Inside tire paint: why bladders stick, what mica does
What inside paint applied to the inner surface of the green tire does during curing; the difference between mica-filled and unfilled types; dilution and dosage errors.
The job of inside paint
During curing the bladder inflates and presses on the inner surface of the green tire. If there is no release film between the two surfaces, they stick to each other at vulcanization temperature. The result: the tire is hard to separate from the bladder, the inner surface tears and bladder life shortens.
Inside paint is applied to this surface before curing. Its job falls under three headings: providing release between the bladder and the tire, helping to carry air trapped inside the tire to the surface, and leaving the inner surface smooth and glossy.
Mica-filled and unfilled types
Traditional inside paints contain mica. Mica gives slip and strengthens release; however, it leaves a dusty residue on the inner surface. This residue can cause problems in inner liner bonding or in later repair operations.
Low-filler types without mica leave a thinner film and keep the inner surface cleaner. Release strength may not be as high as with mica; for this reason dose and drying are managed more carefully. The choice depends on the tire type and on what is done to the inner surface afterwards.
Dilution, dose and application
Some inside paints are ready to use and some are diluted with water; the ratio is in the technical data sheet. Too much paint produces residue and dusting on the inner surface; too little produces bladder sticking. Both show up with the same symptom, an inner surface defect, and are confused.
Application is usually by spray or rotating brush. In spraying, nozzle diameter and pressure determine film thickness; with a brush, brush speed does. If the tire is inflated before the film dries, the paint accumulates on the inner surface and forms local thick areas.
Four common errors
One: residue of a different product in the same tank — the film flakes off. Two: hard process water — the emulsion breaks and release drops. Three: overdose — dust on the inner surface and sticking problems in later operations. Four: insufficient drying — water evaporating during curing leaves steam marks and stains on the inner surface.
None of these four errors is solved by changing the product; preparation and application are checked first.