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The Combustion Basics of High Performance Tire Sealants

The following paragraphs will address the question, “Do tire sealants cause chamber fires?” The belief that the answer to this question is “yes” has a significant following in the retreading industry. We will see that the science of combustion does not support this position regarding high performance tire sealants (HPTS). Some individuals, even in light of the inviolable science which will be presented, maintain the position of “yeah, but I still think that they are involved!”

There is no magic involved with combustion, and ALL combustion is subject to the chemical properties and principles of the components involved.

The majority of HPTS in use today are formulated with a liquid base of approximately half water and half either ethylene glycol (EG) or propylene glycol (PG). This liquid base is essentially the same as common antifreeze. This mixture is thickened with a very small amount of biosynthesized polymer gum. Suspended in the thickened liquid are synthetic fibers which are commonly of the aramid family. A well known example of this type of fiber is DuPont Kevlar, but many other aramids exist in the market. Many HPTS also contain rubber particles, inorganic, particulate fillers and inorganic corrosion inhibitor salts. The combustion characteristics of all components will be addressed.

It will be assumed that casings are not properly cleaned prior to the retreading process, although it would be impossible to properly inspect and repair casings with large amounts of residual sealant in them. Glycol is the component present in the largest quantity and is the most likely suspect. The most common combustion property quoted in a material safety data sheet (MSDS) is the flash point. The flash point is NOT the temperature at which a material bursts into flame! The flash point is the lowest temperature that a material must attain for its vapors to ignite in very close proximity to an open flame. The important issue with flash point is that there must be an open flame ignition source. There are numerous flash point determination procedures with the “closed cup” techniques such as the Pensky Martin Closed Cup (PMCC) giving the lowest, thus most conservative values. The PMCC values for EG and PG are 240.8 deg. F. and 228.2 deg. F., respectively. This means that for either of the glycols to burn at the temperatures encountered in curing chambers, there would have to be an ignition source within millimeters of the glycol. It must be noted that these flash points are the values for pure glycol. The flash point of a 50% water solution of glycol is quoted as, “none due to boiling”.

The less quoted, but more relevant property of glycols is the auto-ignition temperature or kindling point. This is the minimum temperature that a material must attain for combustion to occur. Nothing burns at temperatures below its auto-ignition temperature! Auto-ignition temperatures for EG and PG are 748.4 deg. F. and 699.8 deg. F., respectively. The conclusion is made from the above information that a glycol/water mixture will not burn even if contacted by flame. If all water is evaporated, the remaining glycol will not burn even in contact with flame under its flash point. An ignition source must be present for any combustion to occur. The ignition source must be at least equal to the auto-ignition temperature. From a practical standpoint, there must be an active fire underway in the chamber to ignite glycol.

The preceding discussion eliminates glycol in the casings as a possible cause of chamber fires, but what if glycol vapors are drawn into the electric heater of the chamber, which could act as an ignition source? Combustible vapors in air can only be ignited if their concentration is between the lower explosive limit (LEL) and the upper explosive limit (UEL). The explosive concentration of PG in air is 2.5% to 12.5% volume. The explosive concentration for EG is 3.2% to 15.3% vol. Any concentration outside of these limits will not burn under any condition. The air in a curing chamber is rapidly circulating and mixing. Therefore, for any of the air to reach an explosive (combustible) concentration of glycol, all of the air would have to be at the same concentration. If PG is the glycol present (lowest LEL), the entire chamber would need to have at least 2.5% volume PG vapors to ignite in the electric overhead heater. Assuming that the chamber is approximately 6 ft. diameter and 20 ft. long, its volume is 770 cu. ft. The ideal gas laws determine that it would take 5.6 lbs. of PG vapor to yield a 2.5% vol. concentration in the chamber. This equates to about 5 quarts of HPTS! Remember that this concentration is required for combustion under any circumstance. For every atmosphere of pressure in the chamber, the amount of PG vapor increases proportionally—in other words at 80 psig, the amount of HPTS required to achieve LEL would be over 6 gallons. This assumes that there is total evaporation of the sealant at temperatures several hundred degrees lower than the boiling point of PG at 80 psig. If all of these conditions were met and PG in the chamber environment ignited, it would create a “bomb of great proportion”—not a localized fire in one or two casings.

Now, on to the solid components. The fillers and corrosion inhibitor salts can be eliminated because they are non-flammable. This leaves the thickener, the rubber particles, and the fibers. The thickener is in the HPTS at less than 0.5% wt. It is a non-volatile high molecular weight polymer with essentially no vapor pressure, which coupled with its extremely low concentration, makes its participation in combustion unlikely, if not impossible. The rubber particles are ground tread rubber which is sorted by particle size. These particles have the same composition as the tires in the curing chamber and therefore, the same tendency to “start fires”. The fibers are the only remaining component. They represent less than 10% wt. in the HPTS formulation. Flame tests have been conducted on dried fiber residue collected from improperly cleaned casings. It was observed that the fiber residue would not sustain flame. This would eliminate the possibility that solid residue be drawn into the overhead heater and be expelled as a flaming object, which could ignite materials in the chamber. The only component in the residue that could burn under propane torch conditions was the particulate rubber, which produced a miniscule flame lasting 2 to 3 seconds –insufficient time for contact with materials in the chamber.

The questions of whole sealant residue in casings, flammable glycol vapor concentration in the chamber, and fire transfer via burning dry HPTS residue emanating from the overhead heater have been addressed on a scientific level. Based on well known and understood flammability characteristics of HPTS components and empirical data (in light of the conditions encountered in a curing chamber), the participation of HPTS in chamber fires is extremely unlikely, if not impossible. The HPTS industry wants to find the answer as much if not more than the retreading industry!

 

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Retreading Tires That Have Been Treated With Tire Sealant

Questions have arisen about the relationship between sealant usage and chamber fires in tire retread plants. The following discussion will review the operating environment in modern retread plants and the chemistry of combustion as related to sealants and chambers operations. It will show that it is practically and chemically impossible for a sealant to cause fires in a chamber.

The scope of this paper is limited to modern, water / glycol-based sealants. It is not intended to address other types of sealants or sealant / inflators. A modern sealant is defined as one in which the active components are suspended in a thickened mixture of water and glycol. Two glycols are commonly used: Ethylene Glycol (EG) and Propylene Glycol (PG).

BACKGROUND

Occasionally, statements are made to the effect that “sealants cause chamber fires.” This is something that MULTI SEAL does not wish to ignore or take lightly. Due to the importance of the issue, a somewhat detailed explanation of the retread process and combustion chemistry is necessary. It is important to note that while details may vary from process to process, the fundamental procedures in all retread systems are similar.

RETREAD PROCESS

The first step in the process is cleaning of both the inner and outer surfaces of the tire. Washing and vacuuming are the most common methods used. After washing, the tire should be visually inspected for defects such as sidewall tears, inner liner delamination, tread separation, and so forth.

Following the visual inspection, a check for punctures should be made. A few methods, all of them nondestructive, are available. However, the Hawkinson NDT is probably the most common. In this method, electrically charged chains and wires are dragged along the inner surface of the rotating tire. A puncture will allow a spark to pass from the chains to ground, producing both light and sound to alert the tire technician. If sealant or other conductive solution is present, the charge will be dissipated and there will not be a spark. In at least one process, puncture inspections are augmented with x-ray and laser analysis to find hidden signs of damage.

After defects and punctures have been identified and marked, the tire is mounted vertically on a wheel and slowly rotated against a high speed grinding wheel. This removes remaining tread 2 and brings the wheel down to a precise diameter. If the sealant remains in the tire, the sealant will create a mess by dripping onto the mounting wheel and floor when the tire is removed.

Any punctures or other defects in the tire (now called a casing) are repaired. The type of repair will vary depending on the defect. If it is small enough (3/8 inch or less), it can be repaired with a rubber plug. The hole must be reamed out prior to plug insertion. If the casing is to be patched or sectioned, the inner surface around the hole must be cleaned and buffed. The damage can also be “buzzed out” or “skived” followed by the appropriate filing. In all of the repair procedures, the sealant must be removed from the casing in order to make the repair. Thus, no sealant will be present in either the casing nor the hole.

Following repairs, the casing is again mounted vertically on a wheel and slowly rotated. A thin layer of heat-softened rubber is applied over the buffed surface remaining from tread removal/sizing. Once again, sealant remaining in the tire will create a mess by dripping onto the wheel and floor when the tire is removed.

In some systems, the casing with the rubber layer attached is mounted on another wheel and slowly rotated as the new tread is applied on top of the rubber layer. The tread is cut at a precise length and the space between the ends are filled with rubber and stapled together. As before, sealant remaining in the tire will create a mess by dripping onto the wheel and floor when the tire is removed.

The carcass is then placed inside a rubber covering called an envelope. In the Bandag and Goodyear systems, rings are placed over the tire bead to trap and seal the edge of the envelope between the ring and the bead. The Michelin system uses a second envelope placed inside the carcass. In all cases, a vacuum is pulled between the tread and the envelope(s). This pushes the tread firmly onto the carcass.

The carcass/envelope assembly is placed into the chamber where the vacuum is maintained while pressure in the chamber is raised to approximately 85 psig and the temperature is raised to 250o F-260o F. At this point the vacuum is turned off and the assembly is held at pressure and temperature for around 45 minutes. Note: Some systems use cure temperatures near or slightly below 200o F. At this temperature, cure times increase to as much as 3 hours. After cooling slightly, the retreaded tires are removed for final inspection.

As explained above, depending on the system, there are 4 to 5 steps during which any sealant left in the carcass will cause problems with expensive equipment and sensitive operations. It is unlikely that much, if any sealant could make it to the chamber.

Nevertheless, for purposes of this discussion, we will assume that a tire makes it all the way to the chamber with a full load of sealant. Additionally, we will assume that the full load of sealant is 52 fluid ounces. Now lets take a look at what would be required (by irrefutable laws of chemistry and physics!) for sealant to cause a chamber fire.

COMBUSTION CHEMISTRY

A review of basic combustion chemistry, the chamber and chamber operating conditions will be helpful here.

In order for combustion to take place, two things must be present: Oxygen and fuel. Additionally, the fuel must be in the form of a vapor. Believe it or not, liquid gasoline will not burn! If we could see a cross section of a cup of burning gasoline, we would see that there is a layer of vapor on the surface of the liquid. That vapor is the liquid gasoline that has evaporated. Likewise, for sealant to burn, the glycol must evaporate. Furthermore, it must do it quickly. In order to evaporate quickly enough, the glycol must approach it’s boiling point.

The boiling point of EG at atmospheric pressure, is 388.4o F and for PG it is 372.2o F. It would, of course, be significantly higher at 75-85 psig. Based on readily available boiling point data for water at pressure, we can project that the boiling point of the glycols should be around 580o F at 80 psig. But remember, even at atmospheric pressure, the temperature in a chamber is not high enough to boil either one of the glycols. In fact, at normal operating pressure, it is not hot enough in a chamber to even boil the water! At 85 psig, the boiling point of water is 328o F…63o F higher than the highest temperature in the chamber. But, even if the temperature in the chamber were high enough to boil the water, all the water would have to boil away before the temperature of the glycol could start going up to to it’s boiling point.

There are two other temperatures to consider. They are the flash point and the auto ignition temperature. The flash point is the temperature at which something will burn if an ignition source is present. In other words, it is the temperature to which something (in this case, glycol) must be heated in order to burn if it was touched with a flame. The flash point of propylene glycol is 228.2o F, which is below the temperature in a chamber. However, there is no source of ignition.

The auto ignition temperature is the temperature at which the proper mixture of glycol vapor and air will ignite without a separate source of ignition. For ethylene glycol it is 748.4o F and for propylene glycol it is 699.8o F …almost 3 times the maximum temperature in the chamber.

Finally, both the oxygen and the fuel must be mixed in a somewhat narrow proportion. Think of the air/fuel ratio in an automobile engine. Too much fuel and not enough air…no burn! Too much air and not enough fuel…no burn!

In the double envelope (Michelin) system, as well as older rim/flange/bladder system used on bias ply tires, the inner and outer surfaces of the tire are under a vacuum. No oxygen is present. Thus, even if sealant was present, it could not burn. Once the vacuum is turned off, sufficient air would have to seep into the envelope to support combustion. In the Bandag/Goodyear ring system, the inner surface of the carcass is exposed to 85 psig air…oxygen is present. Nevertheless, for reasons discussed below, sealant would not burn unless large volumes were present and large volumes of the glycol evaporated.

As mentioned above, in order for combustion to take place, it is necessary to have a high temperature, have the glycol in a vapor state and have the right mixture of vapor and air. The range of this mixture is called the explosive limit and it is listed as the “lel” (lower explosive limit) and the “uel” (upper explosive limit). Below the lel there is insufficient vapor for ignition. Above the uel there is insufficient oxygen for ignition. We will discuss the lel. For reasons that will become obvious, we will not need to discuss the uel.

The lel for propylene glycol is 2.6. This means that the vapor must reach a concentration in the air inside the chamber of at least 2.6%. The volume of a chamber in a modern retread plant is approximately 1,130 cubic feet. When compressed to 85 psig, this volume of air weighs about 91 pounds. At 2.6%, we would need 13.73 pounds of propylene glycol or 29.5 lbs. of sealant.

To put it another way, almost 9 tires, each containing 52 ounces of sealant would have to be in the chamber and all the sealant would have to evaporate to total dryness. The circulating fan insures that the air/glycol vapor ratio in the chamber will be uniform. As mentioned above, the percentage of glycol vapor must reach 2.6%. If the circulating fan blows this air over the actual heating coils there would be a source of ignition. But, that is quite a few “if’s.”

SUMMARY

There is no doubt that chamber fires can and do exist, and MULTI SEAL does not imply otherwise. We are not in a position to say what causes chamber fires. We do not know all the things that can get onto or into a tire. Nevertheless, we do know glycol chemistry and we can say what it does not do. Operating conditions of the chamber and the chemical properties of glycol based sealants combine to make combustion caused by the sealant highly unlikely, if not outright impossible. It is not practical to believe that housekeeping would be so poor and technicians so negligent that enough sealant could be left in enough tires to reach the critical percentage of glycol in the air. It just doesn’t make good sense.

Nevertheless, this does not mean that it is acceptable for a recapper to leave residual sealant in the tires. Proper removal is mandatory. Correct procedures and equipment maintenance demand clean tires, free of all foreign material in the casing and any punctures that have been sealed.

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Are All Tire Sealants Created Equal?

If you have ever been stranded by a flat tire on your ATV ten miles from your truck, or a flat on your combine in the middle of harvest, or a flat on your backhoe at just the wrong time, then you know the importance of a good tire sealant. The following paragraphs will explain some of the critical characteristics of a modern, high performance tire sealant which is sure to help you avoid wasting your hard-earned money on a sealant that doesn’t work, and to avoid the horror stories associated with inopportune flats.

But, before we get into what makes a good sealant good, let’s look into how a modern, preventative maintenance sealant works. The key word is “preventative.” The sealant should be installed into the tire as early in its life as possible so punctures can be sealed at the instant that they occur. A modern tire sealant is a suspension of synthetic fibers and fillers in a thickened ethylene or propylene glycol (anti-freeze) and water “carrier system” which allows the fibers and fillers to move within the tire in liquid form.

When a puncture occurs (almost always on the bottom of the tire where the sealant pools when rotating at low and moderate speeds), the air pressure in the tire propels the suspended fibers and fillers into the puncture where the fibers begin to snag on the rubber. In very short order (1 to 3 seconds) the fibers form a tangle in the puncture which stops the escaping air. We’re not done yet! As the tire rotates, the rubber surrounding the wound flexes and packs more fibers into the wound with each rotation until no more fibers will fit. This is where the fillers come into play. The fillers are extremely small 2 particles which pack in-between the fibers in the plug like the mud in a beaver dam to form a permanent airtight seal that will last the life of the tire.

Now that we know how a sealant works, we can consider the performance features that make a good sealant good. The following performance characteristics should be considered when selecting a sealant for your critical equipment. While each of these features might not be specifically mentioned on the package or in the literature, all reputable sealant manufacturers will include a phone number on their package and would welcome questions from their performance-minded customers.

Fibers

Fibers constitute the backbone of the tire sealant. The stronger the fibers, the stronger the tire repair. The strongest fibers are state-of-the-art, synthetic fibers such as those used in tire cord or bulletproof vests. Using a variety of fibers is also very important, especially in the relatively thin carcass of an ATV, golf cart, or lawn implement tire. The more varied the fibers are in both length and in degree of branching, the faster they seal and the more permanent the seal is.

Fillers

Once a strong plug is formed by the fibers, it needs the final seal provided by filler particles, which lodge between the fibers and form an effective, airtight plug. The use of these fillers is imperative for a high performance sealant.

Suspension Stability

The only purpose for the liquid portion of the sealant formulation is to carry the fibers and fillers to the site of the puncture in your pneumatic tire. If the sealant separates in the bottle, it will separate in the tire, and if it separates in the tire it will not act as a sealant! Beware of sealants that show signs of separation in their container, and surely stay away from products that suggest shaking or mixing before using—they are a failure waiting to happen! Beware also of sealants that claim a limited “effective life”…. a high performance sealant should last for the life of the tire—and then some.

Freeze Protection

Not everyone uses their equipment at minus 30 degrees, but most sealant users in this country experience below freezing temperatures at least a few times per year. Be sure that your sealant is freeze protected to prevent the possibility of a horrendous vibration caused by a big lump of ice in your equipment’s tires, not to mention the fact that an “ice cube” will not seal a puncture! Top quality sealants usually provide protection down to minus 20 or 30 degrees.

Inertness

Inertness of a sealant means that the sealant does not react chemically with any part of the tire/wheel assembly. This is critical to maintain the manufacturers’ warranty on both 4 wheels and tires. Any sealant that works by “curing” or “reacting with air” should be avoided like the plague. Any reaction is too much reaction.

Corrosion Protection

Most (if not all) tire sealants contain water, and we all know what water does to metal after prolonged contact—rust on steel and corrosion on aluminum. Corrosion inhibitors are used widely in industry to prevent corrosion and rust in water cooling systems—this same technology works in sealants as well. Be sure that your sealant is effectively rust and corrosion inhibited before you put it into those wheels that could cost hundreds to replace!

While the most dramatic benefit of a high performance tire sealant is the prevention of flats from large punctures, the highest economic impact, especially for the over-the-road user, is the prevention of under-inflation from slow leaks. Undetected under-inflation drastically reduces the life of a tire and is the main cause of catastrophic failures (blowouts) in over-the-road tires and “bead roll-offs” in off-road tires. The financial savings associated with the use of high performance sealants can be over-shadowed by the safety benefits!

Sealants are not sealants are not sealants!! The old adage holds—Only a wealthy man can afford cheap tools (sealant). Check before you buy!