Rubber ozone aging is the deterioration of rubber in the atmosphere. Ozone is a significant factor. Ozone aging first occurs on the surface, particularly at stress concentrations or at the interface between compounding agent particles and the rubber. It typically forms a thin film, which then cracks. If used under dynamic conditions, the film is more likely to break down, exposing a fresh surface. This allows ozone aging to progress further, until complete destruction. Unsaturated rubber is least resistant to ozone because ozone readily undergoes electrophilic addition reactions with double bonds in the main chain.
Characteristics of ozone aging
Studies using ozone aging chambers have revealed distinct characteristics of rubber ozone aging. First, it is a surface reaction, with ozone primarily acting on the rubber surface to induce changes. Second, ozone cracking in rubber requires a certain amount of stress or strain. When unstretched, the aged surface forms a grayish-white, hard, and brittle film, which only cracks when stress or strain is present. Third, the direction of ozone cracking is perpendicular to the direction of force applied to the rubber, providing a basis for assessing the extent of ozone aging in rubber.
Factors Affecting Rubber Ozone Aging
Influence of Rubber Type:
(1) Double bond content: The higher the double bond content, the worse the ozone aging resistance;
(2) Characteristics of substituents on the double bond carbon atoms: Electron-withdrawing substituents reduce the reactivity of the double bond and reduce the ozone reactivity; electron-donating substituents increase the electron cloud density, increase the reactivity of the double bond and increase the ozone reactivity. For example, the ozone aging resistance of CR, BR, and NR rubbers is CR>BR>NR.
Reasons for Testing Rubber and cable insulation materials are closely tied to our daily lives, such as tires, hydraulic brake hoses, door and window glass seals, and cable insulation in public transportation. Increased stress and wear can put lives at risk. Therefore, rubber, tire, and cable insulation must meet stringent global industry testing standards, with durability and mechanical resilience being key parameters in assessing product reliability.
In practical applications, environmental factors such as ozone can affect material durability. Therefore, ozone aging testing is a routine part of material development and quality assurance testing for rubber and elastomer products to ensure product quality. ANSEROS ozone aging chambers in Germany can perform ozone aging testing on all types of rubber materials under a variety of test conditions, including temperature, humidity, and mechanical stress. These test results are crucial for the automotive, transportation, and aerospace industries, helping companies predict the service life and reliability of various elastomers.
Rubber Ozone Resistance Test Method
Purpose of Ozone Resistance Test: This test method can be used to determine the ozone resistance of vulcanized rubber and thermoplastic rubber. This test method is based on testing the ozone cracking resistance of rubber materials exposed to air containing a certain concentration of ozone at a specified temperature and without the direct influence of light. The ozone resistance of different rubber materials varies significantly with ozone concentration and humidity.
1. Test Standard: GB T7762-2003 Static Tensile Test of Vulcanized or Thermoplastic Rubber for Ozone Cracking Resistance
2. Test Equipment: CLM-QL-100 Ozone Aging Test Chamber (Small specimens are used here; larger specimens can be used).
3. Specimens: Three specimens are required. Standard strip specimens must be at least 10 mm wide and 2.0 mm thick. 2 mm. The length of the specimen between the two ends of the clamp before pulling must be no less than 40 mm. The standard dumbbell specimen should consist of a 12 mm x 12 mm square at each end and a 5 mm wide, 50 mm long strip in the middle.
4. Test Conditions: Ozone Concentration: Optimum concentration (50 x 5) × 10-8. Note: Ozone concentration can be expressed as ozone partial pressure MPa. 1 × 10-8 is equivalent to an ozone partial pressure of 1.01 MPa. Temperature: The optimal test temperature is 40°C x 2.2 / 3°C. (Other temperatures may be selected depending on the operating environment, for example, 30°C ± 2°C or 23°C ± 2°C, but the results obtained using these temperatures differ from those obtained using 40°C ± 12°C. Relative humidity: ≤ 65% RH. Elongation: Testing is typically performed using one or more of the following elongation ratios: 5% ± 1%, 10% ± 1%, 15% ± 2%, 20% ± 2%, 30% ± 2%, 40% ± 2%, 50% ± 2%, 60% ± 2%, and 80% ± 2%. Post-stretching specimen conditioning: After stretching, specimens should be conditioned in a lightless, essentially ozone-free atmosphere for 48 to 96 hours. The conditioning temperature should be in accordance with GB/T 2941.
5. Test Method: Method A: After conditioning as specified, specimens with a 20% tensile strain are placed in an ozone chamber for 72 hours. Afterwards, inspect the specimens for cracking. Alternatively, select any elongation and exposure time based on the applicable material properties. Method B Use specimens of one or more elongations as specified and adjust them accordingly. When only one elongation is used, 20% elongation should be used. Unless otherwise specified, inspect the specimens after 2 hours, 4 hours, 8 hours, 16 hours, 24 hours, 48 hours, 72 hours, and 96 hours of exposure. If necessary, the exposure time may be extended appropriately, and the time at which cracking begins to appear for the specimens of each elongation is recorded. Method C Use specimens of at least four elongations and adjust them accordingly. Inspect the specimens after 2 hours, 4 hours, 8 hours, 16 hours, 24 hours, 48 hours, 72 hours, and 96 hours of exposure. If necessary, the exposure time may be extended appropriately, and the time at which cracking begins to appear for the specimens of each elongation is recorded. This allows the critical strain to be determined.
Improving the Ozone Resistance of Rubber Products
When ozone comes into contact with rubber products, it first undergoes an addition reaction with active double bonds, forming molecular ozonides.
① Molecular ozonides are very unstable and quickly decompose to form carbonyl compounds.
② Reacting with zwitterions.
③ In most cases, the zwitterions and carbonyl compounds recombine to form isozonides©. Zwitterions can also polymerize to form diperoxides or peroxides. Furthermore, in the presence of reactive solvents such as methanol, zwitterions can react to form methoxy hydroperoxides.
The activation energy for the reaction between ozone and unsaturated rubber is very low, and the reaction proceeds readily. The reaction continues until all double bonds in the rubber are consumed, forming a silvery-white, inelastic film on the rubber surface. As long as no external force causes cracks in this film, the rubber will not ozonate further. Stretching or dynamically deforming ozonated rubber will cause the resulting ozonated film to crack, exposing new rubber surface that will react with ozone again, causing the cracks to grow. Because saturated rubber lacks double bonds, it can react with ozone, but the reaction proceeds slowly and is less likely to crack.
Many researchers have studied the initiation and growth of cracks in unsaturated rubber during ozonation. Based on their experimental data, these researchers have proposed various mechanisms for the initiation and growth of cracks. For example, some believe that cracks form when the broken molecular chains produced by the decomposition of ozonide under stress tend to separate more than to recombine. Crack growth, on the other hand, is related to the ozone concentration and the mobility of the rubber molecular chains. At a given ozone concentration, the greater the molecular mobility, the faster the crack growth. Others believe that the initiation and growth of ozone cracks are related to the physical properties of the thin ozonide layer formed by ozonation of rubber and its differences from the physical properties of the original rubber surface layer.
For example, the ozonation process of rubber is believed to occur through a combination of physical and chemical processes. When rubber comes into contact with ozone, the double bonds on its surface react rapidly with the ozone, mostly forming ozonides, rapidly transforming the originally flexible rubber chains into rigid chains containing numerous ozonide rings. When stress is applied to rubber, it stretches and expands the rubber chains, exposing more double bonds to ozone. This results in more ozonide rings in the rubber chains, making them more brittle. This brittle surface is susceptible to cracking under stress or dynamic stress.
1. Using a combination of raw rubbers;
2. Adopting a suitable vulcanization system;
3. Choosing the right antioxidant and wax combination.
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