Rubber and rubber products must resist the deterioration of physical properties with time caused by oxidative and thermal aging. Rubber generally experiences possible deterioration due to many reasons and at different degrees. For example, the deterioration of rubber in a tropical climate with high heat and humidity can occur at a faster rate compared to a temperate climate with cooler temperatures and lower humidity. Two processes are of special importance in the preservation of rubber: oxidation, a chemical deterioration, and crystallization, a molecular restructuring that causes the material to lose its elasticity.
Introduction
Rubber is a naturally forming polymer and its products have many applications in the world today. Common rubber products include gloves, tyres, shoe soles, condoms etc. For most modern applications, rubber commonly undergoes a vulcanization process by treating it with sulfur and heat, to harden it while keeping its elasticity.
Rubber and rubber products must resist the deterioration of physical properties with time caused by oxidative and thermal aging. Rubber generally experiences possible deterioration due to many reasons and at different degrees. For example, the deterioration of rubber in a tropical climate with high heat and humidity can occur at a faster rate compared to a temperate climate with cooler temperatures and lower humidity. Two processes are of special importance in the preservation of rubber: oxidation, a chemical deterioration, and crystallization, a molecular restructuring that causes the material to lose its elasticity.
Fig 1.1: Degradation of a rubber tyre for cars.
ASTM D573 – Standard Test Method for Rubber-Deterioration in an Air Oven
This test method covers the procedure to determine the influence of elevated temperatures of vulcanized rubber. Rubber products must resist the deterioration of physical properties with time caused by oxidative and thermal aging. This test method provides a way to assess these performance characteristics of rubber under certain accelerated conditions specified.
In this test method, specimens of vulcanized rubber undergo accelerated ageing under specified elevated temperatures for a specific period. After which, the physical properties are compared between the aged and the original specimen.
Types of ovens according to ASTM E145 Standard Specification for Gravity Convection and Mechanical (Forced-ventilation) Ovens
For ASTM D573, it states that Type IIB ovens specified according to ASTM E145 are suitable for use through 70°C. For higher temperatures Type IIA ovens are required. Forced ventilation ovens are used as they have better temperature uniformity and lower temperature deviation.
Type IA – Gravity Convection oven with uniformity of temperature within ±2% of the differential between oven and ambient temperatures
Type IB – Gravity Convection oven with uniformity of temperature within ±5% of the differential between oven and ambient temperatures
Type IIA – Mechanical Convection oven with uniformity of temperature within ±1% of the differential between oven and ambient temperatures
Type IIB – Mechanical Convection oven with uniformity of temperature within ±2.5% of the differential between oven and ambient temperatures
The sample size shall be sufficient to allow for the determination of the original properties on three specimens and also on three or more specimens for each exposure period of the test. There must also be a time period of at least 24 hours between the completion of the vulcanization of the samples and the start of the ageing test. As far as possible simultaneous aging of a mixed group of different compounds should be avoided due to the possibility of migration. For example high-sulfur compounds should not be aged with low-sulfur compounds.
Dumbbell-shaped specimens (Fig 1.2) prepared as described in Test Methods D 412 shall be considered standard. Their form shall be such that no mechanical, chemical, or heat treatment will be required after exposure. If any adjustments (for example, to thickness) are necessary, they should be performed prior to exposure.
Fig 1.2: Dumbbell-shaped specimens undergoing aging test in an oven.
The aging interval starts when the specimens are placed in the oven. Frequently used intervals are 3, 7 and 14 days depending on the rate of deterioration of the material tested. A suitable aging interval should be used such that deterioration will not be so great to prevent determination of the final physical properties.
Upon completion of the aging interval, the specimens should be removed from the oven and allowed to cool to room temperature on a flat surface. The rest period should be not less than 16 hours and not more than 96 hours before the determination of the physical properties.
Physical properties such as tensile strength, ultimate elongation and stress-strain properties of the specimens shall be determined and compared between the original and the aged specimens.
Conclusion
This two test methods described cover the procedures of using forced-convection ovens to perform accelerated aging testing on rubber products or materials. Such test procedures are important to ensure the durability, performance and lifetime of rubber products under various environmental or stress conditions.
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Rubber and rubber products must resist the deterioration of physical properties with time caused by oxidative and thermal aging. Rubber generally experiences possible deterioration due to many reasons and at different degrees. For example, the deterioration of rubber in a tropical climate with high heat and humidity can occur at a faster rate compared to a temperate climate with cooler temperatures and lower humidity. Two processes are of special importance in the preservation of rubber: oxidation, a chemical deterioration, and crystallization, a molecular restructuring that causes the material to lose its elasticity.
Introduction
Rubber is a naturally forming polymer and its products have many applications in the world today. Common rubber products include gloves, tyres, shoe soles, condoms etc. For most modern applications, rubber commonly undergoes a vulcanization process by treating it with sulfur and heat, to harden it while keeping its elasticity.
Rubber and rubber products must resist the deterioration of physical properties with time caused by oxidative and thermal aging. Rubber generally experiences possible deterioration due to many reasons and at different degrees. For example, the deterioration of rubber in a tropical climate with high heat and humidity can occur at a faster rate compared to a temperate climate with cooler temperatures and lower humidity. Two processes are of special importance in the preservation of rubber: oxidation, a chemical deterioration, and crystallization, a molecular restructuring that causes the material to lose its elasticity.
Fig 1.1: Degradation of a rubber tyre for cars.
ASTM D573 – Standard Test Method for Rubber-Deterioration in an Air Oven
This test method covers the procedure to determine the influence of elevated temperatures of vulcanized rubber. Rubber products must resist the deterioration of physical properties with time caused by oxidative and thermal aging. This test method provides a way to assess these performance characteristics of rubber under certain accelerated conditions specified.
In this test method, specimens of vulcanized rubber undergo accelerated ageing under specified elevated temperatures for a specific period. After which, the physical properties are compared between the aged and the original specimen.
Types of ovens according to ASTM E145 Standard Specification for Gravity Convection and Mechanical (Forced-ventilation) Ovens
For ASTM D573, it states that Type IIB ovens specified according to ASTM E145 are suitable for use through 70°C. For higher temperatures Type IIA ovens are required. Forced ventilation ovens are used as they have better temperature uniformity and lower temperature deviation.
Type IA – Gravity Convection oven with uniformity of temperature within ±2% of the differential between oven and ambient temperatures
Type IB – Gravity Convection oven with uniformity of temperature within ±5% of the differential between oven and ambient temperatures
Type IIA – Mechanical Convection oven with uniformity of temperature within ±1% of the differential between oven and ambient temperatures
Type IIB – Mechanical Convection oven with uniformity of temperature within ±2.5% of the differential between oven and ambient temperatures
The sample size shall be sufficient to allow for the determination of the original properties on three specimens and also on three or more specimens for each exposure period of the test. There must also be a time period of at least 24 hours between the completion of the vulcanization of the samples and the start of the ageing test. As far as possible simultaneous aging of a mixed group of different compounds should be avoided due to the possibility of migration. For example high-sulfur compounds should not be aged with low-sulfur compounds.
Dumbbell-shaped specimens (Fig 1.2) prepared as described in Test Methods D 412 shall be considered standard. Their form shall be such that no mechanical, chemical, or heat treatment will be required after exposure. If any adjustments (for example, to thickness) are necessary, they should be performed prior to exposure.
Fig 1.2: Dumbbell-shaped specimens undergoing aging test in an oven.
The aging interval starts when the specimens are placed in the oven. Frequently used intervals are 3, 7 and 14 days depending on the rate of deterioration of the material tested. A suitable aging interval should be used such that deterioration will not be so great to prevent determination of the final physical properties.
Upon completion of the aging interval, the specimens should be removed from the oven and allowed to cool to room temperature on a flat surface. The rest period should be not less than 16 hours and not more than 96 hours before the determination of the physical properties.
Physical properties such as tensile strength, ultimate elongation and stress-strain properties of the specimens shall be determined and compared between the original and the aged specimens.
Conclusion
This two test methods described cover the procedures of using forced-convection ovens to perform accelerated aging testing on rubber products or materials. Such test procedures are important to ensure the durability, performance and lifetime of rubber products under various environmental or stress conditions.
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