we use rubber compound and we have problem with "Ozone cracking" what is it ? do you know about?

thanks friends..

Most of the above mentioned rubber goods are manufactured using elastomers containing olefinic double bonds that
are sensitive to ozone. C = C bond cracking



Under strain cracks grow at rightangles to the direction of strain. Dynamic operation accelerates reactio

at the wiki;

Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking. The problem was formerly very common, especially in tires, but is now rarely seen in those products owing to preventive measures.

However, it does occur in many other safety-critical items such as fuel lines and rubber seals, such as gaskets and O-rings, where ozone attack is considered unlikely. Only a trace amount of the gas is needed to initiate cracking, and so these items can also succumb to the problem.





Susceptible elastomers

Tiny traces of ozone in the air will attack double bonds in rubber chains, with natural rubber, polybutadiene, styrene-butadiene rubber and nitrile rubber being most sensitive to degradation. Every repeat unit in the first three materials has a double bond, so every unit can be degraded by ozone. Nitrile rubber is a copolymer of butadiene and acrylonitrile units, but the proportion of acrylonitrile is usually lower than butadiene, so attack occurs. Butyl rubber is more resistant but still has a small number of double bonds in its chains, so attack is possible. Exposed surfaces are attacked first, the density of cracks varying with ozone gas concentration. The higher the concentration, the greater the number of cracks formed.

Ozone-resistant elastomers include EPDM, fluoroelastomers like Viton and polychloroprene rubbers like Neoprene. Attack is less likely because double bonds form a very small proportion of the chains, and with the latter, the chlorination reduces the electron density in the double bonds, therefore lowering their propensity to react with ozone. Silicone rubber, Hypalon and polyurethanes are also ozone-resistant.

Ozone cracks form in products under tension, but the critical strain is very small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are very dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. Seals are also susceptible to attack, such as diaphragm seals in air lines. Such seals are often critical for the operation of pneumatic controls, and if a crack penetrates the seal, all functions of the system can be lost. Nitrile rubber seals are commonly used in pneumatic systems because of its oil resistance. However, if ozone gas is present, cracking will occur in the seals unless preventative measures are taken.environmental scanning electron microscope image of ozone cracks in NBR diaphragm seal formed at sharp corners in seal

Ozone attack will occur at the most sensitive zones in a seal, especially sharp corners where the strain is greatest when the seal is flexing in use. The corners represent stress concentrations, so the tension is at a maximum when the diaphragm of the seal is bent under air pressure.
Close-up of ozone crack (using ESEM) in NBR diaphragm seal

The seal shown at left failed from traces of ozone at circa 1 ppm, and once cracking had started, it continued as long as the gas was present. This particular failure led to loss of production on a semi-conductor fabrication line. The problem was solved by adding effective filters in the air line and by modifying the design to eliminate the very sharp corners. An ozone-resistant elastomer such as Viton was also considered as a replacement for the Nitrile rubber. The pictures were taken using ESEM for maximum resolution.

Ozonolysis

The reaction occurring between double bonds and ozone is known as ozonolysis when one molecule of the gas reacts with the double bond:
A generalized scheme of ozonolysis

The immediate result is formation of an ozonide, which then decomposes rapidly so that the double bond is cleaved. This is the critical step in chain breakage when polymers are attacked. The strength of polymers depends on the chain molecular weight or degree of polymerization, the higher the chain length, the greater the mechanical strength (such as tensile strength). By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows steadily until it completes a circuit and the product separates or fails. In the case of a seal or a tube, failure occurs when the wall of the device is penetrated.
EDX spectrum of crack surface
EDX spectrum of unaffected rubber surface

The carbonyl end groups which are formed are usually aldehydes or ketones, which can oxidise further to carboxylic acids. The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected using Energy-dispersive X-ray spectroscopy in the environmental SEM, or ESEM. The spectrum at left shows the high oxygen peak compared with a constant sulfur peak. The spectrum at right shows the unaffected elastomer surface spectrum, with a relatively low oxygen peak compared with the sulfur peak.




Prevention

The problem can be prevented by adding antiozonants to the rubber before vulcanization. Ozone cracks were commonly seen in automobile tire sidewalls, but are now seen rarely thanks to the use of these additives. A common and low cost antiozonant is a wax which bleeds to the surface and forms a protective layer, but other specialist chemicals are also widely used.

On the other hand, the problem does recur in unprotected products such as rubber tubing and seals, where ozone attack is thought to be impossible. Unfortunately, traces of ozone can turn up in the most unexpected situations. Using ozone-resistant rubbers is another way of inhibiting cracking.

For high value equipment where loss of function can cause serious problems, low cost seals may be replaced at frequent intervals so as to preclude failure.

Ozone gas is produced during electric discharge by sparking or corona discharge for example. Static electricity can build up within machines like compressors with moving parts constructed from insulating materials. If those compressors feed pressurised air into a closed pneumatic system, then all seals in the system may be at risk from ozone cracking.

Ozone is also produced by the action of sunlight on volatile organic liquids or VOLs, such as gasoline vapour present in the air of towns and cities, in a problem known as photochemical smog. The ozone formed can drift many miles before it is destroyed by further reactions.


sum of these you have to use antiozonants to the rubber before vulcanization.

What is Ozone Cracking?

Ozone cracking refers to the process by which ozone gas (O₃) in the atmosphere causes cracks to develop on the surface of rubber materials. This phenomenon is particularly significant in applications where rubber is exposed to outdoor environments or industrial conditions with elevated ozone concentrations. Ozone cracking is a form of environmental degradation that can lead to mechanical failure in rubber components, including cable sheaths, seals, gaskets, and tires.

Mechanism of Ozone Cracking

The process of ozone cracking occurs due to the high reactivity of ozone molecules with the double bonds present in the molecular structure of unsaturated elastomers, such as natural rubber (NR) and styrene-butadiene rubber (SBR). The reaction of ozone with these double bonds results in the scission (breaking) of polymer chains, leading to micro-cracks that gradually grow over time.

The chemical reaction involved in ozone cracking can be described as follows:

R-CH=CH-R' + O₃ → R-CH(O)-CH(O)-R' → R-CHO + R'-CHO

Where:
- R and R' represent the polymer chains on either side of the double bond.
- The initial reaction involves the addition of ozone to the double bond, forming an ozonide intermediate.
- The ozonide then decomposes into two aldehyde or ketone fragments, weakening the polymer structure and resulting in cracking.

Factors Influencing Ozone Cracking

Several factors can influence the rate and severity of ozone cracking in rubber materials:

• Ozone Concentration
  Higher concentrations of ozone in the environment lead to faster degradation of rubber materials.
  
  
• Temperature
  Elevated temperatures can accelerate the reaction between ozone and the rubber's double bonds, increasing the rate of cracking.
  
  
• Stress
  Rubber components under mechanical stress are more susceptible to ozone cracking. This is because stressed regions of the rubber are more likely to have exposed double bonds, making them more vulnerable to ozone attack.
  
  
• Type of Rubber
  Unsaturated rubbers, such as natural rubber (NR), styrene-butadiene rubber (SBR), and nitrile rubber (NBR), are more prone to ozone cracking due to their high content of double bonds. Saturated rubbers, such as ethylene propylene diene monomer (EPDM) and silicone rubber, exhibit greater resistance to ozone degradation.
  
  
• Presence of Antioxidants
  Antioxidants and anti-ozonants can significantly reduce the susceptibility of rubber to ozone cracking by neutralizing ozone or by forming a protective layer on the rubber surface.
  

Effects of Ozone Cracking on Cable Sheaths

Cable sheaths are an essential component of electrical and communication cables, providing insulation, mechanical protection, and environmental resistance. Ozone cracking can have severe consequences for cable performance and longevity:

• Reduced Mechanical Strength
  Cracks in the cable sheath compromise its mechanical integrity, making the cable more susceptible to physical damage.
  
  
• Increased Moisture Ingress
  Cracked cable sheaths allow moisture to penetrate into the cable, which can lead to corrosion of metallic conductors and degradation of insulating materials.
  
  
• Electrical Failures
  Moisture ingress and mechanical damage caused by ozone cracking can result in short circuits, electrical leakage, and overall cable failure.
  
  
• Decreased Service Life
  Cables exposed to ozone-rich environments without proper protection may experience a significantly reduced service life, leading to higher maintenance and replacement costs.
  

Applications Where Ozone Cracking is a Concern

Ozone cracking is a critical concern in several applications where rubber components, including cable sheaths, are exposed to ozone-rich environments. These include:

• Outdoor Power Cables
  High-voltage power cables used in outdoor installations are often exposed to sunlight and atmospheric ozone. Without proper protection, the rubber sheaths of these cables can degrade, leading to electrical failures.
  
  
• Automotive Wiring
  In automotive applications, rubber-insulated cables are exposed to high temperatures, mechanical stress, and ozone generated by engine components. Ozone-resistant materials are essential to ensure the reliability of these cables.
  
  
• Industrial Equipment
  Cables used in industrial environments, such as chemical plants and refineries, are exposed to high ozone concentrations generated by industrial processes. The use of ozone-resistant rubber materials is crucial to prevent premature cable failure.
  

Preventive Measures Against Ozone Cracking

Several strategies can be employed to prevent or mitigate ozone cracking in rubber components used in cable sheaths:

• Use of Ozone-Resistant Materials
  Selecting rubbers with high ozone resistance, such as EPDM, silicone rubber, and fluorinated rubbers, can significantly reduce the risk of ozone cracking.
  
  
• Incorporation of Anti-Ozonants
  Adding anti-ozonants to rubber compounds can help neutralize ozone and form a protective barrier on the rubber surface, preventing ozone attack.
  
  
• Surface Coatings
  Applying protective coatings to rubber components can create a physical barrier against ozone, reducing the likelihood of cracking.
  
  
• Proper Design and Installation
  Designing cables with minimal stress concentrations and ensuring proper installation can reduce the risk of ozone cracking by minimizing exposed double bonds.
  

Testing for Ozone Resistance in Cables

To ensure the durability of rubber materials in ozone-rich environments, various testing methods are used to evaluate ozone resistance:

• ASTM D1149 - Standard Test Method for Rubber Deterioration—Cracking in an Ozone Controlled Environment
  This test involves exposing rubber samples to a controlled ozone environment at a specified concentration and temperature. The samples are examined for the presence and severity of cracks after a predetermined exposure period.
  
  
• ISO 1431-1 - Rubber, Vulcanized or Thermoplastic—Resistance to Ozone Cracking
  This international standard specifies methods for determining the resistance of rubber to ozone-induced cracking under static or dynamic strain conditions.
  
  
• IEC 60811-403 - Common Test Methods for Insulating and Sheathing Materials of Electric and Optical Cables—Ozone Resistance Test
  This test method is specifically designed for evaluating the ozone resistance of cable sheathing materials. It involves exposing the cable sheath to ozone under controlled conditions and inspecting it for cracks or other signs of degradation.
  

Chemical Additives to Improve Ozone Resistance

Several chemical additives can be incorporated into rubber compounds to enhance their ozone resistance:

• Anti-Ozonants
  Anti-ozonants are chemicals that react with ozone before it can attack the rubber's double bonds. Common anti-ozonants include waxes and chemical inhibitors.
  
  
• Antioxidants
  Antioxidants help prevent the oxidative degradation of rubber, which can weaken the material and make it more susceptible to ozone cracking.
  
  
• Fillers
  Certain fillers, such as carbon black, can improve the ozone resistance of rubber by reinforcing the polymer matrix and providing a physical barrier to ozone penetration.
  

Conclusion

Ozone cracking is a significant challenge in the design and maintenance of cables, particularly in outdoor and industrial environments where ozone exposure is high. Understanding the mechanisms of ozone cracking and implementing appropriate preventive measures can help ensure the long-term reliability and safety of cables. By selecting ozone-resistant materials, incorporating anti-ozonants, and adhering to international testing standards, manufacturers can produce cables that withstand the harshest environmental conditions, reducing maintenance costs and preventing catastrophic failures.