People who insist on using the correct terminology for everything will probably not approve of the subtitle I’ve chosen. They would prefer something more proper like, "Achieving Repair Unit Vulcanization." Many of them might admit that both titles send the same message, but they just can’t bring themselves to say the words "patch" and "stick" when discussing tire repair.
In some ways, they are entirely correct. Vulcanization involves cross-linking different rubber compounds to change the physical properties in order to create a bond on a molecular level. The result is that a properly installed repair unit actually becomes part of the tire once the process is complete.
Using duct tape or glue to stick a patch on the inside of the tire may appear to work in the short term, but at some point in time it’s going to fail after the tire is returned to service.
The biggest challenge facing all types of tire repairs is the constant heating, cooling and flexing that occurs during a tire’s operation. When you think about it, an air leak can be a most difficult problem to solve because it cannot always be seen with the naked eye or felt on the skin. Dunk tanks and soapy water generally reveal bubbles at the spot where air escapes, but sometimes the leak doesn’t occur until after the tire is loaded.
If the method for restoring the integrity of the air chamber does not take all of these factors into account, the repair method cannot endure the harsh environment of a pneumatic tire.
Getting The Right Start
By removing the damage, filling the void with a suitable material and vulcanizing a repair unit and rubber stem into place, the tire is, by definition, "restored to the original condition."
There’s nothing temporary about cross-linked rubber compounds that are molecularly bonded. But in order to achieve ultimate repair unit vulcanization, there are a number of conditions that must be met.
The first and probably most important/overlooked step, is to remove any contaminants from the inside of the tire. Many tire manufacturers use silicone mold-release lubricants to keep the bladder from sticking to the innerliner during curing. If these lubricants are not removed, they can be buffed into the rubber, which will result in a significant loss of repair unit adhesion.
Other foreign materials such as bead lube, tire sealant and air tool oil can also lead to repair unit failure. By simply applying an approved rubber cleaner to the inside of the tire and scraping the area to be buffed, technicians can remove potential barriers to the vulcanization process.
Once the inside of the tire has been properly cleaned, it should be texturized in order to create the appropriate adhesion platform for the repair unit. Tubeless tires have an innerliner made of butyl rubber at least 2/32-inch thick that should be buffed smooth to eliminate any mold marks. If the pattern on the innerliner is not completely removed, the potential for a loss in repair unit adhesion increases.
On the other hand, if the technician buffs through the innerliner and reaches the rubber-encased body cords, the tire must be referred to a full-service tire repair facility for a section repair. In the field, attempting to repair exposed body cords by covering them with a repair unit is not advisable simply because even the smallest pocket of trapped air can result in an innerliner separation and eventual tire failure.
Trick For Tube-Types
Preparing the inside of a tube-type tire can be more challenging primarily because the thickness of the "innerliner" rubber is minimal at best. A standard buffing wheel will remove the mold marks, but it often leaves exposed fabric. While section repair materials and heat-cured repair units can vulcanize with the remaining rubber present, a typical in-the-field repair that relies on chemical vulcanization will generally fail to reach sufficient adhesion if too much rubber is removed.
The trick is to make sure the inside of the tire is cleaned and texturized without removing any rubber. A simple addition to the tire repair tool inventory will permanently solve that problem.
A soft wire brush mounted on a low-speed buffer with a tool speed of no more than 5,000 RPM allows technicians to prepare the inside surface of bias tube-type tires. After removing any contaminants with an approved rubber cleaner, the area is "buffed" with the wire brush instead of the standard buffing rasp.
The thin layer of rubber between the innertube and the nylon body cords remains intact and can be effectively cemented with the appropriate vulcanizing cement. Once the cement is dry, the repair unit can be installed and the area covered with tire talc to prevent the cushion gum from sticking to the tube.
The same soft wire brush can also be used to clean the surface of tubeless tires after the injured area has been buffed. Excessive buffing dust will contaminate cement and result in a certain degree of adhesion loss. By starting on the right side of the buffed area and working to the left, the dust can be removed from the surface providing a noticeable difference in appearance. When installing one-piece repair units, small steel shavings and dust can be removed without affecting the texture of the previously buffed surface.
Keep in mind that a wire brush used on buffed surfaces should not be used to clean any other components such as rim parts, wheel studs and fasteners.
Cushion gum on the repair unit and the rubber stem are sensitive to light, heat and contamination. All tire repair materials have a specific shelf life that is directly affected by temperature and humidity. Therefore, store repair units and vulcanizing cement in a cool and dry environment.
Technicians should never touch the cushion gum, or leave it exposed to sunlight or dust for more than a few seconds. Any foreign substance between the cemented surfaces of the tire and the repair unit will result in an adhesion loss.
Finally, the repair unit should be installed after the cement is completely dry and the beads are in a relaxed position. The unit should be stitched from the center to the outer edge, being careful not to trap any air. Air bubbles between the repair unit and the tire will expand as the tire generates heat during operation. Repair units closer to the shoulder will experience "bridging" if they are installed when the beads are spread apart.
It’s also important to remember that nail hole repairs can only be installed in the crown area of the tire. Injuries in the shoulder and sidewall areas require section repairs.
Getting the patch to stick is not rocket science as long as the proper procedures are followed. While the engineering behind chemical vulcanization does represent decades of research and development, technicians only have to take the necessary steps to ensure maximum repair unit adhesion. Strict adherence to the guidelines will create repairs that last through multiple retreads and thousands of service miles.
Failing to follow the rules or "customizing" accepted repair procedures generally results in flat tires and and increase in scrap casings. With the right instruction and support, it shouldn’t be difficult for technicians to achieve repair unit vulcanization.
Kevin Rohlwing is TIA’s director of training, and this article originally appeared in TIA’s Commercial Tire Service publication.