find.
There’s no doubt that every manufacturer tries very hard to ensure that every tire they produce remains fully intact, at least until the end of its original tread life – and ideally through subsequent retreads. And the very, very great majority of tires do. But even a very small percentage of a very large number is significant. Fortunately, most tire failures are of limited consequence, but equally, every one is a potential catastrophe.
Manufacturers with relatively high cost bases continue to find ways to compete with those with a cost advantage. And that is totally understandable. The question then becomes, ‘How can we continue to ensure component adhesion is maximized while minimizing cost and scrap, and maximizing productivity?’.
Every manufacturer specifies what they believe to be acceptable limits on every relevant manufacturing parameter – raw material properties, compounding, scorch safety in milling, extrusion, calendaring, and green component storage; bloom avoidance; ply, belt and tread centrality; stitcher pressures, and so on.
In most plants the efforts to minimize the risk of contamination throughout the process are exhaustive. And yet despite all of this knowledge, and all of these efforts, I don’t think there’s one manufacturer who could, hand on heart, swear that not a single tire with less-than-optimal adhesion has ever been released from their warehouse. A controversial call? Perhaps. But the evidence in the market is undeniable.
The science of statistics can help us to understand the challenge. Given the number and diversity of things that can impede maximum component adhesion, in order to be 100% co
nfident that 100% of production meets the minimum required standard it would of course be necessary to destructively test 100% of the population – clearly an absurdity!
So how confident do we need to be? 99%? From a production run population of 1,000 tires you’ll need to test 14. Even a 95% confidence interval will require you to test eight tires. And if you test just two tires, the accuracy of your result can vary from the true result by 14%. It really is an uphill, expensive battle!
But the difficulty and apparent futility do not diminish the necessity. They simply serve to underscore the vital importance of controlling as closely as possible every minute detail that might in some way reduce, even marginally, the number of those long-chain molecules from migrating across the surface of adjoining green components in the minutes immediately following tire assembly. Lives might very well depend on it.
David Southwell is an independent consultant to the tire industry. He has more than 30 years’ experience working with automotive OEMs in design, development, production, quality, technical training, field performance and failure analysis roles with various tire manufacturers and retreaders
in the Asia-Pacific region. Southwell’s expertise spans passenger car, light truck and truck and bus tires, and he holds a master’s degree in Engineering.
David Southwell, independent tire consultant and failure analyst from Australia
