You know those toothbrushes with the blue streak that slowly disappears with use … And when the streak is half gone, time to get a new brush!
a couple of questions arise…
How does the blue stuff work? Is it just a slow process started from the first time it gets wet? (meaning that as long as safe and dry in the box, streak remains, but if gets wet once (even if never used again …) it’ll be half gone in 3-4 months…?)
How was it determined that ‘when it is halfway gone’ is a good system? Instinctively, I feel that “when the blue is all gone” is better or “when the blue turns red” even better?
I have assumed that the bristles of the brush are made of a type of nylon/plastic/whatever that tends to turn white when exposed to stress. I think about pieces of plastic - some ball-point pen caps when I bend the pocket clip, for example - that turn white before cracking or breaking. If the brush bristles are made of similar material, then they are made with blue pigment to provide a starting point, and over time - since they are not bent in half, merely wiggled around during the act of brushing - they would be exposed to the same stresses and would turn white. If this hypothesis is true, then water has nothing to do with it. A way to experiment with this would be to take a new one of these toothbrushes and bend one of the blue bristles - if it turns white at the bend point, then this hypothesis is likely true.
regarding “half-way gone” - I assume they wanted it to be one thing and easy to remember. If the stress hypothesis holds, then when the bristles are half white, it could mean that they have been exposed to enough stress that they have weakened and are therefore no longer stiff enough to do the job.
I think that you are mostly correct. The reason for half-way is that the upper part of the bristle is exposed to the most strain and will lose its blue more quickly. The very bottom part of the bristle may not be exposed to enough strain to change colors at all.
WAG: There is a thin layer of blue nylon/pigment over the ordinary clear nylon. As you use your brush, it abrades away (from the tip down, naturally, like hairs that have been recently shaved grow back square-tipped and gradually abrade to a more pointed form) until the blue is gone and the clear core remains. Oral-B obviously tested various thicknesses to see how much of the blue coating would be enough to withstand three months of brushing.